Dynamical symmetry of strongly light-driven electronic system in crystalline solids

The Floquet state, which is a periodically and intensely light driven quantum state in solids, has been attracting attention as a novel state that is coherently controllable on an ultrafast time scale. An important issue has been to demonstrate experimentally novel electronic properties in the Floquet state. One technique to demonstrate them is the light scattering spectroscopy, which offers an important clue to clarifying the symmetries and energy structures of the states through symmetry analysis of the polarization selection rules. Here, we determine circular and linear polarization selection rules of light scattering in a mid-infrared-driven Floquet system in monolayer MoS2 and provide a comprehensive understanding in terms of the "dynamical symmetry" of the Floquet state

The Atomic and Electronic structure of 0{\deg} and 60{\deg} grain boundaries in MoS2

We have investigated atomic and electronic structure of grain boundaries in monolayer MoS2, where relative angles between two different grains are 0 and 60 degree. The grain boundaries with specific relative angle have been formed with chemical vapor deposition growth on graphite and hexagonal boron nitride flakes; van der Waals interlayer interaction between MoS2 and the flakes restricts the relative angle. Through scanning tunneling microscopy and spectroscopy measurements, we have found that the perfectly stitched structure between two different grains of MoS2 was realized in the case of the 0 degree grain boundary. We also found that even with the perfectly stitched structure, valence band maximum and conduction band minimum shows significant blue shift, which probably arise from lattice strain at the boundary

Short Channel Field-Effect Transistors from Highly Enriched Semiconducting Carbon Nanotubes

ABSTRACT Semiconducting single-walled carbon nanotubes (s-SWNTs) with a purity of ~98% have been obtained by gel filtration of arc-discharge grown SWNTs with diameters in the range 1.2-1.6 nm. Multi-laser Raman spectroscopy confirmed the presence of less than 2% of metallic SWNTs (m-SWNTs) in the s-SWNT enriched sample. Measurement of ~50 individual tubes in Pd-contacted devices with channel length 200 nm showed on/off ratios of >10 4 , conductances of 1.38-5.8 μS, and mobilities in the range 40-150 cm 2 /(V·s). Short channel multi-tube devices with ~100 tubes showed lower on/off ratios due to residual m-SWNTs, although the on-current was greatly increased relative to the devices made from individual tubes. KEYWORDS Single-walled carbon nanotubes, separation, Raman spectroscopy, field-effect transistor One of the long standing hurdles encountered in the practical use of carbon nanotubes for electronics is the surplus of chiral varieties produced by most methods of synthesis. The realization of high performance logic circuits requires s-SWNT field-effect transistors (FETs) exceeding current silicon counterparts. The presence of metallic nanotubes results in significantly lower on/off ratios and should be completely eliminated. Much experimental work has been performed recently to enrich s-SWNTs, and long channel thin-film transistors (TFTs) using these sorted tubes have been studied. Less work has been done on short channel devices due to the more stringent s-SWNT purity requirements. In order to obtain single-walled carbon nanotube (SWNT) transistors with both high on-current and high on/off ratio One of these approaches, solution phase separation, Nano Res. 2012, 5(6): 388-39

Chemically Tuned p- and n-Type WSe2 Monolayers with High Carrier Mobility for Advanced Electronics

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post-silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom-thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p- and n-type semiconductors is essential for various device applications, such as complementary metal-oxide-semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe2 is demonstrated by chemical doping. Two different molecules, 4-nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe2 field-effect transistors (FETs) to p- and n-type, respectively. Moreover, the chemically doped WSe2 show increased effective carrier mobilities of 82 and 25 cm(2) V(-1)s(-1) for holes and electrons, respectively, which are much higher than those of the pristine WSe2. The doping effects are studied by photoluminescence, Raman, X-ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe2 FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (approximate to 0.17 nW). Furthermore, a p-n junction within single WSe2 grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe2 will contribute to the development of TMDC-based advanced electronics

Development of laser-combined scanning multiprobe spectroscopy and application to analysis of WSe2/MoSe2 in-plane heterostructure

By combining scanning multiprobe (MP) microscopy with optical methods such as light-modulated spectroscopy (LMS) and optical pump-probe (OPP) method, we have succeeded in developing a microscopy method for measuring electronic structures and photoinduced carrier dynamics in microscopic structures. We demonstrated its performance by analyzing the electronic structures in a monolayer island of a WSe2/MoSe2 in-plane heterostructure grown on a SiO2/Si substrate. By observing the field-effect transistor characteristics and photocurrent mapping over the heterostructure by LMS, we were able to visualize the band structure. Positional dependence of carrier dynamics was also successfully probed by OPP-MP spectroscopy