The synergetic effects of electromagnetic and chemical enhancements via the combination of semiconductor nanomaterials with noble metal nanoparticles is crucial to the performance of surface-enhanced Raman scattering (SERS). Here, WO3/TiO2 photonic crystal films in the form of three-dimensional inverse opals were fabricated via the co-assembly of polymer colloidal templates with water-soluble precursors in order to simultaneously grow both constituent metal oxides with tailored electronic properties and photonic band gaps. The surface modification of compositionally tuned WO3/TiO2 inverse opals by Ag nanoparticles is demonstrated to be an efficient method to boost SERS efficiency in the detection of 4−mercaptobenzoic acid via the synergy of plasmonic effects with charge transfer and slow-light trapping.

In this work, silicon nanowires were constructed by metal-assisted chemical etching and decorated with silver nanoparticles and used as substrates for the SERS determination of oxidative stress markers, namely glutathione, malondialdehyde and catalase. The assays were sensitive, with detection limits of 50 and 3.2 nM for glutathione and malondialdehyde, respectively, and 0.5 μg/mL for catalase, indicating the capability of the proposed substrates to be implemented for the determination of various oxidative stress markers.

Nanostructured noble metal surfaces enhance the photoluminescence emitted by fluorescent molecules, permitting the development of highly sensitive fluorescence immunoassays. To this end, surfaces with silicon nanowires decorated with silver nanoparticles in the form of dendrites or aggregates were evaluated as substrates for the immunochemical detection of two ovarian cancer indicators, carbohydrate antigen 125 (CA125) and human epididymis protein 4 (HE4). The substrates were prepared by metal-enhanced chemical etching of silicon wafers to create, in one step, silicon nanowires and silver nanoparticles on top of them. For both analytes, non-competitive immunoassays were developed using pairs of highly specific monoclonal antibodies, one for analyte capture on the substrate and the other for detection. In order to facilitate the identification of the immunocomplexes through a reaction with streptavidin labeled with Rhodamine Red-X, the detection antibodies were biotinylated. An in-house-developed optical set-up was used for photoluminescence signal measurements after assay completion. The detection limits achieved were 2.5 U/mL and 3.12 pM for CA125 and HE4, respectively, with linear dynamic ranges extending up to 500 U/mL for CA125 and up to 500 pM for HE4, covering the concentration ranges of both healthy and ovarian cancer patients. Thus, the proposed method could be implemented for the early diagnosis and/or prognosis and monitoring of ovarian cancer.

The potential distribution in a MEMS capacitor with a thin dielectric film on the bottom electrode and under the presence of field emission leakage current is presented for the first time. The paper also demonstrated the build-up of dielectric charging during this process. The investigation is based on obtaining current-voltage characteristics in clockwise and counter clockwise loops and analyzing the transport mechanisms in MIM capacitors. Same procedure is applied to monitor the dielectric charging build-up during field emission in MEMS capacitors. The data of pristine current-voltage characteristics in both MIM and MEMS are used to determine the Voltage drops across the dielectric film and the gap as well as their dependence on the flowing current.

In this work we demonstrate a two-pixel solid-state photoluminescent device able to emit white light covering the entire visible spectrum from 380nm up to 800nm. The device is based on a combination of porous Si, hydrothermally grown ZnO and carbon quantum dots, in a two-pixel formation, with porous Si and ZnO acting independently while the carbon quantum dots are deposited on top of the entire device. All processing is done using standard Si processing techniques. Moreover, the device design allows for tunability of the emitted spectrum simply by choosing the desired combination of the materials. Overall, the demonstrated device is low cost, environmentally safe and biocompatible.

Noble metal nanostructured substrates enhance photoluminescence emitted from molecules immobilized onto their surface, allowing for the development of highly sensitive immunoassays employing fluorescent labels. In this work, nanostructured silver surfaces were implemented as substrates for the immunochemical detection of two ovarian cancer markers, carbohydrate antigen 125 (CA125) and human epididymis protein 4 (HE4). Biotinylated detection antibodies were used to allow for the detection of immunocomplexes through a reaction with streptavidin conjugated to Rhodamine Red-X. The detection limits achieved were 2.5 U/mL and 0.06 ng/mL for CA125 and HE4, respectively, with linear dynamic ranges, covering the concentration ranges of both healthy and ovarian cancer patients.

Early diagnosis and monitoring are essential for the effective treatment and survival of patients with different types of malignancy. To this end, the accurate and sensitive determination of substances in human biological fluids related to cancer diagnosis and/or prognosis, i.e., cancer biomarkers, is of ultimate importance. Advancements in the field of immunodetection and nanomaterials have enabled the application of new transduction approaches for the sensitive detection of single or multiple cancer biomarkers in biological fluids. Immunosensors based on surface-enhanced Raman spectroscopy (SERS) are examples where the special properties of nanostructured materials and immunoreagents are combined to develop analytical tools that hold promise for point-of-care applications. In this frame, the subject of this review article is to present the advancements made so far regarding the immunochemical determination of cancer biomarkers by SERS. Thus, after a short introduction about the principles of both immunoassays and SERS, an extended presentation of up-to-date works regarding both single and multi-analyte determination of cancer biomarkers is presented. Finally, future perspectives on the field of SERS immunosensors for cancer markers detection are briefly discussed.

Glutathione and malondialdehyde are two compounds commonly used to evaluate the oxidative stress status of an organism. Although their determination is usually performed in blood serum, saliva is gaining ground as the biological fluid of choice for oxidative stress determination at the point of need. For this purpose, surface-enhanced Raman spectroscopy (SERS), which is a highly sensitive method for the detection of biomolecules, could offer additional advantages regarding the analysis of biological fluids at the point of need. In this work, silicon nanowires decorated with silver nanoparticles made by metal-assisted chemical etching were evaluated as substrates for the SERS determination of glutathione and malondialdehyde in water and saliva. In particular, glutathione was determined by monitoring the reduction in the Raman signal obtained from substrates modified with crystal violet upon incubation with aqueous glutathione solutions. On the other hand, malondialdehyde was detected after a reaction with thiobarbituric acid to produce a derivative with a strong Raman signal. The detection limits achieved after optimization of several assay parameters were 50 and 3.2 nM for aqueous solutions of glutathione and malondialdehyde, respectively. In artificial saliva, however, the detection limits were 2.0 and 0.32 μM for glutathione and malondialdehyde, respectively, which are, nonetheless, adequate for the determination of these two markers in saliva.

In this study, we developed active substrates consisting of Ag-decorated silicon nanowires on a Si substrate using a single-step Metal Assisted Chemical Etching (MACE) process, and evaluated their performance in the identification of low concentrations of Rhodamine 6G using surface-enhanced photoluminescence spectroscopy. Different structures with Ag-aggregates as well as Ag-dendrites were fabricated and studied depending on the etching parameters. Moreover, the addition of Au nanoparticles by simple drop-casting on the MACE-treated surfaces can enhance the photoluminescence significantly, and the structures have shown a Limit of Detection of Rhodamine 6G down to 10−12 M for the case of the Ag-dendrites enriched with Au nanoparticles.

Charge injection and transport interlayers play a crucial role in many classes of optoelectronics, including organic and perovskite ones. Here, we demonstrate the beneficial role of carbon nanodots, both pristine and nitrogen-functionalized, as electron transport materials in organic light emitting diodes (OLEDs) and organic solar cells (OSCs). Pristine (referred to as C-dots) and nitrogen-functionalized (referred to as NC-dots) carbon dots are systematically studied regarding their properties by using cyclic voltammetry, Fourier-transform infrared (FTIR) and UV–Vis absorption spectroscopy in order to reveal their energetic alignment and possible interaction with the organic semiconductor’s emissive layer. Atomic force microscopy unravels the ultra-thin nature of the interlayers. They are next applied as interlayers between an Al metal cathode and a conventional green-yellow copolymer—in particular, (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1′,3}-thiadiazole)], F8BT)—used as an emissive layer in fluorescent OLEDs. Electrical measurements indicate that both the C-dot- and NC-dot-based OLED devices present significant improvements in their current and luminescent characteristics, mainly due to a decrease in electron injection barrier. Both C-dots and NC-dots are also used as cathode interfacial layers in OSCs with an inverted architecture. An increase of nearly 10% in power conversion efficiency (PCE) for the devices using the C-dots and NC-dots compared to the reference one is achieved. The application of low-cost solution-processed materials in OLEDs and OSCs may contribute to their wide implementation in large-area applications.

Heterostructured WO3/TiO2 photonic crystal films in the form of three-dimensional macroporous inverse opals were developed by single-step, three-phase co-assembly of colloidal templates with water soluble precursors enabling simultaneous growth of both metal oxides into a firmly interconnected periodic pore framework whose skeletal walls consisted of uniformly distributed nanoscale type II WO3-TiO2 heterojunctions. A marked size- and composition-dependence of the binary WO3/TiO2 inverse opal properties was evidenced by controlling the W/Ti molar ratio and macropore diameter that allowed Fermi level and photonic band gap engineering according to optical and photoelectron spectroscopies supplemented by electrochemical measurements. Compositional tuning along with the reduction of WO3 nanocrystallite size and the concomitant W5+ defect growth with increasing TiO2 phase content were explored for the optimization of photocurrent generation by WO3/TiO2 inverse opal photoanodes combining reduced charge carrier recombination and optimal slow light trapping. One step co-assembly of mixed metal oxides is concluded as a promising route for the development of heterostructured inverse opal networks with tailored electronic properties and improved solar light harvesting for photo-induced applications.

This study presents experimental evidence of field emission in MEMS capacitive switches. Devices with dielectric layers of silicon nitride of different thicknesses between 50 and 200 nm were investigated by current-voltage (I-V) measurements. These measurements were performed at room temperature and under a controlled atmosphere pressure of 3 × 10−2 mbar at bias levels below breakdown and corresponding electric fields encountered in MEMS capacitive switches during pull-in (1-2 × 106 V/cm). Field emission although was not always clearly observed, it occurred in all devices and clearly manifested at electric fields larger than 106 V/cm.

Metal-Insulator-Semiconductor micro-capacitors for on-chip energy storage were fabricated and characterized. The capacitors were based on Si nanowires fabricated by Metal Assisted Chemical Etching. 1.2μm long nanowires with 100nm average diameter were created leading to an effective area increase of 6.28, as compared to a flat surface. Nanowires were chemically treated to reduce surface roughness and electronic states and were coated by a HfO2 layer, deposited by Atomic Layer Deposition, to act as the dielectric. Al and Cu were deposited as two possible top metal electrodes. The use of Al as the top electrode was shown to create a parasitic interface oxide between the metal and the dielectric, reducing the measured capacitance. The use of Cu was shown to significantly reduce this problem, leading to more efficient devices. Capacitors with 5.4μF/cm2 capacitance and 8.9x10-7A/cm2 leakage current at -2.5V were demonstrated along with a cutoff frequency of 104Hz. These values make the demonstrated capacitors very attractive for on-chip energy storage applications.

The impact of ambient on the field emission and the resulting breakdown induced damage of rigid MEMS capacitive structure are investigated. The effect of asperities burning due to Joule heating and the resulting explosive breakdown are presented. The breakdown gives rise to almost mirror craters formation on the cathode and anode electrodes. A linear relation between crater diameter and the breakdown current is found when breakdown occurs in vacuum. In ambient atmosphere the breakdown leads to large amplitude current oscillations and the formation of extended damage on both electrodes.

Αn isotype heterojunction n+-ZnO/n-Si photodetector is developed, demonstrating wavelength-selective or broadband operation, depending on the applied bias voltage. Additionally, at self-powered (zero bias) operation, it distinguishes between UV, visible, and near IR (NIR) photons by polarity control of the photocurrent. The photodetector is developed by atomic layer deposition (ALD) of ZnO on n-Si, followed by electric contact deposition and annealing. Photoluminescence measurements reveal high optical quality and improved crystallinity of annealed ZnO on silicon. Photocurrent measurements as a function of illumination wavelength and bias voltage show small negative values in the UV–visible spectral range at zero and positive bias voltage and high positive values in the NIR spectral range. For these measurements, we consider the electric contact to ZnO as the anode and the electric contact to silicon as the cathode. At negative bias voltage, the device shows broadband operation with high photocurrent values across the UV–vis-NIR.

In this study, we developed highly sensitive substrates for Surface-Enhanced-Raman-Scattering (SERS) spectroscopy, consisting of silicon nanowires (SiNWs) decorated by silver nanostructures using single-step Metal Assisted Chemical Etching (MACE). One-step MACE was performed on p-type Si substrates by immersion in AgNO3/HF aqueous solutions resulting in the formation of SiNWs decorated by either silver aggregates or dendrites. Specifically, dendrites were formed during SiNWs’ growth in the etchant solution, whereas aggregates were grown after the removal of the dendrites from the SiNWs in HNO3 aqueous solution and subsequent re-immersion of the specimens in a AgNO3/HF aqueous solution by adjusting the growth time to achieve the desired density of silver nanostructures. The dendrites had much larger height than the aggregates. R6G was used as analyte to test the SERS activity of the substrates prepared by the two fabrication processes. The silver aggregates showed a considerably lower limit of detection (LOD) for SERS down to a R6G concentration of 10−13 M, and much better uniformity in terms of detection in comparison with the silver dendritic structures. Enhancement factors in the range 105–1010 were calculated, demonstrating very high SERS sensitivities for analytic applications.

Tailoring metal oxide photocatalysts in the form of heterostructured photonic crystals has spurred particular interest as an advanced route to simultaneously improve harnessing of solar light and charge separation relying on the combined effect of light trapping by macroporous periodic structures and compositional materials’ modifications. In this work, surface deposition of FeOx nanoclusters on TiO2 photonic crystals is investigated to explore the interplay of slow-photon amplification, visible light absorption, and charge separation in FeOx–TiO2 photocatalytic films. Photonic bandgap engineered TiO2 inverse opals deposited by the convective evaporation-induced coassembly method were surface modified by successive chemisorption-calcination cycles using Fe(III) acetylacetonate, which allowed the controlled variation of FeOx loading on the photonic films. Low amounts of FeOx nanoclusters on the TiO2 inverse opals resulted in diameter-selective improvements of photocatalytic performance on salicylic acid degradation and photocurrent density under visible light, surpassing similarly modified P25 films. The observed enhancement was related to the combination of optimal light trapping and charge separation induced by the FeOx–TiO2 interfacial coupling. However, an increase of the FeOx loading resulted in severe performance deterioration, particularly prominent under UV-Vis light, attributed to persistent surface recombination via diverse defect d-states. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

We develop ZnO/p-Si photodetectors by ALD deposition of ZnO thin films on laser- microstructured Si, which demonstrate high sensitivity and broadband operation (UV-Vis-NIR), due to increased specific surface area of the heterojunction and increased light absorption.

We report on two-step current-induced effects on the electrical, optical, and structural properties of VO2 films around the Metal–Insulator Transition (MIT) in synergy with ambient temperature (T). Simultaneous electrical resistance and transmittance measurements of VO2 semitransparent thin films as a function of T show that the electric current modifies the MIT that takes place in two steps: an abrupt change that increases upon increasing current, implying the formation of larger metallic domains within the current path, accompanied by a smoother change that follows the temperature change. Resistance measurements of thicker bulk-like VO2 films have been also investigated exhibiting similar two-step behavior. By monitoring the specimen temperature (To) during resistance measurements, we show that the abrupt resistance step, accompanied by instantaneous heating/cooling events, occurs at temperatures lower than TMIT and is attributed to current-induced Joule heating effects. Moreover, by monitoring To during current–voltage measurements, the role of T in the formation of two-step current modified MIT is highlighted. X-ray diffraction with in situ resistance measurements performed for various currents at room temperature as a function of To has shown that the current can cause partially MIT and structural phase transition, leading to an abrupt step of MIT. The formation of a rutile metallic phase of VO2 under high applied currents is clearly demonstrated by micro-Raman measurements. By controlling current in synergy with T below TMIT, the VO2 film can be driven to a two-step current-induced MIT as gradually a larger part of the film is transformed into a rutile metallic phase.. © 2021 Author(s).

VO2 thin films were fabricated by thermal evaporation of V2O5 on quartz and glass and subsequent reduction in nitrogen (N2). This Physical Vapor Deposition (PVD) method resulted in high-quality single phase VO2(M1) films with sharp changes in resistance (3–4 orders of magnitude) and transmittance (40–45%) at λ= 1550 nm accompanied by narrow hysteresis loops (  4–5 K) around the Semiconductor-to-Metal Transition (SMT). © 2021 Elsevier B.V.

Photovoltaic devices based on organic semiconductors and organo-metal halide perovskites have not yet reached the theoretically predicted power conversion efficiencies while they still exhibit poor environmental stability. Interfacial engineering using suitable materials has been recognized as an attractive approach to tackle the above issues. We introduce here a zinc porphyrin-triazine-bodipy donor-πbridge-acceptor dye as a universal electron transfer mediator in both organic and perovskite solar cells. Thanks to its "push-pull" character, this dye enhances electron transfer from the absorber layer toward the electron-selective contact, thus improving the device's photocurrent and efficiency. The direct result is more than 10% average power conversion efficiency enhancement in both fullerene-based (from 8.65 to 9.80%) and non-fullerene-based (from 7.71 to 8.73%) organic solar cells as well as in perovskite ones (from 14.56 to 15.67%), proving the universality of our approach. Concurrently, by forming a hydrophobic network on the surface of metal oxide substrates, it improves the nanomorphology of the photoactive overlayer and contributes to efficiency stabilization. The fabricated devices of both kinds preserved more than 85% of their efficiency upon exposure to ambient conditions for more than 600 h without any encapsulation. © 2019 American Chemical Society.

The field emission and resulting breakdown induced damage in the actuation paths of MEMS capacitive switches are investigated. The effect of asperities burning due to Joule heating and the resulting explosive break down are presented. The breakdown gives rise to almost mirror craters formation on the cathode and anode electrodes. A linear relation between crater diameter and the breakdown current is found when breakdown occurs in vacuum. In ambient atmosphere the breakdown leads to large amplitude current oscillations and the formation of extended damage on both electrodes. © 2020 Elsevier Ltd

We develop ZnO/p-Si photodetectors by atomic layer deposition (ALD) of ZnO thin films on laser-microstructured silicon, and we investigate their electrical and optical behavior, demonstrating high sensitivity and broadband operation. Microstructured p-type silicon was obtained by nanosecond laser irradiation in SF6 gas, which results in the formation of quasi-ordered and uniform microspikes on the silicon surface. The irradiated silicon contains sulfur impurities, which extend its absorbance to the near-infrared. A thin film of ZnO was conformally deposited on the microstructured silicon substrates by ALD. Photoluminescence measurements indicate high crystalline quality of the ZnO film after annealing. Current-voltage (I-V) measurements of the ZnO/p-Si heterodiodes in the dark show a nonlinear behavior with unusual high current values in reverse bias. Under illumination photocurrent is observed for reverse bias, even for wavelengths below the silicon bandgap in the case of the laser-microstructured photodetectors. Higher current values are measured for the microstructured photodetectors compared to planar ones. Photoconductivity measurements show enhanced responsivity across the UV-vis-NIR spectral range for the laser-microstructured devices because of their increased surface area and light absorption. © 2020 American Chemical Society.

Photonic crystal structuring has emerged as an advanced method to enhance solar light harvesting by metal oxide photocatalysts along with rational compositional modifications of the materials’ properties. In this work, surface functionalization of TiO2 photonic crystals by blue luminescent graphene quantum dots (GQDs), n–π* band at ca. 350 nm, is demonstrated as a facile, environmental benign method to promote photocatalytic activity by the combination of slow photon-assisted light trapping with GQD-TiO2 interfacial electron transfer. TiO2 inverse opal films fabricated by the co-assembly of polymer colloidal spheres with a hydrolyzed titania precursor were post-modified by impregnation in aqueous GQDs suspension without any structural distortion. Photonic band gap engineering by varying the inverse opal macropore size resulted in selective performance enhancement for both salicylic acid photocatalytic degradation and photocurrent generation under UV–VIS and visible light, when red-edge slow photons overlapped with the composite’s absorption edge, whereas stop band reflection was attenuated by the strong UVA absorbance of the GQD-TiO2 photonic films. Photoelectrochemical and photoluminescence measurements indicated that the observed improvement, which surpassed similarly modified benchmark mesoporous P25 TiO2 films, was further assisted by GQDs electron acceptor action and visible light activation to a lesser extent, leading to highly efficient photocatalytic films. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

Surface functionalization of TiO2 inverse opals by graphene oxide nanocolloids (nanoGO) presents a promising modification for the development of advanced photocatalysts that combine slow photon-assisted light harvesting, surface area, and mass transport of macroporous photonic structures with the enhanced adsorption capability, surface reactivity, and charge separation of GO nanosheets. In this work, post-thermal reduction of nanoGO-TiO2 inverse opals was investigated in order to explore the role of interfacial electron transfer vs. pollutant adsorption and improve their photocatalytic activity. Photonic band gap-engineered TiO2 inverse opals were fabricated by the coassembly technique and were functionalized by GO nanosheets and reduced under He at 200 and 500 °C. Comparative performance evaluation of the nanoGO-TiO2 films on methylene blue photodegradation under UV-VIS and visible light showed that thermal reduction at 200 °C, in synergy with slow photon effects, improved the photocatalytic reaction rate despite the loss of nanoGO and oxygen functional groups, pointing to enhanced charge separation. This was further supported by photoluminescence spectroscopy and salicylic acid UV-VIS photodegradation, where, in the absence of photonic effects, the photocatalytic activity increased, confirming that fine-tuning of interfacial coupling between TiO2 and reduced nanoGO is a key factor for the development of highly efficient photocatalytic films. © 2019 by the authors.

Visible Light Communication (VLC) systems use light-emitting diode (LED) technology to provide high-capacity optical links. The advantages they offer, such as the high data rate and the low installation and operational cost, have identified them as a significant solution for modern networks. However, such systems are vulnerable to various exogenous factors, with the background sunlight noise having the greatest impact. In order to reduce the negative influence of the background noise effect, optical filters can be used. In this work, for the first time, a low-cost optical vanadium dioxide (VO2) optical filter has been designed and experimentally implemented based on the requirements of typical and realistic VLC systems in order to significantly increase their performance by reducing the transmittance of background noise. The functionality of the specific filter is investigated by means of its bit error rate (BER) performance estimation, taking into account its experimentally measured characteristics. Numerous results are provided in order to prove the significant performance enhancement of the VLC systems which, as it is shown, reaches almost six orders of magnitude in some cases, using the specific experimental optical filter. © 2019 by the authors.

We report on the effect of lithium doping of zinc oxide used as electron-transport layer in organic solar cells based on both fullerene and non-fullerene acceptors. The experimental and theoretical results indicate that lithium ions intercalated within the ZnO lattice as dopants replace interstitial zinc defects that act as trap states and give rise to a higher electron conductivity without significantly altering work function and valence band edge. The enhanced electron carrier extraction/collection efficiency, the suppressed bimolecular and interface trap-assisted recombination losses and the higher electron mobility of the photoactive blend synergistically contribute to the superior performance of PTB7-Th:PC71BM-based fullerene devices utilizing doped ZnO layers with an optimized lithium concentration of 5 wt %. Such devices increased their maximum PCE from 8.59% (average 8.05%) to 10.05% (average 9.53%) while, simultaneously, boosting their long-term stability. Moreover, non-fullerene solar cells based on the PTB7-Th:IT-4F blend exhibited PCEs up to 8.96% and maintained more than 80% of their initial efficiency after 1000 h storage in the dark upon using the lithium modified ZnO electron transport layer. © 2019 American Chemical Society.

Photonic crystal-assisted semiconductor photocatalysis has been attracting significant attention as an advanced photon management approach that combines light harvesting with the macro/mesoporous structured materials properties permitting enhanced mass transport and high adsorption. In this work, surface functionalization of well-ordered photonic band gap engineered TiO2 inverse opal films fabricated by the convective evaporation-induced co-assembly method was performed by graphene oxide nanocolloids (nanoGO). The loading of GO nanosheets was determined by the films’ macropore size, with minimal effects on their long range periodicity and photonic properties. While nanoGO deposition reduced mesoporosity of the nanocrystalline titania walls, their surface functionality was greatly improved by the abundant oxygen groups of the GO nanosheets leading to increased pollutant adsorption. Slow photon amplification in the aqueous phase methylene blue photodegradation was identified for the unmodified TiO2 photonic films under both UV–vis and Vis illumination upon spectral overlap of the low energy edge of the inverse opal stop band (in water) with the dye electronic absorption, due to (red) slow photons localized in the titania skeleton that distinctly accelerated dye photodegradation kinetics. The photocatalytic efficiency was further improved for the nanoGO functionalized TiO2 inverse opal films via the synergetic action of interfacial electron transfer from TiO2 to the GO nanosheets. Under UV–vis light, the functionalized photonic films outperformed benchmark mesoporous Aeroxide® P25 TiO2 films where nanoGO modification, despite the enhanced dye adsorption, resulted in adverse effects in photocatalytic degradation due to pore clogging. Combination of the exceptional structural and photonic properties of TiO2 inverse opals with the high adsorption capacity and charge separation afforded by GO nanocolloids is proposed as a promising modification route for the development of efficient photocatalytic films. © 2018 Elsevier B.V.

In the present work, we effectively modify the TiO2 electron transport layer of organic solar cells with an inverted architecture using appropriately engineered porphyrin molecules. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO2, thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend. Upon employing porphyrin-modified TiO2 electron transport layers in PTB7:PC71BM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices, which retained 80% of their initial efficiency after 500 h of storage in the dark. Because of its simplicity and efficacy, this approach should give tantalizing glimpses and generate an impact into the potential of porphyrins to facilitate electron transfer in organic solar cells and related devices. © 2018 American Chemical Society.

Plasma treatment is an environmentally friendly solution for modifying or nanostructuring the surface of several materials including photoactive polymers. The detailed characterization of the effect of plasma treatment on chemical and optoelectronic properties of photoactive polymers is, therefore, of specific interest. Herein, the effect of the exposure of poly(3-hexylthiophene) (P3HT) thin films to plasma created in three different gases (oxygen, argon and hydrogen) was studied. A range of spectroscopic techniques, such as x-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy in conjunction with UV–vis absorption, Fourier transform infrared (FTIR) and photoluminescence (PL) spectroscopies, are employed to quantify the extent of chemical modification occurring in each particular case. It is shown that oxygen plasma treatment leads to the disruption of the π-conjugation via the direct oxidation of the sulfur atom of the thiophene ring while the aliphatic side chain remains nearly unaffected. An oxidation mechanism is proposed according to which the sulfur atom of the thiophene ring is oxidized into sulfoxides and sulfones, which subsequently degraded into sulfonates or sulfonic acids in a relatively small degree. For argon and hydrogen plasma treatments some oxidation products are detected only at the polymer surface. In all cases the polymer surface Fermi level is shifted closer to the highest occupied molecular orbital (HOMO) energy after plasma treatment indicating p-type doping arising from surface oxidation. © 2017 Elsevier Ltd

Combining high efficiency and long lifetime under ambient conditions still poses a major challenge towards commercialization of polymer solar cells. Here we report a facile strategy that can simultaneously enhance the efficiency and temporal stability of inverted photovoltaic architectures. Inclusion of a silanol-functionalized organic-inorganic hybrid polyoxometalate derived from a PW9O34 lacunary phosphotungstate anion, namely (nBu4N)3[PW9O34(tBuSiOH)3], significantly increases the effectiveness of the electron collecting interface, which consists of a metal oxide such as titanium dioxide or zinc oxide, and leads to a high efficiency of 6.51% for single-junction structures based on poly(3-hexylthiophene):indene-C60 bisadduct (P3HT:IC60BA) blends. The above favourable outcome stems from a large decrease in the work function, an effective surface passivation and a decrease in the surface energy of metal oxides which synergistically result in the outstanding electron transfer mediating capability of the functionalized polyoxometalate. In addition, the insertion of a silanol-functionalized polyoxometalate layer significantly enhances the ambient stability of unencapsulated devices which retain nearly 90% of their original efficiencies (T90) after 1000 hours. © 2018 The Royal Society of Chemistry.

We investigated theoretically the effect of introducing Si impurities in a single layer graphene (1LG) that had been deposited on a polar substrate on the transport properties of the graphene layer. We consider in our analysis the scattering effects due to the surface optical (SO) phonons located at the interface of the 1LG with various polar substrates such asSiC, hexagonal BN,SiO2andHfO2. Our results demonstrate a reduction of SO phonon-limited (SOPL) mobility, and SOPL conductivity as well as an increase of the SOPL resistivity and of the scattering rate in the presence of Si impurities in the 1LG. Further, we studied the effect of Si impurities on the electron-surface phonon interaction. For our analysis we used the eigenenergies aquired from the tight-binding Hamiltonian in 1LG. Indeed the presence of the Si impurities induces a decrement in the resonant coupling between the electronic sub-levels and the surface vibration modes in monolayer graphene deposited on polar substrates. Finally, we investigated the effect of Si impurities on the Auger scattering process which affects the carriers relaxation. Our results show an enhancement of the Auger scattering rate in the case of the Si-doped 1LG compared to the undoped 1LG. © 2018 Elsevier B.V.

Here, we use a simple and effective method to accomplish energy level alignment and thus electron injection barrier control in organic light emitting diodes (OLEDs) with a conventional architecture based on a green emissive copolymer. In particular, a series of functionalized zinc porphyrin compounds bearing π-delocalized triazine electron withdrawing spacers for efficient intramolecular electron transfer and different terminal groups such as glycine moieties in their peripheral substitutes are employed as thin interlayers at the emissive layer/Al (cathode) interface to realize efficient electron injection/transport. The effects of spatial (i.e., assembly) configuration, molecular dipole moment and type of peripheral group termination on the optical properties and energy level tuning are investigated by steady-state and time-resolved photoluminescence spectroscopy in F8BT/porphyrin films, by photovoltage measurements in OLED devices and by surface work function measurements in Al electrodes modified with the functionalized zinc porphyrins. The performance of OLEDs is significantly improved upon using the functionalized porphyrin interlayers with the recorded luminance of the devices to reach values 1 order of magnitude higher than that of the reference diode without any electron injection/transport interlayer. © 2018 American Chemical Society.

Motivated by the excellent electron-transfer capability of porphyrin molecules in natural photosynthesis, we introduce here the first application of a porphyrin compound to improve the performance of planar perovskite solar cells. The insertion of a thin layer consisting of a triazine-substituted Zn porphyrin between the TiO2 electron transport layer and the CH3NH3PbI3 perovskite film significantly augmented electron transfer toward TiO2 while also sufficiently improved the morphology of the perovskite film. The devices employing porphyrin-modified TiO2 exhibited a significant increase in the short-circuit current densities and a small increase in the fill factor. As a result, they delivered a maximum power conversion efficiency (PCE) of 16.87% (average 14.33%), which represents a 12% enhancement compared to 15.01% (average 12.53%) of the reference cell. Moreover, the porphyrin-modified cells exhibited improved hysteretic behavior and a higher stabilized power output of 14.40% compared to 10.70% of the reference devices. Importantly, nonencapsulated perovskite solar cells embedding a thin porphyrin interlayer showed an elongated lifetime retaining 86% of the initial PCE after 200 h, while the reference devices exhibited higher efficiency loss due to faster decomposition of CH3NH3PbI3 to PbI2. © 2018 American Chemical Society.

We report on the morphology and optical properties of CuInS2/ZnS core-shell quantum dots in solid films by means of AFM, SEM, HRTEM, steady state and time-resolved photoluminescence (PL) spectroscopy. The amount of aggregation of the CuInS2/ZnS QDs was controlled by changing the preparation conditions of the films. A red-shift of the PL spectrum of CuInS2/ZnS core-shell quantum dots, deposited as solid films on silicon substrates, is observed upon increasing the amount of aggregation. The presence of larger aggregates was found to lead to a larger PL red-shift. Besides, as the degree of aggregation increased, the PL decay became slower. We attribute the observed PL red-shift to energy transfer from the smaller to the larger dots within the aggregates, with the emission being realized via a long decay recombination mechanism (100-200 ns), the origin of which is discussed. © 2016 IOP Publishing Ltd.

A significant contribution to the improvement of efficiency and lifetime of organic solar cells is due to the successful engineering of the metal contact/organic interface by introducing appropriate interlayers. In the current work we show that a short microwave post-annealing treatment in air of an under-stoichiometric molybdenum oxide (MoOx) hole transport layer significantly enhanced the performance and lifetime of an organic solar cell based on a poly(3-hexylthiophene):[6,6]-phenyl-C71-butyric acid methyl ester (P3HT:PC71BM) blend. The enhanced performance is mainly driven by improvement in the short circuit current (Jsc) and the fill factor (FF), caused by, except for an increase of the anode work function, reduced series resistance, and increased shunt resistance and also higher charge generation efficiency, reduced recombination losses and improved hole transport towards the anode contact. In addition, the lifetime of the devices with microwave annealed MoOx interlayers was also significantly improved compared to those with as-deposited MoOx and, especially, those with the PEDOT-PSS interlayer. The above were attributed to effective dehydration which was also followed by structural transformation and crystallization of the MoOx layer during microwave annealing. The removal of absorbed water molecules led to alterations of the structure and microstructure of the MoOx films, visible in the X-ray diffraction patterns, infrared and Raman spectra and atomic force microscopy images recorded on their surface without influencing the oxide's chemical composition as evidenced by X-ray photoelectron spectroscopy. During microwave annealing the substrate remains practically at room temperature, so the method is applicable for films deposited on plastics or other temperature-sensitive substrates. © 2016 The Royal Society of Chemistry.

Transition-metal-oxide hybrids composed of high surface-to-volume ratio Ta2O5 matrices and a molecular analogue of transition metal oxides, tungsten polyoxometalates ([PW12O40]3-), are introduced herein as a charge storage medium in molecular nonvolatile capacitive memory cells. The polyoxometalate molecules are electrostatically self-assembled on a low-dimensional Ta2O5 matrix, functionalized with an aminosilane molecule with primary amines as the anchoring moiety. The charge trapping sites are located onto the metal framework of the electron-accepting molecular entities as well as on the molecule/oxide interfaces which can immobilize negatively charged mobile oxygen vacancies. The memory characteristics of this novel nanocomposite were tested using no blocking oxide for extraction of structure-specific characteristics. The film was formed on top of the 3.1 nm-thick SiO2/n-Si(001) substrates and has been found to serve as both SiO2/Si interface states' reducer (i.e., quality enhancer) and electron storage medium. The device with the polyoxometalates sandwiched between two Ta2O5 films results in enhanced internal scattering of carriers. Thanks to this, it exhibits a significantly larger memory window than the one containing the plain hybrid and comparable retention time, resulting in a memory window of 4.0 V for the write state and a retention time around 104 s without blocking medium. Differential distance of molecular trapping centers from the cell's gate and electronic coupling to the space charge region of the underlying Si substrate were identified as critical parameters for enhanced electron trapping for the first time in such devices. Implementing a numerical electrostatic model incorporating structural and electronic characteristics of the molecular nodes derived from scanning probe and spectroscopic characterization, we are able to interpret the hybrid's electrical response and gain some insight into the electrostatics of the trapping medium. © 2016 American Chemical Society.

Zinc oxide (ZnO) is an important material for polymer solar cells (PSCs) where the characteristics of the interface can dominate both the efficiency and lifetime of the device. In this work we study the effect of fluorine (SF6) plasma surface treatment of ZnO films on the performance of PSCs with an inverted structure. The interaction between fluorine species present in the SF6 plasma and the ZnO surface is also investigated in detail. We provide fundamental insights into the passivation effect of fluorine by analyzing our experimental results and theoretical calculations and we propose a mechanism according to which a fluorine atom substitutes an oxygen atom or occupies an oxygen vacancy site eliminating an electron trap while it may also attract hydrogen atoms thus favoring hydrogen doping. These multiple fluorine roles can reduce both the recombination losses and the electron extraction barrier at the ZnO/fullerene interface improving the selectivity of the cathode contact. Therefore, the fabricated devices using the fluorine plasma treated ZnO show high efficiency and stable characteristics, irrespective of the donor:acceptor combinations in the photoactive blend. Inverted polymer solar cells, consisting of the P3HT:PC71BM blend, exhibited increased lifetime and high power conversion efficiency (PCE) of 4.6%, while the ones with the PCDTBT:PC71BM blend exhibited a PCE of 6.9%. Our champion devices with the PTB7:PC71BM blends reached a high PCE of 8.0% and simultaneously showed exceptional environmental stability when using the fluorine passivated ZnO cathode interlayers. © The Royal Society of Chemistry 2016.

We report on a 20-fold enhancement of the integrated photoluminescence (PL) emission of silicon nanocrystals, embedded in a matrix of silicon dioxide, induced by excited surface plasmons from silver nanoparticles, which are located in the vicinity of the silicon nanocrystals and separated from them by a silicon dioxide layer of a few nanometers. The electric field enhancement provided by the excited surface plasmons increases the absorption cross section and the emission rate of the nearby silicon nanocrystals, resulting in the observed enhancement of the photoluminescence, mainly attributed to a 20-fold enhancement in the emission rate of the silicon nanocrystals. The observed remarkable improvement of the PL emission makes silicon nanocrystals very useful material for photonic, sensor and solar cell applications. © 2015 Elsevier B.V.

The photoluminescence properties of CuInS2/ZnS quantum dots (QDs) dispersed in solutions of different concentrations and solvent polarity and deposited as solid films on quartz substrates by drop-casting and spin-coating are studied. Both steady state and time-resolved photoluminescence spectroscopy have been used. The CuInS2/ZnS QDs in solutions exhibit a red-shift of their absorption and photoluminescence spectra by increasing concentration and solvent polarity. In addition, they exhibit a three-exponential decay with time constants 1-3, 20-40 and 200-300 ns, depending on solvent, concentration and detection wavelength. In films, a red-shifted photoluminescence spectrum is observed for films made by drop-casting compared to those prepared by spin-coating. The time-resolved photoluminescence decays in films, apart from the three mechanisms observed in solutions, also exhibit a fast decay component of <1 ns, which is more pronounced in the spin coated films and especially at long emission wavelengths. The time resolved photoluminescence spectra in the drop-casted films experience a larger transient red-shift than in the spin-coated ones, indicative of a possible energy transfer among adjacent QDs. In general, it is shown that the chemical environment and the presence of defects play a central role in the recombination processes. © 2015 Elsevier B.V. All rights reserved.

The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high-efficiency solar cells. Here a strategy to passivate surface trap states of TiO2 films used as cathode interlayers in organic photovoltaics (OPVs) through applying alumina (Al2O3) or zirconia (ZrO2) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO2 are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open-circuit voltage and the short-circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO2 surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO2 electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO2/ALD metal oxide/organic interface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on the increase of up to 37.5% in conversion efficiency of a Si-based solar cell after deposition of light-emitting Cd-free, CuInS 2/ZnS core shell quantum dots on the active area of the cell due to the combined effect of down-conversion and the anti- reflecting property of the dots. We clearly distinguished the effect of down-conversion from anti-reflection and estimated an enhancement of up to 10.5% in the conversion efficiency due to down-conversion. © 2014 AIP Publishing LLC.

Herein we report on enhanced organic solar cell performance through the incorporation of cathode interfacial layers consisting of self-organized porphyrin nanostructures with a face-on configuration. In particular, a water/methanol-soluble porphyrin molecule, the free base meso-tetrakis(1- methylpyridinium-4-yl)porphyrin chloride, is employed as a novel cathode interlayer in bulk heterojunction organic photovoltaics. It is demonstrated that the self-organization of this porphyrin compound into aggregates in which molecules adopt a face-to-face orientation parallel to the organic semiconducting substrate induces a large local interfacial electric field that results in a significant enhancement of exciton dissociation. Consequently, enhanced photocurrent and open circuit voltage were obtained resulting in overall device efficiency improvement in organic photovoltaics based on bulk heterojunction mixtures of different polymeric donors and fullerene acceptors, regardless of the specific combination of donor-acceptor employed. To highlight the impact of molecular orientation a second porphyrin compound, the Zn-metallated meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, was also studied and it was found that it forms aggregates with an edge-to-edge molecular configuration inducing a smaller increase in the device performance. © The Royal Society of Chemistry.

Highly efficient and stable organic photovoltaic (OPV) cells are demonstrated by incorporating solution-processed hydrogen molybdenum bronzes as anode interlayers. The bronzes are synthesized using a sol-gel method with the critical step being the partial oxide reduction/hydrogenation using an alcohol-based solvent. Their composition, stoichiometry, and electronic properties strongly correlate with the annealing process to which the films are subjected after spin coating. Hydrogen molybdenum bronzes with moderate degree of reduction are found to be highly advantageous when used as anode interlayers in OPVs, as they maintain a high work function similar to the fully stoichiometric metal oxide, whereas they exhibit a high density of occupied gap states, which are beneficial for charge transport. Enhanced short-circuit current, open-circuit voltage and, fill factor, relative to reference devices incorporating either PEDOT-PSS or a solution processed stoichiometric molybdenum oxide, are obtained for a variety of bulk heterojunction mixtures based on different polymeric donors and fullerene acceptors. In particular, high power conversion efficiencies are obtained in devices that employed the s-H xMoO2.75 as the hole extraction layer. The incorporation of solution-processed hydrogen molybdenum bronzes as anode interlayers in organic photovoltaic cells is presented. High power conversion efficiencies are observed in devices based on polymeric donors and fullerene acceptors that include a bronze with a moderate degree of reduction, namely the s-H xMoO2.75, as the anode interlayer. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this study, we investigated the lateral electrical transport and photocurrent mechanisms in multilayers of two-dimensional arrays of silicon nanocrystals (SiNCs), grown on quartz substrates by low pressure chemical vapor deposition (LPCVD) of Si and thermal oxidation. At low voltages, electrical conduction was ohmic, whereas at higher voltages, it was space charge limited in the presence of traps. At temperatures higher than 200 K both dark current and photocurrent were determined by thermal activation of carriers across the energy band gap, with an activation energy depending either on the applied voltage or on illumination. At temperatures lower than 200 K, the rate of current variation with temperature was smaller as transport was realized by carrier hopping, via phonons, between trapping states within the energy band gap, located near in energy and around the Fermi level. However, at the same temperature range, photocurrent was independent of temperature, as it was determined by carrier hopping from higher energy states to progressively lower ones. From this analysis, carrier concentration, an effective carrier mobility and trap density were extracted. © 2013 American Institute of Physics.

We fabricated a silicon-wafer based p-n junction solar cell with conversion efficiency of 11% without conventional doping of the emitter or the use of anti-reflecting coatings. The emitter was originally nanocrystalline, grown on n-type crystalline Si and covered with a thin semi-transparent Al layer. Annealing in nitrogen at 430 °C promoted a simultaneous aluminum (Al)-induced recrystallization and Al-doping of the emitter. The recrystallized emitter consisted of considerably larger Si grains which were epitaxially crystallized on the Si substrate. These two effects led to a considerable improvement of the electrical and photovoltaic properties of the resulting p-n junction. © 2013 AIP Publishing LLC.

We have investigated electrical transport and photocurrent in single and multilayers of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. The films were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. We found that at high voltages, electrical transport is governed by space charge limited currents due to the presence of traps. At low voltages, electrical transport is ohmic. Carrier mobility, carrier concentration, and trap density in the films were extracted from the electrical measurements. Combining photocurrent and absorption measurements for the films with different SiNC sizes, we found a remarkable similarity in the photon energy dependence of the photocurrent and of the absorbed light from the SiNCs, confirming a proportionality relation between the two quantities. Also, from the combined study of electrical transport and photocurrent, minority carrier lifetimes were extracted. © 2012 American Institute of Physics.

We report on the structural and optical characterization of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. Single and multiple SiNC layers were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. The single SiNC layers consisted of one SiNC layer embedded in two silicon dioxide (SiO 2) layers, whereas the multi-layered structure consisted of five SiNC layers of equal thickness separated by SiO 2 layers. SiNC layers with thicknesses ranging from 2 to 25 nm were investigated. A thorough structural characterization of the films was carried out by combining grazing incidence x-ray diffraction, x-ray reflectivity, and transmission electron microscopy (TEM). Both XRD and TEM measurements revealed that the SiNC layers were polycrystalline in nature and composed of SiNCs, separated by grain boundaries, with their vertical size equal to the SiNC layer and their lateral size characterized by a narrow size distribution. The high resolution TEM (HRTEM) images showed that oxidation of the SiNC layers proceeded by consumption of Si from their top surface, without any detectable oxidation at the grain boundaries. Only in the case of the thinnest investigated SiNC layer (2 nm), the SiNCs were well separated by SiO 2 tunnel barriers. From transmission and reflection optical measurements, energy band gaps of the SiNCs were estimated. These results were correlated with the sizes of the SiNCs obtained by HRTEM. A shift of the estimated band gaps with decreasing SiNC size was observed. This was consistent with quantum size effects in the SiNCs. The film containing the smallest SiNCs (2 nm in the growth direction), besides a significant shift of the absorption edge to higher energies, showed light emission at room temperature which is due to radiative recombination of photo-generated carriers in localized SiNCs separated by SiO 2 tunnel barriers. © 2012 American Institute of Physics.

In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent. © 2011 Gardelis et al.

We propose to analyze ellipsometry data by using effective medium approximation (EMA) models. Thanks to EMA, having nanocrystalline reference dielectric functions and generalized critical point (GCP) model the physical parameters of two series of samples containing silicon nanocrystals, i.e. silicon rich oxide (SRO) superlattices and porous silicon layers (PSL), have been determined. The superlattices, consisting of ten SRO/SiO2 layer pairs, have been prepared using plasma enhanced chemical vapor deposition. The porous silicon layers have been prepared using short monopulses of anodization current in the transition regime between porous silicon formation and electropolishing, in a mixture of hydrofluoric acid and ethanol. The optical modeling of both structures is similar. The effective dielectric function of the layer is calculated by EMA using nanocrystalline components (nc-Si and GCP) in a dielectric matrix (SRO) or voids (PSL). We discuss the two major problems occurring when modeling such structures: (1) the modeling of the vertically non-uniform layer structures (including the interface properties like nanoroughness at the layer boundaries) and (2) the parameterization of the dielectric function of nanocrystals. We used several techniques to reduce the large number of fit parameters of the GCP models. The obtained results are in good agreement with those obtained by X-ray diffraction and electron microscopy. We investigated the correlation of the broadening parameter and characteristic EMA components with the nanocrystal size and the sample preparation conditions, such as the annealing temperatures of the SRO superlattices and the anodization current density of the porous silicon samples. We found that the broadening parameter is a sensitive measure of the nanocrystallinity of the samples, even in cases, where the nanocrystals are too small to be visible for X-ray scattering. Major processes like sintering, phase separation, and intermixing have been revealed as a function of annealing of the SRO superlattices. © 2010 Elsevier B.V. All rights reserved.

We report on the lateral transport in a single two-dimensional (2D) array of Si nanocrystals of different sizes grown by low pressure chemical vapor deposition (LPCVD) of silicon on a quartz substrate and subsequent oxidation at high temperature. The initial nanocrystal size in the z-direction was 5 nm, while it was reduced to ∼3 nm after oxidation. The nanocrystals in the x-y plane were connected by grain boundaries and/or by very thin silicon oxide barriers, while a thin oxide layer was formed on their surface. The electrical measurements showed that current in the film is mainly governed by thermionic emission over the barriers (grain boundaries or dielectric barriers) at high temperatures and by tunneling at lower temperatures. Charge traps at the interfaces of the silicon nanocrystals with the oxide and at the grain boundaries cause considerable hysteresis in the current-voltage characteristics. Hydrogen passivation of the charge traps reduces considerably the hysteresis effect and the activation energy of the thermionic emission, while revealing a clear Coulomb gap. © 2011 American Institute of Physics.

In this work, we study theoretically the resonant coupling between longitudinal optical surface vibrations of Si-OH and/or Si-O-Si and electron and hole states in the silicon nanocrystals (Si NCs) within light emitting porous Si (PSi) thin films in the framework of the Fröhlich interaction. The results of this analysis are compared with experimental results, which show considerable enhancement and a redshift of the photoluminescence (PL) spectrum of a fresh as-grown PSi thin film after prolonged laser irradiation or after aging in air. These effects coincide with the formation of Si-OH and Si-O-Si bonds on the surface of PSi. The redshift of the PL spectrum is due to the pinning of the bandgap of the light emitting Si NCs, as both oxidation via laser irradiation in air and aging in air introduce energy states in the Si NC band gaps. According to the theoretical analysis, the PL enhancement is assigned to inhibition of nonradiative channels rather than to an enhancement of radiative channels in the Si NCs within the PSi film, due to a strong coupling of the surface Si-OH and/or Si-O-Si vibrational modes to the electronic sublevels in the Si NCs within the PSi layer. © 2011 American Institute of Physics.

We report that porous anodic alumina (aluminum oxide: Al2O 3) (PAA) thin films directly grown on Si show clear oscillations in their photoluminescence (PL) spectra which are ascribed to PL-induced interferences within the Fabry-Ṕrot optical cavity formed by the PAA film on Si, that involve the air/oxide and oxide/Si interfaces. The existence of the PL-induced oscillations is indicative of the high quality of the interface of the PAA film with Si, which is both planar and smooth. We show that by using these oscillations we can develop a sensitive optical method of measuring the porosity of PAA thin films on Si if we know the film thickness. The method is based on the calculation of the effective refractive index of the PAA film derived from the PL-induced oscillations, which is then introduced into the Bruggeman equation in order to derive the porosity of the film. © 2010 American Institute of Physics.

Different silicon nanocrystal (Si NC) systems in which Si NCs were either entirely isolated or loosely interconnected were studied by photoluminescence (PL) and time-resolved PL decay measurements in the range between 70 and 290 K, in order to investigate the role of exciton migration in the PL properties. We examined three kinds of samples: (a) two light emitting mesoporous Si (PSi) films with different porosities, grown on p -type Si, (b) a heavily oxidized light emitting anisotropic macroporous Si film, and (c) a film consisted of a Si NC superlattice with six Si NC/ SiO2 bilayers, grown by low pressure chemical vapor deposition of amorphous Si (α-Si), followed by high temperature thermal oxidation. In the two mesoporous Si films of the first case, the Si NCs show a degree of interconnection that depends on the porosity, whereas in the two other cases the NCs were isolated by SiO2, the degree of electrical isolation depending on the thickness of the SiO2 interlayer between them. Temperature dependent PL spectra and PL decay times of the different systems correlate well with the ability of excitons to migrate from one NC to another (case of loosely correlated NCs) or remain strongly localized within the Si NCs (case of effectively isolated NCs). © 2009 American Institute of Physics.

We study the effect of a red shift and a considerable enhancement of photoluminescence (PL) intensity from a freshly etched porous Si (PS) thin film after prolonged laser irradiation or after aging in atmosphere. Both effects coincide with the appearance of Si-OH and Si-O-Si vibration modes in the Fourier transform infrared (FTIR) absorption spectra. The red shift is attributed to a pinning of the band gap of the light-emitting Si nanocrystals (NCs) due to the formation of Si-OH and Si-O-Si bonds. Using theoretical calculations, we estimated the electron and hole energy shifts caused by the interaction of the electronic states of the Si NCs with the surface vibrations, and correlated the observed PL enhancement with resonant coupling between the quantized valence sub-levels in the Si NCs and surface vibration modes. © 2008 Elsevier B.V. All rights reserved.

We study theoretically the optical properties of embedded Ge and Si nanocrystals (NCs) in wide band-gap matrix and compared the obtained results for both NCs embedded in SiO2 matrix. We calculate the ground and excited electron and hole levels in both Ge and Si nanocrystals (quantum dots) in a multiband effective mass approximation. We use the envelope function approximation taking into account the elliptic symmetry of the bottom of the conduction band and the complex structure of the top of the valence band in both Si and Ge (NCs). The Auger recombination (AR) in both nanocrystals is thoroughly investigated. The excited electron (EE), excited hole (EH) and biexciton AR types are considered. The Auger recombination (AR) lifetime in both NCs has been estimated and compared. Crown Copyright © 2009.

The aim of a joint research activity in the FP6-ANNA project (http://www.i3-anna.org) is to develop and improve metrologies for the measurement of nanocrystal properties. Within the framework of this cooperation we optimized the sample preparation techniques to obtain a range of structures containing nanocrystals. Based on these samples we have optimized our characterization methods. In this work we focus on ellipsometry and X-ray diffraction measurements for the characterization of nanocrystal sizes in silicon rich oxide and porous silicon. We demonstrate the capabilities of dielectric function parametrizations in the ellipsometric evaluations, revealing the correlation between the broadening parameters of the critical point features and the nanocrystal size. ©The Electrochemical Society.

We calculate the ground and excited electron and hole levels in spherical Si nanocrystals (quantum dots) embedded within SiO 2 in a multiband effective mass approximation. The obtained energies of electron and hole are used to estimate the Auger Recombination (AR) lifetime in Si Nanocrystals (NCs). The excited electron, excited hole and biexciton AR types are considered. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

Ultra-thin nanocrystalline silicon films with varying thickness from 5 to 30nm were grown on quartz by low pressure chemical vapor deposition (LPCVD) of Si. Observations on cross-sectional transmission electron microscopy (TEM) specimens revealed that the films had a columnar growth, i.e. the third dimension of the nanocrystals, perpendicular to the Si/SiO 2 interface, was approximately equal to the film thickness, while the lateral size of nanocrystals was defined during the initial stage of growth and was not very much affected bythe film thickness. The observed columnar growth gives the possibility to obtain two-dimensional nanocrystal arrays on quartz with well defined size in the z-direction. Plane view images showed that the lateral distribution of nanocrystal size presents a well-defined maximum in all the films. The mean lateral size of the nanocrystals did not change very much with the film thickness, being in the range of 11-13 nm. The number of grains with size larger than the mean one tended to increase with the thickness of the film. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

In this work, the morphology, structure, surface chemical composition, and optical properties of very thin (10-70 nm) anodic silicon films grown on a silicon substrate by electrochemical dissolution of bulk crystalline silicon in the transition regime between the porous formation and electropolishing were investigated in detail. Anodization was performed by using short single pulses of anodization current in low and high hydrofluoric acid (HF) concentration electrolytes. A systematic comparison was made between films grown at low and high HF concentration electrolytes. The morphology and structure of the films were investigated by combining atomic force microscopy and transmission electron microscopy (TEM), while x-ray and ultraviolet photoelectron spectroscopies were used to investigate the chemical composition of the films. Photoluminescence was used to investigate the optical properties. It was found that films that formed at low HF concentrations were much thinner than films that formed at high HF concentrations due to surface dissolution of the films during anodization. High resolution TEM images revealed an amorphouslike structure (porous) in all of the films in which discrete Si nanocrystals (NCs) were identified. NC size was, on the average, larger in films fabricated in low HF concentration electrolytes and these films were not luminescent. On the other hand, films fabricated in high HF concentration electrolytes were thicker and contained smaller NCs. A silicon oxide layer covered the internal surface of all films, this oxide being much thinner in films grown at high HF concentrations. This last effect was attributed to self-limiting oxidation of the very small NCs constituting these films. © 2008 American Institute of Physics.

We report on the infrared transmission and light emission of Si-rich nitride (SRN) films prepared by low pressure chemical vapor deposition (LPCVD) from dichlorosilane (SiH2Cl2, DCS) and ammonia (NH3) mixtures. The main absorption band at about 830 cm- 1, attributed to Si-N vibration mode and observed in stoichiometric silicon nitride, shifted to slightly higher wavenumbers with increasing Si content in the SRN films. Annealing at temperatures higher than the deposition temperature induced a further shift of the main band to higher wavenumbers. Additionally, a new band appeared as a "shoulder" at about 1080 cm- 1, attributed to partial oxidation of the silicon nanocrystals. Photoluminescence (PL) obtained from the SRN films increased considerably and shifted to shorter wavelengths as the Si content decreased whereas annealing caused further enhancement and a slight shift to shorter wavelengths in comparison with the as-grown films. © 2007 Elsevier B.V. All rights reserved.

For a better understanding of the physical properties of semiconductor quantum dot ensembles, we have followed the behaviors of the transport and photoluminescence above, at, and below the percolation threshold of ensembles of Si quantum dots that are embedded in a Si O2 matrix. Our study revealed the roles of the interdot conduction, the single dot charging, and the connectivity in such systems. We conclude that while the first two determine the global transport, a connectivity dependent migration determines the coupling between the electrical and optical properties. © 2007 The American Physical Society.

In this study we report on the light-emission characteristics of Si nanocrystals formed at the initial stages of anodization of bulk crystalline Si in the transition regime using extremely short pulses of anodic current. A considerable enhancement and a red-shift of the photoluminescence (PL) peak of the as-grown films under prolonged laser illumination in atmosphere was observed. Ageing or thermal oxidation in atmosphere at 300 °C caused similar red-shift and significant enhancement of PL relative to the as-grown films. In all cases the enhancement and the red-shift of the PL coincided with the appearance of O2-Si-H2 and O2-Si-H(OH) stretching modes in the Fourier-Transform-Infra-Red (FTIR) absorption spectra. Oxidation in dry oxygen at 900 °C caused red-shift of PL without enhancement. Finally removal of the oxide caused blue shift of PL. All these observations are discussed in the context of the quantum confinement model. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

In this study we investigated the magneto-transport properties of the ohmic contact between polycrystal-line NiMnSb thin films grown by pulsed laser deposition and n-type degenerate InSb (100) substrates. An unusual negative giant magnetoresistance (n-GMR) effect is found when the external magnetic field is parallel to the in-plane current direction. A similar effect is also observed when Ni films are deposited on InSb substrates. On the other hand, no n-GMR effect is displayed when the deposited film is nonmagnetic. Grazing-incidence X-ray reflectometry shows the formation of a low-density NiMnSb layer at the interface. The presence of such a layer coincides with the appearance of the n-GMR. We argue that the n-GMR effect is due to magnetic precipitates formed at the interface during the growth of the magnetic films. We propose that these precipitates align their magnetic moments in the direction of the external magnetic field and thus, the spin dependent scattering of the electrons is reduced. The effect of these precipitates on the magnetoresistance depends on the thermal processing of the system. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

An investigation of the optical properties of Si-rich silicon nitride films prepared by low pressure chemical vapor deposition (LPCVD) from dichlorosilane (SiH2Cl2, DCS) and ammonia (NH3) mixtures has been performed. From TEM analysis, it was found that the excess Si forms nanocrystals the size of which depends on the temperature. The real and the imaginary part of the refractive index of the films were calculated using spectroscopic ellipsometry by fitting the ellipsometric data in the range 1000-250 nm using the Tauc-Lorentz model. It was found that the optical constants of the films mainly depend on their chemical composition which can be controlled by the DCS/NH3 flow ratio. Annealing at temperatures up to 1100 °C for 4 h does not considerably affect the refractive index of the films. Depending on their stoichiometry and the annealing conditions applied after growth, some of the films emitted light in the visible at room temperature. This was attributed to the quantum confinement of carriers in the Si nanocrystals contained in the films. © 2007 Elsevier Ltd. All rights reserved.

The optical properties of ultra thin anodic silicon layers containing silicon nanocrystals were investigated by photoluminescence spectroscopy and Fourier transform infrared absorption spectroscopy. The films were grown by electrochemical dissolution of bulk crystalline silicon at the early stages of anodization using short monopulses of anodic currents ranging from the regime of porous silicon formation to the transition regime between porosification and electropolishing. The conditions for obtaining light emitting films and the origin of light emission will be discussed. © 2007 American Institute of Physics.

We review our results on the structural and light-emitting properties of ultra-thin anodic silicon films grown by the electrochemical dissolution of bulk monocrystalline silicon at early stages of anodization. The films were grown using monopulses of anodization current covering the range from the regime of porous silicon formation to electropolishing. The samples were characterized by high resolution transmission electron microscopy, Fourier transform infrared absorption spectroscopy and photoluminescence. © 2007 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.

We review our results on the structural and light-emitting properties of ultra-thin anodic silicon films grown by the electrochemical dissolution of bulk monocrystalline silicon at early stages of anodization. The films were grown using monopulses of anodization current covering the range from the regime of porous silicon formation to electropolishing. The samples were characterized by high resolution transmission electron microscopy, Fourier transform infrared absorption spectroscopy and photoluminescence.

We developed a method for fabricating ultra-thin (18-80 nm) light emitting amorphous films with embedded silicon nanocrystals by anodization of bulk crystalline Si in the transition regime between porosification and electropolishing using short mono-pulses of anodization current. The size of the nanocrystals decreased with increasing current density and it was in the range of 3-7 nm with current densities in the range of 130-390 mA cm-2. At the highest current density used the film/substrate interface was very sharp, while at lower current densities the interface contained nanostructured silicon spikes protruding from the substrate into the amorphous film. The samples were characterized by high resolution transmission electron microscopy, Fourier transform infrared spectroscopy and photoluminescence. © IOP Publishing Ltd.

We present a systematic study of the polarization of the transport current from a variety of NiMnSb and Co2 MnSi thin films and bulk material using point contact Andreev reflection spectroscopy. The simple analysis suggests that the free surface polarization of NiMnSb is within error 10% lower than that of Co2 MnSi. In either material the measured polarization is rather insensitive to key physical and material properties. We use a two channel model to rule out the influence that stray magnetic field from the ferromagnet might have on the measurements presented. © 2006 American Institute of Physics.

We report a detailed study of surface and interface properties of pulsed-laser-deposited NiMnSb films on a Si(100) substrate as a function of film thickness. As the thickness of films is reduced below 35 nm, formation of a porous layer is observed. Porosity in this layer increases with decrease in NiMnSb film thickness. These morphological changes of the ultrathin films are reflected in the interesting transport and magnetic properties of these films. Compositional anomaly and surface and interface roughness are not the source of magnetic and transport properties of the films. © 2006 The American Physical Society.

We report on the electrical and magnetotransport properties of the contact formed between n -type degenerate InSb (100) substrates and polycrystalline NiMnSb thin films grown using pulsed laser deposition. Negative giant magnetoresistance is observed when the external magnetic field is oriented parallel to the in-plane current direction. We attribute the observed phenomenon to magnetic precipitates that are formed during the magnetic film deposition and are confined to a thin layer at the interface. The evidence for the formation of a thin interfacial layer is obtained through x-ray reflectivity measurements. © 2006 The American Physical Society.

We report the growth of ultra-thin indium oxide layers using the dc-magnetron sputtering method. We demonstrate that good quality tunnel barriers made of indium oxide can be routinely fabricated and employed in spintronic-related devices. Simple magnetic tunnel junctions (MTJs) were fabricated in a cross geometry using ex situ thermally evaporated cobalt and permalloy. Our best junctions obey the Rowell criteria for tunneling and exhibit a tunnel magnetoresistance of 15% at 100 K. © 2004 Elsevier B.V. All rights reserved.

We report the growth of single-phase, stoichiometric polycrystalline thin films of the half-Heusler ferromagnet NiMnSb, predicted to be half-metallic, on single crystal (100) InSb substrates heated at 200 °C by pulsed laser deposition (PLD). The NiMnSb target used for the PLD deposition was synthesized by arc melting and had a saturation magnetization, MS (5K) ≤ 4μB/formula unit. The films exhibited saturation magnetization, MS (5K) ≤ 4μB/formula unit and coercive fields of 2 Oe at 300 K, indicating good structural quality. The temperature dependence of the saturation magnetization shows that at low temperature (T < 200 K) the system behaves like a Heisenberg ferromagnet as expected for a half-metal, while at T > 200 K behaves like an itinerant ferromagnet. © 2005 IOP Publishing Ltd.

A review of recent advances in spintronics is presented. We report the structural, magnetic, electrical and thermal properties of the ferromagnetic half-Heusler alloy NiMnSb grown by arcmelting. The bulk material is used to deposit highly crystalline thin films at low temperature (200°C) by Pulsed Laser Deposition (PLD). The structural and magnetic and transport properties of these films are nearly bulk-like suggesting that these films can be grown by PLD in multilayer structures for efficient spin injection in spintronics.

Highly spin polarized Heusler alloys, NiMnSb and Co2MnSi, attract a great deal of interest as potential spin injectors for spintronic applications. Spintronic devices require control of interfacial properties at the ferromagnet: semiconductor contact. To address this issue we report a systematic study of the ordinary and anomalous Hall effect, in Ni 1.15Mn0.85Sb films on silicon, as a function of film thickness. In contrast to the bulk stoichiometric material, the Hall carriers in these films become increasingly electron-like as the film thickness decreases, and as the temperature increases from 50 K toward room temperature. High field Hall measurements confirm that this is representative of the majority transport carriers. This suggests that current injected from a NiMnSb: semiconductor interface may not necessarily carry the bulk spin polarization. The films also show a low temperature upturn in the resistivity, which is linked to a discontinuity in the anomalous Hall coefficient. Overall these trends indicate that the application of Heusler alloys as spin injectors will require strictly controlled interfacial engineering, which is likely to be demanding in these ternary alloys.

The transport properties of pulsed-laser-deposited NiMnSb films on silicon as a function of film thickness were discussed. It was found that a low-temperature upturn was observed in the resistivity for film thicknesses of 130 nm and below. It was observed that as the film thickness decreased, the magnitude of both the resistivity upturn and the magnetoresistance increased. Analysis shows that the evolution of the field dependence of the magnetoresistance appeared similar to the silver chalcogenides and was indicative of a crossover from hole-dominated to electron-dominated transport as the temperature increased.

We have grown films of the half-metallic ferromagnet NiMnSb on single crystals of the narrow gap semiconductor InSb by pulsed laser deposition. NiMnSb is a possible candidate for spin injection applications. The film depositions occurred at 200°C. X-ray diffraction studies indicate a high degree of (220) texture and no secondary phases. A saturation magnetization of four Bohr magnetons, μB. at 5 K and coercive fields down to 5 Oe at 300 K indicate the good quality of the films.

NiMnSb has attracted a great deal of interest as a spin injector/detector in spintronic devices because it has a Curie temperature of 728 K and is predicted to be half-metallic (100% spin polarized). NiMnSb has been reported to have greatly reduced surface polarization, and to lose its half metallicity above 80 K. Here we report the investigation of the surface polarisation and electronic structure of NiMnSb by measurement of the transport spin polarization using point contact Andreev reflection spectroscopy, and anomalous Hall effect in thin films on Si(00 1). A comparison to bulk properties is made. © 2003 Elsevier B.V. All rights reserved.

We report the growth of single-phase, stoichiometric polycrystalline thin films of the half-Heusler ferromagnet NiMnSb, predicted to be half-metallic, on single crystal InSb (100) substrates heated at 200°C by pulsed laser deposition. The films exhibit saturation magnetization of 4 μB/ formula unit at 5 K and coercive fields of 2 Oe at 300 K indicative of their good structural quality. At low temperatures (7<200 K) the system behaves like a Heisenberg ferromagnet as expected for a half-metal, while at T>200 K it behaves like an itinerant ferromagnet. The resistivity of the film at 5 K is 6 μΩ cm. © 2004 American Institute of Physics.

The synthesis and physical properties of arc melted NiMnSb were studied using X-ray diffraction analysis. It was observed that the half metallicity in NiMnSb is supported by the integer saturation magnetization value at 5 K. It was found that the magnetic properties were due to magnetic moments localized at the Mn atoms. The results show that in the temperature region of 80 K to 150 K a crossover takes place from half metallic behavior to normal ferromagnetic behavior.

We report measurements of single-electron polarization in a coupled double-quantum-dot device isolated from current probes and demonstrate that the energetics observed for this process differs from that observed in double dots coupled to reservoirs. The movement of the electrons is detected by a quantum point contact. By analyzing the energy broadening corresponding to the tunneling of a single electron from one dot to the other we estimate a minimum for the intradot scattering time to be 0.2 ns. This energy broadening follows the predicted shot-noise variation in the detector with gate voltage, but is three orders-of-magnitude higher. We speculate that two-level systems could account for the discrepancy.

We present evidence for quantum dot cellular automata action in a cell consisting of four dots defined by submicron metal gates on the top surface of a molecular-beam-epitaxy-grown GaAs/AlGaAs heterostructure in which a two-dimensional electron gas layer was formed approximately 70 nm below the surface. The four-dot cell is separated by a strong barrier in two double-dot sets. We show that by polarizing one of the double-dot sets we can polarize the other set in the cell. The polarization is detected using noninvasive voltage probe without drawing electric currents from the cell.

We demonstrate that a quantum-dot cellular automata device can be fabricated using electron beam lithographically defined gates on GaAs/AlGaAs heterostructure materials, and that by tuning the four quantum dot (J. Phys. C: Solid State Phys. 21 (1988) L893) system polarization of one double dot can lead to polarization in the neighboring double dot (Phys. Rev. B 67 (2003) 033302). The polarization is detected using a 1-D or 0-D channel defined next to one pair of double dots which acts as a non-invasive voltage probe (Phys. Rev. Lett. 70 (1993) 1311). Ultimately a cellular automata device should be isolated from reservoirs to prevent charge fluctuations caused by co-tunneling. The non-invasive voltage probe is used to show that coupled double dots isolated from reservoirs can be made to have a sharper polarization transition. By studying the broadening of the polarization signal from a coupled double dot system isolated from reservoirs, we deduce the charge dephasing times for intra dot scattering to be more than 0.2 ns (Phys. Rev. B 67 (2003) 073302). © 2004 Elsevier Ltd. All rights reserved.

We measure single electron movement between two coupled quantum dots each isolated from a reservoir. The electrons movement is detected using a 1-D channel placed next to the double dot system. The electrostatic energy required to move an electron from one dot to a reservoir is half that required to move the electron from one dot to the other dot. We estimate the maximum intra-dot scattering time at < 0.2 ns. This scattering time corresponds to a lifetime broadening equal to the thermal broadening (80 mK) indicating that the scattering time may be longer than this. © 2002 Elsevier Science B.V. All rights reserved.

We present results on spin-polarized electron transport from a ferromagnet to a two-dimensional electron gas system (2DEG). The investigated device consists of an injector and a collector contact made from ferromagnetic permalloy thin films with different coercive fields. That allows parallel or antiparallel magnetization of the contacts in different applied magnetic fields. The conducting medium is a 2DEG formed in an AlSb/InAs quantum well. Data from this device suggest that its resistance is controlled by two different types of spin-valve effect: the first is due to the ferromagnet-semiconductor contact resistance, determined by the zero-field spin-splitting in InAs, and the second is due to the propagation of electrons with a spin imbalance through the 2DEG without spin-scattering.

We present a spintronic semiconductor field-effect transistor. The injector and collector contacts of this device were made from magnetic permalloy thin films with different coercive fields so that they could be magnetized either parallel or antiparallel to each other in different applied magnetic fields. The conducting medium was a two-dimensional electron gas (2DEG) formed in an AlSb/InAs quantum well. Data from this device suggest that its resistance is controlled by two different types of spin-valve effect: the first occurring at the ferromagnet-2DEG interfaces; and the second occurring in direct propagation between contacts. © 1999 The American Physical Society.

We have investigated the magnetization reversal and magnetoresistance (MR) behavior of a lateral spin-injection device. The device consists of a two-dimensional electron gas (2DEG) system in an InAs quantum well and two ferromagnetic (Ni80Fe20) contacts: an injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and propagating through InAs are collected by the second contact. By engineering the shape of the permalloy film distinct switching fields (Hc) from the injector and the collector have been observed by scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20-60 Oe), at room temperature, over which magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device. © 1999 American Institute of Physics.

The magnetic Compton profiles (MCPs) measured in the [100], [110], [111] and [112] directions in single-crystal nickel with an incident photon beam of energy 224 keV are presented and discussed. The momentum resolution achieved, of 0.43 atomic units, improves on previous studies by almost a factor of two, and facilitates the interpretation of the MCPs in terms of the underlying spin-dependent momentum densities. Calculations have been performed using the linear muffin-tin orbital method, within both the local spin-density approximation (LSDA) and the generalized gradient approximation (GGA). Comparison with experiment reveals the limitations of the LSDA at low momentum, where the GGA is better able to reproduce the contribution of the s- and p-like electrons. All of the calculations overestimate the moment associated with the d-like electrons, for momenta corresponding to the first Brillouin zone. We also confirm the existence of the so-called Umklapp shoulders, which derive from the Fermi surface topology.

The energy dependence of the magnetic Compton cross-section was measured with elliptically polarised synchrotron radiation at five energies from 245.2 to 1000.5 keV at the European Synchrotron Radiation Facility (ESRF) using a recently installed superconducting wavelength shifter, the sample was ferromagnetic iron. These measurements more than double the highest photon energy previously used in synchrotron radiation studies. It was found that the integrated intensity of the spin-dependent scattering was well described by the formulae for the differential cross-section, dσ/dΩ, for free, stationary electrons. The optimisation of experiments designed to yield the spin-dependent Compton profile from the double differential cross-section, d2σ/dΩ dω is discussed.

The spin density in the Heusler alloy Cu2MnAl, has been studied in a Compton scattering experiment with 92 keV circularly polarized synchrotron radiation on the high-energy beamline at ESRF. The conduction electrons were found to have a negative spin polarization of 0.4 μB which is at variance with the deduction of a positive moment from earlier neutron data; neither was any evidence found for a 3d spin moment on the copper site. The spin moment on the Mn site at room temperature was determined as 3.25 μB, which is in agreement with neutron data. The spin-dependent Compton profiles for the [100], [110] and [111] directions, reported here, show anisotropy in the momentum density which is in good agreement with new KKR calculations based on a ferromagnetic ground state. By combining charge- and spin-dependent Compton data the momentum space anisotropies in the majority and minority bands have been analysed. Both the majority and minority spin densities are anisotropic.

Oxidised porous silicon emits luminescence in two distinct bands in the visible region. The fast blue (τ ∼ ns) and slow red (τ ∼ μs at 300K) bands are studied via the separate methods of time-resolved XEOL in single-bunch mode and wavelength-selective and total XEOL in multi-bunch mode. Measurements have been conducted at the silicon K-edge (1840eV) and L2,3-edge for freshly prepared and oxidised porous silicon and related samples. Both methods give conclusive evidence that the fast, blue luminescent site is defects in silica or suboxide formed near to the Si/ SiO2 interface, whereas the slower, red band originates from smaller silicon particles of diameter 14Å or less found in porous silicon. The XEOL process is discussed and range estimates of the spatial separation between the photoionisation event and radiative recombination are made.

In this study we have used high resolution parallel electron energy loss spectroscopy (PEELS) and X-ray excited optical luminescence (XEOL) to investigate the chemical nature of the luminescence centre in fresh and aged porous silicon. We find that regardless of the non-stoichiometric oxides which were observed by PEELS in our fresh porous silicon layers, Si-Si bonded material is involved in the luminescence process. However, in the case of aged porous silicon both Si-Si and Si-O bonded material are involved.

In this study, the structural properties of luminescent porous silicon layers have been investigated. Double x-ray and electron diffraction patterns combined with high resolution electron microscopy (HREM) show a good agreement in that the distortion of the lattice planes in porous silicon is related to the smallness of the crystallites constituting the layers. HREM results indicate that a two-level microstructure is present in porous silicon layers. This structure consists of a crystalline part (nanocrystals) and an amorphous part (a sponge-like structure). The morphology of both parts show differences for samples grown under different conditions. A model based on this two-level microstructure is used to account for these observations. It turns out that this microstructure cannot be characterized, as often attempted, by only the measured values of the porosity and the surface area. Finally, double x-ray diffraction on post-treated porous silicon layers confirms that hydrogen desorption and readsorption or further oxidation and/or reconstruction of the oxide layer in porous silicon induce further distortion of the lattice planes. © 1995, The Electrochemical Society, Inc. All rights reserved.

High resolution scanning transmission electron microscopy (STEM) and synchrotron X‐ray excitation of luminescence (XEOL) is used to probe the chemical nature of the luminescence mechanisms of porous and rapidly oxidised porous silicon. It is concluded that for fresh porous silicon, Si‐Si bonded material is involved in the luminescence. For oxidised material this is probably not the case, the chemical environment being SiO2. Copyright © 1995 WILEY‐VCH Verlag GmbH & Co. KGaA

The structural and optical properties of low and high porosity luminescent porous silicon layers have been studied. It has been shown that the luminescence intensity is strongly dependent on the porosity and not on the surface area. Double-crystal X-ray and electron diffraction patterns indicate that freshly anodized luminescent porous silicon is crystalline. Both techniques show an increasing distortion of the lattice planes with increasing porosity, implying that the distortion is strongly linked to the smallness of the crystallites constituting the layers. © 1995.

Sharp luminescence at 1.54μm from erbium doped porous silicon has been observed. The silicon was made porous after implantation of high doses of erbium and oxygen into p-type Czochralski silicon. The erbium related luminescence from porous silicon is an order of magnitude more intense than that from erbium doped single crystal silicon. © 1995, IEE. All rights reserved.

It has been postulated that light emission from porous silicon is caused by quantum confinement of the electron states within silicon wires formed by anodic electroetching of silicon. In order to investigate this hypothesis we have made measurements of the X-ray excited optical luminescence (XEOL) and the total electron yield (TEY) as a function of X-ray energy for porous silicon at station 3.4 of the SRS at Daresbury laboratory. Results have shown that the luminescence is associated with elemental silicon, and this is true for as prepared and oxidised material. In the latter case the XEOL spectrum is completely different from the TEY. However, by considering the microscopic origin of the excitation together with time-dependent relaxation data, we conclude that the emission comes from silicon surface states and not quantum effects in the nanoparticles. This is in contrast to other similar studies. © 1995.

The role of the surface in the optical properties of porous silicon remains a key issue. Although the burden of evidence points toward some intrinsic radiative mechanism in small silicon particles, the influence of the surface and ways of controlling surface interactions will always be important. We present here the results of surface modification of porous silicon using annealing and rapid oxidation steps. By comparing new results with existing published data we conclude that hydrogen passivation of the surface is not unique in its ability to saturate dangling bonds and hence promote strong luminescence; oxidation, especially at high temperatures, can play a similar role. Oxidation also produces an additional, low energy band which is linked to residual dangling bond related defects at the Si-SiO2 interface. Furthermore, this band suffers a blue shift with increasing porosity in similar fashion to that observed for the visible emission.

The fundamental mechanisms controlling the light emission from porous Si remain unresolved. In this paper we report attempts to modify the luminescence using a variety of surface processing steps, such as vacuum annealing with subsequent anneals in nitrogen and oxygen, exposure to hydrofluoric acid (HF) and rapid thermal oxidation. Luminescence, infrared absorption, and electron spin resonance (ESR) have all been used to gain more information on the link between the optical emission and the localisation of the electrons in this material system. We present evidence that the silicon dangling bond is the key component in the non-radiative recombination. This is based on measurements shown that hydrogen coverage of the surface is significant because of saturation of the dangling bonds and a subsequent reduction in the competing non-radiative paths rather than as an active component in the radiative transition. Finally, we focus our attention upon the lower energy band which appears in the luminescence spectrum of porous Si (approx.0.9eV) by examining its behavior under the surface treatments mentioned above. We found that this luminescence band originates from the surface of the porous layer and its intensity correlates well with increasing oxidation of the porous layer.

We have used anodization techniques to process porous surface regions in p-type Czochralski Si and in p-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy. The SiGe layers were unrelaxed before processing. We have observed strong near-infrared and visible light emission from both systems. Analysis of the radiative and nonradiative recombination processes indicate that the emission is consistent with the decay of excitons localized in structures of one or zero dimensions.