Janus 2D materials via asymmetric molecular functionalization

Abstract

Janus two-dimensional materials (2DMs) are a novel class of 2DMs in which the two faces of the material are either asymmetrically functionalized or are exposed to a different local environment. The diversity of the properties imparted to the two opposing sides enables the design of new multifunctional materials for applications in a broad variety of fields including opto-electronics, energy storage, and catalysis. In this perspective, we summarize the most enlightening experimental methods for the asymmetric chemical functionalization of 2DMs with tailored made (macro)molecules by means of a supratopic binding (one side) or antaratopic binding (two sides) process. We describe the emergence of unique electrical and optical characteristics resulting from the asymmetric dressing of the two surfaces. Representative examples of Janus 2DMs towards bandgap engineering, enhanced photoresponse and photoluminescence are provided. In addition, examples of Janus 2DMs for real applications such as energy storage (batteries and supercapacitors) and generation (photovoltaics), opto-electronics (field-effect transistors and photodetectors), catalysis, drug delivery, self-healing materials, chemical sensors and selective capture and separation of small molecules are also described. Finally, we discuss the future directions, challenges, and opportunities to expand the frontiers of Janus 2DMs towards technologies with potential impact in environmental science and biomedical applications.

Fig. 1. Schematic representation of Janus 2D materials.

Fig. 2. Schematic representation of the three main strategies for the preparation of Janus 2DMs through an antaratopic binding of (macro)molecules. (a) Two-step functionalization assisted by a transfer approach, (b) functionalization of 2DM surfaces at liquid–liquid interfaces and (c) step by step procedure. (a) Adapted with permission from ref. , Copyright 2017 American Chemical Society. (b) FGs stands for functionalized graphenes. Adapted from ref. with permission from Wiley-VHC. (c) Adapted with permission from ref. , Copyright 2017 American Chemical Society.

Fig. 3. (a) Schematic illustration of the preparation of the Janus graphene and the stacked Janus graphene thin film, LbL stands for Layer-by-Layer, (b) scheme of the different intercalation/deintercalation cycle of Na⁺ in AB graphene, and (c) CV curves measured during the first two cycles of Na⁺ intercalation in AB graphene. The sweep rate was 0.042 mV s⁻¹. The first curve shows a sharp cathodic peak, which disappears in the second cycle, corresponding to the formation of a stable solid electrolyte interphase (SEI). Adapted from ref. with permission of Science.

Fig. 4. (a, b) Energy level diagram between DAE and WSe₂, device geometry, and electrical characterization of P(VDF-TrFE) capacitor and DAE/WSe₂/FeFET. (a) Energy level diagram of the electron transport between DAE/WSe₂ and the chemical structures of investigated DAE molecules, (b) schematic diagram of the DAE/WSe₂/FeFET device with double-sided decoration: the bottom surface with DAE film and the top surface with P(VDF-TrFE) layer. (c–h) Multilevel storage of DAE/WSe₂/FeFET. (c) Transfer evolution when partially polarizing the P(VDF-TrFE) from downward to upward direction by the sweeping top gate at various ranges. (d) Corresponding I_ds current evolution behavior at V_bg = 60 V as a function of a programmed sweep. (e, f) Dynamic I_ds current evolution behavior under exposure of short UV pulses at V_bg = 0 V: (e) totally obtained 84 levels and (f) enlarged 5 levels in region I. (g) Multilevel current over five cycles by different stimuli orders. (h) Enlarged levels in region A. Adapted from ref. with permission from Wiley-VHC.

Fig. 5. Characterization of PPh₃-doped Ti–WSe₂/h-BN photodetectors. (a) Schematic diagram and optical images (before and after transfer) of h-BN inserted into the Ti-contacted WSe₂ photodetector that was doped by 7.5 wt% PPh₃. (b) Extracted photoresponsivity as a function of the incident laser power (5, 10, 100, and 1000 pW) before and after transferring to the h-BN/SiO₂ substrate. (c) Normalized temporal photoresponse curves at the rising and decaying edges. (d, e) Rising (d) and decaying (e) times of the photodetector as a function of the gate voltage (V_G = −30, 0, and 30 V), where the drain voltage (V_DS) was 1 V. Adapted from ref. with permission from Wiley-VHC.

Fig. 6. (a) The kinetic profile for nitrobenzene reductions over different catalysts under stirring-free conditions. (b) Comparison of turnover frequencies (TOFs) for different catalysts. (c) Recyclability test of Janus Pd/mSiO₂ nanosheet in biphasic hydrogenation, reaction time: 1.5 h. (d) High-angle annular dark-field imaging-scanning transmission electron microscopy (HAADF-STEM) images of Janus catalyst Pd/mSiO₂ nanosheet. Adapted from ref. with permission from The Royal Society of Chemistry.