Few-layer graphene formation by carbon deposition on polycrystalline Ni surface

Journal Pre-proof Few-layer graphene formation by carbon deposition on polycrystalline Ni surface

A.B. Loginov, I.V. Bozhev, S.N. Bokova-Sirosh, E.D. Obraztsova, R.R. Ismagilov, B.A. Loginov, A.N. Obraztsov PII:

S0169-4332(19)32300-1

DOI:

https://doi.org/10.1016/j.apsusc.2019.07.254

Reference:

APSUSC 43512

To appear in:

Applied Surface Science

Received date:

26 April 2019

Revised date:

13 July 2019

Accepted date:

28 July 2019

Please cite this article as: A.B. Loginov, I.V. Bozhev, S.N. Bokova-Sirosh, et al., Few-layer graphene formation by carbon deposition on polycrystalline Ni surface, Applied Surface Science(2019), https://doi.org/10.1016/j.apsusc.2019.07.254

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© 2019 Published by Elsevier.

Journal Pre-proof Few-layer graphene formation by carbon deposition on polycrystalline Ni surface A.B. Loginov1, I.V. Bozhev2, S.N. Bokova-Sirosh3, E.D. Obraztsova3, R.R. Ismagilov1, B.A. Loginov4, A.N. Obraztsov1,5 [email protected]

1 Department of Physics, Lomonosov Moscow State University, 119991 Moscow, Russia 2 Quantum Technology Centre, Department of Physics, M.V. Lomonosov Moscow State University, 119991 Moscow, Russia 3 Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia

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4 National Research University of Electronic Technology, Zelenograd 124498, Russia

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5 Department of Physics and Mathematics, University of Eastern Finland, 80101 Joensuu,

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Finland

ABSTRACT

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Graphene film formation by carbon deposition from gaseous phase on nickel substrates is

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investigated using scanning tunnel microscope (STM) unit embedded into reactor of the chemical vapor deposition (CVD) system. The microscope was designed to provide STM

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measurements at the same 1µm × 1µm region on the sample surface before and just after CVD synthesis without taking the sample out of the reactor. The peculiarities of graphene deposits formation on polycrystalline nickel substrates are revealed using developed CVD-STM system.

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The topology features are analyzed in combination with Raman spectroscopy data. One of particular features revealed in this study is nanobubbles formation and collapse on graphene film surface. We are discussing possible mechanisms of this phenomenon. Keywords: graphene; STM; CVD; nanobubbles

1. INTRODUCTION Carbon materials possess a number of unique properties attractive for application in electronics, photonics and optoelectronics [1]–[5]. One of the most common methods for reproducible obtaining of thin film carbons with desirable characteristics is chemical vapor deposition (CVD). There are a variety of practical realizations of CVD which are distinguished, among other, by the way of precursor gas activation (see e.g. [6]). Despite potential applicability some gaps in understanding of the deposition process and in revealing conditions for tailoring material properties in accord with practical demands still exist. Particularly, mechanisms of homogeneous large-scale atomic-thick graphene film formation during CVD synthesis on rough

Journal Pre-proof metal substrate surface are not fully understood. According to widely accepted model of graphene catalyst-growth [7] carbon precursor (e.g. methane) molecules are dissociated on substrate heated to certain temperature and created atomic carbon species penetrating into the substrate volume. Subsequent cooling down enforces the carbon atoms to leave the substrate’s body and form few-layered graphene structure at its surface. In the simplest modeling surface of the substrate is assumed to be monocrystalline and atomically flat with defined lattice orientation [8]. However in practical reality metal substrates are mostly polycrystalline and consist of small (down to 10 nm size) grains with different orientations. Even the most accurate grinding of the substrates cannot reduce roughness of its surface down to the value about the grain size. Despite this strong mismatch between the model assumptions and reality, there are a number of

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experimental results that show successful graphene CVD synthesis on the non-ideally flat

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substrates [9], [10].

It is remarkable that as a rule, vast majority of graphene samples obtained by CVD are

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studied after their transfer from a reactor chamber and exposure to air. At the same time microscopic ex-situ studies usually carried out at different regions on the sample before and after

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CVD process and do not allow obtaining detailed information about mechanisms of carbon

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materials growth. The problem may be solved partly in specially designed experiments using marking the surface of the sample (e.g. with scratches) [11]. Alternative in-situ investigations of

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nanomaterials growth performed with use of high resolution microscopy [12] usually carried out for very small surface area which are not representative from the point of view of surface roughness effect on graphene growth process. In this work we present investigation of graphene

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film growth with use of specially designed system which consists of scanning tunnel microscope (STM) unit embedded into CVD reactor chamber. The STM imaging is performed at room temperature and, while is not ‘in-situ’ measurement, it is able to provide high resolution information on surface topology for the same area on the substrate (before its contact with carbonaceous gas precursors) and deposit (just after CVD process without its transfer from the reactor).

2. EXPERIMENTAL DETAILS The scheme of the whole system and a reaction zone of proposed setup are presented in Fig. 1. The main part of the installation is a vacuum chamber with gas and vacuum lines, connected to the carbon gas sources, atmosphere and rotary pump. A self-modified commercially available tunnel microscope SMM-2000 (JSC “Zavod PROTON”, Zelenograd, Russia) was used as embedded STM. A rotary pump allows us to achieve initial vacuum with residual pressure down to 10-2 mbar. Thermal CVD process was realized directly in the STM sample holder.

Journal Pre-proof The main technical problem preventing STM imaging of the same place at substrate before and after heating is spontaneous shifts of the sample. This happens because the thermal expansion center locates at different points at different time moments due to roughness of the interface between the sample and holding body. To prevent this thermal shift we designed special holder providing substrate heating up to temperatures required for CVD processing. The holder was made as a chip of standard 0.5 mm thick Si wafer which was placed on sharpened steel needles (see Fig.1(b)). Each of the needles was inserted by their tip into 10 μm diameter holes made in the Si chip. The opposite (flattened) ends of the needles were fixed on quartz plate by a high temperature epoxy. This construction fully reduces spontaneous shifts during heating and also provides minimization of heat flow from the heater to the sensitive body of the STM.

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The heating was made by applying a DC voltage to the Mo electrodes attached to the Si chip.

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These electrodes also serve as a clamp for the heater due to their own elasticity. The substrates used in this work were made of 50 μm thick Ni foil that allows catalytic graphene growth [7] .

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The resistive heating of the Si chip/holder allows us to change smoothly Ni foil temperature from room temperature to 1100oC.

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The tunnel voltage during STM imaging was supplied to the Ni sample by thin 10 μm

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thick wire. Described design of the sample holder gives the exceptional feature of this system its ability to provide STM measurement at the same point on the sample substrate before and just

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after cooling to room temperature following to the thermal CVD process.

Fig.1. (a) – scheme of CVD system with embedded STM,; (b) – scheme of the sample holder with heating system, 90⁰ turned in respect of scheme in (a) panel

Before being used as a substrate Ni polycrystalline foil was carefully grinded until it gets the mirror shine and then washed in acetone. After grinding the substrate was introduced into STM sample holder and heated up to 300 ºC for 20 seconds in 2∙10-2 mbar vacuum to remove adsorbents. The STM measurements indicate that after this procedure the typical mean roughness (Ra) of substrate surface was equal to 10 nm with average grain lateral size about 30-50 nm. In

Journal Pre-proof some areas the 50 nm-deep scratches were observed on the surface, which were subsequently served as reference coordinate objects for comparison of the surface morphologies before and after CVD processing. The CVD synthesis was carried out by heating the Ni substrate in pure methane atmosphere at 10 mbar pressure. A constant temperature 750 ºC (controlled by optical pyrometer) was maintained for 20 seconds for one of sets of the samples and for 10 seconds for another. At the end of the mentioned time periods the DC current through the heater was smoothly reduced in such a way that estimated cooling rate of Ni substrates was approximately 80 ºC/second. It should be noted, that if the heater is abruptly turned off, then there were no graphitic deposits on Ni surface. Finally, it takes about 10 minutes to automatically bring the scanning needle to the same scanning area.

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After synthesis process all samples of the polycrystalline Ni foil substrates were

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examined using scanning electron microscopy (SEM, Supra 40, Carl Zeiss) and micro Raman spectroscopy (HORIBA LabRAM HR Evolution UV‐ VIS‐ NIR‐ Open). Raman spectra was

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3. RESULTS AND DISCUSSION

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taken with 532 nm laser excitation (second harmonic of 1064 nm Nd:YAG laser) at a power

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Formation of graphitic type deposits using the CVD reactor with embedded STM was found for Ni substrates heated up to 750 ºC for 20 seconds in 10 mbar methane atmosphere and

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following cooling down to the room temperature with the rate about 80 ºC /s . Typical three dimensional (3D) STM images of the Ni substrate surface are presented in Fig. 2. The images were taken at the same area of substrate surface before and after CVD processing. The STM measurements were performed at room temperature in the chamber filled by methane. No changes were detected in the STM images with methane pressure variation from 10 mbar (equal to those used CVD process) to 10-2 mbar (i.e. after pumping out of the chamber). After CVD processing the STM images indicate formation of few-layered graphene film with specific nanobubble structure. The structure looks similar to the blisters observed in graphene films obtained by plasma deposition [13] and in other layered materials [14]. But there are also some differences, especially in the typical size of the bubbles.

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Fig. 2. A typical 3D STM image of the Ni substrate surface before (blue) and after (red)

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CVD process. The images capture area with 50 nm-deep scratch and shows typical 60 nm

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nanobubble size, grown on top of relatively flat (Ra=10 nm) Ni substrate

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Raman spectroscopy inspection of obtained samples confirmed few-layered graphene formation. Typical Raman spectra measured for two of obtained samples are shown in Fig. 3a.

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The spectra looks similar to that obtained for different graphitic materials including graphite[15] , multiwall carbon nanotubes [16], graphene [13] and contains two bands centered

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at 1580 cm-1 (G line) and at 1350 cm-1 (D line). The Raman G line corresponds to the planestretching mode of carbon atoms oscillations in the two-dimensional hexagonal lattice of graphene [17]. The D line is known to appear in presence of defects in sp2 carbon network. The ratio of intensities of these lines (IG and ID, correspondingly) may be used for estimation of average size (La) of the perfectly ordered (defect free) domains of the graphene films as it was proposed in [18]. Following to this approach the defect free single crystal graphene domains size were estimated to be about 9 nm and 1.3 nm (for the spectra marked by “b” and “c”, correspondingly). The Raman spectra contain also relatively sharp and intensive second-order 2D line at 2700 cm-1 which is a distinctive feature of a well-ordered graphene [19]. Typically width of this 2D line in Raman spectra measured for obtained samples was about 90 cm-1 indicating a thickness of deposited film more than 5 layers. It may be also seen in Raman spectra presented in Fig. 3a presence of shoulder on the high frequency sides of the line G. Analysis of this and some other weak spectral features as well as careful determination of peak positions of the Raman lines and their mapping may be used for revealing particular nature of the structural

Journal Pre-proof defects and mechanical stress in graphene material (see e.g. [20], [21]) But this detailed Raman spectra analysis was out of frame of this work and will be performed and reported latter. The SEM observations presented by the typical images on Fig.3b,c for the same samples (the images in panels b and c correspond to the same samples Raman spectra of which are marked by letters b and c in panel (a) of the figure) revealed that on the macroscopic scale the produced deposits possess inhomogeneous morphology which can be observed only at low electron accelerating voltage regime similar to that reported in other works (see e.g. [22]). Latter is caused probably by small (atomically-scaled) thickness of the few-layered graphene and its

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detachment from the Ni substrate surface, which will be discussed more thoroughly below. .

Fig. 3. Raman spectra (a) with and typical SEM images of a few-layered graphene (b, c) obtained with electron accelerating voltage 0.5kV. The Raman spectra marked by letters b

and c correspond to the samples presented by SEM images b and c in Fig.3. The film morphology revealed by SEM is quite similar to that obtained by STM imaging (Fig. 2). Additionally to that both STM and SEM measurements indicate absence of conformity of the film morphology to Ni substrate surface profile. It may be explained by partial or complete detachment of the deposited graphene sheets from the metal substrate. This assumption is confirmed by modifications of graphene samples morphology observed during probe scanning with different tunnel currents. It should be taken into account that graphene has a relatively high polarizability in c-axis direction which leads to its significant attraction to STM probe creating strong non-uniform electric field [23]. In other words, if relatively high potential drop appears between STM tip and graphene, then attraction force appears as a result of electrostatic energy minimization. Hence, when potential difference between graphene and STM tip is increased,

Journal Pre-proof greater attraction between them is appeared. The applied voltage increase up to 2 V caused significant changes in detected surface morphology in comparison with that obtained by STM imaging with low (normal) voltage of 0.2 V. An example of those changes in surface morphology is shown in Fig.4, where, for clarity, some of regions are colored in green and turquoise. As it can be seen from this particular picture, there are two morphologically distinct regions after scanning with probe at high voltage (2V) applied: one of them (green) contains residual nanobubble structure while other (turquoise) has not. The profile curve measured at the border between these regions indicates presence of a 28 nm step between them. This effect of the nanobubble modifications under action of strong electric field during probe scanning was well

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reproducible.

Fig. 4. An example of a set of 3D STM images of the sample surface: blue – original Ni surface; red (lower) – graphene nanobubbles after CVD synthesis; red (upper) – a few-layered graphene film after scanning with increased voltage, where different colors (green and turquoise) indicate pronounced changes in the surface morphology; red line and arrow show

Journal Pre-proof position of morphology section which is represented by profile curve on the top.

The mechanism of the graphene nanobubbles recombination suggested on basis of our numerous STM measurements allow is presented schematically on Fig.5. We believe that graphene nanobubble structure and morphology is stemming from polycrystalline nature of the Ni foil, possessing relatively small grain size (20-50nm). It seems natural to assume that Ni foil is formed by grains with different crystallography orientations. Due to the lattice matching graphene is expected to have the strongest attractive interaction with (111) crystal surfaces of the Ni grains. The difference in the grain crystallography orientations produces a non-uniform attraction of graphene sheets to the Ni substrate surface. Our experimental results show that

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scanning in overvoltage mode (up to 2V) is enough to detach the loosely attached parts of

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graphene sheet from the polycrystalline Ni foil. In particularly it may be used as an effective way for local controlling of the graphene morphology via recombination of the nanobubbles.

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It should be noted, that similar bubble formation was observed previously in few-layered graphene obtained by plasma-enhanced CVD [13]. It was also observed merging of the graphene

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bubbles which was explained in frame of thermodynamics of hydrogen or oxygen [24] that is

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buried between layers of the few-layered graphene. For this buried gas it is thermodynamically profitable to merge because of free energy decrease (entropy increase) with merging two

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volumes of gas separated by surface into one [25]. Moreover, there are no critical restrictions for the merging caused by the graphene atomic structure, since it is believed that the flat hexagonal arrangement of atoms in graphene can be locally changed by converting hexagons to pentagons

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and heptagons. This “macroscopic” concept might be extrapolated to our “nanobubble” case, however for now we are nothing known about presence of particular gas inside bubble and further investigation of this problem is necessary.

Fig. 5. The proposed scheme for nanobubbles recombination process. Lengths of arrows

Journal Pre-proof represent the value of forces. Long blue arrows correspond to attraction to (111) Ni grains, short blue arrow correspond to attraction to Ni grain with other orientation. Red arrows correspond to graphene attraction to the tip.

In other series of our experiments effect of deposition time duration on the number of layers in the few-layered graphene was analyzed. For this purpose the deposition time was reduced from 20 to 10 seconds. Surprisingly, the observed and earlier described phenomena were almost the same, except a two noteworthy cases. In some cases scanning with increased tunnel voltage led to nanobubbles reorganization into the corner-like structures resembling shape of boomerang. The structures were perfectly

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oriented in respect to each other as it is clearly seen from STM images presented in Fig. 6. The

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surface morphology changes with voltage are explicitly indicated by red color. The angle between arms of the nanobubble boomerangs is about 120 degrees. And orientation of one of the

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arms (left) is equal to direction of scratches appeared on Ni substrate surface (similar to those on

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STM images on Figs. 2 and 4) during its grinding.

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Fig. 6. (a) 2D and (b) 3D STM image of appearance of corner-like bubbles after scanning with increased voltage; corner-like bubbles are colored in red on 3D image The deposition time reduction leads to incomplete coverage of the Ni substrate by the few-layered graphene. However nanobubble morphology remains quite similar to continuous film deposits. Thorough inspection the few-layered graphene obtained with reduced deposition time revealed presence ragged, cut boundaries and noisy regions in STM images (see Fig. 7). Presumably, the noisy areas of the image correspond to the free-standing parts of the detached few-layered graphene oscillating during STM scans.

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Fig. 7. 2D STM image of graphene nanobubbles, obtained with short deposition time CVD durations. Red arrows points to ragged and cut boundaries of graphene bubble. Blue

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arrows points to noisy regions which supposedly are free standing graphene sheets.

It should be noted, that the CVD-STM conjugated system developed in this work

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provides graphene growth rate even faster than those in recently reported “ultrafast graphene synthesis” with deposition times close to 1 minute [26]. Moreover, the methodology developed

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in our work can be used as an effective way STM analysis of the films with different compositions (like MoS2, WS2 and others) deposited by CVD on polycrystalline substrates [27].

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Our preliminary results indicate that many of such kind of 2D film materials deposited on polycrystalline and rough surfaces have similar nanobubble structure and demonstrate similar nanobubbles recombination behavior under strong electric field created by the STM probe. This is evidently different from behavior of 3D film materials with grains of the same size as the described graphene nanobubbles. This difference may be used for an experimental methodology to distinguish 2D and 3D structures. Results of our experimental investigations of those kinds will be reported later.

4. CONCLUSION STM unit introduction into CVD reactor chamber allowed investigation of multi-layer graphene formation on rough polycrystalline without their exposure to ambient air after synthesis. The surface morphology of the films with nanobubble structure having typical lateral diameter about 150 nm and height equal to approximately 60 nm was revealed. The size of the bubbles remains the same with shortening of deposition time. The graphene detachment and nanobubbles merging were revealed and investigated in the measurements with increased

Journal Pre-proof potentials applied between the STM tip and sample surface. The qualitative model explaining details of graphene film interaction with substrate is proposed on the base of these experiments. The methodology developed in this work is suitable for investigation of formation of different kinds of 2D structures and allows their distinction from 3D materials.

ACKNOWLEDGMENTS The equipment of the “Educational and Methodical Center of Lithography and Microscopy”, M.V. Lomonosov Moscow State University Research Facilities Sharing Centre used.

This

No.17-72-10173)

work and

was

by

supported

the

Russian

by

Russian

Science

of

was

Foundation

Basic

(grant

Research

(grant

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No. 18-02-01103_А; Raman inspection).

for

Foundation

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Graphical abstract

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Highlights (1) Nanobubble topology on surface of the deposited graphene was obtained and characterized; (2) The phenomena of merging and modification of the nanobubbles during STM tip scanning was observed; (3) CVD process parameters providing ultrafast (10 seconds) few-layer graphene growth was found; (4) Relation between deposition time and homogeneity of graphene film was

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