Atomic layer deposition of Y2O3 on h-BN for a gate stack in graphene FETs.

The combination of h-BN and high-k dielectrics is required for a top gate insulator in miniaturized graphene field-effect transistors because of the low dielectric constant of h-BN. We investigated the deposition of Y(2)O(3) on h-BN using atomic layer deposition. The deposition of Y(2)O(3) on h-BN was confirmed without any buffer layer. An increase in the deposition temperature reduced the surface coverage. The deposition mechanism could be explained by the competition between the desorption and adsorption of the Y precursor on h-BN due to the polarization. Although a full surface coverage was difficult to achieve, the use of an oxidized metal seeding layer on h-BN resulted in a full surface coverage.

The crystal orientation relation and macroscopic surface roughness in hetero-epitaxial graphene grown on Cu/mica.

Clean, flat and orientation-identified graphene on a substrate is in high demand for graphene electronics. In this study, the hetero-epitaxial graphene growth on Cu(111)/mica(001) by chemical vapor deposition is investigated to check the applicability for top-gate insulator research on graphene, as well as graphene channel research, by transferring graphene on to SiO2/Si substrates. After adjusting the graphene growth conditions, the surface roughness of the graphene/Cu/mica substrate and the average smoothed areas are ∼0.34 nm and ∼100 µm(2), respectively. The orientation of graphene in the graphene/Cu/mica substrate can be identified by the hexagonal void morphology of Cu. Moreover, we demonstrate a relatively high mobility of ∼4500 cm(2) V(-1) s(-1) in graphene transferred on the SiO2/Si substrate. These results suggest that the present graphene/Cu/mica substrate can be used for top-gate insulator research on graphene.

Room-temperature quantum emission from interface excitons in mixed-dimensional heterostructures.

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm antibunching. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned.

Resonant exciton transfer in mixed-dimensional heterostructures for overcoming dimensional restrictions in optical processes.

Nanomaterials exhibit unique optical phenomena, in particular excitonic quantum processes occurring at room temperature. The low dimensionality, however, imposes strict requirements for conventional optical excitation, and an approach for bypassing such restrictions is desirable. Here we report on exciton transfer in carbon-nanotube/tungsten-diselenide heterostructures, where band alignment can be systematically varied. The mixed-dimensional heterostructures display a pronounced exciton reservoir effect where the longer-lifetime excitons within the two-dimensional semiconductor are funneled into carbon nanotubes through diffusion. This new excitation pathway presents several advantages, including larger absorption areas, broadband spectral response, and polarization-independent efficiency. When band alignment is resonant, we observe substantially more efficient excitation via tungsten diselenide compared to direct excitation of the nanotube. We further demonstrate simultaneous bright emission from an array of carbon nanotubes with varied chiralities and orientations. Our findings show the potential of mixed-dimensional heterostructures and band alignment engineering for energy harvesting and quantum applications through exciton manipulation.

Micrometer-scale monolayer SnS growth by physical vapor deposition.

Recently, monolayer SnS, a two-dimensional group IV monochalcogenide, was grown on a mica substrate at the micrometer-size scale by the simple physical vapor deposition (PVD), resulting in the successful demonstration of its in-plane room temperature ferroelectricity. However, the reason behind the monolayer growth remains unclear because it had been considered that the SnS growth inevitably results in a multilayer thickness due to the strong interlayer interaction arising from lone pair electrons. Here, we investigate the PVD growth of monolayer SnS from two different feed powders, highly purified SnS and commercial phase-impure SnS. Contrary to expectations, it is suggested that the mica substrate surface is modified by sulfur evaporated from the Sn2S3 contaminant in the as-purchased powder and the lateral growth of monolayer SnS is facilitated due to the enhanced surface diffusion of SnS precursor molecules, unlike the growth from the highly purified powder. This insight provides a guide to identify further controllable growth conditions.

Transport properties of the top and bottom surfaces in monolayer MoS2 grown by chemical vapor deposition.

The advantage of MoS2, compared with graphene, is the direct growth on various oxide substrates by chemical vapor deposition (CVD) without utilizing catalytic metal substrates, which facilitates practical applications for electronics. The carrier mobility is, however, degraded from the intrinsic limit mainly due to short-range scattering caused by S vacancies formed during CVD growth. If the upper limit for the crystallinity of CVD-MoS2 on oxide substrates is determined by the MoS2/substrate interaction during growth, it will hinder the advantage. In this study, we investigated the interaction between monolayer MoS2 and a SiO2/Si substrate and the difference in crystallinity between the top and bottom S surfaces due to the MoS2/substrate interaction. Raman and photoluminescence spectroscopy indicated that doping and strain were induced in MoS2 from the substrate, but they could be removed by transferring MoS2 to a new substrate using polymers. The newly developed polymer-transfer technique enabled selective transfer of the bottom or top surface of CVD-MoS2 onto a new SiO2/Si substrate. The metal-insulator transition was clearly observed for both the normal and inverse transfers, suggesting that the crystallinity of CVD-MoS2 is high and that the crystallinity of the bottom surface interacting with the substrate was similar to that of the top free surface. These results provide positive prospects for the further improvement of the crystallinity of MoS2 on oxide substrates by reconsidering the growth conditions.

Molecularly-thin anatase field-effect transistors fabricated through the solid state transformation of titania nanosheets.

We demonstrate the field-effect transistor (FET) operation of a molecularly-thin anatase phase produced through solid state transformation from Ti0.87O2 nanosheets. A monolayer Ti0.87O2 nanosheet with a thickness of 0.7 nm is a two-dimensional oxide insulator in which Ti vacancies are incorporated, rather than oxygen vacancies. Since the fabrication method, in general, largely affects the film quality, the anatase films derived from the Ti0.87O2 nanosheets show interesting characteristics, such as no photocurrent peak at ∼2 eV, which is related to oxygen vacancies, and a larger band gap of 3.8 eV. The 10 nm thick anatase FETs exhibit superior transport characteristics with a maximum mobility of ∼1.3 cm2 V-1 s-1 and a current on/off ratio of ∼105 at room temperature. The molecularly-thin anatase FET may provide new functionalities, such as field-effect control of catalytic properties.

Photoluminescence of n-type CdGeAs2.

Samples of n-type CdGeAs2 were produced by intentional doping with indium, selenium, or tellurium impurities. A near-edge photoluminescence (PL) band from heavily In-doped CdGeAs2 samples shifts to higher energy and becomes broader with increasing electron concentration. The observed shifts in peak energies are compared to predictions for donor-acceptor pair and free-to-bound (electron-acceptor) recombinations including band filling, band tailing, and band gap shrinkage effects due to the high doping levels. For n>2 × 1018 cm-3, the free-to-bound PL transition related to a shallow 120 meV acceptor level is dominant. A lower energy PL band due to deep acceptors and normally seen for p-type samples is the only emission observed from less n-type samples (n∼1016-1017 cm-3) doped with indium, selenium, or tellurium impurities. Transitions involving the deep acceptor level are not present in the PL for heavily In-doped CdGeAs2 crystals, which suggests that the deep acceptor may be a Cd vacancy.