Bottom up synthesis of multifunctional nanoporous graphene (1804.09670v1)

Abstract: Nanosize pores can turn semimetallic graphene into a semiconductor and from being impermeable into the most efficient molecular sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the one nanometer range. The size, density, morphology and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our measurements further reveal a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ~1 eV coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species.

• The paper introduces a bottom-up synthesis method that achieves atomic precision for creating multifunctional nanoporous graphene with semiconducting and nanosieving properties.
• It employs a hierarchical three-step thermal process on Au(111) to form graphene nanomesh with nearly defect-free structures and high pore density.
• Transport measurements reveal a 1 eV bandgap and an on-off ratio of approximately 10^5, highlighting its potential for FET and sensing applications.

Synthesis of Multifunctional Nanoporous Graphene: Semiconductor and Molecular Sieve Integration

The paper entitled "Bottom up synthesis of multifunctional nanoporous graphene" introduces a novel methodology for synthesizing nanoporous graphene (NPG) with precise control over pore size, density, and chemical composition. Utilizing a bottom-up approach, the authors present an overview process that yields NPG with both semiconducting and nanosieving functionalities, potentially advancing its application in field-effect transistors (FETs) and nanoscale molecular sieves.

Methodology and Findings

The synthesis of NPG described in this paper is accomplished through a hierarchical, on-surface chemical approach, encompassing three thermal reaction steps—debromination, cyclodehydrogenation, and dehydrogenative cross-coupling. The steps are explicitly tuned to yield nanoporous structures from the molecular precursor, DP-DBBA (10,10’-dibromo-9,9’-bianthracene derivative). The process was executed on an Au(111) surface, leveraging the distinct thermal activation onsets for each step to achieve atomic precision and high defect selectivity.

• Step-by-Step Synthesis:
  • T1 (200°C): Initiation with surface-assisted ULLMann coupling forms polymeric chains from debrominated aryl radicals.
  • T2 (400°C): Cyclodehydrogenation transforms these chains into graphene nanoribbons (GNRs) exhibiting a periodic width modulation and maintaining parallel alignment.
  • T3 (450°C): Dehydrogenative cross-coupling interlinks GNRs, creating a nanomesh with precise pore formation and long-range order.

The authors achieved a coupling yield approaching 100%, forming extensive NPG sheets of up to 70 x 50 nm with ultra-high pore density and minimal defects (~2% concentration).

Electronic and Transport Properties

The electronic structure of the developed NPG is characterized by an anisotropic band structure featuring orthogonal one-dimensional bands and a bandgap of approximately 1 eV. Density functional theory (DFT) and scanning tunneling spectroscopy (STS) analyses confirmed the presence of longitudinal bands, transversal bands, and distinct pore bands.

• L and T Bands: The GNRs' bandgap of 1.0 eV, slightly lower than the GW approximation, is conserved in NPG, demonstrating the preservation of semiconducting properties through the hierarchical design.
• Pore Bands: The bonding band formation within pore states provides further modulation capabilities for electronic properties.

Transport measurements utilizing FET configurations yielded encouraging results, demonstrating a significant on-off ratio of ~10^5 and exemplifying the potential of NPG in semiconductor devices. Additionally, the observed Schottky barrier suggests opportunities for optimizing electronic contacts to further enhance device performance.

Implications and Future Prospects

The successful integration of semiconducting and sieving functionalities in NPG opens promising avenues for next-generation graphene-based electronic devices. The ability to precisely control pore properties and band structures suggests vast possibilities in FET sensors, gate-controlled molecular filters, and responsive electronic tracking systems. The presence of pore-localized states presents opportunities for molecular interaction studies, crucial for chemical and biological sensing applications.

Looking forward, further exploration into substrate interactions, scaling synthesis techniques, and refining device fabrication processes could significantly expand the applicability of NPG. Additionally, leveraging its unique band anisotropy may yield exciting advancements in optoelectronics and sensing technologies. The methodology presented could stimulate substantial developments in nanomaterial synthesis and application across diverse technological landscapes.