nanomaterials Review Developments in Synthesis and Potential Electronic and Magnetic Applications of Pristine and Doped Graphynes Gisya Abdi 1,2 , Abdolhamid Alizadeh 3, Wojciech Grochala 1 and Andrzej Szczurek 1,* 1 Centre of New Technologies, University of Warsaw, S. Banacha 2c, 02-097 Warsaw, Poland; [email protected] (G.A.); [email protected] (W.G.) 2 Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Poland 3 Department of Organic Chemistry, Faculty of Physics and Chemistry, Alzahra University, Tehran 1993893973, Iran; [email protected] * Correspondence: [email protected]; Tel.: +48-22-554-08-11 Abstract: Doping and its consequences on the electronic features, optoelectronic features, and magnetism of graphynes (GYs) are reviewed in this work. First, synthetic strategies that consider nu- merous chemically and dimensionally different structures are discussed. Simultaneous or subsequent doping with heteroatoms, controlling dimensions, applying strain, and applying external electric fields can serve as effective ways to modulate the band structure of these new sp2/sp allotropes of carbon. The fundamental band gap is crucially dependent on morphology, with low dimensional GYs displaying a broader band gap than their bulk counterparts. Accurately chosen precursors and synthesis conditions ensure complete control of the morphological, electronic, and physicochemical properties of resulting GY sheets as well as the distribution of dopants deposited on GY surfaces. Citation: Abdi, G.; Alizadeh, A.; The uniform and quantitative inclusion of non-metallic (B, Cl, N, O, or P) and metallic (Fe, Co, Grochala, W.; Szczurek, A. or Ni) elements into graphyne derivatives were theoretically and experimentally studied, which Developments in Synthesis and Potential Electronic and Magnetic improved their electronic and magnetic properties as row systems or in heterojunction. The effect of Applications of Pristine and Doped heteroatoms associated with metallic impurities on the magnetic properties of GYs was investigated. Graphynes. Nanomaterials 2021, 11, Finally, the flexibility of doped GYs’ electronic and magnetic features recommends them for new 2268. https://doi.org/10.3390/ electronic and optoelectronic applications. nano11092268 Keywords: graphyne-like materials; synthesis and doping; electronic and magnetic properties; Academic Editor: José electronic transport; photodetectors Miguel González-Domínguez Received: 23 July 2021 Accepted: 30 August 2021 1. Introduction Published: 31 August 2021 Among all chemical elements, carbon exhibits the greatest flexibility of its first coordi- nation sphere, which is usually presented in textbooks as sp, sp2, and sp3 hybridizations. Publisher’s Note: MDPI stays neutral This plasticity leads to three available types of bonds (single-, double-, and triple-bonded with regard to jurisdictional claims in published maps and institutional affil- C atoms) that may occur in diverse, practically unlimited, connectivities. Altogether, iations. this markedly influences the allotropy of carbon, which is the richest among all chemical elements. Familiar allotropic forms of carbon include graphite, rhombohedral graphite, di- amond, lonsdaleite, amorphous carbon (soot with variable sp2/sp3 carbon atom contents), and a huge variety of human-made high-specific surface area carbons, carbon aerogels [1,2], carbon foams [3,4], glassy carbon [5,6], polyynes [7], diverse fullerenes from C12 to as Copyright: © 2021 by the authors. large as C [8,9], a multitude of single wall nanotubes [10,11], nano-onions [12], and Licensee MDPI, Basel, Switzerland. 960 more. Last but not least, they include graphene, which has unique physical properties [12]. This article is an open access article distributed under the terms and Other exotic forms such as ultra-high pressure BC8 [13,14], Po-32 [15], and ferromagnetic conditions of the Creative Commons carbon [16] have previously been theorized [17] (C18 was even reported [18]), but some are Attribution (CC BY) license (https:// still disputed. Nevertheless, this structural diversity and versatility of chemical bonding creativecommons.org/licenses/by/ brings an enormous pool of physicochemical properties, reactivities, and so on; crystal 4.0/). structures of over 500 periodic allotropes, known and hypothesized, have been collected Nanomaterials 2021, 11, 2268. https://doi.org/10.3390/nano11092268 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 2268 2 of 40 Nanomaterials 2021, 11, 2268 crystal structures of over 500 periodic allotropes, known and hypothesized, have2 of 39 been collected in a unique Sacada database (https://www.sacada.info/) [19]. Despite the long-last- ing research of carbon-based materials, some fundamental issues related to the shape of thein phase a unique diagram Sacada and database mutual (https://www.sacada.info/ stability of polymorphs,)[ or19]. even Despite their the existence, long-lasting remain unresolvedresearch ofto carbon-based this day [20–22]; materials, e.g., someit has fundamental been recently issues claimed related that to thelonsdaleite shape of theis not a phase diagram and mutual stability of polymorphs, or even their existence, remain un- genuine allotropic form but a twin of cubic crystals, which raised controversy [23,24]. One resolved to this day [20–22]; e.g., it has been recently claimed that lonsdaleite is not a illustrationgenuineallotropic of the intensity form but of a the twin research of cubic crystals,field of carbon which raised materials controversy can be[ 23provided,24]. 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The 1. structureThe structure of graphyne of graphyne with with indicated indicated distribution distribution of spsp (red(red rings) rings) and and sp 2sp(blue2 (blue circles) circles) hybridized hybridized carbons carbons (A); The(A); representation The representation of graphdiyne of graphdiyne structure structure (B (B).). WideWide tunability tunability in in structural, structural, mechanical,mechanical, physical, physical, and and chemical chemical properties properties make make GYs fascinating candidates for use in energy storage, solar cells, electronic and spintronic GYs fascinating candidates for use in energy storage, solar cells, electronic and spintronic devices, UV light detectors, as well as adsorbents in the separation of gases [28]. A GY devices,can be UV a useful light catalystdetectors, in wateras well purification as adsorben [28ts]. 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Though the structure of GY was Scientistsfirst proposed have been in 1987 theoretically by Baughman studying et al. [33 graphynes], the demanding since synthesisthe 1980s, of which GYs hindered initially at- tractedtheir attention dynamic developmentafter