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.

Combinatorial discovery of lanthanide-doped nanocrystals with spectrally pure upconverted emission.

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.

Concentrating and recycling energy in lanthanide codopants for efficient and spectrally pure emission: the case of NaYF4:Er3+/Tm3+ upconverting nanocrystals.

In lanthanide-doped materials, energy transfer (ET) between codopant ions can populate or depopulate excited states, giving rise to spectrally pure luminescence that is valuable for the multicolor imaging and simultaneous tracking of multiple biological species. Here, we use the case study of NaYF(4) nanocrystals codoped with Er(3+) and Tm(3+) to theoretically investigate the ET mechanisms that selectively enhance and suppress visible upconversion luminescence under near-infrared excitation. Using an experimentally validated population balance model and using a path-tracing algorithm to objectively identify transitions with the most significant contributions, we isolated a network of six pathways that combine to divert energy away from the green-emitting manifolds and concentrate it in the Tm(3+):(3)F(4) manifold, which then participates in energy transfer upconversion (ETU) to populate the red-emitting Er(3+):(4)F(9/2) manifold. We conclude that the strength of this ETU process is a function of the strong coupling of the Tm(3+):(3)F(4) manifold and its ground state, the near-optimum band alignment of Er(3+) and Tm(3+) manifolds, and the concentration of population in Tm(3+):(3)F(4). These factors, along with the ability to recycle energy not utilized for red emission, also contribute to the enhanced quantum yield of NaYF(4):Er(3+)/Tm(3+). We generalize a scheme for applying these energy concentration and recycling pathways to other combinations of lanthanide dopants. Ultimately, these ET pathways and others elucidated by our theoretical modeling will enable the programming of physical properties in lanthanide-doped materials for a variety of applications that demand strong and precisely defined optical transitions.

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 ß-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 ß-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 ß-phase nanocrystals while having only a modest effect on nanocrystal size. Core/shell ß-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.

Harnessing Chemical Raman Enhancement for Understanding Organic Adsorbate Binding on Metal Surfaces.

Surface-enhanced Raman spectroscopy (SERS) is a known approach for detecting trace amounts of molecular species. Whereas SERS measurements have focused on enhancing the signal for sensing trace amounts of a chemical moiety, understanding how the substrate alters molecular Raman spectra can enable optical probing of analyte binding chemistry. Here we examine binding of trans-1,2-two(4-pyridyl) ethylene (BPE) to Au surfaces and understand variations in experimental data that arise from differences in how the molecule binds to the substrate. Monitoring differences in the SERS as a function of incubation time, a period of several hours in our case, reveals that the number of BPE molecules that chemically binds with the Au substrate increases with time. In addition, we introduce a direct method of accessing relative chemical enhancement from experiments that is in quantitative agreement with theory. The ability to probe optically specific details of metal/molecule interfaces opens up possibilities for using SERS in chemical analysis.

High quantum efficiency of band-edge emission from ZnO nanowires.

External quantum efficiency (EQE) of photoluminescence as high as 20% from isolated ZnO nanowires were measured at room temperature. The EQE was found to be highly dependent on photoexcitation density, which underscores the importance of uniform optical excitation during the EQE measurement. An integrating sphere coupled to a microscopic imaging system was used in this work, which enabled the EQE measurement on isolated ZnO nanowires. The EQE values obtained here are significantly higher than those reported for ZnO materials in forms of bulk, thin films or powders. Additional insight on the radiative extraction factor of one-dimensional nanostructures was gained by measuring the internal quantum efficiency of individual nanowires. Such quantitative EQE measurements provide a sensitive, noninvasive method to characterize the optical properties of low-dimensional nanostructures and allow tuning of synthesis parameters for optimization of nanoscale materials.

Epitaxial growth of InGaN nanowire arrays for light emitting diodes.

Significant synthetic challenges remain for the epitaxial growth of high-quality InGaN across the entire compositional range. One strategy to address these challenges has been to use the nanowire geometry because of its strain relieving properties. Here, we demonstrate the heteroepitaxial growth of In(x)Ga(1-x)N nanowire arrays (0.06 ≤ x ≤ 0.43) on c-plane sapphire (Al(2)O(3)(001)) using a halide chemical vapor deposition (HCVD) technique. Scanning electron microscopy and X-ray diffraction characterization confirmed the long-range order and epitaxy of vertically oriented nanowires. Structural characterization by transmission electron microscopy showed that single crystalline nanowires were grown in the ⟨002⟩ direction. Optical properties of InGaN nanowire arrays were investigated by absorption and photoluminescence measurements. These measurements show the tunable direct band gap properties of InGaN nanowires into the yellow-orange region of the visible spectrum. To demonstrate the utility of our HCVD method for implementation into devices, LEDs were fabricated from In(x)Ga(1-x)N nanowires epitaxially grown on p-GaN(001). Devices showed blue (x = 0.06), green (x = 0.28), and orange (x = 0.43) electroluminescence, demonstrating electrically driven color tunable emission from this p-n junction.

Whispering gallery mode lasing from zinc oxide hexagonal nanodisks.

Disk-shaped semiconductor nanostructures provide enhanced architectures for low-threshold whispering gallery mode (WGM) lasing with the potential for on-chip nanophotonic integration. Unlike cavities that lase via Fabry-Perot modes, WGM structures utilize low-loss, total internal reflection of the optical mode along the circumference of the structure, which effectively reduces the volume of gain material required for lasing. As a result, circularly resonant cavities provide much higher quality (Q) factors than lower reflection linear cavities, which makes nanodisks an ideal platform to investigate lasing nanostructures smaller than the free-space wavelength of light (i.e., subwavelength laser). Here we report the bottom-up synthesis and single-mode lasing properties of individual ZnO disks with diameters from 280 to 900 nm and show finite difference time domain (FDTD) simulations of the whispering gallery mode inside subwavelength diameter disks. These results demonstrate ultraviolet WGM lasing in chemically synthesized, isolated nanostructures with subwavelength diameters.

Imaging single ZnO vertical nanowire laser cavities using UV-laser scanning confocal microscopy.

We report the fabrication and optical characterization of individual ZnO vertical nanowire laser cavities. Dilute nanowire arrays with interwire spacing >10 microm were produced by a modified chemical vapor transport (CVT) method yielding an ideal platform for single nanowire imaging and spectroscopy. Lasing characteristics of a single vertical nanowire are presented, as well as high-resolution photoluminescence imaging by UV-laser scanning confocal microscopy. In addition, three-dimensional (3D) mapping of the photoluminescence emission performed in both planar and vertical dimensions demonstrates height-selective imaging useful for vertical nanowires and heteronanostructures emerging in the field of optoelectronics and nanophotonics.

Optical anisotropy in individual porous silicon nanoparticles containing multiple chromophores.

Polarization anisotropy is investigated in single porous silicon nanoparticles containing multiple chromophores. Two classes of nanoparticles, low current density and high current density, are studied. Low current density samples exhibit red-shifted spectra and contain only one or two chromophores. High current density particles, on average, contain less than four chromophores and display a blue-shifted spectrum. We utilize single-molecule spectroscopy to probe the polarization effects of the particles, and we show that both classes of particles are influenced by a polarized excitation source. These results are exciting at the fundamental level for understanding coupled quantum dot emitters as well as for applications involving single-photon sources or silicon-based polarization-sensitive detectors.

Investigation of the connectivity of hydrophilic domains in Nafion using electrochemical pore-directed nanolithography.

A method of determining the connectivity of ion-conducting hydrophilic channels within the Nafion polymer electrolyte membrane by way of pore-directed nanolithography has been developed. Electrochemical etching of a silicon surface is performed through a Nafion-membrane mask. The resulting silicon surface imaged by tapping-mode atomic force microscopy (TMAFM) provides a footprint of the hydrophilic channels at the Nafion-silicon interface. In a similar fashion, a TMAFM phase-contrast image of the top surface of the Nafion mask prior to etching reveals the spatial distribution of hydrophilic domains at the surface of the polymer membrane. Collectively, these images provide detailed information about the structure of the hydrophilic channels at the top and bottom surfaces of the Nafion membrane. Autocorrelation statistical analysis of these two sets of images shows that only 48% of hydrophilic channels beginning at the Nafion surface connect to the silicon-Nafion interface.