UC Berkeley

The coupling of mechanical, electrical and optical properties of materials becomes more prominent when the dimensionality is reduced to two, such that the external manipulation from the higher dimension can directly apply to the interior, and the Coulomb interaction is greatly enhanced. Among the existing two-dimensional systems, the Group VIB transition metal dichalcogenides (TMDC) are semiconductors with direct band gap. They are potential solutions for field-effect transistors with 5 nanometer node and beyond, as well as broadband nano-optoelectronics. In addition, their broken inversion symmetry and spin-orbit coupling together give rise to a pair of nonequivalent valleys as energy-efficient information carriers.

This dissertation first presents the quasi-static measurement of piezoelectric effect, i.e. the generation of strain through electric field, in ultra-thin freestanding molybdenum disulfide (MoS₂) films by atomic force microscopy. Such van der Waals layered materials overcame the thermodynamic instability when the thickness of traditional bulk materials approaches single-molecular scale. The ambient piezoelectric coefficient of a single layer, e₁₁ = 2.9×10^-10 C/m, represented the intrinsic symmetry breaking and was free from substrate effects. The dependence of piezoelectric response on number of layers and angle of applied electric field agreed with the crystalline symmetry. In complimentary to the natural in-plane piezoelectricity, the single-layer TMDC was chemically engineered to possess out-of-plane piezoelectricity by selectively replacing the chalcogen atoms on one side. The measured piezoelectric coefficient of d₃₃ = 0.1 pm/V could be increased further by varying the substitute atoms. These discoveries are applicable to low-power logic switches for computing and electromechanical sensors at molecular level.

At higher frequency, the electromechanical coupling is replaced by phonon-photon interaction, such as Raman scattering and infrared absorption. We characterized the coupling between quasi-static strain and the optical phonon vibration as a result of mechanical nonlinearity in MoTe₂ by Raman spectroscopy. Small Grüneisen parameters of 0.45 and 0.25 were found for in-plane and out-of-plane optical phonon respectively under in-plane strain. More importantly, the hexagonal lattice enabled unique chiral phonons with intrinsic angular momentum and magnetic moment, that are capable of transporting electrons between valleys. We estimated the phonon energy from resonant Raman scattering, which came from both defect-activation and double-resonance process, and observed the phonon-assisted intervalence band

transition by transient infrared spectroscopy. Infrared spectrum showed distinctive steps from virtual electronic bands projected by the absorption and emission of the valley phonon, in accordance to our theoretical model. The investigation of the phononic chirality and angular momentum conservation by probing the circular polarization of the absorption is ongoing. The valley phonon has potential in information processing and ultrafast optoelectronic modulation.