Using wet-chemical self-assembly, we demonstrate that standard surface reactions can be markedly altered. Although HF etching of Si surfaces is known to produce H-terminated surfaces, we show that up to approximately 30% of a monolayer of stable Si-F bonds can be formed on atomically smooth Si(111) surfaces on HF reaction, when chemically isolated Si atoms are the target of the reaction. Similarly, approximately 30% Si-OH termination can be achieved by immersion of the partially covered F-Si(111) surface in water without oxidation of the underlying Si substrate. Such reactions are possible when H-terminated (111)-oriented Si surfaces are initially uniformly patterned with methoxy groups. These findings are contrary to the knowledge built over the past twenty years and highlight the importance of steric interactions at surfaces and the possibility to stabilize products at surfaces that cannot be obtained on chemically homogeneous surfaces.

Molecular electronics is an attractive option for low-cost devices because it involves highly uniform self-assembly of molecules with a variety of possible functional groups. However, the potential of molecular electronics can only be turned into practical applications if reliable contacts can be established without damaging the organic layer or contaminating its interfaces. Here, a method is described to prepare tightly packed carboxyl-terminated alkyl self-assembled monolayers (SAMs) that are covalently attached to silicon surfaces and to deposit thin metallic copper top contact electrodes without damage to this layer. This method is based on a two-step procedure for SAM preparation and the implementation of atomic layer deposition (ALD) using copper di-sec-butylacetamidinate [Cu(sBu-amd)](2). In situ and ex situ infrared spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrical measurements are used to characterize the chemical modification of the Si/SAM interface, the perturbation of the SAM layer itself, and the metal homogeneity and interaction with the SAM headgroups. This work shows that (i) carboxyl-terminated alkyl monolayers can be prepared with the same high density and quality as those achieved for less versatile methyl-terminated alkyl monolayers, as evidenced by electrical properties that are not dominated by interface defects; (ii) Cu is deposited with ALD, forming a bidentate complexation between the Cu and the COOH groups during the first half cycle of the ALD reaction; and (iii) the Si/SAM interface remains chemically intact after metal deposition. The nondamaging thin Cu film deposited by ALD protects the SAM layer, making it possible to deposit a thicker metal top contact leading ultimately to a controlled preparation of molecular electronic devices.

The initial surface chemistry and growth mechanisms of the atomic layer deposition (ALD) of metallic copper on SiO(2) surfaces are investigated using an amidinate precursor (copper(I) di-sec-butylacetamidinate, [Cu((s)Bu-amd)](2)) and molecular hydrogen. Using in situ Fourier transform infrared spectroscopy together with calculations based on density functional theory, we show that the initial surface reaction of [Cu((s)Bu-amd)](2) with hydroxylated SiO(2) takes place by displacement of one of the sec-butylacetamidinate ligands at a surface -OH site, thus forming a Si-O-Cu-((s)Bu-amd) surface species, evident by the stretching vibrations of Si-O-Cu and the chelating -NCN- bonds. Molecular hydrogen exposure during a subsequent pulse dissociates most of the sec-butylacetamidinate ligands bound to surface Cu, which releases free amidine vapor, leaving Cu atoms free to agglomerate on the surface and thus opening more reactive sites for the next [Cu((s)Bu-amd)](2) pulse. Copper agglomeration is evident in the IR absorbance spectra through the partial recovery of the intensity of SiO(2) optical phonon modes upon H(2) reduction, which was lost after the reaction of [Cu((s)Bu-amd)](2) with the initial SiO(2) surface. The thermally activated ligand rearrangement from a bridging to a monodentate structure occurs above 220 degrees C through hydrogenation of the ligand by surface hydroxyl groups after exposure to a [Cu((s)Bu-amd)](2) pulse. As Cu particles grow with further ALD cycles, the activation temperature is lowered to 185 degrees C, and hydrogenation of the ligand takes place after H(2) pulses, catalyzed by Cu particles on the surface. The surface ligand rearranged into a monodentate structure can be removed during subsequent Cu precursor or H(2) pulses. Finally, we postulate that the attachment of dissociated ligands to the SiO(2) surface during the [Cu((s)Bu-amd)](2) pulse can be responsible for carbon contamination at the surface during the initial cycles of growth, where the SiO(2) surface is not yet completely covered by copper metal.

In situ infrared absorption spectroscopy is used to monitor atomic layer deposition (ALD) of aluminum oxide (Al2O3) on carboxylic acid-terminated self-assembled monolayers (SAMs), Si(111)-(CH2)10-COOH (or COOH-SAMs), directly grafted on silicon (111) at approximately 100 degrees C. The quality of resulting Al2O3 films is comparable to Al2O3 on SiO2. Both the SAM film and the Si/SAM interface remain chemically stable during growth and upon post annealing to 400 degrees C, suggesting that the tight packing of the alkyl chains and COOH-SAM head groups presents a diffusion barrier and promotes ordered nucleation for ALD.