Preparation and Characterization of Silver Sulfide Nanocrystals Generated from Silver(I)-Thiolate Polymers

Brühwiler D., Leiggener C., Glaus S., Calzaferri G.

Silver sulfide clusters are synthesized in the cavities of sodium and calcium zeolite A microcrystals by exposing the activated Ag+-loaded zeolite to H2S. The growth of the silver sulfide clusters during rehydration of the samples is investigated by means of diffuse reflectance spectroscopy. Monomers of Ag2S are formed at low Ag+-loading through reaction of AgSH molecules, yielding colorless composites, which exhibit a characteristic blue-green photoluminescence. Yellow colored samples showing an orange-red luminescence with an average decay time of 81 μs (−160 °C) are obtained at medium silver sulfide content. Further increasing the silver sulfide loading and therefore the cluster size causes a bathochromic shift of this emission which is accompanied by a shortening of the luminescence lifetime. The samples generally exhibit large Stokes-shifts, which can be attributed to HOMO−LUMO transitions with small oscillator strengths. The Ag2S−zeolite host−guest system constitutes a three-dimensional array of sil...

Schaaff T.G., Shafigullin M.N., Khoury J.T., Vezmar I., Whetten R.L.

The broad, intense peak found near 29 kDa in the laser-desorption mass-spectral abundances of various aurothiol (Au:SR) cluster compounds has been used to optimize the preferential formation of the species in that mass range. Recrystallization gives enriched fractions, on the 10 mg scale, that in several cases appear free of species outside that mass range. Elemental analysis and X-ray photoelectron spectra (XPS) confirm the absence of elements other than Au, S, C, and H, while infrared and NMR (1H,13C) spectra are consistent with intact thio groups. The Au 4f7/2 XPS peak is only slightly shifted (∼0.2 eV) and broadened from that of bulk Au(0) metal, and intense optical absorption extends far into the infrared region (0.5 eV), consistent with a metallic Au core. Recrystallized samples of the R = C4, C6, and C12 materials readily form highly diffractive crystalline films, powders, and single crystals on the scale of 10 μm, consistent with perfect ordering in >100 nm grains, and a negligible amorphous conte...

Motte L., Pileni M.P.

We report on the influence of the length of thiol alkyl chains used to coat silver sulfide particles on the self-organization of nanoparticles in monolayers. The variation of the average distance between particles with the length of the thio derivatives alkyl chain is derived from direct measurements of TEM patterns.

Akamatsu K., Takei S., Mizuhata M., Kajinami A., Deki S., Takeoka S., Fujii M., Hayashi S., Yamamoto K.

Composite thin films consisting of nano-sized Ag particles dispersed in nylon 11 thin films have been prepared by using a thermal relaxation technique. The films obtained were characterized by transmission electron microscopy (TEM), electron diffraction (ED), energy-dispersive X-ray (EDX) microanalysis, and optical absorption spectroscopy. The Ag nanoparticles with 4.5–9.1 nm in diameter were found to be isolated individually and dispersed uniformly in the nylon 11 matrix. The volume fraction and the mean size of Ag nanoparticles in the film could be controlled by varying the initial amount of Ag deposition. Optical absorption peak due to the surface plasmon resonance of the Ag particles were observed around 430 nm and shifted to a longer wavelength with increasing the mean size of Ag nanoparticles, and absorbance increased simultaneously. We have also prepared Ag 2 S nanoparticles with 4.7–11.2 nm in diameter in nylon 11 films from exposing the Ag/nylon 11 films to H 2 S/O 2 mixed gas. ED patterns of the particles formed after the reaction with H 2 S were completely indexed as those of β -Ag 2 S with a monoclinic structure. The onset of the optical absorption spectra is similar to that of bulk β -Ag 2 S, suggesting that Ag 2 S nanoparticles with diameter of ca. 5nm preserve the band structure of bulk Ag 2 S.

Templeton A.C., Wuelfing W.P., Murray R.W.

In this report, we evaluate the present state of the rapidly emerging field of monolayer-protected cluster (MPC) molecules with regard to their synthesis and monolayer functionalization, their core and monolayer structure, their composition, and their properties. Finally, we canvass some of the important remaining research opportunities involving MPCs.

Brelle M.C., Zhang J.Z., Nguyen L., Mehra R.K.

A new synthetic method has been developed for preparing silver sulfide, Ag2S, nanoparticles capped with cysteine or glutathione. The average particle diameter has been determined to be around 9 nm using transmission electron microscopy. The ground-state electronic absorption spectra of the Ag2S nanoparticles show a continuous increase in absorption cross section toward shorter wavelengths starting from the red (600−800 nm). Ultrafast dynamics of photoinduced electrons in these nanoparticles have been measured using femtosecond transient absorption/bleach spectroscopy. In most cases studied, the early time transient profiles feature a pulse-width limited (1 ns. One sample (GSH-1) shows a bleach recovery that gradually approaches the baseline with a similar time constant (>1 ns) following the fast 4.5 ps rise. An interesting power dependence was observed for all the samples: the transient absorption contribution becomes more dominant over bleach with increasing excitation intensity. A simple four-state kinetic model developed to account for the main features of the dynamics suggests that initial photoexcitation populates the conduction band and depletes the valence band within the laser pulse (1 ns. This model suggests that the difference in dynamics observed between the different samples is due to different absorption cross sections of deep trap states. The observed excitation intensity dependence of the dynamics is attributed to shallow trap state saturation at high intensities.

Brühwiler D., Seifert R., Calzaferri G.

The reaction of H2S with activated Ag+-loaded zeolite A leads to silver sulfide zeolite A composites. The optical absorption spectra of samples with different loading densities of silver sulfide suggest the formation of small silver sulfide clusters which are stable under ambient conditions. To our knowledge, this is the first report on the preparation and the optical absorption and emission spectra of silver sulfide clusters in the cavities of a zeolite in the size regime below 15 A. The loading densities of the investigated silver sulfide zeolite A composites are 0.05, 0.10, 0.25, 0.50, 1.00, 1.25, 1.50, 1.75, and 2.00 silver ions per pseudo unit cell of zeolite A. The samples are colorless (low loading density) or yellow-green (high loading density). The luminescence is blue-green for samples with low silver sulfide content, yellow or orange at medium silver sulfide content, and red at high silver sulfide content.

Whetten R.L., Shafigullin M.N., Khoury J.T., Schaaff T.G., Vezmar I., Alvarez M.M., Wilkinson A.

Zhang Q.M., Bharti V., Zhao X.

An exceptionally high electrostrictive response ( approximately 4 percent) was observed in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer. The material exhibits typical relaxor ferroelectric behavior, suggesting that the electron irradiation breaks up the coherent polarization domain (all-trans chains) in normal ferroelectric P(VDF-TrFE) copolymer into nanopolar regions (nanometer-size, all-trans chains interrupted by trans and gauche bonds) that transform the material into a relaxor ferroelectric. The expanding and contracting of these polar regions under external fields, coupled with a large difference in the lattice strain between the polar and nonpolar phases, generate an ultrahigh strain response.

Motte L., Pileni M.P.

Silver sulfide Ag2S nanocrystals have been synthesized in reverse micelles and coated with thioalkyl chains with different lengths. With short chains (≤C8) and very long ones (≥C16) the particles cannot be extracted from micelles. For short alkyl chains, this is due to disorder at the interface. For longer chains it is attributed to the highly hydrophobic character of the alkyl chains. For alkyl chains varying from C8 to C14, the long-range order of 2D self-assembly depends on the number of particles deposited on the support; at low concentration, rings are formed and then disappear by increasing the number of particles. When the thiol alkyl chain length is increased, an increase in the average distance between two adjacent particles is observed. It is correlated with the change in the chain conformation and defects, which depend on the chain length. When particles are self-organized in 2D, the distance between adjacent particles centers varies linearly with the particle radius. This indicates that the interparticle distance does not change with the particle size and varies with the length of the alkyl chain. For short chains (≤C6), the gauche defects prevent attachment to the surface, and self-organization in 2D is prevented. Hence, by using short chains, the distance between particles cannot be markedly reduced. By the increase of the particle concentration, 3D superlattices are obtained. The best faceted and largest fcc crystals are formed with the largest particles and densest chain packing. When the chain length is too long (C14) end-gauche defects occur, which show an edge−edge core spacing smaller than one chain length in the all trans zigzag conformation. This prevents the 3D growth.

Shin H.K.

The possibility of producing highly excited OD radicals in the reaction of gas-phase atomic oxygen with deuterium chemisorbed on a tungsten surface has been explored using classical trajectory procedures. The many-body nature of the gas−surface reaction is explicitly considered in constructing a potential energy surface. Nearly all reactive events occur in a direct collision on a subpicosecond scale. The vibrational population distribution of OD radicals is found to be inverted, with the highest population appearing at vibrational level 6. The Eley−Rideal mechanism can account for these results. The calculations are carried out at the gas and surface temperatures of 1000 and 300 K, respectively, where the probability of OD formation is 0.12.

Schaaff T.G., Shafigullin M.N., Khoury J.T., Vezmar I., Whetten R.L., Cullen W.G., First P.N., Gutiérrez-Wing C., Ascensio J., Jose-Yacamán M.J.

Five massive gold-cluster molecules have been isolated in high yield and have undergone separate structural characterization, and their electronic structure has been deduced by optical absorption spectroscopy. These new molecules are distinguished by a crystalline (or quasicrystalline) core of densely packed Au atoms, ranging in size from ∼1.1 nm (∼40 atoms) to ∼1.9 nm (∼200 atoms), surrounded by a compact monolayer of various thio (RS) adsorbates. They are obtained as the thermally and environmentally stable products of the reductive decomposition of nonmetallic (−AuS(R)−) polymer in solution, are separated according to size by fractional crystallization or column chromatography, as monitored by high-mass spectrometry, and are characterized structurally by methods including X-ray diffraction (small and large angle), high-resolution electron microscopy, and scanning tunneling microscopy. The optical absorption spectra of dilute solutions of these molecules show size-dependent steplike structure with an on...

Pileni M.P., Motte L., Billoudet F., Mahrt J., Willig F.

Reverse micelles have been used to make nanosized silver sulfide particles. The size distribution is evaluated from TEM micrographs and from fluorescence spectroscopy experiments. The luminescence spectrum, measured at 2 K, shows that the distribution is narrow but contains also particles with diameters too small for detection with TEM. By coating the particles with dodecanethiol, 2D and 3D self-assemblies composed of silver sulfide nanoparticles are observed. Monolayers organized in a hexagonal network in a very large domain are observed. Furthermore, 3D layers are highly ordered and form a face-centered cubic structure.

Vezmar I., Alvarez M.M., Khoury J.T., Salisbury B.E., Shafigullin M.N., Whetten R.L.

Stable, intense beams of large metal clusters (from 102 to 103 Au or Ag atoms) can be produced by laser desorption of molecular films of passivated nanocrystals that have undergone fractionation by size and a separate structural characterization. The mass onset of the desorbed species corresponds directly to the dimensions of the nanocrystal core, indicating that only the surfactant shell is lost during desorption. Photofragmentation, photoionization, and photodetachment from beams of large metal clusters, generated in this way, have all been observed.

Motte L., Billoudet F., Lacaze E., Douin J., Pileni M.P.

Self-assembly of silver sulfide, Ag2S, nanoparticles is reported for different particles sizes. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced. The observations indicate that the formation of self-assemblies of nanoparticles strongly depends on the preparation conditions of the samples. It is also shown that in the aggregates the particles are highly organized and form pseudocrystals with a face-centered cubic structure for various particles sizes.

Rempel Andrey A., Ovchinnikov Oleg V., Weinstein Ilya A., Rempel Svetlana V., Kuznetsova Yulia V., Naumov Andrei V., Smirnov Mikhail S., Eremchev Ivan Yu., Vokhmintsev Alexander S., Savchenko Sergey S.

Quantum dots are the most exciting representatives of nanomaterials. They are synthesized using advanced methods of nanotechnology pertaining to both inorganic and organic chemistry. Quantum dots possess unique physical and chemical properties; therefore, they are used in very different fields of physics, chemistry, biology, engineering and medicine. It is not surprising that the Nobel Prize in chemistry in 2023 was given for discovery and synthesis of quantum dots. This review addresses modern methods for the synthesis of quantum dots and their optical properties and practical applications. In the beginning, a short insight into the history of quantum dots is given. Many gifted scientists, including chemists and physicists, were engaged in these studies. The synthesis of quantum dots in solid and liquid matrices is described in detail. Quantum dots are well-known owing to their unique optical properties; that is why the attention in the review is focused on the quantum-size effect. The causes for fascinating blinking of quantum dots and techniques for observation of a single quantum dot are considered. The last part of the review describes mportant applications of quantum dots in biology, medicine and quantum technologies.The bibliography includes 772 references.

Masmali N.A., Osman Z., Arof A.K.

Zinc oxide (ZnO) and Zinc ferrite (ZnFe2O4) have unique electrical and optical characteristics. In this work, ZnO and ZnFe2O4 are the working electrodes used in the quantum dots sensitized solar cells (QDSSCs). ZnFe2O4 is able to enhance light absorption. Its band gap is 2.12 eV. ZnO and ZnFe2O4 nanoparticles were synthesized by way of co-precipitation. ZnO and ZnFe2O4 phase structures have been investigated using X-rays. The crystallite size of ZnO and ZnFe2O4 has been determined using the Williamson-Hall method. The morphologies of ZnO and ZnFe2O4 have been examined via electron microscopies and the optical properties of ZnO and ZnFe2O4 have been studied using absorption spectroscopy in the wavelength range from 200 to 800 nm. The ZnO and ZnFe2O4 band gaps were calculated using the Tauc method. Gel polymer electrolyte (GPE) based on methylcellulose, sulfur, and sodium sulfide has been prepared. The GPE has been characterized using electrochemical impedance microscopy (EIS). The bulk resistance of the electrolyte has been determined from the Nyquist plot and enabled the ionic conductivity to be calculated. Mesoporous thin films of ZnO and ZnFe2O4 have been sensitized with Ag2S and CdS and have been used as photoanodes in QDSSCs. The photovoltaic characteristics of four different QDSSCs were investigated at 100 mW cm−2 light intensity. EIS measurements have also been carried out to investigate the kinetic processes at the interfaces within the QDSSC. The performance of QDSSCs using CdS and Ag2S QDs has been compared. Photoanodes using CdS exhibit lower charge carrier resistance, higher fill factor, and longer electron lifetime resulting in cell efficiency enhancement compared to Ag2S-based photoanodes.

Khan M.D., Aamir M., Revaprasadu N.

Synthesis of phase and shape controlled metal chalcogenide nanomaterials or fabrication of thin films is often challenging and requires the use of suitable starting materials. Metal organic precursors have an advantage of pre-formed bonds between metal and the chalcogenide atoms, and the decomposition of these molecular precursors can generally lead to the formation of metal chalcogenide nanomaterials with high phase purity and precise stoichiometry. In this chapter, the use of different metal organic precursors and their suitability in formation of nanomaterials or thin films has been discussed.

Badali Y., Azizian-Kalandaragh Y., Akhlaghi E.A., Altındal Ş.

Au/n-Si metal/semiconductor (MS) Schottky barrier diodes with and without (Ag2S-PVA) interlayer were prepared by the ultrasound-assisted method and their electric and dielectric properties were examined by using current–voltage (I–V) and capacitance–voltage (C–V) measuring devices. The structural, optical and morphological characteristics of the (Ag2S-PVA) were studied by x-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Visible spectroscopy. The observed peaks in the XRD pattern are related to the α-phase of silver sulfide. The UV–Visible spectrum of (Ag2S-PVA) shows the quantum confinement and SEM image proves the grain size in nano-scale. The ideality factor (n) and barrier height (BH) at zero bias (ΦB0(I–V)) were acquired from the I–V data. On the other hand; Fermi energy (EF), donor concentration atoms (ND), and BH (ΦB(C–V)) values were obtained from the reverse bias C–V data. The voltage-dependent resistance profile (Ln(Ri)–V) was obtained by applying Ohm’s law and also by the Nicollian–Brews methods. Also, considering the voltage-dependent n and BH, Nss–(Ec–Ess) profile was acquired from the forward biases I–V characteristics. Depending on high, intermediate and low biases ln(I)–Ln(V) curves have three linear regions with distinct slopes for MS and MPS structures. The predominant current-transport mechanisms were obtained in related regions via trap-charge limited current and space-charge limited current, respectively. These outcomes indicate that the (Ag2S-PVA) interlayer enhanced the performance of the diode considerably in terms of high rectifier rate (RR), shunt resistance (Rsh), and low surface states (Nss) and series resistance (Rs). Thus, the (Ag2S-PVA) interlayer can be used in MS type diode instead of a traditional insulator layer.

Shen Y., Lifante J., Ximendes E., Santos H.D., Ruiz D., Juárez B.H., Zabala Gutiérrez I., Torres Vera V., Rubio Retama J., Martín Rodríguez E., Ortgies D.H., Jaque D., Benayas A., del Rosal B.

A critical analysis of the synthesis routes, properties and optical features of Ag2S nanoparticles is presented. The future perspectives of this material for advanced bioimaging are discussed.

Díez‐Buitrago B., Barroso J., Saa L., Briz N., Pavlov V.

Babeer A.M., Aamir L.

This work presents the synthesis of zinc oxide /silver sulfide (ZnO/Ag2S) core/shell type composite; using combined wet chemical precipitation method and ion exchange mechanism; for wide range absorption of visible spectra by the composite. Synthesis is performed in three steps. In the first step; ZnO nanoparticles (nanocores) are produced, in the second step; ZnS layer is deposited over ZnO nanocores and in the third step; Zn from ZnS is replaced by Ag to form Ag2S shell over ZnO; using ion exchange mechanism. The presence of reflection peaks of ZnO and Ag2S in XRD spectra confirms the formation of ZnO/Ag2S composite. SEM image of ZnO shows the formation of near spherical ZnO nanoparticles of diameter in range 256nm to 584nm with a smooth surface, while SEM image of ZnO/Ag2S composite shows the formation of Ag2S layer over ZnO cores as indicated by rougher and contrasted surface as compared to bare ZnO cores. Also, the size of composite particles became larger than ZnO nanocores (100µm-200µm), which further confirms the formation of Ag2S shell over ZnO nanocores. The optical absorption spectrums of both ZnO and ZnO/Ag2S composite clearly indicate that synthesized composite absorb strongly in UV to IR region of the electromagnetic spectrum as compared to ZnO nanocores; which absorb only in UV region. Therefore, the synthesized composite could be used as a photovoltaic material.

Devangamath S.S., Lobo B.

Organic–inorganic polymer hybrid films were prepared, using polyvinyl alcohol–polyethylene glycol blend as the organic host matrix. Silver sulfide (Ag2S) semiconductor particles have been synthesized in polymer blend matrix, by in-situ method. Polymer hybrid films, with various amounts of Ag2S were prepared and characterized structurally, optically and electronically by using various characterization techniques, such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–Vis absorption spectroscopy, temperature and time dependent DC electrical studies. Hydrogen bonding interactions take place between hydroxyl groups (OH) and carbonyl groups (C=O) of polymer blend molecules with Ag+ ion of Ag2S filler, indicating complex formation between polymer matrix and filler, which is confirmed by FTIR technique. XRD results showed presence of additional multiple peaks (compared to pure blend film) corresponding to αʹ-Ag2S monoclinic crystal system. Changes in degree of crystallinity (%C) are observed for hybrid films with various amounts of Ag2S, showing a highest value of 18.2% for the film containing 0.372 g of Ag2S, when compared to 13.45% (%C) for pure blend film. Uniform distribution of Ag2S nanoparticles in polymer blend matrix is revealed by SEM analysis for hybrid film containing 0.372 g of Ag2S. UV–Vis absorption studies showed multiple absorption peaks due to added filler. Highest value of room temperature conductivity of 9.40 × 10−7 S/cm was obtained for the hybrid film containing 0.372 g of Ag2S. Values of transference numbers (tion and tele) in time dependent electrical studies indicated that the electrical conductivity in pure blend film as well as hybrid films is mainly due to ions.

Andolina C.M., Crawford S.E., Smith A.M., Johnston K.A., Straney P.J., Marbella L.E., Tolman N.L., Hochuli T.J., Millstone J.E.

Guo X., Gao J., Zhang Z., Xiao S., Pan D., Zhou C., Shen J., Hong J., Yang Y.

Ag 2 S quantum dots (QDs) were employed as an interfacial layer between polymer electrolyte and transparent polyaniline counter electrode (PANI CE) in bifacial dye-sensitized solar cells (DSSCs). The Ag 2 S QDs film, which was prepared by successive ionic layer adsorption and reaction (SILAR) could deliver an extra visible light harvesting and enhance the catalytic performance of PANI CE, leading to increased photo-generated electrons and suppressed interfacial recombination. The device performances have been significantly improved as compared to the controlled DSSC without Ag 2 S QDs interfacial layer (QDIL). An optimal power conversion efficiency of 6.50% was achieved in the Ag 2 S QDILs based solid-state bifacial DSSC with 2 SILAR cycles, which was comparable to the highest reported efficiency of 7–8% for solid-state DSSCs, and was almost 130% of the conventional structured DSSC without Ag 2 S QDIL (5.02%). Electrochemical and electron transport/recombination performances have also been demonstrated for this type of device. The results also highlight the general importance of light-active QDs interfacial layer in transparent cathode for other kind of devices.

Jiang Y., Liu D., Yang Y., Xu R., Zhang T., Sheng K., Song H.

In this work, a new photoelectrochemical biosensor based on Ag2S nanoparticles (NPs) modified macroporous ZnO inverse opals structure (IOs) was developed for sensitive and rapid detection of alpha fetal protein (AFP). Small size and uniformly dispersed Ag2S NPs were prepared using the Successive Ionic Layer Adsorption And Reaction (SILAR) method, which were adsorbed on ZnO IOs surface and frame work as matrix for immobilization of AFP. The composite structure of ZnO/Ag2S expanded the scope of light absorption to long wavelength, which can make full use of the light energy. Meanwhile, an effective matching of energy levels between the conduction bands of Ag2S and ZnO are beneficial to the photo-generated electrons transfer. The biosensors based on FTO (fluorine-doped tinoxide) ZnO/Ag2S electrode showed enough sensitivity and a wide linear range from 0.05 ng/mL to 200 ng/mL with a low detection limit of 8 pg/mL for the detection of AFP. It also exhibited high reproducibility, specificity and stability. The proposed method was potentially attractive for achieving excellent photoelectrochemical biosensor for detection of other proteins.

Wang S., Huang Y., Yue J.

A polyurethane/silver sulfide nanocomposite film was synthesized by a biomineralization simulated method. The effect of the Ag2S nanoparticles on the physical properties of the composite was studied by Fourier transform infrared, differential scanning calorimetry (DSC), scanning electron microscopy. The thermal stability of the composite was measured by DSC. The fluorescence emission of the nanocomposite films was found to be very sensitive to Ni(II) ions, with a small amount of Ni(II) ions making the emissions increase dramatically. The films are predicted to have the potential to be developed into excellent sensing films of Ni(II) ions in the water.

Johar M.A., Afzal R.A., Alazba A.A., Manzoor U.

Nanocomposites have a great potential to work as efficient, multifunctional materials for energy conversion and photoelectrochemical reactions. Nanocomposites may reveal more improved photocatalysis by implying the improvements of their electronic and structural properties than pure photocatalyst. This paper presents the recent work carried out on photoelectrochemical reactions using the composite materials of ZnO with CdS, ZnO with SnO2, ZnO with TiO2, ZnO with Ag2S, and ZnO with graphene and graphene oxide. The photocatalytic efficiency mainly depends upon the light harvesting span of a material, lifetime of photogenerated electron-hole pair, and reactive sites available in the photocatalyst. We reviewed the UV-Vis absorption spectrum of nanocomposite and photodegradation reported by the same material and how photodegradation depends upon the factors described above. Finally the improvement in the absorption band edge of nanocomposite material is discussed.

Diware M.S., Ganorkar S.P., Kim J., Bramhe S.N., Cho H.M., Cho Y.J., Chegal W.

Complex optical properties (ε=ε1+iε2) of polycrystalline bulk α-Ag2S are investigated by spectroscopic ellipsometry from 0.5 to 8.5 eV at room temperature. The acanthite structure (monoclinic) of α-Ag2S is confirmed by structural analysis. Multilayer analysis with Tauc-Lorentz dispersion model is applied to draw out ε from ellipsometric data. Ag2S shows absorption in the entire range of the visible spectrum with onset band gap of 1.07 eV; furthermore, six other interband transitions are identified whose energy positions accurately obtained by standard lineshape analysis of second-energy-derivative of point-by-point fit determined ε. Finally, we compared our data with reported theoretically calculated data, and origin of the main optical structures is discussed.