Synthesis of Quasi‐Hexagonal Ordered Arrays of Metallic Nanoparticles with Tuneable Particle Size

Abstract

Noble-metal nanoparticles (Au, Ag, Pt, Pd) are of substantial interest for various scientific and technical applications [1] because they exhibit a number of unique optical, [2][3][4] electronic, [5,6] and catalytic [7,8] characteristics compared to bulk materials. It is important to realize that nearly all of these properties are a function of the particle size and shape at the nanometer scale.The size-dependent interaction of light with small particles, referred to as localized surface-plasmon resonance (LSPR), [9] and coupled-plasmon resonance [10] enables them to be used for various applications such as biosensors [11] and molecular rulers. [12] Spectroscopic techniques such as surface enhancedRaman spectroscopy (SERS), [13,14] nonlinear scattering experiments, for example, second-harmonic generation (SHG), [15,16] and time-resolved methods [17][18][19] have been developed, each of them sensitive to the electronic properties of the metallic nanoparticles and their interaction with electromagnetic waves as a function of the particle diameter and particle-particle spacing. [20] However, the generation of nanostructures with feature sizes smaller than 100 nm relies on conventional lithographic techniques such as electron-beam (e-beam), X-ray, and photolithography. [21] All these methods, summarized as top-down technology, are rather complicated, timeconsuming, and require expensive technical equipment. An alternative route is offered by the so called bottom-up technology. Scanning probe microscopy (SPM) techniques like ''dip-pen'' nanolithography [22] are capable of preparing nanostructures with feature sizes down to 10 nm with excellent spatial resolution. Their disadvantage is low throughput, as a consequence of being a serial process. In contrast, microcontact printing [23] enables a rapid and parallel fabrication of chemical and topological micro-and nanostructures. Here the resolution is limited by the mechanical properties of the stamp and the contact stability between the stamp and the substrate.Lithographic techniques based on self-assembly are particularly attractive since they offer the large-scale fabrication of feature sizes down to a few nanometers with predefined molecular and colloidal properties. Nanosphere lithography [24][25][26] has been reported to be capable of producing highly ordered metallic nanoparticle arrays with well-defined size and shape based on pure self-assembly. Crystalline monolayers of self-assembled polystyrene spheres act as a template for the vapor deposition of metals such as gold or silver. The in-plane diameter, shape, and spacing between the metal clusters are related to the diameter of the colloidal spheres. As a consequence, it is not possible to control the structural parameters independently from each other, which is limiting the general applicability. Therefore a nanofabrication technique that allows a fast, substrate-independent, and inexpensive sample processing with the possibility to independently adjust the structural parameters, such as size and spacing, at a...

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