Electronic,structural and chemical effects of charge-transfer at organic/inorganic interfaces

During the last decade, interest on the growth and self-assembly of organic molecular species on solid surfaces spread over the scientific community, largely motivated by the promise of cheap, flexible and tunable organic electronic and optoelectronic devices. These efforts lead to important advances in our understanding of the nature and strength of the non-bonding intermolecular interactions that control the assembly of the organic building blocks on solid surfaces, which have been recently reviewed in a number of excellent papers. To a large extent, such studies were possible because of a smart choice of model substrate-adsorbate systems where the molecule-substrate interactions were purposefully kept low, so that most of the observed supramolecular structures could be understood simply by considering intermolecular interactions, keeping the role of the surface always relatively small (although not completely negligible). On the other hand, the systems which are more relevant for the development of organic electronic devices include molecular species which are electron donors, acceptors or blends of donors and acceptors. Adsorption of such organic species on solid surfaces is bound to be accompanied by charge-transfer processes between the substrate and the adsorbates, and the physical and chemical properties of the molecules cannot be expected any longer to be the same as in solution phase. In recent years, a number of groups around the world have started tackling the problem of the adsorption, self- assembly and electronic and chemical properties of organic species which interact rather strongly with the surface, and for which charge-transfer must be considered. The picture that is emerging shows that charge transfer can lead to a plethora of new phenomena, from the development of delocalized band-like electron states at molecular overlayers, to the existence of new substrate-mediated intermolecular interactions or the strong modification of the chemical reactivity of the adsorbates. The aim of this review is to start drawing general conclusions and developing new concepts which will help the scientific community to proceed more efficiently towards the understanding of organic/inorganic interfaces in the strong interaction limit, where charge-transfer effects must be taken into consideration.     

Monolithic integration of electroabsorption modulator and DFB laser for 10-Gb/s transmission

A strained InGaAsP-InP multiple-quantum-well DFB laser monolithically integrated with electroabsorption modulator by ultra-low-pressure (22 mbar) selective-area-growth is presented. The integrated chip exhibits superior characteristics, such as low threshold current of 19 mA, single-mode operation around 1550 nm range with side-mode suppression ratio over 40 dB, and larger than 16 dB extinction ratio when coupled into a single-mode fiber. More than 10 GHz modulation bandwidth is also achieved. After packaged in a compact module, the device successfully performs 10-Gb/s NRZ transmission experiments through 53.3 km of standard fiber with 8.7 dB dynamic extinction ratio. A receiver sensitivity of −18.9 dBm at bit-error-rate of 10⁻¹⁰ is confirmed.     

The generation of optical duobinary signal using electroabsorption modulator for optical access networks

We generate the duobinary signal by utilizing the intensity modulation characteristic of the electroabsorption modulator and the phase characteristic of phase shifter for the high-speed transmission system. In addition, compared to those commercial 10 Gbps OOK modulators, lower cost, better performances and easier integration can be achieved by using these duobinary modulators.     

Experimental study of NRZ format wavelength conversion using electroabsorption modulator

In this paper, NRZ format wavelength conversion using cross absorption modulation (XAM) in InGaAsP MQW electroabsorption modulator (EAM) is investigated experimentally in detail for the first time, to the best of our knowledge. The influence of two key operating parameters, input signal power and bias voltage of EAM, on wavelength conversion performance is studied. Experimental results indicate that there exists optimal bias voltage for fixed input signal power and that there exists also optimal input signal power for fixed bias voltage and the lowest power penalty of 0.9 dB is obtained by optimizing these two parameters. The physical explanations are given for these results based on operating characteristic of EAM device. Finally, the probe light wavelength dependence of wavelength conversion is studied for span of 1530–1560 nm.     

Two-photon coincidence studies of highly-charged ion relaxation in solids

We studied the relaxation of hollow atoms by two-photon transitions during the stopping of slow (E=8.5q keV) highly charged Pb^q+ (q=53–58) ions in a thin Ta foil. The X-rays emitted during the relaxation process were detected in coincidence with two Si(Li) detectors. Particular attention was paid to the role of the internal dielectronic excitation (IDE) in the relaxation dynamics. We found a clear contribution to relaxation by the IDE process also for projectiles with initial M-shell vacancies and IDE speeds up relaxation of electrons from high Rydberg states by directly filling vacancies in the N-shell that is accompanied with the excitation of M-shell electrons. We discuss the relaxation mechanism, and present results for the fluorescence yield and the IDE probability of highly charged Pb ions in a solid.     

Performance limits of pico-cell systems using radio-over-fiber techniques with an electroabsorption modulator

We investigate the performance limits of a pico-cell system using the radio- over-fiber technique with an electroabsorption modulator (EAM) in a 2.4 GHz wireless local area network (WLAN) environment. To understand the performance limits of the system using an EAM as the optical transceiver, we vary the optical modulation index (OMI) of the transmitter for different bias voltages to the EAM. Power margins in the links are found to estimate a possible service range, and packet loss rates (PLRs) in the pico-cell system are measured to evaluate the transmission performance. The total service range can be extended up to 3 m by adjusting the OMI of the transmitter for different applied bias voltages to the EAM.     

Raman spectroscopy of the PTCDA–inorganic semiconductor interface: evidence for charge transfer

In the present work, we investigate the vibrational properties of a PTCDA molecule with an additional positive or negative charge using density functional theory. With respect to the calculated vibrational frequencies of the neutral molecule, some modes in particular in the region 1200–1800 cm⁻¹ show large shifts. These calculations are compared with resonant Raman spectra of sub-monolayer PTCDA films on passivated semiconductor surfaces, both before and after annealing the deposited films at elevated temperatures (350 °C). Independent of the sample treatment, the sub-monolayer Raman spectra correspond quite well to reference spectra obtained for thicker films, and we find no evidence for the strong shifts predicted in the calculations for the charged species. From the small changes in the mode frequencies it can be concluded that any charge transfer present involves significantly less than one elementary charge.     

Spectroscopy and energy level location of the trivalent lanthanides in LiYP4O12

The excitation and emission properties of the lanthanides Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Er³⁺, Tm³⁺, and Yb³⁺ in LiYP₄O₁₂ were studied by vacuum ultra-violet spectroscopy at 10 K. It provides information on the energies of 4f-5d excitation and emission bands. In the case of Er³⁺ spin forbidden emission was observed. Charge transfer excitation bands were identified for Eu³⁺, Sm³⁺, Tm³⁺, and Yb³⁺, and in the case of Yb³⁺ charge transfer luminescence is observed. All data appear to be consistent with each other and have been used to construct a level scheme showing the location of the energy levels of all trivalent and divalent lanthanides in LiYP₄O₁₂.     

Time-resolved and cw photoluminescence from strongly coupled organic microcavities

The optical properties of microcavities (MCs) are strongly dependent on both polarization of incident and emitted light and its angle of observation. Here we report the measurements of cw- and time-resolved photoluminescence (PL) observed at negative detuning and at resonance for s- and p-polarization in the strong coupling regime of a planar MC containing J-aggregates of a cyanine dye. Following non-resonant excitation, the emission spectra consist of three types of features: direct J-aggregate exciton emission, polariton emission, and uncoupled monomer emission through the transmission maxima of the distributed Bragg reflector beyond the stop-band. We compare our experimental results with a transfer-matrix calculation of the transmission for s- and p-polarization and explain the different positions of the polariton branches, the stop-band width, and the high- and low energy transmission maxima of the MC. Time-resolved PL experiments show an increase in the decay lifetime of the exciton-like mode when it is positioned far from the cavity mode. Close to resonance, the lower polariton branch decays with the natural lifetime of the J-aggregates.     

Numerical and experimental studies of mixed-host organic light emitting diodes

Electrical characteristic and luminance of three mixed-host organic light emitting diodes (OLEDs): namely the uniformly mixed, step-wise graded and mixed, and continuously graded and mixed, were compared with the conventional hetero-junction OLED in both numerical and experimental studies. These mixed-host OLEDs were fabricated by a mixed-source thermal evaporation process, and half-cell devices were also fabricated to provide some input parameters for OLED simulations. The current efficiencies were largely influenced by their device structures and strongly agreed with the computed current balance factors. The improved mixed-host OLED performances can be discussed with aid from simulations, which include spatial distributions of electron and hole, carrier mobility, electric field profiles, the total recombination rates in the light emitting layer.     

High pressure phase transitions in organic solids I:α → β transition in resorcinol

An experimental program has been started to study polymorphic phase transitions under pressure in organic solids using the Be gasketing technique developed by us. This allows us to obtain x-ray diffraction patterns of low symmetry organic solids with high resolution, employing CuK _α radiation. The first organic solid studied is α-resorcinol. At 0.5 GPa, it transforms to its high temperature and denser modification, β-resorcinol. The transformation mechanism is discussed with the help of molecular packing calculations.     

On the dynamics of the disorder-order transition

Extant theories for nucleation and growth of crystals in molecular systems are adapted to colloidal dispersions and used to compare observations for charged spheres at low ionic strengths; soft, polymerically stabilized spheres: and hard spheres.     

Design and optimization of an arbitrarily segmented traveling wave electrode for an ultrahigh speed electroabsorption modulator

The modulation response of an arbitrarily segmented traveling wave electroabsorption modulator (TW-EAM) with a non-periodic electrode layout is analyzed with an effective approach based on the transmission line theory. An optimization method based on a genetic algorithm is used to optimize the segmented electrode with additional capacitive pads for a TW-EAM. Optimized electrode structures are given for 165 μm-length devices under standard 50 Ω RF termination condition and 210 μm-length devices with smaller integrated resistors. Great enhancement of performances in modulation response is achieved as compared to the periodic design reported previously. The simulation with the parameters extracted from measurement results suggests that a 3 dB-bandwidth over 100 GHz could be achieved. The eye diagram from the simulation demonstrates that such devices would be competent for a 100Gb/s optical network.     

Simultaneous demultiplexing and clock recovery using a single electroabsorption modulator in a novel bi-directional configuration

A single electroabsorption modulator was used to demultiplex a 10 Gbit/s channel from a 40 Gbit/s OTDM data stream, whilst simultaneously recovering the 10 GHz electrical clock. This was achieved using a new bi-directional operation of the EA modulator, combined with a simple phase-locked loop feedback circuit. Excellent system performance was achieved, indicating that operation up to and beyond 100 Gbit/s is possible using current technology.     

Nonequilibrium states of driven disordered polymorphic solids

Condensed matter systems, when driven far from equilibrium, often exhibit a far more varied set of phases than their equilibrium counterparts. The existence of non-equilibrium analogs of ‘solids’ and ‘liquids’ has been demonstrated earlier in the context of models for driven disordered vortex lattices in superconductors. Here we study the effects of a structural (polymorphic) transition in a driven two-dimensional crystal placed in a quenched random background. Such a polymorphic crystal is shown to exhibit a complex sequence of unusual dynamical phases as the external drive is varied, including some which have no analog in the undriven pure system. We propose that such states should be accessible in experiments.     

Microscopic Analysis of 5d States Splitting and Charge Transfer Energies Dependence on Interionic Distance in Alkaline Earth Fluorides Doped with Light Trivalent Lanthanides

A first principles fully relativistic analysis (K. Ogasawara et al., Phys. Rev. B 2001, 64, 115413) of the dependence of 5d orbitals splitting (10Dq) and charge transfer (CT) energies on interionic distance has been performed for light lanthanides (Ce³⁺, Pr³⁺, Nd³⁺) in CaF₂, SrF₂, BaF₂ crystals. The salient feature of the method is that four‐component molecular orbitals (MO) composed of atomic wave functions are used as the basis set. Without any fitting parameter, the power dependencies for 10Dq and linear dependencies for the CT energies on the distance between rare‐earth (RE) ions and ligands were obtained. A comparison with experimental values is discussed.     

Effect of the third component on charge transfer character in ternary organic solar cells with a cascade-type electronic structure

By incorporating a third component into the host donor/acceptor organic components, we theoretically investigate the effects of its electronic structure and spatial location on charge transfer character. It is found that both of the two factors can modulate the distribution of transferred charges, and thus change the role of the third component in charge transfer (i.e., electron donor or acceptor). The binding energy of the formed charge transfer (CT) state is also determined by the two factors. Especially, we find an optimized spatial location of the third component, at which the binding energy of the formed CT state has a minimum value. It suggests that charge separation can be promoted. These findings will provide us a clear direction on how to optimize the electronic structure and spatial location of the third component to further improve the charge transfer and separation in ternary organic solar cells.     

Extending the response spectrum of organic photodetectors by quaternary active layer method with complementary absorption spectra

In this work, we report a new method for extending the response spectra of organic photodetectors (OPDs) by incorporating PBDT-TT-C and PBDT-TT-F in the P3HT:PC₆₁BM. The effects of PBDT-TT-C and PBDT-TT-F incorporation on the optical and electrical properties of OPDs were investigated, It was found that when the mass ratio of P3HT:PBDT-TT-F:PBDT-TT-C:PC₆₁BM was 12:2:2:8, the response spectrum of the active layer was extended to 780 nm. The responsivity (R) and external quantum efficiency (EQE) of the OPDs reached 340, 376, 315 mA/W and 67%, 88%, 85% under 630, 530, and 460 nm illumination and −1 V bias, respectively, and the detectivity (D*) reached 10¹² Jones. The results show that the inclusion of an appropriate amount of donor material with similar chemical structure and complementary absorption spectrum can reduce the influence of the doping material on the micro-morphology of the original film while improving the absorption of the spectrum. The interaction between the donor materials promotes the generation of photogenerated carriers and increases the photocurrent of the OPDs. In addition, the incorporation of the different component promotes crystallization of the film, resulting in a reduction in dark current of the OPDs.     

Bond and charge ordering in low-dimensional organic conductors

We review 35 years of structural studies of quasi-1D organic conductors during which the concepts of 2k_F and 4k_F BOW and CDW have been elaborated. In strongly correlated quarter filled band systems these instabilities give rise to SP, DM and CO ground states. We relate these structural features to the instabilities of the 1D electron gas. To stabilize the different ground states the nature of the electron-phonon coupling has to be considered together with the coupling of the organic stacks with the anion sublattice. New results concerning the classification of the SP phase in connection with the adiabatic or antiadiabatic phonon field and its competition with the CO are also introduced.     

Pressure,temperature and light influence on spin transition solids

Abstract

Present paper is an overview of our efforts during the past few years to understand complicated corelations of physical phenomena related to pressure in Fe(I1) solid state spin transition systems. Some principal results concerning p, T, λ-experiments are extracted. In the context of correlation of the crystallographic phase transition with simultaneous HS → LS relaxation and LS → HS photopopulation, we show the latest results: Brillouin and magnetic measurements on the crystal [Fe(pt₆](BF₆)₂.