Controlling Valley Polarization in 2D Heterostructures

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Electronic information processing devices are currently based on controlling electron charge. However, traditional electronic device architectures are reaching fundamental energy loss and miniaturization limits, which motivate the investigation of transformative nanoscale devices. Electrons in a class of atomically thin two-dimensional semiconductors offer an electronic valley degree of freedom that can be used to encode and potentially process information. Electrons in these materials, specifically monolayer transition metal dichalcogenides, can occupy one of two different electronic valleys, which can be understood as distinguishable pockets in the electronic band structure. Because there are two valleys, they can be used to encode digital information as 1's and 0's. Importantly, excitation of these valleys creates neutral particle species that inherently incur lower energy losses than charged particles. In this project, the PIs will develop optical and electronic devices with the goal of controlling the valley degree of freedom for computing. They will design and fabricate novel electronic devices, which comprise disparate layers of ultra-thin materials, i.e. inorganic and hybrid inorganic-organic devices, which will allow for new valley dependent functionalities. Specifically, they will explore how electronic, magnetic and chemical interactions can be used to engineer spatial transport of charge neutral electronic states carrying valley information. The goal of the project is to understand how to control electronic valley information and to demonstrate valley-based logic gates. The project will advance the field of nanotechnology by demonstrating how valleys can be used in information processing technologies. The project develops new knowledge and trains a new generation of scientists in ultra-small optical and electronic devices that realize the fundamental small-sized limit (atomically thin), and could potentially enable low energy consumption devices that utilize valley instead of charge to encode information.This project focuses on the development of valley-based devices composed of two-dimensional heterostructures. Electrons in monolayer transition metal dichalcogenide (TMD) semiconductors (i.e. WSe2, MoSe2, etc.) can occupy one of two momentum space valleys (+K and +K), which can be used as a binary degree of freedom to encode and potentially process information. In analogy with spintronics, this project seeks to explore device architectures that will leverage the control of valley polarizations for applications to low energy consumption valleytronic information processing. Specifically, heterostructures that host charge-neutral interlayer excitons and can enable micron-scale spatial transport of valley polarized carriers will be explored. These structures will be developed into prototype and transformative valleytronic logic gates that are based on nonlinear valley-dependent interaction effects, and rely on optical injection and readout. The project has three specific aims: 1) controlling pure valley currents in MoSe2/WSe2 devices with interlayer excitons and field-effect structures; 2) controlling valley and ferromagnetic polarizations in ferromagnetic/TMD heterostructures using interfacial exchange-interactions; and 3) developing interlayer valley excitons in TMD/organic semiconductor heterostructures to enable a tunable interlayer exciton system, optical spin-injection, and chemically breaking of the TMD valley degeneracy. The PI and co-PI will utilize a combination of 2D material fabrication, organic semiconductor deposition and electron beam lithography techniques to fabricate the proposed devices. TMD/TMD and ferromagnetic/TMD devices will be investigated using spatially resolved micro-photoluminescence and nonlinear Kerr spectroscopies to probe the exciton energies and valley polarizations. The TMD/organic heterostructures will be characterized by angle-resolved photoemission and advanced x-ray spectroscopies. Valley polarized spatial transport effects will be read out optically, and will be controlled by applying gate voltages in field effect heterostructure devices, applying external magnetic fields, and patterning organic adsorbates. The project will develop novel inorganic and hybrid organic/inorganic devices that have potential to enable transformative technologies based on the electronic valley degree of freedom.