Zhiyong Gu

Integrating nanowires with substrates using directed assembly and nanoscale soldering

This paper describes a new methodology for integrating nanowires with micropatterned substrates using directed assembly and nanoscale soldering. Nanowires containing ferromagnetic nickel segments were fabricated by electrodeposition in nanoporous membranes. The nanowires were released by dissolution of the membrane and subsequently aligned relative to micropatterned substrates using magnetic field-directed assembly. After assembly, the wires were permanently bonded to the substrates using solder reflow to form low-resistance electrical contacts. This is the first demonstration of the use of nanoscale solder reflow to form low-resistance electrical interconnects between nanowires and substrates, and we demonstrated the utility of the strategy by fabricating a nanowire-based functional analog integrator.

Joining and Interconnect Formation of Nanowires and Carbon Nanotubes for Nanoelectronics and Nanosystems

Interconnect formation is critical for the assembly and integration of nanocomponents to enable nanoelectronics- and nanosystems-related applications. Recent progress on joining and interconnect formation of key nanomaterials, especially nanowires and carbon nanotubes, into functional circuits and/or prototype devices is reviewed. The nanosoldering technique through nanoscale lead-free solders is discussed in more detail in this Review. Various strategies of fabricating lead-free nanosolders and the utilization of the nanosoldering technique to form functional solder joints are reviewed, and related challenges facing the nanosoldering technique are discussed. A perspective is given for using lead-free nanosolders and the nanosoldering technique for the construction of complex and/or hybrid nanoelectronics and nanosystems.

Dielectrophoretic assembly of reversible and irreversible metal nanowire networks and vertically aligned arrays

We demonstrate the dielectrophoretic control of metallic nanowires (NWs) in liquid suspensions. By varying a range of parameters including the magnitude and frequency of the applied electric field, the liquid suspending the NWs, and the flow conditions, we demonstrate control over NW network formation and dissolution, as well as ordering of NWs into vertically aligned arrays. These results suggest a straightforward strategy for NW assembly and integration in devices.

Highly sensitive H2S sensor based on template-synthesized CuO nanowires

A sensor device featuring massive aligned CuO nanowires has been fabricated for H2S detection. Metallic copper (Cu) nanowires were first synthesized by a template-assisted electrodeposition method. Dielectrophoresis (DEP) and thermal oxidation were then used to assemble and functionalize/oxidize the nanowires. Topology of the CuO nanowires showed rugged surfaces on the nanowires. H2S sensing properties were characterized by conducting experiments with varying influential factors, including concentration levels from 10 to 1000 ppb and working temperatures from 25 to 420 °C. The CuO nanowire sensor showed good response and repeatability upon H2S exposure with a detection limit of 2.5 ppb and a linear response ranging from 10 ppb to 100 ppb. In addition, the study of temperature influence revealed that the highest response was achieved at 180 °C. Furthermore, with increased working temperature, the CuO nanowire sensor had shorter response and recovery times. An interesting two-stage response was discovered for this CuO nanowire sensor responding to H2S exposure, indicating that two competing reactions existed on the CuO surface.

Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles.

A rapid and efficient method for methyl orange degradation using hollow cobalt (Co) nanoparticles is reported. Hollow Co nanoparticles were fabricated by a galvanic replacement reaction using aluminum (Al) nanoparticles as the template material. The methyl orange degradation characteristics were investigated by measuring the time dependent UV-Vis absorption of the dye solution, which showed a very fast degradation rate under acidic conditions. At an initial methyl orange concentration of 100 mg/L (pH = 2.5) and Co nanoparticle dosage of 0.5 g/L, the azo dye degradation efficiency reached up to 99% within 4 min, and the degradation constant rate was up to 2.444 min(-1), which is the highest value among other studies. A comparison of the decolorization rates at similar conditions with several other azo dyes, including Congo red, Amaranth, and Orange G, showed that the dye with a simpler structure and lower molecular mass decolorized considerably faster than the ones having a more complicated structure (higher molecular mass). The methyl orange degradation was also conducted using hollow nickel (Ni) nanoparticles and commercially available solid spherical Co and Ni nanoparticles. The results showed that Co-based nanoparticles outperformed Ni-based nanoparticles, with the hollow Co nanoparticles exhibiting the fastest degradation rate. Using the hollow Co nanoparticles is a very promising approach for the remediation of methyl orange dye containing wastewater due to the fast degradation rate and high degradation efficiency. In addition, these hollow Co nanoparticles are easily recycled because of their magnetic property.

Paclitaxel/sirolimus combination coated drug-eluting stent: in vitro and in vivo drug release studies.

Paclitaxel and sirolimus are the two major drugs for the treatment of coronary arterial disease in current drug-eluting stents. The two drugs can effectively inhibit the in-stent restenosis through their independent pathways and show synergistic effect in preventing tumor tissue growth. We hypothesize that the combination of the two drugs in a drug-eluting stent (DES) can also effectively suppress the neointima growth in the stented artery. The present work was focused on the investigation of paclitaxel/sirolimus combination release profiles from a novel biodegradable polymer (poly (D, L-lactide-co-glycolide)/amorphous calcium phosphate, PLGA/ACP) coated stent both in vitro and in vivo. For the in vitro, the drug releasing profiles were characterized by measuring the drug concentration in a drug release medium (Dulbeccos phosphate buffered saline, DPBS, pH 7.4) at predetermined time points. For the in vivo, a rat aorta stenting model was employed. The results showed that both paclitaxel and sirolimus had a two-phase release profile both in vitro and in vivo, which is similar to the drug release profile of their individual coated DESs, and there is no evident of interference between two drugs. The data suggest that paclitaxel and sirolimus can be combined pharmacokinetically in a DES for the treatment of coronary arterial diseases.

Formation of PbSe/CdSe Core/Shell Nanocrystals for Stable Near-Infrared High Photoluminescence Emission

PbSe/CdSe core/shell nanocrystals with quantum yield of 70% were obtained by the “successive ion layer adsorption and reaction” technology in solution. The thickness of the CdSe shell was exactly controlled. A series of spectral red shifts with the CdSe shell growth were observed, which was attributed to the combined effect of the surface polarization and the expansion of carriers’ wavefunctions. The stability of PbSe nanocrystals was tremendously improved with CdSe shells.

Effects of Particle Surface Charge, Species, Concentration, and Dispersion Method on the Thermal Conductivity of Nanofluids

The purpose of this experimental study is to evaluate the effects of particle species, surface charge, concentration, preparation technique, and base fluid on thermal transport capability of nanoparticle suspensions (nanofluids). The surface charge was varied by changing the pH value of the fluids. The alumina ( Al 2 O 3 ) and copper oxide (CuO) nanoparticles were dispersed in deionized (DI) water and ethylene glycol (EG), respectively. The nanofluids were prepared using both bath-type and probe sonicator under different power inputs. The experimental results were compared with the available experimental data as well as the predicted values obtained from Maxwell effective medium theory. It was found that ethylene glycol is more suitable for nanofluids applications than DI water in terms of thermal conductivity improvement and stability of nanofluids. Surface charge can effectively improve the dispersion of nanoparticles by reducing the (aggregated) particle size in base fluids. A nanofluid with high surface charge (low pH) has a higher thermal conductivity for a similar particle concentration. The sonication also has a significant impact on thermal conductivity enhancement. All these results suggest that the key to the improvement of thermal conductivity of nanofluids is a uniform and stable dispersion of nanoscale particles in a fluid.

Highly sensitive surface-enhanced Raman scattering using vertically aligned silver nanopetals

Surface-enhanced Raman scattering (SERS) has attracted great attention due to its high sensitivity and specificity in the detection of a variety of molecules. Recently, much effort has been focused on the development of novel nanostructured SERS substrate with reliable and excellent sensing performance. In this work, a transparent Ag thin film composed of vertically aligned, single-crystalline silver nanopetals with uniform distribution is fabricated by galvanic replacement reaction. The growth mechanism of such novel Ag nanopetals is proposed. Furthermore, the outstanding SERS performance of the as-prepared Ag nanostructured thin film is demonstrated using crystal violet as the model compound, and the Raman intensity shows concentration-dependent behavior following the Freundlich equation. Crystal violet as low as 500 pM is facilely detected with high reproducibility (n = 8). The enhancement factor is determined to be 108, which potentially enables the identification of crystal violet at single molecule level on the probed surface. The high sensitivity is attributed to the “hot spots” resulting from proximate edges of Ag nanopetals and the nanocavity architecture bounded by Ag nanopetals.

Nano-soldering of magnetically aligned three-dimensional nanowire networks

It is extremely challenging to fabricate 3D integrated nanostructures and hybrid nanoelectronic devices. In this paper, we report a simple and efficient method to simultaneously assemble and solder nanowires into ordered 3D and electrically conductive nanowire networks. Nano-solders such as tin were fabricated onto both ends of multi-segmented nanowires by a template-assisted electrodeposition method. These nanowires were then self-assembled and soldered into large-scale 3D network structures by magnetic field assisted assembly in a liquid medium with a high boiling point. The formation of junctions/interconnects between the nanowires and the scale of the assembly were dependent on the solder reflow temperature and the strength of the magnetic field. The size of the assembled nanowire networks ranged from tens of microns to millimeters. The electrical characteristics of the 3D nanowire networks were measured by regular current-voltage (I-V) measurements using a probe station with micropositioners. Nano-solders, when combined with assembling techniques, can be used to efficiently connect and join nanowires with low contact resistance, which are very well suited for sensor integration as well as nanoelectronic device fabrication.