Delia J. Milliron

Colloidal nanocrystal heterostructures with linear and branched topology

The development of colloidal quantum dots has led to practical applications of quantum confinement, such as in solution-processed solar cells, lasers and as biological labels. Further scientific and technological advances should be achievable if these colloidal quantum systems could be electronically coupled in a general way. For example, this was the case when it became possible to couple solid-state embedded quantum dots into quantum dot molecules. Similarly, the preparation of nanowires with linear alternating compositions—another form of coupled quantum dots—has led to the rapid development of single-nanowire light-emitting diodes and single-electron transistors. Current strategies to connect colloidal quantum dots use organic coupling agents, which suffer from limited control over coupling parameters and over the geometry and complexity of assemblies. Here we demonstrate a general approach for fabricating inorganically coupled colloidal quantum dots and rods, connected epitaxially at branched and linear junctions within single nanocrystals. We achieve control over branching and composition throughout the growth of nanocrystal heterostructures to independently tune the properties of each component and the nature of their interactions. Distinct dots and rods are coupled through potential barriers of tuneable height and width, and arranged in three-dimensional space at well-defined angles and distances. Such control allows investigation of potential applications ranging from quantum information processing to artificial photosynthesis.

Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals

The development of probes for single-molecule imaging has dramatically facilitated the study of individual molecules in cells and other complex environments. Single-molecule probes ideally exhibit good brightness, uninterrupted emission, resistance to photobleaching, and minimal spectral overlap with cellular autofluorescence. However, most single-molecule probes are imperfect in several of these aspects, and none have been shown to possess all of these characteristics. Here we show that individual lanthanide-doped upconverting nanoparticles (UCNPs)—specifically, hexagonal phase NaYF4 (β-NaYF4) nanocrystals with multiple Yb3+ and Er3+ dopants—emit bright anti-Stokes visible upconverted luminescence with exceptional photostability when excited by a 980-nm continuous wave laser. Individual UCNPs exhibit no on/off emission behavior, or “blinking,” down to the millisecond timescale, and no loss of intensity following an hour of continuous excitation. Amphiphilic polymer coatings permit the transfer of hydrophobic UCNPs into water, resulting in individual water-soluble nanoparticles with undiminished photophysical characteristics. These UCNPs are endocytosed by cells and show strong upconverted luminescence, with no measurable anti-Stokes background autofluorescence, suggesting that UCNPs are ideally suited for single-molecule imaging experiments.

Prospects of Nanoscience with Nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Todays strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

Tunable near-infrared and visible-light transmittance in nanocrystal-in-glass composites

Amorphous metal oxides are useful in optical, electronic and electrochemical devices. The bonding arrangement within these glasses largely determines their properties, yet it remains a challenge to manipulate their structures in a controlled manner. Recently, we developed synthetic protocols for incorporating nanocrystals that are covalently bonded into amorphous materials. This ‘nanocrystal-in-glass’ approach not only combines two functional components in one material, but also the covalent link enables us to manipulate the glass structure to change its properties. Here we illustrate the power of this approach by introducing tin-doped indium oxide nanocrystals into niobium oxide glass (NbOx), and realize a new amorphous structure as a consequence of linking it to the nanocrystals. The resulting material demonstrates a previously unrealized optical switching behaviour that will enable the dynamic control of solar radiation transmittance through windows. These transparent films can block near-infrared and visible light selectively and independently by varying the applied electrochemical voltage over a range of 2.5 volts. We also show that the reconstructed NbOx glass has superior properties—its optical contrast is enhanced fivefold and it has excellent electrochemical stability, with 96 per cent of charge capacity retained after 2,000 cycles.

Tunable Infrared Absorption and Visible Transparency of Colloidal Aluminum-Doped Zinc Oxide Nanocrystals

Plasmonic nanocrystals have been attracting a lot of attention both for fundamental studies and different applications, from sensing to imaging and optoelectronic devices. Transparent conductive oxides represent an interesting class of plasmonic materials in addition to metals and vacancy-doped semiconductor quantum dots. Herein, we report a rational synthetic strategy of high-quality colloidal aluminum-doped zinc oxide nanocrystals. The presence of substitutional aluminum in the zinc oxide lattice accompanied by the generation of free electrons is proved for the first time by tunable surface plasmon absorption in the infrared region both in solution and in thin films.

Dynamically Modulating the Surface Plasmon Resonance of Doped Semiconductor Nanocrystals

Localized surface plasmon absorption features arise at high doping levels in semiconductor nanocrystals, appearing in the near-infrared range. Here we show that the surface plasmons of tin-doped indium oxide nanocrystal films can be dynamically and reversibly tuned by postsynthetic electrochemical modulation of the electron concentration. Without ion intercalation and the associated material degradation, we induce a > 1200 nm shift in the plasmon wavelength and a factor of nearly three change in the carrier density.

Surface oxidation activates indium tin oxide for hole injection

Oxygen plasma treatment of indium tin oxide (ITO) results in a change in work function and electron affinity by ∼0.5 eV. This change correlates with the measured increase in injected current in simple “hole-only” organic devices with O-plasma treated ITO electrodes. Neither addition nor removal of surface hydroxyl functionality accounts for the observed work function and electron affinity changes. X-ray and ultraviolet photoelectron spectroscopies show a new type of oxygen species is formed. Oxidation of surface Sn-OH to surface Sn-O• units is proposed to account for the observed changes in O-plasma treated ITO; this proposal can explain a wide variety of previously described ITO surface activation results.

Organic semiconductor interfaces: electronic structure and transport properties

Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) have been used to investigate a wide range of metal/organic and organic/organic semiconductor interfaces. UPS was used to determine the binding energies of the highest occupied molecular orbitals and vacuum level positions, while XPS was used to find evidence of chemical interactions at these heterointerfaces. It was found that, with a few exceptions, the vacuum levels align at most organic/organic interfaces, while strong interface dipoles, which abruptly offset the vacuum level, exist at virtually all metal/organic semiconductor interfaces. Furthermore, strong dipoles exist at metal/organic semiconductor interfaces at which the Fermi level is completely unpinned within the semiconductor gap implying that the dipoles are not the result of populating or emptying Fermi level-pinning gap states.

Engineering bright sub-10-nm upconverting nanocrystals for single-molecule imaging

Imaging at the single-molecule level reveals heterogeneities that are lost in ensemble imaging experiments, but an ongoing challenge is the development of luminescent probes with the photostability, brightness and continuous emission necessary for single-molecule microscopy. Lanthanide-doped upconverting nanoparticles overcome problems of photostability and continuous emission and their upconverted emission can be excited with near-infrared light at powers orders of magnitude lower than those required for conventional multiphoton probes. However, the brightness of upconverting nanoparticles has been limited by open questions about energy transfer and relaxation within individual nanocrystals and unavoidable tradeoffs between brightness and size. Here, we develop upconverting nanoparticles under 10 nm in diameter that are over an order of magnitude brighter under single-particle imaging conditions than existing compositions, allowing us to visualize single upconverting nanoparticles as small (d = 4.8 nm) as fluorescent proteins. We use advanced single-particle characterization and theoretical modelling to find that surface effects become critical at diameters under 20 nm and that the fluences used in single-molecule imaging change the dominant determinants of nanocrystal brightness. These results demonstrate that factors known to increase brightness in bulk experiments lose importance at higher excitation powers and that, paradoxically, the brightest probes under single-molecule excitation are barely luminescent at the ensemble level.

Controlled Synthesis and Single-Particle Imaging of Bright, Sub-10 nm Lanthanide-Doped Upconverting Nanocrystals

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.