Tuning Equilibrium Compositions in Colloidal Cd_1–<i>x</i>Mn_<i>x</i>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_1–<i>x</i>Mn_<i>x</i>Se nanocrystals by diffusion doping of preformed CdSe nanocrystals. Until recently, Cd_1–<i>x</i>Mn_<i>x</i>Se nanocrystals proved elusive because of kinetic incompatibilities between Mn²⁺ and Cd²⁺ chemistries. Diffusion doping allows Cd_1–<i>x</i>Mn_<i>x</i>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^2–-limited reaction regime is identified, in which Mn²⁺ diffusion into CdSe nanocrystals is gated by added Se^2–, and equilibrium compositions are proportional to the amount of added Se^2–. At large added Se^2– concentrations, a solubility-limited regime is also identified, in which <i>x</i> = <i>x</i>_max = ∼0.31, independent of the amount of added Se^2–. We further demonstrate that Mn²⁺ in-diffusion can be reversed by cation exchange with Cd²⁺ under exactly the same reaction conditions, purifying Cd_1–<i>x</i>Mn_<i>x</i>Se nanocrystals back to CdSe nanocrystals with fine tunability. These chemistries offer exceptional composition control in Cd_1–<i>x</i>Mn_<i>x</i>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 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

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 sp–d 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 Mn²⁺-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 Mn²⁺ 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