Synthesis and characterization of tin sulfide nanomaterials and thin films by pulsed laser ablation in liquid.

Tin monosulfide (SnS) and tin disulfide (SnS2) nanoparticles were synthesized by employing pulsed laser ablation in liquid (PLAL) technique in various organic solvents where the influence of different laser parameters and solvents were investigated. Spray deposition technique has been implemented as a method to fabricate thin films of these materials where the spray parameters were optimized depending on the solvent and material. Combination of pulsed laser ablation in liquid with spray technique was used for the first time in the work. In the case of SnS nanoparticles, they were first prepared by laser ablation in isopropanol and N,N-dimethyl formamide and thin films of the same were deposited by spraying the laser generated nanocolloids onto heated substrates where the films fabricated were characterized for their structure, morphology and opto-electronic and electrochemical properties. Moreover, colloidal graphene oxide was mixed with SnS nanocolloids at different volume concentrations (0.1%, 0.5% and 1%) to obtain SnS: GO or SnS: rGO thin films where the nanocomposite films showed enhanced opto-electronic and electrochemical properties compared to the pristine SnS thin films. A solar cell configuration of glass/ CdS/ Sb2S3/ SnS:rGO was also fabricated using the SnS: rGO layer as the absorber and photoconversion efficiency of 2.3% was achieved. In the case of SnS2 nanoparticles, effect of four different solvents (acetone, isopropanol, ethanol and DMF) and two different laser wavelengths (1064 nm and 532 nm) on the morphologies and properties of SnS2 particles were studied in detail. Furthermore, influence of ablation fluence, liquid medium temperature and post irradiation on the SnS2 nanoparticles were investigated in detail and the hydrogen evolution activity of these nanoparticles were tested in acidic medium. Electrochemical properties of the SnS2 thin films deposited from SnS2 nanoparticles in ethanol and isopropanol and after different post annealing treatments (200, 250 and 300 ℃) were analyzed and the results were compared. For majority of the cases, the prepared nanoparticles and thin films were analyzed for their structure, crystalline nature, composition and morphology by XRD, Raman, XPS, TEM and SEM whereas the optical, opto-electronic and electrochemical properties were elucidated using the UVVisible spectroscopy, I-V measurements under dark and illumination and electrochemical measurements respectively

Tuning electrical properties of hierarchically assembled Al-doped ZnO nanoforests by room temperature Pulsed Laser Deposition

Large surface area, 3D structured transparent electrodes with effective light management capability may represent a key component in the development of new generation optoelectronic and energy harvesting devices. We present an approach to obtain forest-like nanoporous/hierarchical Al-doped ZnO conducting layers with tunable transparency and light scattering properties, by means of room temperature Pulsed Laser Deposition in a mixed Ar:O2 atmosphere. The composition of the background atmosphere during deposition can be varied to modify stoichiometry-related defects, and therefore achieve control of electrical and optical properties, while the total background pressure controls the material morphology at the nano- and mesoscale and thus the light scattering properties. This approach allows to tune electrical resistivity over a very wide range (10^-1 - 10^6 Ohm*cm), both in the in-plane and cross-plane directions. Optical transparency and haze can also be tuned by varying the stoichiometry and thickness of the nano-forests

The pulsed electron deposition technique for biomedical applications: A review

The "pulsed electron deposition" (PED) technique, in which a solid target material is ablated by a fast, high-energy electron beam, was initially developed two decades ago for the deposition of thin films of metal oxides for photovoltaics, spintronics, memories, and superconductivity, and dielectric polymer layers. Recently, PED has been proposed for use in the biomedical field for the fabrication of hard and soft coatings. The first biomedical application was the deposition of low wear zirconium oxide coatings on the bearing components in total joint replacement. Since then, several works have reported the manufacturing and characterization of coatings of hydroxyapatite, calcium phosphate substituted (CaP), biogenic CaP, bioglass, and antibacterial coatings on both hard (metallic or ceramic) and soft (plastic or elastomeric) substrates. Due to the growing interest in PED, the current maturity of the technology and the low cost compared to other commonly used physical vapor deposition techniques, the purpose of this work was to review the principles of operation, the main applications, and the future perspectives of PED technology in medicine

A comprehensive review: SnO2 for photovoltaic and gas sensor applications

184-193Tin oxide is remarkable material in today’s research era due to its unique properties in electrical and optical fields. Due to its wide band gap (3.6 eV), it has been used as a core material in many important applications in the field of optoelectronics, spintronics, photovoltaic, thin-film transistors, photocatalysis, dielectrics, sensors and transparent electronic devices. Thin film technology provides many advantages towards photovoltaic area which includes low cost, less material and energy consumption and easy to access. Fabrication of photovoltaic cells by SnO2 thin films can open the different technological routes for future generation with excellent conversion efficiencies which may range 15% to 20%. It is one of the best candidates for gas sensor applications too with highest sensitivity and selectivity behavior, good oxidizing power, strong chemical bonding, non toxicity and unique transport properties. Tin oxide thin films with various combinations of materials can be synthesized by chemical and physical routes. The detailed advancement in various preparation methods and characterization techniques including X-ray diffraction, atomic force microscopy and X-ray photoelectron spectroscopy have been presented and discussed by authors. Characteristics measurement by Valence Band Structure, Photoluminence Intensity and Scanning Electron Microscope has been also reported with their performance, effect of solar energy conversion efficiency and quick response time in case of gas sensors. Prospective areas of SnO2 research for photovoltaic and gas sensor applications has been discussed and summarized by the authors. The obtained results will illustrate the possibilities of scheming Physical, chemical, magnetic and optical properties of SnO2 for sensing devices and photovoltaic applications

Large area TMD-based van der Waals heterostructures featuring enhanced photoconversion in the flat optics regime

One of the most urgent technological needs of this century concerns the research of innovative nanomaterials for applications in optoelectronics, nanophotonics and photovoltaics for renewable energy conversion. In fact, downscaling of the silicon-based devices (e.g. field-effect transistors) has come to an end due to intrinsic physical limitations such as size, inadequate carrier mobility, short channel effects, atomic-scale interactions, heat generation, and energy consumption at atomic scale thickness, requiring innovative materials to overcome such difficulties. Graphene discovery in 2004 by Novoselov and Geim arose a tremendous interest for the two-dimensional (2D) materials, in which the reduced dimensionality brings new properties with respect to their bulk counterparts. Despite the extraordinary properties exhibited by graphene, its gapless nature strongly limits the fabrication of graphene-based optoelectronic devices. For this reason, the interest of researchers shifted towards the class of 2D semiconductors. Among these materials, Transition Metal Dichalcogenides (TMDs) represent the most important family due to suitable bandgap energy values which match the Schockley-Queisser efficiency criterion for solar photoconversion, and to their extraordinary optical absorption coefficient. Additionally, 2D materials can be vertically stacked to form the so-called van der Waals heterostructures that are endowed by pristine interface thanks to the relatively weak van der Waals interactions that keep the stack together. This offers the opportunity to fabricate heterostructures of arbitrary 2D materials, independently by their crystal structure, with no limitations on the engineering of the optoelectronic and photonic properties of the new stacked metamaterial. In particular, the possibility to realize van der Waals p-n junctions by coupling 2D-TMDs layers is very intriguing for photoconversion and photovoltaic applications. So far, TMDs employment has been mostly limited to the fabrication of prototypical devices such as field-effect transistors and photodetectors, most of which were realized via mechanical exfoliation from single crystal of flakes with thickness ranging from one to few atomic layers. Despite their intriguing properties, these materials do not represent an industrially relevant alternative to traditional semiconductors, being the exfoliation a randomic process with very low yields and limited areas in the micrometer range. Consequently, one of the key frontiers of TMDs research is the large area growth of homogeneous ultra-thin layers with controlled thickness over macroscopic areas. To meet this requirement, several large area techniques have been studied to synthesize TMDs layers extending over cm² areas. However, in contrast with the exfoliation process, these techniques result in polycrystalline layers where the high concentration of grain boundaries degrades the macroscopic conduction. The second crucial issue to be solved for ultra-thin TMD-based devices to be used in photoconductive and photodetection applications is the maximization of the optical absorption. Despite the excellent optical absorption coefficient, an ultra-thin TMD film indeed cannot absorb efficiently the incoming light due to the ultimately reduced thickness, which means a nanometric optical path. It is thus evident that new strategies for the optical absorption amplification in ultra-thin 2D semiconductors need to be developed to allow proper performances in TMD-based photodetection and photovoltaics applications. Because of the atomic thickness of the 2D layer, traditional solutions developed for the light harvesting amplification in conventional silicon-based photovoltaic devices, based on pyramidal microstructuring or on the addition of antireflective coatings, cannot be transferred to these materials. Recently, the research group where I carried out my activity worked on large area ultra-thin MoS₂ films conformally grown on self-organized rippled substrate fabricated by ion beam sputtering. Their results clearly showed a modification of the TMD optical and electronic properties grown on the nanostructured substrate with respect to a flat one. This observation was explained with the stress induced by the substrate morphology in the MoS₂ layer in correspondence to the high curvature regions given by the crests and valleys of the ripples, meaning that by control of the substrate morphology it is possible to engineer the material intrinsic properties. Additionally, the nanostructures anisotropy adds a polarization-dependent optical response, offering a way to engineer the optical absorption of the 2D material in view of photoconversion applications. However, self-organized nanostructures suffer from a relevant size dispersion and long range disorder, whereas more interesting optical effects are expected for periodic nanogratings in which the subwavelength TMD layers reshaping provide them the functionality of flat optic diffractive elements. Starting from here, I devoted the most of my research activity to face the two main challenges described above: developing a growth process for large area ultra-thin TMD films, and studying an efficient light harvesting strategy to maximize the optical absorption in few-layer semiconductor films. An overview of the state-of-the-art regarding 2D materials and nanophotonics approaches for light harvesting in ultra-thin films is given in Chapter 1, while the growth synthesis techniques are postponed in Chapter 2. At the beginning of my PhD, MoS₂ films were grown by external collaborators of my group, thus limiting the activities. In the first phase of my research activity, I developed a novel large area growth process based on the physical deposition of solid precursor films and sulfurization, as I will describe in Chapter 2. This novel technique enabled not only the in-house growth of ultra-thin MoS₂ films for the first time, but also allowed to extend the process to ultra-thin WS₂ layers. Having now the capability to control the deposition of two different TMDs layers, I moved to the growth of large area van der Waals heterostructures. Due to the type-II heterojunction formed by the band structure coupling of MoS₂ and WS₂, such heterostructures are expected to have a high potential in photoconversion applications. In Chapter 3 I will show the nanofabrication of planar MoS₂/WS₂ heterostacks, and their application in photocatalytic experiments and in a prototype of photonic device, featuring first evidence of photovoltage and photocurrent under illumination. This latter application also required me to develop large area transparent electrodes, so that I will dedicate a part of the chapter to indium tin oxide thin films and large area graphene. In the second phase of my research activity, I focused on light harvesting in ultra-thin TMDs layers. Thanks to the conformality achieved by the physical deposition process, I explored a nanophotonic approach based on the optical anomalies arising from periodic modulation of the TMD layer at the subwavelength scale, obtained by conformal growth of the TMD layers onto nanostructured substrates. To this end, periodic nanogratings have been used as a template for the growth of ultra-thin MoS₂ layers. Differently from the self-organized nanostructures mentioned before, the periodicity induces diffractive effects that are exploited to steer the light parallel to the active 2D material enhancing the optical absorption, as demonstrated in Chapter 4 both directly by absorption measurements and indirectly by a photo-to-chemical energy conversion experiment where we detected enhanced photocatalytic performances. In the final part of my project, I started preliminary studies on the elastic scattering properties of subwavelength periodical lattices based on nanostructured tilted TMD layers. By defocused ion beam sputtering, I was able to reshape the morphology of the initial nanograting templates to further engineer the TMD optical response. In particular, by off-normal incidence sputtering it is possible to tailor a specific slope of the tilted nanofacets, on top of which I deposited laterally disconnected MoS₂ nanostripes by grazing angle physical deposition. By developing a custom-made scatterometer, optical characterization of ultra-thin MoS₂ nanostripes and thicker MoS₂ films was performed, giving interesting preliminary results on the directional light scattering properties of these reshaped layers, as reported in Chapter 5. Finally, I adopted a similar deposition approach for the nanofabrication of large area heterostructures nanoarrays based on few-layer WS₂ nanostripes coated by a conformal MoS₂ layer, demonstrating further engineering of the optical response of few-layer TMD films with impact in photoconversion

Laser processing of solution based antimony doped tin oxide thin films

Antimony doped Tin oxide (SnO2:Sb, or ATO) is of interest as an alternative to Indium Tin Oxide (ITO) for large area optoelectronic applications. There is a particular interest in the potential for solution based coatings based on nanoparticulate suspensions of SnO2:Sb. However, solution processed films typically require a high temperature (~700⁰C) annealing step to achieve the desired electrical and optical properties. This is disadvantageous for applications that would benefit from low cost, low temperature/flexible substrates. As an alternative to conventional high temperature annealing, excimer laser processing can provide highly localized energy dissipation, and is an attractive technique to functionalise coated materials. Therefore the work presented in this thesis investigates the use of excimer laser processing to optimise the electrical and optical properties of solution deposited SnO2:Sb thin films for use in electroluminescent display devices. Thin films of SnO2:Sb were deposited using dip coating, inkjet printing and the spin coating technique. By varying the numbers of spin coatings deposited, a series of samples were prepared on Eagle XG glass substrates with different thicknesses of SnO2:Sb (ranging from 0.2 μm to 1.4 μm). The initial sheet resistance, optical transmission and crystal structure of the deposited films was studied. The films were subsequently post processed using three different annealing techniques: (i) Laser Processing: samples were laser processed in air to optimise the sheet resistance and optical transmission

Synthesis And Characterization Of Zero, One And Two Dimensional Metallic And Ceramic Nanostructures

Nanostructured materials are of great interest because the properties of a material at the nanoscale may differ significantly from the properties of the same material in the bulk form. This has led to a lot of new applications for nanomaterials owing to their unique physical, chemical, electrical, optical and magnetic properties. The present work reports on the synthesis and characterization of zero, one, and two dimensional nanostructured materials. Nanostructured materials in the present study were all grown using a pulsed laser deposition technique. Gold (Au) nanodots (zero-dimensional nanostructure) were grown on silicon (Si) substrates and subsequently used in the growth of titanium nitride (TiN) nanowires (one-dimensional nanostructure). TiN nanowires were grown under different conditions; energy entering the chamber (70 mJ, 80 mJ and 90 mJ) and deposition temperature (600 °C, 700 °C and 800 °C) leading to nanowires of varying length (50 nm – 200 nm), diameter (25 nm-50 nm) and spatial density. Corrosion tests run on TiN nanowires, thin films and magnesium (Mg) bulk showed that TiN nanowires degraded faster than TiN thin films but were still better than Mg bulk. The thesis work has also focused on growing nickel (Ni) thin films (two-dimensional nanostructure) sandwiched between an alumina (Al2O3) substrate and thin film. The nickel films were deposited at different substrate temperatures (liquid nitrogen, room temperature and high temperature) keeping all other deposition parameters the same. Magnetic moment versus magnetic field measurements showed that Ni thin film samples deposited at room temperature and liquid nitrogen temperature had almost the same remanent magnetization; however, samples deposited at liquid nitrogen had the highest saturation magnetization and coercivity. The coercivity values at 10K for Ni thin film samples grown at liquid nitrogen, room temperature, and high temparature were found to be 58.92 Oe and 255.15 Oe respectively