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At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Adjustable ferromagnetic resonance frequency in CoO/CoFeB system

Static and dynamic properties of (CoO/CoFeB){sub n} multilayers have been investigated. An anisotropy field enhancement was evidenced when the CoO layer was deposited under the CoFeB layer. Tuning the relative CoFeB and CoO layers thicknesses, high ferromagnetic resonance frequencies up to 4 GHz were achieved. The coupling effect between the CoO and CoFeB layers was induced by a dipolar coupling due to the anisotropic roughness topology of the CoO layer. This anisotropic roughness was induced by the deposition geometry and evidenced by atomic force microscopy. The strength of the dipolar interfacial coupling was calculated thanks to Schlömann's model. Multilayer stacks were fabricated and the magnetic properties observed for the trilayers could be maintained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of bias voltage on structural and optical properties of TiN{sub x} thin films

In the present work, Ti thin films were deposited on Si substrate using DC sputtering technique. Indigenous hot cathode arc discharge plasma system was used for nitriding over these samples, where the plasma parameters and work piece can be controlled independently. A mixture of H{sub 2} and N{sub 2} gases (in the ratio of 80:20) was supplied into the plasma chamber. The effect of bias voltage on the crystal structure, morphology and optical properties was investigated by employing various physical techniques such as X-ray Diffraction, Atomic Force Microscopy and UV-Vis spectrometry. It was found that bias voltage affects largely the crystal structure and band gap which in turn is responsible for the modifications in optical properties of the deposited films.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Topological and thermal properties of polypropylene composites based on oil palm biomass

Roughness on pristine and polymer composite surfaces is of enormous practical importance for polymer applications. This study deals with the use of varying quantity of oil palm ash as a nanofiller in a polypropylene based matrix. The oil palm ash sample was preprocessed to break the particles into small diameter by using ultra sonication before using microfluidizer for further deduction in size and homogenization. The oil palm ash was made to undergo many passes through the microfluidizer for fine distribution of particles. Polypropylene based composites containing different loading percentage oil palm ash was granulated by twin screw extruder and then injection molded. The surface morphology of the OPA passed through microfluidizer was analyzed by Tapping Mode - Atomic Force Microscopy (TMAFM). Thermal analysis results showed an increase in the activation energy values. The thermal stability of the composite samples showed improvement as compared to the virgin polymer as corroborated by the on-set degradation temperatures and the temperatures at which 50% degradation occurred.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Plasma assisted molecular beam epitaxy growth and effect of varying buffer thickness on the formation of ultra-thin In{sub 0.17}Al{sub 0.83}N/GaN heterostructure on Si(111)

This work reports on the detailed plasma-assisted molecular beam epitaxy (PAMBE) growth of ultra-thin In{sub 0.17}Al{sub 0.83}N/GaN heterostructures on Si(111) substrate with three different buffer thickness (600 nm, 400 nm, and 200 nm). Growth through critical optimization of growth conditions is followed by the investigation of impact of varying buffer thickness on the formation of ultra-thin 1.5 nm, In{sub 0.17}Al{sub 0.83}N–1.25 nm, GaN–1.5 nm, In{sub 0.17}Al{sub 0.83}N heterostructure, in terms of threading dislocation (TD) density. Analysis reveals a drastic reduction of TD density from the order 10{sup 10 }cm{sup −2} to 10{sup 8 }cm{sup −2} with increasing buffer thickness resulting smooth ultra-thin active region for thick buffer structure. Increasing strain with decreasing buffer thickness is studied through reciprocal space mapping analysis. Surface morphology through atomic force microscopy analysis also supports our study by observing an increase of pits and root mean square value (0.89 nm, 1.2 nm, and 1.45 nm) with decreasing buffer thickness which are resulted due to the internal strain and TDs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Possibilities of Using Combined Optical and AFM Investigations of Albumin

The results of a complex study of 10% aqueous solution of human serum albumin using methods of optical and atomic force microscopy have been presented. The fine structure of main structures of albumin facies (vitreous matrix and concretions) has been revealed and some observed structural effects have been interpreted from the viewpoint of polymer materials science.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Hydrogen response of porous palladium nano-films

Palladium nanoparticles were synthesized by reducing sodium tetrachloropalladate at 100°C using Ethylene Glycol as reducing agent. The nanoparticles were characterized by TEM (Transmission Electron Microscopy), and optical absorption spectroscopy. The average particle size (<10 nm) was obtained from TEM images, and the palladium SPR (Surface Plasmon Resonance) absorption maxima was observed at ∼220 nm. The porous sensing films were prepared by drying the nanoparticles precipitate on cleaned glass substrates. The high porosity of these films, as revealed by Atomic Force Microscopy (AFM) studies, made these films suitable for hydrogen sensor applications. The resistance of the films, measured by making silver paste contacts on the porous surface, changed upon exposure to 1000 ppm hydrogen in nitrogen. Optimum sensor response was obtained at 50°C, beyond which it deteriorated. The total response comprising of initial rise and subsequent fall in resistance, is due to the formation of Pd-hydrides (whose resistivity is higher relative to Pd), and closure of interparticle gaps due to lattice expansion of palladium, respectively. A detailed analysis of the results based on the sensing mechanism has been discussed in the paper.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Accelerating Structure–Property Relationship Discovery with Multimodal Machine Learning and Self-Driving Microscopy

Microscopy combined with local spectroscopy is widely used to correlate nanoscale structure with functional properties in materials, but conventional measurements rely heavily on human-selected sampling locations and predefined targets, limiting data set diversity and the potential for discovery. Here, we present a framework that integrates autonomous microscopy with dual-novelty deep kernel learning (DN-DKL) for adaptive data acquisition and a dual variational autoencoder (VAE) for representation learning. DN-DKL actively guides the microscopy toward structurally and spectroscopically novel regions, enabling efficient collection of large spectral data sets. Dual-VAE embeds local structures and spectroscopic responses into a shared latent manifold that serves as a structure–property relationship map. We applied this framework for the investigation of halide perovskite films by using conductive atomic force microscopy. The results reveal distinct hysteresis behaviors that are linked to specific nanoscale structural motifs, including grain boundary junction points that show hysteresis under different bias conditions and asymmetric grain boundaries that suppress the charge transport. This framework establishes a general strategy that leverages the complementary strengths of self-driving microscopy, machine learning, and human expertise to accelerate scientific discovery in functional materials.

atomic force microscopy↗
DOE OSTI2010

Formation of nanoscale clusters during the initial stages of CaF{sub 2} growth on miscut Si(111)

The initial stages of high temperature CaF{sub 2} growth by molecular beam epitaxy on Si(111) substrates with a 3 deg. miscut were characterized using atomic force microscopy and low energy electron diffraction. At a growth temperature of 750 deg. C, electron diffraction measurements showed that the surface retained the (3x1) surface reconstruction up to a deposition thickness of at least 1.2 nm. The overall topography of the surface was defined by atomically flat terraces decorated with a large number of clusters. These clusters were confined to step edges and were typically 10-20 nm tall. The clusters appear to nucleate at the top of step edges and then grow in size until they extend across the step onto the neighboring terrace below. These results indicate that in this growth regime, the CaF{sub 2} molecules diffuse across terraces to aggregate into relatively large nanostructures after the formation of a thin wetting layer. The unusually rounded features and large heights seen in these clusters appear to arise from the topography of substrate terraces.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Misfit dislocation gettering by substrate pit-patterning in SiGe films on Si(001)

We show that suitable pit-patterning of a Si(001) substrate can strongly influence the nucleation and the propagation of dislocations during epitaxial deposition of Si-rich Si{sub 1-x}Ge{sub x} alloys, preferentially gettering misfit segments along pit rows. In particular, for a 250 nm layer deposited by molecular beam epitaxy at x{sub Ge} = 15%, extended film regions appear free of dislocations, by atomic force microscopy, as confirmed by transmission electron microscopy sampling. This result is quite general, as explained by dislocation dynamics simulations, which reveal the key role of the inhomogeneous distribution in stress produced by the pit-patterning.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Critical aspects of nano-indentation technique in application to hardened cement paste

Several open questions related to the experimental protocol and processing of data acquired by the nano-indentation (NI) technique are investigated. The volume fractions of mechanically different phases obtained from statistical NI (SNI) analysis are shown to be different from those obtained by back-scattered electron (BSE) image analysis and X-ray diffraction (XRD) method on the same paste. Judging from transmission electron microscope (TEM) images, the representative volume element of low-density calcium-silicate hydrates (C-S-H) can be considered to be around 500 nm, whereas for high-density C-S-H it is about 100 nm. This raises the question how the appropriate penetration depth for NI experiments should be selected. Changing the maximum load from 1 mN to 5 mN, the effect of penetration depth on the experimental results is studied. As an alternative to the SNI method, a 'manual' indentation method is proposed, which combines information from BSE and atomic-force microscopy (AFM), coupled to the NI machine. The AFM allows to precisely indent a high-density C-S-H rim around unhydrated clinkers in cement paste. Yet the results from that technique still show a big scatter.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates

We report a model for metalorganic vapor-phase epitaxy on non-planar substrates, specifically V-grooves and pyramidal recesses, which we apply to the growth of InGaAs nanostructures. This model—based on a set of coupled reaction-diffusion equations, one for each facet in the system—accounts for the facet-dependence of all kinetic processes (e.g., precursor decomposition, adatom diffusion, and adatom lifetimes) and has been previously applied to account for the temperature-, concentration-, and temporal-dependence of AlGaAs nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses. In the present study, the growth of In{sub 0.12}Ga{sub 0.88}As quantum wires at the bottom of V-grooves is used to determine a set of optimized kinetic parameters. Based on these parameters, we have modeled the growth of In{sub 0.25}Ga{sub 0.75}As nanostructures formed in pyramidal site-controlled quantum-dot systems, successfully producing a qualitative explanation for the temperature-dependence of their optical properties, which have been reported in previous studies. Finally, we present scanning electron and cross-sectional atomic force microscopy images which show previously unreported facetting at the bottom of the pyramidal recesses that allow quantum dot formation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Ultra-low density InAs quantum dots

We show that InAs quantum dots (QDs) can be grown by molecular beam epitaxy (MBE) with an ultralow density of sin 10{sup 7} cm{sup -2} without any preliminary or post-growth surface treatment. The strain-induced QD formation proceeds via the standard Stranski-Krastanow mechanism, where the InAs coverage is decreased to 1.3-1.5 monolayers (MLs). By using off-cut GaAs (100) substrates, we facilitate the island nucleation in this subcritical coverage range without any growth interruption. The QD density dependences on the InAs coverage are studied by photoluminescence, atomic force microscopy, transmission electron microscopy, and are well reproduced by the universal double exponential shapes. This method enables the fabrication of InAs QDs with controllable density in the range 10{sup 7}-10{sup 8} cm{sup -2}, exhibiting bright photoluminescence.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Self-organized titanium oxide nano-channels for resistive memory application

Towards developing next generation scalable TiO{sub 2}-based resistive switching (RS) memory devices, the efficacy of 50 keV Ar{sup +}-ion irradiation to achieve self-organized nano-channel based structures at a threshold fluence of 5 × 10{sup 16} ions/cm{sup 2} at ambient temperature is presented. Although x-ray diffraction results suggest the amorphization of as-grown TiO{sub 2} layers, detailed transmission electron microscopy study reveals fluence-dependent evolution of voids and eventual formation of self-organized nano-channels between them. Moreover, gradual increase of TiO/Ti{sub 2}O{sub 3} in the near surface region, as monitored by x-ray photoelectron spectroscopy, establishes the upsurge in oxygen deficient centers. The impact of structural and chemical modification on local RS behavior has also been investigated by current-voltage measurements in conductive atomic force microscopy, while memory application is manifested by fabricating Pt/TiO{sub 2}/Pt/Ti/SiO{sub 2}/Si devices. Finally, the underlying mechanism of our experimental results has been analyzed and discussed in the light of oxygen vacancy migration through nano-channels.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Low-temperature sequential pulsed chemical vapor deposition of ternary B{sub x}Ga{sub 1-x}N and B{sub x}In{sub 1-x}N thin film alloys

In this work, the authors have performed sequential pulsed chemical vapor deposition of ternary B{sub x}Ga{sub 1-x}N and B{sub x}In{sub 1-x}N alloys at a growth temperature of 450 °C. Triethylboron, triethylgallium, trimethylindium, and N{sub 2} or N{sub 2}/H{sub 2} plasma have been utilized as boron, gallium, indium, and nitrogen precursors, respectively. The authors have studied the compositional dependence of structural, optical, and morphological properties of B{sub x}Ga{sub 1-x}N and B{sub x}In{sub 1-x}N ternary thin film alloys. Grazing incidence X-ray diffraction measurements showed that boron incorporation in wurtzite lattice of GaN and InN diminishes the crystallinity of B{sub x}Ga{sub 1-x}N and B{sub x}In{sub 1-x}N sample. Refractive index decreased from 2.24 to 1.65 as the B concentration of B{sub x}Ga{sub 1-x}N increased from 35% to 88%. Similarly, refractive index of B{sub x}In{sub 1-x}N changed from 1.98 to 1.74 for increase in B concentration value from 32% to 87%, respectively. Optical transmission band edge values of the B{sub x}Ga{sub 1-x}N and B{sub x}In{sub 1-x}N films shifted to lower wavelengths with increasing boron content, indicating the tunability of energy band gap with alloy composition. Atomic force microscopy measurements revealed an increase in surface roughness with boron concentration of B{sub x}Ga{sub 1-x}N, while an opposite trend was observed for B{sub x}In{sub 1-x}N thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Controlled Growth of one-dimensional zinc oxide nanostructures in the pulsed electrodeposition mode

Zinc oxide nanostructures are objects of study in the field of optoelectronics, solar power engineering, nanosensorics, and catalysis. For the purpose of the controlled growth of one-dimensional submicrometer zinc oxide structures in the pulsed electrodeposition mode, the effect of the pulse electrolysis parameters on the morphology of ZnO layers, their optical properties, and structural and substructural characteristics is determined using X-ray diffraction, optical spectrophotometry, and atomic-force microscopy. The possibility of fabricating arrays of ZnO nanowires with different geometrical shapes, perpendicular to the substrate surface, by varying the frequency of cathode-substrate potential pulses is shown.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Early stages of plasma induced nitridation of Si (111) surface and study of interfacial band alignment

We report here a systematic study of the nitridation of the Si (111) surface by nitrogen plasma exposure. The surface and interface chemical composition and surface morphology are investigated by using RHEED, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM). At the initial stage of nitridation two superstructures—“8 × 8” and “8/3 × 8/3”—form, and further nitridation leads to 1 × 1 stoichiometric silicon nitride. The interface is seen to have the Si{sup 1+} and Si{sup 3+} states of silicon bonding with nitrogen, which suggests an atomically abrupt and defect-free interface. The initial single crystalline silicon nitride layers are seen to become amorphous at higher thicknesses. The AFM image shows that the nitride nucleates at interfacial dislocations that are connected by sub-stoichiometric 2D-nitride layers, which agglomerate to form thick overlayers. The electrical properties of the interface yield a valence band offset that saturates at 1.9 eV and conduction band offset at 2.3 eV due to the evolution of the sub-stoichiometric interface and band bending.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Polarity control and transport properties of Mg-doped (0001) InN by plasma-assisted molecular beam epitaxy

The authors report on the plasma-assisted molecular beam epitaxy growth and carrier transport of Mg-doped In-face (0001) InN. The 1.2 {mu}m thick InN films were grown on GaN:Fe templates under metal rich conditions with Mg concentration from 1 Multiplication-Sign 10{sup 17}/cm{sup 3} to 3 Multiplication-Sign 10{sup 20}/cm{sup 3}. A morphological transition, associated with the formation of V-shape polarity inversion domains, was observed at Mg concentration over 7 Multiplication-Sign 10{sup 19}/cm{sup 3} by atomic force microscopy and transmission electron microscopy. Seebeck measurements indicated p-type conductivity for Mg-concentrations from 9 Multiplication-Sign 10{sup 17}/cm{sup 3} to 7 Multiplication-Sign 10{sup 19}/cm{sup 3}, i.e., as it exceeded the compensating (unintentional) donor concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Impact of MgO thickness on the perpendicular magnetic anisotropy of Mo/Co2FeAl/MgO/Mo multilayers with improved annealing stability

Highlights: • Robust PMA is demonstrated for thermally stable Mo/Co{sub 2}FeAl(CFA)/MgO/Mo multilayer. • Thickness of CFA and MgO over-layer plays a main role in achieving the strong PMA. • Significant PMA energy of K{sub eff} ≈ 1.67 × 10{sup 6} erg/cm{sup 3} is achieved for film annealed at 400 °C. • Co and/or Fe – 3d{sub Z}{sup 2} and O-2P{sub Z} orbital hybridization at interface is the origin for PMA. - Abstract: In this study, an interfacial perpendicular magnetic anisotropy (PMA) is achieved in Mo/Co{sub 2}FeAl (CFA)/MgO/Mo multilayer with good thermal stability up to 400 °C by tuning the annealing temperature and film thickness of CFA and MgO. For an optimized CFA film thickness (1.0 nm) and annealing temperature (400 °C), the CFA/MgO interface plays a main role in achieving the strong PMA property when the MgO film thickness was tuned from 0.5 nm to 2.0 nm. The effective perpendicular magnetic anisotropy energy density (K{sub eff}) is approximately 1.67 × 10{sup 6} erg/cc when t{sub MgO} = 2 nm. Surface morphology and surface roughness changes in the films are investigated by atomic force microscopy (AFM). An enhancement in surface smoothness is observed for the annealed films. X-ray photoelectron spectroscopy (XPS) reveals that the strong PMA in Mo/CFA/MgO is originated from the adequate oxidation of Co and Fe at CFA/MgO interface.

36 MATERIALS SCIENCE↗