Atomic sheets or layers on surfaces have been significant playgrounds for low-dimensional physics and also promising elements for future technology. There are varieties of such two-dimensional crystal sheets that show rich functionalities. In this special issue, we present selected research topics on various types of atomic sheets.
We present a review on the electronic applications of compound materials nanosheets as semiconductor channel materials. Also, we report our results about the device fabrication and characterization of high-k/metal gate MoS2 metal-oxide-semiconductor field-effect transistors (MOSFETs). To investigate the scattering mechanism to be responsible for the mobility of MoS2 MOSFETs, the effective mobility has been estimated from the capacitance-voltage and current-voltage characteristics of top-gated MoS2 MOSFET with HfO2/TaN gate. Phonon-limited carrier transport probably limited the mobility of the MoS2 device exhibiting the better mobility. In contrast, the transport characteristics of the MoS2 device exhibiting the lower mobility was dominated by charged impurity scattering in MoS2.
The topological aspects in the physics of atomic layers, particularly the valley physics, are briefly reviewed. First, the Berry curvature of band structure is introduced as a basic concept for the k-space topology. The stability of Dirac cones and the valley splitting of Landau levels in gapped graphene are explained topologically. The valley Hall effect in gapped graphene is discussed as a remarkable topological transport phenomenon. The spin-valley correspondence is also mentioned in 2H-MoS2 monolayer with no inversion symmetry.
Electric double layer transistor (EDLT) has attracted attentions over the decade as a useful tool to induce and control fascinating many physical properties at two dimensional (2D) materials surface based on its giant capacitance. This paper highlights another aspect of EDLT: electrochemical reaction as a method to achieve ultrathin films. Precise tuning of reactivity of electrochemical etching between ionic liquid and solid surface makes it possible to peel off 2D materials layer-by-layer. This technique was applied to an iron-based superconductor FeSe in EDLT towards the observation of high temperature superconductivity in ultra-thin film form.
We report on structure determinations of graphene and silicene on metal substrates using total-reflection high-energy positron diffraction (TRHEPD) technique. The magnitude of buckling in graphene (silicene) and the spacing between graphene and substrate are crucial to elucidate the origin of the electronic property of graphene adsorbed on a substrate. In this study, these structure parameters for graphene/Co(0001), graphene/Cu(111), and silicene/Ag(111) were determined with the aid of surface sensitivity of the TRHEPD. The experimental verification of these parameters will promote a better understanding of the properties of graphene and silicene.
Graphene has attracted considerable attention due to its extraordinary carrier transport, optoelectronic, and plasmonic properties originated from its gapless and linear energy spectra enabling various functionalities with extremely high quantum efficiencies that could never be obtained in any existing materials. This paper reviews recent advances in graphene optoelectronics particularly focused on the physics and device functionalities in the terahertz (THz) electromagnetic spectral range. Optical response of graphene is characterized by its optical conductivity and nonequilibrium carrier energy relaxation dynamics, enabling amplification of THz radiation when it is optically or electrically pumped. Current-injection THz lasing has been realized very recently. Graphene plasmon polaritons can greatly enhance the THz light and graphene matter interaction, enabling giant enhancement in detector responsivity as well as amplifier/laser gain. Graphene-based van der Waals heterostructures could give more interesting and energy-efficient functionalities.
The role of promoting materials for the preferential oxidation (PROX) reaction of CO in H2 on Pt-catalysts was elucidated by using the Pt-catalyst supported on a CNT with Ni-MgO at its terminal end and the CNT-p with no Ni-MgO. The role of Ni-MgO is not the synergy effect on the Pt but set up a new catalytic oxidation reaction cycle of CO in the presence of H2O. The dynamic In-situ IR spectroscopy suggested the rate determining slow step of HCOO + OH → CO2 + H2O, where H2O molecule plays like a molecular catalyst in cooperation of Pt and promoting materials. The contribution of H2O is expressed by n(CO + 1/2O2) + H2O → n CO2 + H2O, where n is CO2 molecules by one H2O molecule staying on the surface. According to this new concept, the selectivity is given by n/(n+1), which takes 50%∼100% depending on the staying time of H2O on the catalyst. The mysterious high selectivity reported on the Pt-nano rods in SiO2-nano-tube and the curious selectivity on Au/CeO2 depending on the crystal shape of CeO2 are well rationalized by this new mechanism.