- Email: [email protected]
High performance graphene foam emitter
Accepted Manuscript High Performance Graphene Foam Emitter Taewoo Kim, Jeong Seok Lee, Kunzhou Li, Tae June Kang, Yong Hyup Kim PII:
S0008-6223(16)30089-6
DOI:
10.1016/j.carbon.2016.01.101
Reference:
CARBON 10721
To appear in:
Carbon
Received Date: 7 October 2015 Revised Date:
7 January 2016
Accepted Date: 30 January 2016
Please cite this article as: T. Kim, J.S. Lee, K. Li, T.J. Kang, Y.H. Kim, High Performance Graphene Foam Emitter, Carbon (2016), doi: 10.1016/j.carbon.2016.01.101. 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.
ACCEPTED MANUSCRIPT
High Performance Graphene Foam Emitter
1
RI PT
Taewoo Kim1, Jeong Seok Lee1, Kunzhou Li1, Tae June Kang2,∗, and Yong Hyup Kim1,* School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 08826,
Department of Mechanical Engineering, INHA University, Incheon 22212, South Korea
M AN U
2
SC
South Korea
Abstract
We report a fabrication method for graphene emitter based on simultaneous electrophoretic
TE D
deposition (EPD) and anodic reduction of graphene oxide (GO). The reduction process of GO employs both copper (Cu) electrodes immersed in well dispersed GO solution. Upon applying DC voltage between electrodes, negatively charged GO platelets are rapidly
EP
attracted to an anode, and are simultaneously reduced to graphene (rGO) by the aid of chemically spontaneous oxidation of cuprous to cupric ion and the Kolbe-like
AC C
decarboxylation. The deposition and reduction processes are accomplished at a low voltage (4 V) in a short period of time (10 sec). A rapid vacuum drying process is used after EPD process, which enables to fabricate highly porous rGO structure by vigorous escape of water molecules in an instant. With high electrical conductivity and numerous sharp graphene edges, the rGO emitter shows outstanding field emission properties, such as a low turn-on electric ∗
Corresponding authors. Tel: +82-32-860-7304. E-mail: [email protected] (Tae June Kang), Tel: +82-2-880-7385. E-mail: [email protected] (Yong Hyup Kim)
ACCEPTED MANUSCRIPT field of 1.06 V µm-1, threshold field of 1.42 V µm-1 and long-term emission stability, which
AC C
EP
TE D
M AN U
SC
RI PT
are superior to those of graphene emitters previously reported.
ACCEPTED MANUSCRIPT 1. Introduction With the superior field emission performance such as low turn-on voltage, high emission current density and long-term emission stability, electron field emissions based on sharp and
RI PT
tapered nanomaterials have been extensively studied as an alternative of thermionic emission [1-15]. In particular, the nature of atomically sharp edges of two-dimensional materials reduces greatly turn-on and applied voltages for electron emission owing to the local
SC
amplification of electric field [16-22]. Of these layered materials, graphene and its derivatives have received a great deal of attention with superb emission property and high enhancement
electrical property [23-40].
M AN U
factor, stemming from the unique two-dimensional atomic structure and extraordinary
To fabricate graphene emitters with desirable configuration and orientation suitable for field emission, various methods including direct growth [29-32], screen printing [33, 34],
TE D
electrophoretic deposition (EPD) [35-37], filtration [38], spin coating [39], and freeze-drying [40] have been developed. Among the methods the EPD of graphene is cost effective and a versatile processing technique to fabricate a two-dimensional graphene planar emitter. EPD
EP
has great advantages in obtaining thin films from charged colloidal suspensions, such as high
AC C
throughput, precise thickness control and simplicity of scale up. EPD basically requires charged colloidal particles in a liquid-phase suspension, and the particles are forced to move toward the oppositely charged electrode under an electric field. When adopting negatively charged graphene oxide (GO) suspension for EPD, GO platelets will be deposited to a positive electrode in which the electrochemical reaction is biased toward the oxidation of GO. Therefore, previous EPD studies generally have used the suspension of reduced graphene oxide (rGO) modified with positive charges for deposition on a negative electrode [35, 36, 41]. To render rGO platelets positively charged, metal salts
ACCEPTED MANUSCRIPT were introduced to the suspension so that positively charged metal ions are absorbed onto rGO surface. However, the solubility tolerance of metal-absorbed rGO is typically much lower than that of GO due to the lack of oxygen functional groups [42]. Low concentration of
RI PT
rGO solution leads to low deposition rate in EPD process even though a high electric field (~typically 100-320 V cm-1) is applied [35, 36], which has hindered the use of electrophoretic deposition of graphene in scalable and productive applications.
SC
However, the oxidation of GO at a positive electrode during EPD process is not always true. Reactive metal electrode, such as copper (Cu) and iron, leads to the reduction of GO
M AN U
instead of oxidation. It implies that direct fabrication of rGO structure from GO colloidal solution could be possible in a single step process with high deposition rate. In the present study we report a method to fabricate graphene field emitters based on the anodic reduction of GO at a negative Cu electrode. The process employs both Cu electrodes immersed in
TE D
highly concentrated GO solution. Upon applying DC voltage between electrodes, negatively charged GO platelets are rapidly attracted to the Cu anode, and are simultaneously reduced to graphene (rGO) by the aid of chemically spontaneous oxidation of cuprous (Cu+) to cupric
EP
(Cu2+) ions and the Kolbe-like decarboxylation. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analysis revealed that the oxygen functional
AC C
groups of GO, especially C-O bonds in epoxy/ether and C=O bonds in ketone/carboxylic groups, were effectively removed after EPD of GO. It is noteworthy that the deposition and reduction processes are accomplished at a low voltage (4 V) in a short period of time (10 sec). In the present study a rapid vacuum drying process is demonstrated to obtain the porous structure suitable for field emission by vigorous escape of water molecules from graphene hydrogel in a short time. The fabricated rGO foam on a Cu electrode shows high electrical conductivity and involves numerous sharp edges of graphene, and here we demonstrate its use as a field
ACCEPTED MANUSCRIPT emitter. The rGO foam emitter shows outstanding field emission properties, such as a low turn-on electric field of 1.06 V µm-1, threshold field of 1.42 V µm-1 and long-term emission stability with a current density of 9.2 mA cm-2 for 22 hrs. Our method could be available to
RI PT
various geometries of substrates such as rod, plate, and flexible wire. By employing the flexibility of the electrode, the emitter is applicable to luminescent lighting tube and also
SC
provides a winding structure that requires high-current electron sources [43-45].
2.1 Synthesis of graphene oxide
M AN U
2. Experimental section
GO powders were prepared by a modified Hummer’s method [46] from graphite powder (Bay Carbon, SP-1). A solution mixture of graphite powder, sulfuric acid, and potassium permanganate in a beaker was stirred for 6 hrs at temperature of 45 oC. The
TE D
solution was neutralized by deionized (DI) water and hydrogen peroxide. The obtained brown solution was subjected to dialysis to completely remove any residual acid and salt in the
EP
solution. GO powders in the form of sheet were prepared via a filtration process. This involved vacuum filtering a GO suspension in deionized (DI) water onto a membrane filter
AC C
(Millipore PTFE filter, 0.2 µm pore size, 47 mm diameter), drying in a vacuum chamber, and removal of the formed sheet from the filter. Afterwards, the GO sheet was re-dispersed in deionized (DI) water with a controlled concentration of 1.0 mg mL-1 by sonication. The resulting suspension yields stable colloidal suspensions of individual GO platelets due to the presence of oxygen functional groups [42]. 2.2 Fabrication of reduced graphene oxide foam After the preparation of GO suspension, two identical Cu wire electrodes were immersed in
ACCEPTED MANUSCRIPT the suspension as shown in Fig. 1(a). Then, DC voltage was applied between the electrodes with a range from 4 to 10 V for 10 sec. The inter-electrode spacing was fixed as 1 cm. The GO platelets in the solution migrated toward the positive electrode due to their negatively
RI PT
charged surface which was induced by oxygen functional groups, such as epoxy, hydroxyl, carbonyl, and carboxyl groups, existing on basal plane and edge of GO platelets. During the EPD process, GO platelets were simultaneously reduced to rGO by the aid of the oxidation
SC
process of cuprous (Cu+) to cupric (Cu2+) ion and the Kolbe-like decarboxylation, which will be discussed later. After completing the deposition process, the electrode coated with rGO
M AN U
was dried in two different manners; (1) ambient air drying and (2) rapid vacuum drying at room temperature, as shown in Figs. 1(b) and 1(c). The inset SEM image of Fig. 1(b) shows that the rGO dried in ambient air had a neatly stacked structure similar to GO paper-like materials prepared by filtration [46]. Surface tension of water between the rGO platelets
TE D
might attribute to the packing morphology. On the other hand, the rGO electrode dried in a vacuum chamber exhibited highly porous foam structure as shown in Fig. 1(c) and the inset of the figure. Vigorous escape of water molecules from the rGO hydrogel during the drying
EP
process does not allow enough time to rearrange rGO platelets, resulting in a porous structure with vertically aligned platelets. It is noteworthy that the rapid vacuum drying process is
AC C
completed at a chamber pressure of 1.0×10-2 Torr in a short time (~10 sec).
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 1. Schematics of (a) electrophoretic deposition of rGO, (b) ambient drying and (c) vacuum drying processes. Scale bars in (a) and (b) are 200 nm and 10 µm, respectively.
TE D
2.3 Characterization
Scanning electron microscopy (SEM) analysis was performed using a Hitachi S-4800 fieldemission electron microscope at an acceleration voltage of 10-15 KeV. X-ray photoemission
EP
(XPS) experiments were carried out by using a XPS spectrometer (Kratos, AXIS-HSi).
AC C
Fourier transform infrared spectroscopy (FT-IR) spectra were measured by a FT-IR spectrometer (Thermo Scientific, Nicolet 6700). The Raman spectra were measured using a micro-Raman system (JY-Horiba, LabRam 300) with excitation wavelength of 532 nm. 2.4 Field emission
A voltage between rGO cathode and molybdenum (Mo) anode was applied using a DC power supply (Matsusada Precision Inc.). Field emission current was measured by a multimeter (KEITHLEY 2000).
ACCEPTED MANUSCRIPT 3. Results and Discussion Fig. 2(a) shows the optical images of the rGO coated wires before and after drying processes at ambient air and vacuum conditions, respectively. Even under severe bending deformation,
RI PT
the foam-like rGO layer does not show any delamination from a wire electrode and breakage into small pieces of debris (See Fig. 2(b)). It indicates that strong linkages between rGO platelets in the foam as well as high interfacial strength between rGO and electrode were successfully formed. As long as a substrate is electrically conductive, uniform and
SC
homogeneous rGO foam could be fabricated regardless of its geometry because EPD process
M AN U
is mainly depending on the electric field around the electrode. The present method was applied to a planar Cu plate in the same manner. rGO foam was also successfully fabricated on the plate as shown in Fig. 2(c) and the SEM image of Fig. 3(d). Thickness of the rGO foam could be controlled by varying the applied voltage and processing time. As shown in Figs. S1 and S2 in the supplementary information, the thickness increases almost linearly
AC C
EP
TE D
with the applied voltage ranging from 4 to 10 V at a fixed processing time of 10 sec.
Figure 2. (a) Optical images of rGO deposited copper wires dried in two different manners; ambient air and vacuum dryings at room temperature. Scale bar is 5 mm. (b) Even under
ACCEPTED MANUSCRIPT severe bending deformation, the foam-like rGO layer does not show any delamination from a wire electrode and breakage into small pieces of debris. (c) Optical image of rGO coated
RI PT
planar plate.
Figs. 3(a)-(d) show SEM images of the rGO foam fabricated with an applied voltage of 5V for 10 sec. Individual rGO platelets were well interconnected each other by forming a highly
SC
porous 3-dimensional network. Close observation of rGO foam at an inclined angle (Fig. 3(b)) reveals that rGO platelets with a lateral size of several micrometers stand vertically, providing
M AN U
numerous sharp edges. It is also noteworthy that individual rGO platelets in the foam are spatially separated with distance of several micrometers. We believe that the sharp edges are advantageous by serving as active sites for field emission of electrons and the spatial distribution of rGO could decrease the screen effect felt by individual platelet in the emitter,
AC C
EP
TE D
resulting in lower turn-on and threshold fields.
Figure 3. SEM images of rGO foams fabricated with an applied voltage of 5 V for 10 sec. (a)
ACCEPTED MANUSCRIPT Individual rGO platelets on a copper wire were well interconnected each other by forming a highly porous 3-D network. (b) Close observation of the rGO foam at an inclined angle. (c) Cross-sectional SEM image of the emitter. (d) The cross-sectional SEM image of rGO foam
RI PT
fabricated on a copper plate. Scale bar in the inset of (a) and (b) are 50 µm and 5 µm, respectively.
SC
Fig. 3(c) showed the cross-sectional SEM images of the emitter. The rGO foam shows a homogeneously porous morphology in the structure. We could not observe any delamination
M AN U
and void along the interface between rGO platelets and the electrode. It might be advantageous to robust electrical and thermal conductance at the interface, which is crucial to achieving high performance of field emitter. Fig. 3(d) shows the cross-sectional SEM image of rGO foam fabricated on a planar Cu plate. Likewise the foam structure on the Cu wire, the
TE D
rGO foam also shows uniform morphology along thickness direction on the Cu plate. No delamination or void was observed at the interface between the foam and the plate. XPS analysis was carried out to characterize the reduction state of rGO foam after the EPD
EP
process, which is compared with GO. It was confirmed as shown in Figs. 4(a) and 4(b) that GO was certainly reduced during the EPD process. While two peaks of sp2 C-C bond at 284.6
AC C
eV and C-O (epoxy/ether) bond at 286.7 eV were predominant in the C1s binding energy spectrum of GO film, the intensity of C-O bond was very weak in the spectrum of rGO foam. It indicates that a large amount of epoxy and ether groups in GO were effectively removed during the EPD process. Moreover, carbonyl C=O peak at 288.2 eV and carboxyl C(O)O peak at 289.4 eV also considerably decreased. After annealing the rGO foam at 200 oC under argon (Ar) gas flow for 5 hrs, remaining oxygen-containing functional group peaks almost disappear as shown in Fig. S3. The reduction state shown in the figure is close to that of rGO prepared by chemical reduction of GO with hydrazine [47-49]. FT-IR analysis was also
ACCEPTED MANUSCRIPT performed for further investigation of the reduction state of rGO as shown in Fig. 4(c). The peaks at 1100 cm-1, 1400 cm-1, 1600 cm-1, and 1730 cm-1, correspond to γ(C-O), δ(C-OH), aromatic C=C, and γ(C=O), respectively, are predominant in FTIR spectrum of GO [50].
RI PT
However, δ(C-OH) and γ(C=O) almost disappeared in the rGO foam. It is also confirmed from Raman analysis in Fig. 4(d) that the intensity ratio of G/D bands increased from 0.86 to 1.0 after the EPD process, indicating the reduction process occurred successfully by
AC C
EP
TE D
M AN U
SC
eliminating oxygen functional group of GO platelets.
Figure 4. XPS spectra of (a) GO sheet prepared by filtration and (b) rGO foam fabricated by EPD. (c) FT-IR spectra of GO sheet, rGO foam, and rGO foam annealed at 200 oC. (d) Raman spectra of GO sheet and rGO foam.
When an electrode potential in an electrochemical system is biased relatively more positive than an equilibrium potential (corresponds to the state where the oxidation and reduction processes are balanced), a reaction at the positive electrode will be forced toward the
ACCEPTED MANUSCRIPT formation of oxidized states of an electroactive substance in electrolyte. However, it is quite interesting in this regard that the reduction process of GO was observed at the positive electrode during the EPD of GO. This unusual phenomenon could be addressed as the
RI PT
followings: The oxidation of cuprous ion (Cu+) to cupric ion (Cu2+) in the vicinity of the anode causes the reduction of GO platelets. Upon applying positive voltages to a Cu electrode, the electrode dissolves in an electrolyte by releasing Cu ions. When the dissolved ions encounter GO platelets accumulated near the anode by electrophoretic force, oxidation
SC
of cuprous to cupric ions would be occurred by reducing GO (i.e., 2Cu+ + GO +2H+ ↔ 2Cu2+
M AN U
+ rGO +H2O). This reaction is spontaneous that the standard reduction potential of cupric ion to cuprous ion (0.16 V) is lower than that of GO (0.4~0.6 V) [51]. Moreover, cupric ions could cross link individual GOs by the coordination between the ion and carboxyl, hydroxyl groups on GOs [52]. The cross linking leads to the improvement of structural reliability as well as electrical and thermal conductivity in the rGO foam. The involvement of Cu ions also
TE D
enhanced the film deposition rate of GO up to ~2 µm min-1 for the GO concentration of 1.0 mg mL-1 and the applied voltage of 5 V (the inset of Fig. 1(b)). Moreover, a reaction of
EP
Kolbe-like decarboxylation also contributed to the reduction, in which carboxyl acids in GO are removed by forming unpaired electrons by the Kolbe-like loss of CO2 [53]. The reaction
AC C
could explain the decrease of carboxyl C(O)O peak in the XPS study shown in Figs. 4(a) and 4(b). We believe that these two reactions (i.e., spontaneous oxidation of cuprous to cupric ion and the Kolbe-like decarboxylation) possibly attributes to the reduction of GO during the EPD process.
Investigation on field emission performance of the rGO foam was performed using molybdenum (Mo) tube used as an anode and the experimental setup shown in Fig. 5(a). A 50 µm-thick rGO foam deposited on 2 mm-diameter Cu wire was used as a cathode for field
ACCEPTED MANUSCRIPT emission. Ambient air-dried rGO film was also tested under the same experimental setup. While a planar emitter has an electric field of E = V / d with an inter-electrode distance ( d ), a cylindrical emitter has a different effective distance of r1ln( r2 /r1 ) which results in an
RI PT
electric field of E = V / [r1ln (r2 / r1 )] with a diameter of emitter ( r1 ) and inner diameter of anode ( r2 ) (See also Fig. S4) [45].
Current-voltage (I-V) characteristics of the emitters were investigated in a vacuum
SC
chamber at a base pressure of 3.0×10-7 Torr, as shown in Fig. 5(b). While a rGO film emitter showed a high turn-on electric field (electric field needed to produce a current density of 10
M AN U
µA cm-2) of 10.8 V µm-1 with little current density over an applied electric field (Fig. S5), the rGO foam emitter showed much low turn-on field of 1.42 V µm-1 and threshold field of 2.18 V µm-1 (electric field needed to produce a current density of 1.0 mA cm-2). These field emission characteristics are superior to those of graphene emitters previously reported [32, 34,
TE D
35]. The superb field emission performance of rGO foam emitter is attributed to numerous sharp-edges of rGO platelets at which the electric field intensity is locally enhanced, and correspondingly, electrons can be readily emitted from the edges. For this reason, the rGO
AC C
film emitter.
EP
foam emitter shows significantly improved field emission performance compared to the rGO
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 5. (a) Schematic and optical images of the experimental setup for field emission tests.
TE D
(b) Current-voltage (I-V) characteristics of the emitters in a vacuum chamber at a base pressure of 3.0×10-7 Torr. (c) Fowler–Nordheim (F–N) curves. (d) A long-term emission stability test in which the current density of the emitter was maintained constant at the level
AC C
EP
corresponding to 1.2 and 8.1 mA cm-2 for 12 hrs.
The Fowler–Nordheim (F–N) equation is useful to analyze field emission performance by correlating an applied electric field with the output emission current density, expressed as follows [54].
bΦ 3 / 2 J = aΦ −1 E 2 β 2 exp − βE
(1)
where Φ is the work function of emitting material, J is the current density, E is the local electric field, and β is the local electric field enhancement factor. a and b are constants with
ACCEPTED MANUSCRIPT 1.54 × 10-6 A eV V-2 and 6.83 × 109 eV-3/2 V m-1, respectively. The field enhancement factor (β) is typically used to investigate the geometric effect of field emitters, which is given by:
bΦ 3 / 2 β =− m
(2)
RI PT
where Φ is the work function of emitting material, b is constant (6.83 × 109 eV-3/2 V m-1), and m is the slope of the F-N curve.
SC
Figure 5(c) shows the F-N curves of the rGO foam and film emitters, and the slope of the curve represents the field enhancement factor. The field enhancement factor of 1450 for the
M AN U
rGO foam emitter is higher than that of 1220 for the rGO film emitter. It is obvious that numerous sharp-edges of rGO in the foam are attributed to this improvement. Interestingly, two distinct behaviors regarding the F–N curve of rGO foam emitter were observed. The field enhancement factor of the rGO emitter is almost linear in a low field region; however, it turns
TE D
nonlinear in the region of the high electrical field, exhibiting much higher field enhancement factor of 7140. We believe that the alignment of rGO platelets in the foam toward the direction of applied electric field might be responsible for enhancing the field enhancement
EP
factor, which is caused by electric polarization of graphene [55]. The y-intercept in F-N curve is directly related to actual emission area and work function
AC C
of an emitter. Actual emission area (A) is given from the F-N equation as follows [56, 57]: 3/ 2 J Φ bΦ A ∝ 2 exp E a βE
(3)
A high value of y-intercept in F-N curve of the rGO foam indicates the foam emitter has larger actual emission area compared to the film emitter. By extrapolating the F-N curve, the actual emission area of rGO foam was estimated as 1700 times larger, compared to the rGO film. The rGO foam formed on a Cu plate (i.e., planar-type emitter) also exhibited excellent field
ACCEPTED MANUSCRIPT emission performance with a low turn-on electric field of 1.06 V µm-1, threshold field of 1.42 V µm-1, and high enhancement factor of 7090 (see also Figs. S6(a) and S6(b) in the supplementary information).
RI PT
We compared the field emission performance of graphene emitters fabricated by various method, such as EPD [35], screen printing [33, 34], direct growth (CVD) [30, 32, 58, 59], and etc. [23, 38-40, 60-62] (Fig. S7). The emitter fabricated by our method shows excellent
are better than those of previously reported emitters.
SC
performance with a low turn-on field (1.06 V µm-1) and threshold field (1.42 V µm-1), which
M AN U
The stability of field emission is of practical interest at high current since the emitter can easily be damaged due to evaporation of graphene by Joule heating and ion bombardment of residual gases [60, 63]. A long-term emission stability test was conducted in which the current density of the emitter was maintained constant at the level corresponding to 1.2 and
TE D
8.1 mA cm-2 for 12 hrs. As shown in Fig. 5(d), it is apparent that the current is stably collected with little fluctuation, showing robustness of the graphene foam emitter. The planar rGO emitter also exhibited long-term emission stability with a current density of 9.2 mA cm-2 for
AC C
4. Conclusions
EP
22 hrs. (Fig. S6(c) in the supplementary information)
We developed a method for the direct fabrication of rGO field emitters from GO colloidal solution. A rapid vacuum drying process accomplishes highly porous rGO foam structure which involves numerous sharp edges suitable for field emission. Simultaneous processes of electrophoretic deposition and reduction of GO platelets allowed us to fabricate the rGO foam in a short process time of 10 sec. With high electrical conductivity and numerous sharp graphene edges, the graphene foam emitter shows outstanding field emission properties, such
ACCEPTED MANUSCRIPT as a low turn-on electric field of 1.06 V µm-1, threshold field of 1.42 V µm-1 and long-term emission stability with a current density of 9.2 mA cm-2 for 22 hrs. We envisioned that the present emitter is applicable to luminescent lighting tube and also provides a winding
RI PT
structure that requires high-current electron sources with high mechanical flexibility and robustness.
SC
Acknowledgements
This research was supported by the National Research Foundation of Korea (Grants 2009-
M AN U
0083512, 2014R1A2A1A05007760, and 2014R1A1A4A01008768), Defense Acquisition Program Administration and Agency for Defense Development under Contract UD100048JD, and the Brain Korea 21 Plus Project in 2014. The authors also acknowledge support from the
AC C
EP
TE D
Institute of Advanced Aerospace Technology at Seoul National University.
ACCEPTED MANUSCRIPT References [1] Kakade BA, Pillai VK, Late DJ, Chavan PG, Sheini FJ, More MA, et al. High current density, low threshold field emission from functionalized carbon nanotube bucky paper. Appl
RI PT
Phys Lett. 2010;97(7):073102. [2] Sharma RB, Late DJ, Joag DS, Govindaraj A, Rao CNR. Field emission properties of boron and nitrogen doped carbon nanotubes. Chemical Physics Letters. 2006;428(1–3):102-8.
SC
[3] Jeong Seok L, Taewoo K, Seul-Gi K, Myung Rae C, Dong Kyun S, Minwoo L, et al. High performance CNT point emitter with graphene interfacial layer. Nanotechnology.
M AN U
2014;25(45):455601.
[4] Lee JS, Kim T, Song H, Lee M, Jeong DH, Yoo J-B, et al. Binder-free, high-performance carbon nanotube line emitters fabricated using mechanical clamping process. Journal of Alloys and Compounds. 2015;626:287-91.
TE D
[5] Late D, Misra P, Singh BN, Kukreja L, Joag D, More M. Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W. Appl Phys A. 2009;95(2):613-20.
EP
[6] Ramgir NS, Late DJ, Bhise AB, More MA, Mulla IS, Joag DS, et al. ZnO Multipods, Submicron Wires, and Spherical Structures and Their Unique Field Emission Behavior. The
AC C
Journal of Physical Chemistry B. 2006;110(37):18236-42. [7] Niranjan SR, Dattatray JL, Ashok BB, Imtiaz SM, Mahendra AM, Dilip SJ, et al. Field emission studies of novel ZnO nanostructures in high and low field regions. Nanotechnology. 2006;17(11):2730. [8] Ramgir NS, Mulla IS, Vijayamohanan K, Late DJ, Bhise AB, More MA, et al. Ultralow threshold field emission from a single multipod structure of ZnO. Appl Phys Lett. 2006;88(4):042107. [9] Bhise AB, Late DJ, Walke PS, More MA, Pillai VK, Mulla IS, et al. Sb-doped SnO2 wire:
ACCEPTED MANUSCRIPT Highly stable field emitter. Journal of Crystal Growth. 2007;307(1):87-91. [10] Suryawanshi SR, Bankar PK, More MA, Late DJ. Vapour-liquid-solid growth of onedimensional In2Se3 nanostructures and their promising field emission behaviour. RSC
RI PT
Advances. 2015;5(80):65274-82. [11] Late D, More M, Sinha S, Dasgupta K, Misra P, Singh BN, et al. Synthesis and characterization of LaB6 thin films on tungsten, rhenium, silicon and other substrates and
SC
their investigations as field emitters. Appl Phys A. 2011;104(2):677-85.
[12] Late D, Karmakar S, More M, Bhoraskar S, Joag D. Arc plasma synthesized LaB6
M AN U
nanocrystallite film on various substrates as a field emitter. J Nanopart Res. 2010;12(7):2393403.
[13] Late D, Singh V, Sinha S, More M, Dasgupta K, Joag D. Synthesis of LaB6 micro/nano structures using picosecond (Nd:YAG) laser and its field emission investigations. Appl Phys
TE D
A. 2009;97(4):905-9.
[14] Late D, Kashid R, Sekhar Rout C, More M, Joag D. Low threshold field electron emission from solvothermally synthesized WO2.72 nanowires. Appl Phys A. 2010;98(4):751-
EP
6.
[15] Joag DS, Late DJ, Lanke UD. Field emission from a-GaN films deposited on Si (100).
AC C
Solid State Communications. 2004;130(5):305-8. [16] Suryawanshi SR, Kolhe PS, Rout CS, Late DJ, More MA. Spectral analysis of the emission current noise exhibited by few layer WS2 nanosheets emitter. Ultramicroscopy. 2015;149:51-7.
[17] Late DJ, Shaikh PA, Khare R, Kashid RV, Chaudhary M, More MA, et al. Pulsed LaserDeposited MoS2 Thin Films on W and Si: Field Emission and Photoresponse Studies. ACS Appl Mater Inter. 2014;6(18):15881-8. [18] Kashid RV, Late DJ, Chou SS, Huang Y-K, De M, Joag DS, et al. Enhanced Field-
ACCEPTED MANUSCRIPT Emission Behavior of Layered MoS2 Sheets. Small. 2013;9(16):2730-4. [19] Naik KK, Khare R, Chakravarty D, More MA, Thapa R, Late DJ, et al. Field emission properties of ZnO nanosheet arrays. Appl Phys Lett. 2014;105(23):233101.
RI PT
[20] Pawar MS, Bankar PK, More MA, Late DJ. Ultra-thin V2O5 nanosheet based humidity sensor, photodetector and its enhanced field emission properties. RSC Advances. 2015;5(108):88796-804.
SC
[21] Erande MB, Suryawanshi SR, More MA, Late DJ. Electrochemically Exfoliated Black Phosphorus Nanosheets – Prospective Field Emitters. European Journal of Inorganic
M AN U
Chemistry. 2015;2015(19):3102-7.
[22] Kusha Kumar N, Ruchita TK, Rogerio VG, Mahendra AM, Ranjit T, Dattatray JL, et al. Enhanced electron field emission from NiCo 2 O 4 nanosheet arrays. Materials Research Express. 2015;2(9):095011.
TE D
[23] Jeong HJ, Jeong HD, Kim HY, Kim SH, Kim JS, Jeong SY, et al. Flexible Field Emission from Thermally Welded Chemically Doped Graphene Thin Films. Small. 2012;8(2):272-80.
EP
[24] Khare RT, Gelamo RV, More MA, Late DJ, Rout CS. Enhanced field emission of plasma treated multilayer graphene. Appl Phys Lett. 2015;107(12):123503.
AC C
[25] Khare R, Shinde DB, Bansode S, More MA, Majumder M, Pillai VK, et al. Graphene nanoribbons as prospective field emitter. Appl Phys Lett. 2015;106(2):023111. [26] Rout CS, Joshi PD, Kashid RV, Joag DS, More MA, Simbeck AJ, et al. Enhanced field emission properties of doped graphene nanosheets with layered SnS2. Appl Phys Lett. 2014;105(4):043109. [27] Rout CS, Joshi PD, Kashid RV, Joag DS, More MA, Simbeck AJ, et al. Superior Field Emission Properties of Layered WS2-RGO Nanocomposites. Scientific Reports. 2013;3:3282. [28] Samantara AK, Mishra DK, Suryawanshi SR, More MA, Thapa R, Late DJ, et al. Facile
ACCEPTED MANUSCRIPT synthesis of Ag nanowire-rGO composites and their promising field emission performance. RSC Advances. 2015;5(52):41887-93. [29] Zhang Y, Du J, Tang S, Liu P, Deng S, Chen J, et al. Optimize the field emission
RI PT
character of a vertical few-layer graphene sheet by manipulating the morphology. Nanotechnology. 2012;23(1):015202.
[30] Malesevic A, Kemps R, Vanhulsel A, Chowdhury MP, Volodin A, Haesendonck CV.
SC
Field emission from vertically aligned few-layer graphene. J Appl Phys. 2008;104(8):084301. [31] Behura SK, Mukhopadhyay I, Hirose A, Yang Q, Jani O. Vertically oriented few-layer
M AN U
graphene as an electron field-emitter. physica status solidi (a). 2013;210(9):1817-21. [32] Jiang L, Yang T, Liu F, Dong J, Yao Z, Shen C, et al. Controlled Synthesis of LargeScale, Uniform, Vertically Standing Graphene for High-Performance Field Emitters. Adv Mater. 2013;25(2):250-5.
TE D
[33] Qian M, Feng T, Ding H, Lin L, Li H, Chen Y, et al. Electron field emission from screenprinted graphene films. Nanotechnology. 2009;20(42):425702. [34] Wu C, Li F, Zhang Y, Guo T. Field emission from vertical graphene sheets formed by
EP
screen-printing technique. Vacuum. 2013;94(0):48-52. [35] Wu Z-S, Pei S, Ren W, Tang D, Gao L, Liu B, et al. Field Emission of Single-Layer
AC C
Graphene Films Prepared by Electrophoretic Deposition. Adv Mater. 2009;21(17):1756-60. [36] Liu J, Zeng B, Wu Z, Sun H. Enhanced Field Electron Emission of Graphene Sheets by CsI Coating after Electrophoretic Deposition. ACS Appl Mater Inter. 2011;4(3):1219-24. [37] Chen J, Cui L, Sun D, Yang B, Yang J, Yan X. Enhanced field emission properties from aligned graphenes fabricated on micro-hole patterned stainless steel. Appl Phys Lett. 2014;105(21):213111. [38] Jeong HJ, Kim HY, Jeong HD, Jeong SY, Han JT, Lee G-W. Arrays of vertically aligned tubular-structured graphene for flexible field emitters. J Mater Chem. 2012;22(22):11277-83.
ACCEPTED MANUSCRIPT [39] Baby TT, Ramaprabhu S. Cold field emission from hydrogen exfoliated graphene composites. Appl Phys Lett. 2011;98(18):183111. [40] Kim HY, Jeong S, Jeong SY, Baeg K-J, Han JT, Jeong MS, et al. Chemically doped
RI PT
three-dimensional porous graphene monoliths for high-performance flexible field emitters. Nanoscale. 2015;7(12):5495-502.
[41] Koh ATT, Foong YM, Pan L, Sun Z, Chua DHC. Effective large-area free-standing
SC
graphene field emitters by electrophoretic deposition. Appl Phys Lett. 2012;101(18):183107. [42] Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of
M AN U
graphene nanosheets. Nat Nanotechnol. 2008;3(2):101-5.
[43] Wei Y, Xiao L, Zhu F, Liu L, Tang J, Liu P, et al. Cold linear cathodes with carbon nanotube
emitters
and
their
2007;18(32):325702.
application
in
luminescent
tubes.
Nanotechnology.
TE D
[44] Croci M, Arfaoui I, Stöckli T, Chatelain A, Bonard J-M. A fully sealed luminescent tube based on carbon nanotube field emission. Microelectron J. 2004;35(4):329-36. [45] Bonard J-M, Stockli T, Noury O, Chatelain A. Field emission from cylindrical carbon
EP
nanotube cathodes: Possibilities for luminescent tubes. Appl Phys Lett. 2001;78(18):2775-7. [46] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al.
AC C
Preparation and characterization of graphene oxide paper. Nature. 2007;448(7152):457-60. [47] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon. 2007;45(7):1558-65. [48] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev. 2010;39(1):228-40. [49] Peng-Gang R, Ding-Xiang Y, Xu J, Tao C, Zhong-Ming L. Temperature dependence of graphene oxide reduced by hydrazine hydrate. Nanotechnology. 2011;22(5):055705.
ACCEPTED MANUSCRIPT [50] Zangmeister CD. Preparation and Evaluation of Graphite Oxide Reduced at 220 °C. Chem Mater. 2010;22(19):5625-9. [51] Cao X, Qi D, Yin S, Bu J, Li F, Goh CF, et al. Ambient Fabrication of Large-Area
RI PT
Graphene Films via a Synchronous Reduction and Assembly Strategy. Adv Mater. 2013;25(21):2957-62.
[52] Mathesh M, Liu J, Nam ND, Lam SKH, Zheng R, Barrow CJ, et al. Facile synthesis of
SC
graphene oxide hybrids bridged by copper ions for increased conductivity. J Mater Chem C. 2013;1(18):3084-90.
M AN U
[53] An SJ, Zhu Y, Lee SH, Stoller MD, Emilsson T, Park S, et al. Thin Film Fabrication and Simultaneous Anodic Reduction of Deposited Graphene Oxide Platelets by Electrophoretic Deposition. J Phys Chem Lett. 2010;1(8):1259-63.
[54] Fowler RH, Nordheim L. Electron Emission in Intense Electric Fields. Proceedings of
1928;119(781):173-81.
TE D
the Royal Society of London A: Mathematical, Physical and Engineering Sciences.
[55] Wang Z. Alignment of graphene nanoribbons by an electric field. Carbon.
EP
2009;47(13):3050-3.
[56] Hirakawa M, Sonoda S, Tanaka C, Murakami H, Yamakawa H. Electron emission
AC C
properties of carbon nanotubes. Appl Surf Sci. 2001;169–170(0):662-5. [57] Wong KW, Zhou XT, Au FCK, Lai HL, Lee CS, Lee ST. Field-emission characteristics of SiC nanowires prepared by chemical-vapor deposition. Appl Phys Lett. 1999;75(19):291820.
[58] Qi JL, Wang X, Zheng WT, Tian HW, Hu CQ, Peng YS. Ar plasma treatment on few layer graphene sheets for enhancing their field emission properties. J Phys D: Appl Phys. 2010;43(5):055302. [59] Yu Z, Jiale D, Shuai T, Pei L, Shaozhi D, Jun C, et al. Optimize the field emission
ACCEPTED MANUSCRIPT character of a vertical few-layer graphene sheet by manipulating the morphology. Nanotechnology. 2012;23(1):015202. [60] Stratakis E, Eda G, Yamaguchi H, Kymakis E, Fotakis C, Chhowalla M. Free-standing
RI PT
graphene on microstructured silicon vertices for enhanced field emission properties. Nanoscale. 2012;4(10):3069-74.
[61] Dong J, Zeng B, Lan Y, Tian S, Shan Y, Liu X, et al. Field Emission from Few-Layer
Nanoscience and Nanotechnology. 2010;10(8):5051-5.
SC
Graphene Nanosheets Produced by Liquid Phase Exfoliation of Graphite. Journal of
M AN U
[62] Huang Q, Wang G, Guo L, Jia Y, Lin J, Li K, et al. Approaching the Intrinsic Electron Field-Emission of a Graphene Film Consisting of Quasi-Freestanding Graphene Strips. Small. 2011;7(4):450-4.
[63] Kashid RV, Yusop MZ, Takahashi C, Kalita G, Panchakarla LS, Joag DS, et al. Field
TE D
emission characteristics of pristine and N-doped graphene measured by in-situ transmission
AC C
EP
electron microscopy. J Appl Phys. 2013;113(21):214311-5.
S0008-6223(16)30089-6
DOI:
10.1016/j.carbon.2016.01.101
Reference:
CARBON 10721
To appear in:
Carbon
Received Date: 7 October 2015 Revised Date:
7 January 2016
Accepted Date: 30 January 2016
Please cite this article as: T. Kim, J.S. Lee, K. Li, T.J. Kang, Y.H. Kim, High Performance Graphene Foam Emitter, Carbon (2016), doi: 10.1016/j.carbon.2016.01.101. 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.
ACCEPTED MANUSCRIPT
High Performance Graphene Foam Emitter
1
RI PT
Taewoo Kim1, Jeong Seok Lee1, Kunzhou Li1, Tae June Kang2,∗, and Yong Hyup Kim1,* School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 08826,
Department of Mechanical Engineering, INHA University, Incheon 22212, South Korea
M AN U
2
SC
South Korea
Abstract
We report a fabrication method for graphene emitter based on simultaneous electrophoretic
TE D
deposition (EPD) and anodic reduction of graphene oxide (GO). The reduction process of GO employs both copper (Cu) electrodes immersed in well dispersed GO solution. Upon applying DC voltage between electrodes, negatively charged GO platelets are rapidly
EP
attracted to an anode, and are simultaneously reduced to graphene (rGO) by the aid of chemically spontaneous oxidation of cuprous to cupric ion and the Kolbe-like
AC C
decarboxylation. The deposition and reduction processes are accomplished at a low voltage (4 V) in a short period of time (10 sec). A rapid vacuum drying process is used after EPD process, which enables to fabricate highly porous rGO structure by vigorous escape of water molecules in an instant. With high electrical conductivity and numerous sharp graphene edges, the rGO emitter shows outstanding field emission properties, such as a low turn-on electric ∗
Corresponding authors. Tel: +82-32-860-7304. E-mail: [email protected] (Tae June Kang), Tel: +82-2-880-7385. E-mail: [email protected] (Yong Hyup Kim)
ACCEPTED MANUSCRIPT field of 1.06 V µm-1, threshold field of 1.42 V µm-1 and long-term emission stability, which
AC C
EP
TE D
M AN U
SC
RI PT
are superior to those of graphene emitters previously reported.
ACCEPTED MANUSCRIPT 1. Introduction With the superior field emission performance such as low turn-on voltage, high emission current density and long-term emission stability, electron field emissions based on sharp and
RI PT
tapered nanomaterials have been extensively studied as an alternative of thermionic emission [1-15]. In particular, the nature of atomically sharp edges of two-dimensional materials reduces greatly turn-on and applied voltages for electron emission owing to the local
SC
amplification of electric field [16-22]. Of these layered materials, graphene and its derivatives have received a great deal of attention with superb emission property and high enhancement
electrical property [23-40].
M AN U
factor, stemming from the unique two-dimensional atomic structure and extraordinary
To fabricate graphene emitters with desirable configuration and orientation suitable for field emission, various methods including direct growth [29-32], screen printing [33, 34],
TE D
electrophoretic deposition (EPD) [35-37], filtration [38], spin coating [39], and freeze-drying [40] have been developed. Among the methods the EPD of graphene is cost effective and a versatile processing technique to fabricate a two-dimensional graphene planar emitter. EPD
EP
has great advantages in obtaining thin films from charged colloidal suspensions, such as high
AC C
throughput, precise thickness control and simplicity of scale up. EPD basically requires charged colloidal particles in a liquid-phase suspension, and the particles are forced to move toward the oppositely charged electrode under an electric field. When adopting negatively charged graphene oxide (GO) suspension for EPD, GO platelets will be deposited to a positive electrode in which the electrochemical reaction is biased toward the oxidation of GO. Therefore, previous EPD studies generally have used the suspension of reduced graphene oxide (rGO) modified with positive charges for deposition on a negative electrode [35, 36, 41]. To render rGO platelets positively charged, metal salts
ACCEPTED MANUSCRIPT were introduced to the suspension so that positively charged metal ions are absorbed onto rGO surface. However, the solubility tolerance of metal-absorbed rGO is typically much lower than that of GO due to the lack of oxygen functional groups [42]. Low concentration of
RI PT
rGO solution leads to low deposition rate in EPD process even though a high electric field (~typically 100-320 V cm-1) is applied [35, 36], which has hindered the use of electrophoretic deposition of graphene in scalable and productive applications.
SC
However, the oxidation of GO at a positive electrode during EPD process is not always true. Reactive metal electrode, such as copper (Cu) and iron, leads to the reduction of GO
M AN U
instead of oxidation. It implies that direct fabrication of rGO structure from GO colloidal solution could be possible in a single step process with high deposition rate. In the present study we report a method to fabricate graphene field emitters based on the anodic reduction of GO at a negative Cu electrode. The process employs both Cu electrodes immersed in
TE D
highly concentrated GO solution. Upon applying DC voltage between electrodes, negatively charged GO platelets are rapidly attracted to the Cu anode, and are simultaneously reduced to graphene (rGO) by the aid of chemically spontaneous oxidation of cuprous (Cu+) to cupric
EP
(Cu2+) ions and the Kolbe-like decarboxylation. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analysis revealed that the oxygen functional
AC C
groups of GO, especially C-O bonds in epoxy/ether and C=O bonds in ketone/carboxylic groups, were effectively removed after EPD of GO. It is noteworthy that the deposition and reduction processes are accomplished at a low voltage (4 V) in a short period of time (10 sec). In the present study a rapid vacuum drying process is demonstrated to obtain the porous structure suitable for field emission by vigorous escape of water molecules from graphene hydrogel in a short time. The fabricated rGO foam on a Cu electrode shows high electrical conductivity and involves numerous sharp edges of graphene, and here we demonstrate its use as a field
ACCEPTED MANUSCRIPT emitter. The rGO foam emitter shows outstanding field emission properties, such as a low turn-on electric field of 1.06 V µm-1, threshold field of 1.42 V µm-1 and long-term emission stability with a current density of 9.2 mA cm-2 for 22 hrs. Our method could be available to
RI PT
various geometries of substrates such as rod, plate, and flexible wire. By employing the flexibility of the electrode, the emitter is applicable to luminescent lighting tube and also
SC
provides a winding structure that requires high-current electron sources [43-45].
2.1 Synthesis of graphene oxide
M AN U
2. Experimental section
GO powders were prepared by a modified Hummer’s method [46] from graphite powder (Bay Carbon, SP-1). A solution mixture of graphite powder, sulfuric acid, and potassium permanganate in a beaker was stirred for 6 hrs at temperature of 45 oC. The
TE D
solution was neutralized by deionized (DI) water and hydrogen peroxide. The obtained brown solution was subjected to dialysis to completely remove any residual acid and salt in the
EP
solution. GO powders in the form of sheet were prepared via a filtration process. This involved vacuum filtering a GO suspension in deionized (DI) water onto a membrane filter
AC C
(Millipore PTFE filter, 0.2 µm pore size, 47 mm diameter), drying in a vacuum chamber, and removal of the formed sheet from the filter. Afterwards, the GO sheet was re-dispersed in deionized (DI) water with a controlled concentration of 1.0 mg mL-1 by sonication. The resulting suspension yields stable colloidal suspensions of individual GO platelets due to the presence of oxygen functional groups [42]. 2.2 Fabrication of reduced graphene oxide foam After the preparation of GO suspension, two identical Cu wire electrodes were immersed in
ACCEPTED MANUSCRIPT the suspension as shown in Fig. 1(a). Then, DC voltage was applied between the electrodes with a range from 4 to 10 V for 10 sec. The inter-electrode spacing was fixed as 1 cm. The GO platelets in the solution migrated toward the positive electrode due to their negatively
RI PT
charged surface which was induced by oxygen functional groups, such as epoxy, hydroxyl, carbonyl, and carboxyl groups, existing on basal plane and edge of GO platelets. During the EPD process, GO platelets were simultaneously reduced to rGO by the aid of the oxidation
SC
process of cuprous (Cu+) to cupric (Cu2+) ion and the Kolbe-like decarboxylation, which will be discussed later. After completing the deposition process, the electrode coated with rGO
M AN U
was dried in two different manners; (1) ambient air drying and (2) rapid vacuum drying at room temperature, as shown in Figs. 1(b) and 1(c). The inset SEM image of Fig. 1(b) shows that the rGO dried in ambient air had a neatly stacked structure similar to GO paper-like materials prepared by filtration [46]. Surface tension of water between the rGO platelets
TE D
might attribute to the packing morphology. On the other hand, the rGO electrode dried in a vacuum chamber exhibited highly porous foam structure as shown in Fig. 1(c) and the inset of the figure. Vigorous escape of water molecules from the rGO hydrogel during the drying
EP
process does not allow enough time to rearrange rGO platelets, resulting in a porous structure with vertically aligned platelets. It is noteworthy that the rapid vacuum drying process is
AC C
completed at a chamber pressure of 1.0×10-2 Torr in a short time (~10 sec).
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 1. Schematics of (a) electrophoretic deposition of rGO, (b) ambient drying and (c) vacuum drying processes. Scale bars in (a) and (b) are 200 nm and 10 µm, respectively.
TE D
2.3 Characterization
Scanning electron microscopy (SEM) analysis was performed using a Hitachi S-4800 fieldemission electron microscope at an acceleration voltage of 10-15 KeV. X-ray photoemission
EP
(XPS) experiments were carried out by using a XPS spectrometer (Kratos, AXIS-HSi).
AC C
Fourier transform infrared spectroscopy (FT-IR) spectra were measured by a FT-IR spectrometer (Thermo Scientific, Nicolet 6700). The Raman spectra were measured using a micro-Raman system (JY-Horiba, LabRam 300) with excitation wavelength of 532 nm. 2.4 Field emission
A voltage between rGO cathode and molybdenum (Mo) anode was applied using a DC power supply (Matsusada Precision Inc.). Field emission current was measured by a multimeter (KEITHLEY 2000).
ACCEPTED MANUSCRIPT 3. Results and Discussion Fig. 2(a) shows the optical images of the rGO coated wires before and after drying processes at ambient air and vacuum conditions, respectively. Even under severe bending deformation,
RI PT
the foam-like rGO layer does not show any delamination from a wire electrode and breakage into small pieces of debris (See Fig. 2(b)). It indicates that strong linkages between rGO platelets in the foam as well as high interfacial strength between rGO and electrode were successfully formed. As long as a substrate is electrically conductive, uniform and
SC
homogeneous rGO foam could be fabricated regardless of its geometry because EPD process
M AN U
is mainly depending on the electric field around the electrode. The present method was applied to a planar Cu plate in the same manner. rGO foam was also successfully fabricated on the plate as shown in Fig. 2(c) and the SEM image of Fig. 3(d). Thickness of the rGO foam could be controlled by varying the applied voltage and processing time. As shown in Figs. S1 and S2 in the supplementary information, the thickness increases almost linearly
AC C
EP
TE D
with the applied voltage ranging from 4 to 10 V at a fixed processing time of 10 sec.
Figure 2. (a) Optical images of rGO deposited copper wires dried in two different manners; ambient air and vacuum dryings at room temperature. Scale bar is 5 mm. (b) Even under
ACCEPTED MANUSCRIPT severe bending deformation, the foam-like rGO layer does not show any delamination from a wire electrode and breakage into small pieces of debris. (c) Optical image of rGO coated
RI PT
planar plate.
Figs. 3(a)-(d) show SEM images of the rGO foam fabricated with an applied voltage of 5V for 10 sec. Individual rGO platelets were well interconnected each other by forming a highly
SC
porous 3-dimensional network. Close observation of rGO foam at an inclined angle (Fig. 3(b)) reveals that rGO platelets with a lateral size of several micrometers stand vertically, providing
M AN U
numerous sharp edges. It is also noteworthy that individual rGO platelets in the foam are spatially separated with distance of several micrometers. We believe that the sharp edges are advantageous by serving as active sites for field emission of electrons and the spatial distribution of rGO could decrease the screen effect felt by individual platelet in the emitter,
AC C
EP
TE D
resulting in lower turn-on and threshold fields.
Figure 3. SEM images of rGO foams fabricated with an applied voltage of 5 V for 10 sec. (a)
ACCEPTED MANUSCRIPT Individual rGO platelets on a copper wire were well interconnected each other by forming a highly porous 3-D network. (b) Close observation of the rGO foam at an inclined angle. (c) Cross-sectional SEM image of the emitter. (d) The cross-sectional SEM image of rGO foam
RI PT
fabricated on a copper plate. Scale bar in the inset of (a) and (b) are 50 µm and 5 µm, respectively.
SC
Fig. 3(c) showed the cross-sectional SEM images of the emitter. The rGO foam shows a homogeneously porous morphology in the structure. We could not observe any delamination
M AN U
and void along the interface between rGO platelets and the electrode. It might be advantageous to robust electrical and thermal conductance at the interface, which is crucial to achieving high performance of field emitter. Fig. 3(d) shows the cross-sectional SEM image of rGO foam fabricated on a planar Cu plate. Likewise the foam structure on the Cu wire, the
TE D
rGO foam also shows uniform morphology along thickness direction on the Cu plate. No delamination or void was observed at the interface between the foam and the plate. XPS analysis was carried out to characterize the reduction state of rGO foam after the EPD
EP
process, which is compared with GO. It was confirmed as shown in Figs. 4(a) and 4(b) that GO was certainly reduced during the EPD process. While two peaks of sp2 C-C bond at 284.6
AC C
eV and C-O (epoxy/ether) bond at 286.7 eV were predominant in the C1s binding energy spectrum of GO film, the intensity of C-O bond was very weak in the spectrum of rGO foam. It indicates that a large amount of epoxy and ether groups in GO were effectively removed during the EPD process. Moreover, carbonyl C=O peak at 288.2 eV and carboxyl C(O)O peak at 289.4 eV also considerably decreased. After annealing the rGO foam at 200 oC under argon (Ar) gas flow for 5 hrs, remaining oxygen-containing functional group peaks almost disappear as shown in Fig. S3. The reduction state shown in the figure is close to that of rGO prepared by chemical reduction of GO with hydrazine [47-49]. FT-IR analysis was also
ACCEPTED MANUSCRIPT performed for further investigation of the reduction state of rGO as shown in Fig. 4(c). The peaks at 1100 cm-1, 1400 cm-1, 1600 cm-1, and 1730 cm-1, correspond to γ(C-O), δ(C-OH), aromatic C=C, and γ(C=O), respectively, are predominant in FTIR spectrum of GO [50].
RI PT
However, δ(C-OH) and γ(C=O) almost disappeared in the rGO foam. It is also confirmed from Raman analysis in Fig. 4(d) that the intensity ratio of G/D bands increased from 0.86 to 1.0 after the EPD process, indicating the reduction process occurred successfully by
AC C
EP
TE D
M AN U
SC
eliminating oxygen functional group of GO platelets.
Figure 4. XPS spectra of (a) GO sheet prepared by filtration and (b) rGO foam fabricated by EPD. (c) FT-IR spectra of GO sheet, rGO foam, and rGO foam annealed at 200 oC. (d) Raman spectra of GO sheet and rGO foam.
When an electrode potential in an electrochemical system is biased relatively more positive than an equilibrium potential (corresponds to the state where the oxidation and reduction processes are balanced), a reaction at the positive electrode will be forced toward the
ACCEPTED MANUSCRIPT formation of oxidized states of an electroactive substance in electrolyte. However, it is quite interesting in this regard that the reduction process of GO was observed at the positive electrode during the EPD of GO. This unusual phenomenon could be addressed as the
RI PT
followings: The oxidation of cuprous ion (Cu+) to cupric ion (Cu2+) in the vicinity of the anode causes the reduction of GO platelets. Upon applying positive voltages to a Cu electrode, the electrode dissolves in an electrolyte by releasing Cu ions. When the dissolved ions encounter GO platelets accumulated near the anode by electrophoretic force, oxidation
SC
of cuprous to cupric ions would be occurred by reducing GO (i.e., 2Cu+ + GO +2H+ ↔ 2Cu2+
M AN U
+ rGO +H2O). This reaction is spontaneous that the standard reduction potential of cupric ion to cuprous ion (0.16 V) is lower than that of GO (0.4~0.6 V) [51]. Moreover, cupric ions could cross link individual GOs by the coordination between the ion and carboxyl, hydroxyl groups on GOs [52]. The cross linking leads to the improvement of structural reliability as well as electrical and thermal conductivity in the rGO foam. The involvement of Cu ions also
TE D
enhanced the film deposition rate of GO up to ~2 µm min-1 for the GO concentration of 1.0 mg mL-1 and the applied voltage of 5 V (the inset of Fig. 1(b)). Moreover, a reaction of
EP
Kolbe-like decarboxylation also contributed to the reduction, in which carboxyl acids in GO are removed by forming unpaired electrons by the Kolbe-like loss of CO2 [53]. The reaction
AC C
could explain the decrease of carboxyl C(O)O peak in the XPS study shown in Figs. 4(a) and 4(b). We believe that these two reactions (i.e., spontaneous oxidation of cuprous to cupric ion and the Kolbe-like decarboxylation) possibly attributes to the reduction of GO during the EPD process.
Investigation on field emission performance of the rGO foam was performed using molybdenum (Mo) tube used as an anode and the experimental setup shown in Fig. 5(a). A 50 µm-thick rGO foam deposited on 2 mm-diameter Cu wire was used as a cathode for field
ACCEPTED MANUSCRIPT emission. Ambient air-dried rGO film was also tested under the same experimental setup. While a planar emitter has an electric field of E = V / d with an inter-electrode distance ( d ), a cylindrical emitter has a different effective distance of r1ln( r2 /r1 ) which results in an
RI PT
electric field of E = V / [r1ln (r2 / r1 )] with a diameter of emitter ( r1 ) and inner diameter of anode ( r2 ) (See also Fig. S4) [45].
Current-voltage (I-V) characteristics of the emitters were investigated in a vacuum
SC
chamber at a base pressure of 3.0×10-7 Torr, as shown in Fig. 5(b). While a rGO film emitter showed a high turn-on electric field (electric field needed to produce a current density of 10
M AN U
µA cm-2) of 10.8 V µm-1 with little current density over an applied electric field (Fig. S5), the rGO foam emitter showed much low turn-on field of 1.42 V µm-1 and threshold field of 2.18 V µm-1 (electric field needed to produce a current density of 1.0 mA cm-2). These field emission characteristics are superior to those of graphene emitters previously reported [32, 34,
TE D
35]. The superb field emission performance of rGO foam emitter is attributed to numerous sharp-edges of rGO platelets at which the electric field intensity is locally enhanced, and correspondingly, electrons can be readily emitted from the edges. For this reason, the rGO
AC C
film emitter.
EP
foam emitter shows significantly improved field emission performance compared to the rGO
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 5. (a) Schematic and optical images of the experimental setup for field emission tests.
TE D
(b) Current-voltage (I-V) characteristics of the emitters in a vacuum chamber at a base pressure of 3.0×10-7 Torr. (c) Fowler–Nordheim (F–N) curves. (d) A long-term emission stability test in which the current density of the emitter was maintained constant at the level
AC C
EP
corresponding to 1.2 and 8.1 mA cm-2 for 12 hrs.
The Fowler–Nordheim (F–N) equation is useful to analyze field emission performance by correlating an applied electric field with the output emission current density, expressed as follows [54].
bΦ 3 / 2 J = aΦ −1 E 2 β 2 exp − βE
(1)
where Φ is the work function of emitting material, J is the current density, E is the local electric field, and β is the local electric field enhancement factor. a and b are constants with
ACCEPTED MANUSCRIPT 1.54 × 10-6 A eV V-2 and 6.83 × 109 eV-3/2 V m-1, respectively. The field enhancement factor (β) is typically used to investigate the geometric effect of field emitters, which is given by:
bΦ 3 / 2 β =− m
(2)
RI PT
where Φ is the work function of emitting material, b is constant (6.83 × 109 eV-3/2 V m-1), and m is the slope of the F-N curve.
SC
Figure 5(c) shows the F-N curves of the rGO foam and film emitters, and the slope of the curve represents the field enhancement factor. The field enhancement factor of 1450 for the
M AN U
rGO foam emitter is higher than that of 1220 for the rGO film emitter. It is obvious that numerous sharp-edges of rGO in the foam are attributed to this improvement. Interestingly, two distinct behaviors regarding the F–N curve of rGO foam emitter were observed. The field enhancement factor of the rGO emitter is almost linear in a low field region; however, it turns
TE D
nonlinear in the region of the high electrical field, exhibiting much higher field enhancement factor of 7140. We believe that the alignment of rGO platelets in the foam toward the direction of applied electric field might be responsible for enhancing the field enhancement
EP
factor, which is caused by electric polarization of graphene [55]. The y-intercept in F-N curve is directly related to actual emission area and work function
AC C
of an emitter. Actual emission area (A) is given from the F-N equation as follows [56, 57]: 3/ 2 J Φ bΦ A ∝ 2 exp E a βE
(3)
A high value of y-intercept in F-N curve of the rGO foam indicates the foam emitter has larger actual emission area compared to the film emitter. By extrapolating the F-N curve, the actual emission area of rGO foam was estimated as 1700 times larger, compared to the rGO film. The rGO foam formed on a Cu plate (i.e., planar-type emitter) also exhibited excellent field
ACCEPTED MANUSCRIPT emission performance with a low turn-on electric field of 1.06 V µm-1, threshold field of 1.42 V µm-1, and high enhancement factor of 7090 (see also Figs. S6(a) and S6(b) in the supplementary information).
RI PT
We compared the field emission performance of graphene emitters fabricated by various method, such as EPD [35], screen printing [33, 34], direct growth (CVD) [30, 32, 58, 59], and etc. [23, 38-40, 60-62] (Fig. S7). The emitter fabricated by our method shows excellent
are better than those of previously reported emitters.
SC
performance with a low turn-on field (1.06 V µm-1) and threshold field (1.42 V µm-1), which
M AN U
The stability of field emission is of practical interest at high current since the emitter can easily be damaged due to evaporation of graphene by Joule heating and ion bombardment of residual gases [60, 63]. A long-term emission stability test was conducted in which the current density of the emitter was maintained constant at the level corresponding to 1.2 and
TE D
8.1 mA cm-2 for 12 hrs. As shown in Fig. 5(d), it is apparent that the current is stably collected with little fluctuation, showing robustness of the graphene foam emitter. The planar rGO emitter also exhibited long-term emission stability with a current density of 9.2 mA cm-2 for
AC C
4. Conclusions
EP
22 hrs. (Fig. S6(c) in the supplementary information)
We developed a method for the direct fabrication of rGO field emitters from GO colloidal solution. A rapid vacuum drying process accomplishes highly porous rGO foam structure which involves numerous sharp edges suitable for field emission. Simultaneous processes of electrophoretic deposition and reduction of GO platelets allowed us to fabricate the rGO foam in a short process time of 10 sec. With high electrical conductivity and numerous sharp graphene edges, the graphene foam emitter shows outstanding field emission properties, such
ACCEPTED MANUSCRIPT as a low turn-on electric field of 1.06 V µm-1, threshold field of 1.42 V µm-1 and long-term emission stability with a current density of 9.2 mA cm-2 for 22 hrs. We envisioned that the present emitter is applicable to luminescent lighting tube and also provides a winding
RI PT
structure that requires high-current electron sources with high mechanical flexibility and robustness.
SC
Acknowledgements
This research was supported by the National Research Foundation of Korea (Grants 2009-
M AN U
0083512, 2014R1A2A1A05007760, and 2014R1A1A4A01008768), Defense Acquisition Program Administration and Agency for Defense Development under Contract UD100048JD, and the Brain Korea 21 Plus Project in 2014. The authors also acknowledge support from the
AC C
EP
TE D
Institute of Advanced Aerospace Technology at Seoul National University.
ACCEPTED MANUSCRIPT References [1] Kakade BA, Pillai VK, Late DJ, Chavan PG, Sheini FJ, More MA, et al. High current density, low threshold field emission from functionalized carbon nanotube bucky paper. Appl
RI PT
Phys Lett. 2010;97(7):073102. [2] Sharma RB, Late DJ, Joag DS, Govindaraj A, Rao CNR. Field emission properties of boron and nitrogen doped carbon nanotubes. Chemical Physics Letters. 2006;428(1–3):102-8.
SC
[3] Jeong Seok L, Taewoo K, Seul-Gi K, Myung Rae C, Dong Kyun S, Minwoo L, et al. High performance CNT point emitter with graphene interfacial layer. Nanotechnology.
M AN U
2014;25(45):455601.
[4] Lee JS, Kim T, Song H, Lee M, Jeong DH, Yoo J-B, et al. Binder-free, high-performance carbon nanotube line emitters fabricated using mechanical clamping process. Journal of Alloys and Compounds. 2015;626:287-91.
TE D
[5] Late D, Misra P, Singh BN, Kukreja L, Joag D, More M. Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W. Appl Phys A. 2009;95(2):613-20.
EP
[6] Ramgir NS, Late DJ, Bhise AB, More MA, Mulla IS, Joag DS, et al. ZnO Multipods, Submicron Wires, and Spherical Structures and Their Unique Field Emission Behavior. The
AC C
Journal of Physical Chemistry B. 2006;110(37):18236-42. [7] Niranjan SR, Dattatray JL, Ashok BB, Imtiaz SM, Mahendra AM, Dilip SJ, et al. Field emission studies of novel ZnO nanostructures in high and low field regions. Nanotechnology. 2006;17(11):2730. [8] Ramgir NS, Mulla IS, Vijayamohanan K, Late DJ, Bhise AB, More MA, et al. Ultralow threshold field emission from a single multipod structure of ZnO. Appl Phys Lett. 2006;88(4):042107. [9] Bhise AB, Late DJ, Walke PS, More MA, Pillai VK, Mulla IS, et al. Sb-doped SnO2 wire:
ACCEPTED MANUSCRIPT Highly stable field emitter. Journal of Crystal Growth. 2007;307(1):87-91. [10] Suryawanshi SR, Bankar PK, More MA, Late DJ. Vapour-liquid-solid growth of onedimensional In2Se3 nanostructures and their promising field emission behaviour. RSC
RI PT
Advances. 2015;5(80):65274-82. [11] Late D, More M, Sinha S, Dasgupta K, Misra P, Singh BN, et al. Synthesis and characterization of LaB6 thin films on tungsten, rhenium, silicon and other substrates and
SC
their investigations as field emitters. Appl Phys A. 2011;104(2):677-85.
[12] Late D, Karmakar S, More M, Bhoraskar S, Joag D. Arc plasma synthesized LaB6
M AN U
nanocrystallite film on various substrates as a field emitter. J Nanopart Res. 2010;12(7):2393403.
[13] Late D, Singh V, Sinha S, More M, Dasgupta K, Joag D. Synthesis of LaB6 micro/nano structures using picosecond (Nd:YAG) laser and its field emission investigations. Appl Phys
TE D
A. 2009;97(4):905-9.
[14] Late D, Kashid R, Sekhar Rout C, More M, Joag D. Low threshold field electron emission from solvothermally synthesized WO2.72 nanowires. Appl Phys A. 2010;98(4):751-
EP
6.
[15] Joag DS, Late DJ, Lanke UD. Field emission from a-GaN films deposited on Si (100).
AC C
Solid State Communications. 2004;130(5):305-8. [16] Suryawanshi SR, Kolhe PS, Rout CS, Late DJ, More MA. Spectral analysis of the emission current noise exhibited by few layer WS2 nanosheets emitter. Ultramicroscopy. 2015;149:51-7.
[17] Late DJ, Shaikh PA, Khare R, Kashid RV, Chaudhary M, More MA, et al. Pulsed LaserDeposited MoS2 Thin Films on W and Si: Field Emission and Photoresponse Studies. ACS Appl Mater Inter. 2014;6(18):15881-8. [18] Kashid RV, Late DJ, Chou SS, Huang Y-K, De M, Joag DS, et al. Enhanced Field-
ACCEPTED MANUSCRIPT Emission Behavior of Layered MoS2 Sheets. Small. 2013;9(16):2730-4. [19] Naik KK, Khare R, Chakravarty D, More MA, Thapa R, Late DJ, et al. Field emission properties of ZnO nanosheet arrays. Appl Phys Lett. 2014;105(23):233101.
RI PT
[20] Pawar MS, Bankar PK, More MA, Late DJ. Ultra-thin V2O5 nanosheet based humidity sensor, photodetector and its enhanced field emission properties. RSC Advances. 2015;5(108):88796-804.
SC
[21] Erande MB, Suryawanshi SR, More MA, Late DJ. Electrochemically Exfoliated Black Phosphorus Nanosheets – Prospective Field Emitters. European Journal of Inorganic
M AN U
Chemistry. 2015;2015(19):3102-7.
[22] Kusha Kumar N, Ruchita TK, Rogerio VG, Mahendra AM, Ranjit T, Dattatray JL, et al. Enhanced electron field emission from NiCo 2 O 4 nanosheet arrays. Materials Research Express. 2015;2(9):095011.
TE D
[23] Jeong HJ, Jeong HD, Kim HY, Kim SH, Kim JS, Jeong SY, et al. Flexible Field Emission from Thermally Welded Chemically Doped Graphene Thin Films. Small. 2012;8(2):272-80.
EP
[24] Khare RT, Gelamo RV, More MA, Late DJ, Rout CS. Enhanced field emission of plasma treated multilayer graphene. Appl Phys Lett. 2015;107(12):123503.
AC C
[25] Khare R, Shinde DB, Bansode S, More MA, Majumder M, Pillai VK, et al. Graphene nanoribbons as prospective field emitter. Appl Phys Lett. 2015;106(2):023111. [26] Rout CS, Joshi PD, Kashid RV, Joag DS, More MA, Simbeck AJ, et al. Enhanced field emission properties of doped graphene nanosheets with layered SnS2. Appl Phys Lett. 2014;105(4):043109. [27] Rout CS, Joshi PD, Kashid RV, Joag DS, More MA, Simbeck AJ, et al. Superior Field Emission Properties of Layered WS2-RGO Nanocomposites. Scientific Reports. 2013;3:3282. [28] Samantara AK, Mishra DK, Suryawanshi SR, More MA, Thapa R, Late DJ, et al. Facile
ACCEPTED MANUSCRIPT synthesis of Ag nanowire-rGO composites and their promising field emission performance. RSC Advances. 2015;5(52):41887-93. [29] Zhang Y, Du J, Tang S, Liu P, Deng S, Chen J, et al. Optimize the field emission
RI PT
character of a vertical few-layer graphene sheet by manipulating the morphology. Nanotechnology. 2012;23(1):015202.
[30] Malesevic A, Kemps R, Vanhulsel A, Chowdhury MP, Volodin A, Haesendonck CV.
SC
Field emission from vertically aligned few-layer graphene. J Appl Phys. 2008;104(8):084301. [31] Behura SK, Mukhopadhyay I, Hirose A, Yang Q, Jani O. Vertically oriented few-layer
M AN U
graphene as an electron field-emitter. physica status solidi (a). 2013;210(9):1817-21. [32] Jiang L, Yang T, Liu F, Dong J, Yao Z, Shen C, et al. Controlled Synthesis of LargeScale, Uniform, Vertically Standing Graphene for High-Performance Field Emitters. Adv Mater. 2013;25(2):250-5.
TE D
[33] Qian M, Feng T, Ding H, Lin L, Li H, Chen Y, et al. Electron field emission from screenprinted graphene films. Nanotechnology. 2009;20(42):425702. [34] Wu C, Li F, Zhang Y, Guo T. Field emission from vertical graphene sheets formed by
EP
screen-printing technique. Vacuum. 2013;94(0):48-52. [35] Wu Z-S, Pei S, Ren W, Tang D, Gao L, Liu B, et al. Field Emission of Single-Layer
AC C
Graphene Films Prepared by Electrophoretic Deposition. Adv Mater. 2009;21(17):1756-60. [36] Liu J, Zeng B, Wu Z, Sun H. Enhanced Field Electron Emission of Graphene Sheets by CsI Coating after Electrophoretic Deposition. ACS Appl Mater Inter. 2011;4(3):1219-24. [37] Chen J, Cui L, Sun D, Yang B, Yang J, Yan X. Enhanced field emission properties from aligned graphenes fabricated on micro-hole patterned stainless steel. Appl Phys Lett. 2014;105(21):213111. [38] Jeong HJ, Kim HY, Jeong HD, Jeong SY, Han JT, Lee G-W. Arrays of vertically aligned tubular-structured graphene for flexible field emitters. J Mater Chem. 2012;22(22):11277-83.
ACCEPTED MANUSCRIPT [39] Baby TT, Ramaprabhu S. Cold field emission from hydrogen exfoliated graphene composites. Appl Phys Lett. 2011;98(18):183111. [40] Kim HY, Jeong S, Jeong SY, Baeg K-J, Han JT, Jeong MS, et al. Chemically doped
RI PT
three-dimensional porous graphene monoliths for high-performance flexible field emitters. Nanoscale. 2015;7(12):5495-502.
[41] Koh ATT, Foong YM, Pan L, Sun Z, Chua DHC. Effective large-area free-standing
SC
graphene field emitters by electrophoretic deposition. Appl Phys Lett. 2012;101(18):183107. [42] Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of
M AN U
graphene nanosheets. Nat Nanotechnol. 2008;3(2):101-5.
[43] Wei Y, Xiao L, Zhu F, Liu L, Tang J, Liu P, et al. Cold linear cathodes with carbon nanotube
emitters
and
their
2007;18(32):325702.
application
in
luminescent
tubes.
Nanotechnology.
TE D
[44] Croci M, Arfaoui I, Stöckli T, Chatelain A, Bonard J-M. A fully sealed luminescent tube based on carbon nanotube field emission. Microelectron J. 2004;35(4):329-36. [45] Bonard J-M, Stockli T, Noury O, Chatelain A. Field emission from cylindrical carbon
EP
nanotube cathodes: Possibilities for luminescent tubes. Appl Phys Lett. 2001;78(18):2775-7. [46] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al.
AC C
Preparation and characterization of graphene oxide paper. Nature. 2007;448(7152):457-60. [47] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon. 2007;45(7):1558-65. [48] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev. 2010;39(1):228-40. [49] Peng-Gang R, Ding-Xiang Y, Xu J, Tao C, Zhong-Ming L. Temperature dependence of graphene oxide reduced by hydrazine hydrate. Nanotechnology. 2011;22(5):055705.
ACCEPTED MANUSCRIPT [50] Zangmeister CD. Preparation and Evaluation of Graphite Oxide Reduced at 220 °C. Chem Mater. 2010;22(19):5625-9. [51] Cao X, Qi D, Yin S, Bu J, Li F, Goh CF, et al. Ambient Fabrication of Large-Area
RI PT
Graphene Films via a Synchronous Reduction and Assembly Strategy. Adv Mater. 2013;25(21):2957-62.
[52] Mathesh M, Liu J, Nam ND, Lam SKH, Zheng R, Barrow CJ, et al. Facile synthesis of
SC
graphene oxide hybrids bridged by copper ions for increased conductivity. J Mater Chem C. 2013;1(18):3084-90.
M AN U
[53] An SJ, Zhu Y, Lee SH, Stoller MD, Emilsson T, Park S, et al. Thin Film Fabrication and Simultaneous Anodic Reduction of Deposited Graphene Oxide Platelets by Electrophoretic Deposition. J Phys Chem Lett. 2010;1(8):1259-63.
[54] Fowler RH, Nordheim L. Electron Emission in Intense Electric Fields. Proceedings of
1928;119(781):173-81.
TE D
the Royal Society of London A: Mathematical, Physical and Engineering Sciences.
[55] Wang Z. Alignment of graphene nanoribbons by an electric field. Carbon.
EP
2009;47(13):3050-3.
[56] Hirakawa M, Sonoda S, Tanaka C, Murakami H, Yamakawa H. Electron emission
AC C
properties of carbon nanotubes. Appl Surf Sci. 2001;169–170(0):662-5. [57] Wong KW, Zhou XT, Au FCK, Lai HL, Lee CS, Lee ST. Field-emission characteristics of SiC nanowires prepared by chemical-vapor deposition. Appl Phys Lett. 1999;75(19):291820.
[58] Qi JL, Wang X, Zheng WT, Tian HW, Hu CQ, Peng YS. Ar plasma treatment on few layer graphene sheets for enhancing their field emission properties. J Phys D: Appl Phys. 2010;43(5):055302. [59] Yu Z, Jiale D, Shuai T, Pei L, Shaozhi D, Jun C, et al. Optimize the field emission
ACCEPTED MANUSCRIPT character of a vertical few-layer graphene sheet by manipulating the morphology. Nanotechnology. 2012;23(1):015202. [60] Stratakis E, Eda G, Yamaguchi H, Kymakis E, Fotakis C, Chhowalla M. Free-standing
RI PT
graphene on microstructured silicon vertices for enhanced field emission properties. Nanoscale. 2012;4(10):3069-74.
[61] Dong J, Zeng B, Lan Y, Tian S, Shan Y, Liu X, et al. Field Emission from Few-Layer
Nanoscience and Nanotechnology. 2010;10(8):5051-5.
SC
Graphene Nanosheets Produced by Liquid Phase Exfoliation of Graphite. Journal of
M AN U
[62] Huang Q, Wang G, Guo L, Jia Y, Lin J, Li K, et al. Approaching the Intrinsic Electron Field-Emission of a Graphene Film Consisting of Quasi-Freestanding Graphene Strips. Small. 2011;7(4):450-4.
[63] Kashid RV, Yusop MZ, Takahashi C, Kalita G, Panchakarla LS, Joag DS, et al. Field
TE D
emission characteristics of pristine and N-doped graphene measured by in-situ transmission
AC C
EP
electron microscopy. J Appl Phys. 2013;113(21):214311-5.