Two-Dimensional Nanoparticle Supracrystals: A Model System for Two-Dimensional Melting

Kim, Jin Young,Kwon, S. Joon,Chang, Jae-Byum,Ross, Caroline A.,Hatton, T. Alan,Stellacci, Francesco American Chemical Society 2016 Nano letters Vol.16 No.2

<P>In a Langmuir trough, successive compression cycles can drive a two-dimensional (2D) nanoparticle supracrystal (NPSC) closer to its equilibrium structure. Here, we show a series of equilibrated 2D NPSCs consisting of gold NPs of uniform size, varying solely in the length of their alkanethiol ligands. The ordering of the NPSC is governed by the ligand length, thus providing a model system to investigate the nature of 2D melting in a system of NPs. As the ligand length increases the supracrystal transitions from a crystalline to a liquid-like phase with evidence of a hexatic phase at an intermediate ligand length. The phase change is interpreted as an entropy-driven phenomenon associated with steric constraints between ligand shells. The density of topological defects scales with ligand length, suggesting an equivalence between ligand length and temperature in terms of melting behavior. On the basis of this equivalence, the experimental evidence indicates a two-stage 2D melting of NPSCs.</P>

Square arrays of holes and dots patterned from a linear ABC triblock terpolymer.

Choi, Hong Kyoon,Gwyther, Jessica,Manners, Ian,Ross, Caroline A American Chemical Society 2012 ACS NANO Vol.6 No.9

<P>Microphase separation of a polyisoprene-b-polystyrene-b-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer film during chloroform solvent-annealing formed a 44 nm period square-symmetry array of alternating PI and PFS cylinders in a PS matrix. This nanostructure was converted to either a positive pattern of posts or a negative pattern of holes with tunable diameter by oxygen reactive ion etching or by surface reconstruction in a solvent, respectively, and coexisting post and hole patterns were also formed. Square arrays of silicon posts, pits, and inverted pyramids were fabricated by pattern transfer from the triblock terpolymer film into silicon substrates. The morphology of the triblock terpolymer film varied with the chloroform vapor pressure during solvent annealing, which was explained by selective swelling of the PI block at high vapor pressures. This triblock terpolymer system provides a convenient block copolymer lithography process for generation of nanoscale posts or holes with square symmetry.</P>

Interfacial Energy-Controlled Top Coats for Gyroid/Cylinder Phase Transitions of Polystyrene-<i>block</i>-polydimethylsiloxane Block Copolymer Thin Films

Ryu, In Hyu,Kim, Yong Joo,Jung, Yeon Sik,Lim, Jong Sung,Ross, Caroline A.,Son, Jeong Gon American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.20

<P>Block copolymers (BCPs) with a high Flory-Huggins interaction parameter (chi) can form well-defined sub-10 nm periodic structures and can be used as a template for fabrication of various functional nanostructures. However, the large difference of surface energy between the blocks commonly found in high-chi BCPs makes it challenging to stabilize a useful gyroid morphology in thin film form. Here, we used an interfacial-energy-tailored top-coat on a blended film of a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCP and a low-molecular-weight PDMS homopolymer with a hydrophilic end functional group. The top coat consisted of a random mixture of 40% hydrolyzed poly(vinyl acetate)-random-poly(vinly alcohol) (PVA-r-PVAc, PVA40) and PVAc homopolymer. At the optimized top-coat composition, gyroid nanostructures with sub-10 nm strut width were achieved down to similar to 125 nm film thickness, which is only 3 times the lattice parameter of the gyroid structure. This is in marked contrast with a mixed morphology of gyroid and cylinders obtained for other compositions of the top coat. Self-consistent field theoretic simulations were used to understand the effect of the interfacial energy between the top coat and BCP/homopolymer blends on the phase transition behavior of the BCP/homopolymer films.</P>

Magnetic Phase Formation in Self-Assembled Epitaxial BiFeO₃–MgO and BiFeO₃–MgAl₂O₄ Nanocomposite Films Grown by Combinatorial Pulsed Laser Deposition

Kim, Dong Hun,Sun, XueYin,Kim, Tae Cheol,Eun, Yun Jae,Lee, Taeho,Jeong, Sung Gyun,Ross, Caroline A. American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.4

<P>Self-assembled epitaxial BiFeO3-MgO and BiFeO3-MgAl2O4 nanocomposite thin films were grown on SrTiO3 substrates by pulsed laser deposition. A two-phase columnar structure was observed for BiFeO3-MgO codeposition within a small window of growth parameters, in which the pillars consisted of a magnetic spinel phase (Mg,Fe)(3)O-4 within a BiFeO3 matrix, similar to the growth of BiFeO3 MgFe2O4 nanocomposites reported elsewhere. Further, growth of a nanocomposite with BiFeO3-(CoFe2O4/MgO/MgFe2O4), in which the minority phase was grown from three different targets, gave spinel pillars with a uniform (Mg,Fe,Co)(3)O-4 composition due to interdiffusion during growth) with a bifurcated shape from the merger of neighboring pillars. BiFeO3MgAl2O4 did not form a well-defined vertical nanocomposite in spite of having lower lattice mismatch, but instead formed a two-phase film with in which the spinel phase contained Fe. These results illustrate the redistribution of Fe between the oxide phases during oxide codeposition to form a ferrimagnetic phase from antiferromagnetic or nonmagnetic targets.</P>

Sub‐10 nm Graphene Nanoribbon Array Field‐Effect Transistors Fabricated by Block Copolymer Lithography

Son, Jeong Gon,Son, Myungwoo,Moon, Kyeong‐,Joo,Lee, Byoung Hun,Myoung, Jae‐,Min,Strano, Michael S.,Ham, Moon‐,Ho,Ross, Caroline A. WILEY‐VCH Verlag 2013 ADVANCED MATERIALS Vol.25 No.34

<P><B>Sub‐10 nm Graphene Nanoribbon Arrays</B> are fabricated over large areas by etching CVD‐grown graphene. A mask is used made by the directed self‐assembly of a cylindrical PS‐<I>b</I>‐PDMS block copolymer under solvent annealing guided by a removable template. The optimized solvent annealing process, surface‐modified removable polymeric templates, and high Flory‐Huggins interaction parameters of the block copolymer enable a highly aligned array of nanoribbons with low line edge roughness to be formed. This leads to a higher on/off ratio and stronger temperature dependence of the current for nanoribbon FETs, and a photocurrent which is 30 times larger compared to unpatterned graphene.</P>

Ferromagnetism in single crystal and nanocomposite Sr(Ti,Fe)O₃ epitaxial films

Kim, Hyun-Suk,Bi, Lei,Kim, Dong Hun,Yang, Dae-Jin,Choi, Yoon Jeong,Lee, Jung Woo,Kang, Jeung Ku,Chang Park, Yun,Dionne, Gerald F.,Ross, Caroline A. Royal Society of Chemistry 2011 Journal of materials chemistry Vol.21 No.28

<P>The ferromagnetic properties and electrical leakage current of single-phase SrTi<SUB>1−<I>x</I></SUB>Fe<SUB><I>x</I></SUB>O<SUB>3</SUB> (STF) perovskite films are compared for two different samples: a single-crystal film with a (100) orientation, and a ‘doubly oriented’ nanocomposite film consisting of (110)-oriented nanopillars embedded homoepitaxially in a (100)-oriented matrix. The STF films contain mixed valence Fe ions, with a lower average valence state present in the single crystal film. The films are under an in-plane compressive strain, and exhibit an out-of-plane magnetic easy axis due to magnetoelastic effects. The nanopillars in the double-epitaxial STF films act as single ferromagnetic domains, whereas the single-crystal films show a maze-like domain structure. Composition fluctuations seen in single-crystal films are suppressed in the double-epitaxial structure, which has a lower electrical leakage current. First-principles modeling supports a tendency for Fe ions to occupy adjacent sites. The correlations between the valence state and distribution of the Fe ions, the microstructure, and the magnetic and electrical properties provide a general method of tailoring the properties of perovskite films, which have immense technological value in a range of multiferroic, ferromagnetic, optical, spintronic and hybrid devices.</P> <P>Graphic Abstract</P><P>The ferromagnetic and electrical properties of SrTi<SUB>1−<I>x</I></SUB>Fe<SUB><I>x</I></SUB>O<SUB>3</SUB> films grown on buffered Si are characterized for both (100)-oriented single crystal films and double-epitaxial films consisting of (110)-oriented nanopillars embedded homoepitaxially in a (100)-oriented matrix. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1jm11286d'> </P>

Directed self-assembly of a two-state block copolymer system

Do Hyung Wan,Choi Hong Kyoon,Gadelrab Karim R.,Chang Jae-Byum,Alexander-Katz Alfredo,Ross Caroline A.,Berggren Karl K. 나노기술연구협의회 2018 Nano Convergence Vol.5 No.25

In this work, ladder-shaped block copolymer structures consisting of parallel bars, bends, and T-junctions are formed inside square confinement. We define binary states by the two degenerate alignment orientations, and study properties of the two-state system. We control the binary states by creating openings around the confinement, changing the confinement geometry, or placing lithographic guiding patterns inside the confinement. Self-consistent field theory simulations show templating effect from the wall openings and reproduce the experimental results. We demonstrate scaling of a single binary state into a larger binary state array with individual binary state control.

Templated assembly of Co-Pt nanoparticlesvia thermal and laser-induced dewetting of bilayer metal films

Oh, Yong-Jun,Kim, Jung-Hwan,Thompson, Carl V.,Ross, Caroline A. The Royal Society of Chemistry 2013 Nanoscale Vol.5 No.1

<P>Templated dewetting of a Co/Pt metal bilayer film on a topographic substrate was used to assemble arrays of Co-Pt alloy nanoparticles, with highly uniform particle size, shape and notably composition compared to nanoparticles formed on an untemplated substrate. Solid-state and liquid-state dewetting processes, using furnace annealing and laser irradiation respectively, were compared. Liquid state dewetting produced more uniform, conformal nanoparticles but they had a polycrystalline disordered fcc structure and relatively low magnetic coercivity. In contrast, solid state dewetting enabled formation of magnetically hard, ordered L1(0) Co-Pt single-crystal particles with coercivity >12 kOe. Furnace annealing converted the nanoparticles formed by liquid state dewetting into the L1(0) phase.</P>

Self-assembled multiferroic perovskite-spinel nanocomposite thin films: epitaxial growth, templating and integration on silicon

Kim, Dong Hun,Ning, Shuai,Ross, Caroline A. The Royal Society of Chemistry 2019 Journal of materials chemistry. C, Materials for o Vol.7 No.30

<P>This review describes recent progress in the growth and properties of self-assembled multiferroic perovskite-spinel nanocomposite thin films, in particular, BiFeO3-CoFe2O4, and their integration on silicon or pre-patterned substrates. Vertically aligned nanocomposite thin films, in which ferromagnetic spinel CoFe2O4 nanopillars grow in a ferroelectric perovskite BiFeO3 matrix, have been investigated for applications such as next generation memory devices. The strain transfer between the two oxide phases at the vertical interface enables cross-coupling of the properties, <I>i.e.</I> a magnetic (electric) field can modify the polarization (magnetization) state, making the nanocomposite a two-phase magnetoelectric multiferroic. The ability to grow high quality thin film nanocomposites and the control of their properties through epitaxial strain transfer are prerequisites for their incorporation into practical devices. Most work has been carried out on perovskite substrates using pulsed laser deposition, and integration on semiconductor-compatible substrates using scalable deposition processes presents a challenge. This review focuses on the epitaxial integration of BiFeO3-CoFe2O4 nanocomposite thin films using sputter deposition on silicon substrates, and discusses recent work on the formation of highly ordered nanocomposites using various template patterning methods. This robust, scalable route for large-area fabrication of multifunctional oxide nanocomposites using sputtering brings the exciting prospects offered by these materials closer towards technological realization.</P>

Structure and magnetic properties of pulsed laser deposited SrFe₁₂O₁₉ thin films on SrTiO₃ (100) and (111) substrates

Kim, Dong Hun,Han, Seung Ho,Kang, Young-Min,Yang, Daejin,Ross, Caroline A. ELSEVIER SCIENCE 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.692 No.-

<P><B>Abstract</B></P> <P>Epitaxial SrFe<SUB>12</SUB>O<SUB>19</SUB> films were prepared on SrTiO<SUB>3</SUB> (001) and (111) substrates by pulsed laser deposition and annealed in air at high temperature. As-deposited and annealed SrFe<SUB>12</SUB>O<SUB>19</SUB> thin films were characterized using scanning electron microscopy, x-ray diffraction, and vibrating sample magnetometry. Films annealed at 800 °C and 1000 °C on (111) substrates showed an epitaxial growth whereas films on (001) substrates grew with both in-plane and out-of-plane c-axis. The magnetic hysteresis loops of annealed films on (111) substrates exhibited a strong magnetocrystalline anisotropy with out-of-plane easy axis originating from epitaxial growth while the films on (001) substrates were almost isotropic.</P> <P><B>Highlights</B></P> <P> <UL> <LI> SrFe<SUB>12</SUB>O<SUB>19</SUB> thin films were grown on SrTiO<SUB>3</SUB> substrates by pulsed laser deposition. </LI> <LI> The as-deposited SrFe<SUB>12</SUB>O<SUB>19</SUB> films exhibited low magnetization with poor crystallinity. </LI> <LI> Annealed SrFe<SUB>12</SUB>O<SUB>19</SUB> films showed crystallites with faceted surfaces. </LI> <LI> Annealed SrFe<SUB>12</SUB>O<SUB>19</SUB> films on SrTiO<SUB>3</SUB> (111) substrate had a strong magnetic anisotropy. </LI> </UL> </P>