Substitution engineering of lead-free halide perovskites for photocatalytic applications assisted by machine learning.

Lead-free perovskites (A3B2X9) have drawn much attention in recent years. However, a thorough understanding of these materials is still in its early stages. This is because A3B2X9 perovskites have large-scale component tunability, in which the A+, B3+, and X- ions can be replaced or partially substituted with other elements. Here, based on density functional theory and machine learning techniques we propose a data-driven method to find suitable configurations for photocatalytic water splitting. By replacing atoms in A3B2X9, 3.4 million configurations are constructed and studied. Our results show that the substitutional position plays an important role in determining the photocatalytic performance. Specifically, the co-existence of Br and I elements is favorable for X-sites, while for B-site atoms, it is better to choose atoms from groups IIIB and IIIA with a period greater than 3. Considering their rarity and toxicity, we believe that In is a good choice for B-sites and propose CsRb2BiInBr5I4 as a promising candidate. These results may provide guidance for the discovery of novel lead-free perovskites for photocatalytic applications.

Pure spin current and phonon thermoelectric transport in a triangulene-based molecular junction.

The experimental synthesis and characterization of enigmatic triangulene were reported for the first time recently. Based on this enigmatic molecule, we proposed a triangulene-based molecular junction and presented first principles calculations to investigate the electron and phonon thermoelectric transport properties. Numerical results show that the spin polarized electric transport properties of the triangulene-based molecular junction can be adjusted effectively by bias voltage and gate voltage. Through varying the gate voltage applied on the triangulene molecule, the system can exhibit a perfect spin filter effect. When a temperature gradient is applied between the two leads, spin up current and spin down current flow along opposite directions in the system simultaneously. Thus pure spin current can be obtained on a large scale by changing the temperature, temperature gradient, and gate voltage. When the phonon vibration effect is considered in thermal transport, the figure of merit is suppressed distinctively especially when the temperature is within the 10 K < T < 100 K range. More importantly, a large spin figure of merit can be achieved accompanied by a small charge figure of merit by adjusting the temperature, gate voltage and chemical potential in a wide range, which indicates a favorable application prospect of the triangulene-based molecular junction as a spin calorigenic device.

Photoinduced valley-polarized current of layered MoS2 by electric tuning.

A photoinduced current of a layered MoS2-based transistor is studied from first-principles. Under the illumination of circular polarized light, a valley-polarized current is generated, which can be tuned by the gate voltage. For monolayer MoS2, the valley-polarized spin-up (down) electron current at K (K') points is induced by the right (left) circular polarized light. The valley polarization is found to reach +1.0 (-1.0) for the valley current that carried such a K (K') index. For bilayer MoS2, the spin-up (down) current can be induced at both K and K' valleys by the right (left) circular light. In contrast to monolayer MoS2, the photoinduced valley polarization shows asymmetric behavior upon reversal of the gate voltage. Our results show that the valley polarization of the photoinduced current can be modulated by the circular polarized light and the gate voltage. All the results can be well understood using a simple kp model.

Shot noise of spin current and spin transfer torque.

We report the theoretical investigation of the shot noise of the spin current (S(σ)) and the spin transfer torque (S(τ)) for non-collinear spin polarized transport in a spin-valve device which consists of a normal scattering region connected by two ferromagnetic electrodes (MNM system). Our theory was developed using the non-equilibrium Green's function method, and general nonlinear S(σ) - V and S(τ) - V relations were derived as a function of the angle θ between the magnetizations of two leads. We have applied our theory to a quantum dot system with a resonant level coupled with two ferromagnetic electrodes. It was found that, for the MNM system, the auto-correlation of the spin current is enough to characterize the fluctuation of the spin current. For a system with three ferromagnetic layers, however, both auto-correlation and cross-correlation of the spin current are needed to characterize the noise of the spin current. For a quantum dot with a resonant level, the derivative of spin torque with respect to bias voltage is proportional to sinθ when the system is far away from resonance. When the system is near resonance, the spin transfer torque becomes a non-sinusoidal function of θ. The derivative of the noise of the spin transfer torque with respect to the bias voltage Nτ behaves differently when the system is near or far away from resonance. Specifically, the differential shot noise of the spin transfer torque Nτ is a concave function of θ near resonance while it becomes a convex function of θ far away from resonance. For certain bias voltages, the period Nτ(θ) becomes π instead of 2π. For small θ, it was found that the differential shot noise of the spin transfer torque is very sensitive to the bias voltage and the other system parameters.

A comparative metabolomics study between grain-sized moxibustion and suspended moxibustion on rats with gastric ulcers.

Grain-sized moxibustion (GS-Moxi) and suspended moxibustion (S-Moxi) represent the two typical local heat therapies in Traditional Chinese Medicine (TCM) and have been extensively used in treating gastric ulcers (GU) in China. However, the difference in biological response between the two moxibustion therapies in treating GU remains unclear. Here we investigated the therapeutic effect and potential mechanistic difference underlying the two moxibustion methods. Ethanol-induced GU model was established and was treated with GS-Moxi or S-Moxi at ST36 and ST21 for 5 days separately. And then, gastric histopathological examination, immunohistochemical staining for repair factors (EGFR, VEGF, Ki67), and 1H NMR-based metabolomics analysis of plasma and stomach of rats were conducted. We found GS-Moxi and S-Moxi effectively alleviated gastric damage and significantly increased the expression of related repair factors. However, S-Moxi corrected aberrant energy metabolism and lipids metabolism in GU rats but had little effect on neurotransmitter-related metabolism, while GS-Moxi regulated energy metabolism and neurotransmitter-related metabolism in GU rats but had no effect on lipids metabolism. We further proposed that the main target of S-Moxi may be liver and vasculature, whereas GS-Moxi specially targeted the stomach via regulating nervous system. This study strongly verified the outstanding gastroprotective effects of moxibustion and enriched our understanding of the varied biological responses triggered by different moxibustion methods.

Excitonic energy transfer in light-harvesting complexes in purple bacteria.

Two distinct approaches, the Frenkel-Dirac time-dependent variation and the Haken-Strobl model, are adopted to study energy transfer dynamics in single-ring and double-ring light-harvesting (LH) systems in purple bacteria. It is found that the inclusion of long-range dipolar interactions in the two methods results in significant increase in intra- or inter-ring exciton transfer efficiency. The dependence of exciton transfer efficiency on trapping positions on single rings of LH2 (B850) and LH1 is similar to that in toy models with nearest-neighbor coupling only. However, owing to the symmetry breaking caused by the dimerization of BChls and dipolar couplings, such dependence has been largely suppressed. In the studies of coupled-ring systems, both methods reveal an interesting role of dipolar interactions in increasing energy transfer efficiency by introducing multiple intra/inter-ring transfer paths. Importantly, the time scale (4 ps) of inter-ring exciton transfer obtained from polaron dynamics is in good agreement with previous studies. In a double-ring LH2 system, non-nearest neighbor interactions can induce symmetry breaking, which leads to global and local minima of the average trapping time in the presence of a non-zero dephasing rate, suggesting that environment dephasing helps preserve quantum coherent energy transfer when the perfect circular symmetry in the hypothetic system is broken. This study reveals that dipolar coupling between chromophores may play an important role in the high energy transfer efficiency in the LH systems of purple bacteria and many other natural photosynthetic systems.

Discovery of Two-Dimensional Multinary Component Photocatalysts Accelerated by Machine Learning.

Searching for novel and high-performance two-dimensional (2D) materials is an important task for photocatalytic applications. Although multinary compounds exhibit more diversity in structure and properties in comparison to binary 2D materials, they are comparatively under-studied. Herein, using a machine-learning (ML) technique and high-throughput screening, we develop an efficient approach to accurately predict 2D multicomponent photocatalysts. Over 4000 monolayers are examined, and 75 multinary compounds are identified for photocatalytic applications. Considering our predictions, we find that the ternary and quaternary compounds A2P2X6 and ABP2X6 with A = Cu/Zn/Ge/Ag/Cd, B = Ga/In/Bi, and X = S/Se exhibit superior properties, making them promising candidates for overall water splitting. Thus, our work provides an efficient way to explore novel photocatalysts, which could stimulate further theoretical and experimental investigations on 2D multinary compounds for application in photocatalytic water splitting.

Novel Two-Dimensional Layered MoSi2Z4 (Z = P, As): New Promising Optoelectronic Materials.

Very recently, two new two-dimensional (2D) layered semi-conducting materials MoSi2N4 and WSi2N4 were successfully synthesized in experiments, and a large family of these two 2D materials, namely MA2Z4, was also predicted theoretically (Science, 369, 670 (2020)). Motivated by this exciting family, in this work, we systematically investigate the mechanical, electronic and optical properties of monolayer and bilayer MoSi2P4 and MoSi2As4 by using the first-principles calculation method. Numerical results indicate that both monolayer and bilayer MoSi2Z4 (Z = P, As) present good structural stability, isotropic mechanical parameters, moderate bandgap, favorable carrier mobilities, remarkable optical absorption, superior photon responsivity and external quantum efficiency. Especially, due to the wave-functions of band edges dominated by d orbital of the middle-layer Mo atoms are screened effectively, the bandgap and optical absorption hardly depend on the number of layers, providing an added convenience in the experimental fabrication of few-layer MoSi2Z4-based electronic and optoelectronic devices. We also build a monolayer MoSi2Z4-based 2D optoelectronic device, and quantitatively evaluate the photocurrent as a function of energy and polarization angle of the incident light. Our investigation verifies the excellent performance of a few-layer MoSi2Z4 and expands their potential application in nanoscale electronic and optoelectronic devices.

[Nuclear magnetic resonance-based metabolomics revealed metabolite profiles participate in intervention effect of refined moxibustion in gastric ulcer model rats].

OBJECTIVE: To investigate the effect of refined moxibustion on expression of gastric mucosal epidermal growth factor receptor (EGFR) and vascular endothelial growth factor (VEGF), and changes of metabolite profiles in gastric ulcer (GU) rats, so as to analyze its mechanism underlying improvement of GU. METHODS: Male SD rats were randomized into control, model, acupoint moxibustion groups (n=6 per group). The GU model was induced by cold-restraint stress. The ignited refined moxa was applied to bilateral "Liangmen" (ST21) and "Zusanli" (ST36) for 3 cones/acupoint, once daily for 7 days. Then, we employed 1H NMR-based metabolomics approach to analyze the metabolic profiles of serum and stomach tissue samples. The conventional histopathological changes of the gastric mucosa were observed by H.E. stain and the expressions of EGFR and VEGF in the gastric mucosa were detected by immunohistochemistry. RESULTS: Compared to the control group, the expression levels of EGFR and VEGF were significantly increased in the model group (P<0.01, P<0.05), and further notably up-regulated in the acupoint moxibustion group (P<0.001, P<0.01). Results of H.E. staining showed damage of the folds of gastric mucosa, disordered arrangement of the glands, infiltration of inflammatory cells and unclear structure of gastric mucosa in the model group, which was relatively milder in the acupoint moxibustion group. 1H-NMR technical analysis showed that in comparison with the control group, 11 and 11 metabolites in the stomach extract and plasma were increased, 10 in the gastric tissue and 3 in the plasma were decreased in the GU model group; while in comparison with the model group, 17 differently expressed metabolites in the gastric extract and 10 metabolites in the plasma restored to their levels of control group after the acupoint moxibustion intervention. These metabolites participate in 12 metabolic pathways including glycine, serine and threonine metabolism, glutathione metabolism, glycine metabolism, alanine, aspartic acid and glutamic acid metabolism, purine metabolism, glyoxylic acid and digarboxylic acid metabolism, biosynthesis of aminoacyl-tRNA, amino sugar and nucleotide sugar metabolism, cysteine and methionine metabolism, citrate cycle, pyruvate metabolism, and the mutual conversion of pentose and glucuronate，suggesting their involvement in moxibustion-induced improvement of GU. CONCLUSION: Refined moxibustion at ST21 and ST36 can up-regulate the expression of EGFR and VEGF in the gastric mucosa and lessen gastric mucosal injury, which may be related to its effects in reducing GU-induced metabolic disorders, including sugar, purine, amino acid, and phospholipid metabolism and antioxidant defense system.

[Endogenous metabolites involving effects of moxibustion on injured gastric mucosal tissue in rats with stomach heat or cold syndrome based on nuclear magnetic resonance spectroscopy].

OBJECTIVE: To investigate the profile of metabolites of gastric mucosa involving the effectiveness of moxibustion in the treatment of syndromes of stomach heat (SH) and stomach cold (SC) by 1H-nuclear magnetic resonance (1H-NMR) spectroscopy in rats, so as to reveal its mechanisms underlying improvement of gastric disorders. METHODS: Male SD rats were randomly divided into control, SH-model, SC-model, SH-moxibustion and SC-moxibustion groups (n＝6 rats/group). The SH-model and SC-model were established by gavage of pepper liquid plus ethanol, and ice water plus NaOH, respectively. Moxibustion was applied to bilateral "Zusanli" (ST36) and "Liangmen"(ST21) for 20 min, once daily for 7days. Histopathological changes of the gastric tissue were observed by H．E. staining. Differential metabolites in the gastric mucosal tissue were detected and the relevant metabolic pathways analyzed by using 1H-NMR, pattern recognition method，and online MetPA (http: //www.metaboanalyst.ca). RESULTS: Compared with the control group, the body mass was decreased significantly from the 4th to 14th day after modeling (P<0.05，P<0.01). After the treatment, the body mass was obviously increased from the 10th day on in both SH-EA and SC-EA groups relevant to the SH and SC model group, respectively (P<0.05，P<0.01). H．E. staining showed severe damage of the columnar epithelial structure of the gastric mucosa and inflammatory cell infiltration in the SH group, and inflammatory cell infiltration in the SC model group, which were relatively milder in both moxibustion groups. 1H-NMR analysis displayed a total of 16 potential biomarkers in the injured gastric mucosa of SH syndrome and 14 biomarkers for the SC syndrome after mode-ling, and 13 metabolites related to SH moxibustion and 8 metabolites related to SC moxibustion after moxibustion interventions, respectively. After moxibustion, among the 13 differential metabolites of the SH syndrome, the effectively up-regulated candidates were isoleucine, creatinine, choline and lactate (P<0.05), and the down-regulated ones were choline phosphate, glycine, alanine, urine pyrimidine, tyrosine, phenylalanine, hypoxanthine, adenosine and nicotinamide (P<0.05). Among the 8 metabolites related to the SC syndrome, creatinine, ethanolamine, choline, adenosine and nicotinamide were markedly increased (P<0.05), and glycine, creatine phosphate and tyrosine remarkably decreased in their levels after moxibustion (P<0.05). MetPA showed that moxibustion could regulate 10 metabolic pathways for SH syndrome and 7 metabolic pathways for SC syndrome. Metabolites and metabolic pathways are mainly involved in functions of amino acid metabolism, energy metabolism and inflammatory response. CONCLUSION: The idea of "moxibustion could be used for heat syndrome" has metabolic substance basis, and its efficacy in repairing the injured gastric mucosa involves regulation of amino acid metabolism, energy balance and inflammation response, and moxibustion for SH and SC syndromes has both generality and specificity in regulating metabolic activities.

[Analysis on adolescent depression treated with acupuncture based on the theory of "taking shaoyang as the pivot"].

Depression is related to qi stagnation in the body. At present, the treatment with acupuncture for depression focuses on the theories of the governor vessel, liver, spleen, stomach and the entity of five zang organs, especially for the adult group. However, the attention to adolescent depression is insufficient. It is recorded in Internal Classic that shaoyang meridian is taken as the pivot of three yang meridians, dominates the regulating of the ascending and dispersing of yang qi and plays the key role in treatment. The authors believe that yang qi starts growing at the period of youth, to which shaoyang meridian is corresponded. It is viewed that adolescent depression is closely related to the pivot function of shaoyang. In this paper, based on the theory of "taking shaoyang as the pivot", the mechanism of adolescent depression and the acupoint selection in acupuncture treatment are explored so as to utilize this theory in the treatment of adolescent depression with acupuncture.

Giant tunnel magneto-resistance in graphene based molecular tunneling junction.

We propose and theoretically investigate a class of stable zigzag graphene nanoribbon (ZGNR) based molecular magnetic tunneling junctions (MTJs). For those junctions having pentagon-connecting formations, huge tunnel magneto-resistance (TMR) is found. Different from most of the other proposed molecular junctions, the huge TMR in our structures is generic, and is not significantly affected by external parameters such as bias voltage, gate voltage, length of the molecule and width of the ZGNRs. The double pentagon-connecting formation between the molecule and ZGNRs is critical for the remarkable TMR ratio, which is as large as ∼2 × 10(5). These molecular MTJs behave as almost perfect spin filters and spin valve devices. Other connecting formations of the ZGNR based MTJs lead to much smaller TMR. By first principles analysis, we reveal the microscopic physics responsible for this phenomenon.

Modulation of Electronic Structure of Armchair MoS2 Nanoribbon.

We perform first-principles calculations on electronic structures of armchair MoS2 nanoribbons (AMoS2NRs) passivated by non-metal atoms. In contrast to bare AMoS2NR (AMoS2NR-bare) or purely hydrogen (H) edge-terminated AMoS2NR (AMoS2NR-H), it is found that H and oxygen (O) hybrid edge-terminated AMoS2NR (AMoS2NR-H-O) is more stable. AMoS2NR-H-O exhibits a direct band gap of about 1.43 eV, which is larger than those of pristine AMoS2NR (about 0.61 eV) and AMoS2NR-H (about 0.60 eV), and even exceeds the band gap of bulk MoS2 (about 0.86 eV) and is close to that of monolayer MoS2 (about 1.67 eV). The remarkable band gap of AMoS2NR-H-O is attributed to the charge redistribution on the edge atoms of MoS2 nanoribbon, especially the charges on the edge Mo atoms. Detailed calculations of AMoS2NR-H-O reveal that over 70% of the total density of states (DOS) of the conduction band minimum and the valence band maximum are contributed by the Mo atoms. In particular, edge Mo atoms play a crucial role in modulating the electronic structure. In addition, we have studied a series of functionalized AMoS2NR-H-X with X = S, F, C, N, and P, respectively. It is found that AMoS2NR-H-X with X = S, 2F, C possess remarkable electronic band gaps, while AMoS2NR-H-X with X = F, N, P are metallic. Our studies suggest that non-metal atom hybrid passivation can efficiently tune the electronic band gap of MoS2 nanoribbon and open a new route to obtain MoS2 based practical nanoelectronic device and photo¬voltaic device.

Corrected Article: "ac response of a carbon chain under a finite frequency bias" [J. Chem. Phys. 127, 104701 (2007)].

Based on nonequilibrium Green's function approach and density functional theory, we report first principles investigation on ac transport of four carbon atomic chain connected by two semi-infinite aluminum leads, Al-C(4)-Al. For small alternating external bias voltage, we expanded nonequilibrium Green's function to the first order in the external voltage and calculated the dynamical conductance. The suppression of the dynamic conductance was obtained near the resonant level, while far away from the resonance, the large enhancement of the dynamic conductance was observed. These behaviors can be understood well under the wide-band limit. By changing the coupling distance between the carbon atom and the aluminum leads, the system could change its transport response between capacitivelike and inductivelike.

ac response of a carbon chain under a finite frequency bias.

Based on nonequilibrium Green's function approach and density functional theory, we report first principles investigation on ac transport of four carbon atom chain connected by two semi-infinite aluminum leads Al-C(4)-Al. For small alternating external bias voltage, we expanded nonequilibrium Green's function to the first order in the external voltage and calculated the dynamical conductance. The suppression of the dynamic conductance was obtained near the resonant level while far away from the resonance the giant enhancement of the dynamic conductance was also observed. These behaviors can be well understood under the wide-band limit. By changing the coupling distance between the carbon atom and aluminum leads, the system could change its transport response between capacitivelike and inductivelike.