Spatially Resolved Persistent Photoconductivity in MoS$_2$-WS$_2$ Lateral Heterostructures
Samuel Berweger, Hanyu Zhang, Prasana K. Sahoo, Benjamin M. Kupp, Jeffrey L. Blackburn, Elisa M. Miller, T. Mitch Wallis, Dmitri V. Voronine, Pavel Kabos, Sanjini U. Nanayakkara
The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS$_2$--WS$_2$ lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dissociate into free carriers. These photogenerated carriers are most likely n-type and are seen to persist for up to days, and informed by finite element modeling we reveal that they can increase the carrier density by up to 200 times. This persistent photoconductivity appears to be dominated by contributions from the multilayer MoS$_2$ domains, and we attribute the flake-wide response in part to charge transfer across the heterointerface. Spatial correlation of our SMM imaging with photoluminescence (PL) mapping confirms the strong link between PL peak emission photon energy, PL intensity, and the local accumulated charge. This work reveals the spatially and temporally complex optoelectronic response of these systems and cautions that properties measured during or after illumination may not reflect the true dark state of these materials but rather a metastable charged state.
Vertical and lateral heterostructures of van der Waals materials provide tremendous flexibility for band structure engineering. Since electronic bands are sensitively affected by defects, strain, and interlayer coupling, the edge and heterojunction of these two-dimensional (2D) systems may exhibit novel physical properties, which can be fully revealed only by spatially resolved probes. Here, we report the spatial mapping of photoconductivity in a monolayer-bilayer WSe2 lateral heterostructure under multiple excitation lasers. As the photon energy increases, the light-induced conductivity detec…
We consider the effect of terahertz (THz) radiation on the conductivity of the ungated and gated graphene (G)-phosphorene (P) hybrid structures and propose and evaluated the hot-carrier uncooled bolometric photodetectors based on the GP-lateral diodes (GP-LDs) and GP-field-effect transistors (GP-FETs) with the GP channel. The operation of the GP-LDs and GP-FET photodetectors is associated with the carrier heating by the incident radiation absorbed in the G-layer due to the intraband transitions. The carrier heating leads to the relocation of a significant fraction of the carriers into the P-la…
We report first-principles calculations of electron-phonon coupling in bilayer graphene and the corresponding contribution to carrier scattering. At the phonon $Γ$ point, electrons with energies less than 200 meV are scattered predominantly by LA$^\prime$ and TA$^\prime$ modes while higher-energy electron scattering is dominated by optical phonon modes. Based on a two-temperature model, heat transfer from electrons with an initial temperature of 2000 K to the lattice (phonons) with an initial temperature of 300 K is computed, and in the overall relaxation process, most of this energy scatters …
Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (wavelength = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET…
For many of the envisioned optoelectronic applications of graphene it is crucial to understand the sub-picosecond carrier dynamics immediately following photoexcitation, as well as the effect on the electrical conductivity - the photoconductivity. Whereas these topics have been studied using various ultrafast experiments and theoretical approaches, controversial and incomplete explanations have been put forward concerning the sign of the photoconductivity, the occurrence and significance of the creation of additional electron-hole pairs, and, in particular, how the relevant processes depend on…
Low-energy coherent transport and Goos-Hänchen (GH) lateral shift of valley electrons in planar heterojunctions composed of normal MoS$_2$ and ferromagnetic WS$_2$ monolayers are theoretically investigated. Two types of heterojunctions in the forms of WS$_2$/MoS$_2$/WS$_2$ (type-A) and MoS$_2$/WS$_2$/MoS$_2$ (type-B) with incident electrons in MoS$_2$ region are considered in which the lateral shift of electrons is induced by band alignments of the two constituent semiconductors. It is shown that the type-A heterojunction can act as an electron waveguide due to electron confinement between the…
The multi-scale computational method of combining first-principles calculation and finite element electromagnetic simulations is used to study plasmon-enhanced interlayer charge transfer (ICT) exciton of 2D lateral and van der Waals MoS2/WS2 heterostructure with 2H phase. The weak ICT excitons are observed in the 2H lateral and van der Waals MoS$_2$/WS$_2$ heterostructures. Theoretical results reveal the physical principle of plexciton resulting from strong coupling between plasmonic and ICT exciton. The weak CT exciton can be strongly enhanced by metal plasmon, can provide a new way to observ…
The increasing technological control of two-dimensional materials has allowed the demonstration of 2D lateral junctions, which display unique properties that might serve as the basis for a new generation of 2D electronic and optoelectronic devices. Notably, the chemically doped MoS$_2$ homojunction, the WSe$_2$-MoS$_2$ monolayer and MoS$_2$ monolayer/multilayer heterojunctions, have been demonstrated. Here we report the investigation of 2D lateral junction electrostatics, which differs from the bulk case because of the weaker screening, producing a much longer transition region between the spa…
In this paper we propose two transistor concepts based on lateral heterostructures of monolayer MoS$_2$, composed of adjacent regions of 1T (metallic) and 2H (semiconducting) phases, inspired by recent research showing the possibility to obtain such heterostructures by electron beam irradiation. The first concept, the lateral heterostructure field-effect transistor, exhibits potential of better performance with respect to the foreseen evolution of CMOS technology, both for high performance and low power applications. Performance potential has been evaluated by means of detailed multi-scale mat…
We have studied the finite bias transport properties of a 2H-1T' MoS$_2$ lateral metal-semiconductor (M-S) junction by non-equilibrium Green's functions calculations, aimed at contacting the 2D channel in a field effect transistor. Our results indicate that (a) despite the fundamentally different electrostatics of line and planar dipoles, the Schottky barrier heights respond similarly to changes in doping and applied bias in 2D and 3D M-S junctions, (b) 2H-1T' MoS$_2$ lateral junctions are free from Fermi level pinning, (c) armchair interfaces have superior contacting properties vs.\ zigzag in…
Trion Engineered Multimodal Transistors in Two dimensional Bilayer Semiconductor Lateral Heterostructures
Indian Institute of Technology Kharagpur（印度理工学院卡拉格普尔分校）；Indian Institute of Technology Bombay（印度理工学院孟买分校）；Philipps-University at Marburg（马尔堡菲利普斯大学）；Chalmers University of Technology（查尔姆斯理工大学）
Friedrich Schiller University Jena（耶拿弗里德里希·席勒大学）；Technical University Darmstadt（达姆施塔特工业大学）；Ulm University（乌尔姆大学）；National Institute for Materials Science（国立材料研究所）
The intrinsic spin-dependent transport properties of two types of lateral VS2|MoS2 heterojunctions are systematically investigated using first-principles calculations, and their various nanodevices with novel properties are designed. The lateral VS2|MoS2 heterojunction diodes show a perfect rectifying effect and are promising for the applications of Schottky diodes. A large spin-polarization ratio is observed for the A-type device and pure spin-mediated current is then realized. The gate voltage significantly tunes the current and rectification ratio of their field-effect transistors (FETs). I…
We present a novel methodology to synthesize two-dimensional (2D) lateral heterostructures of graphene and MoS2 sheets with molecular carbon nanomembranes (CNMs), which is based on electron beam induced stitching. Monolayers of graphene and MoS2 were grown by chemical vapor deposition (CVD) on copper and SiO2 substrates, respectively, transferred onto gold/mica substrates and patterned by electron beam lithography or photolithography. Self-assembled monolayers (SAMs) of aromatic thiols were grown on the gold film in the areas where the 2D materials were not present. An irradiation with a low e…
We study the pseudo-spin of electron pair in superconducting transition-metal dichalcogenide monolayers and show that the pseudo-spin affects the electric transport property of lateral heterojunction of the superconducting and metallic monolayers. The pseudo-spins of two electrons forming a Cooper pair are parallel to each other unlike the real spins being anti-parallel. In the lateral heterojunction, the electronic transport with forming a Cooper pair, the Andreev reflection, is suppressed with the Fermi level crossing the valence band near the edge in the metallic monolayer. We numerically i…
We study in-plane lateral heterostructures of commensurate transition-metal dichalcogenides, such as MoS$_{2}$-WS$_{2}$ and MoSe$_{2}$-WSe$_{2}$, and find interfacial and edge states that are highly localized to these regions of the heterostructure. These are one-dimensional (1D) in nature, lying within the bandgap of the bulk structure and exhibiting complex orbital and spin structure. We describe such heteroribbons with a three-orbital tight-binding model that uses first principles and experimental parameters as input, allowing us to model realistic systems. Analytical modeling for the 1D in…
We investigate the electronic transport property of lateral heterojunctions of semiconducting and metallic transition-metal dichalcogenide monolayers, MoSe$_2$ and NbSe$_2$, respectively. We calculate the electronic transmission probability by using a multi-orbital tight-binding model based on the first-principles band structure. The transmission probability depends on the spin and valley degrees of freedom. This dependence qualitatively changes by the interface structure. The heterostructure with a zig-zag interface preserves the spin and the valley of electron in the transmission process. On…
In this work, we study the spin polarization in the $MoS(Se)_{2}-WS(Se)_{2}$ Transition metal dichalcogenide heterostructures by using the non-equilibrium Green's function (NEGF) method and a three-band tight-binding model near the edges of the first Brillouin zone. Although it has been shown that the structures have no significant spin polarization in a specific range of energy of electrons, by applying a transverse electric field in the sheet of the metal atoms, shedding light on the sample, and under a small bias voltage, a significant spin polarization in the structure could be created. Be…
We study the refraction effect of electronic wave in hole-doped lateral heterojunctions of metallic and semiconducting transition-metal dichalcogenide monolayers. This effect is theoretically investigated in 2$H$-type MoSe$_2$-NbS$_2$ and WSe$_2$-NbS$_2$ junctions by combining the first-principles calculation and the lattice Green's function method. We show that the electronic waves change the direction of motion at the interface and collimate the velocity along two different directions depending on the spin. We find that the transmission probability increases with the charge density and that …
Direct nanoscale mapping of band alignment in single-layer semiconducting lateral heterojunctions
Aarhus University（奥胡斯大学）；Indian Institute of Technology, Kharagpur（印度理工学院卡格浦尔分校）；Indian Institute of Science（印度科学学院）；National Institute for Materials Science（日本物质材料研究所）；Stanford University（斯坦福大学）
We report on transport properties of the controllable large area MoSO/Reduced graphene oxide(RGO) heterostructures electrodeposited on FTO substrates and its comparision with theoretical calculations on MoSo/Gr. I-V characteristics of the heterostructure made by P or n-type MoSO, exhibit Schottkey behavior in the interface similar to the MoS$_{2}$/Gr ones. Theoretical calculations show significant effects of lateral layer size as well as layer number in the electronic properties. In monolayer MoS$_{2}$/Gr by increasing the lateral size the energy gap disappears and the Fermi level shifts towar…
