Nanoscale optical and electrical characterization of horizontally aligned single-walled carbon nanotubes
During the recent years, a significant amount of research has been performed on single-walled carbon nanotubes (SWCNTs) as a channel material in thin-film transistors (Pham et al. IEEE Trans Nanotechnol 11:44–50, 2012). This has prompted the application of advanced characterization techniques based on combined atomic force microscopy (AFM) and Raman spectroscopy studies (Mureau et al. Electrophoresis 29:2266–2271, 2008). In this context, we use confocal Raman microscopy and current sensing atomic force microscopy (CS-AFM) to study phonons and the electronic transport in semiconducting SWCNTs, which were aligned between palladium electrodes using dielectrophoresis (Kuzyk Electrophoresis 32:2307–2313, 2011). Raman imaging was performed in the region around the electrodes on the suspended CNTs using several laser excitation wavelengths. Analysis of the G+/G− splitting in the Raman spectra (Sgobba and Guldi Chem Soc Rev 38:165–184, 2009) shows CNT diameters of 2.5 ± 0.3 nm. Neither surface modification nor increase in defect density or stress at the CNT-electrode contact could be detected, but rather a shift in G+ and G− peak positions in regions with high CNT density between the electrodes. Simultaneous topographical and electrical characterization of the CNT transistor by CS-AFM confirms the presence of CNT bundles having a stable electrical contact with the transistor electrodes. For a similar load force, reproducible current–voltage (I/V) curves for the same CNT regions verify the stability of the electrical contact between the nanotube and the electrodes as well as the nanotube and the AFM tip over different experimental sessions using different AFM tips. Strong variations observed in the I/V response at different regions of the CNT transistor are discussed.
KeywordsSingle-walled carbon nanotubes CNT transistor Raman imaging Current sensing AFM Atomic force microscopy
Due to their exceptional properties, carbon nanotubes (CNT) have been the focus of intense research in several fields from spintronics to biosensing [1, 2]. Moreover, recently, CNTs are being explored as active materials for the next generation of sensing devices, solar cells, field effect transistors (FET), and nanoelectronics [3, 4, 5, 6]. Pioneered by the work of Tans et al. , one of the promises of nanotechnology using carbon nanotubes concerns the development of faster, more power-efficient and smaller electronic devices . However, the realization and mass production of CNT electronics have remained elusive so far. It is a complex situation since the large-scale integration of carbon nanotubes into current silicon technology is still under development. One of the main challenges concerns the selective deposition of carbon nanotubes on predefined positions of a circuit such as across a channel in a FET device. In this regard, dielectrophoresis offers a good advantage since it is possible to control the position and alignment of the CNTs along electrodes in an integrated circuit . In addition, dielectrophoresis technology can be made compatible with mass-production processes while allowing deposition directly from CNTs dispersed in liquid [10, 11]. In this work, we undertake the study of semiconducting single-walled CNTs that have been aligned and deposited along two pre-structured palladium electrodes with a channel separation of 2 μm. Using jointly Raman spectroscopy imaging and current sensing AFM (CS-AFM), we aim at investigating the properties of dielectrophoresis-deposited carbon nanotubes in order to find out whether or not the defect concentration in carbon nanotubes increases at the CNT/electrode interface, evaluating at the nanoscale level the quality of the electrical contact between the nanotubes and the electrodes (Ohmic or not) and verifying that a good alignment can be achieved along the channel. In addition to the defect concentration obtained from the intensity ratio of the D/G band, from Raman spectroscopy, the CNT diameter was estimated using the splitting of the G− and G+ peaks .
CS-AFM data were recorded with a 5500 AFM from Agilent Technologies (CA, USA) using Ti/Pt-coated AFM probes (tip radius < 40 nm) with a spring constant of approximately 0.12 N/m.
Raman measurements were performed in the backscattering geometry within the spectral range of 1,100 to 2,800 cm−1, which includes the first and the second order bands using the 488 and 514.5 nm lines of an Ar+ laser and the 632.8 nm line of a HeNe laser. The Raman spectrometer is a LabRam HR800 (HORIBA Scientific, Villeneuve d’Ascq, France) with an optical microscope Olympus BX40 (Olympus Europa Holding GmbH, Hamburg, Germany). A 100× objective (N.A. 0.9) was used to illuminate the sample and to collect the Raman signal with a diffraction limited resolution of λ / (2 N.A.) ≈ 286 nm (λ = 514.5 nm). A liquid nitrogen-cooled back-illuminated charge-coupled device (CCD) was employed for the detection of the Raman signal using a diffraction grating of 600 l/mm yielding a spectral resolution of 4 cm−1. The laser power was limited to the range of 0.5 to 2 mW in order to prevent sample damage. Full Raman spectra were acquired with a Raman imaging stage with a step size of 500 nm.
Results and discussion
CNT resistance values estimated from CS-AFM
While the first and the last contributions are constant and negligible, the contact between the CNT and the metal electrode is of great importance. As can be observed from the bottom part of the topography image in Figure 2a, the contact (which equals the interface path between the CNTs and the metal surface) is different from bundle to bundle. Accordingly, there seems to be a good correlation with the current response. For the marked CNTs, the detected current passing through is gradually decreasing relative to the contact. This is most probably due to different quality of the contact and, therefore, different values for the contact resistance. The average spectra for the investigated CNTs recorded using the same AFM probe are shown within Figure 3b, while the corresponding estimated resistance values are included in Table 1.
Avoiding metallic CNTs in a transistor is of great importance since few metallic carbon nanotubes can create a shortcut, compromising the transistor performance. Giving their clear different signature, in our Raman imaging results, metallic CNTs were not detected but only semiconducting ones . It is possible that the 2% of metallic CNTs present in the original solution were burnt out during the dielectrophoresis deposition  or their amount is not sufficient to be detected. Due to the metallic nature of the Pd electrodes and their roughness, surface-enhanced Raman spectroscopy might appear in regions where the CNT was in direct contact with the electrodes. However, we did not find any visible SERS effect which could be explained by the possible presence of residual photoresist that has also hidden the metallic electrode from the conductive AFM probe evidenced in CS-AFM as discussed above.
Summary of the peak positions and intensity ratios
G− (cm−1); d (nm)
1,571 ± 1; 2.50
1,593 ± 1
1,572 ± 1; 2.75
1,593 ± 1
1,567 ± 5; 1.83
1,592 ± 5
The obtained local intensities of the G+ band are displayed in the Raman map shown in Figure 5. The ID/IG ratio for CNT bundles between the electrodes and on the electrodes is shown in the mapping of Figure 6. The ID/IG ratio appears similar for different excitation wavelengths having a value of 0.29 ± 0.02 for CNTs on the bundles between the electrodes and a ID/IG ratio of 0.30 ± 0.01 for CNTs on the electrode. The shape of the three peaks (D, G+, and G−) does not change throughout the investigated region. Given that the Raman imaging shows a homogeneous CNT quality along the FET, differences in resistance observed by CS-AFM between different bundles can most certainly be attributed to the quality of the Pd electrode/CNT contact, and not to the CNT quality. A slightly higher defect concentration observed at the CNTs on the electrodes might come from welding of the CNT onto the Pd electrode during deposition, although such small difference in ID/IG ratio is within the experimental error.
Raman spectroscopy and imaging in addition to current sensing AFM were used in order to investigate a CNT-based device. Semiconducting single-walled CNTs were deposited and aligned using dielectrophoresis. The semiconducting character of the CNT bundles was proved by Raman spectroscopy, and the SWCNT diameter was determined to be 2.5 ± 0.3 nm. It is shown that an Ohmic contact between the palladium electrodes and the CNTs is realized using this fabrication method without any significant increase in defect density at the CNT/electrode contact.
The work is supported by the following projects: DFG Research Unit 1713 ‘Sensorische Mikro- und Nanosysteme’ and DFG project ZA146/22-1 Raman investigations of In(Ga)As/Al(Ga)As self-assembled quantum dot structures: from ensembles to single quantum dots’. Alexander Villabona is acknowledged for the implementation of the stage for Raman imaging. We also acknowledge the staff of the ZfM for the help with structure fabrication and SEM measurements.
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