Graphene, 2016, 5, 155-167 http://www.scirp.org/journal/graphene

ISSN Online: 2169-3471 ISSN Print: 2169-3439

DOI: 10.4236/graphene.2016.54013 September 21, 2016

Photoluminescence Coupled Raman Spectroscopy of Single-Layer Graphene Sheet

Mohamed G. Al-Khattib1,2*, Ahmed Samir2, Nasser N. Morgan1,2, Safwat Hassablla1,2, M. A. Ahmed1, Abdou A. Garamoon1,2

1Physics Department, Faculty of Science, Al-Azhar University, Cairo, Egypt 2Center of Plasma Technology, Al-Azhar University, Cairo, Egypt

Abstract We present a high quality single-layer graphene (SLG), triple layer graphene (TLG) and few layer graphene (FLG) sheets have been synthesized on cu substrate (metal catalyst) by direct current plasma enhanced chemical vapor deposition (DC-PECVD) at low growth temperature (from 600˚C to 750˚C). Mixture gas ratio, plasma elec-trical current and substrate temperature were investigated to determine the optimum condition of high quality single-layer graphene sheet synthesized. The characteriza-tion of graphene sheet was performed by X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), electron diffraction, Raman spectros-copy and photoluminescence (PL) spectrum. Photoluminescence and Raman result was coupled to investigation and evaluation of single-layer graphene sheet.

Keywords Single-Layer Graphene, Graphene Structure, Photoluminescence, Raman Spectroscopy, High Resolution Transmission Electron Microscope and Electron Diffraction

1. Introduction

Graphene is an atomically thin sheet of carbon atoms tightly packed in a two-dimen- sional (2D) honeycomb with a hexagonal lattice structure. It has been extensively stu-died in the last few years due to possessing many extraordinary properties and potential applications even though it was only isolated for the first time in 2004 [1]. Due to its excellent physical and mechanical properties [2]-[4], it is a good candidate for various applications such as nanoelectronic, sensor, catalysis and photovoltaic [3]-[5]. Gra-

How to cite this paper: Al-Khattib, M.G., Samir, A., Morgan, N.N., Hassablla, S., Ahmed, M.A. and Garamoon, A.A. (2016) Photoluminescence Coupled Raman Spec-troscopy of Single-Layer Graphene Sheet. Graphene, 5, 155-167. http://dx.doi.org/10.4236/graphene.2016.54013 Received: August 26, 2016 Accepted: September 18, 2016 Published: September 21, 2016 Copyright © 2016 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/

Open Access

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phene is classified into three different types: namely: single-layer graphene (SLG), triple-layer graphene (TLG) and few-layer graphene (FLG), with the number of layers (n < 10) [4]-[6], being characterized by its thickness.

Several synthesis methods have been developed for single-layer graphene sheets, in-cluding chemical reduction of exfoliated graphene oxide [7], thermal decomposition of graphite on SiC [8], and chemical vapor deposition (CVD) growth on metals [9]. In re-cently years, many of plasma techniques are introduced to synthesis and processing of graphene sheet, which controlled growth, thickness and morphology of graphene sheet, such as arc discharge of graphite [10], use of graphite as a starting material through an exfoliation reintercalation expansion process [11], substrate free gas phase synthesis of graphene platelets in a microwave plasma reactor [12], radio-frequency plasma en-hanced CVD (RF PE-CVD) [13], microwave plasma enhanced chemical vapor deposi-tion (MW PECVD) [14] [15], and unzipping of CNTs by using an argon plasma etch-ing method [16].

In this work, direct current plasma-enhanced chemical vapor deposition (DC PE- CVD) was developed to grow different structures of graphitic materials and to explore how plasma parameter and substrate temperature affected its properties.

2. Experimental Procedure 2.1. Synthesis of Graphene Sheet

Generally, graphene sheets were synthesized using DC PE-CVD [17] in which gaseous carbon sources (C2H2) in an H2-rich environment were reacted under specific condition and deposited on to metal substrate, as shown in Figure 1. The copper substrate was used as transition metal catalyst with diameter 30 mm and 50 μm thickness. The cupper substrate was electrically treated by Ar plasma to eliminate surface oxides and chemical contaminations. The Cu sheet was mounted in chamber and exposed to DC Ar-plasma with plasma electrical current 50 W for 10 min. Then, the chamber was purged with H2 gas at the flow rate of 100 standard cubic centimeters per minute (sccm) for 2 min to remove residual hydrogen gas.

The chamber is equipped with parallel plate plasma reactor and pumped with diffu-sion pump vacuum system. At that point, the rotary pump (RP) was run for 30 min to bring the pressure to under ~10−3 Torr. The valve of the rotary pump was then shut off in order for the diffusion pump (DP) to run for ~60 min to bring the pressure to under ~10−7 Torr to remove the unwanted particles from inside the chamber. The heater was turned on and temperature rising from room temperature to 700˚C for 120 min due to degassing process of chamber and substrate to remove residual and contaminations gases from bores of chamber and substrate under base pressure ~10−7 Torr, then cham-ber cooling and prepared to film deposition.

Specific properties (such as: substrate temperature, mixture gases ratio and DC plasma electrical current) were investigated for the growth, synthesis, characterization and evaluation of graphene sheets. Substrate temperature is key role of feature and structure of film deposition, which precipitation of carbon atoms on cu substrate is due

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Figure 1. Schematic diagram of the DC PE-CVD chamber.

to the temperature dependence of the solubility of carbon in transition metals [17] [18]. During the graphene growth stage, DC plasma was generated for a specified growth time under a continuous flow of hydrogen (H2) and acetylene (C2H2), while the pres-sure was kept at 5 m Torr. The plasma electrical current was varied from 110 to 150 mA and the substrate temperature from 600˚C to 750˚C, the sample was cooled down ra-pidly to room temperature at a cooling rate of 5˚C/s by turning off the heating electrical current, and then it was taken out for characterization.

2.2. Characterization

The instruments used for identification and characterization of sheets were (i) Phillips X-ray diffractometer for the X-ray diffraction (XRD) analysis was used to analyze crys-tal phase of the material. Cu-Kα irradiation was used at a 2θ scan rate of 2.0˚/min un-der 40 kV accelerating voltage and 20 mA of applied current. (ii) The Raman spectra were obtained using Raman spectroscopy NRS-2100 (Jasco, Japan) with a 514 nm Ar

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ion laser as an excitation source. (iii) The PL spectra were recorded at RT on a Shimadzu RF-5301PC fluorescence spectrophotometer using 150 W Xe lamp as the light source. The excitation and emission slit widths were both 10 nm, and the detector applied voltage was 700 V. (iv) High resolution transmission electron microscopy (HRTEM), JEOL 2100F, operated at 600 kV.

3. Results and Discussion

Raman spectroscopy is an important characterization tool used to probe the phonon spectrum of graphene material. Raman spectroscopy of graphene can be used to deter-mine the number of graphene layers and stacking order as well as density of defects and impurities. Firstly, we investigated the graphene sheets synthesized in different mixture gases ratio (H2:C2H2 = 100:16 sccm, 100:12 sccm, 100:8 sccm and 100:4 sccm) when the substrate temperature, plasma electrical current and synthesis time are fixed at 600˚C, 110 mA and 5 min, respectively. As shown in Figure 2(a), the Raman spectrum of graphene sheets synthesized at different mixture gases ratio exhibit three most promi-nent peaks are the D peak at (~1356 cm−1), the G peak at (~1584 cm−1), and the 2D peak at (~2687 cm−1).

The D peak is a defect induced Raman feature observed in the disordered sample or at the edge of the graphene [19]-[22]. The G peak is known to be associated with the doubly degenerate phonon mode at the Brillouin zone center, indicating sp2 carbon networks in the sample [19]-[22]. The intensity ratio of the D band to the G band, ID/IG, is related to both defect quality and size of graphene [23], and in this case, the high in-tensity of D peak is possibly attributed to the edge region of the graphene sheets in na-nometer scale. The 2D band is a well-known characteristic of graphene structure, and is also sensitive to the number of layers. Moreover, the intensity ratio of I2D/IG has strong correlation with the number of graphene layers [24].

Form Figure 2(b), the intensity ratio of ID/IG reduces from 0.64 to 0.29 > 0.2 as H2 gas ratio increases from 100:16 to 100:4 indicating the crystal quality of graphene im-proved, the disorder and defect density is reduced, with increasing of H2 gas ratio. On the contrary, intensity ratio of I2D/IG increases gradually from 0.85 to 1.28 < 2 with H2 gas ratio increases from 100:16 to 100:4. In addition, the full width at half maximum (FWHM) of the 2D band peak continuously decreases. These clearly suggest that the thickness of the graphene film continues to decrease from stacked graphite structure at 100:16 ratio to few layer of graphene at 100:4 ratio, which can be controllably manipu-lated with percentage of H2 gas.

Accordance of role of hydrogen in graphene growth in H2-rich environmental, the atomic hydrogen generated by DC plasma during the process of the dissociation of C2H2 into active species plays a very important role in determining the dimensions grains and number of layers of graphene because it acts not only as an activator of sur-face-bound carbon but also as controller the thickness of the resulting graphene sheets [25].

From our results, the sheet synthesized at mixture gases ratio (100:16) has high defect

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Figure 2. (a) Raman spectroscopy of graphene sheets synthesized at different mixture gases ratio; (b) Intensity ratio of (ID/IG) and (I2D/IG) dependence on mixture gas ratio obtained from single lorentzian fit; (c) HRTEM of irregular stacked transparent graphite sheets at mixture gas ratio (H2/C2H2 = 100/16 sccm); (d) HRTEM of few layer graphene sheets at mixture gas ratio (H2/C2H2 = 100/4 sccm); (e) XRD of irregular stacked graphite sheets; (f) XRD of few layer graphene sheets.

density (ID/IG ~ 0.64 > 0.2) [10], G peak position at (~1584 cm−1), the FWHM of 2D peak closed to (~37 cm−1 > 30 cm−1) and the intensity ratio I2D/IG (~0.85 < 2), which suggest that the film at this ratio is disorder stacked graphite sheet [24]-[26]. While, the sheet synthesized at mixture gases ratio (100:4) has low defect density (ID/IG ~ 0.29) [24], G peak position (~1584 cm−1), the FWHM of 2D peak closed to (~33 cm−1) and the intensity ratio I2D/IG (~1.28), the suggested film is few layer of graphene FLG [23]-[26].

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HRTEM confirmed the results of Raman spectroscopy. Figure 2(c) shows the HRTEM of graphene sheet synthesis at mixture gases ratio (H2/C2H2 = 100/16 sccm), which define irregular stacked transparent graphite sheets. While, Figure 2(d) exhibit few layer gra-phene sheet (smaller than ten layers) according to mixture gases ratio (H2/C2H2 = 100/4 sccm).

X-ray diffraction patterns of synthesized of graphene sheets synthesized at different mixture gases ratio are shown in Figure 2(e) & Figure 2(f). For sheet at (H2/C2H2 = 100/16 sccm), the sharp and intense diffraction peak appears at 26.52˚ corresponds to (002) plane with interlayer distance 0.373 nm, was attributed to stacked GR film [27]. For sheet at ((H2/C2H2 = 100/4 sccm), the broad and weak diffraction peak appears at 9.85˚ corresponds to (001) plane with interlayer distance 0.896 nm, which indicated few layer of graphene is formed. Similar results have reported by other authors [27] [28], which confirmed of Raman spectroscopy results.

Figure 3(a) illustrate the Raman spectra of synthesized graphene sheets dependence on DC plasma electrical current, when growth temperature, mixture gases ratio and synthesis time at 600˚C, (H2/C2Ha = 100/4 sccm) and 5 min, respectively. As shown in Figure 3(b), the relationship between the intensity ratio of ID/IG and plasma electrical current is inversely proportional, at 150 mA plasma electrical current the ratio (ID/IG) reduced to (~0.15 < 0.2) inducting to high quality of graphene domain and low defect density of sheet [23] [24]. While, the ratio of (I2D/IG) is directly proportional with plas-ma electrical current. The ratio of (I2D/IG) has maximum value (~2.11 > 2) at 130 mA plasma electrical current and from 130 to 150 mA the ratio (I2D/IG) is approximately

Figure 3. (a) Raman spectroscopy of graphene sheets synthesized at various plasma electrical current; (b) Plasma electrical current dependence of intensity ratio of (ID/IG) and (I2D/IG) ob-tained from single lorentzian fit; (c) HRTEM of triple layer of graphene.

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remain constant, which proposed C2H2 gas at this region is fully dissociation into active species [28]. The FWHM and position of 2D peaks define type and number of graphene layers. The FWHM and position of 2D peaks in region of (130 - 150 mA plasma elec-trical current) are approximately constant (as the same behavior of ratio I2D/IG) 30 cm−1 and 2680 cm−1 respectively. This indicator that the film formed at 130 mA plasma elec-trical current is triple-layer graphene sheet [29]. The advantages of plasma, the gra-phene synthesized under DC plasma exhibits better crystallinity and ultrafine sheets at relatively low temperatures. The enhancement of graphene sheet can be attributed to the easier dissociation of C2H2 by DC plasma into active species due to DC plasma high electrical current and formed of ultrafine graphene sheet at low temperature (600˚C) comparing with high substrate temperature (1000˚C) for synthesis ultrafine graphene sheet [29]. HRTEM confirmed the results of Raman spectroscopy. Figure 3(c) exhibit the HRTEM of graphene sheet synthesis at 130 mA plasma electrical current, which shows triple layer graphene sheet.

The effect of substrate temperature on graphene sheets are investigated in a temper-ature range from 600˚C to 750˚C. In this series of growth experiments, the plasma elec-trical current, the synthesis time and mixture gases ratio were maintained at 130 mA, 5 min and (H2/C2H2 = 100/4 sccm), respectively. Figure 4(a) shows Raman spectra of graphene film grown at different substrate temperatures, as the temperature increase from 600˚C to 750˚C the intensity ratio ID/IG decrease from 0.15 to ~ zero and I2D/IG increases from 2.11 to 4. The intensity ratio I2D/IG, position and the FWHM of the 2D band is 2.11, 2680 cm−1 and 31 cm−1 respectively at lower growth temperature 600˚C suggest that triple-layer graphene sheet is formed [27]. Interestingly, at the substrate temperature of 750˚C, the intensity ratio ID/IG, I2D/IG, a position of single-Lorentzian and the FWHM 2D peak are zero, 4 (>2), 2679 cm−1 and 27 cm−1respectivily, indicating that single-layer graphene sheet is synthesized at this temperature [24]-[28].

Our observations suggest that, the single-layer graphene sheet is synthesized at op-timum conditions mixture gases ratio, plasma electrical current and substrate temper-ature are H2/C2H2 = 100/4 sccm, 130 mA and 750˚C respectively. More interestingly, DC PE-CVD has been utilized to successfully grown of the single-layer graphene sheet at low temperature (750˚C) by using H2-rich environmental in reactive species gener-ated in DC plasma at electrical current 130 mA, in opposite to, the high growth tem-perature (~1500˚C) required for thermal decomposition of hydrocarbons in CVD me-thod [30]. Marvelously, the HRTEM and electron diffraction is used as evidence and confirmed tools due to prove and structure study of single-layer graphene sheet. Figure 4(b) shows monolayer graphene sheet with inset graphene’s honeycomb lattice of car-bon hexagons. A recent electron diffraction study of graphene demonstrated that the intensity profiles of graphene diffraction patterns could be used to determine the num-ber of layers in a graphene sheet, as shown in Figure 4(c). The relative intensities of diffraction spots in the inner and outer hexagons were shown to be equivalent in sin-gle-layer graphene. A set of diffraction spots obtained from a synthesized graphene sheet is indicated in Figure 4(d). An intensity profile of equivalent Bragg reflections

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Figure 4. (a) Raman spectroscopy of graphene sheets synthesized at various temperature; (b) HRTEM of monolayer of graphene; (c) Electron diffraction pattern; (d) Intensity profile of the diffraction spots along a line connecting points 1 and 2 in the monolayer graphene diffraction pattern. The uniform intensity profile between the inner and outer spots proves that the graphene sheet consists of a single layer.

with interlayer distance d = 0.14 nm taken along the line denoted by numbers 1 and 2 showed that the intensities of the inner and outer spots were equivalent (Figure 4(d)), indicating that the set of diffraction spots originated from a single-layer graphene sheet [31].

Pure graphene is a semiconductor of zero bandgap containing therefore does not normally show PL [32]-[34]. Figure 5 shows the PL spectra of stacked graphite (SGR) sheet, few layer graphene (FLG), triple layer graphene (TLG) and single-layer graphene (SLG) at a 300 nm excitation wavelength with a 150 W Xe laser. The PL spectrum of SGR sheet clearly shows a strong blue emission band at ~443.5 nm (2.8 eV) and broad band at ~520 nm (2.388 eV), while the PL spectrum of FLG and TLG sheet undergoes significantly quenched and blue shift as compared to that of SGR sheet, as shown in Figure 5. Interestingly, the PL spectrum of SLG exhibits unique behavior of emission. The PL spectrum of SLG sheet shows one weak and broad blue emission band at ~348 nm (3.57 eV), which also undergoes significantly quenched and blue shift as compared to that of SGR sheet. Although the mechanism of light emission of graphene sheets are

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Figure 5. (a) PL spectrum of SGR, FLG, TLG and SLG sheets; (b) The exponential decay of PL intensity of sp2 clusters.

Table 1. Characteristic bands of graphene sheets.

Band Graphene Sheets

SGR FLG TLG SLG

D (cm−1)

G (cm−1)

2D (cm−1)

ID/IG

I2D/IG

FWHM (cm−1)

sp2 domain (nm)

1355

1587

2687

0.64

0.85

37

26. 3

1350

1584

2685

0.29

1.28

33

58

1350

1580

2680

0.15

2.11

30

112.11

1350

1580

2679

~zero

4

29

∞

still unclear and remains unexplored, the PL spectrum and its blue shifted and quench-ing of is possibly explicated due to carbon domains structure.

According to our results of Raman spectroscopy, the graphene sheets (SGR, FLG, TLG and SLG) have two different types of carbon domains, such as sp2 and sp3, ex-pected from value of D, G, and 2D band was recorded in Table 1. Each domain has dif-ferent charge transition pathway inside the domain, wherein the PL emission depends on the π − π* transition within a sp2 and sp3 matrices and between each other [35].

The PL characteristics of graphene derivatives indicate that it originates from the re-combination of electron-hole (e-h) pairs, localized within small sp2 carbon clusters embedded within a sp3 matrix [36] [37]. It possibly gave another evidence of the exis-tence of the sp2 clusters in graphene sheets. The peak intensity ratio I(D)/I(G) is used to evaluate the average size of the sp2 clusters in the graphene materials. In order to de-termine the average size of the sp2 clusters in graphene sheets, we employed the Tuin-stra and Koenig relation, which related the ratio of D and G peak intensities into the crystallite size as follows [38]-[40]:

( ) ( ) ( )10 3 4,where 2.4 10 nmD

G d

CI CI L

λλ λ− −= = × (1)

where λ is the laser wavelength, which is used in Raman spectra (514.5 nm in our case)

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and Ld is the average crystallite size of sp2 domains. The calculated values of sp2 domain size in SGR, FLG, TLG and SLG sheets are recorded in Table 1. It is believed that the PL spectra in graphene sheets depend on the size, shape, thickness, fractions of the graphitic regions (sp2 domains) and defect regions (sp3 domains) [38]. The PL intensity is quenched due to the fraction concentrate of sp3/sp2 (ID/IG) clusters [41]. The exis-tences of these clusters in our sheets were confirmed by Tuinstra and Koenig relation, Raman spectroscopy and XRD results as discussed above. The average size of sp2 do-main was increasing and reaching to infinite size (one domain of sp2 covering sheet) at the intensity ratio ID/IG tend to zero for SLG lead up to reducing of defect, disorder and interstitial domains (vanishing of sp3 domain), which cause quenched of PL intensity. According to the calculation results of Ref. [42], the energy gap of π-π* transition is reduced with the increase of sp3 clusters, i.e., the energy difference between π and π* bands is increasing with the increase of sp2 cluster. As a result, the energy of emitting photons higher with the size increase of sp2 clusters, thus the PL band in the spectrum has a blue shift. Finally, Figure 5(b) shows the PL intensity quenched function of the average size of the sp2 clusters, which represent the decay curves of well exponential functions with lifetime of 0.976 n. Such a short lifetime indicates that the PL mechan-ism is possibly the recombination of excited carriers due to the electron-deficient of graphene sheets [43].

4. Conclusion

In conclusion, graphene sheets with a controllable layers number (FLG, TLG and SLG) were successfully grown by DCPE-CVD method. Mixture gas ratio, DC plasma elec-trical current and growth temperature predominate the structure, thickness and quality of the graphene sheets produced. Based on the Raman spectra, a high quality sin-gle-layer graphene sheet was grown on copper substrate, showing a low ratio of ID/IG (~zero), high ratio of I2D/IG (~4) and low FWHM of the 2D band (~29 cm−1). Our re-sults suggest that DCPE-CVD is an advantageous method for the growth of graphene, providing the ultimate advantage of the controlled synthesis of graphene sheets and low growth temperature (from 600˚C to 750˚C) comparing with high growth temperature (~1500˚C) required for thermal decomposition of hydrocarbons in CVD method (M. L. Terranova, et al.). According to Tuinstra and Koenig relation results, SP2 cluster has in-finite dimension; thus we suggest that the synthesized single-layer graphene has one graphitic domain covering all sheet, as shown in Figure 4(b). Photoluminescence Coupled Raman Spectroscopy has been achieved in characterization and investigation of Single-layer Graphene Sheet.

Acknowledgements

This work has been supported by Center of Plasma Technology, Al-Azhar University, Cairo, Egypt.

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