Investigation of Mechanical and Tribological Properties of Polyaniline Brush by Atomic Force Microscopy for Scanning Probe-Based Data Storage

In this study, the mechanical and tribological properties of polyaniline (PANI) brush are investigated for ultrahigh density data storage system based on atomic force microscope (AFM). A 30-nm-thick PANI brush film is formed on an Au surface by the intermediary of a self-assembled monolayer. The average Young’s modulus of the brush is successfully evaluated to be 3.8 GPa by force-deformation curve measurement. And main cause of the friction force between conductive Pt/Ir-coated tip and the brush in the tip scanning is found to be not adhesion force but the roughness by lateral force microscopy. Moreover, it is demonstrated that the PANI brush has better wear resistivity compared with a spin-deposited PMMA film as the recording medium. The good wear resistivity is attributed to the configuration of the polymer brush without physically-absorbed polymer chains. [DOI: 10.1380/ejssnt.2013.53]


I. INTRODUCTION
Thus far, scanning multiprobe data storage systems based on atomic force microscope (AFM) technology have been developed as one of the future high-density data storage systems with a recording density of 1-10 Tb/inch 2 , as shown in Fig. 1 [1][2][3][4][5][6][7][8].For this system, the contact mode operation is preferable for practical application because this mode needs no complex circuits such as lock-in amplifier, which results in the simple and small storage systems.For putting this system into practical use, one of the most important issues to be solved is the tip wear of the probe.In particular, when the tip is scanned on a hard recording medium composed of inorganic materials, it is worn very easily.The tip wear directly leads to decrease in the bit size and degradation of the reliability of the storage system.
Hence, the various solutions for this problem have been proposed.For example, a conductive-diamond probe with a superior wear resistance has been developed [9].However, diamond is usually deposited at high temperature (> 800 • C), which is not compatible with integration processes of on-chip electric circuits.It is reported that introduction of a thin water layer at the tip-medium interface as a lubricant is effective to prevent from the tip wear [10].But precise humidity control is required in this method.
As one of other solutions, use of a polymer film as the recording medium has been proposed [11][12][13].The flexibility and elasticity of the polymer film are expected to absorb the excess mechanical energy inducing the wear.In this method, a spin-coating technique has been usually employed to fabricate a uniform thin polymer film because of its simplicity of the process.For instance, IBM research group has demonstrated on thermomechanical recording on spin-coated thermoplastic-polymer films such as polymethyl methacrylate (PMMA) by local heating with a heater-equipped probe [14,15].However, the polymer chains in the spin-coated film are usually aggregated due to only the physically interaction among the chains without chemical bonding.Thus, although the tip wear is reduced, the medium is often worn easily.The medium wear particularly becomes a serious problem in the thermomechanical recording.In addition, in this recording, the data transfer rate is speculated to be limited to approximately 1 Mbits/s due to the thermal response time of the heating element [16].The low data transfer rate is a disadvantage of the thermal recording system.Moreover, this system requires the heated AFM tips of which fabrication process is not so easy.
Therefore, we have proposed the simple electrical recording system based on the conductivity change of a conductive-polymer brush film.
In our previous study [17], the reversible conductance switching and dots pattern definition on a polyaniline (PANI) brush was successfully demonstrated by applying an appropriate voltage on the local area with a conductive AFM tip.In this case, the PANI polymer chains are chemically bonded on a gold surface by the intermediary of a self-assembled monolayer (SAM) of 4-aminothiophenol, as shown in Fig. 2. Thus, its wear resistivity is expected to be higher than that of the conventional spin-coated film.The elastic property of the polymer brush is also expected to reduce the tip wear.Hence, this recording medium should reduce the wears of both the tip and medium.In addition, the conductivity change involved with the redox-state change of PANI can be quickly caused due to the electrochemical reaction induced by the voltage application with the tip.Therefore, the electrical recording can probably provide the higher data transfer rate compared to the thermal recording.Moreover, this recording principle is very simple and requires no complex cantilever structures.
However, to the best of our knowledge, the elasticity and wear resistivity of the brush films composed of conductive polymers such as PANI have not been yet investigated comprehensively.Also the frictional property, which significantly influences on the wear, has not been investigated enough.Investigation of these properties is very important to develop the strategy for fabrication of the practicable conductive-polymer recording medium.Therefore, in this study, we evaluate the elastic and tribological properties of the PANI brush on nanometer scale by using AFM.In addition, the good wear resistivity of the brush as the recording medium is empirically demonstrated by wear test.

II. PREPARATION OF A PANI BRUSH ON AU SURFACE
A PANI brush was prepared by surface-graft polymerization, as illustrated in Fig. 2. At first, Ti, Pt and Au films with thicknesses of 20, 60 and 250 nm were formed on a Si substrate by sputter deposition, respectively.Then, the substrate was immersed into an ethanol solution of 1 mM 4-aminothiophenol for 30 h at room temperature.As the result, the SAM of the 4-aminothiophenol was built on the Au surface, which became a binding material between PANI polymer chain and Au surface.Then, the polymerization of PANI was conducted to the SAM by oxidative polymerization of aniline by immersing the substrate in 1 M HCl solution containing 0.15 M aniline and 0.1 M (NH 4 ) 2 S 2 O 8 at 0 • C for 20 h.As a result, the PANI polymer brush was formed on the Au surface.In this brush, many PANI chains were physically absorbed on the surface.Thus, the substrate was immersed in a concentrated ammonia water for dedoping of the PANI film.
After that, the physically absorbed chains were removed in N-methyl-2-pyrrolidinone using an ultrasonic bath and rinsed in deionised water.Finally, the PANI brush was doped by immersing in a 1 M HCl solution, rinsed in deionised water, and dried.In this study, a 30-nm-thick PANI brush film was obtained by repeating this polymerization sequence three times.The surface roughness of the PANI brush was investigated by AFM, as shown in Fig. 3. Compared to the Au-film surface before formation of the PANI brush, many large grains were observed in the PANI brush.The large roughness of the PANI brush was caused by aggregation of the polymer chains.

III. EVALUATION OF THE ELASTIC PROPERTY
The elastic property (i.e.Young's modulus) of the PANI brush was evaluated from force-deformation curves obtained by AFM measurement and theoretical contact model [18].A silicon cantilever with the spring constant of 7.2 N/m was used in this experiment.The loading force was set to 70 nN.The tip radius and the Poisson's ratio of the brush were presumed to be 20 nm and 0.5.The Young's modulus of PANI was speculated to be high and adhesion force was relatively small.The loading force was also small.Thus, considering the adhesion map proposed by Johnson [26] and the simplicity of the contact model, Derjaguin-Muller-Toporov (DMT) model was employed in this study [19].
A representative force-deformation curve to the PANI brush is shown in Fig. 4. The red circles represent the approaching curve.The solid line indicates the fitting result to the DMT model.The PANI brush was deformed about 2 nm at a maximum by applying the loading force.After the measurement, no indentation damages on the brush was observed, which means the brush was deformed not plastically but elastically.In this case, the Young's modulus was calculated to be 4.2 GPa.To visualize the distribution of the Young's modulus, AFM force mapping measurement was conducted.The detailed information about this measurement and analysis is described in the references [20].Figures 5(a  The average Young's modulus was roughly evaluated from the histogram to be 3.8 GPa.Considering that PMMA's value is approximately 3.3 GPa [21], the PANI brush was relatively stiffer than PMMA.But the stiffness differed only slightly.Thus, the PANI brush is expected to absorb the mechanical energy by its elastic deformation as well as the PMMA recording media used in the thermomechanical recording.And strong dependency of the elasticity on the position is seen in Fig. 5(b).The Young's modulus on the valley area among the grains tended to be relatively small compared with that of the top area.It might be attributed to the dependency of the polymer chain density on the position.The polymer chains on the top area are considered to be thickly aggregated.The grains were probably formed due to the strong aggregation of the polymer chains.

IV. INVESTIGATION OF THE FRICTIONAL PROPERTY
Then, the friction force between the PANI brush and Pt/Ir-coated tip was evaluated by lateral force microscopy (LFM) under ambient conditions.The Pt/Ir tip has small tip radius, good electrical conductivity and wear resistance.Thus, use of the tip is one of the best choices at the present time for the conductivity-change recording.The Pt/Ir-coated probe (spring constant: ∼0.2 N/m, tip radius: ∼20 nm) was purchased from NanoWorld.The friction force was calculated from the friction loop empirically obtained by LFM and approximation formulas described as follows [22].
Where F is the friction force, C t is the torsional spring constant of cantilever, d is the tip length, L is the cantilever length, P is the detector sensitivity of AFM equipment in the vertical direction, V is the deflection of output voltage of detector in the horizontal direction, G is the modulus of rigidity of silicon (0.5×10 11 [Pa]), W is the cantilever width, and T is the cantilever thickness.The dimensions of the cantilever and tip were determined by scanning electron microscopy.The thickness and influence of the Pt/Ir film on the torsional spring constant of the cantilever was ignored because the film was very thin.As comparative experiments, the friction forces of a PMMA film and a GeSbTe (GST) film were also measured.The GST film has been one of the candidates as the recording medum for phase-change recording [4,23].
A 30-nm-thick PMMA film was formed on a Si substrate by spin-coating.A 20-nm-thick GST film on a glass substrate was prepared by sputter deposition.The frictional loops of each sample in the loading forces of 10 and 50 nN are shown in Fig. 6.In this experiment, the tip was scanned with the velocity of 5 µm/sec.It can be seen that the dispersion of the friction force of the PANI brush was larger than those of other films.In addition, the average friction force of the brush was the largest of all.Figure 7 graphs the relationship between loading force and friction force of each sample.The friction force was proportionate to the loading force, which was subject to Amonton-Coulomb's law.The friction coefficients of the PANI, PMMA and GST films were evaluated to be 1.00, 0.60 and 0.34, respectively.
Next, the topography and friction images were simultaneously obtained, as show in Fig. 8.The scanning velocity and loading force were set to 1 µm/sec and 50 nN, respectively.In the PANI brush, the large friction force was generated at the edges of the grains, as shown in Fig. 8(a').It was caused by bump of the tip to the grain because the tip motion could not follow the rough surface in the scan speed.This result indicates that the local generation of the large friction force led to increase of not only average but also the large dispersion of friction force.Other samples were very flat compared to the brush.Thus, the local large friction force was not generated.
Then, the adhesion forces between the tip and the brush surfaces were measured by force curve measurement [24,25].Figures 9(a) and (b) show the topography and adhesion force images of the brush, respectively.The adhesion force at the valley area was larger than that of the top of grain, which was attributed to increase of the contact area at the tip-brush interface.As shown in the histogram of the adhesion force (Fig. 9(c)), the average adhesion force could be evaluated to be approximately 4.7 nN.The adhesion forces of the PMMA and GST films were also measured in the same manner as the PANI brush.The measured adhesion forces of these recording media are graphed in Fig. 10   value of the PANI brush was almost equivalent to those of other surfaces in spite of the fact that the large adhesion force was generated at the valley area.Hence, the measurement result indicates that the adhesion force of the PANI brush to the Pt/Ir tip is inherently small.Therefore, the large friction on the PANI brush was mainly attributed to the large roughness.The large friction is undesirable for the data storage application, which has the possibility to enhance the wear of not only the tip but also the media.The crash between the scanning tip and the PANI bump might lead to the significant wears even if the PANI has elasticity.But the friction force is expected to be further reduced by decreasing the roughness of the brush.The grain formation was attributed to strong interaction among PANI polymer chains.Thus, introduction of functional side chains such as alkyl group is expected to weaken the interaction and prevent from the crystalline aggregation.This method will be one of the effective strategies to fabricate a further smooth PANI brush.

V. WEAR TEST OF THE PANI BRUSH
Finally, the wear resistance of the PANI brush was investigated.In the experiment, the Pt/Ir-coated probe was scanned with the speed of 80 µm/s on the square of 20 µm 2 30 times in applying various loading forces.Then, the topographic change due to the wear was observed.As a comparison experiment, the same wear test of a 30nm-thick PMMA film prepared by spin-coating was also conducted.
Figure 11 shows the results of the wear tests.After the scanning with 250-nN loading force, the height of the PMMA film at the right-side rim of the scanned area remarkably increased, which is indicated by a red arrow in Fig. 11(a).The surface on the damaged area was significantly roughened, as shown in Fig. 12.It is speculated that the PMMA polymer chains were chipped off, pushed to the side, and piled up at the rim by the scanning tip.Also in case of 100-nN loading force, the small bump was observed at the rim, as shown in Fig. 11(b).In the scanning with 10-nN loading force, the topographic change was not observed (Fig. 11(c)).On the other hand, such height increase at the rim and roughening on the scanned area were not observed in the PANI brush, as shown in Figs.11(d) and (e).The height of the scanned area just decreased slightly.In the loading force of 250 nN and 100 nN, the height decreased in approximately 2.5 nm and 1.5 nm, respectively.The topography change was not observed in the 10-nN loading force as well as the PMMA film.
The difference of the wear behaviors between the PMMA and PANI brush films was due to the difference of the polymer-chain configuration in the films.The PMMA polymer chains, which were physically attached together, were easily separated the tip scanning.By contrast, the PANI polymer chains were strongly fixed with Au-S bonding.For cutting the bonding, the further large loading force is considered to be required.Of course, also larger force is needed to cut the PANI polymer chain.Thus, the PANI brush was not chipped off.That is, it has strong wear resistance.The slight dent might be caused by the plastic deformation due to compression of the polymer chains, as illustrated in Fig. 13.For the topographicchange recording, the chipping of the recording medium is the critical problem because the recorded data is lost by the surface damage.Meanwhile, in the conductivitychange recording, the slight topographic change is not so severe problem, and the recorded data is expected to be kept.Therefore, it could be demonstrated that the PANI brush is strong for the media wear.And, the use of the conductive-polymer brush as the recording media in the probe-based storage system is considered to be an effec-tive approach for the wear problem of the tip and media.

VI. CONCLUSIONS
In this study, we investigated the mechanical and tribological properties of the PANI brush as the recording medium for the scanning multiprobe data storage system.The average Young's modulus of the 30-nm-thick brush was successfully evaluated to be 3.8 GPa from the forcedeformation curve measurement by AFM.Then, the friction at the interface between the Pt/Ir tip and the brush was found to be mainly caused by not adhesion force but the surface roughness.Introduction of appropriate functional side-chains to PANI would form a further smooth brush, which is expected to provide the lower friction surface.In addition, it was clearly demonstrated that the PANI brush has better wear resistivity than the PMMA spin-deposited film.The good resistivity was owing to the inherent property of the brush which has no physicallyattached polymer chains.These results indicate that the conductive-polymer brush has the potential to be used as the conductivity-change recording medium which can reduce both the tip and medium wears.We believe that the knowledge obtained in this study becomes great help not only to develop the strategy for fabrication of the practicable recording medium but also expand the knowledge about the modification technology with polymer brushes.
Scheme of PANI brush prepared by surface-graft polymerization FIG.2: Scheme of PANI brush prepared by surface-graft polymerization.The polymer chains are bonded on a gold surface through a self-assembled monolayer (SAM).

FIG. 3 :
FIG. 3: Topographic images of (a) the Au-film surface before formation of the PANI brush and (b) PANI-brush surface.(Ra: arithmetic mean roughness) ), (b) and (c) shows the topography image, the Young's modulus distribution image, and the histogram of the Young's modulus, respectively.

Figure 4 .
Figure 4. Representative force-deformation curve on the PANI brush.The red circles Sample deformation (nm)

FIG. 6 :
FIG. 6: Friction loops of (a) PANI brush, (b) PMMA film and (c) GTS film.The left and right loops were measured by applying the loading force of 10 nN and 50 nN, respectively.
FIG. 8: Topography and friction images of the sample surfaces.The left side shows the topography images of (a) PANI, (b) GST, and (c) PMMA films.The right side corresponds to the friction images of (a') PANI, (b') GST, and (c') PMMA films.The scanning velocity and loading force were set to 1 µm/sec and 50 nN, respectively.

FrequencyFIG. 9 :
FIG. 9: (a) Topography image and (b) adhesion force image of the PANI brush.(c) Histogram of the adhesion force in (b).

Figure 10 .
Figure 10.Comparison of the forces of the PANI, GST and PMMA films.

FIG. 10 :
FIG. 10: Comparison of the adhesion forces of the PANI, GST and PMMA films.

Figure 11 .FIG. 11 :
Figure 11.Topographic images of the PMMA film and PANI brush after wear tests.FIG.11: Topographic images of the PMMA film and PANI brush after wear tests.The wear test results of the PMMA film with the loading force of (a) 250 nN, (b) 100 nN, (c) 10 nN.Those of the PANI brush with the loading force of (d) 250 nN, (e) 100 nN, (f) 10 nN.
a) Topographic images of the PMMA film after wear test with the loading force of 250 nN.(b) Topographic profile along A-B line in (a).
FIG. 13: Plastic deformation of the PANI brush due to comof the polymer chains.