Sublimation of Ag nanocrystals and their wetting behaviors with graphene and carbon nanotubes Ruixue Lian a , Han Yu a , Longbing He a, b, * , Lei Zhang a, b , Yilong Zhou a , Xinyang Bu a , Tao Xu a , Litao Sun a, b, ** a SEU-FEI Nano-Pico Center, Key Lab of MEMS of MOE, Southeast University, Nanjing, 210096, PR China b Southeast University-Monash University Joint Research Institute, Suzhou, 215123, PR China article info Article history: Received 29 November 2015 Received in revised form 18 January 2016 Accepted 31 January 2016 Available online 9 February 2016 abstract The wetting and bonding behaviors between electrode nano-metals and carbon materials play an important role in dissimilar joining and solidification especially for graphene/carbon nanotube (CNT) based nano-devices. As an important indicator, the contact angle between them can provide explicit information for the understanding of their interfacial interactions. However, the conventional sessile drop method is still limited to the macroscale, and whether the wettabilities of materials at nanoscale will still follow the rules established based on their bulk counterparts remains unknown. In this paper, we show an in situ approach to investigate the sublimation and wetting behaviors of Ag nanocrystals on graphene/CNTs at elevated temperatures through a high-resolution transmission electron microscope. It is found that, i) Ag nanocrystals soften and sublimate at a lower temperature and possess a higher sublimation speed than theoretical predictions, ii) the intrinsic contact angle between Ag nanocrystals and graphene/CNTs at elevated temperatures mainly maintains a mean value of around 124  e125  , and iii) the contact angle decreases with the decrease of nanoparticle size when it is below 15 nm. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The wetting behavior between a metal and a dissimilar material at elevated temperatures plays an important role in many appli- cations and processes such as brazing, welding, solidification and composite processing [1e 7]. Their interfacial wettability, which is usually characterized as the contact angle, is generally measured by the sessile drop technique [8]. In macroscale and microscale, the sessile drop method can mostly obtain an accurate apparent con- tact angle of a liquid drop on a rigid substrate through analysis of their interfacial configurations with optical systems [8e14]. How- ever, when the sample dimension decreases into nanoscale, this technique is faced with great challenges. On one hand, the con- ventional optical systems can no longer directly measure the apparent contact angle at nanoscale interfaces due to their reso- lution limit. Although electron microscopy meets the resolution requirements, formation of an equilibrium condition of nano- objects inside its high-vacuum chamber, especially for volatile and high-melting-point materials, is still challenging. For example, in macroscale/microscale, formation of an equilibrium state of a molten Ag drop on a substrate for contact-angle measurement can be easily achieved in a heating furnace with different atmospheres [11]. However, this process becomes much more difficult in nano- scale. As reported by Sambles et al. [15], Ag nanoparticles with diameter of ~40 nm would completely evaporate in less than 10 min at 750  C in a TEM chamber (~10 5 Pa). Their dynamic shape variations as well as thermal drift during heating make it difficult to acquire the equilibrium configurations of the nanoscale interfaces. On the other hand, the drastically increased surface-to-volume ratio of nanometals crucially varies their diverse physical proper- ties at nanoscale [16e19]. It has been reported that noble metal nanoparticles with diameter of several to tens of nanometers possess distinct values of surface energy comparing to their bulk counterparts due to size and surface effects [20]. The contraction and reconstruction of their surface, as well as the thermal-induced oscillatory relaxation of the interlayers at elevated temperatures may generate significant influences on the nanoparticle surface energy and stability [21]. Besides, the melting points of nanometals, * Corresponding author. SEU-FEI Nano-Pico Center, Key Lab of MEMS of MOE, Southeast University, Nanjing, 210096, PR China. ** Corresponding author. SEU-FEI Nano-Pico Center, Key Lab of MEMS of MOE, Southeast University, Nanjing, 210096, PR China. E-mail addresses: helongbing@seu.edu.cn (L. He), slt@seu.edu.cn (L. Sun). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon http://dx.doi.org/10.1016/j.carbon.2016.01.105 0008-6223/© 2016 Elsevier Ltd. All rights reserved. Carbon 101 (2016) 368e376