Visualizing the Local Optical Response of Semiconducting Carbon Nanotubes to DNA-Wrapping

Abstract

We studied the local optical response of semiconducting single-walled carbon nanotubes to wrapping by DNA segments using high resolution tip-enhanced near-field microscopy. Photoluminescence (PL) near-field images of single nanotubes reveal large DNA-wrapping-induced red shifts of the exciton energy that are two times higher than indicated by spatially averaging confocal microscopy. Near-field PL spectra taken along nanotubes feature two distinct PL bands resulting from DNA-wrapped and unwrapped nanotube segments. The transition between the two energy levels occurs on a length scale smaller than our spatial resolution of about 15 nm.Semiconducting single-walled carbon nanotubes (SWNTs) as photoluminescent quasi-one-dimensional systems have attracted enormous scientific interest and have large potential for various applications in photonics and opto-and nanoelectronics. 1-3 Photoluminescence (PL) of nanotubes results from exciton recombination and occurs in the near-infrared spectral range with emission energies controlled mainly by the nanotube structure (n,m). [4][5][6][7][8] Since nanotubes consist of surface atoms only, the detected emission energy is very sensitive to the nanotube environment, making them promising candidates for sensing applications. 9 At present, the influence of the environment is described by its relative dielectric constant ε influencing exciton binding energies but also renormalizing the band gap through charge carrier screening. 10-14 As a result, the emission energy of nanotubes is modulated by the dielectric constant, which can be expected to be nonuniform along nanotubes, leading to nonuniform emission energies in single nanotube measurements. 8,15,16 The use of DNA for hybridization of carbon nanotube sidewalls has facilitated sorting nanotubes and building chemical sensors. 9,[17][18][19] Single-strand DNA-wrapping introduces DNA segments with finite length, while the details of the secondary DNA structure will be determined by a complex interplay between π-π stacking interactions between DNA bases and nanotube surface as well as electrostatic interactions of the phosphate backbone. [20][21][22] The effect of helical wrapping by the charged DNA backbone was modeled by applying a helical potential causing symmetry breaking of the nanotube electronic structure and small energetic shifts for semiconducting nanotubes (0.01 meV in water). 23,24 It is well-known that DNA-wrapping red-shifts the PL energy depending on the nanotube chirality by several tens of meV compared to the values reported for micelleencapsulated nanotubes in aqueous solution, 7,25,26 which can be attributed to an increasing ε. 14 The surface coverage with DNA segments of finite length is expected to result in a nonuniform dielectric environment along the nanotubes. 25 Limited by diffraction, the PL information collected in confocal microscopy contains the optical response from a nanotube length of about 300 nm, which is far too large to clarify details of individual DNA-nanotube interactions. Tipenhanced n...

Probing Exciton Localization in Single-Walled Carbon Nanotubes Using High-Resolution Near-Field Microscopy

“…Compared to the average PL energy of DNA-wrapped (9,1)-SWNTs on substrate of 1.332 eV, the localized PL observed in Figure 3d with energies down to 1.310 eV is exceptionally red-shifted. 9 This can account for the strong degree of localization in Figure 3a. To support our model of exciton diffusion and localization, we complemented our experiment with numerical simulations.…”

Raman Spectroscopy in Graphene-Based Systems: Prototypes for Nanoscience and Nanometrology

“…The main challenge for TERS is the development of new and robust instrumentation for achieving a ready-to-use system. TERS in sp 2 nanocarbons started focusing on experimental work [23][24][25][26][27][28][29][30], but recently theoretical studies are appearing in both one-dimensional (SWNTs) [116] and two-dimensional (graphene) [117] materials, thus guiding the way for future developments.…”

“…Due to their low dimensionality and huge optical response, carbons nanotubes started to be widely used as a prototype for the development of TERS. Some results achieved in carbon nanotubes were nanoscale vibrational analysis [25], nanoscale optical imaging of excitons [26], TERS polarization measurements [27], imaging of nanotube chirality changes [28], spectral determination of singlecharged defects [29], and local optical response of semiconducting nanotubes to DNA wrapping [30], among others. A comprehensible review for TERS in carbon nanotubes can be found in [31].…”

Exciton Energy Transfer in Pairs of Single-Walled Carbon Nanotubes

“…The two dashed arrows mark beginning and end of the (9,1) nanotube determined from the simultaneously detected G-band Raman signal (not shown Figure 2, respectively, such shifts can also be explained by nonuniform DNA-wrapping as found for single DNAwrapped nanotubes. 27 Emission from the (9,1) nanotube (Figure 2a) occurs in four bright segments, while emission from the (6,5) nanotube (Figure 2b) is more extended for about 1 µm. This can also be seen in the intensity profiles shown in Figure 2e taken along the two dashed lines in Figure 2a,b.…”