Size Of Energy Gap Research Articles

Highly tunable charge transport in defective graphene nanoribbons under external local forces and constraints: A hybrid computational study

In this paper, we propose a combined modeling of molecular mechanics (MM) and the tight-binding (TB) approach, which enables us to study the effect of factors such as external local forces, constraints, and vacancy defects on electronic transport properties of nanomaterials. Nanostructures selected in this work are armchair graphene nanoribbons (AGNRs). According to this method, the nanostructure is modeled as a frame, and the beam element is applied for illustrating the covalent interatomic interactions in bonds. In our calculations, the terms of torsional, stretching, and bending energies are considered. The selected pristine nanoribbon is a metal, and the purpose of this study is to find the effects of mechanical loading, the vacancy defects and their positions on the electrical conductance of the structure. We observe that the presence of vacancy defects in the structure leads to the opening of an energy gap, which changes the phase of the nanostructure from metal to the semiconductor. We find that with increasing the number of point defects, the energy gap size of the strained system grows. Besides, increasing the magnitude of the local force reduces the conductance, and the energy gap of the system. By changing parameters such as the number of point defects and magnitude local forces, the transport gap of the system can be controlled. The results of this research may be useful in the design of nanoelectromechanical systems.

Valley current splitter in minimally twisted bilayer graphene

We study the electronic transport properties at the intersection of three topological zero-lines as the elementary current partition node that arises in minimally twisted bilayer graphene. Unlike the partition laws of two intersecting zero-lines, we find that (i) the incoming current can be partitioned into both left-right adjacent topological channels and that (ii) the forward-propagating current is nonzero. By tuning the Fermi energy from the charge-neutrality point to a band edge, the currents partitioned into the three outgoing channels become nearly equal. Moreover, we find that current partition node can be designed as a perfect valley filter and energy splitter controlled by electric gating. By changing the relative electric field magnitude, the intersection of three topological zero-lines can transform smoothly into a single zero line, and the current partition can be controlled precisely. We explore the available methods for modulating this device systematically by changing the Fermi energy, the energy gap size, and the size of central gapless region. The current partition is also influenced by magnetic fields and the system size. Our results provide a microscopic depiction of the electronic transport properties around a unit cell of minimally twisted bilayer graphene and have far-reaching implications in the design of electron-beam splitters and interferometer devices.

Atomic scale electronic structure of the ferromagnetic semiconductor Cr2Ge2Te6

Cr2Ge2Te6 is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure, thus represents a promising material for novel electronic and spintronic devices. Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr2Ge2Te6. Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition, as well as a peculiar double-peak electronic state on the Cr-site defect. These features can be quantitatively explained by density functional theory calculations, which uncover a close relationship between the electronic structure and magnetic order. These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr2Ge2Te6.

Adsorption of alanine with heteroatom substituted fullerene for solar cell application: A DFT study

C20 is the most important fullerene cage and alanine is the simplest representation of a backbone unit of the protein. The absorption feasibility of alanine molecule in the Si-doped C20 and B-doped C20 fullerenes has been studied based on calculated electronic properties of fullerenes using density functional theory (DFT). In this work, we explore the ability of Si-doped C20, B-doped C20 fullerene to interact with alanine at the DFT-B3LYP/6-31G, RHF level of theory. We find that noticeable structural change takes place in C20 when one of its carbon is substituted with Si or B. The molecular geometry, electronic properties and vibrational analysis have also been performed on the title compounds. The NMR study reveals the aromaticity of the pure and doped fullerene compounds. Stability of the doped fullerene – alanine compound arises from hyper conjugative interactions. It leads to one of the major property of bioactivity, charge transfer and delocalization of charge and this properties has been analyzed using Natural Bond Orbital (NBO) analysis. The energy gap of the doped fullerene reveals that there is a decrease in the size of energy gap significantly, making them more reactive as compared to C20 fullerene. Theoretical studies of the electronic spectra by using time – dependent density functional theory (TD-DFT) method were helpful to interpret the observed electronic transition state. We aim to optimize the performance of the solar cells by altering the frontier orbital energy gaps. Considering all studied properties, it may be inferred that the applicability of C20 fullerene as the non-linear optical (NLO) material and its NLO property would increase on doping fullerene with Si and B atom. Specifically C19Si would be better among them.

Lithium effects on the mechanical and electronic properties of germanium nanowires

Semiconductor nanowire arrays promise rapid development of a new generation of lithium (Li) batteries because they can store more Li atoms than conventional crystals due to their large surface areas. During the charge–discharge process, the electrodes experience internal stresses that fatigue the material and limit the useful life of the battery. The theoretical study of electronic and mechanical properties of lithiated nanowire arrays allows the designing of electrode materials that could improve battery performance. In this work, we present a density functional theory study of the electronic band structure, formation energy, binding energy, and Young’s modulus (Y) of hydrogen passivated germanium nanowires (H–GeNWs) grown along the [111] and [001] crystallographic directions with surface and interstitial Li atoms. The results show that the germanium nanowires (GeNWs) with surface Li atoms maintain their semiconducting behavior but their energy gap size decreases when the Li concentration grows. In contrast, the GeNWs can have semiconductor or metallic behavior depending on the concentration of the interstitial Li atoms. On the other hand, Y is an indicator of the structural changes that GeNWs suffer due to the concentration of Li atoms. For surface Li atoms, Y stays almost constant, whereas for interstitial Li atoms, the Y values indicate important structural changes in the GeNWs.

Self-amplified photo-induced gap quenching in a correlated electron material.

Capturing the dynamic electronic band structure of a correlated material presents a powerful capability for uncovering the complex couplings between the electronic and structural degrees of freedom. When combined with ultrafast laser excitation, new phases of matter can result, since far-from-equilibrium excited states are instantaneously populated. Here, we elucidate a general relation between ultrafast non-equilibrium electron dynamics and the size of the characteristic energy gap in a correlated electron material. We show that carrier multiplication via impact ionization can be one of the most important processes in a gapped material, and that the speed of carrier multiplication critically depends on the size of the energy gap. In the case of the charge-density wave material 1T-TiSe2, our data indicate that carrier multiplication and gap dynamics mutually amplify each other, which explains—on a microscopic level—the extremely fast response of this material to ultrafast optical excitation.

Energy gap of novel edge-defected graphene nanoribbons

Herein, the effects of width and boundary defects on the energy gap of graphene nanoribbons (GNRs) have been explored and theoretically investigated by means of semi-empirical atomic basis Extended Hückel method. Due to the existence of boundary defects, the energy gap of GNRs is mainly determined by the width of graphene nanoribbons for armchair graphene nanoribbons (AGNRs) or zigzag graphene nanoribbons (ZGNRs). Interestingly, the energy gap of AGNRs with a 120° V-type defect displays the monotone decreasing tendency when the width reaches to 2 nm, while the energy gap of intrinsic AGNRs is oscillatory. At the same time, the energy gap of U-type defected ZGNRs is opened, which differs from the zero energy gap characteristics of the intrinsic zigzag graphene. Furthermore, the size of energy gap of the defected AGNRs and ZGNRs with the same width is proved to be very close. Calculation results demonstrate that the energy gap of GNRs is just inversely proportional to the width and has little to do with the crystallographic direction. All the findings above provide a basis for energy gap engineering with different edge defects in GNRs and signify promising prospects in graphene-based semiconductor electronic devices.

Electronic structure of a narrow-gap semiconductor FeGa3investigated by photoemission and inverse photoemission spectroscopies

We have performed a photoemission and inverse photoemission spectroscopic study of a narrow-gap semiconductor FeGa${}_{3}$, in order to characterize the occupied and unoccupied electronic states. The energy-gap size was found to be $~0.4$ eV, and the valence-band maximum (VBM) was located around the A point of the Brillouin zone. We observed a dispersive Ga $4\mathit{sp}$ derived band near the Fermi level (${E}_{\mathrm{F}}$), and Fe $3d$ narrow bands located at $\ensuremath{-}0.5$ and $\ensuremath{-}1.1$ eV away from ${E}_{\mathrm{F}}$. In contrast to the case of FeSi, there was no temperature-dependent peak enhancement at the VBM on cooling. The observed density of states and band dispersions were reasonably reproduced by the LDA$+$$U$ calculation with the on-site effective Coulomb interaction ${U}_{\mathrm{eff}}~3$ eV to the Fe $3d$ states. Present results indicate that, in spite of sizable ${U}_{\mathrm{eff}}/W~0.6$ ($W$: band width), electron correlation effects are not significant in FeGa${}_{3}$ compared with FeSi since the VBM consists of the dispersive band with the reduced Fe $3d$ contribution, and the energy gap is large.

Controlling Energy Gap of Bilayer Graphene by Strain

Using the first principles calculations, we show that mechanically tunable electronic energy gap is realizable in bilayer graphene if different homogeneous strains are applied to the two layers. It is shown that the size of the energy gap can be simply controlled by adjusting the strength and direction of these strains. We also show that the effect originates from the occurrence of strain-induced pseudoscalar potentials in graphene. When homogeneous strains with different strengths are applied to each layer of bilayer graphene, transverse electric fields across the two layers can be generated without any external electronic sources, thereby opening an energy gap. The results demonstrate a simple mechanical method of realizing pseudoelectromagnetism in graphene and suggest a maneuverable approach to fabrication of electromechanical devices based on bilayer graphene.

Semiconducting Electronic Structure of Graphene Adsorbed on Insulating Substrate: Fragility of the Graphene Linear Dispersion Band

Using the first-principle total-energy procedure within the framework of the density functional theory, we study the electronic structure of graphene adsorbed on hexagonal boron nitride (h-BN). We find that the linear dispersion band of graphene is fragile and that graphene adsorbed on h-BN exhibits semiconducting properties with a narrow energy gap. The energy gap size depends weakly on the relative atomic arrangement between graphene and the h-BN substrate. Nonuniform electron density on the substrate causes an imbalance in the electrostatic potential between the two sublattices on graphene, leading to their semiconducting property when adsorbed on h-BN.

Field-dependent specific heat ofYb4As3: Agreement between a spin-12model and experiment

We report the low-temperature specific-heat measurements on polydomain ${\text{Yb}}_{4}{\text{As}}_{3}$ at magnetic fields up to 20 T. Taking into account the Bethe ansatz results, the zero-field data have been used for the estimation of the lattice specific heat, resulting in a value of the exchange integral for the Heisenberg model of the antiferromagnetic spin $S=\frac{1}{2}$ chain of $J/{k}_{B}=\ensuremath{-}28\text{ }\text{K}$. A quantitative agreement has been achieved between the experimental magnetic specific-heat data in magnetic field and the numerical results obtained by the quantum transfer-matrix (QTM) simulation technique. The finite-size QTM approximants have been analyzed and an extrapolation procedure recovering the known density matrix renormalization group (DMRG) results down to very low temperature has been proposed. On the basis of the data in magnetic field and using the earlier DMRG results, the energy-gap size $\ensuremath{\Delta}$ has been analyzed as a function of the applied magnetic field $B$, leading to an experimental verification of the scaling law $\ensuremath{\Delta}\ensuremath{\propto}{B}^{2/3}$ following from the sine-Gordon model.

Excitonic Instability in the Transition from the Black Phase to the Golden Phase of SmS under Pressure Investigated by Infrared Spectroscopy

We report the pressure-dependent optical reflectivity spectra of a strongly correlated insulator, samarium monosulfide (SmS), in the far- and middle-infrared regions to investigate the origin of the pressure-induced phase transition from the black phase to the golden phase. The energy gap becomes narrow with increasing pressure in the black phase. A valence transition from Sm 2+ in the black phase to mainly Sm 3+ in the golden phase accompanied by spectral change from insulator to metal were observed at the transition pressure of 0.65 GPa. The black-to-golden phase transition occurs when the energy gap size of black SmS becomes the same as the binding energy of the exciton at the indirect energy gap before the gap closes. This result indicates that the valence transition originates from an excitonic instability.

Molecular mechanics and molecular orbital simulations on specific interactions between peroxisome proliferator-activated receptor PPARα and plasticizer

Peroxisome proliferator-activated receptor α (PPARα) has various physiological functions such as lipid and glucose metabolism, inflammation and fibrosis in living organisms. Many types of ligand molecules such as phthalate and adipate esters control these physiological functions. In the present study, to elucidate the dependence of PPARα properties on ligand binding, we investigated stable structures and electronic properties for the complexes of PPARα and phthalate as well as adipate esters, which are used as a plasticizer, by molecular simulations based on molecular mechanics and molecular orbital methods. Furthermore, to elucidate the influence of these esters in vivo, we injected them into male mice and observed the change in the expression of PPARα-related enzymes. The comparison between the calculated and observed results indicates that the change in the expression has a correlation with the size of energy gaps between highest occupied and lowest unoccupied molecular orbitals of the complexes with mouse PPARα and esters.

Sb-NMR/NQR study of CeIrSb

We report Sb-NMR/NQR measurements on a valence fluctuating compound CeIrSb, which is isostructural to the Kondo semiconductors CeRhSb and CeNiSn. The nuclear spin–lattice relaxation rate divided by temperature, 1 / T 1 T has a maximum around 300 K and considerably decreases in proportion to T 2 when cooling down, followed by a 1 / T 1 T ∼ const . behavior below 20 K. These results indicate that CeIrSb has a V-shaped pseudogap with a residual density of states at the Fermi level. The size of pseudogap for CeIrSb is estimated to be about 350 K, which is one order of magnitude larger than the values of 28 K for CeRhSb and 14 K for CeNiSn. The larger size of energy gap in CeIrSb may be attributed to much stronger c– f hybridization than those for CeRhSb and CeNiSn.

Energy gap formation in the valence fluctuating compound CeIrSb probed by Sb NMR and NQR

Sb NMR and NQR studies have revealed the formation of a pseudogap at the Fermi level in the density of states in a valence fluctuating compound CeIrSb. The nuclear spin-lattice relaxation rate divided by temperature, $1∕{T}_{1}T$, has a maximum around $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and decreases significantly as $1∕{T}_{1}T\ensuremath{\sim}{T}^{2}$, followed by a $1∕{T}_{1}T=\mathrm{const}$ relation at low temperature. This temperature dependence of $1∕{T}_{1}T$ is well reproduced by assuming a V-shaped energy gap with a residual density of states at the Fermi level. The size of energy gap for CeIrSb is estimated to be about $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is by 1 order of magnitude larger than those for the isostructural Kondo semiconductors CeRhSb and CeNiSn. Despite the large difference in the size of energy gap, CeIrSb, CeRhSb, and CeNiSn are indicated to be classified into the same group revealing a V-shaped gap due to $c\text{\ensuremath{-}}f$ hybridization. The temperature dependence of the Knight shift measured in a high magnetic field agrees with the formation of this pseudogap.

Exit Routes from the Transition State: Angular Momentum Constraints on the Formation of Products

We have analyzed experimental data from a number of exothermic processes in which molecules in well-defined initial states are deactivated by inelastic, dissociative, or reactive collisions. Further, we analyze deactivation processes that do not occur in molecules despite their containing high levels of excitation. Significant common elements are found among these forms of deactivation. The initial step consists of transition to a product state involving minimum rotation state change (Delta j) consistent with energy conservation. Frequently, this process is near-energy-resonant. More critically, it may frequently require substantial angular momentum (AM) change. Analysis of experimental data indicates that constraints act upon on the formation of products in processes that involve release of excess energy. These constraints are associated with the magnitude of AM that must be generated for the initial transition to occur and this AM "load" increases with the amount of energy to be released. In general, the probability of generating rotational AM falls rapidly as Delta j increases, and this effectively limits the size of energy gap that may be bridged by a given reactant pair and at some point the constraint is sufficient to constitute a barrier that prevents the process from taking place. The choice of reactant species strongly affects the probability of each process that increases (i) when molecules efficiently interconvert momentum and (ii) when many product states are available in the critical near-resonant region. These factors increase the proportion of initial trajectories that possess the energy and momentum necessary to open a "product" channel. Evidence is presented showing that AM load-reduction strategies lead to marked enhancement of rates of collision-induced processes, suggesting that reduction of constraints in the exit channels from the transition state may constitute a previously unrecognized form of catalysis.

Effect of pressure on the size of energy gap in semiconducting mixed-valent rare-earth compounds

We have synthesized CeRhBi and CeRhAs and found that these compounds crystallize in the ϵ-TiNiSi-type structure. It is found that CeRhBi is a heavy-fermion compound, whereas CeRhAs is a mixed-valent semiconductor with an energy gap of 144 K, about 10 times larger than that of CeRhSb. Various experimental results throughout the CeRhX series also vary in the sequence CeRh (BiSbAs). However, the gap observed in CeRhAs is suppressed by external-pressure, suggesting that the chemical and the external pressure seem to cause opposite effects on the size of energy gap in these compounds. These results are compared with those for mixed-valent semiconductors Ce 3Pt 3Bi 4 and Ce 3Pt 3Sb 4.

Electronic transitions from the A 2Πu i(v=3) level of N+2 induced by inelastic collisions with helium atoms

An optical–optical double-resonance technique utilizing two pulsed lasers is used to study collision-induced electronic transitions from the N+2 A 2Πui(v=3) level by helium. Collisional deactivation paths are determined by this technique and found to be between this level and the X 2Σ+g (v=7 and 6) levels. The same propensity for ΔJ≊0 occurs for both of these paths in spite of an electronic energy gap size of approximately 0 cm−1 between the A(v=3) and X(v=7) levels and a large gap size of about 1950 cm−1 between the A(v=3) and X(v=6) levels. The electronic quenching rate from A(v=3) to X(v=7) is found to be only about three times larger than that to the X(v=6) level. We use this branching ratio in an electronic relaxation model to determine the collisional quenching rates between the A(v=3) and X(v=7 and 6) levels. These state specific rates are determined by fitting the model to observed radiative decay curves from the A(v=3) level obtained at various helium pressures. There is excellent agreement between the analytical and observed decay curves. The relatively efficient nature of the collision-induced electronic transition over the large energy gap is somewhat surprising in view of the fact that the nitrogen ions and helium atoms must remove most of this energy as translational kinetic energy. We have also revised our previous rate constants from the A 2Πui(v=4) level for 14N+2 and 15N+2.

Exchange, correlation, and band gaps in conjugated linear chains

AbstractIn the case of conjugated polyenic chains (polyacetylene), the relationships between the long‐ (or short‐) range nature of the restricted Hartree–Fock exchange interaction, the role of correlation effects, and the size of energy gaps are illustrated.