Showing papers by "Andras Kis published in 2016"

Single-layer MoS2 nanopores as nanopower generators

Abstract: Osmotic power generation is a promising renewable energy source. This study demonstrates the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanogenerators. The transport of water through a membrane scales inversely with membrane thickness, so atomically thin materials should provide the ideal medium to host the nanopores in an osmotic power generator. Aleksandra Radenovic and colleagues produced nanopores in two-dimensional MoS2 and, using a salt gradient across a single nanopore, generated a power output per area orders of magnitude greater than that previously reported for nanotubes. They also show that a chemical potential gradient across a single nanopore in MoS2 can generate enough power to operate a single-layer MoS2 transistor.

High Responsivity, Large-Area Graphene/MoS2 Flexible Photodetectors

Abstract: We present flexible photodetectors (PDs) for visible wavelengths fabricated by stacking centimeter-scale chemical vapor deposited (CVD) single layer graphene (SLG) and single layer CVD MoS2, both wet transferred onto a flexible polyethylene terephthalate substrate. The operation mechanism relies on injection of photoexcited electrons from MoS2 to the SLG channel. The external responsivity is 45.5A/W and the internal 570A/W at 642 nm. This is at least 2 orders of magnitude higher than bulk-semiconductor flexible membranes. The photoconductive gain is up to 4 × 105. The photocurrent is in the 0.1–100 μA range. The devices are semitransparent, with 8% absorptance at 642 nm, and are stable upon bending to a curvature of 1.4 cm. These capabilities and the low-voltage operation (<1 V) make them attractive for wearable applications.

Observation of ionic Coulomb blockade in nanopores

Abstract: Ionic Coulomb blockade—the ionic counterpart of the electronic Coulomb blockade—has been observed in a single subnanometre MoS2 pore junction.

Disorder engineering and conductivity dome in ReS2 with electrolyte gating.

Abstract: Atomically thin rhenium disulphide (ReS2) is a member of the transition metal dichalcogenide family of materials. This two-dimensional semiconductor is characterized by weak interlayer coupling and a distorted 1T structure, which leads to anisotropy in electrical and optical properties. Here we report on the electrical transport study of mono- and multilayer ReS2 with polymer electrolyte gating. We find that the conductivity of monolayer ReS2 is completely suppressed at high carrier densities, an unusual feature unique to monolayers, making ReS2 the first example of such a material. Using dual-gated devices, we can distinguish the gate-induced doping from the electrostatic disorder induced by the polymer electrolyte itself. Theoretical calculations and a transport model indicate that the observed conductivity suppression can be explained by a combination of a narrow conduction band and Anderson localization due to electrolyte-induced disorder.

Valley Polarization by Spin Injection in a Light-Emitting van der Waals Heterojunction

Abstract: The band structure of transition metal dichalcogenides (TMDCs) with valence band edges at different locations in the momentum space could be harnessed to build devices that operate relying on the valley degree of freedom. To realize such valleytronic devices, it is necessary to control and manipulate the charge density in these valleys, resulting in valley polarization. While this has been demonstrated using optical excitation, generation of valley polarization in electronic devices without optical excitation remains difficult. Here, we demonstrate spin injection from a ferromagnetic electrode into a heterojunction based on monolayers of WSe2 and MoS2 and lateral transport of spin-polarized holes within the WSe2 layer. The resulting valley polarization leads to circularly polarized light emission that can be tuned using an external magnetic field. This demonstration of spin injection and magnetoelectronic control over valley polarization provides a new opportunity for realizing combined spin and valleytro...

Micro-reflectance and transmittance spectroscopy: a versatile and powerful tool to characterize 2D materials

Abstract: Optical spectroscopy techniques such as differential reflectance and transmittance have proven to be very powerful techniques to study 2D materials. However, a thorough description of the experimental setups needed to carry out these measurements is lacking in the literature. We describe a versatile optical microscope setup to carry out differential reflectance and transmittance spectroscopy in 2D materials with a lateral resolution of ~1 micron in the visible and near-infrared part of the spectrum. We demonstrate the potential of the presented setup to determine the number of layers of 2D materials and to characterize their fundamental optical properties such as excitonic resonances. We illustrate its performance by studying mechanically exfoliated and chemical vapor-deposited transition metal dichalcogenide samples.

Magnetoexcitons in large area CVD-grown monolayer MoS 2 and MoSe 2 on sapphire

Abstract: Magnetotransmission spectroscopy was employed to study the valley Zeeman effect in large area monolayer MoS2 and MoSe2. The extracted values of the valley g factors for both A and B excitons were found to be similar with g(v) similar or equal to -4.5. The samples are expected to be strained due to the CVD growth on sapphire at high temperature (700 degrees C). However, the estimated strain, which is maximum at low temperature, is only similar or equal to 0.2%. Theoretical considerations suggest that the strain is too small to significantly influence the electronic properties. This is confirmed by the measured value of the valley g factor, and the measured temperature dependence of the band gap, which are almost identical for CVD and mechanically exfoliated MoS2.

Electroabsorption in MoS$_2$

Abstract: To translate electrical into optical signals one uses the modulation of either the refractive index or the absorbance of a material by an electric field. Contemporary electroabsorption modulators (EAMs) employ the quantum confined Stark effect (QCSE), the field-induced red-shift and broadening of the strong excitonic absorption resonances characteristic of low-dimensional semiconductor structures. Here we show an unprecedentedly strong transverse electroabsorption (EA) signal in a monolayer of the two-dimensional semiconductor MoS2. The EA spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to the conventional QCSE, the signal increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dBnm-1 bear the scope for extremely compact, ultrafast, energy-efficient EAMs for integrated photonics, including on-chip optical communication.

Vacuum ultraviolet excitation luminescence spectroscopy of few-layered MoS2.

Abstract: We report on vacuum ultraviolet (VUV) excited photoluminescence (PL) spectra emitted from a chemical vapor deposited MoS2 few-layered film. The excitation spectrum was recorded by monitoring intensities of PL spectra at ~1.9 eV. A strong wide excitation band peaking at 7 eV was found in the excitation. The PL excitation band is most intensive at liquid helium temperature and completely quenched at 100 K. Through first-principles calculations of photoabsorption in MoS2, the excitation was explicated and attributed to transitions of electrons from p- and d- type states in the valence band to the d- and p-type states in the conduction band. The obtained photon-in/photon-out results clarify the excitation and emission behavior of the low dimensional MoS2 when interacting with the VUV light sources.

High-frequency, scaled MoS2 transistors

Abstract: The interest in MoS2 for radio-frequency (RF) application has recently increased. However, little is known on the scaling behavior of transistors made from MoS2 for RF applications, which is important for establishing performance limits for electronic circuits based on 2D semiconductors on flexible and rigid substrates. Here, we present a systematic study of top-gated trilayer MoS2 RF transistors with gate lengths scaled down to 70 and 40 nm. In addition, by introducing edge-contacted injection of electrons in trilayer MoS2 devices, we decrease the contact resistance and as a result obtain the highest cutoff frequency of 6 GHz before the de-embedding procedure and 25 GHz after the de-embedding procedure.

THz time-domain spectroscopy and IR spectroscopy on MoS2

Abstract: In the increasing research field of 2D materials such as graphene, molybdenum disulfide MoS2 has attracted great interest due to the existence of a direct bandgap in monolayer MoS2, which gives the possibility of achieving MoS2 field-effect transistors or optoelectronic devices. We analyzed by THz time-domain spectroscopy (THz-TDS) up to 2 THz and infrared (IR) spectroscopy, CVD-obtained MoS2 using either S or H2S gas as a sulfur precursor, grown on a sapphire substrate. From THz-TDS we obtained the transmittance, conductivity, and attenuation. From IR spectroscopy on the same samples, we deduced the transmittance in the IR frequency range. We observed the coherence of both spectroscopic methods. The advantage of the THz-TDS method is that we can get significant parameters related to the sample quality without the need for depositing any electrical contact or sample preparation. Our results show that at high frequencies MoS2 is even better than graphene as a material for optoelectronic devices.

Free-standing electronic character of monolayer MoS 2 in van der Waals epitaxy

Abstract: We have evaluated as-grown $\mathrm{Mo}{\mathrm{S}}_{2}$ crystals, epitaxially grown on a monocrystalline sapphire by chemical vapor deposition (CVD), with direct electronic band-structure measurements by energy-filtered $k$-space photoelectron emission microscopy performed with a conventional laboratory vacuum ultraviolet He I light source under off-normal illumination. The valence states of the epitaxial $\mathrm{Mo}{\mathrm{S}}_{2}$ were mapped in momentum space down to 7 eV below the Fermi level. Despite the high nucleation density within the imaged area, the CVD $\mathrm{Mo}{\mathrm{S}}_{2}$ possesses an electronic structure similar to the free-standing monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ single crystal, and it exhibits hole effective masses of $2.41\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}{m}_{0}$, and $0.81\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}{m}_{0}$, respectively, at \ensuremath{\Gamma} and $K$ high-symmetry points that are consistent with the van der Waals epitaxial growth mechanism. This demonstrates the excellent ability of the $\mathrm{Mo}{\mathrm{S}}_{2}$ CVD on sapphire to yield a highly aligned growth of well-stitched grains through epitaxial registry with a strongly preferred crystallographic orientation.

A Robust Molecular Probe for Ångstrom-Scale Analytics in Liquids

Abstract: Traditionally, nanomaterial profiling using a single-molecule-terminated scanning probe is performed at the vacuum-solid interface often at a few Kelvin, but is not a notion immediately associated with liquid-solid interface at room temperature. Here, using a scanning tunnelling probe functionalized with a single C60 molecule stabilized in a high-density liquid, we resolve low-dimensional surface defects, atomic interfaces and capture Angstrom-level bond-length variations in single-layer graphene and MoS2. Atom-by-atom controllable imaging contrast is demonstrated at room temperature and the electronic structure of the C60-metal probe complex within the encompassing liquid molecules is clarified using density functional theory. Our findings demonstrates that operating a robust single-molecular probe is not restricted to ultra-high vacuum and cryogenic settings. Hence the scope of high-precision analytics can be extended towards resolving sub-molecular features of organic elements and gauging ambient compatibility of emerging layered materials with atomic-scale sensitivity under experimentally less stringent conditions.

High-quality synthetic 2D transition metal dichalcogenide semiconductors

Abstract: We give here an overview on our results on the large-area growth of 2D transition metal dichalcogenide semiconductors MoS 2 , MoSe 2 , WSe 2 using chemical vapor deposition. The growth of MoS 2 on sapphire occurs epitaxially with the crystalline orientation of the MoS 2 film closely matching that of the sapphire substrate, resulting in a high-quality continuous film. The use of H 2 S results in more control over growth morphologies. WSe 2 and MoSe 2 have also been successfully grown using solid-state precursors. Room-temperature mobilities of all these materials exceed 10 cm2V−1s−1. In contrast to MoS 2 , WSe 2 and MoSe 2 show ambipolar behavior.