Substrate tuning of the structural and electronic transition in thin flakes of the excitonic insulator candidate Ta$_2$NiSe$_5$
Ta$_2$NiSe$_5$ continues to draw interest for its $T_\textrm{c}$ = 326 K phase transition, whose dual electronic and structural nature reflects a complex interplay of electron-hole (excitonic) and electron-lattice interactions. The majority of studies that have attempted to decipher the relative importance of these interactions, particularly through charge tuning, have been focused on bulk samples. Here, we utilize an all-dry exfoliation and transfer protocol to isolate ultrathin flakes of Ta$_2$NiSe$_5$ on insulating Al$_2$O$_3$ and conducting Au. Using polarized Raman spectroscopy, we uncover the following substrate dependence: Four layers of Ta$_2$NiSe$_5$ on Al$_2$O$_3$ show a sharp structural and electronic transition that is lowered by roughly 40 K from the bulk $T_\textrm{c}$. Meanwhile, four layers of Ta$_2$NiSe$_5$ on Au undergo a structural and electronic transition that is much more gradual with respect to temperature and finishes roughly 150 K below the bulk $T_\textrm{c}$. The pronounced broadening points to an atomic-scale interface effect, wherein electrostatic screening and charge transfer from Au produces a $T_\textrm{c}$ gradient perpendicular to the layers of the flake. We discuss the role of excitonic physics and suggest the possibility for interface engineering to pattern nanoscale junctions in Ta$_2$NiSe$_5$.