Tomaso Zambelli

Study of the effects of defects in ultrafiltration membranes on the water flux and the molecular weight cut-off

Abstract The aim of this study was to evaluate how defects in a membrane surface can affect the molecular weight cut-off (MWCO) and the membrane permeability as they are traditionally defined. These two parameters are important criteria for the selection of ultrafiltration membranes with regard to efficiency and rejection of dissolved and particulate substances and especially disinfective retention. Tests were conducted with a flat-sheet membrane with an effective filtration area of 1.45×10−3 m−2. Dead-end filtration experiments were performed with various feed solutions: pure water to measure the hydraulic permeability, a mixture of dextrans to determine the apparent MWCO and protein solutions. This was done before and after the membrane integrity was altered by perforating its surface with a sharp tip.

Nanostenciling for fabrication and interconnection of nanopatterns and microelectrodes

Stencil lithography is used for patterning and connecting nanostructures with metallic microelectrodes in ultrahigh vacuum. Microelectrodes are fabricated by static stencil deposition through a thin silicon nitride membrane. Arbitrary nanoscale patterns are then deposited at a predefined position relative to the microelectrodes, using as a movable stencil mask an atomic force microscopy (AFM) cantilever in which apertures have been drilled by focused ion beam. Large scale AFM imaging, combined with the use of a high precision positioning table, allows inspecting the microelectrodes and positioning the nanoscale pattern with accuracy better than 100nm.

Development of UHV dynamic nanostencil for surface patterning

A dynamic nanostencil system based on a movable atomic force microscopy (AFM) cantilever-borne mask has been developed in ultrahigh vacuum environment. This system is conceived to offer an outstanding nanopatterning capability of nanometer precision as well as in situ AFM characterization with a large scanning range. Evaporation experiments in both static and dynamic mode have been performed successfully on this system, and some crucial technical problems of stencilling technique such as resolution and clogging are investigated. As an important application of molecular electronics, a method to fabricate and connect nanoscale structures with microelectrodes by accurately combining it with static stenciling is presented.

Experimental investigation of resonance curves in dynamic force microscopy

A precise experimental investigation of the amplitude and phase resonance curves of a driven dynamic force microscope (DFM) cantilever interacting with an Al2O3(0001) surface in ultra-high vacuum is reported. The large amplitude (a few tens of nanometres), high cantilever stiffness (25 N m−1) and high quality factor (a few 104) characterizing these experiments are typical of the frequency modulation (FM) mode of DFM. The whole range of tip–substrate distances where a stationary oscillation of the cantilever can be maintained is explored. It covers two different regimes: a large distance regime where only long range conservative van der Waals interactions contribute and a small distance regime where short range interactions play a significant role. A comparison is made with frequency shift and excitation amplitude curves as a function of the distance acquired in the FM mode. It is also shown that approach–retract amplitude and phase curves usually obtained in the amplitude modulation mode can be extracted from these data. These experimental results are compared with analytical predictions reported in the literature. An excellent agreement is found in the van der Waals domain, allowing us to evaluate the Hamaker constant for the alumina–vacuum–silicon system.

An experimental investigation of resonance curves on metallic surfaces in dynamic force microscopy: the influence of frozen versus mobile charges

Amplitude resonance curves of a driven dynamic force microscope cantilever interacting with a Cu(100) substrate in ultra high vacuum are derived and analysed, extending a previous study on Al2O3(0001) (Polesel-Maris et al 2003 Nanotechnology 14 1036). It is shown that the charges that are trapped on the oxidized n+-doped silicon tip give rise to long-range electrostatic forces that dominate the van der Waals forces, due to the metallic nature of the substrate. These electrostatic forces cannot be compensated by the usual procedure consisting of applying a constant bias voltage between the tip and the sample. Indeed, the trapped charge does not remain constant on the timescale of the experiment due to charge leakage across the tip oxide layer. Removal of this oxide by electron field emission solves this problem and allows the access of a pure van der Waals tip–substrate interaction regime.

Adsorption of single 1,8-octanedithiol molecules on Cu(100)

Single 1,8-octanedithiol (ODT) molecules adsorbed onto the Cu(100) surface have been characterized by using scanning tunneling microscopy (STM) and studied by semi-empirical calculations. STM images have revealed two types of chiral molecules on the surface upon adsorption and both types of molecules showed two bright spots at the extremities of a small rod due to the enhanced electronic density contrast of the chemisorbed sulfur atoms. In sub-monolayer regime deposition, ODT molecules exhibit preferential adsorption directions and the relaxation mechanism is driven by the chemisorption of the two sulfur atoms in a hollow site of the surface. By means of calculations several conformations of the molecule according to the energetically favorable alkane body stretching constraint have been studied. The comparison between relaxed conformations and between calculated and experimental STM images, followed by an analysis of different orientations, has allowed determining unambiguously the most favorable position of the ODT molecule on Cu(100).