Microscopy scanning probe

Scanning probe microscopy (SPM) can be used to measure the physical, chemical and electrical properties of the sample by scanning the particle surface with a tiny sensor of high resolution. The scanning probe microscope does not measure a force 

Scanning probe microscopy (SPM) is a general term that covers a wide range of techniques within which a physical probe is passed over a surface via piezoelectric actuators in order to reproduce the surface features. The first of these, scanning tunnelling microscopy (STM) technique, was inveuted in 1981. 

Like electron microscopy, scanning probe microscopy (SPM) also opens a window into the world of nanometer-sized specimens and, in some cases, provides details at the atomic level. One version of SPM is scanning tunneling microscopy (STM), in which a platinum-rhodium or tungsten needle is scanned across the surface of a conducting solid. When the tip of the needle is brought very close to the surface, electrons tunnel across the intervening space (Fig. 9.23). 

In atomic force microscopy (AFM), a sharpened tip attached to a cantilever is scanned across the surface. The force exerted by the surface and any molecules attached to it pushes or pulls on the tip and deflects the cantilever (Fig. 9.26). The deflection is monitored by using a laser beam. Because no current needs to pass between the sample and the probe, the technique can be applied to nonconducting surfaces and to liquid samples. 

Two modes of operation of AFM are common. In contact mode, or constant-force mode, the force between the tip and surface is held constant and the tip makes contact with the surface. This mode of operation can damage fragile samples on the surface. In noncontact, or tapping, mode, the tip bounces up and down with a specified frequency and never quite touches the surface. The amplitude of the tip s oscillation changes when it passes over a species adsorbed on the surface. 

Now that we have described motion in one dimension, it is a simple matter to step into higher dimensions. The arrangement we consider is like a particle confined to a rectangular box of side Lx in the x-direction and Ly in the y-direction (Fig. 9.28). The wavefimction varies across the floor of the box, so it is a function of the variables x and y, written as yf x,y). We show in Further information 9.2 that, according to the separation of variables procedure, the wavefunction can be expressed as a product of wavefunctions for each direction 

Introduction What is the Strength of Scanning Probe Techniques  

Self-organized monolayer prepared by solution casting of octyl-decorated Frechet dendritic wedges functionalized by a methyl ester group (30 nm x 30 nm, bias voltage tJbias = 500 mV, tunnelling current /t = 70 pA 0.2 mM in hexane), C. Rohr, LMU 

Munich, Germany and M. Malarek, L. Scherer, C.E. Housecroft, E.C. Constable, Uni. Basel, Switzerland. 

Around 1980 a new method of microscopy known as scanning probe microscopy (SPM) was invented. Within the past ten years, applications have been increasing exponentially in fields like surface physics and chemistry, biology and optics. SPM is also beginning to emerge as a usefvil and popular technique for R D and quality control in several industries. 

Probe microscopes are characterized by two common features. On the one hand, a sharp, tiny probe gets very close to the sample and feels the surface by monitoring some kind of interaction between the probe and the surface, which is very sensitive to distance. On the other hand, the sample is scanned in a raster fashion with near atomic accuracy, and the variation in the interaction is translated into a topographic map of the surface. 

In the contact mode the tip scans the sample in close contact with the surface. The force on the tip is repulsive with a mean value of 10 N. This force is set by pushing the cantilever against the sample surface with a piezoelectric positioning element. In contact mode AFM the deflection of the cantilever is sensed and compared in a DC feedback amplifier to some desired value of deflection. If the measured deflection is different from the desired value, the feedback amplifier applies a voltage to the piezo to raise or lower the sample relative to the cantilever in order to restore the desired value of deflection. The voltage that the feedback amplifier applies to the piezo is a measure of the height of features on the sample surface. It is displayed as a function of the lateral position of the sample. 

Studies on fundamental interactions between surfaces extend across physics, chemistry, materials science, and a variety of other disciplines. With a force sensitivity on the order of a few pico-Newtons, AFMs are excellent tools for probing these fundamental force interactions. Force measurements in water revealed the benefits of AFM imaging in this environment due to the lower tip-sample forces. Some of the most interesting force measurements have also been performed with samples under liquids where the environment can be quickly changed to adjust the concentration of various chemical components. In liquids, electrostatic forces between dissolved ions and other charged groups play an important role in determining the forces sensed by an AFM cantilever. 

Among the family of SPMs the two most commonly used are Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). In STM, a sharp metallic probe and a conducting sample are brought together until their 

The sample limitations imposed by STM provided the motivation to continue to develop SPM with increased versatility. Under a decade after the first STM systems were assembled, atomic force microscopy (ATM) was demonstrated. In AFM, the tip is mounted on a cantilever, typically made of silicon, and brought in close proximity to the sample surface. The ensuing interaction (e.g., Van der Waals forces) pushes the tip back, creating a deflection of the cantilever that can be detected by 

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AFM measures the force instead of the current between the tip attached to a cantilever and the sample. Thus, AFM can operate on an insulator surface. Based on any force that is detectable, instruments similar to AFM can be designed and are categorized as scanning force miaoscopy (SFM). There are two major operating modes for AFM the contact mode and the noncontact mode, according to the interaction of the 

Since the early 1980s, the number of variants to the above that have been developed are manifold and, therefore, only a brief introduction to the technique is possible here. To exploit the potential of STM fully, the sample needs to be both flat and conducting, and hence it is not widely used for the study of polymers. However, a variant of the technique has become very widely used—atomic force microscopy (AFM). In many ways, AFM is derived from surface proliloinelry. in which a stylus is scanned across the 

For the study of non-conducting samples the mechanical interaction between the probe tip and the specimen can be exploited in many ways. 

These include contact force imaging (CFI) mode, in which the tip is scanned across the sample surface at constant force, tapping mode in which the tip oscillates close to the surface enabling either the forces or phase relationships between load and displacement to be used to form the image, and local force spectroscopy or force/volume imaging in which the variation of force with tip/sample separation at a point can be used to study local interachons. 

In the final example, it is possible to modify the chemical nature of the hp to explore specihc interactions,for example, single polymer load extension curves have been explored by, hrst, using the hp to detach some molecules, reattach them elsewhere and, hnally, monitor the force as they are extended. j deed, another use of AFM is as a means of moving atoms and molecules to build structures. Recent developments include a novel highspeed imaging system. 

In situations where the electrical properhes of a material are of interest, a range of SPMs have been developed to explore different effects. Weisendangcr provides a more comprehensive summary of the mulhtude of different SPM techniques than is possible here. 

The probe is scanned across the snrface of the sample, and the height of the probe is adjusted so that the tunneling cnrrent flow is kept constant as it is being scanned, and the height can then be recorded as the probe is scanned aU over the surface of sample. This movement of the probe must necessarily be minute and precise and is accomplished by a devise based on piezoelectricity. The trace of height of the probe may represent the shape and arrangement of atoms and vacancies in between. The scanning result can then be treated by a computer and be visualized. 

By the way, gallium arsenide is expected to function as a semiconductor like silicon, which is the basis of the today s high-technology industry. The diameters of gallium and arsenide can then be estimated from this result. They are about 0.2 and 0.4 nm (2 and 4 A), respectively, and are in agreement with the data obtained by other methods. These results suggest that the resolution of STM is of the order of 0.01 nm (=0.1 A= 10 pm). 

STM is but an example of scanning probe miCToscopes. Another important scanning probe microscope is called atomic force miCToscope (AIM). When a probe is brought close to a sample, a force will be exerted between the probe and the sample. If this force (atomic force) is of a general character, i.e., London dispersion force, then the force will be dependent on the distance and the nature of the probe and the atomic nature of the sample. Hence, a geometrical structure of a sample will be imaged at atomic level by scanning such a probe that responds to atomic force. 

The double helix structure of a DNA has been imaged by AFM. That is shown in Fig. 21.9. A DNA molecule looks like a right-handedly wound ropes. The ropes are the (alpha) helically coiled double strand. These images (Figs. 21.6-21.9) give us 

3 X-Ray Diffraction Atomic Structure of Large Molecular Compounds and Ionic Compounds 

Since the pioneering work of Binnig157 in the 1980s, the family of microscopic techniques collectively known as scanned probe microscopy (or SPM) has become widely available and is extensively used in conducting polymer research. SPM consists of a number of related techniques in which a fine probe is rastered across a sample surface. Interaction between the probe tip and the sample drives a feedback system that allows topographical mapping of the sample surface. Scanning tunnel- 

FIGURE 1.29 Force-distance plot for (a) polypyrrole (NO,-) and (b) polyaniline (HC1) on carbon foil. 

Recent applications of SPM techniques have revealed new details of the electrical properties of conducting polymers. In one example, STM images were taken of the granular structure of electrochemically prepared polyaniline films. Simultane- 

Comparison of cos 0, 0, and A0 Values for Polypyrrole, Poly(3-carboxy-4-methylpyrrole) (PCMP), and Poly(3-carbethoxy-4-methylpyrrole) (PCEMP) on Glassy Carbon 

0n Advancing contact angle 9r Receding contact angle, u = undefined. 

Analogous to record players, a SPM tip is supported by a flexible cantilever. During analysis, the tip is slowly rastered across the surface of a material - either 

The most common operating modes of AFM are contact, noncontact, and tapping, which are self-explanatory in their manner of interrogation of the surface. In contact-mode AFM, there is a repulsive force between the sample and tip (ca. 10 N) the piezoelectric response of the cantilever either raises or lowers the tip to maintain a constant force. Similarly as STM, the best resolution will be obtained under UHV conditions. That is, in an ambient environment, adsorbed 

Without question, AFM exhibits a much greater versatility for surface analysis than STM. In particular, the following variations are possible, through altering the nature of the tip  

Due to the presence of the water meniscus, the mechanism of molecule transport from the tip to the surface is rather complex and not fully understood.Nevertheless, it has been clearly established that the lateral sizes of the patterns are determined by parameters such as RH, molecule solubility in water, contact time (usually as writing speed (usually as and tip functionalization. 

Interestingly, contact force has no influence on patterns, which is an important point for parallelization of the method. In about 10 years, the influence of these parameters has been intensively studied and optimized on model systems such as thioalkanes (octadecylthiol [ODT], mercaptohexadecaneoic acid [MHA]). It comes out that a careful control of experimental conditions, and more particularly of RH, is required to obtain reproducible patterns. Doing so, the best reported spatial resolution is 15 nm, and the typical spot sizes obtained routinely with DPN are of the order of 100 nm. 

During the last decade, a very large range of materials have been patterned using DPN, showing that this technique is technologically relevant in many applications. Systems are now commercially available. Since several reviews have recently listed the applications of DPN, we will not describe them in great detail here and discuss only the two main deposition modes developed. 

Since thioalkanes and thiolated oligonucleotides have shown to be the best suitable molecules for DPN, most of the applications of DPN used patterns of these molecules as templates to attach the molecules of interest in a second step, often performed ex situ. Using specific interactions (chemical or electrostatic, for example) with the functionalized thiols, a wide range of molecules and nano- 

Despite many applications demonstrated using DPN, there is a need for a more general patterning method that could apply to any soluble molecule or nanoobject, independently of its chemical nature. 

Convergent beam microdiffraction uses a convergent rather than a near-parallel beam, and this makes it possible to limit the beam to extremely small regions. The diffracting area is limited spatially by the beam diameter, but few polymers can withstand the focused beam. 

High resolution electron microscopy can provide information that cannot be obtained in any other way, but it requires skill and experience as well as a high resolution TEM. Lattice fringe images have been obtained from a wide range of polymers. This includes polypropylene 

Good information can also be obtained from partially ordered polymers such as poly(p- 

As in the SEM, where the probe is a focused beam of electrons, the resolution of the image is controlled by the region of interaction. The part of the probe that interacts with the sample has to be very small, and in several forms of SPM it is small enough to allow atomic resolution. Extremely precise control of position is required in all SPMs both for (x, y) scanning across the surface and for the z height control, and this is accomplished by use of piezoelectric drivers. Motion control is shown schematically in Fig. 2.7 by double-headed arrows on both probe and specimen, but normally one or the other is moved, not both. Some systems split the control, moving the specimen in the x and y direction and the probe in the z direction. 

The first SPM to be developed was the scanning tunneling microscope (STM) [89]. In the STM the probe is a conductor set at a bias voltage difference from a conducting sample and the signal is a current that passes between them. The probe-sample interaction is the quantum mechanical tunneling current that has a measurable value only when the two conductors are a very small distance apart, typically less than Inm. In the STM the region of 

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The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

The ability to control the position of a fine tip in order to scan surfaces with subatomic resolution has brought scanning probe microscopies to the forefront in surface imaging techniques. We discuss the two primary techniques, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) the interested reader is referred to comprehensive reviews [9, 17, 18]. 

We confine ourselves here to scanning probe microscopies (see Section VIII-2B) scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in which successive profiles of a surface (see Fig. VIII-1) are combined to provide a contour map of a surface. It is conventional to display a map in terms of dark to light areas, in order of increasing height above the surface ordinary contour maps would be confusing to the eye. 

With the exception of the scanning probe microscopies, most surface analysis teclmiques involve scattering of one type or another, as illustrated in figure A1.7.11. A particle is incident onto a surface, and its interaction with the surface either causes a change to the particles energy and/or trajectory, or the interaction induces the emission of a secondary particle(s). The particles that interact with the surface can be electrons, ions, photons or even heat. An analysis of the mass, energy and/or trajectory of the emitted particles, or the dependence of the emitted particle yield on a property of the incident particles, is used to infer infomiation about the surface. Although these probes are indirect, they do provide reliable infomiation about the surface composition and structure. 

Wiesendanger R 1994 Scanning Probe Microscopy and Spectroscopy Methods and Appiications (New York Cambridge University Press)... 

Vansteenkiste S O, Davies M C, Roberts C J, Tendler S J B and Williams P M 1998 Scanning probe microscopy of biomedical interfaces Prog. Surf. Sc/. 57 95... 

Colton R J ef a/ (eds) 1998 Procedures in Scanning Probe Microscopies (New York Wiley)... 

Durig U, Zuger O and Staider A 1992 interaction force detection in scanning probe microscopy methods and appiications J. Appl. Phys. 72 1778... 

Wagner P 1998 Immobilization strategies for biological scanning probe microscopy FEBS Lett. 430 112... 

Bottomley L A, Coury J E and First P N 1996 Scanning probe microscopy Ana/. Chem. 68 185R... 

Gewirth A A and Niece B K 1997 Electrochemical applications of in situ scanning probe microscopy Chem. Rev. 971129... 

A wide variety of measurements can now be made on single molecules, including electrical (e.g. scanning tunnelling microscopy), magnetic (e.g. spin resonance), force (e.g. atomic force microscopy), optical (e.g. near-field and far-field fluorescence microscopies) and hybrid teclmiques. This contribution addresses only Arose teclmiques tliat are at least partially optical. Single-particle electrical and force measurements are discussed in tire sections on scanning probe microscopies (B1.19) and surface forces apparatus (B1.20). 

Monolayers of alkanetliiols adsorbed on gold, prepared by immersing tire substrate into solution, have been characterized by a large number of different surface analytical teclmiques. The lateral order in such layers has been investigated using electron [1431, helium [144, 1451 and x-ray [146, 1471 diffraction, as well as witli scanning probe microscopies [122, 1481. Infonnation about tire orientation of tire alkyl chains has been obtained by ellipsometry [149], infrared (IR) spectroscopy [150, 151] and NEXAFS [152]. 

Schleef D ef a/1997 Radial-histogram transform of scanning probe microscopy images Phys. Rev. B 55 2535... 

Nobel-laureate Richard Feynman once said that the principles of physics do not preclude the possibility of maneuvering things atom by atom (260). Recent developments in the fields of physics, chemistry, and biology (briefly described in the previous sections) bear those words out. The invention and development of scanning probe microscopy has enabled the isolation and manipulation of individual atoms and molecules. Research in protein and nucleic acid stmcture have given rise to powerful tools in the estabUshment of rational synthetic protocols for the production of new medicinal dmgs, sensing elements, catalysts, and electronic materials. 

New types of scanning probe microscopies are continually being developed. These tools will continue to be important for imaging of surfaces at atomic-scale resolution. 

R. Howland and L. Benatar, A Practical Guide to Scanning Probe Microscopy, Park Scientific Instmments, 1996. 

Scanning probe microscopy is a forefront technology that is well established for research in surface physics. STM and SFM are now emerging ftom university laboratories and gaining acceptance in several industrial markets. For topographic analysis and profilometry, the resolution and three-dimensional nature of the data is... 

Atomic Force Microscopy Scanning Probe Microscopy... 

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