Geoffrey F. Strouse

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.

Ligand-passivated Eu:Y2O3 nanocrystals as a phosphor for white light emitting diodes.

Eu(III)-doped Y(2)O(3) nanocrystals are prepared by microwave synthetic methods as spherical 6.4 ± 1.5 nm nanocrystals with a cubic crystal structure. The surface of the nanocrystal is passivated by acetylacetonate (acac) and HDA on the Y exposed facet of the nanocrystal. The presence of acac on the nanocrystal surface gives rise to a strong S(0) → S(1) (π → π*, acac) and acac → Ln(3+) ligand to metal charge transfer (LMCT) transitions at 270 and 370 nm, respectively, in the Eu:Y(2)O(3) nanocrystal. Excitation into the S(0) → S(1) (π → π*) or acac → Ln(3+) LMCT transition leads to the production of white light emission arising from efficient intramolecular energy transfer to the Y(2)O(3) oxygen vacancies and the Eu(III) Judd-Ofelt f-f transitions. The acac passivant is thermally stable below 400 °C, and its presence is evidenced by UV-vis absorption, FT-IR, and NMR measurements. The presence of the low-lying acac levels allows UV LED pumping of the solid phosphor, leading to high quantum efficiency (∼19%) when pumped at 370 nm, high-quality white light color rendering (CIE coordinates 0.33 and 0.35), a high scotopic-to-photopic ratio (S/P = 2.21), and thermal stability. In a LED lighting package luminosities of 100 lm W(-1) were obtained, which are competitive with current commercial lighting technology. The use of the passivant to funnel energy to the lanthanide emitter via a molecular antenna effect represents a new paradigm for designing phosphors for LED-pumped white light.

Involvement of the LSPR spectral overlap for energy transfer between a dye and Au nanoparticle.

Nanometal surface energy transfer (NSET) is a molecular ruler technique that has been utilized to optically probe long distances in biomolecular structures. We investigate the useful spectral range of donor dyes and the importance of overlap between the localized surface plasmon resonance (LSPR) and the donor photoluminescence (520-780 nm) and provide a comprehensive study of the R(0) values for the NSET processes from dyes to 2 nm Au NP (gold nanoparticle). The distance-dependent quenching results provide experimental evidence that the efficiency curve slope, R(0) value, and distance of quenching is best modeled as a surface-mediated NSET process analogous to the predictions of Persson-Lang and Chance-Prock-Silbey (CPS). The results show that the LSPR plays a very important role in the observed quenching of excited-state donors at the surface of the nanometal, and the correlation to the NSET model allows a compilation of the necessary biophysical constants for application within the toolbox of biophysics.

Nanosecond exciton recombination dynamics in colloidal CdSe quantum dots under ambient conditions

In solution, CdSe quantum dot exciton recombination is composed of an intrinsic band edge exciton decay and a photoinduced charged exciton, which produces the observation of biexponential decay dynamics. The nearly identical radiative lifetimes of both intrinsic and extrinsic decays scale with the cube of the size, and the nonradiative rate for the intrinsic decay follows the energy gap law, while the charged exciton exhibits strong lattice perturbations arising from Frolich coupling to optical phonons. The charged exciton, which arises from recombination in a particle that has a photo-induced trapped carrier has a temporally resolved Stark shift of ∼20 meV.

Past, present, and future of gold nanoparticles.

Colloidal gold nanoparticles have been around for centuries. Historically, the use of gold nanoparticles has been predominantly found in the work of artists and craftsman because of their vivid visible colors. However, through research, the size, shape, surface chemistry, and optical properties of gold nanoparticles are all parameters which are under control and has opened the doors to some very unique and exciting capabilities. The purpose of this chapter is to review some of the important discoveries and give background in regard to gold nanoparticles. First, the most common wet chemical methods toward their synthesis are reviewed, specifically discussing routes toward spherical colloidal synthesis and controllable rod formation. Next, because many applications of gold nanoparticles are a result of their magnificent interactions with light, some of the basic optical-electronic properties and the physics behind them are elucidated. Finally, by taking advantage of the optical-electronic properties, numerous proven applications for gold nanoparticles are discussed, as well as their predicted applications in the future.

Charge ordering and phase competition in the layered perovskite LaSr{sub 2}Mn{sub 2}O{sub 7}.

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr{sub 2}Mn{sub 2}O{sub 7} by Raman spectroscopy at temperatures as high as 340 K, and with decreasing temperature they become static, forming a charge-ordered (CO) phase below T{sub CO}=210 K. In the static regime, superlattice reflections are observed by neutron and x-ray diffraction with a propagation vector (1/4, -1/4, 0). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than that in the three-dimensional perovskite manganites. Below T{sub N}=170 K, type-A antiferromagnetism (AF) develops and competes with the charge ordering, causing it to eventually melt below T{sup *}=100 K. High-resolution diffraction measurements suggest that the CO and AF states do not coincide within the same region of material, but rather coexist as separate phases. The transition to type-A antiferromagnetism at lower temperatures is characterized by the competition between these two phases. (c) 2000 The American Physical Society.

Photoinduced electron and energy transfer in soluble polymers

Abstract In soluble polystyrene polymers that contain polypyridyl complexes of Ru II or Os II it has been possible to demonstrate the existence of intrastrand photochemical electron and energy transfer.

Involvement of Carriers in the Size-Dependent Magnetic Exchange for Mn:CdSe Quantum Dots

The magnetic behavior for Mn:CdSe (0.6%) quantum dots (QDs) exhibits size-dependent magnetic exchange mediated by the concentration of intrinsic carriers, which arise from surface states. High temperature paramagnetic behavior that can be fit to a Brillouin function with weak low temperature antiferromagnetic (AFM) coupling is observed for the large Mn:CdSe (5.0 and 5.8 nm) QDs. The 2.8 and 4.0 nm Mn:CdSe QDs display a size-independent blocking temperature (T(B)) at 12 K, decreasing coercivity with increasing size, and a lowering of the activation barrier for spin relaxation as the QD is increased in size. The magnetic behavior is inconsistent with classical domain theory behavior for a superparamagnet (SPM) but can be accounted for in a carrier-mediated RKKY model. Fitting the susceptibility data reveals a Pauli-paramagnetic (PPM) component that is believed to arise from the presence of carriers. The carrier density is observed to scale with the surface to volume ratio in the QDs, indicating the carriers arise from surface states that are weakly localized resulting in the onset of long-distance carrier-mediated RKKY exchange inducing overall ferrimagnetism in the Mn:CdSe QDs when the carrier concentration is above a critical threshold.

C-lysine conjugates: pH-controlled light-activated reagents for efficient double-stranded DNA cleavage with implications for cancer therapy

Double-stranded DNA cleavage of light-activated lysine conjugates is strongly enhanced at the slightly acidic pH (<7) suitable for selective targeting of cancer cells. This enhancement stems from the presence of two amino groups of different basicities. The first amino group plays an auxiliary role by enhancing solubility and affinity to DNA, whereas the second amino group, which is positioned next to the light-activated DNA cleaver, undergoes protonation at the desired pH threshold. This protonation results in two synergetic effects which account for the increased DNA-cleaving ability at the lower pH. First, lysine conjugates show tighter binding to DNA at the lower pH, which is consistent with the anticipated higher degree of interaction between two positively charged ammonium groups with the negatively charged phosphate backbone of DNA. Second, the unproductive pathway which quenches the excited state of the photocleaver through intramolecular electron transfer is eliminated once the donor amino group next to the chromophore is protonated. Experiments in the presence of traps for diffusing radicals show that reactive oxygen species do not contribute significantly to the mechanism of DNA cleavage at the lower pH, which is indicative of tighter binding to DNA under these conditions. This feature is valuable not only because many solid tumors are hypoxic but also because cleavage which does not depend on diffusing species is more localized and efficient. Sequence-selectivity experiments suggest combination of PET and base alkylation as the chemical basis for the observed DNA damage. The utility of these molecules for phototherapy of cancer is confirmed by the drastic increase in toxicity of five conjugates against cancer cell lines upon photoactivation.

Stable efficient solid-state white-light-emitting phosphor with a high scotopic/photopic ratio fabricated from fused CdSe-silica nanocomposites.

1 ] Two approaches to the development of solid-state lighting have been successfully implemented in commercially available white-light light-emitting diodes (LEDs). One method is the use of a phosphor conversion diode, in which a photodiode, emitting UV light, excites a phosphor (or group of phosphors), which then emits, singularly or collectively, white light.