Download Lab II Instruction

AFM Lab
(This is part of the course Nanoscience I, CBGB06)
1. Instrument, Location, and Samples
The atomic force microscope (AFM) instrument (Innova) is located at 21C 215. There are
various samples that one may choose. The samples contain nano-scale objects, such as ZnO
nano particles, thin Ag films (up to tens atomic layers), or even molecular nano crystals.
2. Purpose
The purpose of the AFM experiment is to demonstrate the instrument, its principle, and the
basic operation modes. After an introduction, the students get a chance to put hands on the
equipment. The students shall study the sample with the nano-scale objects and measure
surface topography with different scales, width and height of nano objects, and force-distance
curves.
3. Theory of AFM
AFM is one kind of scanning probe microscopes (SPM). SPM is designed to measure surface
topography with a probe in a nano-scale. It may also measure local properties, such as
conductivity, friction, and magnetism. To acquire an image, the SPM scans the probe over a
small area of the sample, measuring the local property simultaneously.
Fig. 1 AFM operates by measuring force between a probe and the sample (Fig. 1). Normally, the
probe is a sharp tip, which is a 3-6 µm pyramid with a 15-40 nm radius (Fig. 2). Though the
lateral resolution of AFM is low (~30 nm) due to the convolution, the vertical resolution can
be up to 0.1 nm.
To acquire the high resolution, AFM generally measures the vertical and lateral deflections of
the cantilever by using the optical lever. The optical lever operates by reflecting a laser beam
off the cantilever. The reflected laser beam strikes a position-sensitive photo-detector
consisting of four-segment photo-detector. The differences between the segments of photodetector of signals indicate the position of the laser spot on the detector and thus the angular
Fig. 2 AFM tips. (a) New tip. (b) Used tip deflections of the cantilever (Fig. 3).
Fig. 3 The force is continuously calculated by the defection of the cantilever through the spring constant. A piezoelectric ceramic scanner (Fig. 1) positions the sample in Innova (or tip in other
systems) with high resolution. Piezo-ceramics are a class of materials that expand or contract
when in the presence of a voltage gradient. Piezo-ceramics make it possible to create threedimensional positioning devices of high precision.
In a contact (or tapping) mode, AFM uses a feedback signal to regulate the force (or
amplitude) on the sample. The AFM not only measures the force on the sample but also
regulates it, allowing acquisition of images at very low forces. The feedback loop consists of
the tube scanner that controls the height of the sample; the cantilever and optical lever, which
measures the local height of the tip; and a feedback circuit that attempts to keep the cantilever
deflection constant by adjusting the voltage applied to the scanner. A well-constructed
feedback loop is essential to microscope performance.
4. Lab Report
After the experiment, each student should write a report (max. 2 pages), which contains the
basic principle of AFM, one or two recorded images, and some short discussion and
conclusion.