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Atomic-force microscopy study of self-assembled atmospheric contamination on graphene and graphite surfaces
Accepted Manuscript Atomic-force microscopy study of self-assembled atmospheric contamination on graphene and graphite surfaces Alexei Temiryazev, Alexey Frolov, Marina Temiryazeva PII:
S0008-6223(18)31013-3
DOI:
https://doi.org/10.1016/j.carbon.2018.10.094
Reference:
CARBON 13612
To appear in:
Carbon
Received Date: 31 July 2018 Revised Date:
9 October 2018
Accepted Date: 30 October 2018
Please cite this article as: A. Temiryazev, A. Frolov, M. Temiryazeva, Atomic-force microscopy study of self-assembled atmospheric contamination on graphene and graphite surfaces, Carbon (2018), doi: https://doi.org/10.1016/j.carbon.2018.10.094. 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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Atomic-force microscopy study of self-assembled atmospheric contamination on graphene and graphite surfaces.
a
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Alexei Temiryazev *a Alexey Frolov b, and Marina Temiryazeva a
Kotel'nikov Institute of Radioengineering and Electronics of RAS, Fryazino Branch, Vvedensky
Sq. 1, Fryazino 141190, Russia
Kotel'nikov Institute of Radioengineering and Electronics of RAS, Mokhovaya str. 11-7,
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b
Moscow 125009, Russia
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By means of high resolution atomic-force microscopy (AFM) we investigated the surface of graphene and graphite. Our study shows that if the samples were stored in ambient laboratory conditions, an adsorbate of airborne contaminants, presumably hydrocarbons, forms well-ordered layer over the whole area of graphene flakes. In general case, this layer has a stripe structure of 4-5 nm pitch with domains differing
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in the direction of the stripes. The size of one domain can exceed 100 µm2, which means that almost the entire area of a graphene flake can be under a layer with a constant stripe direction. By mechanical impact of AFM probe, we can turn the direction of the stripe structure, while its pitch is preserved. Visualization of the self-
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assembled structure may be hampered by the presence of non-ordered overlayer.
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1. Introduction
A study of atmospheric contamination on the surface of graphene is essential since the
presence of adsorbate changes electrical and optical properties of graphene. It is indicative that one of potential graphene applications is based on its ability to detect gas molecules at extremely low concentrations [1]. When an experiment is carried out in air, the presence of adsorbate is practically inevitable. The question arises as to whether self-assembling of adsorbate can occur and results in nano texturing. Then, a period and orientation of the texture could affect the properties of graphene. It is well known that some organic materials being advisedly coated on graphite surface, self-assemble into nanoscale stripes [2]. As for the self-assembling of the *
Corresponding author. E-mail: [email protected] Tel: +79163069408
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atmospheric pollutants, there are just a few papers where it was reported. By means of high resolution atomic-force microscopy (AFM), it was found on the surface of graphene on 6HSiC(0001) [3, 4] and exfoliated graphene on a variety of substrates: SiO2/Si, gold/mica, hexagonal boron nitride (hBN) [5]. Similar phenomena have also been reported on graphite [6], hBN [5], and epitaxial heterostructures graphene/hBN [5]. A distinguishing feature of the observed self- assembling is existence of domains in the interior of which there is a well-ordered
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stripe structure with a pitch of 4-6 nm. The domains differ in the stripe orientation and have 600 symmetry, which reflects the graphite symmetry. The size of domains observed in [3, 4, 6] was rather small, several hundreds of nanometers. Taking into consideration a great deal of studies on graphene, one would conclude that such self-assembling is rare in occurrence and can hardly
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affect physical properties of graphene, say, its impact on transport in graphene stripe of several micrometers in size should be substantially averaged. However, this is not the case. Gallagher et
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al. [5] have shown that self-assembling results in anisotropy of friction. Thus, we can extend the number of related papers by including the observations of domains of anisotropic friction on the surface of graphene [7 - 10] and graphite [11, 12], despite the fact that in these works two different explanations of the observed phenomena were used. Experiments on friction forces conducted in [7 - 12] did not allow one to directly observe the stripe structure, but they made it possible to observe friction domains on large areas with dimensions of several tens of microns. If
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we associate domains of self-assembling with domains of friction, then based on the results of [5, 7 - 12], must recognize that the whole of the sample surface may be entirely coated with an ordered adsorbate. Moreover, sizes of the individual domains may exceed 10-20 microns that is quite commensurate with the size of the structures commonly used in the study of the transport
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properties of graphene. Here again the question arises, why they are so rarely observed in experiments. The reasons for this may be several. Firstly, the AFM studies should be
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purposefully aimed at obtaining high-resolution images. This implies a small scan size (of the order of 100-300 nm), the use of sharp probes and, most importantly, operating modes that allow obtaining high resolution on rather soft surfaces in air. Secondly, often the structure of the surface was examined on samples that have passed the cleaning procedure using solvents and annealing [13]. Third, scanning tunneling microscopy (STM), which has very high spatial resolution, do not always allow us to reveal the structure of a soft non-conducting adsorbate [2, 5]. In this paper, we show that there is one more reason - the presence of a layer of disordered adsorbate. We will use the methods of high-resolution AFM to detect self-organization on various graphite surfaces. We will estimate the dimensions of self-organization domains, and discuss the reasons that make it difficult to observe such structures experimentally. In addition, on nanoscale, we will investigate the phenomena observed by Gallagher at el. As it was shown in
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[5], with AFM probe, one can switch friction – that is, create a new friction domain, and then erase it. Proceeding from the fact that the friction is determined by the direction of the stripe structure of the adsorbate, this should indicate a reorientation of the periodic structure. We will present direct evidence of such an effect.
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2. Methods
The AFM study was carried out using a SmartSPM atomic force microscope (AIST-NT) in the air. For surface scanning we used a dissipation mode (DM) [14, 15], that is, the probe is driven to oscillate, pump power is fixed, the oscillation amplitude serves to maintain the
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feedback control (as in tapping mode) and the oscillation frequency is maintained at resonance (as in frequency modulation (FM) mode). The surface profile recorded in the DM corresponds to
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the constant dissipation profile, while the change in the resonant frequency recorded simultaneously with the profile reflects the fine structure of the probe-surface interaction force. The DM allows achieving high spatial resolution, about 1 nm, when scanning in air and provides the possibility of scanning soft surfaces [15]. Note that in the works cited above, the FM [3, 4, 6] or the tapping mode [5] were used to visualize the adsorbate self-ordering. Our choice in favor of DM is due to a number of reasons. In air, the FM mode is extremely effective when operating at
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small oscillation amplitudes, about 1 nm or less. This requires the use of stiff force sensors, such as a length-extension resonator [6] or a qPlus sensor [3, 4]. Commercial AFMs usually do not have them; therefore, the tapping mode is commonly applied. When choosing between the DM and the tapping mode, the following should be considered. One of the basic conditions for
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obtaining a high resolution is the exact choice of scanning parameters. The advantage of DM is the ease of selecting these parameters [15]. For the tapping mode, one should choose: a
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frequency of oscillations f, a free-oscillating amplitude A0, and a set-point amplitude Asp i.e., the amplitude at which the scanning is performed. Usually, f is chosen near the resonant frequency of the probe. The choice of the other two parameters is not so obvious, since it depends on the surface properties and the tip sharpness. There are only general recommendations applicable mostly to very sharp tips: to use small amplitudes A0 and to scan in light tapping, that is Asp to be close to A0 [16]. In case of DM, we do not need to select the frequency: a phase locked loop is used to keep the phase of oscillating probe at 90 degrees, that is, the excitation is performed at the resonant frequency. There is a clear procedure for setting other parameters [15]. We believe that in order to obtain a high resolution, it is necessary to set conditions under which it is possible to reliably detect the beginning of the growth of repulsive forces between the probe and the sample. This will correspond to the minimum surface deformations and hence the minimum
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tip-sample contact area. The choice of A0 and Asp is based on dependencies of amplitude A and resonant frequency fres upon a tip-sample distance z. While A(z) monotonically decreases with decreasing z, the shape of the fres(z) curve depends strongly on the initial amplitude A0 and the tip sharpness. If A0 is too small or the probe is blunt, then attractive forces predominate and fres(z) decreases monotonically as the probe approaches the sample. If A0 is too large or the probe is very sharp, then the probe goes into the net repulsive regime and the resonant frequency
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increases at A(z) very close to A0. We choose an intermediate value of the initial amplitude A0, at which a reversal of the frequency curve is observed in the range of A(z) = (0.7-0.9)A0. Then we set Asp equals to A(z) at z corresponding to the frequency turning point. Thus, measurements are made in the net attractive regime, but under conditions in which repulsive forces contribute
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significantly to the interaction. It is important to emphasize that the entire setup procedure is carried out before the scan starts and allows us to select the optimal parameters for each specific
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probe. For instance, the initial amplitude of 10 nm, may be too large for very sharp probe, but insufficient for the other, which in turn will provide a clear image at A0 = 20 nm. The measurements used silicon probes with self-made sharp spikes grown at the tip [17]. Flakes of graphene were prepared by mechanical exfoliation using an adhesive tape [18] under ambient conditions on silicon wafers with 300 nm of thermal oxide. For graphene exfoliation, we used high quality single crystals of natural graphite received from NGS Naturgraphit GmbH. Part of
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the samples were annealed in vacuum at a temperature of 500 o C to remove the adhesive residues. After exfoliation, on the substrate were flakes containing areas of monolayer and multilayer graphene, as well as thicker graphene flakes, 50-200 nanometers in thickness, let us call them “graphite plates”. We started AFM research a few days after the samples were made.
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Some experiments were conducted with highly oriented pyrolitic graphite (HOPG). After cleavage, HOPG samples were also stored in ambient conditions for several days before the
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3. Experimental results
3.1 Self-assembling on graphene flakes on SiO2/Si substrate. Fig. 1 shows typical structures observed on the surface of graphene flakes. There is a clear defined stripe structure (SS) of 4.6 ± 0.2 nm pitch. Peak-to-trough stripe amplitude in height picture (Fig. 1a) is about 150 pm. This value is not very informative, since it strongly depends on the sharpness of the tip and the scanning parameters. The frequency image, as a rule, gives a clearer picture of the periodicity. However, small changes in the setpoint often make it
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possible to change the contrast, increasing it for a height picture due to some reduction in the contrast of the frequency channel. Figures 1 (a, b) show an area containing the boundary of domains differing in the direction of the SS. It can be seen that in this case the presence of the boundary is not related to the change in the number of graphene layers. In some cases, mainly near the edges of the flakes, it is possible to find places where domains with three directions are simultaneously observed (Fig. 1 (c, d)). Fig. 1(e, f) show the scans of the area containing the
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boundary between the monolayer and three-layered graphene. When scanning a relatively large area of 0.5×0.5 µm2, it is only noticeable that the surface of the monolayer has an appreciably higher roughness than three-layered graphene. This is typical for the monolayer graphene, which repeats the roughness of the substrate and the adsorbate sandwiched between graphene and the
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substrate [19, 20]. Against this background, the SS is hardly noticeable. Nevertheless, with a decrease in the scanning area, the presence of the SS is clearly visible (Fig. 1 (g, h)). Here it is
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important to note two features. First, the SS periods for the domains located on monolayer and three-layered graphene coincide. Secondly, the boundary of the domains only partially matches the boundary between monolayer and three-layered areas. In the central part of Fig. 1 (g, h) it is clear that the right domain penetrates into the three-layered graphene region. So far, we have shown scans of areas where we could observe domain boundaries. This was done in order to make the pictures more convincing, and the stripe structure was not
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associated with the hardware effects when scanning. At the same time, it should be noted that the search for domain boundaries on the graphene surface was a rather laborious task. Domain sizes could be several microns, which is much larger than the scan size, on which a fine structure can be demonstrated. To illustrate how large individual domains can be, we performed a series of
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measurements of 100 scans of 50×50 nm2 size, made at the grid nodes covering most of the graphene flake - Fig. 2. The analysis of the obtained images shows that the entire central part of
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the flake is occupied by one domain whose area is more than 100 µm2.
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Fig. 2. AFM image of graphene flake (a) where a series of 100 high-resolution (50×50 nm2) scans (b-d) were performed. Bars show the direction of stripe structures, the places where no structure was observed are marked with circles.
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Fig. 1. Self-alignment on the surface of graphene flakes on silicon dioxide. (a, c, e, g) – topography images, (b, d, f, h) – corresponding frequency images. (a, b) show two domains on the flat area of graphene, (c, d) – demonstrate an area near the edge of graphene flake. Here we can see all three stripe directions separated by 60o. (e, f) show the area containing the boundary between monolayer and three-layer graphene. The region marked in (e) with a square is shown in (g, f). Period of stripe structure p = 4.6±0.2 nm (b), 5.1±0.2 nm (d), 4.9 nm±0.2 (h). (i, j) - height cross sections along black lines in (a) and (e).
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3.2 Self-assembling on graphite plates on SiO2/Si substrate .
The next series of experiments was carried out with graphite plates located on a SiO2/Si substrate. Here, also, SS were found with the same period as on thin graphene flakes on the same substrate. However, in addition to them, other types of structures on the surface were also noticeable. These could be drops (Figures 3(a, b)), which, apparently, are not connected with the
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SS in any way. Horizontal stripes on the image indicate contamination of the probe when scanning the droplets. The probability of such contamination can be significantly weakened by changing the scanning parameters, however, in this case the parameters were chosen for optimal SS visualization. A more interesting picture is shown in Fig. 3(c, d). Domain boundaries are no
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longer visible here. In these areas, there is a higher layer with a completely different ordering structure. In some points - light dots on the frequency image (Fig. (d)), the probe interaction with
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a surface changes abruptly. This can be seen from the change in the resonance frequency. In that case, there frequently was contamination of the probe, resulting in the appearance of stripes on the frequency image. In Fig. 3(e, f), another example of structures found on the surface of thick graphene flakes is presented. Well-ordered regions coexist with regions where long-range order is absent, but there are chaotically scattered elongated objects. It is well known that some organic substances, being applied to the graphite surface, self-assemble into a lamellar structure [2, 16,
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21, 22]. We can assume that the SSs seen in the Figs. 1-3 are lamellar structures formed from the adsorbent, the scattered objects in Fig. 3(e,f) are individual lamellas. The top layer in Fig. 3(c,d) could be formed as a result of another type of ordering and be micelles or fiber-like assemblies [2]. In favor of the SS is associated with the self-organization of the adsorbate, and not with the
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surface bending, we can give the arguments outlined in [5]. The period of the SS was the same for both the monolayer and multilayer graphene flakes, as well as for the graphite plates on the
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same SiO2/Si substrate. If the corrugation were due to mechanical stresses caused by the interaction of the flake with the substrate, it would natural expect a change in the period of corrugation period with a change in the thickness of the flakes. Observation of the SS can be greatly hampered by the presence of a disordered adsorbate
layer on the surface. Figure 4 shows that on the part of the scanned surface, there is an area where we can not observe any fine structure in the height image. Nevertheless, the frequency image allows us to conclude that there is a SS under the layer of unstructured adsorbate.
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Fig. 4. Self-alignment under the layer of unordered adsorbate on the surface of the graphite plate on SiO2. Inside a circle is a magnified view.
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Fig. 3 Adsorbate on the surface of graphite plates on SiO2. (a, c, e) – topography images, (b, d, f) – corresponding frequency images. Color scale in (a) is nonlinear to highlight the stripe structure. Period of stripe structure p = 5±0.2 nm for all images.
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3.3 Self-assembling on HOPG.
On the basis that the SS is a consequence of self-organization of the adsorbate on the surface of graphene, it can be expected that it should be present on the bulk plates of HOPG. We performed many AFM measurements trying to detect the presence of the SS on the HOPG surface. As a rule, such attempts were unsuccessful, but in some cases, the SS was found. These
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observations were quite random; the SS was observed only in certain places and could eventually disappear. Fig. 5 illustrates the procedure of such an experiment. When scanning a large area, we managed to find two spots in which there is a noticeable contrast in the frequency channel (Fig. 5(a, b)). Subsequent scanning of a smaller area shows the presence of the SS in such places (Fig.
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5(c, d)). Note that in this case the direction of the stripes in these regions differs by 17 degrees. This atypical situation is probably due to the mismatch of the crystallographic directions in different layers of HOPG. The height image (Fig. 5(c)) shows the presence of two steps (edges of graphite
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layers) between the areas of self-assembling. This means that the left and right SSs in (Figs. 5(c,d)) are located on different layers of HOPG. Apparently, these layers differ in their crystallographic orientation.
We can see a periodic pattern in Fig. 5(b). It has a period of 38 nm. This is an artifact, a kind of moiré pattern resulting from the digital nature of AFM imaging. When scanning a large
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area, it indicates the presence of the SS. Moiré occurs if the distance between neighboring points of the scan is large and comparable to the period of the SS. This effect is enhanced by a special mode, which we used to scan large areas of the surface. In order to reduce the chance of tip contamination, we used the dissipation mode in combination with a vertical mode [19], that is,
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the movement of the probe from point to point occurred at an elevated setpoint, the height and frequency were measured when the probe approached the surface at a constant speed. In this
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mode, averaging of the measured signals does not occur, the height and resonance frequency are measured at the given points, the distance between which is determined by the scan size and the number of points in the line. When the length of such a step is comparable to the SS period, moiré may appear on the image (Fig. 5(b)). An additional example of moiré structure is shown in the Supplement.
Figs.5(e, f) clearly demonstrate the reasons that make it difficult to observe the SS on HOPG. A rather flat area of the surface is represented. The area where the SS is observed has uneven edges and lays lower than its surroundings. We can assume that the surface outside this region is covered by unstructured and more mobile adsorbate, which, when scanning, can move under the influence of interaction with the probe. This gives grounds to say that in the case when the SS is not observed experimentally, the reason for this may be not the absence of self-
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organization, but the presence of an additional layer of adsorbate covering the layer with selfordering. The probability of finding a SS on HOPG increases if the surface is treated with water. Our experiments showed that if the sample is rinsed with distilled water or condensed on the surface with boiling water, then the adsorbate is redistributed. On the initially flat surface, where only the steps at boundaries of the graphite layers were at first visible, adsorbate clots have been
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formed. They are usually located near layer boundaries, and we can detect the presence of the SS between them (Fig. 6). The results of such experiments should be treated with caution, since contamination could be carried by water. Nevertheless, it seems that interaction with water or
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water vapor can be a very important factor.
Fig. 5. Adsorbate on the surface of HOPG. (a, c, e) – topography images, (b, d, f) – corresponding frequency images. The region marked in (a) with a square is shown in (c, d). Inside a circle in (f) is a magnified view. Moiré pattern with the period of 38 nm can be seen in (b) while the real SS period p = 4.3±0.2 nm (d), p = 4.4±0.2 nm (f).
Fig. 6. Adsorbate on the surface of HOPG rinsed with water. (a, c, e) – topography images, (b, d, f) – corresponding frequency images. Period p = 5±0.2 nm (d), p = 5.1±0.2 nm (f). When scanning a large area, as in (a ,b), the distance between adjacent points of the scan was 5.7 nm. Superimposing this grid on the stripe structure with a period of 5.1 nm led to the appearance of a moiré pattern, clearly visible in (b).
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3.4 Reorientation of stripe structure.
The purpose of the next experiment was to study the change in the SS under the action of the AFM probe. As was shown in [5], scanning in a contact mode can lead to the appearance of a new friction domain. We have studied this effect at nanoscale. The difficulty of such an experiment is the need to maintain the sharpness of the tip after rather hard scanning in the
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contact mode. Therefore, we used a probe with a single-crystal diamond tip [24]. Commercially available probes of this type (D300, SDCprobes) have a stiffness of about 40 N/m and remain sharp enough even after their use to indentate hard surfaces [25]. The radius of curvature of the tip is often somewhat larger than that of special probes for high-resolution scanning. For that
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reason, the quality of height images is lower, and in Fig. 7 we give only frequency images. The experiment was carried out on a graphite plate 80 nm thick on a SiO2/Si substrate. At first, we scanned a relatively large area of 200×200 nm2 in the dissipation mode. Further, a small area of
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50×50 nm2 at the center of this section was scanned in contact mode, that is, 256 lines were performed with a tip pressing force of the order of 80 nN. The tip was in contact only during the forward motion in the fast scan direction; on the return stroke, it was raised. Then again, the large area was scanned in DM. Figure 7(a-e) show the result. There are no traces or flanges from the scraped adsorbate on the surface, but in the area of scanning in the contact, the SS
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reorientation occurred. A new domain has been formed. These changes are reversible, changing the direction of scanning in the contact; we could erase this domain, create it again and again erase it.
Similar experiment was conducted on a monolayer graphene flake. There was also a turn
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of the SS. Interestingly, in some cases reorientation occurred over an area substantially greater than the area scanned in contact. Fig. 7 (f-i) shows that scanning in contact has formed two new
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domains. We did not try to erase them, but watched the dynamics over time. Subsequent scanning at 55 and 90 minutes showed that relaxation was occurring, and one of these new domains gradually disappeared. Such behavior, in some ways, is similar to that was observed in [5] where manipulations with the friction domains were carried out. Over time, some of the “written” friction domains gradually collapsed, while others grew. In our case, the expansion of the new domain into the untreated area could take place during the “writing” procedure. At the next stage, slower processes were observed - the expansion of one of the new domains led to the destruction of the other. We can draw several conclusions. We have received direct proof that the SS can be turned by the action of the tip. The period of structure does not change. This is a confirmation of the fact that the local change in frictional forces observed in [5] can indeed be explained by turning the SS. Changes under the influence of a probe are not related to
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technology known as nanografting, that is, with the removal of the self-assembled monolayer and then forming a layer of other molecules [26, 27]. It is worth noting that the behavior of the layer of ordered adsorbate reminds the properties of liquid crystals that are capable of being repolarized under external action. As for lamellar structures, the possibility of their spontaneous reorientation was demonstrated in [21], where self-assembling of alkanes at high temperatures
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was studied.
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Fig. 7 Reorientation of the stripe structure. (a – e) Frequency images scanned in dissipation mode of the same area (200×200 nm2) on a graphite plate. After the first scan (a) was taken, we conducted contact scanning of the smaller area (50×50 nm2) marked with a square. Arrows between (a) and (b) show fast and low directions of the contact scanning. Re-scan in DM (b) shows that a new domain has been formed. We again conducted contact scanning with the different fast and low directions (arrows between (b) and (c)) to delete this domain (c). Then all operation were repeated. We wrote new domain (d) and deleted it (e). (f – i) Frequency images scanned in dissipation mode of the same area (220×220 nm2) on a graphene flake. On initially single domain area (f) we conducted contact scanning of the smaller area (50×50 nm2) marked with a square. This led to the appearance of two new domains. The bigger one has the area much larger than the area of contact scanning. The second, small domain reduced in time and disappeared. (h) and (i) were taken 55 and 90 minute later the contact scanning. 4. Discussion
We carried out research of one and a half dozen samples with graphene flakes and graphite plates on SiO2/Si substrates. Only on one of the samples did we fail to detect the SS. The quality of the AFM images obtained was very different from the sample to the sample. The cause may be contamination of either the surface or the probe. This question was not specifically
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investigated, we limited ourselves to ascertaining the fact of the existence of the SS. Since in some cases the DM allows us to detect the structure even under the layer of an unordered adsorbate (Fig. 4), we concluded that there is the SS if we could see it in the frequency image. In three cases, primary testing did not reveal the SS, but the stripe structure was recorded on a reexamination conducted after some time. One of these samples was stored in air in laboratory for three months between the measurements, while the other two were exposed to high humidity for
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two weeks.
The period of the SS for samples of graphene and graphite on SiO2/Si substrates was in the range from 3.8 to 5.3 nm. Several series of samples were made at different times within one year. Noticeable changes of the period were observed for different series, showing a relatively
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small spread within the series: 4 ± 0.2 nm, 4.6 ± 0.2 nm, 5.1 ± 0.2 nm.
Detection of the SS on the HOPG surface is a much more complicated task. This is
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probably because the whole surface of the sample can be covered with unstructured adsorbate. The presence of such a coating can be detected by means of Kelvin probe microscopy [28] or by analyzing the mechanical properties of the surface [29]. In Supplement, we give some examples. Such testing will succeed only if there is any contrast in mechanical or electrical properties of different areas. When the entire surface is covered with a uniform layer, this contrast disappears [28, 30]. If the measurements are carried out in the air, the presence of adsorbate on the surface
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is inevitable. In the case, when AFM measurements show a smooth, uniform surface, this only indicates that the technique itself is unable to resolve the structure of molecules on the surface, and not about the absence of such molecules. Some types of pollution are extremely mobile; in the Supplement, we demonstrate an
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example. We believe that in many cases the layer with self-assembling is actually present, but is under a layer of mobile and unstructured adsorbate. In the case when graphene flakes lie on the
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SiO2 support, we have better conditions for observing the SS. This may be due to the mobility of the disordered layer and the hydrophilicity of SiO2. Molecules of various substances are constantly adsorbed on the surface. Some of them form strong bonds with the surface; then selfordering can occur. Others molecules form a layer of disordered adsorbate. This layer is mobile; it can be expected that such molecules will accumulate on the nearby hydrophilic substrate and not on hydrophobic graphene. The disordered layer on graphene will be thinner, which will provide better conditions the AFM study of self-assembling. There are no hydrophilic regions on the surface of HOPG, however, when interacting with water, adsorption can be redistributed, which also increases the probability of SS observation. Note that thermal cycling used in [5] to prepare the samples, could also lead to water condensation.
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The question of the chemical composition of the layer with self-organization is extremely important. It is out of scope of our study, so can only discuss some possible speculations. In [3133], the process of self-ordering of adsorbate on the surface of HOPG in water was investigated. The authors of these studies considered the most probable adsorbate to be nitrogen molecules in water covering the surface. This explanation is also the most common interpretation of experiments in the air [3, 4, 6]. It seems to us that it would be worth taking into account the
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possibility of an alternative explanation, discussed in [5], namely, that hydrocarbon molecules participate in the formation of such a layer. There are several reasons for this. Hydrocarbons are a very common pollutant, which is extremely difficult to escape. It was shown in [30] that even in high vacuum conditions the graphite surface is covered with a layer of hydrocarbon adsorbate
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after several hours. A number of graphite wettability studies [34-38] indicate that the adsorbate, which is a mixture of water and hydrocarbons, has a decisive role in the hydrophobicity of
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graphite. The SS observed by us has an unmistakable similarity with the structure of selforganization of surfactants on graphite [39-41]. Self-assembling of organic molecules into the stripe-superlattice is well explained by the molecular length and Debye screening [40], while the reasons for the formation of an ordered structure from nitrogen molecules are unknown. In addition, it should be noted that the rate of self-assembling growth in water saturated with nitrogen observed in [31] is quite small to be sure, that nitrogen is the main building material.
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Perhaps, the probability of this did not fully exclude the authors of [31], nitrogen only stimulates the precipitation of any pollutants.
Discussion on the explanation of stripe domains has continued in recent papers [42, 43]. The authors of [43] suggested that stripes might be corrugations of surface layer, affected by
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surfactants or intercalants. At the same time, authors of [40] had shown that the domains are
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optically anisotropic, which was attributed to self-assembled adsorbates.
5. Conclusions
Among common air pollutants in laboratories, there are substances, presumably
hydrocarbons, which frequently form self-organization over a large area of the graphitic surface. The dimensions of a single domain with a constant stripe direction can be large, comparable to the size of a graphene flake. Probably, the possibility of forming such a layer should be taken into account when interpreting some other experiments with graphene and graphite. Direct observation of self-organization can be complicated due to the presence of a layer of disordered adsorbate. Under the influence of the AFM probe, the rotation of stripe structure can occur. This process is reversible and is not accompanied by a change in the period of the structure
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Acknowledgment This work was partly supported by the Russian Science Foundation (Project No. 16-12-10411).
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Phys. Chem. Chem. Phys. 2018, 20:13322-30.
S0008-6223(18)31013-3
DOI:
https://doi.org/10.1016/j.carbon.2018.10.094
Reference:
CARBON 13612
To appear in:
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Received Date: 31 July 2018 Revised Date:
9 October 2018
Accepted Date: 30 October 2018
Please cite this article as: A. Temiryazev, A. Frolov, M. Temiryazeva, Atomic-force microscopy study of self-assembled atmospheric contamination on graphene and graphite surfaces, Carbon (2018), doi: https://doi.org/10.1016/j.carbon.2018.10.094. 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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Atomic-force microscopy study of self-assembled atmospheric contamination on graphene and graphite surfaces.
a
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Alexei Temiryazev *a Alexey Frolov b, and Marina Temiryazeva a
Kotel'nikov Institute of Radioengineering and Electronics of RAS, Fryazino Branch, Vvedensky
Sq. 1, Fryazino 141190, Russia
Kotel'nikov Institute of Radioengineering and Electronics of RAS, Mokhovaya str. 11-7,
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b
Moscow 125009, Russia
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By means of high resolution atomic-force microscopy (AFM) we investigated the surface of graphene and graphite. Our study shows that if the samples were stored in ambient laboratory conditions, an adsorbate of airborne contaminants, presumably hydrocarbons, forms well-ordered layer over the whole area of graphene flakes. In general case, this layer has a stripe structure of 4-5 nm pitch with domains differing
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in the direction of the stripes. The size of one domain can exceed 100 µm2, which means that almost the entire area of a graphene flake can be under a layer with a constant stripe direction. By mechanical impact of AFM probe, we can turn the direction of the stripe structure, while its pitch is preserved. Visualization of the self-
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assembled structure may be hampered by the presence of non-ordered overlayer.
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1. Introduction
A study of atmospheric contamination on the surface of graphene is essential since the
presence of adsorbate changes electrical and optical properties of graphene. It is indicative that one of potential graphene applications is based on its ability to detect gas molecules at extremely low concentrations [1]. When an experiment is carried out in air, the presence of adsorbate is practically inevitable. The question arises as to whether self-assembling of adsorbate can occur and results in nano texturing. Then, a period and orientation of the texture could affect the properties of graphene. It is well known that some organic materials being advisedly coated on graphite surface, self-assemble into nanoscale stripes [2]. As for the self-assembling of the *
Corresponding author. E-mail: [email protected] Tel: +79163069408
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atmospheric pollutants, there are just a few papers where it was reported. By means of high resolution atomic-force microscopy (AFM), it was found on the surface of graphene on 6HSiC(0001) [3, 4] and exfoliated graphene on a variety of substrates: SiO2/Si, gold/mica, hexagonal boron nitride (hBN) [5]. Similar phenomena have also been reported on graphite [6], hBN [5], and epitaxial heterostructures graphene/hBN [5]. A distinguishing feature of the observed self- assembling is existence of domains in the interior of which there is a well-ordered
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stripe structure with a pitch of 4-6 nm. The domains differ in the stripe orientation and have 600 symmetry, which reflects the graphite symmetry. The size of domains observed in [3, 4, 6] was rather small, several hundreds of nanometers. Taking into consideration a great deal of studies on graphene, one would conclude that such self-assembling is rare in occurrence and can hardly
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affect physical properties of graphene, say, its impact on transport in graphene stripe of several micrometers in size should be substantially averaged. However, this is not the case. Gallagher et
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al. [5] have shown that self-assembling results in anisotropy of friction. Thus, we can extend the number of related papers by including the observations of domains of anisotropic friction on the surface of graphene [7 - 10] and graphite [11, 12], despite the fact that in these works two different explanations of the observed phenomena were used. Experiments on friction forces conducted in [7 - 12] did not allow one to directly observe the stripe structure, but they made it possible to observe friction domains on large areas with dimensions of several tens of microns. If
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we associate domains of self-assembling with domains of friction, then based on the results of [5, 7 - 12], must recognize that the whole of the sample surface may be entirely coated with an ordered adsorbate. Moreover, sizes of the individual domains may exceed 10-20 microns that is quite commensurate with the size of the structures commonly used in the study of the transport
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properties of graphene. Here again the question arises, why they are so rarely observed in experiments. The reasons for this may be several. Firstly, the AFM studies should be
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purposefully aimed at obtaining high-resolution images. This implies a small scan size (of the order of 100-300 nm), the use of sharp probes and, most importantly, operating modes that allow obtaining high resolution on rather soft surfaces in air. Secondly, often the structure of the surface was examined on samples that have passed the cleaning procedure using solvents and annealing [13]. Third, scanning tunneling microscopy (STM), which has very high spatial resolution, do not always allow us to reveal the structure of a soft non-conducting adsorbate [2, 5]. In this paper, we show that there is one more reason - the presence of a layer of disordered adsorbate. We will use the methods of high-resolution AFM to detect self-organization on various graphite surfaces. We will estimate the dimensions of self-organization domains, and discuss the reasons that make it difficult to observe such structures experimentally. In addition, on nanoscale, we will investigate the phenomena observed by Gallagher at el. As it was shown in
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[5], with AFM probe, one can switch friction – that is, create a new friction domain, and then erase it. Proceeding from the fact that the friction is determined by the direction of the stripe structure of the adsorbate, this should indicate a reorientation of the periodic structure. We will present direct evidence of such an effect.
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2. Methods
The AFM study was carried out using a SmartSPM atomic force microscope (AIST-NT) in the air. For surface scanning we used a dissipation mode (DM) [14, 15], that is, the probe is driven to oscillate, pump power is fixed, the oscillation amplitude serves to maintain the
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feedback control (as in tapping mode) and the oscillation frequency is maintained at resonance (as in frequency modulation (FM) mode). The surface profile recorded in the DM corresponds to
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the constant dissipation profile, while the change in the resonant frequency recorded simultaneously with the profile reflects the fine structure of the probe-surface interaction force. The DM allows achieving high spatial resolution, about 1 nm, when scanning in air and provides the possibility of scanning soft surfaces [15]. Note that in the works cited above, the FM [3, 4, 6] or the tapping mode [5] were used to visualize the adsorbate self-ordering. Our choice in favor of DM is due to a number of reasons. In air, the FM mode is extremely effective when operating at
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small oscillation amplitudes, about 1 nm or less. This requires the use of stiff force sensors, such as a length-extension resonator [6] or a qPlus sensor [3, 4]. Commercial AFMs usually do not have them; therefore, the tapping mode is commonly applied. When choosing between the DM and the tapping mode, the following should be considered. One of the basic conditions for
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obtaining a high resolution is the exact choice of scanning parameters. The advantage of DM is the ease of selecting these parameters [15]. For the tapping mode, one should choose: a
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frequency of oscillations f, a free-oscillating amplitude A0, and a set-point amplitude Asp i.e., the amplitude at which the scanning is performed. Usually, f is chosen near the resonant frequency of the probe. The choice of the other two parameters is not so obvious, since it depends on the surface properties and the tip sharpness. There are only general recommendations applicable mostly to very sharp tips: to use small amplitudes A0 and to scan in light tapping, that is Asp to be close to A0 [16]. In case of DM, we do not need to select the frequency: a phase locked loop is used to keep the phase of oscillating probe at 90 degrees, that is, the excitation is performed at the resonant frequency. There is a clear procedure for setting other parameters [15]. We believe that in order to obtain a high resolution, it is necessary to set conditions under which it is possible to reliably detect the beginning of the growth of repulsive forces between the probe and the sample. This will correspond to the minimum surface deformations and hence the minimum
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tip-sample contact area. The choice of A0 and Asp is based on dependencies of amplitude A and resonant frequency fres upon a tip-sample distance z. While A(z) monotonically decreases with decreasing z, the shape of the fres(z) curve depends strongly on the initial amplitude A0 and the tip sharpness. If A0 is too small or the probe is blunt, then attractive forces predominate and fres(z) decreases monotonically as the probe approaches the sample. If A0 is too large or the probe is very sharp, then the probe goes into the net repulsive regime and the resonant frequency
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increases at A(z) very close to A0. We choose an intermediate value of the initial amplitude A0, at which a reversal of the frequency curve is observed in the range of A(z) = (0.7-0.9)A0. Then we set Asp equals to A(z) at z corresponding to the frequency turning point. Thus, measurements are made in the net attractive regime, but under conditions in which repulsive forces contribute
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significantly to the interaction. It is important to emphasize that the entire setup procedure is carried out before the scan starts and allows us to select the optimal parameters for each specific
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probe. For instance, the initial amplitude of 10 nm, may be too large for very sharp probe, but insufficient for the other, which in turn will provide a clear image at A0 = 20 nm. The measurements used silicon probes with self-made sharp spikes grown at the tip [17]. Flakes of graphene were prepared by mechanical exfoliation using an adhesive tape [18] under ambient conditions on silicon wafers with 300 nm of thermal oxide. For graphene exfoliation, we used high quality single crystals of natural graphite received from NGS Naturgraphit GmbH. Part of
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the samples were annealed in vacuum at a temperature of 500 o C to remove the adhesive residues. After exfoliation, on the substrate were flakes containing areas of monolayer and multilayer graphene, as well as thicker graphene flakes, 50-200 nanometers in thickness, let us call them “graphite plates”. We started AFM research a few days after the samples were made.
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Some experiments were conducted with highly oriented pyrolitic graphite (HOPG). After cleavage, HOPG samples were also stored in ambient conditions for several days before the
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3. Experimental results
3.1 Self-assembling on graphene flakes on SiO2/Si substrate. Fig. 1 shows typical structures observed on the surface of graphene flakes. There is a clear defined stripe structure (SS) of 4.6 ± 0.2 nm pitch. Peak-to-trough stripe amplitude in height picture (Fig. 1a) is about 150 pm. This value is not very informative, since it strongly depends on the sharpness of the tip and the scanning parameters. The frequency image, as a rule, gives a clearer picture of the periodicity. However, small changes in the setpoint often make it
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possible to change the contrast, increasing it for a height picture due to some reduction in the contrast of the frequency channel. Figures 1 (a, b) show an area containing the boundary of domains differing in the direction of the SS. It can be seen that in this case the presence of the boundary is not related to the change in the number of graphene layers. In some cases, mainly near the edges of the flakes, it is possible to find places where domains with three directions are simultaneously observed (Fig. 1 (c, d)). Fig. 1(e, f) show the scans of the area containing the
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boundary between the monolayer and three-layered graphene. When scanning a relatively large area of 0.5×0.5 µm2, it is only noticeable that the surface of the monolayer has an appreciably higher roughness than three-layered graphene. This is typical for the monolayer graphene, which repeats the roughness of the substrate and the adsorbate sandwiched between graphene and the
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substrate [19, 20]. Against this background, the SS is hardly noticeable. Nevertheless, with a decrease in the scanning area, the presence of the SS is clearly visible (Fig. 1 (g, h)). Here it is
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important to note two features. First, the SS periods for the domains located on monolayer and three-layered graphene coincide. Secondly, the boundary of the domains only partially matches the boundary between monolayer and three-layered areas. In the central part of Fig. 1 (g, h) it is clear that the right domain penetrates into the three-layered graphene region. So far, we have shown scans of areas where we could observe domain boundaries. This was done in order to make the pictures more convincing, and the stripe structure was not
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associated with the hardware effects when scanning. At the same time, it should be noted that the search for domain boundaries on the graphene surface was a rather laborious task. Domain sizes could be several microns, which is much larger than the scan size, on which a fine structure can be demonstrated. To illustrate how large individual domains can be, we performed a series of
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measurements of 100 scans of 50×50 nm2 size, made at the grid nodes covering most of the graphene flake - Fig. 2. The analysis of the obtained images shows that the entire central part of
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the flake is occupied by one domain whose area is more than 100 µm2.
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Fig. 2. AFM image of graphene flake (a) where a series of 100 high-resolution (50×50 nm2) scans (b-d) were performed. Bars show the direction of stripe structures, the places where no structure was observed are marked with circles.
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Fig. 1. Self-alignment on the surface of graphene flakes on silicon dioxide. (a, c, e, g) – topography images, (b, d, f, h) – corresponding frequency images. (a, b) show two domains on the flat area of graphene, (c, d) – demonstrate an area near the edge of graphene flake. Here we can see all three stripe directions separated by 60o. (e, f) show the area containing the boundary between monolayer and three-layer graphene. The region marked in (e) with a square is shown in (g, f). Period of stripe structure p = 4.6±0.2 nm (b), 5.1±0.2 nm (d), 4.9 nm±0.2 (h). (i, j) - height cross sections along black lines in (a) and (e).
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3.2 Self-assembling on graphite plates on SiO2/Si substrate .
The next series of experiments was carried out with graphite plates located on a SiO2/Si substrate. Here, also, SS were found with the same period as on thin graphene flakes on the same substrate. However, in addition to them, other types of structures on the surface were also noticeable. These could be drops (Figures 3(a, b)), which, apparently, are not connected with the
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SS in any way. Horizontal stripes on the image indicate contamination of the probe when scanning the droplets. The probability of such contamination can be significantly weakened by changing the scanning parameters, however, in this case the parameters were chosen for optimal SS visualization. A more interesting picture is shown in Fig. 3(c, d). Domain boundaries are no
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longer visible here. In these areas, there is a higher layer with a completely different ordering structure. In some points - light dots on the frequency image (Fig. (d)), the probe interaction with
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a surface changes abruptly. This can be seen from the change in the resonance frequency. In that case, there frequently was contamination of the probe, resulting in the appearance of stripes on the frequency image. In Fig. 3(e, f), another example of structures found on the surface of thick graphene flakes is presented. Well-ordered regions coexist with regions where long-range order is absent, but there are chaotically scattered elongated objects. It is well known that some organic substances, being applied to the graphite surface, self-assemble into a lamellar structure [2, 16,
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21, 22]. We can assume that the SSs seen in the Figs. 1-3 are lamellar structures formed from the adsorbent, the scattered objects in Fig. 3(e,f) are individual lamellas. The top layer in Fig. 3(c,d) could be formed as a result of another type of ordering and be micelles or fiber-like assemblies [2]. In favor of the SS is associated with the self-organization of the adsorbate, and not with the
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surface bending, we can give the arguments outlined in [5]. The period of the SS was the same for both the monolayer and multilayer graphene flakes, as well as for the graphite plates on the
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same SiO2/Si substrate. If the corrugation were due to mechanical stresses caused by the interaction of the flake with the substrate, it would natural expect a change in the period of corrugation period with a change in the thickness of the flakes. Observation of the SS can be greatly hampered by the presence of a disordered adsorbate
layer on the surface. Figure 4 shows that on the part of the scanned surface, there is an area where we can not observe any fine structure in the height image. Nevertheless, the frequency image allows us to conclude that there is a SS under the layer of unstructured adsorbate.
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Fig. 4. Self-alignment under the layer of unordered adsorbate on the surface of the graphite plate on SiO2. Inside a circle is a magnified view.
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Fig. 3 Adsorbate on the surface of graphite plates on SiO2. (a, c, e) – topography images, (b, d, f) – corresponding frequency images. Color scale in (a) is nonlinear to highlight the stripe structure. Period of stripe structure p = 5±0.2 nm for all images.
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3.3 Self-assembling on HOPG.
On the basis that the SS is a consequence of self-organization of the adsorbate on the surface of graphene, it can be expected that it should be present on the bulk plates of HOPG. We performed many AFM measurements trying to detect the presence of the SS on the HOPG surface. As a rule, such attempts were unsuccessful, but in some cases, the SS was found. These
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observations were quite random; the SS was observed only in certain places and could eventually disappear. Fig. 5 illustrates the procedure of such an experiment. When scanning a large area, we managed to find two spots in which there is a noticeable contrast in the frequency channel (Fig. 5(a, b)). Subsequent scanning of a smaller area shows the presence of the SS in such places (Fig.
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5(c, d)). Note that in this case the direction of the stripes in these regions differs by 17 degrees. This atypical situation is probably due to the mismatch of the crystallographic directions in different layers of HOPG. The height image (Fig. 5(c)) shows the presence of two steps (edges of graphite
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layers) between the areas of self-assembling. This means that the left and right SSs in (Figs. 5(c,d)) are located on different layers of HOPG. Apparently, these layers differ in their crystallographic orientation.
We can see a periodic pattern in Fig. 5(b). It has a period of 38 nm. This is an artifact, a kind of moiré pattern resulting from the digital nature of AFM imaging. When scanning a large
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area, it indicates the presence of the SS. Moiré occurs if the distance between neighboring points of the scan is large and comparable to the period of the SS. This effect is enhanced by a special mode, which we used to scan large areas of the surface. In order to reduce the chance of tip contamination, we used the dissipation mode in combination with a vertical mode [19], that is,
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the movement of the probe from point to point occurred at an elevated setpoint, the height and frequency were measured when the probe approached the surface at a constant speed. In this
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mode, averaging of the measured signals does not occur, the height and resonance frequency are measured at the given points, the distance between which is determined by the scan size and the number of points in the line. When the length of such a step is comparable to the SS period, moiré may appear on the image (Fig. 5(b)). An additional example of moiré structure is shown in the Supplement.
Figs.5(e, f) clearly demonstrate the reasons that make it difficult to observe the SS on HOPG. A rather flat area of the surface is represented. The area where the SS is observed has uneven edges and lays lower than its surroundings. We can assume that the surface outside this region is covered by unstructured and more mobile adsorbate, which, when scanning, can move under the influence of interaction with the probe. This gives grounds to say that in the case when the SS is not observed experimentally, the reason for this may be not the absence of self-
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organization, but the presence of an additional layer of adsorbate covering the layer with selfordering. The probability of finding a SS on HOPG increases if the surface is treated with water. Our experiments showed that if the sample is rinsed with distilled water or condensed on the surface with boiling water, then the adsorbate is redistributed. On the initially flat surface, where only the steps at boundaries of the graphite layers were at first visible, adsorbate clots have been
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formed. They are usually located near layer boundaries, and we can detect the presence of the SS between them (Fig. 6). The results of such experiments should be treated with caution, since contamination could be carried by water. Nevertheless, it seems that interaction with water or
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water vapor can be a very important factor.
Fig. 5. Adsorbate on the surface of HOPG. (a, c, e) – topography images, (b, d, f) – corresponding frequency images. The region marked in (a) with a square is shown in (c, d). Inside a circle in (f) is a magnified view. Moiré pattern with the period of 38 nm can be seen in (b) while the real SS period p = 4.3±0.2 nm (d), p = 4.4±0.2 nm (f).
Fig. 6. Adsorbate on the surface of HOPG rinsed with water. (a, c, e) – topography images, (b, d, f) – corresponding frequency images. Period p = 5±0.2 nm (d), p = 5.1±0.2 nm (f). When scanning a large area, as in (a ,b), the distance between adjacent points of the scan was 5.7 nm. Superimposing this grid on the stripe structure with a period of 5.1 nm led to the appearance of a moiré pattern, clearly visible in (b).
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3.4 Reorientation of stripe structure.
The purpose of the next experiment was to study the change in the SS under the action of the AFM probe. As was shown in [5], scanning in a contact mode can lead to the appearance of a new friction domain. We have studied this effect at nanoscale. The difficulty of such an experiment is the need to maintain the sharpness of the tip after rather hard scanning in the
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contact mode. Therefore, we used a probe with a single-crystal diamond tip [24]. Commercially available probes of this type (D300, SDCprobes) have a stiffness of about 40 N/m and remain sharp enough even after their use to indentate hard surfaces [25]. The radius of curvature of the tip is often somewhat larger than that of special probes for high-resolution scanning. For that
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reason, the quality of height images is lower, and in Fig. 7 we give only frequency images. The experiment was carried out on a graphite plate 80 nm thick on a SiO2/Si substrate. At first, we scanned a relatively large area of 200×200 nm2 in the dissipation mode. Further, a small area of
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50×50 nm2 at the center of this section was scanned in contact mode, that is, 256 lines were performed with a tip pressing force of the order of 80 nN. The tip was in contact only during the forward motion in the fast scan direction; on the return stroke, it was raised. Then again, the large area was scanned in DM. Figure 7(a-e) show the result. There are no traces or flanges from the scraped adsorbate on the surface, but in the area of scanning in the contact, the SS
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reorientation occurred. A new domain has been formed. These changes are reversible, changing the direction of scanning in the contact; we could erase this domain, create it again and again erase it.
Similar experiment was conducted on a monolayer graphene flake. There was also a turn
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of the SS. Interestingly, in some cases reorientation occurred over an area substantially greater than the area scanned in contact. Fig. 7 (f-i) shows that scanning in contact has formed two new
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domains. We did not try to erase them, but watched the dynamics over time. Subsequent scanning at 55 and 90 minutes showed that relaxation was occurring, and one of these new domains gradually disappeared. Such behavior, in some ways, is similar to that was observed in [5] where manipulations with the friction domains were carried out. Over time, some of the “written” friction domains gradually collapsed, while others grew. In our case, the expansion of the new domain into the untreated area could take place during the “writing” procedure. At the next stage, slower processes were observed - the expansion of one of the new domains led to the destruction of the other. We can draw several conclusions. We have received direct proof that the SS can be turned by the action of the tip. The period of structure does not change. This is a confirmation of the fact that the local change in frictional forces observed in [5] can indeed be explained by turning the SS. Changes under the influence of a probe are not related to
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technology known as nanografting, that is, with the removal of the self-assembled monolayer and then forming a layer of other molecules [26, 27]. It is worth noting that the behavior of the layer of ordered adsorbate reminds the properties of liquid crystals that are capable of being repolarized under external action. As for lamellar structures, the possibility of their spontaneous reorientation was demonstrated in [21], where self-assembling of alkanes at high temperatures
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was studied.
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Fig. 7 Reorientation of the stripe structure. (a – e) Frequency images scanned in dissipation mode of the same area (200×200 nm2) on a graphite plate. After the first scan (a) was taken, we conducted contact scanning of the smaller area (50×50 nm2) marked with a square. Arrows between (a) and (b) show fast and low directions of the contact scanning. Re-scan in DM (b) shows that a new domain has been formed. We again conducted contact scanning with the different fast and low directions (arrows between (b) and (c)) to delete this domain (c). Then all operation were repeated. We wrote new domain (d) and deleted it (e). (f – i) Frequency images scanned in dissipation mode of the same area (220×220 nm2) on a graphene flake. On initially single domain area (f) we conducted contact scanning of the smaller area (50×50 nm2) marked with a square. This led to the appearance of two new domains. The bigger one has the area much larger than the area of contact scanning. The second, small domain reduced in time and disappeared. (h) and (i) were taken 55 and 90 minute later the contact scanning. 4. Discussion
We carried out research of one and a half dozen samples with graphene flakes and graphite plates on SiO2/Si substrates. Only on one of the samples did we fail to detect the SS. The quality of the AFM images obtained was very different from the sample to the sample. The cause may be contamination of either the surface or the probe. This question was not specifically
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investigated, we limited ourselves to ascertaining the fact of the existence of the SS. Since in some cases the DM allows us to detect the structure even under the layer of an unordered adsorbate (Fig. 4), we concluded that there is the SS if we could see it in the frequency image. In three cases, primary testing did not reveal the SS, but the stripe structure was recorded on a reexamination conducted after some time. One of these samples was stored in air in laboratory for three months between the measurements, while the other two were exposed to high humidity for
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two weeks.
The period of the SS for samples of graphene and graphite on SiO2/Si substrates was in the range from 3.8 to 5.3 nm. Several series of samples were made at different times within one year. Noticeable changes of the period were observed for different series, showing a relatively
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small spread within the series: 4 ± 0.2 nm, 4.6 ± 0.2 nm, 5.1 ± 0.2 nm.
Detection of the SS on the HOPG surface is a much more complicated task. This is
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probably because the whole surface of the sample can be covered with unstructured adsorbate. The presence of such a coating can be detected by means of Kelvin probe microscopy [28] or by analyzing the mechanical properties of the surface [29]. In Supplement, we give some examples. Such testing will succeed only if there is any contrast in mechanical or electrical properties of different areas. When the entire surface is covered with a uniform layer, this contrast disappears [28, 30]. If the measurements are carried out in the air, the presence of adsorbate on the surface
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is inevitable. In the case, when AFM measurements show a smooth, uniform surface, this only indicates that the technique itself is unable to resolve the structure of molecules on the surface, and not about the absence of such molecules. Some types of pollution are extremely mobile; in the Supplement, we demonstrate an
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example. We believe that in many cases the layer with self-assembling is actually present, but is under a layer of mobile and unstructured adsorbate. In the case when graphene flakes lie on the
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SiO2 support, we have better conditions for observing the SS. This may be due to the mobility of the disordered layer and the hydrophilicity of SiO2. Molecules of various substances are constantly adsorbed on the surface. Some of them form strong bonds with the surface; then selfordering can occur. Others molecules form a layer of disordered adsorbate. This layer is mobile; it can be expected that such molecules will accumulate on the nearby hydrophilic substrate and not on hydrophobic graphene. The disordered layer on graphene will be thinner, which will provide better conditions the AFM study of self-assembling. There are no hydrophilic regions on the surface of HOPG, however, when interacting with water, adsorption can be redistributed, which also increases the probability of SS observation. Note that thermal cycling used in [5] to prepare the samples, could also lead to water condensation.
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The question of the chemical composition of the layer with self-organization is extremely important. It is out of scope of our study, so can only discuss some possible speculations. In [3133], the process of self-ordering of adsorbate on the surface of HOPG in water was investigated. The authors of these studies considered the most probable adsorbate to be nitrogen molecules in water covering the surface. This explanation is also the most common interpretation of experiments in the air [3, 4, 6]. It seems to us that it would be worth taking into account the
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possibility of an alternative explanation, discussed in [5], namely, that hydrocarbon molecules participate in the formation of such a layer. There are several reasons for this. Hydrocarbons are a very common pollutant, which is extremely difficult to escape. It was shown in [30] that even in high vacuum conditions the graphite surface is covered with a layer of hydrocarbon adsorbate
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after several hours. A number of graphite wettability studies [34-38] indicate that the adsorbate, which is a mixture of water and hydrocarbons, has a decisive role in the hydrophobicity of
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graphite. The SS observed by us has an unmistakable similarity with the structure of selforganization of surfactants on graphite [39-41]. Self-assembling of organic molecules into the stripe-superlattice is well explained by the molecular length and Debye screening [40], while the reasons for the formation of an ordered structure from nitrogen molecules are unknown. In addition, it should be noted that the rate of self-assembling growth in water saturated with nitrogen observed in [31] is quite small to be sure, that nitrogen is the main building material.
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Perhaps, the probability of this did not fully exclude the authors of [31], nitrogen only stimulates the precipitation of any pollutants.
Discussion on the explanation of stripe domains has continued in recent papers [42, 43]. The authors of [43] suggested that stripes might be corrugations of surface layer, affected by
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surfactants or intercalants. At the same time, authors of [40] had shown that the domains are
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optically anisotropic, which was attributed to self-assembled adsorbates.
5. Conclusions
Among common air pollutants in laboratories, there are substances, presumably
hydrocarbons, which frequently form self-organization over a large area of the graphitic surface. The dimensions of a single domain with a constant stripe direction can be large, comparable to the size of a graphene flake. Probably, the possibility of forming such a layer should be taken into account when interpreting some other experiments with graphene and graphite. Direct observation of self-organization can be complicated due to the presence of a layer of disordered adsorbate. Under the influence of the AFM probe, the rotation of stripe structure can occur. This process is reversible and is not accompanied by a change in the period of the structure
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Acknowledgment This work was partly supported by the Russian Science Foundation (Project No. 16-12-10411).
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