A recent bridge collapse in Minneapolis, Minnesota has prompted renewed interest in the materials science of critical infrastructures like bridges.  Bridges in the northern regions of the country have been weakened by prolonged exposure to deicing salt that finds its way through the concrete to the steel reinforcing beams causing extensive corrosion.  Cathodic protection, a technique developed in the USA and UK during the second half of the 20th century, is a possible means to protect these bridges.  Who would have thought that electronics could play a role in materials science?   If you would like to read more about this technique, visit the Agilent application note library and download this application note.  http://cp.literature.agilent.com/litweb/pdf/5989-6459EN.pdf

As I talk with university professors doing nanotechnology research, many of them are utilizing electronic test equipment in their research on various nanotechnology structures.  Semiconductor analyzers capable of low level voltage and current measurements are at the top of the list.  Network analyzers with the ability to add stimulus and measure responses in the GHz arena are also popular.  And others are using high precision multimeters, high speed oscilloscopes, RF signal generators, MHz function generators, and high precision power supplies. 

What instruments are you using in your research?  Over the course of the next few weeks I will be highlighting applications that I’ve seen using various types of electronic, chemical analysis, bioanalysis, and microscopy products for nanotechnology research.  Feel free to add your own applications to this blog. 

AFM is a powerful characterization tool for polymer science, capable of revealing surface structures with unprecedented spatial resolution. It is extremely useful for studying the local surface molecular composition and mechanical properties of a broad range of polymer materials, including block copolymers, bulk polymers, thin-film polymers, polymer composites, and polymer blends. 

In addition to remarkably high spatial resolution, another key advantage of AFM is simultaneous multichannel data acquisition. In acoustic AC mode, tip-sample force interactions cause changes in the amplitude, phase, and resonance frequency of the oscillating cantilever. The spatial variation of the change can be presented in height (topography) or interaction (amplitude or phase) images. A feedback system monitors the oscillating amplitude of the cantilever at each sample location and tries to maintain a set value (set-point) by moving the scanner up or down based on the surface morphology. 

While the vertical motions of the scanner are used to generate a topographic image, the actual oscillation amplitudes and the phase lag between the AC drive input and the cantilever oscillation output can also be collected simultaneously to produce the corresponding amplitude and phase image, respectively. It has been demonstrated by many research groups that phase contrast is very sensitive to differences in material properties, such as variation of mechanical and adhesive properties. 

The visualization of different components of heterogeneous polymer materials via AFM phase imaging has been demonstrated in numerous studies of block copolymers, semicrystalline polymers, and mesomorphic polymers. For instance, compositional mapping with AFM is often used for observations of microphase separation of block copolymers, which occurs at the sub-100 nm scale. In addition to the compositional imaging of multicomponent polymer samples, visualization of amorphous and crystalline components is an important application of phase imaging. Besides amorphous and crystalline forms, many liquid crystalline polymers such as poly(diethylsiloxane) (PDES) usually exist in a partially ordered or mesomorphic form, which can also be characterized by phase imaging. 

Polymer or plastic materials can be divided into two major groups, thermoplastic or thermosetting, based on their response to heat. Therefore, knowledge of polymer behavior at different temperatures is essential for many practical applications. Although quite a few macroscopic techniques, such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermal mechanical analysis (TMA), are usually employed to probe the temperature changes of polymers, direct nanometer-scale visualization of polymers at different temperatures is still highly desired. With the development of both heating and cooling accessories, the use of AFM on polymer materials can be extended from ambient temperature to temperatures where polymer phase transitions occur. High-resolution AFM temperature studies can provide unique microscopic insight into polymer thermal behavior. 

We have documented many examples of the imaging of different polymer samples with the Agilent 5400 atomic force microscope that demonstrate its capabilities for visualizing important polymer nanostructures and monitoring structural changes caused by thermal transitions. The 5400’s outstanding thermal control is a rare feature for economically priced microscopes and the additional advantage of MAC Mode compatibility provides direct drive imaging in oscillatory mode in liquid and air. 

Please refer to the polymer-related application notes posted on our website. Detailed information about the 5400 atomic force microscope, thermal control, and MAC Mode can also be found on our website. 

In my continuing effort to show you the flexibility of an atomic force microscope (AFM) - let’s look at phase imaging.  

Phase Imaging is a powerful, dynamic force technique that can reveal many unique mechanical and chemical properties of a sample at the nanometer scale. In Phase Imaging, an AFM cantilever is oscillated vertically near its mechanical resonance frequency while it is in close proximity to a sample. As the AFM tip comes in very close proximity to the sample surface, the amplitude of the cantilever’s oscillation is reduced. The change in amplitude is measured and is used to track changes in the surface topography and roughness of the sample. Simultaneously, as the AFM tip encounters regions of different composition, a change in phase, relative to the phase of the drive signal, is measured and recorded. This change in phase is very sensitive to variations in material properties, including surface stiffness, elasticity and adhesion. The phase shifts are measured and displayed in a very straightforward manner that facilitates quantitative analysis and interpretation.

Both inorganic and organic materials have been examined with phase imaging. Phase imaging has been found to be particularly useful to map the various components of composite materials, to study variations in composition and contamination in materials, and to measure adhesion, surface hardness and elasticity. It has been applied to thin film studies, the materials sciences, and composite characterization.

Phase Imaging is included with Acoustic AC Mode and MAC Mode. It can be conducted with or without temperature control, in air, in liquid, and even under controlled atmospheres.

Phase Imaging can reveal material properties that can not be observed in surface topography and it can identify properties that might otherwise be obscured by topography. It is a sensitive, quantitative, high lateral resolution AFM method that is often more convenient and gentler than other surface property methods that are based on contact mode operation.

SPM is the acronym of scanning probe microscopy.  An AFM (atomic force microscope) is a subset of SPM. 

STM is the acronym of scanning tunneling microscopy.  STM is a mode of AFM.  So what is it, you ask? 

STM (Scanning Tunneling Microscopy) was invented in 1981 by G. Binnig and H Rohrer who shared the 1986 Nobel Price in Physics for their invention. STM uses a sharp conducting tip and it applies a bias voltage between the tip and the sample. When the tip is brought close to the sample electrons can “tunnel” through the narrow gap either from the sample to the tip or from the tip to the sample, depending on the sign of the bias voltage. This tunneling current changes with tip-to-sample distance, it decays exponentially with the distance, which gives STM its remarkably high precision in positioning the tip (sub-angstrom vertically and atomic resolution laterally). For the electron tunneling to take place, both the sample and the tip must be conductive. Therefore STM cannot be used on insulating materials.

STM can image a sample surface in either constant-current mode or constant-height mode, as shown in the image above. In constant-current mode, in order to keep the tunneling current constant STM uses feedback to adjust the height of the scanner at each measurement point, e.g. when the system senses a tunneling current increase, it adjusts the voltage applied to the piezoelectric scanner so that the scanner lifts the tip and give an increase in the tip-sample distance. The scanner height measured at each location on the sample surface constitutes the topographic image. The constant-current mode is thus generally used to acquire surface height data, its scan speed is limited by the feedback response and thus it takes longer to image an irregular surface at a larger scan size. In constant-height mode, the tip scans at a constant height above the sample and the tunneling current changes due to the topography and the local surface electronic properties of the sample. The current image is a result of measured tunneling current at each location on the sample surface. The constant-height mode can acquire data faster because the system doesn’t have to move the scanner in the vertical direction, so it is most often used for imaging relatively smooth surfaces.

Strictly speaking STM tunneling current is correlated to the surface electronic density of states, i.e., the number of filled or unfilled electron states near the Fermi level, within an energy range determined by the bias voltage. So STM measures constant tunneling probability instead of the physical topography at the surface. STM tunneling spectroscopy, looking at the current-voltage relationship at a constant tip-sample distance or the current-distance relationship at a constant bias voltage, is a useful tool to study the electronic structure and property of a sample surface at the atomic level.