D. D. D. Ma

Silicon nanowires as chemical sensors

Chemical sensitivity of silicon nanowires bundles has been studied. Upon exposure to ammonia gas and water vapor, the electrical resistance of the HF-etched relative to non-etched silicon nanowires sample is found to dramatically decrease even at room temperature. This phenomenon serves as the basis for a new kind of sensor based on silicon nanowires. The sensor, made by a bundle of etched silicon nanowires, is simple and exhibits a fast response, high sensitivity and reversibility. The interactions between gas molecules and silicon nanowires, as well as the effect of silicon oxide sheath on the sensitivity and the mechanisms of gas sensing with silicon nanowires are discussed.

Excellent photocatalysis of HF-treated silicon nanowires.

HF-treated silicon nanowires exhibited excellent photocatalysis, which were even better than some noble metal-modified ones, such as palladium, gold, silver, and rhodium. This phenomenon was critical in the application of silicon-related materials as they are normally employed as a catalyst carrier. These HF-treated silicon nanowires were also stable in solution over 1 week; consequently, a possible explanation for the stability was proposed.

Structures and energetics of hydrogen-terminated silicon nanowire surfaces

The analysis and density-functional tight-binding simulations of possible configurations of silicon nanowires (SiNWs) enclosed by low-index surfaces reveal a number of remarkable features. For wires along <100>, <110>, and <111> directions, many low-index facet configurations and cross sections are possible, making their controlled growth difficult. The 112 wires are the most attractive for research and applications because they have only one configuration of enclosing low-index facets with a rectangular cross section, enclosed with the most stable (111) facet and the (110) facet next to it. In general, the stability of the SiNWs is determined by a balance between (1) minimization of the surface energy gamma(111)svr(rectangular)>svr(triangular)]. The energy band gaps follow the order of <100>wires > <112>wires > <111>wires > <110>wires. The results are compared with our recent scanning tunneling microscopy and transmission electron microscopy data.

Intrinsic current-voltage properties of nanowires with four-probe scanning tunneling microscopy : A conductance transition of ZnO nanowire

We report intrinsic current-voltage properties of ZnO nanowire measured by a four-tip scanning tunneling microscopy (F-STM). It is found that after bending the nanowire with the F-STM the conductance is reduced by about five orders of magnitude. The cathodoluminescent spectra indicate that the ZnO nanowires contain a sizable amount of defects in the surface region, responsible for their conduction. It is suggested that the observed huge conductance changes are caused by the shifting of the surface defect states in the ZnO nanowires in response to the applied surface strain.

Scanning tunneling microscopic study of boron-doped silicon nanowires

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements have been performed on boron-doped and undoped silicon nanowires (SiNWs). STM images clearly showed the presence of nanoparticle chains and nanowires in the B-doped SiNWs sample. Clear and regular nanoscale domains were observed on the SiNW surface, which were attributed to boron-induced surface reconstruction. STS measurements have provided current–voltage curves for SiNWs, which showed clearly enhancement in electrical conductivity by boron doping.

Photoluminescence and photoconductivity properties of copper-doped Cd1−xZnxS nanoribbons

Copper-doped Cd1?xZnxS (x~0.16) nanoribbons were prepared by controlled thermal evaporation of CdS, ZnS, and CuS powders onto Au-coated silicon substrates. The nanoribbons had a hexagonal wurtzite structure, and lengths of several tens to hundreds of micrometres, widths of 0.6?15??m, and thicknesses of 30?60?nm. Cu doping and incorporation into the CdZnS lattice were identified and characterized by low-temperature photoluminescence (PL) and photoconductivity measurements. Temperature-dependent PL measurement showed that the PL spectra of both Cu-doped and undoped CdZnS nanoribbons have two emission peaks at 2.571 and 2.09?eV, which are assigned to band edge emission and deep trap levels, respectively. In addition, the Cu-doped nanoribbons present two extra peaks at 2.448 and 2.41?eV, which are attributed to delocalized and localized donor and acceptor states in the band gap of CdZnS resulting from Cu incorporation. Photoconductivity results showed the nanoribbons can be reversibly switched between low and high conductivity under pulsed illumination. The Cu-doped CdZnS nanoribbons showed four orders of magnitude larger photocurrent than the undoped ones. The current jumped from ~2 ? 10?12 to ~5.7 ? 10?7?A upon white light illumination with a power density of ~9?mW?cm?2. The present CdZnS:Cu nanoribbons may find applications in opto-electronic devices, such as solar cells, photoconductors, and chemical sensors.

Strong polarization-dependent photoluminescence from silicon nanowire fibers

Fibers of highly oriented Si nanowires (SiNWs) were formed by drawing from a condensed SiNW suspension. The SiNW fiber, excited at 514.5nm, produces a strong photoluminescence (PL) at room temperature. The PL spectrum shows three bands at 565–580, 605–640, and 680–690nm, respectively, which are consistent with the PL of porous silicon. The relative intensity of these bands and the integrated intensity of the PL vary with the angle θ between the electric field of the polarized laser excitation and the fiber axis. The dependence on θ is attributed to the combined effects of the one-dimensional shape of the SiNW and the large dielectric contrast between the SiNW and the ambient.

Nitrogen-doped silicon nanowires: Synthesis and their blue cathodoluminescence and photoluminescence

Nitrogen-doped silicon nanowires were obtained via a high temperature oxide assisted method. Both their cathodoluminescence and photoluminescence exhibited blue emissions, which might attributed to the nitrogen doping. Both the elemental mapping analysis and smooth cathodoluminescence image suggested uniform nitrogen doping in the silicon nanowires.

Surface-enhanced fluorescence of praseodymium ions (Pr3+) on silver/silicon nanostructure

The enhanced fluorescence of praseodymium ions (Pr3+) owing to resonant plasma oscillation on the surface of Ag/Si nanostructure was investigated. When Ag/Si nanomaterials were added, the fluorescence peaks were markedly enhanced. A typical 12- to 40-fold enhancement at 604 nm and 18- to 193-fold enhancement at 640 nm could be achieved over a range of concentration from 0.01 to 0.05 M praseodymium ions, which had larger enhancement factor than that caused by unsupported silver nanoparticles. These results might be explained by the local field overlap originated from the closed and fixed silver nanoparticles on silicon nanowires.

Periodic array of intramolecular junctions of silicon nanowires

The formation of periodic arrays of intramolecular junctions in silicon nanowires from a single growth process is reported. Scanning tunneling microscopic images show intramolecular junctions formed by fusing together two straight wire segments (∼3 nm in diameter) 5 and 10 nm long, respectively, at an angle of ∼30° with respect to each other. The junction repeats itself in a regular pattern along a nanowire, forming a striking superlattice ∼3 nm in diameter and at least several microns long. Scanning tunneling spectroscopic measurements reveal distinctly different current–voltage curves for the two different segments changing sharply across each junction. The segments are most probably formed by a periodic change of growth direction while the different electronic properties of the two segments forming the junction are attributed to the differences in surface structure and segment diameter.