Abnormal Magnetic Phase Transition in Mixed-Phase (110)-Oriented FeRh Films on Al2O3 Substrates via the Anomalous Nernst Effect.

Iron rhodium (FeRh) undergoes a first-order anti-ferromagnetic to ferromagnetic phase transition above its Curie temperature. By measuring the anomalous Nernst effect (ANE) in (110)-oriented FeRh films on Al2O3 substrates, the ANE thermopower over a temperature range of 100-350 K is observed, with similar magnetic transport behaviors observed for in-plane magnetization (IM) and out-of-plane magnetization (PM) configurations. The temperature-dependent magnetization-magnetic field strength (M-H) curves revealed that the ANE voltage is proportional to the magnetization of the material, but additional features magnetic textures not shown in the M-H curves remained intractable. In particular, a sign reversal occurred for the ANE thermopower signal near zero field in the mixed-magnetic-phase films at low temperatures, which is attributed to the diamagnetic properties of the Al2O3 substrate. Finite element method simulations associated with the Heisenberg spin model and Landau-Lifshitz-Gilbert equation strongly supported the abnormal heat transport behavior from the Al2O3 substrate during the experimentally observed magnetic phase transition for the IM and PM configurations. The results demonstrate that FeRh films on an Al2O3 substrate exhibit unusual behavior compared to other ferromagnetic materials, indicating their potential for use in novel applications associated with practical spintronics device design, neuromorphic computing, and magnetic memory.

Enhanced Spin Seebeck Thermopower in Pt/Holey MoS2/Y3Fe5O12 Hybrid Structure.

We first observed the spin-to-charge conversion due to both the inverse Rashba-Edelstein effect (IREE) and inverse spin-Hall effect in a holey multilayer molybdenum disulfide (MoS2) intermediate layer in a Pt/YIG structure via LSSE measurements under nonequilibrium magnetization. We found an enhancement of approximately 238%, 307%, and 290% in the longitudinal spin Seebeck effect (LSSE) voltage, spin-to-charge current, and thermoelectric (TE) power factor, respectively, compared with the monolayer MoS2 interlayer in a Pt/YIG structure. Such an enhancement in the LSSE performance of Pt/holey MoS2/YIG can be explained by the improvement of spin accumulation in the Pt layer by induced spin fluctuation as well as increased additional spin-to-charge conversion due to in-plane IREE. Our findings represent a significant achievement in the understanding of spin transport in atomically thin MoS2 interlayers and pave the way toward large-area TE energy-harvesting devices in two-dimensional transition metal dichalcogenide materials.

Anisotropic temperature-dependent thermal conductivity by an Al2O3 interlayer in Al2O3/ZnO superlattice films.

The thermal conductivity of superlattice films is generally anisotropic and should be studied separately in the in-plane and cross-plane directions of the films. However, previous works have mostly focused on the cross-plane thermal conductivity because the electrons and phonons in the cross-plane direction of superlattice films may result in much stronger interface scattering than that in the in-plane direction. Nevertheless, it is highly desirable to perform systematic studies on the effect of interface formation in semiconducting superlattice films on both in-plane and cross-plane thermal conductivities. In this study, we determine both the in-plane and cross-plane thermal conductivities of Al2O3 (AO)/ZnO superlattice films grown by atomic layer deposition (ALD) on SiO2/Si substrates in the temperature range of 50-300 K by the four-point-probe 3-ω method. Our experimental results indicate that the formation of an atomic AO layer (0.82 nm) significantly contributes to the decrease of the cross-plane thermal conductivity of the AO/ZnO superlattice films compared with that of AO/ZnO thin films. The cross-plane thermal conductivity (0.26-0.63 W m-1 K-1 of the AO/ZnO superlattice films (with an AO layer of ∼0.82 nm thickness) is approximately ∼150%-370% less than the in-plane thermal conductivity (0.96-1.19 W m-1 K-1) of the corresponding film, implying significant anisotropy. This indicates that the suppression of the cross-plane thermal conductivity is mainly attributed to the superlattice, rather than the nanograin columnar structure in the films. In addition, we theoretically analyzed strong anisotropic behavior of the in-plane and cross-plane thermal conductivities of the AO/ZnO superlattice films in terms of temperature dependence.

Lateral heat flow distribution and defect-dependent thermal resistance in an individual silicon nanowire.

Studies aiming to significantly improve thermal properties, such as figure-of-merit, of silicon nanowires (SiNW) have focused on diameter reduction and surface or interface roughness control. However, the mechanism underlying thermal conductivity enhancement of roughness controlled NWs remains unclear. Here, we report a significant influence of stacking faults (SFs) on the lateral thermal conductivity of a single SiNW, using a combination of newly developed in situ spatially-resolved thermal resistance experiments and high-resolution transmission electron microscopy measurements. We used as-grown SiNWs tapered along the growth direction with progressively lower roughness and SFs density. The results clearly confirmed that both surface roughness and twins or SFs densities suppress the thermal conductivity of an individual SiNW. The results and measurement techniques presented here hold great potential for inspecting minute changes in thermal resistance along an individual SiNW, caused by induced SFs on the nanostructure, and for improving one-dimensional nanowire-based thermoelectric device performance.

Dependence of filopodia morphology and the separation efficiency of primary CD4⁺ T-lymphocytes on nanopillars.

Despite significant improvement in separation efficiency using nanostructure-based platforms, the mechanism underlying the high efficiency of rare cell capture remains elusive. Here we report on the first mechanistic study by developing highly controlled nanostructures to investigate cell surface nanomorphology to better understand the cellular response of CD4(+) T-lymphocytes in contact with nanostructured surfaces and to elucidate key mechanisms for enhancing separation efficiency. Our results showed that actin-rich filopodia protruded from T-cells in the early stage of cell capture (<20 min), demonstrate the different morphologies in response to various quartz nanopillar (QNP) arrays functionalized with streptavidin and the generation of sufficient adhesion sites for rendering more stable binding through three-dimensional local nanotopographic interactions between filopodia-QNPs and cell-substrate, leading to synergistic effects for enhancing cell-capture efficiency. This responsive mechanism of T-cells on nanotopographic templates provides new insights to understand the enhanced cell-capture efficiency and specificity from the primary cell suspension on nanostructured substrates.

Electrical transport characterization of PEDOT:PSS/n-Si Schottky diodes and their applications in solar cells.

We demonstrate locally contacted PEDOT:PSS Schottky diodes with excellent rectifying behavior, fabricated on n-type Si substrates using a spin-coating process and a reactive-ion etching process. Electrical transport characterizations of these Schottky diodes were investigated by both current-voltage (I-V) and capacitance-voltage (C-V) measurements. We found that these devices exhibit excellent modulation in the current with an on/off ratio of - 10(6). Schottky junction solar cells composed of PEDOT:PSS and n-Si structures were also examined. From the current density-voltage (J-V) measurement of a solar cell under illumination, the short circuit current (I(sc)), open circuit voltage (V(oc)), and conversion efficiency (eta) were - 19.7 mA/cm2, - 578.5 mV, and - 6.5%, respectively. The simple and low-cost fabrication process of the PEDOT:PSS/n-Si Schottky junctions makes them a promising candidate for further high performance solar cell applications.

Thermally Driven Spin Transport of Epitaxial FeRh Films with a Non-magnetic Pt Layer via the Longitudinal Spin Seebeck Effect.

FeRh has been demonstrated to be an important material for the observation of magnetic phase transitions, such as the first-order transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state, in response to changes in the temperature. This is because of the magnetic moment induced in Rh atoms above the magnetic phase transition temperature. In the present study, we focus on the longitudinal spin Seebeck effect (LSSE), which involves the generation of spin voltage as a result of a temperature gradient in FM materials or FM insulators, and experimentally assess the effect of the crystalline quality of FeRh films and the properties of the substrate on the LSSE thermopower during the FM-AFM phase transition. The measured LSSE thermopower of an epitaxial (110)-oriented FeRh film grown on an Al2O3 substrate is approximately 60 times higher than that of a polycrystalline FeRh film on a SiO2/Si substrate. This can be explained by the high magnetic sensitivity and superior FM properties of (110)-oriented epitaxial FeRh films. Furthermore, by comparing the transverse thermoelectric voltage for in-plane magnetized (IM) and perpendicularly magnetized (PM) configurations, we quantitively evaluate the contribution of the exclusive anomalous Nernst effect (ANE) to the LSSE signals in the FeRh/Al2O3 structure, finding it to be approximately 15-30% over a temperature range of 75-300 K. LSSE measurements in Pt/FeRh films are thus demonstrated to provide a versatile pathway for the development of thermoelectric power generation applications and other practical spintronics and neuromorphic computing devices.

Temperature-dependent thermal conductivities of 1D semiconducting nanowires via four-point-probe 3-ω method.

We report on a systematic study of the thermal transport characteristics of both as-grown zinc oxide and gallium nitride nanowires (NWs) via the four-point-probe 3-ω method in the temperature range 130-300 K. Both as-grown NWs were synthesized by a vapor-liquid-solid growth mechanism, and show clear n-type semiconducting behavior without any defects, which enables both the NWs to be promising candidates for thermoelectric materials. To measure the thermal conductivities of both NWs with lower heat loss and measurement errors, the suspended structures were formed by a combination of an e-beam lithography process and a random dispersion method. The measured thermal conductivities of both NWs are greatly reduced compared to their bulk materials due to the enhanced phonon scattering via the size effect and dopants (impurities). Furthermore, we observed that the Umklapp peaks of both NWs are shifted to a higher temperature than those of their bulk counterparts, indicating that phonon-boundary scattering dominates over other phonon scattering due to the size effect.

Thermal conductivity measurements of single-crystalline bismuth nanowires by the four-point-probe 3-ω technique at low temperatures.

We have successfully investigated the thermal conductivity (κ) of single-crystalline bismuth nanowires (BiNWs) with [110] growth direction, via a straightforward and powerful four-point-probe 3-ω technique in the temperature range 10-280 K. The BiNWs, which are well known as the most effective material for thermoelectric (TE) device applications, were synthesized by compressive thermal stress on a SiO2/Si substrate at 250-270 °C for 10 h. To understand the thermal transport mechanism of BiNWs, we present three kinds of experimental technique as follows, (i) a manipulation of a single BiNW by an Omni-probe in a focused ion beam (FIB), (ii) a suspended bridge structure integrating a four-point-probe chip by micro-fabrication to minimize the thermal loss to the substrate, and (iii) a simple 3-ω technique system setup. We found that the thermal transport of BiNWs is highly affected by boundary scattering of both phonons and electrons as the dominant heat carriers. The thermal conductivity of a single BiNW (d ~ 123 nm) was estimated to be ~2.9 W m(-1) K(-1) at 280 K, implying lower values compared to the thermal conductivity of the bulk (~11 W m(-1) K(-1) at 280 K). It was noted that this reduction in the thermal conductivity of the BiNWs could be due to strongly enhanced phonon-boundary scattering at the surface of the BiNWs. Furthermore, we present temperature-dependent (10-280 K) thermal conductivity of the BiNWs using the 3-ω technique.

Nanowire substrate-based laser scanning cytometry for quantitation of circulating tumor cells.

We report on the development of a nanowire substrate-enabled laser scanning imaging cytometry for rare cell analysis in order to achieve quantitative, automated, and functional evaluation of circulating tumor cells. Immuno-functionalized nanowire arrays have been demonstrated as a superior material to capture rare cells from heterogeneous cell populations. The laser scanning cytometry method enables large-area, automated quantitation of captured cells and rapid evaluation of functional cellular parameters (e.g., size, shape, and signaling protein) at the single-cell level. This integrated platform was first tested for capture and quantitation of human lung carcinoma cells from a mixture of tumor cells and leukocytes. We further applied it to the analysis of rare tumor cells spiked in fresh human whole blood (several cells per mL) that emulate metastatic cancer patient blood and demonstrated the potential of this technology for analyzing circulating tumor cells in the clinical settings. Using a high-content image analysis algorithm, cellular morphometric parameters and fluorescence intensities can be rapidly quantitated in an automated, unbiased, and standardized manner. Together, this approach enables informative characterization of captured cells in situ and potentially allows for subclassification of circulating tumor cells, a key step toward the identification of true metastasis-initiating cells. Thus, this nanoenabled platform holds great potential for studying the biology of rare tumor cells and for differential diagnosis of cancer progression and metastasis.

Cell adhesion and migration on nanopatterned substrates and their effects on cell-capture yield.

With scanning electron microscopy analysis, we investigated the role of nanoscale topography on cellular activities; e.g. cell adhesion and spreading by culturing A549 cells (human lung carcinoma cell line cells) for 1-48 h on three sets of nanostructures; quartz nanopillars (QNPs), silicon nanopillars and silicon nanowire (SiNW) arrays, along with planar glass substrates. We found that cells on QNP arrays developed a longer shape than those on SiNW arrays. In addition, we studied how cell morphologies influence the cell-capture yield on the three sets of nanostructures. This research showed that the filopodial formations were directing the cell-capture yield on nanostructured substrates. This finding implies the possibility of using nanoscale topography features to control the filopodial formation including extension and migration from the cells. Using streptavidin-functionalized SiNW substrate, we further demonstrated a substantially higher yield (~91.8 ± 5.9%) than the planar glass wafers (~24.1 ± 7.5%) in the range of 200-3000 cells.

Enhanced Transverse Seebeck Coefficients in 2D/2D PtSe2/MoS2 Heterostructures Using Wet-Transfer Stacking.

It is very challenging to estimate thermoelectric (TE) properties when applying millimeter-scale two-dimensional (2D) transition metal dichalcogenide (TMDC) materials to TE device applications, particularly their Seebeck coefficient due to their high intrinsic electrical resistance. This paper proposes an innovative approach to measure large transverse (i.e., in-plane) Seebeck coefficients for 2D TMDC materials by placing a low resistance (LR) semimetallic PtSe2 film on high-resistance (HR) semiconducting MoS2 (>10 MΩ), whose internal resistance is too high to measure the Seebeck coefficient, forming a heterojunction structure using wet-transfer stacking. The vertically stacked LR-PtSe2 (3 nm)/HR-MoS2 (12 nm) heterostructure film exhibits a high Seebeck coefficient > 190 µV/K up to 5 K temperature difference. This unusual behavior can be explained by an additional Seebeck effect induced at the interface between the LR-2D/HR-2D heterostructure. The proposed stacked LR-PtSe2/HR-MoS2 heterostructure film offers promising phenomena 2D/2D materials that enable innovative TE device applications.

Abnormal Seebeck Effect in Vertically Stacked 2D/2D PtSe2 /PtSe2 Homostructure.

When a thermoelectric (TE) material is deposited with a secondary TE material, the total Seebeck coefficient of the stacked layer is generally represented by a parallel conductor model. Accordingly, when TE material layers of the same thickness are stacked vertically, the total Seebeck coefficient in the transverse direction may change in a single layer. Here, an abnormal Seebeck effect in a stacked two-dimensional (2D) PtSe2 /PtSe2 homostructure film, i.e., an extra in-plane Seebeck voltage is produced by wet-transfer stacking at the interface between the PtSe2 layers under a transverse temperature gradient is reported. This abnormal Seebeck effect is referred to as the interfacial Seebeck effect in stacked PtSe2 /PtSe2 homostructures. This effect is attributed to the carrier-interface interaction, and has independent characteristics in relation to carrier concentration. It is confirmed that the in-plane Seebeck coefficient increases as the number of stacked PtSe2 layers increase and observed a high Seebeck coefficient exceeding ≈188 µV K-1 at 300 K in a four-layer-stacked PtSe2 /PtSe2 homostructure.

Interface-Induced Seebeck Effect in PtSe2/PtSe2 van der Waals Homostructures.

The Seebeck effect refers to the production of an electric voltage when different temperatures are applied on a conductor, and the corresponding voltage-production efficiency is represented by the Seebeck coefficient. We report a Seebeck effect: thermal generation of driving voltage from the heat flowing in a thin PtSe2/PtSe2 van der Waals homostructure at the interface. We refer to the effect as the interface-induced Seebeck effect. By exploiting this effect by directly attaching multilayered PtSe2 over high-resistance PtSe2 thin films as a hybridized single structure, we obtained the highly challenging in-plane Seebeck coefficient of the PtSe2 films that exhibit extremely high resistances. This direct attachment further enhanced the in-plane thermal Seebeck coefficients of the PtSe2/PtSe2 van der Waals homostructure on sapphire substrates. Consequently, we successfully enhanced the in-plane Seebeck coefficients for the PtSe2 (10 nm)/PtSe2 (2 nm) homostructure approximately 42% compared to that of a pure PtSe2 (10 nm) layer at 300 K. These findings represent a significant achievement in understanding the interface-induced Seebeck effect and provide an effective strategy for promising large-area thermoelectric energy harvesting devices using two-dimensional transition metal dichalcogenide materials, which are ideal thermoelectric platforms with high figures of merit.

Growth time-dependent density and surface evolution of silicon nanowires in a vapor-liquid-solid process.

Single crystalline silicon nanowires (SiNWs) were grown on Si(100) substrate using a gold (Au)-catalyzed vapor-liquid-solid (VLS) approach. The dependence of the growth time (i.e., the time of exposure to the Si source) on the density and surface evolution of the grown SiNWs is considered. It was observed that the density of grown SiNWs on Si substrate increased with increasing growth time. The highest density (approximately 1.1 x 10(6) mm(-2)) was reached at 4 hr. Upon further exposure to the Si source, we observed that the density was maintained for up to 9 hr. We suggest that the increased Si chemical potential in Au-Si droplets with increased growth time enhanced the SiNW growth rate at the interfaces between Au-Si droplets and SiNWs, and enhanced the transition of the NWs from the existing Au-Si droplets onto Si substrate. This allows the SiNW density to increase with increased growth time of up to 4 hr. Moreover, we examined the influence of the growth time on surface evolution including Au diffusion, facet and taper formation, and vapor-solid (VS) growth of the SiNWs. To explain the behavior of the grown SiNWs in the VLS process, we propose a combined model using the VLS and VS growth mechanisms.

Role of Ferromagnetic Monolayer WSe2 Flakes in the Pt/Y3Fe5O12 Bilayer Structure in the Longitudinal Spin Seebeck Effect.

The spin Seebeck effect (SSE) has attracted renewed interest as a promising phenomenon for energy harvesting systems. A noteworthy effort has been devoted to improving the SSE voltage by inserting ultrathin magnetic layers including Fe70Cu30 interlayers in Pt/Y3Fe5O12 (Pt/YIG) systems with increased spin-mixing conductance at the interfaces. Nevertheless, the responsible underlying physics associated with the role of the interlayer in Pt/YIG systems in the SSE is still unknown. In this paper, we demonstrate that with a monolayer tungsten diselenide (ML WSe2) interlayer in the Pt/YIG bilayer system, the longitudinal SSE (LSSE) voltage is significantly increased by the increased spin accumulation in the Pt layer; the spin fluctuation in ML WSe2 amplifies the spin current transmission because the in-plane-aligned WSe2 spins are coupled to thermally pumped spins under nonequilibrium magnetization conditions in the LSSE configuration at room temperature. The thermopower (VLSSE/ΔT) improves by 323% with respect to the value of the reference Pt/YIG bilayer sample in the LSSE at room temperature. In addition, the induced ferromagnetic properties of the ML WSe2 flakes on YIG increase the LSSE voltage (VLSSE) of the sample; the ferromagnetic properties are a result of the improved magnetic moment density in the ML WSe2 flakes and their two-dimensional (2D) ML nature in the LSSE under nonequilibrium magnetization conditions. The results can extend the application range of the materials in energy harvesting and provide important information on the physics of the LSSE with a transition metal dichalcogenide intermediate layer in spin transport.

Giant Thermoelectric Seebeck Coefficients in Tellurium Quantum Wires Formed Vertically in an Aluminum Oxide Layer by Electrical Breakdown.

High efficiency thermoelectric (TE) materials still require high thermopower for energy harvesting applications. A simple elemental metallic semiconductor, tellurium (Te), has been considered critical to realize highly efficient TE conversion due to having a large effective band valley degeneracy. This paper demonstrates a novel approach to directly probe the out-of-plane Seebeck coefficient for one-dimensional Te quantum wires (QWs) formed locally in the aluminum oxide layer by well-controlled electrical breakdown at 300 K. Surprisingly, the out-of-plane Seebeck coefficient for these Te QWs ≈ 0.8 mV/K at 300 K. This thermopower enhancement for Te QWs is due to Te intrinsic nested band structure and enhanced energy filtering at Te/AO interfaces. Theoretical calculations support the enhanced high Seebeck coefficient for elemental Te QWs in the oxide layer. The local-probed observation and detecting methodology used here offers a novel route to designing enhanced thermoelectric materials and devices in the future.

Extrinsic Surface Magnetic Anisotropy Contribution in Pt/Y3Fe5O12 Interface in Longitudinal Spin Seebeck Effect by Graphene Interlayer.

A recent study found that magnetization curves for Y3Fe5O12 (YIG) slab and thick films (>20 µm thick) differed from bulk system curves by their longitudinal spin Seebeck effect in a Pt/YIG bilayer system. The deviation was due to intrinsic YIG surface magnetic anisotropy, which is difficult to adopt extrinsic surface magnetic anisotropy even when in contact with other materials on the YIG surface. This study experimentally demonstrates evidence for extrinsic YIG surface magnetic anisotropy when in contact with a diamagnetic graphene interlayer by observing the spin Seebeck effect, directly proving intrinsic YIG surface magnetic anisotropy interruption. We show the Pt/YIG bilayer system graphene interlayer role using large area single and multilayered graphenes using the longitudinal spin Seebeck effect at room temperature, and address the presence of surface magnetic anisotropy due to magnetic proximity between graphene and YIG layer. These findings suggest a promising route to understand new physics of spin Seebeck effect in spin transport.

Surface Coverage Dependence of Spin-to-Charge Current across Pt/MoS2/Y3Fe5O12 Layers via Longitudinal Spin Seebeck Effect.

The voltage induced by the inverse spin Hall effect (ISHE) is affected by several factors, including the spin Hall angle of the normal metal (NM), the quality and magnetic properties of the ferromagnetic material (FM), and the interface conditions between the NM and FM bilayers in longitudinal spin Seebeck effect (LSSE) measurement. Specifically, the interface conditions in NM/FM systems via LSSE devices play a crucial role in determining the efficiency of spin current injection into the NM layer. In this letter, we report a new approach to controlling the efficiency of spin current injection into a Pt layer across a Pt/Y3Fe5O12 (YIG) interface by surface coverage of the intermediate layer. A continuous, large-area multilayer molybdenum dichalcogenide (MoS2) thin film grown by chemical vapor deposition is inserted between the Pt and YIG layers in the LSSE configuration. We found that, when the large-area multilayer MoS2 film was present, the measured ISHE-induced voltage and theoretically calculated spin current in the Pt/MoS2/YIG trilayer increased by ∼510% and 470%, respectively, compared to those of a Pt/YIG bilayer. The induced voltage and spin current were very sensitive to the surface conductance, which was affected by the surface coverage of the multilayer MoS2 films in the LSSE measurement. Furthermore, the theoretically calculated spin current and spin mixing conductance in the trilayer geometry are in qualitatively good agreement with the experimental observations. These measurements enable us to explain the effect of the interface conditions on the spin Seebeck effect in spin transport.

Achieving Out-of-Plane Thermoelectric Figure of Merit ZT = 1.44 in a p-Type Bi2Te3/Bi0.5Sb1.5Te3 Superlattice Film with Low Interfacial Resistance.

Recently, low-dimensional superlattice films have attracted significant attention because of their low dimensionality and anisotropic thermoelectric (TE) properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. For these superlattice structures, both electrons and phonons show highly anisotropic behavior and exhibit much stronger interface scattering in the out-of-plane direction of the films compared to the in-plane direction. However, no detailed information is available in the literature for the out-of-plane TE properties of the superlattice-based films. In this report, we present the out-of-plane Seebeck coefficient, thermal conductivity, and electrical properties of p-type Bi2Te3/Bi0.5Sb1.5Te3 (bismuth telluride/bismuth antimony telluride, BT/BST) superlattice films in the temperature range of 77-500 K. Because of the synergistic combination of the energy filtering effect and low interfacial resistance of the superlattice structure, an impressively high ZT of 1.44 was achieved at 400 K for the 200 nm-thick p-type BT/BST superlattice film, corresponding to a 43% ZT enhancement compared to the pristine p-BST films with the same thickness.