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Nanoscale goldbeating: Solid-state transformation of 0D and 1D gold nanoparticles to anisotropic 2D morphologies

Abstract Goldbeating is the ancient craft of thinning bulk gold (Au) into gossamer leaves. Pioneered by ancient Egyptian craftsmen, modern mechanized iterations of this technique can fabricate sheets as thin as ∼100 nm. We take inspiration from this millennia-old craft and adapt it to the nanoscale regime, using colloidally synthesized 0D/1D Au nanoparticles (AuNPs) as highly ductile and malleable nanoscopic Au ingots and subjecting them to solid-state, uniaxial compression. The applied stress induces anisotropic morphological transformation of AuNPs into 2D leaf form and elucidates insights into metal nanocrystal deformation at the extreme length scales. The induced 2D morphology is found to be dependent on the precursor 0D/1D NP morphology, size (0D nanosphere diameter and 1D nanorod diameter and length), and their on-substrate arrangement (e.g., interparticle separation and packing order) prior to compression. Overall, this versatile and generalizable solid-state compression technique enables new pathways to synthesize and investigate the anisotropic morphological transformation of arbitrary NPs and their resultant emergent phenomena.
Publications

Diffractive microoptics in porous silicon oxide by grayscale lithography

We demonstrate focusing as well as imaging using diffractive microoptics, manufactured by two-photon polymerization grayscale lithography (2GL), that have been 3D printed into porous silicon oxide. While typical doublet lens systems require support structures that hold the lenses in place, our optics are held by the porous media itself, decreasing both the fabrication time and design constraints while increasing the optically active area. Compared to the typical two-photon polymerization fabrication process, 2GL offers better shape accuracy while simultaneously increasing throughput. To showcase 2GL manufactured optics in porous media, we fabricate singlet diffractive lenses with a diameter of 500 µm and numerical apertures of up to 0.6. We measure the intensity distribution in the focal plane, and along the optical axis. Furthermore, we design and fabricate a doublet lens system for imaging purposes with a diameter of 600 µm and thinner than 60 µm. We examine the imaging performance with a USAF 1951 resolution test chart and determine the resolution to be 287 lp/mm. 3D printing in porous SiO2 thus holds great promise for future complex and unconventional microoptical solutions.
Publications

Evolution of defect formation during atomically precise desulfurization of monolayer MoS2

AbstractStructural symmetry-breaking is a key strategy to modify the physical and chemical properties of two-dimensional transition metal dichalcogenides. However, little is known about defect formation during this process. Here, with atomic-scale microscopy, we investigate the evolution of defect formation in monolayer MoS2 exposed indirectly to hydrogen plasma. At the beginning of the treatment only top-layer sulfur atoms are removed, while vacancies and the molybdenum atomic layer are maintained. As processing continues, hexagonal-shaped nanocracks are generated along the zigzag edge during relaxation of defect-induced strain. As defect density increases, both photoluminescence and conductivity of MoS2 gradually decreases. Furthermore, MoS2 showed increased friction by 50% due to defect-induced contact stiffness. Our study reveals the details of defect formation during the desulfurization of MoS2 and helps to design the symmetry-breaking transition metal dichalcogenides, which is of relevance for applications including photocatalyst for water splitting, and Janus heterostructures.
Publications

Large scale self-assembly of plasmonic nanoparticles on deformed graphene templates

AbstractHierarchical heterostructures of two-dimensional (2D) nanomaterials are versatile platforms for nanoscale optoelectronics. Further coupling of these 2D materials with plasmonic nanostructures, especially in non-close-packed morphologies, imparts new metastructural properties such as increased photosensitivity as well as spectral selectivity and range. However, the integration of plasmonic nanoparticles with 2D materials has largely been limited to lithographic patterning and/or undefined deposition of metallic structures. Here we show that colloidally synthesized zero-dimensional (0D) gold nanoparticles of various sizes can be deterministically self-assembled in highly-ordered, anisotropic, non-close-packed, multi-scale morphologies with templates designed from instability-driven, deformed 2D nanomaterials. The anisotropic plasmonic coupling of the particle arrays exhibits emergent polarization-dependent absorbance in the visible to near-IR regions. Additionally, controllable metasurface arrays of nanoparticles by functionalization with varying polymer brushes modulate the plasmonic coupling between polarization dependent and independent assemblies. This self-assembly method shows potential for bottom-up nanomanufacturing of diverse optoelectronic components and can potentially be adapted to a wide array of nanoscale 0D, 1D, and 2D materials.

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