An execution-backtracking approach to debugging

Spyder, a system for selective checkpointing of computational sequences, is presented. It lets users backtrack from checkpoints without the need to reexecute the program to reach recent prior states. In contrast to more comprehensive (and storage-intensive) checkpointing schemes, backtracking in this approach is constrained to limit storage requirements. The resulting debugger offers a structured view of dynamic events, similar to lexical scope rules' effect on static visibility. The debugger also speeds backtracking to statements before loops and provides what-if capabilities     

Self‐Assembled SERS Substrates with Tunable Surface Plasmon Resonances

The fabrication of surface‐enhanced Raman spectroscopy (SERS) substrates that are optimized for use with specific laser wavelength–analyte combinations is addressed. In order to achieve large signal enhancement, temporal stability, and reproducibility over large substrate areas at low cost, only self‐assembly and templating processes are employed. The resulting substrates consist of arrays of gold nanospheres with controlled diameter and spacing, properties that dictate the optical response of the structure. Tunability of the extended surface plasmon resonance is observed in the range of 520–1000 nm. It is demonstrated that the enhancement factor is maximized when the surface plasmon resonance is red‐shifted with respect to the SERS instrument laser line. Despite relying on self‐organization, site‐to‐site enhancement factor variations smaller than 10% are obtained.     

Determination of band offsets in heterostructured colloidal nanorods using scanning tunneling spectroscopy

The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.     

A unique solid-solid transformation of silver nanoparticles on reactive ion-etching-processed silicon

Processes that combine nanoparticle suspensions with micromechanical or microelectronics platforms can reveal new phenomena unique to nanoscale objects. We report that silver nanoparticles react with silicon wafers that have been patterned by reactive ion etching (RIE) in SF(6)/O(2) plasma. This reaction results in the localized deposition of silver on the patterns. Through the modification of the reaction conditions, the reaction mechanism was explored. Redeposition of the sputtered RIE products is suggested as the key to this transformation. The new silver deposition process was utilized to localize the growth of gold nanoparticles and silicon nanowires on the vertical sidewalls of patterns in silicon, demonstrating a simple route to the fabrication of overhanging nanoscale objects.     

Flexible ordered bundles of infrared transmitting silver-halide fibers: design, fabrication, and optical measurements

Ordered bundles of silver-halide fibers, which are highly transparent in the middle infrared, are fabricated by multiple extrusions from single crystals. We fabricate and characterize the optical properties of thin and flexible bundles of diameters of 0.7-2.0 mm that incorporate 100 individual fibers. The measurements include attenuation, resolution, cross talk, near-field scanning of single fibers in the bundle, and bending losses. Bundles of lengths of several meters transmit thermal images of bodies whose temperature is near room temperature. These bundles would be useful for medical, industrial, and military applications, and in particular for endoscopic thermal imaging.     

Electronic structure and stability of semiconducting graphene nanoribbons

We present a systematic density functional theory study of the electronic properties, optical spectra, and relative thermodynamic stability of semiconducting graphene nanoribbons. We consider ribbons with different edge nature including bare and hydrogen-terminated ribbons, several crystallographic orientations, and widths up to 3 nm. Our results can be extrapolated to wider ribbons providing a qualitative way of determining the electronic properties of ribbons with widths of practical significance. We predict that in order to produce materials with band gaps similar to Ge or InN, the width of the ribbons must be between 2 and 3 nm. If larger bang gap ribbons are needed (like Si, InP, or GaAs), their width must be reduced to 1-2 nm. According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV. For chiral nanoribbons the band gap oscillations rapidly vanish as a function of the chiral angle indicating that a careful design of their crystallographic nature is an essential ingredient for controlling their electronic properties. Optical excitations show important differences between ribbons with and without hydrogen termination and are found to be sensitive to the carbon nanoribbon width. This should provide a practical way of revealing information on their size and the nature of their edges.     

Observation of current reversal in the scanning tunneling spectra of fullerene-like WS2 nanoparticles

Current-voltage characteristics measured using STM on fullerene-like WS2 nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.     

Debonding analysis of fiber-reinforced-polymer strengthened beams: Cohesive zone modeling versus a linear elastic fracture mechanics approach

A linear elastic fracture mechanics (LEFM) approach and a cohesive interface (cohesive zone) modeling approach to the debonding analysis of concrete beams strengthened with externally bonded fiber-reinforced-polymer (FRP) strips are studied and compared. The analytical models that are based on the two approaches are presented and discussed. The cohesive interface model is formulated using a potential function and it takes into account the shear effects, the effect of the peeling stresses, and the coupling of the shear and the peeling effects. This model takes the form of a set on nonlinear differential equations. The LEFM model combines stress analysis using the high order theory and fracture analysis using the concepts of the energy release rate and the J-integral. In addition, an algorithm that converts the results of the LEFM model into the equilibrium path of the debonding process is developed. The main advantages and disadvantages of the two approaches are also discussed. The two approaches are compared in terms of their applicability to quantify and describe the debonding process in various cases that include a single shear test, an edge peeling test, and a beam specimen strengthened with FRP.