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Mechanical exfoliation methods of two-dimensional materials have been an essential process for advanced devices and fundamental sciences. However, the exfoliation method usually generates various thick flakes, and a bunch of thick bulk flakes usually covers an entire substrate. Here, we developed a method to selectively isolate mono- to quadlayers of transition metal dichalcogenides (TMDCs) by sonication in organic solvents. The analysis reveals the importance of low interface energies between solvents and TMDCs, leading to the effective removal of bulk flakes under sonication. Importantly, a monolayer adjacent to bulk flakes shows cleavage at the interface, and the monolayer can be selectively isolated on the substrate. This approach can extend to preparing a monolayer device with crowded 17 electrode fingers surrounding the monolayer and for the measurement of electrostatic device performance.

ScAlMgO4(SAM) has attracted attention as a substrate for nitride semiconductor crystal growth owing to its small a-axis lattice mismatch with GaN and InGaN. In this study, we investigated GaN growth on a SAM substrate via radio-frequency plasma-excited molecular beam epitaxy. By optimizing the growth conditions, GaN with the following epitaxial orientation relations (0001)GaN//(0001)SAMand [11–20]GaN//[11–20]SAMwas successfully grown directly on the SAM substrate. The atomically flat and abrupt interface of GaN directly grown on the SAM substrate was observed via high-resolution transmission electron microscopy, and uniform GaN growth on a two-inch SAM substrate was also demonstrated.

We report distinctive nonlinear behavior of photoluminescence (PL) intensities from localized exciton states embedded in single-walled carbon nanotubes (SWNTs) at room temperature. We found that PL from the local states exhibits strong nonlinear behavior with increasing continuous-wave excitation power density, whereas free exciton PL shows only weak sublinear behavior. The strong nonlinear behavior was observed regardless of the origin of the local states and found to be nearly independent of the local state density. These results indicate that the strong PL nonlinearity arises from a universal mechanism to SWNTs with sparse local states. The significant nonlinear PL is attributed to rapid ground-state depletion of the local states caused by an efficient accumulation of photogenerated free excitons into the sparse local states through one-dimensional diffusional migration of excitons along the nanotube axis; this mechanism is verified by Monte Carlo simulations of exciton diffusion dynamics.

We studied the dispersion-process effects on the photoluminescence (PL) properties of single-walled carbon nanotubes (SWNTs) dispersed using poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO) (PFO-SWNTs). The difference of the synthesis method (CoMoCAT, HiPco, and alcohol chemical vapor deposition) is insignificant for the PL quantum yields (QYs) of PFO-SWNTs. SWNTs processed using a moderate bath-type sonicator showed the maximum PL QY (∼2%) and the QY decreased drastically due to ultrasonic dispersion using a vigorous tip-type sonicator. The very low density of exciton quenching sites (<2 μm–1) introduced by the vigorous sonication was deduced based on the exciton diffusion model, indicating that the nanotube length itself almost limits the PL QYs in PFO-SWNTs.

We studied the thermal conductivity of a van der Waals hetero-bilayer composed of 1L-MoS2and 1L-MoSe2monolayers. We determined the laser heating efficiency of the air-suspended and the h-BN-supported hetero-bilayer to be 1.2 and 0.77 K ?W?1, respectively, through Raman spectroscopy. It was evaluated that the thermal conductivity of the hetero-bilayer was 115 (±25) W mK?1using this result and numerical analysis using a heat equation. The interfacial thermal conductance between the hetero-bilayer and h-BN was also determined to be 0.9 (±0.3) MW m?2K?1.

We demonstrated that repetition of gallium deposition and nitrogen-rich supply is a superior approach to fabricate gallium nitride (GaN) thin films on graphene by electron-cyclotron-resonance plasma-excited molecular beam epitaxy. The gallium layer, which covered the surface at the initial growth stage, reduced nitrogen-plasma-induced damage to the graphene. It thereby might suppress the nucleation of misoriented crystals and enabled the growth of c-axis-oriented GaN films. This approach was more effective on graphene transferred onto a GaN template, which suggests the possibility of remote-homoepitaxy reflecting the electric potential of the GaN template unscreened by graphene.

In this study, the applicability of in situ surface modification by radical beam irradiation method to reduce threading dislocation density in InN was investigated. The dislocation behavior, crystallographic quality, electrical properties, and photoluminescence properties of InN grown with this method were studied. Transmission electron microscopy observation revealed that dislocation density in the InN layer reduced by a factor of 3 in certain regions and showed good agreement with the results from photoluminescence spectroscopy. If this technology is established, high-quality InN with low threading dislocation density can be achieved with a very simple and repeatable growth process.

We studied the effective exciton radiative lifetimes of monolayer two-dimensional transition-metal dichalcogenides MX2(M= Mo, W; X= S, Se). The photoluminescence (PL) quantum yield and PL decay time were measured for monolayer MoS2, MoSe2, WS2, and WSe2at room temperature. Effective exciton radiative lifetimes of >10 ns were determined from the PL quantum yield of 10?2to 10?3and the PL decay time of several hundred picoseconds. The results are explained on the basis of the long radiative lifetime of bright excitons at a low temperature limit, a finite exciton coherence area of several square nanometers, and the population of dark exciton states.

We demonstrate a simple modulation technique of nonlinear optical properties in monolayer (1L) MoS2via chemical doping. The strong nonlinear behavior of the exciton photoluminescence (PL) intensity is observed with increasing excitation power density for low-electron-density 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-doped 1L-MoS2, whereas the exciton PL intensity of as-prepared, heavily electron-doped 1L-MoS2exhibits weak sublinear behavior. These results are attributable to an enhanced exciton-exciton annihilation rate for the excitons in F4TCNQ-doped 1L-MoS2as the dominant decay pathway under strong optical excitation conditions.

We demonstrate microscopic observations of recently discovered efficient upconversion (anti-Stokes) photoluminescence (UCPL) [Nat. Commun. 6, 8920 (2015)] of single-walled carbon nanotubes (SWNTs). UCPL images and spectra of the identical isolated SWNTs were obtained. Unlike previous observations on the SWNT ensemble, some individual SWNTs exhibit UCPL spectra coincident with Stokes PL spectra. The excitation polarization dependence of UCPL intensity indicates that the intermediate states in the upconversion process are extrinsic. These observations are likely to facilitate a comprehensive understanding of the UCPL mechanism towards the realization of highly efficient UCPL of SWNTs.

We studied the enhancement mechanism of the photovoltaic conversion efficiency of single-walled carbon nanotube (SWNT)/Si heterojunction solar cells by nitric acid (HNO3) doping in the dried condition. The conversion efficiency enhanced by HNO3doping strongly depends on the thickness of the SWNT network film. From the analysis of thickness-dependent current-voltage characteristics, we found that the enhancement resulted from not only a decrease in the series resistance of the SWNT network film, but also an increase in the p-n junction density due to an increase in the carrier density of SWNTs.

We studied the enhancement mechanism of the photovoltaic conversion efficiency of single-walled carbon nanotube (SWNT)/Si heterojunction solar cells by nitric acid (HNO3) doping in the dried condition. The conversion efficiency enhanced by HNO3 doping strongly depends on the thickness of the SWNT network film. From the analysis of thickness-dependent current-voltage characteristics, we found that the enhancement resulted from not only a decrease in the series resistance of the SWNT network film, but also an increase in the p-n junction density due to an increase in the carrier density of SWNTs.

We studied the photovoltaic properties of single-walled carbon nanotube/Si (SWNT/Si) heterojunction cells. We observed an optimal thickness of the SWNT network film that maximizes the photovoltaic conversion efficiency. The spectra of incident photon to charge carrier efficiency indicate that the production of carriers in the Si layer mainly contributes to the photovoltaic conversion. The experimental results and loss analysis based on the equivalent circuit model suggest that the fabrication of a high-density semiconducting SWNT network at the interface of Si is the key to improving the conversion efficiency of the SWNT/n-Si heterojunction solar cell.