Tunable Energy Gap Research Articles

Confined state energies in AGNR semiconductor–semiconductor heterostructure

A detailed study of semiconductor–semiconductor heterostructure (SSH) graphene structure, which is combined with the 3m and 3m+1 families AGNRs, is presented in this paper. The research uncovers that the confined state energies of SSH can be modulated continuously between the semiconductor property and the metal property, proved by both the DFT calculation and the Extended Huckel calculation. The results reveal that this tunable energy gap can be attributed to the quantum well localization effect which is induced by the combination of SSH's two different families AGNRs. Furthermore, a brief description of the relationship between SSH's energy gap and Bloch wave of different states is provided. Basing on SSH's tunable energy gap, it is possible to bring a brilliant future for graphene electronic and optical device applications.

RAMAN Characterization of Ge1-XSnX Allows By Using a SPACE Correlation MODEL

Ge1-xSnx alloys present important optoelectronic properties due to the possibility for obtain a direct and tunable energy gap group IV semiconductor. Ge1-xSnx alloys were grown on different substrates by using a R. F. Magnetron Sputtering. Raman spectroscopy and HRXRD were used in order to determinate the structural properties of these alloys. We determined a linear shift of the Raman peak that depends directly on the Sn concentration of the alloy. We also find that the Raman peak has an asymmetrical shape attributed to a small crystal size. In this work we present the characterization of the Raman peaks by fitting a Space Correlation Model. We obtain a strong correlation among FWHM, asymmetrical shape and Sn concentration of the alloys. Keywords: HRXRD, Raman.

Two-dimensional topological crystalline insulator phase in quantum wells of trivial insulators

The realization of two-dimensional (2D) topological insulators (TIs) in HgTe/CdTe quantum wells (QWs) has generated an explosion of research on TIs and novel topologically nontrivial phases. Here we predict, based on first-principles calculations, that the newly discovered 2D topological crystalline insulators (TCIs) phase exists even in the QWs of trivial insulators, e.g. (Sn/Pb)Te and Na(Cl/Br), with mirror Chern number . Tunable nontrivial energy gaps ranging from 4 to 238 meV are obtained, guaranteeing further room-temperature observations and applications. The combined effect of strain and electrostatic interaction that can be engineered by the cladding layers leads to a band inversion, resulting in the phase transition from trivial insulator to 2D TCIs. Our work provides a new strategy for engineering topological states in 2D materials.

Ternary Se[sbnd]Ru[sbnd]Cu clusters: Facile synthesis, electrochemistry, and UV/Vis absorption

New ternary SeRuCu octahedral-based complexes, namely, CuX-capped SeRu5 clusters [SeRu5(CO)14(μ3-CuX)]2− (2a–2c; X = Cl, Br, I), bis-CuX-capped SeRu5 clusters [SeRu5(CO)14(μ3-CuX)2]2− (3a, 3b; X = Cl, Br), Cu4X2-bridged di-SeRu5 clusters [{SeRu5(CO)14}2(μ6-Cu4X2)]2− (4a, 4b; X = Cl, Br), and bis-CuX-incorporated Se2Ru4 clusters [Se2Ru4(CO)10(μ-CuX)2]2− (5a–5c; X = Cl, Br, I), were prepared from the reaction of the binary cluster [SeRu5(CO)14]2− (1) with CuX under appropriate conditions. Their solid-state packing formed cluster-based 1D-supramolecular chains, which were stabilized by CH⋯X/Y (X = halides; Y = O) H-bonding between CuX/COs of cluster anions and the CH moieties of the [PPh4]+ or [PPN]+ cations. In addition, electrochemical studies showed that the successive anodic shifts were found for 1–4b, tuned by the incoming CuBr fragments introduced into the parent SeRu5 cluster, supported by their decreasing HOMO levels and increasing ionization energies. Solid-state diffuse reflectance spectra showed these SeRuCu complexes exhibited semiconducting behaviors with the low and tunable energy gaps of 0.82–1.08 eV. Significantly, the solid-state absorptions of 2b and 4b were red-shifted compared with the corresponding Te analogues, which was explained by the larger numbers of the density of states (DOS) close to the HOMO for the selenide cases.

Electronic Structures of Quaterthiophene and Septithiophene on Cu(111): Spatial Distribution of Adsorption-Induced States Studied by STM and DFT Calculation

The oligothiophene molecule family has a tunable energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) as the function of the number of thiophene units. This tunability is of great use to controlling carrier injection at the molecule/electrode interface in molecule-based electronic devices. We investigate quaterthiophene (4T) and septithiophene (7T) molecules adsorbed on Cu(111) surfaces by scanning tunneling microscopy and spectroscopy (STM and STS) at room temperature. Both oligothiophene molecules form one-dimensional (1D) chain structures on Cu(111), and each molecule in the 1D structures is observed as a row of bright ovals corresponding to thiophene units. Observed features of 4T and 7T molecules differ from those expected from the HOMO and LUMO of the free-standing molecules, and density-functional calculations of a 4T molecule together with a Cu(111) surface reproduce the experimental STM images as they reflect characteristic spatial distribution of adsor...

Tunable electronic and dielectric properties of β-phosphorene nanoflakes for optoelectronic applications

Electronic and dielectric properties of β-phosphorene nanoflakes in various configurations using density functional theory are reported. The tunable energy gap and dielectric response make these nanoflakes potential candidates for optoelectronic device applications.

Direct Observation of a Gate Tunable Band Gap in Electrical Transport in ABC-Trilayer Graphene.

Few layer graphene systems such as Bernal stacked bilayer and rhombohedral (ABC-) stacked trilayer offer the unique possibility to open an electric field tunable energy gap. To date, this energy gap has been experimentally confirmed in optical spectroscopy. Here we report the first direct observation of the electric field tunable energy gap in electronic transport experiments on doubly gated suspended ABC-trilayer graphene. From a systematic study of the nonlinearities in current versus voltage characteristics and the temperature dependence of the conductivity, we demonstrate that thermally activated transport over the energy-gap dominates the electrical response of these transistors. The estimated values for energy gap from the temperature dependence and from the current voltage characteristics follow the theoretically expected electric field dependence with critical exponent 3/2. These experiments indicate that high quality few-layer graphene are suitable candidates for exploring novel tunable terahertz light sources and detectors.

(Invited) Tunable Electronic Energy Structure of ZnSe-AgInSe2 Solid Solution Nanoparticles for Solar Energy Conversion

Size-quantized semiconductor nanoparticles have attracted much attention as an efficient light absorbing material in quantum dot solar cells due to their tunable energy gap (Eg) for light energy conversion systems. Extending the optical absorption range of nanoparticles has been an important research subject to effectively utilize solar light, which contains light from the UV-visible to near-IR wavelength region. As for binary semiconductor nanoparticles (e.g. CdTe, PbS and PbSe), the control of their size has been a useful method to modify their optical properties. Recently, we have reported the syntheses of multinary semiconductor nanoparticles, such as AgInS2[1], ZnS-AgInS2[1], and Ag8SnS6[2], as an alternative to highly toxic binary particles such as CdTe and PbSe. The wide tunability of chemical composition also enabled to control the Eg of these particles. In this study, we report the fabrication and photoelectrochemical characterization of ZnSe-AgInSe2 (ZAISe) solid solution nanoparticles as a novel near-IR-responsive material for a solar energy conversion.[3] ZAISe(x) nanoparticles ((AgIn)xZn2(1-x)Se2; 0≦x≦1) were synthesized by a thermal decomposition of metal acetate and selenourea in oleylamine for 10 min at 250 oC under nitrogen atmosphere. Thus-obtained ZAISe nanoparticles were immobilized on TiO2 porous films deposited on FTO electrodes, in which 3-mercaptopropionic acid was used as a cross-linker between the surfaces of TiO2 and ZAISe(x) nanoparticles. These electrodes were finally surface-coated with a CdS thin layer via SILAR method to remove the surface defect sites on ZAISe particles (TiO2/ZAISe(x)/CdS). A quantum dot-sensitized solar cell (QDSSC) was prepared with the TiO2/ZAISe(x)/CdS photoelectrode and a counter electrode of Cu2S-deposited FTO electrode (Cu2S/FTO). The TiO2/ZAISe(x)/CdS was attached to the Cu2S/FTO with the use of a Himilan polymer film of 50 μm in thickness as a spacer. The cell was filled with an aqueous solution containing S2-/Sxredox species. Current density-voltage curves were measured with AM1.5G irradiation using a solar simulator. TEM observation revealed that spherical nanoparticles were formed with the size of around 4 nm, regardless of x value. As shown in Fig. 1a, the absorption onset of thus-obtained nanoparticles was blue-shifted from 900 nm to 400 nm with an increase in the content of ZnSe, indicating that the Eg of ZAISe(x) nanoparticles could be successfully controlled only by changing the value of x. Electronic energy levels of ZAISe(x) nanoparticles were measured by photoelectron spectroscopy in air. Figure 1b summarizes the electronic energy levels of ZAISe nanoparticles as a function of the value of x. Both levels of the conduction band edge (ECB) and the valence band edge (EVB) varied depending on the chemical composition of ZAISe particles: the EVB levels were slightly shifted from -5.38 to -5.04 eV with a decrease in the x value from 1.0 to 0.25, while the ECB levels were remarkably shifted from -3.94 to -2.97 eV, accompanied by an increase in Eg from 1.44 to 2.07 eV. The ECB levels of ZAISe(x) NCs were higher than that of bulk TiO2(-4.0 eV), regardless of their chemical composition. A QDSSC fabricated with TiO2/ZAISe(x)/CdS photoelectrodes exhibited different light-electricity conversion properties depending on the x value. With a decrease in the x value of ZAISe(x) from 1.0 to 0.5, both the short circuit current (JSC) the open circuit voltage (VOC) increased, resulting in a remarkable increment of power conversion efficiency (PCE). Further decrease of x from 0.5 to 0.25 slightly lessened the values of JSC, VOC, and PCE. The fill factor was almost constant at ca. 0.4 ~ 0.5, regardless of the composition of ZAISe(x) immobilized. Among the kinds of nanoparticles investigated, ZAISe(0.5) particles exhibited the optimal photovoltaic performance, JSC of 8.8 mA cm-2and PCE of 1.9%.

Stimuli‐Responsive Cyclopenta[ef]heptalenes: Synthesis and Optical Properties

AbstractWe report the one‐pot synthesis, 1D and 2D NMR characterization, and UV/Vis study of a series of cyclopenta[ef]heptalenes 4a–c that exhibit strong stimuli‐responsive behavior, with a tunable energy gap as a result of perturbation of HOMO, LUMO, and LUMO+1 energies upon doping/dedoping with TFA/Et3N. The approach employed allows for the extension of conjugation at C‐4 of the cyclopenta[ef]heptalene skeleton from X = H (4a) to X = CN (4b) and X = 2‐thiophenyl (4c), resulting in longer absorption maxima and smaller optical energy gaps of the cyclopenta[ef]heptalenium cations 4a–c+. Additionally, in the presence of a UV‐activated (< 300 nm) photoacid generator (PAG), protonation of 4c can be indirectly achieved by intermolecular photoinduced electron transfer (PeT) from the excited state of 4c to the PAG, upon which the latter undergoes photodecomposition resulting in the generation of acid. This phenomenon facilitates the use of cyclopenta[ef]heptalenes 4a–c as visible sensitizers of commercial PAGs.

Progress of terahertz devices based on graphene

Graphene has unique electronic properties stemming from a linear gapless carrier energy spectrum, and has dominant advantages in the research of devices such as lasers, detectors and modulators in terahertz region due to its tunable energy gap and extremely high carrier mobility. In this review, we summarize its latest progress in applications of terahertz devices such as lasers, detectors and modulators. Terahertz lasers based on graphene can reach a gain as high as 104 cm-1, and terahertz detectors with different structures such as a bilayer graphene field-effect transistor with top gate and buried gate can achieve NEP (noise equivalent power) ～ m nW/Hz. Graphene terahertz modulators, which are equipped with transmission configuration and reflection configuration, can have a very high modulation depth. These results may be helpful for developing the high-efficiency graphene terahertz devices.

Colloidal PbS Nanosheets with Tunable Energy Gaps

ABSTRACTUltrathin colloidal PbS nanosheets are synthesized using organometallic precursors with chloroalkane cosolvents, resulting in tunable thicknesses ranging from 1.2 nm to 4.6 nm. We report the first thickness-dependent photoluminescence spectra from lead-salt nanosheets. The one-dimensional confinement energy of these quasi-two-dimensional nanosheets is found to be proportional to 1/L instead of 1/L2 (L is the thickness of the nanosheet), which is consistent with results calculated using density functional theory as well as tight-binding theory.

Spin manipulation with magnetic semiconductor barriers.

Magnetic semiconductors are a class of materials with special spin-filtering capabilities with magnetically tunable energy gaps. Many of these materials also possess another intrinsic property: indirect exchange interaction between the localized magnetic moments and the adjacent free electrons, which manifests as an extremely large effective magnetic field applying only on the spin degrees of freedom of the free electrons. Novel device concepts can be created by taking advantage of these properties. We discuss in the article the basic principles of these phenomena, and potential ways of applying them in constructing spintronic devices.

Quantum multicriticality in bilayer graphene with a tunable energy gap

We develop a theory for quantum phases and quantum multicriticality in bilayer graphene in the presence of an explicit energy gap in the noninteracting spectrum by extending previous renormalization group (RG) analyses of electron-electron interactions in gapless bilayer graphene at finite temperature to include the effect of an electric field applied perpendicular to the sample, which produces an energy gap in the single-particle electron-hole dispersion. We determine the possible outcomes of the resulting RG equations, represented by ``fixed rays'' along which ratios of the coupling constants remain constant and map out the leading instabilities of the system for an interaction of the form of a Coulomb interaction that is screened by two parallel conducting plates placed equidistant from the electron. We find that some of the fixed rays on the ``target plane'' found in the zero-field case are no longer valid fixed rays, but that all four of the isolated rays are still valid. We also find five additional fixed rays that are not present in the zero-field case. We then construct maps of the leading instability (or instabilities) of the system for the screened Coulomb-like interaction as a function of the overall interaction strength and interaction range for four values of the applied electric field. We find that the pattern of leading instabilities is the same as that found in the zero-field case, namely, that the system is unstable to a layer antiferromagnetic state for short-ranged interactions, to a nematic state for long-ranged interactions, and to both for intermediate-ranged interactions. However, if the interaction becomes too long ranged or too weak, then the system will exhibit no instabilities. The ranges at which the nematic instability first appears, the antiferromagnetic instability disappears, and the nematic instability disappears all decrease with increasing applied electric field. Our main qualitative finding, that the applied electric field opposes the emergence of symmetry-broken phases in general, suppressing, however, the antiferromagnetc phase more strongly compared with the nematic phase, is directly testable experimentally.

Tunable electronic properties of ultra-thin boron-carbon-nitrogen heteronanotubes for various compositions.

A first-principles calculation was carried out to investigate the stability and electronic properties of ultra-thin Cx(BN)y heteronanotubes which were composed by joining pure CNT and BNNT segments with different composition and configurations. We found that the stability of Cx(BN)y heteronanotubes is increased with the increasing number of B and N atoms. In addition, all armchair (3,3) Cx|(BN)y heteronanotubes were found to be semiconductors with tunable energy gaps between 0.45 to 1.62 eV. Whereas zigzag (5,0) Cx|(BN)y heteronanotubes can be metal (y≤4) or semiconductor (y>4), and it is different from the relatively big zigzag Cx|(BN)y heteronanotubes which are always conductors. It indicates that the energy gap of (5,0) Cx|(BN)y heteronanotubes can be tuned by modifying the value of y. Further, zigzag (5,0) and armchair (3,3) C-BN heteronanotubes were found to be metal and semiconductor, respectively; but zigzag and armchair C-BN heteronanotubes with relatively big diameter are always semiconductor and conductor, respectively. Therefore, the electronic properties of ultra-thin Cx(BN)y heteronanotubes are abnormal when comparing with the relatively big ones. Highlights• The stability of Cx(BN)y heteronanotubes is increased with the increasing value of y.• Zigzag (5,0) Cx|(BN)y heteronanotubes can change from metal (y≤4) to semiconductor (y>4) when the value of y increases from 1 to 7.• The band gaps of armchair (3,3) Cx|(BN)y heteronanotubes increase from 0.45 to 1.62 eV when the value of y increases from 1 to 7.• The electronic properties (energy gap) of Cx(BN)y heteronanotubes are tunable and different in comparison with the relatively big ones.

Noncovalent interaction of carbon nanostructures.

The potential application of carbon nanomaterials in biology and medicine increases the necessity to understand the nature of their interactions with living organisms and the environment. The primary forces of interaction at the nano-bio interface are mostly noncovalent in nature. Quantifying such interactions and identifying various factors that influence such interactions is a question of outstanding fundamental interest in academia and industry. In this Account, we have summarized our recent studies in understanding the noncovalent interactions of carbon nanostructures (CNSs), which were obtained by employing first-principles calculations on various model systems representing carbon nanotubes (CNTs) and graphene. Bestowed with an extended sp(2) carbon network, which is a common feature in all of these nanostructures, they exhibit π-π interactions with aromatic molecules (benzene, naphthalene, nucleobases, amino acids), cation-π type of interactions with metal ions, anion-π interactions with anions, and other XH···π type of interactions with various small molecules (H2O, NH3, CH4, H2, etc.). CNTs are wrapped-up forms of two-dimensional graphene, and hence, it is interesting to compare the binding abilities of these two allotropes that differ in their curvature. The chirality and curvature of CNSs appear to play a major role in determining the structural, energetic, and functional properties. Flat graphene shows stronger noncovalent interactions than the curved nanotubes toward various substrates. Understanding the interactions of CNSs with organic molecules and biomolecules has gained a great deal of research interest because of their potential applications in various fields. Aromatic hydrocarbons show a strong propensity to interact with CNSs via the π-π mode of interaction rather than CH···π interaction. As DNA sequencing appears to be one of the most important potential applications of carbon nanomaterials, the study of CNS-nucleobase interactions has become quite important. The nucleobases are physisorbed on the surface of CNSs in the order G > T ≈ A > C > U, exhibiting π-π-stacking type of interaction. These interactions become stronger as the curvature of the CNSs decreases. It is also indispensable to study the interaction of nanomaterials with proteins and especially with amino acids at a molecular level to understand the drug delivery mechanism of CNSs. We have shown that the CNSs interact with small molecules by means of physisorption and thus show potential for sensor applications. The prime requisite for the exploitation of these CNSs in nanoelectronics is the tunable energy gap. We have revealed that metal ion doping modulates the HOMO-LUMO energy gap of the nanotubes significantly and thus provides a handle to tune the electronic and conductivity properties of CNTs. Moreover, metal ions tend to selectively bind with nanotubes of different chirality such as armchair and zigzag nanotubes. The reduction of planar hydrocarbon materials by lithium atoms has also been studied very systematically. We also illustrate the way in which noncovalent interactions can be used to optimize and fine-tune the properties of CNSs.

Photofunctional Materials Fabricated with Chalcopyrite-Type Semiconductor Nanoparticles Composed of AgInS2 and Its Solid Solutions.

Over the past decade, many researchers have made efforts to develop high-quality I-III-VI2 group chalcopyrite-type semiconductor nanoparticles of Cu-based or Ag-based materials, such as CuInS2, Cu(InGa)Se2, and AgInS2, due to their low toxicity, wide range of absorption from UV to visible or near-infrared regions, and tunable energy gap depending on the chemical composition. Ag-based chalcopyrite-type nanoparticles have attracted much attention because they have physicochemical properties that can be controlled in a manner similar to that for Cu-based nanoparticles but can be synthesized easily under milder reaction conditions than those of Cu-based ones. In this Perspective, we review recent works relating to the preparation of low-toxic nanoparticles of Ag-based chalcopyrite-type semiconductors and their solid solutions, and then we introduce their application as photofunctional materials. Tunability of optical properties has considerably contributed to recent advances in photofunctional applications of chalcopyrite-type semiconductor nanoparticles to sensors, photocatalysts, and solar cells.

Giant tunneling magnetoresistance in silicene

We have theoretically studied ballistic electron transport in silicene under the manipulation of a pair of ferromagnetic gate. Transport properties like transmission and conductance have been calculated by the standard transfer matrix method for parallel and antiparallel magnetization configurations. It is demonstrated here that, due to the stray field-induced wave-vector filtering effect, remarkable difference in configuration-dependent transport gives rise to a giant tunneling magnetoresistance. In combination with the peculiar buckled structure of silicene and its electric tunable energy gap, the receiving magnetoresistance can be efficiently modulated by the externally-tunable stray field, electrostatic potential, and staggered sublattice potential, providing some flexible strategies to construct silicene-based nanoelectronic device.

Dark current of infrared photodetectors based on armchair graphene nanoribbons

Armchair graphene nanoribbons (A-GNRs), an alternative material for infrared (IR) photodetectors, attract more attention because of their tunable energy gap by changing the width and number of layers and inducing a transverse electric field on the structure. In this paper, we study the dark current of IR photodetectors based on A-GNRs and its dependence on gate voltage, width of nanoribbon and temperature. The obtained results show that the dark current increases by increasing the temperature. Also, for narrow A-GNRs the dark current increases by increasing the gate voltage and for wider A-GNRs decreases by increasing the gate voltage.

Modeling of the infrared photodetector based on multi layer armchair graphene nanoribbons

Armchair graphene nanoribbons (A-GNRs), an alternative material for Infrared (IR) photodetectors, attract more attention because of those tunable energy gaps by changing the width of nanoribbons and the height of interband transition. In this paper, we calculate the dark current limited detectivity, D*, of the multi layer A-GNR based IR photodetector. For this purpose, we find the band structure of A-GNRs by tight-binding model and by considering the edge deformation, the absorption coefficient using the single electron approximation, the quantum efficiency, and the optical responsivity of photodetector. Then, the dark current of photodetector has been calculated by considering two contributions: (i) The interband tunneling generation and (ii) the thermogeneration due to the optical and acoustic phonon and line edge roughness (LER) scattering in the A-GNRs. Finally, we optimize the dark current limited detectivity of the photodetector for different structural parameters. The obtained results show that for the single layer A-GNR based photodetector with W = 5 nm, L = 20 μm, Vb = 2 V, Vg = 2 V, maximum value of dark current limited detectivity, D*, at T = 300 K is ∼2.2 × 108 (cm Hz1/2/W) and at T = 77 K is ∼2.1 × 1011 (cm Hz1/2/W). Also, for narrow A-GNRs, D* increases with increasing the gate voltage, while for wider A-GNRs decreases with increasing the gate voltage. Moreover, the dark current limited detectivity increases with increasing the number of the A-GNR layers.

Carrier generation and recombination rate in armchair graphene nanoribbons

Armchair graphene nanoribbons (A-GNRs), with a tunable energy gap, are an alternative structure for use in optoelectronic devices. The performance of these optoelectronic devices critically depends on the carrier generation and recombination rates, which have been calculated in this paper. Because of the 1D band structure of A-GNRs, carrier scattering, generation and recombination rates in these structures would be completely different from those in 2D graphene sheets. In this paper, using the tight binding model, and by considering the edge deformation and Fermi golden rule, we find the band structure, and the carrier generation and recombination rates for pure A-GNR due to optical and acoustic phonons, as well as Line Edge Roughness (LER) scatterings. The obtained results show that the total generation and recombination rates increase with increasing A-GNR width and eventually saturate for wide ribbons. These rates increase as the carrier concentration is increased (which has been considered homogenous along ribbon width) and temperature. Also, despite the large LER scattering in narrow ribbons, the generation and recombination rates are less for A-GNRs than for graphene sheets. Using this theoretical model, one can find the suitable A-GNR structure for the design of optoelectronic devices.