Diamond Annual Review 2021/22

44 45 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Reliable on-water synthesis for synthetic van derWaals heterostructures Related publication: K. Liu, J. Li, H. Qi, M. Hambsch, J. Rawle, A. R. Vázquez, A. S. Nia, A. Pashkin, H. Schneider, M. Polozij, T. Heine, M. Helm, S. C. B. Mannsfeld, U. Kaiser, R. Dong, & X. Feng. ATwo-dimensional polyimide-graphene heterostructure with ultra-fast interlayer charge transfer. Angew. Chem. Int. Ed. 60 , 13859 (2021). DOI: 10.1002/anie.202102984 Publication keywords: Heterostructure; 2D polymer; Graphene; Charge transfer; Functional materials S ynthetic van derWaals heterostructures (vdWHs) aremade by stacking different two-dimensional (2D) crystals. They are of interest for their novel functions in phototransistors, photodiodes, memory and tunnelling devices. Surface reconstruction and proximity effects between the neighbouring layers allow the opto-electrical properties of vdWHs to be tailored for tuning carrier density, enhancement of electron-hole separation and accelerated charge transfer. 2D polymers (2DPs) are a new generation of atomically/molecularly thin organic 2D materials, with repeated units linked via covalent bonds with long-range order in two distinct directions. Comparedwith inorganic 2Dmaterials, 2DPs can be readily tailored to amuch higher degree by using abundant building blocks and linkage chemistries. Therefore, 2DP-based vdWHs are highly attractive owing to their tunable physicochemical properties and designable functions. However, it is challenging to synthesise structurally-defined 2D polymers (2DP) and precisely assemble themwith other 2Dmaterials in a defined vdWH sequence. Researchers have nowdemonstrated a general but reliable on-water synthesis and assembly strategy for preparing large-area 2D polyimide (2DPI)-graphene (G) vdWH. The team used Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) on the Surface and Interface Diffraction beamline (I07) to demonstrate the successful formation of vdWHs. The on-water surface synthesis approach holds promise as a general method for preparing organic-inorganic vdWHs. The structures of 2DPs can be adjusted to engineer their band gaps and enhance the charge transfer. Two-dimensional (2D) van der Waals heterostructures (vdWHs) are generated by integration of 2D materials with dangling-bond-free surfaces through the weak interlayer vdW interaction along the vertical direction 1 . 2D polymers (2DPs) have recently emerged as a new generation of atomically/ molecularly thin organic 2D materials, which comprise repeated units linked via covalent bonds with long-range order in two distinct directions 2 . Owing to the homogenous, free-standing, transferable and large-area (up to cm 2 ) characteristics, the resultant 2DPs are attractive for the construction of 2D vdWHs. In this work, the preparation of 2D polyimide (2DPI)-graphene (G) vdWH by the on-water surface synthesis and assembly strategy was demonstrated. The 2DPI was firstly synthesised by polycondensation between 5,10,15,20-tetrakis(4-aminophenyl)porphyrin ( M1 ) and perylene-3,4,9,10- tetracarboxylic dianhydride ( M2 ), as shown in Fig. 1a. The synthesis and fabrication of 2DPI-G involve 6 steps, as illustrated in Fig. 1b. The procedure from Step 1 to Step 4 deals with the synthesis of 2DPI. In Step 1, a chloroform solution of M1 (0.1 ml, 1mgmL −1 ) was spread onto the water surface. After 5 min evaporation of chloroform, the Delrin barriers were driven forward to compress M1 on the water surface at a rate of 1mm.min −1 (Step 2). Then, M2 (20 mL, 1 mg.mL -1 , dissolved in 1 mg.mL -1 LiOH aqueous solution, ~200 molar equivalents to M1 ) was gradually injected into the trough (Step 3). And the subphase was tuned to be alkaline (pH = 11). M2 then slowly diffused to the air-water interface and got adsorbed onto the M1 monolayer and induced the 2D polymerisation. After 30 hours of reaction, we achieved a ~20 cm x 7.5 cm thin film floating on the water surface (Step 4, Fig. 1a). Afterwards, the film was transferred onto a solid substrate and further annealed at 100˚C for 1 hour to complete the reaction of residual functional monomers and remove the water. The Atomic Force Microscopy (AFM) measurement at the film edge revealed a thickness of 0.8 nm, suggesting the single-layer feature. Following the synthesis of the 2DPI, Step 5 and Step 6 are concerned with the fabrication of the 2DPI-G vdWHs. In Step 5, an aqueous suspension of high- quality electrochemically exfoliated graphene was injected into the aqueous subphase (Fig. 1b). Graphene flakes then could attach to the bottom surface of the 2DP monolayer driven by the π-π stacking interaction. To prepare the 2DPI-G, we repeatedly transferred films to other substrates (Step 6 in Fig. 1b). As shown in Fig. 2a, the achieved vdWHs exhibited layered morphology via the repeated transfer method. The EG flakes are distributed in the 2DPI-G without significant aggregation as can be seen in Fig. 2b. We then explored the crystal structure of 2DPI-G. Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) was utilised to characterise the pristine crystal structures of 2DPI-G. Moreover, the 2DPI film is encapsulated by graphene in the case of 2DPI-Gwhich prevents radiation damage during the measurement. The 2D-GIWAXS pattern in Fig. 2c shows diffraction rings at 0.20 Å −1 , 1.48 Å −1 , 2.21 Å −1 and 2.53 Å −1 , which correspond to d-space values of 31.4 Å, 4.24 Å, 2.84 Å and 2.48 Å, respectively. The ring at 31.4 Å can be attributed to the 100 or 010 Bragg peaks of 2DPI (Fig. 2d), consistent with the calculated results (Fig. 2e). The peak at 4.24 Å corresponds to the layer distance of few-layer stacking graphene. 3 The peak at 2.84 Å are mainly distributed along the out-of-plane direction, which could originate from the higher-order Bragg diffraction of interlayer structure. The peak at 2.48 Å is assigned to the lattice constant of graphene ( a = b = 2.46 Å) Moreover, this ring mainly lies in the in-plane direction, suggesting that the EG flakes prefer a face-on orientation in the vdWHs. The relaxation dynamics of charge carriers in 2DPI-G was characterised by theTA spectroscopy.We protonated 2DPI-GwithHCl (37%wt), which is denoted as H-2DPI-G. Figure 3 presents a noticeable acceleration of the relaxation dynamics of H-2DPI-G with near-resonant excitation at 470 nm. Both fast and slow decay processes of H-2DPI-G are influenced by the charge transfer from H-2DPI to graphene. The most remarkable effect is the emergence of the pronounced fast relaxation process caused by the proximity of the graphene layer. Fitting with a bi-exponential decay function gives the relaxation time constants of the fast and slow decays in H-2DPI-G that are equal to 61.56.5 fs and 1700 100 fs. The timescale of 60 fs of the dominating relaxation defines the speed of the interlayer charge transfer in H-2DPI-G. In summary, we demonstrated the novel on-water surface synthesis of 2DPI and its assembly with graphene for the construction of unprecedented 2DPI-based vdWHs. Guided by the strong interlayer cation-π interaction between protonated H-2DPI and graphene, the resultant H-2DPI-G exhibited remarkable ultra-fast charge transfer within 60 fs, one of the fastest in reports. References: 1. Novoselov, K. S. et al . 2D materials and van der Waals heterostructures. Science 353 , aac9439. (2016). DOI: 10.1126/science.aac9439 2. Liu, K. Ion-water surface synthesis of crystalline, few-layer two- dimensional polymers assisted by surfactant monolayers. Nat. Chem. 11, 994. (2019). DOI: 10.1038/s41557-019-0327-5 3. Wen, Y. et al. Expanded graphite as superior anode for sodium-ion batteries. Nat. Commun. 5 , (2014). DOI: 10.1038/ncomms5033 Funding acknowledgement: We acknowledge financial support from EU Graphene Flagship (Core3, No. 881603), ERC Grants on T2DCP and FC2DMOF (No. 852909) and DFG projects (SFB-1415, No. 417590517; SPP 2244, 2DMP) as well as the German Science Council, Centre of Advancing Electronics Dresden, EXC1056 (Center for Advancing Electronics Dresden) and OR 349/1. We acknowledge Diamond Light Source for time on Beamline I07 under Proposal SI25070. Open access funding enabled and organised by Projekt DEAL. Corresponding author: Prof. Xinliang Feng, Technische Universität Dresden, xinliang.feng@tu- dresden.de Structures and Surfaces Group Beamline I07 Figure 1: Reaction scheme of the 2DPI by the LB method and schematic illustration of 2DPI-G vdWH. (a) synthesis of 2DPI on the water surface and face-to-face co-assembly of graphene and 2DPI at the interface; (b) Schematic illustration of the 2DPI-G fabrication on the water surface by LB method. Figure 2: Morphology and structural characterisation of 2DPI-G. (a) SEM image and (b) low-magnification TEM images of 2DPI-G heterostructure after twice deposition. The edges of the graphene flakes were marked by dashed lines; (c) 2D-GIWAXS pattern of 2DPI-G; (d) The profile of integrated intensity of GIWAXS pattern with a zoom-in view of low Qxy region; (e) The model from DFT calculations, showing both top view and side view. Figure 3: Dynamics of transient absorption in multi-layer graphene (blue dots), protonated 2DPI (red dots) and protonated 2DPI-G (green dots) measured using a degenerate pump- probe spectroscopy setup with fs pulses at 470 nm. The inset shows the sub-ps dynamics immediately after the photoexcitation.