Charge transfer induced excitons and nonlinear optical properties of ZnO/PEDOT:PSS nanocomposite films

Highlights

• Composite films were prepared consisting of ZnO and PEDOT:PSS.

• Excitonic interaction affects the linear optical properties through charge transfer.

• Two-photon induced saturable absorption behavior has been observed in the composites.

• Enhancement of saturable absorption behavior has been studied.

Abstract

In this current work, we have prepared zinc oxide (ZnO) nanorods by sol-gel method, and its composite films with a conducting polymer poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) also have been prepared by drop-casting method on the glass substrate. UV–Vis absorption and steady-state fluorescence studies revealed exciton dissociation and recombination at the interface of polymer chain and wide-bandgap semiconductor ZnO. Also, nonlinear optical properties of as-prepared nanocomposite films have been reported by employing an open aperture z-scan technique. A predominantly two-photon induced saturable absorption behavior, when excited with 532 nm, 10 ns laser pulses, appeared in nonlinear optical measurements. These results indicate that our as-synthesized composites can be useful in fabricating optical switch and saturable absorbers.

Introduction

In recent times, a remarkable interest has been found in the development of inorganic semiconductor and polymer hybrid materials in nanoscale range for their extensive applications ranging from electronic devices such as solar cells, amplifying media, light-emitting diodes (LEDs), field-effect transistors (FETs) to other optoelectronic and nonlinear optical devices [[1], [2], [3], [4]]. Although the polymers and inorganic semiconductors have different properties, their composite will give some new synergetic or complementary behavior arising from the molecular level mixing of conductive polymer and semiconductor. Several researchers reported different optoelectronic properties such as charge separation, improved barrier properties, etc. of inorganic semiconductor/polymer nanocomposites and enhancement of these properties for future applications [[2], [3], [4]]. The inorganic semiconductor materials are in constant demand due to their unique physical, chemical, electrical, linear, and nonlinear optical properties [5]. Notably, their nonlinear optical (NLO) properties indicate their prospective application as commercial NLO devices, optical switches, frequency converters, logic elements in the optoelectronic devices, etc. [[1], [2], [3], [4], [5], [6], [7]] Among other investigated inorganic materials, ZnO has attractive linear and NLO properties that can help fabricate NLO-based devices [7]. Due to its several advantages such as wide bandgap (3.37 eV), high exciton binding energy (60 meV), defects induced photoluminescence, it has been extensively investigated [8]. Several research groups have already studied optical properties of different shaped ZnO nanostructures in the form of quantum dots, nanorods, nanowires, etc. [[9], [10], [11]] They are also finding some promising optical applications such as efficient UV emitters and random lasing operating at room temperature [12,13].

In the last two decades, the synthesis of conductive polymers has been focused due to their outstanding applications in the area of electronic and optoelectronic devices, such as flexible LEDs [14], thin film transistors, sensors [15], photovoltaic cells [16], optical limiters, saturable absorber, etc. [17] Among the several nonlinear organic polymers, thiophene based polymers such as PEDOT:PSS are currently under intensive investigation for linear and NLO properties due to their high third-order response, high chemical stability, and willingness of functionalization with others [18]. PEDOT:PSS shows very high optical transparency, high molecular polarizability, and it is due to the strong delocalization of π-electrons in the polymeric backbone. Therefore, large third-order optical nonlinearities have been achieved [19]. Furthermore, the semiconductor/polymer nanocomposite structures are also known to enhance optical nonlinearities substantially. Also, embedding semiconductor nanoparticles in thin polymer films for application purposes is favorable because the polymer matrix serves as a medium to assemble the nanoparticles and stabilize them from getting aggregated [20,21]. In the past, several research groups have studied the third-order NLO properties of a few polymer composites [22,23]. However, a few reports have been devoted to study the optical nonlinearity of nanohybrid film made of PEDOT:PSS with other semiconductors.

Among various NLO responses, saturable absorption (SA) is an interesting nonlinear behavior. SA is a material property where the absorption of light decreases, i.e., the transmission increases with the increase of the intensity of light. The ground state atoms or molecules of a saturable absorber material are excited into upper energy state at enough high incident light intensity. If they are excited rapidly, so that there is more time for them to decay back to the ground state before the depletion of the ground state, the absorption saturates subsequently [24]. The semiconductor/polymer composites are potential candidates that show SA behavior and can be used in optical switching, saturable absorbers, etc.

In this study, we prepared ZnO/PEDOT:PSS composite films by the solution-casting method. The absorbance and fluorescence spectroscopy measurements of the films have been carried out. UV–Vis absorption and PL spectra revealed the charge transfer between ZnO and PEDOT:PSS due to their difference in bandgap energies. Optical nonlinearity was measured by open aperture z-scan method by nanosecond pulsed laser, and the composite samples show saturable absorption behavior, which has been investigated thoroughly.