Andreas Kornowski

Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment

Manufacturing Nanomaterials The exploration of many materials at the nanoscale has revealed properties that only emerge when working at these small dimensions. For device manufacture, materials need to be deposited or assembled in specific patterns. Schliehe et al. (p. 550; see the cover) show the oriented attachment of lead sulfide nanocrystals into two-dimensional sheets. The packing is driven by the choice of solvents that influence the interactions between the nanocrystals. The nanocrystals have excellent photoconductive properties and were incorporated into a photodetector without any additional chemical processing. Self-assembled two-dimensional nanocrystals of lead sulfide have excellent photoconductive properties. Controlling anisotropy is a key concept in the generation of complex functionality in advanced materials. For this concept, 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 two-dimensional oriented attachment of lead sulfide (PbS) nanocrystals into ultrathin single-crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by cosolvents, which alter nucleation and growth rates during the primary nanocrystal formation, and is finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photodetector device without further treatment.

Organization of Matter on Different Size Scales: Monodisperse Nanocrystals and Their Superstructures

Advanced colloidal syntheses enable the preparation of monodisperse semiconductors and magnetic alloy nanocrystals. They can be further used as building blocks for the fabrication of ordered assemblies: two-dimensional and three-dimensional arrays and colloidal supercrystals. This article reviews our recent activities in these fields. A theoretical description of the evolution of an ensemble of nanoparticles in a colloidal solution is applied to the problem of control over the nanocrystal monodispersity.

Liquid‐Phase Synthesis of Colloids and Redispersible Powders of Strongly Luminescing LaPO4:Ce,Tb Nanocrystals

Nanoparticles with high photoluminescence quantum yield have been recently considered as possible biolabels and as emitters in optoelectronic devices. Now gram amounts of nontoxic, chemically stable LaPO4 :Ce,Tb nanocrystals have been obtained in a coordinating solvent. These nanoparticles can be easily redispersed in polar solvents to give scatter-free colloids that exhibit quantum yields of up to 61 %.

Tuning Size and Sensing Properties in Colloidal Gold Nanostars

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.

Synthesis and Thermoelectric Characterization of Bi2Te3 Nanoparticles

Here, a novel synthesis for near monodisperse, sub-10 nm Bi2Te3 nanoparticles is reported. A new reduction route to bismuth nanoparticles is described, which are then applied as starting materials in the formation of rhombohedral Bi2Te3 nanoparticles. After ligand removal by a novel hydrazine hydrate etching procedure, the nanoparticle powder is spark plasma sintered to a pellet with preserved crystal grain sizes. Unlike previous works on the properties of Bi2Te3 nanoparticles, the full thermoelectric characterization of such sintered pellets shows a highly reduced thermal conductivity and the same electric conductivity as bulk n-type Bi2Te3.

Synthesis and surface modification of amino-stabilized CdSe, CdTe and InP nanocrystals

Abstract CdSe, CdTe and InP nanocrystals were prepared by an organometallic synthesis using mixtures of highly boiling primary amines and trioctylphosphine (TOP) as the coordinating solvent, and were characterized by powder XRD, SAXS, HRTEM, absorption and luminescence spectroscopy. The use of amines allowed us to obtain small crystalline nanoparticles for all materials investigated. In all cases, as-prepared colloids show rather narrow particle size distributions which can be further improved by standard size selective precipitation. Amino-capped II–VI nanocrystals show strong size-dependent band edge photoluminescence (PL). CdSe nanocrystals with the mean particle size in the range of 1.2–3.0 nm exhibit emission from blue to green with room temperature quantum yields of 15–20%. CdTe nanocrystals (2.5–5.0 nm size range) show a PL tunable from green to red with quantum yields up to 65% at room temperature. InP nanocrystals (1.5–4.0 nm size range) possess a weak emission (

Micelle and Vesicle Formation of Amphiphilic Nanoparticles

Nanoparticle brushes: Complex nanostructures can be formed by self assembly of amphiphilic CdSe/CdS core-shell nanoparticles that bear a brushlike layer of poly(ethylene oxide) chains. This route is based on controlling the volume fractions of hydrophilic and hydrophobic moieties within the particles and allows the formation of micellar, cylindrical, or vesicular nanoobjects (see picture).

Synthesis and properties of colloidal lanthanide-doped nanocrystals

Colloidal solutions and redispersible powders of nanocrystalline, lanthanide-doped phosphates and vanadates have been prepared in high-boiling coordinating solvents or by hydrothermal means in aqueous solution. Highly crystalline materials were obtained by both methods despite the low temperature of 200°C applied during the synthesis. The materials have been characterized by using high-resolution TEM, powder-XRD, absorption spectroscopy and luminescence spectroscopy. By analyzing line splitting and intensity pattern in the luminescence spectra of the europium-doped samples, we are able to verify that the dopant ions enter the same lattice sites as in the corresponding bulk material. Size-selected samples of hydrothermally prepared lanthanide-doped YVO4 consist of particles ranging in size from about 10 to 30 nm. The nanoparticles exhibit the tetragonal zircon structure known for bulk material. In the case of LaPO4 the hydrothermal method yields nanoparticles or nano-fibers (width of about 9 nm) depending on the synthesis conditions, while the synthesis in high-boiling solvents yields colloids with a very narrow size distribution and a mean particle size of 4.5 nm. All LaPO4-nanomaterials exhibit the monazite-structure, irrespective of their morphology or size. In all systems investigated, energy transfer between the host and the dopant ions is observed. Upon UV excitation, aqueous colloidal solutions of YVO4:Eu nanoparticles show a luminescence quantum yield of 15% at room-temperature. In LaPO4:Ce, Tb energy transfer is observed between cerium and terbium ions.

Wet Chemical Synthesis of Highly Luminescent HgTe/CdS Core/Shell Nanocrystals**

We have recently reported the wet chemical synthesis of colloidal HgTe nanocrystals in aqueous solution at room temperature. This novel material exhibits very strong and broad photoluminescence (PL) in the near infrared, with the exact wavelength depending on the synthetic conditions. The preparation was an extension to the previously published work on cadmium chalcogenide nanocrystals, i.e., CdS, CdSe, and CdTe, and used 1-thioglycerol as the size-regulating capping agent. Particles with a large distribution of sizes in the 3±6 nm diameter range were produced. Luminescence quantum efficiencies (QEs) of around 50 %, which are among the highest ever reported for a colloidal system, and stability towards oxidation indicate that the surface of the HgTe nanocrystals is extremely well-passivated. Typically, the PL from the freshly prepared material is around 1050 nm, but this moves to longer wavelengths with time, accompanied by an apparent drop in QE, until after two weeks the luminescence has shifted out to around 1200 nm. This aagingo process continues indefinitely, albeit at a slower rate, and presents real cause for concern for the long-term stability of such materials in device applications. In addition, if the colloidal nanocrystal solutions are heated, then a similar but more rapid process occurs with the PL shifting to beyond 1500 nm and the QE dropping to < 1 % with just a few minutes refluxing at 100 C. Again, this lack of stability to heating could be problematic in certain applications if high optical pump powers are used in laser or amplifier devices. A similar, although less-pronounced, effect is also observed in 1-thioglycerol-stabilized CdTe nanoparticles, which undergo a red shift of the PL under prolonged reflux. Whether this shift is caused by a gradual aripeningo of the nanocrystals with heat and/or time, or actually represents a compositional change will be discussed at a later date. Needless to say, the effect is undesirable and for HgTe to be a useful material, operating at the strategic near-infrared telecommunications wavelengths, the problem has to be eliminated. A possible way of achieving this is to cap the surface of the nanocrystals with a higher bandgap inorganic layer. These acore/shello composite materials can increase the luminescence quantum yield due to improved passivation of the surface, and also tend to be more physically robust than the abareo organically passivated clusters. This should therefore produce a much more stable material where not only the aagingo process is suppressed, but which is more tolerant to the processing conditions necessary for incorporation into useful devices. Examples of such core/shell structures include CdSe/ZnS, CdS/ZnS, CdSe/CdS, CdSe/ZnSe, and CdS/HgS with reports of roomtemperature QEs of up to 50 % for the CdSe/ZnS material. It was therefore decided to attempt to overcoat our HgTe nanocrystals with a layer of CdS in order to produce a physically stable core/shell heterostructure. The abareo 1-thioglycerol-stabilized HgTe colloidal nanocrystals were prepared in aqueous solution at room temperature using the method described previously. In order to coat these abareo organically capped nanocrystals with a layer of CdS, H2S gas buffered in N2 was passed through a vigorously stirred, dilute aqueous solution of HgTe nanocrystals and cadmium(II) perchlorate at a pH of around 10 in the presence of extra 1-thioglycerol stabilizer. The mixture was initially slightly turbid, but this cleared upon injection of the H2S, resulting in a clear, goldenbrown solution. This solution was then refluxed for about 30 min as a check of the robustness of the HgTe/CdS system. In order to fully characterize the material, optical absorption and photoluminescence spectra were recorded at all stages of the synthesis, as well as X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy (HRTEM) images to confirm the presence of HgTe/CdS core/shell particles in the final solution. The optical absorption data is shown in Figure 1, comparing the freshly prepared abareo HgTe prior to capping, and the subsequent HgTe/CdS material. A clear aexcitonico peak is visible for the bare HgTe sample at 850 nm, which clearly shifts to the red when the CdS capping is introduced. The increased optical density at shorter wavelengths could be indicative of some discrete CdS or HgCdTeS alloyed nanoparticles being formed in addition to the core/shell material. The effect of aging and/or heating the samples, both capped and uncapped, is to red-shift and broaden the absorption edges into the region where water absorption caused by a slight cell-mismatch prevents the recording of good-quality spectra. These are, therefore,

Development of IR-emitting colloidal II-VI quantum-dot materials

The current state-of-the-art of colloidal II-VI nanocrystal formation using the aqueous/thiol synthesis route is described. Work on single component and heterostructures and mixed compound quantum dots is discussed. The purpose of the work is to provide a range of infrared (IR)-emitting materials with high quantum efficiency (QE) as potential gain media for future ultrawideband optical amplifiers for high-capacity wavelength-division multiplexing (WDM) telecommunications systems. Physical and chemical factors influencing particle sizes are described.