The Li-adsorbed C(100)-(1x1):O Diamond Surface
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The Li-adsorbed C(100)-(1x1):O Diamond Surface Kane M. O’Donnell1,2, Tomas L. Martin2,3, Neil A. Fox3 and David Cherns3 1 The Bristol Centre for Nanoscience and Quantum Information, University of Bristol, Bristol, BS8 1FD, United Kingdom. 2 School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1FD 3 H.H. Wills Physics Laboratory, University of Bristol, Bristol, BS8 1FD, United Kingdom. ABSTRACT This paper presents density functional theory results for the Li-adsorbed C(100)-(1x1):O system. Previously it has been shown that at a single monolayer coverage, the binding energy for Li on oxygenated C(100) diamond is substantially higher than that of heavier alkali metals, while at the same time, the presence of the lithium generates a large shift in the diamond workfunction. The system is therefore promising for electronics applications involving diamond. Here, further calculations are presented showing that additional Li atoms above 1ML coverage are far less strongly bound, suggesting the 1ML surface is the most useful for vacuum microelectronic applications. INTRODUCTION The surface of diamond has proven to be highly flexible from the point of view of modification and functionalization. The covalent bonding structure of diamond leads to surfaces with oriented dangling bonds that have been used for a variety of chemical, biological and electronic applications. For example, amination of the diamond surface allows for the chemical attachment and preservation of DNA [1]. A hydrogen termination allows for a form of surface transfer doping, giving surface p-type conductivity [2, 3]. Such surfaces have been used to create surface transistors on diamond [4]. Several diamond surfaces exhibit a negative electron affinity (NEA) when hydrogen-terminated [5] or coated in thin layers of metals such as cobalt [6] and barium [7]. The negative electron affinity of diamond, where the conduction band minimum is higher in energy than the vacuum level, is useful for vacuum microelectronic applications such as photodetection and field emission, and possibly also for the tuning of the interface between diamond and other materials. The NEA of hydrogenated diamond has been extensively studied and can be reproducibly induced on the C(100), C(111) and C(110) surfaces, at least [8], via exposure to a hydrogen plasma. Although the calculated electron affinity of a monohydride-terminated diamond C(100) surface is approximately -2 eV [9, 10, 11], the experimentally measured value is closer to -1.3 eV [12] and is hence a relatively weak NEA. Alternatives to hydrogen for inducing a diamond NEA have been investigated by a number of groups. Metals such as zirconium and cobalt are known to induce a small NEA [13]. Caesium oxide coatings are often used [14] however the caesium is weakly bound, with degradation evident above several hundred degrees centrigrade. Since high temperature operation is one of the possible advantages of using diamond as an electronic material, surface resilience above 500°C is desired for any practical NEA coa
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