Composition engineering and size alteration have been employed as the traditional routes for tuning chemical and physical properties of semiconductor nanocrystals. Phase transformation of nanocrystals leading to the change in size/shape/composition of the nanocrystals is an attractive tool as well for tuning the properties as it opens new possibilities for the fabrication of micro structured optoelectric devices and semiconductor films with controlled morphology. To date, induced phase transformation has been achieved by doping, pressure or high temperature (> 450 °C) treatment of phase pure nanocrystals. Herein, we conduct a comprehensive study on the chemically and thermally induced phase transformation in CZTSSe nanocrystals. We have developed a simple and facile approach to induce phase transformation by post treatment of wurtzite CZTSSe nanocrystals with ligands at 305 °C. Cubic phased nanocrystals of 100-150 nm diameters evolve from pear-shaped wurtzite nanocrystals (length ~20 nm) which forms intermediate polytypic bullet shaped nanocrystals (length ~20 nm) at lower temperatures. Extensive study of the transformation mechanism has been performed which includes Ostwald ripening, shape sharpening and coalescence. To confirm the transitional temperature of the fast phase transformation, ex-situ annealing (hot plate) and in-situ annealing (High temperature XRD chamber) has been performed. The shape, composition and band gap changes at different stages of the chemically and thermally induced phase transformation has been subsequently investigated with transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), Raman spectroscopy, FTIR, UV-vis-NIR techniques, X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV).

Sustainable and cost-effective Group IV elements are potential materials for a broad range of electronic and opto-electronic applications. Silicon and germanium are indirect band gap semiconductors, which make their interactions with light much less efficient than those of direct-band gap semiconductors. This limits their applicability in solar cells, escalating prices and holding utilization down. Nevertheless, the use of these materials could be augmented in solar cells if they can be produced to offer better absorption properties, by creating a direct band gap version. Electronic structure calculations predict that the band structure of Ge can be modified to reduce the energy difference between the first direct and indirect transitions, potentially to the point of “cross-over” by alloying with Sn. The amount of Sn needed to produce fully direct behavior in bulk Ge_xSn_1-x alloys is estimated to be 10-15% and even larger for Si_xSn_1-x formulations. However, the large lattice mismatch between Ge/Si and Sn, higher cohesive energies of Ge and Si compared to Sn, and somewhat preferable octahedral environment for Sn (thereby stabilization of tetragonal Sn lattice over its cubic allotrope at ambient conditions) give rise to the miniscule solubility of tin in bulk Si and Ge. Nevertheless, the high surface-to-volume ratios and degree of curvature present in small NCs allow significant degree of lattice relaxation thereby reducing the lattice strain and enabling a wider range of Ge_xSn_1-x compositions. We have developed a facile colloidal synthesis method for pseudo-spherical colloidal Sn_xGe_1#8209;x nanocrystals with cubic structure in the size range of 5-15nm in almost full range of compositions with x varying from 0 to 0.98. Reaction pathway toward the final product reveals an intriguing interplay between germanium diffusion into tin and the alloy lattice instability. The optical and thermodynamic studies of nanocrystals have been undertaken in combination with electronic structure calculations in order to understand conditions for 1) alloy composition stabilization and 2) optimal tin concentrations and sizes for the indirect-to-direct transition cross-over point. The details of the synthesis methods, reaction pathwaysstructural and optical characterizations will be presented and discussed.

Colloidal semiconductor quantum dots (QDs) and quantum rods (QRs) have been of great interest due to their intriguing properties and potential applications. Their syntheses have been extensively studied and significantly advanced. Growth of CdSe (for example) QRs requires the use of strong binding ligands such as alkylphosphonic acids that have been proposed to 1) strongly bind to the side facets of the rods, allowing the unidirectional growth along the weakly passivated [001] axis, or 2) lower the precursor reactivity that results in a small number of nuclei and a high concentration of remaining nutrients in the solution to promote QR growth. Both mechanisms may coexist in a reaction system. We have found that by using appropriate precursors, ligands, or additives to lower the precursor-conversion kinetics, QRs can be synthesized, whereas QDs are grown at high precursor-conversion kinetics. We propose that control of nucleation and growth kinetics, quantitatively determined by precursor-conversion kinetics, provides a means for the morphology control of colloidal semiconductor nanocrystals having intrinsically anisotropic crystal structures, in favoring the second mechanism. Detailed reaction conditions and analyses will be presented.

In the presentation, we are introducing two homologies of magic-sized II-VI nanoclusters as important semiconductor precursors. These nanoclusters have unique formulation, (II-VI)_n, with precise stoichiometric ratios, n=13 or 34, and neutral-donor ligand (amines only), theoretically reported as cage-core or fullerenes-like conformations. Historically, these clusters can be only observed as transient products spectroscopically until recently elaborated isolations and purifications as the result of stable precursors that successively grow high-quality and bright 2D flat nanocrystals. Such growth pathways into 2D materials (quantum belts and platelets) are structurally derived from bundled lamellar galleries comprising of alkyl chain amines (C=2-16) and these magic-sized nanoclusters. More interestingly, our magic-sized nanoclusters coincidently have very similar electronic transitions to some of non-stoichiometric nanoclusters developed by Owen and coworkers, indicating fundamental puzzles in quantum confinement regimes in these molecular-like semiconductor precursors. By simply tuning inorganic salts and synthetic conditions, we are able to synthesize selected nanoclusters, CdS, CdSe, CdTe, ZnS or ZnSe, via this template-entrained method to further grow 2D thin-film materials for energy-transport applications.

Colloidal nanoplatelets (NPLs), which are also known as colloidal quantum wells, are favorable light-emitting materials exhibiting unique optical properties making them promising for light-emitting diodes [1] and lasers [2, 3]. These properties include narrow photoluminescence full-width at half-maximum (~8 nm) and absence of inhomogeneous broadening. Recently, NPLs have been found to assemble into stacks [4, 5]. However, their emerging characteristics essential to the light-generation applications have not been previously studied nor elucidated.
In this work, we systematically investigate and present excitonic properties of controlled column-like stacked NPL assemblies. Controlled stacking of the CdSe NPLs (4 monolayer thick) is achieved in solution phase via step-by-step addition of ethanol, which is an anti-solvent for the NPLs, as supported by the transmission electron microscopy. As the NPLs form into stacks, surprisingly we observe an order of magnitude decrease in the photoluminescence quantum yield while the transient fluorescence decay of the NPLs is considerably accelerated. These observations are corroborated by ultra-efficient Förster resonance energy transfer (FRET) among the same type of NPLs (homo-FRET). An ultralong Förster radius of ~13.5 nm is found owing to the large extinction coefficient, small Stokes shift and collinear orientation of the transition dipoles in the NPL stacks. Based on the isotropic exciton transport assumption [6], an extremely long exciton migration length of 133 nm is found in the NPL stacks, which is one of the largest among the colloidal semiconductor nanocrystals [7]. Interestingly, the common signatures (i.e., red shift of photoluminescence and broadening of the emission) of homo-FRET are not observed in the NPL assemblies since inhomogeneous broadening is absent [8]. The observed substantial quenching of the photoluminescence intensity and increased fluorescence decay rates as the NPLs form into stacks are excellently explained by trapping of the excitons via non-emissive NPLs within the stacks as crucially assisted by ultra-efficient homo-FRET. Therefore, a single non-emissive NPL within an NPL stack would quench the emission of the stack itself due to rapid trapping and ultrafast nonradiative recombination.
Overall, the stacked assemblies of the NPLs are shown to exhibit a surpassing ultralong exciton transport. These results elucidate that it is critical to control stacking in NPL solids for the purpose of efficient light generation via NPLs.
[1] Z. Chen et al. Adv. Funct. Mater. 24, 295-302 (2014).
[2] B. Guzelturk et al. ACS Nano 8, 6599-6605 (2014).
[3] J. Q. Grim et al. Nat. Nanotech. doi: 10.1038/nnano.2014.213 (2014).
[4] B. Abécassis et al. Nano Lett. 14, 710-715 (2013).
[5] M. D. Tessier et al. ACS Nano 7, 3332-3340 (2013).
[6] S. Menke et al. Nat. Mater. 12, 152-157 (2013).
[7] G. M. Akselrod et al. Nano Lett. 14, 3556-3562 (2014).
[8] C. She et al. Nano Lett. 14, 2772-2777 (2014).

Colloidal nanocrystals based on II-VI semiconductors are well known to possess interesting properties for application in the optoelectronics field. The development and improvement of their synthesis has allowed researchers around the world to produce nanocrystals with different shape, composition and heterostructures, enabling a fine control over their light-emitting properties. Of the vast variety of nanocrystals synthesized, CdSe and CdSe/CdS core-shell systems are currently one of the most promising materials to be used as gain medium in lasers.^[1] CdSe and CdSe/CdS nanocrystals lasers can be fabricated employing distributed Bragg reflectors to form a vertical-cavity structure,^{[2, 3]} or using diffraction gratings to form a distributed feedback structure.^[4] These devices are typically fabricated via conventional solution based methods as spin coating or drop casting of the gain material on top of the photonic structure.
Here, we will show how self-assembly of water-soluble CdSe/CdS nanocrystals represents an alternative route to the fabrication of lasers. In addition, we will discuss how the functionalization of the deposition substrate can be used for the fabrication of non-conventional self-assembled nanocrystals solids.
CdSe/CdS quantum dot-in-rods (QDRs) can be stabilized in water avoiding detrimental effects on their light-emission properties employing an enlarged CdS rod diameter.^[5] This highly luminescent water-soluble nanocrystals can be used for the fabrication of efficient micro-lasers exploiting the coffee-stain effect. ^{[6, 7]} The nanocrystals coffee-rings are able to sustain optical feedback, thus removing the necessity to use an external photonic structure. This class of micro-lasers fabricated from water-soluble CdSe/CdS QDRs present single-mode lasing with a threshold of 11 mu;J/cm².^[5] The low-threshold can be explained by the dense packing of the QDRs in the coffee-ring, which is facilitated by the short-chain surface ligands; as well as the improved total refractive index of the assembly (compared to coffee-rings fabricated from organic soluble QDRs). Moreover, deposition of a water-solution of CdSe/CdS QDRs onto a super-hydrophobic substrate gives rise to a hollow-dome nanocrystals solid after water evaporation. The shell thickness of the dome is similar to the coffee ring cross-section, opening the possibility for these structures to also function as micro-lasers.
References
[1] J.Q. Grim et al., Nature Nanotech.2014, DOI: 10.1038/nnano.2014.213
[2] B. Guzelturk et al., ACS Nano2014, 8, 6599
[3] C. Dang et al., Nature Nanotech. 2012, 7, 335
[4] B. Guilhabert et al., Optics Express2014, 22, 7308
[5] F. Di Stasio et al., Small 2014, DOI: 10.1002/smll.201402527
[6] R. D. Deegan et al., Nature 1997, 389, 827
[7] M. Zavelani-Rossi et al., Laser. Photon. Rev. 2012, 6, 678

Colloidal quantum wells (CQWs) are the latest member of the colloidal semiconductor nanostructures family combining the advantages of size tunable electronic properties with giant oscillator strength and ultra-narrow emission spectra that make them ideal candidates for solution#8209;processed light sources and low-threshold lasers. A further application with great potential impact on life sciences, environmental monitoring and aerospace engineering are solution processed analytical tools, such as luminescent sensing varnishes capable of detecting chemical agents in the atmosphere through their reversible emission response. CQWs with their exceptionally large surface-to-volume ratio and fast radiative lifetimes offer the ideal combination of high emission yield and vast reactive surfaces that are otherwise difficult to achieve using lower dimensional nanostructures and could allow to realize easy-measurable highly-emissive photoresponsive paints to be employed in ambient conditions. In this talk, I will combine spectroelectrochemical (SEC) experiments and spectroscopic studies in a controlled atmosphere to demonstrate the ‘reversed oxygen sensing&’ capability of CdSe CQWs, that is, the exposure to O2 increases their luminescence efficiency, in contrast to conventional sensors that rely on photo-darkening to detect electron withdrawing agents. Reversible photo-brightening has been observed in spherical QDs with un-passivated surfaces that, however, result in low emission effiiciency (~0.4%) and limited detection range (~25%). These systems further showed sensitivity exclusively to water molecules, making them unsuitable for oxygen detection. Side-by-side SEC experiments performed for the first time on CdSe and CdSe/CdS core/shell CQWs allow us to recreate the effects of oxidative or reducing environments in a highly controlled fashion and thereby to directly relate the sensing response to the occupancy of surface states. Time-resolved and magneto-optical measurements demonstrate that, under vacuum, heterostructured CQWs stabilize in their negatively charged trion state, similarly to spherical hetero-nanocrystals. The O₂ sensing process is reversible and could be used to realize novel ‘reversed&’ analytical sensors capable of generating an enhanced light signal when exposed to harsh environments. The high starting emission efficiency of CQWs provides a possible mean to further enhance the environmental sensitivity by partially de-passivating the quantum well surfaces and thereby to enhance the density of available unsaturated sites with a minimal cost in term of luminescence losses. This makes these novel two-dimensional colloidal structures particularly suitable for applications spanning from envonmental sensing to air flow studies in aerodynamic research that currently rely on smoke, viscous fluids or evaporating suspensions for revealing transitions from laminar to turbulent flow as well as flow separation on airplane or car body parts.

Colloidal quantum wells, also known as nanoplatelets (NPLs), are highly promising materials for next generation colloidal optoelectronics. Owing to their similarity between epitaxial quantum wells, NPLs exhibit outstanding optical and electronic properties when compared to other class of colloidal semiconductor nanocrystals. [1] With the ease of colloidal synthesis, NPLs having different thickness can be synthesized and their emission and absorption spectra can be tuned by changing the thickness of NPLs. In addition, thanks to colloidal synthesis of NPLs having atomically flat surface, inhomogeneous emission broadening is almost suppressed and narrower emission bandwidth (< 8 nm) is achieved. Moreover, NPLs exhibit the giant oscillator strength with their ultrafast fluorescence lifetime. Also, assorted structures of NPLs having vertically grown shell region [2] and/or laterally grown crown region [3] can be synthesized in order to achieve superior properties, which is indispensable for various advanced optoelectronic applications. For example, by using core/shell and core/crown NPLs having Type-I electronic structure, light-emitting diodes [4] with a narrower emission bandwidth and solution-processable laser [5] with lower lasing threshold have been demonstrated, respectively. However, NPLs having Type-II electronic structure have not been synthesized nor studied to date.
In this study, we have demonstrated for the first time the synthesis and characterization of CdSe/CdTe core/crown NPLs having Type-II electronic structure. With the core-seeded colloidal synthesis, CdSe/CdTe core/crown NPLs are synthesized having well-controlled CdTe crown region widths. By using TEM images and XRD patterns, uniform and epitaxial growth of CdTe crown region is confirmed. With the increasing width of the CdTe crown region, strongly red-shifted photoluminescence (> 150 nm) is observed originating from the intermediate transition at the core/crown interface. In addition, they exhibit significantly increased radiative lifetime characteristically for semiconductors having Type-II like electronic structures. Finally, we performed ligand exchange with 3-mercaptopropionic acid (MPA), which can allow for enhanced charge extraction efficiency owing to their shorter-chain length and enable high quality film formation by layer-by-layer (LBL) assembly. With all of these promising properties, CdSe/CdTe core/crown NPLs hold great promise for next-generation light harvesting applications including solar cells and photodetectors.
[1] S. Ithurria, et al., Nat. Mater., 12, 936-4, 2011.
[2] B. Mahler, et al., J. Am. Chem. Soc., 45, 18591-8, 2012.
[3] A. Prudnikau, et al., J. Am. Chem. Soc., 39, 14476-9, 2013.
[4] Z. Chen, et al., Adv. Funct. Mater., 24, 295-302, 2014.
[5] B. Guzelturk, Y. Kelestemur, et al., ACS Nano, 8, 6599-6605, 2014.

Colloidal CdSe/CdS quantum dot nanorods (QDNRs) have unique anisotropic optical properties. A passive shell that facilitates functionalization without altering the optical properties of CdSe/CdS QDNRs is needed. Here, we report a method for controlled deposition of SiO₂ overcoatings onto CdSe/CdS QDNRs. While several methods have been developed for SiO₂ overcoatings on spherical CdSe-based quantum dots, successful extension of these approaches to QDNRs has been limited. We report SiO₂ overcoatings of uniform thickness (2-12 nm) on CdSe/CdS QDNRs, which preserves the rod shape of the composite nanoparticle after overcoating. Addition of poly(ethylene oxide) silane (PEG-silane) during growth terminates the reaction and allows for facile control over the shell thickness, especially for obtaining thinner shells. Under different reactions conditions, SiO₂ can be heterogeneously deposited onto the QDNRs, resulting in SiO₂-lobed QDNRs, which can appear as nano-lollipops (single lobe) or nano-dumbbells (double lobes).

(CdSe)₁₃ and other magic-sized nanoclusters have been known as key intermediates and primary building units for low- and high-temperature growths of CdSe 0D, 1D and 2D quantum structures. Recent developments have successfully isolated pure clusters with distinct spectroscopy. Unfortunately, experimental data were not able to confirm these hypothesized crystal structures via X-ray diffraction techniques due to short coherent lengths of packing scales and poor crystallinity from bulky ligands. Theoretical calculations already predict two most stable C_3v and C₁ isomers for (CdSe)₁₃ with relative energy levels. For the C_3v isomer, a cage-core structure was originally proposed in 2004 where Cd₁₃Se₁₂ shaped the cage and one unique Se was situated in the central core position. The cage-core structures have been widely recognized structures in II-IV semiconductor clusters in favor of its stabilization energy. In parallel, less energy-preferred (CdSe)₁₃ present all surface atoms of Cd and Se atoms exposed to amine ligands, resulting C₁ symmetry without inversion center. Naively speaking, two structural isomers may be not spectroscopically equivalent due to slightly different chemical environments. In practical terms, there is some supporting evidence to reveal true structure of intriguing (CdSe)₁₃ that successively match its experimental electronic data. Herein we propose to understand the most stable structure of (CdSe)₁₃ via EXAFS and NMR.
From the EXAFS data, we have obtained a preliminary backscattering results from bulk CdSe (3D structure), quantum belts (QBs, 2D structure) and (CdSe)₁₃ (0D structure). The predominate peak in real-space suggest an existence of Cd-Se bonding between 2.6-2.7 Å among samples with a decreasing trend of Cd-Se coordination numbers from 3D bulk and 2D QBs (1.4 nm) to 0D cluster (0.83 nm). Regression analysis of the real-space data indicate local quantified coordination information between Cd, Se and N. The total Cd coordination numbers in (CdSe)₁₃ around four (3.25 from Cd-Se and 0.62 from Cd-N) are slightly smaller than those of the tetrahedral CdSe in bulk. A small change in Cd coordination numbers suggest that capping agent (amine) can effectively bind to Cd atoms on the surface in exchange of Se bonding in the (CdSe)₁₃ cluster.
Alternatively, we also confirmed a nearly 4-coordination environment in ¹¹³Cd NMR results and a single chemical environment in ⁷⁷Se NMR results. Our results seems to adapt the C1 model in the theoretical predictions as the stable crystal structure at room temperature. We argue that a quick exchange of Cd-Se bond may occur at room temperature or relaxation time (> 3600s) for the unique core-site of Se can be fairly long. As a result, we employ atomistic simulations to study thermally activated exchange between different Cd-Se conformations, which may help us to truly resolve local bonding and chemical environments of the most important cluster.

Recent emerging technologies, especially nanotechnology, allow us to synthesize nanoparticles at easy process and a low cost. Nanotechnology has taken a great attention to fabricate smaller or more compact devices to meet our multiple demands. To improve the performance of nanodevices, scientists put down tremendous efforts to molecularly design nano-materials and then to fabricate novel devices. Nanotechnology is a part of the chemical domain, which produces new materials at the nanoscales. However, there are limitations to produce multifunctional nanomaterials through conventional synthesis. A novel synthetic approach such as a microfluidic synthesis was recently developed to synthesize nanomaterials/particles/colloidal compounds at the nanoscales. Microfluidic synthesis allows us to produce multifunctional nanomaterials/particles, which can satisfy our growing demands in miniaturizations. The technique is also dynamic and continuous productions using a small amount of reagents. In this study, we demonstrate a microfluidic synthesis of CdS colloidal nanoparticles by design of a novel microfluidic reactor. The microreactor operates an alternating droplet generation and controlled dynamic droplet fusion in the micro-channels for synthesis of CdS nanoparticles. A high-efficiency microreactor for droplet generations and fusions was observed. The controlled CdS droplet fusion was managed by passive control based on the channel geometry and liquid phase flow. Synthesis of CdS nanoparticles utilizing each fused droplet as the microreactor for rapid and efficient mixing of two reagents are demonstrated. Following alternating droplet generation, the channel geometry allows the exclusive fusion of alternate droplets with concomitant rapid mixing and produces supersaturated solutions. The spectroscopic properties of the CdS nanoparticles produced by this method are compared with CdS prepared by a simple bulk mixing.

Suitable band gaps, high luminescence efficiencies, and relatively low toxicity make multicomponent chalcopyrite nanocrystals attractive candidates for lighting and biolabeling.¹ However, the synthesis of colloidally-stable I-III-VI materials remains underdeveloped. Here, we present a simple, high-yield colloidal synthesis based on an amide-promoted approach.² This approach differs from conventional hot-injection syntheses of nanocrystals by the presence of amide-anions in the reaction mixture, which speed up the reaction rate at the nucleation stage. Nearly quantitative reaction yields and precise composition control are among benefits of this approach. While amide-promoted syntheses have been successfully applied to make various binary chalcogenides,³ this work presents the first use of amide-promoted synthesis to achieve high performance ternary compounds.
We will present results showing excellent size and composition control for two systems: Cu-In-Se (CISe) and Ag-In-Se (AISe) nanocrystals. To achieve highly emissive nanocrystals, we focus on the size range below 5 nm and an In-rich composition. Luminescence efficiency is further improved by overcoating the CISe and AISe nanocrystals with ZnS or ZnSe shell, and we report record quantum yields of 75-80% for both materials. We will discuss the role of ligands and cation reactivity in tuning composition and defect concentrations, which play a key role in luminescent properties of I-III-VI nanocrystals.
1. H. Zhong et al., J. Phys. Chem. Lett.2012, 3, 3167.
2. O. Yarema et al., Chem. Mater.2013, 25, 375.
3. M. Yarema et al., Nanoscale2013, 5, 8398.

Chemiluminescence (CL) has been considered to be a powerful tool for luminescence imaging due to capabilities of sensing target molecules and of removing background fluorescent signals caused by samples or substrates. Being of particular interest, CL generation from fluorescent semiconductor nanocrystals (NCs) most of which are cadmium chalcogenide was reported with proposed mechanisms of radical-mediated charge injection. Burst CL kinetics within a short time period in previous systems, however, could be addressed as one of the issues that limit practical applications. Here, we present a CL system of CuInS₂ (CIS)-based NCs in combination with oxidants and electron donors, which includes demonstration of peculiar luminescence phenomena and ways to control CL properties. Luminescence was observed from thiolate-capped CIS-based NC colloids in oxidative conditions with electron-donating molecules, and it showed quite slow decay for a period of minute-to-hour scale. Interestingly, the generation of the luminescence was accompanied by partial aggregation of NCs, implying that deterioration of the NC surface occurred. In addition, strong dependence of CL kinetics on compositions of NCs suggested in part that edge regions of a crystal matrix in connection with ligands are where we should focus on. Taken together with XPS and EDS analyses, it was identified that thiolate ligands were detached from the NCs via oxidation during the CL generation, thus likely forming intermittent electron-deficient defects on the surface. By adjusting the combination of oxidants and electron donors, it was able to separate two processes (i.e. hole and electron injection) involved in a chemical excitation of this system, and consequently to control the generation and the kinetics of CL. This presentation will offer detailed discussions on the mechanism of hole/electron injection for CL at the NCs&’ surface and approaches to enhancement of CL signal as well as its duration.

Metal-semiconductor hetero-structured nanomaterials are emerged as a new class of functional materials for their efficient photocatalytic activities in water splitting, photo-current and photo-response studies, facilitating organic reactions and improving the stability and performance in different device based applications. Even, the chemistry of designing several hetero-structures are well documented, but still the core issue of selective facet dependent binding of metal in semiconductors and the formation of hetero-epitaxy at the junction are not yet been widely investigated. Moreover, the possible electron transfer from metal to semiconductor or semiconductor to metal, particularly in the hetero-structures where both materials have the absorption in solar spectral window, is not yet well established. Hence, for proper utilizations of these materials, these core fundamental issues are needed to be understood. Keeping these in mind, we discuss here the formation chemistry of metal Au coupled with low bandgap semiconductors CZTS (Cu₂ZnSnS₄) and SnS, and analyze the hetero-epitaxy formation at their hetero-junctions. For CZTS, confining the epitaxy formation along {111} planes of Au, it is observed that the wurtzite and tetragonal phases of CZTS follow the coincidental site epitaxy with different periodic intervals. Further investigation suggests that the hetero-structures show increase in the photo-current and photo-response activities compared to only CZTS nanostructures. Similarly for SnS, different size of Au particles are attached and peculiarly, here the hetero-structures show the Au particles size dependent tuning in their materials properties. The small Au(0) clusters (< 3 nm) are typically decorated randomly in cube and tetrahedron shapes of SnS but larger plasmonic Au(0) particles form twin structures with coupling at the corner of the SnS cubes. Contrastingly, the non-plasmonic Au cluster-SnS hinders the visible light photocatalytic activity whereas the plasmonic coupled Au-SnS enhances the photo catalytic activity towards the reduction of the organic dye. However, both types of heterostructures show enhanced photoresponse activities and stability of the device during illumination and measurement of photocurrent. From the aspects of the fundamental of the formation of the hetero-structures and also on the issues of new properties or applications, we believe these finding would help the community and also provide guidelines for investing new functional materials and their utilities.

Transition-metal chalcogenides, such as cobalt selenide and nickel selenide, are important for solar cells, catalysts and magnetic studies. Recent study shows that nonstoichiometric cobalt selenide (Co_0.85Se) can be used as transparent counter electrodes for dye-sensitized solar cells [1].There has been much interest in the development of synthetic methods to produce magnetic cobalt selenide systems. However, these processes were complicated and usually accompanied impurity phases. Herein, we present a one-step synthesis to directly and quickly produce the hexagonal Fe-substituted cobalt selenide nanoparticles (NPs), and study their structural and magnetic characteristics. Hexagonal iron-substituted cobalt selenide, (Co_1-xFe_x)_0.85Se (0.0 le; x le; 0.5), NPs were synthesized via thermal decomposition of iron nitrate, cobalt nitrate and elemental selenium in oleylamine (OLA) at temperature between 150 and 350 °C for 0.5-2 hrs. The as-synthesized powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) and SQUID. The broad XRD pattern indicated a hexagonal structure with mean crystallite size of about 5-10 nm and TEM showed spherical particles of similar dimensions.
Magnetic measurements displays that the magnetic susceptibility Chi;_M of (Co_1-xFe_x)_0.85Se (0.0 le; x le; 0.25) NPs is negative at low magnetic field (H= 100 Oe). At low temperatures the magnitude of magnetic susceptibility Chi;_M is seen to decrease quite rapidly with increasing temperature. Chi;_M between 5 and 390 K can be described well by the following relation: Chi;_M=Chi;₀+aT^-α+bT². A change in magnetic properties from diamagnetism for samples with x le; 0.25 to weak ferromagnetism for samples with 0.33 le; x le; 0.42 is obtained. Ferromagnetic behavior is found for samples with x ge; 0.5. This variation of magnetic properties is probably caused by the atomic rearrangement of Fe, Co and Se. Detailed magnetic behavior of (Co_1-xFe_x)_0.85Se NPs during Fe-substitution was discussed in terms of atomic vacancies distribution over the crystal lattice and the sub-lattices unequally populated with magnetic ions.
Reference:
1. Feng Gong, Hong Wang, Xin Xu, Gang Zhou, Zhong-Sheng Wang, J. Am. Chem. Soc. 2012, 134, 23minus;26.

Quantum dots (QDs) are nanometer-sized particles of semiconducting materials having charge carrier confinement with discrete atomic-like energy states. In this way, the size of those structures are sufficiently small which can lead to the interaction between charge carriers and as a consequence the splitting and shifts of the energies [1]. The main advantage of those nanoparticles is the fact that the physical dimensions and shape influence their optical and electrical properties [2]. The quantum tunability have led to many different perspectives and applications ranging from devices [3] to medical diagnoses [4]. An important tendency on the field of QDs synthesis is to move toward environmental friendly procedures.
In this sense we have prepared ZnO QDs via sol-gel route starting with the precursor zinc acetate and the alkaline solution of NaOH, and studied the influence of the addition of ethylene glycol as a stabilizer during the time range of the beginning of the reaction (1-7 min), as well as, the variation in the ethylene glycol and the solvent 2-propanol ratio. The optimization has the main goal to achieve particles with enough stability, and size or shape inside the requirements for quantum confinement effect. During the reaction time of 2 h the nucleation and growth have been accompanied by Uv-vis measurements and the qualitative emission by a handling lamp with excitation energy at 365 nm. For long-term stability, regarding the particle size and monodispersity and good fluorescence yield, seems that the amount of ethylene glycol is around 10 % (v/v) and the time addition of the stabilizer close to 3 minutes. From the spectroscopic measurements it was possible to explore the underlying mechanism of nucleation and growth and to have the control of it. The main techniques which are supporting the results are transmission electron microscopy, fluorescence and Uv-vis spectroscopy. The well stabilized colloidal QDs were further applied as active layers in light emitting devices and as luminescent probes for sensing purposes.
[1] Pohl, U. W.; Rodt, S.; Hoffmann, A. In Semiconductor Nanostructures; Bimberg, D., Ed.; Springer: Berlin, p. 269 (2008).
[2] Kramer, I. J.; Sargent, E. H. Chem Rev, 114, 863(2014).
[3] Talapin, D. V.; Steckel, J. Mrs Bull, 38, 685(2013).
[4] Smith, A. M.; Dave, S.; Nie, S. M.; True, L.; Gao, X. H. Expert Rev Mol Diagn, 6, 231 (2006).

Colloidal quantum dots (QDs) are governed by quantum confinement effect (QCE) according to which the energy band gap of QDs is widened as particle size becomes smaller into a few nanometer regions. QDs show various unique photo-physical properties, such as tunable emission wavelength with changing particle size, excellent color purity, and high photoluminescence (PL) efficiency. For these reasons, QDs have been received lots of attention in various applications for such as solar cells, light-emitting diodes, biological labeling, and chemical sensing. Particularly, extensive research on Cd-chalcogenide QDs (e.g. CdSe,CdS etc.) has been conducted actively due to their high PL quantum yield. However, the CdSe and CdS QDs are restricted to the red and green region of the visible spectrum, respectively. Therefore, many groups tried to develop highly efficient blue-emitting QDs such as ZnCdS, ZnCdSe structures. Nevertheless, those materials are still Cd-based and do not accommodate the health issues, environmental needs, and bio-applications of the future.
In this work, the synthesis of Cd-free blue-emitting ZnTeS/ZnS QDs has been developed. The studies on the synthesis of ZnTeS QDs are still limited, because the metallic tellurium is hardly transferred to anionic species, thus resulting in poor yield of synthesis reaction. We used a strong reducing agent super-hydride (LiBH(CH₂CH₃)₃) solution in THF (1 M) to obtain ZnTeS QDs with a uniform spherical shape. The alloy structure of ZnTeS QDs with different Te molar ratio was consistent with the characteristic X-ray diffraction (XRD) patterns. The shift to larger diffraction angles was detected as Te content increased. PL emission spectra can be controlled from 420 nm to 520 nm by varying the compositions of QDs (ZnTe_1-xS_x, X=0.70, 0.90, 0.95, 0.975). In addition, we employed inductively coupled plasma mass spectrometry (ICP-MS) to further investigate of elemental distribution between tellurium and sulfur in the products. ZnTeS core itself shows very low PL quantum efficiency of less than 1%, and unique dual emission property at 380 nm and at 470 nm under excitation of 330 nm. The introduction of ZnS shell on the ZnTeS core increased the PL QY up to 18% revealing an outstanding achievement among non-Cd based blue-emitting QDs. Additionally, a strong emission peak at 450 nm showed up in the ZnTeS/ZnS core/shell structure which was not observed from the ZnTeS core. At last, the photo-physical study using time-correlated single-photon counting (TCSPC) was investigated to identify the origin of various emission features of ZnTeS and ZnTeS/ZnS structures.

Ever growing need for energy generation and storage applications demands development of materials with high performance and long term stability. A wide variety of copper-based semiconducting chalcogenides have been investigated in recent years to address the need for sustainable solar cell materials. An attractive class of materials consisting of non-toxic and earth abundant elements is the copper-antimony-sulfides. The copper-antimony-sulfide system consists of four major phases, namely CuSbS₂ (Chalcostibite), Cu₁₂Sb₄S₁₃ (Tetrahedrite), Cu₃SbS₃ (Skinnerite) and Cu₃SbS₄ (Femitinite). All four phases are p-type semiconductors having energy band gap between 0.5 and 2 eV, with large absorption coefficient values over 10⁵ cm^-1. We have for the first time developed facile colloidal hot-injection methods for the phase-pure synthesis of nanocrystals of all four phases. Cu₁₂Sb₄S₁₃ and Cu₃SbS₃ are found to have direct band gaps (1.6 eV and 1.4 eV, respectively), while the other two phases display indirect band gaps (1.1 eV and 1.2 eV for CuSbS₂ and Cu₃SbS₄, respectively). The synthesis methods yield nanocrystals with distinct morphology for the different phases. In order to understand the optical and electrical properties, we have calculated the electronic structures of all four phases using the hybrid functional method (HSE 06) and PBE generalized gradient approximation to density functional theory. Consistent with experimental results, the calculations indicate that CuSbS₂ and Cu₃SbS₄ are indirect band gap materials, but with somewhat higher band gap values of 1.6 and 2.5 eV, respectively. On the other hand, Cu₃SbS₃ is determined to be a direct band gap material with a gap of 1.5 eV. The absorption coefficient values at visible wavelengths for all the phases are estimated to range between 10⁴ and 10⁵ cm^-1, confirming their potential for solar energy conversion applications. Exploring the use of these materials for supercapacitors have shown promising specific capacitance values with excellent cyclic stability. The unique properties of Cu-Sb-S nanocrystals make them attractive both for solar energy conversion and energy storage applications. The details of the synthesis methods, structural and optical characterizations, electrochemical measurements and band structure calculation will be presented and discussed

PbS quantum dots (QDs) have attracted much attention in various applications due to their tunable bandgap in the near-infrared region of the electromagnetic spectrum. They are exploited for various applications such as; bioimaging and optoelectronics. PbS QDs have large exciton Bohr radii of 18 nm and charge carriers contribute almost equally to the exciton Bohr radii which leads to a strong confinement. In order to synthesize PbS QDs, hot injection methods are applied which result in particles with large sizes and long emission wavelengths (1200-1600 nm). However, small PbS QDs with wavelengths shorter than 1000 nm have rarely reported. In this work, we examine the effect of various reaction parameters such as; temperature, capping ligands, molar ratio of precursors on the growth of PbS nanoparticles. It is found that small PbS nanoparticles with an emission wavelength of as small as 765 nm is achieved by using TAA (thioacetamide) and PbO as the reaction precursors, ODE (octadecene) as the reaction medium and OA (oleic acid) as the capping ligand. On contrary to the conventional Ostwald Ripening which is dissolution of small particles letting the bigger particles grow, in our system, the smaller ones remain as the larger ones grow. Therefore, a bimodal feature is evidenced both in emission and absorption spectra which can be attributed to the very stable small QDs at wavelength of 765 nm.

Solution processed nanocrystal quantum dots show great promise toward enhancing solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (V_oc) is fundamentally limited due to non-radiative recombination processes in colloidal quantum dot (QD) based solar cells and the high dark current results in a significant reduction in V_oc. In this work, we report dark current reduction in PbS QDs-metal Schottky junction solar cells (ITO/PbS QDs/LiF/Al) throughout careful control of the interfacial LiF layer. Using the diode fitting procedure, the dark current (J₀) of devices was extracted from the measured dark current density-voltage (J-V) curves,. For example, the J₀ of the device with 6 Å thick LiF decreased by one order of magnitude with a 10 Å thick LiF layer, leading to 30% increase in V_oc of the devices. Under simulated 1 sun illumination, the photovoltaic performance was optimized with the device incorporating a 10 Å thick LiF layer, exhibiting the power conversion efficiency of 2.5±0.3% with a high fill factor (FF) of 59.7±1.3%, the V_oc of 483±3 mV, and the short-circuit current density (J_sc) of 8.4±0.7 mA/cm². We also will present the effect of PbS QD synthesis and post-reaction workup on device performance. Diode parameters will be extracted from Schottky devices fabricated with PbS nanocrystals synthesized via the Hines et al. [1] or Weidman et al. [2] method. We will focus on any correlation between dark current reduction and the nature of the reaction, in addition to post-synthesis workup.
[1] Hines, M.A. et al. Advanced Materials, 2003. 15(21): p. 1844-1849
[2] Weidman, M .C. et al., ACS Nano, 2014. 8(6): p. 6363-6371.

Monodisperse ensembles of colloidal semiconductor nanocrystals or quantum dots (QDs) feature a narrow, size-tunable emission spectrum in combination with a broad absorption and excitation spectrum and suitability for solution-based processing. This combination makes QDs emitting in the visible of particular interest for lighting and display applications. To make the use of QDs in these fields feasible, interest is shifting from the well-characterized Cd chalcogenide QDs to Cd-free alternatives such as CuInS₂ or InP, where especially InP QDs have emission characteristics that come close to CdSe QDs. The first synthesis of InP nanocrystals with a reasonable size dispersion was published in 1996.¹ However, this synthesis used tris(trimethylsilyl)phosphine [(TMS)3P] as the phosphorous precursor. This compound has a number of important disadvantages. It has an important cost, it is pyrophoric, produces phosphine, a highly toxic gas, in contact with air and its high reactivity prevents the implementation of efficient size tuning strategies based on, e.g., variation of the reaction rate.² These features explain why InP nanocrystals are considerably less studied than CdSe nanocrystals. These disadvantages also prevent the production of InP nanocrystals at an industrial scale.
Here, we propose protocols based on a new phosphorus precursor that allow for cost efficient, up-scaled syntheses of InP nanocrystals of different sizes. The new phosphorus precursor is considerably cheaper than (TMS)₃P - we have calculated a cost reduction of the synthesis by a factor 10 - and it is air stable. Then our method is considerably easier to implement as we do not have to prevent contact between air and the phosphorus precursor. Most notably, our method involves inventive steps to obtain full chemical yield syntheses (percentage of the initially used indium precursor that is effectively converted into InP nanocrystals at the end of the reaction is 80 % or more), where we achieve size tuning from 2 nm to 4 nm at full chemical yield by straightforward adaptations of the reaction mixture. Finally, we present ZnS and ZnSe shell growth procedures. It allows obtaining InP/ZnS or InP/ZnSe core/shell nanocrystals that emit from 480 nm to 640 nm. Their linewidth are in between 45 nm and 55 nm which are excellent values considering the best ones obtain with (TMS)₃P.³ We also obtain reasonable quantum yield which is in between 30 % and 50 %.
We strongly believe that this new InP nanocrystals synthesis method is an important innovative step as compared to what has already been done in literature. We assume that this synthesis will interest many research groups working on the synthesis of Cd-free colloidal nanocrystals and we hope it could help transferring colloidal nanocrystals from the academic field to product applications.
(1) Micic, I.et al. J. Phys. Chem.1995, 99, 7754-7759.
(2) Abe, S.et al. ACS Nano2012, 6, 42-53.
(3) Li, L.et al. J Am. Chem. Soc.2008, 130, 11588-11589.

We present a comprehensive study of the composition and surface chemistry of InP/ZnS quantum dots (QDs) prepared from the well-established non-coordinating solvent strategy. This work is motivated by a crucial lack of rationalization of the mechanisms involved in current InP QDs synthesis and shelling processes.
Advanced ¹H, ¹³C, and ³¹P solution and solid-state NMR studies combined with XPS were used to probe, at the molecular scale, the composition (of the core, the shell as well as the interface) and the surface chemistry of InP/ZnS core/shell quantum dots prepared via a non-coordinating solvent strategy. The interface between the mismatched InP and ZnS phases is composed of an amorphous mixed oxide phase incorporating InPO_x (with x = 3, and predominantly 4), In₂O₃ and InO_y(OH)_3-2y (y = 0, 1). Thanks to the analysis of the underlying reaction mechanisms, we demonstrate that the oxidation of the upper part of the InP core is the consequence of oxidative conditions brought by decarboxylative coupling (ketonization) and amidation reactions. These side-reactions occur during the core preparation and also the coating process, showing that shelling can be detrimental to the chemical nature of the NCs core.
Thus, the presentation will point out beyond these now well-established synthetic and coating procedures, lie unexpected side-reactions that must be taken into account for designing rational synthesis protocols for complex nano-objects.

Covalent metal phosphide nanocrystals represent an important class of materials in a diverse set of applications, including catalysis, solid-state lighting and solar energy capture. Our ability to harness the unique properties of these quantum confined semiconductors relies on our ability to prepare monodisperse samples in high yield and with near unity quantum efficiency. Addressing these challenges will require controlling the solution synthesis of these nanocrystals with atom-level precision. This is particularly challenging for phosphide semiconductors due both to the nature of the reagents used to make the particles and to the covalency of the particles themselves. This work will outline the progress we have made on understanding the precursor conversion reactions, nucleation and growth mechanisms of indium phosphide and zinc phosphide quantum dots.

Semiconductor nanocrystals attract scientific attention because of their size and shape dependent properties, making them interesting candidates for application in solar energy conversion, lighting, display technology, or biolabelling. However, many of the best studied nanocrystalline materials contain toxic heavy metals; this seriously limits their potential for widespread application. Possible less toxic alternatives to cadmium- or lead-containing semiconductors can be found among copper sulfide-based semiconductors, such as copper indium sulfide [1, 2], copper indium selenide [3], copper indium zinc sulfide, copper tin zinc sulfide, or copper tin sulfide. However, the synthesis of these ternary and quaternary materials requires the adjustment of the reactivities of two or three cationic precursors, which makes finding optimum reaction conditions more challenging compared to the synthesis of binary compounds. Frequently, during the growth process of such ternary and quaternary semiconductors the formation of copper sulfide seeds and/or copper sulfide-containing hybrid nanostructures can be observed. Copper sulfide, which is a solid state superionic conductor, can play the role of the catalyst in such reactions and influence the shape of the resulting ternary and quaternary materials [4].
Here, our recent results in the synthesis of ternary and quaternary copper sulfide-based semiconductor nanocrystals will be presented and the role of the ligand molecules and solvents in the shape control of the resulting materials will be discussed.
[1] Kruszynska, M.; Borchert, H.; Parisi, J.; Kolny-Olesiak, J. Synthesis and Shape Control of CuInS₂ Nanoparticles. J. Am. Chem. Soc.2010, 132, 15976-15986.
[2] Kolny-Olesiak, J.; Weller, H. Synthesis and Application of Colloidal CuInS₂ Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces2013, 5, 12221-12237.
[3] Witt, E.; Kolny-Olesiak, J. Recent Developments in Colloidal Synthesis of CuInSe₂ Nanoparticles. Chem. - A Eur. J.2013, 19, 9746-9753.
[4] Kolny-Olesiak, J. Synthesis of Copper Sulphide-Based Hybrid Nanostructures and Their Application in Shape Control of Colloidal Semiconductor Nanocrystals. CrystEngComm2014, 16, 9381-9390.

Multinary copper based chalcogenide nanocrystals are of great interests in many technical applications specially solar energy conversion due to their inherent high optical absorption coefficients.^1-5 Their synthetic chemistry have been extensively studied and a vast amount of synthetic procedures have been developed in the past years. In this talk, we will introduce our recent progresses on the synthesis and applications of copper based ternary and quaternary nanocrystals. CuInS₂ nanocrystals were successfully synthesized by heating up a mixture of all starting materials in 1-Deodecanethiol. This simple method can produce gram quantities of material with a chemical yield in excess of 90% with minimal solvent waste. The overgrowth of as-prepared nanocrystals with ZnS increases the photoluminescence quantum efficiency up to 50%.^1,2 Using a similar synthetic approach, we synthesized a series of quaternary copper based nanocrystals including (CuIn)_X(CdS)_1-x,(Cu₂Sn)_x(CdS)_1-x, (Cu₂Sn)_x(ZnS)_1-x and (Cu₂Sn)_x(ZnSe)_1-x nanocrystals, they all showed strong tunable emission over a large range of wavelength. Furthermore, we found that their Stokes shifts were very large and tunable via controlling the ratio of Cu(In, Sn)/MS (M=Cd, Zn;S=S, Se). Next, we will introduce our efforts on the applications of copper based ternary and quarternary nanocrystals in LED lighting and solar cells. To improve the photostability of CuInS₂/ZnS nanocrystals for lighting application, a unique passivation way was developed, which significantly improved the life time of CuInS₂/ZnS nanocrystals from hours to hundreds hours when irradiated with blue LED. White light LED device with Luminous efficiency of 165 Lm/W_rad was achieved by using the stable CuInS₂/ZnS nanocrystals as red emitter combining Yttrium aluminum garnet (YAG) as yellow phosphor. Finally, thin film solar cells that showing decent efficiencies were successfully fabricated based on ternary and quaternary nanocrystals by using solution processes.³
References
[1] Li L., Daou T. J., Texier I., Kim Chi T. T., Liem N. Q., Reiss P., Chem. Mater., 21, 2422, 2009.
[2] Li L., Pandey A., Werder D. J., Khanal B. P., Pietryga J. M. Klimov V. I., J. Am. Chem. Soc., 133, 1176, 2011.
[3] Li L., Coates N., Moses D., J. Am. Chem. Soc., 132, 22-23, 2010.
[4] Wang Y. Q., Rui Y. C., Zhang Q. H., Li Y. G., Wang H. Z., ACS Appl. Mater. & Interfaces, 5, 11858, 2013.
[5] Steinhagen C., Panthani M. G., Akhavan V., Goodfellow B., Koo B., Korgel, B. A., J. Am. Chem. Soc., 131, 12554, 2009.

Cation exchange is a reversible chemical reaction widely used to create new materials by replacing one type of cation with another, usually provided from solution. We recently proposed an atomistic model of cation exchange by heterovalent cations in semiconductor nanocrystals. The model was developed for Ag dopants in CdSe and makes use of a small set of microscopic parameters, obtained from density-functional theory (DFT) at zero temperature, to perform dynamical simulations at finite temperature over time scales far beyond the reach of DFT. Our simulations span a wide range of dopant concentrations, from light doping to nearly complete cation exchange, and are consistent with many experimentally reported aspects of both phenomena. Thus our model provides a single conceptual framework in which doping and cation exchange are the two endpoints. We find that the Coulomb interaction plays a central role throughout the range of dopant concentration. Now we have applied the model to the experimentally well-studied system of Cu-doped metal chalcogenides nanocrystals. We find that the feasibility of heterovalent doping and cation exchange is determined by the activation barriers of two key processes: interstitial diffusion of dopants and host cations, and kick-out of the host cation by the dopant. The model also explains the experimentally observed autocatalytic "cooperativity" effect in cation-exchange reactions.

Colloidal semiconductor nanocrystals (NCs) have attracted significant attention due to their unique optical and electrical properties leading to striking potential in various applications including bio-imaging/labeling, light-emitting diodes (LEDs), and photovoltaics (PVs). The challenge remains to control the size and size distribution so that one colloidal NC ensemble exhibit narrow absorption and emission with both homogeneous and in-homogenous line broadening. Single-sized ensembles are in outstanding demand, which exhibit only homogeneous line broadening and are also called magic-sized nanocrystals (MSNCs). Here, we report our synthesis of CdTe MSNCs. Importantly, we will address our understanding of CdTe magic-sized nuclei (MSN) and magic-sized clusters (MSCs), in addition to regular nanocrystals (RNCs). The present study provides insights into the formation of colloidal semiconductor quantum dots (QDs) and enables better design for their syntheses.

Over the last 20 years, the hot injection synthesis has become a well-established method for the formation of monodisperse, sterically stabilized colloidal nanocrystals. The method is based on the initiation of a homogeneous nucleation and growth process by the rapid injection of one or more precursors into an apolar reaction mixture at elevated temperature. Complexing agents or ligands are added, which help dissolve the precursors and stabilize the resulting colloid by steric hindrance. Following the widespread use of colloidal nanocrystals as a model system to explore and understand the properties of nanoscale materials and their implementation as active material in various applications, the reaction mechanism of hot injection syntheses has become an active field of study. This involves the question as to how precursors convert into a precipitant or solute but also how the reaction chemistry is related to the development of the size and size dispersion of the nanocrystals throughout the reaction.
In this contribution, we focus on the second question where we address the possibilities of tuning the diameter attained by the nanocrystals when the precursor conversion is complete. Combining kinetic reaction simulations with experimental studies on different syntheses of CdSe nanocrystals, we identify and rationalize two different approaches that enable nanocrystal sizes to be tuned at full yield. By means of an established synthesis for zincblende CdSe nanocrystals, we show that increasing the concentration of excess carboxylic acid ligands or reducing the length of their aliphatic chain systematically leads to larger nanocrystals at the end of the reaction without affecting the reaction rate.[1,2] Opposite from prevailing interpretations of these effects in the literature, where the formation of larger nanocrystals under these conditions are either link to a lower nucleation rate or an enhanced growth rate, we therefore attribute this size tuning to a faster takeover of nucleation by growth. By combining more in-depth experimental investigations with kinetic reaction simulations, we show that two important parameters determining this balance between nucleation and growth are the solubility and the diffusion coefficient of the solute. Increasing the carboxylic acid concentration raises the former whereas shortening their chain length mainly increases the latter, where both have a similar effect on the nanocrystal size at the end of the reaction.
The possibility to tune nanocrystal sizes at full chemical yield and constant reaction rate is important in view of the economical upscaled production of nanocrystals. In the final part of this contribution, we therefore elaborate on the prospects of applying these tuning strategies for the formation of nanocrystals other than CdSe.
[1] Abe et al, ACS Nano 2013, 7, 943-949
[2] De Nolf et al, submitted

For over 30 years semiconductor nanocrystals (NCs) have been the subject of much interest for fundamental and applied studies. The synthesis of NCs has developed and matured over this time such that production of monodisperse, photostable NCs with close to an exact size and shape are readily achievable. However, an understanding of the chemical reaction mechanism behind the synthesis of NCs has lagged the ability to synthesize high quality nanoparticles. In this presentation, NC synthetic mechanisms that have been proposed for metal-chalcogenide (ME) semiconductor NCs, particularly CdE and PbE, will be discussed. Specifically, for PbE NCs synthesized under relatively low temperatures (< 200 0C) it was found that secondary phosphine chalcogenides are the primary reactive species that determines the formation of NC nuclei. Tertiary phosphine chalcogenides are completely unreactive at these temperatures. Small quantities of secondary phosphine impurities in the conventional tertiary phosphine based synthesis thus play an important role in determining the number of NCs formed and the rates of the reaction. Using this chemistry, it was discovered that the diameter of high-quality colloidal PbSe NCs can be easily tuned through the variation of the side-chains of a secondary phosphine precursor with the reaction also running to thermodynamic completion. Calculations of the P=Se bond strength for secondary phosphines with different side chains show that NC size tunability likely results from the ability of free Pb monomer to displace a phosphine on the NC surface. This new synthetic approach provides an effective route for IV-VI NC syntheses on the large scale while providing precise size control.
For NCs synthesized under much higher temperatures (generally > 250 0C), such as CdE NCs, several other mechanistic pathways start to dominate the reactivity. In particular, strong evidence exists for the formation of highly reactive metal-alkyl species in situ, which fundamentally bears a striking resemblance to some of the original CdSe NC syntheses developed over 20 years ago. Also to be discussed will be how an understanding of fundamental reaction mechanism can lead to improved NCs with controllable surface chemistry and with tailored optical properties.

The blinking of QDs is an intrinsic drawback for some biological and photoelectric applications based on single-dot emission. In this talk, we introduce our recent progress on the blinking suppressions of QDs and preparation of nonblinking QDs.^1-3
We successfully prepared the nonblinking QDs in optimum conditions using thiol ligands as stabilizers. We systematically studied the blinking behaviors of single QDs, investigated the effect of different surface ligand, synthetic conditions or UV irradiation etc. on the blinking suppression. We observed that blinking behaviors of QDs were able to be controlled by the structure and concentration of the thiol compounds. We found that, the suppressed blinking mechanism was mainly attributed to elimination of surface traps, formation of appropriate CdS coating on QDs, and controlling the growth dynamics of QDs. Nonblinking QDs show high quantum yield, small size, and good solubility, and will be applied to some fields that were previously limited by blinking of traditional QDs.
References:
[1] Dong C., Liu H., Zhang A., Ren J. Chem. Eur. J. 20, 1940,2014.
[2] Zan F, Dong C., Liu H., Ren J. J. Phys. Chem. C 116, 3944, 2012.
[3] Zhang A., Dong C., Liu H., Ren J. J. Phys. Chem. C 117, 24592, 2013.
[4] He H., Qian H., Dong C., Wang K., Ren J., Angew. Chem. Int. Ed. 45, 7588, 2006.

Nanocrystal quantum dots with their unique size and shape dependent properties have shown immense potential in various applications including energy harvesting and displays. However, despite much progress, the current applications of quantum dots remain limited largely due to the poor understanding of the parameters that affect their properties. For example, though it was known that surface of the nanocrystal and/or internal microstructure of the quantum dots affect the properties of the material, it is not systematically understood. Although it has been shown that the photo-physical properties like quantum yield, blinking etc are dramatically affected by the microstructure, this has not been extensively exploited. This is primarily due to the fact that though the effects of a single defect or interface on the properties of NQDs could have far-reaching consequences due to the high defect density, the control of defects in NQDs is very challenging as a result of the large number thermodynamic and kinetic factors involved. Similarly, in the case of surface modification, though solution route synthesis has opened up a possibility to passivate the surface traps using ligands, the role of various ligands is poorly understood and passivation till now has largely been carried out by chemical intuition.
In this talk, I will discuss the role of defects and surfaces in nanocrystals. I will demonstrate the role of microstructure and propose a method of tweaking the same leading to a playground to regulate the electron hole overlap at semiconductor interfaces leading to enormous potential in the field of applications taking the example of CdS as a host material in different environments.

There is currently great interest in electron transport in nanocrystal solids, driven by potential applications to electronic and optoelectronic devices. To date, most work in this area has employed disordered assemblies, and focused on enhancement of electronic coupling between nanocrystals through engineering of their ligands. A major goal of the field is to achieve band-type transport of electrons, and despite much progress in this direction, claims of band transport remain controversial.
Recently, the fabrication of quasi-two-dimensional superlattices of oriented nanocrystals with epitaxial inter-dot connections was reported [1,2]. The structures exhibit both short- and long-range order. Such “atomically-coherent” assemblies can exhibit square or honeycomb symmetry. Atomistic tight-binding calculations of the electronic states of the square superlattices of cadmium-salt and lead-salt nanocrystals reveal bandwidths that can exceed 100 meV, which imply promising transport properties [3]. Similar calculations of honeycomb superlattices additionally predict a variety of intriguing features, including Dirac cones and topological edge states [4].
We will report on the synthesis of atomically-coherent square superlattices of PbSe, along with structural characterization by x-ray diffraction and high-resolution electron microscopy. Measurements of the conductivity and mobility of the superlattices in field-effect transistor structures will be presented and compared to the results of theoretical calculations of the electronic structure of the superlattices. Prospects for achieving true band transport in these structures will be discussed.
References
1. W. H. Evers et al., Nano Lett. 13, 2317 (2013).
2. W. J. Baumgardner et al., Nano Lett. 13, 3225 (2013).
3. E. Kalesaki et al., Phys. Rev. B 88, 115431 (2013).
4. E. Kalesaki et al., Phys. Rev. X 4, 011010 (2013).

Advances in synthetic methods allow a wide range of semiconductor nanocrystals (NCs) to be tailored in size and shape and to be used as building blocks in the design of NC solids. However, the long, insulating ligands commonly employed in the synthesis of colloidal NCs inhibit strong interparticle coupling and charge transport once NCs are assembled into the solids state as NC arrays. We employ a range of short, compact ligand chemistries to exchange the long, insulating ligands used in synthesis and to increase interparticle coupling. These ligand exchange processes can have a dramatic influence on NC surface chemistry as well as NC organization in the solids, showing examples of short-range order. Synergistically, we use 1) thermal evaporation and diffusion and 2) wet-chemical methods to introduce extrinsic impurities and non-stoichiometry to passivate surface traps and dope NC solids. NC coupling and doping provide control over the density of states, the carrier statistics and the Fermi energy. Examples of strong coupling and doping in II-VI and IV-VI semiconductor NC solids will be given that yield high-mobility, high-conductivity NC solids. Temperature--dependent transport measurements of these materials are consistent with a transition from localized to extended-state charge transport. These high mobility n- and p-type materials are used as the semiconductors to construct large-area, flexible, field-effect transistors and integrated circuits and for solar photovoltaics.

Colloidal semiconductor nanocrystals, commonly known as quantum dots (QDs), provide solutions to many modern energy problems. They can be used for energy generation as active components of inexpensive solar cells and solar concentrators, but they can also enable efficient utilization of electricity for lighting. Nanocrystal structure, specifically heterostructring, size, and surface chemistry, play crucial roles in adapting their properties for efficient energy harvesting and utilization.
The good solubility of QDs in many liquids enables scalable, room-temperature, non-vacuum deposition approaches to making devices (e.g., spray deposition, dip coating). Unfortunately, most quantum dots (e.g., CdSe, PbS, and InP) are expensive and toxic, which dramatically limit their potential. Dr. McDaniel is the founder and President of UbiQD, LLC, a new company located in Los Alamos, New Mexico that is commercializing inexpensive low-toxicity I-III-VI (e.g., CuInSeS) QDs for various energy applications.
In this talk Dr. McDaniel will present recent results of research he conducted while a postdoc at Los Alamos National Laboratory that ultimately led to the founding of UbiQD. Those results include usage of CuInSeS QDs for photocatalysts, photovoltaics, transistors, light-emitting diodes, and phosphors. In particular, surface modification and cation exchange were found to be extremely useful tools for tailoring carrier dynamics for these purposes.

We report on the precision synthesis of CdSe/CdS/ZnS core-shell-shell nanocrystals using a preparative flow reactor. Experimental design is used to determine the crucial parameters and their influence on particle growth and size distribution.
In the second part of the talk, we will present applications of nanocrystals. In particular, we will report on the development of quantum dot quantum rod particles with fluorescence quantum efficiencies close to unity and applications in lighting and display technology. For biomedical applications we will present a biocompatible encapsulation technique based on amphiphilic poly(isoprene-block-ethylene oxide) (PI-b-PEO) diblock copolymers. We varied block lengths, structure and functional terminal end groups and investigated the effect on unspecific uptake. Fluorescence quenching experiments with encapsulated quantum dots show that best behavior in respect to unspecific cellular uptake is realized in those systems, in which the polymer shell yielded best protection against quenching molecules from the surrounding medium. Combination of micelle encapsulation with block copolymers and seeded emulsion polymerization finally leads to biolables for which unspecific uptake could be almost completely suppressed even under in-vivo conditions. We present various techniques for bio-conjugation with recognition molecules and show examples for specific cell and tissue targeting. In-vitro and in-vivo fluorescence and MRI data will be discussed.

Tin(II) sulfide is a p-type semiconductor with a direct band gap of 1.3 eV and an indirect band gap of 1.07 eV. Composed of earth abundant and environmentally benign elements, SnS is handled as an attractive candidate for use in various applications such as solar cells, photodetectors and transistors but also in the field of energy storage. SnS generally crystallizes in the orthorhombic Pnma structure, which favors in principle the anisotropic growth of 2D nanostructures such as platelets or sheets. However, the majority of reported chemical synthesis methods yield isotropic spherical or facetted nanoparticles. Only few examples of 2D structures exist, essentially in form of large (edge length > 200 nm) nanosheets.
Here we present a new approach for the synthesis of uniform, square SnS nanoplatelets of controlled lateral dimension (edge length: 30-100 nm) and thickness (5-15 nm). As the exciton Bohr radius of SnS is around 7 nm, the obtained thin platelets show quantum confinement in one direction of space, as evidenced by UV-vis spectroscopy. A detailed high-resolution transmission electron microscopy study combined with high angle annular dark field electron tomography, electron diffraction and comparison with simulated diffraction patterns sheds light on the growth mechanism of the anisotropic nanoparticles. Furthermore, this study indicates that the exposed (100) surfaces are sulfur-terminated, which may have important consequences on the oxidation sensitivity of the SnS nanoplatelets. The reaction mechanism has been elucidated by means of systematic control experiments. The gained knowledge allowed us determining the conditions for size- and shape-controlled growth of either spherical SnS nanocrystals or 2D nanoplatelets. Intriguingly, the same synthetic scheme using a different sulfur precursor results in small (2-4 nm) tin(IV) sulfide (SnS₂) nanocrystals. SnS₂ is a larger band gap n-type semiconductor (Eg: 2.4 eV), and promising candidate as buffer layer material in thin-film solar cells, in replacement of ubiquitously used CdS.
Sn(IV) is also constituent of the quaternary metal sulfide semiconductor Cu₂ZnSnS₄ (CZTS), under active research as low cost absorber material in thin-film solar cells (Eg: 1.5 eV). We used in situ synchrotron X-ray diffraction for studying the reaction kinetics and identifying transient crystalline phases occurring during a widely applied one-pot synthesis method of CZTS nanoparticles. Our results, backed up by EDS and TEM analyses, indicate that tin is incorporated into initially formed, off-stoichiometric CZS nanoparticles in a cation exchange reaction. This change in composition leads to an abrupt phase transformation resulting in the final kesterite phase (tetragonal space group I-4), taking place within the first few minutes after reaching the reaction temperature of 280°C.

Doped semiconductor quantum dots (QDs) play an important role in many photonics and other applications. Among the common dopants, Cu is one of the most useful and interesting due to its strong green/blue fluorescence. However, Cu(I)-doping is particularly challenging due to its low solubility and tendency of clustering. Co-doping with anions is a common strategy for improving the solubility and optical properties of the Cu(I) dopant. Recently, we have developed a new strategy to co-dope using trivalent cations such as Al(III), Ga(III) and In(III), that have been found to significantly enhance the photoluminescence (PL) properties of Cu. The enhancement is attributed to better charge balance and increased solubility of Cu(I) ions. ZnSe QDs with zinc blende crystal structure and average particle size of 6 nm were synthesized using a wet chemical route. The local structure of the Cu dopant was studied by extended X-ray absorption fine structure (EXAFS). The optical properties were studied using UV-Vis and PL spectroscopy. Exciton dynamics were investigated using time-resolved PL techniques. In addition, DFT calculations have been used to obtain the density of states of a model system to help explain the optical properties and dynamics processes observed. Our study demonstrates that co-doping using different cations with complementary oxidation states is an effective method to enhance optical properties of doped semiconductor QDs of interest for various applications. Synergistic interaction between the primary dopant and co-dopant is critical for the enhanced and tunable PL, and this interaction is likely facilitated by spatial confinement of QDs.

This talk will focus on the similarities and differences between the syntheses of group II-VI and group IV-VI nanocrystals using trialkylphosphine-based precursors. Our results showed that nucleophilic attack of a metal-activated phosphine chalcogenide is a common mechanism for the production of monomer in both material systems. The factors affecting the reactivity of the precursor pairs will also be discussed.

Chalcogenoureas with adjustable organic substituents are conveniently prepared in single steps from commercially available starting materials. These compounds react with metal surfactant complexes producing metal sulfide nanocrystals in quantitative yields. The rate of precursor conversion can be tuned by adjusting the organic substituents, allowing chalcogenoureas to be used over a wide range of reaction temperatures and together with many metal surfactant precursors. By adjusting the precursor conversion rate a desired nanocrystal size can be prepared in quantitative yield. This tunability allows us to probe the temperature dependence of nanocrystal concentration as well as the size distribution produced by the nucleation time. In particular, we have addressed the mechanisms controlling the link between precursor conversion kinetics and crystal nucleation and demonstrated that these reactions proceed by traditionally conceived homogeneous nucleation and growth pathways.

As-synthesized colloidal quantum dot samples typically or inherently contain large concentrations of molecules that could coordinate the surface. However, applications almost universally require purification and/or surface modification of as-synthesized QDs. Purification methods have frequently been seen to decrease the photoluminescence quantum yield (QY) and also to decrease ligand populations. It is essential to understand whether the changes in QY are reversible, how ensemble QY and decay profiles depend on ligand occupation, and the conditions under which surface structures that support high QY can be maintained or restored. We recently described the use of size-exclusion chromatography with a polystyrene stationary phase to separate natively capped colloidal QDs from small molecules in anhydrous solvents. This has the effect of removing impurities and weakly bound ligands, including phosphines, phosphine oxides, and primary amines, enabling the recovery of QDs with surfaces bearing a low and consistent number of metal carboxylate equivalents. In the present study, we take advantage of GPC purification of core/shell QDs to explore the role of ligand population in maintaining high QY. Our results will be discussed in the context of possible energy loss mechanisms among QDs with low ligand populations.

The high temperatures usually required for the synthesis of quantum dots (QDs) are a major drawback, which, beyond obvious energetic concerns, represents a significant limit in the implementation of simple “routine” synthesis methods to ensure run-to-run reproducibility, automation possibilities, and standardization of the nanomaterials. Synthetic protocol to cadmium phosphide QDs, which is a conspicuous material by virtue of its ability to absorb and emit in the near infrared wavelength window (narrow bandgap -0.55 eV- and large excitonic radius -18 nm) also suffer from this major limitation (T> 250°C). We will show how the design of a suitable precursor allows breaking this technological limitation and allows the room temperature synthesis of air-stable, size-tunable, and high optical quality (quantum yields > 50%) Cd₃P₂/ZnS QDs. A large range of emissivity is easily covered (from ~600 nm to 1400 nm) thanks to the modulation of the concentration of reactants and of the temperature (30°C, 90°C). The strategy followed to achieve this two steps synthesis at low temperature is based on the choice and design of highly reactive and soluble precursors, Cd(OAc)₂(OAm)₂ (OAc = acetate, OAm = octylamine) and (TMS)₃P (tris(trimethylsilyl)phosphine) for the formation of the Cd₃P₂ cores and Zn(OAc)₂(OAm)₂ and C₂H₄S (ethylene sulphide) for the coating process. ¹H, ¹³C and ³¹P solution and solid-state NMR studies will be presented and show the presence of a thin layer of oxide at the interface Cd₃P₂/ZnS and of tightly bond ligands (acetate and octylamine) at the surface of the QDs.

Earth-abundant and low-toxic multicomponent nanocrystals such as Cu₂ZnSnS₄, Cu₂ZnSnSe₄ and their alloys Cu₂ZnSn(SSe)₄ have received much consideration due to their potential applicability in photovoltaics, photo/electro-catalysts and thermoelectricity. The ability to tune the band-gap in the colloidal Cu-Zn-Sn system by compositionally alloying (Se/S) and shape tailoring (nanodots/nanorods/multipods) has created the possibility of building up a rainbow solar cell having good light utilization and charge transport. However, the co-existence of two crystal phases in a single nanocrystal (polytypism) has provided even better opportunities to control the morphological and structural properties. Quaternary and quinary chalcogenides are complex systems due to their intense atomic organization, involvement of various ligands in the synthesis and the wide range of compositional variations. Recently, linear/branched polytypic Cu₂SnSe₃, Cu₂Cd_xSnSe_y, Cu₂CdSn(S_1-xSe_x)₄ and CZTSSe nanocrystals have been reported. However, a thorough investigation on the polytypic nanocrystal formation mechanism, as well as control over the phase ratio and shape of the nanocrystals remains elusive.
Herein, we explore a range of possibilities to form different shapes of linear polytypic CZTSSe nanocrystals, comprised of two separate crystalline domains with a wurtzite core and cubic segments at either/both end. Through variation in the metal precursor, arrow-shaped nanocrystals and ellipsoids were synthesized. Bullet-shaped polytypic nanorods were also achieved via a post treatment reaction of phase pure wurtzite nanocrystals. Exquisite control over the phase ratios and material band gap was also attainable by compositionally alloying the nanocrystals. The study was extended to investigate how minute changes in the reaction can significantly alter the growth kinetics, giving rise to important insights on the formation mechanism. They are well characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Rietveld analysis, energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and UV-vis-NIR techniques.

Adsorption of beneficial foreign ion impurities or better known as dopants onto the surface of nanocrystals can alter fundamental features of the host nanocrystals, e.g, electronic, optical, magnetic properties, and even, it can heavily influence the crystal growth kinetics of the nanostructures. As a consequence, it is often observed that the presence of dopants in the reaction medium makes the difference in the crystals growth pattern, which subsequently leads to nanostructures with different shape and crystal phase. While the role of surface ligands binding, monomer activity and related reaction parameters are well studied in the shape architect of the nanostructure, the role of these foreign ion impurities, and specifically, the impact of their size, charge, concentration and mode of insertion, over the crystals growth have not yet been widely explored. The major issue here is the adsorption of the impurity ions on to the nanocrystals surface, and this requires the compatibility of the host crystal, more specifically, the extent of doping depends on the availability of the energetically favorable facets of the nanocrystals. Hence, selection of proper dopants to accommodate into the host matrix is critically important. In addition, the growth kinetics also regulates the dopant adsorption, and typically a fast growth process reduces the possibility of the dopant insertion. Hence, a co-relation between the growth of the host crystal and adsorption of these dopants is important to study the possible directions of the ongoing crystal growth and shape evolution. Keeping these in mind, we investigate the effect of adsorption of Mn ions onto ZnSe nanocrystals, which is one of the most widely studied dopant-host system, and further, we study how does the insertion of Ag ions impurities into the cuboids shaped PdSe and tubular shapes of Sb₂S₃ affect the growth patterns of the nanostructures. Similarly, with proper control of the impurity ions insertion, one existing shape can be changed into a new dimension. Further, the effect of introduction of the impurities in the fast growth process where the shapes are typically controlled by density of the nucleation has been compared with a typical dopant insertion process in a classical growth process. Our results suggest that all these peculiarities in the shape architect in presence of foreign ions contribute significant inputs to the fundamental of the crystal growth and shape architecture of nanostructures, and it seems still many rooms are vacant in understanding the crystal growth mechanism in solution.

Recently, synthetic efforts in the field of colloidal nanocrystals (NCs) have shifted into the exciting domain of 2D nanocrystals. With thicknesses of about 1-3 nm and large height/width aspect ratios, these materials form a class of NCs that have opto-electronic properties closer to epitaxial 2D quantum wells (QWells) than 0D quantum dots. For example, they possess a distinct quantum well absorption profile, a narrow first exciton absorption peak with negligible heterogeneous broadening (at room temperature), and a large band-edge oscillator strength and corresponding sub-ns emission lifetime.[1-5]
In contrast to solid-state epitaxial growth, with colloidal chemistry we can precisely control all dimensions and even the CQWell crystal structure, not being limited by lattice matching conditions with the inorganic barrier layers. In this presentation, we will show how the synthesis of CdSe CQWells with carefully chosen sizes yields a unique synergy of large exciton oscillator strength and strongly suppressed Auger recombination, allowing us to obtain ultralow-threshold stimulated emission and lasing under continuous-wave pumping conditions at room temperature.[5]
Fitting the absorption spectrum using a CQWell absorption profile,[1] we find an exciton binding energy of Ex = 132 meV, strongly exceeding the expected 4-fold enhancement from bulk (Ex,bulk = 15 meV) that is commonly observed in epitaxial QWells. The value is enhanced due to the large dielectric mismatch between the inorganic core and the organic surroundings,[4] with a possible additional contribution from residual exciton confinement in 9 nm x 35 nm plates. The correspondingly large biexciton binding energy Exx = 30 meV presents a two-fold advantage: with Exx exceeding kT, the biexciton population is stable at room temperature upon intense optical pumping, and in combination with a small but finite 24 meV Stokes shift, Exx further red shifts the biexciton emission band away from the first (exciton) absorption peak into the transparency region of the CQWells.
Most importantly, with the suppressed Auger recombination we maintain the biexciton population and corresponding stimulated emission not only under femtosecond, but also continuous-wave pumping conditions. We demonstrate the feasibility of solution-processed quantum well lasers by embedding thin films between two Bragg mirrors and demonstrating multimode biexciton lasing.
[1] A. Naeem et al. arXiv 2014, arXiv:1403.7798
[2] S. Ithurria et al., Nature Mater.2011, 10, 936-941
[3] C. She et al., Nano Lett.2014, 14, 2772-2777
[4] A. Achtstein et al., Nano Lett.2012, 12, 3151-3157
[5] J.Q. Grim et al., Nature Nanotech.2014, DOI: 10.1038/nnano.2014.213

Understanding the formation mechanism of colloidal nanocrystals is of paramount importance in order to get a rationnal approach towards their synthesis. However, to date, very little is known about the different steps which lead to an assembly of nanocrystals starting from molecular precursors. Mechanisms are often suggested but rarely backed up by solid experimental evidence.
We use time-resolved in situ small and wide-angle X-ray scattering to experimentally monitor in a quantitative fashion the formation of CdSe quantum dots synthesized through the heating-up of inorganic precursors in octadecene. We show that the whole process, from the precursor dissolution to the nucleation and growth of the nanoparticles, can be probed in real time. The treatment of the SAXS patterns using Monte-Carlo simulations yields an ensemble measurement of the concentration and size distribution of the nanoparticles with a time resolution of one second. The nucleation rate is measured and we find that the mechanism can be separated in two phases: a fast nucleation stage, followed by growth with a constant concentration of particles. Our experiments also show that diffraction peaks corresponding to the zinc-blende phase are visible since the onset of the nanoparticle formation. Our data strongly suggests that the rate-limiting process of the quantum dot growth is the generation of monomers and cannot be explained by either diffusion or reaction limited growth models

The versatility of semiconductor nanocrystals has been demonstrated in a variety of electronic and optoelectronic device applications. Not surprisingly, however, the furthest advances have been made in devices that exploit one of their earliest realized and still best known properties, namely efficient, tunable luminescence. This is particularly true for colloidal quantum dots (QDs) of II-VI materials, the most widely studied of QD families. Indeed, QDs made of zinc and cadmium chalcogenides have already made an impact as, e.g., phosphors within televisions and mobile devices, taking advantage of decades of efforts toward optimization of basic QD qualities such as band-gap energy, photoluminescence quantum yield, and photostability.
On the other hand, facile control over more complex phenomena within QDs, such as multi-carrier recombination processes, may give rise to a host of new opportunities. Intriguingly, recent studies have shown that such processes are inherently related to, and are even governed by, the exact structure of a given QD. With this as motivation, this presentation will examine how advances in the synthesis of heterostructured II-VI-based QDs are affording control over structure at the single atom-layer level. Specific attention will be given to examples of how these developments are enabling customized, nanoscale-engineering of QDs for advanced performance in demanding light-emitting applications including QD-based electroluminescence, lasers and single-photon sources.

Composition control for I-III-VI QDs plays decisive role in tuning their optical and electronic properties. Particularly, emission efficiencies of CuInE₂ QDs are composition-dependent with the optimal Cu/In ratio of about 0.6.^1,2 Both concentration and distribution of Cu vacancies are important parameters for optimized CuInE₂ emitters. Introducing Zn into structure of ternary I-III-VI colloidal quantum dots (QDs) is known to improve their luminescence properties and environmental stability.³ Starting from our previously reported highly luminescent Cu-In-Se (CISe) QDs,¹ we examine Zn-alloying effects for quaternary Cu-In-Zn-Se (CIZSe) QDs.
In this study, we employ a solution-processed amide-promoted approach.⁴ An injection of reactive lithium-amide salt into reaction solution promotes high nucleation rates and balances the reactivity of cations. The latter allows us to precisely control the composition while keeping constant the size of colloidal QDs. We are thus able to investigate the optical properties of CIZSe QDs solely as a function of their composition. This includes the optical band gap, emission peak position, Stokes shift, and photoluminescence quantum yield (PLQY). We show that the composition for the most luminescent CIZSe QDs is around Cu_0.6In_1.2Zn_0.2Se_2+x. We attribute the existence of such “best-emitting” composition to a synergy of three effects - a defect ordering,⁵ a structure stabilization by Zn-alloying,³ and high concentration of In in the CIZSe structure. We then use the amide-promoted synthesis to control the QD size, which allows us to demonstrate quantum size effects for chosen Cu-In-Zn ratio in the CIZSe QDs.
1. O. Yarema et al., Chem. Mater.2013, 25, 375.
2. H. Zhong et al., J. Phys. Chem. Lett.2012, 3, 3167.
3. L. De Trizio et al. Chem. Mater.2012, 24, 2400.
4. M. Yarema et al., ACS Nano2011, 5, 3758.
5. S. B. Zhang et al., Phys. Rev. B1998, 57, 9642.

Cu_2-XS nanoparticles are remarkable for exhibiting localized surface plasmon resonances (LSPRs) in the near-infrared range. The dependence of the LSPR on Cu stoichiometric deficiency (carrier concentration), size, and shape make this a varsatile system with potential applications in photovoltaics, photocatalysis, and chemical sensing. A challenge lies in obtaining extremely anisotropic morphologies, which allow for high field localizations and expanded access to LSP frequencies.
Here, we access highly anisotropic morphologies by exploiting the the liquid-crystalline (LC) phases of mesogenic lamellar Cu-alkanethiolates. Cu-alkanethiolates represent single-source precursors for Cu_2-XS nanocrystals upon thermolysis. Thermal treatment of these precursors causes alkyl-chain disordering and structural rearrangement to a columnar mesophase. We find that the LC phase functions as a supramolecular template, confining copper sulfide nanocrystals within columnar assemblies and enhancing radial growth. Long-alkyl chains which support this mesophase transition consistently produce Cu_2-XS nanodisks upon thermolysis. We probe the effects of decreasing alkyl chain length, finding a critical point at 8 carbons where nanodisks become polydisperse. At an alkyl chain length of just 4 carbons, the lamellar structure forms but the mesophase transition is suppressed, and so thermolysis results in elongated nanosheet-like particles resulting from the 2D parent structure. The use of bulky alkyl substituents which prevent mesophase transition yield digenite-phase nanoparticles with undefined morphologies. Thus, we show how highly anisotropic morphologies can be obtained from thermolysis carried out within distinct phases of a supramolecular template.

We present a facile and robust synthetic route to CdSe/Cd_xZn_1-xS core-graded shell nanoparticles (Boldt 2013) prepared by in-situ interface alloying between CdS and ZnS shells at elevated temperatures. The resultant nanoparticles exhibit excellent environmental resistance and extremely high photoluminescence quantum yields (PL QY) up to 97% over a broad range of emission wavelengths. Good control over the composition, shape and shell thickness of the resulting CdSe/Cd_xZn_1-xS nanoparticles is achieved by a slow addition of shell precursors using a syringe pump. The in-situ alloying process provides systematic control of the electronic structure of the nanoparticles and enables switching between Type-I and quasi-Type II configurations. We demonstrate that the penetration of zinc into the CdS shell is a temperature dependent process and can be influenced by the ratio of CdS & ZnS shell thicknesses, affording an unprecedented level of selectivity over the photophysical properties of the nanoparticles by varying simple reaction parameters. Of particular note is the ability to use this alloying process to reverse the emission red-shift associated with growing a CdS shell on CdSe nanoparticles, opening up a new route to ultra-high PL QY green-emitting core-shell quantum dots with a range of optoelectronic applications prospects.
Reference: K. Boldt, N. Kirkwood, G. A. Beane, P. Mulvaney, Chem. Mater., 2013, 25(23), 4731-4738.

We have assembled hybrid nanostructures that couple light harvesting II-VI nanocrystals with redox catalysts that perform proton reduction and water oxidation. The functionality of such hybrid structures depends strongly on the charge transfer kinetics between the photoexcited nanocrystal and the catalyst. We utilize ultrafast transient absorption spectroscopy to examine the dynamics of nanocrystal-catalyst charge transfer and the competitiveness of this process with the energy-wasting charge recombination. Our recent findings identifying kinetic bottlenecks for photochemistry will be discussed. Our ongoing efforts to design nanocrystal structure and surface chemistry to enhance the kinetic competitiveness of the photochemical pathways will be described.
References:
1. M. B. Wilker, K. E. Shinopoulos, K. A. Brown, D. W. Mulder, P. W. King, G. Dukovic. “Electron transfer kinetics in CdS nanorod-[FeFe] hydrogenase complexes an implications for photochemical H2 generation.” Journal of the American Chemical Society, 2014, 136, 4316-4364.
2. H-W. Tseng, M. B. Wilker, N. H. Damrauer, G. Dukovic. “Charge Transfer Dynamics between Photoexcited CdS Nanorods and Mononuclear Ru Water-Oxidation Catalysts” Journal of the American Chemical Society, 2013, 135, 3383-3386.
3. K. A. Brown, M. B. Wilker, M. Boehm, G. Dukovic, P. W. King. “Characterization of photochemical processes for H₂ production by CdS nanorod-[FeFe] hydrogenase complexes.” Journal of the American Chemical Society, 2012, 134, 5627-5636.

The need for materials with improved functional properties based on earth abundant and non-toxic elements is driving the design and development of progressively more complex materials. Nano-heterostructures with increasingly higher number of compounds and nanocrystals with larger number of elements are being produced with an extraordinary level of control not only over size and shape, but also over crystal phase and composition. In particular, the numerous possibilities for chemical substitutions and structural modifications in quaternary materials allow significant range in tuning their fundamental chemical and physical properties. Such ample chemical and structural freedom, permits engineering quaternary chalcogenides, potentially made of abundant and non-toxic elements, to fulfil the requirements of a wide variety of applications. As an example, Cu₂ZnSnSe₄ having a direct band gap in the visible spectrum and a high absorption coefficient, has recently attracted much attention in the field of photovoltaics as alternative absorber materials to CdTe and Cu(In,Ga)Se₂. On the other hand, some quaternary diamond-like chalcogenides has been proven excellent thermoelectric materials because of their layered structures and intrinsically low lattice thermal conductivities.
The possibilities and current limitations in the composition control on such quaternary colloidal nanocrystals will be discussed with special focus on the repercussion this composition has on the functional properties and applications of these materials.

We present by first time a simple single-pot synthesis of cadmium sulfide (CdS) quantum dots using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as capping agent for semiconducting composites for flexible thin film ultraviolet photo-detectors. With this approach, we simplify the synthesis of quantum dots, enable to produce a composite stable in water and provide the control of nanoparticles size, electrical and optical properties without the need of bulky insulating capping agents such as mercaptopropionic acid or thiols. In addition, reduces the necessary preparation steps, like nanoparticle purification. We demonstrated the synthesis of cubic phase CdS around 4 nm diameter (0.64 nm standard deviation) embed in PEDOT:PSS deposited in thin films by micro-dropcasting on plasma treated polyethylene terephthalate. Using a multilayer approach, we obtained films homogeneous with uniform nanoparticle distribution, crack and pinholes free with reduced inter-particle spaces to promote the charge carriers transport through the films.
The calculated optical bandgap (Eg) values were 2.69, 2.79 and 2.69 eV for 0.05, 0.1 and 0.2 M of PEDOT:PSS, respectively. It is higher in comparison with CdS in bulk (Eg= 2.40 eV), which is an indicative of particles in nanometric size and opens the possibility to tune the desired wavelength of detection. The prepared flexible devices exhibited a photoresponse to UV-light, enhancing the photocurrent to a I_UV/dark ratio of 350,239.2 at 5V. Furthermore, was observed that the variation of the photocurrent is bias dependent, it increased with the increase of the voltage and is also polarization dependent, being larger the photoresponse with positive polarization. The observed behavior is satisfactory due to the wavelength selectivity of the quantum dots based thin films.

Quantification of active sites on the surface of colloidal nanocrystals is a challenging task which is of great interest in nanoparticle-based electric-optic devices or catalysts. Super-resolution of organic probes attached to the nanocrystals using Total Internal Fluoresce microscopy (TIRF) has been shown as a powerful tool that can shed light on the surface&’s structure. TIRF allows the stochastic nature of photobleaching in fluorescent dye molecules to be exploited for the localization of single dyes. Here, we have plotted the positions of different dye molecules bonded to the surface of long seeded CdSe/CdS Nanorods (average length=204 nm) to a precision of 20-nm. This mapping shows that the most active sites are localized on the ends of the nanorod. This has interesting implications for the use of nanomaterials in photocatalysis and optoelectronic devices.

Semiconductor nanocrystals (NCs) - quantum dots (QDs) - have been of great interest due to their tunable optical and electronic properties with particle size¹. The optical absorption and emission properties of QDs can easily be tuned due to quantum confinement. IV-VI nanocrystalline materials such as lead sulfide (PbS) have a small band gap and they show superior electronic and optoelectronic properties for near-infrared (NIR) applications¹. Since their luminescence can cover the broad wavelength region of NIR, they have found significant application in Shottky-based solar cells, photodiodes and in vitro/in vivo biological imaging¹.
A variety of flask-based methods have been shown to be successful in producing high-quality IV-VI QDs, applying ‘hot injection&’ or ‘heat-up&’ synthetic approaches². However, macroscale techniques are usually ineffective to provide information regarding nucleation and growth kinetics of nanocrystalline materials because the extraction of absorption and PL spectra is performed off-line (the reaction is quenched and the measurements are carried out at room temperature)². Since the time-scale of the nucleation stage is in the range of milliseconds (ms) to seconds (s), it is essential to perform real time measurements for the extraction of kinetic data². In recent years, the use of microfluidic systems for performing complex chemical experiments for nanoparticle synthesis has become increasingly popular³. The precise and rapid control of reaction temperature, reaction time and reagent concentration together with the integration of on-line analytics⁴ provide an ideal environment for a quantitative analysis on nucleation of colloidal NCs. Accordingly, the development of a microfluidic platform combining the benefits mentioned above can potentially offer opportunities for the fast extraction of kinetic information of semiconductor NCs.
Herein we report a novel approach for millisecond kinetics of PbS quantum dots using droplet-based microfluidics. Monodisperse NIR-emitting PbS with absorption peaks between 680 and 1200 nm can be formed rapidly (0.1-15s) and the temporal evolution of the absorption and PL spectra are monitored in real-time using a microfluidic platform with an on-line absorption and fluorescence optical system. Therefore, this microfluidic platform is able to provide quantitative information about the size, size distribution, concentration and emission characteristics of the generated nanocrystals. To our knowledge, this represents the first microfluidic approach for the study of nucleation and growth in high-temperature colloidal crystallization using in-situ absorption and photoluminescence spectroscopy.
References
1. S. V. Kershaw et al., Chem. Rev., 2013, 42, 3033-3087.
2. N. T. K. Thanh et al., Chem. Rev., 2014, 114, 7610-7630.
3. T. W. Phillips et al., Lab on a Chip, 2014, 14, 3172-3180.
4. I. Lignos et al., Chem. Mater., 2014, 26, 2975-2982.

Ternary I-III-VI₂ semiconductors (where I = Cu, Ag; III = Ga, In, VI = S, Se) have been gaining considerable interest in nanoscale research primarily due to their band structures, which are suitable for light-harvesting and light-emitting applications. While a great deal of attention has been given to the Cu-based systems, there are relatively few reports on their Ag-based counterparts. Much of the studies have been limited to AgInS₂ (AIS),which has been known to exist in the tetragonal and orthorhombic form.Meanwhile, AgGaS₂ (AGS), an attractive material for optoelectronics and photocatalytic applications, is not as well-explored. Unlike in the case of AIS,the orthorhombic phase has never been previously observed for AGS. Using a simple non-injection colloidal synthetic approach,we present the first successfulsynthesis of AGS nanocrystals that have the wurtzite-derived orthorhombic crystal structure. The synthetic strategy involves the co-thermal decomposition of Ag(I) and Ga(III) dithiocarbamates in suitable coordinating solvents. It was found that the formation of the orthorhombic polymorph of AGS can be promoted with the use of long-chain alkanethiols and primary alkylamines as coordinating solvents.Using the crystallographic data for orthorhombic AIS as reference, we have carried out Rietveld analysis of our diffraction data and determined the lattice parameters for orthorhombic AGS. From optical absorption data, we have determined the band gap of orthorhombic AGS to be ~2.7eV, which lies in the visible spectrum. The photocatalytic potential of the orthorhombic AGS nanocrystals for organic dye degradation was investigated under visible-light illumination. We have found that 93% of RhB dye molecules could be degraded within 90 min, demonstrating the effectiveness of orthorhombic AGS nanocrystals as a visible-light-responsive photocatalyst.
References: Chem. Commun., 2014, 50, 7128minus;7131

In order to make use of the shape- and size-dependent properties of colloidal semiconductor nanocrystals (NCs) it is necessary to synthesize them with homogeneous shape and narrow size distribution. Controlling these parameters during colloidal synthesis is facilitated significantly by understanding the NCs early stages of formation, since size and shape are determined during this phase.
Monitoring the nucleation of the NCs, which happens rapidly at the very beginning of their synthesis, and their subsequent growth requires the ability to make size-dependent measurements very quickly after initiating a reaction between their precursors as well as at different points later in time. Suitable time-resolved characterization techniques are UV/VIS absorption spectroscopy and X-ray scattering.
Commercial stopped-flow devices,^[1,2] as well as liquid free jets,^[3] have been reported as suitable tools for in-situ studies of semiconductor NC nucleation and growth kinetics. These devices however are either limited to reactions occurring at low temperatures (stopped-flow), or have very limited observable growth times (free jets).
In our contribution we present our custom-designed continuous-flow device, which allows the conduction of colloidal high-temperature (200-400 °C) NC syntheses, combined with the possibility of performing in-situ time-resolved characterization of the synthesized NCs. For this purpose, the device is fitted with a UV/VIS absorption spectrometer. An X-ray flow-cell allows scattering experiments using synchrotron radiation. By coupling a series of custom-made delay growth ovens of different lengths, it is possible to study the growth kinetics.
Utilizing the very small dead times of our microfluidic setup, we are able to separate nucleation and growth of the synthesized NCs. This allows us to investigate the influence of a wide range of parameters on both processes separately, adding to the devices' versatility.
We also present a time-resolved study of the nucleation and early growth behavior of CdSe NCs, showing the influence of flow rates, mixing behaviour, surfactant type and concentration, as well as precursor concentrations on the NCs' formation.
1) A. L. Brazeau et al., J. Phys. Chem. C2009, 113, 20246.
2) M. Tiemann et al., ChemPhysChem, 2005, 6, 2113.
3) W. Schmidt et al., J. Am. Chem. Soc., 2010, 132, 6822.

Colloidal nanocrystals (NCs) are hybrid nano-objects where the properties of the nanocrystal core and surface both determine the characteristics of the entire NC. The surface is often capped by a shell of (in)organic ligands. Hence the importance to understand and gain control over the ligand shell of inorganic NCs. In depth studies have addressed the surface chemistry of various metal sulfide, selenide and pnictide NCs and the covalent bond classification has provided a framework to describe the NC-ligand interactions.^1,2
Here, we extend this analysis to metal oxide NCs. HfO₂ and ZrO₂ NCs were synthesized from the respective metal chloride using a surfactant-free process in benzyl alcohol.³ As synthesized NCs are charge-stabilized by protons, with chloride acting as the counter-ion. We found that the NCs could be transferred to apolar media using a mixture of carboxylic acids and amines. Using solution ¹H NMR, FTIR and elemental analysis, we studied the transfer reaction and the surface chemistry of the resulting sterically stabilized NCs. We found that first amines remove HCl and subsequently the carboxylic acids bind to the surface. This results in aggregate-free dispersions of NCs, stabilized by carboxylate ligands.
Moreover, titrations with deuterated carboxylic acid show that the charge on the carboxylate ligands is balanced by co-adsorbed protons. Therefore, opposite from reports in literature where X-type ligands are bound to the cation-rich surfaces of, for example, CdSe or PbS, carboxylic acids can dissociatively adsorb to bind to HfO₂ and ZrO₂ NCs. Since proton accommodation is most likely due to the high Broslash;nsted basicity of oxygen, our model could be a general picture for metal oxide NCs.⁴
In addition, we show the practical implications of our fundamental findings. By means of ¹H solution NMR spectroscopy, we show that addition of excess octylamine induces a release of cadmium oleate from CdSe NCs, whereas a direct oleic acid/octylamine exchange is observed with HfO₂ NCs. Moreover, excess oleic acid was found to self-adsorb at the HfO₂ NC surface, a process not observed in the case of CdSe NCs. We argue that these unexpected exchange characteristics of carboxylic acids with respect to metal oxide NCs stem from their dissociative adsorption at oxide NCs. The proton can be seen as a cationic X-type ligand while the oleate is an anionic X-type ligand. Combined they form a binding motif we denote as X₂ that allows for self-adsorption and direct exchange for L-type ligands.
In conclusion, metal oxide NCs can hold protons on the surface, in contrast to metal selenides, sulfides or pnictides. These protons were found to be game changers and completely alter the properties of the ligand shell.
(1) Hens, Z. et al.Chem. Mat.2013, 25, 1211
(2) Anderson, N. C. et al.J. Am. Chem. Soc.2013, 135, 18536
(3) De Roo, J. et al.J. of Nanopart. Res.2013, 15, 778
(4) De Roo, J. et al.J. Am. Chem. Soc.2014, 136, 9650

Achieving fine control over composition, morphology and stoichiometry of complex multi-element oxide nanomaterials is a non-trivial challenge that chemists and material scientists are facing in different application areas, from batteries to fuel cells and resistive switching memories. In the context of solar-to-chemical energy conversion, ternary and quaternary oxides are receiving increasing attention as corrosion-resistant light absorbers to drive multi-electron oxidation reactions (especially water oxidation). Monoclinic bismuth vanadate (BiVO₄) is one example of promising ternary oxide with its 2.4eV band gap.¹ Recently, first principles calculations have identified a series of ternary and quaternary oxynitrides (Ti₃O₃N₂ and La₂TiO₂N₂ as examples) as particularly promising for photostable visible light-driven photocatalysts because of their predicted band gaps (around 2 eV) and band edge positions suitable for water splitting.² However, to date, colloidal approaches for preparing complex oxide semiconductor nanocrystals are still limited. One of the main synthetic challenges consists in identifying reaction conditions that will lead to tunable homogeneous composition rather than favoring phase segregation of the multiple elements.
Herein, we will highlight the importance of a fundamental understanding of the mechanistic paths governing nucleation and growth when developing new synthetic schemes for ternary and quaternary metal oxide nanocrystals. In particular our recent results on three different systems will be presented: N-doped TiO₂, BiVO₄ and Sb-doped BiVO₄ nanocrystals.^3,4,5 For N-doped TiO_2, we show that nitrogen dopants can be selectively positioned in substitutional or interstitial sites by properly choosing the amine type used during the synthesis. Nucleation-doping, in presence of primary amines, and growth-doping, in presence of ternary amines, were hypothesized to explain the observed lattice site selectivity of the nitrogen atoms. For doped and undoped BiVO_4, key for controlling the nanocrystal composition was a one-pot seeded growth approach in which bismuth (or bismuth antimony) nanocrystals were reacted with the vanadium and antimony precursors in properly chosen conditions. Finally, we will present charge carrier dynamics data and preliminary photoelectrochemical measuraments to demonstrate how this atomic-level chemical manipulation provide significant opportunities to build new structure/properties relationships in nanomaterials for multielectron redox reaction, such as water oxidation.
1. Kim, T. W.; Choi, K.-S. Science 2014, 343, 990.
2. Wu, Y.; Lazic, P.; Hautier, G.; Persson, K.; Ceder, G Energy Envion. Sci. 2013, 6, 157
3. Lynch, J.; Giannini, C.; Cooper, J.K.; Sharp, I. D.; Buonsanti, R. submitted
4. Loiudice, A.; Cooper, J. K.; Mattox, T.; Sharp, I. D.; Buonsanti, R. submitted
5. Loiudice, A.; Cooper, J. K.; Mattox, T.; Thao, T.; Drisdell, W.; Ma, J.; Wang, L-W; Yano, J.; Sharp, I. D.; Buonsanti, R. in preparation

Silver nanodots consisting of a few silver atoms are becoming excellent fluorophores thanks to their small size, high brightness and excellent photostability.[1] Initially, such clusters were prepared in noble gas matrices due to their vulnerability to oxidization. New protection groups has improved their stability in aqueous solution, leading to the synthesis of a series of spectrally-pure silver nanodots with emissions ranging from the blue to the near-IR.[2] The exact structures of various silver nanodots remain unclear. We examined the photo-responses of various silver nanodots in oxidizing environments. Oxidants bleaches most of nanodot emitters, but only the red and near-IR emitters correlates with the formation of an oxidant-resistant blue emitter. The HPLC-MS indicates that the blue is an oxidized species. Based on such a unique phenomenon, we have demonstrated the advantage of the blue emitter as an imaging agent in oxidizing environments and designed ratiometric luminescence probes for the ultralow concentration detection.[3] We also investigated the generation of silver nanodots by etching silver nanoparticles. The addition of chelating agents, such as polyamines or ssDNA molecules, accelerates the degradation of large silver nanoparticles to a stable stage of silver nanoparticles, which might be critical to the generation of emissive silver nanodots. The etching process is strongly pH dependent, and the binding between silver and the etching groups is crucial for efficient etching. The active centres are especially important for the formation of silver nanodots. We have shown that etching of silver nanoparticles can be an alternative to generate new silver nanodot emitters.[4]
1. Choi S, Dickson RM, Yu J. Developing luminescent silver nanodots for biological applications. Chem. Soc. Rev. 2012, 41(5): 1867-1891.
2. Choi S, Yu J, Patel SA, Tzeng Y-L, Dickson RM. Tailoring silver nanodots for intracellular staining. Photochem. Photobiol.Sci. 2011, 10(1): 109-115.
3. a) Choi S, Park S, Lee K, Yu J. Oxidant-resistant imaging and ratiometric luminescence detection by selective oxidation of silver nanodots. Chem. Commun. 2013, 49(93): 10908-10910. b) Park S, Choi S, Yu J. DNA-encapsulated silver nanodots as ratiometric luminescent probes for hypochlorite detection. Nanoscale Res. Lett. 2014, 9, 129.
4. Park S, Choi S, Yu J. DNA-encapsulated silver nanodots as ratiometric luminescent probes for hypochlorite detection. Chem. Commun. DOI: 10.1039/C4CC07729F