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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2018)

Semiconducting nanocrystals have shown great use in a variety of light emitting and absorb- ing applications, such as solar cells, light emitting diodes, displays, and even stress sensors. These materials exhibit size-tunable physical properties, such as phase transition pressures and band gaps. Although a lot of work has gone into understanding the structure-property relationships in these materials, a fundamental understanding of how these materials in- teract under various perturbations is necessary to understand the material’s applicability in applications. This dissertation outlines the characterization of various nanocrystals and nanocrystal composites under two different perturbations: self-absorption of luminesced light and elevated pressure.

Chapter 1 outlines the general properties of semiconducting nanocrystals and gives a background on previously studied perturbations. Optoelectronic and structural studies of cadmium chalcogenide nanocrystals are highlighted and the complex phase transitions ob- served in CsPbBr3 are discussed in detail.

In Chapter 2 we investigate how the reabsorption of photoluminesced light, an effect known as the inner filter effect (IFE), can affect the nanocomposite’s properties under illu- mination. The IFE has been well studied in solutions, but has garnered less attention in regards to solid-state nanocomposites. We demonstrate that the IFE can result in a large spectral red-shift of over a third of the linewidth of the photoluminescence of the nanocom- posites over a distance of 100 μm. We then utilize this red-shift to develop a displacement sensor with sub-micrometer resolution that has high-temporal and spatial resolution.

We investigate the effects of pressure on NCs in Chapters 3-4. Chapter 3 outlines the use of a diamond anvil cell for generating gigapascal pressures. This technique is then utilized to understand how CsPbBr3 nanocrystals respond to pressure in Chapter 4. The crystal structure of CsPbBr3 is composed of corner sharing lead bromide octahedra with Cs+ sitting in the cavities. By investigate the optical shifts and structural changes in the material with applied pressure, we find that the material transitions to a high-pressure phase around 1.4 GPa. A crystal structure of this high pressure phase has not been previously reported and we find that it has P21/m symmetry. We further find that CsPbBr3 exhibits a size-dependent compressibility where we find that nanocrystals are at least 40% more compressible than the bulk material.

We finally present an outlook for future studies in Chapter 5. We present ideas for determining additional fundamental properties of the CsPbBr3 high-pressure phase transition and investigating the effect of the inorganic-inorganic interface in nanocrystal core/shell heterostructures on crystallographic phase stability.

    Cover page: Investigations of the Optoelectronic and Structural Properties of Semiconductor Nanocrystals Under Perturbative Environments
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2018)
    Pseudoelasticity in metals is typically associated with phase transformations (e.g., shape memory alloys) but has recently been observed in sub-10 nm Ag nanocrystals that rapidly recovered their original shape after deformation to large strains. The discovery of pseudoelasticity in nanoscale metals dramatically changes the current understanding of the properties of solids at the smallest length scales, and the motion of atoms at surfaces. Yet, it remains unclear whether pseudoelasticity exists in different metals and nanocrystal sizes. The challenge of observing deformation at atomistic to nanometer length scales has prevented a clear mechanistic understanding of nanoscale pseudoelasticity, although surface diffusion and dislocation-mediated processes have been proposed. We further the understanding of pseudoelasticity in nanoscale metals by using a diamond anvil cell to compress colloidal Au nanocrystals under quasihydrostatic and nonhydrostatic pressure conditions. Nanocrystal structural changes are measured using optical spectroscopy and transmission electron microscopy and modeled using electrodynamic theory. We find that 3.9 nm Au nanocrystals exhibit pseudoelastic shape recovery after deformation to large uniaxial strains of up to 20%, which is equivalent to an ellipsoid with an aspect ratio of 2. Nanocrystal absorbance efficiency does not recover after deformation, which indicates that crystalline defects may be trapped in the nanocrystals after deformation.
      Cover page: Pseudoelasticity at Large Strains in Au Nanocrystals
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2016)
      Lead halide perovskites hold promise for photonic devices, due to their superior optoelectronic properties. However, their use is limited by poor stability and toxicity. We demonstrate enhanced water and light stability of high-surface-area colloidal perovskite nanocrystals by encapsulation of colloidal CsPbBr3 quantum dots into matched hydrophobic macroscale polymeric matrices. This is achieved by mixing the quantum dots with presynthesized high-molecular-weight polymers. We monitor the photoluminescence quantum yield of the perovskite-polymer nanocomposite films under water-soaking for the first time, finding no change even after >4 months of continuous immersion in water. Furthermore, photostability is greatly enhanced in the macroscale polymer-encapsulated nanocrystal perovskites, which sustain >1010 absorption events per quantum dot prior to photodegradation, a significant threshold for potential device use. Control of the quantum dot shape in these thin-film polymer composite enables color tunability via strong quantum-confinement in nanoplates and significant room temperature polarized emission from perovskite nanowires. Not only does the high-molecular-weight polymer protect the perovskites from the environment but also no escaped lead was detected in water that was in contact with the encapsulated perovskites for months. Our ligand-passivated perovskite-macroscale polymer composites provide a robust platform for diverse photonic applications.
        Cover page: Encapsulation of Perovskite Nanocrystals into Macroscale Polymer Matrices: Enhanced Stability and Polarization
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2017)
        We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast films of CsPbBr3 nanocubes. By depositing films of nanocrystals on precoated substrates we can control the packing density and resultant radiation pattern of the emitted photons. We develop a technical framework to calculate the average orientation of light emitters, i.e., the angle between the transition dipole moment vector (TDM) and the substrate. This model is applicable to any emissive material with a known refractive index. Theoretical modeling indicates that oriented emission originates from an anisotropic alignment of the valence band and conduction band edge states on the ionic crystal lattice and demonstrates a general path to model the experimentally less accessible internal electric field of a nanosystem from the photoluminescent anisotropy. The uniquely accessible surface of the perovskite nanoparticles allows for perturbation of the normally isotropic emissive transition. The reported sensitive and tunable TDM orientation and control of emitted light will allow for applications of perovskite nanocrystals in a wide range of photonic technologies inaccessible to traditional light emitters.
          Cover page: Tunable Anisotropic Photon Emission from Self-Organized CsPbBr3 Perovskite Nanocrystals
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2018)
          We introduce a general surface passivation mechanism for cesium lead halide perovskite materials (CsPbX3, X = Cl, Br, I) that is supported by a combined experimental and theoretical study of the nanocrystal surface chemistry. A variety of spectroscopic methods are employed together with ab initio calculations to identify surface halide vacancies as the predominant source of charge trapping. The number of surface traps per nanocrystal is quantified by 1H NMR spectroscopy, and that number is consistent with a simple trapping model in which surface halide vacancies create deleterious under-coordinated lead atoms. These halide vacancies exhibit trapping behavior that differs among CsPbCl3, CsPbBr3, and CsPbI3. Ab initio calculations suggest that introduction of anionic X-type ligands can produce trap-free band gaps by altering the energetics of lead-based defect levels. General rules for selecting effective passivating ligand pairs are introduced by considering established principles of coordination chemistry. Introducing softer, anionic, X-type Lewis bases that target under-coordinated lead atoms results in absolute quantum yields approaching unity and monoexponential luminescence decay kinetics, thereby indicating full trap passivation. This work provides a systematic framework for preparing highly luminescent CsPbX3 nanocrystals with variable compositions and dimensionalities, thereby improving the fundamental understanding of these materials and informing future synthetic and post-synthetic efforts toward trap-free CsPbX3 nanocrystals.
            Cover page: Design Principles for Trap-Free CsPbX3 Nanocrystals: Enumerating and Eliminating Surface Halide Vacancies with Softer Lewis Bases
            • 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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