Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon

Aug 10, 2015 - The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constit...
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Nitrogen-Doping Induced Self-Assembly of Graphene NanoribbonBased Two-Dimensional and Three-Dimensional Metamaterials Timothy H. Vo,† U. Gayani E. Perera,‡ Mikhail Shekhirev,† Mohammad Mehdi Pour,† Donna A. Kunkel,§ Haidong Lu,§ Alexei Gruverman,§,∥ Eli Sutter,‡,⊥ Mircea Cotlet,‡ Dmytro Nykypanchuk,‡ Percy Zahl,‡ Axel Enders,§,∥ Alexander Sinitskii,*,†,∥ and Peter Sutter*,‡,# †

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States § Department of Physics, ∥Nebraska Center for Materials and Nanoscience, ⊥Department of Mechanical and Materials Engineering, and #Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States ‡

S Supporting Information *

ABSTRACT: Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties. KEYWORDS: Graphene, nanoribbons, metamaterials, doping, self-assembly

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with bandgaps that strongly depend on the width of the ribbons is realized in narrow armchair GNRs.11−14 The ability to control structural parameters of GNRs, such as their width, edge structure, and termination with atomic precision is the key for the practical realization of these properties.15 While GNRs have widely recognized promise for nanoelectronics, there is little awareness of their potential to serve as nanoscale building blocks for constructing macroscopic metamaterials. Realizing large crystalline GNR assemblies could pave the way toward novel carbon-based materials with widely tunable characteristics. Here we show that the substitutional doping with nitrogen atoms16−20 can trigger the hierarchical self-assembly of GNRs into ordered structures. We focus on GNRs that are synthesized via atomically precise bottom-up approaches

ne of the unique promises of nanoscience is the realization of new families of functional materials from nanometer-sized components whose tunable properties define the characteristics of a macroscopic ensemble. A well-known example of such a metamaterial is the quantum dot crystal, an ordered aggregate of colloidal semiconductor nanoparticles.1 More generally, nanoparticles with interesting optical, magnetic, and plasmonic properties can often be crystallized and even cocrystallized into millimeter-scale three-dimensional crystals.2−5 The physical properties of these crystals are largely governed by the size, composition and shape of the individual nanoparticles, which provides easy tunability of the ensemble characteristics by adjusting the properties of the nanoscale constituents. Graphene nanoribbons (GNRs), high-aspect ratio, fewnanometer-wide strips of graphene, are a class of carbonbased nanostructures with intriguing physical properties.6 GNRs with zigzag edges are predicted to exhibit lowdimensional magnetism,7−10 while semiconducting behavior © XXXX American Chemical Society

Received: May 1, 2015 Revised: August 5, 2015

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DOI: 10.1021/acs.nanolett.5b01723 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Atomically precise bottom-up synthesis of P-GNRs and 8N-GNRs on a surface and in solution. (a) Structure of the precursor monomers 1 and 2. (b) Assembly into chevron-type GNRs on an Au(111) surface (top) and in solution (bottom). In both cases, the assembly proceeds via dehalogenation and polymerization of the monomers, followed by cyclodehydrogenation to form planar GNRs.

involving surface-assisted21−25 or solution polymerization26−36 of molecular precursors, followed by cyclodehydrogenation. Recently, surface-assisted24,25 and solution-based32,35 bottomup approaches with modified precursors were successfully used for the synthesis of nitrogen-doped GNRs. The interaction of doped GNRs is studied by comparing N-doped chevron-like GNRs with eight N atoms per unit cell (referred to as 8NGNRs) with pristine, undoped chevron GNRs (P-GNRs), see Figure 1. Nitrogen substitution causes only a small change in the GNR bandgap.37 In contrast, N doping drives pronounced changes in the interaction between GNRs. Whereas surface-supported PGNRs show no detectable interaction, hydrogen bonding drives 8N-GNRs to self-assemble edge-to-edge into two-dimensional (2D) arrays. Synthesis in solution gives rise to threedimensional (3D) 8N-GNR crystals, in which adjacent ribbons are coordinated in the plane, and the resulting nanoribbon sheets in turn are held together by dispersion forces and πstacking interactions. The resulting 2D and 3D GNR crystals represent hierarchical supramolecular metamaterials built from atomically precise constituents by the interplay of covalent and hydrogen bonding, as well as van der Waals and π−π interactions. The basic scheme of the bottom-up synthesis of P-GNRs and 8N-GNRs on surfaces (here Au(111)) and in solution is shown in Figure 1b. In both cases, the precursor monomers (Figure 1a), 6,11-dibromo-1,2,3,4-tetraphenyltriphenylene (C42Br2H26, 1) and 5,5′-(6,11-dibromo-1,4-diphenyltriphenylene-2,3-diyl)dipyrimidine (C38Br2N4H22, 2) polymerize after dehalogenation and are subsequently converted into planar, chevron-type GNRs by an additional cyclodehydrogenation step. In 2, two phenyl groups are substituted by pyrimidinyls to achieve a doping by eight nitrogen atoms per unit cell of the planar GNRs. On Au(111), the metal substrate catalyzes both the thermal dehalogenation and cyclodehydrogenation at elevated temperatures, whereas suitable chemicals (Ni0, FeCl3) introduced into the solvent perform the analogous function in the solution synthesis of GNRs (Figure 1b). Synthetic details and reaction schemes for precursor monomers and solutionsynthesized nanoribbons are provided in the Supporting Information (Figures S1 and S2; Notes 1 and 2). Microscopic and spectroscopic characterization of solution-synthesized PGNRs has been presented in our previous work;33,36 the characterization of 8N-GNRs by Raman spectroscopy and Xray photoelectron spectroscopy (XPS) is discussed in the Supporting Information (Figure S3 and Note 3).

The surface-assisted synthesis of chevron P-GNRs has been established in several studies,21,24 and recently the surfaceassisted synthesis of 4N-GNRs (i.e., chevron-type ribbons with four N atoms per unit cell)25 and 8N-GNRs37 has been reported. To compare the morphologies of highly N-doped 8NGNRs with undoped P-GNRs on Au(111), we performed surface-assisted synthesis of the two types of ribbons and analyzed them using scanning tunneling microscopy (STM, Figure 2) and spectroscopy (STS, Figure 3). Undoped P-GNRs synthesized from 1 show characteristics similar to those reported previously (Figure 2a,b). They form as predominantly straight, isolated nanoribbons, typically 10−50 nm long and with a clearly observable chevron structure, which occasionally attach to each other end-to-end or end-to-side. In addition to the GNRs formed on substrate terraces, the Au(111) steps are decorated by GNRs attaching parallel to the step edge (Supporting Information Figure S4). The synthesis of 8N-GNRs from 2 gives rise to a different morphology. Long, isolated ribbons are no longer found. Instead, the surface assisted synthesis produces assemblies of 10−20 nm long 8NGNRs, interspersed by disordered arrangements of short (2−5 unit cells) 8N-GNR segments (Figure 2c). A close-up view shows chevron 8N-GNRs arranged in a supramolecular array in which the convex elbows of adjacent GNRs project toward each other (Figure 2d). The observed arrangements suggest hydrogen bonding between adjacent 8N-GNRs as the likely cause for their packing into ordered arrays (Figure 2e), analogous to the supramolecular assembly of a variety of organic molecules on metal surfaces.38−40 Optimal H-bonding should be achieved if the GNRs are not perfectly aligned laterally, but instead shifted along their axis to match the C−H moieties with the lone pair of the substituted N on the adjacent GNR (Figure 2f). Such relative shifts are indeed observed in our STM images of arrays of 8N-GNRs on Au(111) (Figure 2d). A similar aggregation of 8N-GNRs and shifted antiparallel coordination to optimize the N−H interaction has been reported in ref 37. The H-bonding interaction between 8N-GNRs is extremely robust and to suppress it special growth protocols are needed. For example, a sequential synthesis in which P-GNRs are used as seeds that preferentially capture subsequently deposited 8N-GNR precursors provides a reliable route toward isolated chevron GNR heterostructures37 comprising atomically joined undoped and doped segments (see Supporting Information Figure S5 and Note 4). B

DOI: 10.1021/acs.nanolett.5b01723 Nano Lett. XXXX, XXX, XXX−XXX

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Empty-state STM images at low bias emphasize the chevronlike GNR footprint; at higher bias (above ∼2.0 V) protrusions emerge at the inner and outer elbow sites of both undoped (Figure 3a) and N-doped ribbons (Figure 3b). These high-lying conduction band states, which coincide with the H-terminated edge sites, are particularly developed at the inner, concave elbows of the GNRs. Arrays of laterally H-bonded 8N-GNRs show additional localized electronic states within the conduction band with an onset at ∼2.5 eV above the Fermi energy (Figure 3b). The appearance of these states, centered on the junction between the elbows of adjacent 8N-GNRs (i.e., the positions of the hydrogen bonds) and imaged as ring-shaped protrusions between adjacent ribbons, suggests a finite electronic coupling between neighboring ribbons in the 8NGNRs assemblies. STS was used to further explore the electronic interaction between 8N-GNRs. Figure 3c shows local STS spectra, obtained on a ribbon and at the line interface between adjacent ribbons in an 8N-GNR assembly. A characteristic rise in the tunneling conductance (dI/dV ∼ LDOS (local density of states)) at positive and negative sample bias can, by comparison with energy-resolved LDOS maps (Supporting Information Figure S6), be assigned to the conduction and valence band edges in the 8N-GNRs but determining the precise band edges is difficult. We estimate the bandgap to be (1.9 ± 0.5) eV, which is in good agreement with the value reported recently for 8N-GNRs.37 Mapping shows generally low LDOS in the bandgap region, except for features at energies between 0−0.6 eV localized at the “windows” created by neighboring chevrons, which we tentatively assign to a confined Au(111) surface state. Within the bands, the highest LDOS is generally localized near the edges of the GNRs. The valence band maximum and conduction band minimum were assigned to energies at which the LDOS of these edge-localized states begins to rise. For undoped P-GNRs, we find a similar bandgap (2.0 eV) but both valence- and conduction-band edges are shifted to higher energy as compared to the 8N-GNRs, consistent with a previous report.37 While the shift in the band edges between PGNRs and 8N-GNRs in principle could be due to an enhanced interaction with the Au(111) substrate, calculations for freestanding ribbons have shown similar shifts,37 suggesting that they represent the effect of the N dopants on the potential within the ribbons. Discrete resonances due to electronic states in the GNRs are found in the valence (−1.9 eV) and conduction band (+1.6 eV). Within the gap, the spectrum obtained on the ribbons shows a flat LDOS except for a resonance (SS) due to the Shockley surface state of the Au(111) substrate. However, at the junction between adjacent 8N-GNRs an additional state emerges at an energy of +0.6 eV. STM images at this bias (0.5 V, Supporting Information Figure S6b) suggest that this state is associated with a lateral bridge between neighboring 8N-GNRs. The strong tendency toward H-bonded self-assembly paired with relatively minor effects on the confinement-controlled electronic structure presents an opportunity for a possible hierarchical assembly of atomically precise N-doped GNRs into macroscopic metamaterials, whose properties (e.g., the electronic bandgap) are defined by those of their individual nanoscale building blocks. To explore this scenario, we investigated how 8N-GNRs assemble in an unrestricted 3D environment by synthesizing them (along with undoped PGNR control samples) in solution (Figure 1b). Chevron-type GNRs, including nitrogen-substituted 4N-GNRs, have been

Figure 2. Atomically precise bottom-up synthesis of pristine and nitrogen-doped graphene nanoribbons on Au(111). (a) STM image of the morphology of pristine graphene nanoribbons (P-GNRs) on Au(111), after vacuum deposition of (1) followed by cyclodehydrogenation. Scale bar: 10 nm. V = 0.4 V, I = 0.1 nA. (b) Closeup view of one of the isolated P-GNRs. Scale bar: 2 nm. (c) STM image of the morphology of N-doped 8N-GNRs on Au(111), synthesized by evaporation of (2) followed by cyclodehydrogenation. Note the coexistence of short, disordered GNR segments and of larger 2D assemblies of longer GNRs. Scale bar: 10 nm. V = 0.4 V, I = 0.1 nA. (d) Close-up view of one of the 2D assemblies of 8N-GNRs. Scale bar: 2 nm. Overlays in panels b and d show molecular models of the PGNR and 8N-GNR. Contact points between neighboring 8N-GNRs in 2D assemblies are consistently given by N-doped convex elbows projected toward each other. (e) Schematic representation of the 8NGNR arrays, showing the directions of covalent extension of the GNRs and of their self-assembly into ordered arrays. (f) Schematic structure of laterally coordinated 8N-GNRs, which are offset slightly along their axes to enable hydrogen bonding between adjacent ribbons.

The substitution of nitrogen as a possible electron donor and the observed packing of laterally coordinated 8N-GNRs raise questions about the electronic structure of the N-doped GNRs in such assemblies. In conventional semiconductors, the substitutional doping with a higher-valent element introduces excess electrons that remain bound to the impurity in a hydrogen-like orbital with renormalized Bohr radius. Surface supported, doped GNRs experience a very different screening environment and may behave more like doped organic semiconductors. Carrier confinement is responsible for the bandgap opening in narrow graphene nanoribbons, and a possible electronic coupling between tightly packed 8N-GNRs may modify the confinement or introduce new electronic states at the line interfaces between adjacent GNRs. To explore these issues, we performed bias-dependent STM imaging and STS on P-GNRs and on 8N-GNR assemblies on Au(111). C

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Figure 3. Bias-dependent STM imaging on P-GNRs and 8N-GNRs. (a) Bias-dependent STM of a single P-GNR at sample bias 0.1, 2.6, and 3.2 V (I = 0.1 nA). Overlays illustrate the structure of the P-GNR. (b) Bias-dependent STM on an 8N-GNR assembly. STM images at sample bias V = +0.1 V emphasize the footprints of the individual chevron GNRs. Images at bias greater than 2.0 V show well-defined protrusions at concave corner sites. In addition, shallower, ring-shaped protrusions appear where N-doped elbows of adjacent 8N-GNRs meet. Higher bias (V = +3.2 V) STM images show clearly developed ring-shaped protrusions that cover the N-doped elbows of neighboring 8N-GNRs. Scale bars: 2 nm. I = 0.1 nA. (c) STS spectra obtained at two sites centered on one of the 8N-GNRs: within one 8N-GNR (black cross in panel b) and at the contact point of two neighboring 8N-GNRs (red cross in panel b). VB, valence band; CB, conduction band. Both spectra are consistent with a bandgap of ∼1.9 eV. SS, resonance due to the Shockley surface state of Au(111). The spectrum obtained at the contact line shows an additional resonance (R) in the bandgap of the 8N-GNR.

synthesized and investigated in our recent reports33,35 but the solution synthesis of ribbons with such high substitution ratio is reported here for the first time. Similar to on-surface synthesis, the N-doped GNRs in solution should be able to self-assemble into planar H-bonded sheets, which in turn, via van der Waals forces and π-stacking, would tend to arrange as out-of-plane stacks and thus form 3D crystallites (Figure 4a). Nitrogen substitution indeed induces pronounced changes in the crystallization behavior of GNRs. Solution-synthesized 8NGNRs spontaneously form macroscopic crystals with typical overall dimensions of several hundred micrometers (with some crystals reaching up to ∼2 mm in size), much larger than aggregates of undoped P-GNRs (Figure 4b), and show clean surfaces with a characteristic metallic luster (Figure 4c). The mechanical properties of these 8N-GNR crystals were characterized by measuring force−indentation curves. To compare with P-GNRs, which do not form crystals, we prepared pressed pellets of doped and undoped GNRs and measured them along with as-grown 8N-GNR crystals (Figure 4d). Even though the hardness is difficult to extract quantitatively, all N-doped GNR-based materials (crystals and pellets) were significantly harder than those involving P-GNRs. Besides the difference in crystallization, these mechanical properties confirm the stronger interaction between 8NGNRs compared with P-GNRs. Because the interlayer forces between flat aromatic structures (π−π interaction) are nondirectional, undoped GNRs do not tend to form crystalline structures whereas the directional nature of the bonds between the nitrogen lone pairs and hydrogens along the edges of 8NGNRs promotes the formation of hard crystals with maximum interaction achieved when the nanoribbons are aligned in planar sheets, which then stack up to form larger 3D structures. STM provides additional evidence for the self-assembly of 8N-GNRs that results in the formation of 3D crystals. Because P-GNRs do not aggregate as strongly as 8N-GNRs, they can be dispersed in appropriate organic solvents, such as N-methyl-2pyrrolidone, dimethylformamide, toluene, or mesitylene, by intense sonication.33,35 After a droplet of the resulting P-GNR

suspension is deposited on Au(111) and the solvent is evaporated, individual ribbons are routinely observed by STM. Figure 4e shows a typical STM image of an individual P-GNR deposited on Au(111) from mesitylene suspension.33,35 In contrast, when we deposited 8N-GNRs on Au(111) using a similar drop-cast procedure, STM detected no individual ribbons but only large aggregates that remained even after intense sonication of the 8N-GNR samples in mesitylene, again confirming the enhanced interaction between N-doped ribbons. Imaging of these aggregates at high resolution was difficult, but in a few instances we could observe densely packed 8N-GNRs, as shown in Figure 4f,g. STM images show a similar side-byside coordination in solution-synthesized 8N-GNR crystals as found for surface-assembled 8N-GNRs (Figure 4f) with ∼2 nm lateral periodicity given by the width of the chevron GNRs. In other areas, more closely spaced parallel lines were observed (Figure 4g). The height profile in Figure 4g shows a periodicity of ∼0.4 nm, consistent with the distance expected for π−π stacked 8N-GNRs sheets in accordance with the model shown in Figure 4a. This interlayer spacing is consistent with that found in layered van der Waals materials41 but it is larger than the interlayer spacing in graphite (0.335 nm).42 Figure 4g provides a direct visualization of the polycrystalline nature of our 8N-GNR crystals, showing that they consist of π−π stacked nanocrystallites ranging from 5−30 nm in size (see also Supporting Information Figure S9). Despite different orientations, the individual crystallites are in turn densely packed to form macroscopically large structures. The mechanical stability of this packing may benefit from hydrogen bonding between ribbons in adjacent crystallites. We also imaged the 8N-GNR crystals by transmission electron microscopy (TEM, Figure 4h−l). TEM of fragments of 8NGNR crystals dispersed by sonication showed a pronounced flake structure (Figure 4h,i). Within the flakes, a stripe contrast is visible (Figure 4k,l) whose ∼2.1 nm period is consistent with the expected repeat distance of side-by-side coordinated chevron GNRs (Figure 4a-ii.). Images obtained near the electron transparent edges of larger 8N-GNR (Figure 4j) D

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Figure 4. Hierarchical self-assembly of 8N-GNRs in solution. (a) Scheme of the 8N-GNR self-assembly in a 3D solution environment. (b,c) Comparison of optical images of solution-synthesized P-GNRs (b) and crystals of 8N-GNRs (c). Scale bars: 50 μm. (d) Force−indentation curves of GNR crystals. Top inset: Scheme of the experiment. Bottom inset: low force linear region (1 eV) many-body corrections to the single-particle (local density approximation, LDA) bandgaps, giving rise to GW gaps that exceed those in LDA by ∼1.0 to 1.5 eV and are strongly dependent on the GNR width and edge termination (e.g., with/without H).43,44 For GNRs synthesized by surface-assisted methods and supported on metal substrates, the measured bandgaps are typically lower than the quasi-particle gaps due to surface screening.44,45 Reports of measured bandgaps of undoped and N-doped N = 6/N = 9 chevron GNRs on Au(111) range from 2.0 eV (STS for both P- and 8N-GNRs)37 to ∼2.7 eV (high-resolution electron energy loss spectroscopy on 4N-GNRs).24 The bandgap obtained here for 8N-GNRs by STS (1.9 eV) agrees with that reported in ref 37. Beyond many-body effects, the optical properties of GNRs are determined by strong excitonic effects due to confinementinduced modifications of the electron and hole wave functions and reduced screening. For straight N = 10 armchair GNRs, for example, calculations show the first photoabsorption peak at an excitation energy of 1.8 eV compared to a quasi-particle bandgap of 3.2 eV, that is, give an exciton binding energy of ∼1.4 eV.14 A similar calculation for N = 6/N = 9 chevron GNRs shows a quasi-particle gap of 3.74 eV and singlet-exciton binding energy of 1.76 eV, thus predicting an optical bandgap of 1.98 eV and a singlet−triplet splitting of 0.35.46 Our absorption measurements on P-GNR suspensions show discrete peaks at 1.96 and 2.27 eV, very close to these predicted excitonic transitions. A similar doublet of absorption

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MATERIALS AND METHODS Reaction schemes for precursor monomers and solutionsynthesized nanoribbons are shown in Supporting Information Figures S1 and S2, respectively; synthetic details are provided in the accompanying text in the Supporting Information. Surface-assisted bottom-up synthesis of P-GNRs and 8NGNRs was carried out in a Createc low-temperature UHV STM system on samples prepared by sublimation of the precursors 1 and 2 onto a clean Au(111) single crystal at 200 °C, followed by cyclodehydrogenation at 380 °C for 10 min. STM imaging and spectroscopy were carried out at T = 5 K, using the GXSM control system.48 Differential conductance maps were obtained with modulated sample bias (80 mV, ∼ 800 Hz) and lock-in detection. STM on solution-synthesized GNRs was performed in an Omicron UHV LT-STM. GNR powder was sonicated for 1 min in mesitylene and then heated to reflux. The hot suspension was sonicated for additional 30 s and heated back to reflux twice. Then two drops of the suspension were deposited on Au(111) cleaned in UHV, dried for ∼5 min, and the sample subsequently reintroduced into UHV. The sample was annealed in situ in 5 min increments at 40 °C to remove weakly bound adsorbates (total annealing time: 20 min). Electron microscopy was performed in a FEI Titan 80-300 TEM equipped with a CEOS Cs-corrector. Samples were prepared by dispersing ultrasonicated 8N-GNR crystal suspensions onto ultrathin carbon grids, producing exfoliated few-layer flakes as well as larger compact microcrystallites. Planview images were obtained on flakes; cross-sectional TEM was performed near the electron transparent edges of crystals. F

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probe microscopy was supported by the NSF through the Nebraska Materials Research Science and Engineering Center (MRSEC, DMR-1420645). This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

For force−indentation measurements, GNR materials were compacted into pellets (7 mm diameter, ∼ 0.1 mm thick) using a hand press. Force−indentation curves were obtained by measuring force−distance curves using a commercial atomic force microscopy (AFM, MFP3D, Asylum Research). Si probes (PPP-EFM, Nanosensors) with a tip radius of ∼30 nm were used. The spring constant of the AFM cantilever was calibrated by the thermal noise method. The indentation was calculated by subtraction of the cantilever bending distance from the total z-piezo movement in the force−distance curve. UV−vis-NIR spectroscopy was performed using a Jasco V670 spectrometer. For UV−vis-NIR measurements 4 mg of solution-synthesized GNRs were dispersed in 8 mL of mesitylene via 3 min sonication at ambient temperature and heated to reflux for 10 min. Sonication (30 s)/reflux steps were repeated two more times. Then the hot GNR suspension was transferred to the UV−vis cuvettes for measurement. Photoluminescence (PL) was measured at room temperature using a home-built confocal scanning stage inverted microscope (Olympus IX71) equipped with a 0.95 NA, 100× air objective lens (Olympus), with samples excited at 475 nm (1 μW) by a frequency-doubled diode pumped solid-state femtosecond laser (Maitai broadband HP, Spectra Physics). PL spectra were acquired by guiding the collected PL through a multimode fiber coupled to a spectrograph (Acton 2300i, Princeton Instruments) with a charge-coupled device camera (Pixis 100, Roper Scientific).

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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/acs.nanolett.5b01723. Supplementary Figures S1−S8. Figure S1: Synthesis of precursor molecules. Figure S2: Synthesis of P-GNRs and 8N-GNRs. Figure S3: Raman and X-ray photoelectron spectroscopy characterization. Figure S4: Stepedge attachment of GNRs on Au(111). Figure S5: Seeded growth of P-GNR/8N-GNR heterostructures. Figure S6: Tunneling spectroscopy of 8N-GNR assemblies. Figure S7: X-ray scattering of solution synthesized P-GNRs and 8N-GNRs. Figure S8: Estimation of the (0, 0) optical transition. Figure S9: Largescale STM image of an 8N-GNR crystal. Supplementary Notes 1−4: Precursor synthesis; solution synthesis of PGNRs and 8N-GNRs; characterization of solution synthesized GNRs; and seeded growth of GNR heterostructures. Also supplementary references. (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.S.). *E-mail: [email protected] (A.S.). Author Contributions

T.H.V. and U.G.E.P. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research on GNR synthesis was supported by the National Science Foundation (NSF) through Grant CHE-1455330. Characterization of solution-synthesized GNRs by scanning G

DOI: 10.1021/acs.nanolett.5b01723 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.5b01723 Nano Lett. XXXX, XXX, XXX−XXX