We report on the direct visualization of chirality changes in carbon nanotubes by mapping local changes in resonant RBM phonon frequencies with an optical resolution of 40 nm using near-field Raman spectroscopy. We observe the transition from semiconducting-to-metal and metalto-metal chiralities at the single nanotube level. Our experimental findings, based on detecting changes in resonant RBM frequencies, are complemented by measuring changes in the G-band frequency and line shape. In addition, we observe increased Raman scattering due to local defects associated with the structural transition. From our results, we determine the spatial extent of the transition region to be L trans ∼ 40−100 nm.

We report on chemically specific, subsurface imaging with high spatial resolution. Using tip-enhanced Raman spectroscopy, we probe carbon nanotubes buried beneath a host dielectric media. We demonstrate our ability to map and resolve specific vibrational modes with 30 nm spatial resolution for dielectric layers with different thicknesses.

In the newly designed photoswitchable electron transfer compound 2, our previously published donor−bridge−acceptor system (anthracene−CH2−bithiophene−pyridinium) is modified by incorporation of the photoisomerizable dithienylethene (DTE) as a switching unit. In the open-ring form 2a, excitation of the anthracene donor leads to an intramolecular charge separation proved by identification of the anthracene radical cation in transient absorption spectra (ON state). After photocyclization to the closed-ring isomer (2a → 2b), the intramolecular charge separation is completely suppressed (OFF state). The reversibility of the ON/OFF switching is verified. From UV−vis absorption spectra and cyclovoltammetric studies it is deduced that in the open-ring isomer 2a (ON state) conjugation is markedly restricted within the dithienylethene bridge, whereas in the closed-ring isomer 2b (OFF state) the conjugation is extended over the whole dithienylethenepyridinium (DTEP) subunit. Consequently, in 2 an enlarged conjugation within the bridge is not decisive for photoinduced charge separation. Instead the observed transfer properties can be understood by thermodynamic aspects.

We studied spatially isolated single-walled carbon nanotubes (SWNTs) immobilized in a quasi-planar optical λ/2-microresonator using confocal microscopy and spectroscopy. The modified photonic mode density within the resonator is used to selectively enhance or inhibit different Raman transitions of SWNTs. Experimental spectra are presented that exhibit single Raman bands only. Calculations of the relative change in the Raman scattering cross sections underline the potential of our microresonator for the optical control of nonequilibrium phonon populations in SWNT.The control of the vibrational degrees of freedom or, in other words, the phonon populations in molecular nanostructures like single-walled carbon nanotubes (SWNT) is one of the major goals in nanoscience. 1,2 The electronic structure of SWNTs, see, e.g., refs 3 and 4, is highly susceptible to electron-phonon interactions, and nonequilibrium phonon populations can drastically alter device properties. Hot phonon electron scattering, for example, is known to saturate the charge transport along SWNTs.5-14 As a result, the application of SWNTs in electronic and optoelectronic devices, e.g., 15 could be limited. In the following, we demonstrate the potential of a planar microresonator for the optical control of nonequilibrium phonon populations in SWNTs, i.e., phonon populations that are driven out of thermal equilibrium as imposed by the modified photonic mode density in the presence of the microresonator. Moreover, we describe a strategy for controlling hot phonons in electrically driven SWNTs.The efficiency of spontaneous radiative transitions in optical λ/2 microresonators, i.e., resonators with mirror spacings L of one-half emission wavelength λ of embedded emitters, is determined by the local photonic mode density F cav (L) that can be enhanced and inhibited with respect to the corresponding value F 0 in free (nonconfined) space; see, e.g., ref 16 and references therein. Accordingly, both luminescence and Raman scattering processes of embedded emitters can be controlled by enhancement and inhibition of the spontaneous emission rate Γ cav 17,18 or the Raman scattering cross section σ cav , 19,20 respectively, since both scale with the photonic mode density following 21 In Raman scattering processes the radiative transition is accompanied by the creation (destruction) of a phonon. Hence, it should be possible to populate (depopulate) a specific phonon mode in an embedded SWNT using microresonator-controlled Stokes (anti-Stokes) Raman scattering.In Figure 1a, a schematic of the microresonator is shown. The sample was prepared from a poly(methyl methacrylate) (PMMA)/dichloroethane solution containing purified SWNT with a diameter distribution ranging from 0.8 to 1.8 nm (BuckyUSA). A droplet of the solution was enclosed between two silver mirrors. After evaporation of the dichloroethane, the PMMA film fixed the two mirrors and exhibited immobilized SWNT structures spatially separated in the order of 1 µm. Light passing the microresonator in the λ /2-...

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