X-ray Diffraction Photoelectron Research Articles

Development of an efficient electrochemical sensor based on CuAl-LDH using an electrostatic repulsion approach for the selective determination of dopamine in the presence of uric acid and ascorbic acid species

This study provides a unique electrochemical sensor that exhibits both excellent sensitivity and selectivity, while also being environmentally friendly. The CuAl-LDH/GCE sensor, proposed as the recommended sensor, was synthesized using a straightforward one-step co-precipitation procedure. It was first used to measure dopamine levels using differential pulse voltammetry. At the ideal pH level of 8, dopamine has a positive charge, but ascorbic acid and uric acid have a negative charge. LDH, on the other hand, carries a negative charge and exhibits high electrostatic attraction towards dopamine, but is electrostatically repelled by negatively charged ascorbic acid and uric acid. Hence, CuAl-LDH/GCE has the potential to specifically ascertain the existence of dopamine in the presence of these particular species. The examination of the composition and morphology of CuAl-LDH was conducted using various analytical techniques, including scanner electron microscopy (FESEM), transmission electron microscope (TEM), element mapping (MAP), Fourier transform infrared (FTIR), energy-dispersive X-ray spectroscopy (EDX), Brunauer Emmett Teller (BET), X-ray photoelectron diffraction (XRD), and Raman techniques. Under ideal circumstances, the calibration graph of dopamine was generated using differential pulse voltammetry. A linear range of 4.194–1151.54 μM was achieved for dopamine, with a limit of detection of 0.33 μM. The findings of the study indicate that the sensor created for dopamine determination has exceptional stability, repeatability, and reproducibility. The sensor that was presented was effectively used for the measurement of dopamine in both pharmaceutical ampoules and human plasma samples.

X-ray photoelectron diffraction as a modern tool for determining surface stacking sequence in layered materials

We investigated the surface structure of a NbSe2 single crystal at room temperature, using angle-scanned x-ray photoelectron diffraction (XPD) combined with multiple scattering calculations. Different stacking sequences were tested (1T, 2H a , 2H c , and 3R), including possible stacking faults and a mixed 2H–3R stacking proposed earlier in the literature. We confirm the capability of XPD to distinguish different proposed structural models and, unambiguously, determine the true surface structure. Also, our findings provide reliable in-plane and interlayer distances. We observed expansions of the perpendicular distances between atomic planes within the monolayer and between monolayers of 3%–5%. These results are important as accurate experimental input for the development of theoretical methods that involve a quantitative description of van der Waals systems.

An ultrasensitive unlabeled electrochemical immunosensor for the detection of cardiac troponin I based on Pt/Au–B,S,N-rGO as the signal amplification platform

In this study, an ultrasensitive unlabeled electrochemical immunosensor for the detection of cardiac troponin I (cTnI) was developed based on Pt/Au modified B,S,N co-doped reduced graphene oxide (Pt/Au–B,S,N-rGO) as a signal amplification platform. First-principles calculations were employed to analyze the electron density of states of Pt/Au–B,S,N-rGO, revealing an increase in the electron density of the graphene oxide (GO) states. Furthermore, scanning electron microscopy (SEM), X-ray photoelectron diffraction spectroscopy (XPS), and electrochemical detection were used to successfully construct and analyze Pt/Au–B,S,N-rGO. The results showed that B,S,N-rGO exhibited good electrochemical activity, and the Au/Pt NPs demonstrated excellent catalytic properties, which provided a strong foundation for achieving high-sensitivity detection. Moreover, the constructed unlabeled electrochemical immunosensor had an ideal linear range (0.1 pg/mL∼50 ng/mL) and detection limit (0.082 pg/mL). In human serum detection, the results of this immunosensor were essentially similar to the ELISA results for the same samples, which suggested that the immunosensor had a promising clinical application prospect for the detection of cTnI.

Synthesis route and structural, magnetic, optical, and non-linear optical properties of triboluminescent [Cu(NCS)(py)2(PPh3)] complex

Transition-metal coordination compounds exhibit many unique properties, which can be used in a broad range of state-of-the-art technologies. One of these interesting complexes is copper(I) thiocyanate bipyridine triphenylphosphine [Cu(NCS)(py)2(PPh3)]. Herein, we report detailed structural, magnetic, spectroscopic, and nonlinear optical properties, as well as the optimized synthesis route of the [Cu(NCS)(py)2(PPh3)] complex. Performed Raman, infrared (IR), X-ray photoelectron (XPS), and X-ray diffraction (XRD) spectroscopy provide valuable data about the synthesis time dependence and general structural characteristics. Optical studies describe in detail the photoluminescence properties and emission temperature dependence behavior. For the first time, the magnetic and third-order nonlinear optical measurements of the [Cu(NCS)(py)2(PPh3)] complex were performed. Additionally, thermogravimetric analysis was conducted to detail the thermal stability of the complex. The results constitute a detailed description of the physical properties of the [Cu(NCS)(py)2(PPh3)] complex, which is an essential tool for designing sensing solutions based on the triboluminescent properties of the investigated compound.

Circular dichroism in hard X-ray photoelectron diffraction observed by time-of-flight momentum microscopy

X-ray photoelectron diffraction (XPD) is a powerful technique that yields detailed structural information of solids and thin films that complements electronic structure measurements. Among the strongholds of XPD we can identify dopant sites, track structural phase transitions, and perform holographic reconstruction. High-resolution imaging of kll-distributions (momentum microscopy) presents a new approach to core-level photoemission. It yields full-field kx-ky XPD patterns with unprecedented acquisition speed and richness in details. Here, we show that beyond the pure diffraction information, XPD patterns exhibit pronounced circular dichroism in the angular distribution (CDAD) with asymmetries up to 80%, alongside with rapid variations on a small kll-scale (0.1 Å−1). Measurements with circularly-polarized hard X-rays (hν = 6 keV) for a number of core levels, including Si, Ge, Mo and W, prove that core-level CDAD is a general phenomenon that is independent of atomic number. The fine structure in CDAD is more pronounced compared to the corresponding intensity patterns. Additionally, they obey the same symmetry rules as found for atomic and molecular species, and valence bands. The CD is antisymmetric with respect to the mirror planes of the crystal, whose signatures are sharp zero lines. Calculations using both the Bloch-wave approach and one-step photoemission reveal the origin of the fine structure that represents the signature of Kikuchi diffraction. To disentangle the roles of photoexcitation and diffraction, XPD has been implemented into the Munich SPRKKR package to unify the one-step model of photoemission and multiple scattering theory.

Probing the Atomic Arrangement of Subsurface Dopants in a Silicon Quantum Device Platform.

High-density structures of subsurface phosphorus dopants in silicon continue to garner interest as a silicon-based quantum computer platform; however, a much-needed confirmation of their dopant arrangement has been lacking. In this work, we take advantage of the chemical specificity of X-ray photoelectron diffraction to obtain the precise structural configuration of P dopants in subsurface Si:P δ-layers. The growth of δ-layer systems with different levels of doping is carefully studied and verified using X-ray photoelectron spectroscopy and low-energy electron diffraction. Subsequent diffraction measurements reveal that in all cases, the subsurface dopants primarily substitute with Si atoms from the host material. Furthermore, no signs of carrier-inhibiting P-P dimerization can be observed. Our observations not only settle a nearly decade-long debate about the dopant arrangement but also demonstrate how X-ray photoelectron diffraction is surprisingly well suited for studying subsurface dopant structure. This work thus provides valuable input for an updated understanding of the behavior of Si:P δ-layers and the modeling of their derived quantum devices.

Zn Vacancies as Hydrogen Trap Sites in Polar Surfaces: A New Stabilization Mechanism for the ZnO(0001)-(2×2) Surface Reconstruction

The (2×2) reconstruction of the ZnO(0001) surface has been investigated by X-ray photoelectron diffraction (XPD). Comparing the XPD measurements with multiple-scattering simulations, the single Zn vacancy per (2×2) surface unit cell model is confirmed, and structures with O adatoms are ruled out. The analysis indicates an outward relaxation of the topmost Zn layer, in contrast to the usually reported results by density-functional theory (DFT) calculations. On the basis of DFT, we describe a new stabilizing mechanism of the polar ZnO surface through surface reconstruction where the Zn vacancies are occupied by three hydrogens atoms. The DFT surface relaxation of the proposed model is in excellent agreement with the XPD findings. Our DFT simulations also strongly indicate that the migration of hydrogens atoms to the surface, coming from the bulk, may influence the desorption of the surface Zn atom to create the vacancy.

Highly Sensitive p-SmFeO3/p-YFeO3 Planar-Electrode Sensor for Detection of Volatile Organic Compounds

Nanocomposites of SmFeO3/YFeO3 (1:0, 0.8:0.2, 0.6:0.4, 0.4:0.6, 0.2:0.8, and 0:1) with different molar proportions were prepared by the sol–gel method. The material’s properties were characterized by various test methods, such as scanning-electron microscopy (SEM) and X-ray photoelectron-diffraction spectrometry (XPS). The gas-sensing characteristics of the sensor were tested in darkness and under illumination using monochromatic light with various selected wavelengths. The test results show that the SmFeO3/YFeO3 sensor with the molar ratio of 0.4:0.6 had the highest gas response to volatile organic compound (VOC) gases and that the optimum operating temperature was lower (120 °C). The light illumination improved the sensor’s sensitivity to gas. Under the 370-nanometer light illumination, the sensor’s responses to 30 ppm of ethanol, acetone, and methanol gases were 163.59, 134.02, and 111.637, respectively, which were 1.35, 1.28, and 1.59 times higher, respectively, than those without light. The high gas sensitivity of the sensor was mainly due to the adsorption of oxygen on the material’s surface and the formation of a p–p heterojunction. The SmFeO3/YFeO3 sensor, which can respond to different VOC gases, can be used to detect the safety of unknown environments and provide a timely warning of the presence of dangerous gases in working environments.

Amperometric determination of salivary thiocyanate using electrochemically fabricated poly (3, 4-ethylenedioxythiophene)/MXene hybrid film

Thiocyanate (SCN) is a hazardous byproduct of the detoxification of cyanide. Even in minute quantity, the SCN has a negative impact on health. Although there are several ways for SCN analysis, an efficient electrochemical procedure has hardly ever been attempted. Here, the author reports the development of a highly selective and sensitive electrochemical sensor for SCN utilizing Poly (3, 4-Ethylenedioxythiophene) incorporated MXene (PEDOT/MXene) modified screen-printed electrode (SPE). The Raman, X-ray photoelectron (XPS), and X-ray diffraction (XRD) analyses support the effective integration of PEDOT on the MXene surface. Further, scanning electron microscopy (SEM) is employed to demonstrate the formation of MXene and PEDOT/MXene hybrid film. In order to specifically detect SCN in phosphate buffer media (pH 7.4), the PEDOT/MXene hybrid film is grown on the SPE surface via the electrochemical deposition method. Under the optimized condition, the PEDOT/MXene/SPE-based sensor provides a linear response against SCN from 10 to 100 µM and 0.1 μM to 1000 μM with the lowest limit of detections (LOD) of 1.44 μM and 0.0325 μM by differential pulse voltammetry (DPV) and amperometry, respectively. For accurate detection of SCN, our newly created PEDOT/MXene hybrid film-coated SPE demonstrates excellent sensitivity, selectivity, and repeatability. Ultimately, this novel sensor can be used to detect SCN precisely in environmental and biological samples.

Unravelling the surface structure of β-Ga2O3 (100).

The present work is on a comprehensive surface atomic structure investigation of β-Ga2O3 (100). The β-Ga2O3 single crystal was studied by a structural model system in the simulations and in situ characterization via X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) allowed for probing the outermost layers' properties. In situ XPD characterization allows for the collection of valuable element-specific short-range information from the β-Ga2O3 surface, and the results were compared to a systematic and precise multiple scattering simulation approach. The experiments, characterizations, and simulations indicated strong evidence of considerable structural variations in the interatomic layer's distances. Such atomic displacement could clarify the electronic phenomena observed in theoretical studies. The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicated that the β-Ga2O3 surface has two possible terminations. The best fits to the photoelectron diffraction curves are used to calculate the interplanar relaxation in the first five atomic layers. The results show good agreement with previous density functional theory calculations, establishing XPD as a useful tool for probing the atomic structure of oxide surfaces.

X-ray photoelectron diffraction study of the approximant Al5Co2(001) quasicrystal.

The intermetallic Al5Co2 is defined as a structurally complex material and is considered a low-order quasicrystalline approximant. A single crystal of Al5Co2(001) was obtained by the Czochralski method. The sample was characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (PED). The surface composition was also analyzed by XPS, indicating only Al and Co compounds. In the current research, the crystal structure was qualitatively analyzed using the LEED patterns for different incident beam energies indicating a (1 × 1) termination, also in accordance with some literature works. The structure study was performed by applying the standard software MSCD and showed a (1 × 1) pattern. In addition, four different termination models for this termination were tested. The reliability factor indicated that the best termination belongs to the Al-rich surface layer.

Tracking the surface atomic motion in a coherent phonon oscillation

X-ray photoelectron diffraction is a powerful tool for determining the structure of clean and adsorbate-covered surfaces. Extending the technique into the ultrafast time domain will open the door to studies as diverse as the direct determination of the electron-phonon coupling strength in solids and the mapping of atomic motion in surface chemical reactions. Here we demonstrate time-resolved photoelectron diffraction using ultrashort soft x-ray pulses from the free electron laser FLASH. We collect Se $3d$ photoelectron diffraction patterns over a wide angular range from optically excited $\mathrm{Bi}{}_{2}\mathrm{Se}{}_{3}$ with a time resolution of 140 fs. Combining these with multiple scattering simulations allows us to track the motion of near-surface atoms within the first 3 ps after triggering a coherent vibration of the ${A}_{1g}$ optical phonons. Using a fluence of $4.2\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$ from a 1.55 eV pump laser, we find the resulting coherent vibrational amplitude in the first two interlayer spacings to be on the order of 0.01 \AA{}.

Occupancy of lattice positions probed by X-ray photoelectron diffraction: A case study of tetradymite topological insulators

Occupancy of different structural positions in a crystal lattice often seems to play a key role in material properties. Several experimental techniques have been developed to uncover this issue, all of them being mostly bulk sensitive. However, many materials including topological insulators (TIs), which are among the most intriguing modern materials, are intended to be used in devices as thin films, for which the sublattice occupancy may differ from the bulk. One of the possible approaches to occupancy analysis is X-ray Photoelectron Diffraction (XPD), a structural method in surface science with chemical sensitivity. We applied this method in a case study of Sb2(Te1-xSex)3 mixed crystals, which belong to prototypical TIs. We used high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) as a reference method to verify our analysis. We revealed that the XPD data for vacuum cleaved bulk crystals are in excellent agreement with the reference ones. Also, we demonstrate that the anion occupancy near a naturally formed surface can be rather different from that of the bulk. The present results are relevant for a wide range of compositions where the system remains a topological phase, as we ultimately show by probing the transiently occupied topological surface state above the Fermi level by ultrafast photoemission.

Structure Analysis Using Time-of-Flight Momentum Microscopy with Hard X-rays: Status and Prospects

X-ray photoelectron diffraction (XPD) has developed into a powerful technique for the structural analysis of solids. Extension of the technique into the hard-X-ray range (hXPD) gives access to true bulk information. Here we give a status report on hXPD experiments using a novel full-field imaging technique: Time-of-flight momentum microscopy (ToF-MM). A special variant of ToF-MM is capable of recording high kinetic energies (up to >7keV) and enlarged k-fields-of-view. We present applications that are specific for high kinetic energies. The strong site specificity of hXPD is exemplified for NbSe2, the cubic-to-tetragonal transition in SrTiO3 and the zinc-blende structure in epitaxial GaAs films. Bloch-wave calculations show a very good agreement with experiment and reveal fingerprint-like signatures of emitter sites in host lattices. We show a dopant-site analysis in two semiconductors (Mn in GaAs and Te in Si). Hard-X-ray ARPES plus core-level hXPD enable eliminating the strong diffraction signature imprinted in HARPES maps.

Ultrafast X-ray photoelectron diffraction from free molecules: Simulations of diffraction profiles from transient intermediates in the elimination reaction of C2H4I2

We have performed the simulations of C 1 s X-ray photoelectron diffraction (XPD) profiles from C2H4I2, bridged and classical anti-forms of C2H4I intermediates and C2H4 products to capture structures of transient intermediates in the elimination reaction of C2H4I2, under our ultrafast X-ray photoelectron diffraction (UXPD) scheme for free molecules using soft X-ray free-electron laser (SXFEL). In the UXPD scheme, the sample molecules are aligned in advance by near-infrared (NIR) laser with ns pulse duration before applying a pump probe method. Then, we have considered alignment control of C2H4I2 by using the elliptically polarized NIR laser to realize the UXPD experiments for the free molecules. As the results of simulations of XPD profiles from the laser-aligned C2H4I2 molecules, we have demonstrated the two-dimensional (2D) color maps of the C 1 s XPD profiles from C2H4I2, C2H4I, and C2H4. The 2D color maps have revealed that the transient C 1 s XPD profiles from the bridged-form and classical anti-form C2H4I intermediates exhibit remarkable differences, reflecting different intra-molecular scattering pathways of C 1 s photoelectrons within the intermediates. Thus, the present result has proved that UXPD for the free molecules has an advantage, compared with other traditional diffraction methods.

Tracing the structural evolution of quasi-freestanding germanene on Ag(111)

In the last decade, research on 2D materials has expanded massively due to the popularity of graphene. Although the chemical engineering of two-dimensional elemental materials as well as heterostructures has been extensively pursued, the fundamental understanding of the synthesis of 2D materials is not yet complete. Structural parameters, such as buckling or the interface structure of a 2D material to the substrate directly affect its electronic characteristics. In order to proceed the understanding of the element-specific growth and the associated ability of tuning material properties of two-dimensional materials, we performed a study on the structural evolution of the promising 2D material germanene on Ag(111). This study provides a survey of germanium formations at different layer thicknesses right up to the arising of quasi-freestanding germanene. Using powerful surface analysis tools like low-energy electron diffraction, x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction with synchrotron radiation, we will reveal the internal and interfacial structure of all discovered germanium phases. Moreover, we will present models of the atomic and chemical structure of a hbox {Ag}_2hbox {Ge} surface alloy and the quasi-freestanding germanene with special focus on the structural parameters and electronic interaction at the interface.

X-ray photoelectron diffraction (XPED) and X-ray spectro-holography from the contributions of our instruments

This paper reviews our contributions and the development of an instrument for X-ray photoelectron diffraction (XPED) and X-ray holography. XPED and X-ray holography have been extensively applied in materials analysis over the past decades. We made pioneering contributions in this area that fueled the development of the corresponding instrumentation and technology. In this review, we briefly discuss the theory of XPED and X-ray holography instrument, the genesis, and applications. The instruments comprise a high-power soft and hard X-ray source as well as a diffraction aperture; further, they are capable of high-angular-resolution detection and patterning. The applications of these instruments for practical surface analysis are also reviewed for ultrathin insulators, catalysts, and magnetic materials. Finally, possibilities for the differential photoelectron holography (DPH) approach are premiered based on a novel approach.

Observing structural distortions in complex oxides by x-ray photoelectron diffraction

The observation of structural distortions in complex oxides is becoming more important in order to explain their macroscopic behavior. X-ray photoelectron diffraction is an excellent technique to study the crystal structure and potentially observe oxygen distortions in an element-specific fashion. Its surface-sensitive character opens up the possibility to perform experiments on ultra-thin films without probing the substrate underneath. For BaBiO3 thin films, for example recent experimental results point towards a suppression of an oxygen breathing mode with decreasing film thickness resulting in a decreased energy band gap. As another example, a nontrivial electronic band structure is predicted for [111]-bilayers of LaMnO3. However, the likely presence of a Jahn–Teller distortion would suppress the topological phase. Here, the working principle of x-ray photoelectron diffraction is described and the data analysis required afterwards is discussed. X-ray photoelectron diffraction interference patterns are obtained for various thicknesses of BaBiO3 and LaMnO3 thin films. Experimental results are compared with multiple-scattering simulations. Although, indications for the structural distortions are observed, we discuss limitations of the technique in quantifying the displacement of the oxygen atoms.

Energy dependence of interference phenomena in the forward-scattering regime of photoelectron diffraction

X-ray photoelectron diffraction is a powerful spectroscopic technique in the direct legacy of C.S. Fadley that combines high sensitivity to the arrangement of atoms in crystals and element specificity providing unique fingerprints of selected atomic sites in matter. When used with kinetic energies between 500 eV and 1500 eV, its interpretation and description is based on the fact that the atomic scattering factors are strongly forward-peaked in such a way that low-angles scattering and backscattering events are respectively dominant and almost irrelevant in the photoemission process. In this paper we aim to demonstrate with the help of multiple-scattering simulations that energy scans of high-energy (500–1500 eV) forward-scattering photoelectron diffraction can provide valuable structural and chemical information about thin epitaxial films or stacking of two-dimensional materials.

Basic studies toward ultrafast soft x-ray photoelectron diffraction; its application to probing local structure in iodobenzene molecules.

Ultrafast x-ray photoelectron diffraction (UXPD) for free molecules has a promising potential to probe the local structures of the molecules in an element-specific fashion. Our UXPD scheme consists of three steps: (1) near-infrared laser (NIR) with ns pulse duration aligns sample molecules, (2) ultra-violet laser with fs pulse duration pumps the aligned molecules, and (3) soft x-ray free-electron laser (SXFEL) with fs pulse duration probes the molecules by measuring x-ray photoelectron diffraction (XPD) profiles. Employing steps of (1) and (3), we have measured I 3d XPD profiles from ground state iodobenzene aligned by the NIR laser with the SXFEL. Then, we have intensively calculated I 3d XPD profiles with density functional theory, taking degrees of alignments of the molecules into account, to extract a distance between C and I atoms in iodobenzene from the experimental I 3d XPD profiles. Although we have failed to determine the distance from the comparison between the experimental and theoretical results, we have succeeded in concluding that the degeneracies of the initial state eliminate the sensitivity on molecular structure in the I 3d XPD profiles. Thus, the observation of fine structures in the XPD profiles could be expected, if a nondegenerate molecular orbital is selected for a probe of UXPD. Finally, we have summarized our criteria to perform UXPD successfully: (1) to use SXFEL, (2) to prepare sample molecules with the degree of alignment higher than 0.8, and (3) to select a photoemission process from a nondegenerate inner-shell orbital of sample molecules.