Cyclic Carbonate Formation from Epoxides and CO2Catalyzed by Sustainable Alkali Halide-Glycol Complexes: A DFT Study to Elucidate Reaction Mechanism and Catalytic Activity

We provide a comprehensive DFT investigation of the mechanistic details of CO2 fixation into styrene oxide to form styrene carbonate, catalyzed by potassium iodide-tetraethylene glycol complex. A detailed view on the intermediate steps of the overall reaction clarifies the role of hydroxyl substances as co-catalysts for the alkali halide-catalyzed cycloaddition. The increase of iodide nucleophilicity in presence of tetraethylene glycol is examined and rationalized by NBO and Hirshfeld charge analysis, and bond distances. We explore how different alkali metal salts and glycols affect the catalytic performance. Our results provide important hints on the synthesis of cyclic carbonates from CO2 and epoxides promoted by alkali halides and glycol complexes, allowing the development of more efficient catalysts

Photochemical CO2conversion on pristine and Mg-doped gallium nitride (GaN): A comprehensive DFT study based on a cluster model approach

The photochemical reduction of carbon dioxide (CO2) into methanol is very appealing since it requires sunlight as the only energy input. However, the development of highly selective and efficient photocatalysts is still very challenging. It has been reported that CO2 can be spontaneously activated on gallium nitride (GaN). Moreover, the photocatalytic activity for CO2 conversion into methanol can be drastically enhanced by incorporating a small amount of Mg dopant. In this work, density functional theory (DFT) based on a cluster model approach has been applied to further explore the photocatalytic activity of bare GaN towards CO2 adsorption and conversion. We extended the investigation of Mg-doping replacing one Ga atom with Mg on three different sites and evaluated the consequent effects on the band gaps and CO2 adsorption energies. Finally, we explore different routes leading to the production of methanol and evaluate the catalytic activity of bare GaN by applying the energetic span model (ESM) in order to identify the rate-determining states which are fundamental for suggesting modifications that can improve the photocatalytic activity of this promising material

Singular charge fluctuations at a magnetic quantum critical point

Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy–grown thin films of YbRh2Si2, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter.Financial support for this work was provided by the European Research Council (ERC Advanced Grant 227378), the U.S. Army Research Office (ARO W911NF-14-1-0496), the Austrian Science Fund (FWF W1243, P29279-N27, and P29296-N27), and the European Union’s Horizon 2020 research and innovation programme (grant agreement No 824109 – EMP). X.L. and J.K. acknowledge financial support from the National Science Foundation (NSF MRSEC DMR-1720595) and the ARO (W911NF-17-1-0259). Q.S. acknowledges financial support from the NSF (DMR-1920740), the Robert A.Welch Foundation (C-1411), and the ARO (W911NF-14-1-0525), and hospitality of the University of California at Berkeley, the Aspen Center for Physics (NSF grant PHY-1607611), and the Los Alamos National Laboratory (via a Ulam Scholarship from the Center for Nonlinear Studies). This work has also been supported by an InterDisciplinary Excellence Award (IDEA) from Rice University (Q.S., E.R., J.K., S.P.)

Singular charge fluctuations at a magnetic quantum critical point

Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy–grown thin films of YbRh₂Si₂, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter

Singular charge fluctuations at a magnetic quantum critical point

Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy–grown thin films of YbRh2Si2, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter

Microcavity-integrated graphene photodetector

There is an increasing interest in using graphene (1, 2) for optoelectronic applications. (3-19) However, because graphene is an inherently weak optical absorber (only ≈2.3% absorption), novel concepts need to be developed to increase the absorption and take full advantage of its unique optical properties. We demonstrate that by monolithically integrating graphene with a Fabry-Pérot microcavity, the optical absorption is 26-fold enhanced, reaching values >60%. We present a graphene-based microcavity photodetector with responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy

Microcavity-integrated graphene photodetector

The monolithic integration of novel nanomaterials with mature and established technologies has considerably widened the scope and potential of nanophotonics. For example, the integration of single semiconductor quantum dots into photonic crystals has enabled highly efficient single-photon sources. Recently, there has also been an increasing interest in using graphene - a single atomic layer of carbon - for optoelectronic devices. However, being an inherently weak optical absorber (only 2.3 % absorption), graphene has to be incorporated into a high-performance optical resonator or waveguide to increase the absorption and take full advantage of its unique optical properties. Here, we demonstrate that by monolithically integrating graphene with a Fabry-Perot microcavity, the optical absorption is 26-fold enhanced, reaching values >60 %. We present a graphene-based microcavity photodetector with record responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy.Comment: 19 pages, 4 figure

In-depth DFT insights into the crucial role of hydrogen bonding network in CO2 fixation into propylene oxide promoted by Biomass-Derived deep eutectic solvents

The CO2 fixation into epoxide to produce cyclic carbonate is a promising route to realize the large-scale utilization of CO2. In this work, a detailed DFT investigation has been carried out to elucidate the mechanistic details of the CO2 conversion into propylene carbonate (PC) catalyzed by bio-mass derived deep eutectic solvents (bio-DESs), obtained by combining hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) molecules. Our calculations support the known beneficial effect of hydrogen bonds in activating the epoxide. On the other hand, DFT results also show that hydrogen bonds and transfers do also stabilize the negative charge of determining intermediates, with the consequent undesired effect of reducing the nucleophilicity of the oxygen atom involved in the final ring closure step. Moreover, hydrogen bonds are also established between the HBDs and the PC product, thus hampering its release. Our theoretical results offer an in-depth understanding of the CO2 fixation involving bio-DESs, which can guide towards the development of more efficient bio-catalysts