arXiv subjects

Z. V. Vardeny

Publications and source records attributed to Z. V. Vardeny.

At least 19 recordsLinked to original sources

arXiv2021

Spin-transport in an organic semiconductor without free charge carrier involvement

We have experimentally tested the hypothesis of free charge carrier mediated spin-transport in the small molecule organic semiconductor Alq3 at room temperature. A spin current was pumped into this material by pulsed ferromagnetic resonance of an adjacent NiFe layer, while a charge current resulting from this spin current via the inverse spin-Hall effect (ISHE) was detected in a Pt layer adjacent on the other side of the Alq3 layer, confirming a pure spin current through the Alq3 layer. Charge carrier spin states in Alq3, were then randomized by simultaneous application of electron paramagnetic resonance (EPR). No influence of the EPR excitation on the ISHE current was found, implying that spin-transport is not mediated by free charge-carriers in Alq3.

cond-mat.mes-hall↗

arXiv2020

Spin-Dependent Charge-Carrier Recombination Processes in Tris(8-Hydroxyquinolinato) Aluminum

We have studied the nature and dynamics of spin-dependent charge carrier recombination in Tris(8-hydroxyquinolinato) aluminum (Alq$_3$) films in light emitting diodes at room temperature using continuous wave and pulsed electrically detected magnetic resonance (EDMR) spectroscopy. We found that the EDMR signal is dominated by an electron-hole recombination process, and another, weaker EDMR signal whose fundamental nature was investigated. From the pulsed EDMR measurements we obtained a carrier spin relaxation time, $T_2 = 45\pm 25$ ns which is much shorter than $T_2$ in conjugated polymers, but relatively long for a molecule containing elements with high atomic number. Using multi-frequency continuous wave EDMR spectroscopy, we obtained the local hyperfine field distributions for electrons and holes, as well as their respective spin-orbit coupling induced g-factor and g-strain values.

cond-mat.mes-hall↗

arXiv2018

High-Field Magnetoresistance of Organic Semiconductors

The magneto-electronic field effects in organic semiconductors at high magnetic fields are described by field-dependent mixing between singlet and triplet states of weakly bound charge carrier pairs due to small differences in their Landé g-factors that arise from the weak spin-orbit coupling in the material. In this work, we corroborate theoretical models for the high-field magnetoresistance of organic semiconductors, in particular of diodes made of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) at low temperatures, by conducting magnetoresistance measurements along with multi-frequency continuous-wave electrically detected magnetic resonance experiments. The measurements were performed on identical devices under similar conditions in order to independently assess the magnetic field-dependent spin-mixing mechanism, the so-called Δg mechanism, which originates from differences in the charge-carrier g-factors induced by spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2013

Organic magnetoresistance near saturation: mesoscopic effects in small devices

In organic light emitting diodes with small area the current may be dominated by a finite number, N of sites in which the electron-hole recombination occurs. As a result, averaging over the hyperfine magnetic fields, b_h, that are generated in these sites by the environment nuclei is incomplete. This creates a random (mesoscopic) current component, ΔI(B), at field B having relative magnitude ~ N^(-1/2). To quantify the statistical properties of ΔI(B) we calculate the correlator K(B, ΔB)= <δI(B - ΔB/2)δI(B + ΔB/2)> for parallel and perpendicular orientations of ΔB. We demonstrate that mesoscopic fluctuations develop at fields B>>b_h, where the average magnetoresistance is near saturation. These fluctuations originate from the slow beating between S and T_0 states of the recombining e-h spin pair-partners. We identify the most relevant processes responsible for the current fluctuations as due to anomalously slow beatings that develop in sparse e-h polaron pairs at sites for which the b_h projections on the external field direction almost coincide.

cond-mat.mes-hall↗

arXiv2011

Evidence for Excimer Photoexcitations in an Ordered π-Conjugated Polymer Film

We report pressure-dependent transient picosecond and continuous-wave photomodulation studies of disordered and ordered films of 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced absorption (PA) bands in the disordered film exhibit very weak pressure dependence and are assigned to intrachain excitons and polarons. In contrast, the ordered film exhibits two additional transient PA bands in the midinfrared that blueshift dramatically with pressure. Based on high-order configuration interaction calculations we ascribe the PA bands in the ordered film to excimers. Our work brings insight to the exciton binding energy in ordered films versus disordered films and solutions. The reduced exciton binding energy in ordered films is due to new energy states appearing below the continuum band threshold of the single strand.

cond-mat.str-el↗

arXiv2008

Ultrafast dynamics of surface electromagnetic waves in nanohole array on metallic film

We study the ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength holes array by means of transient photomodulation with 100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the dynamics and that is inconsistent with plasmon-polariton mechanism of extraordinary transmission. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron-phonon relaxation dynamics, involving non-equilibrium hot electrons and quasi-equilibrium phonons.

cond-mat.mtrl-sci↗

arXiv2007

Ultrafast response of surface electromagnetic waves in an aluminum film perforated with subwavelength hole arrays

The ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength holes array was studied in the visible spectral range by the technique of transient photomodulation with 100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the optical response of nanohole arrays. The blueshift is inconsistent with plasmonic mechanism of extraordinary transmission and points to the crucial role of interference in the formation of transmission bands. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron-phonon relaxation dynamics, involving non-equilibrium hot electrons and quasi-equilibrium phonons.

cond-mat.mtrl-sci↗

arXiv2006

Nonlinear Optical Spectroscopy of Excited States in Polyfluorene

We used a variety of nonlinear optical (NLO) spectroscopies to study the singlet excited states order, and primary photoexcitations in polyfluorene; an important blue emitting p-conjugated polymer. The polarized NLO spectroscopies include ultrafast pump-probe photomodulation, two-photon absorption, and electroabsorption. For completeness we also measured the linear absorption and photoluminescence spectra. We found that the primary photoexcitations in polyfluorene are singlet excitons.

physics.optics↗

arXiv2006

Metastable polaron supporting phase in poly-p-phenylene films induced by UV illumination

We discovered a new metastable polaron-supporting phase in pristine films of a soluble derivative of poly-p-phenylene vinylene (MEH-PPV) that is induced by UV illumination. In the initial un-illuminated phase A, the films do not show long-lived photogenerated polarons. However, prolonged UV illumination for several hours induces a reversible, me-tastable phase B that shows abundant long-lived photogenerated polarons. Phase B films transform back to the original phase A within 1/2 hour in the dark at room temperature. We propose a reversible mechanism in which UV illumination creates metastable deep defects that substantially increase the photogenerated polaron lifetime.

cond-mat.other↗

arXiv2006

Comment on Frequency response and origin of the spin-1/2 photolumines-cence-detected magnetic resonance in a pi-conjugated polymer

In a recent paper Segal et al. [1] attempted to explain the dynamics of spin 1/2 photoluminescence detected magnetic resonance (PLDMR) in films of a pi-conjugated polymer, namely a soluble de-rivative of poly(phenylene-vinylene) [MEH-PPV] using a model (dubbed TPQ), in which the PLDMR is due to spin dependent triplet-polaron interactions that reduce the polarons density and consequent quenching of singlet excitons. We studied the full PLDMR and photoinduced ab-sorption (PA) dynamics of MEH-PPV films as a function of microwave power at various tempera-tures. We show, firstly, that the TPQ model is incompatible with the full frequency dependent spin 1/2 PLDMR response; secondly, it is not in agreement with the spin-1 PLDMR temperature dependence; thirdly, it predicts a much shorter triplet exciton lifetime than that obtained experimentally; and fourthly, that is in contradiction with the temperature dependencies of spin 1/2 PLDMR and triplet exciton PA. In contrast, an alternative model, namely the spin dependent re-combination of polarons, is capable of explaining the whole body of experimental results, and in particular the PLDMR dynamics.

cond-mat.mtrl-sci↗

arXiv2006

Comment on On the Yield of Singlet Excitons in Organic Light-Emitting Devices: a Double Modulation Photoluminescence-Detected Magnetic Resonance Study

In a recent Letter Lee et al. studied the spin 1/2 photoluminescence-detected magnetic resonance (PLDMR) of a soluble derivative of poly-phenylene vinylene [MEH-PPV] and argued that it originates from the interaction of triplet excitons with polarons. We show that in MEH-PPV as well as in other conjugated polymers the spin 1/2 PLDMR does not depend on the triplet exciton density and thus cannot be associated with triplet excitons.

cond-mat.other↗

arXiv2006

Apparent phonon side band modes in pi-conjugated systems: polymers, oligomers and crystals

The emission spectra of many pi-conjugated polymers and oligomers contain side-band replicas with apparent frequencies that do not match the Raman active mode frequencies. Using a time dependent model we show that in such many mode systems, the increased damping of the time dependent transition dipole moment correlation function results in an effective elimination of the vibrational modes from the emission spectrum; subsequently causing the appearance of a regularly spaced progression at a new apparent frequency. We use this damping dependent vibrational reshaping to quantitatively account for the vibronic structure in the emission spectra of pi-conjugated systems in the form of films, dilute solutions and single crystals. In particular, we show that by using the experimentally measured Raman spectrum we can account in detail for the apparent progression frequencies and their relative intensities in the emission spectrum.

cond-mat.other↗

arXiv2005

Apparent vibrational side-bands in pi-conjugated systems: the case of distyrylbenzene

The photoluminescence (PL) spectra of dilute solution and single crystals of distyrylbenzene show unique temperature dependent vibronic structures. The characteristic single frequency progression at high temperatures is modulated by a low frequency progression series at low temperatures. None of the series side band modes corresponds to any of the distyrylbenzene Raman frequencies. We explain these PL properties using a time dependent model with temperature dependent damping, in which the many-mode system is effectively transformed to two- and then to a single "apparent" mode as damping increases.

cond-mat.other↗

arXiv2005

Universal properties of quasi-one-dimensional excitons in semiconducting single-walled carbon nanotubes and $π$-conjugated polymers

The nature of the primary photoexcitations in semiconducting single-walled carbon nanotubes (S-SWCNTs) is of strong current interest. We have studied the emission spectra of S-SWCNTs and two different $π$-conjugated polymers in solutions and films, and have also performed ultrafast pump-probe spectroscopy on these systems. The emission spectra relative to the absorption bands are very similar in S-SWCNTs and polymers, with redshifted photoluminescence in films showing exciton migration. The transient photoinduced absorptions (PAs) in SWCNTs and $π$-conjugated polymers are also remarkably similar, with a low energy PA$_1$ and a higher energy PA$_2$ in all cases. Theoretical calculations of excited state absorptions within a correlated $π$-electron Hamiltonian find the same excitonic energy spectrum for S-SWCNTs and $π$-conjugated polymers, illustrating the universal features of quasi-one-dimensional excitons in carbon-based $π$-conjugated systems. In both cases PA$_1$ is an excited state absorption from the optically allowed exciton to a two-photon exciton that occurs below the continuum band threshold. PA$_1$ therefore gives the lower limit of the binding energy of the lowest optical exciton. The binding energy of lowest exciton belonging to the widest S-SWCNTs with diameters $\geq$ 1 nm in films is 0.3--0.4 eV, as determined by both experimental and theoretical methods.

cond-mat.mtrl-sci↗

arXiv2005

Electroabsorption spectroscopy of single walled nanotubes

We have measured the electric field modulated absorption of a sample of single-walled nanotubes (SWNT) suspended in a solid polyvinyl alcohol matrix. The electroabsorption (EA) spectrum roughly follows the first derivative of the absorption with respect to photon energy, scales quadratically with the electric field strength, and shows a pronounced anisotropy of light polarization with respect to the applied electric field direction. These findings indicate a quadratic Stark effect caused by a change in the polarizability of the excited states, which is common to quasi-one dimensional (1D) excitons in organic semiconductors. The EA spectrum is well described by calculations involving electron-electron interaction in the model Hamiltonian of both zigzag and chiral nanotubes. We have calculated the EA spectra for both zigzag and chiral nanotubes within a model Hamiltonian that includes electron-electron interactions. The calculations reproduce the observed quadratic Stark shift of the lowest optical exciton, as well as the more complicated behavior of the EA spectrum in the energy region that corresponds to the next higher exciton. Our findings show that the low-lying absorption bands in semiconducting SWNT are excitonic in origin, in agreement with transient optical measurements that identify the primary photoexcitations in SWNT as quasi-1D excitons with a substantial binding energy.

cond-mat.mtrl-sci↗

arXiv2005

Transport Studies of Isolated Molecular Wires in Self-Assembled Monolayer Devices

We have fabricated a variety of novel molecular diodes based on self-assembled-monolayers (SAM) of solid-state mixture of molecular wires (1,4 benzene-dimethane-thiol), and molecular insulator spacers (1-pentanethiol) with different concentration ratios r of wires/spacers, which were sandwiched between two gold (Au) electrodes. We introduce two new methods borrowed from Surface Science to (i) confirm the connectivity between the benzene-dimethane-thiol molecules with the upper Au electrode, and (ii) count the number of isolated molecular wires in the devices. The electrical transport properties of the SAM diodes were studied at different temperatures via the conductance and differential conductance spectra. We found that a potential barrier caused by the spatial connectivity gap between the pentanethiol molecules and the upper Au electrode dominates the transport properties of the pure pentanethiol SAM diode (r = 0). The transport properties of molecular diodes with low r-values are dominated by the conductance of the isolated benzene-dimethane-thiol molecules in the device. We found that the temperature dependence of the molecular diodes is much weaker than that of the pure pentanethiol device indicating the importance of the benzene-dimethane-thiol simultaneous bonding to the two Au electrodes that facilitate electrical transport. From the differential conductance spectra we also found that the energy difference, Delta between the Au electrode Fermi-level and the benzene-dimethane-thiol HOMO (or LUMO) level is ~1.5 eV; whereas it is ~2.5 eV for the pentanethiol molecule. The weak temperature dependent transport that we obtained for the SSM diodes reflects the weak temperature dependence of Delta.

cond-mat.mtrl-sci↗

arXiv2002

Fabrication and properties of gallium metallic photonic crystals

Gallium metallic photonic crystals with 100% filling factor have been fabricated via infiltration of liquid gallium into opals of 300-nm silica spheres using a novel high pressure-high temperature technique. The electrical resistance of the Ga-opal crystals was measured at temperatures from 10 to 280 K. The data obtained show that Ga-opal crystals are metallic network with slightly smaller temperature coefficient of resistivity than that for bulk gallium. Optical reflectivity of bulk gallium, plain opal and several Ga-opal crystals were measured at photon energies from 0.3 to 6 eV. A pronounced photonic stop band in the visible spectral range was found in both the plain and Ga infiltrated opals. The reflectivity spectra also show increase in reflectivity below 0.6 eV; which we interpret as a significantly lower effective plasma frequency of the metallic mesh in the infiltrated opal compare to the plasma frequency in the pure metal.

cond-mat.mtrl-sci↗

arXiv2001

Conjugation-Length Dependence of Spin-Dependent Exciton Formation Rates in Pi-Conjugated Oligomers and Polymers

We have measured the ratio, r = $σ_S/σ_T$ of the formation cross section, $σ$ of singlet ($σ_S$) and triplet ($σ_T$) excitons from oppositely charged polarons in a large variety of $π$-conjugated oligomer and polymer films, using the photoinduced absorption and optically detected magnetic resonance spectroscopies. The ratio r is directly related to the singlet exciton yield, which in turn determines the maximum electroluminescence quantum efficiency in organic light emitting diodes (OLED). We discovered that r increases with the conjugation length, CL; in fact a universal dependence exists in which $r^{-1}$ depends linearly on $CL^{-1}$, irrespective of the chain backbone structure. These results indicate that $π$-conjugated polymers have a clear advantage over small molecules in OLED applications.

cond-mat.soft↗