Manganese(II) in Semiconductor Nanocrystals

Thesis (Ph.D.)--University of Washington, 2014The following chapters describe the state of II-VI semiconductor nanocrystal preparation. Introduction of transition metal dopants gave rise to new materials and provided an insight into both physical and chemical properties of these semiconductor nanocrystals. Mn(II) and Co(II) dopants were used to probe the spatial position of an exciton inside Mn(II):ZnSe/CdSe core/shell nanocrystals and to show that p-d exchange still dominates s-d exchange, consistent with bulk analogues. Energy transfer between Mn(II) and excitonic states in Mn(II):ZnSe/CdZnSe with various compositions was shown to result in temperature dependent dual emission. The use of multishell ZnS/CdS/ZnS heterostructures, were shown to give both stability against chemical degradation and allowed tunability of the excitonic states. The discovery of a method to completely suppress Ostwald ripening at temperatures that still allow cations to assume their thermodynamic distributions inside the samples has laid the first step in the coming revolution of nanocrystal synthesis. Independent control over morphology and composition has settled the debate on "doping impossibility" and nanocrystals seen through the new Overton window can be placed in a dynamic equilibrium with their environment

Absorption and Magnetic Circular Dichroism Analyses of Giant Zeeman Splittings in Diffusion-Doped Colloidal Cd_1–<i>x</i>Mn_<i>x</i>Se Quantum Dots

Impurity ions can transform the electronic, magnetic, or optical properties of colloidal quantum dots. Magnetic impurities introduce strong dopant-carrier exchange coupling that generates giant Zeeman splittings (Δ<i>E</i>_Z) of excitonic excited states. To date, Δ<i>E</i>_Z in colloidal doped quantum dots has primarily been quantified by analysis of magnetic circular dichroism (MCD) intensities and absorption line widths (σ). Here, we report Δ<i>E</i>_Z values detected directly by absorption spectroscopy for the first time in such materials, using colloidal Cd_1–<i>x</i>Mn_<i>x</i>Se quantum dots prepared by diffusion doping. A convenient method for decomposing MCD and absorption data into circularly polarized absorption spectra is presented. These data confirm the widely applied MCD analysis in the low-field, high-temperature regime, but also reveal a breakdown at low temperatures and high fields when Δ<i>E</i>_Z/σ approaches unity, a situation not previously encountered in doped quantum dots. This breakdown is apparent for the first time here because of the extraordinarily large Δ<i>E</i>_Z and small σ achieved by nanocrystal diffusion doping

Valence-Band Mixing Effects in the Upper-Excited-State Magneto-Optical Responses of Colloidal Mn²⁺-Doped CdSe Quantum Dots

We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1S_e, the dominant hole character of each excitonic transition can be identified, making use of the fact that the <i>g</i>-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals