Larger Nanocrystals Research Articles

Controllable growth of BiOCl film with high percentage of exposed {0 0 1} facets

Bismuth oxychloride (BiOCl) granular and flake-like films with specific {001} crystal facets are synthesized by a facile hydrolysis route. The percentage of reactive {001} facets is up to 97.36% in granular film and 91.69% in flake-like film, respectively. It is found that the synthesized granular film is composed by the nano grains with average size around 5–10nm, but the flake-like film contains the flakes originated from the large nano crystals. The percentage of the {001} reactive facets and morphology of the BiOCl film can be tuned by adjusting the concentration or the pH value of the solution during the synthesis process, companied with the appearance of the (101), (102), and (110) facets. The formation mechanism of {001}-preferential orientation has been rationally explained in terms of crystal growth habit and dynamics. Photocatalytic degradation of RhB dye has been observed under UV light, representing that the flake-like film with {001} facets preferred orientation exhibits excellent photoactivity.

Charge Trapping Properties of Au Nanocrystals with Various Sizes for Non-Volatile Memory Applications

Charge trapping properties of Au nanocrystals (NCs) with various sizes and densities embedded in metal oxide semiconductor (MOS) were investigated. Comparing with the Nc size, experimental results show that the NC spacing is more influential in electron trapping. Au NCs with the size of 8~13nm have larger memory window than the Au NCs of 13-17nm and 6-8nm, which can be ascribe to the higher field-enhancement effect caused by the larger NC spacing. Optimized NC size can improve the specific characteristic of memory device effectively.

Spectroscopic investigations of dark Si nanocrystals in SiO2 and their role in external quantum efficiency quenching

The percentage of dark silicon nanocrystals, i.e., the nanocrystals that are not able to radiatively recombine after absorption of a photon, is investigated by combining measurements of external and internal quantum efficiencies. The study is conducted on samples prepared by co-sputtering and subsequent heat treatments. We show that the external quantum efficiency is mainly limited by the presence of dark nanocrystals, which induce losses after direct excitation and also, as we propose, by indirect excitation enabled by energy migration. The percentage of dark nanocrystals can be decreased by high quality surface passivation as a result of low-temperature annealing in ambients of O2 and H2. By using a non-passivated sample as a reference, the relation between the size of a nanocrystal and its probability of being dark is studied. Larger nanocrystals are demonstrated to function more likely as dark centers. The study shows that high external quantum efficiencies of Si nanocrystal ensembles can be realized for small, well passivated Si nanocrystals under suppression of excitation diffusion.

Atomistic Simulations of the Surface Coverage of Large Gold Nanocrystals

Here, the adsorption of alkanethiols (from ethane to dodecanethiol) on icosahedral gold nanocrystals with diameters up to 10 nm is studied by molecular dynamics simulations in a vacuum. The surface coverage of the nanocrystals obtained in the simulations is in good agreement with experimental data. We show that the average surface per adsorbed thiol does not markedly depend on the nanocrystal size and ligand and is only about 10% lower than the value observed on a flat Au(111) surface. We observe two different molecular organizations of the thiolates on the edges and in the centers of the nanocrystal facets. The incompatibility between both organizations explains the fact that the formation of self-assembled monolayers usually observed on flat Au(111) surfaces is hindered for nanocrystals smaller than 6 nm. We also show that the organization of thiolates on the edges is at the origin of the lower average surface per adsorbed thiol found for the nanocrystal.

The influence of different niobium pentoxide precursors on the solid-state synthesis of potassium sodium niobate

Two batches of K0.5Na0.5NbO3 were prepared from the orthorhombic and monoclinic Nb2O5 polymorphs and potassium and sodium carbonates. The influence of the different Nb2O5 precursors on the solid-state synthesis of K0.5Na0.5NbO3 was studied. To reduce the particle size, both types of Nb2O5 were milled prior to use. XRD and TEM analyses showed that the milled orthorhombic Nb2O5 was single phase; however, after milling the monoclinic Nb2O5 consisted of large monoclinic particles and orthorhombic nanocrystals. The latter reacted with the carbonates to form (KxNa1-x)NbO3 solid solutions with varying K/Na molar ratios, while the orthorhombic Nb2O5 reacted to form a homogeneous solid solution of K0.5Na0.5NbO3. Sintering of the two powder compacts resulted in different densification behavior and microstructure. This study shows the important influence of the Nb2O5 precursor phase and the particle size distribution on the homogeneity and further densification of the potassium sodium niobate solid solution.

Theory of Photoinjection of Hot Plasmonic Carriers from Metal Nanostructures into Semiconductors and Surface Molecules

We investigate theoretically the effects of generation and injection of plasmonic carriers from an optically excited metal nanocrystal to a semiconductor contact or to surface molecules. The energy distributions of optically excited hot carriers are dramatically different in metal nanocrystals with large and small sizes. In large nanocrystals, the majority of hot carriers has very small excitation energies, and the excited-carrier distribution resembles the case of a plasmon wave in bulk. For nanocrystal sizes smaller than 20 nm, the carrier distribution extends to larger energies and occupies the whole region EF < ε < EF + ℏω. The physical reason for the above behaviors is nonconservation of momentum in a nanocrystal. Because of the above properties, nanocrystals of small sizes are most suitable for designing opto-electronic and photosynthetic devices based on injection of plasmonic electrons and holes. For gold nanocrystals, the optimal sizes for efficient generation of hot carriers with overbarrier energies are in the range of 10–20 nm. Another important factor is the polarization of the exciting light. For efficient excitation of carriers with high energies, the electric-field polarization vector should be perpendicular to a prism-like nanoantenna (slab or platelet). We also show the relation between our theory for injection from plasmonic nanocrystals and the Fowler theory of injection from a bulk metal. Along with a prism geometry (or platelet geometry), we consider cubes. The results can be applied to design both purely solid-state opto-electronic devices and systems for photocatalysis and solar-energy conversion.

Pronounced Multiferroicity in Oleic Acid Stabilized BiFeO3 Nanocrystals at Room Temperature

We report on the experimental observation of pronounced multiferroicity in BiFeO3 nanocrystals (size approximately 40 nm) at room temperature. Large scale BiFeO3 nanocrystals are synthesized using a low temperature chemical route and further stabilized with oleic acid. The nanocrystals exhibit a significant distortion in lattice parameter c compared to the bulk. Oleic acid plays an important role in reducing oxygen vacancies and Fe2+ ions at the nanocrystal surface giving rise to a high resistivity (approximately 10(10) omega-cm at 300 K) of the sample. The direct band gap of nanocrystals is measured to be approximately 4.2 eV (about 1.5 times the bulk value) suggesting a strong quantum confinement effect. The nanocrystals show a remarkably high spontaneous saturation magnetization approximately 4.39 emu/g along with a prominent ferroelectric hysteresis loop at room temperature. Particle size effect leading to the appearance of large number of uncompensated spins and suppression of modulated spin structure have resulted a strong spontaneous magnetization in such nanoscale multiferroic materials.

Visualizing the electrochemical reaction of ZnO nanoparticles with lithium by in situ TEM: two reaction modes are revealed

The lithiation reaction of ZnO as an anode in a lithium-ion battery (LIB) is unclear. The electrochemical behavior of ZnO was investigated inside a transmission electron microscope (TEM) by constructing a nano-LIB using an individual ZnO/graphene sheet as the electrode. The lithiation reaction of ZnO/graphene was monitored by simultaneous determination of the structure with high-resolution TEM, electron diffraction pattern and electron energy-loss spectroscopy. Two kinds of reaction modes were revealed in terms of different reaction rates. One was the violent reaction mode, in which one particle can evolve into an aggregate of many nanoparticles within the Li2O matrix in 1–2 min. The other was the peaceful evolution mode, in which each ZnO nanoparticle evolves into a core–shell particle with multi-domains constituted of Zn and LiZn nanograins. Abnormally large Zn nanocrystals grow quickly in the violent reaction mode, which can suppress the formation of LiZn and impair the reversible capacity. Our observations give direct evidence and important insights for the lithiation mechanism of metal oxide anodes in LIBs.

Synthesis of chalcopyrite CuIn1−xGaxSe2 alloys for photovoltaic application by a novel melting method

Abstract CuIn 1− x Ga x Se 2 (CIGS) compounds used for the fabrication of thin-film solar cell absorber layers have been synthesized by a novel melting method which is easily controlled and practical. Based on vapor phase transport caused by the temperature gradient built, the quaternary CIGS alloy was prepared from the low-cost elementary Cu, In, Ga and Se in a closed quartz tube at 1100 °C for 3 h. Through various characterization methods, the as-synthesized alloy with crystal grains sized 100–150 µm presented a desirable chemical composition and a single-phase chalcopyrite structure. Furthermore, the CIGS absorber layer made from this material turned out to be high-quality with a correct phase and large nanocrystals, indicating great application potential of the proposed method in low-cost solar cell fabrication.

Ultrahigh throughput plasma processing of free standing silicon nanocrystals with lognormal size distribution

We demonstrate a method for synthesizing free standing silicon nanocrystals in an argon/silane gas mixture by using a remote expanding thermal plasma. Transmission electron microscopy and Raman spectroscopy measurements reveal that the distribution has a bimodal shape consisting of two distinct groups of small and large silicon nanocrystals with sizes in the range 2–10 nm and 50–120 nm, respectively. We also observe that both size distributions are lognormal which is linked with the growth time and transport of nanocrystals in the plasma. Average size control is achieved by tuning the silane flow injected into the vessel. Analyses on morphological features show that nanocrystals are monocrystalline and spherically shaped. These results imply that formation of silicon nanocrystals is based on nucleation, i.e., these large nanocrystals are not the result of coalescence of small nanocrystals. Photoluminescence measurements show that silicon nanocrystals exhibit a broad emission in the visible region peaked at 725 nm. Nanocrystals are produced with ultrahigh throughput of about 100 mg/min and have state of the art properties, such as controlled size distribution, easy handling, and room temperature visible photoluminescence.

Size-Dependent Trap-Assisted Auger Recombination in Semiconductor Nanocrystals

The acceleration of Auger-type multicarrier recombination in semiconductor nanocrystals impedes the development of many quantum-dot photonics, solar-cell, lighting, and lasing technologies. To date, only multiexciton and charged-exciton Auger recombination channels are known to show strong size dependence in nanocrystals. Here, we report the first observation of strongly accelerated "trap-assisted" Auger recombination rates in semiconductor nanocrystals. Trap-assisted Auger recombination in ZnO nanocrystals, involving the recombination of conduction-band electrons with deeply trapped holes via nonradiative energy transfer to extra conduction-band electrons, has been probed using time-resolved photoluminescence and transient absorption spectroscopies. We demonstrate that this trap-assisted Auger recombination accelerates dramatically with decreasing nanocrystal size, having recombination times of >1 ns in the largest nanocrystals but only ~80 ps in the smallest. These trap-assisted Auger recombination rates are shown to scale with inverse nanocrystal radius squared (1/τ(Auger) ~ R(-2)). Because surface carrier traps are ubiquitous in colloidal semiconductor nanocrystals, such fast trap-assisted Auger recombination is likely more prevalent in semiconductor nanocrystal photophysics than previously recognized.

Preparation and photoelectrochemical properties of TiO2 hollow spheres embedded TiO2/CdS photoanodes for quantum-dot-sensitized solar cells

A novel process was developed by introducing TiO2 hollow spheres (THS) into TiO2 nanoparticles film as light scattering centers to improve the light harvesting of CdS sensitized TiO2 (TiO2/CdS) photoanodes for quantum-dot-sensitized solar cells (QDSCs). The THS in TiO2 nanoparticles film can largely enhance the light scattering due to the multi-reflection among or within the THS, leading to the increase of optical path length in TiO2 film. Furthermore, the THS in TiO2 particles favors the penetration of CdS nanocrystals in the porous TiO2 film. As a result, the photocurrent is significantly improved with THS embedding in TiO2/CdS photoanodes. The experiment results suggest that an optimal content of THS is highly desired, too high content of THS in the photoanode will degrade the light to electricity conversion efficiency due to the strong back-scattering in the interfacial area of TiO2 film and the conducting glass, penetration of excess and large CdS nanocrystals, and poor charge transfer between TiO2 particles. The photoelectrochemical measurement shows that the highest photocurrent of 2.4mA/cm2 was obtained with 10% THS in the TiO2/CdS photoanode, which is nearly four times higher than that of the TiO2/CdS photoanode without THS (0.5mA/cm2).

Donor ionization in size controlled silicon nanocrystals: The transition from defect passivation to free electron generation

We studied the photoluminescence spectra of silicon and phosphorus co-implanted silica thin films on (100) silicon substrates as a function of isothermal annealing time. The rapid phase segregation, formation, and growth dynamics of intrinsic silicon nanocrystals are observed, in the first 600 s of rapid thermal processing, using dark field mode X-TEM. For short annealing times, when the nanocrystal size distribution exhibits a relatively small mean diameter, formation in the presence of phosphorus yields an increase in the luminescence intensity and a blue shift in the emission peak compared with intrinsic nanocrystals. As the mean size increases with annealing time, this enhancement rapidly diminishes and the peak energy shifts further to the red than the intrinsic nanocrystals. These results indicate the existence of competing pathways for the donor electron, which depends strongly on the nanocrystal size. In samples containing a large density of relatively small nanocrystals, the tendency of phosphorus to accumulate at the nanocrystal-oxide interface means that ionization results in a passivation of dangling bond (Pb-centre) type defects, through a charge compensation mechanism. As the size distribution evolves with isothermal annealing, the density of large nanocrystals increases at the expense of smaller nanocrystals, through an Ostwald ripening mechanism, and the majority of phosphorus atoms occupy substitutional lattice sites within the nanocrystals. As a consequence of the smaller band-gap, ionization of phosphorus donors at these sites increases the free carrier concentration and opens up an efficient, non-radiative de-excitation route for photo-generated electrons via Auger recombination. This effect is exacerbated by an enhanced diffusion in phosphorus doped glasses, which accelerates silicon nanocrystal growth.

Large area controllable hexagonal close-packed single-crystalline metal nanocrystal arrays with localized surface plasmon resonance response

We demonstrated large-area, high density, hexagonal close-packed, single crystalline metal (Au, Ag) nanocrystal arrays with controllable crystal sizes of tens of nanometers, center-to-center spacing and uniform size distribution by colloidal lithography and the surface energy driven dewetting process. Larger than 30 × 40 μm2 single superlattice domain metal nanocrystal arrays were formed on Si and can be transferred to flexible substrates, illustrating the versatility to realize metal nanocrystal arrays on various substrates. The large-area hexagonal close-packed metal (Au, Ag) nanocrystal arrays with various sizes and center-to-center spacings were prepared and manipulated by regulation of the thickness of metals and the size of colloidal spheres. Appropriate surface treatments and the metal deposition method were shown to be critical for obtaining metal nanocrystal arrays with uniform size and reduced crystal size. Localized surface plasmon resonance (LSPR) responses of these nanocrystal arrays were systematically measured and exhibited high consistency with simulation based on Mie theory, implying that the high controllability of the LSPR wavelength can be achieved by these methods. The process provides an inexpensive and easy route to fabricate a large-scale close-packed single crystalline metal nanocrystal array with controllable sizes compared to the e-beam lithography method for precise regulation of the LSPR wavelength and light scattering cross-sections. It exhibits excellent versatility and controllability to fabricate large-scale metal nanocrystal arrays with various sizes, compositions, morphologies and structures on different substrates. Single crystallinity and long-range order of metal nanocrystal arrays can be achieved to enhance LSPR performance and benefit directional propagation, which can lead to significant applications in surface-enhanced Raman scattering (SERS) based biosensors, nanoantennas and other plasmonic optoelectronics.

Silicon Nanocrystal Superlattices

Silicon Nanocrystal Superlattices

Tunneling of electrons between Si nanocrystals embedded in a SiO2matrix

Based on ab initio density functional calculations of Si nanocrystals (NCs) in a SiO${}_{2}$ matrix we study the impact of NC size and separation on electronic properties and present consequences for inter-NC carrier transport. The NC size and separation significantly influence the electronic structure of three-dimensional superlattice systems with different NC filling. The energy levels for large NC distances are broadened to minibands for small separations due to the wave-function overlap. Their formation is described in terms of inter-NC hopping and carrier tunneling. We find that electron transport in conduction minibands is more likely than hole transport.

Loosening Quantum Confinement: Observation of Real Conductivity Caused by Hole Polarons in Semiconductor Nanocrystals Smaller than the Bohr Radius

We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary. For nanocrystals with radii exceeding 5 nm, we observe the onset of real conductivity, which is attributed to the increasingly smaller separation between the hole states. Remarkably, this onset occurs for a nanocrystal radius significantly smaller than the bulk exciton Bohr radius a(B) ∼ 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement.

Structural, mechanical and optical properties of Ge nanocrystals embedded in superlattices fabricated by in situ low temperature annealing

Ge nanocrystals (Ge-ncs) embedded in SiO2 superlattices were prepared by in situ low temperature annealing. The formation of size controlled nanocrystals was demonstrated by Raman scattering, X-ray diffraction and transmission electron microscopy measurements. A core–shell structure was observed in the in situ grown Ge-ncs as a result of the lower annealing temperatures. The in situ annealing process also produced significant compressive stress in the superlattices, and the calculation result shows that the hydrostatic pressure induced by the compressive stress was about 2.55GPa for the nanocrystals with average size of 7.1nm. The optical absorption edges of the in situ grown superlattices moved to lower wavelength as crystallite size decreased. The presence of Ge-ncs enhanced the film absorption at short wavelength and the absorption coefficient was found to increase in superlattices containing larger nanocrystals. The superlattices exhibited optical band gap energies larger than that of bulk Ge and a blueshift of energies was observed with the decrease of crystallite size. These features are tentatively explained by quantum confinement effect in the Ge-ncs. Our results indicate that in situ low temperature grown Ge-ncs and superlattices are promising candidates for the fabrication of nanoscale devices on low cost substrates.

Theory of hopping conduction in arrays of doped semiconductor nanocrystals

The resistivity of a dense crystalline array of semiconductor nanocrystals (NCs) depends in a sensitive way on the level of doping as well as on the NC size and spacing. The choice of these parameters determines whether electron conduction through the array will be characterized by activated nearest-neighbor hopping or variable-range hopping (VRH). Thus far, no general theory exists to explain how these different behaviors arise at different doping levels and for different types of NCs. In this paper we examine a simple theoretical model of an array of doped semiconductor NCs that can explain the transition from activated transport to VRH. We show that in sufficiently small NCs, the fluctuations in donor number from one NC to another provide sufficient disorder to produce charging of some NCs, as electrons are driven to vacate higher shells of the quantum confinement energy spectrum. This confinement-driven charging produces a disordered Coulomb landscape throughout the array and leads to VRH at low temperature. We use a simple computer simulation to identify different regimes of conduction in the space of temperature, doping level, and NC diameter. We also discuss the implications of our results for large NCs with external impurity charges and for NCs that are gated electrochemically.

Size-Dependent Interaction of the Poly(N-vinyl-2-pyrrolidone) Capping Ligand with Pd Nanocrystals

Pd nanocrystals were prepared by the reduction of a H(2)PdCl(4) aqueous solution with C(2)H(4) in the presence of different amounts of poly(N-vinyl-2-pyrrolidone) (PVP). Their average size decreases monotonically as the PVP monomer/Pd molar ratio increases up to 1.0 and then does not vary much at higher PVP monomer/Pd molar ratios. Infrared spectroscopy and X-ray photoelectron spectroscopy results reveal the interesting size-dependent interaction of PVP molecules with Pd nanocrystals. For fine Pd nanocrystals capped with a large number of PVP molecules, each PVP molecule chemisorbs with its oxygen atom in the ring; for large Pd nanocrystals capped by a small number of PVP molecules, each PVP molecule chemisorbs with both the oxygen atom and nitrogen atom in the ring, which obviously affects the structure of chemisorbed PVP molecules and even results in the breaking of involved C-N bonds of some chemisorbed PVP molecules. Charge transfer always occurs from a chemisorbed PVP ligand to Pd nanocrystals. These results provide novel insights into the PVP-metal nanocrystal interaction, which are of great importance in the fundamental understanding of surface-mediated properties of PVP-capped metal nanocrystals.