Patchy Janus particles with tunable roughness and composition via vapor-assisted deposition of macromolecules Kimberly B. Shepard, Dane A. Christie, Chris L. Sosa, Craig B. Arnold, and Rodney D. Priestley Citation: Applied Physics Letters 106, 093104 (2015); doi: 10.1063/1.4913913 View online: http://dx.doi.org/10.1063/1.4913913 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Formation of magnetic nanocolumns during vapor phase deposition of a metal-polymer nanocomposite: Experiments and kinetic Monte Carlo simulations J. Appl. Phys. 114, 044305 (2013); 10.1063/1.4816252 Spherical magnetic nanoparticles fabricated by laser target evaporation AIP Advances 3, 052135 (2013); 10.1063/1.4808368 Exploitation of thermophoresis effect to prevent re-deposition of expelled particulates in laser assisted surface cleaning J. Appl. 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Priestley1,3,a) 1Chemical and Biological Engineering Princeton University, Princeton, New Jersey 08544, USA 2Mechanical and Aerospace Engineering Princeton University, Princeton, New Jersey 08544, USA 3Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA (Received 24 November 2014; accepted 19 February 2015; published online 2 March 2015) Here, we present a technique for the fabrication of patchy Janus particles utilizing a vapor-assisted macromolecular deposition technique, termed Matrix Assisted Pulsed Laser Evaporation (MAPLE). Using this technique, both inorganic and organic precursor particles, immobilized on a surface, are functionalized on one hemisphere with nanodroplets of a desired polymer, thus forming particles with a patchy Janus morphology and textured surface topology. This fabrication method is flexible with respect to the chemical identity of the precursor particle and the selection of the de- posited polymer. By tuning MAPLE deposition parameters, e.g., target composition or deposition time, the Janus anisotropy and roughness (i.e., patchiness) can be tuned, thus enabling greater con- trol over the particles’ behavior for applications as nanoparticle surfactants for stabilization of emulsions and foams. VC 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4913913] Janus particles, which incorporate two or more “faces” the application of CVD or plasma-polymerization. These tech- with different chemical functionality, have been the subject of niques require specific functional groups and reaction condi- great interest since P. G. de Gennes discussed their vast applica- tions to enable the simultaneous polymerization and tion potential in his 1991 Nobel prize lecture.1 Janus particles deposition of macromolecules.2,21–24 Regarding the latter, cer- can be fabricated from organic and/or inorganic materials in an tain 2D fabrication approaches require multiple processing array of geometries, including spheres, dumbbells, rods, and steps after precursor particle immobilization, such as coating 2 shells. Applications of Janus particles are correspondingly with a chemically appropriate mask layer, etching the mask, 3–5 diverse, for example: as particles active at interfaces, emul- chemical functionalization, and selective removal of the mask 6–9 10,11 sion stabilizers, optically active materials, self-propelling prior to removal of Janus particles from the substrate.20 Here, 12 13–16 particles, and in biological applications. Among the we report the utility of a unique vapor-assisted deposition pro- numerous Janus particle synthetic techniques, one of the most cess to generate both polymer-inorganic and polymer-polymer common is a masking technique in which a mono-functional Janus particles via the 2D scheme. In this approach, the depo- precursor particle is selectively decorated with a second mate- sition of the polymer-coating layer is achieved in the absence rial. In this method, a 2-D array of the precursor particles is im- of polymerization. Only one processing step is required mobilized, then temporarily covered or protected on one side 2,17 between immobilization and particle release. We demonstrate only, thus breaking the symmetry of the precursor particles. a versatile process for the fabrication of Janus nanoparticles In this way, a second material phase can be applied to the im- for broad application in the supramolecular assembly of nano- mobilized precursor particles on the exposed side, thus forming 25 18–20 structured materials, solution-dispersed interfacial stabil- a Janus particle. In cases where the second material is izers,26 and targeted drug-delivery vehicles.27 Uniquely, we applied via a directional deposition method, e.g., physical vapor are able to control both the surface roughness (patchiness) and deposition (PVD), the substrate can act as an effective mask. An particle anisotropy without adding additional complexity to advantage of this family of synthetic techniques is that it enables the fabrication process. the fabrication of Janus particles with diverse material combina- The enabling technology for the Janus particle fabrica- tions: it is feasible to vary both the chemical identity of the pre- tion process presented here is Matrix Assisted Pulsed Laser cursor particle and the second coating material.2,18 Evaporation (MAPLE), a laser-based deposition technique, For 2D synthetic techniques, there exist opportunities for used to create thin films or nanodroplets of polymers, pro- improving the generality and simplicity of material deposi- teins, nanoparticles, and nanocomposites.22,28 Owing to the tion. Regarding the former, the use of PVD in the fabrication technique’s growing popularity since its invention in the late of Janus particles limits the selection of the coating layer to 29 30 atomic and molecular systems. A result is that polymer- 1990s, MAPLE systems are now commercially available. Recently, we have demonstrated that MAPLE can be used to polymer Janus particles cannot be fabricated using PVD. The 31 range of coatings can be expanded to polymeric materials by directionally deposit polymer nanodroplets onto an arbi- trary substrate. Briefly, in the MAPLE technique, a pulsed laser strikes a frozen target made from a dilute polymer solu- a)Author to whom correspondence should be addressed. Electronic mail: tion, inside a high vacuum chamber. The interaction between [email protected]. laser and target material causes nano- to micro-size droplets 0003-6951/2015/106(9)/093104/5/$30.00 106, 093104-1 VC 2015 AIP Publishing LLC This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.112.142.131 On: Tue, 18 Aug 2015 18:32:40 093104-2 Shepard et al. Appl. Phys. Lett. 106, 093104 (2015) of polymer and solvent to be ejected towards the substrate. phase images, respectively, of 700 nm diameter PMMA/ During flight from target to substrate, the solvent evaporates silica patchy Janus particles fabricated by MAPLE. The and the remaining polymer droplets are collected atop a PMMA polymer partially wets the silica surface, forming temperature-controlled substrate. At high substrate tempera- rounded nanodroplets ranging from tens to hundreds of nano- tures, polymer droplets coalesce to create a smooth film. We meters in size. The small size of the nanodroplets relative to take advantage of the deposition process to create Janus par- the precursor particles ensures the formation of patchy Janus ticles. In this way, polymer nanodroplets and films are particles with high patchiness. The small droplet size also MAPLE-deposited onto the target-facing side of a precursor lowers their probability for forming polymer bridges particle film. Because MAPLE is a line-of-sight deposition between two precursor particles. process, only the exposed halves of the particles are coated. Figures 2(c) and 2(d) show AFM height and phase The result is the formation of patchy Janus particles. images for 350 nm PMMA/PS patchy Janus nanoparticles Analogous to other 2D fabrication techniques, a mono- prepared via MAPLE. These particles differ from those layer of precursor particles is necessary to ensure that the shown in Figures 2(a) and 2(b), in both precursor particle depositing material contacts only one hemisphere of the par- size (350 nm vs. 700 nm) and precursor particle chemical ticles. In order to create Janus particles by MAPLE, a uni- identity (PS vs. silica). Despite a twofold difference in pre- form monolayer is used as an effective substrate upon which cursor particle size and dissimilar chemical identity when polymer nanodroplets are deposited. After MAPLE deposi- compared with Figures 2(a) and 2(b), a similar morphology tion, the fabricated Janus particles can be removed from the is obtained for the PMMA/PS Janus particles. These results substrate into a suspension by gentle sonication. In principle, show that the size and precursor particle material can be var- micro and nanosize particles of any composition can be used ied without substantially changing the resulting patchy Janus as precursor particles, so long as a technique exists to immo- morphology. Figures 2(e) and 2(f) show AFM height and bilize the particles into a monolayer on a flat substrate. We phase images of PEO/silica patchy Janus particles prepared demonstrate the technique’s versatility by utilizing both or- in the same manner.