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Evolution of Graphene Growth on Pt(111): from Carbon Clusters to Nanoislands Hyo Won Kim, Wonhee Ko, JiYeon Ku, Yousoo Kim, Seongjun Park, and Sungwoo Hwang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06540 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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The Journal of Physical Chemistry

Evolution of Graphene Growth on Pt(111): From Carbon Clusters to Nanoislands Hyo Won Kim†,§,*, Wonhee Ko†,§, JiYeon Ku†, Yousoo Kim‡, Seongjun Park†, Sungwoo Hwang† †

Device Lab, Samsung Advanced Institute of Technology, Suwon 443-803, Korea

‡

Surface and Interface Science Laboratory, RIKEN, Wako, Saitama 351-0198, Japan

§

These authors contributed equally to this work.

*

E-mail: [email protected]

Abstract

We study the growth of graphene on a Pt(111) surface in stages by varying the annealing temperature of the precursor hydrocarbon decomposition, through an atomic-scale analysis using scanning tunneling microscopy (STM), and studying the geometry-affected electronic properties of graphene nanoislands (GNs) through scanning tunneling spectroscopy. STM reveals that graphene grows on a Pt(111) surface from dome-shaped carbon clusters to flat GNs with the intermediate stages of dome-shaped and basin-shaped hexagonal GN structures. Density functional theory calculations confirm the changes in direction of the concavity upon increase in the size of the GNs. The structural changes are also found to have a significant effect on the electronic properties. Landau levels arise from strain-induced pseudo-magnetic fields because of the large curvature, and the nanoscale-size effect promotes electron confinement. 1 ACS Paragon Plus Environment


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Introduction

The synthesis of graphene on metal surfaces by chemical vapor deposition has emerged as a promising route for the mass production of graphene-based applications.1-5 To achieve the desired structures and at high quality, it is essential to understand the growth mechanism on the metals. Previous studies have identified several possible growth mechanisms depending on the metal substrate. On an Ir surface, for example, dome-shaped C clusters form in the initial state because of the strong interaction between C and the substrate atoms;6 then, the small graphene islands coalesce to a larger island with a diameter of a tens of nanometers through Smoluchowski ripening.7 Graphene islands on Ru(0001) enlarge through the addition of rare clusters of approximately five C atoms rather than monomers,8 and upon further growth, an array of lens-shaped islands of macroscopic size (>100 µm) develops, mediated by the step edges of the substrate. On a Cu(111) surface, with the thermal decomposition of methane, the saturation of various carbon clusters in the intermediate state, such as dimers, rectangles, and chains, drives the formation of defective graphene, which further grows to monolayer graphene.9 Several other theoretical and experimental studies discuss different growth mechanisms depending on the substrate (Ni, Pt, Rh, Au, and Ag).10-19 Despite such extensive studies on growth mechanisms, a precise understanding of the transition from carbon clusters to graphene at the atomic scale is still lacking. Here, we explore, at the atomic scale, the evolution of graphene on a Pt(111) surface from a dome-shaped C cluster to flat graphene nanoisland (GN) and their electronic properties through scanning tunneling microscopy (STM) and density functional theory (DFT) calculations.

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Experimental and Computational Methods

A Pt(111) sample was cleaned by repeated cycles of Ar-ion sputtering and annealing under ultrahigh vacuum. Carbon clusters and GNs were created by exposing clean Pt(111) to ethylene and subsequent annealing at 850–1100 K. STM and scanning tunneling spectroscopy (STS) measurements were performed using two commercial low-temperature STMs, Omicron (in RIKEN) at 4.7 K and Unisoku (in Samsung) at 2.9 K. The STS measurements were performed using a standard lock-in technique with a bias modulation of 30 mV at 797 Hz. DFT calculations were carried out with VASP employing the generalized gradient approximation with the Perdew–Burke–Ernzerhof (PBE) functional and project-augmentedwave (PAW) methods as implemented in the simulation package.20-22 The electronic wave functions were expanded to plane waves with a cutoff energy of 273.9 eV, which is the maximum cutoff energy for the softer version of carbon pseudopotential employed. The longrange dispersion corrections were also tested but not included for the results discussed here since the results well explain the essential chemistry and physics with much better efficacy. Various graphene fragments were placed on a three-layer slab of Pt(111) in a periodically replicated cell with a vacuum spacing of approximately 10 Å in the vertical direction. The number of layers of Pt slab was carefully tested, and three layers of Pt were used as the most effective representation of Pt surface for our investigation. The atomic positions of the Pt atoms of the slab in the top-most layer for Figures 2c–f and the top two layers for Figure 2a , as well as of all the carbon atoms, were relaxed until the residual forces were < 0.02 eV Å–1. (See Supporting Information Figures S3-5 for the selective results of test calculations.) An atomistic calculation was used to extract atom positions from the STM images and calculate the pseudo-magnetic field. The positions of the atoms were determined as peak 3 ACS Paragon Plus Environment


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positions in the STM images, where the current images are used instead of topographs because of their superior contrast. The current images are high-pass filtered to remove the smooth background and to search for local maxima, which are assigned as the atom positions. The height of each atom is assigned as the height at the corresponding position in the topographs. The pseudo-magnetic field is calculated from the extracted atom positions following the methods described in previous studies.23,24

Results and Discussion

Figure 1. Evolution of GNs grown on Pt(111). (a)–(b) STM images of the C clusters obtained after annealing at 850 K (Vs = 0.5 V; It = 0.4 nA). (c)–(e) STM images of the GNs taken after annealing at 950 K (Vs = 0.1 V; It = 1 nA). (f)–(j) Three-dimensional STM images of the C 4 ACS Paragon Plus Environment

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clusters and GNs shown in (a)–(e). (k) STM image of the larger GN obtained after annealing at 1100 K (Vs = 0.1 V; It = 1 nA). (l), (m) Profiles of the height along the lines in (a)–(c) and (c)–(e), respectively.

We probed sequential stages of the graphene growth by suppressing further evolution by varying the annealing temperature for the decomposition of ethylene after adsorption on the Pt(111) surface. Annealing at 850 K yielded dome-shaped C clusters (Figures 1a and b) as the structure in an earlier stage toward graphene formation. The peripheral C atoms strongly interact with the substrate, leading to a highly strained geometry, as seen in the threedimensional STM images (Figures 1f and g) and the high aspect ratio of line profiles (black and red lines in Figure 1l) compared to those of GNs (blue line in Figure 1l). Upon additional C nucleation, the clusters develop a partial hexagonal shape with a 120° angle (Figure 1b). At this stage, the characteristic honeycomb lattice of graphene is not yet observed. Annealing at a higher temperature of 950 K leads to the formation of GNs (Figures 1c, d, and e), as evident in the atomic-scale honeycomb lattice in Figures 1c and h. At this intermediate phase, most of the observed GNs share a similar size and hexagonal shape as in Figures 1c and d (See Supporting Information Figure S1 for more GNs). A subtle increase in the GN size, however, dramatically transforms the three-dimensional geometry from a dome shape with a protruded center to a basin shape with a depressed center (Figures 1h, i, and m). Somewhat larger GNs are also observed at this annealing temperature with a further flattened center, however, still maintaining the basin shape with protruded edges (Figures 1e, j, and m). Such protrusions, as well as the periodic ripples along the edge, are, in fact, typical in GNs on Pt(111) and explained by the strong interaction between graphene and the substrate as well as by the lattice mismatch.23 The same features are clearly found in much larger and flat GNs obtained from annealing at 1100 K (Figure 1k). Larger GNs with a size of tens of nanometers were 5 ACS Paragon Plus Environment


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obtained from annealing above 1200 K, and ultimately, a repetition of exposure to ethylene and annealing above 1250 K leads to the formation of graphene monolayer.25

Figure 2. DFT-optimized structures of graphene fragments on Pt(111) with periodic boundaries in the x- and y- directions. (a) Dome-shaped GN comprised of 96 carbon atoms. (b) Cross-sectional heights of graphene ribbons (c–f) along the y-axis (parallel to dotted lines in (d) and (f)). Graphene ribbons on Pt with zigzag edges and a width of 13 (c,d) and 22 (e,f) graphene unit cells.

The DFT calculation results confirm the formation of dome-shaped GNs as well as the structural transition upon growth from a smaller dome-shaped to larger basin-shaped GN. As shown in Figure 2, the edges of the graphene nano-fragments bend towards the substrate because of the strong interaction between the edge C atoms and underlying metal.23 The resulting curvature in conjunction with the tendency of graphene to retain its two-dimensional geometry drives smaller GNs to exhibit the dome-like shape (Figure 2(a)). As GNs grow, the protrusion toward the center relaxes downward, and our model system of nanoribbons on Pt (Figures 2c–f) effectively captures such a size-dependent transition in the direction of concavity of the GNs, which is also clearly shown in the plot of height profiles of the 6 ACS Paragon Plus Environment

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nanoribbons in Figure 2b.

Figure 3. Electronic structures of the GNs. (a), (b) STM image of the dome-shaped and basin-shaped GN, respectively. (c) Differential conductance (dI/dV) spectra taken at the positions indicated by the red dot in (a) (red line) and the black dot in (b) (black line).

The geometry of the GNs has a significant effect on the electronic property, as shown in the differential conductance dI/dV spectra in Figure 3c. In the dI/dV spectrum taken at the center of a dome-shaped GN (Figure 3a), two peaks are observed near 0.35 and –0.3 V, which are absent in the spectrum at the flat region of a basin-shaped GN (Figure 3b). These two peaks are most likely associated with Landau levels (LLs) arising from strain-induced pseudo-magnetic fields from the large lattice distortion, as reported previously for graphene edges23 and nanobubbles26,27 on metal surfaces. The peak positions, in fact, agree well with the values of the unusual LL energy sequences, , resulting from the relativistic nature of the massless Dirac particles. sgn 2 ħ || ,

n = 0, ±1, ±2, …,

(1)

where En is the nth LL energy with respect to the Fermi level, e is the electron charge, ħ is the reduced Planck constant, νF is the Fermi velocity, Bs is the pseudo-magnetic field, and EDirac 7 ACS Paragon Plus Environment


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is the Dirac point energy. The peak near 0.35 eV coincides with the previously reported Dirac point of graphene grown on Pt(111),4,25,26,28 and it is therefore identified as the n = 0 state. The peak near –0.3 eV is accordingly the n = –1 LL. Though not shown here, from the same origin of strain-induced fields, LLs are observed in the dI/dV spectrum at the curved and protruded edges of the basin-shaped GN in Figure 3b (See Supporting Information Figure S2 and S3). In addition, the peak observed near 1.0 V for the dome-shaped GN demonstrates the n = 1 LL. However, this peak at 1.0 V is also observed in the case of the basin-shaped GN where other LLs are absent, which suggests that this peak may have an additional origin. In fact, it originates from the confinement of the electronic states due to the nanoscale-size effect and will be discussed in more detail in Figure 5.

Figure 4. Spatial dependence of the strain-induced pseudo-magnetic field in the dome- and basin-shaped GNs. (a), (b) Extracted C atom positions overlaid on top of the corresponding STM images of the GNs in Figures 1c and e, respectively. (c), (d) Maps of pseudo-magnetic field calculated from the atom positions in (a) and (b), respectively, overlaid on top of the STM images.

We confirmed by atomistic calculation that the strong strain in the GNs, in fact, 8 ACS Paragon Plus Environment

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generates pseudo-magnetic fields, which significantly affects their electronic properties. To obtain the strain map, we extracted the positions of the atoms from the STM images (see method for detail). Figures 4a and b show the extracted positions of carbon atoms marked over the topographs of the GNs in Figures 1c and e, respectively. Note that the symmetry breaking at the zigzag edges of the GNs makes only one sublattice appear in the topographs, as well as for the extracted atom positions. The pseudo-magnetic field is calculated from the extracted atom positions and corresponding strain,23,24 and is plotted in Figures 4c and d. Both GNs show a large field of a few hundred to a few thousand Teslas, thereupon inducing LLs and peaks in the dI/dV spectra. In particular, the dome-shaped GN exhibits a maximum field at the center (Figure 4c), and the basin-shaped GN exhibits a maximum field at the edge and negligible field near the center (Figure 4d). The calculation results agree well with the dI/dV spectra in Figure 3c, where the spectra taken at the center of the dome-shaped GN exhibit LL peaks, while no LL peaks are observed in the spectra taken at the center of the basin-shaped GN (Figure 3c).

Figure 5. (a), (f) STM image of the dome-shaped and basin-shaped GN, respectively. (b)–(e), (g)–(j) the dI/dV maps.

In addition to evidencing the LLs from the pseudo-magnetic field, the dI/dV spectra of the GNs reveal the confinement of the electronic states induced by nanoscale-size effects. 9 ACS Paragon Plus Environment


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As previously reported,29 the electron confinement effect with free-electron-like nature due to the effect of the Pt substrate is manifest in the change in slope near 1.0 V observed in the dI/dV spectra in Figure 3c for the two GNs in Figures 3a and b. As the slope change is observed at the center of the basin-shaped GN (Figure 3b and 5f) with a negligible field (Figure 4d), the possibility of LL state may be eliminated. Distinct modulation patterns are, in fact, observed in the dI/dV maps (Figure 5h-j)29 showing the confinement effect dominated by the shape of the GN. In the case of the dome-shaped GN, the peak in the dI/dV spectra near 1.0 V interestingly has two different origins. In addition to being the n = 1 state of confined electrons, the energy value of 1.0 V also signifies the n = 1 state of LLs. The absence of a clear standing wave pattern in the dI/dV map taken at 1.0 V (Figure 5c) demonstrates the existence of the two combined effects, as in the similarly unclear pattern in Figure 5d. At 3.5 V, where the effect of the LLs becomes negligible, a clear standing wave pattern is recovered for the dome-shaped GN showing the confinement effect alone. The dI/dV maps were also taken at 0.4 V, which is near n = 0 state of LLs and where the confinement effect may be ignored. As expected, the intensity at the center area of the dome-shaped GN (Figure 5b) is stronger compared to that of the basin-shaped GN signifying the development of LLs in the former structure.

3. Conclusions

In this study, we have successfully demonstrated the process of graphene growth on Pt(111) at the atomic scale and their geometry-driven electronic properties. By varying the annealing temperature, we observed the following stages of GN growth: dome-shaped C clusters, C clusters with partial hexagonal shape, smaller dome-shaped and slightly larger basin-shaped hexagonal GNs, flat hexagonal GNs, and finally even larger GNs with periodic ripples along 10 ACS Paragon Plus Environment

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the edges. The structural changes were found to affect the electronic structures of graphene. LLs arise from strain-induced pseudo-magnetic fields because of the large curvature, and the nanoscale-size effect promotes electron confinement. These results provide a fundamental understanding of the graphene growth at the initial stage, the transition from C clusters to graphene, and the corresponding electronic structures.

Acknowledgement The authors declare no competing financial interest.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional STM images of GNs, dI/dV spectrum at the curved and protruded edges of the basin-shaped GN, the spatially resolved dI/dV spectra taken across a basin-shaped GN plotted in the 2D colormap and DFT-optimized structures calculated under different conditions.

References

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