Automated circuit fabrication and direct characterization of carbon nanotube vibrations

Abstract

Since their discovery, carbon nanotubes have fascinated many researchers due to their unprecedented properties. However, a major drawback in utilizing carbon nanotubes for practical applications is the difficulty in positioning or growing them at specific locations. Here we present a simple, rapid, non-invasive and scalable technique that enables optical imaging of carbon nanotubes. The carbon nanotube scaffold serves as a seed for nucleation and growth of small size, optically visible nanocrystals. After imaging the molecules can be removed completely, leaving the surface intact, and thus the carbon nanotube electrical and mechanical properties are preserved. The successful and robust optical imaging allowed us to develop a dedicated image processing algorithm through which we are able to demonstrate a fully automated circuit design resulting in field effect transistors and inverters. Moreover, we demonstrate that this imaging method allows not only to locate carbon nanotubes but also, as in the case of suspended ones, to study their dynamic mechanical motion.

Figure 1. Preferentially adsorption of p-nitrobenzoic acid on carbon nanotubes.

(a) Top: Chemical structure of p-nitrobenzoic acid (pNBA). Bottom: Schematic illustration of the monoclinic unit cell of pNBA powder as extracted from X-ray diffraction analysis. (b,c) Dark field optical microscopy images of pNBA nanocrystals adsorbed along CVD grown carbon nanotubes (CNTs). Scale bar, 50 and 20 μm, respectively. (d) Amplitude image of AFM of a single CNT with a few pNBA nanocrystals along. Scale bar, 1 μm. Inset: height cross sections along the marked lines of the main figure. (e) Dark field optical microscopy image of pNBA nanocrystals after intensive deposition. Note the black voids along the CNT. Scale bar, 20 μm. (f) Dark field optical microscopy image of pNBA nanocrystals adsorb onto commercial dispersed CNTs. Scale bar, 20 μm.

Figure 2. Sublimation rate of p-nitrobenzoic acid (pNBA) nanocrystals off carbon nanotubes.

(a) A temporal set of AFM amplitude images of pNBA nanocrystals (NCs) along a single carbon nanotube (CNT). The time interval between each image is 11 min. Scale bar, 1 μm. (b) A temporal set of dark field optical microscopy intensity of pNBA NCs along a single CNT. The time interval between each image is 6.5 min. Scale bar is 2 μm. (c) Left axis: NC height as function of time (blue circles) and theoretical fit according to Supplementary Eq. 7 (red line). Right axis: Relative dark field optical microscopy intensity of pNBA NC along CNT as a function of time (green circles). (d) Temporal dependence of the maximum height of different NCs along different CNTs.

Figure 3. Electrical characterization of marked and non-marked carbon nanotubes.

(a–c) AFM images of complete carbon nanotube (CNT) device with metallic source and drain electrodes, before p-nitrobenzoic acid (pNBA) deposition (a) immediately after (b) and after one day (c). Zoom in on the black square areas marked 1 and 2 of the main panel of b are shown as four insets before deposition (a) and after one day (c). The dotted lines mark the locations of the cross sections which are plotted in d. The scale bar of the main panels, (a–c) is 1 μm, and for the insets is 200 nm. (d) Cross sections along the black dotted lines of panels (a–c) of area 1 (left) and area 2 (right). Top panels depict the nanocrystals (NCs) height immediately after deposition (blue for area 1, and red for area 2). Bottom panels plot the CNT diameter before (blue) and long after (red) within area 1 (left) and area 2 (right). (e,f) Transfer characteristic of two CNT devices before deposition (blue), immediately after deposition along the CNT (green), and after cleaning process (red).

Figure 4. Optically imaging of on-surface and suspended carbon nanotube devices.

(a) Dark-field optical microscopy image of marked carbon nanotube (CNT) with p-nitrobenzoic acid (pNBA) nanocrystals (NCs), and overlay of the lithography circuit design. Scale bar is 10 μm. (b) Optical image of the complete fabricated device. The two different colour electrodes are two different metallic contacts. Scale bar is 10 μm. (c) Transfer characteristic of the two CNTs devices 1 (blue), and 2 (red). Inset: SEM image of the completed circuit. Blue arrow points towards the CNT that was marked in (a). Scale bar, 10 μm. (d) Top: Dark field optical microscopy image of marked junction. Bottom: SEM image of the same junction. Scale bar in both images is 3 μm. (e) Main panel: Transfer characteristic of suspended CNT before pNBA deposition (blue), immediately after deposition (green), and after cleaning process (red). Inset: Dark field optical microscopy image of the measured junction marked with pNBA NCs. Scale bar, 3 μm (f,g) Dark field optical images of long suspended CNTs decorated with pNBA NCs. Scale bar, 50 μm.

Figure 5. Optically measured mechanical resonances of marked carbon nanotubes.

(a) Amplitude of vibration of carbon nanotube (CNT) versus excitation frequency of a driven piezoelectric actuator, V_ac. The different coloured curves are for different applied voltages of the piezo as plotted in c (2, 4, 8, 12, 18, 20 and 22 V from bottom to top). The different black dashed lines are best fit to Lorentzian curves. (b) Quality factors as extracted from the Lorentzian fit. (c) Maximum amplitude versus the applied ac piezo voltages (subtracted offset bias originated from the electrical setup, V₀). (d) Frequencies of the maximal response for different excitation voltages. The chronological time of the measurements were from right to left. (e) The same data as in a for three different V_ac's (dotted coloured lines) and resulted fit (colored lines) according to our finite element model (FEM) discussed in Supplementary Note 7. Vib., Vibration.

Figure 6. Hardening to softening behaviour.

(a–e) Amplitude of vibration of carbon nanotube (CNT) versus excitation frequency for different pressures, as marked inside each panel. Blue triangles are data for up sweep and red triangles for down sweep. The yellow and purple dashed lines are the theoretical solutions according to Supplementary Equations 33 and 34, for up and down sweep, respectively. (f) Left axis: Extracted non-linear spring constant, α, as function of pressure. Right axis: Extracted coefficient of linear damping, γ, versus chamber pressure. Vib., Vibration.

Figure 7. Automatic design and fabrication of carbon nanotubes based devices.

(a) Image processing results. Dark field optical microscopy image of marked carbon nanotubes (CNTs) (white dotted lines) superimposed with the image processing analysis results (red lines). Scale bar, 30 μm. (b) Transfer characteristics of automatically designed p- (blue line) and n- (red line) type CNT field effect transistors (FETs). (c) Automatically designed inverter based on p-type CNTFET and external resistor. Inset: schematic diagram for the inverter.