Tunable thermal conductivity of π-conjugated two-dimensional polymers

Two-dimensional (2D) polymers are organic analogues of graphene. Compared to graphene, 2D polymers offer a higher degree of tunability in regards to structure, topology, and physical properties. The thermal transport properties of 2D polymers play a crucial role in their applications, yet remain largely unexplored. Using the equilibrium molecular dynamics method, we study the in-plane thermal conductivity of dubbed porous graphene that is comprised of π-conjugated phenyl rings. In contrast to the conventional notion that π-conjugation leads to high thermal conductivity, we demonstrate, for the first time, that π-conjugated 2D polymers can have either high or low thermal conductivity depending on their porosity and structural orientation. The underlying mechanisms that govern thermal conductivity were illustrated through phonon dispersion. The ability to achieve two orders of magnitude variance in thermal conductivity by altering porosity opens up exciting opportunities to tune the thermal transport properties of 2D polymers for a diverse array of applications.

Interplay of Chemical and Electronic Structure on the Single-Molecule Level in 2D Polymerization

Single layers of covalently linked organic materials in the form of two-dimensional (2D) polymers constitute structures complementary to inorganic 2D materials. The electronic properties of 2D polymers may be manipulated through a deliberate choice of the organic precursors. Here we address the changes in electronic structure-from precursor molecule to oligomer-by scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy. For this purpose, we introduce the polymerization reaction of 1,3,5-tris(4-carboxyphenyl)benzene via decarboxylation on Cu(111), which is thoroughly characterized by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We present a comprehensive study of a contamination-free on-surface coupling scheme and study how dehydrogenation, decarboxylation, and polymerization affect the electronic structure on the molecular level.

Oxygen- and Lithium-Doped Hybrid Boron-Nitride/Carbon Networks for Hydrogen Storage

Hydrogen storage capacities have been studied on newly designed three-dimensional pillared boron nitride (PBN) and pillared graphene boron nitride (PGBN). We propose these novel materials based on the covalent connection of BNNTs and graphene sheets, which enhance the surface and free volume for storage within the nanomaterial and increase the gravimetric and volumetric hydrogen uptake capacities. Density functional theory and molecular dynamics simulations show that these lithium- and oxygen-doped pillared structures have improved gravimetric and volumetric hydrogen capacities at room temperature, with values on the order of 9.1-11.6 wt % and 40-60 g/L. Our findings demonstrate that the gravimetric uptake of oxygen- and lithium-doped PBN and PGBN has significantly enhanced the hydrogen sorption and desorption. Calculations for O-doped PGBN yield gravimetric hydrogen uptake capacities greater than 11.6 wt % at room temperature. This increased value is attributed to the pillared morphology, which improves the mechanical properties and increases porosity, as well as the high binding energy between oxygen and GBN. Our results suggest that hybrid carbon/BNNT nanostructures are an excellent candidate for hydrogen storage, owing to the combination of the electron mobility of graphene and the polarized nature of BN at heterojunctions, which enhances the uptake capacity, providing ample opportunities to further tune this hybrid material for efficient hydrogen storage.

Band-structure engineering in conjugated 2D polymers

The band structures of several conjugated 2D polymers are calculated through DFT and the influence of the polymer's repeat unit on its electronic structure is discussed.

How Much N-Doping Can Graphene Sustain?

Doped, substituted, or alloyed graphene is an attractive candidate for use as a tunable element of future nanomechanical and optoelectronic devices. Here we use the density functional theory, density functional tight binding, cluster expansion, and molecular dynamics to investigate the thermal stability and electronic properties of a binary 2D alloy of graphitic carbon and nitrogen (C(1-x)N(x)). The stability range naturally begins from graphene and must end before x = 1, where pure nitrogen rather forms molecular gas. This poses a compelling question of what highest x < 1 still permits stable 2D hexagonal lattice. Such upper limit on the nitrogen concentration that is achievable in a stable alloy can be found based on the phonon and molecular dynamics calculations. The stability switchover is predicted to between x = 1/3 (33.3%) and x = 3/8 (37.5%), and no stable hexagonal lattice two-dimensional CN alloys can exist at the N concentration of x = 3/8 (37.5%) and higher.

An accurate benchmark description of the interactions between carbon dioxide and polyheterocyclic aromatic compounds containing nitrogen

We assessed the performance of a large variety of modern density functional theory approaches for the adsorption of carbon dioxide on molecular models of pyridinic N-doped graphene.

N-substituted defective graphene sheets: promising electrode materials for Na-ion batteries

ViaDFT calculations, we theoretically demonstrated that the N doped defective structures are beneficial for Na adsorption and that the charge transfer can significantly influence the adsorption energies.