Search

Simple mechanical exfoliation of layered materials is the most frequently employed method for producing high-quality monolayers of two-dimensional semiconducting materials. However, the mechanical exfoliation by human hands is a microscopically sophisticated process with a large number of microscopic parameters, which requires significant operator efforts and limits the reproducibility in achieving high-quality and large-area monolayer semiconducting materials. Herein, we have proposed a new strategy for the mechanical exfoliation by combining a developed robotic system and Bayesian optimization. We have demonstrated that it is possible to explore the optimized experimental conditions among a large number of parameter combinations for mechanical exfoliation in a relatively small number of experimental trials. Moreover, the entire mechanical exfoliation process from preparation to detection of monolayer semiconductors was performed by the developed autonomous robotic system. The optimized experimental condition was determined through only 30 trials of mechanical exfoliation experiments, representing 2.5% of all experimental parameter conditions. As a result, the critical parameters for the efficient fabrication of large-area monolayer WSe$_2$ were elucidated.

Transition metal dichalcogenides (TMDs) exhibit unique properties and potential applications when reduced to one-dimensional (1D) nanoribbons (NRs), owing to quantum confinement and high edge densities. However, effective growth methods for self-aligned TMD NRs are still lacking. We demonstrate a versatile approach for the lattice-guided growth of dense, aligned TMD NR arrays via chemical vapor deposition (CVD) on anisotropic sapphire substrates, without the need for tailored surface steps. This method enables the synthesis of NRs with widths below 10 nm, with their longitudinal axis parallel to the zigzag direction. The growth is influenced by both the substrate and CVD temperature, indicating the role of anisotropic precursor diffusion and substrate interaction. The 1D nature of the NRs was asserted by the observation of Coulomb blockade at low temperature. Pronounced catalytic activity was observed at the edges of the NRs, indicating their promise for efficient catalysis., Comment: 39 pages, 24 figures

Moir\'e potential acts as periodic quantum confinement for optically generated exciton, generating spatially ordered zero-dimensional quantum system. However, broad emission spectrum arising from inhomogeneity among moir\'e potential hinders the exploration of the intrinsic properties of moir\'e exciton. In this study, we have demonstrated a new method to realize the optical observation of quantum coherence and interference of a single moir\'e exciton in twisted semiconducting heterobilayer beyond the diffraction limit of light. A significant single and sharp photoluminescence peak from a single moir\'e exciton has been demonstrated after nano-fabrication. We present the longer duration of quantum coherence of a single moir\'e exciton, which reaches beyond 10 ps and the accelerated decoherence process with elevating temperature and excitation power density. Moreover, the quantum interference has revealed the coupling between moir\'e excitons in different moir\'e potential minima. The observed quantum coherence and interference of moir\'e exciton will facilitate potential application toward quantum technologies based on moir\'e quantum systems., Comment: 42 pages, 4 figures

The moire potential, induced by stacking two monolayer semiconductors with slightly different lattice mismatches, acts as periodic quantum confinement for optically generated excitons, resulting in spatially ordered zero-dimensional quantum systems. However, there are limitations to exploring intrinsic optical properties of moire excitons due to ensemble averaged and broadened emissions from many peaks caused by the inhomogeneity of the moire potential. In this study, we proposed a microfabrication technique based on focused Ga+ ion beams, which enables us to control the number of peaks originating from the moire potential and thus explore unknown moire optical characteristics of WSe2/MoSe2 heterobilayers. By taking advantage of this approach, we reveal emissions from a single moire exciton and charged moire exciton (trion) under electrostatic doping conditions. We show the momentum dark moire trion state above the bright trion state with a splitting energy of approximately 4 meV and clarify that the dynamics are determined by the initial trion population in the bright state. Furthermore, the degree of negative circularly polarized emissions and their valley dynamics of moire trions are dominated by a very long valley relaxation process lasting ~700 ns. Our findings on microfabricated heterobilayers could be viewed as an extension of our groundbreaking efforts in the field of quantum optics application using moire superlattices., Comment: 41 pages, 4 figures

Moir\'e patterns with angular mismatch in van der Waals heterostructures composed of atomically thin semiconducting materials are a fascinating platform to engineer the optically generated excitonic properties towards novel quantum phenomena. The moir\'e pattern as a periodic trap potential can give rise to spatially ordered zero-dimensional (0D) exciton ensembles, which offers the possibility for dense coherent quantum emitters and quantum simulation of many-body physics. The intriguing moir\'e exciton properties are affected by their dynamics and exciton-phonon interaction. However, the moir\'e exciton dynamics and the interaction between the moir\'e exciton and phonon are still elusive. Here, we report the moir\'e exciton and phonon interaction in a twisted WSe$_2$/MoSe$_2$ heterobilayer based on near-resonant photoluminescence excitation spectroscopy. We observed the selective excitation of the ground state of the moir\'e exciton at phonon resonance. The otherwise negligible small absorption below the continuum state is a hallmark of the density of states of a 0D-like system. In addition, the excitation power dependence of the PL spectra reveals the dynamics of moir\'e exciton ensembles between different potential minima with discrete energy levels via the resonant phonon scattering process. The results presented here of the moir\'e exciton dynamics under suppressed phonon interaction could pave a new way for the exploration of novel quantum phenomena of the moir\'e exciton towards potential applications in quantum optics.

We report the first experimental observation of strong coupling effect in one-dimensional moir\'e crystals. We study one-dimensional double-wall carbon nanotubes (DWCNTs) in which van der Waals-coupled two single nanotubes form one-dimensional moir\'e superlattice. We experimentally combine Rayleigh scattering spectroscopy and electron beam diffraction on the same individual DWCNTs to probe the optical transitions of structure-identified DWCNTs in the visible spectral range. Among more than 30 structure-identified DWCNTs examined, we experimentally observed and identified a drastic change of optical transition spectrum in DWCNT with chirality (12,11)@(17,16). The origin of the marked change is attributed to the strong intertube coupling effect in a moir\'e superlattice formed by two nearly-armchair nanotubes. Our numerical simulation is consistent to these experimental findings.

Two-dimensional (2D) layered materials, transition metal dichalcogenides and black phosphorus, have attracted much interest from the viewpoints of fundamental physics and device applications. The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier, owing to their remarkable optical properties and prospects for new devices. Here, we report the anisotropic optical properties of layered 2D monochalcogenide of germanium sulfide (GeS). Three Raman scattering peaks corresponding to the B3g, A1g, and A2g modes with strong polarization dependence are demonstrated in the GeS flakes, which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at around 1.66 eV that originates from the direct optical transition in GeS at room temperature. Moreover, determination of the polarization dependent characteristics of the PL and absorption reveals an anisotropic optical transition near the band edge of GeS, which is also supported by the density functional theory calculations. This anisotropic layered GeS presents the opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties.

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe2/WS2 heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is F\"orster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering., Comment: Main text: 14 pages, 5 figures, Supplementary Information: 18 pages, 12 figures, 1 table, Nano Letters (2016)

Atomically thin layered materials such as graphene and transition-metal dichalcogenides exhibit great potential as active materials in optoelectronic devices because of their high carrier-transporting properties and strong light-matter interactions. Here, we demonstrated that the photovoltaic performances of graphene/Si Schottky junction solar cells were significantly improved by inserting a chemical vapor deposition (CVD)-grown, large MoS2 thin-film layer. This layer functions as an effective passivation and electron-blocking/hole-transporting layer. We also demonstrated that the photovoltaic properties are enhanced with increasing number of graphene layers and decreasing thickness of the MoS2 layer. A high photovoltaic conversion efficiency of 11.1% was achieved with the optimized trilayer-graphene/MoS2/n-Si solar cell., Comment: Main text: 24 pages, 5 figures; Supporting Information: 8 pages, 5 figures, 1 table

We studied multi-exciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density, and long-distance exciton diffusion reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient exciton-exciton annihilation process determined the unusual multi-exciton dynamics in atomically thin layered transition metal dichalcogenides.

Two-dimensional (2D) crystals of semiconducting transition metal dichalcogenides (TMD) absorb a large fraction of incident photons in the visible frequencies despite being atomically thin. It has been suggested that the strong absorption is due to the parallel band or "band nesting" effect and corresponding divergence in the joint density of states. Here, we show using photoluminescence excitation spectroscopy that the band nesting in mono- and bilayer MX$_2$ (M = Mo, W and X = S, Se) results in excitation-dependent characteristic relaxation pathways of the photoexcited carriers. Our experimental and simulation results reveal that photoexcited electron-hole pairs in the nesting region spontaneously separate in the $k$-space, relaxing towards immediate band extrema with opposite momentum. These effects imply that the loss of photocarriers due to direct exciton recombination is temporarily suppressed for excitation in resonance with band nesting. Our findings highlight the potential for efficient hot carrier collection using these materials as the absorbers in optoelectronic devices., Comment: Main text: 19 pages, 5 figures; Supplementary Information: 12 pages, 4 figures, 1 table

We studied the excitation density dependence of photoluminescence (PL) spectra of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes. We found that the PL intensity of trions exhibited a strong nonlinear saturation behavior as the excitation density increased, whereas that of excitons exhibited a weak sublinear behavior. The strong PL saturation of trions is attributed to depletion of doped holes that are captured by excitons in the formation processes. Moreover, the effective radiative lifetime of a trion was evaluated to be approximately 20 ns., Comment: 19 pages, 4 figures in main text and 10 pages, 4 figures in supplemental material

We studied the photoluminescence (PL) properties of graphene quantum dots (GQDs) separated by size-exclusion high performance liquid chromatography. The size separation of GQDs was confirmed by transmission electron microcopy images. PL excitation maps of chromatographically-separated GQDs show four distinct emission peaks at 330, 440, 520, and 600 nm, respectively. The dominant luminescence features of the separated GQDs show discrete change depending on the overall size of GQDs, indicating that PL variation occurs because of differences in density, shape, and size of sp2 fragments available in the GQDs. On the basis of the experimental results of PL peak wavelength and pH dependence, the emission is attributed to quasi-molecular PL from the fragments composed of a few aromatic rings with oxygen containing functional groups.

The photovoltaic properties of carbon nanotube/Si heterojunction solar cells were investigated using network films of high quality single-walled carbon nanotubes (SWNTs) grown by atmospheric-pressure floating-catalyst chemical vapor deposition. Because of the optimization of the device window size and the utilization of SWNT thin films with both low resistivity and high transparency, a high photovoltaic conversion efficiency of greater than 12% was achieved for SWNTs/Si heterojunction solar cells without any post processing, such as carrier doping treatment. In addition, the high stability and reproducibility of the photovoltaic performance of these devices in air was demonstrated.

We demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity, but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier density in 1L-MoS2. Achievement of the extraction and injection of carriers in 1L-MoS2 by this solution-based chemical doping method enables convenient control of optical and electrical properties of atomically thin MoS2.

We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong electron-hole exchange interaction in carbon nanotubes.

We studied the nonlinear time-resolved luminescence signals due to multiexciton recombination processes in single-walled carbon nanotubes (SWNTs) using femtosecond excitation correlation (FEC) spectroscopy. From theoretical analysis of the FEC signals, we found that the FEC signals in the long time range are dominated by the single exciton decay in SWNTs, where the exciton-exciton annihilation process is efficient. Our results provide a simple method to clarify the single exciton decay dynamics in low-dimensional materials., Comment: 21 pages, 5 figures; typos added