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Silicon colloids: A new enabling nanomaterial

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Abstract

We have recently developed a new type of silicon structure that we refer to as a silicon colloid. This new material consists of almost perfectly spherical silicon micro- and nanoparticles with a very smooth surface. They are able to trap light very efficiently in a large-span frequency range covering the visible to the far infrared regions. Silicon colloids can be thought of as a completely new material for scientific and technological purposes, with manifold applications covering electronics, photonics, cosmetics, or paints, among others. Here, we report on the synthesis of polycrystalline, amorphous, and porous silicon colloids, as well as their optical properties, some applications concerning light filters, and photonic bonding.

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Complete original Figure 1 composition extracted directly from the embedded PDF image
Figure 1. Calculated scattering efficiency (Qsca) for 2 μm diameter microspheres made of silica (n = 1.46) and silicon (n = 3.5).

Source: F. Meseguer et al., J. Appl. Phys. 109, 102424 (2011). © American Institute of Physics.

Complete original Figure 2 composition extracted directly from the embedded PDF image
Figure 2. Optical microscopy images at 1000× magnification of (a) porous and (b) amorphous silicon colloids. They were obtained by decomposing disilane at 400 °C for 1 and 5 min, respectively.

Source: F. Meseguer et al., J. Appl. Phys. 109, 102424 (2011). © American Institute of Physics.

Complete original Figure 3 composition extracted directly from the embedded PDF image
Figure 3. (a) SEM image of a polycrystalline silicon colloid about 2 μm diameter, illustrating its spherical perfection and smooth surface. (b) TEM image of a polycrystalline silicon colloid showing well-defined crystalline domains. Such domains can be tens of nanometers in size. The image shows the frontier between the silicon colloid and the surrounding material.

Source: F. Meseguer et al., J. Appl. Phys. 109, 102424 (2011). © American Institute of Physics.

Complete original Figure 4 composition extracted directly from the embedded PDF image
Figure 4. Optical transmittance (black noisy curves) and Mie theory fit [gray (red) curves] for single silicon colloids of diameter (a) Φ = 1885 nm and (b) Φ = 1050 nm. The dips of transmittance correspond to WGM. They are indicated under their corresponding dips in (a) by the labels “b” and “a,” for transversal electric and transversal magnetic modes, respectively. The electric field intensity distribution is shown for modes (from left to right) a21, a31, and b41 in (b).

Source: F. Meseguer et al., J. Appl. Phys. 109, 102424 (2011). © American Institute of Physics.

Complete original Figure 5 composition extracted directly from the embedded PDF image
Figure 5. Transmittance spectra of 2 mg/cm2 layers of o/w emulsions containing, in weight, 1% TiO2 nanoparticles (P-25 Degussa) and 1% silicon colloids (a) in the UV range and (b) in the near-IR range. Both emulsions were spread on PMMA Helioplates-HD6.

Source: F. Meseguer et al., J. Appl. Phys. 109, 102424 (2011). © American Institute of Physics.

Complete original Figure 6 composition extracted directly from the embedded PDF image
Figure 6. The interacting photonic force between two silicon spheres (diameter = 1.5 μm) as a function of the size parameter and wavelength. The spheres’ distance (center to center) is 1.508 μm. Positive values indicate repulsive forces, and negative values indicate attractive forces. The intensity of the incident light is 10−4 W/μm2. The inset shows the geometry considered, indicating the polarization of the electric field, E, as well as the direction, k, of the incident light with respect to the dimer position.

Source: F. Meseguer et al., J. Appl. Phys. 109, 102424 (2011). © American Institute of Physics.

Research fields

Bottom-up

Gas-phase disilane decomposition yields amorphous, polycrystalline or porous silicon microspheres whose density and surface chemistry permit handling as colloids. Their high refractive index adds strong Mie confinement to a particle platform otherwise compatible with solution assembly.

Simulations & fits

Mie-scattering calculations compare silicon and silica spheres and relate resonant efficiency to particle diameter and refractive index. Spectral fits provide an optical route to identify individual colloids and evaluate their potential as microcavities.

Characterization

Optical and electron microscopy, infrared spectroscopy, scattering and transmission measurements distinguish the silicon allotropes and their conversion under heat or oxidation. The combined evidence connects chemical composition, morphology and resonant optical behaviour.

RESEARCH TOPICS

EmissionPorous and polycrystalline silicon microspheres scatter red, orange and yellow light and exhibit infrared optical structure determined by both their material absorption and spherical modes. The paper also tests a macroscopic consequence: surfaces coated with a 1% silicon-colloid sunscreen are compared with uncoated surfaces under a xenon lamp to evaluate heating by ultraviolet, visible and infrared radiation. Emission and scattering are therefore connected to a practical spectral-filtering function rather than described only as colour.MaterialsDisilane decomposition creates spherical silicon particles through gas-phase nucleation and surface-tension-driven growth. Short deposition produces porous material; longer growth gives amorphous spheres, and a one-hour vacuum anneal at 800 °C converts them to polycrystalline silicon while preserving shape. SEM, TEM, X-ray diffraction and mid-infrared spectroscopy follow those structural changes. Figure 3 shows a representative polycrystalline sphere formed after a five-minute decomposition and anneal, tying its optical modes to a documented processing history.ResonatorsIndividual spheres are measured by Fourier-transform infrared transmission and display TE- and TM-like Mie resonances whose positions depend on diameter and refractive index. Their high-index silicon interior confines lower-order modes more strongly than common silica or polymer beads. The paper also models paired spheres, showing optical bonding and antibonding combinations analogous to coupled atomic states. A colloid is thus treated as a photonic microcavity and, when paired, as the elementary unit of a coupled-cavity system.ChemistryThe synthesis begins with thermal decomposition of disilane, Si2H6(g) → 2Si(s) + 3H2(g). Reaction time determines whether the particle remains porous or becomes predominantly amorphous, while subsequent annealing drives crystallisation. This chemistry controls absorption, refractive index and visible appearance: longer-grown amorphous spheres look black because silicon absorbs visible light, whereas porous and annealed particles scatter strongly. The chemical route is therefore inseparable from the photonic response attributed to the final colloid.
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