Biomedical subjects

Jonathan E Spanier

Publications and source records attributed to Jonathan E Spanier.

5 recordsLinked to original sources

PubMed2006

Enhanced Raman scattering from individual semiconductor nanocones and nanowires.

We report strong enhancement (approximately 10(3)) of the spontaneous Raman scattering from individual silicon nanowires and nanocones as compared with bulk Si. The observed enhancement is diameter (d), excitation wavelength (lambda(laser)), and incident polarization state dependent, and is explained in terms of a resonant behavior involving incident electromagnetic radiation and the structural dielectric cross section. The variation of the Raman enhancement with d, lambda(laser), and polarization is shown to be in good agreement with model calculations of scattering from an infinite dielectric cylinder.

Journal Article↗

PubMed2006

Ferroelectric phase transition in individual single-crystalline BaTiO3 nanowires.

We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO3) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling. The diameter at which TC falls below room temperature is determined to be approximately 3 nm, and extrapolation of the data indicates that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining the observed stability of ferroelectricity in small-diameter BaTiO3 nanowires.

Anisotropy↗

PubMed2006

Patterning polyethylene oligomers on carbon nanotubes using physical vapor deposition.

Periodic patterning on one-dimensional (1D) carbon nanotubes (CNTs) is of great interest from both scientific and technological points of view. In this letter, we report using a facile physical vapor deposition method to achieve periodic polyethylene (PE) oligomer patterning on individual CNTs. Upon heating under vacuum, PE degraded into oligomers and crystallized into rod-shaped single crystals. These PE rods periodically decorate on CNTs with their long axes perpendicular to the CNT axes. The formation mechanism was attributed to "soft epitaxy" growth of PE oligomer crystals on CNTs. Both SWNTs and MWNTs were decorated successfully with PE rods. The intermediate state of this hybrid structure, MWNTs absorbed with a thin layer of PE, was captured successfully by depositing PE vapor on MWNTs detached from the solid substrate, and was observed using high-resolution transmission electron microscopy. Furthermore, this hybrid structure formation depends critically on CNT surface chemistry: alkane-modification of the MWNT surface prohibited the PE single-crystal growth on the CNTs. We anticipate that this work could open a gateway for creating complex CNT-based nanoarchitectures for nanodevice applications.

Microscopy, Atomic Force↗

PubMed2006

Instability and transport of metal catalyst in the growth of tapered silicon nanowires.

During metal-catalyzed growth of tapered silicon nanowires, or silicon nanocones (SiNCs), Au-Si eutectic particles are seen to undergo significant and reproducible reductions in their diameters. The reductions are accompanied by the transfer of eutectic droplet mass to adjacent, initially metal catalyst-free substrates, producing secondary nucleation and growth of SiNCs. Remarkably, the catalyst particle diameters on the SiNCs grown on the adjacent substrates are strongly correlated with those on the SiNCs grown on the initially Au-nanoparticle-coated substrate. These post-growth nanoparticle sizes depend on temperature and are found to be independent of the initial nanoparticle sizes. Our modeling and analysis indicates that the size reduction and mass transfer could be explained by electrostatic charge-induced dissociation of the droplet. The reduction in size enables the controlled growth of SiNCs with tip sharpnesses approaching the atomic scale, indicating that metal-catalyst nanoparticles can play an even more dynamic role than previously thought, and suggesting additional modes of control of shape, and of nucleation and growth location.

Catalysis↗

PubMed2005

Diamond-hexagonal semiconductor nanocones with controllable apex angle.

We report on the synthesis of nanostructured and crystalline tapered Si and Ge polyhedra via metal-catalyzed chemical vapor deposition. These Si and Ge nanocones (SiNCs, GeNCs) possess tips with near-atomic sharpness, micron-scaled bases, hexagonal cross-sections, and controllable apex angles. High-resolution transmission electron microscopy, selected-area electron diffraction and Raman scattering spectroscopy and analysis indicate that the SiNCs are of the diamond-hexagonal Si(IV) phase.

Gold↗