Time-dependent Density Functional Theory Simulations Research Articles

Ultrahigh resolution x-ray Thomson scattering measurements at the European X-ray Free Electron Laser

Using an ultrahigh resolution (ΔE∼0.1eV) setup to measure electronic features in x-ray Thomson scattering (XRTS) experiments at the European XFEL in Germany, we have studied the collective plasmon excitation in aluminium at ambient conditions, which we can measure very accurately even at low momentum transfers. As a result, we can resolve previously reported discrepancies between time-dependent density functional theory simulations and experimental observations. The demonstrated capability for high-resolution XRTS measurements will be a game changer for the diagnosis of experiments with matter under extreme densities, temperatures, and pressures, and unlock the full potential of state-of-the-art x-ray free electron laser (XFEL) facilities to study planetary interior conditions, to understand inertial confinement fusion applications, and for material science and discovery. Published by the American Physical Society 2024

Excitation signatures of isochorically heated electrons in solids at finite wave number explored from first principles

Ultrafast heating of solids with modern x-ray free electron lasers (XFELs) leads to a unique set of conditions characterized by the simultaneous presence of heated electrons in a cold ionic lattice. In this work, we analyze the effect of electronic heating on the dynamic structure factor (DSF) in bulk aluminum (Al) with a face-centered cubic lattice and in silicon (Si) with a crystal diamond structure using first-principles linear-response time-dependent density functional theory simulations. We find a thermally induced red shift of the collective plasmon excitation in both materials. In addition, we show that the heating of the electrons in Al can lead to the formation of a double-plasmon peak due to the extension of the Landau damping region to smaller wave numbers. Finally, we demonstrate that thermal effects generate a measurable and distinct signature (peak-valley structure) in the DSF of Si at small frequencies. Our simulations indicate a variety of new features in the spectrum of x-ray-driven solids, specifically at finite momentum transfer, which can be probed in upcoming x-ray Thomson scattering experiments at various XFEL facilities. Published by the American Physical Society 2024

Cycloaddition reaction of methylidyne radical with dipyrromethanol: quantum chemical insights into the formation of di(pyridin-2-yl)methanol

Density functional theory was employed to investigate methylidyne radical reaction on to the unsaturation of dipyrromethanol moiety through cycloaddition forming a six membered di(pyridin-2-yl)methanol followed by H-elimination via ring expansion. Time-dependent density functional theory (TD-DFT) simulations were performed to figure out the ultraviolet–visible spectrum of dipyrromethanol and di(pyridin-2-yl)methanol. Ultraviolet–visible spectrum of dipyrromethanol and di(pyridin-2-yl)methanol shows that two sharp absorption peaks were obtained at λ max = 161 nm and 206 nm for the reactant dipyrromethanol while a sharp peak was observed for the product di(pyridin-2-yl)methanol at shorter wavelength (λ max = 170 nm). The theoretical IR spectrum of dipyrromethanol and di(pyridin-2-yl)methanol shows that a significant red shift of 129 cm−1 was observed in the case of di(pyridin-2-yl)methanol due to presence of two strong intra-molecular hydrogen bonds, which indicates its higher stability than the dipyrromethanol. HOMO–LUMO energy gap calculations and natural bond orbital (NBO) analysis endorse the type of electronic transition for the desired product di(pyridin-2-yl)methanol is π–π*. The global reactivity analysis shows the electron donor and acceptor nature of dipyrromethanol and methylidyne radical, respectively. A potential energy surface is constructed via methylidyne radical addition, ring expansion, and hydrogen elimination transition states. Thermodynamics study and potential energy surface of the title reaction indicates its highly exothermicity and spontaneous in nature.

Effects of structural variations to x-ray absorption spectra of g-C3N4: Insight from DFT and TDDFT simulations

X-ray absorption spectroscopy (XAS) is widely employed for structure characterization of graphitic carbon nitride (g-C3N4) and its composites. Nevertheless, even for pure g-C3N4, discrepancies in energy and profile exist across different experiments, which can be attributed to variations in structures arising from diverse synthesis conditions and calibration procedures. Here, we conducted a theoretical investigation on XAS of three representative g-C3N4 structures (planar, corrugated, and micro-corrugated) optimized with different strategies, to understand the structure–spectroscopy relation. Different methods were compared, including density functional theory (DFT) with the full core-hole (FCH) or equivalent core-hole (ECH) approximation as well as the time-dependent DFT (TDDFT). FCH was responsible for getting accurate absolute absorption energy; while ECH and TDDFT aided in interpreting the spectra, through ECH-state canonical molecular orbitals (ECH-CMOs) and natural transition orbitals (NTOs), respectively. With each method, the spectra at the three structures show evident differences, which can be correlated with different individual experiments or in between. Our calculations explained the structural reason behind the spectral discrepancies among different experiments. Moreover, profiles predicted by these methods also displayed consistency, so their differences can be used as a reliable indicator of their accuracy. Both ECH-CMOs and NTO particle orbitals led to similar graphics, validating their applicability in interpreting the transitions. This work provides a comprehensive analysis of the structure-XAS relation for g-C3N4, provides concrete explanations for the spectral differences reported in various experiments, and offers insight for future structure dynamical and transient x-ray spectral analyses.

Dichroism of plasmonic chiral nanoalloys by rational design.

Time-dependent density functional theory (TDDFT) simulations are conducted on a series of chiral gold/silver alloy nanowires to explore whether silver doping can produce an enhancement of circular dichroism at the plasmon resonance in these systems, and to identify the quantum-mechanical origin of the observed effects. We find a strong plasmonic dichroism when one or two helixes of gold atoms are substituted by silver in a linear chiral nanotube, whose pure gold counterpart does not display any plasmonic dichroism, and we rationalize this finding in terms of "decoupling" the destructive interference of excitations in the pure gold nanotube via alloying. However, further attempts to increase the plasmonic dichroism by considering multi-shell gold nanowires in which one entire shell is doped with silver did not produce the desired effect, but rather a decrease in circular dichroism. We show that this latter result is due to a more severe destructive interference in the dipole excitation contributions, and suggest that further amplification should be possible in principle by properly tuning simultaneously the nanowire structure and chemical ordering.

Carrier injection induced degradation of nitrogen passivated SiC–SiO2 interface simulated by time-dependent density functional theory

The performance of SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs) degrades seriously after a period of continuous operation. To directly understand this issue, we conduct real-time time-dependent density functional theory (TDDFT) simulations on a series of nitrogen passivated SiC–SiO2 interfaces to monitor the interaction between carriers and interface atoms. We find that the nitrogen passivation always leaves behind two local states near the VBM, which gives a chance to the strong interaction between channel carriers and C–N bonds, and finally results in the generation of C dangling bond defects. These processes are vividly presented and confirmed by the TDDFT simulation. Additionally, the results show that the new defects are more easily formed by the passivated C cluster than the passivated Si vacancy. These studies provide physical insights into the degradation mechanisms of working SiC MOSFETs, while simultaneously demonstrating the advantage of TDDFT as a crucial tool for investigating defect generation dynamics in semiconductor devices.

Solid-state high harmonic spectroscopy for all-optical band structure probing of high-pressure quantum states

High pressure has triggered various novel states/properties in condensed matter, as the most representative and dramatic example being near-room-temperature superconductivity in highly pressured hydrides (~200 GPa). However, the mechanism of superconductivity is not confirmed, due to the lacking of effective approach to probe the electronic band structure under such high pressures. Here, we theoretically propose that the band structure and electron-phonon coupling (EPC) of high-pressure quantum states can be probed by solid-state high harmonic generation (sHHG). This strategy is investigated in high-pressure Im-3m H3S by the state-of-the-art first-principles time-dependent density-functional theory simulations, where the sHHG is revealed to be strongly dependent on the electronic structures and EPC. The dispersion of multiple bands near the Fermi level is effectively retrieved along different momentum directions. Our study provides unique insights into the potential all-optical route for band structure and EPC probing of high-pressure quantum states, which is expected to be helpful for the experimental exploration of high-pressure superconductivity in the future.

Plasmon induced hot carrier distribution in Ag20 -CO composite.

The interaction between plasmons and the molecules leads to the transfer of plasmon-induced hot carriers, presenting innovative opportunities for controlling chemical reactions on sub-femtosecond timescales. Through real-time time-dependent density functional theory simulations, we have investigated the enhancement of the electric field due to plasmon excitation and the subsequent generation and transfer of plasmon-induced hot carriers in a linear atomic chain of Ag20 and an Ag20 -CO composite system. By applying a Gaussian laser pulse tuned to align with the plasmon frequency, we observe a plasmon-induced transfer of hot electrons from the occupied states of Ag to the unoccupied molecular orbitals of CO. Remarkably, there is a pronounced accumulation of hot electrons and hot holes on the C and O atoms. This phenomenon arises from the electron migration from the inter-nuclear regions of the C-O bond towards the individual C and O atoms. The insights garnered from our study hold the potential to drive advancements in the development of more efficient systems for catalytic processes empowered by plasmonic interactions.

Electronic stopping in ternary material SiCN: Anomalous behavior of slow channeling helium ions

By performing time-dependent density functional theory simulations, we reveal an anomalous deviation from linear low-velocity electronic stopping for slow channeling helium ions in the ternary material SiCN. We ascribe the deviation behavior that occurs upon helium ion irradiated SiCN to trapped electrons in the final stage of collision. The result show that the electron trapping for the exit-side of the material is more sensitive to the projectile velocity than that for the entrance-side, and that this has a corresponding relationship with nonlinear electronic stopping. In addition, our calculations reveal enhanced nonlinear electronic stopping to the off-center channels, particularly near the C/N layers. We propose the increased electron excitation for the increased charge density channel under a small impact parameter as the most plausible explanation for this behavior.

X-ray induced ultrafast charge transfer in thiophene-based conjugated polymers controlled by core-hole clock spectroscopy.

We explore ultrafast charge transfer (CT) resonantly induced by hard X-ray radiation in organic thiophene-based polymers at the sulfur K-edge. A combination of core-hole clock spectroscopy with real-time propagation time-dependent density functional theory simulations gives an insight into the electron dynamics underlying the CT process. Our method provides control over CT by a selective excitation of a specific resonance in the sulfur atom with monochromatic X-ray radiation. Our combined experimental and theoretical investigation establishes that the dominant mechanism of CT in polymer powders and films consists of electron delocalisation along the polymer chain occurring on the low-femtosecond time scale.

Preparation of high-performance monoazo disperse dyes bearing ester groups based on benzisothiazole and their dyeing performance on polyester fabrics.

To obtain high-performance disperse dyes, a series of azo disperse dyes containing different kinds of ester groups based on benzisothiazole were synthesized by the coupling reaction of diazotization of 3-amino-5-nitro [2,1] benzisothiazole with N-substituted aniline compounds bearing different ester moieties. The structures of the synthesized dyes were evaluated using Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance techniques (1H-NMR), and MS analysis. UV-Vis spectrophotometry methods were applied to study absorption maxima, molar extinction coefficients, and solvatochromic behaviors of the dyes, and time-dependent density functional theory (TD-DFT) simulations were applied to reveal the nature of the absorption spectrum properties. Polyester fabrics were colored using a high-temperature dyeing method under pressure, and the dyed fabrics exhibited deep and bright intense blue hues. In addition, excellent fastness properties, including washing fastness, sublimation fastness, rubbing fastness, and light fastness, were achieved.

A theoretical study on the role of the π-spacer in the thoughtful design of good light-absorbing dyes with phenothiazine for efficient dye-sensitized solar cells (DSSCs).

In this work, we designed ten new organic phenothiazine dyes bridged by different πi-spacers (PTZ1-PTZ10) of D-π-A type based on the synthesized dye CC202-III for their efficacy in dye-sensitized solar cells (DSSC) applications. To learn how various π-spacers affect their performance in DSSCs, these isolated dyes and dye-cluster systems have had their geometries, electronic structures, absorption spectra, dipole moments, and molecular electrostatic potential examined and talked about. Additionally, a number of quantization parameters that affect power conversion efficiency (PCE), including light collection efficiency (LHE), reorganization energy (λtotal), vertical dipole moment (μnormal), strength electron injection driving force (ΔGinject), regeneration driving force (ΔGreg), excited state lifetime (τ), and open circuit voltage (VOC), were calculated in order to identify the organic dyes that would be best suited for DSSC applications. Calculated results revealed that the designed dyes PTZ3, PTZ4, PTZ5, and PTZ10 exhibit a lower energy gap among all dyes compared to the corresponding CC202-III. Additionally, PTZ3, PTZ4, PTZ5, PTZ7, PTZ8, PTZ9, and PTZ10 exhibit significant red-shifted absorption spectra compared to the other dyes with a larger oscillator strength, which improves the photocurrent density of the devices. The findings thus imply that bridge modification is a workable tactic to raise DSSC effectiveness. We used density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to study the electronic and photovoltaic properties of the dyes designed (PTZ1-PTZ10) to assess their effectiveness in DSSCs. DFT and TD-DFT simulations are theoretically used to deeply analyze key characteristics of all organic dyes that affect open-circuit voltage (VOC) and short-circuit current (JSC) to identify structure-property relationships.

Electronic structure and photophysical properties of some promising organic molecules for organic solar cells.

Three novel organic semiconductors (Fig. 1), which are molecule (a) and molecule (c) have the same wing unit molecules (b) and (c) have the same core unit were reported. Thus, the influence of wing units on solar cell device performance parameters such us the opto-electronics properties, non-linear optics (NLO), electronic properties, and natural bond orbitals (NBO) were calculated in order to evincing molecular structure-property relations. The all studied molecules would be promising materials for photovoltaic applications, but molecule (c) could be an excellent candidate for high efficiency organic solar cells with a small energy gap, a lowest ΔGreg, highest Voc, and LHE values. According to all these results, it is seen that the wing units of the molecules affect both the opto-electronic properties and NLO properties more than the core units. These theoretical calculations is expected to obtain new strategies to synthesize efficient materials for organic solar cell devices. Density functional (DFT) and time-dependent density functional (TDDFT) theory simulations for the solar cell device performance parameters, non-linear optics, and natural bond analysis were performed using the Gaussian 09w software. The ground state properties of molecules have been studied with hybrid functional of Beckethree-Lee-Yang-Parr (B3LYP), and excited state properties have been calculated CAMB3LYP and our DFT calculations were performed using 6-31++G(d,p) basis set on fully optimized geometries.

Time-Dependent Density Functional Theory for X-ray Absorption Spectra: Comparing the Real-Time Approach to Linear Response.

We simulate X-ray absorption spectra at elemental K-edges using time-dependent density functional theory (TDDFT) in both its conventional linear-response implementation and its explicitly time-dependent or "real-time" formulation. Real-time TDDFT simulations enable broadband spectra calculations without the need to invoke frozen occupied orbitals ("core/valence separation"), but we find that these spectra are often contaminated by transitions to the continuum that originate from lower-energy core and semicore orbitals. This problem becomes acute in triple-ζ basis sets, although it is sometimes sidestepped in double-ζ basis sets. Transitions to the continuum acquire surprisingly large dipole oscillator strengths, leading to spectra that are difficult to interpret. Meaningful spectra can be recovered by means of a filtering technique that decomposes the spectrum into contributions from individual occupied orbitals, and the same procedure can be used to separate L- and K-edge spectra arising from different elements within a given molecule. In contrast, conventional linear-response TDDFT requires core/valence separation but is free of these artifacts. It is also significantly more efficient than the real-time approach, even when hundreds of individual states are needed to reproduce near-edge absorption features and even when Padé approximants are used to reduce the real-time simulations to just 2-4 fs of time propagation. Despite the cost, the real-time approach may be useful to examine the validity of the core/valence separation approximation.

Gigantic blue shift of two-photon–induced photoluminescence of interpenetrated metal–organic framework (MOF)

Abstract As an important means of modern science and technology, multiphoton fluorescence plays an essential role in high-resolution imaging, photochemistry, micro- and nano-processing and clinical diagnosis. Multiphoton fluorescence usually shares the same radiative channel as its intrinsic fluorescence. Under multiphoton excitation, except for red shift fluorescence caused by the reabsorption effect, gigantic blue shift of multiphoton fluorescence is rarely reported. In this work, metal–organic frameworks (MOFs) with 7-fold and 8-fold interpenetration are successfully synthesized. The synthesized 8-fold interpenetrated MOFs show unexpectedly giant blue-shifted (∼40 nm) two-photon–induced fluorescence compared with its fluorescence emission. Specific optical selection rules lead to different final transition states in one-photon absorption and two-photon absorption. The density functional theory (DFT) and time-dependent density functional theory (TDDFT) simulations show that, under two-photon excitation, electrons and holes can be more delocalized, and intermolecular interactions mainly govern the emission process of 8-fold interpenetrated MOFs. Highly excited electronic states of the interpenetrated MOFs are effectively excited and emitted under two-photon excitation, thus generating the inevitable blue-shifted two-photon–induced fluorescence emission. Our work provides a guide for exploring the excitation mechanism of fluorescent MOFs and offers an access to a tunable all-optical single-crystal device.

Averaging over atom snapshots in linear-response TDDFT of disordered systems: A case study of warm dense hydrogen.

Linear-response time-dependent density functional theory (LR-TDDFT) simulations of disordered extended systems require averaging over different snapshots of ion configurations to minimize finite size effects due to the snapshot-dependence of the electronic density response function and related properties. We present a consistent scheme for the computation of the macroscopic Kohn-Sham (KS) density response function connecting an average over snapshot values of charge density perturbations to the averaged values of KS potential variations. This allows us to formulate the LR-TDDFT within the adiabatic (static) approximation for the exchange-correlation (XC) kernel for disordered systems, where the static XC kernel is computed using the direct perturbation method [Moldabekov et al., J. Chem. Theory Comput. 19, 1286 (2023)]. The presented approach allows one to compute the macroscopic dynamic density response function as well as the dielectric function with a static XC kernel generated for any available XC functional. The application of the developed workflow is demonstrated for the example of warm dense hydrogen. The presented approach is applicable for various types of extended disordered systems, such as warm dense matter, liquid metals, and dense plasmas.

Optically Induced Multistage Phase Transition in Coherent Phonon-Dominated a-GeTe.

Ultrafast photoexcitation can decouple the multilevel nonequilibrium dynamics of electron-lattice interactions, providing an ideal probe for dissecting photoinduced phase transition in solids. Here, real-time time-dependent density functional theory simulations combined with occupation-constrained DFT methods are employed to explore the nonadiabatic paths of optically excited a-GeTe. Results show that the short-wavelength ultrafast laser is capable of generating full-domain carrier excitation and repopulation, whereas the long-wavelength ultrafast laser favors the excitation of lone pair electrons in the antibonded state. Photodoping makes the double-valley potential energy surface shallower and allows the insertion of A1g coherent forces in the atomic pairs, by which the phase reversal of Ge and Te atoms in the ⟨001⟩ direction is activated with ultrafast suppression of the Peierls distortion. These findings have far-reaching implications regarding nonequilibrium phase engineering strategies based on phase-change materials.

Spectroscopic and Theoretical Studies on Conformational Stability of Benzyl Methyl Ether.

Conformer-selected electronic and vibrational spectra of benzyl methyl ether and its terminal methyl group-substituted derivative in a supersonic jet have been measured using ultraviolet (UV)-UV hole burning and fluorescence-detected infrared spectroscopy to investigate the conformational stability of flexible molecules. Various quantum chemical calculations as well as experimental observations reveal the coexistence of three conformers with different CCOC dihedral angles and side-chain orientations relative to the benzene ring plane. Vibrational analysis in the excited state with time-dependent density functional theory and IR simulations containing anharmonic coupling sufficiently reproduce the experimental results, suggesting that these three conformers can be distinguished into one gauche-conformer and two trans-ones with respect to the CCOC dihedral angle. We also observe that the gauche conformer exhibits higher-frequency CH2 modes. The natural bond orbital analysis indicates that this phenomenon is attributed to the electron delocalization from the non-bonding orbitals and the C-O orbitals associated with the neighboring oxygen atom, which leads to a conformer dependence of the methylene C-H bond strength.

Light-Induced Phonon-Mediated Magnetization in Monolayer MoS2

Light-induced ultrafast spin dynamics in materials is of great importance for developments of spintronics and magnetic storage technology. Recent progresses include ultrafast demagnetization, magnetic switching, and magnetic phase transitions, while the ultrafast generation of magnetism is hardly achieved. Here, a strong light-induced magnetization (up to 0.86μ B per formula unit) is identified in non-magnetic monolayer molybdenum disulfide (MoS2). With the state-of-the-art time-dependent density functional theory simulations, we demonstrate that the out-of-plane magnetization can be induced by circularly polarized laser, where chiral phonons play a vital role. The phonons strongly modulate spin-orbital interactions and promote electronic transitions between the two conduction band states, achieving an effective magnetic field ∼ 380 T. Our study provides important insights into the ultrafast magnetization and spin-phonon coupling dynamics, facilitating effective light-controlled valleytronics and magnetism.

All-optical manipulation of charge density waves in kagome metal CsV3Sb5

A major challenge in condensed matter physics is to manipulate collective quantum phases by all-optical methods, which can open enormous new possibilities for materials and device engineering. Here we report an anomalous optical modulation of charge density waves (CDWs) in recently discovered kagome metal $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ using femtosecond-resolved coherent phonon spectroscopy. This optically suppressed CDW phase surprisingly persists for nanoseconds, exhibits a monotonically lowering of transition temperature with increasing fluence, and does not require a critical absorbed photon density, which is intriguingly distinct from those found in the ``optically suppressed potential energy'' scenario. Pump-pump-probe experiments conclusively rule out the laser-induced thermal effect and time-dependent density functional theory simulations confirm that this nonthermal modulation stems from an optical tuning of the energy resonance between the band fermiology and the Van Hove singularity in kagome lattices. These findings open exciting prospects for all-optical engineering of the Van Hove scenario to tune correlated quantum phases in topology networks.