Cole Tait
Research Mentor(s): Adam Alfrey
Mentor Department: Applied Physics
Authors: Cole Tait, Adam Alfrey, Steven Cundiff
Session: Session 1 (9:00am – 9:50am)
Presentation Type: Poster 38
Abstract
Transition-metal dichalcogenides (TMDs) are semiconductors comprised of sheets of atoms weakly held together by van der Waals forces—the same forces that help a gecko cling to vertical surfaces. Like graphene, TMDs can be thinned down to atomically-thin layers, known as monolayers, which provide an ideal platform for exploring fundamental quantum mechanics and the dynamics of quasiparticles, such as electrons and holes. Due to their innate thinness, about 0.7 nanometers, they function as natural quantum wells. Unlike graphene, which lacks a bandgap, TMDs are semiconductors with a direct bandgap in monolayer form, allowing them to be optically addressed and controlled. To study the electron doping behavior of monolayer TMDs, we fabricated heterostructures using conductive graphite, insulating hexagonal boron nitride (hBN), and a monolayer TMD. These structures form capacitors with capacitance C. Graphene act as the metal plates, hBN as the dielectric separating them, and the TMD touching one of the plates. When a bias V is applied between the two graphene plates, a charge Q=CV is moved from one plate to the other, and the contacted TMD will also receive this charge, enabling control of the amount of electrons in the quantum well with the turn of a knob. Using this setup, voltage-dependent photoluminescence (PL) experiments were conducted by employing a laser to excite valence electrons across the bandgap into the conduction band. After these photo-excited electrons slow down sufficiently within the crystal, they reach a minimum energy level in the conduction band. Here, they can encounter a vacancy, or hole, and bind to it temporarily, for about 1 picosecond in TMDs. This electron-hole pair is known as an exciton, which is quantum-mechanically equivalent to a hydrogen atom assuming the system’s center of mass is reduced and dielectric screening is considered. When the electron recombines with the hole, the exciton is annihilated, and light is emitted that we can measure. If there is an excess of electrons, one can bind to an existing exciton, forming a three-particle complex known as a trion. Similarly, in the case of electron depletion or excess holes, a positively charged trion can form. These excitonic species have distinct binding energies; upon annihilation, they emit measurable photons that differ from those of two-particle excitons, creating different peaks in the PL spectrum. Our study aims to analyze the lineshapes of trion photoluminescence as a function of electron density. We hypothesize that the lineshape of ‘free’ trions will be symmetric, whereas in-gap trions—when the Fermi energy lies in the bandgap—will exhibit an asymmetric lineshape. This is because in-gap trions essentially consist of excitons attached to an immobile impurity, while ‘free’ trions can move freely. The symmetry of these peaks depends on the level of localization, and we found that increasing temperature leads to greater localization due to lattice vibrations that scatter the ‘free’ trions.
