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.

Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

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Quantum dot solar cells

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Plasmonics is a research area merging the fields of optics and nanoelectronics by confining light with relatively large free-space wavelength to the nanometer scale - thereby enabling a family of novel devices. Current plasmonic devices at telecommunication and optical frequencies face significant challenges due to losses encountered in the constituent plasmonic materials. These large losses seriously limit the practicality of these metals for many novel applications. This paper provides an overview of alternative plasmonic materials along with motivation for each material choice and important aspects of fabrication. A comparative study of various materials including metals, metal alloys and heavily doped semiconductors is presented. The performance of each material is evaluated based on quality factors defined for each class of plasmonic devices. Most importantly, this paper outlines an approach for realizing optimal plasmonic material properties for specific frequencies and applications, thereby providing a reference for those searching for better plasmonic materials.

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Searching for better plasmonic materials

Searching for Better Plasmonic Materials

This paper reports the effect of homogeneous broadening of the optical gain on lasing spectra in self-assembled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dot lasers. We measured the current-output power characteristics and light-emission spectra for columnar-shaped self-assembled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum-dot lasers that show lasing from the ground state. While lasing occurred with a narrow line including a few longitudinal modes at room temperature, spectra at 80 K showed broad-band lasing emission over a range of 50--60 meV. Based on a coupled set of rate equations taking into account both the size distribution of quantum dots and a series of longitudinal cavity modes, we could clearly explain the measured temperature dependence of lasing spectra in terms of homogeneous broadening of optical gain of a single quantum dot. Dots with different energies start lasing independently at 80 K due to their spatial localization as well as a \ensuremath{\delta}-function-like gain, while at room temperature the dot ensemble contributes to a narrow-line lasing collectively via the homogeneous broadening of optical gain. The magnitude of the homogeneous broadening was found to reach 16 to 19 meV at room temperature, which corresponds to the polarization dephasing of 70 to 80 fs. We present a model on the dephasing process. This paper is a demonstration of the effect of homogeneous broadening on quantum-dot laser characteristics.

Effect of homogeneous broadening of optical gain on lasing spectra in self-assembled In x Ga 1 − x A s / G a A s quantum dot lasers

We grew 1.3-μm emitting self-formed In 0.5 Ga 0.5 As quantum dots on GaAs substrates by supplying InAs and GaAs monolayers alternately during atomic layer epitaxy. The dots were 20 nm in diameter and 10 nm in height, and were surrounded by In 0.5 Ga 0.9 As in the lateral direction and by GaAs perpendicular to the dots. Diamagnetic energy shifts of excitons in the dots clearly demonstrated three-dimensional quantum confinement

Self-Formed $\bf In_{0.5}Ga_{0.5}As$ Quantum Dots on GaAs Substrates Emitting at $\bf 1.3\,{\mbi {\mu}}m$

We demonstrate the first 1.3-/spl mu/m continuous-wave (CW) lasing at room temperature of self-assembled InGaAs-GaAs quantum dots. High-density 1.3-/spl mu/m emission dots were successfully formed by the combination of low-rate growth and InGaAs-layer overgrowth methods of molecular beam epitaxy. The 1.3-/spl mu/m ground-level CW lasing occurred at up to 40/spl deg/C, and the threshold current of 8 mA at 25/spl deg/C is less than one thirtieth of values ever reported for 1.3-/spl mu/m dot pulse lasers. The achievement represents a milestone for creating quantum-dot lasers applicable to fiber-optic communication system.

1.3-μm CW lasing of InGaAs-GaAs quantum dots at room temperature with a threshold current of 8 mA

The effect of phonon bottleneck on quantum-dot laser performance is examined by solving the carrier-photon rate equations including the carrier relaxation process into the quantum-dot ground state. We show that the retarded carrier relaxation due to phonon bottleneck degrades the threshold current and the external quantum efficiency. We also show that quantum-dot lasers are quite sensitive to the crystal quality outside as well as inside quantum dots. Our results clarified that the relaxation lifetime should be less than about 10 ps to fully utilize the laser potential originating from the quantum-dot discrete energy states.

Effect of phonon bottleneck on quantum-dot laser performance

We studied the excitation‐power dependence of photoluminescence (PL) spectra and the current dependence of electroluminescence (EL) spectra of self‐formed InGaAs/GaAs quantum dots. We observed five peaks in the PL and EL spectra. From the size dependence of the peak interval and the carrier‐density dependence of peak energy and peak intensity, we assigned the peaks to the discrete energy levels in the quantum dots. We simulated the EL spectra with rate equations, taking into account the carrier relaxation between the discrete levels, and found that the relaxation lifetime was about 10–100 ps.

Kohki Mukai

Papers

Effect of homogeneous broadening of optical gain on lasing spectra in self-assembled In x Ga 1 − x A s / G a A s quantum dot lasers

Self-Formed $\bf In_{0.5}Ga_{0.5}As$ Quantum Dots on GaAs Substrates Emitting at $\bf 1.3\,{\mbi {\mu}}m$

1.3-μm CW lasing of InGaAs-GaAs quantum dots at room temperature with a threshold current of 8 mA

Effect of phonon bottleneck on quantum-dot laser performance

Emission from discrete levels in self‐formed InGaAs/GaAs quantum dots by electric carrier injection: Influence of phonon bottleneck