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Monitoring the characteristic properties of Ga-doped ZnO by Raman spectroscopy and atomic scale calculations
摘要： We experimentally and theoretically study how the structural and vibrational properties of zinc oxide (ZnO) are modified upon Gallium (Ga) doping. The characteristics of Ga-doped ZnO thin films which are synthesized by sol-gel spin coating method on glass substrates are monitored by using X-ray diffraction (XRD) and Raman scattering measurements. For atomic-level understanding of the experimental findings state-of-the-art density functional theory (DFT) based calculations are also performed. DFT calculations reveal that both the substitution and adsorption of Ga atoms in ZnO are energetically possible and substitutional doping in ZnO is the most favourable scenario. XRD measurements show that all the films are in wurtzite structure and the crystallite size of the films decreases with increasing Ga doping. In addition, Raman analysis show that strong vibrational modes at about 100 and 441 cm?1 are associated with E2low and E2high phonon modes of ZnO, respectively. While the frequency of the E2low downshifts with increasing Ga concentration, the E2high phonon mode is not affected by the Ga doping. Furthermore, EGa phonon branch, stemming from the substituted Ga atoms, emerges at low frequencies. It is also seen that the Raman intensity of the EGa peak linearly increases with increasing Ga concentration. Experimental results on the vibrational properties are in good agreement with the ab initio phonon calculations.
关键词： Thin films,Ga-doped ZnO,XRD,ab initio,Raman spectroscopy
更新于2025-09-23 15:19:57
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On the nanoscale behaviour of single-wall C, BN and SiC nanotubes
摘要： The paper presents a numerical study of defect-free single-wall carbon, boron nitride and silicon carbide armchair and zigzag nanotubes, through a simple stick-and-spring model, based on Morse and cosine potential functions. The study investigates the relaxed configuration of the tubes and gives a comprehensive evaluation of their elastic constants, which is performed by framing tensile, torsional and radial tests within the membrane behaviour of a Donnell thin shell model. Extensive comparisons with reference ab-initio results are given and used to refine some parameters of the potential functions for hexagonal silicon carbide nanomaterials.
关键词： ab-initio comparisons,mechanical behavior,elastic constants,stick-and-spring model,nanotubes
更新于2025-09-19 17:15:36
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Sub-Femtosecond Stark Control of Molecular Photoexcitation With Near Single-Cycle Pulses
摘要： Electric fields can tailor molecular potential energy surfaces by interaction with the electronic state-dependent molecular dipole moment. Recent developments in optics have enabled the creation of ultra-short few-cycle optical pulses with precise control of the carrier envelope phase (CEP) that determines the offset of the maxima in the field and the pulse envelope. This opens news ways of controlling ultrafast molecular dynamics by exploiting the CEP. In this work, we show that the photoabsorption efficiency of oriented H2CSO (sulfine) can be controlled by tuning the CEP. We further show that this control emanates from a resonance condition related to Stark shifting of the electronic energy levels.
关键词： ultrafast photochemistry,Ab initio multiple spawning (AIMS),few-cycle pulse,Stark effect,excited state dynamics
更新于2025-09-19 17:15:36
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with two-dimensional and three-dimensional local environments
摘要： ZnNi(CN)4 is a three-dimensional (3D) framework material consisting of two interpenetrating PtS-type networks in which tetrahedral [ZnN4] units are linked by square-planar [NiC4] units. Both the parent compounds, cubic Zn(CN)2 and layered Ni(CN)2, are known to exhibit 3D and 2D negative thermal expansion (NTE), respectively. Temperature-dependent inelastic neutron scattering measurements were performed on a powdered sample of ZnNi(CN)4 to probe phonon dynamics. The measurements were underpinned by ab initio lattice dynamical calculations. Good agreement was found between the measured and calculated generalized phonon density-of-states, validating our theoretical model and indicating that it is a good representation of the dynamics of the structural units. The calculated linear thermal expansion coefficients are αa = ?21.2 × 10?6 K?1 and αc = +14.6 × 10?6 K?1, leading to an overall volume expansion coefficient, αV of ?26.95 × 10?6 K?1, pointing towards pronounced NTE behavior. Analysis of the derived mode-Grüneisen parameters shows that the optic modes around 12 and 40 meV make a significant contribution to the NTE. These modes involve localized rotational motions of the [NiC4] and/or [ZnN4] rigid units, echoing what has previously been observed in Zn(CN)2 and Ni(CN)2. However, in ZnNi(CN)4, modes below 10 meV have the most negative Grüneisen parameters. Analysis of their eigenvectors reveals that a large transverse motion of the Ni atom in the direction perpendicular to its square-planar environment induces a distortion of the units. This mode is a consequence of the Ni atom being constrained only in two dimensions within a 3D framework. Hence, although rigid-unit modes account for some of the NTE-driving phonons, the added degree of freedom compared with Zn(CN)2 results in modes with twisting motions, capable of inducing greater NTE.
关键词： framework compound,ZnNi(CN)4,negative thermal expansion,lattice dynamics,ab initio calculations,inelastic neutron scattering
更新于2025-09-19 17:15:36
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Structural changes upon electronic excitation in 1,2-dimethoxybenzene from rotationally resolved electronic spectroscopy of various isotopologues
摘要： The structural changes of 1,2-dimethoxybenzene upon electronic excitation have been elucidated from a comparison of ab initio calculations and fits of the structural changes to experimental inertial parameters both in the basis of internal coordinates as well as in vibrational distortion coordinates. It is shown that the use of vibrational distortion coordinates leads to better agreement with the results of ab initio optimized structures compared to the use of internal coordinates in cases, that the inertial data are not sufficient for a complete structure determination. 1,2-dimethoxybenzene has been found to be only very slightly ortho-quinoidally distorted upon electronic excitation, in contrast to former expectations.
关键词： rotational constants,ab initio,pseudo-Kraitchman,evolutionary algorithms,Dimethoxybenzene,high resolution electronic spectroscopy
更新于2025-09-19 17:15:36
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Effect of Net Charge on the Relative Stability of 2D Boron Allotropes
摘要： We study the effect of electron doping on the bonding character and stability of two-dimensional (2D) structures of elemental boron, called borophene, which is known to form many stable allotropes. Our ab initio calculations for the neutral system reveal previously unknown stable 2D ε-B and ω-B structures. We find that the chemical bonding characteristic in this and other boron structures is strongly affected by extra charge. Beyond a critical degree of electron doping, the most stable allotrope changes from ε-B to a buckled honeycomb structure. Additional electron doping, mimicking a transformation of boron to carbon, causes a gradual decrease in the degree of buckling of the honeycomb lattice that can be interpreted as piezoelectric response. Net electron doping can be achieved by placing borophene in direct contact with layered electrides such as Ca2N. We find that electron doping can be doubled by changing from the B/Ca2N bilayer to the Ca2N/B/Ca2N sandwich geometry.
关键词： doping,boron,2D,ab initio calculation,structural stability,electronic structure
更新于2025-09-19 17:15:36
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Effect of the degree of inversion on optical properties of spinel ZnFe <sub/>2</sub> O <sub/>4</sub>
摘要： Spinel ferrites (T[M1?xFex]O[MxFe2?x]O4 with 0 ≤ x ≤ 1, where M is a bivalent metal ion and the superscripts denote tetrahedral and octahedral sites) are materials commonly used in electronics due to their outstanding magnetic properties. Thus, the effect of the degree of inversion, x, on these properties is well known. However, its effect on other properties of these materials has rarely been investigated in detail. Since ferrites gained much attention during the last decade as visible light active photocatalysts and photoelectrocatalysts, understanding the effect of the degree of inversion on the optical properties became necessary. Among photocatalytically and photoelectrocatalytically active spinel ferrites, zinc-ferrite (ZnFe2O4, ZFO) is one of the most widely studied materials. In this work, five ZFO samples with degrees of inversion varying from 0.07 to 0.20 were prepared by a solid-state reaction employing different annealing temperatures and subsequent quenching. Raman and UV-Vis-NIR spectra were measured and analyzed together with theoretical results obtained from ab initio calculations. Changes in the UV-Vis-NIR spectra associated with electronic transitions of tetrahedrally and octahedrally coordinated Fe3+ ions are distinguished. However, the optical band gap of the material remains unchanged as the degree of inversion varies. Based on the experimental and theoretical results, a new assignment for the Raman active internal modes and the electronic transitions of ZFO is proposed.
关键词： ab initio calculations,degree of inversion,zinc ferrite,spinel ferrites,optical properties,UV-Vis-NIR spectroscopy,Raman spectroscopy
更新于2025-09-19 17:15:36
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Dynamics of the photodissociation of ethyl iodide from the origin of the B band. A slice imaging study
摘要： The photodissociation dynamics and stereodynamics of ethyl iodide from the origin of the second absorption B-band have been investigated combining pulsed slice imaging with resonance enhanced multiphoton ionization (REMPI) detection of all fragments, I(2P3/2), I*(2P1/2) and C2H5. The I*(2P1/2) atom action spectrum recorded as a function of the excitation wavelength permits one to identify and select the 00 origin of this band at 201.19 nm (49 704 cm?1). Translational energy distributions and angular distributions for all fragments and semiclassical Dixon’s bipolar moments for the C2H5 fragment are presented and discussed along with high-level ab initio calculations of potential energy curves as a function of the C–I distance. A predissociative mechanism governs the dynamics where in a first step a bound Rydberg state corresponding to the 5ppI - 6sI transition is populated by the 201.19 nm-photon absorption. A curve crossing with a repulsive state located within the Franck–Condon geometry leads to direct dissociation into the major channel C2H5 + I*(2P1/2). A small amount of I(2P3/2) atoms is nevertheless observed and presumably attributed to a second curve crossing with a repulsive state from the A-band.
关键词： ab initio calculations,slice imaging,ethyl iodide,REMPI,predissociation,photodissociation,stereodynamics,dynamics
更新于2025-09-19 17:15:36
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Accelerated Discovery of Two-Dimensional Optoelectronic Octahedral Oxyhalides via High-Throughput <i>Ab Initio</i> Calculations and Machine Learning
摘要： Traditional trial-and-error methods are obstacles for large-scale searching of new optoelectronic materials. Here, we introduce a method combining high-throughput ab initio calculations and machine-learning approaches to predict two-dimensional octahedral oxyhalides with improved optoelectronic properties. We develop an effective machine-learning model based on an expansive data set generated from density functional calculations including the geometric and electronic properties of 300 two-dimensional octahedral oxyhalides. Our model accelerates the screening of potential optoelectronic materials of 5000 two-dimensional octahedral oxyhalides. The distorted stacked octahedral factors proposed in our model play essential roles in the machine-learning prediction. Several potential two-dimensional optoelectronic octahedral oxyhalides with moderate band gaps, high electron mobilities, and ultrahigh absorbance coefficients are successfully hypothesized.
关键词： band gaps,optoelectronic materials,two-dimensional octahedral oxyhalides,absorbance coefficients,electron mobilities,high-throughput ab initio calculations,machine learning
更新于2025-09-19 17:13:59
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Structures and Properties of Methylammonium Iodide Precursors of Halide Perovskites and Implications for Solar Cells: an Ab-Initio Investigation
摘要： Halide perovskite materials based on the prototypical CH3NH3PbI3 received much attention in recent years, with applications ranging from solar cells to light-emitting diodes. However, many fundamental aspects of the halide perovskite materials remain elusive at the moment, such as the structures and impacts of the methyl ammonium precursors on the halide perovskite properties. In this manuscript, we take one step back to theoretically investigate the monomer and dimer structures of methylammonium iodide, a precursor for the synthesis of the halide perovskite CH3NH3PbI3 via the first principles calculations, in order to provide a more complete view of the halide perovskite structures. The calculations show that the hydrogen bonds are found to be the major factor that stabilizes the methylammonium iodide monomers and dimers, while the dimeric structure exhibits geometry with each iodine atom shared by two neighbouring hydrogen atoms in the methylammonium cation molecules. The hydrogen bonds in the methylammonium iodide are proposed to be correlated with the three-dimensional halide perovskite buildup, and the formation of the hydrogen bond that is available in the halide perovskite framework could be “historically” traced back as early as the precursor preparation. The manuscript addresses the importance of the hydrogen bonds in the synthesis and the crystal formation of halide perovskite materials, and anticipates the necessity of the fundamental theoretical understanding of the structures from the halide perovskite precursors to the halide perovskite frameworks.
关键词： ab initio,perovskite solar cell,hydrogen bond,dimer,methylammonium iodide
更新于2025-09-19 17:13:59