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John J Boland

Publications and source records attributed to John J Boland.

11 recordsLinked to original sources
PubMed2006

Measuring the force of interaction between a metallic probe and a single molecule.

Precision current measurements are recorded at 5 K during the approach and contact between a Pt-inked probe and the carbon-carbon double-bond region of an isolated 1,3-cyclohexadiene molecule chemisorbed on a Si(100) surface. Scanning tunneling spectroscopic data reveal systematic features in the current at specific probe-molecule separations. Aided by density functional theory calculations, we show that these features arise from interaction forces between the probe and molecule, which can be interpreted as the relaxation of the probe-molecule system prior to and during contact.

Algorithms↗
PubMed2006

Synthesis and dispersion of isolated high aspect ratio gold nanowires.

We report the assembly properties of high density and high aspect ratio metal nanowire arrays (Au, Cu and Ag with diameters ranging from 40 to 250 nm) after release from the anodic alumina oxide (AAO) templates. Individual Ag and Cu nanowires were observed following release from the template, however, in the case of gold nanowires, the dispersion was dependent on size and aspect ratio. 40-100 nm gold wires aggregated to form bundles or disordered mats. We show that a simple cyanide-mediated release from the AAO template, results in isolated dispersion of wires even for the smallest wire diameters. Possible stabilising mechanisms for observed tendency of nanowires dispersion are discussed.

Journal Article↗
PubMed2006

Microstructure-hardened silver nanowires.

To exploit the novel size-dependent mechanical properties of nanowires, it is necessary for one to develop strategies to control the strength and toughness of these materials. Here, we report on the mechanical properties of silver nanowires with a unique fivefold twin structure using a lateral force atomic force microscopy (AFM) method in which wires are held in a double-clamped beam configuration. Force-displacement curves exhibit super elastic behavior followed by unexpected brittle failure without significant plastic deformation. Thermal annealing resulted in a gradual transition to weaker, more ductile materials associated with the elimination of the twinned boundary structure. These results point to the critical roles of microstructure and confinement in engineering the mechanical properties of nanoscale materials.

Journal Article↗
PubMed2006

A generalized description of the elastic properties of nanowires.

We report a model of nanowire (NW) mechanics that describes force vs displacement curves over the entire elastic range for diverse wire systems. Due to the clamped-wire measurement configuration, the force response in the linear elastic regime can be linear or nonlinear, depending on the system and the wire displacement. For Au NWs the response is essentially linear since yielding occurs prior to the onset of the inherent nonlinearity, while for Si NWs the force response is highly nonlinear, followed by brittle fracture. Since the method describes the entire range of elastic deformation, it unequivocally identifies the yield points in both of these materials.

Computer-Aided Design↗
PubMed2006

Contact formation dynamics: Mapping chemical bond formation between a molecule and a metallic probe.

We present a study that maps out chemical bond formation between a Pt-inked probe and a single 1,3-cyclohexadiene (1,3-CHD) molecule on Si(100). By separating the mechanical and electronic contributions to the current during the approach to contact, we show that there are significant forces between the probe and the C=C of the molecule and we track the relaxation of the molecule, the emergence of a chemical bond feature in the LDOS, and the quenching of specific molecular vibrations during bond formation.

Binding Sites↗
PubMed2006

Ultimate-strength germanium nanowires.

Semiconducting nanowires (NWs) are important "building blocks" for potential electrical and electromechanical devices. Here, we report on the mechanical properties of supercritical fluid-grown Ge NWs with radii between 20 and 80 nm. An analysis of the bending and tensile stresses during deformation and failure reveals that while the NWs have a Young's modulus comparable to the bulk value, they have an ultimate strength of 15 GPa, which is the maximum theoretical strength of these materials. This exceptional strength is the highest reported for any conventional semiconductor material and demonstrates that these NWs are without defect or flaws that compromise the mechanical properties.

Journal Article↗
PubMed2005

Tunable photon lifetime in photonic molecules: a concept for delaying an optical signal.

We experimentally and theoretically studied the photon lifetime spectral distribution in two coherently coupled spherical microcavities of 3-16 microm diameter forming a photonic molecule, which shows a multipeak narrowband modal structure resulting from lifting of the mode degeneracy with respect to the azimuthal quantum number. The results demonstrate the feasibility of photonic molecules as a basis for a multichannel, wavelength-tunable optical delay-line device, which can be used for delay times in the range 10 ps to 1 ns.

Journal Article↗
PubMed2005

Mechanical properties of ultrahigh-strength gold nanowires.

Nanowires have attracted considerable interest as nanoscale interconnects and as the active components of both electronic and electromechanical devices. Nanomechanical measurements are a challenge, but remain key to the development and processing of novel nanowire-based devices. Here, we report a general method to measure the spectrum of nanowire mechanical properties based on nanowire bending under the lateral load from an atomic force microscope tip. We find that for Au nanowires, Young's modulus is essentially independent of diameter, whereas the yield strength is largest for the smallest diameter wires, with strengths up to 100 times that of bulk materials, and substantially larger than that reported for bulk nanocrystalline metals (BNMs). In contrast to BNMs, nanowire plasticity is characterized by strain-hardening, demonstrating that dislocation motion and pile-up is still operative down to diameters of 40 nm. Possible origins for the different mechanical properties of nanowires and BNMs are discussed.

Computer Simulation↗
PubMed2004

Dimer-anticorrelation-induced stabilization of adsorbate clustering on the Si100-(2 x 1) surface.

It is well established that absorbate-absorbate interactions play a key role in determining the distribution of adsorbates on surfaces. In cases where these interactions are repulsive adsorbates frequently arrange so as to minimize these unfavorable interactions. This simple picture, however, neglects the influence of adsorption on the properties on the underlying substrate. Here, using STM, we show that on Si(100) many intrinsically repulsive adsorbates cluster to form surface patches even at low surface coverages. With the aid of density functional theory calculations and Monte Carlo simulations, we show that patch formation is an intrinsic property of the Si(100) surface that is driven by the energy lowering associated with the formation of extended regions of bare dimers. The enhanced attraction between anticorrelated tilted bare dimers is sufficient to offset the repulsions between adsorbates.

Journal Article↗
PubMed2002

Spontaneous roughening: fundamental limits in Si(100) halogen etch processing.

A dynamical scanning tunneling microscopy and density functional theory study of the thermodynamic stability of halogen-terminated Si(100) surfaces is presented. Significant steric repulsion is shown to exist on all halogen-terminated Si(100) surfaces. This repulsion is the driving force for a roughening phenomenon, which is favored for all halogens except fluorine. Since roughening is an intrinsic property of these surfaces, it sets a lower bound on the atomic scale perfection that can be achieved using halogen etch processing.

Journal Article↗