Jing Gao,
Yujin Ji,
Youyong Li,
Jun Zhong* and
Xuhui Sun*
Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, China. E-mail: jzhong@suda.edu.cn; xhsun@suda.edu.cn
First published on 12th April 2017
Aligned one-dimensional carbon nanostructures with different morphologies such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been synthesized by the plasma-enhanced chemical vapor deposition method with different catalyst/underlayer combinations. The electronic structures of CNTs and CNFs have been studied by X-ray absorption near-edge structure spectroscopy (XANES), which reveals that CNTs have much fewer oxidized groups than CNFs. Moreover, the electrical transport properties of a single CNT or CNF have been measured in situ under transmission electron microscopy observation and the results show that CNTs have 2 orders lower resistivity than that of CNFs. A single CNT can be applied with higher voltage and larger current before thermal breakdown compared to a single CNF, which can be related to the electronic structure as revealed by XANES. Our results offer a good example of examining the relationship between morphological structure, electronic structure and electrical transport properties in carbon nanomaterials, which will certainly be useful in the applications of nano-devices.
In this study CNTs and CNFs have been synthesized by PECVD method with different catalyst/underlayer combinations. The electronic structure of CNTs and CNFs has been studied by X-ray absorption near-edge structure spectroscopy (XANES). XANES is an element-specific spectroscopic technique involving the excitation of electrons from a core level to the empty states.13–15 It is an important element-specific characterization tool which is capable of obtaining electronic, structural and bonding information in carbon based systems.13–15 The XANES results reveal that CNTs have much less oxidized groups than CNFs. The electrical transport property measurement shows that 2 orders lower electrical resistivity can be observed in CNTs than that in CNFs and much higher voltage and larger current can be applied on CNTs before the thermal breakdown. Our results offer a good example to examine the relationship of morphological structure, electronic structure and electrical transport property in carbon nanomaterials.
All samples have been shown to be aligned carbon-based nanostructures on substrates.12 High-resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2 F20 S-TIWN) was used to characterize the morphology of the as-synthesized products. The electrical transport properties of individual carbon nanostructure have been in situ measured using the TEM-STM holder (Nanofactory) under TEM observation. XANES experiments were performed on the SGM beamline at the Canadian Light Source (CLS). All the spectra were recorded at room temperature with a resolution of 0.2 eV at the C K-edge with the total electron yield (TEY) detection mode. The spectra were normalized at the pre-edge position and the post-edge position around 300 eV to clearly show the difference between 283 eV and 295 eV.
Fig. 1 TEM images of various carbon nanostructures: (a) Ni-CNTs (10 nm Ni/10 nm Al/Ti underlayer), (b) Ni-CNFs-1 (20 nm Ni/Cr underlayer), and (c) Ni-CNFs-2 (20 nm Ni/Ti underlayer). |
To investigate the electronic structures of the two different structures, C K-edge XANES experiments have been performed and the results are shown in Fig. 2. The XANES spectrum of commercial MWCNTs is also shown for comparison. The spectrum for MWCNTs is characterized by three main features: A, B and C at about 285.5, 288.4 and 292.5 eV, respectively. The spectral shape is in good agreement with the previous reports.13–15 According to the literatures, the feature A is univocally assigned to the π* excitation of C–C bonds in a C ring structure and the feature C to the σ* excitation,13–16 while the feature B to sp3 hybridized states due to various modification groups such as COOH or C–O bonds.13–17 Feature B for MWCNTs is weak, indicating weak modification. However, curve b for Ni-CNTs (10 nm Ni/10 nm Al/Ti underlayer) shows a prominent feature B, indicating increased oxidation groups compared to pure MWCNTs.13–15 Oxidized groups can be expected in the high temperature growth process of CNTs, which could be the origin of increased feature B.13–15 Defects produced in the growth process may also contribute to the formation of feature B. A decreased feature C can also be observed. The feature C stands for a long range order of carbon ring structure according to the ref. 14 The present CNTs prepared by PECVD method have more oxidized groups (a prominent feature B) than MWCNTs, thus the spectrum of Ni-CNTs has less long range order and shows a decreased intensity of feature C.14 A feature at 290.8 eV (labeled as C′ in the revised manuscript) can also be observed, which can be attributed to CO32+ groups produced in the growth process.17 The feature C′ can be found in all the samples (both CNTs and CNFs) with similar intensity.
Fig. 2 Comparison of the C K-edge XANES spectra of (a) commercial MWCNTs, (b) Ni-CNTs, (c) Ni-CNFs-1, and (d) Ni-CNFs-2. |
When observing the XANES spectra of Ni-CNFs-1 and Ni-CNFs-2 in Fig. 2(c) and (d), respectively, even stronger feature B can be found indicating much more oxidized groups compared to Ni-CNTs. Feature B is a dominant feature for the CNF samples (much higher than feature A). The sharply increased intensity of feature B can be attributed to the heavily oxidized nature of the thick CNF wall, which shows a major electronic structure difference when compared to CNTs. The simultaneously decreased feature A for CNFs also confirms the higher oxidization.13–16 The TEM images of CNFs also show irregular stacking cone-like carbon structure of the thick fiber wall. The increased feature B and the decreased feature A clearly show the lose of tube ring structure and the formation of irregular stacking cone-like structure in CNFs, which are the main difference between CNTs and CNFs produced by similar method.
The carbon nanostructures grown with Fe catalysts have also been investigated. Fig. 3 shows the TEM images of carbon nanostructures grown with (a) 5 nm Fe/10 nm Al/Ti underlayer (labeled as Fe-CNTs-1) and (b) 20 nm Fe/Ti underlayer (labeled as Fe-CNTs-2). Both images show a tube-like structure with a hollow center and thin tube walls regardless of the catalyst size or the absence of the additional Al layer, indicating the structural difference between using Ni and Fe as catalysts. Fig. 3(c) shows the high-resolution TEM image of Fe-CNTs-2, which clearly reveals the existence of coherent graphitic layers as the tube wall. For the CNF sample it has a thick wall without coherent graphitic layers. Some stacking cone-like structures in the CNT samples (Fig. 3(a) and (b)) can also be observed similar to that in CNFs. However, they are actually some wrinkle structures on the tube wall. The present CNT has a large inner diameter (around 40 nm) which is much larger than the tube wall thickness, thus it is easy to form wrinkle structures on the tube wall. It can be clearly observed in Fig. 3(c). An internal hollow structure in Fig. 3(c) can also be observed confirming the CNT structure.
To study the electronic structure of CNTs grown by Fe catalysts, C K-edge XANES spectra of Fe-CNTs-1 and Fe-CNTs-2 are shown in Fig. 4 with a comparison of the spectra of Ni-CNFs-2 and commercial MWCNTs. Clearly, both curves b and c for CNTs grown with Fe catalysts show similar spectral shape as that for Ni-CNTs in Fig. 2, with increased feature B compared to that for MWCNTs standing for more oxidized groups. However, the spectra of CNTs are significantly different from that of CNFs (Ni-CNFs-2) which shows a dominant feature B for heavily oxidized fiber wall, confirming the electronic structure difference between CNTs and CNFs.
Fig. 4 Comparison of the C K-edge XANES spectra of (a) commercial MWCNTs, (b) Fe-CNTs-1, (c) Fe-CNTs-2, and (d) Ni-CNFs-2. |
Fig. 5 (a–d) TEM images of the thermal breakdown process of Ni-CNFs-1. (e) I–V curves of Ni-CNFs-1 before (black) and after (red) beam focusing. |
The stability of the CNF as part of the circuit is also tested. A bias is applied on the two electrodes and when it increases to a value of 3.3 V, the structure failure can be observed due to the current-induced joule heating. This process is shown in Fig. 5(b)–(d). In Fig. 5(b) and (c) the aggregation of catalyst particles can be observed due to the increased temperature (we haven't measured the exact temperature) and the Coulomb force. Moreover, the CNF finally breaks off at the center position in Fig. 5(d) due to the current-induced joule heating at the bias of 3.3 V (showed in Fig. 6(e)). The final current density is about 1.4 × 1010 A m−2.
Fig. 6 (a–c) TEM images of the thermal breakdown process of Fe-CNTs-2. (d) I–V curve of Fe-CNTs-2 after beam focusing. (e) Current density curves of Ni-CNFs-1 (red) and Fe-CNTs-2 (black). |
CNTs grown with Fe catalysts are also mounted between two electrodes by using the same method. The TEM image of a CNT on two electrodes is shown in Fig. 6(a) (Fe-CNTs-2). Focused beam is also used for the better connection. I–V curve of the CNT (after beam focusing) is shown in Fig. 6(d) and the measured resistance for the CNT is 33 kΩ. The resistivity of the CNT is about 3.73 × 10−4 Ω·m, which is two orders lower than that of the CNF. Several CNTs (five CNTs) have been measured showing similar resistivity. The results suggest that the present CNTs have better conductivity (or lower resistivity) than the CNFs prepared by similar method. The difference can be attributed to various factors. From the TEM images we know that CNF has a much thick wall than CNT. CNF also loses the coherent graphitic layers and show a stacking cone-like wall. The resistivity difference can be partly attributed to the morphology difference that a coherent graphitic may help for the conductivity. However, although both CNT and CNF are carbon based nanostructures, the composition and their electronic structure can be significantly different. As revealed by the XANES spectra, CNT shows much less oxidized groups than CNF, keeping a good carbon ring structure with less influence from oxidation. Thus a better conductivity can be expected. The heavily oxidized carbon structure as that in CNF may decrease the conductivity. The electrical properties can thus be related to the morphological structure and the electronic structure.
To confirm the relationship between the electronic structure and the electrical property, first-principle calculations were performed by using the software Dmol.3 A pristine armchair (5,5) single-walled carbon nanotube (SWCNT) with the diameter of 6.78 Å was used as a model. To compare with the oxidized state of SWCNT with various surface groups, the pristine SWCNT was then functionalized by four OH– groups, three CH3(CH2)2O– groups and four COOH– groups, respectively. The gradient-corrected functional developed by Perdew et al. (GGA-PBE) are used to describe the approximation of the exchange-correlation energy.18 The convergence criterions of total energy, force, and displacement are set to 10–6 Ha, 0.002 Ha Å−1, and 0.005 Å in the self-consistent calculations, respectively. A thermal smearing parameter 0.005 Ha was applied to accelerate the convergence of orbital occupation. The calculation model and results are shown in Fig. 7. The pristine (5,5) SWCNT keeps a Dirac cone with metallic characterization along the periodic direction (Fig. 7(a)). When decorated with various surface groups, all the modified SWCNT samples show a band gap in Fig. 7(c) and (d), which may lower the conductivity with worse performance. Actually, the oxidized CNTs were also reported in the literature to decrease the conductance due to a larger elastic mean free path of charge.19
Fig. 7 (a–d) The top view and side view of SWCNT (a), OH-SWCNT (b), CH3(CH2)2O-SWCNT (c) and COOH-SWCNT (d) and their corresponding band structure (the valance band maximum is set to zero). |
The stability of the CNT is also measured and the TEM images are shown in Fig. 6(b) and (c). The CNT break off until a bias of 5.9 V is applied, which is much higher than that for the CNF (Fig. 6(e)). The corresponding current density is 4.9 × 1010 A m−2, which is also much lager than that of CNF. Thus higher voltage and larger current can be applied on CNTs when they are used as parts of the nano-devices. Our results reveal the relationship of morphological structure, electronic structure and electrical transport property in carbon-based nanostructures such as CNFs and CNTs, which may favor the rational design of nano-devices.
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