The crystal structure of niobium selenide Nb2Se9 from twin-crystal data

Carrier mobility of one-dimensional vanadium selenide (V2Se9) monolayer and nanoribbon systems: DFT study

Vanadium selenide (V₂Se₉) is a true one-dimensional (1D) crystal composed of atomic nanochains bonded by van der Waals (vdW) interactions. Recent experiments revealed the mechanical exfoliation of newly synthesized V₂Se₉. In this study, we predicted the electronic and transport properties of V₂Se₉through computational analyses. We calculated the intrinsic carrier mobility of V₂Se₉monolayers (MLs) and nanoribbons (NRs) using density functional theory and deformation potential theory. We found that the electron mobility of the two-dimensional (2D) (010)-plane ML of V₂Se₉is highly anisotropic, reachingμ2D,ze=1327cm²V^-1s^-1across the chain direction. The electron mobility of 1D NR systems in a (010)-plane ML of V₂Se₉along the chain direction continuously increased as the thickness increased from 1-chain to 4-chain NR (width below 3 nm). Interestingly, the electron mobility of 1D 4-chain NR along the chain direction (μ1D,xe=775cm²V^-1s^-1) was higher than that of a 2D (010)-plane ML (μ2D,xe=567cm²V^-1s^-1). These results demonstrate the potential of vdW-1D crystal V₂Se₉as a new nanomaterial for ultranarrow (sub-3 nm width) optoelectronic devices with high electron mobility.

Tuning the electronic properties of highly anisotropic 2D dangling-bond-free sheets from 1D V2Se9 chain structures

True one-dimensional (1D) van der Waals materials can form two-dimensional (2D) dangling-bond-free anisotropic surfaces. Dangling bonds on surfaces act as defects for transporting charge carriers. In this study, we consider true 1D materials to be V2Se9 chains, and then the electronic structures of 2D sheets composed of true 1D V2Se9 chains are calculated. The (010) plane has indirect bandgap with 0.757 eV (1.768 eV), while the (111̅) plane shows a nearly direct bandgap of 1.047 eV (2.118 eV) for DFT-D3 (HSE06) correction, respectively. The (111̅) plane of V2Se9 is expected to be used in optoelectronic devices because it contains a nearly direct bandgap. Partial charge analysis indicates that the (010) plane exhibits interchain interaction is stronger than the (111̅) plane. To investigate the strain effect, we increased the interchain distance of planes until an indirect-to-direct bandgap transition occurred. The (010) plane then demonstrated a direct bandgap when interchain distance increased by 30%, while the (111̅) plane demonstrated a direct bandgap when the interchain distance increased by 10%. In mechanical sensors, this change in the bandgap was induced by the interchain distance.

Ternary Chalcogenides BaMxTe2 (M = Cu, Ag): Syntheses, Modulated Crystal Structures, Optical Properties, and Electronic Calculations

Standard solid-state methods produced black crystals of the compounds BaCu_0.43(3)Te₂ and BaAg_0.77(1)Te₂ at 1173 K; the crystal structures of each were established using single-crystal X-ray diffraction data. Both crystal structures are modulated. The compound BaCu_0.43(3)Te₂ crystallizes in the monoclinic superspace group P2(αβ1/2)0, having cell dimensions of a = 4.6406(5) Å, b = 4.6596(5) Å, c = 10.362(1) Å, β = 90.000(9)°, and Z = 2 and an incommensurate vector of q = 0.3499(6)b* + 0.5c*. The compound BaAg_0.77(1)Te₂ crystallizes in the orthorhombic P2₁2₁2(α00)000 superspace group with cell dimensions of a = 4.6734(1) Å, b = 4.6468(1) Å, c = 11.1376(3) Å, and Z = 2 and an incommensurate vector of q = 0.364(2)a*. The asymmetric unit of the BaCu_0.43(3)Te₂ structure comprises eight crystallographically independent sites; that for BaAg_0.77(1)Te₂ comprises four. In these two structures, each of the M (M = Cu, Ag) atoms is connected to four Te atoms to make two-dimensional layers of [M_xTe_4/4]^n- that are separated by layers of Ba atoms and square nets of Te. A Raman spectroscopic study at 298(2) K on a pelletized polycrystalline sample of BaAg_0.8Te₂ shows the presence of Ag-Te (83, 116, and 139 cm^-1) and Ba-Te vibrations (667 and 732 cm^-1). A UV-vis-NIR spectroscopic study on a powdered sample of BaAg_0.8Te₂ shows the semiconducting nature of the compound with a direct band gap of 1.0(2) eV, consistent with its black color. DFT calculations give a pseudo bandgap with a weak value of the DOS at the Fermi level.

Edge Defect-Free Anisotropic Two-Dimensional Sheets with Nearly Direct Band Gaps from a True One-Dimensional Van der Waals Nb2Se9 Material

Dangling-bond-free two-dimensional (2D) materials can be isolated from the bulk structures of one-dimensional (1D) van der Waals materials to produce edge-defect-free 2D materials. Conventional 2D materials have dangling bonds on their edges, which act as scattering centers that deteriorate the transport properties of carriers. Highly anisotropic 2D sheets, made of 1D van der Waals Nb2Se9 material, have three planar structures depending on the cutting direction of the bulk Nb2Se9 crystal. To investigate the applications of these 2D Nb2Se9 sheets, we calculated the band structures of the three planar sheets and observed that two sheets had nearly direct band gaps, which were only slightly greater (0.01 eV) than the indirect band gaps. These energy differences were smaller than the thermal energy at room temperature. The 2D Nb2Se9 plane with an indirect band gap had the shortest interchain distance for selenium ions among the three planes and exhibited significant interchain interactions on the conduction band. The interchain strain induced an indirect-to-direct band gap transition in the 2D Nb2Se9 sheets. These 2D sheets of Nb2Se9 with direct band gaps also had different band structures because of different interactions between chains, implying that they can have different charge mobilities. We expect these dangling-bond-free 2D Nb2Se9 sheets to be applied in optoelectronic devices because they allow for nearly direct band gaps. They can also be used in mechanical sensors because the band gaps can be controlled by varying the interchain strain.

Isolation of Nb₂Se₉ Molecular Chain from Bulk One-Dimensional Crystal by Liquid Exfoliation

The optimum solvent for Nb₂Se₉ dispersion, which is a new type of one dimensional (1D) material, is investigated. Among several solvents (16 solvents in total), strong dispersion was observed in benzyl alcohol, isopropyl alcohol, isobutyl alcohol, and diacetone alcohol, which have medium dielectric constants in the range of 10 to 30 and surface tension in the range of 25 to 35 mJ m^-2. 1D Nb₂Se₉ chains, whose size is less than 10 nm, are well dispersed and it is possible to disperse mono-chains of 1 nm or less in a specific dispersion region. The 1D unit chain with dangling bond free surface and high volume to area ratio is expected to be used in applications that utilize the surface of the material. Such dispersion is an important first step towards various potential applications and is an indispensable scientific goal for the practical applications of Nb₂Se₉.

Mechanical exfoliation and electrical characterization of a one-dimensional Nb2Se9 atomic crystal

A novel semiconductor 1D nanomaterial, Nb₂Se₉, was synthesized on a bulk scale via simple vapor transport reaction between niobium and selenium. Needle-like single crystal Nb₂Se₉ contains numerous single Nb₂Se₉ chains linked by van der Waals interactions, and we confirmed that a bundle of chains can be easily separated by mechanical cleavage. The exfoliated Nb₂Se₉ flakes exhibit a quasi-two-dimensional layered structure, and the number of layers can be controlled using the repeated-peeling method. The work function varied depending on the thickness of the Nb₂Se₉ flakes as determined by scanning Kelvin probe microscopy. Moreover, we first implemented a field effect transistor (FET) based on nanoscale Nb₂Se₉ flakes and verified that it has p-type semiconductor characteristics. This novel 1D material can form a new family of 2D materials and is expected to play important roles in future nano-electronic devices.

A novel semiconductor 1D nanomaterial, Nb₂Se₉, was synthesized on a bulk scale via simple vapor transport reaction between niobium and selenium.

A new mixed group 5 metal selenide, Nb(1.41)V(0.59)Se(9)

The new mixed-metallic phase, niobium vanadium nona-selenide, (Nb(2-x)V(x))Se(9) (0.18≤ x ≤ 0.59) is isostructural with monoclinic V(2)Se(9). The structure is composed of chains of bicapped trigonal-prismatic [MSe(8)] units. The metal (M) site is occupied by statistically disordered Nb [0.706 (5)] and V [0.294 (5)] atoms. Two trigonal prisms are linked by sharing a recta-ngular face composed of two Se(2) (2-) pairs. Through three edging and capping Se atoms, the chains are extended along [101]. The chain shows alternating short [2.8847 (7) Å] and long [3.7159 (7) Å] M-M distances. The structure shows a wide range of Se-Se inter-actions. In addition to the Se(2) (2-) pairs of the recta-ngular face, an inter-mediate Se⋯Se separation [2.6584 (5) Å] is found. The amount of each metal can vary, [(Nb(2-x)V(x))Se(9), 0.18 ≤ x ≤m 0.59] and they seem to form a random substitutional solid solution. The M-M distances increase gradually by increasing the amount of Nb atoms. The classical charge-balance of the compound can be described as [M(4+)](2)[Se(2) (2-)](2)[Se(5) (4-)].