Rational synthesis of ultrathin n-type Bi2Te3 nanowires with enhanced thermoelectric properties

A rational yet scalable solution phase method has been established, for the first time, to obtain n-type Bi(2)Te(3) ultrathin nanowires with an average diameter of 8 nm in high yield (up to 93%). Thermoelectric properties of bulk pellets fabricated by compressing the nanowire powder through spark plasma sintering have been investigated. Compared to the current commercial n-type Bi(2)Te(3)-based bulk materials, our nanowire devices exhibit an enhanced ZT of 0.96 peaked at 380 K due to a significant reduction of thermal conductivity derived from phonon scattering at the nanoscale interfaces in the bulk pellets, which corresponds to a 13% enhancement compared to that of the best n-type commercial Bi(2)Te(2.7)Se(0.3) single crystals (~0.85) and is comparable to the best reported result of n-type Bi(2)Te(2.7)Se(0.3) sample (ZT = 1.04) fabricated by the hot pressing of ball-milled powder. The uniformity and high yield of the nanowires provide a promising route to make significant contributions to the manufacture of nanotechnology-based thermoelectric power generation and solid-state cooling devices with superior performance in a reliable and a reproducible way.     

Electric-field-dependent photoconductivity in CdS nanowires and nanobelts: exciton ionization, Franz-Keldysh, and Stark effects

We report on the electric-field-dependent photoconductivity (PC) near the band-edge region of individual CdS nanowires and nanobelts. The quasi-periodic oscillations above the band edge in nanowires and nanobelts have been attributed to a Franz-Keldesh effect. The exciton peaks in PC spectra of the nanowires and thinner nanobelts show pronounced red-shifting due to the Stark effect as the electric field increases, while the exciton ionization is mainly facilitated by strong electron-longitudinal optical (LO) phonon coupling. However, the band-edge transition of thick nanobelts blue-shifts due to the field-enhanced exciton ionization, suggesting partial exciton ionization as the electron-LO phonon coupling is suppressed in the thicker belts. Large Stark shifts, up to 48 meV in the nanowire and 12 meV in the thinner nanobelts, have been achieved with a moderate electric field on the order of kV/cm, indicating a strong size and dimensionality implication due to confinement and surface depletion.     

Sensitization effect in photoconductivity of CdS

A relative photosensitivity factor,S, of the order of 5000 is observed due to the incorporation of lanthanum in CdS. Dye treatment enhances this value by 10 000. A charge transfer mechanism is considered applicable to this process. Superlinear photoconductivity and thermal quenching observed in the studies from 133 to 366 K are explained in terms of a two-centre model.     

Enhanced photoluminescence and photoconductivity of ZnO nanowires with sputtered Zn

We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the effect of Zn diffusion on the photoluminescence and photoconduction properties of ZnO NWs. Elemental mapping clearly indicates higher Zn concentration in the NWs due to diffusion of Zn. The Zn-sputtered NWs show an enhanced ultraviolet emission with 7 nm red shift. Since the ionization energy of Zni is 51 meV, the enhanced PL emission with a red shift is correlated to the coupling between free exciton and zinc interstitials (Zni) defects. The photocurrent transients show almost 20 times more photocurrent generation in Zn/ZnO NWs compared to the as-grown NWs. In contrast, the thin film shows no significant change in the photoluminescence and photoconductivity. Based on the photoconductivity and photoluminescence results, we predict that Zn diffusion in the NWs occurs easily compared to the films because of the smaller dimensions of the NWs.     

Solvothermal growth of single-crystal CdS nanowires

Cadmium sulfide (CdS) nanowires (NWs) were prepared by the solvothermal method using ethylenediamine as a solvent. Two sets of CdS NWs were synthesized at 160 and 200 °C for various reaction durations (3?5, 7, and 24 h). Scanning/tunneling electron microscopy was used to examine the surface morphology of the grown NWs. Their dimensions are found to depend on the reaction temperature and duration. The CdS NWs grown at 200 °C for all durations are longer than those prepared at 160 °C, with diameters ranging from 15 to 40 nm. A three-armed structure is exhibited by all the samples. The grown CdS NWs display a hexagonal wurtzite phase and grows along the \(\mathbf {\left \langle {001}\right \rangle }\) direction. The optical absorption of the grown NWs shows a sharp absorption edge with a blueshift, which indicates an expansion of the optical band gap. All prepared samples show two emission peaks in their photoluminescence spectra. The emission peak location depends on the reaction temperature and duration. The CdS NWs prepared at 160 °C show a sharp band–band emission compared with those prepared at 200 °C. Raman analysis indicates that the optical properties of the grown NWs are enhanced with increased temperature and reaction duration.     

Nitrogen doped n-type CdS nanoribbons with tunable electrical and photoelectrical properties

Single-crystal nitrogen doped CdS nanoribbons (NRs) with wurtzite structure were synthesized in ammonia atmosphere via a thermal evaporation deposition route. X-ray diffraction patterns reveal a significant contraction of the lattice constants due to the incorporation of nitrogen. Temperature-varied photoluminescence spectra of CdS:N NRs exhibit spectral features near the band edge, which can be ascribed to free excition and neutral acceptor-bound excition emissions. Electrical and photoelectrical properties of the CdS:N NRs were systemically studied by constructing the field-effect transistors based on individual NRs. The conductivity of the NRs can be tuned by two orders of magnitude by controlling the N doping concentration. Moreover, by post-annealing, the device performance is remarkably improved, in particular, the mobility of the CdS:N NRs is increased by nearly three orders of magnitude from approximately 10(-1) to hundreds of cm2/Vs. I(on)I(off) ratio of the annealed device reaches over 10(4). Photoconductive properties of the CdS:N NRs were also studied. The doped NRs show high sensitivity to the light with energy larger than band-gap and the response amplitude and speed depend on the doping concentration.     

Sonochemical synthesis of CdS and CdSe nanowires

A convenient sonochemical route was developed to fabricate one-dimensional (1D) CdS or CdSe assemblies via a simple template method with two-steps: Firstly, the colloid one dimensional cadmium hydroxide particles were prepared as templates under sonication; then, the colloid particles were converted into 1D CdS or CdSe assemblies via a replacement reaction after the surface nucleation and crystal growth processes. The as-prepared CdS and CdSe nanowires were characterized by XRD, TEM, XPS, and UV-visible Spectroscopy. The effects of the ultrasonic irradiation were discussed. It is believed that the ultrasound irradiation played a positive role in both the assembly of the colloid cadmium hydroxide particles into the 1D structure and the growth of CdSe and CdS nanowires. The effects of pH on the morphologies of the cadmium hydroxide template were also discussed. The band gaps of the as-prepared 1D CdSe and CdS assemblies were calculated to be 3.1 eV and 4.9 eV, respectively, indicating the quantum size effect. The as-prepared products might have potential applications in nanodevices in future.     

Optical studies of electrochemically synthesized CdS nanowires

In this paper, electrochemical fabrication and characterization of CdS nanowires having diameter 100 and 200 nm is reported. Nano-channels in anodic alumina membrane were utilized as template. Morphological study of nanowires was made using Scanning electron microscopy (SEM). UV–visible absorption and laser induced time resolved photoluminescence (PL) spectroscopy were used for optical characterization. UV–visible absorption depicts that, there is slight increase in band gap of nanowires with decrease in diameter of nanowires. PL measurements indicate emission band peak of 435 and 420 nm in case of 200 and 100 nm wires respectively. These studies are very important regarding the synthesis and optoelectronic applications of CdS nanowires.     

Low threshold room-temperature lasing of CdS nanowires

The synthesis of CdS nanostructures (bands, wires, irregular structures) was investigated by systematic variation of temperature and gas pressure, to deduce a comprehensive growth phase diagram. The high quality nanowires were further investigated and show stoichiometric composition of CdS as well as a single-crystalline lattice without any evidence of extended defects. The luminescence of individual nanowires at low excitation shows a strong near band edge emission at 2.41?eV indicating a low point defect concentration. Sharp peaks evolve at higher laser power and finally dominate the luminescence spectrum. The power dependence of the spectrum clearly shows all the characteristics of amplified stimulated emission and lasing action in the nanowire cavity. A low threshold was determined as 10?kW?cm(-2) for lasing at room temperature with a slope efficiency of 5-10% and a Q factor of up to 1200. The length and diameter relations necessary for lasing of individual nanowires was investigated.     

Fabrication of n-type mesoporous silicon nanowires by one-step etching

In general, n-type mesoporous silicon nanowires (mp-SiNWs) are exclusively created by the two-step metal-assisted chemical etching (MACE). This work first reports that one-step MACE (in HF and AgNO3) is also capable of producing the n-type mp-SiNWs, and the developed formula is generally adapted to generate SiNWs by etching n-Si(100) with electrical resistivity over a range of 10(-3)-10(1) Ω·cm. Integrating the contribution of silicon intrinsic properties in the existing MACE mechanism explicitly accounts for the new findings and contradictions with previous studies. The as-generated mesoporous structures emit red light under laser excitation at room temperature. The red-color emission sensitively varies with temperature over a range of 16-300 K, attributed to a temperature-dependent photoluminescent mechanism.     

Platinum-nanoparticle-modified TiO2 nanowires with enhanced photocatalytic property

Highly crystalline Pt nanoparticles with an average diameter of 5 nm were homogeneously modified on the surfaces of TiO(2) nanowires (Pt-TiO(2) NWs) by a simple hydrothermal and chemical reduction route. Photodegradation of methylene blue (MB) in the presence of Pt-TiO(2) NWs indicates that the photocatalytic activity of TiO(2) NWs can be greatly enhanced by Pt nanoparticle modification. The physical chemistry process and photocatalytic mechanism for Pt-TiO(2) NWs hybrids degrading MB were investigated and analyzed. The Pt attached on TiO(2) nanowires induces formation of a Schottky barrier between TiO(2) and Pt naonoparticles, leading to a fast transport of photogenerated electrons to Pt particles. Furthermore, Pt incoporation on TiO(2) surface can accelerate the transfer of electrons to dissolved oxygen molecules. Besides enhancing the electron-hole separation and charge transfer to dissolved oxygen, Pt may also serve as an effective catalyst in the oxidation of MB. However, a high Pt loading value does not mean a high photocatalytic activity. Higher content loaded Pt nanoparticles can absorb more incident photons which do not contribute to the photocatalytic efficiency. The highest photocatalytic activity for the Pt-TiO(2) nanohybrids on MB can be obtained at 1 at % Pt loading.     

Fabrication and optical properties of Sb-doped CdS nanowires

Sb-doped CdS nanowires with an average diameter of 30 nm and lengths of up to 20-30 μm are fabricated by chemical vapor approach. The as-synthesized products have a single-crystal phase and grow along the <011> direction. The growth of Sb-doped CdS nanowires is suggested for Quasi-vapor-solid mechanism (QVSM). In particular, the PL spectra show enhancing emission peaks that strongly shift to long wavelength (up to 55 nm redshift) with a doping Sb where Sb-doped CdS nanowires are found to be responsible for the different characteristics. The PL mechanism is explained in detail.     

A precursor nanowire templated route to CdS nanowires

We report the synthesis of the Cd-cysteine precursor nanowires using CdCl₂·2.5H₂O, L-cysteine and ethanolamine in the solvent of water at room temperature. The average diameter and lengths of Cd-cysteine precursor nanowires are 200 nm and several hundred microns, respectively. The precursor nanowires are used as the source materials for cadmium and sulfur and the template for the subsequent preparation of CdS nanowires using ethylene glycol as the solvent by a solvothermal method at 200 °C. The formation process of CdS nanowires is discussed. The samples are characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy.     

Dielectric relaxation behavior of CdS nanoparticles and nanowires

The dielectric relaxation and scaling behavior of CdS nanoparticles and nanowires were investigated in the frequency range 10²–10⁶ Hz and in the temperature range 373–573 K by complex impedance spectroscopy and electric modulus spectroscopy. Studies on the complex permittivity revealed that the dielectric relaxation in CdS nanostructures deviates from Debye like behavior. A detailed study on the grain and grain boundary charge transport was carried out. The charge carrier transport in CdS nanostructures was identified to be hopping of polarons. From the combined analysis of the variation of imaginary part of electric modulus and complex impedance with frequency, it was found that at high temperatures localized conduction is dominant in CdS nanoparticles where as the long range hopping process is dominant with nanowires. It was also found that the scaling behavior of CdS nanoparticles varied considerably from that reported earlier.     

Effects of gold diffusion on n-type doping of GaAs nanowires

The deposition of n-GaAs shells is explored as a method of n-type doping in GaAs nanowires grown by the Au-mediated metal-organic chemical vapor deposition. Core-shell GaAs/n-GaAs nanowires exhibit an unintended rectifying behavior that is attributed to the Au diffusion during the shell deposition based on studies using energy dispersive X-ray spectroscopy, current-voltage, capacitance-voltage, and Kelvin probe force measurements. Removing the gold prior to n-type shell deposition results in the realization of n-type GaAs nanowires without rectification. We directly correlate the presence of gold impurities to nanowire electrical properties and provide an insight into the role of seed particles on the properties of nanowires and nanowire heterostructures.     

High-performance logic circuits constructed on single CdS nanowires

A high-performance NOT logic gate (inverter) was constructed by combining two identical n-channel metal-semiconductor field-effect transistors (MESFETs) made on a single CdS nanowire (NW). The inverter has a voltage gain as high as 83, which is the highest reported so far for inverters made on one-dimensional nanomaterials. The MESFETs used in the inverter circuit show excellent transistor performance, such as high on/off current ratio ( approximately 10(7)), low threshold voltage ( approximately -0.4 V), and low subthreshold swing ( approximately 60 mV/dec). With the assembly of three identical NW MESFETs, NOR and NAND gates have been constructed.     

High-yield fabrication of t-Se nanowires via hydrothermal method and their photoconductivity

Large-scale fabrication of high-purity and uniform trigonal selenium (t-Se) nanowires was obtained by reducing SeO₂ with glucose at 160 °C for 24 h. The resulting t-Se nanowires were characterized and confirmed by means of an X-ray diffractometer, scanning electron microscope, transmission electron microscope and energy dispersive X-ray spectroscope. The results indicated that the high yield uniform t-Se nanowires have high degree of crystallinity. The selected area electron diffraction pattern and high-resolution transmission electron microscopy image demonstrate perfect crystallinity with the growth direction of [001]. Owning to quantum size effect, a significant blue shift could be identified in the absorbance spectroscopy of as-prepared products. The photoconductivity of t-Se nanowires with light was also investigated and reached maximum at the wavelength of 575 nm, which might be useful in the fabrication of micro-devices or photo-switches. Contrast experiments showed that surfactant Tween-40 played a crucial role in the formation of nanowires.     

Investigation of structural and optical properties of the CdS and CdS/PPy nanowires

In this study, cadmium sulfide (CdS), polypyrrole (PPy) nanowires, and their heterojunctions have been electrochemically synthesized. Morphology of the nanowires has been investigated by scanning electron microscopy. Energy dispersed X-ray, X-ray diffraction, UV–Vis, and FTIR analyses have been used to confirm structure of both CdS and PPy nanowires. For the first time with this study, CdS/PPy nanowire heterojunctions have been integrated into photoelectrochemical (PEC) cells. It has been also demonstrated that PEC performance of the nanowires was strongly function of production conditions, such as deposition time and voltage. The maximum power conversion efficiency of the CdS nanowires obtained in this study was 1.36%. Moreover, efficiencies of the CdS/PPy nanowires have been reached to 5.00%, which makes them very favorable for PEC applications.