Size-dependent structure of MoS2 nanocrystals

Molybdenum disulphide nanostructures are of interest for a wide variety of nanotechnological applications ranging from the potential use of inorganic nanotubes in nanoelectronics to the active use of nanoparticles in heterogeneous catalysis. Here, we use atom-resolved scanning tunnelling microscopy to systematically map and classify the atomic-scale structure of triangular MoS2 nanocrystals as a function of size. Instead of a smooth variation as expected from the bulk structure of MoS2, we observe a very strong size dependence for the cluster morphology and electronic structure driven by the tendency to optimize the sulphur excess present at the cluster edges. By analysing of the atomic-scale structure of clusters, we identify the origin of the structural transitions occurring at unique cluster sizes. The novel findings suggest that good size control during the synthesis of MoS2 nanostructures may be used for the production of chemically or optically active MoS2 nanomaterials with superior performance.     

Quantum oscillations in magnetically doped colloidal nanocrystals

Progress in the synthesis of colloidal quantum dots has recently provided access to entirely new forms of diluted magnetic semiconductors, some of which may find use in quantum computation. The usefulness of a spin qubit is defined by its Rabi frequency, which determines the operation time, and its coherence time, which sets the error correction window. However, the spin dynamics of magnetic impurity ions in colloidal doped quantum dots remain entirely unexplored. Here, we use pulsed electron paramagnetic resonance spectroscopy to demonstrate long spin coherence times of ～0.9 μs in colloidal ZnO quantum dots containing the paramagnetic dopant Mn(2+), as well as Rabi oscillations with frequencies ranging between 2 and 20 MHz depending on microwave power. We also observe electron spin echo envelope modulations of the Mn(2+) signal due to hyperfine coupling with protons outside the quantum dots, a situation unique to the colloidal form of quantum dots, and not observed to date.     

Multiple exciton generation in colloidal silicon nanocrystals

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.     

Size-dependent absolute quantum yields for size-separated colloidally-stable silicon nanocrystals

Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1-5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of "green" silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts.     

Size-dependent absorption and defect states in CdSe nanocrystals in various multilayer structures

GeS2-CdSe superlattices and composite films are prepared by consecutive thermal evaporation of CdSe and GeS2 in vacuum. CdSe layer thickness varies between 1 and 10 nm, while the thickness of GeS2 layers is either equal (in superlattices) to or 20 times greater (in composite films) than that of CdSe layers. Standard spectral photocurrent measurements and various constant photocurrent methods are used to study optical absorption of all samples. An overall blueshift is observed with decreasing CdSe layer thickness of superlattices. This shift is related to a size-induced increase of the optical band gap of CdSe due to one-dimensional carrier confinement in the continuous nanocrystalline CdSe layers. A number of features are observed in the absorption spectra of composite films containing CdSe nanocrystals with average radii of approximately 2.5 and approximately 3.3 nm. They are discussed in terms of three-dimensional carrier confinement and are considered a manifestation of excited electron states in CdSe nanocrystals embedded in GeS2 thin film matrix. In addition to these discrete features, the exponential dependence of the optical absorption (Urbach) edge indicates a distribution of "valence band" tail states associated with disorder. Transient photoconductivity measurements made on similarly prepared SiOx-CdSe superlattices exhibit a rapid fall in photocurrent by a power law decay over several orders of magnitude of time, which is consistent with multi-pletrapping transport via an extensive distribution of deep defects.     

Three-dimensional atomic imaging of colloidal core-shell nanocrystals

Colloidal core-shell semiconductor nanocrystals form an important class of optoelectronic materials, in which the exciton wave functions can be tailored by the atomic configuration of the core, the interfacial layers, and the shell. Here, we provide a trustful 3D characterization at the atomic scale of a free-standing PbSe(core)-CdSe(shell) nanocrystal by combining electron microscopy and discrete tomography. Our results yield unique insights for understanding the process of cation exchange, which is widely employed in the synthesis of core-shell nanocrystals. The study that we present is generally applicable to the broad range of colloidal heteronanocrystals that currently emerge as a new class of materials with technological importance.     

Surface activation of colloidal indium phosphide nanocrystals

Against general wisdom in crystallization,the nucleation of InP and Ⅲ-Ⅴ quantum dots (QDs) often dominates their growth.Systematic studies on InP QDs identified the key reason for this:the dense and tight alkanoate-ligand shell around each nanocrystal.Different strategies were explored to enable necessary ligand dynamics—i.e.,ligands rapidly switching between being bonded to and detached from a nanocrystal upon thermal agitation—on nanocrystals to simultaneously retain colloidal stability and allow appreciable growth.Among all the surface-activation reagents tested,2,4-diketones (such as acetylacetone) allowed the full growth of InP QDs with indium alkanoates and trimethylsilylphosphine as precursors.While small fatty acids (such as acetic acid) were partially active,common neutral ligands (such as fatty amines,organophosphines,and phosphine oxides) showed limited activation effects.The existing amine-based synthesis of InP QDs was activated by acetic acid formed in situ.Surface activation with common precursors enabled the growth of InP QDs with a distinguishable absorption peak between ～450 and 650 nm at mild temperatures (140-180 ℃).Furthermore,surface activation was generally applicable for InAs and Ⅲ-Ⅴ based core/shell QDs.     

Two-color antibunching from band-gap engineered colloidal semiconductor nanocrystals

Photon antibunching is ubiquitously observed in light emitted from quantum systems but is usually associated only with the lowest excited state of the emitter. Here, we devise a fluorophore that upon photoexcitation emits in either one of two distinct colors but exhibits strong antibunching between the two. This work demonstrates the possibility of creating room-temperature quantum emitters with higher complexity than effective two level systems via colloidal synthesis.     

An essay on synthetic chemistry of colloidal nanocrystals

The central goal of synthetic chemistry of colloidal nanocrystals at present is to discover functional materials. Such functional materials should help mankind to meet the tough challenges brought by the rapid depletion of natural resources and the significant increase of population with higher and higher living standards. With this thought in mind, this essay discusses the basic guidelines for developing this new branch of synthetic chemistry, including rational synthetic strategies, functional performance, and green chemistry principles.      

Metal-enhanced fluorescence of colloidal nanocrystals with nanoscale control

Engineering the spectral properties of fluorophores, such as the enhancement of luminescence intensity, can be achieved through coupling with surface plasmons in metallic nanostructures. This process, referred to as metal-enhanced fluorescence, offers promise for a range of applications, including LEDs, sensor technology, microarrays and single-molecule studies. It becomes even more appealing when applied to colloidal semiconductor nanocrystals, which exhibit size-dependent optical properties, have high photochemical stability, and are characterized by broad excitation spectra and narrow emission bands. Other approaches have relied upon the coupling of fluorophores (typically organic dyes) to random distributions of metallic nanoparticles or nanoscale roughness in metallic films. Here, we develop a new strategy based on the highly reproducible fabrication of ordered arrays of gold nanostructures coupled to CdSe/ZnS nanocrystals dispersed in a polymer blend. We demonstrate the possibility of obtaining precise control and a high spatial selectivity of the fluorescence enhancement process.     

Annealing of ZnS nanocrystals grown by colloidal synthesis

ZnS nanocrystals (NCs) capped with tetramethylammonium (TMAH) were synthesized from ZnCl₂ · 2H₂O and thiourea using a wet chemical process. Further treatments of the nanocrystals such as aging, and annealing have been conducted to examine the stability of the grown samples. The X-ray diffraction spectra show that the crystal has a zinc blende structure with particle size of about 2 nm. The evidence of nanocrystalline character is also clear in the UV–Vis absorption that shows an excitonic peak at about 236 nm (5.2 eV) arising from band edge transitions. A photoluminescence emission peak centered at about 450 nm (2.7 eV) is attributed to transitions between shallow donors and Zn⁺ vacancies. Both absorption and photoluminescence spectra show that sample aging does not affect the characteristics of the sample, possibly due to protection by TMAH capping. Annealing at 700 °C and 900 °C results in the red shift of the photoluminescence.     

Size-dependent blue luminescent CdS nanocrystals synthesized through a single-source molecular precursor route

In this paper, we report on the synthesis of size-dependent blue luminescent CdS nanocrystals by using a new nonhydrolytic single-source molecular method. The size of the synthesized CdS nanocrystals could be easily controlled by adjusting the ratio of reaction sources under inert atmosphere. The studies on the optical properties reveal an obvious size-dependent photoluminescence characteristic of the synthesized nanocrystals.     

Size-dependent ultrafast magnetization dynamics in iron oxide (Fe3O4) nanocrystals

Optically induced ultrafast demagnetization and its recovery in superparamagnetic colloidal iron oxide (Fe3O4) nanocrystals have been investigated via time-resolved Faraday rotation measurements. Optical excitation with near-infrared laser pulse resulted in ultrafast demagnetization in approximately 100 fs via the destruction of ferrimagnetic ordering. The degree of demagnetization increased with the excitation density, and the complete demagnetization reached at approximately 10% excitation density. The magnetization recovered on two time scales, several picoseconds and hundreds of picoseconds, which can be associated with the initial reestablishment of the ferrimagnetic ordering and the electronic relaxation back to the ground state, respectively. The amplitude of the slower recovery component increased with the size of the nanocrystals, suggesting the size-dependent ferrimagnetic ordering throughout the volume of the nanocrystal.     

Size-dependent oriented attachment in the growth of pure and defect-free hexagonal boron nitride nanocrystals

Pure and defect-free hexagonal boron nitride (hBN) nanocrystals with deep-ultraviolet light emissions at around 215 nm were prepared via a solid state reaction. This involved preparing a precursor from potassium borohydride and ammonium chloride powders, and then heating the precursor and additional ammonium chloride to 1000 °C within a nitrogen atmosphere. The hBN nanocrystals were studied using a variety of characterization techniques (e.g., TEM, AFM, N(2) absorption/desorption). A growth mechanism based on size-dependent oriented attachment was proposed for the nanocrystals.     

Hierarchical self-assembly of suspended branched colloidal nanocrystals into superlattice structures

Self-assembly of molecular units into complex and functional superstructures is ubiquitous in biology. The number of superstructures realized by self-assembly of man-made nanoscale units is also growing. However, assemblies of colloidal inorganic nanocrystals are still at an elementary level, not only because of the simplicity of the shape of the nanocrystal building blocks and their interactions, but also because of the poor control over these parameters in the fabrication of more elaborate nanocrystals. Here, we show how monodisperse colloidal octapod-shaped nanocrystals self-assemble, in a suitable solution environment, on two sequential levels. First, linear chains of interlocked octapods are formed, and subsequently the chains spontaneously self-assemble into three-dimensional superstructures. Remarkably, all the instructions for the hierarchical self-assembly are encoded in the octapod shape. The mechanical strength of these superstructures is improved by welding the constituent nanocrystals together.     

Alignment of colloidal graphene quantum dots on polar surfaces

Controlling the orientation of nanostructures with anisotropic shapes is essential for taking advantage of their anisotropic electrical, optical, and transport properties in electro-optical devices. For large-area alignment of nanocrystals, so far orientations are mostly induced and controlled by external physical parameters, such as applied fields or changes in concentration. Herein we report on assemblies of colloidal graphene quantum dots, a new type of disk-shaped nanostructures, on polar surfaces and the control of their orientations. We show that the orientations of the graphene quantum dots can be determined, either in- or out-of-plane with the substrate, by chemical functionalization that introduces orientation-dependent interactions between the quantum dots and the surfaces.     

Two-dimensional photonic crystal resist membrane nanocavity embedding colloidal dot-in-a-rod nanocrystals

A novel technique for the fabrication of photonic crystal (PC) nanocavities coupled with colloidal nanocrystals is presented. A waveguiding resist membrane embedding highly emitting dot-in-a-rod nanocrystals was patterned through e-beam lithography and released through wet etching process. The proposed approach makes the PC structure independent of fabrication imperfections induced by etching steps. Micro-photoluminescence spectra revealed degenerated resonant modes (Q-factor approximately 700) whose fabrication-induced spectral splitting is comparable to the full width at half-maximum of the peaks. Active nanocavities tunable from visible to infrared spectral range on GaAs or Si substrates can be easily implemented by this technique.     

CuAgSe nanocrystals: colloidal synthesis,characterization and their thermoelectric performance

CuAgSe is a promising thermoelectric (TE) material for its superior carrier mobility and ultralow lattice thermal conductivity. Herein, we present a scalable colloidal method to prepare monodisperse CuAgSe nanocrystals with high yield. The collected powder sample was washed by a sulfur-free reagent of NaNH₂ to remove the surface organic ligands (CuAgSe-W) and then annealed (CuAgSe-W-A). Both kinds of ligand-free samples were then hot pressed into dense pellets to measure the TE property. The results revealed that the crystal structure of both samples changed from low-temperature β-phase to high-temperature α-phase at around 465 K. Sample CuAgSe-W shows interesting temperature-dependent transition from N-type to P-type, which could be potentially used as thermal control transistor. Sample CuAgSe-W-A does not display this transition state but it exhibits potential for intermediate temperature TE applications with a figure-of-merit zT reaching 0.68 at 566 K.     

Self-assembled hexagonal ordering of Si nanocrystals embedded in SiO(2) nanodots

Highly dense hexagonally ordered two-dimensional arrays of Si nanocrystals embedded in SiO(2)?nanodots were fabricated on a silicon substrate by using a self-assembled porous anodic alumina thin film as a masking layer through which electrochemical oxidation of the Si substrate and ultralow energy Si implantation took place. After removal of the alumina film and high temperature annealing of the samples, hexagonally ordered Si nanocrystals embedded within SiO(2) nanodots were obtained, having sizes in the few tens of nanometer range. The fabricated ordered structures show significant potential for applications either in basic physics experiments or as building blocks for nanoelectronic and nanophotonic devices.     

Size- and temperature-dependent quantum confined dielectric effect in colloidal pbse and CdSe nanocrystals

A new method is proposed to calculate the Stark shift induced by surface dielectric effect in colloidal nanocrystals. The effective mass approximation model is revised according to quantum confined dielectric effect. LUMO (the lowest unoccupied molecular orbital), HOMO (the highest occupied molecular orbital), band gap and Stark shift are calculated in CdSe and PbSe nanocrystals that bear significantly different physical properties. The calculated results fit well with the experimental data. The calculation of dielectric effect-induced Stark shift indicates that the quantum confined dielectric effect in PbSe and CdSe nanocrystals is size- and temperature-dependent, which is more notable in PbSe nanocrystals with a narrower band gap and results in the gentle variation of quantum confinement energy with particle size.