Structural characterization. The structural and morphological properties of WSe 2 films, prepared the aSLcS process, were already reported in detail previously. Here, an optimization of the WSe 2 films with respect to the crystallographic properties was carried out by variation of the thickness ratio of the WSe 2 and the Pd-promoter layers, respectively. The XRD patterns of the films, shown in Fig. , indicate the presence of the main phase -WSe 2 and of a promoter phase -PdSe x . First, we optimized the WSe 2 film thickness at a constant Pd promoter thickness of 10 nm. Increasing the WSe 2 thickness from 40 nm to 100 nm leads to an improvement of the crystal quality and the enhancement of the coherently diffracting domain sizes (often called crystallite size), which is obvious from the increasing intensities of the (00.2l)-peaks (l=1-5) about 7 times and from a considerable narrowing of their full widths at half maximum (FWHM), see Fig. . Moreover, increasing the Pd-promoter thickness to 20 nm and the WSe 2 film thickness to 300 nm, respectively, significantly improved the structure of the films resulting in an increase of the thickness d of the hexagonal WSe 2 crystallites from 60 nm to 100 nm. With increasing film thickness the crystallite thickness d also increases (Fig. ), which means that the formed PdSe x effectively promotes the formation of large WSe 2 crystallites. The orientation of the crystallites of the WSe 2 films with respect to the substrate normal was studied by rocking curve measurements (Fig. ). Here, the highest intensities and narrowest FWHM (0.04 °) were observed for the 300 nm thick WSe 2 film, which proves the high texture of this type of film close to that of an epitaxial film (Fig. ). The maximum lateral sizes of the platelets for the best crystallized WSe 2 films amounts to 2-6 µm as revealed from SEM pictures (see Supplementary Fig. ). Surface characterization. The chemical composition and electronic structure of the surfaces of a pristine WSe 2 and an ATM coated WSe 2 films were studied by X-ray photoelectron spectroscopy (XPS), see Fig. . For the pristine WSe 2 film, the W and Se peaks match with those known from single crystals. The peaks at a binding energy of 36 and 38 eV are attributed to the presence of a small amount of oxidized tungsten. The intensities of the elements W and Se almost disappear after coating of the WSe 2 film with an ATM layer, since the ATM layer prevents the escape of photoelectrons from the surface of the WSe 2 film. After a photoelectrochemical treatment, the ATM layer becomes thinner (Supplementary Fig. The ATM layer itself shows the doublet peaks 3d 3/2 /3d 5/2 (232.65 eV and 229.45 eV), which corresponds to molybdenum in oxidation state 4 + (Fig. ). Near to the Mo 3d 5/2 peak a broad sulfur 2s peak is detected, which is attributed to different chemical states of sulfur in the Mo 3 S 13 cluster. The detailed analysis of the S 2p region shows the presence of two 2p 3/2 /2p 1/2 doublets (Fig. ). Following the interpretation given by Müller et al. , the first doublet at 162.1 eV and 163.5 eV can be assigned to the terminal S 2-and S 2 2-ligands while the second doublet at 163.5 eV and 164.8 eV can be addressed as bridging S 2 2-of the Mo 3 S 13 cluster. Our results are in agreement with the structure of (NH 4 ) 2 Mo 3 S 13 , described in detail by Kibsgaard et al. and by Müller et al. After a photoelectrochemical thinning (see Supplementary Fig. ) of the ATM layer, the intensities of the Mo 3d and S 2p peaks are decreased as expected (Fig. ). Surface photovoltage (SPV) analysis and band alignment. The effect of an ATM catalyst layer deposited on WSe 2 films was studied by surface photovoltage measurements (SPV). In Fig. , the SPV amplitude is displayed on a logarithmic scale as a function of the photon energy. For b a c d