Electronic Charge Carriers Research Articles

Eliciting the contribution of TiN to photoelectrochemical performance enhancement of Imma-LaTiO2N at neutral pH

The presence of defects, which act as the recombination hubs for photogenerated charge carriers, hinders the improvement of photocatalytic activity for oxygen evolution reaction of LaTiO2N under visible light irradiation via a four-electron-transfer reaction pathway. Here, we involve titanium nitride (TiN) in a varying content (0–17.8%) to improve the efficiency of charge separation and transport, influencing the photoelectrochemical performance of LaTiO2N. The characterization results confirm the formation of a strong contact between orthorhombic-LaTiO2N and cubic-TiN particles. The photoelectrochemical (PEC) measurements reveal the inverse dependence between electron transfer phenomena and charge carrier recombination, which allows to understand the trend when modifying LaTiO2N with TiN. In fact, the incorporation of 17.8% TiN in the LaTiO2N:TiN material results in a higher photocurrent. Open-circuit potential (OCP) decay and transient absorption spectroscopy (TAS) studies confirm longer lifetimes of charge carriers for increasing amounts of TiN in the synthesized materials. Thus, the main role of TiN is to improve the properties of the semiconductor-electrolyte interface, having verified its impact on the separation and transport of photogenerated charge carriers. Furthermore, computational studies predict that the adsorption of water molecules is favored at the LaTiO2N:TiN surface compared to the individual TiN and LaTiO2N surfaces.

Engineering oxygen into ultrathin graphitic carbon nitride: synergistic improvement of electron reduction and charge carrier dynamics for efficient photocatalysis

The fast separation rate of photogenerated carriers and the high utilization of sunlight are still a major challenge that restricts the practical application of carbon nitride (g-C3N4) materials in the field of photocatalytic hydrogen (H2) evolution. Here, ultrathin oxygen (O) engineered g-C3N4 (named UOCN) was successfully obtained by a facial gaseous template sacrificial agent-induced bottom-up strategy. The synergy of O doping and exfoliating bulk into an ultrathin structure is reported to simultaneously achieve high-efficiency separation of photogenerated carriers, enhance the utilization of sunlight, and improve the reduction ability of electrons to promote photocatalytic H2 evolution of UOCN. As a proof of concept, UOCN affords enhanced photocatalytic H2 evolution (93.78 μmol h−1) under visible light illumination, which was significantly better than that of bulk carbon nitride (named CN) with the value of 9.23 μmol h−1. Furthermore, the H2 evolution rate of UOCN at a longer wavelength (λ = 450 nm) was up to 3.92 μmol h−1 due to its extended light absorption range. This work presents a practicable strategy of coupling O dopants with ultrathin structures about g-C3N4 to achieve efficient photocatalytic H2 evolution. This integrated engineering strategy can develop a unique example for the rational design of innovative photocatalysts for energy innovation.

Investigation on transport property of heterostructure in Bi/Bi2Sr2CaCu2O8+δ composite film

The heterostructure of bismuth (Bi) and Bi2Sr2CaCu2O8+δ (Bi2212) is considered as a potential type of quantum material. The strain-engineering controls the band structure in the composite films, which is applied in the high-temperature quantum spin Hall materials. In this work, Bi/Bi2212 composite film is synthesized using a two-step method to reduce the fabrication cost. The microstructure and transport properties of the composite films are characterized and analyzed. Experimental results demonstrate that the Bi/Bi2212 composite film obtains the high-quality crystallinity and excellent connectivity. The transport property measurement results show that the band structure changes due to the coupled effects of the strain and electric field. The origin is the compensation between the electron and hole charge carriers. The above band gap change leads to a shift in the metal-semiconductor transition temperature. Functionally coupled composite films exhibit new properties in magnetoresistance devices.

Maximizing the electronic charge carriers in donor-doped hematite under oxygen-rich conditions via doping and co-doping strategies revealed by density functional theory calculations

The low electronic conductivity of hematite (α-Fe2O3) limits its best performance in many applications. Though highly reducing conditions induce an intrinsic n-type behavior, reaching extremely low oxygen partial pressure (pO2) values is not practical. Alternatively, certain dopants provide hematite with excess electrons at practical pO2 values. This study employs density functional theory with thermodynamic analysis to compute the concentration of electronic defects in hematite as a function of pO2, upon doping with 1% of 3d, 4d, and 5d transition metals. Isothermal Kröger–Vink diagrams at 1100 K are plotted to reveal the charge compensation mechanism controlling the electronic carriers in doped hematite and the maximum attainable pO2 value, which achieves approximately one electron per dopant. A higher pO2 value is a metric for an effective donor. Ti, Zr, Hf, Nb, Ta, Mo, and W are shown to be effective donors, especially Nb, Ta, and W, which achieve a 1:1 electron/dopant ratio around atmospheric pressure and a maximum electron/dopant ratio greater than one. The latter is a new metric introduced in this study to quantify the doping efficacy of a donor. Moreover, our study shows that W, Ta, and Nb co-doping in specific percentages with any of the other investigated dopants ensures the n-type behavior of the co-doped hematite while opening the possibility of improving other properties via the other dopant. The other dopant can be Ni or Co to enhance the surface catalytic properties or Zn to increase the minority hole carriers. Both properties are desirable in applications such as photoelectrochemical cells.

Synergistic tailoring of band structure and charge carrier extraction in “green” core/shell quantum dots for highly efficient solar energy conversion

Environment-friendly colloidal core/shell quantum dots (QDs) with controllable optoelectronic characteristics are promising building blocks for future commercial solar technologies. Herein, we synergistically tailor the electronic band structure and charge carrier extraction of eco-friendly AgInS2 (AIS)/ZnS core/shell QDs via Mn-alloying and Cu-doping in the core and shell, respectively. It is demonstrated that the Mn-alloying in AIS core can broaden the band gap to facilitate delocalization of photogenerated electrons into the shell and further incorporation of Cu in the ZnS shell enables the creation of Cu-related states that capture the photogenerated holes from core, thus leading to charge carrier recombination and accelerated transfer of photogenerated electrons in the core/shell QDs. As-prepared Mn-AIS/ZnS@Cu QDs were assembled as light harvesters in a photoelectrochemical (PEC) device for light-driven hydrogen evolution, delivering a maximum photocurrent density of ∼ 6.4 mA cm−2 with superior device stability under standard one sun irradiation (AM 1.5G, 100 mW cm−2). Our findings highlight that simultaneously engineering the band alignment and charge carrier dynamics of “green” core/shell QDs endow the feasibility to design future high-efficiency and durable solar hydrogen production systems.

Influence of low-energy magnons on magnon Hanle experiments in easy-plane antiferromagnets

Antiferromagnetic materials host pairs of spin-up and spin-down magnons which can be described in terms of a magnonic pseudospin. The close analogy between this magnonic pseudospin systems and that of electronic charge carriers led to the prediction of fascinating phenomena in antiferromagnets. Recently, the associated dynamics of antiferromagnetic pseudospin has been experimentally demonstrated and, in particular, the first observation of the magnon Hanle effect has been reported. We here expand the magnonic spin transport description by explicitly taking into account contributions of finite-spin low-energy magnons. In our experiments we realize the spin injection and detection process by two Platinum strips and investigate the influence of the Pt-strips on the generation and diffusive transport of magnons in films of the antiferromagnetic insulator hematite. For both a 15 nm and a 100 nm thick film, we find a distinct signal caused by the magnon Hanle effect. However, the magnonic spin signal exhibits clear differences in both films. In contrast to the thin film, for the thicker one, we observe an oscillating behavior in the high magnetic field range as well as an additional offset signal in the low magnetic field regime. We attribute this offset signal to the presence of finite-spin low-energy magnons.

An unreported peak of the electroluminescence turn-on transience from OLEDs with electron or hole potential well

An unreported peak was discovered at the transient electroluminescence (TEL) rising edge from pure emission layer (EML) based OLEDs with an energy potential well for electron or hole charge carriers. A series of control experiments indicated that the formation mechanism of this peak is completely different from the already-reported peak originated from exciton interactions such as singlet-triplet and triplet-triplet annihilations. Herein, the peak at the TEL rising edge is generated by the radiative recombination of injected carriers with pre-stored charges in the energy potential well. Surprisingly, for the pure EML-based OLEDs with simultaneous electron and hole potential wells, the peak at the TEL rising edge disappeared, and contrarily, a super-high spike was observed at the TEL falling edge. Such an already-reported spike at the TEL falling edge was considered to only appear in doped-EML systems. This contradiction leads to the reconsideration for the formation mechanisms of the spike at the TEL falling edge. Accordingly, this work not only deepens the physical understanding of the unprecedented peak at the TEL rising edge and the already-reported spike at the TEL falling edge, but also reveals the effects of the energy potential well on the carrier recombination process in OLEDs.

Valence Band of Rutile TiO2(110) Investigated by Polarized-Light-Based Angle-Resolved Photoelectron Spectroscopy.

Band structure dictates optical and electronic properties of solids and eventually the efficiency of the semiconductor based solar conversion. Compared to numerous theoretical calculations, the experimentally measured band structure of rutile TiO2, a prototypical photocatalytic material, is rare. In this work, the valence band structure of rutile TiO2(110) is measured by angle-resolved photoelectron spectroscopy using polarized extreme ultraviolet light. The effective mass of the hole, which has never been measured before, is determined to be 4.66-6.87 m0 (free electron mass) and anisotropic. The dependence of photoemission intensities on excitation light polarization is analyzed by taking into account of the parity symmetry of molecular orbitals in the blocking unit of rutile TiO2. This work reports a direct measurement of valence band structure and hole effective mass of rutile TiO2(110), which will deepen our understanding of the electronic structure and charge carrier properties of the model material and provide reference data for future theoretical calculations.

Molecular Insight into Efficient Charge Generation in Low-Driving-Force Nonfullerene Organic Solar Cells.

ConspectusFor organic solar cells (OSCs), charge generation at the donor/acceptor interfaces is regarded as a two-step process: driven by the interfacial energy offsets, the excitons produced by light absorption are first dissociated into the charge-transfer (CT) states, and then the CT states are further separated into free charge carriers of holes and electrons by overcoming their Coulomb attraction. Meanwhile, the CT states can recombine through radiative and nonradiative decay. Owing to the emergence of narrow-band-gap A-D-A small-molecule acceptors, nonfullerene (NF) OSCs have developed rapidly in recent years and the power conversion efficiencies (PCEs) surpass 18% now. The great achievement can be attributed to the high-yield charge generation under low exciton dissociation (ED) driving forces, which ensures both high photocurrent and small voltage loss. However, it is traditionally believed that a considerable driving force (e.g., at least 0.3 eV in fullerene-based OSCs) is essential to provide excess energy for the CT states to achieve efficient charge separation (CS). Therefore, a fundamental question open to the community is how the excitons split into free charge carriers so efficiently under low driving forces in the state-of-the-art NF OSCs.In this Account, we summarize our recent theoretical advances on the charge generation mechanisms in the low-driving-force NF OSCs. First, the A-D-A acceptors are found to dock with the D-A copolymer or A-D-A small-molecule donors mainly via local π-π interaction between their electron-withdrawing units, and such interfacial geometries can provide sufficient electronic couplings, thus ensuring fast ED. Second, the polarization energies of holes and electrons are enhanced during CS, which is beneficial to reduce the CS energy barrier and even leads to barrierless CS in the OSCs based on fluorinated A-D-A acceptors. Moreover, the exciton binding energies (Eb) are substantially decreased by the strong polarization of charge carriers for the A-D-A acceptors; especially for the Y6 system with three-dimensional molecular packing structures, the remarkable small Eb can enable direct photogeneration of free charge carriers. Accordingly, the excess energy becomes unnecessary for CS in the state-of-the-art NF OSCs. Third, to simultaneously decrease the driving force and suppress charge recombination via the triplet channel, it is imperative to reduce the singlet-triplet energy difference (ΔEST) of the narrow-band-gap A-D-A acceptors. Importantly, the intermolecular end-group π-π stacking is demonstrated to effectively decrease the ΔEST while keeping strong light absorption. Finally, hybridization of the CT states with local excitation can be induced by small interfacial energy offset. Such hybridization will result in direct population of thermalized CT states upon light absorption and a significant increase of luminescence quantum efficiency, which is beneficial to concurrently promote CS and reduce nonradiative voltage loss. We hope this Account contributes to the molecular understanding of the mechanisms of efficient charge generation with low driving forces and would be helpful for further improving the performance of organic photovoltaics in the future.

Polaronic Conductivity in Cr2Ge2Te6 Single Crystals

AbstractIntrinsic 2D ferromagnetic semiconductors are an important class of materials for spin‐charge conversion applications. Cr2Ge2Te6 retains long‐range magnetic order in the bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X‐ray scattering, electronic, thermal transport and first‐principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr2Ge2Te6 are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr2Ge2Te6—based devices by electron–phonon‐ and spin–orbit coupling‐based tailoring of polaron properties.

Simulation of the Electrical and Thermal Properties of a Graphene Field Effect Transistor

In this paper, the electrical and thermal properties of Graphene field effect transistor (GFET) has been simulated by varying the width of graphene channel. Here, the electrical characteristics, like electron density, hole density, I-V Characteristics and charge carrier velocity profile in the channel region has been studied for three different values of graphene channel width: 1 nm, 2 nm and 5 nm. To analyse the thermal properties of the GFET device, the temperature profile of the graphene channel has been simulated for 100, 300 and 500K. After analysing the simulation of this characteristics, it is concluded that, both electrical and thermal properties of GFET can be improved by fabricating the channel with larger width in the GFET device.

Electrochemical Behavior of Symmetric Electrical Double-Layer Capacitors and Pseudocapacitors and Identification of Transport Anomalies in the Interconnected Ionic and Electronic Phases Using the Impedance Technique.

A double-channel transmission line impedance model was applied to the study of supercapacitors to investigate the charge transport characteristics in the ionic and electronic conductors forming the electrode/solution interface. The macro homogeneous description of two closely mixed phases (Paasch–Micka–Gersdorf model) was applied to study the influence of disordered materials on the charge transport anomalies during the interfacial charge–discharge process. Different ex situ techniques were used to characterize the electrode materials used in electrical double-layer (EDLC) and pseudocapacitor (PC) devices. Two time constants in the impedance model were adequate to represent the charge transport in the different phases. The interfacial impedance considering frequency dispersion and blocked charge transfer conditions adequately described the charge storage at the interface. Deviations from the normal (Fickian) transport involving the ionic and electronic charge carriers were identified by the dispersive parameters (e.g., n and s exponents) used in the impedance model. The ionic and electronic transports were affected when the carbon-based electrical double-layer capacitor was converted into a composite with strong pseudocapacitive characteristics after the decoration process using NiO. The overall capacitance increased from 2.62 F g−1 to 536 F g−1 after the decoration. For the first time, the charge transport anomalies were unequivocally identified in porous materials used in supercapacitors with the impedance technique.

Effect of Ultrafast Broadband Nonlinear Optical Responses by Doping Silver into Ti3C2 Nanosheets at Visible Spectra

Ti3C2 nanosheet is a newly discovered two-dimensional (2D) clan. It turns out to have encouraging applications for electromagnetic shielding and energy storage. Here, Ag@ Ti3C2 hybrids are precisely synthesized by using the one-step solution processing method. Also, their ultrafast broadband nonlinear optical responses in the visible region are studied systematically through nanosecond open-aperture Z-scan and transient absorption techniques. The mechanism of two-photon absorption (TPA) is disclosed in the visible region (409–532 nm). When the laser energy is low and the wavelength is longer than 400 nm, nonlinear absorption cannot happen. Meanwhile, as the laser energy increases, two photons will be absorbed by the electrons in the valence band and the electrons will jump to the conduction band. The process is named as two-photon absorption which will make the specimen show reverse saturable absorption (RSA) properties. What is more, the ultrafast carrier dynamics of the specimen are studied by using the transient absorption. The result shows that the decay contains two phases: the fast and then the slow one. The two phases first come from electron–phonon and then from phonon–phonon interactions, respectively. The electron transfer and charge carrier trapping processes are further verified by the outcomes of similar measurements on Ag@ Ti3C2 hybrids. Besides, the two decay processes increase together with the pump fluence. These results show that Ti3C2 nanosheet has potential applications in broadband optical limiter.

Engineering Oxygen into Ultrathin Graphitic Carbon Nitride: Synergistic Improvement of Electron Reduction and Charge Carrier Dynamics for Efficient Photocatalysis

Engineering Oxygen into Ultrathin Graphitic Carbon Nitride: Synergistic Improvement of Electron Reduction and Charge Carrier Dynamics for Efficient Photocatalysis

Synthesis and characterisation of heteroatom-doped reduced graphene oxide/bismuth oxide nanocomposites and their application as photoanodes in DSSCs.

Semiconductor materials have been recently employed in photovoltaic devices, particularly dye-sensitized solar cells (DSSCs), to solve numerous global issues, especially the current energy crisis emanating from the depletion and hazardous nature of conventional energy sources, such as fossil fuels and nuclear energy. However, progress for the past years has been mainly limited by poor electron injection and charge carrier recombination experienced by DSSCs at the photoanode. Thus, novel semiconductor materials such as bismuth oxide (Bi2O3) have been investigated as an alternative photoanode material. In this study, Bi2O3 was integrated with nitrogen- or boron-doped reduced graphene oxide (N-rGO or B-rGO, respectively) via a hydrothermal approach at a temperature of 200 °C. Various instrumental techniques were used to investigate the morphology, phase structure, thermal stability, and surface area of the resulting nanocomposites. The incorporation of N-rGO or B-rGO into Bi2O3 influenced the morphology and structure of the nanocomposite, thereby affecting the conductivity and electrochemical properties of the nanocomposite. B-rGO/Bi2O3 exhibited a relatively large surface area (65.5 m2 g−1), lower charge transfer resistance (108.4 Ω), higher charge carrier mobility (0.368 cm2 V−1 s−1), and higher electrical conductivity (6.31 S cm−1) than N-rGO/Bi2O3. This led to the fabrication of B-rGO/Bi2O3 photoanode-based DSSCs with superior photovoltaic performance, as revealed by their relatively high power conversion efficiency (PCE) of 2.97%, which outperformed the devices based on N-rGO/Bi2O3, rGO/Bi2O3, and Bi2O3 photoanodes. Therefore, these results demonstrate the promising potential of heteroatom-doped rGO/Bi2O3-based nanocomposites as photoanode materials of choice for future DSSCs.

Electronic transport properties and quantum localization effects monitored by selective functionalization in Bernal bilayer graphene

Monitoring electronic properties of 2D materials is an essential step to open a way for applications such as electronic devices and sensors. From this perspective, Bernal bilayer graphene (BLG) is a fairly simple system that offers great possibilities for tuning electronic gap and charge carriers' mobility by selective functionalization (adsorptions of atoms or molecules). Here, we present a detailed numerical study of BLG electronic properties when two types of adsorption site are present simultaneously. We focus on realistic cases that could be realized experimentally with adsorbate concentration c varying from 0.25% to 5%. For a given value of c, when the electronic doping is lower than c we show that quantum effects, which are ignored in usual semi-classical calculations, strongly affect the electronic structure and the transport properties. A wide range of behaviors is indeed found, such as gap opening, metallic behavior or abnormal conductivity, which depend on the adsorbate positions, the c value, the doping, and eventually the coupling between midgap states which can create a midgap band. These behaviors are understood by simple arguments based on the fact that BLG lattice is bipartite. We also analyze the conductivity at low temperature, where multiple scattering effects cannot be ignored. Moreover, when the Fermi energy lies in the band of midgap states, the average velocity of charge carriers cancels but conduction is still possible thanks to quantum fluctuations of the velocity.

Facile, Low‐Cost and Flexible Ammonia Sensor Arrays Based on Metallic Ion Charge Carriers and Polymer Matrices

AbstractWith the development of industrial and medical technology, chemical sensors based on electron charge carriers are difficult to meet the upping requirements on excellent sensitivity, good selectivity, and portability. Herein, a new type of sensing material is developed via facile preparation method. Metal ions, instead of electrons, are used as charge carriers and distributed in the polymer matrix. As a representative, the ionic conductive gas sensor based on polyacrylic acid and copper ions (PAA‐Cu(II)) is fabricated. The PAA‐Cu(II) sensor exhibits excellent linear response to ammonia (NH3), together with high sensitivity (50.3% for 50 ppm NH3), good selectivity, and fast response/recovery time (10/10.5 s). The sensor also displays high transparency, long‐term air stability, and excellent flexibility. In addition, sensor arrays based on various metal ions (Cu2+, Co2+, Ni2+, Ag+, Fe3+, Mg2+) and polymers (polyacrylic acid, polyvinyl alcohol, polyethyleneimine) are also developed, and the sensing properties are investigated. The results suggest that metal ions‐based capacitive flexible sensors have great potential for low‐cost chemical sensors and provide a promising strategy for developing new sensing materials.

Temperature-dependent structural fluctuation and its effect on the electronic structure and charge transport in hybrid perovskite CH3 NH3 PbI3.

The recently discovered hybrid organic-inorganic perovskites have been suggested for high-performance optoelectronic applications. Owing to the mechanical flexibility of these compounds, they demonstrate structural fluctuation at finite temperatures that have been widely discussed with respect to their optical properties. However, the effect of temperature-induced structural fluctuation is not clear until now, with respect to the equally important charge transport properties. In the present study, through ab initio molecular dynamics simulations of cubic-phase CH3 NH3 PbI3 at different temperatures, the temperature-dependent electronic structure and charge carrier transport properties are examined. Compared with the significant structural fluctuation of organic cations, the structural change of the inorganic framework is minor. In addition, because the band edge states at R point are mainly influenced by the anti-bonding character of the Pb-I bond, CH3 NH3 PbI3 demonstrates relatively small deformation potentials as well as low temperature dependence of band gaps (ΔEg ≈ 50 meV from 330 K to 400 K) and electron-phonon coupling strengths, despite the large structural fluctuation of organic cations. Furthermore, the effective mass of the valence band increases with the increase of temperature. The predicted mobilities of CH3 NH3 PbI3 can reach above 75 cm2 V-1 s-1 near room temperature, exhibiting an appropriate optoelectronic potential, while the temperature dependence is steeper than T-1.5 of the traditional semiconductors because of the enhanced effective masses.

An intensity modulated photocurrent spectroscopy study of the role of titanium in thick hematite photoanodes

In this Letter, the role of Ti addition in thick hematite mesoporous photoanodes was elucidated by performing intensity modulated photocurrent spectroscopy (IMPS) monitoring its charge carrier dynamics during water oxidation. Interface engineering associated with doping of hematite is crucial to develop highly efficient thick photoanodes. Photoelectrochemical data recorded under front- and back-side illumination show that Ti insertion mitigates the collection deficit faced by hematite due to an energy barrier decrease between the grains and a change in the surface chemistry. IMPS reveals that Ti clearly influences the hematite film performance by increasing the charge separation efficiency due to its segregation at the hematite interface. However, Ti insertion does not enhance the kinetics of water oxidation at the solid–liquid interface. These facts indicate that Ti mainly affects the hematite electronic properties instead of accelerating the surface processes. This comprehensive understanding of the electronic transport and charge carrier dynamics in Ti:hematite photoanodes enables the control and design of better interfaces for more efficient photoelectrochemical water splitting.

Oxygen vacancy-enriched Co3O4 as lithiophilic medium for ultra-stable anode of lithium metal batteries

Lithium metal as a promising anode material has been widely applied in many advanced high-energy density batteries. However, uncontrollable Li dendrite formation and infinite volume expansion in lithium anode result in short lifespan and severe safety hazards. In this contribution, we report that utilization of oxygen vacancy-enriched Co3O4 nanowire arrays as lithiophilic material is an effective strategy to obtain ultra-long stable lithium metal anode. The oxygen vacancy can serve as electronic charge carrier to facilitate electron transfer. Moreover, it can induce the generation of local electric field to accelerate lithium ions migration and enable uniform Li+ distribution, which are beneficial for homogeneous lithium nucleation. Combining the excellent ionic/electronic diffusion and the volume buffer offered by the nanowire arrays, the obtained electrode shows high coulombic efficiency of 98.5% at 1 mA cm−2 for 230 cycles.