Keith A. Williams

Nanotechnology: Carbon nanotubes with DNA recognition

Since the discovery of their one-dimensional electronic band structure, the leading candidate that has emerged for nanodevice applications is single-walled carbon nanotubes (SWNTs) . Here we unite their unique properties with the specific molecular-recognition features of DNA by coupling SWNTs to peptide nucleic acid (PNA, an uncharged DNA analogue) and hybridizing these macromolecular wires with complementary DNA. Our findings provide a new, versatile means of incorporating SWNTs into larger electronic devices by recognition-based assembly, and of using SWNTs as probes in biological systems by sequence-specific attachment.

Hydrogen Adsorption in Carbon Materials

Recent reports of very high, reversible adsorption of molecular hydrogen in pure nanotubes, alkali-doped graphite, and pure and alkali-doped graphite nanofibers (GNFs) have aroused tremendous interest in the research community, stimulating much experimental work and many theoretical calculations worldwide. The U.S. Department of Energy (DOE) Hydrogen Plan has seta standard for this discussion by providing a commercially significant benchmark for the amount of reversible hydrogen adsorption. This benchmark requires a system-weight efficiency (the ratio of stored H 2 weight to system weight) of 6.5-wt% hydrogen and a volumetric density of 63 kg H 2 /m. If the encouraging experimental reports (summarized in Table I) are reproducible, it may be possible to reach the goals of the DOE Hydrogen Plan. On the other hand, the community still awaits confirmation of these experimental results by workers in other laboratories. Of additional concern is the fact that theoretical calculations have been unable to identify adsorption mechanisms compatible with the requirements of the DOE Hydrogen Plan. An economical, safe, hydrogen-storage medium is a critically needed component of a hydrogen-fueled transportation system. Hydrogen storage in a carbon-based material offers further advantages associated with its low mass density. Furthermore, fuel cell technology involving the conversion of hydrogen into protons, or hydrogen and oxygen into electric current, is being vigorously researched for both transportation and small power-plant applications.

Experimental probes of the molecular hydrogen–carbon nanotube interaction

Electrical transport (resistance R and thermoelectric power S), Raman scattering, and hydrogen adsorption are used to study the interaction of hydrogen molecules with ropes of single-walled carbon nanotubes. The data are consistent with H2 physisorption under the experimental conditions investigated (4 KoTo500 K; 0.1 atmoPo20 atm). The response of S; R to 1 atm hydrogen at 500 Kis consistent with the introduction of a new scattering channel for electrons/holes in the metallic tubes. Raman scattering from the Q-branch of hydrogen molecules adsorbed on the surface is found shifted only by 1–2 cm � 1 from their frequencies in the free molecule and indicates that two different adsorption sites can be detected. Finally, H2 wt% storage in heavily processed ropes of SWNTs are found to exceed 6% at B1 atm and T ¼ 77 Kand the isosteric heat of adsorption is found to be 120 meV. r 2002 Elsevier Science B.V. All rights reserved.

Gd2@C79N: Isolation, Characterization, and Monoadduct Formation of a Very Stable Heterofullerene with a Magnetic Spin State of S = 15/2

The dimetallic endohedral heterofullerene (EHF), Gd(2)@C(79)N, was prepared and isolated in a relatively high yield when compared with the earlier reported heterofullerene, Y(2)@C(79)N. Computational (DFT), chemical reactivity, Raman, and electrochemical studies all suggest that the purified Gd(2)@C(79)N, with the heterofullerene cage, (C(79)N)(5-) has comparable stability with other better known isoelectronic metallofullerene (C(80))(6-) cage species (e.g., Gd(3)N@C(80)). These results describe an exceptionally stable paramagnetic molecule with low chemical reactivity with the unpaired electron spin density localized on the internal diatomic gadolinium cluster and not on the heterofullerene cage. EPR studies confirm that the spin state of Gd(2)@C(79)N is characterized by a half-integer spin quantum number of S = 15/2. The spin (S = ½) on the N atom of the fullerene cage and two octet spins (S = 7/2) of two encapsulated gadoliniums are coupled with each other in a ferromagnetic manner with a small zero-field splitting parameter D. Because the central line of Gd(2)@C(79)N is due to the Kramers doublet with a half-integer spin quantum number of S = 15/2, this relatively sharp line is prominent and the anisotropic nature of the line is weak. Interestingly, in contrast with most Gd(3+) ion environments, the central EPR line (g = 1.978) is observable even at room temperature in a toluene solution. Finally, we report the first EHF derivative, a diethyl bromomalonate monoadduct of Gd(2)@C(79)N, which was prepared and isolated via a modified Bingel-Hirsch reaction.

EFFECTS OF BEHAVIORAL SKILLS TRAINING ON PARENTAL TREATMENT OF CHILDREN'S FOOD SELECTIVITY

We used behavioral skills training to teach parents of 3 children with autism spectrum disorder and food selectivity to conduct a home-based treatment package that consisted of taste exposure, escape extinction, and fading. Parent performance following training improved during both taste sessions and probe meals and was reflected in increases in childrens acceptance of bites and decreases in their disruptive behavior. Parents also reported that increases in diet variety were maintained at follow-up.

Raman spectroscopic investigation of H2, HD, and D2 physisorption on ropes of single-walled, carbon nanotubes.

We have observed the S- and Q-branch Raman spectra of H2, HD, and D2 adsorbed at 85 K and pressures up to 8 atm on single-walled, carbon nanotubes (SWNT). Comparative data for H2 on graphite and C60 were also collected. Frequency-downshifted and upshifted features were observed in the Q-branch spectra of H2 on C60 and SWNT. These shifts are small and are therefore inconsistent with charge transfer. An H2-surface potential with van der Waals and electrostatic terms was developed and used to estimate the shifts in the frequency of the Q(0) transition of H2 adsorbed in two types of sites. These calculations corroborate the experimental findings and indicate physisorption in multiple sites of the SWNT ropes.

Investigation of Gd3N@C2n (40 n 44) family by Raman and inelastic electron tunneling spectroscopy

The structure and vibrational spectrum of Gd3N@C80 is studied through Raman and inelastic electron tunneling spectroscopy as well as density-functional theory and universal force eld calculations. Hindered rotations, shown by both theory and experiment, indicate the formation of a Gd3N-C80 bond which reduces the ideal icosahedral symmetry of the C80 cage. The vibrational modes involving the movement of the encapsulated species are a ngerprint of the interaction between the fullerene cage and the core complex. We present Raman data for the Gd3N@C2n 40 n 44 family as well as Y3N@C80, Lu3N@C80, and Y3N@C88 for comparison. Conductance measurements have been performed on Gd3N@C80 and reveal a Kondo effect similar to that observed in C60.

Raman study of Fano interference in p-type doped silicon

As the silicon industry continues to push the limits of device dimensions, tools such as Raman spectroscopy are ideal to analyze and characterize the doped silicon channels. The effect of inter-valence band transitions on the zone center optical phonon in heavily p-type doped silicon is studied by Raman spectroscopy for a wide range of excitation wavelengths extending from the red (632.8 nm) into the ultra-violet (325 nm). The asymmetry in the one-phonon Raman lineshape is attributed to a Fano interference involving the overlap of a continuum of electronic excitations with a discrete phonon state. We identify a transition above and below the one-dimensional critical point (E = 3.4 eV) in the electronic excitation spectrum of silicon. The relationship between the anisotropic silicon band structure and the penetration depth is discussed in the context of possible device applications. Copyright

Deep UV pattern definition in PMMA

We have patterned polymethyl methacrylate (PMMA) resist by exposing it to the fifth harmonic (213?nm) of an Nd:YAG source through metalized apertures in contact with the resist. Interference patterns with both near-?and far-field origins were observed. In order to test the contrast and uniformity of exposure, we deposited germanium onto developed areas to form arrays with feature sizes of ~200?nm. We present a straightforward model for interference effects generated in our process, and discuss opportunities for direct-write lithography through single apertures.

Towards DNA‐Mediated Self Assembly of Carbon Nanotube Molecular Devices

We report covalent coupling of peptide nucleic acid (PNA) to ropes of carboxylated, single‐walled, carbon nanotubes. Subsequent hybridization of these functionalized nanotubes with DNA was investigated by atomic force microscopy (AFM). Nanotubes with duplex DNA attached to their ends were observed. We expect that the synthesis of these oligonucleotide‐derivatized nanotubes is an important first step important toward uniting the recognition capabilities of DNA with the unique electronic properties of carbon nanotubes.