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Cite this: Nanoscale, 2015, 7, 1280 Received 14th October 2014, Accepted 29th November 2014 DOI: 10.1039/c4nr06057a
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Breakdown of metallic single-wall carbon nanotube paths by NiO nanoparticle point etching for high performance thin film transistors Shisheng Li,† Shunsuke Sakurai,*† Don N. Futaba and Kenji Hata
www.rsc.org/nanoscale
A selective and highly local etching of the metallic single-wall carbon nanotube (SWCNT) was demonstrated by using a NiO nanoparticle (NP) point etching technique. Following the NiO NP point etching at temperatures ranging from 250 to 350 °C, the current on/off ratios of the SWCNT field effect transistors (FETs) increased over 50-fold from ∼10 s to ∼104. Furthermore, the unavoidable drop in on-state current due to the reduction in current paths could be minimized to within one order of magnitude. Atomic force microscopy and Raman spectroscopy studies supported the view that the improvement in FET performance was attributed to the efficient and localized etching of metallic SWCNT paths solely around the NiO NPs, resulting in minimal damage to the semiconducting SWCNT networks.
Single-wall carbon nanotubes (SWCNTs) promise great potential for high performance nanoelectronics and flexible electronics1–3 due to their high carrier mobility,4 robust mechanical properties5 and chemical stability.6 However, one of the great challenges for such SWCNT-based electronic devices is the co-existence of metallic (m-) and semiconducting (s-) SWCNTs in as-grown samples. The metallic SWCNT paths present in channels lead to poor current modulation of SWCNT field effector transistors (FETs). Over the past decade, great effort has been put into producing pure s-SWCNTs, and both in situ selective growth and post-growth separation techniques have been widely investigated. Because of the small differences in the formation energy of m- and s-SWCNTs, the growth of 100% s-SWCNTs remains a great challenge.7–11 Although post-growth separation methods have been shown to be very effective to obtain high purity s-SWCNTs, these techniques rely on dispersing SWCNTs in aqueous solutions with the aid of surfactants and intense ultrasonication.12,13 As a result, the SWCNTs are not only separated, but also damaged (scission and defects), which leads to a significant degradation
Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST) Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan. E-mail: [email protected] † The first two authors contribute equally to this work.
1280 | Nanoscale, 2015, 7, 1280–1284
of their performance, such as their intrinsic high mobility in nanoelectronic devices. In contrast, the direct etching of m-SWCNTs on insulating substrates that are immediately ready for building high performance FETs is highly preferable. In previous studies, various approaches, such as CH4 or H2 plasma etching,14–16 light irradiation,17 and chemical etching by H2O18 or NiO,19 have been reported. Generally, the principle of all these etching methods is based on the differing electronic structures of m- and s-SWCNTs. Because of their smaller ionization energy, m-SWCNTs are expected to be removed more easily than s-SWCNTs of the same diameter.14,20–22 After these post-growth etching processes, SWCNT FETs did exhibit a great improvement of current on/off ratio from ∼10 s to 103–104. However, in these previous studies, those various stimuli were always exposed to the entire length of every SWCNT on the substrate, causing enormous unwanted damage to the semiconducting SWCNTs. As a result, such strong etching techniques always caused a severe drop in the on-state current of up to 1–3 orders of magnitude.15,16,23,24 For example, in a previous study of using NiO films to etch m-SWCNTs selectively, a large drop of on-state current of more than one order of magnitude was observed due to the destruction of more than half of the SWCNTs after the carbothermic reaction with a NiO film at 350 °C.19 Therefore, this etching method was considered to be detrimental towards making high performance SWCNT FETs. Thus, the development of post-growth etching methods possessing the advantages of efficient scission of metallic paths and minimal damage to SWCNT networks is highly desirable for the application of high performance flexible SWCNT based FETs. In this work, we present a method for selective and highly local etching of the metallic SWCNT by a NiO nanoparticle (NP) point etching method. The carbothermic reaction between NiO NPs and SWCNTs allowed the effective and highly localized scission of the continuous m-SWCNT paths. An improved current on/off ratio of SWCNT FETs, up to ∼103, was achieved after etching at temperatures ranging from 250 to 350 °C. In sharp contrast to the previously reported etching method using a NiO film, a high current on/off ratio up
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Fig. 1 (a) Schematic showing the scission of m-SWCNT paths by NiO NP point etching. AFM images of the (b) original SWCNT network, (c) the SWCNT network with decorated NiO NPs and (d) the SWCNT network after etching by NiO NPs at 350 °C for 5 min. The white arrows indicated the lost SWCNTs. (e) Part of the etched SWCNT in the box area of (d). The white arrows indicated the etching points.
to ∼104 was obtained, and more significantly, the drop in the on-state current could be minimized to within one order of magnitude, particularly at a low etching temperature of ∼250 °C, which is suitable for flexible electronics. By using NiO NPs, in contrast to previous work using a NiO film, the etching could be more localized and confined to the m-SWCNTs, which results in the improvement of the FET performance in the present work. Fig. 1 schematically illustrates the process of breaking m-SWCNT paths by the carbothermic reaction with NiO NPs (see details in Experimental methods). First, SWCNT networks were grown from catalytic dissociation of C2H4 by Fe NPs at 780 °C on Si substrates covered by a thermally grown SiO2 layer. Next, FETs were fabricated using the as-grown SWCNTs as the channel material. Fig. 1b shows the atomic force microscopy (AFM) images of the original SWCNTs at FET channel area. The SWCNTs showed clean and smooth surfaces and formed a continuous network connected by inter-tube cross-junctions. Then, NiO NPs were deposited on the SWCNT sidewalls by a hydroxylamine/NiCl2 chemistry. Fig. 1c shows the SWCNTs with NiO NPs decorating the surfaces. The NiO NPs were uniformly distributed on the walls of SWCNTs and the surface of the Si wafer, and their sizes were in the range of 2–8 nm. Next, the carbothermic reaction was applied to etch the SWCNT network by heating the FET devices to a predetermined reaction temperature (220 to 350 °C) in air. In a previous study using NiO films, m-SWCNTs etched at faster rates than s-SWCNTs.19 Finally, the NiO NPs were removed by a 1 M hydrochloric acid washing. Typical AFM images of the etched SWCNT network are shown in Fig. 1d and 1e. The white arrows in Fig. 1d indicate clearly two etched SWCNTs. Fig. 1e shows a magnified view of the etched SWCNT in the box area of Fig. 1d, where the white arrows indicate the etching points and the lost segments of SWCNTs. To investigate the changes in the current on/off ratio and on-state current of SWCNT FETs, we conducted a NiO NP point etching technique at temperatures ranging from 220 to 350 °C. SWCNT FETs with channel length (L) × width (W) of
This journal is © The Royal Society of Chemistry 2015
Fig. 2 Etching temperature dependent evolution of (a) current on/off ratio and (b) on-state current of SWCNT FETs. (c) Typical Isd–Vgs curves of the original SWCNT FET before (black line) and after (red line) etching by NiO NPs at 250 °C for 20 min. (d) The Ids–Vds characteristic of the SWCNT FET after etching by NiO NPs at 250 °C for 20 min. The Vgs was varied from −30 to 20 V with a step of +10 V.
100 × 100 μm2 were used as starting devices. Since these FETs were fabricated on a single 10 mm × 10 mm Si substrate covered with synthesized SWCNT networks, they had a similar initial current on/off ratio of 20–40. At a lower etching temperature of 220 °C, the change in the device performance was not significant. The current on/off ratio remained small (∼360) (Fig. 2a), and the on-state current dropped to 0.77 (1/1.3) times its original value (Fig. 2b). This result indicates that the conditions were not sufficient for a significant etching of SWCNTs. In contrast, within the temperature range of 250–350 °C, all the SWCNT FETs showed a significant increase in current on/off ratios up to ∼103 (Fig. 2a). The highest current on/off ratio was ∼104 after etching by NiO NPs at 250 °C for 20 min. Furthermore at this temperature, the observed decrease in the on-state current was only within one
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order, or 0.18 (1/5.6) times its original value. The transfer and output characteristics of the SWCNT FETs prior, and subsequent to etching at 250 °C for 20 min showed marked differences (Fig. 2c and d). The original SWCNT FET showed a low current on/off ratio of ∼40. After the carbothermic reaction with NiO NPs, the current on/off ratio increased to ∼11 000, and mobility, which was evaluated by the parallel plate model (see experimental methods), reached 18 cm2 Vs−1. This performance is particularly good when compared with previously reported CNT-based transistors.19,25 However, at the temperatures exceeding 300 °C, sharp decreases in the on-state current were observed. The derived on-state current dropped to 0.05 (1/21) and 0.001 (1/843) times its original value after etching by NiO NPs at 300 and 350 °C, respectively. To clarify the origin of the changes to the on-state current and on/off ratio of the SWCNT FETs, we conducted a detailed Raman study. First, the selective etching of m-SWCNTs by NiO NPs point etching was shown by the evolution of the intensities of radial breathing mode (RBM) in Raman spectra (Fig. 3a–3c). In this study, we chose m- and s-SWCNTs with similar diameters for comparison using the integrated intensity of the RBM peaks corresponding to the m-SWCNTs observed around 210–280 cm−1 with an excitation wavelength of 532 nm (Fig. 3a) and s-SWCNTs observed around 210–280 cm−1 with an excitation wavelength of 785 nm (Fig. 3b). The intensity of change in the signals from both types of SWCNTs with increasing etching temperatures showed that signals from m-SWCNTs dropped more rapidly than s-SWCNT (Fig. 3c). This result indicates that the preferential destruction of m-SWCNTs occurred within a specific reac-
Fig. 3 RBM of SWCNT Raman spectra of original SWCNTs and SWCNTs etched by NiO NPs at temperatures ranging from 220–350 °C, excited with laser wavelengths of (a) 532 nm and (b) 785 nm. The red and blue areas represent m- and s-SWCNTs, respectively. (c) Evolution of integrated RBM peak area of Raman spectra in the wavelength of 210–280 cm−1. (d) Evolution of D-band and G-band of SWCNT Raman spectra of original SWCNTs and SWCNTs etched by NiO NPs at temperatures in the range of 250–350 °C, excited with a laser wavelength of 532 nm. The intensity of all Raman spectra was calibrated by Si (520 cm−1).
1282 | Nanoscale, 2015, 7, 1280–1284
Nanoscale
tion temperature range. For example, at an etching temperature of 250 °C, the intensity of RBM signals from m-SWCNT dropped to 38% of its initial level, while the intensity of signals from s-SWCNT only dropped to 72% (Fig. 3c). In contrast, at an etching temperature of 350 °C, the intensity from the m-SWCNTs was nearly nonexistent (
COMMUNICATION
Cite this: Nanoscale, 2015, 7, 1280 Received 14th October 2014, Accepted 29th November 2014 DOI: 10.1039/c4nr06057a
View Article Online View Journal | View Issue
Breakdown of metallic single-wall carbon nanotube paths by NiO nanoparticle point etching for high performance thin film transistors Shisheng Li,† Shunsuke Sakurai,*† Don N. Futaba and Kenji Hata
www.rsc.org/nanoscale
A selective and highly local etching of the metallic single-wall carbon nanotube (SWCNT) was demonstrated by using a NiO nanoparticle (NP) point etching technique. Following the NiO NP point etching at temperatures ranging from 250 to 350 °C, the current on/off ratios of the SWCNT field effect transistors (FETs) increased over 50-fold from ∼10 s to ∼104. Furthermore, the unavoidable drop in on-state current due to the reduction in current paths could be minimized to within one order of magnitude. Atomic force microscopy and Raman spectroscopy studies supported the view that the improvement in FET performance was attributed to the efficient and localized etching of metallic SWCNT paths solely around the NiO NPs, resulting in minimal damage to the semiconducting SWCNT networks.
Single-wall carbon nanotubes (SWCNTs) promise great potential for high performance nanoelectronics and flexible electronics1–3 due to their high carrier mobility,4 robust mechanical properties5 and chemical stability.6 However, one of the great challenges for such SWCNT-based electronic devices is the co-existence of metallic (m-) and semiconducting (s-) SWCNTs in as-grown samples. The metallic SWCNT paths present in channels lead to poor current modulation of SWCNT field effector transistors (FETs). Over the past decade, great effort has been put into producing pure s-SWCNTs, and both in situ selective growth and post-growth separation techniques have been widely investigated. Because of the small differences in the formation energy of m- and s-SWCNTs, the growth of 100% s-SWCNTs remains a great challenge.7–11 Although post-growth separation methods have been shown to be very effective to obtain high purity s-SWCNTs, these techniques rely on dispersing SWCNTs in aqueous solutions with the aid of surfactants and intense ultrasonication.12,13 As a result, the SWCNTs are not only separated, but also damaged (scission and defects), which leads to a significant degradation
Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST) Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan. E-mail: [email protected] † The first two authors contribute equally to this work.
1280 | Nanoscale, 2015, 7, 1280–1284
of their performance, such as their intrinsic high mobility in nanoelectronic devices. In contrast, the direct etching of m-SWCNTs on insulating substrates that are immediately ready for building high performance FETs is highly preferable. In previous studies, various approaches, such as CH4 or H2 plasma etching,14–16 light irradiation,17 and chemical etching by H2O18 or NiO,19 have been reported. Generally, the principle of all these etching methods is based on the differing electronic structures of m- and s-SWCNTs. Because of their smaller ionization energy, m-SWCNTs are expected to be removed more easily than s-SWCNTs of the same diameter.14,20–22 After these post-growth etching processes, SWCNT FETs did exhibit a great improvement of current on/off ratio from ∼10 s to 103–104. However, in these previous studies, those various stimuli were always exposed to the entire length of every SWCNT on the substrate, causing enormous unwanted damage to the semiconducting SWCNTs. As a result, such strong etching techniques always caused a severe drop in the on-state current of up to 1–3 orders of magnitude.15,16,23,24 For example, in a previous study of using NiO films to etch m-SWCNTs selectively, a large drop of on-state current of more than one order of magnitude was observed due to the destruction of more than half of the SWCNTs after the carbothermic reaction with a NiO film at 350 °C.19 Therefore, this etching method was considered to be detrimental towards making high performance SWCNT FETs. Thus, the development of post-growth etching methods possessing the advantages of efficient scission of metallic paths and minimal damage to SWCNT networks is highly desirable for the application of high performance flexible SWCNT based FETs. In this work, we present a method for selective and highly local etching of the metallic SWCNT by a NiO nanoparticle (NP) point etching method. The carbothermic reaction between NiO NPs and SWCNTs allowed the effective and highly localized scission of the continuous m-SWCNT paths. An improved current on/off ratio of SWCNT FETs, up to ∼103, was achieved after etching at temperatures ranging from 250 to 350 °C. In sharp contrast to the previously reported etching method using a NiO film, a high current on/off ratio up
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Fig. 1 (a) Schematic showing the scission of m-SWCNT paths by NiO NP point etching. AFM images of the (b) original SWCNT network, (c) the SWCNT network with decorated NiO NPs and (d) the SWCNT network after etching by NiO NPs at 350 °C for 5 min. The white arrows indicated the lost SWCNTs. (e) Part of the etched SWCNT in the box area of (d). The white arrows indicated the etching points.
to ∼104 was obtained, and more significantly, the drop in the on-state current could be minimized to within one order of magnitude, particularly at a low etching temperature of ∼250 °C, which is suitable for flexible electronics. By using NiO NPs, in contrast to previous work using a NiO film, the etching could be more localized and confined to the m-SWCNTs, which results in the improvement of the FET performance in the present work. Fig. 1 schematically illustrates the process of breaking m-SWCNT paths by the carbothermic reaction with NiO NPs (see details in Experimental methods). First, SWCNT networks were grown from catalytic dissociation of C2H4 by Fe NPs at 780 °C on Si substrates covered by a thermally grown SiO2 layer. Next, FETs were fabricated using the as-grown SWCNTs as the channel material. Fig. 1b shows the atomic force microscopy (AFM) images of the original SWCNTs at FET channel area. The SWCNTs showed clean and smooth surfaces and formed a continuous network connected by inter-tube cross-junctions. Then, NiO NPs were deposited on the SWCNT sidewalls by a hydroxylamine/NiCl2 chemistry. Fig. 1c shows the SWCNTs with NiO NPs decorating the surfaces. The NiO NPs were uniformly distributed on the walls of SWCNTs and the surface of the Si wafer, and their sizes were in the range of 2–8 nm. Next, the carbothermic reaction was applied to etch the SWCNT network by heating the FET devices to a predetermined reaction temperature (220 to 350 °C) in air. In a previous study using NiO films, m-SWCNTs etched at faster rates than s-SWCNTs.19 Finally, the NiO NPs were removed by a 1 M hydrochloric acid washing. Typical AFM images of the etched SWCNT network are shown in Fig. 1d and 1e. The white arrows in Fig. 1d indicate clearly two etched SWCNTs. Fig. 1e shows a magnified view of the etched SWCNT in the box area of Fig. 1d, where the white arrows indicate the etching points and the lost segments of SWCNTs. To investigate the changes in the current on/off ratio and on-state current of SWCNT FETs, we conducted a NiO NP point etching technique at temperatures ranging from 220 to 350 °C. SWCNT FETs with channel length (L) × width (W) of
This journal is © The Royal Society of Chemistry 2015
Fig. 2 Etching temperature dependent evolution of (a) current on/off ratio and (b) on-state current of SWCNT FETs. (c) Typical Isd–Vgs curves of the original SWCNT FET before (black line) and after (red line) etching by NiO NPs at 250 °C for 20 min. (d) The Ids–Vds characteristic of the SWCNT FET after etching by NiO NPs at 250 °C for 20 min. The Vgs was varied from −30 to 20 V with a step of +10 V.
100 × 100 μm2 were used as starting devices. Since these FETs were fabricated on a single 10 mm × 10 mm Si substrate covered with synthesized SWCNT networks, they had a similar initial current on/off ratio of 20–40. At a lower etching temperature of 220 °C, the change in the device performance was not significant. The current on/off ratio remained small (∼360) (Fig. 2a), and the on-state current dropped to 0.77 (1/1.3) times its original value (Fig. 2b). This result indicates that the conditions were not sufficient for a significant etching of SWCNTs. In contrast, within the temperature range of 250–350 °C, all the SWCNT FETs showed a significant increase in current on/off ratios up to ∼103 (Fig. 2a). The highest current on/off ratio was ∼104 after etching by NiO NPs at 250 °C for 20 min. Furthermore at this temperature, the observed decrease in the on-state current was only within one
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order, or 0.18 (1/5.6) times its original value. The transfer and output characteristics of the SWCNT FETs prior, and subsequent to etching at 250 °C for 20 min showed marked differences (Fig. 2c and d). The original SWCNT FET showed a low current on/off ratio of ∼40. After the carbothermic reaction with NiO NPs, the current on/off ratio increased to ∼11 000, and mobility, which was evaluated by the parallel plate model (see experimental methods), reached 18 cm2 Vs−1. This performance is particularly good when compared with previously reported CNT-based transistors.19,25 However, at the temperatures exceeding 300 °C, sharp decreases in the on-state current were observed. The derived on-state current dropped to 0.05 (1/21) and 0.001 (1/843) times its original value after etching by NiO NPs at 300 and 350 °C, respectively. To clarify the origin of the changes to the on-state current and on/off ratio of the SWCNT FETs, we conducted a detailed Raman study. First, the selective etching of m-SWCNTs by NiO NPs point etching was shown by the evolution of the intensities of radial breathing mode (RBM) in Raman spectra (Fig. 3a–3c). In this study, we chose m- and s-SWCNTs with similar diameters for comparison using the integrated intensity of the RBM peaks corresponding to the m-SWCNTs observed around 210–280 cm−1 with an excitation wavelength of 532 nm (Fig. 3a) and s-SWCNTs observed around 210–280 cm−1 with an excitation wavelength of 785 nm (Fig. 3b). The intensity of change in the signals from both types of SWCNTs with increasing etching temperatures showed that signals from m-SWCNTs dropped more rapidly than s-SWCNT (Fig. 3c). This result indicates that the preferential destruction of m-SWCNTs occurred within a specific reac-
Fig. 3 RBM of SWCNT Raman spectra of original SWCNTs and SWCNTs etched by NiO NPs at temperatures ranging from 220–350 °C, excited with laser wavelengths of (a) 532 nm and (b) 785 nm. The red and blue areas represent m- and s-SWCNTs, respectively. (c) Evolution of integrated RBM peak area of Raman spectra in the wavelength of 210–280 cm−1. (d) Evolution of D-band and G-band of SWCNT Raman spectra of original SWCNTs and SWCNTs etched by NiO NPs at temperatures in the range of 250–350 °C, excited with a laser wavelength of 532 nm. The intensity of all Raman spectra was calibrated by Si (520 cm−1).
1282 | Nanoscale, 2015, 7, 1280–1284
Nanoscale
tion temperature range. For example, at an etching temperature of 250 °C, the intensity of RBM signals from m-SWCNT dropped to 38% of its initial level, while the intensity of signals from s-SWCNT only dropped to 72% (Fig. 3c). In contrast, at an etching temperature of 350 °C, the intensity from the m-SWCNTs was nearly nonexistent (
