spacing Scanning Probe Techniques Heng-Yong Nie STM | AFM | Contact AFM | Force curve | Lateral force | Force modulation | Local modification Non-contact AFM | Phase imaging | Magnetic force | Surface potential | Check AFM tips Cleaning by UVO | Conclusion | Reference & AFM manufacturers | Other info spacing Scanning probe microscopy (SPM) is a mechanical probe microscopy that measures surface morphology in real space with a resolution down to atomic resolution. SPM was originated from the scanning tunneling microscopy (STM), in which electrical current caused by the tunneling of electron through the tip and sample is used to maintain a separation between them. This technique was a totally new one that can image atom arrangement on a surface in real space for the first time. It is so invaluable to science and technology related to surface phenomena that the inventors of STM shared the Nobel Prize in Physics 1986 with the inventor of electron microscope. Because STM requires that the sample surface be conductive, atomic force microscopy (AFM) was developed in 1986 to measure
Transcript
spacing
Scanning Probe TechniquesHeng-Yong Nie
STM | AFM | Contact AFM | Force curve | Lateral force | Force modulation | Local modification
Non-contact AFM | Phase imaging | Magnetic force | Surface potential | Check AFM tips Cleaning by UVO | Conclusion | Reference & AFM manufacturers | Other info
spacing
Scanning probe microscopy (SPM) is a mechanical probe microscopy that measures surface morphology in real space with a resolution down to atomic resolution. SPM was originated from the scanning tunneling microscopy (STM), in which electrical current caused by the tunneling of electron through the tip and sample is used to maintain a separation between them. This technique was a totally new one that can image atom arrangement on a surface in real space for the first time. It is so invaluable to science and technology related to surface phenomena that the inventors of STM shared the Nobel Prize in Physics 1986 with the inventor of electron microscope.
Because STM requires that the sample surface be conductive, atomic force microscopy (AFM) was developed in 1986 to measure surface morphology that are not a good conductor. AFM has since been developed very rapidly and has found much more applications than STM in many fields. Almost all kind of materials can be measured by AFM. Besides surface morphology mapping, SPM has been developed in the past two decades to measure a variety of surface properties, such as electrical, magnetic, and mechanical properties. The diversity of SPM is based on the fact that the probe tip is in contact or close to sample surface so that many interactions between the tip and sample are measurable.
Return to Top
1. STM
The principle of the STM may be simple: tunneling of electrons between two electrodes under electric filed. However, to develop the concept of electron tunneling into
a technology to image atomic resolution on a surface was not simple. To measure the tunneling effect, the distance between the two electrodes must be close to each other on an order of 1 nm. Surface cleanness and vibration-free system are essential to the measurement of the tunneling current. Shown here is an STM inage obtained on an HOPG substrate. Quantum mechanics predicts an exponential dependence of tunneling current with a separate distance between the two electrodes. An observation of this dependence between a W-tip and Pt surface by G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel in 1981 was marked as the invention of STM. Atomic resolution measured on Si(111) 7x7 surface in 1982 might be considered the breakthrough of STM. Since then there have appeared a huge number of papers that carry STM images.
STM, as a research approach, has been mainly used to measure atomic resolution or electronic structure of solid surfaces in UHV. An example of iron atoms located on Cu(111) surface is from an IBM's research laboratory. In many research fields where the sample is usually an insulator, AFM has been used widely.
Return to Top
2. AFM
Examples of AFM images obtained on four different samples
To obtain similar resolution as in STM for insulating surfaces, AFM was invented in 1985 by G. Binnig, C.F. Quate and Ch. Gerber. A sharp tip (apex radius ~20 nm) formed on a soft cantilever is used to probe the interaction (force) between the tip and sample surface, which could be understood through the Lennard-Jones potential which deals with interaction between two atoms: w(r) = -A/r6 + B/r12, where r is the separation of the two bodies, A and B are interaction constants. Then the interaction force is F = -dw(r)/dr= -6A/r7 + 12B/r13. Following a text book, A and B are known to be10-77Jm6 and 10-
134Jm12, respectively. A calculation for the interaction force between two atoms is shown to the right. Around a separation distance of 0.4 nm between the two atoms, a small attractive force is
seen and when the separation distance gets smaller and smaller the repulsive force increases steeply.
For practical AFM probe tip and the sample surface, attractive force between them could be much larger than what is described here for a tow-atoms system. This is because, at least, the size of the tip is much larger than an atom. Typical radius of a commercial tip is ~10 nm. Also, much longer-range forces could occur in practice.
A sharp tip formed on the free end of a cantilever is used to probe the sample surface. The interaction between the tip and the surface is detected by measuring the deflection of the cantilever using a laser diode to radiate the cantilever
and a photodiode to detect the reflected laser beam. The quadrant photodiode is able to measure both the deflection and torsion of the cantilever. The principle of AFM is shown in the figure to the left. Shown in the figure is the case where the tip scans the sample surface. The AFM operates by keeping constant the interaction between the tip and sample surface through a feedback system that adjusts the distance between the tip and the sample surface. Depending on the interaction between the tip and sample surface, which is used as the feedback signal, there are two different imaging modes described as follows
Schematic illustration of AFM principle: while scanning the tip across the sample surface (x, y), the system adjusts the distance (z, which is thus the measure of the height of the sample surface features) between the tip and the sample surface to maintain a constant contact force (contact mode) or oscillation amplitude (dynamic force mode). A 3-D image is thus constructed by the lateral dimension the tip scans and the height the system measures.
Return to Top
2.1 Contact AFM
In the contact mode AFM, the tip is mechanically contacted with sample surface, exerting a force on the surface of the sample. This applied force can be evaluated from a force-distance curve which is measured when the tip is brought to and then retracted from the sample surface, as shown below. Inserts in the figure show the interaction between the tip and sample surface, which is detected by the deflection of the cantilever. There is no interaction between the tip and surface when the tip is far away from the surface (a ). When the tip is brought close enough to the surface there will be an attractive force between them. Usually, the gradient of the attractive force is much larger than the spring constant of the cantilever, so that the tip is snapped to the surface to make a contact between the tip and surface (b). Further extending the tip results in loading (repulsive) forces to the surface (c). This repulsive force is usually used as the feedback parameter for the AFM system to obtain surface morphology. Forces of a couple of nN are used in contact mode AFM. In the retracting cycle (d and e), because of the adhesion established after the contact between the tip and surface, the tip will not detach from the surface until the force used to pull the tip from the surface exceeds the adhesion force between them (f). This pull-off force can serve as a measure of the adhesion force between the tip and surface.
A very soft cantilever with a spring constant of ~ 0.1 N/m is usually used in contact AFM. A photograph of such a cantilever is shown in the optical picture below on the left. The cantilever is so soft that it will be pulled onto the surface because the gradient (~ 10 N/m) of attractive force between them is usually much larger than the spring constant of such soft cantilevers.
After a mechanical contact between the tip and the sample surface, there is a repulsive force between them. This force is used as the feedback parameter (by maintaining a constant force through adjustment of the sample height while the tip scans the surface) to obtain AFM images.
Because the tip is mechanically contacted with surface in the contact mode AFM, many surface properties such as friction force distribution and mechanic properties can be measured simultaneously with the topographic image. Also, nano-lithography on some materials is also available by controlling the applied forces in the contact mode. A working knowledge on force-distance curve is essential for undestanding and interpreting the imaging mechanism of contact mode AFM (especcially when things go wrong).
Return to Top
2.1.1 Force-distance curves
Force-distance curves are obtained by extending the tip to the surface to make a contact between the tip and the sample surface followed by retracting the tip from the surface. The original point for the distance may be defined as the mechanical contact between the tip and surface in the extending cycle. Extending the tip beyond that point will result in load forces applied to the surface. The slope of this load force is a measure of the Young's modulus of the surface, possibly mixed with the spring constant of the cantilever. As a result, a cantilever whose spring constant is comparable with the surface stiffness should be used to measure the elasticit y information.
In the retracting cycle, because of the adhesion properties between the tip and surface, the tip will not depart from the surface until the force used to pull the tip from the surface exceeds the adhesion force between them. This pull-off force can be considered as a measure of the adhesion force between the tip and surface. Adhesion force can be related to surface energies of the tip and sample surfaces, as well as their interfacial energy. Shown above is an example of measruing adhesion force at different regions on a BOPP film. The striped areas have higher adhesion force than the normal surface; we will see later that this is also reflected in the friction force images. Click here to see adhesion force increase for UV/ozone treated polypropylene films. Adhesion force can be related to surface energies of the tip and sample surfaces, as well as their interfacial energy. If there were liquid-like contamination on a surface, the capillary force should be considered. Recently, force-distance curve has been demonstrated to be able to record the event of (a) the breaking of a single molecular bond (single molecule force spectroscopy) and (b) the folding and unfolding of proteins by confining the molecules between the AFM tip and a surface.
Return to Top
2.1.2 Lateral Force Microscopy (LFM)
Lateral force microscopy (LFM) is based on measuring the torsional movement of the cantilever when the tip scans the surface, which is illustrated here. Lateral force detection in AFM is usually used to image different friction forces on a surface. The difference in the bi-directional lateral force images corresponds to the friction force
image. Friction force imaging has the ability to identify such regions of higher hydrophilicity on the basis of increased interaction with the AFM tip. Here is an example showing higher friction force on scratched areas on a BOPP film, which is thought to be due to higher surface energy of the scratched area. Force-distance curves obtained on the normal and striped areas are shown above, revealing that the friction force contrast seen is related to the adhesion force.
Friction force can also be also used to detect chemical functional groups on a surface with the tip modified by a specific chemical functional base (chemical force microscopy). For example, a silicon tip, which is hydrophilic, can be used to d istinguish the chemical amphiphilicity of Octadecylphosphonic acid (OPA) molecular layers. That is, film surface terminated by the hydrophilic headgroup shows larger friction force than that terminated by the hydropho bic tail. Combined with height measurement, one can clearly identify whether a molecular layer is a bilayer (terminated by the headgroup) or a trilayer (terminated by the tail).
On the other hand, there is another effect of lateral force imaging to reveal local topography change by an enhancement of the torsional movement of the cantilever when the tip crosses edges of the surface features. Click here to see an LFM image on 15-min-UV/ozone-treated polypropylene film, where the droplets are distinguished from the surrounding surface. This technique has proven useful in detecting different phases on a surface whose height range is large, which is the case for some practical polymer samples.
Return to Top
2.1.3 Force Modulation
In addition to the topographic feature, one can probe local elastic properties of materials through a mechanical interaction between the surface and tip. This can be done by oscillating the sample height while measuring the response of the cantilever with lock-in amplifier technique. Elasticity difference on a s urface can be distinguished by using this technique. This technique actually measures the slope of the force-distance curves at the repulsive force region.
The oscillation of the sample height may
be realized by applying sinusoidal voltage from a function generator to the Z-direction of the piezo (PZT) scanner on which the sample is fixed. A sinusoidal voltage is applied to the piezo scanner to oscillate the sample height with a peak-to-peak amplitude of about 1 nm. The response of the cantilever to this oscillation is detected with a lock-in amplifier and is used to obtain images relevant to local elasticity of sample surface. The oscillation of the sample height would not influence topographic images as far as its frequency is higher than the cutoff frequency of the feedback loop. Therefore, both topography and elasticity distribution images can be obtained simultaneously. An example of elasticity mapping of PS and PS/PEO blend film coated on mica substrate using a cantilever with a spring constant of 0.75 N/m is shown in the figure to the left.
The mechanism for the force modulation is described using force-distance curves (a) obtained on the mica and the PS film and the simultaneously obtained response (b) of the cantilever to an oscillation of the sample height with an amplitude of 1 nm at 5 kHz. The sprin constant of the cantilever used was 18 N/m and the approachoing and retracting speed for the tip was 3 nm/s. The difference seen for the cantilever response (b) is due to the different slope of the force-distance (a) curves on the different materials, which is a reflection of difference in Young’s modulus for mica (200 GPa) and PS (5GPa).
Return to Top
2.1.4 Locally modifying surface
Surface may be modified by applying large forces through the tip to the surface during an AFM scanning. This technique has a potential application to create nanometer-scale structure on a surface. For example, on the crystallized polyethylene oxide (PEO) thin films, both the surface structure and elasticity were found to be modified locally by the AFM tip. A close look at the modification of the PEO surface is shown here.
Return to Top
2.2 Dynamic force mode AFM techniques
Dynamic force (tapping or non-contact) mode AFM, in which a cantilever oscillated around its resonant frequency is used to probe surface features, was developed initially to eliminate
surface degradation encountered in contact mode AFM, especially for soft materials. For dynamic force mode AFM, silicon cantilevers with a spring constant of 5 ~ 40 N/m are used. A typical 40 N/m cantilever is 125 µm long, 30 µm wide and 3.7 µm thick. Because the variation of the oscillation amplitude is used in the feedback system, the relative change in oscillation amplitude of the cantilever versus distance between the tip and sample surface is shown in the figure below. The amplitude-distance curve shown in the figure was obtained on a BOPP film surface. Interaction between the tip and the surface at different tip-sample distance is indicated by inserts a-c. Arrows indicate the direction of the tip approaching to the sample surface.
The above figure shows that when the tip is far away from the sample surface (a), the oscillation amplitude of the cantilever is a constant, representing a “free space” situation where there is no interaction between the tip and the surface. The amplitude decreases when the tip approaches close enough to the sample surface so that it “feels” attractive and/or repulsive forces (b). The cantilever stops oscillating when the tip is brought in to mechanically contact the surface (c). Dynamic force mode AFM works by scanning the tip across the sample surface and adjusting the distance between the two through maintaining constant damped oscillation amplitude of the cantilever. This adjustment of the separation between the tip and surface allows the AFM to construct the topographic image. There are many modes measuring surface properties based on this dynamic force mode AFM, as described in the following. Usually, a set point at 50% of the oscillation amplitude in free space is a good start. AFM images shown here clearly show formation of mounds on UV/ozone treated polypropylene (PP) film from the
original surface characterized by fiber-like network structure (scan area is 2 micron square and height range is ~ 25 nm) and an increase in adhesion force. This increase in adhesion force indicates an increase in surface energy due to the oxidation of the modified polymer films.
When the oscillating amplitude is large (say, >2 nm), the tip actually taps the surface, which is why it is called dynamic force or tapping mode AFM. In practice of measuring larger scale area, larger amplitude is usually used because it results in more stable imaging. If the oscillating amplitude is very small (say, a couple of nm), then the dynamic force mode could be called non-contact mode because at such small amplitude, the tip would not need to tap the surface to sense
Return to Top
2.2.1 Phase Imaging
The phase shift in the oscillating cantilever is related to tip-surface interaction which is basically material specific. Therefore, phase shift contrast in tapping mode AFM can be used to distinguish different surface compositions on a surface (see the schematic below). There are many surface properties that may have an effect on the phase shift contrast. They could be difference in friction, viscoelasticity, adhesion, material, etc. Phase imaging usually gives clear contrast on a surface if there are detectable differences in surface properties as described above. So, the explanation of a phase shift image should be careful and usually depends on other observations and background knowledge on the sample. It should be noted that phase imaging is a very valuable approach for SPM researchers because they probably find and in fact are finding some new phenomena during their searching answers and explanation to the phase shift measurement. Applications include visualizing phase separation in polymer blends, distinguishing different compositions on surface. Shown below is topography (left) and phase image (right) for a surface of a tonner particle of carbon black matrix with polymer filler (scan area is 3.5 micron square).
Return to Top
2.2.2 Magnetic Force Microscopy (MFM)
Magnetic information on a sample can be measured with a magnetized tip. A local topographic data in each scan line is first obtained by dynamic force mode AFM. Then the tip is lifted up in a certain distance and repeats to scan the same line. The magnetic interaction between the tip and surface will give a change in the magnitude (or phase) of the oscillating cantilever, which gives regional information of magnetic force distribution on a surface. The difference of the magnetic properties can be measured in this method. This technique is useful to image magnetic force distribution on the recorded magnetic media (data storage) and micromagnetic structure on some magnetic materials. For more information on this topic, you may want to visit this web page.
2.2.3 Electric Force Microscopy (EFM)
Similar to MFM, EFM uses a conductive tip to probe the difference of electric filed gradient distribution on a surface. This technique can be used in the failure check on integrated circuit (IC). For more information, visit this web page.
There has appeared recently a new technique which maps local surface potential distribution together with topography using an SPM, by keeping a certain separation between the sample surface and conductive tip to which a sinusoidal voltage is applied. The principle of the SSPM is shown in the figure on the right. In case of that there is a difference in potentials between the tip and sample surfaces, an oscillating electromagnetic force appears between the tip and sample surface at the frequency of the applied sinusoidal voltage, which makes the cantilever oscillate. This oscillation is used as the feedback parameter for the system which tries to stop this oscillation by applying a dc voltage to the tip so as to make the potential difference between the tip and sample surfaces vanish. This applied dc voltage to the tip is thus equal to the surface potential of the sample, which makes the surface potential measurable together with the topography.
As shown in the figure to the left, gold films
deposited on glass substrate were used to confirm the SSPM by making potential difference between the two gold films through applying voltage to one of them and grounding the other. During scanning, the voltage was changed so that different potential difference were recorded. Metal Pd deposited on a semiconductor is an example to give contact potential between the metal and the semiconductor surface. Here is a result showing the contact potential difference between the Pd and semiconductor surface. Other examples of measuring surface potential distribution are a Pd (110) surface and a thin film giving a clear distribution of surface potential.
This technique can be used to map surface voltage distribution, which can be used to detect defects and to measure local work function distribution.
Return to Top
3. A Simple Method to Check AFM Tip Performance Uing a Polymer Film
3.1 BOPP film surface for tip radiu evaluation
An atomic force microscopy (AFM) image of a surface is constructed through the detection of an interaction between the tip apex and the surface features. The interaction, whether it be a contact force, an oscillation amplitude or others, is the feedback signal used to adjust the proximity of the tip and the surface features. Because of this imaging mechanism, an AFM image is, in practice, a convolution of the tip geometry and the surface features. Based on the actual geometry, the tip apex or the surface feature, whichever is sharper, acts as the effective probe.
In practice, there could be a large-sized contaminant on the tip apex, making sharper surface features the effective probe. Therefore, images collected using a contaminated or damaged tip can be dominated by the geometry of the AFM tip itself (i.e., self-imaging of the tip) if the surface features are sharper than the tip. Interpretation of such images can easily be misleading if the tip effect is not taken into account. To ensure that the tip is “good” enough for imaging a surface, one needs reference samples that have known surface features, suitable for checking the tip performance. Introduced here is a simple and effective method of evaluating tip performance by imaging a biaxially-oriented polypropylene (BOPP) film, which is characterized by nanometer-scale sized fibers. The BOPP film surface is appropriate for use as a reference because a contaminated tip will not detect the fiber-like network structure. Imaging the very fine fiber-like structure of the BOPP film surface is a good criterion for the tip performance. Many other samples with known surface features can also be used to chracterize the geometry of AFM probes.
Because the polymer film is soft compared to the silicon tip (Young's modulus for polypropylene is 1-2 GPa, while for silicon it is 132-190 GPa), the polymer will not damage the tip when the tip is pushed into the polymer. This property can be used to clean a contaminated tip, i.e., by pushing the contaminated tip into the polymer, contaminants could be removed from the tip apex. Another important property of the BOPP is that the polymer film is highly hydrophobic and has a very low surface energy of ~ 30 mJ/m2 (The surface energy for Si is ~ 1400 mJ/m2; and the surface tension of water is 72 mJ/m2). These properties prevent contaminants from accumulating on the surface and hence prevent the contamination of the tip in the evaluation process. This method of using BOPP to check AFM tips AND to clean contaminated tips was highlighted in April 1, 2001 issue of Analytical Chemistry.
3.2 Applications to Blind Tip Reconstruction
Considerable effort has been expended to mathematically extract the geometry of the tip based solely on an algorithm derived from a given image. The method is known as blind reconstruction. This methodology is based on an assumption that protrusions in the AFM image represent the self-image of the tip, which is equivalent to the statement that sharper features on the sample surface act as the probe to image the AFM tip. This method has proven useful and successful in estimating tip geometry from an existing image, when appropriate samples were chosen (i.e., some surface features on the sample are sharper than the tip). Once the tip geometry is known, the tip effect may be subtracted from the original image through the mathematical operation of erosion, also known as deconvolution. Dilation is another mathematical operation, which adds a tip effect to an existing AFM image by “scanning” the known tip across the
“surface” of the image. This transformation appears useful in simulating tip effect to a given image, because the mechanism of AFM can be regarded as a dilation between the tip geometry and surface features of the sample.
We have found that a BOPP film is suitable for checking tip performance and for cleaning contaminated tips, thus making it possible to collect images of the same area of a BOPP film surface before and after the tip was cleaned. Therefore, the difference between the two different images is solely due to the contamination of the tip. We took advantage of our ability to collect AFM images of the same area using the same tip, in one instance, contaminated and, in the
other, after being cleaned. Commercial software SPIP (Metrology Image ApS, Denmark) was used to estimate the tip geometry using its “tip characterization module”, in which the blind reconstruction algorithm is implemented. First we used blind reconstruction on the image collected using the contaminated tip. Blind tip reconstruction allows one to extract the geometry of the tip from a given image. Once we had estimated the geometry of the contaminated tip, we used it to simulate the tip effect using the image collected using the cleaned tip. By comparing the simulation result with the image collected using the contaminated tip we showed that the blind reconstruction routine works well. Prior to this, there was no de facto method for testing blind reconstruction algorithms.
Comparison of tip geometry from the blind reconstruction method and from scanning electron microscopy (SEM) images has been made by Dongmoet al. [S. Dongmo, M. Troyon, P. Vautrot, E. Delain, and N. Bonnet, J. Vac. Sci. & Technol. B 14, 1552 (1996)]. We provides a simpler way to test blind reconstruction: comparison of AFM images collected in the same area of the BOPP by clean and contaminated tips. If the estimation of the contaminated tip geometry is reasonable, then one expects to be able to use the estimated tip geometry to dilate the image collected using the clean tip to obtain an image resembling one collected using the contaminated tip. Conversely, one can also determine if the deconvolution works by eroding the image collected with the contaminated tip using the estimated tip geometry to see whether the result resembles the image collected using the clean tip.
Because an AFM image is a convolution of the surface features and the tip geometry, if neither of them is known, there is no way to know, on an unknown sample, if the image is dominated by the surface features or the tip effect. When the tip is much sharper than the surface features, it will collect an image reflecting the “true” surface features. This is the reason why a reference sample is essential to check the tip performance. It is important to note that a tip could be easily contaminated or damaged depending on the chemical and mechanical properties of the sample surface. Using electron microscopes one can evaluate the outlines of the tip shape from specific directions, but it is difficult, if not impossible, to capture the three-dimensional geometry of the tip. In combination with blind reconstruction, using BOPP film to check the tip performance provides a simple and effective protocol to test the estimation of the tip geometry of a contaminated tip. One can do this by comparing the image collected using the contaminated tip with the image generated by dilating the image collected using the clean tip.
On the other hand, when the tip is much larger than the surface features, using such a tip to scan the surface will result in an image that is merely a reflection of the geometry of the tip apex itself. In this case, it is evident that the information about the surface features is physically lost. Therefore, the erosion operation will not lead to the recovery of the “true” surface features, though the mathematic operation may result in an image which is likely closer to the “true” surface features. The degree of the recovery by the erosion operation is dependent on how severely the tip is contaminated. One can imagine that different surface features can have similar images if a large tip is used: they are dominated by the tip effect.
References
H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Atomic Force Microscopy Study of Biaxially-Oriented Polypropylene Films", J. Mater. Eng. Perform., 13, pp.451-460 (2004). H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Use of biaxially-oriented polypropylene film for evaluating and cleaning contaminated atomic force microscopy probe tips: An application to blind tip reconstruction", Rev. Sci. Instrum. 73, pp.3831-3836 (2002). H.-Y. Nie and N.S. McIntyre, "A simple and effective method of evaluating atomic force microscopy tip performance", Langmuir 17, pp.432-436 (2001).
Return to Top
4. A general method to clean contaminated tips using UV/ozone treatment
The tip can be contaminated during scanning some surfaces or just left in air as recognized by the unstable and degraded images obtained by the tip. When the tip was in this condition, we took out the tip for 5 minute treatment in UV/ozone. After that, the imaging condition became stable and images were improved largely. Therefore, the UV/ozone treatment is effective to clean the tip and hence opened a way of recycle-using probe tips. The wavelength of UV light from a mercury lamp is mainly 253.7 nm; with a much lower percentage at 184.9 nm. Photons with those two wavelength are effective for cleaning organic contaminants.
Although ozone can be generated by irradiating oxygen (air) with short wavelength light (184.9 nm; photon energy at this wavelength is 6.70 eV or 154.59 kcal/mol), a separate ozone source (such as an ozone generator) is required to provide enough ozone concentration to clean the contaminants fast enough. What is really doing the cleaning job in short time is the atomic oxygen, which is produced by the decomposition of ozone in the presence of UV light (253.7 nm; photon energy at this wavelength is 4.89 eV or 112.66 kcal/mol). This atomic oxygen oxidizes organic contaminants to form volatile molecules. Meanwhile the UV light also has an effect to excite the contaminant molecules to make them more reactive with ozone and/or atomic oxygen. Ozone itself is reactive with organic contaminant, therefore, ozone alone is also able to clean organic contamination but will take much longer time than UV/ozone combined.
Return to Top
5. Concluding remarks
SPM techniques have been developed extremely fast. It is a promising tool and a base for the new nano science and technology. More and more researchers in many different fields are using SPM. Some want to develop a technology based on SPM to fabricate nano-scale devices. Others are discovering knowledge in physics, chemistry, biology and materials science on nano- and/or meso-scale. SPM promises to provide us new and exciting discovers in surface science and technology, physics, chemistry, materials science and biological technology. This direction is clear if one notes that news media and government agencies are actively involved in reporting and supporting discoveries and development in nanotechnology. The base for such a new field is, of course, SPM.
Return to Top
References
G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Appl. Phys. Lett. 40, 178 (1982).G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 49, 57 (1982).G. Binnig, C.F. Quate and Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986).R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge University Press, 1994.J.N. Israelachvili, Intermolecular and Surface Forces, 2nd ed., Academic Press, 1992.
AFM Manufacturers
Park SystemsVeeco Instruments | Seiko Instruments | Agilent Tchnologies
Asylum Research | NT-MDT | Nanotec ElectronicaJPK Instruments | Surface Imaging Systems | Anfatec
Concentris (cantilever sensor technology)
Publication list on SPM work
33) H.-Y. Nie, J.T. Francis, A.R. Taylor, M.J. Walzak, W.H. Chang, D.F. MacFabe and W.M. Lau, "Imaging subcellular features of a sectioned rat brain using time-of-flight secondary ion mass spectrometry and scanning probe microscopy", Appl. Surf. Sci. 255, 1079-1083 (2008).
32) H.-Y. Nie, N.S. McIntyre, W.M. Lau and J.M. Feng, "Optical properties of octadecylphosphonic acid self-assembled monolayer on a silicon wafer", Thin Solid Films 517, 814-818 (2008).
31) H.-Y. Nie, N.S. McIntyre and W.M. Lau, "Nanolithography of a full-coverage octadecylphosphonic acid monolayer spin coated on a Si substrate", Appl. Phys. Lett. 90, 203114 (2007).
30) H.-Y. Nie, N.S. McIntyre and W.M. Lau, "Selective removal of octadecylphosphonic acid (OPA) molecules from their self-assembled monolayers (SAMs) formed on a Si substrate", Journal of Physcis: Conference Series 61, 869-873 (2007).
29) H.-Y. Nie and N.S. McIntyre, "Unstable amplitude and noisy image induced by tip contamination in dynamic force mode atomic force microscopy", Rev. Sci. Instrum. 78, 023701 (2007).
28) H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Growth and properties of complete monolayer films of octadecylphosphonic acid (OPA) on oxidized aluminum surfaces", ATB Metallurgie 45, 564-568 (2006).
27) H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Scratch resistance anisotropy in biaxially oriented polypropylene and poly(ethylene terephthalate) films", Appl. Surf. Sci., 253, pp.2320-2326 (2006).
26) H.-Y. Nie, M.J. Walzak and N.S. McIntyre,"Delivering Octadecylphosphonic Acid Self-Assembled Monolayers on a Si Wafer and Other Oxide Surfaces", J. Phys. Chem. B, 110, pp.21101-21108 (2006).
25) J.T. Francis, H.-Y. Nie, N.S. McIntyre and D. Briggs, "ToF-SIMS Investigation of Octadecylphosphonic Acid Monolayers on a Mica Substrate", Langmuir, 22, pp.9244-9250 (2006).
24) N.S. McIntyre, H.-Y. Nie, A.P. Grosvenor, R.D. Davidson and D. Briggs, "XPS studies of octadecylphosphonic acid (OPA) monolayer interactions with some metal and mineral surfaces", Surf. Interf. Anal. 37, pp.749-754 (2005).
23) H.-Y. Nie, D.J. Miller, J.T. Francis, M.J. Walzak and N.S. McIntyre, "Robust self-assembled octadecylphosphonic acid monolayers on mica substrate", Langmuir 21 , pp.2773-2778 (2005).
22) H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Atomic Force Microscopy Study of Biaxially-Oriented Polypropylene Films", J. Mater. Eng. Perform., 13, pp.451-460 (2004).
21) H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Use of biaxially-oriented polypropylene film for evaluating and cleaning contaminated atomic force microscopy probe tips: an application to blind tip reconstruction", Rev. Sci. Instrum. 73, pp.3831-3836 (2002).
20) H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Bilayer and odd-numbered multilayers of octadecylphosphonic acid formed on a Si substrate studied by atomic force microscopy", Langmuir 18, pp.2955-2958 (2002).
19) H.-Y. Nie and N.S. McIntyre, "A simple and effective method of evaluating atomic force microscopy tip performance", Langmuir 17, pp.432-436 (2001).
18) H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Atomic force microscopy study of UV/ozone treated polypropylene films", Polymer Surface Modification: Relevance to Adhesion, Vol.2, K.L. Mittal Ed., VSP (Utrecht, The Netherland), pp.377-392 (2000).
17) H.-Y. Nie, M.J. Walzak and N.S. McIntyre, "Draw-ratio-dependent morphology of biaxially-oriented polypropylene films as determined by atomic force microscopy"(Vol. and page in Ref. 3 and 14 should be exchanged), Polymer 41, pp.2213-2218 (2000).
16) H.-Y. Nie, M.J. Walzak, B. Berno and N.S. McIntyre, "Microscopic stripe formation and adhesion force increase introduced by local shear-stress deformation of polypropylene film", Langmuir 15, pp.6484-6489 (1999).
15) H.-Y. Nie, M.J. Walzak, N.S. McIntyre and A.M. EL-Sherik, "Applications of lateral force imaging to enhance topographic features of polypropylene film and photo-cured polymers", Appl. Surf. Sci. 144-145, pp.633-637 (1999).
14) H.-Y. Nie, M.J. Walzak, B. Berno and N.S. McIntyre, "Atomic force microscopy study of polypropylene surfaces treated by UV and ozone: modification of morphology and adhesion force", Appl. Surf. S ci. 144-145, pp.627-632 (1999).
13) H.-Y. Nie and J. Masai, "Surface potentials on Pd/GaAs contacts studied using scanning probe microscopy", Appl. Phys. A 66, pp.s1059-s1062 (1998).
12) H.-Y. Nie, K. Horiuchi, H. Yamauchi and J. Masai, "Local surface potential measurement of Pd/GaAs contact and anodized aluminum films using scanning probe microscopy", Nanotechnology 8, pp.A24-A31 (1997).
11) H.-Y. Nie, M. Motomatsu, W. Mizutani and H. Tokumoto, "Observation of modification and recovery of local properties of polyethylene oxide", J. Vac. Sci. Technol. B15, pp.1388-1393 (1997).
10) M. Motomatsu, T. Takahashi, H.-Y. Nie, W. Mizutani and H. Tokumoto, "Microstructure study of acrylic polymer-silica nanocomposite surface by scanning force microscopy", Polymer 38, pp.177-182 (1997).
9) M. Motomatsu, W. Mizutani, H.-Y. Nie and H. Tokumoto, "Surface structure of a fluorinated thiol on Au(111) by scanning force microscopy", Thin Solid Films 281-282, pp.548-551 (1996).
8) M. Motomatsu, H.-Y. Nie, W. Mizutani and H. Tokumoto, "Surface morphology study of poly (ethylene oxide) crystals by scanning force microscopy", Polymer 37, pp.183-185 (1996).
7) M. Motomatsu, H.-Y. Nie, W. Mizutani and H. Tokumoto, "Scanning force microscopy application to polymer surfaces for novel nano-scale surface characterization", Thin Solid Films 273, pp.304-307 (1996).
6) H.-Y. Nie, M. Motomatsu, W. Mizutani and H. Tokumoto, "Local elasticity measurement on polymers using atomic force microscopy", Thin Solid Films 273, pp.143-148 (1996).
5) H.-Y. Nie, M. Motomatsu, W. Mizutani and H. Tokumoto, "Local modification of elastic properties of polystyrene-polyethyleneoxide blend surfaces", J. Vac. Sci. Technol. B13, pp.1163-1166 (1995).
4) M. Motomatsu, W. Mizutani, H.-Y. Nie and H. Tokumoto, "Lateral force measurements on phase separated polymer surfaces", in Proceedings of Forces in Scanning Probe Microscopies, edited by H.-J. Guntherodt et al., ASI E286 (Kluwer Academic, Dordrecht, 1995), 331-336.
3) M. Motomatsu, H.-Y. Nie, W. Mizutani and H. Tokumoto, "Local properties of phase-separated polymer surfaces by force microscopy", Jpn. J. Appl. Phys. 33, Part 1, pp.3775-3778 (1994).
2) H.Y. Nie, W. Mizutani and H. Tokumoto, "Au(111) reconstruction observed by atomic force microscopy with lateral force detection", Surf. Sci. 311, pp.L649-L654 (1994).
1) H.Y. Nie, T. Shimizu and H. Tokumoto, "Atomic force microscopy study of Pd clusters on graphite and mica", J. Vac. Sci. Technol. B12, pp.1843-1846 (1994).
spacingExamples of AFM images
Last modified on May 16, 2010H.-Y. Nie
Surface Science Western, Room G-1 WSCThe University of Western OntarioLondon, Ontario N6A 5B7, Canada
Phone: (519) 661-2173; Fax: (519) 661-3709 Since March 2002
00000
01
AFM related infomation Analysis Services Polymer AFM Introduction on AFM (PDF) Publication Talks and posters | Return to Top | spacing Since April, 2000
