First-principles studies of graphene antidot lattices on monolayer h-BN substrate

The structures and electronic structures of hetero bilayers composed of graphene antidot lattice (GAL) on monolayer h-BN substrate are studied in first-principles method. Bond lengths, interlayer distances, flatness, biaxial strain effects, and effects of translating the GAL layer are studied and analyzed in detail. Results show that introducing a monolayer BN substrate makes the zero-bandgap $5\times 5$ GAL open a bandgap up to 28 meV, while it makes the semiconducting $6\times 6$ GAL keep its low-energy electronic structure almost intact except a small bandgap change by tens of meV at most. Our studies demonstrate that h-BN is a promising substrate for GAL.     

Thermoelectric properties of gated graphene ribbons in the ballistic regime

We investigate the thermoelectric properties of gated graphene ribbons in the ballistic transport limit using linear response theory and the Landauer formalism. The dependence of the electronic conductance, thermopower as well as electronic thermal conductance on both Fermi level and temperature are clarified and the validity of Wiedemann-Franz law is examined. The electronic part of thermoelectric figure of merit _ZTel${\mathit{ZT}}_{\mathit{el}}$ which gives an upper bound for the thermoelectric efficiency of the gated ribbons, is also calculated. It is shown that _ZTel${\mathit{ZT}}_{\mathit{el}}$ of wide and short gated ribbons is directly related to geometric aspect ratio of the graphene ribbon and for very short ribbons can exceed unity at room temperature. Our results could be useful in the design of efficient graphene-based thermoelectric devices.     

Polaronic effects in one-dimensional quantum antidot arrays

We study the effect of polaronic corrections arising from theelectron-longitudinal optical phonon interaction on the energyspectrum of a two-dimensional electron system with a one-dimensionalperiodic antidot array geometry created by a weak electrostaticmodulation potential, and subjected to a weak magnetic fieldmodulation as well as a uniform strong perpendicular staticmagnetic field. To incorporate the effects of electron-phononinteractions within the framework of Fröhlich polaron theory, wefirst apply a displaced-oscillator type unitary transformation todiagonalise the relevant Fröhlich Hamiltonian, and we thendetermine the parameters of this transformation together with theparameter included in the electronic trial wave function . On thebasis of this technique, it has been shown that the polaroniccorrections have non-negligible effects on the electronic spectrumof a two-dimensional electron system with a quantum antidot array,since switching such an interaction results in shifting thedegeneracy restoring points of Landau levels wherein the flatbandcondition is fulfilled, thus suppressing the Weiss oscillations.     

Electronic and magnetic properties of hydrogenated graphene nanoflakes

We investigated the energetic stability, electronic, and magnetic properties of hydrogenated graphene nanoflakes (GNFs) by using density-functional theory (DFT). Hydrogenated GNFs were found to be the stable heterojunction structures. As the increase of H coverage, a transition of a small-gap semiconductor to wide-gap semiconductor occurs, accompanied with a nonmagnetic (with the coverage χ=0$\chi =0$) → magnetic (with the coverage 0<χ<1$0<\chi <1$) → nonmagnetic (with the coverage χ=1$\chi =1$) transfer for hexagonal nanoflakes and magnetic (with the coverage 0?χ<1$0?\chi <1$) → nonmagnetic (with the coverage χ=1$\chi =1$) transfer for triangular nanoflakes. The efficacious tune of band gaps and the magnetic moments on these nanoflakes by hydrogenation offers an effectual avenue for the applications of C-based nanomagnets.     

Fano resonance in graphene anti-dot and periodic graphene anti-dot arrays

We study the electron transport properties of graphene anti-dot and periodic graphene anti-dot arrays using the nonequilibrium Green?s function method and Landauer–Büttiker formula. Fano resonant peaks are observed in the vicinity of Fermi energy, because discrete states coexist with continuum energy states. These peaks move closer to Fermi energy with increasing the width of anti-dots, but move away from the Fermi energy with increasing the length of anti-dots. When N periodic anti-dots exist in the longitude direction, a rapid fluctuation appears in the conductance with varying resonance peaks, which is mainly from the local resonances created by quasibound state. When P periodic anti-dots exist in the transverse direction, P-fold resonant splitting peaks are observed around the Fermi energy, owing to the symmetric and antisymmetric superposition of quasibound states.     

Noble metals induced magnetic properties of graphene

The adsorption energies, stable configurations, electronic structures, and magnetic properties of the graphene with noble metal (NM=Pt, Ag, and Au) atom adsorption were investigated using first-principles density-functional theory. It is found that the bridge site is the most stable adsorption site for the Pt adatom; the Ag adatom can be stabilized almost equally at the bridge or the top site, while the Au adatom prefers to be adsorbed at top site. The Pt-graphene interaction is stronger than the interaction of Ag-graphene and Au-graphene, since the Pt atom has an unsaturated electronic d-shell (d⁹s¹). While there is no net magnetic moment for the Pt adatom, the Ag and Au adatoms still exhibit magnetic character on the graphene. The magnetic moments of the NM-graphene systems may be quenched (e.g., Pt-graphene), reduced (e.g., Ag-graphene) or not changed (e.g., Au-graphene) as compared with the values before adsorption. Therefore, the magnetic character of the adatom-graphene system can be turned by adsorbing different NM atoms on the graphene.     

High thermoelectric performance of one graphene nanoribbon with line defects

We investigate the quantum transport through zigzag graphene nanoribbons with embedded “5-7-5”-edge line defects, by means of the non-equilibrium Green's function technique. It is found that when two semi-infinite line defects exist in the nanoribbon, notable Fano antiresonance takes place in the quantum transport process, which enables to drive the apparent thermoelectric effect. We propose this structure to be a promising candidate for improving the thermoelectric efficiency based on graphene nanoribbons.     

Transport properties of graphene under periodic and quasiperiodic magnetic superlattices

We study the transmission of Dirac electrons through the one-dimensional periodic, Fibonacci, and Thue–Morse magnetic superlattices (MS), which can be realized by two different magnetic blocks arranged in certain sequences in graphene. The numerical results show that the transmission as a function of incident energy presents regular resonance splitting effect in periodic MS due to the split energy spectrum. For the quasiperiodic MS with more layers, they exhibit rich transmission patterns. In particular, the transmission in Fibonacci MS presents scaling property and fragmented behavior with self-similarity, while the transmission in Thue–Morse MS presents more perfect resonant peaks which are related to the completely transparent states. Furthermore, these interesting properties are robust against the profile of MS, but dependent on the magnetic structure parameters and the transverse wave vector.     

Quantum antidot as an electrometer:Observation of fractional charge

In experiments on resonant tunneling through a quantum antidot in the quantum Hall (QH) regime, we observe periodic conductance peaks both versus magnetic field and a global gate voltage, i.e., electric field. Each conductance peak can be attributed to tunneling through a quantized antidot-bound state. The fact that the variation of the uniform electric field produces conductance peaks implies that the deficiency of the electrical charge on the antidot is quantized in units of charge of quasiparticles of surrounding QH condensate. The period in magnetic field gives the effective area of the antidot state through which tunneling occurs, the period in electric field (obtained from the global gate voltage) then constitutes a direct measurement of the charge of the tunneling particles. We obtain electron charge C in the integer QH regime, and quasiparticle charge C for the QH state.     

Thermoelectric properties of nano-bulk bismuth telluride prepared with spark plasma sintered nano-plates

The pursuit for a high-performance thermoelectric n-type bismuth telluride-based material is significant because n-type materials are inferior to their corresponding p-type materials in highly efficient thermoelectric modules. Herein, to improve the thermoelectric performance of an n-type Bi₂Te₃, we prepared Bi₂Te₃ nano-plates with a homogeneous sub-micron size distribution and thickness range of about a few tens of nanometers. This was achieved using a typical nano-chemical synthetic method, and the prepared materials were then spark plasma sintered to fabricate n-type nano-bulk Bi₂Te₃ samples. We observed a significant enhancement of the anisotropic electrical transport properties for the nano-bulk sample with a higher power factor along the in-plane direction (24.3?μW?cm^?1?K^?2 at 300?K) than that along the out-of-plane direction (8.1?μW?cm^?1?K^?2 at 300?K). However, thermal transport properties were insensitive along the measured direction for the nano-bulk sample. We used a dimensionless figure of merit ZT to calculate the thermoelectric performance. The results showed that the maximum ZT value of 0.69 was achieved along the in-plane direction at 440?K for the nano-bulk n-type Bi₂Te₃ sample, which was however smaller than that of the previously reported n-type samples (ZT of 1.1). We believe that a further enhancement of the ZT value in the fabricated nano-bulk sample could be accomplished by effectively removing the surface organic ligand of the Bi₂Te₃ nano-plate particles and optimizing the spark plasma sintering conditions, maintaining the nano-plate morphology intact.     

The influence of edge defects on the electrical and thermal transport of graphene nanoribbons

The electrical conductance, thermopower, thermal conductance and figure of merit of graphene nanoribbons (GNRs) are investigated using Green function formalism in the linear response regime. The Hamiltonian of GNR is described by the tight-binding approach and the effect of elastic interactions due to the electron–electron interaction or the thermal environmental fluctuations is considered by dephasing approach within the self-consistent Born approximation. The results show that the dephasing process leads to the reduction of the electrical transport of GNRs. Since the edge configuration of GNRs has the significant role in their electronic properties, it is shown that the electrical and thermal transports of the GNRs are decreased by the edge defects while the reduction of thermal conductance is more efficient, therefore, the thermal efficiency of GNRs is increased.     

Surface adhesion properties of graphene and graphene oxide studied by colloid-probe atomic force microscopy

Surface adhesion properties are important to various applications of graphene-based materials. Atomic force microscopy is powerful to study the adhesion properties of samples by measuring the forces on the colloidal sphere tip as it approaches and retracts from the surface. In this paper we have measured the adhesion force between the colloid probe and the surface of graphene (graphene oxide) nanosheet. The results revealed that the adhesion force on graphene and graphene oxide surface were 66.3 and 170.6 nN, respectively. It was found the adhesion force was mainly determined by the water meniscus, which was related to the surface contact angle of samples.     

Transport properties of mesoscopic graphene rings

Based on a recursive Green's function method, we investigate the conductance of mesoscopic graphene rings in the presence of disorder, in the limit of phase coherent transport. Two models of disorder are considered: edge disorder and surface disorder. Our simulations show that the conductance decreases exponentially with the edge disorder and the surface disorder. In the presence of flux, a clear Aharonov-Bohm conductance oscillation with the period Φ₀ (Φ₀=h/e) is observed. The edge disorder and the surface disorder have no effect on the period of AB oscillation. The amplitudes of AB oscillations vary with gate voltage and flux, which is consistent with the previous results. Additionally, ballistic rectification and negative differential resistance are observed in I-V curves, with on/off characteristic.     

Transmission properties of Fibonacci quasi-periodic one-dimensional photonic crystals containing indefinite metamaterials

The transmission properties of Fibonacci quasi-periodic one-dimensional photonic crystals (1DPCs) containing indefinite metamaterials are theoretically studied. It is found that 1DPCs can possess an omnidirectional zero average index (zero-n?) gap which exists in all Fibonacci sequences. In contrast to Bragg gaps, such zero-n? gap is less sensitive to the incidence angle, the scale length and the polarizations of electromagnetic waves. When an impurity is introduced, a defect mode appears inside the zero-n? gap with a very weak dependence on the incidence angle and scaling.     

Vibrational properties and Raman spectra of different edge graphene nanoribbons,studied by first-principles calculations

The vibrational properties and Raman spectra of graphene nanoribbons with six different edges have been studied by using the first-principles calculations. It is found that edge reconstruction leads to the emergence of localized vibrational modes and new topological defect modes, making the different edges identified by polarized Raman spectra. The radial breathing-like modes are found to be independent of the edge structures, while the G-band-related modes are affected by different edge structures. Our results suggest that the polarized Raman spectrum could be a powerful experimental tool for distinguishing the GNRs with different edge structures due to their different vibrational properties.     

Theoretical study on the catalytic properties of single-atom catalyst stabilised on silicon-doped graphene sheets

ABSTRACT

The stable configurations, electronic structures and catalytic activities of single-atom metal catalyst anchored silicon-doped graphene sheets (3Si-graphene-M, M?=?Ni and Pd) are investigated by using density functional theory calculations. Firstly, the adsorption stability and electronic property of different gas reactants (O₂, CO, 2CO, CO/O₂) on 3Si-graphene-M substrates are comparably analysed. It is found that the coadsorption of O₂/CO or 2CO molecules is more stable than that of the isolated O₂ or CO molecule. Meanwhile, the adsorbed species on 3Si-graphene-Ni sheet are more stable than those on the 3Si-graphene-Pd sheet. Secondly, the possible CO oxidation reactions on the 3Si-graphene-M are investigated through Eley–Rideal (ER), Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms. Compared with the LH and TER mechanisms, the interaction between 2CO and O₂ molecules (O₂?+?CO → CO₃, CO₃?+?CO → 2CO₂) through ER reactions (< 0.2?eV) are an energetically more favourable. These results provide important reference for understanding the catalytic mechanism for CO oxidation on graphene-based catalyst.     

Thermoelectric power and magnetic properties of Cu doped NiZn ferrite for technological application

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2-xO₄ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. The thermoelectric power and the magnetic susceptibility were measured. The transition from the ferrimagnetic to the paramagnetic state is accompanied by an increase in the thermo EMF. NiZn ferrite shows n-type conductivity due to the presence of Fe²⁺ ions. The addition of Cu²⁺ ions creates lattice vacancies which give rise to p-type conductivity.

The Tawfik coefficient was determined for NiZn ferrite in the paramagnetic state. This coefficient was reduced by addition of Cu up to x < 0.5.     

Thermoelectric and optical properties of CuAlO2 synthesized by direct microwave heating

A single phase of delafossite CuAlO₂ (CAO) was successfully synthesized by a 600 W microwave radiation for 20 min. The CAO sample was composed of quite distorted single-crystalline plates with 200–350 nm thick. Its atomic vibrations were detected at 760 and 550 cm⁻¹ belonging to Al–O and Cu–O stretching, respectively. The direct and indirect energy gaps were respectively determined to be 3.9 and 2.9 eV. The photoluminescence (PL) at room temperature was at 585 nm (2.12 eV) corresponding to the indirect energy gap and at 760 nm (1.63 eV) corresponding to the p-type native defect. For its thermoelectric (TE) properties, the Seebeck coefficient (S) was positive value, with holes as the majority of charge carriers. By increasing of the test temperature, both the electrical resistivity and absolute value of Seebeck coefficient were decreased, but the power factor was in the opposite manner. The dimensionless figure of merit (ZT) of the crystalline CAO was evaluated to be the maximum of 9 × 10⁻³ at 1073 K.