Work function engineering of two-dimensional (2D) materials by application of polymer coatings represents a research thrust that promises to enhance the performance of electronic devices. While polymer zwitterions have been demonstrated to significantly modify the work function of both metal electrodes and 2D materials due to their dipole-rich structure, the impact of zwitterion chemical structure on work function modulation is not well understood. To address this knowledge gap, we synthesized a series of sulfobetaine-based zwitterionic random copolymers with variable substituents and used them in lithographic patterning for the preparation of negative-tone resists (i.e., "zwitterists") on monolayer graphene. Ultraviolet photoelectron spectroscopy indicated a significant work function reduction, as high as 1.5 eV, induced by all polymer zwitterions when applied as ultrathin films (<10 nm) on monolayer graphene. Of the polymers studied, the piperidinyl-substituted version, produced the largest dipole normal to the graphene sheet, thereby inducing the maximum work function reduction. Density functional theory calculations probed the influence of zwitterion composition on dipole orientation, while lithographic patterning allowed for evaluation of surface potential contrast via Kelvin probe force microscopy. Overall, this polymer "zwitterist" design holds promise for fine-tuning 2D materials electronics with spatial control based on the chemistry of the polymer coating and the dimensions of the lithographic patterning.
Carbonization by rapid thermal annealing (RTA) of precursor films structured by a brush block copolymer-mediated self-assembly enabled the preparation of large-pore (40 nm) ordered mesoporous carbon (MPC)-based micro-supercapacitors within minutes. The large pore size of the fabricated films facilitates both rapid electrolyte diffusion for carbon-based electric double-layer capacitors and conformal deposition of V2O5 without pore blockage for pseudocapacitors. The pores were templated using bottlebrush block copolymers (BBCPs) via cooperative assembly of phenol-formaldehyde resin to produce microphase-segregated carbon precursor films on a variety of substrates. Ultrafast RTA processing (∼50 °C/s) at elevated temperatures (up to 1000 °C) then generated stable, conductive, turbostratic MPC films, resolving a significant bottleneck in rapid fabrication. MPC prepared on stainless steel at 900 °C demonstrated exceptionally high areal and volumetric capacitances of 6.3 mF/cm2 and 126 F/cm3 (at 0.8 mA/cm2 using 6 M KOH as the electrolyte), respectively, and 91% capacitance retention after 10,000 galvanostatic charge/discharge cycles. Post-RTA conformal V2O5 deposition yielded pseudocapacitors with 10-fold increase in energy density (20 μW h cm–2 μm–1) without adversely affecting the high power density (450 μW cm–2 μm–1). The use of RTA coupled with BBCP templating opens avenues for scalable, rapid fabrication of high-performance carbon-based micro-pseudocapacitors.
Porous materials continue to establish critically important roles in applications extending from greenhouse gas capture to thermal superinsulation. Their effective structural control by an array of templating and template-free approaches imparts remarkable properties that are unattainable in the bulk. However, current preparative techniques frequently employ multiple, intricate steps that preclude scalability. Thus, there remains a need to reconcile this trade-off between structural control and procedural simplicity. Herein, a "freeze-burn" process is introduced as a rapid, robust strategy to fabricate porous carbon networks using polymer-templated rapid thermal annealing. Reduced graphene oxide is selected as the model material, templated by a polystyrene/poly(vinyl methyl ether) blend, to generate macropores on the size of phase separation, with the aim of understanding the impact of polymer mobility on templated morphologies. Without changing the template composition or processing conditions, we demonstrate applicability of freeze-burn to other carbon materials, such as graphene oxide, carbon black, carbon nanopowder, and multiwalled carbon nanotubes. This sequential templating and template degradation can be completed in one step in less than 10 min, making freeze-burn an energy-and time-efficient procedure. This work will serve as a powerful platform for the rapid templating of hard materials and will inspire simple, scalable approaches for creating porous structures.
Overcoming throughput challenges in current graphene defect healing processes, such as conventional thermal annealing, is crucial for realizing post‐silicon device fabrication. Herein, a new time‐ and energy‐efficient method for defect healing in graphene is reported, utilizing polymer‐assisted rapid thermal annealing (RTA). In this method, a nitrogen‐rich, polymeric “nanobandage” is coated directly onto graphene and processed via RTA at 800 °C for 15 s. During this process, the polymer matrix is cleanly degraded, while nitrogen released from the nanobandage can diffuse into graphene, forming nitrogen‐doped healed graphene. To study the influence of pre‐existing defects on graphene healing, lattice defects are purposefully introduced via electron beam irradiation and investigated by Raman microscopy. X‐ray photoelectron spectroscopy reveals successful healing of graphene, observing a maximum doping level of 3 atomic nitrogen % in nanobandage‐treated samples from a baseline of 0–1 atomic % in non‐nanobandage treated samples. Electrical transport measurements further indicate that the nanobandage treatment recovers the conductivity of scanning electron microscope‐treated defective graphene at ≈85%. The reported polymer‐assisted RTA defect healing method shows promise for healing other 2D materials with other dopants by simply changing the chemistry of the polymeric nanobandage.
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