Molecular Structure of Soft Matter Interfaces (Research group Prof Gunther Andersson)
Our research focuses on the characterisation and modification of interfaces and their dynamics. We are using surface spectroscopy to investigate and determine the molecular structure of interfaces. This information is used to understand interface properties and to modify on an informed basis the interfaces in technical applications and devices. The techniques we are using are two unique surface science techniques and can be applied for analysing a broad range of liquid and solid surfaces and interfaces: Neutral Impact Collision Ion Scattering Spectroscopy (NICISS) and Metastable Induced Electron Spectroscopy (MIES). NICISS is a method to measure elemental concentration depth profiles with a depth resolution of a few Angstrom in the surface near region. We have developed this method for analysing soft matter surfaces in the last 15 years. MIES is a method to determine quantitatively the composition and electronic structure of the outermost layer of a sample. Combining NICISS and MIES a large number of scientific and technical problems can be addressed. Additionally we are using X-ray reflectivity, surface tension and contact angle measurements.
Solar Cells Based on Organic Compounds
a) Interfaces in Dye-sensitised Solar Cells: Dye-sensitised Solar Cells (DSCs) have a large potential to become part of next generation solar cells. DSCs use a wide band gap semiconductor material, typically titania (TiO2), in combination with an organic sensitising molecular dye. In order to optimise light absorption, a nanoporous matrix of titania is used as electrode on which a layer of dye is adsorbed. It is aimed that the dye layer forms a monolayer with well anchored molecules in order to maximize the efficiency of injection of photoelectrons into the semiconductor material. Photoexcitation of the dye leads to injection of electrons into the titania and leaves behind a hole in the dye. The dye is regenerated by electron donation from the electrolyte; typically an organic solvent containing a lithium salt and an iodide/triodide redox couple. Dye regeneration prevents recapture of the conduction band electron by the oxidized sensitiser. The electron diffuses through the porous matrix to a transparent electrically conductive anode.
The lifetime and efficiency of DSCs currently achievable needs to be improved for using DSCs in commercial products. With our research we are aiming to understand on a molecular basis the processes for charge generation and transport over the interface between the electrolyte, the dye and the titania. NICISS is used to measure concentration depth profiles over the interfaces for understanding the charge distribution at the interfaces and also the morphology of the adsorbed dye layer. For characterising the electronic structure electron spectroscopy is used.
b) Interfaces in Polymer Based Solar Cells:
Similar to DSCs, solar cells based on polymers (OPVs) is a promising technology for future renewable energy sources as the costs of production of OPVs are much lower than those of silicon based solar cells. OPVs have a layered structure consisting of electron and hole conducting polymers. Absorbed light generates excited states of electrons and electrons and holes have to be separated for power production. In order to make these solar cells competitive in large scale production the interfaces have to be optimised and stabilised. Also here NICISS is used measure the composition of the interfaces while electron spectroscopy is used to analyse the electronic structure.
Structure and Dynamics of Thin Foam Films
Understanding the structure and dynamics of thin foam films is of interest in flotation, producing emulsions and in food processing. Self supported foam films have a typical thickness of a few ten nanometres. Foam films are covered on their surface with a dense layer of surfactant molecules while the core is formed by the surfactant solution. The layer of surfactant molecules at the foam film surface is stabilising these thin liquid films. The stability is given by the equilibrium of the internal and external pressure. The internal pressure is the sum of repulsive electrostatic forces, attractive van-der-Waals forces and steric forces. The van-der-Waals forces can be described through empirical models based on measurements with the surface force apparatus. Describing the forces in foam films is so far based on standard models in colloid science. Model free calculation of the electrostatic and van-der-Waals forces can be achieved experimentally when measuring directly the concentration depth profiles across the foam films.
Charges at liquid surfaces
Ions are known to accumulate at liquid surfaces often resulting in distinct layers of positive and negative charge, which is contrary to the bulk of a liquid where the charges are evenly distributed resulting in no charged regions. Understanding the adsorption of charged species (e.g. ions) at liquid surfaces is important to understanding the structure of bulk liquid surfaces, but also the structure of foam films, where the charged surfaces play a major role in stabilising the films. Charged liquid surfaces and foams are used in a wide variety of applications such as detergents/cleaning agents, volatile-gas capture media, chemical solvents/catalysts and froth flotation in the mining industry. Additionally, understanding the role of charges at liquid surfaces is of great fundamental importance in understanding the competition between intermolecular forces in liquids as well as for understanding the interactions of surfaces in solution, such as biological membranes. Our research focuses on two main liquid systems:
1) Foam films: A gas-liquid-gas film, where the liquid core is stabilised by surfactant molecules adsorbed at the liquid-gas interface. Charges are present via dissociation of an ionic surfactant, or in the case of a non-ionic surfactant, adsorption of additional charged species.
2) Ionic liquids: Salts that are in the liquid state at room temperature. The organic nature of cation and/or anion provide additional forces not present in typical inorganic salts (e.g. table salt) that allow them to be in the liquid phase at room temperature. This provides an interesting liquid system that consists only of charged species (unlike aqueous salt solutions containing neutral water molecules).
Investigating these systems using the surface-sensitive techniques NICISS, MIES and UPS allows us to investigate the exact structure of species in the top few nanometres of liquid surfaces, which reveals information on which species prefer to adsorb or push-away from liquid surfaces under different conditions. These results can be combined with other bulk techniques such as surface tension measurements and the thin film pressure balance which give information on the competing forces at liquid surfaces. The combination of these results gives a great deal of insight into the structure of charges at liquid interfaces along with the forces that result from it.
Biochemical Modification of Interfaces in Microfluidic Devices
The interface in microfluidic devices is stepwise modified through surface engineering such that single strained DNA fragments selectively adsorb at the surface. The selective attachment of DNA fragments will make it possible to preselect DNA samples. On long term it is planned to develop a portable instrument for identifying samples on site. The project is part of the new Australian Future Forensics Innovation Network (AFFIN).
Heterogeneous catalysis with atomically precise clusters
It is known that the number of atoms in a metal cluster determines whether or not a cluster is reactive with molecules in the gas phase. The origin of this phenomenon is assumed to be the fact that the valence electron structure of a metal cluster changes with the number of metal atoms forming the cluster and that the valence electron structure is the key factor for the cluster reactivity. This assumption is supported by the observation that the steps on single crystal surfaces are catalytic active. A further observation is that clusters deposited on surfaces are catalytic active as well. However, the relation between the valence electron structure of a cluster in the gas phase and the valence electron structure of a cluster deposited on a surface is not understood. The combination of MIES for analysing the valence electron structure of surfaces and scanning methods like atomic force microscopy and scanning tunnelling microscopy is expected to reveal the key information of this relation.
NICISS is a method to determine the concentration profiles of elements in the interfacial region of soft matter with a depth resolution close to the surface of ~0.2 nm. In a NICISS experiment the target is bombarded with a pulsed beam of inert gas ions - mostly helium ions - with a kinetic energy of several keV. Helium projectiles backscattered from the target loose energy through the collision with the atoms forming the target. The energy loss of the projectiles is determined by measuring the time of flight of the backscattered projectiles from the target to the detector. The projectiles loose energy in two ways. Firstly, energy is transferred from the ion to the target atom during the collisions. The amount of energy transferred in this way depends on the mass of the target atom and is used to identify the elements from which a projectile is backscattered. In the schematic, the projectile backscattered from the outermost layer looses energy only through such a collision. The ions also lose energy as they pass through the target. This second type of energy loss is due to small angle scattering and electronic excitation of the atoms forming the target. This type of energy loss can be approximated as a continuous energy loss and is used to determine the depth of the atom from which a projectile is backscattered. In the schematic, the projectile backscattered from a deeper layer looses energy through this continuous energy loss and one single large angle scattering event. In combination, these two types of energy loss are used to determine the concentration depth profiles of the elements constituting the target.
MIES uses metastable helium atoms to electronically excite molecules and atoms at surfaces and measures the energy spectrum of the emitted electrons. The method has similarities to X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). All these methods map the composition and the electronic structure of surfaces. While XPS is used to determine the core electron structure, UPS and MIES probe the valence electron structure. UPS and MIES use similar excitation energies.
The main difference between MIES and XPS/UPS is their surface sensitivity. In XPS and UPS the exciting radiation penetrates some distance into the sample and the surface sensitivity is determined by the mean free path of the emitted electrons. Thus in UPS and XPS experiments, signals from electrons emitted from both the surface and sub-surface layers (to a few nm depth) are collected. In contrast, the exquisite surface sensitivity of MIES is due to the fact that the object carrying the energy used to excite the target electrons – the metastable atom – cannot penetrate the analyte material in its excited state; it releases its energy to the surface at a distance of a few Å. Thus in a MIES experiment, the surface sensitivity does not originate from the mean free path of the emitted electrons but from the fact that only electrons in the outermost layer can be excited. Therefore, MIES probes the composition of that region which is important for interactions and reactions at surfaces or interfaces. MIES spectra are evaluated quantitatively and directly reveal the density of states (DOS) of the valence electrons.
In the schematic metastable helium atoms impinge on a surface covered with a monolayer. The molecules forming the outermost layer of the sample in the left panel have various orientations while those in the right panel the outermost layer consists of molecules all showing the same orientation. In the left panel metastable atoms can reach all parts of the molecules forming the outermost layer and electrons of various orbitals can be found in the spectrum. On the right side, however, electrons only of the orbitals forming one side of the molecule can be found in the spectrum.
1 'Key World Energy Statistics 2009', publication by the International Energy Agency.
2 V. Bergeron, J. Phys Condens. Matter 11 (1999) R215.