Long and isolated graphene nanoribbons by on-surface polymerization on Au(111)

Abstract

Low electronic gap graphene nanoribbons (GNRs) are used for the fabrication of nanomaterial-based devices and, when isolated, for mono-molecular electronics experiences, for which a well-controlled length is crucial. Here, an on-surface chemistry protocol is monitored for producing long and well-isolated GNR molecular wires on an Au(111) surface. The two-step Ullmann coupling reaction is sequenced in temperature from 100 °C to 350 °C by steps of 50 °C, returning at room temperature between each step and remaining in ultrahigh vacuum conditions. After the first annealing step at 100 °C, the monomers self-organize into 2-monolayered nano-islands. Next, the Ullmann coupling reaction takes place in both 1st and 2nd layers of those nano-islands. The nano-island lateral size and shape are controlling the final GNR lengths. Respecting the above on-surface chemistry protocol, an optimal initial monomer coverage of ~1.5 monolayer produces isolated GNRs with a final length distribution reaching up to 50 nm and a low surface coverage of ~0.4 monolayer suitable for single molecule experiments.

Fig. 1. Effect of initial DBBA coverage on the final GNR lengths on the Au(111) surface.

STM topographs showing the molecular self-assembled phases of DBBA at the initial coverages of (a) 0.5 ML, (b) 1.0 ML, and (c) 1.5 ML, on the Au(111) surface. d–f Corresponding STM topographs after the cyclodehydogenation step revealing the 7-aGNRs and (g–i) their length distribution histograms of the three samples shown in (d–f), respectively. STM set parameters are +2 V/10 pA for (a–c) and −2 V/10 pA for (d–f). Scale bars are 20 nm.

Fig. 2. STM topographies of as-deposited 1.5 ML DBBA on Au(111).

a Large-scale STM topograph showing the ML and BL regions of as-deposited 1.5 ML DBBA covered Au(111) surface. b, c High-resolution STM topographs showcasing the self-assembly patterns of DBBA in the ML and in the 2nd layer of BL regions, respectively. STM parameters are +2 V/10 pA. Scale bars are 100 nm for (a) and 5 nm for (b, c).

Fig. 3. Modification of the nanoscale morphology by stepwise annealing of the same sample.

Large-scale STM topographs revealing the annealing-induced surface modifications and structural phase changes in the 1.5 ML DBBA on the Au(111) surface at (a) 100 °C, (b) 150 °C, (c) 200 °C, (d) 250 °C, (e) 300 °C, and (f) 350 °C. STM set-parameters are +2 V/10 pA (a–c) and −2 V/10 pA (d–f). Scale bars are 100 nm.

Fig. 4. Fractional surface coverage and maximal length of oligomers/7-aGNRs.

Annealing temperature dependent fractional molecular surface coverage remained (red dot and line) after each annealing step of 1.5 ML DBBA on the Au(111) surface and maximum oligomer/7-aGNR lengths (blue circles and line) observed between 200 °C and 350 °C annealing steps.

Fig. 5. Bias polarity-dependent appearance of polyanthryl oligomer nano-islands on the Au(111) surface.

a, b High-resolution STM topographs recorded after the 200 °C annealing step showing the distinct morphology of an oligomer nano-island at the positive bias of (a) + 2.0 V/10 pA and at the negative bias of (b) −2.0 V/10 pA. c Corresponding line profiles drawn across the oligomer island in (a) and (b) are presented with a blue and a red curve, respectively. The presence of two STM apparent heights reveals the bilayered configuration of polyanthryl oligomer islands at 200 °C annealing step. A reconstructed molecular model of two-layered oligomer structure is given in the inset. d–f A similar bias polarity dependent high-resolution STM topograph analysis carried out at the 250 °C annealing step resemble the 200 °C annealing step presented in (a–c), respectively. The presence of uniform and single STM apparent heights indicates the monolayered configuration of polyanthryl oligomer islands at the 250 °C annealing step. Scale bars are 5 nm in (a, b, d, e).

Fig. 6. Reconstruction of BL to ML reorganization of polyanthryl oligomers in a nano-island after the 200 °C and 250 °C annealing steps.

a A scheme overlaid on the high-resolution STM image shows the 1st layer and 2nd layer oligomer chains in a BL configuration of nano-island at 200 °C in blue and red frames, respectively. Each isolated stripe corresponds to an oligomer chain. Molecular model schematics showing (b) plane and (c) cross-sectional views of two-layered oligomer nano-island correlating the experimental STM topograph configuration in (a). d–f Schematic model representations of reorganized ML configuration of oligomer nano-island at 250 °C in blue frames similar to the ones presented in (a–c), respectively. Scale bars are 1 nm in (a, d).

Fig. 7. Large-scale STM image recorded after the final step of on-surface synthesis protocol.

Large-scale STM image after the 350 °C annealing step showing completely transformed, aligned and well-isolated 7-aGNRs on the Au(111) surface. STM set parameters are −2.0 V/10 pA. Scale bar is 25 nm.