The nature of the metal-molecule interface revealed through tunneling spectroscopy
This dissertation describes the fabrication and characterization of molecular tunnel junctions made with a series of carboxylic acid derivatives chemisorbed to amorphous aluminum oxide (AlOx) electrodes with a variety of top metal electrodes. Para-substituted benzoic acids, terminal-substituted long alkyl chain (C16) alkanoic acids, and octadecylphosphonic acid were prepared on the native oxide of aluminum by a solution self-assembly technique. The self-assembled monolayers (SAMs) were characterized via x-ray photoelectron spectroscopy (XPS) and water contact angles. For the benzoic acid derivatives, several trends became apparent. The polarity of the solvent used during self-assembly was critical to monolayer formation. Solvents that were polar and protic in nature produced low coverage monolayers. In contrast, non-polar solvents produced much higher coverage monolayers. The thickness of the monolayers determined via XPS suggests that the plane of the aromatic ring is perpendicular to the AlOx surface with the carboxylate functional group chemisorbed in a bidendate chelating geometry. A similar saturation coverage was achieved for each benzoic acid derivative at approximately 2.7 x 1014 molecules cm-2. At saturation coverage, the benzoic acids and hydroxyls are present in an approximate ratio of 1:1. When Pb was vapor deposited on top of the benzoic acid derivatives and XPS measurements were obtained, only 4-SH benzoic acid showed a chemical reaction in the form of S-Pb. Au also showed a complete chemical reaction with 4-SH benzoic acid with a shift in the S(2s) peak 1.2 eV lower in binding energy indicative of a S-Au bond formation. Contact angle and XPS results of the long alkyl chain SAM derivatives reveal a higher surface coverage of molecules on AlOx when compared to the benzoic acids. Electrical measurements including: I vs. V, dI/dV [normalized as G(V)] vs. V, and IETS were also obtained. G(V) measurements obtained on tunnel junctions without SAMs showed large offsets in the G(V) minimum [G(V)min] conductance due to polar hydroxyl groups native to the AlOx surface introducing large asymmetry in the barrier. Upon addition of the carboxylic and phosphonic acid monolayers, the G(V)min shifted closer to zero bias. For tunnel junctions made with benzoic acid SAMs and Pb, several trends emerged. First, as the coverage of the SAM increased, the G(V)min decreased. Second, as the size of the para-substituent increased, the G(V)min decreased. Finally when a reactive para-substituent such as thiol is used, the barrier asymmetry is completely reversed due to a bond formation at the S-Pb interface. G(V) measurements obtained on various alkanoic acid SAMs showed no offset in the G(V)min. The data obtained was fit to a model to extract the tunnel barrier properties. The results showed that when a reactive terminal group such as thiol is used, the effective thickness of the tunnel barrier is larger than samples made with CH3 terminated SAMs. This suggests that less of the top metal can penetrate through monolayers when a chemical reaction occurs at the metal-molecule interface. Data obtained for tunnel junctions made with octadecylphosphonic acid SAMs showed very little penetration of the top metal likely due to higher saturation coverage and better packing.