Molecular beam epitaxy growth and scanning tunneling microscopy study of ${\mathrm{TiSe}}_2$ ultrathin films

Molecular beam epitaxy is used to grow ${\mathrm{TiSe}}_2$ ultrathin films on a graphitized SiC(0001) substrate. ${\mathrm{TiSe}}_2$ films proceed via a nearly layer-by-layer growth mode and exhibit two dominant types of defects, identified as Se vacancy and interstitial, respectively. By means of scanning tunneling microscopy, we demonstrate that the well-established charge density waves can survive in a single unit-cell (one triple-layer) regime, and find a gradual reduction in their correlation length as the density of surface defects in ${\mathrm{TiSe}}_2$ ultrathin films increases. Our findings offer important insights into the nature of charge density waves in ${\mathrm{TiSe}}_2$, and also pave a material foundation for potential applications based on the collective electronic states.

Figure 1

(a) Schematical crystal structure of ${1T\text{-TiSe}}_2$ in the top (top panel) and side (bottom panel) views. (b)–(d) STM topographic images ($V_{\mathrm{s}}=3$ V, $I=30$ pA, $400\text{nm}\times 400\text{nm}$) at various nominal ${\mathrm{TiSe}}_2$ coverages: (a) 0.4 TL, (b) 1.0 TL, and (c) 3.8 TL. (e) Atomically resolved STM image ($V_{\mathrm{s}}=0.2$ V, $I=100$ pA, $2.5\text{nm}\times 2.5\text{nm}$) taken on the flat ${\mathrm{TiSe}}_2$ terraces, with its unit cell marked by the black rhombus.

Figure 2

(a) Typical STM image of a ${\mathrm{TiSe}}_2$ film grown at $300{}^{\circ }\mathrm{C}(V_{\mathrm{s}}=0.24\text{V},I=100\text{pA},17\text{nm}\times 17\text{nm})$, showing two kinds of defects: dark Se vacancies and bright Se interstitials. (b), (c) Zoom-in STM images of a single Se vacancy and interstitial, respectively ($V_{\mathrm{s}}=0.24$ V, $I=100$ pA, $2\text{nm}\times 2\text{nm}$). (d) Postannealing temperature-dependent densities of surficial Se vacancies (black square) and interstitials (red circles) in the ${\mathrm{TiSe}}_2$ films. The dashed lines are guide to eyes.

Figure 3

(a), (b) Large- and small-energy-scale differential conductance $dI/dV$ spectra on single-, double-, and five-layer ${\mathrm{TiSe}}_2$ films. Set points are stabilized at (a) $V_{\mathrm{s}}=1.0$ V and $I=100$ pA and (b) $V_{\mathrm{s}}=30$ mV and $I=100$ pA, respectively. A smaller lock-in bias modulation of 0.3 meV was used for the spectra in (b).

Figure 4

(a), (b) STM images of a CDW-related $2\times 2$ superstructure in five-layer [(a) $V_{\mathrm{s}}=2.0$ V, $I=100$ pA, $15\text{nm}\times 15\text{nm}$] and single-layer [(b) $V_{\mathrm{s}}=0.1$ V, $I=100$ pA, $20\text{nm}\times 20\text{nm}$] ${\mathrm{TiSe}}_2$ films. (c) Fourier transform pattern of the STM image in (b). The white circles and yellow dashes correspond to the Bragg points and CDW $2\times 2$ modulations, respectively. (d) Autocorrelation of the Bragg-point-filtered STM image in (b) ($6\text{nm}\times 6\text{nm}$). Three white dashes mark the CDW-modulated orientations. (e) Correlation length $\zeta$ as a function of surface defect density in ${\mathrm{TiSe}}_2$ films, with the red dashes as guide to eyes.