Thickness-dependent electronic relaxation dynamics in solution-phase redox-exfoliated MoS2 heterostructures

Electronic relaxation dynamics of solution-phase redox-exfoliated molybdenum disulfide (MoS2) monolayer and multilayer ensembles are described. MoS2 was exfoliated using polyoxometalate (POM) reductants. This process yields a colloidal heterostructure consisting of MoS2 2D sheet multilayers with surface-bound POM complexes. Using two-dimensional electronic spectroscopy, transient bleaching and photoinduced absorption signals were detected at excitation/detection energies of 1.82/1.87 and 1.82/1.80 eV, respectively. Approximate 100-fs bandgap renormalization (BGR) and subsequent defect- and phonon-mediated relaxation on the picosecond timescale were resolved for several MoS2 thicknesses spanning from 1 to 2 L to ∼20 L. BGR rates were independent of sample thickness and slightly slower than observations for chemical vapor deposition-grown MoS2 monolayers. However, defect-mediated relaxation accelerated ∼10-fold with increased sample thicknesses. The relaxation rates increased from 0.33 ± 0.05 to 1.2 ± 0.1 and 3.1 ± 0.4 ps-1 for 1-2 L, 3-4 L, and 20 L fractions. The thicknesses-dependent relaxation rates for POM-MoS2 heterostructures were modeled using a saturating exponential function that showed saturation at thirteen MoS2 layers. The results suggest that the increased POM surface coverage leads to larger defect density in the POM-MoS2 heterostructure. These are the first descriptions of the influence of sample thickness on electronic relaxation rates in solution-phase redox-exfoliated POM-MoS2 heterostructures. Outcomes of this work are expected to impact the development of solution-phase exfoliation of 2D metal-chalcogenide heterostructures.

Photothermal Actuation of Thick 3D-Printed Liquid Crystalline Elastomer Nanocomposites

Liquid crystalline elastomers (LCEs) are stimuli-responsive materials that transduce an input energy into a mechanical response. LCE composites prepared with photothermal agents, such as nanoinclusions, are a means to realize wireless, remote, and local control of deformation with light. Amongst photothermal agents, gold nanorods (AuNRs) are highly efficient converters when the irradiation wavelength matches the longitudinal surface plasmon resonance (LSPR) of the AuNRs. However, AuNR aggregation broadens the LSPR which also reduces photothermal efficiency. Here, the surface chemistry of AuNRs is engineered via a well-controlled two-step ligand exchange with a monofunctional poly(ethylene glycol) (PEG) thiol that greatly improves the dispersion of AuNRs in LCEs. Accordingly, LCE-AuNR nanocomposites with very low PEG-AuNR content (0.01 wt%) prepared by 3D printing are shown to be highly efficient photothermal actuators with rapid response (>60% strain s-1) upon irradiation with near-infrared (NIR; 808 nm) light. Because of the excellent dispersion of PEG-AuNR within the LCE, unabsorbed NIR light transmits through the nanocomposites and can actuate a series of samples. Further, the dispersion also allows for the optical deformation of millimeter-thick 3D printed structures without sacrificing actuation speed. The realization of well-dispersed nanoinclusions to maximize the stimulus-response of LCEs can benefit functional implementation in soft robotics or medical devices.

Product Metrics for the Manufacturability of Single-Crystal Gold Nanorods via Reaction Engineering

Colloidal gold nanorods (AuNRs) are integral to a diverse array of technologies, ranging from plasmonic imaging, therapeutics, and sensors to large-area coatings, catalysts, filters, and optical attenuators. Different lab-scale strategies are available to fabricate AuNRs with a broad range of physiochemical properties; however, this is achieved at the cost of synthetic robustness and scalability, which limit broad adoption in these technologies. To address this, Product Metrics (Structural Precision, Shape Yield, and Reagent Utilization), measurable with UV-vis-NIR spectroscopy, are defined to evaluate the efficiency of AuNR production. The dependency of these metrics on reaction formulation (reagent concentrations, pH, and T) is established and used to develop a two-step method based on optimizing symmetry breaking of seed particles, followed by the controlled extension of AuNR length and volume. Reagent concentrations and their relative molar ratios with respect to HAuCl4 are adjusted for each step to optimize these adversarial processes. Based on these correlations, we successfully demonstrate the production of highly concentrated AuNRs with targeted volume and aspect ratio while reducing particle impurities and shape dispersity to less than 4 and 10%, respectively, by employing a rationalized formulation that maximizes both product quality and Reagent Utilization. This results in a product density of 1.6 mg/mL, which is 20 times higher than that of conventional literature methods, with commensurate reduction in environmental waste products.

Intensity-Dependent Optical Response of 2D LTMDs Suspensions: From Thermal to Electronic Nonlinearities

The nonlinear optical (NLO) response of photonic materials plays an important role in the understanding of light-matter interaction as well as pointing out a diversity of photonic and optoelectronic applications. Among the recently studied materials, 2D-LTMDs (bi-dimensional layered transition metal dichalcogenides) have appeared as a beyond-graphene nanomaterial with semiconducting and metallic optical properties. In this article, we review most of our work in studies of the NLO response of a series of 2D-LTMDs nanomaterials in suspension, using six different NLO techniques, namely hyper Rayleigh scattering, Z-scan, photoacoustic Z-scan, optical Kerr gate, and spatial self-phase modulation, besides the Fourier transform nonlinear optics technique, to infer the nonlinear optical response of semiconducting MoS2, MoSe2, MoTe2, WS2, semimetallic WTe2, ZrTe2, and metallic NbS2 and NbSe2. The nonlinear optical response from a thermal to non-thermal origin was studied, and the nonlinear refraction index and nonlinear absorption coefficient, where present, were measured. Theoretical support was given to explain the origin of the nonlinear responses, which is very dependent on the spectro-temporal regime of the optical source employed in the studies.

Continuous flow synthesis of pyridinium salts accelerated by multi-objective Bayesian optimization with active learning

We report a human-in-the-loop implementation of the multi-objective experimental design via a Bayesian optimization platform (EDBO+) towards the optimization of butylpyridinium bromide synthesis under continuous flow conditions. The algorithm simultaneously optimized reaction yield and production rate (or space-time yield) and generated a well defined Pareto front. The versatility of EDBO+ was demonstrated by expanding the reaction space mid-campaign by increasing the upper temperature limit. Incorporation of continuous flow techniques enabled improved control over reaction parameters compared to common batch chemistry processes, while providing a route towards future automated syntheses and improved scalability. To that end, we applied the open-source Python module, nmrglue, for semi-automated nuclear magnetic resonance (NMR) spectroscopy analysis, and compared the acquired outputs against those obtained through manual processing methods from spectra collected on both low-field (60 MHz) and high-field (400 MHz) NMR spectrometers. The EDBO+ based model was retrained with these four different datasets and the resulting Pareto front predictions provided insight into the effect of data analysis on model predictions. Finally, quaternization of poly(4-vinylpyridine) with bromobutane illustrated the extension of continuous flow chemistry to synthesize functional materials.

Near-infrared ultrafast third-order nonlinear optical response of 2D NbS2, NbSe2, ZrTe2, and MoS2

By employing the optical Kerr gate technique at 800 nm with 180 fs pulses at 76 MHz, we evaluated the third-order nonlinear optical response of two-dimensional (2D) semiconducting MoS₂, semimetallic ZrTe₂, and metallic NbS₂ and NbSe₂. The modulus of the nonlinear refractive index was measured to range from 8.6 × 10^-19 m²/W to 5.3 × 10^-18 m²/W, with all materials' response time limited by the pulse duration. The physical mechanism to explain the ultrafast response time's origin considers the nature of the 2D material, as will be discussed.

Redox exfoliated NbS2: characterization, stability, and oxidation

Niobium disulfide is a layered transition metal dichalcogenide that is being exploited as a two-dimensional material. Although it is a superconductor at low temperatures and demonstrates great potential to be applied as a catalyst or co-catalyst in hydrogen evolution reactions, only a few reports have demonstrated the synthesis of a few-layer NbS₂. However, before applications can be pursued, it is essential to understand the main characteristics of the obtained material and its stability under an atmospheric environment. In this work, we conducted a thorough characterization of redox-exfoliated NbS₂ nanoflakes regarding their structure and stability in an oxygen-rich environment. Structural, morphological, and spectroscopic characterization demonstrated different fingerprints associated with distinct oxidation processes. This led us to identify oxide species and analyse the stability of the redox exfoliated NbS₂ nanosheets in air, suggesting the most likely reaction pathways during the NbS₂ interaction with oxygen, which agrees with our density-functional theory results. The mastery over the stability of layered materials is of paramount importance to target future applications, mainly because the electronic properties of these materials are strongly affected by an oxidizing environment.

Exfoliation procedure-dependent optical properties of solution deposited MoS2 films

The development of high-precision large-area optical coatings and devices comprising low-dimensional materials hinges on scalable solution-based manufacturability with control over exfoliation procedure-dependent effects. As such, it is critical to understand the influence of technique-induced transition metal dichalcogenide (TMDC) optical properties that impact the design, performance, and integration of advanced optical coatings and devices. Here, we examine the optical properties of semiconducting MoS2 films from the exfoliation formulations of four prominent approaches: solvent-mediated exfoliation, chemical exfoliation with phase reconversion, redox exfoliation, and native redox exfoliation. The resulting MoS2 films exhibit distinct refractive indices (n), extinction coefficients (k), dielectric functions (ε1 and ε2), and absorption coefficients (α). For example, a large index contrast of Δn ≈ 2.3 is observed. These exfoliation procedures and related chemistries produce different exfoliated flake dimensions, chemical impurities, carrier doping, and lattice strain that influence the resulting optical properties. First-principles calculations further confirm the impact of lattice defects and doping characteristics on MoS2 optical properties. Overall, incomplete phase reconfiguration (from 1T to mixed crystalline 2H and amorphous phases), lattice vacancies, intraflake strain, and Mo oxidation largely contribute to the observed differences in the reported MoS2 optical properties. These findings highlight the need for controlled technique-induced effects as well as the opportunity for continued development of, and improvement to, liquid phase exfoliation methodologies. Such chemical and processing-induced effects present compelling routes to engineer exfoliated TMDC optical properties toward the development of next-generation high-performance mirrors, narrow bandpass filters, and wavelength-tailored absorbers.

Towards maximum optical efficiency of ensembles of colloidal nanorods

Experimental and theoretical studies of colloidal nanoparticles have primarily focused on accurate characterization and simulation of observable characteristics, such as resonant wavelength. In this paper, we tackle the optimal design of colloidal-nanoparticle ensembles: what is the largest possible per-volume optical cross-section, which designs might achieve them, and can such response be experimentally demonstrated? We combine theory and experiment to answer each of these questions. We derive general bounds on the maximum cross-sections per volume, and we apply an analytical antenna model to show that resonant nanorods should nearly achieve such bounds. We use a modified seed-mediated synthesis approach to synthesize ensembles of gold nanorods with small polydispersity, i.e., small variations in size and aspect ratio. Polydispersity is the key determinant of how closely such ensembles can approach their respective bounds yet is difficult to characterize experimentally without near-field measurements. We show that a certain "extinction metric," connecting extinction cross-section per volume with the radiative efficiencies of the nanoparticles, offers a quantitative prediction of polydispersity via quantities that can be rapidly measured with far-field characterization tools. Our predictions apply generally across all plasmonic materials and offer a roadmap to the largest possible optical response of nanoparticle ensembles.

Plasmon-Mediated Chiroptical Second Harmonic Generation from Seemingly Achiral Gold Nanorods

Throughout nature, simple rules explain complex phenomena, such as the selective interaction of chiral objects with circularly polarized light. Here, we demonstrate chiroptical signals from gold nanorods, which are seemingly achiral structures. Shape anisotropy due to atomic-level faceting and rounding at the tips of nanorods, which are free of chiral surface ligands, induces linear-to-circular polarization modulation during second harmonic generation. The intrinsic nanorod chiroptical response is increased by plasmon-resonant excitation, which preferentially amplifies circularly polarized harmonic signals. This structure-plasmon interplay is uniquely resolved by polarization-resolved second harmonic generation measurements. The material's second-order polarizability is the product of the structure-dependent lattice-normal susceptibility and local surface plasmon field vectors. Synthetically scalable plasmon-supporting nanorods that amplify small circular dichroism signals provide a simple, assembly-free platform for chiroptical transduction.

The materials tetrahedron has a "digital twin"

ABSTRACT

For over three decades, the materials tetrahedron has captured the essence of materials science and engineering with its interdependent elements of processing, structure, properties, and performance. As modern computational and statistical techniques usher in a new paradigm of data-intensive scientific research and discovery, the rate at which the field of materials science and engineering capitalizes on these advances hinges on collaboration between numerous stakeholders. Here, we provide a contemporary extension to the classic materials tetrahedron with a dual framework-adapted from the concept of a "digital twin"-which offers a nexus joining materials science and information science. We believe this high-level framework, the materials-information twin tetrahedra (MITT), will provide stakeholders with a platform to contextualize, translate, and direct efforts in the pursuit of propelling materials science and technology forward.

IMPACT STATEMENT

This article provides a contemporary reimagination of the classic materials tetrahedron by augmenting it with parallel notions from information science. Since the materials tetrahedron (processing, structure, properties, performance) made its first debut, advances in computational and informational tools have transformed the landscape and outlook of materials research and development. Drawing inspiration from the notion of a digital twin, the materials-information twin tetrahedra (MITT) framework captures a holistic perspective of materials science and engineering in the presence of modern digital tools and infrastructures. This high-level framework incorporates sustainability and FAIR data principles (Findable, Accessible, Interoperable, Reusable)-factors that recognize how systems impact and interact with other systems-in addition to the data and information flows that play a pivotal role in knowledge generation. The goal of the MITT framework is to give stakeholders from academia, industry, and government a communication tool for focusing efforts around the design, development, and deployment of materials in the years ahead.

Polarized X-ray scattering measures molecular orientation in polymer-grafted nanoparticles

Polymer chains are attached to nanoparticle surfaces for many purposes, including altering solubility, influencing aggregation, dispersion, and even tailoring immune responses in drug delivery. The most unique structural motif of polymer-grafted nanoparticles (PGNs) is the high-density region in the corona where polymer chains are stretched under significant confinement, but orientation of these chains has never been measured because conventional nanoscale-resolved measurements lack sensitivity to polymer orientation in amorphous regions. Here, we directly measure local chain orientation in polystyrene grafted gold nanoparticles using polarized resonant soft X-ray scattering (P-RSoXS). Using a computational scattering pattern simulation approach, we measure the thickness of the anisotropic region of the corona and extent of chain orientation within it. These results demonstrate the power of P-RSoXS to discover and quantify orientational aspects of structure in amorphous soft materials and provide a framework for applying this emerging technique to more complex, chemically heterogeneous systems in the future.

Tuning Hierarchical Order and Plasmonic Coupling of Large-Area, Polymer-Grafted Gold Nanorod Assemblies via Flow-Coating

Solution-based printing of anisotropic nanostructures is foundational to many emerging technologies, such as energy storage devices, photonic elements, and sensors. Methods to rapidly (>mm/s) manufacture large area assemblies (≫cm²) with simultaneous control of thickness (<10 nm), nanoparticle spacing (<5 nm), surface roughness (<5 nm), and global and local orientational order are still lacking. Herein, we demonstrate such capability using flow-coating to fabricate robust, self-supporting mono- and bilayer films of polystyrene-grafted gold nanorods (PS-AuNRs) onto solid substrates. The relationship among solvent evaporation, deposition speed, substrate surface energy, concentration, and film thickness for solutions of such hairy hybrid nanoparticles spans the Landau-Levich and evaporative film formation regimes. In the Landau-Levich regime, solvent evaporation rapidly concentrates the PS-AuNRs, leading to the formation of thin films with distinct, randomized side-by-side domains. Alternatively, processing at slower velocities in the evaporative regime results in the global alignment of PS-AuNRs. Processing speed and substrate surface energy afford tuning of the film's optical extinction of a given PS-AuNR via fine control of inter-rod distance and subsequent plasmonic coupling between neighboring nanorods. Because the concept of the polymer-grafted nanorod can be expanded to a variety of different polymer canopies, shapes, and core materials, the processing-structure relationships established in this work will have important implications on the future development of anisotropic nanostructure-based applications.

Deep Learning of Binary Solution Phase Behavior of Polystyrene

Predicting binary solution phase behavior of polymers has remained a challenge since the early theory of Flory-Huggins, hindering the processing, synthesis, and design of polymeric materials. Herein, we take a complementary data-driven approach by building a machine learning framework to make fast and accurate predictions of polymer solution cloud point temperatures. Using polystyrene, both upper and lower critical solution temperatures are predicted within experimental uncertainty (1-2 °C) with a deep neural network, Gaussian process regression (GPR) model, and a combination of polymer, solvent, and state features. The GPR model also enables intelligent exploration of solution phase space, where as little as 25 cloud points are required to make predictions within 2 °C for polystyrene of arbitrary molecular weight in cyclohexane. This study demonstrates the effectiveness of machine learning for the prediction of liquid-liquid equilibrium of polymer solutions and establishes a framework to incorporate other polymers and complex macromolecular architectures.

Halogen Etch of Ti3AlC2 MAX Phase for MXene Fabrication

The versatile property suite of two-dimensional MXenes is driving interest in various applications, including energy storage, electromagnetic shielding, and conductive coatings. Conventionally, MXenes are synthesized by a wet-chemical etching of the parent MAX-phase in HF-containing media. The acute toxicity of HF hinders scale-up, and competing surface hydrolysis challenges control of surface composition and grafting methods. Herein, we present an efficient, room-temperature etching method that utilizes halogens (Br₂, I₂, ICl, IBr) in anhydrous media to synthesize MXenes from Ti₃AlC₂. A radical-mediated process depends strongly on the molar ratio of the halogen to MAX phase, absolute concentration of the halogen, the solvent, and temperature. This etching method provides opportunities for controlled surface chemistries to modulate MXene properties.

Projectile Impact Shock-Induced Deformation of One-Component Polymer Nanocomposite Thin Films

Matrix-free assemblies of polymer-grafted nanoparticles (PGNs) enable mechanically robust materials for a variety of structural, electronic, and optical applications. Recent quasi-static mechanical studies have identified the key parameters that enhance canopy entanglement and promote plasticity of the PGNs below T_g. Here we experimentally explore the high-strain-rate shock impact behavior of polystyrene grafted NPs and compare their energy absorption capabilities to that of homopolystyrene for film thicknesses ranging from 75 to 550 nm and for impact velocities from 350 to 800 m/s. Modeling reveals that the initial shock compression results in a rapid temperature increase at the impact site. The uniformity of this heating is consistent with observations of greater kinetic energy absorption per mass (E_p^*) of thinner films due to extensive visco-plastic deformation of molten film around the penetration site. Adiabatic heating is insufficient to raise the temperature at the exit surface of the thickest films resulting in increased strain localization at the impact periphery with less melt elongation. The extent and distribution of entanglements also influence E_p^*. Structurally, each NP acts as a giant cross-link node, coupling surrounding nodes via the number of canopy chains per NP and the nature and number of entanglements between canopies anchored to different NPs. Load sharing via this dual network, along with geometrical factors such as film thickness, lead to extreme E_p^* arising from the sequence of instantaneous adiabatic shock heating followed by visco-plastic drawing of the film by the projectile. These observations elucidate the critical factors necessary to create robust polymer-nanocomposite multifunctional films.

Toward Architected Nanocomposites: MXenes and Beyond

Achieving excellent electromagnetic interference (EMI) shielding combined with mechanical flexibility, optical transparency, and environmental stability is vital for the future of coatings, electrostatic discharge, electronic displays, and wearable and portable electronic devices. Unfortunately, it is challenging to engineer materials with all of these desired properties due to a lack of understanding of the underlying materials physics and structure-property relationships. Nature has provided numerous examples of a combination of properties through precision engineering of hierarchical structures at multiple length scales with selectively chosen ingredients. This inspiration is reflected in a wide range of synthetic architected nanocomposites. In this Perspective, we provide a brief overview of recent advances in the role of hierarchical architectures in MXene-based thin-film nanocomposites in the quest to achieve multiple functionalities, especially focusing on a combination of excellent EMI shielding, transparency, and mechanical robustness. We also discuss key opportunities, challenges, and prospects.

Fifth-order optical nonlinear response of semiconducting 2D LTMD MoS2

The effective fifth-order susceptibility, ${\chi}_{\rm eff}^{(5)}$, of two-dimensional (2D) semiconducting layered transition metal dichalcogenide (LTMD) molybdenum disulfide (${\rm MoS}_2$) is reported here for the first time, to the best of our knowledge. Using the $ Z $-scan technique with a laser operating at 800 nm, 1 kHz, 100 fs, we investigated the nonlinear behavior of ${\rm MoS}_2$ suspended in acetonitrile (concentration, 70 µg/ml). The effective nonlinear refractive index ${{n}_{4,{eff}}} = - ({7.6 \pm 0.5}) \times {10^{- 26}}\; {{\rm cm}^4}/{{\rm W}^2}$, proportional to ${\rm Re}{\chi}_{\rm eff}^{(5)}$, was measured for monolayer ${\rm MoS}_2$ nanoflakes, prepared by a modified redox exfoliation method. We also determined the value of the nonlinear refractive index ${{n}_2} = + ({4.8 \pm 0.5}) \times {10^{- 16}}\;{{\rm cm}^2}/{\rm W}$, which is related to the material's effective third-order optical susceptibility real part, ${Re\chi}_{\rm eff}^{(3)}$. For comparison, we also investigated the nonlinear response of tungsten disulfide (${\rm WS}_2$) monolayers, prepared by the same method and suspended in acetonitrile (concentration, 40 µg/ml), which only exhibited the third-order nonlinear effect in the same intensity range, up to ${120}\;{{{\rm GW}/{\rm cm}}^2}$. Nonlinear absorption was not observed in either ${\rm MoS}_2$ or ${\rm WS}_2$.

Hyper-Rayleigh scattering in 2D redox exfoliated semi-metallic ZrTe2 transition metal dichalcogenide

Nonlinear optical characterization of nanostructured layered transition metal dichalcogenides (LTMDs) is of fundamental interest for basic knowledge and applied purposes. In particular, second-order optical nonlinearities are the basis for second harmonic generation as well as sum or difference frequency generation and have been studied in some 2D TMDs, especially in those with a semiconducting character. Here we report, for the first time, on the second-order nonlinearity of the semi-metallic ZrTe2 monolayer in acetonitrile suspension (concentration of 4.9 × 1010 particles per cm3), synthesized via a modified redox exfoliation method and characterized using the Hyper-Rayleigh scattering technique in the nanosecond regime. The orientation-averaged first-hyperpolarizability was found to be β(2ω) = (7.0 ± 0.3) × 10-24 esu per ZrTe2 monolayer flake, the largest reported so far. Polarization-resolved measurements were performed in the monolayer suspension and indicate the dipolar origin of the generated incoherent second harmonic wave.

Autonomous materials discovery driven by Gaussian process regression with inhomogeneous measurement noise and anisotropic kernels

A majority of experimental disciplines face the challenge of exploring large and high-dimensional parameter spaces in search of new scientific discoveries. Materials science is no exception; the wide variety of synthesis, processing, and environmental conditions that influence material properties gives rise to particularly vast parameter spaces. Recent advances have led to an increase in the efficiency of materials discovery by increasingly automating the exploration processes. Methods for autonomous experimentation have become more sophisticated recently, allowing for multi-dimensional parameter spaces to be explored efficiently and with minimal human intervention, thereby liberating the scientists to focus on interpretations and big-picture decisions. Gaussian process regression (GPR) techniques have emerged as the method of choice for steering many classes of experiments. We have recently demonstrated the positive impact of GPR-driven decision-making algorithms on autonomously-steered experiments at a synchrotron beamline. However, due to the complexity of the experiments, GPR often cannot be used in its most basic form, but rather has to be tuned to account for the special requirements of the experiments. Two requirements seem to be of particular importance, namely inhomogeneous measurement noise (input-dependent or non-i.i.d.) and anisotropic kernel functions, which are the two concepts that we tackle in this paper. Our synthetic and experimental tests demonstrate the importance of both concepts for experiments in materials science and the benefits that result from including them in the autonomous decision-making process.

Resolving Electron-Electron Scattering in Plasmonic Nanorod Ensembles Using Two-Dimensional Electronic Spectroscopy

The use of two-dimensional electronic spectroscopy (2DES) to study electron-electron scattering dynamics in plasmonic gold nanorods is described. The 2DES resolved the time-dependent plasmon homogeneous line width Γ_h(t), which was sensitive to changes in Fermi-level carrier densities. This approach was effective because electronic excitation accelerated plasmon dephasing, which broadened Γ_h. Analysis of Γ_h(t) indicated plasmon coherence times were decreased by 20-50%, depending on excitation conditions. Electron-electron scattering rates of approximately 0.01 fs^-1 were obtained by fitting the time-dependent Γ_h broadening; rates increased quadratically with both excitation pulse energy and frequency. This rate dependence agreed with Fermi-liquid theory-based predictions. Hot electron thermalization through electron-phonon scattering resulted in Γ_h narrowing. To our knowledge, this is the first use of the plasmon Γ_h(t) to isolate electron-electron scattering dynamics in colloidal metal nanoparticles. These results illustrate the effectiveness of 2DES for studying hot electron dynamics of solution-phase plasmonic ensembles.

Monolayer 2D ZrTe2 transition metal dichalcogenide as nanoscatter for random laser action

We demonstrate random laser emission from Rhodamine 6G with ZrTe2 transition metal dichalcogenide (TMD) as nanoscatters, both in powder and 2D nanoflakes liquid suspension. The 2D semimetal ZrTe2 was synthesized by a modified redox exfoliation method to provide single layer TMD, which was employed for the first time as the scatter medium to provide feedback in an organic gain medium random laser. In order to exploit random laser emission and its threshold value, replica symmetry breaking leading to a photonic paramagnetic to photonic spin glass transition in both 2D and 3D (powder) ZrTe2 was demonstrated. One important aspect of mixing organic dyes with ZrTe2 is that there is no chemical reaction leading to dye degradation, demonstrated by operating over more than 2 hours of pulsed (5 Hz) random laser emission.

Uniaxial Deformation and Crazing in Glassy Polymer-Grafted Nanoparticle Ultrathin Films

The deformation behavior of neat, glassy polymer-grafted nanoparticle (PGN) monolayer films is studied using coarse-grained molecular dynamics simulations and experiments on polystyrene-grafted silica. In both the simulations and experiments, apparent crazing behavior is observed during deformation. The PGN systems show a relatively more uniform, perforated sheet craze structure and significantly higher strain at break than reference homopolymers of the same length. Short chain, unentangled PGN monolayers are also simulated for comparison; these are brittle and break apart without crazing. The entangled PGN simulations are analyzed in detail for systems at both high and moderate graft density. Stress-strain curves show three distinct regions: yielding and strain localization, craze widening, and strain hardening preceding catastrophic failure. The PGN stress-strain behavior appears more similar to that of longer chain, highly entangled homopolymer films than to the reference homopolymer films of the same length as the graft chains, suggesting that the particles effectively add additional entanglement points. The moderate graft density particles have higher strain-to-failure and maximum stress than the high graft density particles. We suggest this increased robustness for lower graft density systems is due to their increased interpenetration of graft chains between neighboring particles, which leads to increased interparticle entanglements per chain.

Dynamic Plasmonic Pixels

Information display utilizing plasmonic color generation has recently emerged as an alternative paradigm to traditional printing and display technologies. However, many implementations so far have either presented static pixels with a single display state or rely on relatively slow switching mechanisms such as chemical transformations or liquid crystal transitions. Here, we demonstrate spatial, spectral, and temporal control of light using dynamic plasmonic pixels that function through the electric-field-induced alignment of plasmonic nanorods in organic suspensions. By tailoring the geometry and composition (Au and Au@Ag) of the nanorods, we illustrate light modulation across a significant portion of the visible and infrared spectrum (600-2400 nm). The fast (∼30 μs), reversible nanorod alignment is manifested as distinct color changes, characterized by shifts of observed chromaticity and luminance. Integration into larger device architectures is showcased by the fabrication of a seven-segment numerical indicator. The control of light on demand achieved in these dynamic plasmonic pixels establishes a favorable platform for engineering high-performance optical devices.

Mini Monomer Encapsulated Emulsion Polymerization of PMMA Using Aqueous ARGET ATRP

A new emulsion polymerization based on Activators Regenerated by Electron Transfer Atom Transfer Radical Polymerization (ARGET ATRP) has been developed to produce poly(methyl methacrylate) (PMMA). The critical components of single-pot synthesis via a mini Monomer Encapsulated ARGET ATRP emulsion polymerization will be discussed. In this method, monomer is isolated in a micelle by pre-emulsification in a nonionic surfactant through high power stirring while acetone is introduced to the polymerization to aid in reactant transport and tetrabutylammonium salts are used as phase transfer agents. Polymerizations using tetrabutylammonium bromide (TBAB) were more controlled and demonstrated low Đ (M_w/M_n < 1.17), where those using tetrabutylammonium chloride (TBAC) exhibited higher Đ (M_w/M_n > 1.50). First-order linear kinetics for MMA polymerizations at 100 mM TBAB was demonstrated, while the reactions deviated from linearity at higher concentrations of TBAB and all concentrations of TBAC.

Autonomous Motility of Polymer Films

Adaptive soft materials exhibit a diverse set of behaviors including reconfiguration, actuation, and locomotion. These responses however, are typically optimized in isolation. Here, the interrelation between these behaviors is established through a state space framework, using Nylon 6 thin films in a humidity gradient as an experimental testbed. It is determined that the dynamic behaviors are a result of not only a response to but also an interaction with the applied stimulus, which can be tuned via control of the environment and film characteristics, including size, permeability, and coefficient of hygroscopic expansion to target a desired behavior such as multimodal locomotion. Using these insights, it is demonstrated that films simultaneously harvest energy and information from the environment to autonomously move down a stimulus gradient. Improved understanding of the coupling between an adaptive material and its environment aids the development of materials that integrate closed loop autonomous sensing, actuation, and locomotion.

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites

Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) volume elements] via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a dc field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be electrically triggered. Initial experiments indicate that the SWNT-polymer interfaces play a crucial role in enabling the electrostriction reported herein.

Highly Concentrated Seed-Mediated Synthesis of Monodispersed Gold Nanorods

The extremely large optical extinction coefficient of gold nanorods (Au-NRs) enables their use in a diverse array of technologies, rnging from plasmonic imaging, therapeutics and sensors, to large area coatings, filters, and optical attenuators. Development of the latter technologies has been hindered by the lack of cost-effective, large volume production. This is due in part to the low reactant concentration required for symmetry breaking in conventional seed-mediated synthesis. Direct scale up of laboratory procedures has limited viability because of excessive solvent volume, exhaustive postsynthesis purification processes, and the generation of large amounts of waste (e.g., hexadecyltrimethylammonium bromide(CTAB)). Following recent insights into the growth mechanism of Au-NRs and the role of seed development, we modify the classic seed-mediated synthesis via temporal control of seed and reactant concentration to demonstrate production of Au-NRs at more than 100-times the conventional concentration, while maintaining independent control and narrow distribution of nanoparticle dimensions, aspect ratio, and volume. Thus, gram scale synthesis of Au-NRs with prescribed aspect ratio and volume is feasible in a 100 mL reactor with 1/100th of organic waste relative to conventional approaches. Such scale-up techniques are crucial to cost-effectively meet the increased demand for large quantities of Au-NRs in emerging applications.

Redox Exfoliation of Layered Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) have attracted considerable attention in a diverse array of applications due to the breadth of possible property suites relative to other low-dimensional nanomaterials (e.g., graphene, aluminosilicates). Here, we demonstrate an alternative methodology for the exfoliation of bulk crystallites of group V-VII layered TMDs under quiescent, benchtop conditions using mild redox chemistry. Anionic polyoxometalate species generated from edge sites adsorb to the TMD surface and create Coulombic repulsion that drives layer separation without the use of shear forces. This method is generalizable (MS₂, MSe₂, and MTe₂) and effective in preparing high-concentration (>1 mg/mL) dispersions with narrow layer thickness distributions more rapidly and with safer reagents than alternative solution-based approaches. Finally, exfoliation of these TMDs is demonstrated in a range of solvent systems that were previously inaccessible due to large surface energy differences. These characteristics could be beneficial in the preparation of high-quality films and monoliths.

Stability of Polymer Grafted Nanoparticle Monolayers: Impact of Architecture and Polymer-Substrate Interactions on Dewetting

The stability of polymer thin films is crucial to a broad range of technologies, including sensors, energy storage, filtration, and lithography. Recently, the demonstration of rapid deposition on solid substrates of ordered monolayers of polymer grafted nanoparticles (PGN) has increased potential for inks to additively manufacture such components. Herein, enhanced stability against dewetting of these self-assembled PGN films (gold nanoparticle functionalized with polystyrene (AuNP-PS)) is discussed in context to linear polystyrene (PS) analogues using high throughput surface gradients: surface energy (20-45 mN/m) and temperature (90-160 °C). PGNs exhibit a lower surface (γ_p) and interfacial (γ_sp) energy relative to linear polymers, which results in increased thermal and energetic stability by 10-25 °C and 5-15 mN/m, respectively. This enhanced wetting-dewetting transition is qualitatively consistent with the behavior of star macromolecules and depends on the architecture of the polymer canopy. Increased film stability through canopy architecture expands the manufacturability of thin film hybrids and refines postprocessing conditions to maximize local PGN order.

Frequency-Driven Self-Organized Helical Superstructures Loaded with Mesogen-Grafted Silica Nanoparticles

Adding colloidal nanoparticles into liquid-crystal media has become a promising pathway either to enhance or to introduce novel properties for improved device performance. Here we designed and synthesized new colloidal hybrid silica nanoparticles passivated with a mesogenic monolayer on the surface to facilitate their organo-solubility and compatibility in a liquid-crystal host. The resulting nanoparticles were identified by 1 H NMR spectroscopy, TEM, TGA, and UV/Vis techniques, and the hybrid nanoparticles were doped into a dual-frequency cholesteric liquid-crystal host to appraise both their compatibility with the host and the effect of the doping concentration on their electro-optical properties. Interestingly, the silica-nanoparticle-doped liquid-crystalline nanocomposites were found to be able to dynamically self-organize into a helical configuration and exhibit multi-stability, that is, homeotropic (transparent), focal conic (opaque), and planar states (partially transparent), depending on the frequency applied at sustained low voltage. Significantly, a higher contrast ratio between the transparent state and scattering state was accomplished in the nanoparticle-embedded liquid-crystal systems.

Belousov-Zhabotinsky autonomic hydrogel composites: Regulating waves via asymmetry

Belousov-Zhabotinsky (BZ) autonomic hydrogel composites contain active nodes of immobilized catalyst (Ru) encased within a nonactive matrix. Designing functional hierarchies of chemical and mechanical communication between these nodes enables applications ranging from encryption, sensors, and mechanochemical actuators to artificial skin. However, robust design rules and verification of computational models are challenged by insufficient understanding of the relative importance of local (molecular) heterogeneities, active node shape, and embedment geometry on transient and steady-state behavior. We demonstrate the predominance of asymmetric embedment and node shape in low-strain, BZ-gelatin composites and correlate behavior with gradients in BZ reactants. Asymmetric embedment of square and rectangular nodes results in directional steady-state waves that initiate at the embedded edge and propagate toward the free edge. In contrast, symmetric embedment does not produce preferential wave propagation because of a lack of diffusion gradient across the catalyzed region. The initiation at the embedded edge is correlated with bromide absorption by the inactive matrix, which locally elevates the bromate concentration required for catalyst oxidation. The competition between embedment asymmetry and node geometry was used to demonstrate a repeatable switch in wave direction that functions as a signal delay. Furthermore, signal propagation in or out of the composite was demonstrated via embedment asymmetry and relative dimensions of a T-shaped active network node. Overall, structural asymmetry provides a robust approach to controlling initiation and orientation of chemical-mechanical communication within composite BZ gels.

Plasmon-Mediated Two-Photon Photoluminescence-Detected Circular Dichroism in Gold Nanosphere Assemblies

We report plasmon-mediated two-photon photoluminescence (TPPL)-detected circular dichroism (CD) from colloidal metal nanoparticle assemblies. Two classes of solid gold nanosphere (SGN) dimers--heterodimers and homodimers--were examined using polarization-resolved TPPL, second harmonic generation (SHG), and one-photon photoluminescence (OPPL). Unambiguous CD was detected in both the TPPL and SHG signals, and the magnitudes of the CD responses in these measurements showed agreement for individual nanostructures. Heterodimers gave larger CD responses (average TPPL-CDR = 0.62 ± 0.33; average SHG-CDR = 0.51 ± 0.21) than homodimers (average TPPL-CDR = 0.19 ± 0.04; average SHG-CDR = 0.18 ± 0.06). OPPL-CD was not detected for either structure. Analysis of dimer emission properties suggested the CD responses were determined by properties of the one-photon-resonant mode excited by the laser. Average TPPL signals were (4.3 ± 0.6)× larger than those for SHG. Because signal amplitude is a primary determinant for spatial accuracies and precisions obtained from optical microscopy, CD contrast generated from plasmon-mediated TPPL, which we report for the first time, can extend the suite of super-resolution imaging techniques.

Deterministic Construction of Plasmonic Heterostructures in Well-Organized Arrays for Nanophotonic Materials

Plasmonic heterostructures are deterministically constructed in organized arrays through chemical pattern directed assembly, a combination of top-down lithography and bottom-up assembly, and by the sequential immobilization of gold nanoparticles of three different sizes onto chemically patterned surfaces using tailored interaction potentials. These spatially addressable plasmonic chain nanostructures demonstrate localization of linear and nonlinear optical fields as well as nonlinear circular dichroism.

Topology optimization for the design of folding liquid crystal elastomer actuators

Aligned liquid crystal elastomers (LCEs) are capable of undergoing large reversible shape change in response to thermal stimuli and may act as actuators for many potential applications such as self-assembly and deployment of micro devices. Recent advances in LCE patterning tools have demonstrated sub-millimetre control of director orientation, enabling the preparation of materials with arbitrarily complex director fields. However, without design tools to connect the 2D director pattern with the activated 3D shape, LCE design relies on intuition and trial and error. Here we present a design methodology to generate reliable folding in monolithic LCEs designed with topology optimization. The distributions of order/disorder and director orientations are optimized so that the remotely actuated deformation closely matches a target deformation for origami folding. The optimal design exhibits a strategy to counteract the mechanical frustration that may lead to an undesirable deformation, such as anti-clastic bending. Multi-hinge networks were developed using insights from the optimal hinge designs and were demonstrated through the fabrication and reversible actuation of a self-folding box. Topology optimization provides an important step towards leveraging the opportunities afforded by LCE patterning into functional designs.

Nonlinear chiro-optical amplification by plasmonic nanolens arrays formed via directed assembly of gold nanoparticles

Metal nanoparticle assemblies are promising materials for nanophotonic applications due to novel linear and nonlinear optical properties arising from their plasmon modes. However, scalable fabrication approaches that provide both precision nano- and macroarchitectures, and performance commensurate with design and model predictions, have been limiting. Herein, we demonstrate controlled and efficient nanofocusing of the fundamental and second harmonic frequencies of incident linearly and circularly polarized light using reduced symmetry gold nanoparticle dimers formed by surface-directed assembly of colloidal nanoparticles. Large ordered arrays (>100) of these C∞v heterodimers (ratio of radii R1/R2 = 150 nm/50 nm = 3; gap distance l = 1 ± 0.5 nm) exhibit second harmonic generation and structure-dependent chiro-optic activity with the circular dichroism ratio of individual heterodimers varying less than 20% across the array, demonstrating precision and uniformity at a large scale. These nonlinear optical properties were mediated by interparticle plasmon coupling. Additionally, the versatility of the fabrication is demonstrated on a variety of substrates including flexible polymers. Numerical simulations guide architecture design as well as validating the experimental results, thus confirming the ability to optimize second harmonic yield and induce chiro-optical responses for compact sensors, optical modulators, and tunable light sources by rational design and fabrication of the nanostructures.

Shape-reprogrammable polymers: encoding, erasing, and re-encoding

Shape-reprogramming in a polymer is demonstrated, where prescribed 3D geometric information can be encoded, decoded, erased, and re-encoded. In essence, the shape-reprogrammable polymer (SRP) acts as computer hardware that can be reformatted and reprogrammed repeatedly. Such SRPs have the potential to be repurposed directly without going through material disposal and recycling.

Performance of dielectric nanocomposites: matrix-free, hairy nanoparticle assemblies and amorphous polymer-nanoparticle blends

Demands to increase the stored energy density of electrostatic capacitors have spurred the development of materials with enhanced dielectric breakdown, improved permittivity, and reduced dielectric loss. Polymer nanocomposites (PNCs), consisting of a blend of amorphous polymer and dielectric nanofillers, have been studied intensely to satisfy these goals; however, nanoparticle aggregates, field localization due to dielectric mismatch between particle and matrix, and the poorly understood role of interface compatibilization have challenged progress. To expand the understanding of the inter-relation between these factors and, thus, enable rational optimization of low and high contrast PNC dielectrics, we compare the dielectric performance of matrix-free hairy nanoparticle assemblies (aHNPs) to blended PNCs in the regime of low dielectric contrast to establish how morphology and interface impact energy storage and breakdown across different polymer matrices (polystyrene, PS, and poly(methyl methacrylate), PMMA) and nanoparticle loadings (0-50% (v/v) silica). The findings indicate that the route (aHNP versus blending) to well-dispersed morphology has, at most, a minor impact on breakdown strength trends with nanoparticle volume fraction; the only exception being at intermediate loadings of silica in PMMA (15% (v/v)). Conversely, aHNPs show substantial improvements in reducing dielectric loss and maintaining charge/discharge efficiency. For example, low-frequency dielectric loss (1 Hz-1 kHz) of PS and PMMA aHNP films was essentially unchanged up to a silica content of 50% (v/v), whereas traditional blends showed a monotonically increasing loss with silica loading. Similar benefits are seen via high-field polarization loop measurements where energy storage for ∼15% (v/v) silica loaded PMMA and PS aHNPs were 50% and 200% greater than respective comparable PNC blends. Overall, these findings on low dielectric contrast PNCs clearly point to the performance benefits of functionalizing the nanoparticle surface with high-molecular-weight polymers for polymer nanostructured dielectrics.

Design of polarization-dependent, flexural-torsional deformation in photo responsive liquid crystalline polymer networks

Light responsive materials that exhibit wirelessly actuated, multidimensional deformation are excellent candidates for programmable matter applications such as morphing structures or soft robotics. A central challenge to designing adaptive structures from these materials is the ability accurately predict three dimensional deformations. Previous modeling efforts have focused almost exclusively on pure bending. Herein we examine key material parameters affecting light driven flexural-torsional response in azobenzene functionalized liquid crystal polymer networks. We show that a great deal of control can be obtained by specifying material alignment and actuating the material with polarized light. Insight gained from the theoretical framework here lays the foundation for more extensive modeling efforts to combine polarization controlled flexural-torsional deformations with complex geometry, boundary conditions, and loading conditions.