FABRICATION OF ARRAY OF GOLD NANOPARTICLES THROUGH THERMAL DEWETTING AND FIB PATTERNING

5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT FABRICATION OF ARRAY OF GOLD NANOPARTICLES THROUGH THERMAL DEWETTING AND FIB PATTERNING Goswami A 1 *, Aravindan S 2, Rao P.V 3 1 Department of Mechanical Engineering, IIT Delhi, Delhi-110016 E- Mail: aj87.goswami@gmail.com 2 Department of Mechanical Engineering, IIT Delhi, Delhi-110016 E- Mail: aravindan@mech.iitd.ac.in 3 Department of Mechanical Engineering, IIT Delhi, Delhi-110016 E- Mail: pvrao@mech.iitd.ac.in Abstract This paper details an approach to generate an array of rectangular clusters of gold nanoparticles (Au NPs) on a substrate. A nano layer of gold is deposited through DC sputter coating on a cleaned glass substrate. The coated substrate is annealed in Ar atmosphere at 470 C for 30 minutes which results in breaking of the continuous gold film into individual nanoparticles. For the formation of rectangular clusters of gold nanoparticles, patterns are made on the substrate using focussed ion beam (FIB) machining. The end result is an array of rectangular clusters of Au NPs distributed uniformly over the substrate. Ordered clusters of Au NPs finds wide range of applications in surface enhanced raman spectroscopy (SERS), plasmonics, biological detection etc. Keywords: DC sputter coating, Thermal Dewetting, Gold Nano particle, Array of Gold Nanoparticles 1. Introduction Ordered array of Gold nanoparticles have wide range of applications in fields like plasmonic(fan et al.,2010, Maier et al.,2001), Surface enhanced fluorescence(zhang et al.,2012), magnetic memories, catalysis for growth of nanowire (Kodambaka et al.,2007) etc. Chemical process based on self-assembly and lithography based nano-structuring are the common routes taken for the fabrication of gold nanoparticles(schmid and Simon,2005). Array of nanoparticles with defined position at low throughput has been fabricated using electron beam lithography (EBL)(Lee et al.,2011). But EBL requires stringent processes and expensive facilities. Through the selfassembly methods, spatial control can be exerted on the formation and distribution of the nanoparticles. Dewetting is a simple and a common method used for the fabrication of nanoparticles, whose driving force is derived from the reduction of surface energy of the thin film and the interface energy between the film and the surface. Dewetting can be induced by a number of methods such as pulsed laser heating(favazza et al.,2006, Kondic et al.,2009), electron irradiation (Kojima and Kato,2008), ion irradiation(hu et al.,2000, Hu et al.,2002), and thermal annealing(phuc et al.,2014, Phuc et al.,2013, Tesler et al.,2013, Yoshino et al.,2011). Amongst these methods, thermal annealing is the easiest approach since it does not involve complex steps or elaborate setup and the process is economical. In this, first a layer of gold is deposited on an inert substrate (glass slide in this case) followed by annealing at a constant temperature for a specified period of time. The continuous layer of deposited gold breaks into discrete islands which turn into individual nanoparticles with progress of annealing. The process of thermal dewetting results in formation of particles having wide range of diameters and highly variable inter-particle spacing. In order to control the spatial distribution of the gold nano particles formed after the thermal dewetting, patterns are made on the dewetted film using FIB (Focussed Ion Beam). This can confine the formed nanoparticles in any geometry and location on the substrate wherever required. 2. Experimental methodology The cluster array fabrication process is explained in Figure 1. The chart explains the step by step procedure of the process flow. Glass slides were cleaned in an ultrasonic bath using acetone for 20 minutes, followed by drying under a blower. 15 nm of gold was deposited on the glass slides using Q150RS DC Sputter coater at a sputtering current of 20mA and pressure of 0.27bar. The deposition time took ~ 4minutes. After deposition, thermal dewetting of the film was carried out in a furnace. The temperature of the furnace was made stable at 470 C. Argon was pumped into the chamber at a flow rate of 10 litres per minute for maintaining an inert atmosphere. The glass slide was placed inside the furnace for duration of 30 minutes. 844-1

FABRICATION OF ARRAY OF GOLD NANOPARTICLES THROUGH THERMAL DEWETTING AND FIB PATTERNING The patterns were made using FIB. For milling of the thermally dewetted gold nanoparticles, the parameters had to be optimized first. extraction voltage and the dwell time was kept constant at 30 kv and 1 µs respectively, for all the cases. The film thickness is kept constant throughout. Figure 1 Flow chart of the cluster array fabrication process When the unit cell dimension was of the order of few microns the beam current required was higher as compared to lower unit cell dimensions. After making the patterns, the number of particles and the average diameter of the particles is calculated using Image J software. The SEM images of the various patterns, their dimension, FIB process parameters for their milling, the number of particles, and average diameter of the particles and their range of distribution were evaluated. 3. Results and Discussions Figure 2(a) shows the sputter deposited film which is having good continuity. Figure 2(b) shows the film after thermal dewetting which results in breaking of the film into individual particles. Nanoparticles formed in this case are random. Nanoparticles are useful in plasmonic applications such as plasmonic LEDs (Yeh et al.,2008), plasmonic biosensors,(brolo,2012) plasmonic solar cells (Fang Liu,2011) etc. Gold nanoparticles are also useful in biological imaging (Mi Jung,2011), DNA detection, photo thermal therapy (Huang et al.,2008) etc. Most the applications require the gold nanoparticles to be arranged in a desired configuration. The size of the configuration is of the order of few microns at the most. In order to arrange the randomly distributed gold nanoparticles after thermal dewetting, FIB patterning is done. After number of iterations it is found that when the unit cell sizes is of the order of several microns, comparatively a higher beam current of 30pA is to be used, while for smaller unit cell sizes of 1 micron and below, the current should be reduced to 10pA. The Figure 2 (a) As deposited gold film (b) Thermally dewetted film (c) Gold nanoparticles The particle size is calculated using Image J software. An area is defined for every pattern on the image after the thermal dewetting process. The particle size is calculated within the defined area. SEM images of the arrays are shown in Table 1 for different unit cell size. The range of variation and the average value for the particle size and number of particles formed after thermal dewetting are reported for each of the cases. The number of particle per unit cell, decreases with reduction in cell size which is expected, but an interesting observation is the reduction in average particle size with decrease in the cell size. A unit cell consists of 2 kinds of particles: the nanoparticles which are away from the boundary and their size is same as that of the thermally dewetted one and the nanoparticles which are located at the unit cell boundary. Due to the patterning by the FIB some of the nanoparticles located at the unit cell boundary gets milled and their size is reduced. When the cell size is higher, the number of unmilled particles inside the unit cell is much higher as compared to the milled particles at the boundary. Thus the average particle size is determined mostly by the unmilled particles which are having larger size and it has resulted in a higher value of average particle size. With the reduction in cell size, the number of particles inside the cell decreases and the number of milled particles at the unit cell boundary becomes 844-2

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th 14th, 2014, IIT significant and that has resulted in a decrease in the average particle size. Table 1 SEM image and calculated particle details for different unit cells Sl. No. 1. SEM image of the pattern FIB Process parameters 30pA Particle details in an unit cell Number of particles : 1490; Average particle size : 56nm; Maximum particle size : 133nm; Minimum particle size: 35nm Unit cell size : 5X5 µm2 10pA 2. Number of particles :82; Average particle size : 44nm; Maximum particle size : 138nm; Minimum particle size: 11nm Unit cell size : 1X1µm2 3. 10pA Number of particles : 21; Average particle size : 36nm; Maximum particle size : 61nm; Minimum particle size: 14nm Unit cell size : 500X500nm2 4 16kV 4pA Unit cell size : 250X250nm2 Number of particles : 8; Average particle size : 17nm; Maximum particle size : 35nm; Minimum particle size: 8nm 844-3

FABRICATION OF ARRAY OF GOLD NANOPARTICLES THROUGH THERMAL DEWETTING AND FIB PATTERNING The variation of average particle size with unit cell dimension is shown in Figure 3. It is observed from the SEM image of the 250X250 nm 2 unit cell, the number of whole particles in the unit cell is very less. Mostly the particles are milled during FIB. This imposes a limitation on the smallest geometrical feature in which the nanoparticles can be confined. The nanoparticles after dewetting can be confined in any typical configuration. Figure 3 Typical varation of average particle size The square and circular configurations of nanoparticles are presented in Figure 4. Figure 4: Nanoparticles arranged in a (a) Square configuration (b) Circular configuration 4. Conclusions Thermal dewetting of sputter deposited film leads to formation of nanoparticles, but the distribution of the nanoparticles is random throughout the substrate surface. By making patterns on the thermally dewetted film the location of the particles can be effectively controlled. Four patterns with different unit cell dimensions viz. 5X5µm, 1X1µm, 500X500nm and 250X250nm, were milled. Decrease in unit cell dimension decreases the number of particles and the average diameter of the particles reduces. The nanoparticles at the boundary of the unit cell get milled by the FIB during patterning and thus their size is reduced as compared to their size after thermal dewetting. When the unit cell dimension is low, the contribution of such particles to the average particle size is appreciable and hence a decrease in the average particle size with reduction in unit cell dimension is observed. At unit cell of 250X250 nm, the particles inside the cell is very less and for the used thickness of thermally dewetted film (15nm), it imposes a limitation on the minimum feature size within which the nano particles can be confined. FIB patterning on thermally dewetted film is an efficient approach for generation of micron sized clusters of Au nanoparticle arrays with uniform spatial distribution. References Brolo, A. G. (2012), Plasmonics for future biosensors, Nature Photonics, Vol. 6, pp. 709 713. Fan, J. A., Wu, C., Bao, K., Bao, J., Bardhan, R., Halas, N. J., Manoharan, V. N., Nordlander, P., Shvets, G. and Capasso, F. (2010), Self-Assembled Plasmonic Nanoparticle Clusters, Science, Vol. 328, pp. 1135-1138. Fang Liu, D. Q., Qi Xu, Wanlu Xie, and Yidong Huang (2011), Plasmonic Enhanced Light Absorption of Solar Cells with Metal Nanoparticles, Renewable Energy and the Environment Technical Digest, Vol. pp. JWE11.pdf 1-3. Favazza, C., Trice, J., Krishna, H., Kalyanaraman, R. and Sureshkumar, R. (2006), Laser-induced short- and long-range orderings of Co nanoparticles on SiO2, Applied Physics Letters, Vol. 88, pp. -. Hu, X., Cahill, D. G. and Averback, R. S. (2000), Nanoscale pattern formation in Pt thin films due to ionbeam-induced dewetting, Applied Physics Letters, Vol. 76, pp. 3215-3217. Hu, X., Cahill, D. G. and Averback, R. S. (2002), Burrowing of Pt nanoparticles into SiO2 during ionbeam irradiation, JOURNAL OF APPLIED PHYSICS, Vol. 92, pp. 3995-4000. Huang, X., Jain, P., El-Sayed, I. and El-Sayed, M. (2008), Plasmonic photothermal therapy (PPTT) using gold nanoparticles, Lasers in Medical Science, Vol. 23, pp. 217-228. 844-4

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