Water induced electrical hysteresis in germanium nanowires: a theoretical study

We apply DFT calculations to evaluate the electronic properties of germanium nanowires (GeNWs) upon adsorption of water molecules and reveal the possible causes of the experimentally observed electrical hysteresis in GeNWs based electronic devices. We show that the absorption of water molecules on the GeNW surface would lead to the formation of hydroxyl passivated GeNWs (OH-GeNWs). The first step of the formation mechanism is physisorption of water molecules toward a Ge atom then followed by dissociation of water molecules to form OH-GeNWs, consistent with experimental observation of reversible and irreversible electrical hystereses. More importantly, we also predict that the effective masses of OH-GeNWs depend strongly on their growth orientation and depend nonlinearly on the OH coverage percentage. We propose that the electrical hysteresis phenomenon observed in experiment can be attributed to the formation of OH-GeNWs with different OH coverage percentages, along with different alignments of the OH groups on the GeNW surface, and also the presence of surface trap state defects, during the different stages of I-V measurement.     

Surface modification of silicon nanowires via copper-free click chemistry

A two-step process based on copper-free click chemistry is described, by which the surface of silicon nanowires can be functionalized with specific organic substituents. A hydrogen-terminated nanowire surface is first primed with a monolayer of an α,ω-diyne and thereby turned into an alkyne-terminated, clickable platform, which is subsequently coupled with an overlayer of an organic azide carrying the desired terminal functionality. The reactive, electron-deficient character of the employed diyne enabled a quantitative coupling reaction at 50 °C without metal catalysis, which opens up a simple and versatile route for surface functionalization under mild conditions without any potentially harmful additives.     

Preparation and electrical properties of ultrafine Ga2O3 nanowires

Uniform and well-crystallized beta-Ga2O3 nanowires are prepared by reacting metal Ga with water vapor based on the vapor-liquid-solid (VLS) mechanism. Electron microscopy studies show that the nanowires have diameters ranging from 10 to 40 nm and lengths up to tens of micrometers. The contact properties of individual Ga2O3 nanowires with Pt or Au/Ti electrodes are studied, respectively, finding that Pt can form Schottky-barrier junctions and Au/Ti is advantageous to fabricate ohmic contacts with individual Ga2O3 nanowires. In ambient air, the conductivity of the Ga2O3 nanowires is about 1 (Omega.m)-1, while with adsorption of NH3 (or NO2) molecules, the conductivity can increase (or decrease) dramatically at room temperature. The as-grown Ga2O3 nanowires have the properties of an n-type semiconductor.     

High yield solution-liquid-solid synthesis of germanium nanowires

High yields of crystalline Ge nanowires were synthesized for the first time in a conventional solvent of trioctylphosphine by disproportionating GeI2 in the presence of Bi nanoparticle growth seeds at 350 degrees C and atmospheric pressure.     

P-type 3C-SiC nanowires and their optical and electrical transport properties

We report for the first time the fabrication of p-type SiC nanowire field-effect transistors (FETs) using an individual Al-doped 3C-SiC nanowire with a single crystalline structure. The Raman spectroscopy of the as-grown p-type wire indicates that the linewidth and peak intensity of LO-phonon bands are sensitive to temperature variations.     

In situ fabrication and graphitization of amorphous carbon nanowires and their electrical properties

Individual amorphous carbon nanowires (a-CNWs) were fabricated inside a transmission electron microscope (TEM) by the electron beam induced deposition (EBID) method, and the a-CNWs were graphitized in situ by introducing Fe particles into these a-CNWs and controlled movement of the Fe particles in these CNWs. Detailed structural characterizations and electrical measurements were carried out, and it was found that the current-induced movement of Fe particles has significant effects in purifying the as-fabricated a-CNW, transforming the a-CNW into a graphitized-CNW (g-CNW). Two-terminal current voltage characteristics measurements showed that the g-CNW has a very good electrical conductivity with a resistivity of about 5.3 x 10(-4) Omega cm, a current carrying capacity of at least 4.35 mA, and a current density of 4.6 x 10(8) A/cm(2), and these values are comparable to those of multiwalled carbon nanotubes. Field emission characteristics of both a-CNWs and g-CNWs were also measured, and their respective Fowler-Nordheim plots were found to have basically a linear form.     

Stress induced half-metallicity in surface defected germanium nanowires

Germanium nanowires (GeNWs) with single, double, quadruple and octuple surface dangling bonds (SDBs) are investigated using density-functional-theory calculations. We show that single SDB defected GeNWs remain semiconducting as their non-defected form while double or multiple SDB defects result in either semiconducting or metallic GeNWs, depending on the defect's locations on the surface. More importantly, we show that the electronic properties of surface defected GeNWs can also be fine-tuned by applying tensile and compressive strains. Upon the right loading, the surface defected GeNWs become half-metallic. In addition, we determine that the surface defected GeNWs can be classified into three classes: (1) GeNWs with zero magnetic moment, which are either metallic or semiconducting; (2) GeNWs with net magnetic moments equal to the number of SDBs, which are semiconducting with distinct spin-up and spin-down configurations; and (3) GeNWs with net magnetic moments significantly lower than the number of SDBs. We also find that only the defected GeNWs that fall under (3) are potentially half-metallic. Our results predict that half-metallic GeNWs can be obtained via engineering of the surface defects and the structures without the presence of impurity dopants.     

Anisotropic and passivation-dependent quantum confinement effects in germanium nanowires: a comparison with silicon nanowires

Electronic structures of hydrogen-passivated germanium nanowires (GeNWs) along the [100], [110], [111], and [112] directions are studied by using the density functional theory within the generalized gradient approximation. The band gaps of the fully relaxed GeNWs along the [100], [110], and [111] directions are all direct at the smaller sizes, while those of the wires along the [112] direction remain indirect. The magnitude of the band gaps of the GeNWs for a given size approximately follows the order of E(g)[100] > E(g)[111] > E(g)[112] > E(g)[110]. Compared with silicon nanowires, GeNWs exhibit stronger quantum confinement effects. Replacement of H by the more stable ethine group is found to lead to a weakening of the quantum confinement effects of GeNWs.     

Single-walled carbon nanotube-based coaxial nanowires: synthesis, characterization, and electrical properties

We report herein the template-directed synthesis, characterization, and electric properties of single-walled carbon nanotube- (SWNT-) based coaxial nanowires, that is, core (SWNT)-shell (conducting polypyrrole and polyaniline) nanowires. The SWNTs were first dispersed in aqueous solutions containing cationic surfactant cetyltrimethylammonium bromide (CTAB) or nonionic surfactant poly(ethylene glycol) mono-p-nonyl phenyl ether (O pi-10). Each individual nanotube (or small bundle) was then encased in its own micellelike envelope with hydrophobic surfactant groups orientated toward the nanotube and hydrophilic groups orientated toward the solution. And thus a hydrophobic region within the micelle/SWNT (called a micelle/SWNT hybrid template) was formed. Insertion and growth of pyrrole or aniline monomers in this hybrid template, upon removal of the surfactant, produce coaxial structures with a SWNT center and conducting polypyrrole or polyaniline coating. Raman and Fourier transform infrared (FTIR) spectroscopy and scanning (SEM) and transmission (TEM) electron microscopy were used to characterize the composition and the structures of these coaxial nanowires. The results revealed that the micellar molecules used could affect the surface morphologies of the resulting coaxial nanowires but not the molecular structures of the corresponding conducting polymers. Electric properties testing indicated that the SWNTs played the key roles in the conducting polymer/SWNT composites during electron transfer in the temperature range 77 K to room temperature. Compared with the SWNT network embedded in the conducting polymers, the composites within which SWNTs were coated perfectly by the identical conducting polymers exhibited higher barrier heights during electron transfer.     

Block copolymer mediated deposition of metal nanoparticles on germanium nanowires

Galvanic displacement, mediated by a diblock copolymer, leads to deposition of well dispersed gold and silver nanoparticles on germanium nanowires.     

Nucleation and growth of germanium nanowires seeded by organic monolayer-coated gold nanocrystals

Germanium nanowires, ranging from 10 to 150 nm in diameter, were grown several micrometers in length in cyclohexane heated and pressurized above its critical point. Alkanethiol-protected gold nanocrystals, either 2.5 or 6.5 nm in diameter, were used to seed wire formation. Growth proceeded through a solution-liquid-solid mechanism at growth temperatures ranging from 300 to 450 degrees C. At temperatures exceeding 500 degrees C, large Ge particulates formed due to unfavorable growth kinetics. Temperature, the nature of the precursor, precursor concentration, and the Au:Ge ratio were determining factors in nanowire morphology. The Ge nanowires were characterized using a range of techniques, including XPS, XRD, high-resolution TEM and SEM, nanometer-scale EDS mapping, and DTA.     

Template assisted electrodeposition of germanium and silicon nanowires in an ionic liquid

In this paper we report for the first time on the room temperature template synthesis of germanium and silicon nanowires by potentiostatic electrochemical deposition from the air- and water stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py(1,4)]Tf(2)N) containing GeCl(4) and SiCl(4) as a Ge and Si source, respectively. Commercially-available track-etched polycarbonate membranes (PC) with an average nominal pore diameter of 90-400 nm were used as templates. Ge and Si nanowires with an average diameter corresponding to the nanopores' diameter and lengths of a few micrometres were reproducibly obtained. Structural characterization of the nanowires was performed by EDX, TEM, HR-SEM and Raman spectroscopy. Despite the rough surface of the nanowires, governed mostly by the original shape of the nanopore's wall of the commercially-available PC membrane, preliminary structural characterizations demonstrate the promising prospective of this innovative elaboration process compared to constraining high vacuum and high temperature methods.     

Temperature dependence of the field effect mobility of solution-grown germanium nanowires

The temperature dependence of the field effect mobility was measured for solution-grown single-crystal Ge nanowires. The nanowires were synthesized in hexane from diphenylgermane by the supercritical fluid-liquid-solid process using gold nanocrystals as seeds. The nanowires were chemically treated with isoprene to passivate their surfaces. The electrical properties of individual nanowires were then measured by depositing them on a Si substrate, followed by electrical connection with Pt wires using focused ion beam assisted chemical vapor deposition. The nanowires were positioned over TaN or Au electrodes covered with ZrO2 dielectric that were used as gates to apply external potentials to modulate the conductance. Negative gate potentials increased the Ge nanowire conductance, characteristic of a p-type semiconductor. The temperature-dependent source/drain current-voltage measurements under applied gate potential revealed that the field effect mobility increased with increasing temperature, indicating that the carrier mobility through the nanowire is probably dominated either by a hopping mechanism or by trapped charges in fast surface states.     

Effect of O5+ ion implantation on the electrical and structural properties of Cu nanowires

Ion beam creates changes in the material along their track, not only embody the excellent properties but also tailor new materials. When the ions are implanted into the nanomaterials, they collide with the target atoms and interact through three different phenomena; electron collision, nuclear collision and charge exchange. In the present study, 1 MeV O⁵⁺ ions were implanted in copper nanowires of diameter 80 nm synthesized using template synthesis approach. Electrical and structural properties were recorded using Keithley 2400 series source meter and Rigaku X-ray diffractometer respectively, before and after the implantation. I–V characteristics showed the ohmic behavior with enhancement in conductivity of copper nanowires after implantation. No structural damage in the nanowires was revealed by XRD spectra. The work done can be viewed as a positive aspect of implantation in metallic nanowires especially in 80 nm diameter Cu nanowires and may be utilized to fabricate nanodevices.     

Tuning the electrical transport properties of n-type CdS nanowires via Ga doping and their nano-optoelectronic applications

Gallium-doped n-type CdS nanowires (NWs) were successfully synthesized via a thermal evaporation method. The conductivities of the CdS NWs were dramatically improved by nearly nine orders of magnitude after Ga doping, and could be further tuned over a wide range by adjusting the doping level. High-performance metal-insulator-semiconductor field-effect transistors (MISFETs) were constructed based on the single CdS : Ga NW with high-κ Si(3)N(4) dielectrics and top-gate geometries. In contrast to back-gate FETs, the MISFETs revealed a substantial improvement in device performance. Nano-light emitting diodes (nanoLEDs) were fabricated from the CdS : Ga NWs by using a n-NW/p(+)-Si substrate hybrid device structure. The nanoLEDs showed a bright yellow emission at a low forward bias. It is expected that the Ga-doped CdS NWs with controlled electrical transport properties will have important applications in nano-optoelectronic devices.     

Effects of gold‐induced crystallization process on the structural and electrical properties of germanium thin films

Gold‐induced (Au‐) crystallization of amorphous germanium (α‐Ge) thin films was investigated by depositing Ge on aluminum‐doped zinc oxide and glass substrates through electron beam evaporation at room temperature. The influence of the postannealing temperatures on the structural properties of the Ge thin films was investigated by employing Raman spectra, X‐ray diffraction, and scanning electron microscopy. The Raman and X‐ray diffraction results indicated that the Au‐induced crystallization of the Ge films yielded crystallization at temperature as low as 300°C for 1 hour. The amount of crystallization fraction and the film quality were improved with increasing the postannealing temperatures. The scanning electron microscopy images show that Au clusters are found on the front surface of the Ge films after the films were annealed at 500°C for 1 hour. This suggests that Au atoms move toward the surface of Ge film during annealing. The effects of annealing temperatures on the electrical conductivity of Ge films were investigated through current‐voltage measurements. The room temperature conductivity was estimated as 0.54 and 0.73 Scm⁻¹ for annealed samples grown on aluminum‐doped zinc oxide and glass substrates, respectively. These findings could be very useful to realize inexpensive Ge‐based electronic and photovoltaic applications.     

Thermodynamic properties of germanium and lead tellurites

The temperature dependence of the molar heat capacities of the tellurites PbTeO₃, Pb₂Te₃O₈ and Ge(TeO₃)₂ are determined. By statistical manipulation of the values obtained, the parameters in the equations for the corresponding compounds showing this dependence are determined using the least-squares method. These equations and the standard molar entropies are used to determine the thermodynamic functions Δ₀ ^T S _m ⁰ , Δ_T ^T H _m ⁰ and (Φ_m ⁰+Δ₀ ^T H _m ⁰/T) for T'=298.15 K.     

Surface morphology and electrical properties of copper thin films prepared by MOCVD

Thin copper films have been grown in a vertical MOCVD (Metal-Organic Chemical Vapor Deposition) reactor using bis(2,2,6,6-tetramethyl-3,5-heptanedionato) copper(II), Cu(thd)₂, as precursor. Deposition has been carried out in a pure hydrogen atmosphere (pressure: 3, 20 mbar) at different substrate temperatures (350–750 ° C). The films have been investigated by profilometry, four-point resistivity measurements, ESCA, AES, XRD, AFM, and Normarsky microscopy. An unusual dependence of the film thickness with deposition time has been observed. Rapid growth occurred in the first minutes resulting in badly conducting films (thickness below 1000 Å). Good electrical resistivities have been obtained above 2000 Å. AFM has been used to gain information about the surface morphology of the films with different thicknesses. The grain size and surface roughness increased with increasing film thickness. Small grains grew in the beginning and the electrical properties have been governed by the highly Ohmic bridges between the individual grains.