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Resolving the Heat Generated from ZrO 2 Atomic Layer Deposition Surface Reactions
Abstract In situ pyroelectric calorimetry and spectroscopic ellipsometry were used to investigate surface reactions in atomic layer deposition (ALD) of zirconium oxide (ZrO 2 ). Calibrated and time‐resolved in situ ALD calorimetry provides new insights into the thermodynamics and kinetics of saturating surface reactions for tetrakis(dimethylamino)zirconium(IV) (TDMAZr) and water. The net ALD reaction heat ranged from 0.197 mJ cm −2 at 76 °C to 0.155 mJ cm −2 at 158 °C, corresponding to an average of 4.0 eV/Zr at all temperatures. A temperature dependence for reaction kinetics was not resolved over the range investigated. The temperature dependence of net reaction heat and distribution among metalorganic and oxygen source exposure is attributed to factors including growth rate, equilibrium surface hydroxylation, and the extent of the reaction. ZrO 2 ‐forming surface reactions were investigated computationally using DFT methods to better understand the influence of surface hydration on reaction thermodynamics.
Resolving the Heat Generated from ZrO 2 Atomic Layer Deposition Surface Reactions
Abstract In situ pyroelectric calorimetry and spectroscopic ellipsometry were used to investigate surface reactions in atomic layer deposition (ALD) of zirconium oxide (ZrO 2 ). Calibrated and time‐resolved in situ ALD calorimetry provides new insights into the thermodynamics and kinetics of saturating surface reactions for tetrakis(dimethylamino)zirconium(IV) (TDMAZr) and water. The net ALD reaction heat ranged from 0.197 mJ cm −2 at 76 °C to 0.155 mJ cm −2 at 158 °C, corresponding to an average of 4.0 eV/Zr at all temperatures. A temperature dependence for reaction kinetics was not resolved over the range investigated. The temperature dependence of net reaction heat and distribution among metalorganic and oxygen source exposure is attributed to factors including growth rate, equilibrium surface hydroxylation, and the extent of the reaction. ZrO 2 ‐forming surface reactions were investigated computationally using DFT methods to better understand the influence of surface hydration on reaction thermodynamics.
Ferroelectric Phase Content in 7 nm Hf (1- x ) Zr x O 2 Thin Films Determined by X-Ray-Based Methods
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Frequency locking and monitoring based on Bi-directional terahertz radiation of a 3 rd -order distributed feedback quantum cascade laser
In this study, we have performed frequency locking of a dual, forward reverse emitting third-order distributed feedback quantum cascade laser (QCL) at 3.5 THz. By using both directions of THz emission in combination with two gas cells and two power detectors, we can for the first time perform frequency stabilization, while monitor the frequency locking quality independently. We also characterize how the use of a less sensitive pyroelectric detector can influence the quality of frequency locking, illustrating experimentally that the sensitivity of the detectors is crucial. Using both directions of terahertz (THz) radiation has a particular advantage for the application of a QCL as a local oscillator, where radiation from one side can be used for frequency/phase stabilization, leaving the other side to be fully utilized as a local oscillator to pump a mixer.
Measurement of advective soil gas flux: Results of field and laboratory experiments with CO2
We modified our multi-channel, steady-state flow-through (SSFT), soil-CO2 flux monitoring system to include an array of inexpensive pyroelectric non-dispersive infrared detectors for full-range (0-100%) coverage of CO2 concentrations without dilution, and a larger-diameter vent tube. We then conducted field testing of this system from late July through mid-September 2010 at the Zero Emissions Research and Technology (ZERT) project site located in Bozeman, MT, and subsequently, laboratory testing at the Pacific Northwest National Laboratory (PNNL) in Richland, WA using a flux bucket filled with dry sand. In the field, an array of twenty-five SSFT and three non-steady-state (NSS) flux chambers was installed in a 10x4 m area, the long boundary of which was directly above a shallow (2-m depth) horizontal injection well located 0.5 m below the water table. Two additional chambers (one SSFT and one NSS) were installed 10 m from the well for background measurements. Volumetric soil moisture sensors were installed at each SSFT chamber to measure mean levels in the top 0.15 m of soil. A total flux of 52 kg CO2 d-1 was injected into the well for 27 d and the efflux from the soil was monitored by the chambers before, during, and for 27 d after the injection. Overall, the results were consistent with those from previous years, showing a radial efflux pattern centered on a known “hot spot”, rapid responses to changes in injection rate and wind power, evidence for movement of the CO2 plume during the injection, and nominal flux levels from the SSFT chambers that were up to 6-fold higher than those measured by adjacent NSS chambers. Soil moisture levels varied during the experiment from moderate to near saturation with the highest levels occurring consistently at the hot spot. The effects of wind on measured flux were complex and decreased as soil moisture content increased. In the laboratory, flux bucket testing with the SSFT chamber showed large measured-flux enhancement due to the Venturi effect on the chamber vent, but an overall decrease in measured flux when wind also reached the sand surface. Flux-bucket tests at a high flux (comparable to that at the hot spot) also showed that the measured flux levels increase linearly with the chamber-flushing rate until the actual level is reached. At the SSFT chamber-flushing rate used in the field experiment the measured flux in the laboratory was only about a third of the actual flux. The ratio of measured to actual flux increased logarithmically as flux decreased, and reached parity at low levels typical of diffusive flux systems. Taken together, our results suggest that values for advective CO2 flux measured by SSFT and NSS chamber systems are likely to be significantly lower than the actual values due to back pressure developed in the chamber that diverts flux from entering the chamber. Chamber designs that counteract the back pressure and also avoid large Venturi effects associated with vent tubes, such as the SSFT with a narrow vent tube operated at a high chamber-flushing rate, are likely to yield flux measurements closer to the true values.
Crystal structure and magnetic properties of the Ba{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}, Pb{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}, and Pb{sub 3}TeCo{sub 3}V{sub 2}O{sub 14} langasites
We report the structural and magnetic characterizations of Ba{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}, Pb{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}, and Pb{sub 3}TeCo{sub 3}V{sub 2}O{sub 14}, compounds that are based on the mineral dugganite, which is isostructural to langasites. The magnetic part of the structure consists of layers of Co{sup 2+} triangles. Nuclear and magnetic structures were determined through a co-refinement of synchrotron and neutron powder diffraction data. In contrast to the undistorted P321 langasite structure of Ba{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}, a complex structural distortion yielding a large supercell is found for both Pb{sub 3}TeCo{sub 3}P{sub 2}O{sub 14} and Pb{sub 3}TeCo{sub 3}V{sub 2}O{sub 14}. Comparison of the three compounds studied along with the zinc analog Pb{sub 3}TeZn{sub 3}P{sub 2}O{sub 14}, also characterized here, suggests that the distortion is driven by Pb{sup 2+} lone pairs; as such, the Pb compounds crystallize in a pyroelectric space group, P2. Magnetic susceptibility, magnetization, and heat capacity measurements were performed to characterize the magnetic behavior. All three compounds become antiferromagnetic with Néel temperatures T{sub N}≈21 K (Ba{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}), ≈13 K (Pb{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}), and ≈8 K (Pb{sub 3}TeCo{sub 3}V{sub 2}O{sub 14}), and they exhibit magnetic transitions at high applied magnetic fields, suggesting intrinsically complex magnetic behavior for tetrahedrally coordinated d{sup 7} Co{sup 2+} in this structure type. - Graphical abstract: Ba{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}, Pb{sub 3}TeCo{sub 3}P{sub 2}O{sub 14} and Pb{sub 3}TeCo{sub 3}V{sub 2}O{sub 14} exhibit a triangles-of-triangles arrangement of the magnetic cations. Despite the Pb-induced crystallographic distortion, all of these materials exhibit competing antiferromagnetic and ferromagnetic interactions that lead to complex high field magnetism. - Highlights: • Nuclear and magnetic structures of Ba{sub 3}TeCo{sub 3}P{sub 2}O{sub 14}, Pb{sub 3}TeCo{sub 3}P{sub 2}O{sub 14} and Pb{sub 3}TeCo{sub 3}V{sub 2}O{sub 14}. • Structural characterization shows complex Pb-induced distortion. • Distortion puts Pb-compounds into space group that allows potential multiferroicity. • Magnetic characterization shows competing magnetic interactions. • Triangles of triangles motif results in complex high-field magnetic behavior.
Piezoelectric and ferroelectric properties of lead-free niobium-rich potassium lithium tantalate niobate single crystals
Graphical abstract: - Highlights: • Lead-free K{sub 0.95}Li{sub 0.05}Ta{sub 1−x}Nb{sub x}O{sub 3} single crystals were grown using the top-seeded melt growth method. • The piezoelectric and ferroelectric properties of as-grown crystals were systematically investigated. • The piezoelectric properties are very attractive, e.g. for x = 0.60 composition, k{sub t} ≈ 70%, k{sub 31} ≈ 70%, k{sub 33} ≈ 77%, d{sub 31} ≈ 230 pC/N, d{sub 33} ≈ 600 pC/N. • The coercive fields of P–E hysteresis loops are quite small, about or less than 1 kV/mm. - Abstract: Lead-free potassium lithium tantalate niobate single crystals with the composition of K{sub 0.95}Li{sub 0.05}Ta{sub 1−x}Nb{sub x}O{sub 3} (abbreviated as KLTN, x = 0.51, 0.60, 0.69, 0.78) were grown using the top-seeded melt growth method. Their piezoelectric and ferroelectric properties in as-grown crystals have been systematically investigated. The phase transitions and Curie temperatures were determined from dielectric and pyroelectric measurements. Piezoelectric coefficients and electromechanical coupling factors in thickness mode, length-extensional mode and longitudinal mode were obtained. The piezoelectric properties are very attractive, e.g. for x = 0.60 composition, k{sub t} ≈ 70%, k{sub 31} ≈ 70%, k{sub 33} ≈ 77%, d{sub 31} ≈ 230 pC/N, d{sub 33} ≈ 600 pC/N are comparable to the lead-based PZT composition. The polarization versus electric field hysteresis loops show saturated shapes. In short, lead-free niobium-rich KLTN system possesses comparable properties to those in important lead-based piezoelectric material nowadays.
Energy transduction ferroic materials
Ferroic materials and multiferroics, characterized by their ferroic orders, provide an efficient route for the coupling control of magnetic, mechanical, and electrical subsystems in energy transduction, which aims at converting one form of energy into another. A surge of interest in the ferroic coupling effect has stemmed from its potential use as a new versatile route for energy transduction. Here, the recent progress on the use of (multi)ferroic materials is reviewed, with special emphasis on the fundamental mechanisms that dictate the energy transduction process, including piezoelectricity, pyroelectricity, electrocaloric, magnetostriction, magnetocaloric, elastocaloric, magnetoelectricity, and emerging spin-charge conversion. Research on energy transduction ferroic materials paves the way for ubiquitous energy harvesting through magneto-mechano-electric-thermal coupling mechanisms. Lastly, a summary and the future prospective directions of this field are discussed.
Multifidelity Information Fusion with Machine Learning: A Case Study of Dopant Formation Energies in Hafnia
Cost versus accuracy trade-offs are frequently encountered in materials science and engineering, where a particular property of interest can be measured/computed at different levels of accuracy or fidelity. Naturally, the most accurate measurement is also the most resource and time intensive, while the inexpensive quicker alternatives tend to be noisy. In such situations, a number of machine learning (ML) based multifidelity information fusion (MFIF) strategies can be employed to fuse information accessible from varying sources of fidelity and make predictions at the highest level of accuracy. In this work, we perform a comparative study on traditionally employed single-fidelity and three MFIF strategies, namely, (1) Δ-learning, (2) low-fidelity as a feature, and (3) multifidelity cokriging (CK) to compare their relative prediction accuracies and efficiencies for accelerated property predictions and high throughput chemical space explorations. We perform our analysis using a dopant formation energy data set for hafnia, which is a well-known high- k material and is being extensively studied for its promising ferroelectric, piezoelectric, and pyroelectric properties. We use a dopant formation energy data set of 42 dopants in hafnia—each studied in six different hafnia phases—computed at two levels of fidelities to find merits and limitations of these ML strategies. The findings of this work indicate that the MFIF based learning schemes outperform the traditional SF machine learning methods, such as Gaussian process regression and CK provides an accurate, inexpensive and flexible alternative to other MFIF strategies. While the results presented here are for the case study of hafnia, they are expected to be general. Furthermore, materials discovery problems that involve huge chemical space explorations can be studied efficiently (or even made feasible in some situations) through a combination of a large number of low-fidelity and a few high-fidelity measurements/computations, in conjunction with the CK approach.
Thermodynamics of MgO Atomic Layer Deposition Surface Reactions
The realities of atomic layer deposition (ALD) surface reactions often deviate from the simple ligand exchange frequently used to illustrate the technique. A detailed understanding of these reactions is necessary to develop greater surface synthetic control including chemical selectivity for patterning or to target defects. Here, the thermodynamics of surface reactions relevant to MgO ALD were investigated using pyroelectric calorimetry to measure the time-resolved heat generation. These reactions show exothermic heat generation of 0.12 mJ/cm 2 and 0.15 mJ/cm 2 for alternating Mg(CpEt) 2 and H 2 O reactions, respectively. The total reaction heat closely matches the standard reaction enthalpy for bulk MgO, supplemented with first-principles molecular calculations. First-principles models further reveal that while simple ligand exchange is favorable during surface reactions, the required increase in Mg-coordination number from two in the precursor to six in bulk MgO requires additional coverage-dependent surface reactions, which depend on the availability and stability of proximal surface hydroxyls.
Large polarization gradients and temperature-stable responses in compositionally-graded ferroelectrics
A range of modern applications require large and tunable dielectric, piezoelectric or pyroelectric response of ferroelectrics. Such effects are intimately connected to the nature of polarization and how it responds to externally applied stimuli. Ferroelectric susceptibilities are, in general, strongly temperature dependent, diminishing rapidly as one transitions away from the ferroelectric phase transition (T C ). In turn, researchers seek new routes to manipulate polarization to simultaneously enhance susceptibilities and broaden operational temperature ranges. Here, we demonstrate such a capability by creating composition and strain gradients in Ba 1-x Sr x TiO 3 films which result in spatial polarization gradients as large as 35 μC cm -2 across a 150 nm thick film. These polarization gradients allow for large dielectric permittivity with low loss (ε r ≈775, tan δ<0.05), negligible temperature-dependence (13% deviation over 500 °C) and high-dielectric tunability (greater than 70% across a 300 °C range). The role of space charges in stabilizing polarization gradients is also discussed.
Energy conversion and storage via photoinduced polarization change in non-ferroelectric molecular [CoGa] crystals
To alleviate the energy and environmental crisis, in the last decades, energy harvesting by utilizing optical control has emerged as a promising solution. Here we report a polar crystal that exhibits photoenergy conversion and energy storage upon light irradiation. The polar crystal consists of dinuclear [CoGa] molecules, which are oriented in a uniform direction inside the crystal lattice. Irradiation with green light induces a directional intramolecular electron transfer from the ligand to a low-spin Co III centre, and the resultant light-induced high-spin Co II excited state is trapped at low temperature, realizing energy storage. Additionally, electric current release is observed during relaxation from the trapped light-induced metastable state to the ground state, because the intramolecular electron transfer in the relaxation process is accompanied with macroscopic polarization switching at the single-crystal level. It demonstrates that energy storage and conversion to electrical energy is realized in the [CoGa] crystals, which is different from typical polar pyroelectric compounds that exhibit the conversion of thermal energy into electricity.
Magnetic field-induced ferroelectricity in S = 1/2 kagome staircase compound PbCu 3 TeO 7
Ni 3 V 2 O 8 is an archetypical multiferroic material with a kagome staircase structure of S = 1 (Ni 2+ ) spins whose complex interplay between spin ordering and ferroelectricity has been studied for more than a decade. Here in this paper, we report a new kagome staircase compound PbCu 3 TeO 7 with Cu 2+ (S = 1/2) spins that exhibits two Néel temperatures at T N1 = 36 K and T N2 = 24 K, and a magnetic field (H)-induced electric polarization (P) below T N2 . Pyroelectric and magnetoelectric current measurements in magnetic fields up to 60 T reveal that for H||c of ~ 8.3 T, a spin-flop transition induces a transverse P||a with a magnitude of 15 µC/m 2 below T N2 . Furthermore, for a parallel configuration with P//H//a, two spin-flop transitions occur, the first at ~ 16 T with P//a of 14 µC/m 2 , and the second at ~ 38 T, where P disappears. Monte Carlo simulations based on 12 major exchange interactions uncover that a sinusoidal amplitude modulation of the spins occurs along the b-axis below T N1 and an incommensurate, proper screw-type spin order occurs in the ac-plane below T N2 . The simulation results show that P//a under H//c stems from a spin-flop transition facilitating an ab-plane-type spiral order, while the two successive spin-flop transitions for H//a result in spiral spin orders in the ab- and bc-planes. Based on the experimental and theoretical results, we establish field-induced magnetic and electric phase diagrams for the two H directions, demonstrating that the distorted kagome staircase structure with competing intra–interlayer interactions and lifted frustration creates a plethora of different noncollinear spin textures of S = 1/2 spins that in turn induce electric polarization.
Oriented polyvinylidene fluoride–trifluoroethylene (P(VDF–TrFE)) films by Langmuir–Blodgett deposition: A synchrotron X-ray diffraction study
Langmuir–Blodgett films of polyvinylidene fluoride trifluoroethylene – P(VDF–TrFE)-copolymers possess substantially improved electrocaloric and pyroelectric properties, when compared with conventionally spin-cast films. In order to rationalize this, we prepared single-layered films of P(VDF–TrFE) (70:30) using both deposition techniques. Grazing incidence wide-angle X-ray scattering (GIWAXS), reveals that Langmuir–Blodgett deposited films have a higher concentration of the ferroelectric β-phase crystals, and that these films are highly oriented with respect to the substrate. Based on these observations, we suggest alternative means of deposition, which may substantially enhance the electrocaloric effect in P(VDF–TrFE) films. As a result, this development has significant implications for the potential use of P(VDF–TrFE) in solid-state refrigeration.
Novel multiferroic state and ME enhancement by breaking the AFM frustration in LuMn 1-x O 3
This study provides a comprehensive insight into the effects of controlled off-stoichiometry on the structural and multiferroic properties of the hexagonal manganite LuMn 1-x O 3+δ (x = 0.02; δ ~ 0), supported by neutron powder diffraction measurements confirming single phase P6 3 cm symmetry and evidencing a relevant ferromagnetic component, below T N ~ 90 K, which breaks the archetypal geometrically frustrated antiferromagnetic state typically ascribed to LuMnO 3 . Furthermore, the perturbations in the triangular disposition of spins prompt an additional electric polarization contribution and a clear enhancement of the magnetoelectric coupling which are in good agreement with the results of first principles calculations. In addition, Raman spectroscopy, dielectric permittivity, pyroelectric current and magnetic measurements as a function of temperature point out the precursor effects of the magnetic phase transitions involving a strong coupling between spins, lattice and electric order, even above the Néel temperature.
Optical breakdown threshold in nanosecond high repetition second harmonic generation by periodically poled Mg-doped LiTaO{sub 3} crystal
Our study shows that a local temperature increase of ≈1 K in the crystal lattice caused by second harmonic generation (SHG) and two-photon absorption of 532 nm radiation at the rear of periodically poled stoichiometric LiTaO{sub 3} crystal by changing spontaneous polarization induces a pyroelectric field ≈10 kV/cm, accelerating free electrons to an energy of ≈10 eV, followed by optical breakdown and crystal damage. Theoretical analysis leads to an explicit expression for the threshold laser fluence of optical breakdown giving ≈1.2 J/cm{sup 2} for 1064 nm input radiation in 6.8 kHz pulsed SHG operation, agreeing well with the experimentally found value ≈1.32 J/cm{sup 2}.
Suppression of the quantum-confined Stark effect in Al{sub x}Ga{sub 1−x}N/Al{sub y}Ga{sub 1−y}N corrugated quantum wells
We report comparative studies of 6-nm-thick Al{sub x}Ga{sub 1−x}N/Al{sub y}Ga{sub 1−y}N pyroelectric quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on c-sapphire substrates with a thick AlN buffer deposited under different growth conditions. The Al-rich growth conditions result in a 2D growth mode and formation of a planar QW, whereas the N-rich conditions lead to a 3D growth mode and formation of a QW corrugated on the size scale of 200–300 nm. Time-resolved photoluminescence (PL) measurements reveal a strong quantum-confined Stark effect in the planar QW, manifested by a long PL lifetime and a red shift of the PL line. In the corrugated QW, the emission line emerges 200 meV higher in energy, the low-temperature PL lifetime is 40 times shorter, and the PL intensity is stronger (∼4 times at 4.5 K and ∼60 times at 300 K). The improved emission properties are explained by suppression of the quantum-confined Stark effect due to the reduction of the built-in electric field within the QW planes, which are not normal to the [0001] direction, enhanced carrier localization, and improved efficiency of light extraction.