Report
BORON-DOPED DIAMOND THIN-FILM ELECTRODES arbon-based electrode materials (e.g., carbonfibers,glassy r carbon, and graphite) are used in various electrochemical technologies, including electroanalysis, energy storage devices, and electrosynthesis. These materials have similar microstructures consisting of layers of condensed, six-membered aromatic rings with sp2-hybridized carbon atoms trigonally bonded to one another. The crystallite size and extent of microstructural order can vary from material to material (i.e., edge-to-basal plane ratio), which has important implications for the electron-transfer kinetics of a given redox analyte (1,2). The electrochemical performance of these materials has been studied in detail over the past three decades, and much has been learned about the structurereactivity relationship (1,2). The use of synthetic conductive and semiconductive Jishou Xu Michael C. Granger Qingyun Chen Jerzy W. Strojek Tedd E. Lister Greg M. Swain Utah State University S0003-2700(97)09032-X CCC: $14.00 © 1997 American Chemical Society
Diamond thin films could be an electrochemist's best friend. diamond thinfilmsin electrochemistry has only recently been reported (3-13). However, the relationship between the physical, chemical, and electronic properties of diamond and its electrochemical performance is not well understood and is the subject of much recent interest. The objective of this Report is to acquaint trie reader with trie conditions required to grow highly conductive, borondoped diamond thin films, as well as with some of the analytical methods used to characterize films, electrochemical properties of the films in aqueous media, and some applications. The discussion will be limited to the properties and performance
of highly doped films (>1019 cm"3), although it should be noted that the electrochemical response depends on the doping level. Diamond thin films can possess electronic properties ranging from those of an insulator at low doping levels, to those of a semiconductor at moderate levels, to those of a semimetal at high levels (14,15). Diamond chemistry Diamond possesses several technologically important properties including extreme hardness, high electrical resistance, chemical inertness, high thermal conductivity, high electron and hole mobilities, and optical transparency (14,15). The material is a wide bandgap semiconductor (Eg = 5.5 eV) and offers advantages for electronic applications under extreme environmental conditions. Each carbon atom in diamond is tetrahedrally bonded to four other carbons using sp3-hybrid orbitals. Microstructurally, the atoms arrange themselves in stacked, six-memberedrings,with eachringin a "chair" rather than a planar conformation. In boron-doped films, the boron impurity atoms substitute in place of some of the carbon atoms during film growth.
Analytical Chemistry News & Features, October 1, 1997 591 A
Report
Figure 1 . Scanning electron micrograph of a boron-doped diamond thin film grown on p-Si (100) by microwave-enhanced CVD. The methane-to-hydrogen ratio was 1%, the system pressure was 35 torr, and the plasma power was 1000 W. The growth time was —19 h.
Diamond is one of nature's best insulators; but when doped with boron, the material possesses semimetal electronic properties, making it useful for electrochemical measurements. For example, synthetic diamond thin films grown using hot-filament or microwave-assisted chemical vapor deposition (CVD) can be doped to as high as 10,000 ppm B/C, resulting in films with resistivities ?\ TTip rlptprtion limit" for crlaQQv carhnn is 4.9 nnb
Analytical Chemistry News & Features, October 1, 1997
The surface roughness of diamond (Figure 1) is supposed to play an influential role in improving the analytical performance of the detector. Preliminary scanning electron microscopy studies of the Hg-coated diamond surface indicated that the microcrystallites may influence the deposit size (i.e., volume) by restricting the deposit to the grain boundary regions. The deposits on the diamond surface were observed to be uniformly distributed and possessed a relatively narrow distribution of sizes much than deposits on glassy carbon It is also supposed that the roughness serves to shield the deposits from the shearing forces of the fluid flow as little Ht* was lost even after two wppks of u«:p T-Fo" rlpnnlVC6S jQV tr3€ ^tJCWllCPll
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P u b l i s h e d by t h e A m e r i e a n C h e m i c a l S o c i e t y a n d t n c x\v_-S LJivision of v ^ n c m i c a l IT.ealrn a n d Safetv
Analytical Chemistry News & Features, October 1, 1199 597 A
