K. Andre Mkhoyan

University of Minnesota Duluth, Duluth, Minnesota, United States

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Publications (93)537.7 Total impact

  • E.D. Hintsala · A.J. Wagner · W.W. Gerberich · K.A. Mkhoyan
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    ABSTRACT: Development of more accurate descriptions of dislocation motion requires understanding the actual effective stress driving it. Back stresses from dislocation pile-ups can work against the applied stress resulting in lower stresses acting on moving dislocations. This study presents calculations of back stress derived from in-situ compression of 26–39 nm sized single crystal silicon cubes inside the transmission electron microscope. These initially dislocation free particles exhibited yielding culminating in over 60% plastic strain. The back stress was calculated based on a pile-up model which, when subtracted from the applied stress, suggests a constant effective stress for continuing plasticity.
    No preview · Article · Mar 2016
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    ABSTRACT: Nonlocal spin valves (NLSVs) generate pure spin currents, providing unique insight into spin injection and relaxation at the nanoscale. Recently it was shown that the puzzling low temperature nonmonotonicity of the spin accumulation in all-metal NLSVs occurs due to a manifestation of the Kondo effect arising from dilute local-moment-forming impurities in the nonmagnetic material. Here it is demonstrated that precise control over interdiffusion in Fe/Cu NLSVs via thermal annealing can induce dramatic increases in this Kondo suppression of injection efficiency, observation of injector/detector separation-dependent Kondo effects in both charge and spin channels simultaneously, and, in the limit of large interdiffusion, complete breakdown of standard Valet-Fert-based models. The Kondo effect in the charge channel enables extraction of the exact interdiffusion profile, quantifying the influence of local moment density on the injection efficiency and presenting a well-posed challenge to theory.
    No preview · Article · Jan 2016
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    ABSTRACT: 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. While 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 non-equilibrium synthesis conditions, we obtain NCs in which dopants are not segregated to the surface 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 blueshift 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 in the gas-phase environment influences dopant distribution and activation, defect structure, and impurity phase formation.
    No preview · Article · Nov 2015 · Nano Letters
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    ABSTRACT: Silicon is one of the most technologically important materials, used extensively in electronics, solar cells, micro-electro-mechanical systems (MEMS) based devices and more. Yet its mechanical properties are not well understood at the nanoscale where it is often utilized. Experimental measurements under a variety of loading conditions are needed, and compression experiments are particularly lacking. Here, the elastic-plastic response of 20-65 nm cubic Si nanocubes under uniaxial compression is investigated. The purely elastic limit of these nanocubes is observed to be up to 0.07 true strain at 7 GPa true stress with an upper yield point of 0.20 true strain and 11 GPa true stress. Investigation of the nature of dislocations generated during deformation of these nanocubes using post-mortem analysis in the TEM provides evidence that leading partial dislocations are the dominant source of plasticity at this scale. © 2015 Acta Materialia Inc.Published by Elsevier Ltd. All rights reserved.
    Full-text · Article · Nov 2015 · Acta Materialia
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    ABSTRACT: Two-dimensional (2D) ultra-high carrier densities are of considerable current research interest for novel plasmonic and high charge-gain devices. However, the highest 2D electron density obtained is thus far limited to 3 × 1014 cm-2 (1/2 electron/unit cell/interface) at GdTiO3/SrTiO3 interfaces, and is typically an order of magnitude lower at LaAlO3/SrTiO3 interfaces. We show from experiment and modeling that carrier densities much higher than expected based on resolution of the polar discontinuity at perovskite oxide heterojunctions can be achieved via band engineering. The SrTiO3 (8 u.c.)/NdTiO3 (t u.c)/SrTiO3 (8 u.c.)/LSAT(001) heterostructure shows the expected electronic reconstruction behavior starting at t = 2 u.c., but then exhibits a higher carrier density regime at t ≥ 6 u.c. due to additional charge transfer from band alignment.
    No preview · Article · Nov 2015 · Advanced Materials Interfaces
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    ABSTRACT: Owing to its high room-temperature electron mobility and wide bandgap, BaSnO3 has recently become of significant interest for potential room-temperature oxide electronics. A hybrid molecular beam epitaxy (MBE) approach for the growth of high-quality BaSnO3 films is developed in this work. This approach employs hexamethylditin as a chemical precursor for tin, an effusion cell for barium, and a radio frequency plasma source for oxygen. BaSnO3 films were thus grown on SrTiO3 (001) and LaAlO3 (001) substrates. Growth conditions for stoichiometric BaSnO3 were identified. Reflection high-energy electron diffraction (RHEED) intensity oscillations, characteristic of a layer-by-layer growth mode were observed. A critical thickness of ∼1 nm for strain relaxation was determined for films grown on SrTiO3 using in situ RHEED. Scanning transmission electron microscopy combined with electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy confirmed the cube-on-cube epitaxy and composition. The importance of precursor chemistry is discussed in the context of the MBE growth of BaSnO3.
    No preview · Article · Oct 2015 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
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    ABSTRACT: Three-dimensional (3D) topological insulators are known for their strong spin-orbit coupling (SOC) and the existence of spin-textured surface states that might be potentially exploited for "topological spintronics." Here, we use spin pumping and the inverse spin Hall effect to demonstrate successful spin injection at room temperature from a metallic ferromagnet (CoFeB) into the canonical 3D topological insulator Bi2Se3. The spin pumping process, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) linewidth. Theoretical modeling of spin pumping through the surface of Bi2Se3, as well as of the measured angular dependence of spin-charge conversion signal, suggest that pumped spin current is first giantly enhanced by the surface SOC and then converted into a dc-voltage signal primarily by the inverse spin Hall effect due to SOC of the bulk of Bi2Se3. We find that the FMR linewidth broadens significantly (more than a factor of 5) and we deduce a spin Hall angle as large as 0.43 in the Bi2Se3 layer.
    No preview · Article · Sep 2015 · Nano Letters
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    ABSTRACT: Two-dimensional (2-D) zeolites are near single-unit-cell thick layers of silicon and oxygen atoms with precisely-sized pores of molecular dimensions [1]. It is critical to determine the thickness and structure of 2-D zeolite layers in order to understand their unique transport [2] and catalytic properties [3]. Thickness measurements of 2-D zeolites are often performed by X-ray reflectivity experiments [4] or by imaging cross-sectional samples in a transmission electron microscope (TEM) [5]. However, X-ray reflectivity measurements require fabrication of a periodic multilayer, which is not feasible for zeolite nanosheets with sub-micron lateral dimensions. On the other hand, conventional TEM (CTEM) images of cross-sections require in-depth analysis of thickness and image contrast. Due to these limitations and the electron-beam-sensitive nature of 2-D zeolites [6], a method for unambiguous determination of their thickness and structure has remained elusive. We developed an analytical method based on a set of TEM experiments, which can provide complete quantitative characterization of 2-D zeolites, including crystal structure, uniformity, thickness, and wrinkling. Since intensity modulation of a diffraction spot on tilting is a fingerprint of the thickness, and changes in the spot shape are a measure of the wrinkling, comprehensive three-dimensional (3-D) mapping of the reciprocal lattice was performed to monitor these changes. This was achieved by acquiring a tilt series of selected area electron diffraction patterns (SAED) of MFI-structure type zeolite nanosheets on a FEI Tecnai G2 F30 (S)TEM equipped with a TWIN pole piece at 300 kV accelerating voltage. A MFI-nanosheet suspension in octanol was prepared by following a method reported by Varoon et al [7]. The MFI crystal structure of as-synthesized nanosheets was confirmed by high resolution conventional TEM imaging (Figure 1a) and [010] zone axis SAED pattern (Figure 1b). Comparison of multislice simulated modulation of diffraction spot intensity with tilt to experimental data revealed that the nanosheets are 1.5 unit cells (uc) thick (Figure 1c). In order to investigate quantitatively the deviation from flatness (wrinkling) of the MFI nanosheet, we tracked changes in shape of diffraction spots upon tilting. The wrinkling of nanosheets in real space corresponds to precession of the rel-rods [8] into " cones " in reciprocal space. The diffraction spot shape changes are particularly pronounced at larger tilt angles. Quantification of these wrinkles was done by modeling two independent sine waves superposed in a-and c-directions of a flat nanosheet (Figure 1d-g). Comparison with multislice simulated tilt series of SAED patterns for wrinkled nanosheet models revealed that the wrinkling is larger in the a-direction as compared to the c-direction (due to greater resistance of the MFI nanosheet to bending in c-direction). The average surface roughness of nanosheets was estimated to be 0.8 nm.
    Full-text · Conference Paper · Aug 2015
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    ABSTRACT: Two-dimensional (2-D) zeolites and zeolite nanosheets are porous silicate frameworks desirable for catalytic uses involving bulky molecules [1], thin film separation membranes [2], and low-k dielectric materials [3]. Functionality of such zeolites is highly dependent on their crystal structure, thickness and pore dimensions. Low-dose transmission electron microscopy (TEM) studies at an optimum accelerating voltage have proven particularly useful in crystallographic structure determination of these electron beam sensitive materials [4]. However, it is known that zeolites undergo amorphization through radiolysis at low accelerating voltages, as well as both sputtering and amorphization through knock-on at high accelerating voltages [5]. Being inherently destructive, the examination of zeolites in TEM cannot perfectly reveal as-synthesized structure. Here, we investigate the structural evolution of MFI-zeolite nanosheets over time upon electron beam exposure in the TEM. A time series of selected area electron diffraction (SAED) patterns was acquired using a FEI Tecnai G2 F30 (S)TEM with a TWIN pole piece at 300 kV accelerating voltage. Dose was limited to 1.9 e-/Å 2 /s on the specimen. MFI-nanosheets containing (i) 9 wt% and (ii) 26-28 wt% organic structure directing agent (OSDA) were synthesized using a method described by Varoon et al. [6]. The crystal structure of these nanosheets was confirmed to be MFI-zeolite type within 2 minutes of electron beam exposure, using high-resolution bright-field conventional TEM (BF-CTEM) imaging, high angle annular dark field scanning TEM (HAADF-STEM) imaging, and [010] zone axis SAED (Figure 1a-f). The thickness of these nanosheets was determined to be 1.5 unit cells (or 3.2 nm) by mapping a rel-rod through a tilt series of SAED patterns (Figure 1g-h). The evolution of diffraction spot intensities under continuous electron beam exposure showed that even though there was evidence of amorphization, some spots increased in intensity over time, confirming that other structural rearrangements occur within these nanosheets (Figure 2a-d). The observed increases in spot intensity during TEM illumination are explained by the removal of OSDA upon beam exposure: mass loss of OSDA causes the nanosheets to reduce in volume, as well as allows the crystal structure to relax from monoclinic to orthorhombic type. We relate the observed structural changes to the OSDA fraction in these nanosheets by carefully analyzing the diffraction spot intensities and in-plane lattice parameters.
    Full-text · Conference Paper · Aug 2015

  • No preview · Article · Aug 2015 · Microscopy and Microanalysis
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    ABSTRACT: We report electrical measurements of the current-induced spin polarization of the surface current in topological insulator devices where contributions from bulk and surface conduction can be disen- tangled by electrical gating. The devices use a ferromagnetic tunnel junction (permalloy/Al2O3) as a spin detector on a back-gated (Bi,Sb)2Te3 channel. We observe hysteretic voltage signals as the magnetization of the detector ferromagnet is switched parallel or anti-parallel to the spin polariza- tion of the surface current. The amplitude of the detected voltage change is linearly proportional to the applied DC bias current in the (Bi,Sb)2Te3 channel. As the chemical potential is tuned from the bulk bands into the surface state band, we observe an enhancement of the spin-dependent voltages up to 300% within the range of the electrostatic gating. Using a simple model, we extract the spin polarization near charge neutrality (i.e. the Dirac point).
    Full-text · Article · Jul 2015 · Physical Review B
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    ABSTRACT: Single-unit-cell Sn-MFI, with the detectable Sn uniformly distributed and exclusively located at framework sites, is reported for the first time. The direct, single-step, synthesis is based on repetitive branching caused by rotational intergrowths of single-unit-cell lamellae. The self-pillared, meso- and microporous zeolite is an active and selective catalyst for sugar isomerization. High yields for the conversion of glucose into fructose and lactose to lactulose are demonstrated. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Jul 2015 · Angewandte Chemie International Edition
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    ABSTRACT: BaSnO3 has recently been identified as a high mobility wide gap semiconductor with significant potential for room temperature oxide electronics. Here, a detailed study of the high pressure oxygen sputter-deposition, microstructure, morphology, and stoichiometry of epitaxial BaSnO3 on SrTiO3(001) and MgO(001) is reported, optimized conditions resulting in single-phase, relaxed, close to stoichiometric films. Most significantly, vacuum annealing is established as a facile route to n-doped BaSnO3−δ, leading to electron densities above 1019 cm−3, 5 mΩ cm resistivities, and room temperature mobility of 20 cm2 V−1 s−1 in 300-Å-thick films on MgO(001). Mobility limiting factors, and the substantial scope for their improvement, are discussed.
    Full-text · Article · Jun 2015 · APL Materials
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    ABSTRACT: Some two-dimensional (2D) exfoliated zeolites are single- or near single-unit cell thick silicates that can function as molecular sieves. Although they have already found uses as catalysts, adsorbents and membranes precise determination of their thickness and wrinkling is critical as these properties influence their functionality. Here we demonstrate a method to accurately determine the thickness and wrinkles of a 2D zeolite nanosheet by comprehensive 3D mapping of its reciprocal lattice. Since the intensity modulation of a diffraction spot on tilting is a fingerprint of the thickness, and changes in the spot shape are a measure of wrinkling, this mapping is achieved using a large-angle tilt-series of electron diffraction patterns. Application of the method to a 2D zeolite with MFI structure reveals that the exfoliated MFI nanosheet is 1.5 unit cells (3.0 nm) thick and wrinkled anisotropically with up to 0.8 nm average surface roughness.
    Full-text · Article · May 2015 · Nature Communications
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    ABSTRACT: 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.
    No preview · Article · May 2015 · Applied Physics Letters
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    ABSTRACT: Self-pillared pentasil (SPP) zeolite is a hierarchically-structured zeolite comprised of single-unit cell thick MFI nanosheets arranged in a “house of cards” structure. The nucleation and growth of SPP proceeds through three phases involving the evolution of precursor amorphous nanoparticles to MFI nanosheets and then rotational intergrowth of sheets to produce the SPP morphology. This paper expands upon an earlier report to extend understanding of nucleation and growth events throughout the entire preparation process, from hydrolysis of the silica source to high conversion to crystals. Common aspects with the extensively investigated clear-sol silicalite-1 system are identified. Evaporation of co-solvent ethanol was found to accelerate the crystallization significantly. Furthermore, robust synthesis of SPP with high density of well-developed single-unit cell domains has been achieved with addition of potassium and sodium to the synthesis sols.
    Full-text · Article · May 2015 · Topics in Catalysis
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    ABSTRACT: Stable suspensions of zeolite nanosheets (3 nm thick MFI layers) were prepared in ethanol following acid treatment, which partially removed the associated organic structure-directing agent. Nanosheets from these suspensions could then be dispersed at the air–water interface and transferred to silicon wafers using Langmuir–Schaefer deposition. Using layer-by-layer deposition, control on coating thickness was demonstrated. In-plane X-ray diffraction (XRD) revealed that the deposited nanosheets contract upon calcination similar to bulk MFI crystals. Different methods for secondary growth resulted in preferentially oriented thin films of MFI, which had sub-12-nm thickness in certain cases. Upon calcination, there was no contraction detectable by in-plane XRD, indicating well-intergrown MFI films that are strongly attached to the substrate.
    Full-text · Article · Apr 2015 · Angewandte Chemie
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    ABSTRACT: Stable suspensions of zeolite nanosheets (3 nm thick MFI layers) were prepared in ethanol following acid treatment, which partially removed the associated organic structure-directing agent. Nanosheets from these suspensions could then be dispersed at the air-water interface and transferred to silicon wafers using Langmuir-Schaefer deposition. Using layer-by-layer deposition, control on coating thickness was demonstrated. In-plane X-ray diffraction (XRD) revealed that the deposited nanosheets contract upon calcination similar to bulk MFI crystals. Different methods for secondary growth resulted in preferentially oriented thin films of MFI, which had sub-12-nm thickness in certain cases. Upon calcination, there was no contraction detectable by in-plane XRD, indicating well-intergrown MFI films that are strongly attached to the substrate. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Full-text · Article · Apr 2015 · Angewandte Chemie International Edition
  • Su Yeon Kim · Jong Seok Jeong · K. Andre Mkhoyan · Ho Seong Jang
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    ABSTRACT: Highly efficient downconversion (DC) green-emitting LiYF4:Ce,Tb nanophosphors have been synthesized for bright dual-mode upconversion (UC) and DC green-emitting core/double-shell (C/D-S) nanophosphors-Li(Gd,Y)F4:Yb(18%),Er(2%)/LiYF4:Ce(15%),Tb(15%)/LiYF4-and the C/D-S structure has been proved by extensive scanning transmission electron microscopy (STEM) analysis. Colloidal LiYF4:Ce,Tb nanophosphors with a tetragonal bipyramidal shape are synthesized for the first time and they show intense DC green light via energy transfer from Ce(3+) to Tb(3+) under illumination with ultraviolet (UV) light. The LiYF4:Ce,Tb nanophosphors show 65 times higher photoluminescence intensity than LiYF4:Tb nanophosphors under illumination with UV light and the LiYF4:Ce,Tb is adapted into a luminescent shell of the tetragonal bipyramidal C/D-S nanophosphors. The formation of the DC shell on the core significantly enhances UC luminescence from the UC core under irradiation of near infrared light and concurrently generates DC luminescence from the core/shell nanophosphors under UV light. Coating with an inert inorganic shell further enhances the UC-DC dual-mode luminescence by suppressing the surface quenching effect. The C/D-S nanophosphors show 3.8% UC quantum efficiency (QE) at 239 W cm(-2) and 73.0 ± 0.1% DC QE. The designed C/D-S architecture in tetragonal bipyramidal nanophosphors is rigorously verified by an energy dispersive X-ray spectroscopy (EDX) analysis, with the assistance of line profile simulation, using an aberration-corrected scanning transmission electron microscope equipped with a high-efficiency EDX. The feasibility of these C/D-S nanophosphors for transparent display devices is also considered.
    No preview · Article · Jan 2015 · Nanoscale
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    ABSTRACT: Thin films comprising semiconductor nanocrystals are emerging for applications in electronic and optoelectronic devices including light emitting diodes and solar cells. Achieving high charge carrier mobility in these films requires the identification and elimination of electronic traps on the nanocrystal surfaces. Herein, we show that in films comprising ZnO nanocrystals, an electron acceptor trap related to the presence of OH on the surface limits the conductivity. ZnO nanocrystal films were synthesized using a nonthermal plasma from diethyl zinc and oxygen and deposited by inertial impaction onto a variety of substrates. Surprisingly, coating the ZnO nanocrystals with a few nanometres of Al2O3 using atomic layer deposition decreased the film resistivity by seven orders of magnitude to values as low as 0.12 Ω cm. Electron mobility as high as 3 cm(2) V(-1) s(-1) was observed in films comprising annealed ZnO nanocrystals coated with Al2O3.
    No preview · Article · Dec 2014 · Nature Communications

Publication Stats

2k Citations
537.70 Total Impact Points

Institutions

  • 2009-2016
    • University of Minnesota Duluth
      • • Department of Mechanical and Industrial Engineering
      • • Department of Chemistry and Biochemistry
      Duluth, Minnesota, United States
  • 2003-2009
    • Cornell University
      • School of Applied and Engineering Physics
      Итак, New York, United States