Electric field-dependent phonon spectrum and heat conduction in ferroelectrics.

This article shows experimentally that an external electric field affects the velocity of the longitudinal acoustic phonons (vLA), thermal conductivity (κ), and diffusivity (D) in a bulk lead zirconium titanate-based ferroelectric. Phonon conduction dominates κ, and the observations are due to changes in the phonon dispersion, not in the phonon scattering. This gives insight into the nature of the thermal fluctuations in ferroelectrics, namely, phonons labeled ferrons that carry heat and polarization. It also opens the way for phonon-based electrically driven all-solid-state heat switches, an enabling technology for solid-state heat engines. A quantitative theoretical model combining piezoelectric strain and phonon anharmonicity explains the field dependence of vLA, κ, and D without any adjustable parameters, thus connecting thermodynamic equilibrium properties with transport properties. The effect is four times larger than previously reported effects, which were ascribed to field-dependent scattering of phonons.

Controlling Thermal Conductivity in Mesoporous Silica Films Using Pore Size and Nanoscale Architecture.

This work investigates the effect of wall thickness on the thermal conductivity of mesoporous silica materials made from different precursors. Sol-gel- and nanoparticle-based mesoporous silica films were synthesized by evaporation-induced self-assembly using either tetraethyl orthosilicate or premade silica nanoparticles. Since wall thickness and pore size are correlated, a variety of polymer templates were used to achieve pore sizes ranging from 3-23 nm for sol-gel-based materials and 10-70 nm for nanoparticle-based materials. We found that the type of nanoscale precursor determines how changing the wall thickness affects the resulting thermal conductivity. The data indicate that the thermal conductivity of sol-gel-derived porous silica decreased with decreasing wall thickness, while for nanoparticle-based mesoporous silica, the wall thickness had little effect on the thermal conductivity. This work expands our understanding of heat transfer at the nanoscale and opens opportunities for tailoring the thermal conductivity of nanostructured materials by means other than porosity and composition.

Intrinsic Low Thermal Conductivity and Phonon Renormalization Due to Strong Anharmonicity of Single-Crystal Tin Selenide.

Two-dimensional (2D) van der Waals material tin selenide (SnSe) has recently attracted intensive interest due to its exceptional thermoelectric performance. However, the thermal properties and phonon transport mechanisms in its single-crystal form remain elusive. Here, we measured high-quality SnSe single crystals using nanoscale thermometry based on ultrafast optical spectroscopy and found that its intrinsic thermal conductivity is highly anisotropic in different crystallographic directions. To quantify phonon anharmonicity, we developed a new experimental approach combining picosecond ultrasonics and X-ray diffraction to enable direct measurement of temperature-dependent sound velocity, thermal expansion coefficient, and Grüneisen parameter. The measured Grüneisen parameter suggests an abnormally large temperature effect on phonon dispersion that contributes to over 90% of phonon frequency shifts. Furthermore, we performed ab initio calculations using different methods: in comparison with self-consistent phonon theory, the harmonic and quasi-harmonic models that have been widely used in current phonon calculations fail to accurately predict these important thermophysical properties at room temperature and below. Our study reveals an extremely strong intrinsic anharmonicity in SnSe that introduces phonon renormalization near room temperature. This study represents an important research benchmark in characterizing high-performance thermal energy materials and provides fundamental insight into advancing modern calculation methods for phonon transport theory.

Anisotropic Thermal Boundary Resistance across 2D Black Phosphorus: Experiment and Atomistic Modeling of Interfacial Energy Transport.

Interfacial thermal boundary resistance (TBR) plays a critical role in near-junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal-orientation-dependent interfacial transport study. The measurements show that the metal-semiconductor TBR of the cross-plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction-oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature-dependent TBR using density functional theory (DFT)-derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR-structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces.

High-performance field emission based on nanostructured tin selenide for nanoscale vacuum transistors.

Vacuum field effect transistors have been envisioned to hold the promise of replacing solid-state electronics when the ballistic transport of electrons in a nanoscale vacuum can enable significantly high switching speed and stability. However, it remains challenging to obtain high-performance and reliable field-emitter materials. In this work, we report a systematic study on the field emission of novel two-dimensional tin selenide (SnSe) with rational design of its structures and surface morphologies. SnSe in the form of atomically smooth single crystals and nanostructures (nanoflowers) is chemically synthesized and studied as field emitters with varying channel lengths from 6 µm to 100 nm. Our study shows that devices based on SnSe nanoflowers significantly improve the performance and enable field emission at a reduced voltage due to a surface-enhanced local electrostatic field, and further lead to nonlinear dependent channel scaling when the channel length is shorter than 600 nm. We measured a record-high short-channel field-enhancement factor of 50 600 for a 100 nm device. Moreover, we investigated the emission stability and measured the fluctuations of the emission current which are smaller than 5% for more than 20 hours. Our results demonstrated a high-performance and highly reliable field emitter based on 2D SnSe nanostructures and we developed an important building block for nanoscale vacuum field effect transistors.

Anisotropic thermal conductivity measurement using a new Asymmetric-Beam Time-Domain Thermoreflectance (AB-TDTR) method.

Anisotropic thermal properties are of both fundamental and practical interests, but remain challenging to characterize using conventional methods. In this work, a new metrology based on asymmetric beam time-domain thermoreflectance (AB-TDTR) is developed to measure three-dimensional anisotropic thermal transport by extending the conventional TDTR technique. Using an elliptical laser beam with controlled elliptical ratio and spot size, the experimental signals can be exploited to be dominantly sensitive to measure thermal conductivity along the cross-plane or any specific in-plane directions. An analytic solution for a multi-layer system is derived for the AB-TDTR signal in response to the periodical pulse, elliptical laser beam, and heating geometry to extract the anisotropic thermal conductivity from experimental measurement. Examples with experimental data are given for various materials with in-plane thermal conductivity from 5 W/m K to 2000 W/m K, including isotropic materials (silicon, boron phosphide, and boron nitride), transversely isotropic materials (graphite, quartz, and sapphire), and transversely anisotropic materials (black phosphorus). Furthermore, a detailed sensitivity analysis is conducted to guide the optimal setting of experimental configurations for different materials. The developed AB-TDTR metrology provides a new approach to accurately measure anisotropic thermal phenomena for rational materials design and thermal applications.

Experimental observation of high thermal conductivity in boron arsenide.

Improving the thermal management of small-scale devices requires developing materials with high thermal conductivities. The semiconductor boron arsenide (BAs) is an attractive target because of ab initio calculation indicating that single crystals have an ultrahigh thermal conductivity. We synthesized BAs single crystals without detectable defects and measured a room-temperature thermal conductivity of 1300 watts per meter-kelvin. Our spectroscopy study, in conjunction with atomistic theory, reveals that the distinctive band structure of BAs allows for very long phonon mean free paths and strong high-order anharmonicity through the four-phonon process. The single-crystal BAs has better thermal conductivity than other metals and semiconductors. Our study establishes BAs as a benchmark material for thermal management applications and exemplifies the power of combining experiments and ab initio theory in new materials discovery.

Thermal Properties and Phonon Spectral Characterization of Synthetic Boron Phosphide for High Thermal Conductivity Applications.

Heat dissipation is an increasingly critical technological challenge in modern electronics and photonics as devices continue to shrink to the nanoscale. To address this challenge, high thermal conductivity materials that can efficiently dissipate heat from hot spots and improve device performance are urgently needed. Boron phosphide is a unique high thermal conductivity and refractory material with exceptional chemical inertness, hardness, and high thermal stability, which holds high promises for many practical applications. So far, however, challenges with boron phosphide synthesis and characterization have hampered the understanding of its fundamental properties and potential applications. Here, we describe a systematic thermal transport study based on a synergistic synthesis-experimental-modeling approach: we have chemically synthesized high-quality boron phosphide single crystals and measured their thermal conductivity as a record-high 460 W/mK at room temperature. Through nanoscale ballistic transport, we have, for the first time, mapped the phonon spectra of boron phosphide and experimentally measured its phonon mean free-path spectra with consideration of both natural and isotope-pure abundances. We have also measured the temperature- and size-dependent thermal conductivity and performed corresponding calculations by solving the three-dimensional and spectral-dependent phonon Boltzmann transport equation using the variance-reduced Monte Carlo method. The experimental results are in good agreement with that predicted by multiscale simulations and density functional theory, which together quantify the heat conduction through the phonon mode dependent scattering process. Our finding underscores the promise of boron phosphide as a high thermal conductivity material for a wide range of applications, including thermal management and energy regulation, and provides a detailed, microscopic-level understanding of the phonon spectra and thermal transport mechanisms of boron phosphide. The present study paves the way toward the establishment of a new framework, based on the phonon spectra-material structure relationship, for the rational design of high thermal conductivity materials and nano- to multiscale devices.

A Solution Processable High-Performance Thermoelectric Copper Selenide Thin Film.

A solid-state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low-cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Herein, a flexible thermoelectric copper selenide (Cu2 Se) thin film, consisting of earth-abundant elements, is reported. The thin film is fabricated by a low-cost and scalable spin coating process using ink solution with a truly soluble precursor. The Cu2 Se thin film exhibits a power factor of 0.62 mW/(m K2 ) at 684 K on rigid Al2 O3 substrate and 0.46 mW/(m K2 ) at 664 K on flexible polyimide substrate, which is much higher than the values obtained from other solution processed Cu2 Se thin films (<0.1 mW/(m K2 )) and among the highest values reported in all flexible thermoelectric films to date (≈0.5 mW/(m K2 )). Additionally, the fabricated thin film shows great promise to be integrated with the flexible electronic devices, with negligible performance change after 1000 bending cycles. Together, the study demonstrates a low-cost and scalable pathway to high-performance flexible thin film thermoelectric devices from relatively earth-abundant elements.

Ionic Intercalation in Two-Dimensional van der Waals Materials: In Situ Characterization and Electrochemical Control of the Anisotropic Thermal Conductivity of Black Phosphorus.

Two-dimensional van der Waals materials have shown novel fundamental properties and promise for wide applications. Here, we report for the first time an experimental demonstration of the in situ characterization and highly reversible control of the anisotropic thermal conductivity of black phosphorus. We develop a novel platform based on lithium ion batteries that integrates ultrafast optical spectroscopy and electrochemical control to investigate the interactions between lithium ions and the lattices of the black phosphorus electrode. We discover a strong dependence of the thermal conductivity on battery charge states (lithium concentrations) during the discharge/charge process. The thermal conductivity of black phosphorus is reversibly tunable over a wide range of 2.45-3.86, 62.67-85.80, and 21.66-27.58 W·m-1·K-1 in the cross-plan, zigzag, and armchair directions, respectively. The modulation in thermal conductivity is attributed to phonon scattering introduced by the ionic intercalation in between the interspacing layers and shows anisotropic phonon scattering mechanism based on semiclassical model. At the fully discharged state (x ∼ 3 in LixP), a dramatic reduction of thermal conductivity by up to 6 times from that of the pristine crystal has been observed. This study provides a unique approach to explore the fundamental energy transport involving lattices and ions in the layered structures and may open up new opportunities in controlling energy transport based on novel operation mechanisms and the rational design of nanostructures.

Continuous aerosol size separator using inertial microfluidics and its application to airborne bacteria and viruses.

A microchannel-based aerosol size separator that separates submicron aerosols according to particle inertial differences and Dean vortices in the airflow was developed for use in low-cost, portable, real-time aerosol collectors, detectors, concentrators and other such devices. The microfluidic inertial separator was furthermore applied to simultaneously separate airborne microorganisms by size, such as airborne viruses and bacteria from larger aerosols and viral particles from bacterial cells. The entire system was designed by numerical simulation and analysis. In addition, its performance was evaluated experimentally using airborne standard polystyrene latex (PSL) particles. In addition, two airborne microorganisms, Adenovirus 40 and Staphylococcus epidermidis, were used to verify the performance of the separator. The separation ratios of each bioaerosol were measured using real-time aerosol measurement instruments and quantitative polymerase chain reaction (qPCR) analysis. The system was composed of two 90° curved microchannels and three outlets for separating the virus, bacteria and larger particles. About 70% of 3 µm particles but almost none of the bioaerosols were separated out at the first outlet. In addition, more than 70% of S. epidermidis and ~70% Adenovirus were separated out at the second and third outlets, respectively. Unwanted particle loss in the system was less than 10%. The results indicated not only good separation of bioaerosols but also the potential of our separator for use in bioaerosol applications.

Fast and continuous microorganism detection using aptamer-conjugated fluorescent nanoparticles on an optofluidic platform.

Fast and accurate pathogen detection in aquatic environments is challenging in many biomedical studies and microbial diagnostic applications. In this study, we developed a real-time, continuous, and non-destructive single cell detection method using target specific aptamer-conjugated fluorescent nanoparticles (A-FNPs) and an optofluidic particle-sensor platform. A-FNPs selectively bound to the surfaces of target bacteria (Escherichia coli) and labeled them with high affinity and selectivity so that target bacteria can be countable particles in an optofluidic particle-sensor. A-FNP-labeled target bacterial complexes were detected by the optofluidic particle-sensing system, which provides rapid and continuous single-cell detection. A-FNPs selectively bound to E. coli with a dissociation constant of 0.83 nM, but did not bind Enterobacter aerogenes or Citrobacter freundii strains, which lacked affinity for the aptamer used. We demonstrated that our optofluidic device achieves a detection throughput of ~100 particles per second with high accuracy (~85%) in detecting single bacterial cells conjugated with A-FNPs. This approach can be immediately extended to the real-time, high-throughput detection of other microorganisms such as viruses that are selectively conjugated with A-FNPs. Collectively, these data suggest that optofluidic systems are widely applicable for the fast and continuous detection of microbial cells.

Real-time detection of an airborne microorganism using inertial impaction and mini-fluorescent microscopy.

To achieve successful real-time detection of airborne pathogenic microorganisms, the problem must be considered in terms of their physical size and biological characteristics. We developed an airborne microorganism detection chip to realize the detection of microorganisms, ensuring compactness, sensitivity, cost-efficiency, and portability, using three key components: an inertial impaction system, a cartridge-type impaction plate, and a mini-fluorescent microscope. The inertial impaction system was used to separate microorganisms in terms of their aerodynamic particle size, and was fabricated with three impaction stages. Numerical analysis was performed to design the system; the calculated cutoff diameter at each impaction stage was 2.02 (first stage), 0.88 (second stage), and 0.54 µm (third stage). The measured cutoff diameters were 2.24, 0.91, and 0.49 µm, respectively. A cartridge-type impaction plate was used, composed of molded polydimethylsiloxane (PDMS) and an actual impaction region made of a SYBR green I dye-stained agar plate. A mini-fluorescent microscope was used to distinguish microbes from non-biological particles. Images of the microorganisms deposited at the impaction zone were obtained via mini-fluorescent microscopy, and fluorescent intensities of the images were calculated using in-house image-processing software. The results showed that the developed system successfully identified aerosolized biological particles from non-biological particles in real time.