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Width-Controlled Sub-Nanometer Graphene Nanoribbon Films Synthesized by Radical-Polymerized Chemical Vapor Deposition Hiroshi Sakaguchi,* Yoshiyasu Kawagoe, Yoshitaka Hirano, Taku Iruka, Maki Yano, and Takahiro Nakae* Graphene nanoribbon (GNR), which is a one-dimensional carbon structure, exhibits semiconductor properties, whereas graphene is a two-dimensional carbon sheet with pseudo-metal, zero band gap characteristics.[1] The methods of GNR synthesis can be classified into top-down or bottom-up approaches. Examples of top-down approaches reported to date include the unzipping of carbon nanotubes (CNTs) using an e-beam[2] or chemical oxidation,[3] the etching of graphene by lithography,[4] and plasma chemical vapor deposition (CVD) under a Ni bar.[5] Although top-down methods have advantages for producing GNRs with micrometer length, the edge structures are usually undefined and the width of the ribbon can be larger than the molecular dimensions. In contrast, bottom-up techniques can produce GNRs with a defined edge-structure and narrow widths, less than 1 nm.[1] The bottom-up approaches reported to date include organic synthesis in solution by a combination of aromatic coupling and dehydrogenation of soluble organic building-blocks,[6] the conversion of precursors inside CNTs,[7,8] and surface-assisted polymerization[9–11] with subsequent dehydrogenation in a ultra high vacuum (UHV) environment using the deposition of haloarenes on Au(111) substrate. Although these bottom-up methods provide GNRs with a defined edge structure, the methods reported to date suffer from the systematic synthesis of GNR having different width at larger scales because of the low solubility of GNRs and the need for special environments and instruments. Therefore, a simple bottom-up synthetic method for large-scale production of GNR is eagerly sought not only for complete characterization of the material but also for use in bulk-film devices such as transistors and photovoltaic cells. Here, we demonstrated large-scale growth of all types of armchair-edged GNRs (3p, 3p+1, and 3p+2; p is defined as the number of carbon atoms along the width)[12] on
Prof. H. Sakaguchi, Y. Kawagoe, Y. Hirano, T. Iruka, M. Yano Institute of Advanced Energy Kyoto University Uji 611–0011, Kyoto, Japan E-mail: [email protected] Dr. T. Nakae Graduate School of Science and Engineering Ehime University Matsuyama 790–8577, Ehime, Japan E-mail: [email protected]
DOI: 10.1002/adma.201305034
Adv. Mater. 2014, DOI: 10.1002/adma.201305034
Au(111) even in extremely low-vacuum conditions using our newly developed method, radical-polymerized chemical vapor deposition (RP-CVD). Armchair-edged GNRs with a width of 2, 3, or 4 benzene rings, grown on a large scale, can form the isolated films, which can be used to characterize the experimentally unknown width-dependent band gap and can also be used to fabricate devices such as field effect transistors (FETs) and photoconductive devices. The experimental setup of RP-CVD is presented in Figure 1. The attractive features of this method originate from an independent temperature-control of the tube wall for the precursor path (zone 1) and the growth substrate (zone 2) in order to achieve a high yield of GNR. As shown in Figure 1a, Au(111) on a glass substrate was placed in a quartz tube with a reactor heated by an electric furnace (zone 2) at a temperature of T2 °C. The system was evacuated using a rotary pump with Ar gas, resulting in a pressure of 1 Torr. Solid monomers (1 mg) placed in a quartz boat were vaporized by heating at 200–250 °C, such that they pass through the hot wall of the quartz tube (zone 1) heated to a temperature of T1 °C, and subsequently onto the substrate at T2 = 250–300 °C for 15 min to form prepolymers by radical-polymerization as a first stage. Subsequently, the temperature of T2 was raised to 400–450 °C, and was maintained there for 10 min for the second stage of the reaction, the dehydrogenation of prepolymers to GNR. We found two important parameters for the large-scale growth of GNR by RP-CVD. Only when these conditions are met, an intense Raman signal from poly(perianthracene) GNR was observed when using 10,10’-dibromo-9,9’-bianthryl as a monomer (Figure S1). The obtained Raman spectrum has the G band at 1600 cm−1, the D bands at 1340 cm−1, the edge carbon mode at 1260 and 1220 cm−1, and the radial-breathing-like mode (RBLM) at 398 cm−1. This spectrum is in good agreement with the reported results obtained using UHV deposition.[9] The first requirement is cleaning of the quartz tube by immersion in concentrated nitric acid after heating at 1000 °C. The Raman intensity was enhanced markedly compared to that of the untreated tube (Figure S2). A second requirement is the positioning of the Au(111) substrate in the quartz tube. The Raman signal was more intense when the Au(111) side was placed facing the quartz surface than when it was facing the gas (Figure S3). The dependence of the signal intensity on the height between the center of the Au(111) substrate and the bottom of the quartz tube was also measured. Intense Raman signals were observed at a distance of 1 mm from the reactor
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Figure 1. a) Experimental setup of RP-CVD with an illustration of the presumed GNR growth mechanism when using 10,10′-dibromo-9,9′-bianthryl as a monomer. G-band Raman intensity of poly(perianthracene) GNR as a function of temperature (b) (T1) of zone 1 and (c) (T2) of zone 2. T2 was fixed at 250 °C for (b). T1 was fixed at 350 °C for (c).
surface (Figure S4). It is apparent from Figures 1b and 1c that the independent temperature control of the two zones results in a high yield of GNR at a T1 of 350 °C and T2 of 250 °C. From these results, we propose two possible mechanisms for the high yield of GNR at the given temperatures of T1 and T2. One is the high-density of biradicals generated by the collision of the precursors with the hot wall of the quartz tube (zone 1). The other is the increase in the arrival rate of precursors onto the Au(111) substrate. Although it is necessary to determine the total amount of precursors available on the Au(111) substrate in order to determine which mechanism prevails, it is difficult to do this. In our experience, increasing the supply rate of precursors by raising the temperature from 220 °C to 250 °C at the vaporizing step did not increase the Raman intensity. Therefore, the high-density of biradical generation may be the probable mechanism for the high yield. However, further investigation is necessary to determine the correct mechanism. In addition to the Raman spectroscopy, the GNRs were characterized using STM measured in air, as shown in Figure 2. The STM images of poly(perianthracene) GNR (Figure 2a) produced by RP-CVD (T2 = 250 °C at the first stage and 400 °C at the second stage) showed a multilayered high-density array of linear wires (Figures 2b and 2c). It is apparent from the STM image (Figure S5) that RP-CVD formed multilayer because the under layer was clearly seen. An intense Raman signal observed from the same sample was confirmed to originate from the highly grown multilayered GNR. The monolayer height is 0.22 nm determined from cross-sectional STM image (Figure 2d). The length of the longest GNR is 20 nm, determined from a distribution of sizes from an STM image of 100 nm2 area (Figure 2e). To investigate the GNR growth mechanism, the STM image of RP-CVD-grown sample at T2 = 250 °C at the first stage was acquired. It shows a zigzag chain with a spacing on each side corresponding to 0.82 nm (Figures S6a–S6c), which is in good agreement with that of the alternate anthracene ring in poly(anthrylene) (0.85 nm, Figure S6d). These data suggest that the mechanism of RP-CVD is radical polymerization and dehydrogenation, where the intermediate at the first stage with T2 = 250 °C corresponds to the prepolymer, poly(anthrylene), followed by conversion to the poly(perianthracene) GNR at the second stage at 400 °C. 2
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In fact, RP-CVD can be used with other monomers to produce GNRs of different widths. When using monomer mixtures containing 3,9-dibromoperylene and 3,10-dibromoperylene[13] (Figure 2f), multilayered linear wires of poly(perinaphthalene) GNR, produced by RP-CVD (T2 = 250 °C at the first stage and 450 °C at the second stage), were observed in the STM images (Figures 2g and 2h). The monolayer height was determined to be 0.27 nm using cross-sectional analysis (Figure 2i). A length histogram of the STM image of 100 nm2 area reveals that the length of poly(perinaphthalene) GNR is up to 24 nm (Figure 2j). Production of poly(perinaphthalene) GNR was also confirmed by the Raman spectrum of the RP-CVD grown sample which showed peaks matching those calculated using theoretical simulation (Figure S7). Although several reports have described the synthesis of poly(perinaphthalene) using CVD[14] and pyrolysis[15] from perylene-3,4,9,10-tetracarboxylic dianhydride, no direct evidence has been presented to visualize the wire structure. Moreover, physical properties such as band gaps and carrier mobilities have been unknown. The RP-CVD method has enabled clear visualization of the unknown structure of poly(perinaphthalene). The GNR growth mechanism was evaluated using STM at the first stage of T2 = 250 °C (Figure S8a). It clearly shows the existence of prepolymer and poly(perylenylene). Spacing at each line of the image (1.75 nm, Figure S8b) agrees with alternate spacing (1.7 nm) of the perylene rings in poly(perylenylene) (Figure S8c). Based on these data, the growth mechanism of poly(perinaphthalene) GNR was confirmed as polymerization of biradicals generated from monomers followed by dehydrogenation. Poly(peritetracene) GNR with a width of four benzene rings was produced when using 1,4-Bis(4-bromophenyl)-2,3,6,11tetraphenyltriphenylene[16] (Figure 2k) as a monomer. The multilayered linear wires of poly(peritetracene) GNR, produced by RP-CVD (T2 = 300 °C at the first stage and 450 °C at the second stage), were observed using STM images (Figures 2l and 2m). The monolayer height was determined to be 0.27 nm using cross-sectional analysis (Figure 2n). A length histogram of the STM image reveals that the longest poly(peritetracene) GNR is 7 nm, determined from an STM image of 50 nm2 area (Figure 2o). Production of poly(peritetracene) GNR was also confirmed by the Raman spectrum of the RP-CVD grown
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Adv. Mater. 2014, DOI: 10.1002/adma.201305034
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COMMUNICATION Figure 2. STM of width-controlled graphene nanoribbons. a, f, k) Chemical structures of (a) 10,10′-dibromo-9,9′-bianthryl with poly(perianthracene) GNR, (f) 3,9- and 3,10-dibromoperylene with poly(perinaphthalene) GNR, and (k) 1,4-Bis(4-bromophenyl)-2,3,6,11-tetraphenyltriphenylene with poly(peritetracene) GNR. b, c, g, h, l, m) STM images of (b, c) poly(perianthracene) GNR, (g, h) poly(perinaphthalene) GNR and (l, m) poly(peritetracene) produced by RP-CVD on Au(111). All images were measured at room temperature in air. d, i, n) Cross sections of lines shown in (c), (h), and (m), respectively. e, j, o) Histogram of chain-length from the STM images of (b), (g), and (l), respectively. The ordinate shows a two-dimensional analogue of the weight-average molecular weight, NL ×L/a, where NL refers to the counted number of chains having length L (nm) and a corresponds to the monomer-unit length.
sample, which showed peak positions that were identical with those determined from simulation (Figure S9). Isolation of GNR grown on a Au(111) substrate using a bottom-up fabrication technique is an extremely important process for the characterization of unknown properties and for applications in devices. It was reported that CVD-grown graphene could be transferred from the metal substrate on which it was grown to an insulating substrate by a wet process including metal etching.[17,18] In contrast, the transfer of GNR grown using bottom-up techniques is difficult because it gets distributed easily on the surface of the etching solution because of its small size as a polymer. We developed a GNR isolation process, which includes gold etching and GNR transfer to an insulating substrate, as shown in Figure 3. The isolation process (Figure 3a) involved the following steps: (i) the Au(111) film
Adv. Mater. 2014, DOI: 10.1002/adma.201305034
with the grown GNR was removed from the glass by immersion in water, (ii) the Au(111) was etched using a KI / I2 solution, (iii) the hydrophobic insulating substrate was immersed to provide contact with the Au(111) film with GNR before complete dissolution of gold, and (iv) the substrate with the GNR transferred was withdrawn after completion of the Au(111) etching. Using this method, the GNRs were successfully transferred onto the insulating substrates to form GNR thin films. Poly(perianthracene) GNR films could be transferred using a hydrophobic quartz plate (Figure 3b). Although all peaks in the Raman spectrum of poly(perianthracene) GNR that was, transferred to a heavily doped n++ silicon with a gate insulator (SiO2) having 300 nm thickness (abbreviated as SiO2/Si), were almost identical to those on Au(111), the D band intensity increased (Figure S10b). This could be a result of the change in
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Figure 3. Transferred GNR thin films. a) Schematic illustration of GNR transfer process from Au(111) to the insulating substrate. b) Optical microscope image of transferred poly(perianthracene) GNR films on quartz plate after immersing 12 times. c, d, e) Optical absorption spectra of (c) poly(perianthracene) GNR, (d) poly(perinaphthalene) GNR and (e) poly(peritetracene) GNR films on quartz plate after immersing 12 times. Insets depict the chemical structure of GNR and the Tauc plots showing the band gaps.
GNRs have semiconducting properties, therefore, they can be orientation of the GNR on SiO2/Si compared to that on Au(111). potentially used as a photovoltaic cell. Although several studies The developed isolation technique was applied to the fabricahave reported the photoconduction in top-down fabricated tion of a FET on SiO2/Si substrate. As-grown GNRs by RP-CVD GNRs, comparison with other photoconductors is unknown.[23,24] showed ambipolar transistor characteristics with carrier mobili−5 2 −1 −1 ties of the order of 10 cm V s for poly(perianthracene) Therefore, the photoconductivity of bulk films of RP-CVD grown GNRs was investigated as shown in Figure 4. White light illumiGNR, 10−4 cm2 V−1 s−1 for poly(perinaphthalene) GNR and nation from an LED (1.4 mW cm−2) with a spectrum as shown 10−6 cm2 V−1 s−1 for poly(peritetracene) GNR (Figure S11). in Figure S12 was used as the light source. Poly(perianthracene) The band gap of GNR is believed to depend on the edge GNR and poly(perinaphthalene) GNR showed 7.3% and 4.0% of structure and the width.[19,20] Although there are some reports current-gain upon illumination, respectively. These values are on the dependence of the band gap on the width, especomparable to the value of 2.7% obtained from P3HT, which cially for armchair-edge type GNR,[21,22] it has never been is a conventional p-type photoconductor used in photovoltaic studied systematically. We measured the band gaps of our cells. These results represent the potential of GNR, grown using GNRs using optical measurements. The optical absorption a bottom-up technique, as an excellent photoconductor. Howspectra of the GNR transferred onto a quartz plate ranged ever, further investigation is necessary to elucidate details of the from violet to the infrared region depending on the width mechanism at the electrode interface. (Figures 3c, 3d, and 3e). The band gaps of poly(perianthracene) Our findings demonstrate the potential of GNR grown GNR, poly(perinaphthalene) GNR, and poly(peritetracene) GNR using the bottom-up approach, which can be classified as a were measured as 1.6 eV, 0.8 eV, and 1.3 eV, respectively, from Tauc plots of the optical absorption (insets of Figures 3c, 3d, and 3e). These measurements experimentally established the band-gap value for the armchair-edged GNRs having a width less than 1 nm. These experimental values were compared with those obtained from theory applying the first-principles self-consistent-pseudopotential method using local density approximation (LDA).[20] Theoretical values of band gaps for GNRs of poly(perianthracene), poly(perinaphthalene), and poly(peritetracene) have been reported as 1.57 eV, 0.37 eV, and 0.74 eV, respectively. The experimental band gap of poly(perianthracene) GNR shows good agreement with the theoretical value. However, Figure 4. Photoconductivity of GNR films. a) Effect of white light illumination using LED −2 those of poly(perinaphthalene) GNR and (1.4 mW cm ) on the current-voltage curve of poly(perinaphthalene) GNR film. Black and red lines show conditions before and after the illumination, respectively. Electrode spacing is poly(peritetracene) GNR differ from the the10 µm. Inset depicts the schematic illustration of the experiment. b, c, d) Effect of illumination oretical values. The reason for the deviation on the current change in (b) poly(perianthracene) GNR, (c) poly(perinaphthalene) GNR, and between experiments and theory could be a (d) P3HT. Bias voltage was set to 1 V. GNR films were made by transfer of the RP-CVD grown result of the appropriateness of the approxi- GNRs on Au(111) to SiO2/Si by a single immersion. P3HT film of 50 nm thickness was promation used in theory. duced by spin-coating on SiO2/Si. The inset depicts the chemical structure. 4
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Adv. Mater. 2014, DOI: 10.1002/adma.201305034
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Experimental Section Radical-polymerized Chemical Vapor Deposition: The CVD system consists of a quartz tube (26 mmϕ, 86 cm) as a reactor, a rotary pump which can evacuate the system to
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Width-Controlled Sub-Nanometer Graphene Nanoribbon Films Synthesized by Radical-Polymerized Chemical Vapor Deposition Hiroshi Sakaguchi,* Yoshiyasu Kawagoe, Yoshitaka Hirano, Taku Iruka, Maki Yano, and Takahiro Nakae* Graphene nanoribbon (GNR), which is a one-dimensional carbon structure, exhibits semiconductor properties, whereas graphene is a two-dimensional carbon sheet with pseudo-metal, zero band gap characteristics.[1] The methods of GNR synthesis can be classified into top-down or bottom-up approaches. Examples of top-down approaches reported to date include the unzipping of carbon nanotubes (CNTs) using an e-beam[2] or chemical oxidation,[3] the etching of graphene by lithography,[4] and plasma chemical vapor deposition (CVD) under a Ni bar.[5] Although top-down methods have advantages for producing GNRs with micrometer length, the edge structures are usually undefined and the width of the ribbon can be larger than the molecular dimensions. In contrast, bottom-up techniques can produce GNRs with a defined edge-structure and narrow widths, less than 1 nm.[1] The bottom-up approaches reported to date include organic synthesis in solution by a combination of aromatic coupling and dehydrogenation of soluble organic building-blocks,[6] the conversion of precursors inside CNTs,[7,8] and surface-assisted polymerization[9–11] with subsequent dehydrogenation in a ultra high vacuum (UHV) environment using the deposition of haloarenes on Au(111) substrate. Although these bottom-up methods provide GNRs with a defined edge structure, the methods reported to date suffer from the systematic synthesis of GNR having different width at larger scales because of the low solubility of GNRs and the need for special environments and instruments. Therefore, a simple bottom-up synthetic method for large-scale production of GNR is eagerly sought not only for complete characterization of the material but also for use in bulk-film devices such as transistors and photovoltaic cells. Here, we demonstrated large-scale growth of all types of armchair-edged GNRs (3p, 3p+1, and 3p+2; p is defined as the number of carbon atoms along the width)[12] on
Prof. H. Sakaguchi, Y. Kawagoe, Y. Hirano, T. Iruka, M. Yano Institute of Advanced Energy Kyoto University Uji 611–0011, Kyoto, Japan E-mail: [email protected] Dr. T. Nakae Graduate School of Science and Engineering Ehime University Matsuyama 790–8577, Ehime, Japan E-mail: [email protected]
DOI: 10.1002/adma.201305034
Adv. Mater. 2014, DOI: 10.1002/adma.201305034
Au(111) even in extremely low-vacuum conditions using our newly developed method, radical-polymerized chemical vapor deposition (RP-CVD). Armchair-edged GNRs with a width of 2, 3, or 4 benzene rings, grown on a large scale, can form the isolated films, which can be used to characterize the experimentally unknown width-dependent band gap and can also be used to fabricate devices such as field effect transistors (FETs) and photoconductive devices. The experimental setup of RP-CVD is presented in Figure 1. The attractive features of this method originate from an independent temperature-control of the tube wall for the precursor path (zone 1) and the growth substrate (zone 2) in order to achieve a high yield of GNR. As shown in Figure 1a, Au(111) on a glass substrate was placed in a quartz tube with a reactor heated by an electric furnace (zone 2) at a temperature of T2 °C. The system was evacuated using a rotary pump with Ar gas, resulting in a pressure of 1 Torr. Solid monomers (1 mg) placed in a quartz boat were vaporized by heating at 200–250 °C, such that they pass through the hot wall of the quartz tube (zone 1) heated to a temperature of T1 °C, and subsequently onto the substrate at T2 = 250–300 °C for 15 min to form prepolymers by radical-polymerization as a first stage. Subsequently, the temperature of T2 was raised to 400–450 °C, and was maintained there for 10 min for the second stage of the reaction, the dehydrogenation of prepolymers to GNR. We found two important parameters for the large-scale growth of GNR by RP-CVD. Only when these conditions are met, an intense Raman signal from poly(perianthracene) GNR was observed when using 10,10’-dibromo-9,9’-bianthryl as a monomer (Figure S1). The obtained Raman spectrum has the G band at 1600 cm−1, the D bands at 1340 cm−1, the edge carbon mode at 1260 and 1220 cm−1, and the radial-breathing-like mode (RBLM) at 398 cm−1. This spectrum is in good agreement with the reported results obtained using UHV deposition.[9] The first requirement is cleaning of the quartz tube by immersion in concentrated nitric acid after heating at 1000 °C. The Raman intensity was enhanced markedly compared to that of the untreated tube (Figure S2). A second requirement is the positioning of the Au(111) substrate in the quartz tube. The Raman signal was more intense when the Au(111) side was placed facing the quartz surface than when it was facing the gas (Figure S3). The dependence of the signal intensity on the height between the center of the Au(111) substrate and the bottom of the quartz tube was also measured. Intense Raman signals were observed at a distance of 1 mm from the reactor
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Figure 1. a) Experimental setup of RP-CVD with an illustration of the presumed GNR growth mechanism when using 10,10′-dibromo-9,9′-bianthryl as a monomer. G-band Raman intensity of poly(perianthracene) GNR as a function of temperature (b) (T1) of zone 1 and (c) (T2) of zone 2. T2 was fixed at 250 °C for (b). T1 was fixed at 350 °C for (c).
surface (Figure S4). It is apparent from Figures 1b and 1c that the independent temperature control of the two zones results in a high yield of GNR at a T1 of 350 °C and T2 of 250 °C. From these results, we propose two possible mechanisms for the high yield of GNR at the given temperatures of T1 and T2. One is the high-density of biradicals generated by the collision of the precursors with the hot wall of the quartz tube (zone 1). The other is the increase in the arrival rate of precursors onto the Au(111) substrate. Although it is necessary to determine the total amount of precursors available on the Au(111) substrate in order to determine which mechanism prevails, it is difficult to do this. In our experience, increasing the supply rate of precursors by raising the temperature from 220 °C to 250 °C at the vaporizing step did not increase the Raman intensity. Therefore, the high-density of biradical generation may be the probable mechanism for the high yield. However, further investigation is necessary to determine the correct mechanism. In addition to the Raman spectroscopy, the GNRs were characterized using STM measured in air, as shown in Figure 2. The STM images of poly(perianthracene) GNR (Figure 2a) produced by RP-CVD (T2 = 250 °C at the first stage and 400 °C at the second stage) showed a multilayered high-density array of linear wires (Figures 2b and 2c). It is apparent from the STM image (Figure S5) that RP-CVD formed multilayer because the under layer was clearly seen. An intense Raman signal observed from the same sample was confirmed to originate from the highly grown multilayered GNR. The monolayer height is 0.22 nm determined from cross-sectional STM image (Figure 2d). The length of the longest GNR is 20 nm, determined from a distribution of sizes from an STM image of 100 nm2 area (Figure 2e). To investigate the GNR growth mechanism, the STM image of RP-CVD-grown sample at T2 = 250 °C at the first stage was acquired. It shows a zigzag chain with a spacing on each side corresponding to 0.82 nm (Figures S6a–S6c), which is in good agreement with that of the alternate anthracene ring in poly(anthrylene) (0.85 nm, Figure S6d). These data suggest that the mechanism of RP-CVD is radical polymerization and dehydrogenation, where the intermediate at the first stage with T2 = 250 °C corresponds to the prepolymer, poly(anthrylene), followed by conversion to the poly(perianthracene) GNR at the second stage at 400 °C. 2
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In fact, RP-CVD can be used with other monomers to produce GNRs of different widths. When using monomer mixtures containing 3,9-dibromoperylene and 3,10-dibromoperylene[13] (Figure 2f), multilayered linear wires of poly(perinaphthalene) GNR, produced by RP-CVD (T2 = 250 °C at the first stage and 450 °C at the second stage), were observed in the STM images (Figures 2g and 2h). The monolayer height was determined to be 0.27 nm using cross-sectional analysis (Figure 2i). A length histogram of the STM image of 100 nm2 area reveals that the length of poly(perinaphthalene) GNR is up to 24 nm (Figure 2j). Production of poly(perinaphthalene) GNR was also confirmed by the Raman spectrum of the RP-CVD grown sample which showed peaks matching those calculated using theoretical simulation (Figure S7). Although several reports have described the synthesis of poly(perinaphthalene) using CVD[14] and pyrolysis[15] from perylene-3,4,9,10-tetracarboxylic dianhydride, no direct evidence has been presented to visualize the wire structure. Moreover, physical properties such as band gaps and carrier mobilities have been unknown. The RP-CVD method has enabled clear visualization of the unknown structure of poly(perinaphthalene). The GNR growth mechanism was evaluated using STM at the first stage of T2 = 250 °C (Figure S8a). It clearly shows the existence of prepolymer and poly(perylenylene). Spacing at each line of the image (1.75 nm, Figure S8b) agrees with alternate spacing (1.7 nm) of the perylene rings in poly(perylenylene) (Figure S8c). Based on these data, the growth mechanism of poly(perinaphthalene) GNR was confirmed as polymerization of biradicals generated from monomers followed by dehydrogenation. Poly(peritetracene) GNR with a width of four benzene rings was produced when using 1,4-Bis(4-bromophenyl)-2,3,6,11tetraphenyltriphenylene[16] (Figure 2k) as a monomer. The multilayered linear wires of poly(peritetracene) GNR, produced by RP-CVD (T2 = 300 °C at the first stage and 450 °C at the second stage), were observed using STM images (Figures 2l and 2m). The monolayer height was determined to be 0.27 nm using cross-sectional analysis (Figure 2n). A length histogram of the STM image reveals that the longest poly(peritetracene) GNR is 7 nm, determined from an STM image of 50 nm2 area (Figure 2o). Production of poly(peritetracene) GNR was also confirmed by the Raman spectrum of the RP-CVD grown
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COMMUNICATION Figure 2. STM of width-controlled graphene nanoribbons. a, f, k) Chemical structures of (a) 10,10′-dibromo-9,9′-bianthryl with poly(perianthracene) GNR, (f) 3,9- and 3,10-dibromoperylene with poly(perinaphthalene) GNR, and (k) 1,4-Bis(4-bromophenyl)-2,3,6,11-tetraphenyltriphenylene with poly(peritetracene) GNR. b, c, g, h, l, m) STM images of (b, c) poly(perianthracene) GNR, (g, h) poly(perinaphthalene) GNR and (l, m) poly(peritetracene) produced by RP-CVD on Au(111). All images were measured at room temperature in air. d, i, n) Cross sections of lines shown in (c), (h), and (m), respectively. e, j, o) Histogram of chain-length from the STM images of (b), (g), and (l), respectively. The ordinate shows a two-dimensional analogue of the weight-average molecular weight, NL ×L/a, where NL refers to the counted number of chains having length L (nm) and a corresponds to the monomer-unit length.
sample, which showed peak positions that were identical with those determined from simulation (Figure S9). Isolation of GNR grown on a Au(111) substrate using a bottom-up fabrication technique is an extremely important process for the characterization of unknown properties and for applications in devices. It was reported that CVD-grown graphene could be transferred from the metal substrate on which it was grown to an insulating substrate by a wet process including metal etching.[17,18] In contrast, the transfer of GNR grown using bottom-up techniques is difficult because it gets distributed easily on the surface of the etching solution because of its small size as a polymer. We developed a GNR isolation process, which includes gold etching and GNR transfer to an insulating substrate, as shown in Figure 3. The isolation process (Figure 3a) involved the following steps: (i) the Au(111) film
Adv. Mater. 2014, DOI: 10.1002/adma.201305034
with the grown GNR was removed from the glass by immersion in water, (ii) the Au(111) was etched using a KI / I2 solution, (iii) the hydrophobic insulating substrate was immersed to provide contact with the Au(111) film with GNR before complete dissolution of gold, and (iv) the substrate with the GNR transferred was withdrawn after completion of the Au(111) etching. Using this method, the GNRs were successfully transferred onto the insulating substrates to form GNR thin films. Poly(perianthracene) GNR films could be transferred using a hydrophobic quartz plate (Figure 3b). Although all peaks in the Raman spectrum of poly(perianthracene) GNR that was, transferred to a heavily doped n++ silicon with a gate insulator (SiO2) having 300 nm thickness (abbreviated as SiO2/Si), were almost identical to those on Au(111), the D band intensity increased (Figure S10b). This could be a result of the change in
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Figure 3. Transferred GNR thin films. a) Schematic illustration of GNR transfer process from Au(111) to the insulating substrate. b) Optical microscope image of transferred poly(perianthracene) GNR films on quartz plate after immersing 12 times. c, d, e) Optical absorption spectra of (c) poly(perianthracene) GNR, (d) poly(perinaphthalene) GNR and (e) poly(peritetracene) GNR films on quartz plate after immersing 12 times. Insets depict the chemical structure of GNR and the Tauc plots showing the band gaps.
GNRs have semiconducting properties, therefore, they can be orientation of the GNR on SiO2/Si compared to that on Au(111). potentially used as a photovoltaic cell. Although several studies The developed isolation technique was applied to the fabricahave reported the photoconduction in top-down fabricated tion of a FET on SiO2/Si substrate. As-grown GNRs by RP-CVD GNRs, comparison with other photoconductors is unknown.[23,24] showed ambipolar transistor characteristics with carrier mobili−5 2 −1 −1 ties of the order of 10 cm V s for poly(perianthracene) Therefore, the photoconductivity of bulk films of RP-CVD grown GNRs was investigated as shown in Figure 4. White light illumiGNR, 10−4 cm2 V−1 s−1 for poly(perinaphthalene) GNR and nation from an LED (1.4 mW cm−2) with a spectrum as shown 10−6 cm2 V−1 s−1 for poly(peritetracene) GNR (Figure S11). in Figure S12 was used as the light source. Poly(perianthracene) The band gap of GNR is believed to depend on the edge GNR and poly(perinaphthalene) GNR showed 7.3% and 4.0% of structure and the width.[19,20] Although there are some reports current-gain upon illumination, respectively. These values are on the dependence of the band gap on the width, especomparable to the value of 2.7% obtained from P3HT, which cially for armchair-edge type GNR,[21,22] it has never been is a conventional p-type photoconductor used in photovoltaic studied systematically. We measured the band gaps of our cells. These results represent the potential of GNR, grown using GNRs using optical measurements. The optical absorption a bottom-up technique, as an excellent photoconductor. Howspectra of the GNR transferred onto a quartz plate ranged ever, further investigation is necessary to elucidate details of the from violet to the infrared region depending on the width mechanism at the electrode interface. (Figures 3c, 3d, and 3e). The band gaps of poly(perianthracene) Our findings demonstrate the potential of GNR grown GNR, poly(perinaphthalene) GNR, and poly(peritetracene) GNR using the bottom-up approach, which can be classified as a were measured as 1.6 eV, 0.8 eV, and 1.3 eV, respectively, from Tauc plots of the optical absorption (insets of Figures 3c, 3d, and 3e). These measurements experimentally established the band-gap value for the armchair-edged GNRs having a width less than 1 nm. These experimental values were compared with those obtained from theory applying the first-principles self-consistent-pseudopotential method using local density approximation (LDA).[20] Theoretical values of band gaps for GNRs of poly(perianthracene), poly(perinaphthalene), and poly(peritetracene) have been reported as 1.57 eV, 0.37 eV, and 0.74 eV, respectively. The experimental band gap of poly(perianthracene) GNR shows good agreement with the theoretical value. However, Figure 4. Photoconductivity of GNR films. a) Effect of white light illumination using LED −2 those of poly(perinaphthalene) GNR and (1.4 mW cm ) on the current-voltage curve of poly(perinaphthalene) GNR film. Black and red lines show conditions before and after the illumination, respectively. Electrode spacing is poly(peritetracene) GNR differ from the the10 µm. Inset depicts the schematic illustration of the experiment. b, c, d) Effect of illumination oretical values. The reason for the deviation on the current change in (b) poly(perianthracene) GNR, (c) poly(perinaphthalene) GNR, and between experiments and theory could be a (d) P3HT. Bias voltage was set to 1 V. GNR films were made by transfer of the RP-CVD grown result of the appropriateness of the approxi- GNRs on Au(111) to SiO2/Si by a single immersion. P3HT film of 50 nm thickness was promation used in theory. duced by spin-coating on SiO2/Si. The inset depicts the chemical structure. 4
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Adv. Mater. 2014, DOI: 10.1002/adma.201305034
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Experimental Section Radical-polymerized Chemical Vapor Deposition: The CVD system consists of a quartz tube (26 mmϕ, 86 cm) as a reactor, a rotary pump which can evacuate the system to
