I09-Surface and Interface Structural Analysis
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Benedikt P.
Klein
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Juliana M.
Morbec
,
Markus
Franke
,
Katharina K.
Greulich
,
Malte
Sachs
,
Shayan
Parhizkar
,
Francois C.
Bocquet
,
Martin
Schmid
,
Samuel J.
Hall
,
Reinhard J.
Maurer
,
Bernd
Meyer
,
Ralf
Tonner
,
Christian
Kumpf
,
Peter
Kratzer
,
J. Michael
Gottfried
Diamond Proposal Number(s):
[16259]
Abstract: Interfaces between polycyclic π-electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the π-electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagon-heptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopy (UPS, XPS), and density functional theory (DFT). Coverage-dependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using: (1) the DFT-D3 scheme, (2) the vdWsurf correction based on DFT-TS, (3) a Many-Body Dispersion (MBD) correction scheme, and (4) the D3surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced HOMO-LUMO gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces.
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Nov 2019
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I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[18875]
Abstract: We demonstrate the pressure-induced formation of supercrystals made from PEGylated gold nanorods in aqueous suspension. Utilizing the combined effect of hydrostatic pressure and salt on the solubility of the organic PEG shell that passivates the nanorods, the reversible formation of two-dimensional hexagonal supercrystals has been observed by means of small angle X-ray scattering. The pressure dependence of the crystal lattice's structural parameters is determined. By time-resolved measurements performed after a pressure-jump, the growth process of the crystals is found to be finished already after a few seconds. The presented results demonstrate that by PEGylating nanoparticles pressure-induced homogeneous supercrystals can be formed for different particle shapes, in particular anisotropic nanorods, which determine the resulting lattice type.
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Nov 2019
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David A.
Duncan
,
Peter S.
Deimel
,
Alissa
Wiengarten
,
Mateusz
Paszkiewicz
,
Pablo
Casado Aguilar
,
Robert G.
Acres
,
Florian
Klappenberger
,
Willi
Auwärter
,
Ari Paavo
Seitsonen
,
Johannes V.
Barth
,
Francesco
Allegretti
Abstract: In situ preparation of oxotitanium tetraphenylporphyrin (TiO-TPP) on Ag(111) under ultra-high vacuum conditions was achieved in a multi-step procedure starting from adsorbed free-base tetraphenylporphyrin (2H-TPP). The final product as well as the intermediate titanium tetraphenylporphyrin (Ti-TPP) were characterized by a suite of surface-sensitive spectroscopic tools combined with scanning tunneling microscopy and density functional theory (DFT), and compared against the parent 2H-TPP species. Facile oxidation of Ti-TPP with molecular oxygen was observed at 300 K, with X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) from the Ti 2p core levels supporting a change in the oxidation state from Ti2+ to Ti4+. N K-edge and Ti L-edge NEXAFS suggest that the tetrapyrrole macrocycle conformation is modified upon binding to oxygen, in agreement with DFT calculations that predict a marked change of the local environment of the Ti centers upon oxygen attachment. O K-edge NEXAFS and O 1s energy-scanned photoelectron diffraction from the resulting TiO-TPP monolayer provide strong evidence for the presence of a titanium-oxygen double bond, with the latter technique yielding a bond length of 1.56 ± 0.02 Å. The majority of adsorbed TiO-TPP species have the oxo group pointing away from the surface rather than towards it, and thus the oxygen atom can potentially interact coordinatively with external species. Both the highly reactive, intermediate Ti TPP species and the final product TiO-TPP are of great interest for catalytic applications.
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Nov 2019
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B22-Multimode InfraRed imaging And Microspectroscopy
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Arun S.
Babal
,
Lorenzo
Donà
,
Matthew R.
Ryder
,
Kirill
Titov
,
Abhijeet K.
Chaudhari
,
Zhixin
Zeng
,
Chris S.
Kelley
,
Mark D.
Frogley
,
Gianfelice
Cinque
,
Bartolomeo
Civalleri
,
Jin-chong
Tan
Diamond Proposal Number(s):
[14902]
Abstract: Research on the broadband dielectric response of metal-organic frameworks (MOFs) is an emergent field that could yield exciting device applications, such as smart optoelectronics, terahertz sensors, high-speed telecommunications and microelectronics. Hitherto, a detailed understanding of the physical mechanisms controlling the frequency-dependent dielectric and optical behavior of MOFs is lacking because a large number of studies have focused only on static dielectric constants. Herein we employed high-resolution spectroscopic techniques in combination with periodic ab initio density functional theory (DFT) calculations to establish the different polarization processes for a porous copper-based MOF, termed HKUST-1. We used alternating current measurements to determine its dielectric response between 4 Hz and 1.5 MHz where orientational polarization is predominant, while synchrotron infrared (IR) reflectance was used to probe the far-IR, mid-IR, and near-IR dielectric response across the 1.2 THz to 150 THz range (ca. 40 – 5000 cm-1) where vibrational and optical polarizations are principal contributors to its dielectric permittivity. We demonstrate the role of pressure on the evolution of broadband dielectric response, where THz vibrations reveal distinct blue and red shifts of phonon modes from structural deformation of the copper paddle-wheel and the organic linker, respectively. We also investigated the effect of temperature on dielectric constants in the MHz region pertinent to microelectronics, to study temperature-dependent dielectric losses via dissipation in an alternating electric field. The DFT calculations offer insights into the physical mechanisms responsible for dielectric transitions observed in the experiments and enable us to explain the frequency shifts phenomenon detected under pressure. Together, the experiments and theory have enabled us to glimpse into the complex dielectric response and mechanisms underpinning a prototypical MOF subject to pressure, temperature, and vast frequencies.
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Nov 2019
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[11145]
Abstract: Three new symmetrical donor–acceptor–donor (D–A–D)-type molecules were prepared with phenothiazine (PTZ) as electron donors and 9,9-dimethylthioxanthene (TX) as the electron acceptor. The PTZ groups are attached at different positions on the acceptor core – ortho or meta to the sulfur of TX. The molecules have been characterized by X-ray crystallography, in-depth photophysical studies and theoretical calculations. This series provides new insights into how molecular functionalization and intramolecular charge transfer determines the singlet-triplet gap ΔEST. Two of the molecules have weak D/A decoupling and a relatively large ΔEST of 0.52 eV which prohibits the upconversion of triplet excitons to the singlet state, showing strong room temperature phosphorescence (RTP). When the TX acceptor strength is enhanced by the attachment of benzoyl substituents a very small ΔEST of <0.01 eV is observed. In this case excitons in the triplet state can be efficiently upconverted to the singlet state via reverse intersystem crossing (RISC) resulting in thermally activated delayed fluorescence (TADF). TADF and RTP are unambiguously assigned by distinctive photophysical data, notably a comparison of degassed and aerated luminescence spectra, temperature-dependent time-resolved fluorescence decays and power dependence of the intensity of delayed emission (for the TADF emitter).
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Oct 2019
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Abstract: The Inert Gas Condensation method (IGC) produces multimetallic nanoparticles in a metastable state that may exhibit heterogeneities of size, structure and composition. The deposition of IGC-fabricated nanoparticles on substrates allows for a detailed characterization by combination of aberration-corrected transmission electron microscopy (TEM) and Atom Probe Tomography (APT). Multiple particle monitoring and high-resolution TEM give access to the size distribution of Au-Cu nanoparticles (<10nm in diameter, bimodal distribution). TEM and APT show that the alloying between Cu and Au may stabilize the Ih structure for smaller particles (<4nm). Combining HR-STEM/EDX and 3D composition analysis by APT reveals that an excess of Cu may be present in a shell around the larger particles (>7nm), while Cu is more randomly distributed in smaller particles.
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Oct 2019
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[13560]
Abstract: Non-graphitic carbons (NGCs) represent the most abundant class of \(sp^2\)-hybridized carbon materials (coal, char coal, activated carbon, etc.). These carbons consist of small graphene layer stacks possessing significant structural disorder both in the single graphene sheets and the stacking. In this study an advanced evaluation approach for the wide-angle neutron scattering (WANS) was developed, based on the method introduced by Ruland and Smarsly (2002). In particular, we elucidated if and how the enhanced WANS data quality and larger values of the modulus of the scattering vector \(s\)-range affect the accuracy and the values of the size and disorder parameters - being fitting parameters by themselves - in comparison to wide-angle x-ray scattering (WAXS), which is usually performed by laboratory equipment. We find a reasonable agreement for the parameters \(L_\mathrm{a}\) and \(L_\mathrm{c}\), i.e. the lateral dimension and stack height, within the error bars, while for the disorder parameters different results for WAXS and WANS were found, the origin of which is discussed. Thus, this study addresses the general issue of how reliably microstructural parameters can be determined from WAXS/WANS, by fitting simulated WAXS and WANS curves, which are quality-impaired by added Gaussion noise at different levels and cut-off at different \(s\)-values. From this analysis we estimated the minimal data quality required for a reliable NGC microstructural analysis based on WAXS/WANS. As an important finding, these simulations show that typical, standard WAXS laboratory setups are sufficient to provide reliable values for the most relevant structural parameters. Furthermore, pair-distribution function (PDF) analyses were performed on WAXS data obtained from a Synchrotron facility. Comparing PDF and WAXS/WANS fitting analysis thus suggests the presence of small highly ordered oligoaromatic domains embedded in the larger graphene sheets, questioning the classical view on the NGC microstructure.
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Jul 2019
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Abstract: We report the observation of corrosion of gold at the nanoscale, in which nm-sized gold can be corroded in air in the presence of copper at room temperature during ageing to form a compound of gold with residual copper. The compound is a nanoscale belt-like two-dimensional structure that can be prepared from gold nanoparticles. Using high-resolution electron microscopy and chemical analyses, we show that the structure consists of alternating rows of gold and copper atoms. The atomic columns of gold are arranged in rows separated by 0.5 nm and the structure extends in a perpendicular direction to these. DFT calculations of an atomic model of this two-dimensional material consisting of gold, copper, and oxygen suggest that it is a narrow bandgap semiconductor. This belt-like structure, growing around Au fine wire bonding in nano devices, may cause failure in the electric contact in nanodevices. Thus, this study has direct relevance to the use of gold as a contact material in semiconductor devices. In addition, possible future applications as additives in organic solar cells are discussed.
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Jul 2019
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15132]
Open Access
Abstract: Combining X-ray photoelectron spectroscopy with the standing wave technique we investigated adsorption of a monolayer of water on Ti-oxide terminated SrTiO3(001) in ultra-high vacuum (UHV). At room temperature, the surface is water free but hydroxylated. A quarter monolayer of hydroxyl is tightly bound (1.85 ± 0.06) Å above the TiO2 surface. Deposited at low temperature, a monolayer of water adsorbs with the oxygen located (2.55 ± 0.2) Å above the surface, apparently close to atop Ti, but H2O is unstable at 200K. A fraction desorbs, in part under the X-ray beam, but a major fraction of H2O dissociates immediately, with the liberated hydrogen most likely attaching to a surface oxygen. The produced hydroxyls bind only loosely to the surface, are unstable at 200K and rapidly desorb once the surface is water-free. Although our study was conducted in UHV, the presented results suggests that Ti-oxide terminated SrTiO3(001) may possess a high catalytic activity toward hydrolysis under realistic conditions.
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Jun 2019
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[11320]
Abstract: The understanding of the gypsum (CaSO4∙2H2O) formation pathway from aqueous solutions has been the subject of intensive research in the last couple of years. This interest stems from the fact that gypsum appears to fall into a broader category of crystalline materials whose formation does not follow classical nucleation and growth theories. The pathways involve transitory precursor cluster species, yet the actual structural properties of such clusters are not very well understood. Here, we show how in situ high-energy X-ray diffraction (HEXD) experiments and molecular dynamics (MD) simulations can be combined to derive the structure of small CaSO4 clusters, which are precursors to crystalline gypsum. We fitted several plausible structures to the derived pair distribution functions (PDFs), and explored their dynamic properties using unbiased MD based on both rigid-ion and polarizable force-fields. Determination of the structure and (meta)stability of the primary species is important both from a fundamental and applied perspective; for example, this will allow for improved design of additives for greater control of the nucleation pathway.
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Jun 2019
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