Assembly Of Nanocrystals Research Articles

Oriented assembly of CdS nanocrystals via dynamic surface modification-tailored particle interaction.

The controlled assembly of quantum dots (QDs) remains a challenge due to the lack of in-depth understanding of the roles of ligand dynamics. In this study, tripods, particles, nanorods and nanoflowers of CdS with yellow, red and cyan PL emissions, respectively, were achieved through the coarsening of thioglycolic acid (TGA)-capped CdS QDs with a novel hydroxyl-TGA exchange procedure. Dynamic hydroxyl modification-induced aggregation and coalescence can help to describe the defects and the corresponding photoluminescence characteristics of these nanocrystals. A crystal growth model involving assembly and coalescence was developed to describe the crystal growth and the corresponding PL properties, where hydroxyl-motivated hydrogen-bonding interaction was used to explain the oriented assembly of CdS QDs.

Tunable electronic properties by ligand coverage control in PbS nanocrystal assemblies.

It is well-known that controlling electronic properties of nanocrystal (NC) assemblies can be achieved by the usage of various types of ligands on the NC surface. However, the ligand coverage, which could also tune electronic properties, is always ignored. It is due to the lack of accurate evaluation methods of ligand amounts on the surface of NCs and the difficulty of ligand binding control at the nanoscale. Here, we demonstrate a precise ligand (oleic acid) coverage control of PbS NCs through a modified liquid/air assembly technique. In this way, both the assembly structures and electronic properties can be tuned by ligand coverage at the same time. In particular, the medium oleic acid coverage (2.1 ligand per nm2), which forms a square lattice of PbS NCs, shows the best electronic properties, especially when it is compared with the full coverage (2.4 ligand per nm2) and the sparse coverage (0.7 ligand per nm2) of oleic acid on the NC surface. This ligand coverage controlled electronic properties of NC films will give new insights for finely tuning the properties of electronic devices based on NCs.

On the kinetics of the removal of ligands from films of colloidal nanocrystals by plasmas.

This paper describes the kinetic limitations of etching ligands from colloidal nanocrystal assemblies (CNAs) by plasma processing. We measured the etching kinetics of ligands from a CNA model system (spherical ZrO2 nanocrystals, 2.5-3.5 nm diameter, capped with trioctylphosphine oxide) with inductively coupled plasmas (He and O2 feed gases, powers ranging from 7 to 30 W, at pressures ranging from 100 to 2000 mTorr and exposure times ranging between 6 and 168 h). The etching rate slows down by about one order of magnitude in the first minutes of etching, after which the rate of carbon removal becomes proportional to the third power of the carbon concentration in the CNA. Pressure oscillations in the plasma chamber significantly accelerate the overall rate of etching. These results indicate that the rate of etching is mostly affected by two main factors: (i) the crosslinking of the ligands in the first stage of plasma exposure, and (ii) the formation of a boundary layer at the surface of the CNA. Optimized conditions of plasma processing allow for a 60-fold improvement in etching rates compared to the previous state of the art and make the timeframes of plasma processing comparable to those of calcination.

Self-assembly of Janus Dumbbell Nanocrystals and Their Enhanced Surface Plasmon Resonance.

Self-assembly is a critical process that can greatly expand the existing structures and lead to new functionality of nanoparticle systems. Multicomponent superstructures self-assembled from nanocrystals have shown promise as multifunctional materials for various applications. Despite recent progress in assembly of homogeneous nanocrystals, synthesis and self-assembly of Janus nanocrystals with contrasting surface chemistry remains a significant challenge. Herein, we designed a novel Janus nanocrystal platform to control the self-assembly of nanoparticles in aqueous solutions by balancing the hydrophobic and hydrophilic moieties. A series of superstructures have been assembled by systematically varying the Janus balance and assembly conditions. Janus Au-Fe3O4 dumbbell nanocrystals (<20 nm) were synthesized with the hydrophobic ligands coated on the Au lobe and negatively charged hydrophilic ligands coated on the Fe3O4 lobe. We systematically fine-tune the lobe size ratio, surface coating, external conditions, and even additional growth of Au nanocrystal domains on the Au lobe of dumbbell nanoparticles (Au-Au-Fe3O4) to harvest self-assembly structures including clusters, chains, vesicles, and capsules. It was discovered that in all these assemblies the hydrophobic Au lobes preferred to stay together. In addition, these superstructures clearly demonstrated different levels of enhanced surface plasmon resonance that is directly correlated with the Au coupling in the assembly structure. The strong interparticle plasmonic coupling displayed a red-shift in surface plasmon resonance, with larger structures formed by Au-Au-Fe3O4 assembly extending into the near-infrared region. Self-assembly of Janus dumbbell nanocrystals can also be reversible under different pH values. The biphasic Janus dumbbell nanocrystals offer a platform for studying the novel interparticle coupling and open up opportunities in applications including sensing, disease diagnoses, and therapy.

Continuous Assembly of Cellulose Nanofibrils and Nanocrystals into Strong Macrofibers through Microfluidic Spinning

AbstractMicrofluidic fiber spinning is a promising technique for assembling cellulose nanomaterials into macroscopic fibers. However, its implementation requires upscalabe fabrication processes while maintaining high strength of the fibers, which could not be previously achieved. Herein, a continuous wet spinning process based on microfluidic flow focusing is developed to produce strong fibers from cellulose nanofibrils (CNFs) and nanocrystals (CNCs). Fibers with an average breaking tenacity as high as 29.5 cN tex−1 and Young's modulus of 1146 cN tex−1 are reported for the first time, produced from nonhighly purified CNF grades. Using the same developed method, wet spinning of fibers from CNCs is achieved for the first time, reaching an average Young's modulus of 1263 cN tex−1 and a breaking tenacity of 10.6 cN tex−1, thus exhibiting strength twice as high as that of common CNC films. A rather similar stiffness of CNC and CNF spun fibers may originate from similar degrees of alignment, as confirmed by wide‐angle X‐ray scattering (WAXS) and birefringence measurements, whereas lower strength may primarily arise from the shorter length of CNCs compared to that of CNFs. The benefit of CNCs is their higher solids content in the dopes. By combining both CNCs and CNFs, the fiber properties can be tuned.

Charge Transfer from Upconverting Nanocrystals to Semiconducting Electrodes: Optimizing Thermodynamic Outputs by Electronic Energy Transfer.

Light-harvesting inorganic nanocrystals play an important role in emerging solar energy conversion and optoelectronic devices. We describe here a strategy for a new family of photoelectrodes with upconverting nanocrystal assemblies as the photosensitizer. The assemblies consist of oleic acid-capped cadmium selenide (CdSe) nanocrystals that coordinate directly onto a layer of surface-bound, carboxylic acid-derivatized anthracenes through displacement of the oleic acid capping ligands. Steady-state emission and transient absorption measurements show that the upconverting nanocrystal assemblies, selectively excited by green light, generate singlet excitons that enable efficient charge injection into both the conduction band of TiO2 at the photoanode and the valence band of NiO at the photocathode. The singlet excitons form by sensitized triplet-triplet annihilation within the compact layer of anthracenes on the electrode surfaces. Density of state analysis reveals that the electronic coupling between the anthracene singlet excited states and the oxides provides a thermodynamic basis for light-induced charge transfer. The interplay between the excited-state populations at the surface-bound molecules and the assembled nanocrystals presents new design rules that can potentially overcome the limitations of previous dye-sensitized photoelectrochemical cells for catalytic applications.

Star-like Supramolecular Complexes of Reducing-End-Functionalized Cellulose Nanocrystals.

In this work, we take advantage of the parallel organization of cellulose chains in cellulose I yielding an inherent chemical asymmetry of cellulose nanocrystals, i.e., reducing vs nonreducing end, to selectively modify only one end of these rigid rodlike crystals to be used as a linking point for the formation of supramolecular structures. We have prepared biotin-functionalized tunicate cellulose nanocrystals at the reducing end capable of forming new complex supramolecular hierarchies by the addition of the protein streptavidin. Biotin–streptavidin coupling was chosen because streptavidin has a multivalency of four and the biotin–streptavidin bond is known to be highly selective and stable. Hence, streptavidin molecules would link up to four cellulose nanocrystals through their biotin-modified reducing end. Two biotin derivatives were studied, consisting of an anchoring group, i.e., amine or hydrazine; the biotin moiety; and the linker between them. Results show that the length of the linker significantly affects the bond between the biotinylated cellulose nanocrystals and streptavidin, and a certain chain length is necessary for the supramolecular assembly of several cellulose nanocrystals by streptavidin.

Plasmon-tunable Au@Ag core-shell spiky nanoparticles for surface-enhanced Raman scattering

A facile synthetic approach has been developed to prepare uniform and size-tunable spiky Au@Ag core-shell nanoparticles (NPs) to tailor the localized surface plasmon resonance (LSPR) properties. The gradual assembly of small Au nanocrystals allows the size of spiky Au NPs to be modulated from tens to several hundreds of nanometers by tuning the concentration of initial Au seeds and Au source; and the thickness of the Ag shell can be adjusted with stepwise reduction of Ag(I) ions. The LSPR bands of such spiky Au@Ag core-shell NPs resemble those of pure spiky Au NP cores of similar sizes in near-infrared region, and increasing the Ag shell thickness results in a blue shift and broadening of the LSPR band in the near-infrared region. Additionally, the spiky Au@Ag core-shell NPs exhibit improved surface-enhanced Raman scattering (SERS) activity as compared to the bare spiky Au NPs and spherical Ag@Au NPs. This work has offered a facile route to synthesize plasmonic metal NPs with LSPR band in 650 to 800 nm that show strong enhancement of localized electromagnetic field, which provides an effective SERS substrate for SERS imaging and detection in biological fluids and tissues.

Selective Photocatalytic CO2 Reduction in Water by Electrostatic Assembly of CdS Nanocrystals with a Dinuclear Cobalt Catalyst

In the photocatalytic conversion of CO2 to valuable chemicals under aqueous conditions, the selectivity and the product yield still present the most challenging issues. Against this backdrop, negat...

Synthesis of mesoporous-zeolite materials using Beta zeolite nanoparticles as precursors and their catalytic performance in m-xylene isomerization and disproportionation

Hierarchical Beta nanozeolites with the properties of both Beta nanozeolite and mesoporous material were prepared by two different methods: (1) Direct Hydrothermal Method (DHM) by the assembly of nanocrystals of preformed Beta seeds in the presence of the surfactant P123 under hydrothermal conditions, (2) Acidification of Beta seeds suspension (without the use of the mesopore directing agent). For the two methods, the effect of crystallization time of Beta nanozeolite precursor (seeds) was studied: 6, 12 and 24 h. The obtained materials were characterized using nitrogen sorption measurements, thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), ammonia Temperature Programmed Desorption (NH3-TPD) and adsorption/desorption of pyridine monitored by FTIR. The characterization results show that acidification of Beta seeds suspension leads to best structured nano-Beta (Beta nanoparticles). However, with the DHM method physical mixtures of Beta nanoparticles and mesoporous SBA-15 phases were obtained.The m-xylene isomerization/disproportionation reaction in gas-phase was carried out over all the synthesized solids in order to explore their catalytic properties. It was found that the m-xylene conversion and isomerization/disproportionation ratio were closely related to the crystallization time of Beta zeolite precursors, mesopores creation in hierarchical Beta zeolite nanoparticles and the method used to produce the extra meso-porosity.

Revisiting the Role of Metallic Antennas to Control Light Emission by Lead Salt Nanocrystal Assemblies

Metallic antennas have become ubiquitous tools for extracting light from atoms, molecules, and nanocrystals. Their design is generally based on the understanding that such antennas operate through the Purcell effect: the dependence of the fluorescence rate of individual emitters on their environment. The authors experimentally show that this paradigm fails for ensembles of $c\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}d$ emitters, whose interactions with metallic antennas can be understood with a statistical description of light emission. These findings carry important implications for nano-optics and solution-processed optoelectronics.

Extra Surfactant-Assisted Self-Assembly of Highly Ordered Monolayers of BaTiO3 Nanocubes at the Air–Water Interface

Assembly of nanocrystals into ordered two- or three-dimensional arrays is an essential technology to achieve their application in novel functional devices. Among a variety of assembly techniques, evaporation-induced self-assembly (EISA) is one of the prospective approaches because of its simplicity. Although EISA has shown its potential to form highly ordered nanocrystal arrays, the formation of uniform nanocrystal arrays over large areas remains a challenging subject. Here, we introduce a new EISA method and demonstrate the formation of large-scale highly ordered monolayers of barium titanate (BaTiO3, BT) nanocubes at the air-water interface. In our method, the addition of an extra surfactant to a water surface assists the EISA of BT nanocubes with a size of 15–20 nm into a highly ordered arrangement. We reveal that the compression pressure exerted by the extra surfactant on BT nanocubes during the solvent evaporation is a key factor in the self-assembly in our method. The BT nanocube monolayers transferred to substrates have sizes up to the millimeter scale and a high out-of-plane crystal orientation, containing almost no microcracks and voids.

Metal-organic framework thin films from copper hydroxide nano-assemblies

Potential applications of thin films of metal-organic frameworks (MOFs) cover widespread areas, including advanced optics, electronics, and magnetics. Controlling location and orientation of MOF crystals on a device-scale (over the centimeter scale) substrate is crucial to achieve these applications. Metal hydroxides are advantageous precursors for the MOF synthesis because of their high reactivity with organic linkers, allowing the crystallization of MOFs under mild conditions in a harmless solvent at a room temperature. In addition, the reaction scheme using metal hydroxides as precursors allows to fabricate MOF crystals on polymer, metal and ceramics substrates, where in some cases, substrates with deformability or 3D microstructures can be employed. In this perspective, our recent advances on the fabrication of MOF thin films grown on assemblies of metal hydroxide nanocrystals are reviewed. The present approach enables us to control both position and orientation of MOF crystals on a substrate. The aligned MOF films exhibit anisotropic properties by accommodating functional molecules in their micropores. The materials demonstrated here are considered as good candidates for solid catalysts, bio-sensors, anisotropic electrical/optical devices, and others.

A sustainable approach for scalable production of α-Fe2O3 nanocrystals with 3D interconnected porous architectures on flexible carbon textiles as integrated electrodes for lithium-ion batteries

Herein, we demonstrate a sustainable strategy for scalable production of α-Fe2O3 nanocrystals on flexible carbon textiles via controllable chemical bath deposition process. The assembly of α-Fe2O3 nanocrystals is densely anchored onto the carbon textiles, and the temperature-dependent mass loading and crystal size of α-Fe2O3 are systematically investigated. As an integrated electrode for lithium-ion batteries, the optimized Fe2O3@CTs with ultrathin three-dimensional interconnected porous architectures achieves significantly enhanced cycling performance and rate capability attributing to the improved stability, electronic interconnection, and shortened solid state diffusion pathway for Li+ ions/electrons. After the reactivation of the precursor, the Li-ion batteries based on the recycled Fe2O3@CTs exhibits electrochemical performance with no decay compared to that of the devices based on the fresh products. This sustainable methodology, considering material abundance, eco-efficiency, synthetic approach and scalability, is of crucial importance to both academy and industry for achieving high-performance lithium-ion batteries.

Mechanical Properties of Architected Nanomaterials Made from Organic–Inorganic Nanocrystals

Colloidal nanocrystals that consist of an inorganic core and an organic ligand shell can be assembled into architected nanocomposites. A wide range of mechanical properties can be achieved by tuning structural parameters such as sample dimensions, ligand length, and nanocrystal packing density. This review article describes the fabrication, structural characterization and nanomechanical testing of solid assemblies of nanocrystals. Self-assembly and solution processing techniques can be used to form freestanding membranes, thin films and 3D crystals that consist of ordered arrays, or disordered aggregates of nanocrystals. Membrane deflection, nano-indentation, and thin film buckling have been used to determine elastic properties, hardness and fracture toughness. Nanocrystal solids have elastic moduli of < 1–20 GPa, and hardness of ~ 40–450 MPa. Crosslinking of ligands and chemical sintering of inorganic nanoparticles can lead to dramatic improvements in stiffness, strength and toughness. The self-assembly of anisotropic nanocrystals can be used to make nanostructured materials with ordered porosity and 3D architectures.

Selective Removal of Ligands from Colloidal Nanocrystal Assemblies with Non-Oxidizing He Plasmas

Helium plasmas are attractive reagents for the removal of organics from hybrid materials because of their minimal ablative power and relative inertness, compared to oxidizing feed gases such as O2 and highly ablative inert gases such as Ar. This work describes the use of dilute helium plasmas to selectively remove the organic ligands from films of colloidal nanoparticles (i.e., colloidal nanoparticle assemblies). We determine the relative contribution to etching of different plasma species in a model system consisting of films of ZrO2 nanoparticles capped with trioctylphosphine oxide. Unexpectedly, we find that the strong ultraviolet radiation of He plasma is only a minor contributor to etching (25% of the etched carbon). Excited He species are responsible for most of the etching (75% of the etched carbon). Carbon concentrations as low as 3.5 atom % can be achieved under non-optimized plasma processing conditions.

Challenges and Prospects of Photocatalytic Applications Utilizing Semiconductor Nanocrystals.

Photocatalytic systems based on colloidal semiconductor nanocrystals have gained considerable attention owing to potential benefits that include a visible-range light extinction and a low spatial overlap of photoinduced charges. When coupled to metal catalysts, nanocrystal sensitizers have demonstrated a compelling performance in homogenous photoreduction reactions, including the degradation of organic dyes and hydrogen generation. Going beyond half-cycle reactions, however, the progress in the field of nanocrystal photocatalysis has been rather limited. Here, we review some of the challenges associated with photocatalytic applications of colloidal semiconductor nanocrystals and highlight possible directions aimed toward their resolution. A particular emphasis was made on new paradigms in this field, including the possibility of harvesting triplet excitons and utilizing nanocrystal assemblies to accumulate multiple charges at the reaction site.

Probing Solvent–Ligand Interactions in Colloidal Nanocrystals by the NMR Line Broadening

Although solvent-ligand interactions play a major role in nanocrystal synthesis, dispersion formulation, and assembly, there is currently no direct method to study this. Here we examine the broadening of 1H NMR resonances associated with bound ligands and turn this poorly understood descriptor into a tool to assess solvent-ligand interactions. We show that the line broadening has both a homogeneous and a heterogeneous component. The former is nanocrystal-size dependent, and the latter results from solvent-ligand interactions. Our model is supported by experimental and theoretical evidence that correlates broad NMR lines with poor ligand solvation. This correlation is found across a wide range of solvents, extending from water to hexane, for both hydrophobic and hydrophilic ligand types, and for a multitude of oxide, sulfide, and selenide nanocrystals. Our findings thus put forward NMR line-shape analysis as an indispensable tool to form, investigate, and manipulate nanocolloids.

Quantitative analysis of the reduction kinetics of a Pt(II) precursor in the context of Pt nanocrystal synthesis

In this letter, we report a quantitative analysis of how a Pt(II) precursor is reduced to atoms at different temperatures for the formation of Pt nanocrystals with different morphologies and sizes. Our results suggest that in the early stage of a synthesis, the Pt(II) precursor is reduced to atoms exclusively in the solution phase, followed by homogeneous nucleation to generate nuclei and then seeds. At a relatively low reaction temperature such as 22°C, the growth of the seeds is dominated by autocatalytic surface reduction that involves the adsorption and then reduction of the Pt(II) precursor on the surface of the just-formed seeds. This particular growth pathway results in relatively large assemblies of Pt nanocrystals. When the reaction temperature is increased to 100°C, the dominant reduction pathway will be switched from surface to solution phase, producing much smaller assemblies of Pt nanocrystals. Our results also demonstrate that a similar trend applies to the seed-mediated growth of Pt nanocrystals in the presence of Pd nanocubes.

Guiding kinetic trajectories between jammed and unjammed states in 2D colloidal nanocrystal-polymer assemblies with zwitterionic ligands.

Mesostructured matter composed of colloidal nanocrystals in solidified architectures abounds with broadly tunable catalytic, magnetic, optoelectronic, and energy storing properties. Less common are liquid-like assemblies of colloidal nanocrystals in a condensed phase, which may have different energy transduction behaviors owing to their dynamic character. Limiting investigations into dynamic colloidal nanocrystal architectures is the lack of schemes to control or redirect the tendency of the system to solidify. We show how to solidify and subsequently reconfigure colloidal nanocrystal assemblies dimensionally confined to a liquid-liquid interface. Our success in this regard hinged on the development of competitive chemistries anchoring or releasing the nanocrystals to or from the interface. With these chemistries, it was possible to control the kinetic trajectory between quasi-two-dimensional jammed (solid-like) and unjammed (liquid-like) states. In future schemes, it may be possible to leverage this control to direct the formation or destruction of explicit physical pathways for energy carriers to migrate in the system in response to an external field.