Size Dependence of Charge-Density-Wave Orders in Single-Layer NbSe₂ Hetero/Homophase Junctions

Abstract

Controlling charge-density-wave (CDW) orders in two-dimensional (2D) crystals has attracted a great deal of interest because of their fundamental physics and their demand inse in miniaturized devices. In this work, we systematically studied the size-dependent CDW orders in single-layer hetero/homo-NbSe 2 stacking junctions. We found that the CDW orders in the top 1T-NbSe 2 layer of the junctions are highly dependent on its lateral size. For the 1T/2H-NbSe 2 heterojunction, the critical lateral size of 1T-NbSe 2 islands for the formation of well-defined CDW orders is ∼26 nm, whereas below 15 nm, the CDW orders melt. However, for the 1T/1T-NbSe 2 homojunction, the CDW orders in the islands can persist even with a lateral size of <11 nm. Our findings illuminate the fresh phenomenon of size-dependent CDW orders existing in 2D van der Waals hetero/homojunctions and provide useful information for the control of CDW orders.

Ullmann-Like Covalent Bond Coupling without Participation of Metal Atoms

“…Two-dimensional (2D) materials have a thickness much smaller than the lateral dimension. Many of them have fascinating electronic and optoelectronic properties. − 2D materials, combined with the catalyst reactions, can give rise to many advantageous applications, such as a “chainmail” catalyst, where the catalyst can be protected by the 2D materials. , However, in comparison to the extensive studies on Ullmann-like reactions on metallic substrates, few have focused on the Ullmann-like dehalogenation and coupling reaction on 2D surfaces.…”

Robust Behavior of Charge Density Wave Quantum Motif Star‐of‐David in 2D NbSe2 Nanocrystals

“…These two effects cooperate and become more evident in the small islands, especially when the island size is comparable to several SOD clusters. Compared to CDW melted behavior in the 1T/2H case, [18] in the SL 1T on BLG case, each SOD as a quantum unit is much more robust against the size-limiting effect. This phenomenon may be due to the SL 1T case being closer to the intrinsic free-standing case, and the doping influence from the substrate can be neglected.…”

“…From Figure 1, it is concluded that for the small 1T islands, the SOD array at the interior of the island can be still ordered, in sharp contrast to the melted CDW behavior in the 1T/2H case (Figure S1, Supporting Information). [18] After discussing the ideal triangular packing cases of small 1T-NbSe 2 islands in Figure 1, we further focus on the non-ideal cases of CDW rearrangement with robust SOD clusters. As shown in Figure 2a, there is a small island embracing six SODs in the interior, and four of them show a square-like pattern, as labeled by the red box.…”

“…It has been demonstrated that shrinking the layers vertically from bulk to a few layers can greatly change the transition temperature T c in TaS 2 , [16] the 1D confinement from the edge introduces the quartet Star-of-David (SOD) superstructure in NbSe 2 , [17] 0D confinement for the small single-layer (SL) 1T NbSe 2 on 2H and 1T substrates showing melted and ordered CDW appearance, respectively. [18] In this study, we choose a prototype system of nanoscale 1T-NbSe 2 islands grown on bilayer graphene (BLG) on SiC(0001). We demonstrate that this system provides an ideal platform for studying the relationship between the lateral island size and the CDW ordering as well as the associated electronic states.…”

Observation of Two-Dimensional Type-II Superconductivity in Bulk 3R-TaSe2 by Scanning Tunneling Spectroscopy

“…Transition metal dichalcogenides (TMDs), MX 2 (M = transition metal; X = S, Se, Te), have attracted increasing interest due to their different intriguing quantum states, − such as superconductivity (SC) − and charge density waves (CDWs). − Among these materials, TaSe 2 with several phases, including 2H, 1T, and 3R, has attracted great research interest. The 2H and 1T structures both experience two CDW transitions with incommensurate and commensurate lattice contraction , and show a superconducting transition at a rather low temperature. − Recently, 3R-TaSe 2 , as a very interesting structure, has been arousing wide attention as a candidate for realizing a two-dimensional superconducting system due to its higher superconducting transition temperature ( T c ) compared with bulk 2H-TaSe 2 (0.1 K) .…”