Jared Lodico

Nanofilament Formation and Regeneration During Cu/Al2O3 Resistive Memory Switching

Conductive bridge random access memory (CBRAM) is a leading candidate to supersede flash memory, but poor understanding of its switching process impedes widespread implementation. The underlying physics and basic, unresolved issues such as the connecting filaments growth direction can be revealed with direct imaging, but the nanoscale target region is completely encased and thus difficult to access with real-time, high-resolution probes. In Pt/Al2O3/Cu CBRAM devices with a realistic topology, we find that the filament grows backward toward the source metal electrode. This observation, consistent over many cycles in different devices, corroborates the standard electrochemical metallization model of CBRAM operation. Time-resolved scanning transmission electron microscopy (STEM) reveals distinct nucleation-limited and potential-limited no-growth periods occurring before and after a connection is made, respectively. The subfemtoampere ionic currents visualized move some thousands of atoms during a switch and lag the nanoampere electronic currents.

In Situ STEM of Ag and Cu Conducting Bridge Formation through Al2O3 in Nanoscale Resistive Memory Devices

Non-volatile resistive memory, specifically conducting-bridge RAM or CBRAM, is a potential successor to flash memory. CBRAM requires less power than flash memory, can switch on and off faster, and can withstand a larger number of on/off cycles [1]. In principle CBRAM devices are also expected to be scalable to only a few nanometers. In CBRAM, the memory element switches to an “on” state when a conducting bridge forms through the insulating layer separating two conducting electrodes. Despite recent intense interest in CBRAM, the specifics of the formation and breaking of this bridge is not well understood. We have fabricated horizontally-aligned CBRAM devices specifically designed for high-resolution S/TEM imaging of conducting filaments in situ.

In Situ Video Observations of the Lithiation of Single Microcrystal Graphite

A graphite intercalation compound (GIC) is formed when ions or molecules (intercalant) enter between the weakly bound layers of graphite. Lithium, one of many ions that intercalates graphite, creates a highly reversible GIC that is used in commercial Li-ion batteries [1]. GICs have been studied extensively on the bulk-scale (mg) [2] and more recently on the nano-scale in an effort to understand intercalation mechanisms, electrode/electrolyte interfaces, and the failure modes of batteries [1, 2]. However, little attention has been directed towards observing the cycling process in situ for micro-scale graphite. Here, in preparation for in situ electron microscopy experiments [3, 4], we use optical microscopy to observe the lithiation and delithiation of a single microcrystal graphite flake (0.35 μg) in 1M solution of lithium perchlorate (LiClO4) in ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 volume ratio.

In Situ Scanning Transmission Electron Microscopy (STEM) of Individual Electrochemical Intercalation Events in Graphite

Graphite intercalation compounds (GICs) are formed when ions or molecules (intercalants) are inserted between the carbon layers of a graphite host. With some electrolytes a reversible charge transfer process occurs during intercalation, making GICs attractive materials for batteries. The demand for improved batteries has highlighted the need for in situ measurements probing electrode-electrolyte interactions [1]. With in situ scanning transmission electron microscopy (STEM) we observe the reversible electrochemical intercalation of multi-layered (~20-100 layers) graphene in 96% sulfuric acid (H2SO4).

In Situ Transmission Electron Microscopy of the Electrochemical Intercalation of Graphite in Concentrated Sulfuric Acid

Graphite intercalation compounds, where some atom or molecule is inserted between the carbon layers of the host graphite, are attractive materials for charge storage. In commercial, rechargeable lithium ion batteries, for instance, the graphite anode intercalates and de-intercalates lithium as the battery is cycled [1]. In this work we present our in situ scanning transmission electron microscopy (STEM) observations of a model electrochemical intercalation system, namely graphite and concentrated sulfuric acid (98% mass fraction H2SO4). We cycle the potential between two graphite/gold electrodes that are immersed in the sulfuric acid and sealed in a fluid cell designed for in situ TEM. The construction of the fluid cell and the graphite transfer process have been described previously [2].

In Situ Optical Microscopy of the Electrochemical Intercalation of Lithium into Single Crystal Graphite

The lithium ion battery (LIB) is an energy storage technology that is widely used in consumer electronics [1]. Though many cathode materials have been successfully used in various types of LIB, graphite is still the preferred anode material [2]. A graphite intercalation compound (GIC) is formed during the battery’s charging process as lithium ions are inserted, or intercalated, between the individual graphene layers. Even though GICs have been researched extensively [3], the electrode/electrolyte interface and the kinetics of the intercalant during the intercalation process are not fully understood [1, 4]. We use optical microscopy to observe the intercalation and deintercalation of lithium ions and quantify color changes, which are directly related to the host’s structural evolution, within micron-scale, single-crystal graphite flakes.

Time-Resolved Imaging of Electrochemical Switching in Nanoscale Resistive Memory Elements

FLASH memory is reaching its scaling limit, but resistive random access memory (ReRAM) is considered a promising successor [1]. In ReRAM, metal electrodes sandwiching an insulating electrolyte form a digital memory element, where the presence or absence of a conducting path through the insulator represents one bit of information. The conducting filament is thought to form, atom-by-atom, when subject to a SET voltage applied across the electrodes, and to disintegrate when subject to a RESET voltage. We use scanning transmission electron microscopy (STEM) to image nanoscale ReRAM devices switching in situ. Operating the devices with small current limits slows the rate of filament formation and reduces confounding thermal effects, allowing us to obtain time-resolved images of filament formation and regeneration.

Nanobubbles on Electron Transparent Electrodes

The experimental observation of inhomogeneous nanobubble formation is challenging, and consequently the onset and growth of nanobubbles has yet to be fully localized or observed directly. Conventional methods, such as optical microscopy and atomic force microscopy (AFM), have been used to image small bubbles [1, 2]. However, optical images have limited spatial resolution due to the wavelength of visible light, and AFM is an invasive technique which gives data that can be difficult to interpret. Here we report using an electrical bias applied to graphene electrodes to form nanobubbles in situ for observation with scanning transmission electron microscopy (STEM) .