Nanoscale Phase Engineering of Niobium Diselenide

Abstract

With continuing miniaturization in semiconductor microelectronics, atomically thin materials are emerging as promising candidate materials for future ultra-scaled electronics. In particular, the layered transition metal dichalcogenides (TMDs) have attracted significant attention due to the variety of their electronic properties, depending on the type of transition metal and its coordination within the crystal. Here, we use low-temperature scanning tunneling microscopy (STM) for the structural and electronic phase-engineering of the group-V TMD niobium diselenide (NbSe 2 ). By applying voltage pulses with an STM tip, we can transform the material crystal phase locally from trigonal prismatic (2H) to octahedral (1T), as confirmed by the concomitant emergence of a characteristic (√13x√13)R13.9° charge density wave (CDW) order. At 77 K atomic-resolution STM images of the junction with sub-lattice detail confirm the successful phase-engineering of the material, as we resolve the difference in the Nb coordination evidenced by a slip of the top Se plane. Different 1T-CDW intensities suggest interlayer interactions to be present in 1T-NbSe 2 . Furthermore, a distinct voltage dependence suggests a complex CDW mechanism that does not just rely on a Star-of-David reconstruction as in the case of other 1T-TMDs. Additionally, bias pulses cause surface modifications inducing local lattice strain that favors a one-dimensional charge order (1D-CDW) over the intrinsic 3 x 3 CDW at 4.5 K for 2H-NbSe 2 , which can be reversibly manipulated by STM.

Strain-induced stripe phase in charge-ordered single layer NbSe2

“…S1. The sharp minima in the phonon dispersion of the single layer, located near the √ 13 × √ 13 region, support a recently observed 2H-1T structural transition [42,43].…”

“…Lack of inversion symmetry and van der Waals bonded layers may result in remarkably different behavior with respect to the bulk [39][40][41]. The 2H-1T crystal phase transition at the surface of NbSe 2 [42,43] (the 1T crystal phase is unstable in the bulk [44]), the occurrence of 4 × 4 [3,4] and/or a 2 × 2 [4] modulations, linked to a stripe phase, and the observation of distinct charge density wave structures, determining a factor for the incommensuration character [6], are just a few recent examples. A change from the 3×3 to the 4×4 modulation was previously predicted by ab-initio studies [20,48]; on the other hand, a recent study [5] showed that within a 3 × 3 modulation, uniaxial strain leads to a trigonal-stripe CDW stransition.…”

Strategies on Phase Control in Transition Metal Dichalcogenides

“…Finally, we would like to point out that although here we elaborate the phase transition largely based on group‐6 TMDs, the same mechanisms actually can be adopted to understand the phase transition in other groups of TMDs …”

Phonon-Assisted Magnetic Mott-Insulating State in the Charge Density Wave Phase of Single-Layer 1T−NbSe2

“…Not surprisingly, the ARPES data in the CDW phase are in stark disagreement with electronic structure calculations in the high symmetry 1TNbSe 2 phase [17]. Finally, and most interesting, Bischoff et al [18] recently showed that it is possible to transform the surface of exfoliated 2HNbSe 2 samples into a 1TNbSe 2 phase by applying STM bias pulse of 4V for 100 ms. The 1TNbSe 2 phase in the top layer is then (meta)stable at 77 K and displays the occurrence of the characteristic √ 13 × √ 13 R30 • reconstruction.…”