On-substrate, self-standing Au-nanorod arrays showing morphology controlled properties

doi:10.1016/j.nantod.2010.11.001

Nano Today

Volume 6, Issue 1, February 2011, Pages 12-19

https://doi.org/10.1016/j.nantod.2010.11.001 Get rights and content

Summary

We use supported alumina templates and electrodeposition to fabricate self-standing Au-nanorods (Au-NR) arrays. Depending on electrolyte and deposition conditions two different NR morphologies with either corrugated or smooth topologies are fabricated. We show that the properties of the NR arrays, including amphiphilic and optical, very much depend on their morphology. Smooth NR arrays are hydrophobic while the corrugated ones are hydrophilic. Also the optical reflectance, though it shows plasmon resonances at very similar wave lengths, is much lower for the corrugated NR arrays. The activity of the NR arrays as substrates for molecular detection using Raman scattering, and Rhodamine 6G (R6G) as model dye, also strongly depends on their morphology. R6G concentrations down to 1 pM are detected on the corrugated arrays yielding an effective enhancement factor (EF) of approximately 1 × 10⁹. In contrast an over-estimated (because of their hydrophobic character) EF of 6 × 10⁶ is obtained for the smooth NR arrays.

Graphical abstract

Research highlights

▶ We grow on-substrate gold nanord (NR)arrays with smooth and corrugated morphologies. ▶ We show morphology dependent amphiphilic properties. ▶ Smooth nanorods are hydrophobic. Corrugated nanorods are hydrophilic. ▶ We examine morphology effects on SERS activity. ▶ SERS enhancement factor of corrugated NRs is 3 orders of magnitude higher than that of smooth NRs.

Introduction

Porous-template based processing of nanostructures allows various critical parameters to be controlled to a high degree as the template moulds the nanostructure via its pore ordering, pore size, length and gap that are transferred to it and so determine its properties. A class of nanostructured materials that are now being intensively investigated is noble metal nanostructures as they are most promising for a wide range of applications, including nanobiosensing, molecular detection, catalysis and gas sensing and recently even hyperthermal cancer treatment [1], [2], [3], [4], [5], [6], [7], [8], [9]. Beyond chemical methods that were widely used for the processing of various noble metal nanostructures in solution [1], [2], [3], [4], [5], [6], [7], [8], [9], electrochemical deposition of nanowires (NW), nanorods (NR) and nanotubes (NT) into porous templates certainly offers more freedom for nanostructure control and materials combination, and is additionally versatile and cost-effective. For these reasons electrodeposition was used for the fabrication of nanowires of various metals [10], [11], [12], [13], [14]. However, usual templates may pose serious problems due to their fragility, and thus difficulty of handling, and more importantly their very high aspect ratios of at least 50. Exposure of the nanostructures, e.g. for device fabrication etc., requires dissolving of the membrane which, for membranes with high aspect ratio pores, could result in the collapse of the nanostructures, and the sticking together of the nanowires forming big bundles. Often the character of the nanostructure is lost and with it the suitability for such applications as in sensing, e.g. optical and nanobiological. This constitutes the main draw-back of these “classical” templates. A relatively new approach takes advantage of on-substrate templates that consist of an anodized aluminum film of a thickness ranging between 500 nm and 1 μm. In general, the aluminum film is deposited on silicon, indium–tin oxide (ITO) coated glass wafer or flexible substrates that have been pre-coated with a thin adhesion layer (few nm) and a gold (few nm) electrode [14], [15], [16], [17], [18]. Anodization, pore opening and barrier-layer etching are performed in a similar way to bulk aluminum. These kinds of supported templates were successfully used in few reports for the processing of magnetic nano-hetero-structures, Au-NRs and Ni-NWs [14], [15], [16], [17], [18].

In the present work, self-standing Au-NRs were processed using on-substrate anodic aluminum oxide (AAO) templates and two different electrolytes and deposition conditions. While a number of studies were performed on the electrodeposition of Au-nanostructures into different templates [16], [19], [20], [21], little is known about the effects of process parameters on the microstructure development and ensuing properties. In this paper, we show that morphology, when it is controlled via a proper choice of deposition conditions, keeping otherwise all other parameters, i.e. pore diameter, length and gap, constant, imparts completely different properties. This should in turns offer a valuable tool for an advance control of the properties of Au-NRs.

Section snippets

Template preparation

For substrate preparation, aluminium film was deposited on silicon wafers that had been pre-coated with a thin titanium adhesion layer (a few nm) and gold (tens of nm) underlayer. The subsequent deposition of different layers was carried out via DC-magnetron sputtering (Plassys MP 500) equipment without breaking the vacuum. The anodization was conducted in oxalic acid solution (0.3 M) under constant cell potential and monitoring with a self-written program in Igor Pro. Pore widening and

Microstructure

The Au-NR array that ensues from the first process (NRs-P1) shows specific features displayed in Fig. 1(a and b). While the corrugated character of the NR-tips constitutes at first their most salient feature, the morphology of their inner-parts, as a further salient feature, could only be revealed after mechanical damage to the array. Fig. 1(c) clearly shows that the inner-parts of the NRs are composed of nanoparticle agglomerates. The NR-surfaces seem to be quite smooth so that it may be

Conclusions

Self-standing Au-NR arrays of approximately 500 nm length, 80 nm diameter and 100 nm gap were processed using electrodeposition into supported AAO template films. Two different morphologies were obtained depending on electrolyte and deposition conditions. A chloride containing electrolyte and high deposition voltages resulted in corrugated NRs that appeared to be made of Au-nanotubes filled with Au-nanoparticles. In contrast, a commercial plating electrolyte and pulsed voltage did lead to smooth

Acknowledgement

Financial funding of this work was provided by the “Deutsche Forschungsgemeinschaft (DFG)”, grant #Es119/9-1.

Salah Habouti received his Diploma in Physics at the Christian-Albrechts-University, Kiel, where he worked on photoemission and charge–density waves.

At IMST he has being working on ferroelectric and multiferroic thin films and focuses now for his Ph.D. study on processing and application of nanomaterials. Salah Habouti has authored or co-authored 14 papers in peer reviewed journals.

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Salah Habouti received his Diploma in Physics at the Christian-Albrechts-University, Kiel, where he worked on photoemission and charge–density waves.

Mária Mátéfi-Tempfli received her M.Sc. degree in 1991 from University of Oradea. She is doing research in the field of nanomaterials synthesis and nanostructures fabrication. She possesses more than 28 papers published in peer reviewed journals and contributed to numerous international conferences. Her current research interests include template synthesis of different organic, inorganic and composite nanomaterials and nanostructures. She is particularly interested in the implication of nanomaterials in the field of sensing and bio-sensing as well as in medical applications.

Claus-Henning Solterbeck received his Diploma in Physics from the Christian-Albrechts-University, Kiel, where he continued his research in the field of theoretical surface physics on numerical photoemission. He joined IMST in 2001 where he is since senior scientist and mainly involved in establishing characterization methods of multifunctional materials and thin films. He has authored or co-authored 55 papers in international peer reviewed journals.

Martha Es-Souni received her Diploma (DEA) in Molecular Biology from the University of Strasbourg, France, and her Ph.D. from the University of Hamburg. She is currently senior scientist at the Clinic of Orthodontics, Faculty of Dentistry, Christian-Albrechts University, Kiel. Her research interests are on shape memory alloys, their applications and interactions with live tissues, development of bactericidal coatings and nanomaterials. She authored 20 papers in peer reviewed journals.

Stefan Mátéfi-Tempfli obtained M.Sc. degree in 1985 at the Institute of Technology Cluj and in 1996 at University of Oradea. He received his Ph.D. from the University of Pitesti in 2001. Actually he is Senior Lecturer at University of Namur, Belgium. He possesses more than 120 scientific outputs and holds 3 patents. He has a broad research interest covering multiple domains of physics, chemistry, electronics, medicine, etc. with particular focus on nanomaterials synthesis, especially by template methods, their integration in classical micro- and nano-processing ones and on the investigation of their physical and chemical properties. He is particularly interested in the implication of nanomaterials into various fields of the industry, energy sector, medicine and healthcare.

Mohammed Es-Souni received his “Doctorat es Sciences” in solid state physics at the University of Strasbourg, Institut le Bel, France, where he was working on segregation phenomena at metal surfaces. He subsequently worked with GKSS-Research Center, Institute of Materials Research, Geesthacht (near Hamburg) on the development and characterization of high-temperature titanium aluminides materials. In 1993 he moved to Kiel University of Applied Sciences, where he is since professor for Materials Engineering and director of the Institute of Materials and Surface Technology (IMST). His research interests encompass the broad range of multifunctional materials (ferroelectrics, magnetoelectrics, shape memory alloys), functional coatings and nanomaterials, processing and applications. Mohammed Es-Souni acted as principal editor for the Journal of Materials Research (MRS) and is general chair of the series workshop “International Workshop on Smart Materials & Structures”. He published more than 120 papers in peer reviewed journals and holds 12 patents.

¹: These two authors did their contribution to this work while they were at UCL.

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Rapid communicationOn-substrate, self-standing Au-nanorod arrays showing morphology controlled properties

Summary

Graphical abstract

Research highlights

Introduction

Section snippets

Template preparation

Microstructure

Conclusions

Acknowledgement

Sens. Actuators B: Chem.

Nano Today

J. Adv. Res.

Biosens. Bioelectron.

J. Colloids Interface Sci.

Mater. Chem. Phys.

Curr. Opin. Colloid Interf. Sci.

Coord. Chem. Rev.

Electrochim. Acta

Langmuir

Chem. Soc. Rev.

Science

Science

Science

Adv. Mater.

J. Mater. Chem.

Nano Lett.

Adv. Mater.

Nano Lett.

ACS Nano

J. Phys. D: Appl. Phys.

J. Phys. Chem. B

Langmuir

J. Phys. Chem. B

Rapid communication
On-substrate, self-standing Au-nanorod arrays showing morphology controlled properties