It has also been reported that altering the morphology of a graphene sheet by introducing curves or wrinkles can tune these properties. In this way, we modified the morphology of graphene by applying an external magnetic field. We also demonstrated a microfluidic device that can be turned on and off by magnetic manipulation of graphene.
Although the mechanical properties of graphene show very high values, the measured response at the macroscale is much lower than the theoretical and experimental values at the microscale. In Chapter II, we attempted to determine the nature of graphene's mechanical performance by identifying defect-containing regions to determine their precise effect on mechanical performance using a "Push-to-Pull stage" microelectromechanical system inside a scanning electron microscope.
Graphene manipulation using magnetic tweezer
- Introduction
- Experimental method & Materials
- Materials
- Substrate preparation
- Graphene transfer and fabrication of graphene micro ribbon
- Attachment of iron pad
- Fabrication of microchannel in SU-8 photoresist
- Characterization
- Synthesis of PDMS sheet
- Releasing of Cu layer and graphene manipulation
- Result and Discussions
- Optical images of graphene array and the Raman spectra, SEM, and AFM
- Magnetization of iron patch and Neodymium magnet in magnetic tweezer
- Images of graphene’s behavior according to the rotation of magnetic field
- Conclusion and suggestions for further research
- References
Footage of the graphene ribbon lifting as the permanent magnet approaches the sample. When the direction of the magnetic field was changed by 90˚, the colloidal particles flew faster than before the door was opened. a) Picture of the newly constructed magnetic tweezers. Additional x-, y-, and z-axis actuators are placed in addition to the original magnetic tweezers system for more precise manipulation. a) Two different pathways for micro-scale sorting machine operation. a) Flow of 2 different particles before installing the micro-mixer and (b) expected flow using the mixer.
Mechanical test of graphene-fold using MEMS stage
Introduction
Single-layer graphene has been reported to have very high mechanical performance, but on only the micron length scale, as obtained by various experimental approaches. The authors of a study on the mechanical response to nanoindentation of micron-scale membranes reported Young's modulus (1.0 TPa) and tensile strength (130 GPa) values. 1] On the micron length scale, there is a high probability that the tested area may be defect-free, resulting in very high strength values.
When measurements are made on a macro scale (e.g. centimeter scale), as recently done by the Ruoff group using a new method, both the average values of the modulus at room temperature (793 GPa) and the breaking strength (3.4 GPa) of single-crystalline graphene were lower than the reported values at the microscale. 2] It is clear that at the centimeter scale of chemical-vapor-deposition (CVD)-grown graphene tested so far, there are defects (defects – essentially holes) that reduce the strength of the “ideal value”. While some may suggest that it is the grain boundaries in polycrystalline graphene that extremely limit its strength, this could be the possible case if the introduction of, for example, Thrower-Stone-Wales type defects (pentagon-hexagon pairs replacing hexagon-hexagon, due to to a bond rotation) the strength can change from 130 GPa to 70 GPa.
3, 4] It is also reported that the polycrystalline graphene's mechanical strength (3.33 GPa) is not much different from the single crystalline one (4.50 GPa). For the case of chemical vapor deposition (CVD) grown graphene, the discovery of the presence of parallel linear "folding" structures in the graphene was recently reported by the Ruoff group. 5] They appear as a result of the different thermal contraction between graphene and the copper foil, as the foil on which the graphene was grown at 1070 °C is then cooled to room temperature.
According to studies by other team members of the Ruoff group (Ming Huang, Da Luo, Meihui Wang), fold structures also contain broken regions (cracks). Manuscript in preparation] Such cracks in the graphene fold structure can lower the overall mechanical properties of the CVD-grown single crystal graphene - but it is not known whether these are the critical defects or not. We can estimate this by calculating stress concentration factor, which will be described in the following section.
Study of stress concentration factor
Application of stress concentration factor to the crack in the graphene fold structure
On the other hand, fracture is more likely to occur in the case of scheme 2b than in the case of scheme 2a. Manuscript in preparation] Therefore, based on this model, the fault caused by the discontinuous fold structure can be considered as a linear shape fault. Accordingly, based on this information about the crack, we can also investigate the stress concentration factor of the crack in the fold structure.
Assume that the length of a linear crack at the atomic scale is 20 nm based on Da Luo's research. Manuscript in preparation] From Equation (2), the load that graphene can withstand is 0.131 σc based on QFM. Although we set the value of σc to 130 GPa, which is the highest reported strength of pristine graphene [1], the load bearing capacity of the graphene sample is only 17 GPa, which is drastically lower than its intrinsic strength.
Therefore, we can think that the crack caused by the broken graphene fold structure could be a critical defect that could lower the overall mechanical properties of graphene. Based on the assumption and theoretical calculation, we could estimate the effect of the graphene fold structure defect on the overall mechanical property. We expect that this could be a critical flaw that could reduce the global strength of graphene.
In order to experimentally understand the effect of defects, we tried to locate the exact region of the wrinkles and perform tensile testing in well-defined regions at the micro scale. Although the nanoindentation method is guaranteed to measure the mechanical performance of graphene well, uniaxial tensile loading should be performed to control the direction of stress application and study the effect of cracking. The MEMS Push-to-Pull (PtoP) stage was provided by Professor Juyoung Kim's group (UNIST) based on Hysitron's PtoP stage design.
Transferring graphene with supporting material on the silicon wafer
- Transferring graphene supported by PMMA on the silicon wafer
- Transferring graphene supported by gold on the silicon wafer
10] Polymethyl methacrylate C4 (PMMA) was purchased from Microchem and the gold metal source was purchased from iTasco.
Loading the graphene-gold composite on the MEMS Push-to-Pull device
Model (a) was used to fabricate the dog bone specimen of the graphene-PMMA composite and type (b) was used for the graphene-gold composite. In the case of the graphene-PMMA composite, the same method as described for the graphene-gold composite was used, but since the folding structure in these samples cannot be observed, a random location was chosen to define the test region.
In-situ SEM micro-tensile testing of the graphene-gold composite
Characterization
Stress-Strain curve of graphene-PMMA composite
According to the curve in Figure 3, the measured Young's modulus was 7.72 GPa, the tensile strength was 1.1 GPa, and the sample was linearly elongated until 8% of the strain. 13] If the thickness of the single-layer graphene is approximated to 0.34 nm, we can extract the Young's modulus of the graphene using the mixing rule (equation (3)). According to the calculation, the Young's modulus and tensile strength of the extracted graphene is 2.97 TPa, which is much higher than the 1.0 TPa reported by James Hones' group.
That data is not effective at all because of the following two reasons; First, we have no pure PMMA film's modulus and breaking strength value in our platform, so we cannot use mixture rule with the obtained data. Because although the Young's modulus is an intrinsic property, the modulus value is changed due to the different geometry of the dog bone sample and proportion of defects according to the changed scale. However, the maximum reported Young's modulus of PMMA is 5.0 GPa which is too small (also for its thickness, and therefore more appropriate - for its contribution to the overall stiffness of the composite PMMA/graphene piece), so I write the anomalous high up Young's modulus value versus carbonaceous material deposited on sample in the region being loaded (the "gauge length").
15] If graphene breaks during tensile testing, the change in the slope of the stress-strain curve should be mostly in the strain range of around 0.8%, but nothing was detected. There is another weakness during the operation of the FIB system for cutting, loading and unloading the graphene-PMMA composite sample. Usually, the PMMA layer is on the top side of the sample, so the sample is very sensitive to the electron beam, which resulted in inaccurate production of the dog bone sample.
To avoid this problem, we also reversed the composition sequence so that the graphene was on the top surface. Although we could see the folding structure of the graphene, the PMMA layer under the graphene began to boil, which damaged the graphene. In this respect, gold fits these requirements because it is an electrically conductive metal and has an appropriate Young's modulus (a reported value of 4.43 GPa obtained from testing a 300 nm thick Au film) to extract the information of graphene. a) Rotational problem of graphene-polymer composite under SEM environment and (b) exploding of PMMA under graphene in the reversed sequence.
Stress-Strain curve of gold film and graphene-gold composite
Intrinsic limitation from the instrument
- Poor resolution of SEM
- Broad deposition of platinum and carbon
During the filling of the graphene-gold composite on the PtoP device, carbon and platinum deposition were used. When the sample was picked up, carbon deposition was used to adhere the sample to the microprobe, and platinum was deposited on each corner to attach the sample to the PtoP device. However, the principle of deposition using FIB is that the metal or coal gas (in the case of our instrument (FEI Helios 450s), naphthalene gas) is blown onto the sample so that the deposited area is much wider than the intended area.
This can lead to contamination of the measuring length and consequently inaccurate measurement of mechanical properties due to changes in sample thickness. According to the literature, a certain type of 100 nm thick carbon carbon film had a reported Young's modulus of 400 GPa [17], and platinum was reported to have a Young's modulus of 116 GPa [18], so only 100 nm thick films of these materials would mean that the rule of thumb methods the mixture cannot be used to obtain the mechanical properties of graphene. In addition, the graphene fold structure cannot be found in the in-situ tensile strain SEM because the resolution is insufficient.
If the graphene fold structure were visible in the higher resolution instrument, it would be possible to monitor the area of the graphene that collapsed during tension.
Conclusion and suggestion for further research
Oh, Y., et al., MICRO/NANO-MECHANICAL TESTING SYSTEM USING PUSH-TO-PULL TRANSFORMER TENSILE TEST HOLDER U.S. Jin, S., et al., Colossal grain growth yields single-crystal metal foils by contactless annealing. Park, S.-Y., et al., Nanolaminate of metallic glass and graphene with improved elastic modulus, strength and ductility in tension.
Abbas, K., et al., Nanoscale size affects the mechanical properties of platinum thin films and cross-sectional grain morphology.
