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Image contrast enhancement in field-emission scanning electron microscopy of single-walled carbon nanotubes
Applied Surface Science 255 (2009) 4341–4346
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Image contrast enhancement in field-emission scanning electron microscopy of single-walled carbon nanotubes Y.S. Zhou a, K.J. Yi a, M. Mahjouri-Samani a, W. Xiong a, Y.F. Lu a,*, S.-H. Liou b a b
Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511, USA Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, NE 68588-0111, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 March 2008 Received in revised form 14 November 2008 Accepted 15 November 2008 Available online 21 November 2008
The image contrast enhancement in scanning electron microscopy of single-walled carbon nanotubes (SWNTs) on SiO2 surfaces was experimentally investigated using a field-emission scanning electron microscope (FESEM) using a wide range of primary electron (PE) voltages. SWNT images of different contrasts were obtained at different PE voltages. Image contrast enhancement of SWNTs was investigated by charging SiO2 surfaces at different PE voltages. The phenomena are ascribed to the surface potential difference and charge injection between SWNTs and SiO2 substrates induced by the electron-beam irradiation. ß 2008 Elsevier B.V. All rights reserved.
PACS: 6116Bg 8540Ux Keywords: Single-walled carbon nanotubes Field-emission scanning electron microscopy
1. Introduction Single-walled carbon nanotubes (SWNTs) have attracted extensive interest due to their intriguing structures, superior properties, and promising applications [1–5]. Exhaustive investigations have been made on the fabrication of SWNT-based devices during the past decade [2,4,6–8]. Further development and application of SWNTs require an accurate and nondestructive method for searching and characterizing SWNTs with high efficiency. However, due to the extremely small size and low material volume, searching SWNTs is a tedious and time consuming task. Therefore, a nondestructive technique for searching and locating SWNTs with high efficiency and accuracy is highly desired. Transmission electron microscopy (TEM) is a powerful tool for characterizing structures at nanoscales but requires destructive sample preparation. Scanning probe microscope (SPM) is another alternative, but slow scanning speed and tip-induce damages constrain its application on studying SWNTs. Low-voltage FESEM (LV-FESEM) was reported to observe SWNTs on insulator surfaces with bright contrast due to the electron-
beam-induced current (EBIC) or local potential difference [9–11]. However, the spatial resolution is poor. High-voltage FESEM (HVFESEM) yields much better spatial resolution than LV-FESEM. However, high primary electron (PE) voltages yield SWNT images with reduced contrast, makes it difficult to observe SWNTs. In this study, image contrast enhancement at high PE voltages is investigated. SWNTs are rapidly observed with sharp image contrast at a wide range of PE voltages from 1 to 20 kV. The SWNT image contrast at high PE voltages is sensitive to the surface charging caused by scanning at a different PE voltage. By tuning the PE voltage during the scanning process, SWNTs are observed with enhanced image contrast. The image contrast enhancement is ascribed to the surface potential difference and charge injection between SWNTs and SiO2 substrates induced by the e-beam irradiation. The charging effects in general FESEM observations are proved to be helpful in imaging SWNTs on SiO2 surfaces. Another advantage of the FESEM observation of SWNTs is to discern contact modes between SWNTs and materials underneath, such as electrical contact, nonelectrical contact and noncontact, which is useful for studying SWNTs devices. 2. Experimental details
* Corresponding author at: Department of Electrical Engineering, University of Nebraska-Lincoln, 209N SEC, Lincoln, NE 68588-0511, USA. Tel.: +1 402 472 8323; fax: +1 402 472 4732. E-mail address: [email protected] (Y.F. Lu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.035
The SWNTs were grown in a laser-assisted chemical vapor deposition (LCVD) system, as shown in Fig. 1. An electrical field (about 1 V/mm) was applied between the Mo electrodes to assist
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Fig. 2. Typical Raman spectra of the SWNTs. Fig. 1. Schematic of the self-aligned growth of SWNTs using the LCVD method.
3. Results and discussion the self-aligned growth of SWNTs. A resistor of 1 MV was connected in the circuit to protect SWNTs from being burnt by large currents. A voltmeter was connected to measure the voltage drop across the resistor to detect the formation of SWNTs to bridge the opposite Mo electrodes for in situ monitoring. The reaction process was terminated immediately after detecting the formation of the SWNT-bridge structures. A precursor gas mixture (C2H2 and anhydrous NH3 with a volume ratio of 1:10) was directed to flow from the cathode to the anode to assist the self-aligned growth of SWNTs, as shown in Fig. 1. Heavily doped p-type silicon wafers with a 2-mm-thick thermal oxide layer were used as the substrates. Photolithography, DC sputtering, and lift-off techniques were used for fabricating 100nm-thick Mo electrodes on the SiO2/Si substrates. Mo was selected as the electrode material for its high melting point (2617 8C), high work function (4.20 eV), and low carbon solubility. Shipley 1813 positive photoresist was used for the photolithography. The catalyst for growing SWNTs was Fe–Mo–Al porous mixture prepared by mixing Fe(NO3)39H2O, MoO2(acac)2, and alumina nano-powders in de-ionized water. The catalyst islands were prepared by the photolithography method. A CW CO2 laser (Synrad, firestar v40, wavelength of 10.6 mm) was used to irradiate the substrates. The LCVD chamber was evacuated to a background pressure of 1.0 10 3 Torr. The gas mixture of acetylene and anhydrous NH3 (volume ratio of 1:10) was introduced into the chamber. The anhydrous NH3 is used to dilute acetylene and create an etching environment to remove amorphous carbon. The chamber pressure was maintained at 10 Torr during the reaction process. The SWNTs were locally synthesized between the opposite Mo electrodes at 700 8C. The reaction process generally continued from 5 to 10 min. The vacuum chamber would be kept at 1.0 10 3 Torr after the reaction process, until the samples was cooled down. Scanning electron microscopic images of SWNTs grown on SiO2/Si substrates were taken under a Hitachi 4700 FESEM in the secondary electron (SE) mode. Fixed-point observation technique, i.e. the same area of the sample observed at a wide range of PE voltages, was applied. Raman spectroscopy was carried out using an Argon laser (Coherent, Innova 300, wavelength of 514.5 nm) as the excitation source [12]. The electrical transport measurements were taken on a home-built semiconductor parameter analyzer based on the NI PXI-1000B modules.
Back-gated SWNT field-effect transistors (SWNT-FETs) [13] were fabricated by growing SWNT bridges between the Mo electrodes. Two Mo electrodes serve as the source and drain of the FETs. The Si substrates serve as the gates. The SWNT bridges serve as the channels. The SWNTs were characterized by Raman spectroscopy and electrical transport measurements, and proved to be semiconducting. Fig. 2 shows typical Raman spectra of the SWNTs. Strong G-bands, weak D-bands, and sharp RBM signals indicate the presence of SWNTs. Typical RBM peaks are observed at 166.26 and 170.05 cm 1 from different samples. According to the relationship between the SWNT diameter (dt) and the RBM frequency (vRBM), vRBM = A/dt + B, in which A = 248 cm 1 and B = 0 for isolated SWNTs on SiO2 surfaces [14,15], the SWNTs diameters are estimated to be 1.49 and 1.46 nm, respectively, indicating a narrow diameter distribution. Fig. 3 shows typical IDS–
Fig. 3. Typical IDS–VDS curves of the SWNT-FETs.
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Fig. 4. FESEM micrographs of the same SWNTs observed at different PE voltages, (a) 1.0, (b) 5.0, (c) 10.0, and (d) 15.0 kV, respectively, showing voltage sensitivity of the image contrasts.
VDS curves for the SWNT-FETs. The SWNTs show typical p-type semiconducting features with holes as the major carriers, which is ascribed to the influence of oxygen when exposed in air [16,17]. The asymmetric curves are ascribed to the formation of Schottky barriers at the SWNT/Mo contacts [18,19]. The SEM image contrast of SWNTs on SiO2 surfaces is found to be sensitive to the PE voltage or the incident electron energy. Fig. 4 shows the SEM micrographs of SWNTs observed at different PE voltages. Bright-contrast images were observed at PE voltages of 1 and 15 kV, in which observation at 15 kV shows higher spatial resolution and a reduced contrast. Dark-contrast images were observed at PE voltages of 5 and 10 kV, in which observation at 10 kV shows a higher spatial resolution and a reduced contrast. However, all of the micrographs exhibit larger SWNT diameters than the actual physical size, about 67 nm at 1 kV and 33 nm at 15 kV. Bright and thick lines observed at 1 kV are typical SWNT
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Fig. 5. FESEM micrographs of the same SWNTs observed at different PE voltages, (a) 1.0, (b) 10.0, (c) 15.0, and (d) 10 kV, respectively, showing image contrast enhanced by the tuning PE voltage.
image caused by SE emission enhanced by the EBIC around the SWNTs [9–11]. Observations at high PE voltages above 10 kV, however, exhibit different phenomena from that at 1 kV, as shown in Fig. 5. In HV-FESEM observation (PE voltage > 10 kV), the image contrast of the SWNTs can be enhanced by charging the SiO2 substrate at a different PE voltage (i.e. tuning the PE voltage during the scanning process). Fig. 5(a) shows an SWNT image observed at a PE voltage of 1 kV. When observed at the PE voltages of 10 or 15 kV directly, SWNTs are almost invisible due to the reduced contrast, as shown in Fig. 5(b). By scanning the sample at 10 kV for about 10 s and changing the PE voltage to 15 kV immediately (<5 s), bright lines with an enhanced contrast are observed at the PE voltage of 15 kV, as shown in Fig. 5(c). Vice versa, scanning the sample at the PE voltage of 15 kV for about 10 s and changing the PE voltage to 10 kV within 5 s, dark lines with enhanced contrast are observed at the PE voltage of 10 kV, as shown in Fig. 5(d).
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Generally, the enhanced contrast by tuning PE voltage degrades within 2 min. Interestingly, the image contrast of the SWNTs is sensitive to the scanning sequence of the PE voltage (scanning from low to high or a reverse sequence). Fig. 6((a)–(d)) shows consecutively captured scanning electron micrographs of the same SWNT observed at the PE voltages of 20, 15, 10, and 15 kV, respectively. The SWNT images at the PE voltage of 15 kV show different contrast, as shown in Fig. 6((b)–(d)). A 20–15 kV scanning sequence leads to a dark line with an enhanced contrast, as shown in Fig. 6(b). However, a 10–15 kV scanning sequence
Fig. 6. FESEM micrographs of the same SWNT observed at different PE voltages, (a) 20.0, (b) 15.0, (c) 10.0, and (d) 15 kV, respectively, showing image contrast enhanced by tuning the PE voltage.
leads to a bright line with an enhanced contrast, as shown in Fig. 6(d). Surface contamination caused by amorphous carbon can be excluded in this study. In the LCVD process, anhydrous ammonia was introduced to create an etching environment to remove amorphous carbon. The Raman spectra, as shown in Fig. 2, show large G-band/D-band ratios and sharp RBM peaks, which indicate the purity of SWNTs. FESEM images at different PE voltages, as shown in Fig. 4, further indicate a clean SiO2/Si substrate surface. The enhanced SWNT image contrast can be explained as follows. It is suggested that the image contrast of SWNTs on SiO2 surface stems from the surface potential difference induced by the e-beam irradiation and charge injection between SWNTs and SiO2 substrates. It is well known that e-beam irradiation on SiO2 surfaces causes surface charging due to the electron–hole pairs created during the inelastic collisions between PEs and the surface atom inner shells [20–23]. In this study, a 2-mm-thick oxide layer leads to a negatively charged SiO2 surface [20–23]. Since the SWNTs show p-type semiconducting features after exposed to air, the SWNTs tend to absorb electrons and become negatively charged. Due to their tiny sizes, low material volume, and hollow structure, majority of the incident electrons penetrate the SWNTs and inject into the SiO2 substrate. As a result, the SE emission from the SWNTs is kept at a relatively constant level regardless of the PE voltage. The image contrast of the SWNTs on the SiO2 substrate is determined by the SE emission competition between the SWNTs and the SiO2 substrate. At a PE voltage of 1 kV, the SWNTs are more negatively charged than the SiO2 substrate. The SWNTs emit more SEs than the SiO2 substrate, and hence appear as bright lines. When incident electrons of a higher energy (i.e. higher PE voltage) penetrate the SWNTs and inject into the SiO2 substrate, the SiO2 substrate is more negatively charged and emits more SEs. As a result, the image contrast between the SWNTs and the SiO2 substrate is decreased and eventually reversed with increased PE voltage. Apparently, the large SWNT diameters as observed are ascribed to the electron injection between the SWNTs and the SiO2 substrate due to the surface potential difference. At a PE voltage of 1 kV, the SWNTs are more negatively charged. Electrons are injected from the SWNTs to the SiO2 substrate. As a result, bright thick lines are observed for the SWNTs. At a PE voltage of 5 kV, the SiO2 has more negative potential than the SWNTs do. Electrons are injected from the SiO2 substrate to the SWNTs, which appear as dark lines. When the SiO2 substrate is more negatively charged at a PE voltage of 10 kV, the electron diffusion distance is obviously confined within a narrow region due to a higher surface potential. When the PE voltage goes higher (>15 kV), the image contrast of the SWNTs is immerged by the strong SE emission from THE SiO2 substrate. Therefore, observing SWNTs at high PE voltages is difficult due to the reduced contrast. Different image contrast of SWNTs observed at the same PE voltage, as shown in Fig. 6, is ascribed to the relationship between the SE emission coefficient (d) and the incident electron energy (E0). In the low PE voltage range, an increased E0 (i.e. increased PE voltage) induces emission of more SEs and leads to an increased d. The d reaches a maximum with increased E0. In the high PE voltage range, an increased E0 leads to a longer electron penetration depth, which limits the SE emission from the substrate surface and leads to a decreased d. The SiO2 surface becomes more negatively charged at a PE voltage of 20 kV due to a decreased d. For the SWNTs, the dSWNT is kept at a relatively constant value due to its small size and low material volume. Since the incident electrons can easily penetrate the SWNTs and inject into the SiO2 substrate, fewer electrons are trapped within the SWNTs at a higher PE voltage. Therefore, when charging the SiO2 substrate at a PE voltage of 20 kV, a dark line is observed for the SWNT at the PE voltage of 15 kV, since electrons will be injected from the SiO2 substrate to the SWNT. When charging the
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Fig. 7. FESEM micrographs of the same SWNT observed at different PE voltages, showing the image contrast influenced by the SWNT-substrate contacts: (a) 1.0, (b) 5.0, (c) 10.0, and (d) 15.0 kV, respectively; (e) shows the enlarged part as indicated in (d), in which the SWNT contacts with the SiO2 substrate in Part-1, suspends above the SiO2 substrate in Part-2, and contacts with the Mo electrode in Part-3.
SiO2 substrate at the PE voltage of 10 kV, a bright line is observed for the SWNT at the PE voltage of 15 kV, since electrons will be injected from the SWNT to the SiO2 substrate. The enhanced image contrast will degrade within 2 min when a new electrical balance is established during the scanning process. It should be pointed out that the image contrast enhancement of the SWNTs occurs only on SWNT/SiO2 interfaces, since the enhancement relays on the surface charging effects. Image contrasts for SWNTs suspended above the SiO2 surface and contacting the Mo electrodes are kept constant, since the image contrasts are caused by normal SE emission, as shown in Fig. 7((a)–(d)). The lower grainy areas in the images are the Mo electrodes. The smooth areas are the SiO2 substrate. Fig. 7(e) shows an enlarged area in Fig. 7(d) to elucidate different image contrasts of the SWNT under different contact conditions. When the SWNT contacts directly with the SiO2 substrate (Part-1), the SWNT is observed with an apparently enlarged diameter due to the electrons diffusion between the SWNT
and the SiO2 substrate. The image contrast changes by tuning the PE voltage. When the SWNT is suspended above the SiO2 surface (Part2), a bright-contrast image of the SWNT is observed with a much smaller diameter of 15–20 nm and constant image contrast at different PE voltages. When the SWNT contacts with the Mo electrode (Part-3), the SWNT shows similar contrast as the Mo electrode, and keeps the same image contrast at different PE voltages. The observed results are helpful to distinguishing SWNT/ substrate contacting conditions. 4. Conclusions FESEM is demonstrated to be a powerful tool in studying SWNTs on SiO2 substrates at a wide range of PE voltages. The SWNTs exhibit different image contrast at different PE voltages. The image contrast of the SWNTs is ascribed to the surface potential difference and charging effects induced by the e-beam
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irradiation. The image contrast enhanced by charging the SiO2 substrate at a different PE voltage before observation is ascribed to the charge injection between the SWNTs and the SiO2 substrates under different e-beam irradiation conditions. The PE voltage tuning technique provides a nondestructive and precise characterizing method for studying surface-bounded SWNTs and corresponding devices with high spatial resolutions. Acknowledgments The authors would like to thank National Science Foundation and Nebraska Research Initiative for financially supporting this research. The authors would also like to thank Drs. N.J. Ianno and Y. Zhou for their technical support and valuable discussion during this research. References [1] [2] [3] [4] [5] [6]
P.M. Ajayan, O.Z. Zhou, Carbon Nanotubes 80 (2001) 391. N. Sinha, J.Z. Ma, J.T.W. Yeow, J. Nanosci. Nanotechnol. 6 (2006) 573. R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. C. Liu, H.M. Cheng, J. Phys. D 38 (2005) R231–R252. H.J. Dai, Surf. Sci. 500 (2002) 218. V.N. Popov, Mater. Sci. Eng.-Rep. 43 (2004) 61.
[7] H.J. Dai, Acc. Chem. Res. 35 (2002) 1035. [8] A.P. Graham, G.S. Duesberg, R.V. Seidel, M. Liebau, E. Unger, W. Pamler, F. Kreupl, W. Hoenlein, Small 1 (2005) 382. [9] R.Y. Zhang, Y. Wei, L.A. Nagahara, I. Amlani, R.K. Tsui, Nanotechnolgy 17 (2006) 272. [10] Y. Homma, S. Suzuki, Y. Kobayashi, M. Nagase, D. Takagi, Appl. Phys. Lett. 84 (2004) 1750. [11] T. Brintlinger, Y.F. Chen, T. Durkop, E. Cobas, M.S. Fuhrer, J.D. Barry, J. Melngailis, Appl. Phys. Lett. 81 (2002) 2454. [12] K.J. Yi, H. Wang, Y.F. Lu, Z.Y. Yang, J. Appl. Phys. 101 (2007) 063528. [13] J. Shi, Y.F. Lu, K.J. Yi, Y.S. Lin, S.H. Liou, J.B. Hou, X.W. Wang, Appl. Phys. Lett. 89 (2006) 083105. [14] M.S. Dresselhaus, G. Dresselhaus, R. Saito, A. Jorio, Phys. Rep.-Rev. Sect. Phys. Lett. 409 (2005) 47. [15] A. Jorio, R. Saito, J.H. Hafner, C.M. Lieber, M. Hunter, T. McClure, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. Lett. 86 (2001) 1118. [16] G.U. Sumanasekera, C.K.W. Adu, S. Fang, P.C. Eklund, Phys. Rev. Lett. 85 (2000) 1096. [17] J. Kong, N.R. Franklin, C.W. Zhou, M.G. Chapline, S. Peng, K.J. Cho, H.J. Dai, Science 287 (2000) 622. [18] S. Heinze, J. Tersoff, R. Martel, V. Derycke, J. Appenzeller, P. Avouris, Phys. Rev. Lett. 89 (2002) 106801. [19] C.G. Lu, L. An, Q.A. Fu, J. Liu, H. Zhang, J. Murduck, Appl. Phys. Lett. 88 (2006) 133501. [20] I.A. Glavatskikh, V.S. Kortov, H.J. Fitting, J. Appl. Phys. 89 (2001) 440. [21] C. Legressus, F. Valin, M. Henriot, M. Gautier, J.P. Duraud, T.S. Sudarshan, R.G. Bommakanti, G. Blaise, J. Appl. Phys. 69 (1991) 6325. [22] J. Cazaux, J. Appl. Phys. 85 (1999) 1137. [23] M.A.S. Kalceff, G.J. Thorogood, K.T. Short, J. Appl. Phys. 86 (1999) 205.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Image contrast enhancement in field-emission scanning electron microscopy of single-walled carbon nanotubes Y.S. Zhou a, K.J. Yi a, M. Mahjouri-Samani a, W. Xiong a, Y.F. Lu a,*, S.-H. Liou b a b
Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511, USA Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, NE 68588-0111, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 March 2008 Received in revised form 14 November 2008 Accepted 15 November 2008 Available online 21 November 2008
The image contrast enhancement in scanning electron microscopy of single-walled carbon nanotubes (SWNTs) on SiO2 surfaces was experimentally investigated using a field-emission scanning electron microscope (FESEM) using a wide range of primary electron (PE) voltages. SWNT images of different contrasts were obtained at different PE voltages. Image contrast enhancement of SWNTs was investigated by charging SiO2 surfaces at different PE voltages. The phenomena are ascribed to the surface potential difference and charge injection between SWNTs and SiO2 substrates induced by the electron-beam irradiation. ß 2008 Elsevier B.V. All rights reserved.
PACS: 6116Bg 8540Ux Keywords: Single-walled carbon nanotubes Field-emission scanning electron microscopy
1. Introduction Single-walled carbon nanotubes (SWNTs) have attracted extensive interest due to their intriguing structures, superior properties, and promising applications [1–5]. Exhaustive investigations have been made on the fabrication of SWNT-based devices during the past decade [2,4,6–8]. Further development and application of SWNTs require an accurate and nondestructive method for searching and characterizing SWNTs with high efficiency. However, due to the extremely small size and low material volume, searching SWNTs is a tedious and time consuming task. Therefore, a nondestructive technique for searching and locating SWNTs with high efficiency and accuracy is highly desired. Transmission electron microscopy (TEM) is a powerful tool for characterizing structures at nanoscales but requires destructive sample preparation. Scanning probe microscope (SPM) is another alternative, but slow scanning speed and tip-induce damages constrain its application on studying SWNTs. Low-voltage FESEM (LV-FESEM) was reported to observe SWNTs on insulator surfaces with bright contrast due to the electron-
beam-induced current (EBIC) or local potential difference [9–11]. However, the spatial resolution is poor. High-voltage FESEM (HVFESEM) yields much better spatial resolution than LV-FESEM. However, high primary electron (PE) voltages yield SWNT images with reduced contrast, makes it difficult to observe SWNTs. In this study, image contrast enhancement at high PE voltages is investigated. SWNTs are rapidly observed with sharp image contrast at a wide range of PE voltages from 1 to 20 kV. The SWNT image contrast at high PE voltages is sensitive to the surface charging caused by scanning at a different PE voltage. By tuning the PE voltage during the scanning process, SWNTs are observed with enhanced image contrast. The image contrast enhancement is ascribed to the surface potential difference and charge injection between SWNTs and SiO2 substrates induced by the e-beam irradiation. The charging effects in general FESEM observations are proved to be helpful in imaging SWNTs on SiO2 surfaces. Another advantage of the FESEM observation of SWNTs is to discern contact modes between SWNTs and materials underneath, such as electrical contact, nonelectrical contact and noncontact, which is useful for studying SWNTs devices. 2. Experimental details
* Corresponding author at: Department of Electrical Engineering, University of Nebraska-Lincoln, 209N SEC, Lincoln, NE 68588-0511, USA. Tel.: +1 402 472 8323; fax: +1 402 472 4732. E-mail address: [email protected] (Y.F. Lu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.035
The SWNTs were grown in a laser-assisted chemical vapor deposition (LCVD) system, as shown in Fig. 1. An electrical field (about 1 V/mm) was applied between the Mo electrodes to assist
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Fig. 2. Typical Raman spectra of the SWNTs. Fig. 1. Schematic of the self-aligned growth of SWNTs using the LCVD method.
3. Results and discussion the self-aligned growth of SWNTs. A resistor of 1 MV was connected in the circuit to protect SWNTs from being burnt by large currents. A voltmeter was connected to measure the voltage drop across the resistor to detect the formation of SWNTs to bridge the opposite Mo electrodes for in situ monitoring. The reaction process was terminated immediately after detecting the formation of the SWNT-bridge structures. A precursor gas mixture (C2H2 and anhydrous NH3 with a volume ratio of 1:10) was directed to flow from the cathode to the anode to assist the self-aligned growth of SWNTs, as shown in Fig. 1. Heavily doped p-type silicon wafers with a 2-mm-thick thermal oxide layer were used as the substrates. Photolithography, DC sputtering, and lift-off techniques were used for fabricating 100nm-thick Mo electrodes on the SiO2/Si substrates. Mo was selected as the electrode material for its high melting point (2617 8C), high work function (4.20 eV), and low carbon solubility. Shipley 1813 positive photoresist was used for the photolithography. The catalyst for growing SWNTs was Fe–Mo–Al porous mixture prepared by mixing Fe(NO3)39H2O, MoO2(acac)2, and alumina nano-powders in de-ionized water. The catalyst islands were prepared by the photolithography method. A CW CO2 laser (Synrad, firestar v40, wavelength of 10.6 mm) was used to irradiate the substrates. The LCVD chamber was evacuated to a background pressure of 1.0 10 3 Torr. The gas mixture of acetylene and anhydrous NH3 (volume ratio of 1:10) was introduced into the chamber. The anhydrous NH3 is used to dilute acetylene and create an etching environment to remove amorphous carbon. The chamber pressure was maintained at 10 Torr during the reaction process. The SWNTs were locally synthesized between the opposite Mo electrodes at 700 8C. The reaction process generally continued from 5 to 10 min. The vacuum chamber would be kept at 1.0 10 3 Torr after the reaction process, until the samples was cooled down. Scanning electron microscopic images of SWNTs grown on SiO2/Si substrates were taken under a Hitachi 4700 FESEM in the secondary electron (SE) mode. Fixed-point observation technique, i.e. the same area of the sample observed at a wide range of PE voltages, was applied. Raman spectroscopy was carried out using an Argon laser (Coherent, Innova 300, wavelength of 514.5 nm) as the excitation source [12]. The electrical transport measurements were taken on a home-built semiconductor parameter analyzer based on the NI PXI-1000B modules.
Back-gated SWNT field-effect transistors (SWNT-FETs) [13] were fabricated by growing SWNT bridges between the Mo electrodes. Two Mo electrodes serve as the source and drain of the FETs. The Si substrates serve as the gates. The SWNT bridges serve as the channels. The SWNTs were characterized by Raman spectroscopy and electrical transport measurements, and proved to be semiconducting. Fig. 2 shows typical Raman spectra of the SWNTs. Strong G-bands, weak D-bands, and sharp RBM signals indicate the presence of SWNTs. Typical RBM peaks are observed at 166.26 and 170.05 cm 1 from different samples. According to the relationship between the SWNT diameter (dt) and the RBM frequency (vRBM), vRBM = A/dt + B, in which A = 248 cm 1 and B = 0 for isolated SWNTs on SiO2 surfaces [14,15], the SWNTs diameters are estimated to be 1.49 and 1.46 nm, respectively, indicating a narrow diameter distribution. Fig. 3 shows typical IDS–
Fig. 3. Typical IDS–VDS curves of the SWNT-FETs.
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Fig. 4. FESEM micrographs of the same SWNTs observed at different PE voltages, (a) 1.0, (b) 5.0, (c) 10.0, and (d) 15.0 kV, respectively, showing voltage sensitivity of the image contrasts.
VDS curves for the SWNT-FETs. The SWNTs show typical p-type semiconducting features with holes as the major carriers, which is ascribed to the influence of oxygen when exposed in air [16,17]. The asymmetric curves are ascribed to the formation of Schottky barriers at the SWNT/Mo contacts [18,19]. The SEM image contrast of SWNTs on SiO2 surfaces is found to be sensitive to the PE voltage or the incident electron energy. Fig. 4 shows the SEM micrographs of SWNTs observed at different PE voltages. Bright-contrast images were observed at PE voltages of 1 and 15 kV, in which observation at 15 kV shows higher spatial resolution and a reduced contrast. Dark-contrast images were observed at PE voltages of 5 and 10 kV, in which observation at 10 kV shows a higher spatial resolution and a reduced contrast. However, all of the micrographs exhibit larger SWNT diameters than the actual physical size, about 67 nm at 1 kV and 33 nm at 15 kV. Bright and thick lines observed at 1 kV are typical SWNT
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Fig. 5. FESEM micrographs of the same SWNTs observed at different PE voltages, (a) 1.0, (b) 10.0, (c) 15.0, and (d) 10 kV, respectively, showing image contrast enhanced by the tuning PE voltage.
image caused by SE emission enhanced by the EBIC around the SWNTs [9–11]. Observations at high PE voltages above 10 kV, however, exhibit different phenomena from that at 1 kV, as shown in Fig. 5. In HV-FESEM observation (PE voltage > 10 kV), the image contrast of the SWNTs can be enhanced by charging the SiO2 substrate at a different PE voltage (i.e. tuning the PE voltage during the scanning process). Fig. 5(a) shows an SWNT image observed at a PE voltage of 1 kV. When observed at the PE voltages of 10 or 15 kV directly, SWNTs are almost invisible due to the reduced contrast, as shown in Fig. 5(b). By scanning the sample at 10 kV for about 10 s and changing the PE voltage to 15 kV immediately (<5 s), bright lines with an enhanced contrast are observed at the PE voltage of 15 kV, as shown in Fig. 5(c). Vice versa, scanning the sample at the PE voltage of 15 kV for about 10 s and changing the PE voltage to 10 kV within 5 s, dark lines with enhanced contrast are observed at the PE voltage of 10 kV, as shown in Fig. 5(d).
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Generally, the enhanced contrast by tuning PE voltage degrades within 2 min. Interestingly, the image contrast of the SWNTs is sensitive to the scanning sequence of the PE voltage (scanning from low to high or a reverse sequence). Fig. 6((a)–(d)) shows consecutively captured scanning electron micrographs of the same SWNT observed at the PE voltages of 20, 15, 10, and 15 kV, respectively. The SWNT images at the PE voltage of 15 kV show different contrast, as shown in Fig. 6((b)–(d)). A 20–15 kV scanning sequence leads to a dark line with an enhanced contrast, as shown in Fig. 6(b). However, a 10–15 kV scanning sequence
Fig. 6. FESEM micrographs of the same SWNT observed at different PE voltages, (a) 20.0, (b) 15.0, (c) 10.0, and (d) 15 kV, respectively, showing image contrast enhanced by tuning the PE voltage.
leads to a bright line with an enhanced contrast, as shown in Fig. 6(d). Surface contamination caused by amorphous carbon can be excluded in this study. In the LCVD process, anhydrous ammonia was introduced to create an etching environment to remove amorphous carbon. The Raman spectra, as shown in Fig. 2, show large G-band/D-band ratios and sharp RBM peaks, which indicate the purity of SWNTs. FESEM images at different PE voltages, as shown in Fig. 4, further indicate a clean SiO2/Si substrate surface. The enhanced SWNT image contrast can be explained as follows. It is suggested that the image contrast of SWNTs on SiO2 surface stems from the surface potential difference induced by the e-beam irradiation and charge injection between SWNTs and SiO2 substrates. It is well known that e-beam irradiation on SiO2 surfaces causes surface charging due to the electron–hole pairs created during the inelastic collisions between PEs and the surface atom inner shells [20–23]. In this study, a 2-mm-thick oxide layer leads to a negatively charged SiO2 surface [20–23]. Since the SWNTs show p-type semiconducting features after exposed to air, the SWNTs tend to absorb electrons and become negatively charged. Due to their tiny sizes, low material volume, and hollow structure, majority of the incident electrons penetrate the SWNTs and inject into the SiO2 substrate. As a result, the SE emission from the SWNTs is kept at a relatively constant level regardless of the PE voltage. The image contrast of the SWNTs on the SiO2 substrate is determined by the SE emission competition between the SWNTs and the SiO2 substrate. At a PE voltage of 1 kV, the SWNTs are more negatively charged than the SiO2 substrate. The SWNTs emit more SEs than the SiO2 substrate, and hence appear as bright lines. When incident electrons of a higher energy (i.e. higher PE voltage) penetrate the SWNTs and inject into the SiO2 substrate, the SiO2 substrate is more negatively charged and emits more SEs. As a result, the image contrast between the SWNTs and the SiO2 substrate is decreased and eventually reversed with increased PE voltage. Apparently, the large SWNT diameters as observed are ascribed to the electron injection between the SWNTs and the SiO2 substrate due to the surface potential difference. At a PE voltage of 1 kV, the SWNTs are more negatively charged. Electrons are injected from the SWNTs to the SiO2 substrate. As a result, bright thick lines are observed for the SWNTs. At a PE voltage of 5 kV, the SiO2 has more negative potential than the SWNTs do. Electrons are injected from the SiO2 substrate to the SWNTs, which appear as dark lines. When the SiO2 substrate is more negatively charged at a PE voltage of 10 kV, the electron diffusion distance is obviously confined within a narrow region due to a higher surface potential. When the PE voltage goes higher (>15 kV), the image contrast of the SWNTs is immerged by the strong SE emission from THE SiO2 substrate. Therefore, observing SWNTs at high PE voltages is difficult due to the reduced contrast. Different image contrast of SWNTs observed at the same PE voltage, as shown in Fig. 6, is ascribed to the relationship between the SE emission coefficient (d) and the incident electron energy (E0). In the low PE voltage range, an increased E0 (i.e. increased PE voltage) induces emission of more SEs and leads to an increased d. The d reaches a maximum with increased E0. In the high PE voltage range, an increased E0 leads to a longer electron penetration depth, which limits the SE emission from the substrate surface and leads to a decreased d. The SiO2 surface becomes more negatively charged at a PE voltage of 20 kV due to a decreased d. For the SWNTs, the dSWNT is kept at a relatively constant value due to its small size and low material volume. Since the incident electrons can easily penetrate the SWNTs and inject into the SiO2 substrate, fewer electrons are trapped within the SWNTs at a higher PE voltage. Therefore, when charging the SiO2 substrate at a PE voltage of 20 kV, a dark line is observed for the SWNT at the PE voltage of 15 kV, since electrons will be injected from the SiO2 substrate to the SWNT. When charging the
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Fig. 7. FESEM micrographs of the same SWNT observed at different PE voltages, showing the image contrast influenced by the SWNT-substrate contacts: (a) 1.0, (b) 5.0, (c) 10.0, and (d) 15.0 kV, respectively; (e) shows the enlarged part as indicated in (d), in which the SWNT contacts with the SiO2 substrate in Part-1, suspends above the SiO2 substrate in Part-2, and contacts with the Mo electrode in Part-3.
SiO2 substrate at the PE voltage of 10 kV, a bright line is observed for the SWNT at the PE voltage of 15 kV, since electrons will be injected from the SWNT to the SiO2 substrate. The enhanced image contrast will degrade within 2 min when a new electrical balance is established during the scanning process. It should be pointed out that the image contrast enhancement of the SWNTs occurs only on SWNT/SiO2 interfaces, since the enhancement relays on the surface charging effects. Image contrasts for SWNTs suspended above the SiO2 surface and contacting the Mo electrodes are kept constant, since the image contrasts are caused by normal SE emission, as shown in Fig. 7((a)–(d)). The lower grainy areas in the images are the Mo electrodes. The smooth areas are the SiO2 substrate. Fig. 7(e) shows an enlarged area in Fig. 7(d) to elucidate different image contrasts of the SWNT under different contact conditions. When the SWNT contacts directly with the SiO2 substrate (Part-1), the SWNT is observed with an apparently enlarged diameter due to the electrons diffusion between the SWNT
and the SiO2 substrate. The image contrast changes by tuning the PE voltage. When the SWNT is suspended above the SiO2 surface (Part2), a bright-contrast image of the SWNT is observed with a much smaller diameter of 15–20 nm and constant image contrast at different PE voltages. When the SWNT contacts with the Mo electrode (Part-3), the SWNT shows similar contrast as the Mo electrode, and keeps the same image contrast at different PE voltages. The observed results are helpful to distinguishing SWNT/ substrate contacting conditions. 4. Conclusions FESEM is demonstrated to be a powerful tool in studying SWNTs on SiO2 substrates at a wide range of PE voltages. The SWNTs exhibit different image contrast at different PE voltages. The image contrast of the SWNTs is ascribed to the surface potential difference and charging effects induced by the e-beam
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irradiation. The image contrast enhanced by charging the SiO2 substrate at a different PE voltage before observation is ascribed to the charge injection between the SWNTs and the SiO2 substrates under different e-beam irradiation conditions. The PE voltage tuning technique provides a nondestructive and precise characterizing method for studying surface-bounded SWNTs and corresponding devices with high spatial resolutions. Acknowledgments The authors would like to thank National Science Foundation and Nebraska Research Initiative for financially supporting this research. The authors would also like to thank Drs. N.J. Ianno and Y. Zhou for their technical support and valuable discussion during this research. References [1] [2] [3] [4] [5] [6]
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