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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Microscopic analysis of non-equilibrium dynamics in the semiconductor-laser gain medium

Fully microscopic many-body calculations are used to analyze the carrier dynamics in situations where a strong sub-picosecond pulse interacts with an inverted semiconductor quantum well. Electron-electron and electron-phonon scatterings are calculated on a second Born-Markov level. Intra-subband scatterings on a scale of tens of femtoseconds are shown to quickly re-fill the kinetic holes created in the carrier distributions during the pulse amplification. Even for sub-100 fs pulses, this significantly influences the pulse amplification as well as its spectral dependence. Interband scatterings on a few picosecond timescale limit the possibly achievable repetition rate in pulsed semiconductor lasers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Free vibrations of U-shaped atomic force microscope probes

Contact resonance atomic force microscope (AFM) methods have been used to quantify the elastic and viscoelastic properties of a variety of materials such as polymers, ceramics, biological materials, and metals with spatial resolution on the order of tens of nanometers. This approach involves measurement of the resonant frequencies of the AFM probe both for the free case and the case for which the tip is in contact with a sample. Vibration models of the probe and tip-sample contact models are then used to determine the sample properties from the frequency behavior and to create images of the sample properties. This work has been primarily focused on rectangular, single-beam probes for which the vibration models are relatively simple. Recently, U-shaped AFM probes have been developed to allow local heating of samples and the resonances of these probes are much more complex. In this article, a simplified analytical model of these U-shaped probes is described. This three beam model includes two beams clamped at one end and connected with a perpendicular cross beam at the other end. The beams are assumed only to bend in flexure and twist but their coupling allows a wide range of possible dynamic behavior. Results are presented for the first ten modes and the mode shapes are shown to have complex coupling between the flexure and twisting of the beams, particularly for the higher modes. All resonant frequency results are in good agreement with finite element results for the three probe designs and two values of thickness considered (all wavenumbers are within 3.0%). This work is anticipated to allow U-shaped probes to be used eventually for quantitative measurements of sample material properties during heating using a contact resonance approach.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Real time drift measurement for colloidal probe atomic force microscope: a visual sensing approach

Drift has long been an issue in atomic force microscope (AFM) systems and limits their ability to make long time period measurements. In this study, a new method is proposed to directly measure and compensate for the drift between AFM cantilevers and sample surfaces in AFM systems. This was achieved by simultaneously measuring z positions for beads at the end of an AFM colloidal probe and on sample surface through an off-focus image processing based visual sensing method. The working principle and system configuration are presented. Experiments were conducted to validate the real time drift measurement and compensation. The implication of the proposed method for regular AFM measurements is discussed. We believe that this technique provides a practical and efficient approach for AFM experiments requiring long time period measurement.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Microscopic mechanisms of graphene electrolytic delamination from metal substrates

In this paper, hydrogen bubbling delamination of graphene (Gr) from copper using a strong electrolyte (KOH) water solution was performed, focusing on the effect of the KOH concentration (C{sub KOH}) on the Gr delamination rate. A factor of ∼10 decrease in the time required for the complete Gr delamination from Cu cathodes with the same geometry was found increasing C{sub KOH} from ∼0.05 M to ∼0.60 M. After transfer of the separated Gr membranes to SiO{sub 2} substrates by a highly reproducible thermo-compression printing method, an accurate atomic force microscopy investigation of the changes in Gr morphology as a function of C{sub KOH} was performed. Supported by these analyses, a microscopic model of the delamination process has been proposed, where a key role is played by graphene wrinkles acting as nucleation sites for H{sub 2} bubbles at the cathode perimeter. With this approach, the H{sub 2} supersaturation generated at the electrode for different electrolyte concentrations was estimated and the inverse dependence of t{sub d} on C{sub KOH} was quantitatively explained. Although developed in the case of Cu, this analysis is generally valid and can be applied to describe the electrolytic delamination of graphene from several metal substrates.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

High-speed force mapping on living cells with a small cantilever atomic force microscope

The imaging speed of the wide-spread force mapping mode for quantitative mechanical measurements on soft samples in liquid with the atomic force microscope (AFM) is limited by the bandwidth of the z-scanner and viscous drag forces on the cantilever. Here, we applied high-speed, large scan-range atomic force microscopy and small cantilevers to increase the speed of force mapping by ≈10−100 times. This allowed resolving dynamic processes on living mouse embryonic fibroblasts. Cytoskeleton reorganization during cell locomotion, growth of individual cytoskeleton fibers, cell blebbing, and the formation of endocytic pits in the cell membrane were observed. Increasing the force curve rate from 2 to 300 Hz increased the measured apparent Young's modulus of the cells by about 10 times, which facilitated force mapping measurements at high speed.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

High precision two-dimensional strain mapping in semiconductor devices using nanobeam electron diffraction in the transmission electron microscope

A classical method used to characterize the strain in modern semiconductor devices is nanobeam diffraction (NBD) in the transmission electron microscope. One challenge for this method lies in the fact that the smaller the beam becomes, the more difficult it becomes to analyze the resulting diffraction spot pattern. We show that a carefully designed fitting algorithm enables us to reduce the sampling area for the diffraction patterns on the camera chip dramatically (∼1/16) compared to traditional settings without significant loss of precision. The resulting lower magnification of the spot pattern permits the presence of an annular dark field detector, which in turn makes the recording of images for drift correction during NBD acquisition possible. Thus, the reduced sampling size allows acquisition of drift corrected NBD 2D strain maps of up to 3000 pixels while maintaining a precision of better than 0.07%. As an example, we show NBD strain maps of a modern field effect transistor (FET) device. A special filtering feature used in the analysis makes it is possible to measure strain in silicon devices even in the presence of other crystalline materials covering the probed area, which is important for the characterization of the next generation of devices (Fin-FETs).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Nano-scale luminescence characterization of individual InGaN/GaN quantum wells stacked in a microcavity using scanning transmission electron microscope cathodoluminescence

Using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope at liquid helium temperatures, the optical and structural properties of a 62 InGaN/GaN multiple quantum well embedded in an AlInN/GaN based microcavity are investigated at the nanometer scale. We are able to spatially resolve a spectral redshift between the individual quantum wells towards the surface. Cathodoluminescence spectral linescans allow directly visualizing the critical layer thickness in the quantum well stack resulting in the onset of plastic relaxation of the strained InGaN/GaN system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Macro- and microscopic properties of strontium doped indium oxide

Solid state synthesis and physical mechanisms of electrical conductivity variation in polycrystalline, strontium doped indium oxide In{sub 2}O{sub 3}:(SrO){sub x} were investigated for materials with different doping levels at different temperatures (T = 20–300 °C) and ambient atmosphere content including humidity and low pressure. Gas sensing ability of these compounds as well as the sample resistance appeared to increase by 4 and 8 orders of the magnitude, respectively, with the doping level increase from zero up to x = 10%. The conductance variation due to doping is explained by two mechanisms: acceptor-like electrical activity of Sr as a point defect and appearance of an additional phase of SrIn{sub 2}O{sub 4}. An unusual property of high level (x = 10%) doped samples is a possibility of extraordinarily large and fast oxygen exchange with ambient atmosphere at not very high temperatures (100–200 °C). This peculiarity is explained by friable structure of crystallite surface. Friable structure provides relatively fast transition of samples from high to low resistive state at the expense of high conductance of the near surface layer of the grains. Microscopic study of the electro-diffusion process at the surface of oxygen deficient samples allowed estimation of the diffusion coefficient of oxygen vacancies in the friable surface layer at room temperature as 3 × 10{sup −13} cm{sup 2}/s, which is by one order of the magnitude smaller than that known for amorphous indium oxide films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Characterizing absolute piezoelectric microelectromechanical system displacement using an atomic force microscope

Piezoresponse Force Microscopy (PFM) is a popular tool for the study of ferroelectric and piezoelectric materials at the nanometer level. Progress in the development of piezoelectric MEMS fabrication is highlighting the need to characterize absolute displacement at the nanometer and Ångstrom scales, something Atomic Force Microscopy (AFM) might do but PFM cannot. Absolute displacement is measured by executing a polarization measurement of the ferroelectric or piezoelectric capacitor in question while monitoring the absolute vertical position of the sample surface with a stationary AFM cantilever. Two issues dominate the execution and precision of such a measurement: (1) the small amplitude of the electrical signal from the AFM at the Ångstrom level and (2) calibration of the AFM. The authors have developed a calibration routine and test technique for mitigating the two issues, making it possible to use an atomic force microscope to measure both the movement of a capacitor surface as well as the motion of a micro-machine structure actuated by that capacitor. The theory, procedures, pitfalls, and results of using an AFM for absolute piezoelectric measurement are provided.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Strength of semiconductors, metals, and ceramics evaluated by a microscopic cleavage model with Morse-type and Lennard-Jones-type interaction

An improved microscopic cleavage model, based on a Morse-type and Lennard-Jones-type interaction instead of the previously employed half-sine function, is used to determine the maximum cleavage strength for the brittle materials diamond, tungsten, molybdenum, silicon, GaAs, silica, and graphite. The results of both interaction potentials are in much better agreement with the theoretical strength values obtained by ab initio calculations for diamond, tungsten, molybdenum, and silicon than the previous model. Reasonable estimates of the intrinsic strength are presented for GaAs, silica, and graphite, where first principles values are not available.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A microscopic explanation of the isotonic multiplet at N=90

The shape phase transition from spherical to soft deformed at N=88-90 was observed long ago. After the prediction of the X(5) symmetry, for which analytical solution of the nuclear Hamiltonian is given [1], good examples of X(5) nuclei were identified in the N=90 isotones of Nd, Sm, Gd and Dy, in the recent works. The N=90 isotones have almost the similar deformed level structure, forming the isotonic multiplet in Z=50-66, N=82-104 quadrant. This is explained microscopically in terms of the Nilsson level diagram. Using the Dynamic Pairing-Plus-Quadrupole model of Kumar-Baranger, the quadrupole deformation and the occupancies of the neutrons and protons in these nuclei have been calculated, which support the formation of N=88, 90 isotonic multiplets. The existence of F-spin multiplets in Z=66-82, N=82-104 quadrant, identified in earlier works on the Interacting Boson Model, is also explained in our study.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

In-situ optical transmission electron microscope study of exciton phonon replicas in ZnO nanowires by cathodoluminescence

The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

A new ion sensing deep atomic force microscope

Here we describe a new deep atomic force microscope (AFM) capable of ion sensing. A novel probe assembly incorporates a micropipette that can be used both for sensing ion currents and as the tip for AFM imaging. The key advance of this instrument over previous ion sensing AFMs is that it uses conventional micropipettes in a novel suspension system. This paper focuses on sensing the ion current passively while using force feedback for the operation of the AFM in contact mode. Two images are obtained simultaneously: (1) an AFM topography image and (2) an ion current image. As an example, two images of a MEMS device with a microchannel show peaks in the ion current as the pipette tip goes over the edges of the channel. This ion sensing AFM can also be used in other modes including tapping mode with force feedback as well as in non-contact mode by utilizing the ion current for feedback, as in scanning ion conductance microscopy. The instrument is gentle enough to be used on some biological samples such as plant leaves.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

High-speed imaging upgrade for a standard sample scanning atomic force microscope using small cantilevers

We present an atomic force microscope (AFM) head for optical beam deflection on small cantilevers. Our AFM head is designed to be small in size, easily integrated into a commercial AFM system, and has a modular architecture facilitating exchange of the optical and electronic assemblies. We present two different designs for both the optical beam deflection and the electronic readout systems, and evaluate their performance. Using small cantilevers with our AFM head on an otherwise unmodified commercial AFM system, we are able to take tapping mode images approximately 5–10 times faster compared to the same AFM system using large cantilevers. By using additional scanner turnaround resonance compensation and a controller designed for high-speed AFM imaging, we show tapping mode imaging of lipid bilayers at line scan rates of 100–500 Hz for scan areas of several micrometers in size.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

High resolution transmission electron microscopic in-situ observations of plastic deformation of compressed nanocrystalline gold

Nanocrystalline (nc) metals possess extremely high strength, while their capability to deform plastically has been debated for decades. Low ductility has hitherto been considered an intrinsic behavior for most nc metals, due to the lack of five independent slip systems actively operating during deformation in each nanograin. Here we report in situ high resolution transmission electron microscopic (HRTEM) observations of deformation process of nc gold under compression, showing the excellent ductility of individual and aggregate nanograins. Compression causes permanent change in the profile of individual nanograins, which is mediated by dislocation slip and grain rotation. The high rate of grain boundary sliding and large extent of widely exited grain rotation may meet the boundary compatibility requirements during plastic deformation. The in situ HRTEM observations suggest that nc gold is not intrinsically brittle under compressive loading.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High-stability cryogenic scanning tunneling microscope based on a closed-cycle cryostat

We report on the design and operation of a cryogenic ultra-high vacuum (UHV) scanning tunneling microscope (STM) coupled to a closed-cycle cryostat (CCC). The STM is thermally linked to the CCC through helium exchange gas confined inside a volume enclosed by highly flexible rubber bellows. The STM is thus mechanically decoupled from the CCC, which results in a significant reduction of the mechanical noise transferred from the CCC to the STM. Noise analysis of the tunneling current shows current fluctuations up to 4% of the total current, which translates into tip-sample distance variations of up to 1.5 picometers. This noise level is sufficiently low for atomic-resolution imaging of a wide variety of surfaces. To demonstrate this, atomic-resolution images of Au(111) and NaCl(100)/Au(111) surfaces, as well as of carbon nanotubes deposited on Au(111), were obtained. Thermal drift analysis showed that under optimized conditions, the lateral stability of the STM scanner can be as low as 0.18 Å/h. Scanning Tunneling Spectroscopy measurements based on the lock-in technique were also carried out, and showed no detectable presence of noise from the closed-cycle cryostat. Using this cooling approach, temperatures as low as 16 K at the STM scanner have been achieved, with the complete cool-down of the system typically taking up to 12 h. These results demonstrate that the constructed CCC-coupled STM is a highly stable instrument capable of highly detailed spectroscopic investigations of materials and surfaces at the atomic scale.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Novel scanning electron microscope bulge test technique integrated with loading function

Membranes and film-on-substrate structures are critical elements for some devices in electronics industry and for Micro Electro Mechanical Systems devices. These structures are normally at the scale of micrometer or even nanometer. Thus, the measurement for the mechanical property of these membranes poses a challenge over the conventional measurements at macro-scales. In this study, a novel bulge test method is presented for the evaluation of mechanical property of micro thin membranes. Three aspects are discussed in the study: (a) A novel bulge test with a Scanning Electron Microscope system realizing the function of loading and measuring simultaneously; (b) a simplified Digital Image Correlation method for a height measurement; and (c) an imaging distortion correction by the introduction of a scanning Moiré method. Combined with the above techniques, biaxial modulus as well as Young's modulus of the polyimide film can be determined. Besides, a standard tensile test is conducted as an auxiliary experiment to validate the feasibility of the proposed method.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

High-speed atomic force microscope based on an astigmatic detection system

High-speed atomic force microscopy (HS-AFM) enables visualizing dynamic behaviors of biological molecules under physiological conditions at a temporal resolution of 1s or shorter. A small cantilever with a high resonance frequency is crucial in increasing the scan speed. However, detecting mechanical resonances of small cantilevers is technically challenging. In this study, we constructed an atomic force microscope using a digital versatile disc (DVD) pickup head to detect cantilever deflections. In addition, a flexure-guided scanner and a sinusoidal scan method were implemented. In this work, we imaged a grating sample in air by using a regular cantilever and a small cantilever with a resonance frequency of 5.5 MHz. Poor tracking was seen at the scan rate of 50 line/s when a cantilever for regular AFM imaging was used. Using a small cantilever at the scan rate of 100 line/s revealed no significant degradation in the topographic images. The results indicate that a smaller cantilever can achieve a higher scan rate and superior force sensitivity. This work shows the potential for using a DVD pickup head in future HS-AFM technology.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗