Charles A. Clifford

Multifunctional Nanoprobes for Nanoscale Chemical Imaging and Localized Chemical Delivery at Surfaces and Interfaces

Double take: Double-barrel carbon nanoprobes with integrated distance control for simultaneous nanoscale electrochemical and ion conductance microscopy can be fabricated with a wide range of probe sizes in less than two minutes. The nanoprobes allow simultaneous noncontact topographical (left image) and electrochemical imaging (right) of living neurons, as well as localized K+ delivery and simultaneous neurotransmitter detection.

Quantitative analytical atomic force microscopy: a cantilever reference device for easy and accurate AFM spring-constant calibration

Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules. We have developed a compact and easy-to-use reference artefact for this calibration by bulk micromachining of silicon, which we call a cantilever microfabricated array of reference springs (C-MARS). Two separate reference cantilever structures, each nominally 3 µm thick, are fabricated from a single crystal silicon membrane. A binary code of surface oxide squares (easily visible in light, electron and atomic force microscopy) makes it easy to locate the position of the AFM tip along the length of the cantilevers. Uncertainty in location is the main source of error when calibrating an AFM using reference cantilevers, especially for those having spring constants greater than around 10 N m −1 . This error is effectively eliminated in our new design. The C-MARS device spans the range of spring constants from 25 N m −1 down to 0.03 N m −1 important in AFM, allowing almost any contact-mode AFM cantilever to be calibrated easily and rapidly.

Modelling of nanomechanical nanoindentation measurements using an AFM or nanoindenter for compliant layers on stiffer substrates

Finite element analysis (FEA) is used to model the nanoindentation process for a rigid, spherically shaped indenter acting on an elastic two-phase system of an elastic layer that is more compliant than the underlying elastic substrate. A review of current analytical equations to model this process is made and compared to FEA. The FEA results may be expressed analytically by a simple function that describes the reduced modulus value obtained with Oliver and Pharrs method for any modulus value, thickness of layer or radius of the indenter tip. This function is used to investigate B?ckles rule, that to measure the properties of a layer, the indentation depth should be 10% or less of the total layer thickness. The results show that B?ckles rule is invalid for layer thicknesses below 5??m and a new rule is developed which depends on the layer thickness, the indenter radius and the ratio of the reduced moduli of the substrate and overlayer. This rule is based on FEA data. We present a guide to the analysis of the maximum depth that may be indented in order to keep the uncertainty in the reduced modulus for the layer to better than 10%.

Improved methods and uncertainty analysis in the calibration of the spring constant of an atomic force microscope cantilever using static experimental methods

There are many published methods to calibrate the spring constant of an atomic force microscope (AFM) cantilever needed for quantitative force measurement. Each method has its own merits and falls within three broad categories: dimensional, static experimental and dynamic experimental. We report, here, improved static experimental methods using a one- or a two-step calibration process using readily available equipment. The one-step method uses either a nanoindenter on cantilever method or dimensional means and the two-step method uses the first step to calibrate a reference cantilever which is then used for a cantilever on reference cantilever approach relevant to laboratories with many AFMs. For both static experimental methods, multi-positional methods using four or five positions along the cantilever are described and shown in practice to have lower uncertainty than a one-position method. Full uncertainties in all the methods are discussed and shown to be dominated by the uncertainty in the first step demonstrated here with a standard uncertainty of 8%, although this could be reduced to 5%. In the second, cantilever on reference cantilever step only a further 0.6% is added in quadrature.

Towards easy and reliable AFM tip shape determination using blind tip reconstruction

Quantitative determination of the geometry of an atomic force microscope (AFM) probe tip is critical for robust measurements of the nanoscale properties of surfaces, including accurate measurement of sample features and quantification of tribological characteristics. Blind tip reconstruction, which determines tip shape from an AFM image scan without knowledge of tip or sample shape, was established most notably by Villarrubia [J. Res. Natl. Inst. Stand. Tech. 102 (1997)] and has been further developed since that time. Nevertheless, the implementation of blind tip reconstruction for the general user to produce reliable and consistent estimates of tip shape has been hindered due to ambiguity about how to choose the key input parameters, such as tip matrix size and threshold value, which strongly impact the results of the tip reconstruction. These key parameters are investigated here via Villarrubias blind tip reconstruction algorithms in which we have added the capability for users to systematically vary the key tip reconstruction parameters, evaluate the set of possible tip reconstructions, and determine the optimal tip reconstruction for a given sample. We demonstrate the capabilities of these algorithms through analysis of a set of simulated AFM images and provide practical guidelines for users of the blind tip reconstruction method. We present a reliable method to choose the threshold parameter corresponding to an optimal reconstructed tip shape for a given image. Specifically, we show that the trend in how the reconstructed tip shape varies with threshold number is so regular that the optimal, or Goldilocks, threshold value corresponds with the peak in the derivative of the RMS difference with respect to the zero threshold curve vs. threshold number.

Nanoindentation measurement of Young's modulus for compliant layers on stiffer substrates including the effect of Poisson's ratios

Finite element analysis (FEA) is used to investigate the effect of the Poissons ratios of both the overlayer and the substrate on the nanoindentation of an elastic two-phase system where the elastic overlayer is more compliant than the underlying elastic substrate. A rigid spherical indenter is used as a probe. It is found that nanoindentation results may be expressed analytically using a simple extension of the previously described equation (Clifford and Seah 2006 Nanotechnology 17 5283). This simple function describes the reduced modulus value measured using Oliver and Pharrs method (1992 J. Mater. Res. 7 1564) for any modulus values or Poissons ratio values of the overlayer and substrate, overlayer thickness or indenter tip radius. This equation and the FEA behind it are tested using experimental published data for the nanoindentation of a silicon dioxide layer on silicon.

Simplified drift characterization in scanning probe microscopes using a simple two-point method

A very simple and rapid method of drift evaluation, by monitoring two points in the field of view, is proposed. This method can be used by all analysts and requires no special sample or software programming. Data are shown for a modern commercial atomic force microscope (AFM) with closed loop scanners in which significant image distortion arises from drift. The method is fast and simple to implement and allows the drift in the z axis, the image pseudo-magnification and the image pseudo-rotation to be characterized as well as the drifts in the x and y axes available with other methods. The method is compared with the manufacturers image correlation method for the x and y axes. The demonstrated best precision of the drift observed by both methods is the pixel interval and so the scan area and number of pixels in the scan need to be chosen with this in mind.

Nanomechanical measurements of hair as an example of micro-fibre analysis using atomic force microscopy nanoindentation

The characterisation of nanoscale surface properties of textile and hair fibres is key to developing new effective laundry and hair care products. Here, we develop nanomechanical methods to characterise fibres using an atomic force microscope (AFM) to give their nanoscale modulus. Good mounting methods for the fibre that are chemically inert, clean and give strong mechanical coupling to a substrate are important and here we detail two methods to do this. We show, for elastic nanoindentation measurements, the situation when the tip radius significantly affects the result via a function of the ratio of the radii of the tip and fibre and indicate the importance of using an AFM for such work. A valid method to measure the nanoscale modulus of fibres using AFM is thus detailed and exampled on hair to show that bleaching changes the nanoscale reduced modulus at the outer surface.

Cantilever Spring-Constant Calibration in Atomic Force Microscopy

The measurement of small forces by atomic force microscopy (AFM) is of increasing importance in many applications. For example, in analytical applications where individual molecules are probed, or nanoindentation measurements as a source of information about materials properties on a nanometer scale. The fundamentals of AFM force measurement, and some of these applications, are briefly reviewed. In most cases absolute, not relative, measurements of forces are needed for valid comparisons with theory and other measurement techniques (such as optical tweezers). We review methods of AFM force calibration and the major uncertainties involved. The force range considered in this work is roughly from 10 pN to around 500 nN. We describe some issues of the repeatability of force measurements that can be important in common AFM instruments. In most cases the aspect that then limits the accuracy with which forces can be measured is the uncertainty in the stiffness (more specifically the normal force constant) of the atomic force microscope cantilever at the center of the instrument. It is known that commercially available microfabricated atomic force microscope cantilevers have a wide range of force constant, for cantilevers of nominally the same type and even the same production batch. Calibration is necessary, and many methods have been used over the years. We compare the accuracy that can be achieved and the ease of use of these different methods, including theoretical (dimensional), thermal, static and dynamic methods and their variants. A device developed at NPL should help overcome many of the problems of force constant calibration, at least for the most common AFM configurations. This is a microfabricated silicon device, which, because of its very small mass, is not susceptible to vibration as a larger device would be. A new calibration method based on electrical and Doppler measurements allows the calibration of the force constant of this device traceable to the SI newton. It can then be sent to AFM users for straightforward calibration of AFM force constants. We conclude with a brief discussion of the special problems of calibration of lateral forces, such as those obtained in frictional force measurements.

Microelectromechanical system device for calibration of atomic force microscope cantilever spring constants between 0.01 and 4 N/m

Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurement of nano-newton and pico-newton forces, which are critical to analytical application of AFM in the analysis of polymer surfaces, biological structures and organic molecules. Previously we have described microfabricated array of reference spring (MARS) devices for AFM cantilever spring-constant calibration. Hitherto, these have been limited to the calibration of AFM cantilevers above 0.03 N/m, although they can be used to calibrate cantilevers of lower stiffness with reduced accuracy. Below this limit MARS devices similar to the designs hitherto described would be fragile and difficult to manufacture with reasonable yield. In this work we describe a device we call torsional MARS. This is a large-area torsional mechanical resonator, manufactured by bulk micromachining of a “silicon-on-insulator” wafer. By measuring its torsional oscillation accurately in vacuum we can deduce its torsional spring constant. The torsional ...