In Situ Fabrication of Nano Transistors by Selective ... - ACS Publications

Oct 14, 2015 - Department of Chemistry, Physics, and Engineering Studies, Chicago State University, 9501 South King Drive, Chicago, Illinois. 60628, U...
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In Situ Fabrication of Nano Transistors by Selective Deposition of a Gate Dielectric around Carbon Nanotubes Jae-Hyeok Lee,*,†,⊥ Young K. Jeong,‡,⊥ John A. Peters,§ Gwang-Hyeon Nam,∥ Sunghwan Jin,† and Jae-Ho Kim*,∥ †

Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, United States ‡ Non-Ferrous Materials Group, Korea Institute of Industrial Technology (KITECH), 137-41 Gwahakdanji-ro, Gangneung-si 25440, Republic of Korea § Department of Chemistry, Physics, and Engineering Studies, Chicago State University, 9501 South King Drive, Chicago, Illinois 60628, United States ∥ Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea S Supporting Information *

ABSTRACT: The CNT-SiO2 core−shell structure is particularly appealing because the insulating SiO2 layer wraps around the CNTs, functioning as a gate dielectric. However, it is still a challenge to expose both end-caps of the structure for enabling them to serve as electrodes, which additionally requires complicated postprocesses. Here, we present a unique CNTs-SiO2 core−shell structure where both ends are uncovered with SiO2 in a “peeled-wire” structure. In this structure, SiO2 particles partially encapsulate the CNTs during the synthesis, resulting in both end-caps of the nanotube being self-exposed and electrically conductive. The field-effect transistor build-up with this structure exhibits p-type characteristics with a linear conductance behavior on Id−Vd output performance. This approach for making self-formed electrodes in the CNT-SiO2 core−shell structure provides a simple and efficient way for applying them to future nanodevices in terms of process simplicity and cost effectiveness. KEYWORDS: carbon nanotube, core−shell structure, nanostructures, dielectric layer, self-exposed end-caps, field-effect transistor

1. INTRODUCTION Over the past decade, there has been a resurgence of interest in the understanding and application of carbon nanotubes (CNTs) due to their fascinating physicochemical properties.1−4 CNTs are comprised of single atomic sheets of graphene, and these sheets are rolled at specific and discrete angles. Depending on the combination of the rolling angle (chirality), or the way the graphene sheet is wrapped, CNTs can acquire armchair, zigzag, or chiral structure.5 The distinct structure strongly affects its electrical state and results in a metallic, semiconducting, or semimetallic behavior.6,7 These unique characteristics consequently triggered extensive research on a vast range of applications using CNTs. These applications include single molecular electronics, chemical sensors, hydrogen storage, and field emission devices. For CNTs to be incorporated into various functional devices, surface functionalization has become a critical requirement for modifying the surface structure of CNTs.8−13 By functionalizing the CNT surface, the poor dispersibility of CNTs can be overcome and their solubility and processability improved. In other words, chemical functionalization allows the unique © 2015 American Chemical Society

properties of the CNTs to be effectively coupled to those of other types of materials, thereby making them suitable for device applications. Among the various functional groups, an inorganic silica (SiO2) coating onto CNTs has proven to be effective in dispersing the nanotubes into many solvents.14−19 Furthermore, due to the well-known insulating properties and biocompatibility of SiO2, it is particularly appealing for applications in molecular electronics and biomedicine. There have been several reports regarding the functionalization of CNTs using silica. However, most of the research has focused on multiwalled carbon nanotubes (MWCNTs) rather than single-walled carbon nanotubes (SWCNTs) due to the low solubility of individual SWCNTs in solvents.14−16 Moreover, the many defects that exist on the surface of MWCNTs provide easier ways of functionalization.18 Nevertheless, there has been some success in coating SiO2 onto SWCNTs. For example, Naik et al. reported on using a peptide-mediated SiO2 Received: August 3, 2015 Accepted: October 14, 2015 Published: October 14, 2015 24094

DOI: 10.1021/acsami.5b07137 ACS Appl. Mater. Interfaces 2015, 7, 24094−24102

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ACS Applied Materials & Interfaces coating to form SWNT composites.20 In addition, Wong and others have developed several methods of coating SiO2 onto SWCNTs in covalent21 and noncovalent ways.18 More recently, Liu et al. demonstrated a technique to coat mesoporous SiO2 onto both SWCNTs and MWCNTs by employing cetyltrimethylammonium bromide (CTAB) to increase the surface area of SiO2.19 Despite the research done on the functionalization of SWCNTs using SiO2, several outstanding issues remain. For example, it is still a challenge to develop new ways to uniformly coat the SWCNTs with the silica functional groups in noncovalent ways. The covalent bond between the SWCNTs and the functional groups inevitably causes degradation of the mechanical and electrical properties of the SWCNTs due to the sp3-rehybridized defects.9 In contrast, the noncovalent method is mainly based on multiple weak interactions, such as van der Waals, π−π, and electrostatic interactions.19 These interactions result in nondestructive bonds between the SWCNTs and the functional groups, thereby preserving their nature. In addition to the challenge of coating CNTs with SiO2 functional groups in a noncovalent way, the complete encapsulation of SWCNTs (or MWCNTs) by SiO2 (in a core−shell structure) makes it difficult to use these nanotubes in device applications. For any functional electronic devices, it is necessary to electrically connect at least two electrodes to individual molecules, depending on the device type. Therefore, to utilize the insulating core−shell structures in functional electronics, several postprocesses are additionally required to expose both end-caps of the CNTs and to use them as electrodes.17 Here, we suggest a new approach for reducing the additional fabrication processes by adapting the self-exposed source/drain electrodes for making nanoelectronic devices. We explore a unique CNTs-SiO2 core−shell structure where both ends of the CNTs are uncovered in a “peeled-wire” structure. In this structure, contrary to previous reports, SiO2 particles partially encapsulate the CNTs during the synthesis resulting in both end-caps of the nanotube being exposed and electrically conductive. The as-exposed end-caps make them naturally serve as individualized electrodes that are readily applicable in field-effect transistor (FET) devices without further postprocessing. Moreover, we have been able to prevent unwanted physicochemical destruction of the CNTs by employing SWCNT bundles, which are composed of a few tens of SWCNTs. The outer layer in the bundle is primarily involved in covalent functionalization with SiO2. As a result, the inner SWCNTs inside the bundle can maintain their original properties by preserving the hexagonal network structure of the sp2-hybridized carbon bond.

Triton X-100 surfactant (3 wt %) by sonic agitation for 1 h. After sonication, the SWCNTs solution was centrifuged at 6000 rpm for 1 h. The supernatant was then carefully decanted. This SWCNT solution was filtered through a 200 nm pore AAO membrane and washed with methanol at least three times for the removal of surfactants. The cake of SWCNTs was then redispersed in 500 mL of chloroform or ethanol by sonication for 30 min. The resulting SWCNTs were found to form stable colloidal suspensions in chloroform or ethanol, which contained uniformly sized SWCNT bundles with an average diameter of 40.0 ± 5.0 nm and length of 1.5 ± 0.2 μm, respectively [Figure S1, Supporting Information (SI)]. 2.2. Preparation of the “Peeled-Wire” CNTs-SiO2 Core−Shell Structure. For the “peeled-wire” CNTs-SiO2 core−shell structure to be made, 500 μL of tetraethylorthosilicate (TEOS, 98%) solution was mixed with 2 mL of NH3·H2O (28%), 1.7 mL of H2O, 5 mL of ethanol, and 40 mL of the previously obtained carboxylated SWCNT solution in chloroform under magnetic stirring. Hydrolysis and condensation reactions in the solution were completed at room temperature (24 °C) within 12 h. The precipitates were collected by centrifugation, washed with distilled water and ethanol three times, and redispersed in 100 mL of ethanol. 2.3. Immobilization of the “Peeled-Wire” CNTs-SiO2 Core− Shell Structure on an Amine-Functionalized Substrate for FET Measurements. To attach and increase the absorption yield of the “peeled-wire” CNTs-SiO2 core−shell structure, we used an amide bonding chemistry, which is carboxylated-SWCNTs against an aminefunctionalized substrate. Initially, 300-nm thick SiO2/p-type Si (100) wafers were cleaned in acetone, rinsed with methanol and DI water, and then chemically hydroxylated by a UV/ozone treatment for 10 min (UVO cleaner model 42−220, Jelight Co.). For silane modification, the hydroxylated silicon wafer was immersed in a 1.0% ethanolic solution of 3-(aminopropyl)triethoxysilane (APTES) for 1 h, rinsed with ethanol and DI water, and dried in a flow of purified N2 gas. As a result, APTES-modified silicon substrate was prepared. Subsequently, 100 μL of the “peeled-wire” CNTs-SiO2 core−shell structure solution was dropped on the APTES-modified silicon substrate with 1-ethyl-3-(3-dimethylamino)propyl carbodiimide (EDC)/N-hydroxysuccinimide (NHS) as coupling agents, carboxylates (−COOH) from the self-exposed bundle nanotubes (in our “peeled-wire” CNTs-SiO2 core−shell structure), and primary amines (−NH2) from APTES-modified substrate to form covalent linkages (Figure S2). The current−voltage characteristics of our CNT-SiO2 structure were measured using a Keithly 2612 semiconductor parameter analyzer. Eq 1 was used to calculate carrier mobility. μeff =

Lgd WCoxVDS

(1)

where μeff is the field-effect mobility, L is the channel length (obtained from scanning electron microscopy (SEM) imaging), gd is the transconductance, W is the channel width (obtained from atomic force microscopy (AFM) imaging), Cox is the oxide capacitance (66 nF cm−2 for 50 nm thick SiO2), and VDS is the applied source-drain bias.

3. RESULTS AND DISCUSSION To make the “peeled-wire” CNTs-SiO2 core−shell structure, we prepared SWCNT bundles with relatively thin end-caps prior to SiO2 functionalization. We have previously shown that such bundles can be fabricated by chemical oxidation and purification processes.22 The SWCNT bundles are composed of rigid rodlike nanotube fragments with lengths and diameters in the range of 1.5 ± 0.2 μm and 40.0 ± 5.0 nm, respectively. They are separated from raw pristine-SWCNTs with an oxidation process that involves extensive ultrasonic treatment in an acid solution. This chemical process introduces various oxygen-based-functional groups (mainly carboxyl groups) onto surfaces of the nanotubes, thereby enabling a covalent functionalization with SiO2.11 The resultant SWCNT bundles are found to be rigid enough to maintain a rodlike shape

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. For rigid rod-shaped and stably dispersed SWCNT bundles to be made, chemical functionalization with carboxyl groups (COOH−) was oxidatively introduced to the bundle surfaces. In a typical experiment, 20 mg of SWCNTs (arc discharge, ASP-100F, Hanwha Nanotech Co., Korea) were shortened and carboxylated by chemical oxidation in a mixture of concentrated sulfuric and nitric acid (3:1 v/v, 98% and 70%) in a bath sonicator (Cole-Parmer Instrument Co., Vernon Hills, IL, USA; frequency 55 kHz) at 70 °C for 4 h. The acid treatment was performed in a hood. The reaction mixture was filtered through an alumina oxide (AAO) nanoporous membrane (WhatmanTM; GE Healthcare Life Sciences, Little Chalfont, Buckinghamshire, UK; 200 nm pore size) and washed with deionized (DI) water until the filtrate reached pH 7.0. The resulting filtered SWCNTs were dispersed into 250 mL of aqueous 24095

DOI: 10.1021/acsami.5b07137 ACS Appl. Mater. Interfaces 2015, 7, 24094−24102

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Figure 1. (a) Schematic drawing of the “peeled-wire”-like CNT-SiO2 core−shell structure. (b) SEM images of the as-made core−shell structures that are composed of (top) a single bundle (false-colored) and a double bundle grouped together (bottom). In these structures, both bundle ends appear to be self-exposed during the synthesis, whereas the rest of the body is functionalized with SiO2 nanoparticles. (c) AFM image of the “peeled-wire”like CNT-SiO2 core−shell structure. RMS values were obtained at four different regions in a single rod.

structure appear to be uncovered and without SiO2, whereas the rest of the body is partially encapsulated with SiO2 nanoparticles. In other words, this unique structure is designed to have self-exposed electrodes without further postprocesses such as lithography or etch techniques. Figure 1(c) is a topographic image measured with AFM showing that the unique core−shell structure has both ends that are thin and uncovered. We measured the surface roughness of the coatedSiO2 shell. By using Gwyddion software, RMS (root-meansquare) values were obtained at four different regions (A−D, scanned area is 100 nm2) in the single CNT-SiO2 core−shell structure. The coated-SiO2 shell has a surface roughness in the range of 3.4−6.6 nm and an average value of ∼5.2 nm. Considering that it is essential to build source and drain contacts for utilizing CNTs in many FET devices, the selfexposed electrodes in this core−shell structure are particularly attractive in terms of process simplicity and cost effectiveness. For a detailed characterization of the morphological structure of the “peeled-wire” CNTs-SiO2, transmission electron microscopy (TEM) measurements were carried out. Figure 2 displays the TEM images of the samples taken at different

without bending the structure (Figure S1). The stiffness of the bundle structure allows the potential use of rodlike SWCNT bundles in many device applications. The key factor in making these rigid rodlike SWCNT bundles lies in choosing an appropriate process time for the oxidation treatment. A long oxidation process results in the final SWCNT bundle being cracked or individually separated into single nanotubes, leading to the possibility of severe damage to each tube wall during chemical functionalization. After the preparation of SWCNT bundles, the resultant rodlike structures are then transferred into a chloroform solvent, and a mixture of tetraethylorthosilicate (TEOS) and ethanol (EtOH) is subsequently injected into the solvent for synthesis of the CNTs-SiO2 core−shell structure. Figure 1(a) schematically illustrates how the SiO2 is coated onto the SWCNT bundle in the “peeled-wire” core− shell structure. Figure 1(b) shows SEM images of as-made core−shell samples consisting of a single bundle and a double bundle that are grouped together. We observe that the encapsulating layer is composed of a network of SiO 2 nanoparticles uniformly coated onto the surfaces of the SWCNT bundles. Interestingly, both ends of this core−shell 24096

DOI: 10.1021/acsami.5b07137 ACS Appl. Mater. Interfaces 2015, 7, 24094−24102

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Figure 2. (a) TEM image of the “peeled-wire”-like CNTs-SiO2 core−shell structure showing a cylinder-shaped SiO2 coating. Note that the SiO2 layer uniformly wraps around the SWCNT bundle. (b) TEM image taken at the interface between the SiO2 layer and the bundle end, providing clear evidence of the self-exposed bundle end. (c) Magnified TEM image of the yellow-dotted circle in (b). (d) Magnified TEM image of the white-dotted square in (c). The exposed bundle end at the interface consists of a few tens of single carbon nanotubes with a well-aligned layered structure.

magnifications. As shown in Figure 2(a), the SiO2 shell is shaped like a long cylinder wrapping the inner SWCNT bundle and its thickness is uniform over the shell. The boundary between the inner core of SWCNT bundle and the outer shell of SiO2 is clearly shown in the marked inset of Figure 2(a). The result clearly describes that the SWCNT bundle is covered with a thin SiO2 layer. In most commercial semiconductor devices, achieving a uniform gate dielectric thickness is a prerequisite for improving gate controllability. Thus, the observed structural features of the SiO2 shell offer a great advantage in fabricating a homogeneous gate dielectric with uniform thickness. Figure 2(b) shows a magnified TEM image at the interface between the SiO2 layer and the bundle end, providing clear evidence for the formation of the “peeled-wire” CNTs-SiO2 core−shell structure. From the image, we observe that the SWCNT bundle end is uncovered and without SiO2 coating, implying that the exposed ends of the bundle can be used as electrodes. It is worth noting that the unique “peeled-wire” structure is selfformed during the liquid synthesis without further postprocessing, indicating a new way to simply incorporate source/drain electrodes into CNT-based FETs. Figure 2(c) and (d) show magnified TEM images of the yellow-dotted circle in (b) and the white-dotted square in (c), respectively. From these images,

it is clearly observed that the SWCNT bundles are composed of a few tens of single carbon nanotubes with a well-aligned layered structure. Furthermore, the carbon nanotubes appear to be intact, indicating that they were not destroyed during the synthesis. The question remains as to the nature and origin of such a unique core−shell structure with exposed end-caps. To answer this question, we focus on the role of the chloroform that was used as a cosolvent in the synthesis. As shown in Figure 3(a), in the absence of chloroform, the resulting SWCNT bundles are completely encapsulated with SiO2 nanoparticles in contrast to the bundles treated with chloroform [Figure 3(b)]. This observation indicates that chloroform is crucial for achieving the self-exposed electrodes in the CNT-SiO2 core−shell structure. The schematic illustration in Figure 3(c) is shown to explain the possible scenario regarding the role of chloroform. The SWCNT bundles are uniformly dispersed, forming a stable colloidal suspension in the chloroform solvent. When the mixture of TEOS and EtOH is injected into the chloroform solvent, the polar EtOH is strongly attracted to the polar functional groups on the surface of the SWCNT bundles via hydrogen bonding between COOH- and OH-. Because the SWCNT bundles have needle-shaped ends, a large amount of 24097

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Figure 3. CNT-SiO2 core−shell structures synthesized in the (a) absence and (b) presence of chloroform. SWCNT bundles treated without chloroform are completely encapsulated by SiO2 nanoparticles in contrast to those treated with chloroform. (c) Schematic illustration explaining the chemical mechanism for achieving self-exposed bundle ends.

Figure 4. Raman spectra of (a) completely encapsulated and (b) “peeled-wire” CNTs-SiO2 core−shell structures. Note that typical Raman peaks of SWCNTs were detected with the disorder induced D-band (1341 cm−1) and tangential G-band (1584 cm−1) only in the “peeled-wire” structure.

SWCNT bundle. This mechanism facilitates the heterogeneous nucleation of SiO2 mostly on the central part of the bundle. Simultaneously, chloroform inhibits TEOS from spreading to the end-caps, preventing the nucleation of SiO2 in the vicinity of the bundle ends. To ensure that the bundle ends are truly exposed and conducting, we employ a micro-Raman spectroscopy (632 nm He:Ne laser, inVia Raman microscope, Renishaw) analysis and conductive atomic force microscopy (C-AFM) to measure the intrinsic properties and the electrical conductance of the “peeled-wire” CNTs-SiO2 core−shell structure. For the

functional groups are expected to attach to the central part of the structure than at the tapered ends due to the relatively larger surface area. This distinctive geometric structure renders the polar EtOH more accessible to the central part of the SWCNT bundle. Conversely, chloroform favors a location near the vicinity of the bundle ends owing to the relative lack of affinity to the polar functional groups. The difference in polarity between chloroform and EtOH leads to a nonhomogeneous mixture, forming two different solvent domains, as depicted in the schematic. TEOS tends to diffuse into EtOH and can therefore easily migrate to the surface of the central part of the 24098

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ACS Applied Materials & Interfaces SWCNT bundles completely encapsulated with SiO2, no Raman peak was observed, as shown in Figure 4(a). However, the “peeled-wire” CNTs-SiO2 structure shows a characteristic Raman pattern with D and G bands at 1341 and 1584 cm−1, respectively. Interestingly, the G band appears to be divided into two peaks due to G+ band split, implying that the exposed bundle ends exhibit semiconductor characteristics of SWCNTs.23 C-AFM is known to be an informal tool to electrically construct a local surface profile in terms of electric current. Figure 5(a) shows a schematic drawing of the C-AFM measurement (top) and the SEM image associated with sample preparation. For the C-AFM experiment, the “peeled-wire” CNTs-SiO2 core−shell rods were placed onto a 300-nm thick SiO2/Si substrate (Figure S2). A gold (Au) layer was then locally deposited on the randomly placed rods by employing a photolithography process. Note that the gold layer is deposited on one rod end to act as an electrical contact, whereas the other contact is provided by a conductive AFM tip in contact with the sample (please note that the red-dotted squares in Figure 5(a) indicate the scanning region with C-AFM). Figure 5(b) and (c) show topographic and current images taken from the same area of the sample. These figures allow a correlation between the two images, as well as the identification of conducting or insulating features. The topographic image shows that the CNTs-SiO2 core−shell structures consist of thin ends and thick body parts, as shown in SEM and TEM images. Interestingly, Figure 5(c) exhibits clear line features and these bright lines overlap with the SWCNT bundle ends observed in the topography of Figure 5(b) at the same location (indicated by yellow arrows). By correlating the topographic and current images at the same location, we can conclude that the CNTsSiO2 core−shell structures truly have the “peeled-wire” structure and that the exposed bundle ends are conducting whereas SiO2 shells wrapping the SWCNT bundles are insulating. The inset of Figure 5(c) shows local current− voltage (I−V) characteristics measured at two different areas: (i) at bundle end and (ii) the SiO2 shell, respectively. The I−V curve measured at the bundle end shows a current flow, whereas there is no electric conductance at the SiO2 shell, consistent with the results of the C-AFM map. Though the C-AFM results show that the bundle ends are entirely uncovered and without SiO2 and thus conducting, the associated transport properties of the SWCNT bundles remain unknown. To investigate the electronic transport properties of the bundle, we fabricated a CNT-based FET (CNT/FET) with the “peeled-wire” CNTs-SiO2 core−shell structure that is composed of a single bundle, as shown in Figure 6(a) and Figure S2. In CNT/FETs, a Schottky barrier built at nanotube−metal junctions has been a primary factor that hampers the device performance. It severely limits hole transport through the valence band of nanotubes and reduces the current delivery capability. With the noble metals that have a high work function, however, the potential barrier can be effectively eliminated to obtain high-performance devices.24 Accordingly, we adapted Pt having a high work function of 5.7 eV in making nanotube−metal junctions. Figure 6(b) is an SEM image that shows the Pt-contacted source/drain contacts in our CNT/FET device. Note that the Pt gate contact was also deposited on top of the SiO2 shell to be used as a gate dielectric. Using the focused ion beam (FIB) lithograpy technique, three metal pads were additionally fabricated for electrical connection to these contacted source/drains, and gate

Figure 5. (a) Schematic diagram of the experimental setup for C-AFM measurements (top), and SEM image showing how the Au electrode was prepared at one side of the CNTs-SiO2 core−shell structure (reddotted squares indicate the scanning region with C-AFM). (b) Topographic and (c) C-AFM images taken at the same area in the sample. The C-AFM image clearly shows bright line features on the self-exposed bundle ends (marked with yellow arrows), whereas negligible features are observed on SiO2 shell regions. Insets in (c) exhibit local current−voltage (I−V) characteristics measured at the self-exposed bundle end (area A) and SiO2 shell (area B), respectively. 24099

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Figure 6. SEM image of a “peeled-wire” CNTs-SiO2 core−shell structure (a) before and (b) after the CNT/FET fabrication. Pt-contacted gate and source/drain electrodes are formed on top of the SiO2 shell and at the self-exposed bundle ends, respectively. The CNT/FET device has a channel length of 200 nm and dielectric thickness of 50 nm. (c) Id−Vd curves under various gate voltages showing that the CNT/FET is a p-type semiconductor. Insets in (c) exhibit schematic illustration of the top-gated CNT/FET device. (d) Transfer curve for a “peeled-wire” CNTs-SiO2 core−shell structure FET. The drain current is indicated as a function of gate voltage on a linear scale.

interacted on the outer surface of CNT.26 Finally, the charge screening effects could be a factor to reduce the current flow because our densely packed SWCNT bundles are comprised of metallic and/or semiconducting nanotubes.27 Overall, we conclude that the SWCNT bundles are able to maintain their transport properties and can survive under harsh synthesis conditions. This is possible because the outer nanotube layer of the bundle serves as a protection layer for the inner nanotubes, enabling them to preserve their original semiconducting properties without sp3-hybridized deformation. Among further studies required for the high hole mobility and on/off ratio of CNT/FET structures, we suggest that the use of the highly enriched semiconducting SWCNTs sorted via density-gradient ultracentrifugation (DGU)28 may help enhance the FET property.

electrodes are formed in the CNT/FET structure. This device has a channel length of 200 nm and a dielectric thickness of 50 nm. We have measured the gate response of the current sourcedrain electrodes to determine whether the CNT/FET structure exhibits a semiconducting or metallic behavior. From the Id−Vd curve under various gate voltages in Figure 6(c), the Ptcontacted CNT/FET exhibits highly linear conductance at low Vd due to the reduced height of the Schottky barrier. As the gate voltages moved toward a negative direction, the asymmetric dependence of the conductance on the Vgate polarity indicates that our CNT/FET structure is semiconducting at room temperature (24 °C). In Figure 6(d), the transfer and output curve is shown for our CNT/FET, having a hole mobility of 0.064 cm2/(V s) and Ion/Ioff ratio of ∼2.4 (applied bias voltage = 0.2 V). From the results of Figure 6(c) and (d), we know that our CNT/FET device shows the gate field dependence and behaves like a p-type semiconductor. This device showed little gate field dependence due to high density of SWCNT. This result agrees to the findings of E. S. Snow.25 The electrical transport property of the CNT-FET device is closely related to the SWCNT bundles. For a direct comparison, electrical properties (i.e., on/off ratio and hole mobility) of our CNT/FET devices are shown in Table S1 with the results from the other research groups. There are the possible reasons for the low on/off ratio of the measured CNT/ FET devices. First, the low current on/off ratio is mainly induced from the small area of the gate electrode deposited onto the SiO2 shell, as shown in Figure 6(b). Second, the low current flow might occur from a strong interaction with the SWCNT bundle and SiO2 dielectric layer in the CNT-SiO2 core−shell structure. It has been well-demonstrated that the sol−gel process of the APTES or TEOS can be strongly

4. CONCLUSIONS In conclusion, we have presented a unique CNT-SiO2 core− shell structure where both ends are uncovered and without the SiO2 functionalization in a “peeled-wire” structure. The exposed bundle ends in this structure are designed to be self-formed during the synthesis, making them readily applicable as source/ drain electrodes. Evidence of the “peeled-wire” structure was given using SEM and TEM imaging. Electrical characterization of the SWCNT bundles in the core−shell structure indicated that the transport properties of the CNTs in the bundles remain unchanged. Considering the process simplicity and cost effectiveness, the current approach for making self-formed electrodes in the CNT-SiO2 core−shell structure will provide a simpler and more efficient approach to future nanodevice applications. Our synthetic method can also be expanded to fabricate various kinds of nanoelectronic devices,29−33 such as 24100

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Research Article

ACS Applied Materials & Interfaces

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heterostructured core−shell nanowires, nanosolar cells, and nano-piezo sensors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07137. (SI-1) SEM and AFM images of the rodlike bare SWCNT bundles; (SI-2) immobilization of the “peeledwire” CNTs-SiO2 core−shell structure on APTEStreated substrates for the CNT/FET device; and (Table S1) device performance of various CNT/FET (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ⊥

J.-H.L. and Y.K.J. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-20090093826). In addition, we thank the BK21 program of molecular science and technology at Ajou University.

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ABBREVIATIONS CNTs, carbon nanotubes SWCNT, single-walled carbon nanotube MWCNT, multiwalled carbon nanotube TEOS, tetraethylorthosilicate FET, field-effect transistor SEM, scanning electron microscopy TEM, transmission electron microscopy AFM, atomic force microscopy

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REFERENCES

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DOI: 10.1021/acsami.5b07137 ACS Appl. Mater. Interfaces 2015, 7, 24094−24102

Research Article

ACS Applied Materials & Interfaces Concepts for Nano-enabled Solar Cells. Energy Environ. Sci. 2013, 6, 719−733. (32) Kempa, T.; Lieber, C. M. Semiconductor Nanowire Solar Cells: Synthetic Advances and Tunable Properties. Pure Appl. Chem. 2014, 86, 13−26. (33) Kang, J.; Roh, J. W.; Shim, W.; Ham, J.; Noh, J.-S.; Lee, W. Reduction of Lattice Thermal Conductivity in Single Bi-Te Core/Shell Nanowires with Rough Interface. Adv. Mater. 2011, 23, 3414−3419.

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DOI: 10.1021/acsami.5b07137 ACS Appl. Mater. Interfaces 2015, 7, 24094−24102