Engineering heat transport across epitaxial lattice-mismatched van der Waals heterointerfaces

Artificially engineered 2D materials offer unique physical properties for thermal management, surpassing naturally occurring materials. Here, using van der Waals epitaxy, we demonstrate the ability to engineer extremely insulating ultra-thin thermal metamaterials based on crystalline lattice-mismatched Bi2Se3/MoSe2 superlattices and graphene/PdSe2 heterostructures with exceptional thermal resistances (70-202 m^2K/GW) and ultralow cross-plane thermal conductivities (0.01-0.07 Wm^-1K^-1) at room temperature, comparable to those of amorphous materials. Experimental data obtained using frequency-domain thermoreflectance and low-frequency Raman spectroscopy, supported by tight-binding phonon calculations, reveal the impact of lattice mismatch, phonon-interface scattering, size effects, temperature and interface thermal resistance on cross-plane heat dissipation, uncovering different thermal transport regimes and the dominant role of long-wavelength phonons. Our findings provide essential insights into emerging synthesis and thermal characterization methods and valuable guidance for the development of large-area heteroepitaxial van der Waals films of dissimilar materials with tailored thermal transport characteristics.Comment: 25 page 4 figure

Spectroscopic and Thermal Characterization of Extra Virgin Olive Oil Adulterated with Edible Oils

The substitution of extra virgin olive oil with other edible oils is the primary method for fraud in the olive-oil industry. Developing inexpensive analytical methods for confirming the quality and authenticity of olive oils is a major strategy towards combatting food fraud. Current methods used to detect such adulterations require complicated time- and resource-intensive preparation steps. In this work, a comparative study incorporating Raman and infrared spectroscopies, photoluminescence, and thermal-conductivity measurements of different sets of adulterated olive oils is presented. The potential of each characterization technique to detect traces of adulteration in extra virgin olive oils is evaluated. Concentrations of adulterant on the order of 5% can be detected in the Raman, infrared, and photoluminescence spectra. Small changes in thermal conductivity were also found for varying amounts of adulterants. While each of these techniques may individually be unable to identify impurity adulterants, the combination of these techniques together provides a holistic approach to validate the purity and authenticity of olive oils

Thermal transport in silver-coated polymer sphere composites by the bidirectional 3ω method

The bidirectional 3ω method is an electrothermal technique that is commonly used to obtain the thermal conductivity of materials such as liquids, biological samples and pastes. In this work, an epoxy-based adhesive was filled with monodisperse 10 μm polymethyl methacrylate (PMMA) spheres coated with silver thin films (AgPS), such that a metallic network that dominated the thermal transport was formed through the composite. The bidirectional 3Ω method was used to obtain the thermal conductivity of the conductive adhesive at different volume fractions of AgPS. For 50 vol.% AgPS, corresponding to 3.4 vol.% silver, the thermal conductivity was 2.03 ± 0.21 W/m-1K-1. The results show that the thermal conductivity is strongly correlated with the AgPS volume fraction, while maintaining a volume fraction of silver far below commercial silver paste, which has typical filler fractions of 40 vol.\% silver. The results of this work were compared to thermal measurements of the same material by other techniques, and advantages and disadvantages of the methods were finally discussed

Modification of the Raman Spectra in Graphene-Based Nanofluids and Its Correlation with Thermal Properties

It is well known that by dispersing nanoparticles in a fluid, the thermal conductivity of the resulting nanofluid tends to increase with the concentration of nanoparticles. However, it is not clear what the mechanism behind this phenomenon is. Raman spectroscopy is a characterization technique connecting the molecular and macroscopic world, and therefore, it can unravel the puzzling effect exerted by the nanomaterial on the fluid. In this work, we report on a comparative study on the thermal conductivity, vibrational spectra and viscosity of graphene nanofluids based on three different amides: N, N-dimethylacetamide (DMAc); N, N-dimethylformamide (DMF); and N-methyl-2-pyrrolidinone (NMP). A set of concentrations of highly stable surfactant-free graphene nanofluids developed in-house was prepared and characterized. A correlation between the modification of the vibrational spectra of the fluids and an increase in their thermal conductivity in the presence of graphene was confirmed. Furthermore, an explanation of the non-modification of the thermal conductivity in graphene-NMP nanofluids is given based on its structure and a peculiar arrangement of the fluid

Thermoreflectance techniques and Raman thermometry for thermal property characterization of nanostructures

The widespread use of nanostructures and nanomaterials has opened up a whole new realm of challenges in thermal management, but also leads to possibilities for energy conversion, storage and generation, in addition to numerous other technological applications. At the microscale and below, standard thermal measurement techniques reach their limits and several novel methods have been developed to overcome these limitations. Among the most recent, contactless photothermal methods have been widely used and have proved their advantages in terms of versatility, temporal and spatial resolution, and even sensitivity in some situations. Among them, thermoreflectance and Raman thermometry have been used to measure the thermal properties from bulk materials to thin films, multilayers, suspended structures and nanomaterials. This tutorial presents the principles of these two techniques and some of their most common implementations. It expands to more advanced systems, for spatial mapping and for probing of non-Fourier thermal transport. Finally, this paper concludes with discussing the limitations and perspectives of these techniques and future directions in nanoscale thermometry

Enhancement of Thermal Boundary Conductance of Metal–Polymer System

In organic electronics, thermal management is a challenge, as most organic materials conduct heat poorly. As these devices become smaller, thermal transport is increasingly limited by organic–inorganic interfaces, for example that between a metal and a polymer. However, the mechanisms of heat transport at these interfaces are not well understood. In this work, we compare three types of metal–polymer interfaces. Polymethyl methacrylate (PMMA) films of different thicknesses (1–15 nm) were spin-coated on silicon substrates and covered with an 80 nm gold film either directly, or over an interface layer of 2 nm of an adhesion promoting metal—either titanium or nickel. We use the frequency-domain thermoreflectance (FDTR) technique to measure the effective thermal conductivity of the polymer film and then extract the metal–polymer thermal boundary conductance (TBC) with a thermal resistance circuit model. We found that the titanium layer increased the TBC by a factor of 2, from 59 × 106 W·m−2·K−1 to 115 × 106 W·m−2·K−1, while the nickel layer increased TBC to 139 × 106 W·m−2·K−1. These results shed light on possible strategies to improve heat transport in organic electronic systems

Spectroscopic and thermal characterization of extra virgin olive oil adulterated with edible oils

he substitution of extra virgin olive oil with other edible oils is the primary method forfraud in the olive-oil industry. Developing inexpensive analytical methods for confirming the qualityand authenticity of olive oils is a major strategy towards combatting food fraud. Current methodsused to detect such adulterations require complicated time- and resource-intensive preparation steps.In this work, a comparative study incorporating Raman and infrared spectroscopies, photolumines-cence, and thermal-conductivity measurements of different sets of adulterated olive oils is presented.The potential of each characterization technique to detect traces of adulteration in extra virgin oliveoils is evaluated. Concentrations of adulterant on the order of 5% can be detected in the Raman,infrared, and photoluminescence spectra. Small changes in thermal conductivity were also foundfor varying amounts of adulterants. While each of these techniques may individually be unableto identify impurity adulterants, the combination of these techniques together provides a holisticapproach to validate the purity and authenticity of olive oils.ICN2 is supported by the Severo Ochoa program from the Spanish Research Agency (AEI, grant no. SEV-2017-0706) and by the CERCA Programme/Generalitat de Catalunya. ICN2 authors acknowledge the support from the Spanish MICINN project SIP (PGC2018-101743-B-I00). A.C.-A. acknowledges the support from Fondecyt Iniciacion 11200620. R.C.N. acknowledges funding from the EU-H2020 research and innovation program under the Marie Sklodowska Curie Fellowship (Grant No. 897148)

Comparison of Brillouin Light Scattering and Density of States in a Supported Layer: Analytical and Experimental Study

International audienceWe provide a detailed analytical calculation of the Brillouin light scattering (BLS) intensity of a layer on a substrate, taking into account both photoelastic and moving boundary (ripple effect) mechanisms, and give a comparison between BLS intensity and density of states (DOS) to determine the dispersion curves of longitudinal guided modes in the supported layer. In particular, in the case where the mismatch between the elastic parameters of the substrate and the adsorbed layer is high, such as in a PMMA layer on a Si substrate, we derive closed-form expressions of BLS and DOS and demonstrate a simple relationship between these two quantities. A very good agreement between experimental and theoretical BLS spectra was found and compared to theoretical DOS spectra. In particular, we show that while the peaks in the DOS present a uniform behavior, the BLS spectra follows a sine cardinal (sinc) function shape around a given frequency fixed by the chosen laser wavelength. The theoretical calculation is performed within the framework of the Green’s function approach