Thermally fragile tris(
Nanowires with a few atoms in diameter show completely different properties from those of the three-dimensional bulk materials [
In 1998, it was found that single-wall carbon nanotubes (SWCNTs) are able to encapsulate C60 fullerenes in their inner hollow space [
The encapsulation of organometallic complex molecules, such as Co(C5H5)2 [
Encapsulation of organometallics in SWCNTs, however, has so far been difficult, mainly because of the fact that, under atmospheric condition or at high temperature, such materials normally decompose during the encapsulation process. Here, we have focused on the development of a versatile method to encapsulate fragile organometallic complexes in SWCNTs. To confirm the encapsulation, we have employed a structure determination procedure that is based on high-resolution transmission electron microscope (HR-TEM) observations and HR-TEM image simulation by the multislice method [
SWCNTs were synthesized by the so-called extended direct injection pyrolytic synthesis (e-DIPS) [
ErCp3 is an air- and moisture-sensitive material and decomposes easily upon exposure to the atmosphere [
Figure
TGA profiles of as-produced and purified e-DIPS SWCNT. The sharp drop in sample weight at higher temperature and the smaller amount of residual material show higher quality of purified SWCNTs than that of as-produced ones.
Figures
HR-TEM image of (a) as-produced and (b) purified e-DIPS SWCNTs.
Figures
HR-TEM images of ErCp3 molecules in (a) thinner and (b) broader SWCNTs. The inset of (a) is a simulated HR-TEM image of ErCp3@o-CNT.
When the diameter of SWCNTs is larger than 1.30 nm, ErCp3 molecules aggregate to form clusters (Figure
To confirm the high filling ratio of ErCp3 in o-CNTs, we have measured X-ray diffraction (XRD) patterns. Figures S2(a) and (b) show the XRD patterns of o-CNTs, ErCp3@o-CNTs, p-CNTs, and ErCp3@p-CNTs. As shown in Figures S2(a) and (b), the number of diffraction peaks is limited, so that it is difficult to determine precise filling ratio by pattern fitting. Filling of ErCp3 molecules, however, can be confirmed by changes in intensity of (10) peak. As clearly seen in Figure S2(a), the intensity of (10) peak of ErCp3@o-CNTs is much smaller than that of o-CNTs. This is a clear indication that the inner space of o-CNTs is filled with guest materials (i.e., ErCp3) [
The intensity of (10) peak of ErCp3@p-CNTs is comparable to that of p-CNTs, suggesting that the observed intensity drop in ErCp3@o-CNTs can be attributed to encapsulation of ErCp3 molecules (Figure S2(b)). The high filling ratio is caused by (1) purification and anaerobic handling of ErCp3, (2) an improved preparation method of o-CNTs (i.e., high quality, proper diameter, and optimized cap-opening conditions), and (3) performing the encapsulation reaction under high vacuum at optimized conditions.
Based on the observed HR-TEM images of ErCp3@SWCNT, we have constructed a structure model. Image simulations by the multislice method (at a defocus of 600, 650, 700, 750, and 800 nm) have been carried out based on the structure model constructed. To attain satisfactory agreements between the observed and simulated HR-TEM images, we have superimposed simulated HR-TEM images of ErCp3@SWCNT with different molecular orientations, in which simulated images based on fixed molecular orientations do not match with the observed images.
As illustrated in inset of Figure
To further investigate the electronic structure of ErCp3, X-ray absorption spectrum (XAS) measurements at the Er
X-ray absorption spectra of ErCp3@o-CNTs and Er2O3, confirming the presence of Er3+ in both species.
We have successfully fabricated novel low-dimensional crystalline ErCp3 nanowires encapsulated in SWCNTs with filling yield of ~30% and characterized their structural properties. Encapsulation reactions carried out under high temperature and high vacuum conditions using high quality SWCNTs are necessary in order to obtain ErCp3@SWCNTs. A structure determination method based on the simulated annealing method and HR-TEM image simulation has been shown to be useful in characterizing the crystal structure of metal complex nanowires formed in SWCNTs. The present study may lead to future fabrication of various low-dimensional metal complexes in SWCNTs in high yield.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Hisanori Shinohara acknowledges the Grant-in-Aid for Scientific Research (S) (no. 22225001) of MEXT, Japan, for the partial support of the present study. The XRD experiments were performed at BL02B2 and XAS experiments were performed at BL25SU in SPring-8 with the approval of JASRT.