Quantitative measurement and observation of thermal phenomena on nano-objects at the nanoscale is possible thanks to the Scanning Thermal Microscope. With this local probe microscope, we differentiate the thermal conductivity of nanometer thick films made of two types of molecules of the same family. This experimental and theoretical study allows to correlate the thermal conductivity with the atomic structure of the molecules and their organization in a thin film.
The measurement of thermal properties at the nanometer scale is of increasing interest to characterize nano-objects (individual molecule, monolayer, 2D material or 1D material), but also to monitor the operation of electronic nano-components (transistor). We used the Scanning Thermal Microscope (SThM), a microscope from the large family of local probe microscopes, to characterize thermally and at the nanometer scale organic films with a thickness between 40 and 400 nm. Two types of films have been characterized, each made of a different molecule; one composed of four rings and named BTBT, the other molecule has in addition to BTBT two alkyl chains of 8 carbon atoms added on both sides of the molecule (named C8-BTBT-C8).
These non-commercially available molecules have been synthesized by a chemist partner at the Université Libre de Bruxelles (Belgium). Already known for their performances in organic transistors with electron mobilities reaching sometimes 200 cm² V-1 s-1, we explore here the thermal properties of these promising molecules. Indeed, few studies have focused on their thermal properties, while these molecules are good candidates for thermoelectricity applications (low thermal conductivity).
The SThM study of these materials allows to access both the topography of the surface at the nanometer scale and also to measure the thermal conductivity of the film at a precise location on the surface. This conductivity value is obtained directly after a calibration phase on samples of known thermal conductivity. The BTBT and C8-BTBT films have thermal conductivities of 0.6 -1.3 W m-1 K-1 and 0.3 – 0.7 W m-1 K-1 respectively, showing the impact of the alkyl chain on the vibration modes of the molecules and thus on the thermal conductivity. These differences in thermal conductivities were related to the organization of the molecules in the film using theoretical calculations performed at the University of Mons (Belgium), the Catalan Institute of Nanosciences and Nanotechnologies (Barcelona, Spain) and the University of Cagliari (Italy). This work opens opportunities for the use of SThM in nanotechnology, but also for the study of these organic materials for thermoelectricity.
Read more :
https://dx.doi.org/10.1039/D0NR08619C
https://hal.archives-ouvertes.fr/hal-03135025v1
