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The covalently bonded in-plane heterostructure (HS) of monolayer transition-metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In this study, high-quality monolayer MoSe2-WSe2 lateral HSs are grown by pulsed-laser-deposition-assisted selenization method. The sharp interface of the lateral HS is verified by morphological and optical characterizations. Intriguingly, photoluminescence spectra acquired from the interface show rather clear signatures of pristine MoSe2 and WSe2 with no intermediate energy peak related to intralayer excitonic matter or formation of MoxW(1–x)Se2 alloys, thereby confirming the sharp interface. Furthermore, the discrete nature of laterally attached TMDC monolayers, each with doubly degenerated but nonequivalent energy valleys marked by (KM, K′M) for MoSe2 and (KW, K′W) for WSe2 in k space, allows simultaneous control of the four valleys within the excitation area without any crosstalk effect over the interface. As an ...

In2S3 is one of the fascinating materials for a range of optoelectronic applications due to its suitable bandgap and stability. Nonetheless, its photoluminescence (PL) originating from defect states within its bandgap were hardly reported. In this work, high quality In2S3 thin films on SiO2 substrates were synthesized by using physical vapor deposition of In2S3 powder in a hot-wall Quartz tube-furnace. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy studies of the grown In2S3 films showed that extremely high crystalline quality tetragonal β-In2S3 films were synthesized with preferred growth direction of [1 0 3]. Absorption spectroscopy revealed the In2S3 films have a direct band of ~2.66 eV. Interestingly, our In2S3 films showed an extremely strong PL peak at room temperature at ~1.6 eV. The observed strong PL from our In2S3 films can be attributed to originating from the oxygen isoelectronic substitution at the sulfur site, and its efficiency was affected by oxygen concentrations in In2S3 films. Our In2S3 thin films exhibited remarkably high PL quantum yields (QY) of up to 14% which is comparable to that of GaAs quantum wells (QWs) and CdSe quantum dots (QDs).

We have investigated strong optical nonlinearity of monolayer MoS2(1–x)Se2x across the exciton resonance, which is directly tunable by Se doping. The quality of monolayer alloys prepared by chemical vapor deposition is verified by atomic force microscopy, Raman spectroscopy, and photoluminescence analysis. The crystal symmetry of all of our alloys is essentially D3h, as confirmed by polarization-dependent second-harmonic generation (SHG). The spectral structure of the exciton resonance is sampled by wavelength-dependent SHG (λ = 1000–1800 nm), where the SHG resonance red-shifts in accordance with the corresponding optical gap. Surprisingly, the effect of compositional variation turns out to be much more dramatic owing to the unexpected increase of B-exciton-induced SHG, which indeed dominates over the A-exciton resonance for x ≥ 0.3. The overall effect is therefore stronger and broader SHG resonance where the latter arises from different degrees of red-shift for the two exciton states. We report the corre...

We report a simple and reliable technique to synthesize high-quality In 2 S 3 films on InAs substrates by using thermal sulfurization in a hot-wall tube furnace. X-ray diffraction and energy dispersive X-ray spectroscopy data confirmed that the synthesized films were cubic β-In 2 S 3 or tetragonal β-In 2 S 3 , depending on growth conditions. Field emission scanning electron microscopy analysis and Raman spectroscopy showed that the In 2 S 3 films are of remarkable crystal quality. Especially, by optimizing the growth conditions, we have grown an extremely high-quality tetragonal β-In 2 S 3 thin film firmly remained on the InAs substrate, for the first time.

In this study, we utilized picosecond pulses from an Nd:YAG laser to investigate the nonlinear optical characteristics of monolayer MoSe2. Two-step growth involving the selenization of pulsed-laser-deposited MoO3 film was employed to yield the MoSe2 monolayer on a SiO2/Si substrate. Raman scattering, photoluminescence (PL) spectroscopy, and atomic force microscopy verified the high optical quality of the monolayer. The second-order susceptibility χ(2) was calculated to be ∼50 pm V−1 at the second harmonic wavelength ∼810 nm, which is near the optical gap of the monolayer. Interestingly, our wavelength-dependent second harmonic scan can identify the bound excitonic states including negatively charged excitons much more efficiently, compared with the PL method at room temperature. Additionally, the MoSe2 monolayer exhibits a strong laser-induced damage threshold ∼16 GW cm−2 under picosecond-pulse excitation. Our findings suggest that monolayer MoSe2 can be considered as a promising candidate for high-power, thin-film-based nonlinear optical devices and applications.

CdS nanowires were synthesized using a simple synthesis process using Au nanoparticles (average diameter: 13 nm) as catalyst. The growth conditions were optimized by varying the substrate temperature and the growth time of a thermal chemical-vapor-deposition system. CdS nanowires were successfully grown at temperatures above 480 °C. In fact, high-quality nanowires, with hexagonal wurtzite structures and an average diameter of 25 nm and length of 1.46 μm, were obtained after a 30-min synthesis at a growth temperature of 520 °C. Electron microscope images revealed that our CdS nanowires, grown at relatively lower growth temperatures, have higher average-aspect-ratio and smaller average-diameter than those previously reported in the literature. Our synthesis method resulted in CdS nanowires only without producing nanorods and nanobelts, which make it unnecessary to filter and purify nanowires from the mixture of nanowires, nanorods, and nanobelts.

We have synthesized functionalized graphene by using direct dispersion of graphite flakes with single-stranded DNA in distilled water. The synthesized graphene-DNA complexes were investigated by using Raman spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), and time-of-flight secondary-ion mass spectroscopy (TOF-SIMS). Raman spectra exhibited typical G-band and 2D-band signatures of graphene at ~1590 cm−1 and ~2700 cm−1, respectively. TOF-SIMS result showed that the graphene layers were successfully functionalized with singlestranded DNA. Mono-layer and few-layer graphene were identified by measuring the layer thickness by using AFM and by analyzing the electron diffraction patterns from TEM measurements.

The covalently bonded in-plane heterostructure (HS) of monolayer transition-metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In this study, high-quality monolayer MoSe

We have investigated the effect of annealing of graphene sheets on the electrical properties of Au/graphene/n-type silicon Schottky diode. Large scale graphene sheets were grown by chemical vapor deposition and then annealed at 300, 400, and 500 °C; one sheet was left un-annealed as the control. The diodes were fabricated by transferring the graphene sheets directly onto n-type Si substrates and the current – voltage ( I–V ) and capacitance – voltage ( C–V ) characteristics were evaluated. The average values of the Schottky barrier height (SBH) and ideality factor ( η ) for the as-fabricated Au/graphene/n-type silicon Schottky diode from I–V measurements were determined to be ∼0.8 ± 0.01 eV and ∼1.79 ± 0.05, respectively, whereas the SBH from C–V measurements was ∼0.89 ± 0.01 eV. The electrical transport characteristics measured at room temperature indicated that annealing of graphene sheet prior to the transfer of the graphene onto the n-Si substrates significantly reduces the electric degradation of the Schottky diodes, even though no distinct differences in other electric properties, including ideality factors and SBHs, before or after annealing of the graphene sheets were observed. Thus, by simply annealing the graphene sheets at 500 °C, we found that the Au/graphene/n-type silicon Schottky diode showed an approximately 3.3-fold lower series resistance as compared with the un-annealed Schottky diode under air exposure of up to 7 days. These annealed diodes showed significantly reduced electrical degradation by removing the potentially trapped H 2 O and/or O 2 at the interface between the graphene layer and the n-Si substrate.

There has been growing research interest in the use of molybdenum disulfide in the fields of optoelectronics and energy harvesting devices, by virtue of its indirect-to-direct band gap tunability. However, obtaining large area thin films of MoS2 for future device applications still remains a challenge. In the present study, the amounts of the precursors (S and MoO3) were varied systematically in order to optimize the growth of highly crystalline and large area MoS2 layers by the chemical vapor deposition method. Careful control of the amounts of precursors was found to the key factor in the synthesis of large area highly crystalline flakes. The thickness of the layers was confirmed by Raman spectroscopy and atomic force microscopy. The optical properties and chemical composition were studied by photoluminescence (PL) and X-ray photoelectron spectroscopy. The emergence of strong direct excitonic emissions at 1.82 eV (A-exciton, with a normalized PL intensity of ∼55 × 103) and 1.98 eV (B-exciton, with a normalized PL intensity of ∼5 × 103) of the sample at room temperature clearly indicates the high luminescence quantum efficiency. The mobility of the films was found to be 0.09 cm2/(V·s) at room temperature. This study provides a method for the controlled synthesis of high-quality two-dimensional (2D) transition metal dichalcogenide materials, useful for applications in nanodevices, optoelectronics and solar energy conversion.

Band-like transport behavior of H-doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low-temperature electrical measurements, where MoTe2 , WSe2 , and MoS2 are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n-type conduction and their mobility increases without losing on-state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p- or n-type. Density functional theory calculations show that H-doped MoTe2 , WSe2 , and MoS2 have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H-doped TMD channels is metallic showing band-like transport rather than thermal hopping. These results indicate that H-doped TMD FETs are practically useful even at low-temperature ranges.

Control of living cells on biocompatible materials or on modified substrates is important for the development of bio-applications, including biosensors and implant biomaterials. The topography and hydrophobicity of substrates highly affect cell adhesion, growth, and cell growth kinetics, which is of great importance in bio-applications. Herein, we investigate the adhesion, growth, and morphology of cultured breast cancer cells on a silicon substrate, on which graphene oxides (GO) was partially formed. By minimizing the size and amount of the GO-containing solution and the further annealing process, GO-coated Si samples were prepared which partially covered the Si substrates. The coverage of GO on Si samples decreases upon annealing. The behaviors of cells cultured on two samples have been observed, i.e. partially GO-coated Si (P-GO) and annealed partially GO-coated Si (Annealed p-GO), with a different coverage of GO. Indeed, the spreading area covered by the cells and the number of cells for a given culture period in the incubator were highly dependent on the hydrophobicity and the presence of oxygenated groups on GO and Si substrates, suggesting hydrophobicity-driven cell growth. Thus, the presented method can be used to control the cell growth via an appropriate surface modification.

Graphene layers were synthesized using ultrasonic dispersion of graphite flakes followed by ultracentrifugation in sodium cholate and polyoxyethylene nonylphenyl ether aqueous solutions. The synthesized graphene layers were characterized using Raman spectroscopy and atomic force microscopy (AFM). Typical G-band and 2D-band Raman signatures of graphene at ∼1590 cm−1 and ∼2695 cm−1, respectively, were observed in the Raman spectra of the synthesized graphene samples. The AFM height profiles of the synthesized graphene samples on SiO2 substrates show features that can be attributed to graphenes of a single layer or a few layers.

The interest in layered materials is largely based on the expectation that they will be beneficial for a variety of applications, from low-power-consuming, wearable electronics to energy harvesting. However, the properties of layered materials are highly dependent on thickness, and the difficulty of controlling thickness over a large area has been a bottleneck for commercial applications. Here, we report layer-by-layer growth of SnSe2, a layered semiconducting material, via van der Waals epitaxy. The films were fabricated on insulating mica substrates with substrate temperatures in the range of 210 °C–370 °C. The surface consists of a mixture of N and (N ± 1) layers, showing that the thickness of the film can be defined with monolayer accuracy (±0.6 nm). High-resolution transmission electron microscopy reveals a polycrystalline film with a grain size of ~100 nm and clear Moire patterns from overlapped grains with similar thickness. We also report field effect mobility values of 3.7 cm2 V−1 s−1 and 6.7 cm2 V−1 s−1 for 11 and 22 nm thick SnSe2, respectively. SnSe2 films with customizable thickness can provide valuable platforms for industry and academic researchers to fully exploit the potential of layered materials.

We have investigated the second-order nonlinear optical (NLO) properties of CVD-grown MoS2 monolayer (ML) transferred onto transparent substrates such as fused silica and polyethylene terephthalate. The physical properties of the transferred MLs were characterized by optical and NLO methods. We measured the second-order susceptibility χ(2) in the spectral range of λ= 1064–1600 nm in which the corresponding second harmonic radiation resonates with the exciton levels. It was found that χ(2) is strongly enhanced by up to a factor of 5 near the A- and B-exciton levels due to two-photon resonance. The absolute χ(2) values of our samples determined by both reflection and transmission geometry are on par with that of as-grown MLs. Our results imply that the cavity-confinement scheme can be employed for maximizing the nonlinear optical efficiency of atomically thin transition metal dichalcogenides for transparent/flexible optoelectronics applications, especially when oriented stacking of transferred MLs are contr...