A Tunneling Dielectric Layer Free Floating Gate Nonvolatile Memory Employing Type‐I Core–Shell Quantum Dots as Discrete Charge‐Trapping/Tunneling CentersYan, Chengyuan; Wen, Jiamin; Lin, Peng; Sun, Zhenhua
doi: 10.1002/smll.201804156pmid: 30480357
A nonvolatile memory with a floating gate structure is fabricated using ZnSe@ZnS core–shell quantum dots as discrete charge‐trapping/tunneling centers. Systematical investigation reveals that the spontaneous recovery of the trapped charges in the ZnSe core can be effectively avoided by the type‐I energy band structure of the quantum dots. The surface oleic acid ligand surrounding the quantum dots can also play a role of energy barrier to prevent unintentional charge recovery. The device based on the quantum dots demonstrates a large memory window, stable retention, and good endurance. What is more, integrating charge‐trapping and tunneling components into one quantum dot, which is solution synthesizable and processible, can largely simplify the processing of the floating gate nonvolatile memory. This research reveals the promising application potential of type‐I core–shell nanoparticles as the discrete charge‐trapping/tunneling centers in nonvolatile memory in terms of performance, cost, and flexibility.
Developing 1D Sb‐Embedded Carbon Nanorods to Improve Efficiency and Stability of Inverted Planar Perovskite Solar CellsLi, Zhiqi; Liu, Chunyu; Zhang, Xinyuan; Ren, Guanhua; Han, Wenbin; Guo, Wenbin
doi: 10.1002/smll.201804692pmid: 30480365
To overcome the zigzag pathway transport of the electron diffusion process and eliminate the surface trap states of phenyl‐C61‐butyric acid methyl ester (PCBM) nanofilms in inverted perovskite solar cells, novel 1D N‐type doped carbon nanorods (CNRs) are developed by a stibonium (Sb) auxiliary ball milling method and introduced into the PCBM film to prepare the PCBM:Sb‐CNRs hybrid transport layer. In this way, the N‐type doped Sb‐CNRs can extend the built‐in electric field between CH3NH3PbI3 and PCBM to facilitate the separation of electron/hole pairs. The discontinuous band with the built‐in potential in the PCBM/Sb‐CNRs heterojunction can boost interfacial charge redistribution and promote electrons diffusion from PCBM to electrode through 1D Sb‐CNRs network. As a result, the high device efficiency of 19.26% with enhanced air stability and little hysteresis are achieved. This work demonstrates a simple strategy to improve the efficiency and stability of perovskite photovoltaic devices using low‐cost carbon nanomaterials.
Synthesis of P‐Doped and NiCo‐Hybridized Graphene‐Based Fibers for Flexible Asymmetrical Solid‐State Micro‐Energy Storage DeviceZhou, Caixia; Gao, Taotao; Wang, Yujue; Liu, Qilin; Huang, Zhihan; Liu, Xiaoxia; Qing, Miaoqing; Xiao, Dan
doi: 10.1002/smll.201803469pmid: 30480359
Fiber supercapacitors (FSCs) are promising energy storage devices in portable and wearable smart electronics. Currently, a major challenge for FSCs is simultaneously achieving high volumetric energy and power densities. Herein, the microscale fiber electrode is designed by using carbon fibers as substrates and capillary channels as microreactors to space‐confined hydrothermal assembling. As P‐doped graphene oxide/carbon fiber (PGO/CF) and NiCo2O4‐based graphene oxide/carbon fiber (NCGO/CF) electrodes are successfully prepared, their unique hybrid structures exhibit a satisfactory electrochemical performance. An all‐solid‐state PGO/CF//NCGO/CF flexible asymmetric fiber supercapacitor (AFSC) based on the PGO/CF as the negative electrode, NCGO/CF hybrid electrode as the positive electrode, and poly(vinyl alcohol)/potassium hydroxide as the electrolyte is successfully assembled. The AFSC device delivers a higher volumetric energy density of 36.77 mW h cm−3 at a power density of 142.5 mW cm−3. In addition, a double reference electrode system is adopted to analyze and reduce the IR drop, as well as effectively matching negative and positive electrodes, which is conducive for the optimization and improvement of energy density. For the AFSC device, its better flexibility and electrochemical properties create a promising potential for high‐performance micro‐supercapacitors. Furthermore, the introduction of the double reference electrode system provides an interesting method for the study on the electrochemical performances of two‐electrode systems.
Reactive Inkjet Printing of Functional Silk Stirrers for Enhanced Mixing and SensingZhang, Yu; Gregory, David A.; Zhang, Yi; Smith, Patrick J.; Ebbens, Stephen J.; Zhao, Xiubo
doi: 10.1002/smll.201804213pmid: 30515976
Stirring small volumes of solution can reduce immunoassay readout time, homogenize cell cultures, and increase enzyme reactivity in bioreactors. However, at present many small scale stirring methods require external actuation, which can be cumbersome. To address this, here, reactive inkjet printing is shown to be able to produce autonomously rotating biocompatible silk‐based microstirrers that can enhance fluid mixing. Rotary motion is generated either by release of a surface active agent (small molecular polyethylene glycol) resulting in Marangoni effect, or by catalytically powered bubble propulsion. The Marangoni driven devices do not require any chemicals to be added to the fluid as the “fuel,” while the catalytically powered devices are powered by decomposing substrate molecules in solution. A comparison of Marangoni effect and enzyme powered stirrers is made. Marangoni effect driven stirrers rotate up to 600 rpm, 75–100‐fold faster than enzyme driven microstirrers, however enzyme powered stirrers show increased longevity. Further to stirring applications, the sensitivity of the motion generation mechanisms to fluid properties allows the rotating devices to also be exploited for sensing applications, for example, acting as motion sensors for water pollution.
Solution‐Controlled Conformational Switching of an Anchored Wireframe DNA NanostructureHoffecker, Ian T.; Chen, Sijie; Gådin, Andreas; Bosco, Alessandro; Teixeira, Ana I.; Högberg, Björn
doi: 10.1002/smll.201803628pmid: 30516020
Self‐assembled DNA origami nanostructures have a high degree of programmable spatial control that enables nanoscale molecular manipulations. A surface‐tethered, flexible DNA nanomesh is reported herein which spontaneously undergoes sharp, dynamic conformational transitions under physiological conditions. The transitions occur between two major macrostates: a spread state dominated by the interaction between the DNA nanomesh and the BSA/streptavidin surface and a surface‐avoiding contracted state. Due to a slow rate of stochastic transition events on the order of tens of minutes, the dynamic conformations of individual structures can be detected in situ with DNA PAINT microscopy. Time series localization data with automated imaging processing to track the dynamically changing radial distribution of structural markers are combined. Conformational distributions of tethered structures in buffers with elevated pH exhibit a calcium‐dependent domination of the spread state. This is likely due to electrostatic interactions between the structures and immobilized surface proteins (BSA and streptavidin). An interaction is observed in solution under similar buffer conditions with dynamic light scattering. Exchanging between solutions that promote one or the other state leads to in situ sample‐wide transitions between the states. The technique herein can be a useful tool for dynamic control and observation of nanoscale interactions and spatial relationships.
Functional Femtoliter Droplets for Ultrafast Nanoextraction and Supersensitive Online MicroanalysisLi, Miaosi; Dyett, Brendan; Yu, Haitao; Bansal, Vipul; Zhang, Xuehua
doi: 10.1002/smll.201804683pmid: 30488558
A universal femtoliter surface droplet‐based platform for direct quantification of trace of hydrophobic compounds in aqueous solutions is presented. Formation and functionalization of femtoliter droplets, concentrating the analyte in the solution, are integrated into a simple fluidic chamber, taking advantage of the long‐term stability, large surface‐to‐volume ratio, and tunable chemical composition of these droplets. In situ quantification of the extracted analytes is achieved by surface‐enhanced Raman scattering (SERS) spectroscopy by nanoparticles on the functionalized droplets. Optimized extraction efficiency and SERS enhancement by tuning droplet composition enable quantitative determination of hydrophobic model compounds of rhodamine 6G, methylene blue, and malachite green with the detection limit of 10−9 to 10−11 m and a large linear range of SERS signal from 10−9 to 10−6 m of the analytes. The approach addresses the current challenges of reproducibility and the lifetime of the substrate in SERS measurements. This novel surface droplet platform combines liquid–liquid extraction and highly sensitive and reproducible SERS detection, providing a promising technique in current chemical analysis related to environment monitoring, biomedical diagnosis, and national security monitoring.
Tungsten Nitride Nanodots Embedded Phosphorous Modified Carbon Fabric as Flexible and Robust Electrode for Asymmetric PseudocapacitorDubal, Deepak P.; Chodankar, Nilesh R.; Qiao, Shizhang
doi: 10.1002/smll.201804104pmid: 30609283
Owing to the excellent physical properties of metal nitrides such as metallic conductivity and pseudocapacitance, they have recently attracted much attention as competitive materials for high‐performance supercapacitors (SCs). However, the voltage window for metal nitride‐based symmetric SCs is limited (0.6–0.8 V) in aqueous electrolyte due to the oxidation at high negative potentials. In this respect, ultra‐small tungsten nitride particles onto the phosphorous modified carbon fabric (W2N@P‐CF) are engineered as a promising hybrid electrode for pseudocapacitors. Additionally, the fact that the W2N@P‐CF electrode can operate in the negative potential region is exploited to design asymmetric pseudocapacitors by coupling with a polypyrrole on carbon fabric (PPy@CF) as the positive electrode. Remarkably, the W2N@P‐CF//PPy@CF asymmetric cell can be cycled in a wide voltage window of 1.6 V that is almost two times higher than that of metal nitrides symmetric SCs. The pseudocapacitive behavior with matching different potential regions of W2N@P‐CF and PPy@CF, considerably enhance performance of asymmetric device. The device delivers high volumetric capacity (7.1 F cm−3), high energy (2.54 mWh cm−3), power densities, and good cycling stability (88%) over 20 000 cycles. Thus, pseudocapacitive metal nitride‐based devices hold a great promise to provide high voltage and improved energy density in aqueous electrolyte.