Graphene‐like Molecules with Four Zigzag Edges

In this thesis, I will mainly focus on using Raman spectroscopy and imaging to probe the electronic and crystal structures of graphene that modified both chemically and physically. This thesis is divided into five parts. The first part covers Chapter 1. It reviews the progress on modification of graphene's electronic structure, and introduces the relationship between Raman features and graphene's properties. For modification of graphene's electronic structure, I review some important and common methods, which include applying external electric and magnetic field, changing stacking status, defects, and strain. The second part (Chapter 2) introduces the investigation of graphene edge thermal dynamics. As controlling edge chirality of graphene nanoribbon can tune its electronic structure, and thus it is of great importance to understand the thermal annealing effect on graphene edges as it is one of the inevitable processes for nano device fabrication. In my studies, the armchair edges present much better thermal stability compared to zigzag edges, while the zigzag edges are not thermal stable as revealed by the activation of Raman D band after annealing treatment. The edge atomic structure evolution under annealed at different temperature (from 200 to 500°C) is also studied in detail by comparing the polarized Raman intensity ratio ofD and G bands. In the third part (Chapter 3), the edge dynamics ofbilayer graphene is presented. Motivated by one of the most influentially reported result that the AB-stacked bulk graphite is structurally modified to many AA-stacked bilayer graphene with closed edges at both armchair and zigzag edges after anneal at extremely high temperature (2000 °C) (Phys. Rev. Lett. 102, 015501, 2009), I carried out the work for low temperature edge dynamics study. Based on the conclusions I obtained from single layer graphene edge thermal dynamics, it is indirectly discovered by Raman image and spectroscopy that the zigzag-edge of bilayer graphene is preferred to form closed state when it anneals at low temperature. The formation of closed zigzag edges of bilayer graphene is also supported by the dramatically decreased system energy as revealed by DFT calculation. The armchair edges of bilayer graphene cannot form closed state because of geometrical incompatibility. Hence the thermal dynamic behaviour of the armchair edges of bilayer graphene can be considered as two independent armchair edges oftwo layers. The fourth part (Chapter 4 and 5) introduces the Raman studies ofFeCh-based full intercalation few-layer graphene compounds. Both evolutions of G and 2D bands are discussed in details for identification ofthe modified few-layer graphene crystal structure. The electronic properties of such compound are discussed by its 2D band features. The theoretical calculation results are used to assist the explanation of its Raman features and electronic properties. Followed by the preliminary characterization, various laser energies as excitation source to probe the Raman scattering features for this compound, and the laser dependent G band intensity for graphene layers doped in different level is observed. The mechanism for the enhancement of Raman G band for some specific graphene layers is also revealed theoretically. The fifth part (Chapter 6) introduces another kind of FeCh-based trilayer graphene intercalation compounds. In this type compound, only ·one interlayer space of the trilayer graphene is intercalated and the other interlayer space keeps unchanged. The intercalated FeCh separates the pristine trilayer graphene into two independent parts, which are single layer graphene and bilayer graphene, respectively. This kind of abnormal compound presents obvious different Raman features compared to the full intercalation case. The electronic band structure of this new type intercalation compounds also exhibits exotic property for its opened bandgap arising from inversion symmetry breaking of the separated bilayer graphene.

Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic π-conjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Hückel's rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Möbius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Hückel's rule. When the molecules are excited into the first triplet excited state, the Hückel (anti)aromaticity observed in the ground state will become reversed according to Baird's rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar's aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Hückel's rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Hückel's rule and Baird's rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will mainly discuss their synthesis, global aromaticity, and the fundamental structure-radical character-aromaticity-properties relationships. Various experimental methods (e.g., NMR, X-ray crystallographic analysis, and electronic absorption spectroscopy) and theoretical calculations (e.g., anisotropy of the induced current density, nucleus independent chemical shift, and isochemical shielding surface) have been used to elaborate their (anti)aromatic character. At the end, a perspective on the possible three-dimensional global aromaticity in fully conjugated cagelike diradicaloids or polyradicaloids will be also discussed.

Electronic and magnetic properties of ribbon-shaped nanographite systems with zigzag and armchair edges in a magnetic field are investigated by using a tight-binding model. One of the most remarkable features of these systems is the appearance of edge states, strongly localized near zigzag edges. The edge state in a magnetic field, generating a rational fraction of the magnetic flux $(\ensuremath{\varphi}=p/q)$ in each hexagonal plaquette of the graphite plane, behaves like a zero-field edge state with q internal degrees of freedom. The orbital diamagnetic susceptibility strongly depends on the edge shapes. The reason is found in the analysis of the ring currents, which are very sensitive to the lattice topology near the edge. Moreover, the orbital diamagnetic susceptibility is scaled as a function of the temperature, Fermi energy, and ribbon width. Because the edge states lead to a sharp peak in the density of states at the Fermi level, the graphite ribbons with zigzag edges show Curie-like temperature dependence of the Pauli paramagnetic susceptibility. Hence, there is a crossover from high-temperature diamagnetic to low-temperature paramagnetic behavior in the magnetic susceptibility of nanographite ribbons with zigzag edges.

The structures, stabilities, electronic and magnetic properties of fully and partially hydrogenated germanene nanoribbons: A first-principles investigation

The electronic structure of nanographene having open edges around its circumference crucially depends on its edge shape. The circumference of an arbitrary shaped nanographene sheet is described in terms of a combination of zigzag and armchair edges. According to theoretical suggestions, nanographene has a nonbonding π-electron state (edge state) localized in zigzag edges. This is reminiscent of the nonbonding π-electron state appearing in non-Kekulé-type aromatic molecules. The localized spins of the edge states can give rises to unconventional magnetism in nanographene, such as carbon-only ferromagnetism, magnetic switching phenomenon, spin glass state, etc. STM/STS investigations of well defined graphene edges which are hydrogen terminated in ultra-high vacuum condition confirm the presence of edge states around zigzag edges, in good agreement with theoretical works. The feature of the edge state depends on the detailed edge shape. The edge state in a short zigzag edge embedded between armchair edges becomes less localized due to the state mixing with the adjacent armchair edges. The electrons in the edge state in a finite-length zigzag edge are subjected to electron confinement effect.

We report on microscopic measurements of the low-energy electronic structures both at zigzag and armchair edges of bilayer graphene using scanning tunneling microscopy and spectroscopy (STM and STS). We have found that, both in the absence and in the presence of a magnetic field, an almost zero-energy peak in density of states was localized at zigzag edges, as expected for the surface states at zigzag edges of bilayer graphene. In the quantum Hall regime, we have observed clearly Landau levels bending away from the charge neutrality point near both the zigzag and armchair edges. Such a result is a direct evidence for the evolution of Landau levels into the quantum Hall edge states in graphene bilayers. Our experiment indicates that it is possible to explore rich quantum Hall physics in graphene systems using STM and STS.

By using density functional theory (DFT)-based first-principles calculations, the structural stability and electronic properties for two kinds of silicene domain boundaries, forming along armchair edge and zigzag edge, have been investigated. The results indicate that a linkage of tetragonal and octagonal rings (4|8) appears along the armchair edge, while a linkage of paired pentagonal and octagonal rings (5|5|8) appears along the zigzag edge. Different from graphene, the buckling properties of silicene lead to two mirror symmetrical edges of silicene line-defect. The formation energies indicate that the 5|5|8 domain boundary is more stable than the 4|8 domain boundary. Similar to graphene, the calculated electronic properties show that the 5|5|8 domain boundaries exhibit metallic properties and the 4|8 domain boundaries are half-metal. Both domain boundaries create the perfect one-dimensional (1D) metallic wires. Due to the metallic properties, these two kinds of nanowires can be used to build the silicene-based devices.

Standard spin-polarized density functional theory calculations have been conducted to study the electronic structures and magnetic properties of O and S functionalized zigzag boron nitride nanoribbons (zBNNRs). Unlike the semiconducting and nonmagnetic H edge-terminated zBNNRs, the O edge-terminated zBNNRs have two energetically degenerate magnetic ground states with a ferrimagnetic character on the B edge, both of which are metallic. In contrast, the S edge-terminated zBNNRs are nonmagnetic albeit still metallic. An intriguing coexistence of two different Peierls-like distortions is observed for S edge-termination that manifests as a strong S dimerization at the B zigzag edge and a weak S trimerization at the N zigzag edge, dictated by the band fillings at the vicinity of the Fermi level. Nevertheless, metallicity is retained along the S wire on the N edge due to the partial filling of the band derived from the p(z) orbital of S. A second type of functionalization with O or S atoms embedded in the center of zBNNRs yields semiconducting features. Detailed examination of both types of functionalized zBNNRs reveals that the p orbitals on O or S play a crucial role in mediating the electronic structures of the ribbons. We suggest that O and S functionalization of zBNNRs may open new routes toward practical electronic devices based on boron nitride materials.

By using the first-principles calculations, the electronic properties are studied for the F-terminated SiC nanoribbons (SiCNRs) with either zigzag edges (ZSiCNRs) or armchair edges (ASiCNRs). The results show that the broader F-terminated ZSiCNRs are metallic and the edge states appear at the Fermi level, while the F-terminated ASiCNRs are always semiconductors independent of their width but the edge states do not appear due to the Si-C dimer bonds at the edges. The charge density contours analyses shows that the Si-F and Si-C bonds are all ionic bonds due to the much stronger electronegativities of the F and C atoms than that of the Si atom. However, the C-F bonds display a typical non-polar covalent bonding feature because of the electronegativity difference between the F and C atoms of 1.5 is a much smaller than that of between the F and Si atoms of 2.2, as well as the tighter bounded C 2s 22p 2 electrons with smaller orbital radius than the Si 3s 23p 2 electrons. For both the F- and the H-terminated ZSiCNRs, the ground state is a ferromagnetic semiconductor.

We report a spin-polarized density-functional theory study of electric-field effects on the electronic and spin properties of graphene nanodots. Both undoped and graphene nanodots doped with nitrogen and boron are considered. In the presence of nonlocal exchange-correlation interactions, undoped graphene nanodots are found to be half-semiconductors when a weak electric field is applied across the zigzag edge. At high electric fields these graphene nanodots become nonmagnetic semiconductors. When the electric field is applied across the armchair edge, these graphene nanodots maintain an antiferromagnetic ground state with the energy gap strongly dependent on the magnitude of the electric field. For graphene nanodots doped with nitrogen or boron we find that energetically the most favorable state among all possible configurations is the one in which the dopant replaces the carbon atom at the center of the zigzag edge. The substitutional dopant atom at the zigzag edge leads to a spin-polarized half-semiconducting state in which the spin degeneracy is broken. The spin-dependent energy gaps can be tuned within a wide range by applying electric fields. In addition, we find the half-semiconducting state under doping occurs even when the electric field is very strong. This indicates that edge doping can significantly widen the operating range of applied electric fields for spintronic applications because undoped graphene nanodots become spinless semiconductors under certain applied electric fields.

The electronic states of graphene near the Fermi energy are well described by massless Dirac Fermion. The presence of edges, however, makes strong implications for the spectrum of the electrons. In graphene nanoribbons with zigzag edges, localized states appear at the edge with energies close to the Fermi level. In contrast, edge states are absent for ribbons with armchair edges. In my talk, we focus on edge and nanoscale effect on the electronic properties of graphene nanoribbons. We discuss the following aspects of graphene nanostructured systems. (1) In zigzag nanoribbons, for nonmagnetic long-ranged disorder, a single perfectly conducting channel emerges associated with a chiral mode due to the edge state, i.e., the absence of the localization in this class. (2) We show the electronic transport properties of graphene nanojunctions crucially depend on the peripheral lattice structures. The condition for electron confinement is discussed. (3) We will discuss the effect of edge chemical modification on magnetic properties of nanographene systems. Also, we discuss the hole doping effect on the spin-polarized states appearing along the graphene zigzag edges. Our studies reveal that the peculiar electronic, magnetic and transport properties of graphene nanostructured systems. In addition, we present our recent work on graphene double layer structure (GDLS), where two graphene layers are separated by a thin dielectric. We will discuss the dielectric environment effect on the charged-impurity-limited carrier mobility of the GDLS on the basis of the Boltzmann transport theory. It is found that carrier mobility strongly depends on the dielectric constant of the barrier layer if the interlayer distance becomes larger than the inverse of the Fermi wave vector. Our results suggest effective use of ultra-thin dielectric barriers and a practical design strategy to improve the charged-impurity-limited mobility of the GDLS.

We study hexagonal graphene quantum dots, using density functional theory, to obtain a quantitative description of the electronic properties and their size dependence, considering disk and ring geometries with both armchair and zigzag edges. We show that the electronic properties of quantum dots with armchair edges are more sensitive to structural details than those with zigzag edges. As functions of the inner and outer radii, we find in the case of armchair edges that the size of the band gap follows distinct branches, while in the case of zigzag edges it changes monotonically. This behaviour is further analyzed by studying the ground state wave function and explained in terms of its localisation. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Using first principles calculations, we investigate the electronic and magnetic properties of BNC biribbons, a laterally-heterostructured nanoribbon constructed by joining a graphene nanoribbon (GNR) and a BN nanoribbon (BNNR) with zigzag edges. We find that the spin-polarization and electronic structures of the biribbons can be well-tuned by changing the width of the GNR, undergoing manifold transitions from semiconducting to half-metal and ferromagnetic metal. The critical points of GNR width to induce the transitions depend on the interface type (B/C or C/N) rather than the BNNR width. The ground states of metallic BNC biribbons are spin-polarized, forming non-zero magnetic moments. The tunable electronic structures and ferromagnetic ground states make the BNC biribbons promising candidate nanomaterials for building nanoscaled spintronic devices.

CONSPECTUS: After the discovery of graphene and the development of powerful exfoliation techniques, experimental preparation of two-dimensional (2D) crystals can be expected for any layered material that is known to chemistry. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ∼1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the laboratory as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been observed experimentally and are in excellent agreement with calculations based on density-functional theory. This Account reports theoretical studies of a subgroup of transition metal dichalcogenides with the composition MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external electric field are reported. While all materials of the MoWSeS family share the same qualitative properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external electric fields, bilayers show a linear reduction of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ∼10%. The density-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temperature molecular dynamics simulations. Structural defects in MoS2 result in anisotropy of the electric conductivity. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer.

Transition metal dichalcogenides, mainly focusing on MoS2, have attracted intensive studies in terms of their electronic and optical properties. Their lower-dimensional counterparts, such as nanoribbons and nanoclusters, gradually draw more research attention because of their potential applications in various electronic and spintronic devices. The edge states are essential to determine the intriguing electronic and magnetic properties of the nanocrystals. In this review, we mainly focus on the zigzag edges in the MoS2 system because they are more common in experiments. We first review the physical properties of the unreconstructed edges and then introduce the principles of edge reconstructions, the electron counting model (ECM). Then, based on the ECM, intrinsic edge reconstructions with different periodicities are discussed. The literature we reviewed suggests that the conductivities and magnetism of the edge states are highly related to the periodicities of the edges. Finally, the effects of the edge passivation with extrinsic atoms are reviewed. Edge passivation plays an important role in tuning the electronic and magnetic properties of the edge states and determining the morphology during the crystal growth. Furthermore, MoS2 zigzag edges could be an ideal platform to investigate the interplay between the edge states with different periodicities and magnetic dopants in the future.