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A Classification of SPM Image Techniques

In general, we can categorize Scanning Probe Microscopy (SPM) techniques into three groups: Imaging Techniques, Non-imaging Techniques, and Hybrid techniques which combine the latter two.

Imaging Techniques
Imaging Techniques can be subdivided into two groups of “Modes” we call Primary Imaging Modes, and Derivative Imaging Modes. This classification is arbitrary, some may argue, but it has shown to serve a useful pedagogic purpose by helping us manage our knowledge of the increasingly long and complex list of SPM techniques that continue to be invented.

At this time, we can identify at least five types of Primary Imaging Modes widely in use:

  • Scanning Tunneling Microscopy (STM)
  • Quasi-static or Contact Mode Atomic Force Microscopy (AFM)
  • Dynamic Vertical Mode(s) of Atomic Force Microscopy
  • Dynamic Lateral Mode Atomic Force Microscopy
  • Scanning Near-field Optical Microscopy (NSOM)

Nano-manipulation is sometimes performed in the plane of the sample surface (in-plane) and sometimes out of this plane (out-of-plane nano-manipulation). An example of out-of-plane nano-manipulation is attaching an AFM tip to the end of a macromolecule on the sample surface, and pulling the molecule so that its secondary or tertiary structure unfolds. This is now an extremely active area of research, with applications extending to areas as diverse as drug discovery and composite materials design.

The main features that distinguish these .ve modes each from each are the nature of the probe tip-sample interaction (electrical, mechanical, optical, or a combination of these), the time-scales involved in the interaction, and the proximity of the sample to the probe tip. The time scales are determined in part by the presence or absence of probe tip oscillations, and these oscillations also affect tip-sample proximity or contact.

For example, STM relies primarily on the electrical interaction between the probe tip and the sample, AFM on the mechanical. Contact Mode AFM differs from Dynamic Vertical Modes of AFM in the duration of probe tip-sample interaction. Dynamic Lateral Mode is also different from Dynamic Vertical Modes in the duration of the interaction and in the proximity of the probe tip and the sample. NSOM relies primarily on a combination of optical and mechanical interactions between the probe tip and the sample.

Each of these Primary Imaging Modes enables a host of Derivative Imaging Modes. For example, Dynamic Vertical Modes and Dynamic Lateral Mode have a derivative mode called Phase Imaging which has proven very useful in material research: viscoelasticity, adhesion, electrical, and magnetic properties. Contact Mode AFM has a derivative mode called Lateral Force Microscopy (LFM) which has significantly advanced our understanding of friction and tribology on the nanometer scale.

Non-Imaging Techniques
In addition to imaging techniques (and their classification into primary and derivative modes), we can identify a very broad, second class of techniques, which is related to the imaging techniques in the sense that they rely on the same tip-sample interactions as do the Imaging Techniques, but which extend the utility and applications of the SPM beyone imaging.

Collectively, these can be called Non-Imaging Techniques. Two closely related examples of non-imaging techniques are nano-indenting and nano-scratching with applications in mechanical and wear testing of materials. Here, the AFM tip is used to image the sample surface, then deliverately indent it or scratch it, and then image the altered area to visualize the change. For indenting and scratching hard surfaces, the tip is often made of a very hard material such as diamond-like-carbon-coated silicon, or even a single micro-crystal of diamond.

Nano-indenting and nano-scratching are examples of a broader class of non-imaging techniques that are generally referred to as nano-manipulation.

In some nano-manipulation techniques, we can modify the sample surface with more control and finesse than in nanoindenting and nano-scratching. For example, we can use the AFM or the STM tip to rearrange nanometer-scale objects physisorbed on that surface. Essentially, the tip serves as a nano-scale finger to interact with the sample.

Nano-manipulation is sometimes performed in the plane of the sample surface (in-plane) and sometimes out of this plane (out-of-plane nano-manipulation). An example of out-of-plane nanomanipulation is attaching an AFM tip to the end of a macromolecule on the sample surface, and pulling the molecule so that its secondary or tertiary structure unfolds. This is now an extremely active area of research, with applications extending to areas as diverse as drug discovery and composite materials design.

In Non-Imaging techniques, we can group together several that have much in common, and refer to them as Spectroscopy Modes, or Scanning Probe Spectroscopy. Spectroscopy modes are closely related to their respective modes in the Imaging Techniques as we will discuss elsewhere in this tutorial. In Spectroscopy Modes, raster scanning in an imaging mode is disabled to allow the SPM to record the interaction between the sample and the probe tip at a given point in the sample plane (a given X,Y coordinate), while we change one or more parameters, by either stepping them in discrete quanta, or by ramping them at a rate we control.

Scanning Probe Spectroscopy was recognized early on as a useful technique, but in light of intense interest in protein interactions and research into synthetic and naturally-occurring macromolecules, it has become widely used and highly in demand since the late 1990’s.

Hybrid Techniques
An example of a Hybrid Technique is Force Volume Mode, which combines Force Spectroscopy and Imaging, either in Contact Mode AFM, or in Intermittent Contact Mode AFM.

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