Session S2: Focus Session: Beyond Graphene - Optics in 2D Semiconductors V

Optical and electrical responses of optically excited single layer MoS$_{2}$ depending on the carrier concentration

In this work, we study both optical and electrical responses of a single layer molybdenum disulfide (MoS$_{2})$ connecting source and drain electrodes deposited on SiO$_{2}$ layer. Depending on the gate bias, the incremental rate of the photocurrent is different, implying the carrier concentration is closely involved with the observations. In semiconducting state at high negative gate bias, the increase of the number of carriers is more influential on electrical conductivity of MoS$_{2}$, whereas in metallic state at high positive gate bias, the electron-electron scattering is more dominant. In addition, the photoluminescence (PL) is significantly affected by the carrier concentration as well. At low concentration, PL is stronger than that of at higher carrier concentration that weakens Coulomb interaction between electron-hole pairs.   

Electronic and optoelectronic properties of Few Layer MoS$_{2}$ Flake

We will report on the electronic and optoelectronic properties of few-layers flakes of MoS$_{2}$ obtained by mechanical exfoliation of bulk MoS$_{2}$ crystal. Measurements performed in field effect transistor (FET) geometry show a room temperature mobility $\mu _{\mathrm{FE}}$ $\sim$ 40cm$^{2}$V$^{-1}$s$^{-1}$. Temperature dependent (50K\textless T\textless 300K) photoconductivity measurements investigated using continuous laser of $\lambda =$658nm (E$=$1.88eV), over a broad range of illuminating laser intensity, P (0.1$\mu $W\textless P\textless 2$\mu $W) indicate a fractional power dependence of steady state photocurrent on P. Room temperature responsivity obtained in these samples were found to be $\sim$ 1AW$^{-1}$. Variation and/or dependence of these measured properties with respect to temperature will be presented and compared with similar measurements performed other layered 2D Transition Metal Dichalcogenides. This work is supported by the U.S. Army Research Office through a MURI grant {\#} W911NF-11-1-0362.   

Optical properties of two dimensional gallium selenide

Gallium selenide is a layered metal chalcogenide compound with peculiar but attractive optical properties. Its very high $d_{22}$ coefficient, along with its optical transparency from 0.65 to 18 $\mu$m and high optical damage threshold, makes it an ideal material for non-linear optical applications. The band structure of GaSe presents a pseudo-direct bandgap, with the direct transition sitting $\sim$20 meV above the indirect one, allowing for efficient photoconductivity and photoluminescence. This could prove interesting for various optoelectronic applications. In this study, we have analyzed the angular and polarization dependence of photoluminescence and second harmonic generation of 2D GaSe flakes of varying thicknesses, down to the monolayer. These results are examined in the light of the few-layer structure symmetry and the unusual optical selection rules forbidding light emission perpendicularly to the basal plane.   

Optoelectronics of Transition Metal Dichalcogenide Monolayers and Heterostructures

Monolayer transition metal dichalcogenides (TMDs) contain 2D valley excitons which reside in two degenerate momentum space valleys at the edges of the Brillouin zone. It is crucially important to understand fundamental 2D exciton properties in TMD monolayers and van der Waals heterostructures. By performing coherent nonlinear optical spectroscopy with high spectral resolution, we observe nanosecond decay dynamics in single monolayers of MoSe$_{\mathrm{2}}$, implying the presence of a previously unreported long-lived state that appears to trap the exciton population. In MoSe$_{\mathrm{2}}$-WSe$_{\mathrm{2}}$ vertical heterostructures, we observe intralayer excitons, where the electron and hole are confined to different monolayers, and show evidence of strong exciton-exciton interaction effects and long lifetimes. Based on TMD monolayer excitons, we have also investigated a variety of fundamental quantum devices, including a nano-cavity laser and a second-harmonic generation transistor. Finally, we report a new type of single quantum emitter, based on single localized excitons spatially confined to defects in monolayers of WSe$_{\mathrm{2}}$. The photoluminescence from these localized excitons is spectrally narrow and shows strong anti-bunching, demonstrating the single photon nature of the emission.   

Strong in-plane anisotropic optical properties of monolayer, few-layer and bulk ReSe$_{2}$

Recently, there has been growing interest in the anisotropic properties of certain two-dimensional (2D) materials with reduced lattice symmetry, such as black phosphorus, for developing novel applications in nanoelectronics and infrared optoelectronics. In this work, we report the strong anisotropic optical and electronic properties of monolayer, few-layer and bulk ReSe$_{2}$, an emerging member of the 2D transition metal dichalcogenides (TMDCs) family. With its bulk bandgap around 1.1 eV and potentially tunable with layer number and strain, ReSe$_{2}$ may complement black phosphorus for optoelectronic applications utilizing its anisotropic properties in the near-infrared and visible range. Through careful investigations of the polarization-resolved Raman spectroscopy, photoluminescence (PL), polarization-resolved optical extinction spectrum, angle-resolved DC conductance and first principles calculations, we observed and explained the consistent dependence of phonon, optical and electrical properties of ReSe$_{2}$ on its in-plane crystal orientation. Our results reveal the interesting anisotropic properties of 2D ReSe$_{2}$ and shed light on its potential applications in electronics and optoelectronics.   

Electronic and photo-electronic transport in sputter deposited MoS$_{2}$ film

Here we report on the electrical transport as well as photo response of large area sputter deposited few-layers of thin MoS$_{2}$. Temperature dependent (55 K -275K) electronic conductivity measured on these samples show evidence of 2D Variable Range Hopping (2D-VRH) mechanism within 100K-275K. Photoconductivity measurements investigated using a continuous laser of $\lambda =$658nm (E$=$1.88eV), over a broad range of illuminating laser intensity, P (0.19$\mu $W \textless P \textless 11$\mu $W). The steady state photocurrent (I$_{\mathrm{ph}})$ indicates a fractional power dependence on laser intensity. The highest responsivities obtained in these films are found to be $\sim$ 0.2AW$^{-1}$.The frequency (with Laser pulse frequency range 1Hz-200Hz) dependent photocurrent will be presented and discussed. This work is supported by the U.S. Army Research Office through a MURI grant {\#} W911NF-11-1-0362 and NSF-PIRE OISE-0968405.   

Unusual Electrooptic Response of Colloidal 2D Layered Transition Metal Dichalcogenide Nanodiscs

We have characterized an unusual electrooptic response in colloidal solutions of TiS$_{2}$, WSe$_{2}$, and ZrS$_{2}$ layered transition metal dichalcogenide (TMDC) nanodiscs, where transient orientation order is induced by the time varying component of a square-wave electric field, giving rise to linear dichroism which decays even in the presence of the aligning field. Interestingly, identical electrooptic response were seen from both the rising and falling edges of the field, indicating that particle alignment responds to the absolute value of $\Delta $E, regardless of the DC field offset, essentially performing the optical electric field edge detection. Both the magnitude and decay time of the electrooptic response were sensitive to solvent polarity, which we believe is related to the polarity dependent interparticle interactions previously observed in colloidal TiS$_{2}$ nanodisc solutions. This unusual behavior appears to be a general property of colloidal TMDC nanodiscs, potentially resulting from the time varying anisotropy of the induced dipole moment, in contrast to other anisotropic nanostructures with little to no dipole moment anisotropy, where the electrooptic response is dictated by the magnitude of the electric field.   

Biaxial Strain Engineering in Suspended MoS2

Monolayer MoS2 is a direct gap semiconductor and has attracted significant interest for its potential uses in electronics and optoelectronics. It has also been shown to have a highly strain-sensitive bandgap and can sustain strains of up to 11 percent, making it ideally suited for using strain engineering to tune it's electrical and optical properties. Herein, we fabricate pressurized MoS2 blisters using single and few layer MoS2 membranes suspended over cylindrical microcavities. By applying a pressure difference across the membrane and measuring the changes to it's photoluminescence spectrumwe study the effect of elastic biaxial strain engineering on the bandgap of MoS2.   

Pronounced photovoltaic response from PN-junctions of multi-layered MoSe$_2$ on h-BN

Transition metal dichalcogenides (TMDs) such as MoS$_2$, WSe$_2$, etc., are semiconducting van der Waals bonded solids which are exfoliable down to single atomic layers. Monolayers display unique optical as well as optoelectronic properties, while heterostructures incorporating graphene and multi-layered TMDs display pronounced photoconducting and photovoltaic responses. Here, we report the observation of rectification and enhanced photoconducting as well as photovoltaic, in lateral PN junctions based on multi-layered ambipolar MoSe$_2$ crystals stacked onto h-BN. Our PN junctions composed of $\sim$10 atomic layers are translucent enough to yield photoresponsitivities of 1 A/W, external quantum efficiencies exceeding 30 \%, short circuit currents exceeding 10$^3$ A/cm$^2$, and photovoltaic efficiencies surpassing 5 \% with fill factors of $\sim$70 \%. These values compare favourably with those of transparent photovoltaic cells. Given that TMDs can be grown in large area, that their band gap(s) can be tuned by varying composition, and the available strategies for increasing their efficiency, our results suggest a remarkable potential for semi-transparent photovoltaic cells composed of just a few layers of TMDs.   

Temperature dependent photoconduction in atomically thin Layers of Indium Selenide

We will report on the photo response in few-layers of thin Indium Selenide (InSe) flakes exfoliated from crystals grown using chemical vapor transport technique. Temperature dependent (20 K -300K) photoconductivity measurements investigated using a continuous laser of $\lambda =$658nm (E$=$1.88eV), over a broad range of illuminating laser power, P (0.1 $\mu $W \textless P \textless 4$\mu $W) indicate a power dependence of steady state photocurrent (I$_{\mathrm{ph}})$ on P (I $\sim$ P$^{\mathrm{\beta }}$ with $\beta $ $\sim$ 1). The highest responsivity obtained in these samples were $\sim$ 0.5AW$^{1}$. Variation and/or dependence of these measured properties with respect to temperature will be presented. The frequency (with Laser pulse frequency range 1Hz-200Hz) dependent photocurrent will be presented and discussed. This work is supported by the U.S. Army Research Office through a MURI grant {\#} W911NF-11-1-0362.   

Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS$_{2}$ and Metal/WSe$_{2}$ Heterojunctions

Transition metal dichalcogenides (TMDCs), such as MoS$_{2}$ and WSe$_{2}$, are free of dangling bonds, therefore make more ``ideal'' Schottky junctions than bulk semiconductors, which produce recombination centers at the interface with metals, inhibiting charge transfer. Here, we observe a more than 10X enhancement in the indirect band gap PL of TMDCs deposited on various metals, while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS$_{2}$ and WSe$_{2}$, and subsequently recombine radiatively with minority holes. Since the conduction band at the K-point is 0.5eV higher than at the $\Sigma $-point, a lower Schottky barrier of the $\Sigma $-point band makes electron injection more favorable. Also, the $\Sigma $ band consists of the sulfur $p_{\mathrm{z}}$ orbital, which overlaps more significantly with the electron wavefunctions in the metal. This enhancement only occurs for thick flakes, and is absent in monolayer and few-layer flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures.   

Voltage controlled optics of a monolayer semiconductor quantum emitter

Two-dimensional atomically thin materials are being actively investigated for next generation optoelectronic devices. Particularly exciting are transition metal dichalcogenides (TMDC) since these materials exhibit a band gap, and support valley specific exciton mediated optical transitions. In this work we report the observation of single photon emission in the TMDC tungsten diselenide. We present magneto-optical spectroscopy results and demonstrate voltage controlled photoluminescence of these localized quantum emitters.   

Elucidating the optical properties of MoTe$_{2}$/InN heterostructures for photovoltaic applications

Recently, two-dimensional (2D) atom-thick hexagonal crystals have drawn both experimental and theoretical interest due to their fundamental properties and potential applicability in electronics and optoelectronics. While most studies are focused on 2D crystals with gap in the visible electromagnetic spectrum, the ones with gaps in the near infrared region have not been explored yet. Motivated by this and considering the individual properties of transition metal dichalcogenides and group III-V compounds, we carry out density functional theory (DFT) calculations combine with the GW-Bethe-Salpeter (GW-BSE) methodology to study the optical properties and the power conversion efficiency of MoTe$_{2}$/InN heterostructures. First, we study the geometric and electronic structure of three heterostructures based on different stacking. Secondly, we use the GW-BSE methodology to study the optical spectrum and estimate the power conversion efficiency of the device. Our results indicates that the photoexcited exciton are originated in the range of 1.12 to 1.5 eV. In addition, we estimate the exciton recombination time finding values in the nanosecond range. Finally, we estimated the short-circuit current and power conversion efficiency of the 2 nm thick device.