Time-resolved Kelvin Research Articles

Engineering Domain Variants in 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 Single Crystals Using High-Frequency AC Poling.

Single crystals of (001)-oriented 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 (PMN-30PT) with a composition near the morphotropic phase boundary have attracted considerable attention due to their superior dielectric and electromechanical performance. Recently, a new alternating current (electric field) poling approach used for the enhancement of dielectric and piezoelectric properties. However, the microscopic domain variants that govern the performance, especially under high-frequency alternating current (AC) voltages, remain largely unexplored. In this work, the domain microstructure under AC poling reveals the presence of four monoclinic (MA) domain variants using a suite of scanning probe microscopy methods, and X-ray diffraction (XRD) reciprocal space mapping is tuned. It is reported on the emergence of hierarchical fine domains - needle-shaped, and 109° domain walls under applied high-frequency AC poling. Time-resolved Kelvin probe force microscopy (KPFM) reveals the charge dynamics and relaxation behavior of these needle domains and walls. The findings provide new insight and guidance to the domain engineering by high-frequency AC poling for the development of advanced transducer technology.

Surface Dynamics of Charge Transport in LaAlO3/SrTiO3 with Time-Resolved Kelvin Probe Force Microscopy

Surface Dynamics of Charge Transport in LaAlO₃/SrTiO₃ with Time-Resolved Kelvin Probe Force Microscopy

The dominant role of surfaces in the hysteretic behavior of hybrid perovskites

Hysteresis and memory-effects are an issue for the development of a reliable solar technology based on hybrid perovksites. In this context, we study the effect of surfaces on the electric mobility of charged point-defects at room conditions by molecular dynamics simulations. At variance with the bulk, for which the ionic current is linear and reversible upon inversion of the electric field, we find that the presence of surfaces gives rise to accumulation of charged point-defects and memory effects. Surfaces act as trapping regions for iodine defects with fast accumulation and much slower release kinetics (by at least two orders of magnitude). The calculated release times are of hundreds of millisecond for both vacancies and interstitials at MAI- and PbI2-terminated surfaces, respectively and are consistent with the decays of ionic currents observed in time-resolved Kelvin Probe Force Microscopy experiments. As we do not find ferroelectric polarization and memory effects on ionic currents in perfect crystalline bulks at room conditions, we conclude that surfaces (and likely interfaces or boundaries) are key factors giving rise to hysteresis. Present results are complemented by an analytic model of the effect of surfaces on point defects and of the kinetics of ionic polarization for the design of functional films with better control of hysteretic ionic currents.

Artifacts in time-resolved Kelvin probe force microscopy.

Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias pulses, the resulting time-dependent electrostatic forces are very complex and result in intricate mixing of frequencies that may, in some cases, have a component at the detection frequency, leading to falsified KPFM measurements. Additionally, we performed fast Fourier transform (FFT) analyses that match the results of the numerical dynamics simulations. Small differences are observed that can be attributed to transients and higher-order Fourier components, as a consequence of the intricate nature of the cantilever driving forces. These results are corroborated by experimental measurements on a model system. In the experimental case, additional artifacts are observed due to constructive or destructive interference of the bias modulation with the cantilever oscillation. Also, in the case of light modulation, we demonstrate artifacts due to unwanted illumination of the photodetector of the beam deflection detection system. Finally, guidelines for avoiding such artifacts are given.

Local carrier dynamics in organic thin film transistors investigated by time-resolved Kelvin probe force microscopy

Abstract Understanding the relationship between the microscopic structures and the carrier transport properties in organic electronic materials is of fundamental interest to exploit the potential of the materials and thus to realize highly efficient organic devices. In this study, carrier ejection processes in polycrystalline organic thin film transistors are investigated by time-resolved Kelvin probe force microscopy (tr-KPFM) that is capable of visualizing time evolution of the surface potential triggered by the application of a voltage pulse with a temporal resolution of 3 ms. The obtained tr-KPFM images show that the spatial distribution of the diffusing charge carriers is mostly non-homogeneous, reflecting the complex resistivity distribution in the channel region formed by the crystalline grains. The trapped charges in some grains diffuse to the electrodes on a time scale of several tens of milliseconds due to the low contact resistance of the grain boundaries, while most of the trapped charges diffuse on a time scale of hundreds of milliseconds.

Influence of Nonstoichiometry on Proton Conductivity in Thin-Film Yttrium-Doped Barium Zirconate.

Proton-conducting perovskites have been widely studied because of their potential application as solid electrolytes in intermediate temperature solid oxide fuel cells. Structural and chemical heterogeneities can develop during synthesis, device fabrication, or service, which can profoundly affect proton transport. Here, we use time-resolved Kelvin probe force microscopy, scanning transmission electron microscopy, atom probe tomography, and density functional theory calculations to intentionally introduce Ba-deficient planar and spherical defects and link the resultant atomic structure with proton transport behavior in both stoichiometric and nonstoichiometric epitaxial, yttrium-doped barium zirconate thin films. The defects were intentionally induced through high-temperature annealing treatment, while maintaining the epitaxial single crystalline structure of the films, with an overall relaxation in the atomic structure. The annealed samples showed smaller magnitudes of local lattice distortions because of the formation of proton polarons, thereby leading to decreased proton-trapping effect. This resulted in a decrease in the activation energy for proton transport, leading to faster proton transport.

How the formation of interfacial charge causes hysteresis in perovskite solar cells

In this study, we use time-resolved Kelvin probe force microscopy to investigate current–voltage hysteresis in a hybrid lead-halide perovskite solar cell.

Effects of Microstructure on Electrochemical Reactivity and Conductivity in Nanostructured Ceria Thin Films.

The proton conductivity in functional oxides is crucial in determining electrochemistry and transport phenomena in a number of applications such as catalytic devices and fuel cells. However, single characterization techniques are usually limited in detecting the ionic dynamics at the full range of environmental conditions. In this report, we probe and uncover the links between the microstructure of nanostructured ceria (NC) and parameters that govern its electrochemical reaction and proton transport, by coupling experimental data obtained with time-resolved Kelvin probe force microscopy (tr-KPFM), electrochemical impedance spectroscopy (EIS), and finite element analysis. It is found that surface morphology determines the water splitting rate and proton conductivity at 25 °C and wet conditions, where protons are mainly generated and transported within surface physisorbed water layers. However, at higher temperature (i.e., ≥125 °C) and dry conditions, when physisorbed water evaporates, grain size and crystallographic orientation become significant factors. Specifically, the proton generation rate is negatively correlated with the grain size, whereas proton diffusivity is facilitated by surface {111} planes and additional conduction pathways offered by cracks and open pores connected to the surface.

Time-resolved kelvin probe force microscopy to study population and depopulation of traps in electron or hole majority organic semiconductors

We present a time-resolved Kelvin Probe Force Microscopy (KPFM) technique that can record carrier motion on the scale of milliseconds, appropriate for polycrystalline materials like organic semiconductors. The organic semiconductors are studied in a transistor geometry to which we apply a step voltage to the back-gate. We record the change in potential at a specific location and observe the times associated with filling and emptying majority carrier traps, observed in hole majority carrier poly(3-hexylthiophene-2,5-diyl) (P3HT) and a perylene diimide electron majority carrier, PDI-CN2. We see signs of bias stress with repeated measurements in P3HT.

Electrochemical reactivity and proton transport mechanisms in nanostructured ceria

Electrochemical reactivity and ionic transport at the nanoscale are essential in many energy applications. In this study, time-resolved Kelvin probe force microscopy (tr-KPFM) is utilized for surface potential mapping of nanostructured ceria, in both space and time domains. The fundamental mechanisms of proton injection and transport are studied as a function of environmental conditions and the presence or absence of triple phase boundaries. Finite element modeling is used to extract physical parameters from the experimental data, allowing not only quantification of the observed processes, but also decoupling of their contributions to the measured signal. The constructed phase diagrams of the parameters demonstrate a thermally activated proton injection reaction at the triple phase boundary, and two transport processes that are responsible for the low-temperature proton conductivity of nanostructured ceria.

Spatially Resolved Probing of Electrochemical Reactions via Energy Discovery Platforms.

The electrochemical reactivity of solid surfaces underpins functionality of a broad spectrum of materials and devices ranging from energy storage and conversion, to sensors and catalytic devices. The surface electrochemistry is, however, a complex process, controlled by the interplay of charge generation, field-controlled and diffusion-controlled transport. Here we explore the fundamental mechanisms of electrochemical reactivity on nanocrystalline ceria, using the synergy of nanofabricated devices and time-resolved Kelvin probe force microscopy (tr-KPFM), an approach we refer to as energy discovery platform. Through tr-KPFM, the surface potential mapping in both the space and time domains and current variation over time are obtained, enabling analysis of local ionic and electronic transport and their dynamic behavior on the 10 ms to 10 s scale. Based on their different responses in the time domain, conduction mechanisms can be separated and identified in a variety of environmental conditions, such as humidity and temperature. The theoretical modeling of ion transport through finite element method allows for creation of a minimal model consistent with observed phenomena, and establishing of the dynamic characteristics of the process, including mobility and diffusivity of charged species. The future potential of the energy discovery platforms is also discussed.

Direct Probing of Charge Injection and Polarization‐Controlled Ionic Mobility on Ferroelectric LiNbO3 Surfaces

Mapping surface potential with time-resolved Kelvin probe force microscopy (tr-KPFM) in LiNbO3 periodically poled single crystals reveals activation of the surface ionic subsystem. Electric fields of certain strength induce injection of charge, formation of an active region in its vicinity and uneven distribution of screening charge on the opposite ferroelectric domains. Tr-KPFM technique allows investigating these phenomena in details.

Space- and Time-Resolved Mapping of Ionic Dynamic and Electroresistive Phenomena in Lateral Devices

A scanning probe microscopy-based technique for probing local ionic and electronic transport and their dynamic behavior on the 10 ms to 10 s scale is presented. The time-resolved Kelvin probe force microscopy (tr-KPFM) allows mapping of surface potential in both space and time domains, visualizing electronic and ionic charge dynamics and separating underlying processes based on their time responses. Here, tr-KPFM is employed to explore the interplay of the adsorbed surface ions and bulk oxygen vacancies and their role in the resistive switching in a Ca-substituted bismuth ferrite thin film.