CdS nanocrystalline thin films deposited by the continuous microreactor-assisted solution deposition (MASD) process : growth mechanisms and film characterizations Public Deposited

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  • The continuous microreactor-assisted solution deposition (MASD) process was used for the deposition of CdS thin films on fluorine-doped tin oxide (FTO) glass. The MASD system, including a T-junction micromixer and a microchannel heat exchanger is capable of isolating the homogeneous particle precipitation from the heterogeneous surface reaction. The results show a dense nanocrystallite CdS thin films with a preferred orientation at (111) plane. Focused-ion-beam was used for TEM specimen preparation to characterize the interfacial microstructure of CdS and FTO layers. The band gap of the microreactor-assisted deposited CdS film was determined at 2.44 eV. X-ray Photon Spectroscopy show the bindings of energies of Cd 3d₃/₂, Cd 3d₅/₂, S 2p₃/₂ and S 2p₁/₂ at 411.7 eV, 404.8 eV, 162.1 eV, and 163.4 eV, respectively. The film growth kinetics was studied by measuring the film thickness deposited from 1 minute to 15 minutes in physical (FIB-TEM) and optical (reflectance spectroscopy) approaches. A growth model that accounts for the residence time in the microchannel using empirical factor (η) obtained from previous reported experimental data. Applying this factor in the proposed modified growth model gives a surface reaction rate of 1.61*10⁶ cm⁴ mole⁻¹s⁻¹, which is considerable higher than the surface reaction rates obtained from the batch CBD process. With the feature of separating homogeneous and heterogeneous surface reaction, the MASD process provides the capability to tailor the surface film growth rate and avoid the saturation growth regime in the batch process. An in situ spectroscopy technique was used to measure the UV-Vis absorption spectra of CdS nanoparticles formed within the continuous flow microreactor. The spectra were analyzed by fitting the sum of three Gaussian functions and one exponential function in order to calculate the nanoparticle size. This deconvolution analysis shows the formation of CdS nanoparticles range from 1.13 nm to 1.26 nm using a residence time from 0.26 s to 3.96 s. Barrier controlled coalescence mechanism seems to be a reasonable model to explain the experimental UV-Vis data obtained from the continuous flow microreactor, with a rate constant k' value of 2.872 s⁻¹. Using CFD, low skewness value of the RTD curve at high flow rate (short τ) suggests good radial mixing at high flow rate is responsible for the formation of smaller CdS nanoparticles with a narrower size distribution. The combination of CdS nanoparticle solution with MASD process resulted in the hindrance of CdS thin film deposition. It is hypothesized that the pre-existing sulfide (S²⁻) ions and CdS nanoparticles changes the chemical species equilibrium of thiourea hydrolysis reaction. Consequently, the lack of thiourea slows down the heterogeneous surface reaction. To test the scalability of the MASD process, a flow cell and reel-to-reel (R2R)-MASD system were setup and demonstrated for the deposition of CdS films on the FTO glass (6" x 6") substrate. The film deposition kinetics was found to be sensitive to the flow conditions within the heat exchanger and the substrate flow cell. The growth kinetics of the CdS films deposited by R2R-MASD process was investigated by with a deposition time of 2.5 min, 6.3 min, and 9 min. In comparison with the continuous MASD process, the growth rate in R2R-MASD is higher, however more difficult to obtain a linear relationship with the deposition time.
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