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Experimental Techniques - Scanning Force Microscope

There are a number of ways to analyze a sample. In nanoscience, with samples that are too small to be seen, scanning probe microscopy (SPM) becomes invaluable. SPM is a technique in which one can look at the surface topography of your sample. There are a number of different SPM modes to choose.

First, let us examine how an atomic force microscope (AFM) functions. AFM measurements are the results of interaction forces between a tip and the surface of a sample. These force interactions are measured as a function of position. The cantilever, which is attached to the tip, deflects toward the sample’s surface as the forces begin to act on the tip. In order to control the interaction between the tip and the surface, it is necessary to have both high-resolution positioning of the sample and sensitive detection of the cantilever’s deflection.

The basic design for an AFM system, shown in figure 1, consists of three main components: the system for sample positioning, the force detector, and the feedback control system. Each of these mechanisms will presently be discussed in more detail.

Figure 1: Basic design of an AFM system. [12]

A high-resolution positioning of the sample can be obtained through piezoelectric ceramic devices. Piezoelectric ceramics are capable of easily positioning a sample along the x, y, and z-axes with nanometer resolution. Such devices consist of two metal electrodes with a piece of ceramic placed between them. If a potential difference is placed across the electrodes, the ceramic will expand or contract perpendicular to the applied electric field.

A common piezoelectric device is the piezotube. This is a piece of ceramic, shaped into a hollow cylinder, with one electrode on the inner surface and four electrodes on the outer surface. By biasing the outer electrodes, the tube will move in the x and y directions. Biasing the inner electrode will cause motion of the tube in the z direction. There is a control system that supplies the high voltage required for motion of the sample to each position of the scan.
In a typical AFM scan, the forces are explored in numerous positions within the scan area of the sample surface by sweeping along the x-axis while moving incrementally along the y-axis. Measurements of the forces are made at increasing positions along the x-axis during a sweep. After each sweep is completed, the y position will change by one step. At this new y position, the forces are again measured throughout the tip’s movement during the x-axis sweep. This process repeats until the tip has traveled the extent of the y-axis for the scanning region. It is the size of the scanning region, coupled with the size of these x and y steps that determine the resolution of the scan.

As stated previously, forces interact with the tip as it travels across the sample surface, causing it to deflect. Because the cantilever acts as simple harmonic oscillator, the amount of force acting on the tip can be determined by measuring the amount of the tip’s deflection. A laser beam is bounced off the tip onto a segmented photodiode, allowing the deflection to be detected by the changes in the laser light absorbed by the different quadrants of the photodiode. The photodiode then supplies an output voltage proportional to the deflection of the cantilever.

In a typical AFM scan, the cantilever deflection is held constant for the x and y positions. A feedback system monitors the output of the deflection sensor in order to control its position by adjusting the voltage applied to the inner electrode of the piezotube, thus controlling the motion of the sample in the z direction. This voltage is adjusted until the deflection of the tip returns to the set-point value for the feedback system. The voltage applied to the piezoelectric element is recorded at every position of the scan to supply information in a Cartesian coordinate system.
By recording this applied voltage that was required to maintain a constant deflection of the cantilever, the topographic height at a particular point on a sample can be determined. The applied voltage can be converted into an actual height in nanometers by applying the tube’s z calibration (given in nm/V). The z calibration is the known expansion of the piezotube for a given bias voltage. Accordingly, it is possible to determine the actual x and y distances in nanometers for your scan area by applying their respective calibration values.

There are two main methods of measuring a sample’s surface topography as a function of position: contact mode and non-contact mode. For contact mode, the tip is in actual contact with the sample surface. Any cantilever deflection is governed by the repulsive force interactions between the electron densities of the tip and the sample If the sample consists of weakly bound particles or soft objects, scanning in contact mode may cause damage.

A solution for scanning such objects is to scan in non-contact mode. In this instance, the tip is not in direct contact with the sample surface, and thus will not damage such items. In a non-contact scan, the tip is oscillated near its resonance frequency approximately 5-15 nm above the sample. By driving the cantilever at this frequency, any damping forces from the sample surface will cause the tip’s resonance frequency to shift. This interaction will also cause the amplitude of oscillations to change at the original frequency. This damping of the original resonance frequency can be used in order to maintain a static tip-sample separation distance. To accomplish this, the amplitude of the original resonance frequency force can be measured through a phase sensitive detector (PSD). The feedback system utilizes the output of the PSD to maintain a constant amplitude for the original resonance frequency. Recording any adjustments made by the z piezoelectric element to maintain that constant amplitude generates a topographic profile of the sample surface.

There is a consequence of using non-contact mode. There is a significant loss of lateral resolution due to the fact that the tip is not in physical contact with the surface. Positioning the tip above the surface exposes it long-range forces generated from a much larger area of the sample surface. When in contact mode, the tip-surface interaction dominates the long-range force, but in non-contact mode the tip-surface interaction becomes weak so that the long-range electrostatic force becomes dominant. Though the lateral measurements of the force interactions are complicated, there have been recent advances in AFM technology that have minimized this effect.

Of course, the AFM is an extremely flexible instrument. The two modes mentioned above are not the only possibilities for this instrument. It is also possible to use the AFM control system to explore a wide range of surface forces such as friction, adhesion and electrostatic with different scanning modes.

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