Negative Charge Carriers Research Articles

Modulating the Conduction Band Energies of Si Electrode Interfaces Functionalized with Monolayers of a Bay-Substituted Perylene Bisimide.

The confinement of π-conjugated chromophores on silicon (Si) electrode surfaces is a powerful approach to engineer electroresponsive monolayers relevant to microelectronics, electrocatalysis, and information storage and processing. While common strategies to functionalize Si interfaces exploit molecularly dissolved building blocks, only a handful number of studies have leveraged the structure-function relationships of π-aggregates to tune the electronic structures of hybrid monolayers at Si interfaces. Herein, we show that the semiconducting properties of n-type monolayers constructed on Si electrodes are intimately correlated to the initial aggregation state of π-conjugated chromophore precursors derived from bay-substituted perylene bisimide (PBI) units. Specifically, our study unravels that for n-type monolayers engineered using PBI π-aggregates, the cathodic reduction potentials required to inject negative charge carriers into the conduction bands can be stabilized by 295 mV through reversible switching of the maximum anodic potential (MAP) that is applied during the oxidative cycles (+0.5 or +1.5 V vs Ag/AgCl). This redox-assisted stabilization effect is not observed with n-type monolayers derived from molecularly dissolved PBI cores and monolayers featuring a low surface density of the redox-active probes. These findings unequivocally point to the crucial role played by PBI π-aggregates in modulating the conduction band energies of n-type monolayers where a high MAP of +1.5 V enables the formation of electronic trap states that facilitate electron injection when sweeping back to cathodic potentials. Because the structure-function relationships of PBI π-aggregates are shown to modulate the semiconducting properties of hybrid n-type monolayers constructed at Si interfaces, our results hold promising opportunities to develop redox-switchable monolayers for engineering nonvolatile electronic memory devices.

Micro-Raman characterization of homo-epitaxial n doped GaN layers for vertical device applications

N-doped homo-epitaxial GaN samples grown on freestanding GaN substrates have been investigated by micro-Raman spectroscopy. Quantitative analysis of the E2h and the A1(LO) modes’ behavior has been performed while intentionally increasing the carrier density using silicon doping. We noticed that as the carrier concentration increases up to 1.8 × 1018 cm−3, the E2h mode remains unchanged. On the other hand, when the doping gets higher, the A1(LO) position shifts to a higher frequency range, its width becomes larger, and its intensity drastically diminishes. This change in the A1(LO) behavior is due to its interaction and its coupling with the free negative charge carriers. Furthermore, we calibrated the A1(LO) frequency position shift as a function of the n-carrier concentration. We found out that for low n doping, the change in the A1(LO) position can be considered as a linear variation while in the overall doping range, a sigmoid growth trend with a Boltzmann fit can be tentatively applied to describe the A1(LO) position shift. This calibration curve can also be used to describe the coupling strength between the carriers and the A1(LO) phonon. Eventually, this study shows that micro-Raman spectroscopy is a powerful non-destructive tool to probe the doping concentration and the crystalline quality of GaN material with a microscopic spatial resolution.

On the Conformation of Anionic Peptoids in the Gas Phase.

Although N-(S)-phenylethyl peptoids are known to adopt helical structures in solutions, the corresponding positively charged ions lose their helical structure during the transfer from the solution to the gas phase due to the so-called charge solvation effect. We, here, considered negatively charged peptoids to investigate by ion mobility spectrometry-mass spectrometry whether the structural changes described in the positive ionization mode can be circumvented in the negative mode by a fine-tuning of the peptoid sequence, that is, by positioning the negative charge at the positive side of the helical peptoid macrodipole. N-(S)-(1-carboxy-2-phenylethyl) (Nscp) and N-(S)-phenylethyl (Nspe) were selected as the negative charge carrier and as the helix inductor, respectively. We, here, report the results of a joint theoretical and experimental study demonstrating that the structures adopted by the NspenNscp anions remain compactly folded in the gas phase for chains containing up to 10 residues, whereas no evidence of the presence of a helical structure was obtained, even if, for selected sequences and lengths, different gas phase conformations are detected.

Enhancement in electrical and optical properties of field-effect passivated GaN blue light emitting diodes

This paper investigates the field-effect sidewall surface passivation of III-Nitride blue light emitting diodes (LEDs) using ultrathin films of Al2O3 and ZnO grown using atomic layer deposition. A large density of negative fixed charges existing at the sidewall surface of the passivation layer leads to formation of an electric field at the interface of the passivation layer and the GaN layer. This repels the free negative charge carriers from getting close to the surface of the semiconductor and being trapped there. It also reduces the non-radiative recombination of these charge carriers and eventually leads to enhancement in electrical and optical properties of these devices. Electrical measurements demonstrate a significant decrease in the leakage current and the ideality factor in addition to a notable increase in the parasitic shunt resistance of the field-effect-passivated GaN LEDs compared to non-passivated devices. The field-effect passivation can be especially useful for smaller LED die size. In such a device, the effect of high leakage current at the edges of the device is more detrimental to its performance. The proposed approach can further eliminate the shunt conduction paths along the sidewalls of the LED and increase the efficiency of the devices.

Violation of triboelectric charge conservation on colliding particles.

In microgravity experiments, we quantified the net charge on systems of two identical, 434-μm-diameter glass spheres before and after a collision. We find that charge conservation is significantly violated. Independent of the sign of the total charge, the systems regularly lose some of their net charges, that is, they slightly discharge. This implies that positive as well as negative charge carriers become entrained into the surrounding atmosphere during a collision.

Dependable polysulfone based anion exchange membranes incorporating triazatriangulenium cations

A convenient approach for the fabrication of robust anion exchange membranes designed for electrochemical applications is presented. A triazatriangulenium carbocation (i.e., 4,8,12-tri-ethano-4,8,12-triazatriangulenium tetrafluoroborate) with a pKR+ above 20 was used as hydroxide anion and negative charge carrier in polysulfone anion exchange membranes. Morphological analysis of these membranes using high resolution scanning electron microscopy (HRSEM) and small angle X-ray scattering (SAXS) confirmed a phase separation casting process resulting in electrolyte exposed cations. It appears that 4,8,12-tri-ethano-4,8,12-triazatriangulenium brings about an extensive carbocation network within the polysulfone matrix, allowing smooth transport of hydroxide anions with low activation energy value (6.56 kJ mol−1), substantial ion conductivity (23.5 mS cm−1 at 90 °C), and good ion exchange capacity (2.44 ± 072 meq/g). The high stability of the triazatriangulenium cation contributes to the retention of 88% of the initial membrane conductivity for the tested period of 35 days at 60 °C in a 3 M KOH solution. This class of unique carbocations serve as a model system for stable charge carriers in alkaline exchange membranes for electrochemicaltio applications.

Directional percolating pathways in demixing blends on a wetting substrate

Understanding the dynamical evolution of a percolating network during liquid–liquid demixing is crucial for many technological applications, including plastic electronics, such as organic photovoltaics, whose performance depends on the efficiency to transport the positive and negative charge carriers to the corresponding electrodes. The transport and collection of the charge carriers require sufficient asymmetry between the donor and acceptor phases by attaining a minimum concentration of the majority fluids in it, called the percolation threshold. We investigate demixing in symmetric binary blends on a substrate preferentially wet by one of the fluids from the perspective of such a percolation threshold to achieve directed and connectivity percolation. We also study the influence of the strength of the substrate interaction and the property of the blend right before the quench with respect to the critical point. It is commonly assumed that the bicontinuous morphology of a symmetric blend guarantees percolation, where the average concentration of the blend distinguishes the two phases. However, if the percolation threshold is larger than the average concentration, we find that percolating pathways grow monotonically and a percolating cluster forms only after a time lag. Furthermore, we find that this time lag is characterized by two universal kinetic regimes that can explain all our observations. The first regime is associated with the percolation threshold itself, which grows exponentially. The second regime displays an algebraic growth with an exponent of 1/3 and we argue that it must be associated with the directional connectivity of the wetting phase to the substrate.

Influence of Cu nanoparticles concentration on the electronic transition of pulsed laser deposited TiO2/ ZnO: Cu nanocoposite film

The decoration energy band gap of metal oxide material was a key issues for important applications which has remarkable effects on the electronic transition. in this work, the structure and optical properties of titanium dioxide, zinc oxide and copper nanocoposite were studied. TiO2, ZnO and Cu were successfully deposited on a suitable glass substrate at room temperature using a pulsed laser deposition (PLD) technique N d: YAG laser with = 1064 nm wavelength at energy 500 mJ was used, concentrations of Cu = (4%, 8%, 12%, 16%, 20%). The surface topography was studied using atomic force microscopy (AFM) to ensure that the films had a homogeneous surface and that the minimum grain size obtained was 50.06 nm at 20% of Cu. The absorption spectrum was studied and observed that it increases with increasing in the ratio of Cu, and the energy gap decreases with the percentage of Cu and direct possible transitions in the range of 2.74 to 3.21 eV. Also the electrical properties was studied such as Hall effect and found that these films had of the charge n-type negative charge carriers.

Effective Temperatures of Olivine Dust Impact Plasmas

The effective temperatures of the positive and negative charge carriers in impact plasma are measured experimentally. The measurements are performed using a dust accelerator using polypyrrole (PPy)-coated olivine dust particles impacting onto a tungsten (W) target in four velocity bins: 2–3.6, 5–6.6, 8–12, and 12–18 km/s. The setup measures the retained impact charge as a function of applied bias potential, and the temperatures are calculated by fitting the data. The effective temperatures of the cations are approximately 7 eV and independent of the impact speed. The effective temperature of the negative charge carriers changes from 1 eV for the lowest speed to values comparable to that of the cations at higher speeds. The temperature measurements presented here are significantly different from prior studies using Fe dust particles. The discrepancy is possibly due to a larger fraction of negative ions in the impact plasma that likely originates from the PPy coating.

SEPARAZIONE DI CARICA IN MISCELE DI MATERIALI ORGANICI: VALE LA LEGGE DI COULOMB?

After a brief introduction to organic photovoltaic materials, I discuss the problem of charge separation in these systems. In the best devices, the excitations produced by absorption of a photon can be converted into positive and negative charge carriers (holes and electrons, respectively) with near-100% efficiency. Such efficiencies are apparently at odds with Coulomb’s law, which would lead to change recombination due to their mutual electrostatic attraction. A theoretical model developed within my group overcomes this apparent contradiction, by taking into account quantum mechanical effects such as the delocalization of the charge carriers.

Nanoscale Charge Accumulation and Its Effect on Carrier Dynamics in Tri-cation Perovskite Structures.

Nanoscale investigations by scanning probe microscopy have provided major contributions to the rapid development of organic–inorganic halide perovskites (OIHP) as optoelectronic devices. Further improvement of device level properties requires a deeper understanding of the performance-limiting mechanisms such as ion migration, phase segregation, and their effects on charge extraction both at the nano- and macroscale. Here, we have studied the dynamic electrical response of Cs0.05(FA0.83MA0.17)0.95PbI3–xBrx perovskite structures by employing conventional and microsecond time-resolved open-loop Kelvin probe force microscopy (KPFM). Our results indicate strong negative charge carrier trapping upon illumination and very slow (>1 s) relaxation of charges at the grain boundaries. The fast electronic recombination and transport dynamics on the microsecond scale probed by time-resolved open-loop KPFM show diffusion of charge carriers toward grain boundaries and indicate locally higher recombination rates because of intrinsic compositional heterogeneity. The nanoscale electrostatic effects revealed are summarized in a collective model for mixed-halide CsFAMA. Results on multilayer solar cell structures draw direct relations between nanoscale ionic transport, charge accumulation, recombination properties, and the final device performance. Our findings extend the current understanding of complex charge carrier dynamics in stable multication OIHP structures.

Doping-driven electronic and lattice dynamics in the phase-change material vanadium dioxide

Doping is generally understood as a strategy for including additional positive or negative charge carriers in a semiconductor, thereby tuning the Fermi level and changing its electronic properties in the equilibrium limit. However, because dopants also couple to all of the microscopic degrees of freedom in the host, they may also alter the nonequilibrium dynamical properties of the parent material, especially at large dopant concentrations. Here, we show how substitutional doping by tungsten at the 1 at. % level modifies the complex electronic and lattice dynamics of the phase-change material vanadium dioxide. Using femtosecond broadband spectroscopy, we compare dynamics in epitaxial thin films of pristine and tungsten-doped $\mathrm{V}{\mathrm{O}}_{2}$ over the broadest wavelength and temporal ranges yet reported. We demonstrate that coupling of tungsten atoms to the host lattice modifies the early electron-phonon dynamics on a femtosecond timescale, altering in a counterintuitive way the ps-to-ns optical signatures of the phase transition. Density functional theory correctly captures the enthalpy difference between pristine and W-doped $\mathrm{V}{\mathrm{O}}_{2}$ and shows how the dopant softens critical V-V phonon modes while introducing new phononic modes due to W-V bonds. While substitutional doping provides a powerful method to control the switching threshold and contrast of phase-change materials, determining how the dopant dynamically changes the broadband optical response is equally important for optoelectronics.

Optimization of the electron transport layer in quantum dot light-emitting devices

Quantum dot light-emitting devices have emerged as an important technology for display applications. Their emission is a result of recombination between positive and negative charge carriers that are transported through the hole and electron conductive layers, respectively. The selection of electron or hole transport materials in these devices not only demands the alignment of energy levels between the layers but also balances the flow of electrons and holes toward the recombination sites. In this work, we examine a method for device optimization through control of the charge carrier kinetics. We employ impedance spectroscopy to examine the mobility of charge carriers through each of the layers. The derived mobility values provide a path to estimate the transition time of each charge carrier toward the emitting layer. We suggest that an optimal device structure can be obtained when the transition times of both charge carriers toward the active layer are similar. Finally, we examine our hypothesis by focusing on thickness optimization of the electron transport layer.

A facile approach for enhancing device performance of excitonic solar cells with an innovative SnO2/TCNE electron transport layer

The spread of the Internet of things created the need for huge amounts of off-grid energy sources for tens of billions of electronic devices with low power consumption for indoor applications. Excitonic solar cells may provide a better solution as compared with other solar cells due to their advantages such as low-cost, large-area module, and fabrication by solution printing and coating techniques. However, the organic solar cells (OSCs) or perovskite solar cells (PSCs) should have a low energy loss, suitable absorption spectrum, and minimum trap mediated charge recombination for its proper commercialization. Interface engineering is one of the ways of achieving better performance for these devices. Herein, we report a facile and effective strategy for interfacial modulation to achieve the improved performance of inverted bulk heterojunction (BHJ) solar cells with a solution-processable SnO2 electron transport layer (ETL) modified by an organic small molecule tetracyanoethylene (TCNE). We can say that a few nano-meter thin film of TCNE supports to reduce the energy barrier of SnO2, resulting in the efficient extraction and transport of negative charge carriers toward the cathode. The TCNE also passivates the surface defects of SnO2 and hence decreases the charge recombination rate for iOSCs. Furthermore, it brings the better interfacial contact between SnO2 and BHJ blend in which the polymer PTB7 and fullerene PC71BM are the donor and acceptor, respectively, showing average power conversion efficiencies of ∼4.58%, ∼4.98%, and ∼2.95% with varying concentrations of TCNE (0.5 mg/ml, 1 mg/ml, and 2 mg/ml), respectively, in methanol, on top of SnO2, as ETLs, which are comparably better than that of pristine SnO2 (3.28%). We assumed that the TCNE modified SnO2 method is a potent and easy way to get efficient inverted BHJ solar cells with higher efficiency and may also be an appropriate alternative for other organic semiconducting devices where an ETL is required, such as organic light-emitting diodes and PSCs.

ZnO–PVA Polymer Matrix with Transition Metals Oxide Nano-fillers for High Dielectric Mediums

Polymer-based nanocomposites are considered promising candidates for flexible modern electronic devices. ZnO–PVA polymer nanocomposites have gained recent research interest due to excellent optical and dielectric properties. Transition metals (Cr, Mn, Fe, Ni, Cu) oxide nanoparticles incorporated ZnO–PVA composite matrix are investigated in this work for a comprehensive study of its structure, morphology, optical and dielectric parameters. Crystallographic information revealed the presence of crystalline zinc oxide and transition metals oxides reflections in specific orientations in the PVA matrix. The inter-particle distance and interfacial interaction of transition metal oxide nanoparticles in host polymer are observed as important factors for improved dipole polarization. The nanoparticles of metal oxides in matrix induce the enhanced negative charge carriers and exhibit robust interactions with positive carriers of the polymer surface. A sharp and substantial increment in real dielectric permittivity from 74 for ZnO–PVA to maximum 350 for ZnO–PVA–Fe2O3 composition was observed by the addition of transition metal oxides in the PVA matrix with same loss factor value of 0.1.

Carrier transport in LDPE and its nanocomposites

The mobility of positive and negative charge carriers has been separately measured independently of carrier concentration in low-density polyethylene (LDPE) and its nanocomposites using space charge measurements in a bilayer system. It was found that the carrier mobility of positive charge carriers was larger than that of negative charge carriers for both LDPE and its nanocomposites and that the carrier mobility was one to two orders of magnitude less in the nanocomposite than unfilled LDPE under the same conditions. The mobility is interpreted in terms of a field-assisted hopping model whose estimated activation energies were verified by measurements at different temperatures. The form of the charge traps and carrier transport is interpreted in terms of known features and electron and hole traps and the effect of nano-particles upon these features is discussed.

Fabrication of SiNWs-FET Nanostructure Via Atomic Force Microscopy Lithography

The unique electrical properties of silicon nanowires (SiNWs) is one of the reasons it become an attractive transducer for biosensor nowadays. Positive (holes) and negative (electron) charge carriers from SiNWs can simply interact with either positive or negative charge of sensing target. In this paper, we have studied the fabrication of silicon nanowires field effect transistor (SiNWs-FET) nanostructure patterned on 15 Ω resistivity of p-type silicon on insulator (SOI) wafer fabricated via atomic force microscopy lithography technique. To fabricate SiNWs-FET nanostructure, a conductive AFM tip, Cr/Pt cantilever tip, was used then various value of applied voltage, writing speed and relative humidity were studied. Subsequent, followed by wet etching processes, admixture of tetramethylammonium hydroxide (TMAH) and isopropyl alcohol (IPA) were used to remove the undesired of silicon layer and diluted hydrofluoric acid (HF) was used to remove the oxide layer. From the results, it shows that, cantilever tip at 9 V with 0.4 μm/s writing speed and relative humidity between 55% - 60% gives the best formation of silicon oxide to fabricate SiNWs-FET nanostructure.

Ionization of large helium nanodroplets

SynopsisWe have produced and mass-selected large, highly charged helium droplets containing up to 1010 He atoms and several tens of charges. They are stable with both multiple positive and negative charge carriers and are stabilized by losing small fragments of excess charge instead of disintegrating (highly asymmetric fission).

Sn/SnO hybrid graphene for thermal interface material and interconnections with Sn hybrid carbon nanotubes

Few layer graphene (FLG) was successfully coated with Sn/SnO after reacting with SnCl2 and reducing under H2/N2 gas. X ray diffraction and X ray photoelectron spectroscopy showed the formation of Sn/SnO on the surface. The thermal diffusivity and conductivity of ~1 mm thick pallet of Sn/SnO coated hybrid FLG was surged to 7.91 mm2/s, 14.41 W/m.K from 2.17 mm2/s and 3.27 W/m.K of the pristine FLG, respectively. The increase in thermal properties was attributed to the formation of compact structures in the film and reducing the air gaps by joining hybrid graphene due to Sn. Current-voltage measurements using tuna probe in atomic force microscopy demonstrated the higher number of negative charge carriers in the hybrid FLG compared to pristine FLG because of electron transfer from graphene to Sn. Study using transmission electron microscopy showed the development of interconnection using Sn/SnO hybrid graphene with Sn coated and filled multiwalled carbon nanotubes.

Photosensing System Using Photosystem I and Gold Nanoparticle on Graphene Field-Effect Transistor.

In this study, a light sensor is fabricated based on photosystem I (PSI) and a graphene field-effect transistor (FET) that detects light at a high quantum yield under ambient conditions. We immobilized PSI on a micrometer-sized graphene FET using Au nanoparticles (AuNPs) and measured the I-V characteristics of the modified graphene FET before and after light irradiation. The source-drain current (Isd) increased upon illumination, exhibiting a photoresponsivity of 4.8 × 102 A W-1, and the charge neutrality point of graphene shifted by -12 mV. This system represents the first successful photosensing system based on proteins and a solution-gated graphene FET. The probable mechanism of this negative shift can be explained by the increase in negative charge carriers in graphene induced by a hole trap in the AuNP resulting from electron transfer from the AuNP to PSI. Photoresponses were only observed in the presence of two surface-active agents, n-hexyltrimethylammonium bromide and sodium dodecylbenzenesulfonate, because they caused the formation of a hydrophobic environment on the surface of the graphene. The lipid layer of these agents caused dissociation of ascorbate ions from the graphene sheet, thereby expanding the Debye screening length of the electrolyte solution. The hydrophobic environment above graphene also enhanced hole storage in the AuNP through electron transfer from the AuNP to PSI.