UV-induced grafting of alkenes to silicon surfaces: photoemission versus excitons.

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.

Assembly of nanocrystal arrays by block-copolymer-directed nucleation.

Growing in line: The surface chemistry of self-assembled nanostructured block copolymers is used to control the sites at which semiconducting metal sulfide nanocrystals nucleate and grow on a surface directly from aqueous solutions. This process is a new and general strategy for the bottom-up assembly of functional nanocrystalline materials for a variety of applications.

Photodetector arrays directly assembled onto polymer substrates from aqueous solution.

We report a simple and inexpensive approach to directly assemble arrays of cadmium sulfide (CdS) semiconductors onto transparent flexible poly(ethylene terephthalate) (PET) sheets via a polymer-mediated selective nucleation and growth process from an aqueous solution. This strategy of assembling functional materials onto plastics utilizes the surface functional molecules of the UV photooxidation patterned polymer to direct the nucleation and growth of CdS. We demonstrated that such assembled structures are viable for flexible macroelectronics, as manifested by the fabrication of CdS photodetector arrays on PET that can withstand bending. The best devices exhibited a specific detectivity of 3 x 10(11) cm Hz(1/2) W(-1) at 514-nm excitation wavelength at a modulation frequency of 90 Hz at room temperature. This direct assembly strategy eliminates additional lithography and etching steps during the deposition of the active inorganic semiconductor layer, is general to other inorganic materials and plastic substrates, and can enable low-cost, wearable, and/or disposable flexible electronics.

Dielectrophoretic manipulation and real-time electrical detection of single-nanowire bridges in aqueous saline solutions.

Dielectrophoretic manipulation of nanoscale materials is typically performed in nonionic, highly insulating solvents. However, biomolecular recognition processes, such as DNA hybridization and protein binding, typically operate in highly conducting, aqueous saline solutions. Here, we report investigations of the manipulation and real-time detection of individual nanowires bridging microelectrode gaps in saline solutions. Measurements of the electrode impedance versus frequency show a crossover in behavior at a critical frequency that is dependent on the ionic strength. We demonstrate that by operating above this critical frequency, it is possible to use dielectrophoresis to manipulate nanowires across electrode gaps in saline solutions. By using electrical ground planes and nulling schemes to reduce the background currents, we further demonstrate the ability to electrically detect bridging and unbridging events of individual nanowires in saline solutions. The ability to both manipulate and detect bridging events with electrical signals provides a pathway toward automated assembly of nanoscale devices that incorporate biomolecular recognition elements.