Loren M Picco

Breaking the speed limit with atomic force microscopy

High-speed atomic force microscopy (AFM) is important for following processes that occur on sub-second timescales for studies both in biology and materials science, and also for the ability to examine large areas of a specimen at high resolution in a practical length of time. Further developments of the previously reported high-speed contact-mode AFM are described. Two instruments are presented: (i) a high-speed flexure stage arrangement capable of imaging at a video rate of 30 fps, and (ii) an ultra-high speed instrument using a combined tuning fork and flexure-stage scanning system capable of ultra-high-speed imaging in excess of 1000 fps. Results of imaging collagen fibres under ambient conditions at rates of up to 1300 frames s−1 are presented. Despite tip–specimen relative velocities of up to 200 mm s−1, no significant damage to the collagen specimen was observed even after tens of thousands of frames were acquired in the same area of the specimen.

iTweezers: optical micromanipulation controlled by an Apple iPad

The 3D interactive manipulation of multiple particles with holographic optical tweezers is often hampered by the control system. We use a multi-touch interface implemented on an Apple iPad to overcome many of the limitations of mouse-based control, and demonstrate an elegant and intuitive interface to multi-particle manipulation. This interface connects to the tweezers system hardware over a wireless network, allowing it to function as a remote monitor and control device.

High-speed AFM of human chromosomes in liquid

Further developments of the previously reported high-speed contact-mode AFM are described. The technique is applied to the imaging of human chromosomes at video rate both in air and in water. These are the largest structures to have been imaged with high-speed AFM and the first imaging in liquid to be reported. A possible mechanism that allows such high-speed contact-mode imaging without significant damage to the sample is discussed in the context of the velocity dependence of the measured lateral force on the AFM tip.

High-speed atomic force microscopy in slow motion-understanding cantilever behaviour at high scan velocities

Using scanning laser Doppler vibrometer we have identified sources of noise in contact mode high-speed atomic force microscope images and the cantilever dynamics that cause them. By analysing reconstructed animations of the entire cantilever passing over various surfaces, we identified higher eigenmode oscillations along the cantilever as the cause of the image artefacts. We demonstrate that these can be removed by monitoring the displacement rather than deflection of the tip of the cantilever. We compare deflection and displacement detection methods whilst imaging a calibration grid at high speed and show the significant advantage of imaging using displacement.

Experimental observation of contact mode cantilever dynamics with nanosecond resolution.

We report the use of a laser Doppler vibrometer to measure the motion of an atomic force microscope contact mode cantilever during continuous line scans of a mica surface. With a sufficiently high density of measurement points the dynamics of the entire cantilever beam, from the apex to the base, can be reconstructed. We demonstrate nanosecond resolution of both rectangular and triangular cantilevers. This technique permits visualization and quantitative measurements of both the normal and lateral tip sample interactions for the first and higher order eigenmodes. The ability to derive quantitative lateral force measurements is of interest to the field of microtribology/nanotribology while the comprehensive understanding of the cantilevers dynamics also aids new cantilever designs and simulations.

Mapping real-time images of high-speed AFM using multitouch control

Conventional AFM is highly restricted by its scan rate, a problem that has been overcome by the development of high-speed AFM systems. As the technology to produce higher scan rates has developed it has pushed forward the design of control software. However, the user interface has not evolved at the same rate, limiting the user to sequential control steps. In this paper we demonstrate the integration of HSAFM with a multitouch interface to produce a highly intuitive and responsive control environment. This enables nanometre resolution to be maintained whilst scanning the sample over tens of microns, and arbitrary paths to be traversed. We illustrate this by scanning around two chromosomes in water, before scanning on top of the chromosome, showing the surface structure.

High-speed atomic force microscopy for materials science

Since its inception in 1986, the field of atomic force microscopy (AFM) has enabled surface analysis and characterisation with unparalleled resolution in a wide variety of environments. However, the technique is limited by very low sample throughput and temporal resolution making it impractical for materials science research on macro sized or time evolving samples such as the observation of corrosion. The potential of AFM sparked intense efforts to overcome these limitations shortly after its invention, and has led to the development of high-speed atomic force microscopes (HS-AFMs). Within the last 5 years the technology underpinning these instruments has matured to the point where routine imaging can achieve megapixels per second over scan areas of square millimetres, removing the limitations from AFM for industrial scale materials characterisation. This review explains the technology and looks to the future use of HS-AFMs in materials science.

Error mapping of high-speed AFM systems

In recent years, there have been several advances in the development of high-speed atomic force microscopes (HSAFMs) to obtain images with nanometre vertical and lateral resolution at frame rates in excess of 1 fps. To date, these instruments are lacking in metrology for their lateral scan axes; however, by imaging a series of two-dimensional lateral calibration standards, it has been possible to obtain information about the errors associated with these HSAFM scan axes. Results from initial measurements are presented in this paper and show that the scan speed needs to be taken into account when performing a calibration as it can lead to positioning errors of up to 3%.

Modelling oscillatory flexure modes of an atomic force microscope cantilever in contact mode whilst imaging at high speed

Understanding the modal response of an atomic force microscope is important for the identification of image artefacts captured using contact-mode atomic force microscopy (AFM). As the scan rate of high speed AFM increases, these modes present themselves as ever clearer noise patterns as the frequency of cantilever vibration falls under the frequency of pixel collection. An Euler-Bernoulli beam equation is used to simulate the flexural modes of the cantilever of an atomic force microscope as it images a hard surface in contact mode. Theoretical results are compared with experimental recordings taken in the high speed regime, as well as previous analytical results. It is shown that the model can capture the mode shapes and resonance properties of the first four eigenmodes.

Large area high-speed metrology SPM system

We present a large area high-speed measuring system capable of rapidly generating nanometre resolution scanning probe microscopy data over mm(2) regions. The system combines a slow moving but accurate large area XYZ scanner with a very fast but less accurate small area XY scanner. This arrangement enables very large areas to be scanned by stitching together the small, rapidly acquired, images from the fast XY scanner while simultaneously moving the slow XYZ scanner across the region of interest. In order to successfully merge the image sequences together two software approaches for calibrating the data from the fast scanner are described. The first utilizes the low uncertainty interferometric sensors of the XYZ scanner while the second implements a genetic algorithm with multiple parameter fitting during the data merging step of the image stitching process. The basic uncertainty components related to these high-speed measurements are also discussed. Both techniques are shown to successfully enable high-resolution, large area images to be generated at least an order of magnitude faster than with a conventional atomic force microscope.