Tunable Energy Gap Research Articles

Photonic Berry curvature in double liquid crystal microcavities with broken inversion symmetry

We investigate a photonic device consisting of two coupled optical cavities possessing Rashba-Dresselhaus spin-orbit coupling, TE-TM splitting, and linear polarization splitting that opens a tunable energy gap at the diabolic points of the photon dispersion; giving rise to an actively addressable local Berry curvature. The proposed architecture stems from recent advancements in the design of artificial photonic gauge fields in liquid crystal cavities [K. Rechci\ifmmode \acute{n}\else \'{n}\fi{}ska et al., Science 366, 727 (2019)]. Our study opens perspectives for topological photonics, room-temperature spinoptronics, and studies on the quantum geometrical structure of photonic bands in extreme settings.

Highly sensitive and rapid responding humidity sensors based on silver catalyzed Ag2S-TiO2 quantum dots prepared by SILAR.

We developed a resistive humidity sensor based on a heterojunction of silver sulfide (Ag2S) quantum dots (QDs) and TiO2 because of its specificity to water vapor adsorption and its insensitivity to environmental gases. The QDs were grown on a mesoporous TiO2 layer using the successive ionic layer adsorption and reaction (SILAR) method. The boundary condition between TiO2 and Ag2S provides a tunable energy gap by adjusting the number of SILAR cycles. Besides, the large surface-to-volume ratio of QDs provides a strong water vapor adsorption ability and electron transfer. Nano-silver precipitated during the SILAR process provides free electrons and lowers the Fermi level to between n-type TiO2 and p-type Ag2S. The resistance response increased significantly to 4600 and the reaction equilibrium time decreased greatly to 7 seconds due to the presence of nano-silver. Finally, the Ag2S QDs possess a best sensing range of 13–90%. To sum up, Ag2S QDs are high sensitivity and selectivity humidity sensors.

A comparative study on magnetic order and field-induced magnetic transition in double perovskite iridates: RE2ZnIrO6 and RE2MgIrO6 (RE = Pr, Nd, Sm, Eu, Gd)

We perform a comparative magnetic study on two series of rare-earth (RE) based double perovskite iridates RE2BIrO6 (RE = Pr, Nd, Sm–Gd; B = Zn, Mg), which show Mott insulating state with tunable charge energy gap from ∼330 meV to ∼560 meV by changing RE cations. For nonmagnetic RE = Eu cations, Eu2MgIrO6 shows antiferromagnetic (AFM) order and field-induced spin-flop transitions below Néel temperature (TN) in comparison with the ferromagnetic (FM)-like behaviors of Eu2ZnIrO6 at low temperatures. For magnetic-moment-containing RE ions, Gd2BIrO6 show contrasting magnetic behaviors with FM-like transition (B = Zn) and AFM order (B = Mg), respectively. While, for RE = Pr, Nd and Sm ions, all members show AFM ground state and field-induced spin-flop transitions below TN irrespective of B = Zn or Mg cations. Moreover, two successive field-induced metamagnetic transitions are observed for RE2ZnIrO6 (RE = Pr, Nd) in high field up to 56 T, the resultant field temperature (H–T) phase diagrams are constructed. The diverse magnetic behaviors in RE2BIrO6 reveal that the 4f-Ir exchange interactions between the RE and Ir sublattices can mediate their magnetism.

Phase transition and electronic tuning in gamma-graphynenanoribbons through uniaxial strain and electric field

Gamma-graphyne sheet is a semiconductor with a bandgap of about 0.5 eV. For electronic applications, we need a tunable energy gap. For this purpose, we introduce a tight-binding based method which enables us to study the effects of oriented strains and also electric fields on electronic properties of nanoribbons of monolayer gamma-graphyne. Our results shows that the system has a controllable bandgap in the range of 0–2.9 eV in response to a transverse electric field and strain. Applying a transverse electric field and strain causes a semiconductor-metal phase transition. One can control the bandgap and electronic properties of the system with the help of the above parameters. Our findings indicate that the γ-graphynenanoribbons are promising candidate for applications in high efficiency nanoelectronic devices. • We investigate the electronic properties of armchair and zigzag γ-graphynenanoribbons. • We introduce a tight-binding based method which enables us to study the effects of oriented strains and also electric fields. • Our results shows that the system has a controllable bandgap in the range of 0–2.9 eV. • Due to the giant Stark effect, the band gap of the system is closed at a critical field. • A semiconductor-metal phase transition occurs in the system.

Spin photocurrents in zigzag phosphorene nanoribbons: From infrared to ultraviolet

Using the self-consistent non-equilibrium Green’s function model and the mean-field Hubbard approximation, we studied the possibility of inducing the spin-photovoltaic effects in zigzag phosphorene nanoribbons. We numerically showed that an applied electric field could induce a spin-semiconducting behavior with anisotropic and localized band structures around the Fermi energy in the antiferromagnetic zigzag phosphorene nanoribbons. Moreover, a tunable energy gap with an electric field could induce a spin photocurrent in a wide range of photon energies. Interestingly, increasing the electric field strength induces the spin-valve effect from terahertz to infrared irradiation. Furthermore, ferromagnetic zigzag phosphorene nanoribbons reveal a spin-dependent photoresponsivity, which is induced by infrared to ultraviolet frequencies. These results could enhance photovoltaic effects with a generation of the spin photocurrent in phosphorene junctions.

Manganese Telluride Carbonyl Complexes: Facile Syntheses and Exotic Properties-Reversible Transformations, Hydrogen Generation, Paramagnetic, and Semiconducting Properties.

A novel family of five Mn-Te-CO complexes was prepared via facile syntheses: mono spirocyclic [Mn4Te(CO)16]2- (1), four-membered Mn2Te2 ring-type [Mn2Te2(CO)8]2- (2), hydride-containing square pyramidal [HMn3Te2(CO)9]2- (3), and dumbbell-shaped [Mn6Te6(CO)18]4- (4) and [Mn6Te10(CO)18]4- (5). Electron-precise complexes 4 and 5 exhibit unusual paramagnetism arising from two types of Mn atoms in different oxidation states, as determined by X-ray photoelectron spectroscopy, electron paramagnetic resonance, and density functional theory (DFT) calculations. The structural transformations from small-sized Mn4Te 1 and Mn2Te2 2 to the largest Mn6Te10 5 were controllable, the off/on magnetic-switched transformation between HMn3Te2 3 and 5 was reversible, and the magnetic transformation between Mn6Te6 4 and 5 was observed. Interestingly, the reversible dehydridation and hydridation between the HMn3Te2-based cluster 3 and [Mn3Te2(CO)9]- were successfully accomplished, in which the release of a high yield of H2 was detected by gas chromatography. In addition, upon the addition of CO, cluster 3 first forms a carbonyl-inserted intermediate [HMn3Te2(CO)10]2- (3'), detected by the high resolution ESI-MS, which is readily transformed to a dimeric dihydrido cluster [{HMn3Te2(CO)10}2]2- (6) with the introduction of O2. These low- to high-nuclearity complexes exhibit rich redox properties with semiconducting behavior in solids, possessing low but tunable energy gaps (1.06-1.62 eV) due to efficient electron transport via nonclassical C-H···O(carbonyl) interactions. The structural nature, reversible structural transformations, controllable on/off magnetic switches, electron communication networks, and associated chemical properties for hydrogen generation are discussed in detail and supported by DFT calculations, density of states, band structures, and noncovalent interaction analyses.

Effect of PbS quantum dot-doped polysulfide nanofiber gel polymer electrolyte on efficiency enhancement in CdS quantum dot-sensitized TiO2 solar cells

Quantum dot-sensitized solar cells (QDSSCs) are among the most promising low cost third generation solar cells. Semiconductor quantum dots have unique properties such as high molar extinction coefficients, tunable energy gap by the quantum confinement effect and the ability of multiple exciton generation. In this study, stable CdS QDSSCs were fabricated by using polysulfide liquid electrolyte and also by using cellulose acetate nanofiber-based gel electrolyte. Incorporation of PbS Q dots to the liquid or gel electrolyte showed a significant enhancement in solar cell efficiency. Under the simulated light of 100 mW cm−1 the efficiency of the polysulfide liquid electrolyte based CdS QD solar cells increased from 1.19% to 1.51% and the efficiency of the nanofibre gel electrolyte based CdS QD solar cells increased from 0.94% to 1.46% due to the incorporation of 5% (wt/wt) PbS Q dots into the respective electrolytes. The efficiency increase has been attributed to the increase in short circuit photocurrent density due to increased sulfide ion (S2−) conductivity evidently caused by indirect ionic dissociation facilitated by PbS QDs.

Breakdown of the electron delocalization in hexagonal borophene toward tunable energy gap

The absence of energy gap (Eg) in borophene polymorphs, where multi-center bonds favor electron delocalization and form metallic phases, restricts their potential applications in microelectronic and optoelectronic industry. Herein, we propose to incorporate chalcogenide atoms to breakdown the electron delocalization in hexagonal borophene and introduce an Eg. Based on first principle calculation, we report a group of semiconducting boron chalcogenide monolayers, denoted as B2X (X = O, Se and Te), with an indirect Eg of 2.84, 0.97 and 1.04 eV under HSE06 levels, respectively. Moreover, the introduced Eg can be modulated by external strains (±6%) affordable by substrates, with electronic effective mass being comparable or even lower than those of MoS2 along certain directions. Furthermore, B2X monolayers exhibit desirable light absorption properties under visible light, which can be improved by stacking bilayers. The current study presents an alternative route to introduce the Eg in borophene, endows them with new vitality and highlights their promise in flexible electronics, optoelectronics and photovoltaic devices.

Efficiency enhancement in PbS/CdS quantum dot-sensitized solar cells by plasmonic Ag nanoparticles

Semiconductor quantum dots (Q-dots) are attractive nanomaterials to be used in numerous research areas and device fabrication such as sensors, light-emitting diodes, transistors, and solar cells due to their unique optoelectronic properties. Quantum dot-sensitized solar cells (QDSSCs) have drawn considerable attention due to their cost-effectiveness and ability of multiple exciton generation and tunable energy gap of the quantum dots. In this study, plasmonic Ag colloidal nanoparticle-incorporated plasmonic TiO2 double-layer (nanofiber/nanoparticle) electrodes have been fabricated. These TiO2 electrodes were sensitized with PbS/CdS core-shell quantum dots by successive ionic layer adsorption and reaction (SILAR) technique, and QDSSCs were fabricated with polysulfide electrolyte. Cu2S was formed on brass plate and used as the counter electrode of the QDSSC. A higher power conversion efficiency of 4.09% has been obtained due to the plasmonic effect under the simulated light of 100 mW cm−2 with AM 1.5 spectral filter. The overall efficiency and short-circuit current density of the plasmonic QDSSC are enhanced by 15% and 23%, respectively, with respect to the QDSSC without Ag nanoparticles. The enhanced performance of the plasmonic QDSSC is evidently due to the enhanced optical absorption by localized surface plasmon resonance effect by the Ag nanoparticles in the TiO2 photoanode and the resulting increase in the short-circuit photocurrent.

Room-temperature magnetism and tunable energy gaps in edge-passivated zigzag graphene quantum dots

Graphene is a nonmagnetic semimetal and cannot be directly used as electronic and spintronic devices. Here, we demonstrate that zigzag graphene nanoflakes (GNFs), also known as graphene quantum dots, can exhibit strong edge magnetism and tunable energy gaps due to the presence of localized edge states. By using large-scale first principle density functional theory calculations and detailed analysis based on model Hamiltonians, we can show that the zigzag edge states in GNFs ({mathrm{C}}_{6n^2}H6n, n = 1–25) become much stronger and more localized as the system size increases. The enhanced edge states induce strong electron–electron interactions along the edges of GNFs, ultimately resulting in a magnetic configuration transition from nonmagnetic to intra-edge ferromagnetic and inter-edge antiferromagnetic, when the diameter is larger than 4.5 nm (C480H60). Our analysis shows that the inter-edge superexchange interaction of antiferromagnetic states between two nearest-neighbor zigzag edges in GNFs at the nanoscale (around 10 nm) can be stabilized at room temperature and is much stronger than that exists between two parallel zigzag edges in graphene nanoribbons, which cannot be stabilized at ultra-low temperature (3 K). Furthermore, such strong and localized edge states also induce GNFs semiconducting with tunable energy gaps, mainly controlled by adjusting the system size. Our results show that the quantum confinement effect, inter-edge superexchange (antiferromagnetic), and intra-edge direct exchange (ferromagnetic) interactions are crucial for the electronic and magnetic properties of zigzag GNFs at the nanoscale.

Robust two-dimensional topological insulators in derivatives of group-VA oxides with large band gap: Tunable quantum spin Hall states

Recently, group-VA 2D materials have attracted intense attention due to excellent properties in various aspects. Particularly, the strong spin–orbital coupling effect is a remarkable virtue of group-VA 2D materials, which is more widely concerned by researchers. Here, based on first-principles calculations, we provide a systematical investigation on the electronic and topological properties of a novel series 2D compound VA2OX (VA=Bi, Sb, As, P; X=S, Se, Te). Combining with the nontrivial Z2-type topological invariants, it is confirmed that VA2OX 2D crystals possess topologically nontrivial characteristics. The 2D TIs VA2OX demonstrate large and tunable nontrivial energy gaps, from 0 to 592meV, indicating that the tunable QSH effect can be realized at room temperature. Moreover, as an example of VA2OX systems, the band gap of Bi2OS 2D crystal almost remains unchanged when tensile strain not larger than 8%. Notably, under a large range of strain (±10%), Bi2OS keeps the inversion-asymmetric topological insulator phases, which emerges the robustness of nontrivial topology against mechanical deformation and makes it competent for realizing new topological phenomena. The above results are expected to promote further experimental investigation for fundamental exploration and practical application, which will significantly broaden the scientific and technological impact of the QSH effect.

Aggregation-induced emission (AIE)-active molecules bearing singlet oxygen generation activities: the tunable singlet-triplet energy gap matters.

Herein, a series of photosensitizers were constructed of α, β and γ-isomers of terpyridine and the corresponding N-methylation derivatives. Benefiting from the tunable singlet-triplet energy gap and aggregation-induced emission characteristics, two-photon active photosensitizers L2b and L2c showed relatively strong intersystem crossing facilitating 1O2 generation and cell apoptosis with near-infrared excitation.

Strain engineering the structures and electronic properties of Janus monolayer transition-metal dichalcogenides

We studied the structures and electronic properties of Janus transition-metal dichalcogenide monolayers MXY (M = Mo, W; X ≠ Y = S, Se, Te) by first-principles calculations. The results of the electronic band structures and the density of states reveal that all of the MXY monolayers show semiconducting characteristics. Particular attention has been focused on the bandgap engineering by applying in-plane biaxial compressive and tensile strain. It is observed that the bandgap values of the MXY monolayers decrease with the increase of strain degree under the tension and compression biaxial strain, and a semiconductor-to-metal transition can be undergone at a critical value of strain. The possibility of the tunable energy gap over a wide range makes MXY monolayers potential candidates for nanoelectronics and optoelectronics.

Engineering an Insulating Ferroelectric Superlattice with a Tunable Band Gap from Metallic Components.

The recent discovery of "polar metals" with ferroelectriclike displacements offers the promise of designing ferroelectrics with tunable energy gaps by inducing controlled metal-insulator transitions. Here we employ first-principles calculations to design a metallic polar superlattice from nonpolar metal components and show that controlled intermixing can lead to a true insulating ferroelectric with a tunable band gap. We consider a 2/2 superlattice made of two centrosymmetric metallic oxides, La_{0.75}Sr_{0.25}MnO_{3} and LaNiO_{3}, and show that ferroelectriclike displacements are induced. The ferroelectriclike distortion is found to be strongly dependent on the carrier concentration (Sr content). Further, we show that a metal-to-insulator (MI) transition is feasible in this system via disproportionation of the Ni sites. Such a disproportionation and, hence, a MI transition can be driven by intermixing of transition metal ions between Mn and Ni layers. As a result, the energy gap of the resulting ferroelectric can be tuned by varying the degree of intermixing in the experimental fabrication method.

Tunable energy gaps and mid-infrared optical properties in InAs/GaSb type-II superlattices

We investigate on the infrared (IR) optoelectronic properties in short-period InAs/GaSb type-II superlattices (SLs) by a modified eight-band [Formula: see text] model. The electronic mini-band structures for such SLs are evaluated by the modified eight-band [Formula: see text] model, incorporating the microscopic interface effect. We find that with varying the values around 20/25 Å for the InAs/GaSb layer widths, the tunable mid-IR bandgaps can be achieved effectively. The SL bandgap from 275 to 346 meV can be achieved by decreasing the InAs layer thickness from 23 to 17 Å at a fixed GaSb layer thickness of 24 Å, or from 254 to 313 meV by increasing the GaSb layer thickness from 18 to 27 Å at a fixed InAs layer thickness of 21 Å. Correspondingly the optical absorptions in such systems can be tuned evidently. Our theoretical results are in good agreement with experimental data over a series of SL samples. This study confirms further that short-period InAs/GaSb type-II SLs are of great importance for IR applications.

Spin and valley dependent line-type resonant peaks in electrically and magnetically modulated silicene quantum structures

A barrier with a tunable spin-valley dependent energy gap in silicene could be used as a spin and valley filter. Meanwhile, special resonant modes in unique quantum structure can act as energy filters. Hence we investigate valley and spin transport properties in the potential silicene quantum structures, i.e., single ferromagnetic barrier, single electromagnetic barrier and double electric barriers. Our quantum transport calculation indicates that quantum devices of high accuracy and efficiency (100% polarization), based on modulated silicene quantum structures, can be designed for valley, spin and energy filtering. These intriguing features are revealed by the spin, valley dependent line-type resonant peaks. In addition, line-type peaks in different structure depend on spin and valley diversely. The filter we proposed is controllable by electric gating.

Electronic and optical properties of α-InX (X = S, Se and Te) monolayer: Under strain conditions

Using ab initio study, the structural, electronic and optical properties of α-InX (X=S, Se and Te) are investigated under tensile and compressive strain conditions. The results illustrate that exerting biaxial tensile and compressive strain conditions can lead to a tunable energy gap with a linear trend. The shape of valence band maximum (VBM) and conduction band minimum (CBM) is so sensitive to applying tensile and compressive strain. Besides, a shift in optical spectra toward shorter wavelength (blue shift) occurs under compression. The exerting tensile strain, on the other hand, gives rise to a red shift in optical spectra correspondingly. The results have been presented that InX monolayers can be good candidates for optoelectronic applications as well.

Phosphorus quantum dots as visible-light photocatalyst for water splitting

The discovery and synthesis of phosphorene stimulate the exploration of other two-dimensional (2D) phosphorus allotropes as well as phosphorus nanostructures. The recent successful preparation of phosphorene quantum dots further broadens the application areas of this novel 2D material. Herein we propose, for the first time, to sculpture monolayer black and blue phosphorus into quantum dots for photocatalytic solar water splitting. The size dependency of electronic, optical and photocatalytic properties of monolayer phosphorus quantum dots are systematically investigated. According to our first-principles calculations, these nanostructures possess highly tunable energy gap and work function; their frontier molecular orbitals straddle the water redox potentials with reasonable oxidizing and reducing power; they can harvest the full spectrum of visible light. Therefore, monolayer phosphorus quantum dots are promising candidates of visible-light photocatalysts for water splitting.

サイズと組成で制御する多元半導体ナノ粒子の光機能

Multinary I-III-VI2 group semiconductor nanocrystals, such as CuInS2, Cu(InGa)Se2, AgInS2 and their solid solutions with ZnS, have been attracting increasing attention due to their low toxicity, wide range of absorption from UV to visible or near-infrared regions, and tunable energy gap. In this review, we introduce recent progress of solution-phase syntheses of high-quality multinary nanocrystals, which are candidates for alternative materials of Cd- and Pb-based binary nanocrystals showing high toxicity. Physicochemical properties of multinary nanocrystals are tunable depending on their crystal size and chemical composition. For example, nanocrystals of solid solution between AgInS2 and ZnS exhibited strong photoluminescence, the color of which was controlled from red to green with an increase in the ZnS fraction. The controllability of their photochemical properties is useful to develop novel photofunctional materials for the applications, such as photocatalyst and biomolecule marker.

Engineering of highly mismatched alloy with semiconductor and semi-metallic substituent's for photovoltaic applications

Highly mismatched alloys (HMAs) are getting a substantial interest of researchers because of holding competence of rapid change in physical properties with minor compositional change and consequently showing their potential for solar energy and photovoltaic applications. In the present density functional theory based work, we design HMAs from the extremely dissimilar GaP (semiconductor) and GaBi (semi-metal). The alloying of the two compounds with unmatched electronic characteristics has triggered a rapid reduction in the energy gap of GaPBi. The energy gap is reduced by 39.3 meV for every 1% increase in Bi composition. The semiconductor behavior of GaPBi based HMAs is found to be transformed to semi-metallic for replacing 64.6% of P atoms by Bi. Unlike the conventional alloys, the variation in the electronic energy gap of GaP1-xBix shows deviation from the Vegard's formalism. Where the optical properties are strongly influenced with the narrowing energy gap of GaPBi. For the Bi-rich GaPBi, the notable red shift is observed in optical dielectric function and absorption spectra. Moreover, the larger atomic size of Bi has enhanced the lattice parameters of Bi-rich GaPBi. The GaPBi based HMAs with tunable energy gap in the span of 2.51 eV–0 eV and the interesting optical properties highlight them prospective materials for optoelectronic applications.