Showing papers on "Band gap published in 2021"

Two-dimensional sulfur- and chlorine-codoped g-C3N4/CdSe-amine heterostructures nanocomposite with effective interfacial charge transfer and mechanism insight

Abstract: The poor utilization of visible light and the speedy recombination of photoexcited carriers limit the further development of carbon nitride polymer (CN) photocatalysts. It is a valid means for enhancing the photocatalytic ability to ameliorate the electronic and physicochemical properties via modifying the structure of CN. The sulfur- and chlorine-codoped graphite CN (S/Cl-CN) was successfully fabricated with low-cost ammonium chloride and thiourea as precursors. The introduction of Cl atoms will establish interlayer channels to promote interlayer charge migration and up-shifted conduction-band level. S atom is appropriate to be incorporated into the CN framework to replace N atom, which is beneficial to adjust the band gap. Then, inorganic-organic CdSe-diethylenetriamine (D) grown in situ are employed to fabricate a S/Cl-CN/CdSe-D heterojunction. S/Cl-CN/CdSe-D heterojunction exhibits greater hydrogen evolution activity compared to CN, S-CN, Cl-CN, S/Cl-CN, CdSe-D and CN/CdSe-D. Finally, Step-scheme (S-scheme) photocatalytic mechanism based on S/Cl-CN/CdSe-D heterostructure was proposed.

Boron doping induced charge transfer switching of a C3N4/ZnO photocatalyst from Z-scheme to type II to enhance photocatalytic hydrogen production

Abstract: Heterojunction photocatalysts are very promising for solar hydrogen production due to their high efficiency in photo-driven charge generation and separation. A C3N4/ZnO heterostructure nanocomposite harvests a wide range of solar light from the UV and visible regions and retains a high redox potential due to its Z-scheme band structure. However, since both C3N4 and ZnO have sufficiently high conduction band energies to drive hydrogen photoreduction, a type II heterojunction is more beneficial for enhancing the hydrogen production efficiency in the current system. In this study, we first demonstrated the charge transfer mechanism switching from the Z-scheme to type II by simple boron (B) doping of C3N4/ZnO. The doping of C3N4 with low-electronegativity boron increases its Fermi level by 0.4 V, making it even higher than that of ZnO. As a result, the Fermi level alignment of B-doped C3N4 with ZnO causes a reversed band bending direction at the C3N4/ZnO junction. The resultant charge transfer switching from the Z-scheme (C3N4/ZnO) to type II (B-doped C3N4/ZnO) was confirmed by UPS and ESR analysis. Type II B-doped C3N4/ZnO shows a stable, drastic increase in the photocatalytic hydrogen evolution rate, approximately 2.9 times higher than that of undoped C3N4/ZnO. The decreased bandgap energy of B-doped C3N4/ZnO also contributes to an additional improvement in efficiency through enhanced light harvesting. Our work presents a simple but effective strategy to design highly capable heterojunction photocatalysts via charge transfer switching with a doping method.

Ultrathin Porous Carbon Nitride Bundles with an Adjustable Energy Band Structure toward Simultaneous Solar Photocatalytic Water Splitting and Selective Phenylcarbinol Oxidation.

Abstract: Rational design of photocatalysts with multiple functions, including organic synthesis and water-splitting, is promising and challenging. Herein, we synthesized actiniae-like carbon nitride (ACN) bundles based on the pyrolysis of an asymmetric supramolecular precursor prepared from L-arginine (L-Arg) and melamine. ACN has adjustable band gaps (2.25 eV ~ 2.75 eV) and hollow microtubes with ultrathin pore walls, which enrich reaction sites, improve visible-light absorption and enhance charge separation. In the presence of phenylcarbinol, ACN exhibited excellent pure water-splitting ability (95.3 μmol/h) and in the meanwhile phenylcarbinol was selectively oxidized to benzaldehyde (conversion of 90.9%, selectivity of 99.7%) under solar irradiation. For the concurrent reactions, 2 D isotope labeling, separation and detection were conducted to confirm that the proton source of released hydrogen is water. Further, we theorized the mechanism of water splitting and phenylcarbinol oxidation and hope that it provides inspiration for simultaneous utilization of photogenerated electrons and holes in one system.

Multi-Resonance Deep-Red Emitters with Shallow Potential-Energy Surfaces to Surpass Energy-Gap Law**

Abstract: Efficient organic emitters in the deep-red region are rare due to the "energy gap law". Herein, multiple boron (B)- and nitrogen (N)-atoms embedded polycyclic heteroaromatics featuring hybridized π-bonding/ non-bonding molecular orbitals are constructed, providing a way to overcome the above luminescent boundary. The introduction of B-phenyl-B and N-phenyl-N structures enhances the electronic coupling of those para-positioned atoms, forming restricted π-bonds on the phenyl-core for delocalized excited states and thus a narrow energy gap. The mutually ortho-positioned B- and N-atoms also induce a multi-resonance effect on the peripheral skeleton for the non-bonding orbitals, creating shallow potential energy surfaces to eliminate the high-frequency vibrational quenching. The corresponding deep-red emitters with peaks at 662 and 692 nm exhibit narrow full-width at half-maximums of 38 nm, high radiative decay rates of ca. 108 s-1 , ≈100 % photo-luminescence quantum yields and record-high maximum external quantum efficiencies of ca. 28 % in a normal planar organic light-emitting diode structure, simultaneously.

Local electronic structure variation resulting in Li 'filament' formation within solid electrolytes.

Abstract: Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in potentially catastrophic battery failure. At present, this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system, Li7La3Zr2O12, we discover that Li infiltration in solid oxide electrolytes is strongly associated with local electronic band structure. About half of the Li7La3Zr2O12 grain boundaries were found to have a reduced bandgap, around 1–3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li+ ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design. Solid electrolytes are promising for enabling the use of Li metal anodes but Li infiltration along grain boundaries can lead to battery failure. Li infiltration in a model solid oxide electrolyte is now found to be strongly associated with local electronic band structure.

22.8%-Efficient single-crystal mixed-cation inverted perovskite solar cells with a near-optimal bandgap

Abstract: Expanding the near-infrared (NIR) response of perovskite materials to approach the ideal bandgap range (1.1–1.4 eV) for single-junction solar cells is an attractive step to unleash the full potential of perovskite solar cells (PSCs). However, polycrystalline formamidinium lead triiodide (FAPbI3)-based absorbers, used in record-efficiency PSCs, currently offer the smallest bandgap that can be achieved for lead-halide perovskite thin films (>100 meV larger than the optimal bandgap). Here, we uncover that utilizing a mixed-cation single-crystal absorber layer (FA0.6MA0.4PbI3) is capable of redshifting the external quantum efficiency (EQE) band edge past that of FAPbI3 polycrystalline solar cells by about 50 meV – only 60 meV larger than that of the top-performing photovoltaic material, GaAs – leading to EQE-verified short-circuit current densities exceeding 26 mA cm−2 without sacrificing the open-circuit voltage (VOC), and therefore, yielding power conversion efficiencies of up to 22.8%. These figures of merit not only set a new record for SC-PSCs and are among the highest reported for inverted-structured-PSCs, but also offer an avenue for lead halide PSCs to advance their performance toward their theoretical Shockley–Queisser Limit potential.

Oxygen-doped crystalline carbon nitride with greatly extended visible-light-responsive range for photocatalytic H2 generation

Abstract: Crystalline carbon nitride (CCN) materials with photoresponse of more than 600 nm are rare. Here, we successfully prepared a series of oxygen doped K+ implanted CCN (KCN) with narrowed bandgap (2.71−1.62 eV) for the first time. Compared with most of the O-doped amorphous CN, the optical absorption can only reach 500 nm, and a few more than 500 nm, our oxygen doping strategy is adopted to activate more n→π* transitions to enhance visible light and even near-infrared light harvesting. This active-optimized O-doped KCN accounts for 45- and 10- times promotion than pristine CN and KCN in H2 generation under λ > 500 nm. Most importantly, its maximum active wavelength is up to 650 nm, which has obvious advantages than most CCN-based photocatalysts in the utilization of solar energy. This excellent solar capture and H2 production is attributed to the activated n→π* electron transition, and high crystallinity and conduction band (CB) position caused by oxygen doping.

MXene/Polymer Nanocomposites: Preparation, Properties, and Applications

Abstract: As a new member of the group of promising 2D materials, MXene shows the advantages including metallic conductivity, high charge carrier mobility, tunable band gap, flexibility and diverse surface c

Intertwined Topological and Magnetic Orders in Atomically Thin Chern Insulator MnBi2Te4.

Abstract: MnBi2Te4, a van der Waals magnet, is an emergent platform for exploring Chern insulator physics. Its layered antiferromagnetic order was predicted to enable even-odd layer number dependent topological states. Furthermore, it becomes a Chern insulator when all spins are aligned by an applied magnetic field. However, the evolution of the bulk electronic structure as the magnetic state is continuously tuned and its dependence on layer number remains unexplored. Here, employing multimodal probes, we establish one-to-one correspondence between bulk electronic structure, magnetic state, topological order, and layer thickness in atomically thin MnBi2Te4 devices. As the magnetic state is tuned through the canted magnetic phase, we observe a band crossing, i.e., the closing and reopening of the bulk band gap, corresponding to the concurrent topological phase transition in both even- and odd-layer-number devices. Our findings shed new light on the interplay between band topology and magnetic order in this newly discovered topological magnet.

Z-scheme transition metal bridge of Co9S8/Cd/CdS tubular heterostructure for enhanced photocatalytic hydrogen evolution

Abstract: Narrow band gap semiconductors heterojunction with superior coupling and composition matching can enhance light absorption, reduce carrier recombination and increase redox activity. Here, we report Co9S8/Cd/CdS Z-scheme type heterojunctions with hierarchical tubular heterostructure and inexpensive transition metallic electronic bridge between two distinct semiconductors by a simultaneous immobilization and in-situ reduction strategy. The designed heterostructure greatly promotes the redox activity owing to high-density catalytic sites, excellent visible light capture by small band gap Co9S8/CdS and hollow framework in conjunction with fast charge separation and smooth transfer through intermediary conductive Cd. The optimized photocatalyst exhibits a hydrogen generation rate up to 10.42 μmol h−1 without obvious drop in performance over multiple cycles. The structural design, matching tandem constituent and continuous phase mediator are the pivotal factors to engender an efficient solid-state Z-scheme photocatalysis. The facile synthetic approach and noble metal free tandem structure of this work provide alternative avenues for the development of heterojunction photocatalysts for efficient solar-to-chemical conversion.

Large-scale growth of few-layer two-dimensional black phosphorus

Abstract: Two-dimensional materials provide opportunities for developing semiconductor applications at atomistic thickness to break the limits of silicon technology. Black phosphorus (BP), as a layered semiconductor with controllable bandgap and high carrier mobility, is one of the most promising candidates for transistor devices at atomistic thickness1–4. However, the lack of large-scale growth greatly hinders its development in devices. Here, we report the growth of ultrathin BP on the centimetre scale through pulsed laser deposition. The unique plasma-activated region induced by laser ablation provides highly desirable conditions for BP cluster formation and transportation5,6, facilitating growth. Furthermore, we fabricated large-scale field-effect transistor arrays on BP films, yielding appealing hole mobility of up to 213 and 617 cm2 V−1 s−1 at 295 and 250 K, respectively. Our results pave the way for further developing BP-based wafer-scale devices with potential applications in the information industry. Centimetre-scale growth of few-layer black phosphorous with high crystalline quality and homogeneity is realized by pulsed laser deposition.

Li 4 MgGe 2 S 7 : The First Alkali and Alkaline-Earth Diamond-Like Infrared Nonlinear Optical Material with Exceptional Large Band Gap

Abstract: Large band gap and strong nonlinear optical (NLO) effect are two valuable but contradictory parameters, which are difficult to balance in one infrared (IR) NLO material. Herein, the first alkali and alkaline-earth metal diamond-like (DL) IR NLO material Li4 MgGe2 S7 , presenting a honeycomb-like 3D framework constructed by 6-membered LiS4 rings and GeMgS6 zigzag chains, was rationally designed and synthesized. The introduction of rigid alkali metal and alkaline-earth metal LiS4 and MgS4 tetrahedra effectively broadens the band gap of DL compound to 4.12 eV (the largest one in the reported quaternary metal chalcogenides), generating a high laser damage threshold of 7 × AgGaS2 at 1064 nm. Furthermore, Li4 MgGe2 S7 displays a suitable SHG response (0.7 × AgGaS2 ) with a type I phase-matching behavior. The results indicate that Li4 MgGe2 S7 is a promising IR NLO material for the high-power laser application and it provides an insight into the design of new DL compound with outstanding IR NLO performances.

Self-Driven Perovskite Narrowband Photodetectors with Tunable Spectral Responses

Abstract: Narrowband photodetectors with tunable spectral responses are highly desirable for applications in image sensing, machine vision, and optical communication. Herein, a filterless and self-driven perovskite narrowband photodetector (PNPD) based on the defect-assisted charge collection narrowing (CCN) mechanism is reported, which is enabled by a high-quality thick perovskite film. By adjusting the halide component of the perovskite layer, the bandgap is successfully modulated and the corresponding narrowband photodetectors show a wide spectral response range from the red to the near-infrared (NIR), all with full-widths at half maximum (FWHMs) below 30 nm. Specifically, the methylammonium lead iodide (MAPbI3 ) narrowband photodetector exhibits a characteristic detection peak at 800 nm with a very low noise current of ≈0.02 pA Hz-1/2 , a high specific detectivity up to 1.27 × 1012 Jones, and a fast response speed with rise/fall time of 12.7/6.9 µs. Impressively, these values are among the highest of their kind reported previously, and allow demonstration of narrowband imaging. The excellent performance of self-driven PNPDs lights up their prospect in high-efficiency optoelectronic devices without external power sources.

Why In2O3 Can Make 0.7 nm Atomic Layer Thin Transistors.

Abstract: In this work, we demonstrate enhancement-mode field-effect transistors by an atomic-layer-deposited (ALD) amorphous In2O3 channel with thickness down to 0.7 nm. Thickness is found to be critical on the materials and electron transport of In2O3. Controllable thickness of In2O3 at atomic scale enables the design of sufficient 2D carrier density in the In2O3 channel integrated with the conventional dielectric. The threshold voltage and channel carrier density are found to be considerably tuned by channel thickness. Such a phenomenon is understood by the trap neutral level (TNL) model, where the Fermi-level tends to align deeply inside the conduction band of In2O3 and can be modulated to the bandgap in atomic layer thin In2O3 due to the quantum confinement effect, which is confirmed by density function theory (DFT) calculation. The demonstration of enhancement-mode amorphous In2O3 transistors suggests In2O3 is a competitive channel material for back-end-of-line (BEOL) compatible transistors and monolithic 3D integration applications.

Extensive Benchmarking of DFT+U Calculations for Predicting Band Gaps

Abstract: Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band edge states is a computationally tractable approach to improve the accuracy of band gap predictions beyond that of DFT calculations based on (semi)local functionals. At variance with DFT approximations, which are not intended to describe optical band gaps and other excited-state properties, DFT+U can be interpreted as an approximate spectral-potential method when U is determined by imposing the piecewise linearity of the total energy with respect to electronic occupations in the Hubbard manifold (thus removing self-interaction errors in this subspace), thereby providing a (heuristic) justification for using DFT+U to predict band gaps. However, it is still frequent in the literature to determine the Hubbard U parameters semiempirically by tuning their values to reproduce experimental band gaps, which ultimately alters the description of other total-energy characteristics. Here, we present an extensive assessment of DFT+U band gaps computed using self-consistent ab initio U parameters obtained from density-functional perturbation theory to impose the aforementioned piecewise linearity of the total energy. The study is carried out on 20 compounds containing transition-metal or p-block (group III-IV) elements, including oxides, nitrides, sulfides, oxynitrides, and oxysulfides. By comparing DFT+U results obtained using nonorthogonalized and orthogonalized atomic orbitals as Hubbard projectors, we find that the predicted band gaps are extremely sensitive to the type of projector functions and that the orthogonalized projectors give the most accurate band gaps, in satisfactory agreement with experimental data. This work demonstrates that DFT+U may serve as a useful method for high-throughput workflows that require reliable band gap predictions at moderate computational cost.

Activation Strategy of MoS2 as HER Electrocatalyst through Doping-Induced Lattice Strain, Band Gap Engineering, and Active Crystal Plane Design

Abstract: Doping engineering emerges as a contemporary technique to investigate the catalytic performance of MoS2. Cation and anion co-doping appears as an advanced route toward electrocatalytic hydrogen evolution reaction (HER). V and N as dopants in MoS2 (VNMS) build up a strain inside the crystal structure and narrow down the optical band gaps manifesting the shifting of the absorbance band toward lower energy and improved catalytic performance. FE-SEM, HR-TEM, and XRD analysis confirmed that V and N doping decreases agglomeration possibility, particle size, developed strain, and crystal defects during crystal growth. Frequency shift and peak broadening in Raman spectra confirmed the doping induced strain generation in MoS2 leading to the modification of acidic and alkaline HER (51 and 110 mV @ 10 mAcm-2, respectively) performance. The improved donor density in VNMS was confirmed by the Mott-Schottky analysis. Enhanced electrical conductivity and optimized electronic structures facilities H* adsorption/desorption in the catalytically active (001) plane of cation and anion co-doped MoS2.

Breaking through the "3.0 eV wall" of energy band gap in mid-infrared nonlinear optical rare earth chalcogenides by charge-transfer engineering.

Abstract: Increasing the energy band gap under the premise to maintain a large nonlinear optical (NLO) response is a challenging issue for the exploration and molecular design of mid-infrared nonlinear optical crystals. Utilizing a charge-transfer engineering method, we designed and synthesized a rare earth chalcogenide, KYGeS4. With an NLO effect as large as that in AgGaS2, KYGeS4 breaks through the limitation of energy band gap, i.e., the “3.0 eV wall”, in NLO rare earth chalcogenides, and thus exhibits an excellent comprehensive NLO performance. First-principles electronic structure analysis demonstrates that the large band gap in KYGeS4 is ascribed to the decreased covalency of Y–S bonds by transferring charge from [YS7] to [GeS4] polyhedra. The charge-transfer engineering strategy would have significant implications for the exploration of good-performance NLO crystals.