Periodic Reporting for period 4 - CUHL (Controlling Ultrafast Heat in Layered materials)
Reporting period: 2023-09-01 to 2024-05-31
Layered quantum materials - such as the semimetal graphene, the semiconducting transition metal dichalcogenides MoS2, and the insulator hexagonal BN - exhibit highly interesting electronic, optical, and mechanical properties. These properties have led to several device applications that are being explored and could have a disruptive technological, and therefore important socioeconomic impact. For many of these applications, and specifically for thermal management and thermoelectric applications, it is crucial to develop a proper understanding of the thermal properties of systems based on layered materials. Of particular importance is the nanoscale flow of heat, and the ultrafast interaction between different carriers of heat, in particular electrons and phonons, and light.
The general aim of CUHL was to develop this understanding of heat flow in layered quantum materials and to be able to control the dynamics and transport properties of electron and phonon heat, eventually towards enabling novel and/or improved device capabilities. We focused mainly on 2D layered materials, while also studying samples based on 1D materials, and Van Der Waals stacks. In most of our studies, we used light to study heat flow and dynamics, both employing established optothermal techniques, and by developing novel optothermal and optoelectronic techniques.
During the project we managed to establish a deepened understanding of both electron and phonon heat transport in 2D and 1D layered quantum materials, including several unconventional transport phenomena, and control these properties using thickness and using electrostatic gating. We also managed to understand and control heat dynamics in graphene, twisted bilayer graphene near the magic angle, and topological insulators, using the environment, incident power, electrostatic gating, and twist angle. Finally, we have used this understanding and control towards applications, in particular for photodetection, for the upconversion of terahertz light, and towards thermal management.
We conclude that layered materials indeed exhibit exciting heat transport and heat dynamics properties, including the promising potential to control these properties, and to exploit these controllable properties towards applications.
The general aim of CUHL was to develop this understanding of heat flow in layered quantum materials and to be able to control the dynamics and transport properties of electron and phonon heat, eventually towards enabling novel and/or improved device capabilities. We focused mainly on 2D layered materials, while also studying samples based on 1D materials, and Van Der Waals stacks. In most of our studies, we used light to study heat flow and dynamics, both employing established optothermal techniques, and by developing novel optothermal and optoelectronic techniques.
During the project we managed to establish a deepened understanding of both electron and phonon heat transport in 2D and 1D layered quantum materials, including several unconventional transport phenomena, and control these properties using thickness and using electrostatic gating. We also managed to understand and control heat dynamics in graphene, twisted bilayer graphene near the magic angle, and topological insulators, using the environment, incident power, electrostatic gating, and twist angle. Finally, we have used this understanding and control towards applications, in particular for photodetection, for the upconversion of terahertz light, and towards thermal management.
We conclude that layered materials indeed exhibit exciting heat transport and heat dynamics properties, including the promising potential to control these properties, and to exploit these controllable properties towards applications.
1. Development of novel methodologies
We have developed novel methodologies, and constructed home-built experimental setups, for i) the fabrication of material systems based on layered materials, see [Varghese 2021]; ii) the experimental study of electronic heat flow, see [Block2021]; iii) the experimental study of phononic heat flow, see [Varghese 2023]; and iv) the theoretical understanding of thermal properties of layered materials, see [Saleta2022, Farris2024]. These new methodologies and techniques offer important benefits over the state of the art, in particular: method i) offers the ability to fabricate record-large suspended TMDs with thickness down to the monolayer; technique ii) offers the possibility to follow electronic heat diffusion with ~20 nm spatial accuracy and ~200 fs temporal resolution thanks to a unique spatiotemporal photocurrent microscopy technique; technique iii) offers the possibility to determine the thermal diffusivity of thin films without the need for any material input parameters and with a temperature sensitivity below 1 K, thanks to a unique spatiotemporal optothermal technique; and method iv) offers the possibility to obtain theoretically phonon heat transport properties of systems with many atoms (up to at least 5 molecular TMD layers), using non-zero-Kelvin force constants.
2. Understanding and controlling phononic heat flow
Using these novel methodologies, in combination with established techniques that include Raman thermometry and time-domain thermoreflectance measurements, we have established a deepened understanding of phonon heat flow in layered semiconductors. In particular, we found that the thermal conductivity of TMDs is almost equally large for the monolayer case as for the bulk case, see [Saleta2022, Farris2024]. This is in strong contrast to the case of silicon, where the conductivity dramatically drops for thinner films, and confirms that TMDs are promising materials for (opto)electronic applications, especially if thin devices are required. Our most recent results point towards non-diffusive heat transport in ultrathin layered semiconductors [paper to be submitted]. We also studied phonon heat transport in graphene nanoribbons [paper under review] and in films of carbon nanotubes, which turned out to be useful for extreme ultraviolet lithography applications [Mehew2023].
3. Understanding and controlling electronic heat flow
Using one of the experimental techniques that we have developed (ii), we have followed in space and time the spreading of heat carried by electrons in graphene in the hydrodynamic regime, where heat spreads non-diffusively. In particular, we have found a giant thermal diffusivity of electrons in graphene and have demonstrated a controllable transition between two hydrodynamic regimes: the Fermi liquid regime and the quantum-critical Dirac fluid regime, see [Block2021]. Using a more conventional spatiotemporal transient reflection technique, we also showed that negative apparent diffusion can occur when electronic heat in gold is transferred to phononic heat [Block2023, Brinatti2024].
4. Understanding and controlling ultrafast heat dynamics
Using a variety of ultrafast pump-probe spectroscopy and microscopy techniques, we have studied the coupling between electrons and phonons in graphene-based systems, which allowed us to identify the dominant cooling pathway for hot electrons in graphene, which occurs via its optical phonons see [Pogna2021, Pogna2022, Massicotte2021], or via water modes in its environment [Yu2023]. We also studied hot carrier cooling in twisted bilayer graphene near the magic angle, which at low temperature has an electron-phonon cooling rate that is orders of magnitude larger than the one for non-twisted bilayer graphene, as a result of electron-phonon Umklapp scattering, which we observed for the first time [Mehew2024]. In topological insulators, we discovered a novel cooling mechanism that we call “Coulomb cooling”, see [Kovalev2021a, Principi2022].
5. Exploiting the understanding and control of heat flow and dynamics
Besides photodetection and thermal management applications, we realized that our control over electronic heat transport and dynamics were very useful for the efficient upconversion of light in the terahertz regime, see [Hafez2020, Deinert2021, Kovalev2021b, Tielrooij2022, Ilyakov2023].
We have developed novel methodologies, and constructed home-built experimental setups, for i) the fabrication of material systems based on layered materials, see [Varghese 2021]; ii) the experimental study of electronic heat flow, see [Block2021]; iii) the experimental study of phononic heat flow, see [Varghese 2023]; and iv) the theoretical understanding of thermal properties of layered materials, see [Saleta2022, Farris2024]. These new methodologies and techniques offer important benefits over the state of the art, in particular: method i) offers the ability to fabricate record-large suspended TMDs with thickness down to the monolayer; technique ii) offers the possibility to follow electronic heat diffusion with ~20 nm spatial accuracy and ~200 fs temporal resolution thanks to a unique spatiotemporal photocurrent microscopy technique; technique iii) offers the possibility to determine the thermal diffusivity of thin films without the need for any material input parameters and with a temperature sensitivity below 1 K, thanks to a unique spatiotemporal optothermal technique; and method iv) offers the possibility to obtain theoretically phonon heat transport properties of systems with many atoms (up to at least 5 molecular TMD layers), using non-zero-Kelvin force constants.
2. Understanding and controlling phononic heat flow
Using these novel methodologies, in combination with established techniques that include Raman thermometry and time-domain thermoreflectance measurements, we have established a deepened understanding of phonon heat flow in layered semiconductors. In particular, we found that the thermal conductivity of TMDs is almost equally large for the monolayer case as for the bulk case, see [Saleta2022, Farris2024]. This is in strong contrast to the case of silicon, where the conductivity dramatically drops for thinner films, and confirms that TMDs are promising materials for (opto)electronic applications, especially if thin devices are required. Our most recent results point towards non-diffusive heat transport in ultrathin layered semiconductors [paper to be submitted]. We also studied phonon heat transport in graphene nanoribbons [paper under review] and in films of carbon nanotubes, which turned out to be useful for extreme ultraviolet lithography applications [Mehew2023].
3. Understanding and controlling electronic heat flow
Using one of the experimental techniques that we have developed (ii), we have followed in space and time the spreading of heat carried by electrons in graphene in the hydrodynamic regime, where heat spreads non-diffusively. In particular, we have found a giant thermal diffusivity of electrons in graphene and have demonstrated a controllable transition between two hydrodynamic regimes: the Fermi liquid regime and the quantum-critical Dirac fluid regime, see [Block2021]. Using a more conventional spatiotemporal transient reflection technique, we also showed that negative apparent diffusion can occur when electronic heat in gold is transferred to phononic heat [Block2023, Brinatti2024].
4. Understanding and controlling ultrafast heat dynamics
Using a variety of ultrafast pump-probe spectroscopy and microscopy techniques, we have studied the coupling between electrons and phonons in graphene-based systems, which allowed us to identify the dominant cooling pathway for hot electrons in graphene, which occurs via its optical phonons see [Pogna2021, Pogna2022, Massicotte2021], or via water modes in its environment [Yu2023]. We also studied hot carrier cooling in twisted bilayer graphene near the magic angle, which at low temperature has an electron-phonon cooling rate that is orders of magnitude larger than the one for non-twisted bilayer graphene, as a result of electron-phonon Umklapp scattering, which we observed for the first time [Mehew2024]. In topological insulators, we discovered a novel cooling mechanism that we call “Coulomb cooling”, see [Kovalev2021a, Principi2022].
5. Exploiting the understanding and control of heat flow and dynamics
Besides photodetection and thermal management applications, we realized that our control over electronic heat transport and dynamics were very useful for the efficient upconversion of light in the terahertz regime, see [Hafez2020, Deinert2021, Kovalev2021b, Tielrooij2022, Ilyakov2023].
The new methodologies that we have introduced go well beyond the state of the art, and our scientific findings described above include several major breakthroughs in different fields, ranging from heat transport and phononics to ultrafast dynamics and nonlinear optics, and many more.