Multilayers of sulfur-based anchoring group linkers and CdSe nanocrystals (NCs) were realized on indium-tin oxide substrate via layer-by-layer alternation. The materials were investigated by UV–vis ...and Fourier transform infrared spectroscopy, photoluminescence, and photoconductivity. We found that the change of NC packing during multilayer build-up produces a progressive increase of exciton delocalization. In fact during multilayer deposition, the first excitonic peak of NCs shifts progressively toward longer wavelengths. We found a linear relation between the optical band gap shift (ΔE gn ) and the inverse of the number of CdSe-NC layers (n) for n > 2, in particular, the decrease of its value for the infinite multilayer from one of the solution (2.10 eV) goes from 18 meV for the 1,2-ethylene-bis(dithiocarbamate) linker to 66 meV for sulfide dianions, corresponding to an apparent increase of the exciton radius of 0.20 and 0.85 nm, respectively.
Bis(dithiocarbamate) molecules were reacted with (hexadecylamine/stearate)-capped CdSe nanocrystals (7.5 nm diameter) to form multilayers on ITO glass via layer-by-layer alternation. The new ...materials were investigated by UV–vis and FTIR spectroscopy, photoluminescence, and photoconductivity. FTIR and UV–vis analysis of multilayers showed that the bis(dithiocarbamate) linkers operate the complete removal of caps and cover the nanocrystal layer (ca. 2 × 10–8 mol cm–2 layer–1 in CdSe units) with a dense linker layer (ca. 2 × 10–9 mol cm–2 layer–1). The photoconductivity of the multilayer films obtained with such linkers is strongly enhanced, compared with analogous films produced with bis(carboxylate) linkers of comparable length and structure, attaining record values 10–100 times higher.
A saturated‐red emitting system suitable for solution‐processable LEDs (light‐emitting diodes) is reported. In their approach the authors dope the blue‐emitting conjugated polymer ...poly(9,9‐dioctylfluorene) (see Figure) with the red dye tetraphenylporphyrin, which leads to an efficient single‐step Förster transfer from blue to red. The fabrication and efficiency of LEDs based on this system are also described.
A series of terthiophene and sexithiophene α,ω-dicarboxylic and -diphosphonic acids and some poly(thiophene-carboxylate)s and -sulfonate)s were reacted with hexadecylamine-capped CdSe nanoparticles ...(6 or 7.5 nm diameter) in CHCl3 to form regular polymeric structures on ITO-glass surfaces via layer-by-layer (LBL) alternation. The new materials were investigated by cyclic voltammetry, UV−vis, FTIR and PL spectroscopy, photoelectrochemistry, and photoconductivity. The LBL structures, constituted by quantum dots linked by hole-transporting conjugated chains, display optical and electronic properties useful in photovoltaic devices. The conjugated linker chains ease in fact charge transport between dots but transport of photogenerated carriers is dominated by interdot distance.
We report on excited state dynamics in isolated poly(9,9-dioctylfluorene) chains obtained by embedding the polymer in an inert plastic matrix. Early events (<300 fs) of intrachain photophysics are ...detected by pump-probe spectroscopy using tunable UV 25-fs pump pulses and sub-10-fs visible probe pulses. We show that higher-lying optical states, reached by multiphoton transitions, give rise to on-chain charge separation on the ultrafast time scale. The intrachain charge pair decays geminately within 500 fs to the lowest singlet state. Characteristic time scales for internal conversion and intramolecular vibrational redistribution are also determined.
The modification and control of exciton-photon interactions in semiconductors is of both fundamental and practical interest, being of direct relevance to the design of improved light-emitting diodes, ...photodetectors and lasers. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers. But organic semiconductor microcavities have been studied only in the weak-coupling regime. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons, and suggest that polariton lasing may be possible.
Subpicosecond photoinduced Stark spectroscopy experiments are carried out for measuring charge carrier mobility in organic semiconductors. The technique is demonstrated in state-of-art devices based ...on methanofullerene. The transient mobility of photogenerated charge carriers is measured in the picosecond time domain. Electric field dependent mobility is observed from the earliest time scales. In addition, two distinct transport regimes are revealed: a short-lived state, approximately 10 ps, of high mobility and a transient towards the trap limited transport, associated with the mesoscopic structure of the medium.
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