ATOMIC FORCE MICROSCOPY

Transcription

1 ATOMIC FORCE MICROSCOPY Introduction The atomic force microscope, or AFM, is a member of the family of instruments known as scanning probe microscopes. The AFM operates under a completely different principle from optical microscopes and other instruments including the scanning electron microscope, and provides a method of visualization for objects of nanoscale dimensions with minimal sample preparation. The AFM can be used for both conductive and non-conductive materials, and can perform imaging at atmospheric pressure, vacuum conditions, or even in liquids. In this narrative, the general operation of the AFM is discussed as well as its application in a number of areas of science and technology. Theory of Operation The AFM is, as earlier mentioned, a scanning probe microscope. In operation, a small, often nanoscale dimension probe is used to either hover over a surface or contact the surface to be imaged in either a continuous or intermittent manner. Figure 1 below shows a general representation of a generalized AFM probe, which is often referred to as a cantilever. Figure 1. AFM Block Diagram ( In figure 1, the cantilever appears as a lever with a point at the end. In fact, the cantilever physical properties are, as will later be seen, essential to the operation of the AFM and to the mode in which it is operated. A reflective surface on the backside of the tip of the cantilever provides a point of reflection between a focused laser beam and a photodiode array, usually made up of four or more devices. The cantilever and tip are mechanically scanned across the

2 surface at a constant speed in a sequential, line by line manner with regular intervals between lines. The scanning process is typically performed by a piezoelectric drive system. A common maximum scanning area for an AFM is in the range of µm in the x- and y-directions. Deflection of the cantilever by surface interactions is dependent upon the tip type, geometry, and mode of operation used. Features such as high spots on the surface, electrical/magnetic fields, adhesion, or other atomic-scale forces are conveyed to the system by the change in light pattern on the photodiode array. In most cases, feedback from this array is used with a servo system to adjust the cantilever/tip position to maintain a constant force, constant contact, or other defined relationship between the tip and the surface. Common values for the range of motion in the z- axis are µm, providing allowance for slight variations in sample height due to features. Differences between AFM and Optical Microscopy From the description given above, it can be seen that no optical process, save for the reflection taking place on the back of the cantilever/tip is taking place in scanning the sample. As a result, limitations that exist on optical microscopes, such as low numerical apertures of lenses, object resolution limitations imposed by the wavelength of light required by the human eye for perception, and indices of refraction are eliminated. As a result, an AFM does not rely on any light source other than the laser beam that is focused on the rear face of the cantilever. Ambient lighting conditions may be ignored as long as the sample is not affected by the light (e.g. heating, photoelectric, or other effects). At the same time, it must be noted that no color properties of the object being imaged are conveyed since there is no absorption or reflection of light of any wavelength occurring in the imaging process. As a result, AFM images are often portrayed utilizing a degree of coloration or shade based on the height or depth of a feature found for human perception. Figure 2 below shows an AFM image of a 10 x 10 µm grid of squares on a silicon wafer. The bright spots correspond to the elevated features on the wafer, and the dark lines in-between represent the spaces in between. Figure 2. AFM image of a 10 x 10 µm grid of squares on a silicon wafer.

3 The actual dimensions and heights of the elevated portions are recorded during the imaging process. In the sample above, the elevated portions are approximately 200nM in height above the plane. As noted, the AFM utilizes a line by line scanning motion. This is not unlike other scanning devices such as the scanning electron microscope or imaging devices such as a cathode ray image scanner. However, the AFM is moving a cantilever/tip assembly as compared to an electron beam, and physical and mechanical limitations exist in the rate at which this scanning process takes place. It is not uncommon for an AFM image to require several minutes to complete as opposed to the nearly real-time scanning action of other image scanners. This must be factored into the imaging process. At the same time, since an AFM commonly operates with its cantilever tip in either continuous or intermittent contact with the surface being imaged, properties that may not be disclosed by optical or SEM analysis may be identified during AFM imaging. These properties can include surface roughness, adhesion, electrostatic, magnetic, and weak force interactions between the tip and surface. In later sections, appropriate imaging methods and cantilever/tip selections appropriate to these effects are discussed. Common Modes of Operation The AFM can be operated in different modes to identify specific properties of the object being imaged. The two commonest modes of operation are Contact Mode Dynamic Force or Intermittent Contact Mode Each of these modes will be discussed separately as well as the rationale for their use. Inherent in each mode are system requirements and considerations that need to be factored in to get an acceptable imaging result. Contact Mode: In contact mode, the cantilever/tip is lowered to the surface of the sample to be imaged and it remains in contact with the sample throughout the entire imaging process. Contact mode imaging provides basic surface (topography) information through use of a cantilever that typically is made of lower spring constant material. Since the cantilever is following along the surface, changes in height of surface features will deflect the tip. The feedback system found in the AFM can be used to adjust the height of the tip to follow the contours and maintain a constant tip pressure on the surface. The combination of the low spring constant and feedback serve to preserve, to a degree, the integrity of the tip during imaging. At the same time, an abrupt change in surface feature height may be beyond the capability of the system to handle, and the tip may be damaged or destroyed. Although it is common to see imaging areas (horizontal or vertical) of µm in each axis, the z-axis or height limitation of many AFMs is only in the µm range. This requires that the sample be fairly flat and mounted on a uniformly flat surface. It is also possible to disable the servo feedback systems in some AFMs during contact mode operation. If this is done, high spots on the surface of the sample may be lost or the cantilever/tip

4 may be damaged. At the same time, with or without feedback, the surface may be patterned by the tip if the tip material is harder than the surface of the sample being imaged. AFM lithography is performed by this action, wherein the AFM tip scratches through a thin-film photo-resist layer to create a mask hole in the film for subsequent etching or deposition. Contact mode has the advantage that because of the continuous profiling action, this mode can actually resolve nanoscale features with the appropriate tip. However, there are additional considerations in contact mode that include tip to sample interactions. Weak forces such as Van der Walls and other atomic scale forces can influence the imaging. Mechanical interactions such as abrasion and wear are also factors. If the tip of the AFM cantilever is harder than the surface that it is imaging, excessive force from the tip may scratch or damage the surface of the sample. The damage can occur, either normal to the plane of the scan (scratches following the scan lines with the appearance of a snowplow edge) or laterally due to adhesive properties of the sample. Conversely, a very hard surface material may round or abrade the tip. The consequence of the former situation is a damaged sample, and the consequence of the latter is an imperfect image of the surface. It is not uncommon to see AFM conical or elliptical pattern tips with an end diameter of 10nM. With such a narrow tip, surface features of these dimensions will allow the tip to enter, and these features will be displayed to the user. A damaged conical probe (tip worn, etc.) will not penetrate into smaller feature sizes, and the image provided will either appear flat to the user or, in the limit case when the probe tip is flattened, a smeared image will result. A useful computer simulation of tip-sample interactions can be found in useful links ref [1]. Contact mode can be useful for a variety of measurements associated with material properties. If a sample has adhesive properties, the scanning process will cause the tip to be deflected in the x- or y- (or both) directions. This deflection can be sensed by an AFM with so-called force modulation capabilities, and the deflection used to define these properties. Similarly, force modulation in the z-axis that is due to attraction of the tip to the surface can be measured or quantified by the feedback system, and the result graphed for analysis. In actual practice, operation in contact mode requires setting of the servo feedback parameters in such a way that both provides maximum cantilever/tip life without damage to either the tip or the sample and, at the same time, settings that will disclose the desired information about the sample. In AFM systems, a PID (Proportional Integral Differential) system is used to control the cantilever system. When setting up the AFM, it would be desirable for an error in the loop due to a change in height to be immediately corrected. A feature such as a step change in height would benefit from a high setting of proportional gain. Higher values of proportional gain can correct the error signal that results from the tip movement. However, too much proportional gain in the system may make it unstable, and the cantilever may oscillate when the step occurs. If the proportional gain alone is increased in increments until oscillation occurs and then dropped back, it can be a starting point. The integral gain, as its name implies, is proportional to the integral of the error signal. Increasing the integral gain can decrease this error signal over time and this can be adjusted to reduce the system error that may occur from an overshoot due to proportional gain. The so-called differential gain is proportional to the derivative of the error signal. Increasing the differential gain slows the rate at which the error signal changes, but can, in some cases, increase amplification of noise present. Since the cantilever and tip are key elements of this system, it is advantageous to have a reference sample to work from to roughly establish the gain parameters prior to imaging an

5 unknown sample. This allows the user to observe the system response and identify if the features shown are actual surface features or artifacts from the feedback system being incorrectly set. Since it is difficult to know what features might be present on an unknown sample, some manufacturers specify default parameters or an easy mode of operation to provide a starting point and the user can modify these within a range to determine whether the imaging is representative of the sample or not. In addition to the PID feedback loop settings, another key parameter for contact mode imaging is the so-called set point value. In this context, the set point is the force parameter stored in the feedback system that is to be applied in scanning, and is often given as a percentage of the maximum value of the applied force. This force setting is cantilever-dependent and is part of the specification set for the cantilever. In some AFM application packages, data tables that include spring constant, tip data, applied force, and other parameters for commonly used cantilevers are provided, and the user selects these from the table. In the case of an undocumented cantilever, the user inputs the cantilever data into the software and proceeds from there. In either case, a 100% set point refers to the cantilever/tip operating at 100% of its specified force at all times. When dimensions are within range of the system limits, the feedback PID loop will attempt to maintain whatever percentage of set point is entered. It is in the user s interest to use as low of a set point as practicable to obtain a usable image in most cases. This is due to the fact that higher forces will only cause the tip to become worn or damaged earlier. Higher force contact mode imaging can also cause damage to the surface of the object being scanned. Some instrument manuals recommend a starting point of 50% for this parameter [2]. Increasing the set point and re-imaging with a higher percentage can often disclose if this starting point is sufficiently high. Scan Rate, Scan size, and number of samples/line Many AFM systems are capable of imaging a sample of significant size so long as it is securely mounted. The scanning range in the vertical or horizontal direction is usually limited to µm for general purpose AFM systems. When scanning an image of unknown features for the first time, a full-size scan may be acceptable, followed by a zoom-in view of a feature of interest. It is important to recognize, however, that the number of samples or points per line refers to the number of data points that are taken during the scan. If a low number of points are taken for a given width (say 16 points per line) and the scan width is 16 µm, then the smallest feature that will appear in the image is 1 µm. If the sample has smaller features, they will not all be shown in the scan. Increasing the data points to 64 improves the imaging, putting a pixel on the screen for every 0.25 µm or 250 nm. If the features sought are of nanoscale dimensions, an increase to 256 points/line will increase the resolution to 62.5nm, and so forth. When changing the area to be scanned, examining the setting for the number of samples/line is a good idea. The scan rate can be speeded up to get a faster image. As earlier mentioned, AFM imaging is not a high speed activity due to the mechanical limitations of the cantilever system. In general, slower scan speeds may result in an improved image. This can also be verified during the scan. In practice in many instruments, the cantilever/tip is brought close to the surface by manual adjustment. For a final approach to the surface in many systems, the servo system takes over to slowly put the tip in contact with the surface. The force applied is in this way controlled.

6 Interactions between the surface and cantilever/tip occur due to surface properties. These modes are summarized below. It is noted that in order to utilize these modes, AFM instruments must have the capability to detect these interactions and that not all AFMs may have these capabilities. Force Modulation Microscopy Driving the cantilever with a defined oscillatory excitation is commonly used in intermittent contact mode to avoid constant contact with the surface. It is, however, also possible to drive the cantilever with such excitation while in continuous contact with the sample surface. Contact mode AFM probes normally have lower spring constants and as a result, their resonant frequencies are lower than those of intermittent contact mode probes, but suitable cantilevers are available that will accept drive signals in the 10 KHz range. The change in amplitude of the driving signal that can be measured during the surface scanning can be used to characterize the stiffness or compliance of different areas of a sample surface, and hence describe the uniformity these properties of the sample. Figure 3. Amplitude variation of cantilever deflection according to mechanical properties of the sample as it moves from hard to soft Lateral Force Microscopy During the scanning process, the tip is dragged across the surface of the sample. The resulting lateral drag that occurs due to adhesive or sticky properties of the sample surface pushes the probe back. Measurement of this lateral force can provide information on the adhesive properties of the surface. Lateral surface interactions with the tip can also provide secondary evidence of step features that may not be apparent from a topography scan. As shown below, the lateral force signal shows the edge of the step feature.

7 Figure 4. Lateral force signal showing the edge of the step feature. (Veeco Caliber Operating Manual, C 2008, Veeco Instruments, used with permission) Further Applications of Contact Mode In addition to the imaging and parametric measurement modes discussed above, contact mode can also be used for object manipulation and surface modification purposes. Nanoscale objects such as carbon nanotubes can be manipulated by the cantilever tip by bringing the tip down in the proximity of the object and pushing it into place. Considerations of tip material, possible adhesion between the object and tip, and other factors also need be made if this is contemplated. Surface structures can also be modified by contact mode. As was indicated earlier, damage can occur to a sample if the tip of the AFM is harder than the surface and/or if too high of a set point value is used. At the same time, the AFM tip can act as a precision stylus for writing on the surface, where it is intended that the probe modify the surface. So-called dip-pen lithography uses a liquid that is applied to the tip prior to contact with the surface to apply surface chemicals to the surface. Conductivity of thin films or other samples can also be measured in some AFM systems if a conductive tip/cantilever is used. In this mode of operation, a DC bias is applied to the cantilever/tip with respect to the base upon which the AFM head is placed. The sample must be directly connected to the AFM base (typically via conductive epoxy, pigtail wire, or other means) and the current flow is used to calculate the resistance (or conductance) at a point.

8 Tapping, Dynamic force or Intermittent Contact Mode Imaging The fact that a sample may be damaged in contact mode imaging suggests that an alternative method that may avoid this situation is desirable. In dynamic force (tapping) or intermittent contact mode imaging (tapping is a VEECO trademark), the cantilever is driven by a piezoelectric resonator at a frequency often between KHz. Intermittent contact mode cantilever construction differs from that used in contact mode, typically utilizing higher spring constant material that are stiffer. This results in a higher resonant frequency for the cantilever/tip and the overall system. The AFM setup process typically performs a resonance sweep on the probe and drives it at or near its resonant frequency to minimize the driving force required. In intermittent contact mode operation, the tip is generally brought near to the surface of the sample by manual adjustment as is the case for the contact mode. The servo system brings the cantilever into the proximity of the surface gradually by the approach process that is under computer control. This avoids damage to the tip that might otherwise occur. This process differs from the contact mode approach in that with the oscillating tip, continuous contact is not possible, and a defined force is not applied. Instead, the system monitors the amplitude of the vibration of the cantilever which is a set point parameter. This may be a fixed voltage/amplitude in some instruments or a user-specified level in others, but the general set point value is expressed as a percentage as opposed to the force specified in contact mode. The set point value can be explained in this way. When the cantilever/tip is not in contact with the sample, the amplitude of vibration (default or user-specified) is 100%. Upon contacting the surface of the sample, this amplitude begins to decrease from 100%, and this amplitude decrease provides the system feedback that controls both the approach and the scanning operation during intermittent contact mode. Specifying a set point of, say 50%, programs the servo system to attempt to maintain oscillation levels that do not drop to less than 50% of the free-space value. As such, a higher set point value puts less effective force on the sample. In some cases where the sample integrity may be compromised from higher force, it is advisable to begin at a high set point level of 80% and capture the image. Should the detail not be acceptable, decreasing the set point level will cause more force to be applied. As was the case for contact mode, gain parameters for the PID loop can also be optimized to improve the image, and scan speeds and points per scan line adjusted to provide a more detailed image of the sample. Additional Information Available from Intermittent Contact Mode Since the tip is being driven by an AC signal that is at or near resonance, both the amplitude AND the phase may be changing as a result of tip interactions with the sample surface. Amplitude changes may be due to friction or surface height changes. Variation of the set point is helpful in determining which of these factors is at play. Phase differences between the driving signal and feedback signal to the photodiode array can be due to a dampening effect from the surface due to hardness or stickiness of the surface. Display of the phase mode image as opposed to the topography mode image can show radically different feature details as shown.

9 Phase Imaging Mode Conventional Intermittent Contact Mode Figure 5. PS/PMMA Film Imaging Results Using Phase Imaging as Compared to Intermittent Contact Mode Spectroscopy Analysis (single-point and multi-point or line) This analysis mode is commonly used for weak interactive forces such as Van der Walls, ionic, covalent, or metallic bonds, or others. The attractive or repulsive forces tend to affect the approach of the AFM cantilever. So-called snap-in of the probe occurs from the attractive force. This is a single point measurement mode if only one point on the sample is checked. If a scan is done over several points (e.g. across a line of points), a more general profile is defined. AFM Probe Snap-In Detection of exceptionally small forces via this method is possible. The measurement actually occurs during the approach and not actually during the imaging. Although it is possible to

10 approach and record this information, sample and environmental properties need to be carefully considered when using this mode. As an example, water vapor on the surface can cause a significant variation in the results as can non-uniformity of sample height if the single-point test is conducted over different areas of the sample. As a further example, consider the interaction such as that shown above using spectroscopy mode. In a conventional scan, the z-axis servo control would likely compensate for a weak attractive or repulsive force owing to the sample. Scanning this area with the servo controller active and then disabling the controller and re-scanning can map the deflection. Applied voltages or other stimuli may be provided to the AFM tip for other spectroscopy measurements. Magnetic Force Modulation (MFM) Imaging In this mode, interaction between the tip and surface due to magnetic fields that may be present on the sample occurs. For obvious reasons, the tip must be made of a material that will be affected by a magnetic field. The interaction between the sample and tip actually occurs during the approach and during the scanning probe session. One method used for MFM involves an approach followed by an intermittent contact mode scan of the surface to map the topography of the sample. Following this initial scan, the AFM tip is retracted to a liftoff or non-contact position and the sample is re-scanned. In this way, the magnetic interaction can be separated from surface interactions due to topography. The magnetic interaction between tip and sample can alter the resonance of the cantilever/tip. This effect will both reduce amplitude of the oscillation and phase of the drive signal. The net result can be observed either as a phase image or an amplitude image. In some cases, the phase image may be preferred. Electrostatic Force Modulation (EFM) Imaging EFM imaging relies on electrostatic forces between the tip/cantilever and the sample surface. Given the weak nature of these forces and interactions, care must be taken to isolate the effects of external electrostatic forces or other sources of cantilever deflection. AFM Sample Preparation and Imaging Considerations In some non-optical imaging systems such as Scanning Electron Microscopy, it is necessary to prepare non-conductive samples for imaging by coating them with a conductive material. Sputtering a thin film of gold on such samples is a commonly employed practice. Unfortunately, this irreversibly alters the sample properties and is clearly unacceptable for live specimens. Since the AFM process does not require a conductive sample for imaging, this type of preparation is unnecessary. In fact, some AFM systems are capable of imaging in liquid media, allowing for a variety of sample probing operations that are in situ. However, despite this flexibility, a number of physical considerations must still be taken into account for acceptable AFM imaging. As indicated in the earlier paragraphs, in most common imaging modes, the AFM tip must come into contact with the sample surface, either intermittently or on a constant basis. This requires that the sample remain fixed during the imaging process. This would even be true for MFM and EFM imaging where contact with the sample is not taking place, as the fields encountered would need to remain fixed in place for reliable imaging. Optical microscopy slide preparation

11 techniques to affix the AFM sample to a glass slide or other flat carrier may be used as long as they do not interfere with the scanner. Typical AFM tables can accommodate irregular objects in their base or mounting systems with minimal fixturing. Magnetic pucks in the AFM stage used can be used in conjunction with steel discs to hold samples in place. The sample is affixed to the disk using conventional mounting adhesives. Some AFM tables include micrometer-type moving platforms to position the sample as needed once it is mounted. As earlier noted, in cases where resistivity or other conductive-mode measurements are being made, conductive adhesive of known properties would be required or a wire pigtail would be soldered to the steel disk and grounded to the base. In very thin film samples or nanoscale features as might be present in DNA or other organic samples, it is necessary for the substrate upon which the sample is coated to be exceptionally planar and flat as the sample will, to a degree, follow the substrate contour. This could result in erroneous imaging results. In such cases, a metal disk or even a polished glass microscsope slide may not serve as appropriate substrates. Cleaved mica sheets of defined purity may be used for such samples. As earlier indicated, the sample environment may affect analysis of nanoscale weak forces. Sample temperature, humidity, cleaning methods, and even ambient light levels may need to be considered in such cases. Finally, it is noted that given the operational method and dimensional tolerances of contact, intermittent contact or even non-contact mode, it is essential to isolate the AFM system from external vibration sources. Motion of tables that AFM systems are mounted on, building systems such as heating and air conditioning, and other sources can contribute vibrations that can be seen in scanning processes. The effect may appear in the scan similar to an out of focus photograph or regular vibrational artifact. In some instruments, granite or other composite base materials are used to eliminate vibration or motion. In others, static or dynamic anti-vibration tables using air pressure, balanced masses, or other techniques can isolate the AFM from these effects. Conclusion The AFM provides a method to visualize nanoscale features as well as a tool that can aid in the determination of physical properties of the sample. In this way, AFM differs from many visualization methods. By use of different scanning methods, the AFM can image objects without damaging or physically affecting them. Determination of quantitative vs. qualitative data from these interactions requires identification of the sources of interaction taking place between the sample and the cantilever and an understanding of the physical properties of the visualization system.

12 GLOSSARY OF ATOMIC FORCE MICROSCOPY Atomic Force Microscopy is a high-resolution imaging technique that can resolve features as small as an atomic lattice in real space. It allows researchers to observe and manipulate molecular- and atomic-level features. Application areas include life science, materials science, electrochemistry, polymer science, biophysics, nanotechnology, and biotechnology. Also referred to as scanning probe microscopy (SPM). Cantilever The AFM tip is held at the end of a thin, flexible beam, or "cantilever". This cantilever is made just as the tip was, but its shape is usually triangular ("V" shaped) or long and rectangular (an "I" beam). These are roughly 100 µm long (which is 0.1 millimeters, about the width of a hair) and only a few microns thick. This makes them very flexible but strong enough to securely hold the tips on their end. AFM holder chip AFM cantilever AFM tip Contact mode AFM In this mode, also known as quasi-static 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 is on the order of about nn. Under ambient conditions, two other forces besides van der Waals interactions are also generally present: the capillary force from a thin layer of water on the sample (condensed from the water in the air), as well as the mechanical force from the AFM cantilever itself. Force Modulation AFM This is a contact mode AFM operating mode wherein the feedback from the servo system driving the AFM tip/cantilever is monitored to detect interactions of the AFM tip with the sample. This mode can be used to determine during the physical properties (friction, adhesion, etc.)

13 Nanolithography This is a process where AFM is used to create patterned structures at the nanoscale. Techniques include nanografting, nano-imprinting, and nano-printing. Non-contact mode AFM Non-contact mode (of distances greater than 10Å between the tip and the sample surface), Van der Waals, electrostatic, magnetic or capillary forces produce images of topography, whereas in the contact mode, ionic repulsion forces take the leading role. Because its operation does not require a current between the sample surface and the tip, AFM can move into potential regions inaccessible to the Scanning Tunnelling Microscope (STM) or image fragile samples which would be damaged irreparably by the STM tunnelling current. Insulators, organic materials, biological macromolecules, polymers, ceramics and glasses are some of the many materials which can be imaged in different environments, such as liquids, vacuum, and low temperatures. Phase Imaging AFM A mode of AFM operation using intermittent contact with the sample wherein the change in phase of the AFM tip driving potential is used to disclose sample material properties. Phase imaging is typically used in thin film samples such as PS/PMMA to identify different regions of each entity on the surface and provides imaging that can provide a more detailed picture of the material than topographic imaging alone. PID System The PID abbreviation letters stand for Proportional Integral Differential and refer to the servo feedback system used in AFM to drive the cantilever. With a PID system, errors resulting from interactions in the drive system, components, and sample are reduced to provide a more representative image of the sample being scanned. See also Scanner and Scan Rate. Probe tip Located at the free end of the AFM cantilever, the probe tip interacts with the sample surface. Different tip sizes, shapes, and materials continue to be introduced to the market. Probe tip-sample interaction The interaction between an AFM probe tip and a sample can vary in several fundamental ways, including the type of 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. Scan rate Along with setpoint and servo gain, one of the three most important parameters to

14 change when optimizing the quality of an SPM image in a feedback mode. Faster scan rates require more aggressive feedback, which means higher feedback gain. Note that scan speed is also important. At different scan sizes, the tip velocity relative to the surface differs for the same scan rate. Scanner A critical component in an atomic force microscope that houses the scanning elements as well as associated electronics. Common scanner types include open-loop and closed-loop AFM scanners, as well as STM scanners. There are two ways of moving the sample under the AFM tip in one case you can move the sample and keep the tip in place, or, alternatively, you could move the tip over the stationary sample. In the first case, the scanner is made of piezoelectric crystals (that neat stuff which replaced flint in cigarette lighters and makes the spark used to start a barbeque). This crystal creates a voltage if pressure is applied, or in reverse, can create a pressure by expanding or contracting if a voltage is applied. Using the contraction and expansion of the crystal, the configuration in a scanner allows for the controlled movement on the order of a fraction of a nanometer. Such precise manipulation of the sample could not be possible using traditional mechanical methods with gears and pistons. Scanning tunneling microscope (STM) This is the predecessor of the atomic force microscope. This scientific instrument was invented in 1981 by G. Binnig and H. Rohrer, who subsequently shared the 1986 Nobel Prize in Physics. This technique uses a sharp conducting tip and 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, decaying exponentially as distance increases, thus affording remarkably high precision in positioning the tip (subangstrom vertically and atomic resolution laterally). For the electron tunneling to take place, both the sample and the tip must be conductive. Tapping mode This is a trademarked name (VEECO/Bruker) for intermittent contact mode imaging. This mode of imaging overcomes problems associated with friction, adhesion, electrostatic forces, and other difficulties that an plague conventional AFM scanning methods by alternately placing the tip in contact with the surface to provide high resolution and then lifting the tip off the surface to avoid dragging the tip across the surface. Tapping mode imaging is implemented in ambient air by oscillating the cantilever assembly at or near the cantilever's resonant frequency using a piezoelectric crystal. The piezo-motion causes the cantilever to oscillate with a high amplitude (typically greater than 20nm) when the tip is not in contact with the surface. As the oscillating cantilever begins to intermittently contact the surface, the cantilever oscillation is necessarily reduced due to energy loss caused by the tip contacting the surface. The reduction in oscillation amplitude is used to identify and measure surface features. Topography image

15 This is the most common type of SPM image, and is usually created with atomic force microscopy or scanning tunneling microscopy. Because the AFM tip penetrates a harder sample surface less than it does a softer one, the tip is able to provide higher fidelity when following the height variations of a hard surface. Tunneling current In scanning tunneling microscopy, the error/servo input signal is the tunneling current between the sample and a sharp metal tip (typically hundreds of pico amperes to several nano amperes) when a bias voltage (typically tens or hundreds of mv) is applied between the two. The magnitude of this current is extremely sensitive to (and varies exponentially with) the small gap that separates the nearest atoms between the STM tip and the sample surface Van der Waals forces A term used to describe a number of attractive or repulsive forces between atoms or molecules. In contact mode AFM, these forces become repulsive as the AFM tip comes in close contact with the sample surface. Z-actuator This piezoelectric tube, flexure, or hybrid of the two is responsible for moving the AFM probe in the Z-axis. Useful Links 1. Atomic Force Microscopy Basics (Flash Animation) virtual.itg.uiuc.edu/training/afm_tutorial/ - 2. Atomic Force Microscopy Student Module 3. Powers of Ten. Based on the film by Charles and Ray Eames. An Cached Nanoscience on the tip: University of Washington Using Nanoscale Instrumentation For Quality Undergraduate Education (UNIQUE) in Nanotechnology Undergraduate Education (NUE) 6. Nano-Link (Mid west Regional Center for Nanotechnology Education)

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