Transition metal dichalcogenides (TMDs) are layered compounds which can be thinned down to the single-layer limit. While mechanical exfoliation generates atomically thin TMD flakes possessing an area of a few square microns, chemical and physical methods provide high-quality monolayers on large-area substrates, which are suitable for actual technological applications.
Similar to other two-dimensional materials, TMD monolayers are characterized by extreme surface sensitivity, making it possible to finely tune their optoelectronic properties through electrostatic gating or surface treatments. Molecular functionalization is one of the most promising methods to engineer TMDs, because an accurate choice of convenient functional groups makes it possible to provide programmable doping levels and unique responsivity to light and magnetic fields.
While most studies on molecular functionalization of TMDs demonstrate the engineering of the optoelectronic properties of micron-sized mechanically exfoliated flakes, only a few works focus on technologically relevant large-area TMDs, leaving an open question about the up-scalability of chemical approaches. What is more, the effect of organic adsorbates on other intrinsic properties of TMDs, like superconductivity, has been notably less explored.
Despite its limited stability in air, superconducting Niobium diselenide (NbSe2) has been intensely studied in the past decade because it exhibits intriguing electronic correlated phases. NbSe2 is a superconductor with a critical temperature of 7.2 K. The critical temperature drops when the NbSe2 layers are intercalated by other atoms, or when the sample thickness decreases, being ~1 K in a monolayer. The low-temperature superconducting state is gate tunable and can be modified by molecular functionalization. However, a deterministic manipulation of the superconductivity of TMD monolayers by functionalization with on-purpose molecular adlayers has not yet been reported.
Now, a team of researchers manipulates 1 the critical temperature of large area single-layer NbSe2 in a deterministic way employing ultrathin self-assembled adlayers. Functionalization with a fluorinated or an amine-containing molecule results in a 55% increase and a 70% decrease in the critical temperature of NbSe2 monolayers, respectively.
The researchers use ultraviolet photoemission spectroscopy data to demonstrate that the recorded changes in critical temperature are related to electric fields generated by the molecular adlayers, which act as an effective fixed gate terminal. Importantly, the polarity of the field-effect is determined by an accurate choice of appropriate functional groups.
Interestingly, the presence of the ultrathin adlayer improves the air stability of NbSe2, and the induced critical temperature modification is only minimally affected by storing the sample in air for 60 h.
This functionalization, with the associated improvement of the air stability of NbSe2, is efficient, practical, up-scalable, and suited to functionalize large-area TMDs. These results indicate the potential of hybrid 2D materials as a novel platform for tunable superconductivity.
Author: César Tomé López is a science writer and the editor of Mapping Ignorance
Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research papers.