Geminal Research Articles

Low-Temperature Control of Nanoscale Morphology for High Performance Polymer Photovoltaics

Understanding and controlling nanoscale morphology is crucial to the performance of polymer bulk heterojunction solar cells, as well as other optoelectronic devices such as polymer light-emitting diodes, field-effect transistors, and sensors. In photovoltaic devices, optimum blend morphologies must be commensurate with the nanometer length scales of exciton diffusion and charge separation. We report on a generally applicable method of optimizing the phase segregation in polymer-polymer bulk heterojunctions based on tuning mixtures of low and high boiling point solvents. We have characterized the resulting blend morphologies with nanometer resolution using a transient absorption technique that probes the distribution of paths traveled by the excitons themselves prior to generating charges at an interface. Photovoltaic efficiencies are accounted for in terms of exciton diffusion, geminate pair separation, and polymer ordering, all of which are sensitive to the nanoscale morphology determined by the composition of the solvent mixture.

Monte Carlo modeling of geminate recombination in polymer-polymer photovoltaic devices

A Monte Carlo model is used to examine geminate pair dissociation in polymer-polymer photovoltaic devices. It is found that increasing one or both carrier mobilities aids geminate separation yield eta(GS) particularly at low fields. This, in turn, leads to improved maximum power output from polymer-polymer blend photovoltaics, even when carrier mobilities are unbalanced by a factor of 10. The dynamic behaviors of geminate charges that eventually separate and recombine are examined for the first time. It is shown that geminate pairs in a bilayer become effectively free when separated by approximately 4 nm, which is far smaller than the thermal capture radius of 16 nm here. This may lead one to expect that eta(GS) would not be limited by the separation allowed by the morphology once the domain size has increased above 4 nm. In fact it is found that eta(GS) in a blend improves continuously as the average domain size increases from 4 to 16 nm. We show that although a small degree of separation may be available in a blend, the limited number of possible routes to further separation makes charge pairs in blends more susceptible to recombination than charge pairs in a bilayer.

A templatic approach to gemination in the imperfective of Tashlhiyt Berber.pdf

Tashlhiyt Berber uses, among other processes, gemination to form the imperfective. Most accounts of this phenomenon make reference to syllabic or prosodic structure. In this paper, I diverge from this trend, claiming that imperfective gemination is better analyzed as a templatic-based phenomenon resulting from morphological activity at the skeletal tier. I will argue for the use in the imperfective of a fixed-shape template over which consonant gemination is realized. Moreover, I will show that tri-, bi- and monoconsonantal verbs share the same template. The surface irregularity that bi- and monoconsonantal verbs display is viewed as the consequence of the identification of templatic positions.

Spin-vibrational interactions in geminate radical pairs: Effective spin Hamiltonian including anharmonicity

An equation for the effective spin Hamiltonian including the interaction of radical pair spins with vibrations in both linear and higher approximations was obtained. Even in the linear approximation, the Hamiltonian was found to be non-Heisenberg. The influence of a sound field on the probability of geminate radical pair recombination is considered in the Appendix. The results obtained can be of use for solving certain nanochemistry problems.

Word-initial voiceless geminate stops: production and perception

Quantity contrasts (i.e., singleton vs geminate consonants, short vs long vowels) are common in the languages of the world. Yet, most singleton-geminate contrasts occur in word-medial position. Languages that allow gemination in other than word-medial position (word-initially or word-finally) are quite few. Even less frequent is the occurrence in the languages of the world of word-initial voiceless geminate stops. Pattani Malay (Abramson, 1986), Turgovian Swiss German (Kraehenmann, 2003), Tashlhiyt Berber (Ridouane, 2007), are the only documented languages that allow singleton-geminate contrasts word-initially for all native consonants, including both voiced and voiceless plosives. In this work we deal with word-initial voiceless geminate stops in Tashlhiyt Berber and present results from 3 experiments: 1) a production study including acoustic and electropalatographic measurements 2) a native-listener experiment with labelling and discrimination, and 3) a cross-language perceptual experiment, with listeners of two languages differing in germination: Italian (in which gemination only occurs word-medially) and French (in which gemination only occurs between words in continuous speech).

Effect of consonant gemination on vowel coarticulation

As well known V1CV2 structures are typically characterized by two different phenomena: the first vowel V1 spectral properties are affected by the presence and nature of the second vowel V2 (anticipatory effect), and V2 is influenced by the presence of the first one (cross-over effect). The intensity of anticipatory and the cross-over effects are language dependent and can vary significantly. Both effects have been observed in the past to be present in the Italian language through the analysis of VCV nonsense words. The Italian language has however a unique property, that is distinctive consonant gemination, i.e. consonant length is phonemic; two different words may thus differ by the sole presence or absence of gemination in one of this consonants. The Italian language offers therefore the ideal framework for analyzing the effect of gemination on across-consonant coarticulation. The object of this work is to analyze whether and how the presence of gemination may affect the acoustic properties of vowels in V1CCV2 words. First results of acoustic analysis of V1CV2 as well as their V1CCV2 counterparts, spoken by Italian native speakers, tend to show that gemination may have an effect on across-consonant coarticulation, with specific peculiarities for anticipatory vs. cross-over effects.

Bugotu and Cheke Holo reduplication: in defence of the Emergence of the Unmarked

This paper provides an analysis of the reduplicative systems of Bugotu, Cheke Holo, Tuvaluan and Hoava. In Bugotu and Cheke Holo, intervocalic consonants are not reflected in the reduplicative substring; Tuvaluan reduplication creates initial geminate consonants, while Hoava reduplicants include coda consonants. Blevins (2003, 2005) argues that these languages pose a serious problem for the optimality-theoretic hypothesis of the Emergence of the Unmarked (McCarthy & Prince 1994), which predicts that reduplicated structures should not favour relatively marked outputs. I show that the data in fact are not problematic for the Emergence of the Unmarked hypothesis, because different, conflicting dimensions of markedness can arise in reduplicative phonology. In Bugotu and Cheke Holo, the tolerance of extrasyllabic segments combined with size-penalising constraints helps produce the consonant-deletion pattern. A similar defence of the Emergence of the Unmarked is offered for the reduplicative systems of Tuvaluan and Hoava.

Magnetic Field Effect on Photoinduced Electron Transfer between [Cu(phen)2]2+ and DNA

The magnetic field effect (MFE) on the photoinduced electron transfer (PET) reaction between the [Cu(phen)2]2+ complex and DNA has been studied in homogeneous buffer medium and in reverse micelles. The copper complex on photoexcitation can oxidize DNA in a deoxygenated environment. A prominent MFE is found even in a homogeneous aqueous medium for the triplet born radicals. The process of partial intercalation of [Cu(phen)2]2+ complex within DNA is responsible for such a rare observation. In reverse micelles, the MFE is not very much prominent because of the large separation distance between the component radicals of the geminate radical ion pairs generated through PET.

The phonetics of quantity alternation in Washo

Stress-sensitive quantity alternation is commonplace in the Uralic languages and many of the Germanic languages in and around the Scandinavia region, but few reports have detailed similar types of alternation in Native American languages. This study offers a quantitative analysis of the complementary length alternation between tonic vowels and post-tonic consonants in two generations of speakers of Washo, a severely moribund Hokan language spoken by approximately 13 elderly speakers near the California–Nevada border southeast of Lake Tahoe. The complementary alternation of vowel and consonant length is argued to be motivated by a previously unnoticed requirement in the language to keep the stressed syllable heavy. This paper reports the results of an acoustic study verifying the phonetic reality of this alternation by comparing the speech of two generations of speakers of Washo based on archival audio recordings made in the 1950s and recent fieldwork materials. The results show that the quantity alternation is much more pervasive in the language than it was first described in the 1960s. It is shown that the current generation of Washo speakers retains subtle phonetic alternations, despite the fact they mostly grew up bilingual, if not English-dominant. Their command of Washo phonetics and phonology does not seem to have undergone severe attrition. However, their realization of post-tonic long consonants was not as long in relative duration as the earlier generation. These results show the value of supplementing transcription with direct measurement, even with small numbers of speakers.

Picosecond Kinetic Study of the Photoinduced Homolysis of Benzhydryl Acetates: The Nature of the Conversion of Radical Pairs into Ion Pairs

The conversion of benzhydryl acetate geminate radical pairs to contact ion pairs following photoinduced homolysis in solution is studied using picosecond pump-probe spectroscopy. The dynamics for the decay of the geminate radical pairs into contact ion pairs is modeled within a Marcus-like theory for nonadiabatic electron transfer. A second decay channel for the geminate radical pairs is diffusional separation to free radicals. The kinetics of this latter process reveals an energy of interaction between the two radicals in the geminate pair.

Direct observation of f-pair magnetic field effects and time-dependence of radical pair composition using rapidly switched magnetic fields and time-resolved infrared methods

A rapidly switched (<10 ns) magnetic field was employed to directly observe magnetic fields from f-pair reactions of radical pairs in homogeneous solution. Geminate radical pairs from the photoabstraction reaction of benzophenone from cyclohexanol were observed directly using a pump-probe pulsed magnetic field method to determine their existence time. No magnetic field effects from geminate pairs were observed at times greater than 100 ns after initial photoexcitation. By measuring magnetic field effects for fields applied continuously only after this initial geminate period, f-pair effects could be directly observed. Measurement of the time-dependence of the field effect for the photolysis of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol using time-resolved infrared spectroscopy revealed not only the presence of f-pair magnetic field effects but also the ability of the time dependence of the MARY spectra to observe the changing composition of the randomly encountering pairs throughout the second order reaction period.

A Gestural Approach to Gemination and Nasals in Pali

A Gestural Approach to Gemination and Nasals in Pali

Development of the ability to lexically encode novel second language phonemic contrasts

Second language (L2) learners must often learn to perceive and produce novel L2 phoneme contrasts. Although both research and intuition suggest that these difficulties can be overcome to some extent with exposure to the L2, it is not known what consequences this kind of learning has for the phonological structure of the L2 lexicon. We present an experiment designed to investigate the lexical representations that learners establish for L2 words that contain novel phonemic contrasts. Specifically, we consider the acquisition of Japanese consonant length contrasts by native speakers of English: Japanese contrasts consonants such as /k/ and /kk/ while English does not. The results indicate that native English speakers do not initially encode consonant length consistently in their lexical representations of Japanese words, as reflected in both listening and production tasks. However, after one year of Japanese experience, the phonological structure of their Japanese lexicon more closely approximates that of native Japanese speakers. We conclude that significant changes to the structure of the L2 lexicon can occur even within the first year of L2 learning.

Consonant Gemination in Loanword Phonology and Rhythm Clash

Consonant Gemination in Loanword Phonology and Rhythm Clash

Long‐Range Resonant Energy Transfer for Enhanced Exciton Harvesting for Organic Solar Cells

Exciton harvesting is of fundamental importance for the efficient operation of organic photovoltaic devices. The quantum efficiencies of many organic and hybrid organic-inorganic devices are still limited by low exciton harvesting efficiencies. This problem is most apparent in planar heterostructures that suffer from a direct tradeoff between light absorption and exciton harvesting. The bulk heterojunction concept [1,2] was designed to alleviate the problem of limited exciton migration by intimately blending the donor and acceptor phases on the nanometer length scale. In some polymer/fullerene systems, such as poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4phenylenevinylene) (MDMO-PPV)/(6,6)-phenyl C61-butyric acid methyl ester (PCBM), time resolved spectroscopy shows the photoinduced formation of the radical anions and cations on femtosecond timescales. [3] This ultrafast formation of the polaron signature is only possible if every exciton is formed on a polymer chain segment that is immediately adjacent to one or more fullerene molecules. However, in other systems, very large domains prevail and, consequently, exciton harvesting is inefficient. [4] This has made the fabrication of efficient devices incorporating new materials difficult because each new material leads to a new morphology with its own characteristic length scale. The ordered bulk heterojunction architecture is intended to alleviate these issues by using a pre-patterned nanostructured scaffold [5–7] that has been engineered to have both straight pathways to the electrodes to ensure efficient carrier collection, and controlled domain size to ensure efficient exciton harvesting. Recently, chain alignment has been shown to be promoted for regioregular poly(3-hexyl thiophene) (RR-P3HT) in straight nanopores of anodic alumina leading to a 20-fold increase in hole-mobility in these structures. [8] For solar cell applications, higher mobilities reduce the effects of space charge [9] and increase the probability of separating the geminate pair formed immediately after exciton dissociation. [10–13] While the ordered bulk heterojunction architecture shows promise, existing structures have domains too large [14,15] for efficient exciton harvesting with singlet diffusion lengths only ca. 3–8 nm. [16–18] At this time, few alternatives other than nanostructuring have been proposed to increase exciton harvesting. Triplet excitons have been shown to have large diffusion lengths due to their long lifetimes, [18–20] but except in a few examples [21,22] this usually comes at the expense of a loss in energy of 0.4–0.8 eV associated with intersystem crossing between the photoexcited singlet to the first excited triplet. [23–25]

Limits to accumulation of electric-field-stabilized geminate polaron pairs in an organic semiconductor thin film

We study the dynamics of geminate polaron-pair accumulation using time-dependent electroabsorption spectroscopy in a guest-host system composed of the laser dye, coumarin 6 (C6), doped into the host, $4,{4}^{\ensuremath{'}}$-bis($N$-carbazolyl)biphenyl. Probing the Stark-shifted absorption of C6 at a wavelength of $496\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, we determine the decrease of the internal electric field resulting from buildup of a polarizing charge distribution that accompanies optically induced geminate pair accumulation in an applied electric field. Our model suggests that the drop in internal field qualitatively leads to the observed time-dependent roll-off of accumulated geminate pair density. The results highlight the importance of both nongeminate recombination and reduced geminate pair stability resulting from the decrease in internal electric field in ultimately limiting geminate pair densities in doped organic semiconductors under continuous illumination.

Gemination or fusion?

Gemination or fusion?

Syllabification patterns in Arabic dialects: long segments and mora sharing

In Classical Arabic and many modern Arabic dialects, syllables ending in VVC or in the left leg of a geminate have a special status. An examination of Kiparsky's (2003) semisyllable account of syllabification types and related phenomena in Arabic against a wider set of data shows that while this account explains much syllable-related variation, certain phenomena cannot be captured, and several dialects appear to exhibit conflicting syllable-related phenomena. Phenomena not readily covered by the semisyllable account commonly involve long segments – long vowels or geminate consonants. In this paper, I propose for relevant dialects a mora-sharing solution that recognises the special status of syllables incorporating long segments. Such a mora-sharing solution is not new, but has been proposed for the analysis of syllables containing long segments in a number of languages, including Arabic (Broselow 1992, Broselowet al.1995), Malayalam, Hindi (Broselowet al.1997) and Bantu languages (Maddieson 1993, Hubbard 1995).

Photolysis of the HNO Adduct of Myoglobin: Transient Generation of the Aminoxyl Radical

The photolysis of the HNO-myoglobin adduct has been followed by transient absorption experiments down to the nanosecond time scale. A unique transient assignable to generation of an FeIII/HNO- geminate pair is observed; the transient loss of this absorbance trace, assignable to the recombination, is fit to a single-exponential decay with τ = 7.0 (±0.5) × 10-5 sec. Residual absorbance after the microsecond recombination indicates a small percentage escape of HNO- into solution during pulsed photolysis experiments. Thermal decomposition and bulk photolysis also generate metMb products; analysis of the reaction mixture after photolysis identified low yields of H2NOH, suggesting multiple secondary reactions of the released aminoxyl radical. Photolysis of 15N-labeled HNO-Mb showed only the presence of 15N2O as gaseous product. Both the production of metMb and the microsecond recombination time scale suggest ferric character to the adduct, that is, (HNO-)−FeIIIMb, analogous to similar characterizations of the isoelectronic species, oxyMb as (O2-)−FeIIIMb.

Fortition and reduplication in Leggbo ideophones

Abstract This article describes ideophones in Leggbo, a Cross River language of Nigeria. It focuses on two processes – fortition and reduplication – in the language and their special relation to ideophones. Fortition involves consonant gemination and lengthening and is used for intensification. Reduplication can be either lexical or grammatical. Ideophones can have an inherent repetitive structure but they can also employ reduplication like other modifiers to signal the plurality of nouns, or the diminutive of verbs. Both fortition and reduplication can occur in the same form, especially in ideophones. It is speculated that fortition could have arisen diachronically in ideophones before spreading to other parts of the language. If this scenario is assumed, it argues for treating ideophones in Leggbo, if not in other languages, as central rather than marginal parts of linguistic systems.