Thermal Modification of Nanoscale Mask Openings in Polystyrene Sphere Layers
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Thermal Modification of Nanoscale Mask Openings in Polystyrene Sphere Layers Thomas Riedl1,2,*, Matthias Strake1, Werner Sievers1,2 and Joerg K.N. Lindner1,2 1 University of Paderborn, Department of Physics, Warburger Straße 100, 33098 Paderborn, Germany 2 Center for Optoelectronics and Photonics Paderborn (CeOPP), Warburger Straße 100, 33098 Paderborn, Germany * Corresponding author. Email: [email protected] ABSTRACT An experimental analysis of the morphology changes of hexagonally close packed polystyrene sphere monolayers induced by annealing in air is presented. The triangular interstices between each triple of spheres, which are frequently used as nanoscale mask openings in colloidal lithography, are observed to gradually shrink in size and change in shape upon annealing. Top view scanning electron microscopy images reveal that different stages are involved in the closure of monolayer interstices at annealing temperatures in the range between 110°C and 120°C. In the early stages shrinkage of the triangular interstices is dominated by material transport to and thus shortening of their corners, in the late stages interstice area reduction via displacement of the triangle edges becomes significant. At intermediate annealing times the rate of interstice area reduction displays a maximum before a stabilized state characterized by a rounded isosceles triangular shape forms. INTRODUCTION For periodic nanoscale patterning of surfaces different methods have been developed, including electron beam [1], laser interference [2], nanoimprint [3], and nanosphere lithography [4]. The lithography masks prepared by these techniques enable a position-controlled nanoscale deposition or removal of material as well as chemical modification of solid substrate surfaces. Nanosphere lithography provides an inexpensive and simple patterning method, which allows for good size and shape control of nanoparticles deposited through hexagonally close packed colloidal sphere mono- and bilayers on substrate surfaces, and which can be scaled up to areas in the square centimeter range [4-7]. Such metal or semiconductor nanoparticle arrays have potential applications in the fields of plasmonics [8,9], catalytic nanowire growth [10,11] and optoelectronics. In order to modify the size and shape of the mask interstices and thus to extend the range of accessible nanopatterns microwave heating [12], ion irradiation [13] and plasma treatment [14] were employed. Kosiorek et al. demonstrated that partial microwave sintering of a 540 nm diameter polystyrene (PS) sphere monolayer in a liquid medium leads to circular mask openings of down to about 25 nm diameter [12]. They found that the shrinkage of the mask openings is accompanied by a flattening of the spheres and the formation of sinter necks. Goudy et al. studied the morphological changes of 0.24 µm to 1.05 µm diameter PS sphere monolayers during thermal annealing by means of atomic force microscopy [15]. Their results show that the vertical distance between sphere top and sinter neck decreas
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