Kinetic Insights into Bridge Cleavage Pathways in Periodic Mesoporous Organosilicas.

Bridging functionalities in periodic mesoporous organosilicas (PMOs) enable new functionalities for a wide range of applications. Bridge cleavage is frequently observed during anneals required to form porous structures, yet the mechanism of these bridge cleavages has not been completely resolved. Here, these chemical transformations and their kinetic pathways on sub-millisecond timescales induced by laser heating are revealed. By varying anneal times and temperatures, the transformation dynamics of bridge cleavage and structural transformations and their activation energies are determined. The structural relaxation time for individual reactions and their effective local heating time are determined and compared, and the results directly demonstrate the manipulation of different molecules through kinetic control of the sequence of reactions. By isolating and understanding the earliest stage of structural transformations, this study identifies the kinetic principles for new synthesis and post-processing routes to control individual molecules and reactions in PMOs and other material systems with multi-functionalities.

Ab Initio Studies of the Diffusion of Intrinsic Defects and Silicon Dopants in Bulk InAs.

We expose the predominant diffusional pathways for In and As in InAs, as well as dopant Si atoms in InAs, using Nudged Elastic Band calculations in conjunction with accurate Density Functional Theory calculations of the energy of defective systems. Our results show that As is a very fast diffuser compared to In and Si for both vacancy-assisted and interstitially mediated mechanisms. Larger indium atoms, on the other hand, are very slow diffusers and strongly prefer to remain on the In sublattice. Silicon also prefers to stay in substitutional sites in the In sublattice, in agreement with the fact that Si is used to create n-doped InAs. We find that the mechanism by which Si diffuses within the InAs lattice is very unlikely to proceed via vacancy-assisted jumps, since these routes encounter energy barriers above 2 eV. In contrast, silicon can readily make interstitial jumps since they occur with energy barriers as small as 0.23 eV. This suggests that an interstitial diffusion mechanism is strongly preferred for Si diffusion in InAs which challenges the common presumption made for another similar III-V compound, namely GaAs, that Si diffusion takes place via a vacancy-assisted mechanism.

µ-Rainbow: CdSe Nanocrystal Photoluminescence Gradients via Laser Spike Annealing for Kinetic Investigations and Tunable Device Design.

Much of the promise of nanomaterials derives from their size-dependent, and hence tunable, properties. Impressive advances have been made in the synthesis of nanoscale building blocks with precisely tailored size, shape and composition. Significant attention is now turning toward creating thin film structures in which size-dependent properties can be spatially programmed with high fidelity. Nonequilibrium processing techniques present exciting opportunities to create nanostructured thin films with unprecedented spatial control over their optical and electronic properties. Here, we demonstrate single scan laser spike annealing (ssLSA) on CdSe nanocrystal (NC) thin films as an experimental test bed to illustrate how the size-dependent photoluminescence (PL) emission can be tuned throughout the visible range and in spatially defined profiles during a single annealing step. Through control of the annealing temperature and time, we discovered that NC fusion is a kinetically limited process with a constant activation energy in over 2 orders of magnitude of NC growth rate. To underscore the broader technological implications of this work, we demonstrate the scalability of LSA to process large area NC films with periodically modulated PL emission, resulting in tunable emission properties of a large area film. New insights into the processing-structure-property relationships presented here offer significant advances in our fundamental understanding of kinetics of nanomaterials as well as technological implications for the production of nanomaterial films.

Deposited low temperature silicon GHz modulator.

We demonstrate gigahertz electro-optic modulator fabricated on low temperature polysilicon using excimer laser annealing technique compatible with CMOS backend integration. Carrier injection modulation at 3 Gbps is achieved. These results open up an array of possibilities for silicon photonics including photonics on DRAM and on flexible substrates.

Autonomous materials synthesis via hierarchical active learning of nonequilibrium phase diagrams.

Autonomous experimentation enabled by artificial intelligence offers a new paradigm for accelerating scientific discovery. Nonequilibrium materials synthesis is emblematic of complex, resource-intensive experimentation whose acceleration would be a watershed for materials discovery. We demonstrate accelerated exploration of metastable materials through hierarchical autonomous experimentation governed by the Scientific Autonomous Reasoning Agent (SARA). SARA integrates robotic materials synthesis using lateral gradient laser spike annealing and optical characterization along with a hierarchy of AI methods to map out processing phase diagrams. Efficient exploration of the multidimensional parameter space is achieved with nested active learning cycles built upon advanced machine learning models that incorporate the underlying physics of the experiments and end-to-end uncertainty quantification. We demonstrate SARA's performance by autonomously mapping synthesis phase boundaries for the Bi2O3 system, leading to orders-of-magnitude acceleration in the establishment of a synthesis phase diagram that includes conditions for stabilizing δ-Bi2O3 at room temperature, a critical development for electrochemical technologies.

Block Copolymer Self-Assembly-Directed and Transient Laser Heating-Enabled Nanostructures toward Phononic and Photonic Quantum Materials.

Three-dimensional (3D) periodic ordering of silicon (Si), an inorganic semiconductor, on the mesoscale was achieved by combining block copolymer (BCP) self-assembly (SA) based mesoporous alternating gyroidal network formation with nonequilibrium transient laser heating. 3D continuous and periodically ordered alternating gyroidal mesoporous carbon thin-film networks were prepared from spin coating, SA under solvent vapor annealing (SVA), and thermal processing of mixtures of a triblock terpolymer with resorcinol resols. The resulting mesoporous thin films, acting as structure-directing templates, were backfilled with amorphous silicon (a-Si). Nanosecond excimer laser heating led to transient Si melts conformally filling the template pores and subsequent Si crystallization. The ordered mesostructure of the organic polymer-derived templates was kept intact, despite being thermally unstable at the high temperatures around the Si melting point (MP), leading to high pattern transfer fidelity. As evidenced by a combination of grazing incidence small-angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM), after template removal, the crystalline Si (c-Si) inherited the inverse network topology of the 3D mesoporous thin-film templates, but with reduced F222 space group symmetry (D2 point group symmetry) from compression of the cubic alternating gyroid lattice. Structures with this reduced symmetry have been proposed as photonic and phononic materials exhibiting topologically protected Weyl points, adding to the emerging field of BCP SA-directed quantum materials promising advanced physics and materials properties.

Optical Identification of Materials Transformations in Oxide Thin Films.

Recent advances in high-throughput experimentation for combinatorial studies have accelerated the discovery and analysis of materials across a wide range of compositions and synthesis conditions. However, many of the more powerful characterization methods are limited by speed, cost, availability, and/or resolution. To make efficient use of these methods, there is value in developing approaches for identifying critical compositions and conditions to be used as a priori knowledge for follow-up characterization with high-precision techniques, such as micrometer-scale synchrotron-based X-ray diffraction (XRD). Here, we demonstrate the use of optical microscopy and reflectance spectroscopy to identify likely phase-change boundaries in thin film libraries. These methods are used to delineate possible metastable phase boundaries following lateral-gradient laser spike annealing (lg-LSA) of oxide materials. The set of boundaries are then compared with definitive determinations of structural transformations obtained using high-resolution XRD. We demonstrate that the optical methods detect more than 95% of the structural transformations in a composition-gradient La-Mn-O library and a Ga2O3 sample, both subject to an extensive set of lg-LSA anneals. Our results provide quantitative support for the value of optically detected transformations as a priori data to guide subsequent structural characterization, ultimately accelerating and enhancing the efficient implementation of micrometer-resolution XRD experiments.

A fast ramp rate thermally stimulated current technique to quantify electronic charge dynamics in thin films.

Thermally stimulated current (TSC) techniques have been applied to study thermally activated events in many materials. However, the temperature ramp rates in traditional TSC are typically too slow (few degrees per minute) to monitor materials whose properties are strongly time dependent. A fast ramp rate TSC (FR-TSC) technique was developed with ramp rates of 1-5 K/s. This is up to 100 times faster than traditional TSC, so that material changes can be appropriately quantified in the time scale at which they take place. In this paper, the experimental design and challenges to achieve fast and stable ramp rates and to measure the low-level currents are discussed. The fast ramps were attained using a thermoelectric cooler, controlled by a proportional-integral-derivative feedback loop, for both heating and cooling. FR-TSC measurements (1 K/s and 20-100 degrees C) on poly(vinylidene fluoride-trifluoroethylene) ferroelectric thin films are discussed as an example material. From these measurements, thermally activated currents as well as irreversible and reversible charge dynamics were readily distinguished with multiple thermal cycles. These measurements suggest that this technique holds substantial promise in quantifying charge dynamics in fast response materials.

Synthesis and Formation Mechanism of All-Organic Block Copolymer-Directed Templating of Laser-Induced Crystalline Silicon Nanostructures.

This report describes the generation of three-dimensional (3D) crystalline silicon continuous network nanostructures by coupling all-organic block copolymer self-assembly-directed resin templates with low-temperature silicon chemical vapor deposition and pulsed excimer laser annealing. Organic 3D mesoporous continuous-network resin templates were synthesized from the all-organic self-assembly of an ABC triblock terpolymer and resorcinol-formaldehyde resols. Nanosecond pulsed excimer laser irradiation induced the transient melt transformation of amorphous silicon precursors backfilled in the organic template into complementary 3D mesoporous crystalline silicon nanostructures with high pattern fidelity. Mechanistic studies on laser-induced crystalline silicon nanostructure formation revealed that the resin template was carbonized during transient laser-induced heating on the milli- to nanosecond timescales, thereby imparting enhanced thermal and structural stability to support the silicon melt-crystallization process at temperatures above 1250 °C. Photoablation of the resin material under pulsed excimer laser irradiation was mitigated by depositing an amorphous silicon overlayer on the resin template. This approach represents a potential pathway from organic block copolymer self-assembly to alternative functional hard materials with well-ordered 3D morphologies for potential hybrid photovoltaics, photonic, and energy storage applications.

Ultrafast Self-Assembly of Sub-10 nm Block Copolymer Nanostructures by Solvent-Free High-Temperature Laser Annealing.

Laser spike annealing was applied to PS-b-PDMS diblock copolymers to induce short-time (millisecond time scale), high-temperature (300 to 700 °C) microphase segregation and directed self-assembly of sub-10 nm features. Conditions were identified that enabled uniform microphase separation in the time frame of tens of milliseconds. Microphase ordering improved with increased temperature and annealing time, whereas phase separation contrast was lost for very short annealing times at high temperature. PMMA brush underlayers aided ordering under otherwise identical laser annealing conditions. Good long-range order for sub-10 nm cylinder morphology was achieved using graphoepitaxy coupled with a 20 ms dwell laser spike anneal above 440 °C.

Finite element and analytical solutions for van der Pauw and four-point probe correction factors when multiple non-ideal measurement conditions coexist.

This paper presents an extensive collection of calculated correction factors that account for the combined effects of a wide range of non-ideal conditions often encountered in realistic four-point probe and van der Pauw experiments. In this context, "non-ideal conditions" refer to conditions that deviate from the assumptions on sample and probe characteristics made in the development of these two techniques. We examine the combined effects of contact size and sample thickness on van der Pauw measurements. In the four-point probe configuration, we examine the combined effects of varying the sample's lateral dimensions, probe placement, and sample thickness. We derive an analytical expression to calculate correction factors that account, simultaneously, for finite sample size and asymmetric probe placement in four-point probe experiments. We provide experimental validation of the analytical solution via four-point probe measurements on a thin film rectangular sample with arbitrary probe placement. The finite sample size effect is very significant in four-point probe measurements (especially for a narrow sample) and asymmetric probe placement only worsens such effects. The contribution of conduction in multilayer samples is also studied and found to be substantial; hence, we provide a map of the necessary correction factors. This library of correction factors will enable the design of resistivity measurements with improved accuracy and reproducibility over a wide range of experimental conditions.

Initial performance studies of a wearable brain positron emission tomography camera based on autonomous thin-film digital Geiger avalanche photodiode arrays.

Using analytical and Monte Carlo modeling, we explored performance of a lightweight wearable helmet-shaped brain positron emission tomography (PET), or BET camera, based on thin-film digital Geiger avalanche photodiode arrays with Lutetium-yttrium oxyorthosilicate (LYSO) or [Formula: see text] scintillators for imaging in vivo human brain function of freely moving and acting subjects. We investigated a spherical cap BET and cylindrical brain PET (CYL) geometries with 250-mm diameter. We also considered a clinical whole-body (WB) LYSO PET/CT scanner. The simulated energy resolutions were 10.8% (LYSO) and 3.3% ([Formula: see text]), and the coincidence window was set at 2 ns. The brain was simulated as a water sphere of uniform F-18 activity with a radius of 100 mm. We found that BET achieved [Formula: see text] better noise equivalent count (NEC) performance relative to the CYL and [Formula: see text] than WB. For 10-mm-thick [Formula: see text] equivalent mass systems, LYSO (7-mm thick) had [Formula: see text] higher NEC than [Formula: see text]. We found that [Formula: see text] scintillator crystals achieved [Formula: see text] full-width-half-maximum spatial resolution without parallax errors. Additionally, our simulations showed that LYSO generally outperformed [Formula: see text] for NEC unless the timing resolution for [Formula: see text] was considerably smaller than that presently used for LYSO, i.e., well below 300 ps.

Lateral Temperature-Gradient Method for High-Throughput Characterization of Material Processing by Millisecond Laser Annealing.

A high-throughput method for characterizing the temperature dependence of material properties following microsecond to millisecond thermal annealing, exploiting the temperature gradients created by a lateral gradient laser spike anneal (lgLSA), is presented. Laser scans generate spatial thermal gradients of up to 5 °C/µm with peak temperatures ranging from ambient to in excess of 1400 °C, limited only by laser power and materials thermal limits. Discrete spatial property measurements across the temperature gradient are then equivalent to independent measurements after varying temperature anneals. Accurate temperature calibrations, essential to quantitative analysis, are critical and methods for both peak temperature and spatial/temporal temperature profile characterization are presented. These include absolute temperature calibrations based on melting and thermal decomposition, and time-resolved profiles measured using platinum thermistors. A variety of spatially resolved measurement probes, ranging from point-like continuous profiling to large area sampling, are discussed. Examples from annealing of III-V semiconductors, CdSe quantum dots, low-κ dielectrics, and block copolymers are included to demonstrate the flexibility, high throughput, and precision of this technique.

Processing-Structure-Property Relationships in Laser-Annealed PbSe Nanocrystal Thin Films.

As nanocrystal (NC) synthesis techniques and device architectures advance, it becomes increasingly apparent that new ways of connecting NCs with each other and their external environment are required to realize their considerable potential. Enhancing inter-NC coupling by thermal annealing has been a long-standing challenge. Conventional thermal annealing approaches are limited by the challenge of annealing the NC at sufficiently high temperatures to remove surface-bound ligands while at the same time limiting the thermal budget to prevent large-scale aggregation. Here we investigate nonequilibrium laser annealing of NC thin films that enables separation of the kinetic and thermodynamic aspects of nanocrystal fusion. We show that laser annealing of NC assemblies on nano- to microsecond time scales can transform initially isolated NCs in a thin film into an interconnected structure in which proximate dots "just touch". We investigate both pulsed laser annealing and laser spike annealing and show that both annealing methods can produce "confined-but-connected" nanocrystal films. We develop a thermal transport model to rationalize the differences in resulting film morphologies. Finally we show that the insights gained from study of nanocrystal mono- and bilayers can be extended to three-dimensional NC films. The basic processing-structure-property relationships established in this work provide guidance to future advances in creating functional thin films in which constituent NCs can purposefully interact.

POROUS MATERIALS. Transient laser heating induced hierarchical porous structures from block copolymer-directed self-assembly.

Development of rapid processes combining hierarchical self-assembly with mesoscopic shape control has remained a challenge. This is particularly true for high-surface-area porous materials essential for applications including separation and detection, catalysis, and energy conversion and storage. We introduce a simple and rapid laser writing method compatible with semiconductor processing technology to control three-dimensionally continuous hierarchically porous polymer network structures and shapes. Combining self-assembly of mixtures of block copolymers and resols with spatially localized transient laser heating enables pore size and pore size distribution control in all-organic and highly conducting inorganic carbon films with variable thickness. The method provides all-laser-controlled pathways to complex high-surface-area structures, including fabrication of microfluidic devices with high-surface-area channels and complex porous crystalline semiconductor nanostructures.

Laser-induced sub-millisecond heating reveals distinct tertiary ester cleavage reaction pathways in a photolithographic resist polymer.

Acid-catalyzed, thermally activated ester cleavage reactions are critical for lithographic patterning processes used in the semiconductor industry. The rates of these high-temperature reactions within polymer thin films are difficult to characterize because of the thermal instability of many polymers and a lack of temperature-resolved measurement techniques. Here we introduce the use of transient laser irradiation to heat a methyladamantane-protected acrylate copolymer to 600 °C in less than a millisecond. These conditions mediate the removal of the protecting groups and enable accurate kinetic measurements. At sub-millisecond exposure to high temperatures (∼600 °C), the rate of the ester cleavage reaction exhibits the expected first-order dependence on acid concentration. In contrast, the reaction exhibits more complex kinetics when the polymer film is heated to lower temperatures (115 °C) on a conventional hot-plate. We identify distinct methyladamantane-derived deprotection products under the high- and low-temperature conditions that are consistent with the observed rate differences. The acid-catalyzed dimerization of 1-methyleneadamantane occurs at low temperature, which reduces the acid concentration available for the ester cleavage. This dimerization reaction is minimized during transient laser-induced heating because bimolecular reactions are disfavored under these conditions. We constructed a mathematical model based on these observations that accounts for the competition for the catalyst between the dimerization and ester cleavage processes. This laser-induced, sub-millisecond heating technique provides a means to probe and model temperature and time regimes of thermally activated reactions in polymer films, and these regimes exhibit distinct and advantageous reaction pathways that will inform future advances in high-performance photolithography.

Kinetic rates of thermal transformations and diffusion in polymer systems measured during sub-millisecond laser-induced heating.

Probing chemical reaction kinetics in the near-solid state (small molecules and polymers) is extremely challenging because of the restricted mobility of reactant species, the absence of suitable analytical probes, and most critically the limited temperature stability of the materials. By limiting temperature exposure to extremely short time frames (sub-millisecond), temperatures in excess of 800 °C can be accessed extending kinetic rate measurements many orders of magnitude. Here we demonstrate measurements on a model system, exploiting the advantages of thin-films, laser heating, and chemically amplified resists as an exquisite probe of chemical kinetic rates. Chemical reaction and acid diffusion rates were measured over 10 orders of magnitude, exposing unexpected and large changes in dynamics linked to critical mechanism shifts across temperature regimes. This new approach to the study of kinetics in near-solid state materials promises to substantially improve our understanding of processes active in a broad range of temperature-sensitive, low-mobility materials.

Colloidal self-assembly-directed laser-induced non-close-packed crystalline silicon nanostructures.

This report describes an ultrafast, large-area, and highly flexible method to construct complex two- and three-dimensional silicon nanostructures with deterministic non-close-packed symmetry. Pulsed excimer laser irradiation is used to induce a transient melt transformation of amorphous silicon filled in a colloidal self-assembly-directed inverse opal template, resulting in a nanostructured crystalline phase. The pattern transfer yields are high, and long-range order is maintained. This technique represents a potential route to obtain silicon nanostructures of various symmetries and associated unique properties for advanced applications such as energy storage and generation.

Pulsed laser annealing of thin films of self-assembled nanocrystals.

We investigated how pulsed laser annealing can be applied to process thin films of colloidal nanocrystals (NCs) into interconnected nanostructures. We illustrate the relationship between incident laser fluence and changes in morphology of PbSe NC films relative to bulk-like PbSe films. We found that laser pulse fluences in the range of 30 to 200 mJ/cm(2) create a processing window of opportunity where the NC film morphology goes through interesting transformations without large-scale coalescence of the NCs. NC coalescence can be mitigated by depositing a thin film of amorphous silicon (a-Si) on the NC film. Remarkably, pulsed laser annealing of the a-Si/PbSe NC films crystallized the silicon while NC morphology and translational order of the NC film are preserved.

Block copolymer self-assembly-directed single-crystal homo- and heteroepitaxial nanostructures.

Epitaxy is a widely used method to grow high-quality crystals. One of the key challenges in the field of inorganic solids is the development of epitaxial single-crystal nanostructures. We describe their formation from block copolymer self-assembly-directed nanoporous templates on single-crystal Si backfilled with Si or NiSi through a laser-induced transient melt process. Depending on thickness, template removal leaves either an array of nanopillars or porous nanostructures behind. For stoichiometric NiSi deposition, the template pores provide confinement, enabling heteroepitaxial growth. Irradiation through a mask provides access to hierarchically structured materials. These results on etchable and non-etchable materials suggest a general strategy for growing epitaxial single-crystal nanostructured thin films for fundamental studies and a wide variety of applications, including energy conversion and storage.