Submonolayer copper telluride phase on Cu(111): Ad-chain and trough formation

We present a so-far undetected submonolayer phase of copper telluride on Cu(111) with ${(5\times \sqrt{3})}_{\text{rect}}$ periodicity and coverage of 0.40 ML tellurium (Te), which can be grown with perfect long-range order. It is structurally characterized by a combination of quantitative low-energy electron diffraction (LEED-IV), scanning tunneling microscopy (STM), and density functional theory (DFT). We find that Te induces the formation of two-atom-wide zig-zag troughs within the Cu(111) top layer via replacing 4 Cu atoms per unit cell by two Te atoms. Additionally, the interspace between the troughs is decorated by ${\mathrm{Cu}}_2{\mathrm{Te}}_2$ ad-chains sitting at hcp sites. All Te atoms exhibit the same local sixfold coordination: They occupy threefold hollow sites of the substrate and are one-sided attached to another three Cu atoms. The presented structural model is verified by a LEED-IV analysis with Pendry R factor of R = 0.174 and quantitative agreement of structural parameters with DFT predictions. It also has by far the lowest energy of all models tested by DFT and simulated STM images agree perfectly with experiment. It turns out that the new ${(5\times \sqrt{3})}_{\text{rect}}$ phase is the most dense surface telluride phase possible on Cu(111) before bulk-like copper telluride starts to grow.

Figure 1

LEED pattern (55 eV; top) and corresponding intensity profiles taken in [1 1] $k$-space direction [along the red line shown in (b); bottom] for various Te coverages after subsequent annealing to 750 K. (a) ${(3\times \sqrt{3})}_{\text{rect}}$ phase at $\mathrm{\Theta }=0.33$ ML in three domains (unit cells marked in red, blue, green). (b) Coexistence of ${(3\times \sqrt{3})}_{\text{rect}}$ and the new ${(5\times \sqrt{3})}_{\text{rect}}$ phase for $\mathrm{\Theta }=0.365$ ML. (c) Fully developed ${(5\times \sqrt{3})}_{\text{rect}}$ phase at $\mathrm{\Theta }=0.40$ ML in three domains. For both phases we notice vanishing half-order beams along the main $k$-space axes [see e.g., the $\left(\overline{1/2}0\right)$ spot, whose position is marked by a yellow circle] indicating a glide plane within the structures. For further details see the main text.

Figure 2

STM imaging for 0.365 ML Te on Cu(111) corresponding to the LEED-pattern of Fig. 1. (a) Large scale (150 ${\mathrm{nm})}^2$ image taken at a flat terrace showing the co-existence between two differently imaged phases. Brighter areas are associated with the ${(3\times \sqrt{3})}_{\text{rect}}$ and darker regions with the ${(5\times \sqrt{3})}_{\text{rect}}$ phase. [(b),(c)] Phase area assignment for the image from (a) by means of thresholding and height distribution. (d) Atomically resolved (15 ${\mathrm{nm})}^2$ image with phase boundaries. Both structures exhibit zig-zag chains of protrusions as a basic structural element. (e) Average of 15 line profiles taken parallel to the black line indicated in (d) through identical atomic positions. All interchain distances (given as multiples of the Cu lattice parameter ${\mathrm{a}}_{\mathrm{Cu}}$) within and between the two phases are close to either $3a_{\mathrm{Cu}}$ and $5a_{\mathrm{Cu}}$, errors are obtained via the standard deviation of peak distances within single curves from the mean value. Imaging parameters: (a) $U=1.7$ V, $I=0.50$ nA; (d) $U=-0.05$ V, $I=0.40$ nA.

Figure 3

STM appearance of the fully developed ${(5\times \sqrt{3})}_{\text{rect}}$ phase for 0.40 ML Te on Cu(111), corresponding to the LEED-pattern depicted in Fig. 1. (a) Large scale (150 ${\mathrm{nm})}^2$ image taken at a flat terrace presenting all three domains of an otherwise perfectly ordered ${(5\times \sqrt{3})}_{\text{rect}}$ phase. (b) (25 ${\mathrm{nm})}^2$ atomically resolved image demonstrating that the phase is practically defect-free also at the atomic level. Imaging parameters: (a) $U=0.48$ V, $I=0.15$ nA; (b) $U=-0.33$ V, $I=0.50$ nA.

Figure 4

Phase evolution of Te/Cu(111) for $\mathrm{\Theta }>0.4$ ML. (a) STM-image and height profiles taken along the red and blue lines for a Te coverage slightly above $\mathrm{\Theta }=0.40\mathrm{ML}$. The dark-green area and the red island correspond to the ${(5\times \sqrt{3})}_{\text{rect}}$ phase, the bright areas to a new phase. ($U=1.6$ V, $I=0.15$ nA). (b) Atomically resolved STM-image for the bottom right island in (a) and line profile taken along the green line showing a hexagonal array of protrusions with mutual distance of $\approx \sqrt{3}·a_{\mathrm{Cu}}$ ($U=0.42$ V, $I=0.50$ nA). (c) LEED-pattern for a Te coverage of $\approx 0.7$ ML (left) showing a phase mixture of ${(5\times \sqrt{3})}_{\text{rect}}$ and a “$(\sqrt{3}\times \sqrt{3}){\text{R30}}^{\circ }$” (spots marked by yellow arrows and magnified in the inset). At about 1.5 ML (left) no residues of the ${(5\times \sqrt{3})}_{\text{rect}}$ phase can be found anymore by LEED.

Figure 5

Structural model types in top and side view tested in LEED and DFT together with the respective simulated STM appearance for the respective relaxed DFT structures. For simplicity Te and Cu atoms in the ball models are displayed with the same size. (a) ${\mathrm{Cu}}_2{\mathrm{Te}}_2$ chains alternatingly assuming hcp or fcc sites. (b) ${\mathrm{Cu}}_2{\mathrm{Te}}_2$ chains with extra Te atoms (${\mathrm{Te}}_{\text{hidden}}$) in substitutional sites in between. (c) Same as (b) but with Cu zig-zag row between ${\mathrm{Te}}_{\text{hidden}}$ atoms removed. (d) Same as (c) but with another Cu zig-zag row from the $2^{\text{nd}}$ Cu layer removed. (e) ${\mathrm{Cu}}_2{\mathrm{Te}}_2$ bilayer chains embedded into the substrate's top layer. (f) ${\mathrm{Cu}}_2{\mathrm{Te}}_2$ bilayer chains on a plain substrate. Structure (c) turns out to be the best-fit structure.

Figure 6

Results of the structure analysis for the ${(5\times \sqrt{3})}_{\text{rect}}$ phase of Te on Cu(111). (a) Experimental STM image ($U=-0.33$ V, $I=0.50$ nA) partly overlaid by a DFT-based STM simulation and a ball model. (b) Selection of best-fit LEED IV spectra. [(c),(d)] Ball model of the bestfit structure: (c) top view, (d) side view along the $\left[\overline{2}11\right]$ direction (vertical distances are strongly exaggerated). Red (green) numbers denote average Cu layer spacings, Te-Cu vertical bucklings and bond lengths (all in Å) as derived from LEED-IV (DFT). (e) Visualization of the local binding geometry of Te atoms (left) compared to that in the Weissite (${\mathrm{Cu}}_7{\mathrm{Te}}_4$) structure (right) [43, 44].

Figure 7

Series of diffraction images for $\mathrm{\Theta }=0.40\mathrm{ML}$ Te deposited on Cu(111) at 90 K and successively annealed to the different indicated temperatures for 1 min each. LEED images were taken at an electron energy of $55\mathrm{eV}$ and the sample cooled back to 90 K.