Andrea Centrone

The viscoelastic response of phase-separated polymers systems may deviate from bulk values at the nanoscale. However, current measurement techniques lack the resolution to unmask local variations. We previously demonstrated the advantages of using a focused, pulsed infrared (IR) laser to photothermally heat a sample locally during nanomechanical measurements with an atomic force microscope (AFM), with heating controlled by varying the laser repetition rate. Herein, we demonstrate this IR heating coupled with nano-Dynamic Mechanical Analysis (nDMA). First, we measured the storage and loss modulus of a homogenous copolymer at varying IR repetition rates (i.e., at varying sample temperature), demonstrating a viscoelastic modulus response that is typical near the glass transition temperature. Further, we varied the IR laser repetition rate while modulating the AFM probe between 5 Hz and 100 Hz. With time-temperature superposition, modulus master curves were constructed through the Williams-Landel-Ferry relation, and the shift factors of IR heating-coupled nDMA curves were used to estimate the relation between IR repetition rate and surface temperatures. Lastly, this technique was applied to a phase-separated polymer blend to demonstrate the utility of the technique to decouple the viscoelastic behavior of different phases by leveraging the high spatial resolution of AFM.

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.

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.

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 μm. 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 μm 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 μm thickness range, the signal's approximate linearity for bands of sample thicknesses up 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 ≈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.

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‐MoO ₃ ) 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 ⁹² MoO ₃ ( ⁹² Mo: 99.93 %) and ¹⁰⁰ MoO ₃ ( ¹⁰⁰ Mo: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural α‐MoO ₃ , 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‐MoO ₃ and isotopically enriched analogs within a reststrahlen band (≈820 cm ⁻¹ to ≈972 cm ⁻¹ ). Results show that isotopic enrichment (e.g., ⁹² MoO ₃ and ¹⁰⁰ MoO ₃ ) 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 ⁹² MoO ₃ 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‐MoO ₃ , which is essential to its implementation in nanophotonic applications.

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.

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.