Plasmonic high-entropy carbides

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Plasmonic systems have received considerable attention because they are able to couple collective electron density oscillations with external electromagnetic radiation, concentrating energy down below the diffraction limit.  This unique capability can be profitable in a variety of applications that encompass telecommunications, antennas, molecular sensing and heat engineering. A few applications exploit possibility to transmit, manipulate or harvest light in metal/dielectric interfaces through plasmon excitations, while others take advantage of the energy release associated to the plasmon relaxation as nanoscale heat source. The heterogeneity of applications, structures, physical processes, and working conditions demands for an extended tunability and multi-functionality of the active materials which cannot be restricted to the traditional class of plasmonic noble metals (e.g. Au, Ag, Cu).

In particular, the research for materials that are simultaneously plasmonic in the near-IR-visible range, compositional tunable, mechanically resistant, chemically and thermally stable still remains a challenge. This work tackles this huge task by exploiting the design capability of state-of-the-art computational material science to predict a new class of plasmo-mechanical multifunctional materials (PHECS) that can be used in harsh environments and ultra-high temperatures. New dedicated experiments confirm our theoretical discovery.

Traditionally, concepts like structural order and/or simple chemical composition have been the lighthouse to search or design new materials. However, we renounced the paradigm “the simpler the better” and - paying the cost of a higher structural complexity - we took advantage of the exceptional properties of a new class of disordered multi-compositional materials, known as transition-metal high-entropy carbides (HECs). HECs, discovered in 2018 by some of the authors, exhibit attractive characteristics like extraordinary thermal stability, hardness, strength, toughness, as well as wear and oxidation resistance. Here, we proposed and demonstrated that some HECs have plasmonic properties in the near-IR/vis range, whose characteristics can be tuned by controlling the chemical composition and the stoichiometry.

We evaluated the plasmonic properties of HECs by employing first principles time-dependent density functional theory (TDDFT) to calculate the complex dielectric function, which fully characterizes the material response to an external electromagnetic field. The prerequisite to study the electronic and optical properties of disordered systems is the identification of reliable atomic structures. In this work, we exploited an original approach (Partial Occupation module – POCC) developed within the AFLOW infrastructure, that factorizes structural disorder as the sum of many microscopic configurations, weighted according to Boltzmann statistics. For each optimized POCC structure, we calculated the complex dielectric function and the corresponding electron energy loss (EELS) function. Then, the overall spectra of the disordered material were obtained as an ensemble average on the POCC structures. The accuracy of our combined thermodynamic/TDDFT approach has been firstly demonstrated by comparing the theoretical results for the disordered AuAg testbed case with the corresponding experimental data.

We started the investigation on carbides by considering the optical properties of the archetype HfTa4C5, which is a well-established compound with exceptional thermal and mechanical properties. Simulations predicted a plasmonic resonance in the visible range which remains extremely stable even at ultra-high temperature, making it the first instance of a multifunctional disordered carbide.  Direct STEM/EELS measurements on HfTa4C5 confirmed the theoretical results over a wide temperature range T=300-1500K (Figure 1, panel a), well above the melting point of noble metals. 

Based on our findings, we propose a set of 14 transition-metal high-entropy carbides, whose stability and crystalline phase was recently demonstrated. These are six element alloys: carbon and a combination of 5 different transition metals among {Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W}. All compounds have optical properties similar to parent HfTa4C5 system. In particular, they all exhibit a crossover energy E0 (i.e. energy at which the real part of the dielectric function switches sign upon incoming electromagnetic field) in the infrared and visible range, whose value can be tuned with composition (Figure 1, panel b). For a few systems (HfNbTaTiZrC5, HfNbTaTiWC5) this corresponds to the excitation of a low-loss plasmonic resonance, which is expected to remain stable at ultra-high temperature.

 

Figure 1. a Experimental EELS spectra of HfTa4C5 as a function of temperature. Inset reports the corresponding theoretical spectra, evaluated at the same temperatures. b Trend of the crossover energy (E0) in the simulated plasmonic HECs. Properties of simulated HECs, ordered in classes according the group number gX (X = 4 ,5, 6) of transition metals included in the compound.

The combination of plasmonic activity, high-hardness and extraordinary thermal stability, makes HECs the first example of multifunctional plasmo-mechanical materials. When further confirmed by experiments this would open a new route to high temperature plasmonics and to a new generation of optical devices working in harsh conditions. Overall, the discovery of plasmonic activity in high entropy carbides (PHECS) awakens the exploration of new structure-property correlations in this complex class of systems, opening up exciting avenues for future research.

 

Acknowledgments

This work was supported by DOD-ONR N00014-21-1- 2132, N00014-20-1-2525, N00014-20-1-2299.

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