Controlled, Reversible, and Nondestructive Generation of Uniaxial

May 28, 2014 - ABSTRACT: Theoretical calculations have predicted that extreme strains (>10%) in graphene would result in novel applications. However, ...
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Controlled, Reversible, and Nondestructive Generation of Uniaxial Extreme Strains (>10%) in Graphene H. Hugo Pérez Garza,*,† Eric W. Kievit,† Grégory F. Schneider,‡ and Urs Staufer*,† †

Department of Precision and Microsystems Engineering, Research Group of Micro and Nano-Engineering, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands ‡ Leiden Institute of Chemistry, Faculty of Science, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands S Supporting Information *

ABSTRACT: Theoretical calculations have predicted that extreme strains (>10%) in graphene would result in novel applications. However, up to now the highest reported strain reached ∼1.3%. Here, we demonstrate uniaxial strains >10% by pulling graphene using a tensile-MEMS. To prevent it from slipping away it was locally clamped with epoxy using a femtopipette. The results were analyzed using Raman spectroscopy and optical tracking. Furthermore, analysis proved the process to be reversible and nondestructive for the graphene.

KEYWORDS: Graphene, extreme strains, tensile-MEMS, femtopipette, clamping, graphene applications, van der Waals adhesion is used to fix the graphene to the substrate. Even though the van der Waals adhesion between graphene and its substrate is considered to be strong,26 it has been noticed in all those methods that it is still not strong enough for extreme strains. Furthermore, in most of these experiments where the graphene has been placed as a suspended membrane ready to be strained,11,18,19,22 it has been observed that the graphene slips away from the surface when the van der Waals adhesion is overcome due to a tensile force.18 For that reason, proper clamping would dramatically increase the chances to achieve higher strains. In this paper, thus, we demonstrate the controlled, reversible, and nondestructive generation of uniaxial extreme strains in graphene. We have used an in-plane tensile device, inspired by the work of Espinosa et al.,27 in order to induce uniaxial strain on the graphene. The designing requirements (Figure S-1), analytical estimations (Figure S-2), finite element analysis (Figure S-3 to S-5) and the microfabrication process (Figure S-6) are explained in Supporting Information. The device consisted of two suspended shuttle-beams bridged by the graphene (Figure 1a). The shuttle-beams were aligned to and movable along a single axis. One of the shuttles, namely the thermally actuated shuttle (TAS), could be actively pulled using a thermal in-plane microactuator (TIM), while the other, namely, load shuttle (LS), was fixed to calibrated springs, which allowed measuring

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echanical deformation of graphene offers a tempting prospect of controlling its properties by strain, which opens the path for new applications. Different calculations have predicted that achieving extreme strains (>10%) would unlock the door for novel opportunities.1−7 These include increasing the absorption properties of graphene,8 allowing diffusion through the center of the ring,2 increased formation of bonds onto the graphene surface,9 hydrogen storage,10 novel applications in nanoelectronics devices,11 enhancement of terahertz generation of photonic crystals,12 creation of local gauge fields and even altering graphene’s band structure,13 band gap opening,14 electron−phonon coupling, and pseudo magnetic fields.15,16 Nevertheless, none of these applications have been fully demonstrated because significant strains have not yet been experimentally achieved in a controlled way. As a result, several efforts have been taking to experimentally reach the extreme strains that theoreticians have advised.11,17−23 These efforts include thermal strain of graphene ribbons,17,20 nanoindentation (pressing the membrane with an AFM-19 or diamond-24 tip), straining a suspended membrane by pressure difference (bulge test),25 transferring graphene onto a flexible substrate (i.e., PDMS) and put the substrate under bending-strain,21 straining graphene with dielectric nanopillars,11 and inducing stress by the epitaxial growth of graphene on SiC.23 However, to our knowledge the highest strains reported that have been attained in a controlled, reversible and reproducible way range from 0.6 to ∼1.3%.11,17,23 This is far beyond the theoretically required value to explore the predicted applications. In all © XXXX American Chemical Society

Received: May 6, 2014 Revised: May 28, 2014

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Figure 1. MEMS based in-plane tensile device. (a) A schematic is shown, depicting the main components of the device. (b) SEM picture of the microfabricated device. The parts colored in red highlight the thermal actuator and the blue color indicates the two shuttles. (c) The working principle of the TIM relies on an applied power that creates a displacement. When voltage is applied in between the contact pads, current flows through the thermal beams from one side to the other, generating joule heating that creates an expansion of the beams, inducing a displacement of the TAS. This results in a pulling effect on the graphene, which also pulls on the LS.

the force pulling on it. The graphene was stressed when the TIM was actuated, creating a tensile pulling effect. An SEM picture of the fabricated device is shown in Figure 1b. The microactuator relied on the principle of thermal expansion because of a temperature change ΔT. As a voltage difference was applied across the pads (Figure 1b), a current flowed through the thermal beams from one pad to the other. The high current density in the thermal beams caused joule heating and thermal expansion. The thermal beams were connected to the shuttle at a slight angle such that their expansion caused linear motion of the TAS. The graphene flake was then transferred using the method developed by Schneider et al.28 To start with, the graphene was first mechanically exfoliated (Figure 2a) onto a silicon oxide substrate. When the flake of interest was identified, the chip was coated with a hydrophobic polymer (Figure 2b). The chip was then dipped in water at a slight angle, allowing the water to penetrate the interface formed in between the hydrophobic polymer and the hydrophilic oxide. Because of the differences in surface energies the polymer came off the substrate (Figure 2c), peeling the graphene along with it. When the polymer was floating on the water, a micromanipulator was used to hold it (Figure 2d) while the MEMS device (placed at the bottom of the beaker filled with water) was aligned relative to the position of the graphene (Figure 2e). Once they were aligned, the water was pumped out of the beaker, decreasing the distance in between the polymer/graphene and the chip (Figure 2f). Once in contact, the polymer was left until it dried out and finally dissolved to release the graphene (Figure 2g). To prevent the graphene from slipping away when experiencing high tensile forces, it was clamped using two epoxy patches that were locally deposited along the two edges of the graphene. Once the epoxy is cured it can offer a strong adhesion, high mechanical strength, and good temperature and chemical resistance. For this experiment, the 353ND epoxy (EpoTek technology) was used, containing Bisphenol F as active component. To accurately dispense the epoxy, we have used a femtopipette (Supporting Information Figure.S-7a) previously developed in our group.29 The femtopipette, explained in the Supporting Information, has the capability of

Figure 2. Graphene transferring process. (a) After mechanically exfoliating and localizing the graphene of interest in a SiO2 substrate, (b) a hydrophobic polymer was used to cover the entire chip. When the chip was dipped in water, (c) the polymer peeled off from the chip due to its hydrophobic nature and brought the graphene along with it. The MEMS device was placed (d) inside the same beaker of water, while the polymer membrane containing the graphene was held with a micromanipulator. As the water was pumped out of the beaker, the (e) micromanipulator was used to accurately align the position of the graphene to the position of the shuttle beams of the device. When the water was fully pumped out, the (f) polymer came in contact with the device, ensuring the right location of the graphene. Finally, the (g) polymer was dissolved to end up with the graphene bridging both shuttles.

pipetting and locally dispensing minute volumes (typically in the range of femtoliters, 10−15 L, although under the right circumstances it can dispense volumes as small as few zeptoliters, 10−21 L) of different reagents. To further control the positioning of the femtopipette, it was mounted on a 4 degrees of freedom robot used for nanomanipulation B

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Figure 3. Local deposition of the epoxy. (a) When the tip of the femtopipette snaps in, a controlled amount of epoxy is dispensed. As a result, the process is repeated until the entire edge of the graphene is covered. (b) Because the fresh epoxy is dispensed before it fully polymerizes, the amine linkers (also called curing agent) have the opportunity to chemically bond with the −OH or −COOH groups that are located in the periphery of the graphene. This results in a strongly clamped graphene.

device. We found that the TIM could withstand a maximum power of 2.4 W before the thermal beams get plastically deformed, corresponding to a measured temperature of 850 °C. We started therefore analyzing the sample by optical tracking from which we extracted the original length of the flake, Lo = 22.7 μm. At a maximum applied force of 1.7 mN and based on the visual displacement of the beams, the stretched flake showed an elongation of ΔL = 2.8 μm, denoting an achieved strain of ε = ΔL/Lo ≈ 12.5% (Figure 4a). The stretching was then repeated while observing the in situ shifts of the graphene’s Raman spectra due to increasing strain. From the spectra, the G and 2D-bands were individually analyzed. It is well-known that when a monolayer graphene is under strain, the G-band shows splitting and the 2D-band redshifts.30,31 However, in the case of our multilayer sample splitting of the bands did not occur and the redshift was much smaller in comparison to what is observed in monolayers (Figure 4b,c). The increase in strain was coupled to a noticeable drop of intensity for the G-band, and a redshift from 1584.9 to 1581.8 cm−1, resulting in an average shift of −0.24 cm−1/% (Figure 5a). Additionally, it exhibited an average decrease of 4.4% in intensity per 1% strain. Correspondingly, its 2D-band revealed a pronounced decrease in intensity and reduced redshift, with an average of −0.50 cm−1/%, as a function of strain (Figure 5b). Its intensity decreased 3.3% per 1% strain (Figure 5c). The force needed to strain this graphene to 12.5% was 1.75 mN (Figure 5d). The experiments also revealed that our method is reversible and nondestructive as confirmed from Raman measurements. To demonstrate reversibility, the Raman analysis was

(Supporting Information Figure.S-7b). Once the robot positioned the femtopipette on top of the shuttles, it was slowly brought into contact with the edges of the graphene. After the tip snapped into contact with the surface, the epoxy was correctly dispensed. The process was repeated until the entire edges on both sides were covered (Figure 3a). The chemical bonding between the graphene, the epoxy, and the shuttle is represented in Figure 3b. The chemistry involved (Figure.S-8) is discussed in the Supporting Information. In order to start the straining measurements, the TIM was gradually heated by passing an electrical current through it. The electrical power was swept from 0 to 2.4 W (in steps of ∼200 mW) in order to induce an increasing pulling force on the graphene. For each step, an optical image and the correspondent Raman spectra were taken. The experimental setup (Supporting Information Figure.S-9) comprised a Horiba Labram HR Raman spectroscope and a probe station. The excitation source was 532 nm laser with a power below 0.1 mW on the graphene sample to avoid laser-induced local heating. A 50× objective lens (numerical aperture 0.75) was used to focus the laser to a spot size of around 1 μm. The device containing the clamped graphene was positioned under the laser spot of the Raman such that the TIM could be simultaneously actuated using two probe needles. A digital voltage source, Hameg HMP2020, was used to heat the TIM. For the experiments, we used a multilayer (three-layer graphene) sample. The determination of the number of layers is shown in Supporting Information Figure S-10. Before stretching the transferred/clamped sample, we did some preliminary experiments to find the mechanical limits of our C

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Figure 4. Strain analysis of the graphene sample. (a) The applied power gradually increased from 0 to 2.4 W, generating forces of up to 1.75 mN to reach a strain of 12.5% on the 3LG sample. For each step, the respective optical image was taken. Similarly, the Raman spectra of the strained sample was recorded. Both the (b) G-band and (c) 2D-band were individually analyzed.

Figure 5. Raman analysis of the graphene sample. (a) The G-band showed a very small Raman shift, as opposed from monolayer graphene where the shift is more pronounced. Similarly, the (b) 2D-band of our sample remained almost static. Although no splitting, nor considerable shifting was observed, we did notice a clear affinity to decrease the intensity as a function of strain. During straining, (d) the maximum applied power to reach the ε = 12.5% was 2.4 W, which corresponded to an experienced force of 1.75 mN.

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Figure 6. Reversibility and nondestructive behavior. (a) To demonstrate reversibility, Raman analysis was performed by observing the in situ shifts of the graphene’s spectra due to increasing strain. When the highest strain was reached, the process was repeated in the reverse order (from maximum strain to relaxed state) and the obtained spectra was the same. Furthermore, the Raman analysis of the TIM beams was performed for each of the steps to measure the temperature increase as the actuation power increases. (b) To find out the temperature reached by the TIM, the laser spot was placed on top of the heating beams during actuation. Similarly, the measurements were repeated by placing the laser spot on the end of the shuttle, where the graphene sits, in order to quantify the temperature experienced by the graphene during straining. (c) These results confirmed that at its maximum displacement, the microactuator experienced a temperature of up to 850 °C. However, the presence of the heat sinks prevented the graphene from experiencing such high temperatures. (d) Instead, the graphene experienced a temperature not higher than 38 °C when the maximum power was applied. This ensured that the process was nondestructible for the graphene, nor for the epoxy clamps.

strongly held the edges of the graphene flake and prevented it from slipping away during stretching. We believe that the key to achieving extreme strains relied not only on the mechanical performance of our device but also on the very robust and strong clamping provided by the epoxy. We attribute this to two main factors. On one side, it has been found that the instability of the carbon atoms located on the edge of the graphene often results in the formation of dangling bonds.33 Furthermore, it has been shown that almost all dangling bonds are terminated by foreign species.34 Therefore, the dangling bonds that were in the periphery of the graphene may have been easily terminated by −OH or −COOH in ambient conditions,33,35 taking into account that the graphene was mechanically exfoliated in air. On the other side, these functional groups are precisely similar to those found in the epoxy groups. As a consequence, the amine linkers that were used to promote the polymerization of the epoxy resin could also covalently bond with the hydroxyl or carboxylic groups that were all over the periphery of the graphene. Therefore, there was a chemical reaction that allowed the graphene to chemically bond to the epoxy resin via the hydroxyl groups on the boundaries. Even though graphene has the ability to conform well to a given substrate, it seemed that the graphene was only partially covering the upper part of the peaks formed within the silicon. We believe this was due to the considerable height difference in between the peaks and valleys as measured from the surface roughness (Supporting Information Figure S11). As a result, there were suspended regions of graphene hanging in between the peaks, leaving the depth of the valleys uncovered. Therefore, when the epoxy was locally dispensed, it could have squeezed in between the graphene and the valleys of

performed in the reverse order, meaning that the spectra obtain from maximum-strained to relaxed-state was compared with the original spectra as a function of increasing strain (Figure 6a). The obtained measurements for both cases (ascending and descending strain) were the same, that is, no hysteresis was found. To experimentally demonstrate the high temperatures experienced by the TIM and to confirm that those high temperatures were nondestructive for the graphene and the epoxy, different Raman measurements were also carried. When the temperature of the polycrystalline silicon that makes up the TIM increased, a downshift in the Raman spectrum was expected.32 This shift in the wavenumber is related by ω − ωmeasured T = Tambient − ambient Cstokes (1) where T is the material temperature, Tambient is the reference temperature, ωambient is the reference Stokes peak location (wavenumber), ωmeasured is the measured Stokes peak location at elevated temperature, and CStokes is the material constant for polycrystalline silicon (CStokes = −0.0232 ± 0.0002 cm−1/C). As seen in Figure 6b, the measurement points were located at the heating beams of the TIM and close to the edge of the shuttle, where the graphene and the epoxy clamps sit. On the basis of eq 1 from the previous section, the Raman results in Figure 6c showed that the heating beams experienced a maximum temperature of 849 °C at a maximum applied power of 2.4 W. On the other side, the redshift of the sample’s spectra is neglectable in comparison to that of the thermal beams. The maximum experienced temperature by the suspended graphene was 38 °C (Figure 6d), consequently, the epoxy was also never compromised by the heat, resulting in persistent clamps that E

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the surface and filled the empty regions located below the suspended parts of the graphene. This resulted in a doublesided coverage of the graphene enforcing the clamping. Moreover, the epoxy adhered strongly to the polysilicon due to the robust Si−O−C bonding that was formed. Also, the roughness increased the surface area thus enhancing the contact area between the polysilicon and the epoxy. This represented an additional mechanical anchoring that resulted in an intensified adhesion. During stretching, the experienced tensile strain differentiates between the actual strains on the three bonds in graphene (known as the C3v group-symmetry), which are depicted as 1, 2, and 3 in Supporting Information Figure.S-12. The magnitudes and energies of the strain of each C−C bond should vary with the relative direction between strain and crystal orientation. Thus, it is expected that mechanical strain induces phononband splitting.36 On the basis of the well-established bond order-length-strength (BOLS) correlation theory,36−39 we were able to estimate the individual elongations of the bonds in a layer of our sample and the resulting force constant. This theory proposes to use a strain coefficient, λ, and the force constant of graphene k = 6.283 N/m to define the straininduced redshift and band splitting. Introducing λ and k is more convenient and physically meaningful than using the Gruneisen parameter alone.36 There are a couple of extreme situations defined by the angle (θ = 0° or θ = 30°) shown in Supporting Information Figure S-12. In the first extreme situation, when θ = 0°, the strain is along bond 2, ε1 = ε3 = λε2 < ε2, where λ = 0.31 for the upper branch and λ = 1.0 for the lower branch. This means that bonds 1 and 3 are elongated by 31% compared to bond 2 when strain is along that bond. By using 0.142 nm as the length for C−C bond in graphene36 under relaxed state, it would mean that bond 2 reached a length of 0.159 nm for ε = 12.5% achieved in the experiment reported here, while bonds 1 and 3 reached 0.147 nm. The other extreme situation, when strain is perpendicular to bond 3, at θ = 30°, the strain is ε1 = ε2 > ε3 ∼ 0 and therefore there should be a branch that retains the original frequency as ε3 ∼ 0. We believe that was the case for the sample reported here, because no significant redshift for the 2D-band was observed. Additionally, the three different scattering paths for electrons and holes between the Dirac cones at the K and K′-points contribute equally to the intensity of the 2D-band. Consequently, if the symmetry is disturbed the intensity will decrease as a function of strain,40 which in our case was also observed. To conclude, we have demonstrated a reversible and nondestructive way to induce extreme strains in graphene. Furthermore, the system offered the possibility to tune and accurately reach the desired strain in the suspended graphene. Such control was based on the displacement of the actuated beam due to the applied electrical heating power. To the best of our knowledge, this constitutes the first time that such extreme strains (>10%) were achieved. We consider that the key to accomplish such strains relied not only on the strong forces and the long displacements provided by our MEMS-device but also on the robust clamping given by the epoxy patches. We anticipate that our method can influence electronic properties of graphene very significantly as it breaks the equivalence of sublattice, thus creating a bandgap opening at K-point of Brillouin zone. Furthermore, our experiment can become the starting point for many of the predicted applications where ε > 10% is required. Additionally, strain engineering is also triggering major attention for other two-dimensional materials

like boron nitride41 and molybdenum disulfide42 for which our system and methodology will be of high relevance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information contains the full description on the design, analysis, and fabrication of the MEMS device. It shows the femtopipette device and how it was mounted on a microrobot for extreme accurate positioning during the dispensing of the epoxy required for clamping the graphene. The chemistry involved behind the epoxy clamping is further described. The experimental setup and extra measurements to assess the mechanical behavior of the device are also shown. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (H.H.P.G.) [email protected]. *E-mail: (U.S.) [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Professor Dr. Ernst Sudholter for the very useful discussions to understand better the chemistry behind the clamping method using the epoxy on graphene. The technical staff of the DIMES Technology Center and Kavli Nanolab at TU Delft are also acknowledged. This work was supported by NanoNextNL, a micro and nanotechnology consortium of the government of The Netherlands and 130 partners. Similarly, the research leading to these results received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/ 2007-2013)/ERC Grant Agreement 335879.

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ABBREVIATIONS TIM, thermal in-plane microactuator; TAS, thermally actuated shuttle; LS, load shuttle; SEM, scanning electron microscopy; AFM, atomic force microscopy

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REFERENCES

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