The application of ultrathin two-dimensional (2D) perovskites in near-infrared and visible-range optoelectronics has been limited owing to their inherent wide bandgaps, large excitonic binding energies and low optical absorption at higher wavelengths. Here, we show that by tailoring interfacial band alignments via conjugation with low-dimensional materials like monolayer transition metal dichalcogenides (TMD), the functionalities of 2D perovskites can be extended to diverse, visible-range photophysical applications. Based on the choice of individual constituents in the 2D perovskite/TMD heterostructures, our first principles calculations demonstrate widely tunable type-II band gaps, carrier effective masses and band offsets to enable an effective separation of photogenerated excitons for enhanced photodetection and photovoltaic applications. In addition, we show the possibilities of achieving a type-I band alignment for recombination based light emitters as well as a type-III configuration for tunnelling devices. Further, we evaluate the effect of strain on the electronic properties of the heterostructures to show a significant strain tolerance, making them prospective candidates in flexible photosensors.
Two-dimensional group-VI transition metal dichalcogenide semiconductors, such as MoS2, WSe2 and others, exhibit strong light-matter coupling and possess direct band gaps in the infrared and visible spectral regimes, making them potentially interesting candidates for various applications in optics and optoelectronics. Here, we review their optical and optoelectronic properties with emphasis on exciton physics and devices. As excitons are tightly bound in these materials and dominate the optical response even at room-temperature, their properties are examined in depth in the first part of this a…
One of the most fundamental devices for electronics and optoelectronics is the PN junction, which provides the functional element of diodes, bipolar transistors, photodetectors, LEDs, and solar cells, among many other devices. In conventional PN junctions, the adjacent p- and n-type regions of a semiconductor are formed by chemical doping. Materials with ambipolar conductance, however, allow for PN junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here we report ambipolar monolayer WSe2 devices in …
Despite showing great promise for optoelectronics, the commercialization of halide perovskite nanostructure-based devices is hampered by inefficient electrical excitation and strong exciton binding energies. While transport of excitons in an energy-tailored system via Förster resonance energy transfer (FRET) could be an efficient alternative, halide ion migration makes the realization of cascaded structures difficult. Here, we show how these could be obtained by exploiting the pronounced quantum confinement effect in two dimensional CsPbBr3 based nanoplatelets (NPls). In thin films of NPls of …
The two dimensional layered transition metal dichalcogenides provide new opportunities in future valley based information processing and also provide ideal platform to study excitonic effects. At the center of various device physics toward their possible electronic and optoelectronic applications is understanding the dynamical evolution of various many particle electronic states, especially exciton which dominates the optoelectronic response of TMDs, under the novel context of valley degree of freedom. Here, we provide a brief review of experimental advances in using helicity resolved ultrafas…
Phonon-Bottleneck-Governed Ultrafast Hot-Carrier Super-Diffusion in Transition Metal Dichalcogenides
Key Laboratory of Quantum Materials and Devices of Ministry of Education, School of Physics, Southeast University（东南大学物理学院教育部量子材料与器件重点实验室）；School of Electronic Science and Engineering, Southeast University（东南大学电子科学与工程学院）
Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS$_2$, are promising candidates for nanoscale photonics because of strong-light matter interactions. However, Fermi level pinning due to metal-induced gap (MIGS) states at the metals-monolayer MoS$_2$ interface limits the application of optoelectronic devices based on conventional metals because of the high contact resistance of the Schottky contacts. On the other hand, a semimetal-TMD-semimetal device can overcome this limitation, where the MIGS are sufficiently suppressed and can result in ohmic contacts.…
With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Towards that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics o…
Tuning exciton emission via ferroelectric polarization at a heterogeneous interface between a monolayer transition metal dichalcogenide and a perovskite oxide membrane
Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and optoelectronic applications in the nanoscale, due to their outstanding electrical and optical properties, differing from their bulk state. Currently, 2D perovskite belonging to this group of nanomaterials is widely studied for a wide range of optoelectronic applications. Thanks to their excitonic properties, 2D perovskites are also promising materials for photonics and nonlinear devices working at room temperature. Nevertheless, strong excitonic effects can reduce the photoc…
Density functional theory calculations are used to show that it is possible to dope semiconducting transition metal dichalcogenides (TMD) such as MoS$_2$ and WS$_2$ with electrons and/or holes either by chemical substitution or by adsorption on the sulfur layer. Notably, the activation energies of Lithium and Phosphorus, a shallow donor and a shallow acceptor, respectively, are smaller than 0.1 eV. Substitutional halogens are also proposed as alternative donors adequate for different temperature regimes. All dopants proposed result in very little lattice relaxation and, hence, are expected to …
We theoretically investigate the chiral topological excitons emerging in the monolayer transition metal dichalcogenides, where a bulk energy gap of valley excitons is opened up by a position dependent external magnetic field. We find two emerging chiral topological nontrivial excitons states, which exactly connects to the bulk topological properties, i.e., Chern number =2. The dependences of the spectrum of the chiral topological excitons on the width of the magnetic field domain wall as well as the magnetic filed strength are numerically revealed. The chiral topological valley excitons are no…
Outstanding optoelectronic properties and a facile synthesis render halide perovskite nanocrystals (NCs) a promising material for nanostructure-based devices. However, the commercialization is hindered mainly by the lack of NC stability under ambient conditions and inefficient charge carrier injection. Here, we investigate solutions to both problems, employing methylammonium lead bromide (MAPbBr_3) NCs encapsulated in diblock copolymer core-shell micelles of tunable size. We confirm that the shell does not prohibit energy transfer, as FRET efficiencies between these NCs and 2D CsPbBr_3 nanopla…
We demonstrate near unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (< 15 nm) Van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of th…
Excitation localization involving dynamic nanoscale distortions is a central aspect of photocatalysis, quantum materials and molecular optoelectronics. Experimental characterization of such distortions requires techniques sensitive to the formation of point-defect-like local structural rearrangements in real time. Here, we visualize excitation-induced strain fields in a prototypical member of the lead halide perovskites via femtosecond resolution diffuse x-ray scattering measurements. This enables momentum-resolved phonon spectroscopy of the locally-distorted structure and reveals radially-exp…
Metal-halide perovskite nanocrystals have demonstrated excellent optoelectronic properties for light-emitting applications. Isovalent doping with various metals (M2+) can be used to tailor and enhance their light emission. Although crucial to maximize performance, an understanding of the universal working mechanism for such doping is still missing. Here, we directly compare the optical properties of nanocrystals containing the most commonly employed dopants, fabricated under identical synthesis conditions. We show for the first time unambiguously and supported by first principles calculations …
Tunneling light-emitting devices (LEDs) based on transition metal dichalcogenides (TMDs) and other 2D materials are a new platform for on-chip optoelectronic integration. Some of the physical processes underlying this LED architecture are not fully understood, especially the emission at photon energies higher than the applied electrostatic potential, so-called overbias emission. Here we report overbias emission for potentials that are near half of the optical bandgap energy in TMD-based tunneling LEDs. We show that this emission is not thermal in nature, but consistent with exciton generation …
The ultimate limitations on carrier mobilities in metal dichalcogenides, and the dynamics associated with carrier relaxation, are unclear. We present measurements of the frequency-dependent conductivity of multilayer dichalcogenide MoS2 by optical-pump terahertz-probe spectroscopy. We find mobilities in this material approaching 4200 cm2/Vs at low temperatures. The temperature dependence of scattering indicates that the mobility, an order of magnitude larger than previously reported for MoS2, is intrinsically limited by acoustic phonon scattering at THz frequencies. Our measurements of carrier…
Recent observations indicated that the photoelectric conversion properties of perovskite materials are intimately related to the presence of superlattice structures and other unusual nanoscale features in them. The low dimensional or mixed dimensional halide perovskite family are found to be more efficient materials for device application compared to 3-dimensional halide perovskites. The emergence of perovskite solar cell has revolutionized the solar cell industry because of their flexible architecture and rapidly increased efficiency. Tuning the dielectric constant, charge separation are the …
Hybrid organic-inorganic perovskites have drawn significant interest for applications in optoelectronics over the last few years. Despite rapid progress in understanding the photophysics of perovskite, there remains a need for improved understanding of the effect of microstructure on perovskite photophysical processes. Here, we combine unsupervised machine learning and cathodoluminescence microscopy of a prototypical hybrid perovskite film to decode photophysical processes that are otherwise lost with conventional Gaussian image processing. Hyperspectral maps are decoded with non-negative matr…
Intense light-matter interactions and unique structural and electrical properties make Van der Waals heterostructures composed by Graphene (Gr) and monolayer transition metal dichalcogenides (TMD) promising building blocks for tunnelling transistors, flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and quantum light emitting devices (QLEDs), bright and narrow-line emitters using minimal amounts of active absorber material. The performance of such devices is critically ruled by interlayer interactions which are still poorly understood in many resp…
