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A conventional light management approach on a photo-catalyst is to concentrate photo-intensity to enhance the catalytic rate. We present a counter-intuitive approach where light intensity is distributed below the electronic photo-saturation limit under the principle of light maximization. By operating below the saturation point of the photo-intensity induced hydroxide growth under reactant gaseous H2+CO2 atmosphere, a coating of defect engineered In2O3-x(OH)y nanorod Reverse Water Gas Shift solar-fuel catalyst on an optical waveguide outperforms a coated plane by a factor of 2.2. Further, light distribution along the length of the waveguide increases optical pathlengths of the weakly absorptive green and yellow wavelengths, which increases CO product rate by a factor of 8.1-8.7 in the visible. Synergistically pairing with thinly doped silicon on the waveguide enhances the CO production rate by 27% over the visible. In addition, the persistent photoconductivity behavior of the In2O3-x(OH)y system enables CO production at a comparable rate for 2 h after turning off photo-illumination, enhancing yield with 44-62% over thermal only yield. The practical utility of persistent photocatalysis was demonstrated through outdoor solar concentrator tests, which after a day-and-night cycle showed CO yield increase of 19% over a day-light only period., Concentrating photo-intensities on photocatalyst has diminishing returns. Here the authors show the catalyst on glass rod waveguide at optimal low intensity results in high efficiency in the gas phase reverse water gas shift reaction in an annular glass cylindrical rod photoreactor.

Catalytic reduction of carbon dioxide is a promising strategy for mitigating global carbon emissions. In particular, the abundant and nontoxic material hydroxyapatite demonstrates high activity at ...

Optimizing kinetic barriers of ammonia synthesis to reduce the energy intensity has recently attracted significant research interest. The motivation for the research is to discover means by which a...

Nanostructured forms of stoichiometric In2O3 are proving to be efficacious catalysts for the gas-phase hydrogenation of CO2. These conversions can be facilitated using either heat or light; however, until now, the limited optical absorption intensity evidenced by the pale-yellow color of In2O3 has prevented the use of both together. To take advantage of the heat and light content of solar energy, it would be advantageous to make indium oxide black. Herein, we present a synthetic route to tune the color of In2O3 to pitch black by controlling its degree of non-stoichiometry. Black indium oxide comprises amorphous non-stoichiometric domains of In2O3-x on a core of crystalline stoichiometric In2O3, and has 100% selectivity towards the hydrogenation of CO2 to CO with a turnover frequency of 2.44 s−1., The utilization of white-colored, wide-bandgap CO2 hydrogenation photocatalysts has been hindered by their limited light-harvesting ability. By making stoichiometric white indium oxide non-stoichiometric and black, it is transformed from a highly inactive to a highly active photothermal catalyst.

In this study, we present the effect of silver diffusion into a dielectric medium and its consequent effect on the optical spectra on thin metal-dielectric structures. We show that on a grating pattern, reactive sputter deposition of aluminum nitride immediately following the sputter deposition of a thin layer of silver results in a broadening of the absorption peak of the AlN/Ag gratings which is directly related to diffusion of Ag ions into the overlying AlN layer. Time of Flight Secondary Ion Mass spectroscopy shows that the sample with broadened optical absorption has a significant concentration of Ag within the AlN layer and that the concentration profile follows the error function consistent with Fick’s second law of diffusion. Plasma emission spectroscopy reveal that the use of N2/Ar mixture instead of an Ar only plasma during RF sputtering reduces the emission of neutral silver atoms, implying that the density of singly ionized silver increases in a nitrogen-containing plasma. However, sputter deposition of AlN following exposure of the underlying silver grating film to ambient atmosphere does not give rise to the same broadening effect in the optical spectra for the AlN/Ag gratings due to the increased concentration of oxygen within the AlN and Ag layers, indicating that oxidation at the AlN/Ag interface inhibits silver migration into the dielectric.

Photo-Induced Transient Current Spectroscopy (PICTS) is a versatile technique for measurements of defect state energies and densities in photo-active materials. It is suitable for investigating the surface-gas adsorbate behavior and the defect characteristics of defect laden In2O3-x(OH)y nanorods, having oxygen vacancies and hydroxide surface groups, under in situ reactor conditions of dark ambient temperature, dark 150 °C and photo-illuminated 150 °C, for the photo-assisted Reverse Water Gas Shift reaction. From glovebox-protected X-ray Photoelectron Spectroscopy and in situ PICTS measurements we determined that the reduction of CO2 is associated with heterolytic dissociation of H2 into In-H§- and HO-H§+ centres accompanied by an increase in average carrier trap energies; increased carbonate formation in a photo/thermal reactor state of H2 + CO2, and an average trap energy decrease of 0.11 eV from H2 to a CO2 + H2 mixture, which correlates with binding energy shifts of the OH shoulder of the O1s spectra. These results show the reactivity link between the various OH groups, oxygen vacancies and trap energies of In2O3-x(OH)y in the reactant gas atmosphere components.

The photocatalytic activity of nanostructured In2O3–x(OH)y for the reverse water gas shift (RWGS) reaction CO2 + H2 → CO + H2O can be greatly enhanced by substitution of Bi(III) for In(III) in the ...

Calcium hydroxyphosphate, Ca10(PO4)6(OH)2, is commonly known as hydroxyapatite (HAP). The acidic calcium and basic phosphate/hydroxide sites in HAP can be modified via isomorphous substitution of calcium and/or hydroxide ions to enable a cornucopia of catalyzed reactions. Herein, isomorphic substitution of Ca2+ ions by Cu2+ ions especially at very low levels of exchange created new analogs of molecular surface frustrated Lewis pairs (SFLPs) in CuxCa10− x(PO4)6(OH)2, thereby boosting its performance metrics in heterogeneous CO2 photocatalytic hydrogenation. In situ Fourier transform infrared spectroscopy characterization and density functional theory calculations provided fundamental insights into the catalytically active SFLPs defined as proximal Lewis acidic Cu2+ and Lewis basic OH−. The photocatalytic pathway proceeds through a formate reaction intermediate, which is generated by the reaction of CO2 with heterolytically dissociated H2 on the SFLPs. Given the wealth of information thus uncovered, it is highly likely that this work will spur the further development of similar classes of materials, leading to the advancement and, ultimately, large‐scale application of photocatalytic CO2 reduction technologies., Complementary DRIFTS, UPS, in situ XPS, 1H MAS NMR, and DFT calculations provided insight into a new kind of SFLPs in low Cu2+ exchange CuxCa10− x(PO4)6(OH)2. A formate reaction intermediate is identified, which is generated by the reaction of CO2 with heterolytically dissociated H2 on Ca─OH···Cu lattice pairs to form Ca─OH2 +···CuH−.

This study shows unique capability of deep-subwavelength metal-insulator-metal (MIM) width-graded nano-gratings in offering high intensity electromagnetic field. The plasmonic field is underpinned by the strong coupling of the surface plasmon polaritons on the sidewalls of the nanogrooves. We present quantitative SERS detection of various biomolecule species at an ultra-low concentration corresponding to detection of single molecule. We report limit of detection of a gold coated bullseye width-graded plasmonic nano-grating as a SERS platform in detecting small molecule (such as propylene glycol) or sepsis biomarkers (such as protein c) at multiple wavelengths of light.

In this work, significant enhancement in absorption, especially in the near infra-red region, is achieved for ultra-thin crystalline silicon of various thicknesses using a photonic crystal structure. This is realized by patterning the silicon with close-packed inverted pyramid structuring, wherein the lattice spacing is comparable to the wavelength of the near infra-red light, which results in strong wave-interference-based light trapping. Absorption enhancement of more than a factor of two in the spectral range of 1000–1100 nm is achieved without any other optical enhancments. Details on the techniques used to realize the ultra-thin silicon, the fabrication of the inverted pyramids using standard photolithography and a complete optical analysis are reported in this work.

High minority carrier lifetime of more than 8 ms is achieved by depositing high quality hydrogenated amorphous silicon films on crystalline silicon using the triode plasma enhanced chemical vapor deposition (PECVD) technique. High quality surface passivation is made possible by virtue of introducing a biased grid between the two electrodes of an rf diode. In particular, by choosing an appropriate grid-substrate distance and applying a large negative DC bias on the grid, the passivation quality is substantially enhanced leading to a surface recombination velocity of 2.4 cm/s. In addition, the quality of doped amorphous silicon using the triode PECVD is also improved. An implied open circuit voltage of 710 mV is demonstrated from first silicon heterojunction solar cell devices.

The relationship between the oxygen composition of In2O3 thin films and their photodegradation capabilities has not been elucidated previously. In this investigation, the electronic and material ch...

A novel thin film anodized aluminum oxide templating process was developed and applied to make nanopores with anisotropic etching on crystalline silicon through reactive ion etching, with the purpose of enhancing the anti-reflection of silicon substrates. A unique two-step anodizing method was introduced to create high quality nano-channels and it was demonstrated that this process is superior over a one-step anodization approach. It was found that pore to pore distance and pore density can be tuned by changing the applied potential within a range of 10–80 V. Optical characterization of the nanopatterned silicon showed an average 10% reduction in reflection in the UV–Vis wavelength range.

Summary The production of solar methanol, directly from gaseous CO 2 and H 2 , is important for the development of a sustainable energy economy. Despite growing activity in the field, very few photocatalysts exist that can efficiently and stably hydrogenate gaseous CO 2 to methanol at ambient pressure with high selectivity. Here, we report that a defect-laden indium oxide, In 2 O 3−x (OH) y , with a rod-like nanocrystal superstructure, can photocatalyze the hydrogenation of CO 2 to methanol with 50% selectivity under simulated solar irradiation. Notably, the solar methanol production of the In 2 O 3−x (OH) y nanocrystal superstructures can be stabilized at a rate of 0.06 mmol g cat −1 h −1 at atmospheric pressure. This is 120 times higher than that of the best-known photocatalysts. This discovery bodes well for the development of a low-pressure solar methanol process using CO 2 and renewable H 2 feedstocks.

Solar-driven CO2 hydrogenation can provide a renewable source of fuels and reduce greenhouse gas emissions if operated at industrial scales. Herein we investigate the photomethanation (light-driven Sabatier reaction) rates over Ru films sputtered onto silica opal (Ru/SiO2) and inverted silicon opal photonic crystal (Ru/i-Si-o) supports at ambient temperature under solar-simulated radiation as a function of incident light intensity. Photomethanation rates over both the Ru/SiO2 and Ru/i-Si-o catalysts increase significantly with increasing light intensity, and rates as large as 2.8 mmol g−1 h−1 are achieved over the Ru/i-Si-o catalyst. Furthermore, the quantum efficiency of the photomethanation reaction is almost three times larger when measured over the Ru/i-Si-o catalyst as compared to the Ru/SiO2 catalyst. The large photomethanation rates over the Ru/i-Si-o catalyst are attributed to its exceptional light-harvesting properties. Moreover, we perform DFT analysis to investigate the potential role of photo-induced charges on the Ru surface. The results from the simulation indicate that charged Ru surfaces can destabilize adsorbed CO2 molecules and adsorb and dissociate H2 such that it can readily react with CO2, thereby accelerating the Sabatier reaction.

A periodically structured surface or grating has been used to demonstrate state-of-the-art high efficiency crystalline silicon photovoltaic cells. However, until now the understanding of the complete relationship between the grating periodicity, silicon thickness, and absorption enhancement in silicon solar cell with an inverted pyramidal texture is still unclear. In this paper, we simulate front surface inverted pyramidal grating texture on 2–400-μm thick silicon and optimize it to derive maximum photocurrent density from the cell. We identify a “one-size-fits-all” front grating period of 1000 nm that leads to maximum photo-absorption of normally incident AM1.5g solar spectrum in silicon (configured with a back surface reflector) irrespective of the thickness of the crystalline silicon absorbing layer. With the identification of such universally optimized periodicity for the case of an inverted pyramidal grating texture, a common fabrication process can be designed to manufacture high-efficiency devices on crystalline silicon regardless of the wafer thickness. The measured reflectance from submicron and wavelength scale periodic textures also verifies simulation results.

Metal-dielectric multilayers are versatile optical devices that can be designed to combine the visible transmittance of dielectrics with the electronic properties of metals for plasmonic and meta-material applications. However, their performances are limited by an interfacial optical absorption often attributed entirely to the metal surface roughness. Here, we show that during deposition of AlN/Ag/AlN and SiNx/Ag/SiNx multilayers, significant diffusion of Ag into the top dielectric layer form Ag nanoparticles which excite localized surface plasmon resonances that are primarily responsible for the interfacial optical absorption. Based on experimental depth profiles, we model the multilayer’s silver concentration profile as two complementary error functions: one for the diffused Ag nanoparticles and one for the interface roughness. We apply the Maxwell-Garnett and Bruggeman effective medium theories to determine that diffusion characteristics dominate the experimental absorption spectra. The newfound metal nanoparticle diffusion phenomenon effectively creates a hybrid structure characteristic of both metal-dielectric multilayer and metal-dielectric composite.

In the study reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) and in situ Hall charge carrier measurements provide new insights into the surface physical chemistry of gaseous H2, CO2, and H2+CO2 combined with nanostructured In2O(3&minus, x)(OH)y nanorods, which ensue under photochemical and thermochemical operating conditions. Heterolytic dissociation of H2 in H2-only atmosphere appears to occur mainly under dark and ambient temperature conditions, while the greatest amount of OH shoulder expansion in H2+CO2 atmosphere appears to mainly occur under photoilluminated conditions. These results correlate with those of the Hall measurements, which show that the prevalence of homolytic over heterolytic dissociation at increasing temperatures leads to a steeper rate of increase in carrier concentrations, and that H2 adsorption is more prevalent than CO2 in H2+CO2 photoillumination conditions.

We report on photo-related effects from doping for the reverse water gas shift reaction: enhanced photo-thermalization, and persistent CO yield post illumination generating a ‘memory’ effect. Bismuth substitution doping of Indium in Biz- In2O3-x(OH)y material systems results in significantly increased CO rate with increasing photo-intensities. Urbach analysis reveals that optical-phonon energies in doped systems increase compared to undoped systems by a factor of 1.18–1.41 in CO2+H2 atmosphere, suggesting enhanced photo-thermalization at high photo-intensities. Persistent CO yield post photo-illumination for high-doped In2O3-x(OH)y has a persistent period of 60 min in flow reactor with decay constant of 24.3 h in batch reactor, enhancing yield by 78% and 17%; low-doped and undoped systems present shorter prolonged production times with 30−40 min in flow reactor with longer decay constant of 31.0–31.5 hours in batch reactor. Linked to slow photocurrent decay trends which extends for 1−2 hours in undoped system, and more than 2 h for high doped system.

Nanoscale titanium nitride TiN is a metallic material that can effectively harvest sunlight over a broad spectral range and produce high local temperatures via the photothermal effect. Nanoscale indium oxide-hydroxide, In2 O3- x (OH)y , is a semiconducting material capable of photocatalyzing the hydrogenation of gaseous CO2 ; however, its wide electronic bandgap limits its absorption of photons to the ultraviolet region of the solar spectrum. Herein, the benefits of both nanomaterials in a ternary heterostructure: TiN@TiO2 @In2 O3- x (OH)y are combined. This heterostructured material synergistically couples the metallic TiN and semiconducting In2 O3- x (OH)y phases via an interfacial semiconducting TiO2 layer, allowing it to drive the light-assisted reverse water gas shift reaction at a conversion rate greatly surpassing that of its individual components or any binary combinations thereof.

In this study we investigated, theoretically and experimentally, the unique photoactive behavior of pristine and defected indium oxide surfaces providing fundamental insights into their excited state properties as well as an explanation for the experimentally observed enhanced activity of defected indium oxide surfaces for the gas-phase reverse water gas shift reaction, CO2 + H2 + hν→ CO + H2O in the light compared to the dark. To this end, a detailed excited-state study of pristine and defected forms of indium oxide (In2O3, In2O3-x, In2O3(OH)y and In2O3-x(OH)y) surfaces was performed using time dependent density functional theory (TDDFT) calculations, the results of which were supported experimentally by transient absorption spectroscopy and photoconductivity measurements. It was found that the surface frustrated Lewis pairs (FLPs) created by a Lewis acidic coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site in In2O3-x(OH)y become more acidic and basic and hence more active in the ES compared to the GS. This provides a theoretical mechanism responsible for the enhanced activity and reduced activation energy of the photochemical reverse water gas shift reaction observed experimentally for In2O3-x(OH)y compared to the thermochemical reaction. This fundamental insight into the role of photoexcited surface FLPs for catalytic CO2 reduction could lead to improved photocatalysts for solar fuel production.

A thin film Mo oxide–nitride pseudocapacitive electrode was synthesized by electrodeposition of Mo oxide on Ti and a subsequent low-temperature (400 °C) thermal nitridation. Two nitridation environments, N2 and NH3, were used and the results were compared. Surface analyses of these nitrided films showed partial conversion of Mo oxide to nitrides, with a lower conversion percentage being the film produced in N2. However, the electrochemical analyses showed that the surface of the N2-treated film had better pseudocapacitive behaviors and outperformed that nitrided in NH3. Cycle life of the resultant N2-treated Mo oxide–nitride was also much improved over Mo oxide. A two-electrode cell using Mo oxide–nitride electrodes was demonstrated and showed high rate performance.

Photocarrier radiometry (PCR) with 355 nm laser excitation was used for the study of c-Si covered with intrinsic thin hydrogenated amorphous Si (i-a-Si:H) on one, or both, sides, with thicknesses ranging from 10 nm to 90 nm. Short wave- length excitation allows one to resolve the contribution of the upper i-a-S layer to the PCR signal due to the very small absorption depth (tens of nm) of the excitation beam. As a result, fundamental transport parameters of the composite structure can be evaluated from the PCR frequency dependence. A theoretical model has been devel- oped to describe the diffuse carrier density wave (CDW) in this two-layer system. The model of the one-dimensional CDW fields for composite electronic solids involves front, interface, and back surface recombination velocities, the diffusion coefficient, recombinationlifetimesintheupper andlowerlayers,andtheunoccupied trapdensity at the interface. Simulations of the transport parameter influence on the PCR signal were performed, and the theoretical model was able to describe the experimental data accurately, therefore, making it possible to evaluate the transport parameters of i-a- Si:H and c-Si as well as to elucidate the role of interface electronic traps in the PCR frequency dependence under short wavelength excitation.

A new class of the multi-element catalysis system, Co–Cr–Pt–O, is used for vertically-aligned few-walled carbon nanotube (CNT) forest synthesis by a sandwich-growth technology in electron cyclotron resonance chemical vapor deposition. The effect of catalyst compositions on the resulting particle sizes after pretreatment, and the influence of process parameters on the CNT forest heights and diameters are investigated. Results show that a plasma pretreatment step reduces the metal oxides present in the as-deposited Co–Cr–Pt–O catalyst to their metallic states or to mixed metallic/oxidic states, and the co-catalytic synergistic effect during the reduction process thus leads to the production of uniformly small particles with a size of 3.3–3.9 nm over the substrate. Moreover, with CH4 as the carbon source, vertically-aligned CNT forests of ∼12 μm in height are achieved, and further characterizations indicate that the forests consist of dominant triple-walled nanotubes with an average inner tube diameter of 3.2–3.7 nm mostly. Additionally, field emission measurements show that the turn-on field of the CNT-forest emitters is below 2.6 V/μm, and the emission current density can reach 110 mA/cm2 at 4.5 V/μm. High reproducibility of field emission characteristics and Raman signals suggests the regularity of the CNT forests in structure and size.

article i nfo Asignificant improvement in the photoluminescence of erbium doped amorphous carbon (a-C:H(Er)) is reported. The effects of the RF power on the anode and cathode a-C:H films were investigated in terms of the microstructural and local bonding features. It was determined that Er doped a-C:H fi lms should be placed on the anode to obtain wider bandgap and lower percentage of sp 2 carbon bonding. The metalorganic compound, tris(6,6,7,7,8,8,8- heptafluoro-2,2-dimethyl-3,5-octanedionate) Erbium(+III) or Er(fod)3, was incorporated in-situ into an a-C:H host by metalorganic rf plasma enhanced chemical vapor deposition. This technique provides the capability of doping Er in a vertically uniform profile. The high erbium concentration (3.9 at.%), partial fluorination of the surrounding ligands, and the large optical bandgap of the host a-C:H are the primary factors that enable enhancement of the photoluminescence.

In situ Yb-doped amorphous carbon thin films were grown on Si substrates at low temperatures (

This article reports on the integration of facile native oxide-based passivation of crystalline silicon surfaces within the back amorphous-crystalline silicon heterojunction solar cell concept. The new passivation scheme consists of 1-nm thick native oxide and nominally 70-nm thick PECVD silicon nitride. The low temperature passivation scheme provides uniform high quality surface passivation and low parasitic optical absorption. The interdigitated doped hydrogenated amorphous silicon layers were deposited on the rear side of the silicon wafer using the direct current saddle field PECVD technique. A systematic analysis of a series of back amorphous-crystalline silicon heterojunction cells is carried out in order to examine the influence of the various cell parameters (interdigital gap, n-doped region width, ratio of widths of p, and n-doped regions) on cell performance. A photovoltaic conversion efficiency of 16.7 % is obtained for an untextured cell illuminated under AM 1.5 global spectrum (cell parameters: VOC of 641 mV, JSC of 33.7 mA-cm − 2 and fill factor of 77.3 %). Copyright © 2014 John Wiley & Sons, Ltd.

This article presents the design, fabrication and characterization of a micromachined energy harvester utilizing aluminium nitride (AlN) as a piezoelectric thin film material for energy conversion of random vibrational excitations. The harvester was designed and fabricated using silicon micromachining technology where AlN is sandwiched between two electrodes on top of a silicon cantilever beam which is terminated by a silicon seismic mass. The harvester generates electric power when subjected to mechanical vibrations. The generated electrical response of the device was experimentally evaluated at various acceleration levels. A maximum power of 34.78 μW was obtained for the device with a seismic mass of 5.6 × 5.6 mm2 at an acceleration value of 2 g. Various fabricated devices were tested and evaluated in terms of the generated electrical power as well as the resonant frequency.

A solid pseudocapacitive electrochemical capacitor (EC) enabled by a polymer electrolyte and molybdenum nitride electrodes has been developed and has demonstrated ultra-high rate performance. The solid EC can deliver 1 mF cm−2 at 100 Vs−1 and achieve a 10 ms time constant. This not only exceeds its liquid EC counter-part but also greatly surpasses a similar EC with titanium nitride electrodes as well as nanocarbon based electrochemical double layer capacitors.

Some Like It Hot: Geoffry A. Ozin and co‐workers demonstrate that the reverse water gas shift (RWGS) reaction, driven by a visible and near infrared responsive Pd@Nb2O5 is thermally activated with measured conversion rates as high as 1.8 mmol gcat−1 h−1, in article 1600189. Specifically, the RWGS reaction is enabled by heat generated from thermalization of photoexcited charge carriers in the Pd nanocrystals that function as “nanoheaters” for the entire Pd@Nb2O5 assembly, through non‐radiative relaxation of inter‐band and intra‐band conduction electron transitions. Overall this study advances our understanding of the underlying mechanism of photothermally driven CO2 reduction. Cover illustration by Chenxi Qian.

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.

Very high effective minority carrier lifetime (6.3 ms) and very low surface recombination velocity (2.6 cm/s) have been demonstrated on float-zone 1–2 Ω cm crystalline silicon (c-Si) wafers by depositing a-Si:H films using grid-biased remote radio-frequency plasma enhanced chemical vapor deposition (RF PECVD). This method employs a semi-transparent DC biased-grid within the conventional RF PECVD configuration. The DC-biased grid is positioned between the RF electrodes in order to develop a remote plasma and thus allow control of the flux and type of precursors involved in the growth of hydrogenated amorphous silicon (a-Si:H) film. It is shown that compared to conventional RF diode, the grid-biased remote RF PECVD method produces a-Si:H films with superior passivating properties as well as significantly lower concentrations of void and SiH 2 bonding and a lower overall hydrogen content, all of which contribute to a higher quality a-Si:H–c-Si heterostructure.

Photocarrier radiometry (PCR), microwave photoconductivity decay (μ-PCD) and transient and quasi-steady-state eddy currents were used for the characterization of a crystalline (c-) Si wafer passivated with intrinsic hydrogenated amorphous silicon films on either front or back side, or on both sides of the wafer. All three techniques are capable of measuring recombination lifetimes, however additional fundamental free photoexcited carrier transport parameters (diffusion coefficient, front- and back-surface recombination velocity) can be evaluated from the PCR frequency dependence owing to the free-carrier-density wave's diffusion-length depth-dependence on frequency. The measured recombination lifetimes are compared and differences in values among the three techniques are discussed in terms of the role of surface and interface defects. μ-PCD and eddy currents yield effective or vertically averaged lifetime measurements, as opposed to PCR which yields bulk recombination lifetimes separately from surface recombination effects.

Piezoelectric energy microgenerators are devices that generate continuously electricity when they are subjected to varying mechanical strain due to e.g. ambient vibrations. This paper presents the mathematical analysis, modelling and validation of a miniaturized piezoelectric energy harvester based on ambient random vibrations. Aluminium nitride as piezoelectric material is arranged between two electrodes. The device design includes a silicon cantilever on which AlN film is deposited and which features a seismic mass at the end of the cantilever. Euler–Bernoulli energy approach and Hamilton’s principle are applied for device modeling and analysis of the operation of the device at various acceleration values. The model shows good agreement with the experimental findings, thus giving confidence into model. Both mechanical and electrical characteristics are considered and compared with the experimental data, and good agreement is obtained. The developed analytical model can be applied for the design of piezoelectric microgenerators with enhanced performance.

Molybdenum nitride was developed through thermal nitridation of electrodeposited molybdenum oxide and was evaluated as electrode materials for electrochemical capacitors. Nitrided molybdenum electrode showed significantly enhanced capacitance and improved chemical stability comparing to the as-deposited molybdenum oxide electrode. The added capacitance was attributed from the reversible pseudocapacitance of molybdenum nitride. The study demonstrated an economical alternative process to produce molybdenum nitride electrodes with area specific capacitance comparable to those deposited by CVD.

Hydrogenated amorphous silicon (a-Si H) films deposited on crystalline silicon substrates using the DC saddle field (DCSF) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. We have determined the complex dielectric function, e(E) = e1(E) + ie2(E) for hydrogenated amorphous silicon (a-Si:H) thin films by spectroscopic ellipsometry (SE) in the 1.5–4.5 eV energy range at room temperature. The results indicate that there is a change in the structure of the a-Si:H films as the thickness is increased above 4 nm. This is attributed to either an increase in the bonded hydrogen content and, or a decrease of voids during the growth of a-Si:H films. The film thickness and deposition temperature are two important parameters that lead to both hydrogen content variation and silicon bonding change as well as significant variations in the optical band gap. The influence of substrate temperature during deposition on film and interface properties is also included.

This paper reports on the formation of unique micrometer-scale clusters in erbium-doped silicon-rich silicon oxide (ErSRSO) thin films produced by thermal evaporation. Through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), we identify these micro-clusters as structures where the film curves away from the substrate to encase erbium-rich aggregates. Even though erbium aggregation has been reported to cause quenching, we demonstrate that the micro-clusters as a whole can exhibit significantly enhanced erbium photoluminescence (PL), with emissions over 24 times brighter than the surrounding non-cluster region. Mapping micro-photoluminescence measurements onto SEM images reveals that the erbium aggregates alone do not generate the enhanced PL, but rather the thin film encasing the aggregates is the origin of the emissions. Analysis of the PL Stark splitting shows a correlation between the micro-clusters’ fine structure and the strength of their PL. Taken together, the above evidence indicates that the micro-clusters’ strong PL is produced by changes in the local environment of the Er 3+ inside the thin film surrounding the erbium-rich aggregates.

The DC saddle-field (DCSF) glow discharge method was used to deposit intrinsic a-Si:H onto c-Si to passivate the c-Si surface. The effective minority carrier lifetime in the heterostructures as a function of the excess minority carrier density in the c-Si wafers was measured. The results were then analyzed in the context of recombination associated with interface defect states using three known recombination models. The defect density and the charge density at the interface are inferred. In addition subsequent annealing of the samples was studied. It is shown that for our intrinsic a-Si:H samples improvements in surface passivation are directly correlated with the reduction of interface defects and not the reduction of minority carrier concentration at the interface due to electric field. We have achieved excellent surface passivation with effective carrier lifetime >4 ms for an intrinsic a-Si:H sample deposited at a process temperature of 200 °C and thickness of about 30 nm. It is also demonstrated that subsequent annealing, at 240 °C, of the samples which were prepared at process temperatures

This paper describes the use of chemical cross-linking of ultrathin inorganic nanowires as a bottom-up strategy for nanostructure fabrication as well as a chemical detection platform. Nanowire microfibers are produced by spinning a nanowire dispersion into a cross-linker solution at room temperature. Nanomembranes with thicknesses down to 50 nm were obtained by injecting the nanowire dispersion at the cross-linker-solution/air interface. Furthermore, the sensitivity of the nanowire to amine cross-linkers allowed development of a novel sensing platform for small molecules, like the neurotransmitter serotonin, with detection limits in the picomolar regime.

Room temperature photoluminescence (PL) at 1.54 mm from erbium-doped silicon rich silicon oxide (ErSRSO) films, fabricated on fused quartz by thermal evaporation followed by thermal-annealing in air, is reported. PL measurements show maximum intensity in samples annealed at 1000 � C for 4 h. X-ray diffraction (XRD) structural analyses show that annealing caused the formation of active Er 3þ (Er2O3) centers. XRD and PL results show that increasing Er2O3 concentration does not necessarily lead to an increase in photoluminescence. Compositional analysis using time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth-profiling shows a strong correlation between the presence of contiguous Si rich regions and Er2O3 centers on the one hand and the observed PL on the other. The combination of PL, XRD, and TOF-SIMS results indicate the presence of silicon clusters and its sensitization of erbium. These results are consistent with the literature reported decomposition of SiOx at elevated temperatures which in turn leads to the formation of Si clusters in the silicon oxide matrix. [DOI: 10.1143/JJAP.47.7211]

We report on a simple and versatile method for the integration of tritium in semiconductor materials. A variety of semiconductor materials are exposed to tritium (T{sub 2}) gas at pressures of up to 120 bar and temperatures of up to 250 deg.C Tritiated materials include hydrogenated amorphous silicon (a-Si:H), crystalline silicon (c-Si), silica and carbon nanotubes (CNT). Deep ultra-violet laser irradiation was used to lock tritium in silica films. Effusion measurements show the presence of stable tritium in silicon, silica and CNTs up to 400 deg.C. IR absorption spectra show a Si-T stretching mode at 1200 cm{sup -1} indicating the formation of stable Si-T bonds in a-Si:H. SIMS measurements show that the penetration depth of tritium in a-Si:H and c-Si is 150 and 10 nm, respectively; the concentration of tritium locked in a-Si:H and c-Si is 20 and 4 at.%, respectively. In tritiated silica, 248-nm UV laser irradiation locks the permeated tritium at stable chemical bonding sites in the silica lattice. Thermal effusion measurement shows that 0.5 wt. % tritium can be stably immobilized in CNTs. The application of tritiated silicon as a cold electron source is demonstrated. (authors)

The electrical potential at the substrate surface during the growth of hydrogenated microcrystalline silicon in a direct current saddle field (SF) plasma enhanced chemical vapor deposition (PECVD) system has been previously shown to be a limiting factor for the formation of the microcrystalline phase in the resulting thin films. The authors examine the extension of this concept to large areas under conditions necessary to obtain microcrystalline silicon in a SF-PECVD system—namely the use of hydrogen-diluted silane as a source gas, pressures between 100 and 300 mTorr, and substrate electrical bias between 100 and 250 V. The response of the SF-PECVD system to electrical substrate bias under these conditions is examined in detail, and four regimes of operation are identified, only one of which is useful for the growth of this particular material system. The delineation of these regimes provides new constraints and guidelines for the application of SF-PECVD technology to large-area deposition of thin films sensitive to ion bombardment during growth, such as microcrystalline silicon.

Currently, research is being directed towards thinning conventional 200–300mm thick silicon photovoltaic cells by an order of magnitude or more. The benefits of reducing the cell thickness include decreased material costs, enhanced cell flexibility, and reduced effects of light-induced degradation. However, one of the major challenges associated with reducing the active region to this extent is the corresponding reduction of light absorption. To mitigate this effect it has been proposed that the cell should incorporate enhanced light-trapping strategies. One potential approach to enhance light trapping in thin photovoltaic cells is to structure the back-reflector in the form of a photonic crystal (PC). It has recently been shown that two fundamental attributes of PC back-reflectors optically coupled to thin semiconductor films contribute to enhanced absorption in the semiconductor: (i) the PC backreflector behaves as a perfect mirror, exhibiting complete reflection over stop-gap frequencies; and (ii) the PC–semiconductor film interface couples incident light into resonant states that propagate along the plane of the film, thereby further enhancing the absorption. Although the ability of PC back-reflectors to enhance absorption is encouraging, significant challenges arise when attempting to incorporate this light trapping technique in photovoltaic devices. Herein, we describe the underlying physical mechanisms that give rise to absorption enhancements in thin Si wafers featuring PC back-reflectors, and describe hurdles that will have to be surmounted in order to reduce-to-practice a PC back-reflector into an actual PV device.

Defect creation through tritium decay in tritiated hydrogenated amorphous silicon provides a unique technique for the study of defect dynamics in hydrogenated amorphous silicon (a-Si:H). Isothermal Capacitance Transient Spectroscopy (ICTS) and Constant Photocurrent Method (CPM) were used to measure the positively charged, D+, and negatively charged, D−, states in the gap of a-Si:H:T, respectively. The samples were thermally annealed prior to the measurement of defect state densities. The ICTS experiments showed a decrease in the concentration of positively charged dangling bonds while the CPM measurements showed an increase in the density of D− states with time. This increase was much larger than the concentration of decayed tritium atoms. The CPM results also showed that the Urbach energy decreased with time. The decrease in Urbach energy indicates a decrease in the concentration of weak bonds in the valence band tail and suggests weak bond to dangling bond conversion. CPM experiments carried out under bias confirm the role of excess carriers in the defect creation process. The dynamic defect pool model was developed to quantitatively explain the experimental data. The defect evolution in a-Si:H:T is similar to the evolution of defect states in light exposure studies, supporting the interpretation of the Staebler–Wronski effect in terms of weak bond to dangling bond conversion.

A plasmon waveguide resonance (PWR) sensor is proposed for studying the interaction between gold nanoparticles and proteins. The ability of the PWR sensor to operate in both TM and TE Polarizations, i.e. its polarization diversity, facilitates the simultaneous spectroscopy of the nanoparticles surface reactions using both polarizations. The response of each polarization to streptavidin-biotin binding at the surface of gold nanoparticles is investigated in real time. Finally, using the principles of multimode spectroscopy, the nanoparticle's surface reactions are decoupled from the bulk solution refractive index variations. Schematic diagram of the NP-modified PWR sensor.

With continual thinning of crystalline silicon solar cells the carrier lifetime of the device becomes progressively more dependent on the quality of the surface passivation of the device. Thermal oxide growth can degrade crystalline silicon bulk properties through defect migration and can induce thermal stresses particularly in the context of ultra-thin silicon. This has motivated research into a range of passivation techniques that can be implemented at low temperatures. Here we report on the use of low temperature ozone ambient grown native oxide and PECVD grown silicon nitride bilayer structure to passivate crystalline silicon. The effective minority carrier lifetime is examined as a function of the thicknesses of the bilayers. Inferred interfacial defect and fixed charge densities are investigated vis-a-vis the native oxide and nitride layer thicknesses. Hydrogen content at the interface is determined by elastic recoil detection and Rutherford backscattering. Our results indicate that thicker silicon nitride enhances lifetime through reduced interfacial defect density albeit the trapped charge density decreases marginally with thickness. Preliminary ion spectroscopy results show a link to an increase in the hydrogen content beyond the oxide interface. Also, increasing oxide thickness grown at room temperature enhances the surface passivation.

A “four-wire” configuration thin-film silicon solar cell design is proposed. Front and middle TCO layers are optimized in order to be used, together with a Bragg reflector, to increase the useful current in the top cell (by about 1 mA/cm2 according to initial calculations) and hence the total photocurrent. Preliminary results from both experiments and numerical simulations are presented, showing the potential and challenges of the design. With reasonable assumptions, a stabilized efficiency of 14% could be achieved.

A series of hydrogenated microcrystalline films were grown by a novel thin film deposition method using the Saddle Field Plasma Enhanced Chemical Vapour Deposition system. We show that the surface potential during growth strongly affects the microcrystalline character of the films, as quantified by Raman scattering. This effect can be reproduced on both conductive and non-conductive substrates. Films grown close to the threshold for microcrystalline growth exhibit laser-induced crystallization at low laser intensities.

Characterization results on hydrogenated microcrystalline silicon (μc-Si:H) thin films grown in a Saddle Field (SF) PECVD system are presented. The microcrystalline content of the films is controlled by the application of a positive electrical bias to the film growth surface. The results of photoluminescence, atomic force microscopy, infrared-absorption, and electrical conductivity studies are presented. The results correlate to the changing microcrystalline content of the films in the same way as when microcrystalline content is influenced through growth parameters such as hydrogen dilution in other CVD techniques.