We investigate graphene and graphene layers on different substrates by monochromatic and white-light confocal Rayleigh scattering microscopy. The image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. For a few layers (<6), the monochromatic contrast increases linearly with thickness. The data can be adequately understood by considering the samples behaving as a superposition of single sheets that act as independent two-dimensional electron gases. Thus, Rayleigh imaging is a general, simple, and quick tool to identify graphene layers, which is readily combined with Raman scattering, that provides structural identification.

We show that strong photoluminescence (PL) can be induced in single-layer graphene using an oxygen plasma treatment. The PL is spatially uniform across the flakes and connected to elastic scattering spectra distinctly different from those of gapless pristine graphene. Oxygen plasma can be used to selectively convert the topmost layer when multilayer samples are treated.

We have studied the exciton recombination dynamics of individual (6,4) and (6,5) single-walled carbon nanotubes embedded in aqueous gels or deposited on glass surfaces. CoMoCat nanotubes systematically display short monoexponential photoluminescence (PL) decays presumably due to defects introduced during their synthesis. In contrast HiPco nanotubes can either display mono-or biexponential PL decays depending on the environmental conditions. Transition from bi-to monoexponential decays can be reproduced by a simple three level model taking into account defect-dependent nonradiative decay mechanisms.The photoluminescence (PL) properties of semiconducting single walled carbon nanotubes (SWNTs) have attracted much attention over the last years.1 These properties strongly depend on the structure of each nanotube 2 but also on extrinsic factors resulting for instance from synthesis or environmental factors. 3-7As a result, PL studies performed on ensemble of SWNTs are affected by inhomogeneities that hinder the development of a detailed understanding of the underlying mechanisms. Experiments on individual SWNTs remove part of this heterogeneity by allowing description of the characteristics of each SWNT in its particular environment. 8 Comparisons between values of a physical parameter extracted from different experimental reports are however difficult because, generally, the studied samples differ by synthesis methods or preparation procedures. For example PL decays performed on individual (6,4) and (6,5) nanotubes by different groups have resulted in distinct behaviors, ranging from very short monoexponential decays to longer biexponential ones. 5,7,[9][10][11] This paper aims at understanding this large disparity in PL decay behaviors. We experimentally confirm that PL decays of (6,5) and (6,4) nanotubes can exhibit either mono-or biexponential behaviors and show that these depend on synthesis methods and nanotube environment. These observations are explained by a previous simple three-level model 10 taking now into account the defect-dependent dominant nonradiative decay mechanisms proposed by Pereibenos et al. 12The SWNTs used in this study were either synthesized using HiPco or CoMoCat methods. The nanotubes were dispersed in aqueous suspensions of the anionic surfactants sodium deoxycholate (DOC). For observations, single-molecule wide-field and confocal PL microscopes were used to image SWNTs excited with a continuous wave laser. The SWNTs were immobilized in aqueous agarose gels (5 wt %) or spin-coated on surfaces. SWNTs concentration was kept well below 1 µm -3 such that bright individual nanotubes could be optically resolved. The PL of these bright tubes was sent to a spectrometer for further spectral identifications of individual (6,4) or (6,5) nanotubes. Typically, isolated SWNTs feature narrow PL lines from their bright excitonic state E 11 , with full width at half-maximum (fwhm) in the range of ∼17-22 nm. Following identification of these bright individual nanotubes, the excitation modality was switche...

We show that new low-energy photoluminescence (PL) bands can be created in the spectra of semiconducting single-walled carbon nanotubes by intense pulsed excitation. The new bands are attributed to PL from different nominally dark excitons that are "brightened" because of a defect-induced mixing of states with different parity and/or spin. Time-resolved PL studies on single nanotubes reveal a significant reduction of the bright exciton lifetime upon brightening of the dark excitons. The lowest-energy dark state has longer lifetimes and is not in thermal equilibrium with the bright state.Because of their exceptional electronic properties, singlewalled carbon nanotubes (SWNTs) are promising candidates for future nanoelectronic and biosensing applications, as well as narrow-band nanoscale emitting and detecting devices. [1][2][3][4] Excitons are identified to dominate the absorption and PL emission properties of these one-dimensional systems. 5,6 Enhanced electron-electron interactions due to the reduced dimensionality lead to exceptionally large binding energies of the excitons that are shown to exist even in metallic SWNTs. 7 Theory predicts a manifold of excitonic bands below the free electron-hole continuum of semiconducting SWNTs. [8][9][10][11] Besides the optically active odd-parity singlet excitons, additional even-parity dipole-forbidden singlet excitons as well as triplet excitons are expected to occur. Most importantly, some of these bands form nonemissive states that are lower in energy than the lowest bright state. This complex sequence of excitonic states, and the nonradiative relaxation channels associated with them, presumably have an important impact on the low PL quantum yield of SWNTs and fast exciton decay rates. 12-14 Therefore, detailed studies of the properties of dark excitonic states are essential both for full understanding of the excited-state dynamics and for engineering of the optical properties of the SWNTs.The direct experimental proof for the existence of dark excitonic states in SWNTs was presented, applying twophoton photoluminescence excitation spectroscopy 5,6 and measurements of magnetic brightening of the SWNT PL. 15 Low-energy forbidden states were also used to explain the dynamics observed in pump-probe experiments 16,17 and the temperature dependence of PL intensities. 18 In addition, recent results on ensemble 19 and individual nanotube samples 20 have shown low-energy satellite peaks in the PL spectra. These peaks were attributed to emission from lowlying dark excitonic states, while the mechanisms that enable optically forbidden transitions and the interplay between bright and dark excited states remain to be clarified.In this Letter, we report on the creation of low-energy emission bands in the PL spectra of individual (5,4) and (6,4) SWNTs upon high-power pulsed-laser irradiation at room temperature. The persistence of these bands in subsequent low-power measurements indicates that irreversible distortions of the nanotube structure efficiently "brighten" forbidd...

We studied the exciton decay dynamics of individual semiconducting single-walled carbon nanotubes at room temperature using time-resolved photoluminescence spectroscopy. The photoluminescence decay from nanotubes of the same ͑n , m͒ type follows a single exponential decay function, however, with lifetimes varying between about 1 and 40 ps from nanotube to nanotube. A correlation between broad photoluminescence spectra and short lifetimes was found and explained by defects promoting both nonradiative decay and vibronic dephasing. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2913009͔Based on their exceptional optical properties, singlewalled carbon nanotubes ͑SWNT͒ will eventually play an important role as nanometer-scale building blocks for optoelectronics, nanoelectronics, and biosensing. 1-3 Experimental and theoretical studies confirmed the identification of the photoluminescent state as being excitonic in nature with extremely large exciton binding energies. 4,5 The excited state energies and dynamics of SWNT attracted particular scientific interest motivated by their unique one-dimensional structure combining intriguing optical and transport properties. To date, a large number of ensemble studies using pump-probe and time-resolved photoluminescence ͑PL͒ spectroscopy exist, reporting on mono-or multiexponential relaxation dynamics with decay times ranging from few to several tens of picoseconds for different sample materials. 4,6,7 Temperature dependent PL measurements suggest that the excited state relaxation reflects a complex interplay between excitonic states of different parity that are optically bright or dark depending on their accessibility from the ground state. 8,9 At present, excited state lifetimes are thought to be limited by fast transitions to these dark states but also by quenching related to defect related trap states. Transitions between excitonic states of different parity are expected to require symmetry breaking defects or environmental perturbations. 10 Defects and environmental coupling are spatially localized by nature and a unique property of a given nanotube. As a result, ensemble measurements will always represent an averaging. Previous single nanotube PL measurements revealed a distribution of lifetimes for a single nanotube chirality ͑6,4͒ ranging from sub 20 to 180 ps with an average value of 57 ps at 87 K. 11 These studies were limited to low temperatures because of the time resolution of the utilized system. In this paper, we report on the first time-resolved PL measurements of individual ͑6,4͒ and ͑6,5͒ SWNT at room temperature. PL transients extending over more than four orders of magnitude were found to be monoexponential with lifetimes ranging from about 1 to 40 ps.Single nanotube measurements were performed using an inverted confocal microscope in combination with electronics for time-correlated single-photon counting. Laser excitation is provided by a femtosecond Ti:sapphire laser operating at 760 nm and a repetition rate of 76 MHz. A high numerical aperture obje...

The temporal evolution of photoluminescence in individual single-walled carbon nanotubes (SWNT) under strong laser irradiation is studied and pronounced blinking and bleaching is observed, caused by photoinduced oxidation that subsequently quenches mobile excitons. The nanotubes are isolated with sodium cholate and spun onto either a glass or mica surface. Their bleaching behavior is investigated for variable laser intensities in air and argon atmosphere. The decay rate for luminescence bleaching generally increases with higher laser intensity, however saturating on mica substrates, which is attributed to limited availability of oxygen in the vicinity of the nanotubes. Step-like events in the luminescence time traces corresponding to single oxidation events are analyzed regarding relative step height and suggest an exciton diffusion range of about 105 nm.

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...

No abstract