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).


Scanning Probe Microscope

The scanning tunneling microscopy (STM) is based on the concept of quantum tunneling. When a conducting tip is brought very near to a metallic or semiconducting surface, a bias between the two can allow electrons to tunnel through the vacuum between them. The tip to sample distance is 3-10Ǻ. For low voltages, this tunneling current is a function of the local density of states at the Fermi level, E_f, of the sample. The variations in current as the probe passes over the surface are translated into an image. This technique is used for the characterization of the conducting and semi-conducting samples.

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.

The SPM techniques have a wide range of uses in solid state physics, surface science, nanotechnology and biology.

Omicron UHV VT AFM/STM, model B002645 SPM PROBE VT AFM 25
 

Omicron UHV VT AFM/STM Microscope	SPM head

Technical data
•          Matrix control system
•          Three modes of operation:
            -         Contact mode AFM
            -         Non-contact mode AFM
            -         STM
            These modes provide force/distance and current/voltage spectroscopy measurements. The STM mode provides an atom manipulation facility.
•          Scan (and offset) range X/Y/Z: 10μm x 10μm x 1.2μm
•          Coarse movement X/Y/Z: 10mm x 10mm x 10mm; Step size: 40nm – 500nm
•          Z-resolution: 0.1Ǻ
•          Measurements with atomic resolution
•          Vibration isolation: Internal eddy current dumping
•          Tunneling current: < 1pA – 300nA
•          Gap voltage: ±5mV to ±10V; applied to tip/cantilever, sample grounded
•          Ultra High Vacuum (UHV) chamber: 10^-9-10^-11mbar
•          Working temperature range: 25K - 750K; with LHe or LN₂ cryostat
•          Sample heating in preparation stage through direct or radiative methods up to 1000K


Some results

Highly oriented pyrolitic graphite (HOPG), atomically resolved, AFM contact mode.	Atomically resolved 7x7 reconstruction Si(111) (new surface atom rearrangement obtained by heating the sample to high temperatures under UHV conditions), STM mode.

TwinSNOM system

 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.

Technical data

Sample scan: lateral: 100 x 100 μm², vertical: 20 μm, capacitive x/y/z linearisation, 1 nm resolution.
Sample positioning: 30x30 mm², remote controlled precision piezo drives, step size down to 70 nm.
Tip positioning: remote controlled x/y/z piezo drives, 15x15x10 mm³, step size down to 50 nm.
Aperture diameter: 50 nm nominal.
Shear-force resolution: z: 1 nm, x/y: 10 nm,
Laser: current maximum: 50 mA, max. operating temperature: 50°C, max. radiation flux: ≈1 mW, emission wavelength: 635 nm
Optical microscope: objectives: 50x, 10x (long distance), 10x binocular, conventional mode: upright (bright/dark field), spectral range: 350 nm to 750 nm, detection path: 1700 nm.
Footprint: 55 cm x 55 cm; 60 kg.
 

Some results

Fiber sensor. The sample is latex projection pattern. SNOM image.	Topography obtained by the shear-force AFM technique.

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