The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures and making outstanding progress on fundamental studies, practical applications of 2D materials will require a broader integration strategy. I will present our ongoing and recent work on integration of 2D materials with 3D electronic materials to realize logic switches and memory devices with novel functionality that can potentially augment the performance and functionality of Silicon technology. First, I will present our recent work on gate-tunable diode and tunnel junction devices based on integration of 2D chalcogenides with Si and GaN. Following this I will present our recent work on non-volatile memories based on Ferroelectric Field Effect Transistors (FE-FETs) made using a heterostructure of MoS2/AlScN, and also introduce our work on Ferroelectric Diode (FeD) devices also based on thin AlScN. In addition, I will also present how FeDs provide a unique advantage in compute-in-memory (CIM) architectures for efficient storage, search as well as hardware implementation of neural networks.
I will conclude by providing a broad and optimistic outlook for integration of novel materials and devices in future classical computing chips.
I will focus on the subject of strong light-matter coupling in excitonic 2D and 1D semiconductors. I will then discuss opportunities for light conversion and modulation using superlattices and metastructures made using excitonic materials and also magnetic semiconductors if time permits.
Interface effects in metals-semiconductors heterojunctions are subject of intense research due to the possibility to exploit the synergy between their electronic and optical properties in next-generation opto-electronic devices. In this framework, understanding the carrier dynamics at the metal-semiconductor interface, as well as achieving a coherent control of charge and energy transfer in metal-semiconductor heterostructures, are crucial and yet quite unexplored aspects. Here, we experimentally show that thermionically injected carriers from a gold substrate can drastically affect the dynamics of excited carriers in bulk WS2. By employing a pump-push-probe scheme, where a push pulse excites direct transitions in the WS2, and another delayed pump pulse induces thermionic injection of carriers from the gold substrate into the semiconductor, we can control both the formation and annihilation of excitons. Our findings might foster the development of novel opto-electronic approaches to control charge dynamics using light at ultrafast timescales.
In this talk I will cover our recent results tunable and strong light-matter interactions in van der Waals 2D and other low-dimensional semiconductors. I will first present our results on hybridized exciton-plasmon-polaritons in nanopatterned gratings of WS2. Following that I will present light-trapping and self-hybridized exciton-polariton formation in large area superlattices of WS2 and boron nitride. I will extend the results of self-hybridization to halide perovskites showing exciton-polariton emission at room temperatures and finally I will show the effects of cavity coupling and anti-ferromagnetic phase transition on tunable linear dichroism in van der Waals FePS3 semiconductor.
This talk will discuss our recent work on optical phenomena and electronic phenomena in 2D semiconductors when they are placed in close proximity on plasmonic substrates such as Au, Ag and Al. I will also show how using a plasmonic tip once can use near-field tip based micro-spectroscopy to probe defects, interfaces, strain and hybrid states in 2D excitonic semiconductors and junctions. If time permits, I will also present how 1D nanostructuring of MoS2, WS2, WSe2 etc. into nanophotonic dielectric gratings can enable exploration of new regimes of light-matter confinement including formation of hybrid exciton-plasmon-polariton states. I will extend this concept to 2D hybrid perovskites and superlattices.
This talk will discuss recent work on optical phenomena and photonic devices made from 2D chalcogenides (e.g., halide perovskites and carbon nanotubes) as well as 1D excitonic semiconductors. I will present how 1D nanostructuring of excitonic 2D semiconductors into nanophotonic dielectric gratings can enable exploration of new regimes of light-matter confinement including formation of hybrid exciton-plasmon-polariton states. This discussion will be extended to superlattices which are scalable over large areas. Further we will show that the light-matter hybridization persists in emission of direct gap 2D semiconductors such as hybrid halide pervoskites. Finally I will present our recent work on nanotubes and show dynamic tunability of their optical properties. I will conclude by giving a broad perspective on future prospects of 2D materials from fundamental science to applications.
Two-Dimensional (2D) chalcogenides of Mo and W are semiconductors that show strong excitonic responses due to their highly quantum-confined character. This talk will discuss our recent work on optical phenomena and electronic phenomena in 2D semiconductors when they are placed in close proximity on plasmonic substrates such as Au, Ag and Al. I will present how 1D nanostructuring of Mos2, WS2, WSe2 etc. into nanophotonic dielectric gratings can enable exploration of new regimes of light-matter confinement including formation of hybrid exciton-plasmon-polariton states. I will extend this concept to 2D hybrid pervoskites and superlattices. Finally, I will also show how using a plasmonic tip once can use near-field tip based micro-spectroscopy to probe defects, interfaces and hybrid states in 2D excitonic semiconductors
Efficient doping of 2D materials, including carrier type, concentration and mobility, is challenging but essential for enabling their future electronic and photonic applications. We are developing substitutional n- and p- doping of InSe semiconductor by introducing Sn and Zn, respectively, in the Bridgman bulk crystal growth. Electrical transport properties of undoped vs. n- and p- doped InSe crystals are compared by conducting Hall measurements on bulk crystals and FET transport measurements on exfoliated thin layers. Undoped InSe is intrinsically n-type in both bulk and thin-film forms, with [n]~3.5E14 cm-3 and mu values of up to 1,400 cm2 V-1 s-1 for thick layers at 300K. Carrier concentration in Sn-doped thick layers increases approximately two-fold, while the corresponding mobility reduces ~2 times at 300 K. Zn-doped InSe shows p- behavior for bulk InSe with [p]~7.9E13 cm-3 and mu~43 cm2 V-1 s-1 at 300 K, which reverts to ambipolar/n- type behavior for thin layers in FET devices.
Metal-semiconductor contacts have been the subject of intense investigation and study for several decades. With the advent of atomically-thin semiconductors new opportunities have emerged in investigating these mysterious yet, critical buried interfaces. In this talk we will show our recent advances in probing the nature of these contacts by a variety of scanning probe techniques. The primary model system will be that a bulk noble metal such as gold or silver and a 2D chalcogenides semiconductor such a molybdenum disulfide (MoS2). We will discuss impact of contact type and evaporation technique on the electrical and optical properties of the junction and suggest ways to make idealized contacts. We will then extend our analysis to looking at the same interface from an optical perspective and investigating hybrid states of excitons and plasmons observed via near-field photoluminescence micro-spectroscopy.
KEYWORDS: Imaging spectroscopy, Spectroscopy, Heterojunctions, Near field scanning optical microscopy, Near field, Scanning probe microscopy, Near field optics, Semiconductors, Transition metals, Molybdenum
In this talk we will present the use of near-field scanning probe microscopy and spectroscopy to investigate the electronic and optical quality of excitonic semiconductors. We will use two-dimensional (2D) transition metal dichalcogenides (TMDCs) of Mo and W as prototypical examples but extend our measurements to other low-dimensional excitonic systems including colloidal quantum dots, organic assemblies and layered hybrid perovskites. Via near-field photoluminescence spectroscopy we will show the nanoscale variations in quality of the contact with substrates and disorder at the interface in case of junctions or heterostructures. By placing a plasmonic metal substrate nearby and varying the distance, we will also show exciton hybridization with surface plasmons into propagating hybrid surface modes.
In this talk, we will focus on the subject of strong light-matter coupling in excitonic 2D semiconductors. We will present our recent work on the fundamental physics of light trapping in multi-layer TMDCs when coupled to plasmonic substrates. We systematically demonstrate via calculations and matching experiments that the presence of strong excitonic resonances in multilayers (< 20 nm thickness) combined with surface plasmon excitations of the nearby metals can achieve strongly coupled modes with apparent voided crossings in reflectance spectra. Further, we explore additional light confinement by patterning 1D arrays of rectangular resonators of varying widths and periods (100 nm to 500 nm) showing three mode couplings. We will further present extensions of our studies to resonators with dielectric spaces and optical superlattices in 1D.
I will present our recent works on confining visible frequency photons in heterostructures for plasmonic metals and excitonic transition metal dichalcogenides (TMDCs) of Mo and W. Evidence of strongly coupling between excitonic modes and Fabry-Perot like resonances will be presented in unpatterned case. When the TMDC layer is patterned, plasmonic and dielectric grating modes emerge which lead to further coupling with excitonic modes resulting in tunable strong coupling and light confiementment. Finally, I will extend this notion to monolayer TMDCs and show evidence of near-unity absorption in metamaterials of the same for applications ranging from optical modulators to photodetectors and photovoltaics.
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