Chuang-Han Hsu

Multiphonon Raman scattering in GaN nanowires

UV Raman scattering studies show longitudinal optical (LO) mode up to fourth order in wurtzite GaN nanowire system. Frohlich interaction of electron with the long range electrostatic field of ionic bonded GaN gives rise to enhancement in LO phonon modes. Good crystalline quality, as indicated by the crystallographic as well as luminescence studies, is thought to be responsible for this significant observation. Calculated size dependence, incorporating size corrected dielectric constants, of electron-phonon interaction energy agrees well with measured values and also predict stronger interaction energy than that of the bulk for diameter below ∼3nm.

Surface optical Raman modes in InN nanostructures

Raman spectroscopic investigations are carried out on one-dimensional nanostructures of InN, such as nanowires and nanobelts synthesized by chemical vapor deposition. In addition to the optical phonons allowed by symmetry A1, E1, and E2 (high) modes, two additional Raman peaks are observed around 528 and 560u2002cm−1 for these nanostructures. Calculations for the frequencies of surface optical (SO) phonon modes in InN nanostructures yield values close to those of the new Raman modes. A possible reason for large intensities for SO modes in these nanostructures is also discussed.

Blueshift of yellow luminescence band in self-ion-implanted n-GaN nanowire

Optical photoluminescence studies are performed in self-ion (Ga+)-implanted nominally doped n-GaN nanowires. A 50 keV Ga+ focused ion beam in the fluence range of 1×1014–2×1016u2002ionsu200acm−2 is used for the irradiation process. A blueshift is observed for the yellow luminescence (YL) band with increasing fluence. Donor–acceptor pair model with emission involving shallow donor introduced by point-defect clusters related to nitrogen vacancies and probable deep acceptor created by gallium interstitial clusters is responsible for the shift. High-temperature annealing in nitrogen ambient restores the peak position of YL band by removing nitrogen vacancies.

Mechanism of nanoblister formation in Ga + self-ion implanted GaN nanowires

The formation of voids and bubbles during ion implantation is an important area of material research. Void and bubble formation can result in swelling and embrittlement of metallic or semiconducting materials, and increase catalytic effects in the nanopores of the bubble. Here, we report the observation of metallic nanoblister formation in GaN nanowires under self-ion implantation using a Ga+ focused ion beam. The mechanism of the blister formation was resolved using high-resolution transmission electron microscopy equipped with electron energy loss spectroscopy and plasmon imaging.

Hexagonal-to-Cubic Phase Transformation in GaN Nanowires by Ga+ Implantation

Hexagonal to cubic phase transformation is studied in focused ion beam assisted Ga+-implanted GaN nanowires. Optical photoluminescence and cathodoluminescence studies along with high-resolution transmission electron microscopic structural studies are performed to confirm the phase transformation. In one possibility, sufficient accumulation of Ga from the implanted source might have reduced the surface energy and simultaneously stabilized the cubic phase. Another potential reason may be that the fluctuations in the short-range order induced by enhanced dynamic annealing (defect annihilation) with the irradiation process stabilize the cubic phase and cause the phase transformation.

Spin-Dependent Molecule Symmetry at a Pentacene–Co Spinterface

Incorporating spin-polarized scanning tunneling microscopy (SP-STM) measurements and first-principles calculations, we resolve spin-polarized states and consequent features in a pentacene(PEN)-Co hybrid system. Symmetry reduction of PEN clarifies the PEN adsorption site and the Co stacking methods. Near the Fermi energy, the molecular symmetry is spin-dependently recovered and an inversion of spin-polarization in PEN with respect to Co is observed. The experimental findings and calculation results are interpreted by a pz-d hybridization model, in which spin-dependent bonding-antibonding splitting of molecular orbitals happens at metal-organic spinterfaces.

Electronic structures of group-III–nitride nanorods studied by x-ray absorption, x-ray emission, and Raman spectroscopy

Nitrogen (N) and metal (Al, Ga, and In) K-edge x-ray absorption near-edge structure (XANES), x-ray emission spectroscopy (XES), and Raman scattering measurements were performed to elucidate the electronic structures of group-III–nitride nanorods and thin films of AlN, GaN, and InN. XANES spectra show slight increase of the numbers of unoccupied N p states in GaN and AlN nanorods, which may be attributed to a slight increase of the degree of localization of conduction band states. The band gaps of AlN, GaN, and InN nanorods are determined by an overlay of XES and XANES spectra to be 6.2, 3.5, and 1.9eV, respectively, which are close to those of AlN and GaN bulk/films and InN polycrystals.

Formation and in situ dynamics of metallic nanoblisters in Ga + implanted GaN nanowires

The formation of voids and bubbles in the energetic ion implantation process is an important issue in material science research, involving swelling induced embrittlement of materials in nuclear reactors, catalytic activities in the nanopores of the bubble, etc. We report here the formation and in situ dynamics of metallic nanoblisters in GaN nanowires under self-ion implantation using a Ga + focused ion beam. High-resolution transmission electron microscopes equipped with electron energy loss spectroscopy and energy filtering are used to identify the constituents of the blister. In situ monitoring, with focused ion beam imaging, revealed the translation and rotation dynamics of the blisters.

In situ magnetization switching of magnetic probes applied to spin-polarized scanning tunneling microscopy

Soft magnetic tip was utilized to be the probe of spin-polarized scanning tunneling microscopy. It was demonstrated that the spin contrast can be reversed by in situ switching tip magnetization through varying tip-substrate distance for resolving perpendicular magnetic domain images. With this in situ magnetization direction switching of the soft magnetic tip, it is conceivable to separate magnetic from chemical and topographic contributions without applying external magnetic field. This provides an effective tool for the study of complex magnetic spin structures with various nonmagnetic impurities or compositions involved.

Generation of green frequency comb from chirped χ(2) nonlinear photonic crystals

Spectrally broad frequency comb generation over 510–555u2009nm range was reported on chirped quasi-phase-matching (QPM) χ(2) nonlinear photonic crystals of 12u2009mm length with periodicity stepwise increased from 5.9u2009μm to 7.1u2009μm. When pumped with nanosecond infrared (IR) frequency comb derived from a QPM optical parametric oscillator (OPO) and spanned over 1040u2009nm to 1090u2009nm wavelength range, the 520u2009nm to 545u2009nm up-converted green spectra were shown to consist of contributions from (a) second-harmonic generation among the signal or the idler modes, and (b) sum-frequency generation (SFG) from the neighboring pairs of the signal or the idler modes. These mechanisms led the up-converted green frequency comb to have the same mode spacing of 450u2009GHz as that in the IR-OPO pump comb. As the pump was further detuned from the aforementioned near-degeneracy point and moved toward the signal (1020–1040u2009nm) and the idler (1090–1110u2009nm) spectral range, the above QPM parametric processes were preserved in the chirped QPM de...