Physics

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Solution processable diketopyrrolopyrrole (DPP)-bithiophene polymers (PDBT) with long branched alkyl side chains on the DPP unit are synthesized. These polymers have favourable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for the injection and transport of both holes and electrons. Organic thin film transistors (OTFTs) using these polymers as semiconductors and gold as source/drain electrodes show typical ambipolar characteristics with very well balanced high hole and electron mobilities (μh = 0.024 cm2 V-1 s -1 and μe = 0.056 cm2 V-1 s -1). These simple and high-performing polymers are promising materials for ambipolar organic thin film transistors for low-cost CMOS-like logic circuits. © 2012 Elsevier B.V. All rights reserved.

World University root node

(SoNS) School of Natural Sciences

Organic Electronics

Solution processable poly(2,5-dialkyl-2,5-dihydro-3,6-di-2-thienyl- pyrrolo[3,4-c]pyrrole-1,4-dione) for ambipolar organic thin film transistors

We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti1-xTaxO2 (x ∼0.05) thin films grown by pulsed laser deposition on LaAlO3 substrates. Rutherford backscattering spectrometry (RBS), X-ray diffraction, protoninduced X-ray emission, X-ray absorption spectroscopy (XAS) and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of FM with concomitant large carrier densities was determined by a combination of superconducting quantum interference device magnetometry, electrical transport measurements, soft X-ray magnetic circular dichroism (SXMCD), XAS and optical magnetic circular dichroism, and was supported by first-principles calculations. SXMCD and XAS measurements revealed a 90 per cent contribution to FM from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites, though carrier activation was only 50 per cent at 5 per cent Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (VTi) and Ti3+ was studied via XAS and X-ray photoemission spectroscopy, respectively. It was found that, in films with strong FM, the VTi signal was strong while the Ti3+ signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localized magnetic moments, VTi sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to FM in wide-band-gap semiconducting oxides without any magnetic elements. © 2012 The Royal Society.

Fulltext

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences

Cationic-vacancy-induced room-temperature ferromagnetism in transparent, conducting anatase Ti1-xTaxO2 (x ∼0.05) thin films

We investigated Ru mirror contamination and subsequent EUV reflectivity loss using the IMPACT facility at Purdue University. Because Ru can either be used as a grazing mirror or as a capping layer for multilayer normal mirror, we examined the angular dependency of XPS peak area intensity at the O 1s and Ru 3d regions as well as the effects of sputtering. Although no change in intensity has been observed at lower take-off angles from the target surface, the peak area intensity starts changing with increasing θ(i.e., emission observation angle, representing the angle between the target surface plane and detector entrance). Among different components, the effect of water and oxidized carbon are found to be most notable when viewed at lower θ, and primarily responsible for degrading the reflectivity of the Ru layer. On the other hand, the effect of OH becomes dominant with increasing observation angle θ, and thus plays a key role to suppress optical transmission. Moreover, atomic carbon effect is found to peak when observed at 30°, and most likely plays an important role in degrading both reflectivity and transmission. This is also because of the total photon path length in the Ru film at different angles. During the contamination process, the EUV reflectivity of the Ru film is found to significantly degrade in the presence of additional secondary electrons from the focusing Ru mirror of the EUV setup. This effect could be explained in the light of a competition between oxidation and carbonization processes on Ru surface. © 2012 SPIE.

Proceedings of SPIE - The International Society for Optical Engineering

Mirror contamination and secondary electron effects during EUV reflectivity analysis

The impact of secondary electrons induced contamination of the Ru surface was investigated. Mirror-like Ru sample was bombarded with low energy (100 eV) electrons and the change in surface chemistry was investigated using Xray photoelectron spectroscopy (XPS).Along with XPS studies the corresponding effect on in-situ EUV reflectivity was examined by exposing the Ru surface to photons at a wavelength of 13.5 nm in an ultrahigh vacuum chamber. Detailed XPS analyses showed a sudden increase in carbon concentrations on the Ru surface in the first 60 min, followed by a slow but linear growth in carbon concentration. In parallel, a noticeable decrease in water content was observed during the time of electrons irradiation along with slight oxidation of pure Ru surface. All chemical changes were discussed in terms of the electrons bombardment mediated dissociation of water and hydrocarbon molecules. A time dependent EUV reflectivity measurements show insignificant change in reflectivity up to 510 min of electrons bombardment. The impact of water molecules on the Ru surface and the accumulation of carbon through dissociation of residual hydrocarbons is discussed in details. © 2012 SPIE.

Low energy electron bombardment induced surface contamination of Ru mirrors

Semiconducting transition metal dichalcogenides (TMDs) are emerging as the potential alternatives to graphene. As in the case of graphene, the monolayer of TMDs can easily be exfoliated using mechanical or chemical methods, and their properties can also be tuned. At the same time, semiconducting TMDs (MX 2; M = Mo, W and X = S, Se, Te) possess an advantage over graphene in that they exhibit a band gap whose magnitude is appropriate for applications in the opto-electronic devices. Using ab initio simulations, we demonstrate that this band gap can be widely tuned by applying mechanical strains. While the electronic properties of graphene remain almost unaffected by tensile strains, we find TMDs to be sensitive to both tensile and shear strains. Moreover, compared to that of graphene, a much smaller amount of strain is required to vary the band gap of TMDs. Our results suggest that mechanical strains reduce the band gap of semiconducting TMDs causing an direct-to-indirect band gap and a semiconductor-to-metal transition. These transitions, however, significantly depend on the type of applied strain and the type of chalcogenide atoms. The diffuse nature of heavier chalcogenides require relatively more tensile and less shear strain (when the monolayer is expanded in y-direction and compressed in x-direction) to attain a direct-to-indirect band gap transition. In addition, our results demonstrate that the homogeneous biaxial tensile strain of around 10% leads to semiconductor-to-metal transition in all semiconducting TMDs, while through pure shear strain this transition can only be achieved by expanding and compressing the monolayer of MTe 2 in the y- and x-directions, respectively. Our results highlight the importance of tensile and pure shear strains in tuning the electronic properties of TMDs by illustrating a substantial impact of the strain on going from MS 2 to MSe 2 to MTe 2. © 2012 American Chemical Society.

ACS Nano

Tuning the electronic properties of semiconducting transition metal dichalcogenides by applying mechanical strains

We report a Ni impurity induced reversible ferromagnetism and surface conduction in rutile TiO 2 crystals subjected to specific thermal annealing. For annealing in vacuum at 800 °C, a growing ferromagnetic signal is seen with time while for a similar annealing in air, the magnetism vanishes. The magnetism is concomitant with a surface conductivity which at low temperatures shows tunneling characteristics. Here, we show that Ni magnetic impurity (in TiO 2 crystals at &lt;100 ppm) under vacuum annealing segregates to the surface over a 50 nm layer where the Ni concentration exceeds 10-20 and drops with subsequent air annealing. © 2012 American Institute of Physics.

Applied Physics Letters

Reversible ferromagnetism in rutile TiO 2 single crystals induced by nickel impurities

Furan substituted diketopyrrolopyrrole (DBF) combined with benzothiadiazole based polymer semiconductor PDPP-FBF has been synthesized and evaluated as an ambipolar semiconductor in organic thin-film transistors. Hole and electron mobilities as high as 0.20 cm2 V−1 s−1 and 0.56 cm2 V−1 s−1, respectively, are achieved for PDPP-FBF. © 2012 The Royal Society of Chemistry.

Chemical Communications

A furan-containing conjugated polymer for high mobility ambipolar organic thin film transistors

A new diketopyrrolopyrrole (DPP)-containing donor-acceptor polymer, poly(2,5-bis(2-octyldodecyl)-3,6-di(furan-2-yl)-2,5-dihydro-pyrrolo[3,4-c] pyrrole-1,4-dione-co-thieno[3,2-b]thiophene) (PDBF-co-TT), is synthesized and studied as a semiconductor in organic thin film transistors (OTFTs) and organic photovoltaics (OPVs). High hole mobility of up to 0.53 cm 2 V -1 s -1 in bottom-gate, top-contact OTFT devices is achieved owing to the ordered polymer chain packing and favoured chain orientation, strong intermolecular interactions, as well as uniform film morphology of PDBF-co-TT. The optimum band gap of 1.39 eV and high hole mobility make this polymer a promising donor semiconductor for the solar cell application. When paired with a fullerene acceptor, PC 71BM, the resulting OPV devices show a high power conversion efficiency of up to 4.38\% under simulated standard AM1.5 solar illumination. © 2012 the Owner Societies.

Physical Chemistry Chemical Physics

Poly(2,5-bis(2-octyldodecyl)-3,6-di(furan-2-yl)-2,5-dihydro-pyrrolo[3,4-c] pyrrole-1,4-dione-co-thieno[3,2-b]thiophene): A high performance polymer semiconductor for both organic thin film transistors and organic photovoltaics

We have investigated the photoluminescence and electrical properties of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO 2 layer on sapphire across the phase transition of VO 2. The band edge and defect luminescence of the ZnO overlayer exhibit hysteresis in opposite directions induced by the phase transition of VO 2. Concomitantly the phase transition of VO 2 was seen to induce defects in the ZnO layer. Such coherently coupled interfaces could be of use in characterizing the stability of a variety of interfaces in situ and also for novel device application. © 2012 American Institute of Physics.

Coherently coupled ZnO and VO 2 interface studied by photoluminescence and electrical transport across a phase transition

31P NMR has been done in the paramagnetic state of SmCoPO. Hisohf, Haxhf are 13.4 and -5.19 kOe/$\mu$B respectively have been estimated which indicates the nature of hybridization between P 2p orbitals with Co 3d orbitals. Anisotropic spin-lattice relaxation rate has been observed. In plane spin-correlations are 2D FM and out of plane spin correlations are AFM type. {\textcopyright} 2012 American Institute of Physics.