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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Molecular Mass Growth Processes to Polycyclic Aromatic Hydrocarbons through Radical–Radical Reactions Exploiting Photoionization Reflectron Time-of-Flight Mass Spectrometry

Polycyclic aromatic hydrocarbons (PAHs) represent critical building blocks in molecular mass growth processes to carbonaceous nanoparticles, referred to as interstellar and circumstellar grains along with soot particles in astrophysical environments and combustion systems, respectively. Recent advancements on elucidating elementary steps to PAHs have utilized reactions of aromatic radicals, resonantly stabilized free radicals, and aliphatic radicals with closed shell hydrocarbons. However, the role of radical–radical reactions (RRRs) leading to PAHs has remained largely unexplored on the molecular level due to preceding experimental challenges in producing sufficiently high number densities of radical reactants for isomer-selective detection of products from bimolecular and termolecular reactions. This Account offers the latest developments in our knowledge on the mechanisms and pathways to PAHs via RRRs probed in a chemical microreactor at temperatures as high as 1600 K. Product preservation in a molecular beam coupled with synchrotron vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and photoelectron photoion coincidence spectroscopy enabled isomer-selective detection of PAHs of up to three rings by their photoionization efficiency curves, which were fit with a linear combination of reference curves for identification. Experiments were combined with computational fluid dynamics modeling of the physicochemical processes in the microreactor, as well as high-level electronic structure calculations to reveal the reaction pathways of each system. Six distinct reaction mechanisms were discovered in this work: propargyl addition─benzannulation (PABA), methyl addition─ring expansion (MARE), cyclopentadienyl addition─naphthylization (CPAN), fulvenallenyl addition─cyclization─aromatization (FACA), benzyl addition─aromatization (BAA), and phenyl addition─pentacyclization (PAP). By systematically varying the number of carbon atoms in the radical reactants, molecular mass growth processes involving reactions between radicals with odd numbers of carbon atoms access aromatics carrying one, two, or three six-membered rings, whereas reactions between even- and odd-carbon-numbered radicals produce aromatics combining five- and six-membered rings. Our investigations reveal unconventional cycloadditions on excited state triplet surfaces, additions of radicals to low spin density carbon-centered radicals, spiroaromatic and fulvene-type intermediates, and highly strained bicyclic reaction intermediates, challenging current perceptions of PAH molecular mass growth processes. All of the listed mechanisms, except for FACA, feature endoergic reactions or barriers which lie above the separated reactants and therefore might be central to circumstellar environments of carbon-rich stars and planetary nebulae as their descendants, but they play no role in the gas phase of cold molecular clouds where temperatures as low as 10 K dominate. Altogether, this work provides detailed reaction mechanisms of PAH growth processes, advancing our knowledge of the chemistry of carbonaceous matter in the universe.

Addition reactions↗
DOE OSTI2021

Correlation of Early-Stage Growth Process Conditions with Dislocation Evolution in MOCVD-Based GaP/Si Heteroepitaxy

To identify the complex relationships between early-stage growth processes and the resultant defect microstructure in GaP/Si heteroepitaxy, a holistic study of several key metal-organic chemical vapor deposition (MOCVD) parameters was conducted, focusing on Si surface preparation and GaP atomic layer epitaxy (ALE) based nucleation processes. Here, crystalline defects related to the lattice mismatch and/or interfacial heterovalency, namely misfit dislocations (MD), threading dislocations (TD), and stacking fault pyramids (SFP), were quantitatively characterized via electron channeling contrast imaging (ECCI) and correlated against the different process variations. Choice of Si surface preparation method between the two examined (dilute SiH4 annealing versus Si2H6 based homoepitaxy) had little impact on resultant GaP film morphology and defect content, whereas differing GaP ALE nucleation conditions produced much more substantial changes. In particular, the initial precursor species (tert-butylphosphine versus triethygallium) and ALE cycle purge times both yielded significant influence over threading dislocation densities (TDD) in thin (100 nm), post-critical thickness GaP/Si films, with TDD spanning two orders of magnitude, from 6.7×10 7 cm -2 to 7.1×10 5 cm -2 , depending on the specific process conditions employed. SFP densities were also found to follow a similar trend, ranging from 2.0×10 7 cm -2 to 1.8×10 5 cm -2 , but with no apparent causal relationship between SFP density and TDD. To help explain the dramatic differences observed, detailed, large-area MD network characterization was used to provide statistically-relevant quantitative analyses of the critical dislocation dynamics (introduction rates and glide velocities) associated with the different process variants. These extracted values are then correlated against the ALE process variants to provide insight into the potential mechanistic roles of the different growth processes.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Imaging local luminescence variations in CdSe 0.1 Te 0.9 thin films grown by the colossal grain growth process

Clear visualization and understanding of luminescence properties of grain interiors and grain boundaries in polycrystalline thin-film photovoltaic materials are crucial to achieving high-performance solar cells. Luminescence-based measurements, for example, reveal sub-bandgap energetic disorder or impurity-related radiative transitions in these materials, and when performed in the absolute scale, they can show the extent of voltage-limiting non-radiative recombination. Here we use a hyperspectral imaging technique to investigate inter- and intra-grain photoluminescence variations in polycrystalline CdSe 0.1 Te 0.9 films made by the colossal grain growth process. We observe that grain boundaries show higher luminescence than grain interiors for all energies below the bandgap energy. Multiple distinct sub-bandgap defect transitions are detected and determined to be related to donor-acceptor pair impurities. A comparison of the excitation-intensity-dependent photoluminescence at the grain boundaries with grain interiors suggests that carrier concentration is higher at grain boundaries. This finding is supported by photoemission electron microscopy imaging of the films, showing approximately 70 meV upward energy shift in the band structure at the grain boundaries. Finally, we investigate a dramatic thermal quenching behavior related to the donor-acceptor pairs luminescence over a temperature range and use a rate-equation model to attribute it to the sudden redirection of the radiative recombination channels to non-radiative channels with increasing temperature.

14 SOLAR ENERGY↗
DOE OSTI2020

Understanding KOtBu in atomic layer deposition – in situ mechanistic studies of the KNbO3 growth process

Functional coatings based on alkali metals have become increasingly attractive in the current shift towards sustainable technologies. While lithium-based compounds have a natural impact on batteries, other alkali metal compounds are important as replacements for toxic materials in a range of electronic devices. This is especially true for potassium, being a major component in e.g. K x Na 1-x NbO 3 (KNN) and KTa x Nb 1-x O 3 (KTN), with hope to replace Pb(Zr x Ti 1-x )O 3 (PZT) in piezo-/ferroelectric and electrooptic devices. ALD facilitates functional conformal coatings at deposition temperatures far below what is reported using other techniques and with excellent compositional control. The ALD growth of potassium-containing films using KO t Bu has, however, been unpredictable. Untraditional response to the pulse composition and precursor dose, severe reproducibility issues, and very high growth per cycle are some of the puzzling features of these processes. In this article, we shed light on the growth behavior of KO t Bu in ALD by in situ quartz crystal microbalance and Fourier transform infrared spectroscopy studies. We study the precursor's behavior in the technologically interesting KNbO 3 -process, showing how the potassium precursor strongly affects the growth of other cation precursors. We show that the strong hygroscopic nature of the intermediary potassium species has far-reaching implications throughout the growth. This helps not only to enhance the understanding of alkali metal containing compounds’ growth in ALD, but also to provide the means to control the growth of novel sustainable technological materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Probing unintentional Fe impurity incorporation in MOCVD homoepitaxy GaN: Toward GaN vertical power devices

Unintentional impurity incorporation in GaN drift layers represents a challenging issue that can limit their potential performance in vertical power devices. In this paper, we focus on studying the origins of Fe impurity incorporation in metal-organic chemical vapor deposition (MOCVD) grown GaN materials. Acting as a compensator in n-type GaN drift layers, Fe impurities can reduce the electron mobility in GaN and limit the lowest controllable doping level. Two sources, the sample cleaning process and growth susceptor, were identified as the main mechanisms of Fe incorporation in the MOCVD GaN growth process. It was found that solvent cleaning of the wafer can introduce significant Fe contamination at the growth interface, which would slowly be incorporated into the GaN epilayer, thus causing background Fe impurity as high as 10 17 cm -3 level. Moreover, the Fe impurity in the coating material on the susceptor can introduce additional Fe impurity during the growth process. Our studies revealed that the Fe impurity level could be significantly suppressed by more than two orders when an alternative cleaning process was used and the susceptor surface was fully covered by substrates. Characterization of the Fe impurity concentrations was performed via secondary ion mass spectrometry. The trap level (E C - 0.57) eV from deep-level transient spectroscopy that had previously been attributed to Fe confirmed the carrier compensation effect from Fe. Room temperature Hall mobility as high as 1007 cm 2 /V s was achieved on the MOCVD grown low-Fe GaN. Results from this work will provide guidance for achieving high purity GaN toward high performance GaN vertical power devices.

42 ENGINEERING↗
DOE OSTI2020

Role of ferroelectric polarization during growth of highly strained ferroelectric materials

In ferroelectric thin films and superlattices, the polarization is intricately linked to crystal structure. Here we show that it can also play an important role in the growth process, influencing growth rates, relaxation mechanisms, electrical properties and domain structures. This is studied by focusing on the properties of BaTiO 3 thin films grown on very thin layers of PbTiO 3 using x-ray diffraction, piezoforce microscopy, electrical characterization and rapid in-situ x-ray diffraction reciprocal space maps during the growth using synchrotron radiation. Using a simple model we show that the changes in growth are driven by the energy cost for the top material to sustain the polarization imposed upon it by the underlying layer, and these effects may be expected to occur in other multilayer systems where polarization is present during growth. This motivates the concept of polarization engineering as a complementary approach to strain engineering.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Remote Epitaxial Growth of Compound Semiconductors through Graphene

Epitaxial growth has been widely used for high-quality growth of crystalline materials. However, epitaxial growth techniques usually require lattice match and similar thermal expansion coefficient between growth material and substrate, which blocks this growth technique from wide applicability. Recently, van der Waals (vdW) epitaxy has been demonstrated that allows to relax the limitation of epitaxial growth. vdW epitaxy on layered or two-dimensional (2D) materials is mediated by weak vdW interactions. In addition to these interactions, a substrate below 2D materials also plays a role to intervene in the epitaxial growth process, leaving room to realize a novel epitaxial growth process: remote epitaxy. Potential fluctuation—the difference between potential energy maxima and minima along the surface of the substrate—is influenced by this remote interaction; the fluctuation is predominant over the vdW force, due to the atomic thickness of 2D materials. The remote interactions can be controlled by modulation of polarities of 2D materials and epitaxy materials. Polar-material-based heterostructure, such as GaAs, InP, GaP and GaN with their substrate sandwiching 2D materials, enables the semiconductor film to be exfoliated from the underlying substrates through the 2D materials. This process is expected to open a novel avenue for the field of non-silicon electronics and photonics, where the ability to re-use graphene-coated substrates allows savings on the high cost of non-silicon substrates.

Lee, Kyusang↗
DOE OSTI2023

GaN MOCVD Growth on Native substrates for High Voltage (15-20 KV) Vertical Power Devices

This project aims to develop metalorganic chemical vapor deposition (MOCVD) homoepitaxy of GaN on native substrates with fast growth rate (15-20 μm/hr), low background doping (low-10 15 cm -3 ) and smooth surface morphology via comprehensive understanding of the crystal growth process including high quality GaN substrate development and surface preparation, impurity and native defects control, and their impacts on the breakdown field. The team successfully developed a new growth process by introducing the laser-assisted MOCVD (LA-MOCVD) process to address the limited growth rates of GaN in the traditional MOCVD process. Specifically, the use of the CO2 laser with lasing wavelength of 9.219 μm, the strong coupling between the laser beam with the ammonia (GaN MOCVD precursor) leads to efficient decomposition of NH3 which significantly increases the effective group V/III molar ratio and thus suppresses C impurity incorporation in MOCVD GaN. The reduction in C incorporation in LA-MOCVD GaN is especially prominent when the GaN growth rate is fast (> 10 μm/hr), which allows to develop thick GaN films with high crystalline quality and low controllable doping needed for vertical high power device applications. The results of this project lead to the demonstration of vertical GaN PN diodes with record breakdown voltage of ~ 8kV with high Baliga’s figure of merit.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2022

Growth mechanism of glucose-based hydrochar under the effects of acid and temperature regulation

Improving the tailorability of hydrochar synthesis is an effective way to enhance its performance and utilization efficiency. In this study, the growth rate, morphology, and molecular structure of hydrochar were controlled by regulating the pH and temperature of the hydrothermal carbonization process. Growth process analysis indicates that hydrochar has three growth periods: induction, rapid, and stable growth periods. It is mainly controlled by 5-hydroxymethylfurfural (HMF), which is formed by converting glucose and its transformation products. Further, the regulation of acid can significantly shorten the induction period of hydrochar, even under low-temperature conditions (<180 °C), and increase the growth rate of hydrochar. However, the degree of hydrochar adhesion varies: the lower the temperature, the greater the degree of its adhesion. Molecular structural analysis demonstrates that hydrochar mainly consists of furan structural domains and aromatic clusters, and its surface is rich in oxygen-containing functional groups. The degree at which hydrochar was aromatized was improved by increasing the reaction temperature (160–220 °C); whereas the regulation of acid reduced it and increased the content of oxygen-containing functional groups on the hydrochar surface. Based on these results, it is proposed that hydrochar formation has five stages and three growth periods with or without acid regulation.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Improving parameterization of ice microphysical processes in Arctic clouds using a synergistic modeling and observational approach (Final Report)

The primary goals of the project were to: 1) Improve the fundamental quantitative models of ice growth processes using ARM radar observations. 2) Better understand the growth of ice particle populations through vapor deposition, riming, and aggregation in Arctic clouds using a new particle-property microphysical model. 3) Robustly estimate and constrain key unknown model parameters using ARM radar observations combined with the particle-property model to develop an improved parameterization of ice growth processes for traditional bulk parameterizations used in cloud and climate models.

58 GEOSCIENCES↗
DOE OSTI2021

Revealing the effect of local stresses on twin growth mechanisms in titanium using synchrotron X-ray diffraction

Deformation twinning has a significant impact on the evolution of microstructure and mechanical response in hexagonal close packed (hcp) metals. Understanding the physical mechanisms associated with twin nucleation and growth processes is important to enable the broad use of hcp metals. While both nucleation and growth are conditioned by the local stress state, few studies have quantified it in bulk deformed samples. In this study, we reveal the effects of local stresses on twin thickening using in-situ synchrotron experiments with differential aperture X-ray microscopy. High purity Ti is deformed under four-point bending to activate {1012} tensile twins. Further, 3D stress fields with a spatial resolution of 0.5μm are mapped in the vicinity of low and high macroscopic Schmid factor (MSF) twins growing inside a grain. Reconciling this experimental analysis with recent twin growth models reveals that the growth of low MSF twins is limited by the nucleation of twin growth defects in the high-purity Ti sample. While growth-inducing defect nucleation may occur in many places on the interface of the high resolved shear stress (RSS)/MSF twin, the nucleation rate of twinning disconnections for the low MSF twin is high only at the end of the twin near a stress concentration. Once nucleated, these defects easily propagate into lower RSS regions of the grain resulting in twin growth. This work highlights the importance of stress concentrations not only for twin (embryo) nucleation, but also to the growth process of twins, particularly in low MSF twins.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Low-bandgap mixed tin–lead iodide perovskites with reduced methylammonium for simultaneous enhancement of solar cell efficiency and stability

High-performance perovskite/perovskite tandem solar cells require high-efficiency and stable low-bandgap perovskite subcells. State-of-the-art low-bandgap mixed tin–lead iodide perovskite solar cells exhibit either a high power-conversion efficiency or improved stability, but not both. In this article we report a two-step bilayer interdiffusion growth process to simultaneously meet both requirements for formamidinium-based low-bandgap mixed tin–lead iodide perovskite solar cells. The bilayer interdiffusion growth process allows for the formation of high-quality and large-grained perovskite films with only 10 mol% volatile methylammonium. Additionally, one-dimensional pyrrolidinium perovskite was applied to passivate the perovskite film and improve the junction quality, which resulted in a carrier lifetime of 1.1 µs and an open circuit voltage of 0.865 V for our perovskite film and device with a bandgap of 1.28 eV. Our strategies enabled a power-conversion efficiency of 20.4% for low-bandgap perovskite solar cells under AM 1.5G illumination. More importantly, an encapsulated device can retain 92% of its initial efficiency after 450 h of continuous 1 sun illumination.

14 SOLAR ENERGY↗
DOE OSTI2020

In situ temperature profile measurements with high-energy X-rays as a probe of optical floating zone crystal growth environment

The ability of optical floating zone (OFZ) furnaces to rapidly produce large single crystals of complex emerging materials has had a transformative effect on many scientific fields that require samples of this type. However, the crystal growth process within the OFZ furnace is not well understood owing to the challenges involved in monitoring the high-temperature crystal growth process. Novel beamline-compatible optical furnaces that approximate the inhomogeneous growth environment within an OFZ furnace have been fabricated and tested in high-energy synchrotron beamlines. In this paper, it is demonstrated that temperature profiles can be effectively extracted from powder diffraction data collected on polycrystalline ceramic rods heated at their tip. Furthermore, these measured temperature profiles can be accurately reproduced using a heat-transfer model that accounts for solid-state thermal conduction, partial sample lamp power absorption, convective air cooling and radiative cooling, allowing key thermal parameters such as thermal conductivity to be extracted from experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dynamics and growth rate implications of ribosomes and mRNAs interaction in E. coli

Understanding how cells grow and adapt under various nutrient conditions is pivotal in the study of biological stoichiometry. Recent studies provide empirical evidence that cells use multiple strategies to maintain an optimal protein production rate under different nutrient conditions. Mathematical models can provide a solid theoretical foundation that can explain experimental observations and generate testable hypotheses to further our understanding of the growth process. In this study, we generalize a modeling framework that centers on the translation process and study its asymptotic behaviors to validate algebraic manipulations involving the steady states. Using experimental results on the growth of E. coli under C-, N-, and P-limited environments, we simulate the expected quantitative measurements to show the feasibility of using the model to explain empirical evidence. Our results support the findings that cells employ multiple strategies to maintain a similar protein production rate across different nutrient limitations. Moreover, we find that the previous study underestimates the significance of certain biological rates, such as the binding rate of ribosomes to mRNA and the transition rate between different ribosomal stages. Furthermore, our simulation shows that the strategies used by cells under C- and P-limitations result in a faster overall growth dynamics than under N-limitation. In conclusion, the general modeling framework provides a valuable platform to study cell growth under different nutrient supply conditions, which also allows straightforward extensions to the coupling of transcription, translation, and energetics to deepen our understanding of the growth process.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2025

Control of core–shell nanoparticles properties through plasma synthesis: a computational study

The improved properties of core–shell nanoparticles (CSNPs) over homogeneous nanoparticles (NPs) have expanded and diversified the applications of these nanomaterials. However, controlling the properties of CSNPs can be a challenging task. Low temperature plasmas have proven to be an effective method of producing NPs with uniform size and morphology, and high yield. That said, NP transport and growth dynamics are sensitive to LTP properties. We report on a computational investigation of the evolution of Ge–Si CSNP properties as a function of operating conditions through the modeling of a flowing, two-zone inductively coupled plasma (ICP) reactor. Ar/GeH 4 and Ar/SiH 4 gas mixtures were supplied to separate plasma zones at a pressure of 1 Torr to promote growth of Ge cores and Si shells. The negatively charged CSNPs are trapped electrostatically in the vicinity of the antennas where the plasma is generated and where the majority of particle growth occurs. Particles that grow to a critical size are then de-trapped by fluid drag due to neutral gas flow. A two-dimensional hybrid plasma model coupled with a three-dimensional kinetic NP transport model were utilized to resolve plasma chemistry and NP growth processes that take place on distinct timescales. The trends in CSNP properties and trapping mechanisms associated with flow rate, applied ICP power and inlet precursor fraction are discussed. While the spatial distribution of plasma produced radical species can have significant impact on the NP growth process, the NP transport dynamics are what ultimately dictates the growth environment that is unique to each particle and so determines their final dimension and composition. The key to optimizing reactor conditions involves controlling the spatial density of growth species and plasma profile as a means to tailor particle trapping dynamics suitable to produce CSNPs for a specific application.

36 MATERIALS SCIENCE↗