Yaguang Guo

A New Silicon Phase with Direct Band Gap and Novel Optoelectronic Properties.

Due to the compatibility with the well-developed Si-based semiconductor industry, there is considerable interest in developing silicon structures with direct energy band gaps for effective sunlight harvesting. In this paper, using silicon triangles as the building block, we propose a new silicon allotrope with a direct band gap of 0.61 eV, which is dynamically, thermally and mechanically stable. Symmetry group analysis further suggests that dipole transition at the direct band gap is allowed. In addition, this new allotrope displays large carrier mobility (~104 cm/V · s) at room temperature and a low mass density (1.71 g/cm3), making it a promising material for optoelectronic applications.

Phosphorus K4 Crystal: A New Stable Allotrope.

The intriguing properties of phosphorene motivate scientists to further explore the structures and properties of phosphorus materials. Here, we report a new allotrope named K4 phosphorus composed of three-coordinated phosphorus atoms in non-layered structure which is not only dynamically and mechanically stable, but also possesses thermal stability comparable to that of the orthorhombic black phosphorus (A17). Due to its unique configuration, K4 phosphorus exhibits exceptional properties: it possesses a band gap of 1.54 eV which is much larger than that of black phosphorus (0.30 eV), and it is stiffer than black phosphorus. The band gap of the newly predicted phase can be effectively tuned by appling hydrostastic pressure. In addition, K4 phosphorus exibits a good light absorption in visible and near ultraviolet region. These findings add additional features to the phosphorus family with new potential applications in nanoelectronics and nanomechanics.

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.

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.

Graphdiyne as an ideal monolayer coating material for lithium-ion battery cathodes with ultralow areal density and ultrafast Li penetration

Surface coating of electrodes is an effective way to enhance the performance of lithium-ion batteries (LIBs). It is highly desirable to find ideal coating materials with fast Li penetration and low areal density. Based on first-principles calculations we propose that monolayer sheets can be used as such materials by taking graphdiyne as a test case. We find that the porous structure of graphdiyne can allow fast Li penetration and block the direct contact between the electrode and electrolyte with strong binding between graphdiyne and the LiCoO2 cathode. Graphdiyne has a low areal density, and its electrochemical window is wide enough for it to work at different voltages. In addition, the electronic conductivity of LiCoO2 is improved when coated with graphdiyne as a result of the metallic electronic structure and low interfacial resistance of the graphdiyne coated LiCoO2 electrode. These intriguing theoretical findings would stimulate experimental work on searching for novel coating materials for LIBs.

Quasi-freestanding, striped WS2 monolayer with an invariable band gap on Au(001)

Revealing the structural/electronic features and interfacial interactions of monolayer MoS2 and WS2 on metals is essential to evaluating the performance of related devices. In this study, we focused on the atomic-scale features of monolayer WS2 on Au(001) synthesized via chemical vapor deposition. Scanning tunneling microscopy and spectroscopy reveal that the WS2/Au(001) system exhibits a striped superstructure similar to that of MoS2/Au(001) but weaker interfacial interactions, as evidenced by experimental and theoretical investigations. Specifically, the WS2/Au(001) band gap exhibits a relatively intrinsic value of ∼2.0 eV. However, the band gap can gradually decrease to ∼1.5 eV when the sample annealing temperature increases from ∼370 to 720 °C. In addition, the doping level (or Fermi energy) of monolayer WS2/Au(001) varies little over the valley and ridge regions of the striped patterns because of the homogenous distributions of point defects introduced by annealing. Briefly, this work provides an in-depth investigation into the interfacial interactions and electronic properties of monolayer MX2 on metal substrates.