Nanocrystal Size

[Cre Moonin]

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Interactions between ceria (CeO2) and supported metals greatly enhance rates for a number of important reactions. However, direct relationships between structure and function in these catalysts have been difficult to extract because the samples studied either were heterogeneous or were model systems dissimilar to working catalysts. We report rate measurements on samples in which the length of the ceria-metal interface was tailored by the use of monodisperse nickel, palladium, and platinum nanocrystals. We found that carbon monoxide oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts, clarifying the pivotal role played by the support.

We measure the room-temperature electron and hole field-effect mobilities (micro(FE)) of a series of alkanedithiol-treated PbSe nanocrystal (NC) films as a function of NC size and the length of the alkane chain. We find that carrier mobilities decrease exponentially with increasing ligand length according to the scaling parameter beta = 1.08-1.10 A(-1), as expected for hopping transport in granular conductors with alkane tunnel barriers. An electronic coupling energy as large as 8 meV is calculated from the mobility data. Mobilities increase by 1-2 orders of magnitude with increasing NC diameter (up to 0.07 and 0.03 cm(2) V(-1) s(-1) for electrons and holes, respectively); the electron mobility peaks at a NC size of approximately 6 nm and then decreases for larger NCs, whereas the hole mobility shows a monotonic increase. The size-mobility trends seem to be driven primarily by the smaller number of hops required for transport through arrays of larger NCs but may also reflect a systematic decrease in the depth of trap states with decreasing NC band gap. We find that carrier mobility is independent of the polydispersity of the NC samples, which can be understood if percolation networks of the larger-diameter, smaller-band-gap NCs carry most of the current in these NC solids. Our results establish a baseline for mobility trends in PbSe NC solids, with implications for fabricating high-mobility NC-based optoelectronic devices.

The ability to precisely control the composition of nanocrystals, similar to the way organic chemists control the structure of small molecules, remains an important challenge in nanoscience. Rather than dictating nanocrystal size through the nucleation event, growth of nanocrystals through continuous precursor addition would allow fine structural control. Herein, we present a method of growth for indium oxide nanocrystals that relies on the slow addition of an indium carboxylate precursor into hot oleyl alcohol. Nanocrystal size and structure can be governed at a subnanometer scale, and it is possible to precisely control core size over a range of three to at least 22 nm with dispersities as low as 7%. Growth can be stopped and restarted repeatedly without aggregation or passivation. We show that the volume of the nanocrystal core (and thus molecular weight) increases linearly with added monomer and the number of nanocrystals remains constant throughout the growth process, yielding an extremely predictable approach to size control. It is also possible to place metal oxide shells (e.g., Sn-doped In2O3 (ITO)) at various radial positions within the nanocrystal, and we use this approach to synthesize ITO/In2O3 core/shell nanocrystals as well as In2O3/ITO/In2O3 core/shell/shell nanocrystals.

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Colloidal nanocrystals are made via solution-phase chemistry and consist of an inorganic core with organic ligands bound to its surface. Rapid progress in the field of colloidal nanocrystal synthesis1,2 has led to their use in numerous applications such as LEDs3,4, optoelectronics5,6, electronics7,8, thermal storage9,10 and thermoelectrics11,12. One potential concern for using colloidal nanocrystals in these applications is that their lower melting temperatures may be incompatible with elevated temperatures during device operation and/or fabrication. It is well known that the melting temperature of nanoparticles decreases as their characteristic size decreases and this phenomenon is commonly referred to as melting point depression13,14,15,16,17,18,19,20,21,22. However, the vast majority of melting point depression studies focus on a sub-monolayer of nanoparticles prepared by dewetting of a thin film. Nanoparticles prepared via other methods such as ball milling and/or colloidal synthesis are more commonly used and could exhibit different behavior due to variations in structure-property-processing relations. In addition, nanoparticles are often embedded in matrices, which remove the free surface of dewetted nanoparticles and also change melting behavior.

In this study, we focus on the melting behavior of colloidal nanocrystals, whose phase change properties have been largely unexplored. Goldstein et al.16 used transmission electron microscopy (TEM) to measure colloidal nanocrystal melting temperature, but did not measure melting enthalpy. Calorimetry measurements can determine both melting temperature and melting enthalpy and a few other researchers have used this approach to investigate colloidal nanocrystal melting23,24,25,26. Unfortunately, extracting size-dependent melting behavior from these experiments is problematic because the colloidal nanocrystals were poorly isolated from one another24,26. This led to nanocrystal coalescence during the calorimetry measurements and manifested itself as a melting temperature that drifted toward bulk values during thermal cycling25. In some cases, the nanoscale dimensions of the nanocrystals degraded so fast that bulk melting temperatures were observed during the first melting cycle23,24.

In this expression, h is a characteristic length representing the height of an atomic monolayer on a bulk surface and is estimated from the crystal lattice constant. We obtained reasonable fits to our experimental data on both Sn and In nanocrystals in PI resin with an α value of 1.54. It is intuitive that the same α value can be used to fit both sets of data because their matrices are the same and because Sn and In have the same crystal structure. The α value for Bi nanocrystals in PI resin was 1.42 and differed slightly from Sn and In. We speculate that this slight difference may arise from a difference in crystal structure and hence a change in the value for h in Equation 1. The α value for Bi nanocrystals in an Ag matrix was 1.2 and indicates that the rigid Ag matrix suppresses MSD comparatively more than the soft PI resin matrix.

While reports on size-dependent melting temperature are widespread, reports on size-dependent melting enthalpy are relatively limited17,19,35. One of the primary challenges in measuring size-dependent melting enthalpy stems from the preparation of the experimental sample. Samples for size-dependent melting studies are typically prepared by vapor depositing a thin film on a substrate that subsequently dewets and forms a sub-monolayer of nanoparticles. These samples produce small thermal energy signals during phase change and require sophisticated microfabricated nanocalorimeters to determine their melting enthalpy17,19,20.

The significant difference in depression rates for melting temperature and melting enthalpy means that melting entropy is also size-dependent and decreases with nanocrystal size. Melting entropy represents the difference between the solid state entropy and liquid state entropy. The entropy of a solid increases as nanoparticle diameter decreases due to the growing fraction of surface atoms, which have larger MSD36,37. This brings the entropy of solid nanoparticles closer to the liquid state entropy and leads to decreasing melting entropy as nanocrystal diameter is reduced. Figure S4 illustrates the entropy size dependence for our colloidal In, Sn and Bi nanocrystals.

This work was supported by the National Science Foundation through Grant no. CBET-1236656 and by the Australian Renewable Energy Agency. We gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University. We thank Lenore Dai and Konrad Rykaczewski from Arizona State University for helpful discussions.

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Femtosecond photon-echo techniques are used to probe the dynamics of quantum-confined excitons in nanocrystals of CdSe. Using three-pulse photon echoes, the modulation of the echo signal from the LO-phonon mode is effectively suppressed, and both the electronic dephasing and the couplings to lattice vibrations are probed directly. Detailed measurements are reported as a function of both nanocrystal size and temperature. The dephasing times vary from 85 fs in nanocrystals of 20- diameter to 270 fs in 40- crystals. These rates are determined by several dynamical processes, all of which depend sensitively on the size of the nanocrystal. The time scale of the trapping of the electronic excitation to surface states increases with increasing size. The coupling of the excitation to low-frequency vibrational modes is strongly size dependent as well, in accordance with a theoretical model. The photon echo also gives information on the polar coupling between the electronic state and the LO-phonon mode. This coupling is found to peak at an intermediate size. This phenomenon is interpreted as a manifestation of coupling between the interior confined excitons and localized surface states, which destroys the spherical symmetry of the excited state. Using these data, all of the important contributions to the size-dependent homogeneous linewidths can be enumerated.

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