Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

http://science.sciencemag.org/content/sci/294/5546/1488.full.pdf

Spintronics: a spin-based electronics vision for the future.

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

Semiconductor nanocrystals were prepared for use as fluorescent probes in biological staining and diagnostics. Compared with conventional fluorophores, the nanocrystals have a narrow, tunable, symmetric emission spectrum and are photochemically stable. The advantages of the broad, continuous excitation spectrum were demonstrated in a dual-emission, single-excitation labeling experiment on mouse fibroblasts. These nanocrystal probes are thus complementary and in some cases may be superior to existing fluorophores.

https://science.sciencemag.org/content/sci/281/5385/2013.full.pdf

Semiconductor Nanocrystals as Fluorescent Biological Labels

The size trend for the pressure-induced gamma-Fe2O3(maghemite) to alpha-Fe2O3 (hematite) structural phase transition in nanocrystals has been observed. The transition pressure was found to increase with decreasing nanocrystal size: 7 nm nanocrystals transformed at 272GPa, 5 nm at 343GPa and 3 nm at 372GPa. Annealing of a bulk sample of gamma-Fe2O3 was found to reduce the transition pressure from 352 to242GPa. The bulk modulus was determined to be 2626GPa for 7 nm nanocrystals of gamma-Fe2O3, which is significantly higher than for the value of 1906 GPa that we measured for bulk samples. For alpha-Fe2O3, the bulk moduli for 7 nm nanocrystals (3365) and bulk (30030) were found to be almost the same within error. The bulk modulus for the gamma phase was found to decrease with decreasing particle size between 10 and 3.2 nm particle size. Values for the ambient pressure molar volume were found within 1 percent to be: 33.0 cm3/mol for bulk gamma-Fe2O3, 32.8 cm3/mol for 7 nm diameter gamma-Fe2O3 nanocrystals, 30.7 cm3/mol for bulk alpha-Fe2O3 and 30.6 cm3/mol for alpha-Fe2O3 nanocrystals.

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Size dependence of the pressure-induced γ to α structural phase transition in iron oxide nanocrystals

We present experimental evidence that silver nanoparticles in the size range of 5-10 nm undergo a reversible structural transformation under hydrostatic pressures up to 10 GPa We have used x-ray diffraction with a synchrotron light source to investigate pressure-dependent and size-dependent trends in the crystal structure of silver nanoparticles in a hydrostatic medium compressed in a diamond-anvil cell Results suggest a reversible linear pressure-dependent rhombohedral distortion which has not been previously observed in bulk silver We propose a mechanism for this transition that considers the bond-length distribution in idealized multiply twinned icosahedral particles To further support this hypothesis, we also show that similar measurements of single-crystal platinum nanoparticles reveal no such distortions

/pdf/structural-distortions-in-5-10-nm-silver-nanoparticles-under-5aizhofbuj.pdf

Structural distortions in 5-10 nm silver nanoparticles under high pressure

We have prepared simple photovoltaic devices based on composite materials formed by mixing cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer poly(2-methoxy, 5-(2′-ethyl)-hexyloxy-p-phenylenevinylene) (MEH-PPV). When the surface of the nanocrystals is treated so as to remove the surface ligand, we find that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nanocrystal interface. Transmission electron microscopy (TEM) of these quantum dot/conjugated polymer composites shows clear evidence for phase segregation with length scales in the range 10–200 nm, providing a large area of interface for charge separation to occur. Thin-film photovoltaic devices using the composite materials show quantum efficiencies which are significantly improved over those for pure polymer devices, consistent with improved charge separation. At high concentrations of nanocrystals, where both the nanocrystal and polymer components provide continuous pathways to the electrodes, we find quantum efficiencies of up to 12%. The absorption, charge separation and transport properties of the composites can be controlled by changing the size, material and surface ligands of the nanocrystals.We have prepared simple photovoltaic devices based on composite materials formed by mixing cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer poly(2-methoxy, 5-(2′-ethyl)-hexyloxy-p-phenylenevinylene) (MEH-PPV). When the surface of the nanocrystals is treated so as to remove the surface ligand, we find that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nanocrystal interface. Transmission electron microscopy (TEM) of these quantum dot/conjugated polymer composites shows clear evidence for phase segregation with length scales in the range 10–200 nm, providing a large area of interface for charge separation to occur. Thin-film photovoltaic devices using the composite materials show quantum efficiencies which are significantly improved over those for pure polymer devices, consistent with improved charge separation. At high concentrations of nanocrystals, where both the nanocrystal and polymer components provide continuous pathways t...

A CdSe nanocrystal/MEH-PPV polymer composite photovoltaic

In between the molecular and bulk forms of matter, semiconductor nanocrystals are novel materials with interesting optical and electronic properties. We present a study of the homogeneous optical properties of two nanocrystal systems. First, the homogeneous absorption of InP nanocrystals is studied via hole burning experiments. The optical spectrum consists of a HOMO-LUMO transition with a 10 meV width and a second electronic transition shifted by 0.11 eV. The optical transitions are assigned within a three valence-band model. The CdS/HgS/CdS quantum-dot/quantum-well system is also investigated and a transmission electron microscopy study shows that the growth of the HgS well region and the CdS outer layer is epitaxial. Selective optical techniques are used to study the electronic level structure. In hole burning, a discrete transition (width of 7 meV) with pronounced phonon side bands at a frequency of 250 cm−1 is observed. In fluorescence, the line narrowed spectrum also shows phonon replicas a...

Homogeneous Optical Properties of Semiconductor Nanocrystals

We have used synchrotron radiation photoemission to probe the valence and core level electronic structure of compound‐semiconductor monodisperse clusters (nanocrystals). These clusters exhibited a 10% or less variation relative to the mean diameter and were attached to the metal substrates via alkane chains. Direct evidence of gap broadening due to size variation in CdS clusters was observed. The novel utilization of alkane chain attachment is the key to eliminating the otherwise debilitating problem of sample charging, as occurs with powders. The quality of sample preparation was confirmed by other methods such as transmission electron microscopy, Raman scattering, and x‐ray diffraction. This work provides a direct link between photoemission studies of expitaxial ultrathin films of compound semiconductors, the photon‐spectroscopy measurements of cluster powders and the existing theories of quantum confinement in reduced dimensionality structures.

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A. P. Alivisatos

Papers

Size dependence of the pressure-induced γ to α structural phase transition in iron oxide nanocrystals

Structural distortions in 5-10 nm silver nanoparticles under high pressure

A CdSe nanocrystal/MEH-PPV polymer composite photovoltaic

Homogeneous Optical Properties of Semiconductor Nanocrystals

Photoemission investigation of compound semiconductor monodisperse clusters