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Franco Cacialli

 
    • Organic electronics
      (especially organic light-emitting diodes, OLEDs)
    • Electroabsorption spectroscopy
    • Supramolecular materials for organic electronics
    • Conjugated polyrotaxanes
    • Scanning near-field optical microscopy (SNOM)
    • Scanning near-field optical lithography (SNOL)

Contact details:
Office: Lab 1 Room 3C5
Tel: +44 (0)20 7679 4467
Ext: 34467
Fax: +44 (0)20 7679 1360
Email: f.cacialliucl.ac.uk
Web: UCL Organic Semiconductor Nanostructures

 
Recent Research

Over the years, Franco’s research ranged from the set-up of an original chemical vapour deposition method of heavily doped, conducting polymers, to the study of polymer LEDs and the control of their luminescent properties. Recent interests include supramolecular architectures for the control of the solid-state properties of organic semiconductors, in particular threaded molecular wires (TMWs). This work has been developed in collaboration with Professor Anderson at Oxford (Chemistry) and Dr Samorì (ISIS – Strasbourg) with whom Franco has co-organised other 2 related conferences (EMRS 2004 symposium, RS discussion meeting in 2006). Another strand of activity is on nano-optics, and is aimed at high-resolution fluorescence/photoconductivity imaging and fabrication of organic semiconductors nanostructures by means of scanning near-field optical microscopes.

Franco Cacialli has been teaching a course on Laser and Modern Optics in the Department of Physics (3443) at UCL (both evening and day courses), and has now started his duties as the new Admission Tutor for the MSc course in Physics and Astronomy. He will be teaching a course on Polymer Electronics within both the MSc in Physics and the MSc in Nanotechnology starting in the academic year 2007-2008. Franco is also a Fellow of the Institute of Physics, a member of the EPSRC peer-review college, and a member of the advisory board of Materials Today.

Recent Publications
MS1. F. Cacialli, J.S. Wilson, J. J. Michels, C. Daniel, C. Silva, R. H. Friend, N. Severin, P. Samorì, J. P. Rabe, M. J. O'Connell, P. N. Taylor and H. L. Anderson.
"Cyclodextrin-threaded conjugated polyrotaxanes as electroluminescent insulated molecular wires with reduced interstrand interactions".
Nature materials, 1, 160-164 (2002).                                  

This article reports our investigation on a class of organic semiconductors that is engineered at supramolecular level (i.e. taking advantage of non-covalent interactions between molecular functional sub-units), to improve control over the solid-state properties, especially intermolecular interactions and packing. We have used a variety of tools to characterize the properties of such materials, from mass spectroscopy and optical spectroscopy (continuous wave and time-resolved), to AFM investigations of the films morphology, and electroluminescence in LEDs structure. Our results demonstrate the potential for control of intermolecular interactions in the solid state. The supramolecular approach, based on threading of the conjugated backbone into cyclodextrin rings, is rewarded by a number of advantages. First, reduction of the tendency to aggregation, necessary for the development of molecular electronics nanostructures. Second, increased luminescence efficiency and blue-shifted emission. Third, effective encapsulation of the active groups in a protected environment that increases the stability of the molecule and makes it more resistant to quenching by impurities. Fourth, enhanced solubility in water: this allows spin-coating without toxic solvents, and paves the way for biocompatible processes and structures. In addition, it is particularly attractive for ink-jet printing of the organic semiconductors over large area, since water-based inks may display reduced degradation of the print-heads, compared with organic solvents.
 
 
MS2.    R. Riehn, A. Charas, J. Morgado and F. Cacialli.
"Near-field optical lithography of a conjugated polymer".
Appl. Phys. Lett. 82, 526-528 (2003).                                            
                                    
In this paper we report the fabrication of poly(p-phenylene vinylene), PPV, nanostructures by direct near-field lithography of its soluble precursor, carried out by means of our home-built instrument. The technique is based on the spatially selective inhibition of the precursor solubility by exposure to the ultraviolet optical field present at the apex of near-field fiber probes with aperture diameters between 40 and 80 nm (±5 nm). After development in methanol and thermal conversion under vacuum we obtain features with a minimum dimension of 160 nm. We demonstrate the use of the technique for the direct writing of two-dimensional photonic crystals with intentional defects and a periodicity relevant to applications in the visible range. Using a Bethe-Bouwkamp model, we then discuss the influence of probe size, tip-sample distance, and film thickness on the resolution of the lithographic process.
 
MS3.    T.M. Brown, I.S. Millard, D.J. Lacey, T. Butler, J.H. Burroughes, R.H. Friend and F. Cacialli.
"Electronic line-up in light-emitting diodes with alkali-halide/metal cathodes".
J. Appl. Phys. 93, 6159-6172 (2003).                                                         
         
The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we have reviewed our progress in the understanding of the energy level line-up in finished, blue-emitting, polyfluorene-based light-emitting diodes, which exploit LiF and CsF thin films in combination with Ca and Al to obtain cathodes with low injection barriers. We have used electroabsorption measurements, since they allow the non-invasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/cathode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky-Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g. Bardeen model), except for electrodes which extensively react with the polymer, and introduce deep gap states. In addition, comparison of the built-in voltage, VBI, with the values of the work function of the different electrodes used validates the rigid energy levels approximation, if no significant injection of carriers occurs.

Biographical
PhD (Pisa -1994) New materials for solid state electronics

‘94-‘96 PDRA (Cambridge – Cavendish Laboratory) Brite-Euram programme: POLYLED

’96-‘04 Royal Society University Research Fellow (RS-URF)

’01-‘03 – Lecturer in Physics – UCL (Physics and Astronomy)

’03 –’05 – Reader in Physics – UCL (Physics and Astronomy)

‘05 –   Professor of Physics – UCL (Physics and Astronomy and London Centre for Nanotechnology (LCN, www.london-nano.ucl.ac.uk) )