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CALCULATION OF MONOLAYER STRUCTURES OF HYDROCARBON CHAINS ON TRANSITION-METAL DICHALCOGENIDES - DOTRIACONTANE ON MOSE2
No full textFlexible accumulation and depletion mode organic field effect transistors with tunable threshold voltage
No full textIn this paper we report on the possibility to use organic bulk heterojunctions of two derivatives of a conjugated molecule for tuning and controlling charge carrier population and transport within the active layer. In particular, we report on the fabrication and operation of a series of OFETs, which exhibit (i) accumulation and depletion mode operation, (ii) wide tunability of threshold voltage, (iii) mechanical flexibility. We fabricated OFETs on plastic substrates (Mylar®), acting at the same time as gate dielectric, by co-depositing two organic semiconductor materials, sexithiophene (6T) and α, ω-dihexylsexithiophene (DH6T), at various ratios. Despite the identical conjugated moiety, the two materials gave rise to a very different electrical behaviour, OFETs comprising pure 6T channels exhibited a slightly negative threshold voltage (VT), thus working in p-type accumulation mode. When only DH6T formed the channel, large positive V T was observed, giving evidence that an accumulation of p-type charge carriers was already present without applying any gate bias and that these OFETs work in the depletion regime. DH6T has a 0.15 eV lower ionization energy than 6T, consequently, DH6T is more easily p-doped by atmospheric oxygen than 6T, explaining why pure DH6T OFETs exhibit depletion mode operation (heavily doped) and pure 6T ones are accumulation type (intrinsic). As a result, for OFETs with mixed 6T:DH6T channels we found a linear dependence of VT on the 6T: DH6T ratio. Moreover, the hole mobility was essentially constant for all mixing ratios. 6T and DH6T co-deposited films have very similar structural and morphological properties as the pure materials films, and the two molecules form intercalation compounds. Our work is particularly interesting because it demonstrates the possibility to adjust the device working point and tune its operational mode without negatively affecting charge carriers transport across the channel. A rational approach to precisely control OFET performance has thus been established. Moreover, this approach is not influenced by the particular substrate employed as gate dielectric, and it is therefore suitable for tuning the electrical properties in flexible plastic devices, providing a considerable extension of organic electronics application-potentia
Flexible accumulation and depletion mode organic field effect transistors with tunable onset-voltage
No full textThe emerging appealing possibility of exploiting organic heterojunctions for obtaining new electronics properties derived from the interface between different semiconductors is going to add a further degree of freedom for improving OFET performances. In this paper we report on the possibility to use organic bulk heterojunctions of two derivatives of a conjugated molecule for tuning and controlling charge carrier population and transport within the active layer. In particular, we report on the fabrication and operation of a series of OFETs, which exhibit (i) accumulation and depletion mode operation, (ii) wide tunability of threshold voltage, (iii) mechanical flexibility.
We fabricated OFETs on plastic substrates (Mylar®), acting at the same time as gate dielectric, by co-depositing two organic semiconductor materials, sexithiophene (6T) and α,ω-dihexylsexithiophene (DH6T), at various ratios. OFETs comprising pure 6T channels exhibited a slightly negative threshold voltage (VT), thus working in p-type accumulation mode. When only DH6T formed the channel, large positive VT was observed, giving evidence that an accumulation of p-type charge carriers was already present without applying any gate bias and that these OFETs work in the depletion regime. Photocurrent spectroscopy measurements confirmed that a larger concentration of charge carriers is present in DH6T OFETs.
For OFETs with mixed 6T:DH6T channels we found a linear dependence of VT on the 6T: DH6T ratio. Moreover, the hole mobility was essentially constant for all mixing ratios (≥ 5x10-3 cm2/Vs), and even higher than for pure 6T flexible OFETs (3x10-3 cm2/Vs). X-ray diffraction and atomic force microscopy studies showed that 6T and DH6T co-deposited films (on Mylar® and Si-oxide) have very similar structural and morphological properties as the pure materials films, and that the two molecules form intercalation compounds. This is particularly interesting because it demonstrates the possibility to adjust the device working point and tune its operational mode without negatively affecting charge carriers transport across the channel.
Our work shows that extremely wide OFET function tunability can be achieved by combining molecules with appropriately adjusted properties in molecularly mixed films. A rational approach to precisely control OFET performance has thus been established, providing a considerable extension of the application-potential of organic electronics
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Tunable transport properties of organic field effect devices
No full textOrganic thin film devices based on conjugated small molecules and polymers are receiving huge attention due to their potential in optoelectronic, photovoltaic and sensor applications. The determination of charge transport properties, and of electronic excited states energy and distribution is still an open and controversial issue, despite the fact that it plays a crucial role in the applicability of the material under study.
In the present work, all-organic and transparent devices have been fabricated and characterized. We fabricated flexible thin film field effects transistors (FET) by co-evaporating different proportions of -sexithiophene (6T) and -dihexylsexithiophene (DH6T) as active material on polyethylene terephtalate (Mylar, Du Pont), which acts both as a substrate and as the gate dielectric. The charge injecting properties of metal (Au) and organic (PEDOT:PSS) contacts have been tested.
The morphology and structure of the organic layers have been investigated by X-ray diffraction and atomic force microscopy. The electronic transport properties of the FETs, the energy distribution of the excited electron energy levels, and the major transitions across the energy gap have been investigated as a function of the 6T/DH6T ratio by means of current-voltage measurements and photocurrent spectroscopy, revealing the possibility of tuning the threshold voltage of the device as a function of the ratio of the two molecules.
We focused on the effects of interface states on the device transport properties (such as mobility and threshold voltage). These states are located both at the substrate/semiconductor and at the electrode/semiconductor interfaces, and we identify the hexyl chains of DH6T as the key factor controlling the electrical performance of the device.
Their presence, in fact, affects the energy level alignment at the metal/organic interface and we propose a model to describe the higher charge carrier mobility observed in DH6T, based on the additionally available electron density on the hexyl chains, which induces a localized dipole-like charge distribution
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