arXiv Material Science

/arXiv Material Science

Liquid-phase exfoliated indium-selenide flakes and their application in hydrogen evolution reaction. (arXiv:1903.08967v1 [cond-mat.mtrl-sci])

2019-03-22T02:29:27+00:00March 22nd, 2019|Categories: Publications|Tags: , , |

Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of beta-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g/L. Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. We demonstrate that the obtained InSe flakes have maximum lateral sizes ranging from 30 nm to a few um, and thicknesses ranging from 1 to 20 nm, with a max population centred at ~ 5 nm, corresponding to 4 Se-In-In-Se quaternary layers. We also show that no formation of further InSe-based compounds (such as In2Se3) or oxides occurs during the exfoliation process. The potential of these exfoliated-InSe few-layer flakes as a catalyst for hydrogen evolution reaction (HER) is tested in hybrid single-walled carbon nanotubes/InSe heterostructures. We highlight the dependence of the InSe flakes morphologies, i.e., surface area and thickness, on the HER performances achieving best efficiencies with small flakes offering predominant edge effects. Our theoretical model unveils the origin of the catalytic efficiency of InSe flakes, and correlates the catalytic activity to the Se vacancies at the edge of the flakes.

Published in: "arXiv Material Science".

WS2-graphite dual-ion battery. (arXiv:1903.08961v1 [cond-mat.mtrl-sci])

2019-03-22T02:29:24+00:00March 22nd, 2019|Categories: Publications|Tags: , |

A novel WS2-graphite dual-ion battery (DIB) is developed by combining together a conventional graphite cathode and high-capacity few-layer WS2 flakes anode. The WS2 flakes are produced by exploiting wet-jet milling (WJM) exfoliation, which allows mass production of few-layer WS2 flakes in dispersion, with an exfoliation yield of 100%. The WS2-anodes enable DIBs, based on hexafluorophosphate (PF6-) and lithium (Li+) ions, to achieve charge specific capacities of 457, 438, 421, 403, 295 and 169 mAh g-1 at current rates of 0.1, 0.2, 0.3, 0.4, 0.8 and 1.0 A g-1, respectively, outperforming conventional DIBs. The WS2-based DIBs operate in the 0 to 4 V cell voltage range, thus extending the operating voltage window of conventional WS2-based Li-ion batteries (LIBs). These results demonstrate a new route towards the exploitation of WS2, and possibly other transition metal dichalcogenides (TMDs), for the development of next-generation energy storage devices.

Published in: "arXiv Material Science".

Engineered MoSe2-based heterostructures for efficient electrochemical hydrogen evolution reaction. (arXiv:1903.08951v1 [cond-mat.mtrl-sci])

2019-03-22T02:29:22+00:00March 22nd, 2019|Categories: Publications|Tags: , , |

Two-dimensional transition metal-dichalcogenides are emerging as efficient and cost-effective electrocatalysts for hydrogen evolution reaction (HER). However, only the edge sites of their trigonal prismatic phase show HER-electrocatalytic properties, while the basal plane, which is absent of defective/unsaturated sites, is inactive. Here, we tackle the key challenge that is increasing the number of electrocatalytic sites by designing and engineering heterostructures composed of single-/few-layer MoSe2 flakes and carbon nanomaterials (graphene or single-wall carbon nanotubes (SWNTs)) produced by solution processing. The electrochemical coupling between the materials that comprise the heterostructure effectively enhances the HER-electrocatalytic activity of the native MoSe2 flakes. The optimization of the mass loading of MoSe2 flakes and their electrode assembly via monolithic heterostructure stacking provided a cathodic current density of 10mAcm-2 at overpotential of 100mV, a Tafel slope of 63mVdec-1 and an exchange current density (j0) of 0.203 Acm-2. In addition, electrode thermal annealing in a hydrogen environment and chemical bathing in n-butyllithium are exploited to texturize the basal planes of the MoSe2 flakes (through Se-vacancies creation) and to achieve in situ semiconducting-to-metallic phase conversion, respectively, thus they activate new HER-electrocatalytic sites. The as-engineered electrodes show a 4.8-fold enhancement of j0 and a decrease in the Tafel slope to 54mVdec-1.

Published in: "arXiv Material Science".

Doped-MoSe2 nanoflakes/3d metal oxide-hydr(oxy)oxides hybrid catalysts for pH-universal electrochemical hydrogen evolution reaction. (arXiv:1903.08947v1 [cond-mat.mtrl-sci])

2019-03-22T02:29:20+00:00March 22nd, 2019|Categories: Publications|Tags: , , |

Clean hydrogen production through efficient and cost-effective electrochemical water splitting is highly promising to meeting future global energy demands. The design of Earth-abundant materials with both high activity for hydrogen evolution reaction (HER) and electrochemical stability in both acidic and alkaline environments summarize the outcomes needed for practical applications. Here, we report a non-noble 3d metal Cl-chemical doping of liquid phase exfoliated single/few-layer flakes of MoSe2 for creating MoSe2 nanoflakes/3d metal oxide-hydr(oxy)oxide hybrid HER-catalysts. We propose that the electron-transfer from MoSe2 nanoflakes to metal cations and the chlorine complexation-induced both neutralization, as well as the in situ formation of metal oxide-hydr(oxy)oxides on MoSe2 nanoflake’s surface, tailor the proton affinity of the derived catalysts, increasing the number and HER-kinetic of their active sites in both acidic and alkaline electrolytes. The electrochemical coupling between the doped-MoSe2 nanoflakes/metal oxide-hydr(oxy)oxide hybrids and single-walled carbon nanotubes heterostructures further accelerates the HER process. Lastly, monolithic stacking of multiple heterostructures is reported as a facile electrode assembly strategy to achieve overpotential for a cathodic current density of 10mAcm-2 of 0.081V and 0.064V in 0.5M H2SO4 and 1M KOH, respectively. This opens up new opportunities to address the current density vs. overpotential requirements targeted in pH-universal H2 production.

Published in: "arXiv Material Science".

High thermal conductivity of high-quality monolayer boron nitride and its thermal expansion. (arXiv:1903.08862v1 [cond-mat.mtrl-sci])

2019-03-22T02:29:18+00:00March 22nd, 2019|Categories: Publications|Tags: , |

Heat management becomes more and more critical, especially in miniaturized modern devices, so the exploration of highly thermally conductive materials with electrical insulation and favorable mechanical properties is of great importance. Here, we report that high-quality monolayer boron nitride (BN) has a thermal conductivity (k{appa}) of 751 W/mK at room temperature. Though smaller than that of graphene, this value is larger than that of cubic boron nitride (cBN) and only second to those of diamond and lately discovered cubic boron arsenide (BAs). Monolayer BN has the second largest k{appa} per unit weight among all semiconductors and insulators, just behind diamond, if density is considered. The k{appa} of atomically thin BN decreases with increased thickness. Our large-scale molecular dynamic simulations using Green-Kubo formalism accurately reproduce this trend, and the density functional theory (DFT) calculations reveal the main scattering mechanism. The thermal expansion coefficients (TECs) of monolayer to trilayer BN at 300-400 K are also experimentally measured, and the results are comparable to atomistic ab initio DFT calculations in a wider range of temperatures. Thanks to its wide bandgap, high thermal conductivity, outstanding strength, good flexibility, and excellent thermal and chemical stability, atomically thin BN is a strong candidate for heat dissipation applications, especially in the next generation of flexible electronic devices.

Published in: "arXiv Material Science".

Combined molecular dynamics and phase-field modelling of crack propagation in defective graphene. (arXiv:1903.08390v1 [cond-mat.mtrl-sci])

2019-03-21T02:29:29+00:00March 21st, 2019|Categories: Publications|Tags: , |

In this work, a combined modelling approach for crack propagation in defective graphene is presented. Molecular dynamics (MD) simulations are used to obtain material parameters (Young’s modulus and Poisson ratio) and to determine the energy contributions during the crack evolution. The elastic properties are then applied in phase-field continuum simulations which are based on the Griffith energy criterion for fracture. In particular, the influence of point defects on elastic properties and the fracture toughness are investigated. For the latter, we obtain values consistent with recent experimental findings. Further, we discuss alternative definitions of an effective fracture toughness, which accounts for the conditions of crack propagation and establishes a link between dynamic, discrete and continuous, quasi-static fracture processes on MD level and continuum level, respectively. It is demonstrated that the combination of MD and phase-field simulations is a well-founded approach to identify defect-dependent material parameters.

Published in: "arXiv Material Science".

Confined step-flow growth of Cu intercalated between graphene and a Ru(0001) surface. (arXiv:1903.08397v1 [cond-mat.mtrl-sci])

2019-03-21T02:29:26+00:00March 21st, 2019|Categories: Publications|Tags: , |

By comparing the growth of Cu thin films on bare and graphene-covered Ru(0001) surfaces, we demonstrate the role of graphene as a surfactant allowing the formation of flat Cu films. Low-energy electron microscopy, X-ray photoemission electron microscopy and X-ray absorption spectroscopy reveal that depositing Cu at 580 K leads to distinct behaviors on both types of surfaces. On bare Ru, a Stranski-Krastanov growth is observed, with first the formation of an atomically flat and monolayer-thick wetting layer, followed by the nucleation of three-dimensional islands. In sharp contrast, when Cu is deposited on a graphene-covered Ru surface under the very same conditions, Cu intercalates below graphene and grows in a step-flow manner: atomically-high growth fronts of intercalated Cu form at the graphene edges, and extend towards the center of the flakes. Our findings suggest potential routes in metal heteroepitaxy for the control of thin film morphology.

Published in: "arXiv Material Science".

The Strength of Mechanically-Exfoliated Monolayer Graphene. (arXiv:1903.07695v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:41+00:00March 20th, 2019|Categories: Publications|Tags: |

The deformation and fracture behaviour of one-atom-thick mechanically exfoliated graphene has been studied in detail. Monolayer graphene flakes with different lengths, widths and shapes were successfully prepared by mechanical exfoliation and deposited onto poly(methyl methacrylate) (PMMA) beams. The fracture behaviour of the monolayer graphene was followed by deforming the PMMA beams. Through in-situ Raman mapping at different strain levels, the distributions of strain over the graphene flakes were determined from the shift of the graphene Raman 2D band. The failure mechanisms of the exfoliated graphene were either by flake fracture or failure of the graphene/polymer interface. The fracture of the flakes was observed from the formation of cracks identified from the appearance of lines of zero strain in the strain contour maps. It was found that the strength of the monolayer graphene flakes decreased with increasing flake width. The strength dropped to less than 10 GPa for large flakes, much lower than the reported value of 130 GPa for monolayer graphene, thought to be due to the presence of defects. It is shown that a pair of topological defects in monolayer graphene will form a pseudo crack and the effect of such defects upon the strength of monolayer graphene has been modelled using molecular mechanical simulations.

Published in: "arXiv Material Science".

Physical origin of giant excitonic and magneto-optical responses in two-dimensional ferromagnetic insulators. (arXiv:1903.07787v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:37+00:00March 20th, 2019|Categories: Publications|

The magneto-optical (MO) effects, such as the magneto-optical Kerr effect (MOKE) and the Faraday effect (FE), have been intensively investigated experimentally in a variety of magnetic materials serving as a highly sensitive probe for electronic and magnetic properties. Recent measurements using MOKE have led to the discovery of two-dimensional (2D) magnets, and demonstrated their rich magnetic behaviors. In particular, a giant Kerr response has been measured in mono- and few-layer CrI$_3$. However, the exact microscopic origin of such large MO signals in 2D materials is still unknown, because treating accurately sizable spin-orbit coupling (SOC) and excitonic effects that are essential for such an understanding is beyond the capability of existing methods. With a newly developed full-spinor GW and Bethe-Salpeter equation (GW-BSE) method, we show, for the first time from first principles, that the exceedingly large optical and MO responses in ferromagnetic monolayer CrI$_3$ arise from strongly bound exciton states consisting of spin-polarized electron-hole pairs. These exciton states are shown to have distinct characteristics compared with either the Frenkel excitons in ionic crystals and polymers, or Wannier excitons in other 2D semiconductors. By simulating a realistic experimental setup, we further find that substrate configuration and excitation frequency of the photon strongly shape the MO signals. Our results provide a new conceptual mechanism, explaining quantitatively the recent experiments on CrI$_3$. In addition, comparison between bulk and monolayer CrI$_3$ reveals the pivotal role of quantum confinement in enhancing the MO signals.

Published in: "arXiv Material Science".

Dirac Cones, Can Be Realized Generally in Bilayered Perovskites. (arXiv:1903.07853v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:34+00:00March 20th, 2019|Categories: Publications|Tags: |

We propose that the Dirac cone electronic states can be generally realized in those hexagonal perovskite bilayers sliced out directly from their cubic bulk phases. This is based on the simple analyses that the A-site atoms of these perovskites contribute neither valence nor conduction electrons and after the quasi-atom simplification their hexagonal bilayers indeed possess the honeycomb symmetry. Taking the CsPbBr$_3$ (111) bilayer as an example, with the density functional theory (DFT) calculations, we demonstrate their Dirac cones around the Fermi level and find the corresponding Fermi velocity, $sim$ 2$times$10$^6$m/s, can be almost twice as large as that of graphene. While under ambient conditions the solid ionic compounds normally do not conduct charge carriers very well, such high-velocity ballistic carrier transport in the new layered Dirac materials may revitalize the conventional perovskites and thereby bring new ultra-fast ionic electronics.

Published in: "arXiv Material Science".

Metal to insulator transition in Conducting Polyaniline/Graphene Oxide composites. (arXiv:1903.07954v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:31+00:00March 20th, 2019|Categories: Publications|Tags: , |

Broadband Dielectric Spectroscopy (BDS) measurements of P{omicron}lyaniline/Graphene oxide composites were conducted for an as-prepared and a thermally annealed specimen, respectively, from 15K to room temperature. Electrical conductivity values of the annealed composite display a very modest rise denoting the important contributions of the GO component to achieving electrical stability of the polymer. Patterns of the dc conductivity as a function of temperature also reveal a metal to insulator transition around 75K. The transition is dominated by two key factors; temperature and annealing process. Metal-like and insulating features are subsequently detected, as well, and accordingly described to provide a qualitative inspection of the charge transfer mechanisms involved

Published in: "arXiv Material Science".

Ab initio Investigation of Structural Stability and Exfoliation Energies in Transition Metal Dichalcogenides based on Ti-, V-, and Mo-Group Elements. (arXiv:1903.08112v1 [cond-mat.mtrl-sci])

2019-03-20T02:29:29+00:00March 20th, 2019|Categories: Publications|Tags: |

In this work, we report an ab initio investigation based on density functional theory of the structural, energetic and electronic properties of 2D layered chalcogenides compounds based in the combination of the transition-metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and chalcogenides (S, Se, Te) in three polymorphic phases: trigonal prismatic (2H), octahedral (1T) and distorted octahedral (1T$_{text{d}}$). We determined the most stable phases for each compound, verifying the existence of the 1T$_{text{d}}$ phase for a small number of the compounds and we have also identified the magnetic compounds. In addition, with the determination of the exfoliation energies, we indicated the potential candidates to form one layer material and we have also found a relation between the exfoliation energy and the effective Bader charge in the metal, suggesting that when the materials present small exfoliation energy, it is due to the Coulomb repulsion between the chalcogen planes. Finally, we analyzed the electronic properties, identifying the semiconductor, semimetal and metal materials and predicting the band gap of the semiconductors. In our results, the dependence of the band gap on the $d$-orbital is explicit. In conclusion, we have investigated the properties of stable and metastable phases for a large set of TMD materials, and our findings may be auxiliary in the synthesis of metastable phases and in the development of new TMDs applications.

Published in: "arXiv Material Science".

Discovery of High Dimensional Band Topology in Twisted Bilayer Graphene. (arXiv:1903.07950v1 [cond-mat.str-el])

2019-03-20T02:29:26+00:00March 20th, 2019|Categories: Publications|Tags: , |

Recently twisted bilayer graphene(t-BLG) emerges as a new strongly correlated physical platform near a magic twist angle, which hosts many exciting phenomena such as the Mott-like insulating and unconventional superconducting behavior. Besides the apparent significance of band flatness, band topology may be another critical element in strongly correlated twistronics yet receives much less attention. Here we report the discovery of nontrivial high dimensional band topology in t-BLG moir’e bands through a systematic nonlocal transport study, in conjunction with an examination rooted in $K$-theory. The moir’e band topology of t-BLG manifests itself as two pronounced nonlocal responses in the electron and hole superlattice gaps. We further show that the nonlocal responses are robust to the interlayer electric field, twist angle, and edge termination, exhibiting a universal scaling law. While an unusual symmetry of t-BLG trivializes Berry curvature, we elucidate that two high dimensional $Z_2$ invariants characterize the topology of the moir’e Dirac bands, validating the topological origin of the observed nonlocal responses. Our findings not only provide a new perspective for understanding the emerging strongly correlated phenomena in twisted van der Waals heterostructures, but also suggest a potential strategy to achieve topologically nontrivial metamaterials from topologically trivial quantum materials based on twist engineering.

Published in: "arXiv Material Science".

Excitonic pairing of two-dimensional Dirac fermions caused by long-range Coulomb and short-range Gross-Neveu interactions. (arXiv:1903.07440v1 [cond-mat.str-el])

2019-03-19T02:29:31+00:00March 19th, 2019|Categories: Publications|Tags: |

Two-dimensional Dirac fermions are subjected to two types of interaction, namely the long-range Coulomb interaction and the on-site short-range interaction. The former induces excitonic pairing if its strength $alpha$ is larger than some critical value $alpha_c$, whereas the latter drives an antiferromagnetic Mott transition when its strength $U$ exceeds a threshold $U_c$. Here, we study the impact of the interplay of these two interactions on excitonic pairing by using the Dyson-Schwinger equation approach. We find that the critical value $alpha_c$ is nearly unaffected by weak short-range interaction. As $U$ increases to approach $U_c$, the quantum fluctuation of antiferromagnetic order parameter becomes important and interacts with Dirac fermions via the Yukawa coupling. After treating the Coulomb interaction and Yukawa coupling on equal footing, we show that $alpha_c$ increases substantially as $U rightarrow U_c$. Thus, excitonic pairing is strongly suppressed near the antiferromagnetic quantum critical point. We obtain a global phase diagram on the $U$-$alpha$ plane, and illustrate that the excitonic insulating and antiferromagnetic phases are separated by the gapless semimetal phase. These results may help us understand the strong correlation effects in graphene and other similar systems.

Published in: "arXiv Material Science".

Morphology, ordering, stability, and electronic structure of carbon-doped hexagonal boron nitride. (arXiv:1903.07357v1 [cond-mat.mtrl-sci])

2019-03-19T02:29:29+00:00March 19th, 2019|Categories: Publications|Tags: , , |

We present theoretical studies of morphology, stability, and electronic structure of monolayer hexagonal CBN alloys with rich content of h-BN and carbon concentration not exceeding 50 %. Our studies are based on the bond order type of the valence force field to account for the interactions between atomic constituents and Monte Carlo method with Metropolis algorithm to establish equilibrium distribution of atoms over the lattice. We find out that the phase separation into graphene and h-BN domains occurs in the majority of growth conditions. Only in N-rich growth conditions, it is possible to obtain quasi uniform distribution of carbon atoms over boron sublattice. We predict also that the energy gap in stoichiometric C$_x$(BN)$_{1-x}$ alloys exhibits extremely strong bowing.

Published in: "arXiv Material Science".

Defect-induced magnetically-sensitive valley polarization reversal and revival in WSe2-WS2 heterostructure. (arXiv:1903.06899v1 [cond-mat.mtrl-sci])

2019-03-19T02:29:27+00:00March 19th, 2019|Categories: Publications|Tags: , , |

Manipulating and reserving the valley pseudospin of excitons are one core aim in the transition metal dichalcogenides (TMDs). However, due to the strong electron-hole exchange and spin-orbit coupling interactions, the exciton recombination lifetime is subject to picosecond timescale intrinsically, and the valley polarization is hardly modulated by the moderate magnetic field. It is fortunate that interlayer and defect-localized excitons promise to overcome these difficulties by suppressing these interactions. Here we clearly reveal that the valley polarization can be reversed and revived in the defect-localized excitons with microsecond lifetime in AB-stacked WSe2-WS2 heterobilayer.Specifically, for the interlayer defect-localized exciton, the valley polarization is reversed and can be enhanced efficiently by a weak out-of-plane magnetic field (<0.4T). In sharp contrast, for the intralayer defect-localized exciton, the valley polarization can revive after a fast relaxation process whose sign follows the direction of a strong out-of-plane magnetic field (>1T). We explain the reversed valley polarization with magnetic-field sensitivity by the delocalization of defect-localized holes by a weak magnetic field and the revival of valley polarization by the valley Zeeman effect from a strong magnetic field. Our results raise the prospect to find rich and novel valley dynamics in defect-localized excitons in the TMDs heterobilayer.

Published in: "arXiv Material Science".

Tuning $WS_2$ photoluminescence using polymeric micro-actuators in a van der Waals heterostructure. (arXiv:1903.06732v1 [cond-mat.mes-hall])

2019-03-19T02:29:22+00:00March 19th, 2019|Categories: Publications|Tags: , |

The control of the local strain profile in 2D materials offers an invaluable tool for tailoring electronic and photonic properties of solid-state devices. In this paper, we demonstrate that strain in $WS_2$ based van der Waals heterostructures can be modulated on a local scale by means of polymeric micrometric actuators implemented by electron beam lithography directly on the growth substrate. Thanks to the underlying material structure, i.e. graphene on SiC, $WS_2$ flakes are found to slide with negligible friction, thus circumventing the technical challenges of other approaches requiring layer transfer and production of suspended membranes. As proof of concept, in our experiment we show strain-induced local modulation of the excitonic photoluminescence of such non-suspended $WS_2$ monolayers under application of custom strain profiles.

Published in: "arXiv Material Science".

Inter-layer charge transport controlled by exciton-trion coherent coupling. (arXiv:1903.06437v1 [cond-mat.mes-hall])

2019-03-18T02:30:10+00:00March 18th, 2019|Categories: Publications|Tags: |

The possibility of electrical manipulation and detection of charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. Using a few-layer graphene/monolayer WS$_{2}$/monolayer graphene vertical heterojunction, we report inter-layer charge transport from top few-layer graphene to bottom monolayer graphene, mediated by coherently formed trion state. This is achieved by using a resonant excitation and varying the sample temperature, the resulting change in the WS$_{2}$ bandgap allows us to scan the excitation around the exciton-trion spectral overlap with high spectral resolution. By correlating the vertical photocurrent and in situ photoluminescence features at the heterojunction as a function of the spectral position of the excitation, we show that (1) trions are anomalously stable at the junction even up to 463 K due to enhanced doping, and (2) the photocurrent results from the ultra-fast formation of trion through exciton-trion coherent coupling, followed by its fast inter-layer transport. The demonstration of coherent formation, high stabilization, vertical transportation and electrical detection of trions marks a step towards room temperature trionics.

Published in: "arXiv Material Science".

Edge states and ballistic transport in zig-zag graphene ribbons: the role of SiC polytypes. (arXiv:1903.05185v1 [cond-mat.mes-hall])

2019-03-14T02:29:38+00:00March 14th, 2019|Categories: Publications|Tags: |

Zig-zag edge graphene ribbons grown on 6H-SiC facets are ballistic conductors. It has been assumed that zig-zag graphene ribbons grown on 4H-SiC would also be ballistic. However, in this work we show that SiC polytype matters; ballistic graphene ribbons only grow on 6H SiC. 4H and 4H-passivated ribbons are diffusive conductors. Detailed photoemmision and microscopy studies show that 6H-SiC sidewalls zig-zag ribbons are metallic with a pair of n-doped edge states associated with asymmetric edge terminations, In contrast, 4H-SiC zig-zag ribbons are strongly bonded to the SiC; severely distorting the ribbon’s $pi$-bands. $text{H}_2$-passivation of the 4H ribbons returns them to a metallic state but show no evidence of edge states.

Published in: "arXiv Material Science".

Origin of Strong Correlated Electronic State in Twisted Graphene Bilayer from First Principle Study. (arXiv:1903.05232v1 [cond-mat.supr-con])

2019-03-14T02:29:36+00:00March 14th, 2019|Categories: Publications|Tags: , |

By using the first-principles method based on density of functional theory, we study the electronic properties of twisted bilayer graphene with the rotation angle and the interlayer spacing. With the decrease of the rotation angle (the unit cell becomes larger), the energy band becomes narrower and Coulomb repulsion increases, leading to the enhancement of electronic correlation; On the other hand, as the interlayer spacing decreases and the interlayer coupling becomes stronger, the correlation becomes stronger. By tuning the interlayer coupling, we can realize the strongly correlated state with the band width less than 0.01 eV in medium-sized Moire cell of twisted bilayer graphene. These results demonstrate that the strength of electronic correlation in twisted bilayer graphene is closely related to two factors: the size of unit cell and the distance between layers. Consequently, a conclusion can be drawn that the strong electronic correlation in twisted bilayer graphene originates from the synergistic effect of the large size of Moire cell and strong interlayer coupling on its electronic structure.

Published in: "arXiv Material Science".

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