Nanostructuring of Au/Ru(0001) Thin Films: Height Modulation and Superstructure Formation

Background

The Au(111) surface of bulk crystals is renowned for its 22 × √3 reconstruction, well described as a herringbone pattern that results from elastic strain domains [1–7]. Surface stress governs this reconstruction, and its modification by steps, defects, or thin‑film strain has been extensively documented [8–12]. In Au/Ru(0001) systems, the lattice mismatch (0.271 nm for Ru vs. 0.288 nm for Au) introduces additional interfacial stress that can alter the reconstruction. Prior studies on ultrathin Au films (≤ 3 ML) revealed a giant 100 nm herringbone for 1 ML and a triangular superstructure for 2 ML deposited at ~420 K and flash‑annealed at 790 K [16]. Sub‑monolayer growth at room temperature leads to fractal or dendritic islands [17,18]. However, the behaviour of thicker Au films on Ru(0001) has not been explored. Our aim is to fill this gap by studying 5 ML and 9 ML Au films deposited at RT and annealed at 1050 K, a protocol that yielded atomically flat Au layers in our earlier work [13–15].

Methods

All experiments were performed in a custom ultra‑high vacuum (UHV) chamber (base pressure < 10⁻¹⁰ mbar). A single‑crystal Ru(0001) (5 mm × 5 mm × 5 mm, Mateck) was cleaned by 1.5 keV Ar⁺ sputtering followed by 1100 K annealing. Residual carbon was removed by 5 × 10⁻⁷ mbar O₂ exposure at 1100 K. Gold (99.99 %, Sigma‑Aldrich) was evaporated from a 0.25 mm wire by an electron‑beam evaporator (Omicron) at 1 ML/min, verified by Auger spectroscopy and calibrated against the Au (NVV) / Ru (MNN) peak ratio. After deposition, the samples were annealed at 1050 K for 5 min. STM imaging was carried out with a VT‑STM (Omicron) in constant‑current mode at room temperature, using PtIr (80/20 %) tips prepared by mechanical cutting and conditioned with voltage/current pulses up to 10 V/300 nA. The (2 × 2)‑O/Ru(0001) surface was used to calibrate the STM lateral scale. Data were processed with Gwyddion (gwyddion.net).

Results and Discussion

Figure 1 shows the evolution of the Ru(0001) surface upon Au deposition and annealing. In the clean state (Fig. 1a), atomically flat terraces separated by single atomic steps dominate, with occasional protrusions that correspond to buried Ar bubbles. After deposition of 5 ML (Fig. 1b), the surface becomes rough due to Volmer‑Weber island growth; several atomic layers of Au coexist within the field of view, and the buried bubbles are still discernible. A 9 ML film (Fig. 1c) forms pronounced three‑dimensional islands (~10 nm lateral size), dramatically increasing roughness. Annealing at 1050 K restores atomically flat terraces (Fig. 2a,b). The 5 ML film exhibits random ripples (< 0.05 nm amplitude) with no long‑range order, whereas the 9 ML film shows a highly regular ripple network with triangular motifs.

Figure 2c compares the annealed 9 ML film with a single‑crystal Au(111) surface. The Au(111) displays the familiar 22 × √3 herringbone with comparable height modulation (~0.05 nm). In contrast, the 9 ML film’s ripples lack the long‑range periodicity seen on bulk Au. To probe this structure, we imaged a large terrace (Fig. 3a) and performed a fast Fourier transform (FFT) (Fig. 3b). The first‑order FFT spots correspond to a hexagonal lattice with a period of ~4.6 nm (range 4.44–4.76 nm). A smaller area (Fig. 3d) reveals a superstructure unit cell of ~5 nm, with rippling amplitudes of ~0.03 nm and a slight angular mismatch between the superstructure vectors and the underlying atomic lattice, suggesting a strained topmost layer.

These observations indicate that interfacial strain and annealing temperature dictate the surface reconstruction. The 5 ML film shows disorder, the 9 ML film a short‑range ordered 4.6 nm superstructure, while the bulk Au(111) displays the classic herringbone. The precise atomic model remains unclear because the lattice constants of the first two layers likely differ from bulk values; alloying at the interface may also play a role [26]. Further diffraction studies and controlled STM measurements are needed to resolve these ambiguities.

Potential applications of the 4.6 nm superstructure mirror those of the Au(111) herringbone: as a nanotemplate for regular molecular arrays with a different lateral periodicity and symmetry, which could broaden the scope of surface‑templated nanofabrication.

Conclusions

Our STM study reveals that 5 ML Au on Ru(0001) produces disordered ripples, whereas 9 ML yields a short‑range ordered hexagonal/oblique superstructure with an average in‑plane period of 4.6 ± 0.4 nm. This rippling is analogous to the Au(111) herringbone, yet arises from a distinct strain state in the thin film. Determining the exact atomic geometry will require diffraction experiments and ab initio modeling.

Abbreviations

ML:

Monolayer

RT:

Room temperature

STM:

Scanning tunneling microscopy

UHV:

Ultra‑high vacuum