Electric-Field Induced Activation of Dark Excitonic States in Carbon Nanotubes.

T. Uda,M. Yoshida,Y. K. Kato,A. Ishii

doi:10.1021/acs.nanolett.5b04595

Abstract

Electrical activation of optical transitions to parity-forbidden dark excitonic states in individual carbon nanotubes is reported. We examine electric-field effects on various excitonic states by simultaneously measuring photocurrent and photoluminescence. As the applied field increases, we observe an emergence of new absorption peaks in the excitation spectra. From the diameter dependence of the energy separation between the new peaks and the ground state of E11 excitons, we attribute the peaks to the dark excited states which became optically active due to the applied field. Field-induced exciton dissociation can explain the photocurrent threshold field, and the edge of the E11 continuum states has been identified by extrapolating to zero threshold.

Highlights

Electrical activation of optical transitions to parityforbidden dark excitonic states in individual carbon nanotubes is reported
The strong Coulomb interactions due to the limited screening in quasi-one-dimensional systems result in optical spectra dominated by tightly bound excitons with binding energies of more than a few hundred meV.[2−5] These excitons have a series of excited states in a manner similar to the Rydberg states in atomic hydrogen, and they have either odd (u) or even (g) parity because of the K and K′ valleys in the momentum space being equivalent.[5−8] Since one-photon transitions require a parity change, the odd excitonic series (1u, 2u, 3u, ...) are bright, while the even excitonic series (1g, 2g, 3g, ...) are dipoleforbidden dark states.[6−8]
These dark states have been studied by two-photon excitation spectroscopy that allows for same-parity transitions,[4,6,8] and photoluminescence (PL) measurements under strong magnetic fields have been used as well.[9−12] It is expected that electric fields cause the dark states to become optically active because of wave function mixing,[13] and it gives rise to interesting phenomena such as exciton dissociation, Stark shift, and Franz−Keldysh oscillations.[14−21] In particular, the activation of the dark states could be a key to developing efficient nanotube-based photodetectors and photovoltaic devices, but a well-controlled experiment has been lacking

Summary

Nano Letters

Electric field dependence of PC and PL excitation spectra are investigated (Figure 2). The 2u states are observed at 200 meV above the 1u states by one-photon measurements, while two-photon excitation measurements have shown that the energy difference between the 2g states and the 1u states is 240 meV.[6] Since our nanotubes are airsuspended and environmental dielectric screening is weaker, enhancement of the energy separation is expected.[31,32] we observe the 2u states and the 2g states (X peaks) at 470 and 540 meV above the 1u states, respectively, for d = 1.00 nm tubes (Figure 4d) These results are consistent with the dielectric constant scaling obtained for air-ambient nanotubes.[5] Note that the X peaks are typically identified at F = 5.0 V/μm where redshifts of about 20 meV have occurred.

■ ACKNOWLEDGMENTS

■ REFERENCES