Elastic Properties of Crystalline–Amorphous Core–Shell Silicon Nanowires

The pressure behavior of Raman frequencies and line widths of crystalline core-amorphous shell silicon nanowires (SiNWs) with two different core-to-shell ratio thicknesses was studied at pressures up to 8 GPa. The obtained isothermal compressibility (bulk modulus) of SiNWs with a core-to-shell ratio of about 1.8 is ∼20% higher (lower) than reported values for bulk Si. For SiNWs with smaller core-to-shell ratios, a plastic deformation of the shell was observed together with a strain relaxation. A significant increase in the full width at half-maximum of the Raman LTO-peak due to phonon decay was used to determine the critical pressure at which LTO-phonons decay into LO + TA phonons. Our results reveal that this critical pressure in strained core–shell SiNWs (∼4 GPa) is different from the reported value for bulk Si (∼7 GPa), whereas no change is observed for relaxed core–shell SiNWs

Effect of Catalyst Pretreatment on Chirality-Selective Growth of Single-Walled Carbon Nanotubes

We show that catalyst pretreatment conditions can have a profound effect on the chiral distribution in single-walled carbon nanotube chemical vapor deposition. Using a SiO₂-supported cobalt model catalyst and pretreatment in NH₃, we obtain a comparably narrowed chiral distribution with a downshifted tube diameter range, independent of the hydrocarbon source. Our findings demonstrate that the state of the catalyst at the point of carbon nanotube nucleation is of fundamental importance for chiral control, thus identifying the pretreatment atmosphere as a key parameter for control of diameter and chirality distributions

Homogeneously Alloyed CdSe_1–<i>x</i>S_<i>x</i> Quantum Dots (0 ≤ <i>x</i> ≤ 1): An Efficient Synthesis for Full Optical Tunability

Homogeneously Alloyed CdSe_1–<i>x</i>S_<i>x</i> Quantum Dots (0 ≤ <i>x</i> ≤ 1): An Efficient Synthesis for Full Optical Tunabilit

Electronic Structure and Exciton–Phonon Interaction in Two-Dimensional Colloidal CdSe Nanosheets

We study the electronic structure of ultrathin zinc-blende two-dimensional (2D)-CdSe nanosheets both theoretically, by Hartree-renormalized k·p calculations including Coulomb interaction, and experimentally, by temperature-dependent and time-resolved photoluminescence measurements. The observed 2D-heavy hole exciton states show a strong influence of vertical confinement and dielectric screening. A very weak coupling to phonons results in a low phonon-contribution to the homogeneous line-broadening. The 2D-nanosheets exhibit much narrower ensemble absorption and emission linewidths as compared to the best colloidal CdSe nanocrystallites ensembles. Since those nanoplatelets can be easily stacked and tend to roll up as they are large, we see a way to form new types of multiple quantum wells and II–VI nanotubes, for example, for fluorescence markers

Interfacial Alloying in CdSe/CdS Heteronanocrystals: A Raman Spectroscopy Analysis

We investigate the interface between core and shell in zinc blende CdSe-based CdSe/CdS dot-in-dot heteronanocrystals. Using X-ray diffraction and transmission electron microscopy, we show that a CdS shell grows coherently around the CdSe core. A comparison of the Raman spectrum of bare CdSe nanocrystals and CdSe/CdS heteronanocrystals indicates that the difference in lattice constant leads to compressive and tensile strain in core and shell, respectively. Concomitant continuum mechanical calculations follow this result, yet the calculated strain exceeds the experimental values. Moreover, a detailed analysis of the CdSe/CdS Raman spectra reveals the appearance of additional features upon shell growth. A comparison with pure Cd­(Se,S) alloyed nanocrystals relates these features to alloy vibrations. We show that these observations point toward the presence of a mixed Cd­(Se,S) layer at the CdSe/CdS interface. In this way, this work provides an experimental framework based on Raman spectroscopy to analyze in detail interfacial alloying in heteronanocrystals

Radical Initiated Reactions on Biocompatible CdSe-Based Quantum Dots: Ligand Cross-Linking, Crystal Annealing, and Fluorescence Enhancement

Cross-linking of biocompatible ligand shells significantly improves the stability of nanocrystals in the biological environment. We report a detailed spectroscopic study of radical initiated reactions on poly­(isoprene)-<i>b</i>-poly­(ethelene glycol) encapsulated CdSe/CdS/ZnS core–shell–shell quantum dots. It was found that the radicals not only initiate cross-linking of the polyisoprene moieties but also may anneal the nanocrystal surfaces and improve their crystallinity

Tunable Plasmon Coupling in Distance-Controlled Gold Nanoparticles

Plasmons are resonant excitations in metallic films and nanoparticles. For small enough static distances of metal nanoparticles, additional plasmon-coupled modes appear as a collective excitation between the nanoparticles. Here we show, by combining poly­(<i>N</i>-isopropylacrylamide) micro- and nanospheres and Au nanoparticles, how to design a system that allows controllably and reversibly switching on and off, and tuning the plasmon-coupled mode

“Flash” Synthesis of CdSe/CdS Core–Shell Quantum Dots

We report on the “flash” synthesis of CdSe/CdS core–shell quantum dots (QDs). This new method, based on a seeded growth approach and using an excess of a carboxylic acid, leads to an isotropic and epitaxial growth of a CdS shell on a wurtzite CdSe core. The method is particularly fast and efficient, allowing the controllable growth of very thick CdS shells (up to 6.7 nm in the present study) in no more than 3 min, which is considerably shorter than in previously reported methods. The prepared materials present state-of-the-art properties with narrow emission and high photoluminescence quantum yields, even for thick CdS shells. Additionally, Raman analyses point to an alloyed interface between the core and the shell, which, in conjunction with the thickness of the CdS shell, results in the observed considerable reduction of the blinking rate

Electronic and Vibrational Properties of Diamondoid Oligomers

We analyzed the vibrational and electronic properties of diamondoid oligomers via resonance Raman spectroscopy. The compounds consist of lower diamondoids such as adamantane or diamantane that are interconnected with double bonds. Therefore, all oligomers have ethylene-like centers strongly influencing the character of the optical transitions. The double bond localizes the HOMO (highest occupied moluecular orbital) in between the diamondoids accompanied by a significant decrease of optical transition energies. Comparing Raman spectra of the compounds to pristine diamondoids, we find several characteristic modes originating from the ethylene moieties. Supported by DFT (density functional theory) computations, we attribute these modes to highly localized vibrations that can partially be derived from the vibrational modes of parent ethylene. We further observe two new Raman modes in the compounds: a dimer breathing mode and a rotational mode of the entire ethylene moieties