Tunable Band Gap Energy Research Articles

Reversibly tuning the band structure of high entropy oxides

The first study of a rare earth element-based HEO’s band structure revealed that heat treatment under different atmospheres allows reversible tuning of band gap energy.

MXenes for future nanophotonic device applications

Abstract Two-dimensional (2D) layers of transition metal carbides, nitrides, or carbonitrides, collectively referred to as MXenes, are considered as the new family of 2D materials for the development of functional building blocks for optoelectronic and photonic device applications. Their advantages are based on their unique and tunable electronic and optical properties, which depend on the modulation of transition metal elements or surface functional groups. In this paper, we have presented a comprehensive review of MXenes to suggest an insightful perspective on future nanophotonic and optoelectronic device applications based on advanced synthesis processes and theoretically predicted or experimentally verified material properties. Recently developed optoelectronic and photonic devices, such as photodetectors, solar cells, fiber lasers, and light-emitting diodes are summarized in this review. Wide-spectrum photodetection with high photoresponsivity, high-yield solar cells, and effective saturable absorption were achieved by exploiting different MXenes. Further, the great potential of MXenes as an electrode material is predicted with a controllable work function in a wide range (1.6–8 eV) and high conductivity (~104 S/cm), and their potential as active channel material by generating a tunable energy bandgap is likewise shown. MXene can provide new functional building blocks for future generation nanophotonic device applications.

Experimental and Theoretical Study into Interface Structure and Band Alignment of the Cu2Zn1-x Cd x SnS4 Heterointerface for Photovoltaic Applications.

To improve the constraints of kesterite Cu2ZnSnS4 (CZTS) solar cell, such as undesirable band alignment at p–n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu2Zn1–xCdxSnS4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental–theoretical approach was employed to characterize and assess the optoelectronic properties of Cu2Zn1–xCdxSnS4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu2Zn1–xCdxSnS4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu2Zn1–xCdxSnS4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu2CdSnS4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p–n junction in the ultrafast time scale and highlight a route to improve device performances.

Germanium Tin Surface Passivation and Its Effect on the Optoelectronic Performances

Germanium Tin is an emerging semiconductor, with high carrier mobility and tunable bandgap directness and energy, that has been attracting a great deal of interest for applications in silicon-compatible electronics and monolithic optoelectronics. In contrast to compound semiconductors which have in the limelight to address several challenges in these technologies, this new emerging family of silicon-compatible group IV semiconductors holds the promise to combine both cost effectiveness and performance in several devices such as tunnelling field effect transistors, infrared detectors and emitters. By tuning strain and composition of GeSn, the band structure can be modified, thus allowing to engineer a large variety of low-dimensional heterostructures relevant to these devices. In fact, even though Ge is an indirect bandgap material, the incorporation of Sn in its lattice allows a transition into a direct bandgap material. Thus, GeSn yields to higher rate of radiative transitions and band-to-band tunneling. However, there are still several outstanding challenges at the materials level that must be overcome before harnessing GeSn advantageous properties. For instance, effective processes to effectively passivate its surface at various Sn composition are yet to be established. It is known that the surface of a semiconductor contains electronically active states because of unsaturated surface bonds or dangling bonds states, which act as localized energy levels the band gap that can change the intrinsic electrical behavior of the material. Therefore, understanding and controlling the surface states are of compelling importance. Moreover, the native oxide layer that forms at the surface of GeSn contains defects resulting in trapping charge carriers thus decreasing their mobility. The poor quality of the surface can decrease the rate of radiative transitions and contribute to the dark current. An effective passivation is thus needed to enhance the optoelectronic performances of GeSn. Passivation layer allows improved charge-separation, reduces charge recombination at surface states, passivates the dangling bonds and decreases the reactivity of the surface. To address these issues, this work investigates several chemical passivation processes and evaluate their effects on the optoelectronic properties of GeSn.The GeSn samples investigated in this work were grown on silicon wafers using ~0.6-3 µm-thick Ge interlay – commonly known as virtual substrates (Ge-VS). The epitaxial growth was carried out by the chemical vapor deposition (CVD) using monogermane (GeH4) and tin-tetrachloride (SnCl4) precursors. Strain minimization and the reduced growth temperature below 350 °C are of paramount importance to enhance Sn incorporation in Ge lattice to reach compositions of 7 – 17 at.%, much larger than the equilibrium composition of 1at.%. Several passivation treatments were evaluated and their basic mechanisms were elucidated. We first evaluated the capacity to remove the native oxide, leave the surface clean and passivate the dangling bonds of GeSn. The kinetic of oxide regrowth was studied to assess the chemical stability of the passivation through X-rays photoelectron spectroscopy (XPS) analysis. For instance, for a surface treatment consisting of a dip in HF followed by a dip in (NH4)2S and then by a nitride drying. XPS measurements show that it is effective in cleaning and passivating the surface, it allows removing the major part of Ge and Sn oxides. Recorded spectra reveal that this treatment gives rise to a persistent sulfur bonds at 161.8 eV, which is the signature of sulphide compounds. Sulfur allows surface passivation and slows down the oxide regrowth. Moreover, alternative surface passivation processes will also be discussed, and their performances compared to the treatment above will be elucidated. Besides, to better understand the evolution of the surface state after the treatments, ellipsometric measurements combined with atomic force microscopy studies are conducted and will be presented. Also, complementary electrical measurements and photoluminescence emission studies will also be discussed to highlight the effectiveness of each treatment in improving the optoelectronic performances of GeSn.

Exciton Coherence in Phthalocyanine Crystalline Thin Films

Intermolecular interactions in small molecule crystals play an important role in the exciton delocalization process and can lead to robust exciton coherence. The spatial extension of this delocalization, and the exciton coherence length, significantly influence practical applications that rely on excitonic radiative recombination, dissociation or electron transport. The porphyrins and phthalocyanines derivatives family represent the ideal system for a comprehensive study of excitonic coherence because they represent an iconic example of a one-dimensional J -like aggregate system.At the University of Vermont, the Furis group focuses on exploring excitonic states in solution-processed phthalocyanine thin films using a dual Linear Dichroism/Photoluminescence Laser Scanning Microscope developed in -house. These studies are among the first to directly probe the supramolecular structure-property relationships in these electronic materials. The experiments reported here investigate the effect of alkyl substitutions and dynamic disorder (i.e. exciton-phonon coupling) on exciton coherence in solution-processed crystalline thin films of porphyrin and phthalocyanine derivatives.Specifically, the largest exciton coherence length (approx.15 nm) was measured for an octabutoxy derivative, where the saddle shape of the molecules and the crystalline packing result in weaker coupling to the acoustic phonons (low energy) modes. Enhancing the size of the macrocycle ring to a naphthalocyanine also results in long coherence lengths despite larger intermolecular nearest-neighbor (NN) distances. Most importantly, this study brings evidence that excitonic coherence can engineered in a systematic manner through chemical and physical routes. For example, applying uniaxial strain to the same thin films deposited on Kapton substrates results in continuous bandgap energy tuning, in analogy to inorganic semiconductors.

Synthesis and characterization of pure and Cu doped CeO2 nanoparticles: photocatalytic and antibacterial activities evaluation

This article reports the effect of pure CeO2 and Cu (1%, 3% and 5 mol %) doped CeO2 Nanoparticles (NPs) have been prepared by very simple and improved co-precipitation method. Synthesized NPs has been subjected to several analytical methods, viz. XRD, TEM and UV-Vis spectral analysis. XRD data analysis confirmed that face cubic structure and average size are found to be in the range 5-10nm. Particle size and morphological studies as observed from TEM exhibits almost identical cubical shaped particles with average size 5-8nm. The tuning of band gap energy has been observed from UV-Visible absorption spectrum of cerium oxide NPs upon Cu (1%, 3%, and 5mol %) incorporation. Degradation of the methylene blue (MB) through photocatalysis has been observed for pure and Cu (1%, 3% and 5 mol%) CeO2 nanoparticles under solar spectrum. In addition, pure and Cu (1%, 3% and 5 mol %) cerium oxide nanoparticles has also been subjected to antibacterial response using different strains.

Van der Waals epitaxy and composition control of layered SnSxSe2−x alloy thin films

Abstract

Low cost and green approach in the reduction of graphene oxide (GO) using palm oil leaves extract for potential in industrial applications

Graphene is an exceptional and versatile new material which having worldwide reputation due to its unique properties such as having extremely good electrical conductor, tunable energy band gap, and offers high mechanical strength. The process to derived graphitized carbon into the reduced graphene oxide (rGO) is facing a huge obstacle and the chemical method is proven to be the best choice in order to synthesis rGO in the big scale short amount of time. However, the current chemical method involved the usage of hazardous, toxic and corrosive chemical as reducing agent namely hydrazine to form the sheet layer of graphene. Thus, this will limit the various applications of graphene due to environmental impact and safety issues. As an alternative current research proposed the usage of palm oil leaves extract to replace the hazardous and toxic chemical used as reducing agent. In our project, graphite was used as starting material and was oxidized using modified Hummer’s method. The palm oil leaves extract with the mixture of graphene oxide (GO) solution was refluxed to produce rGO. The as-synthesized green approach rGO material were then characterized using X-ray diffraction (XRD), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), and Raman spectroscopy. The results revealed that the interspace distance between plane increased proportionally as graphite was oxidized, increasing from 0.33 nm to 0.84 nm. The reduction process of GO using palm oil leave extract showed the successful in removing the hydroxyl group and amorphization of sp2 carbon structures. The reduction process found to be increased in C/O ratios from 1:1 to 3:1. Raman spectroscopy revealed that the G band position was restored comparable to graphite as the reduction process successful achieved. TEM images and selected area electron diffraction (SAED) patterns illustrated the confirmation of the successfully synthesized of the monolayer of graphene sheet. Electrochemical studies carried out for both GO and rGO have positively differentiated and concluded a better voltage-current response of rGO in comparison to GO. The as synthesized rGO in the current project holds various potential for further investigation and industrial applications not limited to just supercapacitor and photocatalyst.

Electric field induced band tuning, optical and thermoelectric responses in tetragonal germanene: a theoretical approach.

In this article, we have systematically explored the electronic, optical and thermoelectric properties of tetragonal germanene (T-Ge) using first principles calculations. The ground state geometry of pristine T-Ge is buckled and exhibits nodal line semi-metallic behaviour. In addition, we have proposed a tight binding (TB) model Hamiltonian that efficiently explains the emergence of double Dirac points at the Fermi level of T-Ge. Furthermore, a hopping relation has been explored at which both Dirac points merge and then annihilate resulting in a direct band gap at the Γ point. To exploit the buckling of the system, we have employed a transverse electric field, which invariably breaks the sublattice symmetry and removes the degeneracies at the Fermi surface. Furthermore, the band gap at the Dirac points varies linearly with the external electric field strength. Our TB Hamiltonian adequately satisfies the first principles results even in the presence of an external electric field. Moreover, we have found that T-Ge offers efficient tuning of band gaps at the Dirac points compared to other buckled systems viz. hexagonal silicene and germanene. In addition, the optical behaviour of T-Ge has been explained in accordance with the electronic states of the system. The strong optical responses in a low energy region make the material efficient for optical nanodevice applications. Moreover, T-Ge shows relatively better thermoelectric behaviour than graphene. Therefore, the external electric field induced tunable band gap and intriguing low energy optical signals pave the way to choose T-Ge as a smart choice for optoelectronic device applications. Finally we have suggested probable routes for experimental realization of the T-Ge structure.

Synthesis, characterization and photocatalytic activity of Zn2+, Mn2+ and Co2+ doped SnO2 nanoparticles

Nanomaterials with many improved properties have been used in versatile applications. Herein we have synthesized SnO2 NPs doped with transition metal ions such as Zn2+, Mn2+ and Co2+ through a facile and inexpensive hydrothermal approach. The synthesized nanomaterials were characterized by XRD, FT-IR, SEM and UV-Vis analysis. The optical properties of the NPs were characterized by using UV–vis and photoluminescence spectroscopy (PLS). Their photocatalytic performances were investigated by degrading methylene blue (MB) dye with UV irradiation. Transition metal doping to SnO2 NPs improved the photocatalytic activity to degradation of methylene blue dye due to tuning of band gap energy i.e. lowering of band gap energy compared to undoped SnO2 NPs. The results suggest that the synthesized NPs could be used efficiently for remediation/degradation of environmentally hazardous dyes from waste water or environmental cleanup.

Novel Calcium Carbonate-titania nanocomposites for enhanced sun light photo catalytic desulfurization process

Preparation of active photocatalytic nanostructures to harvest the abundant sunlight energy is a recent worldwide direction for clean energy production and environmental management. Following this target, different calcium carbonate-titania nanostructures were prepared by three different pathways using available raw materials such as limestone as calcium precursor. After characterization of the prepared materials with X-ray diffraction (XRD), X-ray fluorescence (XRF) patterns, Fourier transmission infrared (FT-IR), high resolution transmission electron microscope (TEM), N2 adsorption-desorption isotherm, UV–vis diffuse reflectance and photoluminance (PL), the materials were applied as novel photocatalysts for desulfurization of dibenzothiophene (DBT) and gas oil using different radiation sources at room temperature. It has been obtained that, 95% desulfurization of DBT was possible under 1 h visible light irradiation with linear halogen lamp (LHL) at catalyst/DBT-solution = 10 g/L, while ultra-clean diesel production (99% removal, 3.47 ppm) could be obtained via normal sunlight photochemical desulfurization of diesel fuel by calcium carbonate titania photocatalyst in presence of H2O2 and acetic acid as oxidizing agents and acetonitrile as a solvent. Here, the followed preparation pathway produced highly active calcium titanate photocatalysts with tunable band gap energy (2.05 eV), reduced electrons/hole pairs recombination and stable photocatalytic activity with enhanced visible light removal of organosulfur compounds for economic ultra-clean fuel production, pollution control, and environmental management.

Ultrasound driven mound to self-affine surface structure transition in nanostructured films: one-pot route to cubic CuInS2 quantum dots with controlled structure and tunable band gap energy

In the current study, the first method for synthesis of 3D assemblies composed by close-packed QDs of the cubic modification of CuInS2in thin film form is presented. The developed one-pot synthetic approach enables high tunability of materials’ properties by combining ultrasonic irradiation of the reactor with the one-step colloidal chemical approach or by controlled post-deposition annealing of the deposited samples. The average crystal size of the material can be tuned from ~ 1 up to ~ 36 nm. In-depth studies of the structure, surface morphology and optical properties were carried out. Manifestations of size-quantization effects in the optical properties were analyzed in details. Analysis of the second-order statistical properties of thin films’ surfaces has shown that ultrasonically-supported synthesis enables generation of self-affine surfaces, while the colloidal chemical one leads to mounded surfaces. Such change of the surfaces’ character was attributed to heterogeneous sonochemical effects, leading ultimately to more efficient transport of the clusters formed in the bulk liquid and their improved energizing, enabling them to overcome various diffusion barriers on the growing surface. Sonochemically deposited films are characterized by notably larger values of the Hurst exponent than the chemically deposited ones, and an order of magnitude smaller correlation length. Smaller fractal dimensions in sonochemical films are in line with the smoother surfaces generate by ultrasonically-assisted route.

Studies on Cu2SnS3 quantum dots for O-band wavelength detection

Abstract In this communication, we report on Cu2SnS3 quantum dots synthesized by the solvothermal process using different solvents. The optical properties of the quantum dots are analyzed by UV-Vis-NIR and photoluminescence spectroscopy. The results suggest that Cu2SnS3 material has tunable energy bandgap and appropriate wavelength for fabrication of light emitting diodes and laser diodes as sources for fiber optic communication. They exhibit wide absorption in the near infrared range. Further morphological studies with the use of atomic force microscope confirm the surface topography and the existence of quantum dots. The observed characteristics prove the efficiency of Cu2SnS3 quantum dots for O-band wavelength detection used in fiber optic communication and solar cell applications.

Effect of Monoethanolamine Content on the Crystallinity of ZnO Thin Films

ZnO is one of the most widely used semiconductors for opto-electronic applications due to its tunable band gap energy, high optical transparency in visible region. The effect of different monoethanolamine (MEA) ratio on structural, morphological and optical properties of ZnO thin films was investigated in this study. A nano-fiber like structured ZnO was fabricated through facile sol-gel dip coating process on glass substrates without any further treatment. We observed that the peak intensity of samples increased and the amount of defects in the crystal could be further decreased by rising its of monoethanolamine. Besides, the micro-fiber like grains of the ZnO covered the glass surface quite densely when the monoethanolamine content increased. The optical study showed that the transparency of the films could be enhanced by increasing the amount of stabilizer.

Wide range tuning of band gap energy of A3B2X9 perovskite-like halides

We report the structural, optical, and electrical properties of A3B2X9 halides (where A = Cs, methylammonium (MA), formamidinium (FA), or ethylammonium; B = As, Sb, or Bi; X = Cl, Br, or I). Substitution results in tuning of lattice parameters involving crystal structure change, hence tuning of band gap energy, from ~1.9 to ~3.1 eV. Our results on the band gap energy, type, and crystal structure changes by A, B and X site substitution would give very useful information to develop wide band gap light-absorbers for diverse optoelectronic applications, including highly efficient and stable tandem solar cells.

Phase segregation due to ion migration in all-inorganic mixed-halide perovskite nanocrystals

Semiconductor mixed-halide perovskites featured with a tunable energy bandgap are ideal candidates for light absorbers in tandem solar cells as well as fluorescent materials in light-emitting diodes and nanoscale lasers. These device advancements are currently hindered by the light-induced phase segregation effect, whereby ion migration would yield smaller-bandgap domains with red-shifted photoluminescence. Here we show that upon laser excitation all-inorganic mixed-halide nanocrystals unexpectedly exhibit a blue shift in the photoluminescence peak that can revert back in the dark, thus depicting the processes of ion migration out of and back to the originally excited nanocrystals. Interestingly, this reversible photoluminescence shift can also be induced by electrical biasing of mixed-halide nanocrystals without the injection of charge carriers. The above findings suggest that it is the local electric field that breaks the ionic bonds in mixed-halide nanocrystals, which could be a universal origin for light-induced phase segregation observed in other mixed-halide perovskite materials.

Modelling of electronic and optical properties of Cu2SnS3 quantum dots for optoelectronics applications

Abstract Copper tin sulfide (Cu2SnS3) is a unique semiconductor, whose nanocrystals have attracted researchers’ attention for its tunable energy bandgap and wavelength in visible and near infrared range. Quantum dots which are fabricated from this material are highly suitable for optoelectronics and solar cell applications. This paper discusses the tunable energy bandgap, exciton Bohr radius and wavelength range of wurtzite structure of Cu2SnS3 quantum dots to assess the opportunity to use them in optoelectronics applications. The considerations show that the mole fraction of copper increases as energy bandgap decreases and tunable energy bandgap of this quantum dot material is inversely proportional to the wavelength.

Synchronous Polymerization of 3,4-Ethylenedioxythiophene and Pyrrole by Plasma Enhanced Chemical Vapor Deposition (PECVD) for Conductive Thin Film with Tunable Energy Bandgap

Using a plasma enhanced chemical vapor deposition (PECVD) technique, a synchronous polymerization of 3,4-ethylenedioxythiophene (EDOT) and pyrrole monomers was investigated for the development of thin films with adjusted optoelectronic properties. Maintaining a constant amount of EDOT- and pyrrole-feed in the presence of a carrier gas, the PECVD reaction power was varied in the range of 10–100 W to give different physicochemical states of composite films composed of poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy). The deposition rate gradually increased with the reaction power reaching the highest deposition rate at 30 nm/min (100 W) in this study. The energy bandgap of the plasma-polymerized PEDOT/PPy copolymer films increased from 2.62 to 3.27 eV as the applied power density increased from 10 to 100 W in a continuous way, that could desirably ensure a tunable control of bandgaps in thin films. The electrical conductivity and the surface roughness of the thin films continuously increased from 1.59×10-4 to 2.28×10-2 S/m and from 0.2 to 1.9 nm respectively, as the applied power density decreased. The plasma-polymerized PEDOT/PPy copolymer is expected to find its application in various optoelectronic devices including the hole injection layer (HIL) in organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs) for the improved energy match.

Fabrication and Characterization of Screen-Printed Cu2ZnSnS4 Films for Photovoltaic Applications

Cu2ZnSnS4 (CZTS) is deemed to be one of the most promising photovoltaic materials due to its properties such as tunable band gap energy of 1.4–1.6 eV and an absorption coefficient greater than 104 cm−1. In the present work, CZTS films were deposited onto glass substrate by a less complex and cost efficient screen-printing method and thereafter sintered. The structural, morphological, optical and electrical properties of CZTS films were studied. The x-ray diffraction and Raman studies show that the screen-printed CZTS films were indexed to a single phase kesterite structure. The morphology of the films was examined through scanning electron microscopy. The EDAX analysis indicates the presence of Cu, Zn, Sn, and S elements in the films. Direct type band gap of 1.51 eV was found for the films, which is quite close to the optimum band gap, i.e., between 1.4 eV and 1.5 eV required for a solar absorber layer. The dark electrical resistivity of films was found to be of the order of 102 Ω-cm as evaluated by the two probe method.

Ternary ReS2(1-x)Se2x alloy saturable absorber for passively Q-switched and mode-locked erbium-doped all-fiber lasers

We report Q-switched and mode-locked erbium-doped all-fiber lasers using ternary ReS2(1−x)Se2x as saturable absorbers (SAs). The modulation depth and saturable intensity of the film SA are 1.8% and 0.046 MW/cm2. In Q-switched mechanism output, the pulse was centered at 1531.1 nm with maximum pulse energy and minimum pulse width of 28.29 nJ and 1.07 μs, respectively. In mode-locked operation, the pulse was centered at 1561.15 nm with pulse width of 888 fs, repetition rate of 2.95 MHz, and maximum pulse energy of 0.275 nJ. To the best of our knowledge, this is the first report on the mode-locked Er3+-doped fiber laser using ternary transition metal dichalcogenides. This work suggests prospective 2D-material SAs can be widely used in versatile fields due to their attractive optoelectronic and tunable energy bandgap properties.