Zheng Gai

Room-Temperature Multiferroic Hexagonal LuFeO3 Films

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO(3) films are room-temperature multiferroics.

Multi-functional core-shell hybrid nanogels for pH-dependent magnetic manipulation, fluorescent pH-sensing, and drug delivery.

Remotely optical sensing and drug delivery using an environmentally-guided magnetically-driven hybrid nanogel particle could allow for medical diagnostics and treatment. Such multifunctional hybrid nanogels (<200 nm) were prepared through the first synthesis of magnetic Ni NPs, followed by a moderate growth of fluorescent metallic Ag on the surface of Ni NPs, and then a coverage of a pH-responsive copolymer gel shell of poly(ethylene glycol-co-methacrylic acid) [p(EG-MAA)] onto the Ni-Ag bimetallic NP cores (18 ± 5 nm). The introduction of the pH-responsive p(EG-MAA) gel shell onto the magnetic and fluorescent Ni-Ag NPs makes the polymer-bound Ni-Ag NPs responsive to pH over the physiologically important range 5.0-7.4. The hybrid nanogels can adapt to surrounding pH and regulate the sensitivity in response to external magnetic field (such as a small magnet of 0.1 T), resulting in the accumulation of the hybrid nanogels within the duration from hours to a few seconds as the pH value decreases from 7.4 to 5.0. The pH-dependent magnetic response characteristic of the hybrid nanogels were further integrated with the pH change to fluorescent signal transduction and pH-regulated anticancer drug (a model drug 5-fluorouracil) delivery functions. The hybrid nanogels can overcome cellular barriers to enter the intracellular region and light up the mouse melanoma B16F10 cells. The multiple responsive hybrid nanogel that can be manipulated in tandem endogenous and exogenous activation should enhance our ability to address the complexity of biological systems.

Magnetic iron oxide–fluorescent carbon dots integrated nanoparticles for dual-modal imaging, near-infrared light-responsive drug carrier and photothermal therapy

Multifunctional hybrid nanoparticles (NPs, ∼100 nm) that combine magnetic Fe3O4 nanocrystals and fluorescent carbon dots (CDs) in porous carbon (C) were successfully synthesized using a one-pot solvothermal method by simply increasing the H2O2 concentration. The resultant Fe3O4@C-CDs hybrid NPs not only demonstrate excellent magnetic responsive properties (Ms = 32.5 emu g-1) and magnetic resonance imaging ability (r = 674.4 mM-1 s-1) from the Fe3O4 nanocrystal core, but also exhibit intriguing photoluminescent (quantum yield ∼6.8%) properties including upconversion fluorescence and excellent photostability from the CDs produced in the porous carbon. The hybrid NPs can enter the intracellular region and illuminate mouse melanoma B16F10 cells under different excitation wavelengths. Meanwhile, the mesoporous carbon shell and hydrophilic surface functional groups endow the hybrid NPs with high loading capacity (835 mg g-1) for the anti-cancer drug doxorubicin and excellent stability in aqueous solutions. More importantly, the hybrid NPs can absorb and convert near-infrared (NIR) light to heat due to the existence of CDs, and thus, can realise NIR-controlled drug release and combined photothermo/chemotherapy for high therapeutic efficacy. Such nanostructured Fe3O4@C-CDs hybrid NPs demonstrate great promise towards advanced nanoplatforms for simultaneous imaging diagnostics and high efficacy therapy.

Ultrathin nanosheets of CrSiTe3: a semiconducting two-dimensional ferromagnetic material

Finite range ferromagnetism and antiferromagnetism in two-dimensional (2D) systems within an isotropic Heisenberg model at non-zero temperature were originally proposed to be impossible. However, recent theoretical studies using an Ising model have shown that 2D magnetic crystals can exhibit magnetism. Experimental verification of existing 2D magnetic crystals in this system has remained exploratory. In this work we exfoliated CrSiTe3, a bulk ferromagnetic semiconductor, to mono- and few-layer 2D crystals onto a Si/SiO2 substrate. Raman spectra indicate good stability and high quality of the exfoliated flakes, consistent with the computed phonon spectra of 2D CrSiTe3, giving strong evidence for the existence of 2D CrSiTe3 crystals. When the thickness of the CrSiTe3 crystals is reduced to a few layers, we observed a clear change in resistivity at 80–120 K, consistent with theoretical calculations of the Curie temperature (Tc) of ∼80 K for the magnetic ordering of 2D CrSiTe3 crystals. The ferromagnetic mono- and few-layer 2D CrSiTe3 indicated here should enable numerous applications in nano-spintronics.

Self-Assembly of Nanometer-Scale Magnetic Dots with Narrow Size Distributions on an Insulating Substrate

The self-assembly of iron dots on the insulating surface of NaCl(001) is investigated experimentally and theoretically. Under proper growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting layer. Furthermore, both the vertical and lateral sizes of the dots can be tuned with the iron dosage without introducing apparent size broadening, even though the clustering is clearly in the strong coarsening regime. These observations are interpreted using a phenomenological mean-field theory, in which a coverage-dependent optimal dot size is selected by strain-mediated dot-dot interactions.

Porous carbon protected magnetite and silver hybrid nanoparticles: morphological control, recyclable catalysts, and multicolor cell imaging.

A simple and facile synthetic strategy is developed to prepare a new class of multifunctional hybrid nanoparticles (NPs) that can integrate a magnetic core with silver nanocrystals embedded in porous carbon shell. The method involves a one-step solvothermal synthesis of Fe3O4@C template NPs with Fe3O4nanocrystals in the core protected by a porous carbon shell, followed by loading and in situ reduction of silver ions in the carbon shell in water at room temperature. The core-satellite and dumbbell-like nanostructures of the resulted Fe3O4@C-Ag hybrid NPs can be readily controlled by loading amount of silver ions. The hybrid NPs can efficiently catalyze the reduction reaction of organic dyes in water. The easy magnetic separation and high stability of the catalytically active silver nanocrystals embedded in the carbon shell enable the hybrid NPs to be recycled for reuse as catalysts. The hybrid NPs can also overcome cellular barriers to enter the intracellular region and light up the mouse melanoma B16F10 cells in multicolor modal, with no cytotoxicity. Such porous carbon protected Fe3O4@C-Ag hybrid NPs with controllable nanostructures and a combination of magnetic and noble metallic components have great potential for a broad range of applications in the catalytic industry and biomedical field.

Multifunctional PEG encapsulated Fe3O4@silver hybrid nanoparticles: antibacterial activity, cell imaging and combined photothermo/chemo-therapy

A class of multifunctional hybrid nanoparticles (NPs) that can integrate a magnetic core, silver (Ag) nanocrystals, and a biocompatible poly(ethylene glycol) (PEG) shell were synthesized and characterized and their applications as antibacterial agents, optical labels for cellular imaging and drug carriers were tested. The synthetic strategy involves a one-step solvothermal synthesis of Fe3O4@PEG template NPs (∼60 nm) with magnetic Fe3O4 nanocrystals in the core and porous PEG as the shell, followed by loading and in situ reduction of Ag+ ions to form Ag nanocrystals in the shell. The size and number of the Ag nanocrystals embedded in the PEG shell can be readily controlled via a simple reaction condition change, resulting in different nanostructures and properties of the hybrid NPs. Such designed Fe3O4@Ag-PEG hybrid NPs can combine the properties and functions from each component. While the Fe3O4 core provides an easy magnetic separation and targeting and magnetic resonance imaging (MRI) contrast ability, the Ag nanocrystals provide stable strong fluorescence and antibacterial activity. The porous PEG shell with excellent stability in water and non-cytotoxicity can be used as a drug carrier for combined photothermo/chemo-therapy. The small hybrid NPs can enter the intracellular region and light up the mouse melanoma B16F10 cells. This class of hybrid NPs with rational integration of functional building blocks should offer broad opportunities for external magnetic manipulation, imaging diagnostics, antibacterial applications and as drug carriers.

Multifunctional 1D Magnetic and Fluorescent Nanoparticle Chains for Enhanced MRI, fluorescent Cell Imaging, And Combined Photothermal/Chemotherapy

While the assembled 1D magnetic nanoparticle (NP) chains have demonstrated synergistic magnetic effects from the individual NPs, it is essential to prepare new 1D NP chains that can combine the magnetism with other important material properties for multifunctional applications. This paper reports the fabrication and multifunctional investigation of a new type of 1D NP chains that combine the magnetic properties with fluorescent properties, photothermal conversion ability, and drug carrier function. The building block NPs are composed of magnetic Fe(3)O(4) nanocrystals clustered in the core and fluorescent carbon dots embedded in the mesoporous carbon shell with hydroxyl/carboxyl groups anchored on their surface. These NPs can assemble under the induction of external magnetic field and form stable 1D NP chains of diameter ∼ 90 nm and length ∼ 3 μm via the hydrogen bonding and π-π stacking linkage of the carbon shell. The resulted 1D hybrid NP chains not only demonstrate much higher magnetic resonance imaging (MRI) contrasting ability than the dispersed building block NPs, but also enter into intracellular region and light up the B16F10 cells under a laser excitation with strong and stable fluorescence. While the mesoporous carbon shell provides high drug loading capacity, the embedded fluorescent carbon dots convert near-infrared (NIR) light to heat, and hence kill the tumor cells efficiently and enhance the drug release rate to further improve the therapeutic efficacy under NIR irradiation. Such designed 1D magnetic-fluorescent hybrid NP chains with enhanced MRI contrast, fluorescent imaging ability, and combined chemo-/photothermal therapeutic ability have great potential for various biomedical applications.

Self-assembled FePt nanodot arrays with mono-dispersion and -orientation

For self-assembled nanodots, the ultimate dream is to simultaneously achieve tunable uniformity in size, spatial distribution, chemical composition, and crystallographic orientation. By utilizing the Volmer–Weber growth mode in thin film epitaxy, we have grown self-assembled two-dimensional arrays of FePt alloy nanodots that are uniform in size, chemical composition, and are all crystallgraphically aligned. These dot assemblies are ferromagnetic at room temperature and can be easily transferred onto other templates without destroying the size and orientation uniformity.

Full electroresistance modulation in a mixed-phase metallic alloy

We report a giant, ∼22%, electroresistance modulation for a metallic alloy above room temperature. It is achieved by a small electric field of 2  kV/cm via piezoelectric strain-mediated magnetoelectric coupling and the resulting magnetic phase transition in epitaxial FeRh/BaTiO_{3} heterostructures. This work presents detailed experimental evidence for an isothermal magnetic phase transition driven by tetragonality modulation in FeRh thin films, which is in contrast to the large volume expansion in the conventional temperature-driven magnetic phase transition in FeRh. Moreover, all the experimental results in this work illustrate FeRh as a mixed-phase model system well similar to phase-separated colossal magnetoresistance systems with phase instability therein.