Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Therefore, a microscopic and quantitative understanding of the mechanism and dynamics of molecular adsorption and desorption has to be achieved in order to advance device applications based on these materials. However, recent theoretical calculations have yielded contradictory results, particularly on the magnitude of the adsorption energy. Here, we have experimentally determined the adsorption energy of oxygen molecules on graphene and 2D tungsten disulfide using temperature-programmed terahertz (THz) emission microscopy (TPTEM). The temperature dependence of THz emission from InP surfaces covered with 2D materials reflects the change in oxygen concentration due to thermal desorption, which we used to estimate the adsorption energy of oxygen molecules on graphene (~0.15 eV) and tungsten disulphide (~0.24 eV). Furthermore, we used TPTEM to visualize relative changes in the spatial distribution of oxygen molecules on monolayer graphene during adsorption and desorption. Our results provide much insight into the mechanism of molecular adsorption on the surface of 2D materials, while introducing TPTEM as a novel and powerful tool for molecular surface science.The successful isolation of monolayer graphene in 2004 and its remarkable properties found subsequently have paved the way for a new research field of two-dimensional (2D) atomic layer materials [1][2][3][4] . Many other 2D materials have since been discovered with a wide range of characteristics, from metallic to semiconducting to insulating, opening up exciting new opportunities for the development of devices based on monolayers, bilayers, and heterostructures of 2D materials [5][6][7][8] . However, since these materials typically consist of one or a few atomic layers, their properties are extremely susceptible to perturbations from their environment. Exposure to gases, for example, has been shown to drastically affect their electrical and optical properties [9][10][11][12][13][14][15] , which means that in order to realize 2D-materials-based devices, it is crucial to understand and control the influence of gas adsorption and desorption dynamics on their properties.Of the possible gas adsorbates/contaminants, oxygen (O 2 ) is one of the most important because not only it significantly alters the properties through doping, it is also the second most abundant gas in the atmosphere and is therefore highly likely to affect the performance of devices in practical applications. Though theoretical simulations proved to be useful in understanding the interaction of O 2 molecules and/or O atoms with 2D materials, conflicting results for the adsorption energies were obtained due to the inability of the approximation functionals used to properly describe the dispersion forces [16][17][18][19][20][21][22][23] . Knowing the correct value of the adsorption ...

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

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We proposed a monolithic microcavity wavelength conversion device without a polarity-inverted structure. The device consists of a low birefringence paraelectric material and a dielectric material. A fundamental wave intensity is enhanced significantly in the microcavity with two distributed Bragg reflectors, and a second-harmonic wave is efficiently generated in a very short region close to a coherence length. As a first step of this study, we used GaN as a device material, and succeeded in the blue second harmonic generation with a wavelength of 428 nm.

Physically adsorbed gas molecules have dramatic effects on the inherent properties of 2D nanomaterials due to their atomic/molecular-level thinness. In this work, we applied laser THz emission spectroscopy to study the gas adsorption-desorption dynamics of tungsten disulfide (WS2) nanosheets prepared by liquid phase exfoliation technique. We find that in low vacuum conditions, NIR irradiation promotes desorption of O2 molecules from WS2 nanosheets. Upon exposure to atmospheric air, results suggest slight photo-oxidation caused by NIR irradiation. Strong photo-oxidation by UV illumination is also confirmed, and we also observed subsequent O2 desorption from WS2 nanosheets by NIR irradiation when the UV illumination is turned off. These results are discussed using the Langmuir adsorption isotherm, which explains the variation of gas adsorption with the gas pressure.