Doped Nanowires Research Articles

An oxygen evolution catalyst on an antimony doped tin oxide nanowire structured support for proton exchange membrane liquid water electrolysis

Porous Sb–SnO2nanowires were synthesized as a support for IrO2by an electrospinning method. The thus prepared catalyst exhibits enhanced mass activity toward OER.

Effect of Surface Dangling Bonds and Molecular Passivation on Doped GaAs Nanowires

We have investigated the effect of surface dangling bonds and molecular passivation on the doping of GaAs nanowires by first-principles calculations. Results show that the positively charged surface dangling bond on Ga atom is the most stable defect for both ultrathin and large size GaAs nanowires. It can form the trap centers of holes and then prefer to capture the holes from p-type doping. Thus it could obviously reduce the efficiency of the p-type doping. We also found that the NO2 molecule is electronegative enough to capture the unpaired electrons of surface dangling bonds, which is an ideal passivation material for the Zn-doped GaAs nanowires.

Development of high efficient visible light-driven N, S-codoped TiO2 nanowires photocatalysts

One-dimensional (1D) nanowire material (especially nonmetal doped 1D nanowires) synthesized by a facile way is of great significance and greatly desired as it has higher charge carrier mobility and lower carrier recombination rate. N, S-codoped TiO2 nanowires were synthesized using titanium sulfate as a precursor and isopropanol as a protective capping agent by a hydrothermal route. The obtained doped nanowires were characterized by XRD, SEM, HRTEM, SAED, XPS, BET and UV–vis absorption spectrum. The incorporation of N and S into TiO2 NWs can lead to the expansion of its lattice and remarkably lower its electron-transfer resistance. Photocatalytic activity measurement showed that the N, S-codoped TiO2 nanowires with high quantum efficiency revealed the best photocatalytic performance for atrazine degradation under visible light irradiation compared to N, S-codoped TiO2 nanoparticles and S-doped TiO2 nanowires, which was attributed to (i) the synergistic effect of N and S doping in narrowing the band gap, separating electron–hole pairs and increasing the photoinduced electrons, and (ii) extending the anatase-to-rutile transformation temperature above 600°C.

Effect of Crystalline Quality on Magnetic Properties of Mn-Doped ZnO Nanowires

Mn-doped ZnO nanowires are synthesized by a vapor phase deposition method in air and in a vacuum, respectively. X-ray diffraction results show that all the diffraction peaks correspond to the hexagonal wurtzite structure. X-ray absorption fine structure spectra suggest that a single Mn2+-containing phase exists and Mn2+ions occupy Zn2+ ions in the ZnO lattice. Photoluminescence spectra show that many defects exist in the doped nanowires as the samples grown in air, and the crystalline quality decreases with the increase of Mn. These samples exhibit obvious room-temperature ferromagnetic characteristics, and the magnetization increases with the increase of Mn. The sample with higher crystal quality grown in a vacuum exhibits the paramagnetic behavior at room temperature. As the as-grown samples are annealed, the crystalline quality improves, while the magnetization of the samples grown under the air condition translates from room-temperature ferromagnetism to the paramagnetism behavior. The above results indicate that the magnetic property of the Mn-doped ZnO nanowires can be controlled by the crystalline quality.

Zn-doping of GaAs nanowires grown by Aerotaxy

Nanowires were grown by means of a novel aerosol-based method called Aerotaxy. Here an aerosol of Au catalyst nanoparticles in N2 is mixed with MOVPE precursors in a flow-through reactor at atmospheric pressure, whereby nanowires are produced continuously in high concentrations. We demonstrate the possibility of in situ doping of the NWs and the realization of well-controlled p-type GaAs nanowires using this Aerotaxy method. By controlling the cracking and concentration of the precursors, p-doped GaAs nanowires could be grown exhibiting a wide range of Zn doping levels. DEZn was used as the dopant source and the injected DEZn/TMGa ratio was varied from 0.1% to 3.4%. The morphology, the crystalline structure and the composition of the nanowires were studied using SEM, TEM and XEDS. The nanowires were grown straight without any significant tapering and this ideal morphology could be maintained up to an injected DEZn/TMGa ratio of 3.4%. The nanowires typically grew in the 〈111〉 direction with a pure zincblende structure, but by increasing the DEZn flow the number of twinning defects increased which we ascribe to Zn incorporation. Elemental analysis shows a high Zn content in the catalyst particle and also a gradient in the Zn content along the nanowire. The samples were analyzed optically using photoluminescence (PL). From the result we estimated the free hole concentration induced by Zn acceptors to be 1×1020cm−3 for DEZn/TMGa ratio of 3.4%. To our knowledge this is the first report on in situ doping of GaAs nanowires grown by Aerotaxy.

Broad spectral response photodetector based on individual tin-doped CdS nanowire

High purity and tin-doped 1D CdS micro/nano-structures were synthesized by a convenient thermal evaporation method. SEM, EDS, XRD and TEM were used to examine the morphology, composition, phase structure and crystallinity of as-prepared samples. Raman spectrum was used to confirm tin doped into CdS effectively. The effect of impurity on the photoresponse properties of photodetectors made from these as-prepared pure and tin-doped CdS micro/nano-structures under excitation of light with different wavelength was investigated. Various photoconductive parameters such as responsivity, external quantum efficiency, response time and stability were analyzed to evaluate the advantage of doped nanowires and the feasibility for photodetector application. Comparison with pure CdS nanobelt, the tin-doped CdS nanowires response to broader spectral range while keep the excellect photoconductive parameters. Both trapped state induced by tin impurity and optical whispering gallery mode microcavity effect in the doped CdS nanowires contribute to the broader spectral response. The micro-photoluminescence was used to confirm the whispering gallery mode effect and deep trapped state in the doped CdS nanowires.

Access resistance reduction in Ge nanowires and substrates based on non-destructive gas-source dopant in-diffusion

To maintain semiconductor device scaling, in recent years industry has been forced to move from planar to non-planar device architectures. This alone has created the need to develop a radically new, non-destructive method for doping. Doping alters the electrical properties of a semiconductor, related to the access resistance. Low access resistance is necessary for high performance technology and reduced power consumption. In this work the authors reduced access resistance in top–down patterned Ge nanowires and Ge substrates by a non-destructive dopant in-diffusion process. Furthermore, an innovative electrical characterisation methodology is developed for nanowire and fin-based test structures to extract important parameters that are related to access resistance such as nanowire resistivity, sheet resistance, and active doping levels. Phosphine or arsine was flowed in a Metalorganic Vapour Phase Epitaxy reactor over heated Ge samples in the range of 650–700 °C. Dopants were incorporated and activated in this single step. No Ge growth accompanied this process. Active doping levels were determined by electrochemical capacitance–voltage free carrier profiling to be in the range of 1019 cm−3. The nanowires were patterned in an array of widths from 20–1000 nm. Cross-sectional Transmission Electron Microscopy of the doped nanowires showed minimal crystal damage. Electrical characterisation of the Ge nanowires was performed to contrast doping activation in thin-body structures with that in bulk substrates. Despite the high As dose incorporation on unpatterned samples, the nanowire analysis determined that the P-based process was the better choice for scaled features.

β-Ga2O3 nanowires for an ultraviolet light selective frequency photodetector

The behaviour of β-Ga2O3 nanowires as photoconductive material in deep ultraviolet photodetectors to operate in the energy range 3.0–6.2 eV has been investigated. The nanowires were grown by a catalyst-free thermal evaporation method on gallium oxide substrates. Photocurrent measurements have been carried out on both undoped and Sn-doped Ga2O3 nanowires to evidence the influence of the dopant on the photodetector performances. The responsivity spectrum of single nanowires show maxima in the energy range 4.8–5.4 eV and a strong dependence on the pulse frequency of the excitation light has been observed for undoped nanowires. Our results show that the responsivity of β-Ga2O3 nanowires can be controlled by tuning the chopper frequency of the excitation light and/or by doping of the nanowires. Non-linear behaviour in characteristic current–voltage curves has been observed for Ga2O3 : Sn nanowires. The mechanism leading to this behaviour has been discussed and related to space-charged-limited current effects. In addition, the responsivity achieved by doped nanowires at lower bias is higher than for undoped ones.

High-Performance Sb-Based III-V Nanowires Synthesized on Amorphous Substrates: From the Formation Mechanism to p-Channel Transistor Applications

:In recent years, due to the narrow direct bandgap and outstanding carrier mobility, Sb-base nanowires (NWs), such as GaSb, have been extensively explored for high-performance electronics and optoelectronics. Importantly, these p-channel nanowires can be potentially integrated with n-type InSb, InAs or InGaAs NW devices via different NW transfer techniques to facilitate the III-V CMOS technology. However, there are still very few works till now focusing on the electronic transport properties of GaSb NWs. Here, we successfully demonstrate the synthesis of crystalline, stoichiometric and dense GaSb NWs on amorphous substrates, instead of the commonly used III-V crystalline substrates, InAs or GaAs NW stems as others reported. The obtained NWs are found to grow via the VLS mechanism with a narrow distribution of diameter uniformly along the entire NW length with minimal tapering and surface coating. Notably, when configured into FETs, the NWs exhibit impressive electrical characteristics with the peak hole mobility of ~200 cm2V-1s-1. All these have illustrated the promising potency of such NWs directly grown on amorphous substrates for various technological applications, as compared with the conventional MOCVD grown GaSb NWs.Reference:Yang Z., Wang F.Y., Han N., Lin H., Cheung H.Y., Fang M., Yip S.P., Hung T.F., Wong C.Y., Ho J.C. "Crystalline GaSb nanowires synthesized on amorphous substrates: from the formation mechanism to p-channel transistor applications", ACS Applied Materials & Interfaces, 5, 10946-10952, 2013.Yang Z., Han N., Wang F.Y., Cheung H.Y., Shi X., Yip S.P., Hung T.F., Lee M.H., Wong C.Y., Ho J.C. "Carbon doping of InSb nanowires for high-performance p-channel field-effect-transistors", Nanoscale, 5, 9671-9676, 2013.

New insights into the temperature-dependent photoluminescence of Mg-doped GaAs nanowires and epilayers

The intentional introduction of impurities in semiconductor nanowires is very important in view of device applications. Doping affects the electronic energy level structure which in the case of III–V nanowires can also be strongly influenced by the simultaneous occurrence of two polytypes, zinc-blende (ZB) and wurtzite (WZ). In this work, we report a study on GaAs nanowires with different Mg-acceptor doping levels through temperature dependent photoluminescence. A comparable investigation is presented for Mg-doped GaAs epilayers. For the later, only a band is observed which is ascribed to the involvement of the Mg acceptor due to the observed bandgap energy narrowing effect with increasing the doping level, and the temperature dependent behaviour. A different behaviour is reported for nanowires: several radiative transitions are observed whose temperature dependence follows that of bulk GaAs, in accordance with spatially indirect recombination. Although the polytypic regions mask the role of doping in nanowires it favours the charge separation required for photovoltaic applications.

SERS substrates of doped germanium nanowires decorated with silver nanoparticles

Abstract The microstructure and SERS activity of Ag-decorated Ge nanowires (NWs) doped with boron (B) or phosphorous (P) were investigated. The silver nanoparticles (AgNPs) distribution observed for the both types of doped nanowires differed significantly, as well as before and after the annealing procedures. The deposited strong-packed nonspherical Ag nanoparticles favor the Raman surface enhancement compared to spherical nanoparticles distributed along the nanowires with a larger interparticle distances after annealing. The SERS activity of the 3D nanocomposites is also influenced by the initial shape and the diameter of the nanowires and by the surface effects. The strongest SERS signal of Rhodamine 6G (R6G) was registered for the sample doped with P before annealing. Unlike the 2D AgNPs array the thermal annealing at 300 °C for 1 h destroyed the Raman activity of the B-doped composites and reduced the Raman peak intensities of the P-doped one.

Large enhancement of ferromagnetism by Cr doping in Mn3O4 nanowires

The Mn3O4 nanostructures having low temperature Curie point (45 K) disqualify them for most practical applications. In this work, single-crystalline Cr-doped Mn3O4 nanowires with ferromagnetic Curie point at room temperature (305 K) have been investigated. Our experimental results show an increase in effective magnetic moment per gram as Cr3+ replaces Mn3+ and oxygen vacancies, which result in a transition from paramagnetic (Mn3O4) to ferromagnetic. The doped Cr3+ and oxygen vacancies reveal the remarkable ferromagnetic in Mn3−xCrxO4 nanowires may be ascribed to bound magnetic polarons model. Our experimental results suggest these obtained nanowires are promising nanoscale building blocks in spintronic devices.

Vacancy effect on the doping of silicon nanowires: A first-principles study

The influence of vacancy defect on the doping of silicon nanowires is systematically studied by the first-principles calculations. The atomic structures and electronic properties of vacancies and vacancy—boron (vacancy—phosphor) complexes in H-passivated silicon nanowire with a diameter of 2.3 nm are explored. The results of geometry optimization indicate that a central vacancy can exist stably, while the vacancy at the edge of the nanowire undergoes a local surface reconstruction, which results in the extradition of the vacancy out of the nanowire. Total-energy calculations indicate that the central vacancy tends to form a vacancy—dopant defect pair. Further analysis shows that n-type doping efficiency is strongly inhibited by the unintentional vacancy defect. In contrast, the vacancy defect has little effect on p-type doping. Our results suggest that the vacancy defect should be avoided during the growth and the fabrication of devices.

Magnetoresistance in Mn ion-implanted GaAs:Zn nanowires

We have investigated the magnetoresistance (MR) in a series of Zn doped (p-type) GaAs nanowires implanted with different Mn concentrations. The nanowires with the lowest Mn concentration (∼0.0001%) exhibit a low resistance of a few kΩ at 300 K and a 4% positive MR at 1.6 K, which can be well described by invoking a spin-split subband model. In contrast, nanowires with the highest Mn concentration (4%) display a large resistance of several MΩ at 300 K and a large negative MR of 85% at 1.6 K. The large negative MR is interpreted in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons, separated by intermediate regions of Mn impurity spins. Sweeping the magnetic field back and forth for the 4% sample reveals a hysteresis that indicates the presence of a weak ferromagnetic phase. We propose co-doping with Zn to be a promising way to reach the goal of realizing ferromagnetic Ga1−xMnxAs nanowires for future nanospintronics.

Silicon Nanowires and Nanotrees Electrodes for Pseudocapacitors

Supercapacitors integration in micro-electronic circuit should improve portable devices efficiency [1] and be easier with silicon (Si) electrodes. In our previous work we have realized highly doped Si nanowires (SiNWs) micro-supercapacitor (µ-SC) electrodes showing quasi ideal capacitive behavior [2, 3]. This works studies the elaboration and characterization of highly doped Si nanotrees (SiNTrs) µ-SC and SiNWs micro-pseudocapacitors electrodes. It focuses on our recent advances on SiNTrs growth and use in µ-SC.SiNWs and SiNTrs are grown by CVD on highly doped Si via localized gold catalysis. SiNTrs are grown either by two CVD growths separated by a new gold deposition, or by a single growth in which an annealing (gold pours along NWs) separates trunks and branches growths [4, 5]. Electrochemical properties are evaluated in organic electrolyte (1M, NEt4BF4, PC) and ionic liquid (EMI-TFSI) by EIS, cyclic voltametry and galvanostatic charge/discharge. For PC PEDOT films was electrodeposited on SiNWs.As seen previously, electrode capacity increases with the electrode developed surface, i.e. SiNWs length, density and SiNTrs branches. SiNTrs are tuned by fitting HCl gas ratio during trunk growth, annealing time and temperature. Doping gas/silane ratio monitors their doping level. Highly doped, dense, hyperbranched (L ≈ 3 µm, Ø ≈ 30 nm), 40 µm long SiNTrs electrodes have a 950 µF.cm-2 capacity, i.e. about 158 fold bulk Si one and twice 50µm long SiNWs electrode one. SiNTrs/SiNTrs µ-SCs show highly stable cycle efficiency (97 %) and capacity over at least 100 000 cycles for current densities ranging from 5 to 500 µA.cm-2. Using ionic liquid as electrolyte improves devices voltage and thus performances.[1] J.R. Miller, P. Simon, Science, 2008, 321, 651[2] F. Thissandier, and al, Electrochem. Comm., 2012, 25, 109[3] F. Thissandier, and al, Nanoscale Res. Lett., 2012, accepted[4] P. Gentile, and al, Nanotechnol., 2008, 19, 125608[5] R. Maboudian, and al, J. Mater. Chem., 2008, 18, 5376

Vertical growth of core-shell III-V nanowires for solar cells application.

High density (In)GaAs/GaAs/AIGaAs nanowires (NWs) consisting of n-type core and p-type shell have been vertically grown on (111) GaAs substrate using metal organic chemical vapor deposition (MOCVD) and fabricated into solar cells. Au colloidal nanoparticles (NPs) are employed as a catalyst. High density nanowires were obtained by uniform distribution of Au NPs. Fe-SEM, TEM and HRTEM images show that the morphology of shell is sensitive to p-doping concentration. Increase in the density of p-doping precursor results in "kinking" of NPs and rough shell surface. The origin of kinking has been explained by the GaAs twin phases due to Zn segregation on the surface of shell. It has been observed that the morphology of NPs can be controlled through optimizing various source purge technique of DEZn and deposition temperature. Electrical properties of core-shell doped NWs are carried out using I-V characterization. The core-shell NWs show characteristics of p-n junction as revealed by I-V studies.

Transition metal-doped TiO2 nanowire catalysts for the oxidative coupling of methane

The catalytic oxidative coupling of methane (OCM) on transition metal-doped TiO2 nanowire catalysts was performed and the effects of metal dopants were studied. With transition metal doping, the electric and optical properties of nanowires were adjusted, which seemed to improve the catalytic activity and selectivity of the OCM reaction. A Mn-doped TiO2 nanowire catalyst exhibited the highest C2 yield with the highest (ethylene)/(ethane) ratio because of its moderate oxidation activity, while a highly active Rh-doped TiO2 nanowire catalyst converted methane into fully oxidized CO and CO2. The electric conductivity assessed by UV–vis absorption represented the oxidation activity of the nanowire catalysts.

Synthesis, characterization and optical properties of CdS nanowire/SnO2 nanoparticle nano-heterostructures

In this study, CdS nanowire (NW)/SnO2 nanoparticle (NP) nano-heterostructures were successfully synthesized through a facile two-step chemical solution method. The obtained nano-heterostructures were characterized using scanning electron microscopy and transmission electron microscopy, showing that SnO2 nanoparticles with size of about several nanometers grew on the surface of CdS nanowires. Room temperature photoluminescence spectra of the obtained CdS nanowires and CdS NW/SnO2 NP nano-heterostructures were investigated for comparison. The results showed that the near-band-edge emission at 524nm of CdS has a blue shift for CdS NW/SnO2 NP nano-heterostructure, which probably results from the Sn doping in CdS nanowires.

Investigating thermoelectric properties of doped polyaniline nanowires

Polyaniline (PANI) nanostructures doped with different acids have been prepared through a soft template method. The structures, morphologies and thermoelectric (TE) properties of the as-prepared PANI nanostructures have been carefully investigated. Our results showed that doping level, types of doping acids, and morphologies of as-prepared nanostructures did have strong effect on TE properties. For example, we found that higher doping level in HCl-doped PANI nanowires did not give better thermoelectric performance although it is true in some curtain doping range. AcOH-doped samples became worse than HCl-doped ones due to the poor doping ability of AcOH. As to p-toluenesulfonic acid (p-TSA) doped nanowires, ZT value is 2.75×10−5 at 300K, which is 4 times higher than that of HCl-doped one because doped bulky anions make PANI chains more order. As to the morphology effect, two p-TSA doped samples (nanowires and nanorods) at the same doping level were prepared and the nanowires showed the better Seebeck coefficient and lower thermal conductivity. Comparing with nanorods, Seebeck coefficient of nanowires increased 164% while thermal conductivity reduced by 25% for nanowires. The ZT value for nanowires is 5 times higher than that of nanorods.

Investigation on Mn doping of Ge nanowires for spintronics

AbstractVarious approaches are described to synthesize Mn‐doped Ge NWs using UHV‐CVD growth via the VLS mechanism. We explored in‐situ doping using an organo‐metallic precursor (TCMn) and tested different catalyst seeds such as Au, Au‐Mn or Ge‐Mn. All in‐situ attempts failed to incorporate Mn atoms into the NW. Moreover we observed that the use of TCMn and/or Au‐Mn catalysts have a crippling effect on the growth kinetics. Finally, ex‐situ doping with ion implantation is right now considered as a doping alternative. We report on preliminary results aiming at testing the radiation resistance of Ge NWs embedded in a removable silica matrix. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)