J. C. Poler

Characterization of the Si/SiO2 interface morphology from quantum oscillations in Fowler–Nordheim tunneling currents

As design rules shrink to conform with ultra‐large‐scale integration device dimensions, gate dielectrics for metal–oxide–semiconductor field effect transistor structures are required to be scaled to below ∼60 A, where some properties of the device, such as interface roughness, that are negligible for thicker films become critical. Microroughness at the interface of ultrathin MOS capacitors has been shown to degrade these devices. The present study focuses on the interfacial region of ∼50 A SiO2 on Si, using the quantum oscillations in Fowler–Nordheim tunneling currents as a probe. The oscillations are sensitive to the electron potential and abruptness of the film and interfaces. In particular, inelastic scattering of the electrons will reduce the amplitude of the oscillations. The amplitude of the oscillations is used to examine the degree of microroughness at the interface that results from a preoxidation high temperature anneal in an inert ambient containing various amounts of H2O. Atomic force microsco...

Sonochemical formation of methyl hydroperoxide in polar aprotic solvents and its effect on single-walled carbon nanotube dispersion stability.

Ultrasonication is a common method for dispersing nanoparticles and colloids. We have found that, under certain conditions, unintended sonochemical reactions can be initiated by the incident ultrasonic energy, yielding unwanted byproducts. In this work, we determined that methyl hydroperoxide can be produced by an autoxidation chain reaction when ultrasonicating polar aprotic solvents containing methyl groups. Methyl radicals were detected during ultrasonication by their interaction with lucigenin, which emits sonochemiluminescence. A colorimetric triiodide test was used to confirm the presence of a hydroperoxide. The concentration of methyl hydroperoxide as a function of the ultrasonication time was measured by titration with NaOH. When above the critical coagulation concentration, this sonochemical byproduct collapses the electrical double layer, disrupting the dispersion stability and lowering the dispersion limits. This is significant when developing ultrasonication processes for dispersion of nanoparticles and colloids. There are no other examples of sonochemically initiated solvent autoxidation destabilizing single-walled carbon nanotube dispersions reported in the literature.

Direct Measurement of the Interactions of Amide Solvents with Single-Walled Carbon Nanotubes Using Isothermal Titration Calorimetry

The interaction enthalpy of amide solvents with single-walled carbon nanotube (SWCNT) dispersions is measured using isothermal titration calorimetry (ITC). N,N-Dimethyl-formamide (DMF) and N-methyl-2-pyrilidone (NMP) were used to make dispersions of highly purified (6,5) SWCNTs. Using isothermal titration calorimetry, the ΔH and K(A) terms related to the solvent-nanotube interactions were measured, and ΔG and ΔS of the interaction were determined. It was found that the interaction enthalpy of NMP with SWCNTs dispersed in DMF was exothermic. The addition of a second solvent into a NMP or DMF dispersion produced spontaneous exfoliation of SWCNT bundles as the solvent properties became more favorable. During the titration, a positive change in interaction entropy within the dispersed system due to the unbundling of SWCNTs was measured. From blank titrations of pure DMF into pure NMP and the reverse, dilution enthalpies were also calculated and compared to the literature, along with the corresponding enthalpic interaction coefficients, h(xx) and h(xxx). From our results, ITC appears to be a viable technique for measuring the interaction of solvent molecules with the surface of SWCNTs and for measuring the effect of mixed solvent properties on SWCNT dispersions.

Oxidation and formation mechanisms in disilicides: VSi2 and CrSi2, inert marker experiments and interpretation

The process of atomic transport in the silicide during oxidation of silicide layers formed on Si substrates has been analyzed by means of implanted inert markers. The results confirm that CrSi2 oxidizes via the diffusion of Cr atoms towards the Si substrate, and reveal that the same type of transport occurs in VSi2, which is in opposition to the growth of these disilicides that proceeds via Si motion. Moreover during the oxidation of both VSi2 and CrSi2, the diffusing metal atoms are accompanied by a large proportion of the Si atoms from the silicide which also diffuse towards the substrate. Thus, the experimental evidence now available reveals that all the silicon‐rich silicides, in increasing atomic numbers from TiSi2 to NiSi2, oxidize with the formation of metal‐free SiO2 in a process that involves the diffusion of both metal and Si regardless of the predominant moving species during silicide formation. In CrSi2, VSi2, as well as TiSi2, with similar structures, examination of the structure shows that d...

Photon enhanced aggregation of single walled carbon nanotube dispersions

The authors describe a photon enhanced aggregation of dispersed single walled carbon nanotubes in the presence of electron transfer reagents. A recently synthesized metallodendrimer strongly and specifically binds to the ends of the nanotubes. Upon optical excitation, of the metal to ligand charge transfer absorption, of various ruthenium complexes, the nanotubes rapidly coagulate. The electron transfer mechanism is consistent with observed photon enhancement process. These results support a directed self-assembly paradigm for nanostructured materials.

Novel charge integrating pulsed I(V) technique: A measurement of Fowler–Nordheim currents through thin SiO2 films

The design, characterization, and applications of a novel charge integrating pulsed current‐voltage I(V) measurement are described. Tunneling transport through thin metal‐oxide‐semiconductor capacitors is measured over ten orders of magnitude of current. Short pulse widths (<1 μs) allow electrical characterization of these films under high current densities without significant charge injection. A study of the quantum interference of electrons during Fowler–Nordheim (FN) conduction is used to illustrate the measurement.

Cross Sectional AFM of Oxidized Porous Silicon

Morphological studies of bulk porous-silicon (PS) are presented. Using the atomic force microscope, cleaved cross-sections of electrochemically processed porous-silicon were investigated. The porous-silicon/silicon interface was examined. Using a room temperature UV O3 generator, the porous silicon-samples were oxidized. Oxidation in air was also observed. The morphology of the oxidized porous-silicon is process dependent. AFM contrast was enhanced by selective wet chemical etching. Unusually fast oxide etching in BOE was observed. A possible oxide/PS model is proposed.

Pump‐probe charge integrating technique: A study of trap emission kinetics in silicon dioxide

A new method for the study of charge emission from electrically active traps is described. This technique uses a sequence of electrostatic pulses in a pump‐probe sequencing scheme, to examine the detrapping kinetics of states in the Si/SiO2 metal oxide semiconductor system. We show that with the pump‐probe technique, the initial time regime of the trap emission dynamics is accessed. The results observed on oxidized lightly doped p‐Si(111) substrates show trap interaction at high injection bias, and detrapping kinetics are discussed in the context of an occupation dependent trap interaction model.

Efforts to Implement a PhD degree program in “Nanoscale Science” at UNC Charlotte

UNC Charlotte is a young and growing research university. Most of the Ph.D. programs on our campus have been designed to be interdisciplinary. This strategic choice was made for both economic and pedagogical reasons. At the heart of the drive for interdisciplinary degree programs is the recognition that a lack of educational diversity at the Ph.D. level is limiting for new graduates in today’s research and discovery landscape. This need for educational diversity is even more acute in the sciences. We need more chemists that know more physics, and we need more physicists that know more biology, and we need more engineers that understand matter at a molecular scale. To this end, faculty in the departments of chemistry, optical sciences, mechanical engineering, and electrical engineering have designed and are implementing a new interdisciplinary Ph.D. degree in “Nanoscale Science”. We do not believe that a length scale can institute a philosophy of science. However, research involving nanoscale materials and phenomena do require an educational perspective far broader than traditional academic disciplines currently offer. The question is how to deliver a broad graduate education that enables each student to reach an expertise required for the Ph.D. This is the question that has driven our pedagogical development of this Nanoscale science program. The overall structure of this program will be described and compared to other current efforts in Nanoscale graduate education throughout the United States. Various novel features will be discussed, with the hope for critical feedback and discussion. Details of the educational opportunities we have designed and the method of assessment we will employ will be presented. Background The Need for Ph.D. Programs in Nanoscale Science The National Science Foundation estimates that nanotechnology will be a

Modeling of Supramolecular Systems, Mechanically Docked to Carbon Nanotubes

1 trillion global industry by 2010-2015. This will require about 2 million workers in the field of nanoscale science and engineering, of which 0.8-0.9 million will be in the United Mater. Res. Soc. Symp. Proc. Vol. 931