David Huitink

Photoinduced formation of electrically conductive thin palladium nanowires on DNA scaffolds.

A photochemical method has been exploited for the very fast synthesis of electrically conductive Pd nanowires on DNA. The nanowires have an average diameter of 55-75 nm and a length of approximately 3-5 mum. Our result signifies that the DNA acts both as a reducing agent as well as nonspecific capping agent for the Pd nanowire synthesis. The current voltage (I-V) characterization indicates that the nanowires are continuous and exhibit Ohmic behavior with low contact resistance. The deposition is highly selective on DNA only. Our experiment indicates that Pd nanowires may be valuable as interconnections in nanoscale integrated circuitry, functional nanodevices and in optoelectronics.

Photochemical formation of electrically conductive silver nanowires on polymer scaffolds.

A photochemical method has been exploited for the synthesis of electrically conductive silver (Ag) nanowires in a polymer solution in the presence of negatively charged Au seed particles. The synthesis was completed within 8 min of UV-photoirradiation in ambient conditions. The nanowires were fabricated on a PVA template having an average diameter of 135+/-20 nm and a length of 10-20 microm. The current-voltage (I-V) characterization showed that the PVA-Ag nanowires were continuous, having Ohmic behavior with low contact resistance. Results indicate that the PVA acted as a reducing agent, stabilizing agent, and a template for the nucleation and growth of Ag nanowires. The Ag deposition was highly selective and on the PVA only. Our research indicated that the PVA-Ag nanowires might be useful as interconnects in nanoscale integrated circuitry, functional nanodevices, and in optoelectronics.

A multistage, semi-automated procedure for analyzing the morphology of nanoparticles

This article presents a multistage, semi-automated procedure that can expedite the morphology analysis of nanoparticles. Material scientists have long conjectured that the morphology of nanoparticles has a profound impact on the properties of the hosting material, but a bottleneck is the lack of a reliable and automated morphology analysis of the particles based on their image measurements. This article attempts to fill in this critical void. One particular challenge in nanomorphology analysis is how to analyze the overlapped nanoparticles, a problem not well addressed by the existing methods but effectively tackled by the method proposed in this article. This method entails multiple stages of operations, executed sequentially, and is considered semi-automated due to the inclusion of a semi-supervised clustering step. The proposed method is applied to several images of nanoparticles, producing the needed statistical characterization of their morphology.

Nanoparticle Shape Evolution Identified through Multivariate Statistics

Precise morphological control of nanoparticles (NPs) has been impeded by the lack of in situ techniques enabling the observation of instantaneous growth steps. Fundamentally, understanding in NP nucleation and growth kinetics has yet to achieve. In the present research, morphological characterization is demonstrated using a novel image detection statistical approach for gold NPs. This multivariate statistical technique enhances the recognition of NPs by successfully identifying their morphology in addition to their growth stages. Thermodynamic analysis of those stages is presented relating surface energies to the growth kinetics. Preferred growth of NPs was seen to take place on specific crystallographic surfaces in a correlated manner. Furthermore, the growth steps are dominated by the adsorption of surfactants and the local surface energies. The present approach enabled detailed observation of NP growth kinetics and can be applied to other metallic NPs.

Tribo‐electrochemical surface modification of tantalum using in situ AFM techniques

A scanning-probe-based technique to observe tribo-electrochemically stimulated surface was demonstrated. The configuration consists of an electrochemical cell attached to an atomic force microscope (AFM) scanner. Under an applied electrical potential and in various chemical environments, the surface morphology, roughness, skew, bearing ratio, as well as surface adhesive forces between probes were measured, and the effects of mechano-electrochemical stimuli were evaluated. The effects of mechanical, electrochemical, and mechano-electrochemical stimuli were found to compete during AFM sliding process. Their effects do not follow a linear relationship, implying that the mechanical stimulus promotes electrochemical reactions. Similarly, electrochemically enhanced mechanical removal of surface materials is possible.

In Situ Monitoring of Tantalum during Electrochemical-Mechanically Induced Oxidation

We demonstrate an in situ method to observe the steps of oxidation of tantalum under electrochemical-mechanical stimulation. This was possible through an electrochemical cell incorporated into an atomic force microscope. Using this method, the surface could be concurrently monitored for oxidation by applying an electrochemical potential under mechanical contact. The technique reported here can be used for any other material systems for surface and interfacial studies.

In situ observation of stress-induced Au–Si phase transformation

Nonequilibrium AuSi3 was found due to indenting contact forces at a gold and silicon interface. An in situ transmission electron microscope was used to observe interactions at the material interface during nanoindentation and to identify crystal structures. Furthermore, sliding of a gold-coated atomic-force microscope probe over a silicon substrate forms organized nanostructures. An evaluation of the observed interfacial mechanisms concluded that the contact stress triggered the phase transformation leading to the formation of a metastable interface that mediates the adhesion of contacting materials.

BAYESIAN OBJECT CLASSIFICATION OF GOLD NANOPARTICLES

The properties of materials synthesized with nanoparticles (NPs) are highly correlated to the sizes and shapes of the nanoparticles. The transmission electron microscopy (TEM) imaging technique can be used to measure the morphological characteristics of NPs, which can be simple circles or more complex irregular polygons with varying degrees of scales and sizes. A major difficulty in analyzing the TEM images is the overlapping of objects, having different morphological properties with no specific information about the number of objects present. Furthermore, the objects lying along the boundary render automated image analysis much more difficult. To overcome these challenges, we propose a Bayesian method based on the marked-point process representation of the objects. We derive models, both for the marks which parameterize the morphological aspects and the points which determine the location of the objects. The proposed model is an automatic image segmentation and classification procedure, which simultaneously detects the boundaries and classifies the NPs into one of the predetermined shape families. We execute the inference by sampling the posterior distribution using Markov chain Monte Carlo (MCMC) since the posterior is doubly intractable. We apply our novel method to several TEM imaging samples of gold NPs, producing the needed statistical characterization of their morphology.

Effects of Particle-Induced Crystallization on Tribological Behavior of Polymer Nanocomposites

Similar to conventional engineering fabrication processes, tribological performance of drugs and pills in pharmaceutical manufacturing plays an important role in quality and product yields. In the present research, we investigate the effects of crystal structures of workpiece materials on their tribological performance in conditions typical of pharmaceutical manufacturing processes. Sorbitol composites containing gold nanoparticles were evaluated for material properties and tribological performance. It was found that the control exhibited nonordered gamma forms of sorbitol, while the samples containing rod nanoparticles showed a collection of tiny needlelike crystals of gamma phase. Spherical nanoparticles precipitated beta and alpha phases of sorbitol, which were not seen in the other samples. These variations in the crystal structure resulted in an unusual wear behavior, leading to high friction and softness in the case of the nanocomposites. The nanoparticles were found to influence the crystal structure of the sorbitol matrix, resulting in mechanical and tribological behaviors.

Precise Control of Carbon Nanotube Synthesis of a Single Chirality

This work demonstrates precise control over the synthesis conditions and location during CNT formation, such that single chirality tubes are obtainable. This technique obviates two significant hurdles that prevent the exploitation of CNTs in micro- and nano-devices. Microelectronic applications require precise location and chirality of synthesized CNTs. Conventional CVD synthesis techniques typically yield mixtures of CNTs (semi-conducting and metallic types) that grow at random locations. Dip Pen Nanolithography (DPN) techniques were used to deposit the catalysts at precisely defined locations and to pattern the catalysts on a substrate with specific sizes as well as to control the catalyst composition. After deposition of catalysts, a low temperature Chemical Vapor Deposition (CVD) process was used to synthesize CNT. Various known catalysts were deposited. Characterization studies before and after CVD synthesis of CNT showed that the CNT were of a single chirality as well as uniform diameter (with a very narrow range of variability). The results indicate that the chirality of the synthesized CNT can be controlled by changing the synthesis conditions (e.g., size of the catalyst patterns, composition of the catalysts, temperature of CVD, gas flow rates, etc.).Copyright