Minglei Sun

Tunable Schottky barrier in van der Waals heterostructures of graphene and g-GaN

Using first-principles calculations, we systematically investigated the electronic properties of graphene/g-GaN van der Waals (vdW) heterostructures. We discovered that the Dirac cone of graphene could be quite well preserved in the vdW heterostructures. Moreover, a transition from an n-type to p-type Schottky contact at the graphene/g-GaN interface was induced with a decreased interlayer distance from 4.5 to 2.5 A. This relationship is expected to enable effective control of the Schottky barrier, which is an important development in the design of Schottky devices.

Magnetism in transition metal-substituted germanane: A search for room temperature spintronic devices

Using first-principles calculations, we investigated the geometric structure, binding energy, and magnetic behavior of monolayer germanane substitutional doped with transition metals. Our work demonstrates that germanane with single vacancy forms strong bonds with all studied impurity atoms. Magnetism is observed for Ti, V, Cr, Mn, Fe, and Ni doping. Doping of Ti and Mn atoms results in half-metallic properties, while doping of Cr results in dilute magnetic semiconducting state. We estimate a Curie temperature of about 735 K for Mn-substituted system in the mean-field approximation at impurity concentration 5.56%. Furthermore, when increasing the impurity concentration to 12.5%, Curie temperatures of Ti and Mn-substituted systems are 290 and 1120 K, respectively. Our studies demonstrate the potential of Ti and Mn-substituted germanane for room temperature spintronic devices.

Effects of structural imperfection on the electronic properties of graphene/WSe2 heterostructures

Two-dimensional (2D) material-based van der Waals (vdW) heterostructures have recently attracted much attention because the combined merits of two 2D monolayers enable many novel applications in nanoelectronic and optoelectronic devices. The recently synthesized graphene/WSe2 vdW heterostructures are especially appealing, and understanding the effect of structural imperfection on the electronic properties of the graphene/WSe2 vdW heterostructures is important for their practical application. In this study, we used first-principles calculations to investigate the structural and electronic properties of a graphene monolayer stacked on a WSe2 monolayer with various structural defects: two Se vacancies (WSe2-Vdi-Se) and W vacancy (WSe2-VW). After the structural defects were introduced, the calculated properties of the graphene/WSe2 vdW heterostructure were significantly changed. Therefore, it is extremely important to prepare defect-free WSe2 for the nanoelectronics applications with dedicated function. On the other hand, the artificial generation of defects may provide a new approach to tune the electronic properties of graphene/WSe2 vdW heterostructures. Moreover, by varying the interlayer coupling or by applying a perpendicular electric field, it is possible to further modulate the electronic properties of graphene/WSe2 and graphene/WSe2-Vdi-Se vdW heterostructures. These theoretical results are expected to provide useful guidelines for the design of novel nanodevices based on graphene/WSe2 vdW heterostructures.

Electronic properties of Janus silicene: new direct band gap semiconductors

Using first-principles calculations, we propose a new class of materials, Janus silicene, which is silicene asymmetrically functionalized with hydrogen and halogen atoms. Formation energies and phonon dispersion indicated that all the Janus silicene systems exhibit good kinetic stability. As compared to silicane, all Janus silicene systems are direct band gap semiconductors. The band gap of Janus silicene can take any value between 1.91 and 2.66 eV by carefully tuning the chemical composition of the adatoms. In addition, biaxial elastic strain can further reduce the band gap to 1.11 eV (under a biaxial tensile strain up to 10%). According to moderate direct band gap, these materials demonstrate potential applications in optoelectronics, exhibiting a very wide spectral range, and they are expected to be highly stable under ambient conditions.

Fabrication of HA/PEI-functionalized carbon dots for tumor targeting, intracellular imaging and gene delivery

Carbon quantum dots (CDs) as emerging carbon nano-materials have attracted tremendous attention in biomedical fields due to unique properties. In this study, hyaluronate (HA) and polyethylenimine (PEI) functionalized carbon dots (HP-CDs) were synthesized by a facile bottom-up method for tumor targeting and gene delivery. After HA modification, the HP-CDs exhibited superior dispersibility in water and good biocompatibility and were internalized readily into the cytoplasm of cancer via HA-receptor mediated endocytosis. Meanwhile, the HP-CDs with PEI functionalization were shown to have excellent gene condensation capability via electrostatic attraction and protective capacity by preventing nuclease degradation. By virtue of good photoluminescence properties, the HP-CDs as a gene carrier were successfully applied to intracellular imaging and gene delivery. Taken together, the resultant HP-CDs displayed great potential in tumor targeting, intracellular imaging and gene delivery.

First principles study of silicene symmetrically and asymmetrically functionalized with halogen atoms

Using first principles calculations with the hybrid exchange–correlation functional, we systematically investigated the structural and electronic properties of silicene symmetrically and asymmetrically (Janus) decorated with halogen elements. The calculations show that all the functionalized systems are direct-band-gap semiconductors. Even more remarkable, by carefully selecting the adsorption adatoms on silicene and applying elastic tensile strain, a direct-band-gap semiconductor with any value between 0.98 and 2.13 eV can be obtained. The formation energies indicate that all the silicene derivatives should be formed experimentally. The present study suggests a rather practical way for gap opening and modulation of silicene.

Electronic and optical properties of heterostructures based on transition metal dichalcogenides and graphene-like zinc oxide

The structural, electronic, and optical properties of heterostructures formed by transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se) and graphene-like zinc oxide (ZnO) were investigated using first-principles calculations. The interlayer interaction in all heterostructures was characterized by van der Waals forces. Type-II band alignment occurs at the MoS2/ZnO and WS2/ZnO interfaces, together with the large built-in electric field across the interface, suggesting effective photogenerated-charge separation. Meanwhile, type-I band alignment occurs at the MoSe2/ZnO and WSe2/ZnO interfaces. Moreover, all heterostructures exhibit excellent optical absorption in the visible and infrared regions, which is vital for optical applications.

Few-Layer PdSe2 Sheets: Promising Thermoelectric Materials Driven by High Valley Convergence

Herein, we report a comprehensive study on the structural and electronic properties of bulk, monolayer, and multilayer PdSe2 sheets. First, we present a benchmark study on the structural properties of bulk PdSe2 by using 13 commonly used density functional theory (DFT) functionals. Unexpectedly, the most commonly used van der Waals (vdW)-correction methods, including DFT-D2, optB88, and vdW-DF2, fail to provide accurate predictions of lattice parameters compared to experimental data (relative error > 15%). On the other hand, the PBE-TS series functionals provide significantly improved prediction with a relative error of <2%. Unlike hexagonal two-dimensional materials like graphene, transition metal dichalcogenides, and h-BN, the conduction band minimum of monolayer PdSe2 is not located along the high symmetry lines in the first Brillouin zone; this highlights the importance of the structure–property relationship in the pentagonal lattice. Interestingly, high valley convergence is found in the conduction and valence bands in monolayer, bilayer, and trilayer PdSe2 sheets, suggesting promising application in thermoelectric cooling.

Exceptional Optical Absorption of Buckled Arsenene Covering a Broad Spectral Range by Molecular Doping

Using density functional theory calculations, we demonstrate that the electronic and optical properties of a buckled arsenene monolayer can be tuned by molecular doping. Effective p-type doping of arsenene can be realized by adsorption of tetracyanoethylene and tetracyanoquinodimethane (TCNQ) molecules, while n-doped arsenene can be obtained by adsorption of tetrathiafulvalene molecules. Moreover, owing to the charge redistribution, a dipole moment is formed between each organic molecule and arsenene, and this dipole moment can significantly tune the work function of arsenene to values within a wide range of 3.99–5.57 eV. Adsorption of TCNQ molecules on pristine arsenene can significantly improve the latter’s optical absorption in a broad (visible to near-infrared) spectral range. According to the AM 1.5 solar spectrum, two-fold enhancement is attained in the efficiency of solar-energy utilization, which can lead to great opportunities for the use of TCNQ–arsenene in renewable energy. Our work clearly demonstrates the key role of molecular doping in the application of arsenene in electronic and optoelectronic components, renewable energy, and laser protection.