The development of a reversible chemical sensor based on a CdSe/ZnS nanocrystal (NC) is described. Signal transduction is accomplished by fluorescence resonance energy transfer (FRET) between the NC and a fluorescent pH-sensitive squaraine dye attached to the surface of the NC. The efficiency of FRET, and consequently the relative intensity of NC and dye emissions, is modulated with the pH-dependent absorption cross section of the squaraine dye. The design of a NC sensor based on FRET results in a ratiometric sensor since the emission intensities of dye and NC may be referenced to the isosbestic point between NC and dye emissions. The ratiometric approach allows sensing to be performed, regardless of issues surrounding collection efficiency (scattering environment, light fluctuations, etc.) and dye:NC loadings.

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The reactivity of triplet 16-electron organometallic species has been studied in room-temperature solution using femtosecond UV pump IR probe spectroscopy. Specifically, the Si-H bond-activation reaction of photogenerated triplet Fe(CO)(4) and triplet CpCo(CO) with triethylsilane has been characterized and compared to the known singlet species CpRh(CO). The intermediates observed were studied using density functional theory (DFT) as well as ab initio quantum chemical calculations. The triplet organometallics have a greater overall reactivity than singlet species due to a change in the Si-H activation mechanism, which is due to the fact that triplet intermediates coordinate weakly at best with the ethyl groups of triethylsilane. Consequently, the triplet species do not become trapped in alkyl-solvated intermediate states. The experimental results are compared to the theoretical calculations, which qualitatively reproduce the trends in the data.

Semiconductor nanocrystals (NCs) or quantum dots (QDs) show great promise for use in QD-LED (quantum dot lightemitting device) displays, owing to their unique optical properties and the continual development of new core and core-shell structures to meet specific color needs. [1][2][3][4][5][6][7][8][9][10] This in combination with the recent development of more efficient and saturated QD-LEDs as well as new QD-LED fabrication techniques, [11,12] suggests that QD-LEDs have the potential to become an alternative flat-panel display technology. The ideal red, green, and blue emission spectrum of an LED for a display application should have a narrow bandwidth and a wavelength such that its color coordinates on the Commission Internationale de lEclairage (CIE) chromaticity diagram lie outside the current National Television System Committee (NTSC) standard color triangle (see Figure 2). For a Gaussian emission spectrum with a full width at half maximum (FWHM) of 30 nm and a maximized perceived power, the optimal peak wavelength for display applications is l = 610-620 nm for red, l = 525-530 nm for green, and l = 460-470 nm for blue. For the red pixels, wavelengths longer than l = 620 nm become difficult for the human eye to perceive, while those shorter than l = 610 nm have coordinates that lie inside the standard NTSC color triangle. Optimization of wavelength for the blue pixels follows the same arguments as for the red pixel, but at the other extreme of the visible spectrum. For green pixels, l = 525-530 nm provides a color triangle with the largest area on the CIE chromaticity diagram (and therefore the largest number of colors accessible by a display). Wavelengths longer than l = 530 nm make some of the blue/green area of the triangle inaccessible. Wavelengths shorter than l = 525 nm compromise the yellow display emissions.To date, efficient red-emitting QD-LEDs with a peak emission wavelength optimized for display applications have been realized using (CdSe)ZnS core-shell NCs, [11,13] while blue QD-LEDs with a peak wavelength of emission optimized for display applications have been realized with a (CdS)ZnS core-shell material.[10] To date, although efficient green-emitting core-shell semiconductor NCs that emit at l = 525 nm have been synthesized, they have not been successfully incorporated into a QD-LED suitable for display applications. Previous work using (CdSe)ZnS core-shell NCs gave QD-LEDs that emit at wavelengths no shorter than l = 540-560 nm. [13,14] Using (CdSe)ZnS core-shell NCs to achieve l = 525 nm emission requires making small CdSe cores ( % 2.5 nm in diameter). [15,16] Such small CdSe semiconductor NCs can be difficult to synthesize with narrow size distributions and high quantum efficiencies, and are also more difficult to process and overcoat with a higher-band-gap inorganic semiconductor, which is necessary for incorporation into solid-state structures. A core-shell composite, rather than an organically passivated NC, is desirable in a solid-state QD-LED device owing to the enhanced photolum...