Design of low work function materials using alkali metal-doped transition metal dichalcogenides

Engineering the work function is a key issue in surface science. Particularly, discovering the materials that have work functions less than 1eV is essential for efficient thermionic energy conversion. The lowest work function of materials, reported so far, is in a range of about 1eV. To design low work function materials, we chose MX$_{\mathrm{2}}$ (M$=$Mo and W; X$=$S, Se and Te) as substrates and alkali metals (Li, Na, K, Rb and Cs) as dopants, and studied their electronic structures, charge transfer, induced surface dipole moment, and work function using first-principles calculations. We found that the charge transfer from alkali metals to MX$_{\mathrm{2}}$ substrates decreases as the atomic radius of alkali metals increases. Regardless of the amount of the charge transfer, K on WTe$_{\mathrm{2}}$ exhibits the biggest surface dipole moment, which consequently makes the surface work function the lowest. Also, we found a correlation between the binding distance and the work function.