High-harmonic generation in solids is a unique tool to investigate the electron dynamics in strong light fields. The systematic study in monolayer materials is required to deepen the insight into the ...fundamental mechanism of high-harmonic generation. Here we demonstrated nonperturbative high harmonics up to 18th order in monolayer transition metal dichalcogenides. We found the enhancement in the even-order high harmonics which is attributed to the resonance to the band nesting energy. The symmetry analysis shows that the valley polarization and anisotropic band structure lead to polarization of the high-harmonic radiation. The calculation based on the three-step model in solids revealed that the electron-hole polarization driven to the band nesting region should contribute to the high harmonic radiation, where the electrons and holes generated at neighboring lattice sites are taken into account. Our findings open the way for attosecond science with monolayer materials having widely tunable electronic structures.
High‐resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H‐MoS2 nanosheets, ...MoS2, and WS2 heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS2 nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS2 and WS2 were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS2 nanosheet layers and unveiled the heterogeneous aging state of MoS2 nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.
Nanoscale electrochemical cell: Understanding the hydrogen evolution reaction (HER) mechanism of MoS2 nanosheets requires a comprehensive understanding of the quantitative electrochemical properties and the distribution of active sites in real space. Scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze HER catalytically active sites in 1H MoS2 nanosheets.
The past decades of materials science discoveries are the basis of our present society - from the foundation of semiconductor devices to the recent development of internet of things (IoT) ...technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science '2.5 dimensional (2.5D) materials,' to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of '2.5D materials' and review recent research developments based on this new scientific concept.
Atomic-layer transition metal dichalcogenides (TMDCs) have attracted appreciable interest due to their tunable band gap, spin-valley physics, and potential device applications. However, the quality ...of TMDC samples available still poses serious problems, such as inhomogeneous lattice strain, charge doping, and structural defects. Here, we report on the growth of high-quality, monolayer WS2 onto exfoliated graphite by high-temperature chemical vapor deposition (CVD). Monolayer-grown WS2 single crystals present a uniform, single excitonic photoluminescence peak with a Lorentzian profile and a very small full-width at half-maximum of 21 meV at room temperature and 8 meV at 79 K. Furthermore, in these samples, no additional peaks are observed for charged and/or bound excitons, even at low temperature. These optical responses are completely different from the results of previously reported TMDCs obtained by mechanical exfoliation and CVD. Our findings indicate that the combination of high-temperature CVD with a cleaved graphite surface is an ideal condition for the growth of high-quality TMDCs, and such samples will be essential for revealing intrinsic physical properties and for future applications.
Controlling the direction of exciton-energy flow in two-dimensional (2D) semiconductors is crucial for developing future high-speed optoelectronic devices using excitons as the information carriers. ...However, intrinsic exciton diffusion in conventional 2D semiconductors is omnidirectional, and efficient exciton-energy transport in a specific direction is difficult to achieve. Here we demonstrate directional exciton-energy transport across the interface in tungsten diselenide (WSe2)–molybdenum diselenide (MoSe2) lateral heterostructures. Unidirectional transport is spontaneously driven by the built-in asymmetry of the exciton-energy landscape with respect to the heterojunction interface. At excitation positions close to the interface, the exciton photoluminescence (PL) intensity was substantially decreased in the WSe2 region and enhanced in the MoSe2 region. In PL excitation spectroscopy, it was confirmed that the observed phenomenon arises from lateral exciton-energy transport from WSe2 to MoSe2. This directional exciton-energy flow in lateral 2D heterostructures can be exploited in future optoelectronic devices.
Low‐cost green manufacturing of single‐walled carbon nanotube films via precisely controlled inkjet printing is demonstrated. This type of transistor exceeds the performance of conventional organic ...transistors, both in mobility and the on/off ratio. The production of exclusively inkjet‐printed SWCNT transistors with printable ionic‐liquid gate dielectrics is also shown.
High-harmonic generation (HHG), which is the generation of light with multiple optical harmonics, is an unconventional nonlinear optical phenomenon beyond the perturbation regime. HHG, which was ...initially observed in gaseous media, has recently been demonstrated in solid-state materials. Determining how to control such extreme nonlinear optical phenomena is a challenging subject. Here, we demonstrate the control of HHG through tuning the electronic structure and carrier injection using single-walled carbon nanotubes (SWCNTs). We reveal systematic changes in the high-harmonic spectra of SWCNTs with a series of electronic structures ranging from a metal structure to a semiconductor structure. We demonstrate enhancement or reduction of harmonic generation by more than 1 order of magnitude by tuning the electron and hole injection into the semiconductor SWCNTs through electrolyte gating. These results open a path toward the control of HHG in the context of field-effect transistor devices.
Room‐temperature chiral light sources whose optical helicity can be electrically switched are one of the most important devices for future optical quantum information processing. The emerging valley ...degree of freedom in monolayer semiconductors allows generation of chiral luminescence via valley polarization. However, relevant valley‐polarized light‐emitting diodes (LEDs) have only been achieved at low temperatures (typically below 80 K). Here, a room‐temperature chiral LED with strained transition metal dichalcogenide monolayers is realized. Spatially resolved polarization spectroscopy reveals that strain effects are crucial to yielding robust valley‐polarized electroluminescence. The broken threefold rotational symmetry of strained monolayers induce inequivalent valley drifts at the K/K’ valleys, resulting in different amounts of spin recombination driven by electric fields. Based on this scenario, ideally strained conditions are designed for LEDs on flexible substrates, in which the helicity of room‐temperature valley‐polarized electroluminescence is electrically tuned. The results provide a new pathway for practical chiral light sources based on monolayer semiconductors.
A room‐temperature electrically tunable chiral light‐emitting diode is realized using strained monolayer transition metal dichalcogenides. The strain effects break the threefold rotational symmetry, which results in different amounts of spin recombination at the K/K’ valleys, to generate chiral light emission driven by electric fields. The results provide a new pathway for practical chiral light sources based on monolayer semiconductors.
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