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Nano Microscopy at the NDRL

To study the objects and processes in the nanoscale regime, the Radiation Laboratory has added scanning microscopes to its arsenal of state-of-the-art facilities:

  1. An Atomic Force/Scanning Tunneling Microscope (AFM/STM) from Digital Instruments (Multimode Nanoscope IIIA)
  2. A Near-field/confocal Scanning Optical Microscope (NSOM) from Nanonics Imaging Ltd.

The AFM system is capable of imaging surface topography features on scales from a few angstroms to 100 microns. The technique involves scanning the sample surface using a sharp silicon nitride crystal tip attached to a cantilever. The tip is held within a few nanometers above the surface by using a feedback mechanism. As the tip is scanned over the surface, topographic features on the sample surface bend the cantilever in response to van der Waals forces between the tip and the surface. The bending is detected by following a precisely focused diode laser beam onto the top of the cantilever reflected towards a position sensitive detector (PSD).

The AFM system is also capable of imaging the topography using tapping and non-contact modes. In these modes, pure silicon is the tip material and the cantilever is oscillating at its resonance frequency. As the tip approaches the sample surface, the amplitude and the resonance frequency change drastically. This provides the feedback to the scanner. Imaging in tapping mode may be advantageous since it eliminates adhesion and "scratching" effects of soft samples. Imaging under a liquid medium is possible for both the contact and non-contact/tapping modes using a special cell.

Scanning Tunnel Microscope: A special head is available for using the instrument as a scanning microscope (STM). In STM mode, a very sharp Pt or Pr/Ir tip is used as a probe. The sample must either be a good conductor or a semiconductor. A bias voltage is applied between the tip and the sample. As the tip approaches within a very short distance of the sample surface, electrons from the tip tunnel through the gap and reach the sample surface. This tunneling current is used to provide the feedback to the scanner. Since more than 90% of the tunneling current comes from a single atom from the tip, atomic resolution is achieved in STM. In addition to imaging the surface, the current-voltage characteristics can also be studied.

Near-field Scanning Optical Microscope (NSOM): A state-of-the-art NSOM instrument is now being assembled at the Radiation Laboratory. NSOM is a relatively new technique designed to overcome the diffraction-limited resolution of the light. The Nanonics instrument provides simultaneously topographic (AFM) and optical spectroscopic information of the same area. The optical spectroscopies include Transmission-Absorption, Fluorescence and Raman scattering.

The NSOM tip is a single-mode optical fiber. The walls of the tip are coated with aluminum leaving a 50 nm (or larger) aperture at the tip. Since the size of the aperture is much smaller than the wavelength of UV-Visible-NIR light, it cannot freely propagate through the aperture. The light at the very end of the tip forms an evanescent wave, which resembles a drop of light. In order for the light to propagate freely through the sample surface, the tip must be brought within at least half the distance of the aperture size (also labeled the near field). In the Nanonics system, the tip is bent near its end to provide normal force distance regulation. Therefore, topographic imaging of the surface is possible in contact, non-contact and tapping modes. The lateral resolution in the topographic image taken using the fiber optic is inferior to the silicon tip of the Digital Instruments AFM since the fiber tip is much larger than the silicon tip. However, the height information is precise.

The NSOM scan head is mounted on a Zeiss Axioplot dual microscope. The microscope is needed to collect the light and direct it to the detector. Our system uses a EG&G SPCM Avalanche Photodiode (APD) as the detector. The dual microscope is advantageous as it can also collect the reflected light from the upright microscope and can be imaged simultaneously. The Zeiss upright microscope can also be configured for confocal imaging by inserting a pinhole before the light detector.

 

Supported by the Division of
Chemical Sciences
Office of
Basic Energy Sciences
at the
U.S. Department of Energy

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Radiation Laboratory
Univ. of Notre Dame
Notre Dame, IN 46556

Tel: (574) 631-6163
Fax: (574) 631-8068

Last Modified: 06/28/2010

 

       





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