G. Y. Chen

Adsorption-induced surface stress and its effects on resonance frequency of microcantilevers

It is well known that bimetallic microcantilevers can exhibit static deflection as a result of thermal effects, including exothermic adsorption of chemicals on their surfaces. It is shown here that the resonance frequency of a cantilever can change due to a combination of mass loading and change of spring constant resulting from adsorption of chemicals on the surface. Cantilevers also undergo static bending that is induced by differential surface stress. The magnitude of these effects depends upon the chemical properties of the surface and upon the amount of material adsorbed. Hence cantilever deflection as well as resonance frequency change can be used as the basis for development of novel chemical sensors.

THERMAL AND AMBIENT-INDUCED DEFLECTIONS OF SCANNING FORCE MICROSCOPE CANTILEVERS

The deflection of scanning force microscope cantilevers, metal coated on one side, is significantly influenced by both thermal heating and variations in relative humidity. For constant relative humidity, the deflection of the cantilever drifts due to laser heating and eventually reaches a steady‐state value. For a thermally stabilized cantilever, the deflection varies linearly with relative humidity. Exposure to other vapors, such as mercury, changes the inherent deflection of the cantilever. Relative amounts of adsorbates on the cantilever can be estimated from shifts in the cantilever resonance frequency with picogram mass resolution. The cantilever deflection as well as changes in resonance frequency due to vapor adsorption can be used as basis for novel chemical sensors.

Resonance response of scanning force microscopy cantilevers

A variational method is used to calculate the deflection and the fundamental and harmonic resonance frequencies of commercial V‐shaped and rectangular atomic force microscopy cantilevers. The effective mass of V‐shaped cantilevers is roughly half that calculated for the equivalent rectangular cantilevers. Damping by environmental gases, including air, nitrogen, argon, and helium, affects the frequency of maximum response and to a much greater degree the quality factor Q. Helium has the lowest viscosity, resulting in the highest Q, and thus provides the best sensitivity in noncontact force microscopy. Damping in liquids is dominated by an increase in effective mass of the cantilever due to an added mass of the liquid being dragged with that cantilever.

Viscous drag measurements utilizing microfabricated cantilevers

The influence of viscous drag forces on cantilevers is investigated using standard atomic force microscope (AFM) cantilevers. Viscosity effects on several geometrically different cantilevers manifest themselves as variations in resonance frequencies, quality factors, and cantilever response amplitudes. With this novel measurement, a single cantilever can be used to measure viscosities ranging from η=10−2 to 102 g/cm s.

Harmonic response of near‐contact scanning force microscopy

One‐dimensional harmonic oscillator theory was used to model tapping‐mode atomic force microscope (TMAFM) operation in the near‐contact region in the presence of gases and liquids. The force derivative of the tip‐sample interaction changes the vibration amplitude and frequency at maximum amplitude of the cantilever. Additionally, the interaction is hydrodynamically damped by fluid motion around the tip and between the tip and the surface. Good agreement was found between theoretical and experimental amplitude as a function of height. For a sample‐driven TMAFM operating in fluids, the cantilever can be very soft (spring constant ≪1 N/m) and operated at frequencies well above the fundamental. Under these conditions the cantilever and sample appear to act with a high spring constant, much like that used in a gaseous operation. The tip–sample interaction in the fluid is still mediated through the force derivative of the sample.

Characterization of atomic force microscope tips by adhesion force measurements

The resolution limit in an atomic force microscope image usually is attributed to the finite radius of the contacting probe. Here, it is shown that this assumption is valid only when adhesion forces are minimal. Relative to the tip‐imposed geometrical limit, the resolution and contrast in AFM images can be degraded by increasing adhesion forces. The large adhesion forces observed for some tips at low humidity conditions are shown to be due to tip contamination or poorly formed tip apexes. Methods to determine and to reduce the extent of tip contamination are described. Cleaning carried out using UV‐ozone or oxygen‐plasma etching were found to significantly reduce the minimum adhesion force.

Transient response of tapping scanning force microscopy in liquids

Tapping‐mode scanning force microscopy in liquids is usually accomplished by acoustic excitation of the cantilever because of the strong viscous damping. Contact of the tip with the sample surface results in a damping of the cantilever amplitude with an anharmonic response. This interaction is modeled as a viscous‐damped, one‐dimensional harmonic oscillator periodically perturbed by an exponential surface potential. Experimental results verify the validity of the model.

Enhanced plasticity in a Zr-based bulk metallic glass composite with in-situ formed intermetallic phases

An in situ formed intermetallic phase/bulk metallic glass composite with high strength and good plasticity was fabricated by casting Zr55.0Cu29.0Ni8.0Al8.0 melts. In situ formed tetragonal structured (Zr,Ni,Al)2(Cu,Ni,Al) intermetallic particles with a hardness of 9.6±0.3 GPa improve the fracture strength of the composite. Micrographs of the fractured samples reveal that the shear band spacing is smaller than the intermetallic particles, indicating that they can effectively block shear band propagation before catastrophic fracture.An in situ formed intermetallic phase/bulk metallic glass composite with high strength and good plasticity was fabricated by casting Zr55.0Cu29.0Ni8.0Al8.0 melts. In situ formed tetragonal structured (Zr,Ni,Al)2(Cu,Ni,Al) intermetallic particles with a hardness of 9.6±0.3 GPa improve the fracture strength of the composite. Micrographs of the fractured samples reveal that the shear band spacing is smaller than the intermetallic particles, indicating that they can effectively block shear band propagation before catastrophic fracture.

Improvement of magnetic properties of an Fe-6.5 wt. % Si alloy by directional recrystallization

We report that magnetic properties of an Fe-6.5 wt. % Si alloy can be improved through texture control by using directional recrystallization. Columnar grain structures with column sizes of ∼0.38×1.2 mm2 were developed during directional recrystallization. It was found that there are low energy boundaries between columns and main textures of the specimen were {110}⟨111⟩ and {111}⟨110⟩. As a result, the coercivity of a directionally recrystallized specimen is reduced by a factor of 5 when measured along 60° away from the growth direction, as compared to a specimen consisting of ∼77 μm equiaxed grains.