Sign inversion in the terahertz photoconductivity of single-walled carbon nanotube films

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Sign inversion in the terahertz photoconductivity of single-walled carbon nanotube films

Peter Karlsen, Mikhail V. Shuba, Polina P. Kuzhir, Albert G. Nasibulin, Patrizia Lamberti, and Euan Hendry

Phys. Rev. B 98, 241404(R) – Published 3 December 2018

Abstract

In recent years, there have been conflicting reports regarding the ultrafast photoconductive response of films of single walled carbon nanotubes (CNTs), which apparently exhibit photoconductivities that can differ even in sign. Here, we observe explicitly that the THz photoconductivity of CNT films is a highly variable quantity which correlates with the length of the CNTs, while the chirality distribution has little influence. Moreover, by comparing the photoinduced change in THz conductivity with heat-induced changes, we show that both occur primarily due to heat-generated modification of the Drude electron relaxation rate, resulting in a broadening of the plasmonic resonance present in finite-length metallic and doped semiconducting CNTs. This clarifies the nature of the photoresponse of CNT films and demonstrates the need to carefully consider the geometry of the CNTs, specifically the length, when considering them for application in optoelectronic devices.

Received 10 January 2018
Revised 12 October 2018

DOI:https://doi.org/10.1103/PhysRevB.98.241404

Physics Subject Headings (PhySH)

Dielectric properties Photoconductivity Plasmons

Carbon-based materials Nanotubes

Photoexcitation Time-resolved infrared spectroscopy Ultrafast pump-probe spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Peter Karlsen^1,*, Mikhail V. Shuba^2,3,†, Polina P. Kuzhir^2,3, Albert G. Nasibulin^4,5, Patrizia Lamberti⁶, and Euan Hendry^1,‡

¹School of Physics, University of Exeter, Stocker Road, EX4 4QL, United Kingdom
²Institute for Nuclear Problems, Belarus State University, Bobruiskaya 11, 220050 Minsk, Belarus
³Tomsk State University, Lenin Avenue 36, 634050, Tomsk, Russia
⁴Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, Building 3, Moscow 143026, Russia
⁵Department of Applied Physics, Aalto University, School of Science, P.O. Box 15100, FI-00076 Espoo, Finland
⁶Department of Information and Electrical Engineering and Applied Mathematics, University of Salerno, Fisciano (SA), Italy

^*peterkarlsen88@gmail.com
^†mikhail.shuba@gmail.com
^‡E.Hendry@exeter.ac.uk

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Issue

Vol. 98, Iss. 24 — 15 December 2018

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Images

Figure 1
The photoinduced relative change in the THz transmission $Δ E / E$ due to the 800-nm photoexcitation at 300 K of (a) $met-$ and $sem- CNT$ and (b) $l −, m −$ , and $s − CNT$ vs pump-probe delay time $Δ τ$ and normalized by the absorbed photon density $N$ . The incident fluence is $15 μ J / {cm}^{2}$ for all films except $l − CNT$ , where the fluence is $0.7 μ J / {cm}^{2}$ . The full lines are the experimentally obtained data, and the dashed lines are exponential fits. The decay time $τ$ is found to be 1.8, 1.6, 1.9, and 1.9 ps for $sem-, met-, l −$ , and $m − CNT$ , respectively. For $s − CNT$ , an initial fast decay of 0.7 ps is observed, followed by a slow decay of 4.4 ps.
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Figure 2
Effective conductivity (a) $Re (σ)$ and (b) $Im (σ)$ of $met-$ and $sem- CNT$ at 300 K, and change in effective conductivity (c) $Re (Δ σ)$ and (d) $Im (Δ σ)$ due to 800-nm photoexcitation at pump-probe delay time $Δ τ = 1$ ps. The incident fluence is $15 μ J / {cm}^{2}$ . The frequency spacing of the data points correspond to the Nyquist limit of the measurements, and the error bars indicate the standard deviation of each data point. The negative region of the second axis in (a)–(d) have been shaded to highlight the difference in sign of $σ$ and $Δ σ$ .
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Figure 3
Effective conductivity (a) $Re (σ)$ and (b) $Im (σ)$ of $l −, m −$ , and $s − CNT$ at 300 K, as well as a Drude fit of the $l − CNT$ , and change in effective conductivity (c) $Re (Δ σ)$ and (d) $Im (Δ σ)$ due to 800-nm photoexcitation at pump-probe delay time $Δ τ = 1$ ps. The incident fluence is $0.7 μ J / {cm}^{2}$ for $l − CNT$ and $15 μ J / {cm}^{2}$ for $s −$ and $m − CNTs$ . Note that the values of $l − CNT$ have been scaled by $10^{- 1}$ in (a)–(d). The frequency spacing of the data points correspond to the Nyquist limit of the measurements, and the error bars indicate the standard deviation of each data point. Change in effective conductivity $[Re (Δ σ)$ and $Im (Δ σ)]$ of (e) and (f) $l − CNT$ and (g) and (h) $s − CNT$ due to heating from 10–300 K (filled symbols), compared with the same $Δ σ_{p h}$ data as in (c) and (d) (open symbols). Additionally, the black lines in (e)–(h) show the fitted change in conductivity for three simple Lorentzian resonances at resonance frequency $ω_{0} = 2 π \times 10^{- 2}$ (long, full line), $2 π \times 10$ (short, dashed line), and $2 π \times 8$ THz (short, dash-dotted line), respectively, which have been fitted to $Δ σ_{p h}$ for $l − CNT$ and $Δ σ_{p h}$ and $Δ σ_{heat}$ for $s − CNT$ , respectively. These illustrate the difference in $Δ σ$ when increasing the scattering rate $Δ γ > 0$ and when decreasing the resonance frequency $Δ ω_{0} < 0$ . Here, we have chosen $γ = 2 π \times 1.5$ and $2 π \times 50$ THz, for the long and short resonances, respectively, and $Δ γ = 1$ THz and $Δ ω_{0} = - 1$ THz. Note that all data in (e)–(h) have been normalized by the maximum absolute value of each $Δ σ$ in the displayed frequency region to make the overall frequency behavior more comparable.
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