Effect of Substituents and Initial Degree of Functionalization of Alkylated Single-Walled Carbon Nanotubes on Their Thermal Stability and Photoluminescence Properties.

Alkylated single-walled carbon nanotubes (SWNTs) have been thermally treated to determine the influence of substituents and the degree of functionalization on their thermal stability and photoluminescence (PL) properties. Alkylated SWNTs were prepared by treating SWNTs with sodium naphthalenide and alkyl bromide. The defunctionalization of the alkylated SWNTs was monitored by absorption and Raman spectra. Selective recovery of the characteristic absorption and radial breathing mode peaks was observed during the thermal treatment, which indicates that the thermal stability of the alkylated SWNTs decreases with increases in SWNT diameter and degree of functionalization. n-Butylated and phenethylated SWNTs showed higher thermal stability than sec-butylated and benzylated SWNTs for a similar degree of functionalization, respectively. The diameter selectivity and effect of substituents on the thermal elimination reaction were confirmed by density functional theory. In addition, it was shown that the initial degree of functionalization of the alkylated SWNTs, with the alkyl group and degree of functionalization being kept constant after thermal treatment, strongly affects their PL properties; Stokes shift, and PL peak intensity.

Tuning of the photoluminescence and up-conversion photoluminescence properties of single-walled carbon nanotubes by chemical functionalization.

Single-walled carbon nanotubes (SWNTs) were subjected to alkylation using alkyl bromide and alkyl dibromide, and the photoluminescence (PL) properties of the resulting alkylated SWNTs were characterized. Two new PL peaks were observed along with the intrinsic PL peak at 976 nm when alkyl bromide was used (SWNT-Bu: ∼1095 and 1230 nm, SWNT-Bn: 1104 and 1197 nm). In contrast, the use of α,α'-dibromo-o-xylene as an alkyl dibromide primarily resulted in only one new PL peak, which was observed at 1231 nm. The results revealed that the Stokes shift of the new peaks was strongly influenced by the addition patterns of the substituents. In addition, the time-resolved PL decay profiles of the alkylated SWNTs revealed that the PL peaks possessing a larger Stokes shift had longer exciton lifetimes. The up-conversion PL (UCPL) intensity of the alkylated SWNTs at excitation wavelengths of 1100 and 1250 nm was estimated to be ∼2.38 and ∼2.35 times higher than that of the as-dispersed SWNTs, respectively.

Control of the photoluminescence properties of single-walled carbon nanotubes by alkylation and subsequent thermal treatment.

Dibutylated single-walled carbon nanotubes (Bu-SWNTs-Bu) with varying degrees of functionalization were prepared by two-step reductive alkylation and subsequent thermal treatment. A new photoluminescence peak at around 1240 nm was observed and increased drastically after the thermal treatment. The results indicate that thermally treated Bu-SWNTs-Bu are promising NIR photoluminescent materials.