Structural Evolution of Flower Defects and Effects on the Electronic

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Structural Evolution of Flower Defects and Effects on the Electronic Structures of Epitaxial Graphene Yufeng Cui, Huisheng Zhang, Wei Chen, Zhongqin Yang, and Qun Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04693 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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The Journal of Physical Chemistry

Structural Evolution of Flower Defects and Effects on the Electronic Structures of Epitaxial Graphene Yufeng Cui † §, Huisheng Zhang † ‡, Wei Chen † §, Zhongqin Yang † §, Qun Cai * † § † State Key Laboratory of Surface Physic & Department of Physics, Fudan University, Shanghai 200433, China ‡ College of Physics and Electronic Information, Shanxi Normal University, Linfen 041004, China § Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

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ABSTRACT: The structural evolution processes of flower defects in epitaxial graphene at high temperature annealing are investigated by using scanning tunneling microscopy, Raman spectroscopy and the density functional theory calculations. The experimental results reveal that flower defects can increase in amount with the increasing of annealing time till they nucleate and evolve into complex defect structures. As the direct evidence of the structural development for flower defects evolving into the complex structures, the conjoined-twin defect is detected in epitaxial graphene with two of 2/3 flower defects joined together. The theoretical calculations show that the conjoined-twin defect has a calculated energy 2.7eV smaller than that of two isolated flower defects. And the conjoined-twin defect as a new topological defect in graphene can bring about a bandgap of 50meV. The calculation results also demonstrate that new van Hove singularity states will be generated in the vicinity of +(0.4~0.7)eV and -0.4eV in the density of states for the dislocated carbon rings of the conjoined-twin defects. These results can provide valuable insights into the growth evolution of topological defects and their effects on the electronic structures of graphene.

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■ INTRODUCTION Pristine graphene is a two-dimensional material made of hexagonal lattice of sp2 bonded carbon atoms, and usually exists as an undulate single atomic layer with a thickness of 0.335nm. By reason of its lattice symmetry, graphene is known as a semimetal with the Dirac-cone structures near K points in its Brillouin zone and possesses itself of unique properties, making it highly attractive for many applications 1, 2

. A great deal of techniques can be utilized to prepare graphene, e.g. mechanical

exfoliation, chemical vapor deposition, chemical synthesis and epitaxial growth on SiC 3-6. And intrinsic or extrinsic defects can hardly be avoided to appear in graphene fabricated by means of all the aforesaid methods. The topological defects such as vacancies, Stone-Wales (SW) defects, grain boundary loops and dopant-induced defects are commonly observed in graphene

7-10

.

It is established that these defects can modify the electronic structures of graphene even for rather small concentration, and therefore will influence its chemical, mechanical, transport and magnetic properties

11, 12

. Density functional calculations

have shown that SW defects in graphene locally change the density of π-electrons and open an energy gap in its Dirac band structures which decreases with the increasing of graphene layers

13

. The experimental researches have also demonstrated that

vacancies in graphene can enhance its chemical reactivity, effectively open a bandgap, and probably induce a ferrimagnetic state in multilayered graphene

14, 15

. Due to the

absence of a bandgap, graphene transistors show an on-off switching ratio much lower than the required

16

, which impedes graphene being widely used in post-Si

nanoelectronic devices. Intromitting defects into graphene presumably becomes a promising way for opening bandgap and modulating transport properties to graphene.

3



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For epitaxial graphene grown on SiC substrates, there are usually linear grain boundaries, grain boundary loops and tubular mounds/ridges existing in the hexagonal lattice on the sample surfaces 17-20. A linear grain boundary is usually composed of a chain of carbon pentagon rings separated by heptagon or octagon rings, and when this dislocation chain is arranged into a closed loop, a grain boundary loop will be observed embedded in the pristine graphene lattice

21

. Consequently, a series of

extended topological defects can be obtained, e.g. the SW defect as the smallest loop consists of two pairs of pentagon-heptagon rings

21

. Six carbon pentagon-heptagon

pairs linking together will form the most stable structure among the grain boundary loops

21

. This grain boundary loop with six-fold symmetry is always displayed as a

“flower” pattern (so-called flower defect) in transmission electron microscopy images for graphene grown on Ni substrate

22

(STM) images for graphene on SiC

17, 18

, and also in scanning tunneling microscopy . Recently, STM experiments have verified

that van Hove singularities (VHSs) appear near the Fermi energy in the density of states (DOS) for two kinds of ordered grain boundaries in graphene

23

. These VHS

states can bring about a significant increasing of conductance around the defects, and even superconductivity in the doped graphene, opening a new way to modulate the transport properties of graphene devices

24-27

. It is also found that the thermal

conductivity of graphene with flower defects is diminished due to strong defect scattering and very sensitive to defect concentration 28. Although flower defects have been explored in recent years in the experimental researches, their origin and evolution with sample treatments are still not clear. The ring structure defects usually seen in graphene on SiC were speculated coming from the nucleation of flower defects

18

, but more direct experiment evidences are needed to show its developing

process. The better understandings of these issues will provide insights into reliable 4


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control of defect distributions in graphene, an important step for making new graphene-based devices. In this work, epitaxial graphenes with topological defects were grown on the SiC(0001) substrates via thermal decomposition, and the structural evolutions of flower defects in high temperature annealing were investigated with in situ STM observations and ex situ Raman measurements. Our experimental results have verified that isolated flower defects can nucleate together and evolve gradually into the complex defect structures during the annealing duration. And the fascinating results have revealed a new topological defect in graphene, emerging as the initial stage during the defect evolutions. This new defect structure is composed of two of 2/3 flower defects spliced together, like a pair of conjoined twins (named as “conjoined-twin defect” here). The density functional theory (DFT) calculations were performed to explore the electronic band structures of graphene with flower defects. The calculated results manifest a gap of 50meV opened for the graphene with the conjoined-twin defects. And some new VHS states are shown to appear near the Fermi energy in the DOS for the conjoined-twin defect, which will probably improve the electronic transport properties of graphene-based devices.

■ EXPERIMENTAL SECTION Experimental Methods. The experiments were carried out mainly in an Omicron room-temperature ultrahigh-vacuum STM system with a preparation chamber, and its base pressure was better than 2.0×10-10 mbar. The data processing system was Matrix 3.1. The substrates were cut from commercial Si-terminated 6H-SiC(0001) and 4H-SiC(0001) wafers, with a nitrogen doping density of 1018~19 cm-3. The sample size is 10 × 5.0 × 0.3 mm3, and the miscut angle is less than 0.5°. After being introduced into the vacuum chamber, the samples were degassed at a temperature less than 600℃ 5



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for about 120 min, and then annealed at ~ 1000oC for 30 min via direct current heating to remove oxides and contaminations. Finally, the samples were annealed at ~ 1350oC for more than 60 min to form the defect-furnished graphene on the surface. The STM measurements were performed in situ at room temperature in the constant current mode, and the STM tips were prepared using electrochemical etching of polycrystalline tungsten wires. Raman spectra were measured ex situ in a Dilor LabRam-1B system at room temperature, with a laser of 632.8nm as excitation source and the spot size of 0.7 µm. The ARPES measurements were carried out at 4K in ARPES station of Beijing Synchrotron Radiation Facility, with a photon energy of 21.2 eV. The energy resolution is 7 meV. Density Functional Theory Calculations. The DFT calculations were performed using the Vienna ab initio simulation package code to investigate the electronic structures and densities of states of graphene with flower defects

29, 30

, as well as the

energies of the flower defect formation. The Perdew-Burke-Ernzerhof gradient approximation is used to describe the exchange and correlation functional

31

. The

plane-wave cutoff energy is set to be 500eV and a vacuum space of larger than 15Å is set to avoid the interactions between two adjacent layers. All the atoms in the unit cells are allowed to move until the Hellmann-Feynman force on each atom is smaller than 0.01 eV/Å. A 15 × 15 supercell is used to simulate the graphene with a conjoined-twin defect. The gamma central Monkhorst-Pack K points of 2 × 2 × 1 are set for the systems.

■ RESULTS AND DISCUSSION Flower Defects and Their Evolution with Annealing Duration. The epitaxial graphenes were prepared on 6H- and 4H-SiC(0001) substrates via thermal 6


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decomposition in the ultra-high vacuum chamber. No results with discernible difference were obtained in this work for two kinds of SiC(0001) substrates. Annealing temperature and duration time were the key factors for various defects emerging subsequently in graphene. When the substrates were annealed at 1350oC for more than 60 min, flower defects in graphene could be observed usually in STM images on the graphitized surfaces. The STM images in Figure 1 demonstrate flower defects in epitaxial graphene, the sixfold symmetrical feature with a ring centered. This kind of defect was already observed in the previous experiments and was investigated theoretically

9, 17, 18, 21

.

Three flower defects can be recognized in Figure 1(a), as well as another kind of structural defect shown as a bright dot surrounded by a dark hollow. According to their structural characteristics exhibited in STM images at different bias, we believe that these white-dot defects are probably induced by N dopants 9, although these defects are not within the scope of this work. Figure 1(b) displays a STM image of flower defect in detail, and the inset shows the atomic structure of flower defect proposed by Cockayne et al. 21. It can be seen that flower defect is constituted by six pentagon-heptagon pairs. These six pairs are arranged in a sequential order into a closed loop, making the structural configuration of flower defect very symmetrical. The flower defect always looks like a hexangular star in STM images with its size less than 2nm. As shown in Figure 1(b), the aromatic-ring features are usually found coexisting around the flower defect. These (√3 × √3) R30o patterns are attributed to intervalley scattering between K and K’ states near Fermi level in graphene 17, 20, 21. In STM images obtained on the graphene/SiC samples, the detection of the contrast corrugation induced by the (6√3 × 6√3) R30o reconstructed SiC interface layer can be used to determine the layer thickness of graphene 32. In Figure 1(a), the weak contrast 7



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variation induced by 6√3 interfacial structure can be observed to spread periodically over the terraces. It is indicated that bilayer graphene exists on most of the sample surface after annealing at 1350oC for about 80 min.

a)

b)

Figure 1: STM topographic images obtained on the graphitized SiC(0001) surfaces. The annealing treatments for graphitization were performed at 1350oC for 80 min. a) Three flower defects can be found in epitaxial graphene, as well as the white-dot defects. The scanning area is 32.4 nm × 35.1 nm. Tunneling conditions: Vbias = 1.2 V, Iset = 120 pA. b) A highly magnified STM image of flower defect. The inset shows the atomic structure of a flower defect in detail. The scanning area is 4.3 nm × 4.7 nm. Tunneling conditions: Vbias = -0.3 V, Iset = 80 pA.

The origin of flower defect is still an open question at the moment. Based on the results of DFT calculations, Cockayne et al. suggested the flower defect presumably came from the coalescence of pentagon-heptagon pairs or SW defects at elevated temperature 21. However, no experimental evidence has been obtained till now. When flower defects appear abundantly in graphene under experimental observations, SW defect structures are seldom detected directly even at the initial growth stage. It is necessary to do more investigations for the growth of flower defects. The structural evolution with thermal treatments was studied in this work for flower defects. As 8


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displayed in Figure 2(a), the average areal densities of flower defects are plotted as a function of annealing duration. The densities were calculated according to the number of all flower-like defects observed on the sample surface, and the data were acquired on different samples. It shows that the densities get larger at first with the increasing of annealing time, and then they decrease a little till rise again. Figure 2(b) shows a STM image acquired on the sample annealed at 1350oC for 120 min. It can be seen that one flower defect stays quit close to another one, with a separation of 3.3 nm.

c)

a)

d)

b)

Figure 2: a) The areal densities of flower-like defects acquired on different samples are plotted as a function of annealing time. b) A STM image obtained on the sample annealed for 120 min. c), d) STM images obtained on the sample annealed for 190 min. The scanning area is: b) 14.0 nm × 15.2 nm, c) 8.6 nm × 9.4 nm, and d) 4.9 nm × 5.3 nm, respectively. Tunneling conditions: b) Vbias = 1.2 V, Iset = 100 pA; c) and (d) Vbias = 0.1 V, Iset = 300 pA.

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Figure 2(c) and 2(d) exhibit the typical STM images obtained on the samples annealed at 1350oC for 190 min. It is noticed that many complex defect structures are formed in graphene, very similar to the ring structures in Ref.18. These complex defects bear the similar features to flower defects in some degree (as highlighted by a circle in Figure 2(d)). It is reasonable to suppose that flower defects in graphene can move on the surface at the elevated temperature, and they will be spliced together to form the complex defects at last when they encounter each other

18

. This explains

certainly the reduction of the areal density with the annealing time getting longer. For the sample annealed for 190 min, the corrugation induced by 6√3 reconstructed interface cannot be discerned well in STM images, indicating the layer thickness of graphene increased from bilayer to three-layer or even further 32. In order to gain an insight into the distribution of flower defects in epitaxial graphene, we also performed the measurements of Raman spectrum to the samples. The representative result is shown in Figure 3, obtained on the sample annealed at 1350oC for 120 min. As seen in Figure 3, D, G and 2D peaks can be discerned, as well as the peaks from the SiC substrate. It is well-known that Raman spectroscopy is an efficient tool for graphene to determine the layer thickness, doping type and so on. The position and shape of Raman peaks have been manifested very sensitive to the 33-35

number of graphene layer

. Monolayer graphene usually has a sharp 2D peak in

Lorentz lineshape, while bilayer and multilayer graphene will show a broader and blueshifted 2D peak which can be fitted with several Lorentzian peaks 33. Compared with the exfoliated graphene, the G and 2D peaks of the epitaxial graphene on SiC always blueshift to a higher frequency, and the peak height of G band is larger than that of 2D band

33, 36, 37

. As shown in Figure 3, the D, G and 2D peaks for

graphene/SiC(0001) appear at 1360cm-1, 1600cm-1 and 2720cm-1, respectively. The 10


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full width at half-maximum of the 2D peak is 98 cm-1, much larger than 59 cm-1 for monolayer graphene on SiC 33. The 2D peak here cannot be treated with either single Lorentzian component or four components, and there seems to be a small bulge located at about 2640 cm-1 on the left shoulder of 2D peak. Therefore, the epitaxial graphene with thickness of bilayer and three-layer is believed existing on the most part of our sample surface, in agreement with our STM observations.

b) obtained on the graphene/SiC(0001) sample annealed at Figure 3: Raman spectrum 1350oC for 120min, with the laser excitation of 632.8nm and the spot size of 0.7µm.

The Conjoined-twin Defects. To corroborate the evolution and nucleation of flower defects into the complex defects, our STM investigations were focused on the samples with dense flower defects. As a result, a new topological defect was revealed, which could be viewed as evidence and the initial stage for the growth of complex defect structures from flower defects. Figure 4 exhibits the STM images of the new defect and its atomic structure in detail. As marked by circles in Figure 4(a) and 4(b), the new defect structure is identified as two of 2/3 flower defects combined together. And the structural characteristics of this new defect can be picked out further in Figure 4(c) and 4(d). It can be observed clearly that there is an elongated dark region existing between the central rings of the two 2/3 flower defects. We refer to this new 11



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defect as “conjoined-twin defect” and propose a model for its atomic configuration as illustrated in Figure 4(e). The conjoined-twin defects were usually found emerging in graphene after the annealing at 1350oC for more than 80 min. In Figure 4(a), the “N-dopant-induced defects” are also found existing at the lower right side of the conjoined-twin defect with three-fold symmetry. The thick graphene on the sample annealed for 150 min makes them slightly different in appearance compared with those shown in Figure 1(a).

a)

c)

b)

d)

e)

7 8

5’

5

Figure 4: STM images with the conjoined-twin defects found: a) on the graphene/ 4H-SiC(0001) annealed at 1350oC for 150 min, 16.2 nm × 17.5 nm, Vbias = 0.7 V, Iset = 60 pA; b) on the graphene/6H-SiC(0001) annealed at 1350oC for 80 min, 11.9 nm × 12.9nm, Vbias = 1.2V, Iset = 200pA. c), d) Magnified STM images of the conjoinedtwin defect acquired at 0.6V and -0.6V, respectively. 5.2 nm × 5.9 nm. e) The atomic structural configuration of a conjoined-twin defect. The size of the defect is 1.54 nm × 1.23 nm.

12


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Figure 4(e) shows that a conjoined-twin defect is composed of two of 2/3 flower defects and the two parts are connected with a carbon octagon, making its structure slightly asymmetrical. To further explore the structural developments of flower defects, we calculate the relative energies for a series of structural configurations to simulate the process of the two isolated flower defects approaching each other and finally nucleating into one conjoined-twin defect in graphene, with DFT calculations. Figure 5 shows the simulated structural configurations, and Table 1 gives the corresponding relative energies for the structural configurations shown in Figure 5. Obviously, the calculated results indicate that the energy of the conjoined-twin defect is 2.7eV smaller than that of two isolated flower defects, as given in Table 1. Thus, when the two flower defects encounter in graphene, they tend to nucleate into a more stable structure, i.e. form a conjoined-twin defect. At the end, the complex defect structure is developed on the sample surfaces, as observed in our experiments. Gong et al. have reported direct evidences for the effects of thermal annealing on the movement and the long-distance jump of dislocations in graphene, which were acquired in an aberration corrected transmission electron microscope 38.

Table 1: The relative energies obtained from DFT calculations for the atomic structural configurations in Figure 5.

Configuration

(a)

(b)

(c)

(d)

(e)

Energy (eV)

0

-1.96

-2.28

-2.66

-2.70

13



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a)

b)

c)

d)

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e)

Figure 5: The different structural configurations adopted to simulate the process of two isolated flower defects approaching and finally nucleating into a conjoined- twin defect in graphene.

Electronic Structures of Graphene with Conjoined-twin Defects. The topological defects in graphene have great effects on modifying its electronic structures and properties

9, 21

. The DFT calculations were performed in this work to investigate the

electronic structure of graphene, to reveal the influence of the flower defects and the conjoined-twin defects. The electronic state of the final structural configuration of graphene with a conjoined-twin defect (Figure 5(e)) was studied with DFT 14


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calculations. The obtained band structure of the graphene with a conjoined-twin defect (Figure 5(e)) along high symmetry is given in Figure 6(a). A 50meV band gap is opened at Γ point, rather than Κ point. The reason is due to the folding of the Brillouin zone of the 15 × 15 supercell adopted (Figure 5(e)). Angle-resolved photoemission spectroscopy (ARPES) measurements are also carried out for our graphene samples. The experimental data (as shown in Figure 6(b)) display a gap of 79meV, slightly larger than that of the DFT result. This may be due to following two reasons: the strong interaction from the substrate in experiments

39, 40

and the existence of the

bilayer and trilayer graphene in the experimental samples. Both factors can cause a large band gap to the sample. It is also noticed that the structural configuration in Figure 5(d) is slightly different in energy from that in 5(e). The DFT calculations show the similar electronic state distributions for these two structural models. The VHS states, i.e. a divergence in the DOS resulting from a saddle point in the electronic band, are far away from the Fermi level EF for pristine graphene, making it hard produce new electronic phases by means of chemical doping or gating

25, 26

. As

shown in Figure 4, a conjoined-twin defect consists of 5-, 7- and 8-atom carbon rings besides 6-atom rings. The electronic densities of states of these carbon rings were calculated after relaxation of the atomic structures, and displayed very different electronic properties. Considering the various local atomic structures of a twin defect as shown in Figure 5(e) and the STM features, the average local DOSs for the 5-, 5’-, 7- and 8-atom rings (as marked in Figure 4(e), where the 5-, 5’- and 7-atom rings selected are all closest to the 8-atom ring) are displayed in Figure 6(c). These average DOSs are acquired by adding the partial DOSs of all carbon atoms in the ring and then averaging over the atom number. The averaged DOS for the 6-atom ring is obtained from the hexagonal carbon ring far from the defects. It can be seen in Figure 15



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6(c) that in the energy range of ±1.0eV, the new VHS states emerge in the DOS curves for the pentagon, heptagon and octagon rings. Compared with the carbon hexagon, the prominent VHS peaks are located mainly in the vicinity of +(0.4~0.7) eV, and -0.4 eV for the dislocated carbon rings, quite close to the Fermi energy EF. And the DOS curve of 5-atom ring displays different peak features from that of 5’-atom ring. After comparison of the neighboring atom configurations of these distorted carbon rings, we believe more researches have to be done in the future to understand the origins of these VHS states. It is also worth notice that in STM images the conjoined-twin defect has a darker inner region at 1.2V (Figure 4(b)) than that at 0.7V (Figure 4(a)). As we know, the profile of a STM image is a contour of local DOSs close to the Fermi level for the sample. The calculated DOSs for the defect can provide a proper explanation to the STM features. As shown in Figure 6(c), the intensities of DOS at 0.7eV are much larger than those at 1.2eV for 5’- and 8-atom rings in the defect. But for 7- and 5-atom rings, the intensity of DOS at 0.7eV is almost same as or slightly smaller than that at 1.2eV. Therefore, the dark inner region of the defect shown in the image at 1.2V mainly arises from the octagon and right 5’ pentagon rings.

a)

b)

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c)

Figure 6: a) The electronic band structure of the monolayer graphene with a conjoined-twin defect obtained from the DFT calculations (Figure 5(e)). b) An ARPES intensity map acquired on epitaxial graphene with flower defects on 6H-SiC(0001). c) The calculated average DOSs of some carbon rings, as shown in Figure 4(e), in the conjoined-twins defect. These rings are selected according to the STM experimental features and the local atomic structures of the twin defect different from the flower defect.

■ CONCLUSIONS The epitaxial graphene with flower defects were fabricated on the 6H-SiC(0001) and 4H-SiC(0001) surfaces by means of thermal graphitization of SiC. The structural evolution of flower defects with high temperature annealing was explored by using STM, Raman spectroscopy and DFT calculations. Flower defects are observed to appear in bilayer graphene and increase in amount with annealing duration, finally to nucleate into the complex defect structures. As direct evidence for the structural evolution of flower defects into the complex defect structures found in our experiments, a conjoined-twin defect is recognized made up of two of 2/3 flower defects according to the STM images and DFT simulations, and it has a calculated energy 2.7eV smaller than that of two isolated flower defects. The DFT calculations reveal that 50meV band gap is opened at Γ point in the graphene with conjoined-twin defects, and the new VHS states are also found close to the Fermi level in the DOS of 17



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the conjoined-twin defects. These results will shed light on a new way for the enhancements of electronic properties of future graphene-based devices.

■ AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Qun Cai: 0000-0002-8457-6976 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS Q.C. acknowledges support from the National Natural Science Foundation of China (Grant No. 11374058). Z.Y. acknowledges support from the National Natural Science Foundation of China (Grant No. 11574051) and the Natural Science Foundation of Shanghai (Grant No. 14ZR1403400). Y.C and Q.C thank the experimental assistances from Prof. Kurash Ibrahim and his students at ARPES station of Beijing Synchrotron Radiation Facility.

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Structural Evolution of Flower Defects and Effects on the Electronic

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