Laboratory for nanoscopy
is equipped with state of art equipment for measuring the
properties of materials at nano-level (Omicron variable temperature SPM
and AFM, model B002645 SPM PROBE VT AFM 25 with MATRIX control system,
SNOM, model TwinSNOM R).
An atomic force microscope (AFM) was invented in 1986 by Gerd Binnig, Calvin Quate and Christoph Gerber. The AFM consists of elastic cantilever with a sharp tip (probe) at its end that is used to scan the sample surface. When the tip is brought into proximity of a sample surface, the forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Depending on the situation, the forces that are measured in the AFM include a mechanical contact force, Van der Waals forces, capillary forces, a chemical bonding, etc. The deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes.
The two basic modes of operation of the AFM technique are contact and non-contact mode. The distance between the tip and the sample in the contact mode AFM is 2-3Ǻ. A repulsive interaction is dominant in this area. The reconstruction of the surface topography is done by measuring the deflection of the cantilever. In the non-contact AFM mode the tip is in the region of the attractive forces, at the distance of 10-100 nm from the sample surface. These forces are not strong enough to measure the static deflection of the cantilever directly. It is necessary to induce the oscillations of the cantilever near its resonance frequency. The interaction between the tip and the sample surface leads to frequency shifting (frequency modulation). The reconstruction of the surface topography is effected by measuring the frequency shift.
techniques have a wide range of uses in solid state physics, surface
science, nanotechnology and biology.
The TwinSNOM system
consists of a room-temperature and air-condition Scanning Near-Field
Optical Microscope (SNOM) and an Atomic Force Microscope (AFM). The
optical microscopy is limited in resolution by the diffraction barrier.
The SNOM overcomes this resolution barrier and significantly increases
the resolution of the optical microscopy. For a SNOM operation a tapered
and metal coated optical fiber is used. The fiber aperture diameter is
much smaller than the optical wavelength and this size determines the
resolution of the obtained image. In this case, there is no field
propagation so the light from the fiber aperture is evanescent and the
fiber tip should be brought into the optical near-field of the sample
surface. The fiber tip is held in position at the focus of the optics
(the reflection objective) for light detection. The sample is then moved
with a scanning motion to perform the imaging. A negative feedback loop
should be used in order to control the tip to sample distance.
The TwinSNOM is designed around a stable universal microscope stage equipped with a mechanically decoupled Zeiss Axiotech Vario microscope. The upright microscope is used for reflection mode SNOM and control of tip positioning. The shear-force AFM technique is used to solve the problem of distance regulation. In addition, it provides topographical AFM images together with every SNOM image.
The SNOM fiber tip and the shear-force detector are integrated into a single magnetically mounted and easily exchangeable sensor module. A precise positioning is provided by the use of piezoelectric stepper motors. These motors are used for the remote controlled positioning of the microscope table as well as for the precise positioning of the sensor unit at the focus of the objective. The scanner unit is integrated into the microscope table.
The SNOM control unit contains the laser and the electronics required for light detection. It includes the signal conditioning for the photomultiplier detector as well as a video input selector for the sensor approach monitoring. A highly efficient light collection is achieved in the reflection mode by a specially designed reflection objective.
The TwinSNOM system is placed on a vibrationless table in order to protect the system from the surrounding mechanical vibrations. The table is floated using the air from a high-pressure cylinder.
The needle sensor completes the functionality of the TwinSNOM. The needle sensor allows for a high resolution non-contact mode atomic force microscopy to be used.
lateral: 100 x 100 μm2, vertical: 20 μm, capacitive x/y/z
linearisation, 1 nm resolution.