Atomically Precise 2D Polymers with Kagome Lattice and High Conductivity

February 21, 2024 - Two-dimensional van der Waals heterostructures (2D vdWhs) are of significant interest due to their tunable physical properties defined by the constituent monolayers. Scientists from the Max Planck Institutes of Microstructure Physics and Polymer Research have together with Universities in Dresden, Ulm and Göttingen, Germany and Seoul, Korea and the Hekmholtz Centre Dresden-Rossendorf fabricated a new type of 2D vdWh with defined interfacial band alignment and interlayer coupling.

Two-dimensional van der Waals heterostructures (2D vdWhs) constitute a new class of artificial materials constructed by vertically stacking atomically thin 2D crystals, including graphene, transition metal dichalcogenides (TMDs), and hexagonal boron nitride (h-BN), which are held together by van der Waals (vdW) interactions. Thanks to their unique characteristics, the established vdWhs are versatile platforms for a wide spectrum of applications, including tunneling transistors, and photodetectors. While vdWhs hold great promise for developing new materials and device concepts, the thus-far reported 2D vdWhs are mainly based on the assembly of monolayer inorganic 2D crystals. Exploring novel 2D vdWhs based on monolayer organic 2D materials, characterized by customizable structures and properties [1], is of significant importance but remains rarely explored. This is primarily because of the difficulty in synthesizing organic 2D materials by controlling reactions at the monolayer scale while maintaining large-area uniformity and atomic flatness. Moreover, the limited availability of techniques presents significant challenges in characterizing atomically thin organic 2D layers, due to their high electron beam and X-ray sensitivity [2].

To confirm the crystallinity of the obtained monolayer Cu₃BHT, the authors conducted grazing-incidence X-ray diffraction (GIXD) measurements on the water surface. As shown in Figure 1a, the GIXD pattern unambiguously shows three in-plane Bragg diffraction peaks at Q_xy = 0.87, 1.48, and 1.75 Å⁻¹, corresponding to the 100, 110, and 200 Bragg reflections of a hexagonal lattice (d-spacing, 0.72 nm). The optimized structure model of monolayer Cu₃BHT, obtained by density functional theory (DFT) calculations, aligns well with the experimental observations. Unlike the high electron beam sensitivity of hydrogen-rich monolayer 2D polymers and 2DCPs [2], the absence of hydrogen in the Cu₃BHT framework offers high stability under electron radiation, thus providing the opportunity for further investigation of its crystal structure by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). As presented in Figure 1b, the sharp arcs in the selected area electron diffraction (SAED) pattern reveal the polycrystalline nature of Cu₃BHT. The nearest reflections at 1.37 nm⁻¹ correspond to a real d-spacing of 0.73 nm, confirming the GIXD results. Notably, atomic resolution imaging of Cu₃BHT was achieved with a C_C/C_S-corrected SALVE 3 instrument at 80 kV. As shown in Figure 1c, the AC-HRTEM image presents a highly ordered hexagonal unit cell with a lattice spacing of 0.73 nm, in agreement with the (100) plane of Cu₃BHT. The fast Fourier transform (FFT) image displays Bragg reflection spots to high spatial frequency, characteristic of good crystallinity and revealing an ordered lattice structure in the detectable area (tens of nm). In the enlarged AC-HRTEM image (Figure 1d), the Kagome lattice of Cu₃BHT is visible at the atomic level (resolution, ≈1.2 Å), which qualitatively agrees with the simulated one (Figure 1d, right) and the previously reported multilayer samples [3, 4].

Then the authors fabricated the novel 2D vdWh by combining monolayer Cu₃BHT with large-area chemical vapor deposited (CVD) graphene using a wet transfer technique (Figure 2a). The resulting vdWh was then washed and annealed at 300 °C under an H₂-Ar atmosphere to remove poly(methyl methacrylate) (PMMA) and trapped interfacial contaminants, and to enhance the interaction between Cu₃BHT and graphene. Further, the authors characterized the structures of Cu₃BHT-graphene vdWh using SAED and TEM (Figure 2c, left). Importantly, the SAED pattern from the overlapping Cu₃BHT-graphene region shows two sets of diffraction spots (Figure 2c), with the inner and outer sets corresponding to Cu₃BHT and graphene, respectively. Note that no clear Moiré super-lattice structure was observed using AC-HRTEM imaging, mainly due to the polycrystalline nature of Cu₃BHT [5]. To further explore the electrical characteristics evolution upon vdWh formation, the authors calculated their band structures (Figure 2f) and electrostatic potentials using DFT. The calculated E_F of the vdWh is positioned at 0.315 eV below the graphene Dirac cone, signifying a prominent hole accumulation in graphene. Furthermore, the authors tested three different twist angles, all of which yielded consistent qualitative results, suggesting that the observed effect is independent of interlayer twist and, consequently, the domain position within the polycrystalline material.

The charge density difference (CDD) iso-surfaces highlight a strong interlayer coupling between monolayer Cu₃BHT and graphene (Figure 2g), leading to electron accumulation in Cu₃BHT and hole accumulation in graphene. These results align with our Raman and TRTS observations and identify that the strong interlayer coupling contributes to significant hole transfer from monolayer Cu₃BHT to graphene upon contact. The interfacial charge equilibrium establishes a built-in electric field directed from graphene toward Cu₃BHT. The authors anticipate that the strong interlayer coupling originates from the following two factors: i) intimate contact enabled by their atomically flat surfaces [6, 7]; ii) dense wave-function overlap at the interface contributed by highly delocalized electronic orbitals in Cu₃BHT [8] and massless Dirac fermions in graphene [9].

Further, the authors fabricated gated Hall bar devices to investigate the electronic properties of Cu₃BHT-graphene vdWh (Figure 3a). Furthermore, the authors investigate the magnetic field dependence of normalized longitudinal resistance R_xx/R_{xx_min}, as presented in Figure 3d. These curves are interpolated by data points measured at the neighborhoods of their Dirac points. Close to zero field a small peak is observed, which could be related to weak localization [10]. The remaining curves exhibit positive magnetoresistance, consistent with the Drude model [11]. The full mapping of gate voltage-dependent magneto-resistance of graphene and Cu₃BHT-graphene vdWh is depicted in Figure 3e. In graphene, a 130% positive magneto-resistance is observed around the Dirac point, which diminishes significantly as the system moves away from the Dirac point, aligning with previous reports [12]. However, in Cu₃BHT-graphene vdWh, the magneto-resistance remains largely unchanged over a wide range of gate voltages.

To elucidate the light-induced interfacial charge carrier separation and recombination in Cu₃BHT-graphene vdWhs, the authors measured photoconductivity (Δ𝜎) dynamics using TRTS [13]. Figure 4a,b shows that following photoexcitation by a 1.55 eV ultrashort pulsed laser, bare graphene experiences a reduction in conductivity. This so-called “negative Δ𝜎” has been widely reported in metallic systems including doped graphene [14], due to the enhanced momentum scattering experienced by photo-generated hot carriers. The electronic system relaxes back to equilibrium on the sub-10 ps time scale following hot carrier cooling in bare graphene. Conversely, in monolayer Cu₃BHT the same photoexcitation evokes no discernible Δ𝜎. This is because the intrinsic charge carrier mobility of Cu₃BHT is much lower than that of graphene. Notably, the vdWh manifests negative Δ𝜎 dynamics featuring a rapid decay process within the first few ps, followed by a much slower decay spanning tens of ps. The rapid decay process can be attributed to hot carrier relaxation in graphene, while the subsequent relatively slow decay process can be rationalized by interlayer charge recombination following ultrafast photoinduced CT [15]. Specifically, upon photoexcitation, charge carriers are initially injected into both layers. Subsequently, ultrafast photo-induced CT occurs at the interface between monolayer Cu₃BHT and graphene, modulating the carrier. This is followed by interlayer recombination involving electrons residing in graphene and holes residing in monolayer Cu₃BHT. A single exponential fit to the relatively slow decay process yields an interlayer recombination lifetime of ≈56 ps. The reason for the observed photoinduced net electron (rather than hole) transfer from Cu₃BHT to graphene and the relatively long-lived holes remaining in Cu₃BHT can be tentatively attributed to the static built-in electric field directed from graphene toward Cu₃BHT [16]. This facilitates the transfer of photo-generated electrons from Cu₃BHT to graphene while hindering the transfer of photo-generated holes from Cu₃BHT to graphene. Note that the broadband absorption of Cu₃BHT, effective net electron transfer efficiency of ≈34% from Cu₃BHT to graphene (Figure 4c), and relatively long interlayer charge separation time facilitate conductance modulation through the photogating effect [17], making Cu₃BHT-graphene vdWhs promising candidates for designing broadband photodetectors with ultrahigh photoconductive gain and responsivity [16].

Resource: Wang, Z., Fu, S., Zhang, W., Liang, B., Liu, T.-J., Hambsch, M., Pöhls, J. F., Wu, Y., Zhang, J., Lan, T., Li, X., Qi, H., Polozij, M., Mannsfeld, S. C. B., Kaiser, U., Bonn, M., Weitz, R. T., Heine, T., Parkin, S. S. P., Wang, H. I., Dong, R., & Feng, X. (2024). A Cu₃BHT-Graphene van der Waals Heterostructure with Strong Interlayer Coupling for Highly Efficient Photoinduced Charge Separation. Advanced Materials, 36, 2311454. DOI: 10.1002/adma.202311454