(Invited) Direct Laser Induced Gold Wiring of 2D Materials with Sub-Μm Resolution
Authors: Braun Kai , Geladari Olympia , Eberle Martin , Schnepf Andreas , Meixner Alfred J. 
Published: February 2024

One of the most influential technologies of our modern world are integrated electronics. Almost all parts of our daily life include some bearing with any kind of electrical technologies. The fabrication methods itself, for integrated miniatured electrical devices have had not much innovation in terms of production. Several processing steps still mainly consist of different kinds of lithography. This situation may change with new challenges on the production of smaller quantities with specialized functions, or the use of temperature critical or atomically thin surfaces, where the thermal evaporation of metals damages the sample.
For some decades now, printing of all types of electronics have gained great attraction. While lithography is a time-consuming multi-step process (including resist preparation, illuminating, developing, evaporation and lift-off) with expensive and harmful photoresists and harsh environmental conditions (high-vacuum and thermal treatments), ink printing allows for direct and simple application of the desired structures [1]. Two main types of inks are typically applied: colloidal solutions of metal nanoparticles [2, 3] or metal-organic precursor solutions [4]. In both cases, thermal curing after deposition is required to achieve metallic conduction. To this end, laser writing is often advantageous over thermal sintering due to the overall lower thermal stress applied to the material, particularly on flexible, thermally sensitive substrates [5] whereas metal nanoparticles suffer from a relatively high photothermal stability and the need for a large laser power to achieve sufficient sintering [3]. In contrast, metal-organic precursor solutions feature a relatively low metal content (≤34 wt-%)10, and undergo large volume contractions during laser writing. These challenges have so far prevented the definition of metallic gold contacts with diffraction-limited feature sizes by ink printing and laser writing.
Here, we introduce atomically-precise, metalloid Au32(nBuP12Cl8) nanoclusters (Au32NCs) [6] as an ink for resistless, direct laser writing of metallic gold micro- and nanostructures. The unique advantages of this ink over Au nanoparticles or typical metal-organic Au precursors are their high photosensitivity in combination with a large gold content of 70 wt-%. This allows for writing gold structures with feature sizes of 250 nm and line spacings < 200 nm using 1.86 mW laser power at 488 nm with a scan speed of 0.1 mm/s. We apply this method on a wide range of substrates, including flexible polymer membranes, find metallic conductance with conductivities ~ 1 × 106 S/m and demonstrate that defining such Au contacts on two-dimensional semiconductors results in fully functional optical switches and field effect transistors made out from 2D materials.
[1] Raut NC. et al., Jour. Mat. Chem. C 2018, 6(7): 1618-1641.
[2] Ko E. et al., Nanotechnology 2007, 18(34): 345202.
[3] Hong S. et al., ACS Nano 2013, 7(6): 5024-5031.
[4] Schoner C et al., Thin Solid Films 2013, 531: 147-151.
[5] Skylar-Scott MA et al., PNAS 2016, 113(22): 6137-6142.
[6] Kenzler S et al., Angew. Chem. Int. Ed. 2019, 58(18): 5902-5905.

Figure 1