How Ligands Affect Resistive Switching in Solution-Processed HfO2 Nanoparticle Assemblies

From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

Nonaqueous sol-gel syntheses have been used to make many types of metal oxide nanocrystals. According to the current paradigm, nonaqueous syntheses have slow kinetics, thus favoring the thermodynamic (crystalline) product. Here we investigate the synthesis of hafnium (and zirconium) oxide nanocrystals from the metal chloride in benzyl alcohol. We follow the transition from precursor to nanocrystal through a combination of rheology, EXAFS, NMR, TEM, and X-ray total scattering (PDF analysis). Upon dissolving the metal chloride precursor, the exchange of chloride ligands for benzylalkoxide liberates HCl. The latter catalyzes the etherification of benzyl alcohol, eliminating water. During the temperature ramp to the reaction temperature (220 °C), sufficient water is produced to turn the reaction mixture into a macroscopic gel. Rheological analysis shows a network consisting of strong interactions with temperature-dependent restructuring. After a few minutes at the reaction temperature, crystalline particles emerge from the gel, and nucleation and growth are complete after 30 min. In contrast, 4 h are required to obtain the highest isolated yield, which we attribute to the slow in situ formation of water (the extraction solvent). We used our mechanistic insights to optimize the synthesis, achieving high isolated yields with a reduced reaction time. Our results oppose the idea that nonaqueous sol-gel syntheses necessarily form crystalline products in one step, without a transient, amorphous gel state.

Nonaqueous Chemistry of Group 4 Oxo Clusters and Colloidal Metal Oxide Nanocrystals

We review the nonaqueous precursor chemistry of the group 4 metals to gain insight into the formation of their oxo clusters and colloidal oxide nanocrystals. We first describe the properties and structures of titanium, zirconium, and hafnium oxides. Second, we introduce the different precursors that are used in the synthesis of oxo clusters and oxide nanocrystals. We review the structures of group 4 metal halides and alkoxides and their reactivity toward alcohols, carboxylic acids, etc. Third, we discuss fully condensed and atomically precise metal oxo clusters that could serve as nanocrystal models. By comparing the reaction conditions and reagents, we provide insight into the relationship between the cluster structure and the nature of the carboxylate capping ligands. We also briefly discuss the use of oxo clusters. Finally, we review the nonaqueous synthesis of group 4 oxide nanocrystals, including both surfactant-free and surfactant-assisted syntheses. We focus on their precursor chemistry and surface chemistry. By putting these results together, we connect the dots and obtain more insight into the fascinating chemistry of the group 4 metals. At the same time, we also identify gaps in our knowledge and thus areas for future research.

Resistance Switching Behavior of a Perhydropolysilazane-Derived SiOx-Based Memristor

SiO_x is an important dielectric material layer for resistive switching memory due to its compatibility with complementary metal-oxide semiconductor (CMOS) technology. Here we propose a solution process for a SiO_x dielectric layer based on perhydropolysilazane (PHPS). A series of SiO_x layers with different compositions are prepared by controlling the conversion process from PHPS, then the resistance switching behaviors of typical Ag/SiO_x/Au memristors are analyzed. The effect of oxygen vacancies and Si-OH groups on the formation and rupture of Ag conducting filaments (CFs) in the SiO_x layer was thoroughly investigated. Ultimately, we achieved a high-performance memristor with a coefficient of variation (σ/μ) as low as 0.16 ± 0.08 and an on/off ratio as high as 10⁶, which can rival the performance of the SiO_x memristors based on the high-vacuum and high-cost vapor deposition methods. These findings demonstrate the high promise of the PHPS-derived SiO_x dielectric layer in the field of memristors.

Memristive Behavior of Mixed Oxide Nanocrystal Assemblies

Recent advances in memristive nanocrystal assemblies leverage controllable colloidal chemistry to induce a broad range of defect-mediated electrochemical reactions, switching phenomena, and modulate active parameters. The sample geometry of virtually all resistive switching studies involves thin film layers comprising monomodal diameter nanocrystals. Here we explore the evolution of bipolar and threshold resistive switching across highly ordered, solution-processed nanoribbon assemblies and mixtures comprising BaZrO₃ (BZO) and SrZrO₃ (SZO) nanocrystals. The effects of nanocrystal size, packing density, and A-site substitution on operating voltage (V_SET and V_TH) and switching mechanism were studied through a systematic comparison of nanoribbon heterogeneity (i.e., BZO-BZO vs BZO-SZO) and monomodal vs bimodal size distributions (i.e., small-small and small-large). Analysis of the current-voltage response confirms that tip-induced, trap-mediated space-charge-limited current and trap-assisted tunneling processes drive the low- and high-resistance states, respectively. Our results demonstrate that both smaller nanocrystals and heavier alkaline earth substitution decrease the onset voltage and improve stability and state retention of monomodal assemblies and bimodal nanocrystal mixtures, thus providing a base correlation that informs fabrication of solution-processed, memristive nanocrystal assemblies.

Analog Memristive Characteristics of Square Shaped Lanthanum Oxide Nanoplates Layered Device

Square-shaped or rectangular nanoparticles (NPs) of lanthanum oxide (LaO_x) were synthesized and layered by convective self-assembly to demonstrate an analog memristive device in this study. Along with non-volatile analog memory effect, selection diode property could be co-existent without any implementation of heterogeneous multiple stacks with ~1 μm thick LaO_x NPs layer. Current–voltage (I–V) behavior of the LaO_x NPs resistive switching (RS) device has shown an evolved current level with memristive behavior and additional rectification functionality with threshold voltage. The concurrent memristor and diode type selector characteristics were examined with electrical stimuli or spikes for the duration of 10–50 ms pulse biases. The pulsed spike increased current levels at a read voltage of +0.2 V sequentially along with ±7 V biases, which have emulated neuromorphic operation of long-term potentiation (LTP). This study can open a new application of rare-earth LaO_x NPs as a component of neuromorphic synaptic device.

Recent Progress in Solution-Based Metal Oxide Resistive Switching Devices

Metal oxide resistive switching memories have been a crucial component for the requirements of the Internet of Things, which demands ultra-low power and high-density devices with new computing principles, exploiting low cost green products and technologies. Most of the reported resistive switching devices use conventional methods (physical and chemical vapor deposition), which are quite expensive due to their up-scale production. Solution-processing methods have been improved, being now a reliable technology that offers many advantages for resistive random-access memory (RRAM) such as high versatility, large area uniformity, transparency, low-cost and a simple fabrication of two-terminal structures. Solution-based metal oxide RRAM devices are emergent and promising non-volatile memories for future electronics. In this review, a brief history of non-volatile memories is highlighted as well as the present status of solution-based metal oxide resistive random-access memory (S-RRAM). Then, a focus on describing the solution synthesis parameters of S-RRAMs which induce a massive influence in the overall performance of these devices is discussed. Next, a precise analysis is performed on the metal oxide thin film and electrode interface and the recent advances on S-RRAM that will allow their large-area manufacturing. Finally, the figures of merit and the main challenges in S-RRAMs are discussed and future trends are proposed.