Self-assembly of Silver Nanoparticles and Multiwall Carbon Nanotubes on Decomposed GaAs Surfaces
© The Author(s) 2010
Received: 2 June 2010
Accepted: 13 July 2010
Published: 25 July 2010
Atomic Force Microscopy complemented by Photoluminescence and Reflection High Energy Electron Diffraction has been used to study self-assembly of silver nanoparticles and multiwall carbon nanotubes on thermally decomposed GaAs (100) surfaces. It has been shown that the decomposition leads to the formation of arsenic plate-like structures. Multiwall carbon nanotubes spin coated on the decomposed surfaces were mostly found to occupy the depressions between the plates and formed boundaries. While direct casting of silver nanoparticles is found to induce microdroplets. Annealing at 300°C was observed to contract the microdroplets into combined structures consisting of silver spots surrounded by silver rings. Moreover, casting of colloidal suspension consists of multiwall carbon nanotubes and silver nanoparticles is observed to cause the formation of 2D compact islands. Depending on the multiwall carbon nanotubes diameter, GaAs/multiwall carbon nanotubes/silver system exhibited photoluminescence with varying strength. Such assembly provides a possible bottom up facile way of roughness controlled fabrication of plasmonic systems on GaAs surfaces.
Recently, metals have been intensively studied beyond their reflecting nature by exploiting the collective oscillations of their free electrons in the field called plasmonics [1, 2]. The interaction between free electrons and light, inducing resonant modes, refer as plasmons. Well-established techniques have been realized enabling the development of structures to fully exploit these modes [3–7]. Depending on the properties of the supporting structure, plasmons can be broadly divided into two categories: surface and localized surface plasmon resonance (LSPR). Among others, individual colloidal particles with different shape, size and aggregation determine the LSPR modes [8, 9]. For example, by simply adjusting the size and the shape of the metallic nanoparticles, the frequency at which the localized surface plasmon resonance occurs can be tuned from ultraviolet to infrared region of the electromagnetic spectrum [10–12]. GaAs surfaces have been utilized as high index matrices to lower the metallic localized surface resonance frequencies to near-infrared for telecommunication applications [13, 14]. Arsenic (As) precipitates on GaAs surfaces were found to cause strong damping of the surface plasmon modes and, providing absorption of wavelengths longer than 1.5 μm . Fractal aggregates [15, 16] of nanoparticles where local fields are concentrated can lead to interesting optical phenomena and applications such as harmonic generation, Kerr effect  and surface-enhanced Raman scattering (SERS) [9, 18–22]. Experimental studies on plasmon resonance of silver (Ag) nanoparticles on highly ordered pyrolytic graphite (HOPG) , diamond-like carbon  and fullerene C70 matrices  all lead to fruitful clues of the possible effects of multiwall carbon nanotubes (MWCNTs) in plasmonic system. In addition to the well-known collective excitations of the π-band electrons in MWCNTs [26, 27], their incorporation in any system can be exploited to design hybrid plasmonic system, either by functionalization of Ag nanoparticles on MWCNTs  or by direct filling of MWCNTs with Ag nanoparticles .
Motivated by the respective advantages of adjusting size and shape of the Ag nanoparticles, the benefit of using GaAs as high index matrix and coupling with MWCNTs, we report on a novel system which considers three components: A decomposed GaAs surface, MWCNTs and Ag nanoparticles. Although, the self-assembly of metallic Ag nanoparticles on smooth surfaces has been extensively studied and well known, the co-growth or coupling of Ag and MWCNTs on rough GaAs decomposed surfaces is yet to be explored. The topics of particular importance addressed in this work are the morphology of MWCNTs/Ag grown on the decomposed GaAs surfaces, the role of solvent evaporation on the growth mechanism of the MWCNTs/Ag nano-patterns and to some extent photoluminescence properties of the GaAs/MWCNTs/Ag system.
Samples from commercially available single crystal GaAs (100) wafers were employed as source of substrates. The substrates were cleaned with acetone and ethanol followed by etching using a mixture of sulfuric acid (H2SO4), hydrogen peroxide (H2O2) and water. After etching, the substrates were immediately loaded into the ultra-vacuum preparation chamber and degassed for 48 h at temperature of 200°C prior to an annealing process at 600°C for 30 min. The temperature was measured using an optical pyrometer with an accuracy of ±25°C. Each GaAs substrate was investigated along (110) direction to allow surface structure observations by reflection high energy electron diffraction (RHEED).
Ag nanoparticles and MWCNTs were used as the starting materials to prepare the spin-coated samples. Ag nano-particles with average size of 30 nm and purity of 99.99% were purchased from Xuzhou Hongwu Nanometer Material Co., ltd China. MWCNTs with outer diameter of 8 nm and purity above 95% were supplied by Chengdu Organic Chemicals Co., ltd China. Colloidal suspension of particles (CSP) were prepared with varying concentration of silver and MWCNTs in toluene. The solutions were ultra-sonicated for 15 min, deposited drop wise on the GaAs substrates and spin coated at 8,000 rpm for 30 s. Some of the spin-coated samples were annealed for 30 min at 300°C in a controlled environment.
Surface morphology of the spin-coated samples on the GaAs substrates was investigated by a Nanoscope V Multimode Atomic Force Microscopy (AFM) provided by Veeco Instrument. To minimize the sample damage, AFM imaging was performed in tapping mode, and observations were made with the AFM instrument placed on vibration isolation VT-102 table in order to minimize the effects of vibration. The measurements were taken at well-stabilized room temperature to avoid thermal drift. During the imaging, both the scan rate and the imaging resolution were set at 0.2 Hz and 512 pixels, respectively. Ultra-high resolution tungsten tips purchased from Micro Masch with tip radius <1 nm and back side coated with aluminum were used for the AFM experiments. All tips had a force constant of 75 N/m and a resonance frequency of 400 kHz.
Photoluminescence (PL) measurements were taken at room temperature. The excitation of the samples was achieved by a YAG laser operating at 532 nm owing to a frequency doubler (the natural emission of the YAG laser occurs at 1,064 nm). Using a frequency trippler and appropriate external filters, we extract the third harmonic (355 nm) which is the main PL exciting component, although a weak trace of the second harmonic (532 nm) remains present in the beam. The diameter of the laser spot was 800 μm, and the incidence angle of the incoming laser beam was 45°. The luminescence, collected by spherical mirror, was analyzed in a 5-nm-resolution spectrometer. The spectrometer integrates the PL signal during 5 s. Within this interval of time, the sample is hit by the pulsed laser with a rate of 20 kHz, and this is done 105 times. We collect thus 105 PL emissions and so the signal is collected even if it has a very short lifetime.
Results and Discussion
GaAs Surface Decomposition and Its Subsequent Dewetting by Toluene
Assembling of MWCNTs and Ag Nanoparticles on the GaAs Decomposed Surface
Assembling of MWCNTs on the GaAs decomposed surface is shown in Fig. 2b. A MWCNTs net-like pattern similar to that of toluene can be clearly seen. The image shown was attained after desorption of toluene at 300°C for 30 min. Hence, the observed features are only attributed to the MWCNTs with average thickness of 8 nm and the decomposed surface. This is well reflected from the high-resolution 3D image shown in Fig. 2c.
Decomposition Effects on the Assembly of MWCNTs and Ag Nanoparticles
with A,B,C being parameters involved in describing the RMS roughness and the correlation length in wide frequency range. The PSD profile for the GaAs/Toluene system shown in the inset of Fig. 5b is indeed well fitted with this model.
A Proposed Model for the Growth of GaAs/MWCNTs/Ag System
This contribution shade light on the GaAs/MWCNTs/Ag sample preparation and its self-assembled structures. PSD data analysis suggested strong fractal effects of the decomposed surfaces on the successive MWCNTs and toluene layers. Depending on the MWCNTs diameter, the GaAs/MWCNTs/Ag system exhibits enhanced photoluminescence when MWCNTs diameter is 8 nm, while quenching and blue shift are dominant for the higher diameter (≈50 nm) MWCNTs. However, possible contribution of As-platelets or Ag-oxide to the PL needs further investigation. In addition, comprehensive and quantitative picture is yet to emerge to understand the physics of interaction between different components in the system. In particular, issues such as the impact of the decomposed surface and the MWCNTs diameter PL signal dependent mechanism deserved more attention.
Author Revathy K. P is grateful to Department of Physics, College of Science, Sultan Qaboos University, Oman for providing the experimental facilities and other technical assistance. In addition, Amna Rashid Al-Azri is acknowledged for her AFM measurements contribution.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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