Thermoelectric properties of ultra-thin film formed by molecular self assembly and Langmuir-Blodgett deposition
Creators: name: Lamantia, Angelo
Publisher: name: Lancaster University
Year: 2021
Subjects: Thermoelectric properties
Electric transport
Molecular electronics
Molecular junctions
Scanning probe microscopy
Thermoelectric Force microscopy
Langmuir-Blodgett films
SAMs

description: In the era of nanotechnology and nanosciences, the scalability and miniaturisation of the electronics component in a single chip to obtain superior devices is crucial. This aim creates new challenges for the scientific community in order to develop the materials of the future and understand their behaviour at the nanoscale. Indeed, new phenomena require specific matching tools that, in turn, open new investigation prospects. Among the branches of nanoscience, molecular electronics and, in particular, thermoelectricity of molecular-based structures or devices, have gained a discrete interest especially in the past decade, not only for the basic physical knowledge of the phenomena but also for the perspective of technological developments, making molecules and molecular assembly a particular and interesting substitute to the conventional semiconductors. Whereas conventional techniques to investigate the thermoelectric properties of molecular assembly are usually not efficient or designed to study these "soft" materials, Conductive Atomic Force Microscopy (CAFM) o ers high sensitivity to the nanoscale electric and thermoelectric properties and a spatial resolution of few nm. In addition, AFM-based techniques provide outstanding control of the normal force applied by the nanometric probe on the molecules, preserving their conformation and properties. This thesis aims to find a suitable, non-destructive and reproducible way to measure the thermoelectric effect in molecular self-assembly and molecular thin-films created by Langmuir-Blodgett deposition based on CAFM. The developments of CAFM in intermitting contact mode by combining it with Peakforce AFM (PF-AFM) allows the measurements of the electrical properties and the thermoelectric signature of the molecular films preserving the molecular conformation, thanks to the soft contact ensured by the intermitting contact. Also, by combining those two AFM modes, it was possible acquiring electric and nanomechanical properties simultaneously, which has never been reported before. The developed technique was employed to measure the thermoelectric properties of different families of molecules. The measurement system was initially tested with some well-known alkyl thiolate molecules. More complex structures were also analysed to find the dependency of the thermal power on the thickness of the molecular film and the eventual chemical structure itself. Interestingly, results were found to be quite sensitive to the growth condition of the molecular samples too, revealing outstanding thermal power values for the thicker films growth by LB technique. The research presented in this thesis has then a two-fold impact. On the one hand, the measurement setup development, which represents the major challenge, was answered by developing and studying an intermitting contact technique based on CAFM and PFAFM, opening a new route for further investigations. On the other hand, new insights from the properties studied during the measurements were obtained on the molecular film quality and uniformity dependence. In addition, the information gained from the doping of metallo-porphyrin molecules opens a new way to tailor the thermal power of molecular assembly ex-situ.