Comparison of the electronic structures of layered transition-metal trichalcogenides thallium triselenide, thallium trisulfide and niobium triselenide

Structural approach to charge density waves in low-dimensional systems: electronic instability and chemical bonding

The charge density wave (CDW) instability, usually occurring in low-dimensional metals, has been a topic of interest for longtime. However, some very fundamental aspects of the mechanism remain unclear. Recently, a plethora of new CDW materials, a substantial fraction of which is two-dimensional or even three-dimensional, has been prepared and characterised as bulk and/or single-layers. As a result, the need for revisiting the primary mechanism of the instability, based on the electron-hole instability established more than 50 years ago for quasi-one-dimensional (quasi-1D) conductors, has clearly emerged. In this work, we consider a large number of CDW materials to revisit the main concepts used in understanding the CDW instability, and emphasise the key role of the momentum dependent electron-phonon coupling in linking electronic and structural degrees of freedom. We argue that for quasi-1D systems, earlier weak coupling theories work appropriately and the energy gain due to the CDW and the concomitant periodic lattice distortion (PLD) remains primarily due to a Fermi surface nesting mechanism. However, for materials with higher dimensionality, intermediate and strong coupling regimes are generally at work and the modification of the chemical bonding network by the PLD is at the heart of the instability. We emphasise the need for a microscopic approach blending condensed matter physics concepts and state-of-the-art first-principles calculations with quite fundamental chemical bonding ideas in understanding the CDW phenomenon in these materials.

Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe3and monoclinic-TaS3

NbSe₃and monoclinic-TaS₃(m-TaS₃) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions atTP1andTP2associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron-hole response function. Because of the stronger inter-chain interactions, the FS and electron-hole response function are considerably more complex for NbSe₃thanm-TaS₃; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron-hole response for NbSe₃are found with the (0a^*, 0c^*) and (1/2a^*, 1/2c^*) transverse components atTP1andTP2, respectively, whereas the second maximum is not observed form-TaS₃atTP2. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transversea^*components in NbSe₃. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling alongc^*in NbSe₃and along botha^*andc^*form-TaS₃. Phonon spectrum calculations reveal the formation of a giant 2k_FKohn anomaly form-TaS₃. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides.

Anisotropic quasi-one-dimensional layered transition-metal trichalcogenides: synthesis, properties and applications

The strong in-plane anisotropy and quasi-1D electronic structures of transition-metal trichalcogenides (MX3; M = group IV or V transition metal; X = S, Se, or Te) have pronounced influence on moulding the properties of MX3 materials. In particular, the infinite trigonal MX6 prismatic chains running parallel to the b-axis are responsible for the manifestation of anisotropy in these materials. Several marvellous properties, such as inherent electronic, optical, electrical, magnetic, superconductivity, and charge density wave (CDW) transport properties, make transition-metal trichalcogenides (TMTCs) stand out from other 2D materials in the fields of nanoscience and materials science. In addition, with the assistance of pressure, temperature, and tensile strain, these materials and their exceptional properties can be tuned to a superior extent. The robust anisotropy and incommensurable properties make the MX3 family fit for accomplishing quite a lot of compelling applications in the areas of field effect transistors (FETs), solar and fuel cells, lithium-ion batteries, thermoelectricity, etc. In this review article, a precise audit of the distinctive crystal structures, static and dynamic properties, efficacious synthesis schemes, and enthralling applications of quasi-1D MX3 materials is made.

Low Resistivity and High Breakdown Current Density of 10 nm Diameter van der Waals TaSe3 Nanowires by Chemical Vapor Deposition

Micron-scale single-crystal nanowires of metallic TaSe₃, a material that forms -Ta-Se₃-Ta-Se₃- stacks separated from one another by a tubular van der Waals (vdW) gap, have been synthesized using chemical vapor deposition (CVD) on a SiO₂/Si substrate, in a process compatible with semiconductor industry requirements. Their electrical resistivity was found unaffected by downscaling from the bulk to as little as 7 nm in nanowire width and height, in striking contrast to the resistivity of copper for the same dimensions. While the bulk resistivity of TaSe₃ is substantially higher than that of bulk copper, at the nanometer scale the TaSe₃ wires become competitive to similar-sized copper ones. Moreover, we find that the vdW TaSe₃ nanowires sustain current densities in excess of 10⁸ A/cm² and feature an electromigration energy barrier twice that of copper. The results highlight the promise of quasi-one-dimensional transition metal trichalcogenides for electronic interconnect applications and the potential of van der Waals materials for downscaled electronics.

TaS3 Nanofibers: Layered Trichalcogenide for High-Performance Electronic and Sensing Devices

Layered materials, like transition metal dichalcogenides, exhibit broad spectra with outstanding properties with huge application potential, whereas another group of related materials, layered transition metal trichalcogenides, remains unexplored. Here, we show the broad application potential of this interesting structural type of layered tantalum trisulfide prepared in a form of nanofibers. This material shows tailorable attractive electronic properties dependent on the tensile strain applied to it. Structure of this so-called orthorhombic phase of TaS₃ grown in a form of long nanofibers has been solved and refined. Taking advantage of these capabilities, we demonstrate a highly specific impedimetric NO gas sensor based on TaS₃ nanofibers as well as construction of photodetectors with excellent responsivity and field-effect transistors. Various flexible substrates were used for the construction of a NO gas sensor. Such a device exhibits a low limit of detection of 0.48 ppb, well under the allowed value set by environmental agencies for NO_x (50 ppb). Moreover, this NO gas sensor also showed excellent selectivity in the presence of common interferences formed during fuel combustion. TaS₃ nanofibers produced in large scale exhibited excellent broad application potential for various types of devices covering nanoelectronic, optoelectronic, and gas-sensing applications.

[P6Se12]4-: A Phosphorus-Rich Selenophosphate with Low-Valent P Centers

The new selenophosphate Rb4P6Se12 features the trans-decalin-like, [P6Se12]4- anion, a phosphorus-rich species that possesses three parallel P-P bonds and formally P2+ and P4+ centers. The synthesis of Rb4P6Se12 was accomplished with the reductive addition of P to RbPSe6 and represents an interesting example of how alkali chalcophosphates can serve as starting materials to produce new compounds under mild reaction conditions.

APSe6 (A = K, Rb, and Cs): Polymeric Selenophosphates with Reversible Phase-Change Properties

The ternary alkali selenophosphates KPSe6 and RbPSe6 crystallize in the polar orthorhombic space group Pca2(1) with a = 11.7764(17) A, b = 6.8580(10) A, c = 11.4596(16) A, and Z = 4 for RbPSe6. CsPSe6 crystallizes in the monoclinic space group P2/n with a = 6.877(3) A, b = 12.713(4) A, c = 11.242(4) A, beta = 92.735(7) degrees, and Z = 4. All compounds feature the one-dimensional infinite chain of [PSe2(Se)4-], where each P atom is connected with Se4(2-) bridge. These compounds show reversible glass-crystal transition, and 31P NMR data suggest that crystallization and infinite [PSe(6-)] chain formation are coupled processes.

Unusual spectral behavior of charge-density waves with imperfect nesting in a quasi-one-dimensional metal

Low-temperature electronic properties of the charge-density-wave system NbSe3 are reported from angle-resolved photoemission at 15 K. The effect of two instabilities q(1) and q(2) on the k-resolved spectral function is observed for the first time. With a pseudogap background, the gap spectra exhibit maxima at Delta*(1) approximately 110 meV and Delta*(2) approximately 45 meV. Imperfectly nested sections of the Fermi surface lack a Fermi-Dirac edge, and show the signature of a dispersion that is modified by self-energy effects. The energy scale is of the order of the effective gap 2 Delta*(2). The effect disappears above T2, suggesting a correlation with the charge-density-wave state.

High-temperature symmetry breaking in the electronic band structure of the quasi-one-dimensional solid NbSe3

The electronic band structure of the Peierls compound NbSe3 has been explored for its symmetries with microspot synchrotron photoemission. The Fermi level crossings and deviations from one-dimensional behavior are identified. Density-functional calculations of the Fermi surfaces confirm the nesting conditions relevant for the two phase transitions. The instability along the chains with superstructure periodicity q = 0.44 A(-1) induces a backfolding of the electronic bands, and the Fermi level crossings appear suppressed. This broken symmetry is observed in the fluctuation regime at more than twice the critical temperature, where the correlation length is strongly reduced.