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Synchrotron Radiation Studies of Molecular Building Blocks for Functional Materials
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.ORCID iD: 0000-0001-8739-7773
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The research on new materials is a primary driving force for progress in human society. One of the most significant research topic nowadays is the development of new functional materials for technological applications, like perovskite implemented in solar cells, and graphene as a representative for the new 2D materials family. It is then crucial to fully understand the functionality of such materials from a fundamental point of view, as a complementary and useful guide to develop/design new devices of improved performance and energy efficiency.

In the thesis, comprehensive characterizations of molecular building blocks used in i) novel energy conversion devices (CoPc, TPA, DPTA and m-MTDATA), and ii) in 2D materials (biphenylene and melamine) have been performed by PhotoElectron Spectroscopy (PES), and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy carried out at synchrotron radiation facilities, representing effective, powerful light source dedicated to the front-line materials research of great value in both science and industry. PES and NEXAFS spectroscopy, in combination with Density Functional Theory (DFT) calculations have provided a deep understanding of the electronic structure of the investigated systems in relation to their functionality. The investigations always included the combination and comparison between experimental and theoretical results. The studied molecules were characterized as free and adsorbed on surfaces, from the simple building blocks to more complex molecular systems. The characterizations allowed us to identify the electronic structure modifications due to substitutions (Paper III), increasing complexity of the molecules (Paper V), molecule-substrate interactions (Paper I, II, IV, V) and intra-molecular H-bonding interactions (Paper VI).

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. , p. 97
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1693
Keywords [en]
Synchrotron radiation study, Functional materials, Molecular building blocks, Electron donor, 2D material, Gas-phase, Organic thin film, Electronic structure, Molecule-molecule interaction, Molecule-substrate interaction, Photoelectron spectroscopy, PES, XPS, Near edge X-ray absorption fine structure, NEXAFS, X-ray Absorption Spectroscopy, XAS, Au(111), Cu(111), Surface, Interface, Electronic structure, H-bonding, Cobalt phthalocyanine, CoPc, Triphenylamine, TPA, DPTA, m-MTDATA, Melamine, Biphenylene, Carbon nitride, Graphenylene, Density functional theory, DFT
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-354766ISBN: 978-91-513-0383-3 (print)OAI: oai:DiVA.org:uu-354766DiVA, id: diva2:1228916
Public defence
2018-09-07, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2018-08-14 Created: 2018-06-29 Last updated: 2018-08-28
List of papers
1. Conclusively Addressing the CoPc Electronic Structure: A Joint Gas-Phase and Solid-State Photoemission and Absorption Spectroscopy Study
Open this publication in new window or tab >>Conclusively Addressing the CoPc Electronic Structure: A Joint Gas-Phase and Solid-State Photoemission and Absorption Spectroscopy Study
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2017 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 47, p. 26372-26378Article in journal (Refereed) Published
Abstract [en]

The occupied and empty densities of states of cobalt phthalocyanine (CoPc) were investigated by photoelectron and X-ray absorption spectroscopies in the gas phase and in thin films deposited on a Au(111) surface. The comparison between the gas-phase results and density functional theory single-molecule simulations confirmed that the CoPc ground state is correctly described by the (2)A(1g) electronic configuration. Moreover, photon-energy-dependent valence photoemission spectra of both the gas phase and thin film confirmed the atomic character of the highest occupied molecular orbital as being derived from the organic ligand, with dominant contributions from the carbon atoms. Multiplet ligand-field theory was employed to simulate the Co L-edge X-ray absorption spectroscopy results.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2017
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-344315 (URN)10.1021/acs.jpcc.7b08524 (DOI)000417228500026 ()
Funder
Swedish Research CouncilCarl Tryggers foundation
Available from: 2018-03-08 Created: 2018-03-08 Last updated: 2018-06-29Bibliographically approved
2. Electronic structure investigations of biphenylene films
Open this publication in new window or tab >>Electronic structure investigations of biphenylene films
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(English)Article in journal (Refereed) Submitted
Abstract [en]

Photoelectron Spectroscopy (PES) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy have been used to investigate the occupied and empty density of states of biphenylene films of different thicknesses, deposited onto a Cu(111) crystal. The obtained results have been compared to previous gas phase spectra and single molecule density functional theory (DFT) calculations to get insights into the possible modification of the molecular electronic structure in the film induced by the adsorption on a surface. Furthermore, NEXAFS measurements allowed characterizing the variation of the molecular arrangement with the film thickness and helped to clarify the substrate- molecule interaction. 

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-307784 (URN)
Available from: 2016-11-21 Created: 2016-11-21 Last updated: 2018-06-29
3. Lone-Pair Delocalization Effects within Electron Donor Molecules: The Case of Triphenylamine and Its Thiophene-Analog
Open this publication in new window or tab >>Lone-Pair Delocalization Effects within Electron Donor Molecules: The Case of Triphenylamine and Its Thiophene-Analog
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2018 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 31, p. 17706-17717Article in journal (Refereed) Published
Abstract [en]

Triphenylamine (TPA) and its thiophene-analog, N,N-diphenyl-2-thiophenamine (DPTA), are both well-known as electron-donating molecules implemented in optoelectronic devices such as organic solar cells and LEDs. Comprehensive valence and core level photoelectron spectroscopy, as well as near edge X-ray absorption spectroscopy (NEXAFS), measurements have been performed on gas phase TPA and DPTA. The experimental results have been compared to density functional theory calculations, providing a detailed description of the molecular electronic structure. Specifically, the C 1s photoelectron lines of both TPA and DPTA were resolved in the different C atom contributions and their binding energies explained as the result of two counter-acting effects: (1) the electronegativity of the nitrogen atom (and sulfur atom in DPTA) and (2) the the N (and S in DPTA) lone-pair electrons. In addition, the C K-edge NEXAFS spectrum of DPTA reveals that the lowest unoccupied molecular orbital (LUMO) energy position is affected differently if the core hole site is on the phenyl compared to the thiophene ring. The electron-donating properties of these two molecules are largely explained by the significant contribution of the N lone-pair electrons (p(z)) to the highest occupied molecular orbital. The contribution to the LUMO and to the empty density of states of the sulfur of the thiophene ring in DPTA explains the better performance of donor-pi-acceptor molecules containing this moiety and implemented in photoenergy conversion devices.

National Category
Physical Sciences Theoretical Chemistry Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-355118 (URN)10.1021/acs.jpcc.8b06475 (DOI)000441484600014 ()
Funder
Swedish Research Council, VR 2014-3776Carl Tryggers foundation
Note

Title in thesis list of papers: Lone Pair Delocalization Effect within Electron Donor Molecules:The Case of Triphenylamine (TPA) and Its Thiophene-Analog(DPTA)

Available from: 2018-06-29 Created: 2018-06-29 Last updated: 2018-10-15Bibliographically approved
4. X-ray Spectroscopy Investigations of TPA/Au(111): Charge Redistribution via Core Exitation?
Open this publication in new window or tab >>X-ray Spectroscopy Investigations of TPA/Au(111): Charge Redistribution via Core Exitation?
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Triphenylamine (TPA) is a well-known electron donor molecule largely used in photovoltaics. In this article we analyze the electronic structure modifications due to the adsorption of the molecules at a monolayer coverage on a Au(111) surface. Only a weak interaction was observed between the TPA and the gold during the adsorption process, being impossible to get more than 1ML coverage at room temperature. The characterizations have been performed by core and valence Photoelectron Spectroscopy (PES) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The results were compared with our previous investigations on free TPA, and theoretical models were used to explain the changes of the electronic structure due to the adsorption on the metallic gold surface. The calculation confirms the weak interaction between the adsorbed TPA and the Au(111), with only a slight change of the twisting angle of the TPA phenyl rings. The resulting adsorption geometry can be used to explain the broadening of the C 1s PES line with respect to the gas-phase results and the expected absence of angle dependence in the C K-edge NEXAFS. However, a significant modification was observed in the N K-edge NEXAFS spectra of TPA/Au(111), showing a new pre-edge feature due to transitions involving out-of-plane orbitals. This pre-edge feature is ascribed to the interaction between the molecules and the surface, having a different character and energy position than the pre-edge observed for free TPA. A model, considering a TPA+ cation formed by a charge redistribution process between the adsorbate and the surface valence states seems to give a qualitative explanation of this pre-edge intensity. Since our calculations predict only a weak interaction between the TPA molecules and the gold surface, we propose that such a charge redistribution happens in the core-excited state created by photon absorption.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-355120 (URN)
Available from: 2018-06-29 Created: 2018-06-29 Last updated: 2018-06-29
5. Electronic Structure Study of Free and Adsorbed m-MTDATA
Open this publication in new window or tab >>Electronic Structure Study of Free and Adsorbed m-MTDATA
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The starburst p-conjugated molecule based on triphenylamine (TPA) building block, 4,4',4" -Tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA), is widely used in optoelectronic devices due to its electron-donating properties. The electronic structure of m-MTDATA was investigated in the gas-phase and when deposited in thin films on a Au(111) surface by means of PhotoElectron Spectroscopy (PES) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Density Functional Theory (DFT) calculations were compared to the experimental gas-phase results, providing a comprehensive description of the molecular electronic structure. Moreover, the results were compared with previous TPA measurements, shedding light on the electronic structure modification due to the increased molecular complexity.  Similar to TPA, but more complex, the binding energy of the C 1s photoelectron line of m-MTDATA results from the balance of two counter-acting effects: (1) the electronegativity of the N atoms and (2) the delocalization of lone-pair electrons of the nitrogen. Compared to TPA, the outermost valence PE spectrum of m-MTDATA shows a 3-peak feature with N 2pz character and a lowering of the binding energy of the HOMO. When adsorbed on Au(111),  the changes observed in PES and NEXAFS spectra with respect to the free molecules,  can be explained by a significant modification of m-MTDATA molecular and electronic structure, due to the molecule-substrate interaction.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-355121 (URN)
Available from: 2018-06-29 Created: 2018-06-29 Last updated: 2018-06-29
6. Spectroscopic Fingerprints of Carbon Nitride Functional Groups Locked-up in Intermolecular H-bonding Interactions
Open this publication in new window or tab >>Spectroscopic Fingerprints of Carbon Nitride Functional Groups Locked-up in Intermolecular H-bonding Interactions
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(English)In: Chemistry: A European Journal, ISSN 0947-6539Article in journal (Refereed) Submitted
Abstract [en]

We have investigated the effect of intermolecular H- bonding interactions on the local electronic structure of N- functionalities, amino group and pyridine-like N, which are characteristic of a new class of metal-free polymeric photo-catalysts named graphitic carbon nitrides, g-C3N4. Specifically, we have performed a characterization of the melamine molecule, a building block of g-C3N4, combining X-ray photoemission (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The molecule has been studied in the gas phase, as non-interacting system, and in the solid state within a hydrogen bonded network. With the support of density functional theory (DFT) simulations of the spectra, we have found that the H-bonds mainly affect the N 1s level of the amino group, leaving the N 1s level of the pyridine-like N mostly unperturbed. This fact is responsible for a reduction of the chemical shift between the two XPS N 1s levels, compared to the free melamine. Consequently, N K-edge NEXAFS resonances involving the amino N 1s level also shift to lower photon energies. Moreover, the solid state absorption spectra have shown strong modification/quenching of resonances related with transitions from the amino N 1s level towards σ*orbitals involving the -NH2 terminations. 

National Category
Physical Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-355123 (URN)
Available from: 2018-06-29 Created: 2018-06-29 Last updated: 2018-06-29

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