Ultrafast luminescence kinetics of metallic single-walled carbon nanotubes: Possible evidence for excitonic luminescence

Abstract

In metallic systems, strong screening of the Coulomb interaction between an electron and a hole by free electrons largely prevents the formation of an exciton. In one-dimensional metallic systems, however, the screening effect is significantly reduced. Recent theoretical and experimental studies suggest that an exciton state can be realized in metallic single-walled carbon nanotubes (SWNTs) due to their ideal one-dimensional structure. Here, we experimentally investigate photoluminescence in both metallic-SWNT-enriched and semiconducting-SWNT-enriched samples using femtosecond time-resolved photoluminescence measurements. By comparing luminescence kinetics and time-integrated spectra between the two samples, the observed signal in the metallic-SWNT-enriched sample is attributed to the luminescence from metallic SWNTs. We also show complementary data of photoabsorption signals in the metallic-SWNT-enriched sample measured by transient absorption spectroscopy. A difference in temporal behavior between the luminescence and absorption signals strongly suggests that the luminescence is excitonic in nature, and the exciton lifetime is found to be 40 ± 10 fs. This long lifetime indicates that a large exciton binding energy leads to a relatively stable exciton state in the presence of metallic electrons.

Binding energy and lifetime of excitons in metallic nanotubes

“…Our result indicate otherwise and are also in a good agreement with experiments reported in Ref. [16]. The value of the inverse lifetime now follows from Eq.…”

“…This is consistent with experimental results obtained through ultrafast luminescence. 16 Below we derive our main results, Eqs. ( 4) and ( 5).…”

Ultrafast Charge Transfer and Relaxation Dynamics in Polymer-Encapsulating Single-Walled Carbon Nanotubes: Polythiophene and Coronene Polymer

“…In this scenario, the slow decay component is attributed to the recombination of a hole in the PT with an electron in the SWNT. The decay time constant of 11 ps is reasonable for electron–hole recombination because the distance between the PT and the SWNT wall (∼0.4 nm) is of the same order of magnitude as the exciton Bohr radius in SWNTs, and the exciton lifetime is 10–200 ps depending on competing nonradiative processes. , Ascribing the fast component to electron transfer from the PT to the SWNT, the electron transfer rate is estimated to be 1.9 × 10 12 s –1 , which is very close to the transfer rate (2.3 × 10 12 s –1 ) between P3HT and SWNT for P3HT-wrapped SWNTs …”

“…We can compare these relaxation dynamics with the interband relaxation seen in pristine SWNTs; the experimentally determined relaxation time for E 22 excitons to the E 11 exciton state with phonon emission is as short as 40 fs, 43,47 which is 1 order of magnitude shorter than that observed for our hybrid system (0.38 ps). The E 22 −E 11 relaxation time is well explained by theoretical calculations taking into account the electron−phonon coupling via a deformation-potential interaction.…”

Highly selective sorting of semiconducting single wall carbon nanotubes exhibiting light emission at telecom wavelengths

“…The presence of metallic SWNTs (m-SWNTs) in the as-synthesized mixture of SWNTs having different chiralities can be disadvantageous when specific semiconducting electronic properties are needed, as in the case of most optoelectronic applications. The presence of m-SWNTs could be notably detrimental to the optical emission as they quench the photoluminescence (PL) signal of semiconducting SWNTs (s-SWNTs) by means of inter-nanotube relaxation [11][12][13]. m-SWNTs can also degrade carbon nanotube-FET characteristics as they prevent the use of high density nanotube networks by reducing the ON/OFF current ratio to unacceptable values [14][15][16].…”