Scanning probe microscopy study of chemical vapor deposition grown graphene transferred to Au(111)

Accepted Manuscript Scanning probe microscopy study of chemical vapor deposition grown graphene transferred to Au(111) K. Schouteden, N. Galvanetto, C.D. Wang, Z. Li, C. Van Haesendonck PII:

S0008-6223(15)30152-4

DOI:

10.1016/j.carbon.2015.08.033

Reference:

CARBON 10198

To appear in:

Carbon

Received Date: 4 April 2015 Revised Date:

16 July 2015

Accepted Date: 11 August 2015

Please cite this article as: K. Schouteden, N. Galvanetto, C.D. Wang, Z. Li, C. Van Haesendonck, Scanning probe microscopy study of chemical vapor deposition grown graphene transferred to Au(111), Carbon (2015), doi: 10.1016/j.carbon.2015.08.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

Description: Three-dimensional perspective view of an atomically resolved scanning tunneling microscopy image of single-layer graphene (SLG) on Au(111). The SLG is grown by chemical

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vapor deposition on a copper foil and is subsequently transferred to the Au(111) substrate. The long-rang commensurability of the SLG and Au atomic lattices gives rise to the appearance of

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Moiré patterns, which thereby evidence the high quality of the SLG/Au surface. Fourier-filtering

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was applied to further optimize the experimental image.

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Scanning probe microscopy study of chemical vapor deposition grown graphene transferred to Au(111)

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K. Schouteden,1,* N. Galvanetto,1,2 C. D. Wang, 1,3 Z. Li, 1 and C. Van Haesendonck1 Solid-State Physics and Magnetism Section,

Celestijnenlaan 200D, KU Leuven, BE-3001 Leuven, Belgium

Dipartimento di Fisica ed Astronomia, Università di Padova, Padova 35131, Italy 3

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School of Optical and Electronic Information,

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Huazhong University of Science and Technology, Wuhan 430074, China

Abstract

Graphene was grown by chemical vapor deposition (CVD) on copper films and transferred ex

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situ to atomically flat Au(111) films, after which the sample is annealed in ultra-high vacuum (UHV) prior to scanning tunneling microscopy (STM) investigation. STM imaging at 78 K reveals large, clean and defect-free atomically flat areas that are separated by graphene wrinkles

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and grain boundaries. In addition to the graphene atomic structure, the flat surface regions exhibit patterns with larger periodicity that can be interpreted as Moiré patterns formed by the atomic

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lattices of the graphene and the gold. Our findings show that the CVD growth and ex situ transfer of graphene (G) to atomically flat Au(111) surfaces allows obtaining clean and high-quality G/Au surfaces that are suitable for in situ deposition of, e.g., molecules and atoms, for UHV investigation purposes. This approach may offer a higher degree of freedom in preparing bare and doped graphene on atomically flat surfaces compared to a full in situ approach.

* Corresponding author. Tel: +32 16 32 28 69. E-mail address: [email protected] (K. Schouteden)

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1. Introduction The much investigated versatile properties of graphene [1] make the material a promising candidate for implementation in various applications. In the last decade fundamental and applied

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research has put tremendous effort in the development of novel preparation techniques to obtain and transfer high-quality (few-layer) graphene [2–4]. Nowadays a broad range of preparation techniques exists, including “artisanal” exfoliation from graphite with adhesive tape and

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subsequent transfer to a substrate [5], and direct growth via chemical vapor deposition (CVD) on various substrates. Alternatively, carbon-containing molecules can be used as a precursor for

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obtaining graphene [6, 7]. Moreover, CVD-grown graphene may also be transferred to other substrates [8]. While ex situ preparation obviously has more affinity towards real-life applications, in situ preparation is more desired for the detailed exploration of novel properties of the material that requires highly controlled ultra-high vacuum (UHV) conditions. However, a full

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in situ approach may impose limitations, e.g., to the maximum achievable graphene quality and grain size, as is the case for CVD growth on low-reactive materials [17]. Moreover, a full in situ approach is not always experimentally achievable, since facilities for graphene preparation and

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characterization need to be available in the same UHV setup. A combined ex situ and in situ approach offers more flexibility and may still allow for

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preparation of high-quality graphene on substrates that are suitable for, e.g., studies of the doping behavior of graphene, either resulting from interaction with the substrate or with in situ deposited atomic and molecular species [12]. In this approach large-area high-quality graphene can first be prepared and optimized on a therefore ideally suited substrate (such as the commonly used copper foils) and next be transferred to a substrate that is less suited for graphene growth. Doping may be applied either during synthesis prior to transfer or post-synthesis after transfer to the new

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substrate. The quality of the obtained samples can be ultimately traced with atomic resolution using scanning probe techniques. Previously, we succeeded in obtaining graphene in situ by using the tip of a scanning tunneling microscope (STM) to exfoliate the top atomic layer from a

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graphite substrate [29].

Au(111) surfaces provide very large atomically flat areas and are therefore considered as a model surface for many surface science studies. Previously, graphene was grown on Au(111)

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by deposition of elemental carbon and subsequent annealing, yielding dendritic graphene islands [17] that eventually can evolve into quasi-single crystalline graphene films [27]. Compared to

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more rough, granular substrates [28], graphene on atomically flat substrates, either grown by CVD or obtained by transfer, is interesting for studies of doping with, e.g., nitrogen, as it allows to conveniently retrieve dopants using atomically resolved STM [9–11, 30]. Here, we prepare graphene on Au(111) via ex situ transfer of the graphene (G), grown by

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CVD on copper, to the Au(111) substrate The high quality of the obtained G/Au surface is demonstrated via atomically resolved UHV STM measurements at 78 K after annealing of the sample in UHV. Analysis of Moiré patterns at the G/Au surface allows one to discriminate

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between single-layer graphene (SLG) and twisted bilayer graphene (BLG) regions.

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2. Experimental

2.1 Graphene growth

The growth of graphene is carried out in a CVD system. 25 µm thick Cu foils (Alfa Aesar) of 2 × 2 cm2 in size are used as substrates. After sequential cleaning of the foils with acetone, isopropyl alcohol, and deionized water under sonication, the foils are installed in the CVD chamber. When the temperature reaches 1000 °C, 6 sccm H2 is introduced into the chamber to further clean the

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copper surface for 20 minutes, after which 1.5 sccm CH4 is introduced to provide the carbon material. After growth at 0.1-0.2 mbar for 30 minutes, the CVD system is cooled down to room temperature. The quality of the graphene is verified by Raman spectroscopy (Renishaw Raman

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microscope with 514 nm wavelength laser), which reveals the G peak and the G’ peak (also referred to as the 2D peak) of the graphene around 1585 cm−1 and 2700 cm−1 [13, 14],

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respectively. The Raman spectrum is presented in Fig. 1 (blue curve).

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Figure 1: Blue curve: Raman spectrum of the graphene grown by CVD on a copper foil, revealing the so-called G and G’ (also referred to as 2D) peak of the graphene around 1585 cm−1

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and 2700 cm−1, respectively. Red Curve: Raman spectrum of the graphene after transfer to Au(111), revealing also the G and G’ peak, as well as a defect-related D peak.

2.2. Graphene transfer

Next, the graphene is transferred to SiO2 substrates and atomically flat Au(111) films that are grown on mica by molecular beam epitaxy. For this purpose, 10 ml of PMMA is spin coated onto the G/Cu at 3000 rpm for 60 seconds, after which the sample is annealed at 140 °C for 30

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minutes. Next, a mixed solution of CuSO4 (0.5 mM/ml) and HCl (3 M/L) is used to etch away the copper at room temperature for 4 hours. The remaining film (polymer and graphene) is repeatedly rinsed with deionized water and is placed on the desired substrate. Finally, the polymer is

sample is rinsed with ethanol and with water.

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2.3. Sample characterization

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removed from the graphene layer by drops of acetone and spinning the substrate. After that, the

Atomic force microscopy (AFM) experiments are performed under ambient conditions with an

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Agilent 5500 SPM operated in tapping mode. UHV STM experiments are performed with an Omicron STM operated at 78 K with a pressure in the 10−11 mbar range. The sample is annealed to 400 °C in UHV prior to STM investigation in order to desorb surface contamination resulting from the exposure to ambient conditions. For the STM tips we used mechanically cut PtIr (10%

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Ir) and polycrystalline W wires. The W tips are electrochemically etched and cleaned in situ by thermal treatment. All bias voltages mentioned below are with respect to the sample, and the STM tip is virtually grounded. (dI /dV)(V) spectra that reflect the sample local density of states

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are acquired with a grid size of 200 × 200 points by lock-in detection with open feedback loop (modulation amplitude is 40 mV) at 800 Hz. Experimental images are analyzed using the

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Nanotec WSxM software [15].

3. Results and discussion Figures 2 (a) and (b) present photographs of the CVD-grown graphene (G) after its transfer to a SiO2 and a Au(111) substrate, respectively. The graphene can be recognized as the transparent film on the mirror-like substrates. The AFM image of G/SiO2 in Fig. 2 (c) reveals

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large flat graphene regions as well as the presence of graphene wrinkles (locally up to several nanometers high), which are known to form during CVD growth and still exist after transfer of the graphene to another substrate [16]. Interestingly, the AFM image reveals two graphene

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regions. The light blue and dark blue colored regions in Fig. 2 (c) correspond to SLG and BLG, respectively, as also evidenced by STM measurements that are discussed below. In Fig. 2 (c) approximately 50 % of the surface is covered by BLG. In contrast, Raman spectroscopy

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(performed on three different locations) of the CVD-grown graphene on the copper foil (Fig. 1, blue curve) indicates that the graphene is mainly single-layer [13]. The presence of a relatively

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high amount of bilayer regions should hence be related to the transfer process. SLG flakes are in fact also present on the back side of the copper foil and they might consequently also be transferred to the Au(111) surface together with the complete SLG that is present on the front side of the copper foil. E.g., in case the G flakes at the back side do not fully detach during

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etching, they may eventually stick to the SLG layer if the copper foil in between is fully etched. The presence of BLG on Au(111) is supported by Raman spectroscopy of the G/Au(111) sample. The result is presented in Fig. 1 (red curve). The height of the G peak compared to the G’ peak is

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considerably higher after the transfer to Au(111), pointing to the presence of BLG [13, 14]. Also a small defect-related D peak is resolved, which can be related to the transfer process as well.

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Figure 2 (d) presents a large-scale STM topography image of the G/Au(111) surface, revealing atomically flat regions that are separated by relatively high wrinkles, consistent with the AFM image of the G/SiO2 surface. The wrinkles often are difficult to image stably with STM, which may be explained by the weak coupling of the graphene wrinkles to the Au support. Nevertheless, Fig. 2 (d) illustrates that the ex situ prepared G/Au(111) sample exhibits a large amount of atomically flat graphene regions (up to several hundreds of nanometers in size) that are suitable for STM investigations.

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Figure 3 (a) presents an STM topography image of an atomically flat G/Au terrace, illustrating the cleanliness and good quality of the G/Au surface. The bright lines of the 22xsqrt3 reconstruction of the Au(111) can be clearly resolved, which implies that the ex situ transferred

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graphene follows the height corrugation variations of the reconstruction similar to in situ grown graphene [17]. Apart from the line-shaped defect, no other defects or contaminants can be discerned in spite of the exposure to ambient conditions and to the chemical solvents that are

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involved in the transfer. This also indicates the absence of contaminants between G and Au.

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Water (and other contaminants) may however still be present below the graphene wrinkles [5].

Figure 2: (a), (b) Photographs of graphene transferred to a SiO2 substrate and to a Au(111) film on mica, respectively. (c) AFM topography image of graphene on SiO2. Light blue colored regions correspond to SLG, while dark blue colored regions correspond to BLG. (d) Large-scale STM topography image of graphene on Au(111). Tunneling setpoints are 170 mV and 1.7 nA.

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The line-shaped defect in Fig. 3 (a) can be identified as a grain boundary that separates two different graphene regions, referred to as Region I and Region II hereafter, which have a different orientation with respect to the Au(111) support. Boundaries may exhibit rich physics as

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demonstrated before, e.g., for graphene on SiC [18], yet a more detailed study is beyond the scope of our present research. The different orientation of the two graphene regions can be better resolved in the atomically resolved close-up view images presented in Figs. 3 (b) and (c). Clearly

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there exist striking differences between Region I and Region II: in addition to the graphene honeycomb structure that is resolved in Region II, Region I also exhibits a regular pattern with a

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periodicity of about 1.6 nm. This pattern can be interpreted as a Moiré pattern formed by the different atomic lattices of the graphene and the gold. Indeed, the pattern in (b) can be reproduced by overlaying the atomic lattice of graphene on that of Au(111) (using a C-C distance and Au-Au distance of 0.142 and 0.288 nm, respectively) considering the experimental lattice orientations

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indicated in (b). Though the atomic lattice orientation of the Au(111) cannot be directly resolved, it can be determined from the orientation of the reconstruction lines in (a). The simulated result is presented in Fig. 3 (d). Taking the experimental lattice orientations of Region II indicated in (c),

image.

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no clear pattern is formed in the simulated image in Fig. 3 (e), consistent with the experimental

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Moiré patterns allow obtaining structural information of subsurface atomic layers that are

otherwise not directly accessible by the STM tip, as, e.g., demonstrated for multilayer graphene [19, 20] and double-walled carbon nanotubes [21, 22]. Moreover, Moiré superlattices can also be exploited to intentionally modify surface properties. It has, e.g., been demonstrated that the Moiré-effect related periodic potential can induce the opening of band gaps in the parabolic nearly free-electron dispersion of two-dimensional surface states [23–25]. Furthermore, the Moiré superlattice formed by graphene on a boron nitride substrate recently has been exploited to

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experimentally validate the long-time pursued “Hofstadter butterfly”, a theoretical prediction of the fractal-like behavior of two-dimensional electron systems that are subject to both a periodic electrostatic potential and a magnetic field [24].

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Occasionally, a more complex Moiré pattern is resolved on the G/Au surface. This is illustrated in Fig. 4 (a). Clearly, two Moiré periodicities exist in the image. This is further elucidated in the corresponding Fourier-transform image in Fig. 4 (b), in which arrows point to

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three different sets of periodicities, i.e. the atomic periodicity and the two Moiré periodicities. Figures 4 (d)-(f) present Fourier-filtered images of (a), revealing the atomic periodicity, the

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small-scale Moiré periodicity, and the large-scale Moiré periodicity that exist in Fig. 4 (a). A double Moiré pattern is typically formed by three atomic layers [20, 26] and indicates in this case the presence of BLG on the Au(111) surface, consistent with the AFM image in Fig. 2 (c). On such regions the reconstruction of the underlying Au(111) can no longer be clearly resolved and,

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as a consequence, the Au(111) lattice directions cannot be accurately determined. Figure 4 (c) illustrates the appearance of two Moiré patterns with different periodicity by overlaying a Au(111) atomic lattice and two graphene atomic lattices that are rotated in-plane with respect to

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each other. Alternatively, it cannot be excluded that the double Moiré pattern originates from

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three layers of graphene, e.g., due to local folding of the graphene layer [20, 26].

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Figure 3: (a) STM topography image of an atomically flat region of the SLG/Au surface that

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comprises a grain boundary between two graphene regions with different orientation. Tunneling setpoints are 130 mV and 2.0 nA. Image size is 38 × 19 nm2. (b), (c) Close-up views of the

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regions left and right of the grain boundary in (a), respectively. The reconstruction is filtered out by performing a line-by-line flattening operation to better resolve the Moiré pattern. Tunneling setpoints are 130 mV and 2.5 nA. Image sizes are 6 × 6 nm2. (d), (e) Simulated Moiré pattern formed by overlaying the atomic lattice of graphene on that of Au(111) using the experimental lattice orientations indicated by the black arrows in (a), (b) and (c). C and Au atoms are represented as orange and white solid circles, respectively (placed on a black background). The resulting G/Au structure is smoothed to optimize the visualization of the formed pattern.

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Figure 4: (a) STM topography image of BLG on Au(111). Image size is 15 × 15 nm2. Tunneling setpoints are 475 mV and 1.2 nA. (b) Corresponding Fourier-transform image, revealing bright

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maxima related to the atomic periodicity (blue arrow), the small-scale Moiré periodicity (yellow arrow), and the large-scale Moiré periodicity (green arrow). Image size is 16 × 16 nm-2. (c) Simulation of twisted BLG on Au(111), illustrating the appearance of two Moiré patterns. (d), (e)

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and (f) Fourier-filtered images of (a), revealing the atomic periodicity, the small-scale Moiré

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periodicity, and the large-scale Moiré periodicity, respectively.

Finally, we want to note that scanning tunneling spectroscopy of the SLG/Au surface

(recorded on different areas of the G/Au sample with different STM tips; data not shown) reveals a minimum in the (dI /dV)(V) spectrum (which can be interpreted as the Dirac point) around 20 meV below zero voltage. This points to a n-type doping effect of the graphene by the Au support. This is consistent with previous reports for graphene exfoliated from graphite and transferred ex

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situ to Au(111) [5], yet in contrast to the p-type doping reported for graphene grown in situ on Au(111) via vapor deposition [27]. This discrepancy may be related to a different adsorption distance [28] of the graphene on Au(111) in the different preparation approaches. Intercalation

lead to a stronger n-type doping of graphene on Au(111) [5].

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4. Conclusion

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with water molecules, which occurs during ex situ transfer of the graphene, has been found to

We performed a scanning probe microscopy investigation of CVD-grown graphene (G)

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that was transferred to Au(111) under ambient conditions. UHV STM experiments reveal that the obtained G/Au exhibits large and clean atomically flat regions. Study of the Moiré patterns at the G/Au surface reflect the presence of different graphene domains and allow discriminating between SLG and BLG regions. Our findings demonstrate that CVD-grown graphene transferred

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to Au(111) provides a good candidate substrate for in situ studies of graphene grain boundaries and of graphene interactions with deposited molecules and atoms.

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Acknowledgement

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The research in Leuven has been supported by the Research Foundation - Flanders (FWO, Belgium) and the Flemish Concerted Action research program (GOA/14/007). ZL thanks the China Scholarship Council for financial support (No. 2011624021). KS acknowledges additional support from the FWO.

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List of figure captions Figure 1: Blue curve: Raman spectrum of the graphene grown by CVD on a copper foil,

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revealing the so-called G and G’ (also referred to as 2D) peak of the graphene around 1585 cm−1 and 2700 cm−1, respectively. Red Curve: Raman spectrum of the graphene after transfer to Au(111), revealing also the G and G’ peak, as well as a defect-related D peak.

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Figure 2: (a), (b) Photographs of graphene transferred to a SiO2 substrate and to a Au(111) film

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on mica, respectively. (c) AFM topography image of graphene on SiO2. Light blue colored regions correspond to SLG, while dark blue colored regions correspond to BLG. (d) Large-scale STM topography image of graphene on Au(111). Tunneling setpoints are 170 mV and 1.7 nA.

Figure 3: (a) STM topography image of an atomically flat region of the SLG/Au surface that

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comprises a grain boundary between two graphene regions with different orientation. Tunneling setpoints are 130 mV and 2.0 nA. Image size is 38 × 19 nm2. (b), (c) Close-up views of the regions left and right of the grain boundary in (a), respectively. The reconstruction is filtered out

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by performing a line-by-line flattening operation to better resolve the Moiré pattern. Tunneling

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setpoints are 130 mV and 2.5 nA. Image sizes are 6 × 6 nm2. (d), (e) Simulated Moiré pattern formed by overlaying the atomic lattice of graphene on that of Au(111) using the experimental lattice orientations indicated by the black arrows in (a), (b) and (c). C and Au atoms are represented as orange and white solid circles, respectively (placed on a black background). The resulting G/Au structure is smoothed to optimize the visualization of the formed pattern.

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Figure 4: (a) STM topography image of BLG on Au(111). Image size is 15 × 15 nm2. Tunneling setpoints are 475 mV and 1.2 nA. (b) Corresponding Fourier-transform image, revealing bright maxima related to the atomic periodicity (blue arrow), the small-scale Moiré periodicity (yellow

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arrow), and the large-scale Moiré periodicity (green arrow). Image size is 16 × 16 nm-2. (c) Simulation of twisted BLG on Au(111), illustrating the appearance of two Moiré patterns. (d), (e) and (f) Fourier-filtered images of (a), revealing the atomic periodicity, the small-scale Moiré

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periodicity, and the large-scale Moiré periodicity, respectively.