Low-Temperature, Dry Transfer-Printing of a Patterned Graphene Monolayer

Graphene has recently attracted much interest as a material for flexible, transparent electrodes or active layers in electronic and photonic devices. However, realization of such graphene-based devices is limited due to difficulties in obtaining patterned graphene monolayers on top of materials that are degraded when exposed to a high-temperature or wet process. We demonstrate a low-temperature, dry process capable of transfer-printing a patterned graphene monolayer grown on Cu foil onto a target substrate using an elastomeric stamp. A challenge in realizing this is to obtain a high-quality graphene layer on a hydrophobic stamp made of poly(dimethylsiloxane), which is overcome by introducing two crucial modifications to the conventional wet-transfer method – the use of a support layer composed of Au and the decrease in surface tension of the liquid bath. Using this technique, patterns of a graphene monolayer were transfer-printed on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate and MoO3, both of which are easily degraded when exposed to an aqueous or aggressive patterning process. We discuss the range of application of this technique, which is currently limited by oligomer contaminants, and possible means to expand it by eliminating the contamination problem.

. Optical microscope image of a graphene monolayer transferred onto a PDMS stamp via the conventional wet-transfer method that uses a PMMA support layer.

Water
Graphene/support

(b)
PDMS stamp (hydrophobic surface) Water Blowing N 2 gas Figure S2. Importance of wetting of a substrate in determining the quality of a graphenesupport bilayer on the substrate: schematic illustration. (a) In the conventional wet-transfer case, sufficient wetting of a hydrophilic substrate by water leads to conformal contact between the graphene and the substrate without wrinkles when the sample is blow-dried using N2 gas. (b) When a PDMS stamp, whose surface is hydrophobic, is used instead of a hydrophilic substrate, water does not form a continuous layer between the graphene and the PDMS stamp. Consequently, a blow-drying process in this case causes water droplets trapped between the graphene and the PDMS to burst, damaging the graphene-Au bilayer. (c) In our method, the surface tension of the bath is decreased using a mixture of water and ethanol, which sufficiently wets the PDMS surface. As a result, the graphene-Au bilayer whose quality is comparable to that in the conventional wet-transfer is obtained on the PDMS stamp.
Note 1. Effect of ethanol. To quantify the possible contribution of ethanol in decreasing the sheet resistance (R sh ) of G EtOH-H2O , we performed the following experiment: (i) A graphene layer was transfer-printed onto a Si substrate coated with 300-nm-thick SiO 2 layer using our method.
(ii) The sample was annealed at 400 • C for 3 hr under H 2 and Ar at a total pressure of 90 mTorr, which eliminates ethanol molecules adhered to the graphene.
(iii) the sample was exposed to ethanol for 5 min, and subsequently dried using a N 2 gun and annealed at 40 • C for 4 hr. The sheet resistance values measured before and after Step (iii) using the van der Pauw method are 1334 and 1144 Ω/sq, respectively. Although the sheet resistance was slightly decreased due to exposure to ethanol, the magnitude of change is too small to explain the significant difference in R sh between G EtOH-H2O and G H2O .
Note 2. Effect of Au etchant. To demonstrate whether the Au etchant influences the quality of graphene or not, we performed the following experiment: (i) A graphene layer was transfer-printed onto a Si substrate coated with 300-nm-thick SiO 2 layer using our method.
(ii) The sample was annealed at 400 • C for 3 hr under H 2 and Ar at a total pressure of 90 mTorr, which eliminates possible dopants arising from the Au etchant.
(iii) The sample was exposed to the Au etchant for 5 min, followed by repeated thorough rinsing with clean water.
The R sh values measured before and after Step (iii) using the van der Pauw method are very similar to each other, 770 and 822 /sq, respectively. Figure S4. Importance of wetting of a substrate in determining the quality of a graphene-Au bilayer on the substrate: digital images. These are digital images of a graphene-Au bilayer scooped up from a bath with a PDMS stamp, as shown in Fig. 1(d). (a) When a water bath is used, water dewets the PDMS surface in several locations before a blow-dry process (left). After blow-drying the sample with a N2 gun, the bilayer has many wrinkles, in which water is trapped (center). Although mild annealing on a hot plate at 40 • C for 4 h decreases the heights and number of the wrinkles as it removes the residual water, the wrinkles cannot be completely eliminated (right). (b) In contrast, when the bilayer was scooped up from a mixture of water and ethanol, the mixture liquid forms a continuous lubrication layer between the bilayer and the PDMS stamp throughout the surface (left). As a result, N2 flow aiming at the center of the bilayer displaces the liquid outward, resulting in conformal contact between the bilayer and the PDMS, almost throughout the surface (center). Small number of wrinkles with smaller heights than those in (a) can be removed after heat treatment on a hot plate at 40 • C for 4 h (right).