Kiran G. Sonawane

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.

Electron energy level engineering in Zn1−xCdxSe nanocrystals

Variation in composition provides an additional degree of freedom in nanocrystals design. In a strategic manner, the amount of Zn across the radius of Zn1−xCdxSe nanocrystals (NCs) is varied, resulting in minimal photoluminescence quenching with temperature, hence assuring the least defect density. Further Zn distribution within NCs is made uniform by annealing. Electron energy levels mapped by optical techniques reveal reduced energy level spacing due to Zn incorporation. The Stokes shift attains a remarkably lower value in alloyed Zn1−xCdxSe NCs. Notably, the alloyed NCs concomitantly exhibit a blue shift in the forbidden gap, but a red shift in higher-energy transitions. First-principles electronic structure calculations show enhanced hybridization of Zn d levels with Se p levels in comparison to that of Cd d levels in homogeneously alloyed NCs, leading to decreasing energy difference between the occupied electron energy levels. Varying the size tunes the optical transitions monotonically, while tuning the composition profile engineers the electron energy levels of NCs.

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.

Low power operated highly luminescent ferroelectric liquid crystal doped with CdSe/ZnSe core/shell quantum dots

ABSTRACT We report the enhancement in the molecular ordering of ferroelectric liquid crystal (FLC) doped with CdSe/ZnSe graded core/shell (CZ) quantum dots (QDs) by using optical methods. Significant decrease in operating voltage and enhancement in optical brightness are assigned to the large primary order parameter (θ) and hence anchoring of FLC molecules by CZ QDs. The enhancement in photoluminescence is conjectured to be due to an increase in molecular alignment yielding higher absorption which is confirmed by excitation spectra. These observations would definitely offer a promising tool to get superior core/shell QD incorporated FLC-based display devices. GRAPHICAL ABSTRACT

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.