Scanning transmission ion microscopy of polycarbonate nanocapillaries

Complete text of publication follows. Nanochanneled materials are of a great interest due to their peculiar properties and high potential impact for the fabrication of nanostructures and nanodevices. Polycarbonate membranes are produced by heavy ion irradiation followed by chemical etching of the ion tracks. The irradiation parameters determine the porosity (areal density of the capillaries) and angular spread, while the channel diameters and shapes depend on the chemical process parameters. Such polycarbonate (and other materials) membranes are commercially available from a few manufacturers. The primary use of the filters involves packaging and filtering applications. Moreover, they are used for collecting atmospheric aerosols for environmental research. The nanocapillaries formed in membranes are particularly suitable for ion and electron guiding studies of a recently discovered, but not yet completely understood capillary guiding phenomenon. This interesting guiding effect is very promising for patterning by parallel writing with ions and/or electrons through masks. In order to get a better understanding of this phenomenon, we need a better characterization of the capillaries themselves. This study is addressing the angular distribution of the nanochannels in the polycarbonate filters by using a nuclear microprobe facility and the method of scanning transmission ion microscopy (STIM). The STIM experiments in this work have been performed at ATOMKI. The proton energy was 2 MeV, the beam intensity was about 1000 protons s-1, the beam spot size was about 1 x 1 μm, the scan size was 100 x 100 μm and the beam divergence was smaller than 0.07 deg. A scanning electron microscope (SEM, Hitachi S4300 CFE) was used to measure the capillary diameters and the membrane porosity. The sample thickness was determined by a profilometer (AMBIOS XP-I). We have investigated two different pieces of Millipore IsoporeTM samples. A typical SEM image showed several overlapping holes, while the sample porosity is not as high as it would be characteristic for ordered arrays of capillaries. The angular width of the transparency has been determined by the STIM method. The investigated membranes have shown a larger angular range for ion transmission than it could be expected by their geometrical parameters. This indicates that there is a significant angular spread in the direction of the capillary axes. This method can be useful to provide feedback for improving the ion irradiation process if it is necessary to produce arrays of properly aligned, parallel capillaries. If such samples are intended to be used for ion guiding studies, where high parallelism are required, these types of STIM measurements are always advantageous as input information for the sample characteristics. For the conventional application areas of these samples (filtering) such information are more or less irrelevant. Acknowledgements. The support of EU co-funded Economic Competitiveness Operative Programme (GVOP-3.2.1.- 2004-04-0402/3.0) as well as from the Hungarian Scientific Research Fund (OTKA) under contracts K83886 and CK84225 is gratefully acknowledged.