Michael P. Stewart

Photopatterned Hydrosilylation on Porous Silicon

By illumination with white light porous silicon surfaces can be functionalized with alkynes and alkenes (see scheme). The hydrosilylation reactions are very simple to perform and lead to stable, patterned surfaces. This methodology opens new opportunities in the technological applications of porous silicon in both integrated circuits and sensors.

Inelastic Electron Tunneling via Molecular Vibrations in Single-Molecule Transistors

In single-molecule transistors, we observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of inelastic electron tunneling spectroscopy of single molecules, with the transistor geometry allowing in situ tuning of the electronic states via a gate electrode. The vibrational features shift and change shape as the electronic levels are tuned near resonance, indicating significant modification of the vibrational states. When the molecule contains an unpaired electron, we also observe vibrational satellite features around the Kondo resonance.

Derivatized Porous Silicon Mirrors: Implantable Optical Components with Slow Resorbability

The stability of derivatized mesoporous silicon mirrors in simulated human blood plasma has been assessed. The rate at which they are dissolved in-vivo is predicted to be tunable by surface chemistry over timescales of weeks to years, and high reflectivity can be maintained until the bottom of the multilayer stack starts to corrode. Such biodegradable optical components could be utilized to direct and define optical path lengths for therapeutic treatments and minimally-invasive diagnostics.

New Approaches Toward the Formation of Silicon-Carbon Bonds on Porous Silicon

Porous silicon is a material of intense technological and fundamental interest, due to its high surface area, quantum confinement effects, light emission, and complex nanoscale architecture. There is a strong desire to be able to control the surface properties of this versatile material at will through functionalization with monolayers bound through the stable silicon-carbon bond. Incredibly, the surface chemistry of porous silicon is often very different from that of a silicon-based molecule, or a flat surface, due to interesting electronic effects resulting from both the semiconducting and nanoscale characteristics of the material. Recent results outlining the rapid progress of silicon-carbon bond formation on porous silicon surfaces will be described.

Electrical characterization of metal-molecule-silicon junctions

Abstract Direct assembly of molecules onto silicon surfaces is of particular interest for potential employment in hybrid organic-semiconductor devices. In this study, aryl diazonium salts are used to assemble covalently bound molecular groups onto a hydride-passivated, oxide-free n-type Si(111) surface. The reaction of 4-(trimethylsilylethynyl)benzenediazonium tetrafluoroborate generates a molecular layer of 4-(trimethylsilylethynyl)phenylene (TMS-EP) on the Si surface. The monolayer modifies the electrical properties of the interface and exhibits nonlinear current–voltage characteristics, as compared with the ohmic behavior observed from metal- n++-Si(111) junctions. Results of current–voltage measurements at variable temperatures (from 300 to 10 K) on samples made with the TMS-EP molecules do not show significant thermally-activated transport, indicating tunneling is the dominant transport mechanism for this device structure. The measured data is compared to a tunneling model.

Anodic and cathodic electrografting of alkynes on porous silicon

A versatile electrochemical grafting reaction connects conjugated molecules to porous silicon surfaces with either a positive or negative bias.

A study of the formation, purification and application as a SWNT growth catalyst of the nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98]

The synthetic conditions for the isolation of the iron-molybdenum nanocluster FeMoC [HxPMo12O40 [subset]H4Mo72Fe30(O2CMe)15O254(H2O)98], along with its application as a catalyst precursor for VLS growth of SWNTs have been studied. As-prepared FeMoC is contaminated with the Keplerate cage [H4Mo72Fe30(O2CMe)15O254(H2O)98] without the Keggin [HxPMo12O40]n- template, however, isolation of pure FeMoC may be accomplished by Soxhlet extraction with EtOH. The resulting EtOH solvate is consistent with the replacement of the water ligands coordinated to Fe being substituted by EtOH. FeMoC-EtOH has been characterized by IR, UV-vis spectroscopy, MS, XPS and 31P NMR. The solid-state 31P NMR spectrum for FeMoC-EtOH (delta-5.3 ppm) suggests little effect of the paramagnetic Fe3+ centers in the Keplerate cage on the Keggin ions phosphorous. The high chemical shift anisotropy, and calculated T1 (35 ms) and T2 (8 ms) values are consistent with a weak magnetic interaction between the Keggin ions phosphorus symmetrically located within the Keplerate cage. Increasing the FeCl2 concentration and decreasing the pH of the reaction mixture optimizes the yield of FeMoC. The solubility and stability of FeMoC in H2O and MeOH-H2O is investigated. The TGA of FeMoC-EtOH under air, Ar and H2 (in combination with XPS) shows that upon thermolysis the resulting Fe : Mo ratio is highly dependent on the reaction atmosphere: thermolysis in air results in significant loss of volatile molybdenum components. Pure FeMoC-EtOH is found to be essentially inactive as a pre-catalyst for the VLS growth of single-walled carbon nanotubes (SWNTs) irrespective of the substrate or reaction conditions. However, reaction of FeMoC with pyrazine (pyz) results in the formation of aggregates that are found to be active catalysts for the growth of SWNTs. Activation of FeMoC may also be accomplished by the addition of excess iron. The observation of prior works reported growth of SWNTs from FeMoC is discussed with respect to these results.

Electrical characterization of metal-molecule-silicon junctions.

Abstract: Direct assembly of molecules onto silicon surfaces is of particular interest for potential employment in hybrid organic‐semiconductor devices. In the study we report here, aryl diazonium salts were used to assemble covalently bound molecular groups on a hydride‐passivated, oxide‐free n‐type Si(111) surface. The reaction of 4‐(trimethylsilylethynyl)benzenediazonium tetrafluoroborate generates a molecular layer of 4‐(trimethylsilylethynyl)phenylene (TMS‐EP) on the n++‐Si(111) surface. The monolayer modifies the electrical properties of the interface and exhibits nonlinear current‐voltage characteristics, as compared with the ohmic behavior observed from metal‐n++‐Si(111) junctions. The result of current‐voltage measurements at variable temperatures (from 300 to 10 K) on samples made with the TMS‐EP molecule does not show significant thermally‐activated transport, indicating that tunneling is the dominant transport mechanism. The measured data is compared to a tunneling model.

Functionalization of Porous Silicon Surfaces through Hydrosilylation Reactions

Hydrosilylation of alkynes and alkenes on silicon surfaces utilizing the native Si-H termination can be smoothly and rapidly carried out (30 s to 24 h) at room temperature through hydrosilylation mediated by Lewis acid catalysts or photoinduction with white light. Insertion of alkynes and alkenes into surface silicon hydride bonds yields covalently bound alkenyl and alkyl groups, respectively. Different chemical functionalities can be incorporated through these hydrosilylation reactions, including ester, hydroxy, chloro, nitrile and chiral groups. Hydrophobic porous silicon surfaces demonstrate remarkable stability with respect to boiling aqueous aerated pH 1 to 10 solutions, and protect the bulk silicon from attack. Modification and tailoring of surface properties through this series of reactions induce wide variations in photoluminescent behavior of porous silicon, leading to almost complete quenching in the case of substituted and unsubstituted styrenyl termination, and minor decreases for alkyl and alkenyl functionalization. Because of the broad range of stable, modified surfaces produced using this chemistry, the work described here represents an important step towards technological applications of silicon surfaces.

Functionalization of Porous Silicon with Alkenes and Alkynes via Carbocation-Mediated Hydrosilylation

Efforts to produce stable, derivatized porous silicon have yielded a number of chemical methods capable of functionalizing this interesting material with organic monolayers. Hydrideterminated porous silicon substrates react with alkenes and alkynes in the presence of dilute triphenylcarbenium salt solutions to respectively produce alkyl- and alkenyl-functionalized materials. Characterization by transmission FTIR and solid-state NMR suggests the formation of highly stable silicon-carbon bonds to yield covalently bound organic moieties. Porous silicon passivated in this fashion exhibits a greater resistance than that of the native material to chemical degradation, indicating that the organic functionalities may serve to sterically shield the nanocrystallites from nucleophiles. Hydrosilylation is proposed to proceed via hydride