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Results for “Molecular Layer Deposition”

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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Formation of a Boron-Oxide Termination for the (100) Diamond Surface

A boron-oxide termination of the diamond (100) surface has been formed by depositing molecular boron oxide B 2 O 3 onto the hydrogen-terminated (100) diamond surface under ultrahigh vacuum conditions and annealing to 950 °C. The resulting termination is highly oriented and chemically homogeneous, although further optimization is required to increase the surface coverage beyond the 0.4 monolayer coverage achieved here. This work demonstrates the possibility of using molecular deposition under ultrahigh vacuum conditions for complex surface engineering of the diamond surface, and may be a first step in an alternative approach to fabricating boron doped delta layers in diamond.

36 MATERIALS SCIENCE↗
NASA NTRS1991

Endotaxial growth of CoSi2 within (111) oriented Si in a molecular beam epitaxy system

A new mode of growth is reported in which buried metallic layers can be fabricated within a single-crystal semiconductor through preferential subsurface growth on previously-grown 'seed' regions. The deposition of Co at 800 C at a rate of 0.01 nm/s on (111) Si substrates containing buried CoSi2 columns 40-100 nm below the Si surface results in the growth and coalescence of these subsurface columns. The formation of a CoSi2 layer on the Si surface is suppressed by this growth mode. It is proposed that the high diffusion rate of Co at 800 C, coupled with the high growth rate of CoSi2 at the subsurface columns, is responsible for this preferred 'endotaxial' growth mode. This growth technique was used to produce a continuous buried single-crystal layer of CoSi2 under a single-crystal Si capping layer.

George, T.↗
DOE OSTI2023

Vapor-to-glass preparation of biaxially aligned organic semiconductors

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180 K below the clearing point TLC−iso (and 50 K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility.

Chemistry↗
DOE OSTI2020

Boltzmann machine modeling of layered MoS 2 synthesis on a quantum annealer

Two-dimensional layered materials are usually synthesized by chemical vapor deposition (CVD). Here we use the Restricted Boltzmann Machine (RBM) and its enhancement called Limited Boltzmann Machine (LBM) on a quantum computer to model the chemical vapor deposition (CVD) growth data for a MoS 2 monolayer. The CVD data for Boltzmann machines are generated by reactive molecular dynamics (RMD) simulations. The Boltzmann machines give insight into the semiconducting (2H) and metallic (1T) phases and defects generated during the CVD growth of MoS 2 . We compare the performances of the two types of Boltzmann machine models, i.e. RBM versus augmented topologies with limited intra-layer coupling between the hidden units (LBM). We show that limited topologies have a performance advantage over restricted topologies. We examine connectivity within our topological variants, explore hardware qubit mapping schemes, and discuss what performance differences may imply about locality within the data without prior knowledge.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Selective Hydration of Rutile TiO 2 as a Strategy for Site-Selective Atomic Layer Deposition

In this report the feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO 2 facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H 2 O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO2 single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

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

Over What Length Scale Does an Inorganic Substrate Perturb the Structure of a Glassy Organic Semiconductor?

While the bulk structure of vapor-deposited glasses has been extensively studied, structure at buried interfaces has received little attention, despite being important for organic electronic applications. To learn about glass structure at buried interfaces, we study the structure of vapor-deposited glasses of the organic semiconductor DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene) as a function of film thickness; the structure is probed with grazing incidence X-ray scattering. Here, we deposit on silicon and gold substrates and span a film thickness range of 10–600 nm. Our experiments demonstrate that interfacial molecular packing in vapor-deposited glasses of DSA-Ph is more disordered compared to the bulk. At a deposition temperature near room temperature, we estimate ~8 nm near the substrate can have modified molecular packing. Molecular dynamics simulations of a coarse-grained representation of DSA-Ph reveal a similar length scale. In both the simulations and the experiments, deposition temperature controls glass structure beyond this interfacial layer of a few nanometers.

36 MATERIALS SCIENCE↗
NASA NTRS2005

High Quality GaAs Growth by MBE on Si Using GeSi Buffers and Prospects for Space Photovoltaics

III-V solar cells on Si substrates are of interest for space photovoltaics since this would combine high performance space cells with a strong, lightweight and inexpensive substrate. However, the primary obstacles blocking III-V/Si cells from achieving high performance to date have been fundamental materials incompatabilities, namely the 4% lattice mismatch between GaAs and Si, and the large mismatch in thermal expansion coefficient. In this paper, we report on the molecular beam epitaxial (MBE) growth and properties of GaAs layers and single junction GaAs cells on Si wafers which utilize compositionally graded GeSi Intermediate buffers grown by ultra-high vacuum chemical vapor deposition (UHVCVD) to mitigate the large lattice mismatch between GaAs and Si. Ga As cell structures were found to incorporate a threading dislocation density of 0.9-1.5 x 10 (exp 6) per square centimeter, identical to the underlying relaxed Ge cap of the graded buffer, via a combination of transmission electron microscopy, electron beam induced current, and etch pit density measurements. AlGaAs/GaAs double heterostructures wre grown on the GeSi/Si substrates for time-resolved photoluminescence measurements, which revealed a bulk GaAs minority carrier lifetime in excess of 10 ns, the highest lifetime ever reported for GaAs on Si. A series of growth were performed to ass3ss the impact of a GaAs buffer to a thickness of only 0.1 micrometer. Secondary ion mass spectroscopy studies revealed that there is negligible cross diffusion of Ga, As and Ge at he III-V/Ge interface, identical to our earlier findings for GaAs grown on Ge wafers using MBE. This indicates that there is no need for a buffer to "bury" regions of high autodopjing,a nd that either pn or np configuration cells are easily accomodated by these substrates. Preliminary diodes and single junction Al Ga As heteroface cells were grown and fabricated on the Ge/GeSi/Si substrates for the first time. Diodes fabricated on GaAs, Ge and Ge/GeSi/Si substrate show nearly identical I-V characteristics in both forward and reverse bias regions. External quantum efficiencies of AlGaAs/GaAs cell structures grown on Ge/GeSi/Si and Ge substrates demonstrated nearly identical photoresponse, which indicates that high lifetimes, diffusion lengths and efficient minority carrier collection is maintained after complete cell processing.

Carlin, J. A.↗
DOE OSTI2020

Electrodeposition of Zinc in Aqueous Electrolytes Containing High Molecular Weight Polymers

Strong polarization of the ion distribution in liquid electrolytes subjected to potential differences exceeding the thermal voltage, V T = kT/e, produces a hydrodynamic instability termed electroconvection at ion-selective interfaces. Electroconvection is desirable in some situations (e.g., electrodialysis) because it promotes mixing in a stagnant electrolyte layer, enhancing the ion flux at a fixed potential difference. It is undesirable in others (e.g., electrodeposition of metals) where early experiments show that convective fluid rolls associated with hydrodynamic instability in bounded electrolytes produce preferential metal deposition at localized regions on an electrode. Such localized deposition drives the family of morphological instabilities loosely termed dendritic electrodeposition. We experimentally investigate the effect of ultrahigh molecular weight polymer additives on the onset conditions and physical characteristics of both instabilities. Direct observations of electrodeposit morphology and tracer particle motions used in tandem with indirect electrokinetic measurements reveal that even at moderate concentrations, the polymer additives have a large beneficial effect in extending the range of electric potentials where stable electrodeposition is observed. Additionally, we report that at polymer concentrations above the entanglement threshold, high molecular weight polymers impart elasticity to liquid electrolytes which dampen electroconvective flow at a cation-selective interface but have at most a minimal effect on the bulk ionic conductivity of the liquid.

25 ENERGY STORAGE↗
NASA NTRS1972

Plume backscatter measurements using quartz crystal microbalances in JPL Molsink (molecular sink)

Quantitative evidence of gas flows in the far upstream region of small nozzles with large boundary layer flow are discussed. Gas mass fluxes were measured using quartz crystal microbalances. Both nitrogen and carbon dioxide gases were used as test gases. Gas deposition rates on the order of 100 monolayers per minute were detected 13 inches upstream of the nozzle exit plane. It is significant to note that the crystals detected gases considerably beyond the Prandtl-Meyer turning angle. The data from these tests will be essential in the formulation of scaling laws and analytical prediction methods for viscous plume behavior.

Simon, W.↗
DOE OSTI2022

Growth of ultrathin Bi 2 Se 3 films by molecular beam epitaxy

Bi 2 Se 3 is a widely studied 3D topological insulator having potential applications in optics, electronics, and spintronics. When the thickness of these films decreases to less than approximately 6 nm, the top and bottom surface states couple, resulting in the opening of a small gap at the Dirac point. In the 2D limit, Bi 2 Se 3 may exhibit quantum spin Hall states. However, growing coalesced ultrathin Bi 2 Se 3 films with a controllable thickness and typical triangular domain morphology in the few nanometer range is challenging. Here, we explore the growth of Bi 2 Se 3 films having thicknesses down to 4 nm on sapphire substrates using molecular beam epitaxy that were then characterized with Hall measurements, atomic force microscopy, and Raman imaging. We find that substrate pretreatment—growing and decomposing a few layers of Bi 2 Se 3 before the actual deposition—is critical to obtaining a completely coalesced film. In addition, higher growth rates and lower substrate temperatures led to improvement in surface roughness, in contrast to what is observed for conventional epitaxy. Altogether, coalesced ultrathin Bi 2 Se 3 films with lower surface roughness enable thickness-dependent studies across the transition from a 3D-topological insulator to one with gapped surface states in the 2D regime.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Surface stability of SrNbO 3+δ grown by hybrid molecular beam epitaxy

4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO 3 is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d 1 Nb 4+ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayer structures that may be challenging by PLD. To that end, SrNbO 3 thin films were grown using hybrid MBE for the first time using a tris(diethylamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO 3 substrates. Varying thicknesses of insulating SrHfO 3 capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO 3 films to preserve the metastable surface. Grown films were transferred in vacuo for x-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by x-ray absorption near edge spectroscopy and scanning transmission electron microscopy illustrate that the SrHfO 3 capping plays an important role in preserving the crystalline quality of the material and the Nb 4d 1 metastable charge state under atmospheric conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

PDADMAC/PSS Oligoelectrolyte Multilayers: Internal Structure and Hydration Properties at Early Growth Stages from Atomistic Simulations

We analyze the internal structure and hydration properties of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) oligoelectrolyte multilayers at early stages of their layer-by-layer growth process. Our study is based on large-scale molecular dynamics simulations with atomistic resolution that we presented recently [Sánchez et al., Soft Matter 2019, 15, 9437], in which we produced the first four deposition cycles of a multilayer obtained by alternate exposure of a flat silica substrate to aqueous electrolyte solutions of such polymers at 0.1M of NaCl. In contrast to any previous work, here we perform a local structural analysis that allows us to determine the dependence of the multilayer properties on the distance to the substrate. We prove that the large accumulation of water and ions next to the substrate observed in previous overall measurements actually decreases the degree of intrinsic charge compensation, but this remains as the main mechanism within the interface region. We show that the range of influence of the substrate reaches approximately 3 nm, whereas the structure of the outer region is rather independent from the position. This detailed characterization is essential for the development of accurate mesoscale models able to reach length and time scales of technological interest.

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

Energetic Tug-of-War between Pt and Leaky TiO 2 : Positive and Negative Effects on the Function of Molecularly-Modified p -Si(111)|TiO 2 |Pt Photocathodes

Photoelectrochemical solar energy conversion to chemical products is a promising avenue towards renewable fuels, and TiO 2 deposition on semiconductor photocathodes is a widespread surface protection strategy. However, the electronic, thermodynamic, and kinetic effects of protection layers on interfacial energetics remains poorly understood. Here, electrochemical data (photocurrent onset and flatband potentials) of p-Si(111)|TiO 2 |Pt is collected in contact with an outer-sphere redox couple. We find that onset potentials shift negatively as a function of increasing oxide thickness, but this effect is not reflected in the flatband potentials. This reveals the mechanism of these shifts is kinetic rather than thermodynamic,and suggests that hole leakage through the TiO 2 is responsible. When Pt nanoparticles are deposited onto the TiO 2 , positive shifts in both the onset and flatband potentials are observed, revealing Pt catalysts are beneficial for more than just increased catalytic rates. These results highlight the importance of understanding the effects of surface modifications on the energetics at the interface in both catalytic and simple electron transfer systems.

14 SOLAR ENERGY↗
NASA NTRS1991

Molecular dynamics and quasidynamics simulations of the annealing of bulk and near-surface interstitials formed in molecular-beam epitaxial Si due to low-energy particle bombardment during deposition

The relaxation, diffusion, and annihilation of split and hexagonal interstitials resulting from 10 eV Si irradiation of (2x1)-terminated Si(100) are investigated. Molecular dynamics and quasidynamics simulations, utilizing the Tersoff many-body potential are used in the investigation. The interstitials are created in layers two through six, and stable atomic configurations and total potential energies are derived as a function of site symmetry and layer depth. The interstitial Si atoms are allowed to diffuse, and the total potential energy changes are calculated. Lattice configurations along each path, as well as the starting configurations, are relaxed, and minimum energy diffusion paths are derived. The results show that the minimum energy paths are toward the surface and generally involved tetrahedral sites. The calculated interstitial migration activation energies are always less than 1.4 eV and are much lower in the near-surface region than in the bulk.

Kitabatake, M.↗
NASA NTRS2005

Making AlN(x) Tunnel Barriers Using a Low-Energy Nitrogen-Ion Beam

A technique based on accelerating positive nitrogen ions onto an aluminum layer has been demonstrated to be effective in forming thin (<2 nm thick) layers of aluminum nitride (AlN(x)) for use as tunnel barriers in Nb/Al-AlN(x)/Nb superconductor/insulator/ superconductor (SIS) Josephson junctions. AlN(x) is the present material of choice for tunnel barriers because, to a degree greater than that of any other suitable material, it offers the required combination of low leakage current at high current density and greater thermal stability. While ultra-thin AlN films with good thickness and stoichiometry control are easily formed using techniques such as reactive molecular beam epitaxy and chemical vapor deposition, growth temperatures of 900 C are necessary for the dissociative adsorption of nitrogen from either nitrogen (N2) or ammonia (NH3). These growth temperatures are prohibitively high for the formation of tunnel barriers on Nb films because interfacial reactions at temperatures as low as 200 to 300 C degrade device properties. Heretofore, deposition by reactive sputtering and nitridation of thin Al layers with DC and RF nitrogen plasmas have been successfully used to form AlN barriers in SIS junctions. However, precise control over critical current density Jc has proven to be a challenge, as is attaining adequate process reproducibility from system to system. The present ion-beam technique is an alternative to the plasma or reactive sputtering techniques as it provides a highly controlled arrival of reactive species, independent of the electrical conditions of the substrate or vacuum chamber. Independent and accurate control of parameters such as ion energy, flux, species, and direction promises more precise control of film characteristics such as stoichiometry and thickness than is the case with typical plasma processes. In particular, the background pressure during ion-beam nitride growth is 2 or 3 orders of magnitude lower, minimizing the formation of compounds with contaminants, which is critical in devices the performance of which is dictated by interfacial characteristics. In addition, the flux of incoming species can be measured in situ using ion probes so that the dose can be controlled accurately. The apparatus used in the present ion-beam technique includes a vacuum chamber containing a commercial collimated- ion-beam source, a supply of nitrogen and argon, and an ion probe for measuring the ion dose. Either argon or nitrogen can be used as the feed gases for the ion source, depending on whether cleaning of the substrate or growth of the nitride, respectively, is desired. Once the Nb base electrode and Al proximity layer have been deposited, the N2 gas line to the ion beam is vented and purged, and the ion-source is turned on until a stable discharge is obtained. The substrate is moved over the ion-beam source to expose the Al surface layer to the ion beam (see figure) for a specified duration for the formation of the nitride tunnel barrier. Next, the Nb counter-electrode layer is deposited on the nitride surface layer. The Nb/Al- AlN(x)/Nb-trilayer-covered substrate is then patterned into individual devices by use of conventional integrated-circuit processing techniques.

Kaul, Anupama↗
DOE OSTI2020

Substrate-Dependent Study of Chain Orientation and Order in Alkylphosphonic Acid Self-Assembled Monolayers for ALD Blocking

For years, many efforts in area selective atomic layer deposition (AS-ALD) have focused on trying to achieve high-quality self-assembled monolayers (SAMs), which have been shown by a number of studies to be effective for blocking deposition. In this work, we show that in some cases where a densely packed SAM is not formed, significant ALD inhibition may still be realized. The formation of octadecylphosphonic acid (ODPA) SAMs was evaluated on four metal substrates: Cu, Co, W, and Ru. The molecular orientation, chain packing, and relative surface coverage were evaluated using near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared (FTIR) spectroscopy, and electrochemical impedance spectroscopy (EIS). ODPA SAMs formed on Co, Cu, and W showed strong angular dependence of the NEXAFS signal whereas ODPA on Ru did not, suggesting a disordered layer was formed on Ru. Additionally, EIS and FTIR spectroscopy confirmed that Co and Cu form densely packed, “crystal-like” SAMs whereas Ru and W form less dense monolayers, a surprising result since W-ODPA was previously shown to inhibit the ALD of ZnO and Al 2 O 3 best among all the substrates. This work suggests that multiple factors play a role in SAM-based AS-ALD, not just the SAM quality. Therefore, metrological averaging techniques (e.g., WCA and FTIR spectroscopy) commonly used for evaluating SAMs to predict their suitability for ALD inhibition should be supplemented by more atomically sensitive methods. Finally, it highlights important considerations for describing the mechanism of SAM-based selective ALD.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Construction of Lattice Strain in Bimetallic Nanostructures and Its Effectiveness in Electrochemical Applications

Bimetallic nanocrystals (NCs), associated with various surface functions such as ligand effect, ensemble effect, and strain effect, exhibit superior electrocatalytic properties. The stress-induced surface strain effect can alter binding strength between the surface active sites and reactants as well as their intermediates, and the electrochemical performance of bimetallic NCs can be significantly facilitated by the lattice-strain modification via their morphologies, sizes, shell-thickness, surface defectiveness as well as compositions. Here in this review, an overview of fundamental principles, characterization techniques, and quantitative determination of the surface lattice strain is provided. Various strategies and synthesis efforts on creating lattice-strain-engineered bimetallic NCs, including the de-alloying process, atomic layer-by-layer deposition, thermal treatment evolution, one-pot synthesis, and other efforts are also discussed. It is further outlined how the lattice strain effect promotes electrochemical catalysis through the selected case studies. The reactions on oxygen reduction reaction, small molecular oxidation, water splitting reaction, and electrochemical carbon dioxide reduction reactions are focused. In particular, studies of lattice strain arisen from core–shell nanostructure and defectiveness are highlighted. Lastly, the potential challenges are summarized and the prospects of lattice-strain-based engineering on bimetallic nanocatalysts with suggestion and guidance of the future electrocatalyst design are envisioned.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Peeling back the layers: Investigating the effects of polyelectrolyte layering on surface structure and stability of oil-in-water nanoemulsions

Layer-by-layer deposition of polyelectrolytes is a useful technique for modifying surface functionalities. For drug delivery systems, alternating layers of biopolymers coat nanoemulsions, which house and protect the cargo until the time and destination of delivery. In this report we investigate molecular factors contributing to the stability and interfacial properties of nanoemulsions prepared by a co-adsorption of polymers poly(styrene sulfonate) and polyethylenimine (PEI), and surfactant dodecyltrimethylammonium bromide. We hypothesize the interplay between electrosteric and hydrophobic effects upon multi-polymer co-adsorption contributes to both macroscopic and molecular-level interfacial properties of nanoemulsions. To probe interfacial layering properties, we use vibrational sum frequency scattering spectroscopy with ζ -potential measurements to determine the adsorptive behavior and molecular conformational arrangement of the polymer layers. Complementing these interfacial studies are dynamic light scattering experiments measuring the nanoemulsion size distribution and polydispersity index over a 30-day period. Our light scattering, ζ -potential, and spectroscopic results of the nanoemulsion surface show that the duration of droplet stability and the degree of molecular orientation of adsorbed polymers can be tuned by surfactant concentration, PEI concentration, and pH. These results illustrate how molecular surface properties of multi-polymer coated nanoemulsions contribute to synergistic effects and droplet stability, enabling advancements in applications surrounding biopharmaceuticals, cosmetics, and food sciences.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗