Room-temperature Applications Research Articles

Chemical stability and degradation mechanism of Mg3Sb2-xBix thermoelectrics towards room-temperature applications

Mg3Sb2-xBix compounds have recently attracted surging research attention owing to their low cost and high thermoelectric performance that is comparable with the commercialized Bi2Te3-xSex alloys. For practical applications, chemical stability is another key factor that determines the lifetime of the materials, which is rarely investigated for Mg3Sb2-xBix. Here we show that Mg3Sb2-xBix is more sensitive to moisture, but not to the conventionally considered oxygen. A spontaneous decomposition to crystalline Bi/Sb and amorphous Mg(OH)2 is thermodynamically favorable in the moist ambient air, leading to catastrophically degraded thermoelectric performance. Moreover, such an irreversible degradation was also found to occur in other Bi-contained Zintl phase compounds. Thankfully, if moisture is eliminated, the thermoelectric properties of Mg3Sb2-xBix keep nearly unchanged after two months’ exposure to the dry air. This work provides insight into the degradation mechanism of Mg3Sb2-xBix and other Bi-contained thermoelectric materials, paving the way for their protection in future applications.

High-performance and stable AgSbTe2-based thermoelectric materials for near room temperature applications

AgSbTe2-based ternary chalcogenides show excellent thermoelectric performance at low- and middle-temperature ranges, yet their practical applications are greatly limited by their intrinsic poor thermodynamic stability. In this work, we demonstrate that AgSbTe2-based ternary chalcogenides can be stabilized for service below their decomposition threshold. A series of AgxSb2-xTe3-x (x = 1.0, 0.9, 0.8 and 0.7) samples have been prepared by the melt-quenching method. Among them, phase pure Ag0.9Sb1.1Te2.1 is verified by comprehensive structural characterizations from macroscale by X-ray diffraction to microscale by energy-dispersive spectroscopy and then to sub-nanometer scale by atom probe tomography. This composition is further chosen for the stability investigation. The decomposition threshold of Ag0.9Sb1.1Te2.1 appears around 473 K. Below this temperature, the chemical compositions and thermoelectric properties are barely changed even after 720 h annealing at 473 K. The figure-of-merit (zT) value of Ag0.9Sb1.1Te2.1 below the decomposition threshold is very competitive for real applications even compared with Bi2Te3-based alloys. The average zT of Ag0.9Sb1.1Te2.1 at 300–473 K reaches 0.84, which is higher than most other thermoelectric materials in a similar temperature range, promising applications in miniaturized refrigeration and power generation near room temperature.

VO2 thin film based highly responsive and fast VIS/IR photodetector

The VO2 (M1) thin film has attracted significant attention due to its abrupt semiconductor to metal transition (SMT) near room temperature and diverse optoelectronic applications. In this work, pure VO2 (M1) phase thin film has been grown on the silicon substrate using a pulsed laser deposition technique. The Raman spectroscopy results confirmed the deposition of pure VO2 (M1) phase thin film. The X-ray photoelectron spectroscopy (XPS) showed that the deposited vanadium oxide thin film is mainly composed of VO2. The field emission scanning electron microscopy (FESEM) showed a smooth, continuous, and well-covered surface morphology. The elemental composition analysis of the deposited thin film using energy-dispersive X-ray spectroscopy (EDX) exhibited the presence of V and O in the deposited thin film. The VO2 (M1) thin-film device with a simple planar structure displayed resistance change of more than two orders of magnitude with increasing and decreasing temperature displaying a sharp semiconductor to metal transition (SMT) behavior with a transition width of ∼10 °C and transition temperature of ∼67 °C; a property associated with VO2 (M1). The photo-detection results of the VO2 (M1) based device revealed a better response upon illumination with 650 nm light at 0.15 mW/cm2 compared to 980 nm light at 0.34 mW/cm2. The rise time (tr) and fall time (tf) of ∼1.57 and ∼1.64 ms were achieved upon illumination with 650 nm light at 0.15 mW/cm2 while ∼1.67 and ∼4.12 ms upon illumination with 980 nm light at 0.34 mW/cm2. The photo-responsivity has been measured as ∼353 mA/W and ∼78 mA/W for 650 nm and 980 nm light, respectively. The response time and responsivity of the phase pure VO2 (M1) thin film for the Visible and IR light has been significantly improved compared to the earlier reported vanadium dioxide thin films based photodetectors.

Development of Fe-based amorphous alloy with high iron content for room-temperature magnetic refrigeration application

In this work, in order to develop new Fe-based amorphous alloys with room-temperature magnetocaloric effect and good magnetocaloric performance, high iron content Fe-based amorphous alloys were designed, and their Curie temperatures were adjusted to room temperature through the addition of Mo. As a result, a new Fe83Mo6Si1B7P2C1 amorphous alloy with room-temperature magnetocaloric effect was successfully prepared by melt-spinning technique, and it has the Curie temperature of 300 K, the maximum isothermal magnetic entropy change of 2.74 J kg−1 K−1 and the refrigerant capacity of 485.2 J kg−1 under an applied magnetic field of 5 T. Combining with the advantages of excellent magnetocaloric properties, negligible hysteresis loss, and low material cost, the present Fe83Mo6Si1B7P2C1 amorphous alloy should be a promising candidate as room-temperature magnetic refrigerants.

Tunnel Spin-Polarization of Ferromagnetic Metals and Ferrimagnetic Oxides and Its Effect on Tunnel Magnetoresistance

This work presents an examination and unification of fragmented data on spin polarization in half-metallic, ferrimagnetic oxides. It also includes well understood ferromagnetic metals for comparison. The temperature and disorder dependencies of the spin polarization are evaluated. Both the temperature dependence of the tunnel magnetoresistance and, for the very first time, its temperature coefficient are calculated based on the simplified Julliére model. The tunnel magnetoresistance in the magnetic tunnel junctions deteriorates due to the temperature dependence of the spin polarization the lower the Curie temperature is. As a result, magnetic tunnel junctions—consisting of ferromagnetic oxides with a Curie temperature not far above room temperature—are not promising for room temperature applications. Additionally, ferrimagnetic oxides possessing a Curie temperature below 650 K are not suitable for room temperature applications because of an unacceptable temperature coefficient exceeding −2%.

Talnakhite: A potential n-type thermoelectric sulphide with low thermal conductivity

The mineral talnakhite, Cu18Fe16S32, is an n-type semiconductor with low thermal conductivity (average value of 1.5 ​W ​m−1 ​K−1), making it an attractive candidate for thermoelectric applications. The effect of partial cation substitutions and of deviations from the ideal Cu:Fe ratio on the thermoelectric properties of this material, has been investigated through synthesis of Cu17.58M0.02Fe17.6S32 (M ​= ​Ag, In, Zn) and Cu17.6+xFe17.6-xS32 (−0.03 ≤ x ​≤ ​0.03) by high-temperature methods. The results demonstrate that talnakhite exhibits a narrow range of compositional stability for substitution at the cation sites. X-ray photoelectron spectroscopy (XPS) measurements indicate that all compositions contain Fe3+ and Fe2+ cations, together with Cu+. The electrical and thermal transport properties show two anomalies, at approximately 460 and 510 ​K, which can be related to structural phase transitions. The maximum value of the thermoelectric figure of merit occurs at the temperature of the first structural phase transition, making talnakhite a potential n-type candidate for near room-temperature thermoelectric applications. While substitution with silver, zinc or indium does not lead to any significant improvement in thermoelectric performance, changes in the Cu:Fe ratio result in significant reductions in the total thermal conductivity. This is likely to be associated with increased point defect scattering due to the presence of additional vacancies at the cation sites over which iron and copper are partially ordered. For copper-poor phases, the combination of a slightly improved power factor with a reduced thermal conductivity results in an increase in the figure-of-merit by approximately 20% when compared to the stoichiometric phase.

Giant spin-selective bandgap renormalization in CsPbBr3 colloidal nanocrystals

The spin-dependence of the strongly correlated phenomena and many-body interactions play an important role in quantum information science. In particular, it is quite interesting but unclear how the spin degree of freedom ramifies the bandgap renormalization, one of the fundamental many-body phenomena. We report the first room-temperature observation of giant spin-selective bandgap renormalization (SS-BGR) in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_{3}$ colloidal nanocrystals using time-resolved circularly polarized femtosecond pump-probe spectroscopy. The SS-BGR results from many-body interactions among carriers with the same spin that renormalize their joint density of states by $57\ifmmode\pm\else\textpm\fi{}$ 1 meV, visualized here as photoinduced absorption (PIA) below band-edge transition energy when the pump and probe are co-polarized. The hallmark result of spectrally resolved SS-BGR is in stark contrast to the usually submerged signal in II-VI and III-V semiconductors and is 3 orders of magnitude larger than that observed in Ge/SiGe quantum wells, highlighting the unique and beneficial band structure of the metal-halide semiconductors. We propose that the PIA, due to SS-BGR, can be used to describe the spin-polarization and spin-relaxation dynamics. The experimental and theoretical findings open up new possibilities for optical manipulation of spin degrees of freedom and their many-body interactions in metal-halide perovskite nanocrystals for potential room-temperature applications.

New families of quantum spin Hall insulators with rashba effect in functionalized InBi monolayers

Because quantum spin Hall (QSH) insulators with dissipative electron transport properties have broad application prospect in the field of quantum devices, QSH insulators have aroused great research enthusiasm and many ideal QSH insulators are predicted successfully. Based on density functional calculations, we predict that new families of functionalized InBi monolayers, H–InBi-X (X = F, Cl, Br, I), to be QSH insulators. We find that the pristine InBi monolayer that is simultaneously hydrogenated and halogenated has a maximum band gap of 0.49 eV while preserving the untrivial topological properties, which is large enough for room-temperature applications. Because of the absence of inversion symmetry, the Rashba SOC effect occurs in the band structure and the SOC not only opens the bandgaps but also leads to the band inversion. In addition, the topological properties of these monolayers can survive a wide range of external strain from −6% to 6%. Our findings enrich the inversion-asymmetry topological materials and provide a reliable reference for designing QSH insulators at room temperature.

Spin-Orbit Coupling Electronic Structures of Organic-Group Functionalized Sb and Bi Topological Monolayers.

Electronic band-gap is a key factor in applying two-dimensional (2D) topological insulators into room-temperature quantum spin Hall effect (QSH) spintronic devices. Employing pseudopotential plane-wave first-principles calculations, we investigate spin-orbit coupling (SOC) electronic structures of the novel 2D topological insulator series of antimony (Sb) and bismuth (Bi) monolayers (isolated double atomic layers) functionalized by organic-groups (methyl, amino and hydroxyl). Cohesive energies and phonon frequency dispersion spectra indicate that these organic-group decorated Sb and Bi monolayers possess structural stability in both energetic statics and lattice dynamics. The giant electronic band-gaps adequate for room-temperature applications are attributed to the effective SOC enhancement of group functionalization. The nontrivial topology of these novel 2D monolayer materials is verified by the Z2 invariant derived from wave-function parity and edge-states of their nanoribbons, which is prospective for QSH spintronic devices. The chemical functional group changes the p-orbital component of Fermi level electrons, leading to strong intra-layer spin-orbit coupling, opening a large band-gap of approaching 1.4 eV at Dirac-cone point and resulting in a global indirect band-gap of 0.75 eV, which, even underestimated, is adequate for room-temperature operations. Sb and Bi monolayers functionalized by organic groups are also predicted to maintain stable nontrivial topology under in-layer biaxial strain, which is suitable for epitaxy technology to realize QSH spintronic devices.

Vacancy-Induced Temperature-Dependent Thermal and Magnetic Properties of Holmium-Substituted Bismuth Ferrite Nanoparticle Compacts.

Multiferroics have gained widespread acceptance for room-temperature applications such as in spintronics, ferroelectric random access memory, and transistors because of their intrinsic magnetic and ferroelectric coupling. However, a comprehensive study, establishing a correlation between the magnetic and thermal transport properties of multiferroics, is still missing from the literature. To fill the void, this work reports the temperature-dependent thermal and magnetic properties of holmium-substituted bismuth ferrite (BiFeO3) and their dependencies on oxygen vacancies and structural modifications. Two distinct magnetic transitions on temperature-dependent magnetic and heat capacity responses are identified. Experimental analysis suggests that the excess of oxygen vacancies shifts the magnetic transition temperature by ∼64 K. The holmium substitution-induced structural modification increases BiFeO3 heat capacity by 30% up to the antiferromagnetic phase transition temperature. Furthermore, an unsaturated heat capacity even at temperatures as high as 850 K is observed and is ascribed to anharmonicity and partial densification of the nanoparticles used during heat capacity measurements. The room-temperature thermal conductivity of BiFeO3 is ∼0.33 ± 0.11 W m-1 K-1 and remains unchanged at high temperatures due to defect scattering from porosities.

Strong effects of uniaxial pressure and short-range correlations in Cr2Ge2Te6

Cr$_2$Ge$_2$Te$_6$ is a quasi-2D semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr$_2$Ge$_2$Te$_6$ single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{c}}$ = 24.7 K/GPa and ${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{ab}}$ = $-$15.6 K/GPa) derived from thermodynamic relations. Moderate in-plane strain is thus sufficient to strongly enhance ferromagnetism in Cr$_2$Ge$_2$Te$_6$ up to room temperature. Moreover, unambiguous signs for short-range magnetic order up to 200~K are found.

Enhancing the thermoelectric performance by defect structures induced in p-type polypyrrole-polyaniline nanocomposite for room-temperature thermoelectric applications

Enhancing the thermoelectric performance by defect structures induced in p-type polypyrrole-polyaniline nanocomposite for room-temperature thermoelectric applications

Robust Tunable Large-Gap Quantum Spin Hall States in Monolayer Cu2S on Insulating Substrates.

Quantum spin Hall (QSH) insulators with large band gaps and dissipationless edge states are of both technological and scientific interest. Although numerous two-dimensional (2D) systems have been predicted to host the QSH phase, very few of them harbor large band gaps and retain their nontrivial band topology when they are deposited on substrates. Here, based on a first-principles analysis with hybrid functional calculations, we investigated the electronic and topological properties of inversion-asymmetric monolayer copper sulfide (Cu2S). Interestingly, we found that monolayer Cu2S possesses an intrinsic QSH phase, Rashba spin splitting, and a large band gap of 220 meV that is suitable for room-temperature applications. Most importantly, we constructed heterostructures of a Cu2S film on PtTe2, h-BN, and Cu(111) substrates and found that the topological properties remain preserved upon an interface with these substrates. Our findings suggest Cu2S as a possible platform to realize inversion-asymmetric QSH insulators with potential applications in low-dissipation electronic devices.

Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping.

Vanadium dioxide (VO2) featuring a distinct thermally triggered phase transition is regarded as the most attractive thermochromic material for smart window applications. However, the high transition temperature (∼67 °C) and moderate luminous transmittance (<50%) of the pristine VO2 circumvent room temperature applications. In this work, epitaxial cobalt-doped VO2 thin films were fabricated to tailor the electric and optical properties on a c-plane sapphire substrate. At the highest doping concentration of 10%, the transition temperature of VO2 is reduced to 44 °C, accompanied by a high luminous transmittance of 79% for single-element Co-doped VO2. The roles of cobalt doping and detailed band variation are fully explained experimentally and by modeling (DFT calculation), respectively. Furthermore, the dramatically increased carrier concentration in cobalt-doped VO2 underscores the promising future of cobalt-doped VO2 unveiled by temperature-dependent Hall effect measurement.

Machine learning assisted development of Fe2P-type magnetocaloric compounds for cryogenic applications

Fe2P-type compounds exhibit a giant magnetocaloric effect (MCE) and are extensively studied for room temperature applications. The reduction of their transition temperature below 77 K can pave the way for the potential application of these materials for hydrogen liquefaction using cryogenic magnetic refrigeration. Most of the known magnetocaloric materials with a giant MCE below 77 K are rare-earth-based compounds. In order to explore the possibility of developing rare-earth-free compounds with cryogenic MCE, we collected a dataset by conducting data mining on published experimental results on Fe2P-type magnetocaloric compounds and used machine learning for composition optimization aiming at lowering the transition temperature below 77 K. Guided by the predictions of an artificial neural network, we found a promising composition of Mn1.70Fe0.30P0.63Si0.37 with a transition temperature of 97 K at 1 T magnetic field which was lowered to 73 K by the minor substitution of Fe with Co. The developed rare-earth-free compounds exhibit a large magnetocaloric performance in isothermal magnetic entropy change (∆SM) of 7.5–11.5 J/kgK at the temperatures below 100 K. This study demonstrates that data-driven development of magnetocaloric materials can efficiently boost the optimization of their properties, thus aiding the practical applicability of magnetic refrigeration technology.

High frequency impedance measurements of sodium solid electrolytes

Solid-state sodium batteries are currently gaining enormous interest as a lower-cost and more environmentally friendly alternative to lithium batteries. They contain significantly less critical rare elements in both the electrolyte and the active material. However, to date, there is no efficient material combination of metallic anode, cathode, and solid electrolyte for room temperature applications that does not require an additional liquid electrolyte while maintaining high energy density. NaPSiO-based glass-ceramics show high ionic conductivity at room temperature, good corrosion resistance against ambient humidity and CO2, and stability against metallic sodium. However, the conductivity mechanisms of this promising class of materials are currently poorly understood. Herein, high frequency impedance measurements up to 108 Hz shed light on the contributions of grains and grain boundaries to the total impedance, including the distribution of relaxation time constants of the fully crystallized material. In addition, analysis of the temperature dependence allows separation of electrode contributions and determination of activation energies for grain and grain boundary conductivities. Our study provides the basis for fine-tuning the stoichiometry of NaPSiO-based glass-ceramics in terms of maximizing the conductive phase fraction to optimize the performance of future solid-state sodium batteries.

Piezo-phototronic spin laser based on wurtzite quantum wells

Piezo-phototronic spin lasers are studied based on wurtzite quantum wells. The strain-induced polarization changes the gain spectrum and gain asymmetry at room temperature. In this work, the gain properties of piezo-phototronic spin lasers based on InGaN/GaN quantum wells can be modulated by strain-induced polarization. This work not only paves a new way for the room-temperature applications of quantum piezotronic and piezo-phototronic devices, but also provides new method of regulation for wurtzite spin lasers by strain-induced polarization.

Conductometric NO2 gas sensor based on Co-incorporated MoS2 nanosheets for room temperature applications

Room temperature rapid detection of toxic gases is still ambitious for real-time application. Recently, molybdenum disulfide transition dichalcogenides (MoS2 TMDCs) have been considered as a promising material for gas sensing application, owing to their high surface-to-volume ratio. However low response and incomplete recovery hinder their performance. In this work, we demonstrate MoS2 and cobalt incorporated MoS2 to enhance the sensing response and rapid recovery. Co-incorporated (7 at%) MoS2 showed 546.6% enhancement in response compared to pristine MoS2 for 100 ppm of NO2 gas at room temperature. The enhanced edge site presents in the Co-incorporated samples was the reason for excellent gas adsorption and increased response towards NO2 gas molecules. The stability analysis showed 70% of stability of a fabricated device. These results address a new pathway for an ideal gas sensor for room temperature application.

Ultraflexible and High-Thermoelectric-Performance Sulfur-Doped Ag2Se Film on Nylon for Power Generators.

In this work, we developed a facile method to fabricate low-cost, flexible, and high-thermoelectric-performance n-type Ag2Se1-xSx@(Ag2S1-ySey/S) composite film on a nylon membrane. The composite film was prepared by first performing wet-chemical synthesis of the S-doped Ag2Se powder, then vacuum-assisted filtration of the powder on a nylon membrane, and finally hot-pressing. Transmission electron microscopy (TEM) observation and energy-dispersive system (EDS) analysis of the film revealed that the film had a porous network-like microstructure, in which Ag2Se1-xSx sub-micron grains formed the skeleton and are coated by a ∼15 nm thick layer of S-rich Ag2S1-ySey nanograins mixed with an S amorphous phase. The film showed a power factor of ∼954.7 μW·m-1·K-2 at 300 K and superior flexibility (94.4% of the original electrical conductivity was preserved after bending 2000 times around a rod with a radius of 4 mm). Moreover, a six-leg flexible thermoelectric generator was assembled with the film and produced a maximum power of 6.67 μW (corresponding power density ∼14.8 W/m2) at a temperature difference of 38.7 K. This work reveals a novel approach to explore high-performance and low-cost flexible thermoelectric devices suitable for room-temperature applications.

Effect of Iodine Octahedral Rotations on Dipole Ordering in Organic–Inorganic Hybrid Perovskite CH3NH3PbI3

Organic–inorganic hybrid perovskite MAPbI3 (MA = CH3NH3) has been rapidly growing as one of the most promising materials for future solar cells. One of the mechanisms that contribute to its high efficiency is related to the ferroelectric ordering of the MA molecules inside the inorganic framework. Here, we use first-principles calculations to explore structural distortions that can lead to the dipolar ordering of the organic molecules in the tetragonal phase, which is relevant to room-temperature applications. We find that the existence of antiphase rotation of the PbI6 octahedra has a significant impact, whereas tetragonal change of the lattice parameters is relatively unimportant for either ferroelectric or antiferroelectric configurations.