Xianqi Dai

Trapping of metal atoms in the defects on graphene

The binding of a single metal atom (Pt, Pd, Au, and Sn) nearby a single-vacancy (SV) on the graphene is investigated using the first-principles density-functional theory. On the pristine graphene (pri-graphene), the Pt, Pd, and Sn prefer to be adsorbed at the bridge site, while Au prefers the top site. On the graphene with a single-vacancy (SV-graphene), all the metal atoms prefer to be trapped at the vacancy site and appear as dopants. However, the trapping abilities of the SV-graphene are varied for different metal atoms, i.e., the Pt and Pd have the larger trapping zones than do the others. The diffusion barrier of a metal atom on the SV-graphene is much higher than that on the pri-graphene, and the Pt atom has the largest diffusion barrier from the SV site to the neighboring bridge sites. On the SV-graphene, more electrons are transferred from the adatoms (or dopants) to the carbon atoms at the defect site, which induces changes in the electronic structures and magnetic properties of the systems. This work provides valuable information on the selectivity of lattice vacancy in trapping metal atoms, which would be vital for the atomic-scale design of new metal-carbon nanostructures and graphene-based catalysts.

Absorption of Pt clusters and the induced magnetic properties of graphene

First-principles total energy calculations are performed to investigate the formation and structures of Pt clusters on graphene. It is found that the formation energy of Pt on graphene increases with increasing Pt coverage. The structures of the absorbed Pt are that it is at the bridge site for a single Pt atom absorption, but form a dimerized cluster when two atoms are absorbed on graphene. For three- and four-Pt-atom absorption, linear and tetrahedral structures form, respectively, and the three-dimensional tetrahedral Pt(4) cluster is most stable in all the configurations investigated. There is a strong interatomic interaction among Pt atoms and so they tend to form clusters. While no magnetic behavior is expected after a single Pt atom is absorbed on graphene, the absorption of tetrahedral Pt(4) leads to Fermi level shifting to the valence band and the spin waves of C atoms in graphene become asymmetric and so they exhibit magnetism. The magnetic properties can thus be tuned by Pt absorption on graphene. The ultimate aim is to apply it in catalytic activity and electronic devices.

Preventing the CO poisoning on Pt nanocatalyst using appropriate substrate: a first-principles study

Adsorption energies and stable configurations of CO on the Pt clusters are investigated using the first-principles density-functional theory method. It is found that the adsorption of CO on the top site of the Pt4 cluster is more stable than that on the bridge site. The atomic charges are unevenly distributed within the charged Pt4 cluster, and the structural positions of the Pt atoms determine their charge states and therefore their activity. A systematic study on the effects of electrons and holes doping in the Pt4 clusters suggest an effective method to prevent the CO poisoning through regulating the total charge in Pt4 clusters. The graphene-based substrate is an ideal catalyst support, which makes the Pt catalyst lose electron and weakens the CO adsorption. The results would be of great importance for designing high active nanoscale Pt catalysts used for fuel cells.

High catalytic activity for CO oxidation on single Fe atom stabilized in graphene vacancies

Inspired by the recently discovered dynamics of single Fe atoms in graphene vacancies, we systemically examined the stable configurations, electronic structures, and catalytic activities of Fe-atom-embedded graphene substrates (including monovacancy graphene (MG) and divacancy graphene (DG)) by using first-principles calculations. We found that the doped Fe on the MG sheet (Fe/MG) is more stable than that on the DG sheet (Fe/DG). Doping with Fe atoms provides more transferred electrons to fill the vacancy defects of graphene and allows it to exhibit a more positive charge, which effectively regulates O2 and CO adsorption. Also, the degree of interactions between the reactants and substrates are connected to the reaction pathways and energy barriers. For the Fe/MG sheet, the low coadsorption energy of gas molecules can promote the catalytic reaction through the Langmuir–Hinshelwood (LH) mechanism. In comparison, the initial step for CO oxidation on the Fe/DG sheet is through the Eley–Rideal (ER) mechanism, which is an energetically more favorable process. Moreover, the more stable Fe/MG sheet is a much more efficient catalyst for CO oxidation at low temperature, because the sequential reaction processes (LH and ER) have low enough energy barriers. These results provide valuable guidance on selecting the metal dopant in graphene materials to design effective atomic-scale catalysts.

Single domain Bi2Se3 films grown on InP(111)A by molecular-beam epitaxy

We report the growth of single-domain epitaxial Bi2Se3 films on InP(111)A substrate by molecular-beam epitaxy. Nucleation of Bi2Se3 proceeds at steps, so the lattices of the substrate play the guiding role for a unidirectional crystalline film in the step-flow growth mode. There exists a strong chemical interaction between atoms at the heterointerface, so the growth does not follow the van der Waals epitaxy process. A mounded morphology of thick Bi2Se3 epilayers suggests a growth kinetics dictated by the Ehrlich-Schwoebel barrier. The Schubnikov de Haas oscillations observed in magnetoresistance measurements are attributed to Landau quantization of the bulk states of electrons.

Molecular-beam epitaxy of monolayer MoSe2: growth characteristics and domain boundary formation

Monolayer (ML) transition metal dichalcogenides (TMDs) are of great research interest due to their potential use in ultrathin electronic and optoelectronic applications. They show promise in new concept devices in spintronics and valleytronics. Here we present a growth study by molecular-beam epitaxy of ML and sub-ML MoSe2, an important member of TMDs, revealing its unique growth characteristics as well as the formation processes of domain boundary (DB) defects. A dramatic effect of growth temperature and post-growth annealing on DB formation is uncovered.

Electronic and magnetic properties of n-type and p-doped MoS2 monolayers

The electronic and magnetic properties of n- and p-type impurities by means of group V and VII atoms substituting sulfur in a MoS2 monolayer were investigated using first-principles methods based on density functional theory. Numerical results show that group V and VII atoms can induce magnetic properties, and the magnetic moment mainly originates from the dopant’s p orbital and neighbor Mo atoms’ d orbital. N-, P-, F-, and I-doped (or As-doped) MoS2 exhibits magnetic nanomaterial (or metallic) features, and Cl- and Br-doped systems exhibit half-metallic ferromagnetism (HMF). Moreover, the formation energy calculations also indicate that it is energetically favorable and relatively easier to incorporate group V and VII atoms into a MoS2 monolayer under Mo-rich experimental conditions. The formation energy of the F-doped system is the lowest, the next lowest formation energy is obtained in the Cl-doped system. By comparison, Cl-doped MoS2 is more suitable for spin injection. This finding is important for the achievement of spin devices on MoS2 nanostructures.

Magnetic vanadium sulfide monolayers: transition from a semiconductor to a half metal by doping

Two dimensional crystals, befitting nanoscale electronics and spintronics, can have versatile applications due to their ultrathin and flexible nature. We show by first-principles calculations that suitable doping can influence the exchange splitting of the spin and electronic states in VS2 monolayers using the meta-GGA (MGGA) method by self-consistently introducing the exact-exchange interaction to the sub d shell of the V atoms. Among the large number of non-metallic atoms, H, B, C, N, P, As, O, Se, Te, F, Cl, and Br, As appears to be a candidate for p-type doping of the VS2 monolayer with an appropriate formation energy. The enhancement of the conductivity is limited and the spin polarization of the As doped system is also slightly lowered from the mixed V-s state with an inverse spin orientation. H and halogen atoms (F, Cl, Br) are ideal candidates for n-type doping with very small and even negative formation energy. Particularly, a switch from a half semiconductor to a half metal is realized with enhanced conductivity and spin polarization in VS2 monolayers from the increased magnetic moment of the nearest neighboring V atoms of the dopants under a rather wide range of doping density, which is very promising for spintronic applications.

Robust surface state of intrinsic topological insulator Bi2Te2Se thin films: a first-principles study

Bi(2)Te(2)Se, a ternary tetradymite compound, has recently been identified to be a three-dimensional topological insulator. In this paper, we theoretically study the electronic structures of bulk and thin films of Bi(2)Te(2)Se employing spin-orbit coupling (SOC) self-consistently with density-functional theory. It is found that SOC plays an important role in determining the electronic properties of Bi(2)Te(2)Se. A finite bandgap opens up in the surface states of Bi(2)Te(2)Se thin films due to the hybridization of the top and bottom surface states of films. The intrinsic Bi(2)Te(2)Se thin films of three or more quintuple layers exhibit a robust topological nature of electronic structure with the Fermi energy intersecting the Dirac cone of the surface states only once between time-reversal-invariant momenta. These characteristics of Bi(2)Te(2)Se are similar to the topological behavior of Bi(2)Te(3), promising a variety of potential applications in nanoelectronics and spintronics.

Tuning the Schottky contacts at the graphene/WS2 interface by electric field

Using the first-principle calculations, we study the electronic structures of graphene/WS2 van der Waals (vdW) heterostructures by applying an external electric field (Eext) perpendicular to the heterobilayers. It is demonstrated that the intrinsic electronic properties of graphene and WS2 are quite well preserved due to the weak vdW contact. We find that n-type Schottky contacts with a significantly small Schottky barrier are formed at the graphene/WS2 interface and p-type (hole) doping in graphene occurs during the formation of graphene/WS2 heterostructures. Moreover, the Eext is effective to tune the Schottky contacts, which can transform the n-type into p-type and ohmic contact. Meanwhile, p-type (hole) doping in graphene is enhanced under negative Eext and a large positive Eext is required to achieve n-type (electron) doping in graphene. The Eext can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/WS2 interface. The present study would open a new avenue for application of ultrathin graphene/WS2 heterostructures in future nano- and optoelectronics.