Super low work function of alkali-metal-adsorbed transition metal dichalcogenides

Abstract

Discovering the materials that have work functions less than 1 eV is essential for efficient thermionic energy converter (TEC). The lowest work function of materials reported so far is in a range of about 1 eV. Here, to design low work function materials, we perform first-principles calculations on selected materials of transition metal dichalcogenide as substrates and alkali metals as adsorbates. The work function of our selected materials has a dip ubiquitously independent of the true binding distances of the adsorbates and exhibits contrasting behavior between empty d-shell elements (K, Rb, and Cs) and the others (Li and Na). We show that the interaction of empty d-orbitals of alkali metals and lone pair electrons of chalcogen is a key to the behavior of the work function. From calculated key parameters that determine the work function, we find that, regardless of the amount of charge transfer, K on WTe induces the largest surface dipole moment, which consequently makes the surface work function of as small as 0.8 eV, the smallest reported to date, and that the work function is lowered further to 0.7 eV by lattice strains. We demonstrate that the thermal efficiency of TEC using the low work function material exceeds that of thermoelectric materials with figure of merit of 5-10 in temperature range of 880-1200 K.

Surface Photovoltage-Induced Ultralow Work Function Material for Thermionic Energy Converters

“…Our group has achieved a 1.0 eV work function by electrostatically gating cesium/oxygen covered graphene, which resulted in enhanced TEC efficiency . The lowest theoretically predicted value to date is 0.7–0.8 eV for potassium adsorbed on monolayers of MoTe 2 or WTe 2 …”

“… 25 The lowest theoretically predicted value to date is 0.7–0.8 eV for potassium adsorbed on monolayers of MoTe 2 or WTe 2 . 26 …”

“…25 The lowest theoretically predicted value to date is 0.7−0.8 eV for potassium adsorbed on monolayers of MoTe 2 or WTe 2 . 26 Nearly all development of low work function surfaces for TECs has focused on metal electrodes. Despite the phenomenal progress in semiconductor technology over the last 50 years, semiconductor electrodes for TECs have hardly been explored.…”

Photovoltaic Anodes for Enhanced Thermionic Energy Conversion

“…The former one can be mitigated by using very low work function (ϕ) materials for the anode. 16,17 The latter refers to the accumulation of electrons in the interelectrode vacuum gap, which subsequently creates an electric field that opposes to the electrons' flow. This effect can be eliminated by reducing the interelectrode distance to micrometric scales.…”

“…The most relevant ones are probably related to the thermalization of electrons within the collector and to the voltage losses in the vacuum gap, i.e., the space-charge effect. The former one can be mitigated by using very low work function (ϕ) materials for the anode. , The latter refers to the accumulation of electrons in the interelectrode vacuum gap, which subsequently creates an electric field that opposes to the electrons’ flow. This effect can be eliminated by reducing the interelectrode distance to micrometric scales. − However, there exists a minimum distance between the cathode and the anode at which near-field radiative energy transfer significantly contributes to the energy flux and drastically deteriorates the conversion efficiency …”