Heesuk Rho

Evolution of optical phonons in CdS nanowires, nanobelts, and nanosheets

We report Raman scattering from single and ensemble CdS nanowires, nanobelts, and nanosheets. The Raman spectra of nanobelts and nanosheets are notably different from those of nanowires, exhibiting a strong enhancement of the multiphonon response. Moreover, the first-order longitudinal optical (LO) phonon energy systematically increases with increasing lateral size from nanowires to nanobelts, and to nanosheets. These results suggest that the optical phonons in the CdS nanostructures are influenced by strain, crystallinity, and exciton-LO phonon coupling.

Spatially resolved photoluminescence mapping of single CdS nanosheets

We have utilized spatially resolved low temperature photoluminescence to probe the electronic states and structural symmetries of individual single crystalline CdS nanosheets. Spatially resolved photoluminescence imaging of a single CdS nanosheet reveals a distinctive spectral variation across the nanosheet. We observe A-like exciton states which emit most strongly near the outer edges of the 5μm wide nanosheet, while B-like exciton states emit most strongly along the center region of the 30μm long axis of the nanosheet.

Polarized photoluminescence and time-resolved photoluminescence from single CdS nanosheets

We have utilized polarized low temperature photoluminescence (PL) to probe the electronic states and structural symmetries of individual CdS nanosheets. High resolution transmission electron microscopy measurements indicate highly crystalline material with different nanosheets exhibiting significant variations of the direction of the c axis, which are consistent with polarization measurements of PL from single CdS nanosheets. The quality of the nanosheets is reflected in measurements of exciton lifetimes of ∼200ps, a value significantly longer than observed for CdS nanowires whose diameter is the same as the thickness of these nanosheets, but shorter than that observed in bulk crystals.

Spatially resolved Raman spectroscopy of defects, strains, and strain fluctuations in domain structures of monolayer graphene

We report spatially resolved Raman scattering results of polycrystalline monolayer graphene films to study the effects of defects, strains, and strain fluctuations on the electrical performance of graphene. Two-dimensional Raman images of the integrated intensities of the G and D peaks (IG and ID) were used to identify the graphene domain boundaries. The domain boundaries were also identified using Raman images of ID/IG and I2D/IG ratios and 2D spectral widths. Interestingly, the ID maps showed that the defects within individual domains significantly increased for the graphene with large domain size. The correlation analysis between the G and 2D peak energies showed that biaxial tensile strain was more developed in the graphene with large domain size than in the graphene with small domain size. Furthermore, spatial variations in the spectral widths of the 2D peaks over the graphene layer showed that strain fluctuations were more pronounced in the graphene with large domain size. It was observed that the mobility (sheet resistance) was decreased (increased) for the graphene with large domain size. The degradation of the electrical transport properties of the graphene with large domain size is mainly due to the defects, tensile strains, and local strain fluctuations within the individual domains.

Domain size engineering of CVD graphene and its influence on physical properties

An electrochemical polishing pre-treatment and atmospheric pressure annealing were performed on copper foils in order to obtain large graphene domain sizes by the chemical vapor deposition method. The sizes were confirmed through scanning electron microscopy, optical microscopy and Raman spectroscopy, among other additional characterizations. The larger domain size was expected to improve the quality of the graphene but, on the contrary, the sheet resistance of the samples was found to increase for the larger domain size. We conclude that although the domain size is larger, the sheet resistance of graphene is more affected by the quality of the graphene interdomain itself.

Evaluation of semiconducting/metallic ratio of single wall carbon nanotubes by resonant Raman scatterings-excel

We investigated single wall carbon nanotubes (SWCNTs) synthesized by the HiPCO method and further processed with nitronium hexafluoroantimonate (NO2SbF6 : NHFA) treatment using continuous resonant Raman scattering in the range of 570-900 nm. According to the population ratio calculation results from Raman scattering data, it is obvious that semiconducting SWCNTs with small diameter and metallic SWCNTs were selectively removed by NHFA.

Surface Enhanced Raman Scattering from Single CdS Nanowires and Nanosheets

We report Raman scattering results of single CdS nanowires and nanosheets. Surface enhanced Raman scattering from a single nano wire reveals a strong first-order longitudinal optical (1LO) phonon and a weak 2LO phonon. In contrast, Raman scattering from a single nanosheet shows a significantly enhanced 2LO phonon and several additional multiphonons.