Probing of Thermal Transport in 50 nm Thick PbTe Nanocrystal Films by Time-Domain Thermoreflectance

Abstract

Bottom-up fabrication of thermoelectric (TE) materials from colloidal nanocrystal (NC) building blocks can substantially increase their TE efficiency, e.g., by reducing lattice thermal conductivity. In this work, we first synthesized 10-nm spherical phase-pure oleate-capped PbTe NCs with narrow size distribution and employed them to fabricate the 110-nm thick films on insulating SiO2/Si substrates. Here, we used the spin-coating with subsequent ligand exchange procedure to ensure strong coupling interactions between the NCs. Using the dark conductivity measurements, we confirmed the semiconducting behavior and the Schottky-type electrical field-dependent conductivity mechanism in the resultant thin films. We then probed the thermal transport in the thin-film by means of a time-domain thermoreflectance (TDTR) method. For this purpose, we used a customized state-of-the-art system based on a picosecond thermoreflectance instrument, which enables area-selective analysis with the spatial resolution down to 5 m. The results show that as-fabricated PbTe NC films exhibit ultralow thermal conductivity of ca. 1.52 W m-1 K-1. All in all, the transport properties findings suggest potential of the proposed quick and cost-effective spin-coating strategy for bottom-up fabrication of nanostructured TE films from high-quality colloidal NC building blocks.

Template-directed self-organization of colloidal PbTe nanocrystals into pillars, conformal coatings, and self-supported membranes

“…1b), in turn, provides an example of having to carefully optimize the synthesis procedure, 24 including the surface ligands, to achieve excellent material properties. [22][23][24] Accordingly, we are focusing on self-organization protocols that require minimal chemical modication of the NCs, whether via surface functionalization 8,10 or strong interactions with the matrix. 17,19 Furthermore, these PbTe NCs can be solutionprocessed in large quantities, making them a realistic model of NCs designed and produced for industrial applications.…”

“…These close-packed thin layers of NCs should be suitable for applications that rely on proximal interactions between the NCs. 24,29,30 Uniform conformal coatings over the entire microstructured surface could be useful in optical 31 or energy 32 applications. Alternatively, the coating outside of the patterned cavities could be removed, e.g., by using a strong tape, for applications where a thin continuous NC coating is needed only inside the micropatterned areas.…”

“…Our systematic investigation of template-directed selforganization of model NCs into close-packed microstructured morphologies has explored a phase space of solvent-induced self-organization that lls the gap between the controlled deposition of individual nanoparticles 9,28 onto micro-or nanostructured templates and formation of unstructured thin lms from NCs. 24,26,33 While some of the parameters, such as solvent and evaporation rate, are shared between our work and the template-directed deposition of individual nanoparticles, the underlying mechanisms exploited in such protocols typically are based on a match between the size of an individual particle, template dimensions, and the range of interactions between the particles and template features. 9,28 In contrast, our protocols clearly rely on collective behavior of NCs interacting with the template features that are much larger than the individual NCs.…”

“…Versatility and complexity of self-organized NC structures and morphologies are commonly achieved by taking advantage of the controlled particle-particle and/or particle-solvent interactions produced by surface modication of the constituent NCs and/or the template. 2,10,19,25 Producing NC-based materials with novel or unique properties, however, oen requires maintaining the as-synthesized high quality of the constituent NCs, e.g., in terms of monodispersity, internal structure, or physicochemical characteristics, 1,22,24,26 all of which, in general, may be compromised by post-synthesis modications. Accordingly, we are interested in achieving the diversity of self-organized morphologies without requiring (or assuming) specialized surface properties of NCs, beyond their basic colloidal stability that is intrinsic to the colloidal NC synthesis.…”

Synthesis of Au/SnO2 nanostructures allowing process variable control

“…Only morphologically, Au-decorated SnO 2 nanostructures can be prepared in any number of ways. However, most of these methods refrain from instantaneous processing to increase the cross-sectional area and require various pre-and post-processing techniques [19][20][21] . For example, Kim et al synthesised noble metal-decorated SnO 2 nanowires using thermal activation and employed the large cross-sectional area of these nanoparticles to detect noxious gases 22 .…”