Lithium monolayers on single crystal C(100) oxygen-terminated diamond
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Lithium monolayers on single crystal C(100) oxygen-terminated diamond Tomas L. Martin1,2, Kane M. O’Donnell3, Hidetsugu Shiozawa4, Cristina E. Giusca4, Neil A. Fox1,2, S. Ravi P. Silva4 and David Cherns2 1 School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS 2 H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL 3 Centre for Nanoscience and Quantum Information, University of Bristol, Tyndall Avenue, Bristol, BS8 1FD 4 Advanced Technology Institute, The University of Surrey, Guildford, Surrey, GU2 1XH ABSTRACT Thin lithium layers on oxygenated C(100) boron-doped diamond have been observed using x-ray photoemission spectroscopy. Conductive boron-doped diamond was oxygenterminated using an ozone cleaner. Lithium was evaporated onto the oxygen-terminated C(100) surface and an as-grown hydrogen terminated surface to a thickness of approximately 50 nm. After washing with deionised water, significant lithium signal is still detected on oxygenated diamond, but not on hydrogenated diamond, indicating a strongly bound lithium-oxygen surface layer is formed, as predicted by recent theoretical modeling. INTRODUCTION Adsorbates have been widely studied for their role in changing the chemical, physical and electrical properties of diamond. In particular, the role certain surface terminations or deposited surface layers on diamond play in inducing a negative electron affinity (NEA) property, in which the conduction band sits above the vacuum level, offer a number of potential device applications, including low threshold electron emission, photodetection and in electrochemical cells. Hydrogen termination is common on chemical vapor deposition (CVD) grown diamond and has been widely observed to induce an NEA on both the C(100) [1] and C(111) [2] surfaces. Together the C(111) and C(100) surfaces comprise the majority of surfaces on natural and high pressure, high temperature (HPHT) diamond particles, as well as the surfaces of CVD films [3, 4]. Experimental observations of hydrogen termination show an NEA of around -1.3 eV [5], shifting the bare workfunction of diamond from 5.5-6 eV down to 2.85-3.9 eV [5,6]. More unusual surface coatings such as rubidium fluoride [7] and cesium oxide [8, 9] have also been observed to induce an NEA on diamond. In the case of cesium oxide, the deposition of a cesium submonolayer on an oxygenated diamond surface lowers the workfunction to as little as 1.25eV, making low threshold emission a real possibility. Unfortunately, this surface layer is only weakly bound to the diamond surface, and degrades at temperatures above 400ºC. The relationship between alkali metals and oxygen on the diamond surface is nevertheless of interest, and a workfunction shift has also been observed from potassium on oxygenated diamond. [10] In a previous communication [11] this group has reported ab initio density functional theory calculations that show a lithium monolayer on oxygen-terminated diamond can form an NEA comparable to CsO in strength, but with a much high
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