Publications

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We report on the fabrication and characterization of nanocrystalline ZnO films for use as a random laser physical unclonable function (PUF). Correlation between processing conditions and film microstructure will be made to optimize the lasing properties and random response. We will specifically examine the repeatability and security of PUFs demonstrated in this novel 3 system. This demonstration has promise to impact many of Sandia's core missions including counterfeit detection. 4 4

Charge-transfer materials based on the self-assembly of aromatic donor–acceptor complexes enable a modular organic-synthetic approach to develop and fine-tune electronic and optical properties, and thus these material systems stand to impact a wide range of technologies. Through laser-induction of temperature gradients, in this study, user-defined patterning of strongly dichroic and piezoelectric organic thin films composed of donor–acceptor columnar liquid crystals is shown. Fine, reversible control over isotropic versus anisotropic regions in thin films is demonstrated, enabling noncontact writing/rewriting of micropolarizers, bar codes, and charge-transfer based devices.

In controlling the thermal properties of the surrounding environment, we provide insight into the underlying mechanisms driving the widely used laser direct write method for additive manufacturing. We find that the onset of silver nitrate reduction for the formation of direct write structures directly corresponds to the calculated steady-state temperature rises associated with both continuous wave and high-repetition rate, ultrafast pulsed laser systems. Furthermore, varying the geometry of the heat affected zone, which is controllable based on in-plane thermal diffusion in the substrate, and laser power, allows for control of the written geometries without any prior substrate preparation. These findings allow for the advance of rapid manufacturing of micro- and nanoscale structures with minimal material constraints through consideration of the laser-controllable thermal transport in ionic liquid/substrate media.

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Additive manufacturing offers unprecedented opportunities to design complex structures optimized for performance envelopes inaccessible under conventional manufacturing constraints. Additive processes also promote realization of engineered materials with microstructures and properties that are impossible via traditional synthesis techniques. Enthused by these capabilities, optimization design tools have experienced a recent revival. The current capabilities of additive processes and optimization tools are summarized briefly, while an emerging opportunity is discussed to achieve a holistic design paradigm whereby computational tools are integrated with stochastic process and material awareness to enable the concurrent optimization of design topologies, material constructs and fabrication processes.

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Lead halide perovskites are increasingly considered for applications beyond photovoltaics, for example, light emission and detection, where an ability to pattern and prototype microscale geometries can facilitate the incorporation of this class of materials into devices. Here we demonstrate laser direct write of lead halide perovskites, a remarkably simple procedure that takes advantage of the inverse dependence between perovskite solubility and temperature by using a laser to induce localized heating of an absorbing substrate. We demonstrate arbitrary pattern formation of crystalline CH3NH3PbBr3 on a range of substrates and fabricate and characterize a microscale photodetector using this approach. This direct write methodology provides a path forward for the prototyping and production of perovskite-based devices.

The ability to create optical materials with arbitrary index distributions would prove transformative for optics design and applications. However, current fabrication techniques for graded index (GRIN) materials rely on diffusion profiles and therefore are unable to realize arbitrary distribution GRIN design. Here, we demonstrate the laser direct writing of graded index structures in protein-based hydrogels using multiphoton lithography. We show index changes spanning a range of 10-2, which is comparable with laser densified glass and polymer systems. Further, we demonstrate the conversion of these written density variation structures into SiO2, opening up the possibility of transforming GRIN hydrogels to a wide range of material systems.

We describe a high-resolution patterning approach that combines the spatial control inherent to laser direct writing with the versatility of benchtop chemical synthesis. By taking advantage of the steep thermal gradient that occurs while laser heating a metal edge in contact with solution, diverse materials comprising transition metals are patterned with feature size resolution nearing 1 μm. We demonstrate fabrication of reduced metallic nickel in one step and examine electrical properties and air stability through direct-write integration onto a device platform. This strategy expands the chemistries and materials that can be used in combination with laser direct writing.

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Establishing processing-structure-property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T′ (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔG H), and, with respect to catalysis, the 1T′ transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T′ molybdenum disulfide and a lowering of ΔG H from +1.6 eV for 2H to +0.18 eV for 1T′, comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.

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Lithiation-exfoliation produces single to few-layered MoS₂ and WS₂ sheets dispersible in water. However, the process transforms them from the pristine semiconducting 2H phase to a distorted metallic phase. Recovery of the semiconducting properties typically involves heating of the chemically exfoliated sheets at elevated temperatures. Therefore, it has been largely limited to sheets deposited on solid substrates. We report the dispersion of chemically exfoliated MoS₂ sheets in high boiling point organic solvents enabled by surface functionalization and the controllable recovery of their semiconducting properties directly in solution. Ultimately, this process connects the scalability of chemical exfoliation with the simplicity of solution processing, enabling a facile method for tuning the metal to semiconductor transitions of MoS₂ and WS₂ within a liquid medium.

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