Ch. Rajesh

Magnetic Behavior of Manganese-Doped ZnSe Quantum Dots

Magnetic properties of manganese-doped ZnSe quantum dots with the size of approximately 3.6 nm are investigated. The amount of Mn in the ZnSe quantum dots has been varied from 0.10% to 1.33%. The doping level in the quantum dots is much less than that used in the precursor. The co-ordination of Mn in the ZnSe lattice has been determined by electron paramagnetic resonance (EPR). Two different hyperfine couplings 67.3×10−4 and 60.9×10−4 cm−1 observed in the EPR spectrum imply that Mn atoms occupy two distinct sites; one uncoordinated (near the surface) and other having a cubic symmetric environment (nanocrystal core), respectively. Photoluminescence measurements also confirm the incorporation of Mn in ZnSe quantum dots. From the Curie-Weiss behavior of the susceptibility, the effective Mn-Mn antiferromagnetic exchange constant (J1) has been evaluated. The spin-glass behavior is observed in 1.33% Mn-doped ZnSe quantum dots, at low temperature. Magnetic behavior at a low temperature is discussed.

Synthesis and optical properties of copper-doped ZnSe quantum dots

This paper reports a new method to synthesize Cu-doped ZnSe quantum dots (QDs). Emission properties are tuned from the blue to the green region simply by increasing the size of the QDs. A red shift in optical absorption of Cu:ZnSe QDs compared with undoped ZnSe QDs is observed. The increase in size of QDs is explained by a change in reaction kinematics. PL measurements revealed both a band edge as well as a copper-related emission. Delocalization of electronic wave functions leads to a shift in the copper-related emission with in size. PL excitation spectra recorded at Cu emission shows ZnSe energy levels along with a feature between 350–370 nm. This feature is assigned to excited energy levels of Cu ions. Variation in electron energy levels as a function of size and on Cu incorporation is mapped.

Generation of white light from co-doped (Cu and Mn) ZnSe QDs

White light generation is achieved by single-step co-doping of copper and manganese into the robust ZnSe quantum dots (QDs) which were synthesised using a wet chemical route. Photoluminescence (PL) emission spectra revealed three peaks related to blue (ZnSe), green (copper related) and orange (manganese related). The PL spectra indicated no surface and/or trap state related emission. Photoluminescence excitation (PLE) measurements confirmed co-doping of copper and manganese in the same QD. PLE spectra recorded with emission wavelength fixed at copper and manganese showed a band edge at the same position, indicating the incorporation of both copper and manganese in the same QD. Time-resolved PL measurements suggest an atomic like nature of Mn and Cu in ZnSe QDs.

A case study: Te in ZnSe and Mn-doped ZnSe quantum dots

Photoluminescence (PL) behavior of ZnSe(1-y)Te(y) quantum dots is investigated by varying Te concentration as well as size. The striking effect of quantum confinement is the observation of isoelectronic center-related emission at room temperature in lieu of near-band-edge emission that dominates the optical scenario. ZnSe(0.99)Te(0.01) quantum dots were also doped by Mn(2+) ions. The Mn(2+) ion-related d-d transition is drastically suppressed by Te isoelectronic centers. Incorporation of Mn(2+) at substitutional sites in ZnSe(0.99)Te(0.01) quantum dots is also confirmed by the electron paramagnetic resonance measurements. Effect of Te isoelectronic impurity on the emission behavior is more pronounced than that of Mn(2+) ions. A subtle blueshift in the orange d-d transition is a sign of a decrease in crystal field strength. PL and photoluminescence excitation measurements on Zn(1-x)Se(0.99)Te(0.01)Mn(x) quantum dots indicate that the transition probability from the lowest unoccupied molecular orbital to Te levels is substantially larger than that to Mn(2+) d-d levels.

Isoelectronic Centers in Quantum Dots and Photoluminescence Decay

It is hypothesized that Te forms an isoelectronic trap in ZnSe. These isoelectronic centers show blue and green band luminescence at low temperature. Quantum confinement effects reveal isoelectronic trap related luminescence at room temperature in contrast to bulk ZnSe1 − yTey. To find the effect of these isoelectronic center on Mn2+ d–d transition luminescence, Mn doped ZnSe0.99Te0.01 QDs are synthesized. Mn doped ZnSe shows dominating orange emission related to Mn2+ d–d transitions. This Mn emission increases at the cost of band edge emission. Addition of Te as small as 1 % in ZnSe strongly quenches photoluminescence of Mn-doped ZnSe QDs showing predominance of Te-isoelectronic centers. Orange emission and near band edge luminescence in Mn doped ZnSe0.99Te0.01 are not correlated as they are in case of Mn-doped ZnSe QDs. Time resolved photoluminescence and photoluminescence excitation study revealed these isoelectronic center changes the recombination path ways. The changes in relaxation path ways are responsible for distinct emission behavior of ZnSe0.99Te0.01QDs.