Co-reporter:Pin Ho, Kun-Hua Tu, Jinshuo Zhang, Congli Sun, Jingsheng Chen, George Liontos, Konstantinos Ntetsikas, Apostolos Avgeropoulos, Paul M. Voyles and Caroline A. Ross
Nanoscale 2016 vol. 8(Issue 9) pp:5358-5367
Publication Date(Web):05 Feb 2016
DOI:10.1039/C5NR08865H
Perpendicular magnetic anisotropy [Co/Pd]15 and L10-FePt nanowire arrays of period 63 nm with linewidths 38 nm and 27 nm and film thickness 27 nm and 20 nm respectively were fabricated using a self-assembled PS-b-PDMS diblock copolymer film as a lithographic mask. The wires are predicted to support Néel walls in the Co/Pd and Bloch walls in the FePt. Magnetostatic interactions from nearest neighbor nanowires promote a ground state configuration consisting of alternating up and down magnetization in adjacent wires. This was observed over ∼75% of the Co/Pd wires after ac-demagnetization but was less prevalent in the FePt because the ratio of interaction field to switching field was much smaller. Interactions also led to correlations in the domain wall positions in adjacent Co/Pd nanowires. The reversal process was characterized by nucleation of reverse domains, followed at higher fields by propagation of the domains along the nanowires. These narrow wires provide model system for exploring domain wall structure and dynamics in perpendicular anisotropy systems.
Co-reporter:Dong Hun Kim, XueYin Sun, Tae Cheol Kim, Yun Jae Eun, Taeho Lee, Sung Gyun Jeong, and Caroline A. Ross
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 4) pp:2673
Publication Date(Web):January 11, 2016
DOI:10.1021/acsami.5b10676
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)3O4 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)3O4 composition due to interdiffusion during growth, with a bifurcated shape from the merger of neighboring pillars. BiFeO3–MgAl2O4 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.Keywords: BiFeO3; combinatorial pulsed laser deposition; epitaxy; multiferroic; perovskite; self-assembled oxide nanocomposite; spinel; thin film oxide;
Co-reporter:Christine C. Kathrein, Wubin Bai, Adam Nunns, Jessica Gwyther, Ian Manners, Alexander Böker, Larisa Tsarkova and Caroline A. Ross
Soft Matter 2016 vol. 12(Issue 21) pp:4866-4874
Publication Date(Web):25 Apr 2016
DOI:10.1039/C6SM00451B
We report the effect of electric field on the morphological transitions and ordering behavior of polyferrocenylethylmethylsilane block (PFEMS)-containing copolymers. By analyzing structures in solvent-annealed films of metalorganic sphere- and cylinder-forming diblock copolymers, as well as of 3-miktoarm polyisoprene-arm-polystyrene-arm-PFEMS (3μ-ISF) terpolymers, we decouple two types of responses to the electric field: morphological transformations as a result of an increase in the volume fraction of the PFEMS block by oxidation of the ferrocenyl groups, and the orientation of the dielectric interfaces of microdomains parallel to the electric field vector. In the case of 3μ-ISF, the former effect dominates at high electric field strengths which results in an unexpected cylinder-to-sphere transition, leading to a well-ordered hexagonal dot pattern. Our results demonstrate multiple tunability of ordered microdomain morphologies, suggesting future applications in nanofabrication and surface patterning.
Co-reporter:W. Bai, K.G. Yager, C.A. Ross
Polymer 2016 Volume 101() pp:176-183
Publication Date(Web):28 September 2016
DOI:10.1016/j.polymer.2016.08.055
•The self-assembly of 12–16 kg/mol molecular weight Si-containing block copolymer (BCP) thin films was characterized in situ.•The changes in lattice dimensions and orientation of the self-assembled features was tracked during solvent annealing.•A non-bulk morphology was produced by changing the mixing ratios of the two selective solvents.The room-temperature solvent vapor annealing of polystyrene-b-polydimethylsiloxane (PS-b-PDMS) block copolymer films was studied in situ by grazing incidence small-angle X-ray scattering. Films of cylinder-forming PS-b-PDMS with molecular weight 16 kg/mol and 12.2 kg/mol, annealed under vapors of toluene:heptane with varying composition, exhibited swelling-ratio-dependent evolution of microdomain spacing and orientation. In a vapor made from a toluene:heptane 5:1 volumetric ratio liquid mixture, or from pure toluene, cylindrical microdomains reoriented from majority out-of-plane to in-plane during annealing, while drying led to shrinkage along the film normal and a large distortion of the hexagonal lattice of in-plane cylinders. Annealing under vapor from a toluene:heptane 1:5 volumetric ratio liquid produced a non-bulk lamellar structure in the 16 kg/mol PS-b-PDMS.
Co-reporter:Shuchi Ojha, Wallace C. Nunes, Nicolas M. Aimon, and Caroline A. Ross
ACS Nano 2016 Volume 10(Issue 8) pp:7657
Publication Date(Web):July 19, 2016
DOI:10.1021/acsnano.6b02985
Self-assembled vertically aligned oxide nanocomposites consisting of magnetic pillars embedded in a ferroelectric matrix have been proposed for logic devices made from arrays of magnetostatically interacting pillars. To control the ratio between the nearest neighbor interaction field and the switching field of the pillars, the pillar composition CoxNi1–xFe2O4 was varied over the range 0 ≤ x ≤ 1, which alters the magnetoelastic and magnetocrystalline anisotropy and the saturation magnetization. Nanocomposites were templated into square arrays of pillars in which the formation of a “checkerboard” ground state after ac-demagnetization indicated dominant magnetostatic interactions. The effect of switching field distribution in disrupting the antiparallel nearest neighbor configuration was analyzed using an Ising model and compared with experimental results.Keywords: magnetostatic interactions; self-assembled growth; switching field distribution; templating; vertical oxide nanocomposites
Co-reporter:Wubin Bai, Karim Gadelrab, Alfredo Alexander-Katz, and Caroline A. Ross
Nano Letters 2015 Volume 15(Issue 10) pp:6901-6908
Publication Date(Web):September 21, 2015
DOI:10.1021/acs.nanolett.5b02815
Perpendicular orientation of lamellar microdomains in a high interaction parameter block copolymer was obtained within high aspect ratio gratings functionalized with a preferential sidewall brush. The experiments used polystyrene-block-polydimethylsiloxane (PS-b-PDMS) with molecular weight 43 kg/mol within trenches made using interference lithography. The perpendicular alignment was obtained for both thermal and solvent annealing, using three different solvent vapors, for a range of film thicknesses and trench widths. A platinum (Pt) layer at the base of the trenches avoided the formation of a wetting layer, giving perpendicular orientation at the substrate surface. The results are interpreted using self-consistent field theory simulation and a Ginzburg–Landau analytic model to map the energies of lamellae of different orientations as a function of the grating aspect ratio and the surface energies of the sidewalls and top and bottom surfaces. The model results agree with the experiment and provide a set of guidelines for obtaining perpendicular microdomains within topographic features.
Co-reporter:Dong Hun Kim;Xue Yin Sun;Nicolas M. Aimon;Jae Jin Kim;Michael J. Campion;Harry L. Tuller;Lior Kornblum;Fred J. Walker;Charles H. Ahn
Advanced Functional Materials 2015 Volume 25( Issue 20) pp:3091-3100
Publication Date(Web):
DOI:10.1002/adfm.201500332
A self-assembled three phase epitaxial nanocomposite film is grown consisting of ≈3 nm diameter fcc metallic Cu nanorods within square prismatic SrO rocksalt nanopillars in a Sr(Ti,Cu)O3-δ perovskite matrix. Each phase has an epitaxial relation to the others. The core–shell-matrix structures are grown on SrTiO3 substrates and can also be integrated onto Si using a thin SrTiO3 buffer. The structure is made by pulsed laser deposition in vacuum from a SrTi0.75Cu0.25O3 target, and formed as a result of the limited solubility of Cu in the perovskite matrix. Wet etching removes the 3 nm diameter Cu nanowires leaving porous SrO pillars. The three-phase nanocomposite film is used as a substrate for growing a second epitaxial nanocomposite consisting of CoFe2O4 spinel pillars in a BiFeO3 perovskite matrix, producing dramatic effects on the structure and magnetic properties of the CoFe2O4. This three-phase vertical nanocomposite provides a complement to the well-known two-phase nanocomposites, and may offer a combination of properties of three different materials as well as additional avenues for strain-mediated coupling within a single film.
Co-reporter:Christine C. Kathrein, Wubin Bai, Jean Anne Currivan-Incorvia, George Liontos, Konstantinos Ntetsikas, Apostolos Avgeropoulos, Alexander Böker, Larisa Tsarkova, and Caroline A. Ross
Chemistry of Materials 2015 Volume 27(Issue 19) pp:6890
Publication Date(Web):September 15, 2015
DOI:10.1021/acs.chemmater.5b03354
We report a combined directing effect of the simultaneously applied graphoepitaxy and electric field on the self-assembly of cylinder forming polystyrene-b-poly(dimethylsiloxane) block copolymer in thin films. A correlation length of up to 20 μm of uniaxial ordered striped patterns is an order of magnitude greater than that produced by either graphoepitaxy or electric field alignment alone and is achieved at reduced annealing times. The angle between the electric field direction and the topographic guides as well as the dimensions of the trenches affected both the quality of the ordering and the direction of the orientation of cylindrical domains: parallel or perpendicular to the topographic features. We quantified the interplay between the electric field and the geometry of the topographic structures by constructing the phase diagram of microdomain orientation. This combined approach allows the fabrication of highly ordered block copolymer structures using macroscopically prepatterned photolithographic substrates.
Co-reporter:Nicolas M. Aimon, Dong Hun Kim, XueYin Sun, and C. A. Ross
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 4) pp:2263
Publication Date(Web):January 5, 2015
DOI:10.1021/am506089c
Self-assembled BiFeO3–CoFe2O4 nanocomposites were templated into ordered structures in which the ferrimagnetic CoFe2O4 pillars form square arrays of periods 60–100 nm in a ferroelectric BiFeO3 matrix. The ferroelectricity, magnetism, conductivity, and magnetoelectric coupling of the ordered nanocomposites were characterized by scanning probe microscopy. The insulating BiFeO3 matrix exhibited ferroelectric domains, whereas the resistive CoFe2O4 pillars exhibited single-domain magnetic contrast with high anisotropy due to the magnetoelasticity of the spinel phase. Magnetoelectric coupling was observed in which an applied voltage led to reversal of the magnetic pillars.Keywords: conductive AFM; magnetic force microscopy; magnetoelectric coupling; piezo-response force microscopy; templated self-assembly; vertically aligned nanocomposites
Co-reporter:Saman Safari Dinachali, Wubin Bai, Kun-Hua Tu, Hong Kyoon Choi, Jinshuo Zhang, Melissa E Kreider, Li-Chen Cheng, and Caroline A. Ross
ACS Macro Letters 2015 Volume 4(Issue 5) pp:500
Publication Date(Web):April 17, 2015
DOI:10.1021/acsmacrolett.5b00108
A combined thermal and solvent vapor annealing process for block copolymer self-assembly is demonstrated. Films of cylinder-forming poly(styrene-b-dimethylsiloxane) (SD45, 45.5 kg/mol, fPDMS = 31%) were preheated for 2 min above the glass transition temperature of both blocks, followed by immediate introduction into a chamber containing room temperature saturated vapors of toluene and n-heptane. After quenching in air, microdomains had better order than those obtained from thermal or solvent annealing alone. The short time during which the film is both heated and exposed to solvent vapor played an important role in determining the final morphology.
Co-reporter:A. F. Hannon, W. Bai, A. Alexander-Katz and C. A. Ross
Soft Matter 2015 vol. 11(Issue 19) pp:3794-3805
Publication Date(Web):27 Mar 2015
DOI:10.1039/C5SM00324E
Recent progress in modelling the solvent vapor annealing of thin film block copolymers is examined in the context of a self-consistent field theory framework. Key control variables in determining the final microdomain morphologies include swelling ratio or swollen film solvent volume fraction, swollen film thickness, substrate and vapor atmosphere surface energies, effective volume fraction, and effective Flory–Huggins interaction parameter. The regime of solvent vapor annealing studied is where the block copolymer has a high enough Flory–Huggins parameter that ordered structures form during swelling and are then trapped in the system through quenching. Both implicit and explicit consideration of the solvent vapor is considered to distinguish the cases in which solvent vapor leads to a non-bulk morphology. Block-selective solvents are considered based on the experimental systems of polystyrene-b-polydimethylsiloxane annealed with toluene and heptane. The results of these simulations are compared with these experiments.
Co-reporter:W. Bai, K. G. Yager, and C. A. Ross
Macromolecules 2015 Volume 48(Issue 23) pp:8574-8584
Publication Date(Web):November 18, 2015
DOI:10.1021/acs.macromol.5b02174
Grazing incidence X-ray scattering was used to follow the microphase separation during room-temperature solvent annealing of films of high interaction parameter cylinder-forming 16 kg/mol polystyrene-b-polydimethylsiloxane block copolymer in a toluene–heptane mixed solvent vapor. Microphase separation was observed for swelling ratios above 1.2, but swelling ratios above 1.9 caused the films to disorder. The films exhibited a thickness-dependent microdomain reorientation during annealing, with the initial out-of-plane cylinder orientation converting to an in-plane orientation as a result of preferential surface interactions. Drying led to a reduction in the out-of-plane cylinder spacing, with slower drying leading to greater deswelling and larger distortion of the hexagonal lattice.
Co-reporter:Xue Yin Sun, Qingyang Du, Taichi Goto, Mehmet C. Onbasli, Dong Hun Kim, Nicolas M. Aimon, Juejun Hu, and Caroline A. Ross
ACS Photonics 2015 Volume 2(Issue 7) pp:
Publication Date(Web):June 10, 2015
DOI:10.1021/acsphotonics.5b00026
Photonic integrated circuits require magneto-optical (MO) materials for making nonreciprocal devices such as isolators and circulators. The most successful MO materials are rare-earth-substituted iron garnets, but these can be challenging to grow on silicon without a seed layer, which introduces spacing loss between the waveguide and the MO cladding. A pulsed-laser deposition (PLD) method is used for making MO Ce:YIG (Ce1Y2Fe5O12)/YIG (Y3Fe5O12) bilayer or trilayer films on different substrates, including silicon, quartz, and Gd3Ga5O12 (GGG), in which a multilayer film is deposited in one run and then annealed. A YIG seed layer grown above the MO Ce:YIG facilitates recrystallization during ex situ rapid thermal annealing, which results in a reduced thermal budget and simplified deposition process. A monolithically integrated optical isolator was demonstrated by direct deposition of a bilayer Ce:YIG/YIG capping layer onto a silicon-on-insulator resonator. The device exhibited an insertion loss of 7.4 ± 1.8 dB and an isolation ratio of 13.0 ± 2.2 dB within the telecommunication window (λ = 1564.4 nm), which outperforms previously reported monolithic isolators.
Co-reporter:Caroline A. Ross;Karl K. Berggren;Joy Y. Cheng;Yeon Sik Jung;Jae-Byum Chang
Advanced Materials 2014 Volume 26( Issue 25) pp:4386-4396
Publication Date(Web):
DOI:10.1002/adma.201400386
Thin films of block copolymers are widely seen as enablers for nanoscale fabrication of semiconductor devices, membranes, and other structures, taking advantage of microphase separation to produce well-organized nanostructures with periods of a few nm and above. However, the inherently three-dimensional structure of block copolymer microdomains could enable them to make 3D devices and structures directly, which could lead to efficient fabrication of complex heterogeneous structures. This article reviews recent progress in developing 3D nanofabrication processes based on block copolymers.
Co-reporter:Hong Kyoon Choi;Adam Nunns;Xue Yin Sun;Ian Manners
Advanced Materials 2014 Volume 26( Issue 16) pp:2474-2479
Publication Date(Web):
DOI:10.1002/adma.201305243
Co-reporter:Nicolas M. Aimon;Hong Kyoon Choi;Xue Yin Sun;Dong Hun Kim
Advanced Materials 2014 Volume 26( Issue 19) pp:3063-3067
Publication Date(Web):
DOI:10.1002/adma.201305459
Co-reporter:Adam F. Hannon, Yi Ding, Wubin Bai, Caroline A. Ross, and Alfredo Alexander-Katz
Nano Letters 2014 Volume 14(Issue 1) pp:318-325
Publication Date(Web):December 11, 2013
DOI:10.1021/nl404067s
An inverse design algorithm has been developed that predicts the necessary topographical template needed to direct the self-assembly of a diblock copolymer to produce a given complex target structure. The approach is optimized by varying the number of topographical posts, post size, and block copolymer volume fraction to yield a template solution that generates the target structure in a reproducible manner. The inverse algorithm is implemented computationally to predict post arrangements that will template two different target structures and the predicted templates are tested experimentally with a polydimethylsiloxane-b-polystyrene block copolymer. Simulated and experimental results show overall very good agreement despite the complexity of the patterns. The templates determined from the model can be used in developing simpler design rules for block copolymer directed self-assembly.
Co-reporter:Kevin W. Gotrik;Thomas Lam;Adam F. Hannon;Wubin Bai;Yi Ding;Jonathan Winterstein;Alfredo Alexer-Katz;J. Alexer Liddle
Advanced Functional Materials 2014 Volume 24( Issue 48) pp:7689-7697
Publication Date(Web):
DOI:10.1002/adfm.201402457
Transmission electron microscope (TEM) tomography was used to visualize the morphology and 3D connectivity of a lithographically templated, self-assembled bilayer film of cylinder-forming 45.5 kg/mol polystyrene-block-polydimethylsiloxane. The structure, formed after a 5 min solvothermal anneal, was imaged with a resolution of ≈3 nm in 3D, enabling a comparison between measurement and self-consistent mean-field theory (SCFT) calculations. Images of etched and unetched samples showed that etching collapsed the PDMS microdomain structure and reduced the template dimensions. In addition to the general comparison between modeled and measured dimensions, the tomography revealed connections between the orthogonal layers of cylinders at their crossing points. Comparison with the SCFT model, even under solvothermal annealing conditions, shows that it is helpful in understanding the detailed nanoscale structure of features created by directed self-assembly (DSA), which is essential in developing nanomanufacturing processes based on DSA.
Co-reporter:Dong Hun Kim;Nicolas M. Aimon;Xue Yin Sun;Lior Kornblum;Fred. J. Walker;Charles. H. Ahn
Advanced Functional Materials 2014 Volume 24( Issue 37) pp:5889-5896
Publication Date(Web):
DOI:10.1002/adfm.201401458
Perovskite-spinel epitaxial nanocomposite thin films are commonly grown on single crystal perovskite substrates, but integration onto a Si substrate can greatly increase their usefulness in devices. Epitaxial BiFeO3–CoFe2O4 nanocomposites consisting of CoFe2O4 pillars in a BiFeO3 matrix are grown on (001) Si with two types of buffer layers: molecular beam epitaxy (MBE)-grown SrTiO3-coated Si and pulsed-laser-deposited (PLD) Sr(Ti0.65Fe0.35)O3/CeO2/yttria-stabilized ZrO2/Si. The nanocomposite grows with the same crystallographic orientation and morphology as that observed on single crystal SrTiO3 when the buffered Si substrates are smooth, but roughness of the Sr(Ti0.65Fe0.35)O3 promoted additional CoFe2O4 pillar orientations with 45° rotation. The nanocomposites on MBE-buffered Si show very high magnetic anisotropy resulting from magnetoelastic effects, whereas the hysteresis of nanocomposites on PLD-buffered Si can be understood as a combination of the hysteresis of the Sr(Ti0.65Fe0.35)O3 film and the CoFe2O4 pillars.
Co-reporter:Dong Hun Kim;Nicolas M. Aimon;Xueyin Sun
Advanced Functional Materials 2014 Volume 24( Issue 16) pp:2334-2342
Publication Date(Web):
DOI:10.1002/adfm.201302844
There is great interest in self-assembled oxide vertical nanocomposite films consisting of epitaxial spinel pillars in a single crystal perovskite matrix, due to their tunable electronic, magnetic, and multiferroic properties. Varying the composition or geometry of the pillars in the out-of-plane direction has not been previously reported but can provide new routes to tailoring their properties in three dimensions. In this work, ferrimagnetic epitaxial CoFe2O4, MgFe2O4, or NiFe2O4 spinel nanopillars with an out-of-plane modulation in their composition and shape are grown in a BiFeO3 matrix on a (001) SrTiO3 substrate using pulsed laser deposition. Changing the pillar composition during growth produces a homogeneous pillar composition due to cation interdiffusion, but this can be suppressed using a sufficiently thick blocking layer of BiFeO3 to produce bi-pillar films containing for example a layer of magnetically hard CoFe2O4 pillars and a layer of magnetically soft MgFe2O4 pillars, which form in different locations. A thinner blocking layer enables contact between the top of the CoFe2O4 and the bottom of the MgFe2O4 which leads to correlated growth of the MgFe2O4 pillars directly above the CoFe2O4 pillars and provides a path for interdiffusion. The magnetic hysteresis of the nanocomposites is related to the pillar structure.
Co-reporter:Xue Yin Sun, Chen Zhang, Nicolas M. Aimon, Taichi Goto, Mehmet Onbasli, Dong Hun Kim, Hong Kyoon Choi, and C. A. Ross
ACS Combinatorial Science 2014 Volume 16(Issue 11) pp:640
Publication Date(Web):October 8, 2014
DOI:10.1021/co5000773
Ferromagnetic Sr(GaxTiyFe0.34–0.40)O3−δ (0.1 ≤ x, y ≤ 0.5) films with single-crystal perovskite structure were epitaxially grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates by combinatorial pulsed laser deposition (CPLD) and compared with previous results from films grown from single targets. In CPLD films the Fe was present as both Fe2+ and Fe3+. The distribution of Sr, Ga, Ti, and O was homogeneous, but Fe-rich nanowires with diameter of 3 nm were present perpendicular to the film plane. The unit cell was tetragonally distorted with the ratio of out-of-plane to in-plane lattice parameter decreasing from 1.06 to 1.02 as the Ga content increased. The magnetic easy axis of the films changed from out-of-plane when Ti content y > 0.3 to isotropic as the Ga content increased, consistent with a reduction in magnetoelastic anisotropy. The Ga lowered the Faraday rotation and the magnetization but increased the optical transmittance.Keywords: combinatorial pulsed laser deposition; Faraday rotation; magnetoelastic anisotropy; perovskite
Co-reporter:Wubin Bai, Adam F. Hannon, Kevin W. Gotrik, Hong Kyoon Choi, Karim Aissou, George Liontos, Konstantinos Ntetsikas, Alfredo Alexander-Katz, Apostolos Avgeropoulos, and Caroline A. Ross
Macromolecules 2014 Volume 47(Issue 17) pp:6000-6008
Publication Date(Web):August 26, 2014
DOI:10.1021/ma501293n
Thin film morphologies of a 75.5 kg/mol polystyrene-block-polydimethylsiloxane (PS-b-PDMS) diblock copolymer subject to solvent vapor annealing are described. The PS-b-PDMS has a double-gyroid morphology in bulk, but as a thin film the morphology can form spheres, cylinders, perforated lamellae, or gyroids, depending on the film thickness, its commensurability with the microdomain period, and the ratio of toluene:heptane vapors used for the solvent annealing process. The morphologies are described by self-consistent field theory simulations. Thin film structures with excellent long-range order were produced, which are promising for nanopatterning applications.
Co-reporter:Hong Kyoon Choi, Nicolas M. Aimon, Dong Hun Kim, Xue Yin Sun, Jessica Gwyther, Ian Manners, and Caroline A. Ross
ACS Nano 2014 Volume 8(Issue 9) pp:9248
Publication Date(Web):September 3, 2014
DOI:10.1021/nn503100s
A process route to fabricate templated BiFeO3/CoFe2O4 (BFO/CFO) vertical nanocomposites is presented in which the self-assembly of the BFO/CFO is guided using a self-assembled triblock terpolymer. A linear triblock terpolymer was selected instead of a diblock copolymer in order to produce a square-symmetry template, which had a period of 44 nm. The triblock terpolymer pattern was transferred to a (001) Nb:SrTiO3 substrate to produce pits that formed preferential sites for the nucleation of CFO crystals, in contrast to the BFO, which wetted the flat regions of the substrate. The crystallographic orientation and magnetic properties of the templated BFO/CFO were characterized.Keywords: directed self-assembly; spinel-perovskite nanocomposite; triblock terpolymer;
Co-reporter:Kevin W. Gotrik and C. A. Ross
Nano Letters 2013 Volume 13(Issue 11) pp:5117-5122
Publication Date(Web):October 1, 2013
DOI:10.1021/nl4021683
A two-stage annealing process for block copolymer films was introduced consisting of a solvent vapor exposure followed by a thermal cycle. By heating the film but not the chamber, changes in the ambient vapor pressure of the solvent were avoided. Films of block copolymers and homopolymers showed transient nonmonotonic swelling behavior immediately after solvent exposure that was dependent on how the thin film was cast before the anneal. Thermal cycling of the solvent-swelled block copolymer films during the solvent vapor anneal (SVA) caused the films to deswell in 1–10 s and produced well-ordered microdomains in templated 45.5 and 51.5 kg/mol polystyrene-block-polydimethylsiloxane films annealed in toluene and n-heptane vapors for total process times of 30 s to 5 min.
Co-reporter:Karim Aissou, Hong Kyoon Choi, Adam Nunns, Ian Manners, and Caroline A. Ross
Nano Letters 2013 Volume 13(Issue 2) pp:835-839
Publication Date(Web):January 23, 2013
DOI:10.1021/nl400006c
The directed self-assembly of 3-miktoarm star terpolymer chains (polyisoprene-arm-polystyrene-arm-polyferrocenylethylmethylsilane (3 μ-ISF)) into 2D Archimedean tilings is described. A morphological change from (4.82) to (63) tiling is reported in the 3 μ-ISF thin film blended with PS homopolymer when a greater swelling of PI is achieved during the solvent annealing process. Highly oriented (4.82) tilings were produced by templating the self-assembled three colored structures in blended thin films. The use of (4.82) and (63) tilings as nanolithographic masks to transfer square and triangular hole arrays into the substrate is also demonstrated.
Co-reporter:Yong-Jun Oh, Jung-Hwan Kim, Carl V. Thompson and Caroline A. Ross
Nanoscale 2013 vol. 5(Issue 1) pp:401-407
Publication Date(Web):23 Nov 2012
DOI:10.1039/C2NR32932H
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 L10 Co–Pt single-crystal particles with coercivity >12 kOe. Furnace annealing converted the nanoparticles formed by liquid state dewetting into the L10 phase.
Co-reporter:Amir Tavakkoli K. G.;Adam F. Hannon;Kevin W. Gotrik;Alfredo Alexer-Katz;Karl K. Berggren
Advanced Materials 2012 Volume 24( Issue 31) pp:
Publication Date(Web):
DOI:10.1002/adma.201290190
Co-reporter:Amir Tavakkoli K. G.;Adam F. Hannon;Kevin W. Gotrik;Alfredo Alexer-Katz;Karl K. Berggren
Advanced Materials 2012 Volume 24( Issue 31) pp:4249-4254
Publication Date(Web):
DOI:10.1002/adma.201104895
Co-reporter:Jeong Gon Son, Kevin W. Gotrik, and C. A. Ross
ACS Macro Letters 2012 Volume 1(Issue 11) pp:1279
Publication Date(Web):October 17, 2012
DOI:10.1021/mz300475g
A perpendicular orientation of high-aspect-ratio polystyrene-block-polydimethylsiloxane (PS-b-PDMS) cylindrical and lamellar PDMS microdomains was achieved by solvent annealing and then slowly drying thick PS-b-PDMS films. Perpendicularly oriented microdomains occurred throughout the film thickness, except at the air interface, where a layer of in-plane microdomains formed due to the surface energy difference between PS and PDMS. In contrast, thermal annealing produced in-plane orientation throughout the film thickness. The solvent-annealed perpendicular orientation was observed for cylindrical morphology PS-b-PDMS of 16 and 45 kg/mol, where PDMS is the minority block, and lamellar PS-b-PDMS of 43 kg/mol. To obtain fully perpendicular microdomain patterns, a nonselective high-powered 450 W CF4/O2 reactive ion etching process was performed to remove the top layer of the films. Substrate patterning using electron beam lithography produced local registration of 17 nm period hexagonal cylinder patterns.
Co-reporter:Peng Jiang, Lei Bi, Xueyin Sun, Dong Hun Kim, Daming Jiang, Gaohui Wu, G. F. Dionne, and C. A. Ross
Inorganic Chemistry 2012 Volume 51(Issue 24) pp:13245-13253
Publication Date(Web):December 4, 2012
DOI:10.1021/ic301726w
The structure and magnetic properties of epitaxial (CexSr1–x)(Ti0.6Fe0.4)O3−δ (x = 0, 0.1, 0.2 and 0.3) and (LaxSr1–x)(Ti0.6Fe0.4)O3−δ (x = 0, 0.1, 0.2, 0.3 and 0.4) perovskite-structure thin films deposited by pulsed laser deposition on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates are reported. Both La and Ce ions showed a dominant 3+ valence state and acted as donors on the Sr2+ site (A site) in the perovskite lattice. The optical band gap widened, and the Fermi level moved toward the vacuum level with increased Ce or La content; meanwhile the Ti and particularly the Fe ions were driven to a lower valence state, resulting in a higher Fe2+ concentration. The materials were magnetic at room temperature with up to 0.8 μB/Fe and a magnetoelastic out-of-plane anisotropy. Ce and La lowered the coercivity while raising both the Faraday rotation at 1550 nm and the optical absorption at near-infrared wavelengths.
Co-reporter:Dong Hun Kim, Lei Bi, Nicolas M. Aimon, Peng Jiang, Gerald F. Dionne, and C. A. Ross
ACS Combinatorial Science 2012 Volume 14(Issue 3) pp:179
Publication Date(Web):February 21, 2012
DOI:10.1021/co2001185
Combinatorial pulsed laser deposition (CPLD) using two targets was used to produce a range of transition metal-substituted perovskite-structured Sr(Ti1–xMx)O3−δ films on buffered silicon substrates, where M = Fe, Cr, Ni and Mn and x = 0.05–0.5. CPLD produced samples whose composition vs distance fitted a linear combination of the compositions of the two targets. Sr(Ti1–xFex)O3−δ films produced from a pair of perovskite targets (SrTiO3 and SrFeO3 or SrTiO3 and SrTi00.575Fe0.425O3) had properties similar to those of films produced from single targets, showing a single phase microstructure, a saturation magnetization of 0.5 μB/Fe, and a strong out-of-plane magnetoelastic anisotropy at room temperature. Films produced from an SrTiO3 and a metal oxide target consisted of majority perovskite phases with additional metal oxide (or metal in the case of Ni) phases. Films made from SrTiO3 and Fe2O3 targets retained the high magnetic anisotropy of Sr(Ti1–xFex)O3−δ, but had a much higher saturation magnetization than single-target films, reaching for example an out-of-plane coercivity of >2 kOe and a saturation magnetization of 125 emu/cm3 at 24%Fe. This was attributed to the presence of maghemite or magnetite exchange-coupled to the Sr(Ti1–xFex)O3−δ. Films of Sr(Ti1–xCrx)O3−δ and Sr(Ti1–xMnx)O3−δ showed no room temperature ferromagnetism, but Sr(Ti1–xNix)O3−δ did show a high anisotropy and magnetization attributed mainly to the perovskite phase. Combinatorial synthesis is shown to be an efficient process for enabling evaluation of the properties of epitaxial substituted perovskite films as well as multiphase films which have potential for a wide range of electronic, magnetic, optical, and catalytic applications.Keywords: anisotropy; combinatorial pulsed laser deposition; epitaxial; magnetoelastic
Co-reporter:Hong Kyoon Choi, Jessica Gwyther, Ian Manners, and Caroline A. Ross
ACS Nano 2012 Volume 6(Issue 9) pp:8342
Publication Date(Web):August 21, 2012
DOI:10.1021/nn303085k
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.Keywords: block copolymer; nanolithography; self-assembly; solvent vapor annealing; square pattern; triblock terpolymer
Co-reporter:C. A. Ross;A. Alexander-Katz;K. K. Berggren;A. Tavakkoli K. G.;A. F. Hannon;K. W. Gotrik
Science 2012 Volume 336(Issue 6086) pp:1294-1298
Publication Date(Web):08 Jun 2012
DOI:10.1126/science.1218437
To the Next Level
Block copolymers will spontaneously separate into a range of microstructures that depend on the polymer block lengths and chemical compositions, and have been used as a templating material because one can selectively etch or functionalize one of the blocks. However, creating a template that is more than one layer thick is challenging. Tavakkoli K. G. et al. (p. 1294) used an array of posts to provide independent and simultaneous control of the morphology and orientation of two layers of block copolymers and were able to create local variations in the curvature and spacing of the domains.
Co-reporter:Kevin W. Gotrik, Adam F. Hannon, Jeong Gon Son, Brent Keller, Alfredo Alexander-Katz, and Caroline A. Ross
ACS Nano 2012 Volume 6(Issue 9) pp:8052
Publication Date(Web):August 28, 2012
DOI:10.1021/nn302641z
Solvent vapor annealing of block copolymer thin films can produce a range of morphologies different from the equilibrium bulk morphology. By systematically varying the flow rate of two different solvent vapors (toluene and n-heptane) and an inert gas, phase maps showing the morphology versus vapor pressure of the solvents were constructed for 45 kg/mol polystyrene-block-polydimethylsiloxane diblock copolymer films of different thicknesses. The final morphology was correlated with the swelling of the block copolymer and homopolymer films and the solvent vapor annealing conditions. Self-consistent field theory is used to model the effects of solvent swelling. These results provide a framework for predicting the range of morphologies available under different solvent vapor conditions, which is important in lithographic applications where precise control of morphology and critical dimensions are essential.Keywords: block copolymer; heptane; ps-pdms; self-assembly; self-consistent field theory; solvent vapor annealing; toluene
Co-reporter:Jeong Gon Son;Adam F. Hannon;Kevin W. Gotrik;Alfredo Alexer-Katz
Advanced Materials 2011 Volume 23( Issue 5) pp:634-639
Publication Date(Web):
DOI:10.1002/adma.201002999
Co-reporter:Jeong Gon Son, Jae-Byum Chang, Karl K. Berggren, and Caroline A. Ross
Nano Letters 2011 Volume 11(Issue 11) pp:5079-5084
Publication Date(Web):October 12, 2011
DOI:10.1021/nl203445h
Block copolymer self-assembly generates patterns with periodicity in the ∼10–100 nm range and is increasingly recognized as a route to lithographic patterning beyond the resolution of photolithography. Block copolymers naturally produce periodic patterns with a morphology and length-scale determined by the molecular architecture, and considerable research has been carried out to extend the range of patterns that can be produced from a given block copolymer, but the ability to control the period of the pattern over a wide range and to achieve complex structures with mixed morphologies from a given block copolymer is limited. Here we show how patterns consisting of coexisting sub-10-nm spheres and cylinders and sphere patterns with a range of periods can be created using a combination of serial solvent anneal processes and electron-beam irradiation of selected areas of a film of poly(styrene-block-dimethylsiloxane). These techniques extend the capabilities of block copolymer lithography, enabling complex aperiodic nanoscale patterns to be formed from a single block copolymer thin film.
Co-reporter:Jeong Gon Son, Jessica Gwyther, Jae-Byum Chang, Karl K. Berggren, Ian Manners, and Caroline A. Ross
Nano Letters 2011 Volume 11(Issue 7) pp:2849-2855
Publication Date(Web):June 16, 2011
DOI:10.1021/nl201262f
Square-symmetry patterns are of interest in nanolithography but are not easily obtained from self-assembly of a diblock copolymer. Instead, we demonstrate highly ordered 44 nm period square patterns formed in a thin film of polyisoprene-block-polystyrene-block-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer blended with 15% PS homopolymer by controlling the film thickness, solvent anneal conditions, the surface chemistry and topography of the substrates. The square patterns consist of PFS pillars that remained after removal of the PI and PS with an oxygen plasma. On an unpatterned smooth substrate, the average grain size of the square pattern was increased dramatically to several micrometers by the use of brush layers and specific solvent anneal conditions. Templated self-assembly of well-ordered square patterns was demonstrated on substrates containing nanoscale topographical sidewalls and posts, written by electron beam lithography, in which the sidewalls and base of the substrate were independently chemically functionalized.
Co-reporter:Hyun-Suk Kim, Lei Bi, Dong Hun Kim, Dae-Jin Yang, Yoon Jeong Choi, Jung Woo Lee, Jeung Ku Kang, Yun Chang Park, Gerald F. Dionne and Caroline A. Ross
Journal of Materials Chemistry A 2011 vol. 21(Issue 28) pp:10364-10369
Publication Date(Web):06 Jun 2011
DOI:10.1039/C1JM11286D
The ferromagnetic properties and electrical leakage current of single-phase SrTi1−xFexO3 (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.
Co-reporter:Yeon Sik Jung, Ju Ho Lee, Jeong Yong Lee and C. A. Ross
Nano Letters 2010 Volume 10(Issue 9) pp:3722-3726
Publication Date(Web):August 10, 2010
DOI:10.1021/nl1023518
Metallic nanowires are useful for fabricating highly integrated nanoscale electrical, magnetic, and photonic devices. However, conventional methods based on bottom-up growth techniques are subject to concerns such as broad distributions in their dimension as well and difficulties in precise placement of the nanowires. These issues can be solved by the guided self-assembly of block copolymer thin films that can produce periodic arrays of monodisperse nanoscale features with excellent positional accuracy. Here, we report transfer of high-quality linear block copolymer patterns into various metals, Ti, W, Pt, Co, Ni, Ta, Au, and Al, to fabricate highly ordered nanowire arrays with widths down to 9 nm. This novel patterning process does not require specific film deposition techniques or etch-chemistries. We also describe their structural, magnetic, and electrical properties.
Co-reporter:Yeon Sik Jung, J. B. Chang, Eric Verploegen, Karl K. Berggren and C. A. Ross
Nano Letters 2010 Volume 10(Issue 3) pp:1000-1005
Publication Date(Web):February 10, 2010
DOI:10.1021/nl904141r
The templated self-assembly of block copolymer (BCP) thin films can generate regular arrays of 10−50 nm scale features with good positional and orientational accuracy, but the ordering, registration and pattern transfer of sub-10-nm feature sizes is not well established. Here, we report solvent-annealing and templating methods that enable the formation of highly ordered grating patterns with a line width of 8 nm and period 17 nm from a self-assembled poly(styrene-b-dimethylsiloxane) (PS-PDMS) diblock copolymer. The BCP patterns can be registered hierarchically on a larger-period BCP pattern, which can potentially diversify the available pattern geometries and enables precise pattern registration at small feature sizes. Sub-10-nm-wide tungsten nanowires with excellent order and uniformity were fabricated from the self-assembled patterns using a reactive ion etching process.
Co-reporter:Hyun-Suk Kim, Lei Bi, Hanjong Paik, Dae-Jin Yang, Yun Chang Park, Gerald F. Dionne and Caroline A. Ross
Nano Letters 2010 Volume 10(Issue 2) pp:597-602
Publication Date(Web):December 29, 2009
DOI:10.1021/nl903611t
Thin films of perovskite-structured oxides with general formula ABO3 have great potential in electronic devices because of their unique properties, which include the high dielectric constant of titanates,(1) high-TC superconductivity in cuprates,(2) and colossal magnetoresistance in manganites.(3) These properties are intimately dependent on, and can therefore be tailored by, the microstructure, orientation, and strain state of the film. Here, we demonstrate the growth of cubic Sr(Ti,Fe)O3 (STF) films with an unusual self-assembled nanocomposite microstructure consisting of (100) and (110)-oriented crystals, both of which grow epitaxially with respect to the Si substrate and which are therefore homoepitaxial with each other. These structures differ from previously reported self-assembled oxide nanocomposites, which consist either of two different materials(4-7) or of single-phase distorted-cubic materials that exhibit two or more variants.(8-12) Moreover, an epitaxial nanocomposite SrTiO3 overlayer can be grown on the STF, extending the range of compositions over which this microstructure can be formed. This offers the potential for the implementation of self-organized optical/ferromagnetic or ferromagnetic/ferroelectric hybrid nanostructures integrated on technologically important Si substrates with applications in magnetooptical or spintronic devices.
Co-reporter:Vivian P. Chuang;Jessica Gwyther;Ian Manners
Advanced Materials 2009 Volume 21( Issue 37) pp:3789-3793
Publication Date(Web):
DOI:10.1002/adma.200900756
Co-reporter:Yeon Sik Jung
Advanced Materials 2009 Volume 21( Issue 24) pp:2540-2545
Publication Date(Web):
DOI:10.1002/adma.200802855
Co-reporter:Vivian P. Chuang, Jessica Gwyther, Rafal A. Mickiewicz, Ian Manners and Caroline A. Ross
Nano Letters 2009 Volume 9(Issue 12) pp:4364-4369
Publication Date(Web):2017-2-22
DOI:10.1021/nl902646e
Self-assembly provides the ability to create well-controlled nanostructures with electronic or chemical functionality and enables the synthesis of a wide range of useful devices. Diblock copolymers self-assemble into periodic arrays of microdomains with feature sizes of typically 10−50 nm, and have been used to make a wide range of devices such as silicon capacitors and transistors, photonic crystals, and patterned magnetic media1−3. However, the cylindrical or spherical microdomains in diblock copolymers generally form close-packed structures with hexagonal symmetry, limiting their device applications. Here we demonstrate self-assembly of square-symmetry patterns from a triblock terpolymer in which one organometallic block imparts high etch selectivity and etch resistance. Long-range order is imposed on the microdomain arrays by self-assembly on topographical substrates, and the orientation of both square lattices and in-plane cylinders is controlled by the substrate chemistry. Pattern transfer is demonstrated by making an array of square-packed 30 nm tall, 20 nm diameter silica pillars. Templated self-assembly of triblock terpolymers can generate nanostructures with geometries that are unattainable from diblock copolymers, significantly enhancing the capabilities of block copolymer lithography.
Co-reporter:Yeon Sik Jung, WooChul Jung, Harry L. Tuller and C. A. Ross
Nano Letters 2008 Volume 8(Issue 11) pp:3776-3780
Publication Date(Web):October 28, 2008
DOI:10.1021/nl802099k
Nanostructured conjugated organic thin films are essential building blocks for highly integrated organic devices. We demonstrate the large-area fabrication of an array of well-ordered 15 nm wide conducting polymer nanowires by using an etch mask consisting of self-assembled patterns of cylinder-forming poly(styrene-b-dimethylsiloxane) diblock copolymer confined in topographic templates. The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) nanowires operated as an ethanol vapor sensor, suggesting that the electronic properties of the organic film were preserved during the patterning processes. The higher sensitivity to ethanol vapor, compared to an unpatterned film with the same thickness, was attributed to the enhanced surface-to-volume ratio of the nanowire array.
Co-reporter:Yeon Sik Jung, Wonjoon Jung and C. A. Ross
Nano Letters 2008 Volume 8(Issue 9) pp:2975-2981
Publication Date(Web):August 8, 2008
DOI:10.1021/nl802011w
The formation of well-controlled circular patterns on the nanoscale is important for the fabrication of a range of devices such as sensors, memories, lasers, transistors, and quantum devices. Concentric, smooth ring patterns with tunable dimensions have been formed from a cylinder-forming poly(styrene-b-dimethylsiloxane) (PS-PDMS) diblock copolymer under confinement in shallow circular trenches. The high etch selectivity between PS and PDMS facilitates pattern transfer, illustrated by the fabrication of arrays of ferromagnetic cobalt rings with a density of 1.1 × 109/cm2. The effects of confinement diameter and commensurability on the diameter and period of the concentric rings are analyzed using a free energy model that includes interfacial, strain, and bending energies. This work provides a simple process for the fabrication of nanoscale circular patterns with very narrow line width using a much coarser-scale template, and may facilitate the miniaturization of a variety of microelectronic devices.
Co-reporter:Vivian P. Chuang, Caroline A. Ross, Panayiotis Bilalis and Nikos Hadjichristidis
ACS Nano 2008 Volume 2(Issue 10) pp:2007
Publication Date(Web):September 11, 2008
DOI:10.1021/nn8002345
Although there has been extensive work on the use of self-assembled diblock copolymers for nanolithography, there are few reports of the use of multiblock copolymers, which can form a more diverse range of nanoscale pattern geometries. Pattern transfer from a self-assembled poly(butadiene-b-styrene-b-methyl methacrylate) (PB-b-PS-b-PMMA) triblock terpolymer thin film has been investigated. Polymers of different total molecular weight were synthesized with a predicted morphology consisting of PMMA-core/PS-shell cylinders in a PB matrix. By adjusting the solvent-annealing conditions and the film thickness, thin films with vertically oriented cylinders were formed. The PMMA cylinder cores and the PB matrix were then removed using selective etching to leave an array of PS rings, and the ring pattern was transferred into a silica film by reactive ion etching to form 19 nm high silica rings. This result illustrates the design and use of triblock terpolymers for self-assembled lithography.Keywords: lithography; nanoscale rings; self-assembly; thin films; triblock polymers
Co-reporter:Ion Bita;Joel K. W. Yang;Yeon Sik Jung;Edwin L. Thomas;Karl K. Berggren
Science 2008 Volume 321(Issue 5891) pp:
Publication Date(Web):
DOI:10.1126/science.1159352
Abstract
Self-assembling materials are the building blocks of bottom-up nanofabrication processes, but they need to be templated to impose long-range order and eliminate defects. In this work, the self-assembly of a thin film of a spherical-morphology block copolymer is templated using an array of nanoscale topographical elements that act as surrogates for the minority domains of the block copolymer. The orientation and periodicity of the resulting array of spherical microdomains are governed by the commensurability between the block copolymer period and the template period and is accurately described by a free-energy model. This method, which forms high-spatial-frequency arrays using a lower-spatial-frequency template, will be useful in nanolithography applications such as the formation of high-density microelectronic structures.
Co-reporter:J. Y. Cheng;F. Zhang;H. I. Smith;G. J. Vancso;C. A. Ross
Advanced Materials 2006 Volume 18(Issue 5) pp:597-601
Publication Date(Web):31 JAN 2006
DOI:10.1002/adma.200501936
Pattern registration is achieved in thin films of self-assembling block copolymers by using a topographical template to guide the positions of the polymer domains. The placement accuracy of the polymer domains is related to the edge roughness of the topographical template, and the ultimate placement accuracy is assessed (see Figure). For a block copolymer with spherical morphology, a registered polymer array is achieved by using a two-dimensional template.
Co-reporter:J.Y. Cheng;C.A. Ross;E.L. Thomas;H.I. Smith;G.J. Vancso
Advanced Materials 2003 Volume 15(Issue 19) pp:
Publication Date(Web):7 OCT 2003
DOI:10.1002/adma.200305244
Co-reporter:A.C Ford, T Tepper, C.A Ross
Thin Solid Films 2003 Volume 437(1–2) pp:211-216
Publication Date(Web):1 August 2003
DOI:10.1016/S0040-6090(03)00668-0
Silicon oxide and Si-, GeO2-, B2O3- and P2O5-doped SiO2 films were deposited under various conditions by pulsed laser deposition using a KrF (248 nm wavelength) excimer laser, and were characterized by profilometry, ellipsometry and Rutherford backscattering. Relationships between laser energy, repetition rate, substrate temperature and oxygen pressure, and the composition, optical properties and roughness of the films were examined. SiO2 deposited from a Si target at a substrate temperature of 400 °C, oxygen pressure of approximately 6 Pa, laser energy of 550 mJ/pulse, and laser repetition rate of 25 Hz had a composition and refractive index close to the bulk values. Doped silica was made by ablation from sintered mixed oxide targets. Germanium and phosphorus doping increased the refractive index as compared to pure SiO2, but boron doping decreased the refractive index.
Co-reporter:J. Y. Cheng;C. A. Ross;V. Z.-H. Chan;E. L. Thomas;R. G. H. Lammertink;G. J. Vancso
Advanced Materials 2001 Volume 13(Issue 15) pp:
Publication Date(Web):6 AUG 2001
DOI:10.1002/1521-4095(200108)13:15<1174::AID-ADMA1174>3.0.CO;2-Q
Co-reporter:Hyun-Suk Kim, Lei Bi, Dong Hun Kim, Dae-Jin Yang, Yoon Jeong Choi, Jung Woo Lee, Jeung Ku Kang, Yun Chang Park, Gerald F. Dionne and Caroline A. Ross
Journal of Materials Chemistry A 2011 - vol. 21(Issue 28) pp:NaN10369-10369
Publication Date(Web):2011/06/06
DOI:10.1039/C1JM11286D
The ferromagnetic properties and electrical leakage current of single-phase SrTi1−xFexO3 (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.
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