Direct Band Gap Transition Research Articles

Pressure‐Tailored Band Engineering for Significant Enhancements in the Photoelectric Performance of CsI3 in the Optical Communication Waveband

AbstractThe bandgap and type of optical transition are key factors in determining the functionalities and applications of photoelectric materials. However, it is extremely difficult to modulate the bandgap and indirect‐direct bandgap transition for most materials. This study reports significant enhancements in photocurrents and an extended detection bandwidth resulting from pressure‐regulated indirect–direct bandgap transition in hypervalent CsI3. Furthermore, this study achieves an increase in the photocurrent by almost five orders of magnitude under visible‐light illumination. Impressively, the detection band‐edge shows a successive redshift from visible light to 1650 nm (optical communication waveband) upon compression. And high pressure is conducive to CsI3 operating at an ultralow bias input. Extensive high‐pressure spectroscopy analyses and theoretical calculations suggest that changes in the photoelectric properties of CsI3 are associated with enhanced I–I interactions along the quasi‐endless linear chain directions under compression. These findings offer an effective band engineering strategy for achieving broadband spectral response and high gains with an ultralow bias in photoelectric detectors.

Polarization Selectivity of Aloof-Beam Electron Energy-Loss Spectroscopy in One-Dimensional ZnO Nanorods

Orientation-dependent electronic properties of wurtzite zinc oxide nanorods are characterized by aloof-beam electron energy-loss spectroscopy (EELS) carried out in a scanning transmission electron microscope (STEM). The two key crystal orientation differentiating transitions specific to the in-plane (13.0 eV) and out-of-plane (11.2 eV) directions with respect to the wurtzite structure are examined by first-principles density-functional theory calculations. We note some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV. We demonstrate that good polarization selectivity can be achieved by placing the electron probe at different locations around the specimen with increasing impact parameter while keeping the beam-specimen orientation fixed. The observed results are qualitatively elucidated in terms of the perpendicular electric fields generated by the fast electron (60 kV) used in the microscope. The fact that good polarization selectivity can be achieved by aloof-beam EELS without the requirement of sample reorientation is an attractive aspect from the characterization method point of view in the STEM-EELS community.

Bi/BiPO4 nanocubes supported BiOI-BiOCl nanoplate as a heterostructured blue-light-driven photocatalyst for degradation of Auramine O

Bi surface plasmon resonance (SPR)-promoted BiPO4/BiOI-BiOCl (Bi/BPCI) ternary heterostructure as plasmonic blue-light-driven photocatalyst was prepared by one-pot UV-photoreduction process assisted hydrothermal method with to require any additives. As-prepared photocatalyst was characterized by PL, DRS, FESEM, XRD, EDS and EIS methods and its hierarchical heterostructured exhibits excellent photocatalytic activity toward Auramine O (AO) as typical cationic organic dyes under blue light irradiation. The band structures of the Bi/BiPO4, BiOI and BiOCl are determined combined with the flat potential (Vfb) and bandgap energy (Eg) evaluated by UV–Vis diffuse reflectance spectra. The founding illustrate that the as-synthesized composite of the three Bi-based semiconductors facilitates the photosensitized degradation of AO under blue light as well as we described the optical absorption originated from the SPR influence and direct band gap transition in an individual Bi and transportation of photogenerated carriers at the interface of Bi/BPCI. The photocatalytic mechanism is further proposed by identifying the photogenerated reactive species and intermediates by scavenger test. The OH radical generation confirmed that the OH and h+ were the key species in the present system, which lead to the elimination and part mineralization of the AO.

Direct Growth of van der Waals Tin Diiodide Monolayers (Adv. Sci. 20/2021)

van der Waals SnI2 Monolayers In article number 2100009, Ping Zhang, Shao-Chun Li, and co-workers demonstrate the direct growth of van der Waals SnI2 monolayer, a new semiconducting material. The SnI2 monolayer exhibits a band gap of 2.9 eV, and an indirect- to direct-band gap transition occurs in the SnI2 bilayer, suggesting its potential application in future electronics/optoelectronics.

Development and characterization of (ZnO)0.90(CNT)0.10 thick film for photovoltaic application

In this study we prepared (ZnO)0.90(CNT)0.10 thick film by screen printed technique which is very economical and sophisticated method. ZnO and highly active powdered CNT have been used as basic source of materials under the optimum conditions for preparing the best quality screen printed film have been found. The prepared film has been investigated by X-ray diffraction (XRD), Diffused reflectance spectroscopy and DC conductivity measurement for structural, morphological, optical and electrical characteristics. XRD revealed the presence of CNT with hexagonal structure along a strong (101) orientation of plane. SEM micrograph depicts densely covered ZnO particles by powdered CNT bundles. Direct optical band gap transition of the ordered of 3.15 eV. Negative temperature coefficient of resistance of the sample was confirmed with activation energy 0.23 eV by using two probe method.

Nature of optical excitations and bandgap of Re x Mo1−x S2 alloy at nanoscale probed from high resolution low loss electron energy loss spectroscopy

The two-dimensional (2D) transitional metal dichalcogenides (TMDS) have become an intensive research topic recently. The alloys of these TMDs have offered continuous tunability of the bandstructure and carrier concentration, providing a new opportunity for various device applications. Here the rich variations in optical excitations in Re x Mo1−x S2 alloy at the nanoscale region are shown. The alloy bandgap and charge response are probed by low-loss high-resolution transmission electron energy loss spectroscopy (HR-EELS). Concurrent density functional theory calculations revealed many electronic structures from n-type semiconductors to metallic and p-type semiconducting nature with band bowing effect. The alloying-induced Peierls distortion leads to a change in crystal symmetry and decreased interlayer coupling. These alloys undergo indirect to direct bandgap transition with the function of Re concentration. These unique correlated structural and electronic properties of these 2D alloys can be potentially applicable for various electronic and optoelectronic devices.

Direct Growth of van der Waals Tin Diiodide Monolayers.

Two‐dimensional (2D) van der Waals (vdW) materials have garnered considerable attention for their unique properties and potentials in a wide range of fields, which include nano‐electronics/optoelectronics, solar energy, and catalysis. Meanwhile, challenges in the approaches toward achieving high‐performance devices still inspire the search for new 2D vdW materials with precious properties. In this study, via molecular beam epitaxy, for the first time, the vdW SnI2 monolayer is successfully fabricated with a new structure. Scanning tunneling microscopy/spectroscopy characterization, as corroborated by the density functional theory calculation, indicates that this SnI2 monolayer exhibits a band gap of ≈2.9 eV in the visible purple range, and an indirect‐ to direct‐band gap transition occurs in the SnI2 bilayer. This study provides a new semiconducting 2D material that is promising as a building block in future electronics/optoelectronics.

Strain-tunable band alignment and band gap of GaSe/WTe2 heterojunction for water splitting and light-emitting

Van der Waals heterojunctions (vdWHs) based on transition metal dichalcogenides (TMDs) attract much attention due to their unique electrochemical and physical properties. However, most vdWHs in TMD family are type II with indirect band gap. For vdWHs in TMD family, it is difficult to obtain the direct band gap through band arrangement approaches, limiting their applications in optoelectronics and photocatalysis. In present work, a type-I GaSe/WTe2 heterojunction with direct band gap (1.13 eV) is found by first-principles calculations. We find that this heterojunction has dipoleallowed direct band gap transition and large band offsets, which are beneficial to light emission efficiency. The effects of strain on electronic structure and optical properties of GaSe/WTe2 heterojunction are investigated. We find that band gap of type-I GaSe/WTe2 heterojunction can be tuned in large range by strain and it is a promising platform to develop strain-tunable light-emitting device which can emit various wavelengths. The band alignment type of GaSe/WTe2 heterojunction can be tuned from type-I to type-II by strain. Our results show that, in type-II alignment, it has efficient photocatalyticactivity in water splitting and the photocatalytic efficiency can be enhanced by strain. We hope our work can make a contribution to exploring a strain-tuned platform for photocatalytic and light-emitting devices.

Efficient Direct Band-Gap Transition in Germanium by Three-Dimensional Strain.

Complementary to the development of highly three-dimensional (3D) integrated circuits in the continuation of Moore's law, there has been a growing interest in new 3D deformation strategies to improve the device performance. To continue this search for new 3D deformation techniques, it is essential to explore beforehand, using computational predictive methods, which strain tensor leads to the desired properties. In this work, we study germanium (Ge) under an isotropic 3D strain on the basis of first-principles methods. The transport and optical properties are studied by a fully ab initio Boltzmann transport equation and many-body Bethe-Salpeter equation (BSE) approach, respectively. Our findings show that a direct band gap in Ge could be realized with only 0.70% triaxial tensile strain (negative pressure) and without the challenges associated with Sn doping. At the same time, a significant increase in the refractive index and carrier mobility, particularly for electrons, is observed. These results demonstrate that there is a huge potential in exploring the 3D deformation space for semiconductors, and potentially many other materials, to optimize their properties.

Geometric, electronic, and optical properties of MoS2/WSSe van der Waals heterojunctions: a first-principles study

Van der Waals (vdW) heterojunctions constructed by vertical stacking two-dimensional transition metal dichalcogenides hold exciting promise in realizing future atomically thin electronic and optoelectronic devices. Recently, a Janus WSSe structure has been successfully synthesized by using chemical vapor deposition, selective epitaxy atomic replacement, and pulsed laser deposition methods. Herein, based on first-principles calculations, we introduce the structures and performances of MoS2/WSSe vdW heterojunctions with different interfaces and stacking modes. The vdW heterojunctions possess indirect band gaps for S–S interfaces, while direct band gaps for Se–S interfaces. Besides, the potential drop indicates an efficient separation of photogenerated charges. Interestingly, the opposite built-in electric fields formed in the vdW heterojunctions with a S–S interface and a Se–S interface suggest different charge transfer paths, which would motivate further theoretical and experimental investigations on charge transfer dynamics. Moreover, the electronic property is adjustable by applying external in-plane strains, accomplishing with indirect to direct bandgap transition and semiconductor to metal transition. The findings are helpful for the design of multi-functional high-performance electronic and optoelectronic devices based on the MoS2/WSSe vdW heterojunctions.

Tuning the Direct and Indirect Excitonic Transitions of h-BN by Hydrostatic Pressure.

The pressure dependence of the direct and indirect bandgap transitions of hexagonal boron nitride is investigated using optical reflectance under hydrostatic pressure in an anvil cell with sapphire windows up to 2.5 GPa. Features in the reflectance spectra associated with the absorption at the direct and indirect bandgap transitions are found to downshift with increasing pressure, with pressure coefficients of −26 ± 2 and −36 ± 2 meV GPa–1, respectively. The GW calculations yield a faster decrease of the direct bandgap with pressure compared to the indirect bandgap. Including the strong excitonic effects through the Bethe–Salpeter equation, the direct excitonic transition is found to have a much lower pressure coefficient than the indirect excitonic transition. This suggests a strong variation of the binding energy of the direct exciton with pressure. The experiments corroborate the theoretical predictions and indicate an enhancement of the indirect nature of the bulk hexagonal boron nitride crystal under hydrostatic pressure.

Magnetic, Optical and I-V Characteristics of MoO3 thin films

Molybdenum trioxide (MoO3) thin films with cobalt concentration were grown on quartz and n-type Si (100) by thermal evaporation deposition technique. Then the films were thermally annealed for 2 hours under normal environment. The structure is analyzed by X-Ray Diffraction (XRD) to confirm the orthorhombic α-MoO3 phase confirming the crystallinity of thin films. Furthermore, the morphological study is done by Scanning Electron Microscope (SEM) to confirm granular shaped interconnected films. Thicknesses of films are determined by Ellipsometry which vary from 40nm to 140nm with the decrease in Cobalt concentration of thin films. Modifications in optical properties due to the doping of Cobalt in MoO3 thin films are studied in terms of transmittance data and Taucs plot. All films represent direct band gap transitions. The band-gap of MoO3 comes out be 3.32eV from the Taucs plot. A slight change in the band-gaps is found for various films with different concentration. Vibrating Sample Magnetometer (VSM) results studied at the room temperature confirm the introduced magnetic property as well as the hysteresis loop for the Cobalt doped MoO3 thin films.

Type-II GeAs/GaSe heterostructure as suitable candidate for solar power conversion efficiency

The structural, electronic and optical properties of GeAs and GaSe under monolayer (ML), bilayer (BL) and heterostructure (HS) configurations calculated within the framework of density functional theory are presented. The stability of all configurations was confirmed by calculating the binding energy and phonon dispersion curves. Among all considered configuration, HS4 with AB stacking is most stable configuration and was considered for in-depth investigation. The electronic band structure calculations reveal the nature of all systems with moderate gap magnitude. The GeAs/GaSe HS4 and HS1 are observed to have a type- II band alignment with gap magnitude of 0.98 eV (PBE) and 1.80 eV (HSE) and 1.04 eV (PBE) and 2.01 eV (HSE) respectively, which is favourable for the photovoltaic and photocatalytic applications. Following this, the optical response of the systems suggest enhancement in photon absorption for HS configuration as compared to its constituent MLs. Moreover, the indirect to direct bandgap transition is observed on imposing 2% of tensile strain on the HS. To sum-up, the moderate electronic bandgap with a type-II band alignment and optimal absorption profile of GeAs/GaSe HS result in benchmarking solar power conversion efficiency (PCE) at 4% tensile strain of 22.32% with HSE which is remarkable compared to previously reported two-dimensional (2D) HSs. The present study provides motivation to experimentally explore such 2D HSs for harvesting abundant, green and clean solar energy.

The electronic and optical properties of the Fe,Co,Ni and Cu doped ZnO monolayer photocatalyst

The structural, electronic, and optical properties of optimized Fe, Co, Ni and Cu doped ZnO monolayer in a two-dimensional honeycomb structure are analyzed based on the density functional theory. The stability is testified by analysis of phonon dispersion curves. The variations of electronic and optical properties of Fe, Co, Ni and Cu doped ZnO monolayer under in-plane and out-of-plane strain effect are also investigated. The results show that tunable band structures and optical properties of Fe, Co, Ni and Cu doped ZnO monolayer, such as indirect–direct band gap transition, shrink and enlargement of band gaps can be achieved by applying strain, which indicates a possible new route for tailoring the electronic properties of ultrathin nanofilms through strain engineering.

Strong anisotropic enhancement of photoluminescence in WS2 integrated with plasmonic nanowire array

Layered transition metal dichalcogenides (TMDCs) have shown great potential for a wide range of applications in photonics and optoelectronics. Nevertheless, valley decoherence severely randomizes its polarization which is important to a light emitter. Plasmonic metasurface with a unique way to manipulate the light-matter interaction may provide an effective and practical solution. Here by integrating TMDCs with plasmonic nanowire arrays, we demonstrate strong anisotropic enhancement of the excitonic emission at different spectral positions. For the indirect bandgap transition in bilayer WS2, multifold enhancement can be achieved with the photoluminescence (PL) polarization either perpendicular or parallel to the long axis of nanowires, which arises from the coupling of WS2 with localized or guided plasmon modes, respectively. Moreover, PL of high linearity is obtained in the direct bandgap transition benefiting from, in addition to the plasmonic enhancement, the directional diffraction scattering of nanowire arrays. Our method with enhanced PL intensity contrasts to the conventional form-birefringence based on the aspect ratio of nanowire arrays where the intensity loss is remarkable. Our results provide a prototypical plasmon-exciton hybrid system for anisotropic enhancement of the PL at the nanoscale, enabling simultaneous control of the intensity, polarization and wavelength toward practical ultrathin photonic devices based on TMDCs.

Bandgap engineering of layered mono-chalcogenides via pressure

The layered mono-chalcogenide semiconductors MX (M = In, Ga; X = Se, S) have attracted considerable attention due to their high carrier mobility and tunable bandgap, which gives them potential applications in the development of new optoelectronic devices. We have systematically studied the effect of pressure on the band structure evolution and electronic properties of monolayer indium selenide (InSe) using first-principles calculations. The bandgap of monolayer InSe first increases and then decreases with increasing pressure. In addition, monolayer InSe undergoes an indirect to direct bandgap transition at 6.8 GPa. Increasing the pressure further to 10.4 GPa leads to recovery of the indirect bandgap. This indirect-direct bandgap transition is absent in other monolayer MX compounds. The continuous tuning of band structure of monolayer InSe gives them potential applications for pressure-response optoelectronic devices.

MoS2 Based Photodetectors: A Review.

Photodetectors based on transition metal dichalcogenides (TMDs) have been widely reported in the literature and molybdenum disulfide (MoS2) has been the most extensively explored for photodetection applications. The properties of MoS2, such as direct band gap transition in low dimensional structures, strong light–matter interaction and good carrier mobility, combined with the possibility of fabricating thin MoS2 films, have attracted interest for this material in the field of optoelectronics. In this work, MoS2-based photodetectors are reviewed in terms of their main performance metrics, namely responsivity, detectivity, response time and dark current. Although neat MoS2-based detectors already show remarkable characteristics in the visible spectral range, MoS2 can be advantageously coupled with other materials to further improve the detector performance Nanoparticles (NPs) and quantum dots (QDs) have been exploited in combination with MoS2 to boost the response of the devices in the near ultraviolet (NUV) and infrared (IR) spectral range. Moreover, heterostructures with different materials (e.g., other TMDs, Graphene) can speed up the response of the photodetectors through the creation of built-in electric fields and the faster transport of charge carriers. Finally, in order to enhance the stability of the devices, perovskites have been exploited both as passivation layers and as electron reservoirs.

Semiconductor-to-metal transition in bilayer MoSi2N4 and WSi2N4 with strain and electric field

With exceptional electrical and mechanical properties and at the same time air-stability, layered MoSi2N4 has recently drawn great attention. However, band structure engineering via strain and electric field, which is vital for practical applications, has not yet been explored. In this work, we show that the biaxial strain and external electric field are effective ways for the bandgap engineering of bilayer MoSi2N4 and WSi2N4. It is found that strain can lead to indirect bandgap to direct bandgap transition. On the other hand, electric field can result in semiconductor to metal transition. Our study provides insights into the band structure engineering of bilayer MoSi2N4 and WSi2N4 and would pave the way for its future nanoelectronics and optoelectronics applications.

Quantum-confined blue photoemission in strain-engineered few-atomic-layer 2D germanium

The indirect bandgap (0.67 eV) of bulk germanium (Ge) remains a major bottleneck towards its applications in optoelectronics, enabling poor optical features particularly photoluminescence. Obtaining desired optical functionalities, either by synthesizing few-atoms-thick two-dimensional (2D) germanium on silicon-based substrates, or by inducing an appreciable structural engineering in its crystal lattice, has long remained a formidable challenge yet to be mitigated. Herein, a facile vacuum-tube hot-pressing strategy to synthesize strain-engineered few-atomic-layer 2D germanium nanoplates (Ge-NPts) directly on fused silica substrate (SiO 2 ) is developed. Leveraging from the unique mismatch between coefficient of thermal expansion of Ge and SiO 2 substrate at elevated temperatures (700 °C), and under hydrostatic pressure (~2 GPa), a biaxial compressive strain of ~1.23 ± 0.06% in Ge lattice is engineered, causing a transition from indirect to direct bandgap with an ultra-large opening of 2.91 eV. Strained Ge nanoplates, consequently, display a remarkable 42-fold blue photoluminescence (at 300 K) compared to bulk Ge, accompanied by robust quantum-confinement effects, probed by the quantum-shift ~114 meV with decreasing thicknesses of Ge nanoplates. • Technological advancements of germanium in optoelectronics have remained restricted due to its indirect bandgap (0.67 eV). • Realization of Ge in ultrathin 2D crystals with strain-engineered lattice can exhibit remarkable property modulation. • A vacuum-tube hot-pressing (VT-HP) strategy to synthesize strain-engineered few atomic layer 2D Ge has been introduced. • Coefficient of thermal expansion mismatching causeda biaxial compressive strain of ~1.23 ± 0.06% in 2D Ge lattice. • An ultra-bright 42-fold blue photoemissionin 2D Ge was achieved due to an indirect to direct bandgap transition (~2.91 eV).

Epitaxial growth of ZrSe2nanosheets on sapphireviachemical vapor deposition for optoelectronic application

For the first time the epitaxial growth of ZrSe2layers is realized on sapphireviaCVD, and the ZrSe2photodetector exhibits a stable and appreciable photoresponse in the visible-light region due to the direct band gap transition.