Jacob T. Held

Enhanced tunneling magnetoresistance and perpendicular magnetic anisotropy in Mo/CoFeB/MgO magnetic tunnel junctions

Structural, magnetic, and transport studies have been performed on perpendicular magnetic tunnel junctions (pMTJ) with Mo as the buffer and capping layers. After annealing samples at 300 °C and higher, consistently better performance was obtained compared to that of conventional pMTJs with Ta layers. Large tunneling magnetoresistance (TMR) and perpendicular magnetic anisotropy (PMA) values were retained in a wide range of samples with Mo layers after annealing for 2 h at 400 °C, in sharp contrast to the junctions with Ta layers, in which superparamagnetic behavior with nearly vanishing magnetoresistance was observed. As a result of the greatly improved thermal stability, TMR as high as 162% was obtained in junctions containing Mo layers. These results highlight the importance of the heavy-metal layers adjacent to CoFeB electrodes for achieving larger TMR, stronger PMA, and higher thermal stability in pMTJs.

Nonequilibrium-Plasma-Synthesized ZnO Nanocrystals with Plasmon Resonance Tunable via Al Doping and Quantum Confinement

Metal oxide semiconductor nanocrystals (NCs) exhibit localized surface plasmon resonances (LSPRs) tunable within the infrared (IR) region of the electromagnetic spectrum by vacancy or impurity doping. Although a variety of these NCs have been produced using colloidal synthesis methods, incorporation and activation of dopants in the liquid phase has often been challenging. Herein, using Al-doped ZnO (AZO) NCs as an example, we demonstrate the potential of nonthermal plasma synthesis as an alternative strategy for the production of doped metal oxide NCs. Exploiting unique, thoroughly nonequilibrium synthesis conditions, we obtain NCs in which dopants are not segregated to the NC surfaces and local doping levels are high near the NC centers. Thus, we achieve overall doping levels as high as 2 × 10(20) cm(-3) in NCs with diameters ranging from 12.6 to 3.6 nm, and for the first time experimentally demonstrate a clear quantum confinement blue shift of the LSPR energy in vacancy- and impurity-doped semiconductor NCs. We propose that doping of central cores and heavy doping of small NCs are achievable via nonthermal plasma synthesis, because chemical potential differences between dopant and host atoms-which hinder dopant incorporation in colloidal synthesis-are irrelevant when NC nucleation and growth proceed via irreversible interactions among highly reactive gas-phase ions and radicals and ligand-free NC surfaces. We explore how the distinctive nucleation and growth kinetics occurring in the plasma influences dopant distribution and activation, defect structure, and impurity phase formation.

Nonthermal Plasma Synthesis of Core/Shell Quantum Dots: Strained Ge/Si Nanocrystals

In this work, we present an all-gas-phase approach for the synthesis of quantum-confined core/shell nanocrystals (NCs) as a promising alternative to traditional solution-based methods. Spherical quantum dots (QDs) are grown using a single-stage flow-through nonthermal plasma, yielding monodisperse NCs, with a concentric core/shell structure confirmed by electron microscopy. The in-flight negative charging of the NCs by plasma electrons keeps the NC cores separated during shell growth. The success of this gas-phase approach is demonstrated here through the study of Ge/Si core/shell QDs. We find that the epitaxial growth of a Si shell on the Ge QD core compressively strains the Ge lattice and affords the ability to manipulate the Ge band structure by modulation of the core and shell dimensions. This all-gas-phase approach to core/shell QD synthesis offers an effective method to produce high-quality heterostructured NCs with control over the core and shell dimensions.

Atomic bonding effects in annular dark field scanning transmission electron microscopy. II. Experiments

Quantitatively calibrated annular dark field scanning transmission electron microscopy (ADF-STEM) imaging experiments were compared to frozen phonon multislice simulations adapted to include chemical bonding effects. Having carefully matched simulation parameters to experimental conditions, a depth-dependent bonding effect was observed for high-angle ADF-STEM imaging of aluminum nitride. This result is explained by computational predictions, systematically examined in the preceding portion of this study, showing the propagation of the converged STEM beam to be highly sensitive to net interatomic charge transfer. Thus, although uncertainties in experimental conditions and simulation accuracy remain, the computationally predicted experimental bonding effect withstands the experimental testing reported here.

Effects of small-angle mistilts on dopant visibility in ADF-STEM imaging of nanocrystals

Quantitative ADF-STEM imaging paired with image simulations has proven to be a powerful technique for determining the three dimensional location of substitutionally doped atoms in thin films. Expansion of this technique to lightly-doped nanocrystals requires an understanding of the influence of specimen mistilt on dopant visibility due to the difficulty of accurate orientation determination in such systems as well as crystal movement under the beam. In this study, the effects of specimen mistilt on ADF-STEM imaging are evaluated using germanium-doped silicon nanocrystals as model systems. It is shown that dopant visibility is a strong function of specimen mistilt, and the accuracy of specimen orientation is an important factor in the analysis of three-dimensional dopant location, but the sensitivity to mistilt can be weakened by increasing the STEM probe convergence angle and optimizing ADF detector inner angle.

Quantification of Elemental Distribution in Spherical Core-Shell Nanoparticles Measured by STEM-EDX

Quantum dots exhibit many interesting and useful size-dependent optoelectronic properties and are used extensively in fields ranging from optical displays to biological imaging. By adding shell structures to these quantum dots, many properties may be improved, including mitigation of surface effects, reduction in blinking, and control of spacing between deposited particles. Alternatively, core/shell structures can be fabricated to create various types of band alignment between the core and shell materials: one example is type-II confinement, which has been studied systems such as Ge/Si and CdTe/CdSe core/shell particles. The optical properties of these systems are affected by the quality of the interface between the two materials. As such, interfacial broadening between the core and shell is an important parameter to measure and control in these systems. This elemental distribution, while important, has proven to be difficult to accurately assess. In this study, we use plasma-grown spherical Ge/Si core/shell nanoparticles as a test case for scanning transmission electron microscopy–energy dispersive X-ray (STEM-EDX) analysis of the core/shell elemental distribution.

Quasi continuous wave laser sintering of Si-Ge nanoparticles for thermoelectrics

Silicon-germanium is an important thermoelectric material for high temperature applications. In this study, thin films composed of SiGe nanoparticles were synthesized in a plasma reactor and sintered by a millisecond pulse width, quasi continuous wave, near infrared laser of wavelength 1070 nm. We demonstrate that laser sintered SiGe thin films have high electrical and low thermal conductivity, dependent on the surface morphology and dopant concentration. Substrate wetting of laser heating induced molten SiGe was found to play an important role in the final surface morphology of the films. Interconnected percolation structures, formed when proper substrate wetting occurs, were found to be more conductive than the balling structure that formed with insufficient wetting. Laser power was adjusted to maximize dopant reactivation while still minimizing dopant evaporation. After optimizing laser sintering process parameters, the best electrical conductivity, thermal conductivity, and Seebeck coefficient were found to be 70.42 S/cm, 0.84 W/m K, and 133.7 μV/K, respectively. We demonstrate that laser sintered SiGe thin films have low thermal conductivity while maintaining good electrical conductivity for high temperature thermoelectric applications.Silicon-germanium is an important thermoelectric material for high temperature applications. In this study, thin films composed of SiGe nanoparticles were synthesized in a plasma reactor and sintered by a millisecond pulse width, quasi continuous wave, near infrared laser of wavelength 1070 nm. We demonstrate that laser sintered SiGe thin films have high electrical and low thermal conductivity, dependent on the surface morphology and dopant concentration. Substrate wetting of laser heating induced molten SiGe was found to play an important role in the final surface morphology of the films. Interconnected percolation structures, formed when proper substrate wetting occurs, were found to be more conductive than the balling structure that formed with insufficient wetting. Laser power was adjusted to maximize dopant reactivation while still minimizing dopant evaporation. After optimizing laser sintering process...

Elemental Distribution Analysis of Core/Shell Nanocrystals with STEM/EDX

Semiconductor quantum dots exhibit many interesting and useful size-dependent optoelectronic properties. Recently, considerable effort has been invested in developing reliable methods for producing core/shell quantum dots. By manipulating the band alignment and strain between the core and shell, new avenues of optoelectronic tunability are being explored. However, the properties of these systems are sensitive to the interface between the core and shell materials and characterizing this interface remains a significant challenge. In this study, we build upon previous work and demonstrate a method for quantifying the radial distribution of elements in spherical core/shell nanocrystals by fitting simulated distributions to radially-averaged STEM/EDX data. This approach yields quantitative measurements of interface broadening, outer surface roughness, and core/shell composition.

Quantification of the Effects of Small Mistilts on Dopant Visibility in Nanocrystals

Aberration-corrected annular dark field scanning transmission electron microscopy (ADFSTEM) has been demonstrated to be a powerful technique for the visualization of individual dopant atoms.1 Provided the dopant has a higher atomic number than the host, dopant atoms can be clearly visible in ADF-STEM images of thin, lightly doped specimens and can be readily located to an atomic site in two dimensions given adequate lateral resolution. Channeling of the electron probe down atomic columns in aligned crystalline samples causes the visibility of dopant atoms to change predictably, providing a useful method of determining the dopant atom location also in third dimension.2

Structure evolution of M02C catalysts upon exposure to oxygen

Molybdenum carbide (Mo2C) formulations have recently received renewed interest as catalysts that can be functionally tuned via surface termination alteration (e.g. Mo, C, or O) [1-4]. In this work, we demonstrate oxygen co-processing as a means of reversibly tuning catalyst acid site density in situ. Oxygen co-feed was observed to reversibly alter the dehydration rate of isopropyl alcohol by a factor of about 30 as shown in Figure 1a. Oxygen adatoms both suppressed metallic functionality and generated Brønsted acidity. Nitrogen physisorption measurements in conjunction with XRD peak broadening showed that catalyst particles were composed of 2-5 nm crystallites with BET surface areas of 60 100 m gcat . However, catalyst X-ray diffractograms (Figure 1b) were inadequate for accurate catalyst phase determination due to the indistinguishability of the proposed hexagonal close-packed and orthorhombic Mo2C phases.