Neng-Kai Chang

Fabrication of single-walled carbon nanotube flexible strain sensors with high sensitivity

This work demonstrates a fabrication technique of high sensitivity flexible strain sensors at room temperature. The grown well-aligned millimeter-long single-walled carbon nanotube (SWCNT) was transferred from the silicon substrate to the pretrenched flexible substrate. The sensor design allows effective adhesion between SWCNT and flexible substrate for SWCNT lengthwise strain and piezoresistivity change. Experimental results show that the sensor achieves a high strain resolution of 0.004%. The measured piezoresistive gauge factor of the flexible sensor is 269. The demonstrated fabrication technique of flexible sensors shows advantage of high sensitivity, high quality, and is suitable for mass production.

Determining Mechanical Properties of Carbon Microcoils Using Lateral Force Microscopy

Mechanical properties of amorphous carbon microcoil (CMC) synthesized by thermal chemical vapor deposition method were examined in compression and tension tests, using the lateral force mode of atomic force microscope (AFM). The AFM cantilever tip was manipulated by a piezoelectric scanner to contact, pull, and push an individual CMC. The lateral force that was exerted by the CMC deformation causes the twist of the AFM cantilever. It was monitored by the laser and photodetector of the AFM during the experiments. A linear response of the CMC was observed in the range of 25 nm to 5 mum of tension experiments. The results show that the spring constant of the CMC is reasonably proportional to the coil number. The shear modulus of the amorphous CMC is estimated to be 3 plusmn 0.2 GPa. The proposed method is promising to manipulate the compression and tension of the CMC and to measure the lateral force exerted in an ambient environment.

Fabrication of High Sensitivity Carbon Microcoil Pressure Sensors

This work demonstrates a highly sensitive pressure sensor that was fabricated using carbon microcoils (CMCs) and polydimethylsiloxane (PDMS). CMCs were grown by chemical vapor deposition using various ratios of Fe-Sn catalytic solution. The pressure sensor has a sandwiched structure, in which the as-grown CMCs were inserted between two PDMS layers. The pressure sensor exhibits piezo-resistivity changes in response to mechanical loading using a load cell system. The yields of the growth of CMCs at a catalyst proportion of Fe:Sn = 95:5 reach 95%. Experimental results show that the sensor achieves a high sensitivity of 0.93%/kPa from the CMC yield of 95%. The sensitivity of the pressure sensor increases with increasing yield of CMCs. The demonstrated pressure sensor shows the advantage of high sensitivity and is suitable for mass production.

Coherent longitudinal optical phonon and plasmon coupling in GaAs

The coupling between femtosecond laser-excited coherent LO phonons and plasmon in GaAs was investigated in real time via two-pump time-resolved second-harmonic generation. The coherent LO phonon–plasmon coupling mode dominated the lattice oscillation when the photoexcited plasma density exceeded 3×1018/cm3. Its oscillation intensity showed sinusoidal dependence on the separation time between the two pumps and a phase shift from the initial LO phonon oscillation. These observations reveal the formation time of photoexcited plasmon and its coherent coupling with LO phonon in femtosecond time scale.

Determining modulus of nanowires using nanoindentation technique

This study demonstrates a finite element method for simulating the behavior of nanoindentation of copper nanowires and estimates their mechanical properties. The simulation results reveal that using well-known Oliver-Pharr theory, generally applied for materials with semi-infinite half-space, yields an underestimate of the elastic modulus of the wire materials. Moreover, the size of the indenter tip radius also influences the accuracy of the predicted elastic modulus. Such errors are mainly from the misestimate of contact area between an indenter and a specimen. They can be reduced directly from the observation of real contact area in the numerical modeling. The modified elastic modulus of the wires is very close to the bulk value.

Mechanical Properties of Double Coiled Carbon Nanotubes

The mechanical response of double coiled carbon nanotubes (CNTs) was examined with an atomic force microscopy (AFM) based technique. The compression and tension measurements were carried out in the lateral force image mode. The spring constant of the coiled CNTs and the shear modulus of CNTs have been derived, respectively. We found that the spring constant of the coiled CNT suddenly decreased while the coiled CNT buckled. The shear modulus of CNTs, 3 ± 0.2 GPa, calculated from the spring constant of coiled CNTs is extremely low, compared with well-known high tensile strength of CNTs, ∼1 TPa. The results suggest that the motion of coiled CNTs is mainly relative to the behavior of the slide between the inner tubes of multi-walled CNTs (MWNTs).

Surface Finishing Using Carbon Nanotube Forest

The use of carbon nanotube forest as a brush tool to refine the surface roughness of brass in nanometer scale was reported. The carbon nanotube with a height of 700∼800 μm was first synthesized on silicon wafer by chemical vapor deposition. The carbon nanotube forest on silicon wafer was then transferred and bonded onto a stainless steel wheel. The wheel with carbon nanotube forest was installed on a precision milling machine and used to brush the surface of a brass specimen. The influence of various feed amount, brushing speed and brushing time on the surface roughness of the brass specimen were experimental evaluated. The results show that the increasing feed amount effectively reduces surface roughness of the brass specimen from 20 nanometers to 1 nanometer. Scanning electron microscopy images reveal the evidence of the bushing process that brass chips were melted on the tip of carbon nanotube forest.Copyright

Observation of coherent interfacial optical phonons in III-V semiconductor nanostructures

We report the coherent phonon spectroscopy of InGaP-GaAs-InGaP SQWs, in which a localized interfacial optical phonon is identified. The GaAs-InGaP heterointerface quality can be characterized by measuring the dephasing time of this interfacial phonon mode.

Observation of coherent optical phonons in GaAs/Al/sub x/Ga/sub 1-x/As single heterojunctions

Time-resolved second-harmonic generation was used to generate and detect coherent LO phonons in semiconductor heterojunctions. Three coherent LO/sub GaAs/, LO/sub GaAs-like/, and LO/sub AlAs-like/ phonons were launched by transiently screening the depletion field in the GaAs/Al/sub x/Ga/sub 1-x/ interfacial region.