Tuning competing electronic phases in monolayer VSe$_2$ via interface hybridization

Abstract

Competing electronic phases in two-dimensional transition metal dichalcogenides constitute a fertile platform for uncovering emergent ground states and elucidating the control parameters that govern the correlated electron phases. Among these materials, vanadium diselenide is particularly compelling: while the bulk hosts a well-established charge density wave (CDW), monolayers exhibit markedly different electronic behavior. Here, we identify three distinct electronic regimes in mechanically exfoliated VSe$_2$ flakes on Au(111) substrates, where interfacial hybridization, charge transfer, and strain act as primary tuning parameters of electronic order. Monolayers strongly coupled to gold show complete suppression of the CDW, accompanied by the emergence of moiré modulations. In contrast, bilayers preserve the in-plane $4a \times 4a$ CDW characteristic of the bulk limit. Strained, electronically decoupled monolayers formed in suspended membrane and bubble regions stabilize a $\sqrt{3}a\times\sqrt{7}a$ CDW phase, underscoring the reversible role of substrate interaction and hybridization.

Figures (5)

• Figure 1: (a) STM topography (380 $\times$ 380 nm$^2$, $V_{bias}=500$ mV, $I_t=20$ pA ) of exfoliated ML and BL VSe$_2$ flakes on a Au(111) mosaic substrate. (b) Normal state unit cell of free-standing monolayer VSe$_2$. (c) Density of states (DOS) for free-standing or decoupled monolayer VSe$_2$. (d) Harmonic phonon calculations for free-standing monolayer VSe$_2$. (e) Schematic of the sample with ML and BL flakes of VSe$_2$ (green) on gold (yellow); and possible decoupling scenarios: membrane over a pit in the gold or bubbles due to trapped material.
• Figure 2: (a) 9 $\times$ 9 nm$^2$ STM topography ($V_{bias}=200$ mV, $I_t=200$ pA ) on a BL region showing a weak $4a\times4a$ CDW modulation along with the atomic lattice. The red and yellow dashed lines mark the CDW and atomic directions, respectively. (b) LDOS map (at $E=-70$ meV) over the boundary (grey) between a BL (purple) and ML (green) region. (c) 8 $\times$ 8 nm$^2$ STM topography ($V_{bias}=-100$ mV, I$_T$ = 200 pA) on bulk VSe$_2$ showing the in-plane $4a\times4a$ CDW modulation along with the atomic lattice. (d) DFT simulated image showing $4a\times4a$ CDW modulation.
• Figure 3: STM topography (5 $\times$ 5 nm$^2$, see imaging parameters in Methods) on different ML flakes on Au(111) obtained in a single exfoliation. The red and yellow dashed lines mark the moiré and atomic lattice directions, respectively. Note the same atomic lattice orientation in all four images, as expected for flakes originating from the same single crystal.
• Figure 4: (a) STM topography of the $\sqrt{3}a \times\sqrt{7}a$ ML-CDW developing on ML bubbles (Image size: 3 $\times$ 3 nm$^2$; Setpoint: $V_{bias}=200$ mV, $I_t=50$ pA). (b) Simulated STM images (5 $\times$ 5 nm$^2$) of decoupled VSe$_2$ displaying a $\sqrt{3}a \times\sqrt{7}a$ CDW phase and (c) coupled VSe$_2$ showing the absence of CDW but rather a moiré pattern. (d)-(f) Consecutive STM topographic images of the same area of a ML (see the three defects highlighted in red as references). They show a decoupled ML membrane with its ML-CDW surrounded by moiré regions where the ML is coupled to the substrate. It shows the reversible coupling and decoupling of the membrane to the substrate in the lower left region. (10 $\times$ 10 nm$^2$; $-200$ mV, $1$ nA).
• Figure 5: (a) Differential tunneling conductance spectra measured on a coupled monolayer with a moiré pattern and a bilayer with a $4a \times 4a$ CDW. (b) Differential tunneling conductance spectra measured on a decoupled monolayer with a $\sqrt{3}a\times\sqrt{7}a$ CDW. (c) and (d) show the corresponding DFT model calculations.