Abstract This study yields a comprehensive nanoscale investigation of the composition and molecular orientation of electrospun poly(methyl methacrylate) (PMMA) nanofibers containing halogen-terminated carbon atomic wires (CAWs), a finite-length analog of the carbyne construct. This is accomplished by conducting light-polarization-dependent experiments with advanced techniques such as atomic force microscopy-infrared spectroscopy (AFM-IR), optical photothermal infrared microscopy (O-PTIR), and hyperspectral photoluminescence (PL) microscopy. In PMMA nanofibers, AFM-IR reveals a remarkable, homogeneous uniaxial orientation of PMMA chains along the fiber axis, down to the ≈ 20 nm scale. Semi-quantitative analysis of IR dichroic ratios indicates a nearly orthogonal alignment ( $$\:{\psi\:}_{avg}\approx\:\:79^\circ\:$$ ) of the C = O dipole with respect to the nearly all-trans planar polymer backbone, consistent with expectations of the electrospinning process and in agreement with prior modeling of polymer chain dynamics. Congruous AFM-IR, O-PTIR, and hyperspectral PL data show that incorporation of CAWs into the PMMA nanofibers results in a peculiar compositional heterogeneity, with alternating 3 μm to 4 μm long PMMA-rich and CAW-rich regions across the fiber length. Notably, hyperspectral PL data reveal that CAWs preferentially align along the fiber axis, with sparse, 1.5 μm to 3 μm long domains displaying locally reduced or enhanced molecular orientation. We envision that the widely applicable approach used here will foster engineering of anisotropic nanostructures with advanced functionalities and complex compositions, laying the groundwork for future applications in nanoelectronics, photonics, and energy storage that demand anisotropic molecular orientation and composition.

The performance of holographic photopolymers, one of the most promising material platforms for addressing the critical needs in emerging holographic applications, is governed by spatial variations of chemical composition, which have rarely been assessed at length scales commensurate with the grating pitch. Here, the chemical specificity of AFM-IR, an infrared technique capable of nanoscale resolution, is leveraged to map the local composition of a holographic grating recorded in a thiol-ene/polyurethane photopolymer film. Quantitative analysis enabled by calibration of photopolymer (1584 cm-1) to polyurethane (1532 cm-1) intensity ratios in flood-cured films with systematically varied photopolymer loading reveals variations of the grating local peak-to-valley concentration (averaged through the grating thickness) of ≈1.7% g/g. In addition, the grating structure was probed at different depths by measuring the grating angular diffraction efficiency upon sequential, layer-by-layer material removal with a cryo-microtome. The evolution of the diffraction peak intensity and position with depth (i.e., upon thinning of the grating) indicates through-thickness heterogeneity in both the refractive index modulation and the grating geometry. In combination, these approaches offer a framework for assessing the composition of holographic gratings and for understanding their performance.

Control over the position, orientation, and stacking order of two-dimensional (2D) materials within van der Waals heterostructures is crucial for applications in electronics, spintronics, optics, and sensing. The most popular strategy for assembling 2D materials uses purpose-built stamps with working surfaces made from one of several different polymers. However, these stamps typically require tedious preparation steps and suffer from poor durability, contamination, and limited applicability to specific 2D materials or surfaces. Here, we demonstrate significant improvements upon current 2D flake transfer and assembly practices by using mechanically durable stamps made from polyvinyl chloride (PVC) thin films. These stamps are simpler to prepare compared with existing methods and can withstand multiple transfer cycles without significant degradation, enabling greater reusability and flexibility during 2D flake assembly. We use two commercially available PVC films with distinct pick-up and release temperatures. Together, these films also enable polymer-to-polymer flake transfers and stack-and-flip fabrication of inverted heterostructures in one seamless process. Systematic comparisons of cleaning processes confirm the removal of PVC-derived residue from the assembled structures to create atomically clean interfaces. We demonstrate the utility and versatility of these polymer films and transfer process by fabricating graphene/hexagonal boron nitride heterostructure devices with high-performance electrical characteristics. Further, we demonstrate the ability to pick up and to deposit bulk aluminum gallium arsenide nanostructured films, enabling the creation of heterogeneously integrated devices. These advances enable faster and more versatile assembly of 2D materials than previously reported polymer-assisted methods. Accordingly, this technique increases fabrication rates, improves device quality, and enables more complex structures, thereby facilitating nanomaterial assembly in a broad range of applications.

Abstract By transducing a sample’s photothermal expansion and contraction, the photothermal induced resonance technique (PTIR, also known as AFM-IR) enables characterization of light absorption at scales (≈10 nm) well below the limits imposed by far-field diffraction. However, AFM-IR depth sensitivity on heterogeneous samples remains poorly understood, making quantitative analyses challenging. Here, we examine stratified, three-dimensionally printed, polymer heterostructures engineered to elucidate the AFM-IR signal dependence on detection frequency, light polarization, and measurement modality (i.e., on- or off-resonance). Results show that the signal intensity, derived from absorption in the topmost layer (here, an epoxy-based prism), increases nearly linearly with its thickness regardless of detection frequency or resonance condition, aligning with previous observations in homogeneous (non-stratified) samples. However, the signal from the bottom layer (here, an acrylic photopolymer resin disk beneath the epoxy prism) can be amplified, attenuated, or remain unchanged, depending on the detection frequency and measurement modality. On-resonance, the signal coming from absorption in the bottom, acrylic layer always decays approximately exponentially with the thickness of the top, epoxy layer, with a frequency-dependent decay rate. Notably, absorption from the acrylic layer was detected even beneath the thickest epoxy region (here, ≈440 nm) for all measurement conditions, highlighting the considerable subsurface sensitivity of AFM-IR. We propose adopting a ≈95% signal decay as a convention to describe a practical AFM-IR measurement depth sensitivity (probed depth), as opposed to the previously suggested ≈63% decay. This work advances AFM-IR-based nanoscale chemical imaging by elucidating the relationship between signal attenuation and sample stratification, which represents a critical step toward more-quantitative and three-dimensional compositional analyses.

This work demonstrates direct, rapid 2D thermal mapping measurement capabilities of the ultrawide bandgap semiconductor channel of lateral β-(AlxGa1−x)2O3/Ga2O3 transistors without sample contamination, long acquisition times, or sophisticated thermometry such as developing deep-ultra-violet compatible thermoreflectance systems. The temperature rise in the channel of a β-(Al0.21Ga0.79)2O3/Ga2O3 heterostructure field-effect transistor (HFET) was mapped using thermoreflectance imaging at 470 nm. First, the thermoreflectance response of the HFET channel was measured using a monochromator, revealing a maximum of the reflectance change around 470–480 nm. Thermoreflectance calibrations were then performed at 470 nm (peak of the reflectance change) and yielded an average thermoreflectance coefficient of 1.06 ± 0.07 × 10−4 K−1. Subsequent measurements of the device (power densities of 0.15–1.47 W/mm and gate-source voltage of 0 V) enabled extraction of a device-level thermal resistance of 51.1 mm·K/W in the channel at the drain-side of the gate. High-resolution, in situ scanning thermal microscopy measurements of the channel temperature rise show good agreement with and further support the thermoreflectance measurements. Finally, the thermal profile across the entire device length (metal electrodes and semiconductor channel) and width was simultaneously measured using thermoreflectance imaging at 470 nm, and the peak temperature rise was measured in the channel at the drain-side of the gate electrode.

Hyperbolic phonon polaritons (HPhPs), hybrids of light and lattice vibrations in polar dielectric crystals, empower nanophotonic applications by enabling the confinement and manipulation of light at the nanoscale. Molybdenum trioxide (α-MoO3) is a naturally hyperbolic material, meaning that its dielectric function deterministically controls the directional propagation of in-plane HPhPs within its reststrahlen bands. Strategies such as substrate engineering, nano- and hetero-structuring, and isotopic enrichment are being developed to alter the intrinsic dielectric functions of natural hyperbolic materials and to control the confinement and propagation of HPhPs. Since isotopic disorder can limit phonon-based processes such as HPhPs, here we synthesize isotopically enriched 92MoO3 (92Mo: 99.93 %) and 100MoO3 (100Mo: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural α-MoO3, which is composed of seven stable Mo isotopes. Real-space, near-field maps measured with the photothermal induced resonance (PTIR) technique enable comparisons of in-plane HPhPs in α-MoO3 and isotopically enriched analogs within a reststrahlen band (≈820 cm−1 to ≈972 cm−1). Results show that isotopic enrichment (e.g., 92MoO3 and 100MoO3) alters the dielectric function, shifting the HPhP dispersion (HPhP angular wavenumber × thickness vs. IR frequency) by ≈−7 % and ≈+9 %, respectively, and changes the HPhP group velocities by ≈±12 %, while the lifetimes (≈3 ps) in 92MoO3 were found to be slightly improved (≈20 %). The latter improvement is attributed to a decrease in isotopic disorder. Altogether, isotopic enrichment was found to offer fine control over the properties that determine the anisotropic in-plane propagation of HPhPs in α-MoO3, which is essential to its implementation in nanophotonic applications.

Photothermal induced resonance (PTIR), also known as atomic force microscopy-infrared (AFM-IR), enables nanoscale IR absorption spectroscopy by transducing the local photothermal expansion and contraction of a sample with the tip of an atomic force microscope. PTIR spectra enable material identification at the nanoscale and can measure sample composition at depths >1 mu m. However, implementation of quantitative, multivariate, nanoscale IR analysis requires an improved understanding of PTIR signal transduction and of the intensity dependence on sample characteristics and measurement parameters. Here, PTIR spectra measured on three-dimensional printed conical structures up to 2.5 mu m tall elucidate the signal dependence on sample thickness for different IR laser repetition rates and pulse lengths. Additionally, we develop a model linking sample thermal expansion dynamics to cantilever excitation amplitudes that includes samples that do not fully thermalize between consecutive pulses. Remarkable qualitative agreement between experiments and theory demonstrates a monotonic increase in the PTIR signal intensity with thickness, with decreasing sensitivities at higher repetition rates, while signal intensity is nearly unaffected by laser pulse length. Although we observe slight deviations from linearity over the entire 2.5 mu m thickness range, the signal's approximate linearity for bands of sample thicknesses up to approximate to 500 nm suggests that samples with comparably low topographic variations are most amenable to quantitative analysis. Importantly, we measure absorptive undistorted profiles in PTIR spectra for strongly absorbing modes, up to approximate to 1650 nm, and >2500 nm for other modes. These insights are foundational toward quantitative nanoscale PTIR analyses and material identification, furthering their impact across many applications.

Aluminum gallium nitride/gallium nitride multi-channel superlattice devices are receiving increasing attention as a new paradigm for driving the power density of gallium nitride based transistors toward their theoretical limit. However, the superior electrical performance of superlattice-based transistors is currently limited by excessive Joule-heating. This Letter evaluates what impact the number of superlattice channels and the buffer layer composition has on the reduction of the thermal resistance, i.e., Joule heating, of AlGaN/GaN superlattice devices. A record low thermal resistance (12.51 ± 0.34 K mm W−1) was measured via scanning thermal microscopy for non-castellated superlattice AlGaN/GaN devices with a 100 μm channel width. Overall, the use of a thin gallium nitride buffer layer, in place of a thick aluminum gallium nitride layer, reduced the buffer thermal resistance enabling the accommodation of more superlattice channels (10 vs 6), therefore augmenting the maximum power density of these devices. The superlattice device proposed here not only provides an enhanced thermal dissipation solution for high power density radio frequency electronics, but it also has the benefit of fewer fabrication steps in comparison with previously reported castellated multichannel devices.

Control over the local chemical composition and spatialheterogeneitiesin nanomaterials provides a means to impart new functions and to tailortheir properties in many applications. For two-dimensional (2D) vander Waals materials, intercalation is one useful strategy to exertsuch control, by inserting or removing atomic, molecular, or ionicspecies in their lattice. For example, hydrogen intercalation andthe creation of oxygen vacancies in MoO3 locally increaseelectrical conductivity, optical absorption, and electrochemical activity,thereby enabling its use in electrochromic, photochromic, catalytic,and energy storage applications. Here, after thermal annealing ofMoO(3) crystals in H-2, we show that the resultinghydrogen molybdenum bronzes and/or oxygen-deficient, substoichiometricmolybdenum oxides (MoO3-& delta;) display highlyheterogeneous, optically dark, topographically protruding patternsat the nanoscale. We leverage the nanoscale resolution of photothermalinduced resonance absorption spectra and maps in the visible, mid-,and near-infrared to characterize these features that are spectrallydistinct from the surrounding regions. Subsequent annealing in O-2 enables partial recovery of the initial Mo:O stoichiometry,though some absorption peaks linked to intercalation persist. Thehigh-resolution imaging and spectroscopic analyses employed here enableprecise measurements of nanoscale heterogeneities that foster theadoption of 2D and other materials in advanced applications.

Given the ever increasing, global electricity consumption, improving the efficiency and reliability of high-power electronics is of paramount importance. Ultra-wide band gap (> 3.4 eV) semiconductors have shown the potential to be used in the next generation of power electronics due to their high breakdown field and mobility. Specifically, high-quality growth of doped strontium stannate perovskite-oxide (SSO) has been recently demonstrated. The thermal properties of this novel material, however, have not been fully investigated and could be the limiting factor (excessive junction temperatures) to maximize device performance. In this study, high resolution $(\approx \mathbf{410}\ \mathbf{nm})$ transient thermoreflectance imaging (TTI) is used to obtain temperature maps of SSO two-terminal devices with varying channel width ( $\mathbf{10}\ \boldsymbol{\mu} \mathbf{m}$ to $\mathbf{20}\ \boldsymbol{\mu} \mathbf{m}$ ). The results show that the device thermal resistance increases by $\approx \mathbf{21} {\%}$ due to the increase in thermal spreading resistance with channel width. The observed trend is further confirmed via transient thermal analysis where the thermal time constant is shown to increase from $\mathbf{20}.\mathbf{9}\ \boldsymbol{\mu}\mathbf{s}$ to $\mathbf{29}.\mathbf{9}\ \boldsymbol{\mu}\mathbf{s}$ . Overall, the active layer thicknesses and device geometry (length and width), must be carefully considered to improve device performance and lifetime.

Thermal fluctuations often impose both fundamental and practical measurement limits on high-performance sensors, motivating the development of techniques that bypass the limitations imposed by thermal noise outside cryogenic environments. Here, we theoretically propose and experimentally demonstrate a measurement method that reduces the effective transducer temperature and improves the measurement precision of a dynamic impulse response signal. Thermal noise–limited, integrated cavity optomechanical atomic force microscopy probes are used in a photothermal-induced resonance measurement to demonstrate an effective temperature reduction by a factor of ≈25, i.e., from room temperature down as low as ≈12 K, without cryogens. The method improves the experimental measurement precision and throughput by >2×, approaching the theoretical limit of ≈3.5× improvement for our experimental conditions. The general applicability of this method to dynamic measurements leveraging thermal noise–limited harmonic transducers will have a broad impact across a variety of measurement platforms and scientific fields.

As the demand for computing applications capable of processing large datasets increases, there is a growing need for new in-memory computing technologies. Oxide-based resistive random-access memory (RRAM) devices are promising candidates for such applications because of their industry readiness, endurance, and switching ratio. These analog devices, however, suffer from poor linearity and asymmetry in their analog resistance change. Various reports have found that the temperature in RRAM devices increases locally by more than 1000 K during operation. Therefore, temperature control is of paramount importance for controlling their resistance. In this study, scanning thermal microscopy is used to map the temperature of Au/Ti/HfOx/Au devices at a steady power state and to measure temperature dynamics of the top electrode above the filament location during both resistive switching loops and voltage pulsing. These measurements are used to verify the thermal parameters of a multiphysics finite elements model. The model is then used to understand the impact of thermal conductivities and boundary conductances of constituent materials on resistance change during the first reset pulse in RRAM devices. It is found that the resistance change can be reduced significantly when the temperature in the titanium capping layer is reduced. We find that the greatest temperature reduction and, therefore, the lowest resistance change in the device are afforded by capping layers with increased thermal conductivities. This work links thermal properties to the resistance change in RRAM devices, providing critical insights into engineering devices with improved switching dynamics.

The modular synthesis approach for assembling inorganic nodes and organic multidentate linkers into reticular solids enables rational engineering in porous materials known as metal–organic frameworks (MOFs). Incorporation of two or more linker types in MOF crystals holds great potential for engineering complex pore functionalities, by virtue of chemically heterogeneous domains. However, deciphering linkers distributions in MOFs crystals is challenging because of the insufficient spatial resolution of conventional, chemically sensitive techniques hinders the verification of rational design. Herein, the high spatial resolution and chemical specificity of infrared nanoscopy is leveraged in combination with high‐throughput diffraction‐limited hyperspectral photoluminescence imaging to determine the composition of individual multivariate UiO‐68 MOF crystals (UiO, Universitetet‐i‐Oslo), after linker‐exchange with optically active tetrazine units. The results reveal that the crystals display a three‐layer onion‐like structure composed of a core‐rich in the parent linker, an intermediate multivariate layer with a gradient in linker composition and a proto‐MOF external shell. In this outer layer, a fraction of the linkers’ binding groups is hydrogen bonded rather than coordinated with metal nodes, suggestive of superficial reconstruction during the linker‐exchange. This study advances the analytical capabilities for studying and engineering heterogeneous domains in mixed‐linker MOF crystals down to the nanoscale.

The convergence of optical spectroscopy with scanning probe microscopy (SPM) provides a near-field sensitive platform that circumvents the limitations of light diffraction and can measure highly localized heterogeneity and phenomena. In ultrahigh vacuum (UHV) and at cryogenic temperatures the spatial resolution of these methods can be pushed towards the atomic scale. Here, efforts to use UHV scanning tunneling microscopy (STM) coupled with tip-enhanced Raman spectroscopy (TERS) to investigate the behavior and chemistry of molecular adsorbates on pristine and modified noble metal substrates are highlighted. Spectroscopic imaging provides the ability to visualize highly localized phenomena, while vibrational fingerprints derived from light-matter interactions can be used to understand their effects on a molecule’s or material’s structure. This can provide insight into adsorbate-substrate interactions crucial to the growth of nanostructures through surface chemistry, as well as the functionalization or modification of 2D materials. Moving beyond UHV-STM-TERS, ongoing work to develop a new versatile cryogenic UHV-SPM platform for other measurements will also be discussed. Specifically, STM-induced luminescence and the coupling of infrared light into the tip-sample junction provide the opportunity to investigate otherwise inaccessible properties of materials that include light-matter interactions and quasiparticles at the atomic scale.

Nanophotonic resonator integration and miniaturization decrease detection noise of nanomechanical scanning probe microscopy and increase its throughput. Using pulsed laser excitation, we demonstrate fast imaging (≈500,000× faster than a commercial probe) of thermal properties with 35nm spatial resolution.

This Special Publication represents the work of researchers at professional conferences, as reported by NIST employees in Fiscal Year 2020 (October 1, 2019–September 30, 2020).

Photothermal induced resonance (PTIR), an atomic force microscopy (AFM) analogue of IR spectroscopy also known as AFM-IR, is capable of nanoscale lateral resolution and finds broad applications in biology and materials science. Here, the spectral range of a top-illumination PTIR setup operating in contact-mode is expanded for the first time to the visible and near-IR spectral ranges. The result is a tool that yields absorption spectra and maps of electronic and vibrational features with spatial resolution down to ≈10 nm. In addition to the improved resolution, the setup enables light-polarization-dependent PTIR experiments in the visible and near-IR ranges for the first time. While previous PTIR implementations in the visible used total internal reflection illumination requiring challenging sample preparations on an optically transparent prism, the top illumination used here greatly simplifies sample preparation and will foster a broad application of this method.

Formation and aggregation of metal carboxylates (metal soaps) can degrade the appearance and integrity of oil paints, challenging efforts to conserve painted works of art. Endeavors to understand the root cause of metal soap formation have been hampered by the limited spatial resolution of Fourier transform infrared microscopy (μ-FTIR). We overcome this limitation using optical photothermal infrared spectroscopy (O-PTIR) and photothermal-induced resonance (PTIR), two novel methods that provide IR spectra with ≈500 and ≈10 nm spatial resolutions, respectively. The distribution of chemical phases in thin sections from the top layer of a 19th-century painting is investigated at multiple scales (μ-FTIR ≈ 102 μm3, O-PTIR ≈ 10-1 μm3, PTIR ≈ 10-5 μm3). The paint samples analyzed here are found to be mixtures of pigments (cobalt green, lead white), cured oil, and a rich array of intermixed, small (often ≪ 0.1 μm3) zinc soap domains. We identify Zn stearate and Zn oleate crystalline soaps with characteristic narrow IR peaks (≈1530-1558 cm-1) and a heterogeneous, disordered, water-permeable, tetrahedral zinc soap phase, with a characteristic broad peak centered at ≈1596 cm-1. We show that the high signal-to-noise ratio and spatial resolution afforded by O-PTIR are ideal for identifying phase-separated (or locally concentrated) species with low average concentration, while PTIR provides an unprecedented nanoscale view of distributions and associations of species in paint. This newly accessible nanocompositional information will advance our knowledge of chemical processes in oil paint and will stimulate new art conservation practices.

Thermal properties of materials are often determined by measuring thermalization processes; however, such measurements at the nanoscale are challenging because they require high sensitivity concurrently with high temporal and spatial resolutions. Here, we develop an optomechanical cantilever probe and customize an atomic force microscope with low detection noise ≈1 fm/Hz1/2 over a wide (>100 MHz) bandwidth that measures thermalization dynamics with ≈10 ns temporal resolution, ≈35 nm spatial resolution, and high sensitivity. This setup enables fast nanoimaging of thermal conductivity (η) and interfacial thermal conductance (G) with measurement throughputs ≈6000× faster than conventional macroscale-resolution time-domain thermoreflectance acquiring the full sample thermalization. As a proof-of-principle demonstration, 100 × 100 pixel maps of η and G of a polymer particle are obtained in 200 s with a small relative uncertainty (<10%). This work paves the way to study fast thermal dynamics in materials and devices at the nanoscale.

Photothermal induced resonance (PTIR), also known as AFM-IR, enables nanoscale infrared (IR) imaging and spectroscopy by using the tip of an atomic force microscope to transduce the local photothermal expansion and contraction of a sample. The signal transduction efficiency and spatial resolution of PTIR depend on a multitude of sample, cantilever, and illumination source parameters in ways that are not yet well understood. Here, we elucidate and separate the effects of laser pulse length, pulse shape, sample thermalization time (τ), interfacial thermal conductance, and cantilever detection frequency by devising analytical and numerical models that link a sample's photothermal excitations to the cantilever dynamics over a broad bandwidth (10 MHz). The models indicate that shorter laser pulses excite probe oscillations over broader bandwidths and should be preferred for measuring samples with shorter thermalization times. Furthermore, we show that the spatial resolution critically depends on the interfacial thermal conductance between dissimilar materials and improves monotonically, but not linearly, with increasing cantilever detection frequencies. The resolution can be enhanced for samples that do not fully thermalize between pulses (i.e., laser repetition rates ≳ 1/3τ) as the probed depth becomes smaller than the film thickness. We believe that the insights presented here will accelerate the adoption and impact of PTIR analyses across a wide range of applications by informing experimental designs and measurement strategies as well as by guiding future technical advances.