Transfer-Free Fabrication of Graphene Scaffolds on High-k Dielectrics

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Transfer-Free Fabrication of Graphene Scaffolds on High-k Dielectrics from Metal-Organic Oligomers Qingqing Pang, Deyan Wang, Xiuyan Wang, Shaoguang Feng, Michael B. Clark, Jr., and Qiaowei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08358 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016

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ACS Applied Materials & Interfaces

Transfer-Free Fabrication of Graphene Scaffolds on High-k Dielectrics from Metal−Organic Oligomers Qingqing Pang,† Deyan Wang,*,‡ Xiuyan Wang,§ Shaoguang Feng,§ Michael B. Clark Jr.,ǁ and Qiaowei Li*,† †

Department of Chemistry and iChEM (Collaborative Innovation Center of Chemistry for

Energy Materials), Fudan University, Shanghai 200433, P. R. China ‡

Rohm and Haas Electronic Materials LLC, Marlborough, Massachusetts 01752, United States

§

The Dow Chemical (China) Investment Co., Ltd., Shanghai Dow Center, Shanghai 201203, P. R.

China ǁ

The Dow Chemical Company, 400 Arcola Road, Collegeville, PA 19426, United States

KEYWORDS: graphene scaffold, high-k dielectrics, metal−organic oligomer, electronic materials, transfer-free

ABSTRACT

In situ fabrication of graphene scaffold−ZrO2 nanofilms is achieved by thermal annealing of Zrbased metal−organic oligomers on SiO2 substrates. The structural similarities of the aromatic moieties in the ligand (phenyl-, naphthyl-, anthryl-, and pyrenyl-) compared to the graphene play

1 ACS Paragon Plus Environment


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a major role in the ordering of the graphene scaffolds obtained. The depth profiling analysis reveals ultrathin carbon-pure or carbon-rich surfaces of the graphene scaffold−ZrO2 nanofilms. The graphene scaffolds with ~96.0% transmittance in visible region and 4.8 nm in thickness can be grown with this non-CVD method. Furthermore, the heterogeneous graphene scaffold−ZrO2 nanofilms show a low sheet resistance of 17.0 kΩ per square, corresponding to electrical conductivity of 3197 S m-1. The strategy provides a facile method to fabricate graphene scaffolds directly on high-k dielectrics without transferring process, paving the way for its application in fabricating electronic devices.

1. INTRODUCTION Single- and few-layer graphene,1-5 with their extraordinary electrical conductivity, mechanical flexibility, optical transparency, and thermal conductivity properties, have been studied extensively as platforms for multifunctional materials and devices.6-10 They can be prepared by either mechanical1,11-14 or chemical15-17 exfoliation of graphite, epitaxial growth on SiC single crystals,18-20 or chemical vapor deposition using Cu or Ni as substrates.21-23 However, large-scale graphene production still remains a significant challenge, partially because many of the potential large-scale applications require the process of transferring these graphene to application-specific substrates.24 The multiple-step transfer process, in which polymer deposition, chemical etching, and/or mechanical peeling are usually involved,25-28 can affect the physical properties of the graphene by introducing cracks, tears, and polymer residues.29-30 Moreover, the transfer process is not implementable with the current electronic fabrication processes.


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For practical industrial applications, it is highly desired to fabricate graphene directly on application-specific substrates, such as high dielectric constant materials (high-k dielectrics), to avoid the additional transfer steps in the electronic fabrication process. Several techniques have developed targeting substrate-specific methods for direct graphene growth, but still involved metal etching, dewetting, or peeling with tape, which may degrade the graphene quality.31-34 Herein, we report the transfer-free fabrication of ZrO2 nanofilms with graphene scaffolds on SiO2 substrates by spin coating and thermal annealing of Zr-based metal−organic oligomers (Figure 1). The zirconium carboxylate coordination oligomers coated on SiO2 substrates provide metal source which converts to ZrO2 nanofilm during annealing. Furthermore, the highly conjugated organic moieties from the phenyl-, naphthyl-, anthryl-, and pyrenyl-based ligands in the oligomers (Figure 1a) transpire to the surface of the ZrO2 nanoparticles upon annealing, and reorganize themselves into interconnected graphene scaffolds in situ. Surprisingly, carbon is unevenly distributed in the film, with the very top surface of this film being carbon-pure or extremely carbon-rich (Figure 1b); furthermore, the deposited ZrO2 nanoparticles and the migration of Zr to SiO2 increase the dielectric constant of the substrate. The thickness of the nanofilms can be adjusted by applying various spin coating speed and oligomer concentration in a solution. On the other hand, the thickness and ordering of the graphene scaffolds are easily controlled by the structures of the aromatic ligands employed and the amount of sp2 carbon atoms available from these moieties. By using this approach, graphene scaffolds with 4.8 nm in thickness, and ~96.0% in the white light transmittance can be fabricated directly on ZrO2. The quality of the graphene scaffolds obtained from this non-CVD method are evidenced by the signature fringes observed in the electron microscope and characteristic peaks in Raman spectroscopy. Interestingly, the pyrenyl-based system, which shares more structural similarity



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with graphene compared to the anthryl-, naphthyl-, and phenyl-based systems, has provided as low as 17.0 kΩ per square in sheet resistance, and 3197 S m-1 in electrical conductivity. The facile in-situ strategy has ensured the direct growth of graphene scaffolds on the desired high-k substrates towards electronic applications without further transferring, and provides a practical method to be readily integrated into the continuous processes in electronic device fabrications.

Figure 1. (a) Structure of the Zr-based metal−organic oligomer used as the precursor. (b) Schematic representation of graphene scaffolds fabricated with ZrO2 on the SiO2/Si wafer.

2. RESULTS AND DISCUSSION Specifically, the oligomer of Zr-based coordination complex in ethyl lactate was employed as the coating precursor. In a typical procedure, the oligomer was prepared by mixing Zr(OBu)4 with ethyl lactate at 60 °C with stirring, and precise amount of water was added to facilitate the oligomerization (See the Supporting Information for more details). BuO- groups were further replaced with the aliphatic and aromatic carboxylates by adding octanoic acid and aromatic acid


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(benzoic acid, 2-naphthoic acid, 9-anthracenecarboxylic acid, or 1-pyrenecarboxylic acid) at specific ratios. A typical feeding ratio of the organics in ethyl lactate is BuO-:C7H15COO:ArCOO- being 20:48:32. The Zr−carboxylate oligomer solution shows better film-forming ability and stability compared to the initial monomer structure, and could easily form homogeneous thin film on a SiO2/Si wafer upon spin coating. The use of octanoic acid greatly improves the shelf life of this precursor solution by reducing the steric hindrance from the aromatic acid. This effect was further demonstrated by the fact that more octanoic acid is required for the most bulky 1-pyrenecarboxylic acid, and thus the organic ligand ratio was optimized to 20:60:20 for the pyrenyl-based solution. The coated wafer was then annealed at a temperature as low as 700 °C or as high as 1000 °C under reduction atmosphere of a forming gas (5% H2 in N2) with different annealing time (10-120 min) in a tube furnace. Uniform and continuous films can all be fabricated on the wafers after the annealing of four oligomers from distinct aromatic moieties. Here, the film from the naphthyl-based oligomer (solution in ethyl lactate, with the total weight of all solutes being 15%; spin speed: 2000 rpm; annealing condition: 900 °C for 20 min) is discussed as an illustrative example. While the powder X-ray diffraction (PXRD) of the film is too weak to identify the ZrO2 phase, control studies on the crystalline powders after annealing the precursor in bulk reveal the ZrO2 is mainly tetragonal when annealed at 900 °C, and minor phase of monoclinic ZrO2 appears at higher temperature (Figure S1).35 By further applying different precursor concentrations (3-15 wt%) with different coating conditions (1500-3000 rpm for the spin speed), the average thickness of the continuous film can be carefully controlled between 16.5 and 35.1 nm (Table S1 and S2), as measured in the cross-section SEM images (Figure S2 and S3). It is worth mentioning that the thickness of the film is too thin to be measured when the spin speed reaches up to 3000 rpm.



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X-ray photoelectron spectroscopy (XPS) of the film surface shows rich carbon content, with the atomic ratio of C:Zr being 4.42:1. This intrigued us to perform the depth profiling analysis of the film by XPS. The estimated sampling depth of the XPS measurement was 5 nm, and the Ar+ sputter beam parameters were adjusted to remove materials from the surface with a rate of ~2.4 nm/min. The depth profile of the film is plotted in Figure 2, the atomic percentages of carbon and zirconium remain approximately constant from the depth of 3 nm to 15 nm, and the C:Zr atomic or mole ratio is ~1.6:1 across this depth. These results suggest the film is intergrowth of ZrO2 nanoparticles with carbon at the grain boundaries. However, the depth profile suggests the top 3 nm of the film that the initial carbon concentration starts at 52.2 atomic % on the surface, and drops to 34.7 atomic %. Depth profiling of a ~6.7 nm film obtained from a lower concentration precursor has a similar rich carbon content in first 3 nm (from 54.4 to 35 atomic %), however the carbon content drops to zero quickly due to the thinness of the film (Figure S4). This carbon enrichment is a combination of adventitious carbon and residual carbon from the organic ligands. It is well known that the surface of an inorganic material will be passivated by absorbed carbon when exposed to the atmosphere. However, it is believed the amount of adventitious carbon contamination is low, for the following reasons (i) carbon contamination due to sample handling has much lower concentration of carbon, such as 17% reported by Kwoka et. al.,36 and (ii) control study on a sample prepared with the same procedure but annealed under nitrogen detects little amount of carbon by XPS (4.7 atomic %, Figure S5). Overall, it is reasonable to interpret that the film has an ultrathin, less than 2 nm, layer of either pure carbon or a material with a high concentration of carbon on the surface of the film. We conclude that when the organic moieties were released from the metal and transpired to the surface during the annealing process, some of them were reduced and reorganized into an


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ultrathin layer of carbon deposited on top of the ZrO2/C composite film (Figure 1b). Furthermore, thermogravimetric analysis (TGA) of the solvent-removed naphthyl-based oligomer under N2 atmosphere shows less weight loss at 1000 °C compared to the weight loss under air (Figure S6), confirming the film is a composite of ZrO2 and carbon.

Figure 2. XPS depth profile from film surface to the silicon substrate. Preliminary examination of the carbon structure in the wafers after annealing at 700-1000 °C by Raman spectroscopy reveals characteristic peaks from sp2 carbon, and they are comparable to those obtained from reduced graphene oxide (r-GO) by chemical method (Figure S7).37 The ID/IG intensity ratio increases when higher annealing temperature is applied, with the values being 0.85, 0.86, 0.89, and 0.92 for samples annealed at 700, 800, 900, and 1000 °C, respectively, indicating a possible decrease in the size of the graphitic domain, but an increase in number.17,38 From the Raman spectroscopy, it also can be concluded that the samples annealed for 10, 20, 60, and 120 min give similar peak intensities with ID/IG intensity ratio being 0.85, 0.85, 0.86, and 0.88, respectively (Figure S8).



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This interesting heterogeneity inside the film and the sp2 carbon feature in Raman has intrigued us to further examine the detailed structure of the carbon, especially the carbon on the surface of the film, and to perceive the organic moieties’ roles in the carbon formation. Firstly, to measure the thickness of the carbon-pure or extremely carbon-rich surface of the film, one ZrO2/C on wafer was annealed again in air to remove the carbon. As confirmed by atomic force microscope (AFM) (Figure S9), the thickness change of the film due to second annealing is negligible, indicating the ultrathin nature of this conceivably carbon-pure surface. Secondary, in order to get a comprehensive analysis of the carbon structure, including the carbon on the top surface and the carbon intergrown with ZrO2, the wafers from the pyrenyl-based oligomer (solution in ethyl lactate, with the weight of solute being 15%) were placed in a buffered oxide etchant (BOE) solution (40 wt% HF in water:NH4F:H2O = 3.0 mL:6.0 g:10.0 mL) followed by a 5 wt% HF solution to etch the substrate and the ZrO2 nanoparticles in the film, and thus the carbon layer with the same size of the substrate (1×1 cm2) was detached completely from the wafer (Figure 3a). For convenience, this freestanding carbon layer is named Pyre15 in the context. After washing with deionized H2O, the transparent Pyre15 film was examined using transmission electron microscopy (TEM). TEM images clearly show the uniform texture of the transparent carbon film (Figure 3b and Figure S10). What is more, the graphene characteristics of carbon layer could be clearly seen from the high-resolution TEM (HRTEM) image (Figure 3c). Few layer domains of graphene are evident from the observed fringes, with the average interlayer distance of ~0.40 nm. During the annealing process, carbon reorganizes on the surfaces of the ZrO2 nanoparticles, and thus the final scaffolds, which stay their integrity even after the ZrO2 nanoparticles were removed, show small domains of graphene with wavy and entangled feature. The thickness of the Pyre15 graphene scaffold is 18.4 nm, as measured by the height difference


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at the edge of the layer after it was placed on a new substrate in the AFM (Figure S11a). For a diluted pyrenyl-based system (with the total weight of solute being 5%), named Pyre5, the thickness of the graphene scaffold reduces to 4.8 nm (Figure 3d and S11e).

Figure 3. (a) Photograph of the carbon layer (1×1 cm2) detached from SiO2/Si wafer floating on the surface of BOE solution. (b) TEM image of the freestanding graphene scaffold. (c) HRTEM image of the graphene scaffold, and the yellow lines in the insert indicate the graphene fringes. (d) AFM image and height profile of the graphene scaffold. The Raman spectra of the freestanding Pyre15 and the carbon films from 15 wt% solutions of anthryl-, naphthyl-, and phenyl-based systems, named Anth15, Naph15, and Phen15,



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respectively, are shown in Figure 4a, which are similar with that of r-GO.37 The pronounced peaks are the D peak at 1340, G peak at 1600, 2D peak at 2645, and D+G peak at 2920 cm-1 with ID/IG intensity ratios being 0.91, 0.92, 0.86, and 0.90 for Pyre15, Anth15, Naph15, and Phen15, respectively, suggesting lattice distortions in the graphene obtained.17 To further examine the optical properties of the graphene scaffolds, transmission spectra of them were acquired in the wavelength range of 400 to 700 nm, as presented in Figure 4b. The transmittance of Pyre15 is nearly flat with a value of ~74.7% at 550 nm. For Naph15, although higher content of aromatic ligand was used (32 mol% in naphthyl- vs. 20 mol% in pyrenyl-), less sp2 carbon in each formula (10 in naphthyl- vs. 16 in pyrenyl-) has created thinner graphene scaffold, and ~82.8% white light transmittance was observed. Other parameters, such as the molecular weight of the ligand and preorganization of the aromatic rings, also contribute to the thickness of the layer. For Pyre5, the transmittance is as high as ~96.0%, which is equivalent to that of a ~1.7-layer graphene film based on the reported absorption value of a single layer graphene (about 2.3%),39 being of the thinnest graphene scaffold obtained in our study.


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Figure 4. (a) Raman spectra and (b) transmittance spectra of the freestanding graphene scaffold films. The aromatic moiety coordinated to the metal in the oligomer plays a very important role in the formation of graphene scaffold. Control studies on an oligomer with only aliphatic group were conducted (see the Supporting Information). Freestanding film after etching in BOE solution cannot be obtained, suggesting no continuous carbon layer was fabricated after annealing. It can be concluded that aromatic moieties are more beneficial than simple aliphatic ligand in the growth of graphene scaffolds due to their structural similarity with graphene. While no metal catalyst is present in our process, this similarity in the carbon hybridization becomes essential in the successful graphene scaffold fabrication. Furthermore, a physical mixture of Zr3(OBu)8 trimer, octanoic acid, and 1-pyrenecarboxaldehyde in ethyl lactate, in which the aldehyde just acts as an uncoordinating additive, was employed as the coating precursor to verify the significance of metal−organic coordination in the oligomer (see the Supporting Information). Not surprisingly, no freestanding film was obtained after etching in BOE solution. Thus, the Zr carboxylate coordination bond breaking activates the organic moiety at high temperature, and lets the conjugated aromatic rings fuse together to form carbon layers with graphene characteristics. Recent studies have also shown that the coordination bond breaking plays an important role in the kinetic control of the carbon structure formed on Ni nanoparticle surfaces during the pyrolysis of coordination complex.40 Impressively, the non-CVD and transfer-free method in our study has fabricated graphene scaffolds directly on the high-k dielectrics. What is more, the spin coating approach has ensured the size of the film being as large as that of the wafers, providing viable strategies towards large-scale graphene electrodes and devices fabrication.41



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A positive correlation between the number of sp2 carbon atoms from aromatic moieties and the resulting graphene scaffolds thickness was observed. Phen15, Naph15, and Anth15, from oligomers with 6, 10, and 14 sp2 carbons in the aromatic rings, are measured to be 14.6, 16.4, and 20.4 nm in average thickness from the AFM (Figure S11 and Table 1). The average thickness of Pyre15 is 18.4 nm (Figure S11a), while that of Pyre5 is 4.8 nm (Figure S11e). It’s worth noting that there are 16 sp2 carbon atoms in the pyrenyl rings, however the lower molar content of this ligand in the precursor has made the thickness of Pyre15 slightly thinner than Anth15 (16 vs. 14 in sp2 carbon numbers, and 20 mol% vs. 32 mol% in molar percentages). One of the fascinating features of graphene is its low resistance, with promise in applications such as electronic devices, solar cells, supercapacitors, and flexible organic light emitting diodes (OLEDs).6-10,42 In our system, the surface of the film, which is carbon-pure or extremely carbonrich, may contribute to the low resistance of the film significantly. The phenyl-based oligomer system, Phen15, has the highest sheet resistance in our study (139.1 kΩ per square), as measured by the four-point probe method on the original substrate. Different positions of each wafer show similar sheet resistance values, indicating that the nanofilm throughout the whole wafer is uniform. By increasing the number of aromatic rings in the ligand, the sheet resistance of the scaffold gets reduced (Table 1), indicating less disorder in the graphene scaffolds. Pyre15 has the lowest sheet resistance (17 kΩ per square) and highest conductivity (3197 S m-1) in this work, considering the scaffold thickness to be 18.4 nm. By further decreasing the oligomer concentration, the conductivity of Pyre5 decreases from 3197 to 1965 S m-1, indicating relatively more defects in the graphene scaffold. For further exploration of the electrical conductivity of the film, the top ~4 nm surface was removed by O2 plasma, and the sheet resistance of the film turned to >106 Ω per square, which further confirms the relatively low sheet resistance of the


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film originates from the carbon-pure or carbon-rich surface. Lower annealing temperature has impact on the graphitizaion degree of the scaffolds, resulting in higher sheet resistance (Table S3). The sheet resistances of these scaffolds measured are higher than those of the graphene made from CVD methods with metals as catalysts, or several r-GO after the defect healing process.43 However, they are either at the same range with37,44,45 or better than46-49 several reported r-GO without further healing. Table S4 lists the comparison of the sheet resistances of our graphene scaffolds with other carbon-based materials. What is more, the graphene scaffolds obtained here are fabricated on high-k materials directly, which avoids any potential performance drop by eliminating the transfer process. There have been reports that the nanoscale ZrO2 can act as catalyst for the formation of carbon nanotubes and nanofibers by CVD method, in which the ZrO2 nanoparticles were presynthesized.50-51 However, in our system, the ZrO2 nanofilms were formed in situ from the decomposition of Zr−carboxylate oligomer under a non-CVD process, during which the catalytic effect of ZrO2 is unclear. We have also studied the thermolysis of other metal-based oligomers (Mg-, Zn-, and Ti-) (Figure S12), among which the Ti-based oligomer could follow the same protocol to form the graphene scaffold−TiO2 nanofilm with sheet resistance comparable to the graphene scaffold−ZrO2 nanofilm. Overall, the graphene scaffolds prepared in situ with our strategy has shown low sheet resistance attributed to the relatively better ordering of the carbon arrangement.

Table 1. The relationship between the number of sp2 carbon atoms and the physical properties of the graphene scaffolds



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Graphene Scaffolds

na

mb

T [nm]c

Average Rs [kΩ per square]

Average σ [S m-1]

Phen15

6

6

14.9

139.1

482

Naph15

10

10

16.4

87.7

695

Anth15

14

14

20.4

36.2

1354

Pyre15

16

10

18.4

17.0

3197

Pyre5

16

3.3

4.8

106

1965

a

n is the number of sp2 C atoms in the aromatic rings of each ligand. bm is the normalized number of sp2 C atoms in the precursor. cT is the average thickness of each free graphene scaffold measured by AFM after it was detached from the wafer by BOE/HF solutions.

3. CONCLUSION

In summary, this work presents transfer-free fabrication of graphene scaffolds on desired high-k ZrO2 directly from Zr-based metal−organic oligomers. Upon the coordination bond breaking during the annealing, the sp2 carbon atoms from the aromatic ligands reorganize into carbon layer structures with graphene characteristics in situ. The thickness of the graphene scaffold−ZrO2 nanofilms can be finely tuned by controlling the spin coating parameters and the oligomer solution concentrations. Furthermore, the graphene scaffolds from this non-CVD strategy show ~96.0% transmittance in visible region and 4.8 nm in thickness. Sheet resistance as low as 17.0 kΩ per square, corresponding to 3197 S m-1 in electrical conductivity, is measured on the heterogeneous graphene scaffold−ZrO2 nanofilm from pyrenyl-based oligomer. With specific design on the aromatic ligands, metal−ligand coordination control, and annealing optimization, the strategy presented without further transferring process promises further utilization of these materials in fabricating thin film transistors and capacitors.


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ASSOCIATED CONTENT Supporting Information Experimental details, PXRD, SEM, XPS, TGA, Raman, AFM, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Dow-Fudan Joint Material Research Center. Q. Pang and Q. Li would like to acknowledge Prof. Dongyuan Zhao for fruitful discussions and the contributions to this work. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191. (3) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530-1534.



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(4) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Graphene: The New Two-Dimensional Nanomaterial. Angew. Chem., Int. Ed. 2009, 48, 7752-7777. (5) Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S. I.; Seal, S. Graphene Based Materials: Past, Present and Future. Prog. Mater. Sci. 2011, 56, 1178-1271. (6) Kuila, T.; Bose, S.; Mishra, A. K.; Khanra, P.; Kim, N. H.; Lee, J. H. Chemical Functionalization of Graphene and Its Applications. Prog. Mater. Sci. 2012, 57, 1061-1105. (7) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666-686. (8) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Graphene-Based Liquid Crystal Device. Nano Lett. 2008, 8, 1704-1708. (9) Schwierz, F. Graphene Transistors. Nat. Nanotech. 2010, 5, 487-496. (10) Bonaccorso, F.; Colombo, L.; Yu, G.; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. Graphene, Related Two-Dimensional Crystals, and Hybrid Systems for Energy Conversion and Storage. Science 2015, 347, 1246501. (11) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotech. 2008, 3, 563-568.


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Nonmetallic Catalyst for Graphitization of Carbon and Growth of Single- and Multiwall Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 12144-12154.



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Transfer-free fabrication of graphene scaffold−ZrO2 nanofilms were obtained by thermal annealing of Zr-based metal−organic oligomers on SiO2/Si substrate. Carbon in the film is unevenly distributed with an ultrathin carbon-pure or carbon-rich layer on top of the film surface. The nanofilm shows good electrical conductivity, and can be integrated into the processes of fabricating electronic devices.


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Transfer-Free Fabrication of Graphene Scaffolds on High-k Dielectrics

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