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Science 8 June 2007:
Vol. 316. no. 5830, pp. 1460 - 1462
DOI: 10.1126/science.1141811
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Reports

Monochromatic Electron Photoemission from Diamondoid Monolayers

W. L. Yang1,2, J. D. Fabbri1, T. M. Willey3, J. R. I. Lee3, J. E. Dahl4, R. M. K. Carlson4, P. R. Schreiner5, A. A. Fokin5,6, B. A. Tkachenko5, N. A. Fokina5, W. Meevasana1, N. Mannella1,2, K. Tanaka1,2, X. J. Zhou1,2, T. van Buuren3, M. A. Kelly1, Z. Hussain2, N. A. Melosh1 and Z.-X. Shen1

1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA.
2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA.
3 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
4 Molecular Diamond Technologies, Chevron Technology Ventures, 100 Chevron Way, Richmond, CA 94802, USA.
5 Institut für Organische Chemie, Justus-Liebig University Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany.
6 Kiev Polytechnic Institue, pr. Pobedy 37, 03056 Kiev, Ukraine.


Figure 1 Fig. 1. (A) Polarization-dependent total electron yield NEXAFS spectra collected on [121]tetramantane-6-thiol SAMs prepared on Au. Total electron yield (TEY) is plotted against photon energy at different beam incident angles. The leading-edge peak at 287.6 eV (red oval) is assigned to transitions from the C 1s core level to the unoccupied (C-H) {sigma}* orbitals, and the peak at about 292.5 eV is assigned to (C-C) {sigma}* orbitals (8, 10, 11). The second gap indicated by the arrow is the characteristic signature of diamondoids (11, 12). (B) Comparison between experiments and theoretical simulations. Red squares represent the experimental ratio of (C-H) {sigma}* spectral weight between data at different angles and that at 20°. The black line is the calculated ratio based on the molecular geometry as shown in (C), with a polar angle of 36.5° (6). [View Larger Version of this Image (43K GIF file)]
 

Figure 2 Fig. 2. (A) Photoelectron spectra of [121]tetramantane-6-thiol SAMs grown on Ag substrates, collected with 55-eV photon energy. The strong peak at 1-eV kinetic energy contains 68% of the total photoelectrons. The dotted line is a 50-times enlargement of valence band features. The inset shows the same spectra in a double logarithm plot. (B) Photoelectron spectrum collected on [121]tetramantane-6-thiol SAM covered by C60 sublimed in situ. The strong electron-emission peak disappears after C60 coverage. (C) Photoelectron spectrum collected on an annealed [121]tetramantane-6-thiol SAM. As in (B), the peak observed for the pristine SAM vanishes after the in situ annealing to 550°C. The difference between the spectrum in (C) and a pure Ag PES spectrum could be partially due to residual S atoms still bound to the surface after annealing. [View Larger Version of this Image (23K GIF file)]
 

Figure 3 Fig. 3. (A) Photoelectron spectra of [121]tetramantane-6-thiol SAMs grown on Au substrates. The sharp peak at 1 eV contains about 17% of the total photoelectrons. The inset shows a double logarithm plot. (B) Photoelectron spectra of unsubstituted [121]tetramantane films prepared in situ on Au substrates. The inset is an enlargement of the low–kinetic energy part of the spectrum showing only a small peak. [View Larger Version of this Image (25K GIF file)]
 

Figure 4 Fig. 4. Schematic of the electron-emission process on diamondoid SAM surfaces. EF is the Fermi level of the metal substrate, sitting in the energy gap of diamondoid. The vacuum level (EVacuum) is below the conduction-band minimum of the diamondoid, a characteristic of NEA. The dotted red line depicts the high-probability electron emission. First, electrons in metal substrates are excited by photons into unoccupied states above EF. Second, the excited electrons effectively thermalize in the metal, producing more electrons with lower energy. Third, electrons with energy above the conduction band minimum are transferred to diamondoid moieties. These electrons further lower their energies by exciting phonons in the molecules, and they accumulate at the bottom of the conduction band. Finally, because of NEA, electrons accumulated at the bottom of the conduction band emit into vacuum spontaneously and generate a peak at the low–kinetic energy threshold. Electron emission takes place also at high–kinetic energy levels, but with much lower photoelectron yield. Although this scenario roughly explains the existence of the electron-emission peak, more theoretical understanding is needed to fully explain the results. [View Larger Version of this Image (32K GIF file)]
 

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