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Surface charges and optical characteristic of colloidal cubic SiC nanocrystals.

Li Y, Chen C, Li JT, Yang Y, Lin ZM - Nanoscale Res Lett (2011)

Bottom Line: Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CHx groups.The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals.The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.

View Article: PubMed Central - HTML - PubMed

Affiliation: Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100190, PR China. chen.c.x@sjtu.edu.cn.

ABSTRACT
Colloidal cubic silicon carbide (SiC) nanocrystals with an average diameter of 4.4 nm have been fabricated by anisotropic wet chemical etching of microsized cubic SiC powder. Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CHx groups. UV/Vis absorption and photoluminescence (PL) spectroscopy clearly indicate that water and ethanol colloidal suspensions of the as-fabricated colloidal samples exhibit strong and above band gap blue and blue-green emissions. The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals. The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.

No MeSH data available.


Absorption spectra of the colloidal cubic SiC nanocrystals in water and ethanol. The insert shows the enlarged absorption spectra, taken from the dashed line square area.
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Figure 3: Absorption spectra of the colloidal cubic SiC nanocrystals in water and ethanol. The insert shows the enlarged absorption spectra, taken from the dashed line square area.

Mentions: Previous work has revealed that the UV/Vis absorption spectrum contains much information about the excitation dynamics and the energy levels in the SiC nanocrystals [28]. Figure 3 shows the UV/Vis spectrum of cubic SiC nanocrystals in the water and ethanol suspensions. Below the band gap of bulk cubic SiC (2.2 eV), there is a soft absorption edge for the colloidal cubic SiC nanocrystals in water and an absorption peak at 2.1 eV in ethanol, as shown in the inset of Figure 3. Above the band gap, two suspensions show a strong continuous increase. Pure water and ethanol exhibit no absorption in this range; hence, the light absorption in Figure 3 results from the colloidal SiC nanocrystals. When the incident photon energy is lower than the band gap, colloidal cubic SiC nanocrystals absorb light energy via band tail states, and the above band gap absorption is via band-band transitions in different nanocrystals [9,26]. The colloidal cubic SiC nanocrystals in water show much higher absorption intensity than that in ethanol. The FTIR results indicate that the etched cubic SiC powders posse oxygen-terminated polar surfaces. The oxygen-terminated cubic colloidal SiC nanocrystals have a higher solubility in water, as a result, high absorption intensity is obtained. The cubic SiC nanocrystals in water show a smoother absorption spectrum with the first peak at 5.9 eV. The absorption spectrum show a sharp rise when the incident photo energy is larger than 5 eV. According to the quantum confinement model, the electronic band gap of the nanocrystals increases when its diameters decrease, and the sharp rise in the absorption spectrum arises from a growing number of cubic SiC nanocrystals with band gaps lower than 5 eV. The cubic SiC nanocrystals in ethanol exhibit a step-like absorption spectrum characteristic. There are several sharper absorption peaks (2.1, 4.2, 4.5, 4.9, 5.6 eV) with increasing photo energies. These absorption peaks correspond to the confined energy levels of ultrasmall cubic SiC nanocrystals [29] in ethanol.


Surface charges and optical characteristic of colloidal cubic SiC nanocrystals.

Li Y, Chen C, Li JT, Yang Y, Lin ZM - Nanoscale Res Lett (2011)

Absorption spectra of the colloidal cubic SiC nanocrystals in water and ethanol. The insert shows the enlarged absorption spectra, taken from the dashed line square area.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211874&req=5

Figure 3: Absorption spectra of the colloidal cubic SiC nanocrystals in water and ethanol. The insert shows the enlarged absorption spectra, taken from the dashed line square area.
Mentions: Previous work has revealed that the UV/Vis absorption spectrum contains much information about the excitation dynamics and the energy levels in the SiC nanocrystals [28]. Figure 3 shows the UV/Vis spectrum of cubic SiC nanocrystals in the water and ethanol suspensions. Below the band gap of bulk cubic SiC (2.2 eV), there is a soft absorption edge for the colloidal cubic SiC nanocrystals in water and an absorption peak at 2.1 eV in ethanol, as shown in the inset of Figure 3. Above the band gap, two suspensions show a strong continuous increase. Pure water and ethanol exhibit no absorption in this range; hence, the light absorption in Figure 3 results from the colloidal SiC nanocrystals. When the incident photon energy is lower than the band gap, colloidal cubic SiC nanocrystals absorb light energy via band tail states, and the above band gap absorption is via band-band transitions in different nanocrystals [9,26]. The colloidal cubic SiC nanocrystals in water show much higher absorption intensity than that in ethanol. The FTIR results indicate that the etched cubic SiC powders posse oxygen-terminated polar surfaces. The oxygen-terminated cubic colloidal SiC nanocrystals have a higher solubility in water, as a result, high absorption intensity is obtained. The cubic SiC nanocrystals in water show a smoother absorption spectrum with the first peak at 5.9 eV. The absorption spectrum show a sharp rise when the incident photo energy is larger than 5 eV. According to the quantum confinement model, the electronic band gap of the nanocrystals increases when its diameters decrease, and the sharp rise in the absorption spectrum arises from a growing number of cubic SiC nanocrystals with band gaps lower than 5 eV. The cubic SiC nanocrystals in ethanol exhibit a step-like absorption spectrum characteristic. There are several sharper absorption peaks (2.1, 4.2, 4.5, 4.9, 5.6 eV) with increasing photo energies. These absorption peaks correspond to the confined energy levels of ultrasmall cubic SiC nanocrystals [29] in ethanol.

Bottom Line: Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CHx groups.The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals.The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.

View Article: PubMed Central - HTML - PubMed

Affiliation: Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100190, PR China. chen.c.x@sjtu.edu.cn.

ABSTRACT
Colloidal cubic silicon carbide (SiC) nanocrystals with an average diameter of 4.4 nm have been fabricated by anisotropic wet chemical etching of microsized cubic SiC powder. Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CHx groups. UV/Vis absorption and photoluminescence (PL) spectroscopy clearly indicate that water and ethanol colloidal suspensions of the as-fabricated colloidal samples exhibit strong and above band gap blue and blue-green emissions. The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals. The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.

No MeSH data available.