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Combining scanning probe and optical microscopy / spectroscopy techniques

Lateral molecular force microscopy

Artūras Ulčinas, Valentinas Snitka

Collaboration: Massimo Antognozzi, Mervyn Miles (Bristol University), Giovanni Valdre (University of Bologna), Per Claesson (KTH, Stockholm), Carles Cane, Marta Duch, Jaume Esteve (National Microelectronics Centre, Barcelona)

Scanning probe techniques designed to investigate rheological and tribological properties of materials as well as other interfacial phenomena on the nanoscale by sensing the force in the direction parallel to the surface have been around for many years (lateral force microscopy, friction force microscopy, shear force microscopy, etc.) They have their own particular limitations, such as high torsional stiffness of the probe leading to limited force sensitivity, challenges with calibrating the spring constant in lateral direction, crosstalk in the detection system and so on. Some of these challenges can be efficiently overcome by using the microfabricated cantilevered force probes oriented in the direction perpendicular to the surface [1]. In such configuration, the probe has very high stiffness in vertical direction, allowing a precise control of the separation between the probe and the surface, and high force sensitivity in the lateral direction.
sew_1.jpg
Scheme of the SEW detection system. The evanescent field is created above the surface of optical transparent sample. As the tip of the probe enters the evanescent field, it begins scattering the light which in turn is collected by the objective lens, expanded and projected on the segmented photodetector [2].
langm_scheme.jpg
Representation of the experimental arrangement for investigation of shear response of confined molecular fluids [3].
By using the scattered evanescent wave (SEW) detection system [2], the position of the tip of the probe is detected. This way, virtually any light-scattering probes can be used, opening the way to use the microfabricated cantilevers with extremely small geometrical dimensions and mass, achieving a superior force sensitivity and speed of response. Inherent in the design is the capability of various modes of optical microscopy (including fluorescence, TIRF microscopy, etc.), as well as straightforward coupling with optical spectroscopy techniques.

The applications of lateral molecular force microscopy (LMFM) range from investigation of shear viscoelastic properties of nanoconfined molecular fluids [3] or tracking of molecular motors in action [4] to molecular recognition or rheological behaviour of proteins (biorheology).

sew_2.jpg
LMFM head and detection branch of home-built instrument

At our research centre, the home-built system based on SEW detection system has been built. Innovative micromachined cantilevers with reduced geometric dimensions are being developed in collaboration with National Microelectronics Centre in Barcelona, Spain as part of the FP7 project NANOMAT activities.

References

1. J. A. Vicary, A. Ulcinas, J. K. H. Hoerber, and M. Antognozzi. Microfabricated mechanical sensors for lateral molecular-force microscopy. Ultramicroscopy 111 (2011) 1547.
2. M. Antognozzi, A. Ulcinas, L. Picco, S. H. Simpson, P. J. Heard, M. D. Szczelkun, B. Brenner, M. J. Miles. A new detection system for extremely small vertically mounted cantilevers. Nanotechnology 19 (2008) 384002.
3. A. Ulcinas, G. Valdre, V. Snitka, M. J. Miles, P. M. Claesson, and M. Antognozzi. Shear response of nanoconfined water on muscovite mica: role of cations. Langmuir 27 (2011) 10351.
4. T. Scholz, J. A. Vicary, G. M. Jeppesen, A. Ulcinas, J. K. H. Hoerber, and M. Antognozzi. Processive behaviour of kinesin observed using microfabricated cantilevers. Nanotechnology 22 (2011) 095707.