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Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM)

The materials characterization laboratory is equipped with a Digital Instruments Dimension 3000 Scanning Probe Microscope (SPM). The SPM system includes: 1) a vibration isolation table with acoustic enclosure, 2) a motorized sample stage, 3) a beam deflection SPM head with tip holder, 4) an integrated optical microscope with video controller, 5) a Nanoscope IIIa SPM controller, 6) a Microsoft Windows based computer with dual monitors for simultaneous system control and graphics display, and 7) a photo quality color printer. The SPM produces high-resolution, three-dimensional images by scanning a sharp probe tip over the sample surface.

The SPM brings together all atomic force (AFM) and scanning tunneling (STM) modes and techniques in a single platform (Figure 1). The instrument handles a wide range of sample sizes and types. The rigid, low vibration construction of the SPM ensures the highest quality images and measurements.

Figure 1: The Scanning Probe Microscope brings together all atomic force (AFM) and
scanning tunneling (STM) modes and techniques in a single platform.

Samples up to eight inches in diameter can be scanned in ambient air or fluids using the SPM. The automated stepping feature allows the scanning of multiple areas of a sample while left unattended. The SPM requires little or no sample preparation and the simple vacuum mounting system allows easy and convenient setup. Superior linearity and resolution in all three dimensions is obtained, even for large samples. Integrated top-view video optics with motorized zoom and 1.5 µm optical resolution help identify areas of interest for detailed scanning quickly and easily.

The Digital Instruments TrakScan laser tracking system accurately tracks the probe tip without moving the laser, allowing fewer moving parts and less scanner mass for greater stability and reduction of image bow artifacts. TrakScan technology maintains constant forces on the sample, improving image measurement quality and reducing sample damage.

The NanoScope IIIa system controller provides the software and electronics that drive the microscope. Digital tracking and feedback control insure accuracy for all scan sizes and positions on the sample.

The SPM allows an extensive array of scanning modes. At CAMMP, contact mode, TappingMode, and STM are used. Contact mode measures the topography of a sample by sliding the probe tip across the surface of the sample. Contact mode operation in liquids is the same as in air, with the exception of the hardware used to contain the fluid. Imaging samples in a fluid environment is advantageous due to the reduction of attractive forces between the tip and sample, caused by surface tension effects. The reduction in sample-tip interaction enables the sample surface to be imaged with a minimum of cantilever tip force. This is advantageous when working with delicate materials.

TappingMode measures the topography of a sample with an oscillating probe tip. As the sample is scanned, the tip is maintained at a constant distance. The forces of interaction between the sample and tip dampen the oscillating frequency of the tip. Using a feedback loop, this dampening is converted to topographic information in the SPM controller. This method reduces the shear experienced by the sample during measurement, which can damage soft samples and reduce resolution of the resulting image.

STM measures the topography of surface using a tunneling current, which is directed from a tungsten tip through a highly conductive sample. This current is maintained at constant amplitude, by changing the tip position. The changes in tip position are converted to topographic information in the SPM controller.

At CAMMP, AFM (Figures 2-6) is used to image the crystal growth-induced nanometer-scale surface features in zeolite and zeotype materials.

Figure 2: Tapping mode AFM images of a polygonized single growth spiral located on the upper layer of vicinal faces of zeolite A: (a) top view (phase contrast image) of the upper layers (growth spiral is illustrated in the box); (b) top view (phase contrast image) of a growth spiral shown in (a), illustrating the distances between the single turns of a spiral marked 1 (~215 nm), 2 (~160 nm), 3 (~126 nm); and (c) section analysis along the bold line in (b), showing the height of steps marked 1 (~1.22 nm), 2 (~1.21 nm), 3 (~1.25 nm). Fields of view in Figure 2a and 2b are 10µm x 10µm and 3.5µm x 3.5µm, respectively (Dumrul et al., Microporous and Mesoporous Materials, 2002, 54: 79-88).

Figure 3: Tapping mode (phase contrast) AFM images of pairs of the polygonized growth spirals, located on the upper layers of vicinal zeolite A faces, with: (a) the same sign; and (b) the opposite sign. Fields of view in Figure 3a and 3b are 6µm x 6µm and 4µm x 4µm, respectively (Dumrul et al., Microporous and Mesoporous Materials, 2002, 54: 79-88).

Figure 4: Tapping mode (phase contrast) AFM image of growth terraces on the (100) face of ~25 µm zeolite A crystal. Field of view is 20µm x 20µm (Bazzana et al., Studies in Surface Science and Catalysis, 2002, 142:117-124).

Figure 5: (a) Tapping mode plane-fitted AFM image of the center of a spiral hillock on the (100) surface of a large rectangular ETS-4 crystal; and (b) Tapping mode flattened AFM image of multiple spiral hillocks with circular steps with no preferred orientation on the (100) surface of a large rectangular ETS-4 crystal. The grayscale contrast corresponds to a z-scale topography of 5 nm (Miraglia et al., Microporous and Mesoporous Materials, 2004, 69:71-76).

Figure 6: (a) Tapping mode (phase contrast) AFM image of square-shaped islands and growth terraces on the truncated surface of ETS-10 crystal with a truncated square bipyramidal morphology shown in FE-SEM image(b). The slight deviation from the square shape in (a) is due to distortion caused by inclination of the imaged surface to the horizontal (Ji et al., Microporous and Mesoporous Materials, 2005 81:1-10).