Nanoionics

Nanoionics^[1] is the study and application of phenomena, properties, effects and mechanisms of processes connected with fast ion transport (FIT) in all-solid-state nanoscale systems. Topics of interest include fundamental properties of oxide ceramics at nanometer length scales, and fast ion conductor (advanced superionic conductor)/electronic conductor heterostructures. Potential applications include electrochemical devices (electrical double layer devices) for conversion and storage of energy, charge and information. The term and conception of nanoionics (as a new branch of science) were first introduced by A.L.Despotuli and V.I.Nikolaichik (Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences, Chernogolovka) in January 1992^[1].

There are two classes of solid state ionic nanosystems and two fundamentally different nanoionics: (I) nanosystems based on solids with low ionic conductivity, and (II) nanosystems based on advanced superionic conductors (alpha–AgI, rubidium silver iodide–family, etc.). The second was introduced in ^[2].

The important case of fast ionic conduction in solid states is one in a surface space-charge layer of ionic crystals. Such conduction was first predicted by Kurt Lehovec^[3]. Significance of boundary conditions with respect to ionic conductivity was first experimentally discovered by C.C. Liang (J. Electrochem. Soc. 1973. V.120. P. 1289-1292) who found the anomalously high conduction in the LiI-Al₂O₃ two-phase system. As a space-charge layer with specific properties has nanometer thickness, the effect is directly related to nanoionics (nanoionics-I). The Lehovec’s effect [3] had given a basis for a creation of a multitude of nanostructured fast ion conductors which are used in modern portable lithium batteries and fuel cells.

Being by a branch of science and technology, nanoionics is unambiguously defined by its own objects (nanostructures with FIT), subject matter (properties, phenomena, effects, mechanisms of processes, and applications connected with FIT at nano-scale), method (interface design in the nanosystems of superionic conductors), and criterion (R/L ~1, where R is the nanosize(s) of device structure, and L is the characteristic length on the which the properties, characteristics, and so on (connected with FIT) are change drastically.

Nanoionics-I and nanoionics-II different from each other by design of interfaces. The role of boundaries in nanoionics-I - it is the creation of conditions for high concentrations of charged defects (vacancies and interstices) in the disordered space-charge layer. But in nanoionics-II, it needs to conserve the original highly ionic conductive crystal structures of advanced superionic conductors at the ordered (lattice-matched) heteroboundaries.

The International Technology Roadmap for Semiconductors, known throughout the world as the ITRS, relates the nanoionics-based resistive switching memories to the category of "emerging research devices" ("ionic memory"). The area of close intersection of nanoelectronics and nanoionics can be called by nanoelionics. Now, the vision of future nanoelectronics constrained solely by fundamental ultimate limits is being formed in advanced researches [8-11]. Ultimate physical limits to computation [12] are very far off from the attained «10¹⁰ cm^-2-10¹⁰ Hz» region. What kind of logic switches might be used at the near nm- and sub-nm peta-scale integration? The question was the subject matter already in [13], where the term “nanoelectronics” (Bate R. T., Reed M. A., Frensley W. R, 1987) was not used else. Quantum mechanic constrains the electronic distinguishable configurations by tunneling effect at tera-scale. For overcome 10¹² cm^-2 bit density limit the atomic and ion configurations with characteristic dimension of L <2 nm should be used in information domain and the materials with effective mass of information carriers m* considerable large then electronic one are required: m* =13 m_e at L =1 nm, m* =53 m_e (L =0,5 nm) and m* =336 m_e (L =0,2 nm) [11]. May be future short-sized devices will be nanoionic, i.e. based on the fast ion transport at nanoscale, as it was first stated in [1].

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