Energy transfer from an individual quantum dot to a carbon nanotube

A detailed understanding of energy transduction is crucial for achieving precise control of energy flow in complex, integrated systems. In this context, carbon nanotubes (CNTs) are intriguing model systems due to their rich, chirality-dependent electronic and optical properties. Here, we study the quenching of fluorescence from isolated quantum dots (QDs) upon approach of individual CNTs attached to atomic force microscope probes. Precision measurements of many different CNT/QD pairs reveal behavior consistent with resonant energy transfer between QD and CNT excitons via a Fohrster-like dipole-dipole coupling. The data reveal large variations in energy transfer length scales even though peak efficiencies are narrowly distributed around 96%. This saturation of efficiency is maintained even when energy transfer must compete with elevated intrinsic non-radiative relaxation rates during QD aging. These observations suggest that excitons can be created at different locations along the CNT length, thereby resulting in self-limiting behavior.Comment: 8 pages, 8 figures, with supplementary informatio

Tip-enhanced fluorescence microscopy of high-density samples

Journal ArticleHigh-density samples of fluorescent quantum dots (QDs) were imaged using an apertureless near-field optical microscopy technique. QD fluorescence was modulated by oscillating a silicon atomic force microscope tip above an illuminated sample and a lock-in amplifier was used to suppress background from the excitation laser. Spatial resolution near 10 nm and a peak signal-to-noise ratio (SNR) of ~60 were achieved. Individual QDs within high-density ensembles were still easily resolved (SNR>5) at a density of 14 QDs/µm2. These results have favorable implications for the eventual nanoscale imaging of viable biological systems, such as cellular membranes

Improved localization accuracy in stochastic super-resolution fluorescence microscopy by K-factor image deshadowing

pre-printLocalization of a single fluorescent particle with sub-diffraction-limit accuracy is a key merit in localization microscopy. Existing methods such as photoactivated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM) achieve localization accuracies of single emitters that can reach an order of magnitude lower than the conventional resolving capabilities of optical microscopy. However, these techniques require a sparse distribution of simultaneously activated fluorophores in the field of view, resulting in larger time needed for the construction of the full image. In this paper we present the use of a nonlinear image decomposition algorithm termed K-factor, which reduces an image into a nonlinear set of contrast-ordered decompositions whose joint product reassembles the original image. The K-factor technique, when implemented on raw data prior to localization, can improve the localization accuracy of standard existing methods, and also enable the localization of overlapping particles, allowing the use of increased fluorophore activation density, and thereby increased data collection speed. Numerical simulations of fluorescence data with random probe positions, and especially at high densities of activated fluorophores, demonstrate an improvement of up to 85% in the localization precision compared to single fitting techniques. Implementing the proposed concept on experimental data of cellular structures yielded a 37% improvement in resolution for the same super-resolution image acquisition time, and a decrease of 42% in the collection time of super-resolution data with the same resolution

Fluorescence near-field microscopy of DNA at Sub-10A nm resolution

Journal ArticleWe demonstrate apertureless near-field microscopy of single molecules at sub-10 nm resolution. With a novel phase filter, near-field images of single organic fluorophores were obtained with ~sixfold improvement in the signal-to-noise ratio. The improvement allowed pairs of molecules separated by ~15 nm to be reliably and repeatedly resolved, thus demonstrating the first true Rayleigh resolution test for near-field images of single molecules. The potential of this technique for biological applications was demonstrated with an experiment that measured the helical rise of A-form DNA

Effect of magnetic Gd impurities on the superconducting state of amorphous Mo-Ge thin films with different thickness and morphology

pre-printWe studied the effect of magnetic doping with Gd atoms on the superconducting properties of amorphous Mo70Ge30 films. We observed that in uniform films deposited on amorphous Ge, the pair-breaking strength per impurity strongly decreases with film thickness initially and saturates at a finite value in films with thickness below the spin-orbit scattering length. The variation is likely caused by surface-induced magnetic anisotropy and is consistent with the fermionic mechanism of superconductivity suppression. In thin films deposited on SiN the pair-breaking strength becomes zero. Possible reasons for this anomalous response are discussed. The morphological distinctions between the films of the two types were identified using atomic force microscopy with a carbon nanotube tip

Nanoscale fluorescence imaging using a single-wall carbon nanotube

Journal ArticleA single-wall carbon nanotube attached to an AFM probe is used for near-field optical imaging of 20 nm diameter fluorescent spheres and 5 nm diameter CdSe quantum dots

Nanoscale fluorescence microscopy using carbon nanotubes

Journal ArticleWe demonstrate the first reported use of single-walled carbon nanotubes as nano-optical probes in apertureless nearfield fluorescence microscopy. We show that, in contrast to silicon probes, carbon nanotubes always cause strong fluorescence quenching when used to image dye-doped polystyrene spheres and Cd-Se quantum dots. For quantum dots, the carbon nanotubes induce very strong near-field contrast with a spatial resolution of ∼20 nm. Images of dye-doped spheres exhibit crescent-shaped artifacts caused by distortions in the surface water layer found in ambient conditions

Phase-contrast imaging of atomic superfluid state in degenerate gas of 6Li atoms

Journal ArticleThe recent experimental realizations of Bose-Einstein condensation in trapped ultracold atomic gases composed of bosonic alkali atoms has generated a broad interest in studying different properties of these degenerate atomic Bose systems. Although the degenerate atomic gas composed of fermionic atoms has not yet been demonstrated, theoretical study has predicted that the degenerate Fermi gas can experience BCS-like phase transition to an atomic superfluid state.

Optimizing contrast of tip-enhanced fluorescence microscopy for imaging high-density samples

Journal ArticleQuantum dots are imaged using tip-enhanced fluorescence microscopy. Optimization of the operation parameters leads to high-contrast images of high-density samples and a novel photon analysis improves contrast further