Potassium Doped Single Wall Carbon Nanotubes: Resistance under Pressure

Abstract

We report in situ measurements of four-probe de resistance (R) of K-doped purified single wall carbon nanotube (SWNT) “buckypaper” as a function of quasi-hydrostatic pressure. Doped samples show completely different behavior compared to that of pristine nanotubes in the pressure range up to 90 kbar. The characteristic increase in the resistance of pristine buckypaper above 10 kbar, associated with the formation of kinks or/and twists of tubes, is not observed in K-doped samples. This may originate from 1) a substantial change in electronic band structure of donor intercalated nanotubes, 2) completely different transport properties of defect structures, or 3) higher stiffness of doped SWNT's which prevents formation of kinks and twists in this pressure range. On deintercalation, the pristine behavior of R(P) is restored, establishing the reversibility of potassium vapor-transport doping.     

Erratum

Abstract

The authors regret that in their paper entitled: “Glass-Forming Terephthalic Esters with Lateral Phenylthio Groups and their Relaxation Behavior” by M. Pötter et al. [MCLC 312, 55 (1998)] are two errors on page 5.     

Electron Spin Resonance of K-Doped Single-Wall Carbon Nanohorns and Single-Wall Carbon Nanotubes

Abstract

K-doping into the aggregate of single-wall carbon nanohorns (SWNHs) and into the bundle of single-wall carbon nanotubes (SWNTs) are introduced in this study. Electron spin resonance (ESR) of the pristine SWNHs and SWNTs show that the pristine SWNHs are ESR active and the linewidth (ΔH) is susceptive to the partial pressure of O₂, while the pristine SWNTs are ESR silent. ΔH of K-doped SWNHs becomes wider by a factor of ?2 than that of the pristine one. For K-doped SWNTs, the Dysonian type ESR comes to be observed. Details of the ESR features are discussed.     

Origin of Structural Instability in Layered Intercalation Compounds

Abstract

The mechanisms of the phase transitions in the A_aMX₂-type intercalates (A-metal, MX₂-layered dichalcogenide matrix) occurring upon a change in the stoichiometry (a) are studied. It is shown that maximal stoichiometric ratios, and trends of structural changes in intercalates are determined by stabilization of conduction bands (“electronic stabilization”) of intercalates.     

Interlayer Reaction of Polyamine/α-Zirconium Phosphate Intercalation Compounds with Some Aldehydes

Abstract

The intercalation compound between α-zirconium phosphate and N,N′-bis-(3-aminopropyl)-1, 3-propanediamine has two different modifications with respect to the conformation of the guest amine molecules: one is “bent form” and the other “straight form”. Reactivity of each phase with a series of aldehydes was examined. ¹³C CP/MAS NMR indicated that the tetraamine reacts stereoselectively with aldehydes to form cis-imine in the interlayer spacing. Depending on the molecular size of aldehyde, it is found that a significant difference can be recognized in the yields of the imine products for the two phases of intercalation compounds. It is also noted that residual water molecules in the interlayer spacing play an important role in the imine formation reaction.     

Crystallization tests bench

A model of an air-cooled tube (“air-heat saw”) installed in a laboratory tube furnace is presented. The setup (“crystallization tests bench”) allows easy regulation and simultaneous crystallization tests of a series of different crystallization parameters in crucible columns, enabling fast studies of obtaining crystals from materials with unknown crystallization parameters. The relationships between the crystallization rate and parameters of air-cooled tube are given and numerically analyzed. This method can also be applied in crucible or chamber furnaces.     

Metal-graphene nanostructures for SEIRA spectroscopy

Abstract

Infrared spectroscopy is widely used technique for observing bioorganic materials, but sometimes scientist have to work with small amount of investigated materials and infrared light interacts poorly with nanometric size molecules. Taking into account unique electro-optical properties of graphene and metal we demonstrated possibility to use metal-graphene nanostructures for label-free detection of thymine. It was shown that IR spectra of thymine adsorbed on the composite nanostructures, such as Au “nanostars” with graphene, is more enhanced than whenthese nanoparticles are used without graphene. The enhancement in IR absorption for complex thymine/Au/graphene depends on size of Au nanoparticlesand thymine’s molecular group.     

“Polyaniline”: Interconversion of Metallic and Insulating Forms

Abstract

“Polyaniline” has been synthesized in various forms both chemically and electrochemically in aqueous media. The qulnoid-benzenoid-diimine form, an insulator, is doped by dilute aqueous protonic acids to the metallic regime ([sgrave] = 5 ohm^?1cm^?1; compressed pellet) to give the corresponding iminium salt. This represents a new type of p-doping phenomenon in a conducting polymer. Both these forms are stable in the presence of air and/or water. The doping process is reversed by treatment with aqueous alkali. Cyclic voltammetry studies in an aqueous electrolyte show excellent reversibility between selected reduced and oxidized forms of polyaniline.     

Enhanced persistent photoconduction in CuInS2–ZnIn2S4 alloys single crystals and processes of its relaxation

ABSTRACT

Photoelectrical response in CuInS₂–ZnIn₂S₄ alloys single crystals was analysed in the low temperature region from 30 K up to 100 K. The molar ratio of ZnIn₂S₄ in the alloys was varied in the range 0 mol%?16 mol%. The crystals with up to 12 mol% were the single-crystalline, meanwhile those with 16 mol% were the two phase ones. We have analysed spectral distribution of their photocurrent at different temperatures and the following relaxation towards the stationary values. The photo-induced photoconductivity phenomena were identified. Moreover the long-lasting relaxations with characteristic times exceeding 1.5×10³ sec were observed at lowest temperatures. They used to shorten exponentially with increasing temperature showing thermally activated behaviour. The main parameters of the photoconductivity kinetics and their temperature dependencies were determined. The observed behaviour was explained by the slow multicenter recombination due to the combined effect of different trapping and recombination centers. The effects of both – “fast” and “slow” recombination centers were taken into account.     

Optical defects in barium—strontium niobate single crystals

The distributions of edge dislocations and residual mechanical stresses in Ba_xSr_1-xNb₂O₆ (BSN) crystals are investigated and the explanation of the nature of the “growth column” is proposed. The “growth column” is a defect zone going through all of the crystal and usually repeating in its cross-section the contour of the seed crystal. The “growth column” boundary is the closed contour with extremely high edge dislocation density. These dislocations are connected with thermal stresses due to seed-melt contact or abrupt crystal widening. Under proper crystal seeding and widening conditions one can obtain the BSN crystals with dislocation densities less than 10 cm⁻² and without the “growth column”. The method of chemico-mechanical polishing of BSN crystals not forming a defect layer on the surface of the crystals have been developed. The high temperature diffusion annealing is shown to eliminate the growth striae in BSN crystals.     

Optical and Electrooptical Properties of “Guest-Host” Effect in Twisted Liquid Crystal Layers

Abstract

Optical and electro-optical characteristics of a positive dichroic dye-doped Twisted Nematic Liquid Crystal Layer (TNLCL) for two different orientations of the input light polarization to the NLC director α (α = 0°, α = 90°) and various NLC optical path lengths A = Δnd/λ (Δn = real part of birefringence, d = cell thickness, λ = wavelength) are investigated. In the case of one polarizer, the maximum contrast ratio is obtained for A ? 1.2 (α = 0°) and A ～ 0 (α = 90°), regardless of whether Δn or d is changed. The viewing characteristics of a dye doped TNLCL with one polarizer are shown to be more uniform than those for a “pure” TNLCL between parallel polarizers.     

From Chiral Molecules to Chiral Liquid Crystal Phases

Abstract

During the last decade, the topic of chirality and symmetry in physical sciences has become extremely important, particularly as nature itself is often described as being “handed” or chiral. In liquid crystalline systems many important properties are known to be related to chirality, for instance chirality is reflected in helical structures, in special cases of strongly twisted systems novel frustrated phases may appear such as Blue phases and Twist Grain Boundary phases. In addition, many modern high technology applications, such as ferroelectric displays depend entirely on the presence of chirality in the liquid-crystalline materialsemployed.     

Strukturelle Umklapp-Vorgänge in metastabilen β1-Kupfer-Zink-Mischkristallen bei tiefen und höheren Temperaturen (III). Zur Gitterkinetik der isothermen Phasen-Umwandlungen

Whereas part (I) of the present series treated chiefly lattice-dynamical aspects, being important for the initiation of the β₁/α-transformation processes, and the preceding part (II) discussed explicitely the phase transformation of β₁Cu/Zn-crystals, the submitted third part, (III), handles of a lattice kinetic detailing of the results, exclusively received by X-ray measurements (transformation isotherm 360°C). Keeping the subdivision of the transformation processes into the 3 main periods (A) PREDIFFUSION, (B) “UM-.KLAPPING”, (C) POSTDIFFUSION there are the following results: (A) The quickly running heterogenization of the initial β₁-matrix lattice, β₁ ⟹ β₁ + β₁ takes place with an activation energy Q_w for the wandering of vacancies, frozen- in by quenching: “Primary vacancies”, which will disappear in sinks already after about 2 sec. (B)The first structural actions of “Umklapping”,β₁⟹β₁⟹β₁, occurring in Zn-poorβ₁-districts, will be caused dynamically by longwavy thermo-acoustic sher-vibrations – “soft modes” of the type ({110)}, 〈110〉 with wavelengths of about 86 Ä – and succeeding dissociations of dislocations in the momentary structure β₁. The activation energy of this transformation stage, being isolated from the increase of the lattice period aβ₁, is QII= 0.8 eV> QI; therefore the energy difference QII- QI⋍ 5.10⁻² eV can be interpreted as energy barrier for the structural transitions β₁⟹ β₁⟹ β₂ – As the “mean atomic volume” of the diffusionless period is smaller than that of β₁, “secondary vacancies” will be formed during this period, and so the following diffusion processes become accelerated for some time. In this subperiod, in which the first α-crystals appear and begin to grow, the activation energy is 1.0 eV. the α-crystals are formed with 12-fold crystallographical strong defined orientations relative to the β₁-matrix. (C) During the postdiffusion period, which will begin with the disappearance of the transition lattice β₂ and which is ruled by growth of the original (coherent) fcc α-crystallets, there are only diffusive direct transitions of (Zn-rich) β₁+− and of residual (Zn-poor but not more “umklapping”) β₁-districts; β₁+ α and β₁ α respectively. The growth of the α-crystallets takes place with the activation energy QIV = 1.45 eV. This energy is the sum of the activation energies for the formation and wandering of a vacancy, Q_IV=Q_B + QW, because in the meantime the excessive “secondary vacancies” are annihilated and the “mean atomic volume” of thc reacting associations is increasing; consequently QB = 0.7 eV. Caused by the increase of the “mean atomic volume” and exhibited by X-ray diffraction photographs, near the end of the isotherm reaction the (at least partially) coherent β₁/α-association suffers a self-deformation. This deformation, only taking place in the α-members of the association, will be signified already in much former reaction stages by weak satellite reflections of some α-crystal interferences (to this s. part (II)). A texture-like diffuse prolongation of several a-reflexions shows the β₁/β₂/α-lattice coherence to be removed partially in some micro-regions (very weak X-ray intensities? ) For this there are a few reasons: (1) In the moment of “Umklapping” the concerned micro-lattice regions are superplastic so that the other strong crystallographical Pβ₁α-orientation relationship can easy get lost, at least partially. (2) It is possible that also the so called “massive transformation” contributes to this effect, since this transformation operates without lattice coherency-the rather the lower the Zn-content of thc transforming lattice regions, as it is in fact realized for the Zn-poorest β₁-districts in the moment of “Umk1apping”.     

Ultraviolet Photoelectron Spectra of Potassium Dosed Higher Fullerenes

Abstract

Ultraviolet photoelectron spectra of potassium dosed higher fullerenes are measured with a synchrotron radiation light source. Potassium dosing to higher fullerenes brings a new structure between the spectral onset of pristine fullerenes and the Fermi level. As the spectral edge of the new structure does not cross the Fermi level, potassium dosed higher fullerenes are not metallic but semiconductive. When the potassium is excessively dosed to the fullerenes, the lower binding energy structures above 5 eV become faint. In contrast to this phenomenon, four distinct structures appear between 5 and 14 eV.     

MORPHOLOGICAL TRANSFORMATIONS DURING PHASE-SEPARATION/ORDERING PHENOMENA

Abstract

The quantitative analysis of the morphological transformations in asymmetric and symmetric binary mixtures undergoing the phase separation has been reported. The general features of the bicontinuous morphology evolution have been discussed from the standpoint of the dynamic scaling hypothesis. The method for the quantitative characterization of the percolation transition by calculating the Euler characteristic has been described. It has been shown that the transformation of the bicontinuous morphology into droplets involves formation of the transient “cylindric” morphology composed of highly elongated, disconnected droplets.     

Tunneling Spectra in NbSe3./I/Pb junctions

NbSe₃, goes throught two phases transition inducing SDW states for T₁ = 149 K and T₂ = 59 K. These two transitions bring a partial condensation at Fermi level, the low temperature conductivity being always of metallic type. The evaluation of the width of the “gap” cannot be done simply. We employed this word “gap” althought it is incorrect here. Tunneling effect has permitted us to obtain an estimation of this “gap” for P=l bar and P=1 Kbar. We have kept tunneling spectra only for T < T₂. For T₁ > T > T₂, the width of the “gap” and thermal broadening are such that a measure is too imprecise. We have used the lead superconductivity as a test of good tunneling devices.     

Electronic Structure of TiSe2 and TiSe2 Intercalated with Eu

Abstract

We report on a PE study of the electronic structure of pristine TiSe₂. The experimental data are compared with results of band-structure calculations. It is confirmed that TiSe₂ reveals semimetallic properties. For the first time rare-earth metals were attempted to be incorporated in situ into TiSe₂. It is found that divalent Eu can be easy intercalated, whereas deposition of trivalent Gd leads to strong interfacial reactions. The electronic structure of the Eu-intercalation compound cannot be simply described within a rigid-band approach assuming charge transfer from Eu to unoccupied Ti d states. In a more elaborated approach hybridization phenomena have to be taken into consideration.     

Wide band-gap organic molecules containing benzodithiophene and difluoroquinoxaline derivatives for solar cell applications

Abstract

Two new semiconducting organic small molecules, namely BDTQ-BDT(EH) and BDTQ-BDT(OC), were prepared by attaching electron accepting 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (DTQ) unit on 2,6-position of electron donating 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(EH)) and 4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(OC)) units. Molecule BDTQ-BDT(EH) showed higher thermal stability (5% weight loss temperature, T_d “349 ^оC), slightly lower band-gap (E_g “2.10?eV) and deeper highest occupied molecular orbital energy level (HOMO “–5.36?eV) level compared to those (T_d “336 ^оC, E_g “2.11?eV, and HOMO “–5.30?eV, respectively.) of the molecule BDTQ-BDT(OC). The organic solar cells (OSCs) made with the synthesized molecules as an electron donor and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₀BM) as an electron acceptor gave a maximum power conversion efficiency (PCE) of 1.20% and 0.83%, respectively, for BDTQ-BDT(EH) and BDTQ-BDT(OC). This study confirmed that the substituents attached on the 4,8-position of BDT unit greatly alter the properties of the resulting molecules.     

An Introduction to the Solid State Polymerization of Transition-metal Complexes: A Comparative Study

Preliminary studies in this laboratory^' of topochemical and topotactic polyreactions of NiBr₂[P(CH₂CH₂CN)₃]₂ have shown that the reactions are in many cases characterized by a) product specificity, b) stereospecificity, c) “crystallographic specificity” and d) highly anisotropic “front motion” through individual single crystals. The polymerization reaction under investigation here is     

Small molecular electrolytes as the interlayer for enhanced performance of organic solar cellsExperimental

Abstract

New non-conjugated small molecules (C4-OH and C6-OH) were synthesized and applied to the inverted organic solar cells (OSCs) as the cathode interlayer (CIL). The power conversion efficiencies (PCEs) of 9.35 and 9.21% were obtained in the device based on ZnO/C4-OH and ZnO/C6-OH, which are better than those of pristine ZnO (8.74%). The work function of ZnO/C4-OH and ZnO/C6-OH are also decreased from ?4.5?to ?4.27?eV and ?4.37?eV, respectively. Consequently, OSCs with two small molecules exhibited enhanced photovoltaic properties due to the formation of interface dipole.