UC Berkeley

Tunable properties are essential for a broad range of applications. In photonics systems, modulation of medium and light source plays a fundamental role for transmitting and processing information. For electronics industry, the manipulation of electron spin and magnetization leads to spintronics, currently a major driving force for high density data storage. And the gated ion transport is a critical component for the neuron systems. Often, it is much easier to manipulate a surface than a volume. Thus, two dimensional (2D) materials and systems, such as monolayer or few-layer crystals of van der Waals (vdW) materials and epitaxial-grown quantum wells, are intrinsically advantageous in pursuing tunability. This dissertation presents my work in engineering the tunable optic, electric, magnetic and mechanic properties of 2D materials and systems for various applications.

The first part of the dissertation presents our progress in building high performance optical modulators based on the broadband optical absorption of graphene tunable by carrier density. We achieved a record-high operation speed of 35 GHz with novel optoelectronic design enabled by amorphous silicon waveguide. The device performance is further boosted by a graphene-centered modulator structure. The waveguide mode features a large overlap with graphene corresponding to an absorption coefficient ~ 0.4 dB/m from numerical simulation. Experimentally, we achieved a modulation depth of ~ 5 dB in a 30 m – long device, which is more than twice increased from the previous designs. Our result brings new possibilities for the realization of compact ultrafast wide-band optical modulators. While the carrier density changes the light matter interaction through the Pauli exclusion principle, an external electric field directly modify the electronic levels through Stark effect. In the next part of the dissertation, I report a giant Stark shift of more than 30 meV on the single photons emitted from color centers in hexagonal boron nitride, a vdW layered wide bandgap insulator. We achieved well-shaped nanoscale electrode precisely aligned to the color centers, which allows unprecedented in-plane electric field on the order of 0.1 V/nm to be applied to the single photon emitter, critical for the observation of the giant shift. Furthermore, by controlling the orientation of the electric field, we observed an angle-resolved Stark effect, which shines light on broken inversion symmetry in the color center emitter. The giant room temperature Stark shift and the crucial information on the structural symmetry of the color center may represent an important step towards scalable room-temperature quantum communication and computation system based on vdW materials and their heterostructures. To manipulate the topological property of band structure, magnetic field can be introduced to modify the Hamiltonian of the system. The third part of the dissertation describes a high-frequency kink magnetoplasmons (KMPs) resonating along a topological edge in a 2D electron gas (2DEG) hosted in an AlGaAs/GaAs heterojunction. A magnetic field with a flip of the orientation was applied to a 2DEG to generate a spatial change in the topology of band structure. The experimentally measured field- and electron-concentration-dependent resonant frequency perfectly matches theoretical prediction, which indicates strong potential of using magnetic domain boundaries to develop reconfigurable integrated topological circuits. In the fourth part of the dissertation, I describe the realization of such a magnetic domain boundary in a vdW layered ferromagnetic metal Fe2GeTe3 using its thickness dependent magnetic coercivity. From magneto-transport measurement, we discovered an antisymmetric magnetoresistance and kink Hall resistance as the evidence of the formation of magnetic domain boundary. The theoretical calculation based on our experimental measurement unveiled a perturbation current circulating around thickness step and mediating the electric potential change. Furthermore, such a circulating current is reverted at elevated temperature, which is consistent with the cross-over of magnetic coercivity on samples of different thicknesses. Those findings may lead to novel spintronics and topological electronics devices from vdW materials and their heterostructures. The last part of the dissertation extends the tunability of 2D materials into mechanics and presents the experimental study on tunable ion transport through the interlayer spacing in reduced graphene oxide (rGO) nanoflakes. With a gate voltage applied between the solution and the rGO flake, the ion permeation was considerably modulated, which can be explained by changes of the dehydration process and ion density inside the channel. Strikingly, we found an ultrafast ion diffusion at large gate voltages, with an effective diffusion coefficient orders higher than the value in bulk water. Our results demonstrate the interlayer spacing in rGO as an appealing platform for study the solid-ion and ion-ion interactions, as well as new material phase and chemical reaction in a tightly confined 2D space.