Tunable Band Gap Energy Research Articles

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.

Nonvolatile Charge Injection Memory Based on Black Phosphorous 2D Nanosheets for Charge Trapping and Active Channel Layers

2D van der Waals atomic crystal materials have great potential for use in future nanoscale electronic and optoelectronic applications owing to their unique properties such as a tunable energy band gap according to their thickness or number of layers. Recently, black phosphorous (BP) has attracted significant interest because it is a single‐component material like graphene and has high mobility, a direct band gap, and exhibits ambipolar transition behavior. This study reports on a charge injection memory field‐effect transistor on a glass substrate, where few‐layer BPs act as the active channel and charge trapping layers, and Al2O3 films grown by atomic layer deposition act as the tunneling and blocking layers. Because of the ambipolar properties of BP nanosheets, both electrons and holes are involved in the charge trapping process, resulting in bilateral threshold voltage shifts with a large memory window of 22 V. Finally, a memory circuit of a resistive‐load inverter is implemented that converts analog signals (current) to digital signals (voltage). Such a memory inverter also shows a clear memory window and distinct memory on/off switching characteristics.

Tunable Bandgap Energy and Promotion of H2O2 Oxidation for Overall Water Splitting from Carbon Nitride Nanowire Bundles

Tunable Bandgap Energy and Promotion of H₂O₂ Oxidation for Overall Water Splitting from Carbon Nitride Nanowire Bundles

Effect of sulfurization time on properties of Cu2ZnSnS4 thin films obtained by sol–gel deposited precursors

As a promising and alternative solar absorber material, the Cu2ZnSnS4 (CZTS) has gained a broad interest over recent years due to its excellent material properties such as suitable and tunable band gap energy of 1.4–1.6 eV and absorption coefficient over 104 cm−1 (Jimbo et al. in Thin Solid Films 515(15):5997–5999, 2007; Katagiri et al. in Sol Energy Mater Sol Cells 65(1–4):141–148, 2001). In addition, all components of the CZTS are naturally abundant and non toxic. Nowadays solution based-approaches are being developed for the deposition of CZTS to overcome the problem of expensive and complicated vacuum-based methods. In this work, we report the study of properties of the novel wurtzite CZTS material prepared by sol–gel sulfurization as a function of annealing time. The effect of annealing time on structural and optical properties of the films was investigated. The X-ray diffraction analysis confirmed the formation of the wurtzite CZTS phase. The crystallinity of the films increases with an increasing sulfurization time. The values of the optical absorption coefficients for the film were found to be 104 cm−1. The optical band-gap values were estimated to be between 1.58 and 1.79 eV depending on the sulfurization time.

Cadmium sulphide quantum dots with tunable electronic properties by bacterial precipitation.

We present a new method to fabricate semiconducting, transition metal nanoparticles (NPs) with tunable bandgap energies using engineered Escherichia coli. These bacteria overexpress the Treponema denticola cysteine desulfhydrase gene to facilitate precipitation of cadmium sulphide (CdS) NPs. Analysis with transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy reveal that the bacterially precipitated NPs are agglomerates of mostly quantum dots, with diameters that can range from 3 to 15 nm, embedded in a carbon-rich matrix. Additionally, conditions for bacterial CdS precipitation can be tuned to produce NPs with bandgap energies that range from quantum-confined to bulk CdS. Furthermore, inducing precipitation at different stages of bacterial growth allows for control over whether the precipitation occurs intra- or extracellularly. This control can be critically important in utilizing bacterial precipitation for the environmentally-friendly fabrication of functional, electronic and catalytic materials. Notably, the measured photoelectrochemical current generated by these NPs is comparable to values reported in the literature and higher than that of synthesized chemical bath deposited CdS NPs. This suggests that bacterially precipitated CdS NPs have potential for applications ranging from photovoltaics to photocatalysis in hydrogen evolution.

Acute mechano-electronic responses in twisted phosphorene nanoribbons.

Many different forms of mechanical and structural deformations have been employed to alter the electronic structure of various modern two-dimensional (2D) nanomaterials. Given the recent interest in the new class of 2D nanomaterials - phosphorene, here we investigate how the rotational strain-dependent electronic properties of low-dimensional phosphorene may be exploited for technological gain. Here, using first-principles density-functional theory, we investigate the mechanical stability of twisted one-dimensional phosphorene nanoribbons (TPNR) by measuring their critical twist angle (θc) and shear modulus as a function of the applied mechanical torque. We find a strong anisotropic, chirality-dependent mechano-electronic response in the hydrogen-passivated TPNRs upon vortical deformation, resulting in a striking difference in the change in the carrier effective mass as a function of torque angle (and thus, the corresponding change in carrier mobility) between the zigzag and armchair directions in these TPNRs. The accompanied tunable band-gap energies for the hydrogen-passivated zigzag TPNRs may then be exploited for various key opto-electronic nanodevices.

Intrinsic n- and p-Type MgZnO Nanorods for Deep-UV Detection and Room-Temperature Gas Sensing

MgZnO (MZO) family, like other ternary compounds, offering tunable energy bandgap covering from blue to deep UV emission has attracted attentions for new optoelectronics, where both n- and p-types are essential. Nevertheless, only limited success is reported for p-type only by extrinsic doing. In the work, intrinsic n- and p-type single-crystalline MZO nanorods (NRs) were synthesized using a low-cost multistep process via the solid-state diffusion of Mg into intrinsic n- and p-type ZnO NRs prepared using chemical bath deposition or chemical vapor deposition. Cathodoluminescence spectra demonstrate substantial blue-shifted ultraviolet (UV) emission from 377 to 357 nm for MZO NRs with the highest Mg content. Photosensing measurements of the MZO NR devices show high sensitivity to UVC compared to UVA with bias-dependent sensitivity. Room-temperature gas sensing using MZO NRs exhibits enhanced sensitivity compared to that for ZnO, with opposite responses in current variation, supporting different carrier type...

An easy hydrothermal synthesis of porous CuSySe1-y nanomaterials with broadly tunable near-infrared localized surface plasmon resonance

The ternary Cu2-xSySe1-y nanomaterials have attracted great attention due to their unique optical properties, but they are primarily synthesized by the hot-injection and high temperature solvent method, however they are complex and uneconomical. In the paper, we report a new facile method for the synthesis of monodispersed and uniform porous CuSySe1-y alloyed nanomaterials with tunable composition, size, and morphology. More importantly, the near-infrared localized surface plasmon resonance (NIR LSPR) wavelength of these CuSySe1-y alloyed nanomaterials can be broadly tuned from 1027 to 1400 nm, and the energy bandgap can be simultaneously tuned from 1.42 to 1.91 eV by adjusting the chalcogen ratio. This approach opens up a new route to access to synthesize other porous ternary alloyed nanomaterials with broadly tunable LSPR wavelength and energy bandgap, which will find potential applications in the photothermal therapy and photovoltaic conversion.

Electron Injection Dynamics at the SILAR Deposited CdS Quantum Dot/TiO2 Interface

Semiconductor quantum dots with their tunable band gap energies offer new opportunities for controlling photoresponse and photoconversion efficiency of solar cells. Exciton dissociation, charge injection, and charge recombination are key steps for efficient interfacial electron transfer. Here, electron injection and carrier relaxation dynamics at the CdS quantum dot/TiO2 interface were investigated by using ultrafast transient absorption spectroscopy and global fitting analyses. The CdS/TiO2 composites were prepared by depositing CdS in the TiO2 nanocrystalline films by the successive ionic layer adsorption and reaction (SILAR) technique. Comparing the transient absorption spectra of CdS/TiO2 composites formed by different numbers of CdS coating cycles and CdS/Al2O3 revealed size-dependent electron injection from excited CdS into TiO2 nanoparticles, which occurred on a time scale of 1–9 ps. We also demonstrated that holes are trapped in a localized state with a time constant of 0.3 ps, which was faster th...

Investigation of optical properties of mixed ligand directed ZnO luminescent nanoparticles for application in light emitting diodes

Developing the heavy metal free QD-LEDs is required to curtail the risks to human beings and environment. Nanoparticles that are ecofriendly, chemically stable, easy to synthesize, nontoxic, biocompatible too are the preferred one. ZnO are biocompatible and low cost semiconductor nanoparticles with tunable band gap and large excitonic binding energy, which make them suitable for LED applications. To achieve the same purpose, this research work is focused on the synthesis of surface modified ZnO nanoparticles for their application in light emitting diodes. The condensation reaction is used to synthesize the ligand and wet chemical precipitation method is used to synthesize surface modified ZnO nanoparticles. The optical properties are evaluated using UV–Vis absorption and fluorescence studies. The optical properties are also investigated in the presence of metal ions and an increased band gap is achieved when Fe3+ ion is added to organic receptor coated ZnO nanoparticles. The stability of ZnO coupled receptor is confirmed theoretically using density functional theory.

Temperature-Dependent Optical Properties of PbS/CdS Core/Shell Quantum Dot Thin Films: Probing the Wave Function Delocalization

Colloidal semiconductor quantum dots (QDs) are extraordinarily appealing for the development of cheap and large area solar cells due to high absorption efficiency, tunable bandgap energies, and solution processability. Understanding and controlling electronic wave function delocalization in QD thin films is therefore pivotal for realizing efficient optoelectronic devices. Here, a detailed study of the exciton recombination dynamics in PbS/CdS core/shell films as a function of temperature and surface chemistry is reported. By decreasing the temperature from 295 to 5.4 K, electronic wave function delocalization from the PbS core to the monolayer CdS shell is observed, demonstrating a reduction of the conduction band offset at low temperature and the formation of a quasi-type II band alignment. Interestingly, films made with core–shell particles, where the shell is thicker than a monolayer, display type I alignment also at low temperature.

Prediction of HgCdTe spectral inhomogeneities based on optical and material properties

HgCdTe infrared detectors have been extensively developed over the past few decades. They owe their popularity to the many advantages of HgCdTe material, which has a tunable energy bandgap over the 1–30μm range and high quantum efficiency. They are increasingly used to perform quantitative spectral imaging, which implies a high control of their spectral response. In order to optically model the spectral behavior of this material, it was mandatory to know its refractive index dispersion. The optical properties of this ternary alloy have been particularly studied, as they contribute to detector performance. To the best of our knowledge, however, there are no published measurements of the real part of the refractive index of HgCdTe at low temperatures, above the bandgap energy. We therefore undertook a thorough study of this material at 80K, where we estimated the real part of the refractive index for several alloy compositions in the spectral range of [1800cm-1,6000cm-1]. We used a modified Kramers–Kronig relation, combined with published experimentally measured points. First, we proved the high accuracy of this technique at room temperature, by comparing results with previously published models. We then simulated an HgCdTe bulk layer, in order to extract the dependence of the cut-off wavenumbers on cadmium composition and the layer thickness at low temperature. The influence of these macroscopic parameters on the spectral nonuniformities is discussed.

First principles study of scandium nitride and yttrium nitride alloy system: Prospective material for optoelectronics

Besides many other state of the art promising applications, transition metal (TM) nitride materials are intensively investigated on account of considered potential materials for optoelectronic applications. In this study computations pertaining to structural, electronic as well as the optical properties of Scandium Nitride (ScN), Yttrium Nitride (YN) and their mutual alloying (ScxY1−xN), for x=0.25, 0.50, 0.75, are presented. These computations are carried out by employing the full potential (FP) linearized augmented plane wave (LAPW) plus local orbitals (lo) method designed within density functional theory (DFT). Structural parameters are calculated at the level of Perdew Burke and Ernzerhof (PBE) parameterized generalized gradient approximations (GGA), where to investigate electronic and optical properties, Tran–Blaha modified Becke–Johnson (mBJ) potential is involved. From our calculations, a very small variation is noted in lattice constant values of ScxY1−xN alloying system as a function of Y content, reflecting to appropriate alloying of ScN and YN. Moreover, effect of the site preference for two different configurations is also analyzed. The lower absorption of ScxY1−xN system in the visible light region together with less than 30% reflectivity for entire alloying range lead to their transparent nature. Additionally fascinating characteristics, like high mechanical strength, tunable energy band gap, transparent nature, and lower reflectivity of the ScYN alloying system provoke their further potential in optoelectronics.

MBE growth of entire-indium-composition-tunable In<italic><sub>x</sub></italic>Ga<sub>1-<italic>x</italic></sub>N alloy

As a direct band-gap semiconductor, ternary nitride alloy In x Ga1- x N has a tunable energy band-gap from 3.43 eV (for GaN) to 0.64 eV (for InN) with increasing In content, which covers the spectrum from 0.36 to 1.9 μm. In x Ga1- x N alloy has high electron drift velocity and absorption coefficient, which shows great potential application in devices such as full spectrum solar cells and light-emitting devices. Due to lack of proper substrate, In x Ga1- x N is usually grown on sapphire or GaN template. In this article, we review the growth and properties of InN and InGaN with tuable In composition grown on sapphire substrate and GaN/sapphire template by MBE. A boundary-temperature-controlled method is proposed, which successfully lead to a high-electron-mobility InN layers on sapphire substrate by MBE. The temperature-controlled epitaxy method used to grow In x Ga1- x N layers with tunable In composition on GaN/sapphire template.

(Invited) High Efficiency Solar-to-Hydrogen Conversion on InGaN Nanowire Arrays

Recently, III-nitride nanowire structures have been intensively investigated for applications in solar water splitting and hydrogen generation. Compared to the conventional oxide-based photoelectrodes, III-nitride nanowires offer distinct advantages including tunable energy bandgap across nearly the entire solar spectrum, conduction and valence band edges that can straddle water redox potentials under deep visible and near-infrared light irradiation, and high efficiency charge carrier separation and extraction. Spontaneous overall water splitting under UV, blue, and green light irradiation has been demonstrated on InGaN/GaN nanowire structures. Moreover, multi-band InGaN nanowire photocatalyst and photoanode have been developed to effectively harvest a large part of the solar spectrum. Such nanowire arrays also exhibit a high level of stability in aqueous solution. To further improve the efficiency of solar-to-hydrogen conversion, we have investigated the design, epitaxial growth, and testing of paired InGaN/GaN photoanode and photocathode. The devices exhibit relatively high incident photon-to-current conversion efficiency (up to 65%). The energy conversion efficiency in the wavelength range of 300-600 nm is also significantly improved, compared to a single electrode. Here, we report on the design principle and operation of such novel photoelectrodes. In this design, the photocathode and the photoanode are connected side by side, wherein the solar light can be split spectrally and spatially. Different split light spectra are spread out to strike the corresponding photoelectrodes. n-InGaN/GaN and p-InGaN/GaN decorated with Pt nanoparticles are used as the photoanode and photocathode, respectively. The design of paired InGaN nanowire photoelectrodes can provide high photovoltage without trading off on the harvesting of longer wavelength of sunlight. Catalyst-free, vertically aligned InGaN/GaN nanowire arrays were grown on n-type Si(111) substrate by radio frequency plasma-assisted molecular beam epitaxy under nitrogen rich conditions. After an in situ oxide desorption at ~ 770 °C, A GaN nanowire template with a height of ~ 200 nm was first grown on Si substrate. To enhance the carrier transport between the n-type Si substrate and p-InGaN nanowire, a tunnel junction consisting of n +-GaN/p +-InGaN/p +-GaN was implemented beneath the p-InGaN nanowire. The nanowires are doped n-type or p-type using silicon or magnesium, respectively. The photoelectrochemical (PEC) properties of the photocathode and the photoanodes were investigated separately using a cell with a three-electrode configuration in 1 mol/L HBr. A linear sweep voltammogram for n-InGaN/GaN photoanode was performed under both dark and illuminated conditions. The maximum incident photon-to-current conversion efficiency (IPCE) measured at 1 V vs. Ag/AgCl for the n-InGaN/GaN photoanode is ~ 50% at 350 nm, while the maximum IPCE for p-InGaN/GaN photocathode (at -0.6 V vs. Ag/AgCl) is ~ 65% at 350 nm. The IPCE shows a decreasing trend with increasing wavelength, limited by the effective light absorption of the nanowire structures. In the paired configuration, the incident light was split by using long- and short-pass optical filters. The parallel-connected photoanode was illuminated with light with wavelengths in the range of 300-480nm. The photocathode was illuminated with light with wavelengths of 485-600 nm. Using a two-electrode configuration, the photoanode was considered to be the working electrode, and the photocathode p-InGaN/GaN was connected as the counter electrode in 1 mol/L HCl. With this novel design, the power conversion efficiency in the range of 300-600 nm has been increased by nearly one order of magnitude, compared to efficiency of a single electrode. The maximum power conversion efficiency of this device can reach >5%. Work is currently in progress on the integration of InGaN nanowires with solar cells to achieve further increased efficiency for solar-to-hydrogen conversion.

Thin-film copper indium gallium selenide solar cell based on low-temperature all-printing process.

In the solar cell field, development of simple, low-cost, and low-temperature fabrication processes has become an important trend for energy-saving and environmental issues. Copper indium gallium selenide (CIGS) solar cells have attracted much attention due to the high absorption coefficient, tunable band gap energy, and high efficiency. However, vacuum and high-temperature processing in fabrication of solar cells have limited the applications. There is a strong need to develop simple and scalable methods. In this work, a CIGS solar cell based on all printing steps and low-temperature annealing is developed. CIGS absorber thin film is deposited by using dodecylamine-stabilized CIGS nanoparticle ink followed by printing buffer layer. Silver nanowire (AgNW) ink and sol-gel-derived ZnO precursor solution are used to prepare a highly conductive window layer ZnO/[AgNW/ZnO] electrode with a printing method that achieves 16 Ω/sq sheet resistance and 94% transparency. A CIGS solar cell based on all printing processes exhibits efficiency of 1.6% with open circuit voltage of 0.48 V, short circuit current density of 9.7 mA/cm(2), and fill factor of 0.34 for 200 nm thick CIGS film, fabricated under ambient conditions and annealed at 250 °C.

A promising sputtering route for dense Cu2ZnSnS4 absorber films and their photovoltaic performance

Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) is highly abundant in nature. It is an important absorber material for the development of low-cost and sustainable next-generation I2–II–IV–VI4 thin-film solar cells because it has a the tunable direct band gap energy, inexpensive constituent elements, and a large absorption coefficient in the visible wavelength region. This work develops an efficient one-step vacuum-based approach to depositing CZTS films without the need to supply excess sulfur during/after deposition or to perform any post-sulfurization treatment. This one-step RF sputtering process produces CZTS films that are crystalline, phase-pure, dense, smooth, and continuous. Air Mass 1.5 G power conversion efficiencies of as high as 6% have been achieved with an antireflection coating, demonstrating that this new approach has great potential as a low-cost alternative for high-efficiency CZTS solar cell production.

Preparation and Wide Tunable Band-Gap Energies of PbTe Nanocrystals Using Glycerin as a Capping Ligand

PbTe nanocrystals (NCs) have been synthesized by a novel method at low temperature. Pb acetate being solved in glycerol and Te in TOP solution were used as the precursors for the preparation of PbTe nanocrystals. The as-prepared products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UVvis absorption, Fourier transform infrared spectroscopy (FTIR). These data, together with analysis of the absorption spectra, allowed us to observe the size dependence of the peaks in the absorption spectrum. The size-dependent optical spectra of the PbTe nanocrystals exhibits wide tunable band gap energies varying from UV visible range to near IR region, which corresponds to a huge blue shift of 3.0 eV in comparison to the bulk counterpart. Keywords: 1. PbTe nanocrystals, 2. optical properties, 3. wide tunable band gap, 4. microstructure

Monolithic graphene oxide sheets with controllable composition

Graphene oxide potentially has multiple applications and is typically prepared by solution-based chemical means. To date, the synthesis of a monolithic form of graphene oxide that is crucial to the precision assembly of graphene-based devices has not been achieved. Here we report the physical approach to produce monolithic graphene oxide sheets on copper foil using solid carbon, with tunable oxygen-to-carbon composition. Experimental and theoretical studies show that the copper foil provides an effective pathway for carbon diffusion, trapping the oxygen species dissolved in copper and enabling the formation of monolithic graphene oxide sheets. Unlike chemically derived graphene oxide, the as-synthesized graphene oxide sheets are electrically active, and the oxygen-to-carbon composition can be tuned during the synthesis process. As a result, the resulting graphene oxide sheets exhibit tunable bandgap energy and electronic properties. Our solution-free, physical approach may provide a path to a new class of monolithic, two-dimensional chemically modified carbon sheets.

Wet-chemical synthesis of quaternary chalcopyrite Cu(In1−xAlx)Se2 nanocrystals by triethylenetetramine-assisted triethylene glycol solution process

Abstract Single-phase, near granular shaped quaternary chalcopyrite Cu(In 1− x Al x )Se 2 nanocrystals (0 ⩽ x ⩽ 0.5) were synthesized by a facile, air pressure hot-injection process using triethylenetetramine(TETA)-assisted triethylene glycol solution, CuCl 2 ·2H 2 O, InCl 3 ⋅4H 2 O, AlCl 3 ⋅6H 2 O and Se powder as precursors, polyvinylpyrrolidone as dispersant, at solution temperature 270 °C. The TETA-assisted role, the optimum TETA added amount and the effects of reaction solution temperatures were investigated based on X-ray diffraction, energy dispersive X-ray analyses, transmission electron microscopy, high resolution transmission electron microscopy and selected area electron diffraction. The band gap energy tuning with varying In/Al atomic ratios in feeding solutions was characterized by X-ray diffraction, inductively coupled plasma mass spectrum, Raman spectroscopy and UV–vis–NIR absorption.