The mechanism of photochemical grafting of alkenes to H-terminated silicon has remained poorly understood. Here we demonstrate the importance of a previously unrecognized initiation process, photoelectron ejection (photoemission), as a facile way of initiating photochemical grafting of liquid alkenes to silicon surfaces when using ultraviolet light. A comparison of Si samples with vastly different photocarrier lifetimes showed no difference in the efficiency of alkene grafting. However, differences in the reactivities of different alkenes with different terminal groups that correlate with the electron affinities of these groups were observed. Our results indicate that photoemission is an effective way of initiating grafting because the irreversible nature of photoemission leaves the sample with a net excess of holes that have no corresponding electrons with which to recombine, while in a competing exciton mechanism, the net concentration of holes is limited by recombination processes.

The use of ultraviolet light to functionalize the surface of carbon-based materials with terminal alkenes has emerged as a way to overcome the high chemical stability of these surfaces to create functional interfaces. It was previously shown that surface-bound trifluoroacetic acid protected 10-aminodec-1-ene (TFAAD) can be used to promote the grafting of unreactive alkenes using 254 nm light, but the mechanism by which this enhancement occurs was not understood. Here, we present a detailed study of how surface-bound TFAAD enhances the grafting of organic molecules to the surface of hydrogen-terminated amorphous carbon. Infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy experiments show that pregrafting TFAAD onto carbon surfaces greatly enhances the subsequent grafting of 1-dodecene, dodecane and dodecane-d 26 by locally facilitating photoemission of electrons. Using a photopatterned TFAAD “seed” layer, we demonstrate that subsequent grafting of these hydrocarbons is enhanced in regions immediately adjacent to the seed and proceeds almost exclusively parallel to the carbon surface. The roles of liquid-phase radical species, valence band holes, and the photochemical fragmentation of surface species in controlling the overall reaction mechanism are discussed.

The direct covalent modification of silicon nanowires with DNA oligonucleotides, and the subsequent hybridization properties of the resulting nanowire–DNA adducts, are described. X-ray photoelectron spectroscopy and fluorescence imaging techniques have been used to characterize the covalent photochemical functionalization of hydrogen-terminated silicon nanowires grown on SiO2 substrates and the subsequent chemistry to form covalent adducts with DNA. XPS measurements show that photochemical reaction of H-terminated Si nanowires with alkenes occurs selectively on the nanowires with no significant reaction with the underlying SiO2 substrate, and that the resulting molecular layers have a packing density identical to that of planar samples. Functionalization with a protected amine followed by deprotection and use of a bifunctional linker yields covalently linked nanowire–DNA adducts. The biomolecular recognition properties of the nanowires were tested via hybridization with fluorescently tagged complementary and non-complementary DNA oligonucleotides, showing good selectivity and reversibility, with no significant non-specific binding to the incorrect sequences or to the underlying SiO2 substrate. Our results demonstrate that the selective nature of the photochemical functionalization chemistry permits silicon nanowires to be grown, functionalized, and characterized before being released from the underlying SiO2 substrate. Compared with solution-phase modification, the ability to perform all chemistry and characterization while still attached to the underlying support makes this a convenient route toward fabrication of well characterized, biologically modified silicon nanowires.

We report a mechanistic investigation of the charge transfer processes that occur during photochemical grafting of liquid alkenes to H-terminated surfaces of diamond and amorphous carbon. Spectrally resolved photoelectron yield experiments were performed to directly characterize the photoemission of electrons from the hydrogen-terminated surfaces into liquid alkenes, using trifluoroacetamide-protected 1-aminodec-1-ene (TFAAD) and 10-N-Boc-aminodec-1-ene (tBoc) as model alkenes having different terminal acceptor groups; 1-dodecene was also used as a control. Corresponding X-ray and ultraviolet photoelectron spectroscopy measurements (XPS, UPS) establish a clear correlation between the photoelectron yield, the grafting efficiency at different wavelengths, and the valence electronic structure of the substrate and of the reactant molecule. Direct imaging of the molecular layers via scanning electron microscopy shows that there are substantial differences in the sharpness of molecular patterns that can be produced on single-crystal type Ib (low-mobility) and type IIb (high-mobility) diamond samples. Our results demonstrate that electrons and holes both play important and distinct roles in the photochemical grafting of alkenes to diamond and amorphous carbon surfaces.