D.M. Boye

Nanostructured Gold Architectures Formed through High Pressure-Driven Sintering of Spherical Nanoparticle Arrays

We have demonstrated pressure-directed assembly for preparation of a new class of chemically and mechanically stable gold nanostructures through high pressure-driven sintering of nanoparticle assemblies at room temperature. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle array can be reversibly manipulated allowing fine-tuning of the interparticle separation distance. In addition, 3D nanostructured gold architecture can be formed through high pressure-induced nanoparticle sintering. This work opens a new pathway for engineering and fabrication of different metal nanostructured architectures.

Hydrogen-bonding-assisted self-assembly: monodisperse hollow nanoparticles made easy.

A facile self-assembly process for synthesizing monodisperse hollow spherical nanoparticles that are less than 50 nm in diameter has been developed. Preferential hydrogen bonding between an amphiphilic block copolymer (polystyrene-b-polyvinylpyridine, PS-PVP) and a hydrogen-bonding agent (HA) enables formation of monodisperse spherical solid polymer nanoparticles with the HA residing in the particle core surrounded by the polymer. Removal of the HA results in monodisperse hollow nanoparticles with tunable hollow cavity size and internal surface reactivity. Formation of ordered hollow nanoparticle films with controlled index of refraction for antireflective coating applications is demonstrated.

A facile and general approach to polynary semiconductor nanocrystals via a modified two-phase method

Cu(2)ZnSnS(4) nanocrystals were synthesized through a modified two-phase method and characterized with transmission electron microscopy (TEM), powder x-ray diffraction (XRD) and UV-vis spectroscopy. Inorganic metal salts were dissolved in the polar solvent triethylene glycol (TEG) and then transferred into the non-polar solvent 1-octadecene (ODE) by forming metal complexes between metal ions and octadecylamine (ODA). Since nucleation and growth occur in the single phase of the ODE solution, nanocrystals could be produced with qualities similar to those obtained through the hot-injection route. Balancing the reactivity of the metal precursors is a key factor in producing nanocrystals of a single crystalline phase. We found that increasing the reaction temperature increases the reactivity of each of the metal precursors by differing amounts, thus providing the necessary flexibility for obtaining a balanced reactivity that produces the desired product. The versatility of this synthesis strategy was demonstrated by extending it to the production of other polynary nanocrystals such as binary (CuS), ternary (CuInS(2)) and pentanary (Cu(2 - x)Ag(x)ZnSnS(4)) nanocrystals. This method is considered as a green synthesis route due to the use of inorganic metal salts as precursors, smaller amounts of coordinating solvent, shorter reaction time and simpler post-reaction treatment.

Fluorescence line narrowing and decay dynamics in sol–gel glasses containing Eu3+

Abstract Fluorescence line narrowing and emission decay measurements on Eu3+ doped sol–gel glasses have been used to investigate and compare local Eu3+ environments in binary SiO2–TiO2 glasses, SiO2 glasses containing Al3+, and SiO2 glasses without co-dopant. In particular, clustering of rare-earth ions and the effects of annealing temperature on residual hydroxide concentration are studied. The Al and Ti co-doped samples show pronounced line-narrowing implying little energy migration among Eu3+ ions. The 5D0–7FJ fluorescence decays are faster in the Ti glasses.

Spectral hole burning of Eu2+ in MBE-grown superlattices of CaF2:Eu2+CdF2

Persistent spectral hole burning is observed for the 413 nm 4f7(8S72) → 4f65d(Γ8) zero-phonon transition of Eu2+ in MBE-grown superlattices of CaF2:Eu2+CdF2 on Si(111). Possible mechanisms for the trapping of the photoionized electron are discussed in terms of the band offsets of the superlattice layers.

Cooperative Self-Assembly-Assisted Formation of Monodisperse Optically Active Spherical and Anisotropic Nanoparticles

We report a new method in which spontaneous self-assembly is employed to synthesize monodisperse polymer nanoparticles with controlled size (<50 nm), shape, tunable functionality, and enhanced solvent and thermal stability. Cooperative noncovalent interactions, such as hydrogen bonding and aromatic pi-pi stacking, assist self-assembly of amphiphilic macromolecules (polystyrene-block-polyvinylpyridine, PS--PVP) and structure directing agents (SDAs) to form both spherical and anisotropic solid polymer nanoparticles with SDAs residing in the particle core surrounded by the polymers. Through detailed investigations by scanning electron microscopy and transmission electron microscopy (TEM), we have rationalized nanoparticle morphology evolution and dependence on factors such as SDA concentration and PVP size. By keeping the PS chain size constant, the particle morphology progresses from continuous films to spherical particles, and on to cylindrical nanowires or rods with increasing the PVP chain size. The final nanoparticles are very stable and can be redispersed in common solvents to form homogenous solutions and thin films of ordered nanoparticle arrays through solvent evaporation processes. These nanoparticles exhibit tunable fluorescent colors (or emissions) depending on the choices of the central SDAs. Our method is simple and general without requiring complicated synthetic chemistry, stabilizing surfactants, or annealing procedures (e.g., temperature or solvent annealing), making scalable synthesis feasible.

Two-pulse photon echo studies of optical dephasing in CaF2: Eu2+

Abstract Two-pulse photon echo signals are observed for the 4f 7 → 4f 6 5d transition of Eu 2+ ions in CaF 2 . It is believed that these are the first coherent transient measurements on an f → d transition. The echo decay is studied as a function of Eu 2+ concentration, temperature and magnetic field. For T = 1.5 K and H = 30 kG , we find T 2 = 40 ns for both 0.01% and 0.001% Eu 2+ concentrations, indicating that Eu-Eu interactions are not dominant. T 2 is determined by a combination involving the electron-phonon direct process coupling the excited state spin levels and the Eu-F superhyperfine interaction. Comparison with other rare-earth systems indicates that the electron-phonon coupling in the Γ 8 excited state has matrix elements that are at least an order of magnitude stronger.

Photoionization hole burning on the 4f7(8S72) → 4f65d(Γ8) zero-phonon transition of Eu2+ in CaF2 and Ca1−xLaxF2 + x

Abstract Persistent spectral hole burning due to two-step photoionization of the 413 nm zero-phonon transition of Eu 2+ in CaF 2 and CaF 2 mixed with LaF 3 is described. A pattern of broad holes and a sharp central hole is the result of removing from the absorption, the spectral profile of individual ions which have one of their transitions resonant with the burning laser. The structure of the broad holes is observed in both the pure and mixed crystals and results from the ground state electronic splitting and the hyperfine splittings in the ground and excited states. Efficiencies for the second photoionization step are found to be ∼ 10 −3 in the pure CaF 2 and ∼ 50% in mixed crystals with 1% LaF 3 . Gating of the hole burning is observed with 514nm light with burn rate enhancements of up to 100.

Scalable Assembly of Patterned Ordered Functional Micelle Arrays: Final LDRD Report

In this project, we demonstrated the synthesis of polystyrene-polyvinylpyridyne (PS-PVP) micelles, functionalization of these micelles to form organic/inorganic composite nanoparticles, and template directed assembly of dynamic PS-PVP micelles into features defined via soft nanoimprint lithography. We demonstrated unique assembly properties of dynamic micellar nanoparticles by combining a top down lithographic nanopatterning technique with a solutionbased bottom up self-assembly. The templates for the directed self-assembly of the micelles consisted of arrays of cylindrical recess features fabricated by nanoimprint lithography. Silica was coated on this patterned substrate and subsequently selectively functionalized with a positively charged molecular monolayer (N-(3-Trimethoxysilylpropyl) diethylenetriamine) to regulate the micelle-surface interactions. The self-assembled block co-polymer poly(styrene-b4-vinyl pyridine), (PS480k – PVP145k ) micelles were approximately 325nm in diameter in aqueous solutions (pH = 2.5) and 50nm in diameter in the dry state. The average number of micelles assembled per feature increased from less than 1 to 12 with increasing feature diameter in the range of 200nm – 1μm. Using a 2D model for maximum packing of circles in circular host features, the effective sphere size of the micelles during assembly was calculated to be 250nm in diameter. Thus, the micelles exhibited three characteristic sizes during assembly, 325nm in bulk solution, 250nm during assembly, and 50nm in the dry state. This dramatic variation in nanoparticle diameter during the assembly process offers unique opportunities for forming nanometer scale, multidimensional arrays not accessible using hard sphere building blocks.

Population hole burning on the 4f7 (8S72) → 4f65d (Γ8) zero-phonon transition in MBE films of CaF2:Eu2 +

Abstract Spectral holes due to the redistribution of population among the ground state hyperfine levels have been detected on the 413 nm zero-phonon transition in CaF2:Eu2 + MBE films grown on Si(1 1 1), in magnetic fields in excess of 3 T. These are the first long-lived (up to minutes) population holes reported for a paramagnetic ion. The hole lifetime results from phonon-induced transitions among hyperfine levels of the Eu2 + ground electron spin states. The central hole linewidth of 40 MHz is probably determined by the EuF superhyperfine interaction which leads to spectral diffusion due to F nuclear spin flips. The hole pattern contains a narrow central hole (40 MHz) and side holes and antiholes which result from the hyperfine splittings in the ground and excited states.