• calorimetry
• gold
• mica

This thesis reports the research work on the development of a sensitive atomically flat gold film thermometer which exploits mica as substrate. Besides being an insulator, this mineral boasts a low thermal conductivity that allows an improved thermal decoupling of the sensor from its substrate, if compared to the largely utilized silicon (Si thermal conductivity is about 300 times higher). It is also mechanically soft and elastic, useful for applications in flexible electronics. Moreover, a mica substrate allows the gold re-crystallization, thus permitting the investigation of the processes which can affect the surface of the sensor itself, or of a sample, with atomic resolution, by the use of Scanning Tunneling Microscope (STM).
This entire investigation has been realized at the facilities of Laboratorio NEST of Scuola Normale Superiore. The sample preparation has been performed in clean and controlled conditions (clean room), and the measurements have been taken in an Ultra-High Vacuum environment (UHV chamber of a RHK Technology Scanning Tunneling Microscope).
This study is inserted in the framework of calorimetric techniques, which are useful means to investigate the properties of matter by measuring the heat exchange during a system evolution. Indeed, an energy flux accompanies any evolution of a system, giving invaluable information on the processes underlying the evolution itself. A large number of commercial calorimeters can be found, specialized in studying solid or liquid samples, phase transitions and chemical reactions generally. However, typical commercial calorimeters require a sample mass in the mg range and have a quite limited sensitivity in energy (∼ mJ). Very sensitive thermometric techniques have been successfully developed to measure milliKelvin temperature differences in nano-scale devices, mainly in thermoelectric ones. Such thermometers, however, can operate only at low temperatures (below a few Kelvin) or at least their sensitivity drops to tenths of Kelvin at room temperature.
In this context we present the improvement of a recently reported thermometric technique, which has been utilized to monitor the hydrogen storage in titanium-functionalized mono-layer graphene [1]. The actual focus of our upgrade is the substrate of the sensor: mica has been chosen to replace silicon in order to gain in terms of sensitivity but mostly to exploit its atomically flat surface. Indeed it allows the gold re-crystallization, thus permitting to take advantage of the STM potentiality to investigate the surface of the sample after an experiment of hydrogen adsorption, for example, or any other process which could affect the structure of the sensor/sample.
For testing the new sensor, an experiment of atomic hydrogen adsorption on the gold film itself has been performed. The old sensor has been subjected to the same experiment, as well, in order to compare the performances of the two. This testing has highlighted the potentiality of our sensor: indeed it allowed to study a change in the gold surface reconstruction that only an atomically flat surface could reveal.
The first important result of this thesis work is the improvement, in terms of sensitivity, of gold film thermometers. Indeed the heat transfer coefficient, which represents the heat losses of the sensor (gold layer) towards the substrate, turns out to be more than one order of magnitude lower than previously reported: λ ∼ 2×10^(−7) W/K (to be compared with λ ∼ 5×10^(−6) W/K of silicon), thus improving the thermal decoupling by a factor ∼25.
A drawback of this thermometer is related to the operating temperature range. Indeed, the gold film damages at temperatures higher than ∼250 ◦C. A possible solution to this issue could be an additional Ti layer underneath gold, utilizing the fabrication procedure set up in this thesis work.
The gold surface re-crystallization offers large flat terraces suited to investigate the processes which can affect the surface structure with atomic resolution. This new feature, coupled with a better sensitivity, could be exploited to repeat the experiment of hydrogen storage described in Ref. [1], in order to compare the performances of the two, but also to study the graphene functionalization with different metals (such as alkali metals) or organic molecules.
Moreover, the atomic hydrogen supply experiment could be repeated, in order to better investigate the peculiar reconstruction pattern observed on the gold surface after the hydrogenation, with both STM and LEED analysis (the latter has not been performed yet).
In the present era of “Internet-of-Things”, the demand for flexible, light-weight, low-cost, lowpower consumption, multifunctional and environmentally friendly electronics has moved to the forefront of materials science research. In this framework, mica properties make this sensor suitable to have a remarkable impact on flexible electronics.

[1] L. Basta, S. Veronesi, Y. Murata, Z. Dubois, N. P. Mishra, F. Fabbri, C. Coletti, and S. Heun. A sensitive calorimetric technique to study energy (heat) exchange at the nanoscale. Nanoscale, 10, 10079 (2018).