2017 New's Items

Berkeley Lab Scientists Discover New Atomically Layered, Thin Magnet

April 26, 2017
Sarah Yang | Berkeley Lab

It may not seem like a material as thin as an atom could hide any surprises, but a research team led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) discovered an unexpected magnetic property in a two-dimensional material.

Postdoctoral researcher Cheng Gong (right) and senior faculty scientist Xiang...

https://newscenter.lbl.gov/2017/04/26/scientists-discover-atomically-layered-thi…

Scientists Pull Water Out of Thin Air

April 13, 2017
Sarah Yang | Berkeley Lab

Taking a page out of science fiction playbooks, scientists have demonstrated breakthrough technology capable of generating water out of dry air using the power of the sun.

Omar Yaghi, pictured with models of metal-organic frameworks, porous crystalline sponges into which water in dry air can be trapped, concentrated, and condensed out to deliver clean water....

https://newscenter.lbl.gov/2017/04/13/scientists-make-water-from-air/

Small-Molecule Adsorption in Open-Site Metal-Organic Frameworks: A Systematic Density Functional Theory for Rational Design

K. Lee
J. D. Howe
L. C. Lin
B. Smit
J. B. Neaton
2017

Using density functional theory, we systematically compute and investigate the binding enthalpies of 14 different small molecules in a series of isostructural metal–organic frameworks, M-MOF-74, with M = Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. The small molecules we consider include major flue-gas components, trace gases, and small hydrocarbons, i.e., H2, CO, CO2, H2O, H2S, N2, NH3, SO2, CH...

Reticular Electronic Tuning of Porphyrin Active Sites in Covalent Organic Frameworks for Electrocatalytic Carbon Dioxide Reduction

C. S. Diercks
S. Lin
N. Kornienko
E. A. Kapustin
E. M. Nichols
C. Zhu
Y. Zhao
C. J. Chang
O. M. Yaghi
2017

The electronic character of porphyrin active sites for electro- catalytic reduction of CO2 to CO in a two-dimensional covalent organic framework (COF) was tuned by modification of the reticular structure. Efficient charge transport along the COF backbone promotes electronic connectivity between remote functional groups and the active sites and enables the modulation of the catalytic properties of the system. A series of oriented thinfilms of these COFs was found to reduce CO...

Origins of Singlet Fission in Solid Pentacene from an ab initio Green’s Function Approach

S. Refaely-Abramson
F. H. da Jornada
S. G. Louie
J. B. Neaton
2017

We develop a new first-principles approach to predict and understand rates of singlet fission with an ab initio Green’s-function formalism based on many-body perturbation theory. Starting with singlet and triplet excitons computed from a GW plus Bethe-Salpeter equation approach, we calculate the exciton-biexciton coupling to lowest order in the Coulomb interaction, assuming a final state consisting of two noninteracting spin-correlated...

Prediction of TiRhAs as a Dirac Nodal Line Semimetal via First-Principles Calculations

S. F. Weber
R. Chen
Q. Yan
J. B. Neaton
2017

Using first-principles calculations, we predict that TiRhAs, a previously synthesized compound, is a Dirac nodal line (DNL) semimetal. The DNL in this compound is found to be protected both by the combination of inversion and time-reversal symmetry, and by a reflection symmetry, in the absence of spin-orbit coupling (SOC). Our calculations show that band velocities associated with the nodal line have a high degree of directional anisotropy, with in-plane velocities ...

Local Electronic Structure of a Single-Layer Porphyrin-Containing Covalent Organic Framework

C. Chen
T. Joshi
H. Li
A. D. Chavez
Z. Pedramrazi
P-N. Liu
H. Li
W. R. Dichtel
J-L. Bredas
M. F. Crommie
2017

We have characterized the local electronic structure of a porphyrin-containing single-layer covalent organic framework (COF) exhibiting a square lattice. The COF monolayer was obtained by the deposition of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DMA) and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) onto a Au(111) surface in ultrahigh vacuum followed by annealing to facilitate Schiff-base condensations between monomers. Scanning tunneling spectroscopy (STS) experiments conducted on isolated TAPP precursor molecules and the covalently linked COF networks yield similar...

Orbitally Matched Edge-Doping in Graphene Nanoribbons

R. Durr
D. Haberer
Y-L. Lee
R. Blackwell
A. Kalayjian
T. Marangoni
J. Ihm
S. Louie
F. R. Fischer
2017

A series of trigonal planar N-, O-, and S-dopant atoms incorporated along the convex protrusion lining the edges of bottom-up synthesized chevron graphene nanoribbons (cGNRs) induce a characteristic shift in the energy of conduction and valence band edge states along with a significant reduction of the band gap of up to 0.3 eV per dopant atom per monomer. A combination of scanning probe spectroscopy and density functional theory calculations reveals that the direction and the magnitude of charge transfer between the dopant atoms and the cGNR backbone are dominated by inductive...

Development and Elucidation of Superior Turnover Rates and Selectivity of Supported Molecular Catalysts

R. Ye
W-C. Liu
H-L. Han
G. A. Somorjai
2017

Supported molecular catalysts consist of nanomaterials immobilized on a solid support. Important factors that control catalyst properties (reactivity, product selectivity, and stability) include the structure and compositions of both nanomaterials and supports. This review focuses on recent studies of supported molecular catalysts with controlled activity and selectivity in our group. We will first introduce the development of previously unexplored supported molecular catalysts. We will demonstrate the controllable selectivity of catalysts based on acidified mesoporous silica, metal‐...

Tunable Excitons in Bilayer Graphene

L. Ju
L. Wang
T. Cao
T. Taniguchi
K. Watanabe
S. G. Louie
F. Rana
J. Park
J. Hone
F. Wang
P. L. McEuen
2017

Excitons, the bound states of an electron and a hole in a solid material, play a key role in the optical properties of insulators and semiconductors. Here, we report the observation of excitons in bilayer graphene (BLG) using photocurrent spectroscopy of high-quality BLG encapsulated in hexagonal boron nitride. We observed two prominent excitonic resonances with narrow line widths that are tunable from the mid-infrared to the terahertz range. These excitons obey optical selection rules distinct from those in conventional semiconductors and feature an electron pseudospin winding...