Strain-induced band gap shifts that depend strongly on the chiral angle have been observed by optical spectroscopy in single-walled carbon nanotubes (SWCNTs). Uniaxial and torsional strains are generated by changing the environment surrounding the SWCNTs, using the surrounding D 2 O ice temperature or the hydration state of a wrapping polymer. These methods are used as diagnostic tools to determine the quantum number q and examine chiral vector indices for specific nanotubes. Single-walled

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... n nanotubes (SWCNTs) offer a range of potential applications based on their unique one-dimensional properties combined with a variety of different electronic states that depend on the tube structure. Theoretical calculations [1] [2] [3] show that the chiral vector indices (n; m) determine whether individual tubes are essentially metallic or semiconducting. The graphene wrapping condition n ÿ m 3p q generates metallic (or very small gap semiconducting) tubes for q 0, while semiconducting tubes are found for q 1. The recent development of techniques such as polymer wrapping [4] and nanotube isolation with micelles [5] have allowed the optical properties of spectrally resolved single nanotube species to be studied and led to the observation of band gap luminescence [6, 7] . These techniques ensure that the nanotubes remain isolated in solution and prevent their aggregation into bundles, which quenches their luminescence. As a result, there is now a growing body of evidence to suggest that it is possible to assign a specific set of chiral indices to describe the properties of particular nanotubes. Modifications of the band gap by mechanical deformation also offer a route to characterize the tubes. In previous work, the conductance of some randomly sampled individual SWCNTs has been measured under atomic force microscope -assisted deformation [8, 9] . The results were interpreted on the basis of theoretical predictions [10] that the effect of strain is dependent on the chirality and the quantum number q. In our work, large straininduced shifts of the band gaps have been observed by changing the nanotube environment, by changing the temperature of the surrounding D 2 O ice or the hydration state of the wrapping polymer. These can be used as diagnostic tools to determine the quantum number q and examine chiral vector indices for specific nanotube species, allowing the chiral index assignments that have been proposed in the literature [6, 11] to be tested. Using the strain-induced band gap shifts that have been predicted theoretically [10, 12] we are able to estimate the magnitude of the strain components for the nanotubes induced by the environmental changes. It has been calculated that SWCNTs possess negative linear thermal expansion coefficients for temperatures below 900 K and the linear thermal contraction is estimated to be 0:1% from 80 to 260 K [13]. In contrast, D 2 O ice expands by 0:6% over the same temperature range [14] . It is likely therefore that a combination of uniaxial tension and torsional stretching can be produced in icesurrounded nanotubes by simply lowering the temperature. A similar strain might also be applied to polymerwrapped SWCNTs through conformational changes produced by the contraction of the polymer through drying. We have carried out photoluminescence measurements for PVP (polyvinylpyrrolidone)-wrapped SWCNTs formed from micelle-dispersed aqueous solutions as a function of temperature (4.2 to 300 K) and hydration state (drying from solution). Purified SWCNTs were purchased from Carbon Nanotechnologies which were synthesized by the high pressure CO conversion (HiPCO) method and consist of tubes with a broad band gap distribution. An aqueous surfactant suspension containing individual SWCNTs was prepared by ultracentri- FIG. 1 (color online) . A data map of E 22 excitation-E 11 emission using the semiempirical formula from Bachilo et al. [6] . Three photoluminescence spectra of SDS-suspended SWCNT solution measured at room temperature are shown using selective excitation with 633, 720, and 810 nm radiation.