arXiv subjects

Daniel R. Gamelin

Publications and source records attributed to Daniel R. Gamelin.

26 records · Page 2Linked to original sources

arXiv2017

Excitonic Zeeman Splittings in Colloidal CdSe Quantum Dots Doped with Single Magnetic Impurities

Doping a semiconductor quantum dot with just a single impurity atom can completely transform its physical properties. Here, we report and analyze the magnetic circular dichroism (MCD) spectra of colloidal CdSe quantum dot samples containing on average fewer than one Mn2+ per quantum dot. Even at this sub-single-dopant level, the low-temperature low-field data are dominated by impurity-induced Zeeman splittings caused by dopant-carrier sp-d exchange. Unlike in more heavily doped quantum dots, however, the MCD intensity at the first CdSe exciton shows a field-induced sign flip as the field strength is increased, reflecting competition between sp-d exchange and the intrinsic Zeeman splittings of comparable magnitude. Most unusually, the competition between these two effects leads to a large apparent shift in the first MCD peak maximum, which we show is attributable to a difference in sign of the intrinsic excitonic g factor between the first and second excitons. Finally, the sp-d and intrinsic contributions to the excitonic Zeeman splittings each exhibit unique magnetic-field and temperature dependencies, allowing the MCD spectra of undoped, singly doped, and bi-doped quantum dot sub-ensembles to be analyzed.

cond-mat.mes-hall↗

arXiv2006

Pinned Magnetic Impurity Levels in Doped Quantum Dots

Spectroscopic data demonstrate that impurity D/A levels in doped semiconductor nanostructures are energetically pinned, resulting in variations in D/A binding energies with increasing quantum confinement. Using magnetic circular dichroism spectroscopy, the donor binding energies of Co2+ ions in colloidal ZnSe quantum dots have been measured as a function of quantum confinement and analyzed in conjunction with ab initio density functional theory calculations. The resulting experimental demonstration of pinned impurity levels in quantum dots has far-reaching implications for physical phenomena involving impurity-carrier interactions in doped semiconductor nanostructures, including in the emerging field of semiconductor spintronics where magnetic-dopant-carrier exchange interactions define the functionally relevant properties of diluted magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2005

Charge Transfer Excited State Contributions to Polarity Dependent Ferromagnetism in ZnO Diluted Magnetic Semiconductors

A close link between the charge transfer electronic structures and polarity dependent high-TC ferromagnetism of TM2+:ZnO DMSs (TM2+ = 3d ions) is demonstrated. Trends in ferromagnetism across the 3d series of TM2+:ZnO DMSs predicted from their charge transfer energies reproduce experimental trends well. These results provide a unified basis for understanding both n- and p-type ferromagnetic oxide DMSs.

cond-mat.mtrl-sci↗

arXiv2005

300 K Ferromagnetic Magnetic Circular Dichroism in Co2+-doped ZnO Activated by Shallow Donors

Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300 K ferromagnetic MCD signal was observed between 1.4 and 4.0 eV after introducing shallow donor states by exposure of paramagnetic Co2+:ZnO films to zinc vapor. The new sub-bandgap ferromagnetic MCD intensity is attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic Co2+:ZnO.

cond-mat.mtrl-sci↗

arXiv2005

Critical Nucleus Size, Composition, and Energetics in the Synthesis of Doped ZnO Quantum Dots

The influence of Co2+ ions on the homogeneous nucleation of ZnO is examined. Using electronic absorption spectroscopy as a dopant-specific in situ spectroscopic probe, Co2+ ions are found to be quantitatively excluded from the ZnO critical nuclei but incorporated nearly statistically in the subsequent growth layers, resulting in crystallites with pure ZnO cores and Zn1-xCoxO shells. Strong inhibition of ZnO nucleation by Co2+ ions is also observed. These results are explained using the classical nucleation model. Statistical analysis of nucleation inhibition data allows estimation of the critical nucleus size as 25 +/- 4 Zn2+ ions. Bulk calorimetric data allow the activation barrier for ZnO nucleation containing a single Co2+ impurity to be estimated as 5.75 kcal/mol cluster greater than that of pure ZnO, corresponding to a 1.5x104-fold reduction in the ZnO nucleation rate constant upon introduction of a single Co2+ impurity. These data and analysis offer a rare view into the role of composition in homogeneous nucleation processes, and specifically address recent experiments targeting formation of semiconductor quantum dots containing single magnetic impurity ions at their precise centers.

cond-mat.other↗

arXiv2004

Synthesis of Colloidal Mn2+:ZnO Quantum Dots and High-TC Ferromagnetic Nanocrystalline Thin Films

We report the synthesis of colloidal Mn2+-doped ZnO (Mn2+:ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn2+:ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)2 was found to strongly inhibit oxidation of Mn2+ by O2, allowing the synthesis of Mn2+:ZnO to be performed aerobically. Mn2+ ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn2+:ZnO colloids of nearly spherical shape and uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn2+ with very homogeneous speciation, differing from bulk Mn2+:ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 Bohr magneton/Mn2+ and TC > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn2+:ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.

cond-mat.mtrl-sci↗

arXiv2004

Strong Room-Temperature Ferromagnetism in Co2+-Doped TiO2 made from Colloidal Nanocrystals

Colloidal cobalt-doped TiO2 (anatase) nanocrystals were synthesized and studied by electronic absorption, magnetic circular dichroism, transmission electron microscopy, magnetic susceptibility, cobalt K-shell X-ray absorption spectroscopy, and extended X-ray absorption fine structure measurements. The nanocrystals were paramagnetic when isolated by surface-passivating ligands, weakly ferromagnetic (Ms = 1.5 x 10-3 mB/Co2+ at 300 K) when aggregated, and strongly ferromagnetic (up to Ms = 1.9 mB/Co2+ at 300 K) when spin-coated into nanocrystalline films. X-ray absorption data reveal that cobalt is in the Co2+ oxidation state in all samples. In addition to providing strong experimental support for the existence of intrinsic ferromagnetism in cobalt-doped TiO2, these results demonstrate the possibility of using colloidal TiO2 diluted magnetic semiconductor nanocrystals as building blocks for assembly of ferromagnetic semiconductor nanostructures with potential spintronics applications.

cond-mat.mtrl-sci↗

arXiv2004

Reversible 300K Ferromagnetic Ordering in a Diluted Magnetic Semiconductor

The discovery of reversible 300 K ferromagnetic ordering in a diluted magnetic semiconductor is reported. Switching of room-temperature ferromagnetism between "on" and "off" states is achieved in Co2+:ZnO by lattice incorporation and removal of the native n-type defect, interstitial Zn. Spectroscopic and magnetic data implicate a double-exchange mechanism for ferromagnetism. These results demonstrate for the first time reversible room-temperature ferromagnetic ordering in a diluted magnetic semiconductor, and present new opportunities for integrating magnetism and conductivity in semiconductor sensor or spin-based electronics devices.

cond-mat.mtrl-sci↗