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Facile Fabrication and Field Emission of Metal-Particle-Decorated Vertical N-Doped Carbon Nanotube/Graphene Hybrid Films Duck Hyun Lee, Jin Ah Lee, Won Joon Lee, Dong Sung Choi, Won Jong Lee,* and Sang Ouk Kim* Department of Materials Science and Engineering, KAIST 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea ReceiVed: August 16, 2010; ReVised Manuscript ReceiVed: October 21, 2010
In this study, vertical N-doped carbon nanotube (VNCNT) arrays were decorated with Au, Ru, or Mn nanoparticles, and the effects of the particles on the field-emission properties were investigated. Uniform catalyst nanoparticles were prepared by block copolymer lithography on a graphene film, and the VNCNT arrays were grown from the nanopatterned catalyst particles by plasma-enhanced chemical vapor deposition (PECVD). The surfaces of the VNCNT arrays were subsequently decorated with metal particles, and the vertical alignment of the NCNT arrays was maintained by high-vacuum annealing. The field-emission properties of the metal-particle-decorated VNCNT arrays varied according to the changes in the work-function values, with the Mn-VNCNT field emitter showing the best performance among the emitters tested. Our results revealed that the field-emission properties of VNCNT arrays may be tuned by decoration with metal particles and that particle decoration with a low-work-function material may be used to develop efficient field emitters. 1. Introduction Field emission, which is the extraction of electrons from the surface of a material through the solid-vacuum potential barrier by quantum mechanical tunneling induced by an external electric field,1 has attracted considerable attention because of its numerous applications in technologies such as field-emission displays,2 back-light units,3 and electron guns.4 The extraction of an appreciable field-emission current at a low applied electric field is technologically important for field-emission devices. The key parameter for developing excellent field-emission devices is the use of a suitable low-work-function material capable of forming an emitter structure that concentrates the electric field.1 Carbon nanotubes (CNTs) have been suggested as good candidates for field-emission applications5-8 because of their high aspect ratio, excellent electrical and mechanical properties, and chemical inertness. Practical CNT-based field-emission devices require that the performance of CNT field emitters be improved to yield an appreciable emission current at low applied electric fields. Various methods, such as N-doping,9-11 surface treatment,12-14 and decorating the CNTs,15-19 have been proposed for the fabrication of low-work-function materials for this purpose. In particular, metal-particle decoration of CNTs is an excellent method for tuning the work function and enhancing the longterm stability of CNT-based field emitters. However, metalparticle-decorated CNT field emitters formed tangled CNT structures, and preservation of their vertical alignment during the metal-particle decoration process has not yet been achieved. This disorder degrades the performance for the CNT-based field emitters and limits the practical applications of the CNT arrays. Here, we demonstrate the fabrication and field-emission properties of vertical nitrogen (N)-doped CNT (VNCNT) arrays decorated with Au, Ru, or Mn particles, mounted on a graphene film. Recently, we introduced VNCNT arrays/graphene hybrid * Corresponding authors. E-mail:
[email protected] and sangouk.kim@ kaist.ac.kr; Phone: +82-42-350-3339; Fax: +82-42-350-3310.
films composed of VNCNTs grown on mechanically compliant reduced graphene.11,20 The nanopatterned Fe catalyst particles were prepared on the graphene film via block copolymer lithography, and catalytic plasma-enhanced chemical vapor deposition (PECVD) successfully fostered the high-yield growth of wall-number-selected, N-doped CNTs.11,20-26 In this work, Au, Ru oxide, and MnO2 particles were used to decorate the surfaces of VNCNT arrays via covalent bonding between the CNTs and the carboxyl-group-terminated Au particles,23 RuCl3,27-30 or KMnO4,31,32 respectively. After heat treatment in a H2 environment for 5 min at 400 °C, Ru oxide and MnO2 particles were reduced to Ru and Mn particles, respectively. The decorated metal particles changed the work functions of the VNCNTs, and the field-emission properties followed the changes in work function, revealing that the field emission of VNCNT arrays could be tuned by decoration with metal particles. 2. Experimental Details 2.1. Nanopatterned Catalyst Particle Preparation. A film composed of overlapped and stacked graphene oxide platelets was prepared by spin-coating an aqueous dispersion onto a SiO2 (500 nm)/Si wafer. The graphene oxide surface was neutrally treated by coating with a random copolymer brush. The block copolymer PS-b-PMMA (molecular weight, PS/PMMA-46k/ 21k) was spin-coated onto the wafer surface. After hightemperature annealing at 250 °C, the substrate was rinsed with acetic acid and water. The washes removed the PMMA cylinder cores and cross-linked the PS matrix. The substrate was exposed to oxygen plasma for 20 s to remove residual cylinder cores. An iron catalyst film was deposited (thickness, 2 nm) onto the block copolymer template. After the deposition process, residual PS nanoporous template material was lifted off by sonication in toluene. 2.2. PECVD Growth of VNCNT Arrays. CNT growth was carried out on the prepared substrates by PECVD. Substrates were heated to 600 °C under the flow of a H2/NH3 (80 sccm/20 sccm) gas mixture. When the substrate temperature reached 600
10.1021/jp1077714 2010 American Chemical Society Published on Web 11/10/2010
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Figure 1. (a) SEM image of the as-grown vertical N-doped CNT (VNCNT) arrays on graphene film. (b) Raman spectra of pristine-CNTs (top curve) and N-doped CNTs (bottom curve). Because of the doping with N atoms, the graphitic crystallinity was decreased.
°C, the chamber pressure was adjusted to 5 Torr, and application of 470 V DC power produced a plasma. Slow streaming of the acetylene gas resulted in the growth of VNCNT arrays (growth time, 1 min). 2.3. Metal-Particle Decoration. For the decoration of Au particles on VNCNT surface, carboxyl-group-terminated Au particles (diameter, ∼4 nm) were well dispersed in deionized water, and the pH of the Au-particle suspension was adjusted by addition of dilute H2SO4 solution (pH ) 1.5) to a pH of 5. The as-grown VNCNT arrays were immersed in the Au-particle suspension for 2 h. Ru oxide particles were attached to the VNCNT surfaces by chemical reaction of the VNCNT arrays with RuCl3 and KOH. Small quantities of ethanol were dropped onto the as-grown VNCNT arrays to enhance the wettability, and the VNCNT arrays were immersed in 6 mL 10 mM RuCl3 aqueous solution. Two milliliters of 0.1 M KOH solution were slowly added to the RuCl3 solution, and the Ru oxide particles were synthesized on the surface of the VNCNTs over 6 h at room temperature. For the decoration of MnO2 particles on the VNCNT surface, small quantities of ethanol were dropped onto the as-grown VNCNT arrays, and the VNCNT arrays were immersed in a 10 mM KMnO4 aqueous solution for 2 h at 60 °C. The metal-VNCNT arrays were carefully washed several times with deionized water to remove any residual particles and solution. The washed metal-VNCNT arrays are subsequently dried under vacuum for ∼6 h. For the reduction of the attached Ru oxide and MnO2 particles, the metal-oxide-particle-decorated VNCNT arrays were heat-treated in a H2 environment for 5 min at 400 °C. 2.4. Field-Emission Measurements. Each carbon hybrid film was transferred to an ITO/glass substrate, and an ITO/glass anode was positioned parallel to the top surface of the hybrid film. The spacing between the ITO anode and the top surface of the hybrid film was set to 500 µm. The field-emission properties were measured under a vacuum of ∼10-6 Torr with application of a voltage of 0-2000 V between the two electrodes. 3. Results and Discussion 3.1. CNT Array Growth Kinetics. Figure 1a shows a highmagnification SEM image of VNCNT arrays grown on iron catalyst particles patterned by means of block copolymer lithography on a graphene film after high-temperature heat treatment.11,20-26 The optimized PECVD growth conditions yielded ∼22 µm tall VCNT arrays within 1 min. Figure 1b shows the Raman spectra of pristine CNTs (top curve) and NCNTs (bottom curve), which indicated the crystallinity of the CNT arrays. The Raman spectra of the pristine CNTs and NCNTs showed two major Raman bands at 1340 cm-1 (D band)
and 1577 cm-1 (G band) with IG/ID ratios of 1.8 and 0.9, respectively. The G band corresponded to the original graphite features of the CNTs, but the D band indicated the presence of disordered features in the graphitic sheets.33 Because doping of the N atoms on the CNTs deteriorated the crystallinity of the graphitic sheets,34,35 CNTs with heavier N-doping levels were shown to be less crystalline and to have a higher defect density, which decreased the IG/ID ratio in the Raman spectra. Because NCNTs include partial substitution of C atoms with N atoms in the graphitic side walls, the substitutionally doped quaternary and pyridine N atoms in the graphitic sheets of NCNTs may be exploited as functionalization sites for decoration with metal particles.36,37 The N-doped sites along the CNT backbone permitted the facile decoration of the VNCNT arrays with metal particles. In this work, Au, Ru, and Mn particles were successfully used to decorate the VNCNT surfaces. Decoration by Au particles was enabled by the covalent bonding of the carboxyl groups that terminated the Au particles (diameter, ∼4 nm) during the simple process of incubating in an aqueous solution and subsequently washing the substrate.23 Under weakly acidic conditions, the negatively charged carboxylate group and the positively charged protonated N tightly bonded via ionic interactions. TEM images showed the dense and uniform attachment of Au particles along the NCNTs (Figure 2a,b). Ru and Mn particles were used to decorate the NCNT surfaces through a two-step process: the metal oxide particles were first fabricated (Ru oxide and MnO2) in a chemical reaction, and the particles were subsequently reduced by heat treatment in a H2 environment. The Ru oxide and MnO2 particles were synthesized by reaction of RuCl3 with KOH27-30 and KMnO4,31,32 respectively. The reaction proceeded through the following equations:
RuCl3 + 3KOH f Ru(OH)3 + 3KCl
(1)
24MnO4 + 3C + H2O f 4MnO2 + CO3 + 2HCO3
(2)
Prior to the particle-decoration reaction, a small amount of ethanol was dropped onto the as-grown VNCNT arrays to enhance the wettability with respect to the reactant solution. After the chemical reaction, the metal-oxide-particle-decorated VNCNT arrays were carefully washed several times with deionized water to remove any residual particles or solution, and the arrays were dried under vacuum at elevated temperature (400 °C) for 6 h. The attached Ru oxide and MnO2 particles were reduced by heat treatment of the metal-VNCNT arrays in
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Figure 2. Low (left panel) and high (right panels) magnification TEM images of (a,b) Au-NCNT, (c,d) Ru-NCNT, and (e,f) Mn-NCNT. After the heat treatment process in a H2 environment, Ru oxide and MnO2 particles were reduced, and the metal particles were stably bound to the surfaces of the NCNTs.
a H2 environment for 5 min at 400 °C. Dense and uniform Ru (diameter, ∼4 nm; Figure 2c,d) and Mn (diameter, ∼12 nm; Figure, 2e,f) particles were observed along the NCNTs by TEM imaging. High -performance VNCNT field-emitter fabrication requires that the vertical alignment of the NCNT arrays be maintained. Usually, evaporation of an aqueous solution produces capillary forces among CNTs that collapse the vertical structure of the CNT arrays.38 We avoided the collapse of NCNT arrays by rapidly evaporating the residual solution under high vacuum at elevated temperatures. Figure 3 shows low (left panel) and high (right panel) magnification SEM images of Au-VNCNT (Figure 3a,b), Ru-VNCNT (Figure 3c,d), and Mn-VNCNT (Figure 3e,f) arrays. The vertical alignment of the NCNT arrays was maintained after the particle -decoration process. The vertical structures of the metal-VNCNTs provide emitter structures capable of concentrating the electric field with high efficiency. Figure 4a shows the energy dispersive spectroscopy (EDS) analysis of VNCNT, Au-VNCNT, Ru-VNCNT, and Mn-
VNCNT arrays. The presence of N in the VNCNTs and the decorated metal particles were readily confirmed in the EDS spectra; the contents of Au and Ru showed 2%, and that of Mn showed 3%. The Mn peak showed higher intensity relative to the Au and Ru peaks because of the larger size of the Mn particles (∼12 nm) relative to the Au (∼4 nm) and Ru particles (∼4 nm). Figure 4b shows the work function (Φ) of VNCNT (4.3 eV), Au-VNCNT (5.0 eV), Ru-VNCNT (4.7 eV), and MnVNCNT (4.1 eV) arrays measured by ultraviolet photoemission spectroscopy (UPS). Interestingly, the work function of the metal-VNCNT arrays shifted to work-function values that were comparable to those of the pure decorated metals, clearly showing that the work function of the VNCNT arrays could be tuned by decoration with metal particles. The field-emission properties of 1 cm ×1 cm Au-, Ru-, and Mn-decorated VNCNT arrays were measured in a vacuum chamber under a pressure of 10-6 Torr at room temperature (Supporting Information). The anode electrode was prepared by screen printing of phosphor (ZnS:Ag,Cl) on an ITO/glass
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Figure 3. Low (left panel) and high (right panel) magnification SEM images of (a,b) Au-VNCNT, (c,d) Ru-VNCNT, and (e,f) Mn-VNCNT arrays. The vertical alignment of the NCNT arrays was maintained after the particle-decoration process.
Figure 4. (a) Energy dispersive spectrometer (EDS) analysis and (b) ultraviolet photoemission spectroscopy (UPS) measurements of Au-VNCNT, Ru-VNCNT, Mn-VNCNT, and VNCNT. The work functions (Φ) of the metal-particle-decorated VNCNT arrays shifted to values that were comparable to those of the decorated metals.
electrode. The spacing between the VNCNT tip and the anode (ZnS:Ag,Cl/ITO/glass) was maintained by a glass spacer (500 µm). The field-emission properties were characterized by measuring two parameters: the turn-on electric field, defined as the applied electric field necessary to achieve a current density of 10 µA/cm2, and the electric-field enhancement factor (β) calculated from the Fowler-Nordheim (F-N) plots.1 Because the field-emission current is inversely proportional to the electron work function (Φ) of a field -emitter material, a low-work-function material is generally preferred for the fabrication of high-performance field-emission devices. In this work, metal-particle decoration of the VNCNT arrays enabled tuning of the work function of metal-CNT composite field emitters (Figure 4b). Figure 5a,b shows the current density
versus applied electric field (Figure 5a) and the F-N plots (Figure 5b) used to determine the field-emission properties of VNCNT (purple), Au-VNCNT (blue), Ru-VNCNT (green), and Mn-VNCNT (red). The shape (length, 22 µm; diameter, 9.3 nm; wall number, ∼5) and the areal density of the VNCNT arrays were precisely maintained via catalyst nanopatterning via block copolymer lithography and CNT growth time during PECVD.20 The β values, which were determined by the geometry of the field emitters, decreased slightly upon decoration with Au (7100) and Ru (7200) particles because of the diameter increase (aspectratio decrease), and the Mn-VNCNTs showed the lowest β value (6700) because of the large particle size (Figure 5b). In contrast with the small changes in β for the metal-VNCNT arrays, the values of Eto varied greatly with the decorated metal particles.
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Lee et al. versus Φ1.5 for the various metal-decorated VNCNT field emitters, in good agreement with the theoretical F-N equation. 4. Conclusion In summary, we decorated VNCNT arrays with Au, Ru, or Mn particles and demonstrated their effects on the array fieldemission properties. The field-emission properties of metalparticle-decorated VNCNT arrays clearly followed the changes in work function, and the Mn-VNCNT field emitter, which had the lowest work function, showed the best performance. These results revealed that the field emission of VNCNT arrays may be tuned by decorating with metal particles, and nanoparticle decoration with low-work-function materials may be used to develop efficient field emitters. Synopsis Vertical N-doped carbon nanotube (VNCNT) arrays were decorated with Au, Ru, or Mn nanoparticles, and the effects of the particles on the field-emission properties were investigated. Uniform catalyst nanoparticles were prepared by block copolymer lithography on a graphene film, and the VNCNT arrays were grown from the nanopatterned catalyst particles by plasmaenhanced chemical vapor deposition (PECVD). The surfaces of the VNCNT arrays were subsequently decorated with metal particles, and the vertical alignment of the NCNT arrays was maintained by high-vacuum annealing. The field-emission properties of the metal-particle-decorated VNCNT arrays varied according to the changes in the work-function values, with the Mn-VNCNT field emitter showing the best performance among the emitters tested. Acknowledgment. This work was supported by the National Research Laboratory Program (R0A-2008-000-20057-0), the World Class University (WCU) program (R32-2008-000-100510), and the Basic Science Research Program through NRF funded by the Ministry of Education, Science and Technology (Grant No. 2010-0000862).
Figure 5. (a) Measured turn-on electric field and (b) corresponding field-enhancement factors for the VNCNT, Au-VNCNT, Ru-VNCNT, and Mn-VNCNT arrays. The field-emission properties of the metalparticle-decorated VNCNT arrays followed the changes in work function, and the Mn-VNCNT field emitter showed the best performance. (c) Plot of Vto ln(Ito/Vto2) versus Φ1.5 for the various metaldecorated VNCNT field emitters.
Eto for Au-VNCNT (1.8 V/µm) and Ru-VNCNT (1.4 V/µm) increased relative to that of VNCNT (1.0 V/µm), whereas Eto for Mn-VNCNT (0.7 V/µm) decreased, in agreement with the work-function measurement data. This trend revealed that the field-emission properties of the VNCNT arrays could be tuned by decoration with metal particles, and nanoparticle decoration with low-work-function materials may be used to develop efficient field emitters. The emitted current should theoretically follow the Fowler-Nordheim (F-N) equation,1 and the relation between Eto and the work function may be estimated by using the following equation, 2 Vto(ln(Ito /Vto ) - C1) ) -C2Φ1.5
(3)
where C1 and C2 are constants. Because C1 is lower than ln(Ito/Vto2) by a factor of approximately 10-4, Vto ln(Ito/Vto2) is proportional to Φ1.5. Figure 5c shows the plot of Vto ln(Ito/Vto2)
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