Mohammad Afzaal

Recent developments in II–VI and III–VI semiconductors and their applications in solar cells

Compound semiconductors formed from II–VI or III–VI elements are key materials in plans to harvest energy directly from sunlight in photovoltaic devices. This review summarises the recent advances in thin film deposition and nanocrystallite synthesis. Current work has significant potential for low-cost, scalable solar cells. The close interplay between the properties of the materials and their utility in solar cells is also briefly discussed.

The Chemical Vapor Deposition of Nickel Phosphide or Selenide Thin Films from a Single Precursor

Nickel phosphide, nickel selenide thin films and their heterostructure (Ni0.85Se/Ni2P) were deposited from a newly synthesized single source precursor {Ni[iPr2P(S)NP(Se)iPr2]2} just by altering the deposition temperature using CVD.

Transient Optical Studies of Interfacial Charge Transfer at Nanostructured Metal Oxide/PbS Quantum Dot/Organic Hole Conductor Heterojunctions

We report a transient absorption and luminescence study addressing the charge separation, recombination, and regeneration reactions at nanostructured metal oxide/PbS quantum dot/organic hole conductor heterojunctions. We show that yields of charge separation are significantly higher at PbS/SnO(2) interfaces relative to PbS/TiO(2) interfaces, and conclude that this behavior is a result of the ca. 300-500 meV lower conduction band edge in SnO(2) as compared to TiO(2). We also report a correlation between the PbS particle size and the yield of charge separation at PbS/SnO(2) interfaces, with a smaller PbS particle radius resulting a higher yield of charge separation. Finally we investigated the regeneration of the photooxidized PbS by an organic hole transporting material, namely, spiro-OMeTAD. The overall spiro-OMeTAD(+) yield is found to be 27% at a SnO(2)/PbS (approximately 3 nm diameter)/spiro-OMeTAD heterojunction. The addition of a lithium salt was found to raise the overall spiro-OMeTAD(+) yield from its initial value of 27% (where no Li(+) is present) to 50%. We believe this to be a result of an increase in the primary charge injection yield to near unity as the SnO(2) conduction band is lowered (with increasing [Li(+)]), increasing the driving force for electron injection. The present findings are discussed with relevance to the design of PbS-sensitized metal oxide layers for DSSCs.

The synthesis of amine-capped magnetic (Fe, Mn, Co, Ni) oxide nanocrystals and their surface modification for aqueous dispersibility

Nano-dimensional single-crystals of Fe, Mn, Co or Ni oxides have been prepared by the decomposition of their metal acetylacetonate at elevated temperatures in hexadecylamine. The amine acts as both a solvent and the capping agent. The crystalline structure and morphology of resulting samples have been characterized by TEM, HRTEM, SAED and XRD. The nanocrystals can be dispersed in aqueous solution by overcoating the surface with amphiphilic polyelectrolytes.

Remarkable Magneto-Optical Properties of Europium Selenide Nanoparticles with Wide Energy Gaps

The enhanced magneto-optical properties of nanoscaled lanthanide chalogenide semiconductors which have a wide energy gap were observed at around 500 nm for the first time. The nanoscaled semiconductors, Eu(1-x)Se nanoparticles 1 (cubic shapes) and 2 (spherical shapes), were synthesized by the thermal reduction of Eu(III) ion with organic ligands containing Se atoms. The resulting Eu(1-x)Se nanoparticles were characterized by X-ray diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, superconducting quantum interference devices magnetometer, and microwave induced plasma atomic emission spectroscopy measurements. The particle grain sizes of 1 and 2 were estimated to be 11 and 20 nm, respectively. The concentration-normalized Veldet constants (the magnitude of the Faraday effect) of Eu(1-x)Se nanoparticles were much larger than those of corresponding bulk EuSe and EuS nanoparticles.

Growth of lead chalcogenide thin films using single-source precursors

A series of lead dithiocarbamato complexes, [Pb(S2CNRR′)2] (R = Me, Et; R′ = iPr, nBu) and dichalcogenoimidodiphoshanto complexes, [Pb((EPiPr2)2N)2] (E = S, Se) have been synthesised and characterised. These compounds have been employed as single-source precursors for the growth of lead chalcogenide thin films by aerosol-assisted chemical vapour deposition (AACVD) and low-pressure metal–organic chemical vapour deposition (LP-MOCVD) on glass substrates. X-Ray powder diffraction (XRPD) studies show that the films are composed of cubic PbS/Se. The solid-state structures of the symmetrically substituted complexes, [Pb(S2CNR2)2] (R = C6H10, C5H11), have been determined by single-crystal X-ray crystallography.

Synthesis of ZnO hexagonal single-crystal slices with predominant (0001) and (0001) facets by poly(ethylene glycol)-assisted chemical bath deposition.

ZnO hexagonal single-crystal slices with predominant (0001) and (0001) facets have been fabricated by poly(ethylene glycol)-assisted chemical bath deposition.

Solid state synthesis of tin-doped ZnO at room temperature: Characterization and its enhanced gas sensing and photocatalytic properties

A room temperature solid-state reaction has been used to prepare crystalline tin-doped ZnO. Zinc nitrate hexahydrate, cetyltrimethyl ammonium bromide, stannic chloride pentahydrate and sodium hydroxide with proper ratios were ground together. As-synthesized samples were characterized by inductively coupled plasma analysis (ICP), scanning electron microscopy (SEM) and X-ray powder diffraction (XRD); The products were of different morphologies, well dispersed and exhibited good crystallinity, it is also found that the growth direction and morphology of ZnO depend on the amount of Sn doped, which is mainly caused by the difference in sizes between Zn and Sn atoms as well as the change of pH value. Moreover, gas sensing and photocatalytic properties of the obtained products were studied. The materials showed a high gas response to ethanol vapor, and the gas response can reach a maximum of R(a)/R(g) = 124. In addition, tin-doped ZnO materials exhibited improved photocatalytic performance toward methyl orange (MO) solution under a current density of 0.03 mg L(-1) comparison with undoped ZnO.

The synthesis, X-ray structures and CVD studies of some group 11 complexes of iminobis(diisopropylphosphine selenides) and their use in the deposition of I/III/VI photovoltaic materials

Reaction of NH(PiPr2)2 with elemental selenium in concentrated solvent conditions enables large scale preparation and improved yields of NH(PSeiPr2)2 that may be deprotonated with sodium methoxide to give NaN(PSeiPr2)2. Treatment of the sodium salt with appropriate Group 11 metal salts in methanol yields a range of trinuclear complexes. The protic solvent conditions utilized facilitate the reduction of copper(II) salts resulting in the isolation of copper(I) complexes. These new Group 11 complexes have been characterised by 1H and 31P NMR and IR spectroscopy, APCI mass spectrometry, microanalysis and X-ray crystallography. Thermolytic decomposition of the copper(I) precursors in the presence of the indium precursor, In[(SePiPr2)2N]2Cl, has been carried in the solid state using AA-MOCVD to give copper indium diselenide solid state materials CuInSe2.

Controlled Synthesis of Tuned Bandgap Nanodimensional Alloys of PbSxSe1−x

Truly alloyed PbS(x)Se(1-x) (x = 0-1) nanocrystals (∼5 nm in size) have been prepared, and their resulting optical properties are red-shifted systematically as the sulfur content of the materials increases. Their optical properties are discussed using a modified Vegards approach and the bowing parameter for these nanoalloys is reported for the first time. The alloyed structure of the nanocrystals is supported by the energy-filtered transmission electron microscope images of the samples, which show a homogeneous distribution of sulfur and selenium within the nanocrystals. X-ray photoelectron spectroscopy studies on ligand-exchanged nanocrystals confirmed the expected stoichiometry and various oxidized species.