Showing papers on "Amorphous solid published in 2008"

Electronic transport properties of amorphous indium-gallium-zinc oxide semiconductor upon exposure to water

Abstract: The effect of water exposure on amorphous indium-gallium-zinc oxide (a-IGZO) semiconductors was reported. It was found that water can diffuse in and out of the a-IGZO film, reversibly affecting the transistor properties. Two competing mechanisms depending on the thickness of the active channel were clarified. The electron donation effect caused by water adsorption dominated for the thicker a-IGZO films (⩾100nm), which was manifested in the large negative shift (>14V) of the threshold voltage. However, in the case of the thinner a-IGZO films (⩽70nm), the dominance of the water-induced acceptorlike trap behavior was observed. The direct evidence for this behavior was that the subthreshold swing was greatly deteriorated from 0.18V/decade (before water exposure) to 4.4V/decade (after water exposure) for the thinnest a-IGZO films (30nm). These results can be well explained by the screening effect of the intrinsic bulk traps of the a-IGZO semiconductor.

P‐29: Modeling of Amorphous Oxide Semiconductor Thin Film Transistors and Subgap Density of States

Abstract: We report a model of the carrier transport and the subgap density of states in a representative amorphous oxide semiconductor, amorphous InGaZnO4 (a-IGZO), for device simulation of a-IGZO TFTs. Compared to hydrogenated amorphous silicon, a-IGZO exhibits much lower densities of tail states and deep gap states, leading to the small subthreshold swings and high mobilities.

42.1: Invited Paper: Improved Amorphous In‐Ga‐Zn‐O TFTs

Abstract: We review the features of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs), as well as circuit operation based on these TFTs. We also report a novel TFT structure which improves environmental stability of the TFT operation by taking full advantage of the a-IGZO properties, where a conventional PECVD a-SiNX:H films serve not only as an effective barrier layer but also as a hydrogen source to form the coplanar source and drain.

Resonant bonding in crystalline phase-change materials

Abstract: The identification of materials suitable for non-volatile phase-change memory applications is driven by the need to find materials with tailored properties for different technological applications and the desire to understand the scientific basis for their unique properties. Here, we report the observation of a distinctive and characteristic feature of phase-change materials. Measurements of the dielectric function in the energy range from 0.025 to 3 eV reveal that the optical dielectric constant is 70-200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The optical dielectric constant of the amorphous phases is that expected of a covalent semiconductor, whereas that of the crystalline phases is strongly enhanced by resonant bonding effects. The quantification of these is enabled by measurements of the electronic polarizability. As this bonding in the crystalline state is a unique fingerprint for phase-change materials, a simple scheme to identify and characterize potential phase-change materials emerges.

Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy.

Abstract: We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt % 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C61 (PCBM). Three polymers are observed to give amorphous films, attributed to a nonplanar geometry of their backbone while the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (∼100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt % PCBM resulted in 70−90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissoc...

Hydroxypropyl methylcellulose acetate succinate-based spray-dried dispersions: an overview.

Abstract: Spray-dried dispersions (SDDs) of low-solubility drugs have been prepared using the polymer hydroxypropyl methylcellulose acetate succinate (HPMCAS). For a variety of drug structures, these SDDs provide supersaturation in in vitro dissolution determinations and large bioavailability increases in vivo. In bile-salt/lecithin in vitro solutions, these SDDs provide amorphous drug/polymer colloids and an increased concentration of free drug and drug in micelles relative to crystalline or amorphous drug. As dry powders, the SDDs are a single amorphous phase in which the drug remains amorphous and dispersed and does not crystallize over storage times relevant for practical drug products. A melting temperature (Tm)/glass-transition temperature (Tg) (K/K) versus log P map for 139 compounds formulated as SDDs provides a perspective on an appropriate formulation strategy for low-solubility drugs with various physical properties.

A novel approach for calculating starch crystallinity and its correlation with double helix content: a combined XRD and NMR study

Abstract: A peak fitting procedure has been implemented for calculating crystallinity in granular starches. This methodology, widely used for synthetic polymers, is proposed to better reflect the crystalline content of starches than the method normally used, in which it is assumed that relatively perfect crystalline domains are interspersed with amorphous regions. The new approach takes into account irregularities in crystals that are expected to exist in semicrystalline materials. Therefore, instead of assuming that the amorphous background extends up to the base of diffraction peaks, the whole X-ray diffraction (XRD) profile is fitted to an amorphous halo and several discrete crystalline diffraction peaks. The crystallinity values obtained from the XRD patterns of a wide range of native starches using this fitting technique are very similar to the double helix contents as measured by (13)C solid state NMR, suggesting that double helices in granular starches are present within irregular crystals. This contrasts with previous descriptions of crystalline and noncrystalline double helices that were based on the analysis of XRD profiles as perfect crystals interspersed in a noncrystalline background. Furthermore, with this fitting methodology it is possible to calculate the contribution from the different crystal polymorphs of starch to the total crystallinity.

Controlling Morphology in Polymer–Fullerene Mixtures

Abstract: In the past several years, polymer–fullerene mixtures have been intensely studied for use in organic solar cells because they can be deposited from solution, are compatible with lowcost roll-to-roll fabrication technology, and have shown high power conversion efficiency (g), up to 4–5%. The best devices consist of a single bulk-heterojunction active layer, in which the polymer (donor) and fullerene (acceptor) are deposited from a common solvent. As the solvent dries the donor and acceptor components separate into domains. The final efficiency of the solar cell has been shown to be extremely sensitive to the size, composition, and crystallinity of the formed domains. Improvement of the morphology in devices fabricated with a mixture of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and regioregular poly(3-hexylthiophene) (P3HT) has been achieved by using heat-treatment techniques and long-time solvent curing, with resulting record efficiencies. More recently, a method for increasing the crystallinity of the P3HT component has been introduced which involves filtering preformed nanofibers of P3HT out of solution and mixing the prepared nanofiber dispersion with PCBM to increase the efficiency of as-cast devices. Interestingly, the best device performance was achieved by mixing lower-molecular-weight (MW) amorphous P3HT back into the solution to reduce the crystalline content of the active layer and, thereby, to increase connection between crystalline domains. Studies of the MW impact on P3HT/PCBM solar cells have indicated that a large polydispersity and number-average molecular weight (Mn) over 19000 g mol -1 leads to improved efficiency. Morphology studies of organic field-effect transistor (OFET) devices indicate that the increased MW leads to better network formation between crystalline domains. The morphology of these improved devices has been studied using transmission electron microscopy (TEM), grazing-angle X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), NMR, and a variety of other optical and electrical techniques. The morphology studies give a picture of a device in which the P3HT forms aligned/crystalline domains, between which are amorphous segments of P3HT and PCBM. These domains form with larger size and crystallinity for higher heat-treatment temperatures and longer solvent soaking times. Depending on the fabrication and measurement techniques, the aligned domains of P3HT are depicted as fibers or as shapeless masses. The majority of these studies do not, however, allow quantification of the percentage of the P3HT that is agglomerated/ crystalline in the final device. Only by making use of the nanofiber filtration technique have the authors been given the ability to control the crystalline content of the P3HT in solution and in the final device. The disadvantages of this technique are the necessity of more complicated preparation, and filtered P3HT is restricted to a fibrous form that requires the addition of amorphous P3HT to provide connections between crystalline domains. We present here a simple method to determine the agglomerated–amorphous ratio of the P3HT and to control the degree of agglomeration/crystallinity of the P3HT in the final device by using a solvent mixing method and no further heat-treatment or prepreparation of the polymer. The most obvious change that heat-treatment and solvent soaking yield on a P3HT:PCBM layers is the change in color. It has been widely reported that the P3HT absorption red-shifts and a series of vibronic peaks become visible at k ∼ 600 nm, 550 nm, and 510 nm. This red-shift has been assigned to increased planarization and stabilization of the P3HT chains that accompanies self-stacking of the polymer. The crystal structure for these self-stacking domains has been solved by using XRD and TEM, and shows a herringbone interconnection of the alkyl chains and an a-dimension stacking distance of 1.6 nm. The p–p chain stacking of the P3HT chains in crystallites has been measured to be 0.38 nm. The herringbone structure and planarization of the P3HT with heating has been confirmed using heteronuclear solid-state NMR measurements. The red-shift in the UV-vis spectrum occurs proportionally to the degree of agglomeration of the P3HT. The pure amorphous electronic spectrum of P3HT or a mixture of P3HT and PCBM is simple to measure. A solution UV-vis spectrum can also be measured in the liquid state. If C O M M U N IC A TI O N

Amorphous calcium phosphate is a major component of the forming fin bones of zebrafish: Indications for an amorphous precursor phase

Abstract: A fundamental question in biomineralization is the nature of the first-formed mineral phase. In vertebrate bone formation, this issue has been the subject of a long-standing controversy. We address this key issue using the continuously growing fin bony rays of the Tuebingen long-fin zebrafish as a model for bone mineralization. Employing high-resolution scanning and transmission electron microscopy imaging, electron diffraction, and elemental analysis, we demonstrate the presence of an abundant amorphous calcium phosphate phase in the newly formed fin bones. The extracted amorphous mineral particles crystallize with time, and mineral crystallinity increases during bone maturation. Based on these findings, we propose that this amorphous calcium phosphate phase may be a precursor phase that later transforms into the mature crystalline mineral.

Microscopic origin of the fast crystallization ability of Ge–Sb–Te phase-change memory materials

Abstract: Ge-Sb-Te materials are used in optical DVDs and non-volatile electronic memories (phase-change random-access memory). In both, data storage is effected by fast, reversible phase changes between crystalline and amorphous states. Despite much experimental and theoretical effort to understand the phase-change mechanism, the detailed atomistic changes involved are still unknown. Here, we describe for the first time how the entire write/erase cycle for the Ge(2)Sb(2)Te(5) composition can be reproduced using ab initio molecular-dynamics simulations. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rocksalt structure, form during melt cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge(2)Sb(2)Te(5). As this simulation procedure is general, the microscopic insight provided on crystal nucleation should open up new ways to develop superior phase-change memory materials, for example, faster nucleation, different compositions, doping levels and so on.

Helium induced nanoscopic morphology on tungsten under fusion relevant plasma conditions

Abstract: Polished W discs exposed to pure He plasma in the PISCES-B linear-divertor-plasma simulator at 1120 and 1320 K are found to develop deeply nanostructured surface layers consisting of a conglomerate of amorphous 'nanorods'. The growth of the thickness of the nanostructured layer is explored for exposure times spanning 300–(2.2 × 104) s in He plasmas of density ne ~ 4 × 1018 m−3 and temperature Te ~ 6–8 eV where the average He-ion surface-impact energy is ~60 eV, below the threshold for physical sputtering. A nanostructured layer in excess of 5 µm thick is observed for the longest exposure time explored. The kinetics of the layer growth are found to follow Fick's law, characterized by an effective diffusive mechanism with coefficients of diffusion: D1120 K = 6.6 ± 0.4 × 10−12 cm2 s−1 and D1320 K = 2.0± 0.5 × 10−11 cm2 s−1. The diffusion of He atoms in W is considered too rapid to explain the observed growth of surface modification and points to the interplay of other mechanisms, such as the availability of thermal vacancies and/or the slower diffusion of He through the forming nanostructured layer.

Substrate-limited electron dynamics in graphene

Abstract: We study the effects of polarizable substrates such as $\mathrm{Si}{\mathrm{O}}_{2}$ and SiC on the carrier dynamics in graphene. We find that the quasiparticle spectrum acquires a finite broadening due to the long-range interaction with the polar modes at the interface between graphene and the substrate. This mechanism results in a density dependent electrical resistivity, which exhibits a sharp increase around room temperature, where it can become the dominant limiting factor of electron transport. The effects are weaker in doped bilayer graphene due to the more conventional parabolic band dispersion. Amorphous substrates, such as polymethyl methacrylate, can induce a room temperature resistivity of comparable magnitude, although with a weaker temperature dependence.

Effect of sample size on deformation in amorphous metals

Abstract: Uniaxial compression tests were performed on micron-sized columns of amorphous PdSi to investigate the effect of sample size on deformation behavior. Cylindrical columns with diameters between 8μm and 140nm were fabricated from sputtered amorphous Pd77Si23 films on Si substrates by focused ion beam machining and compression tests were performed with a nanoindenter outfitted with a flat diamond punch. The columns exhibited elastic behavior until they yielded by either shear band formation on a plane at 50° to the loading axis or by homogenous deformation. Shear band formation occurred only in columns with diameters larger than 400nm. The change in deformation mechanism from shear band formation to homogeneous deformation with decreasing column size is attributed to a required critical strained volume for shear band formation.

Transformation mechanism of amorphous calcium carbonate into calcite in the sea urchin larval spicule

Abstract: Sea urchin larval spicules transform amorphous calcium carbonate (ACC) into calcite single crystals. The mechanism of transformation is enigmatic: the transforming spicule displays both amorphous and crystalline properties, with no defined crystallization front. Here, we use X-ray photoelectron emission spectromicroscopy with probing size of 40-200 nm. We resolve 3 distinct mineral phases: An initial short-lived, presumably hydrated ACC phase, followed by an intermediate transient form of ACC, and finally the biogenic crystalline calcite phase. The amorphous and crystalline phases are juxtaposed, often appearing in adjacent sites at a scale of tens of nanometers. We propose that the amorphous-crystal transformation propagates in a tortuous path through preexisting 40- to 100-nm amorphous units, via a secondary nucleation mechanism.

Organic el device

Abstract: An amorphous electrically conductive oxide such as In—Zn—O-containing amorphous oxide or amorphous ITO is used alone, or a laminate having a two- or three-layer structure constituted of the amorphous electrically conductive oxide and at least a thin metal layer is used, to form a transparent electrode whose side surface has a tapered form and whose top layer is formed of a layer of the above amorphous electrically conductive oxide, an organic single-layer portion or an organic multi-layer portion containing at least an organic light-emitting material is formed on the above transparent electrode, and further, an opposing electrode is formed on the above organic single-layer portion or the above organic multi-layer portion, whereby an organic EL device is obtained. A flattening layer is optionally provided so as to abut on the side surface of the transparent electrode for moderating a height-level difference between the above transparent electrode and the surface of the above substrate. Further, the above organic EL device is used to form an organic EL display panel.

Spatial cooperativity in soft glassy flows

Abstract: Amorphous glassy materials of diverse nature-concentrated emulsions, granular materials, pastes, molecular glasses-display complex flow properties, intermediate between solid and liquid, which are at the root of their use in many applications. A general feature of such systems, well documented yet not really understood, is the strongly nonlinear nature of the flow rule relating stresses and strain rates. Here we use a microfluidic velocimetry technique to characterize the flow of thin layers of concentrated emulsions, confined in gaps of different thicknesses by surfaces of different roughnesses. We find evidence for finite-size effects in the flow behaviour and the absence of an intrinsic local flow rule. In contrast to the classical nonlinearities of the rheological behaviour of amorphous materials, we show that a rather simple non-local flow rule can account for all the velocity profiles. This non-locality of the dynamics is quantified by a length, characteristic of cooperativity within the flow at these scales, that is unobservable in the liquid state (lower emulsion concentrations) and that increases with concentration in the jammed state. Beyond its practical importance for applications involving thin layers (for example, coatings), these non-locality and cooperativity effects have parallels in the behaviour of other glassy, jammed and granular systems, suggesting a possible fundamental universality.

Microwave dielectric loss at single photon energies and millikelvin temperatures

Abstract: The microwave performance of amorphous dielectric materials at very low temperatures and very low excitation strengths displays significant excess loss. Here, we present the loss tangents of some common amorphous and crystalline dielectrics, measured at low temperatures (T<100mK) with near single-photon excitation energies, E∕ℏω0∼1, using both coplanar waveguide and lumped LC resonators. The loss can be understood using a two-level state defect model. A circuit analysis of the half-wavelength resonators we used is outlined, and the energy dissipation of such a resonator on a multilayered dielectric substrate is theoretically considered.

Matching glass-forming ability with the density of the amorphous phase.

Abstract: The density of the amorphous phase of metals is generally thought to be related to glass formation, but this correlation has not been demonstrated experimentally to date. In this work, systematic deflection measurements using microcantilevers and a combinatorial deposition method show a correlation between glass-forming ability and the density change upon crystallization over a broad compositional range in the copper-zirconium binary system. Distinct peaks in the density of the amorphous phase were found to correlate with specific maxima in the critical thickness for glass formation. Our findings provide quantitative data for the development of structural models of liquids that are readily quenched to the amorphous state. The experimental method developed in this work can facilitate the search for new glass-forming alloys.

Hexagonal-close-packed, hierarchical amorphous TiO2 nanocolumn arrays: transferability, enhanced photocatalytic activity, and superamphiphilicity without UV irradiation.

Abstract: A hexagonal-close-packed (hcp), hierarchical amorphous TiO2 nanocolumn array was fabricated by pulsed laser deposition (PLD) using a PS colloidal monolayer as a template under a high pressure (6.7 Pa) of background oxygen gas. The formation mechanism was investigated, and a model of multidirection glancing deposition was proposed to explain the formation process. This strategy can be extended to the fabrication of similar structures using different materials. Interestingly, this nanostructured array could be transferred to almost any substrate, avoiding restriction of substrate types in fabrication of nanocolumn arrays, which is helpful in the design and creation of nanodevices on various desired substrates. This hierarchical nanocolumn array exhibits excellent superamphiphilicity with both water and oil contact angles of 0 degrees, without further UV irradiation. More importantly, the amorphous TiO2 nanocolumn array demonstrates better performance in photocatalytic activity than an anatase nanocolumn array due to its large surface area and special microstructures, suggesting that the surface area of the TiO2 is preferable to its crystal structure for enhancing photocatalytic activity. The combination of superamphiphilicity and photocatalytic activity gives the surface an excellent self-cleaning effect.

Modeling of amorphous InGaZnO4 thin film transistors and their subgap density of states

Abstract: We report a model of the carrier transport and the subgap density of states in amorphous InGaZnO4 (a-IGZO) for device simulation of a-IGZO thin-film transistors (TFTs) operated in both the depletion mode and the enhancement mode. A simple model using a constant mobility and two-step subgap density of states reproduced well the characteristics of the a-IGZO TFTs. a-IGZO exhibits low densities of tail states and deep gap states, leading to small subthreshold swings and high mobilities.

The amount of immobilized polymer in PMMA SiO$_{2}$ nanocomposites determined from calorimetric data

Abstract: For semicrystalline polymers there is an ongoing debate at what temperature the immobilized or rigid amorphous fraction (RAF) devitrifies (relaxes). The question if the polymer crystals aremelting first and simultaneously theRAFdevitrifies or the RAF devitrifies first and later on the crystals melt cannot be answered easily on the example of semicrystalline polymers. This is because the crystals, which are the reason for the immobilization of the polymer, often disappear (melt) in the same temperature range as theRAF. For polymer nanocomposites the situation is simpler. Silica nanoparticles do notmelt or undergo other phase transitions altering the polymer–nanoparticle interaction in the temperature range where the polymer is thermally stable (does not degrade). The existence of an immobilized fraction in PMMA SiO2 nanocomposites was shown on the basis of heat capacitymeasurements at the glass transition of the polymer. The results were verified by enthalpy relaxation experiments below the glass transition. The immobilized layer is about 2 nm thick at low filler content if agglomeration is not dominant. The thickness of the layer is similar to that found in semicrystalline polymers and independent from the shape of the nanoparticles. Nanocomposites therefore offer a unique opportunity to study the devitrification of the immobilized fraction (RAF) without interference of melting of crystals as in semicrystalline polymers. It was found that the interaction between the SiO2 nanoparticles and the PMMA is so strong that no devitrification occurs before degradation of the polymer. No gradual increase of heat capacity or a broadening of the glass transitionwas found. The cooperatively rearranging regions (CRR) are either immobilized or mobile. No intermediate states are found. The results obtained for the polymer nanocomposites support the view that the reason for the restricted mobility must disappear before the RAF can devitrify. For semicrystalline polymers this means that rigid crystals must melt before the RAF can relax. 2007 Elsevier Ltd. All rights reserved.

Defect passivation and homogenization of amorphous oxide thin-film transistor by wet O2 annealing

Abstract: Roles of H2O addition to an annealing atmosphere were investigated for amorphous In–Ga–Zn–O thin-film transistors fabricated at room temperature. Although dry O2 annealing improved saturation mobility (μsat) and subthreshold voltage swings (S), wet O2 annealing further improved them to μsat∼12cm2(Vs)−1 and S<0.12Vdecade−1 along with improvement of their uniformity. Desorption of OH-related species caused conductivity increase during thermal annealing at <310°C. Zn–O components started to desorb at ∼300°C for the unannealed and the dry O2 annealed films, while these were suppressed remarkably by the wet O2 annealing.

Subgap states in transparent amorphous oxide semiconductor, In–Ga–Zn–O, observed by bulk sensitive x-ray photoelectron spectroscopy

Abstract: We investigated the electronic states in amorphous In–Ga–Zn–O films with high carrier concentrations by optical absorption and hard x-ray photoelectron spectroscopy (HX-PES). Films having different Hall mobilities were prepared and their annealing effects were examined. All HX-PES spectra showed Fermi edge structures and extra subgap densities of states (DOSs). Tail-like structures observed in the optical spectra originate from subgap DOSs (⪢1020cm−3) near valence band maximas (VBMs). Subgap DOSs near VBMs provide a reason why In–Ga–Zn–O thin film transistors show hard saturation in off states and are difficult to operate in an inversion p-channel mode.

Solid state amorphization of pharmaceuticals.

Abstract: Amorphous solids are conventionally formed by supercooling liquids or by concentrating noncrystallizing solutes (spray-drying and freeze-drying). However, a lot of pharmaceutical processes may also directly convert compounds from crystal to noncrystal which may have desired or undesired consequences for their stability. The purpose of this short review paper is (i) to illustrate the possibility to amorphize one compound by several different routes (supercooling, dehydration of hydrate, milling, annealing of metastable crystalline forms), (ii) to examine factors that favor crystal to glass rather than crystal to crystal transformations, (iii) to discuss the role of possible amorphous intermediates in solid−solid conversions induced by milling, (iv) to address the issue of chemical stability in the course of solid state amorphization, (v) to discuss the nature of the amorphous state obtained by the nonconventional routes, (vi) to show the effect of milling conditions on glasses properties, and (vii) to atte...

Trap densities in amorphous-InGaZnO4 thin-film transistors

Abstract: Trap densities in amorphous-InGaZnO4 (α-IGZO) are extracted directly from the capacitance-voltage characteristics of thin-film transistors at low frequencies. It is found that the trap densities are flat in the energy gap, and are 1.7×1016cm−3eV−1 in the deep energy far from the conduction band edge (Ec), but become larger near Ec. Moreover, postannealing reduces the trap density near Ec, which is associated with the reduction of the hysteresis in the current-voltage characteristics. The annealed α-IGZO does not have a Gaussian-type state and has fewer tail states than amorphous Si.