Porous silicon microspheres: synthesis, characterization and application to photonic microcavities

R. Fenollosa; F. Ramiro-Manzano; M. Tymczenko; F. Meseguer

doi:10.1039/C0JM00079E

Research · Publication & note

Porous silicon microspheres: synthesis, characterization and application to photonic microcavities

Journal of Materials Chemistry · Journal article · 14 May 2010

R. Fenollosa, F. Ramiro-Manzano, M. Tymczenko and F. Meseguer

Journal of Materials Chemistry

Volume 20

Pages 5210-5214

2010

Abstract

Porous silicon microspheres have been synthesized by chemical vapour deposition of disilane gas. Their spectral signatures are similar to those of electrochemically grown porous silicon, in particular they yield photoluminescence and they show an oxidation behaviour upon their exposure to the open air. The particles are highly spherical and poly-disperse in size with diameters of approximately 0.5 to 5 micrometres, and they have a very smooth surface. Because of these reasons they work as optical microcavities with well defined resonating Mie modes. These modes have been identified in the near-infrared range. They blue-shift considerably whenever the microspheres are in contact with air because of the aforementioned oxidation process.

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Figures

Figure 1. Optical microscopy and SEM images of porous silicon microspheres — **Figure 1.** Optical microscopy images at 1000× magnification showing: (a) porous silicon microspheres obtained by decomposing disilane at 400 °C during 1 min and 30 s; the inset is a magnified single microsphere; (b) a:Si-H microspheres synthesized at 400 °C for 5 min; and (c) an SEM image at 45,000× magnification of a porous silicon microsphere approximately 2 µm in diameter, illustrating its spherical perfection and smooth surface.

Absorbance and photoluminescence spectra of porous silicon microspheres — **Figure 2.** Spectral signatures of porous silicon microspheres. (a) Mid-infrared optical absorbance spectra after exposure to open air for several minutes, 2 h and 120 h. (b) Photoluminescence spectrum of a cluster of porous silicon microspheres after several days of exposure to open air.

Figure 3. TEM and HRTEM images of crushed porous silicon microspheres — **Figure 3.** (a) TEM and (b) HRTEM images of crushed porous silicon microspheres. White boundaries surrounding darker areas of about 10 nm correspond to pores. The HRTEM image indicates sub-nanometre pore widths and a polycrystalline nature partly hidden by an amorphous oxide layer.

Figure 4. Optical transmittance spectra and oxidation-induced resonance shift — **Figure 4.** (a) Optical transmittance spectra illustrating the blue shift of resonating modes after exposure to open air. (b) Experimental mode position versus exposure time for the mode marked by the grey strip in (a); the inset shows the derivative of the experimental data.

Research fields

Bottom-up

Controlled disilane decomposition forms micrometre-scale porous silicon spheres with smooth surfaces and intrinsic photoluminescence. Processing time and subsequent oxidation tune porosity and composition while the spherical boundary supplies cavity confinement.

Simulations & fits

Mie-mode analysis assigns the resonances in single-particle transmission and relates their spectral displacement to effective refractive-index changes. This turns oxidation-induced blue shifts into a sensitive probe of the evolving porous sphere.

Characterization

Optical and electron microscopy, mid-infrared absorption, transmission and photoluminescence characterize morphology, silicon-hydride chemistry and cavity response. Correlating these measurements shows how an emitting porous material and a resonator coexist in one particle.

RESEARCH TOPICS

EmissionThe 0.5–5 μm porous silicon spheres scatter red-to-yellow colours under white light and also produce intrinsic photoluminescence. Figure 2 compares their mid-infrared signatures and PL, establishing that they share optical characteristics with electrochemically etched porous silicon despite being free spherical particles. This simultaneous scattering and emission makes the material self-reporting: no separate dye is needed to see the colloids or to interrogate their internal optical evolution.

MaterialsShort disilane decomposition times of about 1 min 30–40 s produce highly porous, polydisperse silicon spheres. TEM and HRTEM in Figure 3 reveal a polycrystalline skeleton, although a rapidly formed amorphous oxide layer complicates direct lattice interpretation. Exposure to air progressively oxidises the porous matrix and lowers its effective refractive index. The experiment uses that ageing process as a material probe, correlating structural chemistry with an optical blue shift rather than treating oxidation as uncontrolled drift.

ResonatorsNear-infrared transmission reveals well-defined Mie modes, demonstrating that even a porous and internally heterogeneous sphere can act as a photonic microcavity. Figure 4 follows the same particle after air exposure: its resonances shift to shorter wavelength as oxidation reduces the effective index. A simple spherical-cavity estimate converts the shift into an approximate refractive-index change. The resonance positions therefore provide a sensitive, non-destructive readout of ageing inside the porous silicon framework.