K Andre Mkhoyan

University of Minnesota Duluth, Duluth, Minnesota, United States

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Publications (59)348.32 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Synthesis of atomically thin MoS2 layers and its derivatives with large-area uniformity is an essential step to exploit the advanced properties of MoS2 for their possible applications in electronic and optoelectronic devices. In this work, a facile method is reported for the continuous synthesis of atomically thin MoS2 layers at wafer scale through thermolysis of a spin coated-ammonium tetrathiomolybdate film. The thickness and surface morphology of the sheets are characterized by atomic force microscopy. The optical properties are studied by UV–Visible absorption, Raman and photoluminescence spectroscopies. The compositional analysis of the layers is done by X-ray photo­emission spectroscopy. The atomic structure and morphology of the grains in the polycrystalline MoS2 atomic layers are examined by high-angle annular dark-field scanning transmission electron microscopy. The electron mobilities of the sheets are evaluated using back-gate field-effect transistor configuration. The results indicate that this facile method is a promising approach to synthesize MoS2 thin films at the wafer scale and can also be applied to synthesis of WS2 and hybrid MoS2-WS2 thin layers.
    Advanced Functional Materials 09/2014; · 10.44 Impact Factor
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    ABSTRACT: Oxygen annealing of thick MoS2 films results in randomly oriented and controllable triangular etched shapes, forming pits with uniform etching angles. These etching morphologies differ across the sample based on the defect sites situated on the basal plane surface, forming numerous features in different bulk sample thicknesses.
    Chemical Communications 08/2014; · 6.38 Impact Factor
  • Ryan J. Wu, Michael L. Odlyzko, K. Andre Mkhoyan
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    ABSTRACT: Multislice simulations were used to analyze the reliability of annular dark field scanning transmission electron microscopy (ADF-STEM) imaging and selected-area electron diffraction (SAED) for determining the thickness of MoS2 and WS2 specimens in the aberration-corrected TEM. Samples of 1 to 4 layers in thickness for both 2 H and 1 T polymorphs were studied and tilts up to 500 mrad off of the [0001] zone axis were considered. All thicknesses including the monolayer showed distortions and intensity variations in their ADF-STEM images and SAED patterns as a result of tilt. Both techniques proved to be applicable to distinguish monolayers from multilayers using tilt. Without tilt, neither technique allows unambiguous thickness determination solely by comparing relative intensities of atomic columns in ADF-STEM images or diffraction patterns oriented along at [0001] zone axis, with the exception of monolayer 2 H WS2. However, differentiation is possible using absolute intensities in ADF-STEM images. The analysis of ADF-STEM images and SAED patterns also allows identification of the 2 H and 1 T polymorphs of MoS2 and WS2.
    Ultramicroscopy 08/2014; · 2.47 Impact Factor
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    ABSTRACT: A structural study of a hierarchical zeolite X, which is similar to the one first synthesized by Inayat et al.,12 was performed using transmission electron microscopy imaging and diffraction. Evidence is provided, by comparison to simulations, that this material is an intergrowth of FAU and EMT and a conceptual model is presented for the growth of the FAU material with a small fraction of EMT in an atypical morphology of assembled sheets with well-defined intersection angles.
    Angewandte Chemie 07/2014;
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    ABSTRACT: A structural study of a hierarchical zeolite X, which is similar to the one first synthesized by Inayat et al.,12 was performed using transmission electron microscopy imaging and diffraction. Evidence is provided, by comparison to simulations, that this material is an intergrowth of FAU and EMT and a conceptual model is presented for the growth of the FAU material with a small fraction of EMT in an atypical morphology of assembled sheets with well-defined intersection angles.
    Angewandte Chemie International Edition 07/2014; · 11.34 Impact Factor
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    ABSTRACT: Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via thermolysis of single-source cation and sulfur precursors copper, zinc and tin diethyldithiocarbamates. The average nanocrystal size could be tuned between 2 nm and 40 nm, by varying the synthesis temperature between 150 °C and 340 °C. The synthesis is rapid and is completed in less than 10 minutes. Characterization by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy confirm that the nanocrystals are nominally stoichiometric kesterite CZTS. The 2 nm nanocrystals synthesized at 150 °C exhibit quantum confinement, with a band gap of 1.67 eV. Larger nanocrystals have the expected bulk CZTS band gap of 1.5 eV. Several micron thick films deposited by drop casting colloidal dispersions of 40 nm CZTS nanocrystals were crack-free, while those cast using 5 nm nanocrystals had micron-scale cracks.
    J. Mater. Chem. A. 06/2014; 2(27).
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    ABSTRACT: A pathway for achieving intense green light emitting LiGdF4:Yb,Er upconversion nanophosphors (UCNPs) via Y(3+) doping is demonstrated. It was revealed that Y(3+) doping initiated the formation of a tetragonal phase and affected the particle size. Single tetragonal-phase LiGd0.4Y0.4F4:Yb(18%),Er(2%) (LGY0.4F:Yb,Er) UCNPs exhibited strong upconversion (UC) green luminescence and tetragonal bipyramidal morphologies. They showed 1325 and 325-fold higher photoluminescence intensity than the 0 and 80 mol% Y(3+)-doped LiGdF4:Yb,Er UCNPs, respectively. Additionally the particle size (edge length) of LiGdF4:Yb,Er-based upconversion tetragonal bipyramids (UCTBs) was controlled from 60.5 nm to an ultrasmall size of 9.3 nm with varying Y(3+) doping concentration. In an LGY0.4F:Yb,Er UCTB, uniform distribution of all constituent elements was directly confirmed by using high-angle annular dark-field scanning transmission electron microscopy and energy-filtered transmission electron microscopy (EFTEM) image analyses. In particular, existence of activator Er(3+) ions with extremely small quantity was clearly seen over a particle on the EFTEM image. Moreover, the LGY0.4F:Yb,Er UCTBs were successfully incorporated into the polydimethylsiloxane (PDMS) polymer and the highly transparent UCTB-PDMS composites showed bright green light under the excitation of 980 nm infrared light.
    Nanoscale 06/2014; · 6.73 Impact Factor
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    ABSTRACT: The mineralogical constitution of the Earth's mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments in laser-heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to 2400 kelvin, we found that such perovskite is unstable; it loses its Fe and disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure. This observation has implications for enigmatic seismic features beyond ~2000 kilometers depth and suggests that the lower mantle may contain previously unidentified major phases.
    Science (New York, N.Y.). 05/2014; 344(6186):877-82.
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    ABSTRACT: When functional films are cast from colloidal dispersions of semiconductor nanocrystals, the length and structure of the ligands capping their surfaces determine the electronic coupling between the nanocrystals. Long chain oleic acid ligands on the surface of IV–VI semiconductor nanocrystals such as PbSe are typically considered to be insulating. Consequently, these ligands are either removed or replaced with short ones to bring the nanocrystals closer to each other for increased electronic coupling. Herein, using high-angle annular dark-field scanning transmission electron microscopy imaging combined with electron energy loss spectroscopy, we show that partial oxidation of PbSe nanocrystals forms conjugated double bonds within the oleic ligands, which then facilitates enhanced plasmonic interaction among the nanocrystals. The changes in the geometric configurations of the ligands are imaged directly and correlated with the changes in the surface plasmon intensities as they oxidize and undergo structural modifications.
    Chemistry of Materials. 05/2014; 26(10):3328–3333.
  • Microporous and Mesoporous Materials 05/2014; 190:152–155. · 3.37 Impact Factor
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    ABSTRACT: The mechanism of secondary hardening in MP35N (Co–35Ni–20Cr–10Mo) alloy due to exposures at elevated temperatures has been studied. It was observed that short exposure to elevated temperatures increased the ultimate tensile strength and yield stress while decreasing the elongation of MP35N wires. Upon aging at temperatures from 300 to 900 °C the elastic modulus increased although no changes in crystallographic orientation or microstructure were observed. The grain size and major texture components were unchanged following aging. Analytical scanning transmission electron microscope investigation showed that MP35N is hardened by preferential segregation of molybdenum to stacking faults and deformation twins. It also revealed that the concentration of molybdenum segregation was proportional to the amount of initial cold work before aging.
    Acta Materialia 01/2014; 63:63–72. · 3.94 Impact Factor
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    ABSTRACT: By controlling stoichiometry via a hybrid molecular beam epitaxy approach, we report on the study of thin film growth and the electronic transport properties of phase-pure, epitaxial NdTiO3/SrTiO3 heterostructures grown on (001) (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) substrates as a function of cation stoichiometry in NdTiO3. Despite the symmetry mismatch between bulk NdTiO3 and the substrate, NdTiO3 films grew in an atomic layer-by-layer fashion over a range of cation stoichiometry; however amorphous films resulted in cases of extreme cation non-stoichiometry. Temperature-dependent sheet resistance measurements were consistent with Fermi-liquid metallic behavior over a wide temperature range, but revealed a remarkable crossover from metal-to-insulator (M-I) type behavior at low temperatures for all compositions. A direct correlation between cation stoichiometry, transport behavior, and the temperature of M-I transition is established.
    Applied Physics Letters 01/2014; 104(8):082109-082109-5. · 3.52 Impact Factor
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    ABSTRACT: We demonstrate a mechanism of solid-phase crystallization (SPC) induced by the presence of nanoscale cavities at the interface between an hydrogenated amorphous silicon film and embedded Si nanocrystals 30-40 nm in size. The nanocavities have the unique property of an internal surface that is part amorphous and part crystalline, enabling capillarity-driven diffusion from the amorphous to the crystalline domain. The nanocavities propagate rapidly through the amorphous phase, up to five times faster than the SPC growth rate, while "pulling behind" a crystalline tail. Using transmission electron microscopy it is shown that twin boundaries exposed on the crystalline surface accelerate crystal growth and influence nanocavity propagation direction.
    Nano Letters 10/2013; · 13.03 Impact Factor
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    ABSTRACT: Large area thin films of few-layered unfunctionalized graphene platelets are developed with fine control over the thickness. The thin films are obtained by a Langmuir-Blodgett assembly at the interface of graphene solution in N-methyl-2-pyrrolidone (NMP) and water, and their optoelectronic properties and conduction mechanism are investigated in relation to lateral flake size and thin film thickness. The electrical conductivity and carrier mobility are affected by the flake size (200 nm to 1 μm) and by the packing of the nanostructure platelet network. General effective medium theory is used to explain the thickness dependent conductivity and to determine the percolation threshold film thickness which was found to be about 10 nm (at a volume fraction of ∼39%) for a Langmuir-Blodgett film of an average platelet lateral size of 170 ± 40 nm. The electronic behaviour of the material shows more similarities with polycrystalline turbostratic graphite than thin films of reduced graphene oxide, carbon nanotubes, or disordered conducting polymers. While in these systems the conduction mechanism is often dominated by the presence of an energy barrier between conductive and non-conductive regions in the network, in the exfoliated graphene networks the conduction mechanism can be explained by the simple two-band model which is characteristic of polycrystalline graphite.
    Nanoscale 10/2013; · 6.73 Impact Factor
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    ABSTRACT: Despite rapid recent progress, controlled dopant incorporation and attainment of high mobility in thin films of the prototypical complex oxide semiconductor SrTiO3 remain problematic. Here, analytical scanning transmission electron microscopy is used to study the local atomic and electronic structure of Nb-doped SrTiO3 both in ideally substitutionally-doped bulk single crystals, and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric undoped SrTiO3, but are nevertheless insulating. The Nb incorporation in such films was found to be highly inhomogeneous on nanoscopic length-scales, with large quantities of what we deduce to be interstitial Nb. Electron energy loss spectroscopy reveals changes in the electronic density of states in Nb-doped SrTiO3 films compared to undoped SrTiO3, but without the clear shift in the Fermi edge seen in bulk single crystal Nb-doped SrTiO3. Analysis of atomic-resolution annular dark-field images allows us to conclude that the interstitial Nb is in the Nb(0) state, confirming that it is electrically inactive. We argue that this approach should enable future work establishing the vitally needed relationships between synthesis/processing conditions and electronic properties of Nb-doped SrTiO3 thin films.
    ACS Nano 04/2013; · 12.03 Impact Factor
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    ABSTRACT: Cu2ZnSnS4 (CZTS) nanocrystals sterically stabilized with oleic acid and oleylamine ligands and dispersed in nonpolar organic liquids have been extracted into, and electrostatically stabilized in, polar liquids by covering their surfaces with S(2-).
    Chemical Communications 03/2013; · 6.38 Impact Factor
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    ABSTRACT: Atomic impurities are critical for many technologies. They are used to engineer the optical and electronic properties of semiconductors for applications such as transistors, solar cells, light-emitting diodes (LEDs), and lasers, as well as to store energy for applications such as batteries and electrochemical cells. While the characterization and understanding of impurities in bulk semiconductors is well developed, new challenges arise at the nanoscale. In particular, methods are needed to characterize structures that may only contain a few impurity atoms. With such techniques, a fundamental understanding of how atomic impurities affect the properties of semiconductor nanostructures could be more fully developed. In this review, we give a brief introduction to the benefits and challenges associated with the incorporation of impurities in nanoscale structures, a process known as doping. We then focus on techniques used to characterize and image atomic impurities in semiconductor nanostructures. Advances in electron microscopy allow researchers to probe the dynamics of impurity incorporation with in situ transmission electron microscopy (TEM), and techniques such as electron energy loss spectroscopy (EELS) coupled with annular dark-field scanning transmission electron microscopy (ADF-STEM) allow individual atomic impurities in semiconductor nanostructures to be detected and imaged. Likewise, techniques such as atom probe tomography (APT) enable the full atomic reconstruction of nanoscale materials.
    Chemistry of Materials. 03/2013; 25(8):1332–1350.
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    ABSTRACT: Localized surface plasmon resonances (LSPRs) enable tailoring of the optical response of nanomaterials through their free carrier concentration, morphology, and dielectric environment. Recent efforts to expand the spectral range of usable LSPR frequencies into the infrared successfully demonstrated LSPRs in doped semiconductor nanocrystals. Despite silicon's importance for electronic and photonic applications, no LSPRs have been reported for doped silicon nanocrystals. Here we demonstrate doped silicon nanocrystals synthesized via a nonthermal plasma technique that exhibits tunable LSPRs in the energy range of 0.07-0.3 eV or mid-infrared wavenumbers of 600-2500 cm(-1).
    Nano Letters 02/2013; · 13.03 Impact Factor
  • Michael L Odlyzko, K Andre Mkhoyan
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    ABSTRACT: Multislice simulations in the transmission electron microscope (TEM) were used to examine changes in annular-dark-field scanning TEM (ADF-STEM) images, conventional bright-field TEM (BF-CTEM) images, and selected-area electron diffraction (SAED) patterns as atomically thin hexagonal boron nitride (h-BN) samples are tilted up to 500 mrad off of the [0001] zone axis. For monolayer h-BN the contrast of ADF-STEM images and SAED patterns does not change with tilt in this range, while the contrast of BF-CTEM images does change; h-BN multilayer contrast varies strongly with tilt for ADF-STEM imaging, BF-CTEM imaging, and SAED. These results indicate that tilt series analysis in ADF-STEM image mode or SAED mode should permit identification of h-BN monolayers from raw TEM data as well as from quantitative post-processing.
    Microscopy and Microanalysis 06/2012; 18(3):558-67. · 2.50 Impact Factor
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    ABSTRACT: Measurements of sub-micron spheres and pillars of silicon single crystals have exhibited a strain hardening capacity equal to or greater than their metallic counterparts. For diameters ranging from 40 to 400 nm, stress - strain characteristics are reported. Evaluations consisted of nanoindentation based atomic force, scanning, and transmission electron microscopies. Values of strain hardening exponents up to unity in nanospheres are attributed to a size effect variation on the rate of increase of contact area with deformation. A surface - mediated dislocation nucleation concept is shown to be consistent with length scale effects partially modified by geometry as well as size. Proposed, but not proven, the modification relates to the greater constraint in compact spheres as opposed to tall pillars.
    Acta Materialia - ACTA MATER. 04/2012;

Publication Stats

471 Citations
348.32 Total Impact Points

Institutions

  • 2011–2014
    • University of Minnesota Duluth
      Duluth, Minnesota, United States
  • 2013
    • Imperial College London
      • Department of Materials
      Londinium, England, United Kingdom
  • 2010–2012
    • University of Minnesota Twin Cities
      • Department of Chemical Engineering and Materials Science
      Minneapolis, MN, United States
  • 2003–2009
    • Cornell University
      • School of Applied and Engineering Physics
      Ithaca, NY, United States