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Kim, Hyeong Jin

Publications and source records attributed to Kim, Hyeong Jin.

14 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Tuning Self-Assembly of Gold Nanoparticles Grafted with Charged Polymers by Varying Temperature and Electrolytes

Here, we report on the assembly of gold nanoparticles (AuNPs, 5 and 10 nm core diameters) grafted with poly(ethylene glycol) (PEG) terminated with either positively (–NH 2 ) or negatively charged (–COOH) end groups. We employ synchrotron-based small-angle X-ray scattering (SAXS) techniques to investigate the effect of electrolytes and temperature on the assembly and crystallization behavior of these grafted AuNPs. Our studies reveal that –NH 2 -grafted AuNPs spontaneously assemble into a layered hexagonal structure that deteriorates upon adding K 2 CO 3 to the suspension. The hexagonal structure recovers after heating the same suspension above 60 °C. On the other hand, the COOH-terminated AuNPs remain dispersed in the suspensions and assemble and crystallize into a face-centered cubic structure only in the presence of K 2 CO 3 at temperatures above 60 °C. Varying the pH of the –NH 2 -terminated AuNP suspensions induces short-range order (SRO) structures with a single characteristic length scale that shrinks upon heating. Similarly, SRO for the –COOH-terminated AuNP is realized only by lowering the pH. This study demonstrates the use of electrolytes and temperature on the surface charge of grafted AuNPs as knobs to tune self-assembly and crystallization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Assembling PNIPAM-Capped Gold Nanoparticles in Aqueous Solutions

Employing small-angle X-ray scattering (SAXS), we explore the conditions under which assembly of gold nanoparticles (AuNPs) grafted with the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or polyelectrolytes with raising the temperature of the suspensions above the lower-critical solution temperature (LCST) of PNIPAM. Our results show that the longer the PNIPAM chain is, the better organization in the assembled clusters. Interestingly, without added electrolytes, there is no evidence of AuNPs assembly as a function of temperature, although untethered PNIPAM is known to undergo a coil-to-globule transition above its LCST. This study demonstrates another approach to assembling potential thermosensitive nanostructures for devices by leveraging the unique properties of PNIPAM.

Nayak, Binay P↗
DOE OSTI2023

Two-dimensional assembly of gold nanoparticles grafted with charged-end-group polymers

We introduced charged terminal groups to polymers that graft nanoparticles enable Coulombic control over their assembly by tuning the pH and salinity of their aqueous suspensions. Gold nanoparticles (AuNPs) are grafted with poly (ethylene glycol) (PEG) terminated with CH 3 (charge-neutral), COOH (negatively charged) or NH 2 groups (positively charged), and characterized with dynamic light scattering, ζ-potential, and thermal gravimetric analysis. Liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) are used to determine the density profile and in-plane structure of the AuNPs assembly at the aqueous surface. Assembly of PEG-AuNPs at the liquid/vapor interface is tunable by adjusting pH or salinity for COOH but less for NH 2 terminals. The distinct assembly behaviors are attributed to the overall charge of PEG-AuNPs as well as PEG conformation. COOH-PEG corona is more compact than those of the other terminal groups, leading to a crystalline structure with a smaller superlattice. The net charge per particle depends not only on the PEG terminal groups but also on the cation sequestration of PEG and the intrinsic negative charge of the AuNP surface. The closeness to overall charge neutrality, and hydrogen bonding in play, brought by NH 2 -PEG, drive NH 2 -PEG-AuNPs to assembly and crystallinity without additives to the suspensions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Lamellar and Hexagonal Assemblies of PEG-Grafted Silver Nanoparticles: Implications for Plasmonics and Photonics

Assembly of silver nanoparticles (AgNPs) grafted with thiolated poly(ethylene glycol) (PEG-SH) with various molecular weights is reported. Synchrotron-based X-ray surface-sensitive diffraction methods are used to determine the nature of two-dimensional (2D) assemblies at liquid/vapor interfaces, and small-angle X-ray scattering (SAXS) is used to monitor the three-dimensional (3D) assemblies in bulk suspensions. We find that assembly can be induced either by the addition of K 2 CO 3 or polyacrylic acid (PAA) to the PEG-AgNP aqueous suspensions. The addition of K 2 CO 3 induces a 2D hexagonal structure at the liquid/vapor interface and a 3D hexagonal columnar structure in solutions, where the columns consist of tethered PEG-AgNP chains. On the other hand, the addition of PAA leads to the formation of a 2D lamella-like distorted hexagonal sheet at the liquid/vapor interface and a 3D lamellar superstructure formed by stacked 2D sheets in the PEG-AgNP suspension. Effects of the core type of nanoparticles on the assembly can be used to control structures for potential applications in plasmonics and photonics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Binary Superlattices of Gold Nanoparticles in Two Dimensions

Here, we have created two-dimensional (2D) binary superlattices by cocrystallizing gold nanoparticles (AuNPs) of two distinct sizes into √3 x √3 and 2 x 2 complex binary superlattices, derived from the hexagonal structures of the single components. The building blocks of these binary systems are AuNPs that are functionalized with different chain lengths of poly(ethylene glycol) (PEG). The assembly of these functionalized NPs at the air–water interface is driven by the presence of salt, causing PEG-AuNPs to migrate to the aqueous surface and assemble into a crystalline lattice. We have used liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) to examine the assembly and crystallization at the liquid interface.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of Polymer Chain Length on the Superlattice Assembly of Functionalized Gold Nanoparticles

We report on the assembly of gold nanoparticle (AuNPs) superlattices at the liquid/vapor interface and in the bulk of their suspensions. Interparticle distances in the assemblies are achieved on multiple length scales by varying chain lengths of surface grafted AuNPs by polyethylene glycol (PEG) with molecular weights in the range 2000–40,000 Da. Crystal structures and lattice constants in both 2D and 3D assemblies are determined by synchrotron-based surface-sensitive and small-angle X-ray scattering. Here, assuming knowledge of grafting density, we show that experimentally determined interparticle distances are adequately modeled by spherical brushes close to the θ point (Flory–Huggins parameter, x ≈ 1/2) for 2D superlattices at a liquid interface and a nonsolvent (χ = ∞) for the 3D dry superlattices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Nanoparticle Superlattices with Negative Thermal Expansion (NTE) Coefficients

In this work, we report on the assembly of gold nanoparticles grafted with poly(ethylene glycol) (PEG–AuNPs) in aqueous solutions into the face-centered cubic superlattice by synchrotron X-ray scattering. At sufficiently high salt concentrations, the lattice constant decreases with increasing temperature, i.e., it exhibits a negative thermal expansion (NTE) coefficient that is 2–4 orders (in magnitude) larger than previously reported in NTE materials. The generality of the effect is demonstrated with different salts and different PEG chain lengths. Theoretical calculations show that PEG–AuNP becomes more insoluble (with the Flory–Huggins parameter χ > (1/2)) as temperature increases through water dehydration, with a decrease in the volume of the superlattice and an increase in the entropy of the system. Implications on the formation of nanoparticle superlattices and NTE properties are also discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

The Effects of Temperature on the Assembly of Gold Nanoparticle by Interpolymer Complexation

Using synchrotron-based small-angle X-ray scattering techniques, we demonstrate that poly(ethylene glycol)-functionalized gold nanoparticles (PEG-AuNPs) are assembled into close-packed structures that include short-range order with face-centered cubic structure, where crystalline qualities are varied by controlling the electrolyte concentration, pH, and temperature of the suspensions. Here, we show that interpolymer complexation with poly(acrylic acid) (PAA) is induced by lowering the pH level of the PEG-AuNPs suspensions, and furthermore, increasing the temperature of the suspension strengthens interparticle attraction, leading to improved supercrystal structures. Our results indicate that this strategy creates robust nanoparticle superlattices with high thermal stability. The effects of PAA and PEG chain lengths on the assemblies are also investigated, and their optimal conditions for creating improved superlattices are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Poly(N-isopropylacrylamide)-grafted gold nanoparticles at the vapor/water interface

Hypothesis: Grafting nanoparticles surfaces with water-soluble polymers modify interparticle interactions that are pivotal for assembling them into ordered phases. By manipulating salt concentrations of gold nanoparticles (AuNPs) that are grafted with poly(N-isopropylacrylamide) (PNIPAM-AuNPs), we hypothesize that various aggregated phases form at the suspension/vapor interface or in the bulk that depend on the molecular weight (MW) of PNIPAM and on salt concentrations. Experiments: AuNPs are grafted with thiolated PNIPAM of molecular weights of 3 or 6 kDa, and grafting is confirmed by dynamic light scattering. Liquid-surfaces X-ray reflectivity and grazing incidence small-angle X-ray scattering are used to determine the density profiles of the suspension/vapor interface and their inplane structure as salt is added to the suspensions. Findings: We observe that surface enrichment is induced by adding NaCl to the suspensions, and that at low salt concentrations, the monoparticle layer formed is dispersed, and above a threshold salt concentration, depending on MW of PNIPAM, the PNIPAM-AuNPs order in a hexagonal structure. We show that the lattice constant of the two-dimensional hexagonal structure varies with salt concentration, and more significantly with MW of PNIPAM.

36 MATERIALS SCIENCE↗