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.

In our continuing series on ways to use an AFM - let’s look at force measurements.  These measurements are important in life science, polymer structures, semiconductors, and composite materials.  Force measurements can be made in air or liquid and under controlled conditions like temperature and humidity. 

Force Modulation mode is a fast, very sensitive imaging method that is especially useful to measure and detect variations in a surface’s mechanical properties, including stiffness and elasticity. In this technique, a modulated driving signal at a constant frequency is applied to the AFM cantilever while the AFM tip is in contact with the sample, and the amplitude variation and phase lag during the scan are measured. Force modulation provides the user with simultaneous surface topography measurements, material elasticity or stiffness (the amplitude of the modulated signal), and energy dissipation characteristics of the sample (from the phase of the cantilever response). When an AFM cantilever is modulated with the driving signal, elastic materials will result in relatively larger modulated amplitude compared to stiffer materials because the AFM tip can indent an elastic material.

If you want more information on Agilent AFM’s go to our website at www.agilent.com/find/afm

Those unfamiliar with the flexibility of an AFM (atomic force microscope) don’t realize that one can oscillate the tip up and down.  Contact mode AFM often has a disadvantage for samples that are either weakly bound or soft because the tip can simply move or damage the surface feature and the resulting images are generally not high resolution. The advent of AC mode AFM, which operates in the intermittent contact regime or in the non-contact regime, provides a solution to this problem. The Agilent AFM’s have two oscillating modes - magnetic and acoustic.   

Magnetic AC Mode (MAC)

To achieve MAC Mode imaging, a cantilever coated with a magnetic material is driven into oscillation by an AC magnetic field generated by a solenoid positioned close to the cantilever housing. The result of MAC Mode™ is a gentle, clean cantilever response that has no spurious background signals (“forest of peaks”) like other AC modes can have.  Because the cantilever (and only the cantilever) is driven directly by the magnetic field, the need to shake the cantilever holder at large amplitudes is eliminated. Background resonance is absent, signal to noise is improved and setup becomes straightforward. Better signal-to-noise means that much smaller amplitudes can be used, This decreases damage to the sample and preserves asperities on the probe, contributing to greatly improved resolution. MAC Mode has even greater advantages when the cantilever is vibrated in liquid. 
 

Acoustic AC Mode (AAC)

In Acoustic AC Mode (AAC) the cantilever is excited by high frequency acoustic vibration from a piezoelectric transducer attached to the cantilever holder.  AAC mode can be classified into two categories, intermittent contact mode and non-contact mode, depending on the force regime and the tip-sample separation distance. The interaction between the tip and the sample is predominately vertical, thus negligible lateral forces are encountered. Consequently, AC mode AFM does not suffer from the tip or sample degradation effects that are sometimes observed after many scans in contact mode AFM, and it is a technique for imaging soft samples. In AC mode, tip-sample force interactions cause changes in amplitude, phase and the resonance frequency of the oscillating cantilever. The spatial variation of the change can be presented in height (topography) or interaction (amplitude or phase) images that can be collected simultaneously. The system monitors the resonant frequency or amplitude of the cantilever and keeps it constant by a feedback circuit that moves the scanner up and down. The motion of the scanner at each probe location is used to generate a topographic data set. The amplitude change at each probe location forms the amplitude image. The phase data is the result of the phase lag between the AC drive input and the cantilever oscillation output at each probe location. Consequently, contrast in phase images, which are due to differences in material properties, can provide very useful information. In addition, fine morphological features are easily observed in amplitude and phase images.

If you would like to learn more about the Agilent AFM’s - go to www.agilent.com/find/afm.

Atomic Force Microscopes (AFM) do more than just provide a topographical image of the surface.  So for the next few blogs I’m going to describe some of the “modes” of the AFM.  Let’s start with contact mode.

In contact mode AFM, interatomic van der Waals forces become repulsive as the AFM tip comes in close contact with the sample surface. The force exerted between the tip and the sample in contact mode is on the order of about 0.1-1000nN. Under ambient conditions two other forces besides van der Waals interactions, are also generally present. These forces include the capillary force from a thin layer of water in the atmosphere, and the mechanical force from the cantilever itself. The capillary force is due to the fact that water can wick its way around the tip, causing the AFM tip to stick to the sample surface. The magnitude of the capillary force should vary with the tip-sample distance. The mechanical force resulting from the cantilever is similar to the force of a compressed spring and its magnitude and sign (repulsive or attractive) is dependent on the cantilever deflection and the cantilever’s spring constant. Consequently, in contact mode AFM, the repulsive van der Waals forces arising for the AFM tip to sample interaction must balance the sum of the forces arising from the capillary force plus the mechanical force from the cantilever.

The thin layer of water present on many surfaces in air exerts an attractive capillary force and holds the tip in contact with the surface. Thus, when scanner pulls the tip away from the surface, the cantilever bends strongly towards the surface and the scanner has to retract further so that the tip can snap off of the surface. The cantilever returns to its original unbent status as the scanner moves the tip away from the surface beyond the snap-out point. In liquid, since the large capillary force is isotropic and the total force that the tip exerts on the sample can be reduced to some extent.

In order to improve imaging in air and liquid environments, Agilent offers Magnetic AC mode (MAC Mode). In MAC Mode, a cantilever that is coated with a magnetic material is driven into oscillation by an AC magnetic field that is generated by a solenoid positioned close by the cantilever. The result is a clean cantilever response that has no artifacts or background signals when the cantilever is vibrated in air or in liquid.

For more information on Agilent AFM’s - visit the www.agilent.com/find/afm

Agilent has a long history of electronic measurements but I often get questions about our newest product line - the atomic force microscope or AFM.  So what is an atomic force microscope and how does it work?

AFM stands for Atomic Force Microscopy or Atomic Force Microscope and is often called the “Eye of Nanotechnology”. AFM, also referred to as SPM or Scanning Probe Microscopy, is a high-resolution imaging technique that can resolve features as small as an atomic lattice in the real space. It allows researchers to observe and manipulate molecular and atomic level features.

How AFM works is illustrated in the figure below. An AFM works by bringing a cantilever tip in contact with the surface to be imaged. An ionic repulsive force from the surface applied to the tip bends the cantilever upwards. The amount of bending, measured by a laser spot reflected on to a split photo detector, can be used to calculate the force. By keeping the force constant while scanning the tip across the surface, the vertical movement of the tip follows the surface profile and is recorded as the surface topography by the AFM.

The predecessor of AFM is STM, Scanning Tunneling Microscopy or the Scanning Tunneling Microscope, was invented in 1981 by G. Binnig and H. Rohrer who shared the 1986 Nobel Price in Physics for their invention. An excellent technique, STM is limited to imaging conducting surfaces.

AFM has much broader potential and application because it can be used for imaging any conducting or non-conducting surface. The number of applications for AFM has exploded since it was invented in 1986 and now encompass many fields of nanoscience and nanotechnology. It provides the ability to view and understand events as they occur at the molecular level which will increase our understanding of how systems work and lead to new discoveries in many fields. These include life science, materials science, electrochemistry, polymer science, biophysics, nanotechnology, and biotechnology.

 AFM has a number of advantages over other techniques making it a favorite among leading researchers. It provides easily achievable high-resolution and three-dimensional information in real space with little sample preparation for low-cost. In-situ observations, imaging in fluids, temperature and environmental controls are all available.  The Agilent AFM’s have a number of modes that let one measure hardness, elasticity, friction, and adhesion in addition to the 3D topology.  Over time we plan to add more modes to our AFM’s - stay tuned. 

If you want more complete information on the Agilent atomic force microscopes - go to the Agilent AFM website at www.agilent.com/find/AFM.

Click on the AFM Image Library on the nanotechnology home page (www.agilent.com/find/nano) to see examples of images and applications that are possible with the Agilent atomic force microscopes. 

The Agilent 4294A Impedance Analyzer is a very popular product for MEMS (micro electromechanical systems) measurements.  Although technically not considered nanotechnology, the measurement principles apply to nanoscale devices.  Agilent has an application note for using this product with a probing station to measure MEMS.  I think you will find this application very appropriate for those of you interested in making impedance measurements at the nanoscale. 

Check out Application Note 1369-3 “Accurate Impedance Measurement with a Cascade Microtech Probe System”.  If you want more information on MEMS and NEMS measurements from Agilent - go to this Agilent web page - www.agilent.com/find/mems

In the June issue of Nature Nanotechnology Harvard University and the University of Hawaii at Manoa have found a way to align carbon nanotubes and nanowires in a cm samples.  Their technique involves suspending the nanodevices in a polymer epoxy and blowing bubbles into the solution to align the nanoscale devices.   The process could lead to a way to produce arrays of transistors. 

 The researchers used an Agilent Semiconductor Analyzer to characterize the current and voltage of these aligned nanotubes and nanowires.  We know that researchers like to use these semiconductor analyzers for nanotaechnology research so we’ve added specific carbon nanotube FET set-ups in the library of functions for the Agilent B1500A Semiconductor Analyzer.  We’ve had a number of positive remarks from researchers who find the touch-screen set-up quite easy to use and flexible to implement. 

What do you use for your electrical characterizations of nanotubes and nanowires?  Post your successes so others can learn from experience. 

I just returned from Santa Clara, California and the NSTI Nanotech 2007 Conference.  Agilent was a gold level sponsor this year with a booth that showed our breadth of offering with bioanalyzers, dielectric analyzers, semiconductor analyzers, and our new MAC III mode on the atomic force microscopes.  In case you missed the show, I inserted a picture of the booth.

Dr. Darlene Solomon, Director of Agilent Labs, gave the keynote speech on Thursday morning.  She talked about how measurements enable us to make breakthroughs in technology which in turn helps us develop better products including new measurement instruments–and the cycle repeats.  She gave some great examples of measurement breakthroughs that have increased resolution and precision by 10x, 100x, and even 1000x.  Nanotechnology is a convergence of electronics, chemistry, biology, and materials science operating at a scale that requires higher resolution measurements.  The breakthroughs will be outstanding and the metrology will need to support those breakthroughs.  “If you can’t measure it … you can;t improve it.” 

The breadth of the work being done in nanotechnology is really amazing.  I sat through papers describing new techniques that will help us dramatically reduce the size of semiconductor electronics.  I saw keynote speakers talk about some very exciting developments in the area of energy including some very important ways we should increase electrical efficiency.  I saw some exciting work being done in disease detection and tissue engineering that will improve our lives in the not too distant future.  We are living in exciting times. 

For those that attended, what were your impressions? 

The NSTI Nanotech 2007 Conference opens on May 21st in the Santa Clara Convention Center.  Agilent will be there on the show floor and giving papers in the sessions.  It’s a good opportunity to see the latest nanoscale measurement products.  Drop by and ask those tough questions in person.