I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[17608]
Abstract: Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure—the (4×4) surface oxide on Pt3Sn(111)–based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.
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Apr 2022
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I07-Surface & interface diffraction
I09-Surface and Interface Structural Analysis
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Philip J.
Mousley
,
Luke A.
Rochford
,
Paul T. P.
Ryan
,
Philip
Blowey
,
James
Lawrence
,
David A.
Duncan
,
Hadeel
Hussain
,
Billal
Sohail
,
Tien-Lin
Lee
,
Gavin R.
Bell
,
Giovanni
Costantini
,
Reinhard J.
Maurer
,
Christopher
Nicklin
,
D. Phil
Woodruff
Diamond Proposal Number(s):
[14884, 4884]
Open Access
Abstract: While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunneling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-ray diffraction (SXRD) uniquely provides unambiguous identification of these metal adatoms. We present the results of a detailed structural study of the Au(111)-F4TCNQ system, combining surface characterization by STM, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy with quantitative experimental structural information from normal incidence X-ray standing wave (NIXSW) and SXRD, together with dispersion-corrected density functional theory (DFT) calculations. Excellent agreement is found between the NIXSW data and the DFT calculations regarding the height and conformation of the adsorbed molecule, which has a twisted geometry rather than the previously supposed inverted bowl shape. SXRD measurements provide unequivocal evidence for the presence and location of Au adatoms, while the DFT calculations show this reconstruction to be strongly energetically favored.
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Apr 2022
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[20426]
Open Access
Abstract: We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the
γ
-ZnPc polymorph thereafter (thickness
≈
1.0
nm). As the growth of the bulk
γ
-ZnPc progresses, a substantial out-of-plane lattice strain (
≈
15
%
relative to
γ
-ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of
≈
13
nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.
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Mar 2022
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[22710]
Open Access
Abstract: The surface region of austenitic stainless steel (SS) is investigated by synchrotron X-ray powder diffraction (XRPD) and X-ray absorption near edge structure (XANES) measurements, because its composition and structure is crucial for the corrosion resistance of SS. Grazing incidence XRPD of a polished AISI 304 bulk steel sample show that the near-surface structure is modified. The concentration of the ferrite phase of Fe, a typical minority phase in AISI 304, increases gradually from 10% to 30% when approaching the surface from 150 nm depth. XANES Fe K-edge investigations of ultra-thin, sputter deposited films also reveal much larger ferrite fractions than expected from the austenitic steel composition of the films. Reasons for the increased ferrite fraction in the surface region of bulk steel and thin films are discussed. However, right at the surface, the trend reverses. Analysis of XANES data for an ultra-thin, 4 nm SS film show that 80% of Fe is oxidized and 20% of metallic Fe is present only in austenite structure, suggesting that ferritic iron is preferentially subject to oxidation. The austenitic Fe is located at more than 2 - 3 nm below the surface where the Ni concentration is > 10%.
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Feb 2022
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I07-Surface & interface diffraction
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[24359, 23666]
Open Access
Abstract: Membranes with high selectivity offer an attractive route to molecular separations, where technologies such as distillation and chromatography are energy intensive. However, it remains challenging to fine tune the structure and porosity in membranes, particularly to separate molecules of similar size. Here, we report a process for producing composite membranes that comprise crystalline porous organic cage films fabricated by interfacial synthesis on a polyacrylonitrile support. These membranes exhibit ultrafast solvent permeance and high rejection of organic dyes with molecular weights over 600 g mol−1. The crystalline cage film is dynamic, and its pore aperture can be switched in methanol to generate larger pores that provide increased methanol permeance and higher molecular weight cut-offs (1,400 g mol−1). By varying the water/methanol ratio, the film can be switched between two phases that have different selectivities, such that a single, ‘smart’ crystalline membrane can perform graded molecular sieving. We exemplify this by separating three organic dyes in a single-stage, single-membrane process.
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Jan 2022
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[13769]
Open Access
Abstract: Antimony ultra-thin films in tensile strain are grown on InAs(111)B substrates and studied in situ using surface X-ray diffraction. The detailed atomic structures of two highly crystalline Sb(0001) films are derived, with thicknesses of 19 and 4 bilayers. Features considered in structural modelling include interfacial intermixing, surface roughness, individual layer relaxations and rotational twin domains (RTDs). The 4-bilayer film shows significant structural relaxation in every layer, while both films include RTDs. The results are discussed in relation to the topological properties of low-dimensional Sb.
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Jan 2022
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[18570]
Open Access
Abstract: Planar organic heterostructures are widely explored and employed in photovoltaic cells, light-emitting diodes, and bilayer field-effect transistors. An important role for device performance plays the energy level alignment at the inorganic–organic and organic–organic interfaces. In this work, incremental ultraviolet photoelectron spectroscopy measurements and real-time X-ray scattering experiments are used to thoroughly investigate the thickness-dependent electronic and structural properties of a perfluoropentacene (PFP)-on-[6]phenacene heterostructure. For both materials an incremental increase of the material work function (positive interface dipole) is found. For [6]phenacene, this can be assigned to a thickness-dependent change of molecular arrangement evident from a change of the unit cell volume and a consequential alteration of the ionization energy. In the case of PFP the interface dipole stems from charge transfer from the substrate into unoccupied molecular orbitals resulting in an electrostatic potential on the surface. The magnitude of this potential can be correlated with an increased gap state density resulting from templated structural defects mediated by the bottom layer.
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Dec 2021
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I07-Surface & interface diffraction
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Bin
Yang
,
Marina
Lledos
,
Riaz
Akhtar
,
Giuseppe
Ciccone
,
Long
Jiang
,
Emanuele
Russo
,
Sunil
Rajput
,
Chunyu
Jin
,
Maria
Angelerou
,
Thomas
Arnold
,
Jonathan
Rawle
,
Massimo
Vassalli
,
Maria
Marlow
,
Dave J.
Adams
,
Mischa
Zelzer
Diamond Proposal Number(s):
[16246]
Open Access
Abstract: Controlling supramolecular self-assembly across multiple length scales to prepare gels with localised properties is challenging. Most strategies concentrate on fabricating gels with heterogeneous components, where localised properties are generated by the stimuli-responsive component. Here, as an alternative approach, we use a spiropyran-modified surface that can be patterned with light. We show that light-induced differences in surface chemistry can direct the bulk assembly of a low molecular weight gelator, 2-NapAV, meaning that mechanical gel properties can be controlled by the surface on which the gel is grown. Using grazing incidence X-ray diffraction and grazing incidence small angle X-ray scattering, we demonstrate that the origin of the different gel properties relates to differences in the architectures of the gels. This provides a new method to prepare a single domain (i.e., chemically homogeneous) hydrogel with locally controlled (i.e., mechanically heterogeneous) properties.
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Oct 2021
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I07-Surface & interface diffraction
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Jeroen
Royakkers
,
Kunping
Guo
,
Daniel T. W.
Toolan
,
Liang-Wen
Feng
,
Alessandro
Minotto
,
Daniel G.
Congrave
,
Magda
Danowska
,
Weixuan
Zeng
,
Andrew
Bond
,
Mohammed
Al-Hashimi
,
Tobin J.
Marks
,
Antonio
Facchetti
,
Franco
Cacialli
,
Hugo
Bronstein
Diamond Proposal Number(s):
[23587]
Open Access
Abstract: Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non-radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide-based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect interpolymer interactions. We reveal that the non-encapsulated analogue P(NDI-2OD-T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing p-interchain stacking distance. This suggests that decreasing p-stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.
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Sep 2021
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I07-Surface & interface diffraction
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Shuai
Yuan
,
Lin-Song
Cui
,
Linjie
Dai
,
Yun
Liu
,
Qing-Weii
Liu
,
Yu-Qi
Sun
,
Florian
Auras
,
Miguel
Anaya
,
Xiaopeng
Zheng
,
Edoardo
Ruggeri
,
You-Jun
Yu
,
Yang-Kun
Qu
,
Mojtaba
Abdi-Jalebi
,
Osman M.
Bakr
,
Zhao-Kui
Wang
,
Samuel D.
Stranks
,
Neil C.
Greenham
,
Liang-Sheng
Liao
,
Richard H.
Friend
Diamond Proposal Number(s):
[17223]
Open Access
Abstract: Metal halide perovskite semiconductors have demonstrated remarkable potentials in solution-processed blue light-emitting diodes (LEDs). However, the unsatisfied efficiency and spectral stability responsible for trap-mediated non-radiative losses and halide phase segregation remain the primary unsolved challenges for blue perovskite LEDs. In this study, it is reported that a fluorene-based π-conjugated cationic polymer can be blended with the perovskite semiconductor to control film formation and optoelectronic properties. As a result, sky-blue and true-blue perovskite LEDs with Commission Internationale de l'Eclairage coordinates of (0.08, 0.22) and (0.12, 0.13) at the record external quantum efficiencies of 11.2% and 8.0% were achieved. In addition, the mixed halide perovskites with the conjugated cationic polymer exhibit excellent spectral stability under external bias. This result illustrates that π-conjugated cationic polymers have a great potential to realize efficient blue mixed-halide perovskite LEDs with stable electroluminescence.
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Sep 2021
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