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

Multivalency-induced structural variations of 2D selenium nanosheets: facile solution-phase synthesis and optical properties

The structural stability of two-dimensional (2D) phases derived from bulk selenium (Se) is intrinsically rooted in the multivalent nature of the material. The emergence of 2D Se, as its morphology evolves from 1D to 2D, was initially inspired by theoretical predictions of various quasi-stable structural phases of 2D Se. Here, we report a facile liquid-phase synthesis of free-standing few-layer selenium nanosheets (SeNS) employing a simple magnetic stirring of their bulk counterpart in N-methyl pyrrolidone (NMP). The synthesized SeNS possess lateral dimensions ranging from several hundreds of nanometers to a few microns, with a minimum thickness of ∼1 nm. High-resolution transmission electron microscopy reveals the existence of α- and β-selenene. Fourier transform infrared analysis suggests that the inherent surface/edge functionalization of 2D SeNS by NMP enhances their dispersion stability. The UV-vis-NIR absorption spectrum of SeNS exhibits a shoulder peak at 330 nm, attributed to surface/edge functionalization, and multiple peaks across the vis-NIR region, stemming from size quantization effects. The functionalized selenium nanosheets generate photoluminescence that spans the blue-green range, while the size quantization of SeNS leads to green-orange luminescence. The non-linear optical studies following Z-scan experiments with an open aperture revealed reverse saturable absorption (RSA) and strong optical limiting in 2D SeNS under 532 nm, 10 ns laser pulses. Notably, a transition from RSA to saturable absorption (SA) has also been observed in samples stirred over an extended period. In this perspective, the results illustrate the first experimental realization of free-standing multivalent 2D selenium in allotropic forms with unique optical properties.

Controlled Bipolar Doping of One-Dimensional van der Waals Nb2Pd3Se8

Tailoring the electrical properties of one-dimensional (1D) van der Waals (vdW) materials is desirable for their applications toward electronic devices by exploiting their unique characteristics. However, 1D vdW materials have not been extensively investigated for modulation of their electrical properties. Here we control doping levels and types of 1D vdW Nb₂Pd₃Se₈ over a wide energy range by immersion in AuCl₃ or β-nicotinamide adenine dinucleotide (NADH) solutions, respectively. Through spectroscopic analyses and electrical characterizations, we confirm that the charges were effectively transferred to Nb₂Pd₃Se₈, and the dopant concentration was adjusted to the immersion time. Furthermore, we make the axial p-n junction of 1D Nb₂Pd₃Se₈ by a selective area p-doping using the AuCl₃ solution, which exhibits rectifying behavior with an I_forward/I_reverse of 81 and an ideality factor of 1.2. Our findings could pave the way to more practical and functional electronic devices based on 1D vdW materials.

Colloidal synthesis of 1-D van der Waals material Nb2Se9: study of synergism of coordinating agent in a co-solvent system

Low-dimensional Nb₂Se₉ nanocrystals were synthesised through a simple one-pot colloidal synthesis with C18 organic solvents (octadecane, oleylalcohol, octadecanethiol, octadecene (ODE), and oleylamine (OLA)) of varied terminal functional groups. The solvent with high reducing power facilitated the nucleation of the nanoparticle, lowering threshold concentration and broadening the concentration spectrum. As a solvent, reducing agent, and capping agent, ODE functions as a primary factor in the synthesis of high-quality Nb₂Se₉ nanorods. We further discuss the unique adhesion role of ODE under the co-solvent system and demonstrate morphology control through synergism between ODE and OLA, verified by the electrochemical measurements.

Liquid Precursor-Assisted Chemical Vapor Deposition of One-Dimensional van der Waals Material Nb2Se9: Tunable Growth for Room-Temperature Gas Sensors

In this study, Nb₂Se₉, a one-dimensional (1D) material with van der Waals (vdWs) bonding, was synthesized by chemical vapor deposition (CVD). A liquid precursor was used to overcome the difficulty of controlling the length and density of Nb₂Se₉ by CVD due to the high melting point of Nb. Growth proceeded horizontally in a nano-ribbon shape on the substrate in the [100] direction, which had the most stable bonding distance, resulting in a preferred orientation of the (010) plane on the out-of-plane axis. Unlike that grown by conventional mechanical or chemical exfoliation, the nanoscale Nb₂Se₉ grown by CVD was uniform and did not have contaminants, such as dispersants, on its surface, meaning it could effectively induce reactions such as gas adsorption and desorption. It exhibited high sensitivity to NO₂ gas adsorption at room temperature (27 °C), and its behavior was confirmed in a high-humidity environment. For the first time, this study demonstrated the possibility of synthesizing a vdWs bonding-based 1D material by CVD, which is expected to be widely used in a variety of low-dimensional materials and devices.

High Breakdown Current Density in Quasi-1D van der Waals Layered Material Ta2NiSe7

We synthesized ternary composition chalcogenide Ta2NiSe7, a quasi-one-dimensional (Q1D) material with excellent crystallinity. To utilize the excellent electrical conductivity property of Ta2NiSe7, the breakdown current density (JBD) according to thickness change through mechanical exfoliation was measured. It was confirmed that as the thickness decreased, the maximum breakdown voltage (VBD) increased, and at 18 nm thickness, 35 MA cm-2 of JBD was measured, which was 35 times higher than that of copper, which is commonly used as an interconnect material. By optimization of the exfoliation process, it is expected that through a theoretical model fitting, the JBD can be increased to about 356 MA cm-2. It is expected that the low-dimensional materials with ternary compositions proposed through this experiment can be used as candidates for current-carrying materials that are required for the miniaturization of various electronic devices.

One-dimensional van der Waals stacked p-type crystal Ta2Pt3Se8 for nanoscale electronics

Recently, ternary transition metal chalcogenides Ta₂X₃Se₈ (X = Pd or Pt) have attracted great interest as a class of emerging one-dimensional (1D) van der Waals (vdW) materials. In particular, Ta₂Pd₃Se₈ has been actively studied owing to its excellent charge transport properties as an n-type semiconductor and ultralong ballistic phonon transport properties. Compared to subsequent studies on the Pd-containing material, Ta₂Pt₃Se₈, another member of this class of materials has been considerably less explored despite its promising electrical properties as a p-type semiconductor. Herein, we demonstrate the electrical properties of Ta₂Pt₃Se₈ as a promising channel material for nanoelectronic applications. High-quality bulk Ta₂Pt₃Se₈ single crystals were successfully synthesized by a one-step vapor transport reaction. Scanning Kelvin probe microscopy measurements were used to investigate the surface potential difference and work function of the Ta₂Pt₃Se₈ nanoribbons of various thicknesses. Field-effect transistors fabricated on exfoliated Ta₂Pt₃Se₈ nanoribbons exhibited moderate p-type transport properties with a maximum hole mobility of 5 cm² V^-1 s^-1 and an I_on/I_off ratio of >10⁴. Furthermore, the charge transport mechanism of Ta₂Pt₃Se₈ was analyzed by temperature-dependent transport measurements in the temperature range from 90 to 320 K. To include Ta₂Pt₃Se₈ in a building block for modern 1D electronics, we demonstrate p-n junction characteristics using the electron beam doping method.

Theoretical Study of Anisotropic Carrier Mobility for Two-Dimensional Nb2Se9 Material

Finding new materials with satisfying all the desired criteria for nanodevices is an extremely difficult work. Here, we introduce a novel Nb2Se9 material as a promising candidate, capable of overcoming some physical limitations, such as a suitable band gap, high carrier mobility, and chemical stability. Unlike graphene, it has a noticeable band gap and no dangling bonds at surfaces that deteriorate transport properties, owing to its molecular chain structure. Using density functional theory (DFT) calculations with deformation potential (DP) theory, we find that the electron mobility of 2D Nb2Se9 across the axis direction reaches up to 2.56 × 103 cm2 V-1 s-1 and is approximately 2.5-6 times higher than the mobility of other 2D materials, such as MoS2, black phosphorous, and InSe, at room temperature. Moreover, the mobility of 2D Nb2Se9 is highly anisotropic (μ a /μ c ≈ 6.5). We demonstrate the potential of 2D Nb2Se9 for applications in nanoscale electronic devices and, possibly, mid-infrared photodetectors.

Ta2 Ni3 Se8 : 1D van der Waals Material with Ambipolar Behavior

In this study, high-purity and centimeter-scale bulk Ta₂ Ni₃ Se₈ crystals are obtained by controlling the growth temperature and stoichiometric ratio between tantalum, nickel, and selenium. It is demonstrated that the bulk Ta₂ Ni₃ Se₈ crystals could be effectively exfoliated into a few chain-scale nanowires through simple mechanical exfoliation and liquid-phase exfoliation. Also, the calculation of electronic band structures confirms that Ta₂ Ni₃ Se₈ is a semiconducting material with a small bandgap. A field-effect transistor is successfully fabricated on the mechanically exfoliated Ta₂ Ni₃ Se₈ nanowires. Transport measurements at room temperature reveal that Ta₂ Ni₃ Se₈ nanowires exhibit ambipolar semiconducting behavior with maximum mobilities of 20.3 and 3.52 cm² V^-1 s^-1 for electrons and holes, respectively. The temperature-dependent transport measurement (from 90 to 295 K) confirms the carrier transport mechanism of Ta₂ Ni₃ Se₈ nanowires. Based on these characteristics, the obtained 1D vdW material is expected to be a potential candidate for additional 1D materials as channel materials.