A molecule detector: Adsorbate induced conductance gap change of ultra-thin silicon nanowire

Inspired by the work of Lieber and co-workers [F. Patolsky, B.P. Timko, G. Zheng, C.M. Lieber, MRS Bull. 32 (2007) 142], we present a general discussion of the possibility of using atomic-chain scaled Si nanowires to detect molecules. Surface-modified Si nanowires were optimized by density functional theory (DFT) calculations. The electronic transport properties of the whole system, including Si nanowires and adsorbed molecules, sandwiched between two gold electrodes are investigated by means of non-equilibrium Green’s function (NEGF) formalism. However, the overall transport properties, including current-voltage (I-V) and conductance-voltage (G-V) characteristics hardly show adsorbate sensitivity. Interestingly, our results show that the conductance gap clearly varies with the different adsorbates. Therefore different molecules can cause differences in the conductance gap compared with the bare Si nanowire. The results provide valuable information regarding the development of atomic-chain scaled molecular detectors.     

Investigation of electrical and optoelectronic properties of zinc oxide nanowires

Zinc oxide (ZnO) nanowires have been synthesized by using tubular furnace chemical vapor deposition technique. The morphology, chemical composition and crystal structure of as-synthesized ZnO nanowires were examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) techniques. Four-terminal current-voltage (I-V) measurements were employed to study the electrical conductance of ZnO nanowires under various testing gas environments for gas sensing purpose. The I-V curves at temperature ranging from 150 to 300 K were recorded in the testing chamber under vacuum. The Arrhenius plot shows perfect linear relationship between the logarithm of the current I and inverse temperature 1/T. The donor level of the semiconducting nanowires is about 326 meV. The I-V behaviors were found to be reversible and repeatable with testing gases. The electrical conductivity was enhanced by a factor of four with ambient CO gas compared to that in vacuum and other testing gases. The optoelectronic properties of the ZnO nanowires were obtained by two-terminal I-V measurement method while the nanowires were illuminated by a ruby laser. The electrical conductivity was increased by 60% when the laser was present in comparison to that when the laser was off. Those significant changes suggest that nano-devices constructed by the ZnO nanowires could be used in gas sensing and optical switching applications.     

A study on structural and electrical properties of low dielectric constant SiOC(–H) thin films deposited via PECVD

Low-dielectric constant SiOC(–H) films were deposited on p-type Si(100) substrates using plasma enhanced chemical vapor deposition (PECVD) at different radio frequency (rf) powers. The structural characteristics of the SiOC(–H) films were characterized using Fourier transform infrared spectroscopy (FTIR) in the absorbance mode. The bonding configurations of the SiOC(–H) films remained unchanged upon annealing, showing their good thermal stability. Electrical characteristics of the SiOC(–H) thin films with Al/SiOC(–H)/p-Si(100)/Al metal-insulator-semiconductor (MIS) structures were analyzed using capacitance–voltage (C–V) and conductance–voltage (G/ω–V) at different frequencies. The conductance and the capacitance measurements were used to extract the interface state density in the MIS structures. From the experimental data and the subsequent quasi-static C–V analysis, the energy distribution of interface state density was obtained. The interface state density of the as-deposited and 400 °C annealed MIS structures increased with increasing rf powers, whereas the fixed charge density decreased with increasing rf powers. The interface state densities and their electrical properties of the SiOC(–H) films strongly affected by the radio frequency power.     

Characterization of Al/Si junctions on Si(100) wafers with chemical vapor deposition-based sulfur passivation

Chemical vapor deposition-based sulfur passivation using hydrogen sulfide is carried out on both n-type and p-type Si(100) wafers. Al contacts are fabricated on sulfur-passivated Si(100) wafers and the resultant Schottky barriers are characterized with current–voltage (I–V), capacitance–voltage (C–V) and activation-energy methods. Al/S-passivated n-type Si(100) junctions exhibit ohmic behavior with a barrier height of <0.078 eV by the I–V method and significantly lower than 0.08 eV by the activation-energy method. For Al/S-passivated p-type Si(100) junctions, the barrier height is ~0.77 eV by I–V and activation-energy methods and 1.14 eV by the C–V method. The discrepancy between C–V and other methods is explained by image force-induced barrier lowering and edge-leakage current. The I–V behavior of an Al/S-passivated p-type Si(100) junction remains largely unchanged after 300 °C annealing in air. It is also discovered that heating the S-passivated Si(100) wafer before Al deposition significantly improves the thermal stability of an Al/S-passivated n-type Si(100) junction to 500 °C.     

Helical Shell Structures of Ni-A1 Alloy Nanowires and Their Electronic Transport Properties

Six kinds of Ni-A1 alloy nanowires are optimized by means of simulated annealing. The optimized structures show that the Ni-A1 alloy nanowires are helical shell structures that are wound by three atomic strands, which is very similar to the case with pure metallic nanowires. The densities of states （DOS）, transmission function T（ E）, current-voltage （I - V） curves, and the conductance spectra of these alloy nanowires are also investigated. Our results indicate that the conductance spectra depend on the geometric structure properties and the ingredients of the alloy nanowires. We observe and study the nonlinear contribution to the I-V characteristics that are due to the quantum size effect and the impurity effect. The addition of Ni atoms decreases the conductance of the Ni-A1 alloy nanowire because the doping atom Ni change the electronic band structures and the charge density distribution. The interesting statistical results shed light on the physics of quantum transport at the nano-scale.     

Electrical characterization of surface and interface potentials on SiC

Contact free vibrating capacitor results have shown that the SiC surface is more stable, compared to Si, and it is possible to identify the different (Si or C) planes on SiC substrates. The surface charge density seems to be higher after compression welding process. Electrostatic (corona) charge on the surface results in accumulation and depletion, and probably avalanche breakdown instead of equilibrium inversion. However, the equilibrium Q–V curve still can be measured starting from the inversion region.Among C–V methods the capabilities of V–Q and mercury C–V have been investigated, as two major electrical measurement techniques for SiC qualification. SiC–silicon-dioxide interfaces and SiC epitaxial layers were characterized with HF/LF C–V and V–Q measurement techniques. These methods were developed basically for Si measurements, but they could easily be adapted for measuring SiC too.     

Study of Schottky contact between Au and NiO nanowire by conductive atomic force microscopy (C-AFM): The case of surface states

In this work, NiO nanowires have been synthesized by a hydrothermal reaction of NiCl₂ with Na₂C₂O₄ in the presence of ethylene glycol at 180 °C for 12 h, then calcinated at 400 °C for 2 h. The NiO nanowires were analyzed by means of scanning electron microscope (SEM), atomic force microscope (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The resulting current–voltage (I–V) characteristics of the NiO nanowires exhibited a clear rectifying behavior. This rectify behavior was attributed to the formation of a Schottky contact between Au coated atomic force microscopy (AFM) tip and NiO nanowires (nano-M/SC) which was dominated by the surface states in NiO itself. Photo-assisted conductive AFM (PC-AFM) was used to demonstrate how the I–V characteristics are influenced by the surface states. Our I–V results also showed that the nano-M/SCs had a good photoelectric switching effect at reverse bias.     

Disorder-induced enhancement of conductance in doped nanowires

A new mechanism is proposed to explain the enhancement of conductance in doped nanowires.It is shown that the anomalous enhancement of conductance is due to surface doping.The conductance in doped nanowires increases with dopant concentration,which is qualitatively consistent with the existing experimental results.In addition,the I-V curves are linear and thus suggest that the metal electrodes make ohmic contacts to the shell-doped nanowires.The electric current increases with wire diameter (D) and decreases exponentially with wire length (L).Therefore,the doped nanowires have potential application in nanoscale electronic and optoelectronic devices.     

Structural,electronic and magnetic properties of Fe(1−x)Cox alloy nanowires encapsulated inside (10,0) boron nitride nanotube

The structural, electronic and magnetic properties of Fe–Co alloy nanowires encapsulated inside zigzag (10,0) boron nitride nanotube (BNNT) are investigated by ab initio calculations. Similar to pristine nanotube, the opposite directional relaxations for the N atoms (move outwards) and B atoms (move inwards) from their initial positions are observed for outside BNNT although with the Fe–Co alloy nanowires inside, but the outward relaxations of the N atoms bonding to the Fe or Co atoms are smaller due to their attractions. The combining processes of Fe–Co/BNNT composites are endothermic when Co concentration x≤0.6 and exothermic x>0.6, and the most stable Fe–Co/BNNT composite is at Co concentration x=0.8. So the semiconducting (10,0) BNNT can be used to shield the Co-rich Fe–Co nanowires. The charges are transferred from Fe–Co nanowires to BNNT and the formed Co–N bonds have covalent bond as well as slight ionic bond characters. Although (10,0) BNNT is nonmagnetic and a decrease in the magnetic moment is found after Fe–Co nanowires are encapsulated inside (10,0) BNNT, the Fe–Co/BNNT composites still have large magnetic moment, reflecting they can be utilized in magnetic storage and ultra high-density magnetic recording devices.     

Electric conductance of metal nanowires at mechanically controllable break junctions under electrochemical potential control

We have developed the mechanically controllable break junction setup with an electrochemical cell (EC-MCBJ) to measure the electric conductance of metal nanowires under electrochemical potential control. The electric conductance of Au nanowires was investigated in 0.1 M Na₂SO₄ solution using EC-MCBJ. The conductance of the Au nanowires was quantized in units of G₀ (=2e²/h), showing clear features in the conductance histogram. The atomic contact with a specific conductance value was kept for >5 s, indicating the relatively high stability of the present EC-MCBJ system.     

The electron distribution in superconducting alloys: II. Room temperature analysis of Cr3Si and comparison with V3Si

The charge distribution in Cr₃Si a non-superconducting alloy of the A-15 family was obtained from accurate X-ray diffraction data. The deformation density in the bond between two adjacent Cr atoms in Cr₃Si is 0.17 eA^?3, about three times lower than the density observed in the isomorphous V₃Si structure. Integration of the charge around the Cr and Si atoms leads to a best estimate of 1.3–1.9 electrons for the charge transfer from the Si to three Cr atoms as compared to 1.8 – 2.4 electrons transfered from the Si to the three V atoms in V₃Si.     

Capacitance–voltage and conductance–voltage characteristics of Ag/n-CdO/p-Si MIS structure prepared by sol–gel method

The frequency dependent electrical properties of Ag/n-CdO/p-Si structure has been investigated using capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics in the frequency range 10 kHz–1 MHz in the room temperature. The increase in capacitance at lower frequencies is observed as a signature of interface states. The presence of the interfaces states (N_SS) is also evidenced as a peak in the capacitance–frequency characteristics. Furthermore, the voltage and frequency dependence of series resistance were calculated from the C–V and G/ω–V measurements and plotted as functions of voltage and frequency. The distribution profile of R_S–V gives a peak in the depletion region at low frequencies and disappears with increasing frequencies. The values of interface state densities and series resistance from capacitance–voltage-frequency (C–V-f) and conductance–voltage-frequency (G/ω–V-f) measurements were obtained in the ranges of 1.44×10¹⁶–7.59×10¹² cm^?2 eV^?1 and 341.49–8.77 Ω, respectively. The obtained results show that the C–V-f and G/ω–V-f characteristics confirm that the interface states density (N_SS) and series resistance (R_S) of the diode are important parameters that strongly influence the electrical parameters in Ag/n-CdO/p-Si structures.     

Development of a miniature STM holder for study of electronic conductance of metal nanowires in UHV-TEM

A new miniature scanning tunneling microscope (STM) holder was developed in order to simultaneously investigate electronic conductance and structure of nanowires in an ultra high-vacuum electron microscope (UHV-TEM). A thin gold wire held between the STM tip and substrate stage of the specimen holder was stretched to form a suspended gold nanowire. The new TEM-STM system allowed us to measure electronic conductance at intervals of 20 ms, and to record high-resolution TEM images on videotape at 30 fps. Suspended gold nanowires formed from [1 1 0] oriented electrodes were well-elongated. In contrast, [1 0 0] and [1 1 1] electrodes produced nanowires with short necks. Electronic conductance was found to change as nanowire structure changed, with conductance quantization in units of 2e²/h, where e is the electron charge and h is Planck’s constant, only being exhibited for well-elongated nanowires.     

Preparation of SiC nanowires with fins by chemical vapor deposition

SiC nanowires with fins have been prepared by chemical vapor deposition in a vertical vacuum furnace by using a powder mixture of milled Si and SiO₂ and gaseous CH₄ as the raw materials. The products were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). These investigations confirm that the nanowires with fins are cubic β-SiC. The diameter of the fins is about 100–120 nm and the diameter of the inner core stems is about 60–70 nm. The formation process of the β-SiC nanowires with fins is analyzed and discussed briefly.     

Negative capacitance of ZnGa2Se4/Si nano-heterojunction diode

The negative capacitance (NC) behavior in p-ZnGa₂Se₄/n-Si nano-crystalline HJD was characterized by admittance-voltage method. The C–V, G–V and R _s –V for the studied diode were analysed in the frequency range of 1–5 MHz and in the temperature range of 303–423 K. The capacitance and conductance plots were interpreted in terms of interface states and series resistance. The C–V and G/ω–V plots exhibit a pounced peak due to the interface states and the series resistance effects. The negative capacitance behavior in the studied diode can be explained in terms of the transient currents.     

Controlling the electrical characteristics of Al/p-Si structures through Bi4Ti3O12 interfacial layer

In this study, the effects of high permittivity interfacial Bi₄Ti₃O₁₂ (BTO) layer deposition on the main electrical parameters; such as barrier height, series resistance, rectifying ratio, interface states and shunt resistance, of Al/p-Si structures are investigated using the current–voltage (I–V) and admittance measurements (capacitance–voltage, C–V and conductance–voltage, G/ω–V) at 1 MHz and room temperature. I–V characteristics revealed that, due to BTO layer deposition, series resistance values that were calculated by both Ohm's law and Cheung's method decreased whereas shunt resistance values increased. Therefore, leakage current value decreased significantly by almost 35 times as a result of high permittivity interfacial BTO layer. Moreover, rectifying ratio was improved through BTO interfacial layer deposition. I–V data indicated that high permittivity interfacial BTO layer also led to an increase in barrier height. Same result was also obtained through C–V data. Obtained results showed that the performance of the device is considerably dependent on high permittivity BTO interfacial layer.     

Effect of surface roughness on nucleation and growth of vanadium pentoxide nanowires

The effect of surface roughness on subsequent growth of vanadium pentoxide (V₂O₅) nanowires is examined. With increasing surface roughness, both the number density and aspect ratio of V₂O₅ nanowires increase. Structures and morphology of obtained nanowires were characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The nanowires are approximately 40-90 nm in diameter and 2 μm in length. X-ray diffraction (XRD) analysis indicates that the obtained nanowires are orthorhombic structure with (0 0 1) out-of-plane orientation. The luminescence property of V₂O₅ nanowires has been investigated by photoluminescence (PL) at 150 K and 300 K. PL results show intense visible emission, which is attributed to different inter-band transitions between the V 3d and O 2p band. This simple fabrication approach might be useful for fabrication of large area V₂O₅ nanowires arrays with high density.     

Photo-induced charge transport in SiO2films containing Si nanocrystals

Photo-induced charge transport is reported in metal–insulator–semiconductor structures containing Si nanocrystals produced by ion implantion and annealing. Successive shifts in current–voltage (I–V) and capacitance–voltage (C–V) curves are shown to be induced by ultra-violet (UV) light exposure under no bias. These shifts are shown to be enhanced by the application of a negative bias voltage during illumination. The application of a positive bias voltage during illumination is shown to reverse the direction of the shifts in both the I–V and C–V curves. This behaviour can be explained by charging of the nanocrystals induced by photoionization of electrons and charge movements in the insulator layer.     

Measurements of Rayleigh–Taylor instability growth of laser-shocked iron–silicon alloy

The Rayleigh–Taylor (RT) instability of liquid iron alloys is important for understanding the core formation mechanism in the Earth. Here we first report the measurement of RT instability growth for a liquid iron–silicon (Fe–Si) alloy, which is one of the major candidate for the material of the Earth’s core, using a high power laser. We optimized the measurement setup and analytical technique to observe the growth of perturbation on an Fe–Si sample surface. The growth of perturbation amplitude on the Fe–Si alloy under high pressure and temperature was successfully observed using in situ X-ray radiography. The growth rate of the RT instability for the Fe–Si alloy on about 1000?GPa was estimated to be 0.3 ns^?1.     

Asymmetric electron tunneling into an amorphous chalcogenide semiconductor

Electron tunneling into the amorphous chalcogenide alloy Tl₂SeAs₂Te₃ was found to occur with a higher conductance G for electrons tunneling out of the semiconductor compared to the opposite polarity. G(V) has a minimum at V=0.070 V with respect to the metal electrode. It rises by a factor 10⁵ as the bias is increased to V=±1 V. This tunnelling asymmetry, which is absent in the case of tetrahedrally bonded semiconductors, may be related to a suggestion by Mott of detecting polaron effects by tunneling.