Molecular electronics is a promising field offering the ability to make devices based on single molecules.  Considerable work has been done in understanding the electrical properties of single molecules(1,2).  However, the molecule-substrate interface plays an integral role in device characteristics, and is rarely addressed.  The molecule in molecular electronics often behaves as a medium for tunneling between two electrodes, with the molecular highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) acting as the hole and electron tunneling barriers respectively(1).  This work focuses on measuring the location of the HOMO and LUMO of two model molecular electronic molecules, styrene and phenylacetylene, with respect to the band edges of the underlying silicon substrates to which the molecules are chemically bound using X-ray and UV photoelectron spectroscopy (XPS, UPS) and scanning tunneling microscopy (STM).  Styrene and phenylacetylene are chosen because they are fully conjugated molecules similar to those used in many studies(2), but have fewer functional groups, and thus fewer potential bonding geometries.  The silicon (111) 7x7 surface is a well characterized surface with surface adatoms and second-layer rest atoms each having single dangling bonds normal to the surface. 

For XPS and UPS, substrates were prepared by first removing the oxide in a dilute HF (~1%) solution, followed by annealing to about 700 C.  The surface reconstruction was confirmed by the presence of characteristic surface features.  The molecules were purified by several freeze-pump-thaw cycles or by pumping directly, and the purity of the molecule was checked by RGA.  Molecular exposures were measured in Langmuir, where 1 L = 10^-6 Torr x 1s.

Bonding of phenylacetylene and styrene on Si (111) 7x7 was confirmed by XPS of the C 1s region (figure 1).  Two features are evident in the spectra: the C 1s transition and a loss peak resulting from the p-p* shakeup.  The C 1s transition can be decomposed into two components, one centered at 285 eV corresponding to C-C and C-H bonds from the styrene, and the lower binding energy component (~283.5 eV) corresponding to C-Si bonds.  The ratio of the C-C to C-Si components is about 4:1, suggesting the molecules bind at two carbon sites.  The presence of the p-p* shakeup suggests the styrene retains aromaticity after bonding on the silicon surface.

UPS measurements of the valence region of silicon (111) 7x7, with increasing exposure to phenylacetylene and styrene, show significant changes in valence band features.  The characteristic 7x7 surface features disappear after 10 L exposure phenylacetylene and after just 2 L exposure styrene, showing the formation of a saturated monolayer.  New molecular features develop with increasing exposure. These features show good agreement with the theoretical density of states calculated by density functional theory.  Extrapolating the start of the valence features for the two molecules after 100 L exposure, the molecular highest occupied molecular orbital is found to be at 0.7 eV below the Fermi level for both molecules. 

To measure unoccupied state information, STM measurements have also been made on these systems.  For STM, silicon 111 (7x7) surfaces were prepared by flash-annealing to 1250 C.  The molecules were dosed as describeed above.  In occupied state images (negative sample bias), both phenylacetylene and styrene appear as protrusions above the silicon surface.  In unoccupied state images, however, the styrene appears as a depression, while phenylacetylene still appears as a protrusion – showing that at the applied biases of 2 V, styrene has few or no molecular states available for tunneling. 

Bias dependent STM has also been performed on the phenylacetylene/silicon systems (figure 2).  Since the apparent height in STM images is a convolution of physical height and electronic structure, tracking the changes in apparent height of the molecule at various applied biases will give an indication of the local electronic structure.  The onset of tunneling into the molecular states occurs between 0.7 and 0.9 V applied sample bias.  The phenylacetylene apparent height then increases with bias until 1.6 V, after which it decreases to a level indistinguishable from the silicon substrate.  Calculating the density of states from the apparent height data, the lowest unoccupied molecular orbital (LUMO) is found to lay approximately 1.7 eV above the Fermi level, consistent with the density of states calculated by density functional theory. 

Using XPS, UPS and STM we have investigated the chemical and physical interface of styrene and phenylacetylene adsorbed on silicon (111) 7x7 surfaces.  XPS and UPS confirm the chemical attachment between molecule and substrate.  The HOMO of both molecules is seen to lay 0.7 eV below the silicon Fermi level from UPS measurements; the LUMO of phenylacetylene is found to be about 1.7 eV above the Fermi level, while that of styrene is greater than 2 eV above.

References:

(1) Linsday, S.  Journal of Chemical Education.  2005, 82, 727-733.

(2) Haiss, W.; et al.  Nature Materials.  2006, 5, 995-1002.

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