Symposium QN08—Colloidal Nanoparticles---From Synthesis to Applications

Colloidal nanostructures have been studied quite extensively in the research area of heterogeneous catalysis long before the introduction of the concept of nanoscience and nanotechnology. The significant progress achieved in the past twenty years in chemical synthesis has enabled precise control over not only the size but also the shape of the metal nanostructures, and therefore attracted intense interest not only in catalysis but also optoelectronics due to the well-known effect of localized surface plasmon resonance. In this presentation, I will introduce our recent progress in the synthesis of colloidal metal nanostructures in confined spaces using various templating methods, their chemical transformation behavior, and further manipulation of their secondary structures. By combining the confinement of templates with the seed-mediated growth strategy, we demonstrate the significant advantages of this general method over the conventional ones in creating a large variety of nanomaterials with novel plasmonic properties.

Quantum dot devices have wide applications in photovoltaics, optoelectronics, and sensors due to quantum dot unique quantum confinement effect, which can be tailed by size, shape, structure, and composition. Therefore, the ultrafast photophysics dynamics of charge carrier in situ devices (electrons, holes, or ions) including carrier transport, injection through optical or electrical, recombination, trapping, and transfer is one of the most fundamental questions.

We developed ultrafast photocurrent spectroscopy and ultrafast electroluminescence spectroscopy to address the ultrafast carrier dynamics in situ devices including solar cells, single-photon detectors, light emitting diodes, and photodetectors. In addition to that we understand ultrafast carrier transport and recombination dynamics, we discover novel photoconductivity phenomena, which cannot be addressed by other laser pump-probe spectroscopies.

Infectious outbreaks caused by foodborne pathogens such as E. coli O157:H7 are still occurring worldwide and cause acute illnesses and significant industrial impact. As a chemical component of organic materials, bisphenol A (BPA) is used in the food and consumer products industries. Despite its benefits, low-dose exposure to BPA adversely affects human health because it is an endocrine disrupting chemical (EDC).
We present on the feasibility of using aptameric DNA sequences, covalently conjugated to colloidal nanoparticles, for the sensitive and highly specific detection of whole cell pathogens and small molecule toxic molecules via surface enhanced Raman spectroscopy (SERS) analysis.
Low concentrations of E. coli O157:H7 were detected (~10¹ CFU/mL) and quantified (~10² CFU/mL) within 20 minutes in both pure culture and ground beef samples. We also report on a new range-extended bisphenol A (BPA) detection method that includes high-SERS-performance gold nanoparticles, i.e., 40 nm gold nanostars (GNSs) and 4-aminothiolphenol (4-ATP) as Raman reporters. The limit of detection for BPA was 0.073 ppb, which is 205 and 20 times more sensitive
than those of visual intensity and color intensity quantifications, respectively.

Iron Oleate has long been one of the most popular precursors for the synthesis of high-quality iron oxide nanoparticles. This is true despite the fact that it is not commercially available and must be custom synthesized. There are a handful of related syntheses that are in common use, but reproducibility of the synthesis is a significant concern. Iron oleate does not readily form a simple complex with one iron atom bonded to three oleate ligands as one might imagine. Instead, iron oleate often exists as a complex mixture of species that includes a series of oligomeric species with iron atoms bonded to each other through oxo bridges as well as other structure. This non-stoichiometric mixture is not easily purified as iron oleate resists the most common purification approaches. Iron oleate can not be recrystallized, as it does not crystallize and instead forms an oil or glass depending upon its temperature. Washing approaches are difficult to reproduce as the oleate ligands can be very labile, and repeated washes will continue to remove oleate ligands until it finally forms an insoluble solid. This lability of the oleates also make it difficult to purify the compound through chromatography as the compound can decompose on the column. Unfortunately, differences in the precursor synthesis can lead to difficulty in reproducing nanoparticles syntheses. We will discuss two approaches that have been demonstrated to yield highly reproducible iron oleate precursors. In the first approach, we produce iron oleate through the decomposition of another organometallic species in an excess of oleic acid yielding a solution of known stoichiometry that can be used without purification. In the second approach, judicious selection of reagents, solvents, and reaction conditions can yield an isolated compound of pure iron oleate with an iron:oleate ratio of exactly 1:3. These compounds can then be used to enhance the reproducibility of iron oxide nanoparticle syntheses. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the DOE’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States government.

Colloidal quantum dots (QDs) have gained considerable attention as a new class of nanomaterials that can potentially enable the next-generation solution-processible electronic and optoelectronic devices (Science 353, aac5523, 2016). Owing to their superior optical properties, intense studies on QD-based color converters, light-emitting diodes (Nature 515, 96, 2014, Nat. Photon. 12, 159, 2018) and solar cells (Nat. Nanotechnol. 13, 456, 2018) have been underway leading to successful commercialization of the QD-display technology. The next frontier is exploration of electronically coupled QD solids in the context of prospective applications in solution-processible flexible electronics toward wearable electronic and sensing systems. One existing challenge in this area is low mobility of non-toxic QDs, which limits performance characteristics of non-toxic QD-based field-effect transistors (FETs) that represent basic building blocks of electronic circuits. Low mobilities lead to low ON-currents of typical QD-FETs. Increase in the current is possible by reducing the FET channel length, however, this would require the use of expensive and nonscalable patterning methods such as e-beam lithography.
Here, we demonstrate high ON-current QD FETs based on environmentally-benign QDs by employing a vertical architecture, which is a vertical stack of a gate electrode, a gate insulator, a source electrode, semiconducting transport layer, and a drain electrode. Using this approach, we reduce a channel length down to sub-micrometer scale, and as a result, achieve an unprecedented ON-current of ~0.1 A/cm² in p-type QD FETs, which is three orders of magnitude higher than in standard lateral FETs. This notably high ON-current of vertical QD FETs is demonstrated using non-toxic Zn-doped CuInSe₂ QDs (J. Am. Chem. Soc. 138, 4201, 2016) and achieved despite their low hole mobility of ~0.001 cm²/Vs. A switchable behavior of a vertical FET is enabled by the porous structure of a source electrode, which allows for the gate-controlled electric field to penetrate into a vertical channel and modulate a charge carrier density. The pattern of the source electrode is defined by optical interferometric lithography, which is a simple, fast, scalable and mask-less patterning method. To improve switching properties of the FET, a charge-blocking layer was applied to the top surface of the patterned source electrodes. This allows us to steer the charge flow in a lateral direction along the gate oxide interface, which is the channel region well-modulated by gate bias. Furthermore, we demonstrate polarity control of QD vertical FETs by employing indium diffusion to enable electron transport. The fabricated n-type vertical FETs exhibit even higher ON-currents approaching 1 A/cm². Owing to high current density with a switchable behavior of the developed vertical QD-FETs, these devices can be used as a new platform to enable a variety of QD-based sensing, electronic and optoelectronic applications such as photodetector, light-emitting transistor, and infrared-to-visible up-converters.

While surfactant assisted synthesis is probably the most common technique for the preparation of colloidal nanoparticles, workers in the field often encounter severe irreproducibility issues. In this talk, we will present several examples, from our work [1,2], as well as by others [3-5], for the role of ‘beneficial impurities’ in nanoparticle synthesis. Furthermore, a new binary phase of cubic nanocrystalline semiconductors has been discovered in recent years in the form of colloidal nanoparticles. The first materials to be discovered were π tin mono-sulfide [6,7] and π tin mono-selenide [8], yet density functional theory calculations predict the existence of additional potentially useful compound semiconductors in the π structure.[9] There is growing experimental evidence that phase control in the tin mono-chalcogenide system is likely to be governed by ‘beneficial impurities’ as well.

References

1. N. Belman, J.N. Israelachvili, Y. Li, C.R. Safinya, J. Bernstein and Y. Golan, Nano Lett. 9 (2009) 2088.
2. N. Belman, J.N. Israelachvili, Y. Li, C.R. Safinya, J. Bernstein and Y. Golan, J. Am. Chem. Soc. 131 (2009) 9107.
3. F. Wang, R. Tang, W.E. Buhro, Nano Lett. 8 (2008) 3521.
4. J.E. Millstone, W. Wei, M.R. Jones, H. Yoo, C.A. Mirkin, Nano Lett. 8 (2008) 2526.
5. D.K. Smith, N.R. Miller, B.A. Korgel, Langmuir 25 (2009) 9518.
6. A. Rabkin, S. Samuha, R.E. Abutbul, V. Ezersky, L. Meshi, Y. Golan, Nano Lett. 15 (2015) 2174-2179.
7. R.E. Abutbul, E. Segev, L. Zeiri, V. Ezersky, G. Makov, Y. Golan, RSC Adv. 6 (2016) 5848-5855.
8. R.E. Abutbul, E. Segev, S. Samucha, L. Zeiri, V. Ezersky, G. Makov, Y. Golan, CrystEngComm 18 (2016) 1918-1923.
9. R. E. Abutbul, E. Segev, U. Argaman, G. Makov, and Y. Golan, Adv. Mater. 30 (2018) 1706285.

Current best-practice lithographic techniques are unable to meet the functional requirements needed to enable on-chip plasmonic devices capable of fully exploiting nanostructure properties reliant on a tailored nanostructure size, composition, architecture, crystallinity, and placement. As a consequence, numerous nanofabrication methods have emerged that address various weaknesses, but none have, as of yet, demonstrated a large-area processing route capable of defining organized surfaces of nanostructures with the architectural diversity and complexity that is routinely displayed in colloidal syntheses. Here, a hybrid fabrication strategy is demonstrated in which nanoimprint lithography is combined with templated dewetting and liquid-phase syntheses that is able to realize periodic arrays of complex noble metal nanostructures over square centimeter areas. The process is inexpensive, can be carried out on a benchtop, and requires modest levels of instrumentation. Demonstrated are three fabrication schemes yielding arrays of core–shell, core–void–shell, and core–void–nanoframe structures using liquid-phase syntheses involving heteroepitaxial deposition, galvanic replacement, and dealloying. With the field of nanotechnology being increasingly reliant on the engineering of desirable physicochemical responses through architectural control, the fabrication strategy provides a platform for advancing devices reliant on addressable arrays or the collective response from an ensemble of identical nanostructures.

Si anode of a lithium ion battery undergoes extreme volume expansions during lithiation and delithiation cycles. Finite element analysis indicates that significant build-up of lithiation-induced stresses can develop that results in fracture of the Si anode material. In this study, we report two different designs for Si microparticle based composite anode that allows for relaxation of the diffusion induced stresses in the Si microparticles. Self-healing polymer and Si microparticle freestanding composite was proposed and analyzed for its cyclic stability that confirmed enhanced retention, with the freestanding composite demonstrating 91.8% capacity retention after 100 cycles at C/10 rate. Second design for the composite anode uses Si microparticles that are embedded in combustion reacted, nanoporous ZnO. The nanoporous ZnO was shown to in itself undergo conversion reaction that hence increased the capacity and also was shown to help in maintaining the coalescence of SiMPs for enhanced electrochemical cycling, which yielded a binder-less design with an initial capacity of ~3,900 mAh/g at C/20 rate and a reversible capacity of ~1,500 mAh/g beyond 200 cycles at C/5 rate. In addition, the Li₂O/Zn matrix derived from conversion-reacted nanoporous ZnO acted as an effective buffer to lithiation-induced stresses from volume expansion and served as a binder-like matrix that contributed to the overall electrode capacity and stability.

The preparation of new nanomaterials requires hierarchical levels of ordering and structuring: from molecular to macroscopic. Much has been demonstrated in metal, metal oxide, inorganic nanoparticles by way of size, shape, and polydispersity control. The Plasmonic and emissive properties dependent on size, shape, and hot spots are well known. Dendrimers and other hyperbranched polymer systems are of interest for their functionality in catalysis, drug delivery, reactivity, etc. Of high interest are dendrimer functionality in electro-optical applications and nanopatterning. This includes dendrimers capable of hierarchical ordering and self-assembly. We highlight the convergent synthesis of a variety of hybrid nanoparticle dendrimers and their electropolymerizability. The synthesis of precise dendrons as modular building blocks for functional dendrimers and the use of star copolymers for various core-shell architectures are demonstrated. Our group has reported a number of these hybrid systems and have reported the step-by-step routes towards structure-property relationships. These has resulted in controlled charge -transfer and energy transfer properties not to mention the ability to provide greater stability for these nanoparticles. What is also important is the use of surface sensitive spectroscopic and microscopic analytical tools applied rationally to highlight evidence of order and function.

Nanostructured materials have been found to be critical in promoting the performance of energy storage and conversion devices, such as batteries. Here, several nanostructured Si materials have been designed for practical anodes of high energy Li-ion batteries (LIBs). In one effort, the large (>20 μm) mesoporous Si sponge prepared have controlled porosity and pore size, which can limit the particle volume expansion at full lithiation to ~30% and prevent pulverization of bulk particles. The porous Si anodes deliver a specific capacity of up to ~750 mAh/g based on the total electrode weight and >80% capacity retention over 1,000 cycles. The electrodes with the loading of ~1.5 mAh/cm2 demonstrated ~92% capacity retention over 300 cycles. The composite electrodes of porous Si and graphite (~3 mAh/cm2 loading) with a specific capacity of ~650 mAh/g demonstrate ~82% capacity retention over 450 cycles. In another effort, hierarchical structured Si/CNT microspheres developed not only have good porous structure to accommodate the volume expansion and achieve ~30% apparent particle swell at full lithiation, but also demonstrate good mechanical integrity with the structure sustained up to ~200 MPa pressure. The anodes deliver a high specific capacity of ~1500 mAh/g and 85% capacity retention over 200 cycles at the areal loading of ~3 mAh/cm2.

Our laboratory has developed a new synthetic procedure for generating periodic arrays of metallic nanostructures shaped as hexagonal or triangular nanoplates using a room temperature light-activated growth mode. Such structures have the potential to act as the active components for the detection of biological and chemical analytes using various sensing modalities (e.g., Surface Enhanced Raman Scattering (SERS)). The synthesis is reliant on the formation of Au seeds exhibiting planar defects – without such defects the growth mode is deactivated. Through the engineering of defects, which have been extensively studied using Titan TEM imaging and electron diffraction, such structures have now been produced in high yield. Here, we will describe the techniques used to generate periodic array of seeds, demonstrate their utility in forming substrate-based metallic nanoplates and provide an understanding of the opportunities and challenges that lie ahead.

Nanocrystalline intermetallic compounds show promise in several applications, however chemical methods giving phase purity and size control are lacking. Using metal precursors previously developed for atomic layer deposition but rarely explored for solution-phase materials synthesis, we demonstrate the synthesis of several phase-pure transition metal alloys including the full library of MSn₂ and MSb (M = Cr, Mn, Fe, Co, Ni) with control over phase and stoichiometry, several of which are reported for the first time in nanocrystalline form. For stannides, the synthesis of which occurs above the melting point of Sn, the alloys formation progresses via an intermediate stage of small Sn⁰ nanocrystals stabilized by the strongly-binding amidinate ligands with subsequent diffusion of transition metal atoms into tin. We find that for metals with higher melting temperatures than Sn (such as Sb), the reductant injection temperature determines the phase purity of the intermetallic product. Temperature-dependent magnetic properties of the intermetallic nanocrystals have been studied, and the effect of annealing the nanocrystals at high temperatures on crystallinity is also explored.

Colloidal semiconductor quantum dots (QDs) are attractive materials for application as laser media because of their high photoluminescence (PL) quantum yields, size-tunable emission colors, and amenability to solution-based processing. Optical amplification in these materials is hampered by nonradiative Auger recombination of gain-active multicarrier states. Recently, we have demonstrated that this problem can be successively addressed by interface-engineering in hetero-structured, compositionally graded QDs. Using these novel structures, we were able to demonstrate both low-threshold lasing with pulsed optical excitation as well as optical gain with d.c. electrical pumping (Nat. Mater. 17, 42, 2018). The next step on the way to practically realizing a QD laser diode is incorporation of a proper feedback system into a high-current-density light-emitting diode (LED).
Towards this goal, we investigate optical properties of photonic structures comprised of a one-dimensional distributed feedback (DFB) resonator and the newest generation of the QD with strongly suppressed Auger decay. These QDs have a CdSe core (~ 2 nm radius) enclosed into a radially graded Cd_xZn_1-xSe layer (~ 5 nm thickness), completed with a final protective thin (< 1 nm) shell of ZnSe_yS_1-y. These structures show a high biexciton PL quantum yield of ~ 45%, which is a result of an exceptionally long biexciton Auger lifetime of > 2ns. QDs are spin-coated onto a DFB grating imprinted onto the SiO₂ substrate via laser interferometric lithography. The grating parameters (pitch, duty cycle, and depth) are selected so as to provide optimal conditions for surface-emitting lasing with the second-order in-plane optical feedback. Applying a Fourier-plane angle-resolved spectroscopy for the analysis of surface emission produced by the first-order Bragg scattering, we observe that our QD-DFB structures are characterized by an X-shaped photonic mode dispersion. In particular, we detect a ca. 10 nm stop band in the direction normal to the DFB plane which indicates strong optical feedback between counter-propagating in-plane modes (coupling coefficient of ~20), a key to obtaining low-threshold lasing. The fabricated structures also exhibit a fairly high quality factor of ~ 600.
Using optical excitation with 400 nm, 100-fs laser pulses, we observe single-mode lasing at the lower-energy edge of the stop band of the TE mode, which corresponds to reduced losses via surface emission due to a destructive character of interference between in-plane counter-propagating waves. The measured thresholds are consistently below 10 μJ/cm², and the lowest value is ~5 μJ/cm². To further lower the lasing threshold, we apply ideas of “zero-threshold gain” by photo-chemically doping the QDs, which suppresses losses due to ground-state absorption (Nat. Nanotech. 12, 1140, 2017). Using this approach, we are able to reduce lasing thresholds to the unprecedented level of ~2 μJ/cm², which corresponds to excitation of only ~0.3 exciton per QD on average. This is a ca. 3-fold reduction compared to the fundamental one-exciton-per-dot limit in the case of the undoped QDs. In our ongoing work, we investigate lasing properties of similar DFB resonators incorporated into state-of-the-art QD-LED stacks.

Colloidal semiconductor quantum dots (QDs) have recently attracted great attention in electric fields sensing via the quantum confined Stark effect (QCSE), but suffering from random local electric field around the charged QDs through Auger-processes. In “giant” CdSe/CdS QDs, the thick CdS shells and alloyed interfacial layer have been proven to be effective ways to restrain Auger recombination and photoluminescence (PL) blinking. [1,2]
In the present work, the QCSE in ensemble of phase-pure wurtzite CdSe/CdS QDs is studied by applying a uniform external electric field. We observe a clear field-dependent PL and absorption modulations, including the spectral shifts and broadening, as well as the changes in its intensity. Besides, we also have performed a clear observation of the robust PL quenching and revival process. The spectral shifts in ensemble emission and absorption are found to be purely quadratic in the electric fields, indicating the presence of polarizable component in both PL emission and first absorbing states, which is consistent with the characteristics of spherically symmetric QDs sample. [3] Finally, the multiexciton states also exhibit obvious optical responses to the electric field, through the amplified stimulated emission (ASE) measurements. The insights into the properties of field-driven multiexcitons emission imply that ASE shows a decrease of 18% in emission intensities and an increase in the ultrafast lifetime from 25 to 31 ps.
In summary, The results imply that the field-induced QCSE in the QD ensemble are attributed to the effective suppression of the primary source of local field in such thick-shell QDs, thus proving that an efficient field control over the optical properties (the changes in emission and absorption intensity, spectral shifts, and PL lifetime in response to the electric field) in these nanoparticles is feasible and open the potential possibility of field-sensitive QDs to achieve real-time electro-optic sensing.

References
[1] Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H.; Fukumura, D.; Jain, R. K.; et al. Nat. Mater. 2013, 12, 445−451.
[2] Liao, C.; Xu, R.; Xu, Y.; Zhang, C.; Xiao, M.; Zhang, L.; Lu, C.; Cui, Y.; Zhang, J. J. Phys. Chem. Lett. 2016, 7, 4968−4976.
[3] Empedocles, S. A.; Bawendi, M. G. Science 1997, 278, 2114–2117.

Colloidal organic/inorganic lead halide perovskite nanocrystals (NCs) are intensely pursued as highly promising, low-cost light-emitting materials with wide color gamut. These NCs exhibit unprecedented luminescent properties – narrow-band emission with high quantum efficiency, covering the whole visible spectral range and extending into near-infrared, all obtained without epitaxial overcoating of the NC surfaces for electronic passivation of the surface states [1]. Their processing and luminescent properties are challenged by the lability of their surfaces, i.e. the interface of the NC core and the ligand shell. Surface and sub-surface atoms are likely directly involved in all possible chemistry equilibria and transformations. Controlling NC surface structure is therefore paramount for mitigating these instabilities.On the example of CsPbBr₃ NCs, we rationalize the typical observation of a degraded luminescence upon aging or the luminescence recovery upon post-synthesis surface treatments using a simple surface-structure model, supported by DFT calculations [2]. Healing of the surface trap states requires restoration of all damaged PbX₆ octahedra and establishing a stable outer ligand shell. Such NCs are halogen-rich. Restoration of such a structure, seen as an increase in the luminescence quantum efficiency to 90-100% and improvement in the overall robustness of CsPbBr₃ NCs, was attained using a facile post-synthetic treatment with a PbBr₂+DDAB (didodecyldimethylammonium bromimde) mixture. In practical terms, we demonstrate that such an approach is useful to obtain purified CsPbBr3 NCs samples, washed up to three times in several solvents, with near unity photoluminescence quantum yields and long-term colloidal stability.
1. M. V. Kovalenko, L Protesescu, M. I. Bodnarchuk. Science 2017, 358, 745-750
2. M. I. Bodnarchuk, S. C. Boehme, S. ten Brinck, C. Bernasconi, Y. Shynkarenko,F. Krieg,R. Widmer, B. Aechlimann, D. Günther, M. V. Kovalenko,I. Infante. ACS Energy Letters 2018, in press

We utilize the combined toolset of low-dose liquid-phase transmission electron microscopy and automated single particle tracking to reveal the fundamental crystallization pathways in nanoparticle superlattice formation. We concern nanoparticle systems involving systematically-changing shapes and surface patchiness, which give rise to a rich diversity of crystallization behaviors, such as hierarchical crystallization, layer-by-layer growth, polymorph interconversion as well as corner-connected open lattice formation. Such behaviors hinge on the specific length scale and time scale of nanoparticle motions, which can be potentially generalizable to the phase behaviors of other nano-sized systems in solution.

Metal-oxide nanocrystals doped with aliovalent atoms can exhibit localized surface plasmon
resonances (LSPRs) that are tunable across the infrared regions of the electromagnetic spectrum.
Yet, the chemical identity and the maximum concentration of active dopants for any specific
metal-oxide nanocrystal have remained limited, which is largely due to the difficulty in
establishing reaction kinetics that favors dopant incorporation with the widely used co-
thermolysis method. In this presentation, I will share our recently developed ion-exchange method for the
incorporation of various cationic dopants into preformed metal-oxide nanocrystals while
preserving their size, shape and crystal structure. We show that successful addition of p-type
dopants into n-type metal-oxide nanocrystals, which may not be
feasible using alternative synthetic methods, leads to programmable LSPR redshifts by virtue of
dopant compensation. We further demonstrate that enhanced n-type doping can be realized via
rationally designed sequential cation-exchange reactions, yielding
metal-oxide nanocrystals with the ensemble LSPR energy surpassing that of beginning
nanocrystals. Our work shows that ion-exchange transformations can add a new
dimension to the chemical design of infrared plasmonic nanocrystals, allowing metal-oxide
nanocrystals synthesized by existing methods to be used as templates to create compositionally
tunable nanocrystals with well-defined LSPR characteristics.

Quantum confined semiconductor nanocrystals have been widely investigated as light harvesting and charge separation components in photovoltaic and photocatalytic devices. The efficiency of these semiconductor nanocrystal-based devices depends on many processes, including light harvesting, carrier relaxation, charge separation and charge recombination. The competition between these processes determines the overall solar energy conversion (solar to electricity or fuel) efficiency. Compared with single component quantum dots (QDs), semiconductor nanoheterostructures, combining two or more materials, offer additional opportunities to control their charge separation properties by tailoring their compositions and dimensions through relative alignment of conduction and valence bands. Further integration of catalysts (heterogeneous or homogeneous) to these materials form multifunctional nanoheterostructures. Using Pt tipped CdSe/CdS dot-in-rod nanorods(NRs) and CdS nanoplatelets as model systems for 1D and 2D hetersotrtuctures, we are examining the mechanism of long-lived charge separation and H₂ generation in the presence of sacrificial electron donor. The rates of carrier transport, interfacial charge (electron and hole) transfer, and charge recombination can be directly monitored by transient absorption and time-resolved fluorescence spectroscopy. In this talk, we will discuss how the rates of these elementary carrier transfer and transport processes depend on the dimension (size and length), morphology and band alignment in these materials, affect the overall H₂ generation efficiency and can be optimized through rational material design.

Despite these impressive developments, instability of perovskite quantum dots (QDs), particularly towards humidity, remains one of the major obstacles for their long-term use in practical optoelectronic materials and devices. Herein, we report a general amphiphilic star-like block copolymer nanoreactor strategy for in-situ crafting a set of hairy perovskite QDs with precisely tunable size and exceptionally high water and colloidal stabilities against decomposition and aggregation, respectively. The selective partition of precursors within the compartment occupied by inner hydrophilic blocks of star-like diblock copolymers imparts in-situ formation of robust hairy perovskite QDs permanently ligated by outer hydrophobic blocks via co-precipitation in nonpolar solvent. Notably, anion exchange of halide ions of perovskite enables the production of perovskite QDs with tunable emissions. These size- and composition-tunable QDs reveal impressive water and colloidal stabilities as the surface of QDs is intimately and permanently ligated by a layer of outer hydrophobic polymer hairs. More intriguingly, the readily alterable length of outer hydrophobic polymers renders the remarkable control over the stability enhancement of hairy perovskite QDs. When assembled into a typical white light emitting diode (WLED) architecture, hairy perovskite QDs exhibit appealing performance, reflecting 116% and 163% color gamut of WLED over national television system committee (NTSC) and standard red green blue (sRGB) standards, respectively. By extension, the amphiphilic star-like block copolymer nanoreactor strategy can afford easy access to a wide range of stable perovskite QDs and other nanocrystals of interest with precise control over dimensions, compositions and surface chemistry for use in solar cells, LEDs, photodetectors, lasers, scintillators, sensors, nanotechnology and biotechnology.

Advanced electron microscopy techniques, i.e., in-situ liquid-cell scanning transmission electron microscopy imaging, and chemically sensitive three-dimensional (3D) electron tomography, have been used to study how the complex spongy structures form in a high-performing fuel cell catalyst, a dealloyed Pd-M catalyst. In-situ observations show that both the strain provided by the alloying metal and the intermetallic ordered structures play a significant role in both the achievable spongy structures and the de/acceleration of the leaching of the Pd content. The results offer fundamental insights regarding the chemical dealloying process in Pd-based fuel cell catalysts.

Nanocrystal (NC) assembly appears as one promising way for controllable design and fabrication of novel functional materials with desired property and functionality. Achievement of a structure-by-design requires not only complete decoding of the structural details of NC assembled supercrystal but also improved understanding of the driving forces for the superlattice formation. In this talk, I will first describe briefly about the synchrotron-based x-ray supercrystallography approach invented at CHESS, and thus explain how this technique can be used to decode the structural details of NC-assembled supercrystals at length levels from atomic through nano to mesoscale. Using several examples of supercrystals made up of spherical, truncate and octahedral NCs, I will demonstrate how to derive the information of dominant driving forces well hidden in various superlattices. Based on the defined translational order and orientational alignment, I will further talk about how various types of interaction forces interact and compete each other with change of anisotropic NC to form typical low-energy superlattices. Taking such insights gained, I will get into how interactions can be designed through delicate control of environment to harvest high quality supercrystals with desired superlattices.

Optoelectronic devices made from colloidal quantum dots (CQDs) often take advantage of the combination of tunable quantum confined optical and electronic properties and carrier mobilities of strongly coupled systems. For lattices of connected CQDs, of interest is the interplay between confinement effects and band-like behavior. In this work, first-principles calculations are applied to investigate the electronic, optical and transport properties of PbS CQD superlattices. Our results show that even in the regime of strong necking and fusing between CQDs, quantum confinement can be generally preserved. In the bandlike regime, computed carrier mobilities for simple cubic and 2D square lattices fused along the {100} facets are 2-4 orders of magnitude larger than those of superlattices fused along other {110} and {111} facets. The relative magnitude of the electron and hole mobilities strongly depends on the crystal and electronic structures. We also find that the carrier mobilities of CQD solids increases as the size of the quantum dot increases due to the stronger coupling between neighboring nanocrystals. Our results illustrate the importance of understanding the effects of crystal structure and connectivity of CQD films.

Functionalized, monolayer-protected nanoparticles (NPs) are versatile materials with surface properties that depend on monolayer composition. Ultrasmall gold NPs (<10 nms in diameter) are of particular interest for drug delivery applications because their small size enables unique interactions with serum proteins and the cell membrane. However, structure-property-interaction relationships correlating NP compositions with their biological interactions remain unclear and can be challenging to determine experimentally.

Here, we present molecular simulation approaches from our group that can guide the design of NP monolayer compositions for biomedical applications. Motivated by prior work that identified NP hydrophobicity as a key parameter driving cellular internalization and protein adsorption, we use atomistic molecular dynamics simulations to quantify NP hydrophobicity as a function of monolayer physical and chemical properties. These results demonstrate how the cooperative interactions of collections of surface-grafted molecules contribute to spatially varying hydrophobic interactions with homogeneous monolayers. We further show that hydrophobic interactions at uniformly nonpolar interfaces can vary in magnitude depending on the fluctuations of surface-grafted alkanethiol ligands, and that these same fluctuations depend on NP core size and shape. These results are further confirmed by atomic force microscopy experiments. Finally, we present a new deep learning methodology to rapidly screen the hydrophobicity of NPs as a function of monolayer composition using only minimal simulation input. This mechanistic insight and new computational tools can be leveraged to design NPs with desired surface properties.

The process by which a crystal sublimates into a gas is a first-order phase transition of considerable fundamental and practical importance in condensed-matter physics, material science and climate change, yet a detailed understanding of its relevant kinetic pathways is still evolving even for the model systems whose equilibrium configuration is known in advance.¹ Transmission electron microscopy (TEM) in principle, enables us to observe dynamic process directly, yet to date many of the TEM studies on sublimation dynamics have been implemented on nanostructures at the nanometer scale, such as nanowires, nanorods and colloidal nanocrystals.² Observing sublimation at the atomic scale has been mostly performed recently on two dimensional layered materials owing to their suitable thickness as a TEM sample in order to exclude the projection issues in electron microscopy.
In case of metals, silver (Ag) nanocrystals with a face-centred cubic (FCC) structure show a low sublimation temperature.³ As such, Ag represents an ideal model material to track heating induced sublimation procedure inside an electron microscope. Nevertheless, due to the limit of spatial resolution of a conventional TEM, many critical details such as structural evolution, dynamic processes, sublimation mechanisms and role of defects during sublimation remain unclear as well as not very well established at the atomic scale. With the development of aberration-corrected TEM (AC-TEM) and advanced heating stage, in-situ dynamic observations at atomic resolution can be achieved even at a low voltage, allowing us to directly resolve extremely small nanostructures that are not clearly visible by a conventional TEM.
Here, we report the in-situ atomic-scale observation of the sublimation dynamics of Ag nanoparticles with a size smaller than ~30 nm under heating conditions using an advanced heating holder within an AC-TEM. The Ag nanoparticles are fabricated directly on the TEM heating chips by plasma irradiation in vacuum system to avoid unwanted solvent effects, and both plan-view and cross-section view of the sublimation dynamics are implemented at the atomic scale. We find surprisingly that the sublimation and atomic rearrangement coexist in small Ag nanoparticles during heating and reveal that the sublimation-induced stable surfaces are the low-energy {111} and {100} planes for the Ag nanocrystal with a size smaller than ~30 nm. In addition to the size-dependent sublimation behaviors, we also demonstrate that surface energy and defects are also of relevance in determining the sublimation pathways, i.e. the Ag nanoparticles of a low surface energy show a uniform sublimation pathway, while those of a high energy or a 5-fold twin grain boundary undergo non-uniform sublimation pathways.⁴
1. P. Cannon, Nature 1959, 183, 1612-1613
2. J. W. Yim, B. Xiang, J. Wu, J. Am. Chem. Soc. 2009, 131, 14526-14530.
3. W. Luo, W. Hu, S. Xiao, J. Phys. Chem. C 2008, 112, 2359-2369.
4. Junjie Li, Zhongchang Wang, Yunping Li and Francis Leonard Deepak, Submitted 2018.

The self-assembly of colloidal nanoparticles into ordered superlattices provides significant expansions for the capability of functional materials design. As the new type of building blocks, the geometric shapes of nanocrystals, such as sphere, cube and octahedron, play critical roles in determining the structure of assembled supercrystal. In the nanocube system, the observed superlattices were dominated by the three phases of simple cubic (SC), rhombohedral (Rh), and face-centered cubic (FCC), but how they nucleate and grow and transform from one to another remain largely unknown. In this work, we applied highly monodispersive magnetite nanocubes with an average side length of 12.5 nm, and controlled various assembly environments, and revealed the formation and growth of three major phases (SC, Rh and FCC). Taking advantage of synchrotron-based SAXS techniques, we also explored the structural correlations between three superlattices phases. With additional simulations and computations, we further discussed the driving forces and constructed a reasonable superlattice phase diagram of nanocube-based assembly. Our results not only dramatically expand our understanding on the shape-dependent phase behaviors of nanocrystal, but also provide insights for design and fabrication of novel functional materials.

We will describe a bottom-up approach to the control of the composition of solid/solid interfaces in nanostructured materials obtained from colloidal nanocrystals. Leveraging the highly selective removal of organics by plasma processing we show that introducing inorganic elements in the ligands of the starting building blocks allows for controlling the interfacial composition of the final all-inorganic ligand-free nanostructured film. We compare the results from ZrO₂ colloidal nanocrystals capped with trioctylphosphine oxide (TOPO) or oleic acid (OA). The interfaces in the plasma processed all-inorganic colloidal nanocrystal assemblies (CNAs) are either nearly bare (for OA-capped nanocrystals) or are terminated with phosphate groups (for TOPO-capped nanocrystals) resulting from the reaction of phosphine oxide groups with plasma species. This modification of the composition of the interfaces has extensive effects on the properties of the material: (i) different growth kinetics indicate different rate limiting processes of growth (surface diffusion for the phosphate-terminated surfaces and dissolution for the “bare” surfaces); (ii) different activation energy of growth indicate three-fold change in interfacial energy; (iii) different phase transformation temperatures show that surface modifications can stabilize the tetragonal phase of ZrO₂ at 900C without the need for doping.

Quantum dot light-emitting diodes (QD-LEDs) is a next-generation display technology with narrow-band, highly efficient and widely tunable emission. For a large area compatible patterning of QD-LEDs, solution process such as inkjet printing is promising but relatively complicated parameters exist. For example, quantum dot nanoparticle dispersion in a common solvent produces a non-uniform dried surface as well as strong coffee-ring at edge boundaries. Therefore, vapor pressure (boiling point), viscosity, and the solid contents of ink for QD must be carefully optimized. In this presentation, various solvents and the combination of mixture were tested for the inkjet printing of QD emitter based on CdSe nanoparticle dispersion. For the solvents of octane and other alkanes, a strong recirculating flow (Marangoni stress) can be expected in the droplets, which helps to reduce the capillary force-induced coffee ring defects of the dried nanoparticle array film. However, the CdSe-ink based on octane are somewhat widely spread in many substrates, so that uniform filling of a confined light emitting pixel geometry is difficult. Using the mixture of various solvents such as 1,2,4-trimethylbenzene, anisole, and cyclohexylbenzene, giant thickness at edge pattern boundaries by coffee-ring effect could be suppressed. However, the reduction of coffee-ring effect sometimes accompanies poor surface morphology. We have controlled the solid contents of CdSe nanoparticles at the condition of proper mixture solvents; addition of small amount of high viscosity and high boiling point solvent was effective method. Evaluation of the printed patterns including a dot/line size variation as well as surface morphology were linked with resultant device efficiency (ITO/PEDOT:PSS/poly-TPD/CdSe quantum dot/ZnO/LiF/Al) for different solid contents of CdSe in inks. Fine-line microscale patterned quantum dot LED (line patterns of 80/120 micrometer linewidth/subpixel pitch; 165ppi and more precise pattern) could be fabricated on the photoresist-processed bank test QD-LED cells.

Among the different plasmonically active assemblies, linear chains of nanoparticles are of special interest. These plasmonic polymers can provide directional, plasmonic waveguiding properties, as well as polarization-dependent field enhancement. An essential parameter governing many optical effects is the number of particles (n) forming a chain. Above the so called infinite chain limit, n_inf [1] , the n dependence of plasmonic properties is limited; however, shorter chains below n_inf (i.e., plasmonic oligomers) are highly responsive to variations in n. Therefore, controlling n below the infinite chain limit is an active topic in nanophotonics.
We present a scalable approach towards the formation of oriented chains consisting of a few plasmonic nanoparticles, according to the described plasmonic oligomers. These plasmonic oligomers are formed by first assembling quasi- infinite chains above the infinite chain limit and subsequently fragmenting them using mechanical deformation of the underlying elastomeric substrate.
Detailed studies of the fragmentation process from quasi-infinite chains are conducted by UV−vis−NIR spectroscopy and in situ atomic force microscopy (AFM). [2] By evaluation of the strain dependent optical properties, we observed a reversible, non-linear shift of the dominant plasmonic resonance. Using electromagnetic simulations, the optical spectra were deconvolved into the corresponding ensemble of oligomers that correspond to the oligomer length distribution and dispersity yielded from the AFM measurements.
Based on the obtained experimental results and mechanical simulations computed by the lattice spring model, we proposed a formation mechanism that explained the observed decrease of chain polydispersity upon increasing strain and provided experimental guidelines in tailoring chain length distribution. Furthermore, we also demonstrated that the chain length can be tuned depending on the ratio of cohesion between the particles and adhesion of the colloids to the supporting elastomeric substrate.
In this work, we demonstrate, that mechanical stimulus is a powerful tool for the scalable fabrication of oriented linear plasmonic oligomers and opens new avenues for strain-dependent optical devises and mechanoplasmonic applications. An example is surface-enhanced sensing, where the reversible switching of the nanostructure may be utilized to trap/infiltrate target molecules into uniform hot-spots

[1] Christoph Hanske et al., Nano Lett., 2014, 14, 6863-6871
[2] Anja Maria Steiner et al. ACS Nano, 2017, 11(9), 8871-8880

One of the two main classes of mechanisms generally used to explain the formation of nanocrystals is the non-classical aggregative growth (e.g. oriented attachment, step-growth crystallization). Better understanding of these mechanism should enable better control over the size, shape, and monodispersity of the products. Even though great strides have been made in using the aggregative growth mechanism to synthesize novel nanostructures (e.g. nanorings, ultrathin nanowires) and in visualizing the process in real time with liquid TEM since the seminal work Penn and Banfield on oriented attachment, there is still a long way to go towards understanding this process mechanistically.
Two major roadblocks are (i) establishing a model reaction system that only involves the aggregative growth (meaning that classical growth mechanisms are not happening or negligible) and (ii) collecting the kinetic data (i.e. size and concentration of each individual population vs time) quantitatively. We will show how we have overcome the two abovementioned roadblocks by using PbS nanocrystals synthesized by a highly reactive sulfur precursor. This system shows no detectable Ostwald ripening or classical supersaturation-induced growth, and allows for the optical characterization of the individual populations.
We show that modeling the aggregative growth kinetics in this system require two separate mechanisms, one whose activation energy is proportional to the radius of the particles and one that is proportional to the volume of the particles. We will then describe how these two mechanisms can be explained within the framework of oriented attachment.

Ceria nanoparticles are used to polish silica due to their high oxide removal rate without the need for added chemicals. Since the early 20^th century ceria has been used to polish glass lenses, and more recently thermal oxides during semiconductor chip fabrication. This second application benefits not only from a high removal rate, but also a high selectivity relative to nitrides. Both of these features are a result of a reversible redox reaction at the surface of the particles allowing them to participate in polishing through both mechanical and chemical means. Ceria’s ability to undergo this reaction is governed by the ratio of Ce³⁺/Ce⁴⁺ on the surface of the particles. As this ratio increases, so does the interaction with the silica surface, resulting in an increased removal rate. This ratio can be affected by both physical factors such as particle size and synthesis conditions, and chemical factors such as interactions with other slurry components. In this study, glass and thermal oxide were polished on a benchtop polisher and the removal rates were determined using a profilometer and ellipsometer respectively. Model slurries with oxidation state ratios between 5% and 25% were prepared by varying the oxidizing agent and surfactant concentrations as well as the surfactant type. A ceria slurry currently being used in industrial thermal oxide polishing was obtained as a reference (oxidation state ratio of 4%) and a silica slurry was used as a pure mechanical removal control. All the ceria slurries tested had a greater removal rate than the silica slurry, even at low oxidation state ratio. The greatest removal rate was found for the model slurry with 24% Ce³⁺ on the particle surfaces, resulting in removal rates at least 1.5 times those of the commercial slurry and 3 times those of the silica slurry for both materials. We discuss these findings and their implications for the specific design of future slurries.

Sodium is necessary for regulating osmotic pressure and assisting the operation of nerve, heart, muscle and various physiological functions in the body. Sodium intake is mainly through food, especially salt. However, excessive sodium intake is harmful because it may lead to the high risk of heart attack, stroke and the development of cardiovascular disease. For the above-mentioned reasons, we worked on developing a rapid, reliable, and sensitive method to detect salt concentration which can be directly monitored by naked eyes. In this study, we utilize the gold nanoparticles (AuNPs) as the colorimetric sensor due to the remarkable characteristic of localized surface plasmon resonance (LSPR) at visible wavelength regions. Based on the concept of LSPR effect, the designed AuNP-based sensor can readily monitor the change of salt concentrations through the color change observed by naked eyes. This phenomenon can be attributed to the ion-induced aggregation of AuNPs, where the response time relative to the salt concentrations can be modulated with the proper control of related detection conditions. Furthermore, in order to improve their detection sensitivity, the organic compound was utilized to further modify the surfaces of AuNPs, and can efficiently reduce the reaction time for showing color change. Such visual detection of salts is expected for readily impact the biological and chemical sensing applications.

Colloidal quantum dots (QDs) that are partially fused together to form highly-coupled, highly-ordered superlattices (SLs) are an exciting new class of materials for optoelectronics. These epitaxially-fused QD SLs promise to combine the band-like carrier transport of bulk semiconductors with the size-tunable photophysics and solution processability of QDs. However, the structure and formation mechanism of these epi-SLs are poorly understood, which limits our ability to fabricate high-performance QD epi-SLs with controlled electronic properties.
In this talk, I present a novel combination of X-ray scattering and correlative electron imaging and diffraction used to rigorously determine the superlattice unit cell of oleate-capped SLs (oleate-SLs) and epi-SLs made from PbSe QDs. This “dual space” method is a powerful and general way to unambiguously assign the structure of 3D colloidal SLs at the single-grain level. We find that oleate-SLs fabricated through self-assembly on a liquid substrate adopt a trigonally-distorted body-centered cubic structure with two major out-of-plane SL orientations. I show that the QDs have full orientational as well as positional order in this structure and that both the lateral size of the SL grains and their degree of distortion from true bcc depend on ligand coverage. The two orientations of oleate-SL grains convert to two distinct orientations of the epi-SL with a very different appearance in the electron microscope. The phase transition occurs almost purely via translation of the QDs with minimal rotational motion, resulting in a trigonally-distorted simple cubic epi-SL with fusion along the PbSe {100} facets. This simple translational phase transition is made possible by the trigonal distortion of the initial oleate-SL and results in >5 μm epi-SL grains free of linear or planar defects. The ligand content, elemental distribution, and impact of atomic layer deposition infilling on the structure of the epi-SLs will also be highlighted.
The powerful correlative imaging and diffraction methodology presented here will help enable detailed understanding of complex nanomaterial superstructures. Furthermore, we show that small distortions in the oleate-SL structure have important ramifications on the epi-SL phase transition and the ability to make highly-perfect epitaxially-fused QD superlattices.

Mint metal nanoparticles are electrically conductive and useful for applications in nanoelectronics. There is considerable interest in formulating inks containing these particles so that they can be applied in the field of printing electronics. NPs are usually stabilized with organic ligands, that are often non-conductive. They act as insulating barriers between the dried NPs. This renders deposited ink non-conductive unless additionally treated after deposition. In order to ensure metal-metal contact and conductivity, ligands have to be removed by sintering that is incompatible with many substrates^{1 2}. In order to avoid this, we developed hybrid NPs that combine gold metal cores and conductive conjugated polymer shells that we will briefly introduce in this talk. Their synthesis is based on a ligand exchange reaction of cetyltrimethylammonium bromide (CTAB) capped gold nanorods (AuNRs) by conjugated polymers such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and poly[2-(3-thienyl)-ethlyoxy-4-butylsulfonate)] (PTEBS). The hybrid particle design allows good electron transport upon ink drying, rendering sintering unnecessary. After deposition, dry films of hybrid NP inks exhibit conductivities on the order of 1.0 x 10⁶ S/m. We use infrared (IR) and Raman spectroscopy to demonstrate the successful ligand exchange and to investigate how metal core interact with polymers and how different particles connect. These polymers serve as ligands and confer high colloidal stability to particles in water and other solvents, too, enabling advanced ink formulations. The resulting inks exhibited good stability of at least one year at high gold concentration of 200 mg/mL.³
Here, we present an extension of the sinter-free hybrid ink concept to core materials and shapes beyond the gold nanorods mentioned above. First, we discuss the large-scale production at high concentrations of faceted gold NPs with diameters below 100 nm and conductive polymer shells. Then, we show that silver can be used instead of gold to reduce the material cost. Silver cores were synthesized in presence of aminohexanoic acid (ε-Ahx) that was then exchanged by PEDOT:PSS. We will discuss the role of the ligand during the synthesis of silver NPs and in the subsequent ligand exchange reaction.
Our hybrid sinter-free inks have rheological properties that are suitable for their deposition by inkjet printing, a method that offers more versatility, design flexibility, and reduced cost when compared to other printing techniques. Appropriate substrate pretreatment and ink formulation lets us to print the ink on different substrates, including glass, silicon, flexible polymer foils, paper, and others. The printed hybrid structures are mechanically flexible: we found no loss in conductivity after 2000 bending cycles, probably because the soft polymer shells act as “hinges” during bending and preserve connectivity. In a final outlook, we will discuss the use of sinter-free inks as one phase in conductive composite materials based on insulating polymer matrices.

1. Perelaer J, Smith PJ, Mager D, et al. J. Mater. Chem., 2010,20, 8446
2. Kamyshny A, Magdassi S., Small, 2014;10(17),3515
3. Reiser B., Gonzalez-Garcia L., Kanelidis I., Maurer J.H.M., Kraus T., Chem. Sci., 2016,7, 4190

Chlorophyll is a sort of magnesium porphyrin as the main component of photosynthetic pigments in chlorophytes. Magnesium porphyrin carries out photosynthesis in the form of self-assemblies, so the porphyrin ordered structures may have better photocatalytic properties than monomer. Molecular self-assembly technology is a powerful means to construct nanomaterials with specific structures and functions. Here, we developed synthesis of self-assembled porphyrin nanostructures by hydrogen bonds. This particular arrangement effectively improved electron transport to enhance the photocatalytic hydrogen production. These nanostructures were synthesized using meso–tetra (4- hydroxylphenyl)porphyrin (THPP) and meso-tetra (4-carboxyphenyl) porphyrin(TCPP) as the building block. The four hydroxyl groups and four carboxyl groups within the porphyrin molecular endow strong hydrogen bonding, which drived porphyrin self-assembly to form assemblies with different morphologies. The self-assembled nanostructures exhibit enhanced hydrogen production rate and the self-assembled nanostructures exhibit structure-dependent photocatalytic hydrogen evolution activity.We considered that molecular arrangement types affected the transmission of photogenerated electrons. Meanwhile, the central metal of the porphyrin molecule also had a great influence on the hydrogen production performance of photocatalytic water splitting. Porphyrins exhibit distrinct photocatalytic properties with the same functional groups and the different central metal.

Stability of nanoparticles in a solution without aggregation is important in practical use of colloidal nanoparticles. Specifically, colloidal nanoparticles to withstand a long-term storage is required in printed electronics. A high stability of up to one year has been demonstrated for metal and oxide nanoparticles by surface passivation using long-length functional groups. On the other hand, silicon nanocrystals (SiNCs) that do not aggregate in a solution for one year has not been reported with particle size measurements. The realization of stable colloidal SiNCs and its application would be useful for optoelectronics because the Si is a bioinert and cost-effective semiconductor material and the devices based on SiNCs have been investigated for energy devices and flexible sensors.
In previous studies, our group synthesized SiNCs using the two different methods of ball milling [1] and pulsed laser ablation in liquid (PLAL) [2-5]. The SiNCs prepared by ball milling in isopropyl alcohol (IPA) showed 4 months stability due to a large zeta potential. However, the mechanism to give the large potential and stability has not yet been clarified due to complex reaction processes. On the other hand, the PLAL consists of a well-defined light source of measurable energy injected into a material. In addition, the PLAL is a facile one-step process conducted at room temperature with a short duration. The SiNCs synthesized by PLAL with long-alkyl passivation show stable optical properties over a period of months.
Here, we show the SiNCs that do not aggregate in IPA for one year. The SiNCs were synthesized by PLAL of Si in IPA, and the particle size and zeta-potential were tracked for one year and the SiNCs structures were quantified using seven experimental methods and theoretical calculations. The SiNCs were stabilized by not long-length passivation but short-length passivation, which would be convenient for carrier injection and transport in the device. In fact, the photogenerated carrier density was enhanced by adding the SiNCs into a free-standing conjugated polymer film prepared via solution process.

References:
[1] H. Sun, S. Miyazaki, H. Tamamitsu, and K. Saitow, Chem. Commun. 49, 10302 (2013).
[2] K. Saitow, In Laser Ablation in Liquids: Principles and Applications in the Preparation of Nanomaterials. G. Yang, Ed. Pan Stanford, Chap. 12 (2012).
[3] T. Kitasako, and K. Saitow, Appl. Phys. Lett. 103, 151912 (2013).
[4] Y. Xin, K. Nishio, and K. Saitow, Appl. Phys. Lett. 106, 201102 (2015).
[5] D. Kajiya, and K. Saitow, Nanoscale 7, 15780 (2015).
[6] Y. Xin, K. Nishio, M. Maeda, and K. Saitow, Chem. Phys. Lett. 674, 90 (2017).

One of the most important challenges in the synthesis of metal and metal oxide nanoparticles (NPs) is the size stability in order to control the coalescence process and avoid the NPs aggregation. Due to the large surface energy of the NPs, they coalesce to each other to produce a thermodynamically stable bulk coalesced crystal. The most common methods used for the size stabilization of NPs involve electrostatic or steric protection by using a capping agent such as a surfactant, polymer, solid substrate, or a binder with a stable molecule among functional groups. The synthesis process involves the presence of a reducing agent and a metallic salt to be reduced. However, in order to accomplish a size control of the NPs, a binder must be added. Metal and metal oxide NPs synthesis using plant extracts and infusions has been widely used due to the simplicity and eco-friendliness of the process to produce a narrow size distribution, stable, and protected NPs. However, in those cases, some of the infusion components act as reducer or catalyst, and others as a stabilizer coating; nevertheless, it is difficult to distinguish which of the active components of the infusion play each role. In the present work, the role of the cinnamaldehyde (Cinn) molecule as reducing and passivating agent using two chemical precursors, AgNO₃ and CuSO₄ respectively, has been studied. Silver and Copper nanoparticles of narrow size distribution were obtained. The synthesis was carried out using AgNO₃ and CuSO₄ respectively in water as the solution phase precursor and Cinn dissolved in methanol as reducing agent. The colloidal NPs system with a narrow size distribution, highly monodispersed, and with a high NPs concentration has been stable for at least two months. The samples were characterized using UV-vis Spectroscopy, High Resolution Electron Microscopy and Mass Spectroscopy.
The authors acknowledge financial ssupport by PAPIIT-IN108019 research grant.

Nanoscale cerium oxide has shown high catalytic activity towards numerous substrates in applications relevant to the chemical and biomedical industries. The utility of this material derives from the Ce³⁺/Ce⁴⁺ redox couple and surface oxygen defects, with the material’s broad applicability across industries arising from the ability to tune surface defect concentrations via synthesis approach. Despite ceria’s high catalytic activity and efficiency: the material’s cost/scarcity may limit its use in industry. The present work focuses on a facile, green synthesis incorporating vanadium into ceria nanoparticles as a means to reduce the cost of catalyst materials, without sacrificing efficiency, while increasing the potential application space. Ultra-small, stable (hydrodynamic radius: 22 nm; zeta potential: 28 mV) cerium/vanadium oxide nanoparticles are produced via hydrolysis through the mixing of cerium and vanadate (as an inorganic redox agent) pre-cursors in water. The particles were washed to remove residual reactants and used without further modifications. The particles were deposited on glassy carbon electrodes and tested using cyclic voltammetry, chronoamperometry, and linear sweep voltammetry to detect various bio-molecules and as a pseudocapacitive material. Further, synthesis parameters were optimized and tested for enzyme-mimetic activity against comparable cerium oxide nanomaterials in standard chemical assays. This material demonstrates high catalytic activity in biomedical and electrochemical applications through a simple green synthesis method while reducing synthesis cost by limiting cerium fraction.

Cadmium-free CuInS₂/ZnS core/shell quantum dots are synthesized through a non-injection synthetic method for both the cores and shelling steps. The resulting CuInS₂/ZnS quantum dots are small (~ 4-7 nm) and spherical with photoluminescence quantum yields (PLQY) in the range of 40-70%. The structure, chemical composition and optical properties of CuInS₂/ZnS quantum dots at both ensemble and single-particle level are carefully investigated as functions of the synthesis conditions. The results show that overcoating CuInS₂ cores using relatively low amounts (0.1 mmol) of Zn(St)₂ precursor during the shelling step leads to an extensive cation exchange process that forms an alloyed core, followed by a thin ZnS shell that significantly increases the quantum yield. Increasing the amount of Zn(St)₂ to 2-4 mmol during shelling leads to more extensive cation exchange/alloying as well as forming thicker shells. Using higher amounts of Zn reduces the QY compared to using lower amounts of Zn, but the fluorescence intermittency (blinking) is significantly reduced. Moreover, we use a single particle subpopulation analysis to gain a better comprehension of the ion exchange/alloying/shelling processes and their role on the blinking mechanism.
Even though fast blinking (short on time, long off time) is a limiting property in many applications, they have great potential in improving super-resolution imaging. Here, we present our preliminary FIONA (fluorescence imaging with one-nanometer accuracy) characterization using bright but fast-blinking CuInS₂/ZnS quantum dots. The results show that the localization precision of CuInS₂/ZnS quantum dots can reach ~1 nm, highlighting the excellent potential of these QDs for super-resolution imaging applications.

PbS quantum dot solar cell is an emerging photovoltaic technology, which is also a versatile platform in emerging memory research due to its well-developed synthesis enabling a wide range of variations in optical bandgap. Moreover, recent developement in leading groups have produced power efficiency of 10%, which sheds lights in its future application areas.
In this presentation, based on the band gap engineering capability of PbS quantum dots, the development route for the enhanced power conversion efficiency of band graded PbS solar cells will be presented. In addition, to further enhance the power output equipped with a good ambient stability, a systematic study on PbS quantum dot solar cells with organic hole transport layers will be presented.
Using above two elemental technologies, it is expected that the state-of-the-art technology of quantum dot solar cells would gain a further advancement.

Nano-crystalline materials, because of their size, exhibit a variety of properties, which are different and often considerably improved in comparison with those of conventional polycrystalline bulk material. Of particular interest are nanoparticles of iron-cobalt because of their unique physical and chemical properties and their industrial importance in preparing magnetic recording materials, pigments, catalysts and inks. The goal of the project is to obtain alloys with the highest possible magnetization value. The metals that we have used for this study are Fe and Co. By varying the composition of the alloy nanoparticles (Fe:Co, 1:1 and 3:1) via a high temperature reduction reaction, highly stable nanoparticles have been synthesized. Their physical parameters have been characterized using SEM, TEM, XRD and composition by ICP-MS and EDX. For measuring magnetization, a home-built vibrating sample magnetometer is used. These alloys are particularly air-stable and have magnetization values that surpass the values of commercial ferrofluids, and ferrite powders. The details of the synthesis and characterization will be presented.
We acknowledge funding from NSF MRI 1626326 and DARPA AMEBA.

Colloidal nanoparticle synthesis has reached a level of empirical maturity with a consensus that growing well-defined colloidal nanoparticles depends on “kinetics control”. However, quantitative study of growth kinetics of colloidal nanoparticles is still missing and virtually unexplored. Rational control over growth kinetics for synthesizing colloidal nanoparticles with desirable properties on demand remains challenging. In this presentation, a strategy has been developed for analyzing growth kinetics of colloidal metal nanoparticle quantitatively by focusing both the very early and the very late growth stages, at which the size of growing nanoparticles and the reaction time follow linear functions. Applying this extreme-condition model to a microwave-assistant synthesis of colloidal silver nanoparticles, for the first time, results in the determination of intrinsic kinetics parameters involving in the growth of the silver nanoparticles. The diffusion coefficient (D) of the precursor species containing Ag⁺ is 4.9×10^–14 m²/s and the surface reaction rate constant (k) of the precursor species on the surface of the growing silver nanoparticles is 8.7×10^-8 m/s in an ethylene glycol solution containing 0.15 M polyvinylpyrrolidone at 140 °C. The extreme-condition model is ready to deconvolute the intrinsic kinetics parameters of growing colloidal nanoparticles once the enlargement rate of the nanoparticles can be experimentally measured in real time and with high temporal resolution. Availability of the high-fidelity values of k and D will provide the crucial information to guide the design and synthesis of colloidal metal nanoparticles with the desirable properties.

Molecular Imaging based on noble metal nanostructures have gained interests in the field of medical applications for the past decades. Gold nanoparticles (AuNPs), also called gold colloids, are considered to be very stable and biocompatible metal nanoparticles. Previously, it was demonstrated that they could be used for in vivo CT imaging. We recently showed that AuNPs modified with glycol chitosan (GC) polymers (GC-AuNPs) have excellent stability without aggregation in serum protein solution and tumor-targeting ability due to the enhanced permeability and retention effect.
Optical / PET and CT / MRI imaging are complementary visulaization techniques, the former providing functional and molecular information and the latter providing anatomical details. Nanoparticles, especially metallic/inorganic nanoparticle, based imaging probes have been extensively developed to evaluate precise information about disease, utilizing their characteristic properties, such as surface plasmon resonance of gold nanoparticles and superparamagnetism of iron oxide. These characteristics provide both anatomic and functional information including the location, size and genetic details of disease by combining with NIRF imaging or PET. In spite of superb nature of these nanoparticles, they have a limitation to employ in vivo diagnosis because of their in vivo stability. Appropriate surface modification method was widely researched.
Here, we introduce several fabrication methods of nanoparticle based imaging probes and their biomedical applications. Gold nanoparticles were decorated with fluorophore labeled peptides for enzyme specific NIRF imaging and demonstrated the potential as an in vivo imaging probe. Gold nanoparticles, stabilized with biocompatible polymer, were prepared for in vivo NIRF/CT imaging and successfully visualized the liver and cancer. The distinguished properties of these probes showed promising potential of the activatable dual imaging probe system as a novel molecular imaging probe.

The assembly of semiconductor nanoparticles, quantum dots (QD), into dense crystalline nanostructures holds great promise for future optoelectronic devices.¹ However, knowledge of the sub-nanometer scale driving forces underlying the kinetic processes of nucleation, growth and final densification during QD assembly remains poor.² Emulsion-templated assembly has recently been shown to provide good control over the bulk assembly of QDs by condensation into highly ordered three-dimensional supercrystals.³ Here, we combine emulsion-templated assembly with in situ small-angle X-ray scattering to obtain direct insight into the nanoscale interactions underlying the nucleation, growth and densification of quantum dot supercrystals.⁴ We show that at the point of supercrystal nucleation, nanoparticles undergo a hard-sphere like crystallization into a hexagonally close-packed lattice slowly transforming into a face-centered cubic lattice. The ligands play a crucial role in balancing steric repulsion against attractive van der Waals forces to mediate the initial equilibrium assembly, but cause the QDs to be progressively destabilized upon densification. The rich detail of our kinetic study elucidates the assembly and thermodynamic properties that define QD supercrystal fabrication approaching single-crystal quality, paving the way towards their use in optoelectronic devices.

(1) Kagan, C. R.; Lifshitz, E.; Sargent, E. H.; Talapin, D. V. Science 2016, 353, aac5523.
(2) Bishop, K. J.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. small 2009, 5, 1600.
(3) De Nijs, B.; Dussi, S.; Smallenburg, F.; Meeldijk, J. D.; Groenendijk, D. J.; Filion, L.; Imhof, A.; Van Blaaderen, A.; Dijkstra, M. Nature materials 2015, 14, 56.
(4) Marino, E.; Kodger, T. E.; Wegdam, G. H.; Schall, P. Advanced Materials 2018, 30, 1803433.

An exquisite sensitivity to the local environment combined with their ability to dramatically concentrate electromagnetic fields makes plasmon-resonant nanoparticles ideal candidates for use in stimulus-responsive materials with potential applications ranging from biosensing to smart optics. In this talk, we explore the synthesis and characterization of reconfigurable plasmonic metachromophores that change their structural organization, and hence their collective optical properties, in response to external stimuli such as light. We explore a variety of approaches to reversibly modulate interparticle plasmonic couplings in response to external triggers, ranging from discrete dimers functionalized with photoswitch-modified DNA, to nanoparticle/polymer hybrids that enable temperature-, and photo-responsive behaviors on multiple timescales. These materials offer tunable optical responses across both the visible and near-IR. We discuss strategies for incorporating multiple stimulus-response functions, and for obtaining self-amplifying and self-limiting feedback behaviors with these materials.

The formation of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, we have quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

Metal-ligand chemistry impacts nearly every aspect of nanoparticle formation, physical properties, and utility. We develop methods to study and leverage these interactions to produce highly tailored multimetallic nanoparticles with dimensions spanning the nanoscale (1-100 nm). Here, we discuss how metal-ligand interactions may be used to mediate the incorporation and distribution of metals in and on discrete, colloidal nanoparticle substrates, as well as their optoelectronic properties once formed. In particular, we demonstrate that nanoparticle ligand chemistry may be used to access previously unobserved mixtures of metals such as continuously tunable Au-Co composition ratios, unique distributions of metals at the surface of a colloidal particle, as well as composition-tunable optoelectronic features. Underpinning these studies are the development of analytical techniques to quantitatively track and ultimately tune the surface chemistry of these nanoparticles in order to create translational insights into the role of nanoparticle surface chemistry in their performance downstream. Together, these results provide mechanistic platforms for the development of nanoscale alloys and other bimetallic structures that we demonstrate are promising for a wide variety of applications ranging from light-driven catalysis to multimodal bioimaging.

Structure and reactivity of super atom gold clusters depends on the electronic interaction between core gold atoms and the ligand. The super atom clusters of the dimension 1.1 nm containing 25 atoms of gold and 18 ligands, can be of two shapes - spherical (icosahedral; comprised of core and shell gold; made of thiol (SR) ligands; Au₂₅-i) and rod shaped (bi-icosahedral; comprised of core and bridging gold atoms; made of SR and triphenlyphosphine (PPh₃) ligands; Au₂₅-bi). Both clusters are photoluminescent (PL) but Au₂₅-bi has higher quantum yield. In this project we push the loading of chromphores on the nanoparticle surface and compare the changes in PL as a function of geometry. Ground state electron transfer is monitored via voltammetry to investigate the direction of electron flow. The influence of the chromophore (modified coumarin and anthracene) will be ascertained by two-photon cross-sectional measurements and steady state measurements.

Among nanocrystals, 2D nanoplatelets appear as a special class of emitters with the best defined (ie the narrowest) photoluminescence signal. The tunability of their color is commonly obtained thanks to quantum confinement. However only discrete values for thickness can be achieved which prevent continuous tuning of the spectrum. Here we demonstrate that doping is an alternative to tune the spectrum of these II-VI semiconductor 2D nanoplatelets from green to red. We develop a synthetic method to achieve silver doping of CdSe nanoplatelets and demonstrate the potential of the material for the design of wide gamut display. Silver doing was already reported on spherical quantum dots, but several questions relative to the electronic structure of these materials were left without firm conclusion. Here we investigate the electronic properties of the silver doped CdSe NPL using a combination of optical spectroscopy, photoemission, transport and simulation. We are finally able to build a consistent picture of the electronic level associated to silver. The Ag state is strongly bound and located 340 meV above the valance of bulk CdSe. Ag dopant tends to reduce the n doping of the non-stoichiometric CdSe.

The formation of Ag octahedron nanoparticles (NPs) from Ag nanocube seeds using a foreign metal ion, Cu²⁺ reported in literature has been attributed to the selective deposition of Ag on Ag(100) over Ag(111) facets, based solely upon observation of the final shape. The driving force for selective deposition of Ag on Ag(100) facets in the presence of the foreign metal ions still remains unknown. We performed a comprehensive study of the seed-mediated synthesis of Ag octahedron in presence of foreign metal ions (Mⁿ⁺ = Ni²⁺, Co²⁺, Cu²⁺, Pd²⁺, and Au³⁺), with the objective of investigating the detailed pathway of transformation from Ag nanocube seeds to octahedron. We demonstrate a generalized mechanism based on experimental observations, periodic table trends, and theoretical calculations using density functional theory (DFT). We propose, the foreign metal ions deposit on the Ag(100) facets through either underpotential deposition (UPD) (Cu²⁺, Ni²⁺ and Co²⁺) or a galvanic exchange reaction (GRR) (Pd²⁺ and Au³⁺) depending on the Mⁿ⁺ reduction potential with respect to Ag. The deposited metal induces a faster growth rate on Ag(100) facets through GRR, in the case of Cu, Ni, and Co, and through UPD in the case of Pd and Au, leading to the disappearance of Ag(100) facets and retention of Ag(111) facets. A faster growth rate on Ag(100) facets is experimentally confirmed from observation of intermediate high-index facets formed on the Ag(100) surface at higher injection rates of Ag⁺. The formation of octahedron is also sensitive to the concentration of foreign metal ions added and the shape of the starting Ag seed template. This work provides a fundamental understanding of the role of foreign metal ions in seed-mediated shape-controlled synthesis of Ag octahedron, which can be easily extended to more complex systems. It also offers a scope for a wide selection of foreign metal ions for shape-controlled synthesis of noble metal NPs, many of which are not explored yet, simply based on standard reduction potential values. Regardless of deposition mechanism, shape transformation from Ag(100)-bound to Ag(111)-bound structures is possible with the simultaneous addition (flux) of new Ag⁺ ions.

The discovery of giant magnetoresistance in 1988 enabled the explosion of hard disk drive storage capacity, and continues to impact various sensing applications today. Granular magnetoresistance, discovered shortly after giant magnetoresistance, offers similar properties in a cheaper package, but has been limited by an inability to prepare well-characterized, uniform,nanoscale magnetic domains. Employing colloidal nanochemistry to design granular magnetoresistance materials allows for explict control of grain structure and expedient testing of different materials. We have investigated the effects of nanoparticle size on magnetoresistance and explored more complex magnetoresistive phenomena in multi-component materials.

We report the use of 2,6-dimethylphenyl isocyanide (2,6-DMPI) as a spectroscopic probe to study the heterogeneous nucleation and deposition of Pd on Ag nanocubes under different conditions by surface-enhanced Raman scattering (SERS). As a competitive advantage, the spectroscopic analysis can be performed in situ and in real time with the nanoparticles still suspended in the reaction solution. The success of this method relies on the distinctive stretching frequencies (n_NC) of the isocyanide group in 2,6-DMPI when it binds to Ag and Pd atoms through s donation and p-back donation, respectively. Significantly, we discovered that n_NC was sensitive to the arrangement of Pd adatoms on the Ag surface. For example, when the isocyanide group bound to one, two, and three Pd atoms, we would observe the atop, bridge, and hollow configurations, respectively, at different n_NC frequencies. As such, the n_NC band could serve as a characteristic reporter for the Pd adatoms being deposited onto different types of facets on Ag nanocubes with atomic-level sensitivity. When 2,6-DMPI molecules were introduced into the reaction solution, we further demonstrated in situ tracking of heterogeneous nucleation and early-stage deposition of Pd on Ag nanocubes by monitoring the evolution of n_NC bands for both Ag and Pd surface atoms as a function of reaction time. This in situ technique opens up the opportunity to investigate the roles played by reaction temperature and the type of Pd(II) precursor in influencing the heterogeneous nucleation and growth of bimetallic nanocrystals. The sensitivity of isocyanide group to Pd atoms helps elucidate some of the details on the reduction, deposition, and diffusion processes involved in heterogeneous nucleation.

Spontaneous assembly of colloidal nanostructures is often viewed as a bottom-up alternative to lithographic techniques when it comes to creating materials with novel collective properties. The intensive research in the past 10-15 years has brought numerous examples of functional superlattices, 3D colloidal crystals, and designed patterns of nanostructures. However, the majority of self-assembly methods suffer from poor reproducibility and limited scale, which often precludes the formation of macroscopically large structures and functional devices. This presentation will focus on the evaporative self-assembly of anisotropic gold nanocrystals and the underlying mechanisms of their colloidal crystallization. In addition, template-assisted evaporative assembly of nanowires, polymers, and collagen fibers into macroscopically-large hexagonal patterns will be discussed.

In this talk the characterization of gold nanoparticles coated with an amphiphilic mixture of ligand molecules will be illustrated. In particular advances in the characterization approach via small angle neutron scattering and mass spectroscopy will be illustrated. A review of the properties will these particles will be presented. Particular attention will be devoted in illustrating the antiviral properties and the ability of these particles to effectively label amyloid fibers.

Tailorable chiroptical effects in chiral plasmonic nanostructures render them promising candidates as high-efficiency broadband circular polarizers and enable enhanced direct asymmetric synthesis of chiral target molecules. Concerning flexibility in design and efficiency (circular dichroism (CD) > 10 deg), fabrication techniques like electron beam lithography and direct laser writing are usually superior to colloidal self-assembly methods (10-100 mdeg). However, using chiral templates (chiral nematic phases, DNA) or ligand-induced clustering of colloidal particles proved to be inevitable for low-cost production on large scales.

Here, we present a facile bottom-up approach to produce a crossed-chains bilayer on centimeter square area that, for the first time, is competitive (CD = 11 deg) to top-down methods. More interestingly, we prove significant spectral tunability (100 nm blue-shift) by mechanical deformation, which remains difficult to achieve in a post-fabrication fashion for other systems known in literature.

Two achiral nanoparticle chain arrays on compliant substrates are placed on top of each other to produce a chiral 3D arrangement of rotated chains. To this end, convective assembly[1] of gold nanoparticles into nanochannels was employed. In this colloidal self-assembly process, a droplet of nanoparticle suspension is dragged over the topographical template in direction perpendicular to the channels. The evaporation of the fluid facilitated at the receding meniscus drives a particle flow toward the three-phase contact line. Geometrical confinement exerted both by the channel geometry as well as the thin liquid film height guide the assembly into particle lines. The resulting small inter-particle and inter-layer distances allow for pronounced chiral plasmonic coupling.

The anisotropic nanoparticle arrangement investigated via scanning electron microscopy (SEM) manifests in optical effects on macroscopic areas easily visible by the bare eye with the help of polarized illumination. The circular dichroism quantified by spectroscopic ellipsometry (SE) is shown to be tuned by post-fabrication change of inter-layer rotation and mechanical deformation. Possible morphological changes upon deformation are discussed to explain the reversible spectral shift, as verified by finite-difference time-domain (FDTD) simulations.

As demonstrated, simple designs cost-effectively fabricated via bottom-up techniques even stand the comparison with e-beam structures for nanoplasmonic devices. Additional tunability of the strong optical effects by external stimuli facilitates broad-band applications and opens the door for sensing applications.

[1] L. Malaquin et al. Langmuir 2007, 23, 11513.
[2] P.T. Probst et al., in preparation.

The exceptional properties of materials at the nano-scale have aroused a huge increase in their demand for a broad variety of applications. During particle engineering, precise control over physicochemical properties of the material plays a key role, allowing for a reproducible fabrication of optimized and highly efficient vectors. Although significant advances have already been made in this area, much optimization is still needed to understand the often very complex interplay between reaction parameters and obtained material characteristics.
Herein, the employment of specific surface ligands as well as controlled variation of reaction parameters during thermal decompositions, coprecipitations and solvothermal reactions enabled the synthesis of iron oxide nanoparticles (IONPs) with highly homogeneous particle morphology (spherical, cube-, ellipsoid-shaped), size (5-200 nm) and phase (Fe₃O₄, Fe_1-xO, α-Fe₂O₃). The stoichiometric ratio of the precursor as well as reaction conditions also determined the formation of complex Fe₃O₄@Fe_1-xO core-shell structures that were proven by X-ray diffraction analyses and Rietfeld refinements. A subsequent functionalization with chemical linkers and biomolecules could be monitored using X-ray photoelectron spectroscopy, infrared spectroscopy as well as UV-vis spectroscopy. This step-by-step approach allowed for a very application-oriented synthesis of highly efficient MRI contrast agents as well as cancer targeting vectors.

Using time-resolved photoluminescence spectroscopy, we show that two-exciton Auger recombination dominates carrier recombination and cooling dynamics in CdSe nanoplatelets, or colloidal quantum wells. The electron−hole recombination rate depends only on the number of electron−hole pairs present in each nanoplatelet, and is consistent with a two-exciton recombination process over a wide range of exciton densities. The carrier relaxation rate within the conduction and valence bands also depends only on the number of electron−hole pairs present, apart from an initial rapid decay, and is consistent with the cooling rate being limited by reheating due to Auger recombination processes. These Auger-limited recombination and relaxation dynamics are qualitatively different from the carrier dynamics in either colloidal quantum dots or epitaxial quantum wells. The results are determined as a function of the shell thickness. We find that the results exhibit a non-monotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2−4 monolayers, and then increasing with further increases in the shell thickness.

Upconversion nanoparticles have matured significantly over the past decade. Preparative and post-preparative techniques have been developed to control migration, tunability, and transfer of energy. These recent advances show promise in the world of photochemical and photovoltaic systems. We demonstrate the ability to improve yield, spectrally tune, and to extinguish the UV luminescence by means of preparative controls. UV luminescence can be extinguished by means of energy migration through excess ytterbium or phonon-assisted energy transfer through secondary activator. We also demonstrate the ability to control the UV luminescence post-preparation by means of pulse-width modulation of excitation. Pulse- width modulation of excitation allows a unique ability to visualize UNPs by means of 1 microsecond pulse-width excitation for NIR-NIR imaging, whereas utilizing 10 millisecond pulse-width excitation can produce UV luminescence and initiate photochemistry. These mechanisms of control of the near-infrared to ultraviolet upconversion enable new applications, especially in the field of biomaterials.

Antibiotics have been used in many parts of the world such as medicine, food preserving, andlivestock. Abuse of antibiotics had led to an increase inantibiotic resistancebacterial infections in modern society. For probiotics to replace antibiotics, enhancing antimicrobial ability against pathogens is one of the key technologies on probiotics. Prebiotics are not digested by human enzymes although their fermented products by probiotics can provide various benefiteffects to host. Polymeric nanoparticles (NPs) have been used for biomedical application because of their interesting characteristics, which enables them to overcome various biological barriers. In this study, we prepared three types of prebiotics NPs usingpolysaccharides. Three type of polysaccharidessuch as inulin as a polyfructose, dextran as a polyglucoseand pullulanas a polymaltose were used. Theprebiotic nanoparticles (NPs) were prepared by conjugating hydrophobic groups such as phthalic anhydride, acetic anhydride, and propionic anhydride to polysaccharides. The effects of phthalylinulin NPs (PINPs), phthalyldextran NPs (PDNPs), and phthalylpllulanNPs (PPNPs) on cellular and antimicrobial activities of probioticsusing Pediococcus acidilactici(PA) and Lactobacillus plantarum(LP) will be mainly reported.

Firstly, PINPs were prepared by varying the molar ratio of phthalic anhydride as a hydrophobic group to inulin, and then dialysis. We found that the internalization of PINPs into PA was size-, glucose transporter- and energy-dependent. Interestingly, PINPs-treated probiotics enhanced the production of antimicrobial peptide (pediocin) which is effective against both Gram-positive and Gram-negative pathogens. Particularly, the antimicrobial activity of PINPs-internalized probiotics was about 9-fold higher than that of untreated probiotics. Gene transcriptional analysis demonstrated the rise in pediocinactivity in PA internalized with PINPs was accompanied with the enhanced expression of genes related to stress response and pediocinbiosynthesis genes.

To confirm the previous concept with a differenttype of polysaccharides, PDNPs were prepared. We observed the internalization of PDNPs into PA varied upon the transporters in probiotics and influenced higher antimicrobial activities against pathogens. The mechanism of enhancing antimicrobialproperty of PA was same as PA internalized with PINPs. Moreover, in PDNPs, the antimicrobialactivity of PA internalized with PDNPs was also proved via animal experiments. PA internalized with PDNPs was able to suppress pathogenic infections in the gutby decreasing pathogens and increasing beneficial bacterial species which are related with prebiotics. Moreover, the composition of gut microbiome altered in PA internalized with PDNPs toward other groups. Therefore, we can conclude that higher pediocinproduction may suppress the pathogenic gut infection by altering the composition of gut microbiota.

Also, we used LP to confirm whether prebiotic NPs enhance the antibacterial ability of lactic acid bacteria as a common phenomenon because the LP is also capable ofproducing natural anti-microbial substances, plantaricin. When PPNPs were added into LP, the PPNPs were internalized into LP through energy-dependent and galactose transporter mechanism. Furthermore, more plantaricinas the natural anti-bacterial peptide was secreted from thePPNPs-internalized LP whichwassimilarmechanismasPINPs and PDNPs.

To the best of our knowledge, this is the first report of the concept of prebiotic NPs using polysaccharides to improve the antimicrobialactivity of aprobiotic bacterium against pathogens. The results suggest that prebiotic NPs are new type of intracellular stimulator of probiotics to produce antimicrobial peptides, and can be used as a potential treatment of many gut disease associated with pathogens.

We have designed nanomaterials that utilize vibrational modes to manipulate electronic interactions. Gold bipyramids with systematically varied localized surface plasmon resonance energies were functionalized with a J-aggregated thiacarbocyanine dye molecule. The resulting systems exhibit two hybridized states with clear anticrossing behavior that have a Rabi splitting energy of 120 meV. Gold bipyramids respond to photoexcitation with the generation of coherent acoustic phonons that cause oscillations in the plasmon resonance energy. In the coupled system, these photogenerated oscillations alter the metal nanoparticle’s energetic contribution to the hybridized system and, as a result, change the plasmon-exciton coupling. These modulations in the hybridization are present across a wide range of bipyramid ensembles. We discuss new analysis methods we have developed to better model this complex behavior. Such oscillatory plasmonic-excitonic nanomaterials offer a route to manipulate and dynamically tune the interactions of plasmonic/excitonic systems and unlock a range of potential applications.

We have developed a straightforward and fast non-aqueous microwave synthesis protocol using acetylacetonate and acetate precursors to produce nanocrystals of the earth-abundant cubic spinel ferrite MgFe₂O₄ [1), which is a promising materials for both photoelectrochemical and photocatalytic water splitting under visible light irradiation due to its narrow band gap (~ 2.0 eV) and matching band positions. Highly-crystalline, single phase nanoparticles with specific surface areas of around 200 m²/g and good colloidal stability in non-polar solvents were achieved, and the crystallite size can be tailored by post-synthetic heat treatment. Colloidal stabilization of the nanocrystals in nonaqueous media was realized either in-situ during synthesis or postsynthetically by surface capping with oleylamine and oleic acid. Phase transfer to aqueous media was performed employing citric acid and betaine hydrochloride, resulting in agglomerate-free and stable dispersions of citrate- or betaine-functionalized MgFe₂O₄ nanocrystals. Furthermore, a one-step synthesis of highly stable, water-dispersible colloids of MgFe₂O₄ was achieved using polyvinylpyrrolidone as stabilizer. Photocatalytic reactions will be presented evaluating the use of such highly stable ferrite colloids for solar energy conversion.
[1] K. Kirchberg, S. Wang, L. Wang, R. Marschall, Chem. Phys. Chem. 19 (2018) 2313.

A post-synthetic method for stabilizing colloidal crystals programmed from DNA has been developed. The method relies on Ag⁺ ions to stabilize the particle-connecting DNA duplexes within the crystal lattice, essentially transforming them from loosely bound structures to ones with very strong interparticle links. Such crystals do not dissociate as a function of temperature like normal DNA or DNA-interconnected superlattices, and they can be moved from water to organic media or the solid state, and stay intact. The Ag⁺-stabilization of the DNA bonds is accompanied by a nondestructive ~25% contraction of the lattice, and both the stabilization and contraction are reversible with the chemical extraction of the Ag⁺ ions, by AgCl precipitation with NaCl. This synthetic tool is important, since it allows scientists and engineers to study such crystals in environments that are incompatible with structures made by conventional DNA programmable methods and without the influence of a matrix such as silica.

Wurtzite-like colloidal CuGaS₂ nanorods and their heterostructures with CuInX₂ (X=S,Se) have been synthesized by solution chemistry. Unusual bent morphologies were observed in TEM images of the initial CuGaS₂ nanorods. Measurement of the bend angle in the nanorods results in a consistent value, suggesting a crystallographic transition between chalcopyrite and wurtzite. However, HRTEM investigation indicates that the nanorods do not go through such a phase change, and are continuous wurtzite throughout. The wurtzite (101) and (100) surface have similar atomic densities, suggesting similar surface energies, which could account for the different growth directions observed during growth via self-sacrificing catalyst/seed Cu_2-xS nanoparticles. We suggest that initial reaction conditions favor growth in the (001) direction, with multiple surface facets accessible, while later reaction conditions favor (100) side facet termination, hence the change in growth direction. This difference in surface termination may also account for the observed semi-selective nucleation of CuInSe₂ onto CuGaS₂ nanorods when sawtooth-like heterostructures are epitaxially grown.

Recently, the quantum-dot enhancement film (QDEF) has been introduced to improve the color gamut for LCD technology. Although this QDEF has been realized approximately zero cross-talk among the main three RGB colors, it leads to the loss in the light power of back light unit due to the resin and barrier sheets. Therefore, we introduce a simple and cost-effect structure for conventional blue LED back light unit (BLU) LCD having no QDEF but higher resolution and wider color gamut by mixing InP-QDs directly into the color filter. This structure is called quantum dot-functional color-filter (QDCF).
To apply this proposed structure of QD-functional CFs to an ultra-high resolution LCD application, the color gamut performance of the LCD-like micro-displays using blue, green, and red CFs were estimated by using the CIE 1931 color space with Rec. 2020 and NTSC 1953 standards. Then it compared with that of the conventional LCD like micro displays using the white LED BLU and conventional CFs. The CIE coordinates of three types of micro-displays using green and red InP-QD color filters were calculated by using the PL spectrum information and the Matlab-based CIE coordinate calculator software. It was confirmed that green and red InP-QD emit a light at the wavelengths of 530, and 632 nm having narrow PL-FWHM of 39, 47 nm and high PL-QY of about 85, 55 %, respectively. The RGB color gamut of the conventional LCD-like micro-displays using the bLED BLU and conventional CFs showed only 73.7 % (NTSC) and 55.1 % (Rec. 2020). On the other hand, the RGB color gamut of the proposed LCD-like micro-displays using the fabricated InP-QD function CFs presented a higher value of 108.8 % (NTSC) and 81.4 % (Rec. 2020) due to the zero cross-talk among PL spectrums of RGB colors.
We will present in detail the RGB color gamut of InP-QD color filters in view point of optical properties of QDs and LCD display operation.

A modified Colvin's method (1) was used to make colloidal “black” TiO₂ nanoparticles with high monodispersity. The absorption and luminescence of these particles were investigated. In order to study the photocatalytic performance of these nanocrystals we used them as building blocks to make, in two steps, TiO₂ mesoporous films by processing spincoated TiO₂ nanocrystal solids with plasmas. Furthermore, to investigate the role of N-doping we demonstrate that plasma processing in N₂ plasma allows to simultaneously remove the ligands and dope homogeneously the mesoporous films with interstitial N. Lastly we will show how doping and processing affects the photocatalytic activity of the mesoporous films under both UV and visible light.

1. T. J. Trentler, T. E. Denler, J. F. Bertone, A. Agrawal, V. L. Colvin, Synthesis of TiO2 nanocrystals by nonhydrolytic solution-based reactions. Journal of the American Chemical Society 121, 1613-1614 (1999).

With the advent of DNA-directed methods to form “single crystal” nanoparticle superlattices, new opportunities for studying the properties of such structures across many length scales now exist. These structure-property relationships rely on the ability of one to deliberately use DNA to control crystal symmetry, lattice parameter, and microscale crystal habit. Although DNA-programmed colloidal crystals consistently form thermodynamically favored crystal habits with a well-defined symmetry and lattice parameter based upon well-established design rules, the sizes of such crystals often vary substantially. For many applications, especially those pertaining to optics, each crystal can represent a single device, and therefore size variability can significantly reduce their scope of use. Consequently, we developed a new method based upon the density difference between two layers of solvents to control nanoparticle superlattice formation and growth. In a top aqueous layer, the assembling particles form a less viscous and less dense state, but once the particles assemble into well-defined rhombic dodecahedra superlattices of a critical size, they sediment into a higher density and higher viscosity sublayer that does not contain particles (aqueous polysaccharide), thereby arresting growth. As a proof-of-concept, this method was used to prepare a uniform batch of Au nanoparticle (20.0 ± 1.6 nm in diameter) superlattices in the form of 0.95 ± 0.20 µm edge length rhombic dodecahedra with body-centered cubic crystal symmetries and a 49 nm lattice parameter (cf. 1.04 ± 0.38 µm without the sublayer). This approach to controlling and arresting superlattice growth yields structures with a three-fold enhancement in the polydispersity index.

It is a critical challenge to develop a single phase phosphor with high luminous efficiency, Ra and suitable correlated color temperature (CCT). Comparing with traditional rare-earth based phosphors, carbon dots (CDs) possess various merits such as broad and tunable emission, excellent photostability and low cost and toxicity. Thus, CDs are ideal single phase phosphor materials, which emission can cover the whole visible spectrum. However, as-prepared CDs display strong blue emission with a broad tail in most cases. Towards making CDs as a single phase white-light phosphor, the emission at a longer wavelength has to be enhanced to balance the PL spectrum by increasing both the content and PL QY of long wavelength emissive CDs. On the other hand, more green and red emissive CDs can be excited by blue and green light, not UV light. Thus, it is necessary to further promote the energy transfer from blue emissive CDs to green and red emissive CDs.In this work, WCDs can be simply synthesized from the o-phenylenediamine (PDA) and the catechol in the mixture of n-amyl alcohol (NAA) and dimethylformamide (DMF) by one-step hydrothermal. The WCDs exhibits broad UV-vis light absorption band and strong excitation-dependent emission covering blue (450nm), green (520 nm) and red (600 nm) light with PL QY of 0.42, 0.55 and 0.67, respectively. We addressed that B-CDs, G-CDs, and R-CDs were from the carbonization of pure catechol, PDA and the complexes of catechol and PDA (named as complexes CP). With the increase of the concentration of WCDs, the B- and G-CDs tend to form J-aggregates, R-CDs can organize in both H- and J- aggregate ways. The aggregates make the PLE spectrum shift, which leads to that the B-CDs and G-CDs can be directly excited by UV light. Although R-CDs cannot be directly excited by UV light, it can emit red light via FRET route due to they have a spectral overlap between donor and acceptor. Finally, WLEDs device is constructed and demonstrate that WCDs can be used as the phosphor for white lighting due to its high CRI of 93, low CCT of 5453 K and high luminous efficiency.

Porphyrins play a very important role in life activities and are known as "living dyes". Porphyrin molecules have high molar extinction coefficient, good bio-source, certain photostability and thermal stability, which have wide application in biological diagnosis and treatment. Porphyrin assemblies obtained by surfactant-assist self-assembly strategy driven non-covalent interaction offsetting the narrow absorption of monomer and improving the dispersity of water. Our research group are dedicated to the research of porphyrin assemblies in biotherapeutics and had achieved some exciting results. We used zinc meso-tetra(4-pyridyl)porphyrin (ZnTPyP) as activity function building block, ZnTPyP@SiO₂ composite structure was conducted through a combined surfactant micelle confined self-assembly and silicate sol–gel process with good biocompatibility and high singlet oxygen yield, and the apoptosis rate reached 90% after folic acid modification. In addition, Nitric oxide (NO) coordinates to central metal Zn ions of ZnTPyP nanoparticles and formed stabilized ZnTPyP@NO nanoparticles. We show that ZnTPyP@NO photosensitizers release highly reactive peroxynitrite molecules under light irradiation and exhibit enhanced antibacterial photodynamic therapy (APDT) activity. Recently, we have used 5,10,15,20-tetrakis (4-aminophenyl) porphyrin (TAPP) as optical active precursor, and the absorption spectrum of TAPP nanoparticles obtained by protonic acid-mediated assembly can broaden to about 1000 nm. The TAPP assemblies exhibit excellent morphology dependent NIR photothermal properties and effects. These consequences provide an important reference for the application of porphyrin self-assemblies in biological therapy.

Fluorescent carbon quantum dots (CQDs) have been attractive from chemical inertness, facile modification, high water-solubility, low photo-bleaching, and excellent biocompatibility [1]. Here, to obtain surface-engineered CQDs with high photoluminescence (PL) quantum yield (QY), we employed a one-step microwave synthesis route based on citric acid and cysteine. For various CQDs synthesized under different cysteine ratios, physical and chemical properties of CQDs were investigated by using the transmission electron microscope (TEM), the Raman, and the X-ray photoelectron spectroscopy (XPS) systems. From the TEM images, CQDs were spherical nanoparticles with the size in the range of 2 - 4 nm and the average lattice spacing of C-dots was 0.21 nm. The Raman spectra showed contributions from both the G-band (in-plane vibration of sp2 carbon) and the D-band (the presence of sp3 defects). In the XPS spectra, we found that the surface amino group of the CQDs increased with increasing the cysteine ratio. These surface-engineered CQDs sample with high density of surface amino groups exhibit high PL QY of ~ 76 %.

Glucose, used in this study is a kind of sugar and an important biological raw material that can be easily extracted in nature without any purification [2]. We fabricated glucose films doped with CQDs by spin-coating of the aqueous solution of CQDs and glucose. We also introduced a periodic nanostructure on the CQD film by using a nanoimprint lithography process [3]. The nanostructured CQD films showed strong enhancement of photoluminescence efficiencies at the specific spectral region depending on the periodicity of the nanostructured films. These nanostructured CQD films can be new light sources and photonic devices for health care.

[1] S. Qu, X. Liu, X. Guo. M. Chu, L. Zhang, D. Shen, Adv. Func. Mater. 24, 2689 (2014).
[2] S. P. Park, Y. J. Tak, H. J. Kim, J. H. Lee, H. Yoo, H. J. Kim, Adv. Mater. 30, 1800722 (2018).
[3] S. H. Lee, N. Y. Ha, Opt. Express 19, 21803 (2011).

To date, Ti-based nanostructures have attracted a considerable amount of attention in photocatalysis, phtovoltaics, electrochromic devices, electrochemistry, biointerfaces and corrosion because of their unique and diverse physico-chemical properties. Among them, potassium (K) incorporated Ti-based nanostructured oxides (KTiO_x) have received particular attention due to their wide range of applications as previously mentioned and because of their remarkable structure. KTiO_x materials are versitle, and through control of the K content, the electrical and optical properties can be fine-tuned. Moreover, whereas other metal oxide particles for photocataysis such as CdS or GaP degrade and produce toxic end-products, the catalytic activity of KTiO_x only involves photoenergy without additional chemicals, resulting in an environmentally friendly product. One of the main bottlenecks in using KTiO_x for diverse applications is the nanostructures fabrication, which generally involves a complicated process with low reproducibility and high cost of chemical modifications. The physical properties of KTiO_xs which are made by chemical modification greatly affected by the incopoarated K in the obtained KTiO_xs, thereby inducing the various the size and morphology of the nanostructures. On the basis of this result, the precious amount of the incopoarated K is a key issue to control the desirable porpperites. Herein, the development of a process that allows simple synthesis and tuning of the desired morphology and properties is currently an important roadblock in this research field.
In this contribution, KTiO_x nanowires were prepared by wet corrosion process (WCP) and had their photocatalytic effects systematically characterized. In the synthesis of the KTiO_xs, the potassium hydroxide (KOH) concentrations were varied durnig the WCP in order to obtain nanostructures with different surface areas and surface charges. Structural and crystalline properties of the KTiO_xs were studied by means of X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Chemical composition was determined through X-ray fluorescence and energy-dispersive X-ray analysis. Photocatalytic performance was investigated as a function of the surface area, pH, and crystalline structures by studying the degradation of methylene blue, cardiogreen, and azorubine red dyes upon UV irradiation. Note that the heat treatment was carried out to increase the crystallinity of KTiO_xs. The results demonstrated that the negatively charged crystalline KTiO_x nanostructures with high surface area showed significantly higher photocatalytic degradation when compared to KTiO_xs with low crystallinity. The produced KTiO_xs also showed greater efficiency for recovery and re-use. The heat treatment belowe 600 °C yielded nanostructures with improved structural properties, leading to well organized, layered structures with improved photocatalysis. We believe that KTiO_x nanostructures produced by WCP are very promising for photocatalysis, due to their high photocatalytic efficiency and potential for high re-useablility.

Colloidal sulfide nanocrystals have shown potential applications in a variety of fields, such as solar cells, biological labeling and imaging, catalysis, sensors etc.. Currently, the translation of these nanocrystals to industrial products is greatly hampered by their lack of sustainability (e.g. low energy efficiency, copious waste, low yield etc.), scalability (e.g. one gram per batch), and high cost of synthesis. As previously identified by other members of the community, the high cost of production is the key bottleneck in the use of sulfide nanocrystals in solar cells.
We will show how a novel renewable ionic liquid sulfur precursor, oleylammonium hydrosulfide, allows for a drastic improvement in the sustainability, scalability, and cost profile of the synthesis of a number of sulfide compositions (PbS, Bi₂S₃, Ag₂S, Cu₂S, etc…). Specifically, this precursor allows for (i) the synthesis of monodisperse nanocrystals under ambient condition (rather than at high temperatures and inert conditions), (ii) facile recycling of excess precursor, (iii) high reaction yield (> 70%) at short reaction times, (iv) high product concentration (90 g L^-1), and facile scalability to >100 g per batch.

An electron microscope can be used to image materials of nanoscale dimensions allowing for examination of size and shape characteristics, and chemical composition. We use an FEI Apreo scanning electron microscope to image magnetic (using electromagnetic mode) and nonmagnetic nanostructures (using electrostatic mode) of various metals (Au and Ag) and metal alloys (Fe/Co, Fe/Au) using different sample preparation methods, and varying system parameters for optimal image quality and resolution. Samples are prepared with spin coating followed by plasma cleaning, with controlled plasma composition. System parameters of spot size and accelerating voltage for different built-in detectors (ETD, EBSD, in-column BSD detectors, and STEM) are studied. In addition parameters for EDX mapping will also be presented, especially for elemental analyis of alloys. Details of the imaging parameters and sample preparation for SEM imaging will be presented.
We acknowledge funding from NSF MRI 1626326.

Designing nanoparticles (NPs) for multifunctional applications towards phosphors, scintillators and white light emitting diodes is highly challenging and demanding. In this work, we explored pyrochlore La₂Hf₂O₇ NPs singly and triply codoped with Eu³⁺, Tb³⁺ and Dy³⁺. The NPs singly doped with Eu³⁺, Tb³⁺ and Dy³⁺ displayed red, green and yellowish-blue emission under ultraviolet irradiation, respectively. The concentration quenching study revealed non-radiative energy transfer in Eu³⁺ doped La₂Hf₂O₇ NPs takes place via dipole-quadrupole mechanism whereas dipole-dipole interaction prevails in Tb³⁺ and Dy³⁺ doped La₂Hf₂O₇ NPs. Furthermore, when triply codoped with Eu³⁺, Tb³⁺ and Dy³⁺ ions, the La₂Hf₂O₇ NPs display beautiful warm white light as a new strategy with color tenability through doping percentage. Lifetime spectroscopy reveals the stabilization of Eu³⁺ and Dy³⁺ ions at La³⁺ site at low doping concentration whereas a fraction of them migrates to Hf⁴⁺ site at high doping concentration. For the La₂Hf₂O₇:Tb³⁺ NPs, Tb³⁺ ions are localized at Hf⁴⁺ site at all doping concentrations. To sum, our complete spectrum of studies on the structure, UV excited photoluminescence, concentration quenching, local site spectroscopy of the La₂Hf₂O₇ NPs suggests that they are potential candidates as single-component multicolor-emitting phosphors.

Two dimensional (2D) semiconductor nanoplatelets (NPLs) have received significant attention because of their unique narrow emission spectra and fast fluorescence lifetimes.^[1] NPLs exhibit strong quantum confinement only along the vertical direction, which can be controlled with atomic precision.^[1] So far, blue (460 nm) and green (510, 550 nm) emitting colloidal CdSe NPLs have been intensely investigated,^[1] however one also requires thicker nanoplatelets, to cover the remaining visible spectrum and for instance obtain narrowband red emitters.
Here we present a novel synthetic approach to obtain thicker CdSe NPLs. The introduction of small amounts of CdCl₂ switches the 2D growth of 4.5 monolayer (ML) NPLs to a more isotropic 3D growth, resulting a series of nearly monodisperse CdSe NPLs with thicknesses from 5.5 ML to 8.5 ML.^[2] The thickness tuning has been achieved by varying the reaction temperature between 280-320°C. The growth mechanism was studied by monitoring the reaction kinetics and control experiments of purified NPL dispersions, which suggest that the thickness increases via Ostwald ripening.^[2,3]
Structural characterization by X-ray diffraction and transmission electron microscopy revealed that the NPLs maintain a zinc-blende crystal structure, despite growing them for 2-5 hours at high temperatures. Optical characterization yielded emission peaks at 554, 585, 605 and 625 nm for thicknesses from 5.5 ML up to 8.5 ML, respectively. They show fast emission lifetimes of 5-11 ns and maintain narrow emission line widths of 9-13 nm (fwhm), making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. Photoluminescence excitation spectra of the thicker NPLs revealed several high-energy peaks, which has been assigned using k.p calculations.
To summarize, we have successfully synthesized CdSe NPLs with increasing thickness by introduction of chloride ions in the synthesis. This has enabled us to obtain narrow, fast emitters over a wide spectral range, and yielded additional insights into both the synthesis mechanism as well as the NPL optoelectronic properties.
Acknowledgement: This project has received funding from the European Research Council (Grant no. 714876 PHOCONA).
References:
[1] M. Nasilowski, B. Mahler, E. Lhuillier, S. Ithurria, B. Dubertret, Two-dimensional colloidal nanocrystals. Chem. Rev. 2016, 116 (18), 10934-10982.
[2] S. Christodoulou, J. I. Climente, J. Planelles, R. Brescia, M. Prato, B. Martin-Garcia, A. H. Khan, I. Moreels, Chloride-Induced Thickness Control in CdSe Nanoplatelets. Nano Lett. 6248-6254, 18, 2018.
[3] F. D. Ott, A. Riedinger, D. R. Ochsenbein, P. N. Knüsel, S. C. Erwin, M. Mazzotti, D. J. Norris, Ripening of Semiconductor Nanoplatelets. Nano Lett. 2017, 17 (11), 6870-6877.

The combination of magnetic and plasmonic functionalities into one nanocomposite material is the basis of many new multifunctional nanomaterials with unique multimodal properties. Such materials retain all functionality of the magnetic component (e.g. for use in magnetic separation, as MRI contrast agents etc¹), but with the added functionality of a plasmonic component (e.g. for use in SERS ²). The majority of magnetic-plasmonic nanomaterials are core-shell in structure, and this results in a number of inherent difficulties. Direct coating of a plasmonic metal onto a magnetic ferrite is difficult due to a mismatch in their crystal lattices. Due to this, a number of intermediate layers and often harsh reagents must be ultilised; which in turn will reduce the magnetic moment, biocompatibility and reproducibility of these nanomaterials³.

In this work, a novel approach to magnetic-plasmonic nanocomposites has been developed through utilisation of two non-toxic and biocompatible polyelectrolytes, polystyrene sulfonate (PSS) and poly(allylamine hydrochloride) (PAH). Negatively charged PSS was used to stabilise superparamagnetic Fe₃O₄ nanoparticles, while positively charged PAH was used to stabilise gold nanoparticles.The subsequent combination of these two moieties resulted in the formation of a magnetic-plasmonic nanocomposite that is bound entirely through electrostatics. This result is achieved through a simple, highly reproducible co-precipitation method which can easily be extended to other magnetic ferrites and plasmonic metals.

This approach was then extended to a number of systems, including gold nanostars with magnetite vests, multilayer magnetic plasmonic microspheres and magnetite coated plasmonic nanowire systems. We plan to explore the applications of these new nanomaterials in magnetic separation and sesning, as well and their potenial applications in new drug delivery systems and in diagnostics and therapy (e.g. hyperthermia).


1. Z. R. Stephen, F. M. Kievit and M. Zhang, Materials today (Kidlington, England), 2011, 14, 330-338.
2. S. Stafford, R. Serrano Garcia and Y. Gun’ko, Applied Sciences, 2018, 8, 97.
3. N. R. Jana, Physical Chemistry Chemical Physics, 2011, 13, 385-396.

Porphyrins are vital pigments involved in biological energy transduction processes. Their abilities to absorb light, then convert it to energy, have raised the interest of using porphyrin nanoparticles as photo-sensitizers in photodynamic therapy. A recent study showed that self-assembled porphyrin-silica composite nanoparticles can selectively destroy tumor cells, but detection of the cellular uptake of porphyrin-silica composite nanoparticles was limited to imaging microscopy. Here we developed a novel method to rapidly identify and semi-quantify porphyrin-silica composite nanoparticles using Atmospheric Solids Analysis Probe-Mass Spectrometry (ASAP-MS). ASAP-MS can directly analyze complex mixtures without the need for sample preparation. Porphyrin-silica composite nanoparticles were vaporized using heated nitrogen desolvation gas, and their thermo-profiles were examined to identify distinct mass-to-charge (M/Z) signatures. HeLa cells were incubated in growth media containing the nanoparticles, and after sufficient washing to remove residual nanoparticles, the cell suspension was loaded onto the end of ASAP glass capillary probe. Upon heating, HeLa cells were degraded and porphyrin-silica composite nanoparticles were released. Vaporized nanoparticles were ionized and detected by MS. The cellular uptake of porphyrin-silica composite nanoparticles was identified using this ASAP-MS method, and their cellular accumulation was quantified. Thus, this study established a quantification toolset for porphyrin nanoparticles.
Sandia National Laboratories is a multimission 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-NA0003525.

Discovery of new complex oxides that exhibit coupled ferromagnetic and ferroelectric properties is of great interest for the design of functional magnetoelectrics, in which research is driven by the long-term technological prospect of controlling ferromagnetic hysteresis with an electric field and vice versa. Multiferroics offer opportunities for sensors, 4-state logic, and spintronics. Single-phase multiferroics are of theoretical interest due to the possibility of a quantum character in such coupling. BaMn₃Ti₄O_14.25 (BMT-134) is a recently discovered single-phase multiferroic complex oxide exhibiting antiferromagnetic and ferroelectric behavior. It is particularly interesting because it introduces the hollandite supergroup as a family of candidate structures for multiferroic behavior. To modulate magnetic properties, ultimately towards a room temperature ferroic order response, BMT-134 was doped with Fe at varying degrees. Using a chemical solution processing approach, the parent compound and four distinct variations of nanocrystals were synthesized; BaMn₃Ti₄O_14.25 (BMT) and BaMn_3-xFe_xTi₄O_14.25 (BMFT) with x = 1, x = 1.5, x = 2, and x = 2.25. All variants were found to belong to the same hollandite crystal class as BMT and Fe was shown to be incorporated into the crystals of each. Using EDS elemental composition was obtained and high product precursor fidelity was observed. Mossbauer spectroscopy was used to probe the local chemical environment of iron present within the crystal; oxidation state and bonding geometry was determined. M-T and M-H curves were obtained using a magnetic property measurement system to determine Curie and Weiss constants through fitting and low temperature hysteresis respectively. The Weiss constant becomes increasingly positive as iron is added until an observed inflection point, indicative of an antiferromagnetic to paramagnetic short-range interaction transition. LCR measurements were used to measure the effect of iron doping on dielectric strength and dissipation factor within a 100 Hz – 2 MHz AC frequency range. Dielectric constants ranging from 100 – 1000 (lower frequencies) to 25 – 40 (higher frequencies) were observed. TEM and SEM were used to determine size distribution and morphology of the nanoparticles and of their packing within capacitor pellets. Effective medium approximations were used to model the contribution of both the ceramic material and void space to the overall effective dielectric of the nanocomposites.

Quantum dot light emittng diode (QD-LED) has attracted a great attention due to its high color purity, and solution processibility. However, understanding on how QD-LED fails is still lacking. Electroluminescence (EL) of QD in the sandwich structure between charge transport layers already reached 20 % external quantum efficiency (EQE), but down conversion display is only available in the consumer market. Due to complex device architecture of QD-LED compared to down coversion type QD emitter, understanding how QD fails is challenging. In this study, we fabricated QD-LEDs with typical core/shell structure and our double heterojunction nanorod. We observed driving bias dependent device lifetime which reflect that there are universal degraded part in QD-LEDs. Also, by measuring EL of QD-LEDs and PL of QD emitter film simultaneously, we revealed that QD film is still emittng PL despite of EL quenching. This clearly indicates that charge transport layer aging is main reason for the QD-LED degradation. Lastly, to overcome organic charge transport layer issue, we also suggested NiO hole transport layer to replace organic charge transport layer. By passivating NiO surface, device performance of NiO HTL based QD-LED was enhanced by factor of 5.

Chemical transformations via postsynthetic modification of colloidal nanocrystals have largely investigated, because those approaches can provide an unusual route to elaborately composition-controlled nanostructures while maintaining the overall structure of template. In particular, the asymmetric cation exchange reaction could be achieved by imposing a significant kinetic hurdle to the cation exchange reaction. The different atomic arrangements of the core and crown in core-crown structures might further differentiate the surface energies of originally identical cation exchange sites, leading to different reactivities of these sites. Herein, we suggest that the synthesis of binary and ternary metal chalcogenide nanoplates with Janus structural motif via asymmetric cation exchange. This synthetic methodology exploiting cation exchange of surface-passivated semiconductor nanoparticles could fabricate the numerous symmetry-controlled Janus heterostructures.

The purpose of this work is the attribution of gunshot residue (GSR) to manufacturer using Surface Enhanced Raman Spectroscopy enabled by a bench top portable spectrometer. We propose nanostructures that span zero- (metal and magnetic clusters), one- (rods), and three- (microbars) dimensions, synthesized in our laboratory. These substrates are chosen because they have charge transfer capabilities with the organic analytes present in the GSR or electromagnetic field enhancements to provide a basis for this approach. This study presents the LOD measurements of the SERS signal from two organic compounds of GSR, diphenyl amine and ethyl centralite, with the gold and silver nanoparticles.

Bimetallic hollow nanostructures have become promising new candidates in biomedicine, fuel cells, and gas sensors due to their porous structures and the synergistic effect between two metals. Currently, galvanic replacement is a powerful strategy to fabricate bimetallic hollow nanostructures. However, most galvanic replacement literatures so far employ costly metal nanostructures as the template and involve complicated purification process. Here, we develop a one-step general synthetic method based on galvanic replacement for the fabrication of Cu-based bimetallic two-dimensional hollow nanostructures. Firstly, the copper nanosheets are prepared at 300 ^oC, and then metal ions (M = Au, Pt, Pd) with higher oxidation potential were injected into Cu nanosheets solution between 80 ^oC to 150 ^oC. Because Cu²⁺/Cu potential is less than that of Mⁿ⁺/M, Cu atoms on nanosheets’ surface are replaced with M due to galvanic replacement; sequentially the interior Cu atoms will diffuse to outside owing to Kirkendall effect, which lead to the formation of hollow nanostructures. Compared to utilize other metal nanosheets as the template, our method consists of three advantages: 1. reduces the cost; 2. avoids the template separating process; 3. enriches two-dimensional bimetallic hollow nanostructures. These two dimensional hollow nanostructures with large surface area can be potentially used for the fabrication of gas sensors and as catalysts for converting CO₂ to chemicals.

Plasmonic noble metal nanoparticles show great promise in a variety of applications, ranging from theranostics to chemical sensing. Chemical sensing is possible through observing changes in the localized surface plasmon resonance (LSPR) in response to changes in environment. Au and Ag structures have notably high refractive index sensitivity (RIS), with Au structures typically being used due to their greater chemical stability relative to Ag. However, due to the high mobility of Au atoms, many structures degrade rapidly due to morphology changes. As we show, the RIS and structural stability of Au structures can be increased through the addition of Pd to such nanostructures. Specifically, Pd is directed to the tips of stellated Au nanostructures through kinetically controlled overgrowth. Interestingly, changing the capping agent switches the Pd deposition mode, producing stellated Au nanostructures conformally coated with Pd. The RIS of these structures were measured by standard methods and compared to all-Au stellated structures. The results were analyzed as a function of structure (branching symmetry and particle size) as well as bimetallic distribution and Pd loading. This analysis is aided by simulations of the anticipated optical properties. This work is coupled with measurements of the benchtop stability of structurally similar all-Au and Au-Pd stellated nanostructures. Collectively, this work demonstrates that the introduction of Pd to Au nanostructures both boosts RIS and structural stability, moving the design of LSPR sensors beyond the all-Au standard.

The silicon wafers used as substrates for semiconductor devices are required to be of increasingly stringent quality because of the downscaling of these devices. Particularly, the wafer surface quality, including the flatness and the local roughness, is critical in avoiding defects in the final products. These necessary surface perfections are achieved via chemical mechanical planarization (CMP) in the wafer manufacturing process, and repeatedly during both FEOL and BEOL flows to maintain global planarity between unit processes. The nanoparticles of the CMP slurry are important for achieving the desired surface finish because the slurry provides both a chemical effect via surface oxidation and a mechanical effect to remove the surface oxide layer of the work piece. In this study, we focus on the silica particles in CMP slurry. Typically, particle sizes in liquid are measured by Dynamic Light Scattering (DLS). However, there are challenges in using DLS to measure particle size and particle size distributions in CMP slurries because the signal is affected by the concentration of solvent and the surface modification of these particles. In addition, the DLS cannot measure the shape of the particles. Although the SEM and TEM image are used to check the particle shape, it’s difficult to get a quantitative value, nor can they provide information about the particle agglomeration in the liquid. Thus, a combination of methods must be used and the results correlated to obtain a full picture of the size, shape, and distribution of slurry nanoparticles; most importantly these features were connected with the final result, i.e. the surface quality of silicon wafers polished with these slurries.
The SEM images were analyzed by Image J, with background modification and binarization by auto setting of local threshold using the Otsu method, and some overlapped edges were separated using Watershed method. Using this optimized filtering and detection process, it was possible to differentiate the edges of even the particles that were overlapped. Image analysis time was also decreased 1/5 by building a macro for these processes.
The wafer polishing quality (the removal rate and the surface roughness) were compared with key particle shape values (diameter, particle size distribution, area, circularity etc…). The removal rate decreased when the particle circularity approached that of a perfect Circle. It seems that the surface oxidation as a chemical effect by particles decreased because the surface area of particles is lowest for a perfect sphere. However, there weren’t any significant differences found relating the wafer quality and the other particle shape values. More subtle effects based on these parameters might be obtained using a more sophisticated analysis method, such as a multiple regression analysis.
In conclusion, by optimized the processing method using the Image J, the quantitative value of particle parameters could be calculated from SEM images, while also improving analysis efficiency by 1/5. This analysis showed that the polishing behavior was inversely related to circularity, with more circular particles yielding a lower material removal rate than imperfect particles.

Continuous microreactor-assisted solution deposition is demonstrated for the deposition of cadmium sulfide (CdS) thin films on fluorine-doped tin oxide (FTO) coated glass. The continuous flow system consists of a microscale T-junction micromixer with the co-axial water circulation heat exchanger to control the reacting chemical flux and optimize the heterogeneous surface reaction. Dense, high quality nanocrystallite CdS thin filmswere deposited at an average rate of 25.2 nm/min, which is significantly higher than the reported growth rate from typical batch chemical bath deposition process. Focused-ion-beam was used for transmission electron microscopy specimen preparation to characterize the interfacial microstructure of CdS and FTO layers. In addition, CdS nanoparticles is formed in the continuous flow microreactor via precipitated reaction in aqueous solutions and the nanoparticle size analyzing is performed at different flow rates by using an in-situ spectroscopy technique. Applying deconvolution fitting method, this result shows the size increases from 1.13 nm to 1.26 nm for the corresponding residence time from 0.26 to 3.96 s. According to the growth curve of CdS nanoparticle, we propose the barrier controlled coalescence mechanism as a reasonable model to describe the CdS nanoparticle growth in the continuous flow microreactor. Also, applying computational fluid dynamics, low skewness value (s = 0.09) of the RTD curve at high flow rate (17 mL/min) suggests good radial mixing in the corresponded short residence time

Among the foreseeable therapeutic approaches at the cellular level, nanoplatform-driven photothermal therapy (PTT) is a thriving tool for the selective eradicationof malignant tissues with minimal side effects to healthy ones, by way of confining the cytotoxic effects of the generated
heat to the area under treatment, simultaneously promoting the accumulation of the photothermal agents at the tumor site¹. A number of near-infrared (NIR) responsive photothermal agents were proposed, as NIR radiation can penetrate deeper in biological tissues allowing for subcutaneous tumor treatment². Hence, chemically versatile, near-infrared absorbing plasmonic nanoparticles are distinctly appealing and most sought after as efficient photothermal agents.

Although many gold-based nanoparticles³ display strong NIR LSPR absorption properties⁴ and corresponding heat conversion efficiencies (HCE) as high as 80%,⁵ they are costly to produce and their size often exceeds 100 nm, a contentious trait in the context of most biological applications.⁶ Moreover, prolonged exposure to heat can result in the modification of the gold nanoparticles’ morphology, leading to changes in their LSPR absorption profile and a partial or complete loss of the heat conversion properties.⁷ These limitations are overcome with copper sulfide plasmonic nanoparticles. They absorb light over a broad NIR spectral range, are smaller, stable under prolonged optical excitation, significantly less expensive to synthesize, and display nearly analogous heat conversion capabilities compared to gold nanoparticles.⁸

In this work, a straightforward method to synthesize monodisperse PEGylated copper sulfide nanoparticles of pure covellite (CuS) phase, featuring strong localized surface plasmonic resonance absorption in the near-infrared and flexible surface chemistry, imparted by monomethyl ether polyethylene glycol molecules, is developed and optimized. These nanoparticles show a remarkable photothermal heat conversion efficiency (HCE) of 71.4% -evaluated in water upon laser excitation at 806 nm—, which is among the highest for CuS systems and rivals that of plasmonic noble metal nanostructures.

A specific merit of the proposed green synthesis approach resides in the dispersibility of the CuS nanoparticles in water and media mimicking biological fluids. A prominent advantage of these nanoparticles as photothermal agents is represented by the minimal amount of optical energy scattered or re-emitted through luminescence, leading to nearly absolute conversion of light into heat.

Altogether, the properties featured by the developed CuS nanoparticles (small size, size monodispersion, colloidal stability in biological media, tunability of the surface chemistry, and outstanding light-to-heat capability) make the system an ideal candidate for a stand-alone photothermal agent and/or a valuable building block for multipurpose biomedical nanomaterials. Another relevant finding for the photothermal community, stemming from a critical evaluation and mathematical modeling of the material’s properties and measurement methodology, is that calculated HCE values drastically depend on the parameters characterizing the experimental setup used to determine them. In particular, the dependence on the solvent absorption and other experimental conditions, such as sol concentration and optical path, calls for some degree of standardization on the measurement of HCE for potential photothermal agents in order to make the data reported universally and reliably comparable.

REFERENCES
¹ N. Bertrand et al Adv. Drug Delivery Rev. 2014, 66, 2.
² E. Hemmer et al Nanoscale Horiz. 2016, 1, 168.
³ J. D. Meyers et al Part. Part. Syst. Charact. 2015, 32, 448.
⁴ J. Zhao et al Nanomedicine 2014, 9, 2041.
⁵ V. Sebastian et al Nanoscale 2014, 6, 13228.
⁶ H. Chen et al Nat. Rev. 2017, 2, 17024.
⁷ D. Jaque et al Nanoscale 2014, 6, 9494.
⁸ B. Li et al Nanoscale 2014,6, 3274.

Magneto-plasmonic nanoparticle has been considered as a promising material in multi-modal applications because such nanoparticles exhibit magnetic and surface enhanced Raman scattering (SERS) effect simultaneously. Plasmonic nanostructures have attracted much attention due to SERS effect. The areas with localized and enhanced electro-magnetic field become “hot spots” where show high SERS activity. Recently, Raman reporter-embedded plasmonic nanostructures have been investigated for advanced signal reproducibility and sensitivity [1] which are affected by uniformity of morphology and size. [2] However, magneto-plasmonic nanoparticles have been rarely reported for SERS bio-applications. Thus, there is a great demand in synthesis of magneto-plasmonic nanoparticles as an attractive SERS nanoprobe.
Here we report a clustered magneto-plasmonic nanoparticle as a candidate for ultra-sensitive and stable SERS bio-sensing. Superparamagnetic iron oxide colloidal nanocrystal clusters (SPIO-CNCs) which are about 150 nm in diameter are synthesized by modified polyol method without surfactant. The SPIO-CNCs are composed of many primary particles approximately 10 nm in size to overcome superparamagnetic-ferromagnetic transition and particle aggregation problem. [3] Subsequently, we cover the thin SiO₂ layer onto the surface of SPIO-CNCs. Au seeds (3~4nm) are attached onto the SiO₂ layer. For obtaining high signal reproducibility and sensitivity, we execute the homogeneous Au seed-mediated growth procedure. We confirm that Au nanoparticles are densely decorated on the shell after growth process with inter-spacing between Au nanoparticles of 1~2 nm. During the growth step, we conjugate the Raman reporter onto the surface of Au nanoparticles. As a result, the Raman dye-embedded magneto-plasmonic nanoparticles exhibit robust and ultra-sensitive SERS signals based on their uniform size, morphology and existence of abundant “hot spots”.
Finally, we show that the magneto-plasmonic nanoparticles can be used efficiently for bio-imaging, bio-detecting and bio-separation about specific biomarkers. This nanoparticle with novel structure offers a great strategy for image-guided diagnosis and separation of target biomarker.

[1] Shen, W et al., Angew. Chem. Int. Ed., 54(25), 7308 (2015)
[2] Li, WY et al., Nano Lett., 9(1), 485 (2009)
[3] Ge, JP et al., Angew. Chem. Int. Ed., 46(23), 4342 (2007)

In recent years, colloidal quantum-dots based light-emitting diodes (QD-LEDs) have been considered as the attractive display device because of remarkable electrical/optical characteristics of colloidal quantum dots. QD-LEDs are of particular interest due to their wide-range color tune ability, high brightness and good color purity by narrow emission bandwidth. Challenges remain, however, in achieving the necessary multilayer device structures using printing process for display applications.
Conventional vacuum evaporate deposition to fabricate OLEDs was unsuitable for heavy metal like QDs materials. Besides, the spin coating has been the most popular approach to fabricating QD-LEDs devices due to the simple method for achieving a near monolayer of the nanoparticle. However, the relatively high cost of QDs and the loss of more than 96% of the QD solution during spin coating are strong reasons to search for an alternative deposition route capable of patterning and consuming minimum amount of QD materials.
QDS were printed by electrohydrodynamic (EHD)-Jet spray technology and were applied to all solution-processed QD-LEDs. Three different spray modes of ‘Big circular type’, ‘Spiral line’, and ‘Straight line’ were investigate to obtain uniform QD thin films. Sprayed QD pattern was also controlled with the various processing parameters to obtain uniform surface morphology.
A QD-LED has a structure of ITO/PEDOT:PSS/PVK/ EHD-sprayed QDs/ZnO/Al. Using a unique spraying technique, a QD-LEDs was successfully fabricated and demonstrated a maximum of luminance of 7801 Cd/m2, a maximum current efficiency of 2.93 cd/A, the maximum EQE is 1.41%. We believed that EHD jet printing will pave the way for the fabrication of multilayer structure devices, allowing for various solvents, toward to a direct patterning, and convenient and optimal for commercial production.

Materials based on nanometer-sized metal particles have received increasing attention due to their novel electronic, magnetic, and optical properties. Metallic nanoparticles have a wide range of applications in biomedicine, optoelectronics, energy, and industrial applications such as catalysis, automotive, aerospace and electronic components. Finely–dispersed metallic nanoparticles in a polymeric matrix can act as dielectric insulators due to their small size and a wide separation of charges. Achieving a uniform dispersion of nanoparticles in the polymer is a crucial step to obtain the optimal electrical properties for the given nanocomposite. In this research work, In situ synthesis of Ag nanoparticles in polyvinylidene fluoride (PVDF) was investigated using different stabilizers such as 3-aminopropyltrimethoxysilane (APS) and 1-dodecanethiol (thiol). Although PVDF is a matrix, it also functions as a stabilizer. Results of UV-vis spectroscopy showed that when APS or PVDF was used, Ag nanoparticles were formed. Yet no formation occurred when thiol was used due to the complexation of Ag⁺ ions by thiol. Polar groups on stabilizers has an important effect on complexation process. APS, a nitrogen-based ligand, has hard base character inhibiting the complexation between Ag⁺ and APS. Consequently, Ag⁺ ions are reduced to Ag nanoparticles in N,N dimethyl formamide (DMF), which acts as a solvent and reducing agent. The electrical properties of flexible nano-metal polymer were tested and the highest improvements in breakdown strength and energy density of 33 and 58 %, respectively were measured with 0.05 wt.% Ag content and APS as a stabilizer.

Lately discovered silver chalcogenide colloidal quantum dots with tetragonal crystal phase exhibit promising optical properties in the mid-wavelength infrared. Although colloidal synthesis of uniform sizes and shapes has been reported, the investigation of their optoelectronic properties as an active layer in photodetector devices has been scarce. Herein, we present silver selenide quantum dot photoconductive photodetectors that exhibit high responsivity in the mid-infrared under room temperature operation. We investigate the effect on the ligands as well as bias-, temperature- and spectral-dependent photoresponses. Our work demonstrate the potential of silver chalcogenide colloidal quantum dots as an promising mid-wavelength infrared absorber for the development of thermal infrared sensors and imagers with low size, weight, power consumption, and cost.

Hedgehog particles (HPs) resembling the spiky geometry of pollen grains reveal surprisingly high dispersion stability in both polar and nonpolar media. This property designated as omnidispersibility, is attributed to the substantial reduction of van der Waals interactions between particles coated with stiff nanoscale spikes compared to particles of the same dimensions with smooth surfaces. One may hypothesize but it remains unknown, however, whether HPs modified with polymers or nanoparticles (NPs) would retain this property. Such modifications will expand the functionalities of HPs making possible their utilization as omnidispersible carriers. We show that HPs carrying dense conformal coatings made by layer-by-layer (LBL) assembly maintain dispersion stability in environments of extreme polarity and ionic strength. Additionally HPs, surface-modified by multilayers of polymers and gold NPs are capable of surface enhanced Raman scattering (SERS) and overcome the limited colloidal stability of other SERS probes. The agglomeration resilience of HPs leads to greater than one order of magnitude increase of SERS intensity compared to colloids with smooth surfaces and enables simultaneous detection of several targets in complex media with high ionic strength. Omnidispersible optically active colloids open the door for rapid multiplexed SERS analysis in biological fluids and other applications.

Photon emission from many materials at low temperature is governed by optically passive “dark” states. For example, cadmium selenide nanocrystals are observed to have “dark exciton” photoluminescence lifetimes of 1 microsecond at liquid helium temperatures. This work presents ultrafast photoluminescence spectroscopy which demonstrates that an electronic excitation followed by subsequent, time-delayed IR excitation of of cadmium selenide nanocrystals converts excitons from the “dark” state, with slow emission, to the spin-allowed “bright” exciton state, which increases the instantaneous rate to <100 ps. The IR control pulse excites organic ligand molecules on the nanocrystal surface, which act as antennae, leading subsequently to heat transfer to the inorganic particle. The IR control pulse can be arbitrarily delayed to induce changes in the instantaneous rate with less than 10 ps temporal resolution. This technique has the ability to dramatically re-shape the distribution of photon emission events in ensembles and single photon sources.

Gold nanoparticles (AuNPs) are highly used in recent times due to their fascinating optical properties. AuNPs have several applications in the medical context as drug delivery, antimicrobial activity, anti-fungal, diagnoses, tissue engineering, regenerative medicine and therapeutic treatments such as cancer. One of the most remarkable phenomena that these nanoparticles have is the collective oscillation of their conduction band electrons when these electrons are illuminated with an appropriate wavelength. This phenomenon is known as Surface Plasmon Resonance, which confer intrinsic optical properties to the AuNPs. Due to the massive use of these nanoparticles in the world, eventually these nanostructures will be interacting with body waters including the sea and it could be causing negative effect in marine organisms. Based on the mentioned, the objectives of this study were (i) synthesize gold nanoparticles of different sizes in presence of sodium citrate, (ii) characterize the morphology and optical properties of AuNPs and, (iii) evaluate the interaction of synthesized nanostructures in marine crustaceans. The generation of AuNPs was realized in presence of gold solution (Au³⁺), and citrate solution as reducing agent. The precursors were heated at 100°C by using a hot plate and stirred for 10 minutes. The production of AuNPs was proved by the color change in the solution; from yellow to red wine. Purified nanoparticles by dialysis membranes, evidenced an absorption peak at 523 nm attributed to the surface Plasmon Resonance which suggested the formation of gold nanoparticles. Furthermore, the full width at half-maximum (FWHM) was estimated at 54 nm; which indicates a narrow distribution of the particle size. High Resolution Transmission Electron Microscopy technique evidenced the small size (20 nm) of the synthesized nanostructures. Also, the particles were mostly spherical and highly crystalline. Peaks at 1.65 keV and 9.87 keV obtained of the energy dispersive X-ray analyses suggest the presence of gold into the crystalline structure of the nanoparticles. Interaction tests will be carried out in presence of marine crustaceans and different concentrations of gold nanoparticles; including a control test. It is expected that gold nanoparticles (at small size) in high concentration decrease the viability of crustaceans due to oxidation from Au⁰ (as AuNPs) to Au³⁺.

Semiconductor nanomaterials like zinc sulphide have interesting potential applications, consequent to their size-dependent optical properties. These nanostructures can be used on optoelectronic, photocatalysis, solar cells, and fluorescence microscopy, among others. Due to the great use of these nanoparticles in society, there is great concern in the scientific community about the potential negative interaction of these nanomaterials in aquatic environments. The present research was addressed on generation of nanostructures of ZnS with modified surface. This work had three goals: 1) morphological, compositional an optical characterization of ZnS nanoparticles; 2) surface chemical modification of ZnS nanoparticles with biocompatible molecules; and 3) Interaction studies of ZnS nanoparticle. A main absorption peak at ~350 nm and a trap emission peak at ~ 425 nm were observed in the emission spectrum of ZnS nanoparticles. The morphology and the size of ZnS were carried out by High Resolution Transmission Electron Microscopy (HR-TEM). In this way, nanoparticles were spherical and with a size around 5 nm. Energy Dispersive X-Ray Spectroscopy evidenced the chemical composition of produced nanostructures. The chemical modification of ZnS nanostructures was corroborated by Infrared Spectroscopy Analysis. The interaction of nanoparticles of ZnS was studied in aquatic systems in presence of marine organisms. The concentrations of nanoparticles for the interaction studies ranged from 0 ppm to 2000 ppm and the contacting time was 24 or 48 hours. Also, Zn²⁺ (as zinc nitrate and zinc sulphate) was used as comparison purposes. The results evidenced that ionic Zn (as zinc nitrate and zinc sulphate) had a negative interaction in marine organisms at concentrations higher than 500 ppm, whereas Zinc sulphide nanoparticles evidenced a negative interaction only at concentrations higher than 1000 ppm. The chemical interaction between the type of organic cover of ZnS and the marine organisms will be discussed.

Thermal resistance across the interface between touching surfaces is critical for many industrial applications. Contacting interfaces are fractally rough, with locally intimate contact separated by regions with a wider gas filled boundary gap. Heat flow across the interface is heterogeneous and thus the contact model is based on a network of thermal resistors representing boundary resistance at local contacts and the access resistance for lateral transport to contacts. In a previous work [1], we developed a network model to predict the macroscopic thermal resistance of mechanically contacting surfaces. The results of molecular dynamics simulations to characterize boundary resistance of Silicon Alumina interfaces and the Boltzmann transport simulations of access resistance in Si in the ballistic transport regime were presented. In the present work, we review some of the current state-of-the-art and highlight the advanced models developed including surface characteristics such as roughness at interfaces. Additionally, we report some of the results of analyses when the components are subjected to variables such as pressure and temperature. This study is of practical significance in understanding the influence of surface roughness in thermal transport across interfaces at nanoscale to macroscale (i.e., multiscale) for applications such as thin films in the semiconductor industry, and mechanical components in industrial equipment.

1. Characterizing Macroscopic Thermal Resistance Across Contacting Interfaces Through Local Understanding of Thermal Transport, MRS Advances, Volume 3, Issue 44, 2018, pp. 2735-2741

Selectively permeable membranes with high flux and selectivity are important for many applications including water desalination, H₂ purification, CO₂ capture, as well as selective ion transport. Reduced membrane thickness and precise pore size/chemistry control are the keys for achieving combined high flux and selectivity. Membranes in natural biological systems can be down to 4 nm in thickness and the pores are precisely constructed by molecular assembly, leading to unbeatable performance when compared to synthetic industrial membranes that are difficult to be fabricated with similar molecular level precision and are typically 100-1000 times thicker. ALD is a layer-by-layer deposition method that builds up a thin layer with atomic precision in structure and compositions. Here we introduce the membrane fabrication by the combination of colloidal self-assembly and a “plasma-defined” ALD process where the location of ALD modification is confined by plasma irradiation. Using this approach, functional groups/species for fast CO2 transport have been successful integrated into the hierarchically structured sub-20nm thick ultra-thin membranes and excellent performance in CO₂ separation was achieved.

Polymer-assisted solution method is an aqueous chemical route. It employs metal ions coordinated to polymers as precursor. The use of polymer as the binding agent for metal ions has several advantages. First, the formation of covalent complexes between the lone pairs on the nitrogen atoms of the polymer and the metal cations makes it possible to prepare almost any metal precursor solutions that are impossible to achieve by the commonly available chemical solution techniques. This feature makes it feasible for physical filtration to remove unbound low molecular weight cations or anions. Secondly, the viscosity of the solution can be effectively controlled by the polymer. Thirdly, the precursor solutions are environmentally friendly as metal salts and commercially available polymers are used as the source materials. The unique chemistry and processing design deliver stable and homogeneous solutions at a molecular level that allows high-quality growth of nanoparticles, thin films, and desirable patterns for a wide range of applications.

As a new fluorescent material, carbon spots have been widely used in biomarkers, imaging, nanopharmaceuticals and so on because of their low toxicity, excellent biological compatibility, and low cost. At the same time, due to its stable structure and high luminous efficiency, it is also expected to be used as a building element for optoelectronic functional materials and devices. The color regulation of fluorescent carbon spot, especially the long wavelength luminescence, needs further study. For carbon dots, they can be regarded as semiconductor quantum dots, and their luminescence can be seen as the transition of electrons from valence band to conduction band under the excitation of light. The electron on the band reacts with the hole in the valence band. The position of the luminescence peak is strongly dependent on the band gap, that is, the band gap. According to the semiconductor theory, the bandgap can be achieved by doping heteroatoms. Therefore, we put forward the concept of hetero-atom doping, and we obtain the doped carbon points by doping the atoms so that the luminous color can be adjusted [4-8]. In addition, the structure of carbon spots is considered to be a kind of graphite fragments coated with a variety of functional groups, in which graphite fragments are graphite or graphene structures with conjugated structures. We know that the light absorption and luminescence of organic compounds are directly related to the length of their conjugated chain breaks. If the length of the effective conjugate chain breaks is increased, the light absorption and luminous redshift will occur. Therefore, we can use the reaction conditions to increase the effective conjugate length of carbon points, or the size of the sp2 domain, or the degree of graphitization to achieve the regulation of luminescence [9-10].
Reference:
[1] S. J. Zhu, Y. Song, & B. Yang, et al.,Nano Research, 2015, 8, 335-382
[2] M. Zheng, Z. Sun, et al., Advanced Materials 2014, 26, 3554-3560
[3] M. Zheng, Z. Sun, et al., ACS Nano 2015, 9, 11455–11461.
[4] D. Qu, Z. Sun, et al.,Nanoscale 2013, 5 (24), 12272-12277
[5] M. Zheng, Z. Sun, et al.,ACS Appl. Mater. Interfaces 2013, 5 (24), 13242-13247
[6]D. Qu, & Z. Sun, et al.,Sci. Rep., 2014, 4, 5294
[7] X. Miao, & Z. Sun, et al.,ACS Applied Materials & Interfaces, 2017, 9, 18549-18556
[8] D. Qu, & Z. Sun ,et al., J. Mater. Chem. B, 2017, 5, 4988-4992.
[9] D. Qu, & Z. Sun et al., Light: Science & Applications 2015, 4, e364
[10] X. Miao & Z. Sun, et al.,Advanced Materials, 2018, 30, 1704740

The self-assembly of colloidal nanocrystals (NCs) into superstructures provides an efficient bottom-up pathway for constructing new functional materials. Among various superstructures, NC superlattices represent an entirely new class of solid-state materials with unique collective properties and wide applications. However, conventional NC superlattices are inappropriate structural motifs for energy applications due to the impeded molecular accessibility of NCs by foreign species. In pursuit of widening the range of applications of NC superlattices, it is desirable to develop new architectures that enable full access of NC constituents while simultaneously facilitating mass transport properties. Herein, I will focus on low-dimensional, hierarchical NC superlattices with rationally designed structures and mesoscale morphologies toward high-performance energy storage and conversion. I will also spend half of the time to cover NC superlattice derivatives, especially ordered graphene-like frameworks, which are emerging as a new class of mesoporous carbon materials. By exploiting the versatile surface chemistry and self-assembly behaviors of colloidal NCs, a rich array of mesoporous graphene-based materials can be designed, which show great promise as material platform for energy applications.

Metal chalcogenide semiconductors continue to show promise as potential low-embedded energy, scalable and inexpensive inorganic alternatives to Si-based photovoltaics (PV). Among these, the quinary I₂-II-IV-VI_x-VI_(4-x) family, which includes Cu₂ZnSnS₄ (CZTS)_, Cu₂ZnSnSe₄ (CZTSe) and Cu₂ZnSn(S/Se)₄ (CZTSSe), are among the most lucrative due to their earth-abundant and non-toxic nature. The fabrication of such materials using nanoparticle routes remains among the most promising for PV due to their potential to be inexpensive and scalable, while maintaining superior phase and compositional control when compared to non-solution based approaches. Here we describe the facile synthesis of aqueous CZTS and CZTSe nanoparticles through a versatile, stoichiometric, one pot synthetic method that produces scalable dispersions without the use of long-chained organic ligands. These materials lend themselves towards the fabrication of carbon-free absorber layers for solar cells. Towards this goal, here we describe a simple process to produce CZTSSe by depositing mixed amounts of CZTS and CZTSe nanoparticle inks followed by a mild heat treatment. The resulting materials exhibit a single-phase kesterite structure and tunable optical bandgaps of between 1.51 and 1.05 eV, which are ideal for single and multi-junction solar cell devices.

To date, it has not been possible to combine the high optical quality of silver particles with the good chemical stability and synthetic convenience in a fully aqueous system, while simultaneously allowing chemical surface functionalization.
We present a general synthetic approach to achieve narrow distributions in size, chemical variability, and oxidant stability, which are well-known for gold nanocrystals, while maintaining the advantageous plasmonic properties of silver.
This is achieved by consequent control of the growth via balancing the reduction potentials and enforcing adequate reaction kinetics (“living reaction conditions”).[1]
Exploiting specific gold crystals as seeds for silver overgrowth via facet-selective capping allows for the preparation of a variety of morphologies with sharp edges/tips and extraordinary narrow distributions. These nanoparticles inherit low dispersity from their masked gold cores, yet simultaneously exhibit the strong plasmonic properties of silver. Here, we will focus on the growth of two differing systems, silver nanocubes and silver nanowires, in order to highlight the generalization of the employed synthesis protocols.
Finally, protecting the silver surface with a thin gold layer (~1 nm) enables oxidant stability and functionality without altering the Ag-controlled optical properties.[2] This combines both worlds, optical quality and chemical stability, and is not limited to a specific particle shape.
This wet-chemical approach opens new possibilities in scalable bottom-up fabrication, especially integration with plasmonics applications requiring long-term stability. Exploiting the advantages of silver-based plasmonics boosts the sensitivity, but the oxidation resistance and chemical functionality of the fully-accessible noble-metal surface is maintained. In combination with the nanometer precision of colloidal self-assembly, a renewed prominence of bottom-up materials for optoelectronics, light harvesting, photocatalysis, sensing and therapeutics can be expected.

[1] M. Mayer, A.M. Steiner, F. Röder, P. Formanek, T.A.F. König, A. Fery, Angew. Chem. Int. Ed. 2017, 56, 15866.
[2] M. Mayer, L. Scarabelli, K. March, T. Altantzis, M. Tebbe, M. Kociak, S. Bals, F.J. García de Abajo, A. Fery, and L.M. Liz-Marzán, Nano Letters 2015 15, 5427.

Photonics and catalysis are at the heart of our future transition to a more sustainable society. Colloidal nanocrystals may contribute to this goal, because many photonic and catalytic processes strongly rely on an exquisite manipulation of materials’ physicochemical attributes (e.g., shapes, sizes, compositions, structures, surface chemistry, etc.), which is an original potential of colloidal chemistry. In practice, reasons for insufficiently realized gains from the superior potential in many areas of sustainable technologies include particularly a lack of fundamental understanding of and thereby incapability of target-orientated engineering of promising nanocrystals. In this talk, after a brief discussion of photonics and catalysis in sustainability and colloidal chemistry, I will present my recent work to illustrate how rationally designed nanocrystals can kindle intriguing photonic and catalytic applications, which include “black-body” nanocrystals for solar steam generation,^1-3 multifunctional nanocrystals for catalytic hydrogenation and surface-enhanced Raman scattering,^4-5 and Cu-based nanocrystals for electrochemical CO₂ reduction.^6-7

References:
1. Huang, J.; Liu, C.; Zhu, Y.; Masala, S.; Alarousu, E.; Han, Y.; Fratalocchi, A. Harnessing Structural Darkness in the Visible and Infrared Wavelengths for a New Source of Light. Nat. Nanotech. 2016, 11(1), 60.
2. Huang, J.; Zhu, Y.; Liu, C.; Shi, Z.; Fratalocchi, A.; Han, Y. Unravelling Thiol’s Role in Directing Asymmetric Growth of Au Nanorod-Au Nanoparticle Dimers. Nano Lett. 2016, 16(1), 617.
3. Liu, C.^#; Huang, J.^# (Equal Contribution); Han, Y.; Fratalocchi, A. High Performance Large-scale Solar Steam Generation with Nanolayers of Reusable Biomimetic Nanoparticles. Adv. Sustain. Systems. 2017, 1, 1600013.
4. Huang, J.; Zhu, Y.; Lin, M.; Wang, Q.; Zhao, L.; Yang, Y.; Yao, K. X.; Han, Y. Site-specific Growth of Au–Pd Alloy Horns on Au Nanorods: a Platform for Highly Sensitive Monitoring of Catalytic Reactions by Surface Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 2013, 135 (23), 8552.
5. Huang, J.; Zhu, Y.; Liu, C.; Zhao, Y.; Liu, Z.; Hedhili, M. N.; Fratalocchi, A.; Han, Y. Fabricating a Homogeneously Alloyed AuAg Shell on Au Nanorods to Achieve Strong, Stable, and Tunable Surface Plasmon Resonances. Small 2015, 11 (39), 5214.
6. Huang, J.; Hormann, N.; Oveisi, E.; Loiudice, A.; De Gregorio, G.; Andreussi, O.; Marzari, N.; Buonsanti, R. Potential-induced Nanoclustering of Metallic Catalysts during Electrochemical CO₂ Reduction. Nat. Commun. 2018, 9, 3117.
7. Huang, J.; Mensi, M.; Oveisi, E.; Mantella, V.; Buonsanti, R. Enhancing Electrochemical Reduction of CO₂ into C₂ products using CuAg Nanodimers. Submitted, 2018.

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.
[3] Wood C, …., Stevens MM, 2018, Bringing Mobile Health Connected Infectious Disease Diagnostics to the Field, Nature, 2018 in press.

Driving forces for oriented attachment-based crystallization and assembly depend on azimuthal crystallographic alignment between nanoparticles. The types of driving forces that can be sensitive to relative orientation include Coulombic, van der Waals (vdW), hydration, and ion correlation forces. The theoretical underpinnings of these anisotropic forces are well established, but techniques that can directly measure them for a given pair of interacting oriented crystal faces have generally been limited to use of macroscopic yet atomically flat single crystals. We report measurement of anisotropic forces between rutile TiO2 (001) nanocrystals as a function of their azimuthal orientation and surface hydration extent using a combined environmental transmission electron microscopy-atomic force microscopy (AFM) technique. Atomically flat rutile TiO2 (001) AFM tips and opposing rutile TiO2 (001) substrates were fabricated by focused ion beam milling to excise nanocrystals from the surface of a single monolith that was pre-oriented, cut, and polished to prepare the (001) face. At tens of nanometers of separation, the attractive forces are weak and show no dependence on azimuthal alignment nor surface hydration. At separations of approximately one hydration layer, attractive forces are strongly dependent on azimuthal alignment and systematically decrease as intervening water density increases. Measured forces closely agree with predictions from Lifshitz theory and show that dispersion forces are capable of generating a torque between particles interacting in solution and between grains in materials. Furthermore, we report the measurement of anisotropic forces between ZnO (0001) and ZnO (0001) nanocrystals in Zn(NO3)2 solution using AFM based dynamic force spectroscopy (DFS). By measuring the jump-from-contact force as a function of tip retraction rate, the rupture force at equilibrium and the corresponding adhesion free energy as a function of azimuthal alignment is directly obtained. Compared with the potential of mean force (PMF) from large-scale molecular dynamics (MD) simulations, the sensitivity to crystallographic registry between nanocrystals at close range arises, at least in part, from the self-organization of intervening water molecules in response to the structures and alignment of the opposing surfaces.

Abilities to control the size and shape of nanocrystals in order to tune functional properties are an important grand challenge. Here we report a surfactant self-assembly induced micelle encapsulation method to fabricate porphyrin nanocrystals using the optically active precursor zinc porphyrin (ZnTPP). Through confined non-covalent interactions of ZnTPP within surfactant micelles, nanocrystals with a series of morphologies including nanodisk, tetragonal rod, and hexagonal rod, as well as amorphous spherical particle are synthesized with controlled size and dimension. A phase diagram that describes morphology control is achieved via kinetically controlled nucleation and growth. Due to the hierarchical ordering of the porphyrins, the nanoparticles exhibit collective optical properties resulted from coupling of molecular porphyrins and photocatalytic activities such as photo-degradation of methyl orange (MO) pollutants, hydrogen production and PDT for cancer. This simple ability to exert rational control over dimension and morphology provides new opportunities for practical applications in photocatalysis, sensing, and nanoelectronics.

N-heterocyclic carbenes (NHCs) are excellent ligands that bind to a variety of metal ions in Group 10/11 with extremely high binding strength via σ donation. However, there are only a few studies of polymer NHC ligands for metal nanoparticles (NPs) in literature, largely due to synthetic challenges of carbine, in particular under aqueous conditions. We present a new method to synthesize metal nanoclusters and NPs with NHC-functionalized polymers as surface ligands. Two approaches will be discussed in this talk, including in situ reduction and transmetallization. NHC-functionalized polymers can be prepared using simple post-polymerization functionalization of polymers capped with halogen atoms (e.g., Cl and Br) synthesized via atom transfer radical polymerization (ATRP). When reacted with N-methyl imidazole, halogen-ended polymers can be converted to yield imidazolium salt and further transferred into Cu(I)-NHC-capped polymers in presence of a weak base, e.g., K₂CO₃ and Cu(I)Cl. We first show that Cu-NHC-polymer can be reduced chemically in solution to form Cu nanoclusters with the bright blue fluorescence. Polymer NHC ligands can stabilize Cu nanoclusters from oxidation as well. In the second approach, we demonstrate that Cu(I)-NHC-capped polymers can be used to modify metal NPs using transmetallization in any good solvents of polymers. The surface modification of citrate-capped gold NPs has been examined using a ligand exchange method in an emulsion solution. We show the successful surface modification of Au/Ag/Cu NPs using both hydrophobic and hydrophilic polymers, including polystyrene, poly(methyl methacrylate)), poly(n-butyl acrylate)) and poly(2-(2-methoxyethoxy)ethyl methacrylate)) functionalized with carbenes.

Interaction with light on the nanoscale is a very active research field, nanophotonics. Commonly, these nanostructures are made from metallic or dielectric materials, while nanostructures from active materials such as semiconductors, for example used to make integrated circuits, require intricate lithography with photoresists as an intermediate fabrication step. Here we show that Colloidal Quantum Dots (CQDs) can be used to directly write patterns on the nanoscale from the active semiconductor material. CQDs are nanocrystals of semiconductor materials, owing their tunable optical, chemical, and electrical properties to quantum confinement size effects and surface chemistry. These tunable properties make CQDs useful for different semiconductor applications including photovoltaics and lighting. We directly pattern quantum dots using e-beam lithography in order to obtain high resolution structures of CQDs at the nanoscale. This technique is applicable to different CQD systems, such as CdSe and PbS without affecting the luminescent properties. We present several examples of nanostructures and show that the resolution can be on the order of tens of nanometers. The chemical mechanism responsible for the observed solubility switch is investigated. We believe this technique shows promise for manufacturing of photonic structures at high volume, directly from the active materials.

Driven forward by advantages that arise with nanoscale structures, a number of synthetic protocols and approaches have been developed to achieve a high level of material control at the nanoscale, particularly through using colloidal approaches. An important, and intensely investigated, class of materials at the nanoscale are quantum confined semiconductor nanoparticles. In recent years quantum confined systems have received intense interest due in large to the potential commercial applications, including solid-state lighting, high-efficiency color displays, light emitting diodes, and bio-imaging, among others. Each of these applications have a number of specific materials performance requirements which can be accessed by a battery of measurements; however, a common parameter for these techniques is the photoluminescent quantum yield (PLQY), a quality factor describing the radiative and non-radiative rates of the material that is limited to 100% (excluding carrier multiplication processes and the like). The culmination of several decades of research has led to the ability to routinely synthesize specific compositions and epitaxial structures of quantum dots with efficiencies up to around 95%. While it would be advantageous to reliably access particle ensembles that present efficiencies as near-unity as possible, the achievable efficiencies at scale have become appropriately high for the commercialization of a number of applications. However, the efficiencies are insufficient for an entire class of potential applications that require a minimum amount of energy to be lost non-radiatively, such as optical refrigeration, thermophotovoltaics, and thermal energy storage in optical cavities. This inquiry naturally leads to interesting questions: What applications become possible when the PLQY is 99%, or 99.9%, or higher? What are the synthetic manipulations required to access these ever more efficient ensemble of particles? And ultimately, how does one measure the performance of these materials as losses become orders of magnitude less than current measurement techniques are capable of measuring? To address these questions we have been working to develop measurement techniques that instead of measuring the successful radiative recombination events, measures the heat generated from non-radiative events directly, events that are increasingly rare as the PLQY approaches unity, offering us access to a measurement technique with 2-3 orders of magnitude less uncertainty than traditional techniques. Informed by feedback from our newly developed measurement technique, we have the unprecedented ability synthetically optimize quantum dot syntheses through a feedback cycle. We have the ability to routinely produce sizable ensembles of wurtzite-CdSe/CdS epitaxially shelled particles with efficiencies in excess of 99%. Moving forward, the exploration of photonics in these materials at truly thermodynamic limits is tied to fundamental aspects of atomically precise materials synthesis and ultimately to the ability to measure progress in increasingly precise methods. To this end we are working to continually improve material quality, informed by newly developed measurement techniques in order to potentially access new applications at the thermodynamic limits of photonics.

The optoelectronic properties of semiconductor nanocrystals (NCs) have led to efforts to integrate them as the active material in light-emitting diodes, solid-state lighting, and lasers. Understanding related high carrier injection conditions is therefore critical as resultant thermal effects have been shown to impact optical properties. We explore the fundamental nature of these thermal phenomena using time resolved X-ray diffraction to probe the structural response of NCs of a range compositions to high fluence photoexcitation. Some NCs reversibly lose crystalline order, or melt, while others undergo photoinduced reversible phase transitions. By understanding these thermal deformations, NCs in display and lasing applications can be designed to be more structurally resilient improving device performances.

Aerogels are highly porous and light-weight materials, which possess many unique properties: low density, short free mean path of diffusion, high porosity, and large surface area. Therefore, they have a wide range of applications in thermal and acoustic insulation, catalysis, water purification, drug delivery, energy storage, and space exploration.
Aerogels are promising in various product markets, but traditional inorganic aerogels such as silica aerogels are mechanically fragile, readily fragmented, and inflexible, which severely impedes their commercialization. Here, we present a flexible inorganic aerogel, germanium aerogel, built from one dimensional germanium nanowires (Ge NWs). Ge NWs were produced by a supercritical fluid-liquid-solid (SFLS) process which enables a large-quantity synthesis. Due to the superior fracture resistance (two orders of magnitude higher than that of bulk Ge) and high aspect ratio of Ge NWs, these aerogels exhibit flexibility and a high porosity (99%). This is done without using cross linkers or surfactants. The gelation is simply achieved by physical entanglement of these Ge NWs when the desired NW concentration has been reached. The van der Waals force at each NW contact point maintains the aerogel integrity. Two drying methods, supercritical drying and freeze drying, have been explored. Freeze dried Ge NW aerogels contain parallel aligned micrometer channels with a typical diameter of ~10 µm and the supercritical dried samples have a more uniform NW network with nanosized pores. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy have shown that the surface of as-synthesized Ge NWs is covered by an organic layer. Therefore, the Ge NW aerogel exhibits superhydrophobicity. Considering that Ge can be oxidized by air to water soluble GeO_x, the protection due to the organic layer greatly increases the resistance to humidity.
In addition, since Ge has unique physical and chemical properties, this work has opened new possibilities for aerogel applications. The quantum confinement in nanosized building enables the synthesis of aerogels with tunable luminescence. The highly porous structure can accommodate the large volume change when Ge NW aerogels are used for lithium or sodium ion batteries. The well-developed surface functionalization chemistry of Ge NWs makes them promising for use in sensor and catalysis applications.

Although excimers—excited dimers—are common in molecular fluorophores, this work presents evidence for the formation of excimers in a new class of materials: atomically-precise two-dimensional semiconductor nanoplatelets. Colloidal nanoplatelets of CdSe display two-color photoluminescence resolved at low temperatures, with one band attributed to band-edge fluorescence and a second, red band attributed to excimer fluorescence. Previously reasonable explanations for two-color fluorescence, such as charging, are shown to be inconsistent with additional evidence. As with excimers in other materials systems, excimer emission is increased by increasing nanoplatelet concentration and the degree of co-facial stacking. Consistent with their promise as low-threshold gain media, amplified spontaneous emission emerges from the excimer emission line.

Counterfeit goods have become a global issue that costs tremendous economic loss and security problems. Here we report on a one-pot photochemical synthetic method to produce dendritic silver nano-particles (AgNPs) promising for physical taggants. Unlike conventional techniques such as holograms and DNA signatures, this method can readily generate large quantities of dendritic AgNPs in 20 minutes under UV illumination and ambient conditions, which possesses great economic value and is desired for large-scale manufacturing. The interplay between silver nucleation and growth could significantly affect the product shape and size. We demonstrate that the tuning of experimental conditions, such as the UV illumination time and the concentration of reducing agent, poly(allylamine), indeed creates unique dendritic structures with distinctive morphological characteristics. All the dendrites possess a singular set of minutiae that lead to a vast number of unclonable features. As the AgNP sizes reach 10-100 μm, the structural information may be quickly read and analyzed by optical microscopy, which has great potential for anti-counterfeiting labels in supply chains.

The promising of anisotropic plasmonics in state-of-art researches lies in orientational control of nanostructures, which is most invariably based on substrate regulation or single-structure measurements. Magneto-optics enables active control yet is lacking general approach towards multifunctional nanostructures. Herein, we report an unconventional templated approach towards coupled magnetic-plasmonic nanostructures. Particularly, the magnetic nanorods inside templates not only regulate the seeded growth but also provide magnetic tunability of their optical properties. By controlling the repining of small Au seeds during the early stage of crystal growth, hybrid Au/Fe₃O₄ nanorods or uniform Fe₃O₄@Au core/shell nanostructures can be produced. This approach can be extended to different metallic nanostructures, like Ag or Cu, other shapes (plates, rods or cubes) or sizes. The plasmonic peak positions of coupled nanostructures can be tuned from visible to infrared region while their absorption/scattering can be finely tuned by the thickness of the metallic nanostructures during synthesis. More interestingly, their physical properties, like plasmonic excitation modes, optical scattering and photothermal conversion can be readily controlled by a magnet, providing vast opportunities for smart nanomaterials. For example, by controlling their orientation in polymer films, they can sense both linear (rotating and bending) and non-linear (twisting) mechanical perturbations. Also, magnetic tuning provides active modulation of imaging signals for biological background-free imaging, which can either enhance or suppress the contrast signals on demand.

Colloidal synthesis of metal nanoparticles has shown tremendous control over size and shape. Understanding how ligands affect nucleation and growth is crucial in controlling size, shape and their distributions. In this talk we show experimental results elucidating the crucial role ligands play in controlling the rates of nucleation and growth and how the ligands contribute to size focusing during the synthesis of Pd and Ir nanoparticles. Using in-situ small angle x-ray scattering, isothermal titration calorimetry and density functional theory calculations, we measured the rates of nucleation and growth, and the M-L binding energies (M = Pd or Ir and L is the ligand) for different ligands. We show that the rates of nucleation and growth are correlated with the M-L binding energies for the metal precursor and metal nanoparticle, respectively. Interestingly, the correlation between nucleation and growth rates and M-L binding energies appears to follow a Brønsted-Evans-Polanyi (BEP) type linear relationship. Moreover, using population balance kinetic modeling, we demonstrate that size focusing of colloidal nanoparticles is largely due to growth rate constant that decreases with ligand coverage on the surface of the nanoparticles through a BEP relationship. This makes growth slower on larger nanoparticles (due to higher coverage), allowing the smaller ones to catch up, thereby focusing the size distribution. The consequences of the M-L binding strength on the size and size distribution will be discussed in relationship to our results and the literature.

Sensing strategies utilizing Förster resonance energy transfer (FRET) are widely used for probing biological phenomena. FRET sensitivity to the donor–acceptor distance makes it ideal for measuring the concentration of a known analyte or determining the spatial separation between fluorescent labels in a macromolecular assembly. The difficulty lies in extracting the FRET efficiency from the acceptor-induced quenching of the donor emission, which may contain a significant non-FRET contribution. Here, we demonstrate a general spectroscopic approach for differentiating between charge transfer and energy transfer (ET) processes in donor–acceptor assemblies and apply the developed method for unraveling the FRET/non-FRET contributions in cyanine dye–semiconductor quantum dot (QD) constructs. The present method relies on correlating the amplitude of the acceptor emission to specific changes in the donor excitation profile in order to extract ET-only transfer efficiencies. Quenching of the donor emission is then utilized to determine the non-ET component, tentatively attributed to the charge transfer. We observe that the latter accounts for 50–99% of donor emission quenching in QD-Cy5 and QD-Cy7 systems, stressing the importance of determining the non-FRET efficiency in a spectroscopic ruler and other FRET-based sensing applications.

Rational design of nanocrystals (NCs) can lead to material systems with fascinating properties. The combination of different materials in single crystalline heterostructures with precisely tunable compositions, size and domain functionalities represents a powerful approach for creating novel functional building blocks with tailored electronic energies and charge relaxation dynamics. These nanoblocks become a unique platform for exploring fundamental semiconductor physics at the nanoscale and can be used to fabricate novel materials with controllable application-targeted characteristics.
Here we report a novel synthetic approach to achieving high-quality heterostructures based on epitaxially joined semiconductor and metal components. Importantly, this synthetic technique enables us to tackle a long-standing problem of uniform epitaxial shell growth around irregularly faceted NCs including synthesis of core/shell nanostructures with an extremely large lattice mismatch. In one example, we target an “inverted” CdSe/PbSe quasi-type II structure wherein an electron is delocalized across the entire NC while a hole is localized within the shell region. This allows one to easily access both charge carriers which is in contrast to well-known PbSe/CdSe NCs wherein holes are core localized and thus unavailable for extraction.¹ In analogy with PbSe/CdSe NCs, CdSe/PbSe NCs show dual emission in the visible and infra-red (IR) with peak positions dependent upon the core and shell dimensions. The observation of visible emission indicates slowed carrier relaxation from higher-energy core to lower- energy shell states. This is known to lead to improved carrier multiplication (CM) or Auger up-conversion depending on energies of core and shell-based excitons.^2-3 Using transient photocurrent spectroscopy, we study charge carrier dynamics in conductive arrays of CdSe/PbSe NCs to evaluate photo-transport characteristics of both electrons and holes.⁴ Additionally, we take advantage of this inverted architecture for creating all-inorganic materials wherein quantum-confined CdSe NCs are encapsulated into a polycrystalline PbSe matrix. These composites show good n-type characteristics upon incorporation into field-effect transistors with mobilities of ca 0.1 cm²/(Vs) and the ON-OFF current ratio of more than 10⁵. Furthermore, we demonstrate that the developed synthetic methodology is highly universal and can be applied to fabricating monocrystalline heterostructures based on combinations of various semiconductors as well as semiconductors and metals (e.g., Au/PbSe).
References:
1. Lin, Q.; Makarov, N. S.; Koh, W.-k.; Velizhanin, K. A.; Cirloganu, C. M.; Luo, H.; Klimov, V. I.; Pietryga, J. M., Design and Synthesis of Heterostructured Quantum Dots with Dual Emission in the Visible and Infrared. ACS Nano 2015, 9 (1), 539-547.
2. Cirloganu, C. M.; Padilha, L. A.; Lin, Q.; Makarov, N. S.; Velizhanin, K. A.; Luo, H.; Robel, I.; Pietryga, J. M.; Klimov, V. I., Enhanced carrier multiplication in engineered quasi-type-II quantum dots. Nature Communications 2014, 5, 4148.
3. Makarov, N. S.; Lin, Q.; Pietryga, J. M.; Robel, I.; Klimov, V. I., Auger Up-Conversion of Low-Intensity Infrared Light in Engineered Quantum Dots. ACS Nano 2016, 10 (12), 10829-10841.
4. Fidler, A. F.; Gao, J.; Klimov, V. I., Electron–hole exchange blockade and memory-less recombination in photoexcited films of colloidal quantum dots. Nature Physics 2017, 13, 604.

Polymer nanocomposites with embedded functional nanoparticles (NPs) are a rich and complex system, where the polymer can drive assembly of the NPs, and the NPs can be used to actuate the polymer. When plasmonic NPs, such as Au NPs, are dispersed within polymers, their organization determines the interparticle distances and extent of coupling of their surface plasmons. The objective of this study was to investigate plasmon coupling of spherical Au NPs with an average diameter of 17 nm in shape memory polymer (SMP) films before and after mechanical stretching and then after thermally driving shape recovery. Scanning electron microscopy confirms that the Au NPs initially form clusters within the SMP. During stretching, a significant polarization-dependent response develops, where the optical absorbance maximum corresponding to the surface plasmon resonance is redshifted by up to 22 nm and blueshifted by up to 12 nm for polarization parallel and perpendicular to the stretching direction, respectively. This result can be explained by the Poisson effect, where stretching pulls the Au NPs apart and pushes them closer together, parallel and perpendicular to the stretching direction, respectively, resulting in the correspondingly weaker and stronger plasmon coupling. Heating or lack of heating during stretching dramatically affects the recovery process. If the sample is not heated during stretching, the polarization dependence vanishes after shape recovery, while polarization-dependent optical properties remain for samples heated during stretching. Simulations of the polarized optical responses of Au NP dimers at different distances are consistent with experimental results for the clusters of Au NPs in the SMP films and allow estimation of the average interparticle spacings. The finding that the stretching temperature affects the assembly and optical properties after shape recovery implies that the reversibility of the effects of stretching can be controlled and adjusted.

Remarkable recent advances on Au₂₅(SR)₁₈ nanoclusters have led to significant applications in catalysis, sensing, and magnetism. However, the existing synthetic routes are complicated such as the use of complex polyacrylamide gel electrophoresis-based nanoparticle separation technique, particularly for the water-soluble Au₂₅(SG)₁₈ nanoclusters. Here we report a simple one-pot concentration and temperature controlled method for rapid synthesis of the Au₂₅(SG)₁₈ nanoclusters in as little as 2 h without the need for a low-temperature reaction or even stirring. A systematic time-based investigation was carried out to study the effects of volume, concentration, and temperature on the synthesis of the nanoclusters. This high-temperature method decreases the synthesis time for the nanoclusters by over 10 folds than the currently existing procedures and does not need an inert atmosphere, low temperature (e.g. 0 °C) or stirring, making it an extremely simple and cost-effective process. Furthermore, we, for the first time, discovered that the Au₂₅(SG)₁₈ nanoclusters exhibit excellent photothermal activities. It was found that 0.75 mg/mL Au₂₅(SG)₁₈ nanoclusters can achieve 100% cell death for MDA-MB-231 breast cancer cells at a power of 10 W/cm² of an 808 nm laser source, which demonstrated great potential of Au₂₅(SR)₁₈ nanoclusters for photothermal therapy.

Various synthetic handles have been identified over the years, while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties.Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. In this presentation, we will describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them.

As a representative example, we will describe a case study wherein pyramid-shaped wurtzite cadmium selenide (CdSe) nanocrystals (NCs) were synthesized by hot injection in the presence of chloride ions as shape-directing agents, which exhibit reversed crystal polarity compared to former reports. Advanced electron microscopy techniques (image-corrected HRTEM with exit wave reconstruction and probe-corrected HAADF-STEM) indicate that the polar [000-1] facet constitutes the triangular base of the pyramids while the opposite [0001] facet had been reduced to a tip signifying the merging point for the three lateral facets. Based on a simple model of binding of Cl atoms to surface Cd atoms, Density Functional Theory calculations support the experimental evidence that chloride ions confer higher thermodynamic stability to the (000-1) facet over the (0001). This finding of differential relative stability of the two polar facets of wurtzite CdSe is contrary to that of previously reported experiments and calculations on NCs in which no chloride-based additives were deliberately used in the synthesis or no chlorine atoms were considered in the binding models. Self-assembly of these pyramids in a peculiar clover-like geometry, triggered by the addition of oleic acid, suggests that the basal (polar) facet has a density and perhaps type of ligands significantly different from the other three facets, since the pyramids interact with each other exclusively via their lateral facets.

Another relevant case study that will form part of this presentation will include the description of colloidal synthesis of fluorine doped indium oxide cubes where fluorine was found to be both a morphology descriptor as well as an aliovalent dopant. Single crystalline 160 nm sized F:In₂O₃ cubes enclosed by (100) facets and concave cubes were synthesized using a colloidal heat-up method. The presence of fluorine was found to impart higher stabilization to the (100) facets through density functional theory (DFT) calculations based on a simple model of F-substitution of surface O-sites. These calculations support the experimentally evident cubic morphology resulting from surface binding of F-atoms as confirmed by electron microscopy and X-ray photoelectron spectroscopy (XPS). These cubes exhibit narrow, shape-dependent multimodal LSPR extinction peaks due to corner- and edge-centered modes that are visualized by electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM).


References:
1. The Many “Facets” of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals. Sandeep Ghosh and Liberato Manna Chem. Rev. 118(16), 7804-7864 (2018).
2. Pyramid-Shaped Wurtzite CdSe Nanocrystals with Inverted Polarity. Sandeep Ghosh, Roberto Gaspari, Giovanni Bertoni, Maria Chiara Spadaro, Mirko Prato, Stuart Turner, Andrea Cavalli, Liberato Manna, and Rosaria Brescia ACS Nano 9 (8), 8537-8546 (2015).

Colloidal semiconductor nanocrystals have many impressive size-tunable properties when considered as individual isolated materials. Building on this, many interesting emergent properties may arise if the electronic states in the individual quantum dots are strongly and controllably coupled. This endeavor is difficult because the native organic ligands on colloidal nanocrystals present a large (>1eV) tunneling barrier preventing strong optical or electronic coupling between quantum dots. Groups have successfully lowered these barriers by removing the organic ligands and letting the inorganic cores crystallographically fuse.(1) While crystallographically coherent interfaces are desirable, it can also lead to the formation of structural defects such as dislocations,(2) which can have deleterious electronic consequences. Considerable research has been done on attaching lead chalcogenide (PbX X=S, Se, Te) nanocrystals since crystallographic attachment is easily achieved since the surface ligands are weakly bound. Recently we have carefully studied dislocation removal from attached PbX nanocrystals using in-situ HRTEM, and proposed design rules to minimize dislocations resulting from imperfect attachment. In this case the attachment of the particles on the {100} facets greatly facilitates dislocation removal due to the favorable orientation of the glide plane of dislocations with the attachment direction.(3)

There is considerable desire to prepare these atomically coherent arrays from other materials or heterostructures, which can access the visible region of the spectrum. While this can be accessed through cation exchange from coherent PbX materials, control over the phase of the daughter material is difficult, and the quality of cation exchanged materials is often inferior to those directly synthesized. To this end, we have studied the atomic attachment of wurtzite CdSe nanocrystals on the {1-100} and {11-20} prismatic facets to determine ideal attachment geometries for minimizing defects or facilitating defect removal. In the case of imperfect attachment on {1-100} facets, we observe the formation of edge dislocations with b=1/3<11-20> which have an unfavorable glide plane dislocation removal. The case of imperfect attachment on {11-20} facets is more complicated. In this case, we again observe b=1/3<11-20> edge dislocations, but with one specific geometry providing a favorable glide plane for removal, and in another case a very unfavorable glide plane. Attaching on {1120} also leads to the formation of b=1/2<-1011> partial dislocations bound by prismatic stacking faults. In this talk, we will explore the relative ease of removal for each type of dislocations using in-situ HRTEM. Based on insights from these mechanistic studies, we can provide guidelines for minimizing the number of dislocations formed during attachment processes, or if they are formed, providing the easiest path towards removal. These studies pave the way to preparation of epitaxially joined arrays of tetrahedrally coordinated wurtzite semiconductors with low defect densities.
References
1. M. P. Boneschanscher et al., Science. 344, 1377–1380 (2014).
2. R. L. Penn, J. F. Banfield, Science. 281, 969–971 (1998).
3. J. C. Ondry, M. R. Hauwiller, A. P. Alivisatos, ACS Nano. 12, 3178–3189 (2018).

Colloidal quantum dot-based infrared photodetectors present a promising path toward fabricating sensors and imagers at significantly reduced cost. Herein, we report on recently discovered silver chalcogenide quantum dots that exhibit distinct optical absorption in the mid-wavelength infrared, which is known to have high penetration through airborne obscurant such as fog and mists, making them optimal for use in long-distance thermal sensing and enhanced night vision applications. These quantum dots are synthesized into a new narrow bandgap tetragonal phase, not available in bulk, and contain excess electrons, allowing intraband optical transitions between the first and the second conduction energy levels. As a new generation of infrared colloidal nanomaterial, their fundamental properties have not been thoroughly investigated to date. This presentation reports the synthesis, characterizations, and photoconductive photodetector device characteristics and discuss potential impacts that this new nanomaterial could bring to mid-infrared optoelectronics.

The fabrication of junctions between colloidal nanomaterials has enabled a wide range of technologies, but, as devices approach the single-nanoparticle scale, the rational assembly of nanomaterials remains challenging. In 2018, Arthur Ashkin won the Nobel Prize for developing optical traps, which offer the ability to manipulate individual nanostructures in three dimensions. However, to date, there are relatively limited reports on optical trapping in non-aqueous media, and thus far, nanoscale inorganic materials have only been optically trapped in vacuum and in aqueous solvents. Here, we present several examples of our recent efforts on the use of light to manipulate and generate inorganic heterostructures in non-aqueous solvent systems, including a novel demonstration of the use of optical trapping to manipulate, align, and assemble metal-seeded semiconductor nanowires in organic dispersion, resulting in the fabrication of arbitrarily long heterostructures consisting of periodically repeating, metal-nanocrystal/semiconductor-nanowire junctions. Heat transport calculations and optical images demonstrate that the metal tip of each optically-aligned nanowire undergoes eutectic melting in the optical trap, facilitating the oriented tip-to-tail “nanosoldering” of successive nanowire building blocks. In addition, some of our exciting new results on the large-scale photothermal modification of nanomaterials will also be presented.

The external quantum efficiencies of state-of-the-art colloidal quantum dot light-emitting diodes (QLEDs) are now approaching the limit set by the out-coupling efficiency. However, the brightness of these devices is constrained by the use of poorly conducting emitting layers, a consequence of the present-day reliance on long-chain organic capping ligands. This presentation will focus on how conductive and passivating halides can be implemented in Zn chalcogenide-shelled colloidal quantum dots to enable high brightness green QLEDs. We use a surface management reagent, thionyl chloride (SOCl₂), to chlorinate the carboxylic group of oleic acid and graft the surfaces of the colloidal quantum dots with passivating chloride anions. This results in devices with an improved mobility that retain high external quantum efficiencies in the high-injection-current region and also feature a reduced turn-on voltage of 2.5 V. The treated QLEDs operate with a brightness of 460,000 cd m⁻², significantly exceeding that of all previously reported solution-processed LEDs.

Reference: X.Y. Li, et al, Nature Photonics, 2018, 12, 159-164

Colloidal quantum dots are size-tunable band gap materials that can be solution processed into thin film optoelectronic devices. However, the device performance is often limitied due to the small particle size and inability to effectively transport photoinduced charges via electron hopping or tunneling. Because of this limitation there is a need to understand the effects that both quantum confinement and particle “grain” size contribute to a films conductivity. For instance, 15 nm CdSe has been shown to have a 10-fold greater conductance than smaller 4 nm CdSe thin films. Here the effect of particle size offers a more substantial effect than quantum confinement to the conductance of a nanocrystal thin film. However, by employing core/shell materials, such as CdS/CdSe, that exhibit confinement only in the shell we can investigate the effect of quantum confinement independently of grain size. This is done by allowing the CdS core to exceed the Bohr radius, which results in large grain size while allowing the CdSe shell to maintain quantum confinement. By adjusting the dimensions of CdS/CdSe materials even greater thin film photoconductivity, than CdSe of the same size, is achieved by taking advantage of both grain size and quantum confinement effects on photoinduced charge transfer.

In this presentation, we provide a synthetic strategy for bright and robust InP based quantum dots (QDs) and their applications to light-emitting diodes. Specifically, we propose a InP emissive core/ZnSe_xS_1-x inner shell/ZnS outermost shell heterostructure that consists of environmentally benign, heavy metal-free elements and conduct comprehensive study involving synthesis, spectroscopy and simulation. The systematic study unveils the role of the geometry and composition of proposed heterostructures on the optical bandgap, spectra linewidth and photoluminescence quantum yield (PL QY). In addition, we evaluate the stability of designed QDs on involving thermal stress, oxidation, high energy photon exposure, and the electrical stresses. The highlight of present work is the realization of green and red-emitting QDs exhibiting peak PL emission wavelength at 525 and 620 nm with narrow spectra linewidth (full width at half maximum (FWHM) below 40 nm), PL QY as high as 90 % and prolonged stability against external stresses. Finally we demonstrate QD based light-emitting diodes (QLEDs) that display color-pure green (peak EL = 531 nm, FWHM = 39 nm) and red (peak EL = 637 nm, FWHM = 45 nm) emission and peak external quantum efficiency as high as 3.78 % and 6.2 %, respectively.

Water dispersibility of photoluminescent semiconductor nanocrystals without a significant drop in photoluminescence quantum yield (QY) is a prerequisite for aqueous applications such as biological labeling or nanocrystal inks. While standard silica coating or ligand exchange phase transfer techniques enable aqueous dispersibility, they often result in low QY (<1%), particle aggregation, or poor scalability. In this contribution, we present a surface engineering approach that enables the phase transfer of aqueous InP/ZnS core/shell nanocrystals with a QY > 35%. Three methodologies were used to transfer as-prepared nanocrystals from the organic to aqueous phases, and each are compared in terms of the resultant aqueous QY, extent of particle aggregation, and scalability. Our approach utilizes the growth of a sacrificial ZnS shell and an optimized exchange ligand that minimize the effects of surface etching that commonly reduces the photoluminescence QY during phase transfer. The scalable approach yields aqueous InP/ZnS nanocrystals that are colloidally stable at room temperature for >3 months with surface functionality amenable to bioconjugation. Synthetic procedures, analytical characterization, and mechanistic insights will be discussed.

We report a new method to assemble nanoparticle-based hollow microstructures utilizing unusual phase separation kinetics in a two-stage nematic nucleation and growth process. This process generates size-tunable closed-cell foams, spherical shells, and hollow tubular networks composed of closely packed nanoparticles. To assemble these structures we dispersed mesogen-modified nanoparticles in a liquid crystal host at a temperature above the nematic-isotropic transition (T_NI). On cooling through T_NI we observe the initial segregation of nanoparticles into the shrinking isotropic phase domains. Subsequently, we observed a second stage nucleation of nematic domains inside the nanoparticle-rich isotropic domains, leading to the formation of a variety of hollow structures whose differentiation is controlled by nanoparticle density and cooling rate. Monte Carlo simulations of the initial nucleation stage support experimental observations that initial phase separation controls the number of nanoparticles in each assembly. Microscopy studies suggest that the secondary nucleation step controls the number and size of internal voids. These novel hollow micro-assemblies may find broad applications in controlled release, metamaterials, catalysis, and sensor applications.

Semiconductor nanocrystals are highly promising material platforms for use in advanced photonic applications. However, conventional device architectures and fabrication methods cannot fully exploit their potential. Colloidal quantum dots (QDs) offer high color purity essential to high-quality liquid crystal displays (LCDs), which enables unprecedented levels of color enrichment in LCD-TVs today.[1] However, for LCDs intrinsically requiring polarized backplane illumination in operation, highly polarized light generation using inherently isotropic QDs remains a fundamental challenge to date. To address the need for their effective utilization, we proposed and demonstrated innovative photon management techniques for photonic applications of QDs focusing on polarized color conversion. To achieve polarized color conversion by using isotropic QDs, we studied the plasmonic interaction mechanisms that can modify the density of optical states. Strong dipole orientation dependence of surface plasmon coupled emission is utilized the in v-shaped grooves defined into the v-shaped backlight unit (v-BLU) structure.[2] Forward scattering of Fano resonance effectively allowed for polarized light generation that can freely radiate into the farfield.[3,4] Unlike many reports in the literature, our experimentally realized high contrast ratio of 9.70 for red quantum dots is measured at the farfield with a home-made measurement setup similar to LCD operation. To analyze the origin of this polarization response, we investigated the emission kinetics of quantum dots using time resolved fluorescence measurements. Here the observed faster decay rate in the resonating polarization state with higher emission intensity is a direct evidence of the strong modification in density of optical states as a part of Purcell enhancement.[5] In addition, the oriented dipoles of coupled quantum dots to this v-BLU surface is observed using back focal plane imaging. The QDs coupled to v-BLU show a distinct back focal plane pattern, which is only reported for strong dipolar emissions observed in single molecule measurements.[6] In contrast, the unpatterned films of isotropic quantum dots on fused silica and plain gold film show no sign of dipolar emission pattern in their back focal plane images. In the light of measurements unveiling the physics behind the polarized emission mechanism, we finally demonstrated a proof-of-concept display under v-BLU back illumination. Our fabrication includes a special mold fabrication and template stripping steps making the developed v-BLU structure commercially appealing. Also changing the plasmonic layer to silver in the v-BLU instead of gold enhances the spectral range of the v-BLU structure from the red to the green. For next-generation QD-TVs, the proposed polarized color-converting isotropic QDs on such v-BLUs can be deployed in bendable electronic displays.

[1] Jang, E.,et al,. Adv. Mater. 2010, 22 (28), 3076–3080.
[2] Gryczynski, I., et al., J. Phys. Chem. B 2005, 109 (3), 1088–1093.
[3] Limonov, M. F., et al., Nat. Photonics 2017, 11 (9), 543–554.
[4] Luk’Yanchuk, B., et al., Nat. Mater. 2010, 9 (9), 707–715.
[5] Novotny, L.; Hecht, B. Principles of Nano-Optics; Cambridge University Press: New York, NY, 2006.
[6] Backer, A. S., et al., Opt. Lett. 2013, 38 (9), 1521–1523.

Iron catalysts play a vital role in the leading chemicals industry, favoring the production of ammonia, hydrogen, and liquid hydrocarbons through the Haber-Bosch, Water-Gas Shift, and Fischer-Tropsch processes, respectively. These catalysts are benefited by an increase in surface area, which can be achieved through reduced dimensionality. To produce efficient catalysts, thermal decomposition routes capable of controlling size and shape of nanoparticles have been reported, enabling fine tuning of the catalytic properties of a given material. Consequently, for iron oxide (Fe₃O₄) nanoparticles, for example, structural control also enables improved magnetic properties. In the context of catalysis, Fe₃O₄ is an interesting material due to its ability to generate localized heat by the conversion of electromagnetic radiation into heat, as seen in magnetic hyperthermia. Therefore, combining the properties of iron catalysts with synthetic control and doping have the potential to make an impact in the chemicals industry.
In this work, thermal decomposition is used to produce monodisperse, cubic shaped, Fe_(3-x)Cr_xO₄ nanoparticles as a strategy to improve the activity of the iron oxide (Fe₃O₄) host nanoparticles via Cr⁺³ doping, while maintaining the magnetic properties of the host lattice. Cr⁺³ replaces Fe⁺³ in the octahedral sites of Fe₃O₄ inverse spinel structure at low doping concentrations (x < 15 mol%), and tetrahedral and octahedral sites are substituted at higher concentrations. Low doping concentration improves activity and redox performance due to coupling of the Fe⁺³/Fe⁺² with Cr⁺²/Cr⁺³, and regeneration of Fe⁺² on the surface. To confirm this regeneration effect and the site occupancy of Cr in Fe₃O₄, L-edge X-ray Absorption Spectra of Fe and Cr are collected. The impact of surface Fe⁺² on the catalytic efficiency of Fe_(3-x)Cr_xO₄ under induction heating catalysis is shown through a butanol condensation/dehydration reaction, and the reaction products are identified via Gas Chromatography-Mass Spectroscopy (GC-MS). The cation regeneration also benefits catalytic performance by increasing the number of active sites, which are measured via CO₂/N₂ adsorption. In addition, Cr doping changes the pore structure from mesopores to micropores, thereby increasing the surface area, demonstrated through BET measurements. To determine the origin of catalytic enhancement with regards to Cr doping or formation of oxygen vacancies, X-ray Photoelectron Spectroscopy (XPS) is performed to elucidate the local bond environment. Finally, magnetic properties are characterized via Superconducting Quantum Interference Device (SQUID) measurements to show how the saturation magnetization, and coercivity are preserved. Induction heating is characterized by determining the Specific Loss Power of these materials under various magnetic field strength.

Control over localized surface plasmon resonance frequency of metal nanoparticles by shaping the geometry of the nanostructure has attracted continued attention. In this presentation, we report a stepwise seed-mediated growth method for the synthesis of self-registered anisotropic plasmonic nanostructures. The synthesis requires using colloidal nanoparticles as substrates for the assembly of the metal seeds, and precise control over the exposure of the metal seeds by surface passivation. The anisotropic seeded growth of metal nanostructure can be induced by creating a physical barrier on the seeds, which partially passivates the exposed area of the seeds. Au dimers, linear trimers and Au-Ag dimers with high purity and distinguishable plasmonic excitation bands have been synthesized. This synthesis method results in rod-like nanostructures perpendicularly registered to the substrates. Taking advantages of this unique feature, we investigated dynamic tuning of the plasmonic excitation of Au-Au and Au-Ag dimers by controlling their orientation relative to the incident light. Such tuning is enabled by switching the substrate to anisotropic magnetic nanoparticles, whose orientation can be magnetically controlled. By tuning the direction of the magnetic field, we show control over the plasmonic resonant modes of the Au-Au dimers and Au-Ag dimers under the incidence of polarized light. The optical switching of Au-Au and Au-Ag dimers exhibits bright colors with high contrast. Finally, we demonstrate the application of such nanostructures as anti-counterfeiting patterns by fixing them in orthogonal orientations in a hydrogel film.

Anisotropic nanoparticles, such as nanorods and nanoprisms, enable packing of complex nanoparticle structures with different symmetry and assembly orientation, which result in unique functions. Despite previous extensive efforts, formation of large areas of oriented or aligned nanoparticle structures still remains a great challenge. Here, we report fabrication of large-area arrays of vertically aligned gold nanorods (GNR) through a controlled evaporation deposition process. We began with a homogeneous suspension of GNR and surfactants prepared in water. During drop casting on silicon substrates, evaporation of water progressively enriched the concentrations of the GNR suspension, which induces the balance between electrostatic interactions and entropically driven depletion attraction in the evaporating solution to produce large-area arrays of self-assembled GNR on the substrates. Electron microscopy characterizations revealed the formation of layers of vertically aligned GNR arrays that consisted of hexagonally close-packed GNR in each layer. Benefiting from the closepacked GNR arrays and their smooth topography, the GNR arrays exhibited a surface-enhanced Raman scattering (SERS) signal for molecular detection at a concentration as low as 10⁻¹⁵ M. Because of the uniformity in large area, the GNR arrays exhibited exceptional detecting reproducibility and operability. This method is scalable and cost-effective and could lead to diverse packing structures and functions by variation of guest nanoparticles in the suspensions.