Two-Dimensional Semiconducting Boron Monolayers

The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations, however, we have firstly found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle G0W0 approach. We demonstrate that the vanished metallicity characterized by the pz-derived bands cross the Fermi level is attributed to the motif of a triple-hexagonal-vacancy, with which various semiconducting boron monolayers are designed to realize the band-gap engineering for the potential applications in electronic devices. The semiconducting boron monolayers in our predictions are expected to be synthesized on the proper substrates, due to the similar stabilities to the ones observed experimentally.Comment: 12 pages, 4 figure

Lifelong Representation Learning for NLP Applications

Representation learning lives at the heart of deep learning for natural language processing (NLP). Traditional representation learning (such as softmax-based classification, pre-trained word embeddings, and language models, graph representations) focuses on learning general or static representations with the hope to help any end task. As the world keeps evolving, emerging knowledge (such as new tasks, domains, entities or relations) typically come with a small amount of data with shifted distributions that challenge the existing representations to be effective. As a result, how to effectively learn representations for new knowledge becomes crucial. Lifelong learning is a machine learning paradigm that aims to build an AI agent that keeps learning from the evolving world, like humans' learning from the world. This dissertation focuses on improving representations on different types of new knowledge (classification, word-level, contextual-level, and knowledge graph) for a myriad of NLP end tasks, ranging from text classification, sentiment analysis, entity recognition, question answering to the more complex dialog system. With the help of lifelong representation learning, models' performance on tasks is greatly improved beyond existing general representation learning

Adsorption Induced Indirect-to-Direct Band Gap Transition in Monolayer Blue Phosphorus

In this work, we systematically studied adsorption induced indirect-to-direct band gap transition in monolayer blue phosphorus from first-principles calculations by combining one-shot GW approximation and the Bethe-Salpeter equation. Our results revealed that surface adsorption (i.e., O₂, −OH, −COOH, and −CN) strongly modifies the conduction and valence band edges, resulting in an indirect-to-direct band gap transition. More importantly, the direct band gap can be dramatically tuned by either the in-plane strain or the coverage ratio of adsorbates, which enables monolayer blue phosphorus to efficiently adsorb visible light. The mechanism of strain effect and surface adsorption on band gap tuning was deeply discussed. Moreover, our results clearly showed that the adsorbates have an important influence on the exciton binding energies (EBE), while the coverage of adsorbates play a crucial role in the linear scaling behavior between EBE and quasi-particle band gap. Our findings suggest that monolayer blue phosphorus has potential applications in electro-optical devices

Intrinsic Role of Excess Electrons in Surface Reactions on Rutile TiO₂ (110): Using Water and Oxygen as Probes

Reactions on catalytically active surfaces often involve complex mechanisms with multiple interactions between adsorbates and various subsequently formed intermediates, and a variable number of excess electrons further complicates the involved mechanisms. Experimental techniques face challenges in precisely tuning or determining the number of excess electrons and in elucidating these complex reactions. In this work, the thermodynamic details and reaction pathways of interactions between the most prevalent and important molecular species, H₂O and O₂, on a prototypical rutile TiO₂ (110) surface are investigated using density functional theory calculations on 10 elementary reaction steps with the intention of gaining further insight into surface catalysis. The results suggest that the final product is independent of the reaction pathway when the number of excess electrons is sufficient. The intrinsic role of excess electrons at the reaction level is thus proposed to extend the understanding of the origin, distribution, and transfer of excess electrons. Such an understanding is beneficial to develop high-performance catalysts

A Practical Criterion for Screening Stable Boron Nanostructures

Due to the electron deficiency, boron clusters evolve strikingly with the increasing size as confirmed by experimentalists and theorists. However, it is still a challenge to propose a model potential to describe the stabilities of boron. On the basis of the 2c-2e and 3c-2e bond models, we have found the constraints for stable boron clusters, which can be used for determining the vacancy concentration and screening the candidates. Among numerous tubular structures and quasi-planar structures, we have verified that the stable clusters with lower formation energies bounded by the constraints, indicating the competition of tubular and planar structures. Notably, we have found a tubular cluster of B₇₆ which is more stable than the B₈₀ cage. We show that the vacancies, as well as the edge, are necessary for the 2c-2e bonds, which will stabilize the boron nanostructures. Therefore, the quasi-planar and tubular boron nanostructures could be as stable as the cages, which have no edge atoms. Our finding has shed light on understanding the complicated electron distributions of boron clusters and enhancing the efficiency of searching stable B nanostructures

Controlling the Reaction Steps of Bifunctional Molecules 1,5-Dibromo-2,6-dimethylnaphthalene on Different Substrates

Using scanning tunneling microscopy, we reveal that the methyl groups of 1,5-dibromo-2,6-dimethylnaphthalene suppress Ullmann-type intermolecular coupling on Au(111), Ag(111), and Cu(111) and steer the reactions toward different final products on the three substrates. On Au(111), the molecules form ordered structures stabilized by intermolecular halogen bonds and desorb from the surface at above 420 K. On Ag(111), the molecules form halogen-bonded structures but are converted into organometallic structures at 360 K and desorb from the surface at above 600 K. On Cu(111), the molecules form organometallic structures at 300 K and undergo an intermolecular cyclodehydrogenation reaction at above 480 K. The reaction yields C–C bonds between debrominated carbons and methyl groups, resulting in dibenz­[<i>a</i>,<i>h</i>]­anthracene derivates and ultranarrow chiral-edge graphene nanoribbon motifs. This comparative study demonstrates a novel concept of using side groups to control the reaction steps that lead to specific final products on different substrates

Oxidation-Induced Topological Phase Transition in Monolayer 1T′-WTe₂

Monolayer (ML) tungsten ditelluride (WTe₂) is a well-known quantum spin Hall (QSH) insulator with topologically protected gapless edge states, thus promising dissipationless electronic devices. However, experimental findings exhibit the fast oxidation of ML WTe₂ in ambient conditions. To reveal the changes of topological properties of WTe₂ arising from oxidation, we systematically study the surface oxidation reaction of ML 1T′-WTe₂ using first-principles calculations. The calculated results indicate that the fast oxidation of WTe₂ originates from the existence of H₂O in air, which significantly promotes the oxidation of ML 1T′-WTe₂. More importantly, this low-coverage oxidized WTe₂ loses its topological features and is changed into a trivial insulator. Furthermore, we propose a fully oxidized ML WTe₂ that can still possess the QSH insulator states. The topological phase transition induced by oxidation provides exotic insight into understanding the topological features of layered transition-metal dichalcogenide materials

The Enhancement of Surface Reactivity on CeO₂ (111) Mediated by Subsurface Oxygen Vacancies

Surface reactivity on metal oxide surfaces and its enhancement play important roles in heterogeneous catalytic reactions. In this work, the interactions of O₂ and H₂O with reduced CeO₂ (111) surface are studied by density-functional theory calculations. The corresponding adsorption geometries, adsorption energies, and reaction barriers are reported. It is found that the diffusion of subsurface oxygen vacancies toward surface can be promoted by the adsorption of O₂ on the CeO₂ (111) surface. Then those oxygen vacancies diffused onto surface sites will be healed by the adsorbed O₂, leaving behind an O adatom on the surface. Interestingly, at moderate temperatures, the surface O adatom will swap positions with surface lattice O dynamically. The adsorption of H₂O may also induce the diffusion of oxygen vacancies from subsurface to surface, leading to the formation of two hydroxyls on the CeO₂ (111) surface. In addition, the interaction between the paired hydroxyl groups and O₂ will result in the formation of water and oxygen adatom on the surface. Our results have revealed important roles played by the subsurface oxygen vacancies in the enhancement of surface reactivity, especially when involving the adsorption of water and oxygen

NiO Matrix Decorated by Ru Single Atoms: Electron-Rich Ru-Induced High Activity and Selectivity toward Electrochemical N₂ Reduction

Developing a single-atom catalyst with electron-rich active sites is a promising strategy for catalyzing the electrochemical N2 reduction reaction (NRR). Herein, we choose NiO(001) as a model template and deposit a series of single transition metal (TM) atoms with higher formal charges to create the electron-rich active centers. Our first-principles calculations show that low-valent Ru (+2) on NiO(001) can significantly activate N2, with its oxidation states varying from +2 to +4 throughout the catalytic cycle. The Ru/NiO(001) catalyst exhibits the best activity with a relatively low limiting potential of −0.49 V. Furthermore, under NRR operating conditions, the Ru site is primarily occupied by *N2 rather than *H, indicating that NRR overwhelms the hydrogen evolution reaction and thus exhibits excellent selectivity. Our work highlights the potential of designing catalysts with electron-rich active sites for NRR

Carbon and Nitrogen Mineralization in Relation to Soil Particle-Size Fractions after 32 Years of Chemical and Manure Application in a Continuous Maize Cropping System

<div><p>Long-term manure application is recognized as an efficient management practice to enhance soil organic carbon (SOC) accumulation and nitrogen (N) mineralization capacity. A field study was established in 1979 to understand the impact of long-term manure and/or chemical fertilizer application on soil fertility in a continuous maize cropping system. Soil samples were collected from field plots in 2012 from 9 fertilization treatments (M₀CK, M₀N, M₀NPK, M₃₀CK, M₃₀N, M₃₀NPK, M₆₀CK, M₆₀N, and M₆₀NPK) where M₀, M₃₀, and M₆₀ refer to manure applied at rates of 0, 30, and 60 t ha⁻¹ yr⁻¹, respectively; CK indicates no fertilizer; N and NPK refer to chemical fertilizer in the forms of either N or N plus phosphorus (P) and potassium (K). Soils were separated into three particle-size fractions (2000–250, 250–53, and <53 μm) by dry- and wet-sieving. A laboratory incubation study of these separated particle-size fractions was used to evaluate the effect of long-term manure, in combination with/without chemical fertilization application, on the accumulation and mineralization of SOC and total N in each fraction. Results showed that long-term manure application significantly increased SOC and total N content and enhanced C and N mineralization in the three particle-size fractions. The content of SOC and total N followed the order 2000–250 μm > 250–53μm > 53 μm fraction, whereas the amount of C and N mineralization followed the reverse order. In the <53 μm fraction, the M₆₀NPK treatment significantly increased the amount of C and N mineralized (7.0 and 10.1 times, respectively) compared to the M₀CK treatment. Long-term manure application, especially when combined with chemical fertilizers, resulted in increased soil microbial biomass C and N, and a decreased microbial metabolic quotient. Consequently, long-term manure fertilization was beneficial to both soil C and N turnover and microbial activity, and had significant effect on the microbial metabolic quotient.</p></div