Gold, colloidal absorption spectra

The lack of a clearly developed peak in the UV-visible absorption spectrum due to plasma resonance places a limit on the collective metallic behavior of the electrons in the cluster. On the other hand, the 5d -y 6s,6p interband absorption is well developed toward that of large colloidal gold particles. 

The electronic absorption spectrum of metal nanocrystals in the visible region is dominated by the plasmon band. This absorption is due to the collective excitation of the itinerant electron gas on the particle surface and is characteristic of a nanocrystal of a given size. In metal colloids, surface plasmon excitations impart characteristic colors to the metal sols, the beautiful wine-red color of gold sols being well-known [6-8]. The dependence of the plasmon peak on the dielectric constant of the surrounding medium and the diameter of the nanocrystal was predicted theoretically by Mie and others at the turn of the last century [9-12]. The dependence of the absorption band of thiol-capped Au nanocrystals on solvent refractive index was recently verified by Templeton et al. [13]. Link et al. found that the absorption band splits into longitudinal and transverse bands in Au nanorods [6, 7]. 

P. Mulvaney M. Giersig A. Henglein, Electrochemistry of multilayer colloids Preparation and absorption spectrum of gold-coated silver particles. J. Phys. Chem. 1993, 97, 7061-7064. 

PL spectra fi-om the samples with different thickness of polyelectrolyte spacer have been measured with the excitation wavelength, corresponding to the maximum of plasmon resonance in the absorption spectrum of the gold colloidal film (T = 550 nm). The enhancement of PL is expected to be most pronounced when excited at the frequency of surface plasmon resonance in gold colloids. 

Figure 16.9 Transient absorption spectra of 15 nm spherical gold nanoparticles after excitation at 400 nm with 100 fs laser pulses, recorded as different delay times. Also shown is the steady-state UV/Vis-absorption spectrum of the colloidal gold solution. The inset shows the decay of the transient bleach when the particles are monitored at the hleach maximum at 520 nm. Fitting of the decay curve yields electron-phonon and phonon-phonon relaxation times of 3.1 and 90 ps, respectively. (Reproduced with permission from S. Link and M. El-Sayed, 1999. J. Phys. Chem. B 103 8410 8426. Copyright 1999 American Chemical Society.)...

Figure 2.13 Absorption spectrum of colloidal gold nanocrystals of 10 nm diameter.

A macroscopic PbS crystal has a band gap of 0.41 eV. Because of the small effective mass of the electrons and holes ( e,eff = h,efF = 0.09 X OTg), Strong size-quantization occurs. The absorption spectrum of an aqueous suspension (of polyvinyl alcohol-capped) PbS nanocrystals, 6.5 nm in diameter (see TEM picture) is shown in Fig. 13. The HOMO-LUMO optical transition occurs at 2.1 eV, and two other absorption peaks are seen at 3.2 and 4.3 eV. When a gold electrode is immersed in this colloidal solution, PbS nanocrystals... 

As described above the procedures according to Frens is a suitable technique to produce colloid solutions. However the aimed red color is obtained after color changes from yellow to black, dark blue, violet, and finally shifts suddenly to red. These earlier stages of colloid synthesis absorb light substantially above 600 nm and can be isolated by a rapid cool-down or addition of metal ions blocking crystallization of the gold colloid. With such glass-type non-crystalline colloids also resonance enhancement at A, > 600 nm is achieved, which enables to shift the desired absorption peak all over the visible spectrum to IR. 

Figure 3. Measured absorption spectrum of a colloid of gold nanoparticles of radius 10 nm. When linked by molecule B, they ate clustered with an absorption spectrum in the blue. When reducing agent molecule A is added, monomers are formed, and the absorption spectmm turns red.

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