Woochul Yang

Large-Area Monolayer Hexagonal Boron Nitride on Pt Foil

Hexagonal boron nitride (h-BN) has recently been in the spotlight due to its numerous applications including its being an ideal substrate for two-dimensional electronics, a tunneling material for vertical tunneling devices, and a growth template for heterostructures. However, to obtain a large area of h-BN film while maintaining uniform thickness is still challenging and has not been realized. Here, we report the systematical study of h-BN growth on Pt foil by using low pressure chemical vapor deposition with a borazine source. The monolayer h-BN film was obtained over the whole Pt foil (2 × 5 cm(2)) under <100 mTorr, where the size is limited only by the Pt foil size. A borazine source was catalytically decomposed on the Pt surface, leading to the self-limiting growth of the monolayer without the associating precipitation, which is very similar to the growth of graphene on Cu. The orientation of the h-BN domains was largely confined by the Pt domain, which is confirmed by polarizing optical microscopy (POM) assisted by the nematic liquid crystal (LC) film. The total pressure and orientation of the Pt lattice plane are crucial parameters for thickness control. At high pressure (∼0.5 Torr), thick film was grown on Pt (111), and in contrast, thin film was grown on Pt (001). Our advances in monolayer h-BN growth will play an important role to further develop a high quality h-BN film that can be used for vertical tunneling, optoelectronic devices and growth templates for a variety of heterostructures.

Polarization-dependent electron affinity of LiNbO3 surfaces

Polar surfaces of a ferroelectric LiNbO3 crystal with periodically poled domains are explored using UV-photoelectron emission microscopy (PEEM). Compared with the positive domains (domains with positive surface polarization charges), a higher photoelectric yield is found from the negative domains (domains with negative surface polarization charges), indicating a lower photothreshold and a corresponding lower electron affinity. The photon-energy-dependent contrast in the PEEM images of the surfaces indicates that the photothreshold of the negative domains is ∼4.6eV while that of the positive domains is greater than ∼6.2eV. We propose that the threshold difference between the opposite domains can be attributed to a variation of the electron affinity due to opposite surface dipoles induced by surface adsorbates.

Full Surface Embedding of Gold Clusters on Silicon Nanowires for Efficient Capture and Photothermal Therapy of Circulating Tumor Cells

We report on rapid thermal chemical vapor deposition growth of silicon nanowires (Si NWs) that contain a high density of gold nanoclusters (Au NCs) with a uniform coverage over the entire length of the nanowire sidewalls. The Au NC-coated Si NWs with an antibody-coated surface obtain the unique capability to capture breast cancer cells at twice the highest efficiency currently achievable (~88% at 40 min cell incubation time) from a nanostructured substrate. We also found that irradiation of breast cancer cells captured on Au NC-coated Si NWs with a near-infrared light resulted in a high mortality rate of these cancer cells, raising a fine prospect for simultaneous capture and plasmonic photothermal therapy for circulating tumor cells.

Large capacitance-voltage hysteresis loops in SiO2 films containing Ge nanocrystals produced by ion implantation and annealing

This work was partially supported by the QuantumFunctional Semiconductor Research Center in Dongguk University and by the National Program for Tera Level Nano Devices through MOST. S.-H.C. acknowledges partial support from the National Research Program for the 0.1 Terabit Non-Volatile Memory Development sponsored by Korea Ministry of Science & Technology. R.G.E. additionally acknowledges the Australian Research Council for their partial financial support of this work.

A free electron laser-photoemission electron microscope system (FEL-PEEM)

We report first results from our effort to couple a high resolution photoemission electron microscope (PEEM) to the OK-4 ultraviolet free electron laser at Duke University (OK-4/Duke UV FEL). The OK-4/Duke UV FEL is a high intensity source of tunable monochromatic photons in the 3–10 eV energy range. This tunability is unique and allows us to operate near the photoemission threshold of any samples and thus maximize sample contrast while keeping chromatic berrations in the PEEM minimal. We have recorded first images from a variety of samples using spontaneous radiation from the OK-4/ Duke UV FEL in the photon energy range of 4.0–6.5 eV. Due to different photothreshold emission from different sample areas, emission from these areas could be turned on (or off) selectively. We have also observed relative intensity reversal with changes in photon energy which are interpreted as density-of-state contrast. Usable image quality has been achieved, even though the output power of the FEL in spontaneous emission mode was several orders of magnitude lower than the anticipated full laser power. The PEEM has achieved a spatial resolution of 12 nm.

Applications of Free‐Electron Lasers in the Biological and Material Sciences

Abstract Free-Electron Lasers (FELs) collectively operate from the terahertz through the ultraviolet range and via intracavity Compton backscattering into the X-ray and gamma-ray regimes. FELs are continuously tunable and can provide optical powers, pulse structures and polarizations that are not matched by conventional lasers. Representative research in the biological and biomedical sciences and condensed matter and material research are described to illustrate the breadth and impact of FEL applications. These include terahertz dynamics in materials far from equilibrium, infrared nonlinear vibrational spectroscopy to investigate dynamical processes in condensed-phase systems, infrared resonant-enhanced multiphoton ionization for gas-phase spectroscopy and spectrometry, infrared matrix-assisted laser-desorption–ionization and infrared matrix-assisted pulsed laser evaporation for analysis and processing of organic materials, human neurosurgery and ophthalmic surgery using a medical infrared FEL and ultraviolet photoemission electron microscopy for nanoscale characterization of materials and nanoscale phenomena. The ongoing development of ultraviolet and X-ray FELs are discussed in terms of future opportunities for applications research.

Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures

Scanning Kelvin probe microscopy (SKPM) and electrostatic force microscopy (EFM) have been employed to measure the surface potentials and the surface charge densities of the Ga- and the N-face of a GaN lateral polarity heterostructure (LPH). The surface was subjected to an HCl surface treatment to address the role of adsorbed charge on polarization screening. It has been found that while the Ga-face surface appears to be unaffected by the surface treatment, the N-face surface exhibited an increase in adsorbed screening charge density (1.6±0.5×1010cm−2), and a reduction of 0.3±0.1V in the surface potential difference between the N- and Ga-face surfaces.

Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures

An intentionally grown GaN film with laterally patterned Ga- and N-face polarities is studied using in situ UV-photoelectron emission microscopy (PEEM). Before chemical vapor cleaning of the surface, the emission contrast between the Ga- and N-face polarities regions was not significant. However, after cleaning the emission contrast between the different polarity regions was enhanced such that the N-face regions exhibited increased emission over the Ga-face regions. The results indicate that the emission threshold of the N-face region is lower than that of the Ga face. Moreover, bright emission was detected from regions around the inversion domain boundaries of the lateral polarity heterostructure. The PEEM polarity contrast and intense emission from the inversion domain boundary regions are discussed in terms of the built-in lateral field and the surface band bending induced by the polarization bound surface charges.

Micro-Raman study of electronic properties of inversion domains in GaN-based lateral polarity heterostructures

The electronic properties of inversion domains in a GaN-based lateral polarity heterostructure were investigated using micro-Raman spectroscopy. The piezoelectric polarization of each domain was calculated from strain determined via Raman scattering. The free carrier concentration and electron mobility were deduced from the longitudinal optical phonon–plasmon coupled mode. The electron concentration in the N-face domain was slightly higher than that in the Ga-face domain. It appears that during growth, a larger number of donor impurities may have been incorporated into the N-face domain than into the Ga-face domain.

Correlation of morphology and electrical properties of nanoscale TiSi2 epitaxial islands on Si (001)

Abstract The morphology and electrical properties of nanoscale epitaxial islands of TiSi 2 are explored. The TiSi 2 islands are prepared by ultra-high vacuum (UHV) deposition of ultra-thin Ti (0.3–2 nm) on both smooth and roughened Si (001) substrates. The roughened substrates are prepared by etching with atomic H produced in a plasma. The island formation is initiated by annealing to 800–1000°C. The morphologies of the substrate before and after island formation are examined by atomic force microscopy (AFM). In particular, the influence of surface-roughness on both the formation of islands and the size distribution of islands is investigated. Islands with a lateral dimension of ~35 nm and a vertical dimension of ~2.5 nm on a roughened substrate (RMS=12 nm) were observed, with a uniform distribution of 120 nm spacing between the islands. It was found that for similar processing conditions the size distribution of islands formed on a rough surface was smaller than islands formed on smooth surfaces. The results are discussed in terms of surface energy, diffusion and the strain field around the islands. The island structures can affect the electrical characteristics of the interface and the Schottky barrier was obtained from diodes formed with a Pt layer deposited over the islanded interface. The Schottky barrier was lowest for interfaces with the smaller TiSi 2 islands.