Periodic Reporting for period 4 - PHONUIT (Phononic Circuits: manipulation and coherent control of phonons)
Berichtszeitraum: 2022-07-01 bis 2023-10-31
The power to control phonons, which are quantized lattice vibrations responsible for the transmission of sound and heat, would lead to extraordinary technological developments in thermal applications. Phonons play a crucial role in solid-state physics also in the interaction with electrons and photons. Therefore, the need to understand and control phonons arises also from optoelectronic and electronic applications, where interaction with phonons is often the bottleneck of device performances.
The controlled modification of phonon dispersion, interactions, and transport define the concept of phonon engineering, which allows controlling heat propagation and tuning electron-phonon and photon-phonon interactions. The combination of phonon engineering and nanostructuring defines the nanophononics field and is one of the most promising routes for thermal management. In this context, new theoretical and experimental methods are required.
With PHONUIT, we tackle the challenges of nanophononics both from the materials point of view, i.e. engineering the phononic properties of nanostructures, as well as from the methodology point of view, i.e. pushing the limits of currently existing experimental methods and developing novel measurements methods and platforms. Specifically, this project aims to realize a phononic integrated circuit, where phonons in coherent and/or incoherent form are generated, routed and detected on chip.
The controlled modification of phonon dispersion, interactions, and transport define the concept of phonon engineering, which allows controlling heat propagation and tuning electron-phonon and photon-phonon interactions. The combination of phonon engineering and nanostructuring defines the nanophononics field and is one of the most promising routes for thermal management. In this context, new theoretical and experimental methods are required.
With PHONUIT, we tackle the challenges of nanophononics both from the materials point of view, i.e. engineering the phononic properties of nanostructures, as well as from the methodology point of view, i.e. pushing the limits of currently existing experimental methods and developing novel measurements methods and platforms. Specifically, this project aims to realize a phononic integrated circuit, where phonons in coherent and/or incoherent form are generated, routed and detected on chip.
We have worked in parallel on the different aspects that are needed for the design and development of integrated phononic circuit:
1. Enhancement of existing methods & development of novel methods
The investigation of phononic properties of nanomaterials is a challenging objective. We use a diverse approach combining inelastic light scattering spectroscopy and thermal conductivity measurements, pushing the limits of currently existing experimental techniques as well as developing new ones.
a. We have realized a set-up to perform space- and time-dependent pump-probe experiments, like time-resolved Raman Spectroscopy and transient reflectivity. This set-up enables us to perform also space dependent pump-probe for hot spot relaxation measurements to extract fundamental heat transport parameters.
b. We have developed a method for measuring thermal conductivity of nanowires (NWs) with a combination of resistive heating and Raman thermometry. We can obtain interesting information about the NW thermal conductivity and heat transfer at the nanoscale.
c. We have developed a method to measure optical absorption of single nanostructures based on accurately measuring the heat flow coming from a wire when it is irradiated by a laser beam.
d. We have realized a device to measure the in-plane thermal properties of nanomaterials in multiple directions.
2. Phonon engineering
In order to excite phonons with desired frequencies, we need to engineer the phononic properties of materials. We tailor the phonon spectrum by means of nanostructuring combined with heterostructuring (i.e. the combination of different materials with different elastic properties). NWs offer the unique possibilities to obtain novel artificial materials controlling the crystal phase and/or combining highly mismatched materials. Both approaches enable the fine tuning of phononic, optical, and electronic properties.
We have implemented superlattices (SLs) into NWs, switching periodically the material (in GaAs-GaP and InAs-InP SL NWs) or the crystal phase (in twinning SL NWs). In twinning SL NWs, we could unveil the relation between local crystal structure, overall lattice symmetry, and vibrational properties. This was the first demonstration of the functionality and tunability of a crystal phase homostructure as an effective phononic material and of phonon engineering in NWs.
We have probed lattice dynamics and electronic resonances in hexagonal phase GaAs-Ge and GaAs-SixGe1-x core-shell NWs. Our work lay the grounds for a deep understanding of the phononic properties of hexagonal SixGe1-x nanostructured alloys and of their relationship with crystal quality, chemical composition, and electronic band structure.
3. Thermal circuits elements
a. Phonon sources and detectors: We came up with an innovative idea to launch and detect phonons with quantum dots (QDs), which support zero-dimensional electronic states and the electron tunnelling is driven by a finite source drain bias. We have achieved the realization of a phonon detector concept with in-built InP barriers in InAs NWs.
b. Thermal diodes: We achieved thermal rectification in telescopic GaAs NWs, where we obtained rectification values ranging from 2 to 8% as a function of base temperature.
All the above-described results have been published in open access journals or are currently being prepared for publication. We have also started to make data available in open data repository. The obtained results have been presented in several international conferences and workshops with Invited or contributed talks as well as with posters.
1. Enhancement of existing methods & development of novel methods
The investigation of phononic properties of nanomaterials is a challenging objective. We use a diverse approach combining inelastic light scattering spectroscopy and thermal conductivity measurements, pushing the limits of currently existing experimental techniques as well as developing new ones.
a. We have realized a set-up to perform space- and time-dependent pump-probe experiments, like time-resolved Raman Spectroscopy and transient reflectivity. This set-up enables us to perform also space dependent pump-probe for hot spot relaxation measurements to extract fundamental heat transport parameters.
b. We have developed a method for measuring thermal conductivity of nanowires (NWs) with a combination of resistive heating and Raman thermometry. We can obtain interesting information about the NW thermal conductivity and heat transfer at the nanoscale.
c. We have developed a method to measure optical absorption of single nanostructures based on accurately measuring the heat flow coming from a wire when it is irradiated by a laser beam.
d. We have realized a device to measure the in-plane thermal properties of nanomaterials in multiple directions.
2. Phonon engineering
In order to excite phonons with desired frequencies, we need to engineer the phononic properties of materials. We tailor the phonon spectrum by means of nanostructuring combined with heterostructuring (i.e. the combination of different materials with different elastic properties). NWs offer the unique possibilities to obtain novel artificial materials controlling the crystal phase and/or combining highly mismatched materials. Both approaches enable the fine tuning of phononic, optical, and electronic properties.
We have implemented superlattices (SLs) into NWs, switching periodically the material (in GaAs-GaP and InAs-InP SL NWs) or the crystal phase (in twinning SL NWs). In twinning SL NWs, we could unveil the relation between local crystal structure, overall lattice symmetry, and vibrational properties. This was the first demonstration of the functionality and tunability of a crystal phase homostructure as an effective phononic material and of phonon engineering in NWs.
We have probed lattice dynamics and electronic resonances in hexagonal phase GaAs-Ge and GaAs-SixGe1-x core-shell NWs. Our work lay the grounds for a deep understanding of the phononic properties of hexagonal SixGe1-x nanostructured alloys and of their relationship with crystal quality, chemical composition, and electronic band structure.
3. Thermal circuits elements
a. Phonon sources and detectors: We came up with an innovative idea to launch and detect phonons with quantum dots (QDs), which support zero-dimensional electronic states and the electron tunnelling is driven by a finite source drain bias. We have achieved the realization of a phonon detector concept with in-built InP barriers in InAs NWs.
b. Thermal diodes: We achieved thermal rectification in telescopic GaAs NWs, where we obtained rectification values ranging from 2 to 8% as a function of base temperature.
All the above-described results have been published in open access journals or are currently being prepared for publication. We have also started to make data available in open data repository. The obtained results have been presented in several international conferences and workshops with Invited or contributed talks as well as with posters.
The emerging concept of phonon engineering is equally powerful as electronic band gap engineering, since it allows controlling heat propagation in materials, and fundamentally tuning electron-phonon and photon-phonon interactions. We have demonstrated for the first time phonon engineering in NWs. We have exploited the possibility offered by NWs to periodically combine materials with a large lattice mismatch leading to large tunability of phononic properties. Furthermore, we have demonstrated for the first time that twinning SL behave as conventional SLs and the phonons keep their coherence over a few periods at room temperature. This is a result of extraordinary importance and pave the way to exciting future developments: the defect-free and atomically sharp interfaces make TSLs the ideal superstructures to study coherent phonons and wave interference effects.
Nowadays, the investigation of the thermal properties of nanomaterials remains challenging. A device capable of measuring heat flow in multiple directions offers significant benefits for studying two-dimensional systems or 3D architectures, enabling the assessment of the physical properties of anisotropic samples. We realised a device designed to measure the thermal properties of nanomaterials in multiple directions on a horizontal plane. This device allows to perform electrical and optical measurements simultaneously and it is suitable for transmission electron microscopy investigation of the samples.
Despite thermal circuits elements were envisioned more than 20 years ago, experimental demonstrations of a solid-state thermal rectifier are still scarce and their efficiency is limited. Nano- and hetero-structuration emerged as effective ways to break the symmetry of various systems to obtain thermal rectification.
We achieved for the first time thermal rectification in telescopic GaAs nanowires, where we obtained rectification values ranging from 2 to 8% as a function of base temperature. We found that the thermal boundary resistance between the thin and the thick part of the nanowire plays a crucial role in the determination of sign of rectification and its dependence on the temperature bias. This has also been confirmed by numerical calculations. It is also worth noticing that we accounted for the effect of thermal contact resistance in the accurate determination of thermal rectification.
Nowadays, the investigation of the thermal properties of nanomaterials remains challenging. A device capable of measuring heat flow in multiple directions offers significant benefits for studying two-dimensional systems or 3D architectures, enabling the assessment of the physical properties of anisotropic samples. We realised a device designed to measure the thermal properties of nanomaterials in multiple directions on a horizontal plane. This device allows to perform electrical and optical measurements simultaneously and it is suitable for transmission electron microscopy investigation of the samples.
Despite thermal circuits elements were envisioned more than 20 years ago, experimental demonstrations of a solid-state thermal rectifier are still scarce and their efficiency is limited. Nano- and hetero-structuration emerged as effective ways to break the symmetry of various systems to obtain thermal rectification.
We achieved for the first time thermal rectification in telescopic GaAs nanowires, where we obtained rectification values ranging from 2 to 8% as a function of base temperature. We found that the thermal boundary resistance between the thin and the thick part of the nanowire plays a crucial role in the determination of sign of rectification and its dependence on the temperature bias. This has also been confirmed by numerical calculations. It is also worth noticing that we accounted for the effect of thermal contact resistance in the accurate determination of thermal rectification.