Youn-Geun Kim

Guidance of Cell Migration by Substrate Dimension

There is increasing evidence to suggest that physical parameters, including substrate rigidity, topography, and cell geometry, play an important role in cell migration. As there are significant differences in cell behavior when cultured in 1D, 2D, or 3D environments, we hypothesize that migrating cells are also able to sense the dimension of the environment as a guidance cue. NIH 3T3 fibroblasts were cultured on micropatterned substrates where the path of migration alternates between 1D lines and 2D rectangles. We found that 3T3 cells had a clear preference to stay on 2D rather than 1D substrates. Cells on 2D surfaces generated stronger traction stress than did those on 1D surfaces, but inhibition of myosin II caused cells to lose their sensitivity to substrate dimension, suggesting that myosin-II-dependent traction forces are the determining factor for dimension sensing. Furthermore, oncogene-transformed fibroblasts are defective in mechanosensing while generating similar traction forces on 1D and 2D surfaces. Dimension sensing may be involved in guiding cell migration for both physiological functions and tissue engineering, and for maintaining normal cells in their home tissue.

The Evolution of the Polycrystalline Copper Surface, First to Cu(111) and Then to Cu(100), at a Fixed CO2RR Potential: A Study by Operando EC-STM

A study based on operando electrochemical scanning tunneling microscopy (EC-STM) has shown that a polycrystalline Cu electrode held at a fixed negative potential, -0.9 V (vs SHE), in the vicinity of CO2 reduction reactions (CO2RR) in 0.1 M KOH, undergoes stepwise surface reconstruction, first to Cu(111) within 30 min, and then to Cu(100) after another 30 min; no further surface transformations occurred after establishment of the Cu(100) surface. The results may help explain the Cu(100)-like behavior of Cu(pc) in terms of CO2RR product selectivity. They likewise suggest that products exclusive to Cu(100) single-crystal electrodes may be generated through the use of readily available inexpensive polycrystalline Cu electrodes. The study highlights the dynamic nature of heterogeneous electrocatalyst surfaces and also underscores the importance of operando interrogations when structure-composition-reactivity correlations are intended.

Dynamically Interchangeable Nanoparticle Superlattices Through the Use of Nucleic Acid-Based Allosteric Effectors

DNA is a powerful tool for programmably assembling colloidal crystals, and has been used to generate nanoparticle superlattices with synthetically adjustable lattice parameters and crystal symmetries. However, the majority of these superlattice structures remain static once constructed, and factors such as interparticle distance cannot be controlled in a facile and rapid manner. Incorporation of these materials into functional devices would be greatly benefitted by the ability to change various aspects of the crystal assembly after the lattice has been synthesized. Herein, we present a reversible, rapid, and stoichiometric on-the-fly manipulation of nanoparticle superlattices with allosteric effectors based upon DNA. This approach is applicable to multiple different crystal symmetries, including FCC, BCC, CsCl, and AlB2.

Electrodeposition of CuInSe2 (CIS) via electrochemical atomic layer deposition (E-ALD).

The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer deposition (E-ALD) is reported here. Parameters for a ternary E-ALD cycle were investigated and included potentials, step sequence, solution compositions and timing. CIS was also grown by combining cycles for two binary compounds, InSe and Cu(2)Se, using a superlattice sequence. The formation, composition, and crystal structure of each are discussed. Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and 1 cycle of Cu(2)Se. The deposits were grown using 0.14, -0.7, and -0.65 V for Cu, In, and Se precursor solutions, respectively. XRD patterns displayed peaks consistent with the chalcopyrite phase of CIS, for the as-deposited samples, with the (112) reflection as the most prominent. AFM images of deposits suggested conformal deposition, when compared with corresponding image of the Au on glass substrate.

Epitaxial Growth of DNA-Assembled Nanoparticle Superlattices on Patterned Substrates

DNA-functionalized nanoparticles, including plasmonic nanoparticles, can be assembled into a wide range of crystalline arrays via synthetically programmable DNA hybridization interactions. Here we demonstrate that such assemblies can be grown epitaxially on lithographically patterned templates, eliminating grain boundaries and enabling fine control over orientation and size of assemblies up to thousands of square micrometers. We also demonstrate that this epitaxial growth allows for orientational control, systematic introduction of strain, and designed defects, which extend the range of structures that can be made using superlattice assembly. Ultimately, this will open the door to integrating self-assembled plasmonic nanoparticle materials into on-chip optical or optoelectronic platforms.

In Situ Visualization of Lithium Ion Intercalation into MoS2 Single Crystals using Differential Optical Microscopy with Atomic Layer Resolution

Atomic-level visualization of the intercalation of layered materials, such as metal chalcogenides, is of paramount importance in the development of high-performance batteries. In situ images of the dynamic intercalation of Li ions into MoS2 single-crystal electrodes were acquired for the first time, under potential control, with the use of a technique combining laser confocal microscopy with differential interference microscopy. Intercalation proceeded via a distinct phase separation of lithiated and delithiated regions. The process started at the atomic steps of the first layer beneath the selvedge and progressed in a layer-by-layer fashion. The intercalated regions consisted of Li-ion channels into which the newly inserted Li ions were pushed atom-by-atom. Interlayer diffusion of Li ions was not observed. Deintercalation was also clearly imaged and was found to transpire in a layer-by-layer mode. The intercalation and deintercalation processes were chemically reversible and can be repeated many times within a few atomic layers. Extensive intercalation of Li ions disrupted the atomically flat surface of MoS2 because of the formation of small lithiated domains that peeled off from the surface of the crystal. The current-potential curves of the intercalation and deintercalation processes were independent of the scan rate, thereby suggesting that the rate-determining step was not governed by Butler-Volmer kinetics.

Self-Assembly of Insoluble Porphyrins on Au(111) under Aqueous Electrochemical Control

Self-assembled monolayers of a water-insoluble porphyrin, tetraphenyl porphyrin (TPP), in the presence of an aqueous electrolyte were characterized in situ with electrochemical scanning tunneling microscopy (EC-STM) at working electrode potentials of between 0.5 and -0.2 V. Isolated domains of TPP monolayers with differing orientation were observed on Au(111) in 0.1 M HClO(4) over this entire potential window. Individual TPP molecules could be resolved over a range of 700 mV, from open circuit potential (OCP) to near the hydrogen evolution potential. The unit cell is square, and the distance between neighboring molecules is about 1.4 ± 0.1 nm. High-resolution images allow the internal molecular structure to be discerned. No changes in the STM contrast of individual molecules were observed as the potential was changed. In a neutral electrolyte (0.1 M KClO(4), pH ~6), the potential range of stability of ordered structures is reduced. On HOPG, TPP forms ordered hexagonal structures with a lattice constant of about 2.6 nm in the double-layer potential region in 0.1 M HClO(4).

Studies of Cu Atomic Layer Replacement, Formed by Underpotential Deposits, to Form Pt Nanofilms Using Electrochemical Atomic Layer Epitaxy

Underpotential deposition (UPD) involves the electrochemical deposition of a surface limited layer of an element onto a substrate [1]. If two elements can be deposited by separate UPD processes, an epitaxial compound can be formed using a cycle. This process is referred to electrochemical atomic layer epitaxy (ECALE). EC-ALE is the electrochemical analog of atomic layer deposition (ALD) and atomic layer epitaxy (ALE). However, metals are known to be difficult to deposit in a layer by layer fashion electrochemically [2]. In this presentation, a novel technique of replacing copper UPD by platinum (IV) will be introduced. In this process, a gold (111) substrate was cleaned by solution and argon ion bombardment, and a (1X1) pattern was observed for the surface using low-energy electron diffraction (LEED), after annealing. This gold sample was then transported from the main analysis chamber to an antechamber, and a monolayer of copper was deposited on the gold from a copper sulfate solution, at an underpotential. The gold sample with the Cu UPD was then immersed in a platinum (IV) solution at open circuit, and the copper UPD was replaced by platinum (IV) spontaneously. The gold sample with the platinum was then transported to the main chamber, and the surface was analyzed and quantified by LEED and Auger electron spectroscopy (AES). This was the first study by this group of metal EC-ALE. Deposition of Cu nanofilms by replacement of Pb UPD will be discussed if time permits. References [1] M.D. Lay and J.L. Stickney, J. Electrochem. Soc. 151, C431 (2004). [2] S.R. Brankovic, J.X. Wang, and R.R. Adzic, Surf. Sci. 474, L173 (2001). 0 100 200 300 400 500 600 700 800 90

Epitaxy: Programmable Atom Equivalents Versus Atoms

The programmability of DNA makes it an attractive structure-directing ligand for the assembly of nanoparticle (NP) superlattices in a manner that mimics many aspects of atomic crystallization. However, the synthesis of multilayer single crystals of defined size remains a challenge. Though previous studies considered lattice mismatch as the major limiting factor for multilayer assembly, thin film growth depends on many interlinked variables. Here, a more comprehensive approach is taken to study fundamental elements, such as the growth temperature and the thermodynamics of interfacial energetics, to achieve epitaxial growth of NP thin films. Both surface morphology and internal thin film structure are examined to provide an understanding of particle attachment and reorganization during growth. Under equilibrium conditions, single crystalline, multilayer thin films can be synthesized over 500 × 500 μm2 areas on lithographically patterned templates, whereas deposition under kinetic conditions leads to the rapid growth of glassy films. Importantly, these superlattices follow the same patterns of crystal growth demonstrated in atomic thin film deposition, allowing these processes to be understood in the context of well-studied atomic epitaxy and enabling a nanoscale model to study fundamental crystallization processes. Through understanding the role of epitaxy as a driving force for NP assembly, we are able to realize 3D architectures of arbitrary domain geometry and size.