Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers

Abstract

Instability of ultrathin surface oxides on alloys under environmental conditions can limit the opportunities for applications of these systems when the thickness control of the insulating oxide film is crucial for device performance. A procedure is developed to directly deposit self-assembled monolayers (SAM) from solvent onto substrates prepared under ultra-high vacuum conditions without exposure to air. As an example, rhenium photosensitizers functionalized with carboxyl linker groups are attached to ultrathin alumina grown on NiAl(1 1 0). The thickness change of the oxide layer during the SAM deposition is quantified by x-ray photoelectron spectroscopy and can be drastically reduced to one atomic layer. The SAM acts as a capping layer, stabilizing the oxide thin film under environmental conditions. Ultraviolet photoelectron spectroscopy elucidates the band alignment in the resulting heterostructure. The method for molecule attachment presented in this manuscript can be extended to a broad class of molecules vulnerable to pyrolysis upon evaporation and presents an elegant method for attaching molecular layers on solid substrates that are sensitive to air.

Comparative Study of the Different Anchoring of Organometallic Dyes on Ultrathin Alumina

“…The 2L-alumina/NiAl(110) samples were rinsed three times with 0.1 mM solution of the rhenium molecules dissolved in acetonitrile (CH 3 CN) to form multilayers. After that, the samples were rinsed three times with pure CH 3 CN to remove physisorbed molecules, and a monolayer of chemisorbed molecules was left on the surface, as described previously by our group . After pump down of the chamber, the samples could be transferred under vacuum back into the UHV system for surface characterization.…”

“…47 Upon SAM deposition, a slight increase of the Al 3+ component between 74 and 75 eV is observed, due to remaining impurities in the solvent and the tendency of 2L-alumina/NiAl(110) to further oxidation, as described in earlier work. 26 This rise is identical for all three molecules, and we quantify the total oxide thickness after molecule deposition to be 2.5 atomic layers.…”

“…Figure shows the chemical structures of the considered dye molecules. The dye 1 is [Re­(NCS)­(CO) 3 (DCO 2 bipy)] (DCO 2 bipy = 2,2′-bipyridine-4,4′-dicarboxylic acid), which was previously used by our group to demonstrate the functionalization of 2L-alumina by self-assembly from solution . Molecule 2 is [Re­(NCS)­(CO) 3 (DMPO 3 bipy)] [DMPO 3 bipy = 2,2′-bipyridine-4,4′-bis­(methylphosphonic acid)], where the carboxyl linkers are replaced by methylphosphonic acid linkers.…”

“…After that, the samples were rinsed three times with pure CH 3 CN to remove physisorbed molecules, and a monolayer of chemisorbed molecules was left on the surface, as described previously by our group. 26 After pump down of the chamber, the samples could be transferred under vacuum back into the UHV system for surface characterization. For the ruthenium dyes, the solution was oversaturated due to the limited solubility of the molecules in CH 3 CN.…”

“…■ RESULTS AND DISCUSSION Self-assembled monolayers (SAMs) of the respective dyes are formed from solutions on clean ultrathin alumina films grown on a NiAl(110) crystal 26 in a custom-built chamber attached to our ultrahigh-vacuum system. The areal densities of the chemisorbed molecules are quantified by means of XPS data, and the coadsorption of the counterions for molecules 3 and 5 is also discussed.…”