Charge Instability in Single-Layer ${\mathrm{TiTe}}_2$ Mediated by van der Waals Bonding to Substrates

Single layers of transition metal dichalcogenides are of interest for emergent properties; an often-neglected issue is substrate effects. Our experiments show that the charge density wave in a single-layer ${\mathrm{TiTe}}_2$ grown on ${\mathrm{PtTe}}_2$ films is strongly suppressed by increasing the ${\mathrm{PtTe}}_2$ substrate thickness. Given that the interfacial bonding remains of the weak incommensurate van der Waals type, the observed changes are correlated with a thickness-dependent metallicity transformation in the ${\mathrm{PtTe}}_2$ substrate. The results illustrate the crucial role of the substrate in single-layer physics.

Figure 1

Lattice structures and ARPES results. (a) Lattice structures of ${\mathrm{TiTe}}_2$ and ${\mathrm{PtTe}}_2$. (b) ($1\times 1$) Brillouin zone (red solid lines) and ($2\times 2$) CDW folded Brillouin zones (red dashed lines) for ${\mathrm{TiTe}}_2$ and ($1\times 1$) Brillouin zone (blue solid lines) for ${\mathrm{PtTe}}_2$. (c),(d) ARPES maps of single-layer ${\mathrm{TiTe}}_2$ along the $\overline{\mathrm{\Gamma }M}$ direction taken at 300 and 25 K, respectively, using 21.22-eV photons. (e),(f) Enlarged second-derivative ARPES maps of (c) and (d) near the conduction band minimum around $\overline{M}$ taken at 300 and 25 K, respectively.

Figure 2

ARPES from single-layer ${\mathrm{PtTe}}_2$ and single-layer ${\mathrm{TiTe}}_2$ on single-layer ${\mathrm{PtTe}}_2$. (a),(b) ARPES maps of single-layer ${\mathrm{PtTe}}_2$ along $\overline{\mathrm{\Gamma }M}$ taken at 300 and 25 K, respectively, using 21.22-eV photons. (c),(d) Corresponding second-derivative maps with superimposed theoretical band structure. (e),(f) ARPES maps of single-layer ${\mathrm{TiTe}}_2$ on single-layer ${\mathrm{PtTe}}_2$ along $\overline{\mathrm{\Gamma }M}$ taken at 300 and 25 K, respectively. (g),(h) Corresponding enlarged second-derivate maps near the conduction band minimum at $\overline{M}$.

Figure 3

ARPES maps for single-layer ${\mathrm{TiTe}}_2$ on $N$-layer ${\mathrm{PtTe}}_2$. (a),(b) ARPES maps along $\overline{\mathrm{\Gamma }M}$ for $N=0,1,\dots ,4$ taken at 300 and 25 K, respectively. The band structure of single-layer ${\mathrm{TiTe}}_2$ (dashed curves) is included in (a) for comparison. (c) Second-derivative maps near the conduction band minimum at 25 K for bare BLG and ${\mathrm{TiTe}}_2/{\mathrm{PtTe}}_2$ samples for $N=0,1,\dots ,4$. (d) ARPES intensity of the folded band [integrated over a box as indicated in (c) for $N=0$] as a function of the temperature for the different $N$’s. The curves are BCS mean-field fits.

Figure 4

Electronic structure and band-match function $\chi$. (a)–(e) Theoretical band structures for single-layer ${\mathrm{TiTe}}_2$ (blue curves) and ${\mathrm{PtTe}}_2/\mathrm{BLG}$ substrates for $N=0,1,\dots ,4$, respectively, along the $\overline{K}-\overline{\mathrm{\Gamma }}-\overline{M}$ directions. The Brillouin zone boundaries for BLG, ${\mathrm{TiTe}}_2$, and ${\mathrm{PtTe}}_2$ are indicated. The horizontal shaded band in (a)–(e) indicates the energy range used to evaluate $\chi$. (f) The band-match function $\chi$ as a function of $N$. The color shading indicates weakening of the CDW state from $N=0$ to 1 and suppression of the CDW state at $N=2$ and beyond.