Fabrication And Properties Of Samarium Sulfides Thin Films By Liquid Phase Deposition Method With Self-assembled Monolayers

The sesquisulfides of the trivalent rare earths are a large family of highband gap semiconductors with potential application as optical windowmaterials. As a P-type semiconductor, Sm₂S₃is an especially interestingmaterial for potential use in the conversion of solar energy to electricity orchemical energy. Sesquisulfide Sm₂S₃exists as a low-temperature phaseα-Sm₂S₃, which is orthorhombic and belongs to Pnma, and ahigh-temperature phase γ-Sm₂S₃, which is cubic （Th₃P₄）. The α-Sm₂S₃â†'γ-Sm₂S₃polymorphic transition temperature is at1445K or1385K.Sm₂S₃thin films were prepared on Si （100） by liquid phase depositionmethod on self-assembled monolayers. The phase compositions, surfacemorphologies and optical properties of the as-deposited Sm₂S₃thin filmswere characterized by X-ray diffraction （XRD）, X-ray photoelectronspectroscopy （XPS）, atomic force microscopy （AFM）, ultraviolet–visible（UV–Vis） and photoluminescence spectrum （PL）. Without template agent,the influence of the n（S₂O₃）^2-/n(Sm³⁺), pH value on as-deposited Sm₂S₃thinfilms were investigated.Under the conditions of using C₆H₈O₇as templateagent, the influence of the （S₂O₃）^2-/(Sm³⁺), pH value，deposition temperatureand deposition time on the phase compositions, surface morphologies andoptical properties of the as-deposited Sm₂S₃thin films were investigated。Wealso discussed the preparation of the SmS thin films by eletrodepositionmethod.Results show that Sm₂S₃thin films have been prepared successfully atlow temperature by liquid phase deposition method on self-assembledmonolayers. The as-deposited thin films exhibit a dense and crystallinesurface morphology. Good transmittance in the visible spectrum and excellent absorbency of ultraviolet light of the thin films are observed. Theas-deposited thin films also exhibit red photoluminescence properties undervisible light excitation. The results show that the optimal syntheticconditions to Sm₂S₃thin films are （S₂O₃）^2-/(Sm³⁺)=2:1, pH=3.0， usingC₆H₈O₇as template agent, deposition temperature is80°C, deposition timeis24h.Under the conditions of no template agent, results show that withincreasing the n（S₂O₃）^2-/n(Sm³⁺), the phase compositions, surfacemorphologies and optical properties of the as-deposited Sm₂S₃thin films hasan great change. When n（S₂O₃）^2-/n(Sm³⁺)=2:1, the obtained films appears todisplay orientation crystallites with parallelepiped-shaped morphology thatparallel to the substrate, further indicating the orientation growth of Sm₂S₃thin film. With increasing pH value, the crystallinity and the grain sizes has aconsiderably change. When pH=3.0, the crystallinity is perfect, and theprepared thin films has good optical property.Under the conditions of using C₆H₈O₇as template agent, results showthat （S₂O₃）^2-/(Sm³⁺), pH value，deposition temperature and deposition timeplays an important role during the deposition process and giving stronginfluence on the crystallization and morphology of the Sm₂S₃thin films.With increasing the （S₂O₃）^2-/(Sm³⁺)，the phase compositions change greatly.When the （S₂O₃）^2-/(Sm³⁺)=2:1, the high purity and uniform layered-likenanostructure with oriented growth perpendicular to the substrate areobtained, suggesting another preferred orientation growth of Sm₂S₃thin filmprepared by using C₆H₈O₇as the template is achieved. The obtained thinfilms possess red photoluminescence properties. With the increase in Sm₂S₃crystallites grain size, the decrease in energy band gap of Sm₂S₃thin filmswas detected. When the （S₂O₃）^2-/(Sm³⁺)=2:1, the energy band gap of theSm₂S₃thin films is calculated to be3.779eV. The pH value has a greatinfluent on Zeta potential of precursor solution, when the pH=3.0, theabsolute value of zeta potential is the minimum, the as-deposited thin filmsgrow along （103） plane and exhibits a dense and crystalline surfacemorphology. Good transmittance in the visible spectrum and excellentabsorbency of ultraviolet light of the thin films are observed, and the band gap of the thin films first decrease and then increase with the increase of thepH value.The deposition temperature affect the rate of migration of ions insolution, when the deposition temperature is80℃，the Sm₂S₃thin films growalong the （103） plane，exhibits flaky crystals and layered grown-mode. Theroughness of the thin films is1.838nm. The film has an excellent absorbencyof ultraviolet light, the band gap of the thin films first decrease and thenincrease with the increase of deposition temperature.With increasing thedeposition time, the thickness of the obtained thin films will be greater. Asthe increase of the thickness, the stress will be maken between the film andthe substrate, so the thin films will come off easily. Deposition time shouldnot be too long; the best deposition time is24h.The SmS was successfully prepared using hexamethyl phosphorustriamide as solvent by electrodeposition.The surface of the thin films isrelatively dense, but undulating, needing for further in-depth study.