Time-domain Density Functional Theory Research Articles

Grain Boundaries Are Benign and Suppress Nonradiative Electron-Hole Recombination in Monolayer Black Phosphorus: A Time-Domain Ab Initio Study.

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that both symmetrical (GB_s) and asymmetrical grain boundaries (GB_a) significantly extend charge-carrier lifetime compared with monolayer black phosphorus. Boundaries create no deep trap states, which decrease electron-phonon coupling. As a result, GB_s increases carrier lifetime by a factor of 22, whereas GB_a extends the lifetime by a factor of 4. More importantly, the interplay between the immobile electron localized at the boundaries in the GB_s and extended excited-state lifetime facilitates a chemical reaction, which is beneficial for photocatalysts. In contrast, GB_a separates electron and hole spatially in different locations, which forms a long-lived charge-separated state and is favorable for photovoltaics. Our simulations demonstrate that grain boundaries are benign and retard nonradiative electron-hole recombination in monolayer black phosphorus, suggesting a route to reduce energy losses via rational choice of defect to realize high-performance photovoltaic and photocatalytic devices.

Hot-electron transfer in quantum-dot heterojunction films

Thermalization losses limit the photon-to-power conversion of solar cells at the high-energy side of the solar spectrum, as electrons quickly lose their energy relaxing to the band edge. Hot-electron transfer could reduce these losses. Here, we demonstrate fast and efficient hot-electron transfer between lead selenide and cadmium selenide quantum dots assembled in a quantum-dot heterojunction solid. In this system, the energy structure of the absorber material and of the electron extracting material can be easily tuned via a variation of quantum-dot size, allowing us to tailor the energetics of the transfer process for device applications. The efficiency of the transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale.

Photoinduced Localized Hole Delays Nonradiative Electron-Hole Recombination in Cesium-Lead Halide Perovskites: A Time-Domain Ab Initio Analysis.

All-inorganic perovskites have attracted intense interest as promising photovoltaic materials due to their excellent performance. Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we demonstrate that a photoinduced localized polaron-like hole greatly delays the nonradiative electron-hole recombination relative to the structure with delocalized free charge of the CsPbBr3. This is because localized charge carriers diminish overlap between electron and hole wave functions and decrease the NA coupling by a factor of 6. In addition, polaron formation increases the band gap of CsPbBr3, slowing down recombination further. The smaller NA coupling and larger band gap compete successfully with the longer decoherence time, extending the recombination to tens of nanoseconds. The calculated recombination times show excellent agreement with experiment. Our study reveals the atomistic mechanisms underlying the suppression of recombination upon formation of localized polaron-like holes and advances our understanding of the excited-state dynamics of all-inorganic perovskite solar cells.

Experimental and theoretical study of fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole in terahertz range

Due to wide variety of biological and pharmacological activities of benzimidazole derivatives, the terahertz fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole are researched by employing terahertz time-domain spectroscopy and density functional theory systematically. Although the substituent at the para position on the benzene moiety are the same family elements (F, Cl) in the periodic table, both experimental and theoretical results show that there are substantial differences in their fingerprint spectra in the range of 0.2–2.5 THz, such as the amount, amplitude and frequency position of absorption peaks. The validity of these results was confirmed by isolated-molecule and solid-state calculations based on density functional theory. The possible reasons of these differences are different intramolecular hyperconjugative interactions, different elongation of the corresponding bond lengths, different HOMO-LUMO energy gaps, as well as different atomic motions within in the unit cell owing to the electron-withdrawing effects of different halogen atoms at the para position on the benzene moiety. These results indicate the importance of this spectral range as a conformational fingerprint region where even minor changes in the molecular configuration lead to major differences in its THz absorption.

Halide Composition Controls Electron-Hole Recombination in Cesium-Lead Halide Perovskite Quantum Dots: A Time Domain Ab Initio Study.

We demonstrate that halide content strongly affects nonradiative electron-hole recombination in all-inorganic perovskite quantum dots (QDs). Using time domain density functional theory and nonadiabatic molecular dynamics, we show that replacing half of the bromines with iodines in a CsPbBr3 QD extends the charge carrier lifetime by a factor of 5, while complete replacement extends the lifetime by a factor of 8. Doping with iodines decreases the nonadiabatic charge-phonon coupling because iodines are heavier and slower than bromines and because the overlap between the electron and hole wave functions is reduced. In general, the nonradiative electron-hole recombination proceeds slowly, on a nanosecond time scale, due to small sub-1 meV nonadiabatic coupling and short sub-10 fs coherence times. The obtained recombination times and their dependence on the halogen content show excellent agreement with experiments. Our study suggests that the power conversion efficiencies of solar cells can be controlled by changing the halide composition in all-inorganic perovskite QDs.

Plasmon-Mediated Electron Injection from Au Nanorods into MoS2: Traditional versus Photoexcitation Mechanism

Summary Using nonadiabatic molecular dynamics simulations combined with time-domain density functional theory, we show that electron injection from gold nanorods into MoS 2 by the traditional mechanism is still faster than energy relaxation causing charge recombination. Plasmon-like excitations decay into free-electron states within 30 fs after photoexcitation of gold nanorods. Electron transfer follows within less than 100 fs, whereas energy relaxation requires 200 fs. Surface plasmons couple to low-frequency phonons of gold, and free charges also couple to higher-frequency phonons of gold and MoS 2 . The contribution of the charge-transfer photoexcitation mechanism to plasmon-driven charge separation depends strongly on the type of donor-acceptor interaction, e.g., chemical versus van der Waals, and more weakly on contact area and system geometry. The simulation generates a detailed time-domain atomistic description of the interfacial plasmon-driven charge separation and relaxation that are fundamental to many applications.

Lewis Base Passivation of Hybrid Halide Perovskites Slows Electron-Hole Recombination: Time-Domain Ab Initio Analysis.

Nonradiative electron-hole recombination plays a key role in determining photon conversion efficiencies in solar cells. Experiments demonstrate significant reduction in the recombination rate upon passivation of methylammonium lead iodide perovskite with Lewis base molecules. Using nonadiabatic molecular dynamics combined with time-domain density functional theory, we find that the nonradiative charge recombination is decelerated by an order of magnitude upon adsorption of the molecules. Thiophene acts by the traditional passivation mechanism, forcing electron density away from the surface. In contrast, pyridine localizes the electron at the surface while leaving it energetically near the conduction band edge. This is because pyridine creates a stronger coordinative bond with a lead atom of the perovskite and has a lower energy unoccupied orbital compared with thiophene due to the more electronegative nitrogen atom relative to thiophene's sulfur. Both molecules reduce two-fold the nonadiabatic coupling and electronic coherence time. A broad range of vibrational modes couple to the electronic subsystem, arising from inorganic and organic components. The simulations reveal the atomistic mechanisms underlying the enhancement of the excited-state lifetime achieved by the perovskite passivation, rationalize the experimental results, and advance our understanding of charge-phonon dynamics in perovskite solar cells.

Research on the differences between 2-(2-Chlorophenyl)benzimidazole and 2-(4-Chlorophenyl)benzimidazole based on terahertz time domain spectroscopy

Due to wide variety of biological and pharmacological activities of benzimidazole derivatives, the differences between 2-(2-Chlorophenyl)benzimidazole and 2-(4-Chlorophenyl) benzimidazole were researched by employing terahertz time-domain spectroscopy and density functional theory systematically. Although the only difference between their molecular configurations is the arrangement of chlorine atom on chlorophenyl ring, there are distinctive differences in their fingerprint spectra in the range of 0.2–2.5THz, such as amount, amplitude, and frequency position of absorption peaks. The validity of these results was confirmed by the theoretical results simulated by using density functional theory. The possible reasons of these differences originate from the different van der Waals forces and the different dihedral angles of the molecules within crystal cell. These results indicate the importance of this spectral range as a conformational fingerprint region where even minor changes in the molecular configuration lead to major differences in its THz absorption.

Prediction of an extremely long exciton lifetime in a Janus-MoSTe monolayer.

The electron-hole separation efficiency is a key factor that determines the performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) and devices. Therefore, searching for novel 2D TMD materials with a long timescale of carrier lifetime has become one of the most important topics. Here, based on time-domain density functional theory (TD-DFT), we propose a brand new TMD material, namely Janus-MoSTe, which exhibits a strong built-in electric field. Our results show that in the Janus-MoSTe monolayer, the exciton consisting of an electron and hole has a relatively wide spatial extension and low binding energy. In addition, a slow electron-hole recombination process is observed, with a timescale on the order of 1.31 ns, which is even comparable to those of van der Waals (vdW) heterostructures. Further analysis reveals that the extremely long timescale for electron-hole recombination could be ascribed to the strong Coulomb screening effect as well as the small overlap of wavefunctions between electrons and holes. Our findings establish the built-in electric field as an effective factor to control the electron-hole recombination dynamics in TMD monolayers and facilitate their future applications in light detection and harvesting.

Mechanistic insights into alginate fouling caused by calcium ions based on terahertz time-domain spectra analyses and DFT calculations

Fouling mechanisms underlying the filtration behaviors of alginate solution caused by calcium addition were investigated by Terahertz time-domain spectroscopy (THz-TDS) and density functional theory (DFT) techniques. Filtration tests showed that specific filtration resistance (SFR) of alginate solution (0.75 g L−1) monotonously increased with calcium addition at a relatively low range of calcium concentration (0–1.0 mM), and SFR (2.61 × 1015 m kg−1) of alginate solution with 1.0 mM calcium addition was extremely high as compared with sludge suspension. Characterizations by X-ray photoelectric spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Thermogravimetric analysis (TGA) showed that the composition of functional groups, elements and thermal stability of alginate was not apparently affected by calcium concentration. Howbeit, THz-TDS spectra showed that calcium addition caused structural variation of alginate polymer in solution. DTF calculation results showed that initial binding of alginate chains induced by calcium ions preferentially occurred in intermolecular other than intramolecular, and moreover, the two alginate chains bridged by a calcium atom tend to stretch in a tetrahedron structure (cross to each other) other than parallel to each other. According to these results, “chemical potential gap” depicted by Flory-Huggins theory was suggested to be responsible for the filtration behaviors of alginate solution caused by calcium addition. This study provided the mechanistic insights into membrane fouling.

Disparity in Photoexcitation Dynamics between Vertical and Lateral MoS2/WSe2 Heterojunctions: Time-Domain Simulation Emphasizes the Importance of Donor-Acceptor Interaction and Band Alignment.

Two-dimensional transition metal dichalcogenides (TMDs) heterojunctions are appealing candidates for optoelectronics and photovoltaics. Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that photoexcitation dynamics exhibit a significant difference in the vertical and lateral MoS2/WSe2 heterojunctions arising from the disparity in the donor-acceptor interaction and fundamental band alignment. The obtained electron transfer time scale in the vertical heterojunction shows excellent agreement with experiment. Hole transfer proceeds 1.5 times slower. The electron-hole recombination is 3 orders of magnitude longer than the charge separation, which favors solar cell applications. On the contrary, the lateral heterojunction shows no band offsets steering charge separation. The excited electron is localized at the interface that attracts holes to form an exciton-like state due to Coulomb interaction, suggesting potential applications in light-emitting devices. The coupled electron and hole wave functions increase NA coupling and the coherence time, accelerating electron-hole recombination by a factor of 3 compared with the vertical case. The atomistic studies advance our understanding of the photoinduced charge-phonon dynamics in TMDs heterojunctions.

Angular momentum transport with twisted exciton wave packets

A chain of cofacial molecules with CN or CNh symmetry supports excitonic states with a screw-like structure. These can be quantified with the combination of an axial wavenumber and an azimuthal winding number. Combinations of these states can be used to construct excitonic wave packets that spiral down the chain with well-determined linear and angular momenta. These twisted exciton wave packets can be created and annihilated using laser pulses, and their angular momentum can be optically modified during transit. This allows for the creation of opto-excitonic circuits in which information, encoded in the angular momentum of light, is converted into excitonic wave packets that can be manipulated, transported, and then re-emitted. A tight-binding paradigm is used to demonstrate the key ideas. The approach is then extended to quantify the evolution of twisted exciton wave packets in a many-body, multi-level time-domain density functional theory setting. In both settings, numerical methods are developed that allow the site-to-site transfer of angular momentum to be quantified.

Research on differences between 2-(2′-pyridyl)benzimidazole and 2-(4′-pyridyl)benzimidazole based on terahertz time-domain spectroscopy

Due to the important pharmaceutical activities of benzimidazole derivatives, the differences between 2-(2′-pyridyl)benzimidazole and 2-(4′-pyridyl)benzimidazole were researched by terahertz time-domain spectroscopy and density functional theory systematically. Although the only difference between their molecular configurations is the different arrangement of nitrogen on pyridine ring, 2PBI and 4PBI have large differences in their experimental absorption spectra in the range of 0.2–2.5THz, such as the amount, amplitude and frequency position of absorption peaks. The validity of these results was confirmed by the theoretical results simulated using density functional theory. The possible reasons of these differences originate from the different dihedral angles between benzimidazole ring and pyridine ring and the different hydrogen-bonding interactions within crystal cell.

The effect of the flexibility of hydrogen bonding network on low-frequency motions of amino acids. Evidence from Terahertz spectroscopy and DFT calculations

Low-frequency modes of L-Asp and L-Asn were studied in the range from 0.1 to 3.0THz using time-domain Terahertz spectroscopy and density functional theory calculation. The results show that PBE-D2 shows more success than BLYP-D2 in prediction of THz absorption spectra. To compare their low-frequency modes, we adopted “vibrational character ID strips” proposed by Schmuttenmaer and coworkers [Journal of Physical Chemistry B, 117, 10444(2013)]. We found that the most intense THz absorption peaks of two compounds both involve severe distortion of their hydrogen bonding networks. Due to less rigid hydrogen bonding network in L-Asp, the side chain (carboxyl group) of L-Asp exhibits larger motions than that (carboxamide group) of L-Asn in low-frequency modes.

Defects Slow Down Nonradiative Electron-Hole Recombination in TiS3 Nanoribbons: A Time-Domain Ab Initio Study.

Layered TiS3 materials hold appealing potential in photovoltaics and optoelectronics due to their excellent electronic and optical properties. Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that the electron-hole recombination in pristine TiS3 nanoribbons (NRs) occurs in tens of picoseconds and is over 10-fold faster than the experimental value. By performing an atomistic ab initio simulation with a sulfur vacancy, we demonstrate that a sulfur vacancy greatly reduces electron-hole recombination, achieving good agreement with experiment. Introduction of a sulfur vacancy increases the band gap slightly because the NR's highest occupied molecular orbital is lowered in energy. More importantly, the sulfur vacancy partially diminishes the electron and hole wave functions' overlap and reduces NA electron-phonon coupling, which competes successfully with the longer decoherence time, slowing down recombination. Our study suggests that a rational choice of defects can control nonradiative electron-hole recombination in TiS3 NRs and provides mechanistic principles for photovoltaic and optoelectronic device design.

Weak Donor-Acceptor Interaction and Interface Polarization Define Photoexcitation Dynamics in the MoS2/TiO2 Composite: Time-Domain Ab Initio Simulation.

To realize the full potential of transition metal dichalcogenides interfaced with bulk semiconductors for solar energy applications, fast photoinduced charge separation, and slow electron-hole recombination are needed. Using a combination of time-domain density functional theory with nonadiabatic molecular dynamics, we demonstrate that the key features of the electron transfer (ET), energy relaxation and electron-hole recombination in a MoS2-TiO2 system are governed by the weak van der Waals interfacial interaction and interface polarization. Electric fields formed at the interface allow charge separation to happen already during the photoexcitation process. Those electrons that still reside inside MoS2, transfer into TiO2 slowly and by the nonadiabatic mechanism, due to weak donor-acceptor coupling. The ET time depends on excitation energy, because the TiO2 state density grows with energy, increasing the nonadiabatic transfer rate, and because MoS2 sulfur atoms start to contribute to the photoexcited state at higher energies, increasing the coupling. The ET is slower than electron-phonon energy relaxation because the donor-acceptor coupling is weak, rationalizing the experimentally observed injection of primarily hot electrons. The weak van der Waals MoS2-TiO2 interaction ensures a long-lived charge separated state and a short electron-hole coherence time. The injection is promoted primarily by phonons within the 200-800 cm-1 range. Higher frequency modes are particularly important for the electron-hole recombinations, because they are able to accept large amounts of electronic energy. The predicted time scales for the forward and backward ET, and energy relaxation can be measured by time-resolved spectroscopies. The reported simulations generate a detailed time-domain atomistic description of the complex interplay of the charge and energy transfer processes at the MoS2/TiO2 interface that are of fundamental importance to photovoltaic and photocatalytic applications. The results suggest that even though the photogenerated charge-separated state is long-lived, the slower charge separation, compared to the electron-phonon energy relaxation, can present problems in practical applications.

Engineering and manipulating exciton wave packets

When a semiconductor absorbs light, the resulting electron-hole superposition amounts to a uncontrolled quantum ripple that eventually degenerates into diffusion. If the conformation of these excitonic superpositions could be engineered, though, they would constitute a new means of transporting information and energy. We show that properly designed laser pulses can be used to create such excitonic wave packets. They can be formed with a prescribed speed, direction and spectral make-up that allows them to be selectively passed, rejected or even dissociated using superlattices. Their coherence also provides a handle for manipulation using active, external controls. Energy and information can be conveniently processed and subsequently removed at a distant site by reversing the original procedure to produce a stimulated emission. The ability to create, manage and remove structured excitons comprises the foundation for opto-excitonic circuits with application to a wide range of quantum information, energy and light-flow technologies. The paradigm is demonstrated using both Tight-Binding and Time-Domain Density Functional Theory simulations.

Hole Trapping by Iodine Interstitial Defects Decreases Free Carrier Losses in Perovskite Solar Cells: A Time-Domain Ab Initio Study

We present a time-domain ab initio study of electron–hole recombination in pristine MAPbI3, and compare it to the trap mediated recombination in MAPbI3 with the iodine interstitial defect. Nonadiabatic molecular dynamics combined with time-domain density functional theory show that the iodine interstitial defect creates a subgap state capable of trapping both electrons and holes. Hole trapping occurs much faster than electron trapping or electron–hole recombination. The trapped hole survives for hundreds of nanoseconds, because, rather surprisingly, recombination of electrons with the trapped hole takes several times longer than recombination of electrons with holes in the valence band. Because the hole trap is relatively shallow, the hole can escape into the valence band prior to recombining with the electron. The differences are rationalized by variation in nonadiabatic electron–phonon couplings, phonon-induced pure-dephasing times and electronic energy gaps. The time-domain atomistic simulations contri...

Strong Interaction at the Perovskite/TiO2 Interface Facilitates Ultrafast Photoinduced Charge Separation: A Nonadiabatic Molecular Dynamics Study

Interfacial electron transfer (ET) plays a key role in the operation of solar cells based on TiO2 sensitized with organohalide perovskites, since it leads to separation of the photogenerated electrons and holes into different materials. The reported experimental ET times range by over 3 orders of magnitude, from sub-200 fs to over 300 ps. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we demonstrate that ET at a CH3NH3PbI3/TiO2 interface can be complete within 100 fs, indicating that the longer time scales reflect other processes, such as charge and exciton diffusion in perovskite bulk. The electron injection is fast because the interaction between the donor and acceptor species is strong at ambient conditions. Photoexcitation directly at the interface can create a charge-separated state. Electrons generated farther away inject by a combination of adiabatic and nonadiabatic mechanisms. Thermally activated low frequency vibrational motions at the interf...

Terahertz Conductivity within Colloidal CsPbBr3 Perovskite Nanocrystals: Remarkably High Carrier Mobilities and Large Diffusion Lengths

Colloidal CsPbBr3 perovskite nanocrystals (NCs) have emerged as an excellent light emitting material in last one year. Using time domain and time-resolved THz spectroscopy and density functional theory based calculations, we establish 3-fold free carrier recombination mechanism, namely, nonradiative Auger, bimolecular electron-hole recombination, and inefficient trap-assisted recombination in 11 nm sized colloidal CsPbBr3 NCs. Our results confirm a negligible influence of surface defects in trapping charge carriers, which in turn results into desirable intrinsic transport properties, from the perspective of device applications, such as remarkably high carrier mobility (∼4500 cm(2) V(-1) s(-1)), large diffusion length (>9.2 μm), and high luminescence quantum yield (80%). Despite being solution processed and possessing a large surface to volume ratio, this combination of high carrier mobility and diffusion length, along with nearly ideal photoluminescence quantum yield, is unique compared to any other colloidal quantum dot system.