Quantum Enhancement of Charge Density Wave in NbS₂ in the Two-Dimensional Limit

Abstract

At ambient pressure, bulk 2H-NbS2 displays no charge density wave instability at odds with the isostructural and isoelectronic compounds 2H-NbSe2, 2H-TaS2 and 2H-TaSe2, and in disagreement with harmonic calculations. Contradictory experimental results have been reported in supported single layers, as 1H-NbS2 on Au(111) does not display a charge density wave, while 1H-NbS2 on 6H-SiC(0001) endures a 3 × 3 reconstruction. Here, by carrying out quantum anharmonic calculations from first-principles, we evaluate the temperature dependence of phonon spectra in NbS2 bulk and single layer as a function of pressure/strain. For bulk 2H-NbS2, we find excellent agreement with inelastic X-ray spectra and demonstrate the removal of charge ordering due to anharmonicity. In the 2D limit, we find an enhanced tendency toward charge density wave order. Freestanding 1H-NbS2 undergoes a 3 × 3 reconstruction, in agreement with data on 6H-SiC(0001) supported samples. Moreover, as strains smaller than 0.5% in the lattice parameter are enough to completely remove the 3 × 3 superstructure, deposition of 1H-NbS2 on flexible substrates or a small charge transfer via field-effect could lead to devices with dynamical switching on/off of charge order.

Weak Dimensionality Dependence and Dominant Role of Ionic Fluctuations in the Charge-Density-Wave Transition of NbSe2

“…It is to be seen whether the enhanced T CDW observed in Raman experiments is a consequence of the sample deterioration, e.g., oxidation, during the exfoliation process or if it is affected by the substrate. Indeed, similar theoretical calculations as those presented here have recently shown that strain or charge doping from the substrate can affect the CDW transition in other TMDs [48,49]…”

“…The harmonic results show a very weak dimensionality dependence in both q CDW and T CDW (the monolayer and bulk temperature differ by 5%). However, while the ordering vector q CDW is in good agreement with experiments, the poor agreement of the calculated T CDW s makes it very difficult to draw any reliable conclusion at the harmonic level, and suggests that anharmonicity could be the key ingredient in the CDW melting, as it happens in other chalcogenides [13,[48][49][50][51][52]. Since the high-temperature undistorted phase is not a minimum of the Born-Oppenheimer potential VðRÞ, anharmonicity cannot be included through standard perturbative approaches on top of the harmonic result.…”

Patterns and driving forces of dimensionality-dependent charge density waves in 2H-type transition metal dichalcogenides

“…1f suggests enhanced ionic bonding upon reducing the layer number in NbS 2 . CDWs, therefore, do not emerge in atomically thin samples, although a recent study predicts otherwise 34 .…”

Optimization Design and Application of Niobium‐Based Materials in Electrochemical Energy Storage

“…[31] The high specific surface area and open layer structure of NbS 2 offer convenient atomic interface contact/interaction pathways for ions, along with its high theoretical capacity and relatively low operating potential, which make it a potential high-energy storage material. [32] Although the aforementioned energy storage advantages, a series of problems of NbS 2 are also obvious. Because of the abundant functional groups on surface, its accumulation and agglomeration is easy to happen.…”