Fancy Qian Wang

An all-carbon vdW heterojunction composed of penta-graphene and graphene: Tuning the Schottky barrier by electrostatic gating or nitrogen doping

The non-zero band gap together with other unique properties endows penta-graphene with potential for device applications. Here, we study the performance of penta-graphene as the channel material contacting with graphene to form a van der Waals heterostructure. Based on first-principles calculations, we show that the intrinsic properties of penta-graphene are preserved in the heterojunction, which is different from the conventional contact with metal surfaces. The stacked system forms an n-type Schottky barrier (Φe) at the vertical interface, while a negative band bending occurs at the lateral interface in a current-in-plane model. From the device point of view, we further demonstrate that a low-Φe or an Ohmic contact can be realized by applying an external electric field or doping graphene with nitrogen atoms. This allows the control of the Schottky barrier height, which is essential in fabricating penta-graphene-based nanotransistors.

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.