Reply to "Comment on 'Bottom-up graphene-nanoribbon fabrication reveals chiral edges and enantioselectivity'".

Abstract

’ Simonov et al. have used a combination of surface-science techniques, including room-temperature (RT) scanning tun- neling microscopy (STM), to investigate 10,10 0 -dibromo-9,9 0 - bianthryl (DBPM), a precursor monomer of graphene nano- ribbons (GNRs), on Cu{111} at high DBPM coverage and slow sample annealing speeds. 1,2 They found that the product is a seven-carbon-wide armchair graphene nanoribbon (7-AGNR), as previously observed on Au{111} and Ag{111}. 3,4 In our recent article, we used low-temperature (LT) STM to probe DBPM/Cu{111} at low DBPM coverage and fast annealing speeds. 5 We found that the product is the (3,1)-chiral-edge GNR ((3,1)-GNR). As described in this Reply, we fabricated GNRs by annealing DBPM/Cu{111} under kinetic conditions close to those in ref 2. Using LT STM, we resolved the atomic structure of the product GNRs. We demonstrate that the GNRs investigated by refs 2 and 5 are likely to be the same structure. Imaging Graphene Nanoribbons Fabricated under the Same Kinetic Conditions as Simonov et al. In their Comment, Simonov et al. propose that DBPM polymerization on Cu{111} may be kineti- cally controlled to yield GNRs of different edge configurations, via different mechanisms. 1 Specifically, for higher coverage DBPM/Cu{111}, a slow annealing rate would successively trigger debromination, polymerization, and cyclodehydro- genation (CDH), forming 7-AGNR (Scheme 1 in Figure 1; i.e., via the Ullmann coupling reaction 3 ); for lower coverage DBPM/Cu{111}, a fast annealing rate would promote island formation followed by debromination and CDH, forming (3,1)- GNR (Scheme 2 in Figure 1). 5 To test this hypothesis, we use our results from ref 5, reproduced below (Figure 2a and blue line in 2c), as reference. We then perform a set of comparative experiments by depositing DBPM on clean Cu{111} at higher coverages, heating the sample at slower rates, and annealing the sample for longer times (Figure 2b and red line in 2c), thus emulating the experimental conditions of Simonov et al. As our STM image in the top panel of Figure 2b indicates, the new fabrication conditions (which attempt to reproduce those from ref 2) yielded a GNR morphology closely resem- bling that shown in Figure 2e of ref 2. Moreover, our high- resolution STM images in Figure 2b, bottom panel, confirm that the “zigzag shape” of our GNR edges is caused by the atomic structure of the (3,1)-GNR. Therefore, we conclude that, as of yet, we find no STM evidence that the polymerization of DBPM on Cu{111} can be kinetically controlled to select the edge conformation of the resulting GNRs. Instead, our current work supports our original conclusion in ref 5 that DBPM polymerization on Cu{111} produces (3,1)-GNRs due to the substrate's surface atomic structure and catalytic prop- erties (Scheme 2). The fact that our newest results still show the formation of (3,1)-GNRs under conditions close to ref 2 Figure 1. Two proposed mechanisms for 10,10 0 -dibromo-9,9 0 -bian- thryl alignment on Cu{111}. The circled numbers denotes indivi- dual mechanistic steps. 1 Surface-catalyzed debromination. 2 Polymerization. 3 Cyclodehydrogenation (CDH). 4 Molecular alignment. 5 Debromination and CDH. LETTER TO THE EDITOR Reply to “Comment on 'Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity'” Figure 2. Effects of 10,10 0 -dibromo-9,9 0 -bianthryl coverage, sample heating speeds, and sample annealing times. (a) Topographic STM images from ref 5 showing (3,1)-GNRs (top panel, 100 nm 100 nm; bottom panel, 20 nm 14 nm). The sample heating speed and annealing time used to obtain (a) are shown as the blue line in (c). (b) Topographic STM images (top panel, 100 nm 100 nm; bottom panel, 20 nm 14 nm) showing (3,1)-GNRs fabricated at higher DBPM coverage, slower sample heating rate, and longer sample annealing time with respect to (a). The sample heating speed and annealing time used to obtain (b) are shown as the red line in (c). The top panel inset of (b) shows the two-dimensional Fourier transform image of the main panel. The bottom panel inset of (b) shows the atomically resolved STM image of the location indicated by the black rectangles. In this latter image, only R-carbons are observed as circular protrusions. (c) Sample heating speeds and annealing rates used to obtain (a) and (b) in terms of power supplied to the heating filament. A filament power of 3.4 W corresponds to ∼500 °C (refer to Figure 3 for the relationship between the substrate temperature and the filament power). precludes the mechanism in Scheme 1 because the polymer structure resulting from 2 cannot form the chiral-edge nanoribbons without substantial rearrangement of CC bonds. We have not observed 7-AGNR postulated by Simonov et al.; 1 thus, we refrain from speculating further on their data interpretation and hypotheses. VOL. 9 NO. 4 www.acsnano.org