Dye loading model for electrochemical self-assembly of CuSCN/stilbazolium dye hybrid thin films

Abstract

We have developed a method to electrochemical self-assemble (ESA) inorganic / organic hybrid thin films in which the inorganic is CuSCN, known as a wide bandgap p-type semiconductor, whereas the organic is 4-N,N-dimethylamino-4’-(N’-methyl)stilbazolium chromophore (abbreviated as DAS⁺) as its salt with tosylate (DAST) known to exhibit second-order nonlinear optical property for terahertz emitters. The CuSCN/DAS hybrid thin films by ESA show concerted photoluminescence based on energy transfer from inorganic CuSCN to organic DAS, making us anticipate its potential use in optoelectronic devices [1]. Understanding the mechanism of ESA to obtain a tuning knob to maximize its functionality is therefore important.

Synthesis and analysis of multiple thin films suggested switching of dye loading mechanism depending on DAS concentration (C_DAS) in the bath [2]. With low C_DAS, the loading is limited by diffusion so that DAS is entrapped within CuSCN crystal grains, while surface reaction of hybridization begins to limit the dye loading with high C_DAS, resulting in formation of unique nanostructures as well as phase separation of inorganic and organic domains. Systematic variation of bulk concentrations of CuSCN precursor [Cu(SCN)]⁺ (C_comp), DAS⁺ (C_DAS) and their flux density by changing angular speed of rotation of the rotating disk electrode has indicated C_DAS/C_comp ratio as the critical value for the switching, and that should be dependent on the stability of the surface complex [3].

In this study, we have completed the dye loading model for ESA of CuSCN/DAS hybrid thin films and also applied it to the ESA employing dyes with enhanced polarities such as 4-methoxy-4’-(N’-methyl)stilbazolium (MTS⁺) and 4-cyano-4’-(N’-methyl)stilbazolium tosylate (CNS⁺). Increased stabilities of surface complex are expected for their DFT calculated dipole moments of 10.1, 14.8 and 23.5 Debye for DAS⁺, MTS⁺ and CNS⁺, respectively. While the validity of the proposed mechanism has been checked for all three dyes, the stability constants of the surface complex has been quantified to describe the difference of their switching behavior.

The electrodeposition of CuSCN is always limited by transport of the precursor complex as,

[Cu(SCN)]⁺ + e^- → CuSCN                                                                                              (1)

Therefore, the current for Eq. (1) can be described by Levich equation as,

                                                                     (2)

where, F is the Faraday constant (96,485 C mol^-1), C_comp and D_comp are the bulk concentration (mol cm^-3) and diffusion coefficient (2.26 × 10^-6 cm² s^-1 in methanol at 298 K) of [Cu(SCN)]⁺, respectively, ν is the kinetic viscosity of the medium (0.687 × 10^-4 cm² s^-1 for methanol at 298 K) and ω is the angular speed of rotation (rad s^-1). Assuming that the Faradic efficiency equals 100% for the electrodeposition of CuSCN, its rate of precipitation (P_CuSCN, mol cm^-2 s^-1) can be re-written as,

                                                                (3)

P_DAS under diffusion limited regime nominated as P_{DAS dif} can also be expressed by taking Levich equation,

                                                                  (4)

for which the diffusion coefficient of DAS, D_DAS, has previously been determined as 1.92 × 10^-6 cm² s^-1 in methanol at 298 K [3].

On the other hand, for the dye loading limited by surface reaction, we can consider a stoichiometry of surface complex between n of DAS and a single CuSCN site as,

                                                                                               (5)

Assuming that such an equilibrium is always maintained during the film growth and by the virtue of constant rate of precipitation both for CuSCN and DAS, the surface concentration of CuSCN site and (CuSCN)(DAS)_n complex are proportional to P_CuSCN and P_DAS, respectively, so that the equilibrium constant K for Eq. (5) can be expressed as,

                                                                                                                (6)

Thus, P_DAS under the surface reaction limited regime nominated as P_{DAS sur} is,

                                                                                        (7)

The best fitting of P_DAS on variation of C_comp and C_DAS yielded n and K as 0.52 and 76.2 mol^-0.47 cm^0.53, respectively. n value close to 0.5 indicates a stoichiometry in which one DAS⁺ binds to two CuSCN sites. By taking n = 0.5, K is recalculated as

Fig.1 shows the plots of P_DAS against C_DAS on variation of C_comp. The straight bold red line indicates diffusion limited P_{DAS dif}, onto which all the plots fall for low C_DAS range irrespective of C_comp, whereas the curves according to Eq. (7) with the above determined values of n and K nicely fit with the plots attenuated to deviate from the red line, well describing C_comp dependent surface reaction limited DAS precipitation, P_{DAS sur}.

At the border of switching,

                                                                                                              (8)

holds, so that the critical concentration of DAS for switching, C_{DAS swi}, is expressed as,

                                                                                                   (9)

ESA of hybrid thin films have been carried out for MTS⁺ and CNS⁺ dyes to test the validity of the above explained model established for DAS⁺. Indeed, the very same behavior as DAS⁺, namely, P_dye linearly increasing against C_dye and deviate from linearity at certain C_dye depending on C_comp, was observed, except that the point of deviation occurred at higher C_dye for MTS⁺ than DAS⁺ and even higher for CNS⁺. While diffusion coefficients, D_MTS and D_CNS were determined as 2.07 × 10^-6 and 2.76× 10^-6 cm² s^-1, respectively, to describe their P_{dye dif} according to Eq. (4), good fittings were obtained for P_{dye sur} according to Eq. (7) and by taking n = 0.5 to determine the K values for MTS⁺ and CNS⁺ as 87.2 and 114.1 mol^-0.5 cm^2.5, respectively. Thus, the switching border as the relationship between C_{dye swi} and C_comp is calculated for all three dyes, DAS⁺, MTS⁺ and CNS⁺, causing upward shift for increased K as shown in Fig. 2.

It has become evident that all these stilbazolium dyes behave in the same manner, for which the maximum achievable diffusion limited dye loading as well as the switching to surface reaction limited loading are dependent on the stability of CuSCN/dye complex quantified as the K values.