Steven K. Buratto

Quantum correlation among photons from a single quantum dot at room temperature

Maxwells equations successfully describe the statistical properties of fluorescence from an ensemble of atoms or semiconductors in one or more dimensions. But quantization of the radiation field is required to explain the correlations of light generated by a single two-level quantum emitter, such as an atom, ion or single molecule. The observation of photon antibunching in resonance fluorescence from a single atom unequivocally demonstrated the non-classical nature of radiation. Here we report the experimental observation of photon antibunching from an artificial system—a single cadmium selenide quantum dot at room temperature. Apart from providing direct evidence for a solid-state non-classical light source, this result proves that a single quantum dot acts like an artificial atom, with a discrete anharmonic spectrum. In contrast, we find the photon-emission events from a cluster of several dots to be uncorrelated.

EXTERNAL QUANTUM EFFICIENCY OF SINGLE POROUS SILICON NANOPARTICLES

We use a combination of single nanoparticle luminescence and scanning force microscopy to determine the quantum efficiency (QE) of single porous Si nanoparticles and to determine the ratio of luminescent nanoparticles deposited on a silica surface to the total nanoparticles. An estimate of the QE of bulk porous Si based on these data compares favorably to the QE measured experimentally. From this we conclude that the 1% QE of bulk porous Si measured experimentally results primarily from a statistical distribution of high QE quantum-confined Si chromophores.

Room-temperature fluorescence characteristics of single dye molecules adsorbed on a glass surface

Measurements of the total fluorescence and fluorescence spectra from single carbocyanine dye molecules (DiIC12) as a function of time reveal a wide range of phenomena. Discrete jumps in the fluorescence intensity from single molecules on a glass surface have been observed with correlation times spanning several orders of magnitude (1 ms–10 s). We propose a model for these fluctuations in which two or more ground state potential minima are accessed by twists of the chromophore backbone that alter the quantum efficiency of emission as well as the emission spectrum. Monte Carlo simulations based on this model are shown which qualitatively match experimental data. In addition, we observed emission spectra which range in shape from narrow, well-separated vibronic bands to a broad, featureless band. The distribution of emission parameters from different molecules, not obtained from ensemble measurements, indicates an abundance of distinct nanoenvironments of the glass surface sampled by the adsorbed molecules.

Single-molecule detection fluorescence of surface-bound species in vacuum

Abstract We have performed single-molecule fluorescence detection of DiIC 12 molecules adsorbed on a solid substrate where the surrounding pressure is controlled. The triplet lifetimes are significantly longer (4–100 ms) and far more easily observed at moderate vacuum pressures than in air (typically

Imaging InGaAsP quantum-well lasers using near-field scanning optical microscopy

The application of near-field scanning optical microscopy (NSOM) to the characterization of InGaAsP multiquantum-well lasers is reported. Collection mode images are collected at varying drive currents from well below to well above the threshold current. The high resolution of NSOM (∼λ/20) provides a detailed mapping of the laser output from the active region as well as additional output from the surrounding mesa. Spectral analysis of the image shows that the extra emission is due to InP electroluminescence. In addition to the emission characteristics of the laser it is also possible to detect local heating of the laser facet via thermal expansion. Topographical images are achieved simultaneously with NSOM images by digitizing the feedback signal which maintains a constant tip-surface gap. It is shown that these data have direct implications on device performance and problems associated with carrier leakage and nonradiative defects.

Near-field photoconductivity: Application to carrier transport in InGaAsP quantum well lasers

A new contrast method in near-field scanning optical microscopy in which the near-field probe is used to excite photocurrent in a semiconductor sample is described and demonstrated. The use of near-field optics results in an order-of-magnitude improvement in spot size and a fivefold improvement in resolution over previous methods of photocurrent imaging. The application of this near-field photoconductivity technique to a multiquantum well laser provides direct visualization of carrier transport throughout the structure, yielding information on growth inhomogeneities, carrier leakage and isolation, and the overall quality of p-n junctions.

Ion mobility spectrometry reveals the mechanism of amyloid formation of Aβ(25-35) and its modulation by inhibitors at the molecular level: epigallocatechin gallate and scyllo-inositol.

Amyloid cascades leading to peptide β-sheet fibrils and plaques are central to many important diseases. Recently, intermediate assemblies of these cascades were identified as the toxic agents that interact with the cellular machinery. The relationship between the transformation from natively unstructured assembly to the β-sheet oligomers to disease is important in understanding disease onset and the development of therapeutic agents. Research on this early oligomeric region has largely been unsuccessful since traditional techniques measure only ensemble average oligomer properties. Here, ion mobility methods are utilized to deduce the modulation of peptide self-assembly pathways in the amyloid-β protein fragment Aβ(25-35) by two amyloid inhibitors (epigallocatechin gallate and scyllo-inositol) that are currently in clinical trials for Alzheimers Disease. We provide evidence that suppression of β-extended oligomers from the onset of the conversion into β-oligomer conformations is essential for effective attenuation of β-structured amyloid oligomeric species often associated with oligomer toxicity. Furthermore, we demonstrate the ease with which ion mobility spectrometry-mass spectrometry can guide the development of therapeutic agents and drug evaluation by providing molecular level insight into the amyloid formation process and its modulation by small molecule assembly modulators.

Landing of size-selected Agn+ clusters on single crystal TiO2 (110)-(1×1) surfaces at room temperature

Mass-selected Ag(n) (+) (n=1,2,3) clusters with impact energy less than 2 eV per atom were deposited from the gas phase onto rutile titania (110)-(1x1) single crystal surfaces at room temperature and imaged using ultra-high vacuum scanning tunneling microscopy. Upon reaching the surface, Ag monomers sintered to form three-dimensional islands of approximately 50 atoms in size, with an average measured height of 7.5 A and diameter of 42 A. This suggests that the monomers are highly mobile on the titania surface at room temperature. Dimers also sintered to form large clusters upon deposition, approximately 30 atoms in size, with an average height of 6.2 A and diameter of 33 A. Clusters formed from monomer deposition appeared approximately three times more frequently at step edges than clusters formed from dimer deposition, indicating that the surface mobility of deposited monomers is higher than that of deposited dimers. In sharp contrast to the deposition of monomers and dimers, the deposition of trimers resulted in a high density of very small clusters on the order of a few atoms in size, indicative of intact trimers on the surface, implying that deposited trimers have very limited mobility on the surface at room temperature.

Amyloid β-Protein Assembly and Alzheimer’s Disease: Dodecamers of Aβ42, but Not of Aβ40, Seed Fibril Formation

Evidence suggests that oligomers of the 42-residue form of the amyloid β-protein (Aβ), Aβ42, play a critical role in the etiology of Alzheimers disease (AD). Here we use high resolution atomic force microscopy to directly image populations of small oligomers of Aβ42 that occur at the earliest stages of aggregation. We observe features that can be attributed to a monomer and to relatively small oligomers, including dimers, hexamers, and dodecamers. We discovered that Aβ42 hexamers and dodecamers quickly become the dominant oligomers after peptide solubilization, even at low (1 μM) concentrations and short (5 min) incubation times. Soon after (≥10 min), dodecamers are observed to seed the formation of extended, linear preprotofibrillar β-sheet structures. The preprotofibrils are a single Aβ42 layer in height and can extend several hundred nanometers in length. To our knowledge this is the first report of structures of this type. In each instance the preprotofibril is associated off center with a single layer of a dodecamer. Protofibril formation continues at longer times, but is accompanied by the formation of large, globular aggregates. Aβ40, by contrast, does not significantly form the hexamer or dodecamer but instead produces a mixture of smaller oligomers. These species lead to the formation of a branched chain-like network rather than discrete structures.

Mesoscale optical properties of conjugated polymers probed by near-field scanning optical microscopy

Abstract Near-field scanning optical microscopy (NSOM) has been used to study the mesoscale optical properties of oriented and non-oriented free-standing thin films of poly( p -phenylene vinylene) (PPV). We report a strong correlation between film morphology and the optical properties on a 100 nm scale. The non-oriented PPV thin film exhibits mesoscale domains of local molecular orientation as well as areas which are characterized by a low photoluminescence efficiency. Upon stretch orientation, PPV thin films show average polymer chain orientation parallel to the stretch axis as well as distinct areas of perpendicular molecular orientation correlated to film topography.