Optical microscopy near field scanning

Near-field scanning optical microscopy is a scanning probe technique that enables optical measurements to be conducted with very high spatial resolution [7-9]. NSOM overcomes the diffraction barrier that restricts the spatial resolution in conventional optical measurements and provides both optical and topographical information on samples with nanometric spatial resolution. 

While the experimental details involved in implementing NSOM can be found elsewhere [7,8], it is instructive to briefly discuss the two main obstacles that must be overcome in order to conduct NSOM measurements. These revolve around aperture formation and implementing a feedback system for tip-sample distance control. For the former, as in all scanning probe techniques, the quality of the measurements is in large part dictated by the quality of the probe. For the latter, as the schematic in Fig. 1 suggests, high resolution requires that the NSOM probe be maintained within nanometers of the sample surface. 

Principles and Characteristics Diffraction limits the spatial resolution of conventional optical microscopy instruments. In practice, the resolution limit is approximately 0.6A., i.e. about 0.5 pm for optical microscopes. Resolution in far-field optical microscopy techniques may be improved (though slightly) by the application of UV (cfr. Chp. 5.3.2) or confocal laser scanning (c/r Chp. 5.3.4). For confocal laser imaging with green light (A. = 500 nm), resolution is limited 

The theory of NSOM is somewhat similar to that of STM, with transport of light (or photons) replacing transport of electrical current (electrons). Instead of the Schrodinger equation, the Maxwell equations for the electromagnetic field must be solved near tip and sample, taking into account the local electromagnetic properties of each medium [270]. The resolution is lower than that attainable with STM. 

Since resolution in the far field is limited by wavelength, conventional optics in the radiofrequency (RE) through far-infrared (PIR) spectrum cannot resolve very small features. With the advent of near-field microscopy, RE and FIR microscopy have gained more attention [343]. 

Near-field microscopy (from visible to high-frequency) was recently reviewed [270,343,349-352]. 

Raman-NSOM has been demonstrated for a variety of samples, including adsorbed dye molecules [358-361] and polymers [357]. By combining NSOM with SERS molecular spectroscopy and imaging with a lateral resolution of 70 nm is possible [337], Also IR-NSOM can be used for chemical imaging [338], Thin him analysis benefits from NSOM. The technique has been used to probe the excitonic transitions in J-aggregates of l,l -diethyl-2,2-cyanineiodide grown in poly(vinyl sulfate) thin films [290]. 

As the fine optical fiber can be inserted close to an object in water, the first applications of this instrument have been directed toward high resolution observation of tissue sections and living cells [167]. The associated photon tunneling microscope has been applied to polymer surfaces [168]. In principle all the contrast modes of normal optical microscopy can be used in the near-field and there may be other unique contrast mechanisms [169]. The light signal can be analyzed spectroscopically to give 

Knutson et al. used NSOM to study the reactivity of nanoporous aluminium alloy 2024 [30]. In particular they used a modified version for determining concurrent topography, fluorescence intensity and fluorescence spectroscopy on alloy surface. For microelectronic and micromagnetic applications, planar and homogeneous surfaces can be formed to provide lower reactivity and longer component working fives. Typically local reactivity varies due to heterogeneous polycrystaUine and local impurities in surfaces. 

Topographic and fluorescence NSOM image for aluminium sample exposed to aqueous solutions with fluorescein. 

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Betzig E, Finn P L and Weiner J S 1992 Combined shear force and near-field scanning optical microscopy/4pp/. Phys. Lett. 60 2484... 

Fischer U Ch, Durig U T and Pohl D W 1988 Near-field scanning microscopy in reflection Appl. Phys. Lett. 52 249 Cline J A, Barshatzky FI and Isaacson M 1991 Scanned-tip reflection-mode near-field scanning optical microscopy Ultramicroscopy 38 299... 

Betzig E and Chichester R J 1993 Single molecules observed by near-field scanning optical microscopy Science 262 1422... 

Figure Bl.22.11. Near-field scanning optical microscopy fluorescence image of oxazine molecules dispersed on a PMMA film surface. Each protuberance in this three-dimensional plot corresponds to the detection of a single molecule, the different intensities of those features being due to different orientations of the molecules. Sub-diffraction resolution, in this case on the order of a fraction of a micron, can be achieved by the near-field scaiming arrangement. Spectroscopic characterization of each molecule is also possible. (Reprinted with pennission from [82]. Copyright 1996 American Chemical Society.)...

Hamann H F, Gallagher A and Nesbitt D J 1999 Enhanced sensitivity near-field scanning optical microscopy at high spatial resolution Appl. Phys. Lett. 75 1469-71... 

Higgins D A and Barbara P F 1995 Excitonic transitions in J-aggregates probed by near-field scanning optical microscopy J. Chem. Phys. 99 3-7... 

Hollars C W and Dunn R C 2000 Probing single molecule orientations in model lipid membranes with near-field scanning optical microscopy J. Phys. Chem 112 7822-30... 

Hayazawa, N Inouye, Y. and Kawata, S. (1999) Evanescent field excitation and measurement of dye fluorescence using a high N.A. objective lens in a metallic probe near-field scanning optical microscopy J. Microsc., 194, 472-476. 

Near-Field Scanning Optical Microscopy (NSOM)... 

For samples thicker than the depth of field, the images are blurred by out-of-focus fluorescence. Corrections using a computer are possible, but other techniques are generally preferred such as confocal microscopy and two-photon excitation microscopy. It is possible to overcome the optical diffraction limit in near-field scanning optical microscopy (NSOM). 

Dunn R. C. (1999) Near-Field Scanning Optical Microscopy, Chem. Rev. 99, 2891— 927. 

Clancy, C. M. R. Krogmeier, J. R. Pawlak, A. Rozanowska, M. Sarna, T Dunn, R. C. Simon, J. D. Atomic Force Microscopy and Near-Field Scanning Optical Microscopy Measurements of Single Human Retinal Lipofuscin Granules. J. Phys. Chem. B 2000, 104, 12098-12101. 

Near-field scanning optical microscopy (NSOM) allows an extension of optical microscopy to near that of electron microscopy. The central feature is the optical element that is similar. 

Brighter, tunable ultrafast light sources would benefit many of the areas discussed in the report, particularly infrared-terahertz (between visible light and radio waves) vibrational and dynamical imaging, near-field scanning optical microscopy (NSOM), and X-ray imaging. 

SOURCE Reprinted with permission from Teetsov, J.A. and D.A. Vanden Bout. 2001. Imaging molecular and nanoscale order in conjugated polymer thin films with near-field scanning optical microscopy. J. Am. Chem. Soc. 123 3605-3606. Copyright 2001 American Chemical Society. 

Schaller, R.D., L.F. Lee, J.C. Johnson, L.H. Haber, R.J. Saykally, J. Vieceli, I. Benjamin, T.-O. Nguyen, and B.J. Schwartz. 2002. The nature of interchain excitations in conjugated polymers Spatially varying interfacial solvatochromism of annealed MEH-PPV films studied by near-field scanning optical microscopy (NSOM). J. Phys. Chem. B 106 9496-9506. 

SOURCE Reprinted with permission from Betzig, E., and R J. Chichester. 1993. Single molecules observed by near-field scanning optical microscopy. Science 262 1422-1425. Copyright 1993 American Association for the Advancement of Science. 

SOURCE Reprinted with permission from Higgins, D.A., D.A. Vanden Bout, J. Kerimo, and P.F. Barbara. 1996. Polarization-modulation near field scanning optical microscopy of mesostructured materials. J. Phys. Chem. 100 13794-13803. Copyright 1996 American Chemical Society. 

Hwang, J., L.K. Tamm, C. Bohm, T.S. Ramalingam, E. Betzig, and M. Edidin. 1995. Nanoscale complexity of phospholipid monolayers investigated by near-field scanning optical microscopy. Science 270 610-614. 

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