Yang Liu 1,2,3,† , Chenglu Liang 1,2,† , Jingjie Wu 4, ⇑ , Sreekanth J. Varma 5,6 , Yusuke Nakanishi 1 , Amir Aliyan 7 , Angel A. Martí 7 , Yan Wang 8 , Banghu Xie 2 , Jitesh Kumar 6 , Katherine Layne 6 , Nitin Chopra 9 , Ihab Odeh 9 , Robert Vajtai 1 , Jayan Thomas 6 , Xiangfang Peng 3 , Wei Yang 2, ⇑ , Pulickel M. Ajayan 1, ⇑ 1 Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, TX 77005, United States 2 College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, Sichuan, China 3 Department of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350108, Fujian, China 4 Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, United States 5 Department of Physics, Sanatana Dharma College, Alappuzha, Kerala 688003, India 6 NanoScience Technology Center, University of Central Florida, Orlando, FL 32826, United States 7 Department of Chemistry, Rice University, 6100 Main Street, Houston, TX 77005, United States 8 School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, PR China 9 SABIC (Saudi Basic Industries Corporation), Sugar Land, TX 77478, United States Although quantum dots (QDs) based on two dimensional (2D) materials have manifested fascinating properties and promising applications in a wide range of fields, a low cost and non-tedious approach for the preparation of atomically thick 2D QDs with high yield remains elusive. Herein, for the first time, we demonstrate a reflux pretreatment-mediated sonication approach to produce a series of ultrathin 2D QDs (e.g., graphene, TiS 2 , MoS 2 , MoSe 2 , WSe 2 , NbS 2 , SnS 2 , and h-BN QDs) from their bulk counterparts. The solvent molecules, confined in the channels of layered materials during reflux, assist the delamination and fracture of 2D sheets in the process of bath sonication. The resulting 2D QDs exhibited uniform lateral size distribution of 2–7 nm and a mean thickness of 0.8–1 nm, almost identical to the monolayer thickness of layered materials. To demonstrate the unique property of these 2D QDs, the optical limiting activity was studied using open aperture z-scan technique. TiS 2 QDs exhibited appreciable nonlinear optical limiting, reaching 50% by a tapered and sharp absorption at input laser fluence of 4.24 GW/cm 2 . Introduction Since graphene was successfully prepared by mechanical exfolia- tion in 2004 [1], two-dimensional (2D) nanomaterials, including transition metal dichalcogenides (TMDs, e.g., MoS 2 , WS 2 ) [2–4], hexagonal boron nitride (h-BN) [5], graphitic carbon nitride (g-C 3 N 4 ) [6], and black phosphorous [7], have attracted tremen- dous attention in the past decade. These outcomes have led to the elucidation of the laminar structures of layered materials and fundamental understanding of their layer-dependent prop- erties [8]. When these 2D materials are transformed into quan- tum dots (QDs, lateral sizes being typically smaller than 10 nm), unique physicochemical properties are observed due to prominent edge exposure and quantum confinement effect [9–11]. QDs can be potentially applied in photodetectors [12], Reflux pretreatment-mediated sonication: A new universal route to obtain 2D quantum dots ⇑ Corresponding authors. E-mail addresses: Wu, J. (jingjie.wu@uc.edu), Yang, W. (weiyang@scu.edu.cn), Ajayan, P.M. (ajayan@rice.edu). † These Authors contributed equally to this work. Materials Today d Volume xxx, Number xx d xxxx 2017 RESEARCH RESEARCH: Original Research 1369-7021/Ó 2018 Published by Elsevier Ltd. https://doi.org/10.1016/j.mattod.2018.06.007 1 Please cite this article in press as: Y. Liu et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.06.007