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It is well known that monolayer transition-metal dichalcogenides (MX$_2$, M = Mo, W and X = S, Se, Te) could exist in three common structures, i.e. 1$T$, 1$T'$, and 1$H$ phases. In order to reveal their possible phase transitions driven by biaxial strain, we use first-principles calculations to determine the energy landscapes associated with these three phases. Due to its intrinsic dynamical instability, the centrosymmetric 1$T$ phase is known to be metastable and will transform into the 1$T'$ phase where pairs of metal atoms pull together toward each other. Moreover, controlling the metallic 1$T'$ and semiconducting 1$H$ phases is of particular interest as this can introduce novel opportunities in a series of applications. When a biaxial strain is simultaneously compressed along the zigzag direction and stretched along the armchair direction，phase transitions from 1$H$ to 1$T'$ have occurred in MSe$_2$ and MoTe$_2$, but for MSe$_2$ the biaxial strain is much difficult to realize in experiments. For WTe$_2$, the 1$T'$ structure will transform into the 1$H$ form when a biaxial strain is just compressed along the armchair direction. The transitions between 1$H$ and 1$T'$ phases could be attributed to the changes of metal-chalcogen bonds along the armchair direction by analyzing the Crystal Orbital Hamilton Population. Only half M-X bonds along the armchair direction is the main factor, because their lengths are robust in 1$T'$ phase and decrease in 1$H$ form with the tensile strain applied along the armchair direction. Our findings provide a guideline for the phase engineering of transition-metal dichalcogenides with biaxial strain.

Skyrmion bags are spin structures with arbitrary topological charges, which consist of a large skyrmion and multiple small skyrmions. In this work, by applying in-plane ac magnetic fields, we investigate the spin-wave modes of skyrmion bags, which behave differently from the clockwise (CW) and counterclockwise (CCW) rotation modes of skyrmions because of their complex spin topological structure. The in-plane excitation power spectral density shows that there are four resonance frequencies for each skyrmion bag. By further calculating the spin dynamics of a skyrmion bag at each resonant frequency, the four spin-wave modes appear as the composition modes of outer skyrmion-inner skyrmions, i.e., a CCW-CW mode, two CW-breathing modes with different resonance strength, and an inner CCW mode. Our results provide an understanding of the in-plane spin excitation of skyrmion bags, which may contribute to the characterization and detection of skyrmion bags, as well as the applications in logic devices.

Higher manganese silicide (HMS) is a P-type medium temperature thermoelectric (TE) material, which has attracted widespread attention over the past few decades due to its remarkable mechanical properties, excellent chemical and thermal stability, as well as the non-toxicity, abundance and competitive price. The peak power factor (PF) of HMS is as high as ∼1.50 × 10−3 W m−1 K−2 because of its intrinsic high electrical conductivity and Seebeck coefficient. However, the thermal conductivity of HMS is also high, resulting in relatively low zT values. Introducing nano-dispersion in the matrix is one of the most effective methods to enhance the TE properties via reducing the lattice thermal conductivity significantly without drastic changes on the other parameters. In this study, CsPbBr3 QDs with uniform size were synthesized and introduced into HMS bulks. The PF (at 823 K) was enhanced to 1.71 × 10−3 W m−1 K−2, which is improved 14.0% approximately compared with that of pure HMS owing to the combined effect of element doping and energy filtering. The lattice thermal conductivity (at 823 K) decreased from 2.56 W m−1 K−1 to 1.99 W m−1 K−1 synchronously (∼22.0%) due to the intensive phonon scattering caused by Cs doping, and the embedding of Pb riched CsPbBr3 QDs and Pb QDs. A maximum zT value of 0.57 (823 K) is achieved in CsPbBr3 QDs/HMS composites, which is 36.0% higher than that of pure HMS. Predictably, for other TE materials, it is also feasible to improve the TE properties via introducing metastable quantum dots.

In order to explore the influence of cold heading equipment based on polyvinylidene fluoride (PVDF) polymer sensing material on steel ball stamping, a new method was proposed to reflect the defects of cold heading forming of steel ball by load signal. PVDF piezoelectric film is used as the sensor design of force and support structure of steel ball cold heading electromechanical sensor model. PVDF piezoelectric thin film force sensor is used for the structural optimization and simulation. The structural parameters affecting the natural frequency are numerically analyzed by MATLAB software. The mapping relationship between the external load and the output load of the sensor is obtained by using ANSYS software, and the simulation curve of the natural frequency of the sensor is compared with the theoretical curve to verify the factors affecting the natural frequency. The results show that the nonlinear error of sensors refers to the measured curve and the maximum deviation between the fitting line and the percentage of full-scale output.

Element doping and nano-inclusion embedding are effective approaches to enhance the electrical conductivities and decrease the lattice thermal conductivities of thermoelectric (TE) materials, respectively. However, the intrinsic low electrical thermal conductivities and high electrical properties are severely sacrificed, and the final figure of merit (ZT) is usually restricted. In this study, Ag doping and Pt quantum dot (QD) embedding were synchronously achieved via embedding Ag/Pt alloy QDs into the higher manganese silicides to avoid the conventional single-element doping strategy. The power factor (at 823 K) was enhanced from 1.57 × 10−3 W m−1 K−2 to 1.82 × 10−3 W m−1 K−2 (∼16%) due to the ∼18% increase in carrier concentration that was derived from the Ag doping effect. Simultaneously, the lattice thermal conductivity (at 823 K) decreased from 2.65 W m−1 K−1–1.92 W m−1 K−1 (∼28%) because of the broadband phonon scattering effect that resulted from the residual Pt QDs inclusions. Synthetically, the optimal ZT value increased by ∼52% from 0.42 to 0.64 at 823 K. This study demonstrated that incorporating metastable alloy QDs to obtain element doping and nano-inclusion embedding effects is a novel and feasible means to enhance the ZT value of HMS. This method is also possibly applicable to other alloy QD/TE composites.

Transition metal oxychlorides (TMOCl) have attracted great attention as promising cathode materials for chloride ion batteries (CIBs). However, current research on TMOCl has been mainly focused on FeOCl and VOCl. On the other hand, the theoretical study of anionic rechargeable batteries faces the difficulty of predicting the discharge voltage of the electrode materials based on the half-cell system. Herein, a reliable theoretical voltage formula for CIBs is proposed based on a full-cell system with Li/LiCl as the reference anode. A high throughput screening method for TMOCl is applied among 16 transition metals. After the screening according to energetic and dynamic stability, Co is identified, which can form a new cathode material of CoOCl. Compared to FeOCl and VOCl, CoOCl has a higher discharge voltage, which is beneficial for achieving a larger energy density. The small crystal field splitting energy and exchange splitting energy of Co3+ result in higher electronic conductivity. In addition, the uniform Cl− binding environment leads to a low Cl− diffusion barrier of 0.37 eV, which is much smaller than that of VOCl (0.65 eV) and FeOCl (0.58 eV).

In order to find low-toxic light-absorbing materials for perovskite solar cells, the electronic structures and optical absorption spectra of doped perovskites CsPb₁–ₓMₓI₂Br (M = Ge, Sn, Sr, and Cu) have been investigated with the first-principles method. Ge- and Sn-doped perovskites display monotonously decreased band gaps with increasing x, opposite to Sr- and Cu-doped ones. Different from group IV Ge and Sn elements, Sr and Cu dopants show distinct electronic characteristics, that is, weak interaction with halogens because of the d orbitals splitting in octahedral complexes and unique density of states and band-decomposed charge densities. Meanwhile, Ge, Sn, and Cu dopants prefer substituting Pb sites in the CsI-terminated surfaces based on thermodynamic consideration, and the dopants accumulating on surfaces will deteriorate the optical absorption of perovskites based on the comparison of the absorption spectra of various doped structures. Therefore, this study provides a theoretical insight into the declined absorption spectra with the increasing concentration of dopants in experimental observations.

Copyright © 2021 The Author(s). Deformation bands (DBs), formed after plastic deformation, are known to have an impact on the recrystallization (RX) process. The exact mechanisms of how DBs influence grain nucleation and grain growth remain unclear. In this paper, deformed single and multicrystal pure aluminium samples are annealed to explore the likely effects of DBs on the grain nucleation and the subsequent grain growth. Regarding the prediction of the recrystallized (RXed) texture, it is noticeable that the orientations of nucleated grains nearby DB are originated from the orientation in DB. Regarding the nucleated positions, it is demonstrated that potential nucleation sites are more likely located in DBs in comparison with the initial grain boundary. Regarding the rate of RX, the number of nucleated grains is also predicted to have a strong positive correlation with the area fraction of DBs, which would consequently affect the kinetics of the grain growth in the deformed microstructure. All the above observations imply that the RX process is strongly controlled by the ensemble characteristics of DBs rather than the initial grain boundaries. EPSRC LightForm program: EP/R001715/1.

Based on the first-principles calculations, the influence of different geometry models on the theoretical results about the electronic properties of LaAlO3/SrTiO3 perovskite heterostructures has been studied. The results show that the superlattice model can indicate n-type conductivity of interfaces, but it is limited to describe the insulator-metal transition of heterostructures. The sandwich and thin film models with the vacuum layer can take into account the surface termination effect on the electronic properties of the interface. The AlO2 or LaO atomic layer as the outmost surface termination is selected during constructing the model, leading to the distinct electronic properties of the heterostructures. The build-in electric filed appears in the AlO2-terminated structure, whereas it does not exist in LaO-terminated structure. Therefore, the AlO2-terminated structure can give a clear description about the insulator-metal transition of heterostructure and the charge transfer mechanism. The build-in electric filed, the atomic rumpling and surface termination effect of perovskite materials codetermine the conductivity of interfaces. The results would be helpful to construct theoretical model and understand the novel electronic properties during investigating other perovskite heterostructures.

Two-dimensional transition metal dichalcogenides (TMDCs) are promising in spintronics due to their spin–orbit coupling, but their intrinsic non-magnetic properties limit their further development. Here, we focus on the energy landscapes of TMDC (MX2, M = Mo, W and X = S, Se, Te) monolayers by rhenium (Re) substitution doping under axial strains, which controllably drive 1H ↔ 1Td structural transformations. For both 1H and 1Td phases without strain, Re-doped TMDCs have an n-type character and are non-magnetic, but the tensile strain could effectively induce and modulate the magnetism. Specifically, 1H-Re0.5Mo0.5S2 gets a maximum magnetic moment of 0.69 μB at a 6% uniaxial tensile strain along the armchair direction; along the zigzag direction it exhibits a significant magnetic moment (0.49 μB) at a 2.04% uniaxial tensile strain but then exhibits no magnetism in the range of [5.10%, 7.14%]. By contrast, for 1Td-Re0.5Mo0.5S2 a critical uniaxial tensile strain along the zigzag direction reaches up to ∼9.18%, and a smaller uniaxial tensile strain (∼5.10%) along the zigzag direction is needed to induce the magnetism in 1Td-Re0.5M0.5Te2. The results reveal that the magnetism of Re-doped TMDCs could be effectively induced and modulated by the tensile strain, suggesting that strain engineering could have significant applications in doped TMDCs.

Bone's hierarchical arrangement of collagen and mineral generates a confluence of toughening mechanisms acting at every length scale from the molecular to the macroscopic level. Molecular defects, disease, and age alter bone structure at different levels and diminish its fracture resistance. However, the inability to isolate and quantify the influence of specific features hampers our understanding and the development of new therapies. Here, we combine in situ micromechanical testing, transmission electron microscopy and phase-field modelling to quantify intrinsic deformation and toughening at the fibrillar level and unveil the critical role of fibril orientation on crack deflection. At this level dry bone is highly anisotropic, with fracture energies ranging between 5 and 30 J/m2 depending on the direction of crack propagation. These values are lower than previously calculated for dehydrated samples from large-scale tests. However, they still suggest a significant amount of energy dissipation. This approach provides a new tool to uncouple and quantify, from the bottom up, the roles played by the structural features and constituents of bone on fracture and how can they be affected by different pathologies. The methodology can be extended to support the rational development of new structural composites.

Tin sulfide (SnS) is a low-cost, earth-abundant, and eco-friendly thermoelectric (TE) material whose layered orthorhombic structure is similar to that of SnSe; however, current studies on SnS are limited because of its difficult synthesis due to the use of volatile sulfur, and its thermoelectric figure of merit (zT) is also low because of its intrinsically low electrical conductivity. Herein, we present a facile wet chemical synthesis to prepare uniform and well-dispersed SnS nanosheet crystals. Low-rate initial production with 3.5 L solvent was performed to obtain high-quality crystals, and also to adapt the process to commercial preparation. The carrier concentrations were enhanced from 3.2 × 1018 cm−3 to 4.8 × 1019 cm−3 (at 873 K) step-by-step via increasing Sn vacancies, adding SnSe, and embedding Ag atoms. The maximum power factor (PF) at 873 K of 0.48 × 10−3 W⋅m−1K−2 was achieved in the selenium-silver co-doped sample, which presented a state-of-the-art value for the polycrystalline SnS. Correspondingly, the lattice thermal conductivities ( κ L ) decreased from 0.8 W⋅m−1K−1 to 0.47 W⋅m−1K−1 (at 873 K) due to enhanced phonon scattering. The highest zT was 0.80 at 873 K, which is about 5 or 6 times higher than those of the pristine SnS(1:1) or SnS(1:0.8) samples. These findings provide a novel method to prepare high-quality and high-performance TE semiconductors via a wet chemical synthesis.

Understanding and controlling phase transition in the transition-metal oxides is interesting from the fundamental physics point of view. The pd hybridization are of considerable issue to tune the phase transition temperature. Here, the transport properties of the NdNiO3 with artificial ultra-thin NiO insert layer have been investigated. The relationship between the essential parameters of the pd hybridization (T) and the metal-insulator transition (MIT) temperature has been clearly elucidated a monotone decreasing relationship. This work realized the combination of the engineering hybridization and the phase transition temperature, which might be significant for the development of multifunctional materials in the transition-metal oxides.

A simple and efficient synthetic approach is reported for the synthesis of 5-trifluoromethyl-5-aryl-3-methylidenepyrrolidin-2-ones in moderate to excellent yields via an acid-catalyzed one-pot multicomponent reaction of aromatic trifluoromethyl ketones, acylhydrazines, and ethyl 2-(bromomethyl)acrylate promoted by tin powder. The protocol is operationally simple. The overall process involves allylation and cyclization of the intermediate of acylhydrazones.

Tin selenide (SnSe) has recently attracted great attention as a new promising thermoelectric material. In this paper, a-axis inclined oriented SnSe thin films have been fabricated by pulsed laser deposition for the development of the transverse thermoelectric effect. Large open-circuit voltage signals along the film plane are all clearly observed when the film surface is illuminated by different lasers with the wavelength ranging from 532 to 2200 nm with no external bias or a built-in electric field, and the amplitude of the output voltage can be enhanced by tuning the inclination angle of the films or the applied temperature difference across the film thickness. Moreover, transverse open-circuit voltage signals are also detected when the film surface is irradiated by a thermal heater. This work demonstrates the novel potential application of SnSe for self-powered broad-band light detectors or thermal sensors.

c-Axis-textured BiCuSeO thin films were grown directly on amorphous glass substrates by pulsed laser deposition. The resistivity (∼27.1 mΩ cm) of the films at room temperature was found to be much lower than those reported for polycrystalline bulk samples with the same nominal composition, and a three dimensional variable-range-hopping conduction process was suggested to govern the electrical transport properties of the films below room temperature. Moreover, detailed microstructural analysis revealed the existence of amorphous grain boundaries throughout the films, which would lead to a significant decrease of thermal conductivity. Thus, the thermoelectric performance of the present BiCuSeO thin films is expected to be greatly enhanced as compared to that of bulk samples with the same nominal composition, demonstrating the potential application in the thermoelectric thin film devices.

A significant enhancement of the light-induced transverse thermoelectric (LITT) effect is obtained in c-axis inclined BiCuSeO films through Ba doping. Upon the irradiation of a 308 nm pulsed laser, the LITT voltage signal in Ba-doped film shows higher sensitivity R and faster response time. The maximum sensitivity R of 5.9 V/mJ is obtained in Bi0.94Ba0.06CuSeO film, which is 4.9 times that of the intrinsic film (1.2 V/mJ). The improvement of LITT performance can be attributed to the increased Seebeck coefficient anisotropy and the decreased film resistivity due to the increase in hole carrier concentration of the Ba-doped films. For the case of irradiation from a continuous laser with wavelength of 532, 1064 and 1550 nm, respectively, obvious LITT voltage signals can be detected in film of Bi0.94Ba0.06CuSeO, and the induced voltage amplitude Vp increases with increasing power or reducing wavelength λ of the irradiated laser. This work demonstrates the potential application of Ba-doped BiCuSeO film in self-powered photodetectors, especially in high sensitivity and fast response UV detectors.

The thermoelectric performances of 1T-ZrX2 (X = S and Se) single layers were investigated using a combination of density functional calculations and semi-classical Boltzmann transport theory. Because of the high hole mobilities at 300 K, ultrahigh power factors ( PF = S 2 σ ) were found in the P-type compounds; these values were ∼ 11.95 and ∼ 13.58 mW K−2 m−1 for 1T-ZrS2 and 1T-ZrSe2 single layers, respectively. However, because of the Lorenz relation between the electrical conductivity ( σ ) and an electron’s thermal conductivity ( κ el ) given by the Wiedemann-Franz law, the electronic figures of merit ( Z el T = PF · T / κ el ) at 300 K were approximately 0.67 and 0.75 for the N- and P-type 1T-ZrSe2, respectively. In addition, the lattice thermal conductivities ( κ ph ) were calculated, giving values of ∼ 1.43 and ∼ 0.97 W K−1 m−1 for 1T-ZrS2 and 1T-ZrSe2 single layers, respectively. Therefore, because of the lower κ ph / κ el ratio, the P-type 1T-ZrX2 single layers possess higher figure-of-merits ( ZT = Z el T / 1 + κ ph κ el ) than their counterparts. This signifies that the P-type samples demonstrate better thermoelectric performance than the N-type ones. The thermoelectric properties of metastable 2H-ZrX2 (X = S and Se) single layers were also investigated.

The lateral photovoltaic effect (LPE) in μc-SiOx:H/a-Si:H/c-Si p-i-n structure is studied at different temperatures under illumination of different lasers ranging from visible to infrared. The LPE improves with increasing laser wavelength with the position sensitivity nearly linearly dependent on the wavelength in the whole temperature range. Though the position sensitivity decreased gradually with decreasing measurement temperature from 296 K to 80 K, the nonlinearity improved a little, which can be ascribed to the decreased resistivity of conductive layer. Considering that the LPE was mainly determined by the Schottky barrier (SB), an external bias voltage was added to enhance the built-in field, it was found that the LPE improved dramatically with position sensitivity linearly proportional to the laser power. More importantly, both the position sensitivity and the nonlinearity were independent of temperature again in the whole laser power range. Our research sheds new light on the bias voltage-modulated LPE at low temperatures.

A gate electrode is usually used to controllably tune the carrier concentrations, further modulating the electrical conductivity and the Seebeck coefficient to obtain the optimum thermoelectric figure of merit (ZT) in two-dimensional materials. On the other hand, it is necessary to investigate how an electric field induced by a gate voltage affects the electronic structures, further determining the thermoelectric properties. Therefore, by using density functional calculations in combination with Boltzmann theory, the thermoelectric properties of bilayer MX2 (M = W, Mo; X = S, Se) with or without a 1 V nm−1 perpendicular electric field are comparatively investigated. First of all, the variations of the electrical conductivity (σ), electron thermal conductivity and Seebeck coefficient (S) with the carrier concentration are studied. Due to the trade-off relationship between S and σ, there is an optimum concentration to obtain the maximum ZT, which increases with the temperature due to the enhancement of the Seebeck coefficient. Moreover, N-type bilayers have larger optimum ZTs than P-type bilayers. In addition, the electric field results in the increase of the Seebeck coefficient in low hole-doped MS2 bilayers and high hole-doped MSe2 bilayers, thus leading to similar variations in ZT. The optimum ZTs are reduced from 2.11 × 10−2, 3.19 × 10−2, 2.47 × 10−2, and 2.58 × 10−2 to 1.57 × 10−2, 1.51 × 10−2, 2.08 × 10−2, and 1.43 × 10−2 for the hole-doped MoS2, MoSe2, and WSe2 bilayers, respectively. For N-type bilayers, the electric field shows a destructive effect, resulting in the obvious reduction of the Seebeck coefficient in the MSe2 layers and the low electron-doped MS2 bilayers. In electron-doped bilayers, the optimum ZTs will decrease from 3.03 × 10−2, 6.64 × 10−2, and 6.69 × 10−2 to 2.81 × 10−2, 3.59 × 10−2, and 4.39 × 10−2 for the MoS2, MoSe2, and WSe2 bilayers, respectively.