www.sciencemag.org/content/346/6213/1092/suppl/DC1

Supplementary Materials for

Dynamic mechanical behavior of multilayer graphene via supersonic

projectile penetration

Jae-Hwang Lee,* Phillip E. Loya, Jun Lou, Edwin L. Thomas*

*Corresponding author. E-mail: leejh@umass.edu (J.-H.L.); elt@rice.edu (E.L.T.)

Published 28 November 2014, Science 346, 1092 (2014)

DOI: 10.1126/science.1258544

This PDF file includes:

Materials and Methods Supplementary Text Figs. S1 to S7

Table S1 Full Reference List

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Materials and Methods Preparation of MLG, gold, and PMMA membranes

Highly ordered pyrolytic graphite (HOPG) was mechanically exfoliated with Scotch™ tape (Fig. S1A). Water-soluble polymer (Clear School Glue™, Elmer’s) was applied and dried to form a 0.3 mm thick film as a temporary substrate (Fig. S1B). After detaching the Scotch tape, several additional exfoliation steps were performed with new tapes until sufficiently thin multilayer graphene (MLG) films were made (Fig. S1C). A non water soluble, liquid adhesive (Scotch Super™ 77, 3M) was diluted five times with toluene and the diluted adhesive was applied to a transmission electron microscopy (TEM) grid. Excessive adhesive was removed by blowing air. After evaporation of the toluene, the now sticky TEM grid was attached to the MLG surface on the thicker, water-soluble substrate film (Fig. S1D). The TEM grid and the now attached MLG -water-soluble composite film was floated on water (Fig. S1E) and the substrate film completely dissolved after 12 hrs at room temperature. After rinsing the sample with deionized water, water on the MLG sample was removed without damage by surface tension of water by touching it on to lens cleaning paper (Fig. S1F).

For free-standing gold samples, a positive tone photoresist (AZ9260, MicroChem) was spin-coated onto a microscope cover glass at 4000 RPM and baked at 110°C for 3 min. The photoresist was flood exposed to ultraviolet light (λ=366nm) to make it soluble to an inorganic developer (AZ 400K 1:4, Clariant). Various thicknesses of gold,

5214 ≤≤ h nm, were then deposited onto the photoresist with a sputter coater (Desk V Sputter System, Denton) and a sticky TEM grid was also attached on it (Fig. S1G). The entire sample was immersed in the developer for 12 hrs to dissolve the photoresist layer (Fig. S1H), and was rinsed with deionized water. Free-standing gold membrane sample was available after removing water with the lens cleaning paper (Fig. S1I).

Poly(methyl methacrylate) (PMMA) of an average molecular weight of Mw~120 kg/mol (Sigma-Aldrich USA) was dissolved in toluene at three different concentrations, 2, 4, and 8 wt% to control the final membrane thickness. The PMMA solution was spun onto a 0.3 mm thick water-soluble polymer film made of the dried paper glue (Clear School Glue™, Elmer’s) at 4000 RPM and then dried for 12 hrs under vacuum. After attaching the sticky TEM grid (Fig. S1J), the entire sample was floated on the surface of deionized water (Fig. S1K). Free-standing PMMA membrane sample was available after removing water with the lens cleaning paper (Fig. S1L).

Optical setup of the α-LIPIT and preparation of µ-bullets Fig. S2 shows the key optical elements of the advanced laser induced projectile

impact test (α-LIPIT) experimental setup. An individual µ-bullet placed on a substrate is selected and aligned to the near focal point of laser ablation using a digital microscope system with camera 1 (Stingray F-146C, 1388x1038 pixels, color) employing illumination from a light emitting diode (LED) (λ=505 nm). Camera 2 (Stingray F-146B, 1388x1038 pixels, monochrome) with a long-working distance objective lens (10x M Plan Apo, NA=0.28, Edmund Optics) is triggered to start image acquisition by a digital delay generator (DG535, Stanford Research Systems). While camera 2 is waiting for the probe pulses, the delay generator successively triggers the Q-switching of two Nd:YAG

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pulsed lasers (Quanta-Ray INDI-40-10, Spectra Physics) with and without a second harmonic generator for the laser ablation (λ=1064 nm) and probe pulse generation (λ=532 nm), respectively. The time difference between the pulses from the two lasers was controlled by the delayed triggering for the Q-switching of the two lasers. A circular pinhole (0.5 mm dia.) was used to make a circular beam profile of the 1064 nm pulse. The 1064 nm laser pulse was generated and focused to the back side of the gold film through the substrate, which led to the ablation of the gold. When the ablation gas/polydimethylsiloxane (PDMS) accelerated the µ-bullet toward the sample, the other Nd:YAG laser was triggered to produce a green laser pulse (λ=532 nm). The initial green laser pulse (dark green line) was first split into two by a polarizing beam splitter and the transverse magnetic (TM)-polarized transmitted pulse (green line) was delayed by propagating around an optical path (10.4 m) in air. The delayed pulse was additionally split again by the beam splitter and a fraction of the pulse (bright green line) is run around the optical delay path once more to make a third pulse. As a result, the three separate pulses were directed to the dye laser cavity for the optical pumping of the gain medium (Rhodamine 640 in ethanol) with time gaps of 34.5 ns defined by the optical delay path. Three light pulses are mainly created by amplified spontaneous emission (ASE) were then used to illuminate the moving µ-bullet and three successive transmission images were recorded by camera 2. As the light pulses created by ASE were highly incoherent compared to the original green pulse, this significantly reduced the laser speckle in the micrograph.

A two-part PDMS kit (Sylgard 184, Dow Chemical) was used with a mixing ratio of 10 to 1 and was spun onto a 50 nm gold coated microscope cover glass (25mm diameter, No. 2). After thermal curing (60 °C for 3 hrs), the physical thickness of the PDMS layer was 20 µm. 3.7 µm diameter solid silica spheres (microParticles GmbH) were mixed with ethanol without modification and a drop of the sphere suspension applied to a PDMS/gold coated cover glass. A strip of lens cleaning paper contacted the droplet and was pulled in one direction to spread the spheres and to prevent clustering. Since the gold film allowed negligible transmission (< 1%) of the ablation laser pulse during the entire ablation time (~10 ns), the µ-bullet was protected from heating by the energetic laser pulse while the gold layer still allowed enough transmission of cyan light (λ=505 nm) for aiming the µ-bullet. Thus, the µ-bullet was indirectly accelerated by the expanding elastomeric PDMS bubble without direct thermal contact to the vaporized gold and the target sample remained uncontaminated from the deposition of any debris as the elastomeric film was not ruptured.

Determination of the thickness of a MLG membrane and thickness inhomogeneity An optical micrograph of a MLG membrane was taken with monochromatic light

(λ=531 nm) prior to α-LIPIT (Fig. S3A). At the same conditions, reference and dark-noise images were also taken. After penetration, another optical micrograph of the MLG membrane was taken to identify the impact region (Fig. S3B). Using the three images of dark-noise, reference, and the sample, a map of transmittance was calculated. The transmittance map was converted to a thickness map by using the relation of transmittance vs. thickness (Fig. S3C). Due to the thickness inhomogeneity, the thickness

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of the penetrated MLG membrane was calculated by taking the average of the thicknesses within the direct impact region given by .

The thicknesses of the gold membranes were also determined via the thickness-dependent transmittance of gold.

Possible grain boundary effects AFM based nanoindentation experiments on monolayer graphene with precise

placement of the tip onto grain boundaries show that the fracture forces are lowered 8-41% depending on grain boundary angles and the resultant fractures show low correlation to the grain boundaries (10,11). Moreover, since sA is smaller than the average grain size, our penetration energy data is likely independent of grain boundary influences.

Potential conditions of a micro-projectile which may affect the results Damage and deformation of a projectile can affect the *

pE values. In this study, we did not recover the silica spheres to examine for damage. However, in our previous study (7), we impacted thick block copolymer samples with the same silica spheres, and each sphere was stopped within a few micrometers. Using a focused ion beam to cross section the target and embedded sphere, this allowed a SEM study of many captured spheres. It was quite rare to find a cracked sphere even though impact speeds were similar and higher (0.5 to 1.5 km/sec) to those used in this study. While bulk graphite is much harder than the copolymer, the likely damage to the sphere during the penetration of an extremely thin freestanding MLG membrane will be less than that for the thick copolymer impact because the force exerted on the sphere, which can be estimated as the kinetic energy change/penetration time, is considerably lower in the case of the MLG target. Thus, we do not consider damage to the silica sphere due to the penetration event. In addition, we did not observe break-up of any of the silica spheres after MLG penetration in our high-speed photographs.

The temperature of the sphere may increase due to air friction. It is very difficult to directly measure the actual temperature of the fast moving micro-projectile just before the moment of impact. However, we have used the same projectiles to impact the lamellar block copolymer as mentioned above. The target material did not show any evidence of thermal degradation in the impacted region as the block copolymer reverted to its initial lamellar morphology upon annealing at 150 ⁰C (see the supplementary information (7)). When considering the additional temperature rise due to adiabatic compression of the block copolymer, the temperature of the projectile at the impact moment should be lower than the degradation temperature of the polymer (approximately 300 ⁰C). Because graphite is stable at this range of temperatures, we do not consider the effect from the temperature of the projectile on our results.

Limitation of the one-dimensional (1D) approximation for the strain and strain rate

We estimated the maximum tensile strain, maxε , and the strain rate, maxε based on a simple 1D model in which the mass of the local volume element responding to the

D

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propagating stress waves is fixed ( hdrρ ), where dr is a line element. However, for a 2D membrane, the mass of local volume elements responding to propagating stress waves increases linearly as a function of the distance ( r ) from the impact origin ( )2( rdrh πρ ). Therefore, the actual radial strain profile is highly concentrated around the impact center and decreases faster than what the 1D model predicts. The 1D approximation therefore underestimates the actual values. This may explain why the estimated maximum strain value (6%) is close to the lower bound of the reported failure strain range, 5-25%. The strain rate jumps initially at impact and then is maintained at a constant level falling at the end of the event.

Nature of penetration through a thin free-standing membrane Penetration mechanism(s) can vary significantly depending on the thickness and

boundary conditions of the target. The thickness of a MLG membrane is significantly thinner than the diameter of a projectile and the membrane is unsupported except at its periphery. Moreover, the thickness is too thin to consider a propagation of compressional waves. Therefore, the entire deformation is essentially caused by tensile stresses. Due to the tensile-dominant deformation, the Hugoniot equation of state, in which a compressional shock wave can propagate faster than speed of sound, is not relevant for the radially propagation tension waves.

Penetration features of MLG membranes Optical micrographs of MLG membranes were taken with monochromatic light

(λ=531 nm) after α-LIPIT experiments to identify the penetration areas. Fig. S4 and S5 show images of various thickness MLG membranes penetrated at the two different velocities, 600 and 900 m/s, respectively, comparable to the typical muzzle velocities of assault rifles such as M-4 or AK-47.

Penetration features of gold membranes For comparison, penetration of polycrystalline gold membranes ( h =14, 24, 52

nm) were studied (10 events for each thickness at iv =900 m/s). As gold is significantly heavier ( Auρ ~ 19.3×103 kg/m3) and also has a much lower Young’s modulus ( AuE ~79 GPa), the speed of sound in gold is substantially slower ~2.0 km/s, about one tenth of

inplanev of graphene. The observed features of gold membranes contrast well with those of the MLG membranes. The penetration holes in gold membranes are hardly wider than the cross sectional area of a µ-bullet and short cracks propagated in straight radial directions (Fig. S6C) which reflects the isotropic nature of the gold membrane arising from the polycrystallinity (grain size < 50 nm) of the sputter coated gold (Fig. S6B). In contrast to the MLG membrane, the gold membrane at the impact region was completely lost by plugging penetration mode with small petals (Fig. S6D). The short crack distance indicates that the penetration was highly localized. Due to insufficient mechanical connectivity arising from gold clusters at h ~10 nm or less (30), the thinnest gold membrane ( h =14 nm) showed a relatively poor mechanical strength compared to the

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thicker membranes. Thus, pE of gold was estimated to h12.0 from experiments on the two thicker membranes.

Microscopic ballistic test of PMMA membranes The penetration behavior of isotropic glassy amorphous PMMA membranes (

=42, 102, 352 nm) was also studied (10 events for each thickness at =930 m/s). The density and Young’s modulus of PMMA are ~ 1.19×103 kg/m3 and ~3 GPa, respectively and its corresponding speed of sound is ~1.6 km/s, similar to that of gold. The penetration holes of the PMMA membranes were similar to the diameter of a µ-bullet (Fig. S7B) and radial cracks were observed. For the 352 nm thick membrane showed wider opening but no radial cracks were observed. Similar to the gold membrane, the impact region was completely lost. As the thickness increases, the slope of tends to increase slightly due to higher plastic deformation. We used the slope for thinner samples and its corresponding pE for PMMA was estimated as h0067.0 .

Comparison of specific energies of penetration The penetration results are sensitive to various experimental parameters, such as the

shape and hardness of the projectile, the impact speed ( ), and the ratio of projectile diameter to target thickness ( hD / ). We searched the literature for suitable comparisons (see Table S1) of our microscopic results with macroscopic ballistic results for which the projectile had a spherical or a hemispherical shape with negligible deformation due to the large ratio of hD / and the high projectile strength. Among the materials, the Kevlar armor is a layered composite made of Kevlar KM2 fabric and polyvinyl butyral. Therefore, the energy delocalization performance of MLG, 2X higher than the Kevlar armor, is significant. An armor system based on rigid ceramic plates can perform well under compression-dominant loading which is only possible when the plates have a substantial thickness or when there is an addition backing material behind the ceramic. It is probable that a thin ceramic membrane in a free-standing form would not perform better than the MLG membrane.

hiv

PMMAρ PMMAE

kE∆

iv

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Fig. S1. Schematic of sample preparation method for free-standing MLG, gold, and PMMA membranes.

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Fig. S2 Optical setup for α-LIPIT for imaging the projectile after penetration of the MLG film. The CCD camera 1 for alignment of a μ-bullet and a target sample; CCD camera 2 for high speed imaging. In the high-speed imaging sequences, camera 2 stays in open shutter status and only the light having wavelengths close to 620 nm can reach its CCD due to the band pass filter. After accelerating the μ-bullet by laser ablation using the infrared laser (λ=1064 nm), three 620 nm light pulses are created from optical pumping of the dye laser cavity excited by three green laser pulses. To make three consecutive green laser pulses, the initial green laser pulse (dark green line) from the laser (λ=532 nm) is successively split into three pulses by circulating along the 10.4 m optical delay path. The time difference among the three pulses is given by the single circulation time of light for the optical delay path. The high-speed illumination using the three red light pulses arriving at the different times, gives consecutive transmittance optical images of the moving μ-bullet recorded in the same frame of camera 2.

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Fig. S3 (A) An optical micrograph of a MLG membrane prior to the α-LIPIT experiment. (B) An optical micrograph of the MLG membrane after the experiment. (C) A map of thickness with the area of penetration (solid line). The thickness inhomogeneities for the two circular areas are given by coefficient of variation (CV) values where the areas are defined by the maximum crack distance ( maxL ) and the radius of a µ-bullet (D/2).

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Fig. S4 Transmittance optical micrographs of free-standing MLG membranes penetrated at 600 m/s. Red triangles point to the penetration holes.

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Fig. S5 Transmittance optical micrographs of free-standing MLG membranes penetrated at 900 m/s. Red triangles point to the penetration holes.

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Fig. S6 (A) Reflective optical micrographs of gold membranes penetrated by a µ-bullet at = 900 m/s. (B) A typical electron diffraction pattern of an undamaged polycrystalline gold membrane. (C) SEM image of a 24 nm thick gold membrane (D) Tilted view of the penetration hole of gold shows plug formation and radial cracks shorter than those of MLG. (E) Energy dissipation plot as a function of thicknesses.

iv

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Fig. S7 (A) Reflective optical micrographs of PMMA membranes penetrated by a µ-bullet at = 930 m/s. (B) SEM images of penetrated PMMA membranes viewed at different angles show no folding and very limited radial cracks. (C) Energy dissipation plot as a function of thickness. The linear relation with , again suggests the primary contributor to kinetic energy loss is just the kinetic energy transfer to the target material.

iv

h

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Target parameters Projectile parameters Impact speed range (vi)

References

PMMA h = 4 – 6 mm

ρ = 1,190 kg/m3

Spherical steel D = 7.8 mm m = 2.05 g

607 – 641 m/s (19)

Aluminum h = 1.27 mm

ρ = 2,700 kg/m3

Spherical steel D = 12.7 mm

m = 8.42 g 151 – 284 m/s (20)

Aluminum h = 1.27 mm

ρ = 2,700 kg/m3

Spherical steel D = 6.35 mm

m = 1.05 g 201 – 853 m/s (20)

Aluminum h = 1 mm

ρ = 2,700 kg/m3

Hemispherical steel D = 19 mm

m = 47 g 92 – 115 m/s (21)

304 stainless steel h = 0.4 mm

ρ = 7,800 kg/m3

Spherical steel D = 8 mm m = 2.0g

176 – 592 m/s (22)

304 stainless steel h = 3 mm

ρ = 7,800 kg/m3

Spherical steel D = 12.5 mm

m = 8.4 g 480 – 997 m/s (23)

Kevlar/PVP composite h = 4.7 mm

ρ = 1,400 kg/m3

Spherical tungsten carbide D = 12.7 mm

m = 16 g 298 – 422 m/s (24)

Table S1 Parameters of macroscopic ballistic tests with 1/ >hD for the different materials compared in Fig. 4C.

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