S. T. Lee

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.

Silicon nanowires for high-sensitivity glucose detection

Silicon nanowires (SiNWs) were investigated as supporting matrices for enzyme immobilization to construct glucose biosensors. Glucose oxidase was adsorbed onto SiNWs after different treatments, either as grown, HF etched, or carboxylic acid (COOH) functionalized. The amperometric biosensor with COOH-functionalized SiNWs performed the best with a detection limit of 0.01mM glucose (signal-to-noise ratio=3). For real-time detection of glucose, SiNW biosensor showed a linear response in the range of 0.1–15mM. This work demonstrates the utility of SiNWs as a biosensor component and provides a general method to modify the surface of semiconducting nanomaterials for potential biomedical applications.

Direct Electrochemistry and Electrocatalytic Activity of Cytochrome c Covalently Immobilized on a Boron-Doped Nanocrystalline Diamond Electrode

Cytochrome c (Cyt c) was covalently immobilized on a boron-doped nanocrystalline diamond (BDND) electrode via surface functionalization with undecylenic acid methyl ester and subsequent removal of the protecting ester groups to produce a carboxyl-terminated surface. Cyt c-modified BDND electrode exhibited a pair of quasi-reversible and well-defined redox peaks with a formal potential (E(0)) of 0.061 V (vs Ag/AgCl) in 0.1 M phosphate buffer solution (pH 7.0) and a surface-controlled process with a high electron transfer constant (ks) of 5.2 +/- 0.6 s(-1). The electrochemical properties of as-deposited and Cyt c-modified boron-doped microcrystalline diamond (BDMD) electrodes were also studied for comparison. Investigation of the electrocatalytic activity of the Cyt c-modified BDND electrode toward hydrogen peroxide (H2O2) revealed a rapid amperometric response (5 s). The linear range of response to H2O2 concentration was from 1 to 450 microM, and the detection limit was 0.7 microM at a signal-to-noise ratio of 3. The stability of the Cyt c-modified BDND electrode, in comparison with that of the BDMD and glassy carbon counterpart electrodes, was also evaluated.

Single-crystal CdSe nanoribbon field-effect transistors and photoelectric applications

Field-effect transistors made of individual CdSe nanoribbons were fabricated and characterized. The CdSe nanoribbon showed n-type semiconducting characteristics, while the transistors revealed a threshold voltage of 20.9V, an on-off ratio >104, and an electron mobility of 9.6cm2∕Vs in the dark. CdSe nanoribbons showed high sensitivity to above-band-gap irradiation with four-orders-of-magnitude increase in conductance and millisecond response speed. The increase of electron mobility due to light irradiation was demonstrated to contribute to increased photoconductance.

Role of nucleation in nanodiamond film growth

Nanodiamond films were deposited using different microwave plasma chemical vapor deposition schemes following several nucleation pretreatment methods. The nucleation efficiency and the films structure were investigated using scanning and transmission electron microscopy and Raman spectroscopy. C2 dimer growth (CH4 and H2 in 90% Ar) cannot nucleate diamond and works only on existing diamond surfaces. The methyl radical process (up to 20% CH4 in H2) allows some nucleation probability on appropriate substrates. Prolonged bias enhanced nucleation initiates both diamond nucleation and growth. C2 dimer growth results in pure nanodiamond free of amorphous carbon, while prolonged bias enhanced nucleation forms an amorphous carbon/nanodiamond composite.

Manganese doping and optical properties of ZnS nanoribbons by postannealing

Manganese (Mn) doping of ZnS nanoribbons was achieved by simple thermal annealing. Upon heating ZnS nanoribbons with MnS powder up to 700°C, the intrinsic photoluminescence (PL) of the annealed nanoribbons disappeared and a new PL peak at 585nm gradually emerged. Significantly, the annealing process induced no detectable change in the morphology and uniform hexagonal wurtzite 2H structure of the single-crystal ZnS nanoribbons. The PL peak at 585nm is attributed to Mn dopant and confirms Mn incorporation in ZnS because (1) the peak appears only when ZnS ribbons were annealed with MnS, but does not appear without MnS, (2) its intensity increases with increasing annealing temperature, which is consistent with increased incorporation of Mn2+ ions, and (3) its position is similar to that of Mn-related emission in ZnS, and is independent of the measuring temperature and excitation power. This work demonstrates the capability of doping nanostructured materials by simple postannealing treatment.

Transport properties of single-crystal CdS nanoribbons

The transport properties of single-crystal CdS nanoribbons were studied by evaluating the characteristics of field-effect transistors made of individual CdS nanoribbons. The nanoribbon transistors exhibited typical normally off n-channel characteristics with an on-off ratio as high as 104 in dark and in air. A barrier of 0.55eV at the metal-semiconductor contact was deduced from the temperature-dependent conductance. The CdS nanoribbons were highly sensitive to visible light and to adsorbed oxygen molecules. The transistor showed substantial improvement in performance under light irradiation and in vacuum.

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.

Contrast improvement of organic light-emitting devices with Sm:Ag cathode

Sm:Ag, easily fabricated via simple thermal evaporation, was investigated as a single-layer light-absorbing cathode to increase the contrast ratio of organic light-emitting devices (OLEDs). The performance of OLEDs with Sm:Ag cathode was found to be comparable to that of the traditional OLEDs with Mg:Ag cathode in terms of brightness, electroluminescence (EL) efficiency, and turn-on voltage. The maximum EL efficiency of the device with Sm:Ag black cathode is 2.72cd∕A at 15V, while the contrast ratio reaches 390:1 at 10V under 140 lx of ambient light, which is 8 times better than that of the traditional device. The lower EL efficiency and enhanced contrast are due to the reduced optical reflectance of Sm:Ag black cathode, which was calculated to be about 0.15.

Surface microstructure analysis of cubic boron nitride films by transmission electron microscopy

A simple coating technique was introduced to preserve the surface structure of samples for transmission electron microscopy (TEM) characterization, and used to study boron nitride (BN) films. A gold film precoated on the surface of BN films served to protect the BN surface against ion damages during sample preparation, and to separate and distinguish the film surface structure from the TEM glue. The technique enabled the observation of detailed surface microstructures of cubic BN (cBN) films, which provided direct evidences for understanding cBN growth mechanisms. The TEM sample technique is expected to be generally applicable to other film systems, particularly those with an amorphous topmost layer.