Graphene oxide as the passivation layer for Cu(x)O photocatalyst on a plasmonic Au film and the corresponding photoluminescence study.

The contribution of graphene oxide (GO) on photocatalytic effects of Cu(x)O on plasmonic Au is investigated. It is found that the H(2) evolution rate from pure water is enhanced 1.4 fold using the visible-active Cu(x)O/GO photocatalyst, as compared with Cu(x)O without GO. In addition, the intensity of photoluminescence of Cu(x)O/GO can be enhanced as much as 2.85 fold as compared with Cu(x)O without GO. The enhancement is due to the negative fixed charge in GO, which can passivate the surface of Cu(x)O and suppress recombination of minority electrons at the surface. The results from optical characterization in this study can help to prove the proposed mechanism of passivation.

Surface Plasmon assisted Cu(x)O photocatalyst for pure water splitting.

In this paper, Cu(x)O photocatalyst on plasmonic nanoporous Au film is proposed to enhancing the H(2) evolution rate of pure water splitting. The nanoporous Au film can simultaneously provide surface-enhanced absorption and built-in potential. The reflection spectrum shows that the surface plasmon (SP) assisted absorption wavelength of the Cu(x)O on the nanoporous Au film can be modified by changing the annealing temperature. It is found that the enhancement of the H(2) evolution rate highly depends on the SP-assisted absorption. As the annealing temperature is 220 ° C, the H(2) evolution rate is 58 µmol hr(-1) under the condition that the device area is 0.25 cm(2).

Enhanced conversion efficiency and surface hydrophobicity of nano-roughened Teflon-like film coated poly-crystalline Si solar cells.

Nano-roughened Teflon-like film coated poly-crystalline Si photovoltaic solar cells (PVSCs) with enhanced surface hydrophobicity and conversion efficiency (η) are characterized and compared with those coated by a Si nanorod array or a standard SiN anti-reflection layer. The Teflon-like film coated PVSC surface reveals a water contact angle increasing from 89.3° to 96.2° as its thickness enlarges from 22 to 640 nm, which is much larger than those of the standard and Si nanorod array coated PVSC surfaces (with angles of 55.6° and 32.8°, respectively). After nano-roughened Teflon-like film passivation, the PVSC shows a comparable η(10.89%) with the standard SiN coated PVSC (η = 11.39%), while the short-circuit current (I(SC)) is slightly reduced by 2% owing to the slightly decreased UV transmittance and unchanged diode performance. In contrast, the Si nanorod array may offer an improved surface anti-reflection with surface reflectance decreasing from 30% to 5% at a cost of optical scattering and randomized deflection, which simultaneously decrease the optical transmittance from 15% to 3% in the visible region without improving hydrophobicity and conversion efficiency. The Si nanorod array covered PVSC with numerous surface dangling bonds induced by 1 min wet-etching, which greatly reduces the open-circuit voltage (V(OC)) by 10-15% and I(SC) by 30% due to the reduced shunt resistance from 3 to 0.24 kΩ. The nano-scale roughened Teflon-like film coated on PVSC has provided better hydrophobicity and conversion efficiency than the Si nanorod array covered PVSC, which exhibits superior water repellant performance and comparable conversion efficiency to be one alternative approach for self-cleaning PVSC applications.

A 533-nm self-luminescent Si-rich SiNx/SiOx distributed Bragg reflector.

A 24-pair Si-rich SiNx/SiOx-based distributed Bragg reflector (DBR) architecture, in situ doped with Si nanocrystals (Si-ncs), is studied to show self-photoluminescence (PL) with narrow-linewidth green-color emission pattern. By cascaded depositing, the broadband luminescent SiNx/SiOx pairs with SiNx and SiOx layer thickness of 45 and 86 nm and corresponding refractive indices of 1.96 and 1.62, respectively, and the transmitted PL linewidth of the in situ Si-nc-doped DBR emitter/filter centered at a wavelength of 533 nm greatly reduces from 150 to 10 nm, which is achieved by blocking the UV and blue luminescence at 400-510 nm with the DBR filter bandwidth up to 95 nm. A multilayer DBR modeling is established to simulate the transmitted PL from the summation of each emissive SiNx/SiOx pair, providing a coincident PL shape with a spectral linewidth of 15 nm.

Nonlinear dependence between the surface reflectance and the duty-cycle of semiconductor nanorod array.

The nonlinear dependence between the duty-cycle of semiconductor nanorod array and its surface reflectance minimization is demonstrated. The duty-cycle control on thin-SiO2 covered Si nanorod array is performed by O(2-) plasma pre-etching the self-assembled polystyrene nanosphere array mask with area density of 4 × 10(8) rod/cm(-2). The 120-nm high SiO2 covered Si nanorod array is obtained after subsequent CF4/O2 plasma etching for 160 sec. This results in a tunable nanorod diameter from 445 to 285 nm after etching from 30 to 80 sec, corresponding to a varying nanorod duty-cycle from 89% to 57%. The TM-mode reflection analysis shows a diminishing Brewster angle shifted from 71° to 54° with increasing nanorod duty-cycle from 57% to 89% at 532 nm. The greatly reduced small-angle reflectance reveals a nonlinear trend with enlarging duty-cycle, leading to a minimum surface reflectance at nanorod duty-cycle of 85%. Both the simulation and experiment indicate that such a surface reflectance minimum is even lower than that of a uniformly SiO2 covered Si substrate on account of its periodical nanorod array architecture with tuned duty-cycle.

Si nanorod length dependent surface Raman scattering linewidth broadening and peak shift.

Enhanced Stoke Raman scattering of large-area vertically aligned Si nanorod surface etched by metal-particle-catalytic is investigated. By enlarging the surface area with lengthening Si nanorods, the linear enhancement on Stoke Raman scattering intensity at 520 cm(-1) is modeled to show well correlation with increasing quantity of surface Si dangling bonds. With Si nanorod length increasing from 0.19 to 2.73 µm, the Raman peaks of the as-etched and oxidized samples gradually shift from -4 cm(-1) and from -4.5 cm(-1) associated with their linewidth broadening from 3 to 9 cm(-1) and from 7 to 18 cm(-1), respectively. The peak intensity of Raman scattering signal from Si nanorod could be enhanced with the increase of interaction area as the number of phonon mode directly corresponds to the tetrahedrally coordinated Si vibrations in the bulk crystal lattice. The asymmetric linewidth broadening and corresponding Raman peak shift is affected by the strained Si nanorod surface caused by etching and the crystal quality. Fourier transform infrared spectroscopy corroborates the dependency between nanorod length and Si-O-Si stretching mode absorption (at 1097 cm(-1)) on oxidized Si nanorod surface, elucidating the increased transformation of surface dangling bonds to Si-O-Si bonds for passivating Si nanorods and attenuating Stoke Raman scattering after oxidation.

Spatially confined synthesis of SiOx nano-rod with size-controlled Si quantum dots in nano-porous anodic aluminum oxide membrane.

By depositing Si-rich SiOx nano-rod in nano-porous anodic aluminum oxide (AAO) membrane using PECVD, the spatially confined synthesis of Si quantum-dots (Si-QDs) with ultra-bright photoluminescence spectra are demonstrated after low-temperature annealing. Spatially confined SiOx nano-rod in nano-porous AAO membrane greatly increases the density of nucleated positions for Si-QD precursors, which essentially impedes the route of thermally diffused Si atoms and confines the degree of atomic self-aggregation. The diffusion controlled growth mechanism is employed to determine the activation energy of 6.284 kJ mole(-1) and diffusion length of 2.84 nm for SiO1.5 nano-rod in nano-porous AAO membrane. HRTEM results verify that the reduced geometric dimension of the SiOx host matrix effectively constrain the buried Si-QD size at even lower annealing temperature. The spatially confined synthesis of Si-QD essentially contributes the intense PL with its spectral linewidth shrinking from 210 to 140 nm and its peak intensity enhancing by two orders of magnitude, corresponding to the reduction on both the average Si-QD size and its standard deviation from 2.6 to 2.0 nm and from 25% to 12.5%, respectively. The red-shifted PL wavelength of the Si-QD reveals an inverse exponential trend with increasing temperature of annealing, which is in good agree with the Si-QD size simulation via the atomic diffusion theory.

Plasma power controlled deposition of SiOx with manipulated Si Quantum Dot size for photoluminescent wavelength tailoring.

Plasma power controlled PECVD of SiO(x) under SiH(4)/N(2)O gas mixture with manipulated Si quantum dot (Si-QD) size for tailoring photoluminescent (PL) wavelength is demonstrated. The incomplete decomposition of N(2)O at high plasma power facilitates Si-rich SiO(x) deposition to enlarge O/Si composition ratio and to shrink Si-QD size. As RF plasma power increases from 20 to 70 W, the O/Si ratio is increased from 1 to 1.6 and the average Si-QD size is reduced from 4.5 to 1.7, which increases Si-QD density from 3.2 x 10(17) to 3.02 x 10(18) cm(-3) and blue-shifts PL wavelength from 780 to 380 nm.

Nearly Gaussian dependence between near-infrared photoluminescence and O/Si composition ratio of Si-rich SiO(x) film with buried Si nanocrystals.

O/Si composition ratio optimized near-infrared (NIR) photoluminescence (PL) from Si nanocrystal (nc-Si) precipitated in Si-rich SiO(x) grown by Ar-diluted SiH4 and N2O is demonstrated. By detuning N2O fluence and N2O/SiH4 ratio to adjust the O/Si composition ratio of SiO(x) from 1.38 to 0.88, a nonlinearly Gaussian-like dependency of PL versus O/Si ratio with its maximum at O/Si of 1.24 is obtained. Maximized Si atomic density of 44.64 atom.% and nc-Si related PL of 25 count/nm is observed from SiO(x) with O/Si = 1.24 grown at N2O fluence of 50 sccm and N2O/SiH4 flowing ratio as low as 5.5. The FTIR analysis clearly indicates that the strong absorption bands at 870 and 2250 cm(-1) contributed by Si-H bonds suggests the hydrogen-passivated nc-Si could efficiently preserve the NIR PL response. Further reduction on O/Si composition ratio inevitably transfers the phase of host matrix from Si2O3 to SiO, which exhausts residual Si atoms and greatly decreases nc-Si density to degrade the PL response.

Manipulative depolarization and reflectance spectra of morphologically controlled nano-pillars and nano-rods.

Depolarization of sub-mum-high Si nano-pillar/nano-rod surface reflectance with morphologically controlled anti-reflection spectrum is demonstrated. Extremely small reflectance dip of 1.5% at 400-450 nm for Si nano-pillars is extraordinary when comparing with Si nano-rods, in which the reflectance vs. L/lambda for Si nano-pillars coincides well with the graded-index multilayer based modeling spectrum. Alternatively, Si nano-rods preserve its flattened reflectance spectrum up to 1700 nm, whereas the Si nano-pillar surface reflectance monotonically increases to approach that of bulk Si. The destructive interference is only induced on Si nano-pillar surface with larger aspect-ratio > or =15 and small sidewall slope <7 to suppress surface reflectance at blue-green wavelength region. Anomalous depolarization observed from disordered Si nano-pillar/nano-rod surface reflection indicates that TM-mode incidence interacts with more bound electrons than TE-mode to preserve its effective dielectric permittivity less deviated from the bulk Si. The degraded depolarization ratio observed under TE-mode incidence which correlates well with a simplified bounded-electron resonance model is elucidated.

Aspect-ratio-dependent ultra-low reflection and luminescence of dry-etched Si nanopillars on Si substrate.

The Si nanopillars with high aspect ratio were fabricated by dry-etching the thin SiO(2)-covered Si substrate with a rapidly self-assembled Ni nanodot patterned mask. Aspect-ratio-dependent ultra-low reflection and anomalous luminescence of Si nanopillars are analyzed for applications in all-Si based lighting and energy transferring systems. The Si nanopillars induce an ultra-low reflectance and refractive index of 0.88% and 1.12, respectively, at 435 nm due to the air/Si mixed structure and highly roughened surface. The reflectance can be <10% with a corresponding refractive index of<1.80 between 190 and 670 nm. Lengthening the Si nanopillars from 150 +/- 15 to 230 +/- 20 nm further results in a decreasing reflectance, corresponding to a reduction in refractive index by Delta n/n = 18% in the visible and near-infrared wavelength region. After dry-etching an Si wafer into Si nanopillars, the weak blue-green luminescence with double consecutive peaks at 418-451 nm is attributed to the oxygen defect (O(2-))-induced radiation, which reveals less relevance with the ultra-low-reflective Si nanopillar surface.