With applications in high performance electronics, photovoltaics and catalysis, two-dimensional transition metal dichalcogenides (TMDs) attract strong attention. Isolated TMDs, which are already ...remarkably complex, can stack in sequence to make even more complex heterostructures. Surprisingly, charge separation is ultrafast in layered TMD heterostructures, even though the interlayer interaction is weak. Also surprisingly, the charge separated state is long-lived, despite the close proximity of electron and hole. Using real-time time-dependent density functional theory combined with nonadiabatic (NA) molecular dynamics, we model hole and electron transfer, and electron–hole recombination at a MoS2/WS2 heterojunction. Hole transfer is ultrafast, in excellent agreement with the experiment, due to significant delocalization of the photoexcited state between the donor and acceptor materials. Electron transfer is 1 order of magnitude longer, due to weaker donor–acceptor and NA couplings, lower density of acceptor states, and shorter quantum coherence. The electron–hole recombination is 3–4 orders of magnitude slower than the charge separation, because the initial and final states are localized strongly within different materials, rationalizing the long-lived charge separation. The computed recombination time scale agrees with the experimental data on the closely related MoSe2/WSe2 system. All electronic processes are coupled to the characteristic out-of-plane 400 cm–1 motion of the MoS2 and WS2 layers. The atomistic, time-domain methodology provides theoretical insights into the photoinduced electron–phonon dynamics in two-dimensional TMD heterostructures, and can be used for in silico design of novel functional materials operating under nonequilibrium conditions.
Two-dimensional transition metal dichalcogenides (TMDs) have appeared on the horizon of materials science and solid-state physics due to their unique properties and diverse applications. TMD ...performance depends strongly on material quality and defect morphology. Calculations predict that sulfur adatom and vacancy are among the most energetically favorable defects in MoS2 with vacancies frequently observed during chemical vapor deposition. By performing ab initio quantum dynamics calculations we demonstrate that both adatom and vacancy accelerate nonradiative charge carrier recombination but this happens through different mechanisms. Surprisingly, holes never significantly populate the shallow trap state created by the sulfur adatom because the trap is strongly localized and decoupled from free charges. Charge recombination bypasses the hole trap. Instead, it occurs directly between free electron and hole. The recombination is faster than in pristine MoS2 because the adatom strongly perturbs the MoS2 layer, breaks its symmetry, and allows more phonon modes to couple to the electronic subsystem. In contrast, the sulfur vacancy accelerates charge recombination by the traditional mechanism involving charge trapping, followed by recombination. This is because the hole and electron traps created by the vacancy are much less localized than the hole trap created by the adatom. Because the sulfur adatom accelerates charge recombination by a factor of 7.9, compared to 1.7 due to vacancy, sulfur adatoms should be strongly avoided. The generated insights highlight the diverse behavior of different types of defects, reveal unexpected features, and provide the mechanistic understanding of charge dynamics needed for tailoring TMD properties and building high-performance devices.
Advances in perovskite solar cells require development of means to control and eliminate the nonradiative charge recombination pathway. Using ab initio nonadiabatic molecular dynamics, we demonstrate ...that charge recombination in perovskites is extremely sensitive to the charge state of the halogen vacancy. A missing iodine anion in MAPbI3 has almost no effect on charge losses. However, when the vacancy is reduced, the recombination is accelerated by up to 2 orders of magnitude. The acceleration occurs due to formation of a deep hole trap in the singly reduced vacancy, and both deep and shallow hole traps for the doubly reduced vacancy. The shallow hole involves a significant rearrangement of the Pb–I lattice, leading to a new chemical species: a Pb–Pb dimer bound by the vacancy charge, and under-coordinated iodine bonds. Hole trapping by the singly reduced iodide vacancy operates parallel to recombination of free electron and hole, accelerating charge losses by a factor of 5. The doubly reduced vacancy acts by a sequential mechanism-free hole, to shallow trap, to deep trap, to free electron, and accelerates the recombination by a factor of 50. The study demonstrates that iodine anion vacancy can be beneficial to the performance, because it causes minor changes to the charge carrier lifetime, while increasing charge carrier concentration. However, the neutral iodine and iodine cation vacancies should be strongly avoided. The detailed insights into the charge carrier trapping and relaxation mechanisms provided by the simulation are essential for development of efficient photocatalytic, photovoltaic, optoelectronic and related devices.
Ultrafast charge recombination in hematite (α-Fe2O3) severely limits its applications in solar energy conversion and utilization, for instance, in photoelectrochemical water splitting. We report the ...first time-domain ab initio study of charge relaxation dynamics in α-Fe2O3 with and without the oxygen vacancy (Ov) defect, using non-adiabatic molecular dynamics implemented within time-dependent density functional theory. The simulations show that the hole trapping is the rate-limiting step in the electron–hole recombination process for both neutral and ionized Ov systems. The electron trapping is fast, and the trapped electron are relatively long-lived. A similar asymmetry is found for the relaxation of free charge carriers: relaxation of photoholes in the valence band is slower than relaxation of photoelectrons in the conduction band. The slower dynamics of holes offers an advantage to water oxidation at α-Fe2O3 photoanodes. Notably, the neutral Ov defect accelerates significantly the charge recombination rate, by about a factor of 30 compared to the ideal lattice, due to the stronger electron-vibrational coupling at the defect. However, the recombination rate in the ionized Ov defect is decreased by a factor of 10 with respect to the neutral defect, likely due to expansion of the local iron shell around the Ov site. The Ov defect ionization in α-Fe2O3 photoanodes is important for increasing both electrical conductivity and charge carrier lifetimes. The simulations reproduce well the time scales for the hot carrier cooling, trapping and recombination available from transient spectroscopy experiments, and suggest two alternative mechanisms for the Ov-assisted electron–hole recombination. The study provides a detailed atomistic understanding of carrier dynamics in hematite, and rationalizes the experimentally reported activation of α-Fe2O3 photoanodes by incorporation of Ov defects.
Air-frying is a novel technology for cooking fried foods through spraying hot air around the raw materials with the aim to promote the homogenous contact between the foods and the mist of oil ...droplets in hot air. The influence of temperature and time of air-frying on the physical, flavor, and lipidomic properties of surimi was investigated. The results indicated that along with the increase of temperature (120, 160, and 200 °C) and time (10 and 15 min), the moisture content of surface decreased dramatically while that of interior was well preserved. The applied texture property analysis (TPA) showed the enhanced hardness, gumminess, and chewiness of air-fried surimi. The lipids were prone to be oxidized under the circulated hot air during processing, which lead to a significant change in TBARS especially when the temperature exceeded 160 °C. In addition, new flavor compounds (aldehydes, ketones, etc.) were produced due to the lipid peroxidation and degradation. The lipidomic property of air-fried surimi was also explored, a total of 30 phospholipid molecular species were examined. Conclusively, in optimum conditions, air-fried surimi exhibit crispy texture, appealing flavor, and low oil content to satisfy consumer preference. Compared with deep fat fried surimi, air frying can be considered as a healthy technique for preparing attractive fried food.
•Surimi was process by air-frying with minimum oil content in hot air.•The influence of temperature and time of air-frying was investigated.•The air-fried surimi was tender with a crispy crust by TPA and moisture analysis.•Flavor compounds were newly produced by peroxidation and degradation of lipids.•TBARS and lipidomics analysis indicated the oxidation of lipids of surface.
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.
Investigation of charge carrier recombination dynamics is central to understanding and further enhancing the photocatalytic activity of water splitting and other photochemical reactions catalyzed by ...TiO2. In this study, we carried out nonadiabatic molecular dynamics calculations combined with real-time time-dependent density functional theory to investigate the effects of size and shape on charge recombination in TiO2 nanoparticles (NPs). Using the Wulff construction method, we considered both octahedral (10, 35, and 84 TiO2 units) and cuboctahedral (29, 78, and 97 TiO2 units) nanoclusters with size varying from 1 to 3 nm. Generally, the recombination rates decreased with increasing NP size. We rationalized the trend in terms of average transition energy, exciton binding energy (ΔE ex), nonadiabatic coupling (NAC), and pure-dephasing time. The relaxation times increased with increasing NP size, as the NAC and ΔE ex decreased. The cuboctahedral clusters showed smaller ΔE ex compared to the octahedral clusters. For the octahedral clusters, the smaller NAC and shorter dephasing time contributed to longer relaxation, despite smaller transition energy, as the size increased. However, the influence of the NAC, transition energy, and dephasing time were intertwined for the cuboctahedral clusters. The smaller NAC of the 97-unit cluster rationalized its longer relaxation time compared to the 29-unit cluster, but the presence of a singly coordinated oxygen atom greatly reduced the transition energy, thus leading to a shorter relaxation time compared to the 78-unit cluster. Our results provide a detailed understanding on the effects of size and shape on the charge carrier dynamics in TiO2 nanoclusters, separating these effects from other factors, such as presence of defects, dopants, and adsorbates that are hard to control precisely in experiments.
•HILIC-MS was used for identifying & quantifying plasmalogen molecular species.•Mild acid hydrolysis was used to degrade plasmalogens for identification.•Three plasmalogen classes were composed of 9 ...plasPC, 7 plasPE, and 3 plasPS.•Statistic models display strong goodness of fit (R2X) and high predictability (Q2).
Shellfishes contain plasmalogens correlating to the functions of brain, heart, etc. Herein, a mild acid hydrolysis and hydrophilic interaction chromatography (HILIC) tandem mass spectrometry method was developed for analyzing plasmalogens in six shellfish species. A total of 19 plasmalogen molecular species were successfully identified, including nine phosphatidylcholine plasmalogen (plasPC), seven phosphatidylethanolamine plasmalogen (plasPE), and three phosphatidylserine plasmalogen (plasPS). The quantitative results indicated that mussel (32 μg·mg−1) possessed the highest content of plasmalogens, followed by oyster (21 μg·mg−1) and razor clam (15 μg·mg−1). The statistic models showed that the plasPE P-18:0/20:5 (m/z 748), plasPE P-16:0/22:2 & P-18:0/20:2 (m/z 754) and plasPS were the most contributing difference between shellfishes. The results indicated that this method was sensitive and precise to determine plasmalogens in shellfish, and mussel was demonstrated to be a good choice for the large-scale preparation of plasmalogens.
Cervical cancer is a common gynecological malignancy with high incidence and mortality. Drugs commonly used in chemotherapy are often accompanied by strong side-effects. To find an anti-cervical ...cancer drug with high effects and low toxicity, luteoloside was used to treat the cervical cancer cell line Hela to investigate its effects on cell morphology, proliferation, apoptosis, and related proteins. The study demonstrated that luteoloside could inhibit proliferation remarkably; promote apoptosis and cytochrome C release; decrease the mitochondrial membrane potential and reactive oxygen species level; upregulate the expression of Fas, Bax, p53, phospho-p38, phospho-JNK, and cleaved PARP; downregulate the expression of Bcl-2 and phospho-mTOR; activate caspase-3 and caspase-8; change the nuclear morphology, and fragmentate DNA in Hela cells. These results strongly suggest that luteoloside can significantly inhibit the proliferation and trigger apoptosis in Hela cells. In contrast, luteoloside had less proliferation inhibiting effects on the normal cell lines HUVEC12 and LO2, and minor apoptosis promoting effects on HUVEC12 cells. Furthermore, the luteoloside-induced apoptosis in Hela cells is mediated by both intrinsic and extrinsic pathways and the effects of luteoloside may be regulated by the mitogen-activated protein kinases and mTOR signaling pathways via p53.
Two-dimensional transition metal dichalcogenides (TMDs) draw strong interest in materials science, with applications in optoelectronics and many other fields. Good performance requires high carrier ...concentrations and long lifetimes. However, high concentrations accelerate energy exchange between charged particles by Auger-type processes, especially in TMDs where many-body interactions are strong, thus facilitating carrier trapping. We report time-resolved optical pump-THz probe measurements of carrier lifetimes as a function of carrier density. Surprisingly, the lifetime reduction with increased density is very weak. It decreases only by 20% when we increase the pump fluence 100 times. This unexpected feature of the Auger process is rationalized by our time-domain ab initio simulations. The simulations show that phonon-driven trapping competes successfully with the Auger process. On the one hand, trap states are relatively close to band edges, and phonons accommodate efficiently the electronic energy during the trapping. On the other hand, trap states localize around defects, and the overlap of trapped and free carriers is small, decreasing carrier–carrier interactions. At low carrier densities, phonons provide the main charge trapping mechanism, decreasing carrier lifetimes compared to defect-free samples. At high carrier densities, phonons suppress Auger processes and lower the dependence of the trapping rate on carrier density. Our results provide theoretical insights into the diverse roles played by phonons and Auger processes in TMDs and generate guidelines for defect engineering to improve device performance at high carrier densities.
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