Article pubs.acs.org/cm
Direct Synthesis and Integration of Individual, Diameter-Controlled Single-Walled Nanotubes (SWNTs) Fatima Z. Bouanis,*,†,‡ Costel S. Cojocaru,*,† Vincent Huc,§ Evgeny Norman,† Marc Chaigneau,† Jean-Luc Maurice,† Talal Mallah,§ and Didier Pribat∇ †
Laboratory of Physics of Interfaces and Thin Films, UMR 7647 CNRS/Ecole Polytechnique, 91767 Palaiseau, France Laboratoire Instrumentation, Simulation et Informatique Scientifique-Université Paris-Est, Ifsttar, 77447 Marne-la Vallée Cedex 2, France § Institut de Chimie Moléculaire et des Matériaux d’Orsay, CNRS, Université Paris Sud 11, 91405 Orsay, France ∇ Department of Energy Science, Sungkyunkwan University, Suwon 4406746, Korea ‡
S Supporting Information *
ABSTRACT: We present a robust and versatile approach for the reproducible and controllable growth of single-walled carbon nanotubes (SWNTs) through a self-assembled monolayer (SAM) technique coupled with an atomic hydrogen (Hat) pretreatment to control the catalytic metallic nanoparticles size and density. The nanoparticles are obtained from a selfassembled monolayer of metal complexes or salts on a SiO2 substrate using a two-step strategy. The oxide is first functionalized by silanization with a coordinating ligand leading to the formation of an anchoring SAM on the substrate. Then, metallic complexes such as ruthenium porphyrin (RuTPP) or metallic salts (FeCl3, RuCl3) are assembled by coordination bonds on the preformed organic SAM. Pyrolysis under radical hydrogen atmosphere of the as-prepared SAM yields metallic nanoparticles whose size and density are controlled and tuned. Using the as-formed nanoparticles as catalysts, SWNTs are grown by double hot-filament-assisted chemical vapor deposition (d-HFCVD). They exhibit a remarkably good crystalline quality, with a diameter (and type) strongly dependent on the nature of the initial catalyst precursor and its preparation. Field-effect transistors (FETs) with excellent characteristics were obtained using such in-place grown SWNTs. The electronic properties of the SWNTs can be tuned: the transistors obtained from Ru(TPP) and FeCl3 exhibit ION/IOFF current ratio up to ∼109, indicative of the direct growth of a high proportion of semiconducting nanotubes over than 98%. Such elevated ION/IOFF values have been reported essentially for CNT-FETs devices based on individual semiconducting SWNTs, so far. By contrast, devices obtained from the RuCl3 salt display ION/IOFF current ratio well below 102, indicating the direct growth of SWNTs highly enriched in metallic specimens.
1. INTRODUCTION
Generally, the synthesis of SWNTs is performed with the assistance of metallic nanoparticles that catalyze the growth of nanotubes from carbon vapors (for laser ablation and arc discharge processes) or through the decomposition of carbonbearing precursor gases for chemical vapor deposition (CVD).9 With only a few exceptions (see below), most of the SWNTs synthesis methods yield variable mixtures of both semiconductor and metallic specimens.9 Even if post-growth separation techniques based on SWNT liquid dispersions10,11 have greatly been improved over the past few years, the cost of such materials will certainly remain prohibitive for quite a while. Moreover, when liquid suspensions are involved in the process,
Single-walled carbon nanotubes (SWNTs) are regarded as excellent candidates for applications in nanoelectronic devices, because of their unique structure and their remarkable electrical properties.1,2 They have been investigated for various applications ranging from single electron transistors3 and field-effect transistors (FETs) to memories and integrated circuits4−6 or chemical and biological sensors.7,8 However, the electronic performances of the devices are strongly dependent on the SWNT diameter and chirality as well as their crystalline quality.1,2 The development of new methods enabling a precise control over the structural properties (and, consequently, over the electronic properties) of SWNTs is of paramount importance for future progress in carbon nanotube (CNT)based electronics. © XXXX American Chemical Society
Received: June 24, 2014 Revised: August 18, 2014
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Figure 1. SAM-based strategy for controlling the diameter of metallic nanoparticles to be used as catalysts for double hot-filament-assisted CVD (dHFCVD) growth of SWNTs: (a) substrate functionalization by a first “anchoring” SAM of a pyridine organic compound and the overlayer formed by a second SAM of a metallic complex, either (1) porphyrin ruthenium(II) carbonyl (Ru(TPP)) or (2) ruthenium(III) chloride (RuCl3). (b−g) Characterization of nanoclusters obtained after annealing under activated hydrogen ((b and e) AFM and (c and f) TEM images of the pyrolyzed SAMs; (d and g) column stack diameter distribution of the nanoclusters obtained by TEM and AFM analysis after pyrolysis in Hat for Ru(TPP) and for RuCl3 complexes, respectively. The solid lines correspond to Gaussian fits.
makes it suitable for large-scale realization of electronic devices based on SWNTs. We have applied this growth strategy on a few tens of 2-in.-diameter oxidized Si substrates, on which we fabricated field-effect transistors (CNT-FETs) from the assynthesized SWNT networks. We have measured ION/IOFF current ratios over 107 and up to 109, which represent some of the best reported performances for such devices to date.6
a thorough sonication step is required to break the SWNTs bundles, which tends to degrade the crystal quality of the SWNTs and yields electronic devices with reduced performances, compared to devices using nonsonicated tubes.11−14 One of the main challenges in this very active area of research is to set novel synthesis routes for SWNTs, allowing an effective control of their diameter and chirality. Recent progresses suggest that CVD methods are among the most promising routes for the versatile and scalable synthesis of SWNTs with desired structure, either by use of activated gas-phase postgrowth treatments12,13 or a specific growth gas phase,14−17 growth kinetics,18 or catalyst type19 and preparation techniques.19−22 So far, however, few studies have been reported on a combination of these strategies for the selective synthesis of SWNTs.14,15 Here, we demonstrate that a joint combination of a selfassembled monolayer (SAM)-based strategy for controlling surface density of metallic complexes and an activated hydrogen-assisted CVD pretreatment can be used to control the density and the size of the nanocluster catalysts. Furthermore, we also demonstrate that such improved control over the nanoparticles’ nature and size, coupled with a controlled double hot-filament-assisted CVD (d-HFCVD) growth gas-phase activation and growth kinetics allow the synthesis of high-crystalline-quality SWNTs with tunable diameters and electronic properties. The high degree of reproducibility and the relative simplicity of our process
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. All reagents were purchased from Aldrich and used as-received. Dichloromethane was freshly distilled over CaH2 prior to use. Before the formation of self-assembled monolayers (SAMs), all substrates were cleaned following a procedure based on piranha solution. They were then rinsed 10 times with deionized water and dried under nitrogen or argon. All the self-assembly experiments were performed in a homemade reactor under an inert atmosphere (argon in this study). The substrates were immersed overnight in a 10−3 mol/L solution of silane in dry toluene. They were then rinsed with dry toluene and annealed for 1 h at 110 °C in an oven. The substrates were then washed under ultrasonic treatment with dry CH2Cl2 and finally dried under nitrogen. The substrates were used immediately for the next steps. In this step, we use two types of substrates: (i) 100-nm thermal silica-coated silicon wafers and (ii) glass substrates. The latter one was used to check the specificity of the anchoring and the surface density of metallic complexes on the sample after the formation of the SAM by ultraviolet−visible light (UV-vis) spectroscopy (when highly absorbing complexes, such as metallic porphyrins are used). In this study, we use B
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Figure 2. (a and d) TEM images of SWNTs grown on TEM grids, using the two types of Ru catalysts obtained from self-assembled monolayers. (b and e) Corresponding histograms of diameters distribution of SWNTs obtained by TEM observations and Raman spectroscopy (532 and 633 nm) for each catalyst. (c and f) Raman spectra (633 nm) of SWNTs grown on Si/SiO2 substrate using the two types of Ru catalysts. RBM features (left panels at low wave numbers), tangential mode (G+- and G−-bands) and disorder-related mode (D-band) of the SWNTs. a long-chain alkylsilanes because it allows the formation of a uniform organic monolayer (limits the condensation occurring in solution). The pyridine-functionalized substrates are subsequently used for anchoring an overlayer of various metallic complexes or salts. The silane-covered substrates were immersed overnight in a solution of metallic complexes/salts (10−3 mol/L of Ru(TPP) in anhydrous CH2Cl2 or 5 × 10−3 mol/L of FeCl3 in ethanol or 5 × 10−3 mol/L of RuCl3 in water). They were then submitted to sonication two times for 5 min with dry dichloromethane and dried under nitrogen. 2.2. Synthesis of Carbon Nanotubes. SWNTs synthesis from previously prepared Ru and Fe nanoparticles on substrates was performed using a homemade experimental d-HFCVD reactor. It mainly consists of a quartz tube enclosed in a cylindrical heater (80 mm in inside diameter and uniform heated lengths up to 250 mm). It is connected to a methane and hydrogen feedstock and to a pumping system (residual base pressure of 10−6 mbar). The methane (CH4) and hydrogen are injected as separate flows and forced to pass over two separate tungsten filaments (0.38 mm in diameter), mounted horizontally near the substrate, and independently electrically driven at variable power (for filament temperature ranging from 1200 °C to 2000 °C). The SAM-loaded substrates were placed at the center of the quartz tube reactor and heated to 900 °C for Ru(TTP) and RuCl3, and to 800 °C for FeCl3 (only the optimal parameters are used in this study). The first step consists of a pretreatment pyrolysis under atomic hydrogen. The substrates are exposed for 5 min to a hot-filamentactivated (160 W, ∼2000 °C, measured with an optical pyrometer) hydrogen flow (100 sccm at 90 mbar). The goal is to both pyrolyze the organic monolayer and transform the metallic atoms contained in the SAM into an assembly of Ru and Fe nanoclusters with controlled size, possibly with monodispersity. After hydrogen annealing, a CH4:H2 gas mixture (9:1 ratio) is introduced into the reactor for 30 min at 900 °C. To efficiently predissociate the CH4 molecules, the methane flow is activated by heating the corresponding tungsten filament at 1700 °C (120 W). The hydrogen flow is also activated by heating the corresponding filament at 2000 °C (160 W, as for the pyrolysis). It is used to remove the undesired carbon particles and amorphous deposits formed on catalyst seeds, tube walls, or substrate surface during CVD. The same procedure in the growth of SWNTs was applied to functionalized SiNx transmission electron microscopy (TEM) grids to obtain TEM images. 2.3. Characterization Techniques. UV-vis spectra were recorded on a Cary 5 spectrophotometer. TEM images were recorded on a
Philips Model CM 30 system operating at 300 keV, and a Topcon Model 002B system operating at 160 keV. Scanning electron microscopy (SEM) observations were carried out on a Hitachi Model S 4800 system. Raman spectra were recorded using a highresolution confocal Raman microscope (Model Labram HR800, Horiba Jobin−Yvon). High resolution with laser excitations of λ = 633 and 532 nm was achieved using a system equipped with a confocal microscope (Olympus Model BX41) and a piezoelectric (PI) XY stage for the μRaman mapping. Atomic force microscopy (AFM) images were acquired using a VEECO Dimension 3100 instrument.
3. RESULTS AND DISCUSSION SWNTs mats have been successfully synthesized both on 2-in. p-type doped Si wafers covered by a 100-nm-thick thermal oxide layer and on SiNx grids for TEM observations. The formation of small size, monodisperse catalytic nanoparticles on the surface of the substrate is one of the key factors for controlling the growth of SWNTs.23 We developed a selfassembled monolayer SAM-based strategy schematized in Figure 1a. The use of SAMs can be considered as a scalable and robust way to control the surface density of metallic atoms, ranging from 1013 to 1014 atoms/cm2, as a function of the steric hindrance of the self-assembled complex/salt grafted on top of a first “anchoring” SAM of pyridine-functionalized silane (for further details, see the Supporting Information (Part I)). In this study, we have used the 5,10,15,20-tetraphenyl-21H,23Hporphyrin ruthenium(II) carbonyl (RuTPP) complex (the structure of this complex is presented in the Supporting Information (Figure S4)), as well as two metallic salts (RuCl3 and FeCl3) as catalyst precursors. The grafted metal complex and salts have been successfully transformed to uniform dispersions of nanoparticles with small diameters through a subsequent pyrolysis step performed at 900 °C for Ru and 800 °C for Fe under hot-filament-activated hydrogen (atomic hydrogen, Hat) flow (a filament temperature of ∼2000 °C was measured with an optical pyrometer; see the d-HFCVD step in the Supporting Information (Part II)). The goal of this thermal treatment is to both pyrolyze the organic layers and transform the metallic ions contained in the SAM into an assembly of C
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shown in Figure 2 for each catalyst preparation. From TEM analysis (examples are given in Figures 2a and 2d), the mean measured diameter of the SWNTs grown on catalyst nanoclusters from Ru(TPP)-functionalized SiNx grids is 1.2 nm with a standard deviation of 0.4 nm and the distribution is quite uniform (Figure 2b). For the SWNTs obtained from RuCl3, the mean SWNTs diameter increases to 1.8 nm with a standard deviation of 0.7 nm and the diameter distribution is broader (Figure 2e). We measured altogether 50 tubes on several different samples. For both catalyst preparations, the growth of relatively straight and low-density SWNTs proceeds according to the base-growth mechanism (the catalyst nanoparticle remains on the supporting substrate throughout the growth process and the nanotubes grow out from the particles with a closed-end26) (see TEM images in Figure 2). The quality and the diameter distribution of the as-grown SWNTs were further investigated by Raman spectroscopy. The Raman spectra have been measured at five random points of each substrate (a few tens of 2-in. substrates were used in this study: 12 for Ru(TPP), 14 for RuCl3, and 50 for FeCl3). Let us first focus on the G- and D-bands of the SWNTs grown from RuTTP and RuCl3 catalyst precursors. As shown in Figures 2c and 2f, an intense and narrow G-band (intensity IG) is clearly observed (two components, G+ and G−, of the tangential mode vibrations located at 1570 and 1590 cm−1, respectively), whereas the intensity ID of the D-band at ∼1310 cm−1 (revealing the crystalline disorder and lattice defects) is very small. The ID/IG ratio of SWNTs synthesized from Ru(TPP) and RuCl3 catalyst precursors are, respectively, 0.09 and 0.05, which is indicative of very high crystalline quality of the asgrown SWNTs. Furthermore, using the “Kataura Plot”, the chirality of SWNTs can be assigned, based on the radial breathing mode (RBM) frequencies and the excitation wavelength.27,28 The frequencies of RBMs are dependent on the diameter of the SWNTs (d), following the relation ω (cm−1) ≅ 248/d (nm).28 The laser excitations used in this study are 633 and 532 nm. For a laser excitation of 633 nm, SWNTs with RBMs frequencies between 120 and 177 cm−1 are considered to be semiconducting tubes while RBMs frequencies from 177 to 221 cm−1 are assigned to metallic tubes.29 Figures 2b and 2e show statistical results of the distribution of the SWNT diameters when synthesized from Ru(TPP) and RuCl3 and deduced from the RBMs position in the Raman spectra and from TEM analysis. When using Ru(TPP) as catalyst precursor, more than 69% of the observed as-grown SWNTs have a mean diameter of 1.3 nm with a standard deviation of 0.2 nm and 31% have a mean diameter of 1.8 ± 0.3 nm. For the SWNTs grown from RuCl3 as catalyst precursor, the mean diameter is ∼1.9 nm (±0.3 nm) for 49% of the SWNTs, 1.3 nm (±0.2 nm) for 43%, and 2.6 nm (±0.2 nm) for 8%. The above results show that nanosized Ru nanoclusters obtained from the decomposition of Ru(TPP) and RuCl3 do exhibit catalytic activity for SWNTs growth. It is important to note that the diameters of the produced SWNTs are correlated with the size distribution of the catalyst nanoparticles. A comparison between the diameter of the produced SWNTs and the size of the nanoclusters obtained from Ru(TPP) and RuCl3 precursors reveals that the diameter of SWNTs is smaller than those of the initial catalyst nanoparticles. These results are in good agreement with previous reports in the literature, showing that the mean diameter of SWNTs grown using Ru as a catalyst ranges between 1.1 nm and 2 nm, while the size of the particle is between 1 nm and 5 nm.30 There are several possible reasons
nanoclusters with a controlled and reproducible size. As previously reported,24 the size and the distribution of the catalyst nanoclusters can be tuned by the Hat pretreatment that creates defects with high trapping energy on the surface of the substrate. Such defects inhibit the surface diffusion of atoms and pin the nanoclusters, thus resulting in well-defined and well-isolated objects. We measured the diameter of those nanoparticles from height profiles in atomic force microscopy (AFM) and also from transmission electron microscopy (TEM) analysis. Figure 1 shows typical AFM analysis (Figures 1b and 1e) and TEM analysis (Figures 1c and 1f) images of nanoclusters obtained after pyrolysis. Gaussian fits have been superimposed on the experimentally measured size distributions (Figures 1d and 1g). For the Ru(TPP) complex, the Hatassisted pyrolysis yields a nanocluster diameter distribution (Figure 1d) with roughly 80% of the diameters centered at 1.9 nm (±0.4 nm) and 20% at 2.9 nm (±0.4 nm). For RuCl3 (Figure 1g), following Hat-assisted pyrolysis, the average diameter is larger and the distribution is broader, compared to those corresponding to nanoclusters obtained from Ru(TPP) under the same conditions: ∼55% of the nanocluster diameters are centered at 2.8 nm (±0.4 nm), 30% at 4 nm (±0.4 nm), and 15% at 1.9 nm (±0.4 nm). Compared to the Ru(TTP)-functionalized samples, the mean diameter of the nanoclusters formed by the Hat pretreatment for the RuCl3 increased by ∼67%, corresponding to a volume increase of ∼289%. These observations tend to indicate that the surface density of metal atoms after pyrolysis is correlated to the size of the molecular structure from which they originate. In other words, assuming a monolayer coverage on top of the anchoring silane for both Ru compounds, because of the larger size and steric hindrance of the Ru(TPP) complex compared to the RuCl3 molecule, the metal clusters obtained from the former are smaller than those obtained from the latter. At this point, we should note the remarkable consistency of the two types of analysis, keeping in mind that TEM observations have been performed on SiNx functionalized grids, while AFM data were recorded on SiO2/Si functionalized wafers. Figures 1d and 1g show statistical results of diameter distribution of Ru nanoclusters annealed in activated H2 and obtained from AFM and TEM analyses. Ruthenium has been relatively poorly investigated as a catalyst for the growth of CNTs; however, it is expected to be active if small nanoparticles (
