Annealing of gold nanostructures sputtered on polytetrafluoroethylene
© Siegel et al; licensee Springer. 2011
Received: 23 August 2011
Accepted: 11 November 2011
Published: 11 November 2011
Gold nanolayers sputtered on polytetrafluoroethylene (PTFE) surface and their changes induced by post-deposition annealing at 100°C to 300°C are studied. Changes in surface morphology and roughness are examined by atomic force microscopy, electrical sheet resistance by two point technique, zeta potential by electrokinetic analysis and chemical composition by X-ray photoelectron spectroscopy (XPS) in dependence on the gold layer thickness. Transition from discontinuous to continuous gold coverage takes place at the layer thicknesses 10 to 15 nm and this threshold remains practically unchanged after the annealing at the temperatures below 200°C. The annealing at 300°C, however, leads to significant rearrangement of the gold layer and the transition threshold increases to 70 nm. Significant carbon contamination and the presence of oxidized structures on gold-coated samples are observed in XPS spectra. Gold coating leads to a decrease in the sample surface roughness. Annealing at 300°C of pristine PTFE and gold-coated PTFE results in significant increase of the sample surface roughness.
Up to now, many efforts have been spent to produce smart materials with extraordinary properties usable in broad range of technological applications. In the last two decades, it has been demonstrated that properties of new prospective materials depend not only on their chemical composition but also on the dimensions of their building blocks which may consist of common materials [1, 2]. Besides other interesting properties of nanostructured gold systems, such as catalytic effects or magnetism [2, 3], which both originate from surface and quantum size effects, they are also extremely usable, those which are closely connected with the average number of atoms in the nanoparticles. The properties and behavior of extremely small gold particles completely differ from those of bulk materials, e.g., their melting point [2, 4, 5], density , lattice parameter [6–8], and electrical or optical properties [6, 7, 9] are dramatically changed. Exceptional properties of gold nanoparticles offer completely new spectrum of applications. For example, the ability to control the size and shape of the particles and their surface conjugation with antibodies allows for both selective imaging and photothermal killing of cancer cells [10–12] due to their excellent biocompatibility  and unique properties in surface plasma resonance . Besides the medicinal applications, gold nanolayers and nanoparticles are nowadays also used in sensor technology  or surface-enhanced Raman spectroscopy .
Recently, new technique has been proposed for modification of Au nanolayer deposited on glass substrate, based on intensive post-deposition annealing [7, 9]. Resulting structures are "hummock-like" isolated gold islands uniformly distributed over the substrate. The formation of new structures may be due to the accelerate diffusion and stress relaxation in gold nanolayer.
In this work, we studied the changes in surface morphology and other physico-chemical properties of gold nanolayers, sputtered on polytetrafluoroethylene surface induced by post-deposition annealing.
Substrate and Au deposition
The present experiments were performed on poly(tetrafluoroethylene) foil (PTFE, thickness of 50 956;m, T g = 126°C, and T f = 327°C) supplied by Goodfellow Ltd., UK. The gold layers were sputtered on polymer foil (2 cm in diameter). The sputtering was accomplished on Balzers SCD 050 device from gold target (supplied by Goodfellow Ltd., Huntingdon, England, UK). The deposition conditions were: DC Ar plasma, gas purity 99.995%, discharge power of 7.5 W, sputtering time 0 to 550 s. Under these experimental conditions, homogeneous distribution of gold over the substrate surface is expected . Post-deposition annealing of Au-covered PTFE was carried out in air at 300°C (± 3°C) for 1 h using a thermostat Binder oven. The heating rate was 5°C.min−1 and the annealed samples were left to cool in air to room temperature (RT).
Electrokinetic analysis (determination of zeta potential) of pristine and Au-coated PTFE foils was accomplished on SurPASS Instrument (Anton Paar, Graz, Austria). Samples were studied inside the adjustable gap cell in contact with the electrolyte (0.001 mol.dm−3 KCl). 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 gap between that is 100 956;m) . All samples were measured four times at constant pH value with the relative error of 10%. The used method was streaming current and zeta potential was calculated by Helmholtz-Smoluchowski equation .
An Omicron Nanotechnology ESCAProbeP spectrometer was used to measure X-ray photoelectron spectroscopy (XPS) spectra . The analyzed areas had dimensions of 2 × 3 mm2. The X-ray source provided monochromatic radiation of 1,486.7 eV. The spectra were measured stepwise with a step in the binding energy of 0.05 eV at each of the six different sample positions. The spectra evaluation was carried out by using CasaXPS software. The composition of the various elements is given in atomic percent disregarding hydrogen, which cannot be assessed by XPS.
Surface morphology of as-sputtered and annealed gold layers deposited for different sputtering times was examined using atomic force microscopy (AFM). The AFM images were taken under ambient conditions on a Digital Instruments CP II set-up working in tapping mode in order to eliminate damage of the sample surface. A Veeco phosphorous-doped silicon probe RTESPA-CP (Veeco, Mannheim, Germany) with spring constant of 20 to 80 N.m−1 was chosen. AFM working in contact mode was also used to determine thickness of sputtered gold by scratch method. The scratch on glass substrate was made in ten different positions on as-sputtered samples and scanned in contact mode . In this case, a Veeco phosphorous-doped silicon probe CONT20A-CP with spring constant 0.9 N.m−1 was chosen. Thickness variations do not exceed 5%. All AFM scans were acquired at scanning rate of 1 Hz. Due to the morphology changes evoked by the annealing, the sputtered layer thickness could only be satisfactorily determined in the case of as-sputtered samples. Thus, in the case of annealed samples, the effective thickness is defined as the thickness of as-sputtered gold which is considered to be the same for annealed structures deposited for the corresponding deposition time.
Sheet resistance (R s) of the gold layers was measured by standard two point method. Two gold contacts, defining measured area (about 50 nm thick) on the layer surface were prepared by sputtering. We define an electrically continuous layer as a layer, where the declining sheet resistance reaches a saturated minimum.
Results and discussion
Besides of the sheet resistance, measurement information on the layer structure and homogeneity can be obtained in another way too. Here, complementary information on the layer homogeneity is obtained from XPS spectra.
AU layer thickness
Atomic concentrations of elements in at.%
The changes in the surface morphology after the annealing were studied by AFM. AFM scans of pristine and Au-coated (20 nm) samples before and after annealing are presented in Figure 4. One can see that the annealing causes a dramatic increase in the surface roughness of the pristine polymer. Since the annealing temperature markedly exceeds PTFE glassy transformation temperature (T g PTFE = 126°C) the increase in the surface roughness is probably due to thermally induced changes of PTFE amorphous phase. The gold sputtering leads to a measurable reduction of the sample surface roughness. The reduction may be due to preferential gold growth in hollows at the PTFE surface. Annealing of the gold-coated sample leads to significant increase of the surface roughness too. In this case, the increase is a result of both, the changes in the surface morphology of underlying PTFE and the changes in the morphology of the gold layer. After annealing, the surface roughness of pristine and gold-coated samples is practically the same. This finding is in contradiction with similar study accomplished on gold layers deposited on glass substrate . Possible explanation of this fact probably lies in much better flatness of the glass substrate and in lower thermal stability of PTFE substrate during annealing.
The properties of thin gold layers sputtered on the PTFE substrate and their changes after annealing at 100°C to 300°C were studied by different methods. Chemical composition, electrical conductivity, surface morphology, and zeta potential of the layers as a function of the layer thickness were determined. Attention was focused on the transition from partial to complete gold coverage of PTFE substrate. From the measurement of the sheet resistance the transition from discontinuous to continuous gold coverage was found at the layer thicknesses 10 to 15 nm for as-sputtered samples. After annealing at 300°C, the transition point increase to about 70 nm, the increase indicating substantial rearrangement of the gold layer. The rearrangement is confirmed also by XPS measurement and an electrokinetic analysis. By XPS measurement, contamination of the gold coated PTFE samples with carbon and the presence of oxidized structures created during gold sputtering were proved. The annealing results in significant increase of the surface roughness of both pristine- and gold-sputtered PTFE.
This work was financially supported by the GA CR under the projects 106/09/0125, P108/10/1106, and P108/11/P337, and AS CR under the project KAN200100801 and by Ministry of Education of the CR under the program LC 06041.
- Rao CNR, Kulkarni GU, Thomas PJ, Edwards PP: Size-dependent chemistry: properties of nanocrystals. Chem Eur J 2002, 8: 25–39.View Article
- Roduner E: Size matters: why nanomaterials are different. Chem Soc Rev 2006, 35: 583–592. 10.1039/b502142cView Article
- Seino S, Kinoshita T, Otome Y, Maki T, Nakagawa T, Okitsu K, Mizukoshi Y, Nakayama T, Sekino T, Niihara K, Yamamoto TA: Gamma-ray synthesis of composite nanoparticles of noble metals and magnetic iron oxides. Scripta Mater 2004, 51: 467–472. 10.1016/j.scriptamat.2004.06.003View Article
- Kan CX, Zhu XG, Wang GH: Single-crystalline gold microplates: Synthesis, characterization, and thermal stability. J Phys Chem B 2006, 110: 4651–4656. 10.1021/jp054800dView Article
- Liu HB, Ascencio JA, Perez-Alvarez M, Yacaman MJ: Melting behavior of nanometer sized gold isomers. J Surf Sci 2001, 491: 88–98. 10.1016/S0039-6028(01)01351-6View Article
- Siegel J, Lyutakov O, Rybka V, Kolská Z, Švorčík V: Properties of gold nanostructures sputtered on glass. Nanoscale Res Lett 2011, 6: 96. 10.1186/1556-276X-6-96View Article
- Švorčík V, Siegel J, Šutta P, Mistrík J, Janíček P, Worsch P, Kolská Z: Annealing of gold nanostructures sputtered on glass substrate. Appl Phys A 2011, 102: 605–610. 10.1007/s00339-010-6167-1View Article
- Solliard C, Flueli M: Surface stress and size effect on the lattice-parameter in small particles of gold and platinium. Surf Sci 1985, 156: 487–494.View Article
- Švorčík V, Kvítek O, Lyutakov O, Siegel J, Kolská Z: Annealing of sputtered gold nano-structures. Appl Phys A 2011, 102: 747–751. 10.1007/s00339-010-5977-5View Article
- Zhao XQ, Wang TX, Liu W, Wang CD, Wang D, Shang T, Shen LH, Ren L: Multifunctional Au@IPN-pNIPAAm nanogels for cancer cell imaging and combined chemo-photothermal treatment. J Mater Chem 2011, 21: 7240–7247. 10.1039/c1jm10277jView Article
- Cobley CM, Chen JY, Cho EC, Wang LV, Xia YN: Gold nanostructures: a class of multifunctional materials for biomedical applications. Chem Soc Rev 2011, 40: 44–56. 10.1039/b821763gView Article
- Au L, Zheng DS, Zhou F, Li ZY, Li XD, Xia YN: A quantitative study on the photothermal effect of immuno gold nanocages targeted to breast cancer cells. ACS Nano 2008, 2: 1645–1652. 10.1021/nn800370jView Article
- Connor EE, Mwamuka J, Gole A, Murphy CJ, Wyatt MD: Gold nanoparticles are taken up by human cells but do not cause acute cytotoxicity. Small 2005, 1: 325–327. 10.1002/smll.200400093View Article
- Jain PK, Huang X, El-Sayed IH, El-Sayed MA: Review of some interesting surface plasmon resonance-enhanced properties of noble metal nanoparticles and their applications to biosystems. Plasmonics 2007, 2: 107–118. 10.1007/s11468-007-9031-1View Article
- Zhang HF, Liu RX, Sheng QL, Zheng JB: Enzymatic deposition of Au nanoparticles on the designed electrode surface and its application in glucose detection. Colloid Surface B 2011, 82: 532–535. 10.1016/j.colsurfb.2010.10.012View Article
- Žvátora P, Řezanka P, Prokopec V, Siegel J, Švorčík V, Král V: Polytetrafluorethylene-Au as a substrate for surface-enhanced Raman spectroscopy. Nanoscale Res Lett 2011, 6: 366. 10.1186/1556-276X-6-366View Article
- Švorčík V, Slepička P, Švorčíková J, Zehentner J, Hnatowicz V: Characterization of evaporated and sputtered thin Au layers on poly(ethylene terephtalate). J Appl Polym Sci 2006, 99: 1698–1704. 10.1002/app.22666View Article
- Švorčík V, Kolská Z, Luxbacher T, Mistrík J: Properties of Au nanolayer sputtered on polyethyleneterephthalate. Mater Lett 2010, 64: 611–613. 10.1016/j.matlet.2009.12.018View Article
- Řezníčková A, Kolská Z, Hnatowicz V, Švorčík V: Nano-structuring of PTFE surface by plasma treatment, etching, and sputtering with gold. J Nanopart Res 2011, 13: 2929–2932. 10.1007/s11051-010-0183-0View Article
- Švorčík V, Hubáček T, Slepička P, Siegel J, Kolská Z, Bláhová O, Macková A, Hnatowicz V: Characterization of carbon nanolayers flash evaporated on PET and PTFE. Carbon 2009, 47: 1770–1778. 10.1016/j.carbon.2009.03.001View Article
- Hodgman CD: Handbook of Chemistry and Physics. Chemical Rubber, Cleveland; 1975.
- Chopra K: Thin Film Phenomena. New York: Wiley; 1969.
- Švorčík V, Kotál V, Siegel J, Sajdl P, Macková A, Hnatowicz V: Ablation and water etching of poly(ethylene) modified by argon plasma. Polym Degrad Stabil 2007, 92: 1645–1649. 10.1016/j.polymdegradstab.2007.06.013View Article
- Siegel J, Řezníčková A, Chaloupka A, Slepička P, Švorčík V: Ablation and water etching of plasma-treated polymers. Radiat Eff Deffect S 2008, 163: 779–788. 10.1080/10420150801969654View Article
- Švorčík V, Řezníčková A, Kolská Z, Slepička P: Variable surface properties of PTFE foils. e-Polymers 2010, 133: 1–6.
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