Controllable Optical Band Gaps Research Articles

Solution processable Zn-salphen containing π-conjugated polymers with unique aggregation behavior as organic optoelectronic materials

Series of π-conjugated polymers containing electron-withdrawing Zn-Schiff base complex (Zn-salphen) were reported. These polymers can be easily obtained with controllable optical bandgap (Egopt) varying from 1.50 to 2.13 eV by changing the π-conjugated comonomers, and together with excellent thermal stabilities. It was found that the introduction of alkyl chains could provide good solution process-ability for these polymers in chlorobenzene solutions. All the polymers exhibit high hydrophilic characteristics compared to traditional semi-conducting polymers. This is caused by the large polarity of Zn-salphen, which results in strong inter-molecular interactions for the polymers analyzed based on their optical properties. Interestingly, these aggregation behaviors are highly dependent on the type of solvents and coordinating additives such as pyridine, which can be observed from their UV–vis absorption and photoluminescence (PL) spectra. In addition, the film morphologies of these polymers can be modified by using pre- and post-treatments such as thermal annealing, solvent annealing and additives. This work introduces a new strategy for constructing semi-conducting polymers by solving the problem of poor solubility of Zn-salphen complex, and preliminarily studies the feasibility of these Zn-salphen containing polymers regarding their solution process-ability, optical adjustment and morphology optimization. These polymers are considered potential candidates for applications in photoelectric devices.

Rational design of a donor-acceptor structured poly(3-bromothiophene) modified g-C3N4 for enhanced photocatalytic degradation of 2-mercaptobenzothiazole

Donor-acceptor (D-A) structure has gained momentous attention due to its controllable optical band gap and impressive carrier separation performance. The intramolecular charge transfer within photocatalysts could be modulated by constructing a D-A structure, thereby enhancing their performance in degrading pollutants. In our recent research, using poly(3-bromothiophene) (PTh-Br) and dicyandiamide (DCD) as precursors, a series of D-A structures (CN-PTh-Br(x)) based on polythiophene donors and carbon nitride acceptors were obtained by one-pot synthesis for first-time. The density functional theory calculations further demonstrate that PTh-Br segment (an electron donor) donates electrons to tri-s-triazine rings (an electron acceptor). The introduction of electron-rich polythiophene groups and electron-withdrawing Br atoms on carbon nitride skeleton can propel the transfer and separation of holes and electrons. Moreover, the polythiophene groups with high conjugation length can optimize the optical absorption of CN-PTh-Br(x) at 450–600 nm, thus markedly heightening the visible-light utilization efficiency of the materials. The photocatalytic performance of CN-PTh-Br(x) was assessed through the photodegradation of 2-mercaptobenzothiazole (MBT) under visible-light illumination. The photodegradation performance of CN-PTh-Br(0.5) stood out among all the samples, demonstrating a remarkable ability to remove 99.8% of MBT within 45 min.

Fabrication of nanostructured CuO thin films with controllable optical band gaps using a mist spin spray technique at 90 °C

A solution-based process, termed the mist spin spray method, was developed to fabricate nanostructured CuO thin films on seed-free glass substrates. In this method, a copper amine complex solution and a NaOH solution are separately atomized by ultrasonication to generate mists. These mists are subsequently sprayed onto a substrate heated to 90 °C and fixed on a rotating platform to grow crystalline CuO thin films at atmospheric pressure. This one-step method enables the generation of phase-pure, dense and crack-free CuO thin films below 100 °C. CuO thin films approximately 30 nm in thickness exhibited good adhesion to glass substrates, and the concentrations of NH3 and NaOH in the initial solutions were found to affect both the film morphology and optical band gap. Specimens prepared using higher NH3 and NaOH concentrations had larger grain sizes, while those fabricated using a low NaOH concentration showed a porous structure. A higher optical band gap was found to correlate with smaller grain sizes in the films.

Tea from the drinking to the synthesis of metal complexes and fabrication of PVA based polymer composites with controlled optical band gap

In the present study black tea extract (BTE) solution which is familiar for drinking was used to prepare cerium metal-complexes (Ce(III)-complex). The prepared Ce(III)-complex was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and UV–Vis spectroscopy. The results indicate that BTE solution is a novel green coordination chemistry approach for the synthesis of metal complexes. The outcomes signify that coordination occurs between cerium cations and polyphenols. The synthesis of metal-complexes with superior absorption performance in the visible region is a challenge for optoelectronic device applications. The suspended Ce(III)-complex in distilled water was mixed with poly (vinyl alcohol) (PVA) polymer to fabricate PVA/ Ce(III)-complex composites with controlled optical properties. The PVA/Ce(III)-complexes composite films were characterized by FTIR, XRD, and UV–Vis spectroscopy. The XRD findings confirms the amorphous structure for the synthesized Ce(III)-complexes. The addition of Ce(III)-complex into the PVA host polymer led to the growth of polymer composites with controllable small optical band gaps. It is shown by the FTIR spectra of the composite films that the functional groups of the host PVA have a vigorous interaction with the Ce(III)-complex. The XRD deconvolution on PVA composites reveals the amorphous phase enlargement with increasing Ce(III)-complex concentration. It is indicated in the atomic force microscopy (AFM) that the surface roughness in the doped PVA films increases with the increase of the Ce(III)-complex. There is a decrease in absorption edge from 5.7 to 1.7 eV. It becomes possible to recognize the type of electron transition by studying both the Tauc's model and optical dielectric loss (ɛi) parameter.

A green synthesis of CISe nanocrystal ink and preparation of quantum dot sensitized solar cells

I–III–VI chalcopyrite copper indium selenium is one of therepresentatives of the light absorbing layer material, and is often used for a thin-film solar cell. With the development of nano-technology, CuInSe2 quantum dots (CISe QDs) which have intermediate belt and excitation effect characteristics are applied to the solar cells as an alternative of Cd- or S-based QDs. Most conventional methods for the synthesis of CISe QDs using solution involve the dangerous and environmentally unfriendly Oleylamine or phosphine coordination compounds. In this work, CISe QDs were synthesized by a green, safe and low-temperature method in triethylene glycol. Through controlling the growth temperature and time, the diameter can be adjusted from 3[Formula: see text]nm to 10[Formula: see text]nm. The samples exhibit quantum confinement effect, and have a controllable optical band gap. QDs were deposited on the surface of ZnO nanorods to obtain a photoanode, which were fabricated into quantum dot-sensitized solar cells. The device exhibits size-dependent performance. And the open circuit voltage shows a fluctuation up to 0.26[Formula: see text]V. When the size is 4[Formula: see text]nm, the short circuit current density is the largest (15[Formula: see text]mA/cm2).

Review on optical, structural and electrical properties of ZnTe thin films: effect of deposition techniques, annealing and doping

Zinc Telluride (ZnTe) has become a very fascinating research material for the scientists over past many years. This p type semiconducting material has a wide band gap, which makes it useful in many optoelectronic applications such as solar cells, light emitting diodes, laser screens etc. With the desire for eco-friendly alternative energy resources, it excites to investigate potential semiconducting materials at nano scale for solar cell applications. Due to direct, wide and controllable optical band gap of ZnTe with easy doping makes it a potential material for photoelectrochemical applications. In the present work, an attempt has been made to compile the work done by various researchers on optical properties (band gap, refractive index and absorption/transmission spectra, etc.), structural properties (crystallite/grain size, lattice constant and Zn:Te, etc.) and electrical properties (resistivity, activation energy, carrier concentration and hall mobility, etc.) of ZnTe thin films with main emphasis on the effect of deposition techniques, doping and annealing on these properties. Most of the thin films reported in literature shown polycrystalline cubic structure. It is found that annealed ZnTe thin films show enhanced optical, structural and electrical properties at different annealing temperatures. Doping methods used by various researchers have been discussed in detail and results show good doping impact on these properties as well.

Self-doped TiO2 nanowires in TiO2-B single phase, TiO2-B/anatase and TiO2-anatase/rutile heterojunctions demonstrating individual superiority in photocatalytic activity under visible and UV light

TiO2 nanowires possess structurally well-defined physical and chemical properties related to their dimensional confinements. However, the obscure phase transformation mechanism and the wide optical band gap of TiO2 nanowires limit their wide applications as a photocatalytic material, particularly under the visible solar spectrum. The present investigation deals with the systematic synthesis of TiO2 nanowires with their different phases e.g., TiO2-B, anatase and rutile, by the hydrothermal technique and subsequent thermal annealing in vacuum at different temperatures, and probing on their characteristic photocatalytic performances. Pure single phase TiO2-B nanowires of high crystallinity have been obtained at TA = 600 °C in which the oxygen vacancies generated via reduction of Ti4+ centers to Ti3+ facilitate enhancing absorption of visible light by virtue of their reduced optical band gaps. In addition, large surface area out of their porous structures provides enhanced adsorptive and reactive sites and accelerates the photocatalytic activity in the visible region, extending a photodegradation efficiency of RhB to ∼88% with a rate constant ∼0.013 min−1. Purely rutile phase of TiO2 formed at TA = 900 °C, exhibiting an intermediate band gap, demonstrates moderate degradation efficiency, ∼55% under visible light and ∼45% under UV exposure on RhB. However, the TiO2-B/anatase heterostructured nanowires grown by partial phase transformation at TA = 700 °C, exhibit a significantly enhanced UV photocatalytic ability by virtue of their wide optical band gaps. Migration of photogenerated holes from the anatase to the TiO2-B facilitated by the differences in two distinct phases reduces the charge recombination and enhances the photocatalytic activity with a high degradation efficiency ∼96% and a rate constant ∼0.058 min−1 under the UV light exposure. The self-transformed TiO2 heterostructured nanowires grown on thermal annealing are significantly promising for advanced photocatalytic applications, by virtue of controllable optical band gap, high porosity and self-doping characteristics.

Modification of WS2 nanosheets with controllable layers via oxygen ion irradiation

As one kind of two-dimensional materials, WS2 nanosheets have drawn much attention with different kinds of research methods. Yet ion irradiation method was barely used for WS2 nanosheets. In this paper, the structure, composition and optical band gap (Eg) of the multilayer WS2 films deposited by chemical vapor deposition (CVD) method on sapphire substrates before and after oxygen ion irradiation with different energy and fluences were studied. Precise tailored layer-structures and a controllable optical band gap of WS2 nanosheets were achieved after oxygen ion irradiation. The results shows higher energy oxygen irradiation changed the shape from triangular shaped grains to irregular rectangle shape but did not change 2H-WS2 phase structure. The intensity of E2g1 (Г) and A1g (Г) modes decreased and have small shifts after oxygen ion irradiation. The peak frequency difference between the E2g1 (Г) and A1g (Г) modes (Δω) decreased after oxygen ion irradiation, and this result indicates the number of layers decreased after oxygen ion irradiation. The Eg decreased with the increase of the energy and the fluence of oxygen ions. The number of layers, thickness and optical band gap changed after ion irradiation with different ion fluences and energies. The results proposed a new strategy for precise control of multilayer nanosheets and demonstrated the high applicability of ion irradiation in super-capacitors, field effect transistors and other applications.

Continuously controllable optical band gap in orthorhombic ferroelectric KNbO3-BiFeO3 ceramics

The optical bandgap of orthorhombic ferroelectric KNbO3 is shown to be continuously controllable via Bi and Fe co-substitution according to a K1-xBixNb1-xFexO3 doping mechanism. The room temperature X-ray diffraction data combined with Raman spectroscopy analysis show the polar orthorhombic crystal structure to persist up to x = 0.25, while the bandgap narrows monotonically by 1 eV (∼33%). In-situ Raman spectroscopy corroborates the polar nature of all compositions in the temperature range of –100 to 200 °C. The ability to control the bandgap while maintaining the spontaneous polarisation makes the K1-xBixNb1-xFexO3 system interesting for photoinduced processes in a wide temperature range.

Enhanced photoelectrochemical properties of ternary Zn1−xCuxO nanorods with tunable band gaps for solar water splitting

Ternary Zn1−xCuxO nanorods with a controllable optical band gap were fabricated by a simple electrochemical route. XRD confirmed that crystalline nanorods with copper concentrations from 1% to 10% had a hexagonal wurtzite structure. The incorporation of copper in the ZnO matrix, the amount of which was determined using XPS, red-shifted the absorption edge of the Zn1−xCuxO nanorods to an extent that was proportional to the copper content. Photoelectrochemical (PEC) measurements clearly revealed that the Zn1−xCuxO nanorods increased the photocoversion efficiency to 2 times that of pure ZnO, and they exhibited a greater photoresponse to visible light because of the band gap narrowing effect. Hence, the Zn1−xCuxO with narrowing band gap illustrated great potential for large light-harvesting device.

Control of optical bandgap energy and optical absorption coefficient by geometric parameters in sub-10 nm silicon-nanodisc array structure

A sub-10 nm, high-density, periodic silicon-nanodisc (Si-ND) array has been fabricated using a new top-down process, which involves a 2D array bio-template etching mask made of Listeria-Dps with a 4.5 nm diameter iron oxide core and damage-free neutral-beam etching (Si-ND diameter: 6.4 nm). An Si-ND array with an SiO2 matrix demonstrated more controllable optical bandgap energy due to the fine tunability of the Si-ND thickness and diameter. Unlike the case of shrinking Si-ND thickness, the case of shrinking Si-ND diameter simultaneously increased the optical absorption coefficient and the optical bandgap energy. The optical absorption coefficient became higher due to the decrease in the center-to-center distance of NDs to enhance wavefunction coupling. This means that our 6 nm diameter Si-ND structure can satisfy the strict requirements of optical bandgap energy control and high absorption coefficient for achieving realistic Si quantum dot solar cells.

Chemical vapor deposition of a PbSe/CdS/nitrogen-doped TiO2 nanorod array photoelectrode and its band-edge level structure

Lead selenide (PbSe) is a candidate for harvesting infrared (IR) light, due to its narrow and controllable optical band gap which can be tuned by its grain size. However, there is still a challenge to assemble a photoelectrode with different cations of selenide and sulphide sensitizers, such as PbSe and cadmium sulphide (CdS), onto the same skeleton by a wet chemical route. In this work, we provide a chemical vapor deposition (CVD) method to assemble a PbSe and CdS nanoparticle (NP) co-sensitized nitrogen (N)-doped TiO2 nanorod array (PbSe/CdS/NT) photoelectrode. This hybrid photoanode showed an increased photoelectrochemical (PEC) performance under simulated sun light irradiation. Electrochemical analysis revealed that a presumable cascade structure of band-edge levels, in this photoanode, could be attributed to the enhanced photocurrent. In addition, the carrier multiplication in PbSe NPs via impact ionization, was also suggested to contribute to both the photo and dark current.

Smooth thin film C/diamond membranes with controllable optical band gaps

Mixed phase carbon-diamond films which consist of small grain diamond in an a:C matrix were deposited on polished Si using a radio frequency CH 4 Ar plasma CVD deposition process. Ellipsometry, surface profilometry, scanning electron microscopy (SEM) and spectrophotometry were used to analyse these films. Film thicknesses were typically 50–100 nm with a surface roughness of ± 30 A ̊ over centimetre length scans. SEM analysis showed the films were smooth and pinhole free. The Si substrate was etched using backside masking and a directional etch to give taut carbon-diamond membranes on a Si grid. Spectrophotometry was used to analyse the optical properties of these membranes. Band gap control was achieved by varying the dc bias of the deposition process. Band gaps of 1.2 eV to 4.0 eV were achieved in these membranes. A technique for controlling the compressive stress in the films, which can range from 0.02 to 7.5 GPa has been employed. This has allowed the fabrication of thin, low stress, high band gap membranes that are extremely tough and chemically inert. Such carbon-diamond membranes seem promising for applications as windows in analytical instruments.