Diamondoid Monolayers as Monochromatic Electron Source
Diamondoids are nanometer-sized molecules that feature diamond-crystal cage structures. Adamantane, the smallest member in the family, consists of one cage structure, diamantane two, triamantane three, tetramantane four, and so on. On all of these, the dangling bonds on the outer surfaces are terminated by hydrogen atoms. Because of their potential to possess novel properties of both diamond and nanomaterial, intensive efforts have been made to synthesize the larger diamondoid molecules, but to no avail. This situation was finally changed in 2003 when significant quantities of higher diamondoids were found in petroleum by researchers in MolecularDiamond Technologies. Now, scientists from Berkeley Lab, Stanford University, Lawrence Livermore National Laboratory, and Germany have used photoelectron spectroscopy at the ALS to reveal an intriguing feature: monochromatized electron emission from a self-assembled monolayer of diamondoids. This discovery has immediately attracted the attention of people who are searching for materials for next-generation electron emitters.
Diamondoids are molecular versions of diamond. The yellow cage embedded in the blue diamond lattice is the smallest diamondoid, adamantane. Materials for electron emitters have long been sought because electrons emitted into vacuum can be precisely controlled and easily integrated into elaborate devices. They lie at the heart of a number of modern technologies, such as field-emission flat-screen displays, electron microscopes, electron lithography, and next-generation free-electron lasers. For electron emitters, one of the biggest challenges is to develop large, uniform surfaces that emit electrons with a sharp energy distribution. In the late 1970s, scientists found that hydrogen-terminated diamond surfaces are characterized by negative electron affinity (NEA), meaning for electrons, the energy level of the vacuum is lower than that of the diamond conduction bands. At surfaces with NEA, electrons excited into the conduction band will spontaneously fall out into vacuum even at low temperature. Thus, NEA-based electron emitters have several advantages over conventional emitters. They exhibit electron emission at extremely low bias voltage (zero in the ideal case), and the energy distribution of the emitted electron is extremely narrow. However, two critical issues prevented NEA semiconductors from being used in commercialized products. One is the nonuniform emission normally observed on diamond surfaces. The other is the difficulty of supplying electrons to the emission surface, because diamond and other NEA semiconductors are wide-gap materials with low electron conductivity. Diamondoids, being diamond-like nanoclusters, provide us with the opportunity to sustain the NEA feature of diamond while avoiding the conventional problems of bulk NEA materials. Toward this end, the collaborators replaced one of the hydrogen atoms on the surface of tetramantane (four-cage diamondoids) with a thiol group (hydrogen + sulfur). This substitution chemically "functionalizes" the tetramantane, i.e., it promotes bonding with other molecules, enabling it to form more complex structures, like nanosized tinker toys. The researchers found that these diamondoid–thiol complexes would then self-assemble into a uniform monolayer on metal surfaces such as silver or gold.
Photoelectron spectroscopy was then performed on the tetramantane–thiol monolayers at Beamline 10.0.1, where a strong, sharp peak was detected. The outstanding peak observed in the spectra is a strong indication of NEA. Furthermore, up to 68% of all the emitted electrons were within this single energy peak, with a width of less than 0.5 eV. This is several times as strong as the same measurement for bulk diamond. Technologically, this means most electrons are emitted from the diamondoid monolayer at the same energy, i.e., speed.
The result directly shows that diamondoid monolayers can be superior to conventional materials as electron emitters. The molecules can be purified and functionalized under precise control. They can be inexpensively self-assembled into large-area, uniform monolayers. More importantly, they perform better than previous materials in terms of the energy distribution of the emitted electrons. Further investigations are under way to fully understand this striking phenomenon, as well as to make real devices based on diamondoids. Research conducted by W.L. Yang, N. Mannella, K. Tanaka, and X.J. Zhou (Stanford University and ALS); J.D. Fabbri, W. Meevasana, M.A. Kelly, N.A. Melosh, and Z.-X. Shen (Stanford University); T.M. Willey, J.R.I. Lee, and T. van Buuren (Lawrence Livermore National Laboratory); J.E. Dahl and R.M.K. Carlson (MolecularDiamond Technologies, Chevron Technology Ventures); P.R. Schreiner, B.A. Tkachenko, and N.A. Fokina (Justus-Liebig University, Germany); A.A. Fokin (Justus-Liebig University, Germany, and Kiev Polytechnic Institute, Ukraine); and Z. Hussain (ALS). Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES. Publication about this research: W.L. Yang, J.D. Fabbri, T.M. Willey, J.R.I. Lee, J.E. Dahl, R.M.K. Carlson, P.R. Schreiner, A.A. Fokin, B.A. Tkachenko, N.A. Fokina, W. Meevasana, N. Mannella, K. Tanaka, X.J. Zhou, T. van Buuren, M.A. Kelly, Z. Hussain, N.A. Melosh, and Z.-X. Shen, "Monochromatic electron photoemission from diamondoid monolayers," Science 316, 1460 (2007). |
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