Soft Lithographic Approach to the Fabrication of Highly Ordered 2D Arrays of Magnetic Nanoparticles on the Surfaces of Silicon Substrates

This paper describes a simple and convenient method that uses patterned monolayers as templates to fabricate highly ordered 2D arrays of magnetic particles (Co, Ni, or α-Fe, and ferrites such as MgFe2O4 or NiFe2O4) with lateral dimensions in the range of 70−460 nm. In this method, the hydrophilic, hydroxyl-terminated surface of a Si/SiO2 wafer was patterned with a hydrophobic monolayer of octadecyltrichlorosilane using microcontact printing with an elastomeric stamp and subsequently used as template to define and deposit a regular 2D array of 2-propanol droplets that contained inorganic salts such as Co(NO3)2, Ni(NO3)2, and Fe(NO3)3, or a combination of these compounds. Evaporation of the solvent led to the formation of a 2D array of nitrate nanoparticles on the hydrophilic, bare regions of Si/SiO2. Each nanoparticle could be well-positioned within the hydrophilic region by withdrawing the substrate from the nitrate solution and by letting the solvent evaporate with the wafer being held at a specified orientation relative to the gravitational field. The nitrate was subsequently converted into metal oxide (Co3O4, NiO, and α-Fe2O3) by thermal decomposition in air at 600 °C, and finally into a magnetic substance (that is, Co, Ni, and α-Fe) through the reduction by hydrogen gas at 400 °C. The dimensions of these particles could be controlled by changing the concentration of the nitrate solution and/or the area of the hydrophilic region. We have also shown that coprecipitation of two (or more) different nitrates within the liquid droplets could lead to the formation of highly ordered 2D arrays of magnetic ferrites such as MgFe2O4 or NiFe2O4. The magnetic properties of these 2D arrays of nanoparticles supported on silicon substrates were studied using magnetic force microscopy.