Effect of Substrate Chemistry on the Bottom-Up Fabrication of

Article pubs.acs.org/JPCC

Effect of Substrate Chemistry on the Bottom-Up Fabrication of Graphene Nanoribbons: Combined Core-Level Spectroscopy and STM Study Konstantin A. Simonov,*,†,‡,∥ Nikolay A. Vinogradov,†,‡,∥ Alexander S. Vinogradov,∥ Alexander V. Generalov,‡,∥ Elena M. Zagrebina,∥ Nils Mårtensson,† Attilio A. Cafolla,§ Tomas Carpy,§ John P. Cunniffe,§ and Alexei B. Preobrajenski*,‡ †

Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden MAX IV, Lund University, Box 118, 22100 Lund, Sweden ∥ V.A. Fock Institute of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia § School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland ‡

ABSTRACT: Atomically precise graphene nanoribbons (GNRs) can be fabricated via thermally induced polymerization of halogen containing molecular precursors on metal surfaces. In this paper the effect of substrate reactivity on the growth and structure of armchair GNRs (AGNRs) grown on inert Au(111) and active Cu(111) surfaces has been systematically studied by a combination of corelevel X-ray spectroscopies and scanning tunneling microscopy. It is demonstrated that the activation threshold for the dehalogenation process decreases with increasing catalytic activity of the substrate. At room temperature the 10,10′-dibromo-9,9′-bianthracene (DBBA) precursor molecules on Au(111) remain intact, while on Cu(111) a complete surface-assisted dehalogenation takes place. Dehalogenation of precursor molecules on Au(111) only starts at around 80 °C and completes at 200 °C, leading to the formation of linear polymer chains. On Cu(111) tilted polymer chains appear readily at room temperature or slightly elevated temperatures. Annealing of the DBBA/Cu(111) above 100 °C leads to intramolecular cyclodehydrogenation and formation of flat AGNRs at 200 °C, while on the Au(111) surface the formation of GNRs takes place only at around 400 °C. In STM, nanoribbons have significantly reduced apparent height on Cu(111) as compared to Au(111), 70 ± 11 pm versus 172 ± 14 pm, independently of the bias voltage. Moreover, an alignment of GNRs along low-index crystallographic directions of the substrate is evident for Cu(111), while on Au(111) it is more random. Elevating the Cu(111) substrate temperature above 400 °C results in a dehydrogenation and subsequent decomposition of GNRs; at 750 °C the dehydrogenated carbon species self-organize in graphene islands. In general, our data provide evidence for a significant influence of substrate reactivity on the growth dynamics of GNRs.

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INTRODUCTION Ever since graphene was first experimentally produced in 2004,1 it has been considered a promising candidate for fast nanoelectronic devices.2 Nevertheless, the absence of band gap in graphene hinders its potential application as an effective component of FET devices for digital logic operations.3 In this context a number of different approaches for generation of a sizable band gap in graphene have been proposed. Recently explored strategies include chemical modification,4−9 bilayer control,10,11 and defects.12 Another promising route for inducing a band gap in the electronic structure of graphene relies on confining the charge carriers within a quasi-onedimensional (1D) ribbon. In the 1990s it was predicted theoretically that narrow GNRs with width below 10 nm possess a sizable band gap.13,14 The magnitude of the band gap depends inversely on the width of the ribbon,15−18 and for subnanometer widths a significant value of more than 1 eV can be expected. In addition, unique electronic and magnetic proper© 2014 American Chemical Society

ties emerge from the boundary conditions imposed by the width, the crystallographic symmetry, and the edge structure of GNRs.19−21 In particular, AGNRs have emerged as possible candidates for introducing device-relevant energy gaps, thus enabling high on/off current ratios.22 In order to produce GNRs with predictable and reproducible electronic structure, a method for fabricating GNRs with atomic precision is highly desirable. A novel bottom-up strategy for synthesizing GNRs with welldefined width and shape from simple molecular precursors has recently been proposed23,24 and employed to synthesize atomically precise AGNRs on Au(111),24−28 Au(788),29 and Ag(111).30 In particular, 7-AGNRs (the integer corresponds to the ribbon width expressed in rows of C atoms) can be grown Received: March 4, 2014 Revised: May 14, 2014 Published: May 16, 2014 12532

dx.doi.org/10.1021/jp502215m | J. Phys. Chem. C 2014, 118, 12532−12540

The Journal of Physical Chemistry C

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annealing in UHV at 550 °C. The cleanliness of the substrate was verified by core-level photoemission spectroscopy (PES) and low-energy electron diffraction (LEED). To prepare the 7AGNRs, DBBA molecules were deposited by sublimation from a Knudsen-cell evaporator onto the clean substrates maintained at room temperature, under UHV conditions. The amount of material deposited was monitored by PES and was always less than a monolayer. The temperature evolution of the system during polymerization and cyclodehydrogenation processes was monitored by NEXAFS and PES. The C K-edge NEXAFS spectra were measured in partial electron yield (PEY) mode. A retarding potential, Uret, of −150 V was applied to the PEY detector in order to increase the signal-to-background ratio. The photon energy resolution at the C K-edge was set to 75 meV. The NEXAFS spectra were normalized to the spectra from the corresponding clean metal surfaces and to the continuum jump. The photoelectron spectra were measured relative to the Fermi level of the system and were taken in a normal emission geometry. The kinetic energy resolution of the SES-200 electron analyzer was set to 125 meV for the C 1s and Br 3d core-level spectra. The fitting procedure for the C 1s PE spectra was performed using the FitXPS software.36 At each stage of the process shown in Figure 1 the samples were also characterized by STM. The STM experiments were performed at room temperature in a separate system, using a commercial instrument (Omicron Nanotechnology GmbH), in a UHV system consisting of an analysis chamber (with a base pressure of 5 × 10−11 mbar) and a preparation chamber (1 × 10−10 mbar). The STM images were recorded in constant current mode using an electrochemically etched polycrystalline tungsten tip. The voltage, Vs, corresponds to the sample bias with respect to the tip. No drift corrections have been applied to the STM images presented in this paper. The WSxM37 software was used for the STM image processing.

by using DBBA molecules as a precursor. Several studies have recently been performed to elucidate the electronic and structural properties of the resulting GNRs.24−31 Calculations of the band gap in 7-AGNRs have predicted values ranging from 1.5 to 3.7 eV,32,33 while reported experimental data yield values of 2.3 eV25 or 2.6 eV.29 The growth mechanism originally described by Cai et al.24 is shown schematically in Figure 1. On Au(111), the surface-

Figure 1. Schematic representation of bottom-up synthesis of 7AGNRs from DBBA precursor.

assisted dehalogenation and formation of linear polyphenylene chains occurs at 200 °C, while further annealing to 400 °C leads to a cyclodehydrogenation reaction that yields, in the case of DBBA precursors, hydrogen terminated 7-AGNRs.26 Evidently, a detailed understanding of the GNR formation process during bottom-up fabrication is critical for growing high-quality GNRs with the desired properties. By analogy with graphene, for which the electronic structure and growth parameters are strongly dependent on the substrate chemistry,34 we can expect that the substrate−molecule interaction will play an important role in the bottom-up fabrication of GNRs at each stage of their formation. Indeed, Li et al. have recently calculated that the electronic and magnetic properties of zigzag GNRs strongly depend on the underlying substrate.35 Nevertheless, the effect of the substrate on the growth mechanism and electronic structure of the GNRs remains to be established. Here we present a comparative study of the 7-AGNR growth process on Au(111) and Cu(111) using near-edge X-ray absorption fine structure (NEXAFS) and photoelectron spectroscopy (PES) in combination with scanning tunneling microscopy (STM). The analysis of spectroscopic results obtained for AGNRs on Au(111) has been performed taking into account the common knowledge gained previously in the STM studies,24−27 and has been used as a starting point for understanding the processes of GNR formation on the more reactive Cu(111) surface. We report on an observed correlation between surface reactivity and growth dynamics of GNRs. Furthermore, we demonstrate that core-level spectroscopy can be used as an effective tool for studying the GNR formation process.

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RESULTS To analyze the debromination dynamics, which is the first key step in the GNR formation process, the Br 3d PE spectrum was recorded as a function of the annealing temperature. The results for the Au(111) and Cu(111) surfaces are presented in Figure 2 I−III. In the case of Au(111) (Figure 2I, II), the spectral shape of the Br 3d PE line undergoes considerable changes upon annealing. Below 100 °C the Br 3d signal is a single broad spin−orbit doublet with the binding energy (EB) of Br 3d5/2 at 69.7 eV. The position of this line coincides with one from a multilayer of DBBA on Au(111) (not shown); therefore, we attribute it to bromine atoms linked to the anthracene units. Above 100 °C a second doublet-component with EB(Br 3d5/2) = 67.8 eV appears, accompanied by a decrease in the intensity of the initial doublet at 69.7 eV. At temperatures of 200 °C and above, only the new low-energy component survives; it can be assigned to Br atoms adsorbed on Au(111) after complete debromination of DBBA molecules.38 Thus, on the Au(111) surface debrominated molecular biradicals may start to form polymer chains at 200 °C (Figure 1 II), in agreement with previously published STM studies.24,26 The total intensity of the Br 3d line at 200 °C is greatly reduced compared to the intensity at RT. This is probably due to an associative desorption of Br atoms through formation of Br2 molecules. At 250 °C all Br species appear to be desorbed from the surface. In the case of Cu(111) (Figure 2 III), the Br 3d line consists of a single spin−orbit split component throughout the entire

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EXPERIMENTAL SECTION All spectroscopic measurements were carried out in situ at the D1011 beamline, MAX IV (Lund, Sweden). The Au(111) and Cu(111) crystals were cleaned by several cycles of Ar+ sputtering (EAr+ = 1 keV) at room temperature and subsequent 12533



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Figure 2. Evolution of the Br 3d PE spectrum of DBBA/Au(111) and DBBA/Cu(111) taken with hv = 170 eV as a function of increasing sample temperature. I: Intensity map of the Br 3d XPS signal from the Au(111) substrate as a function of temperature. II and III: Br 3d XPS spectra corresponding to different substrate temperatures on Au(111) (a−d) and Cu(111) (a−c), respectively.

temperature range up to 550 °C, and rapidly disappears thereafter. The absence of a significant evolution of the Br 3d spectral shape upon annealing allows us to assign this component to atomic Br adsorbed on Cu(111). The corresponding EB(Br 3d5/2) = 68.5 eV is shifted by 0.7 eV to higher binding energies (as compared to the case of Br atoms on Au), in agreement with previous studies.39 Thus, the DBBA molecules already adsorb dissociatively on Cu(111) at RT, and the formation of polyanthracene chains from debrominated fragments can also start at much lower temperatures than on Au(111). The dangling bonds formed after debromination are likely to interact with the more active Cu(111) surface, possibly leading to an additional distortion of the molecules. It is plausible to expect that the differences in debromination dynamics on Au(111) and Cu(111) will cause dissimilarities in the growth and structure of GNRs on these surfaces. Although the formation of GNRs on Au(111) is well documented, the very possibility of GNR growth on Cu(111) remains to be proven. Direct structural evidence can be provided by STM. Therefore, we present in Figure 3 a

Figure 3. STM images of 7-AGNRs on Au(111) obtained after annealing to 400 °C (a), polymer chains formed from debrominated biradicals on Cu(111) surface at 100 °C (b−d) and 7-AGNRs on Cu(111) at 250 °C (e). Inset on (d) shows an enlarged image of the polymer chain superimposed with corresponding ball and stick model. Tunnel parameters (voltage bias, Vs; tunneling current, It): (a) −1.37 V/0.1 nA, (b) −0.61 V/0.1 nA, (c) +0.01 V/0.02 nA, (d) +0.23 V/ 0.02 nA, (e) +1.5 V/0.04 nA. Insets in (a) and (e) are Fourier transforms of corresponding images.

collection of STM images demonstrating GNRs on both Au(111) and Cu(111), with the focus on the latter system as less studied. Figure 3a shows a typical large-scale STM image of 7-AGNRs on Au(111) obtained after adsorbing DBBA and annealing to 400 °C. Detailed STM studies of formation of the 7-AGNRs on Au(111) are reported elsewhere.24,26,27,30 The 12534



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ribbons are randomly oriented and reveal length distribution from 4 to 75 nm, centered at 20 nm. The measured width of GNRs is about 1.5 nm, which is larger than expected for the corresponding atomic model (0.74 nm between C atoms on the opposite edges). This is typically attributed to the finite size of the STM tip and to the H-saturated edges.24,30 Figure 3b−d shows polymer chains produced on Cu(111) after annealing to 100 °C. Protrusions that appear on both sides of the chain axis clearly illustrate the alternating tilt of the adjacent anthracene units within the polymer chain, as shown in the inset of Figure 3d. Further annealing to 250 °C leads to cyclodehydrogenation and the formation of flat 7-AGNRs (Figure 3e). The average width of the GNRs measured from the STM image is the same as that for 7-AGNRs on Au(111), approximately 1.5 nm. It is interesting that the polyanthracene chains on Cu(111) are long (up to 50 nm) and curved, while the resulting GNRs appear shorter (most of them are between 10 and 20 nm) and exhibit sharp edges due to the formation of additional C−C bounds upon cyclodehydrogenation. Upon a closer look at Figure 3b,e, junctions consisting of two or three polymer chains (ribbons) growing from one point in different directions can be noticed (Y-shaped structures). Such behavior may be caused by the additional dehydrogenation of edge carbon atoms, which can randomly occur when polymer chains (ribbons) come close to each other. In some instances, parallel rows, comprising 3 or 4 separate GNRs, are observed. Their orientation is governed by a strong ribbon−substrate interaction, because the rows are oriented along six directions on Cu(111) with the angular separation of 30°, as can be seen from the Fourier transform of the corresponding image (Figure 3e, inset). This is not observed for GNRs on Au(111) (see inset in Figure 3a), where the orientation of nanoribbons is rather random due to a weaker ribbon−substrate interaction. Moreover, stronger interaction with Cu(111) results in a significantly reduced apparent height of 70 ± 11 pm for the GNRs on Cu(111) in comparison with 172 ± 14 pm measured for the GNRs on Au(111). Figure 4 shows an apparent height of the GNRs on

both surfaces from STM images recorded at different sample voltages. It is clear that GNRs on Au(111) and Cu(111) demonstrate a pronounced difference in the apparent height independently of the bias voltage. The local atomic and electronic structure of the adsorbed DBBA molecules, chains, and ribbons has been characterized further by measuring NEXAFS spectra at the C K-edge as a function of annealing temperature (Figure 5). In all cases, the annealing results in dramatic changes in the near-edge fine structure region of the spectra, indicating profound variations in the electronic structure of the GNRs on different surfaces. The overall shape of the C K-edge NEXAFS spectrum of DBBA molecules after deposition on Au(111) (Figure 5a) is complicated but similar to that of the C K-edge spectra recorded from other polyacene molecules.40−44 The π* region of the spectrum consists of three resonances A1 − A3. The same energy interval in NEXAFS spectra of a single anthracene molecule has only two resonance peaks A 1 and A 3 corresponding to the C 1s → π* transitions into the two lowest unoccupied molecular orbitals.44 These two resonances are broadened and structured because the respective transitions are localized on four C atoms with slightly different chemical states.44 The additional resonance, A2, observed in the DBBA spectra is a replica of the A1 resonance in anthracene localized on the C atoms bonded to Br and thus shifted to higher energy. The presence of sharp resonances in the C K-edge spectrum at RT indicates that there is no clear chemical interaction between DBBA and Au(111), which is in agreement with the Br 3d PE data described above. At 200 °C (Figure 5b) the A2 resonance appears as a shoulder, while upon annealing to 400 °C (Figure 5d) the separate absorption resonances merge into a single broad band A. The NEXAFS spectrum at 300 °C (Figure 5c) represents a superposition of those, annealed at 200 and 400 °C (Figure 5b and d, respectively). Such an effect can be naturally explained by the formation of a delocalized π* system characteristic of the nanoribbon upon annealing to 400 °C, while at 200° the formation of polymer chains takes place. These chains retain a molecular-like electronic structure with localized unoccupied states seen in the corresponding C K-edge spectrum as sharp absorption peaks A1 and A3 (Figure 5b). The shift of peak A2 toward A1 is a result of a debromination of the DBBA molecules, as described above. The C K-edge spectra from DBBA and its derivatives on Cu(111) are shown in Figure 5e−n, and demonstrate similar behavior, although with some significant differences. Indeed, the RT spectrum (Figure 5e) is significantly different from the molecular-like spectrum recorded at RT on Au(111) (Figure 5a) due to an interaction with the more active Cu(111) surface.42,45 The A2 resonance, corresponding to the transitions localized on the C atoms bonded to Br, is absent, which is in line with the conclusions made above from the Br 3d PE spectra analysis. The spectrum recorded at 100 °C (Figure 5f) is similar to that obtained from the polymer chains on Au(111) at 200 °C. Upon annealing, the spectrum gradually develops (Figure 5g,k), while at 250 °C (Figure 5m), the spectral shape is close to ribbon-like (Figure 5d). Upon further annealing to 750 °C (Figure 5n), the spectral structure becomes characteristic of graphene weakly bonded to surfaces34 and is also similar to that of bulk graphite.46 We suggest that upon annealing to 750 °C the GNRs on Cu(111) undergo gradual decomposition followed by the formation of graphene islands. Based on a comparison of the temperature dependent PE spectra of Br 3d and the C K-edge NEXAFS spectra for the Au(111) and

Figure 4. Apparent height of the GNRs on Au(111) and Cu(111) surfaces from STM images recorded at different sample voltages. Each data point is an average value for one STM image, while the error bars show the standard deviation in the measured value. 12535



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Figure 5. Evolution of the C K-edge NEXAFS spectra of DBBA on Au(111) (a−d) and Cu(111) (e−n) upon annealing to the noted temperatures for 20 min. For all curves photons are incident on the surface at an angle of 50° between the light polarization vector and the surface normal (see inset in Figure 6).

sites, which have a single hydrogen bond and two neighboring carbon atoms; and two C atoms bound to Br in C[C2Br] sites, colored in black, cyan, and green, respectively, in the inset in Figure 7 (left panel). Within each group the chemical state of the atoms may vary slightly, but the difference in EB of the corresponding C 1s PE lines is too small to be resolved. Thus, at all annealing steps the C 1s PE spectra can be fitted with two major components: C1 (283.7 eV) and C2 (284.1 eV), representing the C[C2H] and C[C3] sites, respectively. It is worth noting that the energy position of the C 1s core level, corresponding to the C atoms in C[C3] sites, is consistent with the C 1s binding energy of epitaxial graphene on Au.47 At RT an additional small high-binding energy component C3 (284.4 eV) corresponding to the C[C2Br] sites is included in the fitting procedure. The high binding energy shift of this component relative to C1 and C2 is due to the transfer of electron density to the more electronegative Br atoms. At RT the C1 component dominates the spectrum; at 200 °C component C3 disappears as a result of debromination, while the C1 and C2 lines have almost equal intensity, reflecting a formation of polyanthracene chains. Upon further annealing, the C1 component decreases, while C2 increases until at 400 °C the C2 component dominates. In Table 1 the contributions of individual components C[C3], C[C2H], and C[C2Br] to the total C 1s signal, determined from the peak fitting analysis at different stages of ribbon formation, are presented and compared with the corresponding values predicted by the simple model for molecules, chains, and ribbons. The results of the C 1s PE peak fitting are in excellent agreement with the model values in the case of Au(111) substrate, and we can conclude that photoemission results quantitatively support the scenario of deposited DBBA molecule transformation into molecular chains at 200 °C and into H-terminated 7-AGNRs at 400 °C. A similar analysis has been performed on the C 1s PE spectra from DBBA and its derivatives on Cu(111). The NEXAFS and STM results show that on Cu(111) the debromination of DBBA molecules occurs at RT, while chains and GNRs form below 100 °C and at 250 °C, respectively. Upon formation of GNRs the C 1s PE line profile evolves gradually, while between 250 and 400 °C the spectrum remains almost unchanged.

Cu(111) surfaces, we suggest that the polymer chains are formed on active Cu(111) much earlier than on the inert Au(111) surface, namely, at 100 °C, while already at 250 °C the GNRs are formed. Angle-dependent C K-edge NEXAFS spectra characteristic of the three consecutive stages of GNR formation on Au(111) and Cu(111) are presented in Figure 6. The insets show schematic ball and stick models of the structures corresponding to each set of data: intact DBBA molecules (on Au) or individual debrominated biradicals (on Cu), polymer chains and 7-AGNRs. An angle dependence can be seen in all cases, with a reduction of the π* resonance intensity at normal incidence. For all systems the angle dependence becomes much more prominent in going from chains to GNRs, while for the first two stages the π* resonances have a nonzero intensity at normal incidence. This behavior is explained by the alternating tilt about the C−C bonds connecting successive anthracene units in the molecular biradical, or along the polymer chain, resulting in a macroscopic deviation from planarity. This tilt observed previously by STM for polyanthracene chains on Au(111)24 and reported above for similar chains on Cu(111) (Figure 3b−d) results from a steric hindrance which prevents polymerization of single planar anthracene molecules. In turn, subsequent cyclodehydrogenation within the polymer chains leads to the formation of predominantly flat GNRs, as can be seen from the vanishing π* resonance intensity at normal incidence. To provide further information on the GNR growth process, a series of C 1s PE spectra were recorded for each system at different annealing temperatures (Figure 7). In both cases the spectra gradually evolve upon annealing. At each step the overall line shape is rather complex, which is common for aromatic compounds and is caused by the presence of a large number of nonequivalent C atoms in such systems.43 In order to establish a reliable curve-fitting procedure, a series of the C 1s PE spectra corresponding to the well-characterized process of GNR formation on inert Au(111) were fitted first. Among 28 atoms constituting the DBBA molecule, three main groups of C atoms can be distinguished (neglecting interaction with the substrate): C atoms with three neighboring carbons and sp2 hybridized valence electron states, C[C3]; C atoms in C[C2H] 12536



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states, while the additional weak component (at 284.0 eV) can be attributed to C atoms on the edges of graphene islands.48 At temperatures lower than or equal to 400 °C the peak fitting analysis can be performed similarly to the case of Au, taking into account that debromination of DBBA on Cu already occurs at RT. Again, three groups of atoms are considered at RT: carbons in C[C3] and C[C2H] sites, and unlike Au(111), the two carbon atoms which have lost their Br neighbors (C[C2] sites). Consequently, the C 1s PE line for the debrominated molecules can be fitted with three components. The binding energy of the sp2 component in graphene on Cu(111) has been chosen as a reference for the energy position of the peak corresponding to carbon in the C[C3] sites, C2 (284.5 eV). The energy separation of 0.4 eV between C1 and C2 in DBBA on Au(111) has been used to set the position of the C1 component (284.1 eV), while component C3 (C[C2] sites) is located at the low-energy side of the spectrum (283.0 eV; see the expanded image in the upper right corner of Figure 7). This pronounced change in the position of C3 (in comparison with DBBA on Au) is caused by an excess of electron density on the two C atoms which have lost their Br neighbors. The dangling bonds formed on these C atoms after debromination are likely to interact with the valence states of active Cu atoms forming C−Cu bonds. The evolution of the C 1s PE spectrum upon annealing to 100 °C, when polymer chains are formed, and further to 250 °C, when 7-AGNRs are formed, is similar to the case of GNR formation on Au(111). The relative intensities of the components, listed in Table 1, are in reasonable agreement with the model structures suggested for each annealing step. Some deviations from the values predicted by the model are most likely due to the interaction with the Cu(111) substrate, which can cause atoms contributing to one and the same spectral component to become more different in chemical state than on Au. Annealing to 500 °C causes a notable change in the spectrum reflecting a decomposition of the GNRs and the formation of a disordered phase. At this temperature only dehydrogenated carbon-containing species remain on the surface. The appearance of a broad low-energy shoulder (fitted with two red components C4) results from the carbon atoms with reduced coordination (various edge atoms), probably interacting with the Cu(111) substrate. This strong interaction is also responsible for the low-energy shift of the dominant C2 component. At 650 °C the carbon atoms start to aggregate leading to a reduction in the amount of the edge C atoms, until eventually graphene islands are formed at about 750 °C.

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Figure 6. Angle dependent C K-edge NEXAFS spectra of DBBA on Au(111) and Cu(111) at different substrate temperatures, where θ is an angle between the light polarization vector and the surface normal. The insets show the schematic representations of the structures corresponding to each spectrum.

DISCUSSION In the following section a comparison of the GNR formation process on Au(111) and Cu(111) is presented. From the C 1s PE spectra (Figure 7) detailed knowledge about the dynamics of the GNRs growth process on each substrate can be obtained. Figure 8 shows the intensity evolution of individual components (C1, C2, and C3) in the C 1s PE signal as a function of annealing temperature. The insets illustrate corresponding stages of DBBA transformation at specific annealing temperatures. Significant differences are observed in the GNR growth dynamics on Au(111) and Cu(111). On Au(111) the C1 and C2 components evolve gradually up to 200 °C, while at 200 °C, where debromination of DBBA molecules and formation of the polymer chains occurs, the active process of cyclodehydrogenation starts and continues up to 300 °C. After this point the relative intensities of C1 and C2

Annealing above 400 °C results in substantial changes in the C 1s PE line shape and binding energy. Particularly, upon annealing from 400 to 500 °C additional broad components appear at lower EB, while the main peak is gradually shifting to lower EB. On further annealing to 650 °C the main peak returns to its initial binding energy, while the low-energy components decrease. The C 1s PE spectrum at 750 °C corresponds to graphene islands on Cu(111), in agreement with the C K-edge NEXAFS data (see Figure 5n). The PE spectrum can be fitted with two components: the dominant component (at 284.5 eV) is due to carbon atoms with sp2 hybridized valence electron 12537



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Figure 7. Evolution of the C 1s PE spectrum of DBBA/Au(111) and DBBA/Cu(111) as a function of increasing sample temperature along with the results of the peak-fit analysis. The spectra are recorded with hv = 380 eV for high surface sensitivity. Insets show ball and stick models of the following structures: DBBA molecule, one unit of the polymer chain, and one unit of the H-terminated 7-AGNR. Colors of the spectral components correspond to the colors of the carbon atoms in the insets, while bromine atoms are yellow and hydrogen atoms are violet.

plausible to suggest that polymer chains do not form actively at this temperature because of the insufficient thermal energy required for the debrominated molecules to overcome the diffusion barrier. Nevertheless, even at temperatures below 100 °C dehalogenated molecules can diffuse on the surface and form short chains (as can be visible in STM), while the formation of long chains can be expected at around 100 °C. From Figure 8 it is clear that in contrast to the Au(111) substrate, active cyclodehydrogenation already occurs on Cu(111) starting from 100 °C until H-terminated 7-AGNRs appear at 200−250 °C. It is worth mentioning that unlike 7-AGNRs on Au(111), nanoribbons on Cu(111) are formed in the presence of adsorbed atomic Br, which remains on the surface up to 500 °C. Potentially, the presence of atomic Br on the surface during GNR formation could have a negative impact on the quality of the GNRs, but we did not observe it directly in STM.

Table 1. Intensities of Individual Components in the C 1s PE Spectra (as a Percentage of the Total Peak Area), Obtained from the Fitting Procedure at Different Steps of GNR Formationa Au(111) Molecule Chain 7-AGNR

C[C3](%)

C[C2H](%)

C[C2Br](%)

35(36) 43(42) 69(71)

57(57) 57(58) 31(29)

8(7) 0(0) 0(0)

Cu(111) Moleculeb Chain 7-AGNR

C[C3](%)

C[C2H](%)

C[C2](%)

30(36) 48(42) 65(71)

67(57) 52(58) 35(29)

3(7) 0(0) 0(0)

a

Values in parentheses show the fraction of atoms in corresponding positions, as expected for the DBBA molecules, chains, and ribbons. b Debrominated molecule.

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CONCLUSIONS To conclude, we have performed a comparative study of the GNR growth process based on DBBA molecule adsorption on Au(111) and Cu(111) substrates, using PES, NEXAFS, and STM techniques. We have shown that H-terminated 7-AGNRs can be grown on a Cu(111) substrate via a bottom-up approach by using DBBA molecules as a precursor. On the basis of a comparison with the analogous process on the Au(111) surface

components do not change significantly. Therefore, we assume that on Au(111) short GNRs can already be found at 300 °C, while annealing to 400 °C leads to the formation of longer ribbons. As demonstrated above, on Cu(111) the precursor molecules lose bromine readily upon adsorption at RT. However, based on the intensity ratio of the C1 and C2 components it is 12538



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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the Swedish Research Council, the Swedish Energy Agency (STEM), the European Research Council (Grant 321319), the St. Petersburg State University (Grant No. 11.38.638.2013), the Russian Foundation for Basic Research (Grant No. 12-0200999), and the Science Foundation Ireland through the Principal Investigator grant SFI P.I.09/IN.1/I2635.

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Figure 8. Intensity evolution of individual components of the C 1s PE signal (C1, C2, and C3) measured as a function of annealing temperature upon GNR formation on Au(111) and Cu(111) substrates. Intensities are taken from the results of fitting and presented in percentages from the total peak area. Insets show ball and stick models of structures, corresponding to different steps of GNRs formation on Au(111) and Cu(111).

the effect of the substrate reactivity on the GNR growth process has been demonstrated. On the more reactive Cu(111) a complete dehalogenation of DBBA already takes place at room temperature, and tilted polymer chains are obtained at slightly elevated temperatures, while for the inert Au(111) substrate debromination of DBBA molecules only occurs at 200 °C. Further annealing of the Cu(111) substrate leads to cyclodehydrogenation along the polymer chain and formation of flat 7-AGNRs at 250 °C, while on the Au(111) surface formation of GNRs completes at around 400 °C. This difference is attributed to the influence of the molecule−substrate interaction on dehalogenation and cyclodehydrogenation processes. On Cu(111) the ribbons are better aligned along the low-index crystallographic direction of the substrate than on Au(111), and the apparent height is smaller, as a consequence of stronger chemical bonding. In general, we have demonstrated that a combination of core-level PES, NEXAFS, and STM can be effectively used to investigate and monitor the process of graphene nanoribbon formation on metal surfaces.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address

Nikolay A. Vinogradov, currently at European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, B.P. 220, FR-38043 Grenoble Cedex, France 12539



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