carbon nanosheet composite film synthesis and field emission performance comparison

Accepted Manuscript Diamond film, single-layer carbon nanosheet film and diamond/carbon nanosheet composite film synthesis and field emission performance comparison Xiao-Ping Wang, Lin-Hong Liu, Li-Jun Wang PII:

S0925-8388(17)32738-X

DOI:

10.1016/j.jallcom.2017.08.010

Reference:

JALCOM 42767

To appear in:

Journal of Alloys and Compounds

Received Date: 8 June 2017 Revised Date:

1 August 2017

Accepted Date: 2 August 2017

Please cite this article as: X.-P. Wang, L.-H. Liu, L.-J. Wang, Diamond film, single-layer carbon nanosheet film and diamond/carbon nanosheet composite film synthesis and field emission performance comparison, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.010. 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.

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Diamond film, single-layer carbon nanosheet film and diamond/ carbon nanosheet composite film synthesis and field emission performance comparison Xiao-Ping Wang1,2*,Lin-Hong Liu1, Li-Jun Wang1 1

College of Science, University of Shanghai for Science and Technology, Shanghai

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200093,China 2

Shanghai Key Lab of Modern Optical System, Shanghai 200093, china

ABSTRACT

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A series of diamond films, single-layer carbon nanosheet (SCNS) films and diamond/carbon nanosheet (D/CNS) composite films have been prepared on the

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heavily-doped n-type silicon substrate by using microwave plasma chemical vapor deposition (MPCVD) techniques and vacuum electron beam evaporation techniques. The films were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction(XRD), Raman spectroscopy. Field emission test results showed that diamond film, SCNS film and D/CNS composite film respective field emission current density is 0.004mA/cm2 (there's almost no electron emission), 0.45

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mA/cm2 and 3.21 mA/cm2 at an electric field of 7.2V/µm. This means that the field emission maximum current density of D/CNS composite films is 7.1 times that of SCNS films at an electric field of 7.2 V/µm. At the same time the D/CNS composite film exhibits the advantage of improved reproducibility and long term stability.

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Diamond layer can effectively improve the field emission characteristics of CNS film. The reason may be due to the diamond film acts as the electron acceleration layer,

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meanwhile, the heterogeneity of diamond and CNS material could lead to lowering of the interfacial potential barrier.

Keywords: diamond/carbon nanosheets composite film; single-layer carbon nanosheet film; diamond film; field emission; chemical vapour deposition PACS: 81.05.uj; 81.05.ug; 81.10.-h; 79.60.Jv ; 79.70.+q *

Corresponding author. E-mail: [email protected]; [email protected] (Xiaoping Wang) Tel:13764442830

Fax: 021-65667144

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1.Introduction. With the development of nano-electronic and nano-optoelectronic devices, excellent carbon-based emitters are increasingly important[1-9]. But until now, the macroscopic properties of carbon-based film emitter have still not been completely achieving the level of practical application.

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Carbon nanosheet(CNS) film is a kind of two-dimensional nano-carbon materials with excellent electrical conductivity and thermal conductivity, large height to

thickness ratio, high specific surface area and rich edges[2,9,10], good toughness, superior mechanical properties, make it can be used as ideal add phase of the

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composite material. Especially in the case of CNS vertical growth substrate surface, it is highly suitable for the preparation of field emission devices. As a crystal carbon-based material, diamond thin films have a variety of excellent properties such

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as high thermal conductivity, high breakdown voltage, radiation resistance, low coefficient of thermal expansion and chemical stability[10,11], which have been considered one of the important candidates in next-generation semiconductor devices[12-14].

Recently many scholars have been trying to combine the diamond and CNT (or

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other nano-carbon) film to improve the field emission performance. [4,6,15-20], and some progress has been made. As far as we know, research on diamond/CNS(D/CNS) composite cathode field emission has not been reported. If diamond and CNS composite together, can use dielectric properties of a diamond to enhance the stability

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of a field emission device, also can use that may exist between diamond layer and CNS layer internal field emission mechanism to improve the field emission properties

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of the composite cathode. So if the diamond and CNS can be combined the advantages of both, are likely to further improve the performance of field emission devices.

In this work, diamond films, single-layer CNS (SCNS) films and D/CNS

composite films are fabricated on heavily-doped n-type silicon substrate by using MPCVD technique, their FE properties were investigated, and the experiment results indicated that diamond film, SCNS film and D/CNS composite film respective field emission current density is 0.004mA/cm2(there's almost no electron emission), 0.45 mA/cm2 and 3.21 mA/cm2 at an electric field of 7.2V/µm. This means that the field emission maximum current density of D/CNS composite films is 7.1 times that of SCNS films at an electric field of 7.2 V/µm,

meanwhile, the field emission stability

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and lifetime of D/CNS composite cathode has been greatly improved. The role of the diamond layer and CNS layer was explained reasonably. 2.Experiments. In order to obtain the CNS film in which the CNS is vertical growth on the substrate, adopt the method of adding catalyst layers, and through the surface

optimization of CNS film preparation process is as follows

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topography characteristic test to optimize the process conditions of SCNS films. The

Firstly, the Al2O3 film and Ni film, which act as a composite catalyst layer, was deposited on the heavily-doped n-type silicon in turn by the vacuum electron beam

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vapor deposition(EBVD) system using an Al2O3(purity 99.9%) target and a Ni(purity 99.9%) target. The main parameters of this process were shown in table 1. Table1-Parameters employed for the EBED process.

Beam current/mA Vacuum degree/Pa Substrate temperature/℃ Voltage/KV

Heater current/A Film thickness/nm

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Deposition time/min

Values of Al2O3

Values of Ni

40

50

1.2×10-2

1.2×10-2

300

300

6

6

5

10

0.6

0.6

10

20

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Parameters of EBVD process

Then the SCNS film layers were deposited on Ni layer by microwave plasma

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chemical vapor deposition (MPCVD) techniques and the main parameters were presented in table 2..

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Table2-Parameters employed for the MPCVD process. CVD process parameters

values

Methane/hydrogen gas flows/ sccm

16/85

Substrate temperature /℃

700

Total gas pressure/kPa

6.9

Microwave/W

1100

Deposition time/h

1

Synthetic diamond film and the D/CNS composite film process are as follows. First, the heavily-doped n-type silicon wafer was preprocessed in the suspension of diamond powder with alcohol by ultrasonic cleaners, so that more nucleation points

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can form on the silicon substrate. This progress was significant for preparation of the better quality diamond. The typical parameters of depositing diamond used for MPCVD process are given in table 3. Table3-Parameters employed for the MPCVD process. values

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CVD process parameters Methane/hydrogen gas flows/ sccm

0.7/100 900

Substrate temperature /℃

6.7

Total gas pressure/kPa

1500

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Microwave/W

10

Deposition time/h

After this step, Al2O3/Ni composite catalyst layer was deposited on diamond films

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by the vacuum electron beam vapor deposition(EBVD) system using an Al2O3(purity 99.9%) target and a Ni(purity 99.9%) target. The main parameters of this process were presented in table 1. Finally, CNS film layers were deposited on Ni layer by the MPCVD techniques and the main parameters were shown in table 2. The surface morphologies and microstructures of films were characterised by X-ray

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diffraction(XRD), field emission scanning electron microscope(FE-SEM), Raman spectroscopy(Raman). FE properties of the samples were measured by using a simple diode configuration in a vacuum chamber under a pressure of 4×10-4Pa at room temperature. The SCNS film, diamond film and D/CNS composite film was used as a

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cathode, respectively, meanwhile phosphor-coated indium-tin oxide(ITO) glass was used as an anode, and cathode and anode was parallel separated by the mica spacer

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with thickness of 150 µm. The current–voltage (I–V) characteristics were recorded using a high voltage power supply and a digital ammeter at room temperature. Field emission current density versus macroscopic electric field (J-E) curve and Fowler–Nordheim (F–N) plots were available from the recorded I-V data. 3.Results and discussion. The FE-SEM images of the diamond film samples were presented in Fig.1.(a), (b) were under different magnification. It is demonstrated that the film was composed of uniform polycrystalline diamond grains.

Fig. 1(c), (d) show the FE-SEM image of

the D/CNS composite film samples after the CNS film layers were deposited. It is can be observed that a lot of CNS vertical growth on the diamond grain surface, with

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sharp edges and large high to thickness ratio. The SCNS film (as showed in Fig.1(e), (f)), the images show that carbon grains uniformly distributed on the surface of the heavily-doped Si substrate, and the CNS randomly distributed on the carbon grains surface. Compared with the CNS in D/CNS composite film, the edge of CNS of

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SCNS film is not sharp and high to thickness ratio is small.

Fig.1 FE-SEM images of surface morphology of the sample. Panels (a) (b)

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corresponding to the diamond film sample, (c) (d) corresponding to D/CNS sample(after the CNS film layers were deposited),(e) (f) corresponding to SCNS sample under different magnifications. The XRD pattern of the diamond film and D/CNS composite film sample are shown in Fig.2. The XRD spectrum of sample diamond film (as showed in Fig.2(a)) in addition to the diamond (111) and (220) crystal plane characteristic peak, show no

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other crystal characteristic peak. It indicates the film is the diamond structure with very few non-diamond phase. According to the XRD spectrum of the D/CNS composite film (as showed in Fig.2(b)), the peaks of (111) and (220) faces of the

300

(a)

80 60 40 20 0 30

40

50

60

70

2-Theta(Degree)

80

200

Diamond

100

0 20

90

(b)

Graphite

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Intensity (arb.units)

(220)

Intensity(arb.units)

(111)

100

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diamond were shown,and the peaks of graphite were also shown clearly.

30

40

Graphite Diamond

50

60

2-Theta(Degree)

70

80

90

Fig.2 XRD spectrum of the film samples.(a) diamond film; (b) D/CNS composite

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film

Fig.3 show the Raman spectrums of the samples. The Raman spectrum of sample

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D/CNS composite film (as showed in Fig.3(a)), shows two broad peaks corresponding to D-band (in 1317 cm-1 location nearby), G-band (in 1582cm-1 location nearby). In addition, in 1758 cm-1 also appeared a small peak. The D-band is assigned as A1gD

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mode, associated with vibrations at defects such as vacancies, grain boundaries, substitution hetero-atoms and impurities[21,22], indicating high quantity disorderly nano-carbon. And the G peak shifting from E2g mode normal value of 1580 to 1598 cm-1 confirms that the film contained a significant amount of nano-crystalline graphite phase [22,23]. The cause of a lesser peak appears around 1758 cm-1 is not definite yet, which may be caused by additional structures of nano-carbon. As it showed in Fig.2(b), around 1312 cm-1 of the SCNS film Raman spectrum appears a D-band peak represents disordered nano-carbon, the G-band peak on behalf of the nanographite phase structure appears at 1591 cm-1. G-band peak is caused by E2g stretching vibration of nano-carbon double bond.

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120

(a)

D

(b)

D

Intensity (arb.units)

80

G

30

G

20

40

1758

10

0 750

1000

1250

1500

1750

2000

-1

0 750

1000

1250

1500

1750

2000

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Intensity (arb.units)

40

-1

Wavenumber(cm )

Wavenumber(cm )

Fig.3 Raman spectrum of the sample (a) a D/CNS film sample. (b) a SCNS film sample.

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Fig.4 shows the current density-electric field intensity(J-E) curves and Fowler-Nordheim(F-N) curves of the samples (such as diamond film, SCNS film and D/CNS composite film). The threshold field E (Eth), defined as the one yielding J=10

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µA/cm2. From Fig.4(a), and the Eth of diamond film, SCNS film, D/CNS composite film is 8.09, 3.75, 2.91V/µm, respectively. And the diamond film, SCNS film and D/CNS composite film field emission current density is 0.004mA/cm2 (there's almost no electron emission), 0.45 mA/cm2 and 3.21 mA/cm2 at an electric field of 7.2V/µm. This means that the field emission maximum current density of D/CNS composite

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films is 7.1 times that of SCNS films at an electric field of 7.2 V/µm, which indicates that the diamond dielectric layer can enhance CNS film field emission performance effectively. Fig.4(b) displays the F-N curves resulting from the experimental data of Fig.4(a). The F-N curves of all the samples are basically linear, it is suggested that the

process).

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3.5

(a)

J(mA/cm2)

3.0 2.5 2.0 1.5

D/CNS composite film cathode SCNS film cathode Diamond film cathode

1.0 0.5 0.0

2

4

6

8

10

E(V/µm)

12

14

ln(J/E2)ln(A/V2)

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acquired electronic currents are due to true field emission (corresponds to tunneling

-28

(b)

-29 -30 -31 -32

D/CNS composite film cathode SCNS film cathode Diamond film cathode

-33 -34 -35

1

2

3

1/E(µm/V)

Fig.4 (a) J–E curve and (b) F–N plot of the samples

4

5

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3.5

J(mA/cm2)

2.5 2.0 1.5

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D/CNS film 1st D/CNS film 3rd D/CNS film 5th SCNS film 1st SCNS film 3rd SCNS film 5th

3.0

1.0

0.0 2

3

4

5

6

E(V/µm)

7

8

9

10

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0.5

Fig.5 J-E curve in the 1st, 3rd and 5th measurements for D/CNS film and SCNS film samples. Finally, the short-term stability of the field emission characteristics was measured for samples after an aging process (as showed in Fig.5). It is indicated that the field

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emission stability of D/CNS composite cathode has been greatly improved. Early field emission research work mainly focused on the use of diamond as the emission layer. The focus of this work is the use of diamond films as intermediate electron acceleration layers, which differ from previous work. Fig.6 and Fig.7 show

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schematic diagram of electron transport and field emission processes. According to Fig.7, the field emission band diagram of diamond film and D/CNS composite film

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can be seen. Because the diamond (111) face has a negative electron affinity, the diamond with (111) face has excellent field emission performance (as showed in Fig.7(a)).But the barrier height of diamond film and vacuum is high and the barrier width is thicker (except for (111) face), so the field emission capability of pure diamond film is limited (as showed in Fig.7(b)). The experiment proved that field emission performance of the D/CNS composite film is preferable to that of CNS film and diamond film field emission performance. The reason is as follows. On the one hand, special surface morphology of CNS film is very beneficial to improve the field emission properties. Many researchs show that the field emission

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performance of the films remarkably depends on its surface microstructure and surface morphology, and sharper and denser protrusions on the film surface can greatly improve the field emission performance. According to Fig.1, compared with the CNS of SCNS film, the CNS of D/CNS composite film not only is perpendicular to the surface of the substrate and at the same time have more sharp edges and bigger

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high to thickness ratio. And this kind of surface morphology is highly advantageous to field emission. On the other hand, diamond layer can be used as electron acceleration layer and plays an important role in D/CNS’s field emission performance. Due to has a low dielectric constant of diamond, the applied electric field mainly effect on the diamond layer, and the electrons, which are injected from the highly doped silicon

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substrate electrode, can be accelerated transmit through the sp2 phase graphite in the grain boundary of polycrystalline diamond layer and get enough energy. According to

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internal field emission model[24-28], the heterogeneity of diamond and CNS material could lead to lowering of the interfacial potential barrier. When the accelerated electron arrived at the diamond layer and carbon layer interface, it easy injection to CNS film (internal field emission), as showed in Fig.7(c). Owing to excellent conductive properties of CNS emission layer, when the electron arrives at the

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boundary between CNS layer and vacuum still has higher energy. And the electron not only can transmit into the vacuum through quantum tunneling, but also can directly across the potential barrier of CNS and vacuum (CNS film has low electron affinity) then enter into vacuum, this will increase the number of transfer into vacuum

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electronics. As a consequence, for the composite cathode, the diamond film layer has a strong field enhancement effect.

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Because in the process of synthesis of CNS film joined the catalytic layer, making the catalytic material residues inevitably exist in CNS film, which directly undermined the field emission performance of the D/CNS composite film and the field emission performance of SCNS film. Therefore, future research direction is under the condition of without the use of a catalyst, can synthesize vertical substrate CNS film.

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graphite grain boundaries

field emission e e e

e

e e

e

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diamond grains e

heavily-doped Si substrate

e e

tunneling electron heavily-dope d

e e vacuu e m e e level e

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EC=EF

e

e

conduction band

diamond grains boundary graphite network conduction band defect electron energy level EC=EF

e

e

e e e e e

vacuum level e

tunneling electron

heavily-dope d

(111) face diamond

e

e

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defect energy level

diamond grains boundary graphite

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Fig.6 The schematic sketch of electron transport and emission processes

(b)

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(a)

diamond film other than

diamond grains boundary defect graphite accelerate energy d e e e vacuu e e e EC=EF m e e e e e tunnelin g heavily-dop ed

diamond CNS

(c) Fig.7 Field emission band diagram of the films. (a) diamond film only (111) face. (b)diamond film other than (111) face.(c)D/CNS composite film

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4.Conclusion A series of diamond films, SCNS films and D/CNS composite films have been fabricated on the heavily-doped n-type silicon substrate by using microwave plasma chemical vapor deposition (MPCVD) techniques and vacuum electron beam evaporation techniques. In order to obtain the vertical growth of carbon, in the process

characterized

by

emission

scanning

electron

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of the experiment the buffer layer Al2O3 and catalyst Ni is used. The films were microscopy(FE-SEM),

X-ray

diffraction(XRD), Raman spectroscopy. Field emission test results showed that diamond film, SCNS film and D/CNS composite film respective field emission current density is 0.004mA/cm2 (there's almost no electron emission), 0.45 mA/cm2

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and 3.21 mA/cm2 at an electric field of 7.2V/µm. This means that the field emission maximum current density of D/CNS composite films is 7.1 times that of SCNS films

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at an electric field of 7.2 V/µm. At the same time the D/CNS composite film exhibits the advantage of improved reproducibility and long term stability. Diamond layer can effectively improve the CNS film field emission performance could be attributed to that the diamond film act as electron acceleration layer and the internal field emission from diamond film to CNS film. This work shows that D/CNS composite field

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emission cathodes are promising candidates for field electron emission cold cathode. This work is financially supported by the Research Innovation Key Project of the Education Committee of Shanghai (No: 14ZZ137). References

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films. ● The heterogeneity between diamond and CNS may result in a reduced interface

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barrier.