Symposium NT6 : Colloidal Nanoparticles---From Synthesis to Applications

Nanoscale semiconductor particles (also known as colloidal quantum dots, cQDs) provide strong, tunable, and photostable electronic transitions. Their properties are potentially useful for many optoelectronic devices. Here, we will explore what happens when cQDs are placed in intense electromagnetic environments. If cQDs are located within a strongly confined optical or plasmonic resonance, their photophysical properties can be modified. For example, their absorption and emission rates can be enhanced via the Purcell effect. They can even enter the “strong coupling” regime in which hybrid light–exciton states known as polaritons form. In this case, excitons that were isoenergetic with the optical or plasmonic mode are then split into an upper and lower polariton due to mixing with the confined electromagnetic field. Interestingly, such states contain both electromagnetic and excitonic components, which can lead to a variety of new physical phenomena. We will describe our recent experiments related to the above effects. We have combined cQDs with state-of-the-art plasmonic structures to influence cQD behavior via tightly localized plasmonic modes. Finally, we will discuss how the observed phenomena can potentially be exploited in optoelectronic devices.

Solar energy is the only source of abundant energy available to humanity that can supply the sheer quantities required for earth's growing, and industrializing, population (DOI: 10.1073/pnas.0603395103). For large scale deployment of photovoltaics (PV) and photocatalysts for splitting of water and generation of solar fuels, the materials requirements are daunting. As a result, there has been great attention devoted towards elements that are abundant in the earth's crust, and economically viable with respect to mining, extracting, and further processing (DOI: 10.1021/es8019534). Using the synthetic tools developed over the past two decades with respect to nanocrystal inks, which can be solution processed and otherwise incorporated into mass manufacturing approaches (DOI: 10.1021/ar3001958), we have focused our attention upon two directions. First, for PV, we have pursued the development of phase pure, single crystalline zinc phosphide (Zn₃P₂) nanoparticles for PV. Secondly, for water oxidation, our focus has been nickel and iron oxide monodisperse nanocrystal solid solutions for water oxidation. Bulk Zn₃P₂ was shown in the late 1970's to have enormous potential for PV because of its high absorptivity and electronic properties, and was abandoned until 2009, when interest was reignited as a very promising earth abundant candidate for solar (DOI: 10.1021/es8019534). Nickel and iron oxide solids have a long history of reactivity for the catalysis of water oxidation (DOI: 10.1021/ja307507a), dating back to Edison.

We sought to synthesize single crystalline, monodisperse Zn₃P₂nanoparticles, which when solubilized, could be applied as a nanoparticle-ink for solar cell manufacture (DOI: 10.1021/nn204381g). In this talk, we will describe two solution-phase, synthetic approaches to Zn₃P₂ nanocrystals, their surface chemistry, and their incorporation into PV devices (DOI: 10.1021/nn4034234). While Zn₃P₂ is sensitive to oxidation, through a fundamental understanding of the 'native' surfaces resulting from the conditions of synthesis, the interfacial chemistry can be controlled, and harnessed, to enable further functionalization (DOI: 10.1021/cm502234k). New directions to enable the development of Zn₃P₂ nanocrystal-based PV will be described.

In another avenue, we will discuss how the synthesis of new earth-abundant nanomaterials can lead to unexpected surprises, such as the discovery of metastable phases that are not known to exist under ambient conditions. Research to develop new iron and nickel oxide nanomaterials led to characterization of single crystal rock salt [Ni,Fe]O nanocrystals, whose composition can be varied from nickel-rich to nickel-poor, with no disruption in crystallinity or phase, and applied for water oxidation catalysis (DOI: 10.1021/cm501881a).

To summarize, we will provide an overview of the fascinating pursuit of earth abundant, inexpensive colloidal "rock" nanocrystals, with respect to their applications in solar energy conversion.

Colloidal gold nanorods with plasmon absorbance Near Infrared were synthesized according to the seed-mediated growth method. and using a HAuCl₄.3H₂O low cost precursor made in home prepared from high purity metallic gold produced in La Libertad Region-Peru. This precursor has been carefully analyzed and validated with other commercial precursor. To grow longer gold nanorods with aspect ratios ranging from 4.0 to 9.0, a binary surfactant mixture composed of benzyldimethylhexadecylammoniumchloride (BDAC) and hexadecyltrimethylammonium bromide (CTAB) was used. Effects of the silver ion and the cosurfactants along with the growth mechanism of gold nanorods are discussed. The gold nanorods were characterized by using transmission electron microscopy, X-ray diffraction, and UV-VIS absorption spectroscopy. It was found that the concentration of the precursors affects the aspect ratio.

Colloidal semiconductor nanoplatelets (NPLs), which are atomically flat nanocrystals, have emerged as favorable light-emitting materials for lasers and LEDs. Recently, CdSe core, CdSe/CdS core/crown and core/shell NPLs have exhibited low threshold single-photon absorption (1PA) and two-photon absorption (2PA) pumped optical gain with record high optical gain coefficients reaching levels of 650 cm^-1 [1, 2]. These gain coefficients represent 2- to 4-fold enhancement as compared to that of the previous best reports in colloidal nanocrystals. Our further investigations suggest that it is possible to achieve larger material gain in the NPLs, which can impressively outperform even that of the epitaxial quantum wells and dots [3]. Additionally, NPLs offer extremely large linear and nonlinear absorption cross-sections, where linear absorption cross-section can reach levels as high as 10^-12 cm² [4] and nonlinear absorption cross-sections can exceed 10⁶ GM [5]. These linear and non-linear absorption cross-sections in the NPLs are at least two orders of magnitude larger than those of the colloidal quantum dots, which could make it possible to achieve optical gain with sub-μJ/cm² thresholds for pulsed and ~100 W/cm² for continuous wave (CW) 1PA pumping together with up-converted lasing thresholds on the order of 100 μJ/cm². Therefore, ultralow-threshold colloidal lasers enabled by the high modal gain coefficients and large absorption cross-sections of the NPLs may become feasible while employing conventional light sources as the optical pump, including laser diodes or LEDs. Though being extremely promising, several significant hurdles are needed to be overcome in the NPLs to fully reach their full potential as an optical gain medium. These mainly arise from the presence of defected nanoplatelet sub-populations within the NPL ensembles causing ultrafast exciton quenching through fast hole trapping in the defected NPLs [6]. This quenching is fostered by ultraefficient and ultrafast dipole-dipole coupling (η_FRET ~99% and γ_FRET ~10 ps) in the close-packed assemblies of the NPLs [6, 7]. Due to the exciton transport assisted hole trapping, gain-active biexcitons are mostly lost that considerably reduces the potential of the gain. Therefore, this necessitates a quest for a better understanding and control of the surface chemistry in the NPLs together with the development of better means of surface passivation towards defect-free NPL ensembles.

[1] B. Guzelturk et al. ACS Nano 14, 673 (2014)
[2] C. She et al. Nano Lett. 14, 2772 (2014)
[3] B. Guzelturk et al. in submission (2015)
[4] A. Yeltik et al. under review (2015)
[5] M. Olutas and B. Guzelturk et al. ACS Nano 9, 5041 (2015)
[6] B. Guzelturk et al. ACS Nano 8, 12524 (2014)
[7] C. E. Rowland et al. Nature Mat. 14, 484 (2015)

Quantum dots (QDs) represent one of the most interesting and versatile systems of modern nanoscience, unravelling the interaction of light with crystalline matter squeezed down to a few nanometers in size. Several synthesis approaches have been proposed featuring different materials, heterostructures, shapes and surface properties [1]. Furthermore, a major effort has been placed in trying to understand how to self - assemble these building blocks in a programmable way in order to access a new class of materials by design [2, 3].
We propose a novel assembly method for colloidal QDs that does not depend on the specific type of material, structure, shape or surface but only on solution conditions and is therefore completely general. We induce controlled nanoparticle assembly by making use of the attractive critical Casimir interaction [4]; this force exploits the role of temperature dependent solvent density fluctuations present in a binary solvent close to its critical point. By carefully tuning magnitude and length scale of attractive Casimir and repulsive Coulombic interactions, we are able to grow large QD superstructures. Our results describe the assembly process as it takes place through scattering techniques (DLS, SAXS) and the growth of superstructures by in-situ electron microscopy (SEM, TEM).
We elucidate an interesting new method to achieve large area self-assembled superstructures that could be beneficial to the preparation of various devices, such as QD solar cells.
[1] Talapin, Dmitri V., et al. "Prospects of colloidal nanocrystals for electronic and optoelectronic applications." Chemical reviews 110.1 (2009): 389-458.
[2] Bishop, Kyle JM, et al. "Nanoscale forces and their uses in self-assembly.", Small 5.14 (2009): 1600-1630.
[3] Hanrath, Tobias. "Colloidal nanocrystal quantum dot assemblies as artificial solids." Journal of Vacuum Science & Technology A 30.3 (2012): 030802.
[4] Nguyen, Van Duc et al. "Controlling colloidal phase transitions with critical Casimir forces." Nature communications 4 (2013): 1584.

Two-Dimensional materials open up great prospects in photodetector applications owing to their sharp optical properties and the ability to combine them in layered heterostructures¹. Among this new class of materials, colloidal nanoplatelets (NPL) made of cadmium chalcogenides readily combine the thickness control at the atomic level together with the large scale production and ease of processing of colloidal materials². Here, we investigate photoconductive devices made from CdSe NPL.

Optoelectronic performances of devices made from a film of colloidal nanocrystals remain restricted by their limited carrier mobilities^3,4. As a first strategy to overcome this, we first play on the photocarrier lifetime by building an electrolyte-gated phototransistor, allowing us to passivate the electron traps⁴. As a consequence, the photoresponse gain is increased by a factor 1000.⁵
As a complementary strategy, we use nanotrench electrodes to decrease the transit time of the carriers and to suppress the influence of film defects⁶. This geometry also provides sufficient electric field to overcome the large exciton binding energy of NPL inherent to 2D materials. Altogether, the enhanced responsivity and lowered noise level lead to specific detectivity up to 10¹¹ Jones.
The last promising route toward improving the harvest of photogenerated charges is to collect them using a graphene transport layer. The performances are thereby restricted to the electrical coupling between the nanoparticles and the graphene channel⁷. We show that the charge transfer from the NPL to the graphene is driven by the large semiconductor exciton binding energy. The latter can be engineered by growing core/shell or core-crown heterostructures at the nanocrystal scale. This allows us to control the magnitude and the direction - electrons or holes - of the charge transfer to graphene.⁸

(1) Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Chem. Soc. Rev. 2015, 44 (11), 3691–3718.
(2) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L. Nat. Mater. 2011, 10 (12), 936–941.
(3) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2010, 110 (1), 389–458.
(4) Lhuillier, E.; Pedetti, S.; Ithurria, S.; Heuclin, H.; Nadal, B.; Robin, A.; Patriarche, G.; Lequeux, N.; Dubertret, B. ACS Nano 2014, 8 (4), 3813–3820.
(5) Lhuillier, E.; Robin, A.; Ithurria, S.; Aubin, H.; Dubertret, B. Nano Lett. 2014, 14 (5), 2715–2719.
(6) Lhuillier, E.; Dayen, J.-F.; Thomas, D. O.; Robin, A.; Doudin, B.; Dubertret, B. Nano Lett. 2015, 15 (3), 1736–1742.
(7) Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.; Bernechea, M.; de Arquer, F. P. G.; Gatti, F.; Koppens, F. H. L. Nat. Nanotechnol. 2012, 7 (6), 363–368.
(8) Robin, A.; Lhuillier, E.; Xu, X. Z.; Ithurria, S.; Aubin, H.; Ouerghi, A.; Dubertret, B. Submitted.

Development of synthetic methods for nanostructures has introduced new approaches for engineering functional materials. Nanocrystal assemblies provide a powerful platform for designing two- and three-dimensional solids with tailored electronic, magnetic, optical and catalytic properties. Unlike atomic and molecular crystals where atoms, lattice geometry, and interatomic distances are fixed entities, the arrays of nanocrystals represent solids made of “designer atoms” with potential for continuous tuning their physical and chemical properties.

The assembly of functional materials from nanoscale building blocks combines advantages of crystalline inorganic semiconductors with inexpensive and scalable solution-based device fabrication. Along these lines, colloidal semiconductor nanocrystals are explored as the functional elements in printable electronics, light emitting devices, photodetectors and solar cells. All the above applications rely on efficient charge transport in nanocrystal arrays. In the recent years significant progress has been achieved in development of chemical approaches to improve electronic transport and control doping in nanocrystal arrays. I will review these developments and discuss recent results on inorganic ligands for colloidal nanomaterials. By using optimized surface chemistries we prepared nanocrystal solids exhibiting carrier mobilities comparable to those in single crystal materials. I will demonstrate the power of “modular” materials fabrication for electronic, thermoelectric and photovoltaic devices.

Metal quantum clusters are an important class of functional nanomaterials with growing applicative potential as size-tunable biocompatible luminescent probes for molecular theranostics and optoelectronics. Here, we demonstrate for the first time that intermolecular interaction between metal quantum clusters, mediated and controlled by the capping ligands, can lead to the formation of stable inter-cluster excimers featuring largely Stokes-shifted emission with no corresponding absorption transition. Based on this newfound motif, we demonstrate a strategy for overcoming the intrinsic limitation of the use of molecular excimers in single-particle applications, that is, their nearly zero collisional formation probability in ultra-diluted solutions. To meet this aim, we use gold clusters as building blocks for fabricating permanent excimer-like colloidal superstructures held together by a network of hydrogen bonds between the capping ligands. Optical spectroscopy experiments and ab initio theoretical modeling reveal that ground state clusters behave as individual photophysical entities, whilst optical excitation results in the formation of inter-cluster excimers. The obtained supramolecular architectures effectively represent a new aggregation state of matter, conveying the photophysics of excimers into self-standing individual particles that find their natural application as non-resonant emitters in cellular imaging and integrated photonic nanotechnologies. In vitro experiments confirm the high biocompatibility of Au₈-pXs and reveal their ability to scavenge cytotoxic reactive oxygen species responsible of premature cellular death, thereby further enhancing their potential as anti-cytotoxic agents for bio-medical applications.

Controlled incorporation of charge carriers able to actively modify the electronic and optical properties of colloidal semiconductor quantum dots (QDs) allows to obtain important functionalities such as largely Stokes-shifted, broad emission spectra, para- and photo-magnetism and controlled electrical behaviours. This regime is achievable by doping the QDs with tiny amounts of transition metal ions aliovalent with respect to the semiconductor host material or capable of exhibiting amphoteric character depending on their substitutional or interstitial position in the host lattice. Generally, QD doping is performed by exposing the QDs to the dopant precursors either during the nucleation or growth phases, or via post synthesis cation exchange reactions. Through these approaches, it is currently possible to control doping to the single impurity level. To date, however, such accuracy is achieved only in organic synthesis routes, whilst the control and pre-tuning of the doping level in QDs produced through aqueous approaches is still challenging. The development of synthetic protocols of highly controlled dQDs in water would therefore be greatly beneficial as it would reduce the use of environmentally harmful organic solvents and would remove the necessity for post synthesis bio-compatibilization procedures for the application of dQDs in bio-imaging and biomedical diagnostics.

In this talk, I will present a new general strategy for the controlled doping of QDs in water at room temperature. This innovative method consists of preselecting the doping level by using magic-sized metal nanoclusters as nucleation seeds for dQDs¹. The “metal cluster seeding” synthesis consists in a two-step procedure with the preparation of the metal nanoclusters (Ag or Cu) followed by the seeded growth of doped QDs (CdS). Compositional and structural analysis confirm that each synthesized dQD incorporates a single metal nanocluster. Optical spectroscopy experiments show the typical signatures of Cu- and Ag- doping and indicate that upon growth of the host particle, the seeding nanoclusters dissolve in their atomic constituents. The metal atoms participate in the recombination mechanism of photoexcited QD excitons as individual doping impurities, analogously to the typical behaviour of dQDs produced through organic based methods. Our approach, demonstrated here for Cu- and Ag-doping of CdS QDs is not QD or metal exclusive, and can in principle be extended to other semiconductor materials and metal impurities for achieving highly controlled dQDs.




References:

1. B. Santiago-González, A. Monguzzi, V. Pinchetti, M. Campione, A. Cassu, M. Prato, F. Meinardi, L. Manna and S. Brovelli. – Nanoletters, under review-

Organometal halide perovskites have attracted great attentions due to their tunable excited states from excitons to free carriers.^1,2 Beyond the remarkable progress toward high performance solar cells, organometal halide also have desired characteristics of color tunability and narrow-band emissions for light-emitting applications.³ Because the photoluminescent properties are strongly correlated with their microstructures, controllable synthesis of perovskite nanocrystals are essential to optimize the PL properties and thus enhance the performance of the devices.
Recently, we developed the ligand-assisted (LARP) technique for the fabrication of highly luminescent CH₃NH₃PbX₃ (X=Br, I, Cl) quantum dots by adapting the fast reprecipitation technique.^4,5 Through adjusting the amount of the surface ligands, we observed interesting shape evolution from nanocubes to nanowires.⁶ To understand the formation mechanism especially the ligand effects on the shape evolution, we adapted thermodynamic thoery to describe the nucleation and growth process of CH₃NH₃PbBr₃ nanocrystals. The formation mechanisms were further discussed by applying the materials characterizations and controllable experiments. Furthermore, we studied the optical properties of resulting CH₃NH₃PbBr₃ nanocrystals and illutrated the size- and shape- dependent PL properties, especially the influence of defects densities on recombination dynamics in these perovskite systems.⁶
References
1. J. S. Manser, and P. V. Kamat. Band filling with free charge carriers in organometal halide perovskites. Nat. Photonics 8, 737-743 (2014)
2. V. D’Innocenzo, G. Grancini, M. J. P. Alcocer, et al. Excitons versus free charges in organo-lead tri-halide perovskites. Nat. commun. 5, (2014).
3. S D.Stranks, and H. J. Snaith. Metal-halide perovskites for photovoltaic and light-emitting devices. Nat. nanotechnol. 10, 391-402 (2015).
4. Y. Niu, F. Zhang, Z. Bai, et al. Aggregation Induced Emission Features of Organometal Halide Perovskites and Their Fluorescence Probe Applications. Adv. Opt. Mater. 3, 112-119. (2015).
5. F. Zhang, H. Z. Zhong, C. Chen, et al. Brightly Luminescent and Color-Tunable Colloidal CH₃NH₃PbX₃ (X= Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 9, 4533-4542. (2015).
6. F. Zhang, C. Chen, W. G. Lu, et al. Morphology Evolution from Nanocubes to Nanowires of CH₃NH₃PbBr₃ Nanocrystals and Their Shape-Dependent Photoluminescence Properties. In preparation.

Intrinsic ability of nanoparticles (NPs) to self-organize can be seen virtually everywhere around us. Although omnipresent, the mechanisms of these processes are not well understood. A step toward adequate description of these mechanisms can be made by juxtaposition of self-organization processes known for biological species and NPs. Analysis of comparative strengths of electrostatic London dispersion, and coordination interactions across all types of building blocks known to self-assemble can be instructive in understanding the special place occupied by inorganic NPs among other self-assembled molecular and nanoscale systems. Comparison between theoretical predictions and experimental data for interactions between semiconductor, metallic, and ceramic NPs indicate the failure of fundamental of typical assumptions put in the foundation of colloidal theories applied at nanoscale. Multiscale collective effects make the NP-NP interactions as complex as those found in biology for proteins and other biomacromolecules. They also open the venue for inquiry about specificity of NP-protein interactions and structural/functional versatility of their hybrid assemblies.

Self-organization of chiral nanoparticles represent, perhaps one of the best examples of biomimetics on inorganic assemblies. The state-of-the-art in the enantioselective synthesis from different groups on chiral nanoscale materials will be described along with their advantages/disadvantages, and current challenges. The latest data on chirality transfer from circularly polarized photons to NPs and their helical assemblies will be included. Special effort will be placed on understanding the pathway for translation of unique structural and optical properties of chiral nanoscale materials to technology. It will be exemplified by the bioanalytical applications for detection of DNA oligomers using CD spectroscopy that enables one-three order of magnitude improvement of detection of protein, DNA oligomers, and small chiral molecules.

Interfacial self-assembly and epitaxial attachment of colloidal Pb-chalcogenide semiconductors has resulted into two-dimensional atomically coherent PbX (X=S, Se, Te) semiconductors with remarkable geometry and strong electronic coupling between the nanocrystal sites. By Cd-for-Pb cation exchange, these honeycomb systems can be transformed into honeycomb semiconductors with a zinc blende CdX atomic lattice. Effective mass- and atomistic theory predict that 2-D semiconductors with honeycomb geometry have a bandgap similar to the 2-D quantum wells, but with Dirac-type electronic conduction and valence bands. Due to strong spin-orbit coupling, the Dirac cone at the K-point can open, and host the quantum spin Hall effect.

I will discuss the theoretically predicted electronic band structure. I will show and discuss the first density-of-state measurements on honeycomb semiconductors and the transport characteristics of electrons and holes in these systems. Finally, the optical properties of freely suspended honeycomb semiconductors will be compared to those of simple quantum wells with the same thickness and same atomic lattice. In this way, the peculiar properties related to the honeycomb geometry will be demonstrated.

References
[1] Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices
M. P. Boneschanscher, W. H. Evers, J. J. Geuchies, T. Altantzis, B. Goris, F. Rabouw, S. van Rossum, H. S. J. van der Zant, L. D. A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A. V. Petukhov, Bals, and D. Vanmaekelbergh, Science 344, 1377 (2014).
[2] Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry. E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allan, and D. Vanmaekelbergh, Phys. Rev. X 4, 011010 (2014)
[3] Low-Dimensional Semiconductor Superlattices Formed by Geometric Control over Nanocrystal Attachment. W. H. Evers, B. Goris, S. Bals, M. Casavola, J. de Graaf, M. Dijkstra, D. Vanmaekelbergh, NanoLett. 13, 2317-2323 (2013)
[4] Topological states in multi-orbital HgTe honeycomb lattices. W. Beugeling et al. Nature Communications 6, doi:10.1038/ncomms7316 (2015).

In optoelectronic devices utilizing colloidal quantum dots (QDs), nonradiative Auger recombination (AR) has been considered as one of the major problems limiting the device performance. One approach to suppressing AR is to smoothen the confinement potential, which reduces the probability of the intra-band transition involving the Auger electron (or hole). Several reports have suggested that the graded interface in core/shell QDs significantly suppresses AR. However, most of these results have been achieved using the QD structures where the interfacial alloy was obtained via uncontrollable anion diffusion during the shell growth. Therefore, based on these studies it is difficult to analyze the actual shape of the confinement potential and correlate the interface structure with the rate of the AR process.
In this study we focus on CdSe/CdS QDs, with a mixed CdSe_xS_1-x layer at the core-shell interface (CdSe/CdSe_xS_1-x/CdS). A prerequisite for this study is to exclude unintentional diffusion and obtain precisely-controlled composition; therefore, we chose a synthetic route based on 1-dodecanethiol and n-trioctylphosphine selenium, allowing for shorter reaction time than successive-ionic layer adsorption and reaction (SILAR) which is often accompanied to uncontrolled alloying. To minimize the influence of a so-called “volume scaling” on the AR rate, and thus, isolate the effects purely due to the CdSe_xS_1-x layer, we fixed the overall radius of the QDs to 5.0 nm. Further, by delicate control of reaction parameters, we were able to produce the CdSe_xS_1-x layer in terms of the tunable thickness (0 ≤ L ≤ 2 nm) and either a radially-flat or graded chemical composition with the controlled atomic ratio (0 ≤ x ≤ 1).
The degree of suppression of AR was evaluated using the average ratio of the photoluminescence (PL) quantum yields (QYs) of biexcitons and single excitons (Q_BX/Q_X), inferred from the second-order intensity correlation function in single-dot measurements. We observe a clear effect of changes in both x and L on the Q_BX/Q_X ratio. For the fixed L = 1.5 nm, a decreasing potential sharpness (larger x) improved the Q_BX/Q_X up to ~0.2. Meanwhile, the increase in L with a fixed x of 0.6 also enhanced the Q_BX/Q_X up to ~0.15. By combing both the thickness and composition control, we obtained the mean Q_BX/Q_X ratio of ~0.3 with highest value up to 0.5. These results imply that “smoothing” of the interfacial potential does lead to the suppression of the AR process, as was originally predicted by Cragg & Efros. (Nano Lett. 2009). This further suggests that “interface engineering” represents a viable strategy for controlling Auger recombination, which should help boosting the performance of QD-based lasers and light-emitting diodes.

Among the materials considered for application towards third generation photovoltaic (PV) devices, quantum dots (QDs) stand out due to their low cost and ability to increase the efficiency of PV devices. The optical and electrical properties of quantum dots can be tuned by controlling their size, surface charge, concentration, quantum yield, and surface coating, making them advantageous for PV applications. The most prevalent materials that have been used to synthesize QDs are cadmium and lead, but the use of such toxic compounds impedes the commercialization of QD-based PV technology. Additionally, common synthesis methods require sensitive, precise injection of volatile and reactive materials, which are difficult to scale up and unfeasible for mass production.
Herein, we present a simple and scalable synthesis of non-toxic Copper Indium Sulfide (CIS) quantum dots as a safer alternative to carcinogenic cadmium- and lead-based QDs. In addition to being a toxicologically favorable option, CIS quantum dots can be produced with high quantum yield, and the ability to cover a wide range of the solar spectrum. In this work, organometallic synthesis methods were used to produce CIS QDs without the complexity of traditional injection-based QD synthesis. The synthesized CIS core QDs were overcoated with ZnS, resulting in surface passivated core/shell CIS/ZnS structures with enhanced photoluminescence (PL) efficiency and stability.
To determine the optimal conditions for CIS QD growth, we investigated the effect of various ratios of the reactants on the PL and lifetime of CIS QDs. For growth of the CIS core, In:Cu ratios were adjusted to examine the effect of different materials on the resulting quantum dots. Overall, it was determined that a 2 mmol:1 mmol In:Cu ratio with a long growth time resulted in the highest quality core CIS QD. This core quantum dot was subsequently overcoated with ZnS. The Zn:S ratios of the overcoating solutions were adjusted to determine the conditions that contributed to the best shell growth. It was ultimately found that overcoating with an excess of zinc resulted in quantum dots with a higher photoluminescence efficiency and longer lifetime. Core CIS QDs synthesized using various In:Cu ratios were also overcoated with ZnS, resulting in notable structures. The structural and optical properties of the synthesized core and core/shell QDs were characterized using Transmission Electron Microscopy (TEM), steady state and decay lifetime photoluminescence, UV-vis, and Fourier Transform Infrared Spectroscopy (FTIR); the results will be reported in the full paper.
The development of scalable synthesis methods for high-throughput production of cadmium- and lead-free, highly photoluminescent quantum dots is key to the realization of commercially-available QD-based photovoltaic devices.

Colloidal inorganic nanocrystals (NCs) are among the most exploited nanomaterials to date due to their extreme versatility. The present talk will highlight the recent progress from our group on the study of chemical transformations in nanostructures, via both cation and anion exchange. Cation exchange is mainly operative in binary ionic compounds and involves replacement of the sublattice of cations with a new sublattice of different cations, while the sublattice of anions remains in place. Some of these transformations, as well as assembly, can now be followed in situ using dedicated transmission electron microscopes with new holder designs and new types of detection systems. An emerging area of research is that of anion exchange, especially in halide perovskites, in which the halide ions exhibit unusually high diffusivities. These reactions have been recently investigated also on perovskite nanocrystals. Finally, new exciting directions have been uncovered recently through the development of plasmonic semiconducting nanoparticles and by the possibility to chemically adjust the density of free carriers in them. The talk will also give an outlook on future developments in these various fields.

We demonstrate a novel strategy for the synthesis of colloidal semiconductor nanocrystals (NCs) exhibiting the size dispersion below 2%. The present approach relies on the sequential deposition of fully saturated cationic and anionic monolayers onto small-diameter clusters, which leads to focusing of nanocrystal sizes with the increasing particle diameter. By using CdS NCs as a model system, we demonstrated that a narrow size dispersion of 1-2% can be achieved through a sequential atomic layer deposition of fully-saturated Cd²⁺ and S^2- half-monolayers onto starting 2-nm CdS cluster “seeds”. With a negligible dispersion in the thickness of the deposited multilayer shell, the primary contribution to the standard size deviation of grown CdS NCs comes from the size variation in the starting CdS clusters (Δd