Tunable Low Crystallinity Carbon Nanotubes/Silicon Schottky Junction Arrays and Their Potential Application for Gas Sensing

Abstract

Highly ordered nanostructure arrays have attracted wide attention due to their wide range of applicability, particularly in fabricating devices containing scalable and controllable junctions. In this work, highly ordered carbon nanotube (CNT) arrays grown directly on Si substrates were fabricated, and their electronic transport properties as a function of wall thickness were explored. The CNTs were synthesized by chemical vapor deposition inside porous alumina membranes, previously fabricated on n-type Si substrates. The morphology of the CNTs, controlled by the synthesis parameters, was characterized by electron microscopies and Raman spectroscopy, revealing that CNTs exhibit low crystallinity (LC). A study of conductance as a function of temperature indicated that the dominant electric transport mechanism is the 3D variable range hopping. The electrical transport explored by I-V curves was approached by an equivalent circuit based on a Schottky diode and resistances related to the morphology of the nanotubes. These junction arrays can be applied in several fields, particularly in this work we explored their performance in gas sensing mode and found a fast and reliable resistive response at room temperature in devices containing LC-CNTs with wall thickness between 0.4 nm and 1.1 nm.

Keywords: Schottky junction arrays; anodic aluminum oxide; electric transport; gas sensor; low crystallinity carbon nanotubes.

Figure 1

SEM micrographs of a sample synthesized with 5 min of growth time. (a) Top view of the pristine sample. (b) top view after Au deposition. (c) Side view, showing the height of the PAM and the thickness of Au-electrode. (d) Backscattered electron image side view, showing the Au penetration inside the nanopores.

Figure 2

TEM micrographs of LC-CNTs growth with 25 sccm C₂H₂ flow at (a) 5 min, (b) 20 min, and (c) 30 min of synthesis time. (d) HR-TEM micrograph of sample growth with 30 min of synthesis time. (e) Wall thickness as a function of the time synthesis plot obtained from the HR-TEM images.

Figure 3

(a) Raman spectra of LC-CNTs as a function of wall thickness. (b) Peak position of resonances 7A₁, D, 5A₁, G. (c) Representative ratio of I(D)/I(G), I(7A₁)/I(D), and I(5A₁)/I(D). (d) FWHM of G and D peaks.

Figure 4

Conductance as a function of temperature and wall thickness of LC-CNTs. Insets show a zoom near 10 K.

Figure 5

(a) Gas sensing behavior of sample with 0.7 nm wall thickness at different H₂ and C₂H₂ concentrations. Insets show the maximum sensitivity percentage as a function of the analyte concentration. (b) Normalized resistive response as a function of bias voltage.

Figure 6

Room temperature dark I–V curves of samples with wall thickness of 0.4 nm, 0.7 nm, and 1.1 nm. Equivalent circuit (inset).

Figure 7

(a–c) I–V curves measured at 300 K, 250 K, 200 K, 150 K, and 50 K of samples that contain LC-CNTs with 0.4 nm, 0.7 nm, and 1.1 nm of wall thickness, respectively. (d–f) are the plot of the conductance σ_p = 1/R_p and σ_s = 1/R_s as a function of temperature for each sample, the insert shows the ideality factor and the average barrier voltage ϕ_B as a function of temperature.