Growth mechanism of nanocrystals in solution: ZnO, a case study

We investigate the mechanism of growth of nanocrystals from solution using the case of ZnO. Spanning a wide range of values of the parameters, such as the temperature and the reactant concentration, that control the growth, our results establish a qualitative departure from the widely accepted diffusion controlled coarsening (Ostwald ripening) process quantified in terms of the Lifshitz-Slyozov-Wagner theory. Further, we show that these experimental observations can be qualitatively and quantitatively understood within a growth mechanism that is intermediate between the two well-defined limits of diffusion control and kinetic control.Comment: 10 pages, 4 figure

Self Assembly and Electronic Structure of ZnO Nanocrystals

In this paper, we report the synthesis and self assembly of various sizes of ZnO nanocrystals. While the crystal structure and the quantum confinement of nanocrystals were mainly characterized using XRD and UV absorption spectra, the self assembly and long range ordering were studied using scanning tunneling microscopy after spin casting the nanocrystal film on the highly oriented pyrolytic graphite surface. We observe self assembly of these nanocrystals over large areas making them ideal candidates for various potential applications. Further, the electronic structure of the individual dots is obtained from the current-voltage characteristics of the dots using scanning tunneling spectroscopy and compared with the density of states obtained from the tight binding calculations. We observe an excellent agreement with the experimentally obtained local density of states and the theoretically calculated density of states

Synergistic Path for Dual Anisotropic and Electrically Switchable Emission From a Nanocomposite of CsPbBr3 Quantum Cuboids and Nematic Liquid Crystal

We report photophysical properties of a nanocomposite consisting of perovskite quantum cuboids (QCs) formed by CsPbBr3 and a wide temperature range nematic liquid crystal. Contrary to observations made with conventional II-VI quantum dots dispersed in a liquid crystal, the used QCs form, under the influence of the nematic orientation, linear assemblies over macroscopic length scales evidenced by polarizing optical microscopy. Interestingly, the linear assembly is actually caused by such an anisotropic arrangement at the nm scale, as seen in TEM images. Thin films of the nanocomposite exhibiting this unique and fascinating character exhibit absorption and emission features, which are quite appealing. These include retention of the sharp bandwidth of emission characteristic of the native QCs and establishment of dual anisotropies, arising from the values being different along the director as well in the two directions orthogonal to it. We also present data on voltage-driven switching between one of the anisotropic limits

Growth Kinetics of ZnO Nanocrystals in the Presence of a Base: Effect of the Size of the Alkali Cation

Following an earlier study (J. Am. Chem Soc. 2007, 129, 4470) describing a very unusual growth kinetics of ZnO nanoparticles, we critically evaluate here the proposed mechanism involving a crucial role of the alkali base ion in controlling the growth of ZnO nanoparticles using other alkali bases, namely, LiOH and KOH. While confirming the earlier conclusion of the growth of ZnO nanoparticles being hindered by an effective passivating layer of cations present in the reaction mixture and thereby generalizing this phenomenon, present experimental data reveal an intriguing nonmonotonic dependence of the passivation efficacy on the ionic size of the alkali base ion. This unexpected behavior is rationalized on the basis of two opposing factors: (a) solvated cationic radii and (b) dissociation constant of the base

Tandem-Layered Quantum Dot Solar Cells: Tuning the Photovoltaic Response with Luminescent Ternary Cadmium Chalcogenides

Photon management in solar cells is an important criterion as it enables the capture of incident visible and infrared photons in an efficient way. Highly luminescent CdSeS quantum dots (QDs) with a diameter of 4.5 nm were prepared with a gradient structure that allows tuning of absorption and emission bands over the entire visible region without varying the particle size. These crystalline ternary cadmium chalcogenides were deposited within a mesoscopic TiO₂ film by electrophoretic deposition with a sequentially-layered architecture. This approach enabled us to design tandem layers of CdSeS QDs of varying band gap within the photoactive anode of a QD solar cell (QDSC). An increase in power conversion efficiency of 1.97–2.81% with decreasing band gap was observed for single-layer CdSeS, thus indicating varying degrees of photon harvesting. In two- and three-layered tandem QDSCs, we observed maximum power conversion efficiencies of 3.2 and 3.0%, respectively. These efficiencies are greater than the values obtained for the three individually layered photoanodes. The synergy of using tandem layers of the ternary semiconductor CdSeS in QDSCs was systematically evaluated using transient spectroscopy and photoelectrochemistry

Self assembly and electronic structure of ZnO nanocrystals

In this paper, we report the synthesis and self assembly of various sizes of ZnO nanocrystals. While the crystal structure and the quantum confinement of nanocrystals were mainly characterized using XRD and UV absorption spectra, the self assembly and long range ordering were studied using scanning tunneling microscopy after spin casting the nanocrystal film on the highly oriented pyrolytic graphite surface. We observe self assembly of these nanocrystals over large areas making them ideal candidates for various potential applications. Further, the electronic structure of the individual dots is obtained from the current-voltage characteristics of the dots using scanning tunneling spectroscopy and compared with the density of states obtained from the tight binding calculations. We observe an excellent agreement with the experimentally obtained local density of states and the theoretically calculated density of states

Current Mapping of Lead Phthalocyanine Thin Films in the Presence of Gaseous Dopants

The structural organization and its effect on conducting pathways in lead phthalocyanine(PbPc) thin films, a nonplanar phthalocyanine, deposited on Si and highly oriented pyrolyticgraphite (HOPG) substrates in the presence of iodine and ammonia vapors are presented. Thetwo-dimensional grazing incidence X-ray diffraction studies revealed that the crystallineordering in pristine PbPc films on Si and HOPG substrates have undergone a drastic molecularrearrangement and surface reconstruction up on iodine doping. The structural rearrangementleads to morphological changes and higher surface roughness in iodine doped PbPc (I-PbPc)films. An obvious enhancement in the current values of I-PbPc is attributed to the introduction ofholes as charge carriers. Nanoscale current mapping reveals the presence of percolationpathways in I-PbPc films, on both Si and HOPG substrates, responsible for the observed highconductance in contrast to the isolated conducting domains in the pristine PbPc system. A broaddistribution of current values across various conducting domains on Si is attributed to a mixtureof crystalline phases and disordered fractions of I-PbPc while a narrow distribution of currentvalues observed in the case of HOPG is attributed to the majorly disordered PbPc molecules.These films also show enhanced sensitivity towards ammonia by almost four times higher inmagnitude than pristine PbPc films. The current maps show that the adsorption of ammoniamolecules disrupts the iodine percolation pathways, thereby imposing a detrimental effect on theconductivity of the PbPc films

Influence of Iodine Doping on the Structure, Morphology, and Physical Properties of Manganese Phthalocyanine Thin Films

Doping with halide ions is a popular method toalter the properties of metal phthalocyanines (MPcs),particularly magnetic and electrical nature of organic semiconductorsfor applications in spintronic or electronic devices.Doping can cause a structural rearrangement in MPc packing,and the physical properties may be correlated with molecularpacking. Films of a planar and magnetic MPc, manganese-(II)phthalocyanine (MnPc), are chosen for iodine dopingstudy. The optical, magnetic, and electrical properties ofpristine- and iodine-doped MnPc thin films are investigatedand can be directly associated with their molecular structure.Two-dimensional grazing incidence synchrotron X-ray diffractionreveals structural disorder in MnPc films upon iodineinfusion induced by the reorientation of ordered, edge-on molecular configuration to tilted and face-on configurations in arandom fashion. The film morphology changes accordingly, where in the uniform crystallites reorganize in a disordered manner.The ferromagnetic nature of the pristine film gets weakened because of iodine species and favors antiferromagnetic coupling.The study of electrical properties at room temperature by conducting atomic force microscopy reveals that the conductance isenhanced independently of the film thickness because of the disorder induced by iodine inclusion

Templating effect of single-layer graphene supported by an insulating substrate on the molecular orientation of lead phthalocyanine

The influence of single-layer graphene on top of a SiO$_2$/Si surface on the orientation of nonplanar lead phthalocyanine (PbPc) molecules is studied using two-dimensional grazing incidence X-ray diffraction. The studies indicate the formation of a mixture of polymorphs, i.e., monoclinic and triclinic forms of PbPc with face-on (lying down) and edge-on (standing up) PbPc orientations, respectively. The formation of monoclinic fractions is attributed to the presence of the graphene layer directing the π interactions between the highly delocalized macrocycles. The competing interfacial van der Waals forces and molecule–molecule interactions lead to the formation of a small fraction of triclinic moieties. The nanoscale electrical characterization of the thin PbPc layer on graphene by means of conducting atomic force microscopy shows enhanced vertical conductance with interconnected conducting domains consisting of ordered monoclinic crystallites through which the charge transfer occurs via tunneling. These results show the importance of a templating layer to induce the formation of a required phase of PbPc suitable for specific device applications

Synchronized Energy and Electron Transfer Processes in Covalently Linked CdSe–Squaraine Dye–TiO₂ Light Harvesting Assembly

Manipulation of energy and electron transfer processes in a light harvesting assembly is an important criterion to mimic natural photosynthesis. We have now succeeded in sequentially assembling CdSe quantum dot (QD) and squaraine dye (SQSH) on TiO₂ film and couple energy and electron transfer processes to generate photocurrent in a hybrid solar cell. When attached separately, both CdSe QDs and SQSH inject electrons into TiO₂ under visible–near-IR irradiation. However, CdSe QD if linked to TiO₂ with SQSH linker participates in an energy transfer process. The hybrid solar cells prepared with squaraine dye as a linker between CdSe QD and TiO₂ exhibited power conversion efficiency of 3.65% and good stability during illumination with global AM 1.5 solar condition. Transient absorption spectroscopy measurements provided further insight into the energy transfer between excited CdSe QD and SQSH (rate constant of 6.7 × 10¹⁰ s^–1) and interfacial electron transfer between excited SQSH and TiO₂ (rate constant of 1.2 × 10¹¹ s^–1). The synergy of covalently linked semiconductor quantum dots and near-IR absorbing squaraine dye provides new opportunities to harvest photons from selective regions of the solar spectrum in an efficient manner