Review of the Electronic, Optical, and Magnetic Properties of

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Review of the Electronic, Optical, and Magnetic Properties of Graphdiyne: From Theories to Experiments Chuannan Ge,†,‡ Jie Chen,† Shaolong Tang,† Youwei Du,† and Nujiang Tang*,† †

National Laboratory of Solid State Microstructures, Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, China ‡ School of Physics & Electronic Engineering, Jiangsu Second Normal University, Nanjing 210013, China ABSTRACT: Graphdiyne (GDY), a two-dimensional artificial-synthesis carbon material, has aroused tremendous interest because of its unique physical properties. The very high activity affords the possibility to chemically dope GDY with metal atoms or lightweight elements such as hydrogen and halogen and so on. Chemical doping has been confirmed to be an effective method to lead to various GDY derivatives with useful physical properties. Thus, this review is intended to provide an overview of the electronic, optical, and magnetic properties of pristine GDY and its derivatives reported from theories to experiments. Because of the importance of pristine GDY and its derivatives in real applications, we also summarize the main physical applications of GDY and its derivatives reported in recent years in this review. We believe that the review will be valuable to all those interested in GDY. KEYWORDS: graphdiyne, electronic properties, optical properties, magnetic properties, theories, experiments

1. INTRODUCTION Carbon material has sp3, sp2, and sp hybrid state, which can form a variety of carbon allotropes. These allotropes present various structures with many novel properties, which have been widely used in different fields. So far diamond and graphite are the only two carbon allotropes which were found in nature, which respectively feature extended networks of sp3- and sp2hybridized carbon atoms.1,2 Some kinds of carbon allotropes such as amorphous carbon, fullerene, carbon nanotube, graphene, and graphdiyne (GDY) and so on, have been successfully synthesized.3−8 Among these allotropes, graphene and GDY are two typical two-dimensional (2D) carbon structures, which respectively composed of sp2 hybrid and sp2−sp hybrid structures (Figure 1).1 GDY can be considered as a hybrid system of graphene (sp2-like carbon atoms) and carbine (sp-like carbon atoms), which contains sp- and sp2hybridized carbon atoms.9−11 The special carbon networks endow GDY with high degrees of π-conjunction, uniformly distributed pores, and tunable electronic properties and so on.12−15 The carbon−carbon triple bonds in GDY make it possible to introduce adatoms (e.g., fluorine, hydrogen, or oxygen) for obtaining various GDY derivatives. In 2010, Li’s group first synthesized GDY via a crosscoupling reaction using hexaethynylbenzene.9,15 The obtained GDY films which compose of GDY multilayers are uniform. This success has been considered as a precedent in artificial chemical synthesis of the new carbon allotropes. Thereafter, tremendous efforts have been dedicated to the study of GDY’s physical properties.8,12−16 Furthermore, the successful synthesis paves the way from the theoretical studies to the experimental © XXXX American Chemical Society

Figure 1. Graphyne (GY)-n varieties. n is the number of carbon− carbon triple bonds in a chain. When n = 2, GY-2 is called GDY. Reprinted with permission from ref 1. Copyright 2016 Nature Publishing. Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: February 27, 2018

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DOI: 10.1021/acsami.8b03413 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Geometrical structure, unit cell (red dashed diamond), coordinate bases, and first Brillouin zone (green hexagon) of GDY. (b) Band structures and density of states (DOS) of GDY at the LDA and GW levels. Optical transitions between the first two Van Hove singularities are indicated. (c) Quasiparticle correction as a function of the LDA energy in GDY. Reprinted with permission from ref 18. Copyright 2011 American Physical Society.

Figure 3. Optimized configurations of bilayer GDY systems named (a) AB(β1) and (b) AB(β2) from top view. Three possible configurations of the trilayer GDY systems from top view: (c) ABA(γ1), (d) ABC(γ2), and (e) ABC(γ3) configurations. Reprinted with permission from ref 19. Copyright 2012 Royal Society of Chemistry.

investigations on GDY. Recent studies indicated that GDYbased materials have shown the great values in the fields such as energy, catalysis, and photoelectric conversion and so on.17 Especially, GDY is attracting much attention for its novel physical properties in recent years. In this review, we summarize a detailed research on the electronic, optical, and magnetic properties of GDY, especially the latest theoretical and experimental results.

The possible configurations of bi- and trilayer GDY systems (shown in Figure 3) were examined by Zheng et al. using the ab initio methods. The calculated minimum band gap induced by electric field of bi- and trilayer GDY with different stacking styles were summarized in Table 1.19 As shown, AB(β1) and AB(β2) configurations, respectively, have the highest and the second highest stability for bilayer system. The band gaps of the AB(β1) and AB(β2) configurations respectively are 0.35 and 0.14 eV. As to the trilayer system, the ABA(γ1), ABC(γ3), and ABC(γ2) configurations respectively have band gaps of 0.32, 0.33, and 0.18 eV. The results clearly showed that the band gap

2. ELECTRONIC PROPERTIES First-principles calculations showed that GY allotropes generally have an intrinsic band gap ranged from 0.46 to 1.22 eV, greatly different from the zero band gap of graphene.14 It is no doubt that the existence of a direct band gap may facilitate the application of GYs in electronic and photoelectronic devices and so on. Luo et al. reported the electronic structure of GDY based on first-principles calculations.18 The structure, unit cell, coordinate bases, and first Brillouin zone of GDY are shown in Figure 2a. The band gap is 0.44 eV at the LDA level, and which will increase to 1.10 eV at the GW level (Figure 2b). Notably, the quasiparticle correction both to the conduction and the valence bands can be divided into two groups, as shown in Figure 2c. The intrinsic direct band gap in GDY predicted by theory is fascinating, which stimulated the following studies because of the potential application as 2D carbon-based semiconductor, which is unexpected for graphene.

Table 1. Calculated Optimal Interlayer Distance (l), Binding Energy (BE), Band Gap under Zero Field (Δ0), and Minimum Band Gap (Δmin) Induced by Electric Field of Biand Trilayer GDY with Different Stacking Styles from Ref 19 bilayer

trilayer

a

B

stacking style

l(Å)

BE (meV/atom)

AB(β1) AB(β2) AA ABA(γl) ABC(γ2) ABC(γ3) AAA

3.42 3.40 3.65 3.40 3.42 3.41 3.64

29.5 29.4 25.2 41.6 41.4 41.2 33.9

Δ0 (eV) 0.35 0.14 0 0.33 0.18 0.32 0

(d) (d) (i) (d) (i)

Δmin (eV) 0.10 0.08 0 0.05 0.05 0.01 0

(d) (d) (i) (d) (i)

d, direct band gap; i, indirect band gap. DOI: 10.1021/acsami.8b03413 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Band gap and effective mass of carriers of (a) AB(β1) and (b) AB(β2) configuration of bilayer GDY as a function of perpendicular electrical field strength. Reprinted with permission from ref 19. Copyright 2012 Royal Society of Chemistry.

Figure 5. Band structures calculated by DFT and the tight-binding approximation (TBA), the partial DOS (PDOS) and the 3D energy bands at the G point for GDY (a) without a strain, and under a symmetrical biaxial tensile strain of (b) 5, (c) 9, and (d) 15%, respectively. Reprinted with permission from ref 22. Copyright 2013 Royal Society of Chemistry.

The results revealed that (i) the band gaps of both systems are significantly decreased with the increase of the electric field, and (ii) the band gap decreases from 0.35 to 0.10 eV for of AB(β1) system while from 0.14 to 0.08 eV for AB(β2) system. Moreover, the carrier mobilities of μh and μe at zero field were estimated respectively to be 1.3 × 104 and 2.1 × 105 cm2 V−1 s−1 for AB(β1) system, and 3.5 × 104 and 3.3 × 105 cm2 V−1 s−1 for the AB(β2) system. However, at the electric-field-induced minimum effective mass point, μh and μe significantly increase to ca. 4.7 × 104 and 5.1 × 105 cm2 V−1 s−1 for AB(β1) system, and 5.2 × 104 and 5.1 × 105 cm2 V−1 s−1 for AB(β2) system. These results revealed that one can tune the band gap and carrier mobility of GDY by applying a perpendicular electrical field, and may help one to know about the electric field-induced change in the band gap of GDY. Like other 2D materials, GDY is sensitive to the external strain. Cui et al. reported that the uniaxial strain could change the electronic structures of GDY.22 It was considered to attribute to the breaking of geometrical symmetry, which can lift the degeneracy of energy bands. As shown in Figure 5, one can find that (i) with the increase of the biaxial tensile strain, the band gap increases from 0.47 to 1.39 eV, whereas (ii) with the increase in the uniaxial tensile strain, the gap decreases from

of GDY is dependent on the stacking arrangements. Furthermore, the band gap of both bi- and trilayer GDY is lower than that of monolayer GDY. Under a vertical electric field, the band gaps of all these configurations decrease with the increase in the field strength. It implies that one can tune the band gap of GDY by a vertical electric field, which will widen the application scope. The electronic structures of several possible GDY bulks with different stacking types were investigated by Luo et al. by using density functional theory (DFT) plus van der Waals (vdW) density functional.20 Bulk GDY can be either semiconductive or metallic due to the stacking styles. Additionally, four threedimensional (3D) GDY systems, namely Tri-C18, Hex-C36, TriC54, and Orth-C72, were studied by first-principles calculations by Hu et al.21 Except for one semimetallic phase of Tri-C18, the other three systems show the direct band gaps of 1.55−1.74 eV, much wider than that of monolayer system. It is clear that despite that, stacking style has some effects on the band gap of bulk GDY, however, which is much wider than only several electron volts for few-layered systems. The dependences of band gap and effective mass of carriers of AB(β1) and AB(β2) systems of bilayer GDY on the electrical field strength were calculated by Zheng et al.19 (see Figure 4). C


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Figure 6. (a) Typical plots of the electron-emission current density (J) as a function of applied electric field (E). (b) Corresponding F−N plots and linear fitting. Reprinted with permission from ref 31. Copyright 2015 American Physical Society.

of oxygen molecules and hydroxyl functional groups, as well as pyridinic nitrogen sites in each sample, and found that the defect type varies with the thicknesses of the samples. The results revealed that the oxidization degree of GDY has the substantial effect on the band gap. Because oxidization is unavoidable for GDY, the results may provide the valuable suggestion to the real application of GDY. Furthermore, metal-doped GDY was investigated by Nayebi et al. using first-principles calculations based on self-consistent charge density functional tight-binding.30 The electronic properties of the optimized GDY sheet, nanoribbons and GDY decorated with metal atoms on the different sites, were investigated in details. The results showed that (i) the structures are direct semiconductor for Ag decorated and indirect semiconductor for Co and Fe decorated for the 6C hole, whereas (ii) are metallic for decorating at top and bridge sites. All the GDY nanoribbons with the size of N = 1, 2, 3, and 4 are nonzero band gaps at the Γ point. Moreover, the band gap decreases with the increase of the nanoribbons size, similar to the case of graphene nanoribbons. These works indicated that one can increase the band gap by decreasing the size. Besides tailoring the size of GDY, chemical doping, and external strain and electrical field, the preparation of GDY isomer is another route to tune its electronic properties. GDY nanowalls synthesized by Zhou et al. using a modified GlaserHay coupling reaction exhibit excellent and stable fieldemission properties (Figure 6).31,32 Moreover, GDY-nanotube (GDYNT), which is another type of GDY, can be formed by rolling a GDY sheet. The nomenclature (n, m) can also be employed for GDYNTs, in which (n, n) and (n, 0) represent zigzag and armchair GDYNTs, respectively. The results showed that the currents in the zigzag GDYNTs are remarkably higher comparing to the armchair GDYNTs.33 Notably, the excellent semiconductors with a conductivity of 1.9 × 103 S m−1 and a mobility of 7.1 × 102 cm2 V−1 s−1 of GDY nanowires (GDNWs) were measured by Qian et al.34 Their results revealed that GDNW may be a promising material in electronic and photoelectric fields.

0.47 eV to zero. This work may help us to further understand the strain-induced change in the electronic structures of GDY. Numerous studies have shown GDY’s various applications for its excellent electronic properties. For example, Sun et al. conformed that GDY is a good thermoelectric material with a high figure of merit.23 Kuang et al. demonstrated that GDY has great potential for applications in photovoltaic field owing to its networks with both delocalized p-systems and unique conductivity.24 Their results showed that doping PCBM layer with GDY can improve the electron transport, and the maximum PCE reaches high up to 14.8%. The enhancement in the performance was attributed to that GDY-doping resulted in improvement in the coverage on the perovskite layer, electrical conductivity, electron mobility, and efficient charge extraction. GDY’s great potential in flexible resistive random access memory devices has been confirmed by Jin et al.25 Therein, the OFF state and the two ON states possess long retention times (above 104 s). In the designed three-state memory device (PET/Ag/PI/GD/PI/Al−Al2O3/PI/Al), GD NPs and Al−Al2O3 NPs, which have different energy levels. work as two types of strong electron traps, resulting in two ON states. This work demonstrated that GDY has great potential applications in future information storage technologies. Similar to the case of other carbon materials, chemical doping also can change GDY’s physical properties. For example, Koo et al. calculated the band gaps of GDY doped with hydrogen or different halogen elements through an ab initio study.26 They found that the band gap of GDY can increases from 0.5 to 5.2 eV by hydrogenation or halogenation. Their results revealed that the band gap of GDY can be tuned in a wide range, which is favorable to its various applications. Bu et al. performed the first-principles calculations on the electronic structures of GDY doped with boron−nitrogen (BN) units.27 Their results showed that with the increase of BN content, the band gap increases first gradually and then abruptly. Or say, one can tailor the band gap of GDY by controlling the content of BN unit, which provides an easy route to engineering the band gap of GDY for widening the scope of applications. Zhong et al. examined the electronic structure of GDY exposed to air for 3 months by using X-ray absorption spectroscopy and scanning transmission X-ray microscopy.28 They found the oxidation in GDY, and the change from carbon−carbon triple bonds at defect sites to double bonds, indicating that GDY has lower air-stability than graphene. The electronic structures of three GDY samples with different thicknesses were investigated by Ketabi et al.29 The band gaps of three GDY samples respectively are 0.6, 0.8, and 0.9 eV. Moreover, they experimentally identified the existence

3. OPTICAL PROPERTIES Compared to the extensive researches have been made in electronic properties of GDY, the research on its optical properties is relatively scarce. In view of the facts that GDY is a semiconductor with a direct band gap, and which can be tuned by intrinsic microstructures (such as size, layer number, and stacking configuration and so on), chemical doping, and external strain and electrical field, studying the optical D


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Figure 7. (a) Imaginary part of polarizability of GDY calculated using the LDA+RPA (black solid line), GW+RPA (green dotted line), and BSE (red solid line) approaches. (b) Experimental absorbance (blue circle) of GDY film and theoretical absorbance at the GW+RPA (green dotted line) and BSE (red solid line) level of GDY. Reprinted with permission from ref 18. Copyright 2011 American Physical Society.

Figure 8. Polarized optical absorption spectra of monolayer GDY and the AA, AB-1, AB-2, AB-3 stackings along the (a) X and (b) Y directions. (c) Unpolarized optical absorption spectra of the AB-2 and AB-3 stackings. Reprinted with permission from ref 20. Copyright 2013 American Chemical Society.

result given by the BSE level most matches the experimental result. Moreover, Chopra studied the optical properties of GY and GDY,35 and found that the absorption range is ca. 1.59− 3.59 eV for GY, much wider than that for GDY (1.91−2.49 eV). All these results indicated that the optical properties of GDY are greatly different from that of graphene due to the great difference in their band structures. Luo et al. discussed the effect of vdW force on the optical absorption of monolayer GDY and four different stacking systems (Figures 8a−c).20 Their results revealed that (i) all the spectra show three major peaks (ca. 1, 1.7, and 4.02 eV), and all the configurations have the same peak at 4.02 eV; however, (ii) with decreasing energy, the difference in the spectrum in the energy region below 1 eV become obvious. For the AA stacking, the spectra along the polarization directions of X and Y are similar, and the lowest peak with the highest intensity is at

properties of GDY and its derivatives is favorable to both to the fundamental physics and applications. Luo et al. calculated the optical properties of GDY.18 As shown in Figure 7a, one can find that (i) the optical absorption spectrum at the LDA+RPA level has three peaks, which are respectively centered at 0.66, 1.77, and 4.02 eV. Herein, the first peak is considered to originate from transitions around the band gap and the others from transitions around the Van Hove singularities at the M and K points. Moreover, quasiparticle effects can shift the spectrum to ca. 1.18, 2.50, and 4.93 eV, respectively. They also found that if the electron hole interactions are considered, the main peaks may locate respectively at 0.74, 1.75, and 3.99 eV. Furthermore, the excitonic spectrum around the Fermi level shows that the electron−hole binding energy is ranged from 0.55 to 1.17 eV. Figure 7b confirms that compared with the other two ones, the E


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Figure 9. Projected DOS for (a) GDY-Cl (n = 0.5) and (b) GDY-Cl (n = 1). Reprinted with permission from ref 41. Copyright 2016 Iranian Chemical Society.

photoacoustic imaging and photothermal therapy in living mice.40 With a large extinction coefficient in near-infrared region under irradiation by 808 nm laser, GDY not only exhibits a stable photothermal performance with a high photothermal conversion efficiency of 42% but also provides an excellent photoacoustic response. This finding implies the novel potential in the application such as photothermo-acoustic wave nanotransducers, and thus widened the application scope of GDY. Because the chemical doping can effectively change the electronic properties of GDY, its optical properties would be changed by doping. Houshmand et al. have studied the optical properties of halogenated GDY, and found that halogenation of GDY could effectively modulate the band gap and thus affect the optical properties of GDY.41 Their results showed that (i) halogenated GDY with n = 0.5 had a remarkable occupied energy state (2.5 eV) around the Fermi level; but (ii) this occupied state in n = 1 shifts toward the Fermi level (1.5 eV). By comparing total projected DOS in valence shell of n = 1 with n = 0.5 (Figure 9), it is found that the injection of electrons from chlorine atoms to sheet obviously increased the occupation of valence bands of carbon atoms. Furthermore, the optical absorption is dominated by excitonic effects with a high electron−hole binding energy within the Bethe−Salpeter equation. Band structures of pristine and halogenated GDY indicate that these nanostructures are semiconductors with a direct band gap of ∼0.5 eV at the center of Brillouin zone. All these results clearly showed that one can tailor the optical properties of GDY for various applications by elemental doping.

0.11 eV. In the case of AB-1 stacking, the lowest peak sits in both directions are at 0.15 eV, but the intensity in the Y direction is ∼40%, much greater than that in the X direction. The spectrum of AB-2 stacking is similar to that of monolayer. In the case of AB-3 stacking, the spectrum shows two peaks (at 0.28 and 0.49 eV) in the X direction combined with a peak at 0.67 eV in the Y direction. Compared to the case of monolayer system (0.68 eV), (i) the first peaks of 0.62 eV for AB-2 and 0.31 eV for AB-3 stacked bulk show red-shift respectively with 0.06 and 0.37 eV; and (ii) the first peaks of the AA and AB-1 stacked bulk show red-shift respectively with 0.57 and 0.53 eV. All these results revealed that the optical absorption of GDY is dependent on both its layer number and stacking arrangements. Or say, the optical absorption spectrum is favorable to distinguish the layer number and stacking arrangements. Due to the special optical properties, GDY shows its great potential in optical related applications. Li et al. has reported a GDYbased metal-free material as hole transfer layer to fabricate quantum dot-sensitized photocathodes for hydrogen production.36 The results showed that (i) upon irradiation by 300 W Xe lamp, this assembled photocathode exhibits nearly −70 μA cm−2 photocurrent, and simultaneously reaches a high speed of 27000 μmol h−1 g−1 cm−2 for hydrogen production, and (ii) the faradic efficiency is high up to 95%. The results clearly showed that GDY is a potential hole transfer material in PEC water splitting cell. Furthermore, GDY also exhibits a high fluorescence quenching ability and can be used as the platform for fluorescence sensing, and the oxidized GDY shows a higher quenching ability than GDY.37 Because of the large specific area, an intrinsic band gap, and high hole mobility, the composite of GDY and other materials may obtain device with improved performance. For example, GDY/ZnO nanocomposites have shown excellent performance in UV photodetectors. Jin et al.38 fabricated the GDY/ZnO film, and the extracted electron mobility in the GDY/ZnO film is 2.4 × 10−4 cm2 V−1 s−1. Furthermore, GDY has also been confirmed to have diverse applications in enhancing optoelectronic devices.39 The current density−voltage characteristics of GDY in PbS CQD solar cells indicated that JSC is 21.74 mA cm−2, VOC is 0.650 V, and FF is 67.34%. The device exhibits higher external quantum efficiency and larger JSC than single GDY device. Moreover, Li et al. used GDY as photothermoacoustic wave nanotransducers for simultaneous effective

4. MAGNETIC PROPERTIES Magnetism of 2D carbon-based materials has attracted remarkable attention for the potential applications in spintronic devices due to their long spin coherence length.42−48 However, these materials are intrinsic nonmagnetism without any local magnetic moments. The discussion on the magnetism of GDY is important but remains very scarce. Theoretically, Kang et al. reported that oxygen adsorption on single-layer GDY could induce magnetism.49 The intrinsic magnetism of the as-prepared GDY (GDY-as) and the annealed GDY samples (GDY-300, GDY-400 and GDY-600, here 300, 400, and 600 refer to the annealing F


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Figure 10. (a) Magnetic moment ΔM measured at 2 K of the as-prepared and annealed GDY as a function of H. Symbols and the solid curves respectively are the measurements and fitted to the Brillouin function. (b) χ−T curves (applied field H = 1 kOe). Insets are the corresponding 1/χ − T curves. Symbols and the solid lines respectively are the measurements and are fitted by the Curie−Weiss law. (c) Spin charge density distribution for hydroxyl absorption. (d) Calculated migration energy barrier of hydroxyl on the GDY sheet migrating from the ring site to the chain site. Reprinted with permission of ref 50. Copyright 2017 AIP Publishing.

temperature of the samples) was first experimentally studied by Zheng et al.50 Their results revealed that GDY-as, GDY-300 and GDY-400 are spin-half paramagnetism (Figure 10a and inset of Figure 10b). Ms of GDY-as, GDY-300, and GDY-400 respectively are 0.12, 0.43, and 0.21 emu g−1. The number of spins N of GDY-as, GDY-300, and GDY-400 respectively are 1.29 × 1019, 4.74 × 1019, and 2.26 × 1019 g−1. An interesting result is antiferromagnetism of GDY-600 with the unsaturated magnetization even at H = 6.5 T and a hump in the χ(T) curve from 50 to 200 K. They proposed that the hydroxyl groups at the ring site are the magnetic source with the magnetic moment of about 1.0 μB (S = 1/2) of GDY, and the spin density mainly results from the odd nearest-neighbor sublattices (Figure 10c). The more interesting result is the high barrier energy of 1.73 eV for hydroxyl groups (Figure 10d), which is much higher than 0.4 eV for the case of graphene. They proposed that the high barrier energy of hydroxyl groups in GDY is the reason for the observed antiferromagnetism in GDY-600. Generating sp3-type functional groups in graphene sheets is confirmed to be an effective route to tune graphene’s magnetism.51−57 However, the low barrier energy for graphene leads to these groups’ clustering together easily, which will cancel its magnetism.51−54 Reasonably, because of its high mobility and similar but higher barrier energy for sp3-type groups than graphene, GDY may be the alternative to graphene.55−57 Zhang et al. studied the magnetism of pristine and N-doped GDY.58 Their results showed that pristine GDY is paramagnetic, and the magnetic source may be the special sphybridized carbon atoms. Their experimental results showed that doping GDY with 5.29% nitrogen could result in the enhancement of magnetic moment. Their calculations results demonstrated that nitrogen on benzene ring of GDY sheet could improve the magnetic moments. These results indicated that N-doping can induce magnetic moments in GDY.

Additionally, several types of Pd-doped GDY were also reported to have magnetic state (Table 2).59 As shown, the Table 2. Energy Band Gaps, Egap, of GDY-Pdn, Change in the Gap with Respect to Clean GDY, and Magnetic Moment M of GDY-Pdn from Ref 59 system

Egap (eV)

ΔEgap (eV)

GDY 0.93 GDY-Pd 0.79 −0.14 GDY-Pd2 0.35 −0.58 GDY-Pd3 0.46 −0.47 GDY-Pd4 0.26 −0.67 GDY-Pd5 0.25 −0.68 GDY-Pd6 0.07 −0.86 GDY-Pd13 0.18 −0.75 Pdn and Pd adsorbed on different triangular holes GDY−Pd-Pd 0.70 −0.23 GDY-Pd3−Pd 0.51 −0.42 Pdn adsorbed on both sides of GDY GDY-2Pd6 0.07 −0.86 GDY-2Pd13 0.03 −0.90

M (μB) 0 0 0 0 0 0 0 0 0 0 2 2

highest magnetic moment induced by Pd13 on pristine GDY can be 4 μB. Moreover, He et al. studied the magnetism of single 3d transition-metal (TM) atom (V, Cr, Mn, Fe, Co, and Ni) adsorbed GDY using DFT.60 They found that the adsorption of TM atom can introduce magnetism, which mainly results from the charge transfer between TM adatom and GDY as well as the electron redistribution of the TM intraatomic s, p, and d orbitals. According to their results, Co-doped GDY have spin-polarized half-semiconductor character, with 100% spin-polarization at the HOMO state. When Cr/Mn/Fe/ Co is adsorbed on GDY, the system become metallic with G


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the physical properties as confirmed by experiments. Therefore, searching an effective method to reduce the oxidized GDY and restore the structural defects, is also urgent and of great significance. Anyway, to stimulate the identification of the fundamental physical properties and the exploration of various related applications of GDY, great efforts from both the experiment and theory are necessary.

nonzero net magnetism. Ni adsorption does not introduce magnetism, but narrows the band gap compared with that of pristine GDY. Their results provided a possible route and some suggestions to tune the electronic and magnetic properties of GDY by TM adatom.

5. CONCLUSIONS In summary, we have reviewed the physical properties from theory to experiments and the corresponding applications of GDY and GDY doped with H, F, Cl and some metal elements. The review of the physical properties and the relative applications should be valuable to further studies. We believe that GDY and its derivatives have the potential applications in physical devices, such as photoelectricity, photocathode, strain sensor, and spintronics devices and so on. The performance of GDY in electrical, optical and magnetic properties is noteworthy. Compared with graphene which is constructed only by rings, GDY with the special rings combined chains structure have many superior physical properties. For example, both GDY and graphene have the very long spin diffusion length and high carrier mobility at room temperature. However, GDY has an intrinsic band gap of 0.46 eV unlike graphene with the zero band gap, which makes GDY to replace graphene in the various applications as 2D semiconductor and spin-based semiconductor. Furthermore, by functionalization or applying an external strain or electric field, the physical properties of GDY show higher turnability than graphene. Both graphene and GDY are expected to be magnetic by sp3-type adatoms, which allow the potential applications in spintronics devices. Nevertheless, GDY has the higher barrier energy than graphene for preventing sp3-type groups clustering together,50−57 which is the superior factor for tuning the magnetism of GDY. Thus, magnetic GDY doped with sp3 groups may have more possibility than graphene in the applications of 2D carbonbased spintronics devices. Overall, due to the coexistence of sp and sp2 hybridization and the unique rings combined chains structure, GDY and its derivatives have many novel physical properties and making them promising materials for various applications. It should be noted that despite that great progresses have been made in GDY, some challenges remain in the research field. In general, identifying and deeply understanding the intrinsic physical properties of the pristine GDY may be the milestone, which is necessary and crucial to its fundamental studies and real applications. Nevertheless, because of the fact that obtaining high-quality GDY have still been difficult until now, the current theoretical studies go well beyond the experimental investigations, and the combination of the experimental investigation and the theoretical simulation of intrinsic GDY is still lacking. For instance, many theoretical studies have predicted the layer number- and stacking styledependent physical properties of pristine GDY. Typically, the theoretical studies have showed the layer number- and stacking style-dependent band gap of GDY. However, the fact that synthesis of such pristine GDY with different layer number and stacking styles remains a great challenge, which no doubt limits the identification of the intrinsic physical properties already predicted by theories. Considering that prepared GDY always is multilayered, exploring the effective routes for synthesizing GDY with different layer numbers is highly desirable. Furthermore, the as-prepared GDY generally contains many O- and N-containing functional groups, which would change

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shaolong Tang: 0000-0002-6705-3209 Nujiang Tang: 0000-0003-4541-2183 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the State Key Program for Basic Research (Grants 2017YFA0206304 and 2014CB921102), NSFC (Grant 51572122), and NSF of the Jiangsu Higher Education Institutions (Grant 14KJB140003), China.

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