Shalinee Kavadiya

Growth of single crystal, oriented SnO2 nanocolumn arrays by aerosol chemical vapour deposition

A single-step, template-free aerosol chemical vapor deposition (ACVD) method is demonstrated to grow well-aligned SnO2 nanocolumn arrays. The ACVD system parameters, which control thin film morphologies, were systematically explored to gain a qualitative understanding of nanocolumn growth mechanisms. Key growth variables include feed rates, substrate temperature, and deposition time. System dynamics relating synthesis variables to aerosol characteristics and processes (collision and sintering) are elucidated. By adjusting system parameters, control of the aspect ratio, height, and crystal structure of columns is demonstrated. A self-catalyzed (SnO2 particles) vapor–solid (VS) growth mechanism, whereby a vapor–particle deposition regime results in the formation of nanocrystals that act as nucleation sites for the preferential formation and growth of nanocolumns, is proposed and supported. Density functional theory (DFT) calculations indicate that the preferential orientation of thin films is a function of the system redox conditions, further supporting the proposed VS growth mechanism. When taken together, these results provide quantitative insight into the growth mechanism(s) of SnO2 nanocolumn thin films via ACVD, which is critical for engineering these, and other, nanostructured films for direct incorporation into functional devices.

Highly Stable Perovskite Solar Cells Fabricated Under Humid Ambient Conditions

Organometallic perovskite solar cells have gained immense attention due to their rapid increase in efficiency and compatibility with low-cost fabrication methods. However, the materials instability in humid ambient conditions has remained a key challenge for the large-scale fabrication and application of such cells. In this paper, we present devices fabricated under 50% humidity with significantly improved long-term stability through three parallel approaches. First, the small molecule hole transport material, 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-MeOTAD) is replaced by a polymeric material Poly(3-hexylthiophene) (P3HT). Second, the device stability is further enhanced by increasing the thickness of the mesoporous titania scaffold. Finally, tetraethyl orthosilicate (TEOS) is used as a processing additive in the perovskite precursor solution to form an in situ protective layer. On our optimized device, a remarkable long-term device stability of more than 1200 h is achieved. X-ray diffraction patterns suggest more than 2500 h of material stability.

Supramolecular self-assembly of bacteriochlorophyll c molecules in aerosolized droplets to synthesize biomimetic chlorosomes

The unique properties of chlorosomes, arising out of the self-assembled bateriochlorophyll (BChl) c structure, have made them attractive for use in solar cells. In this work, we have demonstrated the self-assembly of BChl c in aerosolized droplets to mimic naturally occurring chlorosomes. We compare two different methods for self-assembly of BChl c, one using a single-solvent and the other using two-solvents, and demonstrate the superiority of the two-solvent method. Results show that the self-assembled BChl c sprayed at different concentrations resulted in a varying red shift of 69-75 nm in absorption spectrum compared to the solution, which has peak at 668 nm corresponding to the monomeric BChl c. The sample fluoresces at 780 nm indicating a quality of self-assembly comparable to that observed in naturally occurring chlorosomes. In order to mimic chlorosomes, solution containing BChl c, BChl a, lipids and carotenes in same proportion as in chlorosomes is sprayed. The resulting self-assembly has an absorption peak at 750 nm, shifted by 82 nm compared to that of monomers and the fluorescence peak at 790 nm. Thus in presence of lipids and carotenes, both the absorption and fluorescence peaks are red shifted. Further, using grazing incidence small angle X-ray scattering (GISAXS), we characterized the deposited films, and the 2D X-ray scattering patterns of sample clearly indicate the distinct lamellar structure as present in chlorosomes. The results of this work provide new insights into self-assembly in aerosolized droplets, which can be used for assembling a wide range of molecules.

Hierarchical architecture of CuInS2 microsphere thin films: altering laterally aligned crystallographic plane growth by Cd and V doping

The hierarchical architecture of pristine and cadmium (Cd) and vanadium (V) doped copper indium disulphide (CuInS2 (CIS)) microsphere thin films grown on spray coated seed layers by a wet chemical method is demonstrated. First, nano-flakes of self-assembled porous (NFSAP)-CIS microspheres have been optimized on a CIS seed layer by controlling the synthesis strategy. Later, Cd and V were incorporated as foreign impurity ions into the NFSAP-CIS microspheres. The pristine and doped CIS microsphere films resulted in a body-centered-tetragonal crystal structure which was confirmed from the XRD and SAED patterns. The electron microscope images clearly depict the formation of a solid and an elongated NFSAP-CIS microsphere under Cd and V doping, respectively. The change in morphological structure was attributed to the suppression and expansion of the laterally oriented crystallographic plane. The chemical composition and optical and electrical properties of the pristine and Cd and V doped CIS films were determined by UV-vis, photoluminescence, XPS, and Hall measurements. The Cd and V doped CIS microsphere films have superior photoelectric response compared to the pristine CIS films. The controlled laterally oriented crystallographic plane in CIS microspheres brought about by doping induces the modification in the surface morphological structure that results in improved electrical and photo-physical properties. The results of this study provide a framework for fabricating an optimized CIS absorber layer in photovoltaic devices.

Design of Cerenkov-assisted Photoactivation of TiO2 Nanoparticles and Reactive Oxygen Species Generation for Cancer Treatment

The use of Cerenkov radiation to activate nanoparticles in situ was recently shown to control cancerous tumor growth. Although the methodology has been demonstrated to work, to better understand the mechanistic steps, we developed a mathematic model that integrates Cerenkov physics, light interaction with matter, and photocatalytic reaction engineering. Methods: The model describes a detailed pathway for localized reactive oxygen species (ROS) generation from the Cerenkov radiation–assisted photocatalytic activity of TiO2. The model predictions were verified by comparison to experimental reports in the literature. The model was then used to investigate the effects of various parameters—the size of TiO2 nanoparticles, the concentration of TiO2 nanoparticles, and the activity of the radionuclide 18F-FDG—on the number of photons and ROS generation. Results: The importance of nanoparticle size in ROS generation for cancerous tumor growth control was elucidated, and an optimal size was proposed. Conclusion: The model described here can be used for other radionuclides and nanoparticles and can provide guidance on the concentration and size of TiO2 nanoparticles and the radionuclide activity needed for efficient cancer therapy.

SnO2 Nanostructured Thin Films for Room-Temperature Gas Sensing of Volatile Organic Compounds

We demonstrated room-temperature gas sensing of volatile organic compounds (VOCs) using SnO2 nanostructured thin films grown via the aerosol chemical vapor deposition process at deposition temperatures ranging from 450 to 600 °C. We investigated the films sensing response to the presence of three classes of VOCs: apolar, monopolar, and biopolar. The synthesis process was optimized, with the most robust response observed for films grown at 550 °C as compared to other temperatures. The role of film morphology, exposed surface planes, and oxygen defects were explored using experimental techniques and theoretical calculations to improve the understanding of the room-temperature gas sensing mechanism, which is proposed to be through the direct adsorption of VOCs on the sensor surface. Overall, the improved understanding of the material characteristics that enable room-temperature sensing gained in this work will be beneficial for the design and application of metal oxide gas sensors at room temperature.

Aerosol methods to fabricate perovskite solar cells

Perovskite-based solar cells have shown tremendous increase in the efficiency over the past few years and simultaneously have low processing cost. However, the stability of the material and hence of the device is still a challenge. In order to improve the ambient condition processing and stability of perovskite solar cells, we adopt two strategies - 1) Addition of a protective material tetraethyl orthosilicate (TEOS), in the perovskite layer that protects the perovskite material from exposure to moisture, and 2) Aerosol method to fabricate the perovskite layer. The devices are prepared and tested under ambient humidity (30-50%).