and Applications of Fullerenes, Carbon Dots, Nanotubes, Graphene, Nanodiamonds, and Combined Superstructures Vasilios Georgakilas,† Jason A. Perman,‡ Jiri Tucek,‡ and Radek Zboril*,‡ †Material Science Department, University of Patras, 26504 Rio Patras, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17 listopadu 1192/12, 771 46 Olomouc, Czech Republic

Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures.

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Multiferroicity: the coupling between magnetic and polarization orders

The ground-state structural and electronic properties of ferroelectric $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ are calculated using density functional theory within the local spin-density approximation (LSDA) and the $\mathrm{LSDA}+U$ method. The crystal structure is computed to be rhombohedral with space group $R3c$, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of $90--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$ is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differing by an order of magnitude from early experiments. One possible explanation is that the latter may have suffered from large leakage currents. However, both past and contemporary measurements are shown to be consistent with the modern theory of polarization, suggesting that the range of reported polarizations may instead correspond to distinct switching paths in structural space. Modern measurements on well-characterized bulk samples are required to confirm this interpretation.

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First-principles study of spontaneous polarization in multiferroic Bi Fe O 3

Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents Doping of 2at% Ti4+ ions increased the dc resistivity by more than three orders of magnitude In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Polycrystalline BiFeO3 nanoparticles (size 80-120 nm) are prepared by a simple sol-gel technique. Such nanoparticles are very efficient for photocatalytic decomposition of organic contaminants under irradiation from ultraviolet to visible frequencies. The BiFeO3 nanoparticles also demonstrate weak ferromagnetism of about 0.06 mu(B)/Fe at room temperature, in good agreement with theoretical calculations.

Visible-Light Photocatalytic Properties of Weak Magnetic BiFeO3 Nanoparticles

Single-phased ferroelectromagnet BiFeO3 ceramics with high resistivity were synthesized by a rapid liquid phase sintering technique. Saturated ferroelectric hysteresis loops were observed at room temperature in the ceramics sintered at 880 °C for 450 s. The spontaneous polarization, remnant polarization, and the coercive field are 8.9 μC/cm2, 4.0 μC/cm2, and 39 kV/cm, respectively, under an applied field of 100 kV/cm. It is proposed that the formation of Fe2+ and an oxygen deficiency leading to the higher leakage can be greatly suppressed by the very high heating rate, short sintering period, and liquid phase sintering technique. The latter was also found effective in increasing the density of the ceramics. The sintering technique developed in this work is expected to be useful in synthesizing other ceramics from multivalent or volatile starting materials.

Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering

Single-phase Bi1−xNdxFeO3 (BNFOx) (x=0–02) multiferroic ceramics were prepared to study the effects of Nd substitution on their crystal structure and ferroelectromagnetic behavior Rietveld refinement of x-ray diffraction data showed a continual transformation of crystal structure from the rhombohedral structure of BNFOx=0 (BiFeO3) to a triclinic structure in BNFOx=005–015 and a pseudotetragonal structure in BNFOx=0175–02 Ferroelectromagnetic measurements revealed the existence of ferroelectricity with remnant polarization of ∼9μC∕cm2 in BNFOx=0–0175, paraelectricity in BNFOx=02, and weak ferromagnetism with remnant magnetizations of 007–0227emu∕g in BNFOx=015–02 Magnetoelectric coupling was obvious in BNFOx=015–0175 near the Neel temperature of ∼380°C

Structural transformation and ferroelectromagnetic behavior in single-phase Bi1−xNdxFeO3 multiferroic ceramics

We investigated the energetic stability, electronic, and magnetic properties of the zigzag graphene nanoribbons with one edge saturated by two hydrogen atoms, the other edge saturated by one hydrogen atom by using density-functional theory (DFT). The energy of the ferromagnetic semiconductor state is the lowest state for these nanoribbons. The energy difference between the antiferromagnetic states and the ferromagnetic states varies inversely with the nanoribbon width. Both the band gaps and the magnetic moments in the zigzag graphene nanoribbons with one edge saturated are larger than those of zigzag graphene nanoribbons.

Electronic and magnetic properties of zigzag graphene nanoribbon with one edge saturated

The intriguing electronic and magnetic properties of the semihydrogenated SiC sheet are investigated by means of the first-principles calculations. The semihydrogenated SiC sheet exhibits diverse electronic and magnetic properties: a ferromagnetic semiconductor when Si atoms are hydrogenated, while an antiferromagnetic semiconductor with C atoms hydrogenated. The semihydrogenated SiC sheet with the C atoms hydrogenated is found to be more stable than the sheet with the Si atoms hydrogenated. Thus, controlling the hydrogenation on the different atom sites can precisely modulate the electronic and magnetic properties of the semihydrogenated SiC sheet, which endues the semihydrogenated SiC sheet great potential applications in the future functional nanodevices.

Ferromagnetic and antiferromagnetic properties of the semihydrogenated SiC sheet

Carrier mobilities of the semiconducting single wall carbon nanotubes (SWCNTs) have been studied by using the first-principles calculations with the deformation potential approximation, which only considers the scattering by the longitudinal acoustic phonons based on the adapting Bardeen–Shockley theory [J. Bardeen and W. Shockley, Phys. Rev. 80, 72 (1950)] to one-dimensional case. From the band structures of the semiconducting SWCNTs, we calculated the effective masses, the stretching modulus, the deformation potential constants. We demonstrated that the calculated intrinsic carrier mobility can reach 106 cm2/V s at room temperature, and the carrier mobilities of the semiconducting SWCNTs show the intriguing alternating behavior.

Z. G. Liu

Papers

Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering

Structural transformation and ferroelectromagnetic behavior in single-phase Bi1−xNdxFeO3 multiferroic ceramics

Electronic and magnetic properties of zigzag graphene nanoribbon with one edge saturated

Ferromagnetic and antiferromagnetic properties of the semihydrogenated SiC sheet

The effect of acoustic phonon scattering on the carrier mobility in the semiconducting zigzag single wall carbon nanotubes