Shengyong Xu

Thin-Film Thermocouple Array for Time-Resolved Local Temperature Mapping

This letter reports on a technique to obtain time-resolved 2-D mapping of local temperatures on a solid substrate by employing an array of Cr-Ni thin-film thermocouples (TFTCs). Using 100-nm-thick Cr-Ni TFTCs, a stable and reproducible thermopower of 26.2 ± 1.5 μV/K with temperature resolutions better than 0.1 K was obtained. Investigation showed that the thermopower of these thermocouples had no obvious dependence on their junction area down to 3 × 3 μm2. In an array of 18 individual thermocouples, a relative standard deviation of 0.31% was measured for the thermopower. This technique may have applications in, e.g., thermal failure analysis, real-time and on-site thermal sensing for integrated circuits, and other modern electronic, optical, and thermal devices.

The fabrication of nanoelectrodes based on a single carbon nanotube.

Nanoelectrodes are fabricated from individual carbon nanotubes (CNTs) connected to tungsten probes or carbon fibers. The whole electrode was covered by an insulating HfO(2) layer except for a section of conducting CNT at the apex. The fabrication process includes mounting individual CNTs to the conductive support through nanomanipulation, coating the whole probe by a dielectric layer using atomic layer deposition and removing the dielectric layer at the apex through a nanomanipulation process. Differential pulse voltammetry and cyclic voltammetry measurements show the CNT nanoelectrode has similar electrochemical behavior to the widely used carbon fiber probe, but has a much smaller active electrode area and can provide much higher spatial resolution and signal-to-noise ratio.

Unexpected size effect in the thermopower of thin-film stripes

For low-dimensional materials, size effect of a physical property is usually expected to occur when one (or more) of the dimension sizes decreases to that comparable to or smaller than one of the intrinsic characteristic lengths, e.g., the mean free path. We report here an unexpected size effect, that in centimeter-long stripes of 100-nm-thick metallic thin films, a reduction of the absolute value of thermopower occurs when the stripe width is in the order of 30-50 μm, which is 100–1000 times larger than the intrinsic mean free path of the material. When the stripe width is reduced to 1.5 μm, a relative reduction of thermopower up to 35% is measured in some metals. We suggest that the sidewall scattering due to rough edges of these stripes may be the origin of this unexpected phenomenon. The results may be applied to construct novel thermoelectric devices, such as thermocouples made from a single metal film.

Penetrative imaging of sub-surface microstructures with a near-field microwave microscope

Microwaves have the capability of penetrating through materials with low permittivity. By means of a near-field scanning microwave microscope system, we obtained two-dimensional maps of the incident microwaves reflection coefficient intensity and frequency shift, which correspond well to the spatial distribution and electrical conductance of fluids and metallic thin film structures hidden underneath 15–170 μm thick dielectric covers. The lateral resolution and sensitivity to conductivity for the target samples were found closely related to the thickness of the cover layer. The technique offers a real-time, in-situ, and a non-invasive approach to monitor the local chemical reactions, the motion of fluids, and the distribution or concentrations of ions or bio-materials in lab-on-a-chip systems. This technique also has the potential to be developed for the detection of live cells and tissues.

Towards on-chip time-resolved thermal mapping with micro-/nanosensor arrays

In recent years, thin-film thermocouple (TFTC) array emerged as a versatile candidate in micro-/nanoscale local temperature sensing for its high resolution, passive working mode, and easy fabrication. However, some key issues need to be taken into consideration before real instrumentation and industrial applications of TFTC array. In this work, we will demonstrate that TFTC array can be highly scalable from micrometers to nanometers and that there are potential applications of TFTC array in integrated circuits, including time-resolvable two-dimensional thermal mapping and tracing the heat source of a device. Some potential problems and relevant solutions from a view of industrial applications will be discussed in terms of material selection, multiplexer reading, pattern designing, and cold-junction compensation. We show that the TFTC array is a powerful tool for research fields such as chip thermal management, lab-on-a-chip, and other novel electrical, optical, or thermal devices.

Contact Mechanism of the Ag-doped Trimolybdate Nanowire as An Antimicrobial Agent

Antibacterial Ag-agents are intensively applied as broad spectrum, high-stability, high-efficiency and high-safety inorganic antibacterial agents. We have developed a new kind of antibacterial Ag-agent, namely Ag2−x(NH4)xMo3O10·3H2O nanowires (NWs). Carrying Ag atoms in the lattice and Ag-rich nanoparticles on the surface, the Ag-doped NWs show strong antibacterial effects for a variety of bacteria including E. coli, Staphylococcus aureus, Candida albicans and Aspergillus niger. By performing systematic comparison experiments, we have proven that the main antibacterial effects are neither resulted from the tiny amount of Ag+ ions released from the Ag-doped NWs in aqueous solutions, nor resulted from Ag-rich nanoparticles of fragments of the NWs when they are slowly dissolved in the Martin broth. Instead, the effects are mainly resulted from a contact mechanism, under which, the Ag-doped NWs need to be physically in contact with the bacteria to be eliminated. This is a novel phenomenon observed in the interactions between nanomaterials and live cells, which is worthy of further investigation at the molecular scale. As the Ag-doped NWs are not dissolved in pure water or weak acids, one may find practical antibacterial applications in textile industry and food storage industry for these unique nanomaterials.

Current sustainability and electromigration of Pd, Sc and Y thin-films as potential interconnects

The progress on novel interconnects for carbon nanotube (CNT)-based electronic circuit is by far behind the remarkable development of CNT-field effect transistors. The Cu interconnect material used in current integrated circuits seems not applicable for the novel interconnects, as it requires electrochemical deposition followed by chemical-mechanical polishing. We report our experimental results on the failure current density, resistivity, electromigration effect and failure mechanism of patterned stripes of Pd, Sc and Y thin-films, regarding them as the potential novel interconnects. The Pd stripes have a failure current density of (8∼10)×106 A/cm2 (MA/cm2), and they are stable when the working current density is as much as 90% of the failure current density. However, they show a resistivity around 210 μΩ·cm, which is 20 times of the bulk value and leaving room for improvement. Compared to Pd, the Sc stripes have a similar resistivity but smaller failure current density of 4∼5 MA/cm2. Y stripes seem not suitable for interconnects by showing even lower failure current density than that of Sc and evidence of oxidation. For comparison, Au stripes of the same dimensions show a failure current density of 30 MA/cm2 and a resistivity around 4 μΩ·cm, making them also a good material as novel interconnects.

Dissolvable Trimolybdate Nanowires as Ag Carriers forHigh-Efficiency Antimicrobial Applications

Elimination of bacteria and other microbes effectively is important to our daily life and a variety of medical applications. Here, we introduce a new kind of trimolybdate nanowires, namely Ag2−x(NH4)xMo3O10⋅3H2O, that carry a large amount of Ag atoms in the lattice and Ag-rich nanoparticles on the surface. These nanowires can eliminate bacteria of E. coli, Staphylococcus aureus, and unknown microbes in raw natural water with high efficiency. For example, they can inactivate more than 98% of E. coli with a nanowire concentration of only 5 ppm in the solution. The excellent sterilization performance is attributed to the combined effects of Ag ions, Mo ions, and Ag-rich nanoparticles of the Ag2−x(NH4)xMo3O10⋅3H2O nanowires. These nanowires are not dissolvable in deionized water but can be dissolved by the metabolic materials released from bacteria, making them attractive for many biological applications.

Cyclic Stress–strain Relation and Cyclic Yeild Behavior on Cyclic Deformation

ABSTRACT Based on the experimental results and analysis of the cyclic deformation, We find that there is an obvious yield stage on the cyclic stress-strain curve at the stage of small plastic deformation (in room or low temperatures). This phenomenon is similar to that of monotonic tensile curve case. But for the former the deformation amount at which the yield begins is much smaller than that for the latter. The cyclic stress-strain constitutive relation needs to be further studied according to the actual cyclic stress-strain curve. The tensile-compressive symmetric permanent limit can be determined by the stress corresponding to the cross point of the cyclic stress-strain curve and the elastic line.