An inexpensive system for the deterministic transfer of 2D materials

After the isolation of graphene and other two-dimensional (2D) materials in 2004–2005 [1, 2], the development of the transfer methods to deterministically place 2D materials at specific locations with high accuracy is one of the most importants breakthrough in 2D materials research [3–8]. In fact, these deterministic transfer methods have made possible, the fabrication of artificial materials by the stacking of dissimilar 2D materials on top of each other in what is called nowadays van der Waals heterostructures [9–15].

The implementation of experimental setups for the deterministic placement of 2D materials, however, typically requires costly optical and mechanical components, hampering their implementation in labs with modest budget and their use in science education and public demonstrations. Although, previously, some of the authors of this work reported all the details to build up a transfer setup with an approximate cost of 7000–8000 € (much cheaper than commercially available systems or conventional transfer setups based on retrofitted metallurgical microscopes or probe stations) [8] this cost can be still a big handicap for the applicability of the system. Here, we report all the details to install a fully functional deterministic transfer setup at a total cost under 900 € and with a very compact footprint. The performance of this system is illustrated by placing a few-layer thin InSe flake bridging two electrodes.

Figure 1(a) shows a picture of the presented system to transfer 2D materials. The system is basically composed of a zoom lens with coaxial illumination, a XY + rotation manual stage (sample/substrate stage) and a XYZ manual stage (stamp stage). The zoom lens is supplemented with a 21-megapixel digital camera with HDMI output and all the components are mounted on a magnetic breadboard. The manual stages are attached to the breadboard through magnets glued at the base of the stages. Figure 1(b) shows a close-up picture of the sample and stamp stages where it is shown how the stamp is mounted. We employ a rectangular piece (5 mm by 10 mm approx.) of Gel-Film (WF x4 6.0 mil, by Gel-Pak^®) as viscoelastic stamp. Unlike in our previous work, in which we used the PF Gel-Film, we now use the WF Gel-Film that has the polydimethylsiloxane (PDMS) -based gel material bonded to a flexible and quasi transparent backing polyester substrate. The Gel-Film stamp is glued to a glass slide with Scotch tape, leaving most of the stamp overhanging (protruding from the glass slide as shown in figure 2(c)). We have found that this 'cantilever-like' geometry of the stamp is more advantageous for the deterministic placement of 2D materials than the stamp geometry used in our early work in [8]. Then double-side tape (Scotch^® Restickable Tabs) is used to fix the glass slide to the stamp stage and the sample to the sample stage.

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Table 1 summarizes all the different parts needed to implement the system and the reader will find a thorough step-by-step guide to assemble it in the supporting information is available online at stacks.iop.org/JPMATER/3/016001/mmedia. A list of consumables required to use the deterministic transfer system is shown in table 2.

In order to illustrate the potential of this inexpensive transfer setup, figure 2 shows how one can use this deterministic transfer setup to fabricate a 2D material based field effect transistor. The process starts by exfoliating a bulk crystal of a layered material (in this case a synthetic bulk crystal of InSe, an n-type semiconductor) with Nitto SPV224 tape. The crystals adhered to the surface of the tape are then transferred to the surface of the Gel-Film stamp by gently pressing the tape against the Gel-Film surface and suddenly peeling off the tape. Then, the surface of the Gel-Film stamp is inspected under an optical microscope to find the flake that the operator wants to transfer. Note that the stamp can be inspected under the zoom lens imaging system of the transfer setup to identify the flakes if a metallurgical microscope is not available. See the supplementary material for a comparison between the image quality obtained with the zoom lens system of the transfer setup and with a metallurgical microscope.

As a target substrate we use a SiO₂/Si substrate (285 nm of SiO₂) with pre-patterned electrodes (Ossila^®) separated by 30 μm (step 1). The InSe flake that we want to transfer can be found under the inspection of the zoom lens imaging system by focusing on the surface of the stamp (step 2). The stamp is brought closer to the surface of the sample and the flake is aligned with the electrodes employing the XYZ stamp stage micro-positioner (step 3). The stamp is gently lowered until it contacts the surface of the sample (step 4). Then the stamp is slowly peeled off (steps 5 to 7) until it is completely removed and the InSe has been successfully transferred bridging the two electrodes (step 8). The peeling off process takes place by lifting up the Z-micromanipulator of the stamp stage at an approximate speed of 0.25–0.50 mm min⁻¹. Note that these values are a rough indication as the precise lifting speed is usually adjusted by inspecting the peeling off process with the imaging system during the lifting.

Another widespread use of systems for the deterministic placement of 2D materials is the fabrication of van der Waals heterostructures. In figure 3 we demonstrate that our inexpensive deterministic transfer system can be also used to fabricate van der Waals heterostructures by fabricating a fully encapsulated InSe flake between two hexagonal boron nitride (h-BN) flakes. Figure 3(a) shows the sequence of transfer. First, a h-BN flake is transferred in the middle of a pre-pattern cross-hair marker structure (left column). Second, a InSe flake is transferred onto the bottom h-BN flake (middle column). Finally, another h-BN flake is transferred sandwiching the InSe flake between h-BN sheets (right column in figure 3(a)). Figure 3(b) shows the resulting van der Waals heterostructure.

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Finally, we demonstrate how one can use this inexpensive transfer setup to fabricate not only simple devices nor isolated van der Waals heterostructures but also fully-encapsulated electronic devices. Hexagonal boron nitride (h-BN) is an atomically flat insulator with a very low density of Coulomb scattering centers and thus by sandwiching a 2D material between two layers of h-BN provides a very clean dielectric environment preserving the intrinsic electronic properties of the encapsulated material. Because of this, the best performances have been reported for electronic devices based on 2D materials encapsulated between h-BN layers [16, 17]. figure 4 shows the steps needed for the assembly of a fully encapsulated InSe device using our transfer setup. A bottom narrow h-BN flake is transferred bridging two pre-patterned gold contacts, then a long InSe flake is transferred on top of the h-BN flake protruding and contacting the two gold contacts. Finally, a large h-BN flake is transferred on top. The result is a gold contacted InSe device, fully encapsulated between h-BN flakes.

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Conclusions

Acknowledgments