主页

资料库

Metrology & Instrumentation: Polymer Imaging


Operational Modes

In AFM, imaging of a sample surface is performed with a use of a miniature probe, which is microfabricated in a shape of a cantilever with a sharp tip at its end. Historically, AFM has been introduced as the contact mode technique, in which a deflection of the cantilever has been used as a measure of the tip-sample forces. The cantilever deflection is measured with an optical detection scheme that magnifies the cantilever motion using a laser beam reflected from the cantilever backside. In this mode, an engagement of the tip on the sample surface is completed as soon as the cantilever deflection reaches a set-point level. After this happened, the tip is rastered over the sample surface under a feedback control that adjusts the probe-sample vertical distance to keep the tip force at the set-point level. The voltage applied to the scanner, which controls these vertical displacements, is reproduced in an AFM height image. A grey-scale or color-coded contrast reflects surface corrugations. In addition to the height image, there are deflection and lateral force images, which reflect instantaneous deflection (error signal) and variations in lateral forces, respectively. Deflection (error) images are highly sensitive to fine surface features such as edges and steps. Lateral force images might correlate to local friction variations.


Applications of the contact mode showed that at ambient conditions one could reach atomic lattice resolution while imaging flat crystalline surfaces. Operation at low-forces is essential for studies of soft samples in order to avoid sample damage. Force minimization can be achieved by use of the AFM probes with low stiffness and by performing imaging under liquid (typically, under water). For recognition of surface locations with different adhesive and mechanical properties, lateral force imaging and force modulation can be applied. Force modulation, the probe preloaded into a sample is oscillating at the resonance frequency of an external actuator. Changes of the deflection amplitude will be larger on stiffer surface locations than on softer ones. When a multi-component sample is examined, the contrast variations of the lateral force and deflection amplitude and in force modulation technique reveal a presence and distribution of different components. Studies of many polymer samples with the contact mode showed that this mode is not always applicable because of surface damage caused by shearing deformation, which is developing during permanent tip-sample contact.


An introduction of the TappingMode, which is based on oscillatory motion of the probe and which is characterized by intermittent tip-sample contact, has revolutionized AFM applications for polymers. A broad range of soft materials became accessible for AFM examination. This happened because lateral force interactions, which lead to shearing of the sample surface in the contact mode, were practically eliminated in the oscillatory mode. In TappingMode, a drop of the amplitude of the oscillating probe which initially is driven into oscillation at (or close to) its resonance frequency due to the tip-sample force interactions is employed for the feedback mechanism during scanning. In some sense, amplitude in TappingMode plays the role of deflection in contact mode. A cantilever probe is driven into its resonance oscillation by a small actuator and its amplitude is registered using the optical detection scheme common for atomic force microscopes.


It is worth noting that whilst imaging at ambient conditions resolution of TappingMode is generally related to the tip-sample contact area. Structures of a few nanometers in size can be routinely detected in AFM images even using probes with 10-20 nm apexes. This happens because under appropriate tip-sample force level only a part of the tip apex actually comes into contact with the sample.


Height images in TappingMode are usually accompanied by the amplitude (error) images or phase images. The phase images reflect differences between the phase of a driving actuator (the phase of the freely oscillating cantilever is the same) and the phase of the cantilever oscillation when the probe is interacting with the sample. The phase images became useful in AFM of polymers for several reasons. First of all, the phase changes indicate either imaging is performed in overall attractive or repulsive force interactions. This information could not be obtained from amplitude changes, which do not reflect tip-sample force interaction completely. Second, the phase changes are extremely sensitive to the tip-sample interactions and can be used for precise adjustment of a tip-sample force level. Third, similar to error images (deflection images in the contact mode and amplitude images in TappingMode) the phase changes on homogeneous samples are extremely sensitive to fine surface features, which are seen with higher contrast as compared to the height images. Fourth, the phase images of heterogeneous polymer samples often present compositional maps of these samples with highest contrast.


In imaging of polymer samples, it is quite important to realize that stiffness of the commercial AFM probe is comparable to the stiffness of the samples. This circumstance provides definite advantages for compositional mapping but also requires an additional attention of an operator in order to obtain the most valuable experimental data and to conduct their rational analysis. The general recipes for imaging of polymer samples are quite easy to understand. In operation at low forces, the AFM tip will be imaging the topmost surfaces and this can be realized even for soft materials, such as polymers and biological objects. At this condition, the tip-sample contact area will be minimal and this, in principle, will provide the highest image resolution. In practice, the situation can be more complicated and fine nanometer-scale structures, such as polymer lamellae, can be hidden under a featureless top material (usually amorphous polymer in rubbery state or melted semicrystalline material). In this case, the probe should penetrate this top layer before its apex reaches the lamellae.


In addition to sample topography, a researcher can be interested in learning the composition of polymer sample. The images, which best suit this purpose, are usually obtained at elevated tip-sample forces. In this condition, the image resolution might be sacrificed but the contrast will better reflect different sample components and their spatial distribution. Material-related contrast appears in imaging modes related to contact mode operation (e.g. force modulation, lateral force microscopy) as well as in phase imaging TappingMode. In the contact mode, the variation of the set-point deflection is used for the tip-force adjustment. In TappingMode, an increase of the free amplitude oscillation (A0) and a decrease of the set-point amplitude (Asp) lead to elevated forces. Stiffness of the probe is another parameter influencing tip-sample forces in both modes, and, therefore, optimization of imaging conditions includes a choice of the most appropriate probes.


Phase imaging is a most popular technique applied for compositional imaging of polymer multi-component materials. Analysis of the phase images, however, is not necessary straightforward. The same locations or individual components can appear with different contrast depending on the imaging parameters (driving frequency, A0 and Asp, cantilever stiffness). Unfortunately, there is only a general understanding that the phase changes reflect the energy of the oscillating probe dissipated in the sample. The complex nature of the tip-sample interactions, which are taking place during tapping of a non-linear viscoelastic polymer material, complicates analysis of the phase contrast in terms of specific sample properties such as elastic moduli, viscoelasticity, adhesion, etc. A practically valid approach to this problem is the study of model systems with known composition of components. By performing imaging at different parameters, a researcher might find optimal imaging conditions for a particular group of polymer materials. For example, imaging of blends of polyolefines and elastomers provides high-contrast phase images when experiments are performed with ~40 N/m stiff probes and following tapping amplitudes: A0 = 80-100 nm and Asp = 0.4�0.5 A0. In such images, stiffer components are seen as locations with brighter contrast.


Compositional mapping of heterogeneous polymer materials can be based not only on differences of the mechanical properties of individual components, but also on differences in sample local conductivity. In studies of conducting polymer materials, Electric Force Microscopy (EFM) can be successfully applied. When the conducting probe is biased with respect to the conducting sample, its resonant frequency is shifted to lower frequency due to long-range attractive forces. This frequency shift and the related shift of the phase are caused by the gradient of the electric field between the probe and the sample. However, these forces are weak and might be screened by the tip-sample force interactions occurring during tapping. Lift mode, or a two-pass technique is used to separate the effects of the electric and mechanical forces in this mode. For each scan line, the height profile is recorded during the first pass. At the end of the first pass the probe is lifted typically 5-50 nanometers above the surface, and then it is moved along a path without the feedback but reproducing the just-learned height profile. During this second pass, the changes of the probe resonant frequency and the phase, which are caused by the sample conducting locations, are monitored. Dark areas of the phase images, which are recorded in the second pass, correspond to regions of the stronger attractive force (with higher electric field gradients).