Poor Man's Atomic Layer Deposition of LiF for Additive-Free Growth of Lithium Columns.

Lithium metal is an ideal material for high-energy, cost-effective rechargeable energy storage systems. The thermodynamically unfavorable solid-liquid interface between the lithium metal and organic electrolyte necessitates the formation of an interlayer (SEI) which is known to have significant impact on lithium morphologies. Less well understood is the impact of the current collector substrate on the morphology of electrodeposited lithium. Here we report on the morphology of electrodeposited lithium as a function of the chemical pretreatments of the working electrode. We find that a copper substrate pretreatment with acidic solutions (sulfuric, oxalic, or acetic acid) results in the deposition of close-packed lithium columns with a uniform diameter. A controlled study of the pre-electrodeposited copper surface indicates that the formation of a 5-8 nm thick LiF protective layer on copper substrate from a chemical reaction between adsorbed surface water layer in acidic solutions and LiPF6 electrolyte is the key process in the electrochemical growth of lithium columns. We anticipate that this simple chemical approach can be generalized as a scalable, low-cost, additive-free substrate treatment method for depositing a LiF protective layer, broadly applicable in the development of uniform lithium films.

Control of Growth Front Evolution by Bi Additives during ZnAu Electrodeposition.

The performance of many electrochemical energy storage systems can be compromised by the formation of metal dendrites during charging. Additives in the electrolyte represent a useful strategy to mitigate dendrite formation, but understanding the mechanisms involved requires knowledge of the nanoscale effects of additives during electrochemical deposition. Here we quantify the effects of an inorganic additive on the morphology of an evolving electrochemical growth front, using liquid cell electron microscopy to provide the necessary spatial and temporal resolution. We examine deposition of ZnAu on Au in the presence of Bi additive, and show that low concentrations of Bi delay but do not prevent the formation of growth front instabilities. We describe a model in which Bi segregates at the growth front and promotes the surface diffusion and relaxation of Zn, allowing better coverage of the initial Au electrode surface. A more precise knowledge of the mechanism of inorganic additive effects may help in designing electrolyte chemistry for battery and other applications where morphology control is essential.

Strain-driven mound formation of substrate under epitaxial nanoparticles.

We observe the growth of crystalline SiC nanoparticles on Si(001) at 900 °C using in situ electron microscopy. Following nucleation and growth of the SiC, there is a massive migration of Si, forming a crystalline Si mound underneath each nanoparticle that lifts it 4-5 nm above the initial growth surface. The volume of the Si mounds is roughly five to seven times the volume of the SiC nanoparticles. We propose that relaxation of strain drives the mound formation. This new mechanism for relieving interfacial strain, which involves a dramatic restructuring of the substrate, is in striking contrast to the familiar scenario in which only the deposited material restructures to relieve strain.

3D printed quantum dot light-emitting diodes.

Developing the ability to 3D print various classes of materials possessing distinct properties could enable the freeform generation of active electronics in unique functional, interwoven architectures. Achieving seamless integration of diverse materials with 3D printing is a significant challenge that requires overcoming discrepancies in material properties in addition to ensuring that all the materials are compatible with the 3D printing process. To date, 3D printing has been limited to specific plastics, passive conductors, and a few biological materials. Here, we show that diverse classes of materials can be 3D printed and fully integrated into device components with active properties. Specifically, we demonstrate the seamless interweaving of five different materials, including (1) emissive semiconducting inorganic nanoparticles, (2) an elastomeric matrix, (3) organic polymers as charge transport layers, (4) solid and liquid metal leads, and (5) a UV-adhesive transparent substrate layer. As a proof of concept for demonstrating the integrated functionality of these materials, we 3D printed quantum dot-based light-emitting diodes (QD-LEDs) that exhibit pure and tunable color emission properties. By further incorporating the 3D scanning of surface topologies, we demonstrate the ability to conformally print devices onto curvilinear surfaces, such as contact lenses. Finally, we show that novel architectures that are not easily accessed using standard microfabrication techniques can be constructed, by 3D printing a 2 × 2 × 2 cube of encapsulated LEDs, in which every component of the cube and electronics are 3D printed. Overall, these results suggest that 3D printing is more versatile than has been demonstrated to date and is capable of integrating many distinct classes of materials.

Structure and performance of dielectric films based on self-assembled nanocrystals with a high dielectric constant.

Self-assembled films built from nanoparticles with a high dielectric constant are attractive as a foundation for new dielectric media with increased efficiency and range of operation, due to the ability to exploit nanofabrication techniques and emergent electrical properties originating from the nanoscale. However, because the building block is a discrete one-dimensional unit, it becomes a challenge to capture potential enhancements in dielectric performance in two or three dimensions, frequently due to surface effects or the presence of discontinuities. This is a recurring theme in nanoparticle film technology when applied to the realm of thin film semiconductor and device electronics. We present the use of chemically synthesized (Ba,Sr)TiO3 nanocrystals, and a novel deposition-polymerization technique, as a means to fabricate the dielectric layer. The effective dielectric constant of the film is tunable according to nanoparticle size, and effective film dielectric constants of up to 34 are enabled. Wide area and multilayer dielectrics of up to 8 cm(2) and 190 nF are reported, for which the building block is an 8 nm nanocrystal. We describe models for assessing dielectric performance, and distinct methods for improving the dielectric constant of a nanocrystal thin film. The approach relies on evaporatively driven assembly of perovskite nanocrystals with uniform size distributions in a tunable 7-30 nm size range, coupled with the use of low molecular weight monomer/polymer precursor chemistry that can infiltrate the porous nanocrystal thin film network post assembly. The intercrystal void space (low k dielectric volume fraction) is minimized, while simultaneously promoting intercrystal connectivity and maximizing volume fraction of the high k dielectric component. Furfuryl alcohol, which has good affinity to the surface of (Ba,Sr)TiO3 nanocrystals and miscibility with a range of solvents, is demonstrated to be ideal for the production of nanocomposites. The nanocrystal/furfuryl alcohol dispersions are suitable for the fabrication of thin films by chemical deposition techniques, including spin-coating, printing or a spraying process. To demonstrate the application of this technique to device fabrication, a multilayer capacitor with capacitance of 0.83 nF mm(-2) at 1 MHz is presented.

Electrochemical electron beam lithography: Write, read, and erase metallic nanocrystals on demand.

We develop a solution-based nanoscale patterning technique for site-specific deposition and dissolution of metallic nanocrystals. Nanocrystals are grown at desired locations by electron beam-induced reduction of metal ions in solution, with the ions supplied by dissolution of a nearby electrode via an applied potential. The nanocrystals can be "erased" by choice of beam conditions and regrown repeatably. We demonstrate these processes via in situ transmission electron microscopy using Au as the model material and extend to other metals. We anticipate that this approach can be used to deposit multicomponent alloys and core-shell nanostructures with nanoscale spatial and compositional resolutions for a variety of possible applications.

Nanoscale evolution of interface morphology during electrodeposition.

Control of interfacial morphology in electrochemical processes is essential for applications ranging from nanomanufacturing to batteries. Here, we quantify the evolution of an electrochemical growth front, using liquid cell electron microscopy to access unexplored length and time scales. During galvanostatic deposition of copper from an acidic electrolyte, we find that the growth front initially evolves consistent with kinetic roughening theory. Subsequently, it roughens more rapidly, consistent with diffusion-limited growth physics. However, the onset of roughening is strongly delayed compared to expectations, suggesting the importance of lateral diffusion of ions. Based on these growth regimes, we discuss morphological control and demonstrate the effects of two strategies, pulse plating and the use of electrolyte additives.

Flexible and stretchable power sources for wearable electronics.

Flexible and stretchable power sources represent a key technology for the realization of wearable electronics. Developing flexible and stretchable batteries with mechanical endurance that is on par with commercial standards and offer compliance while retaining safety remains a significant challenge. We present a unique approach that demonstrates mechanically robust, intrinsically safe silver-zinc batteries. This approach uses current collectors with enhanced mechanical design, such as helical springs and serpentines, as a structural support and backbone for all battery components. We show wire-shaped batteries based on helical band springs that are resilient to fatigue and retain electrochemical performance over 17,000 flexure cycles at a 0.5-cm bending radius. Serpentine-shaped batteries can be stretched with tunable degree and directionality while maintaining their specific capacity. Finally, the batteries are integrated, as a wearable device, with a photovoltaic module that enables recharging of the batteries.