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Open Access
Abstract: Chiral materials formed by aggregated organic compounds play a fundamental role in chiral optoelectronics, photo- and spin-tronics. Nonetheless, a precise understanding of the molecular interactions involved remains an open problem. Here we introduce magnetic circular dichroism (MCD) as a new tool to elucidate molecular interactions and structural parameters of a supramolecular system. A detailed analysis of MCD together with electronic circular dichroism spectra combined to ab initio calculations unveils essential information on the geometry and energy levels of a self-assembled thin film made of a carbazole di-bithiophene chiral molecule. This approach can be extended to a generality of chiral organic materials and can help rationalizing the fundamental interactions leading to supramolecular order. This in turn could enable a better understanding of structure-property relationships, resulting in a more efficient material design.
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Nov 2023
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[27952]
Abstract: The oxygen substoichiometry of as-prepared WO3 (tungsten trioxide) nano-powders (NPs) from a polyol process was tuned via peptization of the NPs in aqueous solutions of different Cr2O72− oxidizing concentrations. The as-synthesized materials have been characterized by X-ray scattering (XRD and PDF), transmission electron microscopy (TEM), X-ray photoelectron (XPS), and UV-VIS photochromic activity. The local atomic structure of WO3-X was investigated using total scattering atomic pair distribution function (PDF) analysis based on X-ray total scattering data collected on powder at ambient conditions. The PDF analysis confirms that the crystal structure of all studied samples can be described in terms of very small crystallites with a P21/n space-group monoclinic framework but with anomalous unit cell distortion parameters, indicating that small crystallite sizes resulted in a larger monoclinic distortion (as measured by the beta angle of the unit cell). The nanometer dimension of the crystallites combined as well as the oxygen-tungsten stoichiometric ratio control, are key features for optimized photochromic properties. Moreover, we present the fabrication of a WO3-x thin film based UV photosensor, which was carried out on silica glass substrates via the dip-coating method. The obtained films exhibited a UV photoresponse and photoelectric characteristics at 5 V bias voltages able to detect very low UV doses, inferior to 10 W/m2. The photo-detection measurements prove the usability of our device as a UV photodetector with a good responsivity of 0.37 A/W and external quantum efficiency of more than 100% even at a very low power density (9.2 W/m2) of UV illumination.
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Nov 2023
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[30315]
Open Access
Abstract: In a recent study we showed that record open circuit voltage (1.1 V) can be obtained for wide band gap full Germanium kesterite Cu2ZnGeS4 (CZGS) thin film solar cells by substituting the standard CdS buffer with a Zn1−xSnxOy (ZTO) buffer layer grown by atomic layer deposition [N. Saini et al., Sol. RRL 2021, 2100837]. However, a low short-circuit current and a very small variation in device performance with ZTO properties (band gap or thickness) were observed, suggesting similar interface properties. As a chemical surface etching was not possible for the CZGS absorber due to peeling, it was suggested that the presence of an oxide interlayer could influence the device performance. To prove this, we have studied non-destructively the chemical and electronic properties of the near surface region of CZGS absorber and its interface with ZTO buffers with varying properties by employing excitation dependent X-ray photoemission spectroscopy. The results indicate the formation of oxide species at the absorber surface during device fabrication (GeO2), which are preserved during the ALD deposition. Additional Na+COx species are also observed to be present at the near buffer/absorber interface. Further, the results show that the ZTO/CZGS interface properties are similar and not influenced by variations in ZTO properties (thickness and band gap energy), which could explain the small variation in device performance with buffer layer band gap or thickness. The results suggest that attention shall be given to possible absorber surface cleaning treatment typically applied before the buffer layer deposition for Ge containing CZGS absorbers, and an optimization of the ALD ZTO buffer layer growth shall be considered for a non-etched absorber.
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Nov 2023
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[20857]
Open Access
Abstract: Ultrathin Co𝑥
Fe3−𝑥
O4
films of high structural quality and with different Co content (x = 0.6–1.2) were prepared by reactive molecular beam epitaxy on MgO(001) substrates. Epitaxy of these ferrite films is extensively monitored by means of time-resolved (operando) X-ray diffraction recorded in out-of-plane geometry to characterize the temporal evolution of the film structure. The Co ferrite films show high crystalline ordering and smooth film interfaces independent of their Co content. All Co𝑥
Fe3−𝑥
O4
films exhibit enhanced compressive out-of-plane strain during the early stages of growth, which partly releases with increasing film thickness. When the Co content of the ferrite films increases, the vertical-layer distances increase, accompanied by slightly increasing film roughnesses. The latter result is supported by surface-sensitive low-energy electron diffraction as well as X-ray reflectivity measurements on the final films. In contrast, the substrate–film interface roughness decreases with increasing Co content, which is confirmed with X-ray reflectivity measurements. In addition, the composition and electronic structure of the ferrite films is characterized by means of hard X-ray photoelectron spectroscopy performed after film growth. The experiments reveal the expected increasing Fe3+
/Fe2+
cation ratios for a higher Co content.
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Nov 2023
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[29451]
Abstract: Smart windows change their properties in response to external factors, with glazing that can switch between transparent and opaque depending on temperature, light levels or an applied voltage. They can be used for privacy and visual effects or to improve energy efficiency. Smart windows using thermochromic materials, for example, can change to block infrared transmission as temperatures rise, remaining transparent to visible light. The thermochromic properties of vanadium dioxide (VO2) offer great potential for energy-saving smart windows. However, depositing VO2 films and coatings through sputtering, chemical or physical vapour deposition can be time-consuming and requires complex and expensive equipment. Solution-based methods are a simpler option, but usually require using a furnace to heat the materials above around 400°C to achieve the necessary crystalline structure, limiting the materials that can be used as a substrate. In work recently published in Applied Surface Science, an international team of researchers crystallised VO2 thin film using laser annealing, substantially reducing the annealing time and crystallisation temperature. Their results showed that the thermochromic properties were comparable with those of furnace-treated samples and that pulsed laser annealing of VO2 could be exploited for a range of applications, including smart windows, metamaterials and flexible electronics.
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Nov 2023
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[25808]
Abstract: Among the functional materials suitable for the realization of solar cells and light emission diodes (LEDs), perovskites, especially fully inorganic perovskites, are considered among the most promising. CsPbBr3 is the most studied due to its favorable band gap value and stability under mild atmospheric conditions: moisture and UV visible radiation exposure. Using this material, highly efficient thin film solar cells have been produced at the lab scale, and researchers aim to scale the process up to the industrial level. Here, ultrathin films of CsPbBr3 were obtained by magnetron sputtering, and the texture and morphology of the deposited films were evaluated as a function of their thickness and the substrate's nature. The obtained films are composed of highly textured structures of submicrometric crystals homogeneously distributed all over the surface. Finally, the texture decreases as the deposit thickness increases.
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Oct 2023
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I09-Surface and Interface Structural Analysis
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P.
Ding
,
M.
Braim
,
A. L.
Hobson
,
L. A.
Rochford
,
P. T. P.
Ryan
,
D. A.
Duncan
,
T.-L.
Lee
,
H.
Hussain
,
G.
Costantini
,
M.
Yu
,
D. P.
Woodruff
Diamond Proposal Number(s):
[21622, 29928]
Open Access
Abstract: The results of a quantitative experimental structural investigation of the adsorption phases formed by 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane (F4TCNQ) on Cu(111) are reported. A particular objective was to establish whether Cu adatoms are incorporated into the molecular overlayer. A combination of normal incidence X-ray standing waves, low-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy measurements, complemented by dispersion-inclusive density functional theory calculations, demonstrates that F4TCNQ on Cu(111) does cause Cu adatoms to be incorporated into the overlayer to form a two-dimensional metal–organic framework (2D-MOF). This conclusion is shown to be consistent with the behavior of F4TCNQ adsorption on other coinage metal surfaces, despite an earlier report concluding that the adsorption structure on Cu(111) is consistent with the absence of any substrate reconstruction.
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Oct 2023
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I05-ARPES
I10-Beamline for Advanced Dichroism - scattering
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Gesa-R.
Siemann
,
Seo-Jin
Kim
,
Edgar
Abarca Morales
,
Philip A. E.
Murgatroyd
,
Andela
Zivanovic
,
Brendan
Edwards
,
Igor
Markovic
,
Federico
Mazzola
,
Liam
Trzaska
,
Oliver J.
Clark
,
Chiara
Bigi
,
Haijing
Zhang
,
Barat
Achinuq
,
Thorsten
Hesjedal
,
Matthew D.
Watson
,
Timur K.
Kim
,
Peter
Bencok
,
Gerrit
Van Der Laan
,
Craig M.
Polley
,
Mats
Leandersson
,
Hanna
Fedderwitz
,
Khadiza
Ali
,
Thiagarajan
Balasubramanian
,
Marcus
Schmidt
,
Michael
Baenitz
,
Helge
Rosner
,
Phil D. C.
King
Diamond Proposal Number(s):
[28412, 31035]
Open Access
Abstract: In half-metallic systems, electronic conduction is mediated by a single spin species, offering enormous potential for spintronic devices. Here, using microscopic-area angle-resolved photoemission, we show that a spin-polarised two-dimensional hole gas is naturally realised in the polar magnetic semiconductor AgCrSe2 by an intrinsic self-doping at its CrSe2-terminated surface. Through comparison with first-principles calculations, we unveil a striking role of spin-orbit coupling for the surface hole gas, unlocked by both bulk and surface inversion symmetry breaking, suggesting routes for stabilising complex magnetic textures in the surface layer of AgCrSe2.
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Oct 2023
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B23-Circular Dichroism
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Diamond Proposal Number(s):
[31951]
Open Access
Abstract: Obtaining hierarchical structures with arbitrarily controlled chirality remains a challenge since the commonly used chiral building blocks favor chiral randomness throughout length scales. Here, we obtain chiroptically bipolar patterned thin films by introducing an autonomous device for microscale photothermal re-melting of materials exhibiting chirality synchronization. The device operates on an algorithm that reads, identifies, and controls the recrystallization of the film, facilitating the homochiral growth of supramolecular organic helices. Owing to the molecular achirality of the individual molecules and local chirality synchronization, both handednesses of the helices are available, enabling unrestricted chiral writing in the film. The collective chiroptical response of assembled molecules is used for guiding the patterning process, thus creating a foundation for optically secured information. Established methodology enabled achieving dissymmetry factor values for circular dichroism a magnitude higher than previously reported, as confirmed with state-of-the-art, synchrotron-based Mueller matrix polarimetry. Moreover, we apply the developed method to nanocomposites comprising gold nanoparticles, providing the opportunity to tune the circular dichroism towards the plasmonic region. Our strategy uncovers the potential of photothermal processing by laser-directed melting, widening the selection of nanostructured materials with precisely designed chiroptical functionalities for photonic applications.
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Oct 2023
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I06-Nanoscience (XPEEM)
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Xinqi
Liu
,
Puyang
Huang
,
Yunyouyou
Xia
,
Lei
Gao
,
Liyang
Liao
,
Baoshan
Cui
,
Dirk
Backes
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Yuchen
Ji
,
Peng
Chen
,
Yifan
Zhang
,
Fan
Wu
,
Meixiao
Wang
,
Junwei
Zhang
,
Guoqiang
Yu
,
Cheng
Song
,
Yulin
Chen
,
Zhongkai
Liu
,
Yumeng
Yang
,
Yong
Peng
,
Gang
Li
,
Qi
Yao
,
Xufeng
Kou
Abstract: To harness the intriguing properties of 2D van der Waals (vdW) ferromagnets (FMs) for versatile applications, the key challenge lies in the reliable material synthesis for scalable device production. Here, the epitaxial growth of single-crystalline 1T-CrTe2 thin films on 2-inch sapphire substrates are demonstrated. Benefiting from the uniform surface energy of the dangling bond-free Al2O3(0001) surface, the layer-by-layer vdW growth mode is observed right from the initial growth stage, which warrants precise control of the sample thickness beyond three monolayer and homogeneous surface morphology across the entire wafer. Moreover, the presence of the Coulomb interaction at the CrTe2/Al2O3 interface plays an important role in tailoring the anomalous Hall response, and the structural optimization of the CrTe2-based spin-orbit torque device leads to a substantial switching power reduction by 54%. The results may lay out a general framework for the design of energy-efficient spintronics based on configurable vdW FMs.
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Sep 2023
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