Anal. Chem. 1997, 69, 749-754
Optical Spectroscopy and Laser Desorption on a Nanometer Scale Dieter Zeisel, Bertrand Dutoit, Volker Deckert, Thomas Roth, and Renato Zenobi*
Laboratorium fu¨ r Organische Chemie, ETH Zu¨ rich, Universita¨ tsstsse 16, CH-8092 Zu¨ rich, Switzerland
Optical fiber tips for scanning near-field optical microscopy (SNOM or NSOM) produced by etching with the protection layer method and subsequent metalization show an optical transmission of up to 0.5%. This throughput is 2-4 orders of magnitude higher than that of conventional pulled fiber tips. The high light transmission permits SNOM-based surface analytical and spectroscopic applications with high spatial resolution (10. Rapid heating of chemically etched probes with pulsed laser radiation can be used for thermal desorption of molecules from organic crystals and for modification of polymer surfaces. A lateral resolution of as little as 75 nm fwhm is achieved in the latter experiments. New and powerful methods of micro- and nanoanalysis for the investigation of clusters, domains, and aggregates of surface adsorbates require both high spatial resolution and high sensitivity. At the same time such analyses should be performed at ambient conditions and nondestructively. These requirements are met by scanning near-field optical microscopy (SNOM or NSOM),1-3 a method that circumvents the fundamental diffraction limit by scanning a nanometer-sized optical probe in the near field (roughly several nanometers) over the surface under investigation. The emitted, reflected, or scattered light from the sample is collected and imaged with conventional far-field optics onto a sensitive photodetector. The image is built up in a sequential manner point by point. The unique advantage of this technique in comparison to other scanning probe microscopies,4 such as scanning tunneling microscopy (STM) or atomic force microscopy (AFM), is the addition of a spectral dimension, which allows chemical identification of surface adsorbates. The sensitivity reached so far has permitted the detection of fluorescence from single molecules5-8 with a high signal-to-noise ratio. The meas(1) Betzig, E.; Trautman, J. K.; Harris, T. D.; Weiner, J. S.; Kostelak, R. L. Science 1991, 251, 1468-1470. (2) Pohl, D. W.; Denk, W.; Lanz, M. Appl. Phys. Lett. 1984, 44, 651-653. (3) Lewis, A.; Isaacson, M.; Harootunian, A.; Muray, A. Ultramicroscopy 1984, 13, 227-231. (4) Louder, D. R.; Parkinson, B. A. Anal. Chem. 1995, 67, 297A. (5) Betzig, E.; Chichester, R. J. Science 1993, 262, 1422-1425. (6) Ambrose, W. P.; Goodwin, P. M.; Martin, J. C.; Keller, R. A. Phys. Rev. Lett. 1994, 72, 160-163. S0003-2700(96)00943-2 CCC: $14.00
© 1997 American Chemical Society
ured fluorescence lifetimes vary with the position of the aluminized probe relative to the position of the molecule.9-11 Few investigations combined SNOM with a spectroscopic analysis of surfaces. Spectrally and spatially resolved analysis of single quantum wires12 and multiple quantum well structures13 at low temperatures showed the origin of the photoluminescence along excitons localized at interfaces. Trautman et al. observed narrowing effects and shifts of the bands of single-molecule spectra in the near field.14 Spatially resolved near-field fluorescence spectra of molecular crystals have been obtained with a lateral resolution of ∼100 nm to identify nanoaggregates15 and crystals of different crystallographic forms,16 repectively. SNOM has also been applied to ablation of photoresists using micropipets instead of fibers17 and to Raman imaging.18 However, true chemical analysis and identification has never been achieved. In this article we describe the use of chemically etched and metalized optical fiber tips for scanning near-field optical spectroscopy and demonstrate thermal desorption of molecules from surfaces as well as surface modification on a nanometer scale. EXPERIMENTAL SECTION A commercially available near-field scanning optical microscope (Aurora, Topometrix, Santa Clara, CA) is the basic unit of our experimental setup (see Figure 1). The sample stage is scanned relative to the tip in order to maintain a fixed optical pathway. Laser light penetrating through the nanometer-sized aperture of the probe illuminates the surface under investigation. The emitted or scattered light from the sample is collected with high numerical aperture microscope objectives either in transmission or in reflection (not shown) and imaged via a dichroic mirror onto a 200 µm silica glass fiber or onto a photomultiplier, respectively. (7) Dunn, R. C.; Allen, E. V.; Joyce, S. A.; Anderson, G. A. Xie, X. S. Ultramicroscopy 1995, 57, 113-117. (8) Meixner, A. J.; Zeisel, D.; Bopp, M. A.; Tarrach, G. Opt. Eng. 1995, 34, 2324-2332. (9) Ambrose, W. P.; Goodwin, P. M.; Martin, J. C.; Keller, R. A. Science 1994, 265, 364-367. (10) Xie, X. S.; Dunn, R. C. Science 1994, 265, 361-364. (11) Bian, R. X.; Dunn, R. C.; Xie, X. S.; Leung, P. T. Phys. Rev. Lett. 1995, 75, 4772-4775. (12) Harris, T. D. et al. Appl. Phys. Lett. 1996, 68, 988-990. (13) Hess, H. F.; Betzig, E.; Harris, T. D.; Pfeiffer, L. N.; West, K. W. Science 1994, 264, 1740-1745. (14) Trautman, J. K.; Macklin, J. J.; Brus, L. E.; Betzig, E. Nature (London) 1994, 369, 40-42. (15) Birnbaum, D.; Kook, S.-K.; Kopelman, R. J. Phys. Chem. 1993, 97, 30913094. (16) Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F. J. Phys. Chem. 1996, 100, 11843-11849. (17) Lewis, A.; Liebermann, K. Anal. Chem. 1991, 63, 625-638. (18) Jahncke, C. L.; Paesler, M. A.; Hallen, H. D. Appl. Phys. Lett. 1995, 67, 2483-2485.
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Figure 2. Electron micrograph of a chemically etched fiber tip. Chemical etching produces sharp tips with short taper regions and large cone angles. For SNOM applications, the tips are coated with a thin layer of aluminum to prevent light leakage except at the very end, where a tiny (
