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Atomic and Molecular Nanosystems

Two decades after its invention, scanning probe microscopy has become a widely used method in laboratories as diverse as industrial magnetic storage development or structural biology. Consequently, the community of users ranges from biologists and medical researchers to physicists and engineers all of them exploiting the unrivalled resolution. Since its invention in 1982, scanning tunnelling microscope (STM) has become an important surface science tool. Conducting surfaces and thin non-conducting atomic or molecular layers on conducting surfaces can be imaged with atomic resolution. The density of states near the Fermi edge or molecular oscillations can be explored with a sub-nanometer spatial resolution using various spectroscopic methods. While the use of the STM is restricted to conducting surfaces, the scanning force microscope (SFM) or atomic force microscope (AFM) is in principle capable of determining the topography of any surface, conducting or not. The versatility of the SFM has led to a rapid growth in its application in various fields of fundamental and applied research, which has in turn resulted in progress in the attainable resolution.

High resoution SFM

Although, the SFM or AFM was originally intended to be a tool capable of measuring the forces acting between a single pair of atoms but has only recently evolved into an instrument capable of producing atomically resolved images of surfaces with characteristic features and defects and to measure various types of chemical bonds between the tip apex and specific surface sites. However, in contrast to the STM, the SFM is still not well established as a surface science tool. This is mainly due to the complexity of operation and to the limited signal-to-noise ratio. Both limitations can be overcome with the new UHV low temperature scanning force / field ion microscope setup that was recently designed and is currently manufactured at Empa and in collaboration with suitable partners (Createc GmbH, Cryovag AG, NanoScan Ltd)

Fig.1. UHV low temperature scanning force microscope / field ion microscope system that is presently assembled at Empa.

An ideal scanning force microscope would allow high speed atomic resolution imaging, the simultaneous measurement of vertical and lateral forces on the atomic scale, and the distinction of the various energy loss mechanisms, but would still be easy to operate. One of the key factors to reach this goal is the use of hard cantilevers, i.e. > 100N/m (about 10 times stiffer than those presently used). Such cantilevers can be used with ultrasmall oscillation amplitudes below 0.1nm (about 10 to 50 times smaller than in most present experiments) without snapping to the surface of the sample. The tip-sample force (derivative) then becomes directly proportional to the measured frequency shift, and no complicated deconvolution procedure is required. However, such small oscillation amplitudes must remain detectable with a sufficiently high signal-to-noise ratio, and the decreased sensitivity of the cantilever oscillating due to the low amplitudes must be compensated. The latter can be done by decreasing the geometrical dimensions of the cantilever (Fig.2, Table 1) >[->Yang04-1]. Such cantilevers were developed in a collaboration between Prof. Hug’s research group at the Uni Basel with IBM Zurich Research Laboratory within the Top Nano 21 project 5988.1 (ETH Board) and the NCCR on Nanoscale Science at the University of Basel.

FIG. 2: (a) SEM image of a cantilever and part of the recess step. (b) Close view of the cantilever tip. The tip diameter is < 10 nm.

In order to measure the deflection of an ultrasmall cantilever with a new type of deflection sensor was required. In collaboration with FISBA Optik we have developed and tested a new Fabry-Perot type of deflection sensor [->Hoogenboom04-1] (Fig.3).

Table 1: Typical results of the characterisation of cantilevers at room temperature in vacuum. aFor the small cantilevers, Q can probably be enhanced, and thus the thermal noise of the cantilever can be further reduced, by annealing the cantilevers in UHV. bConventional cantilever (Team Nanotec GmbH, Villingen-Schwenningen, Germany).

 

A deflection sensitivity of 1fm/sqrt(Hz) at 1MHz was obtained and the light could be focussed into a spot of only 3um diameter. The sensor is thus well suited to measure deflections of ultrasmall cantilevers (L: 10um, W: 3um) or to be focussed slightly of the long axis of a conventional cantilever. Then the flexural and torsional oscillation modes of the cantilever can be observed (Fig.2a and b).

The new Fabry-Perot sensor and the recently developed ultrasmall cantilevers [->Yang04-1] will be the key elements of a new UHV low temperature scanning force microscope system that is presently assembled at Empa (Fig.1).
With the new UHV-LTSFM/FIM System many fundamental questions in the field atomic and molecular nanostructures can be addressed. Among these are:

  • Measure the vertical and lateral force during the manipulation of atoms and molecules on surfaces, and the lateral diffusion barrier on flat and stepped substrates of different chemical reactivity.
  • Map the surface charge distribution by measuring the interaction of a surface with a polarizeable but otherwise chemically unreactive tip. The measurement of other ultrasmall tip-sample interaction forces such as magnetic exchange force and the interaction of tips with saturated and dangling bonds with molecules with end-groups of various electronegativities are also of high interest.
  • Distinguish energy loss processes due to instabilities of atomic positions and stochastic fluctuations of the inter-atomic force. Note that the sensitivity of present SFM's is not sufficient to measure the energy loss caused by the latter process. The interplay between molecular vibrations and the measured energy loss will be investigated.  In addition, the energy loss generated by the stochastically distributed force pulses generated by small tip-sample tunneling currents may become measurable.
  • Simultaneously measure vertical and lateral force and energy loss on the atomic scale. This allows the study of the subtle transition of energy loss occurring between two bodies in the non-contact, near-contact and contact regime.
  • Build nanostructures from single atoms and to map their physical properties. As an example the spatial localization of a single electron deposited onto a specific atomic site of a nanostructure build from metal atoms will allow certain conclusions on the electrical conductivity of the nanostructure. The magnetic ordering occurring in nanostructures build from atoms with a net-spin will be studied by high-resolution MFM and exchange force microscopy.
  • Measure the electrical conductivity and its dependence on the confirmation of molecules on surfaces. Suitable molecules will allow a confirmation by the site-specific application of forces.
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