Fan Sun

Deciphering chemical order/disorder and material properties at the single-atom level

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling ‘real’ materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure–property relationships at the fundamental level.

Enhanced valley splitting in monolayer WSe2 due to magnetic exchange field

Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition-metal dichalcogenide (TMDC) with a broken inversion symmetry possesses two degenerate yet inequivalent valleys, which offers unique opportunities for valley control through the helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest (∼0.2 meV T-1). Here we show greatly enhanced valley spitting in monolayer WSe2, utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5 meV is demonstrated at 1 T by magnetoreflectance measurements and corresponds to an effective exchange field of ∼12 T. Moreover, the splitting follows the magnetization of EuS, a hallmark of the MEF. Utilizing the MEF of a magnetic insulator can induce magnetic order and valley and spin polarization in TMDCs, which may enable valleytronic and quantum-computing applications.

Robust ferromagnetism in Mn-doped MoS2 nanostructures

Layered transition metal dichalcogenides (TMDs) have attracted extensive attention due to their interesting properties originating from an effective honeycomb lattice and strong spin-orbit coupling, and have potential applications in catalysis, lithium batteries, photonic, electronic, and valleytronic devices. Introducing magnetism in the TMDs can lead to the interesting functionalities such as magnetic order and carrier spin polarization with potential applications in spintronics. Here, we demonstrate an effective approach to induce robust ferromagnetism in MoS2 nanostructures by transition metal doping. After doping with a few percent Mn2+, the magnetism of MoS2 nanostructures is enhanced dramatically. Moreover, the magnetic properties are strongly temperature dependent, which is clearly different from the behavior of defect-induced magnetism. Our approach opens up the possibility for tuning the spin and magnetic properties in two-dimensional nanostructures.

Maximizing Specific Loss Power for Magnetic Hyperthermia by Hard–Soft Mixed Ferrites

Maximized specific loss power and intrinsic loss power approaching theoretical limits for alternating-current (AC) magnetic-field heating of nanoparticles are reported. This is achieved by engineering the effective magnetic anisotropy barrier of nanoparticles via alloying of hard and soft ferrites. 22 nm Co0.03 Mn0.28 Fe2.7 O4 /SiO2 nanoparticles reach a specific loss power value of 3417 W g-1metal at a field of 33 kA m-1 and 380 kHz. Biocompatible Zn0.3 Fe2.7 O4 /SiO2 nanoparticles achieve specific loss power of 500 W g-1metal and intrinsic loss power of 26.8 nHm2 kg-1 at field parameters of 7 kA m-1 and 380 kHz, below the clinical safety limit. Magnetic bone cement achieves heating adequate for bone tumor hyperthermia, incorporating an ultralow dosage of just 1 wt% of nanoparticles. In cellular hyperthermia experiments, these nanoparticles demonstrate high cell death rate at low field parameters. Zn0.3 Fe2.7 O4 /SiO2 nanoparticles show cell viabilities above 97% at concentrations up to 500 µg mL-1 within 48 h, suggesting toxicity lower than that of magnetite.

Signature of structural distortion in optical spectra of YFe2O4 thin film

We report structural, optical, and electro-optical properties of polycrystalline YFe2O4 thin films, deposited on (0001) sapphire substrates using the electron-beam deposition technique. The optical spectra of a 120 nm YFe2O4 show Fe d to d on-site and O 2p to Fe 3d, Y 4d, and Y 5s charge-transfer electronic excitations. Anomalies in the temperature dependence data of the charge-transfer excitations and the splitting of the 4.46 eV charge-transfer peak strongly suggest a structural distortion at 180 ± 10 K. Evidence of such a structural distortion is also manifested in the surface resistance versus temperature data. In addition, the YFe2O4 thin film at low temperatures shows strong electro-optical properties, as high as 9% in the energy range of 1 - 2.5 eV, for applied electric fields up to 500 V.cm−1.

Ferromagnetic Cr2Te3 nanorods with ultrahigh coercivity

Ferromagnetic Cr2Te3 nanorods were synthesized by a one-pot high-temperature organic-solution-phase method. The crystalline phases and magnetic properties can be systematically tuned by varying the molar ratio of the Cr and Te precursors. A magnetically hard phase, identified as chemically ordered Cr2Te3, is the dominating one at the precursor ratio between Cr : Te = 1 : 1.2 and 1 : 1.8. A magnetically soft phase, attributed to chemical disorder due to composition inhomogeneity and stacking faults, is present under either Cr-rich or Te-rich synthesis conditions. A large coercivity of 9.6 kOe is obtained for a Cr : Te precursor ratio of 1 : 1.8, which is attributed to the large magnetocrystalline anisotropy of ordered Cr2Te3 nanorods, and verified by density-functional theory calculations. The hard and soft phases sharing coherent interfaces co-exist in a seemingly single-crystalline nanorod, showing an unusual transition from exchange-coupled behavior at higher temperatures to two-phase behavior as the temperature is lowered.

Corn-like, recoverable γ-Fe 2 O 3 @SiO 2 @TiO 2 photocatalyst induced by magnetic dipole interactions

Corn-like, γ-Fe2O3@SiO2@TiO2 core/shell heterostructures were synthesized by a modified solvothermal reduction combined with a sol-gel method. SiO2 shells were first deposited on monodisperse Fe3O4 microspheres by a sol-gel method. Fe3O4@SiO2@TiO2 corn-like heterostructures were then obtained by sequential TiO2 coating, during which the magnetic dipolar interactions induced the anisotropic self-assembly process. After annealing at 350 °C, the crystalized TiO2 enhanced photocatalytic activity, while Fe3O4 was converted to γ-Fe2O3. The corn-like γ-Fe2O3@SiO2@TiO2 photocatalyst can be recycled and reused by magnet extraction. Despite the photocatalytic activity decreased with each cycle, it can be completely recovered by moderate heating at 200 °C.

Atomic Electron Tomography: Probing 3D Structure and Material Properties at the Single-Atom Level

1. Dept. of Physics and Astronomy and California NanoSystems Institute, UCLA, CA, USA. 2. Dept. of Physics, National Sun Yat-sen University, Kaohsiung, Taiwan. 3. NCEM, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 4. Dept. of Physics, University at Buffalo, the State University of New York, Buffalo, NY, USA. 5. Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, UK. 6. National Center for Computational Sciences, ORNL, Oak Ridge, TN, USA. 7. Computer Science and Mathematics Division, ORNL, Oak Ridge, TN, USA. 8. Center for Nanophase Materials Sciences, ORNL, Oak Ridge, TN, USA. 9. Dept. of Physics, University of Nebraska at Omaha, Omaha, NE, USA.