The opportunity of using electron induced dissociation of molecules as a tool for chemical control has been recognised for several years and is widely adopted by the low temperature radio frequency plasma community to prepare reactive species for surface processing. For example, in the semiconductor industry the electron temperature (energy) in a fluorocarbon plasma is optimised to produce CFx (x =1 to 3) radicals which subsequently etch SiO2 substrates. However, one of the most exciting advances of recent molecular science has been the discovery of the ability of low energy electrons to initiate and effectively drive selective bond cleavage processes in molecular systems. The selectivity is being controlled by the kinetic energy of the electron within the range of few electron volts (eV) or by the choice of functional groups of the target molecule.
This process, which is termed dissociative electron attachment (DEA) is one of the most basic
chemical reactions, i.e., a cleavage of a chemical bond preceded by electron attachment to the
Here, XY# - is a transient negative ion (TNI) that leads to the production of a negatively charged fragment X- and its neutral counterpart(s) Y. In the condensed phase or on a surface the neutral, and generally highly reactive radical products, may react further with neighbouring molecules or with the surface itself;
In contrast to direct electron impact where an excess energy of several eV (e.g. 4 to 5 eV) is required to fragment a molecule, DEA can dissociate a molecule with an almost zero electron energy threshold (i.e., it is an example of a barrierless chemical reaction). Hence in numerous cases DEA can occur at electron impact energies of meV, that is, at thermal energies (kT). Generally, these low energy processes have very large cross-sections of 100s to 1000s of Å2. DEA can therefore lead to a new form of very low energy but extremely efficient chemical reaction dynamics. Furthermore DEA is bond selective, each TNI often having a preferred dissociation channel.
Electron attachment in the energy range of few meV to about 10 eV (close to or just below the ionisation energy) has been the subject of substantial experimental and theoretical research during the last two decades, and considerable knowledge has accumulated on the dependence of the reaction pathways of different molecules as a function of chemical composition, structure and incident energy of the attaching electron [1-5].
One example is a systematic study on the halogenated pentafluorobenzenes C6F6X with X = F, Cl, Br or I under single collision conditions and in clusters [1,6]. Upon interaction with electrons with few meV incident energy, hexafluorobenzene forms exclusively the molecular ion, while chloropentafluorobenzene forms exclusively Cl- and the C6F5• radical while the mono-brominated analogue forms the anions bromide and C6F5- in 4/1 ratios and hence the corresponding radicals with an inverse ratio. Finally, for the mono-iodinated fluorobenzene, the I-/C6F5- ratio is 1/20. All of these processes have very high cross sections and are correspondingly very efficient and stay efficient in clusters despite the fast relaxation pathways offered to the transient negative ion in the condensed medium.
This example shows one of many possibilities of controlling reactivity with low energy electrons. By para-substitution of the monochloropentafluorobenzene with an aliphatic chain or any other desirable substituent, the chosen substituent may be linked to an activated surface via the R-C6F4• radical formed upon low energy electron attachment:
R-C6F4• + DB-Surface -> R-C6F4-B-Surface
with DB standing for dangling bond.
This reaction may therefore be one of many reactions induced by low energy electrons that are well suited to link a variety of substituents (R) to activated surfaces, thereby offering the possibility of pattering the surface with virtually any chemical or physical properties desired.
The selectivity of dissociative low energy induced chemistry was first observed more than two decades ago. In DEA to CFCl3, electrons with meV incident energies cleave only the C-Cl bond producing Cl- and the CCl2F radical with very high cross-section, whereas at 3.2 eV incident energy, electrons cleave only the C-F bond resulting in the anion/radical pairs F-/CCl3 or F/CCl3 -. Hence DEA releases reactive fragments that are free to initiate further chemical reactions. Such examples from the literature include intra- and iter-molecular Cl2 formation [8,9], nucleophilic substitution reactions [5,10-12] and even polymerisation reactions . A good example of such a reaction is the nucleophilic substitution in mixed cluster of NF3 and CH3Cl . The chloride production from CH3Cl by direct electron impact is negligible. However in a mixed cluster of CH3Cl and NF3 molecules, fluoride ions may be liberated from NF3 and react with CH3Cl by nucleophilic displacement (F- + CH3Cl -> CH3F + Cl-). An identical process may occur on a surface in a mixed multilayer of co-deposited CH3Cl and NF3. The chloride ions can then be liberated from the film and the molecular species CH3F left on the surface or in the film. Hence it is possible to pattern a surface with the molecular species of your choice.
Such chemical reactions have also been used to bind specific functional groups to the surface of a solid material in a controlled way. As has been shown earlier , DEA of acetonitrile (CH3CN + e- (2eV) -> CH2CN- + H) may be used in the functionalisation of hydrogenated diamond. In this case dissociative electron attachment at 2 eV incident electron energy induces exclusively covalent attachment of about 1 monolayer of H2CCN fragments on the diamond surface through C-diam-C and C-diam-N linkages. This clearly demonstrates the potential that such electron induced reactions have to form arrays of functionalised chemical groups with specific chemical and/or physical properties on surfaces.
Low energy electron induced reactions have also opened new perspectives for the formation of chemical nanostructures on surfaces with relevance to biochips, relying on the immobilisation of proteins or nucleotides. A selective transformation according to
has been established as a basis for these applications and entitled chemical lithography . In this specific case, the nanostructures are transferred to the surface either by irradiation through a mask or by using electron beam techniques.
Since the introduction of the scanning tunnelling microscope (STM) and the atomic force microscope (AFM) in 1981 and 1986, respectively, great and exciting advance has been made in imaging and manipulation at the molecular level. It is now a well established approach to use the STM to manoeuvre individual atoms and molecules on a surface [16,17] and more recently inelastic tunnelling from the STM tip has been shown viable to selectively induce vibrational excitation and/or dissociation of molecules [18-20].
For example, by dosing the target molecule with ‘tunnelling’ electrons in the energy range from 2 to 5 eV, controlled dissociation of C-H bonds in benzene has been induced ; the controlled stepby- step dissociation of single iodobenzene molecules has been demonstrated on a Cu(111) surface by using 1.5 eV tunnelling electrons ; individual hydrogen bond breaking on a Si(100) surface has also been used to pattern the surface with atomic resolution  and electrons from the STM tip have been used to induce C–Cl bond cleavage in chlorobenzenes [24,25] with subsequent attachment of the remaining organic radical to the surface .
These and many other examples from the recent literature have clearly demonstrated the potential that STM has to allow tailored synthesis of molecules on surfaces, an important step towards molecular engineering. Hence such ‘molecular surgery’ with low energy electrons from the STM tip introduces the prospect of designer synthesis on the nanoscale and electron controlled manipulation of both the physical and chemical properties of surfaces with nanometre resolution. The adoption of STM technology together with the technique of scanning a tuned beam of low energy electrons across a pre-prepared surface to form arrays of functionalised chemical groups with specific chemical and/or physical properties, provide the basis of the next generation of electron controlled chemical lithography (ECCL) with spatial resolution ranging from the millimetre down to the nanometre scale.
The European research community is currently regarded as leading international studies of electron attachment processes and has an active research community in the use of STM technology as a chemical tool. This Action provides an opportunity to enact a strategy that will ensure that the EU will retain and develop a vibrant research community capable of providing international leadership in this new and exciting field of ECCL, and the opportunity to ensure that the EU plays a leading role in the industrial/commercial exploitation of such a field.
There are three factors that are essential in achieving such goals: communication, coordination and last, but not least, to maintain continuity of progress, interest and knowledge within the field. This COST Action provides the means to achieve this by providing an instrument, which is solely dedicated to structuring the communication, to organizing the research efforts in the most efficient way and to pave the path for young researchers starting in the field.
Being a rapidly developing field of research, this is an area in which younger researchers often provide the lead with new innovative ideas. However, many of the current EU forums (e.g. Framework Programmes) are often too complex for younger researchers requiring extensive administrative support and existing contacts/knowledge of EU schemes. This COST Action provides an excellent forum for younger researchers to meet and develop ideas and projects, and gain experience in international and trans-disciplinary research projects.
In order to successfully develop scientific and technological applications of ECCL, collaboration over both discipline and national boundaries is essential. To date, there are few opportunities for such networking, EU RTNs being focused on narrower aspects of the research. COST, through its ability to support targeted meetings and to support short-term scientific missions (STSMs), provides an ideal framework for the initialisation of such a research programme that may subsequently be developed under other EU initiatives (e.g. Framework 7 and ESF Eurocore programmes).
Last updated: 28th September 2007