LETTER pubs.acs.org/JPCL
Gold-Substrate-Enhanced Scanning Electron Microscopy of Functionalized Single-Wall Carbon Nanotubes Yin Zhang†,‡ and YuHuang Wang*,†,§ †
Department of Chemistry and Biochemistry and §Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, United States ‡ Department of Physics, Xi’an JiaoTong University, Xi’an, China
bS Supporting Information ABSTRACT: Functionalized regions of a single-wall carbon nanotube were resolved by scanning electron microscopy at 1 kV when the functionalized nanotube was placed on a gold substrate. Beam energy and substrate dependence studies suggest that the sharp imaging contrast arises from an increase in the yield of secondary electrons as compared to gold due to covalent modification of the nanotube. Using this surprisingly simple technique, it becomes possible to rapidly map surface functionalization on individual carbon nanotubes with a spatial resolution better than 10 nm. This new functionalization imaging technique may facilitate spatial control of surface chemistry and defect engineering in carbon nanomaterials. SECTION: Nanoparticles and Nanostructures
C
hemical functionalization is widely used to tailor the materials properties of carbon nanostructures, including carbon nanotubes and graphene.111 Both the electrical and optical properties of a carbon nanotube sensitively depend on the degree of functionalization or defect density.1,4,5 Controlled covalent modification can tune the optical properties5 and significantly improve nanotube solubility,68 mechanical strength,9 and materials interfaces in composites.10,11 Recent theoretical works further predict that some of these properties are substantially affected by the spatial distribution of the defects or functional groups.12,13 However, controlling and imaging functionalization at the nanometer scale remains experimentally challenging. Transmission electron microscopy (TEM) and scanning probe methods such as scanning tunneling microscopy (STM) represent two of the high-resolution techniques that have been used to study functionalized carbon nanomaterials.14,15 The functional groups often give low TEM contrast over a carbon background and may cause beam damage, while STM typically requires electrically conductive samples that are strongly adhered to the substrate to prevent sample stability issues.14 Additionally, these processes are time-consuming and low-throughput. Despite a lower spatial resolution compared to STM and TEM techniques, scanning electron microscopy (SEM) techniques are widely used to characterize carbon nanomaterials.1618 Sample preparation is convenient for SEM, and the large dynamic scale of imaging makes it possible to provide a rapid overall view of the specimen. Brintlinger et al.16 show that it is even possible to image smalldiameter carbon nanotubes on oxidized silicon substrates based on differential charging of the conducting nanotubes and insulating substrate. However, these SEM techniques are insensitive to surface defects and covalent modifications.16 r 2011 American Chemical Society
Herein, we report a gold-substrate-enhanced SEM imaging technique for locating and identifying clusters of functional groups that were covalently attached on individual single-wall carbon nanotubes (SWNTs). Sharp imaging contrasts among the gold substrate and the functionalized and nonfunctionalized regions of a single nanotube are easily attainable at low beam energies (∼1 kV). We attribute the image contrast to a substrateenhanced mechanism involving local variation in the yield of secondary electrons of different materials. HiPco-SWNTs were covalently functionalized with (CH2)5 COONa using an alkylcarboxylation method, as we have reported previously.7 This chemistry allowed us to create functional bands along the length of a nanotube by a reaction propagation mechanism, which we will report separately in detail. High degrees of covalent functionalization were confirmed by Raman spectroscopy and other methods.7 Figure 1a shows a representative SEM image of individual, (CH2)5COONa functionalized SWNTs on a gold substrate using a beam energy of 1 kV. The most striking feature was the alternating pattern of bright and dark segments along the nanotube length with a pitch of approximately 200 nm. By contrast, nonfunctionalized SWNTs imaged under the same conditions showed a dark contrast along the entire length, with no evidence of the bright/dark alternation pattern (Figure 1b). In a second control, circular dot arrays of HS(CH2)17COOH or HS(CH2)17COONa self-assembled monolayers (SAMs) formed on a gold substrate by microcontact printing19 exhibited a similarly bright contrast at the same beam energy. From these experiments, Received: February 26, 2011 Accepted: March 28, 2011 Published: March 30, 2011 885
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we can unambiguously assign the dark regions in Figure 1a to intact nanotubes (nonfunctionalized) and the bright regions to the alkylcarboxylated portions of the nanotube surface. The observed contrast was strongly dependent on the beam energy. For images taken at higher beam energies (Figure 2), the alternating contrast gradually decreased and almost completely disappeared at 2 kV, at which point, the nanotubes, both
functionalized and nonfunctionalized, became darker compared to the gold substrate. The difference between the functionalized and pristine nanotube segments was indistinguishable at this condition. The sharp contrast observed here is opposite to those from SiO2/Si substrates (Supporting Information, Figures S1 and S2). We observed much lower and completely different contrast on SiO2/Si substrates, where raw SWNTs are brighter rather than being darker on gold at 1 kV, consistent with previous SEM studies of SWNTs using SiO2/Si as the substrate.1618 The image contrast of SWNTs on SiO2/Si substrates is attributed to electron-beaminduced current18 or surface potential difference.16,17 Both mechanisms invoke surface charging of the insulating SiO2/Si substrate when scanned by the electron beam. In the current experiment, the gold substrate is electrically conductive; therefore, the charging effect, if any, is not a dominating mechanism. We attribute the observed contrast in Figure 1 to a material contrast mechanism due to local variations in the yield of secondary electrons. This hypothesis is confirmed by substrate and beam energy dependence studies, as described below. The functionalized nanotube segments and pristine regions as well as
Figure 1. Secondary electron SEM images of SWNTs, (a) functionalized with bands of (CH2)5COONa and (b) nonfunctionalized on a gold substrate. The images were taken using a beam energy of 1 kV.
Figure 2. Image contrasts depend strongly on beam energy. 886
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others20 that an insulator typically has a higher δSE than a metal or semiconductor at low beam energies. As the beam energy increases above 2 kV, the imaging contrast between functionalized and intact nanotube segments is lost because the secondary electrons from the gold substrate become overwhelming. This materials dependence makes it possible to distinguish different functional groups on carbon nanotubes (Table 1 and Supporting Information, Figures S3 and S4). All three functional groups, (CH2)5COONa, (CH2)5NH2, and (CH2)5CH3, that were investigated showed brighter contrasts relative to gold at 1 kV. However, the contrast of (CH2)5CH3 was not as sharp as other functional groups, and above 1 kV, the contrast reversed. The lower contrast of (CH2)5CH3 is correlated with its lower δSE among the three functional groups. The δSE of (CH2)5COONa and (CH2)5NH2 peaked at approximately 0.6 kV with about 1.5 times more yield than gold at 1 kV. This contrast was reversed at approximately 1.5 kV. Finally, on the basis of known δSE data for metals,21 gold seems uniquely suited for simultaneous resolving of the alkylcaboxylated and nonfunctionalized regions. However, the different δSE and beam energy dependence among different functional groups (Figures S3 and S4, Supporting Information) suggests the possibility of choosing a metal substrate to enhance the imaging contrast for a particular functional group. The measured δSEEPE trend of nonfunctionalized SWNTs is remarkably similar to that of graphite,21 suggesting the extension of this substrate-enhanced imaging method to other types of carbon nanomaterials, including multiwall carbon nanotubes and graphene. We will verify this hypothesis in follow-up experiments.
Figure 3. The dependence of secondary electron yields on beam energy and materials. The optimal image contrast was observed at approximately 1 kV, where the differences in δSE among the three materials are the highest. The data for gold is adapted from ref 21.
Table 1. The Relative Maximum Secondary Electron Yields m δm SE and the Corresponding Beam Energies EPE from Fitting Experimental Data to Equation 1 δm SE Em PE
SWNT
Au
COONa
NH2
CH3
0.70
1.28
1.81
2.13
1.43
0.38
0.68
0.67
0.74
0.64
the gold substrate are effectively three different materials. It is experimentally confirmed that the yield of secondary electrons (δSE) is dependent on both materials and the primary electron beam energy (EPE). The δSEEPE function has a similar shape for all materials; δSE first increases with EPE, reaches a maximum, and then drops rapidly.20 This trend can be described by a semiempirical equation a b EPE 2:3ðEPE =EmPE Þ 1e ð1Þ δmSE δSE ¼ 1:11 m EPE where a and b are material-dependent constants. We measured the δSE as a function of beam energy for all three materials (Figure 3). The experiments were perfermed by measuring the image contrast or secondary electron current relative to gold. The data were then rescaled by δSE of gold at the corresponding beam energy, which is known in the literature.21 The curves clearly show a different maximum δSE for each material, and there is a notable material dependence in both the position and relative intensity (Table 1). At low beam energies, the δSE follows (CH2)5COONa ≈ (CH2)5COOH > gold > raw nanotubes. As the beam energy increases, the δSE of functionalized nanotubes quickly drops below that of gold and then maintains at a similar level as that of pristine nanotubes. This strong beam energy dependence is consistent with a material contrast mechanism because the δSE of insulators (e.g., functionalized nanotubes) fall sharply after reaching the maximum, while those of conductors (e.g., gold) decrease much more slowly.20,21 At approximately 1 kV, the differences are the largest, giving the optimal imaging contrast as observed in Figure 1. Raw SWNTs (unfunctionalized) produce a darker contrast as compared to gold, while alkylcarboxylated nanotubes are brighter. Covalent modification dramatically reduces the electric conductivity of nanotubes,4 ultimately converting them from semiconductors or metals to insulators. Hence, the observed trends are very consistent with previous theoretical work and experimental observations by
’ EXPERIMENTAL METHODS HiPco-SWNTs were covalently functionalized with (CH2)5 COONa, (CH2)5CH3, and (CH2)5NH2 using a similar experimental procedure as detailed elsewhere.7 After reaction, the (CH2)5 COONa and (CH2)5NH2 functionalized SWNTs were dispersed in Nanopure water. The (CH2)5CH3 functionalized SWNTs were dispersed in 1,2-dichlorobenzene by brief bath sonication. SEM samples were prepared by adding a drop of each solution on a silicon, gold, carbon, or copper substrate with a glass pipet and allowed to dry in air. The gold substrate was prepared using a Metra thermal evaporator by depositing 2 nm of Cr on a silicon wafer (with a native oxide layer) followed by 15 nm of Au. The substrate was chemically cleaned in a H2O2/H2SO4 (v/v 1:3) solution at room temperature. The carbon substrates were prepared by sputter coating 50 nm of carbon on silicon. The copper substrates were prepared by e-beam deposition of 100 nm of copper on silicon. The samples were imaged using a SU-70 SEM (Hitachi) with an Everhart Thornly secondary electron detector. ’ ASSOCIATED CONTENT
bS
Supporting Information. SEM images of functionalized and nonfunctionalized SWNTs on copper, silicon, and carbon substrates and the effects of different functional groups on the secondary electron yield and beam energy dependence. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 887
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’ ACKNOWLEDGMENT This work was supported by a NSF CAREER Award (CHE1055514), a PRF Doctoral New Investigator Award, and the University of Maryland. Y.Z. acknowledges fellowship support provided by the State Scholarship Council of China. The support of the Maryland NanoCenter and shared experimental facilities support from the NSF MRSEC under Grant DMR 0520471 are also gratefully acknowledged.
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