Most often, this is the simplest technique to produce nanoscale structures, and this is the main reason of the recent wide interest, as revealed by comprehensive compilations. Some reviews [1–4] exhaustively describe the different existing technologies, mainly based on electrophoretic forces [5], capillary forces [6, 7], dip coating [8, 9], and ink-jet printing [10], among others. Top-down approaches, such as lithography or ion sputtering, have smaller chances to be able to produce large-scale low cost materials than bottom-up wet methods, despite the limitations of techniques such as spinning or sedimentation. Mono- and multilayers of

nanospheres have a huge number of promising electrical GDC-0068 and optical applications [11–14]; some benefiting from the high surface-to-volume ratio to, for example, foster a new generation of ultrafast bulk battery electrodes [15], scaffolds

of macroporous materials [16, 17], while others benefit from the dimension of the periodicity of three-dimensional (3D) structures making them suitable for photonic [18–20] or terahertz applications [21]. The technique used in this work is known as electrospray. It consists of producing a fine aerosol by dispersion of a liquid by application of a high electric field between an emitter, usually a thin needle, and a flat electrode. Above a given voltage threshold, a Taylor Olaparib cone develops [22] and the liquid tip becomes unstable breaking into small droplets. The main application of electrospray is found in the ion source of mass spectrometers, although it has also been recently used as a nanoparticle deposition method [23–25], polymer thin film deposition [26], or to create photonic balls [27]. To our knowledge, electrospraying of nanofluids or colloidal solutions of nanometer-size spheres to produce full 3D

self-assembled crystals has not been reported so far. A very comprehensive work on state-of-the-art colloidal crystals has recently been published [1] where a few indicators of the crystal quality produced by the various techniques are summarized and compared, namely the thickness, area, deposition time, and optical quality. We have drawn in Figure 1 a radial plot of selected information from Table MRIP one in [1] for some of the deposition techniques reported there. We have not included the indicators concerning four techniques, namely motor-drawing, sedimentation, cell confinement, and air-water interface due to the poor results compared to the rest. Figure 1 Radial plot of quality indicators for some of the most relevant colloidal crystal fabrication techniques. Deposition time, area, thickness, and quality of the photonic crystal are compared. The technology introduced in this work is the electrospray, in solid black.