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.

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Room-temperature ferroelectricity in strained SrTiO3.

Optimized Sputtering Parameters for ITO Thin Films of High Conductivity and Transparency

The BiFeO3 is a largely studied compound with magnetoeletric coupling at room temperature. This is due its potential technological applications that can be directly used in advanced technologies. BiFeO3 is an oxide with perovskite structure and R3c space group (rhombohedral symmetry). In this work, synchrotron radiation X-ray powder diffraction data was used to obtain a detailed description of BiFeO3 crystal structure. Thereafter, experimental electron density distribution calculations were obtained using by the maximum entropy method (MEM) statistical approach. The ionic/covalent nature of the bonding and the interaction between the ions are clearly revealed by constructing electron density maps. The Fe3+-O2− and Bi3+-O2− bonding were studied in 1 and 2 dimensional electron density distributions. Due the high resolution of the obtained maps, it can be regarded as the most precise electron density distributions seen inside the material.

Maximum Entropy Method Applied in the Experimental Visualization of Electron Density Distributions in BiFeO3

Very important advantage of ZnO thin films is an opportunity of use in the composite heterostructures opening
opportunities for development of ZnO-based optoelectronics devices. In this work we report the preparation of
ferroelectric crystal - ZnO thin film heterostructures by vacuum deposition method and creation of new type of
pyroelectric photodetector. The ferroelectric field effect transistor has been prepared using ZnO:Li films as transistor
channel and LiNbO3 and TGS crystals as pyroelectric sensitive element. The photoelectric properties (currents ratio,
charge carriers mobility, ampere-watt sensitivity in IR diapason, NEP sensitivity, and photocurrent kinetics) of prepared
heterostructures were investigated and first samples of novel pyroelectric photodetector with high sensitivity and
detectability were prepared.

Pyroelectric photodetector based on ferroelectric crystal-semiconductor thin film heterostructure

Complex dielectric properties of PbZr 1 m x Ti x O 3 ceramics with compositions in the vicinity of the morphotropic phase boundary (x=0.46, 0.48, and 0.51), as a function of dc bias fields, were measured under small ac signal conditions. In monoclinic phase, dielectric permittivity shows hysteresis behavior and decreases with increasing bias field when E bias >E c . For compositions of tetragonal phase and near the on-set of the monoclinic phase transitions, dielectric permittivity increases with increasing bias electric field, indicating a continued polarization switching process. Such anomalous behavior is attributed to the E-field induced monoclinic phase, which allows polarization rotation without domain reversal. Temperature dependence of the dc biased dielectric property between 12K and 300K is also reported. Dielectric dispersion is mostly clamped out for both PZT 54:46 and PZT 52:48 as temperatures approaching 12K; however, remains significant in PZT 49:51.

DC Biased Dielectric Property of MPB PZT Ceramics with Monoclinic Distortion at Low Temperatures

The advantages of producing a periodically poled lithium niobate (PPLN) structure during crystal growth process are the possibility of obtaining thicker and wider structures that leads to greater useful surfaces in addition to the elimination of the subsequent poling process. We report in this paper a new technology of creating bulk periodically poled LiNbO 3 single crystals with antiparallel ferroelectric domains, by direct electric field poling during growth processes. Growth system configuration, crystal composition and geometry selection are explored to allow successful control of the direction of spontaneous polarization using external electric field. PPLN crystal samples with periodicity 20-100 μm were grown using this electric field modulated growth technique.

Ruyan Guo

Papers

Optimized Sputtering Parameters for ITO Thin Films of High Conductivity and Transparency

Maximum Entropy Method Applied in the Experimental Visualization of Electron Density Distributions in BiFeO3

Pyroelectric photodetector based on ferroelectric crystal-semiconductor thin film heterostructure

DC Biased Dielectric Property of MPB PZT Ceramics with Monoclinic Distortion at Low Temperatures

Creation of periodical antiparallel domain structure in lithium niobate crystals during growth process