Intrinsic Band Gap Research Articles

Highly Efficient Blue Electroluminescent Materials Based on Phenanthro[9,10-d] imidazole

Phenanthro[9,10-d]-imidazole( PI) unit has attracted extensive attentions in organic optoelectronic field due to its intrinsic wide band gap,high luminescence efficiency,good thermal stability,balanced charge carrier injection and transport property as well as low fabrication cost. In this manuscript,the recently reported blue electroluminescent PI derivatives were reviewed. The structure characteristics of PI were demonstrated,and the structure-property relationships of some PI derivatives were summarized systemically. At last,all the reported highly efficient nondoped deep blue emitters were summed up in order to comprehensively evaluate the potentials of PI derivatives in organic light emitting diodes( OLEDs) in the future.

Density Functional Theory Study of Li3VO4-δ

The large intrinsic band gap of Li3VO4 hinders its potential application as anode material for lithium ion batteries. Here, we perform first-principles calculations to investigate structural relaxation, formation energy, surface energy and electronic structure. The O vacancy in the Li3VO4-(110) surface can enhance charge transfer and to improve the electron conductivity. This work indicates that Li3VO4-δ would be a potential anode material for batteries due to greater electron conductivity than the pristine Li3VO4.

Optical Properties of MBE-Grown Hg1−xCdxSe

In this work, we present a study on the temperature-dependent infrared absorption spectra of Hg1−xCdxSe grown by molecular beam epitaxy (MBE) on GaSb (211) substrate, having a nominal x-value of 0.21. For temperatures below 200 K, the observed optical bandgap is found to correspond to the Fermi energy level rather than the intrinsic bandgap, which is quantitatively explained by the Burstein–Moss shift due to the presence of a background electron concentration of 3.5 × 1016 cm−3. In addition, empirical formulae for calculating both the absorption edge and intrinsic absorption coefficient in the Kane region have been derived from the previously reported absorption spectra of Bridgeman-grown Hg1−xCdxSe samples (0.15 ≤ x ≤ 0.3). By employing the empirical expressions, the infrared transmission spectra have been modeled based on the characteristic matrix method, and the x value profile of the sample along the growth direction has been determined, which is in good agreement with the experimental results obtained from secondary ion mass spectrometry (SIMS) depth profiling combined with Rutherford backscattering spectrometry (RBS).

Point defects in group III nitrides: A comparative first-principles study

One of the main challenges in the development of wide bandgap semiconductor devices is to understand the behavior of defects and avoid their harm. Using density-functional theory calculations with hybrid functional, we systematically investigated the neutral and charged native point defects (vacancy, interstitial, and antisite defect) in GaN, AlN, and InN crystals in terms of local geometry relaxation, formation energies, and electronic and diffusion properties. By comparing the defect configuration and transition levels as a function of the Fermi level, we show that Ga interstitial (Gaoc, Gate) in GaN, N vacancy (VN), N interstitial (Ni), In antisite (InN), and In interstitial (Inte, Inoc) in InN can exist stably only in the positive charge states with donor level and VIn is stable in the neutral state, while the other defects exhibit both donor and acceptor behavior. Among them, the most stable defects are identified as VN for p-type nitrides and VGa, VAl for n-type nitrides. These results, providing a mechanism for self-compensation effects, explain the reduced doping efficiencies for both n-type and p-type nitrides due to defects. Moreover, it is also demonstrated that N interstitial diffuses faster than vacancy, which are mainly responsible for the low concentration of N interstitials and N-based defect complexes produced in nitrides. Significantly, the trends of formation energy, transition level, and migration barrier of nitrides are also consistent with their intrinsic atomic size and bandgap. Our study is important for the identification and control of point defects in nitrides, which have a profound impact on device performance and reliability.

Theoretical investigations of electronic and thermodynamic properties of Ce doped La2Zr2O7 pyrochlore

With ab-initio calculations, the geometrical structure, electronic band structure, optical and thermodynamic properties of pure and Ce doped La2Zr2O7 are calculated. Doping Ce at Zr site modified the intrinsic band gap of La2Zr2O7. However, increasing the Ce doping concentration has no considerable effect on the band structure. With the increase in Ce doping concentration, the absorption spectra of the doped La2Zr2O7 is shifted to higher wavelengths and the peaks intensity is reduced. Some of the peaks associated with the intrinsic La2Zr2O7 are disappeared and new peaks are introduced in 3.40% Ce doped La2Zr2O7. Perturbations in the phonons density of states are reduced with the increasing Ce doping concentration. The La16Zr13Ce3O56 system with 3.40% of Ce doping provided the most stable phonons curve among the simulated systems. Interesting behavior is observed in the thermal conductivity of the doped models with the increasing temperature. The thermal conductivity of La16Zr15CeO56 model increases compared to the pure La2Zr2O7. However, further increase in the Ce doping concentration drastically reduced the thermal conductivity values in the range 373–1673 K. The La16Zr13Ce3O56 system (3.4% Ce) provided the lowest thermal conductivity attributed to the high concentration of point defects affecting the phonon scattering.

Preparation of ZnSe powder by vapor reaction during combustion synthesis

Abstract ZnSe powders have been prepared by vapor reaction during combustion synthesis. The powders contain both cubic and hexagonal ZnSe, where the fraction of the hexagonal phase increases with increasing distance from the evaporation source. The powders consist of spherical particles and rod-like crystals with sizes from submicron to a few microns. In UV–Vis region, the ZnSe powders have two absorption edges at 450–500 nm and 650–700 nm, likely connected with intrinsic band-gap transition and defect energy levels. From the optical absorption data, the band-gap of the ZnSe powders is estimated to be 2.4–2.5 eV. Under an UV or blue excitation, the ZnSe powders show very weak near-band-edge emission but strong defect-induced red emission. It appears that the ZnSe powders synthesized by vapor reaction clearly differ from the bulk ZnSe sample by melt solidification in both phase composition and optical properties.

Effect on Schottky Barrier of Graphene/WS2 Heterostructure With Vertical Electric Field and Biaxial Strain

Graphene has been widely used in many applications, such as sensors, field‐effect transistors, and integrated circuits due to its high strength and excellent electrical and thermal conductivity. Its lack of a band gap, however, means that applications in some fields are limited. In this paper, using first principles calculations based on the density functional theory method, the electronic properties of graphene/WS2 heterostructures under electric field and in‐plane biaxial strain are investigated. It is found that the two materials are subject to weak van der Waals forces after stacking. The band gap of WS2 in the graphene/WS2 heterostructure is reduced by 0.57 eV compared to the intrinsic WS2 band gap. An n‐type Schottky contact with a barrier height of 0.22 eV is formed. In addition, an n‐type to p‐type Schottky contact transition can be achieved by increasing the applied vertical electric field on the graphene/WS2 heterostructure. The variation of the barrier heights ϕBn and ϕBp of the graphene/WS2 heterojunction with strain is sensitive. However, plane strain can only change the height of the Schottky barrier; the Schottky contact cannot change from n‐type to p‐type with in‐plane biaxial strain. The results suggest that there is a potential application of graphene/WS2 heterostructures in Schottky devices.

Electronic Properties of the Co3O4 Photoelectrode: Predictions from First-Principles Calculations

Cobalt Oxide (Co3O4) has emerged as a highly promising material for hydrogen generation through photoelectrochemical cells (PECs); however, a detailed understanding of the electronic properties of this material is largely lacking. This includes the band gap and electrical conductivity of the photoelectrode, which are important factors in determining the performance of PECs. For example, contradicting experimental results have been reported for the optical gap of Co3O4, leading to two commonly reported values of 0.8 eV and 1.6 eV. In this work, we have employed first-principles calculations based on Density Functional Theory to support that the intrinsic band gap of Co3O4 is 1.6 eV. Meanwhile, we show that the 0.8 eV transition found experimentally is due to the presence of polaron or defect states. In particular, our calculations predict the spontaneous formation of electron and hole polarons, that in turn exhibit significant contribution to the absorption spectra of the material, and are responsible for the optical excitation at 0.8 eV. Finally, we resolve the nature of the stable spin states of electron and hole polarons, and we discuss how the interaction between polarons with n-type dopants, such as carbon, could improve the electrical conductivity of this intrinsic p-type material. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Large bandgap of pressurized trilayer graphene

Graphene-based nanodevices have been developed rapidly and are now considered a strong contender for postsilicon electronics. However, one challenge facing graphene-based transistors is opening a sizable bandgap in graphene. The largest bandgap achieved so far is several hundred meV in bilayer graphene, but this value is still far below the threshold for practical applications. Through in situ electrical measurements, we observed a semiconducting character in compressed trilayer graphene by tuning the interlayer interaction with pressure. The optical absorption measurements demonstrate that an intrinsic bandgap of 2.5 ± 0.3 eV could be achieved in such a semiconducting state, and once opened could be preserved to a few GPa. The realization of wide bandgap in compressed trilayer graphene offers opportunities in carbon-based electronic devices.

Origin of Intrinsic Direct Band Gap of Janus Group‐III Chalcogenide Monolayers

Using first‐principles calculations, the evolution of electronic properties of InSe based Janus structures has been investigated. The obtained results suggest that the In2SeTe Janus structure has an intrinsic direct band gap which is beneficial for optoelectronic applications. The direct band gap nature of In2SeTe Janus structure is due to the strong coupling of pz orbitals between the Se and Te sublayers, which emphasizes the importance of the intra‐layer interactions. In addition, the effective mass of the holes in In2SeTe is one order of magnitude smaller than that of intrinsic InSe monolayer, which makes it a good candidate for p‐type semiconductor. The broken mirror symmetry of the Janus structures induces out‐of‐plane dipolar polarization, yielding additional built‐in electric field and offering extra channel to tune its electronic band structure. The electric field has dominated effluences on the charge polarization on anions thus exhibiting stronger band tunability on the valence bands. These results indicate that the preparation of In2SeTe Janus structure provides a new way for the future use of two‐dimensional materials in optoelectronic nanodevices.

Oxygen Vacancy Generation and Stabilization in CeO2–x by Cu Introduction with Improved CO2 Photocatalytic Reduction Activity

Introducing O vacancies into the lattice of a semiconductor photocatalyst can alter its intrinsic electronic properties and band gap, thus enhancing the visible light absorption, promoting the sepa...

Novel N-TiO2/SiO2/Fe3O4 nanocomposite as a photosensitizer with magnetism and visible light responsiveness for photodynamic inactivation of HeLa cells in vitro

Semiconductor photocatalyst materials based on TiO 2 are potential photosensitizers for photodynamic therapy of cancer. Despite its effectiveness under ultraviolet light, pure TiO 2 shows almost no photocatalytic activity under visible light because of its wide intrinsic band gap energy (anatase, 3.2 eV). Besides, TiO 2 has no magnetic delivery property. In this study, an inorganic nanocomposite, N-TiO 2 /SiO 2 /Fe 3 O 4 , is successfully prepared in three steps by means of co-precipitation, Stöber and sol-gel methods. Deep characterization results by XRD, TEM, XPS, UV–vis DRS, FL and magnetic hysteresis loop reveal that N-TiO 2 /SiO 2 /Fe 3 O 4 has core-shell structure, magnetic property and visible-light-response capability. As a potential photosensitizer for photodynamic therapy, it shows a stronger photodynamic suppression effect on proliferation of HeLa cells than TiO 2 under the irradiation of xenon light.

High-Performance Ultraviolet Light Detection Using Nano-Scale-Fin Isolation AlGaN/GaN Heterostructures with ZnO Nanorods.

Owing to their intrinsic wide bandgap properties ZnO and GaN materials are widely used for fabricating passive-type visible-blind ultraviolet (UV) photodetectors (PDs). However, most of these PDs have a very low spectral responsivity R, which is not sufficient for detecting very low-level UV signals. We demonstrate an active type UV PD with a ZnO nanorod (NR) structure for the floating gate of AlGaN/GaN high electron mobility transistor (HEMT), where the AlGaN/GaN epitaxial layers are isolated by the nano-scale fins (NFIs) of two different fin widths (70 and 80 nm). In the dark condition, oxygen adsorbed at the surface of the ZnO NRs generates negative gate potential. Upon UV light illumination, the negative charge on the ZnO NRs is reduced due to desorption of oxygen, and this reversible process controls the source-drain carrier transport property of HEMT based PDs. The NFI PDs of a 70 nm fin width show the highest R of a ~3.2 × 107 A/W at 340 nm wavelength among the solid-state UV PDs reported to date. We also compare the performances of NFI PDs with those of conventional mesa isolation (MI, 40 × 100 µm2). NFI devices show ~100 times enhanced R and on-off current ratio than those of MI devices. Due to the volume effect of the small active region, a much faster response speed (rise-up and fall-off times of 0.21 and 1.05 s) is also obtained from the NFI PDs with a 70 nm fin width upon the UV on-off transient.

Versatile Crystal Structures and (Opto)electronic Applications of the 2D Metal Mono‐, Di‐, and Tri‐Chalcogenide Nanosheets

AbstractEmerging 2D metal chalcogenides present excellent performance for electronic and optoelectronic applications. In contrast to graphene and other 2D materials, 2D metal chalcogenides possess intrinsic bandgaps, versatile band structures, and superior atmospheric stability. The many categories of 2D metal chalcogenides ensure that they can be applied to various practical scenarios. 2D metal monochalcogenides, dichalcogenides, and trichalcogenides are the three main categories of these materials. They have distinct crystal structures resulting in different characteristics. Some basic device characteristics, such as the charge carrier characteristics, scattering mechanisms, interfacial contacts, and band alignments of heterojunctions, are vital factors for practical device applications that ensure that the desired properties can be achieved. Various electronic, optoelectronic, and photonic applications based on 2D metal chalcogenides have been extensively investigated. 2D metal chalcogenides are considered as competitive candidates for future electronic and optoelectronic applications.

3D self-branched zinc-cobalt Oxide@N-doped carbon hollow nanowall arrays for high-performance asymmetric supercapacitors and oxygen electrocatalysis

This study reports the design and fabrication of ultrathin zinc-cobalt oxide nanoflakes@N-doped carbon hollow nanowall arrays (ZnCo2O4@NC NWAs) from vertically aligned 2D Co-MOF solid nanowall arrays by controllable cation ion-exchange and post annealing strategies. The unique 3D self-branched nanostructure anchored on flexible carbon textiles (CTs) can offer short ion diffusion length, fast and continuous electron transport pathway, and abundant reaction active sites. More importantly, the rational incorporation of Zn2+ generates hollow structure as well as reduces the intrinsic band gap, which further enhance the ion transportation efficiency and electronic conductivity. The above superiorities endow the 3D self-branched ZnCo2O4@NC/CTs electrodes with remarkable performances in terms of flexible asymmetric supercapacitor and oxygen electrocatalysis. When evaluated as a flexible cathode for asymmetric supercapacitor, the as-fabricated ZnCo2O4@NC/CTs electrode exhibits outstanding electrochemical performance with a wide work voltage up to 2.0 V, high areal energy density of 0.278 mWh cm−2 (or volumetric energy density of 2.32 mWh cm−3) and long-term cycling stability (∼85.89% capacitance retention over 6000 cycles). Additionally, the ZnCo2O4@NC/CTs electrode shows excellent oxygen evolution reaction (OER) performance with a small overpotential of 196.4 mV at 10 mA cm−2 and long-term durability (over 45 h). This work provides a rational design strategy for controllable synthesis of 3D self-branched hollow nanostructure on flexible substrate for energy storage and conversion applications.

Interface structure prediction via CALYPSO method

The atomistic structures of solid–solid interfaces are of fundamental interests for understanding physical properties of interfacial materials. However, determination of interface structures faces a substantial challenge, both experimentally and theoretically. Here, we propose an efficient method for predicting interface structures via the generalization of our in-house developed CALYPSO method for structure prediction. We devised a lattice match toolkit that allows us to automatically search for the optimal lattice-matched superlattice for construction of the interface structures. In addition, bonding constraints (e.g., constraints on interatomic distances and coordination numbers of atoms) are imposed to generate better starting interface structures by taking advantages of the known bonding environment derived from the stable bulk phases. The interface structures evolve by following interfacially confined swarm intelligence algorithm, which is known to be efficient for exploration of potential energy surface. The method was validated by correctly predicting a number of known interface structures with only given information of two parent solids. The application of the developed method leads to prediction of two unknown grain boundary (GB) structures (r-GB and p-GB) of rutile TiO2 Σ5(2 1 0) under an O reducing atmosphere that contained Ti3+ as the result of O defects. Further calculations revealed that the intrinsic band gap of p-GB is reduced to 0.7 eV owing to substantial broadening of the Ti-3d interfacial levels from Ti3+ centers. Our results demonstrated that introduction of grain boundaries is an effective strategy to engineer the electronic properties and thus enhance the visible-light photoactivity of TiO2.

T4,4,4-graphyne: A 2D carbon allotrope with an intrinsic direct bandgap

A novel two-dimensional (2D) structurally stable carbon allotrope is proposed using first-principles calculations, which is a promising material for water purification and for electronic devices due to its unique porous structure and electronic properties. Rectangular and hexagonal rings are connected with acetylenic linkages, forming a nanoporous structure with a pore size of 6.41 Å, which is known as T4,4,4-graphyne. This 2D sheet exhibits a direct bandgap of 0.63 eV at the M point, which originates from the pz atomic orbitals of carbon atoms as confirmed by a tight-binding model. Importantly, T4,4,4-graphyne is found to be energetically more preferable than the experimentally realized β-graphdiyne, it is dynamically stable and can withstand temperatures up to 1500 K.

Theoretical insight into the hydrogen adsorption on MoS2 (MoSe2) monolayer as a function of biaxial strain/external electric field

The hydrogen adsorptions on 2H-phased MoX2 (X = S, Se) monolayers as a function of biaxial strain or external vertical electric field were investigated on the basis of density-functional theory. The results show the intrinsic bandgap and electron transfer effects on the hydrogen adsorption of MoX2 monolayer, which reveal the relationship between the electronic structures of MoX2 and corresponding hydrogen adsorption behaviors. Both biaxial strain and external vertical electric field have the ability to change bandgap of MoX2 monolayer. Bandgap narrowing can lead to hydrogen adsorption strengthening. The hydrogen adsorption on MoX2 monolayer can also be finely tuned by an electron transfer between H adatom and substrate by means of vertical electric field. Therefore, we suggest that these two factors can possibly be regulated and synergistically tune the hydrogen adsorption on MoX2 monolayer.

Thermochromic Lead‐Free Halide Double Perovskites

AbstractLead‐free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature‐derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs2AgBiBr6 double perovskites is demonstrated. Along with the thermochromism, temperature‐dependent changes in the bond lengths of AgBr (RAgBr) and BiBr (RBiBr) are observed. The first‐principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the RAgBr and RBiBr at high temperatures. The synergy of anharmonic fluctuations and associated electron–phonon coupling, and the peculiar spin–orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs2AgBiBr6 shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead‐free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation‐induced bandgap narrowing of Cs2AgBiBr6 provide new insights for the further development of optoelectronic devices based on the lead‐free halide double perovskites.

Bandgap tuning of C3N monolayer: A first-principles study

The newly found graphene-like material C3N exhibits great potential in a variety of important applications, due to its unique topological and electronic structures. To extend the utilization, a crucial challenge is to make its intrinsic bandgap (1.03 eV) tunable. Here we performed first-principles calculations to investigate the band structure variations of C3N monolayer under various surface modification treatments, including defect engineering, surface decoration and substitutional doping. Results show that those treatments can induce impurity states, orbital rehybridization, and n- or p-type doping simultaneously, and therefore enable effective band structure adjustment. Importantly, some linear relationships between the bandgap and doping concentration are revealed, paving the way for precise control of C3N bandgap.