E. Lochner

Interdiffusion study of magnesium in magnetite thin films grown on magnesium oxide (001) substrates

Magnetite (Fe3O4) films and multilayers were grown using plasma-assisted molecular beam epitaxy and result in single-phase films grown in registry with a MgO substrate. No evidence of interdiffusion is detected on as-grown films. Both structural and magnetic probes indicate behaviors expected for a magnetite thin film. A thermal stability study of these films was performed by annealing these films under ultrahigh vacuum conditions at temperatures below 900 K. Bulk techniques such as x-ray diffraction, superconducting quantum interference device magnetometry, and energy dispersive spectroscopy confirm that the magnesium interdiffuses throughout the entire film, and surface techniques such as x-ray photoelectron spectroscopy and scanning tunneling microscopy/ion scattering spectroscopy show changes in the surface structure and stoichiometry of the film caused by the magnesium intermixing.

Aqueous Growth of Gold Clusters with Tunable Fluorescence Using Photochemically Modified Lipoic Acid-Based Ligands

We report a one-phase aqueous growth of fluorescent gold nanoclusters (AuNCs) with tunable emission in the visible spectrum, using a ligand scaffold that is made of poly(ethylene glycol) segment appended with a metal coordinating lipoic acid at one end and a functional group at the other end. This synthetic scheme exploits the ability of the UV-induced photochemical transformation of LA-based ligands to provide DHLA and other thiol byproducts that exhibit great affinity to metal nanoparticles, obviating the need for chemical reduction of the dithiolane ring using classical reducing agents. The influence of various experimental conditions, including the photoirradiation time, gold precursor-to-ligand molar ratios, time of reaction, temperature, and the medium pH, on the growth of AuNCs has been systematically investigated. The photophysical properties, size, and structural characterization were carried out using UV-vis absorption and fluorescence spectroscopy, TEM, DOSY-NMR, and X-ray photoelectron spectroscopy. The hydrodynamic size (RH) obtained by DOSY-NMR indicates that the size of these clusters follows the trend anticipated from the absorption and PL data, with RH(red) > RH(yellow) > RH(blue). The tunable emission and size of these gold nanoclusters combined with their high biocompatibility would make them greatly promising for potential use in imaging and sensing applications.

Crosslinking of extracellular matrix scaffolds derived from pluripotent stem cell aggregates modulates neural differentiation

UNLABELLED At various developmental stages, pluripotent stem cells (PSCs) and their progeny secrete a large amount of extracellular matrices (ECMs) which could interact with regulatory growth factors to modulate stem cell lineage commitment. ECMs derived from PSC can be used as unique scaffolds that provide broad signaling capacities to mediate cellular differentiation. However, the rapid degradation of ECMs can impact their applications as the scaffolds for in vitro cell expansion and in vivo transplantation. To address this issue, this study investigated the effects of crosslinking on the ECMs derived from embryonic stem cells (ESCs) and the regulatory capacity of the crosslinked ECMs on the proliferation and differentiation of reseeded ESC-derived neural progenitor cells (NPCs). To create different biological cues, undifferentiated aggregates, spontaneous embryoid bodies, and ESC-derived NPC aggregates were decellularized. The derived ECMs were crosslinked using genipin or glutaraldehyde to enhance the scaffold stability. ESC-derived NPC aggregates were reseeded on different ECM scaffolds and differential cellular compositions of neural progenitors, neurons, and glial cells were observed. The results indicate that ESC-derived ECM scaffolds affect neural differentiation through intrinsic biological cues and biophysical properties. These scaffolds have potential for in vitro cell culture and in vivo tissue regeneration study. STATEMENT OF SIGNIFICANCE Dynamic interactions of acellular extracellular matrices and stem cells are critical for lineage-specific commitment and tissue regeneration. Understanding the synergistic effects of biochemical, biological, and biophysical properties of acellular matrices would facilitate scaffold design and the functional regulation of stem cells. The present study assessed the influence of crosslinked embryonic stem cell-derived extracellular matrix on neural differentiation and revealed the synergistic interactions of various matrix properties. While embryonic stem cell-derived matrices have been assessed as tissue engineering scaffolds, the impact of crosslinking on the embryonic stem cell-derived matrices to modulate neural differentiation has not been studied. The results from this study provide novel knowledge on the interface of embryonic stem cell-derived extracellular matrix and neural aggregates. The findings reported in this manuscript are significant for stem cell differentiation toward the applications in stem cell-based drug screening, disease modeling, and cell therapies.

Robust Manipulation of Magnetism in Dilute Magnetic Semiconductor (Ga,Mn)As by Organic Molecules.

We firstly demonstrated the organic molecular manipulation of the magnetism of (Ga, Mn)As. Mn-doped GaAs thin films with various thicknesses were grown by low-temperature molecular-beam epitaxy (LT-MBE), and organic charge-transfer molecules were deposited on the surface of (Ga, Mn)As films by either solution-based self-assembly or vacuum thermal evaporation, which led to large carrier density modulation, and significant changes in the Curie temperature (Tc) and magnetization (Ms). Electron donor (acceptor) molecules were found to decrease (increase) both Tc and Ms. Moreover, through proper preparation of the (Ga, Mn)As surface, self-assembled monolayer (SAM) patterns of organic molecules with sub-75 nanometer line width were successfully created via dip-pen nanolithography (DPN). These results could open a new pathway to control nano-scale manipulation of magnetism in DMS, with potential applications in reconfigurable, non-volatile and hybrid molecular nano-spintronics.

Magnetic structure determination for Fe3O4/NiO superlattices

Neutron diffraction measurements reveal the nature of the magnetic structure in Fe3O4/NiO superlattices grown by molecular beam epitaxy. Taking advantage of differences between the Fe3O4 and NiO crystalline symmetries, we have determined independently the magnetic order parameters of the bilayer components. The NiO antiferromagnetic order propagates coherently through several superlattice bilayers, while the magnetic coherence of the ferrimagnetic Fe3O4 is restricted to a single interlayer due to the random stacking of the spinel unit cells at the interfaces. A model for the diffraction data, based upon a Hendricks–Teller description of the interfacial disorder, demonstrates that the observed broadening of selected reflections originates directly from these stacking faults.

Investigations of the interplay between crystalline and magnetic ordering in Fe3O4/NiO superlattices

Using SQUID magnetometry and both x‐ray‐ and neutron‐diffraction techniques, we have studied the structural and magnetic ordering of a series of Fe3O4/NiO superlattices grown by MBE. X‐ray diffraction reveals that the superlattices are coherent, single phase crystals with narrow interfaces. Symmetry differences between the Fe3O4 spinel and NiO rocksalt structures lead to interfacial stacking faults, manifested in some diffraction intensities. Analysis of the neutron‐diffraction spectra show that the NiO antiferromagnetic ordering is coherent through several superlattice bilayers, while the Fe3O4 magnetic ordering is confined to individual interlayers by stacking faults in all superlattices but those with thinnest (≤10 A) NiO interlayers. Neutron diffraction and SQUID magnetometry have been used to study the Fe3O4 Verwey phase transition in thin‐layered superlattices. The charge ordering in superlattices such as [Fe3O4 (75 A)‖NiO (9 A)]500, below the Verwey transition, directly observable in (4, 0, 1/2) ne...

Magnetic ordering in layered oxide structures: Fe3O4 thin films and Fe3O4/NiO superlattices

Abstract We report magnetic ordering studies of iron oxide and nickel oxide layered structures using SQUID magnetometry and neutron and X-ray diffraction techniques. This work focuses on the influence of interlayer coupling on the Neel ordering in the NiO layers and the Verwey ordering in the Fe 3 O 4 layers, and well as field dependence of moments in each layer.

Magnetic and crystallographic properties of molecular beam epitaxially grown Fe3O4/NiO superlattices and Fe3O4 films

Ferromagnetic resonance, SQUID magnetometry, and x‐ray diffraction have been used to characterize a set of [Fe3O4(68 A)/NiO(17 A)]N superlattices (SL) with N=3, 10, 30, and 100, as well as a 1.5‐μm‐thick Fe3O4 film. For this NiO thickness, Fe3O4 layers are strongly coupled and the in‐plane anisotropy is much less than the 330‐Oe ferromagnetic resonance (FMR) linewidth at 35 GHz. Both in‐plane and perpendicular FMR at 9.5 and 35 GHz have been used, with the 9.5‐GHz data showing significant hysteresis associated with the sample magnetization. X‐ray diffraction indicates that both the film and SL’s are nearly cubic single‐crystalline structures with long‐range coherence. The 300 K magnetization data indicate the presence of small cubic anisotropy in the SL’s, although bulklike Fe3O4 magnetic ordering in the thick single film. When the Fe3O4 film is cooled below the Verwey transition in a 10 kOe field (aligned along 〈100〉), the FMR shows that the sample develops a large uniaxial (Ku=1.8 kOe) in‐plane anisotro...

Magnesium outdiffusion through magnetite films grown on magnesium oxide (001) (abstract)

Scanning tunneling microscopy (STM) studies of 1 μm thick films of single crystalline Fe3O4 grown on MgO(001) indicate that repeated annealing of the sample in UHV causes Mg diffusion through the Fe3O4 film. The onset of this effect was clearly seen by STM at room temperature for samples raised above 400–430 °C. It appears that the annealing process causes the migration of Mg from the substrate entirely through the Fe3O4 lattice, and that the migration tends to fill the surface layer first, with lower layers filling as anneal time is increased. Upon detection of this effect, several complementary sample analysis techniques were employed to determine the extent of the changes observed. X-ray diffraction studies indicate shifts in the lattice constant from the cubic constant of magnetite, Fe3O4, (8.396 A), which is already strained in thin-film growth on a substrate, further toward the cubic lattice constant of magnesioferrite, MgFe2O4, (8.375 A) in order to accommodate the Mg that has migrated to the surfa...

Robust manipulation of magnetism in dilute magnetic semiconductor (Ga, Mn)As by organic molecules

We firstly demonstrated the organic molecular manipulation of the magnetism of (Ga, Mn)As. Mn-doped GaAs thin films with various thicknesses were grown by low-temperature molecular-beam epitaxy (LT-MBE), and organic charge-transfer molecules were deposited on the surface of (Ga, Mn)As films by either solution-based self-assembly or vacuum thermal evaporation, which led to large carrier density modulation, and significant changes in the Curie temperature (T c ) and magnetization (M s ). Electron donor (acceptor) molecules were found to decrease (increase) both T c and M s . Moreover, through proper preparation of the (Ga, Mn)As surface, self-assembled monolayer (SAM) patterns of organic molecules with sub-75 nanometer line width were successfully created via dip-pen nanolithography (DPN). These results could open a new pathway to control nano-scale manipulation of magnetism in DMS, with potential applications in reconfigurable, non-volatile and hybrid molecular nano-spintronics.