Showing papers on "Amorphous solid published in 2009"

Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays.

Abstract: Hydrogenated amorphous Si (a-Si:H) is an important solar cell material. Here we demonstrate the fabrication of a-Si:H nanowires (NWs) and nanocones (NCs), using an easily scalable and IC-compatible process. We also investigate the optical properties of these nanostructures. These a-Si:H nanostructures display greatly enhanced absorption over a large range of wavelengths and angles of incidence, due to suppressed reflection. The enhancement effect is particularly strong for a-Si:H NC arrays, which provide nearly perfect impedance matching between a-Si:H and air through a gradual reduction of the effective refractive index. More than 90% of light is absorbed at angles of incidence up to 60° for a-Si:H NC arrays, which is significantly better than NW arrays (70%) and thin films (45%). In addition, the absorption of NC arrays is 88% at the band gap edge of a-Si:H, which is much higher than NW arrays (70%) and thin films (53%). Our experimental data agree very well with simulation. The a-Si:H nanocones functio...

Crystalline-amorphous core-shell silicon nanowires for high capacity and high current battery electrodes.

Abstract: Silicon is an attractive alloy-type anode material for lithium ion batteries because of its highest known capacity (4200 mAh/g). However silicon’s large volume change upon lithium insertion and extraction, which causes pulverization and capacity fading, has limited its applications. Designing nanoscale hierarchical structures is a novel approach to address the issues associated with the large volume changes. In this letter, we introduce a core−shell design of silicon nanowires for highpower and long-life lithium battery electrodes. Silicon crystalline-amorphous core−shell nanowires were grown directly on stainless steel current collectors by a simple one-step synthesis. Amorphous Si shells instead of crystalline Si cores can be selected to be electrochemically active due to the difference of their lithiation potentials. Therefore, crystalline Si cores function as a stable mechanical support and an efficient electrical conducting pathway while amorphous shells store Li+ ions. We demonstrate here that these...

Atomic and Electronic Structure of Graphene-Oxide

Abstract: We elucidate the atomic and electronic structure of graphene oxide (GO) using annular dark field imaging of single and multilayer sheets and electron energy loss spectroscopy for measuring the fine structure of C and O K-edges in a scanning transmission electron microscope. Partial density of states and electronic plasma excitations are also measured for these GO sheets showing unusual π* + σ* excitation at 19 eV. The results of this detailed analysis reveal that the GO is rough with an average surface roughness of 0.6 nm and the structure is predominantly amorphous due to distortions from sp3 C−O bonds. Around 40% sp3 bonding was found to be present in these sheets with measured O/C ratio of 1:5. These sp2 to sp3 bond modifications due to oxidation are also supported by ab initio calculations.

P-9: Numerical Analysis on Temperature Dependence of Characteristics of Amorphous In-Ga-Zn-Oxide TFT

Abstract: We fabricated inverted-staggered amorphous In-Ga-Zn-O (a-IGZO) TFTs and measured temperature dependence of the TFT characteristics. A Vth shift between 120°C and 180°C was as large as about 4 V. In the analysis with 2-D numerical simulation, we could reproduce the measured result by assuming two kinds of donor-like states as carrier generation sources.

Nanomoulding with amorphous metals

Abstract: Nanoimprinting promises low-cost fabrication of micro- and nano-devices by embossing features from a hard mould onto thermoplastic materials, typically polymers with low glass transition temperature. The success and proliferation of such methods critically rely on the manufacturing of robust and durable master moulds. Silicon-based moulds are brittle and have limited longevity. Metal moulds are stronger than semiconductors, but patterning of metals on the nanometre scale is limited by their finite grain size. Amorphous metals (metallic glasses) exhibit superior mechanical properties and are intrinsically free from grain size limitations. Here we demonstrate direct nanopatterning of metallic glasses by hot embossing, generating feature sizes as small as 13 nm. After subsequently crystallizing the as-formed metallic glass mould, we show that another amorphous sample of the same alloy can be formed on the crystallized mould. In addition, metallic glass replicas can also be used as moulds for polymers or other metallic glasses with lower softening temperatures. Using this 'spawning' process, we can massively replicate patterned surfaces through direct moulding without using conventional lithography. We anticipate that our findings will catalyse the development of micro- and nanoscale metallic glass applications that capitalize on the outstanding mechanical properties, microstructural homogeneity and isotropy, and ease of thermoplastic forming exhibited by these materials.

Modification of graphene properties due to electron-beam irradiation

Abstract: The authors report micro-Raman investigation of changes in the single and bilayer graphene crystal lattice induced by the low and medium energy electron-beam irradiation (5–20 keV). It was found that the radiation exposures result in the appearance of the strong disorder D band around 1345 cm−1, indicating damage to the lattice. The D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests graphene’s transformation to the nanocrystalline and then to amorphous form. The results have important implications for graphene characterization and device fabrication, which rely on the electron microscopy and focused ion beam processing.

Estimation of Drug–Polymer Miscibility and Solubility in Amorphous Solid Dispersions Using Experimentally Determined Interaction Parameters

Abstract: The amorphous form of a drug may provide enhanced solubility, dissolution rate, and bioavailability but will also potentially crystallize over time. Miscible polymeric additives provide a means to increase physical stability. Understanding the miscibility of drug–polymer systems is of interest to optimize the formulation of such systems. The purpose of this work was to develop experimental models which allow for more quantitative estimates of the thermodynamics of mixing amorphous drugs with glassy polymers. The thermodynamics of mixing several amorphous drugs with amorphous polymers was estimated by coupling solution theory with experimental data. The entropy of mixing was estimated using Flory–Huggins lattice theory. The enthalpy of mixing and any deviations from the entropy as predicted by Flory–Huggins lattice theory were estimated using two separate experimental techniques; (1) melting point depression of the crystalline drug in the presence of the amorphous polymer was measured using differential scanning calorimetry and (2) determination of the solubility of the drug in 1-ethyl-2-pyrrolidone. The estimated activity coefficient was used to calculate the free energy of mixing of the drugs in the polymers and the corresponding solubility. Mixtures previously reported as miscible showed various degrees of melting point depression while systems reported as immiscible or partially miscible showed little or no melting point depression. The solubility of several compounds in 1-ethyl-2-pyrrolidone predicts that most drugs have a rather low solubility in poly(vinylpyrrolidone). Miscibility of various drugs with polymers can be explored by coupling solution theories with experimental data. These approximations provide insight into the physical stability of drug–polymer mixtures and the thermodynamic driving force for crystallization.

Amorphous and crystalline aerosol particles interacting with water vapor: conceptual framework and experimental evidence for restructuring, phase transitions and kinetic limitations

Abstract: . Interactions with water are crucial for the properties, transformation and climate effects of atmospheric aerosols. Here we present a conceptual framework for the interaction of amorphous aerosol particles with water vapor, outlining characteristic features and differences in comparison to crystalline particles. We used a hygroscopicity tandem differential mobility analyzer (H-TDMA) to characterize the hydration and dehydration of crystalline ammonium sulfate, amorphous oxalic acid and amorphous levoglucosan particles (diameter ~100 nm, relative humidity 5–95% at 298 K). The experimental data and accompanying Kohler model calculations provide new insights into particle microstructure, surface adsorption, bulk absorption, phase transitions and hygroscopic growth. The results of these and related investigations lead to the following conclusions: (1) Many organic substances, including carboxylic acids, carbohydrates and proteins, tend to form amorphous rather than crystalline phases upon drying of aqueous solution droplets. Depending on viscosity and microstructure, the amorphous phases can be classified as glasses, rubbers, gels or viscous liquids. (2) Amorphous organic substances tend to absorb water vapor and undergo gradual deliquescence and hygroscopic growth at lower relative humidity than their crystalline counterparts. (3) In the course of hydration and dehydration, certain organic substances can form rubber- or gel-like structures (supramolecular networks) and undergo transitions between swollen and collapsed network structures. (4) Organic gels or (semi-)solid amorphous shells (glassy, rubbery, ultra-viscous) with low molecular diffusivity can kinetically limit the uptake and release of water and may influence the hygroscopic growth and activation of aerosol particles as cloud condensation nuclei (CCN) and ice nuclei (IN). Moreover, (semi-)solid amorphous phases may influence the uptake of gaseous photo-oxidants and the chemical transformation and aging of atmospheric aerosols. (5) The shape and porosity of amorphous and crystalline particles formed upon dehydration of aqueous solution droplets depend on chemical composition and drying conditions. The apparent volume void fractions of particles with highly porous structures can range up to ~50% or more (xerogels, aerogels). (6) For efficient description of water uptake and phase transitions of aerosol particles, we propose not to limit the terms deliquescence and efflorescence to equilibrium phase transitions of crystalline substances. Instead we propose generalized definitions according to which amorphous and crystalline components can undergo gradual or prompt, partial or full deliquescence or efflorescence. We suggest that (semi-)solid amorphous phases may be important not only in the upper atmosphere as suggested in recent studies of glass formation at low temperatures. Depending on relative humidity, (semi-)solid phases and moisture-induced glass transitions may also play a role in gas-particle interactions at ambient temperatures in the lower atmosphere.

The role of viscous flow of oxide in the growth of self-ordered porous anodic alumina films.

Abstract: Porous anodic alumina (PAA) films are widely used as templates for functional nanostructures, because of the high regularity and controllability of the pore morphology. However, growth mechanisms have not yet been developed that can explain quantitative relationships between processing conditions and oxide layer geometry. Here, we present a model for steady-state growth of these amorphous films, incorporating the novel feature that metal and oxygen ions are transported by coupled electrical migration and viscous flow. The oxide flow in the model arises near the film-solution interface at the pore bottoms, in response to the constraint of volume conservation. The hypothesis of viscous flow was successfully validated through detailed comparisons to observations of the motion of tungsten tracers in the film. Predictions of localized tensile stress near nanoscale ridges at the metal-film interface were supported by observations of voids at these sites. We suggest that the ordering of PAA may be explained by a mechanism in which metal-film interface motion is regulated by the combination of ionic migration in the oxide and stress-driven interface diffusion of metal atoms.

Dynamic Polarization of Single Nuclear Spins by Optical Pumping of Nitrogen-Vacancy Color Centers in Diamond at Room Temperature

Abstract: We report a versatile method to polarize single nuclear spins in diamond, based on optical pumping of a single nitrogen-vacancy (NV) defect and mediated by a level anticrossing in its excited state. A nuclear-spin polarization higher than 98% is achieved at room temperature for the 15N nuclear spin associated with the NV center, corresponding to microK effective nuclear-spin temperature. We then show simultaneous initialization of two nuclear spins in the vicinity of a NV defect. Such robust control of nuclear-spin states is a key ingredient for further scaling up of nuclear-spin based quantum registers in diamond.

Development of suitable interatomic potentials for simulation of liquid and amorphous Cu–Zr alloys

Abstract: We present a new semi-empirical potential suitable for molecular dynamics simulations of liquid and amorphous Cu–Zr alloys. To provide input data for developing the potential, new experimental measurements of the structure factors for amorphous Cu64.5Zr35.5 alloy were performed. In this work, we propose a new method to include diffraction data in the potential development procedure, which also includes fitting to first-principles and liquid density and enthalpy of mixing data. To refine the new potential, we used first-principles and liquid enthalpy of mixing data published earlier combined with the densities of liquid Cu64.5Zr35.5 measured over a range of temperatures. We show that the potential predicts a liquid-to-glass transition temperature that agrees reasonably well with experimental data. Finally, we compare the new potential with two previously developed semi-empirical potentials for Cu–Zr alloys and examine their comparative and contrasting descriptions of structure and properties for Cu64.5Zr35...

Liquid crystalline ordering and charge transport in semiconducting materials.

Abstract: Organic semiconducting materials offer the advantage of solution processability into flexible films. In most cases, their drawback is based on their low charge carrier mobility, which is directly related to the packing of the molecules both on local (amorphous versus crystalline) and on macroscopic (grain boundaries) length scales. Liquid crystalline ordering offers the possibility of circumventing this problem. An advanced concept comprises: i) the application of materials with different liquid crystalline phases, ii) the orientation of a low viscosity high temperature phase, and, iii) the transfer of the macroscopic orientation during cooling to a highly ordered (at best, crystalline-like) phase at room temperature. At the same time, the desired orientation for the application (OLED or field-effect transistor) can be obtained. This review presents the use of molecules with discotic, calamitic and sanidic phases and discusses the sensitivity of the phases with regard to defects depending on the dimensionality of the ordered structure (columns: 1D, smectic layers and sanidic phases: 2D). It presents ways to systematically improve charge carrier mobility by proper variation of the electronic and steric (packing) structure of the constituting molecules and to reach charge carrier mobilities that are close to and comparable to amorphous silicon, with values of 0.1 to 0. 7 cm 2 . V -1 . s -1 . In this context, the significance of crosslinking to stabilize the orientation and liquid crystalline behavior of inorganic/organic hybrids is also discussed.

Nanoscale cellulose films with different crystallinities and mesostructures--their surface properties and interaction with water.

Abstract: A systematic study of the degree of molecular ordering and swelling of different nanocellulose model films has been conducted. Crystalline cellulose II surfaces were prepared by spin-coating of the precursor cellulose solutions onto oxidized silicon wafers before regeneration in water or by using the Langmuir−Schaefer (LS) technique. Amorphous cellulose films were also prepared by spin-coating of a precursor cellulose solution onto oxidized silicon wafers. Crystalline cellulose I surfaces were prepared by spin-coating wafers with aqueous suspensions of sulfate-stabilized cellulose I nanocrystals and low-charged microfibrillated cellulose (LC-MFC). In addition, a dispersion of high-charged MFC was used for the buildup of polyelectrolyte multilayers with polyetheyleneimine on silica with the aid of the layer-by-layer (LbL) technique. These preparation methods produced smooth thin films on the nanometer scale suitable for X-ray diffraction and swelling measurements. The surface morphology and thickness of th...

Turning carbon nanotubes from exceptional heat conductors into insulators.

Abstract: Thermal conductivity ($\ensuremath{\kappa}$) of isolated carbon nanotubes (CNTs) is higher than the $\ensuremath{\kappa}$ of diamond; however, in this Letter we show that the $\ensuremath{\kappa}$ of a packed bed of three-dimensional random networks of single and multiwall CNTs is smaller than that of thermally insulating amorphous polymers. The thermoelectric power ($\ensuremath{\Sigma}$) of the random network of CNTs was also measured. The $\ensuremath{\Sigma}$ of a single wall nanotube network is very similar to that of isolated nanotubes and, in contrast with $\ensuremath{\kappa}$, $\ensuremath{\Sigma}$ shows a strong dependence on the tube diameter.

Controlled synthesis of WO3 nanorods and their electrochromic properties in H2SO4 electrolyte

Abstract: Uniform crystalline WO3 nanorods were synthesized by a lithium- or sulfate-free hydrothermal process with NaCl as a capping agent. It is found that variations in the pH and amount of capping agent have critical influence on the morphologies of the resultant WO3 nanostructures. The electrochromic film comprised of the as-synthesized crystalline WO3 nanorods exhibits a fast coloration/bleaching switching, coloration efficiency comparable to that of conventional amorphous WO3 films, and a high H+ ion-insertion ability in 0.5 mol/L H2SO4 solution without degradation.

Atomistic simulations of heat transport in silicon nanowires.

Abstract: We present a series of molecular dynamics, lattice dynamics, and Boltzmann transport equation calculations aimed at understanding heat transport in Silicon nanowires. In agreement with recent experiments, we find that the computed thermal conductivity strongly depends on the surface structure. It may be as high as that of bulk Si for crystalline wires, while wires with amorphous surfaces have the smallest thermal conductivity, about 100 times lower than the bulk. Two, combined effects are responsible for this dramatic difference: the presence, at disordered surfaces, of extended, nonpropagating modes analogous to heat carriers in amorphous Si, together with decreased lifetimes of propagating modes.

Toward High-Performance Amorphous GIZO TFTs

Abstract: This work analyzes the role of processing parameters on the electrical performance of GIZO (Ga 2 O 3 :In 2 O 3 :ZnO) films and thin-film transistors (TFTs). Parameters such as oxygen partial pressure, deposition pressure, target composition, thickness, and annealing temperature are studied. Generally, better devices are obtained when low oxygen partial pressure is used. This is related to the damage induced by oxygen ion bombardment and very high film's resistivity when higher oxygen partial pressures are used. Low deposition pressures and targets with richer indium compositions led to films with high carrier concentration, resulting in transistors with field-effect mobility as high as ∼ 80 cm 2 /Vs but poor channel conductivity modulation, becoming ineffective as switching devices. Nevertheless, it is demonstrated that reducing the GIZO thickness from 40 to 10 nm greatly enhances the switching behavior of those devices, due to the lower absolute number of free carriers and hence to their easier depletion. Annealing also proves to be crucial to control device performance, significantly modifying GIZO electrical resistivity and promoting local atomic rearrangement, being the optimal temperature determined by the as-produced films' properties. For the best-performing transistors, even with a low annealing temperature (150°C), remarkable properties such as μ FE = 73.9 cm 2 /Vs, on/off ratio as 7 × 10 7 , V T ≈ 0.2 V, and S = 0.29 V/dec are achieved.

Core/CdS Quantum Dot/Shell Mesoporous Solar Cells with Improved Stability and Efficiency Using an Amorphous TiO2 Coating

Abstract: Here we present a CdS quantum dot sensitized solar cell based on a mesoporous TiO2 film with remarkable stability using I−/I3− electrolyte. Chemical Bath Deposition (CBD) was used to deposit the CdS quantum dots within the porous network. We show that a thin coating of the QD sensitized film with an amorphous TiO2 layer strongly improves the performance and photostability of the solar cell. We propose that the coating passivates QD surface states which act as hole traps and are responsible for photodegradation of the device. In addition, this coating decreases the recombination of electrons from the CdS quantum dots and the mesoporous TiO2 into the electrolyte solution. We obtain a significant improvement of all cell parameters resulting in a total light to electric power conversion efficiency of 1.24%.

High performance solution-processed amorphous zinc tin oxide thin film transistor

Abstract: Thin film transistors (TFTs) with amorphous zinc tin oxide (ZTO) channel layer were fabricated by a simple and low-cost solution process. The ZTO thin films are highly transparent (>90% transmittance) in the visible region. The ZTO TFTs fabricated at 400 and 500 °C are operated in enhancement mode. The TFT annealed at 500 °C shows a mobility of 14.11 cm2 V−1 s−1, a threshold voltage of 1.71 V, a subthreshold slope of 0.4 V dec−1 and an on–off current ratio greater than 108. In addition, we investigated the gate bias stability of the TFT. Positive gate bias results in a positive shift of the threshold voltage due to the charge trapping in the channel/dielectric interface.

Remarkable capacity retention of nanostructured manganese oxide upon cycling as an electrode material for supercapacitor

Abstract: Electrochemical capacity retention of nearly X-ray amorphous nanostructured manganese oxide (nanoMnO2) synthesized by mixing directly KMnO4 with ethylene glycol under ambient conditions for supercapacitor studies is enhanced significantly. Although X-ray diffraction (XRD) pattern of nanoMnO2 shows poor crystallinity, it is found that by Mn K-edge X-ray absorption near edge structure (XANES) measurement that the nanoMnO2 obtained is locally arranged in a δ-MnO2-type layered structure composed of edge-shared network of MnO6 octahedra. Field emission scanning electron microscopy and XANES measurements show that nanoMnO2 contains nearly spherical shaped morphology with δ-MnO2 structure, and 1D nanorods of α-MnO2 type structure (powder XRD) in the annealed (600 °C) sample. Volumetric nitrogen adsorption−desorption isotherms, inductively coupled plasma analysis, and thermal analysis are carried out to obtain physicochemical properties such as surface area (230 m2 g−1), porosity of nanoMnO2 (secondary mesopores of diameter 14.5 nm), water content, composition, etc., which lead to the promising electrochemical properties as an electrode for supercapacitor. The nanoMnO2 shows a very high stability even after 1200 cycles with capacity retention of about 250 F g−1.

Two-dimensional numerical simulation of radio frequency sputter amorphous In–Ga–Zn–O thin-film transistors

Abstract: We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10−3 Ω cm2.

Printable Ferroelectric PVDF/PMMA Blend Films with Ultralow Roughness for Low Voltage Non‐Volatile Polymer Memory

Abstract: Here, a facile route to fabricate thin ferroelectric poly(vinylidene fluoride) (PVDF)/poly(methylmethacrylate) (PMMA) blend films with very low surface roughness based on spin-coating and subsequent melt-quenching is described. Amorphous PMMA in a blend film effectively retards the rapid crystallization of PVDF upon quenching, giving rise to a thin and flat ferroelectric film with nanometer scale β-type PVDF crystals. The still, flat interfaces of the blend film with metal electrode and/or an organic semi-conducting channel layer enable fabrication of a highly reliable ferroelectric capacitor and transistor memory unit operating at voltages as low as 15 V. For instance, with a TIPS-pentacene single crystal as an active semi-conducting layer, a flexible ferroelectric field effect transistor shows a clockwise I–V hysteresis with a drain current bistability of 103 and data retention time of more than 15 h at ±15 V gate voltage. Furthermore, the robust interfacial homogeneity of the ferroelectric film is highly beneficial for transfer printing in which arrays of metal/ferroelectric/metal micro-capacitors are developed over a large area with well defined edge sharpness.

Measuring the crystallinity index of cellulose by solid state 13C nuclear magnetic resonance

Abstract: The crystallinity index of cellulose is an important parameter to establish because of the effect this property has on the utilization of cellulose as a material and as a feedstock for biofuels production. However, it has been found that the crystallinity index varies significantly depending on the choice of instrument and data analysis technique applied to the measurement. We introduce in this study a simple and straightforward method to evaluate the crystallinity index of cellulose. This novel method was developed using solid state 13C NMR and subtraction of the spectrum of a standard amorphous cellulose. The crystallinity indexes of twelve different celluloses were measured and the values from this method were compared with the values obtained by other existing methods, including methods based on X-ray diffraction. An interesting observation was that the hydration of the celluloses increased their crystallinity indexes by about 5%, suggesting that addition of water increased cellulose order for all the cellulose samples studied.