Bistable nanoparticles for thermal imaging at a high spatial and temporal resolution – THERMOSPIN

This fundamental research project aims at developing thermal nanocharacterization techniques with a high spatial and temporal resolution based on the use of bistable nanostructured molecular materials with predictable and controllable physical properties. This bistability is accompanied by a spectacular change of various physical properties (magnetic, mechanical and optical). Depending on the size and the composition of the molecular materials, the thermally activated spin state switching can occur with a rate from the nanosecond to the microsecond (around room temperature) enabling a very fast temperature characterization. In this project, metal nanowires, Joule-heated under various current injections in terms of amplitude and frequency modulation, will be used as thermal sources to explore a wide range of spatial and temporal windows and to validate the proposed techniques for solid and liquid phases.

Three major challenges will be considered to attain on-chip single nanoparticle detection to reach the targeted spatial and temporal resolution (less than 100 nm and a few hundreds of nanoseconds):

(i) Synthesis of molecular probes and fabrication of metallic nanosources along with their concomitant use to design functional thermal nanodevices :

• Bistable molecular probes will be synthesized as (re)dispersable nanoparticles with controlled size, shape, composition, surface chemistry and physical properties. The nanoparticle synthesis will be carried out by different methods including the use of capping ligands, surfactants, polymers and host matrices. The nanoparticles will be characterized using static and time-resolved spectroscopic methods.
• Metallic nanowires will be fabricated by e-beam lithography or metal electrodeposition into porous membranes. In the latter case, dedicated assembly techniques, based on dielectrophoresis or capillary-driven flows, will be used to obtain functional nanowires-based thermal nanodevices.
• The molecular probes will be attached on the surface of metallic nanowires for thermal characterization in solid phase using directed assembly methods. Alternatively, colloidal suspension of molecular probes will be used for thermal characterization in liquid phases.

(ii) Modeling and simulation :
• Physical properties of nanostructured bistable molecular materials will be simulated using microscopic elastic models on account of the elastic origin of the inter-molecular interactions in these materials.
• Heat transfer within metallic nanowires in steady-state and transient regimes for solid and liquid phases will be obtained both from analytical approaches or finite element methods.

(iii) Thermal nanocharacterization :
• In a first phase, fluorescence techniques will be developed by taking advantage of the fluorescence quenching effect which takes place within a given range of emission wavelengths for fluorophores in the presence of bistable molecular materials during spin transition. It is interesting to note that in many cases one can observe spectral changes in opposite directions within the same material (depending on the probe wavelength) providing thus a straightforward way for two-color measurements. In order to achieve single-particle detection we will design systems with efficient (resonant) energy transfer and implement state-of-the art detection techniques (colloidal lenses, FCS).
• In a second, complementary approach, surface Plasmon resonance (SPR)-based detection techniques will be employed. To this aim, core-shell systems comprising a nobel metal (Au, Ag) core and a bistable shell will be designed, which are expected to display intense light scattering with strong temperature dependence.
• In parallel, thermal characterization based on electrical measurements will be performed to get additional information on the transient regime.