UC Berkeley

Two dimensional (2D) materials have unique mechanical, optical, electrical and magnetic properties, which are strong and flexible, exhibit large exciton binding energy, cover a broad spectrum of bandgaps, have intensive light matter interactions and are particularly prone to many-body instabilities etc. The diversity provides unlimited possibilities to build various devices achieving different functionalities. To further tailor the properties of 2D materials, many methods have been utilized, including reducing dimensions, intercalation, alloying, gating, strain engineering and heterostructures. Heterostructures of 2D materials have demonstrated rich physics, and wide device applications. For lateral heterostructures with covalent bonding, the formation of misfit dislocations could be a big concern, which may dramatically affect the properties. In vertical heterostructures of 2D/2D and 2D/other dimensions, weak van der Waals interaction enables much wider material choices and functionalities.

This dissertation primarily consists of two projects related to 2D heterostructures. The 1st project is focusing on a large lattice mismatched (~6%) lateral Bi2Se3-Bi2Te3 heterostructure with periodic rippling. Taking advantage of the large lattice mismatch between the constituents, we demonstrate a 3D heterogeneous architecture combining a basal Bi2Se3 nanoplate and wavelike Bi2Te3 edges buckling up and down forming periodic ripples. Unlike 2D heterostructures directly grown on substrates, the solution-based synthesis allows the heterostructures to be free from substrate influence during the formation process. The balance between bending and in-plane strain energies gives rise to controllable rippling of the material. Our experimental results show clear evidence that the wavelengths and amplitudes of the ripples are dependent on both the widths and thicknesses of the rippled material, matching well with continuum mechanics analysis. The rippled Bi2Se3/Bi2Te3 heterojunction demonstrates a unique way to release the strain energy from lattice mismatch through out-of-plane deformation, and also provides a platform to enable nanoscale structure generation and associated photonic/electronic properties manipulation for optoelectronic and electro-mechanic applications.

The 2nd project is related to an ultracompact far-infrared (far-IR) spectrometer based on vertical hybrid heterostructure of InAs, h-BN and graphene. The principle of this ultracompact spectrometer relies on a new surface plasmon polaritons (SPPs) coupling mechanism on the non-structured InAs surface with gradient carrier concentration. When free space light illuminates on the InAs surface with a gradient of negative permittivity, SPPs can be excited and propagates along the planar interface. For every single wavelength, there would be a corresponding location, where ε_m=-ε_d and the surface wave vector reaches its maximum point, and both the group velocity and phase velocity approach zero. As a result, the electric field magnitude is strongly enhanced and localized at the nanoscale. With h-BN and graphene as dielectric spacer and photodetection media, the localized near-field enhancement from the gradient doped surface can be transformed into photocurrent generation, enabling applications such as photodetectors and wavelength sorting spectrometers. We have evaluated the performance of such spectrometer computationally, designed and fabricated the device from scratches, and for measurement, freestanding cross-shaped band pass filters are also designed and made. The ultracompact spectrometer demonstrates a direct application of the unique light coupling theory for rainbow trapping and enables new designs of plasmonic devices. More importantly, the compact spectrometer works within far-IR range without the need for complicated optical components and active cooling, which could be applied in wearable devices and smart phones.