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Graphene multilayer as nano-sized optical strain gauge for polymer surface relief gratings Giuseppe Di Florio, Erik Brundermann, Nataraja Sekhar Yadavalli, Svetlana A Santer, and Martina Havenith Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl502631s • Publication Date (Web): 22 Sep 2014 Downloaded from http://pubs.acs.org on September 28, 2014
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Graphene multilayer as nano-sized optical strain gauge for polymer surface relief gratings
G. Di Florio,a E. Bründermann , a N. S. Yadavalli ,b S. Santer b M. Havenith a*
a
Ruhr-Universität Bochum, Physical Chemistry II, Universitätsstr. 150, 44780 Bochum, Germany b
Universität Potsdam, Institute for Physics and Astronomy, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
[*] Ruhr-Universität Bochum, Physical Chemistry II, Universitätsstr. 150, 44780 Bochum, Germany. E-mail:
[email protected] Abstract In this paper we show how graphene can be utilized as a nanoscopic probe in order to characterize local opto-mechanical forces generated within photosensitive azobenzene containing polymer films. Upon irradiation with light interference patterns, photosensitive films deform according to the spatial intensity variation, leading to the formation of periodic topographies such as surface relief gratings (SRG). The mechanical driving forces inscribing a pattern into the films are supposedly fairly large, since the deformation takes place without photo-fluidization; the polymer is in a glassy state throughout. However, until now there has been no attempt to characterise these forces by any means. The challenge here is that the forces vary locally, on a nanometer scale. Here we propose to use Raman analysis of the stretching of the graphene layer adsorbed on top of polymer film under deformation in order to probe the strength of the material transport spatially resolved. With the
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well-known mechanical properties of graphene, we can obtain lower bounds on the forces acting within the film. Based upon our experimental results we can deduce that the internal pressure in the film due to grating formation can exceed 1 GPa. The graphene based nanoscopic gauge opens new possibilities to characterize opto-mechanical forces generated within photosensitive polymer films.
Keywords: Surface Relief Grating; Optomechanical Forces, Photosensitive Polymer Films; Multilayer graphene Deformation; Confocal Raman Microscopy
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Azobenzene-containing photosensitive polymer materials have attracted broad interest for applications such as data storage, diffractive optical elements, micro- and nanofabrication of complex periodic structures for plasmonic studies, and trapping of light in solar cells.1,2,3,4,5,6 Especially photosensitive polymer films exhibit a peculiar behaviour during optical irradiation with strongly inhomogeneous distribution in intensity or polarization at microscopic scales. The film topography supposed to be glass-like forms so-called surface relief gratings (SRG).7,8,9,10 Although this phenomenon is known since 20 years now, there is still no comprehensive model of the physical origin of the process. The puzzling question here is what kind of forces are generated within the polymer films during irradiation that could account for the reshaping of the glassy material without photo-induced softening. At the molecular scale this phenomenon is triggered through reversible photo-isomerisation of the azobenzene groups under irradiation. This in turn enforces alignment of azobenzene molecules according to the direction of the local electrical field.11,12,13 The opto-mechanically induced forces during the re-orientation of azobenzenes vary as a function of the position along the surface relief grating in both, magnitude and direction. The response of the polymer matrix, in which the azobenzene molecules are embedded, is a mass transport of the polymer materials from regions of maximum to those of minimum illumination intensities. The opto-mechanical stresses generated within the polymer film are expected to be at least larger than the yield point of the polymer, usually of the order of 50 MPa.14,15,16 However, this is presumably only a lower limit of the stresses that can potentially be build up during irradiation of azobenzene-containing polymer films. Recently, we demonstrated that even covalent bonds within the polymer chains can be ruptured, in addition to metal layers that were placed on top of the films.17,18,19,20,21,22 Metal films up to a thickness of several tens of nanometres are easily ruptured periodically over the entire irradiation area during surface relief grating formation. With standard models we are able to estimate the magnitude of the forces, which the polymer film exerts on the adsorbed metal layer put 3 ACS Paragon Plus Environment
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under tensile stress to several hundreds of MPa.21 While useful to illustrate the action of optically generated stresses in general, there is a lack of experimental methods to probe stress on the micrometer and nanometer scale. In a previous paper it was possible to map deformations of a graphene monolayer on a polymer composite by Raman spectroscopy.23 Here we introduce a method to use graphene multilayers as a sensitive nano-probe for displaying opto-mechanical action within photosensitive polymer films after formation of a surface relief grating. The position of the graphene/graphite G-band peak can be directly correlated to strain in the material:24,25,26 The frequency of the G-band phonon is frequency shifted under strain, resulting in a softening (“red” shift to lower frequencies) when the strain is tensile and in hardening (“blue” shift to higher frequencies) when the strain is compressive.24,25,26 The magnitude of the phonon frequency shift is found to be proportional to the strain in the material.24,25,26 Here we report on Raman spectroscopic measurements of the G-band of graphene multilayers (GML) placed on the surface of a surface relief grating. Based upon the measured frequency shifts of the G-band of grapheme after grating inscription of surface relief grating we can conclude that the local opto-mechanical stresses generated within the photosensitive polymer films corresponds to at least 1 GPa. Commercially available azobenzene-containing photosensitive polymer PAZO (pol{1-[4-(3carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl,sodiumsalt}) (Mn = 1.4104 g/mol) is purchased from Sigma-Aldrich. The polymer solution is prepared by dissolving PAZO in a mixture of 95% methoxyethanol and 5% ethyleneglycol. To prepare a polymer film of 1 µm thickness, a polymer solution of 250 mg/ml concentration is spin-casted onto a glass substrate (thickness ~130 µm from Carl Roth GmbH, Germany) at 7000 rpm during 1 minute.
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Graphene multilayers (G-ML) have been transferred onto the polymer film surface using mechanical exfoliation from high purity and highly oriented pyrolytic graphite (HOPG) purchased from MomentiveTM.27 Details of the experimental set-up used to inscribe surface relief gratings (SRGs) in the photosensitive polymer films have been reported elsewhere.28 Briefly, a continuous wave-diode pumped solid state laser (CW-DPSSL) operating with 50 mW output power at a wavelength of 491 nm is used (Cobalt CalypsoTM). The laser beam is expanded, divided in two beams by a nonpolarizing beam splitter and then collimated onto the sample. The polarization state of the laser beams is controlled with a set of half wave plates and polarizers. An interference pattern with 100 mW/cm2 intensity and ±45° beams polarization is generated on the surface of the photosensitive polymer in order to inscribe surface relief gratings with 4 µm periodicity. All atomic force microscopy measurements were performed using tapping mode Atomic Force Microscopy (NTEGRA, NT-MDT, Russia) with 1 Hz scan speed. We have investigated surface relief gratings (SRG) inscribed by optical beams in addition to graphene multilayers (G-ML) deposited prior to inscription on the polymer (SRG+G). The experimental details of the Raman microscope and measurements of polymer surface relief gratings have been described elsewhere.29 Briefly, a commercial confocal Raman microscope (model alpha300 RAS, WITec GmbH, Ulm, Germany) has been used. As a laser source we used a continuous wave diode laser at 785 nm, avoiding the azobenzene absorption band around 500 nm. The laser light is focused on the sample with a 100x/0.9 NA objective, the scattered radiation is collected by the same objective in a backscattering configuration and redirected into a monochromator. The Raman intensity is measured with a CCD camera. In this configuration the microscope provides a diffraction limited lateral resolution of ca. 500 nm and a spectral resolution of less than 2 cm-1 with a spacing between subsequent pixels corresponding to 0.5 cm-1. Raman images have been recorded using a step size of 5 ACS Paragon Plus Environment
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600 nm in x and y directions with a total scan area of 28 × 14 µm2 and a total integration time of 10 s per pixel. For further discussion we have visualized the results in two ways, putting the focus either on the integrated intensity or on position at the band centre in order to analyze strain specific frequency shifts. The centre of mass (CM) of a specific band is defined as:
CM =
∑xI ∑x
i i
i
i
,
i
where xi are Raman shifts and Ii the relative Raman intensities. An in-house written MATLAB script for multi-component analysis has been used to fit single spectra. We assumed Lorentzian lineshapes to deconvolute congested bands. An AFM micrograph of a graphene multilayer of 10 nm thickness adsorbed on 1 µm thick azobenzene-containing polymer layer is shown in Fig. 1. During irradiation the polymer topography is affected and forms a surface relief grating. Using our in-situ experimental setup we can study the evolution of the surface relief grating as a function of irradiation time.30 The height of the 4 µm grating increases gradually during irradiation until a maximum height of (850 ± 50) nm (Fig. 1) after 12 hours is reached, which corresponds to 85% of the polymer film thickness of 1 µm. After adsorption of graphene multilayer the maximum grating height is 500 nm (Fig. 1(a)). In order to estimate the change in surface area, we assume a sinusoidal profile: z ( x ) = Ao sin
2πx , where Ao Λ
and Λ are the amplitude and the periodicity of the SRG, respectively. The arc length S is given by: Λ
Λ
S = ∫ 1 + [z´(x)] dx = ∫ 2
0
0
2
4π 2 2πx 1 + A 2 cos dx . Λ Λ 2 0
Solving this integral numerically yielded a maximum increase in surface area of 10% for the nonconstrained polymer layer and 4% for polymer with graphene multilayer adsorbed (Fig. 1(a)). 6 ACS Paragon Plus Environment
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Figure 1. (a) AFM micrograph of the graphene multilayer piece on the polymer layer after inscription of the surface relief grating. The AFM cross-section profile of the naked polymer surface (black line) was recorded along the line marked by a white dotted line and the polymer/graphene surface by a red dotted line. The change in the surface area is denoted as S. (b) 3D view of (a).
The graphene layer is found to remain tightly bound to the polymer surface upon irradiation and subsequent deformation which is an indication of strong adhesion. The increase of surface area of the graphene multilayer by 4% is caused by the mechanical stress of the polymer upon adsorption. In the following we will show that the local strain can be quantitatively characterized by changes in the phonon G-band frequency of the graphene multilayer using Raman spectroscopy. In Fig. 2 we show the Raman spectrum of the photosensitive polymer film. The azobenzene groups can be either found in trans or cis configuration (for details of assignments see29,31). However after laser illumination the azobenzene groups relax back to trans configuration. In addition, we display the Raman spectra of a polymer film with adsorbed grappe multilayer comparing a flat polymer surface (graphene is unstrained), with the same plane afert inscription of a surface relief grating (SRG) (strained graphene). Since the adsorbed graphene multilayer is very thin (layer thickness less than 10 nm) it is possible to observe both, the graphene multilayer and the polymer (Fig. 2). Prominent vibrational modes of the polymer were selected for further evaluation (Fig. 2): νC-N at 1143 cm-1, νN=N at 1440 cm-1, and νC-C at 1586 cm-1. In the presence of graphene multilayers in the 7 ACS Paragon Plus Environment
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band noted as band at 1143 cm-1 in Fig. 2 two small additional peaks at 1139 and 1170 cm-1 appear. This is caused by the molecular interaction between the two materials at the interface, where azobenzene moieties are in tight contact with the graphene hexagonal structure.30 With unstrained graphene multilayer on top of the polymer an additional strong line is observed at 1581 cm-1. The peak position of this line is red-shifted to 1577 cm-1 for strained graphene multilayer. This peak, which is observed in all graphitic materials, is assigned to a first order Raman process of a doubly degenerate E2g phonon mode (G-band) at the centre of the Brillouin zone.32,33,34,35 The G-band vibration is characterized by an in-plane bond stretching of the carbon atoms in the hexagonal structure of graphene and graphite (Fig. 2 inset). Position of the 2D band of graphene (not shown) around 2700 cm-1 (not shown) should also be sensitive to strain in graphene. In the present study 2D band has been proved to be inadequate as a diagnostic signal for detecting strain in the multilayergraphene on polymer substrate. However, upon excitation with the laser at 785 nm, the signal in the 2D spectral region cannot be detected with a sufficient signal to noise ratio. Excitation by Raman lasers at a shorter wavelengths caused the deterioration of the polymer substrate during the measurement, particullary when using photon energies close to trans-cis photoisomerization.
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Figure 2. Raman spectra of flat bare polymer (blue), flat polymer with adsorbed graphene multilayer (G-ML) (red) and after imprinting of a surface relief grating (SRG+G, green). The inset shows the assignment of the most prominent lines of the graphene multilayer and azobenzene and the evolution of the G band line shape upon additional strain.
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Figure 3. (a) Optical image of the surface relief grating polymer with a piece of a deposited graphene multilayer. The investigated area is marked in red. (b) and (c) show the Raman intensity of the vibrational band at 1143 cm-1 and around 1580 cm-1, respectively. Plot of the center of mass at (d) 1143 cm-1 (νC-N), (e) 1440 cm-1 (νN=N), and (f) 1580 cm-1, the G-band and chromophore ring stretch region. The x-y dimensions of (b-f) are given in µm, the intensity color scales (b-c) are plotted in arbitrary units and the color scales (d-f) correspond to wavenumbers (cm-1).
In Fig. 3 we show (a) the optical image, (b-c) the Raman scattering intensity, and (d-f) the centre of mass of distinct vibrational bands of the polymer chromophore and the G-band. In Fig. 3(b) we display the scattered intensity of the surface relief grating at the νC-N vibration. The adsorbed graphene multilayer attenuates the signal (visible as a "shadow"). At 1580 cm-1 , both, the G-band and chromophore ring stretching vibrations will contribute to the observed signal (Fig. 3(c)). The center of mass of the νC-N vibration is red shifted by up to 2 cm-1, when the polymer film is covered 10 ACS Paragon Plus Environment
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with graphene multilayer. A red shift is observed independent of the position along the surface relief grating (Fig. 3(d)). This red shift of the center of mass frequency is attributed to an intermolecular interaction between the aromatic ring of the azobenzene-containing polymer in the upper surface and honey comb structured graphene in the lower surface of the graphene multilayer which affects the νC-N vibration and thus changes the overall lineshape .30 The central double bond νN=N at 1440 cm-1 of azobenzene is not affected (Fig. 3(e)). In Fig. 3(f) dark blue corresponds to the bare polymer film probing the aromatic azobenzene ring frequency νC-C centred at 1586 cm-1.29,31,36 Along the graphene multilayer flake the center of mass varies between 1579 cm-1 and 1585 cm-1 (Fig. 3(f)) and, in contrast to the νC-N vibration shown in Fig. 3(d), the frequency depends on the position along the grating. Since the G-band and the azobenzene ring deformation vibration νC-C overlap we used spectral deconvolution of the overall line shape to deduce the frequency dependence of the G-band peak with the position along the grating. As a reference for the unstrained G-band we recorded separate spectra of a graphene multilayer on a flat polymer sample, lacking surface relief grating inscription (Fig. 2). The G-band and the polymer vibrational bands have been fitted assuming Lorentzian line shapes (Fig. 4(a-c)).
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Figure 4. Multi-component spectral fitting at (a) top hills, (b) midslopes, and (c) bottom valleys of the surface relief grating. The chromophore vC-C peak is coloured in green and the graphene G-band in black. (d) Summary of the G-band shift for the respective positions on the SRG (a-c) in reference to unstrained graphene (see also Table 1).
Several spectra have been recorded in three distinctive areas of the surface relief grating, on top hill, midslope and valley positions, respectively (Fig. 4(a-c)). The band profiles have been analysed by means of multi-component spectral fitting. As a result we obtained a frequency of 1586 cm-1 for the νC-C chromophore peak. The variation in the peak position was less than 1.5 cm-1 (0.1%) and less
then 10% for the peak width, independent whether, the polymer film is covered or not covered with graphene multilayer. Figure 4(d) summarizes the results of the G-band peak positions along with
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the respective standard deviations at different positions of the surface relief grating in comparison to those of the unstrained graphene multilayer. For the unstrained graphene multilayer we recorded a peak position of (1581.6 ± 0.5) cm-1 (see Fig. 2 and Table 1), which is in good agreement with G-band centre frequencies (~1581 cm-1) reported previously.24,25,26,32,37 For all positions along the surface relief grating we observe a softening of the phonon G-band (Fig. 4(d)), resulting in a red shift of the peak. The morphology of the grating changes upon inscription. The relative stress in the polymer film is transferred to the graphene multilayer, which tries to adapt to the surface corrugations and consequently reflects the deformation. This causes a general tension on the surface of the film with variations at top hill and valley positions. The results for frequency shift ∆ν between the G-band of the graphene multilayer on the surface relief grating (SRG) polymer (νp) and the unstrained graphene multilayer (G-ML) (ν0) are summarized in Table 1. The C-C ring vibration of the chromophore νC-C at 1586 cm-1 is close to ν0 (1582 cm-1). This results in an increased error for the result of the fit for the peak position of ν0 (±0.5 cm-1) compared to the standard deviation of νp (±0.1 cm-1 to ±0.2 cm-1). However, the difference in ∆ν between top and bottom positions (-4.0 versus -5.4 cm-1) clearly exceeds the experimental uncertainty (see Table 1). SRG + G-ML
νp (cm-1)
σ (cm-1)
∆ν (cm-1)
ε (%)
p (GPa)
top, hill
1577.6
0.1
-4.0 ± 0.5
0.33 ± 0.04
0.85 ± 0.10
Midslope
1577.8
0.1
-3.8 ± 0.5
0.31 ± 0.04
0.81 ± 0.10
bottom, valley
1576.2
0.2
-5.4 ± 0.5
0.45 ± 0.04
1.15 ± 0.10
Table 1: Fitted peak position νp and standard deviation of the mean σ, frequency shift ∆ν = νp −ν0 compared to ν0 = (1581.6 ± 0.5) cm-1 of unstrained graphene multilayers (G-ML) on a flat polymer prior to inscription of a SRG. Shown is also the deduced values for strain ε (∆ν/ε = (-12.1 ± 0.6) cm-1/% 26), and pressure p (∆ν/p = 4.7 cm-1/GPa 38).
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Based on our experimental results we conclude that the strain in valleys of the surface relief grating exceeds the strain at other positions. This agrees with a prior observation that in 10-15 nm deposited gold on polymer film cracks were exclusively found in the valleys of the grating, indicating increased stress at the valley positions.21 Using ∆ν we can quantify the strain ε and the internal pressure p. For strained graphene, a linear relation between ∆ν and ε holds 24,25,26: For monolayer26 graphene a value of ∆ν/ε = (-14.2 ± 0.7) cm-1/% and for three-layer26 graphene a value of (-12.1 ± 0.6) cm-1/% has been reported. Here, for the multilayer graphene layer, we use the value for the three-layer graphene to deduce a lower limit of the strain in our probe (see Table 1). In case of uniaxial strain the G-band of a monolayer graphene is split into two sub-bands G+ and G− with a value of (-5.6 ± 1.2) cm-1/% and (-12.5 ± 2.6) cm-1/%, respectively, due to symmetry breaking.24 Therefore we have also tested the stability of our fit by assuming two lines, G+ and G− ,24 as well as a potential third contribution of unstrained graphene in the upper layer of the graphene multilayer. However, in all cases the best result could be achieved when anticipating a single, red-shifted Lorentzian curve for the G band. Observation of a single line is in agreement with the assumption of biaxial strain25,37 but is in contrast to uniaxial strain as proposed by other authors,24,25,26. We can try to correlate this strain (e.g. 0.45% at the SRG valley position) with the increase in surface area of the whole graphene multilayer (4% as deduced by the AFM measurement). In general, an increase in surface area can be caused either by relative shifts between adjacent graphene layers or by a stretching of the graphene rings. Only the latter process directly influences and dominates the phonon absorption spectrum probed by Raman spectroscopy and corresponds to internal strain within the graphene layers on the nanoscale. Furthermore the observed red shift ∆ν can be compared to experimentally observed red-shifts of the G-band due at a given, externally applied pressure.38,39 In previous work38 the G-band shift ∆ν of 14 ACS Paragon Plus Environment
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graphite under hydrostatic pressure p up to 14 GPa was determined with an initial slope of ∆ν/p = 4.7 cm-1/GPa. We use this value to estimate an internal pressure for the graphene multilayer/polymer SRG films investigated here. The results of about 1 GPa (see Table 1) are well above typical values for yield stress in glassy polymers,14,15,16 which is a necessary condition for mass transport in polymer films.21 We should point out that the recorded high stress values have to be attributed to photo-generated changes in a broader sense. This includes a local change in the fraction of the cis- and transisomers, but also local change in the orientation of the azobenezene chromophors along the surface relief grating. It is well known that after subsequent photoswitches the chromophores are expected to align preferentially perpendicular to while polymer chains tend to hinder a reorientation. 11,12,13,29 As a consequence, the electric field induces a local, spatially varying alignment of the azomolecules. In a previous publication29 we have characterized the orientation of azo-molecules in PAZO films at the free surface as well as inside the film using polarization Raman spectroscopy. Most probably the mechanical response on the macroscale is a multi-scale chain of physicochemical phenomena, including small scale motions of azo-molecules due to the local change in their chemical potential upon isomerization, the collective alignment of azo-molecules which are coupled to the polymer backbone, and finally the macroscopic mechanical response due to a change of elastic properties of the polymer, i.e. a mass transport.
In conclusion, we could show that small graphene multilayer flakes can serve as a nano-strain gauge in photosensitive polymer films. The sensitivity of the graphene/graphite G-band to deformations allowed us to characterize the strain at distinct positions: hills, slopes, and valleys in optically inscribed surface relief gratings. The measurement accuracy allowed us to distinguish positiondependent strain variations with an accuracy of 0.1% corresponding to a deformation difference of 0.4 nm in the focal spot of the confocal Raman microscope. The strain in the valleys is increased 15 ACS Paragon Plus Environment
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compared to the strain at other positions. We deduce a lower limit of the strain of 0.45% and an internal pressure exceeding 1 GPa in glassy polymers. The graphene multilayer nano-strain gauge allows for the first time to quantify the mechanical forces involved in light driven processes, e.g. during the formation of surface relief gratings in polymers. Thus Raman microspectroscopy of graphene based strain gauges provides a new tool for future applications and engineering of these important material systems. The authors declare no competing financial interest. Acknowledgment. The work is supported by the DFG priority program SPP-1369 (project A4). M. Mischo is acknowledged for her help in the early stage of this research, E. G. Sorrentino for help with the graphics, and D.A. Schmidt for helpful discussions.
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