Nanoherding: Plasma-Chemical Synthesis and Electric-Charge-Driven Self Organization of SiO₂ Nanodots

We report on the chemical synthesis of the arrays of silicon oxide nanodots and their self-organization on the surface via physical processes triggered by surface charges. The method based on chemically active oxygen plasma leads to the rearrangement of nanostructures and eventually to the formation of groups of nanodots. This behavior is explained in terms of the effect of electric field on the kinetics of surface processes. The direct measurements of the electric charges on the surface demonstrate that the charge correlates with the density and arrangement of nanodots within the array. Extensive numerical simulations support the proposed mechanism and prove a critical role of the electric charges in the self-organization. This simple and environment-friendly self-guided process could be used in the chemical synthesis of large arrays of nanodots on semiconducting surfaces for a variety of applications in catalysis, energy conversion and storage, photochemistry, environmental and biosensing, and several others. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis A and controlled patterns of low-dimensional nanostructures on semiconducting surfaces have recently been a subject of intense research efforts due to their unique size-dependent properties. Large arrays of self-organized nanodots (NDs) are of particular interest, along with the arrays of highly ordered and densely packed silicon oxide NDs; however, it is very challenging to identify and control the most effective driving forces for the ND self-organization. Electric forces have long been pursued as highly promising and easy-tocontrol driving forces for the arrangement of NDs on the surface. Despite decades of intense experimental and theoretical studies, this control still remains elusive. Here we solve this problem by using chemically active lowtemperature plasma, which ensures the chemical synthesis of NDs on surface and simultaneously guides the ND selforganization via physical processes triggered by surface charges. We demonstrate the effectiveness of this approach for silicon oxide NDs on silicon, one of the most commonly used ND systems. The ordered arrays of silicon oxide (SiO2) NDs chemically synthesized directly on the surface are of a special interest due to their importance for various applications such as biosensing, photochemistry, biomedical applications, and others. In these applications, the surface density and ordering of the NDs on a substrate surface are the key parameters, which determine performance of the devices. Besides, the use of the silica ND arrays as a catalyst pattern for growing ordered forests of vertically aligned carbon nanotubes requires a very high level of process controllability. High-temperature silicon oxidation in atmospheric-pressure oxygen-enriched environment results in SiO2 NDs with many structural defects, which are not tolerated in the electronic and photonic devices. Moreover, thermal oxidation is not effective in controlling ND size distribution, surface density, and ordering on the surface. Other possible methods such as nanosphere lithography and liquid-phase deposition are very expensive and difficult to control. The Stranski−Krastanow growth is also a very complex process with a very low controllability. Recently, it was demonstrated that inductively coupled radio frequency (ICRF) plasmas are very effective for the self-organized synthesis of SiO2 NDs at very low (not exceeding 100 °C) temperatures, thus ensuring fabrication of the defect-free silicon−silica interfaces. The plasma-based methods of silicon oxidation and synthesizing SiO2 ND patterns offer a promise for better controllability, mainly due to the effects of the electric charges and fields at the plasma-surface interface. Although the idea of controlling self-organization of the ND array formation by surface charges on the surface was proposed, there has been no convincing experimental demonstration and numerical modeling that can explain these phenomena. In our previous work, we have reported on the kinetic of SiO2 NDs formation on the plasma-exposed surfaces. In this Received: January 15, 2013 Accepted: February 5, 2013 Letter