Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 75
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Phys Chem Lett ; 2020 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-32693591

RESUMO

Carrier dynamics across the interface of heterostructures have important electronic, photovoltaic, and catalytic implications. Using first-principles time dependent density functional theory, we have systematically investigated the charge transfer of excited carriers from CdS to MoS2 and found that two interdependent mechanisms are responsible for the transfer, one slow and one fast. While the slower process may be attributed to typical electron-phonon coupling, the interfacial dipole resulting from this transfer enables a fast secondary process involving a level crossing of the excited carrier state in CdS with receiving states in MoS2. An analysis based on the interfacial binding energy reveals that the Cd-terminated (001) interface is by far the most energetically favorable. Which in addition to its calculated fast resonant electron transfer suggests it is a good candidate to explain the experimentally observed charge transfer between CdS and MoS2.

2.
Phys Rev Lett ; 124(16): 166401, 2020 Apr 24.
Artigo em Inglês | MEDLINE | ID: mdl-32383949

RESUMO

While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a semihydrogenated graphene (known as graphone), however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. The critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in the semihydrogenated graphene, which offers yet another test by experiment.

3.
Phys Chem Chem Phys ; 2020 Apr 09.
Artigo em Inglês | MEDLINE | ID: mdl-32270831

RESUMO

Critical topological phases, possessing flat bands, provide a platform to study unique topological properties and transport phenomena under a many-body effect. Here, we propose that critical nodal points and nodal lines or rings can be found in Kagome lattices. After the C3 rotation symmetry of a single-layer Kagome lattice is eliminated, a quadratic nodal point splits into two critical nodal points. When the layered Kagome lattices are stacked into a three-dimensional (3D) structure, critical nodal lines or rings can be generated by tuning the interlayer coupling. Furthermore, we use Kagome graphene as an example to identify that these critical phases could be obtained in real materials. We also discuss flat-band-induced ferromagnetism. It is found that the flat band splits into two spin-polarized bands by hole-doping, and as a result the Dirac-type critical phases evolve into Weyl-type phases.

4.
Adv Mater ; 32(14): e1907565, 2020 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-32091144

RESUMO

Parity-time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd2 As2 . As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time-reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c-axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system.

6.
Adv Mater ; 31(52): e1903491, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31725182

RESUMO

Metal oxides, as one of the mostly abundant and widely utilized materials, are extensively investigated and applied in environmental remediation and protection, and in energy conversion and storage. Most of these diverse applications are the result of a large diversity of the electronic states of metal oxides. Noticeably, however, many metal oxides present obstacles for applications in catalysis, mainly due to the lack of efficient active sites with desired electronic states. Here, the fabrication of single-tungsten-atom-oxide (STAO) is demonstrated, in which the metal oxide's volume reaches its minimum as a unit cell. The catalytic mechanism in the STAO is determined by a new single-site physics mechanism, named as quasi-atom physics. The photogenerated electron transfer process is enabled by an electron in the spin-up channel excited from the highest occupied molecular orbital to the lowest unoccupied molecular orbital +1 state, which can only occur in STAO with W5+ . STAO results in a record-high and stable sunlight photocatalytic degradation rate of 0.24 s-1 , which exceeds the rates of available photocatalysts by two orders of magnitude. The fabrication of STAO and its unique quasi-atom photocatalytic mechanism lays new ground for achieving novel physical and chemical properties using single-metal-atom oxides (SMAO).

7.
J Phys Chem Lett ; 10(22): 6996-7001, 2019 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-31652068

RESUMO

As an intensively studied electrode material for secondary batteries, TiS2 is known to exhibit high electrical conductivity without extrinsic doping. However, the origin of this high conductivity, either being a semimetal or a heavily self-doped semiconductor, has been debated for several decades. Here, combining quasi-particle GW calculations, density functional theory (DFT) study on intrinsic defects, and scanning tunneling microscopy/spectroscopy (STM/STS) measurements, we conclude that stoichiometric TiS2 is a semiconductor with an indirect band gap of about 0.5 eV. The high conductivity of TiS2 is therefore caused by heavy self-doping. Our DFT results suggest that the dominant donor defect that is responsible for the self-doping under thermal equilibrium is Ti interstitial, which is corroborated by our STM/STS measurements.

8.
J Chem Phys ; 151(12): 124703, 2019 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-31575162

RESUMO

Improving electronic structure calculations for practical and technologically important materials has been a never-ending pursue. This is especially true for transition and post-transition metal oxides for which the current first-principles approaches still suffer various drawbacks. Here, we present a hierarchical-hybrid functional approach built on the use of pseudopotentials. The key is to introduce different amounts of exact exchange to core and valence electrons. It allows for treating the delocalization errors of sp and d electrons differently, which have been known to be an important source of error for the band structure. Using wurtzite ZnO as a prototype, we show that the approach is successful in simultaneously reproducing the bandgap and d-band position. Importantly, the same approach, without having to change the hybrid mixing parameters from those of Zn, works reasonably well for other binary 3d transition and post-transition metal oxides across board. Our findings thus point out a new direction of systematically improving the exchange functional in first-principles calculations.

9.
Phys Chem Chem Phys ; 21(39): 22160, 2019 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-31552964

RESUMO

Correction for 'Significance of hydrogen bonding networks in the proton-coupled electron transfer reactions of photosystem II from a quantum-mechanics perspective' by Jun Chai et al., Phys. Chem. Chem. Phys., 2019, 21, 8721-8728.

10.
Phys Rev Lett ; 122(23): 236402, 2019 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-31298916

RESUMO

First-principles calculations reveal an unusual electronic state (dubbed as half excitonic insulator) in monolayer 1T-MX_{2} (M=Co, Ni and X=Cl, Br). Its one spin channel has a many-body ground state due to excitonic instability, while the other is characterized by a conventional band insulator gap. This disparity arises from a competition between the band gap and exciton binding energy, which exhibits a spin dependence due to different orbital occupations. Such a state can be identified by optical absorption measurements and angle-resolved photoemission spectroscopy. Our theory not only provides new insights for the study of exciton condensation in magnetic materials but also suggests that strongly correlated materials could be fertile candidates for excitonic insulators.

11.
Nano Lett ; 19(6): 3612-3617, 2019 06 12.
Artigo em Inglês | MEDLINE | ID: mdl-31096752

RESUMO

We show that non-equilibrium dynamics plays a central role in the photoinduced 2H-to-1T' phase transition of MoTe2. The phase transition is initiated by a local ordering of Te vacancies, followed by a 1T' structural change in the original 2H lattice. The local 1T' region serves as a seed to gather more vacancies into ordering and subsequently induces a further growth of the 1T' phase. Remarkably, this process is controlled by photogenerated excited carriers as they enhance vacancy diffusion, increase the speed of vacancy ordering, and are hence vital to the 1T' phase transition. This mechanism can be contrasted to the current model requiring a collective sliding of a whole Te atomic layer, which is thermodynamically highly unlikely. By uncovering the key roles of photoexcitations, our results may have important implications for finely controlling phase transitions in transition metal dichalcogenides.

12.
Phys Chem Chem Phys ; 21(17): 8721-8728, 2019 Apr 24.
Artigo em Inglês | MEDLINE | ID: mdl-30968099

RESUMO

The photosynthetic protein complex, photosystem II (PSII), conducts the light-driven water-splitting reaction with unrivaled efficiency. Proton-coupled electron transfer (PCET) reactions at the redox-active tyrosine residues are thought to play a critical role in the water-splitting chemistry. Addressing the fundamental question as to why the tyrosine residue, YZ, is kinetically competent in comparison to a symmetrically placed tyrosine residue, YD, is important for the elucidation of the mechanism of PCET in the water-splitting reaction of PSII. Here, using all-quantum-mechanical calculations we study PCET at the YZ and YD residues of PSII. We find that when YZ is in its protein matrix under physiological conditions, the HOMO of YZ constitutes the HOMO of the whole system. In contrast, the HOMO of YD is buried under the electronic states localized elsewhere in the protein matrix and PCET at YD requires the transfer of the phenolic proton, which elevates the HOMO of YD to become the HOMO of the whole system. This leads to the oxidation of YD, albeit on a slower timescale. Our study reveals that the key differences between the electronic structure of YZ and YD are primarily determined by the protonation state of the respective hydrogen-bonding partners, D1-His190 and D2-His189, or more generally by the H-bonding network of the protein matrix.


Assuntos
Modelos Moleculares , Complexo de Proteína do Fotossistema II/química , Transporte de Elétrons/efeitos da radiação , Ligação de Hidrogênio/efeitos da radiação , Cinética , Oxirredução , Fotossíntese/efeitos da radiação , Conformação Proteica , Prótons , Teoria Quântica , Tirosina/química , Água/química
13.
Adv Mater ; 31(4): e1804919, 2019 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-30422346

RESUMO

Phase transition is a fundamental physical phenomenon that has been widely studied both theoretically and experimentally. According to the Landau theory, the coexistence of high- and low-temperature phases is thermodynamically impossible during a second-order phase transition in a bulk single crystal. Here, the coexistence of two (α and ß) phases in wedge-shaped nanosized single-crystal Cu2 Se over a large temperature range are demonstrated. By considering the surface free-energy difference between the two phases and the shape effect, a thermodynamic model is established, which explicitly explains their coexistence. Intriguingly, it is found that with a precise control of the heating temperature, the phase boundary can be manipulated at atomic level. These discoveries extend the understanding of phase transitions to the nanoscale and shed light on rational manipulation of phase transitions in nanomaterials.

14.
Appl Phys Rev ; 5(1)2018 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-30397419

RESUMO

We review the concept of stochasticity-i.e., unpredictable or uncontrolled fluctuations in structure, chemistry, or kinetic processes-in materials. We first define six broad classes of stochasticity: equilibrium (thermodynamic) fluctuations; structural/compositional fluctuations; kinetic fluctuations; frustration and degeneracy; imprecision in measurements; and stochasticity in modeling and simulation. In this review, we focus on the first four classes that are inherent to materials phenomena. We next develop a mathematical framework for describing materials stochasticity and then show how it can be broadly applied to these four materials-related stochastic classes. In subsequent sections, we describe structural and compositional fluctuations at small length scales that modify material properties and behavior at larger length scales; systems with engineered fluctuations, concentrating primarily on composite materials; systems in which stochasticity is developed through nucleation and kinetic phenomena; and configurations in which constraints in a given system prevent it from attaining its ground state and cause it to attain several, equally likely (degenerate) states. We next describe how stochasticity in these processes results in variations in physical properties and how these variations are then accentuated by-or amplify-stochasticity in processing and manufacturing procedures. In summary, the origins of materials stochasticity, the degree to which it can be predicted and/or controlled, and the possibility of using stochastic descriptions of materials structure, properties, and processing as a new degree of freedom in materials design are described.

15.
Phys Rev Lett ; 121(19): 196802, 2018 Nov 09.
Artigo em Inglês | MEDLINE | ID: mdl-30468617

RESUMO

The built-in potential is of central importance to the understanding of many interfacial phenomena because it determines the band alignment at the interface. Despite its importance, its exact sign and magnitude have generally been recognized as ill-defined quantities for more than half a century. Here, we provide a common energy reference of bulk matter which leads to an unambiguous definition of the built-in potential and innate (i.e., bulk) band alignment. Further, we find that the built-in potential is explicitly determined by the bulk properties of the constituent materials when the system is in electronic equilibrium, while the interface plays a role only in the absence of equilibrium. Our quantitative theory enables a unified description of a variety of important properties of interfaces, ranging from work functions to Schottky barriers in electronic devices.

16.
Phys Chem Chem Phys ; 20(35): 23106-23111, 2018 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-30168546

RESUMO

Two-dimensional (2D) B-C-N alloys have recently attracted much attention but unfortunately, Chemical Vapor Deposition (CVD) B-C-N alloys typically phase separate. In spite of that, our analysis of the B-C-N alloy fabricated by electron-beam irradiation suggests that non-phase-separated B-C-N may in fact exist with a carbon concentration up to 14 at%. While this analysis points to a new way to overcome the phase-separation in 2D B-C-N, by first-principles calculations, we show that these B-C-N alloys are made of motifs with even numbers of carbon atoms, in particular, dimers or six-fold rings (in a molecule-like form), embedded in a 2D BN network. Moreover, by tuning the carbon concentration, the band gap of the B-C-N alloys can be reduced by 35% from that of BN. Due to a strong overlap of the wavefunctions at the conduction band and valance band edges, the non-phase-separated B-C-N alloys maintain the strong optical absorption of BN.

17.
Nat Commun ; 9(1): 3719, 2018 09 13.
Artigo em Inglês | MEDLINE | ID: mdl-30213927

RESUMO

Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions, and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we resolve the conflict by employing low-temperature photoluminescence spectroscopy to identify the biexciton state in BN-encapsulated single-layer WSe2. The biexciton state only exists in charge-neutral WSe2, which is realized through the control of efficient electrostatic gating. In the lightly electron-doped WSe2, one free electron binds to a biexciton and forms the trion-exciton complex. Improved understanding of the biexciton and trion-exciton complexes paves the way for exploiting the many-body physics in TMDs for novel optoelectronics applications.

18.
Phys Rev Lett ; 121(9): 096401, 2018 Aug 31.
Artigo em Inglês | MEDLINE | ID: mdl-30230862

RESUMO

A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered Fe_{3}Sn_{2} crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.

19.
Nanotechnology ; 29(44): 445702, 2018 Nov 02.
Artigo em Inglês | MEDLINE | ID: mdl-30124437

RESUMO

In this work, we show that remote heteroepitaxy can be achieved when Cu thin film is grown on single crystal, monolayer graphene buffered sapphire(0001) substrate via a thermal evaporation process. X-ray diffraction and electron backscatter diffraction data show that the epitaxy process forms a prevailing Cu crystal domain, which is remotely registered in-plane to the sapphire crystal lattice below the monolayer graphene, with the (111) out-of-plane orientation. As a poor metal with zero density of states at its Fermi level, monolayer graphene cannot totally screen out the stronger charge transfer/metallic interactions between Cu and substrate atoms. The primary Cu domain thus has good crystal quality as manifested by a narrow crystal misorientation distribution. On the other hand, we show that graphene interface imperfections, such as bilayers/multilayers, wrinkles and interface contaminations, can effectively weaken the atomic interactions between Cu and sapphire. This results in a second Cu domain, which directly grows on and follows the graphene hexagonal lattice symmetry and orientation. Because of the weak van der Waals interaction between Cu and graphene, this domain has inferior crystal quality. The results are further confirmed using graphene buffered spinel(111) substrate, which indicates that this remote epitaxial behavior is not unique to the Cu/sapphire system.

20.
Nanoscale ; 10(29): 14298-14303, 2018 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-30015343

RESUMO

The recent discovery of ferromagnetic single-layer CrI3 creates ample opportunities for studying the fundamental properties and the spintronic applications of atomically thin magnets. Through first-principles calculations and model Hamiltonian simulations, here we build for the first time a substantial magnetic phase diagram under lateral strain and charge doping, the two factors that are easily modulated in single-layer CrI3via substrate and gating controls. We demonstrate that both lateral strain and charge doping efficiently change the coupling between the local spins and thus have unexpected effects on the magnetic properties of CrI3. In particular, the strain tunes the magnetic order and anisotropy: a compressive strain leads to a phase transition from a ferromagnetic insulator to an antiferromagnetic insulator, while a tensile strain can flip the magnetic orientation from off-plane to in-plane. Furthermore, we find that the phase transition under compressive strain is insensitive to charge doping, whereas the phase transition under tensile strain is modulated by electron doping significantly. Our predicted magnetic phase diagram and rationalized analysis indicate the single-layer CrI3 to be an ideal system to harness both basic magnetic physics and building blocks for magnetoelastic applications.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA