Zhiyu Hu

Multiple-input microcantilever sensors

A surface-micromachined micro-electro-mechanical-system (MEMS) process has been used to demonstrate multiple-input chemical sensing using selectively coated cantilever arrays. Cantilever motion due to absorption-induced stress was readout using a custom-designed, eight-channel integrated circuit. Combined hydrogen and mercury vapor detection was achieved with a palm-sized, self-powered module with spread-spectrum telemetry reporting.

Investigation of adsorption and absorption-induced stresses using microcantilever sensors

The interaction between a vapor and a thin film adsorbed on one side of a bimaterial microcantilever produces differential stress, resulting in readily measurable curvatures of the cantilever structure. Depending upon the system studied, there exist two types of gas–solid interaction: bulk-like absorption and surface-like adsorption. The absorption of hydrogen into palladium results in film expansion whose magnitude is governed by hydrogen partial pressure. The bending of a bimaterial microcantilever (palladium/silicon) due to hydrogen absorption depends on the thickness of the palladium film and is reversible but rate limited by a surface barrier. In contrast, the stress induced by adsorption of mercury onto a bimaterial (gold/silicon) cantilever is irreversible at room temperature, is rate limited by surface coverage, and is independent of the gold–film thickness.

Mercury vapor detection with a self-sensing, resonating piezoelectric cantilever

A microcantilever with an integrated piezoelectric film is demonstrated as a mercury vapor detector. The cantilever is self-sensing and self-actuating, and therefore does not need alignment of an external, optical detection system. This gives the new sensor system an advantage in array applications. Mercury vapor, when adsorbed onto gold on the cantilever, causes the stiffness, and therefore the natural frequency, of the cantilever to increase as a result of mercury gold amalgamation. This shift is detected using the piezoelectric portion of the cantilever in conjunction with a bridge circuit and amplifier. A mercury concentration of 93 ppb in nitrogen is detected.

Detection of pH variation using modified microcantilever sensors

A micromechanical technique for measuring solution pH using modified silicon (SiO2) and silicon nitride (Si3N4) microcantilevers is described. As the modified surface of the cantilever accumulates charge proportional to the pH of the surrounding liquid, the cantilever undergoes bending due to the differential surface stress. Results are presented for chemically modified (4-aminobutyltriethoxysilane, 11mercaptoundecanoic acid) and metal-modified (Au/Al) surfaces over a pH range 2‐12. Aminosilane-modified SiO2/Au cantilevers performed robustly over pH range 2‐8 (49 nm deflection/pH unit), while Si3N4/Au cantilevers performed well at pH 2‐6 and 8‐12 (30 nm deflection/pH unit). The influences of other ions on cantilever bending were found to be negligible below 10 ˇ2 M concentration.

Low‐temperature photoluminescence of detector grade Cd1−xZnxTe crystal treated by different chemical etchants

Low‐temperature photoluminescence (PL) spectra of detector grade Cd1−xZnxTe (x=0.1) have been measured to obtain information about shallow level defect concentration introduced during mechanical polishing and chemical etching processes. We present here a comparative PL study of Cd0.9Zn0.1Te crystals treated by different chemical solutions used for nuclear detector surface treatment. The results show that the 5% Br–MeOH+2%Br–20% lactic acid in ethylene glycol treatment combines the advantages of bromine and lactic acid for chemical etching and results in the best surface condition, as evidenced by the largest I(D0,X)/Idef intensity ratio and the narrowest full width at half‐maximum of the main peak (D0,X). Changes in the surface morphology were also analyzed by atomic force microscopy and correlated with the PL results. Current–voltage (I–V) curves and the room‐temperature 55Fe spectral response of the sample etched by the best treatment are also presented and discussed.

Chemical sensing in Fourier space

Chemical sensing using optical diffraction from an array of microcantilevers is demonstrated. Properly fashioned arrays of micromachined silicon-nitride cantilevers containing embedded deformable diffraction gratings are functionalized with chemically selective coatings. Adsorption of specific molecules on the cantilever leads to bending, which changes the diffraction pattern of a laser beam reflecting off the array. Quantitative chemical information can be obtained by monitoring the displacement of diffraction peaks as a function of analyte exposure.

A piezoresistive microcantilever array for surface stress measurement: curvature model and fabrication

This paper presents a procedure for the fabrication of a piezoresistive microcantilever array for surface-stress-based chemical and biochemical sensing applications. All existing microcantilever surface stress sensors that are based on single-crystal silicon use p-doped piezoresistors. In this work, the advantages of using n-doped silicon piezoresistors for surface stress sensing have been demonstrated. Further, a new model for surface-stress-sensitive cantilevers, based on classical laminated plate theory, is presented. This model allows for the estimation of the deformation and piezoresistive response of a multilayered microcantilever to surface stresses during analyte measurement and residual stresses in the structural layers due to fabrication processes. Also, the model accounts for bending–stretching coupling in the microcantilever response to the stresses. The utility of the model as a design tool for control of cantilever curvature during the fabrication process has been demonstrated.

Study of electroless Au film deposition on ZnCdTe crystal surfaces

The ‘‘electroless’’ deposition method of Au thin films on n‐type ZnCdTe crystal surfaces has been investigated by atomic force microscopy, x‐ray photoelectron spectroscopy, and low temperature photoluminescence. The blocking contact behavior of these films was strongly dependent on post deposition annealing treatments which were also found to induce modifications in the surface morphology and surface chemical composition. Heat treatments (at 300 °C) in vacuum eliminates most of the interfacial tellurium oxide introduced during the deposition. Annealing also reduces the radiative recombination at defects in the region below the interface and increases the barrier height of the contact.

Calibration of optical cantilever deflection readers

Because of its ultrahigh sensitivity, the optical lever detection method similar to that used in the atomic force microscope (AFM) has been widely employed as a standard technique for measuring microcantilever deflection. Along with the increasing interest in using the microcantilever as a sensing platform, there is also a requirement for a reliable calibration technique. Many researchers have used the concept of optical lever detection to construct microcantilever deflection readout instruments for chemical, physical, and biological detection. However, without an AFM piezo z scanner, it is very difficult to precisely calibrate these instruments. Here, we present a step-by-step method to conveniently calibrate an instrument using commercially available piezoresistive cantilevers. The experimental results closely match the theoretical calculation. Following this procedure, one can easily calibrate any optical cantilever deflection detection system with high reproducibility, precision, and reliability. A de...

Current advances in precious metal core–shell catalyst design

Abstract Precious metal nanoparticles are commonly used as the main active components of various catalysts. Given their high cost, limited quantity, and easy loss of catalytic activity under severe conditions, precious metals should be used in catalysts at low volumes and be protected from damaging environments. Accordingly, reducing the amount of precious metals without compromising their catalytic performance is difficult, particularly under challenging conditions. As multifunctional materials, core–shell nanoparticles are highly important owing to their wide range of applications in chemistry, physics, biology, and environmental areas. Compared with their single-component counterparts and other composites, core–shell nanoparticles offer a new active interface and a potential synergistic effect between the core and shell, making these materials highly attractive in catalytic application. On one hand, when a precious metal is used as the shell material, the catalytic activity can be greatly improved because of the increased surface area and the closed interfacial interaction between the core and the shell. On the other hand, when a precious metal is applied as the core material, the catalytic stability can be remarkably improved because of the protection conferred by the shell material. Therefore, a reasonable design of the core–shell catalyst for target applications must be developed. We summarize the latest advances in the fabrications, properties, and applications of core–shell nanoparticles in this paper. The current research trends of these core–shell catalysts are also highlighted.