Diffraction gratings based on a multilayer silicon nitride waveguide with high upward efficiency and large effective length.

Diffraction gratings with high upward diffraction efficiency and large effective length are required for chip-scale light detection and ranging. We propose a diffraction grating based on a multilayer silicon nitride waveguide, which theoretically achieves an upward diffraction efficiency of 92%, a near-field effective length of 376 µm, and a far-field divergence angle of 0.105° at a wavelength of 850 nm. The diffraction grating has a high tolerance to process variations based on Monte Carlo analysis. When the conditions are ±5% layer thickness variation, ±50nm lithographic variation, and ±20nm wavelength drift, more than 71% of the grating samples have a diffraction efficiency higher than 80%, and 100% of the samples have an effective length larger than 200 µm (corresponding to a far-field divergence <0.2∘). Furthermore, the near-field effective length of the grating with an upward diffraction efficiency above 90% can be adjusted from hundreds of microns to centimeters by changing the etching layer thickness and the grating duty cycle. This diffraction grating has a potential application in optical sensing and imaging from visible to near-IR wavelengths.

As(III) Removal from Drinking Water by Carbon Nanotube Membranes with Magnetron-Sputtered Copper: Performance and Mechanisms.

Current approaches for functionalizing carbon nanotubes (CNTs) often utilize harsh chemical conditions, and the resulting harmful wastes can cause various environmental and health concerns. In this study, magnetron sputtering technique is facilely employed to functionalize CNT membranes by depositing Cu onto premade CNT membranes without using any chemical treatment. A comparative evaluation of the substrate polymeric membrane (mixed cellulose ester (MCE)), MCE sputtered with copper (Cu/MCE), the pristine CNT membrane (CNT), and CNT membrane sputtered with Cu (Cu/CNT) shows that Cu/CNT possesses mechanically stable structures and similar membrane permeability as MCE. More importantly, Cu/CNT outperforms other membranes with high As(III) removal efficiency of above 90%, as compared to less than 10% by MCE and CNT, and 75% by Cu/MCE from water. The performance of Cu/CNT membranes for As(III) removal is also investigated as a function of ionic strength, sputtering time, co-existing ions, solution pH, and the reusability. Further characterizations of As speciation in the filtrate and on Cu/CNT reveal that arsenite removal by Cu/CNT possibly began with Cu-catalyzed oxidation of arsenite to arsenate, followed by adsorptive filtration of arsenate by the membrane. Overall, this study demonstrates that magnetron sputtering is a promising greener technology for the productions of metal-CNT composite membranes for environmental applications.

Fabrication of ultra-thin silicon nanowire arrays using ion beam assisted chemical etching.

Uniform dispersion of Au-Ag alloy nanoparticles underneath the surface of a Si wafer is realized via Au film pre-deposition and Ag ion implantation. The Au-Ag nanoparticles are used as catalysts in metal assisted chemical etching for fabricating Si nanowire arrays with average diameters of less than 10 nm. We find that the alloy catalysts introduced by ion implantation are the key to obtaining thin nanowire arrays and we also demonstrate that SiNWAs with various diameters could be simply produced by changing the thickness of the pre-deposited Au layer. Compared with the traditional process, ion beam assisted chemical etching is proven to be a convenient and efficient approach to fabricate ultra-thin SiNWAs on a large scale.

Simulation Analysis on Photoelectric Conversion Characteristics of Silicon Nanowire Array Photoelectrodes.

Semiconductor nanowire photoelectrochemical cells have attracted extensive attention in the light-conversion field owing to the low-cost preparation, excellent optical absorption, and short distance of carrier collection. Although there are numbers of experimental investigations to improve the device performance, the understanding of the detailed process of photoelectric conversion needs to be further improved. In this work, a thorough optoelectronic simulation is employed to figure out how the nanowire diameter, doping concentration, and illumination wavelength affect the photoelectric conversion characteristics of the silicon nanowire array photoelectrodes. We find that two balances should be carefully weighted between optical absorption and photogenerated-carrier collection, along with between short-circuit photocurrent density and open-circuit voltage. For the small-diameter nanowire array photoelectrodes, the overall absorption is higher than that of the larger-diameter ones with the most contribution from the nanowires. However, the substrate shows increasing absorption with increasing illumination wavelength. Higher doping density leads to a larger open-circuit voltage; while lower doping density can guarantee a relatively higher short-circuit photocurrent. To obtain high-light-conversion-efficiency photoelectrodes, the doping density should be carefully chosen with considerations of illumination wavelength and surface recombination. Suppressing the surface recombination velocity can effectively enhance the short-circuit photocurrent (open-circuit voltage) for the lightly (heavily) doped nanowire array photoelectrodes. Our systematical results provide a theoretical guidance for the photoelectrochemical devices based on semiconductor nanostructures.

High current density and longtime stable field electron transfer from large-area densely arrayed graphene nanosheet-carbon nanotube hybrids.

Achieving high current and longtime stable field emission from large area (larger than 1 mm(2)), densely arrayed emitters is of great importance in applications for vacuum electron sources. We report here the preparation of graphene nanosheet-carbon nanotube (GNS-CNT) hybrids by following a process of iron ion prebombardment on Si wafers, catalyst-free growth of GNSs on CNTs, and high-temperature annealing. Structural observations indicate that the iron ion prebombardment influences the growth of CNTs quite limitedly, and the self-assembled GNSs sparsely distributed on the tips of CNTs with their sharp edges unfolded outside. The field emission study indicates that the maximum emission current density (Jmax) is gradually promoted after these treatments, and the composition with GNSs is helpful for decreasing the operation fields of CNTs. An optimal Jmax up to 85.10 mA/cm(2) is achieved from a 4.65 mm(2) GNS-CNT sample, far larger than 7.41 mA/cm(2) for the as-grown CNTs. This great increase of Jmax is ascribed to the reinforced adhesion of GNS-CNT hybrids to substrates. We propose a rough calculation and find that this adhesion is promoted by 7.37 times after the three-step processing. We consider that both the ion prebombardment produced rough surface and the wrapping of CNT foot by catalyst residuals during thermal processing are responsible for this enhanced adhesion. Furthermore, the three-step prepared GNS-CNT hybrids present excellent field emission stability at high emission current densities (larger than 20 mA/cm(2)) after being perfectly aged.

Irradiation damage determined field emission of ion irradiated carbon nanotubes.

Figuring out the underlying relationship between the field emission (FE) properties and the ion irradiation induced structural change of carbon nanotubes (CNTs) is of great importance in developing high-performance field emitters. We report here the FE properties of Si and C ion irradiated CNTs with different irradiation doses. It is found that the FE performance of the ion irradiated CNTs ameliorates before and deteriorates after an irradiation-ion-species related dose. The improved FE properties are ascribed to the increased amount of defects, while the degraded FE performance is attributed to the great shape change of CNTs. These two structural changes are further characterized by a structural damage related parameter: dpa (displacement per atom), and the FE performance of the ion irradiated CNTs is surprisingly found to be mainly dependent on the dpa. The optimal dpa for FE of the ion irradiated CNTs is ∼0.60. We ascribe this to the low irradiation doses and the low substrate temperature that make the ion irradiation play a more important role in producing defects rather than element doping. Furthermore, the ion irradiated CNTs exhibit excellent FE stability, showing promising prospects in practical applications.

Significant reduction of thermal conductivity in silicon nanowire arrays.

Vertically aligned single-crystal silicon nanowire arrays (SiNWs) with various lengths, surface roughnesses and porosities were fabricated with the metal-assisted chemical etching method. Using the laser flash technique and differential scanning calorimetry, we characterized the thermal conductivities of bulk SiNWs/Si/SiNWs sandwich-structured composites (SSCs) at room temperature (300 K). The results demonstrate that the thermal conductivities of SSCs notably decrease with increases in the length, surface roughness and porosity of SiNWs. Furthermore, based on the series thermal-resistance model, we calculated the thermal conductivity of porous SiNWs to be as low as 1.68 W m(-1) K(-1) at 300 K. Considering the remarkable phonon scattering from the diameter, surface roughness and porosity of SiNWs, leading to a significant reduction of the thermal conductivity, SSCs and SiNWs could be applied to high-performance thermoelectric devices.

Self-assembled growth of multi-layer graphene on planar and nano-structured substrates and its field emission properties.

Vertical multi-layer graphenes (MLGs) have been synthesized without a catalyst on planar and nano-structured substrates by using microwave plasma enhanced chemical vapor deposition. The growth of MLGs on non-carbon substrates is quite different from that on carbon-based substrates. It starts with a pre-deposition of a carbon buffer layer to achieve a homo-epitaxial growth. The nucleation and growth of MLGs was found to be strongly influenced by the surface geometry and topography of substrates. Planar substrates suitable for atom diffusion are favorable for growing large-scale MLGs, and defect-rich substrates are beneficial for quick MLG nucleation and thus the growth of densely distributed MLGs. The field emission properties of MLGs grown on planar and nano-structured substrates were studied and are found to be strongly dependent on the nature of substrates. Substrates having good conductivity and large aspect ratios such as carbon nanotubes (CNTs) have good field emission properties. The best field emission properties of MLG/CNT composites with optimal shapes were observed with a low turn-on electric field of 0.93 V µm(-1), a threshold field of 1.56 V µm(-1), a maximum emission current density of 60.72 mA cm(-2), and excellent stability.

Surface morphology-dependent photoelectrochemical properties of one-dimensional Si nanostructure arrays prepared by chemical etching.

Maximizing the optical absorption of one-dimensional Si nanostructure arrays (1DSiNSAs) is desirable for excellent performance of 1DSiNSA-based optoelectronic devices. However, a quite large surface-to-volume ratio and enhanced surface roughness are usually produced by modulation of the morphology of the 1DSiNSAs prepared in a top-down method to improve their optical absorption. Surface recombination is mainly determined by the surface characteristics and significantly affects the photogenerated carrier collection. In this paper, we systematically investigated the photoelectrochemical characteristics of 1DSiNSAs with various morphologies prepared by the metal-assisted chemical etching of Si wafers. Our results show that the saturation photocurrent density and photoresponsivity of 1DSiNSAs first increased and then gradually decreased with an increasing etching time, while the reflection spectrum was gradually suppressed to the measurable minimum. To identify the behaviors of the photoresponsivity and optical absorption of the various 1DSiNSAs, we analyzed the morphology, structure, and minority-carrier lifetime. Additionally, device physics simulations were used to confirm the significance of surface recombination. We proposed that future directions for the design of nanostructure-based optoelectronic devices should include not only strong optical absorption but also low surface carrier recombination. High-performance devices could be obtained only by balancing the requirements for light absorption and photogenerated carrier collection.

Enhanced electron field emission from carbon nanotubes irradiated by energetic C ions.

The field emission performance and structure of the vertically aligned multi-walled carbon nanotube arrays irradiated by energetic C ion with average energy of 40 keV have been investigated. During energetic C ion irradiation, the curves of emission current density versus the applied field of samples shift firstly to low applied fields when the irradiation doses are less than 9.6 x 10(16) cm(-2), and further increase of dose makes the curves reversing to a high applied field, which shows that high dose irradiation in carbon nanotube arrays makes their field emission performance worse. After energetic ion irradiation with a dose of 9.6 x 1016 cm(-2), the turn-on electric field and the threshold electric field of samples decreased from 0.80 and 1.13 V/microm to 0.67 and 0.98 V/microm respectively. Structural analysis of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy indicates that the amorphous carbon nanowire/carbon nanotube hetero nano-structures have been fabricated in the C ion irradiated carbon nanotubes. The enhancement of electron field emission is due to the formation of amorphous carbon nanowires at the tip of carbon nanotube arrays, which is an electron emitting material with low work function.

Structures and field emission properties of silicon nanowire arrays implanted with energetic carbon ion beam.

Structures and field emission properties of silicon nanowire arrays (SiNWAs), which were fabricated by using of electroless-chemical etching method and post-implanted by the energetic carbon ion beam with an average energy of 20 keV at various doses, have been investigated. Structural analysis of SEM and XPS shows that SiC compound had been formed at the top of SiNWAs, and Si-C/Si composite nanostructure had been obtained. Compared to as-grown SiNWAs, the C ion implanted SiNWAs have better field emission characteristics. The turn-on field and the applied field at 100 microA/cm2 are reduced from 5.01 V/microm and 5.93 V/microm for as-grown SiNWAs to 4.45 V/microm and 5.40 V/microm for SiNWAs implanted at the dose of 1 x 10(16) cm(-2), respectively. However, large implanting amounts made serious structural damages at the top of nanowires, and impaired the field emission characteristics. The influence of energetic C ion implantation on the structures and field emission properties of SiNWAs has been discussed.

Vapor-solid growth of few-layer graphene using radio frequency sputtering deposition and its application on field emission.

The carbon nanotube (CNT) and graphene hybrid is an attractive candidate for field emission (FE) because of its unique properties, such as high conductivity, large aspect ratio of CNT, and numerous sharp edges of graphene. We report here a vapor-solid growth of few-layer graphene (FLG, less than 10 layers) on CNTs (FLG/CNT) and Si wafers using a radio frequency sputtering deposition system. Based on SEM, TEM, and Raman spectrum analyses, a defect nucleation mechanism of the FLG growth was proposed. The FE measurements indicate that the FLG/CNT hybrids have low turn-on (0.956 V/µm) and threshold fields (1.497 V/µm), large field enhancement factor (∼4398), and good stability. Excellent FE properties of the FLG/CNT hybrids make them attractive candidates as high-performance field emitters.

Field emission enhancement of Au-Si nano-particle-decorated silicon nanowires.

Au-Si nano-particle-decorated silicon nanowire arrays have been fabricated by Au film deposition on silicon nanowire array substrates and then post-thermal annealing under hydrogen atmosphere. Field emission measurements illustrated that the turn-on fields of the non-annealed Au-coated SiNWs were 6.02 to 7.51 V/µm, higher than that of the as-grown silicon nanowires, which is about 5.01 V/µm. Meanwhile, after being annealed above 650°C, Au-Si nano-particles were synthesized on the top surface of the silicon nanowire arrays and the one-dimensional Au-Si nano-particle-decorated SiNWs had a much lower turn-on field, 1.95 V/µm. The results demonstrated that annealed composite silicon nanowire array-based electron field emitters may have great advantages over many other emitters.

Structures and field emission characteristics of ion irradiated silicon nanowire arrays.

Silicon nanowire (SiNW) arrays irradiated by energetic Si ions were fabricated by metal vapor vacuum arc (MEVVA) ion implantation method. Hetero-structure of amorphous/crystalline nanowire was formed in which structure of the implanted region on the top of the nanowires was amorphous while the structure of unimplanted region on the bottom remained crystal. Field emission (FE) properties of the SiNW arrays could be improved and modulated by different implantation doses. A low turn-on field of 4.63 V/microm was observed in the SiNWs irradiated by 21 keV Si ion with a dose of 7.86 x 10(16)/cm2, and the applied field for the emission current density reaching 100 microA/cm2 is only 5.52 V/microm. The main reason for the efficient emission is attributed to the formation of amorphous SiNWs and structure defects after implantation. The ion irradiated SiNWs after post-annealing at high temperature had better FE property due to eliminating the restrain effect to electrons.

Fabrication and Properties of Ag-nanoparticles Embedded Amorphous Carbon Nanowire/CNT Heterostructures.

Carbon nanotubes were subjected to doping with an energetic Ag ion beam, and the carbon nanotubes on the top of the array were transformed into amorphous carbon nanowires with embedded Ag-nanoparticles. The field emission characteristics of these nanowires were investigated. The minimum turn-on and threshold fields were 0.68 and 1.09 V/mum, respectively, which were lower than those of the as-grown carbon nanotubes. This was probably because Ag-nanoparticles embedded in the carbon nanowires reduced the effective work function from 4.59 to 4.23 eV. Large doping amounts produced serious structural damage at the top of the nanowires and impaired the field emission characteristics.

Fabrication of silicon carbide nanowires/carbon nanotubes heterojunction arrays by high-flux Si ion implantation.

An array of silicon carbide nanowire (SiCNW)-carbon nanotube (CNT) heterojunctions was fabricated by high-flux Si ion implantation into a multi-walled carbon nanotube (MWCNT) array with a metal vapor vacuum arc (MEVVA) ion source. Under Si irradiation, the top part of a CNT array was gradually transformed into an amorphous nanowire array with increasing Si dose while the bottom part still remained a CNT structure. X-ray photoelectron spectroscopy (XPS) analysis shows that the SiC compound was produced in the nanowire part even at the lower Si dose of 5 × 10(16) ions cm(-2), and the SiC amount increased with increasing the Si dose. Therefore, the fabrication of a SiCNW-CNT heterojunction array with the MEVVA technique has been successfully demonstrated. The corresponding formation mechanism of SiCNWs was proposed.

[A comparative Raman study of carbon nanotubes allays].

A series of comparative Raman study of carbon nanotubes arrays prepared by thermal chemical vapor deposition are reported. The results suggest that the G mode and D mode of carbon nanotubes (CNTs) arrays are all downshifted as compared to that of polycrystalline graphite, and the shifted number in well-aligned CNTs arrays is more than that in misaligned CNTs arrays. Moreover, the intensity ratio ID/IG indicates the ordering in CNTs arrays. A lower ID/IG means a higher graphitization and less amorphous carbon in CNTs arrays.

[Investigation of the layers doped with rare earth elements in Si substrate and it's key problems].

The photoluminescence properties of rare earth doped silicon were investigated with ion beam technique. The photoluminescence spectra in the layers doped with ions of La, Ce and Nd were obtained at room temperature. At the same time, the up-conversion luminescence in the doped layers was observed. The intensities of both the luminescence and the up-conversion luminescence increased with increasing the doping dose and the treatment temperature. However, the intensities of the luminescence decreased with increasing the exciting wavelength between 220 nm and 300 nm; the intensities of the up-conversion luminescence increased with increasing the exciting wavelength between 600 nm and 800 nm. It was indicated that the luminescence and the up-conversion luminescence in the layers doped with ions of La, Ce and Nd depended on the doping dose, the heat treatment temperature and the exciting wavelength.

[Photoluminescence from Si and Er dual-implanted Si-rich thermal oxidation SiO2/Si thin films].

Si and Er ions were implanted into Si-rich thermal oxidation SiO2/Si thin film using metal vapor vacuum arc (MEVVA) ion source implanter, which could produce ion beams with high-fluence and strong-flux. Rutheroford backscattering spectra show that Er concentration in as-implanted sample is attained to 10 at. % corresponding to the level of 10(21) atoms.cm-3. Needle nanocrystal Si in the surface of annealed samples have been formed during ion implantation followed by rapid thermal annealing. Photoluminescence around 1.54 microns from Er-doped Si-rich thermal oxidized SiO2/Si thin film were observed at 77 K and room-temperature (RT). Light emission around the wavelength of 1.54 microns were closely related to many conditions including substrate, ion implantation and annealing etc.