Session Q7: Focus Session: Computational Design of Materials: Graphene - Strain, defect and interface engineering

Formation of hydrogenated graphene nanoripples by strain engineering and directed surface self-assembly

We propose a class of semiconducting graphene-based nanostructures: hydrogenated graphene nanoripples (HGNRs), based on continuum-mechanics analysis and first-principles calculations. They are formed via a two-step combinatorial approach: first by strain-engineered pattern formation of graphene nanoripples, followed by a curvature-directed self-assembly of H adsorption. It offers a high level of control of the structure and morphology of the HGNRs, and hence of their band gaps, which share common features with graphene nanoribbons. A cycle of H adsorption (desorption) at (from) the same surface locations completes a reversible metal-semiconductor-metal transition with the same band gap.   

Engineering electronic properties of armchair graphene nanoribbons using strain and functional species

First principles density-functional theory calculations were performed to study effects of strain, edge passivation, and surface functional species on structural and electronic properties. Particularly band gap and work function, of armchair graphene nanoribbons (AGNRs), are addressed. It was found that the band gap of the O-passivated AGNRs experiences a direct-to-indirect transition with sufficient tensile strain. The indirect band gap reduces to zero with further increased strain. The work function was found to increase with uniaxial tensile strain while decreasing with compression. The variation of the work function under strain is primarily due to the shift of the Fermi energy with strain. For AGNRs with edge carbon atoms passivated by oxygen, the work function is higher than that of nanoribbons with edge passivated by hydrogen under a moderate strain. The difference between work functions in these two edge passivations is enlarged (reduced) under a sufficient tensile (compressive) strain. Furthermore, the effect of surface species decoration, such as H, F, or OH with different covering density, was investigated. It was found the work function varies with the type and coverage of surface functional species. F and OH decoration increase the work function while H decreases it. The surface functional species were decorated on either one side or both sides of AGNRs. The difference in the work functions between one-side and two-side decorations was found to be relatively small, which may suggest the introduced surface dipole plays a minor role.   

Electronic Band Engineering of Epitaxial Graphene by Atomic Intercalation

Using calculations from first principles, we have investigated possible ways of engineering the electronic band structure of epitaxial graphene on SiC. In particular, intercalation of different atomic species, such as Hydrogen, Fluorine, Sodium, Germanium, Carbon and Silicon is shown to modify and tune the interface electronic properties and band alignments. Our results suggest that intercalation in graphene is quite different from that in graphite, and could provide a fundamentally new way to achieve electronic control in graphene electronics.   

Interplay of Defects, Magnetism, Ripples and Strain in Graphene

We present a comprehensive study based on first-principles calculations about the interplay of four important ingredients on the electronic structure of graphene: defects + magnetism + ripples + strain. So far they have not been taken into account simultaneously in a set of ab initio calculations. Furthermore, we focus on the strain dependence of the properties of carbon monovacancies, with special attention to magnetic spin moments. We demonstrated that such defects show a very rich structural and spin phase-diagram with many spin solutions as function of strain. At zero strain the vacancy shows a spin moment of 1.5 Bohrs that increases up to 2 Bohrs with stretching. Changes are more dramatic under compression: the vacancy becomes non-magnetic under a compression larger than 2{\%}. This transition is linked to the structural modifications associated with the formation of ripples in the graphene layer. Our results suggest that such interplay could have important implications for the design of future spintronics devices based on graphene derivatives, as for example a spin-strain switch based on vacancies.   

Spin switching in organic molecules by strain engineering in graphene

One of the primary objectives in molecular nano-spintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. Here, we demonstrate by first principles density functional calculations, as well as second order perturbation thoery, that a strain induced change of the spin state, from S=1 $\to$ S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in a sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP. We propose that the spin switching should be detected by x-ray magnetic circular dichroism experiments through the contributions from spin dipole and magnetic anisotropy.   

The magnetism in graphene under strain

We theoretically study the magnetic features in graphene dot under mechanical deformation using the mean field Hubbard model. The edge local magnetic moment (ELMM) is considerably modified in accordance with an effective quantum well originating from a strain-induced gauge field. Applying a uniaxial strain along the zigzag or armchair directions enhances or dampens the ELMM due to the development of the edge quantum wells. Whereas a circular arc bending deformation is applied, the inner and outer edge display ELMM caused by nonuniform gauge field, a direct consequence of the presence of the bulk localized states. These states suggest that an effective single well potential is introduced by a nonuniform pseudo-magnetic field.   

Vacancy-induced order-to-disorder transition in single-layer graphene

Defect engineering provides a potential route for tuning the mechanical, electronic, and chemical properties of graphene. While individual defects in single-layer graphene have been investigated in much detail, the outcomes of collective interactions of multiple defects remain elusive. In this work, we address the collective interaction of populations of vacancies in single-layer graphene using classical molecular-dynamics simulations based on reliable bond-order potentials; we examine random vacancy distributions with the vacancy concentration and temperature being the key parameters in the analysis. We demonstrate that a crystalline-to-amorphous structural transition occurs as the vacancy concentration in single-layer graphene increases beyond a critical level; the transition leads to complete loss of long-range order in the graphene layer. The onset of this order-to-disorder transition typically occurs over the vacancy concentration range from 10 to 20\% and is independent of the details of the interatomic interactions in the classical potentials employed. We present a systematic parametric study of the phenomenon and discuss the implications of our findings for the mechanical and electronic properties of single-layer graphene.   

Two magnetic impurities in graphene

We theoretically investigate two magnetic impurities in graphene. We mainly study the indirect interaction between the two magnetic impurities mediated by conducting electrons, which is so called RKKY interaction. The spin-spin and charge-charge correlation functions are calculated by quantum Monte Carlo simulations when the Fermi energy of the system is changed by gate voltage. The spectral density of the two impurities is also studied by maximum entropy methods.   

Power-Law Correlated Disorder in Graphene and Square Nanoribbons

Two dimensional metal-insulator transitions have remained an active topic in condensed matter physics do to the lack of a general model that can predict when an MIT occurs. With the creation of truly 2D crystals and nanostructures, the question has become increasingly relevant. Though initially predicted to not contain a MIT transition, the inclusion of electron-electron interactions and/or spatial disorder can drive a MIT in some 2D systems. In the case of graphene, correlated ripples are present even when the nanoribbons are freestanding and can have an effect on the transport properties while electron-electron interactions are normally considered negligible. To explore the effect of ripples, we model graphene with a long-range power-law spatial correlation of the form $\langle \epsilon_i^2 \rangle = 1/(1 + |\vec{r_i}/a|)^\alpha$ where $\epsilon_i$, $\vec{r}$, $a$, and $\alpha$ are the on-site energy, position, lattice constant, and strength of the correlation respectively. It should be noted that much work has been completed on short-range correlations but little on truly long-range correlations. We also present our finding for the square lattice for comparison.\\[4pt] [1] Abrahams E. Phys. Rev. Lett. 42, 673-676 (1979)\\[0pt] [2] Abrahams E. Ann. Phys. 8 (1999) 7-9, 539-548   

Thermally-driven Isotope Separation Across Nanoporous Graphene

Experiment and theory indicate that a single graphene sheet is impermeable to gases even as small as helium; pores are required for transmission of atoms and molecules. Nanoporous forms of graphene, such as two-dimensional polyphenylene (2D-PP), consist of a regular array of sub-nanometer pores which can be used for separating atoms and molecules by size. Because the nanoporous graphene barrier is only an atom-thick, quantum tunneling plays a role in the the transmission of atoms through the nanoporous barrier, even at room temperature. This talk describes how the mass-dependence of the tunneling, combined with a temperature gradient, can be used to separate isotope mixtures under conditions where classical transmission cannot. Using transition state theory, we show that the zero-point and tunneling contributions lead to isotopic separations in opposite directions with respect to the temperature gradient. We examine the separation of $^{3}$He/$^{4}$He across a 2D-PP membrane under modest temperature and pressure conditions. We will also describe 2D-PP bilayer structures that yield resonant tunneling of helium atoms, and new nanoporous graphene structures suitable for separating heavier noble-gas isotopes.   

Phonon instabilities in graphene

We study the thermal distribution of phonons in graphene and compare in-plane and out-of-plane contributions with a focus on two in-plane modes that represent intervalley scattering. Due to the two electronic bands there are also two out-of-plane phonon modes with respect to the two sublattices. The electron-phonon interaction softens the phonon modes with a tendency to create instabilities for a sufficiently large electron-phonon coupling. The instabilities are characterized by phase transitions, where one of the out-of-plane mode undergoes an Ising transition by spontaneously breaking the sublattice symmetry. The in-plane modes undergo a Berezinskii-Kosterlitz-Thouless transition. We calculate the critical points of the instabilities, the renormalization of the phonon frequencies and the phonon frequency splitting for in-plane modes. The possibility to observe these instabilities in doped graphene and their consequences for transport are discussed.   

Electronic Properties of Epitaxial Graphene on Si(111)-7$\times$7 and 3CS-SiC(100) Substrates

The synthesis of epitaxial graphene is an attractive method for industrial-scale fabrication and mass production of graphene-based electronic devices. Recently, efforts of epitaxial growth of graphitic carbon films on Si(111)-7$\times$7 and 3CS-SiC(100) substrates have been undertaken in order to explore new potential substrates compatible with conventional Si technology. In this talk, we will discuss the electronic properties of epitaxial graphene on Si (111)-7$\times$7 and 3CS-SiC(100) substrates using calculations from first principles based on Density Functional Theory. In particular, we found that a single graphene layer on Si(111)-7$\times$7 displays ripples of about 0.5 {\AA} and shows an n type electronic behavior. We also calculated the band structures for a graphene bilayer on 3CS-SiC(100) surface with different staking sequences. While AA stacking on Si face and turbostratic stacking on C face show n type behavior, turbostratic stacking on Si face and AA stacking on C face show p type behavior. On both faces, the first carbon plane is covalently bonded to the substrate and serves as a buffer layer similarly to the graphene/6H-SiC(0001) system previously studied.   

Graphene-nickel interface: hybridization and magnetization

The unique properties of graphene have opened up a new avenue for fundamental research as well as technological applications. Whereas the in-plane $sp^{2}$ bonding is primarily responsible for the overall structural stability and mechanical strength of graphene, the out-of-plane pp-$\pi$ states control its transport and interfacing properties. In this talk, we present a first principles study of the of single layer graphene/Ni(111) interface. We discuss how hybridization between the carbon pp-$\pi$ and nickel d orbitals modifies the electronic and magnetic properties of the interface. \\ \\ We acknowledge the computational support provided by the Center for Computational Research at the University at Buffalo, SUNY. This work is supported by the Department of Energy under Grant No. DE-SC0002623 and by the National Science Foundation under Grant No. DMR-0946404.   

Correlated magnetic states in domain and grain boundaries in graphene

Ab initio calculations indicate that while the electronic states introduced by grain boundaries in graphene are only partially confined to the defect core, a domain boundary introduces states near the Fermi level that are very strongly confined to the core of the defect, and that display a ferromagnetic ground state. The domain boundary is fully immersed within the graphene matrix, hence this magnetic state is protected from reconstruction effects that have hampered experimental detection in the case of ribbon edge states. Furthermore, our calculations suggest that charge transfer between one-dimensional extended defects and the bulk in graphene is short ranged for both grain and domain boundaries. http://arxiv.org/abs/1109.6923   

A Nanocluster Based Study of Silicon Carbide Nanocones: Existence and Stability

A systematic study of silicon carbide nanocones of different disclination angles and different tip geometries using the finite cluster approximation is presented. The geometries of the nanocones have been spin optimized using the hybrid functional B3LYP (Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all electron 3-21G* basis set. The study indicates that the binding energy per atom or the cohesive energy of the nanocones depends not only on the size of the nanocones but also on the disclination angle of the nanocones. The electronic properties of nanocones depend on disclination angles, size of the nanocone clusters and the edge structure of the nanocones. Given similar cluster size, silicon carbide appears to favor tubular structures over two dimensional graphene-like structures. For relatively smaller clusters the B.E./atom oscillates in all cases except in nanocones of disclination angle 300$^{\circ}$. This indicates the greater stability of nanocones of some particular size as compared with its neighboring sizes. A study of binding energies, NBO charge, density of states and HOMO-LUMO gaps has been performed for all nanocones from disclination angles of 60$^{\circ}$ to 300$^{\circ}$.