Bryan D. Huey

Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films.

A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelectric, almost nothing is known about their ferroelectric properties experimentally. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelectric domains in high-quality β-CH3NH3PbI3 perovskite thin films that have been synthesized using a new solution-processing method. The size of the ferroelectric domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelectric domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelectric behavior of hybrid perovskites, in particular in situ photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics.

Deterministic switching of ferromagnetism at room temperature using an electric field

The technological appeal of multiferroics is the ability to control magnetism with electric field. For devices to be useful, such control must be achieved at room temperature. The only single-phase multiferroic material exhibiting unambiguous magnetoelectric coupling at room temperature is BiFeO3 (refs 4 and 5). Its weak ferromagnetism arises from the canting of the antiferromagnetically aligned spins by the Dzyaloshinskii–Moriya (DM) interaction. Prior theory considered the symmetry of the thermodynamic ground state and concluded that direct 180-degree switching of the DM vector by the ferroelectric polarization was forbidden. Instead, we examined the kinetics of the switching process, something not considered previously in theoretical work. Here we show a deterministic reversal of the DM vector and canted moment using an electric field at room temperature. First-principles calculations reveal that the switching kinetics favours a two-step switching process. In each step the DM vector and polarization are coupled and 180-degree deterministic switching of magnetization hence becomes possible, in agreement with experimental observation. We exploit this switching to demonstrate energy-efficient control of a spin-valve device at room temperature. The energy per unit area required is approximately an order of magnitude less than that needed for spin-transfer torque switching. Given that the DM interaction is fundamental to single-phase multiferroics and magnetoelectrics, our results suggest ways to engineer magnetoelectric switching and tailor technologically pertinent functionality for nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.

Scanning probe microscopy method for nanosuspension stabilizer selection.

An atomic force microscopy (AFM) method was successfully developed and utilized for investigating the interaction of polymeric stabilizers with ibuprofen to determine their suitability for the preparation and stabilization of ibuprofen nanosuspensions. Images obtained clearly showed that HPMC and HPC interacted strongly with the ibuprofen resulting in extensive surface adsorption, confirming their suitability for ibuprofen nanosuspension preparation. In addition, differences in the morphology of the adsorbed HPMC and HPC molecules were observed, which may be attributed to their variable degree of substitution. Consistent with their poor performance in stabilizing the ibuprofen nanosuspensions, images obtained with PVP and Poloxamers depicted inadequate adsorption on the ibuprofen surface. Careful analysis of the AFM images and the ibuprofen crystal structure gave valuable insight into the success of top-down processing for the preparation of nanosuspensions as compared to bottom-up processing. On the basis of the relationship observed between nanosuspension stability and adsorption characteristics of specific polymers, such AFM studies can aid in the selection of suitable nanosuspension stabilizers. This method provides the basis for a scientific rationale for nanosuspension stabilizer selection rather than the trial and error method which is currently practiced.

High speed piezoresponse force microscopy: <1 frame per second nanoscale imaging

An atomic force microscopy (AFM) based technique is described for mapping piezoactuation with nanoscale resolution in less than a second per complete image frame. “High speed piezo force microscopy” (HSPFM) achieves this >100× increase in acquisition rates by coupling a commercial AFM with concepts of acoustics. This allows previously inaccessible dynamic studies, including measuring ferroelectric domain nucleation and growth during in situ poling. Hundreds of consecutive images are analyzed with 49 μs temporal resolution per pixel per frame, revealing 32 nucleation sites/μm2 with 36 μm/s average domain velocities. HSPFM images acquired in as fast as 1/10th s are also presented.

Enhanced piezoelectric response from barium strontium titanate multilayer films

Multilayered and homogeneous thin films of BaxSr1−xTiO3 (BST) were grown on Pt-coated Si substrates via metal-organic solution deposition. The multilayer 220nm thick BST heterostructure consisted of Ba0.60Sr0.40TiO3, Ba0.75Sr0.25TiO3, and Ba0.90Sr0.10TiO3. A single composition 220nm thick Ba0.60Sr0.40TiO3 was also grown for comparison. The piezoelectric properties were measured using piezoresponse force microscopy. There is approximately a 50% improvement in the piezoelectric response of the multilayered heterostructure compared to the homogeneous sample, with some spatial inhomogeneity. This enhancement can be attributed to the internal potential that arises from the polarization gradient and the commensurate built-in strain in the multilayer sample.

Mapping the Photoresponse of CH3NH3PbI3 Hybrid Perovskite Thin Films at the Nanoscale

Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) hold unprecedented promise for low-cost, high-efficiency photovoltaics (PVs) of the future. While PV performance parameters of PSCs, such as short circuit current, open circuit voltage, and maximum power, are always measured at the macroscopic scale, it is necessary to probe such photoresponses at the nanoscale to gain key insights into the fundamental PV mechanisms and their localized dependence on the OTP thin-film microstructure. Here we use photoconductive atomic force microscopy spectroscopy to map for the first time variations of PV performance at the nanoscale for planar PSCs based on hole-transport-layer free methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) thin films. These results reveal substantial variations in the photoresponse that correlate with thin-film microstructural features such as intragrain planar defects, grains, grain boundaries, and notably also grain-aggregates. The insights gained into such microstructure-localized PV mechanisms are essential for guiding microstructural tailoring of OTP films for improved PV performance in future PSCs.

Virtual piezoforce microscopy of polycrystalline ferroelectric films

An innovative methodology is presented that utilizes the experimental results of electron backscattered diffraction to map the crystallographic orientation of each grain, the finite element method to simulate the local grain-grain interactions, and finally piezoforce microscopy to infer the local properties of polycrystalline ferroelectric materials by comparing the output of the numerical calculation(s) with the experimental results. The proposed combined method resolves the local hysteretic and electromechanical interactions in polycrystalline ferroelectric films, thus quantifying the effects of grain corners and boundaries on the polycrystal’s macroscopic response. For a polycrystalline lead zirconate titanate sample, a finite range of crystallographic orientations and epitaxial strains is found to enhance the out-of-plane electrical response of the film with respect to its single-crystal, stress-free counterpart. Results show that {111} oriented grains parallel to the normal of the surface of the film...

Correlations between adhesion hysteresis and friction at molecular scales

Correlations between adhesion hysteresis and local friction are theoretically and experimentally investigated. The model is based on the classical theory of adhesional friction, contact mechanics, capillary hysteresis, and nanoscale roughness. Adhesion hysteresis was found to scale with friction through the scaling factor containing a varying ratio of adhesion energy over the reduced Youngs modulus. Capillary forces can offset the relationship between adhesion hysteresis and friction. Measurements on a wide range of engineering samples with varying adhesive and elastic properties confirm the model. Adhesion hysteresis is investigated under controlled, low humidity atmosphere via ultrasonic force microscopy. Friction is measured by the friction force microscopy.

Effects of coherent ferroelastic domain walls on the thermal conductivity and Kapitza conductance in bismuth ferrite

Ferroelectric and ferroelastic domain structure has a profound effect on the piezoelectric, ferroelectric, and dielectric responses of ferroelectric materials. However, domain walls and strain field effects on thermal properties are unknown. We measured the thermal conductance from 100–400 K of epitaxially grown BiFeO3 thin films with different domain variants, each separated primarily by 71° domain walls. We determined the Kapitza conductance across the domain walls, which is driven by the strain field induced by the domain variants. This domain wall Kapitza conductance is lower than the Kapitza conductance associated with grain boundaries in all previously measured materials.

Physical mechanisms of megahertz vibrations and nonlinear detection in ultrasonic force and related microscopies

Use of high frequency (HF) vibrations at MHz frequencies in Atomic Force Microscopy (AFM) advanced nanoscale property mapping to video rates, allowed use of cantilever dynamics for mapping nanomechanical properties of stiff materials, sensing μs time scale phenomena in nanostructures, and enabled detection of subsurface features with nanoscale resolution. All of these methods critically depend on the generally poor characterized HF behaviour of AFM cantilevers in contact with a studied sample, spatial and frequency response of piezotransducers, and transfer of ultrasonic vibrations between the probe and a specimen. Focusing particularly on Ultrasonic Force Microscopy (UFM), this work is also applicable to waveguide UFM, heterodyne force microscopy, and near-field holographic microscopy, all methods that exploit nonlinear tip-surface force interactions at high frequencies. Leveraging automated multidimensional measurements, spectroscopic UFM (sUFM) is introduced to investigate a range of common experimenta...