Blown-Bubble Assembly and in Situ Fabrication of Sausage-like

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Blown Bubble Assembly and in situ Fabrication of SausageLike Graphene Nanotubes Containing Copper Nano-Blocks Shiting Wu, Long Yang, Mingchu Zou, Yanbing Yang, Mingde Du, Wenjing Xu, Liusi Yang, Ying Fang, and Anyuan Cao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01490 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016

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Blown Bubble Assembly and in situ Fabrication of Sausage-Like Graphene Nanotubes Containing Copper Nano-Blocks Shiting Wu,1 Long Yang,2 Mingchu Zou,1 Yanbing Yang,3 Mingde Du,2 Wenjing Xu,1 Liusi Yang,1 Ying Fang,2 Anyuan Cao1* 1

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P.

R. China 2

National Center for Nanoscience and Technology, 11 Beiyitiao Street, Zhongguancun, Beijing 100190, P. R.

China 3

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China

* Corresponding author: [email protected]

Abstract We use a blown bubble method to assemble Cu nanowires, and in situ fabricate graphene-based one-dimensional heterostructures, including versatile sausage-like configurations consisting of multi-layer graphene nanotubes (GNTs) filled by single or periodically arranged Cu nano-blocks (CuNBs). This is done by first assembling Cu nanowires among a polymer-based blown bubble film (BBF), and then growing graphene on the nanowire substrate using the polymer matrix as a solid carbon source by chemical vapor deposition. The formation of sausage-like GNT@CuNB nanostructures is due to partial melting and breaking of embedded Cu nanowires during graphene growth, which is uniquely related to our BBF process. We show that the GNT skin significantly slows down the oxidation process of CuNBs compared with bare Cu nanowires, while the presence of stuffed CuNBs also reduces the linear resistance along the GNTs. The large-scale assembled graphene-based heterostructures, achieved by our BBF method, may have potential applications in heterojunction electronic devices and high-stability transparent conductive electrodes. Keywords: Blown bubble assembly, graphene nanotube, Cu nano-blocks, graphene-based heterostructure, sausage-like nanostructure.

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Introduction Graphene, a two-dimensional (2D) honeycomb network of carbon atoms, has fascinating mechanical and electronic properties.1-4 Large-scale assembly and controlled synthesis of graphene-based structures are key steps toward practical applications. Microscale graphene oxide sheets have been assembled into monolithic aerogels through a solution process, with potential applications in energy and environmental fields.5-9 On the other hand, functional graphene materials can be directly synthesized by exploring the structure and dimension of substrate materials. First planar substrates (e.g. Cu and Ni foils) have been used to synthesize large-area single- to few-layer graphene films by chemical vapor deposition (CVD), which can serve as flexible transparent conductive electrodes.10-15 Later three-dimensional porous graphene foams and fishnets were obtained from metallic foams or meshes consisting of interconnected Ni or Cu wires, and then embedded into elastomers to make stretchable composites and strain sensors.16-19 Recently, there were efforts in down-sizing the growth substrate to nanoscale range such as Cu nanowires. For example, Y. Lee’s group adopted a plasma-enhanced CVD process to reduce the reaction temperature and produce Cu nanowire-graphene core-shell structures, which could resist thermal oxidation and serve as transparent electrodes for polymer solar cells.20 However, their films were simply made by spray-coating, and there still lacks an efficient method to assemble those core-shell structures into pre-defined arrangements. Many techniques have been developed to assemble nanomaterials into predefined orientations. Our group has reported a polymer-based blown bubble film (BBF) technique to align carbon nanotubes and semiconducting nanowires into parallel arrays or cross-junctions21-25, 35. Here, we

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explore a BBF system based on polymethylmethacrylate (PMMA) and Cu nanowires to assemble and fabricate graphene-based one-dimensional (1D) hererostructures, that is, sausage-like graphene nanotubes (GNTs) containing Cu nano-blocks (CuNBs). The reason to choose this combination lies in that: the PMMA can be blown into large bubbles and also act as a solid carbon source, while the Cu nanowire is an ideal (nanoscale) substrate for growing graphene by CVD. Although graphene growth on Cu nanowires has been realized by plasma-enhanced CVD operated at relatively low temperature (500 °C) to avoid melting of the nanowires20, the synthesis temperature is critical for high quality graphene. The high-temperature stability of our BBF-assembled Cu nanowires is greatly improved since they are embedded within the polymer matrix. Thus, our PMMA-Cu nanowire BBF represents a distinct system that not only allows large-scale assembly of nanowires but also facilitates subsequent synthesis of graphene and graphene-based heterostructures. Results and discussions The nanowire assembly and graphene fabrication process involves the following steps, as illustrated in Figure 1a (see Experimental Methods for details). Single-crystalline Cu nanowires with diameters of ∼150 nm and lengths of ∼30 µm were first synthesized by a hydrothermal method in large scale (Supporting Information, Figure S1).26 Then a homogeneous BBF solution was prepared from PMMA, acetone, and dispersed Cu nanowires, from which a single bubble was blown and transferred to silicon wafer (with oxide). Introduction of high concentration Cu nanowires (3.33 mg/mL) was evident from the red color solution and the tinged bubble (Figure 1b). The PMMA-Cu nanowire BBF anchored on a Si wafer showed an assembled areal density of about 1.37 × 105 nanowires per cm2 (Figure 1b). Then the BBF was subjected to thermal annealing process at 900 oC for 15 minutes, during which the PMMA matrix was evaporated leaving graphene-coated Cu 3


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nanowires at original positions with the same areal density (Figure S2a-2c). Here the annealing temperature is important for graphene formation. When the temperature is higher than 900 oC, multi-layered graphene layer is formed as the 2D band emerges in Raman spectra (Figure S5a). At 1000 oC, most Cu nanowires evaporate without forming the GNT@CuNB sausage nanostructures. Therefore, a suitable temperature of 900 oC is adopted in our process. Scanning electron microscopy (SEM) characterization reveals versatile sausage-like configurations featured by graphene nanotubes (GNTs) containing melted segments of Cu nanowires, which we call as Cu nano-blocks (CuNBs). Typically, we see white-contrast regions (GNT@CuNB) connected by black lines (GNT) at one side, in the middle, or at both ends (Figure 1c). Close view shows that the GNTs are partially filled by CuNBs, and the empty portion lacking any support becomes a collapsed tube with longitudinal wrinkles (Figure 1d). The formation of partially filled GNTs is due to the partial melting and breaking of embedded Cu nanowires during graphene growth, as evidenced from the shrinkage of CuNBs (more than 25% length reduction, Figure 1c) and increase of diameters (from ~150 nm in original nanowires to ~200 nm in the resulting nano-blocks, Figure S3). Even the nanowires has broken apart, they are still connected by the GNT skin outside. Depending on the melting position and shrinking direction, versatile sausage-like structures are observed including GNTs with single CuNB in the middle or one side, and GNTs encapsulating multiple CuNBs in periodic arrangement (Figure 1c). In addition, by transferring double-layer BBFs with perpendicular nanowires (Figure S2d) for thermal annealing process, we obtain crossed sausages with different contact positions (filled or empty portion). Three types of junctions exist which include 1) GNT-CuNB portions crossed with each other, 2) empty GNTs in contact with each other, and 3) CuNB-stuffed portion overlapped with empty GNT. (Figure 1e) Since the polymer 4


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matrix has been removed during thermal annealing process, the resulting sausages in the top and bottom layers come to direct contact. This provides a route to form continuous two-dimensional networks through a layer-by-layer BBF stacking process to increase the nanowire density and inter-wire contacts. The resulting 2-layer and 4-layer GNT-CuNB networks remain highly transparent, with optical transmittances of 98.395%, 97.598% at 550 nm, respectively (Figure S4). Comprehensive characterization on those sausage-like nanostructures has been carried out by SEM, transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Raman spectroscopy, as summarized in Figure 2. As seen from SEM, the inside CuNB is wrapped by an external conformal graphene layer. The GNT grown on the nanowire surface is very thin and cannot sustain itself as a freestanding tube. Once the nanowire shrinks, the remaining empty GNT portion collapses at the end of CuNBs, resulting in a nearly flattened, wrinkled hollow tube with a diameter of about 130 nm (Figure 2a). TEM image on the surface of CuNB reveals a graphene coating with a uniform thickness of ~2 nm (Figure 2b), and the high resolution TEM image of the hollow GNT with the inner CuNBs dissolved (Figure S5b) reveals 6 to 7 graphene layers. From the corresponding SAED results, in addition to the pattern of single-crystalline Cu along the axis, discrete diffraction spots stemming from the graphitic carbon shell are also observed (Figure 2c), and the electron diffraction spots of an individual hollow GNT without CuNBs are shown in Figure S5c. We also performed elemental mapping on both stuffed and empty portion in a sausage (Figure 2d). The signal of Cu-L is confined within the nano-block region whereas the signal of carbon spread along the length direction, which confirms the core-shell structure in the stuffed region and the empty GNT next to it. XRD result shows sharp peaks from crystalline faces in the encapsulated CuNB (same peaks as original Cu 5


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nanowires) and a relatively broad band belonging to the carbon layer (at 21.84°) (Figure 2e). The graphene skin grown on Cu nanowire is also characterized by Raman spectrum measured at an excitation wavelength of 514 nm, which shows typical peaks of graphene at 1580 cm-1 (G-band) and 1340 cm-1 (D-band). The 2D band emerges at 2880 cm-1 with a lower intensity than the G-band, indicating that the outer skin is multi-layered graphene (Figure 2f), which corresponds to the number of graphene layers measured by Figure S5b. In addition, XPS analysis (Figure 2g) exhibits an intense sp2 peak located at 284.5 eV, indicating that most carbon atoms in the GNTs are arranged in sp2-hybridization (repeating carbon hexagon-like honeycombs)27, although a small sp3 band (at 285.4 eV) owing to the intrinsic sp3 defect and the edge of graphene20, and peaks of C-O and O-C=O are also present.28 The above characterization results confirm that multi-layer graphene with sp2 character has been grown on the surface of Cu nanowires. We have analyzed the growth mechanism of the sausage-like GNT@CuNB structures. It appears that both the PMMA coating (introduced by the BBF process) and the morphology evolution of Cu nanowires during thermal annealing process are important underlying factors. Here, PMMA plays a triple role for the formation of nanosized GNTs which includes: 1) acting as the polymer matrix to blow bubbles and assemble Cu nanowires over large-area, 2) protecting the embedded Cu nanowires from melting under high temperature, and maintaining the cylindrical nanowire structure as the growth substrate, and 3) serving as a solid carbon source for synthesizing graphene by conventional thermal annealing process. On the other hand, bare Cu nanowires have a strong tendency to melt and agglomerate due to the effect of Rayleigh instability.29-32 Here, we observed their morphology transformation through four stages from a straight nanowire to nanonecklace,31 nanobottle, then nanocoreshell (Figure S6a-6d). Since the graphene layer simultaneously grew on the 6


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melting Cu surface (as the PMMA matrix gradually consumes), it acted as a rigid tubular skin to limit the deformation of Cu nanowires. Thus, the nanowires had to contract along the length direction (accompanied with certain radial swelling), forming graphene@nanowire coreshell structures. This is the appropriate stage where diverse sausage-like configurations can be obtained. Apart from the nanowire contraction, thermal evaporation of Cu also happens during the graphene growth process. If the thermal annealing process continues for a prolonged time, shrinking and evaporation of nanowires would be severe, resulting in residual nanospheres with much reduced volume along the empty GNTs (Figure S6e). After nanowire shrinkage, only tube-like graphene (Figure S6f) was present to connect separate Cu segments. Sometimes we found cracks on these GNTs (Figure S6g) and even totally unzipped nanoribbons at the end of the CuNBs (Figure S6h). Such longitudinal cracking and unzipping of GNTs is attributed to the swelling of melting Cu nanowires (with increase of nanowire diameter) that splits the outer graphene skin. Thus, the growth of sausage-like structures is owing to simultaneous formation of a tubular graphene skin and consequently limited spatial deformation (mainly lengthwise contraction) of Cu nanowires. Cu nanowires are potential candidates as high-conductivity electrical interconnects, but one of the concerns is their tendency of being oxidized. Here, since the CuNBs have been encapsulated within the GNTs, we investigated their oxidation stability by exposing pristine Cu nanowires (as control sample) and the GNT@CuNB sausages to air for 30 days (Figure 3a, 3b), and monitoring their surface morphology and composition evolution by SEM and XPS. Both samples showed a pronounced Cu 2p3/2 peak initially, and SEM images revealed very smooth surface of nanowires (Figure 3a1, 3b1). After 10 days, the surface of bare Cu nanowires became rough, an indication of 7


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oxidation (Figure 3a2). Correspondingly, an obvious CuO peak appeared (at 934.05 eV) together with two shakeup satellites at 941.25 and 943.75 eV (Figure 3a2).33 The intensities of those peaks related to CuO increased considerably after 30 days, indicating that strong oxidation has occurred on the (rougher) nanowire surface (Figure 3a3). In contrast, the Cu sections in our sausage-like structures maintained smooth walls during the entire period, without producing CuO peaks in XPS spectra (Figure 3b1-3b3). Since the crystallinity of Cu nanowires remains the same after thermal annealing process (Figure 2e, Figure S1c), the only difference between the two samples lies in the presence of a GNT skin in the sausage structure. And single-layer graphene has also proven to be an impermeable barrier to gas and water molecules, protecting the inner materials from oxidation.34 Therefore, we conclude that our sausage-like GNTs are stable against oxidation, and the conformal GNTs serve as an impermeable coating, significantly enhances the stability of the enclosed CuNBs. Since the GNT@CuNB sausages maintain the 1D structure, we have performed electrical measurements on individual sausages as well as their cross-junctions obtained from two-step BBF assembly. We have put a particular focus on how the encapsulated CuNBs influence the electrical behavior of the GNTs, which has not been investigated before. To do this, E-beam lithography-defined Au strips were deposited on selected positions along a sausage, either the empty GNT or the Cu-filled region. By this way, we constructed various two-probe devices consisting of empty (denoted as G-G), partially stuffed (Cu-G), or completely filled (Cu-Cu) GNT channels (Figure 4). All of those devices show ohmic behavior with linear current-voltage (I-V) curves within a certain voltage sweeping range (typically from -5 to 5 V). However, the calculated linear resistance varies considerably depending on whether the GNT channel is empty or filled. For example, an empty GNT channel (G-G) shows a linear resistance of 96.46 kΩ/µm, whereas a partially filled GNT 8


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channel (Cu-G) shows a much reduced value of 29.08 kΩ/µm (Figure 4a, 4b). Furthermore, the resistance drops to 6.99 kΩ/µm in a completely filled channel (Cu-Cu) where the CuNB spans the entire distance between the two electrodes (Figure 4c, 4d). There is saturation in current flow at relatively high voltage (around ±2.5 V) in the I-V curve of the Cu-Cu channel, and similar behavior was observed during repeated sweeping. We suppose that under high voltage, the high current through a single sausage channel would produce considerable heat, which concentrates around the Cu-Cu channel and increases the channel resistance, resulting in the saturation behavior. For partially filled GNT devices, the linear resistance (RL) decreases with increasing length of the filled CuNBs between the two electrodes. In order to quantify this effect, we define a filling ratio (L1/L2) as the length of CuNB (L1) relative to the electrode distance (L2). Measurements on several devices with different filling ratios reveal that the linear resistance and the filling ratio follow a reversed proportional relationship as: RL=-0.5379L1/L2+62.9282 (Table S1, Figure S7-S8). Single-layer graphene synthesized on planar Cu substrate has demonstrated extremely high electrical conductivity, but the electrical conductivity of a nanoscale tubular graphene remains to be studied. The above electrical tests provide important insights underlying the charge transport along the 1D heterostructure. For our sausage-like structure, the graphene layer and the inner CuNB make a parallel arrangement to pass the electrical current simultaneously. Taking the completely filled (Cu-Cu) channel in the Cu-Cu(G)-G device as a parallel connection (Figure 4c, 4d), it is possible to derive the electrical conductivity of the outer GNT and the inner CuNB. First we supposed that the quality of the whole GNT in the Cu-Cu (G)-G device is the same, so the linear resistance of the outer GNT (RL(GNT)) in the Cu-Cu channel can be equal to that of the G-G channel (97.64 kΩ/µm) in the same device, and then the resistance of the outer GNT (RGNT) in the Cu-Cu channel is about 272.42 9


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kΩ, according to RGNT=RL(GNT)×L, where the Cu-Cu channel length (L) is 2.79 µm. Similarly, the total resistance of the CuNB-filled GNT (RGNT@Cu = 19.50 kΩ) in the Cu-Cu channel could be obtained by its linear resistance (6.99 kΩ/µm) and the same channel length. Therefore, the resistance of the inner CuNB (RCu) in the Cu-Cu channel can be calculated to be 21.00 kΩ according to Equation (1). 1 R @

=

1 R

+

1 (1) R

Based on the above resistance values, the electrical conductivity of the outer GNT (σGNT) and the inner CuNB (σCu) can be computed by: σ = 1 σ

=

L (2) R × S

1 (3) R × S

where SCu and SGNT is the cross-sectional area of the inner CuNB (a solid cylinder with diameter of 300 nm) and the outer GNT (a hollow tube with an inner diameter of 300 nm and a layer thickness of 2 nm), respectively. As a result, the nanoscale tubular graphene synthesized by our BBF method has an electrical conductivity of about 5400.15 S/m. Furthermore, the encapsulated CuNB demonstrates an electrical conductivity of 1878.97 S/m. Although σCu is lower than σGNT, the solid cross-section of CuNB enables large current flow and thus greatly reduces the linear resistance of the GNT@CuNB sausages. Under a certain bias voltage (1-5 V), sweeping the gate (back silicon) voltage from -80V to 80V does not change the source-drain current, indicating that those sausages just work as a resistor (Figure S9). This is because both the graphene layer and Cu nanowire are metallic materials, and there seems to be no barrier between the graphene-Cu interfaces. In this regard, these Cu-enhanced 10


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1D GNTs may work as nanoscale conducting units for building 2D conductive networks. In such a 2D network, charge carriers must transport through individual GNTs as well as their junctions. We have done a preliminary test on several inter-GNT cross-junctions assembled by stacking two BBFs with perpendicular nanowire orientations. It shows that the electrical current can flow through one GNT to another across their junction, maintaining an ohmic behavior (linear I-V curve) (Figure 4e-4f, Figure S10, S11). In addition, the cross-junctions coated by graphene layers are highly stable when stored in air, and we observed only 10% degradation of electrical current through the junction after 165 days (Figure S10g-S10i). Also the electrical conductivity of these cross-junctions depends on whether the GNT channel is empty or filled at the contact point. For example, the GNT-GNT crossed sausages show a linear resistance of 87.13 kΩ/µm from one sausage to another through the junction (calculated by I-V curve between A-B in Figure 4f), whereas the GNT-CuNB crossed sausages show a much reduced value of 31.13 kΩ/µm (calculated by I-V curve between B-C in Figure S10c). Compared with other metallic nanowires, our graphene coated sausages are electrically stable in air, and can be configured into various junctions with tunable conductivities. Thus the assembled GNT@CuNB sausages are potential candidates for constructing transparent conducting films. Conclusion In summary, we demonstrate a way to fabricate graphene nanotubes in situ on Cu nanowire arrays assembled by a blown bubble method, and obtain versatile sausage-like GNT@CuNB nanostructures. The outer impermeable GNT skin enhances the oxidation stability of encapsulated CuNBs, meanwhile the stuffed CuNBs also reduces the linear resistance of the GNT sausages. It might be possible to tailor the electrical conductivity of graphene nanotubes by filling metals. Other catalytic metal nanowires such as Ni may be involved to produce graphene-based magnetic 11


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nanocables. The large-scale fabrication of assembled graphene-based 1D structure and cross-junctions by our BBF method has potential applications in nanoelectronics and transparent conducting films with high stability.

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Experimental Methods Synthesis of Cu nanowires. All the chemicals are of analytical grade. We adopted a previous solution method26 to synthesize Cu nanowires, with some modification on parameters. First 384 g of NaOH was dissolved into 640 mL distilled water, mixed with 0.3871 g Cu(NO3)2•3H2O dissolved in 32 mL distilled water. And then 165 µL hydrazine hydrate (85%, w/w%) and 2.4 mL ethylenediamine were added. The solution turned homogeneous after magnetic stirring for about 5 minutes. The solution was maintained at 60 °C for 4 hours. Finally the upper red suspension was washed with water and ethyl alcohol, and was centrifuged at the rate of 9000 r/min for 5 min. About 0.1 g Cu nanowires were produced. Characterization of Cu nanowires. Scanning electron microscope (SEM) images were recorded on HITACHI S-4800. Transmission electron microscope (TEM) images, high-resolution TEM images and selected area electron diffraction (SAED) images were recorded on Tecnai T20. The X-ray diffraction patterns (XRD) were measured on Philips X pert pro, and the sweep speed was 2°/min from 15° to 95°. Preparation of PMMA bubble solution. 3.6 g PMMA (Mw ≈ 996,000) and 30 mL acetone were mixed together under ultrasonication for 30 minutes with constant stirring. Then ~0.1 g Cu nanowires dispersed in absolute ethyl alcohol (~10 mL) were added to the PMMA-acetone solution, which became homogeneous under magnetic stirring for 1 hour. Assembly of Cu nanowire arrays and crossed configurations by blown bubble method. A small portable tool with an air outlet of 12.5 mm diameter was used to blow bubbles manually. After dipping the air outlet into the PMMA bubble solution, a thin membrane was formed on the outlet. By supplying a smooth air flow, a bubble was expanding upward and mainly along its long axis (this is the direction of aligned Cu nanowires). A Si wafer was placed close to the side bubble surface to let the bubble film cover the entire wafer gradually, to accomplish the transfer of PMMA bubble film. Then Cu nanowires aligned in one direction were placed on top of the first layer containing Cu nanowires along the perpendicular direction. Therefore, through the layer-by-layer process, bubble films were put on the Si wafer sequently, while keeping Cu nanowire arrays in the upper layer perpendicular to those in the layer below. Fabrication of assembled sausage-like GNT@CuNB nanostructures. Under H2/Ar atmosphere (H2: 10 sccm, Ar: 190 sccm), bubble films on the Si wafer was heated from room temperature to 900 oC in 72 minutes, and then annealed at 900 oC for 15 minutes. Finally, the Si wafer was cooled down to room temperature and sausage-like GNTs containing CuNBs were synthesized. Characterization of GNT@CuNB sausages. The optical microscope images were recorded under Olympus BX51M. Scanning electron microscope (SEM) images were recorded on HITACHI S-4800. Transmission electron microscope (TEM) images, high-resolution TEM images and selected area electron diffraction (SAED) images were recorded on Tecnai T20. The Raman spectra were tested on Renishaw inVia plus at 514 nm excitation on Si wafer. The X-ray diffraction patterns (XRD) were measured on Philips X pert pro, and the sweep speed was 2°/min from 15° to 95°. The X-ray photoelectron spectroscopy (XPS) results were measured on Imaging Photoelectron Spectrometer, Axis Ultra. The optical transmittance of network was measured on Cary 5000 UV-Vis-NIR (agilent) spectrometer. Electrical measurement of GNT@CuNB sausages and crossed confugurations. We coated MMA layer (spin-coated at the rate of 500 rpm for 5 seconds and then 2200 rpm for 60 seconds) and PMMA950 A4 layer (MicroChem Corp, spin-coated at the rate of 500 rpm for 5 seconds and then 2200 rpm for 60 seconds) on top of 13


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the assembled sausage-like nanostructures, and then electron beam lithography (SEM/FIB dual-beam instrument， Nova 200 NanoLab, FEI) and thermal evaporation (5-nm-layer of Cr and 100-nm-layer of Au) were carried out to deposit Cr/Au electrodes on the nanostructures in predefined positions. Electrical properties of the devices were measured at room temperature with the LakeShore Model TTP4 Cryogenic Probe Station and Keithley 4200-SCS.

Associated Content Supporting Information. Details of the original Cu nanowires synthesized through hydrothermal method, assembled Cu nanowire arrays and sausage-like GNT@CuNB nanostructures, diameter distribution histogram of Cu nanowires before and after thermal annealing process, optical transmittances of Cu nanowire and GNT@CuNB networks, characterization of the outer graphene layer in the sausage-like nanostructures, and formation of the sausage-like GNTs@CuNB nanostructures. Additional details of electrical measurements on different devices, and relation between the filling ratio of CuNB in the GNT sausage and the linear resistance of the sausage.

Author Information Corresponding Author * [email protected] Notes The authors declare no competing financial interest.

Acknowledgement This work was financially supported by the National Nature Science Foundation of China (NO. 91127004 and 51325202). We would like to thank W. L. Yang from Peking University for measuring the XPS spectra of the samples.

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Figure 1. BBF-Assembly of Cu nanowires and fabrication of sausage-like GNT@CuNB nanostructures. (a) Illustration of the process including bubble assembly and thermal annealing growth of GNT@CuNB sausages. (b) Photos of the bubble solution, a blown PMMA bubble, BBF transferred to silicon wafer and after thermal annealing process. Optical image shows the Cu nanowire arrays assembled in the bubble film. (c) Various configurations of the sausage-like nanostructures, such as CuNB filled in one side of GNT (Cu-G), at both ends (Cu-G-Cu), in the middle (G-Cu-G), a periodical arrangement (Cu-and-G), and nearly complete GNT (G). (d) Enlarged SEM images of the GNT@CuNB sausages, showing the filled and empty portions. (e) SEM images of three junctions crossed at different regions. Insets are the enlarged images of the junctions and corresponding models.

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Figure 2. Characterization of the sausage-like GNT@CuNB nanostructures, including (a) Enlarged SEM image, (b) TEM image, (c) selected area diffraction pattern (red and blue dashed circles represent the pattern of the graphene layer), (d1-d3) SEM image and the EDS mapping of Cu and C elements, (e) XRD result (inset is XRD of as-synthesized Cu nanowires as reference sample), (f) Raman spectrum (inset shows the position for Raman analysis), and (g) XPS spectra of the sausage-like nanostructures.

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Figure 3. Oxidation stability of the GNT@CuNB sausages compared with as-synthesized Cu nanowires. (a) Model of the as-synthesized Cu nanowire exposed in air. (a1-a3) SEM images and XPS spectra of the original Cu nanowires, and Cu nanowires after exposed in air for 10 days, 30 days, respectively. (b) Model of the GNT@CuNB sausage exposed in air. (b1-b3) SEM images and XPS spectra of the original GNT@CuNB sausages, and GNT@CuNB sausages after exposed in air for 10 days, 30 days, respectively.

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Figure 4. Electrical measurements on the BBF-assembled GNT@CuNB sausages. (a) SEM image of a G-G-Cu device, containing an empty GNT channel (G-G) on the left and a partially filled channel (Cu-G) on the right. (b) I-V curves of the two channels in the G-G-Cu device. (c) SEM image of a Cu-Cu(G)-G device, containing a completely filled channel (Cu-Cu) on the left and an empty GNT channel (G-G) on the right . (d) I-V curves of the two channels in the Cu-Cu(G)-G device. Inset shows the cross section of the Cu-Cu channel and a model of parallel circuit. (e) SEM image of the GNT-GNT crossed sausages. (f) I-V curves of the junction measured at different electrodes (A, B, C).

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Table of contents

A blown bubble method is used to assemble Cu nanowires and fabricate graphene nanotubes filled by Cu nano-blocks in various sausage-like configurations. The oxidation stability of Cu is enhanced and the linear resistance of graphene is reduced.

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Blown-Bubble Assembly and in Situ Fabrication of Sausage-like

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