Samples of the spontaneously ordered Ga0.5In0.5P alloys were grown by the MOCVD method on [001]-oriented GaAs substrates. The thickness of the epitaxal layer is about 2 μm. Raman scattering and photoluminescence spectra have been measured at room temperature. The result from photoluminescence measurement indicates that the direct-band gaps of the spontaneously ordered samples are lower than that of the disordered sample. Three scattering peaks have been observed in the Raman spectra, corresponding to the GaP-like LO, InP-like LO and InP-like TO modes in the alloys, respectively. The frequencies of the GaP- and InP-like LO modes increase with the decrease of the band-gap of the ordered alloys. It is related to the formation of the (GaP)1/(InP)1 monolayer superlattice along [111] direction in the ordered alloys. The polarization properties of the ordered alloys are similar to those of the bulk III-V semiconductors with the zinc-blende structure.

We describe the growth of (Zn,Mn)(S,Se) epitaxial layers of high structural quality on (100) GaAs substrates. Double crystal x-ray diffraction (DCXRD) measurements indicate that quaternary epilayers nearly lattice-matched with GaAs are characterized by DCXRD curves with a foil width at half maximum in the range 30 – 60 arc seconds. Photoluminescence (PL) spectroscopy is employed to map the variation of the energy gap of the quartenary alloys over a wide range of alloy compositions. Finally, temperature dependent PL is used to examine the viability of (Zn,Mn)(S,Se) alloys as confining layers for ZnSe and (Zn,Cd)Se quantum wells. While efficient exciton confinement is demonstrated through the observation of robust PL from such quantum wells up to high temperatures, the renormalization of the quartenary band gap by spin fluctuations leads to a rapid decrease in confinement with increase in temperature.

We have developed a nanogrowth technology for the fabrication of periodic arrays of semiconductor nanostructures on silicon that is currently being investigated for silicon based x-ray detectors. The semiconductor nanostructures are formed by chemical synthesis in pores of a template created by the anodization of aluminum on a silicon substrate. The use of the silicon substrate allows greater control over the aluminum thin film properties, better in situ monitoring of the pore formation process, and the direct integration of nanostructure arrays with conventional silicon technology.

Addition of methacrylic acid (MA) during preparation of ZnS doped with Mn2+ (ZnS:Mn) increased the photoluminescence (PL) due to 4T1-6A1 transition of Mn2+. According to X-ray fluorescence analysis and electron paramagnetic resonance spectroscopy, ion exchange between Zn2+ and Mn2+ through a preferential dissolution of Mn2+ was promoted by acidic additives. This caused that Mn ions were isolatedly incorporated into ZnS. The X-ray photoelectron spectra show that the intensity of S 2p3/2 peak due to S6+ increased relative to that of S2- by virtue of carboxylic groups. The intensities of PL peaks at 450 and 580 nm, corresponding to polymethacrylic acid and Mn2+, respectively, increased after heating at 80°C for 1 week. We conclude that MA plays important roles on selective leaching to increase the amount of isolated Mn2+ ions, chemical interaction between ZnS:Mn and MA and energy transfer to Mn2+, leading to the increase in PL intensity.

Mn2+-doped CdS nanocrystals (1.2 to 2.4 nm in diameter) embedded in organic-inorganic silica xerogels have been synthesized. Extensive studies (EXAFS, ESR and ENDOR) allow us to localize the ions responsible for the bright luminescence observed in such materials (quantum yield of 7%). The average number of Mn2+ per nanocrystal is in the 0.2–0.8 range, and the emission arises from an energy transfer from surface trapped carriers to Mn2+ ions.

Application of two complementary femtosecond techniques (time-resolved nonlinear transmission and photoluminescence up-conversion) allows us to observe separately the electron and the hole relaxation paths in CdS nanocrystals. The obtained data indicate that hole relaxation channels are different at low and high pump fluences which is attributed to an Auger-process-assisted hole trapping at surface/interface states activated at high excitation intensities.

CdSe quantum dots (QDs) with several different ligands were prepared by ligand exchange reactions of trioctylphosphine oxide (TOPO)/trioctylphosphine selenide (TOPSe) passivated QDs with an excess of new ligands, such as pyridine, 4-picoline, thiophenol, 4-(trifluoromethyl)-thiophenol, and tris(2-ethylhexyl)phosphate. We find that 85–90% of ligands passivating the surface of the QDs are the newly introduced capping species and 10–15% are native TOPO/TOPSe ligands, reflecting the incomplete exchange of the QD surface. Thermal gravimetric analysis (TGA) and quantitative proton NMR measurements show that on average only 30% of surface cadmium atoms are passivated by TOPO/TOPSe. Approximately 1/3 of these native ligands occupy sites too strongly bound to be replaced by ligand exchange reactions.

Pressure tuned photoluminescence and Raman scattering were employed to study the structural phase transition from wurtzite to rocksalt structure of CdSxSei1-x nanocrystals embedded in a glass matrix as a composite material and in the colloidal form. In both the composite and the colloidal form the increased phase stability of the wurtzite to rocksalt structure is reversible and size dependent. In contrast the phase transition in bulk semiconductors of cadmium-sulfide is sharp but for the nanocrystals it is broad and occurs at higher pressures. A size dependent study of the phase stability was conducted and a correlation between size and increase in phase stability was definitely established. For the nanocrystals in the composite form the difference in surface tension in both phases allowed for a model that predicted the tendencies observed in optical spectra. For the colloidal nanocrystals a model based on surface tension and volume effects (defects) gave good agreement in explaining the observations. The above results are discussed with respect to surface tension and deep level defects and their pressure dependence.

We collect and spectrally resolve photoluminescence from single CdSe nanocrystallite quantum dots. By eliminating spectral inhomogeneities, we reveal resolution limited linewidths < 120μev at 10K. These lines are more than fifty times narrower than what has previously been reported using ensemble measurements. Light driven spectral diffusion is seen as a form of power broadening and may be the cause of surprisingly broad linewidths at room temperature. In addition, we see no evidence of excited state emission or coupling to acoustic phonons.

We present results on photoluminescence excitation spectra (PLE) of wurtzite CdSe nanocrystals (NCs) embedded in a glass with effective radii in the range 15–35 Å. Information on the near band-gap absorption of an assembly of NCs is obtained by selecting a narrow energy range in the inhomogeneously broadened photoluminescence band. The size and shape dependence of the lowest exciton states are calculated for slightly non-spherical wurtzite NCs. The experimental results are in good agreement with the theoretical predictions when both shape and size dispersions are taken into account.

We study the band edge luminescence of CdSe nanocrystallites to determine the origin of this emission. Previous studies have attributed the band edge emission to the recombination of photo-generated carriers trapped in localized surface states. Recently a number of “dark exciton” theories have been proposed which explain the luminescence in terms of recombination through internal core states. To address this issue we modify the surface of CdSe nanocrystallites with a number of organic/inorganic ligands and monitor the effect this has on the energetics of the resonant and non-resonant band edge luminescence. Our results for nanocrystallites passivated with trioctylphosphine oxide (TOPO), ZnS, 4-Picoline, 4-(trifluoromethyl)thiophenol, and tris(2-ethylhexyl)phosphate are in agreement with a dark exciton description of the band edge luminescence.

We use High Resolution Transmission Microscopy (HRTEM) to study CdSe, CdS/HgS/CdS quantum-dot quantum well (QDQW), and CdSe/CdS core-shell nanocrystals, grown by wet-chemical techniques. The nanocrystals have faceted Wulff polyhedron shapes.

In addition to HRTEM, we employ multi-wavelength anomalous dispersion (MAD) x-ray diffraction. We use computer modeling to help interpret the experimental data.

Growth of a heterogeneous phase proceeds epitaxially preserving the overall shape and point-group symmetry of the original seed nanocrystals, both for wurtzite (CdSe/CdS) and zincblende (CdS/HgS/CdS) type structures.

Recently prepared InAs nanocrystals also show evidence of faceting as observed by HRTEM and may lend themselves equally well to epitaxy.

We report the synthesis of a series of nearly monodisperse ZnS-overcoated CdSe quantum dots whose room temperature photoluminescence quantum yield approaches 50%. This spectrally narrow (FWHM < 40nm) band edge luminescence spans the visible region from blue to red light. We use a two-step synthesis based on the high temperature decomposition of organometallic precursors in a coordinating solvent. We characterize our composite quantum dots using optical spectroscopies, wavelength dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, high resolution transmission electron microscopy, small angle x-ray scattering, and wide angle x-ray diffraction. The data indicate that samples displaying the highest quantum yield are those which have just achieved a closed shell of ZnS encapsulating the CdSe core. As more ZnS is added, the quantum yield decreases somewhat, possibly due to the many defects present in larger ZnS shells.

We report cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals of 33 and 42 Å diameter imbedded in a ZnS matrix. The thin film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and on the voltage and current density applied. Electron beam irradiation caused a decrease of the luminescence which may be explained by electron ionization of the quantum dots. Quenching of the cathodoluminescence at low temperatures is attributed to a more efficient trapping of the ionized electrons in deep traps inside the ZnS matrix.

Highly crystalline nanoclusters of MoS2 were synthesized and their optical absorption and photoluminescence spectra were investigated. Key results include: (1) strong quantum confinement effects with decreasing size; (2) preservation of the quasiparticle (or excitonic) Nature of the optical response for clusters down to ∼2.5 nm in size which are only two unit cells thick; (3) demonstration that 3-D confinement produces energy shifts which are over an order of magnitude larger than those due to 1-D confinement; (4) observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size; and (5) observation of photoluminescence due to both direct and surface recombination.

The nanoparticles of PbI2 layered semiconductor, embedded in Sio2 films, were prepared by the sol-gel method. The low-temperature luminescence of PbI2 nanoparticles consists of a series of exciton lines in the region of ∼2.5eV and additional donor-acceptor recombination luminescent bands at lower energies, centered at 2.44eV (Green) and 2.07 eV (red), respectively. This paper reports the investigation of the nonexcitonic transitions, associated with of the Green and Red bands. The correlation between donor-acceptor recombination emission processes and lattice imperfections was examine, utilizing optically detected magnetic resonance (ODMR) spectroscopy. The results identified the following imperfection sites of both the Green and Red bands: an acceptor site associates with an isotropie Lead vacancy defect, [V]Pb2+ and a donor site, associate with an anisotropie Iodine vacancy, [V°]jodine. However, the results pointed on differences in relaxation processes of these bands. The relaxation processes correspond to the existence of competitive nonradiative recombination, spin-lattice relaxation and thermalization among the spin substrates. Thus, the results suggest that the Red band corresponds to stoichiometric defects at the surface, while the Green band corresponds to the same stoichiometric defects at the interior part of the nanoparticles.

Optical properties of size restricted Cu2O obtained through aqueous and non aqueous preparations are compared with those of commercially available bulk crystals. One method of synthesis involved using polyvinyl alcohol as a restraining agent in an aqueous preparation to produce nanocrystals having diameters with a mean of 11 nm and a standard deviation of 6 nm. Low-temperature spectroscopie studies indicate size restriction effects are manifest by a decrease in exciton luminescence and a 3.0 to 9.0 meV blue shift in 0,0 transition. No noticeable changes in the dynamics were observed for the nanocrystals, indicating no alteration or relaxation of the selection rules for the direct forbidden transition.

ZnO nanoparticles are produced by the laser vaporization-controlled condensation technique. These particles are connected in a web-like agglomeration. Their properties are compared to those of ZnO nanoparticles produced by solution sol-gel and reverse micelle techniques. All particles have the bulk crystal structure and show quantum size effects in absorption and emission. They show emissions that consist of a blue bandgap feature with a sub-nanosecond lifetime and a green feature with multiexponential lifetime decays. Emission from the stearate coated particles produced by the reversed micelle method is particularly intense.

This paper presents the results of microstructural studies of terbium-doped yttria quantum nanocrystals by imaging and diffraction in a transmission electron microscope. The nanocrystals are typically found in clusters of varying size. Electron energy-loss spectroscopy confirms that the particles consist of yttrium and oxygen. Analysis of selected-area electron diffraction patterns shows that the nanocrystals largely have the cubic structure found in bulk yttria. These patterns furthermore suggest that within each cluster the nanocrystals align themselves with a preferred orientation relative to the incident electron beam as well as to each other.

Device-grade, boron-doped amorphous hydrogenated silicon can be made microporous by anodization in ethanoic HF. The thickness of the porous layer is limited by an instability due to the high resistivity of the material. Amorphous porous silicon exhibits strong room-temperature photoluminescence around 1.5 eV even in samples containing a high density of non-radiative recombination centers. This demonstrates the presence of a spatial confinement effect, as opposed to quantum confinement effect for crystalline porous silicon. The temperature dependence of the luminescence intensity is also accounted for on the same grounds.