Search

We report on the formation of epitaxial perovskite oxide superlattice structures by atomic layer deposition (ALD), which are integrated monolithically on Si wafers using a template layer of SrTiO3 deposited by hybrid molecular beam epitaxy. ALD film growth was carried out at 360 °C, which is significantly lower than the typical deposition temperatures for epitaxial perovskite thin films. The high control over the stacking sequence of different constituents is demonstrated in a series of (BaTiO3)m/(SrTiO3)n superlattices with various m/n cycle ratios. All superlattice structures were coherently strained to the virtual substrate layer of SrTiO3 on Si. Irrespective of the m/n superlattice sequence, SrTiO3 sublayers retain slight compressive strain which is transmitted to the BaTiO3 layers.

Ferroelectrics can exhibit the bulk photovoltaic effect (BPVE), which enables a switchable photoresponse and above-band-gap open-circuit voltages (Voc). For these systems a large remnant polarization (Pr) combined with a narrow band gap (Eg) is believed to be critical in optimizing the photovoltaic performance. Here we investigate the PbTiO3-BiFeO3-Bi(Ni1/2Ti1/2)O3 system which exhibits a large bulk Pr and a tunable narrow Eg. To optimize the ferroelectric polarization, ceramic samples were prepared in the vicinity of the ternary morphotropic phase boundary (MPB) and their photovoltaic properties were characterized over a broad range of optical wavelengths. MPB compositions with a direct optical gap ranging from 2.25 - 2.85 eV and Pr = 32 - 39 μC/cm2 show a BPVE with Voc = 6 V at room temperature and a wavelength-dependent switchable photoresponse. Under 1 sun AM1.5 G illumination the short-circuit photocurrent (jsc) increased by an order of magnitude as Eg was lowered from 2.85 eV to 2.25 eV, with ~ 0.1 ...

Epitaxial thin-film growth enables novel functionalities, particularly if significant barriers to integration with existing technologies, scalability and excessive temperature of films, can be addr...

We report on the high temperature thin film growth of BaTiO3 on Ti3C2Tx MXene flakes using van der Waals epitaxy on a degradable template layer. MXene was deposited on amorphous and crystalline substrates by spray- and dip-coating techniques, while the growth of BaTiO3 at 700 °C was accomplished using pulsed laser deposition in an oxygen rich environment. We demonstrate that the MXene flakes act as a temporary seed layer, which promotes highly oriented BaTiO3 growth along the (111) direction independent of the underlying substrate. The lattice parameters of the BaTiO3 films are close to the bulk value suggesting that the BaTiO3 films remains unstrained, as expected for van der Waals epitaxy. The initial size of the MXene flakes has an impact on the orientation of the BaTiO3 films with larger flake sizes promoting a higher fraction of the polycrystalline film to grow along the (111) direction. The deposited BaTiO3 film adopts the same morphology as the original flakes and piezoresponse force microscopy shows a robust ferroelectric behavior for individual grains. Transmission electron microscopy results indicate that the Ti3C2Tx MXene fully decomposes during the BaTiO3 deposition and the surplus Ti atoms are readily incorporated into the BaTiO3 film. Electrical measurements show a similar dielectric constant as a BaTiO3 film grown without the MXene seed layer. The demonstrated process has the potential to overcome the longstanding issue of integrating highly oriented complex oxide thin films directly on any desired substrate.

Domain walls separating regions of ferroelectric material with polarization oriented in different directions are crucial for applications of ferroelectrics. Rational design of ferroelectric materials requires the development of a theory describing how compositional and environmental changes affect domain walls. To model domain wall systems, the discrete microscopic Landau-Ginzburg-Devonshire (dmLGD) approach with A- and B-site cation displacements serving as order parameters is developed. Application of dmLGD to the classic BaTiO3 , KNbO3 and PbTiO3 ferroelectrics shows that A-B cation repulsion is the key interaction that couples the polarization in neighboring unit cells of the material. dmLGD decomposition of the total energy of the system into the contributions of the individual cations and their interactions enables the prediction of different properties for a wide range of ferroelectric perovskites based on the results obtained for BaTiO3 , KNbO3 and PbTiO3 only. It is found that the information necessary to estimate the structure and energy of domain-wall "defects" can be extracted from single-domain 5-atom first-principles calculations, and that "defect-like" domain walls offer a simple model system that sheds light on the relative stabilities of the ferroelectric, antiferroelectric, and paraelectric bulk phases. The dmLGD approach provides a general theoretical framework for understanding and designing ferroelectric perovskite oxides. This article is protected by copyright. All rights reserved.

Thin films of ≈50 nm thickness with Ba/Ti-ratios ranging from 0.8 to 1.06 were prepared by depositing alternating layers of Ba(OH)2 and TiO2. Annealing at 750 °C promoted the solid-solid transformation into polycrystalline BaTiO3 films containing a mixture of the perovskite and the hexagonal polymorphs with average crystallite sizes smaller than 14 nm and without impurity phases. This, together with an increase of the cubic lattice parameters for Ba-rich films, suggests an extended metastable solubility range for the perovskite-phase in these nanocrystalline thin films on both sides of the stoichiometric composition. Mapping of the cation distribution utilizing energy-filtered transmission electron microscopy corroborates defect accommodation within the BaTiO3 grains. While the cation off-stoichiometry in thermodynamic equilibrium is negligible for BaTiO3, the metastable extended solubility range in the thin films can be directly correlated to the low annealing temperature and nanocrystalline nature. The leakage current behavior can be explained by the formation of Schottky defects for nonstoichiometric films, and the cation ratio has a distinct impact on the dielectric properties: while excess-BaO has a marginal detrimental effect on the permittivity, the dielectric constant declines rapidly by more than 50% towards the Ti-rich side. The present findings highlight the importance of compositional control for the synthesis of nanocrystalline BaTiO3 thin films, in particular for low annealing and/or deposition temperatures. Our synthesis approach using alternating layers of Ba(OH)2 and TiO2 provides a route to precisely control the cation stoichiometry.

An investigation of the influence of annealing conditions on the carrier transport, leakage current and dielectric properties of ALD-grown amorphous Bi–Fe–O thin films after their crystallization into BiFeO3 is presented. Whereas the interface-limited Schottky emission mechanism is dominant in 70 nm thick Fe-rich films after relatively short annealing, a space-charge-limited conduction mechanism is dominant in stoichiometric films with a thickness of 215 nm independent of the annealing conditions. Interestingly, prolonged annealing of the thin films also results in space charge limited conduction. Analysis of the changes in dielectric properties, on one hand, and the film composition, microstructure and morphology, on the other hand, reveal the key role of grain boundary interfaces for the conductivity of the polycrystalline ALD-grown BiFeO3 thin films. Extended annealing in oxygen results in 2–3 orders-of-magnitude reduction in leakage current accompanied by decreases in dielectric loss, highlighting the importance of optimizing annealing conditions for any applications of BiFeO3 thin films.

A superlattice approach for the atomic layer deposition of polycrystalline BaTiO3 thin films is presented as an example for an effective route to produce high-quality complex oxide films with excellent thickness and compositional control. This method effectively mitigates any undesirable reactions between the different precursors and allows an individual optimization of the reaction conditions for the Ba–O and the Ti–O subcycles. By growth of nanometer thick alternating Ba(OH)2 and TiO2 layers, the advantages of binary oxide atomic layer deposition are transferred into the synthesis of ternary compounds, permitting extremely high control of the cation ratio and superior uniformity. Whereas the Ba(OH)2 layers are partially crystalline after the deposition, the TiO2 layers remain mostly amorphous. The layers react to polycrystalline, polymorph BaTiO3 above 500 °C, releasing H2O. This solid-state reaction is accompanied by an abrupt decrease in film thickness. Transmission electron microscopy and Raman spect...

We report on the growth of polycrystalline BiFeO3 thin films on SiO2 /Si(001) and Pt(111) substrates by atomic layer deposition using the precursors ferrocene, triphenyl-bismuth, and ozone. By growing alternating layers of Fe2 O3 and Bi2 O3 , we employ a superlattice approach and demonstrate an efficient control of the cation stoichiometry. The superlattice decay and the resulting formation of polycrystalline BiFeO3 films are studied by in situ X-ray diffraction, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. No intermediate ternary phases are formed and BiFeO3 crystallization is initiated in the Bi2 O3 layers at 450 °C following the diffusion-driven intermixing of the cations. Our study of the BiFeO3 formation provides an insight into the complex interplay between microstructural evolution, grain growth, and bismuth oxide evaporation, with implications for optimization of ferroelectric properties.

Recently, it has been shown how shift and ballistic currents in piezoelectric sillenite crystals Bi$_{12}$GeO$_{20}$ and Bi$_{12}$SiO$_{20}$ can be separated experimentally under the assumption that the shift component of the circular current is small. However, it has been claimed that the shift and ballistic currents cannot be quantified by this method, due to the magneto-photovoltaic effect caused either by the change of the crystal's spatial symmetry in the magnetic field or by the breaking of time-reversal symmetry. Presently, we report observations of photovoltaic currents in Bi$_{12}$SiO$_{20}$, excited by linearly- and circularly-polarized light under weak external magnetic field, as well as measurements of the corresponding photo-Hall signals. We demonstrate that the magneto-photovoltaic current constitutes a significant fraction of the measured current in the Hall direction for Bi$_{12}$SiO$_{20}$ under specific experimental conditions., Comment: 9 pages, 3 figures

Electrically tunable dielectric thin films in active circuits and systems are challenged by capacitance-induced delays and impedance matching requiring a lower dielectric constant. Here an approach to increasing the intrinsic tunability of compounds containing TiO6 octahedra by considering the influence of different connectivity among these octahedra is presented. Such connectivity variants in nanocrystalline monoclinic BaTi2O5 thin films enable a two orders of magnitude enhancement in Ti anharmonic interaction, thereby permitting a ≈ 65% decrease in dielectric constant to 70 at room temperature without sacrificing tunability. Edge-sharing TiO6 octahedra possess a much shorter Ti-Ti distance of only 2.91 A as compared to the perovskite structure (~4 A), permitting large field-induced structural re-arrangement and intrinsic tunability.

We report on the phase evolution of pure and Fe-doped bismuth oxide (Bi–O and Bi–Fe–O) thin films grown by atomic layer deposition. In situ X-ray diffraction was used to map the evolution of phase formation and orientational growth of the oxides during the gradual crystallization of the films from the amorphous state on single-crystalline substrates. The formation of (001)-oriented Bi2O2.3 was observed at temperatures as low as 300 °C on (001)SrTiO3, with a gradual transformation into Bi2O3. A similar crystallization of Fe-doped Bi2O3 on (111)ZrO2(Y2O3) leads to the formation of (111)-oriented δ-Bi2O3 above 400 °C instead of the sillenite phase. We thus demonstrate that epitaxial stabilization of the metastable phases can take place during atomic layer deposition of oxides as well as post-deposition annealing. In situ crystallization of the amorphous Bi–Fe–O films revealed the evolution of oxide phases in the film with the sillenite composition. Our results are important for the design of annealing procedures to obtain phase-pure epitaxial BiFeO3 thin films from amorphous stoichiometric Bi–Fe–O grown by atomic layer deposition.

We report on thin-film nanocrystalline polymorphic BaTiO3 for enabling high dielectric and low leakage response. BaTiO3-Al2O3 bi-layer thin films based on nanocrystalline BaTiO3 containing the tetragonal and hexagonal polymorphs were deposited by atomic layer deposition. Polymorphic BaTiO3 film combined with a ≈ 3.5-nm thick Al2O3 layer located between the BaTiO3 film and top electrode exhibits dielectric constant of 108 or 130 and leakage currents 2.2 × 10−8 A/mm2 or 1.3 × 10−7 A/mm2 at 1 MV/cm, respectively. Where x-ray photoemission analysis of barrier heights for the metal-BaTiO3-Al2O3-metal structure point to using the polymorphic BaTiO3 interspersed between Al2O3 layers in tri-layered dielectric thin film capacitors, the outstanding combination of high dielectric constant and low leakage current for the polymorphic BaTiO3 - Al2O3 thin film stacks is superior to common high-κ polycrystalline thin film materials.

Shift and ballistic photovoltaic currents, previously indistinguishable experimentally, can now be separated., The quantum phenomenon of shift photovoltaic current was predicted decades ago, but this effect was never observed directly because shift and ballistic currents coexist. The atomic-scale relaxation time of shift, along with the absence of a photo-Hall behavior, has made decisive measurement of shift elusive. Here, we report a facile, direct-current, steady-state method for unambiguous determination of shift by means of the simultaneous measurements of linear and circular bulk photovoltaic currents under magnetic field, in a sillenite piezoelectric crystal. Comparison with theoretical predictions permits estimation of the signature length scale for shift. Remarkably, shift and ballistic photovoltaic currents under monochromatic illumination simultaneously flow in opposite directions. Disentangling the shift and ballistic contributions opens the way for quantitative, fundamental insight into and practical understanding of these radically different photovoltaic current mechanisms and their relationship.

We report on the high temperature thin film growth of BaTiO3 on Ti3C2Tx MXene flakes using van der Waals epitaxy on a degradable template layer. MXene was deposited on amorphous and crystalline substrates by spray- and dip-coating techniques, while the growth of BaTiO3 at 700 °C was accomplished using pulsed laser deposition in an oxygen rich environment. We demonstrate that the MXene flakes act as a temporary seed layer, which promotes highly oriented BaTiO3 growth along the (111) direction independent of the underlying substrate. The lattice parameters of the BaTiO3 films are close to the bulk value suggesting that the BaTiO3 films remains unstrained, as expected for van der Waals epitaxy. The initial size of the MXene flakes has an impact on the orientation of the BaTiO3 films with larger flake sizes promoting a higher fraction of the polycrystalline film to grow along the (111) direction. The deposited BaTiO3 film adopts the same morphology as the original flakes and piezoresponse force microscopy shows a robust ferroelectric behavior for individual grains. Transmission electron microscopy results indicate that the Ti3C2Tx MXene fully decomposes during the BaTiO3 deposition and the surplus Ti atoms are readily incorporated into the BaTiO3 film. Electrical measurements show a similar dielectric constant as a BaTiO3 film grown without the MXene seed layer. The demonstrated process has the potential to overcome the longstanding issue of integrating highly oriented complex oxide thin films directly on any desired substrate.

Traditionally, the ferroelectric Curie temperature can be manipulated by chemical substitution, e.g., in Ba1−xSrxTiO3 as one of the archetypical representatives. Here, we show a novel approach to tune the ferroelectric phase transition applicable for nanostructured thin films. We demonstrate this effect in nano-grained BaTiO3 films. Based on an enhanced metastable cation solubility with Ba/Ti-ratios of 0.8 to 1.06, a significant shift of the phase transition temperature is discovered. The transition temperature increases linearly from 212 K to 350 K with increasing Ba/Ti ratio. For all Ba/Ti ratios, a completely diffused phase transition is present resulting in a negligible temperature sensitivity of the dielectric constant. Schottky defects are identified as the driving force behind the off-stoichiometry and the shift of the phase transition temperature as they locally induce lattice strain. Complementary temperature dependent Raman experiments reveal the presence of the hexagonal polymorph in addition to the perovskite phase in all cases. Interestingly, the hexagonal BaTiO3 influences the structural transformation on the Ba-rich side, while on the Ti-rich side no changes for the hexagonal polymorph at the ferroelectric transition temperature are observed. This concerted structural change of both polymorphs on the Ba-rich side causes a broad phase transition region spanning over a wide range up to 420 K including the transition temperature of 350 K obtained from dielectric measurements. These findings are promising for fine adjustment of the phase transition temperature and low temperature coefficient of permittivity.

Thin films of ≈50 nm thickness with Ba/Ti-ratios ranging from 0.8 to 1.06 were prepared by depositing alternating layers of Ba(OH)2 and TiO2. Annealing at 750 °C promoted the solid-solid transformation into polycrystalline BaTiO3 films containing a mixture of the perovskite and the hexagonal polymorphs with average crystallite sizes smaller than 14 nm and without impurity phases. This, together with an increase of the cubic lattice parameters for Ba-rich films, suggests an extended metastable solubility range for the perovskite-phase in these nanocrystalline thin films on both sides of the stoichiometric composition. Mapping of the cation distribution utilizing energy-filtered transmission electron microscopy corroborates defect accommodation within the BaTiO3 grains. While the cation off-stoichiometry in thermodynamic equilibrium is negligible for BaTiO3, the metastable extended solubility range in the thin films can be directly correlated to the low annealing temperature and nanocrystalline nature. The leakage current behavior can be explained by the formation of Schottky defects for nonstoichiometric films, and the cation ratio has a distinct impact on the dielectric properties: while excess-BaO has a marginal detrimental effect on the permittivity, the dielectric constant declines rapidly by more than 50% towards the Ti-rich side. The present findings highlight the importance of compositional control for the synthesis of nanocrystalline BaTiO3 thin films, in particular for low annealing and/or deposition temperatures. Our synthesis approach using alternating layers of Ba(OH)2 and TiO2 provides a route to precisely control the cation stoichiometry.

The phase transition evolution for ${[{\mathrm{KNbO}}_{3}]}_{1\ensuremath{-}x}{[{\mathrm{BaNi}}_{1/2}{\mathrm{Nb}}_{1/2}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}]}_{x}\phantom{\rule{0.16em}{0ex}}(x=0,0.1)$ is determined via complementary dielectric permittivity and Raman-scattering measurements. Raman scattering by optical phonons over the range of 100--$1000{\mathrm{cm}}^{\ensuremath{-}1}$ for 83 K $lTl873$ K reveals six discernible zone-center optical phonon modes. Mode behaviors are observed through structural and ferroelectric phases in the solid solution $x=0.1$ and compared with those for end member $x=0$ and with the results of temperature-dependent dielectric permittivity. Rigorous peak fitting analyses of spectra collected from the solid solution and end member indicate structural and ferroelectric phase transition temperatures that are close to those for the ${\mathrm{KNbO}}_{3}$ end member despite the inclusion of 5 atomic % of ferroelectrically inactive Ni cations. Density functional theory calculations were performed in the solid solution and end member using both cation displacement and Berry phase-based methods. Differences in the electronic and polar properties between the solid solution and the end member highlights local and nonlocal characteristics, which are discussed in relation to the experimental data.

Ordering of ferroelectric polarization1 and its trajectory in response to an electric field2 are essential for the operation of non-volatile memories3, transducers4 and electro-optic devices5. However, for voltage control of capacitance and frequency agility in telecommunication devices, domain walls have long been thought to be a hindrance because they lead to high dielectric loss and hysteresis in the device response to an applied electric field6. To avoid these effects, tunable dielectrics are often operated under piezoelectric resonance conditions, relying on operation well above the ferroelectric Curie temperature7, where tunability is compromised. Therefore, there is an unavoidable trade-off between the requirements of high tunability and low loss in tunable dielectric devices, which leads to severe limitations on their figure of merit. Here we show that domain structure can in fact be exploited to obtain ultralow loss and exceptional frequency selectivity without piezoelectric resonance. We use intrinsically tunable materials with properties that are defined not only by their chemical composition, but also by the proximity and accessibility of thermodynamically predicted strain-induced, ferroelectric domain-wall variants8. The resulting gigahertz microwave tunability and dielectric loss are better than those of the best film devices by one to two orders of magnitude and comparable to those of bulk single crystals. The measured quality factors exceed the theoretically predicted zero-field intrinsic limit owing to domain-wall fluctuations, rather than field-induced piezoelectric oscillations, which are usually associated with resonance. Resonant frequency tuning across the entire L, S and C microwave bands (1-8 gigahertz) is achieved in an individual device-a range about 100 times larger than that of the best intrinsically tunable material. These results point to a rich phase space of possible nanometre-scale domain structures that can be used to surmount current limitations, and demonstrate a promising strategy for obtaining ultrahigh frequency agility and low-loss microwave devices.

Until now, metal–organic hybrid perovskites (MOHPs) have been known mainly for their remarkable electronic (1⇓–3) and optical properties, generating immense interest worldwide and offering the potential to realize the goal of highly efficient and inexpensive solar power conversion (1, 4, 5) as well as a wide variety of other optoelectronic applications (6, 7). In an exciting development that promises to electrify the already hot field of MOHPs, Rakita et al. (8) for the first time demonstrate unambiguously that a tetragonal trihalide perovskite is ferroelectric, associated with a spontaneous and reconfigurable ion ordering. This opens the door for the realization of multiple functionalities derived from ferroelectric behavior and a wide range of devices that can be enabled by the juxtaposition of ferroelectricity and the excellent semiconducting and electronic structure properties of MOHPs. Ferroelectricity, defined as the presence of a spontaneous and switchable bulk dipole moment (polarization), has a long history of turning up in unexpected classes of materials. Ferroelectric behavior was first discovered in 1920 in the Rochelle salt compound (9). Upon the discovery of ferroelectricity in compounds such as potassium dihydrogen phosphate (KDP) in the 1930s (10), ferroelectricity was thought to be a curiosity restricted to compounds with hydrogen bonds and complicated unit cells. Approximately twenty years later, the discovery of ferroelectric behavior and excellent piezoelectric properties of man-made BaTiO3 (with a simple five-atom unit cell) (11, 12) spurred an upsurge of ferroelectric research. Ferroelectric behavior has been observed in a wide range of perovskite oxides, including the lead zirconate titanate solid solution (13) that has formed the mainstay of piezoelectric devices for several decades. Theoretically, ferroelectric behavior was placed in the wider context of symmetry-breaking materials by Devonshire (14). This worked to unify the different mechanisms of ferroelectricity (e.g., hydrogen-bond-driven in … [↵][1]1To whom correspondence should be addressed. Email: spanier{at}drexel.edu. [1]: #xref-corresp-1-1

We report on the growth of polycrystalline BiFeO

The perovskite phase of cesium lead iodide (α-CsPbI3 or “black” phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional “yellow” phase (δ-CsPbI3). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state. Using differential scanning calorimetry, it is shown herein that the metastable state is maintained in the absence of moisture, up to a temperature of 100 °C, and a reversible phase-change enthalpy of 14.2 (±0.5) kJ/mol is observed. The presence of atmospheric moisture hastens the black-to-yellow conversion kinetics without significantly changing the enthalpy of the transition, indicating a catalytic effect, rather than a change in equilibrium due to water adduct formation. These results delineate the conditions for trapping the desired phase and highlight the significant magnitude of the entropic stabilization of thi...

Group III-V coaxial core-shell semiconducting nanowire heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic, and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in nanowires are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual nanowires. Here, we report on, in the GaAs/AlxGa 1-xAs and GaAs/AlAs core-shell nanowire systems, how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets.

The introduction low levels of oxygen during the vapor–liquid–solid growth (VLS) of germanium nanowires causes an oxide sheath to form at the catalyst/nanowire/vapor interface during growth. This results in extremely high aspect ratio nanowires due to the removal of homoepitaxial deposition and the finite energy required for heterogeneous nucleation of germanium on its oxide. With the removal of oxygen, the catalyzed oxide sheath terminates and conventional growth with finite sidewall deposition dominates subsequent growth. The successful transition between oxide-stabilized and conventional VLS regimes can be deliberately manipulated to grow finite conical nanowire segments with discontinuous changes in diameter.

We show how finite-size scaling of a bulk photovoltaic effect-generated electric field in epitaxial ferroelectric insulating BaTiO_{3}(001) films and a photo-Hall response involving the bulk photovoltaic current reveal a large room-temperature mean free path of photogenerated nonthermalized electrons. Experimental determination of mesoscopic ballistic optically generated carrier transport opens a new paradigm for hot electron-based solar energy conversion, and for facile control of ballistic transport distinct from existing low-dimensional semiconductor interfaces, surfaces, layers, or other structures.

The synthesis of cylindrical silicon-core and ferroelectric oxide perovskiteshell nanowires and their response characteristics as individual threeterminal nanoscale electronic devices is reported. The co-axial nanowire geometry facilitates large ferroelectric fi eld-effect modulation ( > 10 4 ) of nanowire conductivity following sequential application and removal of an applied dc fiSource-drain current‐voltage traces collected during sweeps of ferroelectric gate potential and switching of the component of shell outward and inward polarization provide direct evidence of ferroelectric coupling on nanowire channel conductance. Despite a very small (1:20) ferroelectric-tosemiconductor channel thickness ratio, an unexpectedly strong electrostatic coupling of ferroelectric polarization to channel conductance is observed because of the co-axial gate geometry and curvature-induced strain enhancement of ferroelectric polarization.

A vapor transport method is employed to synthesize single crystalline tellurium nanowires with tailorable size using tellurium powder in an inert atmosphere. The average nanowire diameter is tunable between 50 and 3000 nm with an associated length of 1 to 22 μm. Growth temperature and time are used to specifically control the behavior of supersaturation, leading to nucleation and morphological control in this vapor−solid growth regime. Analysis of the resulting nanowire product provides insight into the dominant reaction kinetics involved in these growths and suggests routes by which to control growth product. This methodology provides a practical approach to synthesizing high-quality tellurium nanowires of various sizes and their incorporation into two-dimensional mesh-like structures through controlled nucleation and growth dynamics.

Thin films of LaAlO3 were deposited on TiO2-terminated (100) SrTiO3 crystals by atomic layer deposition (ALD), using tris(iso-propylcyclopentadienyl)lanthanum and trimethyl aluminum precursors. Water was used as the oxidizer. The film composition was shown to be controlled by the ratio of La/Al precursor pulses during ALD, with near-stoichiometric LaAlO3 resulting at precursor pulse ratios of 4/1 to 5/1. Films near the stoichiometric LaAlO3 composition were shown to crystallize on subsequent annealing to form epitaxial LaAlO3/SrTiO3 heterostructures. Electrical characterization of these structures was done by two-terminal direct-current (DC) current–voltage scans at room temperature and under high-vacuum conditions. The results show electrical conductivity for the ALD-deposited epitaxial LaAlO3/SrTiO3 heterostructures, which turns on for thickness above four unit cells for the LaAlO3 film.

This paper describes an MOCVD technique for producing both monolithic and compositionally graded BaxSr1-xTiO3 (BST) thin films for tunable RF device applications. Specifically, we describe a “multiple injection flash evaporation” technique, which can easily produce BST thin films with a wide variety of compositional grading and dopant profiles. Preliminary results show the films to have excellent voltage standoff properties and tunability of dielectric permittivity. These materials are presently being used in the development of tunable RF filters for communication and radar applications.

In this paper, we report the results of third order combline inter-digited band pass filters implemented with parallel plate barium strontium titanate (BST) capacitors on sapphire substrates with platinum electrodes. The frequency characteristics of the filters were simulated using Sonnet software package. The resonators were implemented on FR-4 substrates using a LPKF rapid prototyping machine. The resonators were loaded with BST capacitors prepared on sapphire substrates by metalorganic decomposition (MOD). The measured center frequency of the filters was 1.35 GHz with a 3 db bandwidth of 0.15 GHz for zero bias voltage to the ferroelectric capacitors. The center frequency can be tuned from 1.35 GHz to1.45 GHz for an applied tuning voltage of 3 Volts.

We present the results of characterization and analysis of the optical dielectric function of monolithic Ba x Sr1-x TiO 3 films prepared by metal-organic solution deposition (MOSD). Lorentz Oscillator + Drude parameters and band gap for selected compositions are determined from variable-angle spectroscopic ellipsometry. Variation of the complex optical dielectric function is seen, and the results suggest that spectroscopic ellipsometry can be an effective means of both ex situ analysis and in situ monitoring of film composition during other BST and related film material growth processes.

We demonstrate that ferroelectric (FE) polarizations oriented along the finite thickness direction in ultrathin films are enhanced by the introduction of extreme curvature, thereby suppressing the finite-size-driven evolution of the FE phase transition temperature T(C). The measured responses within individual nanoshells possess magnitudes nearly three times that for their planar counterparts while exhibiting finite curvature-dependent offsets in FE switching hystereses. In stark contrast to the expected scaling of a depression of T(C) with inverse thickness, results based on modified Landau-Ginzburg model calculations indicate geometric curvature-driven polarization gradients in ultrathin films result in significant increases in T(C).

We report on the chemical synthesis, structural and compositional characterization, and hierarchical organization of Co-core, concentric CoO–Au multi-shell nanocrystals (Co@CoO@Au). Based on electron microscopy, magnetometry and spectroscopy experiments, we present compelling evidence for the formation of the Au outer shell on CoO, challenging the common assumption that the reduction reaction to form Au shells can only occur if the cobalt surface has not oxidized. Our findings suggest that the presence of a metal shell surrounding a transition-metal core nanocrystal following such a reduction reaction cannot be taken as evidence that the transition metal oxide is absent from the surface of the nanocrystal core. We find that Au shell growth can produce Co@CoO@Au nanocrystals possessing five-fold twinning symmetry and we suggest that their growth is facilitated through self nucleation and coalescence of Au particles.

Using a synchrotron X-ray radiation source and a diamond anvil cell we measured the dependences of the lattice parameters on quasi-hydrostatic pressure of the order of 50 GPa of the following MAX phases: Ti2InC, (Ti0.5,Zr0.5)2InC, Zr2InC and Ti2SnC, Nb2InC and Hf2InC. Like other MAX phases, the phases studied herein were all stable up to ≈50 GPa. In both series, the substitution of Ti ( r = 1.32 A ) by the larger sized metals, Zr or Nb (with r varying between 1.34 and 1.55 A) resulted in larger unit cell parameters and volumes. At 152±3 and 148±3 GPa, the respective bulk moduli, K o , of Ti2SnC and Ti2InC are quite comparable. Replacing Ti by Hf or Nb in Ti2SnC leads to increases in K o by 11% and 18%, respectively. Conversely, replacing half the Ti by Zr in Ti2InC leads to a 13% drop in K o ; replacing all of the Ti drops it by 16.5%. Most of these trends are reproduced in our ab initio calculations of K o . For all compositions, the compressibilities along the c -direction were greater than those along the a -direction. For the M2SnC series, the compressibilities along the c -axes were quite similar; the compressibilities along the a -axis of Ti2SnC were greater than in the Nb- or Hf-containing ternaries. The c -axis compressibilities of the In-containing compounds were almost indistinguishable. The compressibilities along the a -axes of (Ti0.5,Zr0.5)2InC and Zr2InC were also quite comparable; those of Ti2InC was less compressible.

Renewed interest in the evolution of the ferroelectric phase transition temperature TC and the character of ordering of ferroelectric polarizations with finite size and shape is driven in part by several recent developments. An expanding array of pathways for producing nano-structured ferroelectric oxides with control of size, shape, and composition has emerged. Experimental characterization methods originally developed for thin films have been extended to ensemble-free investigations of functional properties of individual nanostructures. Progress in understanding the origin and nature of ferroelectric stability in ultra-thin films and nanostructures is reviewed. Specifically, we discuss evidence for a new surface adsorbate-driven mechanism for stabilizing ferroelectricity in nanostructures owing to a combination of recent proximal probe analysis and model calculation results, along with a new experimental paradigm for investigating and exploiting these effects and effects of finite curvature.

We have fabricated and characterized 2-D gas-based, including 2-D electron gas (2DEG) and 2-D hole gas (2DHG), heterostructure metal-semiconductor-metal (MSM) photodetectors on GaAs. Both the high-speed measurement of time response and the simulation results show that a vertical field developed in the active absorption region due to the delta-doping layer facilitates one type of photogenerated carrier transport. In addition, the confined carriers facilitate collection of the optically generated carriers that reach them. The vertical field in the MSM. structure that is created by a 2-D gas transforms a traditional lateral MSM device to a vertical one, although remaining as a planar structure, thus allowing a device design for high-speed performance without sacrificing the external quantum efficiency.

The interaction between light and reduced-dimensionality silicon attracts significant interest due to the possibilities of designing nanoscaled optical devices, highly cost-efficient solar cells, and ultracompact optoelectronic systems that are integrated with standard microelectronic technology. We demonstrate that Si nanowires (SiNWs) possessing metal-nanocluster coatings support a multiplicatively enhanced near-field light-matter interaction. Raman scattering from chemisorbed probing molecules provides a quantitative measure of the strength of this enhanced coupling. An enhancement factor of 2 orders of magnitude larger than that for the surface plasmon resonance alone (without the SiNWs) along with the attractive properties of SiNWs, including synthetic controllability of shape, indicates that these nanostructures may be an attractive and versatile material platform for the design of nanoscaled optical and optoelectronic circuits.

The Shockley–Queisser limit for solar cells is overcome in the ferroelectric insulator BaTiO3.

We report on the one-phonon Raman scattering spectra from the following M2AC MAX-phase ternary carbides: Ti2AlC, V2AlC, Cr2AlC, Nb2AlC, Ta2AlC, Ti2InC, Hf2InC, V2GeC, Cr2GeC, V2AsC, and Nb2AsC. We also report the results of calculations of the Γ-point, Raman-active phonon energies for these phases based on density functional theoretical simulations, including the effect of the k-point sampling on the convergence of phonon energies. Good agreement between all measured and calculated Γ-point Raman-active optical phonon energies is obtained.

We present a short review of recent work on the unusual Raman scattering characteristics in semiconductor nanowires, focusing on the dependence of the intensity, spectral and spatial character of the scattering response on the size, shape and composition of the nanowires. Results of Raman scattering collected from individual Si and nanowires and nanocones indicate that the observed enhancement is dependent on the diameter (d), the excitation wavelength (λlaser) and the incident polarization state. Strong agreement between experimental data and model calculations of scattering from an infinite dielectric cylinder is found. This size dependence is in accordance with other recent findings in which a diameter-dependent dipolar ‘antenna’ pattern for the scattered intensity and a cross-sectional shape dependence of the surface optic (SO) phonon dispersions were seen. Further providing evidence of the nanowire shape and size dependence on Raman scattering, these results suggest manipulation of classical electromagnetic scattering signatures, thereby expanding opportunities for engineering the photonic and sensing properties of nanowires. Copyright © 2007 John Wiley & Sons, Ltd.

We investigate the origin of the depolarization rates in ultrathin adsorbate-stabilized ferroelectric wires. By applying density functional theory calculations and analytic modeling, we demonstrate that the depolarization results from the leakage of charges stored at the surface adsorbates, which play an important role in the polarization stabilization. The depolarization speed varies with thickness and temperature, following several complex trends. A comprehensive physical model is presented, in which quantum tunneling, Schottky emission, and temperature-dependent electron mobility are taken into consideration. This model simulates experimental results, validating the physical mechanism. We also expect that this improved tunneling-Schottky emission model could be applied to predict the retention time of polarization and the leakage current for various ferroelectric materials with different thicknesses and temperatures.

We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO3) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling. The diameter at which TC falls below room temperature is determined to be approximately 3 nm, and extrapolation of the data indicates that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining the observed stability of ferroelectricity in small-diameter BaTiO3 nanowires.