Contents lists available at ScienceDirect Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb Alloyed monolayers of Cu, Ag, Au and Pt in hexagonal phase: A comprehensive first principles study Pooja Kapoor a, ⁎ , Arun Kumar b , Munish Sharma a , Jagdish Kumar c , Ashok Kumar d , P.K. Ahluwalia a, ⁎ a Department of Physics, Himachal Pradesh University, Shimla, Himachal Pradesh, India b Department of Physics, Swami Vivekananda Govt College, Ghumarwin, Bilaspur, Himachal Pradesh, India c Department of Physics and Astronomical Science, Central University of Himachal Pradesh, Dharamshala, Himachal Pradesh, India d Department of Physical Sciences, School of Basic and Applied Sciences, Central University of Punjab, Bathinda, Punjab, India, ARTICLE INFO Keywords: DFT Monolayers Band structure Mechanical properties STM images NDR region ABSTRACT We present density functional theory (DFT) based comprehensive study of two-dimensional (2D) alloyed monolayers of noble metals (AgCu, AgPt, AgAu, AuCu, AuPt and CuPt) in hexagonal phase within numerical atomic orbitals and plane wave basis sets methods. The monolayers considered exhibit positive phonon fre- quencies suggesting them to be dynamically stable. The Pt containing alloyed monolayers have superior structural stability (binding energy and tensile strength) and exhibit metallic and ferromagnetic character amongst all the alloyed monolayers. Interestingly, alloying of Au monolayer with Cu and Ag show semi- conducting behavior whereas alloyed AgCu monolayer posseses Dirac-cone like features at high symmetry points. These distinct features in electronic structures of alloyed 2D monolayers have been captured in STM like set up. An anisotropic behavior has been observed in dielectric spectra for all the considered structures. Tunneling characteristics show NDR region for Pt containing alloyed monolayers. The considered alloyed monolayers may potentially be useful as a building blocks for the applications in nano- and opto-electronics. 1. Introduction Two-dimensional (2D) form of noble-metals with thickness in few nanometer range, possesses unique physical [1] and chemical proper- ties [2]. These have numerous technological applications such as in catalysis, micro electromechanical and nano-electromechanical sys- tems, as interconnects in molecular circuits, sensors, devices for surface enhanced Raman spectroscopy, as protective coatings and biomedical area [3–6]. Noble-metal nanostructures are reported to be synthesized in different polymorphs such as triangular, hexagonal and helical structures as compared to FCC structure in their bulk counterpart [7]. It is known that the properties of metal nanostructures are closely related to their crystal structure [8,9]. Noble metals exhibit various crystal structures in the bulk form, for example, Ag possesses two different polymorphs 9R- [10] and 4H-polytype [11]. The occurrence of hex- agonal phase in one-dimensional (1D) hetero-nanostructures [12], thin films [13], nanorods [14–17], and nanowires of Silver has been re- ported in previous studies [18,19]. A crystallization pathway for metastable HCP phase in gold has recently been reported by Marshall et al. [20]. In last decade it has also been possible to synthesize Gold in various nanostructures such as na- noparticles [21], nanowires [22], nanobelts or nanoribbons [23,24], nanoplates [6] nanosheets [25] and nanotubes [26] with HCP structure. This occurrence of nanostructures of Gold in HCP structure makes un- derstanding of their properties interesting from basic research as well as technological point of view. Despite wide applications of noble metal nanoplates and na- nosheets, their use is limited due to their scarce reserves and high costs. Alloying of noble metals with earth abundant metals has enhanced their properties in a cost effective manner [27]. Alloyed nanomaterials in which d-band vacancies of a Group 8 and 10 (Fe, Pt) metal are pro- gressively filled by a Group 11 (Au, Ag and Cu) metal have attracted wide interest for its application in the field of electronics [28–31]. Note that the heterogeneous Au–Pt [32,33], Au-Ag [34] and Au-Cu [35] nanostructures with much better properties than their pristine coun- terparts have been synthesized for applications in various fields like optics, electronics, protective coatings, bio-sensing, drug delivery and catalysis [6,30–35]. Though hexagonal structure is not an obvious choice for noble metal alloyed sheets, but motivated by the above mentioned recent reports, https://doi.org/10.1016/j.mseb.2017.11.011 Received 25 July 2017; Received in revised form 31 October 2017; Accepted 9 November 2017 ⁎ Corresponding authors. E-mail addresses: pupooja16@gmail.com (P. Kapoor), pk_ahluwalia7@yahoo.com (P.K. Ahluwalia). Materials Science & Engineering B 228 (2018) 84–90 0921-5107/ © 2017 Elsevier B.V. All rights reserved. T