140 research outputs found
Introducing a nonvolatile N-type dopant drastically improves electron transport in polymer and small-molecule organic transistors
KGaA, Weinheim Molecular doping is a powerful yet challenging technique for enhancing charge transport in organic semiconductors (OSCs). While there is a wealth of research on p-type dopants, work on their n-type counterparts is comparatively limited. Here, reported is the previously unexplored n-dopant (12a,18a)-5,6,12,12a,13,18,18a,19-octahydro-5,6-dimethyl- 13,18[1′,2′]-benzenobisbenzimidazo [1,2-b:2′,1′-d]benzo[i][2.5]benzodiazo-cine potassium triflate adduct (DMBI-BDZC) and its application in organic thin-film transistors (OTFTs). Two different high electron mobility OSCs, namely, the polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8- bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2′-bithiophene)] and a small-molecule naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) are used to study the effectiveness of DMBI-BDZC as a n-dopant. N-doping of both semiconductors results in OTFTs with improved electron mobility (up to 1.1 cm2 V−1 s−1), reduced threshold voltage and lower contact resistance. The impact of DMBI-BDZC incorporation is particularly evident in the temperature dependence of the electron transport, where a significant reduction in the activation energy due to trap deactivation is observed. Electron paramagnetic resonance measurements support the n-doping activity of DMBI-BDZC in both semiconductors. This finding is corroborated by density functional theory calculations, which highlights ground-state electron transfer as the main doping mechanism. The work highlights DMBI-BDZC as a promising n-type molecular dopant for OSCs and its application in OTFTs, solar cells, photodetectors, and thermoelectrics
First principle study of intrinsic defects in hexagonal tungsten carbide
The characteristics of intrinsic defects are important for the understanding
of self-diffusion processes, mechanical strength, brittleness, and plasticity
of tungsten carbide, which present in the divertor of fusion reactors. Here, we
use first-principles calculations to investigate the stability of point defects
and their complexes in WC. Our calculation results confirm that the formation
energies of carbon defects are much lower than that of tungsten defects. The
outward relaxations around vacancy are found. Both interstitial carbon and
interstitial tungsten atom prefer to occupy the carbon basal plane projection
of octahedral interstitial site. The results of isolated carbon defect
diffusion show that the carbon vacancy stay for a wide range of temperature
because of extremely high diffusion barriers, while carbon interstitial
migration is activated at lower temperatures for its considerable lower
activation energy. These results provide evidence for the presumption that the
800K stage is attributed by the annealing out of carbon vacancies by long-range
migration.Comment: Submitted to Journal of Nuclear Material
Temperature effect on low-k dielectric thin films studied by ERDA
Low-k dielectric materials are becoming increasingly interesting as alternative to SiO2 with device geometries shrinking beyond the 65 nm technology node. At elevated temperatures hydrogen migration becomes an important degradation mechanism for conductivity breakdown in semiconductor devices. The possibility of hydrogen release during the fabrication process is, therefore, of great interest in the understanding of device reliability. In this study, various low-k dielectric films were subjected to thermal annealing at temperatures that are generally used for device fabrication. Elastic recoil detection analysis (ERDA) was used to investigate compositional changes and hydrogen redistribution in thin films of plasma-enhanced tetraethylortho-silicate (PETEOS), phosphorus doped silicon glass (PSG), silicon nitride (SiN) and silicon oxynitride (SiON). Except for an initial hydrogen release from the surface region in films of PETEOS and PSG, the results indicate that the elemental composition of the films was stable for at least 2 hours at 450◦C
Addition of the lewis acid Zn(C6 F5 )2 enables organic transistors with a maximum hole mobility in excess of 20 cm2 V-1 s-1
Incorporating the molecular organic Lewis acid tris(pentafluorophenyl)borane [B(C6 F5 )3 ] into organic semiconductors has shown remarkable promise in recent years for controlling the operating characteristics and performance of various opto/electronic devices, including, light-emitting diodes, solar cells, and organic thin-film transistors (OTFTs). Despite the demonstrated potential, however, to date most of the work has been limited to B(C6 F5 )3 with the latter serving as the prototypical air-stable molecular Lewis acid system. Herein, the use of bis(pentafluorophenyl)zinc [Zn(C6 F5 )2 ] is reported as an alternative Lewis acid additive in high-hole-mobility OTFTs based on small-molecule:polymer blends comprising 2,7-dioctyl[1]benzothieno [3,2-b][1]benzothiophene and indacenodithiophene-benzothiadiazole. Systematic analysis of the materials and device characteristics supports the hypothesis that Zn(C6 F5 )2 acts simultaneously as a p-dopant and a microstructure modifier. It is proposed that it is the combination of these synergistic effects that leads to OTFTs with a maximum hole mobility value of 21.5 cm2 V-1 s-1 . The work not only highlights Zn(C6 F5 )2 as a promising new additive for next-generation optoelectronic devices, but also opens up new avenues in the search for high-mobility organic semiconductors
First principles study of point defects in titanium oxycarbide
We have performed first principles density functional theory calculations to study the formation energy of point defects
in TiC, TiO and TiCO compounds. The formation energy of isolated vacancies were obtained for different equilibrium
conditions. For binary compounds, we have also calculated the formation energy of antisite defects. It was found that
the defect formation energies strongly depend on the chemical environment. Our results show that C vacancies are
easily formed in TiC and TiCO. For the TiO compound, Ti vacancies are highly probable to occur and O vacancies
are also easily formed under titanium rich atmosphere.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – CONC-REEQ/443/EEI/2005, POCTI/CTM/69362/200
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
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