Kin Mun Wong

Spatial distribution of neutral oxygen vacancies on ZnO nanowire surfaces: An investigation combining confocal microscopy and first principles calculations

mechanism behind the green emission spectral intensity and the characteristics of an individual ZnO NW. The highly accurate density functional theory (DFT)-based full-potential linearized augmented plane-wave plus local orbitals (FP-LAPW þ lo) method is used to compute the defect formation energy (DFE) of the SSs. Previously, using these SS models, it was demonstrated through the FP-LAPW + lo method that in the presence of oxygen vacancies at the (0001) surface, the phase transformation of the SSs in the graphite-like structure to the wurtzite lattice structure will occur even if the thickness of the graphite-like SSs are equal to or less than 4 atomic graphite-like layers [Wong et al., J. Appl. Phys. 113, 014304 (2013)]. The spatial profile of the neutral VO DFEs from the DFT calculations along the ZnO [0001] and [10 � directions is found to reasonably explain the spatial profile of the measured confocal luminescence intensity on these surfaces, leading to the conclusion that the green emission spectra of the NWs likely originate from neutral oxygen vacancies. Another significant result is that the variation in the calculated DFE along the ZnO [0001] and [10 � directions shows different behaviors owing to the non-polar and polar nature of these SSs. These results are important for tuning and understanding the variations in the optical response of ZnO NW-based devices in different geometric configurations. V C 2013 Author(s). All article

First-principles investigation of the size-dependent structural stability and electronic properties of O-vacancies at the ZnO polar and non-polar surfaces

In this paper, all electron full-potential linearized augmented plane wave plus local orbitals method has been used to investigate the structural and electronic properties of polar (0001) and non-polar (101¯0) surfaces of ZnO in terms of the defect formation energy (DFE), charge density, and electronic band structure with the supercell-slab (SS) models. Our calculations support the size-dependent structural phase transformation of wurzite lattice to graphite-like structure which is a result of the termination of hexagonal ZnO at the (0001) basal plane, when the stacking of ZnO primitive cell along the hexagonal principle c-axis is less than 16 atomic layers of Zn and O atoms. This structural phase transformation has been studied in terms of Coulomb energy, nature of the bond, energy due to macroscopic electric field in the [0001] direction, and the surface to volume ratio for the smaller SS. We show that the size-dependent phase transformation is completely absent for surfaces with a non-basal plane termi...

Study of the Electronic Structure of Individual Free-Standing Germanium Nanodots Using Spectroscopic Scanning Capacitance Microscopy

High spatial resolution spectroscopic scanning capacitance microscopy (SCM) measurements at room temperature based on the differential capacitance (d C=dV ) versus probe tip-to-substrate bias characteristics, is directly used to characterize the electronic structure of a pyramidal and an ellipsoidal shaped germanium (Ge) nanodot (with physical sizes smaller than the Bohr exciton radius of Ge), among the arrays of Ge nanodots fabricated on a highly doped silicon substrate using an anodic alumina membrane as an evaporation mask. The ability to study the individual nanodots under the probe tip circumvents and isolates the statistical disorders encountered when studying large ensembles of size-dispersed nanodots. It is demonstrated that the spectra of the positive d C=dV peaks are reflective of the energetics of electron trapping in the quantized energy states inside the Ge nanodots. Furthermore, the spectroscopic SCM d C=dV profile is observed to be substantially influenced by the nanodot shape where the pyramidal Ge nanodot shows three distinct shells which are interpreted as the s-like ground state, the first excited p-like state and the second excited d-like state. On the other hand, the elliptical deformation of the ellipsoidal Ge nanodot breaks the shell structure which significantly affects its electronic structure. Using the spacing between the d C=dV peaks from the spectroscopic SCM measurements, an analytical expression is derived to calculate the energy separation between the different energy states in the Ge nanodots. # 2009 The Japan Society of Applied Physics

Scanning capacitance microscopy detection of charge trapping in free-standing germanium nanodots and the passivation of hole trap sites

An array of freestanding germanium (Ge) nanodots was fabricated on a highly doped silicon substrate using an anodic alumina membrane as an evaporation mask. Approximately half of the Ge nanodots exhibit contrast reversal and hole trapping characteristics during scanning capacitance microscopy (SCM) measurements, as shown by a negative peak in the SCM differential capacitance (dC∕dV) versus probe tip-to-substrate bias profile. The disappearance of the negative dC∕dV characteristic peak after forming gas anneal at 450°C show the complete passivation of the hole trap sites by hydrogen. This is a demonstration on the spectroscopic detection of hole trapping in Ge nanodots and hydrogen passivation of hole trap sites using SCM. Partial passivation of electron trap sites after the relatively low temperature forming gas anneal was also observed as compared with complete passivation of hole trap sites. This suggests that hole traps sites are possibly located at or closer to the surface of the Ge nanodots as compar...

Fabrication and characterization of well-aligned, high density ZnO nanowire arrays and their realizations in Schottky device applications using a two-step approach

We report a two-step general and viable approach for preparing a large area of high density and horizontally well-aligned arrays of zinc oxide nanowires (ZnO NWs) for the realization of Schottky device applications on inexpensive, flexible polymer substrate. A modified chemical vapor deposition (CVD) process is initially used for synthesizing the highly efficient and rapid growth of vertical ZnO NW arrays along their [0001] direction, which is perpendicular to the donor substrate surface without using any metal catalyze. This is followed by transferring the NWs to a receiver substrate by a dry contact printing method. Utilizing the ZnO NWs synthesized by our method, a fully controllable and relatively large separation between the adjacent rows of silver (Ag) electrodes for the electrical contact with the NWs can be obtained using a photolithographic process. The printed ZnO NWs are well aligned along their c-axis, resulting in a spontaneous polarization which leads to a potential gradient along the length of the individual NW. This coupled with the effect of the surface states in the ZnO NWs result in the formation of a Schottky contact at the Ag/ZnO NW interface. Hence, virtually all of the ZnO NW arrays are functional as Schottky diodes which display non-linear current–voltage characteristics with good rectifying diode-like behaviour.

Theoretical model of interface trap density using the spread of the differential capacitance characteristics in scanning capacitance microscopy measurements

In this letter, we propose a theoretical model for the calculation of interface trap density (Dit) in a metal-oxide-semiconductor structure using data from scanning capacitance microscopy (SCM) measurements. The model is based on the correlation of Dit with the change in the full width at half maximum of the SCM differential capacitance (dC∕dV) characteristics. The good agreement between the calculated Dit values from the SCM theoretical model and the experimental midgap Dit values obtained from conductance measurements shows the validity of the proposed model. The model opens up possibilities for obtaining the spatial distribution (with nanometers resolution) of interfacial traps on a device using SCM measurements.

Spatial distribution of interface trap density in strained channel transistors using the spread of the differential capacitance characteristics in scanning capacitance microscopy measurements

In this letter, the authors obtain, with nanometer spatial resolution, the local spatial distribution of interface trap density (Dit) at the gate oxide-substrate interface across the channel region of a transistor with silicon germanium source/drain (S/D) stressors. The Dit values were extracted using a theoretical model which is based on the correlation of Dit with the change in the full width at half maximum of the scanning capacitance microscopy differential capacitance characteristics. Results show that Dit increases with spatial location, measured from the center of the channel, towards the S/D regions of the strained channel transistor. The large value of Dit near the S/D regions is possibly due to germanium diffusion into the channel region and other resulting structural defects.

Selective growth and piezoelectric properties of highly ordered arrays of vertical ZnO nanowires on ultrathin alumina membranes

A well controlled and cost effective method of fabricating highly ordered arrays of vertical zinc oxide (ZnO) nanowires or nanopores is demonstrated where an ultrathin alumina membrane (UTAM) itself is utilized as a substrate for the selective growth of the ordered arrays. A thin film of gold was thermally evaporated on the UTAM followed by the growth of highly regular ZnO nanowires using chemical vapor deposition (CVD). Alternatively, highly ordered ZnO nanopores arrays were also grown by CVD on the bare UTAM. Additionally, piezoelectric currents were generated from the ZnO nanowires during the conductive atomic force microscopy probe tip scan across the array.

Synthesis and field emission properties of different ZnO nanostructure arrays

In this article, zinc oxide (ZnO) nanostructures of different shapes were fabricated on silicon substrate. Well-aligned and long ZnO nanowire (NW) arrays, as well as leaf-like ZnO nanostructures (which consist of modulated and single-phase structures), were fabricated by a chemical vapor deposition (CVD) method without the assistance of a catalyst. On the other hand, needle-like ZnO NW arrays were first fabricated with the CVD process followed by chemical etching of the NW arrays. The use of chemical etching provides a low-cost and convenient method of obtaining the needle-like arrays. In addition, the field emission properties of the different ZnO NW arrays were also investigated where some differences in the turn-on field and the field-enhancement factors were observed for the ZnO nanostructures of different lengths and shapes. It was experimentally observed that the leaf-like ZnO nanostructure is most suitable for field emission due to its lowest turn-on and threshold field as well as its high field-enhancement factor among the different synthesized nanostructures.

Ni/Au hybrid nanoparticle arrays as a highly efficient, cost-effective and stable SERS substrate†

A large-area cost-effective Ni/Au hybrid nanoparticle array is synthesized with a proposed versatile and simple process by depositing Au on the pre-prepared arrays of Ni particles with ultra-thin alumina membranes as shadow mask during the deposition. A highly efficient and stable surface enhancement Raman scattering (SERS) substrate could be obtained from utilizing the resulting regular pattern of Ni/Au NP arrays. As compared with the single Au NP arrays, a largely decreased Au evaporation thickness and much lesser Au is needed for achieving the same Raman enhancement factor for the Ni/Au NP arrays. Subsequent SERS spectra measurement of the crystal violet (CV) molecule detection indicate a good SERS-active sensitivity with a detection limit of 10−10 M concentration, a large Raman enhancement factor at 108 was obtained, excellent SERS signal reproducibility with a relative standard deviation (RSD) as low as 6–7% as well as a great long term stability at 10 months.