Maksym Yarema

Monodisperse and Inorganically Capped Sn and Sn/SnO2 Nanocrystals for High-Performance Li-Ion Battery Anodes

We report a facile synthesis of highly monodisperse colloidal Sn and Sn/SnO2 nanocrystals with mean sizes tunable over the range 9-23 nm and size distributions below 10%. For testing the utility of Sn/SnO2 nanocrystals as an active anode material in Li-ion batteries, a simple ligand-exchange procedure using inorganic capping ligands was applied to facilitate electronic connectivity within the components of the nanocrystalline electrode. Electrochemical measurements demonstrated that 10 nm Sn/SnO2 nanocrystals enable high Li insertion/removal cycling stability, in striking contrast to commercial 100-150 nm powders of Sn and SnO2. In particular, reversible Li-storage capacities above 700 mA h g(-1) were obtained after 100 cycles of deep charging (0.005-2 V) at a relatively high current of 1000 mA h g(-1).

Infrared Emitting and Photoconducting Colloidal Silver Chalcogenide Nanocrystal Quantum Dots from a Silylamide-Promoted Synthesis

Here, we present a hot injection synthesis of colloidal Ag chalcogenide nanocrystals (Ag(2)Se, Ag(2)Te, and Ag(2)S) that resulted in exceptionally small nanocrystal sizes in the range between 2 and 4 nm. Ag chalcogenide nanocrystals exhibit band gap energies within the near-infrared spectral region, making these materials promising as environmentally benign alternatives to established infrared active nanocrystals containing toxic metals such as Hg, Cd, and Pb. We present Ag(2)Se nanocrystals in detail, giving size-tunable luminescence with quantum yields above 1.7%. The luminescence, with a decay time on the order of 130 ns, was shown to improve due to the growth of a monolayer thick ZnSe shell. Photoconductivity with a quantum efficiency of 27% was achieved by blending the Ag(2)Se nanocrystals with a soluble fullerene derivative. The co-injection of lithium silylamide was found to be crucial to the synthesis of Ag chalcogenide nanocrystals, which drastically increased their nucleation rate even at relatively low growth temperatures. Because the same observation was made for the nucleation of Cd chalcogenide nanocrystals, we conclude that the addition of lithium silylamide might generally promote wet-chemical synthesis of metal chalcogenide nanocrystals, including in as-yet unexplored materials.

Low Driving Voltage and High Mobility Ambipolar Field-Effect Transistors with PbS Colloidal Nanocrystals

PbS colloidal nanocrystals (NCs) are promising materials for optoelectronic devices, due to their size-tunable properties. However, there is still minimal understanding of their charge transport mechanism. Through a combination of ligand selections, ambipolar transistor structure optimization, and electrochemical gating usage, high carrier mobility is achieved. The outstanding device characteristics open possibility to investigate the intrinsic transport properties of PbS NCs.

Highly Monodisperse Bismuth Nanoparticles and Their Three-Dimensional Superlattices

A simple and reproducible synthesis of highly monodisperse and ligand-protected bismuth nanoparticles (Bi NPs) is reported. The size of the single-crystalline and spherically shaped NPs is controlled between 11 and 22 nm mainly by the reaction temperature. The high uniformity of the NPs allows their self-assembly into long-range-ordered two- and three-dimensional superstructures.

Highly Luminescent, Size- and Shape-Tunable Copper Indium Selenide Based Colloidal Nanocrystals

We report a simple, high-yield colloidal synthesis of copper indium selenide nanocrystals (CISe NCs) based on a silylamide-promoted approach. The silylamide anions increase the nucleation rate, which results in small-sized NCs exhibiting high luminescence and constant NC stoichiometry and crystal structure regardless of the NC size and shape. In particular, by systematically varying synthesis time and temperature, we show that the size of the CISe NCs can be precisely controlled to be between 2.7 and 7.9 nm with size distributions down to 9–10%. By introducing a specific concentration of silylamide-anions in the reaction mixture, the shape of CISe NCs can be preselected to be either spherical or tetrahedral. Optical properties of these CISe NCs span from the visible to near-infrared region with peak luminescence wavelengths of 700 to 1200 nm. The luminescence efficiency improves from 10 to 15% to record values of 50–60% by overcoating as-prepared CISe NCs with ZnSe or ZnS shells, highlighting their potential for applications such as biolabeling and solid state lighting.

From Highly Monodisperse Indium and Indium Tin Colloidal Nanocrystals to Self-Assembled Indium Tin Oxide Nanoelectrodes

Indium tin oxide (ITO) nanopatterned electrodes are prepared from colloidal solutions as a material saving alternative to the industrial vapor phase deposition and top down processing. For that purpose highly monodisperse In(1-x)Sn(x) (x < 0.1) colloidal nanocrystals (NCs) are synthesized with accurate size and composition control. The outstanding monodispersity of the NCs is evidenced by their self-assembly properties into highly ordered superlattices. Deposition on structured substrates and subsequent treatment in oxygen plasma converts the NC assemblies into transparent electrode patterns with feature sizes down to the diameter of single NCs. The conductivity in these ITO electrodes competes with the best values reported for electrodes from ITO nanoparticle inks.

Soft surfaces of nanomaterials enable strong phonon interactions

Phonons and their interactions with other phonons, electrons or photons drive energy gain, loss and transport in materials. Although the phonon density of states has been measured and calculated in bulk crystalline semiconductors, phonons remain poorly understood in nanomaterials, despite the increasing prevalence of bottom-up fabrication of semiconductors from nanomaterials and the integration of nanometre-sized components into devices. Here we quantify the phononic properties of bottom-up fabricated semiconductors as a function of crystallite size using inelastic neutron scattering measurements and ab initio molecular dynamics simulations. We show that, unlike in microcrystalline semiconductors, the phonon modes of semiconductors with nanocrystalline domains exhibit both reduced symmetry and low energy owing to mechanical softness at the surface of those domains. These properties become important when phonons couple to electrons in semiconductor devices. Although it was initially believed that the coupling between electrons and phonons is suppressed in nanocrystalline materials owing to the scarcity of electronic states and their large energy separation, it has since been shown that the electron–phonon coupling is large and allows high energy-dissipation rates exceeding one electronvolt per picosecond (refs 10, 11, 12, 13). Despite detailed investigations into the role of phonons in exciton dynamics, leading to a variety of suggestions as to the origins of these fast transition rates and including attempts to numerically calculate them, fundamental questions surrounding electron–phonon interactions in nanomaterials remain unresolved. By combining the microscopic and thermodynamic theories of phonons and our findings on the phononic properties of nanomaterials, we are able to explain and then experimentally confirm the strong electron–phonon coupling and fast multi-phonon transition rates of charge carriers to trap states. This improved understanding of phonon processes permits the rational selection of nanomaterials, their surface treatments, and the design of devices incorporating them.

Monodisperse colloidal gallium nanoparticles: synthesis, low temperature crystallization, surface plasmon resonance and Li-ion storage.

We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable in the range of 12–46 nm and with excellent size distribution as small as 7–8%. When stored under ambient conditions, Ga nanoparticles remain stable for months due to the formation of native and passivating Ga-oxide layer (2–3 nm). The mechanism of Ga nanoparticles formation is elucidated using nuclear magnetic resonance spectroscopy and with molecular dynamics simulations. Size-dependent crystallization and melting of Ga nanoparticles in the temperature range of 98–298 K are studied with X-ray powder diffraction, specific heat measurements, transmission electron microscopy, and X-ray absorption spectroscopy. The results point to delta (δ)-Ga polymorph as a single low-temperature phase, while phase transition is characterized by the large hysteresis and by the large undercooling of crystallization and melting points down to 140–145 and 240–250 K, respectively. We have observed size-tunable plasmon resonance in the ultraviolet and visible spectral regions. We also report stable operation of Ga nanoparticles as anode material for Li-ion batteries with storage capacities of 600 mAh g–1, 50% higher than those achieved for bulk Ga under identical testing conditions.

Crystal Phase Transitions in the Shell of PbS/CdS Core/Shell Nanocrystals Influences Photoluminescence Intensity

We reveal the existence of two different crystalline phases, i.e., the metastable rock salt and the equilibrium zinc blende phase within the CdS-shell of PbS/CdS core/shell nanocrystals formed by cationic exchange. The chemical composition profile of the core/shell nanocrystals with different dimensions is determined by means of anomalous small-angle X-ray scattering with subnanometer resolution and is compared to X-ray diffraction analysis. We demonstrate that the photoluminescence emission of PbS nanocrystals can be drastically enhanced by the formation of a CdS shell. Especially, the ratio of the two crystalline phases in the shell significantly influences the photoluminescence enhancement. The highest emission was achieved for chemically pure CdS shells below 1 nm thickness with a dominant metastable rock salt phase fraction matching the crystal structure of the PbS core. The metastable phase fraction decreases with increasing shell thickness and increasing exchange times. The photoluminescence intensity depicts a constant decrease with decreasing metastable rock salt phase fraction but shows an abrupt drop for shells above 1.3 nm thickness. We relate this effect to two different transition mechanisms for changing from the metastable rock salt phase to the equilibrium zinc blende phase depending on the shell thickness.

Hole Mobility in Nanocrystal Solids as a Function of Constituent Nanocrystal Size

Solids of semiconductor nanocrystals (NCs) are semiconductors in which the band gap can be controlled by changing the size of the constituent NCs. To date, nontrivial dependencies of the carrier mobility on the NC size have been reported. We use the time-of-flight (TOF) technique to measure the carrier mobility as a function of the NC size and find that the hole mobility of the NC solid increases dramatically with decreasing NC radius. We show that this result is in agreement with an analytic model for carrier mobility in NC solids. We further implement Monte Carlo simulations to aid in understanding the transient measurements in the context of models of dispersive transport. This work highlights that changing NC size in a device has important implications for charge transport.