Discovering Materials With Ultra-Low Work Functions for Thermionics Energy Conversion

The work function is one of the most fundamental properties of a metallic surface, important in determining the material’s applicability as a contact electrode [1, 2] or electron emitter [3]. Surfaces with ultra-low work functions could therefore improve technologies requiring precise control of contact barriers such as organic and printed electronics [2] or a variety of devices based on electron emission, ranging from fluorescent light bulbs [4, 5] to THz sources [6], thermionic energy converters (TECs) [7], and the photon-enhanced thermionic emission converters (PETEC) [8]. For TEC and PETEC, in particular, discovery of thermally stable materials with work functions of less than 1 eV would allow thermionic conversion of high-temperature (> 500 °C) heat or solar radiation directly to electricity with efficiencies exceeding 50%.

We present a model based on adsorbate atomic orbital interactions which offers new insights into the interfacial phenomena that control the work function [9]. In addition to the model, we will discuss ab-initio studies of surfaces with alloyed alkali metal and alkali-earth metal oxides. These adsorbed compound structures can enhance the emissivity of refractory metal substrates commonly used in current electron-emissive devices. By visualizing the electrostatic charge distributions, surface dipole formations, and the corresponding changes in the work function, we can devise design rules for low-work function materials to be incorporated into high-efficiency thermionic energy conversion devices.

[1] Lindell L, Burquel A, Jakobsson F L E, Lemaur V, Berggren M, Lazzaroni R, Cornil J, Salaneck W R and Crispin X 2006 Chem. Mater. 18, 4246

[2] Zhou Y et al 2012 Science 336, 327

[3] Yamamoto S 2006 Rep. Prog. Phys. 69, 181–232

[4] Wada M, Gotoh S and Kurumada S 2009 J. Plasma Fusion Res. Series 8, 1366

[5] Watanabe S, Watanabe T, Ito K, Miyakawa N, Ito S, Hosono H and Mikoshiba S 2011 Sci. Technol. Adv. Mater. 12, 034410

[6] Snapp J P, Lee J-H, Provine J, Bargatin I, Maboudian R, Lee T H and Howe R T 2012 Sidewall silicon carbide emitters for terahertz vacuum electronics Solid-State Sensor and Actuator Workshop (San Diego, CA, June)

[7] Lee J-H, Bargatin I, Provine J, Clay W, Schwede J, Liu F, Maboudian R, Melosh N, Shen Z-X and Howe R T 2009 Tech. Dig. Power MEMS

[8] Schwede J W et al 2010 Nature Mater. 9, 762

[9] Chou S H, Voss J, Bargatin I, Vojvodic A, Howe R T and Abild-Pedersen F 2012 J. Phys.: Condens. Matter 24, 445007