Co-reporter:Zhiyong Pang, Dali Sun, Chuang Zhang, Sangita Baniya, Ohyun Kwon, and Zeev Valy Vardeny
ACS Photonics August 16, 2017 Volume 4(Issue 8) pp:1899-1899
Publication Date(Web):July 31, 2017
DOI:10.1021/acsphotonics.7b00567
Exciplex organic light-emitting diodes (XOLEDs) utilize nonemissive triplet excitons via a reverse intersystem crossing process of thermally activated delayed fluorescence. The small energy difference between the lowest singlet and triplet levels of exciplex also allows a magnetic field to manipulate their populations, thereby achieving ultralarge “intrinsic” magneto-electroluminescence (MEL) in XOLEDs. Here we incorporate it into a hybrid type of spintronic device (“hybrid spin-XOLED”), where the XOLED is connected to a magnetic tunnel junction with large magnetoresistance, to introduce an “extrinsic” MEL response that interferes with the “intrinsic” MEL. The ratio between two MEL contributions, the MEL value, and the field response were altered by changing the exciplex layer thickness or actively manipulated by adding another current source that drives the XOLED. Most importantly, by involving two XOLEDs (green and red) in the same circuit, the hybrid spin-XOLED shows a color change when sweeping the magnetic field, which provides an alternative way for future OLED display technologies.Keywords: exciplex; magneto-electroluminescence; organic light-emitting diodes; organic spintronics; thermally activated delayed fluorescence;
Co-reporter:Chuang Zhang, Dali Sun, Xiaojie Liu, Chuan-Xiang Sheng, and Zeev Valy Vardeny
The Journal of Physical Chemistry Letters April 6, 2017 Volume 8(Issue 7) pp:1429-1429
Publication Date(Web):March 16, 2017
DOI:10.1021/acs.jpclett.7b00353
Organo-lead halide perovskites show excellent optoelectronic properties; however, the unexpected inconsistency in forward–backward I–V characteristics remains a problem for fabricating solar panels. Here we have investigated the reasons behind this “hysteresis” by following the changes in photocurrent and photoluminescence under electric field poling in transverse CH3NH3PbI3-based devices from 300 to 10 K. We found that the hysteresis disappears at cryogenic temperatures, indicating the “freeze-out” of the ionic diffusion contribution. When the same device is cooled under continuous poling, the built-in electric field from ion accumulation brings significant photovoltaic effect even at 10 K. From the change of photoluminescence upon polling, we found a second dipole-related mechanism which enhances radiative recombination upon the alignment of the organic cations. The ionic origin of hysteresis was also verified by applying a magnetic field to affect the ion diffusion. These findings reveal the coexistence of ionic and dipole-related mechanisms for the hysteresis in hybrid perovskites.
Co-reporter:Yaxin Zhai;Chuan Xiang Sheng;Chuang Zhang;Zeev Valy Vardeny
Advanced Functional Materials 2016 Volume 26( Issue 10) pp:1617-1627
Publication Date(Web):
DOI:10.1002/adfm.201505115
Studying the room temperature broadband ultrafast transient response of photoexcitations in three perovskite films, namely MAPbI3, MAPbI1.1Br1.9, and MAPbI3−xClx (MA = CH3NH3), allowed unravelling the branching ratio between photogenerated carriers and excitons, a key factor for optoelectronic applications of perovskites. An instantaneously generated mid-IR photoinduced absorption (PA) band, PA1 is observed in all three perovskites, as well as a strong derivative-like band of photoinduced bleaching (PB) and PA (PA2) close to the corresponding absorption band edge. From the distinguished different decay dynamics of the PA bands in MAPbI3, PA1 is interpreted as due to the exciton transition, whereas PA2 and PB are due to band-filling effect caused by the photocarriers. In contrast, all bands in MAPbI1.1Br1.9 and MAPbI3−xClx share the same dynamics and are therefore due to the same species, namely photogenerated excitons. The transient photoinduced polarization memory (POM) for both excitons and photocarriers as well as the steady-state photoluminescence (PL) emission are observed in MAPbI3, but not in MAPbI1.1Br1.9 and MAPbI3−xClx because they possess cubic symmetry at room temperature. The estimated long excitons diffusion length (≈150 nm) in MAPbI3 opens up the possibility of photocarriers generation at interfaces and grain boundaries even when the exciton binding energy is large compare to kBT.
Co-reporter:ChuanXiang Sheng, Yaxin Zhai, Uyen Huynh, Chuang Zhang, Z. Valy Vardeny
Synthetic Metals 2016 Volume 216() pp:31-39
Publication Date(Web):June 2016
DOI:10.1016/j.synthmet.2015.09.013
•Application of ultrafast photomodulation spectroscopy in three classes of semiconductors.•Universal excitonic transitions in π-conjugated polymers, nanotubes and hybrid perovskites.•Exciton/free-carrier duality nature in hybrid perovskites such as CH3NH3PbI3.•The exciton binding energy is crucial in determining the primary photoexcitations.We compare the ultrafast dynamics of the primary photoexcitations in various π-conjugated organic semiconductors, semiconducting single-walled carbon nanotubes (S-NTs) and organometal trihalide perovskites including CH3NH3PbI3 (MAPbI3) and CH3NH3PbI1.1Br1.9 (MAPbI1.1Br1.9), using broadband pump–probe photomodulation spectroscopy in the spectral range of 0.2–2.7 eV with 300 fs time resolution. The primary photoexcitations in single polymer chains and isolated S-NTs have been found to be quasi-one-dimensional (q-1D) excitons, with characteristic photoinduced absorption (PA) band due to intra-band transitions. This conclusion is in agreement with the large exciton binding energy, Eb in polymers and S-NTs (where Eb > 200 meV), illustrating the universal optical characteristic features of q-1D excitons. In three dimensional (3D) semiconductors of organometal trihalide perovskites such as MAPbI3 we found that with above-gap excitation both photo-carriers and excitons are photogenerated; but only photocarriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in MAPbI1.1Br1.9. The contrast between MAPbI3 and MAPbI1.1Br1.9 is ascribed to the difference of Eb, which is ∼20 meV and ∼110 meV, respectively. Our work shows that Eb is one of the crucial parameters that determine the photophysics characteristic of semiconductors, that result in universal occurrence of an exciton PA band, regardless if the compound is q-1D carbon based π-conjugated semiconductors, NTs, or 3D crystalline perovskite semiconductors. At the same time, our work also shows that the broadband ultrafast photomodulation spectroscopy is a powerful tool in analyzing the photophysics of semiconductors, and emphasizes the need for a broad probe spectral range in order to decipher the primary photoexcitation species.
Co-reporter:Tek Basel;Dali Sun;Sangita Baniya;Ryan McLaughlin;Hyeonho Choi;Ohyun Kwon
Advanced Electronic Materials 2016 Volume 2( Issue 2) pp:
Publication Date(Web):
DOI:10.1002/aelm.201500248
Reverse intersystem crossing (RISC) from triplet to singlet states has been recently introduced to photophysics of organic chromophores. One type of RISC occurs in donor (D)–acceptor (A) composites that form an exciplex manifold in which the energy difference, ΔEST between the lowest singlet (S1) and triplet (T1) levels of the exciplex is small (<100 meV) thus allowing RISC at room temperature. This adds a delayed component to the photoluminescence emission that is widely known as thermally activated delayed fluorescence. Here, it is found that the electroluminescence in organic light-emitting diodes (OLED) based on electron D–A exciplex is significantly enhanced (up to ≈40%) by applying magnetic field, due to the existence of an additional spin-mixing channel between singlet and triplet states in the exciplex. The large magneto-electroluminescence (MEL) in N,N,N′,N′-tetrakis(4-methoxyphenyl)benzidine: tris-[3-(3-pyridyl)mesityl]borane [D–A] based OLEDs is demonstrated. These results are supported by magneto-photoluminescence (MPL) measurements, and density functional theory calculations. Importantly, it is found that both MEL and MPL are thermally activated indicating the dominant role of the RISC process. The MEL(B) response using the Δg mechanism is analyzed, where the electron and hole g-values are different from each other because they reside in different molecules.
Co-reporter:Evan LafalceChuang Zhang, Xiaojie Liu, Zeev Valy Vardeny
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 51) pp:
Publication Date(Web):December 5, 2016
DOI:10.1021/acsami.6b11925
We studied steady state and transient photocurrents in thin film and single-crystal devices of MAPbBr3, a prototype organic–inorganic hybrid perovskite. We found that the devices’ capacitance is abnormally large, which originates from accumulation of large densities of Pb2+ and Br– in the active perovskite layer. Under applied bias, these ions are driven toward the opposite electrodes leading to space-charge fields close to the metal/perovskite interfaces. The ion accumulation, in turn, causes photocurrent reversal polarity that depends on the history of the applied bias and excitation photon energy with respect to the optical gap. Furthermore, the large capacitive response dominates the transient photocurrent and, therefore, obscures the weaker contribution from the photocarriers’ drift. We show that these properties depend on the ambient conditions in which the measurements are performed. Understanding these phenomena may lead to better control over the stability of perovskite photodetectors for visible light.Keywords: charge-carrier transport; hybrid perovskites; ionic impurities; I−V hysteresis; photodetectors;
Co-reporter:Tek Basel;Uyen Huynh;Tianyue Zheng;Tao Xu;Luping Yu
Advanced Functional Materials 2015 Volume 25( Issue 12) pp:1895-1902
Publication Date(Web):
DOI:10.1002/adfm.201403191
The charge photogeneration and recombination processes in organic photovoltaic solar cells based on blend of the low bandgap copolymer, PTB7 (fluorinated poly-thienothiophene-benzodithiophene) with C60-PCBM using optical, electrical, and magnetic measurements in thin films and devices is studied. A variety of steady state optical and magneto-optical techniques were employed, such as photoinduced absorption (PA), magneto-PA, doping-induced absorption, and PA-detected magnetic resonance (PADMR); as well as picosecond time-resolved PA. The charge polarons and triplet exciton dynamics in films of pristine PTB7, PTB7/fullerene donor–acceptor (D–A) blend is followed. It is found that a major loss mechanism that limits the power conversion efficiency (PCE) of PTB7-based solar cell devices is the “back reaction” that leads to triplet exciton formation in the polymer donor from the photogenerated charge-transfer excitons at the D–A interfaces. A method of suppressing this “back reaction” by adding spin½ radicals Galvinoxyl to the D–A blend is presented; this enhances the cell PCE by ≈30%. The same method is not effective for cells based on PTB7/C70-PCBM blend, where high PCE is reached even without Galvinoxyl radical additives.
Co-reporter:Dali Sun, Eitan Ehrenfreund and Z. Valy Vardeny
Chemical Communications 2014 vol. 50(Issue 15) pp:1781-1793
Publication Date(Web):16 Dec 2013
DOI:10.1039/C3CC47126H
The first decade of organic spintronics research has benefitted from the analogy and previous experience of the inorganic spintronics field, coupled with the unlimited versatility of organic materials synthesis. At the same time, the field of organic spintronics has developed into an attractive and promising field of its own, with rich physics and promising unique potential applications. We review here a set of significant milestones achieved in organic spintronic devices such as organic spin valves, bipolar spin-valves, and hybrid organic/inorganic light emitting diodes in comparison with representative inorganic spintronic devices. We also point out acute problems that need to be resolved before the young field of organic spintronics can mature.
Co-reporter:B. Pandit, B.R. Gautam, T.P. Basel, Z.V. Vardeny
Organic Electronics 2014 Volume 15(Issue 6) pp:1149-1154
Publication Date(Web):June 2014
DOI:10.1016/j.orgel.2014.03.021
•PCE of various P3HT/PCBM solar cells is correlated with ps transient photophysics in blend films.•The relative transient charge-transfer exciton (CTE) density is highest for the optimum blend.•The CTE dissociation rate across the blend agrees well with Isc of OPV devices based on same blend.Ultrafast transient spectroscopy was applied to various films of regio-regular polythiophene (RR-P3HT, donor-D) and C60 derivative (PCBM, acceptor-A) blends, in conjunction with organic photovoltaic (OPV) solar cell fabrication and evaluation based on the same blends, for investigating the existence of a correlation between the device efficiency and the transient photophysics characteristics. For our transient spectroscopy measurements we used the ps pump–probe transient photomodulation (PM) technique having a unique probe spectral range in the mid-IR (0.25–1.05 eV). We found that the transient PM spectra contain photoinduced absorption bands of excitons in the donor polymer, charge transfer excitons (CTE) at the D–A interfaces, and free polarons. We compared the relative density of photogenerated CTE in D–A blends having various D–A weight ratio with the photocurrent density of fabricated solar cells based on the same blends. We found that the dissociation of CTE into free charges correlates well with the optoelectronic measurements of the corresponding solar cell. The more efficient CTE dissociation occurs in films having the optimum D–A weight ratio (which is 1.2:1 for the P3HT/PCBM system) that shows the highest OPV power conversion efficiency; this is due to the lowest CTE binding energy for this blend that results from the most suitable D- and A- grain sizes. We also show that the exciton lifetime is the shortest for the optimum blend, and this helps boosting the device efficiency by reducing energy loss.Graphical abstract
Co-reporter:Tek Basel, Dali Sun, Bhoj Gautam, Z. Valy Vardeny
Journal of Luminescence 2014 155() pp: 89-94
Publication Date(Web):
DOI:10.1016/j.jlumin.2014.06.015
Co-reporter:Tho D. Nguyen, Eitan Ehrenfreund, Z. Valy Vardeny
Synthetic Metals 2013 Volume 173() pp:16-21
Publication Date(Web):1 June 2013
DOI:10.1016/j.synthmet.2012.11.015
We report magnetic field, voltage and temperature dependent studies on recently realized spin-polarized organic light emitting diode (spin-OLED) with ferromagnetic (FM) electrodes that acts as a bipolar organic spin valve (OSV). In the double-injection regime the device shows ∼1% spin-valve related magneto-electroluminescence (MEL) response that follows the FM electrode coercive fields. In stark contrast to the properties of homopolar OSV devices, both the MEL and magneto-conductivity responses in the double-injection regime are practically bias voltage independent, and their temperature dependence follows that of the FM electrode magnetization. Careful comparative studies of devices with various FM/non-FM electrodes rule out the possibility that the observed MEL is due to the effect of stray magnetic fields associated with the FM electrodes. The spin-OLED behavior is explained by the spin-aligned space charge-limited current operation upon reaching double-injection conditions during bipolar operation.Highlights► We report magnetic field, voltage and temperature dependent studies on spin-polarized organic light emitting diodes. ► In the double-injection regime the device shows ∼1% spin-valve related magnetoelectroluminescence (MEL) response. ► We rule out the possibility that the MEL is due to stray magnetic field associated with the ferromagnetic electrodes.
Co-reporter:Ye Zhang, Bhoj R. Gautam, Tek P. Basel, Debra J. Mascaro, Z. Valy Vardeny
Synthetic Metals 2013 Volume 173() pp:2-9
Publication Date(Web):1 June 2013
DOI:10.1016/j.synthmet.2012.12.035
One of the major losses in organic photovoltaic (OPV) devices has been the recombination of polaron pairs (PP) at the donor–acceptor (D–A) domain interfaces that prevent photogeneration of free charges. Here we report our study on a new method, namely doping the device active layer with spin ½ radical galvinoxyl, to suppress PP recombination at the D–A domain interfaces, and thereby improve the OPV solar cell efficiency. We demonstrate that at an optimal doping level of 3 wt.%, the efficiency of a standard regio-regular poly(3-hexylthiophene) (P3HT) and 1-[3-(methoxycarbonyl)propyl]-1-1-phenyl)[6,6]C61 (PCBM) solar cell improves by 18%. We found that this enhancement is not due to morphology change upon galvinoxyl doping, but rather is directly related to the spin degree of freedom of the radical, by reducing PP recombination rate at the P3HT/PCBM interfaces via spin flipping interaction. The enhancement is more significant in OPV systems that are PCBM-rich. We also conclude that the existence of D–A interface is crucial for the enhancement to occur. In addition, we also found that the galvinoxyl radical additives improve the efficiency of solar cells based on other D–A blends with of other donor polymers, but only with the acceptor fullerene PCBM. However, we believe this method may work with other D–A blends using other spin ½ radical additives.Highlights► P3HT/PCBM OPV PCE is enhanced by 18% upon doing spin ½ radical galvinoxyl. ► The enhancement is directly related to the radical's spin degree of freedom. ► The method works with OPV cells based on other donor polymers. ► Exchange interaction converts PP from singlet to triplet and reduces recombination.
Co-reporter:C.-X. Sheng, T. Basel, B. Pandit, Z.V. Vardeny
Organic Electronics 2012 Volume 13(Issue 6) pp:1031-1037
Publication Date(Web):June 2012
DOI:10.1016/j.orgel.2012.02.023
We used the transient and steady state photomodulation spectroscopies for studying the photoexcitations dynamics in blends of regio-regular poly(3-hexylthiophene) (RR-P3HT) and fullerene in a broad spectral range from 0.13 to 2.25 eV. We found that both localized polarons and singlet excitons are instantaneously photogenerated in the blends. However the photogeneration process of delocalized polarons which contribute to the photocurrent proceeds in two steps: first, within a couple of ps the excitons generated in the polymer domains populate the charge transfer complex states at the RR-P3HT/fullerene interfaces; this is followed by the charge transfer ionization into delocalized charge polarons in the polymer and fullerene constituents within ∼20 ps. In contrast, the localized polaron dynamics are unrelated with the excitons and delocalized polarons dynamics. We also report on the occurrence of ultrafast quantum interference anti-resonances between photoinduced infrared-active vibrations and the delocalized polaron band in the blends, which shows the delocalization character of the photogenerated charges that contribute to the photocurrent.Graphical abstractHighlights► Localized polarons and excitons are instantaneously photogenerated in D–A blends. ► The generation of delocalized polarons occurs at the expense of excitons. ► The dynamics of localized polarons are unrelated with those of excitons. ► Ultrafast quantum interference anti-resonances are photogenerated.
Co-reporter:Tho D. Nguyen;Eitan Ehrenfreund
Science 2012 Volume 337(Issue 6091) pp:204-209
Publication Date(Web):13 Jul 2012
DOI:10.1126/science.1223444
Spin-Dependent Light Emission
Spintronic devices exploit electronic currents that are spin polarized, which have an excess of one spin current over the other. One way to detect this polarization would be to create a light-emitting diode that is sensitive to spin polarization. Along these lines, Nguyen et al. (p. 204) constructed a bipolar device in which an organic semiconductor was sandwiched between two ferromagnetic contacts whose relative polarization could be controlled by an applied magnetic field. Magneto-electroluminescence of the order of ∼1% was observed at a bias voltage of ∼3.5 V. The use of a deuterated organic polymer interlayer improved spin transport relative to polymers with hydrogen side groups, and a thin LiF buffer layer on the ferromagnetic cathode improved electron injection efficiency.
Co-reporter:T.D. Nguyen, B.R. Gautam, E. Ehrenfreund, Z.V. Vardeny
Synthetic Metals 2011 Volume 161(7–8) pp:604-607
Publication Date(Web):April 2011
DOI:10.1016/j.synthmet.2010.11.051
The magnetoconductance (MC) of unipolar and bipolar organic diodes based on active layers made of the π-conjugated polymer DOO-PPV is presented in the range of magnetic field strength, B below about 100 mT, and for various polymer exchange isotopes. The MC(B) response is composed of two main regions: (i) a “sign-reversal” region for |B| < 1–2 mT, where MC(B) reverses its sign reaching a maximum absolute value |MC|m at B = Bm; and (ii) a monotonic region for |B| > 2 mT, where MC(B) monotonically increases (decreases) for bipolar (unipolar) devices with an approximate Lorentzian line shape of width, ΔB. Similar behavior has been observed for the magneto-electroluminescence in bipolar devices. We found that MC(B) in both field regions is strongly isotope dependent; in particular both Bm and ΔB are larger for nuclei with larger hyperfine interaction (HFI) constant. The complete MC(B) response, including the novel sign-reversal component is explained by a model of spin-coupled pairs of either same or opposite charge polarons, in which the HFI is explicitly included in the electronic spin Hamiltonian.Research highlights▶ The magnetoconductance (MC) of unipolar and bipolar organic diodes. ▶ The MC(B) response is composed of two main regions: |B| < 1–2 mT and |B| > 2 mT. ▶ MC(B) in both field regions is strongly isotope dependent. ▶ The MC(B) response is explained by a model of spin-coupled pairs.
Co-reporter:Tho D. Nguyen, Golda Hukic-Markosian, Fujian Wang, L. Wojcik, Xiao-Guang Li, Eitan Ehrenfreund, Z. Valy Vardeny
Synthetic Metals 2011 Volume 161(7–8) pp:598-603
Publication Date(Web):April 2011
DOI:10.1016/j.synthmet.2010.12.013
Although the hyperfine interaction (HFI) has been foreseen to play an important role in organic spin response, clear experimental evidence for it has been scarce. We studied and compared spin dynamics in films and organic spin-valve (OSV) devices based on π-conjugated polymers made of protonated, H-, deuterated, D-hydrogen (having a weaker HFI strength, aHFI), and 13C-rich chains (having stronger aHFI). We demonstrate that HFI indeed plays a crucial role in the spin dynamics of all three polymer isotopes. Films based on the D-polymer show substantial narrower optically detected magnetic resonance of spin ½ polarons; whereas due to the longer spin diffusion, OSV devices based on D-polymers show substantially larger magnetoresistance. We also found that the giant magnetoresistance (GMR) steep temperature dependence in OSV devices is isotope independent, showing that is due to the magnetic response of the ferromagnetic electrodes. In addition, we found that the GMR steep voltage dependence is isotope dependent indicating that is due to a spin injection process at the electrodes, rather than spin transport through the organic interlayer. Finally we report GMR response in OSV devices made of C60 interlayer having very weak HFI. These devices show sharp GMR response, and may therefore be excellent candidates for room temperature operation.
Co-reporter:
Nature Physics 2010 6(4) pp:
Publication Date(Web):2010-01-24
DOI:10.1038/nphys1509
Random lasing consists of a sequence of narrow, coherent spectral lines formed when stimulated emission in a disordered gain medium is excited above threshold excitation intensity, Ith. We studied the Ith distribution function in π-conjugated polymer films, and its dependence on the excitation area size and polymer mixture with TiO2 nanoparticles; optical pictures of the excited film area support the statistical measurements. At Ith we found that a single dominant random lasing resonator appears in the picture, with good agreement between its diameter and the cavity size extracted from the power Fourier transform analysis of the emission spectrum. This was directly confirmed by a new technique of space/spectrum cross-correlation of the laser emission lines from the excited area with 10×10 μm2 spatial resolution. The statistical results coupled with the imaging data provide strong evidence for the model of random resonators in the gain medium for explaining random lasing in π-conjugated polymer films.
Co-reporter:Fujian Wang, Z. Valy Vardeny
Synthetic Metals 2010 Volume 160(3–4) pp:210-215
Publication Date(Web):February 2010
DOI:10.1016/j.synthmet.2009.10.014
Organic Spintronics has been considered to be the physics and applications of spin polarized electron injection, transport, manipulation and detection in organic diodes by the application of an external magnetic field. The prototype device is the organic spin-valve (OSV), which is based on an organic semiconductor spacer placed in between two ferromagnetic electrodes having different coercive fields, of which magnetoresistance changes with the applied field. Immense progress has been achieved in the past few years in fabricating, studying and understanding the underlying physics of these devices. We highlight the most significant advance in OSV research at the University of Utah, including the magnetoresistance response temperature and bias voltage dependencies; and show significant room temperature operation using LSMO/C60/Co structure. We also report positive OSV-related magnetoresistance at low temperature, which was achieved using LSMO/polymer/Co OSV structure, where the polymer is a poly[phenylene-vinylene] derivative.
Co-reporter:Ye Zhang, Golda Hukic-Markosian, Debra Mascaro, Zeev Valy Vardeny
Synthetic Metals 2010 Volume 160(3–4) pp:262-265
Publication Date(Web):February 2010
DOI:10.1016/j.synthmet.2009.10.005
Co-reporter:Tatsunosuke Matsui,
Amit Agrawal,
Ajay Nahata
&
Z. Valy Vardeny
Nature 2007 446(7135) pp:517
Publication Date(Web):2007-03-29
DOI:10.1038/nature05620
Resonantly enhanced light transmission through periodic subwavelength aperture arrays perforated in metallic films1 has generated significant interest because of potential applications in near-field microscopy, photolithography, displays, and thermal emission2. The enhanced transmission was originally explained by a mechanism where surface plasmon polaritons (collective electronic excitations in the metal surface) mediate light transmission through the grating1, 3. In this picture, structural periodicity is perceived to be crucial in forming the transmission resonances. Here we demonstrate experimentally that, in contrast to the conventional view, sharp transmission resonances can be obtained from aperiodic aperture arrays. Terahertz transmission resonances are observed from several arrays in metallic films that exhibit unusual local n-fold rotational symmetries, where n = 10, 12, 18, 40 and 120. This is accomplished by using quasicrystals with long-range order, as well as a new type of ‘quasicrystal approximates’ in which the long-range order is somewhat relaxed. We find that strong transmission resonances also form in these aperiodic structures, at frequencies that closely match the discrete Fourier transform vectors in the aperture array structure factor. The shape of these resonances arises from Fano interference4 of the discrete resonances and the non-resonant transmission band continuum related to the individual holes5. Our approach expands potential design parameters for aperture arrays that are aperiodic but contain discrete Fourier transform vectors, and opens new avenues for optoelectronic devices.
Co-reporter:F.J. Wang, Z.H. Xiong, D. Wu, J. Shi, Z.V. Vardeny
Synthetic Metals 2005 Volume 155(Issue 1) pp:172-175
Publication Date(Web):15 October 2005
DOI:10.1016/j.synthmet.2005.07.345
We have embarked on studying magneto-transport response of organic spin-valves made of evaporated Alq3 spacer sandwiched between two ferromagnetic (FM) electrodes with spin-injecting capability. Recently, we have fabricated and completed studies on organic spin-valve devices using half-metallic manganites as one of the spin-injecting FM electrode, which have shown giant magnetoresistance (GMR) of up to 40% at 11 K. Also we found that the GMR response decreases at higher temperatures, and actually disappears at temperatures above ∼180 K, partially because the FM manganite loses its magnetic properties at ambient temperature. In order to realize room temperature organic spin-valve devices, we have begun studying spin-valve devices where both spin-injecting FM electrodes have high Curie temperatures, and thus maintain their magnetic properties at ambient temperature. In this paper we report our preliminary investigations of Fe/Alq3/Co spin-valves, where both electrodes are regular, d-band metallic FM's. We found that these devices show GMR with maximum of about 5% at 11 K. However, at elevated temperatures the GMR value steeply decreases, and in fact vanishes at 90 K. We attribute this decrease to the increase of the spin-lattice relaxation rate of the injected spin-1/2 carriers in the Alq3 spacer at elevated temperatures, where there is also change in the spin carrier injection mechanism at the Fe/Alq3 interface.
Co-reporter:M.N. Shkunov;Z.V. Vardeny;M.C. DeLong;R.C. Polson;A.A. Zakhidov;R.H. Baughman
Advanced Functional Materials 2002 Volume 12(Issue 1) pp:
Publication Date(Web):9 JAN 2002
DOI:10.1002/1616-3028(20020101)12:1<21::AID-ADFM21>3.0.CO;2-S
A photonic crystal laser that is tunable throughout the visible in three-dimensionally switchable directions is demonstrated. This photo-pumped laser utilizes a dye-infiltrated, single-crystal SiO2 opal having incomplete bandgaps. Our results support a gap-state-enhanced distributed feedback mechanism for lasing. Three different types of wavelength tunability are demonstrated, each applicable over a different frequency range and involving either single or multiple bandgaps. The many independent laser cavities that exist in one photonic crystal are demonstrated by simultaneously obtaining lasing in various colors and directions from an opal crystal. The observation of characteristic laser emission lines provides a new spectroscopy for characterizing intra-gap photonic states, which may be useful for developing the photonic crystal analogues of electronic circuitry.
Co-reporter:X.M. Jiang;R. Österbacka;O. Korovyanko;C.P. An;B. Horovitz;R.A.J. Janssen;Z.V. Vardeny
Advanced Functional Materials 2002 Volume 12(Issue 9) pp:
Publication Date(Web):16 SEP 2002
DOI:10.1002/1616-3028(20020916)12:9<587::AID-ADFM587>3.0.CO;2-T
Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.
Co-reporter:R. C. Polson;A. Chipouline;Z. V. Vardeny
Advanced Materials 2001 Volume 13(Issue 10) pp:
Publication Date(Web):17 MAY 2001
DOI:10.1002/1521-4095(200105)13:10<760::AID-ADMA760>3.0.CO;2-Z
The properties of random lasers in π-conjugated polymer films and solutions infiltrated into opal photonic crystals are reviewed. We show that random lasing is a generic phenomenon that occurs in disordered gain media at an excitation intensity regime higher than that giving rise to amplified spontaneous emission. The emission radiation is coherent as demonstrated by photon statistics methods, and its spectrum contains many laser modes from which a typical cavity length can be obtained using Fourier transform spectroscopy. Since the random cavities are independent from each other, we show that laser emission in several colors is possible when mixing different dyes in the same random cavities. In addition, it is demonstrated that random lasing is formed in many disordered media with various scattering properties ranging from a regime of light prelocalization to that of weak scattering.
Co-reporter:M. Wohlgenannt,
Kunj Tandon,
S. Mazumdar,
S. Ramasesha
and
Z. V. Vardeny
Nature 2001 409(6819) pp:494
Publication Date(Web):
DOI:10.1038/35054025
Electroluminescence in organic light-emitting diodes arises from a charge-transfer
reaction between the injected positive and negative charges by which they
combine to form singlet excitons that subsequently decay radiatively. The
quantum yield of this process (the number of photons generated per electron
or hole injected) is often thought1 to have a statistical upper
limit of 25 per cent. This is based on the assumption that the formation cross-section
of singlet excitons, &bitsigma;S, is approximately the same as
that of any one of the three equivalent non-radiative triplet exciton
states, &bitsigma;T; that is, &bitsigma;S/&bitsigma;
T 1. However, recent experimental2 and theoretical3 work suggests that &bitsigma;S/&bitsigma;T
may be greater than 1. Here we report direct measurements of &bitsigma;
S/&bitsigma;T for a large number of -conjugated polymers
and oligomers. We have found that there exists a strong systematic, but not
monotonic, dependence of &bitsigma;S/&bitsigma;T on
the optical gap of the organic materials. We present a detailed physical picture
of the charge-transfer reaction for correlated -electrons, and quantify
this process using exact valence bond calculations. The calculated &bitsigma;
S/&bitsigma;T reproduces the experimentally observed trend.
The calculations also show that the strong dependence of &bitsigma;S
/&bitsigma;T on the optical gap is a signature of the discrete
excitonic energy spectrum, in which higher energy excitonic levels participate
in the charge recombination process.
Co-reporter:Dali Sun, Eitan Ehrenfreund and Z. Valy Vardeny
Chemical Communications 2014 - vol. 50(Issue 15) pp:NaN1793-1793
Publication Date(Web):2013/12/16
DOI:10.1039/C3CC47126H
The first decade of organic spintronics research has benefitted from the analogy and previous experience of the inorganic spintronics field, coupled with the unlimited versatility of organic materials synthesis. At the same time, the field of organic spintronics has developed into an attractive and promising field of its own, with rich physics and promising unique potential applications. We review here a set of significant milestones achieved in organic spintronic devices such as organic spin valves, bipolar spin-valves, and hybrid organic/inorganic light emitting diodes in comparison with representative inorganic spintronic devices. We also point out acute problems that need to be resolved before the young field of organic spintronics can mature.
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