Two photon absorption in Mn<SUP>2+</SUP>-doped ZnSe quantum dots

The nonresonant third order nonlinear optical properties of three different sized Mn2+-doped ZnSe quantum dots (QDs) are investigated. The nonlinear absorption is measured at 532 nm using 35 picosecond laser pulses in an open-aperture Z-scan setup. Two photon absorption (2PA) cross-section in ZnSe QDs is found to be three orders of magnitude higher than its bulk value. These nanostructures show size dependent nonlinear absorption coefficients. It is found that the 2PA further enhances with decrease in size of the QD and is twice that of its undoped counterpart due to change in the local electric field

Two-photon absorption in ZnSe and ZnSe/ZnS core/shell quantum structures

The third order nonlinear optical properties of two different sized ZnSe and ZnSe/ZnS quantum dots (QDs) are investigated. The nonlinear absorption is measured at 806 nm using Ti:sapphire 100 fs laser pulses in an open aperture Z-scan setup. Two-photon absorption (2PA) is found to be dominant in core and core shell QDs. 2PA cross section is enhanced by three orders of magnitude compared bulk ZnSe. 2PA cross section is observed to increase with reduction in QD diameter, due to strong confinement effect. ZnSe/ZnS QDs exhibit higher 2PA cross section compared with corresponding ZnSe QDs, indicating better passivation of the QD surface

Ferromagnetic ZnO Nanocrystals and Al-induced Defects

ZnO nanocrystals (NCs) capped with polyvinyl pyrrolidone reveal room temperature ferromagnetism. Incorporation of Al3+-ions induce defects in ZnO NCs leading to quenching of excitonic luminescence of ZnO at the cost of an increase in the intensity of oxygen vacancy related emission. Photoluminescence excitation spectra exhibit an additional hump like feature attributed to Al-doping. Saturation magnetization of Al3+-doped ZnO NCs is the same as that of the undoped ZnO NCs. However, a remarkable decrease in the coercivity associated with change in the nature of M (T) curve and electron paramagnetic resonance signal with g = 1.96 is observed consequent to Al doping. The results provide direct evidence of the defects within the core of NCs that are responsible for the ferromagnetic ordering in the Al3+-doped ZnO. The M(T) curve unravels a typical exchange mechanism

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

Chemical reactivity and band offset at the CdS/Si interface

Transport phenomena at heterojunctions are critically governed by chemical reactivity and the valence-band offset at the interface. Here we report our investigations carried out on the CdS/Si(111) interface using X-ray and ultraviolet photoelectron spectroscopy and glancing-incidence X-ray diffraction. Weak reactivity between CdS and Si(111) has been observed. A partial charge transfer interaction is suggested to explain the weak reactivity at the interface on the basis of the electronegativity difference. Hexagonal phase of CdS is preferentially grown at 700 K. The valence-band offsets obtained for the interfaces grown at 300 and 700 K are 1.6 and 1.3 eV, respectively. The effects of chemical reactivity and disorder are discussed

Strain-Induced Hierarchy of Energy Levels in CdS/ZnS Nanocrystals

Aside of size and shape, the strain induced by the mismatch of lattice parameters between core and shell in the nanocrystalline regime is an additional degree of freedom to engineer the electron energy levels. Herein, CdS/ZnS core/shell nanocrystals (NCs) with shell thickness up to four monolayers are studied. As a manifestation of strain, the low temperature radiative lifetime measurements indicate a reduction in Stokes shift from 36 meV for CdS to 5 meV for CdS/ZnS with four monolayers of overcoating. Concomitant crossover of S- and P-symmetric hole levels is observed which can be understood in the framework of theoretical calculations predicting flipping the hierarchy of ground hole state by the strain in CdS NCs. Furthermore, a nonmonotonic variation of higher energy levels in strained CdS NCs is discussed

Low-Temperature Photoluminescence Studies of CsPbBr₃ Quantum Dots

Cesium lead halide pervoskite semiconductors are being extensively studied due to unprecedentedly high luminescence efficiency and concomitant narrow emission line width. Here, we report photophysical properties of CsPbBr₃ quantum dots having different sizes. Notably 5.5 nm sized CsPbBr₃ quantum dots reveal 95% photoluminescence (PL) quantum yield at room temperature. Moreover, signature of the stimulated emission is observed at low temperature for excitation fluence as low as ∼4.16 μW (Xe lamp excitation). Even though CsPbBr₃ quantum dots reveal the red shift in band gap at low temperature, similar to the single crystal, the exciton–phonon interaction is profoundly affected by the quantum size effects. Temperature-dependent optical studies reveal an anomalous decrease in exciton-LO phonon coupling in small-sized quantum dots aside from expected higher exciton binding energy. Observed stimulated emission in low-sized CsPbBr₃ quantum dots has implications in realizing a quantum dot based laser