Article pubs.acs.org/Langmuir
Size Control of Porous Silicon-Based Nanoparticles via Pore-Wall Thinning Emilie Secret, Camille Leonard, Stefan J. Kelly, Amanda Uhl, Clayton Cozzan, and Jennifer S. Andrew* Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *
ABSTRACT: Photoluminescent silicon nanocrystals are very attractive for biomedical and electronic applications. Here a new process is presented to synthesize photoluminescent silicon nanocrystals with diameters smaller than 6 nm from a porous silicon template. These nanoparticles are formed using a pore-wall thinning approach, where the as-etched porous silicon layer is partially oxidized to silica, which is dissolved by a hydrofluoric acid solution, decreasing the pore-wall thickness. This decrease in pore-wall thickness leads to a corresponding decrease in the size of the nanocrystals that make up the pore walls, resulting in the formation of smaller nanoparticles during sonication of the porous silicon. Particle diameters were measured using dynamic light scattering, and these values were compared with the nanocrystallite size within the pore wall as determined from X-ray diffraction. Additionally, an increase in the quantum confinement effect is observed for these particles through an increase in the photoluminescence intensity of the nanoparticles compared with the as-etched nanoparticles, without the need for a further activation step by oxidation after synthesis.
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INTRODUCTION Micro- and nanoparticles made from porous silicon are widely studied for biomedical applications1−3 due to their biocompatibility and biodegradability.4−6 Porous silicon nanoparticles studied for this type of application usually have sizes ranging from 50 to 200 nm; however, for some applications, silicon nanoparticles with diameters smaller than 6 nm are of interest. For example, smaller luminescent silicon nanoparticles have the potential to be used as fluorescent dyes in biological imaging and are advantageous compared with II−VI semiconductor quantum dots, which have been shown to be toxic.7 Several groups have reported on the formation of silicon nanoparticles from porous silicon, where the particles are usually fabricated by breaking up a porous silicon layer directly after formation. This can be achieved by sonication,8 mechanical pulverization,9 high current pulses10 or by boiling the porous silicon in a solvent.11 Silicon nanoparticles can also be prepared directly by etching a silicon wafer in the presence of an oxidizing catalyst, such as polyoxometalates.12 Although each of these methods is capable of producing silicon nanoparticles, the processes tend to result in particles that are polydisperse. Additionally, in our experience, we were not able to produce silicon nanoparticles that were smaller than 6 nm by sonication of different porous silicon layers. We hypothesize that this is due to the increased surface energy barrier for forming smaller and smaller particles via sonication, and as a result sonication is not powerful enough to further break up the nanocrystals within the porous layer to form smaller particles. To overcome this, we have developed a process to obtain small silicon nanoparticles with diameter smaller than 6 nm by © 2016 American Chemical Society
thinning the walls of porous silicon prior to sonication. This is based on the idea that the pore-wall thickness will determine the minimum particle size that can be formed by sonication or other mechanically driven top-down approaches. This pore-wall thinning process for the fabrication of silicon nanoparticles is shown schematically in Figure 1. In this process, a porous silicon layer is first formed via an electrochemical etch of a p++ silicon wafer in an ethanolic hydrofluoric acid solution. After etching, the porous layer is removed from the wafer and a low-temperature oxidation is performed. The purpose of this low-temperature oxidation is to form a thin oxide layer on the surface of the pores. This thin oxide layer is subsequently removed by dipping the layer in hydrofluoric acid, leaving behind thinned pore walls. The porous silicon layer with thin walls is then sonicated, resulting in the formation of small diameter silicon nanoparticles, where the particle diameter is ultimately determined by the crystallite size within the pore wall. This process has been previously described as a route to increase the pore size in porous silicon films and has also been shown to control the rough texture of the porous silicon walls.13 Here we have adapted this process to form silicon nanoparticles with an average diameter smaller than 6 nm while simultaneously increasing the quantum confinement effect in the particles, resulting in luminescence in the as-synthesized particles without the need of a further oxidation step. Received: November 16, 2015 Revised: January 10, 2016 Published: January 22, 2016 1166
DOI: 10.1021/acs.langmuir.5b04220 Langmuir 2016, 32, 1166−1170
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Langmuir
Figure 1. Schematic of the process to form small diameter silicon nanoparticles from porous silicon via a pore-wall thinning process, where the final nanoparticle diameter is related to the pore wall thickness. In this method, thermal oxidation followed by oxide dissolution in a hydrofluoric acid solution results in thinning of the pore walls.
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EXPERIMENTAL SECTION
RESULTS AND DISCUSSION To demonstrate the effectiveness of the pore-wall thinning process to form silicon nanoparticles with small diameters, we etched samples at a variety of current densities, followed by a low-temperature thermal oxidation. Samples were etched at current densities ranging from 20 to 200 mA/cm2, with the anticipation that samples etched at higher current densities would have thinner pore walls than those etched at lower current densities, resulting in the formation of smaller diameter nanoparticles. Figure 2a shows the average diameter, determined by dynamic light scattering (DLS), of silicon nanoparticles formed as a function of etching current density, where the nanoparticles were prepared by direct sonication for 24 h of the as-etched porous layer. As-predicted samples etched at the lowest current density (20 mA/cm2) had the largest diameters (∼20 nm) compared with 15 nm for those etched at 200 mA/cm2. Although increasing current density lead to decreased particle size the overall decrease in size was minimal over the current densities tested. To further test our hypothesis that pore-wall thickness is correlated with nanoparticle size and to obtain smaller diameter nanoparticles, a previously described method for the thinning and smoothing of the walls of porous silicon was adapted.13 The pore walls were thinned by first performing a partial oxidation of the pore walls at low temperatures, ranging from 400 to 500 °C, on a lifted off porous silicon layer (Figure 1). These temperatures were chosen because they are well below 800 °C, which is the temperature required for complete oxidation of the silicon to silica.14 During partial oxidation, a silica layer forms on the pore walls, which is subsequently removed by dissolution in a hydrofluoric acid (HF) solution, resulting in thinning of the pore walls as the unoxidized silicon portion of the walls remains. After pore-wall thinning the porous layers were sonicated to form nanoparticles. Figure 2b shows the average silicon nanoparticle diameter, as determined by DLS, for particles formed by sonication for 24 h of partially oxidized samples (T = 400 °C, 1 h) etched at a range of current densities (20−200 mA/cm2) and dipped in a solution of HF. As anticipated the average nanoparticle diameter decreased as a function of etching current density; however, it is important to note that the pore-wall thinning process had the greatest effect on nanoparticles etched at the highest current densities. For samples etched at current densities of 100, 150, and 200 mA/ cm2, the wall-thinning process resulted in particles with an average diameter of 2.8, 2.1, and 3.4 nm, respectively; however, for lower current densities, the diameter of the silicon nanoparticles did not significantly decrease compared with
Materials. Single-crystal boron-doped p++ silicon wafers with resistivities
