Correlating Optical Reflectance with the Topology of Aluminum Nanocluster Layers Growing on Partially Conjugated Diblock Copolymer Templates.

Marc Gensch,Oleksandr Polonskyi,Suzhe Liang,Jonas Drewes,Stephan V Roth,Thomas Strunskus,Simon J Schaper,Peter Müller-Buschbaum,Franz Faupel,Matthias Schwartzkopf,Wei Chen,Calvin J Brett,Nian Li

doi:10.1021/acsami.1c18324

Abstract

Large-scale fabrication of metal cluster layers for usage in sensor applications and photovoltaics is a huge challenge. Physical vapor deposition offers large-scale fabrication of metal cluster layers on templates and polymer surfaces. In the case of aluminum (Al), only little is known about the formation and interaction of Al clusters during sputter deposition. Complex polymer surface morphologies can tailor the deposited Al cluster layer. Here, a poly(methyl methacrylate)-block-poly(3-hexylthiophen-2,5-diyl) (PMMA-b-P3HT) diblock copolymer template is used to investigate the nanostructure formation of Al cluster layers on the different polymer domains and to compare it with the respective homopolymers PMMA and P3HT. The optical properties relevant for sensor applications are monitored with ultraviolet-visible (UV-vis) measurements during the sputter deposition. The formation of Al clusters is followed in situ with grazing-incidence small-angle X-ray scattering (GISAXS), and the chemical interaction is revealed by X-ray photoelectron spectroscopy (XPS). Furthermore, atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) yield topographical information about selective wetting of Al on the P3HT domains and embedding in the PMMA domains in the early stages, followed by four distinct growth stages describing the Al nanostructure formation.

Highlights

We investigated the interaction of Ag with polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(3-hexylthiophen-2,5-diyl) (P3HT) polymer surfaces using surface-sensitive methods such as grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray photoelectron spectroscopy (XPS) to reveal the topological changes and chemical interactions.[36,37]
Www.acsami.org broad range of Al plasmon absorption from various Al nanoparticles obtained by chemical synthesis or lithographic procedures were reported to be located in the UV−vis spectral region, depending on the size, shape, amount of oxygen, local arrangement, and surrounding medium.[12,29,51−56] Hitherto, a comprehensive investigation of tuning the morphology and collective optical reflectance of Al layers sputter-deposited on nanostructured diblock copolymers (DBCs) thin films is still missing
To highlight the relative changes in the UV−vis reflectance during sputter deposition originating from Al layer growth on the DBC template, the reflectance at an incident angle of 55° of the pristine 20 nm PMMA-b-P3HT thin film on a Si wafer is set as reference to 100%

Summary

■ INTRODUCTION

The exploitation of the optoelectronic properties of organic and inorganic nanostructures and cluster layers relies on the ability to direct their self-assembly using chemically or topographically tailored organic and inorganic templates.[1−8] The utilization of abundant and low-cost metals such as aluminum (Al) is of high interest for surface-enhanced Raman scattering (SERS)-type sensors due to the high absorption in the ultraviolet (UV) spectral range and lower material costs compared to, e.g., silver (Ag) or gold (Au).[9−13] thin metal layers of Al or Ag are often used as an electrode material in organic electronics such as in organic photovoltaics (OPVs).[14−19] A common and facile method to prepare functional metal layers on large scales is sputter deposition.[20−27] the intrinsic physicochemical and nonequilibrium processes during sputter deposition are complex, in particular, when using reactive metals such as Al.[28]. A different growth of Al on PMMA-b-P3HT is seen compared to Ag on the DBC template, which formed cylindrically shaped Ag clusters on the P3HT domain of the DBC.[37] At the early stages of the Al sputter deposition (δAl < 1 nm), Al forms small clusters on the P3HT domain, which connect very fast to wormlike Al nanostructures growing in their size (Figure 1e), comparable to earlier results.[21] Figure 1c (for δAl = 1 nm) shows wormlike nanostructures on the P3HT domain and small isolated clusters next to the P3HT domains. The Al clusters on the Al merged Al layer continuously grow in size, as seen on the fibers of the P3HT homopolymer thin film, to a nanogranular Al layer (III)

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES