Tunable Energy Gap Research Articles

Tunable band gap and hydrogen adsorption property of a two-dimensional porous polymer by nitrogen substitution

After substitution of carbon by nitrogen, the electronic structures of the porous graphene have been studied by the density functional theory calculations. The N substitutional site without hydrogen passivation leads to a tunable energy gap of the two-dimensional porous polymer, depending on the number of N atoms in a unit cell. Moreover, the increasing number of N in an aromatic ring enhances the binding energies between hydrogen molecules and pre-adsorbed Li atoms from 1H(2) to 3H(2). Thus, porous polymer materials through controllable synthesis techniques will improve their potential applications in photosplitting of water as well as hydrogen storage.

Kinetic Isotope Effect in the Hydrogenation and Deuteration of Graphene

AbstractTime‐dependent photoemission spectroscopy is employed to study the kinetics of the hydro‐genation/deuteration reaction of graphene. Resulting in an unusual kinetic isotope effect, the graphene deuteration reaction proceeds faster than hydrogenation and leads to substantially higher maximum coverages of deuterium (D/C≈35% vs H/C≈25%). These results can be explained by the fact that in the atomic state H and D have a lower energy barrier to overcome in order to react with graphene, while in the molecular form the bond between two atoms must be broken before the capture on the graphene layer. More importantly, D has a higher desorption barrier than H due to quantum mechanical zero‐point energy effects related to the C–D or C–H stretch vibration. Molecular dynamics simulations based on a quantum mechanical electronic potential can reproduce the experimental trends and reveal the contribution of the constituent chemisorption, reflection, and associative desorption processes of H or D atoms onto graphene. Regarding the electronic structure changes, a tunable electron energy gap can be induced by both deuteration and hydrogenation.

Physical Properties of Zn<SUB>x</SUB>Cd<SUB>1-x</SUB>S Nanocrytalline Layers Synthesized by Solution Growth Method

In recent years, zinc cadmium sulphide (ZnxCd1-xS) alloy compounds have paid much attention in the fields of opto-electronics, particularly in photovoltaic devices because of its tunable energy gap and the lattice parameters. The energy band gap of ZnxCd1-xS is controlled by the change of Zn-composition in order to suit the material properties with that of absorber material in solar cells. In this paper, we report on the effect of Zn-composition on physical properties of ZnxCd1-xS thin films deposited on corning glass substrates by solution growth method. The layers were prepared for different 'x' values that vary in the range, 0 - 1.0 at. %. The as-grown layers were characterized using EDAX, XRD, SEM, and UV-Vis-NIR spectrophotometers. All the layers showed a strong (002) plane as the preferred orientation that exhibited the hexagonal crystal structure. The composition of the layers agrees approximately with that of the elements in the solution. The films showed an average optical transmittance of 72 % at a zinc composition of 0.75 with a band gap of 3.88 eV.

The optical responsivity in IR-photodetector based on armchair graphene nanoribbons with p–i–n structure

Armchair graphene nanoribbons (A-GNRs) are an alternative material to use in novel infrared photodetectors, because of their tunable energy gap in the infrared spectrum, and their high quantum efficiency. In this paper, an A-GNR p–i–n structure with all three structural families, different width, and different number of layers to use in IR detectors have been investigated. With calculating the band structure and energy gap using the tight-binding model and by including the edge deformation, the optical absorption in the single electron approximation has been obtained by calculating the optical conductance. Finally, we have calculated the quantum efficiency and the optical responsivity of A-GNR based IR photodetector as a function of incident photon energy, temperature, nanoribbon width and the number of layers. Results show that the responsivity of the A-GNR based IR photodetector increase by increasing the width and number of layers and decrease by increasing the temperature.

Flux qubit with a large loop size and tunable Josephson junctions

We present the design of a superconducting flux qubit with a large loop inductance. The large loop inductance is desirable for coupling between qubits. The loop is configured into a gradiometer form that could reduce the interference from environmental magnetic noise. A combined Josephson junction, i.e., a DC-SQUID is used to replace the small Josephson junction in the usual 3-JJ (Josephaon junction) flux qubit, leading to a tunable energy gap by using an independent external flux line. We perform numerical calculations to investigate the dependence of the energy gap on qubit parameters such as junction capacitance, critical current, loop inductance, and the ratio of junction energy between small and large junctions in the flux qubit. We suggest a range of values for the parameters.

Near‐Infrared Thermochromic Diazapentalene Dyes

A series of 2,5-diazapentalene containing dyes with tunable energy gaps are visible and near-infrared halochromic towards various acids and their protonated counterparts represent a new class of thermochromic materials with the near-infrared absorption being switched on at room temperature and off above 50 °C.

Nonmagnetic and magnetic thiolate-protected Au25superatoms on Cu(111), Ag(111), and Au(111) surfaces

Geometry, electronic structure, and magnetic properties of methylthiolate-stabilized Au${}_{25}$L${}_{18}$ and MnAu${}_{24}$L${}_{18}$ (L = SCH${}_{3}$) clusters adsorbed on noble-metal (111) surfaces have been investigated by using spin-polarized density functional theory computations. The interaction between the cluster and the surface is found to be mediated by charge transfer mainly from or into the ligand monolayer. The electronic properties of the 13-atom metal core remain in all cases rather undisturbed as compared to the isolated clusters in gas phase. The Au${}_{25}$L${}_{18}$ cluster retains a clear highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap in the range of 0.7 to 1.0 eV depending on the surface. The ligand layer is able to decouple the electronic structure of the magnetic MnAu${}_{24}$L${}_{18}$ cluster from the Au(111) surface, retaining a high local spin moment of close to 5 ${\ensuremath{\mu}}_{B}$ arising from the spin-polarized Mn($3d$) electrons. These computations imply that the thiolate monolayer-protected gold clusters may be used as promising building blocks with tunable energy gaps, tunneling barriers, and magnetic moments for applications in the area of electron and/or spin transport.

Fluorinating Hexagonal Boron Nitride/Graphene Multilayers into Hybrid Diamondlike Nanofilms with Tunable Energy Gap

Using ab initio calculations and quantum molecular dynamics simulations, we demonstrate that a few layers of graphene sandwiched between hexagonal boron nitride (h-BN) layers can undergo spontaneous transformation into hybrid cubic BN-diamond (c-BN/Dmd) nanofilms upon fluorination. This spontaneous transformation stems from the remarkably higher stability of thin c-BN/Dmd nanofilm with sp3 hybridization over the precursor multilayer with sp2 hybridization and is promoted by strong selectivity of fluorination with the boron atoms of the coating BN layers. Upon increasing the total number of multilayers, however, the transformation is no longer spontaneous due to emergence of the energy barrier. Nevertheless, adding more h-BN layers to the hybrid nanofilm can assist the transformation into c-BN/Dmd nanofilms upon fluorination. The electronic properties of the c-BN/Dmd nanofilms can be tuned by controlling the ratio of the BN component and film thickness, which can yield narrow-gap semiconductors for novel electronic applications. In addition, the energy gap in the nanofilms can be modulated linearly by applying external electric fields.

Spatially resolving edge states of chiral graphene nanoribbons

A central question in the field of graphene-related research is how graphene behaves when it is patterned at the nanometer scale with different edge geometries. Perhaps the most fundamental shape relevant to this question is the graphene nanoribbon (GNR), a narrow strip of graphene that can have different chirality depending on the angle at which it is cut. Such GNRs have been predicted to exhibit a wide range of behaviour (depending on their chirality and width) that includes tunable energy gaps and the presence of unique one-dimensional (1D) edge states with unusual magnetic structure. Most GNRs explored experimentally up to now have been characterized via electrical conductivity, leaving the critical relationship between electronic structure and local atomic geometry unclear (especially at edges). Here we present a sub-nm-resolved scanning tunnelling microscopy (STM) and spectroscopy (STS) study of GNRs that allows us to examine how GNR electronic structure depends on the chirality of atomically well-defined GNR edges. The GNRs used here were chemically synthesized via carbon nanotube (CNT) unzipping methods that allow flexible variation of GNR width, length, chirality, and substrate. Our STS measurements reveal the presence of 1D GNR edge states whose spatial characteristics closely match theoretical expectations for GNR's of similar width and chirality. We observe width-dependent splitting in the GNR edge state energy bands, providing compelling evidence of their magnetic nature. These results confirm the novel electronic behaviour predicted for GNRs with atomically clean edges, and thus open the door to a whole new area of applications exploiting the unique magnetoelectronic properties of chiral GNRs.

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.

Dynamical response functions and collective modes of bilayer graphene

Bilayer graphene (BLG) has recently attracted a great deal of attention because of its electrically tunable energy gaps and its unusual electronic structure. In this Rapid Communication we present analytical and semianalytical expressions, based on the four-band continuum model, for the layer-sum and layer-difference density-response functions of neutral and doped BLG. These results demonstrate that BLG density fluctuations can exhibit either single-component massive-chiral character or standard two-layer character, depending on energy and doping.

Versatile, Benzimidazole/Amine‐Based Ambipolar Compounds for Electroluminescent Applications: Single‐Layer, Blue, Fluorescent OLEDs, Hosts for Single‐Layer, Phosphorescent OLEDs

AbstractA series of compounds containing arylamine and 1,2‐diphenyl‐1H‐benz[d]imidazole moieties are developed as ambipolar, blue‐emitting materials with tunable blue‐emitting wavelengths, tunable ambipolar carrier‐transport properties and tunable triplet energy gaps. These compounds possess several novel properties: (1) they emit in the blue region with high quantum yields; (2) they have high morphological stability and thermal stability; (3) they are capable of ambipolar carrier transport; (4) they possess tunable triplet energy gaps, suitable as hosts for yellow‐orange to green phosphors. The electron and hole mobilities of these compounds lie in the range of 0.68–144 × 10−6 and 0.34–147 × 10−6 cm2 V−1 s−1, respectively. High‐performance, single‐layer, blue‐emitting, fluorescent organic light‐emitting diodes (OLEDs) are achieved with these ambipolar materials. High‐performance, single‐layer, phosphorescent OLEDs with yellow‐orange to green emission are also been demonstrated using these ambipolar materials, which have different triplet energy gaps as the host for yellow‐orange‐emitting to green‐emitting iridium complexes. When these ambipolar, blue‐emitting materials are lightly doped with a yellow‐orange‐emitting iridium complex, white organic light‐emitting diodes (WOLEDs) can be achieved, as well by the use of the incomplete energy transfer between the host and the dopant.

Fermi Condensates for Dynamic Imaging of Electromagnetic Fields

Ultracold gases provide micrometer size samples whose sensitivity to external fields may be exploited in sensor applications. Bose-Einstein condensates of atomic gases have been demonstrated to perform excellently as magnetic field sensors in atom chips. Here we propose that condensates of fermions can be used for noninvasive sensing of time-dependent and static magnetic and electric fields, by utilizing the tunable energy gap in the excitation spectrum as a frequency filter. Perturbations by the field create collective excitations and quasiparticles. The latter requires the frequency of the perturbation to exceed the gap. The frequencies of the field may be selectively monitored from the amount of quasiparticles which is measurable, e.g., by rf spectroscopy. We analyze the method by calculating the density-density susceptibility and discuss its sensitivity and spatial resolution.

Direct energy gap group IV semiconductor alloys and quantum dot arrays in Sn xGe 1− x/Ge and Sn xSi 1− x/Si alloy systems

The narrow gap semiconductor alloys Sn x Ge 1− x and Sn x Si 1− x offer the possibility for engineering tunable direct energy gap Group IV semiconductor materials. For pseudomorphic Sn x Ge 1− x alloys grown on Ge (001) by molecular beam epitaxy, an indirect-to-direct bandgap transition with increasing Sn composition is observed, and the effects of misfit on the bandgap analyzed in terms of a deformation potential model. Key results are that pseudomorphic strain has only a very slight effect on the energy gap of Sn x Ge 1− x alloys grown on Ge (001) but for Sn x Ge 1− x alloys grown on Ge (111) no indirect-to-direct gap transition is expected. In the Sn x Si 1− x system, ultrathin pseudomorphic epitaxially-stabilized α-Sn x Si 1− x alloys are grown on Si (001) substrates by conventional molecular beam epitaxy. Coherently strained α-Sn quantum dots are formed within a defect-free Si (001) crystal by phase separation of the thin Sn x Si 1− x layers embedded in Si (001). Phase separation of the thin alloy film, and subsequent evolution occurs via growth and coarsening of regularly-shaped α-Sn quantum dots that appear as 4–6 nm diameter tetrakaidecahedra with facets oriented along elastically soft 〈100〉 directions. Attenuated total reflectance infrared absorption measurements indicate an absorption feature due to the α-Sn quantum dot array with onset at ∼0.3 eV and absorption strength of 8×10 3 cm −1, which are consistent with direct interband transitions.

Spectroscopical evidences of photoinduced charge transfer in blends of C 60 and thiophene-based copolymers with a tunable energy gap

We report a study of the photoinduced charge transfer in the blends of buckminsterfullerene (C 60) and thiophene-based copolymers with tunable energy gap. The photoluminescence and steady state photoinduced absorption spectra evidence the electron transfer from the photoexcited copolymer to C 60. The efficiency of the charge transfer depends on the degree of miscibility of the two components which is easily controlled by the copolymer composition. In the blends obtained with a copolymer with an high energy absorption onset, the spectral signatures of the photoinduced hole transfer to the polymer, arising from the selective excitation of the C 60, are also detected.

Photoexcitations of thiophene-based copolymers with tunable energy gap

The photoluminescence and the cw photoinduced absorption spectra of a series of conjugated polymers obtained by the copolymerisation of 3-butylthiophene and 3,4-dibutylthiophene in different ratio are reported. By varying the copolymer chemical composition it is possible to tune the intensity and the colour of the luminescence spectra in the whole visible range. The PA bands, associated to charged species and triplet states, are also affected by the copolymer composition. We show that disorder, controlled by interring rotations, is the tuning factor for the photoexcitations in these materials.

Formation of Direct Energy Gap Group IV Semiconductor Alloys and Quantum Dot Arrays in SnxSi1−x /Si and SnxGe1−x/Ge Alloy Systems

AbstractThe narrow gap semiconductor alloys SnxGe1−x, and SnxSi1−x offer the possibility for engineering tunable direct energy gap Group IV semiconductor materials. For pseudomorphic SnxGe1−x, alloys grown on Ge (001) by molecular beam epitaxy, an indirect-to-direct bandgap transition with increasing Sn composition is observed, and the effects of misfit on the bandgap analyzed in terms of a deformation potential model. Key results are that pseudomorphic strain has only a very slight effect on the energy gap of SnxGe1−x, alloys grown on Ge (001) but for SnxGe1−x alloys grown on Ge (111) no indirect-to-direct gap transition is expected. In the SnxSi1−x system, ultrathin pseudomorphic epitaxially-stabilized α-SnxSi1−x alloys are grown on Si (001) substrates by conventional molecular beam epitaxy. Coherently strained oa-Sn quantum dots are formed within a defect-free Si (001) crystal by phase separation of the thin SnxSi1−x layers embedded in Si (001). Phase separation of the thin alloy film, and subsequent evolution occurs via growth and coarsening of regularly-shaped α-Sn quantum dots that appear as 4–6 nm diameter tetrakaidecahedra with facets oriented along elastically soft [100] directions. Attenuated total reflectance infrared absorption measurements indicate an absorption feature due to the α-Sn quantum dot array with onset at ˜0.3 eV and absorption strength of 8 × 103 cm−1, which are consistent with direct interband transitions.

Carbon fiber polymer-matrix structural composite as a semiconductor and concept of optoelectronic and electronic devices made from it

An epoxy-matrix composite with continuous crossply carbon fibers was found to be a semiconductor in the through-thickness direction, with a tunable energy gap of - eV (infrared). The higher the pressure during composite fabrication by lamination, the higher the interlaminar stress and the greater the energy gap, which is the activation energy for electron jumping from one lamina to the adjacent one in the composite. The semiconducting behavior involves the contact electrical resistivity between adjacent laminae in the composite decreasing reversibly with increasing temperature. The concept of optoelectronic and electronic devices made from carbon fiber polymer-matrix composites is provided. Devices include solar cells, light emitting diodes, lasers, infrared detectors and transistors. Thus, a new dimension is added to smart structures and a new field of electronics (`structural electronics') is born.

Photoluminescence of thiophene-based copolymers with tunable energy gap

The photoluminescence spectra and the quantum yields of a series of conjugated polymers obtained by the copolymerisation of 3 butylthiophene and 3,4-dibutylthiophene in different ratio are reported. The emission maxima, like the absorption maxima, are tunable in the visible by varying the fraction f of disubstituted monomers. By increasing f there is a blue shift of the spectra while the Stokes shift exhibits a considerable increase both in the solid state and in solution. The role of ring rotations in the structural relaxation of the excited state and in the non radiative recombination channels is discussed.

Synthesis and characterization of mixed CdSZnS nanoparticles in reverse micelles

Abstract Three different types of mixed CdSZnS semiconductor nanoparticles were synthesized in reverse micelles and were characterized by optical absorption and photoluminescence (PL) spectroscopy. The coprecipitated ZnxCd1−xS nanoparticles, comprised of homogeneously mixed crystals, show a continuously tunable energy gap from single CdS nanoparticles to ZnS nanoparticles. Coating the CdS core particles with a layer of ZnS in reverse micelles leads to ZnS-coated CdS (CdS/ZnS) nanoparticles with a core-shell structure, which show absorption and PL characteristics which differ considerably to those of either the coprecipitated particles or the sum of the separate particles. The intensity of the PL band centered at 630 nm, which arises from the coated CdS/ZnS nanoparticles, shows a maximum at a Zn/Cd molar ratio of about 1:1 on increasing the Zn/Cd ratio when the total concentration of CdS and ZnS is kept constant. CdS-coated ZnS (ZnS/CdS) nanoparticles synthesized in reverse micelles have absorption spectra similar to those of the coated ZnS/CdS nanoparticles but show no significant luminescence activation.