Atomically flat platinum films grown on synthetic mica

Although platinum has superior physicochemical properties such as high melting point, resistance to corrosion, catalytic activity, as well as thermal and electrical conductivities, its applications have been limited owing to its high price. Since the platinum surface is important for reactions involving electrochemistry and catalysis, using a thin film in lieu of a bulk material to reduce the amount of platinum required for an application is an effective cost-saving strategy. For example, as a substitute for a bulk platinum single crystal, the use of a platinum thin film deposited on a solid substrate would be reasonable. In that case, it is desirable that the surface of the substrate for such thin film growth is atomically flat. It is known that yttria-stabilized zirconia (YSZ)^1,2) and sapphire³⁾ substrates can easily be atomically flattened by annealing in air. We have already succeeded in making Pt single-crystalline thin films on YSZ²⁾ and sapphire⁴⁾ substrates by the sputtering. However, anticipated savings by making the Pt thinner will not be realized if the price of the thin film growth substrate is high. Mica is well known as an atomically flat, easy to prepare, and inexpensive substrate for van der Waals epitaxy.^5–8) Mica can be cleaved easily to prepare an atomically flat clean surface and can be used as a thin substrate with thickness ranging from millimeters to sub-micrometers. Therefore, mica is frequently used as substrates for scanning probe microscope (SPM) observation of biomolecules or for the deposition of thin films, and is commercially available.⁹⁾ Since commonly used muscovite mica thermally decomposes at temperatures greater than 600 °C,¹⁰⁾ it is unsuitable for the thin film growth of refractory metals such as Pt, which has a melting point of 1768 °C. According to literature reports of platinum film growth on muscovite mica, the heating temperature was limited to less than 700 °C.^11,12) As far as we know, there are no reports of SPM observations of atomically flat platinum thin films grown on mica substrates. Recently, we have succeeded in obtaining an atomically flat nickel thin film grown on thermally stable synthetic mica¹³⁾ (a fluorophlogopite mica).^14–16) Here, we report on the successful growth of atomically flat single-crystalline platinum thin films on mica using synthetic mica substrates.

A platinum film was deposited on a freshly cleaved fluorophlogopite mica [KMg₃(AlSi₃O₁₀)F₂, Itoh Kikoh] substrate by inductively coupled plasma (ICP)-RF sputtering in an argon atmosphere with a gas pressure of 0.11 Pa and a flow rate of 3.7 sccm. The RF and ICP powers were held constant at 40 and 20 W, respectively. The thickness of the film was greater than 200 nm. When the film thickness was less than 200 nm, hexagonal voids were formed after annealing. All air atomic force microscope (AFM) images were obtained using a Pico-Plus AFM (Molecular Imaging) system and an MFP-3D (Asylum Research) device in the AC mode (known as the tapping mode).

Figures 1(a)–1(d) show AFM images of a platinum film deposited by ICP-assisted RF sputtering on a synthetic mica substrate heated at 700 °C (without postdeposition annealing) with different gas mixtures: (a, b) Ar (100%)+O₂ (0%) and (c, d) Ar (95%)+O₂ (5%). Note that ca. 700 °C is the maximum heating temperature of the ICP-assisted RF sputtering system used in this study. As shown in Figs. 1(a)–1(d), the surface roughness improved for the oxygen-doped sputtered film. Moreover, an fcc(111)-like morphology of the facets with hexagonal symmetry can be recognized in Fig. 1(d). These results agree well with our previous report of platinum film on Al₂O₃(0001) prepared by oxygen-doped sputtering deposition.⁴⁾ The introduction of oxygen led to the generation of twin single-crystalline epitaxial Pt(111) films with a typical fcc(111) surface morphology of hexagonal symmetry.

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In an effort to investigate the crystallinity of these Pt films on synthetic mica substrates, XRD data were obtained under atmospheric conditions at room temperature, as shown in Fig. 2. We performed θ = 2θ scans around the 111 peak of platinum (supplementary data), φ scans of the {220} peaks (Fig. 2, upper), and φ scans of the {111} peaks at χ = 70.5° (Fig. 2, lower). An ideal fcc(111) single crystalline film will give six equally separated sharp [220] peaks and three equally separated sharp {111} peaks. Although all of the θ = 2θ scans (see the online supplementary data at http://stacks.iop.org/JJAP/57/048001/mmedia) indicated (111)-oriented films, platinum films on synthetic mica produced by ICP-RF sputtering were found to have sharp and strong peaks, as shown in Figs. 2(a) and 2(b). Although Pt films on synthetic mica produced by ICP-RF sputtering without oxygen being introduced were found to be polycrystalline [Fig. 2(a)], those films with oxygen introduced were found to be twin single crystal [Fig. 2(b)]. These results agree well with our previous report on single-crystalline epitaxial platinum film on Al₂O₃(0001) prepared by oxygen-doped sputtering deposition.⁴⁾ Since we do not currently have sufficient evidence to explain the oxygen effect, we consider that one possible explanation for the observed improvement may involve the oxygen ashing of impurities such as sulfur and carbon.¹⁷⁾ Consequently, we also consider that observed broad satellite peaks in Fig. 2(a) may correspond to the presence of impurity-induced rotational twinning.

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Unlike Al₂O₃(0001) or YSZ, mica has a two-dimensional sheet structure with pseudohexagonal Si₂O₅ sheets formed by SiO₄ tetrahedrons.¹⁴⁾ Owing to the layered structure, mica can be perfectly cleaved into few-micrometer-thick sheets, which are atomically smooth. There is only weak interaction between the mica substrate and the deposited atoms. This results in a very small lattice-mismatch distortion. Thus we consider that the deposited platinum atoms can form a (111)-oriented twin single crystal, when impurities are eliminated with oxygen ashing. It must be mentioned that we cannot reveal the orientation relationship between synthetic mica(001) and Pt(111) using the XRD apparatus described in the experimental section. We are currently planning to perform further precise XRD measurements and report the results in the near future.

In conclusion, atomically flat platinum thin films ware heteroepitaxially formed on synthetic mica under various deposition and growth conditions. Platinum films deposited on fluorophlogopite mica substrate by ICP-assisted RF sputtering on synthetic mica substrate without oxygen doping resulted in the growth of polycrystalline platinum films, whereas 5% oxygen doping of the sputtering gas resulted in the formation of atomically flat twin single-crystalline platinum films. Considering that our results can be reproduced easily and with the use of relatively inexpensive synthetic mica, we believe that this report offers one of the cheapest and convenient methods of preparing atomically flat platinum films. We hope that this work encourages the investigation and application of SPM research, thin film growth and graphene growth.

Acknowledgments