3 research outputs found
Tuning Equilibrium Compositions in Colloidal Cd<sub>1β<i>x</i></sub>Mn<sub><i>x</i></sub>Se Nanocrystals Using Diffusion Doping and Cation Exchange
The
physical properties of semiconductor nanocrystals can be tuned dramatically <i>via</i> composition control. Here, we report a detailed investigation
of the synthesis of high-quality colloidal Cd<sub>1β<i>x</i></sub>Mn<sub><i>x</i></sub>Se nanocrystals by
diffusion doping of preformed CdSe nanocrystals. Until recently, Cd<sub>1β<i>x</i></sub>Mn<sub><i>x</i></sub>Se
nanocrystals proved elusive because of kinetic incompatibilities between
Mn<sup>2+</sup> and Cd<sup>2+</sup> chemistries. Diffusion doping
allows Cd<sub>1β<i>x</i></sub>Mn<sub><i>x</i></sub>Se nanocrystals to be prepared under thermodynamic rather than
kinetic control, allowing access to broader composition ranges. We
now investigate this chemistry as a model system for understanding
the characteristics of nanocrystal diffusion doping more deeply. From
the present work, a Se<sup>2β</sup>-limited reaction regime
is identified, in which Mn<sup>2+</sup> diffusion into CdSe nanocrystals
is gated by added Se<sup>2β</sup>, and equilibrium compositions
are proportional to the amount of added Se<sup>2β</sup>. At
large added Se<sup>2β</sup> concentrations, a solubility-limited
regime is also identified, in which <i>x</i> = <i>x</i><sub>max</sub> = βΌ0.31, independent of the amount of added
Se<sup>2β</sup>. We further demonstrate that Mn<sup>2+</sup> in-diffusion can be reversed by cation exchange with Cd<sup>2+</sup> under exactly the same reaction conditions, purifying Cd<sub>1β<i>x</i></sub>Mn<sub><i>x</i></sub>Se nanocrystals back
to CdSe nanocrystals with fine tunability. These chemistries offer
exceptional composition control in Cd<sub>1β<i>x</i></sub>Mn<sub><i>x</i></sub>Se NCs, providing opportunities
for fundamental studies of impurity diffusion in nanocrystals and
for development of compositionally tuned nanocrystals with diverse
applications ranging from solar energy conversion to spin-based photonics
Valence-Band Mixing Effects in the Upper-Excited-State Magneto-Optical Responses of Colloidal Mn<sup>2+</sup>-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<sub>e</sub>, 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
Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device
Electrical
spin manipulation remains a central challenge for the realization
of diverse spin-based information processing technologies. Motivated
by the demonstration of confinement-enhanced spβd exchange
interactions in colloidal diluted magnetic semiconductor (DMS) quantum
dots (QDs), such materials are considered promising candidates for
future spintronic or spin-photonic applications. Despite intense research
into DMS QDs, electrical control of their magnetic and magneto-optical
properties remains a daunting goal. Here, we report the first demonstration
of electrically induced magnetic polaron formation in any DMS, achieved
by embedding Mn<sup>2+</sup>-doped CdSe/CdS core/shell QDs as the
active layer in an electrical light-emitting device. Tracing the electroluminescence
from cryogenic to room temperatures reveals an anomalous energy shift
that reflects current-induced magnetization of the Mn<sup>2+</sup> spin sublattice, that is, excitonic magnetic polaron formation.
These electrically induced magnetic polarons exhibit an energy gain
comparable to their optically excited counterparts, demonstrating
that magnetic polaron formation is achievable by current injection
in a solid-state device