Characterisation of Ion Track-Etched Solid-State Nanopores using Small Angle X-ray Scattering

Abstract

When a highly energetic heavy ion passes through a target material, the damaged region left in its wake exhibits preferential chemical etching. The etching process can leave very high aspect ratio channels up to tens of microns in length, with pore openings of just a few nanometres. Membranes with ion track etched pores potentially have new industrial applications including chemical and biological sensing, templates for nanowire growth, energy technology, and filtration at the molecular scale. While ion track etched polymer membrane technology is reasonably well established, the processes needed to produce porous membranes in silicon dioxide and silicon nitride are not. Ion track etching in silicon dioxide has previously been studied extensively, but not for the formation of permeable membranes. Because these materials are chemically and thermally stable, the ability to produce nanopores in them is highly desirable. In this thesis techniques for characterising the pores were developed and used to better understand the influence of irradiation and etching parameters on pore formation. Small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) were used to quantitatively study the morphology of aligned, monodisperse conical etched ion tracks. Samples were irradiated with 1.1GeV Au ions at the GSI UNILAC, and with 185, 89 and 54MeV Au ions at the Heavy Ion Accelerator Facility at ANU. They were subsequently chemically etched using different concentrations over a series of etch times to reveal conically shaped etched channels of various sizes. To accurately reconstruct the pore shapes several complementary analysis methods were developed. Firstly, a two-dimensional model to fit the SAXS scattering intensity images was developed and used to extract the cone heights, base radii and cone opening angles. This method enabled both the radial and axial etch rates of ion tracks to be deduced with high precision. Secondly, an angle fit function method was developed where the geometric relationship between the sample tilt angle with respect to the incident X-ray beam and features in the recorded scattering patterns was used to deduce the cone opening angles. By plotting data from a tilt angle sequence of measurements it was possible to accurately measure the cone opening angles and identify any off-set angles introduced by the initial sample alignment with the X-ray beam that could potentially affect the deduced cone opening angle, ensuring the accuracy of the results. Finally, a one-dimensional model assuming an abrupt boundary between the etched region and surrounding unetched region of the sample was used to determine the cone radii, providing a useful check for the 2D fitting method. To improve the signal from smaller conical pores some samples were measured in grazing incidence configuration. The results of the ion energy series for silicon dioxide show that at higher ion energies, etching in the axial direction is slower despite a higher electronic energy loss. This is most likely due to the velocity effect and the concomitant reduced energy density deposited at the higher ion energies. It was possible to accurately measure the very shallow cone opening angles in very long and narrow pores in polycarbonate membranes using the angle fit function method. The SAXS techniques developed were so sensitive that is was possible to deduce from the data that the conical etched pores resembled a slightly truncated cone and that the radii of the truncated part compared with previously measured radii of unetched ion tracks. The accuracy of the cone opening angle measurements were far better than has been achieved previously using SEM or AFM because the SAXS techniques are far more accurate, and because SAXS is a non-destructive technique which does not introduce any measurement artefacts. This thesis work will enable the fabrication of pores which can be tailored at the nanometre scale for specific applications. Show more