Tso-Fu Mark Chang

Fully Depleted Ti–Nb–Ta–Zr–O Nanotubes: Interfacial Charge Dynamics and Solar Hydrogen Production

Poor kinetics of hole transportation at the electrode/electrolyte interface is regarded as a primary cause for the mediocre performance of n-type TiO2 photoelectrodes. By adopting nanotubes as the electrode backbone, light absorption and carrier collection can be spatially decoupled, allowing n-type TiO2, with its short hole diffusion length, to maximize the use of the available photoexcited charge carriers during operation in photoelectrochemical (PEC) water splitting. Here, we presented a delicate electrochemical anodization process for the preparation of quaternary Ti-Nb-Ta-Zr-O mixed-oxide (denoted as TNTZO) nanotube arrays and demonstrated their utility in PEC water splitting. The charge-transfer dynamics for the electrodes was investigated using time-resolved photoluminescence, electrochemical impedance spectroscopy, and the decay of open-circuit voltage analysis. Data reveal that the superior photoactivity of TNTZO over pristine TiO2 originated from the introduction of Nd, Ta, and Zr elements, which enhanced the amount of accessible charge carriers, modified the electronic structure, and improved the hole injection kinetics for expediting water splitting. By modulating the water content of the electrolyte employed in the anodization process, the wall thickness of the grown TNTZO nanotubes can be reduced to a size smaller than that of the depletion layer thickness, realizing a fully depleted state for charge carriers to further advance the PEC performance. Hydrogen evolution tests demonstrate the practical efficacy of TNTZO for realizing solar hydrogen production. Furthermore, with the composition complexity and fully depleted band structure, the present TNTZO nanotube arrays may offer a feasible and universal platform for the loading of other semiconductors to construct a sophisticated heterostructure photoelectrode paradigm, in which the photoexcited charge carriers can be entirely utilized for efficient solar-to-fuel conversion.

Micro-Compression Test of Nanocrystalline Nickel Deposited by Supercritical Carbon Dioxide Emulsion

This paper reports experimental results of compression test on non-tapered rectangular shaped micro-pillar fabricated by focused ion beam techniques. The pillar is composed of electrodeposited nickel in additive-free Watts bath emulsified with supercritical carbon dioxide. We found that the electroplated film does not contain any defects or pores and has grain size of 8 nm. Maximum compression flow stress exceeds 3.5 GPa without any failure up to 9 % of permanent strain. This is 10 times higher than the strength of the single crystal nickel counterparts fabricated using the same focused ion beam techniques loaded along . This is because of the enhanced mechanical properties by grain boundary strengthening in nanocrystalline nickel and defect-free nickel film. Carbon impurity observed in the nickel film fabricated by electroplating with supercritical carbon dioxide emulsion enhances cohesion of the grain boundary and inhibits grain boundary sliding, which is the predominant deformation mechanisms in this grain size regime.

A study on young's modulus of electroplated gold cantilevers for MEMS devices

This paper presents evaluation of the effective Youngs modulus of electroplated gold micro-cantilevers. Youngs modulus is one of the fundamental parameters to design MEMS (microelectromechanical systems) components. Gold electroplated MEMS structures can be used to develop highly-sensitive MEMS sensors, such as accelerometers. With a resonant frequency method, we evaluate gold-electroplated Ti/Au cantilevers by varying both the length and the width. The experimental results showed that the Youngs modulus was dependent on the width, and it was found that the Youngs modulus increased with an increase in the width, which would be useful to design MEMS devices.

Long-term vibration characteristics of MEMS inertial sensors by multi-layer metal technology

This paper presents the long-term vibration characteristics of MEMS inertial sensors developed by multi-layer metal technology based on gold electroplating. We evaluate the change of resonant frequencies of the MEMS inertial sensors and tip displacements of multi-layer metal cantilevers during long-term vibration tests. The vibration tests employ a cyclic input acceleration with the amplitude of 1G. The experimental results show that the inertial sensors have a mechanical tolerance to the vibration up to 107 cycles. Moreover, the cantilever tests suggest that the structure stability of multi-layer metal devices could be higher when the structure thickness becomes thicker. In conclusion, we confirmed that the Ti/Au multi-layer metal structures can be promising components for MEMS inertial sensors.

Size Effect on the Electrodeposited Nickel Investigated by Micro-compression Test

Effect of the grain size and sample size were examined with different grain size below 20 nm and sample size from 30 to 5 μm fabricated from electroplated nickel. TEM observation confirmed smallest grain size of 8 nm obtained at applied pressure of 15 MPa. On the contrary to the Hall–Petch relationship reported before, inverse Hall–Petch was not observed in our nanocrystalline nickel even when the grain size at 8 nm. Sample size effect on the 8 nm grained nickel was smaller than that of single crystal nickel as Hall–Petch exponent of −0.25 and −0.125 respectively. Suppressed Hall–Petch breakdown and sample size effect were explained by the physics of grain boundaries in association with impurity carbon.

Study on Ti/Au Two-Layered Cantilevers with Different Aspect Ratio for MEMS Devices

We report the structure stability of Ti/Au two-layered micro-cantilevers with the various ratios of length and width for the first time. The cantilevers were fabricated by gold electroplating, a key technology for post-CMOS process for CMOS-MEMS devices. Ti is conventionally used for adhesion layers of gold electroplating, while the mechanical properties of Ti/Au structures have not been investigated. To evaluate the structure stability, the lengths of Ti/Au cantilevers were varied from 100 μm to 1000 μm. The experimental results showed that the cantilever of less than 500 μm in length and 5 μm in width made the structure more stable. Moreover, the cantilever of 1000 μm-length and 15-μm width showed high flatness. These results reveal the potential of Ti/Au layers to be applied to MEMS structures.

Mechanical Properties of Electrodeposited Gold for MEMS Device

Mechanical properties of electrodeposited gold materials for MEMS device were evaluated by a micro-compression test. The gold electroplating method is considered as a key technology for post-CMOS fabrication process. The micro-compression specimens were 15×15×30 μm3 micro-pillars fabricated by focused ion beam. The gold micro-pillars showed a high compressive strength of 600 MPa. The high strength was suggested to be mainly caused by size of the gold grains, which was found to be about 14.7 nm.

Mechanical Property Evaluation of Electrodeposited Nanocrystalline Metals by Micro-testing

Electrodeposition is a very important technology in the fabrication of micro-compo‐ nents for micro-electro-mechanical systems (MEMS) or integrated circuits. Evalua‐ tions of the materials used in these devices as 3D components should be conducted using micro-sized specimens due to the sample size effect on the practical use of the components. Nanocrystalline metals could be deposited using an electrodeposition method with supercritical CO2 emulsion. Our experiment on the micro-specimens provides information on micro-mechanical testing of electrodeposited metals in‐ cluding the effect of sample size, grain size, and anisotropic structures on mechani‐ cal properties. In this chapter, recent studies on crystal growth in electrodeposition of metals and its evaluation using micron-sized testing will be presented.

Crystal Growth by Electrodeposition with Supercritical Carbon Dioxide Emulsion

A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, as shown in Fig. 1, where distinct liquid and gas phases do not exist [1]. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a SCF to be fine-tuned between a gas and a liquid. SCFs are suitable as a substitute for organic solvents in a range of industrial and laboratory processes.

Intact Ultrathin Ni Films Fabricated by Electroplating with Supercritical CO2 Emulsion

Ultrathin (2 emulsion (SCE). Incomplete coverage of the Cu plate, the working electrode, by electroplated Ni and non-uniform Ni films with defects were obtained when conventional electroplating at 1 A/dm2 with 30 sec of deposition time was used. When electroplating with SCE (ESCE) was applied, complete coverage, defect-free and uniform UTNFs were obtained. SEM and AFM showed surface morphology of the UTNFs was covered by spherical-shaped particles with ~10 nm in diameter, which was expected to be individual Ni grains because the size was consistent with grain size of Ni films reported when ESCE was applied. High H2 solubility in CO2, periodic-plating-characteristic after applying ESCE, and improved transport efficiency of the reactive species are believed to be the main reasons to cause effects of grain refinement and suppression in formation of the defects. Thickness of the UTNFs was 11.97±1.82 nm when the deposition time was 15 sec, and the thickness increased to 38.45±1.71 nm when the deposition time was increased to 45 sec.