The Oxygen Reduction Reaction Mechanism on FeN3 Doped Divacancy Graphene: A Theoretical Perspective

2018-02-17T16:28:01+00:00 February 17th, 2018|Categories: Publications|Tags: , |

In the past years, non-precious transition metal and nitrogen co-doped graphene electrocatalysts for oxygen reduction reaction (ORR) have attracted great attention because of their high catalytic activity and stability. In this work, the reaction mechanism for ORR on FeN3 doped divacancy graphene (FeN3-Gra) has been investigated by using the density functional theory. The results showed that FeN3-Gra is thermodynamically stable. The ORR elementary reactions could take place within a small region around FeN3 and its adjacent eleven carbon atoms. HOOH is unstable on the catalyst surface, indicating that ORR is a four-electron process. O2 hydrogenation is much easier than its dissociation. The most favorable pathway is the OOH hydrogenation to form O + H2O. The formation of the second water molecule is the rate-determining step with an energy barrier of 0.48 eV. This barrier is much lower than 0.80 eV for pure Pt, also lower than 0.56 eV for FeN4 doped divacancy graphene. In addition, the calculated Tafel slope is also smaller than pure Pt in both low and high overpotential regions. The above results indicated that FeN3-Gra is a potential ORR catalyst.

Published in: "Journal of the Electrochemical Society".

Elastic Ag Anchored N-doped Graphene/Carbon Foam for the Selective Electrochemical Reduction of Carbon Dioxide to Ethanol

2018-02-17T03:17:58+00:00 February 17th, 2018|Categories: Publications|Tags: |

J. Mater. Chem. A, 2018, Accepted ManuscriptDOI: 10.1039/C7TA10802H, PaperKuilin Lv, Yanchen Fan, Ying Zhu, Yi Yuan, Jinrong Wang, Ying Zhu, Qianfan ZhangElectrochemical reduction of CO2 is considered as an efficient strategy for converting CO2 emission into the valued-added carbon compounds. However, it often

Published in: "Journal of Materials Chemistry A".

Atomic-like high-harmonic generation from two-dimensional materials

2018-02-16T20:32:13+00:00 February 16th, 2018|Categories: Publications|Tags: |

The generation of high-order harmonics from atomic and molecular gases enables the production of high-energy photons and ultrashort isolated pulses. Obtaining efficiently similar photon energy from solid-state systems could lead, for instance, to more compact extreme ultraviolet and soft x-ray sources. We demonstrate from ab initio simulations that it is

Published in: "Science Advances".

Survival of spin state in magnetic porphyrins contacted by graphene nanoribbons

2018-02-16T20:32:11+00:00 February 16th, 2018|Categories: Publications|Tags: |

We report on the construction and magnetic characterization of a fully functional hybrid molecular system composed of a single magnetic porphyrin molecule bonded to graphene nanoribbons with atomically precise contacts. We use on-surface synthesis to direct the hybrid creation by combining two molecular precursors on a gold surface. High-resolution imaging

Published in: "Science Advances".

Double Indirect Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure. (arXiv:1802.05310v1 [cond-mat.mtrl-sci])

2018-02-16T19:59:19+00:00 February 16th, 2018|Categories: Publications|Tags: , , |

An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as the transition metal dichalcogenides (TMDs) to form van der Waals heterostructures. In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron-hole pairs with the electron in one TMD layer and the hole in an adjacent layer. We are able to clearly resolve two distinct emission peaks separated by 24 meV from an ILE in a MoSe2/WSe2 heterostructure fabricated using state-of-the-art preparation techniques. These peaks have nearly equal intensity, indicating they are of common character, and have opposite circular polarizations when excited with circularly polarized light. Ab initio calculations successfully account for these observations – they show that both emission features originate from excitonic transitions that are indirect in momentum space, are split by spin-orbit coupling, and that including interlayer hybridization is essential in correctly describing the ILE transition. Although well separated in momentum space, we find that in real space the electron has significant weight in both the MoSe2 and WSe2 layers, contrary to the commonly assumed model. This is a significant consideration for understanding the static and dynamic properties of TMD heterostructures.

Published in: "arXiv Material Science".

Non-perturbative vacuum polarization effects in planar QED with $ alpha_g sim 1$. (arXiv:1802.05336v1 [cond-mat.mes-hall])

2018-02-16T19:58:47+00:00 February 16th, 2018|Categories: Publications|Tags: , , |

The non-perturbative vacuum polarization effects are explored for a planar supercritical Dirac-Coulomb system with strong coupling (similar to graphene and graphene-based heterostructures) and an extended external Coulomb source. Both the induced charge density $rho_{VP}(vec{r})$ and the Casimir (vacuum) energy $mathcal{E}_{VP}$ are considered. The main result is that in the overcritical region $mathcal{E}_{VP}$ turns out to be negative and behaves like $- eta_{eff}, Z^3/R_0, $ with $eta_{eff}>0$ and $Z, , R_0$ being the charge and size of the external source. First we consider the evaluation and behavior of the induced density $rho_{VP}$, which further is used for calculation of the Casimir energy, with emphasis on the renormalization, convergence of the partial expansion for $rho_{VP}^{ren}$ and $mathcal{E}_{VP}^{ren}$ and behavior of the renormalized integral induced charge $Q_{VP}^{ren}$ and $mathcal{E}_{VP}^{ren}$ in the overcritical region. The dependence of the induced density and vacuum energy on the screening of the Coulomb asymptotics of the external field at some $R_1>R_0$ is also explored in detail.

Published : "arXiv Mesoscale and Nanoscale Physics".

Singlet and triplet trions in WS$_2$ monolayer encapsulated in hexagonal boron nitride. (arXiv:1802.05538v1 [cond-mat.mes-hall])

2018-02-16T19:58:46+00:00 February 16th, 2018|Categories: Publications|Tags: , |

Embedding a WS$_2$ monolayer in between flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negative trions in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of electron-electron exchange interaction for the trion triplet in a WS$_2$ monolayer, which involves two electrons with the same spin but from different valleys. Polarization resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observed even at ambient conditions whereas the emission due to triplet (intervalley) trions is only efficient at low temperatures.

Published : "arXiv Mesoscale and Nanoscale Physics".

Landau level diagram and the continuous rotational symmetry breaking in trilayer graphene. (arXiv:1802.05691v1 [cond-mat.mes-hall])

2018-02-16T19:58:45+00:00 February 16th, 2018|Categories: Publications|Tags: |

Sequence of the zeroth Landau levels (LLs) between filling factors ($nu$) -6 to 6 in ABA-stacked trilayer graphene (TLG) is ambiguous because it depends sensitively on the non-uniform charge distribution on the three layers of ABA-TLG. Using the sensitivity of quantum Hall (QH) data on the electric field (textit{E}$^perp$) and magnetic field (textit{B}), in an ultraclean ABA-stacked TLG sample, we quantitatively estimate the non-uniformity of the electric field and determine the sequence of the zeroth LLs. We also observe anticrossings between some LLs differing by 3 in LL index, which result from the breaking of the continuous rotational to textit{C}$_3$ symmetry by the trigonal warping. Finally, we report the observation of a few fractional quantum Hall (FQH) states at low magnetic field.

Published : "arXiv Mesoscale and Nanoscale Physics".

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