Workplan of the project

This highly interdisciplinary project (it involves elements of chemistry, material science, condensed matter physics, electronics) will exploit recent advances in growth and characterization of novel TMD based nanostructures, as well as state-of-the-art theoretical modelling. It will be divided in three scientific and three management workpackages (WP). The researcher’s activities will span from material synthesis and device fabrication (WP1), to their structural and chemical characterization (WP2) and the measurements of thermal and thermoelectric properties by means of Scanning Probe Microscopy (SPM) (WP3). The results from WP2 and 3, as well as the theoretical modelling (WP1), will be used as feedback to optimize the fabrication procedure.

WP1. Materials design, synthesis and device fabrication. This task constitutes the principal technological objective of the project. Thermal and thermoelectric transport mechanisms depend on numerous parameters, hence the importance of fabricating the structures as per the designed specifications to properly identify the parameters impacting transport properties. In this WP the researcher will perform and optimize the growth of the TMD nanoscale films (PdSe2, MoSe2 and SnSe2) and graphene using MBE and CVD, respectively, at the host institute. Several substrates such as sapphire, AlN/Si, InAs/Si will be used to grow TMDs and the optimal one will be chosen so as to be compatible with the transport measurement requirements. For the MBE growth, the refractory metals Mo, Pd etc. will be evaporated by e-gun, while Se will be thermally evaporated from an effusion cell, using high Se overpressure to ensure sufficient Se incorporation to achieve stoichiometric TMD material. The growth conditions (growth temperature, growth rate etc.) and the epitaxial MBE growth of TMD heterostructures have already been optimized (see our publications in Part B_2), which will help to make a fast start in the project. Mechanical exfoliation will also be investigated to obtain flakes of materials, such as graphene and hexagonal Boron Nitride (h-BN), which will be initially grow in a copper foil. The h-BN will be used as electrically insulating layer on top of the investigated TMD material. The graphene-TMD lateral interfaces will be patterned by using polymer-based transfer methods and the vdW pick up method. This task also includes standard cleanroom fabrication, i.e., e-beam lithography, to pattern metal contacts and graphene ribbons on top of graphene and TMD materials, respectively, which will be used as electrodes (see Fig. 1). To explore the impact of electrical contacts and sheet edges on thermal and thermoelectric transport measurements, molecular dynamic (MD) simulations will be conducted in collaboration with Prof. C. Lambert (ULANC). This will help to optimize the design of the 2D nanostructures, fabrication procedure and the application properties of the proposed devices. 

WP2. Structural and chemical characterization. The researcher will use a set of characterization techniques at the host institute to optimize the quality and geometry of the fabricated TMD nanostructures by giving feedback for WP1 and 2, but also to single out the impact of structural and chemical parameters on thermal transport. The studies will include AFM, SEM and TEM to evaluate the quality and geometry of the interfaces, including size, shape and rugosity, i.e., surface roughness, known to strongly impact thermal transport in 2D nanostructures. The stoichiometry of the TMD materials will be monitored by in-situ XPS. Raman spectroscopy will complete the structural investigation, as it gives structural information, i.e., number of TMD layers. In parallel, the researcher will adapt the existing in-situ STM of the host institute for thermal mapping selected heterostructures by implementing a resistive tip sensor/heater and a proper electronic bridge circuit. Combined with STM measurements, this will allow to simultaneously explore the impact of the interface quality and defects density of the fabricated epitaxial heterostructures on heat transport measurements. Hence, the researcher will assess the robustness of transport properties as a function of the fabrication conditions. Finally, the electronic band structure of the TMDs will be imaged by Angle-Resolved Photoemission Spectroscopy (ARPES).

WP3. Thermal and thermoelectric characterization of 2D nanostructures. This task is based in the WP1 and WP2 developments and constitutes the ultimate scientific objective of the project. The researcher will use SPM methods to characterize the fabricated TMD based structures, for which the collaborating University (ULANC) has leading expertise. The custom-built ultra-high-vacuum (UHV) SPM set up at ULANC, will allow the operation of highly sensitive thermal and thermoelectric measurement at the fabricated nanostructures with sub-nW heat flux resolution, sub-mK and sub-10 nm temperature and spatial resolution, respectively. The thermal microscope was proven to be of great value in the direct investigation of the local Seebeck coefficient, Joule heating and Peltier effects in 2D material nanostructures without compromising their surface quality. The measurements will beperformed at room temperature in UHV experimental conditions (pressure ~ 10-9 mbar), where parasitic heateffects are eliminated. In particular, in the THERMIC project we propose two kind of measurements: 

(A) Joule heating and Peltier effects: Imaging nanoscale temperature fields in self-heating TMD nanostructures will be essential to investigate local Joule and Peltier effects and reveal nanoscopic hotspots and heating/cooling effects. To perform these measurements an AC voltage bias will be applied in the investigated structure in order to generate an AC current bias that will result in Joule heating and Peltier heating/cooling. The Peltier effect is expected to show a linear dependence with the electrical current, which in this case is in the same frequency with the applied voltage bias (𝑓𝑒𝑥𝑐), while the Joule heating a quadratic response that can be measured in the higher harmonic response (2nd harmonic) of the tip. Therefore, by measuring the temperature response of the SPM tip as it will be scanned over the AC biased sample and modulating it at the first (Peltier) and second (Joule) harmonic, it is possible to decouple the two effects and extract the respective heating/cooling values (see Fig. 1).

(B) Seebeck coefficient: For local Seebeck measurements the SPM tip will be heated by applying a high AC voltage to it while the global voltage drop (𝑉𝑡h) over the device will be measured at the second harmonic during scanning conditions (new mode of STGM pioneered by ULANC). The generated heat from the tip will modify the potential voltage drop across the two electrical terminals and a 2D nanoscale map of the thermoelectric response will be recorded.

Nanoscale mapping of local Joule heating, Seebeck and Peltier effects in nanostructures proposed in this action (e.g., graphene-TMD lateral interface)