Fundamentals of nanoscience Spectroscopy of nano-objects Mika Pettersson
1. Non-spatially resolved spectroscopy Traditionally, in spectroscopy, one is interested in obtaining information on the energy level structure or dynamics of the studied systems whereas spatially resolved information is of less importance Can this be relevant in nanoscience??? Much of nanoscience deals with imaging of nanostructures with great spatial resolution But still One of the main aspects in the whole nanoscience is that the properties of the systems depend on their size, especially in the range 1 atom 100 nm.
A spectroscopic experiment contains information on the whole ensemble of objects in the probed volume -distribution of particles/objects -inhomogeneity How to relate signal in the spectrum to the particular particle in the sample -combine with information from sample preparation -combine with information from other means of sample characterization Example: electronic level structure of metal particles depends on the particle size
The colour of the cup originates from colloidal gold and silver nanoparticles Lycurgus cup Source: British museum, www.britishmuseum.org/explore/highlights/highlight_objects/pe_mla/t/the_lycur gus_cup.aspx
Colour is due to electronic absorption in the visible range Absorption shifts with the average particle size Au/Ag, increasing Au concentration Au, increasing aspect ratio Ag, increasing lateral size Materialstoday Feb. 2004, 26 Simultaneous absorption measurement and characterization of the particle size/shape gives specific information on the electronic energy level structures of the nanoparticles
2. Example: Electronic absorption and Raman spectra of carbon nanotube samples Individual peaks belong to individual tube types Careful analysis and combination of methods can yield tube-specific information
2. Spatially resolved spectroscopy More powerful tool is offered by a combination of spectroscopy and microscopy -spectrally and spatially resolved information 2.1 Basics of optical microscopy A lens system is used to form a magnified image of the object Magnification cannot be increased arbitrarely -diffraction limit
Imaging of a point source through a circular aperture (e.g. lens) yields not a point-like image but a characteristic pattern of intensity distribution (Airy pattern). This is due to diffraction and it sets a limit to the practical resolution achieved
Ref. www.olympusmicro.com Numerical aperture (NA)
Airy disc size determines the resolution Measure of the disc size: d d = 0.61λ = nsin µ 0.61λ NA λ 2 ( in best case)
Microscopy can yield spatially resolved information with a resolution of ~200-300 nm in the best case -visualization of micro/nanostructures, cells, bacteria etc. The idea of combining microscopy and spectroscopy is to use spectral information to determine the spatial distribution of specific molecules or chemical groups
2.1.1 Fluorescence microscopy -Emission from dye molecules, quantum dots or naturally fluorescent chemical groups is detected through a microscope -very high sensitivity -good for biological samples More on this in the biology part
2.1.2 Micro raman spectroscopy Raman spectrum is measured point by point from the sample Note, Raman spectrum contains information on the chemical composition choose a peak corresponding to a particular chemical group (for example OH) plot the peak intensity at each spot Raman image raster scanning (x-y) spectrum
Raman images of carbon nanotube bundles Vertical polarization Horizontal polarization 15 µm Phys Rev. Lett. 90. 095503 (2003)
2.2 Near field optical microscopy The dimensions of most nano-objects are below the diffraction limit can optical methods go below diffraction limit yes, near field optics Idea: the sample is illuminated by placing it within a few nm of an aperture Scanning near field optical microscope (SNOM)
Practical solution: tapered optical fiber
Sample or tip scanning
Applications Single molecule imaging Veerman et al. J. Microsc. 194, 477 (1999)
Apertureless SNOM: tip enhanced Raman spectroscopy
Mapping of individual carbon nanotubes N. Anderson, A. Hartschuh, S. Cronin, L. Novotny. J. Am. Chem. Soc. 127, 2533 (2005)
Conclusions Non-spatially resolved optical spectroscopy can yield specific information on nano-objects when combined with information from sample preparation and/or characterization techniques Combining spectroscopy with microscopy yields chemical information with a spatial resolution down to diffraction limit Scanning near field optical microscopy (SNOM) provides spatial resolution below the diffraction limit Combining SNOM with spectroscopy provides chemical information with high spatial resolution Apertureless SNOM combined with Raman spectroscopy is a particularly promising technique e.g for carbon nanotube research