Hong Jiang

Localized and itinerant states in lanthanide oxides united by GW @ LDA + U

Many-body perturbation theory in the GW approach is applied to lanthanide oxides, using the local-density approximation plus a Hubbard U correction (LDA+U) as the starting point. Good agreement between the G0W0 density of states and experimental spectra is observed for CeO2 and Ce2O3. Unlike the LDA+U method G0W0 exhibits only a weak dependence on U in a physically meaningful range of U values. For the whole lanthanide sesquioxide (Ln2O3) series G0W0 @ LDA+U reproduces the main features found for the optical experimental band gaps. The relative positions of the occupied and unoccupied f states predicted by G0W0 confirm the experimental conjecture derived from phenomenological arguments.

Structural and electronic properties of ZrX2 and HfX2 (X = S and Se) from first principles calculations

Early transition metal dichalcogenides (TMDC), characterized by their quasi-two-dimensional layered structure, have attracted intensive interest due to their versatile chemical and physical properties, but a comprehensive understanding of their structural and electronic properties from a first-principles point of view is still lacking. In this work, four simple TMDC materials, MX(2) (M = Zr and Hf, X = S and Se), are investigated by the Kohn-Sham density functional theory (KS-DFT) with different local or semilocal exchange-correlation (xc) functionals and many-body perturbation theory in the GW approximation. Although the widely used Perdew-Burke-Ernzelhof (PBE) generalized gradient approximation (GGA) xc functional overestimates the interlayer distance dramatically, two newly developed GGA functionals, PBE-for-solids (PBEsol) and Wu-Cohen 2006 (WC06), can reproduce experimental crystal structures of these TMDC materials very well. The GW method, currently the most accurate first-principles approach for electronic band structures of extended systems, gives the fundamental band gaps of all these materials in good agreement with the experimental values obtained from optical absorption. The minimal direct gaps from GW are systematically larger than those measured from thermoreflectance by about 0.1-0.3 eV, implying that excitonic effects may be stronger than previously estimated. The calculated density of states from GW quasi-particle band energies agrees very well with photo-emission spectroscopy data. Ionization potentials of these materials are also computed by combining PBE calculations based on the slab model and GW quasi-particle corrections. The calculated absolute band energies with respect to the vacuum level indicate that that ZrS(2) and HfS(2), although having suitable band gaps for visible light absorption, cannot be used for overall water splitting as a result of mismatch of the conduction band minimum with the redox potential of H(+)/H(2).

Impact of widely used approximations to the G0W0 method: an all-electron perspective

Focusing on the fundamental band gaps in Si, diamond, BN, LiF, AlP, NaCl, CaSe and GaAs, and the semicore d-state binding energies in ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe and GaN, we study the differences between the all-electron (AE) and the pseudopotential (PP)-based G0W0 method. Leaving aside issues related to the choice of PPs within PP-G0W0, we analyze in detail the well-known discrepancies between AE-G0W0 and PP-G0W0 band gaps by separately addressing the approximations underlying PP-G0W0, i.e. the frozen-core approximation, the core-valence partitioning and the use of pseudo-wavefunctions. The largest differences, of the order of eV, appear in the exchange part of the self-energy and the exchange-correlation potential due to the core-valence partitioning. These differences cancel each other and, in doing so, make the final core-valence partitioning effect on the band gaps controllable when the semicore states are treated as valence states. This cancelation, however, is incomplete for semicore d-state binding energies, due to the strong interaction between these semicore states and the deep core. From our comprehensive analysis, we conclude that reliably describing the many-body interactions at the G0W0 level and providing benchmark results require an AE treatment.

A Single Crystal with Multiple Functions of Optical Waveguide, Aggregation-Induced Emission, and Mechanochromism

A novel single crystal, PyB, is produced in a high yield by the simple method of connecting a pyrene unit and a rhodamine B moiety together. PyB shows multiple functions of aggregation-induced emission, low-loss optical waveguiding, and tricolored mechanochromism. The crucial point for fabricating such a multifunctional single crystal is selecting the C═N group as a spacer, which simplifies the synthetic procedure, confines the molecular conformation to develop single crystals, and allows one to dynamically observe the color variation in situ and quantitatively analyze the effect of applied pressures. Such a simple approach may be extended to other fluorophores, thus providing a new opportunity for the real world application of mechanochromic materials for mechanical sensors, optical encoding, and optoelectronic devices, etc.

FHI-gap: A GW code based on the all-electron augmented plane wave method

Abstract The G W method has become the state-of-the-art approach for the first-principles description of the electronic quasi-particle band structure in crystalline solids. Most of the existing codes rely on pseudopotentials in which only valence electrons are treated explicitly. The pseudopotential method can be problematic for systems with localized d - or f -electrons, even for ground-state density-functional theory (DFT) calculations. The situation can become more severe in G W calculations, because pseudo-wavefunctions are used in the computation of the self-energy and the core–valence interaction is approximated at the DFT level. In this work, we present the package FHI-gap , an all-electron G W implementation based on the full-potential linearized augmented planewave plus local orbital (LAPW) method. The FHI-gap xa0code can handle core, semicore, and valence states on the same footing, which allows for a correct treatment of core–valence interaction. Moreover, it does not rely on any pseudopotential or frozen-core approximation. It is, therefore, able to handle a wide range of materials, irrespective of their composition. Test calculations demonstrate the convergence behavior of the results with respect to various cut-off parameters. These include the size of the basis set that is used to expand the products of Kohn–Sham wavefunctions, the number of k points for the Brillouin zone integration, the number of frequency points for the integration over the imaginary axis, and the number of unoccupied states. At present, FHI-gap xa0is linked to the WIEN2k code, and an implementation into the exciting xa0code is in progress.

Hydrothermally stable Ru/HZSM-5-catalyzed selective hydrogenolysis of lignin-derived substituted phenols to bio-arenes in water

In the context of arenes generated from bioresources, a hydrothermally stable catalyst Ru/HZSM-5 is reported for the quantitative conversion of lignin-derived phenol, anisole, guaiacols, and syringols into bioaromatic hydrocarbons in a one-pot aqueous-phase process in moderate conditions (240 °C, 2 bar H2), with a high carbon balance of 98.6%. The selective hydrodeoxygenation reaction pathway from guaiacol proceeded via an initial hydrogenolysis route to phenol with a rate of 33 mmol g−1 h−1, and subsequently phenol was directly hydrogenolyzed to benzene at a rate of 10 mmol g−1 h−1 over Ru/HZSM-5 in selected conditions. Moreover, routes via the hydrogenation of guaiacol, as well as the hydrogenolysis of catechol and anisole, have been rationally excluded via comparative kinetic studies of the individual steps of the hydrodeoxygenation of catechol, anisole, and phenol over Ru/HZSM-5 in identical conditions. The selective cleavage of guaiacol at the C–O position of the Csp3–OAr group is influenced by space hindrance and the environment of the reactant (gas phase, liquid phase, or acidic aqueous phase). In the condensed liquid phase and gas phase, catechol rather than phenol is formed as the major product from guaiacol over Ru/HZSM-5. However, in the highly diluted aqueous phase, phenol alone (without detected catechol) is formed as the dominant product. In addition, the pathway for guaiacol conversion is sensitive to the hydrogen pressure and temperature, which demonstrates that a relatively high temperature and a low hydrogen pressure are crucial for manipulating the coupled tandem hydrogenolysis routes. Among various oxide-loaded Ru catalysts, Ru/HZSM-5 has been confirmed to be the most efficient catalyst for the target selective hydrodeoxygenation of guaiacol, and the metal sites and support were found to be highly stable for four successive catalytic runs in the near-critical aqueous phase that was used.

Systematic investigation on topological properties of layered GaS and GaSe under strain

The topological properties of layered β-GaS and ε-GaSe under strain are systematically investigated by ab initio calculations with the electronic exchange-correlation interactions treated beyond the generalized gradient approximation (GGA). Based on the GW method and the Tran-Blaha modified Becke-Johnson potential approach, we find that while ε-GaSe can be strain-engineered to become a topological insulator, β-GaS remains a trivial one even under strong strain, which is different from the prediction based on GGA. The reliability of the fixed volume assumption rooted in nearly all the previous calculations is discussed. By comparing to strain calculations with optimized inter-layer distance, we find that the fixed volume assumption is qualitatively valid for β-GaS and ε-GaSe, but there are quantitative differences between the results from the fixed volume treatment and those from more realistic treatments. This work indicates that it is risky to use theoretical approaches like GGA that suffer from the band gap problem to address physical properties, including, in particular, the topological nature of band structures, for which the band gap plays a crucial role. In the latter case, careful calibration against more reliable methods like the GW approach is strongly recommended.

Theoretical studies of the work functions of Pd-based bimetallic surfaces.

Work functions of Pd-based bimetallic surfaces, including mainly M/Pd(111), Pd/M, and Pd/M/Pd(111) (M = 4d transition metals, Cu, Au, and Pt), are studied using density functional theory. We find that the work function of these bimetallic surfaces is significantly different from that of parent metals. Careful analysis based on Bader charges and electron density difference indicates that the variation of the work function in bimetallic surfaces can be mainly attributed to two factors: (1) charge transfer between the two different metals as a result of their different intrinsic electronegativity, and (2) the charge redistribution induced by chemical bonding between the top two layers. The first factor can be related to the contact potential, i.e., the work function difference between two metals in direct contact, and the second factor can be well characterized by the change in the charge spilling out into vacuum. We also find that the variation in the work functions of Pd/M/Pd(111) surfaces correlates very well with the variation of the d-band center of the surface Pd atom. The findings in this work can be used to provide general guidelines to design new bimetallic surfaces with desired electronic properties.

Ab initio molecular dynamics with enhanced sampling for surface reaction kinetics at finite temperatures: CH2⇌ CH + H on Ni(111) as a case study

A comprehensive understanding of surface thermodynamics and kinetics based on first-principles approaches is crucial for rational design of novel heterogeneous catalysts, and requires combining accurate electronic structure theory and statistical mechanics modeling. In this work, ab initio molecular dynamics (AIMD) combined with the integrated tempering sampling (ITS) method has been explored to study thermodynamic and kinetic properties of elementary processes on surfaces, using a simple reaction CH2⇌CH+H on the Ni(111) surface as an example. By a careful comparison between the results from ITS-AIMD simulation and those evaluated in terms of the harmonic oscillator (HO) approximation, it is found that the reaction free energy and entropy from the HO approximation are qualitatively consistent with the results from ITS-AIMD simulation, but there are also quantitatively significant discrepancies. In particular, the HO model misses the entropy effects related to the existence of multiple adsorption configurations arising from the frustrated translation and rotation motion of adsorbed species, which are different in the reactant and product states. The rate constants are evaluated from two ITS-enhanced approaches, one using the transition state theory (TST) formulated in terms of the potential of mean force (PMF) and the other one combining ITS with the transition path sampling (TPS) technique, and are further compared to those based on harmonic TST. It is found that the rate constants from the PMF-based TST are significantly smaller than those from the harmonic TST, and that the results from PMF-TST and ITS-TPS are in a surprisingly good agreement. These findings indicate that the basic assumptions of transition state theory are valid in such elementary surface reactions, but the consideration of statistical averaging of all important adsorption configurations and reaction pathways, which are missing in the harmonic TST, are critical for accurate description of thermodynamic and kinetic properties of surface processes. This work clearly demonstrates the importance of considering temperature effects beyond the HO model, for which the AIMD simulation in combination with enhanced sampling techniques like ITS provides a feasible and general approach.

Mechanical activation of a dithioester derivative-based retro RAFT-HDA reaction

A new mechanophore was synthesized from a dithioester derivative 1 and an open-chain diene 2 by the HDA reaction. It was embedded in the polymer chain by initiating the polymerization of methyl acrylate (MA). It was found that the PMA with Mn > 30 kDa could undergo a mechanically induced retro RAFT-HDA reaction rapidly at ambient temperature. The released species was evidenced chemically and spectroscopically, and the liberated dithioester derivative 1 maintained the activity to react with cyclopentadiene (Cp) or to copolymerize with styrene (St).