Quantum modelling of photo-excited processes

In the framework of quantum field theory and the dipole approximation, a self-consistent quantum molecular dynamics method is used to investigate the effect of chain length on the probability of formation or decay of both singlet and triplet excitons due to photon absorption or emission in isolated poly( p-phenylene vinylene) (PPV) chains. We found that the probability of the photoinduced intra-molecular singlet exciton formation and decay increases linearly with chain length and the probability for triplet exciton formation and decay does not depend on the chain length. Polymers with long chains have thus an advantage over small molecules in solar cell and light-emitting diode (LED) applications because their efficiency depends on the number of intramolecular singlet excitons formed or emitted in the device, which is expected to increase with the conjugation length.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001, SFRH/BD/11231/2002

Electric field induced charge transfer through single and double-stranded DNA polymer molecules

The charge transfer through single-stranded and double-stranded DNA polymer molecules has been the subject of numerous experimental and theoretical studies concerning their applications in molecular electronics. However, the underlying mechanisms responsible for their different electrical conductivity observed in the experiments are poorly understood. Here we use a self-consistent quantum molecular dynamics method to study the effect of an applied electric field along the molecular axis on charge transfer through single-stranded and double-stranded DNA polymer molecules with an injected electron or hole and assess the consequences for electronic applications. Charge transfer through both single-stranded and double-stranded DNA polymer molecules is predicted, regardless of the sign of the injected charge, the molecular structure and the base sequence. The amount of charge transfer through a double-stranded DNA polymer molecule is slightly lower than through the corresponding two isolated single-strands as a result of the lower charge transport through the purine-pyrimidine base-stacking as compared with through DNA nucleobase-stacking. These results suggest that each DNA polymer strand can act as a molecular wire with both the sugar-phosphate backbone and the bases playing an important role in charge transfer, which opens new perspectives for molecular electronics applications.In this work, the calculations were performed on SeARCH (Services & Advanced Computing with HTC/HPC) funded FEDER funds through COMPETE program and by the Portuguese Foundation for Science and Technology (FCT) funds, under contract CONC-REEQ/443/EEI/2005,. One of us (H.M.G.C.) is also indebted to FCT for financial support under the post-doctoral grant no. SFRH/BPD/64554/2009

Modeling charge transport properties of cyano-substituted PPV

In recent years, poly (p-phenylenevinylene) (PPV) and its derivatives have attracted much interest due to their applications in light-emitting diodes (LEDs). One of the issues that determine device performance is the transport of charge carriers along the polymer strands. For that reason, we investigate the influence of cyano substitution on geometry and electronic behaviour of PPV chains using self-consistent quantum molecular dynamics simulations. Our results suggest that substitution by cyano groups induce distortion in the PPV chains and a charge rearrangement among the polymer atoms. Specifically addressed is the issue concerning estimates of charge (electron and hole) mobility by computer experiments. Significant differences have been found both in the strength of the electric field needed to move positive and negative charge carriers along the polymer chain as well as in charge mobility.Fundação para a Ciência e a Tecnologia (FCT) - Programa Operacional "Ciência, Tecnologia e Inovação" (POCTI) - POCTI/CTM/41574/2001

Modelling the effect of structure and base sequence on DNA molecular electronics

DNA is a material that has the potential to be used in nanoelectronic devices as an active component. However, the electronic properties of DNA responsible for its conducting behaviour remain controversial. Here we use a self-consistent quantum molecular dynamics method to study the effect of DNA structure and base sequence on the energy involved when electrons are added or removed from isolated molecules and the transfer of the injected charge along de molecular axis when an electric field is applied. Our results have shown that the DNA molecules of poly(C)-poly(G) on B-form and poly(A)-poly(T) on A-form have the highest energy released when one electron is added or removed from them and their Z-form has the lowest energy released. Besides, when an electric field is applied to a charged DNA molecule along its axis there is electron transfer through the molecule, regardless of the number and sign of the injected charge, the molecular structure and the base sequence. Results from these simulations provide useful information that is hard to obtain from the experiments and needs to be considered for a further modelling aiming to improve charge transport efficiency in nanoelectronic devices based on DNA.Programa Operacional “Ciência ,Tecnologia, Inovação” – POCTI/CTM/41574/2001Fundação para a Ciência e a Tecnologia (FCT)REEQ/443/EEI/2005 e SFRH/BD/11231/200

Quantum modelling of poly(vinylidene fluoride)

Although extensive studies have been conducted on poly(vinylidene fluoride) (PVDF) because of its ferroelectric, pyroelectric and piezoelectric properties, the effects of inverted monomer units, on the molecular scale properties of this polymer are not fully understood. Therefore, we have used a method combining molecular dynamics with a self-consistent semi-empirical quantum mechanical method to study the effects of both chain length and monomer inversion on the electronic properties of individual PVDF chains, such as the dipole moment and the polarizability. The effects of monomer inversion on the infrared spectra are also discussed. Our results suggest that alpha and beta polymorphs of PVDF have dipole moment per monomer unit that varies (increases for beta-PVDF and decreases for alpha-PVDF) with the chain length but converges to a nearly constant value for chain lengths greater than a certain value, whereas chain length does not seem to produce any significant effect on molecular polarizability of both polyrnorphs. Our calculations also suggest a decrease of the dipole moment with increasing inversion monomer defect concentration but no significant effect has been predicted for molecular polarizability, except when two or more isolated defects are present in the same chain. The presence of monomer inverted defects decreases the intensity of the infrared peaks attributable to defect-free chains and gives rise to new peaks. The number and frequency of the defect peaks depends both on the isolated defect concentration and molecular chain structure.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER).Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001, SFRH/BD/11231/2002

What can we learn from vibrational analysis calculations of defective polymer chains?

The possibility of using infrared (IR) spectroscopy to determine the concentration of inversion monomer defects in polymers depends on the knowledge of the relationship between the spectral properties and the polymer microstructure. This can easily be achieved by performing vibrational analysis. In order to investigate the changes in IR spectra of poly(vinylidene fluoride) resulting from the presence of monomeric units in “head-to-head” and “tail-to-tail” positions, we calculated the frequencies and intensities of IR-active vibrations for individual molecules in alpha and beta form with a defect concentration up to 15% and compared them with the ones obtained for a defect-free molecule.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/2001 e SFRH/BD/11231/2002.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Modelling the effects of mesostructure on electronic applications of polymer thin layers

There is considerable experimental evidence that the microstructure has important consequences for polymer-based electronic and optoelectronic applications, but few theoretical and computational models account for it. We produced several realisations of polymer networks exhibiting specific arrangements of C4n+2 H2n+4 molecules at the mesoscopic scale and we carried out computer experiments in which bipolar charge carriers were injected in the polymer system from the appropriate electrodes. Our results show that polymer microstructures resulting from different arrangements of polymer molecules have significant effects on the competition between charge trapping, current transport and recombination within the polymer layer. It was found that current efficiency increases non-linearly with the external applied electric field, the effect being more pronounced for molecular orientations parallel to the electrode surface. In contrast, recombination efficiency shows an opposite behaviour since no significant charge accumulation within the polymer layer is predicted. However, the space-charge effects due to electrons and holes are responsible for most of the recombination events not occurring in neither long or short chains.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001 e SFRH/BD/11231/200

Effects of electric field and charge distribution on nanoelectronic processes involving conducting polymers

The injection of charge carriers in conducting polymer layers gives rise to local electric fields which should have serious implications on the charge transport through the polymer layer. The charge distribution and the related electric field inside the ensemble of polymer molecules, with different molecular arrangements at nanoscale, determine whether or not intra-molecular charge transport takes place and the preferential direction for charge hopping between neighbouring molecules. Consequently, these factors play a significant role in the competition between current flow, charge trapping and recombination in polymer-based electronic devices. By suitable Monte Carlo calculations, we simulated the continuous injection of electrons and holes into polymer layers of polydiacetylene with different microstructures and followed their transport through those polymer networks. Results of these simulations provided a detailed picture of charge and electric field distribution in the polymer layer and allowed us to assess the consequences for current transport and recombination efficiency as well as the distribution of recombination events within the polymer film. In the steady state we found an accumulation of electrons and holes near the collecting electrodes giving rise to an internal electric field which is greater than the external applied field close to the electrodes and lower than that one in the central region of the polymer layer. We also found that a strong variation of electric field inside the polymer layer leads to an increase of recombination events in regions inside the polymer layer where the values of the internal electric field are lower.Fundação para a Ciência e a Tecnologia (FCT) ; Programa Operacional “Ciência , Tecnologia, Inovação” ; POCTI/CTM/41574/2001 e SFRH/BD/11231/200

Modelling molecular transformations in ferroelectric polymers induced by mechanical and electric means

We describe a new approach to model the dynamical structural modifications of ferroelectric polymer molecules induced by uniaxial stretching and by the application of electric fields of various strengths and orientations. Our approach combines a self-consistent quantum mechanical method with molecular dynamics, removing the need to use inter-atomic potentials in the dynamic calculations. Here we present results obtained for individual molecules of poly(vinylidene fluoride) in its alpha and beta form. The effects of structural disorder due to the presence of inverted monomer units in a “headto- head” and “tail-to-tail” position are also discussed.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/2001 e SFRH/BD/11231/2002.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Teoria de processos em materiais orgânicos e biológicos

Tese de Doutoramento em Ciências - Ramo do Conhecimento FisícaOs processos electrónicos de injecção, transferência e recombinação de carga em materiais orgânicos e biológicos são muito importantes, quer em contextos tecnológicos quer em contextos biológicos. No entanto, estes processos são ainda pouco conhecidos, o que limita a nossa capacidade de compreender o funcionamento desses sistemas. Neste trabalho é apresentado um estudo teórico, às escalas atomística e mesoscópica, sobre alguns processos que ocorrem em materiais orgânicos e biológicos electroactivos, com potencialidades de aplicação em dispositivos moleculares como, por exemplo, díodos, transístores e actuadores. Os processos intramoleculares, relacionados com a injecção e transferência de carga em moléculas orgânicas conjugadas (com ou sem polimerização) e biológicas, a electroluminescência em moléculas poliméricas conjugadas e os efeitos de aplicação de campos eléctricos e/ou deformações mecânicas em moléculas poliméricas piezoeléctricas, foram estudados usando um método de dinâmica molecular quântica autocoerente. Os resultados obtidos, a nível atomístico, permitiram não só explicar alguns resultados experimentais publicados na literatura, mas também forneceram informações sobre as propriedades moleculares destes materiais, que são necessárias para uma descrição do seu comportamento à escala mesoscópica e impossíveis de obter experimentalmente. Os efeitos das propriedades moleculares e do arranjo espacial das moléculas no transporte de carga bipolar em díodos poliméricos semicondutores foram estudados utilizando modelos à escala mesoscópica, que incluem os resultados atomísticos relacionados com o transporte intramolecular e intermolecular de electrões e lacunas. Estes estudos permitiram identificar a influência das propriedades moleculares e da orientação das moléculas no funcionamento de díodos poliméricos electroluminescentes, bem como sugerir estratégias para o aumento da eficiência destes dispositivos.Electronic processes such as charge injection, transfer and recombination are very important in both biological and technological contexts. Despite their importance, many of these processes in organic and biological materials are not well understood. In this work a theoretical study is presented, both at atomistic and mesoscopic scale, concerning some processes that occur in electroactive organic and biological materials, which have the potential to be applied in molecular devices such as diodes, transistors and actuators. The intramolecular processes, related with charge injection and transfer through biological and conjugated organic molecules (with and without polymerization), electroluminescence in conjugated polymeric molecules and the effect of applied electric fields and/or mechanical stretching on piezoelectric polymeric molecules, were studied using a self-consistent quantum molecular dynamics method. The results obtained, at atomistic level, allowed not only to explain some experimental results published in the literature but also provided information concerning the molecular properties of these materials, that are needed for a description of their behaviour at mesoscopic scale and are impossible to obtain experimentally. The effects of molecular properties and the spatial molecular arrangements on bipolar charge transport through semiconducting polymer diodes were studied using mesoscopic models, taking into account the atomistic results concerning both intramolecular and intermolecular transport of electrons and holes. These studies allowed us to identify the influence of molecular properties and molecular alignment on the functioning of electroluminescent polymer diodes, as well as to suggest viable strategies to improve the efficiency of these devices.Fundação para a Ciência e Tecnologia (FCT) - Bolsa de Doutoramento - SFRH/BD/11231/2002Phantoms Foundatio