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Low temperature synthesis and field emission characteristics of single to few layered graphene grown using PECVD
Accepted Manuscript Title: Low temperature synthesis and field emission characteristics of single to few layered graphene grown using PECVD Author: Avshish Kumar Sunny Khan M. Zulfequar Harsh Mushahid Husain PII: DOI: Reference:
S0169-4332(17)30044-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.044 APSUSC 34839
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-10-2016 1-1-2017 6-1-2017
Please cite this article as: Avshish Kumar, Sunny Khan, M.Zulfequar, Harsh, Mushahid Husain, Low temperature synthesis and field emission characteristics of single to few layered graphene grown using PECVD, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Low temperature synthesis and field emission characteristics of single to few layered graphene grown using PECVD Avshish Kumar1, Sunny Khan1, M. Zulfequar1, Harsh1, Mushahid Husain1# 1
Department of Physics, Jamia Millia Islamia (A Central University), New Delhi, India
# Corresponding author email: [email protected]
Highlights of the work Single-layered Graphene (SLG) to few-layered graphene (FLG) by were synthesized by PECVD system at low temperature. From different characterization techniques, the presence of single and few layered graphene were confirmed. X-ray diffraction pattern of the graphene showed single crystalline nature of the film. Based on Electronic band structure of graphene and concept of Dirac point, an intensive study of field emission properties conclude that SLG to FLG are extremely good field-emitters with long term emission current stability. It’s a combined study of synthesis of graphene and its application in FEDs.
Abstract: In this work, high-quality graphene has successfully been synthesized on copper (Cu) coated Silicon (Si) substrate at very large-area by plasma enhanced chemical vapor deposition system. This method is low cost and highly effective for synthesizing graphene relatively at low temperature of 6000C. Electron microscopy images have shown that surface morphology of the grown samples is quite uniform consisting of single layered graphene (SLG) to few layered graphene (FLG). Raman spectra reveal that graphene has been grown with highquality having negligible defects and the observation of G and G' peaks is also an indicative of stokes phonon energy shift caused due to laser excitation. Scanning probe microscopy image also depicts the synthesis of single to few layered graphene. The field emission characteristics of as-grown graphene samples were studied in a planar diode configuration at room temperature. The graphene samples were observed to be a good field emitter having low turn-on field, higher field amplification factor and long term emission current stability.
Keywords: graphene; chemical vapor deposition; field emission; scanning probe microscopy; emission current.
1. Introduction: A one atom thick planar sheet of sp2 hybridised carbon atoms arranged in a hexagonal fashion makes the wonder material of the 21st century. This wonder material forms the elementary building block of graphite and termed as the graphene. Ever since its discovery in 2004, it has attracted the attention of numerous researchers. The first graphene was extracted from graphite using a technique known as micro mechanical cleavage [1]. Graphene is a unique material because of its stand apart electronic, optical and mechanical properties. It has been found experimentally that graphene has very high electron mobility which is in excess of 15000 cm2 V-1s-1[1] at room temperature. The single layered graphene is also called as semi-metal or zero band gap semiconductor. It’s resistivity is of the order of 10-6 ohm cm which is even less than that of silver at room temperature [2]. It may prove to be an excellent replacement of ITO as transparent electrode material in solar cells and liquid crystal display devices [3] because of startlingly low absorption ratio of 2.3% of white light [4]. The 2D structure of graphene provides it a large aspect ratio i.e., surface area to volume ratio, which renders it exceptional sensing abilities leading to applications in sensory devices and detectors. The 2D structure is also useful for large current transferring and more rapidly heat dispersion, which significantly deteriorate the Joule heating induced burning of energetic emission sites in graphene and consequently improves its field emission stability [5, 6]. Therefore, graphene has aroused many field emission applications [7], such as liquid crystal displays [8], flat panel displays [9-11], lighting lamps [12], X-ray sources [13] etc due to its distinctive characteristics such as exceptional mechanical strength, assorted electrical properties, and its chemical inertness [14,15]. Although, efforts have been made by some researchers to synthesize graphene which have promising application as field emitters but maximum field emission characteristics of graphene have not been obtained so far [16-18]. The required maximum current density (J) could only be achieved when graphene with
controlled and repetitive properties can be synthesized on the desired substrate, preferably silicon, at desired locations. Therefore, first of all, the synthesis process needs to be optimized and developed to grow graphene with highest order of purity and the process should be capable of growing monolayer to few layered graphene required for specific applications. The graphene can be synthesised by a number of techniques which include chemical cleavage from graphite, epitaxial growth on SiC substrates, soft chemical route like Hummer and Offeman method [19] and Chemical Vapor Deposition (CVD). Out of these methods CVD has the potential of large-scale area production [20-23]. The CVD is a method of thin film deposition from a vapour precursor on to a heated substrate by means of chemical reaction on it. The graphene can be grown by CVD method using transition metals like Ni and Cu as catalysts [24]. We have used Cu as the catalyst in our experiment and the methane has been used as the source gas. The process involves decomposition of methane on a heated Si substrate coated with Cu. The Cu has almost zero solubility for carbon because of which the carbon atoms, originating from decomposed hydrocarbon gas, directly nucleate on to the Cu surface leading to graphene growth and the process is self limiting as well [24,25]. The CVD method has a number of variants including Atmospheric Pressure Chemical Vapor Deposition (APCVD), Low Pressure Chemical Vapor Deposition (LPCVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD). We have employed PECVD for the growth of graphene. The inherent predicament with thermal CVD techniques is that the graphene grows at higher temperatures ranging between 800°C - 1000°C [26-32] depending upon the source hydrocarbon. This handicap has been overcome by PECVD technique. The graphene can be grown by PECVD at temperatures as low as 600°C reason being the enhancement of energy by plasma which in turn leads to early dissociation of hydrocarbon gas.
In this work, the synthesis of single layered graphene (SLG) to few layered graphene (FLG) has successfully been carried out using PECVD technique at low temperature of 6000C. The as-grown samples were characterized using different techniques such as field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and scanning probe microscopy (SPM). For device point of view, the field emission characteristics of graphene samples were analysed in the frame work of Fowler-Nordheim equation. 2. Experimental: 2.1 Synthesis of Graphene: As the preliminary step, first of all, Copper (Cu) film (catalyst) was deposited on ultrasonically cleaned Si(100) substrate by RF-sputtering system. The thickness of Cu film was 10 nm and argon (Ar) as the plasma forming gas was used during deposition. The forward power of RF was set at 100 W and reverse at 0 W for a 1 min catalyst coating. Thereafter, the Cu coated Si substrate was put upon substrate holder fitted inside the quartz bell jar chamber of PECVD system (Black Magic 2 inch system, AIXTRON, Cambridge, UK). The chamber was then evacuated to a pressure ~ 30 mbar. Afterwards, two steps were followed to synthesize graphene. In the first step, substrate was pre-treated for 10 min at 5500C in the nitrogen atmosphere and the temperature was monitored using thermocouple connected to substrate holder. In the second step, after pre-treatment, methane (CH4) at a flow rate of 15 sccm was inserted into the chamber in continuation with NH3 at a flow rate of 500 sccm. During growth process, the heater temperature was quickly raised to 6000C. The growth time was kept 15 min. In this process, DC plasma at a power of 40 W was used, to assist uniform growth of the graphene. The plasma in our system is dc pulsed plasma.
2.2 Characterization techniques used: To ascertain that the as grown sample is graphene, a number of characterizations have been done which include HRTEM, FESEM and Raman Spectroscopy. FESEM of FEI (model: Nova Nano) keeping beam energy 5 keV was used to analyze the morphology of graphene. The sample scanned by SEM was taken to HRTEM, Tecnai G2 F30 S-Twin (FEI; Super Twin lens with Cs = 1.2 mm) operating at an accelerating voltage of 300 kV, was used to assess the structure of graphene. For the structural analysis of grown samples of graphene, Raman Spectrometer of HORIBA Jobin Yvon (LABRAM HR 800 JY) at a wavelength of 633 nm (energy 19.6 eV) was used. Bruker, multimode 8 SPM was used for high resolution imaging of the graphene samples. 2.3 Field emission studies of graphene The field emission characteristics of the as-grown graphene samples were measured using indigenously designed set-up. The system was operated at room temperature in a high vacuum chamber using cathode-anode arrangement. The current density was measured by plotting J-E curve and field enhancement factor was calculated using Fowler-Nordheim (F-N) plots. 3. Results and Discussions: 3.1 Electron Microscopic studies 3.1.1 Scanning Electron Microscopy
FESEM is a powerful tool to have an idea of morphology of the as grown sample. SEM is similar to optical microscopy but it has an edge over latter one. Geim’s group did compare optical and SEM micrographs of graphene deposited over large areas, only to conclude that certain portions of the film were clearly visible in SEM rather than in optical images [33]. It could be noticed from the images shown in Fig. 1 that we have a good deposition of layery structure. The film is quite continuous and has a good spatial expansion. Undulating
appearance of the film shows that we have a combination of single to few layered growth of graphene. 3.1.2
Transmission Electron Microscopy
TEM micrograph shown in Fig. 2 depicts the variation in graphene layers. It could be noticed from this figure that there are few light color patches which are single layered graphene whereas some dark shaded portions are few layered graphene. It is also evident that there are some folded patterns at the edges of films, as graphene films have a tendency to get folded at the edges. These folded patterns may be considered as few layered graphene but the growth of such layers is quite less. The HRTEM micrograph exhibits the loosely stacked single layered graphene as the distance between the lattice fringes is in the range 0.7 – 1.2 nm which is larger than 0.156 nm for strongly stacked layers of graphene as reported in Ref. [34]. It is also evident from this micrograph (Fig.3) that the layers of graphene are almost unidirectional and loosely stacked over each other. It is because of the fact that suspended graphene layers have the tendency to stack or coagulate with each other and therefore, graphene layers get piled up at some places. The presence of single and few layered graphene has also been substantiated by Raman spectra which clearly show prominent G' peak vis-à-vis G peak. 3.2 High-resolution Scanning Probe Microscopy Scanning probe microscope in atomic force microscopy (AFM) mode with atomic resolution was used to characterize the graphene samples. It is an influential technique mostly for nanomaterials because it can reveal the information at atomic level with 3 D view. It can be seen from Fig. 4 that the planar nature of graphene makes it positioned in close contact with the substrate as well as the stacking of single to few layered graphene. The respective line profile (Fig. 4 b) indicates that the noticeable heights of the layer steps are about 0.5 nm to
1.5 nm, which is more than the distance between two consecutive layers of multi layered graphene i.e. 0.156 nm. Therefore, it may be suggested that the prepared sample possesses loosely stacked pattern of single layered graphene.
3.3 Raman Spectroscopy:
The most robust and effective technique to be sure of graphene growth is Raman spectroscopy. It has been widely used to investigate electronic and structural characteristics of graphene. The Raman spectra of graphene comprises of three major peaks namely D band, G band and G' band (or 2D) (Fig. 5 (a, b)). Each has its own significance. D band stands for the degree of defects in crystalline structure [35] and usually observed at around 1350 cm-1.
It was observed from the Raman spectrum (Fig. 5a) that there is no noteworthy D peak which shows negligible defects in the grown samples. Therefore, as-grown graphene structure is found crystalline in nature [36]. G and G' peaks are indicative of stokes phonon energy shift caused due to laser excitation of the sample. G peak corresponds to primary in-plane vibrational mode of sp2 carbon atoms. This peak is doubly degenerated phonon mode at the Brillouin zone center which has been observed at 1588 cm-1. The prominent peak intensity of the G band is also used to conclude the number of layers of graphene. G' peak also known as 2D band originates from second overtone of a different in-plane vibrations of D band and has been extensively used to verify the existence of single layered graphene (SLG). In single layered graphene, the G' band can be fitted with a sharp peak. Since G' peak originates from a two phonon double resonance process therefore, this peak is closely linked with the band structure of graphene layers. From Fig. 5 (a), G' peak is observed at 2737cm-1[37] and it can also be observed from the Raman spectrum that G' band is quite intense and sharp in comparison with G band and therefore presence of single layered graphene is relatively high.
At some other point of the sample Fig. 5 (b), it was also observed that the intensity of G band is high and sharp in comparison with G' band and therefore exhibits the few layered graphene.
3.4 X-ray diffraction (XRD) analysis: The crystallinity of the graphene can be assured using X-Ray diffraction analysis (Fig. 6). A single prominent sharp peak is obtained at an angle 2𝜃 = 26.080 corresponding to the (002) plane which exhibits that the prepared sample is single crystalline in nature. The XRD measurements are in good agreement with the previous reported work [38]. 3.5 Field Emission Studies: To understand the mechanism of graphene field emission, we need to have a look on its electronic structure. The band structure is most likely studied from the perspective of a relationship between energy and momentum of electrons of any material [39]. The plot of energy vs momentum dispersion relation has been shown by many researchers theoretically in which a well known fact is proved using tight binding approximation [40]. Without indulging into intense mathematics, we can easily say that graphene is a zero-gap semiconductor because the conduction and valance bands meet at a point termed as Dirac point. The Dirac points are locations at the edge of Brillouin Zone. The charge carriers i.e., electrons at this region are effectively mass less. [18, 40]; that is why, in graphene, charge carriers behave like relativistic particles with an effective speed of light [41].
The point that we take out of this discussion of band structure is that the charge carriers i.e., electrons, present at this k point, have an effective barrier height of just the work function [42]. When the macroscopic field is applied, the local field at the edges of graphene layer gets enhanced as the electrons begin to accumulate at edges which bring into play the screening effects and the local field in its neighbourhood is enhanced. The vacuum potential in front of
the edge gets reduced and chances of emission of electrons into vacuum, through quantum tunnelling, become bright. The possibilities of such tunnelling of charge carriers lead to application of Fowler Nordeim theory [43,44] of field emission in the case of graphene as well. Fowler Nordeim (FN) theory of field emission was the beginning of quantum tunnelling phenomenon and is still being widely used in order to study the field emission process in a material. It is the best means to describe field emission from carbon based electron emitters and specially graphene. Therefore, keeping in view of the field emission mechanism from graphene, the field emission characteristics of the as-grown graphene samples were measured using indigenously designed set-up (Fig. 7). Field emission studies were performed using a diode arrangement by applying negative voltage on the cathode (copper plate) with respect to stainless steel anode plate. Graphene film as electron emitter source was pasted on the copper plate with silver epoxy. The effective area of anode was ~ 78.5 mm2 for the measurements. The results of emission characteristics were performed in the vacuum of the order of 10-6 Torr to minimize the electron scattering which enhanced the electron collection at the anode plate. The distance between cathode and anode was kept 250 µm (constant) during complete studies. 3.5.1 J-E Plot According FN theory, emission current density ( J) from the surface of emitting material can be expressed as a function of the electric field (E) and work function (ϕ) of the emitting material i.e. B 3 / 2 J AE 2 exp E
(1)
Where, A & B are constants having values 1.56 x 10-6 A eV V-2 and 6.83 x 107 eV3/2 Vcm-1 respectively. Applied electric field (E) is defined as βV/d, where V is the voltage between
anode and cathode (CNT emitters), d is the distance between cathode and anode, and β is the field enhancement factor. Since, current density is directly proportional to the applied electric field, therefore, a plot between current density (J) versus applied electric field (E) was drawn as shown in Fig. 8. As observed from the J-E plot (Fig. 8), a current density of 2.4 mA/cm2 at 3.0 V/µm and at low turn-on field (Eto) of 2.2 V/µm were observed from graphene samples. 3.5.2 Field enhancement factor According to FN theory, 𝐽
𝐴𝛽2
𝑉
𝜑𝑑 2
𝑙𝑛 ( 2 ) = 𝑙𝑛 (
)−
𝐵𝜑3/2 𝑑
(2)
𝛽𝑉 𝐽
1
𝑉
𝑉
with the help of eqn. 2, A plot known as FN plot between 𝑙𝑛 ( 2 ) versus
were drawn (Fig.
9). FN plot illustrates almost straight line and confirms existence of FE mechanism in graphene samples. Field emission (FE) takes place from the multiple emitters of the materials and therefore, the measured current is an average of currents due to all field emitters. The exact analysis of FE behaviour of the graphene is quite typical. The value of β can be determined from the slope of F-N plot (Fig. 9) by using the following relation
B 3 / 2 d m
(3)
β was estimated from slope of FN plots which comes out to be 705 for graphene samples by assuming work function (ϕ) to be 5 eV as for carbon which is in good agreement with the reported values [45,46]. 3.5.3 Field emission current stability
In fact, emission current stability is an important aspect for field emission display devices and therefore, graphene samples were tested for the same. Emission current stability was recorded in terms of degradation in current density at constant applied voltage. The measurement was recorded for 10 h. The variation in current density with respect to time (J-T plot) is shown in Fig. 10. The figure (inset image) also reveals an extremely less degradation ~ 10% in the current density, after 10 h. This shows that grown graphene samples are very good field emitters even for the long period of time.
4. Conclusions Single to few layered graphene have been synthesized by PECVD with high purity, good control and accuracy, at very low temperature of 600°C. From different characterization techniques, the presence of single and few layered graphene were confirmed. The negligible D peak intensity exhibits that as-grown graphene film is crystalline in nature. X-ray diffraction pattern of the graphene showed single crystalline nature of the film. It has not only been shown a well controlled graphene synthesis method but also shed light on how the same could be used as a good field emitter. Using FN equation, an intensive study of field emission properties of as grown samples led to conclude that it is a good field emitter with current density of 2.4 mA/cm2 and a field enhancement factor of 705. As-grown graphene samples were found extremely good field emitters with long term emission current stability. Acknowledgements One of the authors (Sunny Khan) is thankful to UGC for providing financial assistance in the form of SRF (Senior Research Fellowship) under Maulana Azad National Fellowship for Minority Students (Award letter no. F1-17.1/2012-13/MANF-2012-13-MUS- DEL-15956). Dr. Avshish Kumar is also thankful to Council of Scientific and Industrial Research (CSIR, grant no. 363401/2k14/1) for providing Research Associateship.
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(a)
(b)
Fig.1 (a,b): FESEM micrographs of graphene grown on Cu/Si substrate
Fig.2: TEM micrograph of single to few layered graphene
d = 0.7 nm
d = 1.2 nm Fig.3: HRTEM micrograph of single to few layered graphene
(a)
(b)
Fig.4: (a) High-resolution AFM imaging of single to few layered graphene. (b) The line profile corresponding to the black line drawn in micrograph. Scan size: 3μm × 3μm
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Fig. 7: Schematic diagram of field emission setup
Fig. 8: J-E Plots of as-grown graphene samples
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Fig. 9: FN Plots of as-grown graphene samples.
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Fig. 10: Field emission current stability of grown single to few layered graphene
S0169-4332(17)30044-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.044 APSUSC 34839
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-10-2016 1-1-2017 6-1-2017
Please cite this article as: Avshish Kumar, Sunny Khan, M.Zulfequar, Harsh, Mushahid Husain, Low temperature synthesis and field emission characteristics of single to few layered graphene grown using PECVD, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Low temperature synthesis and field emission characteristics of single to few layered graphene grown using PECVD Avshish Kumar1, Sunny Khan1, M. Zulfequar1, Harsh1, Mushahid Husain1# 1
Department of Physics, Jamia Millia Islamia (A Central University), New Delhi, India
# Corresponding author email: [email protected]
Highlights of the work Single-layered Graphene (SLG) to few-layered graphene (FLG) by were synthesized by PECVD system at low temperature. From different characterization techniques, the presence of single and few layered graphene were confirmed. X-ray diffraction pattern of the graphene showed single crystalline nature of the film. Based on Electronic band structure of graphene and concept of Dirac point, an intensive study of field emission properties conclude that SLG to FLG are extremely good field-emitters with long term emission current stability. It’s a combined study of synthesis of graphene and its application in FEDs.
Abstract: In this work, high-quality graphene has successfully been synthesized on copper (Cu) coated Silicon (Si) substrate at very large-area by plasma enhanced chemical vapor deposition system. This method is low cost and highly effective for synthesizing graphene relatively at low temperature of 6000C. Electron microscopy images have shown that surface morphology of the grown samples is quite uniform consisting of single layered graphene (SLG) to few layered graphene (FLG). Raman spectra reveal that graphene has been grown with highquality having negligible defects and the observation of G and G' peaks is also an indicative of stokes phonon energy shift caused due to laser excitation. Scanning probe microscopy image also depicts the synthesis of single to few layered graphene. The field emission characteristics of as-grown graphene samples were studied in a planar diode configuration at room temperature. The graphene samples were observed to be a good field emitter having low turn-on field, higher field amplification factor and long term emission current stability.
Keywords: graphene; chemical vapor deposition; field emission; scanning probe microscopy; emission current.
1. Introduction: A one atom thick planar sheet of sp2 hybridised carbon atoms arranged in a hexagonal fashion makes the wonder material of the 21st century. This wonder material forms the elementary building block of graphite and termed as the graphene. Ever since its discovery in 2004, it has attracted the attention of numerous researchers. The first graphene was extracted from graphite using a technique known as micro mechanical cleavage [1]. Graphene is a unique material because of its stand apart electronic, optical and mechanical properties. It has been found experimentally that graphene has very high electron mobility which is in excess of 15000 cm2 V-1s-1[1] at room temperature. The single layered graphene is also called as semi-metal or zero band gap semiconductor. It’s resistivity is of the order of 10-6 ohm cm which is even less than that of silver at room temperature [2]. It may prove to be an excellent replacement of ITO as transparent electrode material in solar cells and liquid crystal display devices [3] because of startlingly low absorption ratio of 2.3% of white light [4]. The 2D structure of graphene provides it a large aspect ratio i.e., surface area to volume ratio, which renders it exceptional sensing abilities leading to applications in sensory devices and detectors. The 2D structure is also useful for large current transferring and more rapidly heat dispersion, which significantly deteriorate the Joule heating induced burning of energetic emission sites in graphene and consequently improves its field emission stability [5, 6]. Therefore, graphene has aroused many field emission applications [7], such as liquid crystal displays [8], flat panel displays [9-11], lighting lamps [12], X-ray sources [13] etc due to its distinctive characteristics such as exceptional mechanical strength, assorted electrical properties, and its chemical inertness [14,15]. Although, efforts have been made by some researchers to synthesize graphene which have promising application as field emitters but maximum field emission characteristics of graphene have not been obtained so far [16-18]. The required maximum current density (J) could only be achieved when graphene with
controlled and repetitive properties can be synthesized on the desired substrate, preferably silicon, at desired locations. Therefore, first of all, the synthesis process needs to be optimized and developed to grow graphene with highest order of purity and the process should be capable of growing monolayer to few layered graphene required for specific applications. The graphene can be synthesised by a number of techniques which include chemical cleavage from graphite, epitaxial growth on SiC substrates, soft chemical route like Hummer and Offeman method [19] and Chemical Vapor Deposition (CVD). Out of these methods CVD has the potential of large-scale area production [20-23]. The CVD is a method of thin film deposition from a vapour precursor on to a heated substrate by means of chemical reaction on it. The graphene can be grown by CVD method using transition metals like Ni and Cu as catalysts [24]. We have used Cu as the catalyst in our experiment and the methane has been used as the source gas. The process involves decomposition of methane on a heated Si substrate coated with Cu. The Cu has almost zero solubility for carbon because of which the carbon atoms, originating from decomposed hydrocarbon gas, directly nucleate on to the Cu surface leading to graphene growth and the process is self limiting as well [24,25]. The CVD method has a number of variants including Atmospheric Pressure Chemical Vapor Deposition (APCVD), Low Pressure Chemical Vapor Deposition (LPCVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD). We have employed PECVD for the growth of graphene. The inherent predicament with thermal CVD techniques is that the graphene grows at higher temperatures ranging between 800°C - 1000°C [26-32] depending upon the source hydrocarbon. This handicap has been overcome by PECVD technique. The graphene can be grown by PECVD at temperatures as low as 600°C reason being the enhancement of energy by plasma which in turn leads to early dissociation of hydrocarbon gas.
In this work, the synthesis of single layered graphene (SLG) to few layered graphene (FLG) has successfully been carried out using PECVD technique at low temperature of 6000C. The as-grown samples were characterized using different techniques such as field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and scanning probe microscopy (SPM). For device point of view, the field emission characteristics of graphene samples were analysed in the frame work of Fowler-Nordheim equation. 2. Experimental: 2.1 Synthesis of Graphene: As the preliminary step, first of all, Copper (Cu) film (catalyst) was deposited on ultrasonically cleaned Si(100) substrate by RF-sputtering system. The thickness of Cu film was 10 nm and argon (Ar) as the plasma forming gas was used during deposition. The forward power of RF was set at 100 W and reverse at 0 W for a 1 min catalyst coating. Thereafter, the Cu coated Si substrate was put upon substrate holder fitted inside the quartz bell jar chamber of PECVD system (Black Magic 2 inch system, AIXTRON, Cambridge, UK). The chamber was then evacuated to a pressure ~ 30 mbar. Afterwards, two steps were followed to synthesize graphene. In the first step, substrate was pre-treated for 10 min at 5500C in the nitrogen atmosphere and the temperature was monitored using thermocouple connected to substrate holder. In the second step, after pre-treatment, methane (CH4) at a flow rate of 15 sccm was inserted into the chamber in continuation with NH3 at a flow rate of 500 sccm. During growth process, the heater temperature was quickly raised to 6000C. The growth time was kept 15 min. In this process, DC plasma at a power of 40 W was used, to assist uniform growth of the graphene. The plasma in our system is dc pulsed plasma.
2.2 Characterization techniques used: To ascertain that the as grown sample is graphene, a number of characterizations have been done which include HRTEM, FESEM and Raman Spectroscopy. FESEM of FEI (model: Nova Nano) keeping beam energy 5 keV was used to analyze the morphology of graphene. The sample scanned by SEM was taken to HRTEM, Tecnai G2 F30 S-Twin (FEI; Super Twin lens with Cs = 1.2 mm) operating at an accelerating voltage of 300 kV, was used to assess the structure of graphene. For the structural analysis of grown samples of graphene, Raman Spectrometer of HORIBA Jobin Yvon (LABRAM HR 800 JY) at a wavelength of 633 nm (energy 19.6 eV) was used. Bruker, multimode 8 SPM was used for high resolution imaging of the graphene samples. 2.3 Field emission studies of graphene The field emission characteristics of the as-grown graphene samples were measured using indigenously designed set-up. The system was operated at room temperature in a high vacuum chamber using cathode-anode arrangement. The current density was measured by plotting J-E curve and field enhancement factor was calculated using Fowler-Nordheim (F-N) plots. 3. Results and Discussions: 3.1 Electron Microscopic studies 3.1.1 Scanning Electron Microscopy
FESEM is a powerful tool to have an idea of morphology of the as grown sample. SEM is similar to optical microscopy but it has an edge over latter one. Geim’s group did compare optical and SEM micrographs of graphene deposited over large areas, only to conclude that certain portions of the film were clearly visible in SEM rather than in optical images [33]. It could be noticed from the images shown in Fig. 1 that we have a good deposition of layery structure. The film is quite continuous and has a good spatial expansion. Undulating
appearance of the film shows that we have a combination of single to few layered growth of graphene. 3.1.2
Transmission Electron Microscopy
TEM micrograph shown in Fig. 2 depicts the variation in graphene layers. It could be noticed from this figure that there are few light color patches which are single layered graphene whereas some dark shaded portions are few layered graphene. It is also evident that there are some folded patterns at the edges of films, as graphene films have a tendency to get folded at the edges. These folded patterns may be considered as few layered graphene but the growth of such layers is quite less. The HRTEM micrograph exhibits the loosely stacked single layered graphene as the distance between the lattice fringes is in the range 0.7 – 1.2 nm which is larger than 0.156 nm for strongly stacked layers of graphene as reported in Ref. [34]. It is also evident from this micrograph (Fig.3) that the layers of graphene are almost unidirectional and loosely stacked over each other. It is because of the fact that suspended graphene layers have the tendency to stack or coagulate with each other and therefore, graphene layers get piled up at some places. The presence of single and few layered graphene has also been substantiated by Raman spectra which clearly show prominent G' peak vis-à-vis G peak. 3.2 High-resolution Scanning Probe Microscopy Scanning probe microscope in atomic force microscopy (AFM) mode with atomic resolution was used to characterize the graphene samples. It is an influential technique mostly for nanomaterials because it can reveal the information at atomic level with 3 D view. It can be seen from Fig. 4 that the planar nature of graphene makes it positioned in close contact with the substrate as well as the stacking of single to few layered graphene. The respective line profile (Fig. 4 b) indicates that the noticeable heights of the layer steps are about 0.5 nm to
1.5 nm, which is more than the distance between two consecutive layers of multi layered graphene i.e. 0.156 nm. Therefore, it may be suggested that the prepared sample possesses loosely stacked pattern of single layered graphene.
3.3 Raman Spectroscopy:
The most robust and effective technique to be sure of graphene growth is Raman spectroscopy. It has been widely used to investigate electronic and structural characteristics of graphene. The Raman spectra of graphene comprises of three major peaks namely D band, G band and G' band (or 2D) (Fig. 5 (a, b)). Each has its own significance. D band stands for the degree of defects in crystalline structure [35] and usually observed at around 1350 cm-1.
It was observed from the Raman spectrum (Fig. 5a) that there is no noteworthy D peak which shows negligible defects in the grown samples. Therefore, as-grown graphene structure is found crystalline in nature [36]. G and G' peaks are indicative of stokes phonon energy shift caused due to laser excitation of the sample. G peak corresponds to primary in-plane vibrational mode of sp2 carbon atoms. This peak is doubly degenerated phonon mode at the Brillouin zone center which has been observed at 1588 cm-1. The prominent peak intensity of the G band is also used to conclude the number of layers of graphene. G' peak also known as 2D band originates from second overtone of a different in-plane vibrations of D band and has been extensively used to verify the existence of single layered graphene (SLG). In single layered graphene, the G' band can be fitted with a sharp peak. Since G' peak originates from a two phonon double resonance process therefore, this peak is closely linked with the band structure of graphene layers. From Fig. 5 (a), G' peak is observed at 2737cm-1[37] and it can also be observed from the Raman spectrum that G' band is quite intense and sharp in comparison with G band and therefore presence of single layered graphene is relatively high.
At some other point of the sample Fig. 5 (b), it was also observed that the intensity of G band is high and sharp in comparison with G' band and therefore exhibits the few layered graphene.
3.4 X-ray diffraction (XRD) analysis: The crystallinity of the graphene can be assured using X-Ray diffraction analysis (Fig. 6). A single prominent sharp peak is obtained at an angle 2𝜃 = 26.080 corresponding to the (002) plane which exhibits that the prepared sample is single crystalline in nature. The XRD measurements are in good agreement with the previous reported work [38]. 3.5 Field Emission Studies: To understand the mechanism of graphene field emission, we need to have a look on its electronic structure. The band structure is most likely studied from the perspective of a relationship between energy and momentum of electrons of any material [39]. The plot of energy vs momentum dispersion relation has been shown by many researchers theoretically in which a well known fact is proved using tight binding approximation [40]. Without indulging into intense mathematics, we can easily say that graphene is a zero-gap semiconductor because the conduction and valance bands meet at a point termed as Dirac point. The Dirac points are locations at the edge of Brillouin Zone. The charge carriers i.e., electrons at this region are effectively mass less. [18, 40]; that is why, in graphene, charge carriers behave like relativistic particles with an effective speed of light [41].
The point that we take out of this discussion of band structure is that the charge carriers i.e., electrons, present at this k point, have an effective barrier height of just the work function [42]. When the macroscopic field is applied, the local field at the edges of graphene layer gets enhanced as the electrons begin to accumulate at edges which bring into play the screening effects and the local field in its neighbourhood is enhanced. The vacuum potential in front of
the edge gets reduced and chances of emission of electrons into vacuum, through quantum tunnelling, become bright. The possibilities of such tunnelling of charge carriers lead to application of Fowler Nordeim theory [43,44] of field emission in the case of graphene as well. Fowler Nordeim (FN) theory of field emission was the beginning of quantum tunnelling phenomenon and is still being widely used in order to study the field emission process in a material. It is the best means to describe field emission from carbon based electron emitters and specially graphene. Therefore, keeping in view of the field emission mechanism from graphene, the field emission characteristics of the as-grown graphene samples were measured using indigenously designed set-up (Fig. 7). Field emission studies were performed using a diode arrangement by applying negative voltage on the cathode (copper plate) with respect to stainless steel anode plate. Graphene film as electron emitter source was pasted on the copper plate with silver epoxy. The effective area of anode was ~ 78.5 mm2 for the measurements. The results of emission characteristics were performed in the vacuum of the order of 10-6 Torr to minimize the electron scattering which enhanced the electron collection at the anode plate. The distance between cathode and anode was kept 250 µm (constant) during complete studies. 3.5.1 J-E Plot According FN theory, emission current density ( J) from the surface of emitting material can be expressed as a function of the electric field (E) and work function (ϕ) of the emitting material i.e. B 3 / 2 J AE 2 exp E
(1)
Where, A & B are constants having values 1.56 x 10-6 A eV V-2 and 6.83 x 107 eV3/2 Vcm-1 respectively. Applied electric field (E) is defined as βV/d, where V is the voltage between
anode and cathode (CNT emitters), d is the distance between cathode and anode, and β is the field enhancement factor. Since, current density is directly proportional to the applied electric field, therefore, a plot between current density (J) versus applied electric field (E) was drawn as shown in Fig. 8. As observed from the J-E plot (Fig. 8), a current density of 2.4 mA/cm2 at 3.0 V/µm and at low turn-on field (Eto) of 2.2 V/µm were observed from graphene samples. 3.5.2 Field enhancement factor According to FN theory, 𝐽
𝐴𝛽2
𝑉
𝜑𝑑 2
𝑙𝑛 ( 2 ) = 𝑙𝑛 (
)−
𝐵𝜑3/2 𝑑
(2)
𝛽𝑉 𝐽
1
𝑉
𝑉
with the help of eqn. 2, A plot known as FN plot between 𝑙𝑛 ( 2 ) versus
were drawn (Fig.
9). FN plot illustrates almost straight line and confirms existence of FE mechanism in graphene samples. Field emission (FE) takes place from the multiple emitters of the materials and therefore, the measured current is an average of currents due to all field emitters. The exact analysis of FE behaviour of the graphene is quite typical. The value of β can be determined from the slope of F-N plot (Fig. 9) by using the following relation
B 3 / 2 d m
(3)
β was estimated from slope of FN plots which comes out to be 705 for graphene samples by assuming work function (ϕ) to be 5 eV as for carbon which is in good agreement with the reported values [45,46]. 3.5.3 Field emission current stability
In fact, emission current stability is an important aspect for field emission display devices and therefore, graphene samples were tested for the same. Emission current stability was recorded in terms of degradation in current density at constant applied voltage. The measurement was recorded for 10 h. The variation in current density with respect to time (J-T plot) is shown in Fig. 10. The figure (inset image) also reveals an extremely less degradation ~ 10% in the current density, after 10 h. This shows that grown graphene samples are very good field emitters even for the long period of time.
4. Conclusions Single to few layered graphene have been synthesized by PECVD with high purity, good control and accuracy, at very low temperature of 600°C. From different characterization techniques, the presence of single and few layered graphene were confirmed. The negligible D peak intensity exhibits that as-grown graphene film is crystalline in nature. X-ray diffraction pattern of the graphene showed single crystalline nature of the film. It has not only been shown a well controlled graphene synthesis method but also shed light on how the same could be used as a good field emitter. Using FN equation, an intensive study of field emission properties of as grown samples led to conclude that it is a good field emitter with current density of 2.4 mA/cm2 and a field enhancement factor of 705. As-grown graphene samples were found extremely good field emitters with long term emission current stability. Acknowledgements One of the authors (Sunny Khan) is thankful to UGC for providing financial assistance in the form of SRF (Senior Research Fellowship) under Maulana Azad National Fellowship for Minority Students (Award letter no. F1-17.1/2012-13/MANF-2012-13-MUS- DEL-15956). Dr. Avshish Kumar is also thankful to Council of Scientific and Industrial Research (CSIR, grant no. 363401/2k14/1) for providing Research Associateship.
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(a)
(b)
Fig.1 (a,b): FESEM micrographs of graphene grown on Cu/Si substrate
Fig.2: TEM micrograph of single to few layered graphene
d = 0.7 nm
d = 1.2 nm Fig.3: HRTEM micrograph of single to few layered graphene
(a)
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Fig.4: (a) High-resolution AFM imaging of single to few layered graphene. (b) The line profile corresponding to the black line drawn in micrograph. Scan size: 3μm × 3μm
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Fig. 7: Schematic diagram of field emission setup
Fig. 8: J-E Plots of as-grown graphene samples
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Fig. 9: FN Plots of as-grown graphene samples.
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Fig. 10: Field emission current stability of grown single to few layered graphene