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Graphite patterning in a controlled gas environment
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Nanotechnology 22 335304 (http://iopscience.iop.org/0957-4484/22/33/335304) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 22 (2011) 335304 (7pp)
doi:10.1088/0957-4484/22/33/335304
Graphite patterning in a controlled gas environment Joonkyu Park, K B Kim, Jun-Young Park, T Choi and Yongho Seo Faculty of Nanotechnology and Advanced Material Engineering, and Graphene Research Institute, Sejong University, Seoul, 143-747, Korea E-mail:
[email protected]
Received 23 April 2011, in final form 2 July 2011 Published 26 July 2011 Online at stacks.iop.org/Nano/22/335304 Abstract Although a number of methods using scanning probe lithography to pattern graphene have already been introduced, the fabrication of real devices still faces limitations. We report graphite patterning using scanning probe lithography with control of the gas environment. Patterning processes using scanning probe lithography of graphite or graphene are normally performed in air because water molecules forming the meniscus between the tip and the sample mediate the etching reaction. This water meniscus, however, may prevent uniform patterning due to its strong surface tension or large contact angle on surfaces. To investigate this side effect of water, our experiment was performed in a chamber where the gas environment was controlled with methyl alcohol, oxygen or isopropanol gases. We found that methyl alcohol facilitates graphite etching, and a line width as narrow as 3 nm was achieved as methyl alcohol also contains an oxygen atom which gives rise to the required oxidation. Due to its low surface tension and highly adsorptive behavior, methyl alcohol has advantages for a narrow line width and high speed etching conditions.
the performance of a graphene device, and the plasma etching causes defects in graphene which induce localization of charge carriers [14]. Other ways to fabricate GNRs have been reported, such as unzipping carbon nanotubes lengthwise [15–18] and crystallographic orientation dependent etching of a graphene sheet [19–21]. However, these methods are only allowed in specific etching conditions, and the location or direction of the GNR is not controllable. For these reasons, scanning probe lithography (SPL) techniques have received attention [22]. Recently, several methods using SPL have been reported: an indentation or scratch method on a soft substrate [23, 24], dip-pen nanolithography [25, 26] using diffusion of organic molecules [27, 28], and an anodic oxidation method with local electrical fields [24, 29, 30]. The atomic force microscope (AFM) and scanning tunneling microscope (STM) are the most commonly used fabrication tools for SPL. Even though AFM lithography is commonly used for the graphene patterning in ambient conditions, the line widths of patterns are no narrower than 20 nm. On the other hand, STM lithography is preferable in terms of accuracy [11, 31–33], but the local oxidation of graphene without water in a vacuum chamber is almost impossible [34–36].
1. Introduction The use of graphene as a key material for ballistic transport devices [1–4] is hampered by its zero band gap, which makes it difficult to develop graphene-based transistors as semiconductor devices. It was expected that quasi-onedimensional size confinement of the graphene could open the band gap [5–10]. Much research is in progress to modify the band structure and enhance the electronic properties of graphene by confining the path in which carriers move. Theory predicts that tailoring the graphene sheet into a nanoribbon a few nanometers wide will open the gap. Several groups have already fabricated such narrow ribbons and confirmed the realization of gap opening [11–13]. Han et al [13] fabricated graphene nanoribbons (GNRs) with varying widths in the range of 10–100 nm by using e-beam lithography. They found that the energy gap scales inversely with the ribbon width, and energy gaps as high as 0.2 eV were achieved with a 15 nm ribbon width. A common way to pattern graphene is using e-beam lithography. However, the line widths of GNRs fabricated by e-beam lithography are equal to or larger than 10 nm [12, 13], which seems to be the limit of this technique. Also, chemical contamination due to the e-beam resist could degrade 0957-4484/11/335304+07$33.00
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© 2011 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 22 (2011) 335304
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Figure 1. (a) A line was patterned with a 5 V sample bias and 3 nA tunneling current in air. The line has a linewidth of 150 nm and a depth of 3 nm. The inset shows the line profile along the white line. (b) Patterning with circle and hexagon shapes was performed in ambient conditions. The two insets show patterned images for the same condition and magnified scale.
GNR patterns on highly oriented pyrolytic graphite (HOPG) can be transferred to other substrates by several methods [23, 37–39], which would open the way to mass production of graphene devices. Although many SPL studies on HOPG have been performed for over 20 years, it has not been applied as an industrial fabrication method due to low yield and environmental vulnerability. Our graphite patterning study using SPL in a controlled atmosphere was motivated in this context.
required for the carbon atom in graphite to gasify in a vacuum chamber. This chemical reaction can be described as [36]:
2. Experimental methods
C (graphite) + H2 O (liq.) → CO (g) + H2 (g),
C (graphite) → C(g),
H 0f = 717 kJ mol−1 .
(1)
In air, on the other hand, the atoms react with water molecules and create carbon oxides, forming adducts of hydrogen simultaneously. At this time, about 175.3 kJ mol−1 of enthalpy is required to break the carbon chains and carve a pattern [33]:
H 0f = 175.3 kJ mol−1 C (graphite) + H2 O (liq.) → CO (g) + H2 (g), H 0f = 178.2 kJ mol−1 .
A thin graphite film was prepared by mechanical exfoliation [4] from HOPG. A commercially available Pt–Ir tip with 10 nm tip radius was used as the STM probe. A homemade STM unit for the controlled atmosphere was manufactured [40] and a similar type of microscope to that published earlier [41]. The STM unit was installed in a cylindrical vacuum chamber of diameter 7.5 cm and height 39.5 cm with a neck having an inner diameter of 1.2 cm and a height of 90 cm. A bellowstype vacuum tube with diameter 2.5 cm and length 1.5 m was connected between the vacuum chamber and the vacuum pumping system. Before every single experiment, the chamber was evacuated to lower than 10 mTorr, and each gas was injected. After liquids were injected, it took several minutes for the chamber pressure to stabilize. In this time, quick gasification and slow adsorption processes occurred sequentially. In the adsorption process, the pressure decrements were about a dozen torr. All pressure values given in this paper are the values measured after equilibrium was reached. While the tip moved and drew some patterns on the sample surface, a digital feedback loop was activated by controlling the tunneling current as a set point to maintain the distance between the tip and the sample. Different bias voltages and tunneling currents were applied in the patterning process.
(2) (3)
As shown in equations (1)–(3), for the endothermic reactions a much larger energy is required in a vacuum to etch the graphite than in air. Therefore, a much higher bias voltage should be applied at the tip in a vacuum. It is well known that adsorbed water molecules facilitate etching of carbon-based materials in SPL [34, 35]. Most SPL studies on graphene have been conducted in air rather than in a vacuum because of the need for water. Figure 1(a) shows a patterned line which was drawn with a 5 V sample bias and 3 nA tunneling current in air. The line has a line width of 214 nm and a depth of 35 nm, as shown in the inset. However, the line is irregular and some swollen hills remain at the edge. This is deemed to be a result of the peeling off graphene layers due to the electrostatic force or the formation of graphene oxide. It is suspected that the irregularity of the line shape is due to the strong surface tension of the water meniscus formed between the tip and the sample. Water is crucial for the lithography of carbon-based materials in air, but is not suitable for locations that must be controlled with nanometer resolution. The water molecules may agglomerate with large contact angles in random directions on a hydrophobic surface. The random nature of the shape and location of the meniscus causes the etched line to have an irregular shape. Circular or hexagonal shape patterning was attempted at bigger scales and the reproducibility was tested, as shown in
3. Results and discussion 3.1. Ambient conditions The binding energy of a carbon atom in graphite is about 7.43 eV [36], and this amount of electrostatic energy is 2
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Figure 2. In a humid ( p ≈ 0.75 p0 ) environment (a) an unintended broaden pattern and (b) an unrealistic image of the patterned surface were obtained. At the saturation vapor pressure ( p ≈ p0 ), (c) the etched line width was more broadened and (d) an overlapped pattern was found occasionally.
by more than two meniscuses formed randomly. As a result, the etching took place near the meniscus connecting the tip and the sample.
figure 1(b). The two insets in figure 1(b) show patterns drawn in air at a magnified scale. The patterns were not regular, depth and width were not controlled well, and oxidation in air caused partial protrusions.
3.3. Oxygen and isopropyl alcohol (IPA) atmosphere 3.2. In a water vapor atmosphere
Since water-assisted SPL causes line broadening of the etched pattern, etching experiments were performed in other gas environments. The carbon atoms in graphite can be burned in an O2 environment as below:
Line widths patterned by other groups in air are much larger in scale than a few nanometers [24, 29, 30] and the quality of patterning depends on the humidity in air. Consequently, the next step was to control the water vapor pressure. After a small amount of water was infused into the vacuum chamber, the water evaporated and the equilibrium vapor pressure was reached within several minutes. In the meantime, patterning was attempted with a bias voltage of 4.5 V and currents in the range of 1–3 nA. No reaction was perceived until the water molecules became adsorbed on the graphite surface after several minutes. Figure 2 shows the drawbacks of waterassisted SPL under wet conditions. An unintended broadened shape was made, as shown in figure 2(a), or an unreliable patterned image would be scanned (figure 2(b)) at relative pressure p ≈ 0.75 p0 , where the saturation vapor pressure p0 ≈ 17.5 Torr. At the saturation vapor pressure ( p ≈ p0 ), the etched line width was broadened further, as shown in figures 2(c) and (d). In particular, an overlapped hexagon pattern in the right side of figure 2(d) was found which was not intentionally drawn. It is suspected that it was doubly patterned
C (graphite) + 12 O2 (g) → CO (g)
H 0f = −110.5 kJ mol−1
(4)
C (graphite) + O2 (g) → CO2 (g)
H 0f = −393.5 kJ mol−1 .
(5)
Even though these are exothermic reactions, a large amount of energy is required to etch a graphite surface in an O2 environment, similar to vacuum conditions [42]. It was found that reactions of graphite in an O2 environment (i.e. combustion or oxidation of carbons) usually occur at defects or edges of the graphite rather than the middle of the surface [42–46]. Figure 3(a) shows a topographic image after a line pattern trial with a bias voltage of 5 V and an O2 pressure of 10 Torr. It shows an irregularly shaped residue of 30 nm scale by oxidation which occurred at the edge of a graphene grain. Even though O2 was expected to facilitate combustion 3
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Figure 3. (a) Line patterning was attempted in an O2 atmosphere of 10 Torr, but residues were found at the edge of the graphene. (b) An initial image obtained at 6 Torr methyl alcohol pressure before patterning. Patterning was attempted consecutively at pressures of 10 Torr (c) and 30 Torr (d) of IPA, but residues remained near the edges of the graphene.
While the pressure of IPA was increased gradually from 10 to 30 Torr, some patternings were attempted, but the residues remained more and more at the edge of the graphene layers. Generally, graphene edges have dangling bonds and are likely to be chemically reactive. As seen in figures 3(c) and (d), some dirt gradually grew on the graphene edges in consecutive patterning trials, which are presumed to be graphene oxides.
of carbon, in fact it was not etched but partially oxidized. In this O2 atmosphere experiment, both protrusions and etchings were found. The protrusions are suspected to be some residues which remained after the oxidation reaction, and they were found mainly at 10–100 Torr of oxygen. The etching, on the other hand, was rarely found at pressures higher than 100 Torr. Because the number of O2 molecules adsorbed near the tip can be ignored at room temperature, compared with the liquid phase molecules, the patterning was not efficient in the O2 environment as a result. After the failure of fine patterning in an oxygen environment, IPA was chosen because its surface tension is low and graphene can disperse well in it [47]. Therefore, less aggregation of adsorbed molecules on the surface was expected. Possible etching reactions are given by
3.4. In a methyl alcohol (MA) atmosphere Finally, MA was chosen as its surface tension (22.5 dyn cm−1 @ 20 ◦ C) is much lower than that of water (72.8 dyn cm−1 @ 20 ◦ C), and graphene can disperse well in liquid MA. MA consists of polar molecules, like water, and is likely to strongly disperse on a graphite surface [48]. Its adsorption behavior is stronger due to the –CH3 group than water adsorption by the – H group [48]. Possible reactions for etching the graphene are indicated as below, only if activation energies are supplied:
(CH)3 CHOH (liq.) + C (graphite) → C3 H8 (g) + CO (g), H 0f = 102.7 kJ mol−1
(6)
2(CH)3 CHOH (liq.)+C (graphite) → 2C3 H8 (g) + CO2 (g),
H 0f = 32.9 kJ mol−1
CH3 OH (liq.) + C (graphite) → CH4 (g) + CO (g),
(7)
H 0f = 53.4 kJ mol−1
−1
where the minimum required enthalpy is 32.9 kJ mol , which is much lower than that in water-assisted etching reactions. Notwithstanding these advantages, fine patterning in an IPA atmosphere was not successful, as shown in figures 3(b)–(d). Figure 3(b) shows the initial topography before patterning which was obtained at a pressure of less than 10 Torr of IPA.
(8)
2CH3 OH (liq.) + C (graphite) → 2HCHO (g) + CH4 (g),
H 0f = 185.4 kJ mol−1 .
(9)
As shown in these chemical formulae (8) and (9), the minimum required enthalpy is 53.4 kJ mol−1 , which is much lower than that for water (175 kJ mol−1 ). This relatively small 4
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Figure 4. (a) A topographic image was taken before patterning at a pressure of 15 Torr of MA. (b) Three line drawings along black lines were attempted with different applied voltages (6, 7 and 8 V), and only the 8 V application caused partial etching. At a MA pressure 50 Torr, a line pattern was attempted with 5 V bias voltage, and topographic images were taken before (c) and after (d) the drawing. The inset shows the line width of about 8.6 nm.
amount of enthalpy and these simple chemical reactions were expected to make electrochemical etching easier. Two topographic images at 15 Torr of MA pressure were taken before and after patterning, as shown in figures 4(a) and (b), respectively. Three line drawings were attempted with different applied voltages (6, 7 and 8 V). Each line drawing along the black line in figure 4(b) was repeated 10 times, moving the tip back and forth. While no changes appeared with 6 and 7 V applications, an irregular line pattern appeared with 8 V, as shown in figure 4(b). Then, the MA pressure was increased to 50 Torr, and the line pattern was attempted as shown in figures 4(c) and (d). For this patterned feature, drawing was done three times back and forth at a bias voltage of 5 V. An etched line appeared in figure 4(d), and the line width was about 8.6 nm as indicated in the inset. Even though the narrowed line width was a noticeable achievement, it was unsatisfactory that the surrounding protrusions were caused by etching. Unclear edges in patterning can obstruct the process of fabricating graphene-based devices. Instead of reducing bias voltage, the pressure of MA was increased for more precise patterning. Figures 5(a)–(d) show the hexagonal patterns which were etched at a MA pressure of 70 Torr with the same bias voltage, 4.5 V. The tunneling currents were controlled to be 3, 3, 2 and 2 nA, respectively, in the pattern writing process. The writing was a single line drawn without repetition, implying that the patterning yield was much
higher than in the other gas environments. White arrows in the insets indicate the line widths which are 14.8, 4.8, 2.9 and 3.9 nm, respectively. The bias voltage and tunneling current were in the same range as for etching in the water environment, but the line width of the etched line was much narrower than those for water. The line drawing speed was about 100 nm s−1 on average, which is 20 times faster than Tapaszto et al’s [11] result. This improvement in patterning speed contributed to the lower viscosity of MA and lower enthalpy change of the reaction than with water. 3.5. Dependence of etching on voltage and current The factors which are easily controllable and mainly discussed in SPL are the bias voltage and tunneling current, as these are considered to determine the etching quality. Figure 6(a) shows the voltage dependence of etched lines which were patterned by applying bias voltages of 4.0, 4.3 and 4.5 V in air. The same tunneling current was set at 3 nA, but the etched depth was deeper as a higher voltage was applied. It was confirmed that a voltage lower than 4 V does not etch the graphite surface in our setup. This implies that there are threshold voltages to activate the reactions for electrochemical etching. This time, the bias voltage was set at 4 V, and the tunneling current was increased from 3 to 5 nA, as shown in figure 6(b). It seems that the line widths are similar but the depth is slightly dependent 5
Nanotechnology 22 (2011) 335304
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Figure 5. Hexagonal patterns were etched at a MA pressure of 70 Torr with a bias voltage of 4.5 V and tunneling currents of 3 (a), 3 (b), 2 (c) and 2 nA (d), respectively. The insets indicate the line profiles, where the line widths were estimated to be 15.7, 4.8, 2.7 and 3.2 nm. The writing speeds were about 122, 96, 48 and 15 nm s−1 , respectively.
Figure 6. (a) In ambient conditions, the dependence on etching on voltage was investigated by applying bias voltages of 4.0, 4.3 and 4.5 V with the same tunneling current of 3 nA. (b) The bias voltage was set at 4 V, and the tunneling current was increased from 3, 4 and 5 nA. The insets in the images show the line profiles.
on the current, which can be interpreted that the line width is mainly determined by the meniscus size and the depth can be controlled by the amount of tunneling current.
at a nanometer scale. While the large surface tension of water degrades pattern quality in etching, MA with a low surface tension and high solubility on graphene provides a suitable environment for nanometer scale patterning. In a MA environment, line patterning with a 3 nm line width was achieved, and a fast patterning speed (∼100 nm s−1 ) was confirmed in the MA environment. By using the top layer transfer technique for graphite, SPL in a controlled gas environment can be used for the fabrication of graphene-based nanodevices.
4. Conclusions The line patterning on a thin graphite layer was performed by using the SPL technique in a controlled gas environment. Among the various gas environments such as H2 O, O2 , IPA, MA and air, MA was the best choice for narrow line patterning 6
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Acknowledgments
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This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean Government Ministry of Education, Science and Technology (MEST) grants: R01-2008-000-20185-0, 331-2008-1-C00102, 20090070725, 2010-0005393, Priority Research Centers Program (2010-0020207), and Ministry of Knowledge Economy grant no. 10033728.
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