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

Here, we will first briefly summarize the general principles of QD synthesis using our previous work on InP as an example. Then we will focus on QDs of the IV-VI Pb chalcogenides (PbSe, PbS, and PbTe) and Si QDs because these were among the first QDs that were reported to produce multiple excitons upon absorbing single photons of appropriate energy (a process we call multiple exciton generation (MEG)). We note that in addition to Si and the Pb-VI QDs, two other semiconductor systems (III-V InP QDs(56) and II-VI core-shell CdTe/CdSe QDs(57)) were very recently reported to also produce MEG. Then we will discuss photogenerated carrier dynamics in QDs, including the issues and controversies related to the cooling of hot carriers and the magnitude and significance of MEG in QDs. Finally, we will discuss applications of QDs and QD arrays in novel quantum dot PV cells, where multiple exciton generation from single photons could yield significantly higher PV conversion efficiencies.

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Semiconductor quantum dots and quantum dot arrays and applications of multiple exciton generation to third-generation photovoltaic solar cells.

Publisher Summary Quantum transport is conveniently studied in a two-dimensional electron gas (2DEG) because of the combination of a large Fermi wavelength and large mean free path. Semiconductor nanostructures are unique in offering the possibility of studying quantum transport in an artificial potential landscape. This is the regime of ballistic transport in which scattering with impurities are neglected. The chapter presents a self-contained account of these three novel transport regimes in semiconductor nanostructures. The study of quantum transport in semiconductor nanostructures is motivated by more than scientific interest. The fabrication of nanostructures relies on sophisticated crystal growth and lithographic techniques that exist because of the industrial effort toward the miniaturization of transistors. Conventional transistors operate in the regime of classical diffusive transport, which breaks down on short length scales. The discovery of novel transport regimes in semiconductor nanostructures provides options for the development of innovative future devices.

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Quantum Transport in Semiconductor Nanostructures

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

Blueshifts in the photoluminescence emission energies from an ensemble of self-assembled InAs quantum dots are observed as a result of postgrowth thermal annealing. Enhancement of the integrated photoluminescence emission and narrowing of the full width half-maxima (from 55 to 12 meV) occur together with blueshifts up to 300 meV at annealing temperatures up to 950 °C. Evidence that the structures remain as dots comes form the observation of level filling and photoluminescence excitation studies which reveal LO phonon peaks occurring at multiples of ∼30 meV from the detection energies.

Tuning self-assembled InAs quantum dots by rapid thermal annealing

We have investigated the exciton kinetics in self‐assembled InAs quantum dots and ultrathin quantum wells grown by molecular beam epitaxy on (001) oriented GaAs substrates At low temperatures, the photoluminescence decay time of the quantum wells increases almost linearly while the decay time of the quantum dot system is independent of temperature However, above 50 K there is a linear increase in the decay time of the dots which may be due to electrons escaping into the wetting layer or occupation of nonradiative exciton states Under the conditions of high injection, relaxation from the excited states has a time constant of about 500 ps

Time resolved study of self‐assembled InAs quantum dots

We present results obtained from a scanning transmission-electron microscopy study of InAs/GaAs quantum dot (QD) layers. It is shown that the QD's are embedded within an ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ confining layer following overgrowth with GaAs. Using energy dispersive x-ray analysis (EDX) the QD dimensions can be measured with reasonable accuracy and are not affected by strain contrast. In QD bilayers where the dots are uncorrelated along the growth direction, a comparison of the indium EDX signals from the confining layer and a dot allow us to estimate the compositions of these regions as ${\mathrm{In}}_{0.07}{\mathrm{Ga}}_{0.93}\mathrm{As}$ and ${\mathrm{In}}_{0.31}{\mathrm{Ga}}_{0.69}\mathrm{As},$ respectively.

Scanning transmission-electron microscopy study of inas/gaas quantum dots

We have investigated the growth conditions necessary to achieve strong room temperature emission at 13 µm for InAs/GaAs self-assembled quantum dots (QDs) using conventional solid source molecular beam epitaxy (MBE) A relatively high substrate temperature and very low growth rate (LGR) result in long wavelength emission with a small linewidth of only 24 meV Atomic Force Micrographs obtained from uncapped samples reveal several differences between the LGRQDs and those grown at higher growth rates The former are larger, more uniform in size and their density is lower by a factor of about 4 LGRQDs have been incorporated in p-i-n structures and strong room temperature electroluminescence detected The light output of the QD p-i-n diodes is found to be significantly higher than a quantum well (QW) sample at least for current densities up to 05 kAcm-2

https://iopscience.iop.org/article/10.1143/JJAP.38.528/pdf

1.3 µm room temperature emission from InAs/GaAs self-assembled quantum dots

The authors have grown (Al,Ga)As/GaAs two-dimensional electron gas (2DEG) structures with lightly doped regions of (Al,Ga)As and superlattices in the undoped GaAs. Using this technique, they have obtained ultralow density (2*1010 cm-2), high-mobility samples where phonon scattering at 4 K is the dominant factor in limiting the mobility; reducing the measurement temperature to below 1.5 K gives mobilities of up to 107 cm2 V-1 s-1.

https://iopscience.iop.org/article/10.1088/0268-1242/4/7/016/pdf

C. Roberts

Papers

Tuning self-assembled InAs quantum dots by rapid thermal annealing

Time resolved study of self‐assembled InAs quantum dots

Scanning transmission-electron microscopy study of inas/gaas quantum dots

1.3 µm room temperature emission from InAs/GaAs self-assembled quantum dots

Optimisation of (Al,Ga)As/GaAs two-dimensional electron gas structures for low carrier densities and ultrahigh mobilities at low temperatures