Comment on 'Temperature dependence of the energy dissipation in dynamic force microscopy'

A recent article in this journal by Roll et al (2008 Nanotechnology 19 045703) presents experimental results of the temperature dependence of dissipation in dynamic force microscopy which they use to elucidate the mechanisms of such a dissipation signal in the PTCDA on KBr system. We argue here that dissipation results are often highly dependent upon the tip structure, and urge caution in the interpretation of single sets of experimental data.     

Measuring phase shifts and energy dissipation with amplitude modulation atomic force microscopy

By recording the phase angle difference between the excitation force and the tip response in amplitude modulation AFM it is possible to image compositional variations in heterogeneous samples. In this contribution we address some of the experimental issues relevant to perform phase contrast imaging measurements. Specifically, we study the dependence of the phase shift on the tip-surface separation, interaction regime, cantilever parameters, free amplitude and tip-surface dissipative processes. We show that phase shift measurements can be converted into energy dissipation values. Energy dissipation curves show a maximum (～10?eV/cycle) with the amplitude ratio. Furthermore, energy dissipation maps provide a robust method to image material properties because they do not depend directly on the tip-surface interaction regime. Compositional contrast images are illustrated by imaging conjugated molecular islands deposited on silicon surfaces.     

Chaos in dynamic atomic force microscopy

In tapping mode atomic force microscopy (AFM) the highly nonlinear tip-sample interaction gives rise to a complicated dynamics of the microcantilever. Apart from the well-known bistability under typical imaging conditions the system exhibits a complex dynamics at small average tip-sample distances, which are typical operation conditions for mechanical dynamic nanomanipulation. In order to investigate the dynamics at small average tip sample gaps experimental time series data are analysed employing nonlinear analysis tools and spectral analysis. The correlation dimension is computed together with a bifurcation diagram. By using statistical correlation measures such as the Kullback-Leibler distance, cross-correlation and mutual information the dataset can be segmented into different regimes. The analysis reveals period-3, period-2 and period-4 behaviour, as well as a weakly chaotic regime.     

Energy dissipation effects on imaging of soft materials by dynamic atomic force microscopy: A DNA-chip study

Using amplitude-mode AFM (AM-AFM), we have obtained valuable information during these recent years through the study of amplitude and phase shift dependence on tip–sample separation, leading to a comprehensive understanding of the interaction processes. Two imaging regimes, attractive and repulsive, have been identified and a relationship between phase and dissipative energy was established, providing information on observed material properties. Most of the previous studies have concerned model systems: either hard or soft materials. In this paper, we present the analysis of a mixed system of soft structures on a hard substrate. This is a DNA chip for biological applications consisting of oligonucleotides covalently linked by a layer of silane to a silicon substrate. A detailed study of amplitude-phase curves as a function of the tip–sample separation allowed us to define the best experimental conditions to obtain specific information: we got reliable conditions to minimize noise during topographic imaging and an understanding of the processes of energy dissipation involved in the DNA breaking for DNA arrays. By calculating the energy dissipated as a function of the amplitude of oscillation, we have demonstrated a transition from an energy dissipation process governed by localized viscoelastic interactions (due to the soft layer) to a process governed by extended irreversible deformations (due to the hard substrate).     

Quantitative force and dissipation measurements in liquids using piezo-excited atomic force microscopy: a unifying theory

The use of a piezoelectric element (acoustic excitation) to vibrate the base of microcantilevers is a popular method for dynamic atomic force microscopy. In air or vacuum, the base motion is so small (relative to tip motion) that it can be neglected. However, in liquid environments the base motion can be large and cannot be neglected. Yet it cannot be directly observed in most AFMs. Therefore, in liquids, quantitative force and energy dissipation spectroscopy with acoustic AFM relies on theoretical formulae and models to estimate the magnitude of the base motion. However, such formulae can be inaccurate due to several effects. For example, a significant component of the piezo excitation does not mechanically excite the cantilever but rather transmits acoustic waves through the surrounding liquid, which in turn indirectly excites the cantilever. Moreover, resonances of the piezo, chip and holder can obscure the true cantilever dynamics even in well-designed liquid cells. Although some groups have tried to overcome these limitations (either by theory modification or better design of piezos and liquid cells), it is generally accepted that acoustic excitation is unsuitable for quantitative force and dissipation spectroscopy in liquids. In this paper the authors present a careful study of the base motion and excitation forces and propose a method by which quantitative analysis is in fact possible, thus opening this popular method for quantitative force and dissipation spectroscopy using dynamic AFM in liquids. This method is validated by experiments in water on mica using a scanning laser Doppler vibrometer, which can measure the actual base motion. Finally, the method is demonstrated by using small-amplitude dynamic AFM to extract the force gradients and dissipation on solvation shells of octamethylcyclotetrasiloxane (OMCTS) molecules on mica.     

The role of the tip in non-contact atomic force microscopy dissipation images of ionic surfaces

In this paper we use simulations to investigate the role of the tip in nc-AFM measurements of dissipated energy. Using a virtual AFM we simulate the experiment focusing on the atomic scale energy dissipation on an NaCl(100) flat surface. The non-conservative interaction was treated with the theory of dynamic response and all the calculations were carried out using an atomistic model; several sets of tips were tested using ionic crystals (NaCl, KBr, MgO), each in different configurations (ideal, vacant, divacant, doped). Using an MgO-doped tip we were able to calculate a dissipation signal comparable to what is typically measured in experiments. It was not possible to see any dissipation with ideal tips, although they still have a significant interaction with the surface and give atomic contrast in the frequency shift signal. The effect of the scanning speed on measured frequency shift and dissipation is also calculated and discussed.     

Mechanical manifestations of rare atomic jumps in dynamic force microscopy

The resonance frequency and the excitation amplitude of a silicon cantilever have been measured as a function of distance to a cleaved KBr(001) surface with a low-temperature scanning force microscope (SFM) in ultrahigh vacuum. We identify two regimes of tip-sample distances. Above a site-dependent critical tip-sample distance reproducible data with low noise and no interaction-induced energy dissipation are measured. In this regime reproducible SFM images can be recorded. At closer tip-sample distances, above two distinct atomic sites, the frequency values jump between two limiting curves on a timescale of tens of milliseconds. Furthermore, additional energy dissipation occurs wherever jumps are observed. We attribute both phenomena to rarely occurring changes in the tip apex configuration which are affected by short-range interactions with the sample. Their respective magnitudes are related to each other. A specific candidate two-level system is also proposed.     

Energy dissipation distributions and dissipative atomic processes in amplitude modulation atomic force microscopy

Instantaneous and average energy dissipation distributions in the nanoscale due to short and long range interactions are described. We employ both a purely continuous and a semi-discrete approach to analyze the consequences of this distribution in terms of rate of heat generation, thermal flux, adhesion hysteresis, viscoelasticity and atomic dissipative processes. The effects of peak values are also discussed in terms of the validity of the use of average values of power and energy dissipation. Analytic expressions for the instantaneous power are also derived. We further provide a general expression to calculate the effective area of interaction for fundamental dissipative processes and relate it to the energy distribution profile in the interaction area. Finally, a semi-discrete approach to model and interpret atomic dissipative processes is proposed and shown to lead to realistic values for the atomic bond dissipation and viscoelastic atomic processes.     

Temperature dependence of the dynamic properties of phenolic-fiber-reinforced thermoplastics

The temperature dependence of the dynamic elastic modulus, loss modulus and tan δ of phenolic-fiber-reinforced polystyrene, polypropylene and Nylon 66 (FRTP) has been measured by a resonant bending system and the reinforcing effect examined from the standpoint of the fiber content and/or the fiber size. A considerable improvement of the dependence of E′ for PS and Nylon 66 can be seen. For PP, however, a large decrease of the value of E′, which is attributed to a shrinkage due to a morphological change in the matrix, can be seen in the higher temperature region for a low fiber content. The fiber content dependence of E′ and E″ for these matrices shows a nearly linear relation except in the decreasing region in the case of PP. It is found that the system of fiber mixed with fiber powder is a better reinforcing method for a wide temperature range for PS and PP resins.     

Application of dynamic impedance spectroscopy to atomic force microscopy

Abstract

Atomic force microscopy (AFM) is a universal imaging technique, while impedance spectroscopy is a fundamental method of determining the electrical properties of materials. It is useful to combine those techniques to obtain the spatial distribution of an impedance vector. This paper proposes a new combining approach utilizing multifrequency scanning and simultaneous AFM scanning of an investigated surface.     

Frequency dependence of the electrochemical activity contrast in AC-scanning electrochemical microscopy and atomic force microscopy-AC-scanning electrochemical microscopy imaging

Alternating current mode scanning electrochemical microscopy (AC-SECM) enables local detection of electrochemical surface activity without any redox mediator present in solution. Z-approach curves toward the substrate result in a negative feedback curve of the ac signal for insulating samples. On conducting samples, however, the shape of the feedback curve was found to be dependent on the ac perturbation frequency. Approach curves over a wide range of frequencies were performed, and the results were applied to interpret laterally resolved frequency-dependent measurements obtained with combined atomic force microscopy-AC-SECM (AFM-AC-SECM). For the first time, this frequency dependence of the signal was utilized to fine-tune the electrochemical contrast in lateral imaging in AC-SECM. An array of gold microelectrodes embedded in silicon nitride displaying significant changes in electrochemical activity as well as in topography was investigated using a bifunctional AFM-SECM tip with an integrated recessed ring microelectrode. Due to the unique geometrical conditions the electrochemical contrast between the conducting gold spots and the insulating SixNy is reversed, crosses zero, and inverts as a function of the applied ac frequency.     

Temperature dependence of ultrasonic backscattered energy in motion-compensated images

Noninvasive temperature imaging would enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. Previously, we predicted monotonic changes in backscattered energy (CBE) of ultrasound with temperature for certain subwavelength scatterers. We also measured CBE values similar to our predictions in bovine liver, turkey breast, and pork muscle in one dimension (1-D). Those measurements were corrected manually for changes in the axial position of echo signals with temperature. To investigate the effect of temperature on CBE in 2-D, we imaged 1-cm thick samples of bovine liver, turkey breast, and pork muscle during heating in a water bath. Images were formed by a phased-array imager with a 7 MHz linear probe. Using radio frequency (RF) signals permitted the use of cross correlation as a similarity measure for automatic tracking of feature displacement as a function of temperature. Feature displacement across the specimen was nonuniform with typical total displacements of 0.5 mm in both axial and lateral directions. Apparent movement in eight image regions in each specimen was tracked from 37 to 50 degrees C in 0.5 degrees C steps. Envelopes of motion-compensated image regions were found then smoothed with a 3 x 3 running average filter before forming the backscattered energy at each pixel. Our measure of CBE compared means of both the positive and negative changes in the backscattered energy (BE) images. CBE was monotonic and differed by about 4 dB at 50 degrees C from its value at 37 degrees C. Relatively noise-free CBE curves from tissue volumes of less than 1 cm3 supports the use of CBE for temperature estimation.     

Temperature dependence of friction force acting on dislocations in silicon crystals

The characteristics of the dislocation motion in silicon crystals during annealing have been investigated by means ofin situ X-ray topographic observations in the temperature range from 1173 to 1273 K. When annealed at elevated temperatures, the small displacements in position of the dislocation segments took place so as to balance the friction force acting on these segments with the elastic interaction force between dislocations. From the analysis of (i) repulsive interaction between parallel screw dislocations with the same sign, and of (ii) configurational changes of dislocation half-loops, the friction force acting on dislocations can be determined at each annealing temperature by estimating the repulsive interaction force between parallel dislocations, and the force due to line tension for curved dislocation. On the basis of the analysis of the temperature dependence of friction force, the activation energy for dislocation motion is evaluated to be 2.4 eV for a screw dislocation and 2.2 eV for a 60° dislocation.     

Micro-scale energy dissipation mechanisms during dynamic fracture in natural polyphase ceramic blocks

The dynamic fracture of natural polyphase ceramic (granite) blocks by high-speed impact at 207 m/s, 420 m/s and 537 m/s has been investigated. An electromagnetic railgun was used as the launch system. Results reveal that the number of fragments increases substantially, and the dominant length scale in their probability distributions decreases, as the impact energy is increased. Micro-scale studies of the fracture surfaces reveals evidence of localized temperatures in excess of 2000 K brought on by frictional melting via fracturing and slip along grain boundaries in orthoclase and plagioclase, and via transgranular fracture (micro-cracking) in quartz. The formation of SiO₂- and TiO₂-rich spheroids on fracture surfaces indicates that temperatures in excess of 3500 K are reached during fracture.     

On the dependence of crack surface morphology and energy dissipation on microstructure in ductile plate tearing

The two distinct tearing mechanisms observed in ductile metal plates are the void-by-void advance of the crack tip, and the simultaneous interaction of multiple voids on the plane ahead of the crack tip. Void-by-void crack advance, which leads to a cup-cup crack surface morphology, is the dominant mechanism if the plate contains a low number of small void nucleation sites (i.e., second phase particles). Conversely, a large number and/or size of nucleation sites trigger the simultaneous interaction of multiple voids resulting in a slanted crack. The present work aims to provide further insight into the parameters controlling the mechanisms and energy dissipation of plate tearing by focusing on the shape and, thereby, the orientation of the nucleation sites. The study uses a two-dimensional plane strain finite element domain to model the cross section of a plate, subject to mode I tearing, with discretely modeled, randomly distributed, finite-sized elliptic void nucleation sites. The developed finite element setup can capture the dependence of the crack surface morphology on the microstructure of the plate. The simulation results confirm that cup-cup crack propagation develops by intense plastic straining throughout the thinning region of the plate. Conversely, slanted and cup-cone cracks propagate in thin localized shear deformation bands. The energy dissipation is, therefore, greater for cup-cup cracks. The study demonstrates that the damage-related microstructure has a significant role in determining the overall hardening capacity of a plate, which in turn dictates the tearing mode and energy.