Silicon is a poor light emitter, unless reduced to nanoscale dimensions. Recently, synthesis of Si quantum dots and quantum rods has been demonstrated. This presentation will cover the synthesis of bright Si quantum dots with widely controlled diameter and bright Si quantum rods made using a colloidal approach. A synthesis of colloidal silicon (Si) nanocrystals, or quantum dots, with widely tunable average diameter, from less than 3 nm up to 90 nm and peak photoluminescence (PL) from visible wavelengths to the bulk band gap of Si at 1100 nm has been developed that relies on the high temperature (>1100C) decomposition of hydrogen silsesquioxane (HSQ). The Si quantum dots are crystalline with tunable size and uniform size distribution, and can be hydrosilylated with alkenes for stable and bright photoluminescence (PL). H-terminated Si nanocrystals isolated from the HSQ-derived product can also undergo room temperature hydrosilylation with bifunctional alkenes with distal polar moieties—ethyl-, methyl-ester or carboxylic acids—without the aid of light or added catalyst to yield Si nanocrystals with bright PL, dispersable in polar solvents, including water. In this case, a reaction mechanism is proposed in which ester or carboxylic acid groups facilitate direct nucleophilic attack of the highly curved Si surface of the nanocrystals by the alkene. Fluorescent Si nanorods can also be synthesized. In this case, the nanorods are produced by a ligand-controlled solution-liquid-solid (SLS) growth using tin (Sn) nanocrystal seeds. The nanorods are hydrosilylated with organic ligands to stabilize the PL and prevent oxidation with PL quantum yields of 4-5%.

The combination of nanoparticles with well-established thin-film technologies is a promising way to profit from the advantages of both technological approaches: Thin-film technologies based e.g. on silicon or organics on the one hand have proven to provide excellent devices on very large scales with extraordinary high material quality at low process temperatures. Nanoparticles on the other hand can be processed prior to device fabrication and thus open up the possibility to use higher temperatures than compatible with cheap substrate materials such as glass and plastic. Additionally the short diffusion length of defects towards the surface of nanoparticles is beneficial since defects can be be passivated there. The approach of combining materials with large absorption and suitable bandgaps like CuO, Cu2O or Cu2S prepared as nanoparticles with a thin-film matrix will benefit from both technologies.
One major challenge regarding the inclusion of nanoparticles is their dispersion for a well-controlled deposition. Mono-dispersion in solution is usually achieved by stabilizing the particles with a shell of organic ligands. This shell acts as a source of impurities and can lead to the generation of an insulating layer during the inclusion into matrix material and has hence to be removed prior to device fabrication. Plasma mitigated processes can be used to remove ligands and passivate the surface of nanoparticle absorbers.
In the present paper we present results on the plasma mediated ligand removal and surface passivation of copper sulfide and copper oxide nanoparticles. Radio frequency at 13.56 MHz is applied to generate plasmas in Argon and Hydrogen gases. Plasma parameters like gas pressure and in coupled power have been varied.
The influence of plasma parameters on the morphology of the particles has been studied using scanning electron microscopy. The treatment conditions were optimized to prevent any sintering or change of the shape of the particles.
Additionally, the removal of attached ligands is studied by measuring their specific infrared absorption with fourier transform infrared spectroscopy. Using optimized conditions the organic shell that was initially present around the nanoparticles was removed.
The opto-electronic quality of the particles before and after ligand removal and passivation was studied using photoluminescence spectroscopy. To study the influence of plasma treatments on the total absorption of nanoparticle layers photothermal deflection spectroscopy was applied. With this method the removal of ligands was verified by a reduced absorption in the near IR.
Solar cell devices based on an amorphous silicon p-i-n structure with nanoparticles embedded into the intrinsic layer have been prepared. The influence of the plasma treatment on solar cell performance has been studied.

Silicon quantum dots functioning as luminescent downshifters have the potential to become a simple means for enhancing silicon solar cell efficiencies. As a result of their indirect band gap, silicon quantum dots absorb high energy photons—light that cannot be effectively utilized by the solar cell—and emit the photons at lower energy, in the spectral range where the cell is more efficient. As successful luminescent downshifters, silicon quantum dots must possess high photon conversion efficiencies, or photoluminescent quantum yields; this has been achieved in the past by passivating the surface of silicon quantum dots produced in a non-thermal plasma with organic ligands through a post-synthesis, thermally-activated hydrosilylation process. This method, however, produces quantum dots with quantum yields that degrade upon exposure to ultraviolet light. This is a significant obstacle as it indicates that sunlight can diminish the effectiveness of the luminescent downshifting system, rendering its implementation impractical. To overcome this obstacle, silicon quantum dots are produced in a process known as photochemical hydrosilylation, where organic ligands are attached onto the quantum dot&’s surface using ultraviolet light, instead of heat. This work will show that not only do the resulting quantum dots possess equally high quantum yields, but also stability against ultraviolet irradiation. Furthermore, as a gentler technique, it offers the possibility of a gas-phase quantum dot synthesis, passivation, and deposition process, eliminating the use of solvents.
Acknowledgement: This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-0819885. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program.

Semiconductor quantum dots (QDs) constitute very promising candidates as light emitters for numerous applications in the field of biotechnology, such as cell labeling or other bioimaging techniques. For such applications, semiconductor QDs represent an attractive alternative to classic organic fluorophores as they exhibit a far superior photostability by several orders of magnitude and a higher brightness thanks to large absorption cross-sections. Within this family of materials, core-shell heterostructures such as CdSe/CdS QDs are especially of interest. In particular CdSe/CdS QDs with relatively thick CdS shells offer several properties essential to biolabeling, including high photoluminescence quantum yields, low blinking behavior and robustness towards aggressive environments. We recently developed a new, fast and very efficient method for the synthesis of such QDs, denoted ‘flash&’ CdSe/CdS, which can feature up to 20 monolayers of CdS after no more than 3 minutes of synthesis. These ‘flash&’ CdSe/CdS QDs show state-of-the-art optical properties (sharp emission spectra, high photoluminescence quantum yields, low blinking behavior), and the CdS shell thickness can be easily controlled thanks to the full chemical yield of the reaction. These QDs were encapsulated in silica nanoparticles through a water-in-oil microemulsion process, a technique that allows a high control on the morphology of the resulting QD@SiO2 nanoparticles. All the nanoparticles contain one single QD located in its center and the thickness of the silica shell can be varied from only a few nanometers up to several tens of nanometers. The silica matrix provided the QDs with enhanced colloidal stability in polar solvents, but also enhanced photo-physical and chemical stability under irradiation. More importantly, QD@SiO2 nanoparticles based on ‘flash&’ CdSe/CdS QDs fully retain their photoluminescence quantum yield even after more than a year of storage in water, whereas QD@SiO2 nanoparticles based on ‘classical&’ SILAR grown core-shell QDs typically lose their luminescence after a few weeks or even days. Thereafter, these ‘flash&’ CdSe/CdS@SiO2 nanoparticles have proven to be very promising nanoprobes for bioimaging techniques. Indeed, the rapid uptake of high levels of these nanoparticles by live cells was evidenced by confocal fluorescence microscopy. Furthermore, thanks to the high stability of their optical properties but also to their low toxicity after silica encapsulation, these nanoparticles are particularly appropriate for long term cell labeling and tracking. Thus, in this contribution we will report from the synthesis and characterization of these ‘flash&’ CdSe/CdS@SiO2, all the way to the study of their toxicity and their application to cell labeling.

Tina Zhang*, Mary Ann Go+, Vincent Daria+ and Antonio Tricoli*
*Nanotechnology Research Laboratory, Research School of Engineering,
College of Engineering and Computer Sciences, Australia National University,
Canberra, Australia
+John Curtin School of Medical Research, The Australian National University,
Canberra, ACT, Australia
Highly crystalline, pure TiO2 nanoparticles of tunable size were synthesized by scalable flame spray pyrolysis of organometallic precursor[1] resulting in up to 86wt% Anatase content. A novel protocol is presented for the controlled functionalization of these particles with fluorescein isothiocyanate (FITC), an important biomedical dye. The dissociation of water into hydrogen and an active hydroxyl group is first promoted at the particle surface via UV exposure at room temperature. The adsorbed hydroxyl group is subsequently reacted with the carboxylic group of amino acid L-Lysine to form covalently functionalized, amine-capped nanoparticles. The pH, stoichiometry and time of reaction were set to control the final surface density of active amine groups and degree of polylysine formation. Functionalization in excess L-lysine pH > 7 yielded large agglomerates (~ 3mu;m) of nanoparticles inside a polypeptide matrix of 5 - 20 nm thickness, while reducing the pH to 1.5 at the same reaction stoichiometry reduced the maximal agglomerate size to 1 mu;m and organic layer became irresolvable by TEM inspection. Under stoichiometric and sub-stoichiometric conditions, the agglomerate size reached 300 nm, indistinguishable from the hard agglomeration of the nanoparticles due to sintering during flame spray synthesis, while retaining active amine groups for further reaction. Subsequent reaction with FITC in aqueous solution at room temperature produced fluorescent nanoparticles with a surface coverage of up to 0.3 dye molecules per Ti-atom. The dye attachment proved sufficiently strong and able to survive ultra-sonication and low temperature drying. The resulting particles were suitable for use as neuron imaging tools, showing high fluorescence and sufficient dispersion to flow through 0.5 mu;m needles. Functionalized particles were suspended in intracellular fluid (pH 7.25), containing K-methylsulfate, KCl, HEPES, NaCl, Mg-ATP, Na-GTP, and Alexa Fluor 594, and injected into neurons of 350 mu;m thick coronal slices from 14-day-old C57BL/6 mouse. The wide band gap (3.2 eV) semiconductor characteristic of TiO2 was exploited for simultaneous imaging and in cell photo catalysis. The effect of inducing photocatalytic reactions in the neuron by electron-hole separation in the TiO2 nanoparticles was investigated. Additionally, the cytotoxicity of these novel nanocomposites was investigated showing good biocompatibility.
1.Tricoli, A., M. Righettoni, and S.E. Pratsinis, Minimal cross-sensitivity to humidity during ethanol detection by SnO2-TiO2 solid solutions. Nanotechnology, 2009. 20(31): p. 315502.

We report synthesis and optical properties of PbSe/PbS core-shell, PbSe/CdSe/PbS, and PbS/CdS/PbS core-barrier-shell colloidal quantum dots (QDs). Engineering barrier layers between inner core and outer shell of these structures are monitored by TEM and spectroscopic signatures, and their band structures are simulated using effective mass model. While electrons are delocalized over entire QD structures, holes are confined in either core or shell depending on their geometries. This is also affected by core and shell materials, where PbSe/CdSe/PbS causes more asymmetric distribution of excitons while PbS/CdS/PbS generates more symmetric distribution of excitons by stronger coupling between core and shell. Lifetime, quantum yield, and photoluminescence excitation spectroscopy suggest how to engineer core/barrier/shell nanostructures to control exciton dynamics, which will impact on optoelectric applications such as photovoltaics and photodetectors.

A systematic study of the temperature dependence of Cd-cation replacement in PbSe and PbS quantum dots reveals energetic and dynamical insights into this important process for creating stable infrared-active quantum dots. Through appropriate choice of conditions, we demonstrate the synthesis of PbSe/CdSe and PbS/CdS core/shell quantum dots featuring very thick shells (up to 2.5 nm) and discrete core-shell interfaces without alloying. In addition to core-based infrared emission, PbSe/CdSe quantum dots with shells thicker than ~2 nm exhibit measurable visible emission. Static and ultrafast time-resolved photoluminescence studies are interpreted using a simple effective-mass calculations to suggest that the visible emission can be ascribed to 1S-electron to 2S-hole transition that arises as a result of slow intraband cooling of holes excited in the quantum dots shell. Finally, nonlinear optical studies of dual-emitting PbSe/CdSe quantum dots reveals surprisingly efficient two-photon up-conversion, with effective two-photon cross sections 1-2 orders of magnitude greater than those observed in pure CdSe quantum dots.

Cationic exchange (CE) has become a widely used method for the formation of heteronanocrystals (HNC). It proved possible for example to create multiple dot-in-rod structures featuring a series of Ag2S discs or PbS dots inside a CdS rod.1 Moreover, it has been used most extensively to from PbX/CdX (X=S,Se,Te) dot-in-dots. Even if CE is often implemented, little is known about the actual mechanism of the reaction. Recently, a mechanism based on vacancy diffusion was put forward to account for the strong tendency of the CE process to from {111} interfaces in PbSe/CdSe HCN.2 This however raises the question as to how these vacancies are formed.
Here, we analyze the formation of PbS/CdS core/shell HCN by CE starting from PbS NC synthesized using two different methods, the main difference between both being the amount of excess surface Pb. For 6.8 nm PbS NC, with the Abel synthesis3 (PbS-1), a Pb:S ratio of 1.45 was found, which amounts to a full surface coverage by excess Pb, while with the Cademartiri synthesis4 (PbS-2), an excess of only 1.28 is obtained, indicating that these NC still have vacant excess Pb2+ sites on their surface.
In the case of PbS-1 NC, the extend of the CE reaction follows a trend that is very much in line with the expectations. Thicker CdS shells are obtained for longer exchange times, higher Cd:Pb ratios in the exchange bath and higher temperatures. Nevertheless, even using the most favorable exchange conditions, shell thicknesses of only 1 nm at the most are obtained. Opposite from this, in the case of PbS-2, there is a remarkable dependence on reaction time and Cd:Pb ratio. First, even at room temperature, a CdS shell with a thickness of asymp;0.2 nm is obtained within seconds, which only grows thicker at elevated temperature. Second, the exchange proceeds further for lower Cd:Pb ratios. In fact, at Cd:Pb ratios of 0.5 and 0.25, part of the PbS NC can be completely transformed into CdS, while at high Cd:Pb ratios, similar results as with PbS-1 particles are obtained, i.e., 1 nm thick shells.
These observations show that unoccupied excess Pb2+ sites at the PbS NC surface enhance CE reaction by providing (1) adsorption sites for dissolved Cd2+ species and (2) vacancies for the transfer of Pb2+ from the inside of the NC to the surface. Especially the latter point is well demonstrated by the anomalous dependence of exchange rate on the Cd:Pb ratio. When the amount of Cd is too low, obviously too little Cd is provided for CE to happen. However, when the amount of Cd is too high, adsorbed Cd2+ will block all empty surface sites, thus hampering the transfer of Pb2+ cations. By stressing the importance of the surface reaction, this result provides an essential missing link in the mechanistic understanding of CE reactions and may thus help to achieve better control over these processes and the resulting HCN.
1 JACS, 2012, 134 (12), 5484
2 Chem. Mater., 2012, 24 (2), 294
3 Chem. Mater. 2008, 20, 3794
4 J. Phys. Chem. B 2006, 110, 671

In semiconductors, heterostructure architectures enable modulation of electrical and optical properties in devices. However in nanoparticles (NPs), methods to create 2D quantum well heterostructures in spherical particles are not well-developed. Here, we demonstrate dual interface formation in NPs through cation exchange, creating epitaxial heterostructures within isotropic, spherical NPs. To achieve this, we perform a cation exchange reaction on copper sulfide NPs, converting them into a hexagonal zinc sulfide (ZnS). Interfaces symmetrically form between the {100} planes of copper sulfide and the {001} planes of ZnS from opposing sides of the NPs due to the similar sulfur sublattice shared by copper sulfide and ZnS. By controlling the reaction time, the copper sulfide region can be confined to a 2D layer 1 nm to 10 nm in width in the center of the NP forming a 2D quantum well-type structure. Upon full conversion, the ZnS nanoparticles become single crystalline and preserve the original shape and size of the initial copper sulfide nanoparticles. Additionally, cation exchange from copper sulfide to hexagonal cadmium sulfide (CdS) yields similar interface formation. However, the interface curvature for the copper sulfide-CdS NPs differs from that of the copper sulfide-ZnS NPs due to opposite interfacial strains. The smaller lattice constant of ZnS induces concave interfaces between copper sulfide and ZnS whereas the larger lattice constant of CdS creates convex interfaces between copper sulfide and CdS. These heterostructured NPs exhibit surface plasmon resonance due to the intrinsically high carrier concentration of the copper sulfide phase. The surface plasmon resonance can be tuned over a broad range of wavelengths from 1270 nm to 1800 nm by controlling the thickness of the 2D copper sulfide layer confined between either ZnS or CdS. These NPs with heterostructures might show unique charge transfer and photoluminescence since they build type-II energy band alignments. Our interface formation using chemical transformations may provide a new approach to creating heterostructures in semiconductor nanomaterials.

In binary semiconductor nanocrystals (e.g. CdSe, PbS) the optical and electronic properties can be precisely tuned by controlling the size. In ternary compounds like CuInS2, on the other hand, changing the stoichiometry adds a further degree of freedom, which enables to switch from p-type (Cu-rich) to n-type (In-rich) behavior. However, widely applied synthesis methods relying on the use of dodecanethiol (DDT) as the sulfur source and surface ligand do not allow for the independent control of composition and size of the nanocrystals. We carried out an in situ X-ray study of the nucleation and growth of CuInS2 nanocrystals using synchrotron radiation. SAXS measurements during the pre-nucleation stage indicate the auto-assembly of the precursors in a lamellar phase. The formation of this phase has direct influence on the growth kinetics of the nanocrystals. In any case the reaction proceeds through an initial copper-rich phase, while indium is subsequently incorporated into the nanocrystals.
Ligand exchange with small organic or inorganic ligands is a general strategy for reducing the barrier for charge transfer and transport imposed by initial insulating surface ligands. We present conductivity measurements on assemblies of CuInS2 nanocrystals as a function of surface ligands, showing a dramatic increase in conductivity when replacing DDT with tetrafluoroborate or sulfide ligands. As an alternative, we propose a novel synthesis method, which directly yields CuInS2 nanocrystals capped with very short ligands.
Finally, the use of CuInS2 nanocrystals in hybrid solar cells combined with P3HT and PCBM will be discussed. As compared to neat P3HT:PCBM devices a remarkable improvement of the efficiency is observed for P3HT:PCBM:CuInS2 blends of 1:1:0.5 wgt ratio. By means of light-induced electron spin resonance measurements we identify enhanced exciton dissociation and charge carrier generation to be at the origin of this behavior.

Current synthetic techniques for III-V colloidal quantum dots (QDs) lag substantially behind those for other similar materials such as CdSe. Colloidal III-V QDs are attractive as visible emitters for lighting and display technology (InP) or stable infrared fluorophores for deep tissue in-vivo imaging (InAs). However, these materials have proven more challenging to synthesize with the narrow size distributions that are readily obtained using II-VI and IV-VI materials such as CdSe and PbSe. The superior size distributions of II-VI and IV-VI materials can be explained by the size focusing that occurs as molecular precursors react and add to growing nanocrystals. In contrast, previous studies showed that the III-V precursors react quickly, and particles grow in the absence of molecular precursors.
Here we supplement previous reports of mechanistic studies and novel precursors for III-V synthesis with the development of additional group-V precursors for both InP and InAs that have reaction rate constants that span several orders of magnitude. We performed a systematic of the effect of rate constant on III-V QD growth. This systematic study illuminates differences between the effect of precursor reaction rates on InP growth and InAs growth.

Semiconductor colloidal nanocrystals (NCs) are promising materials in many research fields including biomedicine, biosensing, or optoelectronics.1,2 During the past years, an exceptional research effort has been accomplished in order to develop well-defined methods to obtain and characterize highly monodisperse and efficient NCs. However, a better knowledge about the nucleation events determining the final structure and properties of these NCs is still missing. This is of high importance to understand the final crystal structure and size and shape of the nanostructures, since the stable nuclei for the formation of these NCs is not necessarily identical to the small clusters formed in the first nucleation stages. Likely, these initial structures form other intermediate products that evolve to form the final NCs. Parameters such as initial precursors ratio, temperature and capping surface ligands are known to strongly influence the nucleation and growth pathways and thus, their influence in the nucleation process should be deeply studied.
CdTe represents a model system to study the early nucleation events and the influence of different reaction parameters, such as ligand type and concentration, since the energy difference between the cubic (zinc blende) and hexagonal (wurtzite) crystal structures is large enough to promote (by the hot injection method) the nucleation in the cubic phase followed by a growth in the wurtzite phase. As a result, cubic-wurtzite tetrapod CdTe NCs are observed. 3
The aim of this work is to study the nucleation events on CdTe NCs, with special attention to the influence of different reaction parameters, such as the nature and concentration of different phosphonic capping ligands. Optical measurements as well as X-ray diffraction experiments have been used to study the structure and optical properties of the CdTe NCs. Moreover, with the aim to investigate the dynamics and kinetics of NCs formation, and to overcome the limited time-resolution of these conventional techniques, we are developing a continuous flow reactor, which allows studying different reaction times. Its combination with either optical lasers or X-ray beam, allows to deeply study the first nucleation stages of the mentioned model system. From this knowledge, more complex systems could be studied.
1Gill, R.; Zayats, M.; Willner, I. Angew. Chem. Int. Ed. 2008, 7602-7625.
2Talapin, D.V.; Lee, J.S.; Kovalenko, M.V.; Shevchenko, E.V. Chem. Rev. 2010, 389-458.
3Manna, L.; Milliron D.J.; Meinsel, A.; Scher, E.C.; Alivistaos, A.P. Nature Materials, 2003, 382-385.

Lead sulfide (PbS) nanocrystals have a size-dependent, tunable bandgap in the infrared region, making them particularly interesting for photovoltaics, photodetectors, and infrared communication. In most of these applications it is beneficial to have monodisperse nanocrystals- whether to provide a flat energy landscape for charge transport or to ensure narrow absorbance / emission spectra. To date, the synthetic recipes for PbS nanocrystals have not been able to achieve the same degree of monodispersity as nanocrystals of CdSe or PbSe. In this work, we find that by controlling the stoichiometry of our lead and sulfur precursor species we can synthesize monodisperse PbS nanocrystals (size distribution standard deviation < 5% of mean diameter) over a wide range of sizes. Specifically, we employ a large excess of Pb precursor to achieve monodisperse nanocrystals from 4.5 - 8.0 nm in diameter, corresponding to absorbance peaks from 1000 - 1800 nm, or 1.25 - 0.70 eV. We hypothesize that the large Pb-to-S ratio delays the onset of Ostwald ripening, as indicated by the time evolution of the absorption linewidth for varying Pb concentrations. We observe absorbance peak half-width at half-max (HWHM) values as small as 20 meV, and obtain evidence that such numbers reflect an ensemble that is almost entirely homogeneously broadened. This degree of monodispersity enables accurate measurements of interparticle spacing and the formation of well-ordered nanocrystal superlattices. Moreover, the nanocrystals are air-stable, exhibiting no change in absorbance features for over three months.