Presence Of Charge Carriers Research Articles

Impact of Dielectric Environment on Trion Emission from Single-Walled Carbon Nanotube Networks.

Trions are charged excitons that form upon optical or electrical excitation of low-dimensional semiconductors in the presence of charge carriers (holes or electrons). Trion emission from semiconducting single-walled carbon nanotubes (SWCNTs) occurs in the near-infrared and at lower energies compared to the respective exciton. It can be used as an indicator for the presence of excess charge carriers in SWCNT samples and devices. Both excitons and trions are highly sensitive to the surrounding dielectric medium of the nanotubes, having an impact on their application in optoelectronic devices. Here, the influence of different dielectric materials on exciton and trion emission from electrostatically doped networks of polymer-sorted (6,5) SWCNTs in top-gate field-effect transistors is investigated. The observed differences of trion and exciton emission energies and intensities for hole and electron accumulation cannot be explained with the polarizability or screening characteristics of the different dielectric materials, but they show a clear dependence on the charge trapping properties of the dielectrics. Charge localization (trapping of holes or electrons by the dielectric) reduces exciton quenching, emission blue-shift and trion formation. Based on the observed carrier type and dielectric material dependent variations, the ratio of trion to exciton emission and the exciton blue-shift are not suitable as quantitative metrics for doping levels of carbon nanotubes.

(Invited) Simulating Dynamic Excitons Via Quantum Molecular Dynamics: A Case Study in Lead Halide Perovskites

The ultrafast electronic and structural dynamics invoked by photoexcitation, i.e., “dynamic exciton” phenomena, not only are important in the context of practical applications such as solar cells, but also raise many questions from the viewpoint of fundamental science. Experimental measurement, computational simulations, and theoretical interpretation will be the three pillars for deciphering the dynamic exciton phenomena.From the viewpoint of computational simulations, molecular dynamics (MD) techniques combined with quantum chemical calculations, i.e., quantum molecular dynamics (QMD), has been the popular tool to simulate the dynamic exciton phenomena. The quantum chemical calculations, which are typically conducted on the basis of the density-functional theory (DFT), require the large computational resources and time. These have been the limiting factors for the accessible spatial and time scales by the QMD simulations.To extend the coverage of simulations to more complex, large-scale systems, we have developed efficient excited-state QMD methods that can include nonadiabatic effects. Our method combines the density-functional tight binding (DFTB) method, which is an approximate DFT, and the surface hopping method, which is a theoretical framework to incorporate the nonadiabatic effects into the QMD simulations. The method was further improved to be suitable for condensed-phase simulations explicitly including the environment, i.e., solvent, by using a “divide-and-conquer” style quantum chemical calculation technique. These theoretical framework enables us to simulate the coupled electronic–structural dynamics in excited states of systems consisting of 102–103 atoms[1,2,3].In addition, using the developed method, we conducted the real-time simulations of the ultrafast processes invoked by photoexcitation of lead iodide perovskites, which are known as the key materials for perovskite solar cells. The dissociation of the exciton into the positive and negative charge carriers was observed. Moreover, the hot carrier cooling, where the charge carriers dissipate excess energy via the electron–phonon coupling and relax to the band edges, was also tracked. Finally, the direct evidence of the polaron formation, where the structural deformation is induced by the presence of charge carriers, was observed. These results highlight the importance of the coupling between electronic and structural degrees of freedom.In the talk, recent improvements in the methodology and future perspectives will also be presented[5].

Investigation of degradation processes in perovskite under the influence of external factors

The article describes a number of new fundamental knowledge about mechanisms of degradation processes occurring in photoactive perovskite materials based on complex lead halides and solar cells based on them, modern methods and approaches to increasing the operational stability of perovskite photovoltaic devices are considered. The revealed paths of degradation processes occurring in complex metal halides (lead and tin) under the influence of light and elevated temperatures are important for further developments in the field of creating highly efficient and stable perovskite solar cells of a new generation. The investigated models of degradation are described both under the action of moisture and as a result of radiation ionization processes. The importance of solving the Dexter-Varley paradox, which takes into account the competition between the processes of displacement of IS0 states, as well as the delocalization of the resulting hole in the valence band, is emphasized. It was shown that by changing the force of pressure of the tape on the perovskite film, it was possible to achieve the maximum values of the light conversion efficiency of about 12.7%. It was found that the presence of charge carriers in the form of polarons can significantly affect the assessment of the degradation efficiency towards its increase. The data obtained can radically change the traditional ideas about the efficiency of photochemical reactions.

Evidence of attraction between charge carriers in a doped Mott insulator

Recent progress in optically trapped ultracold atomic gases is now making it possible to access microscopic observables in doped Mott insulators, which are the parent states of high-temperature superconductors. This makes it possible to address longstanding questions about the temperature scales at which attraction between charge carriers are present, and their mechanism. Controllable theoretical results for this problem are not available at low temperature due to the sign problem. In this work, we employ worm-algorithm Monte Carlo to obtain completely unbiased results for two charge carriers in a Mott insulator. Our method gives access to lower temperatures than what is currently possible in experiments, and provides evidence for attraction between dopants at a temperature scale that is now feasible in ultracold atomic systems. We also report on spin-correlations in the presence of charge carriers, which are directly comparable to experiments.

Time-Resolved Changes in Dielectric Constant of Metal Halide Perovskites under Illumination.

Despite their impressive performance as a solar absorber, much remains unknown on the fundamental properties of metal halide perovskites (MHPs). Their polar nature in particular is an intense area of study, and the relative permittivity (εr) is a parameter widely used to quantify polarization over a range of different time scales. In this report, we have exploited frequency-dependent time-resolved microwave conductivity (TRMC) to study how εr values of a range of MHPs change as a function of time, upon optical illumination. Further characterization of charge carriers and polarizability are conducted by femtosecond transient absorption and stimulated Raman spectroscopy. We find that changes in εr are roughly proportional to photogenerated carrier density but decay with a shorter time constant than conductance, suggesting that the presence of charge carriers alone does not determine polarization.

Palm-based cationic polyurethane membranes for solid polymer electrolytes application: A physico-chemical characteristics studies of chain-extended cationic polyurethane

Polyurethane (PU) having a great potential to be used as a polymeric host in polymer electrolytes. However, it still suffers from low conductivity and high glass transition temperatures (Tg). In this work, chain-extended palm-based cationic PU using polyethylene glycol (PEG) were synthesized at different composition by a polyaddition polymerization reaction. Infrared analysis proved the complete reaction of the limiting monomer by the disappearance of isocyanate peak. The glass transition temperature and crystallinity were decreased to 35 °C. The molecular weight increased by 22 % along with the composition of PEG. Thermal studies proved that all samples are thermally stable up to 190 °C. Further modification on PU structure by alkylation reaction enhanced overall conductivity of the polymeric host up to two orders of magnitude without the presence of charge carriers. These observations indicate this palm-based cationic PU possessed favourable properties and great potential as a polymeric host material for solid-state electrolytes application.

Time-resolved imaging of streamer formation inside gaseous bubbles in liquids

Bulk liquid conditions can impact how plasma discharges behave in bubbles and affect the future implementation of plasma technologies for water treatment. A 2D plasma-in-liquid apparatus was used to investigate the effects of liquid conductivity on the propagation of nanosecond pulsed streamers in bubbles. ANSYS Maxwell was used to model the resulting static electric field. Electric field enhancement was found at the contact point between the bubble surface and the plexiglass, and can direct propagation of observed discharges. Nanosecond exposure time intensified charge-coupled device imaging was used to photograph the temporal development of the streamers in the bubble. At high liquid conductivity, surface hugging streamers stalled out axial streamers. However, at lower conductivity, increase in charge relaxation time and presence of charge carriers became important. Deposited charges on the interface led to development of opposing electric field and terminated streamer development. Presence of organic contaminants can increase conductivity of surface water. To maximize energy efficiency of future plasma-based water treatment reactor, it is important to take liquid conductivity into consideration. Shorter pulse width and rise time can potentially minimize development of surface hugging streamers, which is a major source of charge dissipation and thermal effects.

Structural, optical, thermal, and dielectric properties of polyethylene oxide/carboxymethyl cellulose blend filled with barium titanate

Nanocomposite films of polyethylene oxide/carboxymethyl cellulose (PEO/CMC)/barium titanate (BaTiO3) were prepared using the solution casting technique. The results of X-ray diffraction (XRD) analysis depicted the semicrystalline nature of PEO/CMC, which was largely reduced due to BaTiO3 loading. Fourier transform infrared (FT-IR) spectroscopy showed the interaction between the components of blend, PEO and CMC, through the formation of hydrogen bond and confirmed the interaction between BaTiO3 and the functional groups of PEO/CMC, in particular C-O-C for PEO and COO− for CMC. From the Ultraviolet/visible (UV/vis.) absorption spectra, the sharp absorption edge in the UV range for the filled samples was red shifted and the optical energy gap was also calculated. The effect of BaTiO3 on the glass transition, melting point, and decomposition temperatures of the prepared samples was investigated using differential scanning calorimetry (DSC) technique. From the AC conductivity results, the electrical properties were improved depending on temperatures, frequencies, and the BaTiO3 content within the polymeric matrix. This indicated the presence of charge carriers that transported within defect sites by hopping. Furthermore, the mechanism for AC conduction and its dielectric parameters had been studied. Transmission electron micrographs revealed the cubic shape for BaTiO3 with an average size range of 15–50 nm. These observations indicate that these nanocomposite samples are potential candidates in various industrial applications, such as pyroelectric sensors and electromechanical transducers, and in ceramic capacitors as a dielectric material inserted between the two electrodes.

Electronic contribution in heat transfer at metal-semiconductor and metal silicide-semiconductor interfaces

This work presents a direct measurement of the Kapitza thermal boundary resistance Rth, between platinum-silicon and platinum silicide-silicon interfaces. Experimental measurements were made using a frequency domain photothermal radiometry set up at room temperature. The studied samples consist of ≈50 nm of platinum and ≈110 nm of platinum silicide on silicon substrates with different doping levels. The substrate thermal diffusivity was found via a hybrid frequency/spatial domain thermoreflectance set up. The films and the interfaces between the two layers were characterized using scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. X-ray diffraction was also used to determine the atomic and molecular structures of the samples. The results display an effect of the annealing process on the Kapitza resistance and on the thermal diffusivities of the coatings, related to material and interface changes. The influence of the substrate doping levels on the Kapitza resistance is studied to check the correlation between the Schottky barrier and the interfacial heat conduction. It is suggested that the presence of charge carriers in silicon may create new channels for heat conduction at the interface, with an efficiency depending on the difference between the metal’s and substrate’s work functions.

Photocurrent enhancement of graphene photodetectors by photon tunneling of light into surface plasmons

We demonstrate that surface plasmon resonances excited by photon tunneling through an adjacent dielectric medium enhance the photocurrent detected by a graphene photodetector. The device is created by overlaying a graphene sheet over an etched gap in a gold film deposited on glass. The detected photocurrents are compared for five different excitation wavelengths, ranging from to Although the device is not optimized, the photocurrent excited with incident p-polarized light (which excites resonant surface plasmons) is significantly amplified in comparison with that for s-polarized light (without surface plasmon resonances). We observe that the photocurrent is greater for shorter wavelengths (for both s- and -polarizations) with increased photothermal current. Position-dependent Raman spectroscopic analysis of the optically-excited graphene photodetector indicates the presence of charge carriers in the graphene near the metallic edge. In addition, we show that the polarity of the photocurrent reverses across the gap as the incident light spot moves across the gap. Graphene-based photodetectors offer a simple architecture which can be fabricated on dielectric waveguides to exploit the plasmonic photocurrent enhancement of the evanescent field. Applications for these devices include photodetection, optical sensing and direct plasmonic detection.

Charging and Release Mechanisms of Flexible Macromolecules in Droplets.

We study systematically the charging and release mechanisms of a flexible macromolecule, modeled by poly(ethylene glycol) (PEG), in a droplet by using molecular dynamics simulations. We compare how PEG is solvated and charged by sodium Na+ ions in a droplet of water (H2O), acetonitrile (MeCN), and their mixtures. Initially, we examine the location and the conformation of the macromolecule in a droplet bearing no net charge. It is revealed that the presence of charge carriers do not affect the location of PEG in aqueous and MeCN droplets compared with that in the neutral droplets, but the location of the macromolecule and the droplet size do affect the PEG conformation. PEG is charged on the surface of a sodiated aqueous droplet that is found close to the Rayleigh limit. Its charging is coupled to the extrusion mechanism, where PEG segments leave the droplet once they coordinate a Na+ ion or in a correlated motion with Na+ ions. In contrast, as PEG resides in the interior of a MeCN droplet, it is sodiated inside the droplet. The compact macro-ion transitions through partially unwound states to an extended conformation, a process occurring during the final stage of desolvation and in the presence of only a handful of MeCN molecules. For charged H2O/MeCN droplets, the sodiation of PEG is determined by the H2O component, reflecting its slower evaporation and preference over MeCN for solvating Na+ ions. We use the simulation data to construct an analytical model that suggests that the droplet surface electric field may play a role in the macro-ion-droplet interactions that lead to the extrusion of the macro-ion. This study provides the first evidence of the effect of the surface electric field by using atomistic simulations. Graphical Abstract ᅟ.

Electron-phonon Coupling and the Superconducting Phase Diagram of the LaAlO3-SrTiO3 Interface.

The superconductor at the LaAlO3—SrTiO3 interface provides a model system for the study of two-dimensional superconductivity in the dilute carrier density limit. Here we experimentally address the pairing mechanism in this superconductor. We extract the electron—phonon spectral function from tunneling spectra and conclude, without ruling out contributions of further pairing channels, that electron—phonon mediated pairing is strong enough to account for the superconducting critical temperatures. Furthermore, we discuss the electron—phonon coupling in relation to the superconducting phase diagram. The electron—phonon spectral function is independent of the carrier density, except for a small part of the phase diagram in the underdoped region. The tunneling measurements reveal that the increase of the chemical potential with increasing carrier density levels off and is zero in the overdoped region of the phase diagram. This indicates that the additionally induced carriers do not populate the band that hosts the superconducting state and that the superconducting order parameter therefore is weakened by the presence of charge carriers in another band.

Molecular Dynamics Simulation of Aggregates in the Dodecane/span80 System and Their Behaviour in an Electric Field

Molecular dynamics simulation of self-assembly of surfactant span80 molecules to form reverse micelles in nonpolar liquid dodecane is carried out. Simulations are performed using a united atom model for dodecane and a hybrid model for span80 molecules. Various physical characteristics of reverse micelle are measured, and the same are compared with available experimental results. Presence of charge carriers in the form of solvated ions in the core of reverse micelles is confirmed by the simulation. Movement of reverse micelles under the effect of uniform external electric field is also discussed.

Phase transition enhanced thermoelectric figure-of-merit in copper chalcogenides

While thermoelectric materials can be used for solid state cooling, waste heat recovery, and solar electricity generation, low values of the thermoelectric figure of merit, zT, have led to an efficiency too low for widespread use. Thermoelectric effects are characterized by the Seebeck coefficient or thermopower, which is related to the entropy associated with charge transport. For example, coupling spin entropy with the presence of charge carriers has enabled the enhancement of zT in cobalt oxides. We demonstrate that the coupling of a continuous phase transition to carrier transport in Cu2Se over a broad (360–410 K) temperature range results in a dramatic peak in thermopower, an increase in phonon and electron scattering, and a corresponding doubling of zT (to 0.7 at 406 K), and a similar but larger increase over a wider temperature range in the zT of Cu1.97Ag.03Se (almost 1.0 at 400 K). The use of structural entropy for enhanced thermopower could lead to new engineering approaches for thermoelectric materials with high zT and new green applications for thermoelectrics.

Electrochemical properties of GaN nanowire electrodes—influence of doping and control by external bias

We report on the electrochemical characteristics of GaN nanowire (NW) ensembles grown by plasma-assisted molecular beam epitaxy on Si(111) substrates and on the influence of Si and Mg doping. The NW electrochemical properties in terms of surface capacitance (CS), surface resistance (RS) are extracted from electrochemical impedance spectra. While Mg doping of GaN NWs does not cause a significant variation of these quantities, an increase of the Si concentration leads to an increase of CS and a simultaneous decrease of RS, indicating the presence of charge carriers in the NWs. According to the extracted values for RS and CS the NWs are classified into resistive and conductive. For conductive NWs charge transfer to a ferricyanide redox couple in the electrolyte is demonstrated and the ensemble average of the flatband voltage was determined. Variation of the lateral surface potential due to application of an external bias via the electrolyte is demonstrated.

Effect of a magnetic field on charge transport in polyaniline-manganese acetylacetonate-lanthanum chloride-praseodymium chloride composites

New composite materials containing polyaniline, sulfur, manganese acetylacetonate, lanthanum chloride, and praseodymium chloride are obtained. The components are blended and subjected to mechanochemical treatment in a ball mill; then, the composites are molded into thin films. Application of an external magnetic field with a strength of 6000 Oe leads to an increase in the conductivity of the films by more than 3000% to reach the properties of materials possessing giant magnetoresistance. An analysis of the time dependences of transient current under application of direct voltage to the samples indicates the presence of charge carriers of three types: presumably, e−, H+, and Cl−. A possible mechanism for the development of the giant magnetoresistance effect in the obtained composites is discussed.

Photochemistry and kinetics of single organic nanoparticles in the presence of charge carriers

This paper describes a preliminary experimental study on charge injection in MEH-PPV polymer nanoparticles incorporated in an OLED type device. The nanoparticles have been investigated with single molecule fluorescence spectroscopy recorded simultaneously with charging and discharging of the particles. Structure and dynamics of an anion/MEH-PPV complex, proposed to be responsible for discrete photobleaching in conjugated polymers, have been investigated with this technique. Device physics and kinetics have been elucidated by using the fluorescent conjugated polymer nanoparticles as probes for detecting the presence of hole polarons, and have been related to the electrically induced oxidation and reduction of the anion/MEH-PPV complex.

Magnetic properties of zig-zag ladders

We analyze the phase diagram of a system of spin-1/2 Heisenberg antiferromagnetic chains interacting through a zig-zag coupling, also called zig-zag ladders. Using bosonization techniques we study how a spin-gap or more generally plateaux in magnetization curves arise in different situations. While for coupled XXZ chains, one has to deal with a recently discovered chiral perturbation, the coupling term which is present for normal ladders is restored by an external magnetic field, dimerization or the presence of charge carriers. We then proceed with a numerical investigation of the phase diagram of two coupled Heisenberg chains in the presence of a magnetic field. Unusual behaviour is found for ferromagnetic coupled antiferromagnetic chains. Finally, for three (and more) legs one can choose different inequivalent types of coupling between the chains. We find that the three-leg ladder can exhibit a spin-gap and/or non-trivial plateaux in the magnetization curve whose appearance strongly depends on the choice of coupling.

Application of the Wigner-Function Formulation to Mesoscopic Systems in Presence of Electron-Phonon Interaction

A theoretical and computational analysis of the quantum dynamics of charge carriers in presence of electron-phonon interaction based on the Wigner function is here applied to the study of transport in mesoscopic systems. Numerical applications are shown for a) a wave packet scattering with phonons while crossing a potential profile and b) electrons scattering with phonons in a finite device with open boundary conditions.

Wigner Function for Open Systems with Electron–Phonon Interaction

The Wigner-function (WF) formalism is used to analyze the quantum dynamics of charge carriers in presence of electron–phonon (e–p) interaction in open mesoscopic systems making use of the perturbation theory with the external fields incorporated into the unperturbed Hamiltonian. Proper boundary conditions have been chosen and the transport phenomenon inside the device is treated in such a way that boundary conditions at finite positions are kept unaltered, even though the basis of extended scattering states is used.