Ballistic electron spectroscopy of individual buried molecules

George Kirczenow

Phys. Rev. B 75, 045428 – Published 22 January 2007

Abstract

A theoretical study is presented of the ballistic electron emission spectra (BEES) of individual insulating and conducting organic molecules chemisorbed on a silicon substrate and buried under a thin gold film. It is predicted that ballistic electrons injected into the gold film from a scanning tunneling microscope tip should be transmitted so weakly to the silicon substrate by alkane molecules of moderate length (decane, hexane) and their thiolates that individual buried molecules of this type will be difficult to detect in BEES experiments. However, resonant transmission by molecules containing unsaturated C-C bonds or aromatic rings is predicted to be strong enough for BEES spectra of individual buried molecules of these types to be measured. Calculated BEES spectra of molecules of both types are presented and the effects of some simple interstitial and substitutional gold defects that may occur in molecular films are also briefly discussed.

Received 17 September 2006

DOI:https://doi.org/10.1103/PhysRevB.75.045428

Authors & Affiliations

George Kirczenow

Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6

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Issue

Vol. 75, Iss. 4 — 15 January 2007

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Images

Figure 1
(Color online) The extended molecule studied in the BEEM transport calculations for the case where the molecule is ethylmethylbenzene (Ref. 25). The STM tip is represented by ten Au atoms in a (111) geometry and terminates in a single atom. The Au film is represented by a cylindrical cluster of 591 Au atoms in a bulk crystal geometry that presents (111)-oriented facets to the STM tip and the molecule. The substrate is represented by an approximately hemispherical crystallite of 390 Si atoms. The Si dangling bonds are passivated with H atoms and a flat (111) surface faces the molecule. The molecule bonds to the Si substrate through a single C-Si covalent bond. A large number of single-channel ideal leads (most of them not shown) representing the source and drain electrodes are attached to the atomic valence orbitals of appropriate surface atoms of the clusters representing the STM tip, Au film, silicon substrate, and passivating H atoms.Reuse & Permissions
Figure 2
(Color online) Calculated electron transmission probabilities $T_{BEEM} (E, V)$ from electron source to drain via STM tip, gold film, molecule, and and silicon substrate vs electron energy $E$ . Solid (dotted) curves are for zero $(2 V)$ bias $V$ between STM tip and gold film. The gold film Fermi energy is at $E = 0$ and the Si conduction band edge at the Si surface $E_{C}$ is shown by the dashed vertical line. The lower plots are for the decane thiolate $C_{10} H_{20} S$ molecule in the lower inset. The upper plots are for the $C_{10} H_{18} S$ molecule in the upper inset that exhibits resonant transmission due to the presence of a double C-C bond.Reuse & Permissions
Figure 3
(Color online) Calculated BEEM to film current ratios vs tip bias voltage $V$ for some saturated molecules: decane $(C_{10} H_{21})$ , decanethiolate $(C_{10} H_{20} S)$ , and hexanethiolate $(C_{6} H_{12} S)$ . The red horizontal dashed line is the nominal BEEM sensitivity threshold. Solid curves are for a separation of $4.33 Å$ between the STM tip atom and the gold film. The (black) dotted curve for $C_{6} H_{12} S$ is for a $5.05 Å$ separation.Reuse & Permissions
Figure 4
(Color online) Calculated BEEM to film current ratios vs tip bias voltage $V$ for some unsaturated chain molecules $C_{10} H_{18} S$ (shown in the upper inset of Fig. 2) and $C_{6} H_{10} S$ , and for ethylmethyl benzene $C_{9} H_{11}$ shown in Fig. 1. Arrows indicate values of the bias voltage at which the STM tip Fermi level crosses the energies of molecular resonant states. The red horizontal dashed line is the nominal BEEM sensitivity threshold. Solid curves are for a separation of $4.33 Å$ between the STM tip atom and the gold film. The (black) dotted curve for $C_{6} H_{10} S$ is for a $5.05 Å$ separation.Reuse & Permissions

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