Graphene Nanoislands Research Articles

Non-equilibrium magnetic properties of a mixed spin (1/2, 1) Ising graphene nanoisland.

Based on Glauber-type stochastic dynamics intertwined with mean field theory, we have investigated a graphene nanoisland's out-of-equilibrium magnetic and thermomagnetic behaviors with a mixed ferrimagnetic spin (1/2, 1) configuration in the presence of an oscillating magnetic field. The influence of the oscillating external magnetic field, single-ion anisotropy and temperature on the dynamic magnetic properties (dynamic phase transitions, hysteresis loop areas, correlations, and phase diagrams) of the system is studied in detail. The results illustrate that single ion anisotropy and the magnetic field play important roles in the system's dynamic order parameters, phase transitions and phase diagrams. Dynamic phase diagrams display some dynamic special points such as dynamic tricritical points, quadruple points and some sort of reentrant phenomena, which strongly depend on interaction parameters. Our predicted results may provide a reference for future experimental and theoretical studies of the nanostructures.

Spin state engineering of triangulene graphene embedded in h-BN nanoflake

Zigzag-edged graphene nanoislands hold great potential in nanoelectronics and spintronics applications, however, controlling its edge chemistry is still challenging from the experimental perspective. One promising strategy to circumvent the issues associated with the highly reactive graphene zigzag edges is to incorporate it into an insulator matrix (e.g., hexagonal boron nitride). In light of this, stability, electronic and magnetic properties of hybrid graphene/BN nanoflakes are investigated through the Density functional theory (DFT). The results reveal that the formation of separated graphene and h-BN domains would be feasible in hybrid flakes. The multiplicity of the finite heterostructures is dictated by the carbon domain, regardless the position, size, and whether the carbon islands are connected or not. To investigate the feasibility of spin state engineering, changes in the highest orbital occupations were evaluated by increasing the applied electric or magnetic field magnitude. The results indicate that an external electric field above a critical value can induce a degeneracy of ground and first excited spin state, which might pave the way for a spin state transition triggered by a magnetic field perturbation.

Atomistic simulations of graphene origami: Dynamics and kinetics

Origami offers two-dimensional (2D) materials with great potential for applications in flexible electronics, sensors, and smart devices. However, the dynamic process, which is crucial to construct origami, is too fast to be characterized by using state-of-the-art experimental techniques. Here, to understand the dynamics and kinetics at the atomic level, we explore the edge effects, structural and energy evolution during the origami process of an elliptical graphene nano-island (GNI) on a highly ordered pyrolytic graphite (HOPG) substrate by employing steered molecular dynamics simulations. The results reveal that a sharper armchair edge is much easier to be lifted up and realize origami than a blunt zigzag edge. The potential energy of the GNI increases at the lifting-up stage, reaches the maximum at the beginning of the bending stage, decreases with the formation of van der Waals overlap, and finally reaches an energy minimum at a half-folded configuration. The unfolding barriers of elliptical GNIs with different lengths of major axis show that the major axis should be larger than 242 Å to achieve a stable single-folded structure at room temperature. These findings pave the way for pursuing other 2D material origami and preparing origami-based nanodevices.

Structural identification of graphene films and nanoislands on 6H-SiC(0001) by direct height measurement

By combining non-contact atomic force microscopy (nc-AFM) and Kelvin probe microscopy (KPFM) in ultra high vacuum environment (UHV), we directly measure the height and work function of graphene monolayer on the Si-face of 6H-SiC(0001) with a precision that allows us to differentiate three different types of graphene structures : zero layer graphene (ZLG), Quasi free-standing monolayer graphene (QFMLG) and bilayer graphene (BLG). The height and work function of ZLG are 2.62 ± 0.22 Å and 4.42 ± 0.05 eV respectively, when they are 4.09 ± 0.11 Å and 4.63 ± 0.05 eV for QFMLG. The work function is 4.83 ± 0.05 eV for the BLG. Unlike any other available technique, the local nc-AFM/KPFM dual probe makes it possible to directly identify the nature of nanometer-sized graphene islands that constitute the early nuclei of graphene monolayer grown on 6H-SiC(0001) by chemical vapor deposition.

Theory of triangulene two-dimensional crystals

Abstract Equilateral triangle-shaped graphene nanoislands with a lateral dimension of n benzene rings are known as [n] triangulenes. Individual [n] triangulenes are open-shell molecules, with single-particle electronic spectra that host n − 1 half-filled zero modes and a many-body ground state with spin S = ( n − 1 ) / 2 . The on-surface synthesis of triangulenes has been demonstrated for n = 3 , 4 , 5 , 7 and the observation of a Haldane symmetry-protected topological phase has been reported in chains of [3]triangulenes. Here, we provide a unified theory for the electronic properties of a family of two-dimensional honeycomb lattices whose unit cell contains a pair of triangulenes with dimensions n a , n b . Combining density functional theory and tight-binding calculations, we find a wealth of half-filled narrow bands, including a graphene-like spectrum (for n a = n b = 2 ), spin-1 Dirac electrons (for n a = 2 , n b = 3 ), p x , y -orbital physics (for n a = n b = 3 ), as well as a gapped system with flat valence and conduction bands (for n a = n b = 4 ). All these results are rationalized with a class of effective Hamiltonians acting on the subspace of the zero-energy states that generalize the graphene honeycomb model to the case of fermions with an internal pseudospin degree of freedom with C 3 symmetry.

Strong multipolar transition enhancement with graphene nanoislands

For a long time, the point-dipole model was a central and natural approximation in the field of photonics. This approach assumes that the wavelength is much larger than the size of the emitting atom or molecule so that the emitter can be described as a single or a collection of elementary dipoles. This approximation no longer holds near plasmonic nanostructures, where the effective wavelength can reach the nanometer-scale. In that case, deviations arise and high-order transitions, beyond the dipolar ones, are not forbidden anymore. Typically, this situation requires intensive numerical efforts to compute the photonic response over the spatial extent of the emitter wavefunctions. Here, we develop an efficient and general model for the multipolar transition rates of a quantum emitter in a photonic environment by computing Green’s function through an eigen permittivity modal expansion. A major benefit of this approach is that the position of the emitter and the permittivity of the material can be swept in a rapid way. To illustrate, we apply the method on various forms of graphene nanoislands, and we demonstrate a local breakdown of the selection rules, with quadrupolar transition rates becoming 100 times larger than dipolar ones.

Direct Visualization of Atomic-Scale Graphene Growth on Cu through Environmental Transmission Electron Microscopy.

The functionalities of two-dimensional (2D) materials are solely determined by their perfect single-layer lattice or precisely stacking of multiple lattice planes, which is predominately determined during their growth process. Although the growth of graphene has been successfully achieved on different substrates with a large area up to millimeters, direct visualization of atomic-scale graphene growth in real time still lacks, which is vital to decipher atomistic mechanisms of graphene growth. Here, we employ aberration-corrected environmental transmission electron microscopy (AC-ETEM) to visualize the nucleation and growth of graphene at the atomic scale in real time. We find a unique lateral epitaxial growth process of graphene on Cu edges under the CO2 atmosphere with a ledge-flow process. The nucleation of graphene nuclei from amorphous carbon atoms also has been found to proceed with a gradual ordering of in-plane carbon atoms. The coalescence of smaller graphene nanoislands to form large ones is thermodynamically favored, and the evolution of atomic structures at grain boundaries is also revealed in great details. These atomic insights obtained from real-time observations can provide direct evidence for the growth mechanisms of graphene, which can be extended to other 2D materials.

Fabrication and manipulation of nanosized graphene homojunction with atomically-controlled boundaries

Controlling the atomic configurations of structural defects in graphene nanostructures is crucial for achieving desired functionalities. Here, we report the controlled fabrication of high-quality single-crystal and bicrystal graphene nanoislands (GNI) through a unique top-down etching and post-annealing procedure on a graphite surface. Low-temperature scanning tunneling microscopy (STM) combined with density functional theory calculations reveal that most of grain boundaries (GBs) formed on the bicrystal GNIs are 5-7-5-7 GBs. Two nanodomains separated by a 5-7-5-7 GB are AB stacking and twisted stacking with respect to the underlying graphite substrate and exhibit distinct electronic properties, forming a graphene homojunction. In addition, we construct homojunctions with alternative AB/twisted stacking nanodomains separated by parallel 5-7-5-7 GBs. Remarkably, the stacking orders of homojunctions are manipulated from AB/twist into twist/twist type through a STM tip. The controllable fabrication and manipulation of graphene homojunctions with 5-7-5-7 GBs and distinct stacking orders open an avenue for the construction of GBs-based devices in valleytronics and twistronics.

Unidirectional growth of graphene nano-islands from carbon cluster seeds on Ge(1 1 0)

The anisotropic twofold symmetry of Ge(110) makes it a unique substrate for the growth of single-crystalline graphene. However, the underlying mechanism during the initial stage of growth of graphene on Ge(110) surface is not well understood. Here, we use in-situ cryogenic scanning tunneling microscopy (STM) and spectroscopy (STS), to study the initial growth properties of graphene synthesized on Ge(110) surface with ethylene precursor gases. The STM results reveal that the unidirectional growth of the graphene nanoribbons (GNRs) initiates from carbon cluster seeds on terraces during the early stage of the growth. The orientations of the GNRs show the same directions which are nearly parallel to the 1¯10direction of the Ge(110) surface. This orientation is the same as that of the wafer-scale graphene grown on Ge(110). Subsequent growth, transformed the GNRs to graphene nanoislands (GNIs), which eventually coalesced to form single crystalline monolayer graphene. STS measurements demonstrated that the GNRs have small bandgaps induced by the confinement effect of graphene. This study provides an in-depth understanding of the growth mechanism of graphene on Ge(110) surface in various synthesis conditions.

Dynamic magnetic properties of a double-layer core–shell graphene nanoisland in an oscillating magnetic field

By using Monte Carlo simulations, we discuss the dynamic magnetic properties of a double-layer core–shell graphene nanoisland with mixed-spin (2, 5/2) configuration in an oscillating magnetic field. We investigate the influence of exchange couplings, oscillating magnetic field, and temperature on the dynamic magnetic and thermodynamic properties of the system. We obtain the various characteristic curves of the dynamic order parameter, susceptibility, and internal energy as well as hysteresis loops. To better understand the impact of these physical parameters on the phase transition of the system, the phase diagrams are depicted. It should be noticed that the system can exhibit multiple-loop hysteresis behaviors under certain conditions. Besides, we plot the coercivity and remanence of the system under the influence of various parameters.

Magnetic properties of a triangle double-layer graphene nanoisland: a Monte Carlo study

By using Monte Carlo simulations, we investigated a triangle double-layer graphene nanoisland with mixed-spin (3/2, 2) configuration. We focused on the effects of crystal-fields, exchange couplings, longitudinal magnetic fields, and temperature on the magnetic properties of the system. Various curves of magnetization, susceptibility, and internal energy as a function of temperature were obtained. We plotted the phase diagrams of blocking temperature in the different parameter planes. We found that increasing |Jab/JZ|,Jbb/JZ, and h/JZ or weakening |Db/JZ| is beneficial to improving the TB/JZ, but the TB/JZ is very insensitive to changes in the Da/JZ and Jaa/JZ. In addition, it is worth noting that the system can exhibit abundant multiple-loops hysteresis behaviors for certain physical parameters, such as triple, quadruple, quintuple, septuple hysteresis loops.

Atomically precise, custom-design origami graphene nanostructures.

The construction of atomically precise carbon nanostructures holds promise for developing materials for scientific study and nanotechnology applications. Here, we show that graphene origami is an efficient way to convert graphene into atomically precise, complex nanostructures. By scanning tunneling microscope manipulation at low temperature, we repeatedly fold and unfold graphene nanoislands (GNIs) along an arbitrarily chosen direction. A bilayer graphene stack featuring a tunable twist angle and a tubular edge connection between the layers is formed. Folding single-crystal GNIs creates tubular edges with specified chirality and one-dimensional electronic features similar to those of carbon nanotubes, whereas folding bicrystal GNIs creates well-defined intramolecular junctions. Both origami structural models and electronic band structures are computed to complement analysis of the experimental results. The present atomically precise graphene origami provides a platform for constructing carbon nanostructures with engineered quantum properties and, ultimately, quantum machines.

Precisely folding nanographene

Nanomaterials Graphene nanostructures that would result from folding or rolling graphene monolayers or bilayers have been predicted to have a number of interesting electronic properties, but control over such folding processes has been limited. Chen et al. used a scanning tunneling microscope tip to fold and unfold graphene nanoislands etched on graphite surfaces at low temperatures (4 kelvin). The fold angle could be precisely controlled to create different twist angles in bilayer graphene and a tubelike edge in folded graphene. They also folded 5 ring–7 ring defects and explored this heterojunction with scanning tunneling spectroscopy. Science , this issue p. [1036][1] [1]: /lookup/doi/10.1126/science.aax7864

Single-Plasmon Thermo-Optical Switching in Graphene

While plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.

Magnetic behaviors in a bilayer graphene nanoisland structure: A Monte Carlo study

The Monte Carlo simulation has been applied to study the magnetic behaviors of a bilayer graphene nanoisland structure on a triangular lattice by modeling the ferrimagnetic Ising system with spin-5/2 and spin-2. The temperature dependence of magnetization, susceptibility and internal energy curves of the system and two sublattices with the influence of the anisotropy, the exchange coupling, the external magnetic field and the size index have been discussed in detail. The diagrams of the blocking temperature under the effects of different parameters have been obtained. The results show the anisotropy plays a significant role in controlling the magnetic properties of the bilayer graphene nanoisland structure.

Recovery of edge states of graphene nanoislands on an iridium substrate by silicon intercalation

Finite-sized graphene sheets, such as graphene nanoislands (GNIs), are promising candidates for practical applications in graphene-based nanoelectronics. GNIs with well-defined zigzag edges are predicted to have spin-polarized edge-states similar to those of zigzag-edged graphene nanoribbons, which can achieve graphene spintronics. However, it has been reported that GNIs on metal substrates have no edge states because of interactions with the substrate.We used a combination of scanning tunneling microscopy, spectroscopy, and density functional theory calculations to demonstrate that the edge states of GNIs on an Ir substrate can be recovered by intercalating a layer of Si atoms between the GNIs and the substrate. We also found that the edge states gradually shift to the Fermi level with increasing island size. This work provides a method to investigate spin-polarized edge states in high-quality graphene nanostructures on a metal substrate.

Nonlinear optical response in graphene nanoribbons: The critical role of electron scattering

Nonlinear nanophotonics has many potential applications, such as in mode locking, frequency-comb generation, and all-optical switching. The development of materials with large nonlinear susceptibility is key to realizing nonlinear nanophotonics. Nanostructured graphene systems, such as graphene nanoribbons and nanoislands, have been predicted to have a strong plasmon-enhanced nonlinear optical behavior in the nonretarded regime. Plasmons concentrate the light field down to subwavelength scales and can enhance the nonlinear optical effects; however, plasmon resonances are narrowband and sensitive to the nanostructure geometry. Here we show that graphene nanoribbons, particularly armchair graphene nanoribbons, have a remarkably strong nonlinear optical response in the long-wavelength regime and over a broad frequency range, from terahertz to the near infrared. We use a quantum-mechanical master equation with a detailed treatment of scattering and show that, in the retarded regime, electron scattering has a critical effect on the optical nonlinearity of graphene nanoribbons, which cannot be captured via the commonly used relaxation-time approximation. At terahertz frequencies, where intraband optical transitions dominate, the strong nonlinearity (in particular, third-order Kerr nonlinearity) stems from the jagged shape of the electron energy distribution, caused by the interband electron scattering mechanisms along with the intraband inelastic scattering mechanisms. We show that the relaxation-time approximation fails to capture this quantum-mechanical phenomenon and results in a significant underestimation of the intraband nonlinearity. At the midinfrared to near infrared frequencies, where interband optical transitions dominate, the Kerr nonlinearity is significantly overestimated within the relaxation-time approximation. These findings unveil the critical effect of electron scattering on the optical nonlinearity of nanostructured graphene, and also underscore the capability of this class of materials for nonlinear nanophotonic applications.

Phase diagram and magnetization of a graphene nanoisland structure

Magnetic properties of a graphene nanoisland structure have been studied by the effective-field theory with correlations. It consists of two triangular interpenetrating sublattices described by Ising model with mixed spin. The results show the exchange coupling and the anisotropy have important influence on the magnetization. The reentrant and staircase effect have been found within a certain parameter range.

The Electronic Structure of Graphene Nanoislands: A CAS-SCF and NEVPT2 Study

This paper presents a tight binding and ab initio study of finite graphene nanostructures. The attention is focused on three types of regular convex polygons: triangles, rhombuses, and hexagons, which are the most simple high-symmetry convex structures that can be ideally cut out of a graphene layer. Three different behaviors are evidenced for these three classes of compounds: closed-shells for hexagons; low-spin open-shells for rhombuses; high-spin open-shells for triangles.

Evolution of Graphene Growth on Pt(111): From Carbon Clusters to Nanoislands

We study the growth of graphene on a Pt(111) surface in stages by varying the annealing temperature of the precursor hydrocarbon decomposition through an atomic-scale analysis using scanning tunneling microscopy (STM) and studying the geometry-affected electronic properties of graphene nanoislands (GNs) through scanning tunneling spectroscopy. STM reveals that graphene grows on a Pt(111) surface from dome-shaped carbon clusters to flat GNs with the intermediate stages of dome-shaped and basin-shaped hexagonal GN structures. Density functional theory calculations confirm the changes in direction of the concavity upon increase in the size of the GNs. The structural changes are also found to have a significant effect on the electronic properties. Landau levels arise from strain-induced pseudomagnetic fields because of the large curvature, and the nanoscale-size effect promotes electron confinement.