Eitan Ehrenfreund

Bio: Eitan Ehrenfreund is an academic researcher from Technion – Israel Institute of Technology. The author has contributed to research in topics: Raman scattering & Photoluminescence. The author has an hindex of 37, co-authored 215 publications receiving 5391 citations. Previous affiliations of Eitan Ehrenfreund include University of Utah & University of Pennsylvania.

Isotope effect in spin response of pi-conjugated polymer films and devices.

Abstract: The origin of the effect that a magnetic field has on various electronic properties of organic semiconductors is still controversial. It is now shown that substituting hydrogen for deuterium in conducting polymers changes the response to a magnetic field substantially, proving the essential part played by hyperfine interaction in this effect.

Giant Rashba splitting in 2D organic-inorganic halide perovskites measured by transient spectroscopies

Abstract: Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural "multiple quantum wells" that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to "Rashba splitting" close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C6H5C2H4NH3)2PbI4. The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·A, which are among the highest Rashba splitting size parameters reported so far. This finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.

Multiexciton Spectroscopy of a Single Self-Assembled Quantum Dot

Abstract: We apply low temperature confocal optical microscopy to spatially resolve, and spectroscopically study, a single self-assembled quantum dot. By comparing the emission spectra obtained at various excitation levels to a theoretical many body model, we show that (a) single exciton radiative recombination is very weak, and (b) sharp spectral lines are due to optical transitions between confined multiexcitonic states among which excitons thermalize within their lifetimes. Once these few states are fully occupied, broadbands appear due to transitions between states which contain electrons in the continuum.

Amplitude and phase modes in trans -polyacetylene: Resonant Raman scattering and induced infrared activity

Abstract: The resonant Raman scattering (RRS) from the three ${A}_{g}$ modes of trans-polyacetylene and the charged-induced ir modes are analyzed with use of the amplitude- and phase-mode theories. It is shown that the observed phonon frequencies and the relative intensities of all modes obtained at various laser excitation energies \ensuremath{\Elzxh}${\ensuremath{\omega}}_{L}$ is accounted for by a single phonon propagator which also describes the charge-induced infrared-active modes. The dispersion of the RRS frequencies with \ensuremath{\Elzxh}${\ensuremath{\omega}}_{L}$ exhibits inhomogeneity of the sample which in turn provides the functional dependence of the \ensuremath{\pi}-electron gap ${E}_{g}$ on an effective coupling parameter \ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{}. We show that inhomogeneity in both the electron-phonon and the electron-electron interaction parameters yields inhomogeneity in \ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{}. The experimental gap-versus-\ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{} relation is consistent with the Peierls model but allows for weak electron-electron interactions which enhance the gap. We propose a method by which the distribution in \ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{}, P(\ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{}), is directly derived from the experimental spectra. It appears that different samples show different breadth for the distribution function; samples with sharper RRS features have narrower P(\ensuremath{\lambda}\ifmmode \tilde{}\else \~{}\fi{}). We give an experimental estimate of the \ensuremath{\sigma}-bond contribution to the force constant of the carbon-carbon stretching mode and the electron-phonon interaction parameter. The pinning parameter of the charged carriers and its distribution are derived directly from the infrared absorption spectra induced either by doping or by photogeneration. The pinning of the doping-induced carriers is stronger and its distribution is wider; giving thus rise to the broader lines in the doping-induced infrared spectra. The mass of the photogenerated solitons is estimated from the relative strength of the infrared spectra and is approximately equal to the band effective mass of the electrons.

Spin-Polarized Light-Emitting Diode Based on an Organic Bipolar Spin Valve

Abstract: The spin-polarized organic light-emitting diode (spin-OLED) has been a long-sought device within the field of organic spintronics. We designed, fabricated, and studied a spin-OLED with ferromagnetic electrodes that acts as a bipolar organic spin valve (OSV), based on a deuterated derivative of poly(phenylene-vinylene) with small hyperfine interaction. In the double-injection limit, the device shows ~1% spin valve magneto-electroluminescence (MEL) response, which follows the ferromagnetic electrode coercive fields and originates from the bipolar spin-polarized space charge–limited current. In stark contrast to the response properties of homopolar OSV devices, the MEL response in the double-injection device is practically independent of bias voltage, and its temperature dependence follows that of the ferromagnetic electrode magnetization. Our findings provide a pathway for organic displays controlled by external magnetic fields.

The use of nanocrystals in biological detection

Abstract: In the coming decade, the ability to sense and detect the state of biological systems and living organisms optically, electrically and magnetically will be radically transformed by developments in materials physics and chemistry. The emerging ability to control the patterns of matter on the nanometer length scale can be expected to lead to entirely new types of biological sensors. These new systems will be capable of sensing at the single-molecule level in living cells, and capable of parallel integration for detection of multiple signals, enabling a diversity of simultaneous experiments, as well as better crosschecks and controls.

Quantum dot solar cells

Abstract: Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

A Quantum Dot Single-Photon Turnstile Device

Abstract: Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond

Abstract: Raman spectroscopy is a standard characterization technique for any carbon system. Here we review the Raman spectra of amorphous, nanostructured, diamond-like carbon and nanodiamond. We show how to use resonant Raman spectroscopy to determine structure and composition of carbon films with and without nitrogen. The measured spectra change with varying excitation energy. By visible and ultraviolet excitation measurements, the G peak dispersion can be derived and correlated with key parameters, such as density, sp(3) content, elastic constants and chemical composition. We then discuss the assignment of the peaks at 1150 and 1480 cm(-1) often observed in nanodiamond. We review the resonant Raman, isotope substitution and annealing experiments, which lead to the assignment of these peaks to trans-polyacetylene.

A toxicologic review of quantum dots: toxicity depends on physicochemical and environmental factors.

Abstract: As a growing applied science, nanotechnology has considerable global socioeconomic value, and the benefits afforded by nanoscale materials and processes are expected to have significant impacts on almost all industries and all areas of society. A diverse array of engineered nanoscale products and processes have emerged [e.g., carbon nanotubes, fullerene derivatives, and quantum dots (QDs)], with widespread applications in fields such as medicine, plastics, energy, electronics, and aerospace. With the nanotechnology economy estimated to be valued at $1 trillion by 2012, the prevalence of these materials in society will be increasing, as will the likelihood of exposures. Importantly, the vastness and novelty of the nanotechnology frontier leave many areas unexplored, or underexplored, such as the potential adverse human health effects resulting from exposure to novel nanomaterials. It is within this context that the need for understanding the potentially harmful side effects of these materials becomes clear. The reviewed literature suggests several key points: Not all QDs are alike; engineered QDs cannot be considered a uniform group of substances. QD absorption, distribution, metabolism, excretion, and toxicity depend on multiple factors derived from both inherent physicochemical properties and environmental conditions; QD size, charge, concentration, outer coating bioactivity (capping material and functional groups), and oxidative, photolytic, and mechanical stability have each been implicated as determining factors in QD toxicity. Although they offer potentially invaluable societal benefits such as drug targeting and in vivo biomedical imaging, QDs may also pose risks to human health and the environment under certain conditions.