Quasi-One-Dimensional Transition-Metal Chalcogenide Semiconductor (Nb₄Se₁₅I₂)I₂

Abstract

The discovery of new low-dimensional transition-metal chalcogenides is contributing to the already prosperous family of these materials. In this study, needleshaped single crystals of a quasi-one-dimensional (1D) material, (Nb 4 Se 15 I 2 )I 2 , were grown by chemical vapor transport, and the structure was solved by single-crystal X-ray diffraction (XRD). The structure has 1D (Nb 4 Se 15 I 2 ) n chains along the [101] direction, with two I − ions per formula unit directly bonded to Nb 5+ . The other two I − ions are loosely coordinated and intercalated between the chains. Individual chains are chiral and stack along the b axis in opposing directions, giving space group P2 1 /c. The phase purity and crystal structure were verified by powder XRD. Density functional theory calculations show (Nb 4 Se 15 I 2 )I 2 to be a semiconductor with a direct band gap of around 0.6 eV. Resistivity measurements of bulk crystals and micropatterned devices demonstrate that (Nb 4 Se 15 I 2 )I 2 has an activation energy of around 0.1 eV, and no anomaly or transition was seen upon cooling. Low-temperature XRD shows that (Nb 4 Se 15 I 2 )I 2 does not undergo a structural phase transformation from room temperature to 8.2 K, unlike related compounds (NbSe 4 ) n I (n = 2, 3, or 3.33), which all exhibit charge-density waves. This compound represents a well-characterized and valenceprecise member of a diverse family of anisotropic transition-metal chalcogenides.

Exfoliable Transition Metal Chalcogenide Semiconductor NbSe2I2

“…This distance range confirms the Nb−I bonding, which is seen in some but not all Nb−Se−I compounds. 29 Figure S3 in Supporting Information shows the visualization of the Nb−I distance comparison. 39 All of the selenium ions are dimerized, forming Se 2 2− dimers between niobium ions.…”

“…The CVT synthesis of NbSe 2 I 2 gained its insight from the CVT synthesis of (TaSe 4 ) 2 I 38 as well as the discovery of (Nb 4 Se 15 I 2 )I 2 . 29 Thin sheets of NbSe 2 I 2 crystals were first discovered from a trial CVT synthesis with the starting ingredients at 460 °C and the crystals grown at a lower temperature of 420 °C. This unusual CVT growth from higher temperature to lower temperature matched with the published synthesis of many Nb−Se−I ternary compounds.…”

“…The starting ingredients were the same as those for the synthesis of (Nb 4 Se 15 I 2 )I 2 . 29 Elements were ground in an agate mortar with a molar ratio of Nb (99.8%, powder):Se (99.999%, ground shots):I (99.8%, crystalline solid) = 2:8:10.3. The excess 0.3 ratio of iodine was intentionally added to compensate for the loss from sublimation, and the ratio of iodine was larger than any known Nb−Se−I ternary compounds to ensure enough iodine was available to act as the transport agent.…”

“…For many 2D materials, large crystals were first grown by CVT, such as CrAs and TaSe 2 , and it is a viable method to discover new materials due to the straightforward setup and the large number of tunable experimental variables. Iodine is the most widely used transport agent, in addition to a large library of possible transport agents, and this will especially lead to the discovery of new iodine-containing compounds . The crystals synthesized by CVT are bulky, but often they can be exfoliated down to a few layers or monolayers if the planes are separated by van der Waals interactions.…”

“…Iodine is the most widely used transport agent, in addition to a large library of possible transport agents, 28 and this will especially lead to the discovery of new iodine-containing compounds. 29 The crystals synthesized by CVT are bulky, but often they can be exfoliated down to a few layers or monolayers if the planes are separated by van der Waals interactions. Recently, pristine graphene came back under the spotlight after breakthrough studies on twisted bilayer graphene (TBG) in 2018.…”