An inexpensive system for the deterministic transfer of 2D materials

The development of systems for the deterministic transfer of two-dimensional (2D) materials have undoubtedly contributed to a great advance in the 2D materials research. In fact, they have made it possible to fabricate van der Waals heterostructures and 2D materials-based devices with complex architectures. Nonetheless, as far as we know, the amount of papers in the literature providing enough details to reproduce these systems by other research groups is very scarce in the literature. Moreover, these systems typically require the use of expensive optical and mechanical components hampering their applicability in research groups with low budget. Here we demonstrate how a deterministic placement system for 2D materials set up with full capabilities can be implemented under 900 Eur which can be easily implemented in low budget labs and educational labs.

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 important breakthrough in the 2D materials research. [3][4][5][6][7][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][10][11][12][13][14][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, some time ago 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.  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 closeup 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 where 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 Ref. [[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. 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. A list of consumables required to use the deterministic transfer system is shown in Table 2.   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 3a 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 3a). Figure 3b shows the resulting van der Waals heterostructure.

Conclusions
In summary, we presented an inexpensive system to transfer 2D materials that can be easily implemented in labs with low budget and educational labs and it can be used for public demonstrations. Moreover, we believe that this transfer system can also help to reduce the entry threshold to work in the field of van der Waals heterostructures. The whole system can be assembled for less than 900 € and the final system has a very compact footprint (easy to transport for educators and public demonstrations). We demonstrate that despite the low cost of the system, it has a functionality comparable to that of more expensive setups. In fact, we show how the system can be used to fabricate devices based on 2D materials and it could even allow a motivated physics teacher to build basic, yet functional devices like transistors or solar cells out of two-dimensional crystals, together with his class. Figure S1. Optical images acquired with the transfer setup imaging system before (a) and after (b) transferring an ultrathin MoS2 flake in the middle of the pre-patterned crosshair. Figure S2. Comparison of optical images of the same MoS2 flake shown in Figure S1, acquired with different imaging systems: (a) zoom-lens and camera used in this work (~500 €, WD = 20 mm), (b) Optem 7x zoom lens with 3x mini-tube and Canon EOS 1200D camera (~3100 €, WD = 32 mm) and (c) Motic BA 310 MET-T metallurgical microscope with AMScope MU18 camera (~4500 €, WD = 11 mm). Note that features smaller than 2-3 microns cannot be resolved with our low-cost imaging system and can be barely resolved with more expensive zoom lens systems (see the cracks and fold in the single-layer part of the flake indicated with a white arrow in (c)).