Intrinsic Band Gap Research Articles

Scalable and atom economic preparation of red-near-infrared emitted N-doped graphene quantum dots with a high quantum yield

The development of an environmental-friendly and scalable preparation approach for graphene quantum dots (GQDs) with high quantum yield is a focus area for GQDs. Hence, we describe an atom economic green chemistry strategy for nitrogen-doped GQDs (N-GQDs). The yield of N-GQDs is 98.5%. Moreover, due to the simple reaction and processes, N-GQDs can be synthesized on a large scale (the throughput of N-GQDs is 10 g). Besides, the intrinsic bandgap PL of N-GQDs can be adjusted by size modulation. The emission wavelength of N-GQDs is modulated from 405 nm to 698 nm with a high quantum yield (0.89–0.62). Furthermore, with the general nucleophilic addition reaction of indigo, the PL response-ability of the obtained GQDs can be further tuned. Different precursors with heteroatoms (such as S and Se) can be used for the preparation of dual- heteroatoms co-doped GQDs (N, S-GQDs and N, Se-GQDs). We also confirmed the utilization of the red-near-infrared emitted N-GQDs (emission wavelength: 698 nm) with high quantum yield (0.62) in bio-imaging.

Narrowing Bandgap of HfS2 by Te Substitution for Short‐Wavelength Infrared Photodetection

AbstractInfrared photodetectors are widely used in the field of remote sensing, communications, biomedical imaging, etc. Most photodetection based on 2D transition‐metal dichalcogenides (TMDs) is limited to the visible (Vis) to near‐infrared (NIR) due to large intrinsic bandgaps (≈1.2–2 eV). Here, a bandgap engineering of HfS2 by a tellurium (Te)‐replacement strategy is obtained via chemical vapor transport method. The bandgap values of HfS2(1−x)Te2x decrease from 1.7 to 0.88 eV with Te composition changing from 0 to 0.095. Few‐layer HfS1.81Te0.19 based field‐effect transistors exhibit a high current on/off ratio of 106 and decent electron mobility of 12.6 cm2 V−1 s−1 at room temperature. The photodetectors show a responsivity of 2 A W−1 with a remarkable photocurrent of ≈3 μA and a fast response speed of 8.8/75 ms at 830 nm simultaneously. Further, the response spectrum of HfS2(1−x)Te2x based photodetectors is broadened from Vis to short‐wavelength infrared (SWIR), covering the free‐space laser communications wavelength and the second NIR region in medicine. Bandgap engineering of 2D TMDs proposed in this work offer a promising route to develop bandgap‐variable 2D materials for infrared photodetection applications.

Advances on Graphyne-Family Members for Superior Photocatalytic Behavior.

Graphyne (GY) and graphdiyne (GDY) have been employed in photocatalysis since 2012, presenting intriguing electronic and optical properties, such as high electron mobility and intrinsic bandgap due to their high π‐conjugated structures. Authors are reporting the enhanced photocatalytic efficiency of these carbon allotropes when combined with different metal oxides or other carbon materials. However, the synthesis of graphyne‐family members (GFMs) is still very recent, and not much is known about the true potential of these photocatalytic materials. In this review article, the implications of different synthesis routes on the structural features and photocatalytic properties of these materials are elucidated. The application of GFMs in the nicotinamide adenine dinucleotide (NADH) regeneration, hydrogen and oxygen evolution, and carbon dioxide reduction is discussed, as well as in the degradation of pollutants and bacteria inactivation in water and wastewater treatment.

Chiral interface states and related quantized transport in disordered Chern insulators

In this Letter, we study an Anderson-localization-induced quantized transport in disordered Chern insulators (CIs). By investigating the disordered CIs with a step potential, we find that the chiral interface states emerge along the interfaces of the step potential, and the energy range for such quantized transport can be manipulated through the potential strength. Furthermore, numerical simulations on cases with a multi-step potential demonstrate that such chiral state can be spatially shifted by varying the Fermi energy, and the energy window for quantized transport is greatly enlarged. Experimentally, such chiral interface states can be realized by imposing transverse electric field, in which the energy window for quantized transport is much broader than the intrinsic band gap of the corresponding CI. These phenomena are quite universal for disordered CIs due to the direct phase transition between the CI and the normal insulator.

Sintering‐driven effects on the band gap of (Pb,La)(Ti,Ni)O3 photovoltaic ceramics

AbstractIn this work, the influence of the sintering temperature on the physical properties of (Pb0.8La0.2)(Ti0.9Ni0.1)O3 (PLT‐Ni) ceramics is reported. The experimental data revealed that the energy band gap of PLT‐Ni ceramics could be tailored from approximately 2.7 to 2.0 eV by changing the sintering temperature from 1100°C to 1250°C. It is demonstrated that the simple substitution of Ti4+ by Ni2+ cations is effective to decrease the intrinsic band gap while increasing the tetragonality factor and the spontaneous polarization. However, the additional red‐shift observed in the absorption edge of the PLT‐Ni with increasing the sintering temperature was associated with a continuous increase in the oxygen vacancies () amount. It is believed that the impact of the creation of these thermally induced is manifold. The presence of and Ni2+ ions generate the Ni2+‐ defect‐pairs that promoted both a decrease in the intrinsic band gap and an additional increase of the tetragonality factor, consequently, increasing the spontaneous polarization. The creation of Ni2+‐ defects also changed the local symmetry of Ni2+ ions from octahedral to a square pyramid, thus lifting the degeneracy of the Ni2+ 3d orbitals. With the increase in the sintering temperature, lower‐energy absorbing intraband states were also formed due to an excess of , being responsible for an add‐on shoulder in the absorption edge, extending the light absorption curve to longer wavelengths and leading to an additional absorption in “all investigated” spectrum as well.

Ultrafast transient sub-bandgap absorption of monolayer MoS2

The light–matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications, which is intrinsically determined by the bandgap of the materials involved. To extend the applications beyond the bandgap limit, it is of great significance to study the light–matter interaction below the material bandgap. Here, we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from ~0.86 µm to 1.4 µm. Even though this spectral range is below the bandgap, we observe a significant absorbance enhancement up to ~4.2% in the monolayer molybdenum disulfide (comparable to its absorption within the bandgap region) due to pump-induced absorption by the excited carrier states. The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states (i.e., real carrier states in the short-wavelength region of ~<1 µm, and exciton states in the long wavelength region of ~>1 µm). Our results elucidate the fundamental understanding regarding the optical properties, excited carrier states, and carrier dynamics in the technologically important near-infrared region, which potentially leads to various photonic and optoelectronic applications (e.g., excited-state-based photodetectors and modulators) of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.

One-Dimensional van der Waals Heterostructures as Efficient Metal-Free Oxygen Electrocatalysts.

Two-dimensional covalent organic frameworks (2D-COFs) may serve as an emerging family of catalysts with well-defined atomic structures. However, the severe stacking of 2D nanosheets and large intrinsic bandgaps significantly impair their catalytic performance. Here, we report coaxial one-dimensional van der Waals heterostructures (1D vdWHs) comprised of a carbon nanotube (CNT) core and a thickness tunable thienothiophene-pyrene COF shell using a solution-based in situ wrapping method. Density functional theory calculations and operando and ex situ spectroscopic analysis indicate that carbon-sulfur regions in thienothiophene groups in the COF serve as an active catalytic site for oxygen reduction and evolution reactions. The coaxial structure enables n-doping from the CNT core to the COF shell, which is controllable by varying COF shell thickness. The charge transfer from CNTs lowers COF's bandgap and work function, which reduces the charge transfer barrier between the active catalytic sites and adsorbed oxygen intermediates, resulting in dramatically enhanced catalytic activity. The 1D vdWHs were applied as a bifunctional oxygen electrocatalyst in rechargeable zinc-air batteries, delivering a high specific capacity of 696 mAh gZn-1 under a high current density of 40 mA cm-2 and excellent cycling stability. The 1D vdWHs based on the coaxial structure of 2D COF wrapped around CNT cores may be further used as versatile building units to create multidimensional vdWHs for exploring fundamental physics and chemistry as well as practical applications in electrochemistry, electronics, photonics, and beyond.

Structural and electronic properties of AlY (Y B, N, O) dual-doped twin graphene: A density functional theory study

Twin graphene is a novel two-dimensional semiconducting carbon allotrope with an intrinsic direct bandgap. To explore the excellent properties and potential applications of twin graphene, we performed first-principle density functional theory calculations on the structural, electronic, and magnetic properties of twin graphene with dual-doping of Al and Y (YB, N, O) atoms at different sites (ortho, meta, and para). The combined processes of the formation of all Al–Y dual-doped twin graphene (AlY-TG) systems are exothermic and form stable dual-doped structures, and the most stable structure is the AlB-TG system. For the B, N, or O atoms at the same doped site, the stability decreases in the order of AlB-TG, AlN-TG, and AlO-TG. Dual-doping regulates the bandgap of twin graphene in the cases of AlB and AlN doping. A transition from a semiconducting material into a metal is observed when AlO doping is used, while a transition from a direct semiconductor into an indirect semiconductor is observed for AlB doping at the ortho site. Different magnetic moments of the AlY-TG systems are observed for different Y atoms placed at different doping sites. The AlN-TG system remains nonmagnetic, while the AlB-TG system has a magnetic moment of 1.01 μB only in the meta site, and the AlO-TG systems have magnetic moments of 0.50 μB and 0.56 μB in the ortho and meta sites, respectively. Moreover, different doping concentrations can effectively affect the electronic structures and magnetisms of the AlY-TG systems. For doping concentrations of 5.6% and 11.1% at the meta site, the AlB-TG systems exhibit metal characteristics with a high spin polarization. The results show that the electronic and magnetic properties of twin graphene can be modulated via Al–Y (YB, N, O) dual-doping, indicating that twin graphene has potential applications in nanomagnets and spintronics devices.

Research progress of puckered honeycomb monolayers

Graphene, as the representative of two-dimensional materials, has varous novel physical properties and potential applications. The intrinsic zero band gap of graphene limits its application in semiconductor devices, and thus the search for new semiconducting alternative materials has become a current research hotspot. Phosphorene is the monolayer of black phosphorus and has a puckered honeycomb structure. Its advanced properties, such as adjustable direct band gap, high carrier mobility and in-plane anisotropy and so on, have recently aroused great research interest, thus opening up the research field of puckered honeycomb monolayers in group V elements. In this article, we first focus on the structure, synthesis and physical properties of five single-element two-dimensional materials (nitrogen, phosphorus, arsenic, antimony and bismuth) each with puckered honeycomb structure. Second, some binary two-dimensional materials with puckered honeycomb structure are discussed, including IV-VI and V-V compounds. These materials have their own unique crystal symmetry, and the properties can be controlled by changing their structures and dimensions. Finally, we also make a summary on some current challenges that need to be solved, and the possible future applications of these two-dimensional materials are also presented.

Asymmetric carrier transport and weak localization in few layer graphene grown directly on a dielectric substrate.

Temperature-dependent electrical and magneto-transport measurements have been performed on devices composed of few layer (4L) graphene grown directly on SiO2/Si substrates using the CVD method. An intrinsic energy band-gap of 4.6 meV in 4L graphene is observed, which primarily dictates the current transport at T <50 K. Unusual temperature dependent electron-hole conduction asymmetry is observed at T >50 K, which can be explained in the framework of the defect scattering of relativistic charge carriers. Magneto-transport measurements reveal a weak localization effect sustainable till T >200 K. The coexistence of phonon mediated carrier mobility and defect induced weak localization effects in measuring devices suggests low disorder and impurity scattering.

Nonlinear optics of graphdiyne

Graphdiyne features a high π-conjugation degree and an intrinsic natural bandgap, which guarantee a large optical refractive index and broadband absorption, and thus promises a wide range of application prospects in nonlinear optics.

A reduced-dimensional polar hybrid perovskite for self-powered broad-spectrum photodetection.

Polar hybrid perovskites have been explored for self-powered photodetection benefitting from prominent transport of photo-induced carriers and the bulk photovoltaic effect (BPVE). However, these self-powered photodetection ranges are relatively narrow depending on their intrinsic wide bandgaps (>2.08 eV), and the realization of broad-spectrum self-powered photodetection is still a difficult task. Herein, we successfully obtained a polar multilayered perovskite, (I-BA)2(MA)2Pb3I10 (IMP, MA+ = methylammonium and I-BA+ = 4-iodobutylammonium), via rational dimension reduction of CH3NH3PbI3. It features the narrowest bandgap of 1.71 eV in a BPV material. As a consequence, the integration of narrow bandgap and BPVE causes the self-powered photodetection to extend to 724 nm for IMP, and a repeatable photovoltaic current reaching 1.0 μA cm−2 is acquired with a high “on/off” ratio of ∼103 and photodetectivity (∼109 Jones) at zero bias. This innovative research provides a foothold for adjusting the physical properties of hybrid perovskites and will expand their potential for self-powered broad-spectrum detection.

Recent development in graphdiyne and its derivative materials for novel biomedical applications.

Graphdiyne (GDY), which possess sp- and sp2-hybridized carbon and Dirac cones, offers unique physical and chemical properties, including an adjustable intrinsic bandgap, excellent charge carrier transfer efficiency, and superior conductivity compared to other carbon allotropes. These exceptional qualities of GDY and its derivatives have been successfully used in a variety of fields, including catalysis, energy, environmental protection, and biological applications. Herein, we focus on the potential application of GDY and its derivatives in the biomedical domain, including biosensing, biological protection, cancer therapy, and antibacterial agents, demonstrating how the biomimetic behavior of these materials can be a step forward in bridging the gap between nature and applications. Considering the excellent biocompatibility, solubility and selectivity of GDY and its derived materials, they have shown great potential as biosensing and bio-imaging materials. The unusual combination of properties in GDY has been used in biological applications such as "OFF-ON" DNA detection and enzymatic sensing, where GDY has a greater adsorption capacity than graphene and other 2D materials, resulting in increased sensitivity. GDY and its derivatives have also been used in cancer treatment due to their high doxorubicin (DOX) loading capacity (using-stacking) and photothermal conversion ability, and radiation protection since their initial biological use. The poor biodegradation rate of graphene demands the search for new nanomaterials. Accordingly, GDY has better biocompatibility and bio-safety than other 2D nanomaterials, especially graphene and its oxide, due to its absence of aggregation in the physiological environment. Thus, GDY-based nanomaterials have become promising candidates as bio-delivery carriers. Besides, GDY and GDY-based materials have also shown interesting applications in the fields of cell-culture, cell-growth and tissue engineering. Herein, we present a comprehensive review on the applications of GDY and its derivatives as biomedical materials, followed by their future perspectives. This review will provide an outlook for the application of graphene and its derivatives and may open up new horizons to inspire broader interests across various disciplines. Finally, the future prospects for GDY-based materials are examined for their potential biological use.

Silicon‐Based Intermediate‐Band Infrared Photodetector Realized by Te Hyperdoping

AbstractSi‐based photodetectors satisfy the criteria of being low‐cost and environmentally friendly, and can enable the development of on‐chip complementary metal‐oxide‐semiconductor (CMOS)‐compatible photonic systems. However, extending their room‐temperature photoresponse into the mid‐wavelength infrared (MWIR) regime remains challenging due to the intrinsic bandgap of Si. Here, we report on a comprehensive study of a room‐temperature MWIR photodetector based on Si hyperdoped with Te. The demonstrated MWIR p‐n photodiode exhibits a spectral photoresponse up to 5 µm and a slightly lower detector performance than the commercial devices in the wavelength range of 1.0–1.9 µm. The correlation between the background noise and the sensitivity of the Te‐hyperdoped Si photodiode, where the maximum room‐temperature specific detectivity is found to be 3.2 × 1012 cmHz1/2 W−1 and 9.2 × 108 cmHz1/2 W−1 at 1 µm and 1.55 µm, respectively, is also investigated. This work contributes to pave the way towards establishing a Si‐based broadband infrared photonic system operating at room temperature.

Superexchange Coupling-Induced Enhancements of Thermoelectric Performance in Saturated Molecules.

To develop thermoelectric devices, it is of the utmost importance to design organic building blocks to have efficient thermopower. Whereas conjugated and aromatic molecules with intrinsic narrow band gaps are attractive candidates to achieve efficient thermoelectric properties, saturated molecules are usually avoided owing to intrinsically poor thermopower. Here we demonstrate that thermopower of saturated molecules can be enhanced by superexchange coupling. Specifically, thermoelectric properties of large-area junctions that contain self-assembled monolayers of oligo(ethylene glycol) thiolates and alkanethiolates are compared. Through large-area thermopower measurements using a liquid metal top electrode, we show that the superexchange coupling enhances the Seebeck coefficient and counterintuitively leads to an increase in the Seebeck coefficient with increasing the length in a certain conformation. The improved thermoelectric performance is attributed to the superexchange-induced enhanced ability to mediate metal wave function in junctions. Our work offers new insights for improving the thermoelectric performance of nonconjugated, saturated molecules.

Enhanced spectral splitting in a novel solar spectrum optical splitter based on one dimensional photonic crystal heterostructure

Controlling the distribution of solar spectrum in different bands would boost the energy harvesting efficiency and optimize the energy dispatchability. 1D photonic crystal with intrinsic optical band gap can be used to split the solar spectrum for hybrid photovoltaic/thermal solar applications. Here, we designed an efficient solar spectrum optical filter based on a cermet layer, Si/SiO2 1D photonic crystal, and top heterostructure layer. Compared with 1D photonic crystal structure, the 1D photonic crystal heterostructure with top YSZ layer can realize the reflectance of greater than 92% in PV band and the low average reflectance in two thermal bands by tuning the effective impedance of multilayer films. The enhanced reflectance in PV band results from the huge mismatching of impedance between free space and the heterostructure structure. The top dielectric layer can also be extended to other oxides.

Ultrahigh carrier mobility of penta-graphene: A first-principle study

Within the formalism of density functional theory and using the concept of effective mass approximation, the electron-acoustic phonon scattering principle along with the deformation potential theory of Bardeen and Shockley, we have investigated the intrinsic charge carrier mobility of penta-graphene. The electron mobility of penta-graphene is significantly large compared to hole mobility and it is reasonably higher than other novel two-dimensional materials e.g. penta-X 2 C (X = P, As, Sb), Si-based pentagonal monolayer (SiX, X = B, C, N), penta-stanene monolayer and CaP 3 which possess the highest carrier (electron) mobility among phosphorene derivatives. This high carrier mobility originates from the low effective mass, low deformation potential and high stiffness constant of this material. As our main focus is to investigate the carrier mobility which depends on band edges, a detailed study about orbital contribution to the electronic states in the vicinity of band edges has been carried out. The intrinsic wide band gap and ultrahigh carrier mobility show the possible signature of using it in the fabrication of microelectronic devices, mainly in FETs and logic circuits and in optoelectronic devices . • The electron mobility of PG is significantly large compared to hole mobility. • Ultrahigh mobility originates from low effective mass and high stiffness constant. • The electron mobility of unstrained PG is 22.74 × 10 3 cm 2 /V at 300 K. • The electron mobility of PG varies from 11.72 to 40.90 × 10 3 cm 2 /V for +4% to −4% strain at 300 K. • PG may be suitable for microelectronics applications.

Structural, Electronic and Mechanical Properties of Few-layer Porous Nanosheet from Spheroidal Cage-like ZnO Polymorph

The low-dimensional II-VI group semiconductors have recently emerged as interesting candidate materials for the tailoring of two dimensional (2D) layered structures. Herein, a series of the cage-like nanoporous composed of spheroidal hollow cages (ZnO)12, cutting from the high symmetrical cubic SOD cage-like polymorph as building block, is proposed. We have performed the density-functional tight binding (DFTB+) calculations on the structural, electronic and mechanical properties of this few-layer SOD-cage-block nanosheet series, to investigate the effects of structural modification and sheet thickness on their structural, electronic, and mechanical properties. Optimized geometries, formation energy, phonon spectra, electronic band structure, and elastic tensor calculation has ensured the energetically, dynamical and mechanical stability for the sheets. Furthermore, the theoretically found nanosheet series possess an intrinsic wide direct band gap preserving from wurtzite tetragonal-based bonding. This high symmetry wide bandgap semiconductor nanosheet series and their derivatives are expected to have broad applications in photocatalysis, and biomedicine.

Bandgap Renormalization in Monolayer MoS2 on CsPbBr3 Quantum Dots via Charge Transfer at Room Temperature

AbstractMany‐body effect and strong Coulomb interaction in monolayer transition metal dichalcogenides lead to intrinsic bandgap shrinking, originating from the renormalization of electrical/optical bandgap, exciton binding energy, and spin‐orbit splitting. This renormalization phenomenon has been commonly observed at low temperature and requires high photon excitation density. Here, the augmented bandgap renormalization (BGR) in monolayer MoS2 anchored on CsPbBr3 perovskite quantum dots at room temperature via charge transfer is presented. The amount of electrons significantly transferred from perovskite gives rise to the large plasma screening in MoS2. The bandgap in heterostructure is red‐shifted by 84 meV with minimal pump fluence, the highest BGR in monolayer MoS2 at room temperature, which saturates with a further increase of pump fluence. Further, it is found that the magnitude of BGR inversely relates to Thomas–Fermi screening length. This provides plenty of room to explore the BGR within existing vast libraries of large bandgap van der Waals heterostructure toward practical devices such as solar cells, photodetectors, and light‐emitting‐diodes.

Fe(III)-C3N4 hybrids photocatalyst for efficient visible-light driven nitrogen fixation

Solar-driven reduction of nitrogen to ammonia is a promising strategy for energy storage and conversion. Herein, a new strategy of modifying Fe(III) onto carbon nitride is reported showing enhanced photocatalytic nitrogen fixation. The Fe(III) and nitrogen vacancies narrow down the carbon nitride intrinsic bandgap and suppress the radiative electron-hole recombination. Fe(III)-C3N4 therefore shows potential as a new high-efficiency artificial photosynthesis ammonia.