Ultrafast Photophysics of Single‐Walled Carbon Nanotubes

Abstract

Single-walled carbon nanotubes (SWNTs) are nano-cylinders obtained by wrapping one layer of graphene; due to their very high aspect ratio, they are the prototypical quantum confined one-dimensional systems. The unique mechanical, electronic and optical properties of SWNTs open up transversal application possibilities in many fields of science and technology, with particular emphasis on optoelectronics and photonics. A prerequisite for many of these applications is a thorough understanding of the nature and dynamics of their elementary excitations. This review aims at summarizing the current understanding of the ultrafast photophysics of SWNTs, based on two decades of experimental investigations. After discussing the morphological and electronic properties of SWNTs and introducing the different photogenerated species, we will briefly describe the ultrafast spectroscopic techniques most commonly used for their characterization. We will then present the experimental evidence that has led to establish the nature (singlet and triplet excitons, biexcitons, trions and free charges) and the relaxation pathways of photoexcitations in SWNTs.

Bandgap renormalization in single-wall carbon nanotubes

“…Figure 2 shows the differential transmission spectra (Δ T / T 0 ) at time zero for both measurements. While the excitonic peaks are always characterized by strong PB signals regardless of degenerate or non-degenerate configurations as expected from previous literature 25 , the PA signals are found to be more complicated. A generalizable pattern however is discernible, i.e.…”

“…Furthermore, the ultrafast dynamics of SWNTs offers reliable optical switching mechanism for laser mode-locking 3 , 4 , 10 – 12 . Although there are numerous reports on ultrafast carrier dynamics of carbon nanotubes in the literature 13 – 25 , correlating the experimental results with the underlying physical mechanisms has proved a challenge as it is affected by a number of factors, including SWNT chirality, dielectric medium surrounding the tubes and the experimental configurations for the transient absorption spectroscopy. For example, while ample experimental evidence has confirmed the excitonic nature of optical excitation of SWNTs 26 – 28 , photo-generation of free carriers has been demonstrated by broadband pump-probe experiments 25 , and the actual role of free carriers in the relaxation processes of SWNTs has not been well understood.…”

“…Although there are numerous reports on ultrafast carrier dynamics of carbon nanotubes in the literature 13 – 25 , correlating the experimental results with the underlying physical mechanisms has proved a challenge as it is affected by a number of factors, including SWNT chirality, dielectric medium surrounding the tubes and the experimental configurations for the transient absorption spectroscopy. For example, while ample experimental evidence has confirmed the excitonic nature of optical excitation of SWNTs 26 – 28 , photo-generation of free carriers has been demonstrated by broadband pump-probe experiments 25 , and the actual role of free carriers in the relaxation processes of SWNTs has not been well understood. In addition, theoretical studies indicate that the electron-hot optical phonon coupling can lead to bandgap renormalization effect in carbon-based materials 29 , 30 , however this effect has been largely neglected by previous ultrafast spectroscopy investigations.…”

Ultrafast saturable absorption of large-diameter single-walled carbon nanotubes for passive mode locking in the mid-infrared

“…By the open aperture Z-scan method and degenerate pump-probe absorption measurements, a small saturation fluence of tens of µJ/cm 2 and subpicosecond recovery dynamics are characterized. These findings extend the past knowledge on the ultrafast photoresponse of semiconductor SWCNTs [31][32][33][34] to the mid-IR range. We also study the performance of a mode-locked Cr:ZnS laser using various thickness SWCNT film.…”

Excitons in Carbonic Nanostructures

“…High local dielectric screening of charges needed for formation of loosely bound excitons is the characteristic of low bandgap inorganic semiconductors [210]. However, such type of electron delocalization may exist in ensembles of aromatic hydrocarbons when their surface is exposed to a polar environment, e.g., in short carbon nanotubes that can be fluorescent [211]. It is not clear if this effect may operate in three-dimensional carbon nanostructures.…”