Porous Silicon Microcavities: Synthesis, Characterization, and Application to Photonic Barcode Devices

F. Ramiro-Manzano; R. Fenollosa; E. Xifré-Pérez; M. Garín; F. Meseguer

doi:10.1186/1556-276X-7-497

Research · Publication & note

Porous Silicon Microcavities: Synthesis, Characterization, and Application to Photonic Barcode Devices

Nanoscale Research Letters · Journal article · 3 September 2012

F. Ramiro-Manzano, R. Fenollosa, E. Xifré-Pérez, M. Garín and F. Meseguer

Nanoscale Research Letters

Volume 7

Issue 1

Article 497

2012

Abstract

We have recently developed a new type of porous silicon we name as porous silicon colloids. They consist of almost perfect spherical silicon nanoparticles with a very smooth surface, able to scatter (and also trap) light very efficiently in a large-span frequency range. Porous silicon colloids have unique properties because of the following: (a) they behave as optical microcavities with a high refractive index, and (b) the intrinsic photoluminescence (PL) emission is coupled to the optical modes of the microcavity resulting in a unique luminescence spectrum profile. The PL spectrum constitutes an optical fingerprint identifying each particle, with application for biosensing. In this paper, we review the synthesis of silicon colloids for developing porous nanoparticles. We also report on the optical properties with special emphasis in the PL emission of porous silicon microcavities. Finally, we present the photonic barcode concept.

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Article access and reuse

The article is open access under the Creative Commons Attribution 2.0 licence. Figures 3 and 6 are original to this article and are reused under CC BY 2.0. Figures 1, 2, 4 and 5 were reprinted in the article from earlier publications; they are shown here under the verified author-reuse terms of their original RSC, AIP and Wiley sources, with the original publication credited below each figure.

Figures

Figure 1. Optical microscopy images of porous and amorphous silicon microspheres and an SEM image of a porous silicon microsphere.

Figure 1. Microscopy images. (a) Optical image of porous silicon microspheres and (b) optical image of a:Si-H microspheres. Porous silicon microspheres scatter yellow, orange and red colours under white light, whereas a:Si-H microspheres appear black. (c) SEM image of a porous silicon microsphere about 2 μm in diameter.

From R. Fenollosa et al., J. Mater. Chem. 20, 5210–5214 (2010), DOI 10.1039/C0JM00079E. © The Royal Society of Chemistry 2010. Reproduced by an author with acknowledgement.

Figure 2. Optical transmittance spectra and Mie-theory fits for two polycrystalline silicon colloids.

Figure 2. Optical properties of polycrystalline silicon colloids. Optical transmittance (black) and Mie-theory fits (red) of single silicon colloids with diameters Φ = 1885 nm (a) and Φ = 1050 nm (b). Transmittance dips correspond to whispering-gallery modes; the insets show electric-field intensity distributions.

From F. Meseguer et al., J. Appl. Phys. 109, 102424 (2011), DOI 10.1063/1.3581880. © American Institute of Physics. Reused on an author’s personal, non-commercial website with full citation.

Experimental and calculated transmittance spectra of a porous silicon colloid — **Figure 3.** Optical properties of porous silicon colloids. Measured optical transmittance of a 1.910-μm-diameter porous silicon sphere (black curve). The Mie theory fit (red curve) gives a refractive index of 1.8 in all the measured range. The resonant modes are indicated under their corresponding deeps by letters ‘a’ (for transversal magnetic modes) and ‘b’ (for transversal electric modes) and two sub-indexes that account for the different electric field intensity distributions.

Figure 4. Transmittance and photoluminescence spectra of a porous silicon microsphere.

Figure 4. Transmittance (a) and photoluminescence (b) spectra of a porous silicon microsphere about 4.1 μm in diameter. Dips in the transmittance spectrum correspond to photoluminescence peaks associated with the optical resonances of the microcavity.

From F. Ramiro-Manzano et al., Advanced Materials 23, 3022–3025 (2011), DOI 10.1002/adma.201100986. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Reused by the authors under Wiley’s contributor reuse terms.

Figure 5. Two photonic barcodes with photoluminescence spectra and optical microscopy images.

Figure 5. Photonic barcodes (a,e) produced by two porous silicon microspheres. Bar positions and widths are determined by the resonances of their photoluminescence spectra (b,f). Optical microscopy images were taken in reflection (c,g) and transmission (d,h).

From F. Ramiro-Manzano et al., Advanced Materials 23, 3022–3025 (2011), DOI 10.1002/adma.201100986. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Reused by the authors under Wiley’s contributor reuse terms.

Photoluminescence spectra showing the stability of porous silicon colloids — **Figure 6.** PL spectra of an about 3.4-μm diameter porous silicon colloid. After being immersed in 200-mM NADPH aqueous solution during 1 (bottom spectrum) and 5 h (top spectrum).

Research fields

Bottom-up

Disilane decomposition and subsequent material conversion produce amorphous, polycrystalline and porous silicon colloids with smooth spherical boundaries. In the porous form, the particle combines an intrinsic luminescent network with a self-contained optical microcavity.

Simulations & fits

Mie fits to single-sphere transmission determine effective refractive index and assign the resonant modes. Matching those modes with the porous-silicon emission explains why each particle develops a diameter- and porosity-dependent photonic barcode.

Characterization

Optical and electron microscopy, infrared chemical analysis, transmission and photoluminescence follow composition, size and resonant response. Exposure-dependent spectral shifts also reveal how oxidation changes the optical fingerprint of one microsphere over time.

RESEARCH TOPICS

EmissionPorous silicon supplies its own broad photoluminescence, but the spherical boundary redistributes that light into a comb of narrow resonant peaks. In Figure 4, several peaks reach full widths near 1 nm and are substantially sharper than features in ordinary transmission. The result is not merely brighter emission: each particle acquires a distinctive spectral fingerprint that can function as an optical barcode.

InterferenceThe narrow lines arise because light circulating inside the high-index sphere interferes constructively only at discrete wavelengths. Figure 4 compares transmission and photoluminescence from the same 4.1 μm particle; the much stronger structure in the luminescence spectrum demonstrates that the internal emitter couples selectively to the allowed cavity fields.

MaterialsPorosity changes the sphere from homogeneous polycrystalline silicon into an effective medium with a radially non-uniform refractive index. That is why the porous-particle spectra cannot be fitted by a simple homogeneous-silicon Mie model. Figures 2 and 3 contrast the optical response of polycrystalline and porous colloids, linking the change in mode positions and scattering to the internal material structure.

ResonatorsEach microsphere acts as a three-dimensional optical microcavity supporting transverse-electric and transverse-magnetic mode families. The mode labels and field distributions are used to interpret spectra from particles around 3.4-4.1 μm in diameter. Because the resonance pattern depends sensitively on size and internal refractive index, two visually similar spheres can be distinguished spectrally.

SetupsIndividual spheres are inspected at 1000× magnification in reflection and transmission before their spectra are recorded. Figure 5 places optical micrographs beside the corresponding transmission and photoluminescence data, allowing diameter and appearance to be checked against the spectral barcode rather than analysing an ensemble average.

ChemistryThe particles are grown by chemical-vapour decomposition of disilane, and decomposition time controls their internal state. At 400 °C, a short treatment of about 1 minute 30 seconds produces porous spheres, whereas a longer treatment of about 5 minutes under otherwise similar conditions produces amorphous particles. The optical cavity is therefore linked directly to the gas-phase growth history.