Non-centrosymmetric Crystal Structure Research Articles

Beyond Conventional Catalysts: Monoelemental Tellurium as a Game Changer for Piezo-Driven Hydrogen Evolution.

The increasing demand for clean hydrogen production over fossil fuels necessitates the development of sustainable piezoelectrochemical methods that can overcome the limitations of conventional electrocatalytic and photocatalytic approaches. In this regard, existing piezocatalysts face challenges related to their low piezoelectricity or active site coverage for hydrogen evolution reaction (HER). Driven by global environmental concerns, there is a compelling push to engineer practical materials for highly efficient HER. Herein, monoelemental 2D tellurium (Te) is presented as a class of layered chalcogenide with a non-centrosymmetric crystal structure (P3121 space group). The refined Te nanosheets demonstrate an unprecedented highly efficient H2 production rate ≈9000µmolg-1h-1 under ultrasonic mechanical vibration due to built-in piezo-potential in the system. The remarkable piezocatalytic performance of Te nanosheets arises from a synergistic interplay between their semi-metallic nature, favorable free energy landscape, enhanced electrical conductivity and outstanding piezoelectricity. As a proof of concept, the theoretical approach based on Density Functional Theory (DFT) validates the findings due to the gradual exposure of active sites on the Te nanosheets leading to a self-optimized catalytic performance for hydrogen generation. Therefore, mechanically driven Te emerges as a promising piezocatalyst with the potential to revolutionize highly efficient and sustainable technology for futuristic applications.

Out-of-Plane Ferroelectricity in Two-Dimensional 1T‴-MoS2 Above Room Temperature.

Two-dimensional (2D) molybdenum disulfide (MoS2), one of the most extensively studied van der Waals (vdW) materials, is a significant candidate for electronic materials in the post-Moore era. MoS2 exhibits various phases, among which the 1T‴ phase possesses noncentrosymmetry. 1T‴-MoS2 was theoretically predicted to be ferroelectric a decade ago, but this has not been experimentally confirmed until now. Here, we have prepared high-purity 2D 1T‴-MoS2 crystals and experimentally confirmed the room-temperature out-of-plane ferroelectricity. The noncentrosymmetric crystal structure in 2D 1T‴-MoS2 was convinced by atomically resolved transmission electron microscopic imaging and second harmonic generation (SHG) measurements. Further, the ferroelectric polarization states in 2D 1T‴-MoS2 can be switched using piezoresponse force microscopy (PFM) and electrical gating in field-effect transistors (FETs). The ferroelectric-to-paraelectric transition temperature is measured to be about 350 K. Theoretical calculations have revealed that the ferroelectricity of 2D 1T‴-MoS2 originates from the intralayer charge transfer of S atoms within the layer. The discovery of intrinsic ferroelectricity in the 1T‴ phase of MoS2 further enriches the properties of this important vdW material, providing more possibilities for its application in the field of next-generation electronic devices.

Observation of Anisotropic Second Harmonic Generation in Two-Dimensional Niobium Diselenide.

The intrinsic anisotropy of NbSe2 provides a favorable prerequisite of second harmonic generation (SHG) and rich possibilities for tailoring its nonlinear optical (NLO) properties. Here we report the highly efficient SHG of mechanically exfoliated NbSe2 flakes. The nonlinear optical response changes with excitation wavelengths, layer thicknesses, and polarizations of the excitation laser. The anisotropic SHG response further exhibits the intrinsic non-centrosymmetric crystal structure and could effectively assign the crystalline orientation of NbSe2 flakes. Interestingly, although NbSe2 flakes with tens of nanometers thickness experience attenuations in SHG performance, more efficient SHG anisotropy ratios were obtained, which are around 4 times higher than that of the 5-layer counterpart. The determined second-order nonlinearities of NbSe2 flakes (monolayer: ∼1.0 × 103 pm/V; 3-layer: ∼73 pm/V) are comparable to those of the commonly reported two-dimensional materials (e.g., MoS2, WSe2, graphene) with the same number of layers and much higher than those of commercial nonlinear optical crystals.

Transport studies in piezo-semiconductive ZnO nanotetrapod based electronic devices

ZnO nanotetrapods (ZnO NTs) with a non-centrosymmetric crystal structure consisting of four 1-D arms interconnected together through a central crystalline core, introduce interesting piezoelectric semiconducting responses in nanorods in the bent state. Considering the widespread applications of nanotetrapods in semiconductor devices, it becomes very crucial to establish a coupled model based on piezoelectric and piezotronic effects to investigate the carrier transport mechanism, which is being reported here in detail for the first time. In this work, we established a multiphysics coupled model of stress-regulated charge carrier transport by the finite element method (FEM), which considers the full account of the wurtzite (WZ) and zinc blende (ZB) regions as well as the spontaneous polarization dependence and the dependence of the material properties on the arm orientation. It is discovered that the forward gain of ZnO NT in the lateral force working mode is almost 50 % higher than that in the nanorod or in the normal force working mode while the reverse current is reduced to negligible. Through the simulation calculations and corresponding analysis, it is confirmed that the developed piezoelectric polarization charges are able to regulate the transport and distribution of carriers in ZnO crystal, which lays a theoretical foundation for the application of piezo-semiconductive ZnO NT devices in advanced technologies.

Research on the Stability of Different Polar Surfaces in Aluminum Nitride Single Crystals

Wurtzite aluminum nitride (AlN) crystal has a non-centrosymmetric crystal structure with only a single axis of symmetry. In an AlN crystal, the electronegativity difference between the Al atom and N atom leads to a distortion of electron cloud distribution outside the nucleus and a spontaneous polarization (SP) along the c-axis direction. The N-polar surface along the directions of [000-1] has higher surface energy than the Al-polar surface along the directions of [0001]. Due to the different atomic arrangement, Al atoms on the Al-polar surface bond with O and OH− in the environment to generate Al2O3·xH2O, which prevents the reaction from occurring inside the crystal. After the Al2O3·xH2O dissolve in an alkaline environment, N atoms have three dangling bonds exposed on the surface, which can also protect OH− from destroying the internal Al-N bonds, so the Al-polar surface is more stable than the N-polar surface.

Van der Waals quaternary oxides for tunable low-loss anisotropic polaritonics.

The discovery of ultraconfined polaritons with extreme anisotropy in a number of van der Waals (vdW) materials has unlocked new prospects for nanophotonic and optoelectronic applications. However, the range of suitable materials for specific applications remains limited. Here we introduce tellurite molybdenum quaternary oxides-which possess non-centrosymmetric crystal structures and extraordinary nonlinear optical properties-as a highly promising vdW family of materials for tunable low-loss anisotropic polaritonics. By employing chemical flux growth and exfoliation techniques, we successfully fabricate high-quality vdW layers of various compounds, including MgTeMoO6, ZnTeMoO6, MnTeMoO6 and CdTeMoO6. We show that these quaternary vdW oxides possess two distinct types of in-plane anisotropic polaritons: slab-confined and edge-confined modes. By leveraging metal cation substitutions, we establish a systematic strategy to finely tune the in-plane polariton propagation, resulting in the selective emergence of circular, elliptical or hyperbolic polariton dispersion, accompanied by ultraslow group velocities (0.0003c) and long lifetimes (5 ps). Moreover, Reststrahlen bands of these quaternary oxides naturally overlap that of α-MoO3, providing opportunities for integration. As an example, we demonstrate that combining α-MoO3 (an in-plane hyperbolic material) with CdTeMoO6 (an in-plane isotropic material) in a heterostructure facilitates collimated, diffractionless polariton propagation. Quaternary oxides expand the family of anisotropic vdW polaritons considerably, and with it, the range of nanophotonics applications that can be envisioned.

An atomically tailored chiral magnet with small skyrmions at room temperature

Creating materials that do not exist in nature can lead to breakthroughs in science and technology. Magnetic skyrmions are topological excitations that have attracted great attention recently for their potential applications in low power, ultrahigh density memory. A major challenge has been to find materials that meet the dual requirement of small skyrmions stable at room temperature. Here we meet both these goals by developing epitaxial FeGe films with excess Fe using atomic layer molecular beam epitaxy (MBE) far from thermal equilibrium. Our atomic layer design permits the incorporation of 20% excess Fe while maintaining a non-centrosymmetric crystal structure supported by theoretical calculations and necessary for stabilizing skyrmions. We show that the Curie temperature is well above room temperature, and that the skyrmions have sizes down to 15 nm as imaged by Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The presence of skyrmions coincides with a topological Hall effect-like resistivity. These atomically tailored materials hold promise for future ultrahigh density magnetic memory applications.

Enhancing nonlinear optical responses via Methoxy Positional Isomerism in Chalcone-Based Materials

The functional moieties present in chalcone derivatives play a pivotal role in finely adjusting the thermal, optical, structural, and nonlinear optical (NLO) characteristics of the chalcone material. This paper details the synthesis of chalcone derivatives where three distinct trimethoxy substituents are integrated into the benzoyl moiety through carboxyl bonding with 2-acetyl pyridine. The synthesis employs the Schmidt condensation method, and the resultant samples are subject to scrutiny through FTIR and FT-Raman techniques to ascertain phase purity. The outcomes reveal substantial alterations in the structural, thermal, optical, and NLO attributes of the samples. The electron-donating property of the trimethoxy groups augments electron density within the chalcone molecule, thereby effecting multiple shifts in the material's characteristics. Interestingly, a methoxy group located at positions 2, 4, and 5 forms crystals with centrosymmetric properties. Conversely, relocating the methoxy groups to positions 3, 4, 5, and 2, 4, 6 induces a transformation from centrosymmetric to non-centrosymmetric crystal structures, resulting in a remarkable enhancement of NLO properties, encompassing second and third harmonic generation. Notably, the compound featuring trimethoxy substitution at positions 3, 4, and 5 demonstrates a 27-fold increase in second harmonic generation compared to the urea crystal. The detailed discussion encompasses the intricate relationship between the molecular structure and nonlinear properties, encompassing third harmonic generation.

A perspective on piezotronics and piezo-phototronics based on the third and fourth generation semiconductors

The rapid development of semiconductor materials and devices has brought tremendous development opportunities to optoelectronics, intelligent manufacturing, Internet of Things, power electronics, and even innovative energy technologies. Among them, the third and fourth generation semiconductors represented by ZnO, GaN, SiC, and Ga2O3 are two kinds of emerging strategic material systems. Due to their large energy bandgaps, they exhibit excellent performance in application scenarios of high voltage, high frequency, and high temperature resistance, making them great candidates in high-power, radio frequency, and optoelectronic devices. The third and fourth generation semiconductors usually possess non-centrosymmetric crystal structures, which makes the piezoelectric polarization effect a fundamental characteristic for the third and fourth generation semiconductors in contrast to the first and second generation semiconductors as represented by Si, Ge, and GaAs. Research studies on the coupling of piezoelectricity, semiconductor, and light excitation properties were coined as piezotronics and piezo-phototronics in 2007 and 2010, respectively, by Zhong Lin Wang. The piezotronic and piezo-phototronic effects open another avenue for further improvement of the performance of electronic and optoelectronic devices. This Perspective will first introduce the basic concepts and principles of piezotronics and piezo-phototronics and the basic characteristics of the third and fourth generation semiconductors. Then, progress, challenges, and opportunities of ideal materials, comprehensive physical models, and outstanding applications based on piezotronics and piezo-phototronics are presented with emphasis. Finally, conclusions and outlooks are drawn for the piezotronics and piezo-phototronics based on the third and fourth generation semiconductors.

B20 Weyl Semimetal CoSi Film Fabricated by Flash-Lamp Annealing.

B20-CoSi is a newly discovered Weyl semimetal that crystallizes into a noncentrosymmetric crystal structure. However, the investigation of B20-CoSi has so far been focused on bulk materials, whereas the growth of thin films on technology-relevant substrates is a prerequisite for most practical applications. In this study, we have used millisecond-range flash-lamp annealing, a nonequilibrium solid-state reaction, to grow B20-CoSi thin films. By optimizing the annealing parameters, we were able to obtain thin films with a pure B20-CoSi phase. The magnetic and transport measurements indicate the appearance of the charge density wave and chiral anomaly. Our work presents a promising method for preparing thin films of most binary B20 transition-metal silicides, which are candidates for topological Weyl semimetals.

Kramers nodal lines and Weyl fermions in SmAlSi

Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been experimentally established. Here we gather the experimental evidence pointing at the presence of KNLs in SmAlSi, a non-centrosymmetric metal that develops incommensurate spin density wave order at low temperature. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and magneto-transport methods, we provide evidence suggesting the presence of KNLs, together with observing Weyl fermions under the broken inversion symmetry in the paramagnetic phase of SmAlSi. We discuss the nesting possibilities regarding the emergent magnetic orders in SmAlSi. Our results provide a solid basis of experimental observations for exploring correlated topology in SmAlSi

Large anomalous Hall effect and intrinsic Berry curvature in magnetic Weyl semimetal NdAlGe

Weyl semimetals driven by both time-reversal (TRS) and inversion symmetries (IS) are rare and be a good platform to investigate the relationship between chiral current and magnetic order. NdAlGe compound, grown in a single crystalline form by the flux method, has a noncentrosymmetric I41md crystal structure, which breaks inversion symmetry. The magnetic susceptibility χ(T) shows anisotropic behavior; ferromagnetic with divergence of field cooling (FC) and zero field cooling (ZFC) in H∥c, and antiferromagnetic transition for magnetic field H⊥c. The band structure calculation has revealed that NdAlGe is the type-II Weyl semimetal with four types of Weyl nodes in the ab-plane with a probable Fermi surface nesting vector. The ferromagnetic spin alignment gives a significant anomalous Hall coefficient Rs and large anomalous Hall conductivity σxyA, for H∥c. The giant anomalous Hall conductivity σxyA, constant SH=σxyA/M (H = 50 kOe) value, and their Karplus and Luttinger (KL) analysis support that the giant AHE is associated with the intrinsic Berry curvature in NdAlGe compound. The intrinsic Berry curvature is also confirmed by the planar Hall effect over a wide temperature range, implying that the Weyl semimetallic property is not relevant with the magnetic order.

Superconducting Gap Structure of the Noncentrosymmetric Topological Superconductor Candidate HfRuP

We investigate the gap symmetry of the topological superconductor candidate HfRuP, which crystallizes in a noncentrosymmetric hexagonal crystal structure, using muon spin rotation/relaxation (μSR) measurements in transverse-field (TF) geometry. The temperature and magnetic field dependencies of the superconducting relaxation rate derived from the TF-μSR spectra can be well described by an isotropic s-wave gap. The superconducting carrier density ns = 1.41(1) × 1026 m−3 and the magnetic penetration depth, λ(0) = 603(2) nm, were calculated from the TF-μSR data. Interestingly, the ratio between the superconducting transition temperature and the superfluid density, Tc/λ−2(0) ∼ 3.3, is very close to those of unconventional superconductors. Further, our zero-field (ZF) μSR results do not show any considerable change in the muon spin relaxation above and below the superconducting transition temperature, suggesting that time-reversal symmetry is preserved in the superconducting state of this superconductor.

Low-temperature phase transition in polar semimetal Td−MoTe2 probed by nonreciprocal transport

Nonreciprocal transport, which is a rectification effect reflecting noncentrosymmetric crystal structures, is investigated in semimetallic MoTe\penalty1000\-\allowhyphens{}${}_{2}$. It is clarified that nonreciprocal transport is a powerful probe of structural and/or electronic phase transition.

Polarization-Independent Second Harmonic Generation in 2D Van Der Waals Kagome Nb3 SeI7 Crystals.

Second harmonic generation (SHG) of 2D crystals has been of great interest due to its advantages of phase-matching and easy integration into nanophotonic devices. However, the polarization-dependence character of the SHG signal makes it highly troublesome but necessary to match the laser polarization orientation relative to the crystal, thus achieving the maximum polarized SHG intensity. Here, it is demonstrated a polarization-independent SHG, for the first time, in the van der Waals Nb3 SeI7 crystals with a breathing Kagome lattice. The Nb3 triangular clusters and Janus-structure of each Nb3 SeI7 layer are confirmed by the STEM. Nb3 SeI7 flake shows a strong SHG response due to its noncentrosymmetric crystal structure. More interestingly, the SHG signals of Nb3 SeI7 are independent of the polarization of the excitation light owing to the in-plane isotropic arrangement of nonlinear active units. This work provides the first layered nonlinear optical crystal with the polarization-independent SHG effect, providing new possibilities for nonlinear optics.

Rubrene/CuInP2S6 organic/inorganic heterojunctions for ferroelectric organic transistor memory devices

Ferroelectric organic field-effect transistor (Fe-OFET) memory devices exhibit low power consumption, low cost and good flexibility, which are indispensable to next generation flexible and wearable memory devices. However, up until the present, the ferroelectric materials used in Fe-OFETs have been limited to ferroelectric polymers, i.e. poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], and suffer from large surface roughness and high-risk of depolarization. This work integrates a new two-dimensional CuInP2S6 crystal into Fe-OFET, which can introduce excellent ferroelectric property due to its single crystalline smoothness and noncentrosymmetric crystal structure. Meanwhile, the high-mobility rubrene crystal is selected as the channel material to construct rubrene/CuInP2S6 organic/inorganic van der Waals heterojunctions. Associated with synergetic effect of single-crystalline nature of the heterojunction and atomically flat/defect-free interface, the memory devices possess a high programming speed of 0.5 s and long retention time of 104 s with a current ratio larger than 103. Furthermore, the performance of flexible devices is also demonstrated, including a programming/erase cycle of 1000 with a current on/off ratio approaching 103, data retention time of 104 s with a current on/off ratio of 103 and a current on/off ratio maintained at 102 even after 100 bending cycles under a bending radius of 25 mm, which are qualified for memory buffer usage or short-term data storage especially when they are integrated with other flexible electronics. The proposed configuration of memory transistors using organic/inorganic heterojunctions provides a rational design of device structure toward flexible Fe-OFET non-volatile memory applications.

Electronic structure and physical properties of the candidate topological material GdAgGe

We grew needle-shaped single crystals of GdAgGe, which crystallizes in a noncentrosymmetric hexagonal crystal structure with space group $P\overline{6}$ $2m$ (189). The magnetic susceptibility data for $H \ensuremath{\perp} c$ reveal two pronounced antiferromagnetic transitions at ${T}_{N1}$ = 20 K and ${T}_{N2}$ = 14.5 K. The magnetic susceptibility anomalies are less prominent for $H \ensuremath{\parallel} c$. The transition at ${T}_{N1}$ is accompanied by a pronounced heat capacity anomaly confirming the bulk nature of the magnetic transition. Below ${T}_{N1}$, the electrical resistivity data follows a ${T}^{3/2}$ dependence. In the magnetically ordered state, GdAgGe shows positive transverse magnetoresistance, which increases with decreasing temperature and increasing field, reaching a value of $\ensuremath{\sim}27%$ at 9 T and 10 K. The Hall resistivity data and electronic band structure calculations suggest that both the hole and the electron charge carriers contribute to the transport properties. The electronic band structure displays linear band crossings near the Fermi level. The calculations reveal that GdAgGe has a nodal line with drumhead surface states coupled with a nonzero Berry phase, making it a nontrivial nodal-line semimetal.

Electroactive properties and piezo-tribo hybrid energy harvesting performances of PVDF-AlFeO3 composites: role of crystal symmetry and agglomeration of fillers.

Inorganic filler-loaded PVDF-based composites have been very widely used for electrical and energy harvesting applications in recent times. In this regard, the effects of different parameters of fillers like size, shape, chemical states, distribution, functional properties, and many others on the output performance of PVDF have been widely studied. However, the effect of another important parameter, namely the crystal symmetry of the filler, in tuning the energy harvesting performance of PVDF has been rarely explored. Therefore, to explore this fact, here we develop PVDF-based composite films by using two types of AlFeO3 fillers, one with rhombohedral R3̄c symmetry (AFRH) and another with an orthorhombic Pc21n structure. Ferrite-based oxides have been chosen here as fillers due to their good dielectric compatibility with PVDF. On the other hand, AlFeO3 has been chosen due to the simplicity of synthesizing it with both centrosymmetric and non-centrosymmetric crystal structures and the scarcity of reports exploring the energy-harvesting performance of AlFeO3-based polymer composites. A significant difference in particle agglomeration has also been observed here between the mentioned two types of AlFeO3 fillers which was mainly due to their specific synthesis conditions. The electroactive properties of PVDF have been observed to be mostly dependent on filler agglomeration. However, the crystal symmetry has shown a strong effect on the piezoelectric energy harvesting performances. As a result of these facts, the piezo-tribo hybrid energy harvesting performance, which depends on both the dielectric permittivity and piezoelectric activity, has been observed to be better for the AFRH5-based hybrid device (AFRH5H) (with ∼72 V open circuit voltage and ∼45 μW cm-2 power density) compared to that of the AFOR5-based hybrid device (AFOR5H). The real-life applications of all the energy harvesting devices have also been demonstrated here.

Time reversal invariant single-gap superconductivity with upper critical field larger than the Pauli limit in NbIr2B2

Recently, compounds with noncentrosymmetric crystal structure have attracted much attention for providing a rich playground in search for unconventional superconductivity. NbIr$_2$B$_2$ is a new member to this class of materials harboring superconductivity below $T_{\rm c} = 7.3(2)$~K and very high upper critical field that exceeds Pauli limit. Here we report on muon spin rotation ($\mu$SR) experiments probing the temperature and field dependence of effective magnetic penetration depth in this compound. Our transverse-field -$\mu$SR results suggest a fully gaped $s$-wave superconductvity. Further, the estimated high value of upper critical field is also supplemented by high field transport measurements. Remarkably, the ratio $T_{\rm c}$/$\lambda^{-2}(0)$ obtained for NbIr$_2$B$_2$ ($\sim$2) is comparable to those of unconventional superconductors. Zero-field $\mu$SR data reveals no significant change in the muon spin relaxation rate above and below $T_{\rm c}$, evincing that time-reversal symmetry is preserved in the superconducting state. The presented results will stimulate theoretical investigations to obtain a microscopic understanding of the origin of superconductivity with preserved time reversal symmetry in this unique noncentrosymmetric system.

Solution-Grown Ternary Semiconductors: Nanostructuring and Stereoelectronic Lone Pair Distortions in I–V–VI2 Materials

Alkali pnictogen dichalcogenides─I–V–VI2 or APnCh2─have been identified as promising semiconducting materials for energy conversion devices. However, the controlled nanoscale synthesis and our understanding of the effects of cation ordering and stereochemically active lone pairs on the structures of these ternary compounds remain underdeveloped. Here, we use solution-phase chemistry to synthesize a family of APnCh2 materials, including LiSbSe2, NaSbS2, NaSbSe2, NaBiS2, and NaBiSe2. Our approach utilizes alkali metal hydrides (AH) or carboxylates, A(O2CR), PnPh3, and elemental chalcogens as synthetic precursors and oleylamine or 1-octadecene as solvents. Synthetic manipulation via fine-tuning of reaction temperature enables control over the degree of ordering caused by the Sb 5s2 lone pair-induced distortions in NaSbS2. Pair distribution function analysis demonstrates that the structure of the Sb-containing phases deviates much more from a disordered rock salt structure than that of the Bi-containing phases. This local distortion, induced by the Sb lone pair, leads to a previously unreported noncentrosymmetric NaSbS2 crystal structure, which is additionally supported by second-harmonic generation measurements. Infrared and multinuclear solid-state NMR spectroscopies show that oleylamine or chelating carboxylates and, in some cases, unreacted precursors (LiH and PnPh3) remain bound to the nanocrystalline surfaces. A deeper understanding of the local atomic environment, long-range ordering, surface chemistry, and optoelectronic properties of these materials may speed up their fundamental study and application.