E. R. White

Charged nanoparticle dynamics in water induced by scanning transmission electron microscopy

Using scanning transmission electron microscopy we image ~4 nm platinum nanoparticles deposited on an insulating membrane, where the membrane is one of two electron-transparent windows separating an aqueous environment from the microscopes high vacuum. Upon receiving a relatively moderate dose of ~10(4) e/nm(2), initially immobile nanoparticles begin to move along trajectories that are directed radially outward from the center of the field of view. With larger dose rates the particle motion becomes increasingly dramatic. These observations demonstrate that, even under mild imaging conditions, the in situ electron microscopy of aqueous environments can produce electrophoretic charging effects that dominate the dynamics of nanoparticles under observation.

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.

Imaging interfacial electrical transport in graphene–MoS2 heterostructures with electron-beam-induced-currents

Heterostructure devices with specific and extraordinary properties can be fabricated by stacking two-dimensional crystals. Cleanliness at the inter-crystal interfaces within a heterostructure is crucial for maximizing device performance. However, because these interfaces are buried, characterizing their impact on device function is challenging. Here, we show that electron-beam induced current (EBIC) mapping can be used to image interfacial contamination and to characterize the quality of buried heterostructure interfaces with nanometer-scale spatial resolution. We applied EBIC and photocurrent imaging to map photo-sensitive graphene-MoS2 heterostructures. The EBIC maps, together with concurrently acquired scanning transmission electron microscopy images, reveal how a devices photocurrent collection efficiency is adversely affected by nanoscale debris invisible to optical-resolution photocurrent mapping.

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 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).

STEM EBIC to Study 2D Materials

Since its discovery in 2004 graphene has become a very widely studied material. The electronic excitations in single layer graphene can be described as two-dimensional massless Dirac particles, resulting in desirable electronic properties for nanoelectronic devices. Adding layers allows the electronic properties to be tuned, thus multilayer graphene is better suited for some device applications. Characterizing defects in this material is critical for understanding device performance. Recently Butz et al. have used dark field TEM to image dislocations in bilayer graphene, furthering our understanding of its electronic properties and the key importance of defects [1]. Here, we report using electron beam induced current (EBIC) measurements in a scanning transmission electron microscope (STEM) to image local changes in the conductivity of multilayer graphene.

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].

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.

Introduction to Plasmon Energy Expansion Thermometry

1. Department of Physics and Astronomy & California NanoSystems Institute, University of California, Los Angeles, CA, USA 2. Department of Electrical Engineering, University of Southern California, Los Angeles, CA, USA 3. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 4. Center for Electron Microscopy and Microanalysis, University of Southern California, Los Angeles, CA, USA

Applications of Plasmon Energy Expansion Thermometry

1. Center for Electron Microscopy and Microanalysis, University of Southern California, Los Angeles, CA, USA. 2. Department of Physics and Astronomy & California NanoSystems Institute, University of California, Los Angeles, CA, USA. 3. Department of Electrical Engineering, University of Southern California, Los Angeles, CA, USA 4. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.