Fabrication of Complexly Patterned Wavy Structures Using Self-Organized Anisotropic Wrinkling

Pil J. Yoo 대한금속·재료학회 2011 ELECTRONIC MATERIALS LETTERS Vol.7 No.1

In this work, we review a novel means of self-organized anisotropic wrinkling, which enables the patterning of wavy microstructures that are rarely achievable with conventional patterning approaches. The surface wrinkling phenomenon usually occurs in a system of stacked layers in the course of relieving the accumulated stress therein. For example, the polymer/metal bilayer system supported on a rigid substrate can generate random wrinkles on the surface due to the compressive stress resulting from thermal annealing. However,these random and isotropic wrinkles can be self-organized and ordered by an external confinement effect being guided in periodic microstructures of a patterned mold material. By exploiting the competitive wave interactions between the intrinsic wrinkling wavelength and periodic spacing of the externally imposed mold,complex wavy structures can be created with designed shape of the wrinkles. In addition, manipulating the contact condition of the external mold can allow controllability over the phase of patterned wrinkles. Therefore,the proposed technique is anticipated to be useful in a broad range of applications in micro-optics, microfluidic devices, and flexible electronics.

멀티스케일 아키텍쳐링 기반 역오팔상 구조체 기능성 멤브레인 기술

Pil J. Yoo 한국막학회 2016 멤브레인 Vol.26 No.6

최근 들어 정렬구조의 나노구조체를 이용한 분리막 응용기술이 큰 관심을 받고 있다. 나노구조체 분리막은 낮은 흐름저항을 통해 높은 투습성을 유지하면서도 매우 균일한 기공크기 특성으로 인해 높은 분리선택비를 가질 수 있다는 장점 을 지닌다. 특히 콜로이드 입자의 자기조립체인 오팔상 및 그 역구조인 역오팔상 구조체를 이용한 분리막 기술이 각광을 받 고 있는데, 기공크기를 자유롭게 제어하면서도 내부에 다양한 기능기의 도입이 가능하여 크기선별 분리 뿐 아니라 반응성 분 리막의 응용에까지 폭넓게 적용이 가능하다. 더불어 다양한 멀티스케일 구조화 기술을 이용하여 기존의 분리막 소재에서는 다룰 수 없었던 다양한 형태의 기공 및 채널구조를 도입할 수 있어, 차세대 고부가가치 분리막 소재기술에 있어 큰 활용이 기대된다. 본 기고에서는 다양한 소재를 활용한 역오팔상 구조체 분리막 기술과 더불어 계층구조화를 통한 기능성 분리막의 개발에 대해 총괄적으로 살펴보고 논의하고자 한다. Novel membrane technologies that harness ordered nanostructures have recently received much attention because they allow for high permeability due to their reduced flow resistance while also maintaining high selectivity due to their isoporous characteristics. In particular, the opaline structure (made from the self-assembly of colloidal particles) and its inverted form (inverse-opal) have shown strong potential for membrane applications on account of several advantages in processing and the resulting membrane properties. These include controllability over the pore size and surface functional moieties, which enable a wide range of applications ranging from size-exclusive separation to catalytically-reactive membranes. Furthermore, when combined with multiscale architecturing strategies, inverse-opal-structured membranes can be designed to have specific pores or channel structures. These materials are anticipated to be utilized for next-generation, high-performance, and high-value-added functional membranes. In this review article, various types of inverse-opal-structured membranes are reviewed and their functionalization through hierarchical structuring will be comprehensively investigated and discussed.

Multiscale Architectured Core/Shell Colloidal Particle Assemblies for Optical and Electrochemical Applications

Pil J. Yoo(유필진) 한국고분자학회 2019 한국고분자학회 학술대회 연구논문 초록집 Vol.44 No.2

이중층 박막에서의 응력 유도 패턴의 변화: 스피노달 좌굴현상

유필진 ( Pil J. Yoo ),이홍희 ( Hong H. Lee ) 한국화학공학회 2007 화학공학의이론과응용 Vol.10 No.1

Cumulative energy analysis of thermally-induced surface wrinkling of heterogeneously multilayered thin films

Yoo, Seong Soo,Choi, Gwan H.,Lee, Wooseop,Park, Juhyun,Yi, Gi-Ra,Ryu, Du Yeol,Yoo, Pil J. The Royal Society of Chemistry 2018 SOFT MATTER Vol.14 No.5

<P>Wrinkling is a well-known example of instability-driven surface deformation that occurs when the accumulated compressive stress exceeds the critical value in multilayered systems. A number of studies have investigated the instability conditions and the corresponding mechanisms of wrinkling deformation. Force balance analysis of bilayer systems, in which the thickness of the capping layer is importantly considered, has offered a useful approach for the quantitative understanding of wrinkling. However, it is inappropriate for multilayer wrinkling (layer number > 3) consisting of heterogeneous materials (<I>e.g.</I> polymer/metal or inorganic), in which the thickness variation in the substrate is also crucial. Therefore, to accommodate the additive characteristics of multilayered systems, we thermally treated tri- or quad-layer samples of polymer/metal multilayers to generate surface wrinkles and used a cumulative energy balance analysis to consider the individual contribution of each constituent layer. Unlike the composite layer model, wherein the thickness effect of the capping layer is highly overestimated for heterogenously stacked multilayers, our approach precisely reflects the bending energy contribution of the given multilayer system, with results that match well with experimental values. Furthermore, we demonstrate the feasibility of this approach as a metrological tool for simple and straightforward estimation of the thermomechanical properties of polymers, whereby a delicate change in the Young's modulus of a thin polymeric layer near its glass transition temperature can be successfully monitored.</P>

Selective Water Vapor Separation Using Hierarchically Developed Nanochannels inside Particle-Nested Inverse Opal Structures

Eunjin Kim,Pil J. Yoo 한국고분자학회 2016 한국고분자학회 학술대회 연구논문 초록집 Vol.41 No.2

Binder-free heat dissipation films assembled with reduced graphene oxide and alumina nanoparticles for simultaneous high in-plane and cross-plane thermal conductivities

Hong, Sunghwan,Yoo, Seong Soo,Yoo, Pil J. The Royal Society of Chemistry 2019 Journal of Materials Chemistry C Vol.7 No.30

<P>Reduced graphene oxide (rGO) is an attractive material for heat dissipation films due to its high thermal conductivity. However, stacked rGO nanosheets generally suffer from low cross-plane thermal conductivity owing to inter-sheet gaps, which is an obstacle for implementing effective heat dissipation films. To resolve this issue, here, we propose multilayered films consisting of GO nanosheets and alumina nanoparticles using a spin-assisted layer-by-layer deposition method. Charged moiety-supplemented alumina nanoparticles are uniformly inserted between horizontally oriented GO nanosheets to compensate for the mitigated thermal transport encountered in GO-only stacked films. By virtue of successful electrostatic binding between GO nanosheets and functionalized alumina nanoparticles, alternately stacked films are readily grown up to ∼10 μm in thickness. After thermal reduction treatment, in particular, thermally non-conductive residual functional groups are completely removed while securing the structural intactness in layered stacking, and rGO/alumina stacked films exhibit a remarkably high in-plane thermal conductivity of 565 W m<SUP>−1</SUP> K<SUP>−1</SUP>, which is even greater than that of Cu films by a factor of 1.4. In particular, due to the formation of a well-interconnected, ladder-like architecture of alumina nanoparticles between rGO nanosheets, the assembled film exhibits a cross-plane thermal conductivity of 18.1 W m<SUP>−1</SUP> K<SUP>−1</SUP>, which outperforms rGO-stacked layers by two orders of magnitude. As a result, rGO/alumina films can show efficient heat dissipation performances by significantly reducing the maximum operating temperature of high-power light-emitting devices.</P>

Influence of Ionic Strength on the Deposition of Metal–Phenolic Networks

Guo, Junling,Richardson, Joseph J.,Besford, Quinn A.,Christofferson, Andrew J.,Dai, Yunlu,Ong, Chien W.,Tardy, Blaise L.,Liang, Kang,Choi, Gwan H.,Cui, Jiwei,Yoo, Pil J.,Yarovsky, Irene,Caruso, Frank American Chemical Society 2017 Langmuir Vol.33 No.40

<P>Metal–phenolic networks (MPNs) are a versatile class of self-assembled materials that are able to form functional thin films on various substrates with potential applications in areas including drug delivery and catalysis. Different metal ions (e.g., Fe<SUP>III</SUP>, Cu<SUP>II</SUP>) and phenols (e.g., tannic acid, gallic acid) have been investigated for MPN film assembly; however, a mechanistic understanding of the thermodynamics governing MPN formation remains largely unexplored. To date, MPNs have been deposited at low ionic strengths (<5 mM), resulting in films with typical thicknesses of ∼10 nm, and it is still unclear how a bulk complexation reaction results in homogeneous thin films when a substrate is present. Herein we explore the influence of ionic strength (0–2 M NaCl) on the conformation of MPN precursors in solution and how this determines the final thickness and morphology of MPN films. Specifically, the film thickness increases from 10 nm in 0 M NaCl to 12 nm in 0.5 M NaCl and 15 nm in 1 M NaCl, after which the films grow rougher rather than thicker. For example, the root-mean-square roughness values of the films are constant below 1 M NaCl at 1.5 nm; in contrast, the roughness is 3 nm at 1 M NaCl and increases to 5 nm at 2 M NaCl. Small-angle X-ray scattering and molecular dynamics simulations allow for comparisons to be made with chelated metals and polyelectrolyte thin films. For example, at a higher ionic strength (2 M NaCl), sodium ions shield the galloyl groups of tannic acid, allowing them to extend away from the Fe<SUP>III</SUP> center and interact with other MPN complexes in solution to form thicker and rougher films. As the properties of films determine their final performance and application, the ability to tune both thickness and roughness using salts may allow for new applications of MPNs.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/langd5/2017/langd5.2017.33.issue-40/acs.langmuir.7b02692/production/images/medium/la-2017-026926_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/la7b02692'>ACS Electronic Supporting Info</A></P>

Solution-Processable Electrical Conductors Using Percolation-Regulated Metal/Polyelectrolyte Complexation

Min Jun OH,Pil J. YOO 한국고분자학회 2016 한국고분자학회 학술대회 연구논문 초록집 Vol.41 No.2

Interconnected assembly of ZrO2@SiO2 nanoparticles with dimensional selectivity and refractive index tunability

Rhee, Do Kyung,Yoo, Pil J. The Royal Society of Chemistry 2019 Journal of Materials Chemistry C Vol.7 No.27

<P>Self-assembled alignment of functional nanoparticles is an important strategy to create complexly architectured nanostructures. While there has been significant progress in nanoparticle self-assembly over the past decade, elaborate control over the assembled structures and long-term structural stability have yet to be fully realized. In this study, we have presented a novel synthetic approach for synchronizing the pre-assembly of primary nanoparticles and their subsequent shell-coating, eventually implementing dimensionally controlled ordering of nanoparticles while securing permanent structural stability. In particular, simply by adjusting the dielectric constant of the dispersion medium and accordingly varying the electrostatic repulsion between particles, the aligned structure of nanoparticles was modulated to have different dimensional orderings (<I>i.e.</I>, 1D, 2D, or 3D-like assembly). The obtained results were successfully interpreted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Finally, to demonstrate the practical applicability of aligned structures of nanoparticles, 1D-like structured ZrO2@SiO2 was synthesized and incorporated inside a polymeric resin as a filler. By virtue of the tuning of the effective refractive index by varying the relative fractions of the ZrO2 core and the SiO2 shell, the optical transparency of filler-incorporating polyethersulfone composite films was retained even including a 3 wt% amount of ZrO2@SiO2. Also, due to the facilitated structural interconnectivity between the 1D-chain-like filler and the encompassing polymeric matrix, the thermomechanical properties of the composite films were also greatly improved.</P>