Graphene is a two-dimensional material consisting of a carbon atom with a sp2 hybrid orbital to form a hexagonal honeycomb-like lattice, and has a thickness of about one in 100,000 (~0.335 nm) of the radius of the hair. Graphene is also the basic structural unit for constructing some carbon materials (such as fullerenes, carbon nanotubes, and graphite) (Fig. 1). Researchers have studied the uniaxial direction of graphene, and found that single-layer graphene nanoribbons (GNR) with a certain width and infinite length can be obtained by etching graphene and epitaxial growth, and can be divided according to the edge. Armchair and sawtooth (Fig. 2). Graphene nanoribbons have a quasi-one-dimensional spatial structure and extend infinitely in a uniaxial direction, and have similar mechanical and electrical properties to graphene. Since the graphene nanoribbon has an open edge, the material has a quantum confinement effect similar to carbon nanotubes. Recently, the synthesis and study of graphene nanoribbons on metal surfaces has attracted the attention of researchers, but this process requires surface synthesis to ensure the stable presence of graphene nanoribbons, which limits their application in three dimensions.
Figure 1. Schematic diagram of the structure of graphene and its derivatives. Image from the network
Figure 2. Graphene nanoribbons with two different boundaries: armchair and sawtooth. Image from the network
Recently, Colombia University Professor Thomas J. Sisto made a breakthrough, they based on graphene nanoribbons Stable three-bladed propeller-like three-dimensional nanostructures were prepared with triptycene. This three-dimensional graphene nanomaterial exhibits a new property different from graphene nanobelts and triptycene, and as an optoelectronic material, it can enhance light absorption and reduce contact resistance. The authors use it as the electron extraction layer of perovskite solar cells, and the photoelectric conversion efficiency (PCE) reaches 18.01%, which is equivalent to the best non-fullerene electron extraction layer reported. The work was published on J. Am. Chem. Soc. .
The preparation process of the propeller-like graphene nanomaterial is simply to assemble a triptycene molecule as a central “hub” and three graphene nanoribbons as “blades”. Together (Figure 3a). As shown in Figure 3b, 1, 2, and 3 are three prepared propeller-like graphene nanomaterials, respectively, “The size increases in turn. 4 is a schematic view showing the preparation of propeller-like graphene nanomaterials of different sizes. These three nanomaterials, even the largest one (width ~5 nm), have good solubility in organic solvents.
Figure 3. Propeller-like graphene nanomaterial structure. Image credit: J. Am. Chem. Soc.
Figure 4. Preparation of propeller-like graphene nanomaterials of different sizes. Image credit: J. Am. Chem. Soc.
UV-visible spectra from 1, 2, and 3 (Fig. 5) It can be seen that their longest wavelength absorption peaks are 504 nm, 570 nm and 619 nm, respectively, and thus their optical band gaps can be calculated to be 2.46 eV, 2.18 eV and 2.00 eV, respectively. From the results of UV absorption spectroscopy, it can be seen that the molar absorptivity of 3 is very high, reaching 635,000 M-1 cm-1 at ~410 nm. In addition, for 2 and 3, their molar absorptivity is greater than the sum of the molar absorptivity of the respective three “blade” molecules. For example, the molar absorptivity of 3 (455,000M-1 cm-1) is the sum of the molar absorptivity of the three “blade” molecules hPDI3 (3 × 83,000 M-1 cm-1= 250,000 M-1 cm-1), which is more than 80% higher, the total absorption has also increased by 46%. The authors believe that the increase in the absorption coefficient is partly due to the benzene cyclization of the reactants and is demonstrated by methods such as density functional theory. It can be seen from Figure 6 that many electrons can be obtained for these three nanomaterials in a small voltage range, and the authors have also studied this property by electron paramagnetic resonance spectroscopy. These excellent properties make them an important potential application value in optoelectronic devices.
Figure 5. UV-visible spectra of three propeller-like graphene nanomaterials. Image credit: J. Am. Chem. Soc.
Figure 6. Cyclic voltammograms of three propeller-like graphene nanomaterials. Image source: J. Am. Chem. Soc.
The author then uses 2 and 3 as the electron extraction layer for perovskite solar cells. It is also believed that the propeller-like structure facilitates the entry of the nanobelts into the perovskite layer and reduces the contact resistance. The photoelectric conversion efficiency of 3 as the electron extraction layer is up to 18.01%, which is equivalent to the other excellent non-fullerene electron extraction layers reported.
Figure 7. Propeller-like graphene nanomaterials as electron extraction layers of perovskite solar cells and their properties. Image: J. Am. Chem. Soc.
In general, a three-dimensional propeller-like graphene nanomaterial is prepared, which is not only refreshing in structure, but also in terms of performance. 1+1+1 > 3″ performance. These results also demonstrate the importance of preparing three-dimensional graphene materials and provide an interesting direction for future research.