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. 

As Agilent was building its own image library around its Atomic Force Microscopes, I had the opportunity to check out a variety of websites that also presented imagery. While the engineer in me is certainly wowed by the cool factor that surrounds these images and their place in nanotechnology research, I am also struck by how artful they really are. This observation was recently reinforced when I received in the mail the quarterly alumni magazine from my undergrad alma mater, the University of Dayton. As I flipped the magazine open, I was immediately struck by an article about UD’s Nanoscale Science and Technology Laboratory (the NEST Lab). While it was a fairly short article about the lab itself, the 2 pages of images generated by the lab was what really catches one’s attention. The article indicated that the university’s art students were working with scientists in the NEST lab to “uncover the hidden worlds of repeating patterns and incongruous forms”. Pretty cool intersection of art and science I think! Check out their image library. My interest now peaked, I began searching the web for things like ‘nanotechnology artwork’ which leads you to cool sites like the Nanotechnology Art Gallery at the Nanotechnology Now web site and many more. Agilent’s image library focuses on images captured by AFM products and while it is a fairly recent addition to our website we think it will continue to grow.

If you have found a particularly cool repository of nanotechnology imagery, whether it is your own or someone else’s, please share them!

Over the past two decades, the use of scanning probe microscopy to directly visualize electrochemical processes in situ at the molecular and atomic levels has increased dramatically. To demonstrate the high resolution and utility of ECSPM techniques for interfacial investigations, we have presented a number of original experiments (please refer to the application notes posted on our website).

In one instance, we conducted an in situ ECSTM experiment to watch the order-disorder transition of 2,2´-bipyridine (22BPY) on Au(111) surface under potential control. Individual bipyridine molecules in the ordered phase and their orientations were resolved, helping to understand the polymerization and ordering process of 22BPY at the molecular level.

2,2´-bipyridine was dissolved in 100 mM NaClO₄ to a final concentration of 1 mM. Deionized water (18.0 MW cm) was used throughout the experiment. A small Teflon cell used had an exposed electrode area of 0.28 cm². Ag/Ag⁺ was the quasi-reference electrode. Apiezon-wax-coated Pt/Ir tips had typical leaking current of 10 pA or less. Before each experiment, the fluid cell and electrodes were cleaned with H₂SO₄/H₂O₂ mixture and thoroughly rinsed with deionized water. After a gold substrate was hydrogen flame annealed, it was immediately transferred to a sample stage and covered with the electrolyte. Typical bias and setpoint current used were 200 mV and 0.2 nA, respectively.

The ordering process of 22BPY molecules on Au(111) at different potentials was then demonstrated. At 0.0 V (versus Ag/Ag⁺), molecules tended to be loosely in contact with the surface, randomly orientated. STM did not resolve either molecular rows or single molecules. At slightly higher potential, molecules started to bind to the surface and became observable.

When the surface potential was changed to 0.15 V, the adsorbate began to form short, parallel rows. At 0.20 V, over half of the molecules on the surface appeared to be ordered. Domains formed by groups of the same orientated molecular rows began to appear. At 0.27 V, the adsorbate showed long-range ordering. Three distinct orientations perfectly fit the underlying atomic lattice of the Au(111) surface. Domains and domain boundaries were visible. The measured chain-chain spacing was around 9 Å. Individual bipyridine molecules closely packed along polymeric chains were clearly resolved with a period of 3.3 ± 0.3 Å. The disorder-to-order transition was reversible and images were stable over several hours.

We then used in situ ECAFM to repeat the experiment of Cu underpotential deposition (UPD) on Au(111) with both molecular and atomic resolution.

The electrolyte was 100 mM H₂SO₄ containing 5 mM CuSO₄. A piece of Cu wire was used as the quasi-reference electrode. The AFM fluid cell had an exposed electrode area of 0.57 cm². Si₃N₄ cantilevers used had a typical spring constant of 0.5 nN/nm. The cell and electrodes were thoroughly cleaned before the experiment. Similar to the STM experiment, a gold substrate was flame annealed right before being covered with the electrolyte.

A cyclic voltammogram of Au(111) in 100 mM H₂SO₄ containing 5 mM CuSO₄ showed very distinct UPD peaks at 0.275 V versus Cu/Cu₂⁺ (DER = 9 mV). AFM images were acquired both below and above the peaks. At high potential (prior to Cu deposition), the atomic lattice of bare Au(111) surface was repeatedly observed. The unreconstructed Au atomic structure on the (111) plane with the familiar threefold symmetry was clearly resolved, showing an atomic spacing of 2.9 ± 0.2 Å.

Further ramping up the potential (to 0.70 V) did not significantly change the atomic image. Ramping down, however, showed a new lattice after passing the UPD peak. The measured lattice constant was 5.0 ± 0.3 Å and the orientation was 30º ± 1º relative to Au(111) lattice. These parameters suggested that the new lattice was (√3 × √3)R30º. When the potential was ramped below 0.060 V, the lattice disappeared and a full monolayer of deposited Cu was formed. When the potential was returned, the lattice reappeared. The (√3 × √3)R30º structure was very stable at a constant potential, indicating a strongly bound layer of molecules.

With continued advances in AFM instrumentation and in situ technologies, ECSPM is being utilized in an increasingly broad range of application areas.

We invite you to share any AFM-related comments, queries, suggestions, and ideas with us, as well as with your fellow researchers, on this blog.

Welcome to the Agilent Nanotechnology WebLog. As the premier measurement company we feel it is important for us to start the dialogue around measurements being made in nanotechnology research. We use the word “measurements” in a broad sense to include the imaging, manipulation and characterization of nanoscale devices and structures with microscopy products. We include the sensing and sourcing of electrical signals to characterize nanoscale devices. And we include the chemical and biological analysis of molecules and compounds at the nanoscale. All measurements are fair game – shape, size, hardness, conductance, capacitance, current flow, voltage, chemical make-up, bioanalysis, fluorescence, etc. It’s your choice what we talk about.

It’s an exciting time to be on the leading edge. In basic research you collaborate on projects. You learn together, you share expertise, and you succeed as a team. Why not take this same approach on the measurement side? That’s the focus of this blog – the measurements (in the broad sense) that you need in your nanotechnology projects. It’s the measurements that lead to breakthroughs in nanotechnology which I hope will make a better life for us and our children.

This blog is about measurements made with any instrument. It’s about learning from each other what works and what doesn’t. It’s about success—finding ways to successfully complete nanotechnology projects, whether that be in basic research or the engineering of products for the market. Other web sites and blogs will provide us with information on breakthroughs in nanotechnology research – this blog is all about the nuts and bolts of the measurements that lead to the breakthrough. It’s the HOW and not the WHAT.

I encourage you to sign up, learn from others, and participate by sharing your experience.

Grant Drenkow
Agilent Nanotechnology Program Manager
Grant_drenkow@agilent.com