Scot A.C. Gould

Professor of Physics

Email: sgould@kecksci.claremont.edu
Office: Keck Science Center 113
Phone: 909-607-3197
Office Hours: https://sgould.youcanbook.me
Web Site: https://faculty.jsd.claremont.edu/sgould

Educational Background

Ph.D. University of California, Santa Barbara – Physics (1991)
A.B. Middlebury College – Physics, Mathematics and Computer Science (1985)

Courses Taught

Physics 33,34,35,102,106,107,114
Scripps Core I Pitzer First-year seminar

Research Interests

• Ultrastructure of spider silk
• Surface properties of fluidized cracking catalysts
• Synthetic polymer formation and liquid crystals
• Characterization of surfaces using fractal techniques
• Physics education: computer integration with Maple
• Economics based: systemic risk in financial institutions

Selected Publications

1. M.L. Occelli and S.A.C. Gould. (2004). The use of atomic force microscopy (AFM) to study the surface topography of commercial fluid cracking catalysts (FCCs) and pillared interlayered clay (PILC) catalysts. Proc. of the Am. Chem. Soc. Petro. Div. Conf.: Fluid Cracking Catalysts.
2. D.A. Schiraldi, M.L. Occelli, S.A.C. Gould. (2002). Applications of Atomic Force Microscopy to Current Problems in Industrial Polyester Chemistry. Polymer News   27 (6): 195-200.
3. S.A.C. Gould, S.A. Naftilan S.J. Khoury and D.J. Wright*. (2001). Systemic Risk: A More Rigorous and Realistic Simulation. Financial Innovations and the Welfare of Nations (L.L. Jacque, P.M. Vaaler, eds.) Tufts University, Kluwer Academic Press. 86-106.
4. D.A. Schiraldi, M.L. Occelli, S.A.C. Gould. (2001). Atomic force microscopy (AFM) study of poly(ethylene terephthalate-co-4, 4′-bibenzoate): A polymer of intermediate structure. J. Appl. Polymer Sci.  82(11): 2616-2623.
   Abstract – An atomic force microscope (AFM) operating in both tapping and contact modes was used to study the surface topography and the molecular organization of a molded flexural test bar prepared from a poly(ethylene terephthalate-co-4,4′-bibenzoate) copolymer containing a terephthalate:4,4-bibenzoate molar ratio of 45:55 (PETBB-55). Micrometer-scale (15 × 15 m) contact-mode AFM images revealed that the PETBB surface contains a deep indentation that forms trenches that extend over the entire surface examined. In addition, the surface may appear as an overlay of fibrils having different orientation. At greater magnification (1 × 1 m), it is possible to observe the existence of micropores. These results were also observed in images obtained while operating the AFM in the tapping mode. The side of the part is more homogeneous and ordered than is its top surface. It has the appearance of a stacked lamellar structure in which missing fibrils can originate cracks 0.5-m wide. Fine surface details were observed in nanometer-scale images, showing the presence of chains of white spots that could represent molecules or a cluster of molecules. These chains can form domains in which they are almost parallel to each other and have a preferred orientation; this structural organization was generated without any orientation other than that produced during a mold flow. Alternatively, chain lengths are interrupted and white spots form, distorted by easily recognizable hexagonal arrangements. The degree of lamellar order observed in the side of the bar, the area of greatest flow orientation in the part, was not been observed for the PET homopolymer in the past and bears some resemblance to previously imaged liquid crystalline polyester (LCP) structures. Combined with some previously reported LCP-like mechanical properties, we propose that this PETBB copolymer is, in fact, a frustrated LCP, one that with some driving force could be converted to liquid crystallinity.
   Article – URL not found
5. M.L. Occelli, S.A.C. Gould. (2001). Examination of coked surfaces of pillared rectorite catalysts with the atomic force microscope. J. Catalysis  198(1): 41-46.
   Abstract – The reaction of a sample of natural rectorite with an aluminum chlorhydroxide solution containing the dodecameric [Al13O4(OH)24 (H2O)12]7+ ion generates a heat-stable microporous structure with surface area available to sorption and catalysis. An atomic force microscope (AFM) operating in contact mode has produced images showing that the pillared rectorite surface is free from Al species, implying that all the [Al13O4(OH)24(H2O)12]7+ ions are located inside the clay montmorillonite-like layers. As expected, nanometer-scale images of the clay tetrahedral sheet show a surface that consists of rings of basal oxygens arranged in a hexagonal symmetry. White spots, representative of the three basal oxygens of SiO4 tetrahedra, have repeat distances of 0.54 and 0.97 nm, respectively, in close agreement with the rectorite unit cell dimensions. Thermal and hydrothermal treatments used to prepare the pillared rectorite for microactivity testing (MAT) have little effect on the molecular parameters of the clay surface. After cracking gas oil under MAT conditions, AFM images of the spent clay catalyst reveal that surface carbon preferentially deposited on or near the three basal oxygens of the SiO4 units that constitute the clay silicate layer. As a result, nanometer-scale images of the coked pillared clay surface contain well-defined hexagonal arrangements of white spots having next-neighbor and lateral distances of 0.70 and 1.23 nm that exclude the possibility of graphite formation. After regeneration in flowing air at 760°C/2 h to remove carbon deposits, the pillared clay catalyst resumes its original coloration and the nanometer-scale parameters of the rectorite surface are restored.
   Article – URL not found
6. D.A. Schiraldi, J.J. Lee*, S.A.C. Gould, M.L. Occelli. (2001). Mechanical properties and atomic force cross sectional analysis of injection molded poly(ethylene terephthalate-co-4, 4′-bibenzoate). J. Ind. Eng. Chem. 7(2): 67-71.
7. M.L. Occelli, M. Kalwaei, A. Wolker, H. Eckert, A. Auroux and S.A.C. Gould. (2000). The Use of Nuclear Magnetic Resonance, Microcalorimetry and Atomic Force Microscopy to Study the Aging and Regeneration of Fluid Cracking Catalysts. Journal of Catalysis 195.
8. M.L. Occelli, J.A. Bertrand, S.A.C. Gould, J.M. Dominguez. (2000). Physicochemical Characterization of a Texas montmorillonite pillard with polyoxocations of aluminum. Microporous and Mesoporous Materials  34(2): 195-206.
   Abstract – Atomic force microscopy (AFM), nitrogen and argon porosimetry, high resolution electron microscopy (HREM) together with powder X-ray diffraction (XRD) have been used to characterize a heat stable pillared interlayered clay (PILC). The clay catalyst was prepared by replacing the charge compensating cations in a Texas Na-montmorillonite with [Al13O4(OH)24(H2O)12]7+ (Al13) ions from an aluminum chlorhydroxide (ACH) solution using an ACH/clay and water/clay (wt/wt) ratio of 1.0 and 100, respectively. The reaction product was extensively washed with deionized water to ensure a complete hydrolysis of the interlamellar Al species and formation of stable Keggin ions. Molecular scale AFM images have shown the absence of adsorbed surface Al species indicating that, at the synthesis conditions used, the added Keggin ions are located in the clay interlamellar space where they are believed to hydrogen bond with basal oxygens. Model calculations have been used to estimate its dimensions to be 0.97 nm×(0.89 nm×1.09 nm). After heating in air at 500°C, the Keggin ions lose their water ligands forming shorter Al13 blocks 0.84 nm in height that become the structure supporting pillars. Thermal and hydrothermal treatments can transform ACH powders into gamma-alumina, a transition phase having a spinel-like structure, as identified by XRD. However, gamma-alumina formation could not be observed by XRD in similarly treated Al13-PILCs. Pore size distribution data from nitrogen and argon porosimetry experiments have indicated that the Al13-PILC under study is essentially a microporous material with a pore volume (PV) in the 0.10–0.12 cm3 g−1 range and with a Langmuir surface area (SA) of 300–320 m2 g−1, well in agreement with SA values measured from density functional theory methods based on cylindrical-like pore geometry. Discrepancies between calculated and measured SA and PV values have been attributed mainly to the incomplete expansion of all the Na-montmorillonite platelets, as observed in HREM images. Moreover, AFM and HREM images have shown that steam aging for 5 h at 760°C with 100% steam decomposes the clay aluminosilicate layers, thus providing evidence that during this hydrothermal treatment the Al13-PILC structure will collapse irrespective of the stability of its Al13 pillars.
9. S. M. Thomas, J.A. Bertrand, M.L. Occelli, J.M. Stencel and S.A.C. Gould. (1999). The Synthesis and Characterization of Expanded Smectites Containing Trinuclear Co-Complexes. Chemistry of Materials 11(4): 1153-1164.
   Abstract – Several smectites were pillared with metal complexes, such as the trimeric cobalt cation [Co3(OC2H4NH2)6]3+ and its divalent analogue [Co3(OC2H4NH2)6]2+, containing clusters of one to three cobalt cations separated by bridging ligands. In pillared interlayered clays (PILC) prepared by reacting Na-montmorillonite with Co3(H2NCH2CH2O)6(ClO4) 3 solutions, pillar orientation is controlled by the trimer concentration in the interlamellar space. Initially the Co trimer (0.72 nm × 0.48 nm in size) lies parallel to the clay surface. Then as more trimer is introduced, it forms an angle of ~48 relative to the clay surface that increases to 61 as the clay reached its highest Co loadings. FT IR and FT LRS spectra together with chemical analysis have been used to ascertain the presence of pristine trimer ions in all the PILC samples. XPS results indicate that reduction of the trivalent trimer to its divalent analogue [Co3(OC2H4NH2)6]2+ has occurred during PILC preparation. XRD patterns and molecular scale AFM images have revealed the presence of trimer molecules also on the clay external surface. Thermogravimetric analyses of [Co3(OC2H4NH2)6]3+ PILC are all qualitatively similar despite the variance in d spacing seen at different Co trimer loadings. Ligand decomposition occurs in two steps in the 250-450 C temperature range. At T > 250 C, ligand losses cause a total structural collapse of the pillared structure. In contrast, prior to 250 C, the interlamellar expansion decreases and the trimer seems to adopt a position parallel to the clay surface. The d spacing changed very little over the temperature range where initial ligand losses occur. Partial decomposition of the trimeric complex can thus be achieved while a 0.45 nm layer expansion is maintained, indicating that these materials could be considered as cobalt catalysts for low temperature (T < 200 C) applications.
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10. S.M. Thomas, J.A. Bertrand, M.L. Occelli, F. Huggins, S.A.C. Gould. (1999). Microporous montmorillonites expanded with alumina clusters and M[mu-OH)Cu(mu-OCH2CH2NEt2)] (6)(ClO4)(3), (M = Al, Ga, and Fe), or Cr[(mu-OCH3)(mu-OCH2CH2NEt2)CUCl](3) complexes. Inorganic Chemistry   38(9): 2098-2105.
    Abstract – Expanded clays bipillared with [Al13O4(OH)24(H2O)12]7+ ions and with hexameric Cu complexes such as M[(-OH)Cu(-OCH2CH2NEt2)]6(ClO4)3, or with M[(-OH)Cu(-OCH2CH2NEt2)]6(PF6)3 where M = Fe, Al, Ga, form microporous materials whose stability and microporosity depend mainly on the identity of the hexamer central metal atom. In fact, a general decrease in thermal stability, interlamellar heights, surface areas, and pore volumes was noted when, in the (Cu,M) hexamer, M changed from gallium to aluminum to iron. Mossbauer results have indicated that only Fe3+ in octahedral coordination is present in the iron-containing bi-PILC samples (bi-PILC = bipillared interlayered clays). It is believed that metals such as Fe3+ and Cu2+ can interact with the interlamellar Keggin ions thereby decreasing the stability of the alumina pillars. In contrast, the intermediate Al13-PILC structure is least affected when the more stable Cr complex is used. Bi-PILC materials containing 2.7-3.4% Cr stable to 500 C have been obtained. The low polarity of the chosen solvent (acetonitrile) appears to inhibit the back-exchange of the intermediate PILC’s Keggin ions with the hexameric Cu complexes. Elemental analysis together with XRD results suggests that the primary intercalation pathway was diffusion or ion exchange when Cr[(-OCH3)(-OCH2CH2NEt2)CuCl]3 or M[(-OH)Cu(-OCH2CH2NEt2)]6(ClO4)3, respectively, was used. In all preparations, bi-PILC were produced containing complexes that suffered ligand losses during the synthesis reaction. Molecular scale AFM images have shown that these complexes can be found also outside the clay interlamellar space.
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11. S.A.C. Gould, K. Tran*, J. Spagna, A. M. F. Moore, J.B. Shulman*. (1999). Short and Long Range Order of the Morphology of Silk from Latrodectus hesperus (Black Widow) as Characterized by Atomic Force Microscopy. International Journal of Biological Macromolecules 24: 151-7.
    Abstract – The surfaces of both stretched and unstretched silk threads from the cobweb weaver, Latrodectus hesperus (Black Widow) have been examined by atomic force microscopy (AFM). AFM images of cobweb scaffolding threads show both unordered and highly ordered regions. Two types of fibers within the threads were observed: thicker (~300 nm in diameter) fibers oriented parallel to the thread axis and thinner (10–100 nm) fibrils oriented across the thread axis. While regions which lacked parallel fibers or fibrils were observed on threads at all strain values, the probability of observing fibers and/or fibrils increased with strain. High-resolution AFM images show that with increasing strain, both mean fiber and fibril diameters decrease and that fibrils align themselves more closely with the thread axis. The observation of fibers and fibrils within the cobweb threads has implications for current models of the secondary and tertiary structure and organization of spider silk.
12. S.A.C. Gould, J.B. Shulman*, D.A. Schiraldi, M.L. Occelli. (1999). Atomic Force Microscopy (AFM) Studies of Liquid Crystalline Polymer (LCP) Surfaces. Journal of Applied Polymer Science  74: 2243-2254.
    Abstract – An atomic force microscope (AFM) operating in tapping or contact mode was used to study the surface topography and the molecular organization of Vectra-A and Vectra-B films. Large-scale (15 × 15 m) AFM images revealed that ribbonlike fibrils with a width/height 1.0 are the dominant surface features of these liquid crystalline polymers (LCPs). The region of local disorder, surface debris, and interfibrillar debris as well as possible amorphous regions were observed in both LCP samples. Large fibrils, 5.0-10.0 m in width, can be thought of as formed by smaller microfibrils capable of forming ordered structures. Microfibrils can bend upward, forming raised surface features; bend inward, originating cracks 1-2 m wide on the film surface; or divide and subdivide into smaller units. Longitudinal and lateral stresses are believed responsible for the variation in fibril size, shape, and orientation. AFM images containing molecular-scale details showed that microfibrils consists of chains of molecules coiled around a central axis and that they can be only about 2.0 nm wide. These submicron surfaces consist of white spots (representing molecules) that can form ordered structures or that can cluster to form agglomerates distributed in a random manner. Submicron fibrils are believed to represent the LCP basic structural unit. AFM results indicate that the surface topography of Vecta-B is more ordered and uniform than is the one observed for Vectra-A. Seemingly, amorphous particles form debris on Vectra-A surfaces. Short rods oriented crosswise on the fibril surface are instead what increases the Vectra-B roughness. These LCPs can have a surface topography similar to the one observed in AFM images of a spiderweb. However, the spiderweb fibrils are formed by more uniform microfibrils that are oriented parallel to each other.
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13. S.A.C. Gould, D.A. Schiraldi, M.L. Occelli. (1998). Imaging the Surface of PET Films. Chemtech  Jan: 35-39.
14. S.A.C. Gould, D. Schiraldi and M.L. Occelli. (1997). Analysis of Polyethylene Terephthalate (PET) Films using Atomic Force Microscopy. J. of Applied Polymers 67(7): 1237-43.
15. S.A.C. Gould. (1996). All the Threads, All the Time. OS/2 Mag 1: 28-34.
16. M.L. Occelli, S.A.C. Gould, F. Baldiraghi and S. Leonicini. (1996). Atomic Force Imaging and Porosimetry Characterization of a Fluid Cracking Catalyst (FCC) Before and After Thermal and Hydrothermal Treatments. Proc. of the Am. Chem. Soc. Petro. Div. Conf.: Fluid Cracking Catalysts (M.L. Occelli, P. O’Connor, eds.) : 203-216.
17. H. Kinney*, M.L.Occelli and S.A.C. Gould. (1996). Surface Roughness Measurements of Vanadium Contaminated Fluidized Cracking Catalysts by Atomic Force Microscopy. Proc: Microscopy and Microanalysis 1996 Conf. (G.W. Baily, et. al., eds.)
18.  M.L. Occelli, S.A.C. Gould, J.M. Tsai* and B. Drake. (1995). Examination of Vanadium-contaminated Pillared Rectorite Catalyststs with the Atomic Force Microscope. J. of Molecular Catalysis A: Chemical 100: 161-166.
    Abstract – When vanadyl naphthanate solutions in toluene are used to V-contaminate rectorite pillared with alumina clusters, V migration from the pillared clay microporous structure to the outer surface occurs during the thermal treatments used in catalyst preparation. The presence of V on the clay catalyst surface can be inferred from atomic force microscopy (AFM) images showing a marked decrease in surface roughness resulting from the presence of vanadia. AFM images show what are believed to be islands as well as stacks of vanadia layers on the clay surface. Irrespective of the thermal and hydrothermal treatments used to prepare these catalysts, atomic scale details of V-contaminated clay samples retain the surface parameters characteristic of the silicate layers in smectites. These results suggest that V on the clay surface is present as V=O groups attached to the three basal oxygens of the SiO4 units that form the clay silicate layers. These VO units form an hexagonal arrangement of white spots having next neighbor (dc) and lateral distance (dl) of 5.2 Å and 9.1 Å, respectively.
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19. F.E. Huggens, M.L. Occelli, J.M. Dominguez, J.M. Sencel and S.A.C. Gould. (1995). Characterization of Iron Impurities in Pillared Rectorite Catalysts. Microporous Materials  4: 291-300.
    Abstract – X-Ray mapping, energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS) and Mössbauer effect spectroscopy (MES) together with atomic force microscopy (AFM) have been used to examine iron impurities in natural rectorites pillared with alumina clusters. EDS results have shown that Fe is preferentially located near the uppermost 0.5 m of the clay crystal and that high-temperature thermal or hydrothermal treatments reduced by almost 50% the Fe concentration in this region. However, the presence of surface Fe impurities could not be inferred from AFM images nor by XPS measurements indicating that Fe, on the top 2.5 nm of the clay crystal is present below the 0.5 wt.-% level. Mössbauer spectroscopy has shown that, before pillaring, iron in rectorite is predominantly ferric and in octahedral coordination. After pillaring and upon heating, a fraction of the ferric iron exhibits a lower coordination number, and eventually, at the highest temperatures, hematite is formed from the clay structure. Thus, Fe appears to migrate from the octahedral to the tetrahedral layer and onto the clay silicate layer where it can form hematite. Hydrothermal treatment at 760°C converts the octahedral ferric iron in the pillared clay to octahedral ferrous, tetrahedral ferric, and hematite, in approximately equal amounts.
20. S.A.C. Gould, M.L. Occelli. (1995). Analysis of fluidized cracking catalysts by atomic force microscopy. Proc: Microscopy and Microanalysis 1995 Conf. 424-425.
21. M.L. Occelli, S.A.C. Gould. (1994). The Surface Architecture of an FCC Catalyst. Chemtech 24(5): 24-27.
22. M.L. Occelli, S.A.C. Gould and B. Drake. (1994). Atomic Scale Imaging of Pillard Rectorite Catalysts with the Atomic Force Microscope. Microporous Materials  2: 205-215.
23.  M.L. Occelli, S.A.C. Gould, and B. Drake. (1994). Atomic Force Microscopy Examination of the Topography of a Fluidized Cracking Catalyst Surface. Proc. of the Am. Chem. Soc. Petro. Div. Conf.: Fluid Catalytic Cracking III (G.W. Baily, A.J, eds.) Garrat-Reed (SF:SFP): 272-293.
24. A.N. Nguyen*, A.M.F. Moore, S.A.C. Gould. (1994). High Resolution Images of Cobweb Threads of Black Widow Spider Latrodectus Mactans by Atomic Force Microscopy. Proc. of the Microscopy Society of America 1994 Conf. (G.W. Baily, A.J, eds.) Garrat-Reed (SF:SFP): 1074-75.
25. M.L. Occelli, B. Drake, and S. A. C. Gould. (1993). Characterization of Pillared Montmorillonites with the Atomic Force Microscope (AFM). Journal of Catalysis 142: 337-348.
    Abstract – An atomic force microscope (AFM) has been used to investigate the surface features of samples of Ca-bentonite from Texas and Na-bentonite from Wyoming that were pillared with alumina clusters. Atomic-scale-resolution images of the clay surface consist of hexagonal arrays of bright spots. The nearest-neighbor distance in the two parent clays was found to be consistently greater after pillaring in numerous images, suggesting that the bulky Al13 clusters stretched the clays′ silicate layers. A number of images showed possible atomic resolution of oxygen atoms on the clay surface, suggesting that it can be possible to obtain resolution below the scale of the unit cell. Molecular-scale-resolution images of the cross-sectional area of extrudates formed using pillared Wyoming bentonite powder showed platelets about 9.0 Å apart, in agreement with X-ray diffraction (XRD) results. The presence of alumina debris or clusters on the silicate layer was not observed in any image examined, suggesting that the expended clay coking tendency during gas oil cracking can be attributed mainly to the strong Lewis-type acidity of the alumina pillars between the clay silicate layers.
26. J. Garnaes, H. Lindgreen, P.L. Hansma, S.A.C. Gould and P.K. Hansma. (1992). Atomic Force Microscopy of Ultrafine Clay Articles. Ultramicroscopy  42B: 1428-32.
    Abstract – Atomic force microscopy (AFM) was applied to small clay particles of illite/smectite from North Sea Jurassic rock, which is the most important source rock for North Sea oil. The clay is dispersed in water and a drop is then dried on graphite or mica. The images show flat particles with areas from 1000 to 30 000 nm2 and sides of equal length. Details of the surface topography of the illite/smectite particles can be imaged and it is possible to resolve both the distance between unit-cells (0.51 nm) and the distance between oxygen atoms (0.3 nm). Systematic thickness measurements show that the particles are 2 to 4 nm thick on average. The measurements also showed that the material contains 1 nm thick single silicate 2:1 layers. Information about the thickness, size and surface structure of mixed layers of illite/smectite can provide new insight into their formation.
27. H. Lindgreen, J. Garnaes, P.L. Hansen, F. Besenbacker, E. Laegsgard, I. Stensgaard, S.A.C. Gould, and P.K. Hansma. (1991). Ultrafine Particles of North Sea Illite/Smectite Clay Minerals Investigated by STM and AFM. American Mineralogist 76: 1218.
28. H.G. Hansma, A.L. Weisenhorn, S.A.C. Gould, R.L. Sinsheimer, H.E. Gaub, G.D. Stucky, C.M. Zaremba, and P.K. Hansma. (1991). Progress in Sequencing DNA with an Atomic Force Microscope. J. Vac. Sci. Tech. B9:
    Article: URL not found
29. J. Garnaes, S.A.C. Gould, P.K. Hansma, and R.V. Coleman. (1991). Atomic Force MIcroscopy of Charge-Density Wave and Atoms on 1T – TaSe2, 1T – TaS2, 1T – TiSe2 and 2H – NbSe2. J. Vac. Sci. Technol. B.
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30. H.G. Hansma, S.A.C. Gould, P.K. Hansma, J.A.N. Zasadzinski, M.L. Longo, and H.E. Gaub. (1991). Imaging and Creating Holes in Organic Monolayers with the Atomic Force MIcroscope. Langmuir  7: 1051.
31. A.L. Weisenhorn, M. Eggar, F. Ohnesorge, S.A.C. Gould, S.P. Heyn, H.G. Hansma, R.L. Sinsheimer, H.E. Gaub, and P.K. Hansma. (1991). Molecular-Resolution Images of Langmuir Blodgett Films and DNA by Atomic Force Microscopy. Langmuir   7: 8.1282.
32. A.L. Weisenhorn, M. Eggar, F. Ohnesorge, S.A.C. Gould, S.P. Heyn, H.G. Hansma, R.L. Sinsheimer, H.E. Gaub, and P.K. Hansma. (1991). Using Force Modulation To Image Surface Elasticities With The Atomic Force Microscope. Nanotechnology   2(103).
    Abstract – Using a new mode of scanning, the force modulation mode, surfaces are imaged by the atomic force microscope. The new contrast mechanism relies on variation in the surface elasticity. The cross section of a carbon fibre and epoxy composite is imaged, showing contrast between the two materials. Surface elasticity variations across the cross section of the fibre are revealed. A lateral modulation mode is used to highlight atomic steps in gold.
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33. M.L. Occelli, S.A.C. Gould, G.D. Stucky. (1984). The Study of the Surface Topograph of Microporous Materials Using Atomic Force Microscopy. Zeolites and Related Microporous Mat.: State of the Art. 1994: 485-491.