Evolution of structural features in GO/CdS multilayer films for advanced optoelectronic devices

In this work graphene oxide/cadmium sulphide [GO/CdS]n multilayer films were obtained onto FTO/SLG substrates by cyclic electrophoretic deposition (EPD) and successive ion layer adsorption and reaction (SILAR) methods, with 3, 5, 7 and 10 cycles. The optical characterization for 3 cycles sample exhibits an optical band gap of ∼2.9 eV due the nanocrystalline nature of CdS, from 5 to 10 cycles a blue shift is observed in the band gap around ∼2.15 eV by the electronic disorder induced by graphene oxide intercalation and the mixture of hexagonal/cubic phases of CdS. Raman spectra shows the longitudinal optical modes LO, 2LO and 3LO, the LO peak position shows a blue shift in comparison to the bulk CdS. The multilayer films of 3 and 5 cycles shows a wide range of photoluminescence emission from 350 to 700 eV, for samples of 7 and 10 cycles a quenching and red shift is observed attributed to the CdS crystals growth with a peak emission at 615 eV from bulk CdS, which make the materials suitable for photovoltaic solar cells and UV-Vis diodes for optoelectronic devices.


Introduction
GO/CdS multilayers are being widely studied due to the excellent physicochemical properties of GO and CdS, since CdS is a semiconductor with a band gap of ~2.4 eV, high absorption coefficient (~10 4 cm -1 ) in the visible region, high electrochemical stability, good electron affinity and high photoconductive behaviour.GO, for its part, has extraordinary properties in thermal and electrical conductivity, in addition, it was proven that the set of GO/CdS obtained by EPD/SILAR produces a chemical bond between carbon or sulphur (C-S) [1].This hybrid (organic/inorganic) systems are currently being studied as potential materials in optical, electronic, and photocatalytic applications, due to GO matrix can enhance the charge separation of the photogenerated carriers overcoming the high intrinsic recombination rate of CdS nanoparticles [2].As usual, the synthesis method used for CdS thin films is crucial for tune the desired properties depending on the final application, typically exist two major ways to synthetize CdS thin films, vacuum, and non-vacuum methods.Which includes RF magnetron sputtering [3], pulsed laser deposition [4], thermal evaporation [5], spray pyrolysis [6], chemical bath deposition [7], electrodeposition [8], among others.While for high quality graphene thin films obtention, methods such as chemical vapor deposition [9], liquid phase exfoliation [10], and plasma chemical vapor deposition [11] are used.For graphene oxide thin film deposition is achieving by electrophoretic deposition (EPD) departing of aqueous dispersion of GO powders, which oxidation degree could be tune varying the EPD parameters.
In the present work, GO/CdS films prepared by the successive ionic layer adsorption and reaction (SILAR) method were deposited onto GO prepared by electrophoretic deposition onto SnO2:F/Sodalime glass (FTO/SLG) substrates forming hybrid multilayer structures at 3, 5, 7 and 10 cycles of EPD/SILAR respectively, finishing the systems with a GO layer.These hybrid multilayer systems were characterized the optical band gap, and photoluminescence behaviour depending on the cycle number and structural disorder in this hybrid system to evaluate the feasibility of this materials for optoelectronic devices such as UV-Vis LEDs or photovoltaic materials.

Multilayer preparation
The preparation of graphene oxide powders and multilayers films of GO/CdS by a cyclic electrophoretic deposition/SILAR was as follows, GO was obtained by graphite oxidation using a modification of the Hummers method as reported elsewhere [12].First 1 mg mL -1 solution of GO was made for the electrophoretic deposition (EPD) onto the FTO/SLG substrate, in a two-electrode configuration electrochemical cell, where FTO was used as working electrode and a graphite bar was used as a counter electrode.The gap between the two electrodes was kept at 1 cm for all the samples and the electrodes were connected to a regulated DC power supply.A constant current density of 0.5 mA cm -2 was applied during 75 s to produce the GO film onto the FTO substrate, then the substrate was removed from the GO solution and a 0.05 M CdCl2 and 0.05 M Na2S solutions were set into separated beaker to perform the successive ion layer adsorption and reaction (SILAR) deposition.After the 10 SILAR cycles, the GO deposition was performed again in the aforementioned conditions.The GO/CdS deposition cycle was repeated 3, 5, 7 and 10 times, finishing with a GO layer, the whole process is showed in Figure 1.After deposition, GO/CdS QDs multilayer films were rinsed with deionized water and dried at room temperature.Figure 1 shows the GO synthesis and multilayer synthesis by EPD and SILAR cyclic deposition.

Multilayer characterization
The characterization of GO/CdS multilayer films was performed as follows; Raman spectra were collected using a MicroRaman i-Plus in a spectral range of 100-3000 cm -1 using a laser line of 532 nm with a fluence of 6 mW/cm2.UV-Vis transmittance was perform in a Stellarnet system composed of a tungsten-halogen source, a bifurcated 600 µm optical fiber and a Blue Wave spectrometer.Photoluminescence (PL) emission was measured by exciting the samples with a 325 nm line of He-Cd laser at room temperature operated at a maximum output power of 18 mW, and a 500 mm focal length monochromator (SPEX 500 M) was used to collect the PL emission in the range from 330 to 900 nm, with a slit width of 100 mm and notch filters for blocking laser emission.

Results and discussion
The optical characterization shows a band gap difference from 3 cycles to 5, 7 and 10, the blue shift in band gap for 3 cycles multilayers is attributed to cubic zinc blende structure in CdS nanoparticles, after 5 cycles hexagonal and cubic crystalline structures are present in the films.In Figure 2 the transmittance and Tauc plot are presented, is noticeable that with the increase of the number of cycles, increase the film thickness and reduces the optical transmission spectra from 80% a 3 cycles to almost 20% with 10 cycles, the absorption edge is blue shifted, which is seen in the difference in band gap from ~2.90 to ~2.15 eV, that is attributed to electronic disorder induced by CdS nanoparticles phase distribution and graphene oxide intercalation [1], [13].Theoretical calculations are ongoing to predict the size of CdS nanoparticles based in the absorption coefficient, also Urbach tail will be deduced [14].Raman spectroscopy is presented in Figure 3, longitudinal optical modes from CdS are observed, at 296 cm -1 peak 1LO appears, with the increase on cycles increase the intensity of 1LO peak, also 2LO and 3LO are shown at 600 and 904 cm -1 respectively, the blue shift of 1LO peak in comparison with bulk CdS (~305 cm -1 ) is due nanocrystalline structure [13], also the peaks signals belonging to FTO substrate disappear with the increase of cycles as were observed in UV-Vis spectra.Defects and graphitic bands (D and G bands) from graphene oxide are visible at 1345 and 1598 cm -1 .Increasing the number of deposition cycles increases the intensity of the CdS peaks, this leads off the use of Raman spectra as additional information in the film thickness deduction as well UV-Vis spectra.Photoluminescent spectra are shown in Figure 4, films with 3 and 5 cycles shows a blue shift in the PL spectrum, attributed to nanocrystalline nature, and point defects in CdS films.It is obvious that 5 cycles are the limit for the quantum confinement of CdS films.After 7 cycles, photoluminescence behaviour present a quenching in the spectra, and also a red shift in PL emission that occurs in the bulk CdS [15]. of the films, which would include modifying the concentration, deposition time in SILAR, voltage and current for the electrophoretic deposition of GO.These results are interesting for proposed GO/CdS multilayer architecture as promising new optoelectronic devices for example UV-Vis light emitting diodes for systems with 3 and 5 cycles and new photovoltaic materials for GO/CdS multilayers with cycles beyond 7.Further electrochemical characterization will be carried out on the multilayers, to evaluate the electrochemical potential and deduce the band structure of the systems, also theoretical approaches to the system will be carried out.

Conclusions
The obtention of GO/CdS multilayers were achieved, from 3 to 10 deposition cycles by alternate EPD and SILAR methods.Blue shifted PL behaviour is observed in a wide range from 350 to 700 eV from samples at 3 and 5 cycles, and a quenching and red shift to the bulk CdS obtained at cycles beyond 7.By Raman spectroscopy CdS peaks attributed to longitudinal optical modes 1LO, 2LO and 3LO were observed.UV-Vis spectra shows the electronic disorder at low cycles (3 and 5), which is attributed to nanocrystalline nature and GO/CdS interactions, confirmed too by the blue shift of 1LO peak in comparison with the bulk one in Raman spectra.

Figure 1 .
Figure 1.a) Synthesis scheme of GO and b) cyclic deposition GO and CdS films by EPD and SILAR.