# First-Principles Study of the Contact Resistance at 2D Metal/2D Semiconductor Heterojunctions
## 5 T/2H MoS\({}_{2}\) Edge Contacts  
We considered 1T/2H-MoS\({}_{2}\) heterostructures (also based on MoS\({}_{2}\) monolayers) with armchair or zigzag edges [62], as shown in Figure 9. The armchair-edge (198 atoms) and zigzag-edge (216 atoms) interface models were obtained by using (35 \(\times\) 16.5) A\({}^{2}\) and (34 \(\times\) 19) A\({}^{2}\) rectangular supercells, respectively, with a vacuum layer of 20 A; the residual strain in the MoS\({}_{2}\) layers was typically less than 0.25%. The relaxed 1T/2H-MoS\({}_{2}\) interface models, with armchair and zigzag edges, are shown in Figure 9a,b, respectively. Since the lattice parameters of 1T- and 2H-MoS\({}_{2}\) are almost identical, atomically sharp and defect-free interfaces were obtained, in agreement with previous DFT simulations [26]. The computed R\({}_{\text{c}}\)W at the pristine 1T/2H-MoS\({}_{2}\) interfaces was about 3-4 \(\times\) 10\({}^{4}\)\(\Omega\cdot\)um for the armchair and zigzag edges (see Table 2). The high contact resistance was due to the large interface energy barrier (\(\Phi_{\text{B}}\)\(\approx\) 0.8 eV), which corresponds to the difference between the computed work function of 1T-MoS\({}_{2}\) (\(\Phi_{\text{m}}\)\(\approx\) 5 eV) and the computed electron affinity of 2H-MoS\({}_{2}\) (\(\chi\) 2248 4.2 eV), as schematically illustrated in Figure 10. The calculated interface barrier \(\Phi_{\text{B}}\)\(\approx\) 0.8 eV is in very good agreement (\(\Phi_{\text{B}}\) = 0.82 eV) with other DFT simulations performed on similar 1T/2H-MoS\({}_{2}\) heterojunctions [26]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Interface Model** & **R\({}_{\text{c}}\)W (\(\Omega\cdot\)um)** & **E\({}_{\text{ads}}\) (eV)** & **Electron Transfer to 1T-MoS\({}_{2}\) (e)** \\ \hline Pristine armchair contact & \(3.8\times 10^{4}\) & / & / \\ Pristine zigzag contact & \(2.9\times 10^{4}\) & / & / \\ H-adsorbed contact & \(2.8\times 10^{2}\) & \(-0.75\) & 0.05 \\ Li-adsorbed contact & \(1.9\times 10^{2}\) & \(-1.46\) & 0.18 \\ H\({}_{2}\)O-adsorbed contact & \(4.7\times 10^{2}\) & \(-2.43\) & 0.08 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Computed contact resistance, adsorption energies E\({}_{\text{ads}}\) and electron transfer (Hirshfeld population analysis) of the different 1T/2H-MoS\({}_{2}\) interface models. The adsorption energies and charge transfer were computed for one adsorbed atom or molecule on a (4 \(\times\) 4) 1T-MoS\({}_{2}\) supercell. Figure 9: Top views of the relaxed atomic configurations of lateral 1T-MoS\({}_{2}\)/2H-MoS\({}_{2}\) interface models with (**a**) armchair and (**b**) zigzag edge contacts. Blue and yellow spheres correspond to Mo and S atoms, respectively. It should be pointed out that the calculated R\({}_{\rm c}\)W was about two orders of magnitude larger than experimental values reported on 1T-MoS\({}_{2}\)/2H-MoS\({}_{2}\) edge contacts [16; 20]. The discrepancy may be explained by assuming that 1T-MoS\({}_{2}\) has a lower effective work function, which could arise from its interaction with chemical species during the local semiconducting (2H) to metallic (1T) MoS\({}_{2}\) transformation. During device processing, 2H-MoS\({}_{2}\) is indeed exposed to \(n\)-butyllithium solutions, to induce its transformation to the 1T metallic phase [16; 20; 63]. During this chemical treatment, chemical species like H, Li, and/or H\({}_{2}\)O could be adsorbed onto the 1T-MoS\({}_{2}\) surface. We, thus, investigated the effect of these adsorbates on the 1T-MoS\({}_{2}\) work function. An H atom was adsorbed onto 1T-MoS\({}_{2}\) by forming an S-H bond, with a bond length of 1.37 A (see Figure 11a). On the other hand, Li was most favorably adsorbed onto of an Mo atom, with average Li-Mo distance of 3.07 A, as indicated in Figure 11b, as in 2H-MoS\({}_{2}\)[64].