We report on the spatial and temporal evolution of photoinduced charge generation and carrier separation in heteroepitaxial BiFeO3 thin films deposited on Nb:SrTiO3 as measured in ambient at room temperature with Kelvin probe and piezoresponse force microscopy. Contributions from the self-poled and ferroelectric polarization charge are identified from the time evolution of the correlated surface potential and ferroelectric polarization in films as grown and following poling, and at different stages and intensities of optical illumination. Variations in the surface potential with bias voltage, switching history, and illumination intensity indicate how both bulk ferroelectric photovoltaic and the domain wall offset potential mechanisms contribute to the photogenerated charge.

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Dynamics of Photogenerated Surface Charge on BiFeO3 Films

This paper presents an in-situ study by combining contact dc-writing in biased scanning probe microscopy and non-contact Kelvin probe force microscopy to characterize the effect of bias-induced Li+ insertion/extraction on the changes of surface potential of TiO2 anode in an all-solid-state thin film Li-ion battery at nanoscale. With single layer TiO2 film as the reference, the factors affecting the changes of the surface potential, including the Fermi energy level shift and charge screening related to TiO2 semiconductor behavior, and bias-induced Li+ intercalation/ de-intercalation related to battery behavior are investigated. Surface potential hysteresis loop can be formed for TiO2 anode film, in which the potential value increases under the positive polarization and decreases under the negative polarization. In addition, TiO2 anode film after Li+ insertion/extraction cycles has a lower surface potential, indicating the decrease of charge sustaining capacity and the surface electrical degradation. In add...

In-situ nanoscale mapping of surface potential in all-solid-state thin film Li-ion battery using Kelvin probe force microscopy

In this paper, improvements of resistive random access memory (RRAM) using doping technology are summarized and analyzed. Based on a Cu/ZrO2/Pt device, three doping technologies with Ti ions, Cu, and Cu nanocrystal, respectively, are adopted in the experiments. Compared to an undoped device, improvements focus on four points: eliminating the electroforming process, reducing operation voltage, improving electrical uniformity, and increasing device yield. In addition, thermal stability of the high resistance state and better retention are also achieved by the doping technology. We demonstrate that doping technology is an effective way of improving the electrical performance of RRAM.

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Improving the electrical performance of resistive switching memory using doping technology

The prospect of tuning and enhancing multiple properties of ZnO from optical, electrical, piezo to ferroelectricity/magnetism with Cu dopants will certainly spur the pursuit of facile doping methodology to immensely advance this field of research. Here, a one-step aqueous synthesis of Cu-doped ZnO nanostructured materials with effective controllability over the morphology (film to nanowire) and doping concentrations both on rigid and flexible substrates has been developed. High structural integrity Cu-doped ZnO films and nanowires were achieved without multiple/harsh post-processing which tends to degrade their functional properties. Comprehensive investigations of varying doping concentrations on the enhancement and tunability of room temperature piezo/ferroelectricity to gas/photosensing multifunctional properties were systematically reported for the first time.

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Tuning of multifunctional Cu-doped ZnO films and nanowires for enhanced piezo/ferroelectric-like and gas/photoresponse properties

Zinc oxide (ZnO) is a remarkable inorganic semiconductor with exceptional piezoelectric properties compared to other semiconductors. However, in comparison to lead-based hazardous piezoelectric materials, its properties have undesired limitations. Here we report a 5∼6 fold enhancement in piezoelectric features via chemical doping of copper matched to intrinsic ZnO. A flexible piezoelectric nanogenerator (F-PENG) device was fabricated using an unpretentious solution process of spin coating, with other advantages such as robustness, low-weight, improved adhesion, and low cost. The device was used to demonstrate energy harvesting from a standard weight as low as 4 gm and can work as a self-powered mass sensor in a broad range of 4 to 100 gm. The device exhibited a novel energy harvesting technique from a wind source due to its inherent flexibility. At three different velocities (10∼30 m s-1) and five different angles of attack (0∼180 degrees), the device validated the ability to discern different velocities and directions of flow. The device will be useful for mapping the flow of air apart from harvesting the energy. The simulation was done to verify the underlining mechanism of aerodynamics involved.

https://iopscience.iop.org/article/10.1088/1361-6528/aaa6bd/pdf

Enhancement of ZnO-based flexible nano generators via a sol-gel technique for sensing and energy harvesting applications.

The surface potential of undoped and copper-doped zinc oxide (ZnO:Cu) films has been studied using the Kelvin probe force microscopy at ambient condition. In contrast to the undoped ZnO with unipolar behavior, the ZnO:Cu film exhibits a bipolar surface potential behavior under a dc bias. The localized hole trapping phenomenon is attributed to the presence of Cu ions in ZnO films. With an appropriate amount of the Cu ions (∼8 at. %), the charge trapping is reasonably stable over a period of 20 h, which can be associated with the presence of oxygen vacancies. This coexistence of Cu ions and oxygen vacancies in ZnO gives rise to stable bipolar behavior, paving way to potential charge storage application.

Zhongkun Zhang

Papers

Stable bipolar surface potential behavior of copper-doped zinc oxide films studied by Kelvin probe force microscopy