Huaxiang Fu

Bio: Huaxiang Fu is an academic researcher from University of Arkansas. The author has contributed to research in topics: Ferroelectricity & Quantum dot. The author has an hindex of 31, co-authored 73 publications receiving 5290 citations. Previous affiliations of Huaxiang Fu include National Renewable Energy Laboratory & Rutgers University.

Polarization rotation mechanism for ultrahigh electromechanical response in single-crystal piezoelectrics

Abstract: Piezoelectric materials, which convert mechanical to electrical energy (and vice versa), are crucial in medical imaging, telecommunication and ultrasonic devices. A new generation of single-crystal materials, such as Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), exhibit a piezoelectric effect that is ten times larger than conventional ceramics, and may revolutionize these applications. However, the mechanism underlying the ultrahigh performance of these new materials-and consequently the possibilities for further improvements-are not at present clear. Here we report a first-principles study of the ferroelectric perovskite, BaTiO3, which is similar to single-crystal PZN-PT but is a simpler system to analyse. We show that a large piezoelectric response can be driven by polarization rotation induced by an external electric field. Our computations suggest how to design materials with better performance, and may stimulate further interest in the fundamental theory of dielectric systems in finite electric fields.

Unusual phase transitions in ferroelectric nanodisks and nanorods

Abstract: Bulk ferroelectrics undergo structural phase transformations at low temperatures, giving multi-stable (that is, multiple-minimum) degenerate states with spontaneous polarization. Accessing these states by applying, and varying the direction of, an external electric field is a key principle for the operation of devices such as non-volatile ferroelectric random access memories (NFERAMs). Compared with bulk ferroelectrics, low-dimensional finite ferroelectric structures promise to increase the storage density of NFERAMs 10,000-fold. But this anticipated benefit hinges on whether phase transitions and multi-stable states still exist in low-dimensional structures. Previous studies have suggested that phase transitions are impossible in one-dimensional systems, and become increasingly less likely as dimensionality further decreases. Here we perform ab initio studies of ferroelectric nanoscale disks and rods of technologically important Pb(Zr,Ti)O3 solid solutions, and demonstrate the existence of previously unknown phase transitions in zero-dimensional ferroelectric nanoparticles. The minimum diameter of the disks that display low-temperature structural bistability is determined to be 3.2 nm, enabling an ultimate NFERAM density of 60 x 10(12) bits per square inch-that is, five orders of magnitude larger than those currently available. Our results suggest an innovative use of ferroelectric nanostructures for data storage, and are of fundamental value for the theory of phase transition in systems of low dimensionality.

Size-Dependent Spectroscopy of InP Quantum Dots

Abstract: The spectroscopic behavior of colloidal InP quantum dots (QDs) has been investigated as a function of the mean QD diameter (which ranged from 26 to 60 A). Absorption spectra show up to three peaks or shoulders which reflect excited state transitions in the QDs. Global photoluminescence (PL) spectra (excitation well to the blue of the absorption onset and which consequently excites most of the QDs in the size distribution) show broad PL emission. The emission and absorption features shift to higher energy with decreasing QD size. Resonant PL spectra (size-selective excitation into the tail of the absorption onset) show increasing fluorescence line narrowing with increasing excitation wavelength; PL and photoluminescence excitation spectroscopy were used to derive the PL red shift as a function of QD size. The resonant red shifts for QDs of a single size were extracted from PL data that reflect the emission from an ensemble of QD diameters. An analysis of the single-dot resonant red shift (difference betwee...

Many-body pseudopotential theory of excitons in inp and cdse quantum dots

Abstract: We present a pseudopotential approach to the calculation of the excitonic spectrum of semiconductor quantum dots. Starting from a many-body expansion of the exciton wave functions in terms of single-substitution Slater determinants constructed from pseudopotential single-particle wave functions, our method permits an accurate and detailed treatment of the intraconfiguration electron-hole Coulomb and exchange interactions, while correlation effects can be included in a controlled fashion by allowing interconfiguration coupling. We calculate the exciton fine structure of InP and CdSe nanocrystals in the strong-confinement regime. We find a different size dependence for the electron-hole exchange interaction than previously assumed (i.e., ${R}^{\ensuremath{-}2}$ instead of ${R}^{\ensuremath{-}3})$. Our calculated exciton fine structure is compared with recent experimental results obtained by size-selective optical spectroscopies.

The First Covalent Organic-Inorganic Networks of Hybrid Chalcogenides: Structures That May Lead to a New Type of Quantum Wells

Abstract: Rigidity and stability, in addition to superior electronic, magnetic, and optical properties, have made inorganic frameworks highly attractive in the search for new, functional materials, 1 while organic and coordination compounds built upon molecular building-blocks hold great promises for processability, flexibility, structural diversity, and geometrical control, such as size, shape, and symmetry. 2 Incorporation of the two counterparts into a single structure may generate organic -inorganic hybrid composites that enhance or combine the useful properties of both components, as found in zeolites and other mesoporous oxides. 3 For example, an intensively pursued area in the miniaturization of electronic devices is the investigation of composite materials that combine the semiconducting functionality of the inorganic constituent with the lower weight and volume of the organic component. 1,4 Many examples have been reported in which organic species enter the inorganic structures through either ionic bonding or relatively weak H-bonding and van der Waals interactions. However, organic-inorganic covalent architectures with explicit bonding directions are much needed, but rare. 5 The novel periodic mesoporous organosilicas (PMOs) recently reported have shed light on the preparation of hybrid materials of this type. 6 The ability to change or modify physical properties, auch as optical absorption edges, is of equal significance to the synthesis of new materials. This can be achieved, for example, by controlling the alloy composition7 and the size of confined systems such as quantum dots (QD)8 and quantum well (QW). 9 Currently, semiconductor dots are preferred because a very large variation is achievable in these systems. 10 However, it is a great challenge to generate uniform and periodic lattices of dots. 11 Here, we report a new type of covalently bonded hybrid composites that not only possess a uniform and periodic structure, but simultaneously offer a significant variation of optical properties. The three novel compounds, [ R-ZnTe(en) 1/2] (I), [â-ZnTe(en) 1/2] (II ), and [ZnTe(pda) 1/2](III ), represent the first examples of chalcogenide-based hybrid materials of which uniform structures are formed via direct, covalent bonds between the inorganic host (the II-VI semiconductor ZnTe) and the organic spacers. I and II were synthesized in ethylenediamine (en) and III in 1,3propanediamine (pda). Both en and pda serve as solvents and as a source of bifunctional ligands. 12 I-III are stable in air for a long period of time. Upon heating they are converted to ZnTe by separating out the organic component, L. 12 The formation of the title compounds is, therefore, reversible: ZnTe + L (solvent)T ZnTe(L)1/2, L ) en, pda. The conversion between the two groups of compounds may also be achieved under mild conditions: ( I , II ) T III .12 X-ray diffraction analysis reveals that the crystal structure ofI is a three-dimensional network 13 containing 2D [ZnTe] slabs and en molecules, as illustrated in Figure 1a. The [ZnTe] slabs stack along the c-axis and are interconnected by en molecules, each bridged to two Zn metal centers from the adjacent slabs. As shown in Figure 1b, the inorganic slab is a puckered 6 3

Multiferroic and magnetoelectric materials

Abstract: A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

Revival of the Magnetoelectric Effect

Abstract: Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.

Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Abstract: Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

Quantum dot solar cells

Abstract: Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

Large Piezoelectric Effect in Pb-Free Ceramics

Abstract: We report a non-Pb piezoelectric ceramic system Ba(Ti(0.8)Zr(0.2))O(3)-(Ba(0.7)Ca(0.3))TiO(3) which shows a surprisingly high piezoelectric coefficient of d(33) approximately 620 pC/N at optimal composition. Its phase diagram shows a morphotropic phase boundary (MPB) starting from a tricritical triple point of a cubic paraelectric phase (C), ferroelectric rhombohedral (R), and tetragonal (T) phases. The high piezoelectricity of the MPB compositions stems from the composition proximity of the MPB to the tricritical triple point, which leads to a nearly vanishing polarization anisotropy and thus facilitates polarization rotation between 001T and 111R states. We predict that the single-crystal form of the MPB composition of the present system may reach a giant d(33) = 1500-2000 pC/N. Our work may provide a new recipe for designing highly piezoelectric materials (both Pb-free and Pb-containing) by searching MPBs starting from a TCP.