Sequential reactions with amine-terminated monolayers and isolated molecules on H/Si(111).

A simple method for preparing monolayers with terminal amine functionality is demonstrated. A gas-phase photochemical reaction of 1,3-diaminopropane with a H-terminated Si(111) surface results in the molecules covalently attaching to the surface, primarily through the formation of a Si--N bond. These monolayers are characterized by scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). The reactivity of the terminal amine is confirmed by exposing the monolayer to benzaldehyde, resulting in the formation of an imine link and the grafting of phenyl rings onto the surface. For short irradiation times, this reaction leads to the formation of isolated amine groups on an otherwise pristine H-terminated surface. STM and HREELS studies of the benzaldehyde reaction on these low-coverage surfaces (less than 0.005 monolayers) indicate that the reaction is restricted to the reactive amine groups, leaving the remainder of the surface unaffected. This simple approach for a sequential coupling reaction is expected to facilitate attachment of more complex molecules (molecular switches, biomolecules) for single-molecule STM studies.

Reaction of alkenes with hydrogen-terminated and photooxidized silicon surfaces. A comparison of thermal and photochemical processes.

Reagentless micropatterning of hydrogen-terminated Si(111) via UV irradiation through a photomask has proven to be a convenient strategy for the preparation of ordered bicomponent monolayers. The success of this technique relies upon the differential rate of reaction of an alkene with the hydrogen-terminated and photooxidized regions of the surface. Monolayer formation can be accomplished under either thermal or photochemical conditions. It was observed that, after 3 h, reaction in neat alkene solution irradiation (Rayonet, 300 nm) afforded the expected patterned surface, while thermal conditions (150 degrees C) resulted in a partial loss of pattern fidelity. Monolayer properties and formation were studied on oxidized and hydrogen-terminated silicon under thermal and photochemical initiation, by contact angle, ellipsometry, Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. Results show that alkenes add to silanol groups on the silica surface in a manner consistent with acid catalysis: once attached to the surface, the silica oxidized the hydrocarbon.

Formation of organic monolayers on silicon via gas-phase photochemical reactions.

A new method for the formation of molecular monolayers on silicon surfaces utilizing gas-phase photochemical reactions is reported. Hydrogen-terminated Si(111) surfaces were exposed to various gas-phase molecules (hexene, benzaldehyde, and allylamine) and irradiated with ultraviolet light from a mercury lamp. The surfaces were studied with in situ Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. The generation of gas-phase radicals was found to be the initiator for organic monolayer formation via the abstraction of hydrogen from the H/Si(111) surface. Monolayer growth can occur through either a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds.

Selective adsorption of pyridine at isolated reactive sites on Si(100).

Dative bonding of nitrogen-containing heterocycles offers a strategy for the controlled attachment of aromatic molecules to silicon surfaces. However, while scanning tunneling microscopy shows that pyridine on clean Si(100) initially binds via a dative bonding configuration, slow conversion to a more stable bridging state, destroying the aromaticity, is observed. To restrict adsorption to the dative bonded form, we investigated the interaction of pyridine with isolated reactive sites on partially H-terminated Si(100). While dative bonding on isolated clean dimers is observed, single dangling bonds remain unreacted. This selectivity can be accounted for by the ability of the Si-Si dimers to act as electron acceptors that stabilize the dative bonded species. This observation has important implications for the controlled positioning of single molecules on silicon via dative bonding.

Photochemical attachment of organic monolayers onto H-terminated Si(111): radical chain propagation observed via STM studies.

Photochemical reactions of terminal alkenes with hydrogen-terminated silicon surfaces are being used by many groups to produce covalently attached organic monolayers with a wide range of terminal functionalities. Despite the considerable activity in this area, the mechanism for these reactions has not been definitively established. Here we present STM and HREELS data on a sequence of partially reacted samples, showing the progress of the reaction. The attachment reaction is found to proceed via formation of irregularly shaped islands that appear to grow by a pseudorandom walk process. These data support a radical chain propagation mechanism previously suggested for this reaction. However, since the photons employed here (447 nm) lack sufficient energy for Si-H bond cleavage, an alternate mechanism for initiating the chain reaction appears to be required.