Atomic Force Microscopes

Micro-discharge plasma has been widely used in many fields, but the Thomson discharge theory can't reasonably explain mesoscopic-scale air discharge phenomenon. In order to further explore the micro-gap air discharge characteristics and to determine the effect of field emission on micro-scale air discharge, the precision displacement plate and atomic force microscope (AFM) were used to construct the pin-plate electrode structure which can realize the micrometer distance control. The phenomena and mechanism of micro-gap discharge were investigated when the DC voltage was applied in atmosphere environment. The results show that the micro-gap discharge mechanism is very different from that between the long gap. When the gap is about 15 μm, non-uniform electric field discharge generates continuous glow discharge or intermittent spark discharge. When the gap is less than 2 μm, there is no glow discharge occurs. Fowler-Nordheim formula describes the same phenomena that breakdown voltage does not change with the gap distance, denoting that is the field emission mechanism. Cathode field emission

The authors developed a tip-enhanced near field Raman microscope that can precisely regulate longitudinal distance between a metallic tip and sample molecules. This was done by employing a time-gated photoncounting scheme that enabled us to observe exponentially decaying near field Raman intensity with the tip-sample distance. The exponential decay shows a characteristic of the enhanced field generated by the localization of the surface plasmon polaritons near the tip apex. This microscope was applied to evaluate metal-coated tips and also to investigate confinement of the field generated at a gap between two metal nanostructures from the decay curves.

We describe the design and performance of a relative humidity (RH) control chamber for use with atomic force microscopes (AFM) in which the tip is scanned across the stationary sample. The small volume (~9 cm3) chamber encloses the sample, the cantilever holder, and a commercial humidity/temperature sensor. The RH is controlled by passing a controlled ratio of dry and humid nitrogen gas across the sample. This unique design prevents exposure of the scanner assembly to humid gas and maintains all of the functionalities of the AFM system with no measurable degradation of its performance. Using this system, the RH at the sample position can be varied between 5% and 95% and controlled to within +/-0.2% during an AFM measurement. To demonstrate the performance of the RH control chamber in imaging and force spectroscopy modes, we have characterized the RH-dependent swelling of small chitosan droplets with diameters of 3-40 μm, and the RH dependence of capillary forces between the AFM tip and a mica surface.

We adjust the transient dynamics of a piezoactuated bimorph atomic force microscopy (AFM) probe using a state feedback controller. This approach enables us to adjust the quality factor and the resonance frequency of the probe simultaneously. First, we first investigate the effect of feedback gains on dynamic response of the probe and then show that the time constant of the probe can be reduced by reducing its quality factor and/or increasing its resonance frequency to reduce the scan error in tapping mode AFM.

Torsional harmonic cantilevers allow measurement of time varying tip-sample forces in tapping-mode atomic force microscopy. Accuracy of these force measurements is important for quantitative nanomechanical measurements. Here we demonstrate a method to convert the torsional deflection signals into a calibrated force waveform with the use of non-linear dynamical response of the tapping cantilever. Specifically the transitions between steady oscillation regimes are used to calibrate the torsional deflection signals.

ABSTRACT We report on the design and performance of a quartz tuning-fork-based dynamic mode atomic force microscope for the imaging of biological samples under ambient conditions. The instrument uses a stiff cantilever that maintains stable oscillations at low amplitudes even in the presence of capillary and other long range forces that can overwhelm more flexible cantilevers. The performance of the microscope is demonstrated with high resolution measurements of double-stranded DNA on cleaved mica substrates using Si and Si3N4 tips. © 2001 American Institute of Physics.