Haiwei Du

Recent developments in black phosphorus transistors

The discovery of graphene has inspired great research interest in two-dimensional (2D) layered nanomaterials during the past decade. As one of the newest members in the 2D layered nanomaterial family, black phosphorus (BP), with puckered structure similar to graphene, has shown great potential in novel nanoelectronics owing to its thickness-dependent bandgap. Especially, the unique in-plane anisotropy and high carrier mobility enable BP to be a promising candidate for field-effect transistor (FET) applications. In addition, monolayer or few-layer BP can be combined into van der Waals heterostructures and this opens up a pathway for overcoming existing problems such as impurity scattering and surface degradation or achieving functionalities. In this article, we will review the typical physical and chemical properties of BP and provide an overview of the recent developments in BP-based transistors. In this review, we also discuss the current challenges in BP transistors and future research directions.

Electric double-layer transistors: a review of recent progress

With the miniaturization of electronic devices, it is essential to achieve higher carrier density and lower operation voltage in field-effect transistors (FETs). However, this is a great challenge in conventional FETs owing to the low capacitance and electric breakdown of gate dielectrics. Recently, electric double-layer technology with ultra-high charge-carrier accumulation at the semiconductor channel/electrolyte interface has been creatively introduced into transistors to overcome this problem. Some interesting electrical transport characteristics such as superconductivity, metal–insulator transition, and tunable thermoelectric behavior have been modulated both theoretically and experimentally in electric double-layer transistors (EDLTs) with various semiconductor channel layers and electrolyte materials. The present article is a review of the recent progress in the EDLTs and the impacts of EDLT technology on modulating the charge transportation of various electronics.

Engineering Silver Nanowire Networks: From Transparent Electrodes to Resistive Switching Devices

Metal nanowires (NWs) networks with high conductance have shown potential applications in modern electronic components, especially the transparent electrodes over the past decade. In metal NW networks, the electrical connectivity of nanoscale NW junction can be modulated for various applications. In this work, silver nanowire (Ag NW) networks were selected to achieve the desired functions. The Ag NWs were first synthesized by a classic polyol process, and spin-coated on glass to fabricate transparent electrodes. The as-fabricated electrode showed a sheet resistance of 7.158 Ω □-1 with an optical transmittance of 79.19% at 550 nm, indicating a comparable figure of merit (FOM, or ΦTC) (13.55 × 10-3 Ω-1). Then, two different post-treatments were designed to tune the Ag NWs for not only transparent electrode but also for threshold resistive switching (RS) application. On the one hand, the Ag NW film was mechanically pressed to significantly improve the conductance by reducing the junction resistance. On the other hand, an Ag@AgOx core-shell structure was deliberately designed by partial oxidation of Ag NWs through simple ultraviolet (UV)-ozone treatment. The Ag core can act as metallic interconnect and the insulating AgOx shell acts as a switching medium to provide a conductive pathway for Ag filament migration. By fabricating Ag/Ag@AgOx/Ag planar structure, a volatile threshold switching characteristic was observed and an on/off ratio of ∼100 was achieved. This work showed that through different post-treatments, Ag NW network can be engineered for diverse functions, transforming from transparent electrodes to RS devices.

Growth of Lithium Lanthanum Titanate Nanosheets and Their Application in Lithium-Ion Batteries.

In this work, lithium-doped lanthanum titanate (LLTO) nanosheets have been prepared by a facile hydrothermal approach. It is found that with the incorporation of lithium ions, the morphology of the product transfers from rectangular nanosheets to irregular nanosheets along with a transition from La2Ti2O7 to Li0.5La0.5TiO3. The as-prepared LLTO nanosheets are used to enhance electrochemical performance of the LiCo1/3Ni1/3Mn1/3O2 (CNM) electrode by forming a higher lithium-ion conductive network. The LiCo1/3Ni1/3Mn1/3O2-Li0.5La0.5TiO3 (CNM-LLTO) electrode shows better a lithium diffusion coefficient of 1.5 × 10(-15) cm(2) s(-1), resulting from higher lithium-ion conductivity of LLTO and shorter lithium diffusion path, compared with the lithium diffusion coefficient of CNM electrode (5.44 × 10(-16) cm(2) s(-1)). Superior reversibility and stability are also found in the CNM-LLTO electrode, which retains a capacity at 198 mAh/g after 100 cycles at a rate of 0.1 C. Therefore, it can be confirmed that the existence of LLTO nanosheets can act as bridges to facilitate the lithium-ion diffusion between the active materials and electrolytes.

Recent Developments in Oxide-Based Ionic Conductors: Bulk Materials, Nanoionics, and Their Memory Applications

ABSTRACT Oxide-based ionic conductors have attracted tremendous research interests due to their wide applications in energy storage and conversion devices, such as photovoltaics, fuel cells, batteries, and supercapacitors. Extensive efforts have been undertaken to improve the ionic conductivity of existing materials along with the development of novel conductors. The recent advance of ionic conductors in nanoscale demonstrated their ultra–high ionic conductivity for the promising applications in energy sector. In this work, recent progresses of conventional oxide conductors and the development of novel conductors are reviewed in details. The strategy to exploit the nanoionics of enhancing the ionic conductivity is discussed. Furthermore, the novel applications of nanoionics for the resistive switching memories are summarized.

Tunable resistance switching in solution processed chromium-doped strontium titanate nanoparticles films

In this work, resistance switching behaviours in solution processed chromium (Cr)-doped strontium titanate (SrTiO3) films have been investigated. Undoped SrTiO3 film shows I-V characteristics of typical nonlinear resistor and no resistance hysteresis loops are observed. On the contrary, Cr-doped SrTiO3 films show stable and reversible hysteresis loops, which can be controlled by applying different voltage bias. Based on a series of characterization results, including X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS), we infer that Ti4+ is substituted by Cr3+, giving rise to increased concentration of oxygen vacancies. Therefore, the observed resistance switching phenomenon is attributed to voltage driven oxygen vacancy migration. Furthermore, gradually decreased overall resistance is also realized under repeated sweeping cycles.

Interfacial Redox Reactions Associated Ionic Transport in Oxide-Based Memories

As an alternative to transistor-based flash memories, redox reactions mediated resistive switches are considered as the most promising next-generation nonvolatile memories that combine the advantages of a simple metal/solid electrolyte (insulator)/metal structure, high scalability, low power consumption, and fast processing. For cation-based memories, the unavailability of in-built mobile cations in many solid electrolytes/insulators (e.g., Ta2O5, SiO2, etc.) instigates the essential role of absorbed water in films to keep electroneutrality for redox reactions at counter electrodes. Herein, we demonstrate electrochemical characteristics (oxidation/reduction reactions) of active electrodes (Ag and Cu) at the electrode/electrolyte interface and their subsequent ions transportation in Fe3O4 film by means of cyclic voltammetry measurements. By posing positive potentials on Ag/Cu active electrodes, Ag preferentially oxidized to Ag+, while Cu prefers to oxidize into Cu2+ first, followed by Cu/Cu+ oxidation. By sweeping the reverse potential, the oxidized ions can be subsequently reduced at the counter electrode. The results presented here provide a detailed understanding of the resistive switching phenomenon in Fe3O4-based memory cells. The results were further discussed on the basis of electrochemically assisted cations diffusions in the presence of absorbed surface water molecules in the film.

Correlating morphology and doping effects with the carbon monoxide catalytic activity of Zn doped CeO2 nanocrystals

The effects of Zn-doping on CeO2 nanocrystals were investigated for the catalytic oxidation of carbon monoxide (CO). Incorporating Zn2+ into CeO2 nanocubes with an isotropic 3D structure can effectively regulate oxygen vacancy concentration, compared with CeO2 nanorods showing an anisotropic one dimensional 1D structure. The catalytic activity was shown to be governed by a morphology-dependent doping effect.

Tailoring the multi-functionalities of one-dimensional ceria nanostructures via oxygen vacancy modulation

Lattice defects, for example oxygen vacancies in cerium oxide (CeO2), usually play a vital role in determining physical and chemical properties, including catalytic performance and resistance switching behaviour. Here, tin (Sn) was introduced as a dopant in one dimensional CeO2 nanostructures to investigate oxygen vacancy modulation and achieve improved catalytic properties and a tunable electrical performance. Our findings revealed that the Sn-doped CeO2 nanorods maintained their morphology while the aspect ratio decreased gradually with increasing Sn content. The variation in oxygen vacancy concentration with Sn doping was confirmed by Raman and X-ray photoelectron spectroscopies and enhanced thermal catalytic and photo-catalytic performances were attained for the Sn-doped CeO2 nanorods. The variation in oxygen vacancy concentration with Sn doping was also found to influence its electrical properties. Hysteresis loops expressing resistance switching behaviour were observed in Sn-doped CeO2-δ nanorods. The results detailed in this study can help to rationally design nanostructures with the potential to provide desirable multi-functionalities.

Digital to analog resistive switching transition induced by graphene buffer layer in strontium titanate based devices

Resistive switching behaviour can be classified into digital and analog switching based on its abrupt and gradual resistance change characteristics. Realizing the transition from digital to analog switching in the same device is essential for understanding and controlling the performance of the devices with various switching mechanisms. Here, we investigate the resistive switching in a device made with strontium titanate (SrTiO3) nanoparticles using X-ray diffractometry, scanning electron microscopy, Raman spectroscopy, and direct electrical measurements. It is found that the well-known rupture/formation of Ag filaments is responsible for the digital switching in the device with Ag as the top electrode. To modulate the switching performance, we insert a reduced graphene oxide layer between SrTiO3 and the bottom FTO electrode owing to its good barrier property for the diffusion of Ag ions and high out-of-plane resistance. In this case, resistive switching is changed from digital to analog as determined by the modulation of interfacial resistance under applied voltage. Based on that controllable resistance, potentiation and depression behaviours are implemented as well. This study opens up new ways for the design of multifunctional devices which are promising for memory and neuromorphic computing applications.