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TaS2 is Molybdenite-like structured and crystallizes in the orthorhombic Fmm2 space group. The structure is two-dimensional and consists of two TaS2 sheets oriented in the (0, 0, 1) direction. Ta4+ is bonded to six equivalent S2- atoms to form distorted edge-sharing TaS6 pentagonal pyramids. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a distorted T-shaped geometry to three equivalent Ta4+ atoms.
TaS2 is trigonal omega-like structured and crystallizes in the trigonal R3m space group. The structure is two-dimensional and consists of six TaS2 sheets oriented in the (0, 0, 1) direction. Ta4+ is bonded to six equivalent S2- atoms to form edge-sharing TaS6 octahedra. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a distorted T-shaped geometry to three equivalent Ta4+ atoms.
TaS2 is trigonal omega structured and crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of one TaS2 sheet oriented in the (0, 0, 1) direction. Ta4+ is bonded to six equivalent S2- atoms to form edge-sharing TaS6 octahedra. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a distorted T-shaped geometry to three equivalent Ta4+ atoms.
TaS2 is Molybdenite-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is two-dimensional and consists of two TaS2 sheets oriented in the (0, 0, 1) direction. Ta4+ is bonded to six equivalent S2- atoms to form distorted edge-sharing TaS6 pentagonal pyramids. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a 3-coordinate geometry to three equivalent Ta4+ atoms.
TaS2 is trigonal omega-like structured and crystallizes in the monoclinic C2/m space group. The structure is two-dimensional and consists of one TaS2 sheet oriented in the (0, 0, 1) direction. Ta4+ is bonded to six equivalent S2- atoms to form edge-sharing TaS6 octahedra. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a distorted T-shaped geometry to three equivalent Ta4+ atoms.
TaS2 is trigonal omega-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of six TaS2 sheets oriented in the (0, 0, 1) direction. Ta4+ is bonded to six equivalent S2- atoms to form edge-sharing TaS6 octahedra. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a distorted T-shaped geometry to three equivalent Ta4+ atoms.
TaS2 is Molybdenite-like structured and crystallizes in the orthorhombic Cmcm space group. The structure is two-dimensional and consists of two TaS2 sheets oriented in the (0, 0, 1) direction. Ta4+ is bonded to six equivalent S2- atoms to form distorted edge-sharing TaS6 pentagonal pyramids. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a distorted T-shaped geometry to three equivalent Ta4+ atoms.
Abstract The influence of intrinsic defects of 1T-TaS 2 on charge density waves (CDWs) is studied using scanning tunneling microscopy and spectroscopy (STM, STS), angle-resolved photoelectron spectroscopy (ARPES), and density functional theory (DFT). We identify several types of structural defects and find that most have a local character limited to a single CDW site, with a single exception which effectively behaves as a dopant, leading to band-bending and affecting multiple neighboring sites. While only one type of defect can be observed by STM topographic imaging, all defects are easily resolved in STS mapping. Our results indicate modulation of the Mott band gap commensurate with the CDW and breaking of the three-fold symmetry of electronic states. DFT calculations (with included Coulomb interactions) are used to investigate the electronic structure, focusing on both sulfur vacancy and oxygen-sulfur substitution. The sulfur vacancy system, characterized with a metallic behavior, is identified as the origin of one of the experimentally observed defects. Additionally, the effect of oxidation of 1T-TaS 2 depends on the substitution site, leading to the heterogeneity of electronic properties.
1T-TaS2 is a layered charge density wave (CDW) crystal exhibiting sharp phase transitions and associated resistance changes. These resistance steps could be exploited for information storage, underscoring the importance of controlling and tuning the CDW states. Given the importance of out-of-plane interactions in 1T-TaS2, modulating interlayer interactions by heterostructuring is a promising method for tailoring CDW phase transitions. In this work, we investigate the optical and electronic properties of heterostructures comprising 1T-TaS2 and monolayer 1H-WSe2. By systematically varying the thickness of 1T-TaS2 and its azimuthal alignment with 1H-WSe2, we find that intrinsic moiré strain and interfacial charge transfer introduce CDW disorder in 1T-TaS2 and modify the CDW ordering temperature. Furthermore, our studies reveal that the interlayer alignment impacts the exciton dynamics in 1H-WSe2, indicating that heterostructuring can concurrently tailor the electronic phases in 1T-TaS2 and the optical properties of 1H-WSe2. This work presents a promising approach for engineering the optoelectronic behavior of heterostructures that integrate CDW materials and semiconductors.
Ta2MoS4(TaS2)2 is Molybdenite-derived structured and crystallizes in the trigonal P-3m1 space group. The structure is two-dimensional and consists of one Ta2MoS4 sheet oriented in the (0, 0, 1) direction and two TaS2 sheets oriented in the (0, 0, 1) direction. In the Ta2MoS4 sheet, Ta+3.50+ is bonded to six S2- atoms to form distorted TaS6 pentagonal pyramids that share corners with six equivalent MoS6 octahedra, edges with six equivalent TaS6 pentagonal pyramids, and a faceface with one MoS6 octahedra. The corner-sharing octahedral tilt angles are 45°. There are three shorter (2.48 Å) and three longer (2.49 Å) Ta–S bond lengths. Mo2+ is bonded to six equivalent S2- atoms to form MoS6 octahedra that share corners with twelve equivalent TaS6 pentagonal pyramids, edges with six equivalent MoS6 octahedra, and faces with two equivalent TaS6 pentagonal pyramids. All Mo–S bond lengths are 2.51 Å. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Ta+3.50+ atoms. In the second S2- site, S2- is bonded in a 6-coordinate geometry to three equivalent Ta+3.50+ and three equivalent Mo2+ atoms. In each TaS2 sheet, Ta+3.50+ is bonded to six S2- atoms to form distorted edge-sharing TaS6 pentagonal pyramids. There are three shorter (2.47 Å) and three longer (2.48 Å) Ta–S bond lengths. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Ta+3.50+ atoms. In the second S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Ta+3.50+ atoms.
The goals of this fundamental science project, aimed at understanding the physical mechanisms and developing methods for electric-bias control of phase transitions in quasi-2D CDW materials, have been achieved. We focused on 1T-TaS2, one of the most interesting materials of this type, and demonstrated electrical gating of the I-V characteristics and hysteresis in this material. The demonstration of electrical gating of CDW phases in quasi-2D material was performed at RT. We have conducted experiments to separate the electric-field CDW switching from Joule heating-induced switching. This was an important development for 2D CDW materials. The project has led to a better understanding of the physical mechanisms behind the phase transitions and CDW depinning in quasi-2D van der Waals materials. We established that the CDW domain depinning in 1T-TaS2 does not lead to a strong increase in the collective current and accompanying narrow-band noise. We developed a technique that utilized the low-frequency noise measurements in such materials for monitoring the CDW phase transitions. In the experiments where 1T-TaS2 flakes were used in polymeric matrices, we verified the robustness of the phase transitions between the nearly commensurate and incommensurate CDW phases.
Quasi-two-dimensional (quasi-2D) materials hold promise for future electronics because of their unique band structuresthat result in electronic and mechanical properties sensitive to crystal strains in all three dimensions. Quantifying crystal strain is a prerequisite to correlating it with the performance of the device, and calls for high resolution but spatially resolved rapid characterization methods. Here we show that using fly-scan nano X-ray diffraction we can accomplish a tensile strain sensitivity below 0.001% with a spatial resolution of better than 80 nm over a spatial extent of 100 µm on quasi 2D flakes of 1T-TaS2. Coherent diffraction patterns were collected from a ~100 nm thick sheet of 1T-TaS2 by scanning 12keV focused X-ray beam across and rotating the sample. We demonstrate that the strain distribution around micron and sub-micron sized ‘bubbles’ that are present in the sample may be reconstructed from these images. The experiments use state of the art synchrotron instrumentation, and will allow rapid and non-intrusive strain mapping of thin film samples and electronic devices based on quasi 2D materials.
HgTaS2 is H-Phase-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of three hydrargyrum molecules and three TaS2 sheets oriented in the (0, 0, 1) direction. In each TaS2 sheet, Ta3+ is bonded to six equivalent S2- atoms to form edge-sharing TaS6 octahedra. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a distorted T-shaped geometry to three equivalent Ta3+ atoms.
Ta2CrS4Ta2NiS4TaS2 crystallizes in the trigonal P3m1 space group. The structure is two-dimensional and consists of two Ta2CrS4 sheets oriented in the (0, 0, 1) direction; two Ta2NiS4 sheets oriented in the (0, 0, 1) direction; and two TaS2 sheets oriented in the (0, 0, 1) direction. In each Ta2CrS4 sheet, Ta3+ is bonded to six S2- atoms to form distorted TaS6 pentagonal pyramids that share corners with six equivalent CrS6 octahedra, edges with six equivalent TaS6 pentagonal pyramids, and a faceface with one CrS6 octahedra. The corner-sharing octahedral tilt angles are 46°. There are three shorter (2.48 Å) and three longer (2.52 Å) Ta–S bond lengths. Cr3+ is bonded to six equivalent S2- atoms to form CrS6 octahedra that share corners with twelve equivalent TaS6 pentagonal pyramids, edges with six equivalent CrS6 octahedra, and faces with two equivalent TaS6 pentagonal pyramids. All Cr–S bond lengths are 2.43 Å. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded to three equivalent Ta3+ and three equivalent Cr3+ atoms to form a mixture of distorted corner and edge-sharing STa3Cr3 pentagonal pyramids. In the second S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Ta3+ atoms. In each Ta2NiS4 sheet, there are two inequivalent Ta3+ sites. In the first Ta3+ site, Ta3+ is bonded to six S2- atoms to form distorted TaS6 pentagonal pyramids that share corners with six equivalent NiS6 octahedra, edges with six equivalent TaS6 pentagonal pyramids, and a faceface with one NiS6 octahedra. The corner-sharing octahedral tilt angles are 47°. All Ta–S bond lengths are 2.48 Å. In the second Ta3+ site, Ta3+ is bonded to six S2- atoms to form distorted TaS6 pentagonal pyramids that share corners with six equivalent NiS6 octahedra, edges with six equivalent TaS6 pentagonal pyramids, and a faceface with one NiS6 octahedra. The corner-sharing octahedral tilt angles are 47°. All Ta–S bond lengths are 2.48 Å. Ni2+ is bonded to six S2- atoms to form NiS6 octahedra that share corners with twelve TaS6 pentagonal pyramids, edges with six equivalent NiS6 octahedra, and faces with two TaS6 pentagonal pyramids. All Ni–S bond lengths are 2.38 Å. There are four inequivalent S2- sites. In the first S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Ta3+ atoms. In the second S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Ta3+ atoms. In the third S2- site, S2- is bonded to three equivalent Ta3+ and three equivalent Ni2+ atoms to form a mixture of distorted corner and edge-sharing STa3Ni3 pentagonal pyramids. In the fourth S2- site, S2- is bonded to three equivalent Ta3+ and three equivalent Ni2+ atoms to form a mixture of distorted corner and edge-sharing STa3Ni3 pentagonal pyramids. In each TaS2 sheet, Ta3+ is bonded to six equivalent S2- atoms to form distorted edge-sharing TaS6 pentagonal pyramids. All Ta–S bond lengths are 2.48 Å. S2- is bonded in a 3-coordinate geometry to three equivalent Ta3+ atoms.
Noncentrosymmetric transition metal dichalcogenide (TMD) monolayers offer a fertile platform for exploring unconventional Ising superconductivity (SC) and charge density waves (CDWs). However, the vulnerability of isolated monolayers to structural disorder and environmental oxidation often degrade their electronic coherence. Herein, an alternative approach is reported for fabricating stable and intrinsic monolayers of 1H-TaS2 sandwiched between SnS blocks in a (SnS) 1.15 TaS 2 van der Waals (vdW) superlattice. The SnS block layers not only decouple individual 1H-TaS 2 sublayers to endow them with monolayer-like electronic characteristics, but also protect the 1H-TaS 2 layers from electronic degradation. The results reveal the characteristic 3 × 3 CDW order in 1H-TaS 2 sublayers associated with electronic rearrangement in the low-lying sulfur p band, which uncovers a previously undiscovered CDW mechanism rather than the conventional Fermi surface-related framework. Additionally, the (SnS) 1.15 TaS 2 superlattice exhibits a strongly enhanced Ising-like SC with a layer-independent T c of ≈3.0 K, comparable to that of the isolated monolayer 1H-TaS 2 sample, presumably attributed to their monolayer-like characteristics and retained Fermi states. These results provide new insights into the long-debated CDW order and enhanced SC of monolayer 1H-TaS 2 , establishing bulk vdW superlattices as promising platforms for investigating exotic collective quantum phases in the 2D limit.
Charge density waves (CDWs), electronic crystals that form within a host solid, have long been speculated to melt into a spatially textured electronic liquid. Though they have not been previously detected, liquid CDWs may nonetheless be fundamental to the phase diagrams of many correlated electron systems, including high temperature superconductors and quantum Hall states. In one of the most promising candidate materials capable of hosting a liquid CDW, 1T-TaS2, a structural phase transition impedes its observation. Here, by irradiating the material with a femtosecond light pulse, we circumvent the structural phase transition to reveal how topological defect dynamics govern the otherwise invisible CDW correlations. Upon photoexcitation, the CDW diffraction peaks broaden azimuthally, initially revealing a hexatic state. At higher temperatures, photoexcitation completely destroys translational and orientational order and only a ring of diffuse scattering is observed, a key signature of a liquid CDW. Our work provides compelling evidence for a defect-unbinding transition to a CDW liquid and presents a protocol for uncovering states that are hidden by other transitions in thermal equilibrium.
In this work, we report on the preparation of inks containing fillers derived from quasi-two-dimensional charge-density-wave materials, their application for inkjet printing, and the evaluation of their electronic properties in printed thin-film form. The inks were prepared by liquid-phase exfoliation of CVT-grown 1T-TaS 2 crystals to produce fillers with nm-scale thickness and μm-scale lateral dimensions. Exfoliated 1T-TaS 2 was dispersed in a mixture of isopropyl alcohol and ethylene glycol to allow fine-tuning of filler particles thermophysical properties for inkjet printing. The temperature-dependent electrical and current fluctuation measurements of printed thin films demonstrated that the charge-density-wave properties of 1T-TaS 2 are preserved after processing. The functionality of the printed thin-film devices can be defined by the nearly commensurate to the commensurate charge-density-wave phase transition of individual exfoliated 1T-TaS 2 filler particles rather than by electron-hopping transport between them. The obtained results are important for the development of printed electronics with diverse functionality achieved by the incorporation of quasi-two-dimensional van der Waals quantum materials.
Here we report on the electrical gating of the chargedensity- wave phases and current in h-BN-capped three-terminal 1T-TaS 2 heterostructure devices. It is demonstrated that the application of a gate bias can shift the source–drain current– voltage hysteresis associated with the transition between the nearly commensurate and incommensurate charge-density-wave phases. The evolution of the hysteresis and the presence of abrupt spikes in the current while sweeping the gate voltage suggest that the effect is electrical rather than self-heating. We attribute the gating to an electric-field effect on the commensurate charge-density-wave domains in the atomic planes near the gate dielectric. The transition between the nearly commensurate and incommensurate charge-density-wave phases can be induced by both the source– drain current and the electrostatic gate. Since the charge-densitywave phases are persistent in 1T-TaS 2 at room temperature, one can envision memory applications of such devices when scaled down to the dimensions of individual commensurate domains and few-atomic plane thicknesses.