ABSTRACT OF THE DISSERTATION

Strongly Interacting Electrons and Holes in Ultrathin MoTe2 Heterostructures

by

Dennis Pleskot

Doctor of Philosophy, Graduate Program in Materials Science and Engineering

University of California, Riverside, March 2019

Dr. Nathaniel Gabor, Chairperson

Heterostructures composed of atomic layer materials (ALMs) bonded through van der Waals (vdW) interactions have demonstrated great potential for use in next generation optoelectronic devices. However, the role of strong charge carrier interactions in the interlayer photocurrent in such devices is not fully understood. One such ALM, molybdenum ditelluride (MoTe2), has a band gap energy near 1.0 eV, suggesting that MoTe2 is a promising material for ultrasensitive infrared optical photodetectors and next-generation solar photocells. Like other ALMs, the photoresponse in MoTe2 should give experimental access to strong many-body phenomena. Here, we report on the advanced fabrication and dynamic photoresponse of graphene-MoTe2-graphene vdW heterostructure photocells. We find that the power dependence of the interlayer photocurrent is well described by a single power law at low power, where the power law exponent parameterizes the nonlinearity of the photoresponse. We develop a detailed 2-particle Auger recombination model that accounts for careful time integration of the dynamics, resulting in an analytic solution that reproduces the non-linear power dependence. Additionally, we observed sharp suppression of conventional power law behavior above a critical power threshold, accompanied with a critical phase transition in the spatial expansion of the suppressed photocurrent region. We attribute this transition to the emergence of an electron-hole liquid phase that is remarkably stable at room temperature and moderate operating conditions. Additional measurements of the dynamic photoresponse support the existence of this phase, allowing us to fully explore the evolution of electron-hole pairs with increasing density in MoTe2.

Nanoscale fabrication methods are generating ever smaller and more precise devices that exhibit exciting physics but also require intense care to measure accurately. To address this need, we develop a modular optical microscope capable of rapidly measuring multiple parameters and generating datasets consisting of dozens of dimensions. We use this microscope to measure three very different physical systems. In the first, we spatially map excitation-emission characteristics of the laser medium neodymium aluminum nitride. We next measure interlayer charge transport in a van der Waals layered heterostructure consisting of monolayer MoSe2 and bilayer WSe2. Our experiments show carrier multiplication as current is driven between materials and we develop a charge-transport model that explains the responsible mechanism and replicates the observed temperature dependence. Finally, in a platinum-yttrium iron garnet device, we use the interaction between the spin-Seebeck effect and the Shockley-Ramo theorem to spatially map electric field lines in arbitrary device geometries.

Physical systems driven out of thermal equilibrium may exhibit complex behavior before eventually relaxing to thermal equilibrium. The prototypical non-equilibrium state in quantum materials is the photoexcited electron, where an electron is imparted excess energy and may exhibit a variety of interesting phenomena before eventually recombining with a hole. In nanoscale systems, quantum confinement makes the behavior of non-equilibrium electrons more complex and experimentally accessible. To measure non-equilibrium dynamics, we develop a technique for data-intensively imaging photoresponse that efficiently samples phenomenological parameter space. The result is a large set of images which we condense down to dynamical parameters that can be visualized and physically interpreted. Using this data intensive methodology, we explore the non-equilibrium physics of photoexcited electrons on multiple scales. Firstly, on the microscopic scale we study the interactions of excitons and electron-phonon coupling in TMD heterostructures. Then, zooming out to the mesoscopic scale, we observe an electron-hole liquid phase in MoTe2 and hot carrier regime in graphene heterostructures. Finally, at the statistical scale we explore how a simple model for quieting a noisy antenna can decrease noise in the non-equilibrium states that power photosynthesis. In sum, we demonstrate that data intensive imaging is a powerful and versatile tool for exploring non-equilibrium dynamics of quantum materials and biophysical systems.

We stack-engineer an optically gated thermionic valve which acts as a thermometer for hot electronic charge carriers. Through pulsed photoexcitation of encapsulated graphene-boron nitride-graphene heterostructure devices, we trigger picosecond charge carrier transit through an ultrathin BN barrier. The resultant interlayer photoconductance exhibits extraordinary enhancement near the charge neutrality (Dirac) point of graphene. We attribute this to ultrafast thermionic cascade arising from an elevated electronic temperature from the rapidly thermalizing electrons and holes. The electronic temperature was found to peak at the Dirac point and reaches well above 2000 K. This Dirac point thermionic cascade can also be dramatically quenched via electrical control at low temperatures, which indicates unconventional-yet highly efficient- cooling pathways that serve as a hallmark of Dirac electron-hole plasma in graphene.