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Core-hole Clock Spectroscopy Using Hard X-rays: Exciting States in Condensed Matter
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.ORCID iD: 0000-0002-6471-1093
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is about how electrons move from one place to another, that is charge transfer dynamics. Charge transfer dynamics is an important property governing chemical and physical changes that form the base for many applications such as electronics, optoelectronics and catalysis. The fundamental aspect is how charge transfer manifests in the constituent materials and their interfaces building up these devices. The basic method used is synchrotron radiation based electron spectroscopies.

Using core-hole clock spectroscopy it is possible to study dynamic processes in the femtosecond and attosecond regimes - here we study the if the core-excited electron decays back into the core hole (local decays), or if the core excited electron have been tunneled away from the atomic site before the core-hole decays. Spectroscopically we can discern the two situations since one of the processes is photon energy dependent and one is not. Knowledge of the life-time of the core hole, and measuring the probability of the core-excited system decaying one way or the other makes it possible to calculate a charge transfer time. Using hard X-rays to create excited state with deep core-holes allow us to study high kinetic energy Auger electrons, also deep core-holes tend to be short lived, which gives access to short time-scales.

Bulk crystals of 2D materials have been used as model systems here owing to their well-known properties. Using those it has been demonstrated that the regime of observable times using the mentioned method can be extended with an order of magnitude compared to previous studies. Our results present themselves on time-scales on par with the atomic unit of time. The highly selective nature of resonant X-ray excitations allows the anisotropic unoccupied electronic structure of bulk 2D crystals to be mapped out, here the example of SnS2 is presented. This shows that this is a direct probe of the unoccupied band structure.

With core-hole clock spectroscopy the charge transfer time dependence on relative concentrations of blends between the low band-gap polymer PCPDTBT, with PCBM (functionalized fullerenes). This is a common prototypical system for organic photovoltaics. The charge transfer time decreases with increasing intermixing, up to a point where is starts getting slower, the same trend as the efficiency of solar cell devices made with the same mixing. The method employed here is chemically specific and probes the local surrounding energy landscape at the site of excitation – this is different from other techniques that utilize optical excitations which are non-local in character.

The synthetization of bulk heterostructures and thin films, and the disentanglement of core-ionized states are also investigated using spectroscopic and scattering techniques.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. , p. 104
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1921
Keywords [en]
core-hole clock, resonant Auger, XPS, black phosphorous, TMDC, perovskite, graphene, coincidences spectroscopy, synchrotron radiation, HAXPES
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-407540ISBN: 978-91-513-0915-6 (print)OAI: oai:DiVA.org:uu-407540DiVA, id: diva2:1416922
Public defence
2020-08-28, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2020-04-27 Created: 2020-03-26 Last updated: 2020-05-19
List of papers
1. Resonant Auger Spectroscopy on Solid Xe on Cu and Au
Open this publication in new window or tab >>Resonant Auger Spectroscopy on Solid Xe on Cu and Au
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-407534 (URN)
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2020-03-26
2. Gone in 23 Attoseconds: Charge Transfer in Resonantly Core Excited Black Phosphorous
Open this publication in new window or tab >>Gone in 23 Attoseconds: Charge Transfer in Resonantly Core Excited Black Phosphorous
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-407535 (URN)
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2020-03-26
3. Interlayer Charge Transfer in Tin Disulphide: Orbital Anisotropy and Temporal Aspects
Open this publication in new window or tab >>Interlayer Charge Transfer in Tin Disulphide: Orbital Anisotropy and Temporal Aspects
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2020 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 3, article id 035165Article in journal (Refereed) Published
Abstract [en]

An investigation of the unoccupied electronic structure of the transition-metal dichalcogenide tin disulphide has been conducted using core-hole clock spectroscopy. Polarization-dependent x-ray absorption in the tender x-ray regime at the S K edge and maps of the resonant Auger spectra in the S KLL Auger kinetic energy range have been recorded. Supported with ab initio calculations of the unoccupied electronic structure, these allow us to relate resonances in the absorption cross section to excitations along various directions in the Brillouin zone. We observe anisotropy in the x-ray absorption cross section in polarization directions in plane and out of plane of the crystal. There is also anisotropy in the charge transfer dynamics as inferred from the coherent and noncoherent parts of the resonant Auger spectra. This approach can be generally used to interpret dynamics in unoccupied states, e.g., in layered structure or heterogenous interfaces.

Place, publisher, year, edition, pages
American Physical Society (APS), 2020
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-407536 (URN)10.1103/PhysRevB.102.035165 (DOI)000554409900002 ()
Funder
Swedish Research Council, 2014-6463Swedish Research Council, 2018-05336Knut and Alice Wallenberg Foundation
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2020-10-06Bibliographically approved
4. Intralayer Charge Transfer Anisotropy in Black Phosphorus
Open this publication in new window or tab >>Intralayer Charge Transfer Anisotropy in Black Phosphorus
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-407537 (URN)
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2020-03-26
5. Tailoring Ultra-fast Charge Transfer in MoS2
Open this publication in new window or tab >>Tailoring Ultra-fast Charge Transfer in MoS2
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2020 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 22, no 18, p. 10335-10342Article in journal (Refereed) Published
Abstract [en]

Charge transfer dynamics are of importance in functional materials used in devices ranging from transistors to photovoltaics. The understanding of charge transfer in particular of how fast electrons tunnel away from an excited state and where they end up, is necessary to tailor materials used in devices. We have investigated charge transfer dynamics in different forms of the layered two-dimensional material molybdenum disulphide (MoS2, in single crystal, nanocrystalline particles and crystallites in a reduced graphene oxide network) using core-hole clock spectroscopy. By recording the electrons in the sulphur KLL Auger electron kinetic energy range we have measured the prevalence of localised and delocalised decays from a state created by core excitation using X-rays. We show that breaking the crystal symmetry of the single crystal into either particles or sheets causes the charge transfer from the excited state to occur faster, even more so when incorporating it in a graphene oxide network. The interface between the MoS2 and the reduced graphene oxide forms a Schottky barrier which changes the ratio between local and delocalised decays creating two distinct regions in the charge transfer dependent on the energy of the excited electron. Thereby we show that ultra-fast charge transfer in MoS2 can be tailored, a result which can be used in the design of emergent devices.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-407538 (URN)10.1039/d0cp00857e (DOI)000535921700040 ()32364190 (PubMedID)
Funder
Swedish Research Council, 2014-6463Swedish Research Council, 2018-05336
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2020-06-26Bibliographically approved
6. Femtosecond and Attosecond Electron-Transfer Dynamics in PCPDTBT: PCBM Bulk Heterojunctions
Open this publication in new window or tab >>Femtosecond and Attosecond Electron-Transfer Dynamics in PCPDTBT: PCBM Bulk Heterojunctions
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2018 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 24, p. 12605-12614Article in journal (Refereed) Published
Abstract [en]

Charge separation efficiency is a crucial parameter for photovoltaic devices-polymers consisting of alternating electron-rich and electron-deficient parts can achieve high such efficiencies, for instance, together with a fullerene electron acceptor. This offers a viable path toward solar cells with organic bulk heterojunctions. Here, we measured the charge-transfer times in the femtosecond and attosecond regimes via the decay of sulfur is X-ray core excited states (with the core-hole clock method) in blends of a low-band gap polymer {PCPDTBT [poly[2,6-(4,4-bis (2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-1/1 dithiophene)-alt-4,7- (2,1,3-benzothiadiazole)]]} consisting of a cyclopentadithiophene electron-rich part and a benzothiadiazole electron-deficient part. The constituting parts of the bulk heterojunction were varied by adding the fullerene derivative PCBM ([6,6]-phenyl-C-61-butyric acid methyl ester) (weight ratio of polymer/PCBM as 1:0, 1:1, 1:2, and 1:3). For low-energy excitations, the charge-transfer time varies to the largest extent for the thiophene donor part. The charge-transfer time in the 1:2 blend is reduced by 86% compared to that of pristine PCPDTBT. At higher energy excitations, the charge-transfer time does not vary with the chemical environment, as this regime is dominated by intramolecular conduction that yields ultrafast charge-transfer times for all blends, approaching 170 as. We thus demonstrate that the core-hole clock method applied to a series with changing composition can give information about local electron dynamics (with chemical specificity) at interfaces between the constituting parts the crucial part of a bulk heterojunction where the initial charge separation occurs.

National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-359439 (URN)10.1021/acs.jpcc.8b02453 (DOI)000436381600004 ()
Available from: 2018-09-03 Created: 2018-09-03 Last updated: 2020-03-26Bibliographically approved
7. Defect formation in graphene during low-energy ion bombardment
Open this publication in new window or tab >>Defect formation in graphene during low-energy ion bombardment
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2016 (English)In: APL Materials, E-ISSN 2166-532X, Vol. 4, no 4, article id 046104Article in journal, Letter (Refereed) Published
Abstract [en]

This letter reports on a systematic investigation of sputter induced damage in graphene caused by low energy Ar+ ion bombardment. The integral numbers of ions per area (dose) as well as their energies are varied in the range of a few eV's up to 200 eV. The defects in the graphene are correlated to the dose/energy and different mechanisms for the defect formation are presented. The energetic bombardment associated with the conventional sputter deposition process is typically in the investigated energy range. However, during sputter deposition on graphene, the energetic particle bombardment potentially disrupts the crystallinity and consequently deteriorates its properties. One purpose with the present study is therefore to demonstrate the limits and possibilities with sputter deposition of thin films on graphene and to identify energy levels necessary to obtain defect free graphene during the sputter deposition process. Another purpose is to disclose the fundamental mechanisms responsible for defect formation in graphene for the studied energy range.

National Category
Materials Chemistry Nano Technology
Identifiers
urn:nbn:se:uu:diva-284702 (URN)10.1063/1.4945587 (DOI)000375846100007 ()
Funder
Knut and Alice Wallenberg Foundation, 2011.0082Swedish Research Council, 2014-5591 2014-6463
Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2023-10-17Bibliographically approved
8. Minimizing sputter-induced damage during deposition of WS2 onto graphene
Open this publication in new window or tab >>Minimizing sputter-induced damage during deposition of WS2 onto graphene
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2017 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 110, no 9, article id 091601Article in journal (Refereed) Published
Abstract [en]

We demonstrate the sputter-deposition of WS2 onto a single-layer graphene film leaving the latter disorder-free. The sputtering process normally causes defects to the graphene lattice and adversely affects its properties. Sputtering of WS2 yields significant amounts of energetic particles, specifically negative S ions, and reflected neutral Ar, and it is therefore used as a model system in this work. The disorder-free sputtering is achieved by increasing the sputteringpressure of Ar thereby shifting the kinetic energy distribution towards lower energies for the impinging particle flux at the substrate. Raman spectroscopy is used to assess the amount of damage to the graphene film. Monte Carlo simulations of the sputteringprocess show that W is completely thermalized already at relatively low sputtering pressure, whereas Ar and S need a comparably higher pressure to thermalize so as to keep the graphene film intact. Apart from becoming completely amorphous at 2.3 mTorr, the graphene filmremains essentially disorder-free when the pressure is increased to 60 mTorr. The approach used here is generally applicable and readily extendable to sputter-deposition of other material combinations onto sensitive substrates. Moreover, it can be used without changing the geometry of an existing sputtering setup.

National Category
Other Physics Topics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-316602 (URN)10.1063/1.4977709 (DOI)000397871600010 ()
Available from: 2017-03-04 Created: 2017-03-04 Last updated: 2020-06-04Bibliographically approved
9. Quantitative analysis of plasmon excitations in hard x-ray photoelectron spectra of bulk black phosphorus
Open this publication in new window or tab >>Quantitative analysis of plasmon excitations in hard x-ray photoelectron spectra of bulk black phosphorus
2020 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 505, article id 144385Article in journal (Refereed) Published
Abstract [en]

Black phosphorus (BPh) is a layered material with strong in-plane anisotropy of its structural and electronic properties; in spite of the great potential of BPh for conceptually new devices in optoelectronics and plasmonics, its fundamental electronic excitations have not yet been fully elucidated. In order to discriminate collective (plasmons) and single-particle (inter band transitions) excitations, we investigate the energy-loss distribution of P 1s photoelectrons in hard X-ray photoelectron spectra of BPh over a wide energy range. The energy-loss function (ELF), averaged over the principal directions of the BPh crystal, has been retrieved by using a Fourier Transform analysis to eliminate multiple inelastic scattering events. At low loss energies (1-8 eV), weak unresolved energy loss peaks are well described by DFT calculated inter band transitions, showing some anisotropy in the dielectric function epsilon(omega, q) tensor of BPh. At high loss energies, the ELF is dominated by the collective excitation of valence electrons with a peak energy at 20.1 +/- 0.2 eV, and weak anisotropy is found in the DFT calculated Im(- 1/epsilon) tensor. The anomalously small peak energy (9.0 +/- 0.5 eV) of a weak surface plasmon resonance is attributed either to low surface electron density in the terminal phosphorene layer or to some anisotropic surface plasmon propagation.

Keywords
Black phosphorus, XPS, HAXPES, Energy loss, Plasmon
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-407533 (URN)10.1016/j.apsusc.2019.144385 (DOI)000510846500037 ()
Funder
Swedish Research Council, 2014-6463
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2020-04-01Bibliographically approved
10. In Situ Formation of Ge Nanoparticles by Annealing of Al-Ge‑N ThinFilms Followed by HAXPES and XRD
Open this publication in new window or tab >>In Situ Formation of Ge Nanoparticles by Annealing of Al-Ge‑N ThinFilms Followed by HAXPES and XRD
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2019 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 58, no 16, p. 11100-11109Article in journal (Refereed) Published
Abstract [en]

Ge nanoparticles embedded in thin films have attracted a lot of attention due to their promising optical and electronic properties that can be tuned by varying the particle size and choice of matrix material. In this study, Ge nanoparticle formation was investigated for Al-Ge-N based thin films by simultaneous measurements of HAXPES and grazing incidence XRD during in situ annealing in vacuum conditions. As-deposited Al-Ge-N thin films, synthesized by reactive dc magnetron sputtering, consisted of a nanocrystalline (Al1–xGex)Ny solid solution and an amorphous tissue phase of Ge3Ny. Upon annealing to 750 °C, elemental Ge was formed shown by both HAXPES and XRD measurements, and N2 gas was released as measured by a mass spectrometer. Postannealed ex situ analysis by SEM and TEM showed that the elemental Ge phase formed spherical nanoparticles on the surface of the film, with an average size of 210 nm. As the annealing temperature increased further to 850 °C, the Ge particles on the film surface evaporated, while the phase segregation of Ge still could be observed within the film. Thus, these results show the possibility for a controlled synthesis of Ge nanoparticles through annealing of Al-Ge-N thin films to produce materials suitable for use in electronic or optoelectronic devices.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-392698 (URN)10.1021/acs.inorgchem.9b01631 (DOI)000482173300075 ()31381309 (PubMedID)
Funder
Swedish Research Council, 2014-6463EU, FP7, Seventh Framework Programme, INCA 600398
Available from: 2019-09-08 Created: 2019-09-08 Last updated: 2020-03-26Bibliographically approved
11. Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells
Open this publication in new window or tab >>Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells
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2017 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 40, p. 34970-34978Article in journal (Refereed) Published
Abstract [en]

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAP-bI(3))(0.85)(MAPbBr(3))(0.15)) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2017
Keywords
photoelectron spectroscopy, laser illumination, lead halide perovskite, ion migration, phase separation, stability
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-340141 (URN)10.1021/acsami.7b10643 (DOI)000413131500043 ()28925263 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 321319Swedish Research Council, 2014-6019Swedish Research Council, 2014-6463StandUpSwedish Foundation for Strategic Research , RMA15-0130
Available from: 2018-01-26 Created: 2018-01-26 Last updated: 2021-10-07Bibliographically approved
12. Auger- and Photoelectron Coincidences of Molecular O2 Adsorbed on Ag(111)
Open this publication in new window or tab >>Auger- and Photoelectron Coincidences of Molecular O2 Adsorbed on Ag(111)
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2022 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 256, article id 147174Article in journal (Refereed) Published
Abstract [en]

The oxygen on Ag(111) system has been investigated with Auger electron–photoelectron coincidence spectroscopy (APECS). The coincidence spectra between O 1s core level photoelectrons and O KLL Auger electrons have been studied together with Ag3d/AgM4,5NN coincidences. We also describe the electron–electron coincidence spectrometer setup, CoESCA, consisting of two angle resolved time-of-flight spectrometers at a synchrotron light source. Contributions from molecular oxygen and chemisorbed oxygen are assigned using the coincidence data, conclusions are drawn primarily from the O 1s/O KLL data. The data acquisition and treatment procedure are also outlined. The chemisorbed oxygen species observed are relevant for the catalytic ethylene oxidation.

Place, publisher, year, edition, pages
Elsevier, 2022
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-407539 (URN)10.1016/j.elspec.2022.147174 (DOI)000782654500004 ()
Available from: 2020-03-25 Created: 2020-03-25 Last updated: 2022-05-09Bibliographically approved

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