Anna Jolene Mork

Bio: Anna Jolene Mork is an academic researcher. The author has contributed to research in topics: Raman spectroscopy. The author has an hindex of 1, co-authored 1 publications receiving 9 citations.

Exploring excitations and vibrations in semiconductor nanocrystals through fluorescence and Raman spectroscopy

Abstract: Semiconductor nanocrystals, also known as quantum dots (QDs) have been used in solid state light emission applications ranging from fluorescent downconverters to LEDs and lasers, as well as energy generation devices such as solar photovoltaics and thermoelectrics. In order to realize these myriad applications, the fundamental physics of both electronic and vibrational energy transfer must be understood to engineer better device performance. This thesis begins with a general introduction to the physics and chemistry of QDs as well as an introduction to lattice vibrations, including a proposed model for understanding thermal conductivity in solid state QD-based devices. It continues with a discussion of the methods used to understand the photoluminescence and vibrational characteristics of QDs, including spectrallyresolved time-correlated single photon counting measurements to understand QD photoluminescence lifetime as a function of emission wavelength, and low-frequency Raman spectroscopy to measure acoustic phonons in nanocrystal solids. These two chapters serve as an introduction to the ideas and methods used throughout the thesis. In Chapter 3, Förster theory is used in conduction with spectrallyand temporallyresolved photoluminescence spectroscopy to understand the rates of excitonic energy transfer in CdSe/CdZnS core/shell QDs through a calculation of the effective dipoledipole coupling distance known as the Förster radius. This work demonstrated energy transfer rates between donor and acceptor QDs between 10-100 times faster than the predictions based on standard applications of Förster theory, corresponding to an effective Förster radius of 8-9 nm in close packed QD films. Several possible effects, including an enhanced absorption cross section, ordered dipole orientations, or dipolemultipole coupling, can explain the observed difference between our measurements and the Förster theory predictions, demonstrating that several standard assumptions commonly used for calculating QD resonant energy transfer rates must be carefully considered when the QDs are in a thin-film geometry. Chapters 4-5 involve the use of low-frequency Raman spectroscopy to probe acoustic phonons in QDs. These low-frequency acoustic vibrations affect the

Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

Abstract: Photoluminescence blinking—random switching between states of high (ON) and low (OFF) emissivities—is a universal property of molecular emitters found in dyes, polymers, biological molecules and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires. For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon. The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge, which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism). However, this ‘charging’ model was recently challenged in several reports. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept ‘hot’ electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.

Thermophysical properties of materials

Abstract: Report on Fifth European Conference on Thermophysical Properties at High Temperatures, 15-21 May 1976 at the High Temperature Institute, Academy of Sciences of the USSR, Moscow.

Vibrational Properties of Metal Nanoparticles: Atomistic Simulation and Comparison with Time-Resolved Investigation C

Abstract: Knowledge of the vibrational spectrum of metal clusters and nanoparticles is of fundamental interest since it is a signature of their morphology, and it can be used to determine their mechanical, thermodynamical, and other physical properties It is expected that such a vibrational spectrum depends on the material, size, and shape of clusters and nanoparticles In this work, we report the vibrational spectra and density of states of Au, Pt, and Ag nanoparticles in the size range of 05–4 nm (13–2057 atoms), with icosahedral, Marks decahedral, and FCC morphologies The vibrational spectra were calculated through atomistic simulations (molecular dynamics and a normal-mode analysis) using the many-body Gupta potential A discussion on the dependence of the vibrational spectrum on the material, size, and shape of the nanoparticle is presented Linear relations with the nanoparticle diameter were obtained for the periods of two characteristic oscillations: the quasi-breathing and the lowest frequency (acoustic gap) modes These linear behaviors are consistent with the calculation of the periods corresponding to the breathing and acoustic gap modes of an isotropic, homogeneous metallic nanosphere, performed with continuous elastic theory using bulk properties Additionally, experimental results on the period corresponding to isotropic volume oscillations of Au nanoparticles measured by time-resolved pump–probe spectroscopy are presented, indicating a linear variation with the mean diameter in the size range of 2–4 nm These, and similar results previously obtained for Pt nanoparticles with size between 13 and 3 nm, are in good agreement with the calculated quasi-breathing mode periods of the metal nanoparticles, independently of their morphologies On the other hand, the calculated period of the mode with the highest (cutoff) frequency displays weak size and shape dependencies up to ∼4 nm, for all nanoparticles under study In contrast with the behavior of other physicochemical properties, the clear consistency between experiments with atomistic and continuous media approaches resulting from this work indicates the existence of simple relations with size and weak dependence with the material and shape, for vibrational properties of metal nanoparticles

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots : assignment of the first electronic transitions

Abstract: CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

Self-organization of CdSe nanocrystallites into three-dimensional quantum dot superlattices

Abstract: The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superiattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.