Hyeon Kook Seo

Anisotropic Lithiation Onset in Silicon Nanoparticle Anode Revealed by in Situ Graphene Liquid Cell Electron Microscopy

Recent real-time analyses have provided invaluable information on the volume expansion of silicon (Si) nanomaterials during their electrochemical reactions with lithium ions and have thus served as useful bases for robust design of high capacity Si anodes in lithium ion batteries (LIBs). In an effort to deepen the understanding on the critical first lithiation of Si, especially in realistic liquid environments, herein, we have engaged in situ graphene liquid cell transmission electron microscopy (GLC-TEM). In this technique, chemical lithiation is stimulated by electron-beam irradiation, while the lithiation process is being monitored by TEM in real time. The real-time analyses informing of the changes in the dimensions and diffraction intensity indicate that the very first lithiation of Si nanoparticle shows anisotropic volume expansion favoring the ⟨110⟩ directions due to the smaller Li diffusion energy barrier at the Si-electrolyte interface along such directions. Once passing this initial volume expansion stage, however, Li diffusion rate becomes isotropic in the inner region of the Si nanoparticle. The current study suggests that the in situ GLC-TEM technique can be a useful tool in understanding battery reactions of various active materials, particularly those whose initial lithiation plays a pivotal role in overall electrochemical performance and structural stability of the active materials.

Self-Assembled Incorporation of Modulated Block Copolymer Nanostructures in Phase-Change Memory for Switching Power Reduction

Phase change memory (PCM), which exploits the phase change behavior of chalcogenide materials, affords tremendous advantages over conventional solid-state memory due to its nonvolatility, high speed, and scalability. However, high power consumption of PCM poses a critical challenge and has been the most significant obstacle to its widespread commercialization. Here, we present a novel approach based on the self-assembly of a block copolymer (BCP) to form a thin nanostructured SiOx layer that locally blocks the contact between a heater electrode and a phase change material. The writing current is decreased 5-fold (corresponding to a power reduction by 1/20) as the occupying area fraction of SiOx nanostructures is increased from a fill factor of 9.1% to 63.6%. Simulation results theoretically explain the current reduction mechanism by localized switching of BCP-blocked phase change materials.

Reliable Control of Filament Formation in Resistive Memories by Self-Assembled Nanoinsulators Derived from a Block Copolymer

Resistive random access memory (ReRAM) is a promising candidate for future nonvolatile memories. Resistive switching in a metal-insulator-metal structure is generally assumed to be caused by the formation/rupture of nanoscale conductive filaments (CFs) under an applied electric field. The critical issue of ReRAM for practical memory applications, however, is insufficient repeatability of the operating voltage and resistance ratio. Here, we present an innovative approach to reliably and reproducibly control the CF growth in unipolar NiO resistive memory by exploiting uniform formation of insulating SiOx nanostructures from the self-assembly of a Si-containing block copolymer. In this way, the standard deviation (SD) of set and reset voltages was markedly reduced by 76.9% and 59.4%, respectively. The SD of high resistance state also decreased significantly, from 6.3 × 10(7) Ω to 5.4 × 10(4) Ω. Moreover, we report direct observations of localized metallic Ni CF formation and their controllable growth using electron microscopy and discuss electrothermal simulation results based on the finite element method supporting our analysis results.

Freeze-Dried Sulfur–Graphene Oxide–Carbon Nanotube Nanocomposite for High Sulfur-Loading Lithium/Sulfur Cells

The ambient-temperature rechargeable lithium/sulfur (Li/S) cell is a strong candidate for the beyond lithium ion cell since significant progress on developing advanced sulfur electrodes with high sulfur loading has been made. Here we report on a new sulfur electrode active material consisting of a cetyltrimethylammonium bromide-modified sulfur-graphene oxide-carbon nanotube (S-GO-CTA-CNT) nanocomposite prepared by freeze-drying. We show the real-time formation of nanocrystalline lithium sulfide (Li2S) at the interface between the S-GO-CTA-CNT nanocomposite and the liquid electrolyte by in situ TEM observation of the reaction. The combination of GO and CNT helps to maintain the structural integrity of the S-GO-CTA-CNT nanocomposite during lithiation/delithiation. A high S loading (11.1 mgS/cm2, 75% S) S-GO-CTA-CNT electrode was successfully prepared using a three-dimensional structured Al foam as a substrate and showed good S utilization (1128 mAh/g S corresponding to 12.5 mAh/cm2), even with a very low electrolyte to sulfur weight ratio of 4. Moreover, it was demonstrated that the ionic liquid in the electrolyte improves the Coulombic efficiency and stabilizes the morphology of the Li metal anode.

Fabrication of high-density In3Sb1Te2 phase change nanoarray on glass-fabric reinforced flexible substrate

Mushroom-shaped phase change memory (PCM) consisting of a Cr/In(3)Sb(1)Te(2) (IST)/TiN (bottom electrode) nanoarray was fabricated via block copolymer lithography and single-step dry etching with a gas mixture of Ar/Cl(2). The process was performed on a high performance transparent glass-fabric reinforced composite film (GFR Hybrimer) suitable for use as a novel substrate for flexible devices. The use of GFR Hybrimer with low thermal expansion and flat surfaces enabled successful nanoscale patterning of functional phase change materials on flexible substrates. Block copolymer lithography employing asymmetrical block copolymer blends with hexagonal cylindrical self-assembled morphologies resulted in the creation of hexagonal nanoscale PCM cell arrays with an areal density of approximately 176xa0Gb/in(2).

Atomic visualization of a non-equilibrium sodiation pathway in copper sulfide

Sodium ion batteries have been considered a promising alternative to lithium ion batteries for large-scale energy storage owing to their low cost and high natural abundance. However, the commercialization of this device is hindered by the lack of suitable anodes with an optimized morphology that ensure high capacity and cycling stability of a battery. Here, we not only demonstrate that copper sulfide nanoplates exhibit close-to-theoretical capacity (~560u2009mAhu2009g–1) and long-term cyclability, but also reveal that their sodiation follows a non-equilibrium reaction route, which involves successive crystallographic tuning. By employing in situ transmission electron microscopy, we examine the atomic structures of four distinct sodiation phases of copper sulfide nanoplates including a metastable phase and discover that the discharge profile of copper sulfide directly reflects the observed phase evolutions. Our work provides detailed insight into the sodiation process of the high-performance intercalation–conversion anode material.Copper sulfide allows for high-performance sodium ion storage, yet its sodiation mechanism is poorly understood. Here, the authors examine the atomic structures of sodiated phases via in situ transmission electron microscopy, showing a non-equilibrium reaction pathway.

In Situ High-Resolution Transmission Electron Microscopy (TEM) Observation of Sn Nanoparticles on SnO2 Nanotubes Under Lithiation

We trace Sn nanoparticles (NPs) produced from SnO2 nanotubes (NTs) during lithiation initialized by high energy e-beam irradiation. The growth dynamics of Sn NPs is visualized in liquid electrolytes by graphene liquid cell transmission electron microscopy. The observation reveals that Sn NPs grow on the surface of SnO2 NTs via coalescence and the final shape of agglomerated NPs is governed by surface energy of the Sn NPs and the interfacial energy between Sn NPs and SnO2 NTs. Our result will likely benefit more rational material design of the ideal interface for facile ion insertion.

Enhanced self-assembly of block copolymers by surface modification of a guiding template

AbstractThe formation of highly ordered patterns of block copolymers (BCPs) with high χ is important for next-generation lithography applications. We demonstrate here a surface-engineering methodology to enhance the self-assembly of poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) BCPs with high χ by employing a hydroxyl-terminated polystyrene (PS-OH) brush. By precisely controlling the molecular weight (MW) and weight percent of PS-OH, well-ordered sub-20-nm BCP patterns were obtained over a large area in a short annealing time (<10u2009min) with the use of guiding templates. We systemically analyzed how the PS-OH brush affects the self-assembly kinetics of BCPs with various MWs and volume fractions. Moreover, the transmission electron microscopy (TEM) results strongly support that the PS-modulated surface plays an important role in the ordering of BCP patterns. We also achieved well-aligned 12u2009nm line and 18u2009nm dot patterns within 3u2009min by means of binary solvent vapor annealing at a moderate temperature under the optimum PS-OH brush conditions. These results provide a new platform for effective engineering and manipulation of the self-assembly of other BCPs for advanced BCP nanotechnologies.A facile and practical surface-engineering methodology to enhance the self-assembly of PS-b-PDMS BCPs with high χ by precisely controlling the molecular weight and weight percent of PS brushes. Highly ordered sub-20-nm BCP patterns were successfully obtained in a few minutes under the optimum PS brush condition.n

In Situ Transmission Electron Microscopy Graphene Liquid Cell on Chemical Sodiation of Nickel Oxide Nanoparticle

Sodium-ion batteries (SIBs) are highly treated to be the complementary alternatives to lithium ion batteries (LIBs) with response to the abundance of cost-effective sodium material. Both sodium and lithium belong to the same main group, showing most similar chemical characteristics although Na has relatively large ionic radius compared to lithium, which limits its electrochemical performance [1]. For SIBs to fulfil its promise of being alternative to LIBs, there is a necessity to innovate effective and appropriate strategies to design various electrodes materials with functional structures [2]. Among the widely studied electrodes are the transition metal oxides (TMOs) as they exhibit high theoretical capacities for sodium storage via similar mechanisms to lithium [3].

In Situ TEM Observation on the Agglomeration of Nanoparticles in the Interface of SnO 2

Direct observation of nanoparticles in high resolution has attracted considerable attention, as it can provide the fundamental understanding that is crucial to manufacturing the nanoscale materials that have different morphologies (e.g. sizes and shapes). Recently, very feasible in situ TEM platform called ‘graphene liquid cell’ (GLC) has been developed, which is easy to fabricate while maintaining high resolution imaging to better understand the dynamics of nanoparticles [1].