D3 Absorption and Emission Spectroscopy

Topic

Ultraviolet-visible (UV-Vis) absorption spectroscopy—the first of the two experimental methods described by this model—illuminates a suspension of metal nanoparticles with light of various wavelengths and records the amount of light absorbed by the sample at each one; a pronounced absorption peak at a specific wavelength reveals the surface plasmon resonance of these nanoparticles—the collective oscillation of their conduction electrons, a phenomenon already established by another model in this set. The fact that these metal nanoparticles exhibit surface plasmon resonance causes them to absorb significantly more light at that resonance wavelength than the same metal would in its bulk form.

The exact position of this surface plasmon resonance peak within the spectrum is not fixed; instead, it shifts linearly with the size of the nanoparticles: the larger the metal nanoparticle in the sample, the further the plasmon peak shifts toward longer wavelengths. Thus, an increase in nanoparticle size causes the position of the plasmon peak to shift—approximately linearly—toward a longer wavelength.

Photoluminescence (PL)—the second experimental method described by this model—illuminates a suspension of semiconductor nanoparticles with light possessing sufficient energy to excite their electrons, and records the light re-emitted by the nanoparticles as they relax; the energy of this emitted light directly reveals the nanoparticles' bandgap energy, and the photoluminescence itself experimentally confirms that this bandgap energy depends on nanoparticle size, just as predicted by the quantum confinement effect in another model within this set. The smaller size of these semiconductor nanoparticles results—in accordance with the quantum confinement confirmed by this photoluminescence—in a wider bandgap and, consequently, higher-energy light emission.

For this reason, the model—which incorporates UV-Vis spectroscopy and photoluminescence—relies on visible light; although its resolution limit is far inferior to that of the electron microscopy used in another model within this set, it nonetheless enables the indirect yet highly sensitive detection of both the size and electronic structure of these nanoparticles via their surface plasmon resonance and bandgap energy. The fact that this photoluminescence experimentally confirms the size-dependent bandgap energy predicted by the quantum confinement described in another model of the set means that this model also serves as a way to verify whether that quantum confinement remains intact or has been destroyed by the thermal coalescence described in yet another model of the set.

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