Confirming the Dual Role of Etchants during the ... - ACS Publications

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Confirming the Dual Role of Etchants during the Enrichment of Semiconducting Single Wall Carbon Nanotubes by Chemical Vapor Deposition Imad Ibrahim,†,‡ Jana Kalbacova,§,∥ Vivienne Engemaier,† Jinbo Pang,† Raul D. Rodriguez,§,∥ Daniel Grimm,† Thomas Gemming,† Dietrich R. T. Zahn,∥,§ Oliver G. Schmidt,†,‡ Jürgen Eckert,†,‡,⊥ and Mark H. Rümmeli*,†,‡,#,∇ †

IFW Dresden, P.O. Box 270116, 01171 Dresden, Saxony, Germany Center for Advancing Electronics Dresden (cfaed), Dresden University of Technology, 01062 Dresden, Saxony, Germany § Semiconductor Physics and ∥Center for Advancing Electronics Dresden (cfaed), Technische Universität Chemnitz, D-09107 Chemnitz, Saxony, Germany ⊥ Institute of Materials Science, TU Dresden, 01062 Dresden, Saxony, Germany # IBS Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS) and ∇Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 23, 2015 | http://pubs.acs.org Publication Date (Web): August 17, 2015 | doi: 10.1021/acs.chemmater.5b02037

‡

S Supporting Information *

ABSTRACT: The search for ways to synthesize single wall carbon nanotubes (SWCNT) of a given electronic type in a controlled manner persists despite great challenges because the potential rewards are huge, in particular as a material beyond silicon. In this work we take a systematic look at three primary aspects of semiconducting enriched SWCNT grown by chemical vapor deposition. The role of catalyst choice, substrate, and feedstock mixture are investigated. In terms of semiconducting yield enhancement, little influence is found from either the binary catalyst or substrate choice. However, a very clear enrichment is found as one adds nominal amounts of methanol to an ethanol feedstock. Yields of up to 97% semiconducting SWCNT are obtained. These changes are attributed to two known etchant processes. In the first, metal SWCNT are preferentially etched. In the second, we reveal etchants also preferentially etch small diameter tubes because they are more reactive. The etchants are confirmed to have a dual role, to preferentially etch metallic tubes and narrow diameter tubes (both metallic and semiconducting) which results in a narrowing of the SWCNT diameter distribution.

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INTRODUCTION Within the search for materials suitable for electronic devices beyond silicon, single-walled carbon nanotubes (SWCNT) are regarded as a leading candidate due to their outstanding electrical and physical properties, i.e. high mobility and high current-carrying capacities.1−4 More interestingly, closely packed arrays of parallel aligned SWCNT, as the active channel material,5,6 are attractive for the scalable fabrication of highly integrated circuits.2,7 Such SWCNT circuits will provide obvious improvements to the on-driving current, charge carrier mobility, cutoff frequency, and device-to-device consistency, as well as offer compatibility with existing Si fabrication technology.1,3 In order to realize this goal, the SWCNT should possess the following essential characteristics; be in highdensity and be well-aligned with a controlled orientation and, ideally, all be high-purity semiconducting nanotubes of a controlled conductivity type, ultimately of a single chirality.3,7,8 While dense SWCNT arrays can carry higher currents in thin film transistors (TFT) and are more robust for integrated © XXXX American Chemical Society

circuits, it is semiconducting-rich SWCNT arrays that guarantee high on/off ratios enabling efficient switching.3,5,7,8 Therefore, the reproducible fabrication of type-selected, semiconducting (sc-) SWCNT on different substrates is crucial.4,8 The current bottleneck is that most current SWCNT synthesis routes yield a mix of both metallic (m-) and semiconducting (sc-) SWCNT.2,4,7 This negatively affects the performance of devices based on mixed metallic and semiconducting tubes.7−9 Currently, there are two main competing approaches for the preparation of semiconducting-rich SWCNT; in the first, preselected semiconducting nanotubes are deposited on the target substrate from solution.10,11 This strategy has had success regarding separation of the nanotubes according their length, chirality, and electronic type.12,13 However, this approach still has many limitations related to the obtained low yield, poor upReceived: May 30, 2015 Revised: July 14, 2015

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DOI: 10.1021/acs.chemmater.5b02037 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

squares. While the sapphire and silicon substrates were used asreceived without any further treatment, the ST-cut quartz substrates were first subjected to thermal annealing at 750 °C for 15 h in a 1 in. diameter horizontal homemade oven prior to drop coating the catalyst source.6 This thermal annealing is shown to improve the alignment of the later grown nanotubes.17 Once coated, the substrates (one per each type) plus coating were inserted to the middle of a 1 in. purposebuilt horizontal tube oven, where they were annealed in H2 (H2 = 99.9%) at 900 °C with a flow of 0.2 SLPM to decompose the ferrocene and cobalt(II) phthalocyanin yielding Fe:Co catalyst nanoparticles.18 Thereafter, the optimized CVD reaction was conducted in a gaseous environment consisting of H2, Ar bubbled through ethanol, and Ar bubbled through methanol. For simplicity, the Ar flow through ethanol and that bubbles through methanol and hereafter is referred to as the ethanol:methanol flow. The gas flow rates varied, as shown in Table 1,

scalability, and low quality end-products due to the various harsh treatment steps involved in separating the tubes.14,15 Alternatively, horizontally aligned SWCNT arrays can be synthesized directly by chemical vapor deposition (CVD).2,6 Chemical vapor deposition (CVD) is believed to be a significant, versatile, and promising approach for the in-place growth of high-quality type-selected SWCNT, due to its upscalability and technology compatibility.8 Significant improvements have been shown recently in the field toward the development of reliable CVD routes for growing in-place well aligned SWCNT on different substrates.3,7,8 For example, horizontally aligned semiconducting-rich SWCNT can be grown on single crystal quartz substrates by using an optimized CVD process.2 The type selectivity is attributed either to selective etching of metallic SWCNT by etchants such as H2O or *OH introduced to the CVD reaction2,3,16 or, as others argue, due to the strong interaction (affinity) between the SWNT and the quartz surface.7 It is argued that the strong interface interaction increases the yield of semiconducting tubes. Though some CVD approaches have demonstrated the potential to bias the population of one type of nanotube during synthesis, there is only a limited understanding of what exactly determines a tubes’ chirality during synthesis, in particular the contribution of CVD process parameters versus interactions with the support substrate is poorly understood. Moreover, the importance of etchants in the process needs further investigation. To better answer these questions we set about a systematic investigation on the synthesis parameters to better comprehend their role in sc-enrichment of SWCNT produced by CVD. At the same time we also explore the use of different substrates to enable an evaluation of the role of the substrate. Moreover, we explore the role of methanol as a source of etchants/radicals. Our results point to process parameters playing a more important role than substrate selection. In particular, we show through a systematic adaption of different characterization techniques, mainly the correlated AFM-Raman spectroscopy and in situ SEM identification of CNT type, that etchants play a dual and critical role in the enrichment of semiconducting SWCNT. In the first they selectively etch metallic species in agreement with previous studies.2,3 Second we show the preferential etching of small diameter tubes. The combined effect of radicals is to drive the system to a narrow diameter distribution while preferentially etching away metallic tubes. Both of these roles are positive toward enrichment of scnanotubes.

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Table 1. Flow Rates Introduced to the CVD Oven in the Growth Phase H2/ SLPM

Ar bubbled through ethanol/SLPM

Ar bubbled through methanol/SLPM

case I

0.1

0.50

0.00

case II

0.1

0.35

0.15

case III

0.1

0.25

0.25

shown in text as/SLPM Eth:Meth = 0.50:0.00 Eth:Meth = 0.35:0.15 Eth:Meth = 0.25:0.25

in order to investigate the effect on the SWCNT type selectivity. The CVD reaction is run for a period of 15 min at a temperature of 900 °C. Finally, the gas flows were turned off, and the samples were cooled down to room temperature in the presence of Ar (flow rate 0.2 SLPM).19 Fabrication of Thin Film Transistor (TFT) Devices. Typically, a set of source-drain electrode pairs, with a channel length of 1−4 μm and a width of 50 μm, were fabricated using standard photolithography on the ST-cut quartz substrates where the nanotubes were grown. This was followed by preparation of a 20 nm thick insulating material (Al2O3) serving as a top gate oxide. Later, a third electrode (gate) was fabricated to form the gate. The three electrodes were fabricated by ebeam evaporation and contained a thin layer of Cr (10 nm) followed by 50 nm of Au. Characterization. The as-grown SWCNTs were characterized in terms of their morphology, density, length, alignment, and homogeneity using a SEM (FEI, NOVA NanoSEM 200, with typical acceleration voltage of 2−3 kV). The electronic properties and quality of the SWCNT were also characterized using microRaman spectroscopy (Horiba) with excitation laser wavelengths of 633 and 515 nm. While the samples characterized with SEM were investigated as-is on the original support substrates, the Raman spectroscopy were performed on the as-grown as well as transferred SWCNT onto target Si substrates.6 The as-grown SWCNTs were transferred from the hosting substrates onto Si substrates for Raman investigations, respectively, using a protocol described elsewhere. The SWCNT, grown under different CVD conditions, were further characterized with atomic force microscopy (AFM), and their diameter distribution for each case is compared to those from the Raman spectroscopy.32 to ensure any changes in radial breathing modes are due to changes in electronic type rather than shifts in mean diameter distributions of the tubes. A further technique to ensure reliable quantification of enriched semiconducting tubes in the samples involved investigating the aligned tubes with in situ voltage contrast SEM. This in situ bias SEM route leads to a differing contrast between semiconducting and metallic tubes and so provides a visual route to check on the sc-/m-SWNT ratios.20 A detailed flowchart for the performed experiments and characterization is shown in Figure S1 in the Supporting Information. Thermodynamic Calculations. Thermodynamic calculations were performed using the NASA computer program CEA (Chemical Equilibrium with Applications).

MATERIALS AND METHODS

Synthesis of Single Wall Carbon Nanotubes. Mixtures of ferrocene (Sigma-Aldrich, ≥ 98%) and cobalt(II) phthalocyanin (Fluka Chemie, >97%) with different molar ratios (1:1, 1:2, and 2:1) were used as the catalyst source. The mixtures were then dissolved in ethanol (Merck, ≥99.5%) and sonicated overnight resulting in a homogeneous solution, with a concentration of 0.01%, a catalyst source which was later drop coated on the selected support surface. Quartz, sapphire, and silicon/silicon oxide were used as support substrates. Stable-temperature (ST)-cut single crystal quartz wafers (100 mm diameter, 0.5 mm thickness, angle cut 38° 00′, seeded, single side polished from Hoffman Materials, LLC) were cut into 5 × 6 mm rectangles. Sapphire substrates rectangles (4 × 5 mm) were cut from an R-plane sapphire wafer (75 mm diameter, 0.45 mm thickness, single side polished from Epistone). Commercial silicon wafers (100 mm diameter, 0.5 mm thickness, single side polished, 300 nm thermal oxide from Silicon Materials) were cut to have 4 × 4 mm B


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Figure 1. Effect of catalyst mix ratio: A. representative Raman spectra for the SWCNT grown on ST-cut quartz with pure (0.50 standard liter per minute (SLPM)) after transfer to silicon substrates and B. corresponding distribution of RBM peaks collected from more than 10 measurements for each case. Green shaded areas corresponded to regions where semiconducting tubes are in resonance, while patterned red areas are those for regions in which metallic tubes are expected.

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RESULTS AND DISCUSSION Different catalyst combinations (Fe:Co = 2:1, 1:1, and 1:2 molar ratio) were used to nucleate the growth of aligned SWCNT under different carbon precursor mixes (ethanol:methanol). Initially, scanning electron microscopy was used to characterize the as-grown SWCNT in terms of their density, length, and alignment. There was no observable effect in terms of density, length, and alignment with respect to the catalyst ratio in agreement with refs 6 and 21. The effect of the binary catalyst ratio on the electronic type of the as-grown SWCNT was studied by Raman spectroscopy. Raman spectroscopy has become a widely used and powerful tool to identify the physical properties of carbon nanotubes and to assess their functionalization. The Raman spectra presents different features which are also sensitive to the SWCNT chirality (n,m), viz. the way that a sheet of graphene is rolled-up to form a tube. The primary Raman spectroscopy features are a sharp G mode (ca. 1590 cm−1) and the D mode (ca. 1350 cm−1); the defect density can be evaluated from the intensity ratio between the G and D modes and finally the well-known radial breathing modes (RBM).8 RBM allows type identification of the SWCNT in resonance with the used laser excitation energy. We performed the Raman spectroscopy investigation of SWCNT under different laser excitation wavelengths, 632.8, 514.7, and, in some cases, with 780 nm. Five to ten different spots on each

sample were characterized on the whole surface in order to guarantee homogeneity and reproducibility of our results. The range of RMB resonant for a given laser excitation energy can be obtained from the well-known Kataura plot (Figure S2).22 To minimize the interference from the substrate, the samples were transferred onto Si/SiO2 prior to measurement.6 In Figure 1A representative Raman spectra for the different catalyst mixes are provided (when pure ethanol was used as the precursor). Regions shaded in green correspond to RBMs from semiconducting SWCNT, while those in patterned red are from metallic tubes (see Figure S2). Panel B shows the corresponding distribution of the RBM peaks obtained from the many spectra. Again, the areas corresponding to semiconducting and metallic SWCNT are shown in green and patterned red shading, respectively. The corresponding Raman spectra for the as-grown SWCNT on ST-cut quartz substrates are shown in Figure S3, and the diameter distribution, calculated using RBMs for the SWCNT nucleated from different catalyst mixtures, is shown in Figure S4. The SWCNT diameter was calculated from the RBM mode using the following equation, dCNT = 248/ωRBM, where dCNT is the SWCNT diameter and ωRBM is the RBM position.23 It is obvious that a mixture of sc- and m-SWCNT was grown regardless of the catalyst nanoparticle system. The data suggest that different Fe:Co ratios do not significantly affect the diameter distribution of the grown SWCNT. Subsequent C


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Figure 2. Effect of carbon precursor: A. representative Raman spectra and SEM images (inset) for the SWCNT grown over ST-cut quartz using a binary catalyst system (Fe:Co = 2:1 molar ratio) and different combinations of C precursors and B. corresponding distribution of RBM peaks collected from more than 10 spots for each case. Green shaded areas corresponded to regions where semiconducting tubes are in resonance, while patterned red areas are those for regions in which metallic tubes are expected.

(Figure 2). Panel A of Figure 2 shows the Raman spectra for the SWCNT catalyzed by the binary catalyst system, Fe:Co = 2:1 molar ratio, and grown with different flow rates of Ar bubbling through ethanol and methanol for a constant total flow rate of 0.5 standard liter per minute (SLPM). The SWCNT were grown on ST-cut quartz and later transferred onto silicon substrates. The Raman spectra for the as-grown SWCNT are shown in Figure S5. The corresponding distribution of the RBM peaks obtained from the Raman spectra for the as-grown as well as transferred SWCNT are shown in panel B of Figure 2. Again, regions shaded in green correspond RBMs from semiconducting SWCNT, while those in patterned red are from metallic tubes. One can observe that the RBM peaks for the SWCNT grown with only ethanol as the precursor are distributed over the whole range covering both semiconducting as well as metallic regions, indicating the growth of a mixture of semiconducting and metallic SWCNT with a wide diameter distribution. Once methanol was mixed with ethanol, the RBM peaks became fewer and mostly located in semiconducting regions. Notably, no peaks are visible in the range where RBM peaks from metallic tubes are expected for the best ethanol:methanol mix, i.e. 0.25:0.25 SLPM, suggesting the presence of no or few metallic nanotubes.2 Furthermore, the reduction excited RBMs might be attributed to a narrower diameter distribution. This is more obvious in panel B, where

diameter distribution studies using atomic force microscopy (AFM) confirmed little change in the diameter distribution for different catalyst mixes (Figure S4). This is in contrast to other reports, where an enrichment of semiconducting SWCNT by adjusting the catalyst mix was observed. In that case plasma enhanced CVD was implemented,24 whereas here we use thermal CVD and this might account for the difference.9 Investigation of the effect of the carbon precursor ethanol to methanol/ethanol mix ratio on the resultant sc-/m-SWCNT ratio was also conducted. The SWCNT were grown on quartz substrates using mixtures of ethanol and methanol as C feedstock (see Table 1 in the Methods section). The insets of Figure 2 A show representative SEM images of the SWCNT grown on ST-cut quartz substrates using ethanol and methanol with different flows. The as-grown SWCNT are aligned following the surface atoms of the support ST-cut quartz substrate.6 The inclusion of methanol with ethanol, as the C precursor, does not observably affect either the yield or alignment of the as-grown SWCNT. With pure methanol as the feedstock no SWCNT were observed.7 This is attributed to the high decomposition temperature of methanol.24 However, methanol can help by etching amorphous C when partially decomposed as it provides (OH) radicals.9,16 However, the introduction of methanol to ethanol improved the sc-/mSWCNT ratio, as shown in the corresponding Raman spectra D


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Figure 3. Effect of substrate: A. representative Raman spectra collected from SWCNT grown with Fe:Co = 2:1 and C precursor ethanol:methanol = 0.25:0.25 SLPM on different substrates. Inset: Representative SEM images for the grown tubes under the given conditions and B. corresponding distribution of RBM peaks. Green shaded areas corresponded to regions where semiconducting tubes are in resonance, while patterned red areas are those for regions in which metallic tubes are expected.

molar ratio of 2:1. The G− peaks exhibit a sharp and symmetric Lorentzian line shape with the best methanol flow (Eth:Meth = 0.25:0.25 SLPM) as compared to a wide and asymmetric line shape when ethanol alone served as the feedstock (Eth:Meth = 0.50:0.00 SLPM). This observation further reinforces the above hints for the enrichment of semiconducting SWCNT.27 The same observation is applicable when different catalyst mixtures were implemented as shown in Figure S6. This further confirms that the enrichment of semiconducting tubes is more related to the C feedstock rather than the catalyst system. The influence of the support substrate on the type of grown SWCNT is a point of debate.2,3,7 Some reports attribute the enrichment of semiconducting SWCNT to the choice of crystalline support due to their affinity.7 Others report that substrate selection does not influence the sc-/m-SWCNT ratio.2 In this study, crystalline and amorphous substrates were explored as supports. Representative SEM images of the grown SWCNT on ST-cut quartz, silicon, and sapphire are provided as insets in Figure 3A. While the tubes grown on silicon substrates are randomly oriented, those grown on quartz and sapphire are horizontally aligned. The alignment is attributed to tube− substrate van der Waals interactions which is different for amorphous substrates than so for crystalline supports since crystalline supports have an ordered atomic surface structure which can serve as channels to guide and align the SWCNT. In amorphous substrates no such channels exist. The Raman

the patterned red areas corresponding to metallic tubes are free of peaks with the best methanol flow. The same improvement trend for sc-/m-SWCNT ratios is also observed when different catalyst ratios were used, i.e. Fe:Co = 1:2 molar ratio, as presented in Figure S6. Furthermore, neither semiconducting nor metallic RBM peaks were observed under λ = 780 nm laser excitation for the SWCNT grown under the best ethanol/ methanol mix. Again, the lack of RBM peaks corresponding to metallic tubes further supports a low percentage of metallic SWCNTs present in the sample. On the other side, the absence of the sc-SWCNT peaks is attributed to the diameter of the tubes being out of resonance since only tubes with small diameters (

Confirming the Dual Role of Etchants during the ... - ACS Publications

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