Louisa Meshi

Defect reduction in GaN/(0001)sapphire films grown by molecular beam epitaxy using nanocolumn intermediate layers

Transmission and scanning electron microscopies are used to examine the epitaxial lateral overgrowth of GaN on GaN nanocolumns grown on AlN/(0001)sapphire by molecular beam epitaxy. Initially, N-rich growth gave a bimodal morphology consisting of defect-free Ga-polar nanocolumns emanating from a compact, highly defective N-polar layer. Under subsequent Ga-rich conditions, the nanocolumns grew laterally to produce continuous Ga-polar overlayers. Threading dislocation (TD) densities in the overlayer were in the range of 108–109cm−2, up to two orders of magnitude less than in the N-polar underlayer. It is proposed that the change in polarity is a key factor controlling the reduction in TD density.

New nanocrystalline materials: a previously unknown simple cubic phase in the SnS binary system.

We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (a = 11.7 Å, P2(1)3) was deduced and found to be associated with the rocksalt-type structure. Kramers-Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS.

Direct Imaging of the Ligand Monolayer on an Anion-Protected Metal Nanoparticle through Cryogenic Trapping of its Solution-State Structure

According to Derjaguin−Landau−Verwey−Overbeek (DLVO) theory, anions stabilize solutions of metal(0) nanoparticles by binding to the metal surface. However, the structure of the metal−anion interface in these colloidal systems has eluded direct observation. To address this, we deployed a 1.2-nm sized heteropolytungstate cluster anion, α-AlW11O399− (1), which features 11 W atoms (Z = 183) for effective imaging by electron microscopy, and a high negative charge to enhance binding to a prototypical Ag(0) nanoparticle. Cryogenic methods were then used to “trap” the 1-protected Ag(0) nanoparticles at liquid-N2 temperatures within a water−glass matrix. The “solution-state” structures thus captured provide the first reported TEM images of self-assembled monolayers (SAMs) of anions on a colloidal metal(0) nanoparticle.

Assembly of mesoscale helices with near-unity enantiomeric excess and light-matter interactions for chiral semiconductors

Mesoscale CdTe helices with near-unity enantiomeric excess offer insight into design rules for chiroptical semiconductor materials. Semiconductors with chiral geometries at the nanoscale and mesoscale provide a rich materials platform for polarization optics, photocatalysis, and biomimetics. Unlike metallic and organic optical materials, the relationship between the geometry of chiral semiconductors and their chiroptical properties remains, however, vague. Homochiral ensembles of semiconductor helices with defined geometries open the road to understanding complex relationships between geometrical parameters and chiroptical properties of semiconductor materials. We show that semiconductor helices can be prepared with an absolute yield of ca 0.1% and an enantiomeric excess (e.e.) of 98% or above from cysteine-stabilized cadmium telluride nanoparticles (CdTe NPs) dispersed in methanol. This high e.e. for a spontaneously occurring chemical process is attributed to chiral self-sorting based on the thermodynamic preference of NPs to assemble with those of the same handedness. The dispersions of homochiral self-assembled helices display broadband visible and near-infrared (Vis-NIR) polarization rotation with anisotropy (g) factors approaching 0.01. Calculated circular dichroism (CD) spectra accurately reproduced experimental CD spectra and gave experimentally validated spectral predictions for different geometrical parameters enabling de novo design of chiroptical semiconductor materials. Unlike metallic, ceramic, and polymeric helices that serve predominantly as scatterers, chiroptical properties of semiconductor helices have nearly equal contribution of light absorption and scattering, which is essential for device-oriented, field-driven light modulation. Deconstruction of a helix into a series of nanorods provides a simple model for the light-matter interaction and chiroptical activity of helices. This study creates a framework for further development of polarization-based optics toward biomedical applications, telecommunications, and hyperspectral imaging.

Defect-controlled growth of GaN nanorods on (0001)sapphire by molecular beam epitaxy

Transmission electron microscopy is used to reveal threading defects in single crystal c-oriented GaN nanorods grown on (0001)sapphire by molecular beam epitaxy. The defects are shown to be planar faults lying on {101¯0} planes and bounded by opposite partial screw dislocations with Burgers vectors of 1/2⟨0001⟩. The faults nucleate, as dislocation half-loops, from points close to the GaN/(0001)sapphire interface. It is proposed that the spiral growth of the partial atomic step joining the emerging dislocations controls nanorod growth and accounts for the growth surface morphology. The significance of these defects for nanorod growth and applications is discussed.

Size-dependent spin state and ferromagnetism in La0.8Ca0.2CoO3 nanoparticles

Magnetic and structural properties of nanocrystalline low-doped La0.8Ca0.2CoO3 cobaltites with particle size of 8, 13, 23, and 50 nm, prepared by the glycine-nitrate method, were investigated in temperature range 5–320 K, magnetic field up to 50 kOe and under hydrostatic pressure up to 10 kbar. With particle downsizing, a noticeable expansion of unit cell, with concomitant changes in the rhombohedral structure toward the cubic one was observed. It was found that the increased surface-disorder effect strongly suppresses the ferromagnetic state in La0.8Ca0.2CoO3 nanoparticles leading to a decrease, by factor of about 2, both in spontaneous magnetization, MS, and Curie temperature, TC, when particle’s size decreases from 23 to 8 nm. The effective magnetic moment μeff was found also to decrease distinctly due to the strong interdependence between Co–O–Co interactions and Co spin state. The size-induced magnetic disorder drives the La0.8Ca0.2CoO3 nanoparticles to a dominant glassy behavior for 8 nm particles. This is evidenced by the fact that the freezing temperature varies with magnetic field in a strict conformity with the de Almeida–Thouless law for spin glasses and also by the observation of characteristic slowing down in the spin dynamics. The applied pressure suppresses TC, MS, and coercive field HC, like it is observed for bulk La0.8Ca0.2CoO3. Nevertheless, in nanoparticles the pressure effect on TC is noticeably stronger, while HC diminishes with pressure much slower then in bulk material.

Identification of the structure of a new Al-U-Fe phase by electron microdiffraction technique

The particles of an unknown intermetallic phase with the approximate composition Al10Fe2U were observed in a ternary Al–Fe–U alloy. The structure of this phase was investigated in a transmission electron microscope using a microdiffraction technique based on analysis of the symmetry and relative positions of reflections in the zero-order and high-order Laue zones. The phase has an orthorhombic C-centered unit cell with lattice parameters a=8.900, b=10.190 and c=8.993 A; its crystal symmetry can be described by the Cmcm space group.

Electrochemical Intercalation of Lithium Ions into NbSe2 Nanosheets

Transition metal dichalcogenides (TMDCs) have been known for decades to have unique properties and recently attracted broad attention for their two-dimensional (2D) characteristics. NbSe2 is a metallic TMDC that has been studied for its charge density wave transition behavior and superconductivity but is still largely unexplored for its potential use in engineered devices with applications in areas such as electronics, optics, and batteries. Thus, we successfully demonstrate and present evidence of lithium intercalation in NbSe2 as a technique capable of modifying the material properties of NbSe2 for further study. We demonstrate successful intercalation of Li ions into NbSe2 and confirm this result through X-ray diffraction, noting a unit cell size increase from 12.57 to 13.57 Å in the c lattice parameter of the NbSe2 after intercalation. We also fabricate planar half-cell electrochemical devices using ultrathin NbSe2 from platelets to observe evidence of Li-ion intercalation through an increase in the optical transmittance of the material in the visible range. Using 550 nm wavelength light, we observed an increase in optical transmittance of 26% during electrochemical intercalation.

New ordered phase in the quasi-binary UAl3–USi3 system

The industrial importance of the U-Al-Si system stems from the fact that during processing the Al-based alloy (containing Si as impurity), used for the cladding of U (fuel in nuclear reactors), undergoes heat treatment which stimulates diffusion between the fuel and the cladding. One of the possible ways to represent the ternary U-Al-Si phase diagram is the construction of an UAl3-USi3 quasi-binary phase diagram. On the one hand, since the UAl3 and USi3 phases are isostructural, an isomorphous phase diagram is expected; on the other hand, some researchers observed a miscibility gap at lower temperatures. During our study of the UAl3-USi3 quasi-binary phase diagram, a new stable U(Alx,Si1 - x)3 phase was identified. The structure of this phase was determined, using a combination of electron crystallography and powder X-ray diffraction methods, as tetragonal [I4/mmm (No.139) space group], with lattice parameters a = b = 8.347 (1), c = 16.808 (96) Å. Its unit cell has 64 atoms and it can be described as an ordered variant of the U(Al,Si)3 solid solution. A Bärnighausen tree was constructed using the original U(Al,Si)3 structure as an aristotype.

Uniting electron crystallography and powder diffraction

Preface.- Part 1 - Powder Diffraction.- Powder Diffraction: By Decades W.I.F. David.- Rietveld Refinement P. W.Stephens.- Structure Solution - an Overview L. B. McCusker, Ch. Baerlocher.- Inorganic Materials R. Cerny.- Organic Compounds K. Shankland.- Laboratory X-ray Diffraction P. Whitfield.- Synchrotron X-Ray Powder Diffraction F. Gozzo.- Ultra Fast Powder Diffraction A. Fitch, C. Curfs.- Taking it to Extremes - Powder Diffraction at Elevated Pressures D. I. A. Millar, C. R. Pulham.-Structure Solution by Charge Flipping L. Palatinus.- Structure Solution: Global Optimisation Methods K. Shankland.- Proteins and Powders: Technical Developments J. P. Wright.- Proteins and Powders: An Overview I. Margiolaki.- Parametric Powder Diffraction W.I.F. David.- Powder Diffraction + Computational Methods L. Smrcok.- Information on Imperfections M. Leoni.- Pair Distribution Function Technique: Principles and Methods S. J. L. Billinge.- Debye Analysis Y. G. Andreev.- Quantitative Phase Analysis I. C. Madsen et al.- Quantifying amorphous phases A. Kern et al.- Quantitative phase analysis: method developments L. Lutterotti.- Texture - an Overview R.B. Von Dreele.- The future of powder diffraction is 2-D R. Dinnebier.- Part 2 - Electron crystallography.- Electron Crystallography - New methods to explore Structure and Properties of the Nano World U. Kolb.- Image formation in the Electron Microscope L.Meshi.- Models for Precession Electron Diffraction L. D. Marks.- Structure Solution using HRTEM S. Hovmoller.- Combination of X-ray Powder Diffraction, Electron Diffraction and HRTEM data Ch. Baerlocher, L. B. McCusker.- Automated Electron Diffraction Tomography U. Kolb.- Automated Quantitative 3D Electron Diffraction Rotation Tomography P. Oleynikov.- Introduction to ADT/ADT3D T. E. Gorelik et al.- Electrostatic Potential determined from Electron Diffraction Data A. Avilov.- Domino Phase Retrieval Algorithm for Structure Solution F. N. Chukhovskii.- LARBED: Exploring the 4th dimension in Electron Diffraction Ch. T. Koch.- Shadow Imaging for Charge Distribution Analysis Y. Zhu.- Electron Diffraction of Protein 3D Nanocrystals J. P. Abrahams et al.- Parallel-beam Diffraction and Direct Imaging in an aberration-corrected STEM O. L. Krivanek.-Electron diffraction of commensurately and incommensurately modulated Materials J. Hadermann, A. M. Abakumov.- Detection of Magnetic Circular Dichroism using TEM and EELS S. Rubino et al.- Index.-