Tunable Band Gap Energy Research Articles

Semiclassical Boltzmann transport theory of few-layer black phosphorus in various phases

Black phosphorus (BP), a two-dimensional (2D) van der Waals layered material composed of phosphorus atoms, has been one of the most actively studied 2D materials in recent years due to its tunable energy band gap (tunable even to a negative value) and its highly anisotropic electronic structure. Depending on the sign of the band gap tuning parameter, few-layer BP can be in a gapped insulator phase, gapless Dirac semimetal phase, or gapless semi-Dirac transition point between the two phases. Using the fully anisotropic multiband Boltzmann transport theory, we systematically study the dc conductivity of few-layer BP as a function of the carrier density and temperature by varying the band gap tuning parameter, and determine the characteristic density and temperature dependence corresponding to each phase.

Utilizing MVAD method to optimize crystallization and nanostructured surface of the perovskite film: Toward electroluminescent and ultraviolet photodetective bifunctional optoelectronics

Metal-halide perovskite of MAPbBr3, by exhibiting several remarkable optoproperties such as tunable energy bandgap, high absorption coefficients, high color purity, and long charge carrier diffusion lengths, has made great contributions to photovoltaic and electroluminescent (EL) technologies, i.e., solar cell, photodetector, light-emitting diode, and newly developed bifunctional optoelectronics. To realize high performance bifunctional optoelectronics, it is imperative to promote the balance of the trade-off between effective radiative recombination during EL process and fast exciton dissociation in photodetective process. Herein, we optimize the crystallization and nanostructured surface of MAPbBr3 film through applying multistep vapour assisted deposition (MVAD) method. Then, we enhance the optical field distribution and charge transfer feature of MAPbBr3 film by establishing an optical simulation model. Based on our modifed MAPbBr3 active layer, finally, we demonstrate a high performance device with EL and ultraviolet photodetective bifunctions, which exhibits a 6.27-fold improvement of brightness and a 3 orders of magnitude increase of detectivity. To the best of our knowledge, this work not only reports high bifunctional perovskite optoelectronics performance, but also illustrates the optical field redistribution process in horizontal orientation surface of MAPbBr3 films for relevant researchers.

Band gap engineering, electronic state and local atomic structure of Ni doped CeO2 nanoparticles

In cerium dioxide (CeO2) as semiconductor compound, the tuning of band gap energy is a pivotal feature for visible light applications. In this report, nanoparticles (NPs) of Ce1 − xNixO2 (0.0 ≤ x ≤ 0.10) were synthesized through co-precipitation route. The structural, electronic state and optical band gap were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, Raman scattering and UV–Vis spectroscopy. XRD results revealed the single-phase nanocrystalline behavior of CeO2 with cubic fluorite structure. The electronic configuration of the samples, probed via NEXAFS spectra at Ce M5, 4 edges, demonstrated that the cerium exists in Ce4+ and Ce3+ mixed valence states. The formation of Ce3+ ions in the vicinity of oxygen vacancies (Ov) were assessed also via Raman scattering and XANES spectra at Ce L3 edge. The estimated concentrations of Ce3+ were increased from 6 to 13% as the Ni content increase from 0 to 10% respectively. Here, the inclusion of Ni2+ ions into CeO2 network induced additional Ov and simultaneously reduced the optical band gap from 3.9 eV for pure CeO2 to 2.6 eV for 10% Ni doped CeO2 NPs. Therefore, the oxygen loss population seem to be responsible for the band gap reduction. The role of Ov for creating profound donor band near the conduction band and narrowing the band gap energy were discussed.

Full-color-emitting (CuInS2)ZnS-alloyed core/shell quantum dots with trimethoxysilyl end-capped ligands soluble in an ionic liquid.

Zinc-copper-indium sulfide (ZCIS)-alloyed quantum dots are emerging as a new family of low toxic I–III–VI semiconductors due to their broad and color-tunable emissions as well as large Stokes shifts. Here, we fabricated a series of ZCIS QDs with tunable PL wavelengths and band-gap energies via a facile strategy by varying the ratio of A1–3 stock (Cu+/In3+) to the B stock (Zn2+) content. The ZnS shell was formed to improve the PL emission efficiency of the core nanoparticles and the PL emission wavelength of the resulting ZCIS/ZnS NCs gradually blue-shifted with an increase in the number of shell layers, resulting in a wide range of emissions from 800 nm to 518 nm that can be tuned by the core compositions or shell layer numbers for ZCIS/ZnS. Finally, the long-chain ligands dodecanethiol/octadecylamine on the quantum dots' surface were efficiently replaced by (3-mercaptopropyl)trimethoxysilane, thus enabling their solubility in an ionic liquid, which was confirmed via GC-MS. It also benefited for the co-dissolution of the polymers and chemical binding with other materials through the reactive silanol group, which provide stable and well-distributed ZCIS/ZnS QDs composites or surface coating by the QDs.

An In0.42Ga0.58N tunnel junction nanowire photocathode monolithically integrated on a nonplanar Si wafer

Group III-nitride semiconductors exhibit many ideal characteristics for solar water splitting, including a tunable energy bandgap across nearly the entire solar spectrum and suitable band edge positions for water oxidation and proton reduction under visible and near-infrared light irradiation. To date, however, the best reported energy conversion efficiency for III-nitride semiconductor photocathodes is still below 1%. Here we report on the demonstration of a relatively efficient p-type In0.42Ga0.58N photocathode, which is monolithically integrated on an n-type nonplanar Si wafer through a GaN nanowire tunnel junction. The open pillar design, together with the nonplanar Si wafer can significantly maximize light trapping, whereas the tunnel junction reduces the interfacial resistance and enhances the extraction of photo-generated electrons. In addition, photodeposited Pt nanoparticles on InGaN nanowire surfaces significantly improve the cathodic performance. The nanowire photocathode exhibits a photocurrent density of 12.3 mA cm−2 at 0 V vs. RHE and an onset potential of 0.79 V vs. RHE under AM 1.5 G one-sun illumination. The maximum applied bias photon-to-current efficiency reaches 4% at ~0.52 V vs. RHE, which is one order of magnitude higher than the previously reported values for III-nitride photocathodes. Significantly, no performance degradation was measured for over 30 h solar water splitting with a steady photocurrent density ~12 mA cm−2 without using any extra surface protection, which is attributed to the spontaneous formation of N-terminated surfaces of InGaN nanowires to protect against photocorrosion.

Quantum Dots for Solar Energy Harvesting

Owing to their versatile optical and electrical properties, semiconductor quantum dots are attracting attention as a material of choice for solar energy conversion. The quantum dot sensitized solar cells are considered as one of the most promising next-generation solar cells as they have the advantage of tunable band-gap energy and multiple exciton generation. We present here a study on quantum dot sensitized solar cells considering their construction and working, impact of incorporation of nanomaterials in solar cells and various structures for improving the performance of solar cells.

CuSbSe2 thin film solar cells with ~4% conversion efficiency grown by low-temperature pulsed electron deposition

CuSb(S,Se)2 is emerging as an alternative absorber for thin film photovoltaics, for its intrinsic p-type conductivity, tunable energy bandgap (1.0÷1.6 eV), high absorption coefficient > 104 cm−1 and very low cost of the constituent elements. In this work, we studied the structural, compositional and electro-optical properties of CuSbSe2 (CASe) films grown by Low Temperature Pulsed Electron Deposition (LTPED), a developing technology in thin film photovoltaics. We proved that stoichiometric or Cu-poor CASe films can be obtained at low temperature (< 200 °C) from a stoichiometric target with no need for post-deposition treatments to adjust the stoichiometry. Solar cells based on Cu-poor and stoichiometric CASe absorbers exhibit a similar open circuit voltage (∼290÷310 mV), while the JSC increases from ∼3 mA/cm2 up to ∼20 mA/cm2 as the Cu/Sb ratio increases from 0.55 to 0.95. A low FF ~40% still limits the performance of all the cells indicating that the standard architecture of thin film solar cells, currently used for CIGS and CdTe devices, need to be significantly reviewed. The results of the structural and optoelectrical characterization presented in this paper show that JSC, VOC and FF can all be improved as a result of the optimization of the device architecture. The best of the LTPED-grown CASe cells obtained so far, reached an efficiency of 3.8%, very close to the state of the art reported so far in literature.

Laser Q-switching with PtS2 microflakes saturable absorber.

Numerous studies have been conducted to explore the performance of two-dimensional (2D) layered nano-materials based saturable absorber (SA) for pulsed laser applications. However, fabricating materials in nanoscale requires complicated preparation processes, high energy consumption, and high expertise. Hence, the study of pulsed laser performance based on the saturable absorber prepared by layered materials with bulk-micro size have gained a great attention. Platinum disulfide (PtS2), which is newly developed group 10 2D layered materials, offers great potential for the laser photonic applications owing to its high carrier mobility, broadly tunable natural bandgap energy, and stability. In this work, the first passively Q-switched Erbium (Er) doped fiber laser is demonstrated with an operational wavelength of 1568.8 nm by using PtS2 microflakes saturable absorber, fabricated by a simple liquid exfoliation in N-Methyl-2-pyrrolidone (NMP) and then incorporated into polyvinyl alcohol (PVA) polymer thin film. A stable Q-switched laser operation is achieved by using this PtS2-SA within a fiber laser ring cavity. The maximum average output power is obtained as 1.1 mW, corresponding to the repetition rate of 24.6 kHz, the pulse duration of 4.2 μs, and single pulse energy of 45.6 nJ. These results open up new applications of this novel PtS2 layered material.

(Invited) Artificial Photosynthesis on III-Nitride Nanowire Arrays

High efficiency artificial photosynthesis, that can convert solar energy directly into chemical fuels, has been extensively investigated. To date, however, success in finding abundant visible-light active photocatalyst has been very limited. Recently, metal-nitrides (e.g. InGaN) have attracted considerable attention for applications in artificial photosynthesis, due to their excellent stability and tunable energy bandgap across nearly the entire solar spectrum. Moreover, InGaN is the only known material whose energy bandgap can straddle the redox potential of water under deep visible and near-infrared light irradiation. In this context, we have investigated the design, fabrication, and performance characterization of multi-band InGaN/GaN nanowire photocatalysts and photoelectrodes for solar water splitting and carbon dioxide reduction. In this work, InGaN nanowires are grown on Si substrate by molecular beam epitaxy. The water splitting reaction takes place on the nonpolar surface (m-planes) of InGaN nanowire photocatalysts. By optimizing the surface charge properties through controlled Mg dopant incorporation, the efficiency for solar-to-hydrogen conversion is enhanced by nearly two orders of magnitude. The absorbed photon conversion efficiency reaches ~90% with optimum Mg doping concentration. The significantly enhanced efficiency is directly related to the optimized surface electronic properties that lead to both efficient water oxidation and proton reduction. A solar-to-hydrogen conversion efficiency of 3.3% and stability >500 hours was demonstrated in photocatalytic overall water splitting. High stability of these nanowires is attributed to the N-rich surfaces of GaN nanowire structures, which protects against photocorrosion and oxidation. We have further demonstrated multi-band InGaN/GaN nanowire photoelectrodes monolithically integrated on a Si solar cell wafer. The tandem PEC device consists of a planar n+-p Si solar cell wafer and p-InGaN nanowire segments. The p-InGaN nanowire arrays are designed to absorb the ultraviolet and visible solar spectrum. The remaining photons with wavelengths up to 1.1 µm are absorbed by the underlying planar Si p-n junction. Such a monolithically integrated photocathode promises solar-to-hydrogen conversion efficiency >20%. With the use of such a photoelectrode, we have also demonstrated that syngas, a key feedstock to produce methanol and liquid fuels in industry, can be produced from a CO2 and H2O with a benchmark turnover number of 1330 and a desirable CO/H2 ratio of 1:2. Work is currently in progress to achieve high efficiency syngas and methanol generation in an aqueous photoelectrochemical cell.

Improved Stability and Performance of Visible Photoelectrochemical Water Splitting on Solution-Processed Organic Semiconductor Thin Films by Ultrathin Metal Oxide Passivation

Solution-processable organic semiconductors have potentials as visible photoelectrochemical (PEC) water splitting photoelectrodes due to their tunable small band gap and electronic energy levels, but they are typically limited by poor stability and photocatalytic activity. Here, we demonstrate the direct visible PEC water oxidation on solution-processed organic semiconductor thin films with improved stability and performance by ultrathin metal oxide passivation layers. N-type fullerene-derivative thin films passivated by sub-2 nm ZnO via atomic layer deposition enabled the visible PEC water oxidation at wavelengths longer than 600 nm in harsh alkaline electrolyte environments with up to 30 μA/cm2 photocurrents at the thermodynamic water-oxidation equilibrium potential and the photoanode half-lifetime extended to ∼1000 s. The systematic investigation reveals the enhanced water oxidation catalytic activity afforded by ZnO passivation and the charge tunneling governing the hole transfer through passivation l...

A simple synthesis of transparent and highly conducting p-type Cu x Al1-x S y nanocomposite thin films as the hole transporting layer for organic solar cells.

Inorganic p-type films with high mobility are very important for opto-electronic applications. It is very difficult to synthesize p-type films with a wider, tunable band gap energy and suitable band energy levels. In this research, p-type copper aluminum sulfide (CuxAl1−xSy) films with tunable optical band gap, carrier density, hole mobility and conductivity were first synthesized using a simple, low cost and low temperature chemical bath deposition method. These in situ fabricated CuxAl1−xSy films were deposited at 60 °C using an aqueous solution of copper(ii) chloride dihydrate (CuCl2·2H2O), aluminium nitrate nonohydrate [Al(NO3)3·9H2O], thiourea [(NH2)2CS], and ammonium hydroxide, with citric acid as the complexing agent. Upon varying the ratio of the precursor, the band gap of the CuxAl1−xSy films can be tuned from 2.63 eV to 4.01 eV. The highest hole mobility obtained was 1.52 cm2 V−1 s−1 and the best conductivity obtained was 546 S cm−1. The CuxAl1−xSy films were used as a hole transporting layer (HTL) in organic solar cells (OSCs), and a good performance of the OSCs was demonstrated using the CuxAl1−xSy films as the HTL. These results demonstrate the remarkable potential of CuxAl1−xSy as hole transport material for opto-electronic devices.

Self-doped colloidal semiconductor nanocrystals with intraband transitions in steady state.

The tunable bandgap energy has been recognized as a prominent feature of the colloidal semiconductor nanocrystal, also called the colloidal quantum dot (CQD). Due to the broken degeneracy caused by the quantum confinement effect, the electronic states of the conduction band (CB) are separated by a few hundred meV. The electronic transition occurring in the conduction band is called the intraband transition and has been regarded as a fast electron relaxation process that cannot be readily observed under steady state. However, recent progress in the studies of intraband transitions allowed the observation of the mid-IR intraband transition in steady state and ambient condition, providing a pathway to exploit the mid-IR electronic transition for various optoelectronic applications. The observation of the steady state intraband transitions has been possible due to the electron filling of the lowest electronic state (1Se) of the conduction band in the semiconductor nanocrystal. Specifically, the nanocrystals are "self-doped" with electrons through chemical synthesis - that is, without the need of adding heterogeneous impurity or applying an electrical potential. In this feature article, we summarize the recent advances in the study on intraband electronic transitions along with the interesting findings on the magnetic and electronic properties of the self-doped colloidal metal chalcogenide semiconductor nanocrystals. The mid-IR intraband transitions of non-toxic nanocrystals, which exclude the toxic mercury and cadmium constituents, are also highlighted, which hold promise for safer applications utilizing the higher quantum states of nanocrystals.

Multicomponent Electron-Hole Superfluidity and the BCS-BEC Crossover in Double Bilayer Graphene.

Superfluidity in coupled electron-hole sheets of bilayer graphene is predicted here to be multicomponent because of the conduction and valence bands. We investigate the superfluid crossover properties as functions of the tunable carrier densities and the tunable energy band gap E_{g}. For small band gaps there is a significant boost in the two superfluid gaps, but the interaction-driven excitations from the valence to the conduction band can weaken the superfluidity, even blocking the system from entering the Bose-Einstein condensate (BEC) regime at low densities. At a given larger density, a band gap E_{g}∼80-120 meV can carry the system into the strong-pairing multiband BCS-BEC crossover regime, the optimal range for realization of high-T_{c} superfluidity.

Noble metal-coated MoS2 nanofilms with vertically-aligned 2D layers for visible light-driven photocatalytic degradation of emerging water contaminants

Two-dimensional molybdenum disulfide (2D MoS2) presents extraordinary optical, electrical, and chemical properties which are highly tunable by engineering the orientation of constituent 2D layers. 2D MoS2 films with vertically-aligned layers exhibit numerous 2D edge sites which are predicted to offer superior chemical reactivity owing to their enriched dangling bonds. This enhanced chemical reactivity coupled with their tunable band gap energy can render the vertical 2D MoS2 unique opportunities for environmental applications that go beyond the conventional applications of horizontal 2D MoS2 in electronics/opto-electronics. Herein, we report that MoS2 films with vertically-aligned 2D layers exhibit excellent visible light responsive photocatalytic activities for efficiently degrading organic compounds in contaminated water such as harmful algal blooms. We demonstrate the visible light-driven rapid degradation of microcystin-LR, one of the most toxic compounds produced by the algal blooms, and reveal that the degradation efficiency can be significantly improved by incorporating noble metals. This study suggests a high promise of these emerging 2D materials for water treatment, significantly broadening their versatility for a wide range of energy and environmental applications.

Transparent Large-Area MoS2 Phototransistors with Inkjet-Printed Components on Flexible Platforms.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have gained considerable attention as an emerging semiconductor due to their promising atomically thin film characteristics with good field-effect mobility and a tunable band gap energy. However, their electronic applications have been generally realized with conventional inorganic electrodes and dielectrics implemented using conventional photolithography or transferring processes that are not compatible with large-area and flexible device applications. To facilitate the advantages of 2D TMDCs in practical applications, strategies for realizing flexible and transparent 2D electronics using low-temperature, large-area, and low-cost processes should be developed. Motivated by this challenge, we report fully printed transparent chemical vapor deposition (CVD)-synthesized monolayer molybdenum disulfide (MoS2) phototransistor arrays on flexible polymer substrates. All the electronic components, including dielectric and electrodes, were directly deposited with mechanically tolerable organic materials by inkjet-printing technology onto transferred monolayer MoS2, and their annealing temperature of <180 °C allows the direct fabrication on commercial flexible substrates without additional assisted-structures. By integrating the soft organic components with ultrathin MoS2, the fully printed MoS2 phototransistors exhibit excellent transparency and mechanically stable operation.

Tunable dielectric behaviour and energy band gap range of ZnAl2O4 ceramics mediated by Mg substitution

In this work, we report a systematic variation in the band gap of stoichiometric Zn1−xMgxAl2O4 ceramics with varying contents of Mg (x), x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, synthesized by the microwave-assisted sol–gel combustion technique. X-ray diffraction confirmed the cubic spinel structure of all the prepared samples. Real and imaginary dielectric constants were in full agreement with the general trend of the spinel series, showing a decrease with increasing frequencies, in accordance with non-Debye characteristics. For this purpose, a modified form of the Debye relation was used to explain the problem. Gradual substitution of Mg at Zn site affected the modulus characteristics and resulted in an increase in the resistance of the samples. The Cole–Cole plots were explained in the light of grain size distribution and dielectric polarisation. UV–Visible (UV–Vis) diffuse reflectance spectroscopy determined a wide range of band gap energies, ranging from 4.46 to 5.71 eV, obtained without any significant change in lattice parameters. The present work provides a detailed overview of Mg–Zn-based aluminates featuring stepped or graded energy band gaps that can be used for optoelectronic devices.

Graphene/BiVO4/TiO2 nanocomposite: tuning band gap energies for superior photocatalytic activity under visible light

A facile, ultrasonic wave-assisted one pot hydrothermal method has been developed to fabricate reduced graphene oxide/bismuth vanadate/titanium oxide (RGO/BiVO4/TiO2) ternary nanocomposites. By utilizing graphene oxide (GO) as multifunctional structure, RGO/BiVO4/TiO2 (GBT) with diverse percentage composition possessing varying band gap energies is obtained. XRD and Raman spectroscopy evince formation of tetragonal and monoclinic phases of BiVO4. The band gap energies of the components of the composite were determined by applying modified Kubelka–Munk function on UV–Vis DRS data. Tuning of band gap energy of the BiVO4 and TiO2 were simultaneously achieved by modifying the concentrations of GO and TiO2 during synthesis. The GBT exhibited enhanced photocatalytic degradation of methylene blue (MB) under visible light irradiation. The relative photocatalytic activity rates of the composites in GBT series are in agreement with the photoluminescence data. The mechanism behind the activity suggests GO acting as an electron trapper and TiO2 behaving as an efficient mediating co-catalyst. The band gap energy tuning led to reduction in time needed for complete MB degradation from 40 min with RGO/BiVO4 to 10 min with the ternary composite GBT. It is expected that the work would encourage new vistas to engineer different combinations of graphene based ternary composites which might lead to potential applications guided by band gap tuning.

Electrically Tunable Energy Bandgap in Dual-Gated Ultra-Thin Black Phosphorus Field Effect Transistors**Supported by the National Basic Research Program of China under Grant Nos 2013CB921900 and 2014CB920900, the National Natural Science Foundation of China under Grant No 11374021) (S. Yan, Z. Xie, J.-H. Chen). K.W. and T.T. acknowledge the support from the Elemental Strategy Initiative conducted by the MEXT, Japan and

The energy bandgap is an intrinsic character of semiconductors, which largely determines their properties. The ability to continuously and reversibly tune the bandgap of a single device during real time operation is of great importance not only to device physics but also to technological applications. Here we demonstrate a widely tunable bandgap of few-layer black phosphorus (BP) by the application of vertical electric field in dual-gated BP field-effect transistors. A total bandgap reduction of 124 meV is observed when the electrical displacement field is increased from 0.10 V/nm to 0.83 V/nm. Our results suggest appealing potential for few-layer BP as a tunable bandgap material in infrared optoelectronics, thermoelectric power generation and thermal imaging.

Epitaxial growth of rocksalt Zn1−xMgxO on MgO (100) substrate by molecular beam epitaxy

Zn1−xMgxO films with x=0.22–0.87 were grown on MgO (100) substrates by molecular beam epitaxy at 400 and 600°C respectively. The films containing 85% or less ZnO grow epitaxially and retain entirely the rocksalt (rs) crystal structure. The rs-Zn1−xMgxO epilayers have a tunable bandgap energy of 4.5–6.2eV. In addition, the rs-Zn1−xMgxO epilayer grown at 600°C exhibits a lower FWHM value of its (200) rocking curve as compared to its low- temperature counterpart. The lattice constant of rs-ZnO at ambient pressure and temperature is obtained to be 4.2766Å. The sticking coefficient of Mg atoms on rs-ZMO is approximately four times higher than that of ZnO atoms regardless of the growth temperature in the range of 400–600°C.

Strain effect on graphene nanoribbon carrier statistic in the presence of non-parabolic band structure**Project supported by the Ministry of Higher Education (MOHE), Malaysia under the Fundamental Research Grant Scheme (FRGS) (Grant No.Q.J130000.7823.4F477). We also thank the Research Management Center (RMC) of Universiti Teknologi Malaysia (UTM) for providing an excellent research environment.

The effect of tensile uniaxial strain on the non-parabolic electronic band structure of armchair graphene nanoribbon (AGNR) is investigated. In addition, the density of states and the carrier statistic based on the tight-binding Hamiltonian are modeled analytically. It is found that the property of AGNR in the non-parabolic band region is varied by the strain. The tunable energy band gap in AGNR upon strain at the minimum energy is described for each of n-AGNR families in the non-parabolic approximation. The behavior of AGNR in the presence of strain is attributed to the breakable AGNR electronic band structure, which varies the physical properties from its normality. The linear relation between the energy gap and the electrical properties is featured to further explain the characteristic of the deformed AGNR upon strain.