Emerging interactive sensor electronics requires metal oxide electrodes that possess long-term atmospheric stability and electrical conductivity to function under harsh conditions (e.g., high temperatures) in air. In this study, we report a rational method to accomplish the long-term thermal stability of conductive Al-doped ZnO (AZO) nanofilms, which have been thermally unstable due to inevitable crystal defects. Our method utilizes a sequential thermal annealing in air and Zn vapor atmosphere. An initial annealing was performed in air, followed by a second annealing in a Zn vapor atmosphere. Air tolerance tests on the resulting AZO nanofilms revealed the stable electrical resistivity (∼10 −4 Ω•cm) in air, even at temperatures up to 500 °C. Conversely, when annealing was performed in the reverse sequence, the electrical resistivity of the AZO nanofilms significantly increased by 5 orders of magnitude during tolerance tests. Photoluminescence data further supported the results of the air tolerance tests. The unusual effect of the annealing−atmosphere sequence is discussed in terms of the presence of dual anion/cation vacancies and the sequential benefits when these vacancies are compensated during annealing. The applicability of these thermally stable AZO electrodes for use in nanochannel sensor devices is demonstrated. Furthermore, we show that the proposed sequential annealing method is applicable for Ga-doped ZnO films, supporting its use as a platform fabrication method. Thus, the proposed fundamental concept for tailoring thermally stable conductive metal oxide electrodes provides a foundation for designing interactive electronic devices that are stable for a long period. KEYWORDS: conductive (Al, Ga)-doped ZnO nanofilms, crystal defects, long-term thermally stable electrode, sequential effect of annealing−atmospheres

Extracellular ATP (eATP) plays an essential role in plant growth, development, and stress tolerance. Here, we report that eATP participated in Nicotiana tabacum pollen germination (PG) and pollen tube growth (PTG) by regulating K and Ca influx. Exogenous ATP or ADP effectively promoted PG and PTG in a dose-dependent manner; weakly hydrolysable ATP analog (ATPγS) showed a similar effect. AMP, adenosine, adenine, and phosphate did not affect PG or PTG. Within a certain range, higher concentrations of K or Ca in the medium increased the effect of ATP in promoting PG and PTG. However, in mediums containing K or Ca concentrations above this range, the effect of ATP was reversed, resulting in PG and PTG inhibition. Ca chelators (EGTA), Ca channel blockers, and K channel blockers suppressed ATP-promoted PG and PTG. Results from a patch clamp showed that ATP activated a K and Ca influx in pollen protoplasts. These results suggest that, as an apoplastic signal, eATP may be involved in PG and PTG via regulating Ca and K absorption.

Our understanding of crystal growth mechanisms has changed deeply in the past few decades. Particularly, the oriented attachment of intermediate nanoparticles has been accepted to be a crucial crystal growth mechanism that is distinct from the traditional one involving nucleation and Ostwald ripening. However the details of the oriented attachment process are not readily observed experimentally, and little is known about the driving force and the dynamics involved in oriented attachment. In this respect, ZnO is an ideal material because it possesses strong spontaneous polarization which may easily drive oriented attachment during its crystal growth. We study experimentally and theoretically the complete crystal growth process (from primitive amorphous nanoclusters to ultimate single nanocrystals) of one-dimensional ZnO nanocrystals growing in water/ethanol at high temperatures. The results reveal that both axial (along the direction of the polarization axis) and lateral oriented attachment of the intermediates occurs during the growth process of the one-dimensional ZnO nanocrystals. Calculation based on the force and interaction model reveals that the axial oriented attachment driven by the spontaneous polarization force dominates the crystal growth of ZnO nanocrystals, and the van der Waals force also plays a role in driving oriented attachment. The study shows that oriented attachment of intermediate nanoparticle ensembles induces formation of the symmetric twin-nanorods. These results improve our understanding of the growth mechanism of nanocrystals in a liquid medium.

The structure and luminescence mechanisms of the CuInS2 quantum dots (QDs) after epitaxial growth of ZnS shell are in debate. The light absorption/emission spectroscopy reveals that after ZnS shell growth the cation diffusion at the CuInS2/ZnS interface results in formation of the alloyed CuxZn1− xInS2/ZnS:Cu QDs. These core/shell QDs exhibit dual-color photoluminescence with abnormal blue shift with decreasing excitation photon energy. The results show that the green and orange emissions originate separately from defects in the core and the shell. The absorption tail of the ZnS QDs turns from Urbach to Halperin-Lax type after Cu doping.

The underlying mechanism behind the blue/red color-switchable luminescence in the C8 carbon quantum dots (CQDs)/organic hybrid light-emitting devices (LEDs) is investigated. The study shows that the increasing bias alters the energy-level spatial distribution and reduces the carrier potential barrier at the CQDs/organic layer interface, resulting in transition of the carrier transport mechanism from quantum tunneling to direct injection. This causes spatial shift of carrier recombination from the organic layer to the CQDs layer with resultant transition of electroluminescence from blue to red. By contrast, the pure CQDs-based LED exhibits green–red electroluminescence stemming from recombination of injected carriers in the CQDs.

Chemically stable and electrically conductive metal oxide nanofilms are promising as robust electrodes for chemical/ biosensors and for photoelectrochemical applications, which require harsh conditions (e.g., acidic or basic environments). Among the various conductive metal oxides, impurity-doped ZnO nanofilms deposited on substrates are chemically nonresistive to acidic and basic environments because of the inevitable etching effects. Herein, we demonstrate a strategy to enhance the pH tolerance of Al-doped ZnO (AZO) nanofilms using a sequential annealing technique for film preparation. This technique involves first annealing in air followed by annealing in Zn vapor atmosphere. Although the as-grown AZO nanofilms rapidly dissolved in acidic and basic solutions, the sequentially annealed AZO nanofilms exhibited excellent pH tolerance toward the chemical etching rate and electrical resistance in buffer solutions, with pH ranging from 3 to 11. This enhancement effect of pH tolerance was considerably weakened when sequential annealing was performed in reverse (Zn vapor/air). The origin of the enhanced pH tolerance of the sequentially annealed AZO nanofilms is discussed in terms of the compensation of the anion/cation vacancies and the surface polarity of the ZnO(0001) surface.

Harsh electronics", which are designed to operate under harsh environments, have garnered significant attention to collect various physical and chemical information in surroundings toward the Internet of Things era. Among various electronic materials and structures, ZnO thin films, which consist of an abundant resource, have been intensively investigated because of their unique electrical and optical properties. However, ZnO thin films have been regarded as chemically nonresistive to harsh environments (e.g., high temperatures, high humidity, and acidic and basic conditions). Herein, we present recent progress and advances in electrically conductive ZnO thin films and nanostructures for applications in harsh electronics. First, various fabrication methods and progresses for achieving high-quality ZnO nanomaterials are introduced. Subsequently, previously reported approaches for enhancing the reliability and stability of ZnO nanostructures in harsh electronics are compared. Strategies for fabricating robust ZnO materials and ZnO-based electronics are discussed on the basis of several proposed mechanisms. Finally, we describe the current limitation, perspective, and outlook for future developments of ZnO nanostructures for use in harsh electronics.

As-grown undoped n-type semiconducting and annealed undoped semi-insulating ͑SI͒ liquid encapsulated Czochralski ͑LEC͒ InP has been studied by temperature dependent Hall measurement, photoluminescence spectroscopy, infrared absorption, and photocurrent spectroscopy. P-type conduction SI InP can frequently be obtained by annealing undoped LEC InP. This is caused by a high concentration of thermally induced native acceptor defects. In some cases, it can be shown that the thermally induced n-type SI property of undoped LEC InP is caused by a midgap donor compensating for the net shallow acceptors. The midgap donor is proposed to be a phosphorus antisite related defect. Traps in annealed SI InP have been detected by photocurrent spectroscopy and have been compared with reported results. The mechanisms of defect formation are discussed.