Abstract
Ion transport is an essential process for various applications including energy storage, sensing, display, memory and so on, however direct visualization of oxygen ion motion has been a challenging task, which lies in the fact that the normally used electron microscopy imaging mainly focuses on the mass attribute of ions. The lack of appropriate understandings and analytic approaches on oxygen ion motion has caused significant difficulties in disclosing the mechanism of oxides-based memristors. Here we show evidence of oxygen ion migration and accumulation in HfO2 by in situ measurements of electrostatic force gradient between the probe and the sample, as systematically verified by the charge duration, oxygen gas eruption and controlled studies utilizing different electrolytes, field directions and environments. At higher voltages, oxygen-deficient nano-filaments are formed, as directly identified employing a CS-corrected transmission electron microscope. This study could provide a generalized approach for probing ion motions at the nanoscale.
Highlights
Ion transport is an essential process for various applications including energy storage, sensing, display, memory and so on, direct visualization of oxygen ion motion has been a challenging task, which lies in the fact that the normally used electron microscopy imaging mainly focuses on the mass attribute of ions
Depending on the moving ion species, memristive systems can be further classified into electrochemical metallization memory (ECM) driven by the transport of metal cations[13,26] and valence change memory (VCM) where the resistance switching is presumably caused by the migration of oxygen ions/ vacancies[9,27,28]
While the switching mechanism of ECM cells has been well understood, for example, through real time, in situ transmission electron microscopy (TEM) observations[11,12,13,29,30,31] as well as scanning tunnelling microscopy investigations[8], the mechanism of VCM devices and their internal ion transport dynamics still remain largely elusive, which may be ascribed to the low atomic number of oxygen ions and easy adsorption of oxygen contaminations from the ambient, imposing significant difficulties on acquiring unambiguous evidence regarding oxygen-deficient filaments and oxygen ion motion like that achieved in ECM cells
Summary
Ion transport is an essential process for various applications including energy storage, sensing, display, memory and so on, direct visualization of oxygen ion motion has been a challenging task, which lies in the fact that the normally used electron microscopy imaging mainly focuses on the mass attribute of ions. Ion transport in solid-state materials is a fundamentally important process that moves charges simultaneously with mass transfer, enabling numerous applications from lithium batteries[1], solid-oxide fuel cells[2] to sensors[3], electrochromic displays[4] and lately, memristive devices[5,6,7,8,9,10]. While the switching mechanism of ECM cells has been well understood, for example, through real time, in situ transmission electron microscopy (TEM) observations[11,12,13,29,30,31] as well as scanning tunnelling microscopy investigations[8], the mechanism of VCM devices and their internal ion transport dynamics still remain largely elusive, which may be ascribed to the low atomic number of oxygen ions and easy adsorption of oxygen contaminations from the ambient, imposing significant difficulties on acquiring unambiguous evidence regarding oxygen-deficient filaments and oxygen ion motion like that achieved in ECM cells. Instead, exploiting the charge attribute of the ions may lead to brand new opportunities in tackling this problem
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