Image of the Month

May 2015

Assembling molecular Sierpiński triangle fractals


The whole family of the observed B4PB molecular STs. a,b, Models of the STs(a) and the corresponding STM images of B4PB-ST-n (n =0, 1, 2, 3or4)(b). The number in each image in b indicates the number (Tn) of the B4PB molecules that participate in the corresponding molecular ST. The STM image for n =4 (b) is a cropped section of an imperfect B4PB-ST-4 structure (the largest one achieved in experiments to date). To illustrate the ST-4 structure clearly, the missing bottom-right corner, separated by the dashed line, was artificially added to the structure and is composed of modeled molecules. The length given below each STM image in b indicates the horizontal size of the corresponding STM image. Imaging conditions: constant height, Vbias =20 mV, I =1 nA (n = 0, 1 and 2); constant current, Vbias =50 mV, I =10 pA (n = 3); constant current, Vbias = −100 mV, I =15 pA (n =4).

Credits: Images and data graciously provided by Professor Kai Wu. Jian Shang, Yongfeng Wang, Min Chen, Jingxin Dai1, Xiong Zhou, Julian Kuttner, Gerhard Hilt, Xiang Shao, J. Michael Gottfried and Kai Wu.

SKLSCUSS, BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. 2Key Laboratory for the Physics andChemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China. 3Fachbereich Chemie, Philipps-Universität Marburg,Hans-Meerwein-Strasse, Marburg 35032, Germany. 4Department of Chemical Physics, School of Chemistry and Materials Science, University of Scienceand Technology of China, Hefei 230026, China. 5SPURc, 1 CREATE Way, #15-01, CREATE Tower, Singapore 138602, Singapore.

Microscope: Unisoku low-temperature STM (USM-1200S)

Control System: RHK Technology R9 Control System

Fractals, being “exactly the same at every scale or nearly the same at different scales” as defined by Benoit B. Mandelbrot, are complicated yet fascinating patterns that are important in aesthetics, mathematics, science and engineering. Extended molecular fractals formed by the self-assembly of small-molecule components have long been pursued but, to the best of our knowledge, not achieved. To tackle this challenge we designed and made two aromatic bromo compounds (4,4″-dibromo-1,1′:3′,1″-terphenyl and 4,4‴-dibromo-1,1′:3′,1″:4″,1‴-quaterphenyl) to serve as building blocks. The formation of synergistic halogen and hydrogen bonds between these molecules is the driving force to assemble successfully a whole series of defect-free molecular fractals, specifically Sierpiński triangles, on a Ag(111) surface below 80 K. Several critical points that govern the preparation of the molecular Sierpiński triangles were scrutinized experimentally and revealed explicitly. This new strategy may be applied to prepare and explore various planar molecular fractals at surfaces.