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Essay: CuS thin films

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  • Published: 5 November 2015*
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Special attention is now given for the study of CuS thin films probably due to the discovery of heterojunction solar cell Pathan and Lokhande (2004), and therefore, due to variable optical and electrical properties this found to have many potential applications in various fields as for solar cells, fluorescent devices Sartale and Lokhande (2000), Lindroos et al. (1995), Grozdanov and Najdoski (1995), Dhar and Chakraborti (1997), Nickless. G (1968), Greenwood and Earnshaw (1990), Partain et al. (1995), Nair and Nair (1991)1, photo thermal conversion, microwave shielding coatings, electro conductive electrodes, catalyst Yamaguchi et al. (1996), Teteris. J (2003), Savadogo. O (1998), Sang et al. (2002), selective radiation filters, photodetectors, as polarizers of IR radiation Nair and Nair (1991)2, active absorbent of radio waves Grozdanov. I (1994), semiconductors, electroconductive coatings, low temperature gas sensor applications Pathan and Lokhande (2004), field emission Feng et al. (2007), switching Sakamoto et al. (2003), sensing devices Sagade and Sharma (2008), Zaman et al. (2014), themoreflecting coatings Nair et al. (1989), optical sensors Nagaraju. G (2008), eyeglass coatings, antireflecting coatings Ilenikhena. P.A (2008), thermo-electric cooling materials Shen et al. (2009), optical filters, optical recording materials, nanoscale switches, and superionic materials Abdullaeva et al. (2013), as electrocatalyst on counter electrodes to fabricate photoanode cosensitized solar cells Thanh et al. (2014), etc. Having such immense varied applications, with passing time CuS nanoparticles with various structures have been synthesized like nanodisks Wang et al. (2010), nanorods Singh et al. (2007), nanoplatelets Du et al. (2007), nanotubes Wu et al. (2006)1, nanowires Chen et al. (2008), hollow spheres Kim et al. (2004), dendrites and nanowires Han et al. (2011), dendrites Fang et al. (2010), snowflakes Wu et al. (2006)2, polygonal rings and wires Vasquez et al. (2011), flowers and hollow spheres Li et al. (2007), nanowalls Feng et al. (2007), nanosheets thin films obtained on copper, glass, ITO substrates from the facile wet chemical route at low temperature (60 ‘C or less), nanowhiskers Puspitasari et al. (2007), ball-flowers Cheng et al. (2010), tube-like structures Gong et al. (2006), flower-like Gorai et al. (2005), and urchin-like patterns Zhu et al. (2004). Various methods have been known to produce nanostructures of CuS such as solid-state reaction Thongtem et al. (2009), microemulsion Chen et al. (2010), hydrothermal method Saranya et al. (2014), pulsed plasma in liquid method Abdullaeva et al. (2013), microwave Liao et al. (2003), microwave hydrothermal method Phuruangrat et al. (2013), ultrasonic irradiation route Xu et al. (2003)1, solvothermal method Shen et al. (2009), sonochemical Xu et al. (2006), electrochemical synthesis Cordova et al. (2002), one-step soft solution route from reflux condensation method Mageshwari et al. (2011) and polyol route Ding et al. (2008) etc. CBD has attracted researchers due to its promising nature to produce good quality films from convenient and simple to use process, in turn giving large area of deposition as been discussed within starting chapters.
It’s worth mentioning that even after over viewing the work done by CBD method to synthesize CuS nanostructure, no particular two preparative methods and characteristics or properties thus exhibited by the compound can be comparable to each other. All the preparation which has been done already and mentioned hereby can vary from the chemical composition of reactants (precursor ion source, molar concentration, % composition, volume, etc.), the combination of reactants in the reaction bath (chemical reactivity of reactants to be considered, stirring time of reactants, time of adding reactants, etc.), chemical bath composition, physical parameters (substrates used, substrate cleaning process, distance between two substrates, position and placement of substrates, pH, temperature, deposition time, etc.). The condition implied, directly or indirectly effect the final composition and properties exhibited by the final product. At this time an effortless and brief attempt taken to mention specific progress to deposit CuxS nanomaterial (specifically thin film), emphasizing CBD method. Different preparation method to fabricate CuxS phases and results obtained discussed here:
Kundu et al. (2008) attempted deposition of CuS by the UHV method. They studied that structural properties were dependent on the amount of sulphur in the final phase of CuS thin films. Thin film when exposed to an ambient atmosphere causes degradation of thin films having a higher concentration of sulphur, i.e. roxbyite- and covellite-phase films because of which these can oxidize in ambient atmospheric condition having higher oxygen uptake and ultimately degrade faster. Spray Pyrolysis Method opted time to time: – Naumov et al. (2002) have deposited thin films of CuS on quartz and glass ceramic substrate at 230’C and 300’C. Copper sulfide thin films may consist of Cu2-xS (x = 0 – 0.25) phases, including non-equilibrium ones. It was also observed that the deposits tend to retain the inherent structural features of the parent TC (thiourea complex). Wang et al. (2003) without using a complexing agent obtained polycrystalline or a mixture of amorphous and polycrystalline CuxS (x = 1, 2) thin films deposited at a relatively low temperature via an APUSP technique. The process proves that crystalline nature of thin film is dependent on the molar ratio of CuCl2 to (NH2)2CS and temperature at which reaction sustaining. Isac et al. (2007)1 successfully achieved deposition of CuxS thin films on transparent conductive SnO2:F glass substrates with CuS, Cu2S and Cu1.8S phases, in a range of 235??C to 285??C temperatures. The digenite phase found to be prominent phase between the mixtures of phases obtained. It was found that increasing copper precursor ions lead to increase in grain size, while uniformity and denser surface morphology obtained by doing a variation in the amount of solvents (water: alcohol). The number of spraying sequences when increased to double results in increase of grain size in thin films which were deposited at 285??C. Isac et al. (2007)2 proposed that Cu2S synthesized can be used as absorber material for solid state solar cells, which is obtained from the spray pyrolysis method. This achieved by rigorously controlling the precursor solution concentration having an optimized condition of Cu: S molar ratio of 1:3, using a mixture of water: ethanol: glycerin as solvents with volume ratio: 7:2:1 and by optimizing the spraying time longer than 40 min at 285??C. Other than this, it was seen substrate temperature also plays an important role to finalize the CuS composition. Isac et al. (2007)3 leads to the formation of covellite CuS thin films when deposition temperatures of substrates were maintained between 185??C to 285??C. It was found that as amount of alcohol increased in aqueous precursor, it effects the morphology of finally deposited thin film at 235??C and at 285??C. The resulting thin films have porous, dense surface with large aggregates and crystallite’s formation. This implies that crystal growth is limiting step when the deposition temperature is controlled at lower values. Isac et al. (2013) for organic pollutants (example methylene blue) photodegradation, CuxS (x = 1.8, 2) thin films deposited via robotic spray pyrolysis method having direct Eg = 2.11’2.78 eV. Adelifard et al. (2012)1 opted two substrates, glass and FTO coated glass substrate, involving two different Cu to S molar ratios (0’33 and 0’43) at a substrate temperature of 285’C. The deposition process leads to the formation of CuS (covellite) individually on a layer/glass sample, while the layer/FTO sample leads to two additional phases of Cu2S (chalcocite) and Cu1’8S (digenite). The Eg for thin films deposited for Cu to S molar ratios (0’33 and 0’43) on glass and FTO substrate being 2.58 eV (0.33), 2.54 eV (0.43) and 2.57 (0.33), 2.51 eV (0.43) respectively. All samples showing p-type high electrical conductivity. Adelifard et al. (2012)2 – The spray pyrolysis process involves a reaction mixture with different Cu to S molar ratios (0.33 and 0.43) at various glass substrate temperatures of 260, 285 and 310’C. CuS single phase (covellite) obtained showing polycrystalline nature with preferred orientation along (1 0 2) plane, except the one with amorphous nature. The crystallite size found to be around 20-40 nm having Eg value in range of ‘2.4’2.6 eV. The thin films show the higher absorption coefficient and have characteristic p-type conductivity with hole concentration of ~1.8 ??1020 to 1.7 ??1021 cm-3.
Another method hydrothermal deposition chosen by Chu et al. (2008) synthesized hexagonal nanoplates of CuS by variations done in concentration of CTAB, while thickness up to 18 ‘ 38 nm and mean plane size of 75-179 nm of nanoplates can be controlled by the quantity of HNO3 used and sustaining reaction temperature. Kalanur et al. (2013) obtained effective counter electrode for quantum dot solar cells in the form of the thin layer of Cu1.8S/CuS deposited on FTO. This thin layer of Cu1.8S/CuS shows power conversion efficiency of 1.66%. This will widen the possibility of manufacturing large-scale transparent QDSCs in the future.

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