Self‐Assembled In‐Plane Growth of Mg2SiO4 Nanowires on Si Substrates Catalyzed by Au Nanoparticles

In‐plane growth of Mg₂SiO₄ nanowires on Si substrates is achieved by using a vapor transport method with Au nanoparticles as catalyst. The self‐assembly of the as‐grown nanowires shows dependence on the substrate orientation, i.e., they are along one, two, and three particular directions on Si (110), (100), and (111) substrates, respectively. Detailed electron microscopy studies suggest that the Si substrates participate in the formation of Mg₂SiO₄, and the epitaxial growth of the nanowires is confined along the Si <110> directions. This synthesis route is quite reliable, and the dimensions of the Mg₂SiO₄ nanowires can be well controlled by the experiment parameters. Furthermore, using these nanowires, a lithography‐free method is demonstrated to fabricate nanowalls on Si substrates by controlled chemical etching. The Au nanoparticle catalyzed in‐plane epitaxial growth of the Mg₂SiO₄ nanowires hinges on the intimate interactions between substrates, nanoparticles, and nanowires, and our study may help to advance the developments of novel nanomaterials and functional nanodevices.     

Heterojunction based hybrid silicon nanowire solar cell: surface termination,photoelectron and photoemission spectroscopy study

Silicon nanowires (SiNWs) combined with a conducting polymer are studied to constitute a hybrid organic/inorganic solar cell. This type of cell shows a particularly high interfacial area between SiNWs and the polymer so that interfacial control and interface optimization are required. For that purpose, we terminated the SiNW surfaces with well selected functional groups (molecules) such as native oxide (hereinafter SiO₂‐SiNW), hydrogen (hereinafter H‐SiNW) and methyl (hereinafter CH₃‐SiNW). A radial hetero‐junction solar cell is formed, and the cell parameters with and without interface control by functionalization with molecules are compared. Electronically, the three surfaces were close to flat‐band conditions. The CH₃‐SiNW, H‐SiNW and SiO₂‐SiNW produced a surface dipole of −0.12, +0.07 and 0.2 eV and band bending of 50, 100 and 170 meV, respectively. The surface properties of functionalized SiNWs are investigated by photoelectron yield (PY) and photoemission spectroscopy. PY studies on functionalized SiNWs are presented for the first time, and our results show that this type of measurement is an excellent option to carry out interface optimization of NWs for envisaged nano‐electronic and photonic applications. The solar cell efficiency is increased dramatically after terminating the surface with CH₃ molecules due to the decrease of the defect emission. The differently functionalized SiNW surfaces showed identical absorbance, reflectance and transmission so that a change in PY can be attributed to the Si–C bonds at the surface. This finding permits the design of new solar cell concepts. Copyright © 2013 John Wiley & Sons, Ltd.     

Low‐Temperature Formation of Well‐Aligned Nanocrystalline Si/SiOx Composite Nanowires

Well‐aligned nanocrystalline (nc)‐Si/SiO_x composite nanowires have been deposited on various substrates at 120 °C using SiCl₄/H₂ in a hot‐filament chemical vapor deposition reactor. Structural and compositional analyses reveal that silicon nanocrystals are embedded in the amorphous SiO_x nanowires. The nc‐Si/SiO_x composite nanowires are transparent in the range 500–900 nm. Photoluminescence spectra of the nc‐Si/SiO_x composite nanowires have a broad emission band, ranging from 420 to 525 nm. Water vapor from the chamber wall plays a crucial role in the formation of the well‐aligned nanowires. A possible mechanism for the formation of the composite nanowires is suggested.     

Surface‐Dominated Transport Properties of Silicon Nanowires

Surface effects are widely recognized to significantly influence the properties of nanostructures, although the detailed mechanisms are rarely studied and unclear. Herein we report for the first time a quantitative evaluation of the surface‐related contributions to transport properties in nanostructures by using Si nanowires (NWs) as a paradigm. Critical to this study is the capability of synthesizing SiNWs with predetermined conduction type and carrier concentration from Si wafer of known properties using the recently developed metal‐catalyzed chemical etching method. Strikingly, the conductance of p‐type SiNWs is substantively larger in air than that of the original wafer, is sensitive to humidity and volatile gases, and thinner wires show higher conductivity. Further, SiNW‐based field‐effect transistors (FETs) show NWs to have a hole concentration two orders of magnitude higher than the original wafer. In vacuum, the conductivity of SiNWs dramatically decreases, whereas hole mobility increases. The device performances are further improved by embedding SiNW FETs in 250 nm SiO₂, which insulates the devices from atmosphere and passivates the surface defects of NWs. Owing to the strong surface effects, n‐type SiNWs even change to exhibit p‐type characteristics. The totality of the results provides definitive confirmation that the electrical characteristics of SiNWs are dominated by surface states. A model based on surface band bending and carrier scattering caused by surface states is proposed to interpret experimental results. The phenomenon of surface‐dependent transport properties should be generic to all nanoscale structures, and is significant for nanodevice design for sensor and electronic applications.     

利用金作为催化剂在不同衬底上制备二氧化硅纳米线

利用金作为催化剂分别在二氧化硅及硅衬底上制备出二氧化硅纳米线。用扫描电子显微镜（SEM）及x射线光电子能谱（XPS）对纳米线进行了结构表征。SEM结果表明二氧化硅纳米线的长度为几个纳米,直径为20-150纳米。XPS结果给出硅与氧的原子比为1:2,说明所得到的为二氧化硅纳米线。二氧化硅纳米线的生长机理为气-液-固（VLS）机制。实验发现退火时间影响二氧化硅纳米线的形貌。我们也讨论了衬底对纳米线生长的影响。     

High-quality oxide formed by evaporation of SiO nanopowder: Application to MOSFETs on plastic substrates and GaN epilayers

Although good gate oxide of SiO₂ is usually formed by high-temperature thermal oxidation, lowering the temperature for formation of SiO₂ is mandatory for future Si VLSIs, in particular, for flexible ICs, the demand for which has been increasing every year. Vacuum evaporation of SiO powder is an ideal technique not only to form oxide at low temperature but also to form an abrupt interface with the substrate. The latter feature of evaporation is suitable to form thin gate oxide for Si MOSFETs and gate oxide on compound semiconductors. High-quality SiO₂ on compound semiconductors helps development of MOSFETs made of compound semiconductors, which were longed for to be commercially available. The evaporation is not much used to form SiO₂ for MOSFETs in spite of its many advantages, because quality of SiO₂ formed by evaporation of SiO is too poor to be used as gate oxide. Unlike the commercial SiO powder, the newly developed SiO nanopowder, made by thermal CVD using SiH₄ and O₂, consists of spherical particles with sizes less than 50 nm. It does not contain any Si nanocrystals but small molecular Si networks. Such molecular Si networks are easily thermally or optically decomposed. This makes the deposited oxide more free from Si nanocrystals, which usually degrade the insulating property of the oxide. The SiO₂ thin films formed by evaporation of the SiO nanopowder have demonstrated great potential for application to MOSFETs on plastic substrates and GaN epilayers.     

ZnSe–Si Bi‐coaxial Nanowire Heterostructures

We report on the fabrication, structural characterization, and luminescence properties of ZnSe/Si bi‐coaxial nanowire heterostructures. Uniform ZnSe/Si bi‐coaxial nanowire heterostructures are grown on silicon substrates by the simple one‐step thermal evaporation of ZnSe powder in the presence of hydrogen. Both ZnSe and silicon are single‐crystalline in the bi‐coaxial nanowire heterostructures, and there is a sharp interface along the nanowire axial direction. Furthermore, secondary nanostructures of either ZnSe nanobrushes or a SiO_x sheath are also grown on the primary bi‐coaxial nanowires, depending on the ratio of the source materials. The experimental evidence strongly suggests that bi‐coaxial nanowires are formed via a co‐growth mechanism, that is, ZnSe terminates specific surfaces of silicon and leads to anisotropic, one‐dimensional silicon growth, which simultaneously serves as preferential nucleation sites for ZnSe, resulting in the bi‐coaxial nanowire heterostructures. In addition, the optical properties of ZnSe/Si nanowires are investigated using low‐temperature photoluminescence spectroscopy.     

A High‐Performance Nitro‐Explosives Schottky Sensor Boosted by Interface Modulation

A high‐performance Schottky sensor boosted by interface modulation is fabricated for the detection of trace nitro‐explosives vapors. The interface modulation strategy results in a silicon nanowires (SiNWs) array/TiO₂/reduced graphene oxide (rGO) sensor with sensitive and selective response toward nitro‐explosives vapors. The response of the SiNWs array/TiO₂/rGO sensor toward nitro‐explosives vapors, such as 9 ppb 2,4,6‐trinitrotoluene, 4.9 ppt hexogen, and 0.25 ppq octagon, is boosted by 2.4, 7.5, and 5 times with the insertion of TiO₂. Superior selectivity is shown even compared with interfering gases of 10 ppm. Such good sensing performance can be attributed to the good sensing performance of the Schottky heterojunction‐based sensor, the Schottky barrier height modulation with the insertion of TiO₂, SiNWs array structure enhanced diffusion, and TiO₂ nanoparticles enhanced adsorption. This is believed to be the first Schottky heterojunction‐based sensor for nitro‐explosives vapors detection. This work would open a new way to develop highly sensitive and selective sensors.     

Single‐Crystalline p‐Type Zn3As2 Nanowires for Field‐Effect Transistors and Visible‐Light Photodetectors on Rigid and Flexible Substrates

Zn₃As₂ is an important p‐type semiconductor with the merit of high effective mobility. The synthesis of single‐crystalline Zn₃As₂ nanowires (NWs) via a simple chemical vapor deposition method is reported. High‐performance single Zn₃As₂ NW field‐effect transistors (FETs) on rigid SiO₂/Si substrates and visible‐light photodetectors on rigid and flexible substrates are fabricated and studied. As‐fabricated single‐NW FETs exhibit typical p‐type transistor characteristics with the features of high mobility (305.5 cm² V^?1 s^?1) and a high I_on/I_off ratio (10⁵). Single‐NW photodetectors on SiO₂/Si substrate show good sensitivity to visible light. Using the contact printing process, large‐scale ordered Zn₃As₂ NW arrays are successfully assembled on SiO₂/Si substrate to prepare NW thin‐film transistors and photodetectors. The NW‐array photodetectors on rigid SiO₂/Si substrate and flexible PET substrate exhibit enhanced optoelectronic performance compared with the single‐NW devices. The results reveal that the p‐type Zn₃As₂ NWs have important applications in future electronic and optoelectronic devices.     

An effective way to simultaneous realization of excellent optical and electrical performance in large‐scale Si nano/microstructures

Despite the optical advantage of near‐zero reflection, the silicon nanowire arrays (SiNWs)‐based solar cells cannot yet achieve satisfactory high efficiency because of the serious surface recombination arising from the greatly enlarged surface area. The trade‐off between reflection and recombination fundamentally prevents the conventional SiNWs structure from having both minimal optical and electrical losses. Here, we report the simultaneous realization of the best optical anti‐reflection (the solar averaged reflectance of 1.38%) and electrical passivation (the surface recombination velocity of 44.72 cm/s) by effectively combining the Si nano/microstructures (N/M‐Strus) with atomic‐layer‐deposition (ALD)‐Al₂O₃ passivation. The composite structures are prepared on the pyramid‐textured Si wafers with large‐scale 125 × 125 mm² by the two‐step metal‐assisted chemical etching method and the thermal ALD‐Al₂O₃ treatment. Although the excellent optical anti‐reflection is observed because of the complementary contribution of Si N/M‐Strus at short wavelength and ALD‐Al₂O₃ at long wavelength, the low recombination has also been realized because the field effect passivation is enhanced for the longer and thinner SiNWs through the more effective suppression of the minority carrier movement and the reduction of the pure‐pyramid‐textured surface recombination. We have further numerically modeled the Al₂O₃‐passivated Si N/M‐Strus‐based solar cell and obtain the high conversion efficiency of 21.04%. The present work opens a new way to realize high‐efficiency SiNWs‐based solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Monte Carlo simulation of the effect of silicon monoxide on silicon-nanocluster formation

Silicon-nanocluster formation upon the annealing of SiO _x (1 ≤ x < 2) layers is studied with the use of the lattice Monte Carlo model. The simulation is performed taking into account an additional mechanism of silicon transport due to the diffusion of silicon-monoxide (SiO) particles. It is demonstrated that the presence of SiO in the system leads to the growth of a critical silicon-nanocluster nucleus and can increase the nanocluster growth rate. Silicon-nanocluster formation upon the annealing of SiO _x layers occurs only for the composition with x < 1.8. Upon the annealing of SiO _x layers on a silicon substrate, a region depleted of silicon nanoclusters is observed in the layer adjacent to the substrate, which allows the formation of silicon nanoclusters in the SiO₂ matrix at a certain distance from the Si/SiO₂ interface.     

A systematic study of the impact of etching time to the sensitivity of SiNW sensor fabricated by MACEtch process

In this paper, silicon nanowires (SiNWs) was fabricated by a combination of metal-assisted chemical etching (MACEtch) and nanosphere lithography. We get the silicon nanowires with different specific surface area by changing the etching time. The microscopic structure of the silicon nanowires is observed by field emission scanning electron microscope (FESEM). The gas sensing performances of the SiNWs with different specific surface area have been systematically examined by measuring the resistance change towards the concentrations of NO₂ in the range of 1–5 ppm at room temperature (RT, 300 K), the gas sensor composed of SiNWs showed perfect gas sensitive property and possessed a short response–recovery time. The main reason of these excellent attributes is quite likely that high specific surface area of the SiNWs, and NO₂ sensing mechanism of the SiNWs was also further explained, which can be attributed to the oxygen in the air and detected NO₂ extract electrons from the surface of the SiNWs, and the resistivity of SiNWs changed with the changing of space-charge layer under the of SiNWs surface.     

Hydrogen‐Terminated Si Nanowires as Label‐Free Colorimetric Sensors in the Ultrasensitive and Highly Selective Detection of Fluoride Anions in Pure Water Phase

The detection of anions in pure water phase with colorimetric sensor is a long standing challenge. As one of the most important anions, F^– is associated with nerve gases and the refinement of uranium for nuclear weapons. However, limited by its anions nature, few of the reported colorimetric sensors can successfully applied to detect F^–1 in pure water phase. This work designs a colorimetric sensor for F^–1 pure water phase detection by taking the advantages of the strong specific binding between F and Si, as well as the color‐changing property of H‐terminated Si nanowires (SiNWs). The sensor demonstrates ultra‐sensitivity, high selectivity, and good stability. The results reveal particular interest for the development of new type aqueous phase anions sensors with SiNWs.     

Vertically-tapered silicon nanowire arrays prepared by plasma enhanced chemical vapor deposition: Synthesis,structural characterization and photoluminescence

Vertically aligned silicon nanowires (SiNWs) have been successfully synthesized using pure silane gas as a precursor by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The effect of the growth temperature on the morphology, structure and photoluminescence properties of SiNWs has been studied. The SiNWs were needle-liked materials with the length of a few microns having the diameters of tens of nanometers near the bottom and a few nanometers at the top. Thinner nanowires have been obtained at the higher growth temperature process. High resolution transmission electron microscopy confirms that the nanowires are composed of a crystalline silicon core with an oxide shell. The PL spectrum of the Si nanoneedles have shown two emission bands around 450 nm and ~750, which originate from the defects related to oxygen fault in the oxide shell and interfaces between the crystalline Si core and the oxide shell, respectively.     

Fully coherent Ge islands growth on Si nano-pillars by selective epitaxy

Our recent experimental results of Ge nanoheteroepitaxy (NHE) on Si nanopillars (NPs) are reviewed to confirm the possibility of relaxed Ge growth on Si without misfit dislocations (MDs) formation by elastic deformation. Selective Ge growth is performed by using reduced pressure chemical vapor deposition (CVD) on two types of Si NPs with thermal SiO₂ or CVD SiO₂ sidewalls and on Si nanoislands (NIs) on SiO₂. By using thermal SiO₂ sidewall, compressive strain is generated in the Si pillar and fixed by the thermal SiO₂. This results in an incoherent Ge growth on Si NPs due to MD formation. By using CVD SiO₂ sidewall, tensile strain formation due to thermal expansion during prebake for Ge epi process is observed. However, strain in Si due to Ge growth is not dominant. By introducing a Si_0.5Ge_0.5 buffer layer, no MD and stacking faults are observed by cross section TEM. The shape of Ge on Si NPs becomes more uniform due to improved crystal quality. On Si NIs on SiO₂, a clear compliance effect is observed after Ge growth. Coherent growth of Ge on Si is also realized on Si NIs by using Si_0.5Ge_0.5 buffer.     

Improved nanocrystal formation,quantum confinement and carrier transport properties of doped Si quantum dot superlattices for third generation photovoltaics

An all‐Si tandem solar cell has the potential to achieve high conversion efficiency at low cost. However, the selection and synthesis of candidate material remain challenging. In this work, we show that the conventional ‘Si quantum dots (Si QDs) in SiO₂ matrix’ approach can lead to the formation of over‐sized Si nanocrystals especially when doped with phosphorous, making the size‐dependent quantum confinement less effective. Also, our investigation has shown that the high resistivity of this material has become the performance bottleneck of the solar cell. To resolve these matters, we propose a new design based on Si QDs embedded in a SiO₂/Si₃N₄ hybrid matrix. By replacing the SiO₂ tunnel barriers by the Si₃N₄ layers, the new material manages to constrain the growth of doped Si QDs effectively and enhances the apparent band gap, as shown in X‐ray diffraction, Raman, photoluminescence and optical spectroscopic measurements. Besides, electrical characterisation on Si QD/c‐Si heterointerface test structures indicates the new material possesses improved vertical carrier transport properties. Copyright © 2011 John Wiley & Sons, Ltd.     

Large-Area Direct Hetero-Epitaxial Growth of 1550-nm InGaAsP Multi-Quantum-Well Structures on Patterned Exact-Oriented (001) Silicon Substrates by Metal Organic Chemical Vapor Deposition

We employ a simple two-step growth technique to grow large-area 1550-nm laser structures by direct hetero-epitaxy of III–V compounds on patterned exact-oriented (001) silicon (Si) substrates by metal organic chemical vapor deposition. Densely-packed, highly uniform, flat and millimeter-long indium phosphide (InP) nanowires were grown from Si v-grooves separated by silicon dioxide (SiO₂) stripes with various widths and pitches. Following removal of the SiO₂ patterns, the InP nanowires were coalesced and, subsequently, 1550-nm laser structures were grown in a single overgrowth without performing any polishing for planarization. X-ray diffraction, photoluminescence, atomic force microscopy and transmission electron microscopy analyses were used to characterize the epitaxial material. PIN diodes were fabricated and diode-rectifying behavior was observed.     

High‐Performance Hybrid Supercapacitor Enabled by a High‐Rate Si‐based Anode

A hybrid supercapacitor constructed of a Si‐based anode and a porous carbon cathode is demonstrated with both high power and energy densities. Boron‐doping is employed to improve the rate capability of the Si‐based anode (B‐Si/SiO₂/C). At a high current density of 6.4 A/g, B‐Si/SiO₂/C delivers a capacity of 685 mAh/g, 2.4 times that of the undoped Si/SiO₂/C. Benefiting from the high rate performance along with low working voltage, high capacity, and good cycling stability of B‐Si/SiO₂/C, the hybrid supercapacitor exhibits a high energy density of 128 Wh/kg at 1229 W/kg. Even when power density increases to the level of a conventional supercapacitor (9704 W/kg), 89 Wh/kg can be obtained, the highest values of any hybrid supercapacitor to date. Long cycling life (capacity retention of 70% after 6000 cycles) and low self‐discharge rate (voltage retention of 82% after 50 hours) are also achieved. This work opens an avenue for development of high‐performance hybrid supercapacitors using high‐performance Si‐based anodes.     

White‐Light Emitting Diode Array of p+‐Si/Aligned n‐SnO2 Nanowires Heterojunctions

We report on the fabrication and optoelectronic properties of p‐n heterojunction arrays of p⁺‐type Si and aligned n‐type SnO₂ nanowires with high rectification ratios of >10⁴ at ±15 V. The electrical stability of the p‐n heterojunction devices was improved by coating the junction with poly(methylmethacrylate) to minimize the degradation of the interface layer at the junction. As a photodiode an enhanced UV photosensitivity higher than 10² was recorded under reverse bias. Using a large forward bias in the light‐emitting diode mode white light was emitted from the large‐scale heterojunction devices with at least three broad peaks in the visible range, which can be attributed to the interband transitions of the injected electrons or holes mediated by an interfacial SiO₂ layer with a contribution of trap‐level energies. These results indicate the high potential of Si/SnO₂ nanowires heterojunctions as optoelectronic devices with proper tuning of the recombination center at the junctions.     

Synthesis and growth mechanism of oriented amorphous SiO2 nanowires

Based on the catalytic property of gold, seemingly oriented amorphous SiO₂ nanowires were synthesized by radio frequency (RF) magnetron sputtering technique and annealing at 1100 °C for 40 min in the presence of a nitrogen flow using Silicon(Si) powder as the Si source. The length and diameter of nanowires growth are almost uniform, which are about 30 μm and 35 nm respectively. The growth of nanowires is consistent with vapor–liquid–solid mechanism, with Au particles being observed to remain at the tip of nanowires. They were characterized by scanning electron microscopy, transmission electron microscopy, selected area electron diffraction and photoluminescence. A sharp intensive green emission peak at around 519 nm was observed with an excitation wavelength of 325 nm;this has been ascribed to an oxygen deficiency.