POROUS MATERIALS

Transient laser heating inducedhierarchical porous structures fromblock copolymer–directed self-assemblyKwan Wee Tan,1 Byungki Jung,1* Jörg G. Werner,1,2 Elizabeth R. Rhoades,3

Michael O. Thompson,1 Ulrich Wiesner1†

Development of rapid processes combining hierarchical self-assembly with mesoscopicshape control has remained a challenge. This is particularly true for high-surface-areaporous materials essential for applications including separation and detection, catalysis,and energy conversion and storage. We introduce a simple and rapid laser writing methodcompatible with semiconductor processing technology to control three-dimensionallycontinuous hierarchically porous polymer network structures and shapes. Combiningself-assembly of mixtures of block copolymers and resols with spatially localized transientlaser heating enables pore size and pore size distribution control in all-organic andhighly conducting inorganic carbon films with variable thickness. The method providesall-laser-controlled pathways to complex high-surface-area structures, includingfabrication of microfluidic devices with high-surface-area channels and complex porouscrystalline semiconductor nanostructures.

Hierarchically porous materials that bridgenano- to macroscopic length scales areexpected to find use in a wide variety ofapplications including catalysis, energyconversion and storage, and membrane

filtration (1, 2). Well-ordered nanoporous ma-terials derived from small-molecule surfactantand block copolymer (BCP) self-assembly havebeen explored in the form of amorphous (3–7),polycrystalline (8, 9) and single-crystal (10) sol-ids. Multiple time-consuming processing stepsare typically required to generate the final struc-tures. For example, removal of organic componentsby conventional thermal processing to create po-rosity typically takes several hours (6–8). Methodsto fabricate hierarchical porous polymer scaf-folds using BCPs have recently been devised thatcircumvent some of these complexities (11, 12).Further developments are still desirable, however,as hierarchical structures provide high surfacearea combined with high flux beneficial for anumber of applications.Laser-induced transient heating of organic ma-

terials has been explored as an alternative ther-mal processing approach to alter morphology, aswell as materials properties, and to enable directpattern transfer. For example, by laser heatingin the millisecond to submillisecond time frames,the temperature stability of a methacrylate-basedphotoresist was enhanced by ~400°C relative toconventional hot-plate heating (13). CO2 laserablation of polyimide and polyetherimide resultedin the direct formation of three-dimensional (3D)

porous graphitic carbon network structures, butwork did not involve a bottom-up self-assemblyprocess, resulting in little control over mesostruc-ture and pore size (14).Three-dimensional porous structure fabrication

by optical lithography has also been demonstratedusing multiphoton absorption polymerization(MAP) and stimulated emission depletion (STED)–inspired direct laser writing techniques (15–19).However, direct laser writing of such 3D contin-uous structures with sub-100-nm feature size andperiodicity remains challenging due to the inher-ent serial mode of operation and sophisticatedinstrumentation, diffraction limitations of MAPexcitation lasers, and stringent materials criteria—e.g., for STED resists (16–19).We demonstrate a simple and rapid method for

direct generation of 3D continuous hierarchical-ly porous structures with meso- to macroscopicshapes by combining BCP self-assembly directedresols structure formation with CO2 laser-inducedtransient heating. A schematic of the BCP-basedwriting induced by transient heating experiments(B-WRITE) is shown in Fig. 1A (20). Lab-synthesizedABC triblock terpolymers [poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide), ISO-38/69]as well as commercially available ABA di-BCPs(Pluronic F127) were used as examples (20, 21).Oligomeric resols were synthesized by the polymer-ization of resorcinol/phenol and formaldehydeunder basic conditions (22). The structure-directingBCP was first mixed with resols to form an all-organic hybrid thin film by spin-coating on asilicon substrate. Samples are designated asBCP-M-R, where M represents BCP molar massand R denotes the resols additives. As-depositedor thermally cured films were then heated by acontinuous wave CO2 laser (l = 10.6 mm), focusedinto a ~90 mm by 600 mm line beam, and scannedto generate a 0.5-ms dwell in air. Although the

organic layer does not interact strongly withthe far-infrared photons, the boron-doped Sisubstrate absorbs most of the laser energy, therebyheating the film. As the laser passes, cooling oc-curs by thermal conduction on submillisecondtime frames. Laser heating temperatures weredetermined using a procedure described in (13).Well-defined porous resin structures were formedby the simultaneous thermopolymerization ofresols (negative-tone) and decomposition of BCPs(positive-tone) during transient heating by se-quential CO2 laser irradiations at locations pre-defined by the beam. Material in nonirradiatedregions could be removed by rinsing with sol-vent, leaving resulting porous resin structureson the substrate.Thermal stability was compared for organic

films prepared from 5 weight percent (wt %)ISO-69, 5 wt % resols, and 6 wt % ISO-38/69-Rsolutions in tetrahydrofuran (THF) spin-coatedon Si (Fig. 1B). The 6 wt % ISO-38/69-R hybridthin films were prepared by mixing 2 to 3 partsISO to 1 part resols in THF by mass (see table S1for hybrid compositions). After thermal curingin vacuum at 100°C overnight (>12 hours), filmswere heated in air by either a single CO2 laserirradiation for 0.5-ms dwell or furnace heatingfor 1 hour. Curves in Fig. 1B show film thicknessas a function of temperature (and correspond-ing laser power). Under CO2 laser heating for0.5-ms dwell, ISO-69 (black) remained stable upto 550°C (35 W) and thermally decomposed en-tirely above 670°C (>40 W). Resols films (orange)extensively cross-linked, retaining more than50% of the original film thickness at 670 to805°C (40–45 W). For ISO-69-R hybrid films(blue), mixed behavior was observed. This sug-gested a laser heating process window to decoupleBCP decomposition but retain the cross-linkedresols (resin).Initial experiments to generate nanoporous

resin structures in ISO-69-R films by a singleCO2 laser irradiation with ISO decomposition at670°C (40 W) for 0.5-ms dwell confirmed theseconcepts, albeit with poor uniformity and cover-age (fig. S1). Thermal stability of ISO-69-R filmswas increased dramatically by more than 400°Cwhen heating duration was reduced from 1 hourin a furnace to 0.5 ms with CO2 laser-inducedheating (compare dashed and solid blue lines inFig. 1B, respectively). Submillisecond time frameCO2 laser heating suppresses oxidation reactionsin ambient environments, enabling organic sys-tems to reach temperatures inaccessible by con-ventional heating.For more controlled polymerization of resols

and to inhibit thermocapillary dewetting effects(23), we introduced sequential CO2 laser irradi-ations of increasing laser powers yielding thickerresin structures with similar enhanced thermalstability (Fig. 1C), but high film uniformity andcoverage (Fig. 1, D and E). Scanning electronmicroscopy (SEM) (Fig. 2 and fig. S2) and Fouriertransform infrared (FTIR) spectroscopy (fig. S2)were employed to investigate film morphologyand chemical composition, respectively. Upon se-quential irradiations, cured ISO-resols hybrid films

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1Department of Materials Science and Engineering, CornellUniversity, Ithaca, NY 14853, USA. 2Department of Chemistryand Chemical Biology, Cornell University, Ithaca, NY 14853,USA. 3Cornell NanoScale Science and Technology Facility,Cornell University, Ithaca, NY 14853, USA.*Present address: Intel Corporation, Hillsboro, OR 97124, USA.†Corresponding author. E-mail: ubw1@cornell.edu

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(fig. S2, A and C) densified due to cross-linkingof resols with increasing transient temperatures(22), whereas above 550°C, BCP decompositionoccurred (fig. S2, B and D) and was completedat about 670°C (Fig. 2, A and C). This is cor-roborated by decreasing IR signal intensities ofhydroxyl stretching vibrations in the 3500 to3200 cm−1 band and a sharp reduction of IR sig-nal intensities of alkyl stretching vibrations inthe 3000 to 2800 cm−1 band, respectively (22, 24)(fig. S2, E and F). Resulting resin structures re-tained ~30 to 55% of the original film thickness(Fig. 1C).Optical images in Fig. 1D (and fig. S3) show

macroscopic linear trenches in ISO-38/69-R hy-brid thin films generated by sequential CO2 laserirradiations for 0.5 ms in air (B-WRITE method).Each ISO-38-R–based trench was ~2 cm long, re-quiring ~4.5 min to fabricate in air (Fig. 1D). SEM(Fig. 1E) and profilometry (fig. S3B) show thatthe ~150-mm-wide B-WRITE trench indeed con-sists of a ~200-nm-thick nanoporous resin thinfilm. Laser-heated macroscopic shape dimensionsare tunable, as demonstrated for ISO-69-R hybridfilms with ~350-nm-thick structures in ~550-mm-wide B-WRITE trenches (fig. S3F). Cross-sectionalSEM at lower magnifications in Fig. 2, E and F,shows highly uniform structure formation underthese conditions over large areas, as defined bythe CO2 laser.

Resulting pore size and pore size distributioncould be tailored by tuning hybrid film compo-sition. Sequential CO2 laser-heated resin struc-tures formed in the submillisecond time frameswere compared with mesoporous structuresformed by furnace heating under N2 for ≥1 hour(fig. S4). The average pore size of the N2-furnace-heated ISO-69-R structures at 450°C is 39 T 9 nmwith an in-plane lattice spacing of ~47 nm (fig.S5). Laser-heated structures obtained from ISO-69-R hybrid films with the same ISO-to-resolsmass ratio of 2.4:1 show a wide pore size distri-bution with values ranging from ~30 to 200 nm(Fig. 2C). The increase in pore size relative to theN2-furnace-heated sample is ascribed to rapidevolution and release of gaseous decompositionproducts during transient heating, resulting inlocal pore expansion (14, 25). Hybrid film com-position provides a convenient lever to tune poresize of B-WRITE structures. For example, Fig. 2Dshows that the pore diameter of laser-heatedISO-69-R films was reduced to ~20 to 50 nm bydecreasing the ISO-to-resols mass ratio to 1.5:1(see Fig. 2B for ISO-38-R). From fast Fouriertransform analysis, the in-plane lattice spacingwas ~52 nm—i.e., close to the value obtainedafter furnace heating under N2 (fig. S5).Terpolymers are synthetically challenging, so

BCPs were varied to also include the commer-cially available Pluronics family (22). Smaller

molar mass Pluronic F127 copolymer mixed withresols at 1:1 mass ratio in ethanol led to theformation of resin structures with pore sizes of20 to 50 nm (Fig. 2G). Pore size and pore sizedistribution were further reduced to 10 to 30 nmby decreasing the F127-to-resols mass ratio to 1:2(Fig. 2H), thereby approaching the 5- to 10-nmlimit of size control in conventional BCP phaseseparation (26). Film thickness could be tunedvia solution concentration. For example, main-taining ISO-38-to-resols mass ratio at 3:1, sam-ple thickness was increased with the mixedsolution concentration (table S1) from ~500 nm(6 wt %) to 1.0 mm (10 wt %) and 1.6 mm (12.5 wt %).Figure 2, I and J, show thicker laser-heated struc-tures with hierarchical pore size distributions of50 to 400 nm and 50 to 600 nm for 10 wt % and12.5 wt % solution–derived ISO-38-R samples,respectively. B-WRITE–based porous structuresmay be accessible for films thicker than 1.6 mmby further optimizing laser heating protocols.Skipping the 100°C thermal curing step and

directly laser-heating as-deposited ISO-resolssamples achieved further process simplification.Figure 3A displays macroscopic porous resin lineson a clean Si substrate prepared by subjectingas-deposited ISO-69-R samples to an increasednumber of sequential CO2 laser irradiations andremoval of nonirradiated components by rinsingwith THF (see stage III in Fig. 1A and SEM in

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Fig. 1. Transient laser heating of BCP-directed resols hybrid samples.(A) Schematic of the B-WRITE method. (B) Thickness plots of 5 wt %ISO-69, 5 wt % resols, and 6 wt % ISO-69-R hybrid thin films heated by asingle CO2 laser irradiation of 0.5-ms dwell (solid lines) compared withfurnace heating for 1 hour (dashed line), all in air. (C) Thickness plots of6 wt % ISO-38/69-R hybrid thin films heated by sequential CO2 laser

irradiations of 0.5-ms dwell (solid lines) compared with furnace heatingfor 1 hour (dashed lines), all in air. (D and E) Optical image of macroscopictrenches in sample ISO-38-R after sequential CO2 laser irradiations (D)with corresponding SEM cross section and plan view (inset) of the re-sulting structures in a trench (E). Each B-WRITE trench was generated inless than 5 min.

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fig. S6A). Selective removal of resin materialwith a single CO2 laser irradiation at 30 W of25-ms dwell enabled macroscopic line-width ad-justments and sidewall smoothening (see SEMin Fig. 2F and depth profile in Fig. 3B). Morecomplex shapes were generated by removal ofresin using laser scans in other directions, asshown for orthogonal scans in Fig. 3A.A final furnace heating step under N2 at tem-

peratures up to 800°C resulted in carbonizedand electrically conductive nanoporous carbonfilms without structure collapse. Raman spec-troscopy shown in Fig. 3C indicated negligi-ble graphitic carbon in laser-heated ISO-69-Rsamples, corroborating the organic nature ofthe B-WRITE porous resin structures. After fur-nace heating at 600°C, D and G bands centeredat ~1325 and 1588 cm−1, respectively, appearedin the Raman spectrum, indicating the conver-sion of phenolic resins into disordered carbon(22). At 800°C, D and G bands narrowed andshifted to ~1314 and 1598 cm−1, respectively, sug-

gesting a higher degree of graphitization (22).Cross-sectional SEM (fig. S6B) revealed that B-WRITE structures contracted by 50 to 60% inthe out-of-plane direction during carbonizationat 800°C, but otherwise stayed intact. The elec-trical conductivity of B-WRITE films carbonizedat 600°C and 800°C under N2 was ~8 S/cm (fig.S6C) and 270 to 750 S/cm (Fig. 3D), respectively.This electrical conductivity is higher than thatof dense carbon samples heated under identicalconditions at 800°C (~52 S/cm) (fig. S6D) butlower than polycrystalline graphite (1250 S/cm)(27). The high electrical conductivity of B-WRITEcarbon structures after furnace heating is at-tributed to the connectivity of the 3D continuousnetwork. We speculate that porosity facilitatescarbonization and release of oxygen/hydrogenspecies to form graphitic-like clusters (fig. S6E).We constructed proof-of-concept microfluid-

ic devices with highly porous organic channelfloors for potential sensing or catalysis applica-tions (Fig. 4, A to C, and fig. S7). A dense resols

film was spin-coated onto Si and cured at 100°C.A trench was then formed with a single CO2

laser irradiation at 30 W of 25-ms dwell, com-pletely decomposing the film, followed by spin-coating a 6 wt % ISO-38-R film as an overlayer.B-WRITE application formed nanoporous resinstructures in the trench. The device was finallysealed with a polydimethylsiloxane film (Fig.4A). From optical and electron microscopy, aswell as profilometry, the microfluidic devicechannel was ~13 mm long (port-to-port), 630 mmwide, and 1.4 mm tall, containing ~200-nm-thicknanoporous resin structures at the channel bottom,increasing surface area (fig. S7). Bright field andfluorescence optical micrographs in Fig. 4, B andC, confirmed that tetramethylrhodamine (TRITC)dye dissolved in dimethyl sulfoxide (DMSO) wasflowing through the microfluidic channel usinga pressure-controlled pump.We further demonstrated compatibility of the

B-WRITE method with complex substrates ob-tained from established semiconductor processing

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Fig. 2. SEM of B-WRITE resin structures. (A to D) Plan views and cross sections (insets) of laser-heated 6 wt % ISO-38-R [(A) and (B)] and 6 wt % ISO-69-R [(C)and (D)] solution–derived structures of 2 to 3:1 [(A) and (C)] and 1.5:1 [(B) and (D)] ISO/resols mass ratios, respectively. (E and F) Lower-magnification images ofsame sample as in (C). (G andH) Plan views and cross sections (insets) of laser-heated 20 wt% F127-R solution–derived structures of 1:1 (G) and 1:2 (H) F127/resolsmass ratios, respectively. (I and J) Plan views and cross sections (insets) of laser-heated 10 wt % (I) and 12.5 wt % (J) ISO-38-R solution–derived structures.

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Fig. 3. Macroscopic pattern inversion and carboni-zation of B-WRITE structures. (A and B) Opticalimage of complex shapes (A) and correspondingdepth profile (B) of laser-heated ISO-69-R after THFrinsing. Depth profile was acquired in the region de-noted in (A). (C) Raman spectra (785-nm excitation)of laser-heated ISO-69-R formed by B-WRITE (lightgray) and after carbonization at 600°C (gray) and800°C (black). (D) Current/voltage plot of laser-heated porous carbon sample carbonized at 800°C.

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Fig. 4. Hierarchical porous resin structure for-mation and use in microfluidic device, on lith-ographically defined surfaces, as well as forpattern transfer into Si. (A) Optical image of amicrofluidic device fabricated by CO2 laser writingon the probe station. (B and C) Bright-field (B) andfluorescence (C) optical micrographs of TRITC dyedissolved in DMSO passing through the micro-fluidic channel. (D toN) SEMmicrographs. Plan view(D) and cross sections [(E) to (G)] of hierarchical struc-tures from coupling photolithography, BCP-directedself-assembly, and CO2 laser heating. Inset in (E)

shows the identified area at higher magnification. Cross section of excimer laser-induced crystalline Si nanostructures employing B-WRITE–derived organictemplate (H). Higher resolution cross section (I) and plan view (J) of area as indicated in (H). Plan views [(K) and (M)] and cross sections [(L) and (N)] ofperiodically ordered mesoporous gyroidal resin template [(K) and (L)] and resulting excimer laser-induced crystalline Si nanostructures after templateremoval [(M) and (N)].

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technologies. Figure 4D displays a SEM plan viewmicrograph of an ISO-69-R hybrid sample depo-sited on a photolithographically patterned Sisubstrate and transformed into porous resin struc-tures by the B-WRITE method. Cross-sectionalSEM in Fig. 4, E to G, shows excellent conformaladhesion of the laser-derived structures despitethe complex Si surface topography.Finally, we demonstrated an all-laser-induced

pattern transfer to create several-hundred-nanometer-thick 3D porous crystalline Si nano-structures using an excimer laser-induced Simelt-crystallization process described previous-ly (10). To that end, B-WRITE–derived ISO-69-Rstructures were further heated for 1 hour at400°C under N2 to enhance adhesion to the sub-strate. The resin pore network was then back-filled with amorphous Si (a-Si) by chemical vapordeposition (CVD) (28) and irradiated with a sin-gle 40-ns pulsed XeCl excimer laser (l = 308 nm)at 550 mJ/cm2 to induce a transient 40- to 50-nsSi melt, which subsequently solidified into poly-crystalline Si (fig. S8). SEM in Fig. 4, H to J,shows nanoporous polycrystalline Si nanostruc-tures after template removal, using CF4 reactiveion and wet acid etching treatments. What isnotable about this experiment is the use of anorganic BCP-directed resin template [as opposedto BCP-directed inorganic templates employed in(10)] that is not only stable for a-Si CVD at 350°Cbut also survives the transient melt of a-Si attemperatures ~1250°C, enabling high pattern-transfer fidelity (10, 29). All-organic BCP-directedstructures open pathways to templates with highdegrees of periodic order obtained—e.g., via ther-mal or solvent annealing before porous struc-ture formation (30, 31). In a proof-of-conceptexperiment, SEM micrographs in Fig. 4, M andN, demonstrate that periodically ordered andnetworked mesoscopic crystalline Si nanostruc-tures can be obtained after a-Si CVD into meso-porous gyroidal resin structures prepared byevaporation-induced self-assembly and pyrolysis(Fig. 4, K and L, and fig. S9) (21), excimer laserannealing, and template removal. Plan view (Fig. 4,K and M) and cross-sectional (Fig. 4, L and N)SEM images suggest that the interconnected andperiodic pores of the mesoporous gyroidal resintemplate were conformally backfilled by thenetwork struts of the resulting laser-inducedcrystalline Si nanostructures. Furthermore, themesoporous all-organic resin template is inertto hydrofluoric (HF) acid, facilitating removal ofnative SiO2 layers on the Si substrate using di-luted HF (or HF vapor) before a-Si deposition andenabling formation of periodically ordered poroussingle-crystal epitaxial nanostructures (10).

REFERENCES AND NOTES

1. M. E. Davis, Nature 417, 813–821 (2002).2. U.S. Department of Energy Basic Energy Sciences Advisory

Committee Mesoscale Science Committee W, From Quanta tothe Continuum: Opportunities for Mesoscale Science(Government Printing Office, Washington, DC, 2012).

3. T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem.Soc. Jpn. 63, 988–992 (1990).

4. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli,J. S. Beck, Nature 359, 710–712 (1992).

5. M. Templin et al., Science 278, 1795–1798 (1997).

6. D. Zhao et al., Science 279, 548–552 (1998).7. S. H. Joo et al., Nature 412, 169–172 (2001).8. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky,

Nature 396, 152–155 (1998).9. S. C. Warren et al., Science 320, 1748–1752 (2008).10. H. Arora et al., Science 330, 214–219 (2010).11. K.-V. Peinemann, V. Abetz, P. F. W. Simon, Nat. Mater. 6,

992–996 (2007).12. H. Sai et al., Science 341, 530–534 (2013).13. B. Jung et al., ACS Nano 6, 5830–5836 (2012).14. J. Lin et al., Nat. Commun. 5, 5714 (2014).15. S. Kawata, H.-B. Sun, T. Tanaka, K. Takada, Nature 412,

697–698 (2001).16. S. Maruo, J. T. Fourkas, Laser & Photon. Rev. 2, 100–111 (2008).17. L. Li, R. R. Gattass, E. Gershgoren, H. Hwang, J. T. Fourkas,

Science 324, 910–913 (2009).18. J. Fischer, M. Wegener, Laser & Photon. Rev. 7, 22–44 (2013).19. Z. Gan, Y. Cao, R. A. Evans, M. Gu, Nat. Commun. 4, 2061 (2013).20. Materials and methods are available as supplementary

materials on Science Online.21. J. G. Werner, T. N. Hoheisel, U. Wiesner, ACS Nano. 8, 731–743

(2014).22. Y. Meng et al., Chem. Mater. 18, 4447–4464 (2006).23. J. P. Singer et al., Adv. Mater. 25, 6100–6105 (2013).24. R. M. Silverstein, G. C. Bassler, T. C. Morrill, Spectrometric

Identification of Organic Compounds (Wiley, New York, 1991).25. M. F. El-Kady, V. Strong, S. Dubin, R. B. Kaner, Science 335,

1326–1330 (2012).26. F. S. Bates, Science 251, 898–905 (1991).27. R. O. Grisdale, J. Appl. Phys. 24, 1288–1296 (1953).28. S. A. Rinne, F. García-Santamaría, P. V. Braun, Nat. Photonics

2, 52–56 (2008).29. M. O. Thompson et al., Phys. Rev. Lett. 52, 2360–2363 (1984).30. S. Park et al., Science 323, 1030–1033 (2009).31. G. Deng et al., Chem. Commun. (Camb.) 50, 12684–12687 (2014).

ACKNOWLEDGMENTS

This work was supported by the National Science Foundation(NSF) Single Investigator Award (DMR-1409105). K.W.T. gratefullyacknowledges the Singapore Energy Innovation ProgrammeOffice for a National Research Foundation graduate fellowship.This work made use of research facilities at the Cornell Centerfor Materials Research (CCMR), with support from the NSFMaterials Research Science and Engineering Centers (MRSEC)program (DMR-1120296); Cornell NanoScale Science andTechnology Facility (CNF), a member of the National NanotechnologyInfrastructure Network, which is supported by the NSF (grantECCS-0335765); Nanobiotechnology Center shared researchfacilities at Cornell University; and Center for Energy andSustainability at Cornell University. We acknowledge experimentalassistance from P. Carubia and T. Yuasa for attenuated totalreflectance FTIR spectroscopy measurements; D. Lynch forelectrical conductivity measurements; Y. Sun and Q. Zhangfor microfluidic device fabrication; and T. Corso (CorSolutionsLLC) for microfluidic probe station setup. We also gratefullyacknowledge the expertise and experimental assistance fromM. Goodman and P. Braun (University of Illinois at UrbanaChampaign) for amorphous silicon chemical vapor deposition.We thank L. Estroff, Y. Gu, T. Porri, H. Sai, and M. Weathers forhelpful discussions. A patent application related to this researchhas been filed by Cornell University.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/349/6243/54/suppl/DC1Materials and MethodsTable S1Figs. S1 to S9References (32, 33)

11 March 2015; accepted 14 May 201510.1126/science.aab0492

SULFUR CHEMISTRY

Gas phase observation and microwavespectroscopic characterization offormic sulfuric anhydrideRebecca B. Mackenzie, Christopher T. Dewberry, Kenneth R. Leopold*

We report the observation of a covalently bound species, formic sulfuric anhydride (FSA),that is produced from formic acid and sulfur trioxide under supersonic jet conditions.FSA has been structurally characterized by means of microwave spectroscopy andfurther investigated by using density functional theory and ab initio calculations. Theoryindicates that a p2 + p2 + s2 cycloaddition reaction between SO3 and HCOOH is aplausible pathway to FSA formation and that such a mechanism would be effectivelybarrierless. We speculate on the possible role that FSA may play in the Earth’s atmosphere.

There is an extensive literature on the chem-istry of sulfur oxides and their derivatives.The area is rich in fundamental science andfinds applications ranging from industrialchemistry to laboratory synthesis. Sulfur

compounds are also active species in the atmo-sphere (1), and in particular, the oxides andoxyacids are important players in the formationof atmospheric aerosol (2). Here, we present amicrowave spectroscopic study of the anhydridederived from formic and sulfuric acids, producedin a supersonic jet containing HCOOH and SO3.

The present work was stimulated by a seriesof studies concerned with the formation of sul-furic acid in the atmosphere. The acid, whichcan form via both gas-phase and aqueous-phaseprocesses, is generated in the gas phase by theoxidation of SO2 to SO3, which is subsequentlyhydrated to give H2SO4

SO3 + H2O → H2SO4 (1)

Both theoretical and experimental studies ofthis reaction indicate that viable mechanismsin the gas phase involve a facilitator molecule.For example, using ab initio theory, Morokumaand Muguruma (3) showed that the addition of

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Department of Chemistry, University of Minnesota, 207Pleasant Street SE, Minneapolis, MN 55455, USA.*Corresponding author. E-mail: kleopold@umn.edu

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self-assemblydirected−Transient laser heating induced hierarchical porous structures from block copolymer

Kwan Wee Tan, Byungki Jung, Jörg G. Werner, Elizabeth R. Rhoades, Michael O. Thompson and Ulrich Wiesner

DOI: 10.1126/science.aab0492 (6243), 54-58.349Science 

, this issue p. 54Sciencedistributions.block copolymer. This method allows direct patterning of the films on a local scale, with tunable pore sizes and size exposure to ultraviolet light, rapid heating of the substrate causes polymerization of the resols and decomposition of thecopolymers mixed with phenol-formaldehyde resins (resols) on silicon substrates using a simple laser process. On

prepared porous thin films of blocket al.the ability to finely control both the pore sizes and their connectivity. Tan Porous materials are useful for membranes, filters, energy conversion, and catalysis. Their utility often depends on

Laser patterning polymer membranes

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