Symposium NM05 : Colloidal Nanoparticles---From Synthesis to Applications

The few lateral flow assays (LFAs) established for detecting the endocrine disrupting chemical (EDC) bisphenol A (BPA) have employed citrate-stabilized gold nanoparticles (GNPs), which have inevitable limitations and instability issues. To address these limitations, we developed more stable sensitive lateral flow assay by designing strategies for modifying the surfaces of gold nanostructures with polyethylene glycol and then testing their effectiveness and sensitivity toward BPA in an LFA.
To further enhance detection, the work was extended to the incorporation of gold nannostructures into the design of a surface-enhanced Raman scattering lateral flow assay (SERS-LFA). The assay includes high-SERS-performance gold nanoparticles, i.e., 40 nm gold nanostars (GNSs), and 4-aminothiolphenol (4-ATP) as a Raman reporter. To demonstrate the performance of this SERS-LFA kit toward BPA detection, we tested BPA stock solutions with concentrations ranging from 0 to 32 ppb. Without the application of any enhancement strategy, this modified BPA LFA can achieve a naked-eye limit of detection (LOD) of 0.8 ng/mL, which is 12.5 times better than the LOD of other reported BPA LFAs, and a quantitative LOD of 0.472 ng/mL. The limit of detection for the SERS signals was 0.074, which was 202 and 8 times more sensitive than those of visual intensity and color intensity quantification, respectively. The range of quantification of the SERS signals doubled compared to that of color intensity quantification.

This talk will provide an overview of our recent developments in the design of nanomaterials for ultrasensitive biosensing. Bio-responsive nanomaterials are of growing importance with potential applications including drug delivery, diagnostics and tissue engineering (1). Using enzyme-mediated signal readouts we have developed a suite of nanoparticle based ultrasensitive biosensing approaches as well as a cell/tissue-interfacing nanoneedle platform (2). We are applying these biosensing approaches both in high throughput drug screening and to diagnose diseases ranging from cancer to global health applications.
[1] Stevens MM, George JH, Exploring and engineering the cell surface interface., Science, 2005, Vol:310, Pages:1135-1138.
[2] Howes PD, Chandrawati R, Stevens MM, 2014, Colloidal nanoparticles as advanced biological sensors, Science, 2013, Vol:346, DOI: 10.1126/science.1247390.

Platinum-nickel (Pt-Ni) octahedral based nanomaterials represent an emerging class of highly active electrochemical catalysts, but can not meet the increasing activity demand for broad application and suffer from stability issue. We introduced Cu into octahedral Pt-Ni with controlled morphology. The octahedral Pt-Ni-Cu shows significantly enhanced activity and stability compared to octahedral Pt-Ni catalysts for cathode oxygen reduction reaction (ORR) , which is demonstrated to be a potential electrochemical catalyst. Computational simulation confirmed that the presence of Cu can improve the transition metal (Ni and Cu) retention thus improve both activity and stability.

Ni nanoparticles (NPs) catalyze many chemical reactions, in which they can become contaminated or agglomerate, resulting in poorer performance. We report deposition of silica (SiO₂) onto Ni NPs from tetraethyl orthysilicate (TEOS) through a reverse microemulsion approach, which is accompanied by an unexpected etching process. Ni NPs with an initial average diameter of 27 nm were embedded in composite SiO₂-overcoated Ni NPs (SiO₂-Ni NPs) with an average diameter of 30 nm. Each SiO₂-Ni NP contained a ~7-nm oxidized Ni core and numerous smaller oxidized Ni NPs with diameters of ~2 nm distributed throughout the SiO₂ shell. Etching of the Ni NPs is attributed to use of ammonium hydroxide as a catalyst for deposition of SiO₂. Aliquots acquired during the deposition and etching process reveal agglomeration of SiO₂ and Ni NPs, followed by dissociation into highly uniform SiO₂-Ni NPs. This etching and embedding process may also be extended to other core materials. The stability of SiO₂-Ni NPs was also investigated under high-temperature oxidizing and reducing environments. The structure of the SiO₂-Ni NPs remained significantly unchanged after both oxidation and reduction, which suggests structural durability when used for catalysis.

Hafnium oxide (HfO₂) with a k-edge of 50 keV is being explored as a clinical X-ray contrast agent. HfO₂ NPs are also in clinical trials as radiosensitizers that induce immune action against tumor sites. However, the unpredictable stability of commercially available HfO₂ makes it hard to predict their pharmacokinetics and their size range of ₍(50-100 nm) results in zero clearance from an in vivo system. Therefore, in this study we have executed a modular approach for the design and scalable synthesis of novel, non-sintered, 4-8 nm metal oxide nanoparticles (e.g. Hafnium oxide, Gadolinium oxide) with 40-60 fold higher X-ray attenuation cross-sections than the NPs diameter. Contrast-enhanced computed tomography (CT) and spectral (color) X-ray CT with the aid of these new class of probes have the potential to enable targeted image guided therapeutics with CT as a lower cost and higher resolution alternative to PET and MRI.
Amorphous HfO₂ seed NPs, ~1.5 nm in size, embedded in a polycationic mesh and immobilized inside a silica template, were prepared by the simultaneous reduction and stabilization of tetrakis dimethyl amido hafnium by oleylamine at >250°C in a polyol solvent. Seed NPs were subsequently subjected to high-temperature calcination (>650°C), followed by alkaline digestion of the silica template, to prepare crystalline HfO₂ NPs. As-prepared HfO₂ NPs were surface functionalized with a monolayer of fluorescent silane to enable fluorescence imaging. The hydrodynamic diameter and zeta potential were measured using dynamic light scattering. In vitro cytocompatibility was characterized using established methods which included Live/Dead assay and confocal laser scanning microscopy after incubating NPs with HeLa cancer cells and THP-1 macrophage cells at 25-200 mg NPs/100,000 cells
We achieved >95% efficient conversion of the amorphous seed NPs to crystalline orthorhombic-HfO₂ NPs. HfO₂ NPs were ~4-5 nm in diameter and formed non-sintered flocculants, 220-290 nm in diameter. As-prepared CY5 tagged-HfO₂ NPs exhibited simultaneous X-ray contrast and fluorescence in multispectral imaging. Both HfO₂ and CY5 tagged-HfO₂ NPs remained well-dispersed over 24 h. The measured zeta potential was -13 to -16 mV. Encapsulating the HfO₂ NPs in a SiO₂ shell reduced the hydrodynamic diameter to ~30 nm and inhibited the formation of flocculants. MTT and Live/Dead assays confirmed that the HfO₂ NPs were neither cytotoxic nor pro-inflammatory. Confocal microscopy confirmed highly efficient uptake of Cy5-HfO₂ NPs by HeLa cells and THP-1 cells.
HfO₂ NPs were prepared using a novel templated synthesis resulting in crystalline NPs, ~4-5 nm in diameter, which flocculate into 220-290 nm clusters. Therefore, these NPs provide both long blood circulation and eventual renal clearance through phagocytic breakdown after delivery as radiographic imaging probe or radiosensitizer.

The future of semiconductor quantum dots (QDs) in biomedical imaging lies not with well-established CdSe-based materials that raise issues of toxicity and limited tissue penetration depths, but rather with heavy metal-free compositions that emit in the near infrared (NIR) and short wave infrared (SWIR) in addition to visible wavelength ranges. With this motivation, we have developed semiconductor quantum shells (QSs) comprising non-toxic constituents. These ZnSe/InP/ZnS core/shell/shell structures are dubbed QSs because their Inverted-Type I bandgap structure yields quantum confinement-based emission from the InP shell. The excitonic emission from the QSs is tunable from the visible through the NIR with shell thickness, yielding emission peaks ranging from 515 – 850 nm. This tunablility range is wider than that seen for Type I InP QDs, particularly expanding emission deeper in the first optical tissue window, which spans from 650 – 950 nm. In this talk, I will detail the synthetic control of these heterostructured nanomaterials. In depth photophysical characterization has elucidated the structure/function relationship, enabling the concerted design of these emitters. Of particular note is our ability to match the brightness of emitters of various colors by tuning both the core size and shell thickness. The presentation will include early nanotoxicity and animal imaging results.

In this talk, I will summarize the recent research on the crystal phase-engineering of novel nanomaterials in my group. It includes the first-time synthesis of hexagonal-close packed (hcp) Au nanosheets (AuSSs) on graphene oxide, surface-induced phase transformation of AuSSs from hcp to face-centered cubic (fcc) structures, the first-time synthesis of 4H hexagonal phase Au nanoribbons (NRBs) and their phase transformation to fcc Au RNBs, and the epitaxial growth of 4H Ag, Pt, Pd, PtAg, PdAg, PtPdAg, Rh, Ir, Ru, Os and Cu on 4H Au NRBs. Importantly, the concept of crystal-phase heterostructure is proposed.

Traditional approaches to nanoparticle size control generally attempt to control size by controlling the nucleation step and varying the number of nuclei formed. I will present several approaches to nanoparticle size control and systematic variation that seeks to control and systematically vary nanoparticle size using identical nucleation events, but varying the nanoparticle growth. The approaches have in common the constant addition of nanoparticle precursor that leads to a steady state reaction. This method, referred to as the Extended LaMer mechanism, leads to a linear increase in nanoparticle volume with time. The reaction can be extended for as long as the nanoparticles remain colloidally stable, allowing for systematic variation of nanoparticle sized through a wide range. The approach is general and can be applied to a range of synthetic systems to produce nanoparticles with exceptional reproducibility in size. One can also take advantage of the loss in colloidal stability to design a reaction that precipitates at a desired size. Since this loss of solubility is essentially a phase transition, the nanoparticle size is controlled by thermodynamics and not kinetics. This improve the ease of reproducibility of nanoparticle size with or without careful control of the reaction kinetics. A continuous reaction using this precipitation approach in magnetic nanoparticles will be discussed as will scale up and applications of these nanoparticles. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.

Well-defined, organically-modified ZnO nanoparticles were prepared via an efficient hydrolysis route, without the need for surfactant co-ligands, washing or size-selection steps. The products have a narrow size distribution and are soluble in organic solvents. The synthesis involves reacting a mixture of alkylzinc carboxylate complex and excess diethylzinc with water to yield carboxylate-capped ZnO nanoparticles. Varying the ratio of the different organometallic species enables control of either size or degree of surface modification.
The bottom-up synthesis of ligand-stabilized functional nanoparticles from molecular precursors is widely applied but is difficult to study mechanistically. In this organometallic system, which avoids excess ligand, ³¹P NMR spectroscopy can be used to follow the trajectory of phosphinate ligands during the synthesis. Initially, we established the structures of a range of ligated zinc oxo clusters, containing 4, 6 and 11 zinc atoms, showing that the clusters interconvert rapidly and self-assemble in solution based on thermodynamic equilibria rather than nucleation kinetics. Subsequently, we identified these clusters in situ during the synthesis of phosphinate-capped zinc oxide nanoparticles. Unexpectedly, the ligand is sequestered to a stable Zn₁₁ cluster during the majority of the synthesis and only becomes coordinated to the nanoparticle surface, in the final step. In addition to a versatile and accessible route to (optionally doped) zinc clusters, the findings provide an understanding of the role of well-defined molecular precursors during the synthesis of small (2–4 nm) nanoparticles.
These stabilised ZnO nanoparticles can be combined with copper nanoparticles to form colloidal catalysts. In a slurry phase reactor, these systems have proven to be highly active in methanol synthesis compared to the analogue heterogeneous Cu/ZnO/Al₂O₃. The increased catalytic activity appears to correlate with a higher exposed surface area and with the selection of the capping ligand, which plays a role in supporting re-structuring and stability.

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing nanoscale semiconductors in high yield. While much progress has been made towards the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products’ inherent carrier transport properties. This is often due to their irregular morphology or small dimensions, which usually demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report a novel means for obtaining a comprehensive analyses of the electronic transport properties of individual colloidal semiconductor nanocrystals. First, we present the development of a novel solution chemistry-based synthetic approach to produce nearly monodisperse µm-scale 2D tin(II) sulfide (SnS) nanoribbons and square nanosheets using a one-pot, one-step, easily-scalable synthetic route. These syntheses represent a rare example in this size regime of essentially uniform, single-crystalline, 2D nanocrystals produced using colloidal chemistry. Next, we detail the structural characterization of these SnS materials, and describe how they are processed from solution to fabricate back-gated, top-contact solid-state devices from individual colloidal crystals. Finally, we interrogate their electronic transport properties by a combination of multi-point contact probe electrical transport measurements and time-resolved terahertz spectroscopy. These studies allow for the determination of carrier concentration, carrier mobility, conductivity, and the majority carrier type within an individual 2D colloidal nanocrystal. Our findings illustrate that minor manipulation of solution chemistries may afford products with substantively disparate charge carrier parameters, which are challenging to extract using common experimental practices, and underpins that this metrological strategy represents a significant and valuable advancement in the characterization of colloidally-synthesized semiconductors.

Carbon dots, as a rising fluorescent materials have attracted continuously attention for potential applications in LED, solar cells, sensor, bioimaging and photocatalyst. However, its photo luminescent quantum yield (PL QY) is still quite low, especially emission in long wavelength region like red light. Herein, we proposed increasing the PL QY by doped carbon dots with N or S, N. The PL QY of carbon dots dramatically rises up after doping with N. It can reach over 90%. The carbon dots prepared via bottom-up route show excitation independent emission. In order to extend the absorption in the visible light region, S element is further introduced into the carbon dots to form S, N co-doped carbon dots. Due to the introduction of S and N, there is another S state was introduced into the band gap. That results in the new emission at red light region. Blue, green and red light emissions were obtained from carbon dots. Due to the excellent biocompatibility and low cytotoxicity, we further conjugated the cisplatin with carbon dots to obtain the theranostic agent. We explored to adding more function onto the carbon dots, like self-targeting and therapeutic function to construct the nanomedicine integrating with targeting, bio-imaing and therapy function together.

Platonic particles are promising materials with nanoscale light-matter interactions in plasmonics and biosensing, due to their unique structure caused by vertices, edges and facets. The position and orientation of Platonic particles play a crucial role in determining the resultant assembled structures at a liquid/liquid interface. Therefore, it is desirable to develop a reliable theory that can predict the interfacial configuration of an isolated Platonic nanoparticle from nanoparticle-solvent interactions and solvent-solvent interactions. Here, we numerically explored all possible orientations of a Platonic nanoparticle, including three specific orientations: vertex up, edge up, and facet up orientations. We found that a specific orientation is more preferred than random orientations. We also demonstrated that the free energy change theory could quantitatively predict the position and orientation of an isolated Platonic nanoparticles at a liquid/liquid interface, where the surface wettability of the nanoparticle determined the most stable position and the preferred orientation. Molecular dynamics simulations were used to test our theory where the surface wettability of a Platonic solid was adjusted from extremely hydrophobic to extremely hydrophilic by changing the charge amount on the Ag surface. The molecular dynamics simulation results were in excellent agreement with our theoretical prediction for an isolated Ag Platonic nanoparticle at a hexane/water interface. Our proposed theory bridges molecular-level simulations and assembly structure of Platonic nanoparticles in experiments, in which the insights from nanoparticle wettability in solvents can be used to predict macroscopic superlattice structure in experiments. This work advances our ability to precisely predict the final structures of the Platonic nanoparticle assemblies at a liquid/liquid interface.

Significance of the surface passivation of II-VI colloidal quantum dots (QDs) on their photophysical properties is discussed. Optically forbidden nature of surface-associated states makes their direct measurements challenging. Our DFT-based simulations of CdSe QDs ligated by anionic ligands, such as carboxylates, thiolates, and hydrides, provide insights into the role the acidic ligands play during synthesis and ligand exchange, as well as in manipulating QD’s optical response. Thus, our calculations reveal much more complicated exchange mechanism of the native surface ligands of CdSe QDs with phenyl-dithiocarbamates (PTCs) as it was thought before. PTCs decompose during exchange with native ligands, while only a small portion of deprotonated PTCs covalently bounds to the Cd-enriched surface. Our calculations also reveal that attachment of the hydride to Se sites results in strong distortions of Cd-Se bounds leading to ‘cleaning’ out of extra Se ions from the QD surface (in a form of SeH₂ gas) and eliminating Se-associated trap states. On the other hand, adsorption of H^- on Cd, when the surface is enriched by metal ions, results in blue-shifted lower-energy transitions with very high oscillator strength, which likely responsible for experimentally observed emission enhancement of CdSe QDs treated by hydrides. The calculated results allow for explanations of experimental trends and observables sensitive to surface defects and ligand passivation and offering guidance for controlling the optical response of II-VI nanostructures by means of surface ligand engineering.

Complex nanoparticles, which contain multiple elements, crystal structures, and architectures, are useful for elucidating principles of fundamental thermodynamics and can be applied in fields spanning catalysis to plasmonics. However, the combinations of elements that have been explored thus far in these systems are limited by a lack of combinatorial methods for preparing multiplexed nanoparticle systems. In this presentation, we will discuss how correlative electron microscopy can be used to study the structures and dynamics of complex nanoparticles generated in scanning probe-deposited polymer nanoreactors. Metal, oxide, and hybrid metal-oxide nanoparticles of a variety of compositions and structures can be rapidly generated on electron-transparent substrates with exquisite site-specificity. These particles can then be easily relocated multiple times, enabling repeated, correlated characterization at the individual nanoparticle level. Moreover, polymer nanoreactors provide a viable platform for studying coarsening and solvent-particle interactions at the nanoscale and can be analyzed using in situ electron microscopy. This work demonstrates how polymer nanoreactors, when combined with advanced electron microscopy techniques, can be used as a versatile methodology for understanding the structure–function relationships as well as the formation mechanisms of complex nanoparticles, especially during their synthesis and processing.

The precise control of hetero-interface and doping induced band gap engineering, in colloidal semiconductor based hetero-nanocrystals (metal/semiconductor) and doped nanocrystals, is very important for the efficient energy or charge transfer through hetero-interface and then their novel optoelectronic properties exploration and their new energy, new optoelectronic devices applications. Growth of monocrystalline semiconductor based metal/semiconductor hybrid nanocrystals (core/shell and heterodimer) with modulated composition, morphology and interface strain are the prerequisite for exploring their plasmon-exciton coupling, efficient electron/hole separation, and enhanced photoctalysis properties. We realized nanoscale monocrystalline growth of the semiconductor shell on metal nanocrystals, the precise relative positions and hetero-interface between original building blocks to precisely synthesize metal/semiconductor hetero-nanostructures and hetero-valent doped semiconductor nanostructures, in particular the hetero-valent dopant engineering. These controls enable the fine tuning of doped level, plasmon-exciton coupling, Plasmon enhanced photocatalytic performance and enhanced photovoltaic, electrical properties applications.
References:
1.J. Zhang, Y. Tang, K. Lee, M. Ouyang, Nature 2010, 466, 91.
2.J. Zhang, Y. Tang, K. Lee, M. Ouyang, Science 2010, 327, 1634.
3.Q. Zhao, J. Zhang*, etc., Adv. Mater. 2014, 26, 1387.
4.H.Qian, J. Zhang*, etc., NPG Asian Mater. (2015) 7, e152; doi:10.1038/am.2014.120.
5.J. Gui, J. Zhang*, etc., Angew. Chem. Int. Ed. 2015, 54，3683-3687.
6.J. Liu, Q. Zhao, J. Zhang*, etc., Adv. Mater. 2015, 27，2753-2761.
7.M. Ji, M. Xu, J. Zhang*, etc. Adv. Mater. 2016, 28, 3094–3101.
8.J. Zhang*, Q. Di, etc. J. Phys. Chem. Lett.2017, 8, 4943-4953. (Invited perspective)

Selectively permeable membranes with high flux and high selectivity have important implications in many areas, such as water desalination, CO2 capture, H2 removal or purification, O2 separation, as well as selective ion transport for fuel cells and lithium batteries. Reduced membrane thickness and precisely constructed pore size/chemistry are the two keys for achieving combined high flux and high selectivity. In natural biological system, membranes can be as thin as 4 nm, and the pores/chemistries are elaborately constructed via molecular assembly, resulting in unbeatable performance compared to synthetic membranes. A manufacturing approach that ensures the structural and compositional precision is critical for high performance membranes. ALD is a layer-by-layer deposition method that builds up a thin layer with atomic precisions, and colloidal self-assembly is a method of building up materials in nm-level precisions. The combination of the two provides a new approach for membrane fabrication. Using this approach, hierarchically structured sub-50nm thick ultra-thin membranes with precisely defined pore size and pore surface chemistry have been successfully fabricated. Excellent performances in CO2 and oxygen separation have been achieved. The authors would like to acknowledge the support from DOE SBIR award number DE-SC0017178.

Colloidal photonic crystals are photonic crystals made by bottom-up physical chemistry strategies from monodisperse spherical colloidal particles. The self-assembly process is leading to inherently three-dimensional structures with optical properties determined by the periodicity, induced by this ordering process, in the dielectric properties of the material. Apart from the optical properties, the nanoscopic periodicity, exemplified by SANS, can be transferred if the crystal is used as a template for depositing or removing material as e.g. vortex pinning in Nb thin films deposited on such a crystal.

The use of hollow spheres as building blocks has brought about a whole realm of unique possibilities. One, again derived from the periodicity, is the fabrication of three-dimensional hierarchical structures at the nanoscale. It is also possible to convert the hollow nanospheres to open nanorings, which can be used as templates for the unique core-shell nanoring topology. This topology leads to tunable plasmon resonances.

The best-known optical effect, though, is the photonic band gap, the range of energies, or wavelengths, that is forbidden for photons to exist in the structure. This photonic band gap is analogous to the electronic band gap of electronic semiconductor crystals. We have previously shown how with the proper photonic band gap engineering, we can insert allowed pass band defect modes and use the suppressing band gap in combination with the transmitting pass band to induce spectral narrowing of emission and improved energy transfer. We show now how with a high-quality narrow pass band in a broad stop band, it is possible to achieve photonic crystal lasing in self-assembled colloidal photonic crystals with a planar defect. In addition, with proper surface treatment in combination with patterning, we prepare for addressable integrated photonics. Finally, by incorporating a water in- and outlet, we can create optomicrofluidic structures on a photonic crystal allowing the optical probing of microreactors or micro-stopped-flow in the lab-on-an-optical-chip.

Plasmonic harvesting of solar energy to drive off-grid fuel generation is hindered by reaction selectivity, short (~100 fs) hot electron lifetimes, and catalyst stability. Au nanorods allow reaction specificity under solar irradiation via their tunable localized surface plasmon resonance (LSPR) and can be synthesized in gram-level quantities. This work exploits the longitudinal LSPR of Au nanorods to photodeposit Pt at targeted locations as a co-catalyst to expand hot electron lifetimes and promote reactant desorption. Time-resolved reaction kinetics between (i) the Pt(IV) precursor and (ii) plasmonic hot electrons at the Au nanorod surfaces were monitored by transmission UV-vis spectroscopy. The method is amenable to other metallic precursors. Discrete dipole computation of the Pt-decorated Au nanorods allow a priori design of photocatalysts with optimal energetics. These efforts provide a foundation towards economical manufacturing of plasmon-sensitized, bimetallic photoanodes that can spatially target specific photochemical reactions, such as C-C bond cleavage during ethanol oxidation.

For many years the semiconductor industry has been driven by decreasing the structural dimensions thus increasing the device density and boosting compute and memory performance. Today, true nanoscale dimensions are reached with, e.g., transistor fin-width of smaller than 10 nm in the current technology node. The semiconductor industry research agenda is no longer driven by exclusively scaling dimensions but by integrating new materials and devices offering additional functionality like sensing and transmission for IoT applications. In this regard, the integration of nanoscale, high quality crystalline materials with precise control of dimension and location on the silicon platform is crucial. We have developed the Template-Assisted Selective Epitaxy (TASE) approach to monolithically integrate a broad range of III-V compounds on Si for the applications in nanoelectronics and nanophotonics. In TASE the III-V semiconductor is grown within the confined space of an oxide template and results in an III-V-on-insulator structure which can be used for fabricating devices. High mobility semiconductors like InAs, GaAs and GaSb were grown by TASE and the structural and electronic properties were investigated in detail. TASE-grown III-V nanostructures seem to have superior properties compared with conventional grown nanowires. This is indicated by photoluminescence spectroscopy and also by low-temperature transport measurements. In a recent experiment we demonstrated ballistic 1D quantum transport in single InAs nanowires and cross-junctions. Characteristic 1D conductance plateaus are resolved in field-effect measurements across up to four nanowire junctions in series. Furthermore, the effect of size and symmetry of the nanowire cross section on the quantum confinement is investigated. In general, our investigations of the TASE materials as well as the fabricated nanoscale electronic and optical devices demonstrate the attractiveness of TASE to integrate nanoscale materials on silicon.

Silver nanoparticles (AgNPs) are among the most extensively used engineered nanomaterials in consumer products and biomedical applications. This is due to their ability to slowly release antibacterial Ag+ ions. Rising demand for AgNPs, especially in healthcare, is projected to drive the global AgNP market to approach USD 2.5 billion over the next 5 years. However, with increased use of AgNPs, their release into the environment raises both health and environmental concerns. Several studies have sought to evaluate the potential environmental outcomes of released AgNPs. Investigations of AgNP sulfidation have drawn particular attention due to the prevalence of sulfidation in aquatic environments,¹ and also due to interest in selective sulfidation for optical sensing and photonics applications. However, mechanistic details of the reaction pathway transforming AgNPs to silver sulfide (Ag₂S) remain controversial topics with conflicting results reported,^1,2 and knowledge regarding sulfidation kinetics is limited. Our in situ studies have sought to elucidate the fundamental mechanisms associated with AgNP aggregation behavior, sulfidation and crystallization kinetics by combining high-resolution synchrotron ultra-small-angle X-ray scattering (USAXS), small angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS)/X-ray diffraction (XRD) measurements focused on a well-controlled sulfidation process for polyvinylpyrrolidone (PVP)-coated monodisperse AgNP suspensions in the presence of fulvic acid. The combined methods simultaneously monitor a length-scale range from sub-Å to several micrometers,³ hence allowing quantitative characterization of the evolution in both atomic structure and nanoparticle suspension morphology (i.e., microstructure). Our in situ measurements have been complemented by ex situ high-resolution scanning transmission electron microscopy (STEM) of AgNPs previously subjected to the same sulfidation history. Under the real-time experimental conditions used, we find aggregation of the nanocrystalline AgNPs to be minimal with the AgNPs remaining spherical, or at least globular with facets, throughout the sulfidation process. Incorporation of sulfur ions into the AgNPs causes a monotonic increase in mean nanoparticle diameter, and the nanoparticles exhibit well-defined growth kinetics. We find that the degree of sulfidation is directly related to the availability of sulfur ions in solution, and the crystallization kinetics of Ag₂S correlate well with the nanoparticle growth kinetics. We also find that Ag+ ions do not leach into the solution during the sulfidation process.⁴

[1] B. Thalmann, A. Voegelin, E. Morgenroth & R. Kaegi; Environ. Sci.: Nano, 3, 203-212 (2016).
[2] J.M. Pettibone & J. Liu; Environ. Sci. Technol., 50, 11145-11153 (2016).
[3] N.M. Martin, A.J. Allen, R.I. MacCuspie & V.A. Hackley; Langmuir, 30, 11442-11452 (2014).
[4] F Zhang, A.J. Allen, A.C. Johnston-Peck, J. Liu & J.M. Pettibone; ACS Nano, submitted (2018).

Thin gold nanowires (NWs) represent ideal objects for fundamental studies as well as potential applications in sensing, catalysis, and electronic contacts for molecular devices. Various methods have been developed to grow and characterize thin Au NWs. Though exciting, the reported methods did not univocally address the questions of the nucleation and growth mechanism, which is of prime interest to tune the size of the NWs and thus their physicochemical properties. Furthermore, the atomic-scale structure of thin Au NWs is still under debate. We will present results of in situ high-energy XRD and atomic PDF studies on the nucleation, growth and 3D structure of Au NWs in solution. XRD data were taken in an interval of 5 min for up to 30 hours at room temperature. Gold clusters (size < 1 nm) are seen to nucleate fast and then continuously grow forming NWs with a diameter of < 2 nm and length of many tens of micrometers. A priori, one could anticipate that Au NWs of such a size would exhibit a weak tendency to depart from the bulk fcc structure, if at all. We find though that, due to competition between achieving optimal surface energy and atomic packing, the resulting Au NWs do not necessarily possess an fcc-type structure at atomic level. We elucidate how the unusual 3D structure of Au NWs evolves, including the unusually short Au-Au bonding distances, and show that the degree of its departure from the bulk fcc structure can be controlled through adjusting particular details in the synthesis protocol.

Copper nanoparticles (CuNPs) are attractive as a low-cost alternative to their silver and gold analogues for numerous applications, including emerging electronic devices based on organic and perovskite semiconductors, plasmonic hot-electron devices and nano-electrodes for molecular electronics. However, their potential has hardly been explored due to their higher susceptibility to oxidation in air. Here we present the unexpected findings of an investigation into the correlation between the stability of CuNPs in air and the structure of the thiolate capping ligand. The experiment design is based on monitoring (in real time) the oxidation of largely isolated CuNPs tethered to a solid substrate via the evolution of the localised surface plasmon resonance band - a direct approach which simplifies the interpretation of the data. Remarkably, of the 8 different ligands screened those with the shortest alkyl chain, -(CH₂)₂- , and a hydrophilic carboxylic acid end group are found to be the most effective at retarding oxidation in air. We also show that CuNPs are not etched by thiol solutions as previously reported, and address the important fundamental question of how the work function of small supported metal particles scales with particle size. Taken together these findings set the stage for greater utility of CuNPs for emerging electronic applications.

Solution-processing of semiconductor materials is an emerging strategy that can potentially reduce the cost of thin-film photovoltaic devices. The fundamental challenge accompanying this effort lies in processing of nanoparticles inks into high-performance solids that show high crystallinity and low defect density. This task becomes exceedingly difficult with a decreasing nanoparticle size. Here, we develop the ionic intercalation strategy by which nanostructured films are being forced to undergo an inter-particle ion exchange towards reducing the inter-surface tension (improves crystallinity) and achieving overall charge neutrality at boundaries (reduces defects). Such non-thermal interparticle fusion has been enabled by raising the ion solubility in deposited nanoparticles through establishing the lattice-solvent equilibrium regime. The degree of inter-particle intercalation has been varied towards achieving either a complete sintering of the film or a partial fusion that preserves the quantum confinement of individual dots. Thus, the ionic intercalation approach improves electrical characteristics of nanoparticle solids and provides a reliable starting platform for developing photovoltaic and other photoconducting solids.

The noble atom catalysts have attracted rapidly increasing attention due to their unique catalytic properties and maximized utilization for low-cost. However, the noble metal catalysts, downsized to clusters or single-metal atoms, are structurally unstable due to the natural tendency for metal atoms to diffuse and agglomerate, resulting in the formation of larger particles. In practical applications, it require that the single atom not only have higher activity, but also have satisfying stability. Moreover, the high density of single atom catalysts also was required to meet the practical applications such as fuel cells¹. We used atomic layer deposition technique (ALD) to produce high density of isolated single Pt atom catalysts with high stability on different supports. Those single Pt atom catalysts have been investigated for the different reactions such as methanol oxidation reaction (MOR)² and hydrogen evolution reaction (HER)³, where they exhibit significantly enhanced catalytic activity and high stability in comparison to their nanoparticle counterparts. We also studied the mechanism for design of high stable single atom catalysts by ALD^3-5 and revealed the mechanisms of the unexpectedly high electrocatalytic activity of the single atom², which can deliver atomic-level understanding of catalytic active sites and provide insights into the design of high performance of novel catalytic systems. We will used area-selected ALD to prepare Pt-based high stable catalysts for fuel cells⁶.


References:
N. Cheng, Y. Shao, J. Liu and X. Sun, Electrocatalysts by Atomic Layer Deposition for Fuel Cell Applications. Nano Energy (2016) In press doi:10.1016/j.nanoen.2016.01.016 (review paper)
S. H. Sun, G. X. Zhang, N. Gauquelin, N. Chen, J. G. Zhou, S. L. Yang, W. F. Chen, X. B. Meng, D. S. Geng, M. N. Banis, R. Y. Li, S. Y. Ye, S. Knights, G. A. Botton, T. K. Sham and X. L. Sun, Sci Rep-Uk, 2013, 3, 1775.
Niancai Cheng , Samantha Stambula, Da Wang , Mohammad Norouzi Banis , Jian Liu , Adam Riese , Biwei Xiao , Ruying Li , Tsun-Kong Sham, Li-Min Liu, Gianluigi A. Botton and a. X. Sun, Nat. Commun., 7 (2016) 13638.
N. C. Cheng, M. N. Banis, J. Liu, A. Riese, S. C. Mu, R. Y. Li, T. K. Sham and X. L. Sun, Energ Environ Sci, 2015, 8, 1450-1455.
S. Stambula, N. Gauquelin, M. Bugnet, S. Gorantla, S. Turner, S. H. Sun, J. Liu, G. X. Zhang, X. L. Sun and G. A. Botton, J Phys Chem C, 2014, 118, 3890-3900.
N. Cheng, M. Banis, J. Liu, A. Riese, X. Li, R. Li S. Ye, S. Knights and X. Sun, Extremely stable platinum nanoparticles encapsulated in zirconia nanocages by area-selective atomic layer deposition for oxygen reduction reaction. Adv. Mater., 27 2 (2015) 277

Colloidal semiconductor nanomaterials offer unique opportunities for electronic and photonic applications. However, many of these materials contain elements that are toxic, exhibit poor stability, or are chemically incompatible with a target application. A potential alternative to conventional quantum dots that could overcome these challenges are colloidal group IV semiconductors with nanoscale dimensions. Quantum-confined Group IV nanomaterials exhibit optical properties that can be tuned from the visible with potential for mid-infrared properties by modifying composition. Here, I will present our recent results related to the synthesis of Group IV alloy nanocrystals as well as atomically-thin sheets of Group IV materials (e.g., silicane) with unique optical properties. I will address synthetic challenges that are specific to Group IV nanomaterials, surface functionalization strategies, and our progress in characterizing the optical properties of these materials.

Quantum Dots (QDs) are semiconductor nanocrystals with a wide range of potential applications including displays, photovoltaics, and biological labeling and tracking. Understanding the charge carrier dynamics of emerging quantum dot materials and correlating them to their structure, composition, and surface chemistry allows for QDs to be developed for different applications. One such system is InP QDs, which are being developed as a cadmium-free alternative to traditional CdSe QDs. InP offers size-tunable emission across the visible and near infrared spectral range. Shelling an InP core with a wide band gap material such as ZnSe increases the quantum yield from 2-3% up to ~50%, improves the QD’s photostability, and suppresses blinking. Many additional properties of InP QDs are yet to be fully studied and understood. Ultrafast fluorescence upconversion spectroscopy is a powerful technique used to characterize QDs by providing valuable information about their ensemble charge carrier and trapping dynamics. Femtosecond decay constants and their relative amplitudes provide insight into how these QDs’ trapping dynamics differ from those of traditional cadmium-based core-shell QDs. Traditionally, shelling a QD core leads to confinement of the electron and hole leading to improved photoluminescent properties by reducing the number of defects observed on the QD’s surface. Initial ultrafast measurements on a thick-shell InP-ZnSe QD sample show a fast initial 6.2 ± 1.7 picoseconds decay, traditionally associated with hole trapping. A second longer lived component represents radiative recombination.

References
1. Underwood, D. F.; Kippeny, T.; Rosenthal, S. J., Ultrafast Carrier Dynamics in CdSe Nanocrystals Determined by Femtosecond Fluorescence Upconversion Spectroscopy. J. Phys. Chem. B 2001, 105 (2), 436-443.
2. Keene, J. D.; McBride, J. R.; Orfield, N. J.; Rosenthal, S. J., Elimination of Hole–Surface Overlap in Graded CdS_xSe_1–x Nanocrystals Revealed by Ultrafast Fluorescence Upconversion Spectroscopy. ACS Nano 2014, 8 (10), 10665-10673.

Colloidal CdSe nanoplatelets (NPLs) are considered to be excellent candidates for many applications in nanotechnology. One of the current challenges is to self-assemble these colloidal quantum wells into large ordered structures to control their collective optical properties. We describe a simple and robust procedure to achieve controlled face-to-face self-assembly of CdSe nanoplatelets into micron-long polymer-like threads made of up to ∼1000 particles. These structures are formed by addition of oleic acid to a stable colloidal dispersion of platelets, followed by slow drying and re-dispersion. We could control the average length of the CdSe nanoplatelet threads by varying the amount of added oleic acid. These 1-dimensional structures are flexible and feature a “living polymer” character because threads of a given length can be further grown through the addition of supplementary nanoplatelets at their reactive ends. [1] We also show that these ribbons of stacked board-shaped NPL twist upon the addition of oleic acid ligand, leading to chiral ribbons that reach several micrometers in length and display a well-defined pitch of ~400 nm. We demonstrate that the chirality originates from surface strain caused by the ligand because isolated NPLs in dilute solution undergo a transition from a flat to a twisted shape as the ligand coverage increases. When the platelets are closely stacked within ribbons, the individual twist propagates over the whole ribbon length. These results show that a ligand-induced mechanical stress can strongly distort thin NPLs and that this stress can be expressed at a larger scale, paving the way to stress engineering in assemblies of nanocrystals. Such a structural change resulting from a simple external stimulus could have broad implications for the design of sensors and other responsive materials. [2]

[1] S. Jana, P. Davidson, B. Abécassis, Angew. Chem. Int. Ed. , 55, 9371, (2016)
[2] S. Jana, M. de Frutos, P. Davidson, B. Abécassis, Ligand-induced twisting of nanoplatelets and their self-assembly into chiral ribbons. Sci. Adv. 3, e1701483 (2017).

Now a day, advanced optical spectroscopy studies at the level of individual nanocrystals can be perform routinely. In parallel, advances in Hi-Res transmission electron microscopy allow imaging not only the size and shape of the nanocrystals but also their internal composition. However, for most of the times, these two powerful characterization experiments were applied on two different sets of a nanocrystal ensemble. As a result, establishing direct optical-structural property correlation becomes impossible. Here in this talk we will review our recent experiments, in which single nanocrystal, time tagged, time correlated photon counting experiments and z contrast scanning TEM experiments were performed on the same set of individual core/thick-shell nanocrystals quantum dots (i.e. giant QD or g-QDs). Direct correlation of individual g-QDs’ emission characteristics (i.e. lifetimes and photon emission statistics etc.) with their size, shape and composition allows us to identify photo-charging of the g-QDs, not the existence of dark g-QD subpopulation, as the physical origin of the imperfect QY of the core/thick-shell nanocrystals. Furthermore, by extending the experiment to high temperature and high light flux regime, we investigated PL bleaching issue of the nanocrystals that is hindering the application of nanocrystals in solid-state lighting. The experiment allows us to eliminate heat and light induced changes in shell-thickness and composition of the g-QD as a mechanism responsible for the PL bleaching. The experiment further shows that while the charging of g-QDs is partly responsible, the creation of hot carrier traps that intercept the excitons before they relax to the ground state is mainly responsible for permanent PL bleaching. More interestingly, we also observed that the g-QDs that are prone to charging are more resistant to the creation of hot carrier traps and hence permanent PL bleaching. These fundamental understandings on structure-function relationships open path toward nano-engineering fine internal structure of g-QDs for higher emission efficiency and thermal stability.

The device performance of PbS heterojunction quantum dot (QD) solar cells is hindered by defects at the pn-junction interface, which forms trap states and leads to interface recombination. We design an advanced architecture for the device by introducing a CdSe QD buffer layer at junction between the PbS QDs and ZnO nanoparticles (NPs). We passivate the CdSe QD layer by CdI₂ to achieve n-type doping shown by current-voltage and capacitance-voltage measurement. We optimize the band alignment by exploiting the size-dependent band structure of CdSe QDs to facilitate carrier transport across the junction studied under different illumination conditions. We also show that the CdSe QD buffer layer not only reduces interface recombination but also provides additional photo-generated carriers through external quantum efficiency (EQE) and time-resolved microwave conductivity (TRMC) measurements, which is consistent with the calculated device parameters. Therefore, we fabricate the ZnO NP/CdSe QD/PbS QD solar cell that has significantly higher open circuit voltage and short circuit current density resulting in a 25% enhancement in power conversion efficiency compared to the reference device without the CdSe QD layer.

Quantum dots (QDs) sensitized solar cells (QDSCs)^[1] are achieving considerable attention, due to versatile optoelectronic properties of QDs such as size-tunable band gap, high absorption coefficient, large dipole moment, solution processability and the possibility of multiple exciton generation^[2-4]. However, the record photoconversion efficiency (PCE) of QDSCs reaches to only 12.07%, still lower than the commercial silicon solar cells (typical in the range of 20-40%) and new emerging star perovskite solar cells ^[5]. The possible reasons for this relatively low PCE of QDSCs are mainly associated with narrow light harvesting range of QDs and carrier recombination at the interfaces or within the QDs. This demands the further development of highly efficient QDs to be applied as light harvester to boost the PCE of QDSCs, as the QDs have the immense potential and easy to control their optoelectronic properties. The surface passivation of QD core with shell layer of different materials/thickness has been shown to be an effective approach, which offers a significantly enhanced quantum yield (QY), prolonged PL lifetime and improved chemical, thermal and photochemical/physical stability compared to bare QDs due to the reduced density of surface trap states/defects and optimized electronic band alignment between core and shell ^[6-8]. Herein, we explore a nano-engineering approach to highlight the influence of CdSe_xS_1-x interfacial alloyed layers between the core and shell on the optoelectronic properties of CdSe/(CdS)₆ “giant” core/shell QDs. We will discuss the benefits of interface engineering of core/shell QDs in terms to improve the optoelectronic properties as well as the PCE of liquid junction QDSCs. Resulting newly engineered alloyed core/shell QDs based QDSCs, yielding a maximum PCE of 7.12%, which is mainly attributed to broadening of the absorption spectrum and higher electron-hole transfer rate in favorable stepwise electronic band alignment with respect to reference core/shell QDs, offers a new path to improve PCE of liquid junction QDSCs will be also presented and discussed.
References
[1] S. Rühle, M. Shalom, A. Zaban, Chem. Phys. Chem, 2010, 11, 2290.
[2] S. V. Kershaw, A. S. Susha, A. L. Rogach, Chem. Soc. Rev., 2013, 42, 3033.
[3] I. J. Kramer, E. H. Sargent, Chem. Rev., 2014, 114, 863.
[4] A. J. Nozik, M. C. Beard, J. M. Luther, M. Law, R. J. Ellingson, J. C. Johnson, Chem. Rev., 2010, 110, 6873.
[5] S. Jiao, J. Du, Z. Du, D. Long, W. Jiang, Z. Pan, Y. Li, X. Zhong, J. Phys. Chem. Lett. 2017, 8, 559.
[6] H. Zhao, D. Wang, T. Zhang, M.Chaker, D. Ma, Chem. Commun., 2010, 46, 5301.
[7] P. Reiss, M. Protiere, L. Li, Small, 2009, 5, 154.
[8] Y. Chen, J. Vela, H. Htoon, J. L. Casson, D. J. Werder, D. A. Bussian, V. I. Klimov, J. A. Hollingsworth, J. Am. Chem. Soc., 2008, 130, 5026.

Porphyrin and its derivatives with macrocyclic aromatic conjugation system were excellent organic semiconductor materials and ideal building blocks based on the unique planar, rigid molecular geometry. Molecular self-assembly is a powerful method to the synthesis of nanostructured materials with fine-tuning of the morphology and size. Through designing molecules and supramolecular entities, desired structure and function can be achieved. Here, we report a general microemulsion-based approach to the synthesis of a wide variety of porphyrin nanocrystals with controlled size and morphologies by using different porphyrin as building blocks. This method is based on a designed oil-in-water (O/W) normal microemulsion system. The porphyrin molecules are gathered, assembled, and fixed together spontaneously by the hydrophobic Vander Waals and π-π interaction during controlled evaporation of low boiling point oil solvent in the restricted, micrometer 3-D space provided by microemulsion droplets. The size, shape, component and surface charge of the porphyrin nanocrystals can be controlled by designed experiment parameters. This bottom-up assembly approach opens the way to constructing different monodispersed porphyrin nanocrystals by using oil-soluble porphyrin as building blocks, which may serve as larger building blocks for constructing integrated macroscopic architectures or devices for the fundamental study and practical applications of nanoscience and nanotechnology.

Producing hollow nanoparticles of different size and shape is a challenging job. Galvanic replacement reaction (GRR) is considered as one of the reliable and reproducible synthesis technique to produce hollow nanoparticles. The number of available template materials for GRR is limited. In this work, As(0) nanoparticles are used as template materials which are capable of producing almost similar sized hollow gold nanoparticles (HGNPs). Here two different size ranges (viz., 50±7 and 70±10 nm) As(0) nanoparticles are synthesized by sodium borohydride reduction of arsenite under controlled pH (7-9), and controlled temperature (10° C and 40° C). Further, the reducing property of these As(0) nanoparticles are exploited to form two different sized HGNPs with average diameter 55±7 and 72±7 nm. These HGNPs are designated as AuNP1 and AuNP2. The controlled medium pH restricts the reduction of arsenite to As(0) and not to AsH₃. The catalytic reduction of 4-nitrophenol (4-NP), which is although a toxic compound but used in many industries, to 4-aminophenol (4-AP), an industrially important compound, is a well studied model reaction. The size-dependent catalytic activities of AuNP1 and AuNP2 have been examined on the reduction of 4-NP to 4-AP in the presence of sodium borohydride. While both the nanoparticles exhibit excellent catalytic activity, the smaller particles (AuNP1) are observed to be more effective. The turn over frequency (TOF) of AuNP1 shows much higher value of 300 h^-1 in terms of molar ratio of Au:4-NP:NaBH₄ (1:50:50000) as compared to other gold catalysts used for 4-NP reduction. The reaction is carried out with various catalyst doses and various initial 4-NP concentrations. In all cases the reaction follows first order kinetics. The TOF for the catalytic reaction using AuNP1 and AuNP2 suggests that AuNP1 bears ~6 times higher catalytic activity compared to that of AuNP2. Both the nanocatalysts can be reused up to fourth cycle with good efficiency. In addition, it is also possible to control the size of As(0) by varying the reaction time which ultimately can lead to the formation of different sized HGNPs.

Naturally occurring responsive systems such as folding and unfolding in self-assembled DNA bundles prove natural designs are hierarchical, with structures and property on multiple scales through interactions of subunits or building blocks. Mimicking these designs in fabrication of active materials requires a clear picture of energy landscaping that governs local interactions such as hydrogen bonding, van der Waals interactions, dipole-dipole interaction, capillary forces, etc, which will provide correct thermodynamic end points as well as facile kinetics for precise control of hierarchical structure for responsive functions. To date, fabrications of active and responsive nanostructures have been conducted at ambient pressure and largely relied on these specific chemical or physical interactions. Using our recently developed stress-induced assembly (SIA) as an artificial actuator, we can emulate natural folding and unfolding processes to explore energy landscaping that govern local interactions. Through SIA, we can design new classes of active materials with controlled structure and function and investigate new properties resulting from the folding and unfolding processes. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle arrays can be manipulated to reversibly shrink and swell during compression and release of pressure, allowing precise tuning of interparticle symmetry and spacing, ideal for controlled investigation of distance-dependent energy couplings and collective chemical and physical property such as surface plasmon resonance. Moreover, beyond a threshold pressure, nanoparticles are forced to contact and sinter, forming new classes of chemically and mechanically stable 1-3D nanostructures that cannot be manufactured by current top-down or bottom-up methods. Depending on the orientation of the initial nanoparticle arrays, 1-3D ordered nanostructures (Au, Ag, etc.) including nanorods, nanowires, nanosheets, and nanoporous networks can be fabricated. The SIA method mimics embossing and imprinting manufacturing processes and opens exciting new avenues for the study of responsive behaviors of active materials during compression (folding) and pressure release (unfolding). Exerting stress-dependent control over the structure of nanoparticle or building block arrays provides a unique and robust system to understand collective chemical and physical characteristics of nanocrystal superlattices.