Excitonic physics in two-dimensional semiconductors

Thumbnail Image
Wang, Tianmeng
Issue Date
Electronic thesis
Chemical engineering
Research Projects
Organizational Units
Journal Issue
Alternative Title
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) represent a new class of atomically thin semiconductors with superior optical and optoelectronic properties. TMDCs have extensively been investigated for potential applications in valleytronics, field-effect transistors, logic circuits, phototransistors, photodetectors, quantum information, and quantum computing. With the reduced dimension in one direction, 2D TMDCs show strong Coulomb interactions compared with bulk materials. The enhanced electron-electron interaction enables a new platform to study excitonic fine structures of different quasi-particles. Moreover, monolayer TMDCs possess a valley degree of freedom due to lack of spatial inversion symmetry, giving rise to applications like valleytronics. The strong spin-orbital coupling (SOC) results in spin-valley locking, leading to unique optical and electronic properties under the magnetic field. Besides, the angular controlled stacking of 2D TMDCs opens a new era for manipulating the electron-electron interactions in the artificial 2D structures, introducing a universal platform to study correlated states like Mott insulators and generalized Wigner crystals. A fundamental understanding of the excitonic states in TMDCs and their artificial structures is crucial for both fundamental physics studies and potential applications in the future.First, the excitonic fine structures due to strong electron-electron Coulomb interaction are investigated with helicity resolved photoluminescence (PL), reflectance and photocurrent spectroscopy under a high magnetic field in the hexagonal boron nitride (hBN) encapsulated monolayer tungsten diselenide (WSe2). The true biexciton state is identified in charge neutral WSe2 through the control of efficient electrostatic gating, indicating a strong Coulomb interaction in the 2D material. The new dark trion states are unveiled with magneto photoluminescence (PL) and back focal plane imaging technique. The helicity-resolved magneto-photocurrent spectroscopy is introduced to study the excited states of neutral exciton in which the excited exciton states are observed to 11s, the highest excited state of exciton ever reported for any 2D semiconductor. Besides, exciton fine features and exciton-polaron are observed under a high magnetic field. The quantized excitonic resonance using the optical method is proof of strong many-body interactions in the 2D system. Second, the phonon-exciton interactions in monolayer WSe2 are explored. A phonon-assisted circularly polarized replica has been discovered by magneto PL, where the spin-forbidden dark exciton is brightened by phonons. The exciton-phonon replica inherits large magneto-tunability and a long lifetime from dark exciton. The replica has an efficient radiative recombination channel, providing a chirality dictated emission channel for both phonons and photons. In addition, the momentum-dark intervalley exciton is observed through the interaction between intervalley exciton and chiral phonon. The pseudo angular momentum (PAM) of the chiral phonon is shown to play an important role in determining the helicity of the emitted photon through phonon-exciton interaction. The phonon-exciton interaction in a high magnetic field also shows new excitonic states, including the phonon replica of the dark trions. The inter-LL transition selection rule of dark trions is modified by the exciton-phonon interaction. Finally, the excitonic physics of hetero-bilayers of TMDCs is studied. An electrical switchable effect, transferring between exciton dissociation and funneling, is observed in MoSe2/WS2 heterostructures by using a highly efficient ionic back gate substrate. In WSe2/MoSe2 heterostructures with magneto-PL, a near-unity valley polarization of the interlayer exciton is observed, inspiring future exploration of applications in valleytronics and spintronics. In angle aligned WSe2/WS2 heterostructures, a series of correlated insulating states at integer fillings and a series of fractional fillings are investigated with scanning microwave impedance microscopy (MIM) and PL spectra of interlay exciton which interacts with correlated insulating states.
August 2021
School of Engineering
Full Citation
Rensselaer Polytechnic Institute, Troy, NY
PubMed ID