波 高 - Academia.edu (original) (raw)
波 高
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Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor dep... more Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.
We present femtosecond transient absorption measurements of individual metallic single-wall carbo... more We present femtosecond transient absorption measurements of individual metallic single-wall carbon nanotubes (SWNTs) to elucidate environmental effects on their spectroscopy and dynamics. Isolated suspended SWNTs were located using atomic force microscopy, and Raman spectroscopy was employed to determine the chiral index of select nanotubes. Transient absorption spectra of the SWNTs were obtained by recording transient absorption images at different probe wavelengths. This unique experimental approach removes sample heterogeneity in ultrafast measurements of these complex materials and provides a direct means to unravel the role of the substrate. The results show a ∼40 meV red shift of the lowest exciton transition, which is attributed to dielectric screening effects by the substrate. Energy relaxation in individual metallic nanotubes was observed with decay constants of a few hundred fs and about 10 ps. We attributed the fast and slow decay components to carrier scattering by optical and acoustic phonons, respectively. SECTION:
Raman spectra of individual SWNTs were systematically investigated by comparing the behavior of 2... more Raman spectra of individual SWNTs were systematically investigated by comparing the behavior of 21 semiconducting and 32 metallic SWNTs. It is found that, compared to semiconducting SWNTs, the RBM of metallic ones is softened, which exhibits chirality dependence. There are significant differences in the line shape of the G-band between semiconducting and metallic SWNTs, which was attributed to electron-phonon coupling previously. However, we found that the differences cannot be explained only by the electron-phonon coupling via the Kohn anomaly mechanism. Curvature effect and other unknown reasons appear to also contribute to the dissimilarities. As for the D-band, the frequency of metallic SWNTs does not show a softening effect when compared with that of semiconducting SWNTs, which is not consistent with theoretical predictions.
Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynam... more Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si 3 N 4 substrate-supported monolayer and few-layer MoS 2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electronÀhole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirectÀdirect band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS 2 . Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS 2 along with controlling their dimensions.
Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor dep... more Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.
We present femtosecond transient absorption measurements of individual metallic single-wall carbo... more We present femtosecond transient absorption measurements of individual metallic single-wall carbon nanotubes (SWNTs) to elucidate environmental effects on their spectroscopy and dynamics. Isolated suspended SWNTs were located using atomic force microscopy, and Raman spectroscopy was employed to determine the chiral index of select nanotubes. Transient absorption spectra of the SWNTs were obtained by recording transient absorption images at different probe wavelengths. This unique experimental approach removes sample heterogeneity in ultrafast measurements of these complex materials and provides a direct means to unravel the role of the substrate. The results show a ∼40 meV red shift of the lowest exciton transition, which is attributed to dielectric screening effects by the substrate. Energy relaxation in individual metallic nanotubes was observed with decay constants of a few hundred fs and about 10 ps. We attributed the fast and slow decay components to carrier scattering by optical and acoustic phonons, respectively. SECTION:
Raman spectra of individual SWNTs were systematically investigated by comparing the behavior of 2... more Raman spectra of individual SWNTs were systematically investigated by comparing the behavior of 21 semiconducting and 32 metallic SWNTs. It is found that, compared to semiconducting SWNTs, the RBM of metallic ones is softened, which exhibits chirality dependence. There are significant differences in the line shape of the G-band between semiconducting and metallic SWNTs, which was attributed to electron-phonon coupling previously. However, we found that the differences cannot be explained only by the electron-phonon coupling via the Kohn anomaly mechanism. Curvature effect and other unknown reasons appear to also contribute to the dissimilarities. As for the D-band, the frequency of metallic SWNTs does not show a softening effect when compared with that of semiconducting SWNTs, which is not consistent with theoretical predictions.
Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynam... more Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si 3 N 4 substrate-supported monolayer and few-layer MoS 2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electronÀhole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirectÀdirect band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS 2 . Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS 2 along with controlling their dimensions.