An in situ study on the coalescence of monolayer-protected Au-Ag nanoparticle deposits upon heating
© Song et al.; licensee Springer. 2014
Received: 10 June 2014
Accepted: 15 August 2014
Published: 27 August 2014
The structural evolution of thiolate-protected nanoparticles of gold, silver, and their alloys with various Au/Ag ratios (3:1, 1:1, and 1:3) upon heating was investigated by means of in situ synchrotron radiation X-ray diffraction. The relationships between the coalescence and composition of nanoparticles, as well as the surfactant reactions, were clarified. Experimental results show that there existed a critical temperature ranging from 120°C to 164°C, above which the tiny broad X-ray diffraction peaks became sharp and strong due to particle coalescence. The coalescence temperatures for alloy nanoparticle deposits were clearly lower than those for pure metals, which can be ascribed to the rivalry between the thermodynamic effect due to alloying and the interactions between surface-assembled layers and the surface atoms of the nanoparticles. The strong affinity of thiolates to Ag and thus complex interactions give rise to a greater energy barrier for the coalescence of nanoparticles into the bulk and subsequent high coalescence temperature. The influences of particle coalescence on the optical and electrical properties of the nanoparticle deposits were also explored.
KeywordsSelf assembled monolayer Alloyed nanoparticle Coalescence In situ synchrotron radiation X-ray diffraction
One of the important applications of nanomaterials metallic nanoparticles (NPs) is to manufacture fine-pitch electrical line patterns for organic transistors, radio frequency identification (RFID) antennas, or ultra- large- scale integration (ULSI) interconnections not only because of the high electrical conductivity and flexibility in handling, but also the low processing temperature [1, 2]. The reduced processing temperature is due to the large surface-to-volume ratio of the particles leading to a dramatic lowering of the melting point and sintering transition. Unlike copper NPs which suffer from easy oxidation, the NPs of noble metals, gold and silver, are stable and widely used in the aforementioned interconnect applications [3–9]. However, the price of gold is high, while silver tracks are plagued by electrochemical migration. Strategies such as alloying and core-shell structure have been proposed to achieve better performance. Nanoalloys of gold and silver metals, which have attracted much attention due to high catalytic activities and unique optical properties [10–13], exhibit essentially identical lattice constants and are completely miscible , presenting new opportunities for the development of interconnect materials [15–17].
With respect to ligand-protected NPs, the protect shell must be thermally or chemically eliminited, and the NPs need to join together to form continuous conductive networks in order to generate electrical conductance . Coalescence of gold nanoparticles has been studied by means of simulation, surface plasmon resonance absorption, and thermogravimetric analysis [18–21]. Recently, synchrotron X-ray radiations, powerful probing sources to study the structural, physical, and chemical properties of nano-materials , were applied to study the morphological and phase transitions of NP deposits [23, 24]. Using synchrotron radiation X-ray diffraction (SR-XRD) and small-angle X-ray scattering (SAXS), Ingham et al. proposed the mechanisms of coalescence; in sequence, they are desorption or melting of the capping ligands, aggregation of nanocrystals, necking of particles, and subsequent grain growth. However, there is still a lack of insight regarding the alloying effect on the coalescence of NPs.
In this report, a real-time and systematic study into the coalescence of binary gold-silver alloy NPs was performed. The phase evolution upon heating of thiol-protected NPs of gold, silver, and their alloys with various Au/Ag ratios (3:1, 1:1, and 1:3) was monitored by synchrotron radiation XRD. The interactions between ligands and surface atoms of alloy NPs as well as their influence on the coalescence and related properties were investigated.
The preparation of the octanethiolate-stabilized gold-silver alloy nanoparticles followed a modified two-phase protocol proposed by Murray , which has been described in a previous work . The nanoparticles were synthesized with varying initial Au/Ag molar ratios (0:1, 0.25:0.75, 0.5:0.5, 0.75:0.25, and 1:0) and designated as Au, Au3Ag, AuAg, AuAg3, and Ag, respectively.
The UV-visible spectra of the nanoparticle solutions were measured by a spectrophotometer (Varian Cary 100 UV-Visible spectrometer, Palo Alto, CA, USA) with a 10-mm quartz cell. A transmission electron microscope (FEI-TEM, Philips Technai G2, Amsterdam, Netherlands) with an accelerating voltage of 200 kV was used to observe the morphology of the NPs and the particle size was measured using Scion Image 4.0.2 image analysis software.
NPs were suspended in tolune solvent with the proportion of 20% by weight. The suspensions were dropped on 5 × 5 mm2 Si wafers with a native oxide layer on the surface, which were ultrasonically cleaned in alcohol and acetone, and then dried in an oven at 50°C for 30 min. Each NP deposits/substrate combination was prepared by pipetting NPs suspensions (approx. 30 ± 0.9 μL) onto the substrates with subsequent spin-coating at 500 rpm for 3 s and then 2,000 rpm for 15 s. In situ high-temperature synchrotron radiation X-ray diffraction (SR-XRD) was performed at the wiggler beamline BL-17B1 of the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The incident X-rays were focused vertically by a mirror and monochromatized to 8 keV (λ = 1.5498 Å) by a Si(111) double-crystal monochromator. In this experiment, two pairs of slits positioned between sample and detector were used, which provided the typical wave vector resolution in the vertical scattering plane of about 0.003 nm-1. The temperature-dependent XRD patterns of all the samples were collected on a resistive heating copper stage at a heating rate of 5°C/min in air. To minimize the collection time, the patterns were collected only in the 33° to 43° 2θ range back and forth at a scan rate of 5°/min and the evolution of the diffraction peaks was monitored simultaneously. The surface morphology observations were performed by scanning electron microscopy (SEM, JEOL JSM-6460, Akishima-shi, Japan). The chemical valence states of the elements on the surface of the NP deposits were examined using X-ray photoelectron spectroscopy (XPS) with Al sources.
To evaluate the electrical performance of the NP deposits, four-point probe measurement of the deposit resistivity after being heated to different temperatures was performed. The corresponding optical absorption properties were also examined using a UV-vis spectrophotometer.
Results and discussion
Characteristics of nanoparticles
Phase transition of nanoparticle deposits upon heating
Optical and electrical properties of nanoparticle deposits subjected to heating
Electrical resistivity of the NP deposits
1.75 × 103
2.5 × 103
3.75 × 103
Factors affecting the coalesence of the thiol-protected AuAg nanoparticles
where G(s) is the mole free energy of solid phase, Λ1 is the latent heat of component 1, Λ2 is the latent heat of component 2, N2 is the mole fraction of component 2, and T is the equilibrium temperature of an alloy. Accordingly, the solid-liquid transition temperature in the gold-silver binary system decreases with an increasing silver fraction, and thus, it can be inferred that the coalescence temperature follows the same tendency due to alloying, as marked in the lower left circle (at the low silver side) in Figure 11a.
As to the ascending coalescence temperature at the high silver side, we should consider the ligand shells on the particle surface and their influence on coalescence kinetics, as marked in the lower right square in Figure 11a. A study on ionic monolayer-protected nano-Au and nano-Ag inks by Anto et al.  proposed that the coalescence temperature of nanoparticles is not determined by the thermodynamic size melting or by the surface area effect, as previously thought, but by the temperature when a large portion of the dense monolayer is eliminated. In other words, the coalescence temperature depends on the thermal stability and packing density of the shell, rather than the size of the metal core. As reported, the sulfur of octanethiol on Au NPs thermally decomposed at elevated temperatures and the amount was reduced to half of the initial value when heating to around 125°C . This explains why the coalescence of octanethiolate-protected NPs can occur at a low temperature of 120°C.
where R - S- is the absorbing species, thiolates in this case, and M represents Au or Ag. Ulman suggested that thiolate monolayers on Ag(111) are more densely packed due to the shorter S…S distance (4.41 Å for Ag(111) and 4.97 Å for Au(111)) . If we take alkanethiolates for example, there are two possible bonding locations for thiolates on Ag(111), i.e., hollow sites and on-tope sites, while thiolates can only be bonded at the hollow sites in the case of Au(111). As illustrated in Figure 11b, it can be deduced that the strong affinity of thiolates for Ag and thus complex interactions gives rise to a greater energy barrier (ΔG*) for the coalescence of nanoparticles into the bulk and subsequent high colescence temperature.
In this study, the evolution of thiolate-protected binary gold-silver NP deposits with a wide compositional range upon heating in air was studied via in situ synchrotron radiation X-ray diffraction and the characteristics of NP deposits before and after heating were investigated. Particle coalescing can be revealed by the sudden intensification of the diffractions, and the coalescence temperature for alloy nanoparticle deposits are clearly lower than those for pure metals. It is suggested that the coalescence of nanoparticles strongly depends on the rivalry between the thermodynamic and kinetic factors, which are respectively due to alloying effect and the ligand/surface atom interactions. Subjected to annealing, gold-silver alloy NP deposits exhibit low electrical resistivity and the ability to avoid abnormal grain growth, showing the great potential as interconnect materials.
JMS is a professor with Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan. IGC is a Professor with Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan. WTC and KHH are former graduate students supervised by JMS. THK is a former graduate student supervised by IGC and JMS. HYL and SJC are researchers with National Synchrotron Radiation Research Center, Hsinchu, Taiwan.
This work was supported primarily by National Science Council of R.O.C. through contracts No. NSC101-2120-M-006-003 and No. NSC 101-2120-M-006-007-CC1, from which the authors are grateful.
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