Khalid M. Hanif

Evidence for Ligand-Induced Paramagnetism in CdSe Quantum Dots

We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the surface chemistry. Using SQUID magnetometry and X-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand end-group functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed. The results suggest that the paramagnetism is induced via pi back-bonding between Cd 4d orbtials and ligands with empty pi* orbitals.

Structure determination and a vibrational study for the hexagonal elpasolite Cs2NaGaF6:Cr3+

Single crystals of 0.5% Cr3+-doped Cs2NaGaF6 were studied by means of x-ray diffraction and polarized Raman scattering. The crystal exhibits a unique stacking interaction, with a hexagonal structure with Rm symmetry. Polarized Raman spectra recorded at 16 and 300 K reveal sidebands that are ascribed to the local vibrational of the [CrF6] coordination unit, which differ markedly from those of the Cs2NaGaF6 host lattice. The results are compared to findings for other elpasolite lattices, and the room temperature luminescence quantum yields are discussed in terms of a delicate balance between electron–phonon strength and lattice distortion.

Structural and vibrational study of chromium doped elpasolite crystals Cs2NaAlF6

The influence of site-symmetry and electron phonon coupling in Cs2NaAlF6:Cr3+ is probed by correlation of optical and structural measurements. Based on neutron and x-ray analysis the structure is a distorted R3m¯, exhibiting a unique stacking interaction. Selection rules have been used to assign the Raman-active zone-center vibrations. Although local and bulk mode frequencies differ slightly, indicating that the guest Cr3+ ion does not significantly perturb the host structure, vibrational analysis indicates the Cr3+ ion is coupled to the first coordination sphere of the Al–F lattice.

Synthesis and Characterization of Cd 1−x Cu x Se Quantum Dots

Cu:CdSe nanoparticle alloys were synthesized using inorganic cluster precursor via a lyothermal method. The dots synthesized were approximately 4 nm in size and exhibited doping levels as high as 20% for Cu(I). Cu(I) cations are randomly doped on the Cd-T d site based on a Vegards law analysis, suggesting that doping arises from a random ion substitution mechanism. The resulting dots exhibited essentially no band edge photoluminescence due to a high level of trap site inclusions in these materials resulting from Se vacancy formation in order to compensate for the Cu + replacing a Cd 2+ ion. Se vacancy formation is evident in the XPS analysis of the Cd:Cu:Se ratios for these materials.