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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Non-stoichiometry Governs the Pathway from Amorphous to Crystalline Calcium Carbonate

The controlled crystallization of calcium carbonate underlies the elaborate architectures of marine corals, the design of biomimetic materials, and the global carbon cycle. Despite its ubiquity, the chemical mechanisms that govern the crystallization of calcium carbonate from its amorphous precursor, amorphous calcium carbonate (ACC), remain elusive, largely due to the difficulty in resolving the structure and chemistry of this transient amorphous state. Here, we use time-lapse photography, image analysis, spatially resolved pH determination, in situ synchrotron pair distribution function (PDF) analysis, and dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) spectroscopy to reveal pervasive compositional variability in ACC that underpins its metastability and crystallization. By evaluating the kinetics of the amorphous-to-crystalline transition across different solution chemistries, we demonstrate that ACC is non-stoichiometric and CO 3 -deficient. Counterions from the precursor (e.g., NO 3 - from Ca(NO 3 ) 2 ) substitute into the ACC network, displacing CO 3 ions and mediating both ACC stability and transformation. Crystallization proceeds through refinement of the stoichiometry toward CaCO 3 and uptake of free CO 3 2- anions. Furthermore, these findings are consistent across a wide range of concentrations and different carbonate sources, explaining the diverse behaviors observed for ACC and providing a chemical framework for controlling calcium carbonate crystallization.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Effect of structure and composition on the electronic excitation induced amorphization of La2Ti2−xZrxO7 ceramics

Abstract Understanding the response of ceramics operating in extreme environments is of interest for a variety of applications. Ab initio molecular dynamic simulations have been used to investigate the effect of structure and B -site (=Ti, Zr) cation composition of lanthanum-based oxides (La 2 B 2 O 7 ) on electronic-excitation-induced amorphization. We find that the amorphous transition in monoclinic layered perovskite La 2 Ti 2 O 7 occurs for a lower degree of electronic excitation than for cubic pyrochlore La 2 Zr 2 O 7 . While in each case the formation of O 2 -like molecules drives the structure to an amorphous state, an analysis of the polyhedral connection network reveals that the rotation of TiO 6 octahedra in the monoclinic phase can promote such molecule formation, while such octahedral rotation is not possible in the cubic phase. However, once the symmetry of the cubic structure is broken by substituting Ti for Zr, it becomes less resistant to amorphization. A compound made of 50% Ti and 50% Zr (La 2 TiZrO 7 ) is found to be more resistant in the monoclinic than in the cubic phase, which may be related to the lower bandgap of the cubic phase. These results illustrate the complex interplay of structure and composition that give rise to the radiation resistance of these important functional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Amorphizing noble metal chalcogenide catalysts at the single-layer limit towards hydrogen production

Rational design of noble metal catalysts with the potential to leverage efficiency is vital for industrial applications. Such an ultimate atom-utilization efficiency can be achieved when all noble metal atoms exclusively contribute to catalysis. In this work, we demonstrate the fabrication of a wafer-size amorphous PtSex film on a SiO 2 substate via a low-temperature amorphization strategy, which offers single-atom-layer Pt catalysts with high atom-utilization efficiency (~26 wt%). This amorphous PtSex (1.2 < x < 1.3) behaves as a fully activated surface, accessible to catalytic reactions, and features a nearly 100% current density relative to a pure Pt surface and reliable production of sustained high-flux hydrogen over a 2 inch wafer as a proof-of-concept. Furthermore, an electrolyser is demonstrated to generate a high current density of 1,000 mA cm –2 . Such an amorphization strategy is potentially extendable to other noble metals, including the Pd, Ir, Os, Rh and Ru elements, demonstrating the universality of single-atom-layer catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Fe-rich X-ray amorphous material records past climate and persistence of water on Mars

X-ray amorphous material comprises 15-73 wt.% of sedimentary rocks and eolian sediments in Gale crater. This material is variably siliceous and iron rich but aluminum poor. The presence of volatiles is consistent with the existence of incipient weathering products. To better understand the implications of this material for past aqueous conditions on Mars, here we investigate X-ray amorphous material formation and longevity within terrestrial iron rich soils with varying ages and environmental conditions using bulk and selective dissolution methods, X-ray diffraction, and transmission electron microscopy. Results indicate that in situ aqueous alteration is required to concentrate iron into clay-size fraction material. Cooler climates promote the formation and persistence of X-ray amorphous material whereas warmer climates promote the formation of crystalline secondary phases. Iron rich X-ray amorphous material formation and persistence on Mars are therefore consistent with past cool and relatively wet environments followed by long-term cold and dry conditions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Large-deformation and high-strength amorphous porous carbon nanospheres

Carbon is one of the most important materials extensively used in industry and our daily life. Crystalline carbon materials such as carbon nanotubes and graphene possess ultrahigh strength and toughness. In contrast, amorphous carbon is known to be very brittle and can sustain little compressive deformation. Inspired by biological shells and honeycomb-like cellular structures in nature, we introduce a class of hybrid structural designs and demonstrate that amorphous porous carbon nanospheres with a thin outer shell can simultaneously achieve high strength and sustain large deformation. The amorphous carbon nanospheres were synthesized via a low-cost, scalable and structure-controllable ultrasonic spray pyrolysis approach using energetic carbon precursors. In situ compression experiments on individual nanospheres show that the amorphous carbon nanospheres with an optimized structure can sustain beyond 50% compressive strain. Both experiments and finite element analyses reveal that the buckling deformation of the outer spherical shell dominates the improvement of strength while the collapse of inner nanoscale pores driven by twisting, rotation, buckling and bending of pore walls contributes to the large deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Local structure of ion pair interaction in lapatinib amorphous dispersions characterized by synchrotron x-ray diffraction and pair distribution function analysis

For many years, the idea of analyzing atom-atom contacts in amorphous drug-polymer systems has been of major interest, because this method has always had the potential to differentiate between amorphous systems with domains and amorphous systems which are molecular mixtures. In this study, local structure of ionic and noninonic interactions were studied by High-Energy X-ray Diffraction and Pair Distribution Function (PDF) analysis in amorphous solid dispersions of lapatinib in hypromellose phthalate (HPMCP) and hypromellose (HPMC-E3). The strategy of extracting lapatinib intermolecular drug interactions from the total PDF x-ray pattern was successfully applied allowing the detection of distinct nearest neighbor contacts for the HPMC-E3 rich preparations showing that lapatinib molecules do not cluster in the same way as observed in HPMC-P, where ionic interactions are present. Orientational correlations up to nearest neighbor molecules at about 4.3 Å were observed for polymer rich samples; both observations showed strong correlation to the stability of the systems. Lasty, the superior physical stability of 1:3 LP:HPMCP was consistent with the absence of significant intermolecular interactions in (ΔD inter LP (r)) in the range of 3.0 to 6.0 Å, which are attributed to C-C, C-N and C-O nearest neighbor contacts present in drug-drug interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Oxygen vacancy and hole conduction in amorphous TiO 2

The amorphous titanium dioxide (a-TiO2) has drawn attention recently due to the finding that it holds promise for coating conventional photoelectrodes for corrosion protection while still allowing the holes to transport to the surface. The mechanism of hole conductivity at a level much higher than the edge of the valence band is still a mystery. In this work, an amorphous TiO2 model is obtained from molecular dynamics employing the “melt-and-quench” technique. The electronic properties, polaronic states and the hole conduction mechanism in amorphous structure were investigated by means of density functional theory with Hubbard's energy correction (DFT + U) and compared to those in crystalline (rutile) TiO2. The formation energy of the oxygen vacancy was found to reduce significantly (by a few eV) upon amorphization. Our theoretical study suggested that the oxygen vacancies and their defect states provide hopping channels, which are comparable to experimental observations and could be responsible for hole conduction in the “leaky” TiO2 recently discovered for the photochemical water-splitting applications.

Marcus theory↗
DOE OSTI2018

Deformation behavior of an amorphous zeolitic imidazolate framework – from a supersoft material to a complex organometallic alloy

Zeolitic imidazolate frameworks (ZIFs)—a subset of metal–organic frameworks (MOFs)—have recently attracted immense attention. Many crystalline ZIFs (c-ZIFs) have highly porous zeolite structures that are ideal for molecular encapsulation. Recently emerging non-crystalline or amorphous ZIFs (a-ZIFs) with a similar short-range order are of interest because they can be converted from c-ZIFs for large-scale production. Here, we present a computational study of the deformation behavior of a unique a-ZIF model by simulating step-wise compression and expansion with strains between –0.389 and +0.376. An insulator-to-metal transition is observed at 51 GPa leading to a multicomponent light amorphous alloy of only 3.68 g (cm) –3 . A high-density amorphous-to-amorphous phase transition is observed due to the sudden formation of N–N bond pairs. Here, the systematic expansion of the a-ZIF retains the framework softness until it fractures at high strain. Based on the expansion data, we propose an empirical formula for super-soft materials, which is in line with available experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Giant spontaneous polarization for enhanced ferroelectric properties of biaxially oriented poly(vinylidene fluoride) by mobile oriented amorphous fractions

Poly(vinylidene fluoride) (PVDF) and its random copolymers exhibit the most distinctive ferroelectric properties; however, their spontaneous polarization (60–105 mC m -2 ) is still inferior to those (>200 mC m -2 ) of the ceramic counterparts. In this work, we report an unprecedented spontaneous polarization ( P s = 140 mC m -2 ) for a highly poled biaxially oriented PVDF (BOPVDF) film, which contains a pure β crystalline phase. Given the crystallinity of ~0.52, the P s for the β phase ( P s,β ) is calculated to be 279 mC m -2 , if a simple two-phase model of semicrystalline polymers is assumed. This high P s,β is invalid, because the theoretical limit of P s,β is 185 mC m -2 , as calculated by density functional theory. To explain such a high P s for the poled BOPVDF, a third component in the amorphous phase must participate in the ferroelectric switching to contribute to the P s . Namely, an oriented amorphous fraction (OAF) links the lamellar crystal and the mobile amorphous fraction. From the hysteresis loop study, the OAF content was determined to be ~0.28, more than 50% of the amorphous phase. Because of the high polarizability of the OAFs, the dielectric constant of the poled BOPVDF reached nearly twice the value of conventional PVDF. Overall, the fundamental knowledge obtained from this study will provide a solid foundation for the future development of PVDF-based high performance electroactive polymers for wearable electronics and soft robotic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Amorphous Interface Layer in Thin Graphite Films Grown on the Carbon Face of SiC

Cross-sectional transmission electron microscopy (TEM) is used to characterize an amorphous layer observed at the interface in graphite and graphene films grown via thermal decomposition of C-face 4H-SiC. The amorphous layer does not cover the entire interface, but uniform contiguous regions span microns of cross-sectional interface. Scanning transmission electron microscopy (STEM) images and electron energy loss spectroscopy (EELS) demonstrate that the amorphous layer is a carbon-rich composition of Si/C. The amorphous layer is clearly observed in samples grown at 1600 C for a range of growth pressures in argon, but not at 1500 C, suggesting a temperature-dependent formation mechanism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Enhanced magnetic and electrical properties in amorphous Ge:Mn thin films by non-magnetic codoping

Amorphous Ge{sub 1-x}Mn{sub x} thin films have been prepared by co-depositing Ge and Mn on SiO{sub 2}/Si using an ultrahigh vacuum molecular beam epitaxy system. Across a range of growth temperatures and Mn concentrations (2.8 at. %, 10.9 at. %, and 21.3 at. %), we achieved enhanced magnetic and electrical properties with non-magnetic codopants dispersed in the films. Self-assembled Mn-rich amorphous nanostructures were observed in the amorphous Ge matrix, either as isolated nanoclusters or as nanocolumns, depending on Mn concentration. The ferromagnetic saturation moments were found to increase with Mn concentration and reached a maximum of 0.7 {mu}{sub B}/Mn in the as-grown samples. Two magnetic transition temperatures around 15 K and 200 K were observed in these amorphous MBE-grown samples. Coercivity is considered within the context of local magnetic anisotropy. The anomalous Hall effect confirmed a strong correlation between the magnetization and transport properties, indicating that global ferromagnetic coupling was carrier-mediated rather than through direct exchange. In addition, negative magnetoresistance was detected from 5 K to room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Arsenic redistribution after solid phase epitaxial regrowth of shallow pre-amorphized silicon layers

The behavior of ultra shallow ion implants of arsenic in Si following solid phase epitaxial re-growth process is reported. A 16 nm amorphous layer was created by ion implantation of Si{sup +} at energy 5 keV and a dose 1 Multiplication-Sign 10{sup 15} at/cm{sup 2}. As ion were implanted at 2 keV using 3 different doses: 1 Multiplication-Sign 10{sup 14}, 5 Multiplication-Sign 10{sup 14} and 1 Multiplication-Sign 10{sup 15} at/cm{sup 2}. The resulting As distributions, confined in the amorphous layer, were thermally treated at 550 Degree-Sign C for 5-300 s in order to electrically activate dopant atoms. Crystal re-growth and As redistribution was investigated by secondary ion mass spectrometry and medium energy ion scattering. A growth rate depending on the As concentration was observed, the rate being slower for higher As content. Arsenic re-distribution to the surface and at the end-of-range defects was observed and a segregation model was developed. Finally, the substitutional fraction of As atoms was related to sheet resistance measurements revealing a higher fraction of electrically active dopant atoms in pre-amorphized samples compared to not pre-amorphized.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Local structure of amorphous GaN{sub 1-x}As{sub x} semiconductor alloys across the composition range

Typically only dilute (up to {approx}10%) highly mismatched alloys can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. We have overcome the miscibility gap of the GaN{sub 1-x}As{sub x} system using low temperature molecular beam epitaxy. In the intermediate composition range (0.10 < x < 0.75), the resulting alloys are amorphous. To gain a better understanding of the amorphous structure, the local environment of the As and Ga atoms was investigated using extended x-ray absorption fine structure (EXAFS). The EXAFS analysis shows a high concentration of dangling bonds compared to the crystalline binary endpoint compounds of the alloy system. The disorder parameter was larger for amorphous films compared to crystalline references, but comparable with other amorphous semiconductors. By examining the Ga local environment, the dangling bond density and disorder associated with As-related and N-related bonds could be decoupled. The N-related bonds had a lower dangling bond density and lower disorder.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Intrinsic nature of visible-light absorption in amorphous semiconducting oxides

To enlighten microscopic origin of visible-light absorption in transparent amorphous semiconducting oxides, the intrinsic optical property of amorphous InGaZnO{sub 4} is investigated by considering dipole transitions within the quasiparticle band structure. In comparison with the crystalline InGaZnO{sub 4} with the optical gap of 3.6 eV, the amorphous InGaZnO{sub 4} has two distinct features developed in the band structure that contribute to significant visible-light absorption. First, the conduction bands are down-shifted by 0.55 eV mainly due to the undercoordinated In atoms, reducing the optical gap between extended states to 2.8 eV. Second, tail states formed by localized oxygen p orbitals are distributed over ∼0.5 eV near the valence edge, which give rise to substantial subgap absorption. The fundamental understanding on the optical property of amorphous semiconducting oxides based on underlying electronic structure will pave the way for resolving instability issues in recent display devices incorporating the semiconducting oxides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Optical bandgap of ultra-thin amorphous silicon films deposited on crystalline silicon by PECVD

An optical study based on spectroscopic ellipsometry, performed on ultrathin hydrogenated amorphous silicon (a-Si:H) layers, is presented in this work. Ultrathin layers of intrinsic amorphous silicon have been deposited on n-type mono-crystalline silicon (c-Si) wafers by plasma enhanced chemical vapor deposition (PECVD). The layer thicknesses along with their optical properties –including their refractive index and optical loss- were characterized by spectroscopic ellipsometry (SE) in a wavelength range from 250 nm to 850 nm. The data was fitted to a Tauc-Lorentz optical model and the fitting parameters were extracted and used to compute the refractive index, extinction coefficient and optical bandgap. Furthermore, the a-Si:H film grown on silicon was etched at a controlled rate using a TMAH solution prepared at room temperature. The optical properties along with the Tauc-Lorentz fitting parameters were extracted from the model as the film thickness was reduced. The etch rate for ultrathin a-Si:H layers in TMAH at room temperature was found to slow down drastically as the c-Si interface is approached. From the Tauc-Lorentz parameters obtained from SE, it was found that the a-Si film exhibited properties that evolved with thickness suggesting that the deposited film is non-homogeneous across its depth. It was also found that the degree of crystallinity and optical (Tauc) bandgap increased as the layers were reduced in thickness and coming closer to the c-Si substrate interface, suggesting the presence of nano-structured clusters mixed into the amorphous phase for the region close to the crystalline silicon substrate. Further results from Atomic Force Microscopy and Transmission Electron Microscopy confirmed the presence of an interfacial transitional layer between the amorphous film and the underlying substrate showing silicon nano-crystalline enclosures that can lead to quantum confinement effects. Quantum confinement is suggested to be the cause of the observed increase in the optical bandgap of a-Si:H films close to the a-Si:H/cSi interface.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Characterization of amorphous multilayered ZnO-SnO{sub 2} heterostructure thin films and their field effect electronic properties

Multilayered ZnO-SnO{sub 2} heterostructure thin films were produced using pulsed laser ablation of pie-shaped ZnO-SnO{sub 2} oxides target, and their structural and field effect electronic transport properties were investigated as a function of the thickness of the ZnO and SnO{sub 2} layers. The films have an amorphous multilayered heterostructure composed of the periodic stacking of the ZnO and SnO{sub 2} layers. The field effect electronic properties of amorphous multilayered ZnO-SnO{sub 2} heterostructure thin film transistors (TFTs) are highly dependent on the thickness of the ZnO and SnO{sub 2} layers. The highest electron mobility of 37 cm{sup 2}/V s, a low subthreshold swing of a 0.19 V/decade, a threshold voltage of 0.13 V, and a high drain current on-to-off ratio of ∼10{sup 10} obtained for the amorphous multilayered ZnO(1.5 nm)-SnO{sub 2}(1.5 nm) heterostructure TFTs. These results are presumed to be due to the unique electronic structure of an amorphous multilayered ZnO-SnO{sub 2} heterostructure film consisting of ZnO, SnO{sub 2}, and ZnO-SnO{sub 2} interface layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Preferred orientations of laterally grown silicon films over amorphous substrates using the vapor–liquid–solid technique

A novel method has been optimized so that adhesion layers are no longer needed to reliably deposit patterned gold structures on amorphous substrates. Using this technique allows for the fabrication of amorphous oxide templates known as micro-crucibles, which confine a vapor–liquid–solid (VLS) catalyst of nominally pure gold to a specific geometry. Within these confined templates of amorphous materials, faceted silicon crystals have been grown laterally. The novel deposition technique, which enables the nominally pure gold catalyst, involves the undercutting of an initial chromium adhesion layer. Using electron backscatter diffraction it was found that silicon nucleated in these micro-crucibles were 30% single crystals, 45% potentially twinned crystals and 25% polycrystals for the experimental conditions used. Single, potentially twinned, and polycrystals all had an aversion to growth with the (1 0 0) surface parallel to the amorphous substrate. Closer analysis of grain boundaries of potentially twinned and polycrystalline samples revealed that the overwhelming majority of them were of the 60° Σ3 coherent twin boundary type. The large amount of coherent twin boundaries present in the grown, two-dimensional silicon crystals suggest that lateral VLS growth occurs very close to thermodynamic equilibrium. It is suggested that free energy fluctuations during growth or cooling, and impurities were the causes for this twinning.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Optical bandgap of single- and multi-layered amorphous germanium ultra-thin films

Accurate optical methods are required to determine the energy bandgap of amorphous semiconductors and elucidate the role of quantum confinement in nanometer-scale, ultra-thin absorbing layers. Here, we provide a critical comparison between well-established methods that are generally employed to determine the optical bandgap of thin-film amorphous semiconductors, starting from normal-incidence reflectance and transmittance measurements. First, we demonstrate that a more accurate estimate of the optical bandgap can be achieved by using a multiple-reflection interference model. We show that this model generates more reliable results compared to the widely accepted single-pass absorption method. Second, we compare two most representative methods (Tauc and Cody plots) that are extensively used to determine the optical bandgap of thin-film amorphous semiconductors starting from the extracted absorption coefficient. Analysis of the experimental absorption data acquired for ultra-thin amorphous germanium (a-Ge) layers demonstrates that the Cody model is able to provide a less ambiguous energy bandgap value. Finally, we apply our proposed method to experimentally determine the optical bandgap of a-Ge/SiO{sub 2} superlattices with single and multiple a-Ge layers down to 2 nm thickness.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗