Xiaoyin Li

ψ-Graphene: A New Metallic Allotrope of Planar Carbon with Potential Applications as Anode Materials for Lithium-Ion Batteries

Using state-of-the-art first-principles calculations, we propose a new two-dimensional (2D) carbon allotrope constructed by polymerizing the carbon skeletons of s-indacenes, named PSI (ψ)-graphene. We show that ψ-graphene has the lowest energy among all hitherto reported 2D allotropes of carbon composed of 5-6-7 carbon rings and is dynamically and thermally stable. This structure is metallic with robust metallicity against external strain. In addition, we find that the adsorption of Li atoms on ψ-graphene is exothermic, and the diffusion energy barrier is low and comparable to that of graphene. Furthermore, ψ-graphene can achieve a maximum Li storage capacity equivalent to that of LiC6, suggesting its potential as an anode material for Li-ion batteries (LIBs). In addition, we show that increasing the number of hexagons in this structure can enhance the thermodynamic stability of the sheet while maintaining its metallicity. This study provides new insights into the design of new metallic carbon for nanostructured anode materials in the next generation of LIBs.

Parallel molecular stacks of organic thin film with electrical bistability

A thin film of 1,1-dicyano-2,2-(4-dimethylaminophenyl) ethylene (DDME) has been grown by a modified vacuum deposition. The films were characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning tunneling microscopy (STM). Highly ordered parallel molecular stacks were repeatedly observed with STM under ambient conditions. The dc current–voltage (I–V) characteristics of device Au/DDME/Au were measured, and the film was found to possess good electrical bistability and electrochromic properties. Nanometer-scale recording was realized on the film by applying pulse voltage between the STM tip and substrate. The possible switching mechanism is discussed.

Weak interlayer dependence of lattice thermal conductivity on stacking thickness of penta-graphene

Penta-graphene (PG), as a novel carbon allotrope, has attracted considerable attention because of its unique atomic structure and outstanding intrinsic properties. Here, we systematically investigate the effect of layer numbers on the lattice thermal conductivity of the stacked PG structures by solving exactly the linearized phonon Boltzmann transport equation combined with first-principles calculations. We find that the lattice thermal conductivity of the stacked PG is insensitive to the number of layers, which is in sharp contrast to that of graphene. Such a layer-independent thermal conductivity is attributed to the buckled structure of PG which breaks the two-dimensional selection rule of three-phonon scattering and the weak van der Waals interlayer interactions that hardly have any effect on the lattice thermal conductivity. This mechanism can be generalized to other van der Waals layered materials with buckled or puckled structures, which may also show the layer-independent lattice thermal conductivity.

A C20 fullerene-based sheet with ultrahigh thermal conductivity

A new two-dimensional (2D) carbon allotrope, Hexa-C20, composed of C20 fullerene is proposed. State-of-the-art first principles calculations combined with solving the linearized phonon Boltzmann transport equation confirm that the new carbon structure is not only dynamically and thermally stable, but also can withstand temperatures as high as 1500 K. Hexa-C20 possesses a quasi-direct band gap of 3.28 eV, close to that of bulk ZnO and GaN. The intrinsic lattice thermal conductivity κlat of Hexa-C20 is 1132 W m-1 K-1 at room temperature, which is much larger than those of most carbon materials such as graphyne (82.3 W m-1 K-1) and penta-graphene (533 W m-1 K-1). Further analysis of its phonons uncovers that the main contribution to κlat is from the three-phonon scattering, while the three acoustic branches are the main heat carriers, and strongly coupled with optical phonon branches via an absorption process. The ultrahigh lattice thermal conductivity and an intrinsic wide band gap make the Hexa-C20 sheet attractive for potential thermal management applications.

Physical Properties and Photovoltaic Application of Semiconducting Pd2Se3 Monolayer

Palladium selenides have attracted considerable attention because of their intriguing properties and wide applications. Motivated by the successful synthesis of Pd2Se3 monolayer (Lin et al., Phys. Rev. Lett., 2017, 119, 016101), here we systematically study its physical properties and device applications using state-of-the-art first principles calculations. We demonstrate that the Pd2Se3 monolayer has a desirable quasi-direct band gap (1.39 eV) for light absorption, a high electron mobility (140.4 cm2V−1s−1) and strong optical absorption (~105 cm−1) in the visible solar spectrum, showing a great potential for absorber material in ultrathin photovoltaic devices. Furthermore, its bandgap can be tuned by applying biaxial strain, changing from indirect to direct. Equally important, replacing Se with S results in a stable Pd2S3 monolayer that can form a type-II heterostructure with the Pd2Se3 monolayer by vertically stacking them together. The power conversion efficiency (PCE) of the heterostructure-based solar cell reaches 20%, higher than that of MoS2/MoSe2 solar cell. Our study would motivate experimental efforts in achieving Pd2Se3 monolayer-based heterostructures for new efficient photovoltaic devices.

A New Anisotropic Dirac Cone Material: A B2S Honeycomb Monolayer

Different from the isotropic Dirac cones existing in other two-dimensional (2D) materials, anisotropic Dirac cones have the merit of anisotropic carrier mobility for applications in direction-dependent quantum devices. Motivated by the recent experimental finding of an anisotropic Dirac cone in borophene, here we report a new 2D anisotropic Dirac cone material, B2S monolayer, identified by using a global structure search method and first-principles calculation combined with a tight-binding model. The B2S monolayer is found to be stable mechanically, thermally, and dynamically and exhibits an anisotropic Dirac cone exactly at the Fermi level, showing a Fermi velocity of 106 m/s in the same order of magnitude as that of graphene. Moreover, B2S monolayer is the first anisotropy Dirac cone material with a pristine honeycomb structure stabilized by S in free-standing conditions where each atom has four valence electrons on average being isoelectronic to C. This study would expand the Dirac cone material family with new features.

A new 3D Dirac nodal-line semi-metallic graphene monolith for lithium ion battery anode materials

Due to the limited specific capacity of graphite, it is highly desirable to develop new alternatives for high performance lithium ion battery (LIB) anode materials. Motivated by the recent synthesis of three-dimensional (3D) graphene (Phys. Rev. Lett., 2016, 116, 055501), here we propose a new 3D graphene monolith, termed HZGM-42. State-of-the-art theoretical calculations uncover that HZGM-42 not only possesses high thermodynamic stability, anisotropic elasticity and high specific strength, but also exhibits a unique electronic band structure with Dirac nodal-lines. Equally important, HZGM-42 shows excellent electrical conductivity and uniformly distributed channels for the transport of Li ions with fast kinetics. As compared to the commercially used graphite anode, HZMG-42 possesses a much higher theoretical specific capacity (637.71 mA h g−1), a much lower energy barrier (0.02 eV) for Li ion diffusion along the one-dimensional channels, and a much smaller volume change (2.4%) during charging and discharging operation. This study expands the family of 3D porous carbon materials with great potential for LIBs.