Patterning two-dimensional chalcogenide crystals of Bi2Se3 and In2Se3 and efficient photodetectors

Patterning of high-quality two-dimensional chalcogenide crystals with unique planar structures and various fascinating electronic properties offers great potential for batch fabrication and integration of electronic and optoelectronic devices. However, it remains a challenge that requires accurate control of the crystallization, thickness, position, orientation and layout. Here we develop a method that combines microintaglio printing with van der Waals epitaxy to efficiently pattern various single-crystal two-dimensional chalcogenides onto transparent insulating mica substrates. Using this approach, we have patterned large-area arrays of two-dimensional single-crystal Bi2Se3 topological insulator with a record high Hall mobility of ∼1,750 cm2 V−1 s−1 at room temperature. Furthermore, our patterned two-dimensional In2Se3 crystal arrays have been integrated and packaged to flexible photodetectors, yielding an ultrahigh external photoresponsivity of ∼1,650 A W−1 at 633 nm. The facile patterning, integration and packaging of high-quality two-dimensional chalcogenide crystals hold promise for innovations of next-generation photodetector arrays, wearable electronics and integrated optoelectronic circuits.

Inset: Enlarged image of 2D In2Se3 crystal array captured from the white box area of (a). Bi2Se3 arrays grown on exposed mica surface using the NaCl crystal template. f, Typical AFM image of 2D layered Bi2Se3 crystals grown on the mica substrate defined by the 2D NaCl crystal template. Optical images of 2D Bi2Se3 crystals before and after transfer, respectively. c-f, Optical images of 2D Bi2Se3 crystals transferred onto thin Au film, a soft PDMS matrix, S11 transparent PET plastic, a TEM grid, respectively. g, TEM image of 2D Bi2Se3 crystals on a TEM grid. h, TEM image of 2D In2Se3 crystals transferred onto a TEM grid.

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By finely tuning the growth conditions and taking advantage of the uniform crystallization and layer-by-layer growth mode of 2D Bi2Se3 crystals, we can readily prepare 2D Bi2Se3 arrays with domains of ~10 μm in size and a uniform thickness of as thin as ~2 nm. (Fig. 1d, Supplementary Figure 4) The r.m.s. roughness of the nanoplate surface shown in the green dashed square is as small as 0.091 nm, comparable to the roughness of freshly cleaved mica surface. This ultra-flat surface confirms the layer-by-layer growth mechanism.

Supplementary Note 4. Patterning of 2D crystals using an aqueous solution ink
We printed an aqueous solution ink of NaCl (~0.1 mol/L) to form an intaglio pattern containing heat-stabilized inorganic crystal imprint on mica using a PDMS stamp with relief structures (Supplementary Figure 8). This observation confirms the micro-intaglio printing process based on PDMS stamps. With the assistance of NaCl crystal template on mica, ordered array of 2D Bi2Se3 crystals was grown on exposed mica surface using the selective-area van der Waals epitaxy, which is faithfully reproduced from the relief structure of the PDMS stamp.

Supplementary Note 5. Faithful transfer of 2D crystals from mica onto arbitrary substrates
We have transferred the array of 2D chalcogenide crystals grown on mica substrates onto arbitrary substrates using a PMMA-mediated transfer technique. 2

Supplementary Note 6. Merging of aligned 2D crystal islands to a single crystal
Van der Waals epitaxy facilitates the growth of orientation-defined 2D crystals with six-fold symmetry on the surface of layered mica that has a pseudo-hexagonal structure.
Due to the symmetry matching, 2D hexagonal crystals with identical orientations were predominantly grown on mica substrates. After transferring of 2D crystals onto the TEM grid, we observed that the orientations of the 2D nanoplates' edge were predominantly at multiples of ∼60°, consistent with the 2D hexagonal lattice. From the extensive HRTEM and SAED analyses, we found that the discrete 2D hexagonal crystal islands were unidirectionally aligned and then merged to uniform single-crystal nanoplate rings with indicating that the photocurrent generation originated from the photoconductive mechanism. The spatial photocurrent map was recorded by a focused laser beam scanning over the whole photodetector under a fixed bias. It is distinct that the photocurrent was strongly generated in the whole 2D In2Se3 crystal channel, weakly in the two electrodes, which can attribute to the ohmic contacts between the In2Se3 channel and metallic electrodes. A stable and repeatable operation of dynamic photoresponse was observed in the 2D In2Se3 crystal. Supplementary Figure 17d presents the good photoresponse properties of the In2Se3 device with light illumination off and on. The photoresponse ratio (Ilight/Idark) is ~860.