rus
Issue #7/2017
G.Meshkov, O.Sinitsyna, Sh.Rajabzoda, A.Grigoryeva, I.Yaminsky
Scanning resistance microscopy of graphene oxides
Scanning resistance microscopy of graphene oxides
Using the scanning resistance microscopy, the local electrical conductivity of graphene oxides grown by the local anodic oxidation on the graphite surface and obtained by the chemical method was measured.
Теги: graphene oxide local anodic oxidation scanning resistance microscopy локальное анодное окисление оксид графена сканирующая резистивная микроскопия
A promising material for the creation of an electronic nose based on resistive sensors, elements of accumulators and supercapacitors is graphene oxide [1], a non-stoichiometric compound that consists of partially oxidized graphene grids containing epoxy, hydroxyl groups, and carbonyl and carboxyl groups at the edges. When molecules are adsorbed, the charge is transferred to the carbon matrix of graphene oxide, which significantly affects its electrical conductivity. Advantages of this material: high surface-to-volume ratio, which provides high sensitivity of sensors and efficiency of energy storage; low cost; the possibility of varying the physical-chemical characteristics (electrical conductivity, adsorption) by changing the degree of oxidation. In this connection, it is of interest to study spatial inhomogeneities in the distribution of the local electrical conductivity of graphene oxides. For these purposes, the use of scanning resistance microscopy (SRM) is appropriate [2].
One of the widely used methods for the formation of nanostructures is local anodic oxidation (LAO), which can be used to form graphene oxide on graphite and graphene [3, 4]. In this paper, the topography and local electrical conductivity of grown graphene oxide films were studied. The results were compared with the data obtained for graphene oxide particles synthesized by the chemical method.
SCANNING PROBE MICROSCOPY
The FemtoScan scanning probe microscope produced by the Advanced Technologies Center was used in the project. The received data were processed and analyzed in the FemtoScan Online software. Surface topography was studied by atomic force microscopy (AFM) in the tapping mode in air at room temperature. The cantilevers HR11 with resonant frequencies of 380 and 230 kHz were used, with a tip radius of about 10 nm. Images of the distribution of lateral forces were obtained in the AFM contact mode with the help of CSG11 cantilevers.
By the SRM method, the measurements were carried out in a contact mode with a constant force. Cantilevers CNC/Au11 with a gold conductive coating and a stiffness of 1.0 and 1.5 N/m and FM/W2C11 with conductive coating of tungsten carbide and a stiffness of 6 and 3.5 N/m were used. At each point of the surface, simultaneously with the topography, an electric current was measured, flowing through the contact between the probe and the sample. The contact resistance was calculated as a modulus of the ratio of the voltage applied between the probe and the sample to the current.
LOCAL ANODIC OXIDATION
LAO of the surface was performed in contact mode when a potential difference from 3 to 9 V was applied between the probe and the sample (the sample was positively charged), the probe’s impact on the surface was selected in the range of 1–10 nN depending on the features of the cantilever used, the probe movement speed was 2 μm/s. Relative air humidity was about 60%.
Conductive cantilevers CNC/Au11 and CSG11Au with a gold coating and average stiffness of 0.03 and 0.11 N/m with a tip radius of about 35 nm were used. Before the experiment, the conductive coating was removed directly from the point of the tip by rubbing against the surface. Increasing the distance between the electrodes (the edges of the conductive coating on the probe and the surface of the graphite) led to partial oxidation of the surface with the formation of graphene oxide and graphite oxide structures instead of complete oxidation with formation of gaseous carbon oxides and etching pits. The surface of freshly cleaved highly oriented pyrolytic graphite (HOPG) with the mosaicism of 0.8° produced by Atomgraf-crystal was subjected to local oxidation.
PREPARATION OF GRAPHENE OXIDE SAMPLES OBTAINED BY CHEMICAL SYNTHESIS
A sample of graphene oxide provided by A.Grigorieva (Lomonosov Moscow State University), was obtained by the Tour method [5]. The concentration of graphene oxide in a solution of dimethylformamide is 0.001 g/ml. Samples for probe microscopy were prepared on the surface of mica with a deposited conductive layer (Cr – 5 nm, Au – 50 nm).
RESULTS OF PROBE MICROSCOPY
OF GRAPHITE SURFACE
The main defects observed in topography on the surface of the cleaved HOPG are the cleavage steps, edge and screw dislocations with the Burgers vector perpendicular to the basal plane, grain boundaries.
According to the SRM data, the contact resistance for graphite and cantilever depended both on the initial state of the conductive coating on the tip and on the wear of the probe and varied in the range from 1 MΩ to 1 GΩ. The images obtained by the SRM reveal all the elements of topography with a sharp change in the relief. A characteristic feature is also the variation in the resistance of atomic terraces: terraces with a higher height correspond to a lower resistance (Fig.1). This effect is caused by the hysteresis of the contact area when the impact of the probe changes during the passage of sharp relief features, on which the feedback does not ensure the constancy of the force of the probe’s impact on the surface. At the locations of the grain boundaries, the local electric resistance decreases relative to the atomic terraces, which allowed to estimate the linear dimensions of the crystallites (1,5–10 µm).
TOPOGRAPHY AND LOCAL ELECTRICAL CONDUCTIVITY
OF GRAPHENE OXIDE FILMS
OBTAINED BY LAO
Analysis of AFM data for oxidized areas by the LAO method allowed us to distinguish two stages of partial oxidation of the graphite surface depending on the applied voltage. At low voltages (less than 4–6 V), the surface height remains practically unchanged, but the friction coefficient between the probe and the surface increases by a factor of 1.5–2. With increasing voltage, the increase in the surface height occurs non-uniformly, and individual convex points and lines are formed. With a further increase in the voltage to 7–9 V, homogeneous regions are formed, the height of which increases linearly on the voltage. When the height of the oxidized regions is 1–1.5 nm, cracking of the upper carbon layer is observed. It can be assumed that in the first stage of partial oxidation, only the chemical modification of the upper graphite layer with the addition of oxygen-containing groups occurs. In the second stage of oxidation, an increase in the height of the surface can be explained by the intercalation of water molecules into the interlayer space.
The results of the SRM show that at the first stage of partial oxidation, areas of 10–30 nm in size appear, in which the contact resistance increases by approximately 10 times in comparison with the unoxidized areas of graphite (Fig.2). Such areas occupy less than 50% of the area of the modified surface. It can be assumed that at the first stage of oxidation the aromatic structure of the graphene grid is destroyed only locally. In other areas, the contact resistance remains the same as for unoxidized graphite.
In the second stage of partial oxidation, when the voltage is increased, the combination of individual sections with an increased contact resistance is observed, and when the height of the oxidized regions is about 1 nm, these areas occupy almost the entire area of the surface being modified. It is interesting to note that with a significant spatial inhomogeneity of the contact resistance of oxidized areas with a height in the range from 0.3 to 1.0 nm, their topography remains fairly smooth with a RMS roughness of about 0.1 nm.
Comparison of the data of AFM and SRM allows to make the assumption that an increase in the distance between the surface layer of graphite and the adjacent layer, and, consequently, the intercalation of water molecules, occur evenly throughout the entire modifiable region, whereas the destruction of the aromatic structure of the upper carbon layer is of an islet character. There were no changes in the nature of the LAO process in the areas of passage of the cleavage steps.
PROBE MICROSCOPY OF GRAPHENE OXIDE FLAKES OBTAINED
BY CHEMICAL METHOD
The AFM study showed that the dimensions of flakes of graphene oxide in the substrate plane were in the range from 400 to 1 700 nm, the height of the flakes was 1–15 nm. The surface of the particles was characterized by a RMS roughness of 0.5–0.9 nm, contained protrusions with a height of 1–3 nm and a size in the flake plane of 50–90 nm. The study of samples by the SRM method showed that the contact resistance when the probe passed particles of graphene oxide increased from 80 MΩ to 3 GΩ and the spatial distribution of the contact resistance being homogeneous (Fig.3). According to the morphology of the surface and the nature of the distribution of local electrical conductivity, the graphene oxide obtained by the chemical method is close to the layers of graphene oxide formed by the LAO method at high voltages.
СONCLUSIONS
It is established that at the initial stages of formation of graphene oxide by the LAO, island surface oxidation occurs with the formation of areas with a reduced local electrical conductivity. Between these areas the structure of the top layer of graphite is preserved. With more intense oxidation, the surface roughness increases, and a decrease in the electrical conductivity is observed for the whole modification region. The morphology and local electrical conductivity of the obtained graphene oxide layers are close to those parameters for the graphene oxide obtained by chemical method.
The study was carried out with the financial support of the RFBR in the framework of scientific projects No. 16-33-00866 (O.Sinitsyna), 17-52-560001 (G.Meshkov) and 16-29-06290 (I.Yaminsky). ■
One of the widely used methods for the formation of nanostructures is local anodic oxidation (LAO), which can be used to form graphene oxide on graphite and graphene [3, 4]. In this paper, the topography and local electrical conductivity of grown graphene oxide films were studied. The results were compared with the data obtained for graphene oxide particles synthesized by the chemical method.
SCANNING PROBE MICROSCOPY
The FemtoScan scanning probe microscope produced by the Advanced Technologies Center was used in the project. The received data were processed and analyzed in the FemtoScan Online software. Surface topography was studied by atomic force microscopy (AFM) in the tapping mode in air at room temperature. The cantilevers HR11 with resonant frequencies of 380 and 230 kHz were used, with a tip radius of about 10 nm. Images of the distribution of lateral forces were obtained in the AFM contact mode with the help of CSG11 cantilevers.
By the SRM method, the measurements were carried out in a contact mode with a constant force. Cantilevers CNC/Au11 with a gold conductive coating and a stiffness of 1.0 and 1.5 N/m and FM/W2C11 with conductive coating of tungsten carbide and a stiffness of 6 and 3.5 N/m were used. At each point of the surface, simultaneously with the topography, an electric current was measured, flowing through the contact between the probe and the sample. The contact resistance was calculated as a modulus of the ratio of the voltage applied between the probe and the sample to the current.
LOCAL ANODIC OXIDATION
LAO of the surface was performed in contact mode when a potential difference from 3 to 9 V was applied between the probe and the sample (the sample was positively charged), the probe’s impact on the surface was selected in the range of 1–10 nN depending on the features of the cantilever used, the probe movement speed was 2 μm/s. Relative air humidity was about 60%.
Conductive cantilevers CNC/Au11 and CSG11Au with a gold coating and average stiffness of 0.03 and 0.11 N/m with a tip radius of about 35 nm were used. Before the experiment, the conductive coating was removed directly from the point of the tip by rubbing against the surface. Increasing the distance between the electrodes (the edges of the conductive coating on the probe and the surface of the graphite) led to partial oxidation of the surface with the formation of graphene oxide and graphite oxide structures instead of complete oxidation with formation of gaseous carbon oxides and etching pits. The surface of freshly cleaved highly oriented pyrolytic graphite (HOPG) with the mosaicism of 0.8° produced by Atomgraf-crystal was subjected to local oxidation.
PREPARATION OF GRAPHENE OXIDE SAMPLES OBTAINED BY CHEMICAL SYNTHESIS
A sample of graphene oxide provided by A.Grigorieva (Lomonosov Moscow State University), was obtained by the Tour method [5]. The concentration of graphene oxide in a solution of dimethylformamide is 0.001 g/ml. Samples for probe microscopy were prepared on the surface of mica with a deposited conductive layer (Cr – 5 nm, Au – 50 nm).
RESULTS OF PROBE MICROSCOPY
OF GRAPHITE SURFACE
The main defects observed in topography on the surface of the cleaved HOPG are the cleavage steps, edge and screw dislocations with the Burgers vector perpendicular to the basal plane, grain boundaries.
According to the SRM data, the contact resistance for graphite and cantilever depended both on the initial state of the conductive coating on the tip and on the wear of the probe and varied in the range from 1 MΩ to 1 GΩ. The images obtained by the SRM reveal all the elements of topography with a sharp change in the relief. A characteristic feature is also the variation in the resistance of atomic terraces: terraces with a higher height correspond to a lower resistance (Fig.1). This effect is caused by the hysteresis of the contact area when the impact of the probe changes during the passage of sharp relief features, on which the feedback does not ensure the constancy of the force of the probe’s impact on the surface. At the locations of the grain boundaries, the local electric resistance decreases relative to the atomic terraces, which allowed to estimate the linear dimensions of the crystallites (1,5–10 µm).
TOPOGRAPHY AND LOCAL ELECTRICAL CONDUCTIVITY
OF GRAPHENE OXIDE FILMS
OBTAINED BY LAO
Analysis of AFM data for oxidized areas by the LAO method allowed us to distinguish two stages of partial oxidation of the graphite surface depending on the applied voltage. At low voltages (less than 4–6 V), the surface height remains practically unchanged, but the friction coefficient between the probe and the surface increases by a factor of 1.5–2. With increasing voltage, the increase in the surface height occurs non-uniformly, and individual convex points and lines are formed. With a further increase in the voltage to 7–9 V, homogeneous regions are formed, the height of which increases linearly on the voltage. When the height of the oxidized regions is 1–1.5 nm, cracking of the upper carbon layer is observed. It can be assumed that in the first stage of partial oxidation, only the chemical modification of the upper graphite layer with the addition of oxygen-containing groups occurs. In the second stage of oxidation, an increase in the height of the surface can be explained by the intercalation of water molecules into the interlayer space.
The results of the SRM show that at the first stage of partial oxidation, areas of 10–30 nm in size appear, in which the contact resistance increases by approximately 10 times in comparison with the unoxidized areas of graphite (Fig.2). Such areas occupy less than 50% of the area of the modified surface. It can be assumed that at the first stage of oxidation the aromatic structure of the graphene grid is destroyed only locally. In other areas, the contact resistance remains the same as for unoxidized graphite.
In the second stage of partial oxidation, when the voltage is increased, the combination of individual sections with an increased contact resistance is observed, and when the height of the oxidized regions is about 1 nm, these areas occupy almost the entire area of the surface being modified. It is interesting to note that with a significant spatial inhomogeneity of the contact resistance of oxidized areas with a height in the range from 0.3 to 1.0 nm, their topography remains fairly smooth with a RMS roughness of about 0.1 nm.
Comparison of the data of AFM and SRM allows to make the assumption that an increase in the distance between the surface layer of graphite and the adjacent layer, and, consequently, the intercalation of water molecules, occur evenly throughout the entire modifiable region, whereas the destruction of the aromatic structure of the upper carbon layer is of an islet character. There were no changes in the nature of the LAO process in the areas of passage of the cleavage steps.
PROBE MICROSCOPY OF GRAPHENE OXIDE FLAKES OBTAINED
BY CHEMICAL METHOD
The AFM study showed that the dimensions of flakes of graphene oxide in the substrate plane were in the range from 400 to 1 700 nm, the height of the flakes was 1–15 nm. The surface of the particles was characterized by a RMS roughness of 0.5–0.9 nm, contained protrusions with a height of 1–3 nm and a size in the flake plane of 50–90 nm. The study of samples by the SRM method showed that the contact resistance when the probe passed particles of graphene oxide increased from 80 MΩ to 3 GΩ and the spatial distribution of the contact resistance being homogeneous (Fig.3). According to the morphology of the surface and the nature of the distribution of local electrical conductivity, the graphene oxide obtained by the chemical method is close to the layers of graphene oxide formed by the LAO method at high voltages.
СONCLUSIONS
It is established that at the initial stages of formation of graphene oxide by the LAO, island surface oxidation occurs with the formation of areas with a reduced local electrical conductivity. Between these areas the structure of the top layer of graphite is preserved. With more intense oxidation, the surface roughness increases, and a decrease in the electrical conductivity is observed for the whole modification region. The morphology and local electrical conductivity of the obtained graphene oxide layers are close to those parameters for the graphene oxide obtained by chemical method.
The study was carried out with the financial support of the RFBR in the framework of scientific projects No. 16-33-00866 (O.Sinitsyna), 17-52-560001 (G.Meshkov) and 16-29-06290 (I.Yaminsky). ■
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