Raman spectra. In order to study whether chemical bonds between WSe 2 and ATM are formed, Raman spectroscopy of the ATM/WSe 2 double layer and the separate films was performed (Fig. ). The Raman spectrum of the ATM/WSe 2 heterojunction shows a clear combination of separate ATM and WSe 2 layers, where no considerable change in chemical bonding was observed indicating a van der Waals bonding type, similar to the combination of a MoS 2 and WS 2 heterojunction. The WSe 2 film exhibits the main in-plane E 1 2g vibration mode slightly merged with the A 1g out-of-plane mode at 250 cm -1 as the main peak. The ATM Raman spectra are characteristic for its terminal, bridging and apical chemical bonds, which are similar to the Raman spectra of ATM reported previously. However, the positions of the Raman peaks for ATM on WSe 2 are shifted in comparison to a pristine ATM film, which means that a bonding on WSe 2 is formed. The Mo-Mo, Mo-S and Mo 3 -S a bonds in the ATM are strong and not significantly changed after combination with the WSe 2 . The peaks at 362.7 and 384.3 cm -1 for the ATM/WSe 2 appear due to the overlay of the Mo-S peaks at 352.6 and 378.6 cm -1 with 3L(AM) and +LA(M) peaks at 371.28 and 392.63 cm -1 . However, the ν(S-S) t and ν(S-S) br modes at 507.72 and 543.3 cm -1 for the ATM are considerably shifted to higher vibration modes at 520 and 553.11 cm -1 for the ATM/WSe 2 , i.e. by about 12.3 and 10 cm -1 (difference in the Raman shift of 2.3 cm -1 indicates the weaker terminal bond than that of the bridging bond). This significant shift can be explained by the weaker S-S bond compared to the Mo-Mo and Mo-S bonds, which can be easily changed by adjacent in-phase vibrations of A 1g and/or E 1 2g modes of WSe 2 . In addition to this, the ATM layer also influences the vibration modes of WSe 2 , resulting in the shift of the A 1g and E 1 2g modes to higher vibration frequencies. This means, that the adjacent Se atomic layer of the WSe 2 forms the vdW bonding with S layer in the ATM. However, it is not excluded that the S atoms in ATM can create chemical bonds at the edge steps of the Sedeficient site of the WSe 2 , i.e. it forms Mo-S-W bonds. On the other hand also the Se atoms of the vdW planes of WSe 2 can create a chemical bonding at the S-deficient site of the ATM changing its vibration modes, i.e. forming W-Se-Mo bonds. PEC performance. A pristine 120 nm thick WSe 2 film on TiN:O exhibits only a very weak photoactivity; the measured photocurrent density under AM 1.5 illumination was less than 0.1 mA cm -2 at 0 V vs. RHE due to prevailing vdW planes exposed to the electrolyte (Fig. ) where the electron transfer is hindered. Also a thin ATM film on TiN:O is only weakly photoactive upon illumination, exhibiting a negative photocurrent density upon chopped light (Supplementary Fig. ), which has an onset potential starting from 0.7 V vs. RHE similar to the WSe 2 indicating a p-type behavior. On the other hand, the combination of ATM and WSe 2 film results in a signif-icant increase of the photocurrent density up to 2.7 mA cm -2 at 0 V vs. RHE, which is in agreement with the observed increased SPV signal when depositing an ATM layer onto WSe 2 (Fig. ). Increasing the WSe 2 film thickness to 350 nm, leads to a further enhancement of the photocurrent density to a maximum of 5 mA cm -2 at 0 V vs. RHE (Fig. ). The highest measured photocurrent density for an ATM/WSe 2 film at 0 V vs. RHE is 5.6 mA cm -2 (Supplementary Fig. ). The increase of the photocurrent density with increasing WSe 2 thickness (up to about 350 nm) can be explained by the larger number of absorbed photons and hence generated electron-hole pairs. For even larger thicknesses, the photocurrent decreases, caused by the limited carrier diffusion length in the order of 400 nm (Fig. ). This value is much smaller than the diffusion length of 1-2 µm, evaluated recently by us from time-resolved microwave conductivity (TRMC) measurements. This, however, can be explained by the strong anisotropy of the carrier b d c a transport in these layer-type materials; the mobility along the c-axis, i.e. towards the SEI, is one to two orders of magnitude less than the conductivity perpendicular to the c-axis due to the weak vdW bonding between the S-W-S layers. The thickness dependence was fitted with an analytical model taking into account the increased absorption with increasing thickness and the decreasing carrier collection if the absorber is thicker than the diffusion length. Based on an old model of Hecht for the carrier collection efficiency in AgCl single crystals, Abeyweera et al. derived a simple analytical formula for the carrier collection efficiency of thick films where a carrier generation depth profile exists which can be expressed as: