Growth of tungsten bronze phase out of niobate perovskite phase for opto‐ferroelectric applications

Abstract

Engineering the optical bandgaps of classic ferroelectrics from the typical ultraviolet range down to the visible range is an emerging methodology of developing the next-generation optoelectric and opto-ferroelectric devices including ferroelectric solar cells, light-driven transistors and modulators, and multi-sensors/energy harvesters. Recently, a material interface comprised of a pseudo-morphotropic phase boundary between the tungsten bronze and perovskite phases of the KNBNNO [(K,Na,Ba) x (Ni,Nb) y O z ] has been reported to be an effective approach for bandgap engineering while retaining excellent ferroelectricity and piezoelectricity of the perovskite-phased KNBNNO. However, this approach requires the compositions of the materials to be determined at the synthesis stage, leaving little room for any further modification of the microstructure and functional properties at the post-processing stage. This paper presents a post-processing method, that is, atmospheric annealing in N 2 and O 2 , to grow the necessary tungsten bronze phase out of the perovskite phase in the KNBNNO. This method is advantageous over the previously reported because it enables to grow the tungsten bronze-perovskite interface region independent of the initial composition. The distinctive electrical properties and the giant tunability of photoconductivity of the tungsten bronze phase, the perovskite phase, and the interface are characterized in detail in this paper, supporting the exploitation of fabricating opto-ferroelectric devices using the reported method which is compatible and comparable with some of the post-processing methods applied in the silicon industry.

Electromechanical properties of paper‐derived potassium sodium niobate piezoelectric ceramics

“…17,18 It should be noted that the formation of the TTB secondary phase influences the electromechanical response and conductivity of KNNbased compositions. 19,20 The diffraction data indicate that sintering at 1150 • C induces minor secondary phases in both cases, that is, 3% and 5% for conventional and paperderived KNN, respectively, highlighting that the impurity phases are not due to the paper-derived ceramic processing technique. Nevertheless, further optimization of the sintering condition, such as atmospheric sintering with reduced oxygen partial pressure, can be used to prevent the formation of the secondary phases in KNN.…”

“…Such a secondary phase in KNN‐based compositions generally indicates the existence of a tetragonal tungsten bronze (TTB) phase with the formula K 2 Nb 4 O 11 , possibly due to the volatilization of alkali metal oxides, that is, Na 2 O and/or K 2 O, during the sintering 17,18 . It should be noted that the formation of the TTB secondary phase influences the electromechanical response and conductivity of KNN‐based compositions 19,20 . The diffraction data indicate that sintering at 1150°C induces minor secondary phases in both cases, that is, 3% and 5% for conventional and paper‐derived KNN, respectively, highlighting that the impurity phases are not due to the paper‐derived ceramic processing technique.…”

Filterless Visible‐Range Color Sensing and Wavelength‐Selective Photodetection Based on Barium/Nickel Codoped Bandgap‐Engineered Potassium Sodium Niobate Ferroelectric Ceramics

“…The compositional designs and studies are detailed in previous works. [14,18,19] Material Characterization: Some sintered ceramics were crushed in a mortar and then measured under XRD for phase identification. The complete ceramic samples were polished to the thickness of 500-600 μm with P1200 silicon carbide abrasive paper and then a polishing plate and diamond suspension (grain size 1 μm, MD-Nap and DiaPro, respectively, Struers, USA) to achieve a surface roughness of 50-60 nm.…”

“…The rationale of studying the specific composition in this work is that the family of Ba-and Ni-codoped (K,Na)NbO 3 (referred to as KNBNNO, KNN-BNN, or KNN-BNNO) has been systematically and thoroughly optimized from several chemical and physical aspects in our previous works. [13][14][15][18][19][20][21][22][23][24][25][26][27] Thus, it is much better known than other bandgap-engineered counterparts. In brief, two approaches could achieve to reduce the bandgap and retain reasonable ferroelectricity of the widely researched parental composition, (K,Na)NbO 3 , simultaneously.…”

“…[11,12] It is proven that the electrical properties, including conductivity and photoinduced/stimulated polarization switching, will drastically change when both incident light with above-gap photon energy and an external electric field are applied simultaneously, known as the optoelectrical (or photoferroelectric) cumulative effect, compared to the cases where only individual light or electric field is present. [13][14][15] This forms the fundamental basis of using "bandgap-engineered ferroelectrics" to distinguish the wavelength via the application of proper electric bias, given the fact that most ferroelectrics have bandgaps wider than 2.7 eV which is beyond the photon energy of most visible light and hence are not suitable for visible-range photodetection. [16,17] This work demonstrates the filterless, wavelength-selective photodetection using a bandgap-engineered ferroelectric ceramic made from the oxide perovskite compound of Ba-and Ni-codoped (K,Na)NbO 3 .…”