Applied Surface Science 539 (2021) 148208 Available online 24 October 2020 0169-4332/© 2020 Elsevier B.V. All rights reserved. The common and intrinsic skin electric-double-layer (EDL) and its bonding characteristics of nanostructures Yuan Peng a , Zhibo Tong a , Yezi Yang a , Chang Q. Sun b, a, * a EBEAM, School of Materials Science and ENgineering, Yangtze Normal University, Chongqing 408100, China b Micro- and Nanoelectronic Research Center, School of Electronic and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore A R T I C L E INFO Keywords: Nanostrcture Atomic undercoordination BOLS-NEP theory Phonon spectroscopy Single atom catalysis Superconductivity ABSTRACT We show that nanocrystals share a common and intrinsic skin electric-double-layer (EDL). The EDL is determined to be 2.14 regular-bond-length thick using differential phonon spectroscopy that distills phonon abundance transiting from the core region to the EDL of the sized crystals. Theoretical reproduction of the size-resolved Raman shift for Si, CeO 2 , and SnO 2 nanocrystals, elasticity of ZnO, and the XPS 2p energy shift, band gap expansion and melting point shift of Si crystals confrmed the universality of the EDL of which bonds are shorter and stronger than those inside the bulk or the particle core interior. The EDL bond contraction and the associated electron entrapment and polarization originate, and the EDL volume quantifes the size dependency of nano- structures while the electron entrapment or polarization entitles the undercoordinated single or edge atoms with properties that a bulk does never show. 1. Introduction As an important degree-of-freedom for engineering substance for advanced functional materials, atomic or molecular undercoordination entitled defects, edges, surfaces, foams, and low-dimensional systems two types of fascinations that can be categorized as atomic under- coordination derivacy and nanocrystal size dependency [1]. The undercoordination derivacy means properties that the bulk substance never demonstrate [2] such as the single-atom catalysis [3], topological insulator edge superconductivity and monolayer and skin high-T C su- perconductivity [4–6], and the atomic site resolved extreme reactivity and toxicity [7], etc. Structure miniaturization in terms of dimension- ality and crystal size turns the known properties of the bulk constant such as conductivity, dielectricity, elasticity, electron and photon emissivity, and critical temperatures for phase transitions, etc., into their size dependency [8–11]. The nanostructure size dependency and atomic-undercoordination derivacy laid the foundations of surface sci- ence and nanoscience and technology [12,13]. Considerable progress has been made on engineering nanomaterials and devices, which provide enormous impact to life and nature sciences and industrial sectors, such as application of single-atom catalysis. Comparatively, fundamental insight and characterization of the bonding and electronic response to atomic undercoordination remains their infancy despite efforts made from perspectives of classical continuum and contemporary quantum approaches. Deeper insight into the bonding and electronic dynamics pertained to the undercoordinated atoms is critical to promoting the effciency of nanotechnology and nanoscience. Besides characterization using electronic and scanning tunneling microscopies, spectroscopies of electron, phonon, and photon have been used to evaluate the size distribution of nanocrystals of semiconductors [14–16], foods and drugs [17]. Quantifcation of the atomic undercoordination-resolved bonding, electron, and molecular performance remains challenging. With the aid of the recently disclosed differential phonon spectro- metrics (DPS) [18], we show that an intrinsic skin electric-double-layer (EDL) of 2.14 bond-length thick and its (12–16)% bond contraction initiate the unusual performance of nanostructures. Theoretical repro- duction of the size dependent phonon frequency shift of Si, CeO 2 and SnO 2 nanocrystals, elastic modulus of ZnO, XPS 2p level shift and band gap expansion, melting point of Si further confrmed the EDL formation and bond contraction and its impact to the unusual behavior of materials at the nanometer scale. The intrinsic EDL is completely different from the traditionally known EDL that appears on the surface of an object when it is exposed to a fuid [19–22]. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The EDL refers to two parallel layers of * Corresponding author. E-mail addresses: ypeng@yznu.edu.cn (Y. Peng), tongzhi.bo@yznu.edu.cn (Z. Tong), 20161042@yznu.edu.cn (Y. Yang), ecqsun@ntu.edu.sg (C.Q. Sun). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc https://doi.org/10.1016/j.apsusc.2020.148208 Received 27 May 2020; Received in revised form 10 October 2020; Accepted 15 October 2020