Larger Nanocrystals Research Articles

Diffusion of Pb in carbon ion-implanted silicon: Discovery of a new crystalline phase after electron beam annealing

A novel phase has been discovered by dual low-energy ion implantation and high vacuum electron beam annealing. (100) p-type Si was implanted with (a) 20 keV 12C + ions to the fluence of 6 × 10 16 cm −2 and (b) 7 keV Pb + ions to the fluence of 4 × 10 15 cm −2. The 12C ion implantation results in an understoichiometric shallow SiC x layer that intersects with the surface. The implanted Pb ions decorate a shallow subsurface region. High vacuum electron beam annealing at 1000 °C for 15 s using a temperature gradient of 5 °C s −1 leads to the formation of large SiC nanocrystals on the surface with RBS measurements showing Pb has diffused into the deeper region affected by the 12C implantation. In this region, a new crystalline phase has been discovered by XRD measurements.

Structure and energetics ofc−Si∕a−SiO2systems: Planar interfaces and embedded Si nanocrystals

We use Monte Carlo simulations to study the properties of the interfaces between $c\text{\ensuremath{-}}\mathrm{Si}$ and $a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$, more specifically planar interfaces and interfaces between $a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ and embedded Si nanocrystals (NCs). We find that the $\mathrm{Si}(111)∕a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ interface with suboxide ${\mathrm{Si}}^{+1}$ only has the lowest interfacial energy among the planar cases considered. We also find that embedded NCs smaller than $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ have even lower interfacial energy, suggesting that a faceted shape is favorable only for large NCs. No significant differences are observed between normal and twinned NCs, indicating that the experimentally observed stacking faults might result from the coalescence of small defect-free NCs.

Reversible and Hierarchical Composite Gels with CdSe Nanocrystals

To synthesize composite solid materials of metal salt and CdSe nanocrystals by a simple one-step method has been described. These solids can form stable gel in some organic solvent, such as benzene, cyclohexane and 1-butanol, especial in n-decane even below 0.1 wt/vol.%. Further-more, these gels appear strong fluorescence which can be easily adjusted by the gel concentration. Temperature-dependent fluorescence spectra of composite gels suggested that the CdSe NCs aggregate together in gel state which would induce the energy transfer between nanocrystals and these aggregates could be reversibly disintegrated when gel was heated to form sol. TEM observations provided the further evidence of the energy transfer and suggested that the CdSe NCs were enchased regularly not only on the surface of self assembly of metal salt, but also embedded inside of self assembly in composite gel with small size nanocrystals. In contrast, in composite organogel with large nanocrystals they were only enchased on the edge of self assembly.

Interband, intraband, and excited-state direct photon absorption of silicon and germanium nanocrystals embedded in a wide band-gap lattice

Embedded Si and Ge nanocrystals (NCs) in wide band-gap matrices are studied theoretically using an atomistic pseudopotential approach. Small clusters to large NCs containing the order of several thousand atoms are considered. Effective band-gap values as a function of NC diameter reproduce very well the available experimental and theoretical data. It is observed that the highest occupied molecular orbital for both Si and Ge NCs and the lowest unoccupied molecular orbital for Si NCs display oscillations with respect to size among the different irreducible representations of the ${C}_{3v}$ point group to which these spherical NCs belong. Based on this electronic structure, first, the interband absorption is thoroughly studied, which shows the importance of surface polarization effects that significantly reduce the absorption when included. This reduction is found to increase with decreasing NC size or with increasing permittivity mismatch between the NC core and the host matrix. Reasonable agreement is observed with the experimental absorption spectra where available. The deformation of spherical NCs into prolate or oblate ellipsoids is seen to introduce no pronounced effects for the absorption spectra. Next, intraconduction and intravalence band absorption coefficients are obtained in the wavelength range from far-infrared to visible region. These results can be valuable for the infrared photodetection prospects of these NC arrays. Finally, excited-state absorption at three different optical pump wavelengths, 532, 355, and $266\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are studied for 3 and $4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ diameter NCs. This reveals strong absorption windows in the case of holes and a broad spectrum in the case of electrons, which can especially be relevant for the discussions on achieving gain in these structures.

Formation and stability of rocksalt ZnO nanocrystals in MgO

Coimplantation of Zn and O ions into a single crystalline MgO and subsequent thermal annealing were applied in the synthesis of ZnO nanocrystals. Electron microscopy showed that rocksalt instead of wurtzite ZnO stabilizes for relatively large nanocrystals up to ∼15nm, resulting from its small lattice mismatch with MgO of ∼1.7%. The vacancies initially created by implantation induce favorable nanocrystal growth kinetics and are effectively absorbed during the nucleation and growth processes. The optical band edge of the ZnO nanocrystals was detected at ∼2.8eV.

Electrochemical Deposition of Bi2S3 Thin Films Using Dimethylsulfoxide as a Solvent

thin films with different morphology and crystallite size were electrodeposited on conducting glass substrates from a dimethylsulfoxide solution of bismuth chloride and elemental sulfur at various temperatures. Porous films, constituted of open-packed nanoparticles , were obtained at low temperature . Dense films were obtained in the temperature range of . They were formed of larger close-packed nanocrystals arranged in columnar agglomerates. Electrodeposition at higher temperatures resulted in open-packed large nanocrystals that constitute a microstructured film. The stoichiometry of the films was confirmed by chemical analysis. However, X-ray diffraction indicated distortions in lattice. Deposition temperature was found to influence the distortion, with the smallest distortion observed for the highest temperature . Optical gaps in the range were estimated for the obtained films. The influence of the morphology and structural defects on the optical properties of films was also discussed.

Aqueous Synthesis of Thiol-Capped CdTe Nanocrystals: State-of-the-Art

We report on the state-of-the art synthesis and improved luminescence properties of thiol-capped CdTe nanocrystals (NCs) synthesized in water. The optimized pH (12) and molar ratio of thiol to Cd ions (1.3:1) increases the room-temperature photoluminescence quantum efficiency of as-synthesized CdTe NCs capped by thioglycolic acid (TGA) to values of 40−60%. By employing mercaptopropionic acid (MPA) as a stabilizer, we have synthesized large (up to 6.0 nm in diameter) NCs so that the spectral range of the NCs' emission currently available within this synthetic route extends from 500 to 800 nm. Sizing curve for thiol-capped CdTe NCs is provided. In contrast to CdTe NCs capped by TGA, MPA-capped CdTe NCs show up to 1 order of magnitude longer (up to 145 ns) emission decay times, which become monoexponential for larger particles. This phenomenon is explained by considering the energetics of the Te-related traps in respect to the valence-band position of CdTe NCs. The correlation between luminescence quantum ef...

Structural and magnetic properties of MnAs/GaAs ferromagnetic semiconductor nanocomposite material

Self-organized (Ga,Mn)As nanoclusters, embedded in GaAs and formed during thermal annealing of Ga1-xMnxAs layer at 500 °C or 600 °C, were studied using Transmission Electron Microscopy (TEM) and Magnetic Force Microscopy (MFM). We found that 10–20 nm large NiAs-type hexagonal MnAs nanocrystals gave magnetic contrast in MFM images, whereas smaller zinc-blende nanoinclusions were not visible by means of this technique. Theoretical simulations showed that MFM contrasts reflect interaction between magnetic tip and many randomly distributed MnAs nanocrystals.

Multiple Exciton Generation in Colloidal Silicon Nanocrystals

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

Silicon nanocrystal growth in the long diffusion length regime using high density plasmachemical vapour deposited silicon rich oxides

In this study, silicon nanocrystal (Si-nc) growth is studied in a relatively long thermal budget regime, 3 hat 1100–1200 °C, to examine large diameter nanocrystals (i.e. average diameters greater than 5 nm).Morphology, defects within the nanocrystals and size dependence as a functionof thickness in the oxide are exaggerated in this regime and are more readilycharacterized in the longer diffusion length regime. In particular, nearby surfaces, thesilicon substrate and oxide surface, appear to deplete the excess silicon in theoxide, leading to a strong nanocrystal size dependence with position in the oxide.To pursue this work, silicon naocrystals were formed through a combination of high densityplasma enhanced chemical vapour deposition (HDP-CVD) of silicon rich oxides (SRO)followed by phase separation of the SRO into Si-ncs and stoichiometric oxide(SiO2). Details of the characterization of the as-grown HDP-CVD SROs are included,and differences in the Si–O–Si stretch mode peak position dependence on [O]between HDP-CVD and previously reported plasma enhanced CVD are discussed.

Nonthermal Plasma Synthesis of Faceted Germanium Nanocrystals

AbstractLow‐pressure plasmas are known for their ability to synthesize nanoparticles via gas‐phase nucleation. While in the past this particle formation has often been considered from the viewpoint of a contamination problem in semiconductor processing, we here describe a low‐pressure plasma that enables the synthesis of faceted germanium nanocrystals. The germanium nanocrystals are generally single crystals without any planar defects such as twin boundaries. A change in the morphology with particle size is observed. Small germanium particles up to about 30 nm in size are mostly cubes with (100) crystal facets. Larger crystals between 30–60 nm are primarily cuboctahedrons. Even larger nanocrystals are mainly spherical. The shape evolution is interpreted in terms of the equilibrium shape of hydrogen‐coated germanium crystals. Possible applications of these crystals are discussed.

Structure and magnetism of MnAs nanocrystals embedded in GaAs as a function of post-growth annealing temperature

Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1−xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed.

Open-to-air synthesis of monodisperse CdSe nanocrystals via microfluidic reaction andits kinetics

Monodisperse zinc blende CdSe nanocrystals (NCs) ranged from 2.5 to 4.3 nm have beenreproducibly synthesized directly open to air in a microfluidic reactor with PTFEcapillaries acting as the reaction channels. The foregoing features led to superioroperational stability, allowing the assessment of kinetic data such as temperature, dwellingtime and ligand concentration. Red shifts of the absorption spectra were observed withincreasing residence time and temperature. ‘Crossover time’ of size distribution was firstreported in a microfluidic reactor preceded by encouraged Ostwald ripening at highreaction temperature. Oleic acid concentration influenced the formation of CdSe NCs, andhigh OA to Cd ratio resulted in large NCs size and decreased CdSe concentration.By systematic optimization of the parameters, CdSe NCs with FWHM for thephotoluminescence peak in the range from 35 to 42 nm were thus made possible.

Nanocrystallites in Bi–As–S system

Bi2S3–As2S3 composite formation was performed by two methods: by the direct insertion of Bi2S3 nanocrystals into a molten As2S3 glass which was further solidified and by the crystallization of a rapidly quenched (As2S3)1−x (Bi2S3)x glasses with x=0.005, 0.01, 0.02 and 0.04 at different conditions. Fine tuning of the annealing of the quenched glass as well as the mixing of nanocrystals in to the molten glass resulted glass-crystalline composites with different amounts and distribution of 20–50nm large Bi2S3 nanocrystals as well as larger, up to few micrometer long, needle-like crystals. Structural and optical investigations support the presence of the Bi2S3 crystalline phase in all composites. Optical absorption and the photoconductivity of bulk composite samples follow the structural changes of the structure in the amorphous and amorphous-crystalline phase. In addition, the 290cm−1 characteristic band in Raman spectra may be used for tracing the formation of the nanocomposites.

Deuteron microprobe analysis of carbon in the transition region between SiC and Si nanostructures grown on Si

Carbon ion implantation and high vacuum electron beam annealing have been applied to produce 200–300 nm large SiC nanocrystals on silicon. Simultaneously, Si nanostructures, called silicon nanowhiskers, grow in the unimplanted region of the (1 0 0) Si substrate during the annealing treatment. The lateral transition region between the SiC nanocrystals and Si nanostructures has been investigated using optical microscopy, deuteron microscopy and atomic force microscopy. Carbon distributions were measured via the 13C(d, p 0) 14C nuclear reaction with a focused scanning deuteron beam with an energy of 1.05 MeV. It has been found that the carbon distribution is directly linked to the growth of the SiC nanostructures. At the boundary between the implanted and un-implanted region, the amount of carbon decreases over 50 μm which causes an abrupt depletion of SiC nanocrystals at this transition region. Specific results outline the important role of nuclear microscopy in nanotechnology process development.

Role of local fields in the optical properties of silicon nanocrystals using the tight binding approach

The role of local fields in the optical response of silicon nanocrystals is analyzed using a tight binding approach. Our calculations show that, at variance with bulk silicon, local field effects dramatically modify the silicon nanocrystal optical response. An explanation is given in terms of surface electronic polarization and confirmed by the fair agreement between the tight binding results and that of a classical dielectric model. From such a comparison, it emerges that the classical model works not only for large but also for very small nanocrystals. Moreover, the dependence on size of the optical response on the nanocrystal size is discussed, in particular treating the limit of large size nanocrystals.

X‐ray diffraction study of low‐energy carbon‐ion implanted Si(001)

AbstractIn the search for Si‐ and C‐based crystalline phases in low‐energy ion implanted and electron‐beam annealed Si surface layers, X‐ray diffraction (XRD) measurements were performed at grazing incidence on samples of large SiC nanocrystals grown on a 90 nm thick Si layer containing C atoms. Diffraction patterns and reciprocal space maps did not reveal XRD patterns originating from the nanocrystals or the implanted layer, but did show that distortions of the Si crystal structure were introduced into the implanted layer. After annealing, the strain in the implanted layer is reduced, possibly by carbon atoms that have moved to locations close to dislocations and dislocation loops. This investigation underpins the growth theory of the SiC nanocrystals on Si, with carbon atoms migrating to form the nanostructures. Copyright © 2007 John Wiley & Sons, Ltd.

Polymerization of Nanocrystal Quantum Dot–Tubulin Bioconjugates

Nanocrystal quantum dot (NQD)-tubulin bioconjugates were prepared using a two-step crosslinking procedure. NQD-decorated microtubules were successfully polymerized directly from the NQD-tubulin conjugates to form nonaggregated, full-length (several micrometers) biopolymers. However, polymerization kinetics were slowed in comparison with unmodified tubulin and tubulin modified with small-molecule dyes or biotin. Association with the relatively large nanocrystals, therefore, interferes to some extent with tubulin's ability to polymerize. These results suggest that before NQDs are used extensively as fluorescent labels in studies of biomolecular activity, the impact of NQD bioconjugation must be well understood.

Design Optimization of Metal Nanocrystal Memory—Part I: Nanocrystal Array Engineering

The three-dimensional (3D) electrostatics together with the modified Wentzel-Kramers-Brillouin tunneling model has been implemented to simulate the programming and retention characteristics of the metal nanocrystal (NC) memories. Good agreements with experimental data are first demonstrated to calibrate the transport parameters. In contrast to previous works, the 3D electrostatic effects investigated in this paper are proven very significant in the memory operations. Therefore, new design criteria of metal NC memories are investigated. Part I presents the physical model and the NC array design optimization. A sparse and large-size NC array, which is suitable for the one-dimensional narrow-channel memories, provides higher program/erase tunneling current density due to the field-enhancement effect and lower charging energy due to the large NC capacitance. On the other hand, to achieve a sufficient memory window, fast programming speed, and long retention time in the typical two-dimensional channel memories, a dense and large-size NC array is favorable while taking the tradeoff with the NC number density into account. Based on the same theoretical model, the authors continue in Part II to consider the design optimization when high-K dielectrics can be employed

Shape and faceting of Si nanocrystals embedded ina−SiO2: A Monte Carlo study

We study the equilibrium shape and faceting of Si nanocrystals embedded in $a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$, using continuous-space Monte Carlo simulations supplemented by the Wulff construction method. Our aim is to explain in atomistic terms why large nanocrystals are often observed to be faceted, while smaller ones are always found to be spherical. By developing and analyzing realistic structural models, we show that faceting is indeed the equilibrium state of large nanocrystals, and that it breaks down as the size shrinks. The decomposition of interface energies into bond contributions reveals that the key factors driving the destabilization of facets are the distortions of Si-O-Si bridge bonds at the edges and apeces of the unstable facets. The transition region from a faceted to a spherical shape is in the range of $4--6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.