UC Berkeley

Laser with selective and sharp range of photon energies has offered considerable advantages in site-selective, on-demand and fast modifications in materials properties and structures. In the presence of gas molecules, the laser excitation results in combined photo-thermo-chemical processes, which enable doping, synthesis, crystal growth, deposition, and etching. Laser also offers substantial potential for non-invasive, vacuum-free, sensitive, and high-resolution optical probing of various physicochemical phenomena. Integrated with tip-based scanning near-field optical microscopy (SNOM), nanoscale optical diagnostics can be possible by overcoming diffraction limit of far-field laser beam. Further, strong light coupling at the junction between the tip and the sample can induce enhanced electric field, which leads to various exotic light matter interactions. We explored all-laser chemical processing of two-dimensional (2D) van der Waals (vdW) materials including graphene and transition metal dichalcogenides (TMDCs) using our developed laser chemical processing apparatus, including doping, thinning, and synthesis of those materials. 2D vdW material was a choice of material system as it possesses highly sensitive, and delicate atomically thin structure with random shape, which demands reliable site-selective processing tools. Our study demonstrated that laser chemical processing indeed offers substantial capabilities in modifications of material properties and structures. Dual laser beam was developed to control optoelectronic properties of 2D vdW materials by chemical doping. The use of two laser beams in selected geometries, photon energies, and pulse widths can separately address the photochemical and photothermal processing routes. Atomic precision thinning of 2D vdW materials using laser induced chemical etching was studied with in-situ optical probing method. Integrated with tip-based processing, the nanoscale processing of 2D vdW materials was achieved by overcoming diffraction limit of far-field laser processing. Synthesis of 2D vdW materials was also demonstrated by laser assisted mass diffusion of transition metal and chalcogenide at the interface of thin layers consisted of those materials. Thus, the presented studies pave the way to reliable and facile modification of optoelectronic properties and structures of 2D vdW materials. We also demonstrated tip-based nanoscale diagnostics of low dimensional materials, including tip-enhanced photothermal nanoscale mapping of exciton polariton (EP) transport in TMDCs and tip-enhanced near-field second harmonic generation (SHG) imaging of semiconductor and metal nanowires. The tip-enhanced photothermal probing interrogates the thermal dissipation of evanescent electromagnetic field, which can potentially track non-radiative pathways of exciton polaritons in TMDC materials. We found that the use of near infrared wavelength renders thermal expansion of the tip large, which amplifies the electromagnetic field exerted at the junction between the tip and the sample. Next, we explored tip-enhanced visualization of SHG in low dimensional materials. We proposed a resonant energy transfer model to explain the experimentally observed nanoscale SHG contrast. Our technique was applied in in-situ imaging of corrosion process of zinc oxide nanowire, multipolar SHG resonance in silicon nanowire, and SPP mode in silver nanowire.