Comparison of influence of intercalation and substitution of Cu on electrical and thermoelectric transport properties of InSe alloys.

Layered post-transition-metal chalcogenides, such as InSe, In4Se3, SnSe, and SnSe2, have recently been investigated as semiconducting electronic materials and thermoelectric materials owing to their adjustable electrical transport properties either by doping or alloying. Herein, the influence of intercalation doping and substitutional doping of Cu in layered InSe alloys on electrical and thermoelectric transport properties was investigated and compared by synthesizing varied compositions of CuxInSe and In1-yCuySe. It was found that Cu was intercalated in CuxInSe samples (x = 0.01 and 0.02) and behaved as an electron donor, resulting in an increase in the electron concentration and a decrease in the activation energy. Therefore, the power factor of CuxInSe samples was increased compared to that of InSe. In contrast, the substituted Cu in the In site of In1-yCuySe samples (y = 0.01 and 0.02) acted as an acceptor, and the power factor decreased owing to a decrease in the electron concentration and activation energy. Moreover, a decrease in thermal conductivity was seen for CuxInSe and In1-yCuySe samples due to increased phonon scattering after the addition of Cu. Consequently, an enhanced thermoelectric figure of merit (zT) was only observed for intercalated CuxInSe samples due to the increased power factor and decreased thermal conductivity, while substituted In1-yCuySe samples only show degraded zT. A maximum zT value of 0.062 was observed for the CuxInSe (x = 0.02) sample at 700 K, which showed a 77% enhancement compared to that of InSe.

Investigation of the Effect of Double-Filler Atoms on the Thermoelectric Properties of Ce-YbCo4Sb12.

Skutterudite compounds have been studied as potential thermoelectric materials due to their high thermoelectric efficiency, which makes them attractive candidates for applications in thermoelectric power generation. In this study, the effects of double-filling on the thermoelectric properties of the CexYb0.2-xCo4Sb12 skutterudite material system were investigated through the process of melt spinning and spark plasma sintering (SPS). By replacing Yb with Ce, the carrier concentration was compensated for by the extra electron from Ce donors, leading to optimized electrical conductivity, Seebeck coefficient, and power factor of the CexYb0.2-xCo4Sb12 system. However, at high temperatures, the power factor showed a downturn due to bipolar conduction in the intrinsic conduction regime. The lattice thermal conductivity of the CexYb0.2-xCo4Sb12 skutterudite system was clearly suppressed in the range between 0.025 and 0.1 for Ce content, due to the introduction of the dual phonon scattering center from Ce and Yb fillers. The highest ZT value of 1.15 at 750 K was achieved for the Ce0.05Yb0.15Co4Sb12 sample. The thermoelectric properties could be further improved by controlling the secondary phase formation of CoSb2 in this double-filled skutterudite system.

Three-dimensional ternary NixCuyZnz(CO3)(OH)2 electrodes for supercapacitors: electrochemical properties and applications.

Transition metal-based binary and ternary compound arrays were directly grown on a porous Ni foam substrate using a facile one-step hydrothermal method. Transition metals are considered ideal electrode materials for faradaic capacitors because they exhibit a wide range of oxidation states enabling effective redox charge transfer. Furthermore, compounds in which two or more transition metals react can help increase the number of active sites for charge-discharge reactions and provide more valence changes for improved charge transfer. In this work, we fabricated ternary electrodes with Ni, Cu, and Zn ions, exhibiting a larger surface area and higher entropy than those made with binary compounds. The NixCuyZnz-based ternary electrode had a shorter diffusion path for the electrolyte ions owing to its larger surface area. Ternary compounds can increase the entropy of the electrode because of the reaction between atoms of different sizes, bringing about a synergistic effect for high characteristic electrochemical values. The optimized NixCuyZnz(CO3)(OH)2 compound showed a maximum specific capacity of 344 mA h g-1 at a current density of 3 A g-1, which was remarkably higher than that of the binary electrode, and a cycling stability of 84.9% after 5000 cycles. An asymmetric supercapacitor produced with this compound as the positive electrode and graphene as the negative electrode exhibited a high energy density of 36.2 W h kg-1 at a power density of 103.1 W kg-1 and a current density of 2 A g-1. The asymmetric supercapacitor fabricated using the NixCuyZnz(CO3)(OH)2 compound as the positive electrode exhibited excellent electrical properties when used in an illuminated LED device.

Analysis of the Response Characteristics of Toluene Gas Sensors with a ZnO Nanorod Structure by a Heat Treatment Process.

The sensing characteristics of toluene gas are monitored by fabricating ZnO nanorod structures. ZnO nanostructured sensor materials are produced on a Zn film via an ultrasonic process in a 0.01 M aqueous solution of C6H12N4 and Zn(NO3)2∙6H2O. The response of the sensors subjected to heat treatment in oxygen and nitrogen atmospheres is improved by 20% and 10%, respectively. The improvement is considered to be correlated with the increase in grain size. The relationship between the heat treatment and sensing characteristics is evaluated.

Electron-beam induced damage process for Ca2Na2Nb5O16nanosheets.

Dielectric two-dimensional oxide nanosheets are attractive because of their thermal stability and high-k property. However, their atomic structure characterization has been limited since they are easily degraded by electron-beams. This study aimed to investigate the electron-beam induced damage mechanisms for exfoliated Ca2Na2Nb5O16(CNNO) nanosheets. Knock-on damage dominantly occurred at high voltages, leaving short-range order in the final amorphous structure. On the other hand, a series of chemical reactions predominantly occurred at low voltages, resulting in random elemental loss and a fully disordered amorphous structure. This radiolysis was facilitated by insulated CNNO nanosheets that contained a large number of dangling bonds after the chemical solution process. The radiolysis damage kinetics was faster than knock-on damage and induced more elemental loss. Based on our understanding of the electron beam-induced degradation, atomic-scale imaging of the CNNO nanosheets was successfully performed using Cs-corrected scanning transmission electron microscopy at 300 keV with a decreased beam current. This result is of particular significance because understanding of electron-beam damage in exfoliated and insulating 2D oxide sheets could improve identification of their atomic structure using electron microscopy techniques and lead to a practical guide for further extensive characterization of doped elements and layered structures to improve their properties.

Thermoelectric Properties of Cu2Te Nanoparticle Incorporated N-Type Bi2Te2.7Se0.3.

To develop highly efficient thermoelectric materials, the generation of homogeneous heterostructures in a matrix is considered to mitigate the interdependency of the thermoelectric compartments. In this study, Cu2Te nanoparticles were introduced onto Bi2Te2.7Se0.3 n-type materials and their thermoelectric properties were investigated in terms of the amount of Cu2Te nanoparticles. A homogeneous dispersion of Cu2Te nanoparticles was obtained up to 0.4 wt.% Cu2Te, whereas the Cu2Te nanoparticles tended to agglomerate with each other at greater than 0.6 wt.% Cu2Te. The highest power factor was obtained under the optimal dispersion conditions (0.4 wt.% Cu2Te incorporation), which was considered to originate from the potential barrier on the interface between Cu2Te and Bi2Te2.7Se0.3. The Cu2Te incorporation also reduced the lattice thermal conductivity, and the dimensionless figure of merit ZT was increased to 0.75 at 374 K for 0.4 wt.% Cu2Te incorporation compared with that of 0.65 at 425 K for pristine Bi2Te2.7Se0.3. This approach could also be an effective means of controlling the temperature dependence of ZT, which could be modulated against target applications.

No evidence for global decrease in CO2 concentration during the first wave of COVID-19 pandemic.

Numerous studies have reported that CO2 emissions have decreased because of global lockdown during the first wave of the COVID-19 pandemic. However, previous estimates of the global CO2 concentration before and after the outbreak of the COVID-19 pandemic are limited because they are based on energy consumption statistics or local specific in-situ observations. The aim of the study was to explore objective evidence for various previous studies that have claimed the global CO2 concentration decreased during the first wave of the COVID-19 pandemic. There are two ways to measure the global CO2 concentration: from the top-down using satellites and the bottom-up using ground stations. We implemented the time-series analysis by comparing the before and after the inflection point (first wave of COVID-19) with the long-term CO2 concentration data obtained from World Meteorological Organization Global Atmosphere Watch (WMO GAW) and Greenhouse Gases Observing Satellite (GOSAT). Measurements from the GOSAT and GAW global monitoring stations show that the CO2 concentrations in Europe, China, and the USA have continuously risen in March and April 2020 compared with the same months in 2019. These data confirm that the global lockdown during the first wave of the COVID-19 pandemic did not change the vertical CO2 profile at the global level from the ground surface to the upper layer of the atmosphere. The results of this study provide an important foundation for the international community to explore policy directions to mitigate climate change in the upcoming post-COVID-19 period.

Modulation of Conductivity and Contact Resistance of RuO2 Nanosheets via Metal Nano-Particles Surface Decoration.

We studied the variation in electrical conductivity of exfoliated RuO2 nanosheets and the modulation in the contact resistance of individual nanosheet devices using charge transfer doping effects based on surface metal nanoparticle decorations. The electrical conductivity in the monolayer and bilayer RuO2 nanosheets gradually increased due to the surface decoration of Cu, and subsequently Ag, nanoparticles. We obtained contact resistances between the nanosheet and electrodes using the four-point and two-point probe techniques. Moreover, the contact resistances decreased during the surface decoration processes. We established that the surface decoration of metal nanoparticles is a suitable method for external contact engineering and the modulation of the internal properties of nanomaterials.

Silver Nanowire Network Hybridized with Silver Nanoparticle-Anchored Ruthenium Oxide Nanosheets for Foldable Transparent Conductive Electrodes.

Facile strategies in flexible transparent conductive electrode materials that can sustain their electrical conductivities under 1 mm-scale radius of curvature are required for wider applications such as foldable devices. We propose a rational design as well as a fabrication process for a silver nanowire-based transparent conductive electrode with low sheet resistance and high transmittance even after prolonged cyclic bending. The electrode is fabricated on a poly(ethylene terephthalate) film through the hybridization of silver nanowires with silver nanoparticles-anchored RuO2 nanosheets. This hybridization significantly improves the performance of the silver nanowire network under severe bending strain and creates an electrically percolative structure between silver nanowires and RuO2 nanosheets in the presence of anchored silver nanoparticles on the surface of RuO2 nanosheets. The resistance change of this hybrid transparent conductive electrode is 8.8% after 200,000 bending cycles at a curvature radius of 1 mm, making it feasible for use in foldable devices.

Strong Thermopower Enhancement and Tunable Power Factor via Semimetal to Semiconductor Transition in a Transition-Metal Dichalcogenide.

Electronic band engineering is a promising approach to enhance the thermopower of thermoelectric materials. In transition-metal dichalcogenides (TMDCs), this has so far only been achieved using their inherent semiconducting nature. Here, we report the thickness-modulated band engineering of nanosheets based on semimetallic platinum diselenide (PtSe2) resulting in a thermopower enhancement of more than 50 times than that of the bulk. We obtained this by introducing a semimetal to semiconductor (SMSC) transition resulting in the formation of a bandgap. This approach based on semimetallic TMDCs provides potential advantages such as a large variation of transport properties, a decrease of the ambipolar transport effect, and a high carrier density dependence of the transport properties. Our observations suggest that the SMSC transition in TMDCs is a promising and straightforward strategy for the development of two-dimensional nanostructured thermoelectric materials.

Enhancing Thermoelectric Performances of Bismuth Antimony Telluride via Synergistic Combination of Multiscale Structuring and Band Alignment by FeTe2 Incorporation.

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi2Te3-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi0.4Sb1.6Te3 materials with Fe-rich nanoprecipitates and sub-micron FeTe2 inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe2 incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively high thermoelectric figure of merit ZT of 1.52 is achieved at 396 K for p-type Bi0.4Sb1.6Te3-8 mol % FeTe2, which is a 43% enhancement in ZT compared to the pristine Bi0.4Sb1.6Te3. This work demonstrates not only the effectiveness of multiscale structuring for lowering lattice thermal conductivities, but also the importance of interfacial band alignment between matrix and inclusions for maintaining high carrier mobilities when designing high-performance thermoelectric materials.

Ambipolar thermoelectric power of chemically-exfoliated RuO2 nanosheets.

The electrical conductivity and Seebeck coefficient of RuO2 nanosheets are enhanced by metal nanoparticle doping using Ag-acetate solutions. In this study, RuO2 monolayer and bilayer nanosheets exfoliated from layered alkali metal ruthenates are transferred to Si substrates for device fabrication, and the temperature dependence of their conductivity and Seebeck coefficients is investigated. For pristine RuO2 nanosheets, the sign of the Seebeck coefficient changes with temperature from 350-450 K. This indicates that the dominant type of charge carrier is dependent on the temperature, and the RuO2 nanosheets show ambipolar carrier transport behavior. By contrast, the sign of the Seebeck coefficient for Ag nanoparticle-doped RuO2 nanosheets does not change with temperature, indicating that the extra charge carriers from metal nanoparticles promote n-type semiconductor behavior.

Electrically tunable two-dimensional metasurfaces at near-infrared wavelengths.

A novel, electrically tunable metasurface comprising a periodic array of disk-shaped silicon resonators is proposed. The dielectric resonators can be individually manipulated by applying external bias, inducing the complex permittivity modulation of indium tin oxide (ITO) embedded in the middle of the silicon nanodisks. Simulation data shows a reflectance shift from 61% to 8% at λ = 1111nm and a phase shift of 272.9° at λ = 1127nm with an applied voltage in the range of -4 ~4V. In addition, by simply adjusting the resonator geometry, any operating wavelength from 850nm to 1150nm can be achieved with the metasurface.

High-Performance Screen-Printed Thermoelectric Films on Fabrics.

Printing techniques could offer a scalable approach to fabricate thermoelectric (TE) devices on flexible substrates for power generation used in wearable devices and personalized thermo-regulation. However, typical printing processes need a large concentration of binder additives, which often render a detrimental effect on electrical transport of the printed TE layers. Here, we report scalable screen-printing of TE layers on flexible fiber glass fabrics, by rationally optimizing the printing inks consisting of TE particles (p-type Bi0.5Sb1.5Te3 or n-type Bi2Te2.7Se0.3), binders, and organic solvents. We identified a suitable binder additive, methyl cellulose, which offers suitable viscosity for printability at a very small concentration (0.45-0.60 wt.%), thus minimizing its negative impact on electrical transport. Following printing, the binders were subsequently burnt off via sintering and hot pressing. We found that the nanoscale defects left behind after the binder burnt off became effective phonon scattering centers, leading to low lattice thermal conductivity in the printed n-type material. With the high electrical conductivity and low thermal conductivity, the screen-printed TE layers showed high room-temperature ZT values of 0.65 and 0.81 for p-type and n-type, respectively.

Effect of Substitutional Pb Doping on Bipolar and Lattice Thermal Conductivity in p-Type Bi0.48Sb1.52Te3.

Cation substitutional doping is an effective approach to modifying the electronic and thermal transports in Bi2Te3-based thermoelectric alloys. Here we present a comprehensive analysis of the electrical and thermal conductivities of polycrystalline Pb-doped p-type bulk Bi0.48Sb1.52Te3. Pb doping significantly increased the electrical conductivity up to ~2700 S/cm at x = 0.02 in Bi0.48-xPbxSb1.52Te3 due to the increase in hole carrier concentration. Even though the total thermal conductivity increased as Pb was added, due to the increased hole carrier concentration, the thermal conductivity was reduced by 14-22% if the contribution of the increased hole carrier concentration was excluded. To further understand the origin of reduction in the thermal conductivity, we first estimated the contribution of bipolar conduction to thermal conductivity from a two-parabolic band model, which is an extension of the single parabolic band model. Thereafter, the contribution of additional point defect scattering caused by Pb substitution (Pb in the cation site) was analyzed using the Debye-Callaway model. We found that Pb doping significantly suppressed both the bipolar thermal conduction and lattice thermal conductivity simultaneously, while the bipolar contribution to the total thermal conductivity reduction increased at high temperatures. At Pb doping of x = 0.02, the bipolar thermal conductivity decreased by ~30% from 0.47 W/mK to 0.33 W/mK at 480 K, which accounts for 70% of the total reduction.

Strong enhancement of electrical conductivity in two-dimensional micrometer-sized RuO2 nanosheets for flexible transparent electrodes.

The enhancement in electrical transport properties of exfoliated individual RuO2 NSs was systemically investigated for their application in flexible electronics and optoelectronics. Decoration of Ag NPs on the surface of the RuO2 NSs provides donor electrons and dramatically increases the electrical conductivity of the monolayer RuO2 NSs by up to 3700%. The n-type doping behavior was confirmed via Hall measurement analysis of the doped RuO2 NSs. The layer number- and temperature-dependence of the conductivity were also investigated. Moreover, carrier concentration and mobility were obtained from Hall measurements, indicating that the undoped RuO2 NSs had ambipolar transport and semi-metallic characteristics. Moreover, the Ag-doped RuO2 NS multilayer films on polycarbonate substrates were demonstrated by the Langmuir-Blodgett assembly methods, showing one-third reduction in the sheet resistance and extraordinarily high bending stability that the change in the resistance was less than 1% over 50 000 cycles.

Synthesis of Multishell Nanoplates by Consecutive Epitaxial Growth of Bi2Se3 and Bi2Te3 Nanoplates and Enhanced Thermoelectric Properties.

We herein demonstrate the successive epitaxial growth of Bi2Te3 and Bi2Se3 on seed nanoplates for the scalable synthesis of heterostructured nanoplates (Bi2Se3@Bi2Te3) and multishell nanoplates (Bi2Se3@Bi2Te3@Bi2Se3, Bi2Se3@Bi2Te3@Bi2Se3@Bi2Te3). The relative dimensions of the constituting layers are controllable via the molar ratios of the precursors added to the seed nanoplate solution. Reduction of the precursors produces nanoparticles that attach preferentially to the sides of the seed nanoplates. Once attached, the nanoparticles reorganize epitaxially on the seed crystal lattices to form single-crystalline core-shell nanoplates. The nanoplates, initially 100 nm wide, grew laterally to 620 nm in the multishell structure, while their thickness increased more moderately, from 5 to 20 nm. The nanoplates were pelletized into bulk samples by spark plasma sintering and their thermoelectric properties are compared. A peak thermoelectric figure of merit (ZT) ∼0.71 was obtained at 450 K for the bulk of Bi2Se3@Bi2Te3 nanoplates by simultaneous modulation of electronic and thermal transport in the presence of highly dense grain and phase boundaries.

Thermoelectrics. Dense dislocation arrays embedded in grain boundaries for high-performance bulk thermoelectrics.

The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi(0.5)Sb(1.5)Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.

Fe-Doping Effect on Thermoelectric Properties of p-Type Bi0.48Sb1.52Te3.

The substitutional doping approach has been shown to be an effective strategy to improve ZT of Bi2Te3-based thermoelectric raw materials. We herein report the Fe-doping effects on electronic and thermal transport properties of polycrystalline bulks of p-type Bi0.48Sb1.52Te3. After a small amount of Fe-doping on Bi/Sb-sites, the power factor could be enhanced due to the optimization of carrier concentration. Additionally, lattice thermal conductivity was reduced by the intensified point-defect phonon scattering originating from the mass difference between the host atoms (Bi/Sb) and dopants (Fe). An enhanced ZT of 1.09 at 300 K was obtained in 1.0 at% Fe-doped Bi0.48Sb1.52Te3 by these synergetic effects.

Surfactant-free scalable synthesis of Bi2 Te3 and Bi2Se3 nanoflakes and enhanced thermoelectric properties of their nanocomposites.

Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.