Emergent interface vibrational structure of oxide superlattices.

As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices1-9. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity10,11, are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries12,13, the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate-calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses.

Photoelectrochemistry of Self-Limiting Electrodeposition of Ni Film onto GaAs.

Gallium arsenide (GaAs) provides a suitable bandgap (1.43 eV) for solar spectrum absorption and allows a larger photovoltage compared to silicon, suggesting great potential as a photoanode toward water splitting. Photocorrosion under water oxidation condition, however, leads to decomposition or the formation of an insulating oxide layer, which limits the photoelectrochemical performance and stability of GaAs. In this work, a self-limiting electrodeposition method of Ni on GaAs is reported to either generate ultra-thin continuous film or nanoislands with high particle density by controlling deposition time. The self-limiting growth mechanism is validated by potential transients, X-ray photoelectron spectroscopy composition and depth profile measurements. This deposition method exhibits a rapid nucleation, forms an initial metallic layer followed by a hydroxide/oxyhydroxide nanofilm on the GaAs surface and is independent of layer thickness versus deposition time when coalescence is reached. A photocurrent up to 8.9 mA cm-2 with a photovoltage of 0.11 V is obtained for continuous ultrathin films, while a photocurrent density of 9.2 mA cm-2 with a photovoltage of 0.50 V is reached for the discontinuous nanoislands layers in an aqueous solution containing the reversible redox couple K3 Fe(CN)6 /K4 Fe(CN)6 .

Photoconductive mechanism of IR-sensitive iodized PbSe thin films via strong hole-phonon interaction and minority carrier diffusion.

Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole-phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers (Vk center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3d5/2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron-hole recombination in KI hole traps).

Nonequivalent Atomic Vibrations at Interfaces in a Polar Superlattice.

In heterostructures made from polar materials, e.g., AlN-GaN-AlN, the nonequivalence of the two interfaces is long recognized as a critical aspect of their electronic properties; in that, they host different 2D carrier gases. Interfaces play an important role in the vibrational properties of materials, where interface states enhance thermal conductivity and can generate unique infrared-optical activity. The nonequivalence of the corresponding interface atomic vibrations, however, is not investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein, the nonequivalence of AlN-(Al0.65Ga0.35)N and (Al0.65Ga0.35)N-AlN interface vibrations is experimentally demonstrated using monochromated electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and density-functional-theory (DFT) calculations are employed to gain insights in the physical origins of observations. It is demonstrated that STEM-EELS possesses sensitivity to the displacement vector of the vibrational modes as well as the frequency, which is as critical to understanding vibrations as polarization in optical spectroscopies. The combination enables direct mapping of the nonequivalent interface phonons between materials with different phonon polarizations. The results demonstrate the capacity to carefully assess the vibrational properties of complex heterostructures where interface states dominate the functional properties.

Direct Visualization of Localized Vibrations at Complex Grain Boundaries.

Grain boundaries (GBs) are a prolific microstructural feature that dominates the functionality of a wide class of materials. The functionality at a GB results from the unique atomic arrangements, different from those in the grain, that have driven extensive experimental and theoretical studies correlating atomic-scale GB structures to macroscopic electronic, infrared optical, and thermal properties. In this work, a SrTiO3 GB is examined using atomic-resolution aberration-corrected scanning transmission electron microscopy and ultrahigh-energy-resolution monochromated electron energy-loss spectroscopy, in conjunction with density functional theory. This combination enables the correlation of the GB structure, nonstoichiometry, and chemical bonding with a redistribution of vibrational states within the GB dislocation cores. The new experimental access to localized GB vibrations provides a direct route to quantifying the impact of individual boundaries on macroscopic properties.

Observation of grain boundary plasmon and associated deconvolution techniques for low-loss electron energy-loss (EEL) spectra acquired from grain boundaries.

Spatially resolved valence electron energy-loss spectroscopy (VEELS) was used to acquire low-loss EEL spectra from Al grain boundaries (GBs) with different GB energies. The loss signal from the GB is highly delocalized and is mixed with the bulk loss, therefore requiring separation. Three different separation techniques, i.e., Fourier-log, Fourier-ratio deconvolution and direct subtraction, were employed to extract the GB response from the low-loss spectra and produced similar results. The GB response consists of a positive intensity peak from the excitation of GB plasmons (GBP) and a negative intensity begrenzungs (Bgs) peak from reduced scattering from bulk oscillations. Also, lower electron density at the GB reduces the inelastic scattering of the bulk plasmon. The intensity of GBP scattering and begrenzungs peak is found to increase toward the GBs, with maximum intensity when the electron probe is positioned on the GB, connecting the begrenzungs effect with the creation of a GBP.

Observation of solid-state bidirectional thermal conductivity switching in antiferroelectric lead zirconate (PbZrO3).

Materials with tunable thermal properties enable on-demand control of temperature and heat flow, which is an integral component in the development of solid-state refrigeration, energy scavenging, and thermal circuits. Although gap-based and liquid-based thermal switches that work on the basis of mechanical movements have been an effective approach to control the flow of heat in the devices, their complex mechanisms impose considerable costs in latency, expense, and power consumption. As a consequence, materials that have multiple solid-state phases with distinct thermal properties are appealing for thermal management due to their simplicity, fast switching, and compactness. Thus, an ideal thermal switch should operate near or above room temperature, have a simple trigger mechanism, and offer a quick and large on/off switching ratio. In this study, we experimentally demonstrate that manipulating phonon scattering rates can switch the thermal conductivity of antiferroelectric PbZrO3 bidirectionally by -10% and +25% upon applying electrical and thermal excitation, respectively. Our approach takes advantage of two separate phase transformations in PbZrO3 that alter the phonon scattering rate in different manners. In this study, we demonstrate that PbZrO3 can serve as a fast (<1 second), repeatable, simple trigger, and reliable thermal switch with a net switching ratio of nearly 38% from ~1.20 to ~1.65 W m-1 K-1.

Interface controlled thermal resistances of ultra-thin chalcogenide-based phase change memory devices.

Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data processing to overcome the von Neumann bottleneck. In PCMs, data storage is driven by thermal excitation. However, there is limited research regarding PCM thermal properties at length scales close to the memory cell dimensions. Our work presents a new paradigm to manage thermal transport in memory cells by manipulating the interfacial thermal resistance between the phase change unit and the electrodes without incorporating additional insulating layers. Experimental measurements show a substantial change in interfacial thermal resistance as GST transitions from cubic to hexagonal crystal structure, resulting in a factor of 4 reduction in the effective thermal conductivity. Simulations reveal that interfacial resistance between PCM and its adjacent layer can reduce the reset current for 20 and 120 nm diameter devices by up to ~ 40% and ~ 50%, respectively. These thermal insights present a new opportunity to reduce power and operating currents in PCMs.

Suppressed electronic contribution in thermal conductivity of Ge2Sb2Se4Te.

Integrated nanophotonics is an emerging research direction that has attracted great interests for technologies ranging from classical to quantum computing. One of the key-components in the development of nanophotonic circuits is the phase-change unit that undergoes a solid-state phase transformation upon thermal excitation. The quaternary alloy, Ge2Sb2Se4Te, is one of the most promising material candidates for application in photonic circuits due to its broadband transparency and large optical contrast in the infrared spectrum. Here, we investigate the thermal properties of Ge2Sb2Se4Te and show that upon substituting tellurium with selenium, the thermal transport transitions from an electron dominated to a phonon dominated regime. By implementing an ultrafast mid-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in these materials, we find that this reduction in thermal conductivity is a result of a drastic change in electronic lifetimes of Ge2Sb2Se4Te, leading to a transition from an electron-dominated to a phonon-dominated thermal transport mechanism upon selenium substitution. In addition to thermal conductivity measurements, we provide an extensive study on the thermophysical properties of Ge2Sb2Se4Te thin films such as thermal boundary conductance, specific heat, and sound speed from room temperature to 400 °C across varying thicknesses.

Thermal conductivity measurements of sub-surface buried substrates by steady-state thermoreflectance.

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump-probe spectroscopies due to their limited thermal penetration depths. Here, we experimentally and numerically investigate the TPD of the recently developed optical pump-probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD (i.e., the depth at which temperature drops to 1/e value of the maximum surface temperature) does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump-probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries.

High In-Plane Thermal Conductivity of Aluminum Nitride Thin Films.

High thermal conductivity materials show promise for thermal mitigation and heat removal in devices. However, shrinking the length scales of these materials often leads to significant reductions in thermal conductivities, thus invalidating their applicability to functional devices. In this work, we report on high in-plane thermal conductivities of 3.05, 3.75, and 6 µm thick aluminum nitride (AlN) films measured via steady-state thermoreflectance. At room temperature, the AlN films possess an in-plane thermal conductivity of ∼260 ± 40 W m-1 K-1, one of the highest reported to date for any thin film material of equivalent thickness. At low temperatures, the in-plane thermal conductivities of the AlN films surpass even those of diamond thin films. Phonon-phonon scattering drives the in-plane thermal transport of these AlN thin films, leading to an increase in thermal conductivity as temperature decreases. This is opposite of what is observed in traditional high thermal conductivity thin films, where boundaries and defects that arise from film growth cause a thermal conductivity reduction with decreasing temperature. This study provides insight into the interplay among boundary, defect, and phonon-phonon scattering that drives the high in-plane thermal conductivity of the AlN thin films and demonstrates that these AlN films are promising materials for heat spreaders in electronic devices.

Polymersome Poration and Rupture Mediated by Plasmonic Nanoparticles in Response to Single-Pulse Irradiation.

The self-assembly of amphiphilic diblock copolymers into polymeric vesicles, commonly known as polymersomes, results in a versatile system for a variety of applications including drug delivery and microreactors. In this study, we show that the incorporation of hydrophobic plasmonic nanoparticles within the polymersome membrane facilitates light-stimulated release of vesicle encapsulants. This work seeks to achieve tunable, triggered release with non-invasive, spatiotemporal control using single-pulse irradiation. Gold nanoparticles (AuNPs) are incorporated as photosensitizers into the hydrophobic membrane of micron-scale polymersomes and the cargo release profile is controlled by varying the pulse energy and nanoparticle concentration. We have demonstrated the ability to achieve immediate vesicle rupture as well as vesicle poration resulting in temporal cargo diffusion. Additionally, changing the pulse duration, from femtosecond to nanosecond, provides mechanistic insight into the photothermal and photomechanical contributors that govern membrane disruption in this polymer-nanoparticle hybrid system.