Evolution of Self-Assembled Au NPs by Controlling Annealing Temperature and Dwelling Time on Sapphire (0001)
© Lee et al. 2015
Received: 18 August 2015
Accepted: 14 December 2015
Published: 24 December 2015
Au nanoparticles (NPs) have been utilized in a wide range of device applications as well as catalysts for the fabrication of nanopores and nanowires, in which the performance of the associated devices and morphology of nanopores and nanowires are strongly dependent on the size, density, and configuration of the Au NPs. In this paper, the evolution of the self-assembled Au nanostructures and NPs on sapphire (0001) is systematically investigated with the variation of annealing temperature (AT) and dwelling time (DT). At the low-temperature range between 300 and 600 °C, three distinct regimes of the Au nanostructure configuration are observed, i.e., the vermiform-like Au piles, irregular Au nano-mounds, and Au islands. Subsequently, being provided with relatively high thermal energy between 700 and 900 °C, the round dome-shaped Au NPs are fabricated based on the Volmer-Weber growth model. With the increased AT, the size of the Au NPs is gradually increased due to a more favorable surface diffusion while the density is gradually decreased as a compensation. On the other hand, with the increased DT, the size and density of Au NPs decrease due to the evaporation of Au at relatively high annealing temperature at 950 °C.
In this experiment, the size, density, and configuration of the self-assembled Au nanostructures were investigated with the variation of annealing temperature and dwelling time. At the beginning of the experiment, a 430-μm-thick sapphire (0001) substrate with off-axis ±0.1o from the iNexus Inc. (South Korea) was cleaved. After the RCA cleaning, the substrate was indium-bonded to an Inconel holder and introduced in a pulsed laser deposition (PLD) chamber for degassing at 350 °C for 1800 s under 1 × 10−4 Torr to remove the contaminants. Additional file 1: Figure S1 shows the Raman spectra of bare sapphire in the range between 154 and 1388 cm−1 excited by a continuous wave (CW) laser of 532 ± 1 nm wavelength at room temperature. Additional file 2: Figure S2 shows the AFM image and line profile of the bare sapphire (0001) and its crystal structure.
In this work, the effects of AT between 300 and 900 °C as well as the DT from 150 to 3600 s on the evolution of the Au NPs were systematically investigated by annealing the samples in a PLD system. After degassing, the samples were deposited on a 3-nm Au film, at a growth rate of 0.05 nm/s with the ionization current of 3 mA under the vacuum of 1 × 10−1 Torr in an ion-coater sputter. Subsequently, in order to study the AT effect with the fixed DA and DT, the sample temperature was systematically ramped up to each target temperature (300, 400, 500, 600, 700, 800, 850, 900, 950) in a PLD chamber at a ramping rate of 1.83 °C/s under 1 × 10−4 Torr with the computer-operated recipes, and then the samples were dwelt at the target temperature of 450 s to ensure the heat conduction. After that, the sample temperature was immediately quenched down to the ambient temperature to avoid further Ostwald ripening. In order to investigate the effect of the DT on the resulting Au NPs with the identical AT, diverse DTs of 150, 1800, and 3600 s were dwelt when the samples reached the target AT of 950 °C with the DA of 3 nm.
Characterization of Au Nanostructures and NPs
After the successful growth of each sample, the surface morphology was investigated utilizing an AFM. The AFM with a non-contact mode was used to characterize the sample surface morphologies. The tips used in the AFM has a height of 17 μm with a radius curvature of ~10 nm, a force constant of 40 N/m, and a resonant frequency of ~270 kHz. In order to minimize the tip effect on the measurement of the resulting Au NPs, the tips utilized were from the same batch. The acquired data were analyzed by XEI software (Park Systems) to investigate the size and density of observed Au NPs in terms of the AFM images, 2-D Fourier filter transform (FFT) power spectra, and cross-sectional line profiles. Also, an EDS system (Thermo Fisher Noran System 7) was employed to perform the elemental analysis under the vacuum. Additionally, the Raman spectrum of the bare sapphire was acquired by a charge-coupled device (CCD) detector excited by a CW laser with the wavelength of 532 ± 1 nm.
Results and Discussion
In this paper, the evolution of the self-assembled Au nanostructures and NPs were systematically investigated by the control of AT and DT. In general, below the 700 °C of AT, the evolution of the Au nanostructures was divided into three distinct regimes based on their configuration: the vermiform Au piles; irregular Au nano-mounds; and Au islands, and the evolution can be attributed to the enhanced surface diffusion due to the increased AT. When the AT was elevated to 700 °C, being provided with the sufficient thermal energy, the isolated Au islands evolved into the self-assembled Au NPs based on Volmer-Weber growth model due to the more favorable surface diffusion. In addition, with the increased AT between 700 and 900 °C, the size of the Au NPs was gradually increased accompanied with the decreased AD. On the other hand, with the increased DT from 150 to 3600 s, the Ostwald ripening showed a mild effect on the Au NPs evolution. The size and density of Au NPs were decreased with increased dwelling time which contradicts with the Ostwald ripening, may be due to the Au evaporation at relatively high annealing temperature (950 °C).
Financial support from the National Research Foundation of Korea (nos. 2011-0030821 and 2013R1A1A1007118) is gratefully acknowledged. This work reported in this paper was conducted during the sabbatical year of Kwangwoon University in 2015.
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