Kyoung-Duck Park

Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging of Monolayer WSe2 with Local Strain Control

Many classes of two-dimensional (2D) materials have emerged as potential platforms for novel electronic and optical devices. However, their physical properties are strongly influenced by nanoscale heterogeneities in the form of edges, twin boundaries, and nucleation sites. Using combined tip-enhanced Raman scattering and photoluminescence (PL) nanospectroscopy and nanoimaging, we study the associated effects on the excitonic properties in monolayer WSe2 grown by physical vapor deposition. With ∼15 nm spatial resolution, we resolve nanoscale correlations of PL spectral intensity and shifts with crystal edges and internal twin boundaries associated with the expected exciton diffusion length. Through an active atomic force tip interaction we can control the crystal strain on the nanoscale and tune the local bandgap in reversible (up to 24 meV shift) and irreversible (up to 48 meV shift) fashion. This allows us to distinguish the effect of strain from the dominant influence of defects on the PL modification at the different structural heterogeneities. Hybrid nano-optical spectroscopy and imaging with nanomechanical strain control thus enables the systematic study of the coupling of structural and mechanical degrees of freedom to the nanoscale electronic and optical properties in layered 2D materials.

Variable-Temperature Tip-Enhanced Raman Spectroscopy of Single-Molecule Fluctuations and Dynamics

Structure, dynamics, and coupling involving single-molecules determine function in catalytic, electronic or biological systems. While vibrational spectroscopy provides insight into molecular structure, rapid fluctuations blur the molecular trajectory even in single-molecule spectroscopy, analogous to spatial averaging in measuring large ensembles. To gain insight into intramolecular coupling, substrate coupling, and dynamic processes, we use tip-enhanced Raman spectroscopy (TERS) at variable and cryogenic temperatures, to slow and control the motion of a single molecule. We resolve intrinsic line widths of individual normal modes, allowing detailed and quantitative investigation of the vibrational modes. From temperature dependent line narrowing and splitting, we quantify ultrafast vibrational dephasing, intramolecular coupling, and conformational heterogeneity. Through statistical correlation analysis of fluctuations of individual modes, we observe rotational motion and spectral fluctuations of the molecule. This work demonstrates single-molecule vibrational spectroscopy beyond chemical identification, opening the possibility for a complete picture of molecular motion ranging from femtoseconds to minutes.

Mechanical and oxidation properties of Ti–xFe–ySi alloys

Abstract New Ti-based alloys having reasonable mechanical properties and good oxidation resistance were designed by alloying with low cost elements of iron and silicon. From tensile tests at room temperature and 400 °C, Ti–4 wt.% Fe–(0.5–2)wt.% Si alloys were found to have the optimum combination of strength and ductility, which are comparable to the Ti–6Al–4V alloy. The precipitation of fine Ti-silicides and the reduction in α/β colony sizes are primarily responsible for the obtained mechanical behavior. The air oxidation resistance of Ti–4 wt.% Fe–(0.5–2)wt.% Si alloys between 700 and 1000 °C is far superior to that of the Ti–6Al–4V alloy, and even better than that of TiAl intermetallics. The formation of amorphous silica within the TiO2-rich oxide layers increased the oxidation resistance significantly.

Probing Bilayer Grain Boundaries in Large-Area Graphene with Tip-Enhanced Raman Spectroscopy.

The bilayer grain boundaries (GBs) in chemical-vapor-deposition-grown large-area graphene are identified using multispectral tip-enhanced Raman imaging with 18 nm spatial resolution. The misorientation angle of the bilayer GBs is determined from a quantitative analysis of the phonon-scattering properties associated with the modified electronic structure.

Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect

Excitons, Coulomb-bound electron–hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons (XD) show anti-parallel spin configuration with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of XD states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce, switch and programmably modulate the XD emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe2 on a gold substrate, we demonstrate ~6 × 105-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with a large Purcell factor of ≥2 × 103 of the tip–sample nano-cavity. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.Tip-enhanced photoluminescence spectroscopy allows for modulation and radiative control of dark exciton emission at room temperature.

Operation of a wet near-field scanning optical microscope in stable zones by minimizing the resonance change of tuning forks

A resonant shift and a decrease of resonance quality of a tuning fork attached to a conventional fiber optic probe in the vicinity of liquid is monitored systematically while varying the protrusion length and immersion depth of the probe. Stable zones where the resonance modification as a function of immersion depth is minimized are observed. A wet near-field scanning optical microscope (wet-NSOM) is operated for a sample within water by using such a stable zone.

Scanning absorption nanoscopy with supercontinuum light sources based on photonic crystal fiber

We have experimentally demonstrated a scanning absorption nanoscopy system combining a near-field scanning optical microscope with an absorption spectroscope using supercontinuum radiation generated by coupling a mode-locked Ti:sapphire pulse laser to a nonlinear photonic crystal fiber as a light source. For the performance test of the system, the absorption spectrum and near-field absorption image of Rhodamine 6G were observed. As this system allows us to investigate the absorption properties and distribution of materials with high spatial resolution, it is expected to be effectively applied in various research areas.

Sensitivity maximized near-field scanning optical microscope with dithering sample stage

We developed a new scheme for a higher sensitivity near-field scanning optical microscope (NSOM) by using a dithering sample stage rather than a dithering probe for the constant gap control between probe and sample. In a conventional NSOM, which use tip dithering feedback mechanism, the Q factor drastically decreases from 7783 to 1000 (13%) or even to 100 (1%) because harmonic oscillating characteristic is deteriorated owing to the large change of stiffness and mass of one prong of tuning fork when a probe is attached to it. In our proposed scheme, on the other hand, we use sample dithering feedback mechanism, where the probe is not attached to the tuning fork and the sample is loaded directly onto the surface of dithering tuning fork. Thus, the Q factor does not decrease significantly, from only 7783 to 7480 (96%), because the loaded sample hardly changes the stiffness and mass of tuning fork. Accordingly, gap control between the immobile fiber probe and the dithering sample is performed precisely by detecting the shear force with high sensitivity. Consequently, the extremely high Q factor enables clear observation of graphene sheets with sub-nanometer vertical resolution, which is not possible with a conventional NSOM setup.

Polarization Control with Plasmonic Antenna Tips: A Universal Approach to Optical Nanocrystallography and Vector-Field Imaging

Controlling the propagation and polarization vectors in linear and nonlinear optical spectroscopy enables us to probe the anisotropy of optical responses providing structural symmetry selective contrast in optical imaging. Here, we present a novel tilted antenna-tip approach to control the optical vector-field by breaking the axial symmetry of the nanoprobe in tip-enhanced near-field microscopy. This gives rise to a localized plasmonic antenna effect with significantly enhanced optical field vectors with control of both in-plane and out-of-plane components. We use the resulting vector-field specificity in the symmetry selective nonlinear optical response of second-harmonic generation (SHG) for a generalized approach to optical nanocrystallography and imaging. In tip-enhanced SHG imaging of monolayer MoS2 films and single-crystalline ferroelectric YMnO3, we reveal nanocrystallographic details of domain boundaries and domain topology with enhanced sensitivity and nanoscale spatial resolution. The approach is applicable to any anisotropic linear and nonlinear optical response and enables the optical nanocrystallographic imaging of molecular or quantum materials.

Ultrafast Anisotropic Optical Response and Coherent Acoustic Phonon Generation in Polycrystalline BaTiO3-BiFeO3

Abstract We optically study the as-yet little explored multiferroic material, BaTiO3-BiFeO3 (BTO-BFO), that has demonstrated enhanced magnetic properties, a higher DC resistance in comparison to BFO, and improved magnetoelectric coupling. Our studies include: ultrafast time resolved differential reflection, optically induced birefringence, and second-harmonic nano-imaging of the ferroic order. We observe a strong sensitivity to pump/probe polarizations, photo-induced ferroelectric poling on a picosecond timescale, as well as the generation of photo-induced coherent acoustic phonons with a frequency of ∼11