Je-Geun Park

Large-Scale Synthesis of Uniform and Extremely Small-Sized Iron Oxide Nanoparticles for High-Resolution T1 Magnetic Resonance Imaging Contrast Agents

Uniform and extremely small-sized iron oxide nanoparticles (ESIONs) of < 4 nm were synthesized via the thermal decomposition of iron-oleate complex in the presence of oleyl alcohol. Oleyl alcohol lowered the reaction temperature by reducing iron-oleate complex, resulting in the production of small-sized nanoparticles. XRD pattern of 3 nm-sized nanoparticles revealed maghemite crystal structure. These nanoparticles exhibited very low magnetization derived from the spin-canting effect. The hydrophobic nanoparticles can be easily transformed to water-dispersible and biocompatible nanoparticles by capping with the poly(ethylene glycol)-derivatized phosphine oxide (PO-PEG) ligands. Toxic response was not observed with Fe concentration up to 100 μg/mL in MTT cell proliferation assay of POPEG-capped 3 nm-sized iron oxide nanoparticles. The 3 nm-sized nanoparticles exhibited a high r(1) relaxivity of 4.78 mM(-1) s(-1) and low r(2)/r(1) ratio of 6.12, demonstrating that ESIONs can be efficient T(1) contrast agents. The high r(1) relaxivities of ESIONs can be attributed to the large number of surface Fe(3+) ions with 5 unpaired valence electrons. In the in vivo T(1)-weighted magnetic resonance imaging (MRI), ESIONs showed longer circulation time than the clinically used gadolinium complex-based contrast agent, enabling high-resolution imaging. High-resolution blood pool MR imaging using ESIONs enabled clear observation of various blood vessels with sizes down to 0.2 mm. These results demonstrate the potential of ESIONs as T(1) MRI contrast agents in clinical settings.

Nanoparticulate Iron Oxide Tubes from Microporous Organic Nanotubes as Stable Anode Materials for Lithium Ion Batteries

During the last several decades, diverse porous materials have been prepared for a wide range of applications, such as adsorbents, gas storage materials, and solid supports for catalytic materials. These materials can be classified into three groups according to their components: inorganic materials, metal–organic composites, and purely organic systems. Among these porous materials, organic porous materials have recently attracted special attention because of their low densities and robustness. The accumulated organic synthetic methods can also be easily applied for the designed synthesis of organic porous materials with tailored functionalites. Thus, in a short period, diverse microporous organic networks have been prepared through diverse C C bond-forming reactions. In the synthesis of porous organic networks, the rigid building blocks are chosen so that the connection of these building blocks through covalent bonds induces the intrinsic porosity of the materials. Related studies have focused on the inner porosity and the resultant high surface area of materials. However, porous organic systems with well-defined outer shapes are rare. In particular, the template-free synthesis of hollow organic materials is quite rare. It is noteworthy that in the synthesis of secondary target inorganic materials using porous materials, the organic templates can be easily removed by combustion in air. In these cases, the outer shapes of materials along with their inner porosity are very critical for obtaining well-defined materials. Moreover, inorganic materials with a particulate surface could be obtained from the microporosity of organic network. Recently, Cooper and others have shown that Sonogashira coupling between alkynes and arylhalides is a very efficient method for the preparation of microporous organic materials. The resultant materials themselves showed promising gas-adsorption capacities. It can be expected that more diverse functional sites can be introduced into materials by designing the organic building blocks. During our trials for introduction of viologen groups into microporous organic materials, we observed the unexpected formation of microporous organic nanotubes. Herein, we present the preparation of microporous organic nanotubes and the template synthesis of iron oxide nanotubes with particulate walls and their application as anode materials for high-performance lithium ion batteries. Figure 1a shows the synthesis of microporous organic nanotubes (MONTs). For preparation of the MONT, two building blocks, N,N’-di(4-iodophenyl)-4,4’-bipyridinium dichloride (2 equiv) and tetra(4-ethynylphenyl)methane (1 equiv) were dissolved in a 3:2:2 mixture of toluene, methanol, and triethylamine. After adding catalytic amount of bis(triphenylphosphine)palladium dichloride and copper iodide, the reaction mixture was heated at 90 8C for 72 h to form precipitates. After cooling to room temperature, the solid was retrieved by centrifugation and washed with excess dimethyl sulfoxide, methanol, dichloromethane, and diethyl ether. The resultant materials were dried under a vacuum for a day. The obtained precipitates were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in typical SEM images(Figure 1b), the obtained materials have a 1D character with mild ending-parts. Interestingly, a careful investigation of the materials by TEM revealed the hollow inner space and dark-contrasted walls (Figure 1c–e; Supporting Information, Figure S1). The average diameter and thickness of the wall of MONT were (92 19) nm and (31 4) nm, respectively. Brunauer– Emmett–Teller (BET) analysis showed the microporous character of materials with type I N2 isotherm at 77 K, 765.0 mg 1 surface area, and 1.01 cmg 1 pore volume (P/P0=0.995; Figure 2a). Powder X-ray diffraction (PXRD) studies revealed the amorphous character of MONT that has been observed previously (Supporting Information, Figure S2). Thermogravimetric analysis (TGA) of the materials showed that they are stable up to 205 8C and then slowly decomposed at a higher temperature (Figure 2b). Solidstate C NMR spectroscopy showed signals at d= 62 ppm, d= 90 ppm, and d= 120–160 ppm for benzyl, alkyne, and aryl groups, respectively (Figure 2c). Elemental analysis of mate[*] N. Kang, Dr. J. H. Park, J. Choi, J. Jin, J. Chun, Dr. I. G. Jung, Prof. S. U. Son Department of Chemistry and Department of Energy Science Sungkyunkwan University, Suwon 440-746 (Korea) E-mail: sson@skku.edu

Ising-Type Magnetic Ordering in Atomically Thin FePS3

Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order-disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with TN ∼ 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering.

Size Dependence of Metal–Insulator Transition in Stoichiometric Fe3O4 Nanocrystals

Magnetite (Fe3O4) is one of the most actively studied materials with a famous metal-insulator transition (MIT), so-called the Verwey transition at around 123 K. Despite the recent progress in synthesis and characterization of Fe3O4 nanocrystals (NCs), it is still an open question how the Verwey transition changes on a nanometer scale. We herein report the systematic studies on size dependence of the Verwey transition of stoichiometric Fe3O4 NCs. We have successfully synthesized stoichiometric and uniform-sized Fe3O4 NCs with sizes ranging from 5 to 100 nm. These stoichiometric Fe3O4 NCs show the Verwey transition when they are characterized by conductance, magnetization, cryo-XRD, and heat capacity measurements. The Verwey transition is weakly size-dependent and becomes suppressed in NCs smaller than 20 nm before disappearing completely for less than 6 nm, which is a clear, yet highly interesting indication of a size effect of this well-known phenomena. Our current work will shed new light on this ages-old problem of Verwey transition.

Exfoliation and Raman Spectroscopic Fingerprint of Few-Layer NiPS3 Van der Waals Crystals

The range of mechanically cleavable Van der Waals crystals covers materials with diverse physical and chemical properties. However, very few of these materials exhibit magnetism or magnetic order, and thus the provision of cleavable magnetic compounds would supply invaluable building blocks for the design of heterostructures assembled from Van der Waals crystals. Here we report the first successful isolation of monolayer and few-layer samples of the compound nickel phosphorus trisulfide (NiPS3) by mechanical exfoliation. This material belongs to the class of transition metal phosphorus trisulfides (MPS3), several of which exhibit antiferromagnetic order at low temperature, and which have not been reported in the form of ultrathin sheets so far. We establish layer numbers by optical bright field microscopy and atomic force microscopy, and perform a detailed Raman spectroscopic characterization of bilayer and thicker NiPS3 flakes. Raman spectral features are strong functions of excitation wavelength and sample thickness, highlighting the important role of interlayer coupling. Furthermore, our observations provide a spectral fingerprint for distinct layer numbers, allowing us to establish a sensitive and convenient means for layer number determination.

Negative magnetostrictive magnetoelectric coupling of BiFeO3

How magnetoelectric coupling actually occurs on a microscopic level in multiferroic BiFeO3 is not well known. By using high-resolution single crystal neutron diffraction techniques, we have determined the electric polarization of each individual element of BiFeO3, and concluded that magnetostrictive coupling suppresses the electric polarization at the Fe site below T-N. This negative magnetoelectric coupling appears to outweigh the spin current contributions arising from the cycloid spin structure, which should produce positive magnetoelectric coupling.

Temperature-Dependent Interplay of Dzyaloshinskii-Moriya Interaction and Single-Ion Anisotropy in Multiferroic BiFeO 3

Low-energy magnon excitations in multiferroic BiFeO3 were measured in detail as a function of temperature around several Brillouin zone centers by inelastic neutron scattering experiments on single crystals. Unique features around 1 meV are directly associated with the interplay of the Dzyaloshinskii-Moriya interaction and a small single-ion anisotropy. The temperature dependence of these and the exchange interactions were determined by fitting the measured magnon dispersion with spin-wave calculations. The spectra best fit an easy-axis type magnetic anisotropy and the deduced exchange and anisotropy parameters enable us to determine the anharmonicity of the magnetic cycloid. We then draw a direct connection between the changes in the parameters of spin Hamiltonian with temperature and the physical properties and structural deformations of BiFeO3.

Antiferromagnetic ordering in Li2MnO3 single crystals with a two-dimensional honeycomb lattice

Li(2)MnO(3) consists of a layered Mn honeycomb lattice separated by a single layer of LiO(6) octahedra along the c-axis. By using single crystal Li(2)MnO(3) samples, we have examined the physical properties and carried out both powder and single crystal neutron diffraction studies to determine that Mn moments order antiferromagnetically at T(N) = 36 K with an ordered magnetic moment of 2.3 μ(B) perpendicular to the ab plane. We have also discovered that about 35% of the full magnetic entropy is released in the supposedly simple paramagnetic phase, indicative of unusual spin dynamics at higher temperature.

Opportunities and challenges of 2D magnetic van der Waals materials: magnetic graphene?

There has been a huge increase of interests in two-dimensional van der Waals materials over the past ten years or so with the conspicuous absence of one particular class of materials: magnetic van der Waals systems. In this Viewpoint, we point it out and illustrate how we might be able to benefit from exploring these so-far neglected materials.

Localized character of 4f electrons in CeRhx (x=2,3) and CeNix (x=2,5).

We have measured Ce 4f spectral weights of extremely alpha-like Ce transition metal intermetallic compounds CeRhx (x=2,3) and CeNix (x=2,5) by using the bulk-sensitive resonant photoemission technique at the Ce M5(3d(5/2)-->4f) edge. High energy resolution and longer escape depth of photoemitted electron at this photon energy enabled us to distinguish the sharp Kondo resonance tails at the Fermi level, which can be well described by the Gunnarsson-Schönhammer calculation based on the Anderson impurity Hamiltonian. On the other hand, the itinerant 4f band description shows big discrepancies, which implies that Ce 4f electrons retain localized characters even in extremely alpha-like compounds.