Free Nanocrystals Research Articles

Co-wide-bandgap conformally heteroepitaxial perovskite–monolithic SiC nanowire array with quantum-confined blue luminescence

SiC is a wide-bandgap semiconductor with excellent mechanical and electrical properties and is a crucial template for epitaxially growing other semiconductors. We report the conformally epitaxial growth of the lead halide perovskites on the red-luminescent monolithic 6H-SiC nanowire arrays. The small lattice mismatch (0.8%) between SiC and CsPbBr3 ensures perfect heteroepitaxial growth of the CsPbBr3 quantum dots and nanosheets over the SiC nanowire arrays. The heteroepitaxial perovskites show intense multiband blue luminescence stemming from the strongly quantum-confined excitons with a twice prolonged lifetime compared with free nanocrystals. These blue-luminescent heteroepitaxial semiconductor–semiconductor nanostructures are promising nanophotonic device units.

Embedding laser generated nanocrystals in BiVO4 photoanode for efficient photoelectrochemical water splitting

Addressing the intrinsic charge transport limitation of metal oxides has been of significance for pursuing viable PEC water splitting photoelectrodes. Growing a photoelectrode with conductive nanoobjects embedded in the matrix is promising for enhanced charge transport but remains a challenge technically. We herein show a strategy of embedding laser generated nanocrystals in BiVO4 photoanode matrix, which achieves photocurrent densities of up to 5.15 mA cm−2 at 1.23 VRHE (from original 4.01 mA cm−2) for a single photoanode configuration, and 6.22 mA cm−2 at 1.23 VRHE for a dual configuration. The enhanced performance by such embedding is found universal owing to the typical features of laser synthesis and processing of colloids (LSPC) for producing ligand free nanocrystals in desired solvents. This study provides an alternative to address the slow bulk charge transport that bothers most metal oxides, and thus is significant for boosting their PEC water splitting performance.

Enhancing Upconversion Luminescence of LiYF4:Yb,Er Nanocrystals by Cd2+ Doping and Core–Shell Structure

In this work, we report a phenomenon that upconversion emission intensity of Er3+ was enhanced by cadmium. Cd2+ was taken with 0, 3, 10, 20, 30, and 40 mol % as a new dopant to codope LiY0.78Yb0.20Er0.02F4, meanwhile the doping effects on the crystal structure, morphology, and upconversion fluorescence emission are investigated by XRD, Rietveld refinement, TEM, and upconversion spectroscopy analysis methods in detail. The green upconversion emission intensity was enhanced, and the maximum emission intensities were enhanced to twice when 10 mol % Cd2+ was codoped into tetragonal LiYF4, while its decay lifetime was reduced. Additionally, we further designed the synthesis of the homogeneous LiYF4:20%Yb,2%Er,10%Cd@LiYF4 of which the emission intensity is 11 times higher than that of the Cd2+-free nanocrystals, meanwhile an intriguing morphology of core–shell nanocrystals was obtained. This investigation may be useful for the design and improvement of upconversion materials for fulfilling the prerequisite of t...

ChemInform Abstract: From Atomistic Surface Chemistry to Nanocrystals of Functional Chalcogenides

AbstractReview: [focus on PbSe and SnSe; 59 refs.

From Atomistic Surface Chemistry to Nanocrystals of Functional Chalcogenides.

Synthesis and utilization of nanocrystals are highly active fields of current research, but they require a thorough understanding of the underlying crystal surfaces. In this Minireview, we span the arc from surfaces to free nanocrystals, and onward to their chemical synthesis, using as examples lead selenide (PbSe), tin telluride (SnTe), and their direct chemical relatives. Besides experimental insights, we highlight the increasingly influential role played by quantum-chemical simulations of surfaces and nanocrystals. What can theory do today, or possibly tomorrow; where are its limits? Answering these questions, and skillfully linking them to experiments, could open up new atomistically (that is, chemically) guided perspectives for nanosynthesis.

Study of surfactant-free lead sulfide nanocrystals-P3HT hybrid polymer solar cells

Surfactant free nanocrystals (NCs) of lead sulfide (PbS) prepared using Continuous Spray Pyrolysis (CoSP) technique had been dispersed in poly (3-hexylthiophene) (P3HT) and used in hybrid polymer solar cells (HSCs) to study the role of surfactant free NCs in these devices. Absence of surfactant on PbS NCs improves the polymer–NCs interface and is responsible for an efficient electronic interaction between the two materials. FTIR results show a shift in the characteristics band for the blends confirming the proper bonding between the polymer and NCs. Hybrid polymer solar cells with configuration ITO/PEDOT:PSS/P3HT:PbS/Al have been fabricated with different PbS loading. With the increase in PbS NC content, the device efficiency increases first and then decreases, the device having 30wt% of PbS NCs shows the best performance with an energy conversion efficiency of 1.6%, an open circuit voltage of 0.42V, a short-circuit current density of 9.90mA/cm2, and a fill factor of 0.40 under Air Mass 1.5G illumination with the irradiation of 100mW/cm2. Structural and morphological investigations show that better performance is achieved with the optimum amount of PbS NCs due to good distribution and interconnection in the polymer layer.

Simultaneous Interfacial and Precipitated Supracrystals of Au Nanocrystals: Experiments and Simulations

Under solvent saturation, a precipitation of full-grown supracrystals on the one hand and the formation of well-defined supracrystalline films at the air-liquid interface on the other hand were previously observed for the first time (J. Am. Chem. Soc.2012, 134, 3714-3719). Here, these two simultaneous growth processes are studied by additional experiments and by Brownian dynamics simulations. The thickness of the supracrystalline films and the concentration of free nanocrystals within the solution are measured as a function of the nanocrystal size. The simulations show that the first process of supracrystal growth is due to a homogeneous nucleation favored by solvent-mediated ligand interactions, while the second one is explained in terms of a diffusion process caused by a decrease in the surface energy when the particles penetrate the air-liquid interface. It is also verified that the presence of thiol molecules at the air-solution interface does not hinder the formation of supracrystalline films.

ZnO Hollow Quantum Dot: A Promising Deep-UV Light Emitter

We establish an analytic model to illustrate the energy bandgap of ZnO hollow quantum dots (HQDs) with negative curvature surface from the perspective of nanothermodynamics. It was found that the bandgap of ZnO HQDs shows a pronounced blue-shift as comparable to those of bulk counterpart and free nanocrystals. Furthermore, the photoelectric properties of ZnO HQDs can be effectively modulated by three independent dimensions, including the outer surface, the inner surface and the shell thickness. Strikingly, the emission wavelength of ZnO HQDs can be extended into the deep-ultraviolet (DUV) region, which suggests this kind of nanostructure could be expected to be applicable for the new-generation, compact, and environmentally friendly alternative DUV light emitter.

Stability Investigation of CuInS2 based heavy-metal free nanocrystals

ABSTRACTIn the present work the photostability of high-quality CuInS2 based nanocrystals (Zn-Cu-In-S/ZnSe/ZnS and CuInS2/ZnS core/shell nanocrystals) of different sizes and concentrations were investigated at ambient condition both under UV irradiation and in the darkness. The photostability of commercial CdSe/ZnS core/shell nanocrystals were used as reference to compare to that of CuInS2 based nanocrystals. The half-life times of the CuInS2 base nanocrystals are 2-8 times that of the reference which indicates the CuInS2 base NCs we obtained in the present work are very stable, reliable and competent for the application in biomedical fields.

Monoclinic symmetry of twin-free nanocrystals in theBiScO3-PbTiO3solid solution as revealed by aberration-corrected TEM

Ferroelectric perovskite solid solutions with morphotropic phase boundary (MPB) have been studied extensively as best candidates for electromechanical transduction due to their exceptionally high piezoelectric coefficients. However, despite the high number of publications about these oxides, there is still ambiguity about the actual symmetry of the phases at the MPBs. Here, we present a detailed analysis of the crystalline structure of $0.39{\text{BiScO}}_{3}\text{\ensuremath{-}}0.61{\text{PbTiO}}_{3}$ twin free nanocrystals, using the combination of aberration-corrected high-resolution electron microscopy and multislice simulations. Our study indicates the presence of a monoclinic symmetry for crystals with an average size of 15 nm. This is strong evidence in favor of the actual existence of this polymorph at the MPB, which has been recently questioned. It also shows that perovskite distortion is still present in nanocrystals, which is important from the point of view of the size limit of ferroelectricity and consistent with previous electrical measurements in nanostructured ceramics.

Low‐Frequency Raman Scattering from Nanocrystals Caused by Coherent Excitation of Phonons

Since the first observation of surface acoustic modes from silicon nanocrystals (NCs) embedded in silica byDuval et al., low-frequency Raman scattering from NCs has become an important researcharea inmanyfields including semiconductor technologies, medical therapeutics, and biophysics. Recent research has indicated that low-frequency Raman spectroscopy is a feasible non-destructive technique for investigating virus functionalization, for example, introducing viruses on different materials, attaching viruses to quantum dots and carbon nanotubes, and forming multiple superstructures. These superstructures are expected to have important applications in biological science and medicine. However, the current assignment of the low-frequency Raman modes is based on Lamb’s theory that mainly focuses on the mode frequencies of an elastic vibration of a free isotropic sphere. Many studies have demonstrated that the surface acoustic-mode frequencies observed from free NCs and NCembedded matrix systems are consistent with the theoretical prediction. This is understandable because a large number of frequency values can be selected to match the experimental results. At the same time, other studies have unequivocally

Size-dependent melting of nanocrystals: a self-consistent statistical approach

A self-consistent statistical method is used to describe size effects on melting of free nanocrystals. The melting transition is assumed to be directly related to evolution of high-temperature instability of the phonon subsystem of the crystal, caused by strong anharmonicity of atomic vibrations. We show that depression of the melting temperature of small free particles is mainly due to presence of surface atoms which are bound to smaller numbers of atoms than those of the interior. The melting temperatures of spherical nanocrystals of Ar and Au were calculated as functions of the inverse of their radii and compared with experimental and molecular dynamics data.

High resolution spectroscopy of self‐assembled single GaAs/AlGaAs quantum dots

AbstractIn this contribution we report the results of polarization resolved, high resolution photoluminescence spectroscopy of single lattice‐matched, GaAs/Al0:3Ga0:7As quantum dots. The investigated samples are self assembled, strain free nanocrystals grown by Modifed Droplet Epitaxy technique. The lack of strain reflects in a nominal lack of piezoelectric field making such nanostructures potentially highly symmetric and suitable systems as polarization entangled photon sources.The high resolution, polarization resolved photoluminescence from isolated emitters present a clear linearlypolarized fine spectral splitting between the neutral exciton and biexciton lines while charged complexes are completely degenerate. By measuring the fine structure splitting and the line broadening of the excitonic photoluminescence we get new insights on the overlap of the opposite polarized lines (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Line broadening of excitonic complexes in self‐assembled GaAs/AlGaAs single quantum dots

AbstractWe present a detailed experimental study of the line broadening of the different excitonic complexes inside single GaAs/AlGaAs quantum dots. The investigated nanostructures are self assembled quantum dots (QDs), strain free nanocrystals grown by Modified Droplet Epitaxy technique. They show a typical PL spectrum with a triplet structure and by time correlated spectroscopy each contribution can be unambiguously assigned to neutral exciton, biexciton and trion.By high resolution spectroscopy, we have studied the line broadening for several single GaAs/AlGaAs QDs. The measured linewidth is found to vary between several tens to few hundreds of meV and we attribute this to spectral diffusion. The striking feature of the PL spectrum is that the trion and the biexciton line are always sharper with respect to the neutral exciton (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Size effect on thermodynamic properties of free nanocrystals

We demonstrate that the discrete character of the vibrational spectrum of a small crystal accounts for size dependence of its thermodynamic properties and melting temperature. Using a self-consistent statistical method [Phys. Rev. B 66, 054302 (2002)] we derive the Gibbs free energy of free nanocrystalline plates and calculate the thermodynamic parameters as functions of plate thickness for Cu.

Surfactant-induced enhanced room temperature ferromagnetism in Zn 0.96Mn 0.03Li 0.01O nanoparticles: Prepared by solid-state pyrolitic reaction

We report the synthesis of nanoparticles of Zn 0.96Mn 0.03Li 0.01O by a low-temperature solid-state pyrolitic reaction, followed by a surfactant-assisted calcination at 400 °C. The X-ray diffraction and transmission electron microscopy analyses showed the formation of impurity free nanocrystals of Mn doped Li co-doped ZnO with wurtzite structure. XPS data revealed that Mn exists in +2 oxidation state. DC magnetization measurements as a function of field and temperature showed enhanced room temperature ferromagnetism for the surfactant-treated Zn 0.96Mn 0.03Li 0.01O. FMR signal observed in the EPR spectrum further confirmed its ferromagnetic nature.

TEM characterization of organic nanocrystals grown in sol–gel thin films

The tetracene molecule (2,3-benzanthracene, C8H12) was used to synthesize nanocrystals grown in sol–gel thin films, ranging from 10 to 100 nm of diameter. This confined nucleation and growth was compared to microcrystallizations of the same molecule in free solution. Transmission electron microscopy (TEM) was used to characterize these two kinds of tetracene crystals. The observation was performed under low-dose illumination to avoid amorphization of the samples during electron irradiation. Spatial confinement and size distribution of micro- and nanocrystals were compared. Using electron microdiffraction and diffraction patterns simulations, we showed that free microcrystals and nanocrystals confined in gel glasses exhibit the same triclinic $$ P\overline 1 $$ structure. In addition, time-resolved spectroscopy was used to record fluorescence decays, showing a monoexponential fluorescence decay for nanocrystals while microcrystals exhibit a multiexponential decay. The simple signature of nanocrystals luminescence is promising for the future development of chemical or biological sensors.

Recombination lifetime of single GaAs/AlGaAs quantum dots

AbstractIn this contribution we experimentally demonstrate, by means of time resolved microphotoluminescence on single GaAs quantum dot (QD), that the lifetime is reduced when decreasing the dot size. The samples are grown by modified droplet epitaxy and are strain free nanocrystals. The analysis of the recombination kinetics as a function of the excitation power allows us to resolve the exciton and biexciton contribution, leading to a clear interpretation of the experimental data. The broad inhomogeneous size distribution of QDs grown by droplet epitaxy (O. Stier et al., Phys. Rev. B 59, 5688 (1999), [4]) is used to investigate QDs of different size in a single sample. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Grain growth of CuO nanocrystal activated by high energy ball milling

X-ray Diffraction (XRD), small-angle X-ray scattering, scanning electron microscopy and energy dispersive X-ray Analysis were used to investigate the effect of controlled high energy ball milling (HEBM) on the average volume weighted crystallite size, 〈 D〉 V and weighted average microstrain, 〈 ε〉, of nanostructures of CuO prepared by solid state reaction. The starting material, S 0, consists of almost strain free nanocrystals of monoclinic CuO with 〈 D〉 V ≈ 20nm , as determined by XRD data Rietveld analysis. It was found that after an initial decrease of 〈 D〉 V and increase of 〈 ε〉, the values of these parameters go through a steady-state stage followed by an increase of an order of magnitude in 〈 D〉 after a period of only 120 m of HEBM. According to the results here presented, the presence of small amounts of contaminants in the starting material can have an influence on the kinetics of crystal growth in HEBM CuO.

MELTING-THERMODYNAMIC CHARACTERISTICS OF Fe, Co, Ni MAGNETIC NANOCRYSTALS

A simplified model that describes the size and shape dependence of melting thermodynamics of full free nanocrystals was established. Critical sizes of Fe , Co , Ni magnetic nanocrystals when the crystals keep their crystallinity were calculated and the corresponding minimum melting temperature was predicted. Theoretical predictions were consistent with experimental results.