Session H12: Computational Materials Design - Carbon-Related Materials

Bubble-Wrap Carbon: An Integration of Graphene and Fullerenes

Graphene and fullerene are the two most prominent carbon allotropes. They all have unique structures and exhibit peculiar physical and chemical properties. It will be a great advantage to combine the geometry features of the two. Herein, we report a series of novel two-dimensional carbon allotropes that possess fullerene-like hollow structures (bubbles) embedded in a graphene sheet. These carbon allotropes are both thermally and dynamically stable. Calculations using hybrid functionals show that these two-dimensional carbon allotropes could be metals or semiconductors depending on the sizes and the patterns of the bubbles. The band gap can be as large as 1.66 eV. Due to the unique atomic configuration, some bubble-wrap carbons have unusual negative Poisson's ratios. The combination of graphene and fullerene provides an appealing approach to design carbon-based materials with dexterous properties. For example, the insertion of the metal atoms inside the bubbles may greatly enhance the functions of such materials in photovoltaic and catalysis.   

Design of in-plane graphene metasurfaces for generating strong mid-infrared circular dichroism

We show that graphene plasmonic metasurfaces can create strong circular dichroism (CD) in mid-infrared due to the nanostructure's geometrical chirality and the plasmonic coupling between the elements in a chiral unit. This is best demonstrated by the plasmonic CD yielded by graphene nanodisks aligned along a 3D helical line [1]. In contrast with the 3D nanodisks assemblies, which pose fabrication difficulty, in-plane graphene nanostructures are more favorable for the plasmonic CD generation. The interaction between the in-plane graphene nanoscale elements can be enhanced by placing them above a graphene sheet. In this way, the in-plane chiral structures (metasurfaces) can exhibit comparable differential absorption of the left and right circular polarizations of light to that of the graphene nanodisks assemblies. The strong plasmonic CD generated by the graphene nanostructures and metasurfaces provides us a possibility to enhance the vibrational optical activity of important biomolecules, such as DNA and peptides, whose vibrational modes overlap well with the frequency interval of graphene plasmons.

[1] Kong X.-T., Zhao R., Wang Z. & Govorov A. O. Nano Lett. 2017, 17, 5099   

Theoretical and Experimental Studies of Thermal Conductivity in Functionalized Carbon Nanotubes Optimized via Genetic Algorithms

The use of carbon nanotubes (CNTs) to improve the thermal conductivity of composite materials is thwarted by their large thermal boundary resistance. We study how to overcome this Kapitza resistance by functionalizing CNTs with mixed molecular chains. Certain configurations of chains improve the transmission of thermal vibrations through our systems by decreasing phonon mismatch between the CNTs and their surrounding matrix. We explore the space of possible designs using a genetic algorithm (GA) that evolves the molecular sidechain as its DNA and optimize the GA search procedure. We show how different configurations of attached chains affect the samples' $\kappa$ values by considering various permutations of over a dozen molecular units from our chemical library. We vary the composition to maximize $\kappa$. To validate and optimize these designs, we perform molecular dynamics simulations for comparison. We also present experimental results of composites enhanced with functionalized CNTs and make comparisons to the theory. We find that functionalization can substantially improve $\kappa$.   

Inhomogeneous Strain-induced Spin-splitting in Bent Zigzag Graphene Nanoribbons

Atomic Realization of half-metallicity in low dimensional materials is a fundamental challenge for nano spintronics, which is a critical component for next-generation information technology. Using the method of generalized Bloch theorem, we show that an in-plane bending can induce inhomogeneous strains, which in turn lead to spin-splitting in zigzag graphene nanoribbons and results in the highly desired half-metallic state. Unlike the previously proposed scheme that requires unrealistic strong external electric fields, the obtained half-metallicity with sizeable half-metallic gap and high energetic stability of magnetic order of edge states requires only relatively low-level strain in the in-plane bending. Given the superior structural flexibility of graphene and the recent experimental advances in controllable synthesis of graphene nanoribbons, our design provides a hitherto more practical approach to the realization of half-metallicity in low dimensional systems.
D. B. Zhang and S.-H. Wei, npj Computational Materials 3, 32 (2017).   

Prediction of a thermodynamically stable carbon-based clathrate

Carbon-based clathrates are predicted to exhibit a range of exceptional properties that are of fundamental and technological interest. Although several candidate structures have been proposed, the experimental synthesis of carbon clathrates remains unrealized due large formation energies, even under high-pressure conditions. Here, we predict, through first-principles calculations and unbiased automatic structure searching methods, a carbon-based clathrate in the bipartite sodalite structure, SrB₃C₃, that is thermodynamically stable at high pressure and dynamically stable at ambient pressure. Strikingly, SrB₃C₃ clathrate is predicted to be a superconductor with an estimated T_c of 42 K at ambient pressure. Calculated stress-strain relations for SrB₃C₃ clathrate demonstrate its intrinsic hard nature with Vickers hardness of 24-31 GPa. Our results reveal that boron substitution aids in the stabilization of SrB₃C₃ clathrate, and offers valuable insights into design guidelines for various carbon-based materials.   

Van der Waals Heterojunctions Containing Penta-graphene as Channel Material: Tuning the Schottky Barrier by Electrostatic Gating or Nitrogen Doping

The non-zero band gap together with other unique properties endows penta-graphene (PG) with potential for device applications. We have studied the performance of penta-graphene as the channel material contacting with graphene (G) and penta-BN₂ (P-BN₂)_, respectively, to form van der Waals heterostructures. Based on first-principles calculations, we found that the intrinsic properties of penta-graphene are preserved after stacking together, which is different from the conventional contact with metal surfaces. The stacked PG/G and PG/P-BN₂ heterostructures form n-type Schottky barriers at their vertical interfaces, while there is a negative band bending at their lateral interfaces between the contact regions and the freestanding PG, respectively. From the device point of view, we further demonstrated that an Ohmic contact can be realized by applying an external electric field or doping graphene with nitrogen atoms in PG/G system.

[1] Penta-graphene: A new carbon allotrope, S.H Zhang et al, Proc. Natl. Acad. Sci. 112, 2372-2337 (2015)
[2] An all-carbon vdW heterojunction composed of penta-graphene and graphene: tuning the Schottky barrier by electrostatic gating or nitrogen doping, Y.G Guo, et al. Appl. Phys. Lett. 111, 073503 (2017)   

Strain induced bandgap engineering on two-dimensional SiC/GeC in-plane heterostructures

The emergence of in-plane (monolayer stitched together seamlessly) heterostructures from transition-metal di-chalcogenides family (Nature Materials 13, 1135 (2014)) has opened up new realms in materials science, device physics, and engineering. Bandgap engineering by introducing strain is a promising method for tuning bandgaps and band alignments in designing such in-plane heterostructures. Our preliminary studies on the two-dimensional SiC/GeC in-plane heterostructures found that bandgaps of SiC and GeC sheets vary with the strain and a transition between direct and indirect bandgap occurs under certain strains. The interface effect, on the other hand, will also play an important role when SiC and GeC sheets are combined to form in-plane heterostructures and will be discussed in our presentation.   

Online Pattern Search Tools for Organic Materials Database

We present two newly developed online search tools for electronic structure calculations contained within the Organic Materials Database (OMDB) [http://omdb.diracmaterials.org]. The tools allow for automatic online analysis of a large collection of electronic structures where the amount of data makes its manual inspection impracticable. The first tool is capable of finding user-specified graphical patterns in the collection of thousands of band structures, which can be used to find realizations of functional materials characterized by a specific pattern in their electronic structure, for example, Dirac materials, characterized by a linear crossing of bands or topological insulators, characterized by a "Mexican hat" pattern. The second tool is designed to retrieve materials with similar density of states to a reference material. Using this tool we present search results for novel organic high-Tc superconductors. As a reference material, we use p-terphenyl, which was recently reported to exhibit transition temperatures in the order of 120 K.   

Structure and Property Prediction of Hybrid Inorganic-Organic Systems: Tetracene and Pentacene at H:Si(111)

Hybrid organic inorganic systems allow to combine the properties of organic and inorganic substances and hence open up a wide field for the construction of new materials with tunable properties. In particular the organic compound offers a large range of functionalities and can be designed to be environmentally benign. As a key example of aromatic molecule-interface systems, we here present a first-principle based structure search and electronic property investigation of pentacene and tetracene adsorbed at the hydrogenated Si(111) surface. The adsorption of single molecules is explored at different thin film coverage conditions with a structure search routine that includes grid- and random-structure search elements. This enables us to identify the most favorable orientation of the acene molecules and, using hybrid functionals, to investigate the level alignment between the organic and inorganic compounds with respect to the possibility of charge transfer: Single tetracene and pentacene molecules prefer to lie flat on the surface. For both organic molecules, the HOMO lies at the top of the valence band maximum allowing at best little charge transfer. For increasing coverage, we expect a transition to a dense geometry of standing molecules.   

Construction of Interatomic Potential for Fe–C Systems using Evolutionary Algorithm

Due to the complexity of Fe-C bonding structure in iron alloys, to date, potentials have limitation on describing correctly either stable position of carbon or the diffusion barrier/pathway of carbon in BCC iron. Here, we developed an interatomic potential for Fe-C based on the bond-order framework and tested on diffusion of carbon in BCC iron. We found that, carbon diffuses from octahedral (O) site to the nearest neighbor O-site via tetrahedral (T) site with an energy barrier of 0.74eV, in agreement with experiment (O-site to O-site via T-site with energy barrier of about 0.81-0.87eV). Other theoretical works found similar diffusion mechanism with energy barrier of about 0.72-0.92eV. With this, we verified the transferability of the newly developed potential in the study of the effect of carbon on the BCC/FCC transformation in iron following the Bain mechanism. We found that the presence of carbon leads to the decrease of the BCC/FCC transformation energy barrier, consistent with our own first-principles calculations. Thus, the new potential can be a good candidate for modeling transformation processes in Fe-C.   

Tunable Photonic Band Gaps in an Entropic Crystal

Materials adopting the diamond structure exhibit many useful properties across atomic and colloidal systems, and therefore have been a strong research focus in materials science. The desirable properties of the diamond structure pose an interesting opportunity for reconfigurable materials: can we create a material that naturally transitions reversibly to and from the diamond structure? Hypothetically, such a material could exhibit two states, an on state, in which it exhibits the striking properties tied to the diamond structure, and an off state, where one or more of these properties are no longer present. In this talk, we show that we can design a colloidal crystal from convex polyhedral particles with a complete photonic band gap in the on state, which disappears when the material is switched off by a shape- and pressure-induced phase transition.   

On the Origin of High Hydrogen Evolution Catalytic Activity and Stability in Cobalt-Embedded C2N

Transition metal-embedded two-dimensional materials have recently drawn significant research interest because they can serve as excellent catalysts in fuel cells, hydrogen production devices, etc. Especially, cobalt oxide encapsulated in C₂N (<a href="mailto:Co@C2N">Co@C₂N</a>) was recently shown to exhibit excellent catalytic performance for hydrogen evolution reaction (HER) together with high stability [1], but their precise understanding still remains elusive. Based on first-principles calculations, we herein scrutinize the mechanisms of enhanced HER performance of <a href="mailto:Co@C2N">Co@C₂N</a>, particularly focusing on the role of C₂N. We explore three plausible mechanisms that include (i) single-atom catalyst, (ii) quantum tunneling from substrate, and (iii) atomic-cluster catalyst. Among the three, we find that the atomic-cluster catalyst mechanism, in which few-atom Co cluster encapsulated within the holey region formed between two C₂N layers explains the experimentally-observed high HER activity. The atomic-cluster catalyst might prove to be a general mechanism for the TM-embedded two-dimensional catalysts.
[1] Mahmood, J. et al, Chem. Mater. 27, 4860 (2015)