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Open Access Publications from the University of California

Scholarly Works (11 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2019)

Semiconducting quantum dots hold a great deal of promise in a variety of light-based applications like displays and bioimaging due to their bright and relatively narrow emissions. For each of these applications, an understanding of the surface of the quantum dots is important to maximize their efficiency, as the surface is home to most of the trap states in a quantum dot. One can either make use of these states as intermediates for energy transfer, or remove them to maximize the quantum dot’s native emission. This dissertation will investigate both the coupling of a luminescent species to the surface of InP quantum dots, as well as investigate ligand-based passivations of the surface of InP quantum dots.

Chapter 1 of this dissertation gives an overview of the variety of materials that have been made into quantum dots, along with some of their applications. The coupling of quantum dots to emissive dopants is discussed, along with two passivation strategies that have been studied in nanoparticles: the growth of a passivating shell material around the emissive core and the use of ligands bound to the surface of the quantum dot to passivate trap states.

Chapter 2 details the coupling of InP quantum dots to emissive Yb3+ or Nd3+ contained in a YF3 shell around the quantum dot, which are finally passivated with a LuF3 or YF3 shell to reduce non-radiative quenching. This geometry allows for the broad absorbance of the quantum dot core to be coupled to the narrow emission from the Yb 3+ f – f transition at ∼976 nm. This work utilizes a wide variety of electron microscopy and x-ray characterization methods to investigate the structure of this novel geometry. Optical characterizations were used to show that excitation of the quantum dot core led to a transfer to an intermediate trap state before transfer to the emissive rare earth ion. Though the total system quantum efficiency was only around 0.5%, this work introduces a novel method of creating extremely narrow emissions from quantum dot based systems.

Chapter 3 details initial investigations on the thermodynamics of ligand interactions with the surface of InP quantum dots. Isothermal titration calorimetry was used to investigate the heats of reactions of a variety of organic ligands with InP quantum dots, while quantitative nuclear magnetic resonance spectroscopy was used to investigate the extent of ligand binding to the surface of the quantum dots. It was found that carboxylate ligands, which are the most commonly used ligand in InP quantum dot synthesis, are not necessarily fully passivating the surface, but amine ligands strongly bind to the quantum dots and passivate more of the surface than the carboxylates do.

    Cover page: Studies of Energy Transfer and Ligand Binding at the Surface of Indium Phosphide Quantum Dots
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2017)
    Precise morphology and composition control is vital for designing multifunctional lanthanide-doped core/shell nanocrystals. Herein, we report controlled isotropic and anisotropic shell growth techniques in hexagonal sodium rare-earth tetrafluoride (β-NaLnF4) nanocrystals by exploiting the kinetics of the shell growth. A drastic change of the shell morphology was observed by changing the injection rate of the shell precursors while keeping all other reaction conditions constant. We obtained isotropic shell growth for fast sequential injection and a preferred growth of the shell layers along the crystal's c-axis [001] for slow dropwise injection. Using this slow shell growth technique, we have grown rod-like shells around different almost spherical core nanocrystals. Bright and efficient upconversion was measured for both isotropic and rod-like shells around β-NaYF4 nanocrystals doped with Yb3+/Er3+ and Yb3+/Tm3+. Photoluminescence upconversion quantum yield and lifetime measurements reveal the high quality of the core/shell nanocrystal. Furthermore, multishell rod-like nanostructures have been prepared with optically active cores and tips separated by an inert intermediate shell layer. The controlled anisotropic shell growth allows the design of new core/multishell nanostructures and enables independent investigations of the chemistry and physics of different nanocrystal facets.
      Cover page: Controlled Isotropic and Anisotropic Shell Growth in β‑NaLnF4 Nanocrystals Induced by Precursor Injection Rate
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2018)
      Semiconductor quantum dot (QD)-sensitized lanthanide ions hold great promise in producing a broadly absorbing and sharply emitting luminophore, but their synthesis has proven to be difficult. We report the first synthesis of core/shell/shell InP/Ln xY1- xF3/ShF3 (Ln = Yb, Nd; Sh = Lu, Y) nanocrystals that exhibit a broad visible absorption coupled to a sharp near-infrared emission. Additionally, this is the first report of Nd being coupled to a QD absorber. We characterize the system with a variety of electron microscopy and X-ray techniques that prove this unique structure. Optical measurements confirm the correlation of the Ln3+ emission to the QD absorption, while the presence of a trap-state emission gives a clue as to the mechanism of energy transfer between the dot and the lanthanide.
        Cover page: Broadband Sensitization of Lanthanide Emission with Indium Phosphide Quantum Dots for Visible to Near-Infrared Downshifting
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2018)
          Cover page: Addition to “Essentially Trap-Free CsPbBr3 Colloidal Nanocrystals by Postsynthetic Thiocyanate Surface Treatment”
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2017)
          We demonstrate postsynthetic modification of CsPbBr3 nanocrystals by a thiocyanate salt treatment. This treatment improves the quantum yield of both freshly synthesized (PLQY ≈ 90%) and aged nanocrystals (PLQY ≈ 70%) to within measurement error (2-3%) of unity, while simultaneously maintaining the shape, size, and colloidal stability. Additionally, the luminescence decay kinetics transform from multiexponential decays typical of nanocrystalline semiconductors with a distribution of trap sites, to a monoexponential decay, typical of single energy level emitters. Thiocyanate only needs to access a limited number of CsPbBr3 nanocrystal surface sites, likely representing under-coordinated lead atoms on the surface, in order to have this effect.
            Cover page: Essentially Trap-Free CsPbBr3 Colloidal Nanocrystals by Postsynthetic Thiocyanate Surface Treatment
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2016)
            Lanthanide-doped nanocrystals are of particular interest for the research community not only due to their ability to shape light by downshifting, quantum cutting, and upconversion but also because novel optical properties can be found by the precise engineering of core-shell nanocrystals. Because of the large surface area-to-volume ratio of nanocrystals, the luminescence is typically suppressed by surface quenching. Here, we demonstrate a mechanism that exploits surface quenching processes to improve the luminescence of our core-shell lanthanide-doped nanocrystals. By carefully tuning the shell thickness of inert β-NaLuF4 around β-NaYF4 nanocrystals doped with Yb3+ and Er3+, we unravel the relationship between quantum yield and shell thickness, and quantify surface quenching rates for the relevant Er3+ and Yb3+ energy levels. This enhanced understanding of the system's dynamics allowed us to design nanocrystals with a surface quenching-assisted mechanism for bright NIR to NIR downshifting with a distinctive efficiency peak for an optimized shell thickness.
              Cover page: Precise Tuning of Surface Quenching for Luminescence Enhancement in Core–Shell Lanthanide-Doped Nanocrystals
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2019)
              Lead toxicity has sparked interest into alternative halide nanomaterials with properties similar to CsPbX 3 perovskites. A promising alternative suggested from bulk studies is the family of double perovskites of the form Cs 2 AgMX 6 . Here, we report the synthesis of colloidal Cs 2 AgInCl 6 and Cs 2 AgSbCl 6 nanocrystals via injection of acyl halides into a metal acetate solution under atmospheric conditions and relatively mild temperatures. We demonstrate the synthesis of single-crystalline cubic nanocrystals of ca. 10 nm side length and their morphological similarities to other double perovskite nanostructures in terms of their [200] facet termination and decoration with Ag (0) smaller nanocrystallites. To compare the stabilities of the synthesized materials, we develop a titration assay based on the degradation of nanocrystals with amines as a proxy for degradation by humidity, which provides a quantifiable stability metric. This measurement shows that Cs 2 AgSbCl 6 releases more than twice the decomposition energy compared to Cs 2 AgInCl 6 or CsPbCl 3 and degrades in the presence of approximately one molar equivalent of amine, whereas the other two materials require more than a 100-fold excess. Using facile chemical titration to quantitatively determine chemical stability provides an additional tool to aid in the basic understanding of what makes some of these materials more environmentally stable than others.
                Cover page: Probing the Stability and Band Gaps of Cs2AgInCl6 and Cs2AgSbCl6 Lead-Free Double Perovskite Nanocrystals
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2018)
                Replacing lead in halide perovskites is of great interest due to concerns about stability and toxicity. Recently, lead free double perovskites in which the unit cell is doubled and two divalent lead cations are substituted by a combination of mono- and trivalent cations have been synthesized as bulk single crystals and as thin films. Here, we study stability and optical properties of all-inorganic cesium silver(I) bismuth(III) chloride and bromide nanocrystals with the double perovskite crystal structure. The cube-shaped nanocrystals are monodisperse in size with typical side lengths of 8 to 15 nm. The absorption spectrum of the nanocrystals presents a sharp peak, which we assign to a direct bismuth s-p transition and not to a quantum confined excitonic transition. Using this spectroscopic handle combined with high-resolution transmission electron microscopy (TEM) based elemental analysis, we conduct stoichiometric studies at the single nanocrystal level as well as decomposition assays in solution and observe that Ag+ diffusion and coalescence is one of the pathways by which this material degrades. Drying the nanocrystals leads to self-assembly into ordered nanocrystal solids, and these exhibit less degradation than nanocrystals in solution. Our results demonstrate that Cs2AgBiX6 (X = Cl, Br) nanocrystals are a useful model system to study structure-function relationships in the search for stable nontoxic halide perovskites.
                  Cover page: The Making and Breaking of Lead-Free Double Perovskite Nanocrystals of Cesium Silver–Bismuth Halide Compositions
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2017)
                  Lead halide perovskite nanocrystals (NCs) have emerged as attractive nanomaterials owing to their excellent optical and optoelectronic properties. Their intrinsic instability and soft nature enable a post-synthetic controlled chemical transformation. We studied a ligand mediated transformation of presynthesized CsPbBr3 NCs to a new type of lead-halide depleted perovskite derivative nanocrystal, namely Cs4PbBr6. The transformation is initiated by amine addition, and the use of alkyl-thiol ligands greatly improves the size uniformity and chemical stability of the derived NCs. The thermodynamically driven transformation is governed by a two-step dissolution-recrystallization mechanism, which is monitored optically. Our results not only shed light on a decomposition pathway of CsPbBr3 NCs but also present a method to synthesize uniform colloidal Cs4PbBr6 NCs, which may actually be a common product of perovskite NCs degradation.
                    Cover page: Ligand Mediated Transformation of Cesium Lead Bromide Perovskite Nanocrystals to Lead Depleted Cs4PbBr6 Nanocrystals
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2017)
                    The reabsorption of photoluminescence within a medium, an effect known as the inner filter effect (IFE), has been well studied in solutions, but has garnered less attention in regards to solid-state nanocomposites. Photoluminescence from a quantum dot (QD) can selectively excite larger QDs around it resulting in a net red-shift in the reemitted photon. In CdSe/CdS core/shell QD-polymer nanocomposites, we observe a large spectral red-shift of over a third of the line width of the photoluminescence of the nanocomposites over a distance of 100 μm resulting from the IFE. Unlike fluorescent dyes, which do not show a large IFE red-shift, QDs have a component of inhomogeneous broadening that originates from their size distribution and quantum confinement. By controlling the photoluminescence broadening as well as the sample dispersion and concentration, we show that the magnitude of the IFE within the nanocomposite can be tuned. We further demonstrate that this shift can be exploited in order to spectroscopically monitor the vertical displacement of a nanocomposite in a fluorescence microscope. Large energetic shifts in the measured emission with displacement can be maximized, resulting in a displacement sensor with submicrometer resolution. We further show that the composite can be easily attached to biological samples and is able to measure deformations with high temporal and spatial precision.
                      Cover page: Characterizing Photon Reabsorption in Quantum Dot-Polymer Composites for Use as Displacement Sensors
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