Chemistry of Doped Colloidal Nanocrystals

Garcia G., Buonsanti R., Llordes A., Runnerstrom E.L., Bergerud A., Milliron D.J.

A plasmonic electrochromic effect in which electrochemical doping reversibly modulates near-infrared surface plasmon absorption of aluminium-doped zinc oxide and tin-doped indium oxide nanocrystals is reported. Optical performance, switching kinetics, and cycling durability point to high-performance NIR selective plasmonic electrochromic coatings based on earth-abundant materials.

Della Gaspera E., Bersani M., Cittadini M., Guglielmi M., Pagani D., Noriega R., Mehra S., Salleo A., Martucci A.

We present a new colloidal synthesis of gallium-doped zinc oxide nanocrystals that are transparent in the visible and absorb in the near-infrared. Thermal decomposition of zinc stearate and gallium nitrate after hot injection of the precursors in a mixture of organic amines leads to nanocrystals with tunable properties according to gallium amount. Substitutional Ga(3+) ions trigger a plasmonic resonance in the infrared region resulting from an increase in the free electrons concentration. These nanocrystals can be deposited by spin coating, drop casting, and spray coating resulting in homogeneous and high-quality thin films. The optical transmission of the Ga-ZnO nanoparticle assemblies in the visible is greater than 90%, and at the same time, the near-infrared absorption of the nanocrystals is maintained in the films as well. Several strategies to improve the films electrical and optical properties have been presented, such as UV treatments to remove the organic compounds responsible for the observed interparticle resistance and reducing atmosphere treatments on both colloidal solutions and thin films to increase the free carriers concentration, enhancing electrical conductivity and infrared absorption. The electrical resistance of the nanoparticle assemblies is about 30 kΩ/sq for the as-deposited, UV-exposed films, and it drops down to 300 Ω/sq after annealing in forming gas at 450 °C, comparable with state of the art tin-doped indium oxide coatings deposited from nanocrystal inks.

McLaurin E.J., Fataftah M.S., Gamelin D.R.

Alloyed Zn(1-x-y)Cd(x)Mn(y)Se nanocrystals exhibiting bright intrinsic dual emission attractive for ratiometric optical nanothermometry are reported. Relative to earlier materials with related dual emission, these alloy nanocrystals require shorter syntheses, contain less Cd(2+), and show dual emission that is less sensitive to nanocrystal shape, size, or surfaces.

Rivest J.B., Jain P.K.

Cation exchange is an age-old technique for the chemical conversion of liquids or extended solids by place-exchanging the cations in an ionic material with a different set of cations. The technique is undergoing a major revival with the advent of high-quality nanocrystals: researchers are now able to overcome the limitations in bulk systems and fully exploit cation exchange for materials synthesis and discovery via rapid, low-temperature transformations in the solid state. In this tutorial review, we discuss cation exchange as a promising materials synthesis and discovery tool. Exchange on the nanoscale exhibits some unique attributes: rapid kinetics at room temperature (orders of magnitude faster than in the bulk) and the tuning of reactivity via control of nanocrystal size, shape, and surface faceting. These features make cation exchange a convenient tool for accessing nanocrystal compositions and morphologies for which conventional synthesis may not be established. A simple exchange reaction allows extension of nanochemistry to a larger part of the periodic table, beyond the typical gamut of II-VI, IV-VI, and III-V materials. Cation exchange transformations in nanocrystals can be topotactic and size- and shape-conserving, allowing nanocrystals synthesized by conventional methods to be used as templates for production of compositionally novel, multicomponent, or doped nanocrystals. Since phases and compositions resulting from an exchange reaction can be kinetically controlled, rather than governed by the phase diagram, nanocrystals of metastable and hitherto inaccessible compositions are attainable. Outside of materials synthesis, applications for cation exchange exist in water purification, chemical staining, and sensing. Since nanoscale cation exchange occurs rapidly at room temperature, it can be integrated with sensitive environments such as those in biological systems. Cation exchange is already allowing access to a variety of new materials and processes. With better mechanistic understanding and control, researchers may be able to advance the field to a stage where a custom nanostructure of arbitrary complexity would be achievable by simple cation exchange chemistry and a basic understanding of the periodic table.

Pandey A., Brovelli S., Viswanatha R., Li L., Pietryga J.M., Klimov V.I., Crooker S.A.

Nanoscale materials have been investigated extensively for applications in memory and data storage. Recent advances include memories based on metal nanoparticles1, nanoscale phase-change materials2 and molecular switches3. Traditionally, magnetic storage materials make use of magnetic fields to address individual storage elements. However, new materials with magnetic properties addressable via alternative means (for example, electrical or optical) may lead to improved flexibility and storage density and are therefore very desirable. Here, we demonstrate that copper-doped chalcogenide nanocrystals exhibit not only the classic signatures of diluted magnetic semiconductors4—namely, a strong spin-exchange interaction between paramagnetic Cu2+ dopants and the conduction/valence bands of the host semiconductor—but also show a pronounced and long-lived photoinduced enhancement of their paramagnetic response. Magnetic circular dichroism studies reveal that paramagnetism in these nanocrystals can be controlled and increased by up to 100% when illuminated with above-gap (blue/ultraviolet) light. These materials retain a memory of the photomagnetization for hour-long timescales in the dark, with effects persisting up to ∼80 K. Magnetic circular dichroism reveals an enhancement of paramagnetism in copper-doped ZnSe–CdSe nanoscrystals and that the enhancement persists for hours in the dark.

Schimpf A.M., Ochsenbein S.T., Buonsanti R., Milliron D.J., Gamelin D.R.

The "extra" electrons in colloidal n-type ZnO nanocrystals formed by aliovalent doping and photochemical reduction are compared. Whereas the two are similar spectroscopically, they show very different electron-transfer reactivities, attributable to their different charge-compensating cations (Al(3+)vs. H(+)).

Rossell M.D., Ramasse Q.M., Findlay S.D., Rechberger F., Erni R., Niederberger M.

Dopant atoms are used to tailor the properties of materials. However, whether the desired effect is achieved through selective doping depends on the dopant distribution within the host material. The clustering of dopant atoms can have a deleterious effect on the achievable properties because a two-phase material is obtained instead of a homogeneous material. Thus, the examination of dopant fluctuations in nanodevices requires a reliable method to chemically probe individual atoms within the host material. This is particularly challenging in the case of functionalized nanoparticles where the characteristic length scale of the particles demands the use of a high-spatial-resolution and high-sensitivity technique. Here we demonstrate a chemically sensitive atomic resolution imaging technique which delivers direct site-specific information on the dopant distribution in nanoparticles. We employ electron energy-loss spectroscopy imaging in a scanning transmission electron microscope combined with multivariate statistical analysis to map the distribution of Ba dopant atoms in SrTiO3 nanoparticles. Our results provide direct evidence for clustering of the Ba dopants in the SrTiO3 nanoparticles outlining a possible explanation for the presence of polar nanoregions in the Ba:SrTiO3 system. The results we present constitute the first example of site-specific atomic resolution spectroscopy of foreign atoms in doped nanoparticles and suggest a general strategy to ascertain the spatial distribution of impurity atoms in nanocrystals and hence improve the performance of nanoparticle-based devices.

Chan E.M., Han G., Goldberg J.D., Gargas D.J., Ostrowski A.D., Schuck P.J., Cohen B.E., Milliron D.J.

Nanoparticles doped with lanthanide ions exhibit stable and visible luminescence under near-infrared excitation via a process known as upconversion, enabling long-duration, low-background biological imaging. However, the complex, overlapping emission spectra of lanthanide ions can hinder the quantitative imaging of samples labeled with multiple upconverting probes. Here, we use combinatorial screening of multiply doped NaYF(4) nanocrystals to identify a series of doubly and triply doped upconverting nanoparticles that exhibit narrow, spectrally pure emission spectra at various visible wavelengths. We then developed a comprehensive kinetic model validated by our extensive experimental data set. Applying this model, we elucidated the energy transfer mechanisms giving rise to spectrally pure emission. These mechanisms suggest design rules for electronic level structures that yield robust color tuning in lanthanide-doped upconverting nanoparticles. The resulting materials will be useful for background-free multicolor imaging and tracking of biological processes.

Pucci A., Willinger M., Liu F., Zeng X., Rebuttini V., Clavel G., Bai X., Ungar G., Pinna N.

A simple one-pot approach based on the “benzyl alcohol route” is introduced for the fabrication of highly ordered supercrystals composed of highly uniform 3–4 nm zirconia and rare-earth stabilized zirconia nanoparticles. The as-fabricated supercrystals reach sizes larger than 10 μm and present well-defined 3D morphologies such as flower-like, rhombic dodecahedron, and bipyramids. This system is unique in that the supercrystals are formed in one-step directly in the reaction medium where the nanoparticles are synthesized. The uniformity in nanocrystal shape and size is attributed to the in situ formation of benzoate species that directs the nanoparticle growth and assembly. The low colloidal stabilization of the benzoate-capped nanoparticles in benzyl alcohol promotes the formation of supercrystals in solution by π–π interaction between the in situ formed benzoate ligands attached to neighboring particles. By varying the reaction temperature and the nature of the doping the way the nanobulding blocks assemble in the supercrystals could be controlled. Standard FCC superlattice packings were found together with more unusual ones with P6/mmm and R3̅m symmetries.

Cohn A.W., Kittilstved K.R., Gamelin D.R.

Colloidal reduced ZnO nanocrystals are potent reductants for one-electron or multielectron redox chemistry, with reduction potentials tunable via the quantum confinement effect. Other methods for tuning the redox potentials of these unusual reagents are desired. Here, we describe synthesis and characterization of a series of colloidal Zn(1-x)Mg(x)O and Zn(0.98-x)Mg(x)Mn(0.02)O nanocrystals in which Mg(2+) substitution is used to tune the nanocrystal reduction potential. The effect of Mg(2+) doping on the band-edge potentials of ZnO was investigated using electronic absorption, photoluminescence, and magnetic circular dichroism spectroscopies. Mg(2+) incorporation widens the ZnO gap by raising the conduction-band potential and lowering the valence-band potential at a ratio of 0.68:0.32. Mg(2+) substitution is far more effective than Zn(2+) removal in raising the conduction-band potential and allows better reductants to be prepared from Zn(1-x)Mg(x)O nanocrystals than can be achieved via quantum confinement of ZnO nanocrystals. The increased conduction-band potentials of Zn(1-x)Mg(x)O nanocrystals compared to ZnO nanocrystals are confirmed by demonstration of spontaneous electron transfer from n-type Zn(1-x)Mg(x)O nanocrystals to smaller (more strongly quantum confined) ZnO nanocrystals.

Sahu A., Kang M.S., Kompch A., Notthoff C., Wills A.W., Deng D., Winterer M., Frisbie C.D., Norris D.J.

We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core-shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.

Yarema M., Pichler S., Kriegner D., Stangl J., Yarema O., Kirchschlager R., Tollabimazraehno S., Humer M., Häringer D., Kohl M., Chen G., Heiss W.

Indium tin oxide (ITO) nanopatterned electrodes are prepared from colloidal solutions as a material saving alternative to the industrial vapor phase deposition and top down processing. For that purpose highly monodisperse In(1-x)Sn(x) (x < 0.1) colloidal nanocrystals (NCs) are synthesized with accurate size and composition control. The outstanding monodispersity of the NCs is evidenced by their self-assembly properties into highly ordered superlattices. Deposition on structured substrates and subsequent treatment in oxygen plasma converts the NC assemblies into transparent electrode patterns with feature sizes down to the diameter of single NCs. The conductivity in these ITO electrodes competes with the best values reported for electrodes from ITO nanoparticle inks.

Navulla A., Huynh L., Wei Z., Filatov A.S., Dikarev E.V.

The first single-source molecular precursor for a lithium-manganese cathode material is reported. Heterometallic β-diketonate LiMn(2)(thd)(5) (1, thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) was obtained in high yield by simple one-step solid-state reactions employing commercially available reagents. Substantial scale-up preparation of 1 was achieved using a solution approach. The crystal structure of the precursor contains discrete Li:Mn = 1:2 trinuclear molecules held together by bridging diketonate ligands. The complex is relatively stable in open air, highly volatile, and soluble in all common solvents. It was confirmed to retain its heterometallic structure in solutions of non-coordinating solvents. The heterometallic diketonate 1 was shown to exhibit clean, low-temperature decomposition in air/oxygen that results in nanosized particles of spinel-type oxide LiMn(2)O(4), one of the leading cathode materials for lithium ion batteries.

Ostrowski A.D., Chan E.M., Gargas D.J., Katz E.M., Han G., Schuck P.J., Milliron D.J., Cohen B.E.

Phosphorescent nanocrystals that upconvert near-infrared light to emit at higher energies in the visible have shown promise as photostable, nonblinking, and background-free probes for biological imaging. However, synthetic control over upconverting nanocrystal size has been difficult, particularly for the brightest system, Yb(3+)- and Er(3+)-doped β-phase NaYF(4), for which there have been no reports of methods capable of producing sub-10 nm nanocrystals. Here we describe conditions for the controlled synthesis of protein-sized β-phase NaYF(4): 20% Yb(3+), 2% Er(3+) nanocrystals, from 4.5 to 15 nm in diameter. The size of the nanocrystals was modulated by varying the concentration of basic surfactants, Y(3+):F(-) ratio, and reaction temperature, variables that also affected their crystalline phase. Increased reaction times favor formation of the desired β-phase nanocrystals while having only a modest effect on nanocrystal size. Core/shell β-phase NaYF(4): 20% Yb(3+), 2% Er(3+)/NaYF(4) nanoparticles less than 10 nm in total diameter exhibit higher luminescence quantum yields than comparable >25 nm diameter core nanoparticles. Single-particle imaging of 9 nm core/shell nanoparticles also demonstrates that they exhibit no measurable photobleaching or blinking. These results establish that small lanthanide-doped upconverting nanoparticles can be synthesized without sacrificing brightness or stability, and these sub-10 nm nanoparticles are ideally suited for single-particle imaging.

Wills A.W., Kang M.S., Wentz K.M., Hayes S.E., Sahu A., Gladfelter W.L., Norris D.J.

Al- and In-doped CdSe nanocrystals were synthesized using a three-part core-shell synthesis. CdSe core nanocrystals were first prepared, then allowed to react with dopant precursors in the presence of weakly binding ligands, and finally overcoated with an additional shell of CdSe. The addition of Al dopants quickened shell overgrowth and led to more monodisperse nanocrystals while the addition of In dopants produced more polydisperse particles, as seen by absorption spectroscopy. Elemental analysis combined with chemical etching revealed the dopants were inside the particles and solid state 27Al nuclear magnetic resonance (NMR) spectra indicated that the Al impurities were well dispersed. When the Al-doped nanocrystals were processed into thin-film transistors, enhanced n-type transport was observed with a rise in the Fermi level compared to undoped particles.

Oli F., Feyen S., Alqahtani Z., Al-Ahmary K.M., Al-Mhyawi S.R., Al-Otaibi J.S., Tsegaye D., Abebe B.

Nasr R.A., El‑Sayed A.F., El Komy G.M., El‑Bassyouni G.T., Mousa S.M.

Abstract This study focuses on the fabrication of zinc-doped manganese oxide (Mn2O3) nanoparticles (NPs) to unleash their potential as high-performance photocatalysts. Zn-doped Mn2O3 nanoparticles (NPs) were prepared via a precipitation method, with fine adjustment of Zn content (3%, 5%, and 10%). The structural evolution from cubic Mn2O3 to tetragonal ZnMn2O4 was confirmed by X-ray diffraction (XRD). Energy dispersive X-ray (EDX) analysis confirmed the smooth Zn incorporation, while transmission electron microscopy (TEM) and FESEM revealed the transformative effect of Zn on the particle size and shape. Optical characterizations showed impressive results:UV-Vis DRS revealed a significant reduction in the band gap from 2.26 eV to 1.89 eV, enhancing light absorption. Meanwhile, the photoluminescence (PL) spectra showed vibrant emission peaks at 425, 466, 563, and 623 nm, with the intensity increasing along with the zinc content. The optical prowess of these nanoparticles was validated by the nearly complete degradation of methyl green (MG) dye under visible light. Also, Zn-doped Mn2O3 samples were evaluated against harmful pathogens such as S. aureus, E. faecalis, K. pneumoniae, P. aeruginosa, E. coli, and C. albicans.

Lin H., Talebi S., MacSwain W., Vanshika V., Chakraborty A., Zheng W.

Long T., Luo H., Sun J., Lu F., Chen Y., Xu D., Yuan Z.

Ultrafast reaction kinetics is essential for rapid detection, synthesis, and process monitoring, but the intrinsic energy barrier as a basic material property is challenging to tailor. With the involvement of nanointerfacial chemistry, we propose a carbonization-based strategy for achieving ultrafast chemical reaction. In a case study, ultrafast Griess reaction within 1 min through the carbonization of N-(1-naphthalene)ethylenediamine (NETH) was realized. The carbonization-mediated ultrafast reaction is attributed to the synergic action of reduced electrostatic repulsion, enriched reactant concentration, and boosted NETH nucleophilicity. The enhanced reaction kinetics in o-phenylenediamine-Cu2+ and o-phenylenediamine-ascorbic acid systems validate the universality of carbonization-engineered ultrafast chemical reaction strategy. The finding of this work offers a novel and simple tactic for the fabrication of multifunctional nanoparticles as ultrafast and effective nanoreactants and/or reporters in analytical, biological, and material aspects.

Yu J., Demir H.V., Sharma M.

Lattice strain plays a vital role in tailoring the optoelectronic performance of colloidal nanocrystals (NCs) with exotic geometries. Although optical identifications of lattice strain in irregular-shaped NCs or hetero-structured NCs have been well documented, less is known about optical signatures of the sparsely distributed lattice mismatch in chemically-doped NCs. Here, we show that coherent acoustic phonons (CAPs) following bandgap optical excitations in Cu-doped CdSe colloidal quantum wells (CQWs) offer a unique platform for indirectly measuring the dopant-induced lattice strain. By comparing the behavior of CAPs in Cu-doped and undoped CQWs (i.e., vibrational phase/lifetime/amplitude), we have revealed the driving force of CAPs related to the optical screening of lattice strain-induced piezoelectric fields, which thus allows to determine the strain-induced piezoelectric field of ~102 V/m in Cu-doped CdSe CQWs. This work may facilitate a detailed understanding of lattice strain in chemically-doped colloidal NCs, which is a prerequisite for the design of favorable doped colloids in optoelectronics. Chemical doping induces lattice strain in semiconductor nanocrystals which modulates their optical properties. Here, the authors show that the study of coherent acoustic phonons can explain this behavior in Cu-doped colloidal quantum wells.

Ghosh S., Medda A., Kalauni D., Patra A.

Increasing the Ag dopant percentage reduces the thermalization rate and enhances charge carrier separation, improving optoelectronic device efficiency.

Liu J., Zhang M., Dong N., Leng J., Cheng C., Ma H.

de Mello Donega C.

The essential feature of nanomaterials is that their physical and chemical properties are size dependent, making it possible to engineer the material properties not only by defining its chemical composition, but also by tailoring the size, shape, and surface chemistry of the nanostructures, and the way in which individual building blocks are assembled. This chapter addresses the origin of the size dependence of the properties of nanomaterials, which can be traced to two fundamental nanoscale effects: (a) the increase in the surface/volume ratio with decreasing size, and (b) spatial confinement effects. Furthermore, the definition and classification of nanomaterials is introduced, and the techniques used to fabricate and study them are briefly addressed, with emphasis on nanoparticles of inorganic materials.

Liu J., Song J., Zhang M., Dong N., Leng J., Cheng C., Ma H.

Ji S., Abbas H.G., Kim S.Y., Lee H.C., Lee K., Li S., Choe S., Ahn H., Ringe S., Yang J.

Doping quantum‐confined semiconductor nanocrystals offers an effective way to tailor their unique properties. However, the inherent challenges of nanoscale doping processes, such as the low probability of successful doping, have hindered their practical applications. Nucleation‐controlled doping has emerged as a potential solution, but a comprehensive mechanistic understanding of this process is lacking. Herein, the nucleation‐controlled doping process facilitated by magic‐sized cluster intermediates is elucidated. This approach enables the synthesis of 2D ZnSe quantum nanoribbons with two distinct doping sites. Remarkably, the identity of the dopants plays a critical role in determining the chemical pathways of nucleation‐controlled doping. Substitutional doping of magic‐sized clusters with Mn2+ ions leads to successful substitutional doping of the final 2D nanocrystals. Conversely, Co2+ ions, initially occupying substitutional positions in the magic‐sized cluster intermediates, relocate to alternative sites, such as interstitial sites, in the final nanocrystals. First‐principle calculations of dopant formation energies support these experimental findings, demonstrating the thermodynamic favorability of specific dopant site preferences. Moreover, a consistent tendency is observed in CdSe nanocrystals, suggesting that the proposed doping mechanism is generally applicable to II–VI semiconductors. This study will advance the controlled synthesis of various doped semiconductor nanocrystals using nucleation‐controlled doping processes.

Wei J., Liu G., Fu H., Zheng W., Ma L., Chen X.

Gallucci N., Cangiano A., Russo S., Pota G., Di Girolamo R., Martinez E., Vaxelaire N., Paduano L., Vitiello G.

F/ZnO-QDs of R ∼ 3 nm and relative quantum yield of 22% are obtained via wet-precipitation at 5 at% nominal F content. F/ZnO-NCs of R ∼ 30 nm, high surface defects and photoactivity are obtained via the solvothermal route at 5 at% nominal F content.

Gebretsadik A., Kefale B., Sori C., Tsegaye D., Ananda Murthy H.C., Abebe B.

The doped porous heterostructure material was synthesised using the combustion approach. It has better photocatalysis potential than the separate constituents.

Abebe B., Kefale B., Amenu G., Guta L., Ravikumar C.R., Hamdalla T.A., Giridhar Reddy S., Tsegaye D., Murthy H.C.

AbstractDoping enhances the optical properties of high‐band gap zinc oxide nanoparticles (ZnO NPs), essential for their photocatalytic activity. We used the combustion approach to synthesize cobalt‐doped ZnO heterostructure (CDZO). By creating a mid‐edge level, it was possible to tune the indirect band gap of the ZnO NPs from 3.1 eV to 1.8 eV. The red shift and reduction in the intensity of the photoluminescence (PL) spectra resulted from hindrances in electron‐hole recombination and sp‐d exchange interactions. These improved optical properties expanded the absorption of solar light and enhanced charge transfer. The field emission scanning electron microscopy (FESEM) image and elemental mapping analysis confirmed the CDZO′s porous nature and the dopant‘s uniform distribution. The porosity, nanoscale size (25–55 nm), and crystallinity of the CDZO were further verified by high‐resolution transmission electron microscopy (HRTEM) and selected area electron image analysis. The photocatalytic activity of the CDZO exhibited much greater efficiency (k=0.131 min−1) than that of ZnO NPs (k=0.017 min−1). Therefore, doped heterostructures show great promise for industrial‐scale environmental remediation applications.

Oli F., Tilahun D., Ravikumar C.R., Avinash B., Tsegaye D., Abebe B.