Electron Injection Mechanism Research Articles

Distinguishing Inner and Outer-Sphere Hot Electron Transfer in Au/p-GaN Photocathodes.

Exploring nonequilibrium hot carriers from plasmonic metal nanostructures is a dynamic field in optoelectronics, with applications including photochemical reactions for solar fuel generation. The hot carrier injection mechanism and the reaction rate are highly impacted by the metal/molecule interaction. However, determining the primary type of reaction and thus the injection mechanism of hot carriers has remained elusive. In this work, we reveal an electron injection mechanism deviating from a purely outer-sphere process for the reduction of ferricyanide redox molecule in a gold/p-type gallium nitride (Au/p-GaN) photocathode system. Combining our experimental approach with ab initio simulations, we discovered that an efficient inner-sphere transfer of low-energy electrons leads to an enhancement in the photocathode device performance in the interband regime. These findings provide important mechanistic insights, showing our methodology as a powerful tool for analyzing and engineering hot-carrier-driven processes in plasmonic photocatalytic systems and optoelectronic devices.

DFT/TD-DFT study of novel triphenylamine-based dyes with azo moieties and π-spacer variations for enhanced dye-sensitized solar cell performance

This study involves a computational analysis of new D-π-A dyes obtained from triphenylamine (TPA), which contain various azo-dye components. The structural, molecular, electrical, and optical properties of these dyes were computed using Density Functional Theory (DFT) and Time-Dependent DFT, utilizing the B3LYP/6–31 G model. Our research specifically aimed to investigate the effects of incorporating different azo dye constituents in the para position of two phenyl groups of TPA. The results indicate that these alterations lead to notably broadened and red-shifted absorption spectra, as well as improved optoelectronic properties that are subject to additional tuning through the manipulation of the π-spacer. The excitation energies and HOMO-LUMO energy levels that have been estimated indicate the presence of effective electron injection and dye regeneration mechanisms. The results concerning the nonlinear optical (NLO) properties suggest that these dyes are likely to demonstrate superior performance in NLO applications. The factors encompassed in this study consist of light-harvesting efficiency (LHE), open-circuit photovoltage (VOC), electron injection driving force (ΔGinj), dye regeneration driving force (ΔGreg), excited state lifetime (τ) and reorganization energy (λtotal), which has a strong correlation with the electrical current density in a short-circuit (JSC) and DSSC's overall effectiveness. This scientific attempt contributes to the systematic advancement of efficient dyes, demonstrating the possibility for enhanced efficiency in DSSCs. Further validation of computational forecasts and advancement of renewable energy technology necessitate future experimental synthesis and testing.

Evidence‐Based Understanding of Lateral Hole Transport During OFF‐State Stress Completing Dynamic GaN‐on‐Si Buffer Charging Model

Gallium nitride (GaN)‐on‐Si high electron mobility transistors require insulating GaN buffers, which are prone to charge trapping and result in dynamic ON‐state resistance (dR DS,on) that negatively impacts performance and reliability. Herein, simultaneous measurements of threshold voltage shift (dV TH) and dR DS,on during OFF‐state stress with microsecond time resolution are employed. Ohmic p‐GaN gate contacts enable the use of dV TH to probe charge accumulation under the gate, while dR DS,on probes charge accumulation in the gate‐drain access region. Comparison of dV TH and dR DS,on provides direct evidence of lateral hole transport in the GaN buffer when exposed to a lateral electric field in OFF‐state. This lateral hole transport causes positive charge accumulation in the buffer under the gate and triggers a newly proposed electron injection mechanism into the same region. Only by considering the combination of lateral hole transport and electron injection under the gate the observed up to fivefold dR DS,on increase in OFF‐state stress compared to back‐gating at low biases can be explained. Furthermore, another electron spillover mechanism is introduced that occurs for large positive charge accumulation under the gate and limits the maximum negative dV TH. All known and newly introduced processes during OFF‐state are summarized in a concise dynamic buffer charging model.

355 nm Nanosecond Ultraviolet Pulsed Laser Annealing Effects on Amorphous In-Ga-ZnO Thin Film Transistors.

Bottom-gate thin-film transistors (TFTs) with n-type amorphous indium-gallium-zinc oxide (a-IGZO) active channels and indium-tin oxide (ITO) source/drain electrodes were fabricated. Then, an ultraviolet (UV) nanosecond pulsed laser with a wavelength of 355 nm was scanned to locally anneal the active channel at various laser powers. After laser annealing, negative shifts in the threshold voltages and enhanced on-currents were observed at laser powers ranging from 54 to 120 mW. The energy band gap and work function of a-IGZO extracted from the transmittance and ultraviolet photoelectron spectroscopy (UPS) measurement data confirm that different energy band structures for the ITO electrode/a-IGZO channel were established depending on the laser annealing conditions. Based on these observations, the electron injection mechanism from ITO electrodes to a-IGZO channels was analyzed. The results show that the selective laser annealing process can improve the electrical performance of the a-IGZO TFTs without any thermal damage to the substrate.

Rationalization of the light-induced electron injection mechanism in a model 1D ZnO nanowire-dye complex: insights from real-time TD-DFTB simulations.

Zinc oxide nanowires (ZnO NWs) possess a unique one-dimensional (1D) morphology that offers a direct pathway for charge transport. In this article, we present the first application of the real-time time-dependent density functional tight-binding (real-time TD-DFTB) method for a model hybrid system consisting of a catechol molecule adsorbed on a ZnO nanowire. The rationalization of the photoinduced electron injection to the 1D nanostructure is attained through quantum dynamics simulations, stressing the role of charge transfer in the new optical transitions upon dye adsorption. We provide a momentum-resolved picture of the photoexcitation dynamics, highlighting the charge accumulation in certain k-points, which could improve our understanding of these ultrafast processes. Finally, in the context of dye-sensitized solar cells (DSSCs) based on ZnO NW arrays, we provide a method to calculate the photoresponse obtaining similar results to experiments. This work paves the way towards the fast and accurate theoretical design of 1D optoelectronic nanodevices.

An artificial dead-layer to protect the ferroelectric thin films from electron injection

For ferroelectric (FE) thin films, working in the harsh environment of a high electric field (E) or high temperature (T) remains a great challenge. As a post-treatment approach, dead-layer engineering exhibits a certain generality and could improve the dielectric strength (Eb) via depositing a specially designed artificial “dead-layer” on most of the as-prepared FE thin films. However, physical essence of the artificial dead-layer needs further analysis. Great challenges are the abundant and complicated conduction mechanisms in the FE-based thin films and the lack of adequate research on modulating these conduction mechanisms by this artificial dead-layer. As a vital part of the conduction mechanisms, electron injection can be easily triggered under E and thermal excitation that almost appeared in all FE-based thin films. Here, the ultrathin artificial dead-layer of Ca0.2Zr0.8O1.8 (CSZ) was used to restrain multiple electron injection in low-Eb LaNiO3/Ba0.58Sr0.42TiO3 (LNO/BST) FE thin films, including thermal emission and tunneling effect, under a high E and T, even under an opposite E. It was found that the suppressing effect on the multiple electron-injection mechanisms via the artificial dead-layer mainly comes from its wall-like energy-barrier structure, which is composed of two opposite and high interface energy barriers (BST/CSZ: 2.95 eV, CSZ/Au: 3.92 eV) and a wide depletion layer. The generality of protecting the ferroelectric thin films from electron injection via the artificial dead-layer was discussed.

Quasi-Periodic Pulsations in an M-Class Solar Flare

We have studied the quasi-periodic pulsations (QPPs) of the M2.3 flare that occurred in the active region NOAA 12172 on 23 September 2014. Through the fast Fourier transform (FFT) method, we decompose the flare light curves into fast- and slowly-varying components, and the cut-off threshold is 100 s. We find that the QPPs have a period of 40 s at soft X-ray (SXR), hard X-ray (HXR), radio and ultraviolet (UV). Based on the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), we find that the QPPs take place at the same time interval as the flare ribbon separation, and that the QPPs seem to originate from the flare ribbons. Our observations tend to support the mechanism of the periodic nonthermal electron injection during the flare eruption.

The role of laser chirp in relativistic electron acceleration using multi-electron gas targets

The role of multi-10 TW chirped laser pulses interacting with N2 gas jet targets, as a test case for multi-electron targets, is experimentally examined. Complementary measurements using He gas jet targets, which are fully ionized well before the laser pulse peak, are also presented for comparison with the measurements for the multi-electron N2 targets. It is found that for both gases positively chirped laser pulses accelerate electrons more efficiently compared to the Fourier transform-limited and negatively chirped pulses. Furthermore, multi-electron targets offer additional electron injection mechanisms for efficient electron acceleration as a function of the chirp, due to the dynamic ionization of inner-shell electrons near the peak of the laser pulse. Finally, we show that the background plasma density value plays a critical role in the efficient acceleration of positively chirped pulses as well as in the tuning of the positive chirp value for maximizing the electron energy. We clearly observe that larger plasma density values require higher positive chirp values for efficient electron acceleration.

Evaluation of darrow red-organosilane composite as a photosensitizer for application in dye-sensitized zinc oxide photocatalysts: DFT and TD-DFT studies.

Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of darrow red covalently attached to 3-glycidyloxypropyltrimethoxysilane (DR-GPTMS) were conducted, and it was shown that DR-GPTMS can be applied as a photosensitizer in dye-sensitized zinc oxide (ZnO) photocatalysts. The frontier molecular orbital (FMO) levels of DR-GPTMS simulated using a conductor-like polarizable continuum model in water were suitable for electron injection from photoexcited DR-GPTMS to ZnO. Additionally, the oxidized DR-GPTMS produced by electron injection can be regenerated using triethanolamine. UV-visible absorption spectra, FMOs, and density of states spectra of DR-GPTMS adsorbed on the Zn2O3 cluster were also investigated to understand the mechanism of electron injection. The results suggest that DR-GPTMS induces good visible-light absorption and efficient electron injection to ZnO. DFT and TD-DFT calculations were performed using the Gaussian 09W package (Gaussian, Inc., Wallingford, CT, USA). The density of states spectra was obtained using GaussSum (3.0.2). The B3LYP/6-31G** level of theory was employed for all calculations except for the UV-visible absorption spectra simulation. In calculations involving ZnO, the LanL2DZ or SDD basis set was assigned to the zinc atoms. TD-DFT calculations were performed at the eight different functionals (B3LYP, PBE0, M06-L, M06, M06-2X, M06-HF, CAM-B3LYP, and ωB97XD) with the 6-31+G** basis set. A conductor-like polarizable continuum model (CPCM) using water was employed determining the solvent effect.

Nonthermal electron acceleration at collisionless quasi-perpendicular shocks

Shock waves propagating in collisionless heliospheric and astrophysical plasmas have been studied extensively over the decades. One prime motivation is to understand the nonthermal particle acceleration at shocks. Although the theory of diffusive shock acceleration (DSA) has long been the standard for cosmic-ray acceleration at shocks, plasma physical understanding of particle acceleration remains elusive. In this review, we discuss nonthermal electron acceleration mechanisms at quasi-perpendicular shocks, for which substantial progress has been made in recent years. The discussion presented in this review is restricted to the following three specific topics: The first is stochastic shock drift acceleration (SSDA), which is a relatively new mechanism for electron injection into DSA. The basic mechanism, related in-situ observations and kinetic simulations results, and how it is connected with DSA will be discussed. Second, we discuss shock surfing acceleration (SSA) at very high Mach number shocks relevant to young supernova remnants (SNRs). While the original proposal under the one-dimensional assumption is unrealistic, SSA has now been proven efficient by a fully three-dimensional kinetic simulation. We discuss the multidimensional nature of SSA and its role in electron injection. Finally, we discuss the current understanding of the magnetized Weibel-dominated shock. It is essentially a magnetized shock in which the reflected-gyrating ions dominate the formation of the shock structure but with a substantial magnetic field amplification by the ion-Weibel instability. Spontaneous magnetic reconnection of self-generated current sheets within the shock structure is an interesting consequence of Weibel-generated strong magnetic turbulence. Although the exact condition for active magnetic reconnection has not been clarified, we argue that high Mach number shocks with both Alfvén and sound Mach numbers exceeding 20–40 will likely behave as a Weibel-dominated shock. Despite a number of interesting recent findings, the relative roles of SSDA, SSA, and magnetic reconnection for electron acceleration at collisionless shocks and how the dominant particle acceleration mechanisms change depending on shock parameters remain to be answered.

Trap state-assisted electron injection in blue quantum dot light-emitting diode

We report trap state-assisted electron injection in a blue quantum dot light-emitting diode (QLED) in this work. By replacing an electron transport layer and a quantum dot emission layer, we identify trap states are indeed on blue quantum dots. We also analyze the equivalent circuit model and the density of trap state distribution by impedance spectroscopy. Furthermore, the trap states induce charge transfer in the blue QLED and lower the device efficiency, suggesting the competition between electron injection and trapping in a working device. Our work shows a distinct electron injection mechanism in blue QLEDs that has not been shown in red and green QLEDs.

Tuning the electron injection mechanism by changing the adsorption mode: the case study of Alizarin on TiO2

Functionalized titanium dioxide (TiO2) nanoparticles (NPs) with intense fluorescent dyes are a promising tool for several technological applications ranging from photochemistry, photocatalysis, photovoltaics, photodynamic therapy, or bioimaging. Here, we present the case study of Alizarin adsorption on TiO2 NPs of different shapes and increasing size up to 2.2 nm (700 atoms), by means of density functional theory calculations. We find that Alizarin can bind in three different ways, depending on the number and the type of bonds between Alizarin and TiO2: ‘tridented’, ‘bidented’, and ‘chelated’. Next, we investigate the optical properties of these systems by time-dependent density functional theory calculations. Based on the absorption spectra and the Kohn–Sham orbitals analysis, we discovered that the mechanism of electron injection depends on the Alizarin binding mode to the TiO2 surface. While for bidented and chelated adsorption modes, a direct charge transfer is observed; for the tridented one, an indirect mechanism governs the charge transfer process following the excitation. Our results are in good agreement with existing experimental data and suggest that by tailoring the shape of the TiO2 NPs and, thus, determining the type of undercoordinated Ti atoms prevalently exposed at the surface, it is possible to control the predominant injection mechanism.

Effect of carboxylic acid and cyanoacrylic acid as anchoring groups on Coumarin 6 dye for dye-sensitized solar cells: DFT and TD-DFT study

Starting with Coumarin-6 dye, two novel D-π-A organic dyes C6X and C6N have been designed by attaching carboxylic acid and cyanoacrylic acid groups as anchoring groups to Coumarn-6 dye, respectively, to understand their potential use in dye-sensitized solar cells (DSSCs). The electronic structure and photophysical and photovoltaic properties of the novel designed dyes were studied using density functional theory DFT and time-dependent density functional theory TD-DFT with the Becke3-Parameter-Lee–Yang–Parr (B3LYP) functional and the 6-31G (d, p) basis set. Optimized structure and electronic properties (highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital (ELUMO), and energy difference (Eg) between HOMO and LUMO) were calculated showing that C6N has the smallest band gap with the larger absorption region. Density of states (DOS), molecular electrostatic potential (MEP), natural bond orbital (NBO) analysis, non-linear optical (NLO) properties, UV–vis spectra, as well as some crucial parameters affecting the photovoltaic performance of DSSCs, such as light-harvesting efficiency (LHE), electron injection driving force (ΔGinject), dye regeneration driving force(ΔGreg), and the excited state life time(τe), were calculated to study the effect of the anchoring group on the DSSC performance. Additionally, the adsorption of C6X and C6N dyes on the TiO2 anatase (101) surface and the mechanism of electron injection were also investigated using a dye–(TiO2)9 cluster model using TD-B3LYP calculation. The calculated adsorption energies of the dyes suggest a strong adsorption of dyes to a TiO2 surface. The results show that C6N may be theoretically a good candidate as sensitizer of DSSC application.

Optoelectronic properties of Ag2S/graphene and FeS2/graphene nanostructures and interfaces: A density functional study including dispersion forces

The optoelectronic properties of several Ag2S/graphene and FeS2/graphene nanostructures are examined through density functional theory calculations including dispersion forces. First, we analyzed the electronic structure of Ag2S and FeS2 nanocluster models, as prototypes of quantum dots, focusing on the electronic structure and absorption spectra. In the case of FeS2 nanoparticle, both ferro- and antiferromagnetic arrangements were considered. Then, Ag2S and FeS2 nanoclusters were adsorbed on top of both graphite and graphene slab models to study the stability of the systems as well as the effect of the nanocluster–surface interaction in the electronic structure, including an insight into the electron injection mechanisms in these systems. Finally, interface models, consisting of Ag2S or FeS2 thin films adsorbed on a graphene layer, were built and their optoelectronic properties examined. Overall, the results obtained in this work, support the suitability of these systems for their usage in solar cells.Graphical abstract

Electron Injection Improvement of n-Type Organic Field-Effect Transistors With Indium Contact Interlayer

The organic field-effect transistors (OFETs) have developed rapidly in recent years. However, compared with the p-type OFETs, the n-type devices always show poorer performance due to their hard electron injection and low stability. It is necessary to further improve the charge injection properties of n-type OFETs. In this work, indium (In) layers, as an interlayer above contacts, were adopted for electron injection improvement in the n-type poly((N, $\text{N}^{\prime }$ -bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl)-alt-5, $5^{\prime }$ -(2, $2^{\prime }$ -bithiophene)) (N2200) OFETs. The injection barrier, such as Schottky barrier, at the metal/semiconductor interface was effectively weakened with the In interlayers thickness ( ${T}_{In}$ ) increasing from 0 to 6 nm. The mechanism of electron injection changed from thermionic emission to tunneling, as the ${T}_{In}$ reaching 6 nm. It results in a marked improvement in electrical properties for N2200 OFETs. The mobility increased from 0.09 to 0.29 cm ${}^{2}\cdot \text{V}^{-1}\cdot \text{s}^{-1}$ while the subthreshold swing decreased from 2.7 to 0.8 $\text{V}\cdot $ dec−1 with the ${T}_{In}$ . This work provides an efficient approach for the electron injection improvement in n-type OFETs.

New Erratic Program Disturbance Mechanism of 2T-SONOS Embedded Nonvolatile Memory

We developed a 32 kB embedded nonvolatile memory (NVM) intellectual property (IP) using 2T-SONOS cells. Although SONOS cells possess intrinsic defect immunity, we discovered abnormal memory cell failure during the probe test of the IP. The major failed items are checkerboard (CKBD), inverse CKBD, and gate-with-source (G-S) disturbance at the erased state. The threshold voltage distributions and electrical failure analysis (FA) reveal that all major failures occurred because of an abnormally weak G-S disturbance immunity. In addition, the temperature dependency of the G-S disturbance implies that the failure mechanism is related to Co-spike rather than midgap trap-induced junction leakage. Although abnormal defects are not detected through physical FA, by using simple failure modeling and a process split test, we verified the root cause of a new type of erratic failure. The single-bit disturbance failure can be explained as electrons generated in the floating junction area being accelerated in the G-S stress mode, resulting in soft programming by the channel hot electron (CHE) injection mechanism.

In silico investigation of catechol-based sensitizers for type II dye sensitized solar cells (DSSCs)

A series of catechol-based sensitizers, Cat-I to Cat-XV, for a possible application in type II DSSCs is presented. The electronic structures and excited state properties of free and bound to (TiO2)9 dyes have been analysed and compared by means of Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) methods. In particular, the effect of introducing electron-donating and electron-withdrawing substituents, as well as the effect of introducing an ethylene spacer as π bridge between the substituents and the catechol unit to increase the conjugation has been investigated. Moreover, key aspects that could strongly influence the direct (type II) electron injection mechanism are analyzed and discussed. The results of the calculations suggest that the introduction of the π bridge contributes to enhance the dye-to-TiO2 charge transfer (DTCT) efficiency regardless of the presence of electron-donating or electron-withdrawing substituents.

Low voltage operating organic light emitting transistors with efficient charge blocking layer

Charge injection/blocking layers play important roles in the performances of organic electronic devices. Their incorporation into organic light emitting transistors has been limitted, due to generally high operating voltages (above 60 V) of these devices. In this work, two hole blocking molecules are integrated into tris-(8-hydroxyquinoline) aluminum (Alq 3 ) based light emitting transistors under operating voltage as low as 5 V. The effects of hole blocking and electron injection are decoupled through the differences in the energy levels. Significantly improved optical performance is achieved with the molecule of suitable energy level for electron injection. Surprisingly, a decreased performance is observed in the case of another hole blocking molecule evidencing that charge injection overweighs charge blocking in this device architecture. • Low Voltage (5 V) operating OLETs are reported. • Hole blocking and electron injection mechanisms are decoupled. • Significantly improved OLET performance is achieved with suitable hole blocking layer.

Water-processable n-type doping of carbon nanotubes via charge transfer with imidazolium chloride salt

Imidazolium chloride salt was demonstrated as a novel water-processable, strong base-free electron dopant for single-walled carbon nanotubes (SWCNTs). By thermoelectric evaluation and various photoelectron spectroscopy analyses associated with the comparison of the redox potentials, the valence change of the anion (Cl-/Cl3-) was presented as a possible mechanism for electron injection into the nanotube conduction band. The obtained n-type SWCNT films presented good thermoelectric power factor exceeding 100 μW m−1 K−2, nearly identical to that of the original p-type film.

Redox-Reactive Field-Effect Transistor Nanodevices for the Direct Monitoring of Small Metabolites in Biofluids toward Implantable Nanosensors Arrays.

Chemically modified field-effect transistor (FET) nanodevices were shown to be a selective and extremely sensitive detection platform. In FET-based sensors, signal amplification and transduction is based on electrostatic gating of the nanometric semiconductor channel by analyte–receptor interactions, which measurably affect the transconductance of the device. However, chemically modified FETs must overcome several fundamental limitations before they can be effectively deployed as real-time sensors for bioevents occurring on their surface in complex biofluids. Here, we demonstrate the development of amperoFET devices for the real-time continuous monitoring of small molecular metabolites in biofluids. The surface of the nanowires is covalently modified with a redox reversible moiety, which is easily oxidized in the presence of H2O2. The reversible redox transformation of the surface-confined molecules is carried out by a hot electron injection mechanism, conducted simply by the modulation of the source–drain current through the nanoFET sensing device. By this approach, electrons may be injected by the nanowire element into the surface-confined redox moiety and thus maintain a whole-electrically actuated redox system in which the oxidation state is completely controlled by the current applied to the amperoFET system. The modulation of the source–drain current allows the control of the reduced versus oxidized redox moieties population on the nanowire surface, and this, in turn, is applied as the main sensing mechanism. At a given constant source–drain and gate voltage, the chemical perturbation exerted by the presence of chemical oxidants in the tested biofluid will lead to a measurable conductance change. Alteration in the concentration of the specific metabolite will chemically regulate the extent of perturbation applied to the redox system, which can be utilized for the quantification of the molecular metabolite of interest. These ‘equilibrium’-type sensors are fully electrically operated and can be further used in implantable sensing applications.