Room-temperature Ferromagnetism in Sulfur-doped Graphdiyne

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Room-Temperature Ferromagnetism in Sulfur-Doped Graphdiyne Semiconductor Mingjia Zhang, Huijuan Sun, Xiaoxiong Wang, Huiping Du, Jianjiang He, Yun-Ze Long, Yanliang Zhang, and Changshui Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10507 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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The Journal of Physical Chemistry

Room-temperature Ferromagnetism in Sulfur-doped Graphdiyne Semiconductor Mingjia Zhang,a, † Huijuan Sun,b, † Xiaoxiong Wang,b Huiping Du,a Jianjiang He,a Yunze Long,b Yanliang Zhangc and Changshui Huanga,* aQingdao

Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences,

Qingdao 266101, P.R. China. bCollege

of Physics, Qingdao University, Qingdao 266071, P.R. China.

cThermo

Fisher Scientific Ltd, Shanghai 201206, P.R. China.

Abstract: The realization of magnetic ordering in two-dimensional graphitic semiconductor, graphdiyne, has attracted great interest due to its promising potential application in semiconductor devices involving spin. Here, we propose a simple and feasible sulfuration strategy to induce robust ferromagnetic ordering into graphdiyne and realize the coexistence of room-temperature ferromagnetism and semiconductivity in graphdiyne without extrinsic magnetic impurity. The robust residual magnetization of more than 0.047 emug−1 at room temperature and the transition temperature of up to 460 K indicate great potential for application in magnetic storage. The subsequent spin-polarized density functional theory calculation reveals that the intrinsic ferromagnetic ordering originates from the enhanced local magnetic moment and non-local electron transfer between carbon atoms and sulfur atoms, which is well confirmed in our electrical measurements. This synthetic strategy could spur studies of two-dimensional

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magnetic semiconductor based on graphdiyne and even accelerate the potential application of graphdiyne in spintronic devices.

1. Introduction Since the unique quantum manipulating capability and low power consumption, twodimensional (2D) carbon materials exemplified by graphene have been placed great hopes on next-generation electron devices.1 Especially in the fabrication of spintronic devices, 2D carbon materials have received much attention2 because these metal-free materials with s/p electrons present long spin diffusion length3-4 as well as weak spin-orbit coupling.5 Due to many spintronic devices such as spin light emitting diode and spin transistor need to introduce room-temperature ferromagnetism into a semiconductor,6 novel material systems with semiconductor properties need to be developed. Recently, graphdiyne, a semiconducting 2D carbon material with planar structure has been proposed as a promising material for many potential applications due to its fascinating properties such as moderate direct bandgap of about 1eV,7-8 high mobility comparable with graphene9 and so on. In particular, previous studies have shown that graphdiyne is characterized by intrinsic paramagnetism.10 If we can endow graphdiyne with ferromagnetism, it will be expected to take full advantage of both charge and spin of electrons and thus become an ideal candidate for spintronics.11 Although we have reported that nitrogen doping and transition metal hybridization can significantly improve local magnetic moment and thus induce paramagnetism10 and even extrinsic ferromagnetism,12 intrinsic ferromagnetic ordering has not yet been realized in graphdiyne up to now, which becomes a critical issue that restricts its application in spintronics. To obtain ferromagnetic two-dimensional carbon materials, the chemical modifications based on element doping or substitution are widely used to introduce local magnetic moment.13-15


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For example, hydrogen,13 fluorine14 and nitrogen15 doped graphene are found to have a greater number of local magnetic moments, resulting in macro paramagnetism or ferromagnetism. In particular the chemical modification methods using sulfur element demonstrate the unique spin regulatory effect. In graphite-sulfur composites, sulfur adsorption induces the coexistence of ferromagnetic and superconducting-like magnetizations within some sort of domains.16 For graphene, R. Zboril et al. found that only 4.2 % sulfur doping can realize strong ferromagnetic properties in graphene below T≈62 K due to the appearance of C-S bond in the carbon matrix.17 Similarly, a surface modulation strategy related to sulfur-concentration has also been successfully

used

to

realize

ferromagnetism

as

well

as

large

room-temperature

magnetoresistance.18As for graphdiyne, recently sulfur doping has been first used to successfully improve its electrochemical performances.19 More importantly, first-principles calculations predict that the energy gap of graphdiyne can be efficiently controlled by varying the S-doping level in nitrogen/sulfur dual doping system.20 Motivated by the above results, we anticipate that doped sulfur atoms can play an important role in the manipulation of local magnetic moment to realize the ferromagnetic graphdiyne semiconductor. In this work, the sulfuration strategy is employed to induce local magnetic moment into the graphdiyne material. By this method, we successfully realize the room-temperature ferromagnetism in graphdiyne semiconductor, which is investigated in detail through the measurement of magnetization curves and hysteresis loops. From the temperature dependencies of saturation magnetization and coercive field, we can obtain the ferromagnetic transition temperature Tc~460 K. Compared with the pristine graphdiyne annealed in the same process, as well as combined with theoretical calculation, we further reveal the important role of sulfur atoms in improving local magnetic moment and charge transfer. Considering that sulfur-doped


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graphdiyne (SGDY) demonstrates a typical semiconductor transport property, the coexistence of semiconductivity and room-temperature ferromagnetism in SGDY may encourage us to explore the application of graphdiyne in spintronics. 2. Experimental Section 2.1 Synthesis of sulfuretted graphdiyne. The graphdiyne was synthesized via a cross-coupling reaction on the surface of copper based on the previous report.21 Then the pristine graphdiyne films were obtained by the corrosion and removal of copper (the average thickness of the pristine graphdiyne film is about 1.70 m, see Figure S1). After multiple cleanings, the collected and dried graphdiyne flakes were ground into powders using an agate mortar. Then GDY powder and sublimed sulfur were thoroughly mixed with a mass ratio of 1:1 by means of ball-milling. Subsequently, the black mixture was heat-treated at 350 oC for 3h with Ar atmosphere to dissociate sublimed sulfur into short-chain sulfur. CS2 solution was used to remove the residual and unreacted sublimed sulfur. The final composite was dried in a vacuum oven at 100 oC for 2h and then annealed at 500 oC for 2h with vacuum-sealed in quartz tubes, which was marked as SGDY. As a comparison, another pristine graphdiyne powder was treated with the same annealing process, which was marked as GDY 350. The mass of these powders was accurately determined by High Precision Electronic Balances with a sensitivity of 0.01 mg. To perform the electrical conductivity measurement, the GDY 350 and SGDY dispersions were prepared by adding 2 mg of powder into 1 mL isopropyl alcohol under ultrasonic treatment for 72 hours respectively. The corresponding dispersion was dropwise added onto the one of the electrodes and then it was dried off. The measuring electrode was composed of two Ag-coated glass substrates assembled face to face configuration.


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2.2 Characterization of the samples. The X-Ray photoelectron spectroscopy (XPS, ULVACPHI) was measured to investigate the bonding environment and chemical component. The structure characteristics of GDY-350 and SGDY were analyzed by using Raman spectra (NTMDT NTEGRA Spectra system). Morphological information was measured using scanning electron microscope (Hitachi S-4800 FESEM). The transmission electron microscopy (TEM) was carried out on a JEOL 2011F and 2100F field emission electron microscope. UV−vis measurement was performed using the Hitachi U-4100 spectrometer. Magnetic moment measurements were recorded using a vibrating sample magnetometer (PPMS-VSM, Quantum Design) with temperature changing from 2 K to 300 K. The temperature dependent conductivities were also measured by PPMS with a two electrode configuration. 2.3 The DFT calculation methods. Spin-polarized DFT based on the Vienna ab initio simulation package (VASP) was used to perform the calculation of density of states (DOS). Projector augmented wave (PAW) method was adopted to describe the electron-nuclear interactions and the exchange correlation energy was described by the Perdew-Burke-Ernzerho (PBE) function. The energy cutoff for the truncation of the plane wave basis was taken to be 500 eV. The geometries were optimized with all forces ≤0.01 eV/Å, and the criterion of convergence for electronic structure was set to be 10-6 eV. Three stable structural models for the S-doping were constructed with different doping site of sulfur atom in the carbon matrix. 3. Results and Discussion The pristine graphdiyne films on the surface of copper were synthesized via a crosscoupling reaction. After cleaning and obtaining GDY flakes, the sulfur-doped graphdiyne (hereinafter to be referred as SGDY) was obtained by a thermal synthetic strategy, as depicted schematically in Figure 1a. Herein, the full grinding was used to realize a homogeneous mixing


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of GDY and sublimed sulfur (S8). Subsequently the composites were heat-treated at 350 oC to dissociate the S8 rings into short-chain sulfur and then annealed at 500 oC to obtain the S-doped GDY. In order to reveal the role of sulfur doping in structure and properties of graphdiyne,

Figure 1. (a) Schematic illustration for the synthesis of defective GDY (GDY 350) and S doped GDY (SGDY) respectively. (b) Raman spectra of GDY 350 and SGDY. (c) XPS result for SGDY. (d) C 1s and (e) S 2p spectrum of SGDY. another sample was prepared by treating pristine graphdiyne with the same annealing process, which was marked as GDY-350. Subsequently, detailed structural characterizations including Raman and X-ray photoelectron spectroscopy (XPS) measurements were carried out, as shown in Figure 1b-e respectively. Both the Raman spectra of GDY 350 and SGDY show two characteristic peaks related to graphdiyne at 1360 cm-1 and 1567 cm-1, which respectively corresponds to the structural defects (D band) and E2g vibrational modes (G band). It is should be noted that the acetylenic linkage at 2162 cm-1 21 becomes unapparent. Compared with pristine graphdiyne, the ID/IG ratios of both GDY 350 and S-GDY sample are significantly enhanced, suggesting the introduction of disorder or defect such as vacancy, which results in the


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suppression of the vibration of conjugated diine links.10, 22 The weakening and disappearance of this acetylenic band can be attributed to the heat-treatment process for graphdiyne.23 For SGDY, new peaks at 358 and 525 cm-1 can be assigned to C-S bonds and S-S bonds,24 indicating the interaction between sulfur atom and graphdiyne. Further, the overall chemical composition and bonding states of SGDY are analyzed based on the XPS spectrum shown in Figure 1c. No metal impurities such as Cu or Fe are present in our samples. Except for the essential C 1s peak as well as O 1s peak mainly arising from the adsorbent oxygen,22 a new peak at 164 eV assigned to S 2p peak can be observed,25 suggesting the efficient introduction of sulfur in GDY. Figure 1d shows the high resolution C 1s XPS spectrum of SGDY, which can be fitted and divided into four bonds including C-C (sp2), C≡C (sp), C-S (or C-O) and C=S (or C=O).26 As for the S 2p peak, it can be divided into two different peaks at 163.5 eV and 164.5 eV as Figure 1e shown, which could be attributed to S 2p3/2 and S 2p1/2 peaks of C-S-C respectively,25 further confirming the CS bonding characteristics. In particular, these two major peaks observed in S 2p spectrum reveals the formation of a thiophene structure between sulfur atom and its adjacent carbon atoms by spin–orbit coupling.27 Figure 2 shows the morphologies of GDY 350 and SGDY. The scanning electron microscopy (SEM) (Figure 2a) and transmission electron microscope (TEM) (Figure 2b) images of pristine GDY powder indicate the particle aggregation morphology and layer structure respectively. Through the high-resolution transmission electron micrographs (HRTEM) in the inset of Figure 2b, we can get the layer spacing of graphdiyne as 0.364 nm. As for SGDY (Figure 2c-e), though its overall morphology shows irregularly and randomly agglomerated features, at a higher resolution scale, the similar uniformly distributed nano-size particles can be observed compared with the GDY 350, demonstrating that doping of sulfur does not change the


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morphology obviously.10 Herein we also performed the scanning electron microscope chemical mapping to reveal the distribution of carbon and sulfur elements in SGDY, which is shown in Figure 2f and g. We can see that the sulfur atoms are effectively introduced into GDY by the thermal synthesis procedure. Subsequently, the TEM image is also obtained to confirm the microstructure with stacked layers for SGDY, which is shown in Figure 2h. Especially the HRTEM image with almost unchanged layer spacing (0.368 nm) shown in Figure 2i further excludes the obviously large cluster or impurity particles coming from the sulphur source, suggesting that our sulfuration strategy is relatively uniform due to the bonding between sulfur and carbon atoms.


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Figure 2. (a) SEM, (b) TEM images of GDY 350. (c)-(e) SEM images of SGDY with different scale bars changing from 2 m to 200 nm. (f), (g) Distribution of carbon and sulfur atoms on the SGDY sheet respectively. (h) TEM, (i) HRTEM images for SGDY respectively. The ultraviolet-visible reflectivity spectra of both raw GDY 350 and SGDY powders were measured to reveal the effect of sulfur doping on the band structure of graphdiyne (Figure 3a). Based on Kubelka–Munk transformation28-29, we obtain Eg values of 0.77 eV and 0.70 eV for GDY 350 and SGDY respectively (Figure 3b), indicating the narrowing of bandgap by sulfur doping. The suppression of bandgap can be attributed to the appearance of donor impurity30 or rehybridization of the carbon atoms.31 To further reveal the influence of sulfur doping on the transport properties of graphdiyne, experiments about the conductivity were also performed with a two electrode configuration. Figure 3c shows the I-V curves of GDY 350 and SGDY respectively, demonstrating a significantly enhanced charge transfer characteristic after sulfur doping. It also can be seen that the current increases with voltage almost linearly, which can be attributed to the Ohmic contact between Ag and GDY.32 Based on this linear relationship, we can obtain the conductivity of GDY 350 and SGDY as 1.35×10-4 Sm-1 and 1.49×10-3 Sm-1 respectively. What’s more, for SGDY, the temperature dependent resistivity curve shown in Figure 3d increases with decreasing temperature, suggesting a typical semiconducting transport property. These results confirm the semiconductivity of SGDY and demonstrate the enhanced conductivity due to the electronic structure modification of GDY by sulfur doping.


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Figure 3. (a) The ultraviolet-visible reflectivity spectra of both raw GDY 350 and SGDY powders. (b) The Kubelka-Munk spectra of GDY 350 and SGDY based on UV-vis absorption. (The error of the band gap value is less than 5 %.) (c) I-V curves of the raw GDY 350 and SGDY. (d) Temperature dependent resistivity of SGDY. Next, for investigating the influence of sulfur doping on magnetic properties of graphdiyne, we measured the temperature dependent magnetic susceptibility -T curves (the applied magnetic field H=500 Oe) and magnetization M-H curves for GDY 350 and SGDY by VSM (Quantum Design), which are shown in Figure 4a-d. Figure 4a shows that the magnetic susceptibility of GDY 350 increases with the decreasing of temperature and no obvious magnetic transition can be found. Especially the temperature dependence of χ can be well described by the Curie law, confirming the typical paramagnetic characteristic. Further, the magnetization curve for GDY 350 at 2 K was also measured (Figure 4b), which can be described by the paramagnetic Brillouin function as follows: 33


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M  M s[

2J 1 2J 1 1 x cont ( x)  cont ( )] 2J 2J 2J 2J

where Ms is the saturation magnetization, x=gJBH/kBT, kB is the Boltzmann constant, B is the Bohr magneton, J is the angular momentum quantum number and g is the g-factor. As the solid line shown, the experimental data can be well fitted with g=2 and J=1/2, suggesting the contribution of defects such as vacancy or edge on magnetization. Different from GDY 350, the

Figure 4. (a) Typical χ-T curve measured from 2 K to 300 K for GDY 350. (b) M-H curve measured at 2 K for GDY 350. (c) Temperature dependence of magnetization curves for SGDY. (d) Hysteresis loops of SGDY. (e) Temperature dependence of coercivity for SGDY. (f) Temperature dependence of the magnetization. magnetic behavior of SGDY measured under field cooling (FC) and zero field cooled (ZFC) modes displays a typical ferromagnetic ordering with a Curie temperature Tc above 350 K, which is shown in Figure 4c. Below 100 K, the dramatically increased magnetization may arise from the intrinsic paramagnetic background of graphdiyne.10 Besides, the ferromagnetic behavior was also confirmed by magnetic hysteresis (M-H) loop measurements with different temperatures


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(see Figure 4d). For T=300 K, the remanent magnetization Mr, coercive field Hc and saturation magnetization Ms are 0.047 emu/g, 450 Oe and 0.20 emu/g, respectively, demonstrating a robust room-temperature ferromagnetic ordering. Taking into account the intrinsic semiconducting properties of graphdiyne, SGDY can be expected to combine the advantages of 2D carbon materials and dilute magnetic semiconductors, which is crucial to the potential application of graphdiyne materials in nonmetallic magnet or magnetic memory. At low temperature, the magnetization at high field shows significant temperature dependence (Figure S2). Especially at 2 K, with the applied magnetic field increases gradually, the magnetization tends to saturate. This indicates that the effect of paramagnetism at low temperatures (less than 100 K) cannot be neglected. To gain further insight on the ferromagnetic characteristics of SGDY, we plot the temperature evolution of Hc and Ms extracted from the magnetization curves, which is shown in Figure 4e and f. Herein Hc(T) in the temperature range of 100 K-350 K can be described by Kneller’s law34 to obtained a Tc of 465 K. To verify the obtained Tc, we further use the modified Block law35 to describe the temperature dependent saturation magnetization in the temperature range of 75–350 K and also obtain the Tc value as 459 K. As for the deviation at low temperature, it can be associated with surface paramagnetic spins of SGDY. These results provide a reasonable and credible Tc value about 460 K, indicating a robust ferromagnetic coupling in SGDY.


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Figure 5. (a) DFT calculation models for three typical unit cells. (b)-(d) The corresponding spinresolved DOS of SGDY for adsorbed S, thiophene-like S and substituted S respectively. (e), (f) The calculated spin density of SGDY for unit cell-2 and unit cell-3 respectively. The orange and blue iso-surfaces are for spin up and down, respectively. For an in-depth understanding of the unique ferromagnetic behavior of S-doped graphdiyne, the electronic structure calculations based on spin-polarized density functional theory were performed. Considering the special structural characteristics of graphdiyne,36-38 we define the three typical structures with surface absorbed S atom, embedded S atom in the interior angle (which may be constructed as thiophene-like sulfur) and substituted S atom on benzene as Unit cell-1, Unit cell-2 and Unit cell-3 respectively (Figure 5a). In these simulation models, the S:C


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ratios are 1:18 (Unit cell-1, Unit cell-2) and 1:12 (Unit cell-3), respectively, which is close to the actual S/C ratio of 6.52 % obtained in the XPS measurement. As for the situation that replacing carbon atom in acetylene linkage by sulfur atom, it results in unstable structure and binding energy, suggesting that the probability of substitutional doping in acetylenic carbon is relatively low. The calculated density of states (DOS) are displayed in Figure 5b-d. Unlike the nonmagnetic DOS exhibited in pure graphdiyne (Figure S3.) and graphdiyne with absorbed sulfur atom (Figure 5b), an obvious local magnetic moment indicated by asymmetry between the spinup states and spin-down states appears in SGDY composed by unit cell-2 or unit cell-3. The calculated magnetic moment values are 2.02 μB and 1.37 μB respectively, confirming sulfur doping as an efficient strategy to introduce significant local magnetic moment in graphdiyne. Considering the long spin-diffusion length in carbon material, improved local magnetic order is beneficial to long-range ferromagnetic coupling. Furthermore, to analyze the origin of ferromagnetism, we must take the excellent charge transfer characteristic of graphdiyne39 into account. The DFT simulation process also indicates the existence of charge transfer between carbon atom and sulfur atom, which has been diagramed in the form of electron cloud distribution in Figure 5e and f. The quantitative charge transfer is studied by bader charge analysis. Corresponding to Unit cell-1, Unit cell-2 and Unit cell-3, 0.09 e, 0.22 e and 0.36 e are found transferred from the S atom to the neighbouring C atoms respectively indicating the enhanced carrier transport by sulfur doping, which is consistent with the conductivity measurement results. It should be pointed out here that the GGA correlation is used in the above DFT calculation. Even if it is replaced by LDA or LDA+U (Figure S4 and Figure S5), the calculated local magnetic moment and electron transfer values have little difference.(see Table S1) With the enhancement of local magnetic moment and charge transfer, a robust ferromagnetic


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coupling occurs by exchange interaction such as Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction among different local magnetic orders40. The magnetic coupling formed by the RKKY interaction decays with D-r, where D is the distance between the magnetic moments and r is the decay exponent. The introduced local magnetic moments will decrease the distance and the enhanced electron transfer may decrease r,41 thus the magnetic coupling between the magnetic moments is enhanced to form ferromagnetic order. When the S/C ratio is low, the distance between the magnetic moments becomes longer, while for the high S/C ratio, the instability of the structure leads to unzipping of the carbon matrix17 in graphdiyne, both of which are not conducive to the formation of ferromagnetic coupling. As for our sulfur-doped graphdiyne, further experiments are still needed in the future to reveal the effects of different sulfur contents on the electromagnetic properties of graphdiyne. Up to now, our experiments and theoretical analysis both verify the coexistence of ferromagnetis and semiconducting conductivity in SGDY, which makes SGDY to be a vital candidate in novel spintronics. 4. Conclusion In conclusion, we synthesized sulfur-doped graphdiyne with a simple and feasible method and investigated its magnetic and transport properties. Structural and morphological characterization indicates that the sulfur atom is effectively incorporated into the graphdiyne. Based on the measurement of M-T curves and M-H loops, we demonstrate experimentally that SGDY is a typical ferromagnetic material with Tc=460 K. Spin-polarized DFT calculation further reveals the important role of doped S atom in improving local magnetic moment. Because of the enhanced non-local charge transfer induced by S doping, long-range ferromagnetic ordering occurs, resulting in the outstanding ferromagnetism. Considering the intrinsic semiconductor


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properties of SGDY, we expect to apply this kind of 2D carbon materials in future spintronic devices. ASSOCIATED CONTENT Supporting Information. Cross-sectional SEM image of pure graphdiyne, M-H curves at high field of SGDY, DOS of pristine GDY, DOS of SGDY with LDA correlation, DOS of SGDY with LDA+U correlation and the comparison of calculation results with different corrections. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (51802324, 21790050, 21790051, 51822208, 21771187), the Frontier Science Research Project (QYZDBSSW-JSC052) of the Chinese Academy of Sciences, and the Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (JQ201610). REFERENCES (1) Wei, D. S.; Van der Sar, T.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; JarilloHerrero, P.; Halperin, B. I.; Yacoby, A., Mach-Zehnder interferometry using spin- and valleypolarized quantum Hall edge states in graphene. Sci. Adv. 2017, 3, 1700600.


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Table of Contents Graphic and Synopsis


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Figure 1 147x76mm (220 x 220 DPI)



Figure 2 146x110mm (220 x 220 DPI)


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Figure 3 55x42mm (300 x 300 DPI)



Figure 4 289x144mm (300 x 300 DPI)


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Figure 5 705x715mm (72 x 72 DPI)


Room-temperature Ferromagnetism in Sulfur-doped Graphdiyne

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