Bhavin N. Jariwala

In Situ Gas-Phase Hydrosilylation of Plasma-Synthesized Silicon Nanocrystals

Surface passivation of semiconductor nanocrystals (NCs) is critical in enabling their utilization in novel optoelectronic devices, solar cells, and biological and chemical sensors. Compared to the extensively used liquid-phase NC synthesis and passivation techniques, gas-phase routes provide the unique opportunity for in situ passivation of semiconductor NCs. Herein, we present a method for in situ gas-phase organic functionalization of plasma-synthesized, H-terminated silicon (Si) NCs. Using real-time in situ attenuated total reflection Fourier transform IR spectroscopy, we have studied the surface reactions during hydrosilylation of Si NCs at 160 °C. First, we show that, during gas-phase hydrosilylation of Si NCs using styrene (1-alkene) and acetylene (alkyne), the reaction pathways of the alkenes and alkynes chemisorbing onto surface SiH(x) (x = 1-3) species are different. Second, utilizing this difference in reactivity, we demonstrate a novel pathway to enhance the surface ligand passivation of Si NCs via in situ gas-phase hydrosilylation using the combination of a short-chain alkyne (acetylene) and a long-chain 1-alkene (styrene). The quality of surface passivation is further validated through IR and photoluminescence measurements of Si NCs exposed to air.

Atomic hydrogen interactions with amorphous carbon thin films

The atomic-scale interactions of H atoms with hydrogenated amorphous carbon (a-C:H) films were identified using molecular dynamics (MD) simulations and experiments based on surface characterization tools. Realistic a-C:H films developed using MD simulations were impinged with H atoms with a kinetic energy corresponding to a temperature of 700 K. The specific chemical reactions of the H atoms with the a-C:H surface were identified through a detailed analysis of the MD trajectories. The MD simulations showed that hydrogenation occurs primarily at the sp2 sites and converts them to sp3-hybridized C atoms. Depending on the hybridization of the next-nearest neighbor, a dangling bond may or may not be created. The hydrogenation reaction is highly exothermic, >2.5 eV, and proceeds with a negligible activation energy barrier via a mechanism similar to Eley–Rideal. In certain cases hydrogenation may also cleave a C–C bond. The reaction events observed through MD simulations are consistent with the surface characte...

Carbon monoxide-induced reduction and healing of graphene oxide

Graphene oxide holds promise as a carbon-based nanomaterial that can be produced inexpensively in large quantities. However, its structural and electrical properties remain far from those of the graphene sheets obtained by mechanical exfoliation or by chemical vapor deposition—unless efficient reduction methods that preserve the integrity of the parent carbon-network structure are found. Here, the authors use molecular dynamics and density functional theory calculations to show that the oxygen from the main functional groups present on graphene oxide sheets is removed by the reducing action of carbon monoxide; the energy barriers for reduction by CO are very small and easily overcome at low temperatures. Infrared and Raman spectroscopy experiments confirm the reduction in CO atmosphere and also reveal a strong tendency for CO to heal vacancies in the carbon network. Our results show that reduced graphene oxide with superior properties can be obtained through reduction in CO atmosphere.

Silicon quantum dot optical properties and synthesis: Implications for photovoltaic devices

We study key optical properties for designing an absorber layer with silicon quantum dots (Si-QDs), including the absorptivity of the material, whether the character of the bandgap is direct or indirect, and the relation between absorption and photoluminescence. We report necessary synthesis conditions in order to control size, size distribution, and crystallinity of Si-QDs. This is important for applications of Si-QDs in photovoltaics [1,2], where they excite interest due to their size-tunable bandgap [3], potentially cheap fabrication, and possible enhancement of solar energy conversion efficiency through mechanisms such as multiple exciton generation [4].