There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

/pdf/room-temperature-ferroelectricity-in-strained-srtio3-4u1ustezxf.pdf

Room-temperature ferroelectricity in strained SrTiO3.

The preparation of Ba(Mg1/3Ta2/3)O3 (BMT) ceramics and the first successful growth of BMT single crystals by the laser heated pedestal growth technique (LHPG) is reported. The single crystal has a simple cubic perovskite structure in comparison to the ordered hexagonal structure normally found in ceramics of the same phase. The dielectric properties of single crystals are examined, the dielectric Q value increases with the increase in B‐site ordering, while the dielectric constant κ is relatively independent of ordering. Our results also show that BMT is a candidate substrate for high Tc superconductor thin films as it has one of the highest Q values known for the oxide perovskite family, along with its matching thermal expansion coefficient α=9.0×10−6/ °C and twin‐free cubic perovskite structure with a=4.0877 A. BMT single crystals grown by the LHPG technique are probably the highest melting oxide compounds grown to date.

Ba(Mg1/3Ta2/3)O3 single crystal fiber grown by the laser heated pedestal growth technique

ZnO single-crystal microtubes were fabricated using an encapsulated microwave growth method. The ZnO crystals are grown in hexagonal hollow tube form with a well faceted end and side surfaces, which have cross-sectional dimensions of 100to250μm, lengths of 3–5mm, and wall thickness of 1–2μm. Under optical excitation, a strong near-band-edge emission was obtained at a peak wavelength of 377.8nm with a full width at half maximum of 11nm. The ZnO microtubes exhibited a highly selective UV light response with a cut-off wavelength at ∼370nm, and excellent electron field emission properties with an emission current density of 11mA∕cm2 at an applied field of ∼20V∕μm.

https://www.mri.psu.edu/sites/default/files/file_attach/109.pdf

Zinc oxide single-crystal microtubes

Synthesis and characterization of lead strontium titanate thin films by sol-gel technique

We have studied lattice dynamical properties of ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$ thin films and single crystals with $x=0.05,$ 0.1, 0.2, 0.35, and 0.5 by Raman spectroscopy in the temperature range 5--300 K. In the crystals, only the second order Raman features are observed above the phase transition temperature due to crystal symmetry. In the films the first-order phonon modes are observed both below and above the ferroelectric phase transition temperature. The soft modes in films are harder than those in single crystals and show larger overdamping over a broader temperature range near the phase transition. The overdamping is correlated with the broad peak in the temperature dependence of the dielectric constant. The lattice dynamical behaviors in the films are strikingly similar to those in relaxor ferroelectrics. The similarities are explained by the existence of polar nanoregions in the ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$ thin films.

Raman study of Ba x Sr 1 − x TiO 3 films: Evidence for the existence of polar nanoregions

We have deposited CaTiO3 thin films on SrRuO3 electrode layers on SrTiO3 and LaAlO3 substrates by pulsed laser deposition. The low-frequency dielectric properties were measured from 4 to 300 K. We found that the dielectric constants of 1-μm-thick CaTiO3 films are very close to that observed in the bulk materials. The dielectric loss depends on the temperature and is on the order of 10−4 at low temperature. The dielectric constants of the films can be tuned by electric field by 6% at room temperature and 10% at 200 K.

Ruyan Guo

Papers

Ba(Mg1/3Ta2/3)O3 single crystal fiber grown by the laser heated pedestal growth technique

Zinc oxide single-crystal microtubes

Synthesis and characterization of lead strontium titanate thin films by sol-gel technique

Raman study of Ba x Sr 1 − x TiO 3 films: Evidence for the existence of polar nanoregions

Dielectric properties of pulsed-laser-deposited calcium titanate thin films