Nanocrystal Volume Research Articles

Superconducting parity effect across the Anderson limit

How small can superconductors be? For isolated nanoparticles subject to quantum size effects, P.W. Anderson in 1959 conjectured that superconductivity could only exist when the electronic level spacing δ is smaller than the superconducting gap energy Δ. Here we report a scanning tunnelling spectroscopy study of superconducting lead (Pb) nanocrystals grown on the (110) surface of InAs. We find that for nanocrystals of lateral size smaller than the Fermi wavelength of the 2D electron gas at the surface of InAs, the electronic transmission of the interface is weak; this leads to Coulomb blockade and enables the extraction of electron addition energy of the nanocrystals. For large nanocrystals, the addition energy displays superconducting parity effect, a direct consequence of Cooper pairing. Studying this parity effect as a function of nanocrystal volume, we find the suppression of Cooper pairing when the mean electronic level spacing overcomes the superconducting gap energy, thus demonstrating unambiguously the validity of the Anderson criterion.

Dissecting charge relaxation pathways in CdSe/CdS nanocrystals using femtosecond two-dimensional electronic spectroscopy.

Exciton relaxation dynamics of CdSe and quasi-type-II CdSe/CdS core/shell nanocrystals were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The use of 2DES allowed for determination of structure-specific and state-resolved carrier dynamics for CdSe nanocrystals formed with five, or fewer, CdS passivation monolayers (ML). For CdSe and CdSe/CdS nanocrystals formed with one through three MLs of CdS, excitation using broad bandwidth femtosecond visible laser pulses generated electron-hole pairs among the |X1〉 = 2.14 eV and |X2〉 = 2.27 eV exciton states. For both excitations, the electron is promoted to the lowest energy excited (1Se) conduction-band state and the hole is in the 1S3/2 (X1) or 2S3/2 (X2) valence-band state. Therefore, the relaxation dynamics of the hot hole were isolated by monitoring the-time-dependent amplitude of 2DES cross peaks. The time constant for hot hole relaxation within the CdSe valence band was 150 ± 45 fs. Upon passivation by CdS, this hole relaxation time constant increased to 170 ± 30 fs (CdSe/CdS-3ML). This small increase was attributed to the formation of a graded, or alloyed, interfacial region that precedes the growth of a uniform CdS capping layer. The small increase in hole relaxation time reflects the larger nanocrystal volume of the CdSe/CdS system with respect to the CdSe nanocrystal core. In contrast, the dynamics of larger core/shell nanocrystals (≥4ML CdS) exhibited a picosecond buildup in 2DES cross-peak amplitude. This time-dependent response was attributed to interfacial hole transfer from CdS to CdSe valence-band states. Importantly, the 2DES data distinguish CdSe exciton relaxation from interfacial carrier transfer dynamics. In combination, isolation of structurally well-defined nanocrystals and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.

Radiative defects, emission and structure of ZnO nanocrystals obtained by electrochemical method

Radiative defects, emission and structure of ZnO nanocrystals (NCs) have been studied by means of the scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL) techniques. ZnO NCs were prepared by etching Zn sheets and annealing at 200–400°C in ambient air. The NC size and volume of ZnO crystalline phase increase with annealing temperature rising from 200°C up to 400°C. Simultaneously, XRD parameters and inter-planar distances in ZnO NCs vary no monotonically: decreasing upon annealing at 200–360°C and increasing at 400°C annealing. The PL intensity variation of orange, yellow, green and blue PL bands correlates with the change of XRD parameters. The study of PL band thermal decays, within the range 10–300K, permits to estimate decay activation energies and to analyze the nature of donors and acceptors responsible for the orange, yellow and green PL bands in ZnO NCs.The electrochemical method allows varying the PL intensity of different PL bands by a controllable way that is interesting for ZnO NC film application in “white” light emitting device structures.

(Invited) Upconverting Fluoride Nanocrystals - Synthesis and Optical Properties

Over the past decade, high-quality lanthanide doped upconverting nanocrystals (UPCNCs) have been successfully synthesized with the rapid development of nanotechnology. Several materials (i.e. CaF2, NaYF4, NaGdF4) have been successfully applied so far as potential matrix for Yb3+-RE3+combinations, where the low crystal field symmetry and low phonon frequency are the main factors defining the best matrix selection. These materials have already found several potential applications in biology, medicine as well as in photovoltaics and photocatalysis – as the most common examples. However, to change the commercial potential of UPCNCs to real devices still a several problems must be solved, where week emission quantum efficiency (EQE typically < 2%) is one of the most important one. The EQE is directly related to other challenges in this field: growth of small size <10 nm UPCNCs, UPCNCs shape and architecture control and control of UPCNCs surface properties. To solve all abovementioned problems the correlation between technological, morphological and physical parameters is needed. This is however a difficult task because the system itself is very complicated and consist number of correlated parameters. Thus, still a better understanding of physical processes responsible for the optical properties of UPCNCs in needed to bring this fascinating material to commercial use. In this work, we will discuss synthesis and optical properties of NaGdF4-based up-converting nanocrystals. We will discuss how to control size, shape and core-shell architecture of lanthanide doped NaGdF4 nanocrystals focusing mainly on Yb-Er co-doping. We will discuss how doping influencing the nanocrystals size, shape and their optical properties (Fig.1.). This will be done based on experimental results obtained from photoluminescence (PL) measurements, PL-excitation spectroscopy, PL-decay measurements, microscopic imaging and structural data obtained from HRTEM and XRD. Our conclusions will be supported by results obtained from numerical Monte Carlo modeling used to model the kinetic properties of nanocrystals emission and to model the ions distribution within the nanocrystals volume. Finally, based on our work on nanocrystals surface functionalization and their toxicity tests we will briefly discuss the potential use of UPCNCs in biology and medicine. Figure 1. ß-NaGdF4:Yb, Er nanocrystals with different shapes and architectures. Figure 1

Redox Potentials of Colloidal n-Type ZnO Nanocrystals: Effects of Confinement, Electron Density, and Fermi-Level Pinning by Aldehyde Hydrogenation.

Electronically doped colloidal semiconductor nanocrystals offer valuable opportunities to probe the new physical and chemical properties imparted by their excess charge carriers. Photodoping is a powerful approach to introducing and controlling free carrier densities within free-standing colloidal semiconductor nanocrystals. Photoreduced (n-type) colloidal ZnO nanocrystals possessing delocalized conduction-band (CB) electrons can be formed by photochemical oxidation of EtOH. Previous studies of this chemistry have demonstrated photochemical electron accumulation, in some cases reaching as many as >100 electrons per ZnO nanocrystal, but in every case examined to date this chemistry maximizes at a well-defined average electron density of ⟨Nmax⟩ ≈ (1.4 ± 0.4) × 10(20) cm(-3). The origins of this maximum have never been identified. Here, we use a solvated redox indicator for in situ determination of reduced ZnO nanocrystal redox potentials. The Fermi levels of various photodoped ZnO nanocrystals possessing on average just one excess CB electron show quantum-confinement effects, as expected, but are >600 meV lower than those of the same ZnO nanocrystals reduced chemically using Cp*2Co, reflecting important differences between their charge-compensating cations. Upon photochemical electron accumulation, the Fermi levels become independent of nanocrystal volume at ⟨N⟩ above ∼2 × 10(19) cm(-3), and maximize at ⟨Nmax⟩ ≈ (1.6 ± 0.3) × 10(20) cm(-3). This maximum is proposed to arise from Fermi-level pinning by the two-electron/two-proton hydrogenation of acetaldehyde, which reverses the EtOH photooxidation reaction.

(Invited) Colloidal Nanocrystal Composites with High and Low Thermal Conductivity

We present our work on thermal transport in colloidal nanocrystal composites within the context of thermoelectrics and thermal energy management. Colloidal nanocrystals consist of an inorganic crystalline core with short ligand molecules bound to the surface. These nanocrystals can be made with excellent control over size, shape, and composition, thereby enabling microstructural control over thermal energy transport. Our first two topics focus on low thermal conductivity materials for thermoelectricity: semiconductor nanocrystal thin films and semiconductor matrix – semiconductor nanoparticle composites. Our last topic focuses on metal matrix – metal nanoparticle composites with high thermal conductivity, which can be used to thermally manage transient power spikes in high power electronics. For the first topic, we systematically study the effect of nanocrystal diameter and surface chemistry on thermal transport in lead chalcogenide nanocrystal thin films. These samples exhibit very low thermal conductivities ranging from 0.1 to 0.5 W/m-K and we find that larger nanocrystals and shorter ligands generally lead to higher thermal conductivities. Our measurements also show that the effect of surface chemistry on thermal transport becomes more pronounced as nanocrystal diameter decreases. For the second topic, we focus on polycrystalline In2Se3 matrices with embedded CdSe nanocrystals ranging from 0 to ~100 vol%. We find that the thermal conductivities of these composites are very low over the entire nanocrystal volume fraction range and vary from 0.3 to 0.5 W/m-K. We also find that the presence of CdSe nanocrystals strongly effects the formation of the In2Se3 matrix (i.e. grain orientation and size as well as ternary phase formation). Lastly, we present our work on phase change Bi nanocrystals embedded in a solid Ag-matrix. The latent heat of the phase change Bi nanocrystals can absorb large quantities of transient heat generated during power spikes in power electronics operation. In addition, fast thermal energy transport is facilitated by the Ag-matrix, which leads to high composite thermal conductivities ranging from 101 to 102 W/m-K. We also use size-dependent melting of the phase change nanocrystals as a design variable for the thermal energy absorption temperature.

Efficient Solid‐State Light‐Emitting CuCdS Nanocrystals Synthesized in Air

AbstractSemiconductor nanocrystals (NCs) possess high photoluminescence (PL) typically in the solution phase. In contrary, PL rapidly quenches in the solid state. Efficient solid state luminescence can be achieved by inducing a large Stokes shift. Here we report on a novel synthesis of compositionally controlled CuCdS NCs in air avoiding the usual complexity of using inert atmosphere. These NCs show long‐range color tunability over the entire visible range with a remarkable Stokes shift up to about 1.25 eV. Overcoating the NCs leads to a high solid‐state PL quantum yield (QY) of ca. 55 % measured by using an integrating sphere. Unique charge carrier recombination mechanisms have been recognized from the NCs, which are correlated to the internal NC structure probed by using extended X‐ray absorption fine structure (EXAFS) spectroscopy. EXAFS measurements show a Cu‐rich surface and Cd‐rich interior with 46 % CuI being randomly distributed within 84 % of the NC volume creating additional transition states for PL. Color‐tunable solid‐state luminescence remains stable in air enabling fabrication of light‐emitting diodes (LEDs).

Far-infrared spectroscopic study of CeO2 nanocrystals

We present the far-infrared reflectivity spectra of 5 nm-sized pure and copper-doped Ce1−xCuxO2−y (x = 0; 0.01 and 0.10) nanocrystals measured at room temperature in the 50–650 cm−1 spectral range. Reflectivity spectra were analyzed using the factorized form of the dielectric function, which includes the phonon and the free carriers contribution. Four oscillators with TO energies of approximately 135, 280, 370, and 490 cm−1 were included in the fitting procedure. These oscillators represent local maxima of the CeO2 phonon density of states, which is also calculated using the density functional theory. The lowest energy oscillator represents TA(L)/TA(X) phonon states, which become infrared-active Eu modes at the L and X points of the Brillouin zone (BZ). The second oscillator originates from TO(Γ) phonon states. The oscillator at ~400 cm−1 originates from Raman mode phonon states, which at the L point of BZ also becomes infrared-active Eu mode. The last oscillator describes phonons with dominantly LO(Γ) infrared mode character. The appearance of phonon density of states related oscillators, instead of single F2uinfrared-active mode in the far-infrared reflectivity spectra, is a consequence of the nanosized dimension of the CeO2 particles. The best fit spectra are obtained using the generalized Bruggeman model for inhomogeneous media, which takes into account the nanocrystal volume fraction and the pore shape.

3D visualization of TiO2 nanocrystals in mesoporous nanocomposite using energy filtered transmission electron microscopy tomography.

IntroductionMesoporous silica, SBA-15, is one of the best candidate for the supporting material of catalytic nanoparticles because of its relative large and controllable pore size and large specific surface area [1]. So far, various nanoparticles, such as Au, Pt and Pd, have been introduced into the pore for catalytic application [2]. The size of nanoparticles supported inside SBA-15 is restricted by that of the pore, and they are usually ranging from 2 nm and 50 nm in space.It is necessary to anchor the nanoparticles within pores to avoid segregation / sintering of them. However, it is difficult to anchor them within pores in the case of use of deposition-precipitation method due to extreme low iso-electric point (IEP) of silica (∼2). Therefore, TiO2 nanocrystals (IEP 6-8) were then introduced to anchor AuNPs [3].In this study, EFTEM tomography was applied to examine the effectiveness of TiO2 for AuNPs. Materials and methodAu/TiO2-SBA-15 was embedded into epoxy resin for electron microscopy and microtomed to about 30 nm thickness. EFTEM-tomography was operated at 120 kV and using Ti-L ionization edge via three-window method. Prior to EFTEM, STEM-HAADF tomography was also carried out for visualizing AuNPs and for comparison. Result and discussionFigure1 shows 3D-volume of AuNPs and TiO2 nanocrystals from EFTEM-tomography. TiO2 nanocrystals in the porous material were successfully visualized using EFTEM -tomography, and local relationship between AuNPs and TiO2 nanocrystals were revealed. A large number of TiO2 nanocrystals were randomly distributed in the SBA-15. It was found that most AuNPs were directly on the exposed TiO2 nanocrystals. It implies that TiO2 nanocrystals were exposed on the surface of the pore and anchored AuNPs inside the pores.jmicro;63/suppl_1/i27/DFU081F1F1DFU081F1Fig. 1.3D volume of AuNPs and TiO2 nanocrystals.

Tin induced a-Si crystallization in thin films of Si-Sn alloys

Effects of tin doping on crystallization of amorphous silicon were studied using Raman scattering, Auger spectroscopy, scanning electron microscopy, and X-ray fluorescence techniques. Formation of silicon nanocrystals (2–4 nm in size) in the amorphous matrix of Si1−xSnx, obtained by physical vapor deposition of the components in vacuum, was observed at temperatures around 300 °C. The aggregate volume of nanocrystals in the deposited film of Si1−xSnx exceeded 60% of the total film volume and correlated well with the tin content. Formation of structures with ∼80% partial volume of the nanocrystalline phase was also demonstrated. Tin-induced crystallization of amorphous silicon occurred only around the clusters of metallic tin, which suggested the crystallization mechanism involving an interfacial molten Si:Sn layer.

Measurement of Thermal Conductivity of PbTe Nanocrystal Coated Glass Fibers by the 3ω Method

Fiber-based thermoelectric materials can conform to curved surfaces to form energy harvesting devices for waste heat recovery. Here we investigate the thermal conductivity in the axial direction of glass fibers coated with lead telluride (PbTe) nanocrystals using the self-heated 3ω method particularly at low frequency. While prior 3ω measurements on wire-like structures have only been demonstrated for high thermal conductivity materials, the present work demonstrates the suitability of the 3ω method for PbTe nanocrystal coated glass fibers where the low thermal conductivity and high aspect ratio result in a significant thermal radiation effect. We simulate the experiment using a finite-difference method that corrects the thermal radiation effect and extract the thermal conductivity of glass fibers coated by PbTe nanocrystals. The simulation method for radiation correction is shown to be generally much more accurate than analytical methods. We explore the effect of nanocrystal volume fraction on thermal conductivity and obtain results in the range of 0.50-0.93 W/mK near room temperature.

Controlling Carrier Densities in Photochemically Reduced Colloidal ZnO Nanocrystals: Size Dependence and Role of the Hole Quencher

Photodoped colloidal ZnO nanocrystals are model systems for understanding the generation and physical or chemical properties of excess delocalized charge carriers in semiconductor nanocrystals. Typically, ZnO photodoping is achieved photochemically using ethanol (EtOH) as a sacrificial reductant. Curiously, different studies have reported over an order of magnitude spread in the maximum number of conduction-band electrons that can be accumulated by photochemical oxidation of EtOH. Here, we demonstrate that this apparent discrepancy results from a strong size dependence of the average maximum number of excess electrons per nanocrystal, <nmax>. We demonstrate that <nmax> increases in proportion to nanocrystal volume, such that the maximum carrier density remains constant for all nanocrystal sizes. <nmax> is found to be largely insensitive to precise experimental conditions such as solvent, ligands, protons or other cations, photolysis conditions, and nanocrystal or EtOH concentrations. These results reconcile the broad range of literature results obtained with EtOH as the hole quencher. Furthermore, we demonstrate that <nmax> depends on the identity of the hole quencher, and is thus not an intrinsic property of the multiply reduced ZnO nanocrystals themselves. Using a series of substituted borohydride hole quenchers, we show that it is possible to increase the nanocrystal carrier densities over 4-fold relative to previous photodoping reports. When excess lithium and potassium triethylborohydrides are used in the photodoping, formation of Zn(0) is observed. The relationship between metallic Zn(0) formation and ZnO surface electron traps is discussed.

Direct characterization of nanocrystal size distribution using Raman spectroscopy

We report a rigorous analytical approach based on one-particle phonon confinement model to realize direct detection of nanocrystal size distribution and volume fraction by using Raman spectroscopy. For the analysis, we first project the analytical confinement model onto a generic distribution function, and then use this as a fitting function to extract the required parameters from the Raman spectra, i.e., mean size and skewness, to plot the nanocrystal size distribution. Size distributions for silicon nanocrystals are determined by using the analytical confinement model agree well with the one-particle phonon confinement model, and with the results obtained from electron microscopy and photoluminescence spectroscopy. The approach we propose is generally applicable to all nanocrystal systems, which exhibit size-dependent shifts in the Raman spectrum as a result of phonon confinement.

Atomic-scale dynamic process of deformation-induced stacking fault tetrahedra in gold nanocrystals

Stacking fault tetrahedra, the three-dimensional crystalline defects bounded by stacking faults and stair-rod dislocations, are often observed in quenched or irradiated face-centred cubic metals and alloys. All of the stacking fault tetrahedra experimentally observed to date are believed to originate from vacancies. Here we report, in contrast to the classical vacancy-originated ones, a new kind of stacking fault tetrahedra formed via the interaction and cross-slip of partial dislocations in gold nanocrystals. The complete atomic-scale processes of nucleation, migration and annihilation of the dislocation-originated stacking fault tetrahedra are revealed by in situ high-resolution observations and molecular dynamics simulations. The dislocation-originated stacking fault tetrahedra can undergo migration and annihilation due to mechanical loading in a manner that is not expected in bulk samples. These results uncover a unique deformation mechanism via dislocation interaction inside the confined volume of nanocrystals and have important implications regarding the size effect on the mechanical behaviour of small-volume materials.

Two-Photon-Induced Blue Shift of Core and Shell Optical Transitions in Colloidal CdSe/CdS Quasi-Type II Quantum Rods

The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied via two-photon-induced luminescence excitation spectroscopy. We verified that the two-photon absorption in these samples is a purely nonlinear phenomenon, excluding the contribution from multistep linear absorption mediated by defect states. A large absorption cross section was observed for CdSe/CdS core/shell quantum rods, in the range of 10(5) GM (1 GM = 10(-50) cm(4) s phot(-1)), scaling with the total nanocrystal volume and thus independent of the core emission wavelength. In the two-photon luminescence excitation spectra, peaks are strongly blue-shifted with respect to the one-photon absorption peaks, for both core and shell transitions. The experimental results are confirmed by k·p calculations, which attribute the shift to both different parity selection rules that apply to one-photon and two-photon transitions and a low oscillator strength for two-photon transitions close to the ground-state one-photon absorption. In contrast with lead chalcogenide quantum dots, we found no evidence of a breakdown of the optical selection rules, despite the presence of band anisotropy, via the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry via the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P-states in the case of a large CdSe core encapsulated in a thin CdS shell. Hence, tuning of the core and shell dimensions and the concurrent transition from type I to quasi-type II carrier localization enables unprecedented control over the band-edge two-photon absorption.

Optical gap of P-doped Si nanocrystals

The optical gap of Si nanocrystals strongly doped with phosphorus has been calculated. The energy levels of the electron and hole ground states in the Si nanocrystal were found as functions of the nanocrystal size and the P concentration in the framework of the envelope function approximation and under the assumption of uniform impurity distribution over the nanocrystal volume. It is shown that introducing phosphorus into the nanocrystal leads to a decrease in its optical gap. This decrease is related to a strong shift of the ground electron level produced by the short-range part of the Coulomb electron-P-ion interaction.

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Phase Transformation of Colloidal In2O3 Nanocrystals Driven by the Interface Nucleation Mechanism: A Kinetic Study

The kinetics of phase transformation of colloidal In(2)O(3) nanocrystals (NCs) during their synthesis in solution was explored by a combination of structural and spectroscopic methods, including X-ray diffraction, transmission electron microscopy, and extended X-ray absorption fine structure spectroscopy. Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) and the interface nucleation models were used to analyze the isothermal kinetic data for the phase transformation of NCs in the temperature range of 210-260 °C. The results show that NCs are initially stabilized in the metastable corundum (rh-In(2)O(3)) phase. The phase transformation occurs via nucleation of cubic bixbyite (bcc-In(2)O(3)) phase at the interface between contacting rh-In(2)O(3) NCs, and propagates rapidly throughout the NC volume. The activation energy of the phase transformation was determined from the Arrhenius expression to be 152 ± 60 kJ/mol. The interface nucleation rate is maximal at the beginning of the phase transformation process, and decreases over the course of the reaction due to a decrease in the concentration of rh-In(2)O(3) NCs in the reaction mixture. In situ high-temperature XRD patterns collected during nonisothermal treatment of In(2)O(3) NCs reveal that phase transformation of smaller NCs occurs at a faster rate and lower temperature, which is associated with their higher packing density and contact formation probability. Because NC surfaces and interfaces play a key role in phase transformation, their control through the synthesis conditions and reaction kinetics is an effective route to manipulate NC structure and properties.

Origin of size-dependent photoluminescence decay dynamics in colloidal γ-Ga2O3 nanocrystals

We studied size-dependent dynamics of defect-based photoluminescence of colloidal γ-Ga2O3 nanocrystals in the framework of the donor-acceptor pair model. Two theoretical models were developed based on relative positioning of donor and acceptor sites: (1) for random distribution of defects throughout the nanocrystal volume and (2) for surface segregation of defects. The results of the modeling indicate that defect sites are predominantly located in the vicinity of nanocrystal surfaces and that the density of defects increases with decreasing nanocrystal size. The donor Bohr radius obtained as a fitting parameter suggests an increase in the donor binding energy with decreasing nanocrystal size.

Influence of surface segregation on the luminescence properties of doped YVO4 nanocrystals

For the first time the segregation of doped ions in YVO4:Pr3+ and YVO4:Er3+,Yb3+ nanocrystals was revealed by means of spectroscopic techniques. Luminescence spectra are modified due to doped ions redistribution within nanocrystal volume. Variation of heat treatment temperature allows us to determine the diffusion-depended nature of observed doped ions redistribution within the volume of nanocrystal and to trace this process.