Camphor‐Enabled Transfer and Mechanical Testing of Centimeter‐Scale Ultrathin Films

Abstract

Camphor is used to transfer centimeter-scale ultrathin films onto custom-designed substrates for mechanical (tensile) testing. Compared to traditional transfer methods using dissolving/peeling to remove the support-layers, camphor is sublimed away in air at low temperature, thereby avoiding additional stress on the as-transferred films. Large-area ultrathin films can be transferred onto hollow substrates without damage by this method. Tensile measurements are made on centimeter-scale 300 nm-thick graphene oxide film specimens, much thinner than the ≈2 μm minimum thickness of macroscale graphene-oxide films previously reported. Tensile tests were also done on two different types of large-area samples of adlayer free CVD-grown single-layer graphene supported by a ≈100 nm thick polycarbonate film; graphene stiffens this sample significantly, thus the intrinsic mechanical response of the graphene can be extracted. This is the first tensile measurement of centimeter-scale monolayer graphene films. The Young's modulus of polycrystalline graphene ranges from 637 to 793 GPa, while for near single-crystal graphene, it ranges from 728 to 908 GPa (folds parallel to the tensile loading direction) and from 683 to 775 GPa (folds orthogonal to the tensile loading direction), demonstrating the mechanical performance of large-area graphene in a size scale relevant to many applications.

Ultrastiff, Strong, and Highly Thermally Conductive Crystalline Graphitic Films with Mixed Stacking Order

“…However, the intrinsic strength has been found only at the micrometer‐length scale. We recently used a new method to measure the mechanical strength of multi‐centimeter single‐layer graphene grown by chemical vapor deposition (CVD); the average strength of those macroscale samples was 4.5 GPa, and the average Young's modulus was 793 GPa . To date, macroscale samples made of only stacked and overlapped graphene platelets (no other constituents) had Young's moduli lower than 50 GPa and fracture strengths of around 100 MPa .…”

“…We found it was not possible to realize the transfer by the widely used “polymer‐assisted” wet transfer method as the high surface tension of the liquid (water or acetone) broke the film in the regions it was not supported by the substrate. Our recently discovered “camphor‐enabled” transfer method was instead used to transfer the film from Cu foil to the hollow graphite plate successfully as shown in Figure a . A solid camphor layer was first deposited on the SG400 film on a Cu foil as a support, and the Cu was etched away.…”

“…Due to the ≈35 nm thickness of SG2800 , the measurement of a freestanding macroscale film is challenging. A camphor‐enabled transfer method for tensile testing of large‐scale ultrathin films was recently invented by our group, and it was used to measure the properties of the thin SG2800 films. A polycarbonate (PC) film with thickness of 240 nm was used as a support and the details are in the Methods section (Supporting Information).…”

Substrate Engineering for CVD Growth of Single Crystal Graphene

“…As revealed by a Raman optothermal technique, [ 147 ] the bilayer graphene has a high thermal conductivity of ≈2300 W m −1 K −1 (Figure 13o). Its mechanical properties were characterized after being transferred using a camphor‐assisted method, [ 148 ] showing an average Young's modulus of 478 GPa and a fracture strength of 3.31 GPa. In addition, such AB‐stacked bilayer graphene film on single‐crystal Cu/Ni alloy substrate can be converted into a “diamond‐like” single‐layer carbon material (diamane) by fluorination.…”

Synthesis of Large-Area Single-Crystal Graphene

“…By patterning graphene field effect transistors with the device channel in the 'clean' regions between adjacent folds, high-performance devices with carrier mobilities of around 1.0 × 10 4 cm 2 V -1 s -1 have been achieved without the disturbance of adlayers, GBs, and folds, which are all found to decrease the carrier mobilities. Using the camphor-assisted transfer method, the mechanical properties of a centimeter-scale single-crystal graphene film were measured, revealing a Young's modulus of 728-908 GPa and an average fracture strength of 4.5 GPa [25]. Such a graphene film, without any GBs and adlayers, is essential for constructing 'stacked graphene single crystals' with controlled rotation angles and number of layers by a layer-by-layer assembly method, which we will discuss later.…”