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.

We have proposed and developed a scanning laser terahertz (THz) near-field imaging system using a 1.56μm femtosecond fiber laser for high spatial resolution and high-speed measurement. To obtain the two-dimensional (2D) THz images of samples, the laser pulses are scanned over a 2D THz emitter plate [DASC: 4’-dimenthylamino-N-methyl-4- stilbazolium p-chlorobenzenesulfonate] by a galvano meter. In this system, THz wave pulses locally generated at the laser irradiation spots transmit through the sample set on the emitter, and the amplitude of the transmitted THz wave pulse is detected by using a typical THz time-domain spectroscopy (THz-TDS) technique. Using this system, we have succeeded in obtaining THz transmission images of a triangle shaped metal sheet of millimeter-size and a human hair sample with a spatial resolution of sub-wavelength order up to ~27μm (~λ_THz/28) at an imaging speed of about 47 seconds/image for 512 x 512 pixels.

The structural dependence of magnetization in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin films deposited on ${\left(\mathrm{La}\mathrm{Al}{\mathrm{O}}_3\right)}_{0.3}{\left({\mathrm{Sr}}_2\mathrm{Al}\mathrm{Ta}{\mathrm{O}}_6\right)}_{0.7}$ (001) substrate has been investigated in the film thickness $\left(t\right)$ range of $40–500\mathrm{nm}$. High-resolution x-ray-diffraction studies reveal a thickness dependent structural transition from a fully strained tetragonal phase for films with $t⩽75\mathrm{nm}$ to a partially strained and/or relaxed rhombohedral phase in films thicker than $110\mathrm{nm}$, via a coexistence of fully and partially strained phases in the thickness of $75>t>110\mathrm{nm}$. The fully strained phase could also be partially induced in $110\text{−}\mathrm{nm}$ films by increasing the post-deposition ${\mathrm{O}}_2$ annealing pressure. The films $⩾110\mathrm{nm}$ were found to possess a thickness independent bulklike magnetic moment. A weak enhancement in magnetic moment (up to $\sim 0.1{\mu }_{\mathrm{B}}$) is observed in fully strained films $(t⩽75\mathrm{nm})$ and in excessive ${\mathrm{O}}_2$-annealed $110\text{−}\mathrm{nm}$ films having a fraction of strained phase. These results suggest that by a considerably weak magnetic moment in the pseudomorphic strained phase arises from the epitaxial strain induced canted antiferromagnetic structure. This study resolves a much debated issue of strain induced magnetic moment in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin films.

We present a nonlinear optical crystal (NLOC)-based terahertz (THz) microfluidic chip with a few arrays of split ring resonators (SRRs) for ultra-trace and quantitative measurements of liquid solutions. The proposed chip operates on the basis of near-field coupling between the SRRs and a local emission of point like THz source that is generated in the process of optical rectification in NLOCs on a sub-wavelength scale. The liquid solutions flowing inside the microchannel modify the resonance frequency and peak attenuation in the THz transmission spectra. In contrast to conventional bio-sensing with far/near-field THz waves, our technique can be expected to compactify the chip design as well as realize high sensitive near-field measurement of liquid solutions without any high-power optical/THz source, near-field probes, and prisms. Using this chip, we have succeeded in observing the 31.8 fmol of ion concentration in actual amount of 318 pl water solutions from the shift of the resonance frequency. The technique opens the door to microanalysis of biological samples with THz waves and accelerates development of THz lab-on-chip devices.