Advances and trends in chemically doped graphene

www.advmatinterfaces.de 2000999 (1 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH REVIEW Advances and Trends in Chemically Doped Graphene Sami Ullah, Qitao Shi, Junhua Zhou, Xiaoqin Yang, Huy Q. Ta, Maria Hasan, Nasir Mahmood Ahmad, Lei Fu, Alicja Bachmatiuk, and Mark H. Rümmeli* DOI: 10.1002/admi.202000999 conductivity, eminent mechanical strength, amazing chemical stability, flexibility, light weight, large surface area, matchless thinness, and transparency all in one material.[1–3] It has been extensively explored in 0–3D (e.g., graphene quantum dots (GQDs), carbon nanotubes, and porous graphene), in many functionalized forms, as well as in combination with other materials (composites).[4–9] The typical goals are as follows: 1) exploit the unique behavior of graphene; 2) overcome the gapless symmetry of graphene as a zero bandgap material (i.e., the conduction and valence states meet at some point) that hinders its use in the electronic industry; 3) enhance the reactivity of graphene, which is naturally stable under ambient conditions owing to the delocalized π-electron system that resists chemical modifications, for use in synthetic industry; and 4) design new materials with ideal attributes.[10–15] Among the graphene derivatives, a focus area is chemically doped graphene—more specifically covalently doped graphene, in which a controlled number of foreign atoms (heteroatoms) are intentionally introduced to customize the properties of graphene. The potential uses of chemically doped graphene include: Chemically doped graphene materials are fascinating because these have different desirable attributes with possible synergy. The inert and gapless nature of graphene can be changed by adding a small number of heteroatoms to substitute carbon in the lattice. The doped material may display superior catalytic activities; durable, fast, and selective sensing; improved magnetic moments; photo responses; and activity in chemical reactions. In the current review, recent advances are covered in chemically doped graphene. First, the different types of heteroatoms, their bonding configurations, and briefly their properties are discussed. This is followed by the description of various synthesis and analytical methods essential for assessing the characteristics of heterographene with specific focus on the selected graphene materials of different dopants (particularly, single dopants, including N, B, S, P, first three halogens, Ge, and Ga, and codopants, such as N/O), and more importantly, up-to-date applications enabled by the intentional doping. Finally, outlook and perspectives section review the existing challenges, future opportunities, and possible ways to improve the graphitic materials. The goal is to update and inspire the readers to establish novel doped graphene with valuable properties and for current and futuristic applications. S. Ullah, Q. Shi, J. Zhou, X. Yang, Prof. A. Bachmatiuk, Prof. M. H. Rümmeli College of Energy Soochow Institute for Energy and Materials Innovations Soochow University Suzhou 215006, China E-mail: [email protected] S. Ullah, Q. Shi, J. Zhou, X. Yang, Prof. A. Bachmatiuk, Prof. M. H. Rümmeli Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province Soochow University Suzhou 215006, China Dr. H. Q. Ta, Prof. A. Bachmatiuk, Prof. M. H. Rümmeli Institute for Complex Materials IFW Dresden 20 Helmholtz Strasse, Dresden 01069, Germany M. Hasan School of Natural Sciences National University of Sciences and Technology Islamabad 44000, Pakistan Prof. N. M. Ahmad Polymer Research Lab School of Chemical and Material Engineering National University of Sciences and Technology Islamabad 44000, Pakistan Prof. L. Fu College of Chemistry and Molecular Sciences Wuhan University Wuhan 430072, P. R. China Prof. A. Bachmatiuk Polish Center for Technology Development (PORT) Ul. Stabłowicka 147, Wrocław 54-066, Poland Prof. A. Bachmatiuk, Prof. M. H. Rümmeli Centre of Polymer and Carbon Materials Polish Academy of Sciences M. Curie-Sklodowskiej 34, Zabrze 41-819, Poland Prof. M. H. Rümmeli Institute of Environmental Technology VŠB-Technical University of Ostrava 17. listopadu 15, Ostrava 708 33, Czech Republic The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202000999. © 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 1. Introduction Since its isolation, graphene is often called a substance of wonder due to its notable features, such as improved Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (2 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH cost-effective, long-term, high energy density, grid-scale, and portable energy generation; industrial catalysis and synthesis; as well as bio-related adaptable and wearable applications.[16–19] Compared to other approaches, such as functionalization and compositing, the chemical doping of graphene is exceptionally efficient because a small amount of dopant can significantly alter the material properties. Additionally, the doping agents can be added into the graphene lattice using conventional synthesis methods, and most synthetic doped graphenes are highly stable owing to the presence of real chemical bonds between the graphene and dopant atoms.[11,20–23] Recently reported approaches for doping graphene include hydrothermal,[24] calcination,[25] solvothermal,[26] and ionimplantation[27] methods, as shown in Figure1a–h. These synthetic methods have produced graphene materials with monodopants[28,29] and codopants.[30,31] Importantly, experimentalists[32,33] and theoreticians[34] have joined forces to understand the mechanism of controlled growth with rationally designed doping features and to develop synthetic routes that are simple, economical, green, and scalable.[16,35–37] These issues are critical to the reliable and large-scale production and application of graphene derivatives. There is considerable research on graphene doping from different aspects. In this review, we will address recent developments in doped graphene, including new synthesis routes for 2D doped graphene (single dopants, such as N, B, S, P, first three halogens, Ge, and Ga, and codopants, such as N/O), common heteroatom-induced bonding configurations, and properties of the doped graphene. Different analytical methods and more importantly the up-to-date applications enabled by intentional doping will be discussed. We hope this review would have potential interests for researchers in this field. Figure 1. a–h) Summary of current synthetic routes developed for heteroatom-doped graphene with the corresponding example SEM micrographs. a) S-doped graphene developed by solvothermal method. Adapted with permission.[65] Copyright 2017, Elsevier. b) N/S double-doped graphene via calcination route. Adapted with permission.[25] Copyright 2016, Wiley-VCH. c) Boron heterographene by supercritical fluid processing procedure. Adapted with permission.[169] Copyright 2019, Wiley-VCH. d) Nitrogen and sulfur codoped by polymerization process. Adapted with permission.[54] Copyright 2017, Elsevier. e) N-doped graphene via hydrothermal method. Adapted with permission.[110] Copyright 2017, Elsevier. f) N & S dual doped plasma CVD method. Adapted with permission.[31] Copyright 2019, American Chemical Society. g) Bromine doped by thermal CVD route. Adapted with permission.[46] Copyright 2019, Royal Society of Chemistry. h) Phosphorous-doped graphene via pyrolysis technique. Adapted with permission.[32] Copyright 2017, American Chemical Society. i) Library of bonding configurations induced by various doping agents. j) Plot of drain current (Ids) versus drain voltage (Vds) at different gate voltages (Vg) for nitrogen-doped graphene field effect transistors (FETs). The direct relation of Ids with Vg reveals that nitrogen heterographene exhibits n-type doping effect. Adapted with permission.[60] Copyright 2009, American Chemical Society. k) Ids versus Vds curve. The voltage at Dirac point shifts toward the positive value, indicating p-type doping. Adapted with permission.[197] Copyright 2016, Royal Society of Chemistry. l) Voltammograms for oxygen reduction reaction (ORR) based on nitrogen-doped graphene electrode in comparison to graphitic and Pt/C based electrodes, showing the catalytic feature induced by nitrogen doping. Adapted with permission.[21] Copyright 2010, American Chemical Society. m) Schematic illustration of Si-doped graphene with three different dye probe molecules: crystal violet (CRV), rhodamine B (RhB), and methylene blue (MB), demonstrating the sensing property of heterographene. Adapted with permission.[198] Copyright 2014, Wiley-VCH. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (3 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH 2. Dopant-Induced Bonding Configurations and Properties 2.1. Bonding Configurations Foreign dopant atoms can alter the properties of graphene. Hence, doping has further opened the door to many graphene-based applications including electronics[38] and sensing[39] owing to the distinct nature of each dopant.[40,41] These impurity atoms display unique bonding configurations in the graphene lattice (see Figure1i), endowing the graphene with distinctive properties. Nitrogen typically forms three types of bonding structures: graphitic, pyridinic, and pyrrolic. The former two are planar arrangements with sp2 hybridization, while the sp3 configuration of the last one causes distortion in the planar symmetry of graphene.[42] Boron can form two planar bonding moieties that are equivalent to graphitic and pyridinic, plus one outof-plane configuration in which the B atom sticks out from a single carbon atom.[22] Theoretically, sulfur can form several bonding arrangements, and the commonly observed configurations are the thiophenic (generating a pentagonal vacancy), and bonding similar to pyridinic and graphitic arrangements (by binding, respectively, with two and three carbon atoms). In addition, sulfur oxide structures are frequently observed.[40] Note that some foreign atoms can display the same valence but different physical arrangements: some stay in the graphene plane, whereas others are out-of-plane or even overhanging.[14] For instance, P exhibits the same three bonding forms on graphene as N (graphitic, pyridinic, and pyrrolic), however the graphitic form of P is a pyramidal overhang, whereas that of N is planar.[41,43] In the case of Si doping, an analogous overhang structure was observed, accompanied by a remarkable distortion of the planar structure of graphene. This endows Si-doped graphene with (metal-free) catalytic activity for a variety of reactions (e.g., CO oxidation). In addition, Si can establish a planar structure forming a divacancy bond with four C atoms.[44] Halogens (F, Cl, and Br) are only one electron short of fulfilling the octet, and so they are expected to bond with one C in graphene and stick out from the basal plane.[41] Such a configuration would change the hybridization of carbon from planar sp2 to trigonal planar sp3, thereby introducing curvature into the graphene sheets. This effect is especially pronounced for bromine due to its large size.[45–48] On the other hand, the smaller F tends to fully cover the graphene sheet to form fluorographene (CF)n. Unlike other types of halographene, this is the only stable form for fluorine doping, and (CF)n acts as an insulator by restricting the delocalized electrons.[41,49] Metals displaying different bonding states with graphene have also been reported, such as Fe and Cr.[50,51] Analogous to Si, Fe forms a divacancy planar or out-of-plane graphitic bonding system. Cr has demonstrated pyridinic and pyrrolic configurations, but the disruption caused by its doping is stronger (viz., the structural distortion is greater) because of the large size.[52,53] One thing to bear in mind is that the graphitic bonding configurations are generally more stable than others, especially for smaller dopant atoms. The reason is likely because of the preservation of the graphitic hexagonal lattice which is stable.[41] In addition to the commonly bonding configurations derived theoretically or observed experimentally for chemical dopants, other bonding arrangements are also possible, and they can be interesting in many ways. Currently, codoped heterographene has drawn considerable interest.[54,55] The goal of codoping is to create superior synergy by joint contributions to a particular property.[27] For instance, P/N codoping is useful for designing improved n-type effect, which is not seen the individually doped materials. As another example, when B and N atoms are doped alone in graphene, they tend to assume a scattered and random distribution. However, when doped together they preferentially form B-N bonds and even hexagonal patches (h-BN) in pristine graphene system.[56,57] These integrated B-N bonds cancel out the doping effect, and the material behaves more like pristine graphene. 2.2. Properties Provided by Doping Agents Owing to their valence electronic configurations and atomic size (especially for bulky atoms, such as S and Br), dopants cause two major transformations: destruction of the pristine graphene’s hexagonal symmetry, and alteration of the electronic structure. These changes in turn influence the properties of the doped graphene such as the bandgap (Eg), magnetic moment, thermal stability, electron mobility, spin densities, reactivity, optical characteristics, and photoresponses.[11,41,58] Some of the typical properties induced by dopants are summarized in Figure1j–m. Depending on the type of dopant(s), bonding configuration, and the degree of doping, new or improved properties may arise that are advantageous for particular applications. The ability to customize graphene properties by heteroatom doping is vital for the design and discovery of novel characteristics in graphene materials and further extension of their exploitations. The lattice parameters of graphene are slightly altered upon doping. For example, the BC bond length is greater than the CC bond (≈1.5 and 1.41 ± 0.01, respectively). Some dopants present n-type doping (N in the graphitic state, P, Ga, Fe, etc.),[50,59–61] while others display p-type effect accompanied with a downshift of the Fermi level toward the Dirac point (e.g., N in the pyridinic state, B, S, Si, O, and halogens).[46,62–64] It is generally thought that the opening of the bandgap is due to symmetry breaking induced by the doping atoms. Therefore, the bandgap (Eg) is usually a function of the doping level.[41] S-doped graphene has a high catalytic activity (e.g., for the oxygen reduction reaction, ORR), because of a valence orbital mismatch to carbon despite their comparable electron affinities (≈2.58).[65] Meanwhile, metal doping endows graphene with single-atom catalytic features, as pointed out by Ta et al. for chromium doping.[66] Importantly, different bonding configurations of the same dopant can introduce distinct properties. Graphitic N is conductive,[59] while pyridinic N is attractive for storage purposes (particularly, high energy capacity in batteries).[67] N-doped graphene is promising in spintronics because of its magnetic properties. Specifically, pyrrolic and pyridinic N-doped graphene both have nonbonding electrons, the former displays strong magnetic properties and the latter displays poor ones. Compared to N, P as a doping impurity causes a stronger magnetic effect.[11,41] Moreover, since P has a lower electron affinity than carbon (2.11vs 2.55), the electron density in the CP bond shifts more toward carbon as compared to the CN system, where the electron density shifts toward N. As mentioned Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (4 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH earlier, halogens (F, Cl, and Br) tend to be covalently doped into graphene and transform the sp2 carbon into sp3 hybridization (tetrahedral symmetry). Iodine, however, tends to form a charge transfer complex (ionic bond) rather than covalent bond with graphene, and this leaves the sp2 symmetry intact. Previous studies demonstrated that graphene doped with F and Cl are direct bandgap semiconductors, whereas brominated graphene is an indirect bandgap semiconductor.[49,68] Additionally, smaller dopant atoms (such as B and N) induce less structural deformation, while bigger atoms (such as Br, S, and Fe) promote larger structural defects. These transformations may be deliberately utilized to modify graphene.[22,40,69] In the case of codoping, B/N is of special interest. Graphene, with h-BN patches, behaves like pristine graphene, because B and N have opposing effects— N creates conducting electrons, while B create holes upon doping into graphene. Furthermore, B/N codoped graphene is thermally less stable than N-doped graphene but more stable than B-doped graphene. Unlike the B/N system, other codoped atoms, such as N/S and N/P tend to be distributed randomly in the material. On the other hand, one of the doped species may favor a particular bonding configuration in the presence of another specific dopant. For example, P and S promote pyridinic and graphitic nitrogen configurations, respectively.[41] It is worth noting that, when one of the codopants is strong (i.e., more tightly bound with carbon, such as nitrogen) and another is weak (especially heavy atoms like aluminum), the codoping system is more robust than doping with a single weak dopant. An example is the N/Al codoping structure, which is more stable than the Al-doped structure[70–72] because of a reduced formation energy in the codoped system.[73] 3. Analytical Approaches To exploit the potential of graphene doping to the maximum, it is critical to understand the different attributes of doped graphene in detail, such as the topography, morphology, quality, number of layers, and the doping system (such as doping chemical state, doping level, and distribution), because these characteristics strongly impact the material’s properties.[41,69,74] Thus, various spectroscopic and microscopic techniques have been used to comprehensively examine the chemical derivatives of graphene, and some of them are reviewed below. Raman analysis is an efficient and simple spectroscopic technique for inspecting the structural features (e.g., number of graphene layers, layer stacking order, structural disorders, details about the quality and crystallinity, and evidence of doping).[60,75,76] The intensity ratio of the 2D and G peaks (I2D/IG) is generally used to estimate the number of layers.[77] For monolayer graphene, the 2D peak is narrow and symmetric in shape, with a higher intensity than that of the G mode. In single layer graphene, I2D/IG is often around ≥1.5 (without being rigorous), and this ratio decreases with additional layers[78–80] as shown in Figure2a. The full width at half maximum (FWHM) Figure 2. a) Raman spectra of pristine graphene with different thicknesses. 1L (pristine graphene), 2L, and few-L stand for mono-, bi-, few-layers of graphene, respectively. Adapted with permission.[79] Copyright 2010, Springer Nature. b) TEM micrograph of double and trilayer boron/nitrogen dualdoped graphene. Adapted with permission.[57] Copyright 2010, Springer Nature. c) AFM image of single layer B-doped graphene. Adapted with permission.[88] Copyright 2013, Wiley-VCH. d) SEM image showing the various thicknesses in graphene (1L: single layer, MLRs: multilayer graphene regions). Adapted with permission.[199] Copyright 2017, Elsevier. e) Broadview XPS spectrum of N 1s and the different states. Adapted under the terms of the Creative Commons Attribution 4.0 License.[42] Copyright 2018, The Authors, published by Springer Nature. f) EELS spectrum of B-doped graphene. Inset: magnified view of the dashed square region. Adapted with permission.[13] Copyright 2012, American Chemical Society. g) EDS results of pristine and halogenated graphene (Cl, Br, and I). Adapted with permission.[92] Copyright 2013, Springer Nature. h) FTIR spectra of different samples of B-doped graphene. Adapted with permission.[94] Copyright 2018, Elsevier. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (5 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH of the 2D peak also reflects the number of layers. Monolayer graphene often has a FWHM value below 40 cm−1 (again, this value is changeable).[46,81] Raman mapping can be used to evaluate the uniformity and homogeneity of graphene thickness, as well as the distribution of defects and/or doping within the measured sample region.[82] Moreover, Raman analysis helps confirm doping by the emergence of D (which might originate from either intentional doping or other structural disorders) and D΄ shoulder peaks near the G mode. The position, shape, and shift of Raman peaks can also provide information about the doping and the number of graphene layers.[38] A blueshift and/or relative reduction in intensity of the 2D peak with respect to pristine graphene can be attributed to the doping and an enhanced number of layers, respectively.[12,46,77] In contrast, transmission electron microscopy (TEM) can precisely identify the number of graphene layers by counting the folds at the graphene edge in micrographs (Figure 2b), which is also backed by fast Fourier transform of the micrograph.[33,83] High-resolution TEM (HRTEM) can assess the material quality and crystalline hexagonal symmetry with atomic resolution.[57,84,85] In addition, selective area electron diffraction (SAED) analysis is useful for determining the lattice parameters, crystal orientation, stacking order, number of graphene layers, and more importantly the sample crystallinity over a small region. A single set of hexagonal spots obtained from the sample suggests better crystal symmetry, whereas continuous circles or diffuse spots indicate an amorphous region.[86] The atomic force microscopy (AFM) technique can reveal accurate details about the sample topography, such as film continuity, cracks, wrinkles, folds, smoothness, and contaminants on the surface.[87,88] AFM provides 3D imaging of the surface up to the nanoscale, and the number of graphene layers can be determined from the height profile. The commonly reported AFM intensity profile height for single layer graphene is <1nm (Figure2c), and the height increases when there are more layers.[13] Both conductive and nonconductive surfaces can be measured by this technique. Similar to AFM, scanning electron microscopy (SEM) uses an incident/primary electron beam to strike the sample, creating secondary electrons and several other signals that provide different types of information. By creating a 3D micrograph of the sample, SEM can be a supportive characterization technique for graphitic materials at low resolution and large areas, primarily providing the topographical and morphological information (such as, grain shape, grain boundaries, patches, contaminations, and number of layers). This technique is also applicable to both conductive and nonconductive surfaces: the former is measured directly, while the latter needs special treatment such as gold coating (Figure2d).[75,89] Various aspects of heterodoped graphene can be evaluated, such as the doping agent/s-induced bonding configurations, degree of doping, and elemental distribution/s. Several techniques exist to measure these characteristics, with pros and cons for each. X-ray photoelectron spectroscopy (XPS) uses X-rays to eject core level electrons, whose binding energy is specific to the elemental atom and the bonding state. It is a sensitive way to measure the surface doping configurations, oxidation states, as well as the amount of doping agent present (Figure2e). Electron energy loss spectroscopy (EELS), which is integrated with TEM and scanning TEM (STEM), is based on the kinetic energy loss of electrons upon inelastic collision with atoms in the sample. EELS is also sensitive to the chemical composition, bonding configuration, and electronic structure in thin samples because of its transmission-based mechanism. Thus, it is usually considered the best tool for establishing the chemical identities and bonding at the atomic level (Figure2f).[15,90,91] Moreover, EELS mapping can reveal the elemental distribution even at trace amounts.[13] For analysis of the chemical composition inside the sample to a depth of a few micrometers and mapping the elements over a broader region, the energy dispersive X-ray (EDS) technique can be used, which measures the characteristic X-ray emitted by each atom in the sample under an electron beam. EDS can be integrated with both SEM and TEM (see Figure2g).[92] On the other hand, unlike XPS and EELS, EDS is not appropriate for detecting the bonding states.[93] Another sensitive, simple, fast, and universal analytical approach is Fourier transform infrared (FTIR) analysis, which is able to recognize chemical bonds, chemical reactions, contamination in the specimens, and even chemical structure of the analyte (Figure2h).[94,95] FTIR is, however, unsuitable for detecting monoatomic moieties (e.g., H* radical or Na+ ion), homoatomic molecules (e.g., N2 and Cl2), and the chemical states, as compared to XPS and EELS. Besides those mentioned above, many other analytical methods could be exploited for various purposes. Overall, careful characterization helps to reveal the growth mechanism and screen for desired characteristics in the materials, thus contributing to the development of scale-up practical applications. More significantly, the joint use of atomic resolution microscopy and spectroscopy has made it possible to directly spot impurity atoms in the graphene lattice together with their precise chemical nature, even in the presence of camouflage contaminations. A usual problem with heteroatom-doped graphene structures is contamination comprised of carbon-based structures, whose bonding environment is analogous to that expected for atomic dopants inside the graphene lattice (Figure3a,b).[15] Even detecting the atomic bonding environment is not enough to resolve this, as the contaminant atoms can easily be mistaken for doping agents. In most cases, successful doping is evaluated indirectly from nonlocal spectroscopic signatures, e.g., by comparing XPS data with existing literature. This was reported by Susi etal. for phosphorous-implanted graphene, where silicon impurities cause identical signals.[96] Since the contamination hinders adequate assessment of heterographene as well as reducing the sample quality, advanced atomic-scale characterization methods would provide significant in-depth insight into heterographene and its application in various devices. For instance, scanning tunneling microscopy is a powerful tool for local characterization.[97] Although lacing direct chemical sensitivity, it has been used to confirm the local bonding of light impurities (e.g., N and B) in graphene. Recent advances in aberration corrected STEM have similarly enabled the direct visualization of individual atoms in low-dimensional heterographene materials.[98] When atomic resolution STEM is used for EELS, even the precise nature of atomic bonding can now be resolved. For instance, researchers have directly observed nitrogen and oxygen atoms in N-doped graphene lattice and graphene oxide, respectively, along with their chemical forms via atomic resolution imaging (Figure3c,d).[99] (Surprisingly, oxygen bound with three carbon atoms was also visualized, which had been rarely Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (6 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH reported before.) In addition to that, EELS can provide adequate electronic structural information down to the single atom scale, as presented by Kepaptsoglou etal. for boron-implanted graphene.[44,100] Finally, thanks to recent progresses in electron beam manipulation, it is possible to controllably create and heal defects in graphene and to move them.[98,101–103] This concept was presented by Tripathi etal. (who attempted to displace Ge atoms in graphene lattice) and other researchers.[15,66,104,105] Atomic resolution study has further advanced noncarbon elemental doping of graphene (either singly or as a codopant), which is important for understanding the novel attributes of heterographene and developing related applications. 4. Synthesis 4.1. N-Doped Graphene Because of its comparable atomic size with carbon and greater electron affinity (3.04), nitrogen is highly suited to substitute carbon in graphene without causing serious structural effects.[40,41] As a result, it is the most comprehensively investigated dopant from both theoretical[14,106] and experimental perspectives. The reported synthetic methodologies range from chemical vapor deposition (CVD)[33,107] to hydrothermal[108–111] and ion implantation[112] methods. Different synthesis parameters can be adjusted to tailor the N-doped graphene, e.g., the number of layers, quality, doping content, and doping configurations. Recently, Jia et al. produced continuous single layer N-doped graphene by the CVD approach at several growth temperatures (230–600°C), as illustrated in Figure4a. This approach uses a single solid precursor (pentachloropyridine) that simultaneously supplies both carbon and nitrogen. When there are two or more precursors, the mechanism for controlled growth becomes complicated due to the different properties of carbon and doping agent, such as the thermal properties, decomposition rate, and chemical compatibility of decomposed moieties with each other as well as with the growth substrate. In contrast, a sole precursor containing both carbon and doping elements can simultaneously introduce the different chemical species in a controlled manner for the large-scale growth of uniform and high-quality doped graphene.[88] Jia et al. found that the quality of the grown material is degraded when raising the growth temperature, as can be observed in Figure 4b–f. The proposed reason is that the chlorine radical drastically etches the copper surface, and the resultant coarse surface is not conductive to the deposition of high-quality graphene. They also showed that the growth temperature can be used to produce single layer N-doped graphene (Figure 4g,h) with a particular doping configuration. For instance, dominant graphitic and pyrrolic N-doped graphene was grown at low (230–300°C) and at high (400–600°C) temperatures, respectively.[38] Similarly, another group conducted CVD at ambient pressure using gaseous precursors (ammonia as the N source and methane as the C source), and reported Figure 3. a,b) Two and one Ge atoms (bright spots) bound to graphitized contaminations and graphene lattice, respectively. c,d) Atomic resolution STEM image, the corresponding filtered and simulated images, and model structures of Ge implanted in graphene and bound by three and four carbon atoms, respectively. These images suggest the potential of atomic resolution STEM for the direct observation of doping atoms along with the bonding configurations in graphene, as well as the ability to distinguish dopant in the graphene lattice from that in graphitized contaminations. Adapted with permission.[15] Copyright 2018, American Chemical Society. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (7 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH that the growth temperature as well as growth time could be exploited for controlled N doping.[33] Maddi etal. implemented a laser-assisted strategy, by depositing a Ni thin film (60nm) onto a SiO2 substrate followed by the deposition of an amorphous carbon nitride thin film (a-C:N, 10nm). Afterward, the a-C: N/ Ni/SiO2 hybrid film was thermally treated to grow N-doped graphene. The prepared sample consisted of a few layers with dominant pyridinic doping structure, as can be viewed in Figure2e. The doping content (2–3 at%) was shown to be reproducible and stable, being independent of the N concentration in the a-C: N film.[42] While N-doped graphene with minor defects are not common, it was produced by Sarau etal. via ex situ doping of pristine graphene. In this two-step method, CVD-grown pristine graphene was treated with NH3 after transfer to a sapphire substrate. According to the Raman study, the produced samples were predominantly monolayers. XPS analysis revealed the coexistence of pyrrolic and dominant pyridinic states (11 at% in total). This study points out that graphene doping can be achieved without causing significant defects following an ex situ route.[107] Ion implantation is another important and mature technique for achieving adequate graphene doping, and it has shown great promise.[112–114] One research team had proposed that heteroatoms can be successfully incorporated in the graphene lattice by ion irradiation under the following conditions: i) the energy of incident ion should be enough to knock out the carbon(s); ii) kinetic energy remaining in the ion after collision with carbon should be small enough to allow it to be lodged in the graphene lattice; and iii) the implanted ion should form covalent bond with atoms of the vacancy to reach a stable configuration.[15] However, contradicting the above conditions, Bai etal. found that the ion incident angle is more important than ion energy for the productivity and quality of doping. Particularly, compared to the perpendicular direction, ion bombardment tilted at an angle to the normal is more favorable. At the optimal angle of 70°, the productivity was boosted by more than 25%, together with enhanced quality of substitution.[27] This observation could lead to a deeper insight about the ion implantation mechanism that is useful for realizing controlled doping. Zhao etal. used such a method to obtain single-layer nitrogen-implanted graphene Figure 4. a) Growth schematic of N-doped graphene at different temperatures. A low temperature (230–300°C) leads to dominant graphitic nitrogen doping, while a high temperature (400–600°C) mainly results in pyrrolic N-doping. b–f) The corresponding optical images over silicon substrates. g,h) HRTEM micrographs of monolayer N-doped graphene grown at 300 and 500°C, respectively. Adapted under the terms of the Creative Commons Attribution 4.0 License.[38] Copyright 2016, The Authors, published by Springer Nature. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (8 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH with improved quality (Figure5a–c). Notably, they used a multilayer substrate (Ni/Cu/amorphous-SiC/SiO2/Si) in which amorphous-SiC provided carbon feedstock, and control over nitrogen content was achieved by monitoring the N ion fluence.[115] The advantages of this method are the compatibility with microelectronic industry and the feasibility for even heavy atomic dopants (e.g., germanium).[15,115,116] Scardamaglia and his colleagues came up with a similar implanting method. Unlike Zhao etal., these authors employed a two-step approach of CVD growth of graphene followed by nitrogen ion treatment (ion implantation), achieving a high nitrogen content (≈8 at%) without formation of graphene lattice defects. Furthermore, thermal annealing was used to tune the doping configuration, such as converting pyridinic to graphitic nitrogen. This investigation suggests that the ion implantation method can achieve a high doping content as well as thermal interconversion among different bonding configurations. It will facilitate the understanding of distinctive properties of foreign atoms in a specific bonding form and relate them to the corresponding applications.[64] Recently, Hofer etal. demonstrated nitrogen doping by irradiating nitrogen ions onto graphene on a TEM grid, and atomic resolution imaging confirmed that the doped nitrogen prefers to bind with three carbon atoms (graphitic form).[99] Based on these observations from different research groups, high-quality tunable nitrogen doping of graphene is possible; and particular bonding configurations, a large continuous area, and single-layer thickness can be achieved by controlling the synthesis parameters such as, the growth temperature, growth duration, precursor sublimation temperature (for solid precursors only), and even by laser deposition and ion implantation methods. 4.2. Boron-Doped Graphene Similar to nitrogen, B-doped graphene has been reported with different numbers of layers, doping content, and bonding configurations. The synthetic approaches include pyrolysis[117] and hydrothermal methods.[24,118] Wu etal. used novel precursors (9,10-dimesityl-9,10-diboraanthracene (DBA(Mes)2) as the boron source and toluene as the carbon source) to customize the quality of B-doped graphene using a bubbler-assisted CVD method. The resultant sample was of high quality and consisted of single layers with a significant B content up to 4.6 at%.[119] Solid precursors are generally preferred as the growth feedstock due to the large range of choices, their benign property, stability, and ease of handling.[120] Srivastava and colleagues used B powder as the boron source and liquid ethanol as the carbon source to produce B-doped graphene by low pressure CVD. The as-grown sample was few-layered according to Raman analysis, whereas the XPS result revealed two dominant bonding configurations (BC3 (graphitic form) and BC4) with a doping concentration up to 4.9 at%.[39] Incorporation of boron into the graphene framework has also been achieved via a two-step CVD procedure. Pristine graphene is deposited by CVD, transferred to a target substrate, and then treated with a boron source solution. XPS analysis verified the presence of boron, and Raman mapping concluded that the as-grown sample was comprised of double layers on average.[81] Additionally, the novel route developed by Bleu and collaborators used a solid precursor to control the doping concentration. They used a laser deposition method identical to that for nitrogen doping (already described in Section4.1,[42]). The growth schematic is depicted in Figure6a. Figure 5. Schematic illustration of ion implantation growth method for nitrogen heterographene (NG). b) Corresponding Raman spectra of pristine (PG) and nitrogen-doped graphene. c) HRTEM micrograph of monolayer NG. Adapted with permission.[115] Copyright 2018, Elsevier. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (9 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH In the as-prepared sample, up to 2 at% of boron was distributed in few layered graphene as demonstrated in Figure6b,c.[121] This study demonstrates the versatility of the laser deposition route, which could be extended beyond boron and nitrogen doping. Dianat etal. reported an economical and simple way to integrate boron into graphene. In their ex situ doping approach, pristine monolayer graphene was grown by CVD and transferred to a silicon substrate predoped with boron. Finally, the substrate was heated at 400 K to allow boron integration into graphene. This route using a relatively low temperature may be applied to many other types of heterodoping.[122] In addition, argon flow can be used to control the growth, as recently established by Boukhvalov etal. in a bubbler-assisted CVD procedure. Argon (as the buffer gas) at different flow rates (50 and 100 sccm) was passed through the bubbler containing boron source. In general, different parameters in CVD can be optimized for tunable growth.[123] Romani et al. used a single liquid precursor (isopropyl borate) and cracked it to grow B-doped graphene by the CVD route. At the optimal growth temperature (1000 °C), pressure (75 mTorr), and growth duration (5min), single layer B-doped graphene can be achieved, and the presence of doping was shown by XPS and Raman analyses.[124] Significantly, this study demonstrates the usefulness of a single liquid precursor, whereas single solid precursors are commonly reported.[88] 4.3. Sulfur-Doped Graphene Sulfur is a bulky dopant, and its low electronegativity means that it does not bind very strongly with graphitic carbon.[40] Among numerous recent studies, Guo et al. employed a two-step CVD technique to grow S-doped graphene with excellent quality. First, pristine graphene was deposited via CVD and transferred to a substrate mediated by polymethyl methacrylate (PMMA). The graphene was then once again subjected to CVD in the presence of H2S as the S source under medium heating (500°C). The goal was to preserve the quality of the grown film by limiting the number of transfer steps to one. The high quality of the product was confirmed by both SAED and Raman studies.[86] This study shows that, in addition to optimized synthesis parameters, the number of transfer steps should be reduced to ensure excellent quality and crystallinity in the doped graphene. 4.4. Phosphorous-Doped Graphene Generally, preparing phosphorous-doped graphene is considered challenging because the large size of P hinders its incorporation into the graphene system. This is why the growth of such materials is rarely reported as compared to other dopants, such as nitrogen and boron.[11,40,125] However, we have already mentioned that ion implantation method is adequate for inserting large atoms into graphene. He et al. successfully implanted phosphorous into graphene with minimal defects. First, they employed a gold coating on CVD graphene as protection layer against damage by the high-energy ions. Second, a subsequent posthealing step further restored the structural integrity of heterographene. The authors claimed a record phosphorous doping content of 4.22 at%.[126] It is challenging to correctly identify the dopants because of the identical signal produced by contaminations. Significantly, Susi et al. effectively addressed this issue for phosphorous-doped graphene developed by the ion implantation route. They used atomic resolution imaging along with EELS to precisely confirm the integration of Figure 6. a) Schematic illustration of B-doped graphene synthesis by pulse laser codeposition method. b,c) Histograms for the intensity ratio between the 2D and G peaks (I2D/IG) and the 2D peak width (FWHM), respectively. Both histograms indicate few-layer B-doped graphene. Adapted with permission.[121] Copyright 2020, Elsevier. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (16 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH properties toward the oxidation of DA and UA, and the respective detection limits were 0.006 and 0.038 ×10−6m. With its excellent stability and reproducibility, this electrode could satisfactorily detect DA and UA in samples.[189] 5.5. Synthetic Uses Chemical doping of carbon nanomaterials is a useful route to obtain reactive species that function as a secondary precursor for the post synthesis of useful grafted graphene derivatives. Halogenated graphene has attracted massive attention for this reason. Covalently bound bromine, for instance, is an attractive synthetic handle and has been used to prepare PMMA grafted graphene. In addition, treatment with water and poly(ethylene glycol) methyl ether (mPEG) can functionalize bromine-doped graphene with hydroxyl (OH) and PEG groups (the as-grown materials are referred to as BrG-OH and BrG-PEG), respectively. This approach provides an easy way to access modified few layer graphene with improved and tunable dispersibility. Such modification is critical in bulk applications that require compatibility with common organic solvents, polymer matrices, and more importantly biocompatibility.[190] In addition, fluorineincorporated graphene has shown comparable performance Figure 13. a) Schematic of sulfur-based sensor for 8-OHdG detection. b) Differential pulse voltammetry (DPV) responses of the sensor at different 8-OHdG concentrations. The inset is the expanded form of the square region in the DPV plot. c) Calibration curve for 8-OHdG concentrations between 0.002 and 20 ×10−6m. Both plots indicate high sensitivity of the as-fabricated sensor. Adapted with permission.[35] Copyright 2016, Elsevier. Figure 12. a) Schematic of OLED fabrication process. b) External quantum efficiency versus luminance for the boron-doped graphene OLED, ITO, and pristine graphene. Inset is the optical image of the OLED device. c) Sheet resistance ratio versus bending cycle of boron-doped graphene on PET. Inset: optical image of the test samples after different cycles of bending. Adapted with permission.[76] Copyright 2017, American Chemical Society. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (17 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH in the nucleophilic exchange of fluorine with aromatic thiol group. This approach could be used to modify the gapless electronic structure of graphene for electronic applications.[130] Ye et al. successfully modified fluorine-doped graphene (FG) by treatment with urea. The FG stayed on top of water when it is dispersed in accordance with its hydrophobic nature. On the other hand, the urea-functionalized FG became hydrophilic and could be properly dissolved in water. This suggests potential use in lubrication.[28] In light of this study, the authors concluded that heterodoped graphene (more precisely, halogenated graphene) materials are suitable secondary precursors for the synthesis of many useful graphene derivatives. Ammonia, which is used in synthetic chemistry, agriculture, food industry, etc., is conventionally prepared using harsh conditions. To develop feasible, inexpensive, clean, and eco-friendly synthetic routes of ammonia, Yu etal. used graphene with covalently incorporated boron as electrocatalyst to promote the conversion of N2 to NH3, as demonstrated in Figure14a. The heterodoped graphene catalyst was highly efficient, with fivefold higher NH3 yield and superior stability compared to pure graphene (Figure13b,c).[94] Heterodoped graphene composited with other materials has also presented tremendous potential as catalysts for producing useful chemicals. Cobalt-decorated N-doped graphene turned out to be unparalleled and robust for reducing quinoline compounds to the corresponding hydroquinoline derivatives, which are essential chemicals in agriculture and pharmaceuticals.[191] 5.6. Other Common Applications Doped graphene has shown significant potential for many applications including in environmental, health, water, and energy sectors. For example, due to its comparable size to carbon and reasonable electronegativity, boron has been used as a dopant in graphene for a wide range of applications. In particular, it was used for the photocatalytic degradation of organic pollutants such as methyl orange (MO) and methylene blue (MB). The heterosystem displayed the highest degradation potential, with complete degradation of MO and MB in 100 and 50min, respectively. A theoretical study also emphasized the potential of boron-modified graphene as a catalyst in photodegradation, and this property can be exploited for other photocatalytic-based applications.[24] The applications of doped graphene continues to grow in number, attracting more and more attention from researchers. Recently, Arihanan etal. exploited the large surface area and storage capacity of nitrogen heterodoped graphene as a hydrogen (H2) storage material. At the N doping concentration of 7.5 at%, the H2 storage capacity at room temperature and 90bar of pressure was 1.5 wt%.[192] 6. Future Outlook and Perspectives Chemically doped heterographene has been studied in countless applications including electronics, catalysis, energy science, synthesis, sensing, and biorelated applications. Despite the significant advances, there remain questions in their synthesis and analysis. The conducting behavior and electronic characteristics also require further explanation (Figure15a–c). In terms of synthesis, a great deal of research effort was dedicated to growing continuous monolayer graphene with good quality and large area.[77] However, previous studies tend to be limited in scale and the results are often unsatisfactory. Figure 14. a) Schematic of the working mechanism for the electrocatalytic reduction of N2 to NH3. b) NH3 production rates over BG-1, BOG, BG-2, and G at different potentials. c) Stability test of the optimal catalyst (BG-1) versus RHE. The boron-doped graphene was synthesized by mixing H3BO3 and graphene oxides in different mass ratios (5:1, 1:10, and 0:1), and the respective samples were assigned as BG-1, BG-2, and G. Another BG catalyst grown under a high oxygen content is referred to as BOG. Adapted with permission.[94] Copyright 2018, Elsevier. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (18 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH Specifically, no researcher has demonstrated the continuous growth of defectless monolayer graphene over a large area. The quality and thickness of graphene also strongly influence its conductivity, which in turn impacts the electrical and catalytic properties.[46,77] The ability to continuous grow graphene with a large area is helpful toward governing its conducting properties. In addition, the as-formed sample often contains a large amount of contamination and impurities from the growth and cleaning.[15,193] Therefore, it remains difficult to establish a reproducible synthetic route for a particular structure with a particular property.[41] For instance, reports demonstrated that the catalytic functions can be promoted by chemical dopants, topographical defects, and/or other impurities, such as metal residues. Thus, a single characteristic is affected by several factors, and it is difficult to establish a one-to-one relationship between them.[40] Similarly, the introduction of doping agents causes inevitable defects that usually lower the conductivity and also destabilize the graphene. In other words, we need the ability to control a target property in graphene by doping while preserving other intrinsic characteristics. Other urgent issues in synthesis include controlling the doping mechanism and achieving a particular doping configuration, a degree of doping, and a tailored dopant distribution. To establish logical doping, various routes and the associated growth parameters have been optimized. However, many groups claimed controlled doping but none has been able to truly monitor these characteristics in the doped graphene. In the case of codoping, the scenario becomes even more complex due to the combined and synergistic/antagonistic effects from cobonding configurations.[18,57,160] There is great interest in developing viable and clean synthesis systems that are capable of such control and with high reproducibility.[41] Many analytical techniques with high sensitivity are available to effectively evaluate the sample from various aspects. However, a thorough understanding of the structural information, specific doping features, and the corresponding properties remains elusive. In some cases, the instrument failed to detect Figure 15. a–d) Further research question in synthesis: a) Growth pathway for nitrogen doped graphene at high and low temperatures in CVD mechanism. Adapted with permission.[200] Copyright 2014, American Chemical Society. b) HRTEM of single layer of nitrogen doped graphene. Adapted with permission.[201] Copyright 2015, American Chemical Society. c) AFM image of single layer nitrogen heterographene. Adapted with permission.[87] Copyright 2011, American Chemical Society. d) XPS spectrum of nitrogen incorporated graphene, showing the coexistence of graphitic and pyridinic N with the corresponding concentration. Control of the growth mechanism toward high-quality, large area, and single layer doped graphene with a particular doping configuration is critical for large-scale and reproducible applications, so, this area need the major focus from research communities. e,f) Further research question in analysis: e) Aberration-corrected TEM (AC-TEM) of iron doped graphene and, f) Corresponding DFT image. Adapted with permission.[127] Copyright 2013, American Chemical Society. Effective and more sensitive characterization methods up to the atomic scale (e.g., atomic resolution microscopy (STEM) and spectroscopy (EELS)) are extremely useful for obtaining accurate information about the structure, topographical features, and doping. g,h) Questions about conduction and charge redistribution: g) This plot highlight the change in resistance with respect to temperature of boron-based graphitic material. Adapted with permission.[195] Copyright 2012, Wiley-VCH. h) This figure indicates the band structure and corresponding bandgap (≈0.2eV) generated upon nitrogen integration to graphene. d,h) Adapted with permission.[59] Copyright 2011, American Chemical Society. These two properties significantly depend on the quality of graphene and the doping agent. Fundamental insight about them is necessary for developing the corresponding applications. Adv. Mater. Interfaces 2020, 2000999 www.advancedsciencenews.com www.advmatinterfaces.de 2000999 (19 of 23) © 2020 The Authors. Published by Wiley-VCH GmbH trace amounts of doped content, even though such a small amount would still affect the microelectronic properties.[59] Therefore, critical progresses are needed to enhance the sensitivity and precision of the analysis down to the atomic scale. The third question regarding heterographene is understanding their conductive and electronic attributes in detail (Figure 15c). Despite extensive theoretical and experimental studies, the dependence of electron mobility and bandgap on the structural and doping attributes is still not fully known. In fact, multiple factors (such as the number of layers and the doping) jointly determine the conductivity,[194,195] whereas the electronic band structure is affected by the degree of doping and the different bonding configurations. Thus, the exact relationships are difficult to extract.[50] The rational design and reproducible production of given electronic structures should greatly facilitate the commercialization of heterographene materials. We believe that the research of chemically doped graphene will continue to prosper, as a result of new opportunities presented by heteroatom doping. Besides the advancement of synthetic and analytical techniques, artificial intelligence (AI) can integrate self-rationality and automate graphene technology at the synthetic, characterization, and fabrication levels. Specifically, AI systems can automatically carry out/monitor heterographene growth and asses/correct the faults in order to produce high-quality graphene materials. While such incorporation of AI in graphene technology is still at its infancy, Leong et al. and Mendes etal. have suggested using AI in Raman and TEM studies, respectively.[91,196] 7. Conclusions During chemical doping, noncarbon atoms are deliberately introduced into graphene. With their different electronegativity and size, the dopants cause rearrangement of the electronic band structure and activate the naturally inert graphene, resulting in the generation of a bandgap and a reactive structure that are key for the diverse applications of doped graphene. This article reviews various aspects of doped graphene, including the common analytical techniques and finally recent reported applications. Many doping agents could form multiple bonding configurations, each with unique properties. In terms of synthesis, the CVD method can produce continuous single layer doped graphene with excellent quality and large area, as well as exerting some control over the bonding configuration and rational design of the topographical defects. Similarly, ion implantation method could successfully introduce noncarbon atoms into the graphene lattice. For characterization purposes, atomic resolution microscopic (STEM) and spectroscopic studies (EELS) are favorable for assessment of graphene heterostructures by revealing information down to the atomic scale and even the electronic structure. Nevertheless, many questions about heterographene remain to be resolved, probably through the combination of theoretical, experimental, and analytical studies as well as artificial intelligence. The application prospects of heterographene will be greatly advanced if doped graphene with optimal structural and doping characteristics could be cheaply and reliably grown, and their structure–property relationships in the in situ state become firmly established. Acknowledgements This work was financially supported by the National Science Foundation China (NSFC, Project No. 51672181) and the Czech Republic under the ERDF program “Institute of Environmental Technology—Excellent Research” (No. CZ.02.1.01/0.0/0.0/16_019/0000853). M.H.R. and L.F. thank the Sino-German Research Institute for support (Project No. GZ 1400). M.H.R., J.C., and R.Y. are grateful for the funding provided by the Soochow Institute for Energy and Materials Innovations (SIEMIS) and the School of Energy, Suzhou University. Open access funding enabled and organized by Projekt DEAL. Conflict of Interest The authors declare no conflict of interest. Keywords 2D doped graphene, analytical approach, bonding configurations, properties, synthesis, various applications Received: June 5, 2020 Revised: August 11, 2020 Published online: [1] A. C.Ferrari, F.Bonaccorso, V.Fal’Ko, K. S.Novoselov, S.Roche, P. Bøggild, S. Borini, F. H. Koppens, V. Palermo, N. Pugno, Nanoscale 2015, 7, 4598. [2] K. S.Novoselov, V.Fal, L.Colombo, P.Gellert, M.Schwab, K.Kim, Nature 2012, 490, 192. [3] W.Ren, H.-M.Cheng, Nat. Nanotechnol. 2014, 9, 726. [4] K.Chen, L.Shi, Y.Zhang, Z.Liu, Chem. Soc. Rev. 2018, 47, 3018. [5] M. 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Nasir Mahmood Ahmad is full professor and head of Polymer research Lab and Focal person of Polymer and Composites Research Group in School of Chemical and Materials Engineering, National University of Sciences and Technology (NUST), Islamabad. He has over 25-years extensive experiences both in academia and industry in the R & D of advanced materials and processes. He has been involved in studies, which include polymerization of acrylate, ethylene, and vinyl acetate monomers to manufacture primary resins, extrude plastic films and develop master batches. Mark H. Rummeli heads the electron microscopy and LIN labs at the Soochow Institute for Energy and Materials Innovations (SIEMIS), Soochow University, where he is a full professor. He is also director of the characterization center at the College of Energy and SIEMES. He earned his Ph.D. from London Metropolitan University and then worked as a postdoc at the German Aerospace Center and the Leibniz Institute, the IFW-Dresden. His research focuses on the growth mechanisms of 2D nanostructures and their functionalization. Adv. Mater. Interfaces 2020, 2000999