A novel method for biopolymer surface nanostructuring by platinum deposition and subsequent thermal annealing
© Slepička et al.; licensee Springer. 2012
Received: 1 October 2012
Accepted: 5 December 2012
Published: 12 December 2012
A novel procedure for biopolymer surface nanostructuring with defined surface roughness and pattern dimension is presented. The surface properties of sputtered platinum layers on the biocompatible polymer poly(l-lactic acid) (PLLA) are presented. The influence of thermal treatment on surface morphology and electrical resistance and Pt distribution in ca. 100 nm of altered surface is described. The thickness, roughness and morphology of Pt structures were determined by atomic force microscopy. Surface sheet resistance was studied by a two-point technique. It was the sequence of Pt layer sputtering followed by thermal treatment that dramatically changed the structure of the PLLA’s surface. Depending on the Pt thickness, the ripple-like and worm-like patterns appeared on the surface for thinner and thicker Pt layers, respectively. Electrokinetic analysis confirmed the Pt coverage of PLLA and the slightly different behaviour of non-annealed and annealed surfaces. The amount and distribution of platinum on the PLLA is significantly altered by thermal annealing.
Polymer-metal composites are becoming an attractive subject because of their unique surface morphology and electric properties. They can be made on the base of polymeric films metalized from one or both sides with a noble metal (gold or platinum)[2, 3]. These structures can be applied in many different parts of advanced electromechanical application, such as biomimetic robots or actuators. Metal nanolayers on polymers were used in applications for LCD technology. Metal-polymer composites were studied as a key material for transistor construction in electronics industry. The increasing interest in the field of nanomaterials has created novel or advanced analytical techniques capable of characterizing the materials in the nanostructure scale and preparing functionalized nanostructured materials. Methods for metal nanolayer preparation can be divided into several branches, involving, e.g. sputtering[8, 9], evaporation or electrochemical methods. It has been reported frequently that platinum thin film patterns, like temperature sensors or heaters, degrade at temperatures in the range of 500°C to 900°C, which can lead to functional failure of the microdevices on which these patterns are deposited. Polymer-metal structures are of great importance in thermal management of microsystems. The biopolymer poly(l-lactic acid) (PLLA) in combination with various types of treatment procedures, grafting or metal layer deposition, can be applied in the preparation of substrates for tissue engineering[12, 13]. Different polymer substrates (e.g. PDMS) can, on the contrary, play a significant role in metal superlattice preparation. Thermal treatment of metal nanostructures such as Pt and Au may also lead to bimetallic nanostructures. The potential applications of polymer-metal nanocomposites and nanoparticles can be found in electronics or biomedical engineering[16–22].
In this paper, we present a simple and cheap method for biopolymer surface nanostructuring by platinum nanostructure deposition and subsequent thermal annealing of the biopolymer surface. The electrical properties, zeta potential, chemical structure and surface morphology of ripple composites are introduced.
The biopolymer PLLA (density 1.25 g cm−3, glass transition temperature (Tg) = 60°C, crystallinity 60% to 70%, 50-μm-thick foils, supplied by Goodfellow, Ltd., Huntingdon, UK) was used for the present experiments. The platinum layers on the PLLA substrate were deposited from a Pt target (99.999%) by means of diode sputtering technique (BAL-TEC SCD 050 equipment, BalTec Maschinenbau AG, Pfäffikon, Switzerland). The theoretical sputtering rate of the SCD 050 for platinum is 0.15 nm s−1. Typical sputtering conditions were as follows: room temperature, time 5 to 500 s, total argon pressure of about 5 Pa, electrode distance of 50 mm and current of 20 mA. For the measurement of the Pt layer thickness, the metal was deposited under the same conditions on a glass substrate and measured with an atomic force microscope (AFM). Typically, five measurements on three scratches each were accomplished on each sample.
The electrical discontinuity/continuity of the as-sputtered and as-sputtered + heated (at 60°C) platinum layers was examined by measuring electrical sheet resistance (Rs). For determination of Rs by standard Ohm’s method, a KEITHLEY 487 pico-ammeter (Cleveland, OH, USA) was used. Two Au contacts (about 50 nm thick) were sputtered on the layer’s surface for resistance measurement. Typical error of the measurement was ±5%. Thermal treatment of the polymers was accomplished in a BINDER thermostat (Tuttlingen, Germany). The samples were heated for 60 min at 60°C, and then they were cooled down to room temperature.
The surface morphology was examined using an AFM. The AFM images were taken under ambient conditions on a Bruker Corporation CP-II setup (Santa Barbara, CA, USA). Ra represents the arithmetic average of the deviations from the centre plane of the sample. Four areas of each sample were scanned in order to obtain representative data.
Rutherford back scattering (RBS) analyses were performed on a Tandetron 4130MC accelerator using 1.7-MeV 4He ions (High Voltage Engineering Europa, Amersfoort, The Netherlands). The measurements were performed in IBM geometry with an incident angle of 0° and a laboratory scattering angle of 170°. The typical energy resolution of the spectrometer was FWHM = 15 keV. The RBS spectra were evaluated using SIMNRA and GISA software. The RBS measurement was realized at the CANAM infrastructure.
The zeta potential of the samples was determined using the SurPASS Instrument (Anton Paar, Graz, Austria). The samples were studied inside an adjustable gap cell with an electrolyte (0.001 mol dm−3 KCl) at a temperature of 25°C and pH = 6.0. For each measurement, a pair of polymer foils with the same top layer was fixed on two sample holders (with a cross section of 20 × 10 mm2 and a gap of 100 μm). All samples were measured four times with a relative error of ±10%. For zeta potential determination, we applied two methods (streaming potential and streaming current) and two equations for zeta potential calculation (Helmholtz-Smoluchowski and Fairbrother-Mastins).
Results and discussion
As it is apparent from Figure 1, thermal annealing leads to the shift of the point of electrical continuity. The Pt becomes electrically continuous in the thickness above 5 nm, while in the PLLA/Pt heated at 60°C, the point of electrical continuity has shifted to the Pt thickness of 10 nm. The Pt deposited on PLLA and heated exhibits in the area of electrical continuity lower values than the layer on the non-heated sample; for higher thicknesses, the convergence is apparent. The changes of electrical continuity are probably caused by the formation of the PLLA/Pt composite, which will be discussed further (RBS analysis), and by changes in surface morphology. The errors of measurement are introduced, but they are within the graph points (did not exceed 3%). The dependence of Pt thickness on sputtering time was proved to be linear (see embedded graph in Figure 1). The dependence was fitted with standard mathematical regression with a confidence interval better than 0.99.
We have developed a simple and cheap procedure for biopolymer surface nanostructuring. The ripple patterns with a defined chemical structure, dimension and roughness were prepared. The platinum nanolayer sputtered on pristine PLLA surface becomes electrically continuous for a thickness of 5 nm. Thermal annealing results to the shift of the point of electrical continuity up to 10 nm. The annealing of the Pt-deposited polymer surface results in the formation of a ripple-like nanostructure. The surface roughness of the composite structure and its properties strongly depend on the thickness of previously sputtered Pt. The annealing of PLLA-Pt samples leads to a significant increase of metal concentration in the upper polymer layer and plays a significant role in ripple-like structure formation. Electrokinetic analysis confirmed the slightly different behaviour of as-sputtered and annealed Pt nanostructures, the highest differences being observed for lower Pt thicknesses.
This work was supported by GACR under project P108/12/G108.
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