The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Suitable labels are at the core of Luminescence and fluorescence imaging and sensing. One of the most exciting, yet also controversial, advances in label technology is the emerging development of quantum dots (QDs)--inorganic nanocrystals with unique optical and chemical properties but complicated surface chemistry--as in vitro and in vivo fluorophores. Here we compare and evaluate the differences in physicochemical properties of common fluorescent labels, focusing on traditional organic dyes and QDs. Our aim is to provide a better understanding of the advantages and limitations of both classes of chromophores, to facilitate label choice and to address future challenges in the rational design and manipulation of QD labels.

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Quantum dots versus organic dyes as fluorescent labels

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...

http://wiki.phy.queensu.ca/shughes/images/d/d1/Quantum_Dot_Solar_Cells._Semiconductor_Nanocrystals_as_Light_Harvesters.pdf

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters

We report the synthesis and characterization of highly luminescent colloidal nanocrystals consisting of CdSe cores protected with double inorganic shells (core−shell−shell nanocrystals). The outer ZnS shell provides efficient confinement of electron and hole wave functions inside the nanocrystal as well as high photochemical stability. Introducing the middle shell (CdS or ZnSe) sandwiched between CdSe core and ZnS outer shell allows considerable reducing strain inside nanocrystals because CdS and ZnSe have the lattice parameter intermediate to those of CdSe and ZnS. In contrast to CdSe/ZnS core−shells, in the core−shell−shell nanocrystals ZnS shell grows nearly defect free. Due to high quality of the ZnS shell, the core−shell−shell nanocrystals exhibit PL efficiency and photostability exceeding those of CdSe/ZnS nanocrystals. Preferential growth of the middle CdS shell in one crystallographic direction allows engineering the shape and luminescence polarization of the core−shell−shell nanocrystals.

CdSe/CdS/ZnS and CdSe/ZnSe/ZnS Core−Shell−Shell Nanocrystals

As an expansion to the wet chemical route for the preparation of quantum-sized II−VI semiconductor materials, a series of thiol-capped crystalline CdSe nanoparticles has been synthesized in aqueous solution using mercapto- alcohols (2-mercaptoethanol, 1-thioglycerol), and mercapto acids (thioglycolic acid, thiolactic acid) as stabilizers The smaller (app 14−22 nm diameter) CdSe particles were obtained using thioalcohols as capping agents; the use of thioacids as stabilizers produced larger (21−32 nm diameter) CdSe particles CdSe nanoparticles were separated from the crude solutions as redissolvable powder samples with narrow size distributions using a size-selective fractionation and have been characterized by UV−vis absorption and photoluminescence spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, and energy-dispersive X-ray analysis A calculation of the HOMO−LUMO gap of CdSe particles as a function of their size has been done using an extended effective mass appr

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Synthesis and Characterization of a Size Series of Extremely Small Thiol-Stabilized CdSe Nanocrystals

Optical fibers have revolutionized the telecommunications industry to such an extent that the network capacity available today was unthinkable 20 years ago. Even so, with the advent of the datawave, and the exponential increase of network traffic predicted to continue indefinitely, the generation of bandwidth remains a challenge. One of the major limitations to the implementation of future high-capacity, ultra-broadband optical networks is the expansion of the fiber bandwidth beyond that available from the current state-of-the-art signal amplification device—the erbium-doped fiber amplifier (EDFA). Although there is currently a large effort to expand the flat-gain bandwidth of the EDFA, most of these efforts involve sophisticated engineering, exotic glass fibers, or multicomponent cascaded systems. In a radically different approach, we are attempting to use the unique properties of semiconductor nanocrystals, or quantum dots, as "designer atoms" in order to produce an ultra-broadband optical amplifier with complete coverage of the telecommunications wavelengths. In this paper we review the synthesis of thiol-stabilized mercury chalcogenide nanocrystals via an aqueous colloidal route, which demonstrate extremely intense photoluminescence all the way across the spectral region of interest, i.e., from 1000 to over 1700 nm.

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Colloidal nanocrystals for telecommunications. Complete coverage of the low-loss fiber windows by mercury telluride quantum dot

The current state-of-the-art of colloidal II-VI nanocrystal formation using the aqueous/thiol synthesis route is described. Work on single component and heterostructures and mixed compound quantum dots is discussed. The purpose of the work is to provide a range of infrared (IR)-emitting materials with high quantum efficiency (QE) as potential gain media for future ultrawideband optical amplifiers for high-capacity wavelength-division multiplexing (WDM) telecommunications systems. Physical and chemical factors influencing particle sizes are described.

A. Kornowski

Papers

CdSe/CdS/ZnS and CdSe/ZnSe/ZnS Core−Shell−Shell Nanocrystals

Synthesis and Characterization of a Size Series of Extremely Small Thiol-Stabilized CdSe Nanocrystals

Colloidal nanocrystals for telecommunications. Complete coverage of the low-loss fiber windows by mercury telluride quantum dot

Development of IR-emitting colloidal II-VI quantum-dot materials

Strongly Photoluminescent CdTe Nanocrystals by Proper Surface Modification.