Electrochemistry at the Nanoscale

Ilia Valov

Dr. rer. nat. Ilia Valov is Senior Scientist at the Research Centre Juelich and RWTH-Aachen University, Germany. His research interests and activities are concentrated on electrochemical and, in general, physicochemical phenomena at the nano and sub-nanoscale, such as mass and charge transport, point defects, surfaces and interfaces with a focus on resistive switching memories, memristive devices and energy conversion.

Wei D. Lu

Wei Lu is a professor at the Electrical Engineering and Computer Science Department of the University of Michigan, Ann Arbor. His research interests include high-density memory based on two-terminal resistive devices (RRAM), memristor-based neuromorphic circuits, aggressively scaled nanowire transistors, and electrical transport in low-dimensional systems.

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Advances in science and technology have had a strong impact on the development of modern society and the improvement of quality of life. However, the exponential growth of technology, famously coined as “Moore's law” in the semiconductor industry and similarly observed in many other fields, faces an increasing number of challenges. Among others, issues of high societal relevance are renewable “green” energy conversion, energy storage, health/medicine, and information and communication technology, where ever-growing demands for improving efficiency, lowering power consumption, saving costs by avoiding precious/expensive materials, and making systems/devices smaller and smarter need to be met. New materials and devices are constantly being developed to meet these challenges. For example, a special recent trend is the development of artificial neuromorphic systems, and alternative logic and memory concepts towards energy-efficient computing and artificial intelligence beyond the conventional von Neumann computing architecture. Remarkably, at the root of these material and device developments, across disciplines in physics, chemistry and engineering, are effects strongly related to and based on electrochemical fundamentals and processes.

In the meantime, the advances in nanoscale materials and devices, along with new measurement and characterization techniques, have significantly advanced our understanding of electrochemical processes. Nanoscale materials with a broad variety of forms and properties have been developed – nanoparticles, nm thick films or film stacks, nanotubes, embedded (nano)structured materials, etc. Some of them are reaching even atomic dimensions, in the case of 2D materials and atomic switches. At such scales the properties of matter can significantly deviate from those of their macroscopic descriptions. In addition to well-known size effects such as energy quantization, conductance quantization and Coulomb blockades, additional phenomena arise from the different thermodynamic and kinetic factors of nanosized systems. For example, for nanoscale systems space-charge regions can become larger than the electrolyte thickness, inducing fully depleted or fully enriched regions. Additionally, lattice misfit and specific surface termination occurring at interfaces can induce 2D electron gas phenomena. These material and technology advances in turn pose opportunities and challenges for the understanding and control of the physical and physicochemical processes of matter at the atomic and mesoscopic levels.

The special issue “Electrochemical processes at the nanoscale: from fundamentals to applications” of Nanoscale aims to present the latest advances by leading experts across different disciplines and provide in-depth and essential background reviews in the field of nanoelectrochemistry. In particular, in recent years different directions of electrochemistry have independently developed, covering specific applications and forming focused communities, i.e. in areas of “classical” electrochemistry with liquids (including galvanics, electrolysis, and corrosion), bio-electrochemistry, solid state ionics (solid state electrochemistry), Li-ion batteries and resistive switching materials and devices (memristors or memristive devices). In all cases, however, the underlying processes are driven by the same fundamental physical and chemical forces, and the understanding and control of these fundamental processes at the nanoscale are key to continued performance improvements or the discovery of new functionalities. The objective of this issue is thus to reach across the boundaries of these sub-fields and to get together leading scientists from different fields to discuss the latest discoveries or provide deep insights that can help understand and control electrochemical processes at the nano- or subnanoscale.

The issue includes 6 review articles covering topics from fundamentals and theoretical analysis, to new experimental approaches, applications and future developments. They include:

• The fundamental atomistic picture of electrochemical phase formation and growth (DOI: 10.1039/C6NR02354A and DOI: 10.1039/C6NR01547F, covering theoretical and applied aspects, respectively) where the emphasis is on interfacial thermodynamics and electrode kinetics.

• New understandings of structures at the liquid metal–electrolyte interface (DOI: 10.1039/C6NR01571A). This review was selected in the 2016 Nanoscale HOT Article Collection.

• New functionalities based on nanoionic solid state devices and systems (DOI: 10.1039/C6NR00956E).

• The challenges and opportunities of electrochemistry with insulating materials (dielectrics) as electrolytes (DOI: 10.1039/C6NR01383J). This review was selected in the 2016 Nanoscale HOT Article Collection.

• The latest developments and applications of scanning probe microscopy techniques for spatially resolved electrochemical analysis (DOI: 10.1039/C6NR01524G). This review was selected in the 2016 Nanoscale HOT Article Collection.

In addition to the review articles, the special issue offers 18 research articles presenting the latest studies in the fields of electrocatalysis, Li-ion batteries, deposition/dissolution from liquid phases, electrochemical nanostructuring and resistive switching memories with a focus on microscopy understandings of nanoscale electrochemical processes.

Electrochemistry using liquid/aqueous electrolytes is represented by research articles on a novel, CO tolerant catalyst for the hydrogen oxidation reaction by Shi et al. (DOI: 10.1039/C6NR00778C); transformations of model electrodes of hydrous RuO₂ catalyst deposited Ru (0001) substrates by Krause et al. (DOI: 10.1039/C6NR00732E); processes of the 2D de-alloying of NiPd monoatomic layers by Damian et al. (DOI: 10.1039/C6NR01390B); the electrodeposition of dysprosium by Berger et al. (DOI: 10.1039/C6NR01351A); the speciation of gold nanoparticles (AuNPs) and their electrochemical detection using the concept of “nanoparticle imprinted matrices” by Hitrik et al. (DOI: 10.1039/C6NR01106C); and nanoelectrode array formation by electrolytic nanoparticle impacts by Bartlett et al. (DOI: 10.1039/C5NR08872K).

The solid state electrochemistry articles report on mass and charge transport in Fe-doped SrTiO₃ by Taibl et al. (DOI: 10.1039/C6NR00814C) and on the surface electronic structure of a Pt₃Ti electrocatalyst by Paßens et al. (DOI: 10.1039/C5NR08420B).

Nanoscale processes in Li-ion batteries are reported by: Seidl et al. (DOI: 10.1039/C6NR00825A) studying, by in situ EC-STM, solid electrolyte interface (SEI) formation at the C electrode and Breitung et al. showing in situ and in operando AFM characterization of nano-silicon based electrodes (DOI: 10.1039/C6NR03575B).

Research on resistive switching/memristive phenomena provides insight into observed Pd switching in SiO₂ cells (Wang et al., DOI: 10.1039/C6NR01085G). Detailed analysis and simulations on the origin of ultra-fast processes during filament formation are presented by Onofrio et al. (DOI: 10.1039/C6NR01335J). Krishnan et al. discuss the kinetic factors controlling the filament formation in polymer electrolytes (DOI: 10.1039/C6NR00569A). A 3D AFM tomography technique was used by Celano et al. to characterize the shape and stages of Cu filament formation in Al₂O₃ based ECM/CBRAMs (DOI: 10.1039/C5NR08735J). The same technique was also employed by Baeumer et al. to visualize and discuss the reactions and retention/failure issues in oxygen-based memory cells using Nb-SrTiO₃ (DOI: 10.1039/C6NR00824K). Finally ferroelectric/antiferroelectric-like polarization switching in a Hf_0.4Zr_0.6O₂ system is shown by Park et al. in DOI: 10.1039/C5NR08346J.

Learning and brain-inspired neuromorphic computing are highlighted by Wang et al. (DOI: 10.1039/C6NR00476H) and atomic switch-based “tug of war” for decision making is demonstrated by Lutz et al. (DOI: 10.1039/C6NR00690F).

By presenting views, theories and experimental techniques developed in different communities in a common platform, we hope the issue will serve as a bridge between different electrochemical communities irrespective of applications, generate interest, and stimulate discussions and interactions among researchers in different sub-fields to continue the relentless drive for new material and device developments to satisfy society's need for cheaper, better and pervasive energy, computing, and health/medicine products.

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