An ordinary optical microscope is capable of
viewing objects as small as approximately 0.5 micrometers (in the order of the
wavelength of light). This is sufficient for many uses.
Other techniques need to be employed for viewing
objects that are smaller than 0.5 micrometers. A range of scanning probe
microscopies such as atomic force microscopy (AFM) and near field scanning
probe optical microscopy (NSOM) have been developed to determine the shape and
the appearance of such small objects.
Near field Scanning Optical Microscopy is unique in
allowing researchers to simultaneously observe optical properties and the
topography of their sample with the precision of tens of nanometres (1
nanometre =10-9 m). The
optical part of the microscope works by irradiating a sample with light waves
emitted by a very sharp, conical optical fibre tip. The end of the tip has the diameter much smaller than the light
wavelength, for example in the order of 50 nm. The tip is carefully located in
the proximity of the sample surface, at a distance comparable to the tip size.
The sample is then moved in the horizontal plane using precision actuators,
with nanometre precision. The movement can follow a raster scan (like the movement
of the electron beam on the TV screen). During this movement the control
electronics keeps the fibre at a constant separation from the sample surface.
During the scan the computer records the sample’s vertical position, which is
stored and displayed producing a topographical map. At the same time the
optical signal collected during the movement is also stored separately and
produces another map, of light reflected from the sample surface. The second
map gives an NSOM image, while the first one gives an AFM image.
The Nanonics NSOM scanning head allows viewing of
the sample through a conventional optical microscope as well as through any
spectroscopic systems that are microscope based. This feature makes it possible
to identify a region of interest at a low magnification first, and then apply
the scanning probe to that selected region only. That is not possible if the
region is identified using a standard microscope and then moved to a scanning
probe microscope for further examination – the exact region to be imaged could
not generally be found again…Importantly spectroscopic measurements either
Raman or fluorescence or fluorescence excitation from a region examined by
NSOM/AFM are possible as well. These features are unique to the Nanonics
system.
The Nanonics system uses a special optical fibre probe
design. The cantilevered fibre is held between the objective lens and the
sample without obstructing any aspect of the conventional microscope. The tip
of the fibre is exposed allowing direct viewing of the scanned region either
through the eyepieces or through a video viewer. This is not possible in a
standard AFM that uses a silicon micromachined tip which obscures the scanned
region, and is also impossible with straight near-field optical probes.
The Nanonics NSOM hardware and software is fully
integrated with the Renishaw Raman system. Several modes of operation are
possible.
AFM/NSOM with far field Raman.
In this mode an AFM image of the region of interest
is taken, and points of interest identified on the image (separated by more
than the spatial resolution of the Raman probe of 1-2 micrometers, depending on
wavelength. Far-field Raman (and fluorescence) spectra can then be taken at
selected points.
NSOM Raman (fluorescence)
The exciting laser light is delivered through the
fibre probe and the emitted/scattered light is collected in far-field.
Additionally the sample can be scanned relative to the fibre aperture at the
near-field scale.
It needs to be noted that the force-sensing
capabilities of the optical tip enable these data to be correlated with the
motion/mechanical/topographical variations.
Surface –enhanced NSOM Raman
(fluorescence)
Over 20 years ago it was discovered that the weak Raman signal could be enhanced by many orders of magnitude (up to 1014 times) when small metal particles are located in the immediate proximity to the molecules being studied. This approach can be used to generate an efficient near-field Raman scattering. This approach can be used to generate near-field Raman scattering using a specially fabricated optic fibre tips. The tip of a scanning probe with an isolated metal nanoparticle can be brought into a close proximity of the surface being studied leading to the surface-enhanced Raman effect, which can then be observed far-field. The published studies suggest that it is possible to probe, with nanometre precision the Raman spectra of surfaces.