Growth and crystallographic structure of TiTe$_2$ on Au(111): From sub-monolayer structures to single- and multi-layer films

Abstract

We investigated the initial growth of TiTe$_2$ on Au(111) from sub-monolayer to multi-layer coverage by scanning tunneling microscopy (STM), low-energy electron diffraction intensity analysis (LEED-IV), and density functional theory (DFT). In the submonolayer regime we find a stable and well-ordered $(5\times\sqrt{3})_{\mathrm{rect}}$ superstructure consisting of separated TiTe$_2$ molecules, whereby the Ti atoms substitute Au atoms of the first substrate layer as proven by LEED-IV. By adding further Ti and Te in a 1:2 ratio and proper annealing dealloying sets in and a homogeneous 1T-TiTe$_2$ monolayer film on an unreconstructed substrate is formed. The resulting moiré structure is close to a $(4 \times 4)$ superstructure w.r.t. Au(111) and has a slightly expanded in-plane lattice parameter compared to the 1T-TiTe$_2$ bulk value. With further stoichiometric deposition, thicker 1T-TiTe$_2$ films grow. Surprisingly, a five layer thick film exhibits an even larger lattice-parameter (1.5 % larger than the bulk value). All LEED-IV analyses are based on best-fit R-factors of $R \le 0.13$.

Figures (8)

• Figure 1: (a) After deposition of $\Theta_{Te}=\qty{0.40}{\ML}$ and $\Theta_{Ti}=\qty{0.20}{\ML}$ onto Au(111) and annealing to $T=\qty{470}{\K}$, the surface is covered by a well-ordered $(5\times \sqrt 3 )_\textrm{rect}$ superstructure (green rect. unit cell). The missing spot at the (1/2 0) position is marked by a yellow square in the LEED pattern. The circular yellow markers refer to the intensity spectra shown in Fig. \ref{['Fig3_v1']} (a). (b) The close-up STM image shows the atomic basis of the $(5\times \sqrt 3 )_\textrm{rect}$ superstructure. The yellow line indicates the glide plane. (c) After deposition of $\Theta_{Te}=\qty{0.44}{\ML}$ and $\Theta_{Ti}=\qty{0.11}{\ML}$ and post-annealing to 420 LEED reveals a $\left(3\sqrt{3}\times3\sqrt{3}\right)\mathrm{R}30^{\circ}$ pattern. The first-order integer spots and the reciprocal superstructure cell are highlighted with yellow circles and with the green diamond, respectively. (d) STM image revealing defect-rich $\left(3\sqrt{3}\times3\sqrt{3}\right)\mathrm{R}30^{\circ}$ domains. The $\left(3\sqrt{3}\times3\sqrt{3}\right)\mathrm{R}30^{\circ}$ unit cell is composed of building blocks formed by three protrusions in a triangular arrangement (upper inset) which are the building blocks of a kagome-like structure (see overlay). (e) Exceeding the maximum Ti amount required for the $\left(3\sqrt{3}\times3\sqrt{3}\right)\mathrm{R}30^{\circ}$ structure results in the formation of a defect-rich $(3\times 3)$ phase, see upper left area. (f) Such an atomically resolved $(3\times 3)$ patch features three protrusions in the unit cell (annotated in blue). STM parameters: (b) $U=\qty{2.0}{\V}$, $I=\qty{0.2}{\nA}$; (d) $U=\qty{0.1}{\V}$, $I=\qty{0.5}{\nA}$; (e) $U=\qty{-1.0}{\V}$, $I=\qty{0.1}{\nA}$; (f) $U=\qty{0.01}{\V}$, $I=\qty{2.0}{\nA}$
• Figure 2: (a) LEED pattern at $\qty{60}{\eV}$ after evaporation of $\Delta\Theta_{Te}=\qty{0.7}{\ML}$ and $\Delta\Theta_{Ti}=\qty{0.35}{\ML}$ onto the $(5\times \sqrt 3 )_\textrm{rect}$ structure and post-annealing to $\qty{620}{\K}$. Broadened $(0\;3/4)$ spots with centers still in direction of the substrate spots indicate only small misalignment angles of the TiTe2 domains. (b) For the approach of stepwise preparation the LEED pattern ($\qty{60}{\eV}$) exhibits sharp spots. The structure is denoted as quasi-$(4\times4)$ (for details see text). (c) Atomically resolved STM image of the quasi-$(4\times4)$ reveals a moiré-like height modulation. The two solid gray $(3\times 3)$ meshes together with the dashed lines serve as a guide for observing the contrast variations across the surface. (d) STM height profile along the green line displayed in (c). (e) The STM image for stepwise preparation reveals an almost perfectly grown layer as well as mirror domains. The insets show a zoom-in of equivalent defects in differently oriented domains. The orientation indicated by white three-pointed stars appears mirrored as expected for two different stacking sequences. (f) The STM close-up at the position of the dashed rectangle marked in (e) shows the defect-rich $(3\times 3)$ phase with the TiTe2 layer. Imaging parameters: (c) $U=\qty{0.01}{\V}$, $I=\qty{2.00}{\nA}$; (e) $U=\qty{0.72}{\V}$, $I=\qty{0.21}{\nA}$ (inset: $U=\qty{0.03}{\V}$, $I=\qty{0.93}{\nA}$); (f) $U=\qty{0.72}{\V}$, $I=\qty{0.21}{\nA}$
• Figure 3: (a) LEED pattern taken at 60 of a nominally 4.5 layer thick 1T-TiTe2 film on Au(111). Due to the thickness of the film the quasi-$(4\times4)$ superstructure w.r.t. Au(111) is no more visible and only the integer spots of the growing film remain. (b) Large-scale RT-STM image revealing a flat and homogeneously grown film. The green line annotates the position of the line profile displayed in (c). The observed step heights fit to either TiTe2 and Au(111) layer distances, respectively. Imaging parameters: (b) $U=\qty{2.05}{\V}$, $I=\qty{0.36}{\nA}$
• Figure 4: (a) Three exemplary intensity spectra (out of 71 seesm) and comparison with the calculated bestfit spectra underline the quality of the fit. The positions of the displayed beams are marked in Fig. \ref{['Fig1_v1']} (a). (b) Schematic ball model of the LEED-IV best-fit structure in top and side view with a Pendry R factor of $R + \mathrm{var}(R) = 0.106+0.006$ reveals incorporated Ti atoms in the topmost Au(111) layer and two Te atoms next to them residing approximately in hollow positions. The $(5\times \sqrt 3 )_\textrm{rect}$ unit cell is indicated in green. The side view (vertical distances exaggerated) visualizes the $\qty{28}{\pm}$ buckling of the chemically mixed top-layer and vertical relaxations down to the fourth Au(111) layer. The corresponding results of the DFT calculation (in brackets) match the LEED results within few picometers. (c) Exemplary error curves for the (x,y,z) positions of the Te atom in nearly fcc position and the incorporated Ti atom resulting from the LEED-IV analysis. Vertical lines indicate the error margins. The full set of error curves is provided in the SM seesm.
• Figure 5: (a) The LEED-IV domain fit approach finds a preference of 82(9) for stacking sequence A while the rest is stacking sequence B. (b) The optimization of the lateral lattice parameter results in a minimum Pendry R factor of $R+\mathrm{var}(R)=0.134+0.021$ for a value of $\qty{3.800(24)}{\AA}$. The commensurate lattice constant $4/3\cdot a_{\mathrm{Au(111)}}$ is clearly outside the variance interval of the R factor. The best-fit values for the parameters specified in (a) are listed in Tab. \ref{['tab1']}. (c) The two first order beams of the growing TiTe2 film are well represented by the calculated best-fit spectra and underline the quality of the fit. The full set of spectra is provided in the SM seesm.