Chemical surface and 3D analysis
Submicronic molecular surface analysis with enhanced sensitivity
Group of Arnaud Delcorte
For two decades, A. Delcorte and collaborators have been contributing to the development of secondary ion mass spectrometry (SIMS) for the molecular characterization and imaging of surfaces, with a special emphasis on organic materials such as polymers or proteins. The SIMS technique is inherently a nanoscale chemical characterization technique because of its very limited information depth (~1 nm).
a) Secondary ion images of polyelectrolyte/collagen tubes (500nm width) adsorbed on indium
b) SEM image of the metal islets condensed on a polymer surface in MetA-SIMS
c) 3D rendition of the MetA-SIMS image of polymer chain ends in a microstructured film (75x75µm2)
d) Enhancement of the molecular ion yield of an antioxidant molecule using H2O flooding of the sample surface
Owing to recent developments of the method, including better focusing of ion beams down to 50 nm in size, the technique has reached the level required for nanoscale molecular imaging. For molecular analysis, the limiting factor nowadays is no longer the primary beam focus but the actual sensitivity of the method. In addition to optimizing high-resolution molecular imaging for various applications, our contribution in that field consists in devising new approaches to enhance the molecular secondary ion yields, among which the so-called Metal-Assisted SIMS (or MetA-SIMS) variant of the technique and the flooding of the samples with water vapor, which both induced yield increases of one to several orders of magnitude in the best analysis conditions. The current developments in molecular imaging are strongly supported by our fundamental investigations of cluster beam interactions with organic surfaces, both experimental and theoretical.
Damageless molecular depth-profiling using slow massive clusters
Group of Arnaud Delcorte
With the advent of new “exotic” sputtering beams, such as massive noble gas (e.g. Ar500-5000) or water clusters, coupled to a finely focused analysis beam (e.g. Bi3-5), sub-micrometric 3D molecular imaging was made possible in secondary ion mass spectrometry (SIMS). Indeed, large clusters with a low energy per constituent atom induce soft desorption and minimal sample degradation so that the integrity of the molecules and the stability of the molecular signals are maintained upon depth profiling of fragile organic samples.
The contribution of A. Delcorte and collaborators to this exciting step forward in the chemical characterization of organic materials is twofold: (i) the experimental investigation of large noble gas cluster interactions with molecular solids and polymers, and the optimization of the bombardment conditions for damageless depth-profiling, including the comparison with other types of ion beams; (ii) the microscopic study of the sputtering processes using classical molecular dynamics simulations.
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a) Molecular depth profile of the active layers of an organic photovoltaic heterojunction b) Experimental (dashed green line) and calculated (points) sputter yields upon Ar cluster bombardment of molecular solids c) Snapshot of the molecular dynamics showing the impact of a 5 keV Ar1000 cluster impinging on a polymeric substrate with adsorbed macromolecules |
Modelling energetic cluster-surface interactions
Group of Arnaud Delcorte
The development of high-resolution surface analysis and 3D molecular imaging by cluster secondary ion mass spectrometry (SIMS) requires a continued effort of modelling, in order to guide the experiments and to understand the observed effects. Energetic cluster impacts with surfaces involve many physical parameters which can be tuned to optimize the outcome of the sputtering process or even to move into a completely different regime of interaction.
The recent replacement of atomic projectiles first by small molecular ions (SF5 and C60) and, more recently, by massive cluster ions, leading to damageless molecular depth profiling of organic samples, is a good example of the constructive interplay of theory and experiment. The Delcorte group is part of that movement, with a substantial contribution in the modelling of the interaction of various cluster types (Bin, C60, Arn, [H2O]n), essentially with organic materials. Detailed studies of the effect of the cluster nature, nuclearity, energy and incidence angles on the projectile penetration, energy deposition, induced chemistry and sputtering of organic solids have provided a wealth of information useful for the experimental developments. For cluster-surface impacts, involving many body interactions, the binary collision approximation-based models and codes (SRIM) are inappropriate. Therefore we perform molecular dynamics simulations, either with a full atomistic or a coarse-grained description of the systems, using the SPUT code developed by B. Garrison and collaborators [1,2]. The sputtering yields of organic solids could be predicted for a wide range of cluster sizes and energies, as well as the transition between induced fragmentation/crosslinking and damageless emission, thereby providing a mechanistic basis to understand and further direct the experiments of molecular depth profiling.
a-b) Polystyrene oligomers desorbed from a silver substrate and from a molecular solid by an Ar atom;
c) Au400 bombardment of a polyethylene surface decorated with gold nanoparticles;
d) Massive organic cluster impact in soft matter;
e) Crater induced by a 5 keV fullerene impinging on a polymer surface.
Images taken from the molecular dynamics (selected journal covers).
Scanning probe microscopies
Nanochemistry: SPM chemical and physico-chemical characterisation and modification of surfaces
Group of Sophie Demoustier, Karine Glinel, Alain Jonas, Bernard Nysten
We developped or are still developping various grafting methods in order to chemically functionalize AFM probes (tips) and to chemically nanopattern surfaces. These probes are used to map chemical functionalities on surfaces. They are also used to realise single-molecule force spectroscopy. Presently, we develop functionalized probes with polymer brush able to complex metals to realise the local catalysis of surface reactions.
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| Principles of surface chemical nano-patterning | Lateral force microscopy image of COOH-ended nanodots in a CF3-terminated background |
Some applications:
Additives dispersion on the surface of polymers: AFM probes were functionalized with hydrophobic or hydrophilic moities and were used to measure and map the adhesion force on neat additives (antioxidants, UV stabilizers) as well as on surfaces of polymers containing these additives. These analyses allowed to map the surface distribution of these additives on the polymer surface and their progressive migration toward the surface during the ageing process (Langmuir 2001, 17, 6351).Chemically nano-patterned surfaces: The same procedures as those used for AFM tip functionalization (self-assembly of silanes on silicon oxide or of thiols on gold) were combined with nanolithography techniques to prepare chemically nanopatterned surfaces (see figures below) (NanoLett 2004, 4, 365).
Those surfaces were then used to guide and study the local adsorption of different systems such as copolymers (AdvMater 2007, 19, 4453), polyelectrolyte layer-by-layer assemblies, proteins (collagen and antigen), oligomer nanocrystals, as well as for the local grafting of polymer.
Nano-biosensors: With the purpose of developing biosensors, the reliable proof of the biological activity of two new sensor systems was obtained by atomic force microscopy (AFM) in both the imaging and the single-molecule force spectroscopy modes. Antigens or antibodies of pharmacological interest were grafted onto self-assembled monolayers of thiols on gold, and AFM imaging demonstrated that the grafting process produced homogeneous submonolayers of isolated proteins. (AnalChem 2007, 79, 6489).
Nanomechanics: SPM mechanical characterisation of surfaces and nanomaterials
Group of Christian Bailly, Bernard Nysten
Using various SPM-based methods, we are studying the mechanical properties of polymer blends, nanocomposites, and nanomaterials.
Some applications:
Quantitative measurement and mapping of elastic and viscoelastic properties in polymers: We developed methodologies that allowed to quantitatively measure at the nanoscale the elastic modulus of organic materials based on the measurement and the analysis of force-curves.
The validity of the method was first demonstrated on standard polymer samples (Nanotechnology 1998, 9, 305). The method was then applied to characterise elastomers, polypropylene samples, cell membranes, … In parallel, we developed and used FMM to characterise the elastic and viscoelastic properties in polymer blends. The technique was applied to the study of the effect of synthesis procedures on the toughening of polypropylene resins (JAP 1995, 78, 5953), the effects of processing conditions on the surface morphology of polymer blends, paint adhesion of blends. Presently, we are using the newly developed HarmoniX™ and Peak-Force Tapping™ with Quantitative Nanomechanical Mapping™ (PFT-QNM) modes to characterise the nanoscale and the interface mechanical properties in polymer blends and polymer-based nanocomposites. These modes allow the simultaneous mapping of the topography, the phase-shift, the adhesion force, the energy dissipation, and the elastic modulus.
Mechanical properties of nanomaterials (nanowires and nanotubes): Resonant contact atomic force microscopy (resonant C-AFM) and Force spectroscopy were used to quantitatively measure the elastic modulus of polymer nanotubes and metallic nanowires. The obtained results for the larger nanostructures fairly agree to the values reported in the literature for the macroscopic elastic modulus of the corresponding materials. The measured modulus of the nanomaterials with smaller diameters is significantly higher than that of the larger ones. The increase of the apparent elastic modulus for the smaller diameters was attributed to the surface tension effects (PRL 2000, 85, 1690; PRB 2004, 69, 165410). The method is now applied to the measurement of the mechanical properties of halloysite (clay) nanotubes (Nanotechnology 2013, 24, 105704) and cellulose nanocrystals.
Polypyrrole nanotube on the pore of a supporting membrane, apparent elastic modulus of polypyrrole nanotubes and apparent elastic modulus of metallic (Ag & Pb) nanowires
Nanophysics: SPM characterisation of the electrical and magnetic properties of functional nanostrucures
Group of Alain Jonas, Bernard Nysten, Luc Piraux
We are applying electrical and magnetic modes of AFM to characterise the physical properties of nanomaterials. Among those modes, we mainly use Magnetic Force Microscopy (MFM), Current-Sensing AFM (CS-AFM), Kelvin Probe Force Microscopy (KPFM) and Piezoresponse Force Microscopy (PFM). They are or were applied in various projects and applications such as organic electronics, charge dissipation in antistatic felts, magnetic reversal process in nanowires, development of plastic ferroelectrics, …
Some applications:
Organic electronics: KPFM is performed in situ on OTFTs to characterise the electronic properties in the channel of the transistors (potential distribution, boundary resistances, charge trapping) (Thin Solid Films 2013, 536, 295).
Magnetic nanowires: We are studying the magnetic reversal process in arrays of single domain magnetic nanowires (Ni, NiFe, Co, CoFe) or nanotubes (Ni) as a function of parameters such as the NW (NT) diameter and the density of the arrays. The NWs (NTs) are electrochemically grown within the pores of polycarbonate membranes. The magnetic properties are analyzed by MFM. For that purpose, we modified our AFM equipment with an electromagnet allowing to perform in-situ measurements. (Nanotechnology 2014, 25, 245707).
MFM images at various applied magnetic fields of an array of CoFe NWs (70 mm diametre)
Comparison of magnetization loops obtained from macroscopic measurements and from MFM images.
Ferroelectric polymers: PFM is developed and used to characterise the local ferroelectric properties on nanopatterned PVDF-TrFE films. This work is realised in collaboration with the group of Prof. A.M. Jonas and aims at developping plastic ferroelectric memories. It is based on the fact that the nano-confinement of PVDF-TrFE improves its ferroelectric properties (NatureMat 2009, 8, 62). The present work aims at understanding the parameters allowing to obtain the best ferroelectric performances and in the development of functional devices (Macromol. 2014, 47, 4711).
Charge dissipation in antistatic felts: The various electrical modes of AFM (CS-AFM, KPFM, EFM, …) are used to determine the parameters involved in electrical conduction, electrical charge repartition on the surface of textile and conducting fibres as well as between fibres in galvanic and non-galvanic contacts (Appl. Surf. Sci. 2015, 330, 65).
STM and STS: Structure and electronic properties of nanomaterials and thin films
Group of Bernard Nysten
Scanning Tunnelling Microscopy (STM) and Scanning Tunnelling Spectroscopy are used to resolve the atomic or molecular structure of nanomaterials and thin films.Recently, STM at the liquid-solid interface was used to study the self-assembly of oligothiophenes on MoS2 substrates (JPCC 2013, 117, 21143).
Dioctylterthiophene self-assembled monolayer on MoS2 imaged by STM at the liquid/solid interface