Chemical and physical interactions at metal/self-assembled organic monolayer interfaces

Abstract

The purpose of research on metals (M) deposited onto self-assembled monolayers (SAMs) is to understand the interactions between metal (M) and eventually metal oxide overlayers on well-ordered organic substrates. Application of WSAM and inorganic/SAM research results to the understanding of real inorganic/ organic interfaces in vacuum and under environmental conditions can potentially play a key role in the development of advanced devices with stable interfacial properties. The M/SAM approach to interface research is delineated as a new subfield in surface science in the context of other approaches to inorganic/organic interface research. Current issues in M/SAM research are outlined, including chemical compound formation, the morphology (spreading, clustering. or penetration) of the metal species, the kinetics of the metal morphology, the effect of the metal on the degree of order in the SAM, and the rate of metal penetration into the SAM. Probes are recommended that are suitable for M/SAM research. The results of M/SAM studies to date are reviewed, and M/SAM combinations are ranked according to reactivity and penetration. Key probes for addressing gaps in the research results are identified. The effects of defects, disordering, air exposure, and X-ray and electron beam exposure on the experimental results to date are evaluated. Thus far, the results have successfully revealed qualitative relationships of M/SAM chemistry, temperature, and penetration. The chemical interactions that have been found are applicable to real M/polymer interfaces as formed in vacuum. It has yet to be shown that WSAM research will yield quantitative understanding of interface formation or that WSAM interfaces are entirely analogous to M/polymer interfaces in the details of interface formation. The future of this subfield of surface science lies in its expansion from M/SAM interfaces in vacuum to other inorganic/SAM interfaces in vacuum and. eventually, under environmental conditions. KEY WORDS: self-assembled monolayer, metal/organic interface, surface chemistry. LIST OF ACRONYMS AND ABBREVIATIONS* USED IN THIS ARTICLEAES, auger electron spectroscopy; AFh4, atomic force microscopy; BE, binding energy; BIS, bremsstrahlung isochromat spectroscopy; FITR, Fourier transform infrared spectroscopy; FT'-RAIRS, Fourier transform reflection absorption infrared spectroscopy; HAS, helium atom scattering; HREELS, high-resolution electron energy loss spectroscopy; IFM, interfacial force microscopy; ISS. ion scattering spectroscopy; LB, Langmuir-Blodgett; LEED. low-energy electron diffraction; M or MO, metal or metal oxide; MMHD. methyl 16-mercaptohexadecanoate. HS(CH,),,COOCH,; M/SAM, metayself-assembled monolayer;

Copper Electrodeposition on Alkanethiolate Covered Gold Electrodes*

“…The question we have to address is whether copper atoms can penetrate a thiolate layer; in section 4.4 we will show that this is indeed possible. Here we mention that silver evaporated on thiolate/Au(l 11) in vacuum has indeed been observed to penetrate the layer [11,82]. Let us also mention that thiolates readily adsorb on Cu(lOO) [83] and that even multilayer copper thiolates can form [84].…”

“…Several studies of vacuum metal deposition on thiolate-covered metal substrates were conducted; Jung and Czanderna compiled a valuable review [11]. The [12a, 18-23].…”

Molecular Nanoelectronics

“…If the metal atoms are not too reactive (e.g. Al with CH 3 or OCH 3 …) [58][59][60][61][62][63], they can penetrate into the organic monolayer, diffusing to the bottom interface where they can eventually form an adlayer between this electrode and the monolayer (in addition to metallic filamentary short circuits). In a practical way for device application using organic monolayers, the metal evaporation is generally performed onto a cooled substrate (~100 K).…”

“…Al with COOH or OH groups, Ti with COOCH 3 , OH or CN groups….) [58][59][60][61][62][63], a chemical reaction occurs forming a molecular overlayer on top of the monolayer. This overlayer made of organometallic complexes or metal oxides may perturb the electronic coupling between the metal and the molecule, leading, for instance, to partial or total Fermi-level pinning at the interface [64].…”

“…Several studies [58][59][60][61][62][63] have analyzed (by X-ray photoelectron spectroscopy, infra-red spectroscopy,…) the interaction (bond insertion, complexation…) between the metal atoms and the molecules. When the metal atoms are strongly reactive with the end-groups of the molecules (e.g.…”