Elsevier

Ultramicroscopy

Volume 184, Part A, January 2018, Pages 46-50
Ultramicroscopy

Imaging surface acoustic wave dynamics in semiconducting polymers by scanning ultrafast electron microscopy

https://doi.org/10.1016/j.ultramic.2017.08.011Get rights and content

Highlights

  • Scanning ultrafast electron microscopy was used to image surface acoustic waves dynamics photo-generated in a polymer.

  • Mechanical properties of the polymer were characterized.

  • Numerical simulation was used to validate the observations.

Abstract

Understanding the mechanical properties of organic semiconductors is essential to their electronic and photovoltaic applications. Despite a large volume of research directed toward elucidating the chemical, physical and electronic properties of these materials, little attention has been directed toward understanding their thermo-mechanical behavior. Here, we report the ultrafast imaging of surface acoustic waves (SAWs) on the surface of the Poly(3-hexylthiophene-2,5-diyl) (P3HT) thin film at the picosecond and nanosecond timescales. We then use these images to measure the propagation velocity of SAWs, which we then employ to determine the Young's modulus of P3HT. We further validate our experimental observation by performing a semi-empirical transient thermoelastic finite element analysis. Our findings demonstrate the potential of ultrafast electron microscopy to not only probe charge carrier dynamics in materials as previously reported, but also to measure their mechanical properties with great accuracy. This is particularly important when in situ characterization of stiffness for thin devices and nanomaterials is required.

Introduction

The exceptional characteristics of organic semiconductors allow their adoption in innovative technologies such as solar cells, microelectronics, and chemical sensors [1], [2], [3], [4]. In comparison with traditional inorganic semiconductors, these materials offer added technological value due to their processability, reduced weight, and low production cost. However, to be fully exploited for commercial applications, they must also possess sufficient mechanical strength to retain performance under thermomechanical stresses generated during operation.

The mechanical properties of thin films are different than those of the same materials in the bulk due to their reduced dimensions, enhanced surface effects, and the constraints imposed by the substrate. Thus, in the design of thin devices, such deviations should be included to ensure performance and durability [5]. The optoelectronic aspects of organic semiconductors have been thoroughly investigated in the past few decades, but their mechanical properties, especially those exclusive to the surface, are generally overlooked. This is particularly troublesome for applications that require these materials to survive prolonged exposures to the harsh outdoor environment. The stiffness of thin films is commonly evaluated by instrumented indentation, which induces both elastic and inelastic strain [6]. The extraction of information concerning elastic behavior, though, is usually nontrivial and involves the use of intricate mathematical models. Surface acoustic wave (SAW) spectroscopy is an alternative approach that derives the stiffness from the velocity of ultrasound waves generated by a piezoelectric device and detected by a transducer placed at a preset distance [7]. These waves are classified as Rayleigh waves and travel along the surface of the elastic material with an amplitude which exponentially decays into the bulk [8]. Since the longitudinal and transverse components of Rayleigh waves couple with any medium in contact with the surface, variations in the mass and stiffness caused by the presence of defects are reflected in the SAW profile [9]. In addition, SAWs are technologically relevant in a variety of applications such as chemical and biological sensors where they are used for accurate analysis of chemical and biological reactions [10], [11], [12], [13], [14]. They also provide the basis for devices used in high-frequency applications in the range of 100 MHz to a few GHz [15]. Acoustic charge transport (ACT) is a relatively recent application of SAWs, which involves coupling between SAWs and the two-dimensional electron gas (2DEG) within a piezoelectric semiconductor [16], [17], [18]. SAWs propagating on a piezoelectric crystal generate electric potential which confines the electrons in the 2DEG within the moving quantum wells; thus, allowing quantized and energy efficient charge transport over large distances [16], [17], [18].

To characterize the mechanical behavior of supported thin films and multilayers, picosecond ultrasonic spectroscopy is usually preferred because it allows a quick, non-invasive and precise measurement by exciting merely a small region of the surface that does not extend far into the bulk [19]. This technique can also characterize the polymer-substrate interaction by comparing the mechanical properties of the thin film at various thicknesses, especially in the nanometer regime where such effects are pronounced. In this technique, a focused ultra-short laser pulse induces transient expansion or ablation of an absorbing material due to an impulsive heating, which results in thermoelastic and elastodynamic responses, respectively. Such processes are limited to only a few nanometers in metals, but become volumetric in semiconductors and polymers due to the larger penetration depth of the optical pulse. The propagation of the thermoelastic waves is then probed by measuring the transient reflectivity of a weaker laser pulse hundreds of micrometers away from the laser/sample crossover. The transient change in the reflectivity measures the acoustic strain caused by interface displacement as well as variation in the refractive index by the acousto-optic coupling. Since wave propagation is mainly dependent upon material properties near the surface, optically generated SAWs are an excellent tool to investigate surface elasticity [9].

The use of SAWs to study organic materials is somewhat challenging because the velocity of these waves is slow and their detection requires a near field detection scheme due to their strong attenuation. Thus, there is a need for a time-resolved microscopy technique that will allow the characterization of SAWs in polymers with simplicity and precision. Here, we investigate the formation and propagation of optically generated SAWs on the surface of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin film by scanning ultrafast electron microscopy (SUEM). P3HT is a p-type semiconductor, which is widely used in organic opto-electronic and photovoltaic devices. SUEM, which combines the spatial resolution of the electron probe with the temporal resolution of the femtosecond laser, was previously employed to study charge carriers in semiconductors by imaging their dynamics in both space and time [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. In this work, we use SUEM to image the spatiotemporal behavior of radially propagating SAWs and we measure their propagation velocity and wavelength to be 550 ± 40m/s and 50μm, respectively. By employing the thermoelastic transport equation, we then calculate the Young's modulus to be 1.03 ± 0.01 GPa, which is in agreement with the reported values in the literature for P3HT [30]. Two-dimensional finite element analysis on a semi-infinite model well mimics the experimental observation and validates the calculated Young's modulus. This work illustrates the potential of SUEM for the in-situ characterization of organic thin films and nanomaterials. Furthermore, SUEM can operate at low vacuums, allowing a similar study in the presence of inert and reactive gasses.

Section snippets

Experimental method

For this experiment, P3HT with the average Mn=54,00075,000 was purchased from Sigma-Aldrich and used without further chemical treatments. The powder was fully dissolved in dichlorobenzene at 50 °C under continuous magnetic stirring to produce a concentrated solution at 40 mg/ml. The solution was then spun-cast on the surface of a heavily doped p-type silicon (1019 B/cm3) at 500 rpm, which resulted in a film with an average thickness of 1μm. Such a relatively large thickness ensured that the

Results and discussions

Fig. 1 schematically shows our experimental setup for the generation and detection of SAWs on the P3HT surface. First, the green pulse at 0.05 mJ/cm2 excites the sample and impulsively increases the local temperature (T), whose extent depends on the optical absorption coefficient (α). The thermal response of the sample is described by the heat diffusion equation [33], [34]:ρcT(r,t)tk2T(r,t)=Q(r,t)where ρ, c, k are density, heat capacity and heat conductivity, respectively, and Q is the

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgments

This work was supported by NSF grant DMR-0964886 and Air Force Office of Scientific Research grant FA955011-1-0055 in the Physical Biology Center for Ultrafast Science and Technology at California Institute of Technology, which is supported by the Gordon and Betty Moore Foundation.

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