Structural, Optical and Transport Properties of Copper Chalcogenide Nanocrystal Superlattices

This cumulative thesis is based on three publications. It investigates the self-assembly of nanocrystal (NC) superlattices, charge transport in NC assembly, and application of these superlattices in optoelectronic and vapor sensing. The materials of choice are copper chalcogenide NCs such as binary copper sulfide Cu1.1S NCs, binary copper selenide Cu2Se NCs and ternary Cu2-xSeyS1-y NCs and the organic semiconductors metal (Cu or Co) centered -4,4′,4″,4″,4‴-tetraaminophthalocyanine (Cu/CoTAPc). Macroscopic superlattices of NCs are prepared by Langmuir-type self-assembly at the air/liquid interface followed by simultaneous ligand exchange with an organic semiconductor. To enhance interparticle coupling, we cross-link the nanocrystals with the organic π-system Cu-4,4′,4″,4″,4‴-tetraaminophthalocyanine and observe a significant increase in electrical conductivity. Ultraviolet-visible-near-infrared (UV-vis-NIR) and Raman spectroscopy are used to track the chemical changes on the nanocrystals’ surface before and after ligand exchange and develop a detailed picture of the various components which dominate the surface chemistry of this material. Grazing-incidence small-angle X-ray scattering (GISAXS) serve to study the importance of electronic conjugation in the organic π-system vs interparticle spacing for efficient charge transport. Transport measurements reveal that Cu4APc provides efficient electronic coupling for neighboring Cu1.1S NCs. The electrical properties of monolayers of this hybrid ensemble are consistent with a two-dimensional semiconductor and exhibit two abrupt changes at discrete temperatures (120 and 210 K), which may be interpreted as phase changes. This material provides the opportunity to apply the hybrid ensemble as a chemiresistor in organic vapor sensing. The vapor sensing experiments exhibits a strong selectivity between polar and nonpolar analytes, which we discuss in light of the role of the organic π-system and its metal center. Next, we choose ternary alloyed Cu-based chalcogenide NCs Cu2SeyS1–y and checked the effect of ligand exchange with the organic π-system Cobalt β-tetraaminophthalocyanine (CoTAPc) along with its binary counterpart Cu2Se NCs. We analysed changes in the structural, optical as well as electric properties of thin films of these hybrid materials. Strong ligand interaction with the surface of the NCs is revealed by UV/vis absorption and Raman spectroscopy. GISAXS studies show a significant contraction in the interparticle distance upon ligand exchange. For copper-deficient Cu2-xSe, this contraction has a negligible effect on electric transport, while for copper-deficient Cu2-xSeyS1-y, the conductivity increases by eight orders of magnitude and 8 results in metal-like temperature-dependent transport. We discuss these differences in the light of varying contributions of electronic vs. ionic transport in the two materials and highlight their effect on the stability of the transport properties under ambient conditions. With photocurrent measurements, we demonstrate high optical responsivities of 200-400 A/W for CoTAPc-capped Cu2SeyS1–y and emphasize the beneficial role of the organic π-system in this respect, which acts as an electronic linker and an optical sensitizer at the same time. Finally, we report on the in-situ monitoring of the formation of conductive superlattices of Cu1.1S nanodiscs via cross-linking with semiconducting Co-4,4′,4″,4″,4‴-tetraaminophthalocyanine (CoTAPc) molecules at the liquid/air interface by real-time grazing incidence small angle X-ray scattering (GISAXS). We determine the structure, symmetry and lattice parameters of the superlattices, formed during solvent evaporation and ligand exchange on the self-assembled nanodiscs. Cu1.1S nanodiscs self-assemble into two-dimensional hexagonal superlattice with a minor in-plane contraction (~ 0.2 nm) in the lattice parameter. A continuous contraction of the superlattice has been observed during ligand exchange, preserving the initial hexagonal symmetry. We estimate a resultant decrement of about 5% in the in-plane lattice parameters. The contraction is attributed to the continuous replacement of the native oleylamine surface ligands with rigid CoTAPc. The successful cross-linking of the nanodiscs is manifested in terms of the high electrical conductivity observed in the superlattices. This finding provides a convenient platform to understand the correlation between the structure and transport of the coupled superstructures of organic and inorganic nanocrystals of anisotropic shape

Design and Development of Domestic Solar Dryer with Comparative Analysis of Nutritional Aspect of Dried Raisins

Solar dryer technology is simple and can be implemented by households and small communities due to its simplicity. The suitable design of the absorber/collector is very vital for any solar drying system as the collector efficiency plays a key role in determining the overall system drying efficiency. To study the practical applicability of a developed solar dryer, grapes were dried in the drying chamber of a designed and developed solar dryer and for comparison, in open sunlight. Faster drying was noted for the grapes dried in the unit. The study on nutritional aspects indicated that solar drying process retained the major nutritional components like total sugars, total proteins and total lipids in raisins. The comparison between the solar dried raisins and open sun dried ones showed a higher ash content of 2.71% with solar dried ones and 1.95% in case of open sun dried raisins. The quantity of MUFA content was also affected by the varying drying practices implemented with 10.95% and 7.12% MUFA in solar dried and open sun dried raisins respectively. The drying technique also affects the bacterial load on raisins as observed in our current study with negligible bacterial growth on on solar dried raisins compared to open dried ones

Dye-sensitized ternary copper chalcogenide nanocrystals : optoelectronic properties, air stability, and photosensitivity

We report on the effect of ligand exchange of Cu2SeyS1-y and Cu2Se nanocrystals (NCs) with the organic pi system cobalt beta-tetraaminophthalocyanine (CoTAPc) and analyze the changes in the structural, optical, and electric properties of thin films of these hybrid materials. A strong ligand interaction with the surface of the NCs is revealed by UV/vis absorption and Raman spectroscopy. Grazing incidence small-angle X-ray scattering studies show a significant contraction in the interparticle distance upon ligand exchange. For copper-deficient Cu2-xSe, this contraction has a negligible effect on electric transport, whereas for copper-deficient Cu2-xSeyS1-y, the conductivity increases by 8 orders of magnitude and results in metal-like temperature-dependent transport. We discuss these differences in the light of varying contributions of electronic vs ionic transport in the two materials and highlight their effect on the stability of the transport properties under ambient conditions. With photocurrent measurements, we demonstrate high optical responsivities of 200-400 A W-1 for CoTAPc-capped Cu2SeyS1-y and emphasize the beneficial role of the organic pi-system in this respect, which acts as an electronic linker and an optical sensitizer at the same time

In-situ formation of electronically coupled superlattice of Cu1.1S nanodiscs at the liquid/air interface

We report on the in situ monitoring of the formation of conductive superlattices of Cu1.1S nanodiscs via cross-linking with semiconducting cobalt 4,4′,4′′,4′′′-tetraaminophthalocyanine (CoTAPc) molecules at the liquid/air interface by real-time grazing incidence small angle X-ray scattering (GISAXS). We determine the structure, symmetry and lattice parameters of the superlattices, formed during solvent evaporation and ligand exchange on the self-assembled nanodiscs. Cu1.1S nanodiscs self-assemble into a two-dimensional hexagonal superlattice with a minor in-plane contraction (∼0.2 nm) in the lattice parameter. A continuous contraction of the superlattice has been observed during ligand exchange, preserving the initial hexagonal symmetry. We estimate a resultant decrement of about 5% in the in-plane lattice parameters. The contraction is attributed to the continuous replacement of the native oleylamine surface ligands with rigid CoTAPc. The successful cross-linking of the nanodiscs is manifested in terms of the high electrical conductivity observed in the superlattices. This finding provides a convenient platform to understand the correlation between the structure and transport of the coupled superstructures of organic and inorganic nanocrystals of anisotropic shape.by Sonam Maiti, Santanu Maiti, Andre Maier, Rupak Banerjee, Chen Shen, Bridget Mary Murphy, Marcus Scheele and Frank Schreibe

Moisture Effect on the Threshold Switching of TOPO-Stabilized Sub-10 nm HfO 2 Nanocrystals in Nanoscale Devices

The enduring demand for ever-increasing storage capacities inspires the development of new few nanometer-sized, high-performance memory devices. In this work, tri-n-octylphosphine oxide (TOPO)-stabilized sub-10 nm monoclinic HfO2 nanocrystals (NC) with a rod-like and spherical shape are synthesized and used to build up microscale and nanoscale test devices. The electrical characterization of these devices studied by cyclic current–voltage measurements reveals a redox-like behavior in ambient atmosphere and volatile threshold switching in vacuum. By employing a thorough spectroscopic and surface analysis (FT-IR and NMR spectroscopy and XPS), the origin of this behavior was elucidated. While the redox behavior is enabled by residual moisture present during clean-up of the NC and thin film preparation, which leads to a partial desorption of TOPO from the NC surface, threshold switching is obtained for dry TOPO-stabilized HfO2 NC in microchannel as well as in nanoelectrode devices addressing only a few sub-10 nm TOPO-stabilized HfO2 NC. The results show that integration of sub-10 nm HfO2 NC in nanoscale devices is feasible to build up switching elements

Understanding the Formation of Conductive Mesocrystalline Superlattices with Cubic PbS Nanocrystals at the Liquid/Air Interface

We report the formation of conductive mesocrystalline superstructures of cubic PbS nanocrystals (NCs) through directional cross-linking with organic semiconductors at the liquid/air interface monitored simultaneously by in situ grazing incidence small angle X-ray scattering and grazing incidence X-ray diffraction. We determine the superlattice type, its symmetry and parameters, and the atomic orientation of NCs from the time-resolved scattering patterns. The superlattice contraction follows an exponential decay during ligand exchange, preserving always the two-dimensional square geometry. We attribute the contraction to the continuous replacement of oleic acid with smaller cobalt/copper 4,4′,4″,4‴-tetraaminophthalocyanine molecules. In these superlattices, the NCs are directed with a [100]AL axis perpendicular to the liquid surface for the whole assembly period. The kinetics and structural results provide a direct correlation between the superstructure and their atomic orientation on the liquid surface during self-assembly followed by ligand exchange