Narrow Energy Gap Research Articles

Semiconductorlike photocarrier dynamics in Dirac-semimetal Cd3As2 films probed with transient terahertz spectroscopy

The topological three-dimensional Dirac semimetal ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ has drawn great attention for the novel physics and promising applications in optoelectronic devices operating in the infrared and terahertz (THz) regimes. Among the extensive studies in the past decades, one intriguing debate is the underlined mechanism governing the nonequilibrium carrier dynamics following photoexcitation. In this study, the temperature-dependent photocarrier dynamics in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ film has been investigated with time-resolved terahertz spectroscopy. The experimental results demonstrate that photoexcitation results in abrupt increase in THz photoconductivity, and the subsequent relaxation shows a single exponential relaxation for various temperatures and pump fluences. The relaxation time increases from 4.7 ps at 5 K to 7.5 ps at 220 K, while the lifetime remains almost constant at $\ensuremath{\sim}7.5$ ps with temperature above 220 K. A Rothwarf-Taylor model was employed to fit the temperature-dependent relaxation time, and a narrow energy gap of $\ensuremath{\sim}35\ifmmode\pm\else\textpm\fi{}6$ meV is obtained, which occurs around the Dirac node. Our THz spectroscopy results demonstrate that the photocarrier relaxation in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ shows a semiconductorlike behavior, rather than hot-carrier scatterings in graphene and most of metals.

Highly-dispersed nickel on 2D graphitic carbon nitrides (g-C3N4) for facilitating reaction kinetics of lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are promising next-generation energy storage devices due to high theoretical energy density and low-cost. Nevertheless, the practical applications are hindered by polysulfide shuttling effect, low electrical conductivity of sulfur, and slower conversion kinetics. Here, the graphited g-C3N4 assembled with highly-dispersed nickel (HDNi@g-C3N4) is designed as a catalyst to accelerate the reaction kinetics of lithium polysulfide. The oxidized Ni sites of HDNi@g-C3N4 molecules significantly accommodate the orbital for the electron clouds of polysulfide by forming Sn2–‧‧‧Ni-N active site, thus efficiently improving redox kinetics and mitigating shuttle effects. Based on density functional theory (DFT) calculations, HDNi@g-C3N4 exhibits a superior metallicity with increased density of states (DOS) at the Fermi energy level. Then, the narrowed energy gap between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) level contributes to the enhanced conductivity of catalyst molecular and fast combination between electrons and Li+ ions. Moreover, the positive Gibbs free energy change is significantly decreased for the HDNi@g-C3N4 cathode. The Li-S battery exhibits a high reversible capacity of 1, 271.6 mAh g−1 at 0.1 C and a high rate capacity of 571.96 mAh g−1 at 2.0 C, a preferable cycling stability with a capacity retention of 53 % even after 500 cycles at a 1.0 C, and an average decay rate of 0.733 % per cycle.

Robust Chalcogenophene Viologens as Anolytes for Long-Life Aqueous Organic Redox Flow Batteries with High Battery Voltage.

A series of chalcogenophene viologens ([(NPr)2FV]Cl4, [(NPr)2TV]Cl4, and [(NPr)2SeV]Cl4) as anolytes for neutral aqueous organic redox flow batteries (AORFBs) via a combination of chalcogenophenes (furan, thiophene, and selenophene) and viologens are reported. The chalcogenophene viologens showed narrow HOMO-LUMO energy gap, high solubility, and stable electrochemical properties. Compared with the parent [(NPr)2V]Cl4, the introduction of π-conjugated chalcogenophene groups reduced the redox potential and enhanced the stability of their free radical state, which endowed the chalcogenophene viologens/FcNCl-based AORFBs with a higher theoretical battery voltage of 1.20 V and enhanced stability for one-electron storage. In particular, the [(NPr)2FV]Cl4/FcNCl-based AORFB exhibited excellent long-cycle stability for 3000 cycles with 0.0006% capacity decay per cycle for one-electron storage and 300 cycles with 0.06% capacity decay per cycle for two-electron storage at a charge voltage of 1.9 V (1.42 V theoretical battery voltage). This work provided a new strategy for regulating the voltage and improving the performance of neutral AORFBs.

Ce2(MoO4)3 synthesized with oleylamine and oleic acid as additives for photocatalysis: effect of preparation method.

Ce2(MoO4)3 was prepared using dielectric barrier discharge (DBD) plasma method, co-precipitation method and hydrothermal method, respectively, with water/ethanol (W/O) as solvent, oleylamine (OAm) and oleic acid (OAc) as additives. Preparation method showed significant influence on the morphological and structural properties, as well as photocatalytic performance. Ce2(MoO4)3 synthesized with DBD plasma (MO-P) was mainly flowerlike nanosheets, which were beneficial to promoting electron transfer and providing more space for catalytic activity. Also, MO-P samples exhibited more oxygen vacancies, which were conducive to the photocatalytic performance. What's more, MO-P showed lower PL intensity and narrow energy gap, which implied a slow photoelectron-hole pair recombination rate and an increased electron transfer rate. The degradation rate of methyl orange (50mg/L) could achieve 98% within 12min with 0.5g/L MO-P. Hydroxyl radicals (·OH) and superoxide radicals (·O2-) played a major effect. Plasma synthesis method exhibited potential application prospect in photocatalysts preparation.

Detection of excited triplet species from photolysis of carbonyls: Direct evidence for single oxygen formation in atmospheric environment.

Excited triplet species play an important role in the photolytic formation of 1O2 from carbonyls, but the related mechanism is still uncertain, due to lack of direct evidence. In this study, steady-state and transient photolysis of eleven carbonyls to produce 1O2 was investigated. Dicarbonyl displayed greater 1O2 production ability than monocarbonyl, while dicarbonyl containing both ketone and carboxyl groups connected by CC bond (i.e., pyruvic acid (PA)) showed the highest 1O2 steady-state concentration ([1O2]SS). For the first time, the production of 3PA* from PA with narrow energy gap was confirmed by laser flash photolysis technique and the second-order decay rate constant of 3PA* was 2.78 × 107 M-1 s-1. Quenching results verified the dominant contribution of 3PA* to 1O2 production from PA. Addition of inorganic salt or increase in solution pH showed negligible effect on 3PA*, but significantly decreased the [1O2]SS of PA by up to two orders of magnitude, due to reduction of hydrate content. Photolysis of methylglyoxal and dimethylamine mixture led to higher content of excited triplet species at pH ≈ 11 and remarkably enhanced [1O2]SS, which was 2.3 times of that from PA and dimethylamine mixture. These findings provide direct evidence for the contribution of transient species from carbonyls or their product to 1O2 formation in atmospheric environment.

Organic Charge-Transfer Cocrystals toward Large-Area Nanofiber Membrane for Photothermal Conversion and Imaging.

Organic photothermal materials integrating a high-efficiency light-heat conversion effect and high flexibility have generated immense interest in fundamental research and practical applications. Nevertheless, their practical applications still remain a challenge, owing to the complicated design, tedious synthesis, and limited programmable substrates. Herein, an organic charge-transfer cocrystal with a narrow energy gap of 0.33 eV and a high photothermal conversion efficiency (PCE) of 69.3% was rationally designed and synthesized via a facile self-assembly process, which was introduced into polyurethane for forming a large-area photothermal nanofiber membrane via electrospinning technology. Femtosecond transient absorption spectroscopy elucidates that the excellent PCE is attributed to the nonradiation transition process, including internal conversion and charge dissociation processes. Furthermore, the temperature of the as-prepared photothermal nanofiber membrane could quickly rise to 52 °C under laser irradiation with a power density of 0.183 W/cm2, suggesting a high PCE of 53.7%. This work successfully achieves the fabrication of a large-area photothermal membrane and the development of photothermal imaging.

Dipyrido[3,2-a:2′,3′-c]phenazine acceptor based thermally activated delayed fluorescence emitters

A rigid π-conjugated and electron-deficient moiety, dipyrido[3,2-a:2′,3′-c]phenazine (DPPZ), was employed as the electron acceptor for thermally activated delayed fluorescence (TADF) emitters with a donor–acceptor structure. Two TADF emitters, DCz-DPPZ and DDPhCz-DPPZ, were prepared by adopting carbazole derivatives as the donor and DPPZ as the acceptor. Compared to DCz-DPPZ without substituents at the carbazole group, DDPhCz-DPPZ showed a narrower energy gap, a longer emission wavelength, and a higher photoluminescence quantum efficiency. The TADF organic light-emitting diodes based on DDPhCz-DPPZ exhibited a higher quantum efficiency of 12.4% than 4.6% of devices based on DCz-DPPZ. These results demonstrate that DPPZ is a suitable acceptor for constructing D–A type TADF emitters.

Highly efficient singlet oxygen generation of AIE luminogens enable mitochondria-targeted photodynamic therapy

The development of aggregation-induced emission luminogens (AIEgens) with high brightness and efficient photodynamic therapy (PDT) is of great value for the precise diagnosis and treatment of cancers. In this contribution, two novel AIEgens, TBBCyP and TBTCyP with D-A-π-A structure, were designed and prepared based on molecular rational design of electron-donating receptors. Experimental and theoretical results indicated that both TBBCyP and TBTCyP own bright AIE feature, satisfy photos ability, and generate efficient singlet oxygen (1O2). Notably, TBBCyP exhibited brighter emission and higher 1O2 yield in compared with TBTCyP, which could be ascribed to its weaker twisted intramolecular charge transfer (TICT) effect and narrower singlet-triplet energy gap (ΔEST). Moreover, it was proved that TBBCyP and TBTCyP showed a great mitochondrial specificity and desired intracellular ROS productivity for realizing the effective mitochondria-targeted PDT of cancer cells.

A-type antiferromagnetic order in semiconducting EuMg2Sb2 single crystals

Eu-based Zintl-phase materials EuA$_2$Pn$_2$ (A = Mg, In, Cd, Zn; Pn = Bi, Sb, As, P) have generated significant recent interest owing to the complex interplay of magnetism and band topology. Here, we investigated the electronic, magnetic, and electronic properties of the layered Zintl-phase single crystals of EuMg$_2$Sb$_2$ with the trigonal CaAl$_2$Si$_2$ crystal structure (space group $P\bar{3}m1$). Electrical resistivity measurements complemented with angle-resolved photoemission spectroscopy (ARPES) studies find an activated behavior with the intrinsic conductivity at high temperatures indicating a semiconducting electronic ground state with a narrow energy gap of 370 meV. Magnetic susceptibility and zero-field heat-capacity measurements indicate that the compound undergoes antiferromagnetic (AFM) ordering at the Neel temperature $T_{\rm N}$ = 8.0(2) K. Zero-field neutron-diffraction measurements reveal that the AFM ordering is A-type where the Eu ordered moments (Eu$^{2+}$, S= 7/2) arranged in ab-plane layers are aligned ferromagnetically in the ab plane with the Eu moments in adjacent layers aligned antiferromagnetically. We also find that Eu-moment reorientation in the trigonal AFM domains within the ab planes occurs below $T_{\rm N}$ at low fields < 0.05 T due to very small in-plane anisotropy. Although isostructural semimetallic EuMg$_2$Bi$_2$ is reported to host Dirac surface states, the observation of narrow-gap semiconducting behavior in EuMg$_2$Sb$_2$ implies a strong role of spin-orbit coupling in tuning the electronic states of these materials.

A novel 2D sulfide gallium heterojunction as a high-performance electrocatalyst for overall water splitting

It is an effective way to split water to produce high-clean hydrogen energy for solving energy crisis in the world. However, overall water splitting is still suffering from high voltage and sluggish kinetics. Herein, we firstly synthesize sulfide gallium heterojunction modified with abundant S vacancies in a one-pot hydrothermal way, showing the low OER overpotential of 345 ​mV with good stability as well as HER overpotential of 209 ​mV at 10 ​mA ​cm−2. Furthermore, we assemble the sulfide gallium heterojunction into cathode and anode for overall water splitting, which possesses a low voltage of 1.43 ​V in an alkaline solution. The density functional theory calculations also reveal that the sulfide gallium heterojunction with S vacancy has narrower band energy gap in sulfide gallium. This designed heterojunction has great potential for being used as a novel electrocatalyst to replace the commercial RuO2//Pt/C system in water splitting field.

Multiplying the efficiency of red thermally activated delayed fluorescence emitter by introducing intramolecular hydrogen bond

As one of the three primary colors, red organic light-emitting diodes (OLEDs) are indispensable in practical applications. However, red emitters are generally subject to severe non-radiative exciton loss due to their narrow energy gap. In this work, three new thermally activated delayed fluorescence (TADF) emitters were developed, namely 4-(acenaphtho[1,2-b]quinoxalin-9-yl)-N,N-diphenylaniline (TPA-AP), 4-(acenaphtho[1,2-b]pyrido[2,3-e]pyrazin-10-yl)-N,N-diphenylaniline (TPA-APy), and 4-(acenaphtho[1,2-b]pyrazino[2,3-e]pyrazin-9-yl)-N,N-diphenylaniline (TPA-APm), employing a series of finely modified acenaphtho[1,2-b]quinoxaline (AP) derivatives as acceptor units. Among three TADF emitters, an intramolecular hydrogen bond is formed between the donor (D) and acceptor (A) units in TPA-APm. Consequently, the overlap of the frontier molecular orbitals (FMOs) of TPA-APm can increase appropriately, and the fluorescence radiative rate (kF) of TPA-APm is nearly twofold than that of TPA-AP and TPA-APy. Furthermore, the non-radiative decay rate (knr) of TPA-APm is less than that of TPA-AP and TPA-APy by an order of magnitude, which is attributed to the improved molecular rigidity caused by intramolecular hydrogen bond. As a result, TPA-APm-based OLEDs achieved a multiplied external quantum efficiency (EQE) of 21.1% with the electroluminescence peak at 590 nm, comparing to only 7.0% and 11.5% for the TPA-AP-based and TPA-APy-based devices, respectively. These results demonstrate appropriate intramolecular hydrogen bond can suppress the influence of non-radiative decay by simultaneously enhancing molecular rigidity and facilitating the fluorescence process, and have great potential in the design of efficient red TADF emitters.

Large Conjugated Bis/Triimidazolium Derivatives Directed Iodobismuthates(III): Syntheses, Structures, and Visible-Light-Induced Photocatalytic Properties

Under the direction of large conjugated organic species, a series of novel iodobismuthates(III), [Me3BIP][BiI6] (1, Me3BIP = N,N′,N″-trimethyl-3,5-bis(imidazole-1-yl)pyridine), [Me3BIP][Bi2I9] (2), [Me3TIP][Bi2I9] (3, Me3TIP = N,N′,N″-trimethyl-tris[4-(1H-imidazole-1-yl)-phenyl]amine), and [Me3TIB][Bi2I9] (4, Me3TIB = N,N′,N″-trimethyl-1,3,5-tris(1-imidazolyl)benzene), were synthesized via the solvothermal method. These iodobismuthates display two types of structures: 1 contains a [BiI6]3– monometallic unit, and 2–4 possess binuclear [Bi2I9]3– clusters, which are formed by two BiI6 octahedra via face-sharing. The reactants BIP, TIP, and TIB were in situ transformed into [Me3BIP]3+ in 1 and 2, [Me3TIP]3+ in 3, and [Me3TIB]3+ in 4 through the direct N-methylation reaction. UV–visible spectra analyses imply that all compounds possess potential semiconductor properties with narrow band gaps of 2.14, 2.02, 2.09, and 2.07 eV for 1–4, respectively. Compound 3 has high visible-light-driven photocatalytic degradation activity and good recyclability to organic pollutants. Radical trapping experiments combined with theoretical calculations show that the conjugated organic cations contribute to the narrow band gaps of the semiconductors, which can efficiently restrain the recombination of photogenerated electron–hole pairs, resulting in a good photocatalytic efficiency and stability under visible-light irradiation. It is expected that the work will shed useful insights in exploitation of novel halometallates with narrow energy gaps by employing large conjugated organic templates.

S⋯N Conformational Lock Acceptor Based on Indacenodithiophene (IDT) Structure and High Electronegative Terminal End Group.

High-performance organic semiconductors should have good spectral absorption, a narrow energy gap, excellent thermal stability and good blend film morphology to obtain high-performance organic photovoltaics (OPVs). Therefore, we synthesized two IDTz-based electron acceptors in this research. When they were blended with donor PTB7-Th to prepare OPV devices, the PTB7-Th:IDTz-BARO-based binary OPVs exhibited a power conversion efficiency (PCE) of 0.37%, with a short-circuit current density (Jsc) of 1.24 mA cm−2, a fill factor (FF) of 33.99% and an open-circuit voltage (Voc) of 0.87 V. The PTB7-Th:IDTz-BARS-based binary OPVs exhibited PCE of 4.39%, with Jsc of 8.09 mA cm−2, FF of 54.13% and Voc of 1.00 V. The results show the strong electronegativity terminal group to be beneficial to the construction of high-performance OPV devices. Highlights: (1) Two new acceptors based on 5,5′-(4,4,9,9-tetrakis (4-hexylphenyl)-4,9-dihydro-s-indaceno [1,2-b:5,6-b′] dithiophene-2,7-diyl) dithiazole (IDTz) and different end groups (BARS, BARO) were synthesized; (2) BARS and BARO are electron-rich end groups, and the electron acceptors involved in the construction show excellent photoelectric properties. They can properly match the donor PTB7-Th, and show the appropriate surface morphology of the active layer in this work; (3) Compared with IDTz-BARO, IDTz-BARS has deeper LUMO and HOMO energy levels. In combination with PTB7-Th, it shows 4.39% device efficiency, 8.09 mA cm−2 short-circuit current density and 1.00 V open circuit voltage.

Theoretical approach to modeling the early nonadiabatic events of ESIPT originating from three-state conical intersection in quinophthalone.

We explore the excited-state intramolecular proton transfer process of quinophthalone theoretically. This molecule possesses three low-lying singlet excited states ([Formula: see text] and [Formula: see text]) in a narrow energy gap of less than the N-H stretching frequency. Dynamics simulations show nonadiabatic wavepacket transfer to [Formula: see text] and [Formula: see text] upon initiating the wavepacket on [Formula: see text]. Multiple accessible conical intersections that lie in the Franck-Condon region facilitate the nonadiabatic wavepacket transfer. Nuclear densities associated with the proton transfer promoting vibrations would start accumulating on [Formula: see text] and [Formula: see text] within a few tens of femtoseconds, validating the involvement of these vibrations in the nonadiabatic events that occur before the proton transfer process. Our findings emphasize the necessity of refined kinetic models for assigning the time constants of ultrafast transient spectroscopy measurements due to the simultaneous evolution of nonadiabatic events and proton transfer kinetics in quinophthalone.

Unveiling a Quinoidal 2,3:10,11-Dibenzoheptazethrene

Parent 2,3:10,11-dibenzoheptazethrene is a singlet diradicaloid polycyclic hydrocarbon in the ground state that did not change its diradical character upon substitution (methyl and triisopropylsilylethynyl). Described herein are the synthesis and characterization of an ethoxy/3,5-(CF3)2C6H3-substituted 2,3:10,11-dibenzoheptazethrene 3 that prefers to retain its p-quinoidal core and shows zero diradical character, as determined by single-crystal analysis and density functional theory calculations. Negative solvatochromism, π-π interactions, Csp2-H···O hydrogen bonding, intramolecular charge transfer, redox amphotericity, and a narrow HOMO-LUMO energy gap make 3 a potential candidate for application in optoelectronics.

Benzo-Extended Cyclohepta[def]fluorene Derivatives with Very Low-Lying Triplet States.

Open‐shell non‐alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene‐based diradicaloids (1–3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (E g opt=0.52–0.69 eV) and persistent stability under ambient conditions (t 1/2=11.7–33.3 h). More importantly, this new type of diradicaloids possess a low‐lying triplet state with an extremely small singlet–triplet energy gap, as low as 0.002 kcal mol−1, with a clear dependence on the molecular size. This family of compounds thus offers a new route to create non‐alternant open‐shell PHs with high‐spin ground states, and opens up novel possibilities and insights into understanding the structure–property relationships.

Benzo‐Extended Cyclohepta[def]fluorene Derivatives with Very Low‐Lying Triplet States

AbstractOpen‐shell non‐alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene‐based diradicaloids (1–3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (Egopt=0.52–0.69 eV) and persistent stability under ambient conditions (t1/2=11.7–33.3 h). More importantly, this new type of diradicaloids possess a low‐lying triplet state with an extremely small singlet–triplet energy gap, as low as 0.002 kcal mol−1, with a clear dependence on the molecular size. This family of compounds thus offers a new route to create non‐alternant open‐shell PHs with high‐spin ground states, and opens up novel possibilities and insights into understanding the structure–property relationships.

Revealing the vibronic coupling effects in the size-dependent optical properties of cycloparaphenylene dications

We explain the visible and near-infrared optical properties of low-lying singlet excited states (S and S) of [5]-, [7]- and [9]cycloparaphenylene dications (CPPs). A narrow energy gap (<150 cm) separates these states at the Franck–Condon point of the individual molecule. The energy gap increases upon increasing the molecular size. Consequently, [5]CPP possesses highly overlapping S and S absorption bands. The overlap between these bands decreases and the resulting composite spectrum appears broader, particularly for [9]CPP. Theoretical spectra generated using quantum wavepacket calculations within the linear vibronic coupling approach match well with the experiment. Our findings reveal near-degenerate S and S minima, highlighting the relevance of both these states while assigning experimental fluorescence emission properties of CPPs.

Half-metallic Characteristics of the Novel Half Heusler Alloys XCrSb (X=Ti, Zr, Hf )

Ab-initio calculations are performed to examine the structural, mechanical, electronic, magnetic and thermodynamic properties of the half-Heusler ternary alloys XCrSb (X = H f , Ti, Zr). In this study, the spin-polarized density functional theory (DFT) method that is spin-polarized with generalised gradient approximation (GGA) are used to perform ab-initio calculations to investigate the physical properties of a novel half-Heusler ternary alloys XCrSb (X = H f , Ti, Zr). It was confirmed that the alloys are stable mechanically and exhibit ferromagnetic states (FM). The study reveals that the alloys portray half-metallic character with narrow energy gaps. And it also shows that they have a total magnetic moment of approximately 3ub. From the formation energy calculation, it shows that the alloys can be synthesized experimentally. Also, it was observed that they are mechanically stable. The heat capacities and Debye temperatures were also computed and they show high thermodynamic stability.

Electrochromism of Nanographenes in the Near-Infrared Region.

Nanographene (NG) is a potential candidate for organic EC materials because of its large π-conjugated system, chemical stability, absorption band covering the visible region, and tunable optical properties by postsynthetic modification. We show that NGs carrying redox-active triphenylamine (TPA) units covalently linked to the NG edge function as EC materials in the NIR region. The hybrid materials can be obtained by the installation of TPA units onto the NG edge and display changes in the absorption spectrum in the NIR region extending to a wavelength of over 2000 nm upon one-electron oxidation and reduction at low potentials (<1.1 V). Time-dependent unrestricted density functional theory calculation of a model NG at the UB3LYP/6-31G(d,p) level of theory suggests that a narrow energy gap between the basal plane and the oxidized TPA unit is responsible for the observed EC function in the NIR region.