Oriented attachment is the key: Single crystalline ZnO nanorods with lengths up to 500 nm could be prepared in a stepwise manner from quasi‐spherical nanoparticles. Only after the formation of pearl‐chain‐like structures (left), do the aggregated particles fuse upon heating to form nanorods (center and right).

New approaches to synthesize photostable thiol-capped CdTe nanocrystals are reported. Post-preparative sizeselective precipitation and selective photochemical etching have been developed as methods providing an increase of photoluminescence quantum efficiency of the nanocrystals of up to 40%. Some advantages of thiol-capping in comparison to conventional organometallic syntheses of quantum dots are discussed.

Highly monodisperse CdSe nanocrystals were prepared in a three-component hexadecylamine−trioctylphosphine oxide−trioctylphosphine (HDA−TOPO−TOP) mixture. This modification of the conventional organometallic synthesis of CdSe nanocrystals in TOPO−TOP provides much better control over growth dynamics, resulting in the absence of defocusing of the particle size distribution during growth. The roomtemperature quantum efficiency of the band edge luminescence of CdSe nanocrystals can be improved to 40−60% by surface passivation with inorganic (ZnS) or organic (alkylamines) shells.Chemically grown CdSe nanocrystals (also referred to as quantum dots) are probably the most extensively investigated object among semiconductor nanoparticles since the introduction of the concept of the "size quantization effect" in the earlier eighties. 1,2 This is caused to a large extent by the existence of a very successful preparation method for highquality CdSe nanocrystals, i.e., arrested precipitation in high boiling mixtures of trioctylphosphine oxide (TOPO) and trioctylphospine (TOP). 3,4 The term "high-quality quantum dots" has been recently defined as follows: 5 the achievement of desired particle sizes over the largest possible range, narrow size distributions, good crystallinity, desired surface properties, and in the case of luminescent materials, high quantum yield. CdSe nanocrystals prepared by the TOPO-TOP route and size-separated after synthesis meet all these requirements apart from the lastshigh luminescence quantum yield, which does not exceed 5-15% for as-prepared particles. 6-8 The luminescence quantum efficiency can be sufficiently improved by growing heteroepitactically an inorganic shell of the wide-band gap semiconductor around the particles. 6-9 The conventional techniques used for the wide-band gap shell growth, however, allow us to prepare only very small amounts of core-shell nanoparticles. Another problem of the TOPO-TOP synthesis is the irreproducibility of the growth dynamics and the shape of the CdSe nanocrystals conditioned by an uncertain composition of the coordinating solvent. Technical grade TOPO (90%, Strem or Aldrich), for instance, provides better conditions for the growth of CdSe nanocrystals than distilled TOPO. 10 Recent developments of the organometallic synthetic routes to II-VI semiconductor nanocrystals included an introduction of hexylphosphonic acid to the TOPO-TOP mixture 10-12 and use of hexadecylamine (HDA) as the capping agent for pure and doped ZnSe nanocrystals. 13,14 Taking into account the growing demand on highly luminescent semiconductor nanocrystals for light-emitting devices 15-17 and tagging applications, 18,19 we tried to improve the conventional organometallic TOPO-TOP synthesis by introducing an additional coordinating component (HDA) to the TOPO-TOP mixture. In this mixture focusing of the size distribution is observed during particle growth so that no postpreparative size-selective precipitation is required. The surface of as-prepared CdSe nanocrystals can be passivated wi...

Controlling anisotropy is a key concept to generate complex functionality in advanced materials. For this, oriented attachment of nanocrystal building blocks, a self assembly of particles into larger single crystalline objects, is one of the most promising approaches in nanotechnology. We report here the 2D oriented attachment of PbS nanocrystals into ultra-thin single crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by co-solvents which alter nucleation and growth rates during the primary nanocrystal formation and finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photo-detector device without further treatment.Controlled assembly leading to anisotropic nanostructures poses a conceptual challenge in materials research. Penn and Banfield [1, 2] described crystal growth, in which oxide nanoparticles coalesce in well defined crystalline orientations. Their method of oriented attachment of nanocrystals is now one of the most favorable techniques to grow linear or zig-zag-type one-dimensional nanostructures. In addition to strong size quantization effects occurring in these structures, their big advantage is solution processability making them attractive candidates for optoelectronic and thermoelectric applications in low-cost integrated systems. One-dimensional assemblies of oriented attachment have been reported, and in most cases the anisotropy during self-assembly is caused by crystal planes with preferred reactivity and dipole moments in the crystallites. Systems with cubic crystal symmetry, however, like PbS and PbSe, where beautiful one-dimensional oriented attachment occurs, are somewhat more difficult to explain. Oriented attachment, in this case, should result in three-dimensional networks rather than one-dimensional structures. The common explanation assumes that despite the strict monodispersity of the samples inhomogeneities in the chemical composition of surface planes exist and result in dipole moments within the nanocrystals. On the other hand organic ligand molecules play a crucial role in such processes by capping nanoparticle surfaces selectively and may hinder, modify, or trigger an oriented attachment [3]. In this work we show that the formation of ordered and densely packed ligand surface layers of oleic acid on {100} PbS surfaces can drive the normally isotropic crystal growths into a two-dimensional oriented attachment of nanocrystals. Hereby the presence of chlorine containing co-solvents during the initial nucleation and growth process of the nanocrystals plays a prominent role.

We demonstrate that efficient shape control may be achieved in the shell of colloidally grown semiconductor nanocrystals (independent of the core), allowing the combination of a 0-D spherical CdSe core with a 1-D rodlike CdS shell. Besides exhibiting linearly polarized emission with a room-temperature quantum efficiency above 70%, these mixed-dimensionality colloidal heterostructures display large, length-dependent Stokes shifts as well as giant extinction coefficients approaching 10 7 cm -1 M -1 .

We report the synthesis and characterization of highly luminescent colloidal nanocrystals consisting of CdSe cores protected with double inorganic shells (core−shell−shell nanocrystals). The outer ZnS shell provides efficient confinement of electron and hole wave functions inside the nanocrystal as well as high photochemical stability. Introducing the middle shell (CdS or ZnSe) sandwiched between CdSe core and ZnS outer shell allows considerable reducing strain inside nanocrystals because CdS and ZnSe have the lattice parameter intermediate to those of CdSe and ZnS. In contrast to CdSe/ZnS core−shells, in the core−shell−shell nanocrystals ZnS shell grows nearly defect free. Due to high quality of the ZnS shell, the core−shell−shell nanocrystals exhibit PL efficiency and photostability exceeding those of CdSe/ZnS nanocrystals. Preferential growth of the middle CdS shell in one crystallographic direction allows engineering the shape and luminescence polarization of the core−shell−shell nanocrystals.

CdTe nanoclusters were prepared in aqueous solution by the reaction between Cd 2+ and NaHTe in the presence of thioglycolic acid. Under reflux, the clusters start to crystallize and show a narrow band emission. The photoluminescence efficiency of CdTe nanocrystals strongly depends on the pH value of the colloidal solution.The maximum quantum yield at room temperature is approximately 18% when the pH value of the CdTe solution is brought to 4.5 by using thioglycolic acid. The optical spectroscopy studies imply that the pHdependent behavior of the CdTe nanocrystals' fluorescence is caused by structural changes on the surface rather than the size of the nanocrystals. Systematic absorption and fluorescence studies on dialyzed samples suggest that in the acidic range a shell of cadmium thiol complexes is formed around the CdTe core. Thus, the fluorescence quantum yield is enhanced dramatically when the solution is made acidic. In contrast, such a shell can also be produced in the alkaline range, but only after the CdTe nanocrystal crude solution is purified by dialysis.

Gold nanostars are multibranched nanoparticles with sharp tips, which display extremely interesting plasmonic properties but require optimization. We present a systematic investigation of the influence of different parameters on the size, morphology, and monodispersity of Au nanostars obtained via seeded growth in concentrated solutions of poly(vinylpyrrolidone) in N,N-dimethylformamide. Controlled prereduction of Au(3+) to Au(+) was found to influence monodispersity (narrower plasmon bands), while the [HAuCl(4)]/[seed] molar ratio significantly affects the morphology and tip plasmon resonance frequency. We also varied the size of the seeds (2-30 nm) and found a clear influence on the final nanostar dimensions as well as on the number of spikes, while synthesis temperature notably affects the morphology of the particles, with more rounded morphologies formed above 60 °C. This rounding effect allowed us to confirm the importance of sharp tips on the optical enhancing behavior of these nanoparticles in surface-enhanced raman scattering (SERS). Additionally, the sensitivity toward changes in the local refractive index was found to increase for larger nanostars, though lower figure of merit (FOM) values were obtained because of the larger polydispersity.