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The diffusion of photogenerated excitons in organic semiconductors represents often a rate-determining process in the exciton population evolution in ultrafast transient absorption experiments, due to an increased mutual exciton interaction probability at typical excitation densities. A simple description of a diffusion process with a time-independent diffusion coefficient often fails to explain the observed kinetics in the highly non-equilibrium system. In this work, we used the concept of time-dependent diffusion and determined the evolution of the diffusion coefficient of photogenerated excited species in thin films of bis(terpyridine-4’-yl)terthiophene from the experimental ultrafast transient absorption spectroscopic data in a sub-nanosecond time scale. We show that the diffusion coefficient decreases in time more rapidly with increasing intensity of the photoexcitation. We explain the dependences by the planarization of photoexcited molecules and excimer formation.

Fluorophores functionalized with heavy elements show enhanced intersystem crossing due to increased spin-orbit coupling, which in turn shortens the fluorescence decay lifetime (tau(PL)). This phenomenon is known as the heavy-atom effect (HAE). Here, we report the observation of increased tau(PL) upon functionalisation of near-infrared photoluminescent gold nanoclusters with iodine. The heavy atom-mediated increase in tau(PL) is in striking contrast with the HAE and referred to as inverse HAE. Femtosecond and nanosecond transient absorption spectroscopy revealed overcompensation of a slight decrease in lifetime of the transition associated with the Au core (ps) by a large increase in the long-lived triplet state lifetime associated with the Au shell, which contributed to the observed inverse HAE. This unique observation of inverse HAE in gold nanoclusters provides the means to enhance the triplet excited state lifetime.

Gadolinium bisphthalocyanine (GdPc2) has been placed among the highest ranked molecular materials considered namely for modern optoelectronic applications including organic solar cells. To improve understanding of the correlation between GdPc2 magnetic properties and its electronic functionality, we experimentally and theoretically studied charge carrier concentration, charge mobility, and influence of local magnetic field on charge carrier transport. For better clearance, all the main studied properties of GdPc2 bisphthalocyanine were compared with Zn phthalocyanine (ZnPc) as a reference material. Conductivity and charge carrier mobility were measured in materials incorporated in FET active channels. UV Vis spectroscopy, Electron Paramagnetic Resonance Spectroscopy, and IR spectroscopy were also applied. The narrow band gap together with small ionization potential of GdPc2 lead to high free charge carrier concentration. Among parameters affecting charge carrier mobility, molecular arrangement and intermolecular and intramolecular charge carrier pathways were highlighted. The possibility that the interaction of the mobile charge carriers with the local magnetic field of GdPc2 molecules reduces charge carrier mobility is also discussed.

On the basis of the analysis of the experimental data of the transient absorption spectroscopy of thin films of regioregular poly(3-hexyl thiophene) we suggest a mechanism of the polaron pairs decay based on the coherent up and down conversion cycling between hot polaron pairs and free polarons. Compared to previously adopted models, based on single molecular processes or bimolecular collisions, the suggested model reproduced well both the polaron pairs and polaron population kinetics at short time after photoexcitation. For longer times, when the polaron pairs are already thermalized, they decay via 1-D diffusion limited bi-molecular collision processes.

Correction for ‘Inverse heavy-atom effect in near infrared photoluminescent gold nanoclusters’ by Goutam Pramanik et al., Nanoscale, 2021, DOI: 10.1039/d1nr02440j.

Electronic memory elements represent indispensable components of most electronic circuits. For some applications, like RFID tags used for smart packaging, for manufacturing and warehouse automation or security systems, simple low-level performance devices are required, which could be produced by cheap large scale production technology, as e.g. printing. We focus on resistor-type memory (ReRAM) made from switchable resistive materials. Unlike transistor and capacitor memory devices, ReRAM does not require a specific cell structure, which is difficult to prepare by printing. ReRAM is a simple two electrode device based on a sandwich structure, in which data are stored as different electrical conductivity states. Electrical bistability usually arises from changes of the intrinsic materials properties, such as charge transfer, phase change, conformation change and reduction–oxidation (redox) reaction, in response to an applied electric field. The basic memory cells can be integrated into a cross-point memory array, where each memory cell is unambiguously identified by its x and y coordinates, suitable for easy programming and reading the addressed cell. We show the preparation and properties of simple organic memory elements based on thin films of soluble polymers, which can be prepared using common laboratory casting techniques as spin casting, slot die or dip coating, or screen and inkjet printing in the manufacturing scale. Two systems are presented: (i) Molecularly-doped polymer based on a soluble derivative of polydiketopyrrolopyrrole mixed with a soluble low-molecular weight derivative of perylene sandwiched between ITO and Al electrodes shows pronounced hysteresis in its current voltage characteristics. Depending on the composition, the maximum achieved ON/OFF ratio was about 5 with writing/erasing voltage +/-5 V and reading voltage 0.5 V. The functionality of the structure as rewritable resistive memory is well documented on repeated read-write cycles and a retention time exceeding several hours. The properties of the memory element were further improved by the incorporation of a very thin insulating layer of high-k dielectrics containing molecular groups with high permanent dipole. Such elements reached the ON/OFF ratio about 103. (ii) A single component polymer rewritable resistive memory was prepared using a newly synthesized poly(N-(3-(9H-carbazol-9-yl)propyl)methacrylamide) sandwiched as a thin film between ITO and Al electrode. The polymer was prepared by a radical polymerization of the N-(3-(9H-carbazol-9-yl)propyl)methacrylamide using azobisisobutyronitrile as a radical initiator. The starting monomer was obtained by three-step reaction from carbazole. The molar mass of the polymer was influenced in limited extent by the reaction temperature but it was still relatively low (M w = 6000, M n = 4000, Ð = 1.50). However, the film forming abilities were sufficient, particularly in thin films (typical thickness used in the memory elements was below 100 nm). The polymer is stable at higher temperatures up to 300 °C, glass transition temperature of the polymer is 130 °C. The device showed good hysteresis in I-V characteristics with switching effects, maximum ON/OFF ratio ~ 200..A typical write/erase cycle is shown in Fig. 1 showing a good retention time exceeding 2 hours even during permanent reading in both ON and OFF states. Here, writing was at the applied voltage +5 V erasing with -5 V and reading at 0.5 V (sign refers to the polarity of the ITO electrode). After some initial instability during memory element forming, the ON and OFF current was stable during more than 40 repeated write/erase cycles. Work supported by the Grant Agency of the Czech Republic, project No.17-03984S Figure 1

Unsubstituted 2,5-bis(2,2′-bithiophene-5-yl)-1,3,4-thiadiazole and four of its derivatives containing solubilizing octyl groups in different positions of the terminal thiophene ring were synthesized (3 new compounds and 2 already reported). Their UV–vis absorption and emission spectra turned out to be strongly dependent on the position of the substituent and showed significant bathochromic shifts of the dominant transition for compounds with the substituents attached to Cα of the terminal ring (λmax > 425 nm). A good correlation was found for the experimentally determined and theoretically calculated excitation energies employing the time-dependent Coulomb-attenuated hybrid exchange-correlation functional using the 6-31G* basis set (TD CAM-B3LYP/6-31G*). The calculations showed, in addition, that the alkyl substituents improved the planarity of the molecule and its aromaticity, and that they raised the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels (the lat...

An electrophoretic deposition (EPD) procedure was adopted for the cathodic preparation of thin films of conducting polymer/metal oxide nanocomposites with a core-shell structure. The deposition process was investigated at different potentials and in various solvents. The mechanism and kinetics of the electrophoretic deposition were studied via quartz crystal microbalance (QCM) and zeta-potential measurements. The properties of the composite layers were studied by electrochemical methods (cyclic voltammetry, impedance spectroscopy) and photocurrent measurements. The reversible redox potential of polythiophene films was about 0.75 V SCE . The p-type semiconducting behaviour of the reduced polythiophene was studied by photocurrent measurements. In the case of using TiO 2 (n-type semiconductor) as a core material, an n/p heterojunction was observed. In the photocurrent spectra the maximum of the cathodic peak of polythiophene was found around λ = 500 nm (2.5 eV), depending on the applied potential. It is in agreement with the results of UV–vis optical spectra of deposited layers and of pressed pellets. The flatband potential of polythiophene in the heterojunction with TiO 2 , obtained from photocurrent measurements, was 0.53 V SCE .

A composite of polythiophene (PTh) and nanoscopic titanium dioxide (TiO 2 ), possessing core-shell structure, was prepared via oxidative polymerization of thiophene by iron (III) chloride in the presence of TiO 2 particles. The morphology of the obtained composite particles was studied by transmission electron microscopy (TEM), proving the core-shell structure of the prepared nanocomposite. The chemical structure of the composites was investigated by Raman spectroscopy. The electrophoretic deposition (EPD) technique was used as a method to prepare composite layers on various conducting substrates (platinum and ITO plate). The characteristics of the composite layer after EPD were studied by electrochemical methods. The photoelectrical properties of PTh and TiO 2 in the nanocomposites were studied as well. In the photoelectrochemical spectra (depending on the applied potential), the characteristic anodic peak of TiO 2 at λ = 340 nm and cathodic peak of PTh around λ = 530 nm were observed. A bandgap energy, E g = 1.95 eV and a direct electron transition of PTh were found. A cathodic peak in the photocurrent spectra was detected unexpectably also around 340 nm, and ascribed to the photoelectrical activity of the TiO 2 core. The redox processes in PTh were investigated via cyclic voltammetry (CV) and electrochemical impedance microscopy (EIS).

Two semiconducting naphthalene bisimides were comparatively studied: NBI-(TAA)(2), symmetrically N-substituted with triaryl amine and asymmetric NBI-TAA-Oc with triaryl amine and octyl N-substituents. Both compounds show very similar spectroscopic and redox properties but differ in their supramolecular organization. As evidenced by STM, in monolayers on HOPG they form ordered 2D structures, however of different packing patterns. NBI-(TAA)(2) does not form ordered 3D structures, yielding amorphous thin films whereas films of NBI-TAA-Oc are highly crystalline. DFT calculations predict the ionization potential (IP) of 5.22 eV and 5.18 eV for NBI-TAA-Oc and NBI-(TAA)(2), respectively, as well as the electron affinity values (EA) of -3.25 eV and -3.22 eV. These results are consistent with the cyclic voltammetry data which yield similar values of IP (5.20 eV and 5.19 eV) and somehow different values of EA (-3.80 eV and -3.83 eV). As judged from these data, both semiconductors should exhibit ambipolar behavior. Indeed, NBI-TAA-Oc is ambipolar, showing hole and electron mobilities of 4.5 × 10(-5) cm(2)/(V s) and of 2.6 × 10(-4) cm(2)/(V s), respectively, in the field effect transistor configuration. NBI-(TAA)(2) is not ambipolar and yields field effect only in the p-channel configuration. This different behavior is rationalized on the basis of structural factors.

A simple and facile method was used to synthesize polyaniline (PANI) nanocomposites with sodium montmorillonite clay (Na+-MMT) using in situ intercalative oxidative polymerization. Aniline was admixed with Na+-MMT at various concentrations, keeping the aniline monomer in the reaction mixture constant. The intercalation of PANI into the clay layers was confirmed by X-ray diffraction studies in conjugation with electron microscope techniques and FTIR spectra, particularly by the narrowing of the Si–O stretching vibration band confirmed the interaction between PANI and the clay. The employed route offers the possibility to improve the thermal properties with simultaneously controlled electrical conductivity. Thermal studies show an improved thermal stability of the nanocomposites relative to the pure PANI. Depending on the loading of the clay, the room temperature conductivity values of these nanocomposites varied between 2.0 × 10−4 and 7.4 × 10−4 S cm−1, with the maximum at 44 wt% PANI concentration. The decrease of electrical conductivity at high PANI concentration was ascribed to the decrease of the structural ordering of PANI in the nanocomposite.

Evaluation methods based on dispersive kinetics were applied to derive kinetic parameters for the rearrangement of differently α-substituted nitronic acids generated by flash photolysis (λ inc = 347 nm) of o-nitrobenzyl esters. The rearrangements were performed in polymer matrices of different chemical nature: poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), poly(styrene) (PSt), and poly(dimethylsiloxane) (PDMS). A nonexponential decay behavior was observed in rigid and highly viscous matrices which can be characterized by the average rate constant v and the dispersion factor α

The results of degenerate four‐wave mixing studies performed on evaporated films of metal‐free phthalocyanine and Langmuir–Blodgett films of a silicon phthalocyanine using subpicosecond 602 nm pulses are reported. The third‐order nonlinearity of all the samples is high permitting the observation of the degenerate four‐wave mixing signal from even a monolayer. In both cases, the third‐order nonlinearity has a resonant character and the observed time‐resolved phase conjugate signals provide information on the dynamics of excitons. Simultaneously monitoring first‐ and second‐order diffractions from the transient gratings formed in our samples allowed us to obtain more‐detailed information on the factors influencing the temporal behavior of the nonlinear response. It is concluded that the dynamics of excitons determining the nonlinearity is mainly governed by the presence of bimolecular exciton–exciton interaction. Saturation behavior is observed at high light intensities.

Downscaling metallic fillers in polymer composites to nanometers opens a new field of applications as, for example, in photochemical catalysis, optical sensing or nonvolatile rewritable memories. Combination of plasmonic properties of NPs with semiconducting properties of p-conjugated polymers brings a new functionality for advanced optoelectronic applications due to the synergy effect of surface plasmons and delocalized p-electrons in the polymer backbone. It can be used e.g. for better harvesting of absorbed light for photovoltaic energy conversion by the enhancement of charge transfer near the NP surface.1 Various procedures of plasmonic composites preparation will be shown on the example of poly(3-alkylthiophene), P3AT and the electrical and optical properties of these composites will be discussed. By mixing negatively charged Ag or Au NPs with cationic derivatives of P3AT in a composition ratio near to the charge balance between NPs and cationic chains aggregates were formed, in which interparticle distances of NPs enable strong interactions of their plasmons, as it was demonstrated by their optical extinction and high SERS activity, which is even preserved in dry thin solid films prepared from such sol. The surface plasmon resonance of metal nanoparticles contributed markedly to the overall extinction of the composite showing a red shift of optical absorption with increasing number of layers due to an increased electronic coupling of plasmons. For understanding phenomena that accompany the effect of plasmonic nanoparticles on the optoelectronic behavior of the composites we prepared a more defined system consisting of a metal NPs layer prepared by physical vapor deposition onto surface of spin-cast thin films of poly(3-hexylthiophene). Gold or silver deposited in a very thin layer formed a nanoparticles-like island structure with the morphology depending on the effective thickness of the deposited layer and on its subsequent thermal treatment. Particularly, we focused on the changes of the electrical conductivity of the polymer/NPs composite system measured during temperature cycling (annealing), and on its relation to morphology of the same composite structures monitored by TEM and extra-high resolution SEM microscopy. Comparison to the analogous polystyrene/AuNP layers was made in order to distinguish the role of the polymer support on the morphology and electrical properties of the nanoparticles assembly. The electrical conductivity and optical properties were found to be strongly related to the morphology of the Au NP surface layer, reflecting the thermodynamical behavior of the polymer during thermal annealing, namely the glass transition and melting of the polymer structure. A stabilizing effect of the thiophene-gold interaction on the nanoparticles morphology was observed. By embedding the Au or Ag NPs, having strong optical extinction in the wavelength interval from 450 nm to 570 nm, in the active layer of P3HT-fullerene bulk heterojunction solar cell, we changed the photovoltaic spectrum of incident photon to collected charge ratio. Besides that, it was shown that in the cells with Au NPs the power conversion efficiency under white light illumination was improved by about 25%. By careful control of the nanoparticles formation we were able to exclude the effects of variation of the plasmonic resonance frequency and possible effects of incident light scattering that would increase the interaction pathway of light within the active polymer. We attributed the observed increase in photovoltaic conversion efficiency to a strong local field enhancement around metal nanoparticles that locally increases optical absorption in a surrounding semiconducting polymer. Figure 1 shows the influence of the Ag and Au NPs on the IPCE and volt-amp characteristics of P3HT/PCBM solar cells. In the case of Au nanoparticles, a clear increase in the power conversion efficiency under white light illumination was detected. The observed increase in the photovoltaic conversion efficiency was attributed to a strong local optical field enhancement around metal nanoparticles that locally increases light absorption in the surrounding semiconducting polymer. The maximum of this optical absorption was found at the position of the respective plasmon resonance peak. It indicates that the enhancement of the solar cells efficiency occurred due to the localized surface plasmon phenomena.2 Acknowledgement: This work was funded by the Czech Science Foundation under the project No. P108/12/1143 and the Ministry of Education, Youths and Sports of the Czech Republic (COST LD 14011). References [1] P.V. Kamat, J.Phys. Chem. C 111 (2007) 2834. [2] G.D. Spyropoulos et.al, Photonics and Nanostructures - Fundamentals and Applications 9, 2 (2011) 184. Figure 1

Transient photocurrent measurements were performed for a series of layers of discotic liquid crystals and of perylene derivative prepared in different ways with an aim to characterize the charge carrier transport in configurations corresponding to photovoltaic devices and field effect transistors. The course of the transient photocurrent was found to be strongly dependent on the layer preparation procedure: the non-dispersive transport, and well defined transit times, were found only for “squeezed” sandwich-type sample of triphenylene derivative in which homeotropic orientation of the discotic columns was induced assuring formation of continuous paths connecting the opposite electrodes. In all other cases dispersive transport of charge carriers was observed, due to the spatial and/or energy disorder of hopping states and fast trapping by localized states. From the asymmetry of the transient photocurrents for different polarity of the illuminated electrode it was concluded, that holes are more mobile in the hexabenzocoronene derivatives and in the triphenylene derivative, while electron transport dominates in the perylene derivative.

The work is focused on the refinement of the laser ablation method for a nanosized silver colloid preparation with regards to the nanoparticle size distribution, as well as to the reproducibility and stability of the ablated hydrosol. In the adopted procedure, additionally to the variation of the laser pulse energy, beam focusation and duration of laser ablation, we developed a technique based on further fragmentation of the ablated colloid solution by subsequent treatment with the 1064 and 532 nm nanosecond laser pulses. The method yields redistribution of nanoparticle diameters to a smaller mean size, as it was observed by transmission electron microscopy, quasielastic light scattering studies and surface plasmon (SP) optical extinction. It was shown that a similar fragmentation procedure is also effective in reduction of nanoparticle size and polydispersity of silver hydrosol prepared by routine chemical procedures and leads to a narrower SP extinction profile.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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Monolayer film formation at the air/water interface was investigated for both electrochemically and chemically prepared poly(3-dodecylthiophene) using surface pressure-molecular area isotherms. Only the electrochemically prepared polymer formed a stable monolayer, which was successfully transferred using the horizontal lifting method. The transferred Langmuir-Blodgett films were characterized by u.v.-visible spectroscopy, quartz crystal microbalance measurements, electrical conductivity measurements and femtosecond degenerate four-wave mixing studies of the third-order optical non-linearity. The third-order optical susceptibility of undoped poly(3-dodecylthiophene) was found to be χ (3) ∼ 10 −9 , esu, large enough to allow the first reported observation of a degenerate four-wave mixing signal from ultrathin Langmuir-Blodgett films. In situ iodine-doping studies of u.v.-visible absorption, electrical conductivity and third-order non-linear optical susceptibility were carried out. Upon doping, the conductivity increased by more than eight orders of magnitude and the χ (3) value decreased to within ten percent of the original value.

Monolayer and multilayer Langmuir-Blodgett films of two systems: (i) polyalkylthiophene and (ii) tetrakis cumylphenoxy phthalocyanines in pure form and mixed phase with octadecanol were prepared. We report here the first observation of a degenerate four wave mixing signal from a monolayer Langmuir-Blodgett film that of phthalocyanine, due to a large resonant third order optical nonlinearity, x(3) . The effective x(3) behaviors of both phthalocyanine and polythiophene Langmuir-Blodgett films were investigated by time-resolved femtosecond degenerate four wave mixing. The polythiophene Langmuir-Blodgett film when doped with iodine was found to be highly conductive. We have also observed a dramatic reduction of the nonlinear reflectivity as a consequence of iodine doping of polythiophene Langmuir-Blodgett films.