Colloidal core/shell nanocrystals contain at least two semiconductor materials in an onionlike structure. The possibility to tune the basic optical properties of the core nanocrystals, for example, their fluorescence wavelength, quantum yield, and lifetime, by growing an epitaxial-type shell of another semiconductor has fueled significant progress on the chemical synthesis of these systems. In such core/shell nanocrystals, the shell provides a physical barrier between the optically active core and the surrounding medium, thus making the nanocrystals less sensitive to environmental changes, surface chemistry, and photo-oxidation. The shell further provides an efficient passivation of the surface trap states, giving rise to a strongly enhanced fluorescence quantum yield. This effect is a fundamental prerequisite for the use of nanocrystals in applications such as biological labeling and light-emitting devices, which rely on their emission properties. Focusing on recent advances, this Review discusses the fundamental properties and synthesis methods of core/shell and core/multiple shell structures of II-VI, IV-VI, and III-V semiconductors.

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Core/Shell semiconductor nanocrystals.

The potent analgesic effects of cannabis-like drugs and the presence of CB1-type cannabinoid receptors in pain-processing areas of the brain and spinal cord, indicate that endogenous cannabinoids such as anandamide may contribute to the control of pain transmission within the central nervous system (CNS). Here we show that anandamide attenuates the pain behaviour produced by chemical damage to cutaneous tissue by interacting with CB1-like cannabinoid receptors located outside the CNS. Palmitylethanolamide (PEA), which is released together with anandamide from a common phospholipid precursor, exerts a similar effect by activating peripheral CB2-like receptors. When administered together, the two compounds act synergistically, reducing pain responses 100-fold more potently than does each compound alone. Gas-chromatography/mass-spectrometry measurements indicate that the levels of anandamide and PEA in the skin are enough to cause a tonic activation of local cannabinoid receptors. In agreement with this possibility, the CB1 antagonist SR141716A and the CB2 antagonist SR144528 prolong and enhance the pain behaviour produced by tissue damage. These results indicate that peripheral CB1-like and CB2-like receptors participate in the intrinsic control of pain initiation and that locally generated anandamide and PEA may mediate this effect.

Control of pain initiation by endogenous cannabinoids

We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminesce...

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state (∼10 5 cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening. Stark shifts of the lowest excited state more than two orders of magnitude larger than the linewidth were observed, suggesting the potential use of these dots in electro-optic modulation devices.

Quantum Confined Stark Effect in Single CdSe Nanocrystallite Quantum Dots

Biexcitons in strongly confined, colloidal CdSe quantum dots were investigated with excitonic state selectivity combined with 10 fs temporal precision. Within the first 50 fs, the first excited state of the biexciton was observed. By 100 ps, mixed character biexcitons were observed, comprised of a core exciton and a surface trapped exciton. The size dependence of the biexciton binding energies is reported for these specific biexcitons. Analysis of the spectral signatures of each biexcitonic state yields a quantitative measure of enhanced excited state trapping rates at the surface of the quantum dots. By comparing the biexcitonic signals to the state-filling signals, we show that it is primarily the holes which are trapped at the interface on the 100 ps time scale.

State-resolved studies of biexcitons and surface trapping dynamics in semiconductor quantum dots.

Size dependent hole dynamics are measured in colloidal CdSe quantum dots for a specific state-to-state excitonic transition. These experiments show that the hole energy loss rate increases for smaller quantum dots, contradicting known relaxation mechanisms for holes. These experiments reveal a new mechanism for hole relaxation in colloidal quantum dots which circumvents the expected phonon bottleneck for holes. The data are consistent with a nonadiabatic surface channel as the dominant pathway for hole relaxation in colloidal semiconductor quantum dots.

Breaking the Phonon Bottleneck for Holes in Semiconductor Quantum Dots

Size dependent electron and hole relaxation dynamics were measured in colloidal CdSe quantum dots with state-to-state specificity. These experiments reveal the electron and hole state-to-state relaxation dynamics with a precision of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, allowing quantitative evaluation of the manifold of pathways by which an exciton relaxes in strongly confined quantum dots. These experiments corroborate previously observed confinement induced femtosecond Auger relaxation channels for electrons, but with sufficient precision to quantitatively and unambiguously determine the size dependence of the Auger mechanism. These experiments also show that the hole energy loss rate increases for smaller quantum dots, contradicting known relaxation mechanisms for holes. We propose a confinement enhanced mechanism for hole relaxation in colloidal quantum dots, overcoming the predicted phonon bottleneck for holes. The relative contributions of the relaxation pathways are identified for electrons and for holes. These state selective experiments produce a unified picture of the manifold of relaxation pathways available to both electrons and holes in strongly confined colloidal quantum dots.

Unified picture of electron and hole relaxation pathways in semiconductor quantum dots

Size dependent, state-resolved studies of exciton-phonon couplings in strongly confined semiconductor quantum dots

Femtosecond carrier relaxation dynamics in colloidal CdSe quantum dots are measured which focus on the early time dynamics for different initial excitonic states. Selecting the initial excitonic states produces a clear influence upon the early time dynamics. The difference between the dynamics when initially populating different excitonic states provides a direct measurement of excitonic relaxation times in strongly confined colloidal semiconductor quantum dots with specificity to electrons and holes. These results furthermore show state specific biexciton interactions.

Eva A. Dias

Papers

State-resolved studies of biexcitons and surface trapping dynamics in semiconductor quantum dots.

Breaking the Phonon Bottleneck for Holes in Semiconductor Quantum Dots

Unified picture of electron and hole relaxation pathways in semiconductor quantum dots

Size dependent, state-resolved studies of exciton-phonon couplings in strongly confined semiconductor quantum dots

State-to-state exciton dynamics in semiconductor quantum dots