Large Scale Synthesis, Characterization And Properties Of Chalcogenide Based Semiconductor Nanomaterials

Chalcogenide based semiconductor materials have attracted a great attention due totheir unique physical and chemical properties for wide range applications in electronics,optoelectronics, solar energy and thermoelectrics. Due to excellent properties, differentChalcogenides such as AB type and A2B3where (A=Zn, Cd, Bi, In and B=Se) have beenconsidered as promising materials for device fabrication. This thesis presents the researchwork on chalcogenides specially selenides covering preparation, characterization, growthmechanism and their properties. Advanced techniques such as FESEM, XRD, XPS, EDX,TEM, HRTEM and SAED have been employed for the characterization of novelmorphologies obtained for chalcogenides based semiconducting selenides. Potentialproperties of these materials such photoluminescence, solar energy conversion by solar cellfabrication, electrochemical performance for application in lithium ion batteries,thermoelectric properties, and photodetection capabilities for Vapour-Solid (VS) basedgrowth mechanism have been studied. New growth strategy has been employed to preparethese materials by using elemental powders as precursors in CVD technique.Initially, the simultaneous growth of ZnSe cactus-like structures with nanorods emergingfrom their surface and selenium microflowers using Zn and Se powders as precursors wereprepared. ZnSe cactuses were obtained by solid state chemical reaction whereas seleniummicroflowers were obtained by thermal evaporation and condensation process. ZnSecactuses consist of spheres with diameter range2–3μm having nanorods with diameter inthe range60–90nm on the exterior surface. Room temperature photoluminescence (PL)measurement of as-synthesized products show two intensive peaks centered at404nm and427nm (2.85eV), which are attributed to the band edge emission of ZnSe. These two strong emission bands are blue shifted; this blue shift is because of quantum sizeconfinement effect of nanostructures. A very weak peak centered at464nm (2.67eV) isalso observed in our sample which is exactly equivalent to the peak associated with the bulkZnSe, and is attributed to near band edge emission (NBE) for the ZnSe nanostructures.Later on, thermochemical approach was employed for controlling the catalystassisted and catalyst free growth of CdSe cactus like structures with sharply pointednanorods (SPNR). The cactuses were simultaneously obtained on the silicon substrate andin semicircular alumina boat using cadmium and selenium powders as precursors underammonia gas flow. Vapor–liquid–solid (VLS) growth mechanism is proposed for thecatalyst assisted while solid state chemical reaction is suggested for the catalyst free growthof microcactuses. Room temperature photoluminescence (PL) studies exhibit sharp peak at672nm for catalyst assisted CdSe microcactuses. A broad peak at596nm was observed forcatalyst free growth of CdSe SPNR. In this work we have demonstrated that PL emissionfrom the catalyst free growth of nanostructured CdSe is broad and intense. These two peaksare blue shifted from the bulk. Average diameter in the middle of nanorods is estimated inthe range of250–450nm. Photo electrochemical (PEC) solar cell is fabricated on ITOcoated glass substrate with the help of PVDF in NMP solution. The efficiency and fillfactor for as synthesized solar cell are0.47%and0.35respectively.Afterwards, a novel idea of using thermoelectric material in lithium ion batterieswas proposed. The reason behind this idea is to improve the efficiency of batteriesespecially when heat dissipation and exothermic reaction of cathode and anode with theelectrolyte at higher temperatures restricts their continuous operation due to thermalrunaway. In order to execute this idea we developed a new synthetic technique forfabrication of Bi2Se3(thermoelectric material) and also compared its efficiency by preparing this material by existing synthetic technique. Charge/discharge comparison ofboth the samples indicated that the synthesis technique has a profound effect onelectrochemical performance. Bismuth selenide (Bi2Se3) was intentionally selected for thispurpose. Bi2Se3rectangular nanosheets (BRNS) with thicknesses of200-500nm weresynthesized by a simple thermochemical method in which we used bismuth and seleniumpowders as precursors. Self assembled nanosheets (SANS) in spherical shape wereprepared by conventional hydrothermal technique. Charge discharge experiments showedthat BRNS synthesized with this technique have reasonable performance as compared tonanosheets synthesized by conventional hydrothermal technique. It showed the firstdischarge capacity is up to725.6mAh g-1for BRNS and419.6mAh g-1for SANS.Next, investigations were carried out on the effect of synthetic technique onthermoelectric properties of high-quality Bi2Se3nanoplatelets through simplethermochemical method in a horizontal tube furnace using elemental bismuth and seleniumpowders as precursors. Morphological and chemical synthetic parameters were investigatedthrough a series of experiments; thickness and composition of the product are wellcontrolled in the large scale fabrication and subsequently spark plasma sintering (SPS) tofabricate n-type nanostructured bulk thermoelectric materials. Raman Spectroscopy of thetwo selected products with thickness of50and100nm shows three vibrational modes. Asthe thickness decreases from100to50nm, the maximum red shift of about2.17cm-1andmaximum broadening of about10cm-1is observed by in-plane vibrational mode E2g. Theseobservations are in contrast with already reported observations which suggest thatbroadening and red shift are only observed below20nm thickness. Similarly we obtainedthe maximum value of figure of merit~0.41reported to date for pure phase bismuthselenide. We observed metallic conduction behaviour while others have reported semiconducting behaviour for nanostructured bismuth selenide. These results clearlysuggest that synthetic technique has profound effect on the electronic and thermoelectrictransport properties of this material.At the end, catalyst-free growth of In2Se3microwires was carried out in horizontaltube furnace. The synthesized microwires showed γ-phase, high quality, single crystallineand grown along [1120] direction. The wires have uniform diameter of~1μm and lengthsin several micrometers. Photodetector fabricated from synthesized micro wires showedreliable and stable photoresponse exhibiting a photoresponsivity of0.54A/W, externalquantum efficiency (EQE) of1.23at633nm with4V bias. The photodetector had areasonable response time of0.11s and specific detectivity of3.94x1010Jones at633nm witha light detection range from350nm to1050nm, covering the UV–visible–NIR region. Thephotoresponse shown by single wire is attributed to direct band gap (Eg=1.3eV) andsuperior single crystalline quality. The photoresponsive studies of single microwire, clearlysuggest the use of this new and facile growth technique without using catalysts forfabrication of indium selenide microwires in next generation sensors and detectors forcommercial and military applications.