1903391 (1 of 21) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com REVIEW Ga-Based Liquid Metal Micro/Nanoparticles: Recent Advances and Applications Hyunsun Song, Taekyung Kim, Seohyun Kang, Haneul Jin, Kwangyeol Lee,* and Hyo Jae Yoon* H. Song, T. Kim, S. Kang, H. Jin, Prof. K. Lee, Prof. H. J. Yoon Department of Chemistry and Research Institute for Natural Sciences Korea University Seoul 02841, Republic of Korea E-mail: kylee1@korea.ac.kr; hyoon@korea.ac.kr The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201903391. DOI: 10.1002/smll.201903391 room temperature could be tuned below 0 °C. In particular, EGaIn has been used to replace solid metal contacts in electronic devices due to its conductivity in the liquid state. Unlike solid metal electrodes, EGaIn could function as a nondamaging contact in large-area junctions. [3] Furthermore, its rheological characteristics [4] prohibit EGaIn from flowing out of a micro- channel, thus allowing EGaIn to function as a soft electrode mold in large-area junc- tions. [5] A conical micrometer-scale tip of EGaIn, shown in Figure 1, is the most widely exploited morphology of EGaIn as an electrode in large-area tunneling junc- tions and requires only a microsyringe and micromanipulator to be reproducibly fabricated in the micrometer regime. In addition to the use of LM in electronics, micro/nano-sized LMs have recently been shown to have poten- tial in drug delivery systems, [6] imaging, [7] tumor therapeutics, [8] and catalysis. [9] There is, however, no review that focuses on this emerging material class; this review could thus help clarify the state of the art and initiate the efforts of nanotechnologists, materials chemists, inorganic/organometallic chemists, supra- molecular chemists, and physical chemists to further develop the application of LMs. In this review, we highlight the recent developments [10] in the rapidly emerging field of gallium- and its alloy-based LMs with micro/nano-scaled size (Figure 2). Synthesizing micro/nano LMs while controlling the shape and size is challenging, as their high surface tension and ultrathin native oxide skin (typically, gallium oxide) often induce the fusion of small LM particles. Herein, recent top-down and bottom-up synthetic approaches to liquid metal micro/nano-sized particles (LMPs) are summarized to guide researchers interested in the utilization of LMs to meet their specific technological needs. The properties of LMPs and their representative up-to-date applications are then also dis- cussed. Finally, an outlook of future research on these materials is provided to help guide innovative scientific endeavors. 2. Syntheses of Liquid Metal Micro/Nano Particles The self-passivating gallium oxide skin enables the forma- tion of geometrically defined shapes of Ga-based LMPs and determines their size, scale, and properties. The nature of this oxide layer is discussed in detail in Section 3.1. In this section, Liquid metals are emerging as fluidic inorganic materials in various research fields. Micro- and nanoparticles of Ga and its alloys have received particular attention in the last decade due to their non toxicity and accessibility in ambient conditions as well as their interesting chemical, physical, mechanical, and electrical properties. Unique features such as a fluidic nature and self-passivating oxide skin make Ga-based liquid metal particles (LMPs) distinguishable from conventional inorganic particles in the context of synthesis and applications. Here, recent advances in the bottom-up and top- down synthetic methods of Ga-based LMPs, their physicochemical properties, and their applications are summarized. Finally, the current status of the LMPs is highlighted and perspectives on future directions are also provided. 1. Introduction Liquid metals (LMs) are metals and metal alloys that can exist as a liquid near room temperature due to their low melting points; well-known pure LMs include cesium (Cs, melting point = 28.5 °C), francium (Fr, 27 °C), rubidium (Rb, 39.3 °C), mercury (Hg, -38.8 °C), and gallium (Ga, 29.8 °C) [1] (see Tables 1 and 2). Despite their interesting melting behavior, most of these metals are of limited utility due to their physicochemical properties. For example, Cs and Rb are very reactive in the air, Fr is radio- active, and Hg is highly toxic. Among known LMs, Ga has low toxicity and high chemical stability; as such, researchers from a variety of fields have focused on Ga, revealing interesting elec- trical, thermal, mechanical, and fluidic properties. In particular, the formation of a self-limited, native, thin oxide layer on liquid Ga in air allows for the geometric construction of well-defined Ga structures. The application horizon of Ga has been dramatically expanded by the formation of Ga-based eutectic alloys, such as the eutectic gallium–indium alloy (EGaIn, 78.6 wt% Ga and 21.4 wt% In) and gallium–indium–tin alloy (Galinstan or EGaInSn, 68.5 wt% Ga, 21.5 wt% In, and 10.0 wt% Sn). [2] Depending on their composition, the melting points around Small 2019, 1903391