Periodic Reporting for period 1 - PhoMOFs (Accessing Electron-Phonon interactions of two-dimensional Metal Organic Frameworks by Ultrabroadband Terahertz Spectroscopy based on the Spintronic Trilayer Emitter)
Berichtszeitraum: 2021-05-01 bis 2023-04-30
The recent discovery of semiconducting MOFs, has ignited the interest on these hybrid materials, with tailor-made properties, as alternatives for applications where long-range charge transport is demanded. However, for tailoring the semiconductive properties of a MOF for a specific application, we need a deep understanding of the relationships between the structure and the chemistry and its charge transport properties. One of the most important questions is the identification of phonons in MOFs and how these affect the charge carrier transport and ultimately, whether they can be tailored on-demand, by introducing changes in the chemistry of the material.
The host group introduced a powerful non-contact AC technique, namely the Time Resolved THz Spectroscopy (TRTS) technique, for studying carrier dynamics in MOFs. In a TRTS experiment, an optical pulse is used to excite the electrons of the semiconducting material from the valence band to the conduction band. Then, a THz pulse transverses the material with an adjustable time delay and due to the sensitivity of THz radiation to free charge carriers, it allows accessing the sample’s complex conductivity in the AC limit, providing precious information on the charge transfer mechanism, charge carrier density and mobility. However, one of the most important drawbacks for the study of optical phonons using traditional electro-optic crystals for the THz emission is their limited bandwidth, usually covering frequency windows of not more than ~0.1-2THz. This limits substantially the amount of information one can harvest from TRTS experiments, since it does not allow the characterization of IR-active lattice vibrations (phonons), which usually extend in the frequency range between 60 cm-1 and 500 cm-1.
Here, I proposed the use of the recently developed Spintronic Trilayer Emitter (STE) ultrabroadband THz source, in order to achieve the desirable large bandwidth. Its simple geometry includes a ferromagnetic (FM) thin layer, enclosed between two non-magnetic (NM) layers. Upon laser excitation an ultrashort THz pulse (~100-200fs) is emitted, covering frequencies between ~0.3 and 30 THz.
The main objective of PhoMOFs was to introduce the ultrabroadband THz STE for the investigation of phonons and their interplay with charge carrier dynamics and transport properties in the rapidly emerging field of 2D semiconducting MOFs. We aimed at establishing neat correlations between chemistry and structure of the material and its electronic structure, conductivity and carrier mobility.
The host group introduced a powerful non-contact AC technique, namely the Time Resolved THz Spectroscopy (TRTS) technique, for studying carrier dynamics in MOFs. In a TRTS experiment, an optical pulse is used to excite the electrons of the semiconducting material from the valence band to the conduction band. Then, a THz pulse transverses the material with an adjustable time delay and due to the sensitivity of THz radiation to free charge carriers, it allows accessing the sample’s complex conductivity in the AC limit, providing precious information on the charge transfer mechanism, charge carrier density and mobility. However, one of the most important drawbacks for the study of optical phonons using traditional electro-optic crystals for the THz emission is their limited bandwidth, usually covering frequency windows of not more than ~0.1-2THz. This limits substantially the amount of information one can harvest from TRTS experiments, since it does not allow the characterization of IR-active lattice vibrations (phonons), which usually extend in the frequency range between 60 cm-1 and 500 cm-1.
Here, I proposed the use of the recently developed Spintronic Trilayer Emitter (STE) ultrabroadband THz source, in order to achieve the desirable large bandwidth. Its simple geometry includes a ferromagnetic (FM) thin layer, enclosed between two non-magnetic (NM) layers. Upon laser excitation an ultrashort THz pulse (~100-200fs) is emitted, covering frequencies between ~0.3 and 30 THz.
The main objective of PhoMOFs was to introduce the ultrabroadband THz STE for the investigation of phonons and their interplay with charge carrier dynamics and transport properties in the rapidly emerging field of 2D semiconducting MOFs. We aimed at establishing neat correlations between chemistry and structure of the material and its electronic structure, conductivity and carrier mobility.
The fellow during the grant was active in mainly three different areas, developed simultaneously during the period of the grant, to guarantee that the output was maximum, always pointing toward the objectives set in the proposal. More specifically:
Firstly, the fellow during the whole duration of the grant has been engaged with setup development. Initially, we found out that the amplified laser system in our labs at the moment, did not have the appropriate pulse temporal duration for obtaining the ultrabroadband spectrum we were hoping for, but we were limited at ~3THz. For this reason, we decided to use the laser facilities of the Max Planck Institute for Polymer research (MPIP), in Mainz Germany, since the host group is assigned as an official Max Planck partner group. By using their ultrabroadband THz spectrometer, based on air-plasma, we characterized several families of MOFs with THz time-domain spectroscopy (THz-TDS) and for one sample, namely the Cu-HHTP (HHTP=hexahydroxy triphenylene) we were able to identify a phonon, residing at 1.8THz. The low frequency where this phonon appeared allowed us to investigate it at IMDEA, with the resources we had, observing a clear electron-phonon interaction. In the meantime, we obtained a new amplified laser system, with an oscillator output of a duration of <10fs, which will allow us reaching the desired bandwidth and consequently will allow us to study more conductive MOFs with phonons appearing at higher frequencies. Moreover, the new setup design also allows measurements in transmission and reflection, which will give us the opportunity to study any samples independently of its opacity at THz frequencies.
Secondly, following the research lines of the host group, the fellow engaged in other projects, closely related to the proposal’s objectives. He performed temperature dependent THz-TDS measurements on the HHTP MOFs with different metal centers and found out that the electrical properties of the samples is greatly affected by the nature of the metal center. Moreover, the fellow also performed temperature dependent THz-TDS measurements on the Cu2(OHPTP) MOF, a sample of great interest since it shows a bandgap in the near IR, making it very relevant for logic and solar cell applications. From these measurements he found out that this material has excellent mobility and also a large scattering time, very beneficial for these applications where long range charge migration is needed.
Finally, measurements on other side projects took place, due to the scientific relation of the fellow with colleagues at Fritz Haber Institute in Berlin, producing a total of three extra publications. In brief the fellow used a Lithium Niobate high-field THz source for performing THz-pump/Raman probe on electrolyte solutions. He used a newly built SFG/DFG spectrometer to experimentally derive the nonlinear Fresnel Factors of metals for the first time and did complementary measurements on z-cut alpha-quartz crystals proving its applicability on THz applications.
The project has been very productive scientifically, with 4 publications in high impact journals, and 4 additional ones in peer-review or preparation phases. Furthermore, it has been presented 3 conferences and one internal IMDEA seminar.
Firstly, the fellow during the whole duration of the grant has been engaged with setup development. Initially, we found out that the amplified laser system in our labs at the moment, did not have the appropriate pulse temporal duration for obtaining the ultrabroadband spectrum we were hoping for, but we were limited at ~3THz. For this reason, we decided to use the laser facilities of the Max Planck Institute for Polymer research (MPIP), in Mainz Germany, since the host group is assigned as an official Max Planck partner group. By using their ultrabroadband THz spectrometer, based on air-plasma, we characterized several families of MOFs with THz time-domain spectroscopy (THz-TDS) and for one sample, namely the Cu-HHTP (HHTP=hexahydroxy triphenylene) we were able to identify a phonon, residing at 1.8THz. The low frequency where this phonon appeared allowed us to investigate it at IMDEA, with the resources we had, observing a clear electron-phonon interaction. In the meantime, we obtained a new amplified laser system, with an oscillator output of a duration of <10fs, which will allow us reaching the desired bandwidth and consequently will allow us to study more conductive MOFs with phonons appearing at higher frequencies. Moreover, the new setup design also allows measurements in transmission and reflection, which will give us the opportunity to study any samples independently of its opacity at THz frequencies.
Secondly, following the research lines of the host group, the fellow engaged in other projects, closely related to the proposal’s objectives. He performed temperature dependent THz-TDS measurements on the HHTP MOFs with different metal centers and found out that the electrical properties of the samples is greatly affected by the nature of the metal center. Moreover, the fellow also performed temperature dependent THz-TDS measurements on the Cu2(OHPTP) MOF, a sample of great interest since it shows a bandgap in the near IR, making it very relevant for logic and solar cell applications. From these measurements he found out that this material has excellent mobility and also a large scattering time, very beneficial for these applications where long range charge migration is needed.
Finally, measurements on other side projects took place, due to the scientific relation of the fellow with colleagues at Fritz Haber Institute in Berlin, producing a total of three extra publications. In brief the fellow used a Lithium Niobate high-field THz source for performing THz-pump/Raman probe on electrolyte solutions. He used a newly built SFG/DFG spectrometer to experimentally derive the nonlinear Fresnel Factors of metals for the first time and did complementary measurements on z-cut alpha-quartz crystals proving its applicability on THz applications.
The project has been very productive scientifically, with 4 publications in high impact journals, and 4 additional ones in peer-review or preparation phases. Furthermore, it has been presented 3 conferences and one internal IMDEA seminar.
PhoMOFs has been consolidated as a new research line at IMDEA nanoscience. The STE-based THz setup, developed during the action’s period is the first ultrabroadband THz pulse situated in Spain, that can offer unparalleled electrical characterization and reveal the electron-phonon interactions in other families of MOFs, reported monthly or even other types of materials, like perovskites.
However, our plans do not stop there. Nowadays, a high-power THz source based on Lithium Niobate (LN) is being built by the fellow next to the current ultrabroadband setup. The idea is to combine these two sources in a THz-pump (LN)/THz-probe (UBB-STE) configuration, that will allow us to study the phonon-phonon interactions and obtain the whole lattice vibration landscape of MOFs and other technologically relevant samples.
For this research line the fellow has already applied and will continue to apply for funding, in the national and international level.
However, our plans do not stop there. Nowadays, a high-power THz source based on Lithium Niobate (LN) is being built by the fellow next to the current ultrabroadband setup. The idea is to combine these two sources in a THz-pump (LN)/THz-probe (UBB-STE) configuration, that will allow us to study the phonon-phonon interactions and obtain the whole lattice vibration landscape of MOFs and other technologically relevant samples.
For this research line the fellow has already applied and will continue to apply for funding, in the national and international level.