Bottom-Up Synthesis of Graphene Monolayers with Tunable Crystallinity and Porosity

Electron-Beam-Induced Modification of N-Heterocyclic Carbenes: Carbon Nanomembrane Formation

Electron irradiation of self-assembled monolayers (SAMs) is a versatile tool for lithographic methods and the formation of new 2D materials such as carbon nanomembranes (CNMs). While the interaction between the electron beam and standard thiolate SAMs has been well studied, the effect of electron irradiation for chemically and thermally ultrastable N-heterocyclic carbenes (NHCs) remains unknown. Here we analyze electron irradiation of NHC SAMs featuring different numbers of benzene moieties and different sizes of the nitrogen side groups to modify their structure. Our results provide design rules to optimize NHC SAMs for effective electron-beam modification that includes the formation of sulfur-free CNMs, which are more suitable for ultrafiltration applications. Considering that NHC monolayers exhibit up to 100 times higher stability of their bonding with the metal substrate toward electron-irradiation compared to standard SAMs, they offer a new alternative for chemical lithography where structural modification of SAMs should be limited to the functional group.

Emerging Advances around Nanofluidic Transport and Mass Separation under Confinement in Atomically Thin Nanoporous Graphene

Membrane separation stands as an environmentally friendly, high permeance and selectivity, low energy demand process that deserves scientific investigation and industrialization. To address intensive demand, seeking appropriate membrane materials to surpass trade-off between permeability and selectivity and improve stability is on the schedule. 2D materials offer transformational opportunities and a revolutionary platform for researching membrane separation process. Especially, the atomically thin graphene with controllable porosity and structure, as well as unique properties, is widely considered as a candidate for membrane materials aiming to provide extreme stability, exponentially large selectivity combined with high permeability. Currently, it has shown promising opportunities to develop separation membranes to tackle bottlenecks of traditional membranes, and it has been of great interest for tremendously versatile applications such as separation, energy harvesting, and sensing. In this review, starting from transport mechanisms of separation, the material selection bank is narrowed down to nanoporous graphene. The study presents an enlightening overview of very recent developments in the preparation of atomically thin nanoporous graphene and correlates surface properties of such 2D nanoporous materials to their performance in critical separation applications. Finally, challenges related to modulation and manufacturing as well as potential avenues for performance improvements are also pointed out.

Nanoporous Amorphous Carbon Monolayer Derived from Fullerene Film

Carbon materials derived from fullerene are reported recently with unique structures and properties. However, only micrometer size samples can be obtained that limits the studying and exploration for membrane applications. Here, the preparation of a centimeter-size nanoporous amorphous carbon monolayer is reported by rapid pyrolyzing a Langmuir-Blodgett film of fullerene. The sample is fully characterized and the results indicate that the amorphous carbon monolayer derived from fullerene is metastable and insulating. The ionic transmembrane transport study demonstrates that the membrane is porous and cation selective with a selectivity of 26%. This work provides new insights into the controlled synthesis of large-size metastable amorphous carbon monolayer.

Critical Point Drying of Graphene Field-Effect Transistors Improves Their Electric Transport Characteristics

A critical point drying (CPD) technique is reported with supercritical CO2 as a cleaning step for graphene field-effect transistors (GFETs) microfabricated on oxidized Si wafers, which results in an increase of the field-effect mobility and a decrease of the impurity doping. It is shown that the polymeric residues remaining on graphene after the transfer process and device microfabrication are significantly reduced after the CPD treatment. Moreover, the CPD effectively removes ambient adsorbates such as water therewith reducing the undesirable p-type doping of the GFETs. It is proposed that CPD of electronic, optoelectronic, and photonic devices based on 2D materials as a promising technique to recover their intrinsic properties after the microfabrication in a cleanroom and after storage at ambient conditions.

Innovations in the synthesis of graphene nanostructures for bio and gas sensors

Sensors play a significant role in modern technologies and devices used in industries, hospitals, healthcare, nanotechnology, astronomy, and meteorology. Sensors based upon nanostructured materials have gained special attention due to their high sensitivity, precision accuracy, and feasibility. This review discusses the fabrication of graphene-based biosensors and gas sensors, which have highly efficient performance. Significant developments in the synthesis routes to fabricate graphene-based materials with improved structural and surface properties have boosted their utilization in sensing applications. The higher surface area, better conductivity, tunable structure, and atom-thick morphology of these hybrid materials have made them highly desirable for the fabrication of flexible and stable sensors. Many publications have reported various modification approaches to improve the selectivity of these materials. In the current work, a compact and informative review focusing on the most recent developments in graphene-based biosensors and gas sensors has been designed and delivered. The research community has provided a complete critical analysis of the most robust case studies from the latest fabrication routes to the most complex challenges. Some significant ideas and solutions have been proposed to overcome the limitations regarding the field of biosensors and hazardous gas sensors.

Advances in analysis of pharmaceuticals by using graphene-based sensors

Safe and effective use of drugs relies on proper pharmaceutical analysis. Graphene has been extensively used to construct sensors for this purpose. Over the years, a large variety of pharmaceutical sensors have been developed from graphene or its derivatives. This articles reviews the current status of sensor development from graphene and its derivatives, and discusses the use of graphene-based sensors in pharmaceutical analysis. It is hoped that this article cannot only offer a snapshot of recent advances in the fabrication and use of graphene-based sensors, but can also provide insights into future engineering and optimization of the sensors for effective pharmaceutical analysis.

Graphene-derived antibacterial nanocomposites for water disinfection: Current and future perspectives

Antimicrobial nanomaterials provide numerous opportunities for the synthesis of next-generation sustainable water disinfectants. Using the keywords graphene and water disinfection and graphene antibacterial activity, a detailed search of the Scopus database yielded 198 and 1433 studies on using graphene for water disinfection applications and graphene antibacterial activity in the last ten years, respectively. Graphene family nanomaterials (GFNs) have emerged as effective antibacterial agents. The current innovations in graphene-, graphene oxide (GO)-, reduced graphene oxide (rGO)-, and graphene quantum dot (GQD)-based nanocomposites for water disinfection, including their functionalization with semiconductor photocatalysts and metal and metal oxide nanoparticles, have been thoroughly discussed in this review. Furthermore, their novel application in the fabrication of 3D porous hydrogels, thin films, and membranes has been emphasized. The physicochemical and structural properties affecting their antibacterial efficiency, such as sheet size, layer number, shape, edges, smoothness/roughness, arrangement mode, aggregation, dispersibility, and surface functionalization have been highlighted. The various mechanisms involved in GFN antibacterial action have been reviewed, including the mechanisms of membrane stress, ROS-dependent and -independent oxidative stress, cell wrapping/trapping, charge transfer, and interaction with cellular components. For safe applications, the potential biosafety and biocompatibility of GFNs in aquatic environments are emphasized. Finally, the current limitations and future perspectives are discussed. This review may provide ideas for developing efficient and practical solutions using graphene-, GO-, rGO-, and GQD-based nanocomposites in water disinfection by rationally employing their unique properties.

Photoactive ultrathin molecular nanosheets with reversible lanthanide binding terpyridine centers

In recent years, functional molecular nanosheets have attracted much attention in the fields of sensors and energy storage. Here, we present an approach for the synthesis of photoactive metal-organic nanosheets with ultimate molecular thickness. To this end, we apply low-energy electron irradiation induced cross-linking of 4'-(2,2':6',2''-terpyridine-4'-yl)-1,1'-biphenyl-4-thiol self-assembled monolayers on gold to convert them into functional ∼1 nm thick carbon nanomembranes possessing the ability to reversibly complex lanthanide ions (Ln-CNMs). The obtained Ln-CNMs can be prepared on a large-scale (>10 cm²) and inherit the photoactivity of the pristine terpyridine lanthanide complex (Ln(III)-tpy). Moreover, they possess mechanical stability as free-standing sheets over micrometer sized openings. The presented methodology paves a simple and robust way for the preparation of ultrathin nanosheets with tailored photoactive properties for application in photocatalytic and energy conversion devices.

Exponentially selective molecular sieving through angstrom pores

Two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates. No such pores have been demonstrated experimentally. Here we study gas transport through individual graphene pores created by low intensity exposure to low kV electrons. Helium and hydrogen permeate easily through these pores whereas larger species such as xenon and methane are practically blocked. Permeating gases experience activation barriers that increase quadratically with molecules' kinetic diameter, and the effective diameter of the created pores is estimated as ∼2 angstroms, about one missing carbon ring. Our work reveals stringent conditions for achieving the long sought-after exponential selectivity using porous two-dimensional membranes and suggests limits on their possible performance.

Chip-Scalable, Room-Temperature, Zero-Bias, Graphene-Based Terahertz Detectors with Nanosecond Response Time

The scalable synthesis and transfer of large-area graphene underpins the development of nanoscale photonic devices ideal for new applications in a variety of fields, ranging from biotechnology, to wearable sensors for healthcare and motion detection, to quantum transport, communications, and metrology. We report room-temperature zero-bias thermoelectric photodetectors, based on single- and polycrystal graphene grown by chemical vapor deposition (CVD), tunable over the whole terahertz range (0.1-10 THz) by selecting the resonance of an on-chip patterned nanoantenna. Efficient light detection with noise equivalent powers <1 nWHz^-1/2 and response time ∼5 ns at room temperature are demonstrated. This combination of specifications is orders of magnitude better than any previous CVD graphene photoreceiver operating in the sub-THz and THz range. These state-of-the-art performances and the possibility of upscaling to multipixel architectures on complementary metal-oxide-semiconductor platforms are the starting points for the realization of cost-effective THz cameras in a frequency range still not covered by commercially available microbolometer arrays.

Halogen Functionalization in the 2D Material Flatland: Strategies, Properties, and Applications

Given the electronegativity and bonding environment of halogen elements, halogenation (i.e., fluorination, chlorination, bromination, and iodination) serves as a versatile strategy for chemical modifications of materials. The combination of halogens and 2D materials has triggered extensive interests since the first report on graphene fluorination in 2008. Subsequently, scholars consistently conduct pre-, in-process, or posthalogenation modifications of emerging 2D materials to achieve desired properties and broad device applications. They also continuously explore the role of halogens in 2D material functionalization. The multiple advantages introduced by halogen decoration make 2D materials outstanding from each subclass. In this review, an overall retrospect is provided on the research advances in the area of 2D material halogenation, including experimental halogenation strategies, halogen-triggered novel physics and properties, and advanced applications across the studied objects. Future research directions in this area are also proposed.

Graphene coupled TiO2 photocatalysts for environmental applications: A review

Nanostructured photocatalysts have always offered opportunities to solve issues concerned with the environmental challenges caused by rapid urbanization and industrialization. These materials, due to their tunable physicochemical characteristics, are capable of providing a clean and sustainable ecosystem to humanity. One of the current thriving research focuses of visible-light-driven photocatalysts is on the nanocomposites of titanium dioxide (TiO₂) with carbon nanostructures, especially graphene. Coupling TiO₂ with graphene has proven more active by photocatalysis than TiO₂ alone. It is generally considered that graphene sheets act as an electron acceptor facilitating the transfer and separation of photogenerated electrons during TiO₂ excitation, thereby reducing electron-hole recombination. This study briefly reviews the fundamental mechanism and interfacial charge-transfer dynamics in TiO₂/graphene nanocomposites. Design strategies of various graphene-based hybrids are highlighted along with some specialized synthetic routes adopted to attain preferred properties. Importantly, the enhancing interfacial charge transfer of photogenerated e¯_CB through the graphene layers by morphology orientation of TiO₂, predominated exposure of their high energy crystal facets, defect engineering, enhancing catalytic sites in graphene, constructing dedicated architectures, tuning the nanomaterial dimensionality at the interface, and employing the synergism adopted through various modifications, are systematically compiled. Portraying the significance of these photocatalytic hybrids in environmental remediation, important applications including air and water purification, self-cleaning surfaces, H₂ production, and CO₂ reduction to desired fuels, are addressed.

Selective Etching of Graphene Membrane Nanopores: From Molecular Sieving to Extreme Permeance

Two-dimensional materials are the essential building blocks of breakthrough membrane technologies due to minimal permeation barriers across atomically thin pores. Tunable pore size fabrication combined with independently controlled pore number density is necessary for outstanding performance but remains a challenge. There is a great need for parallel, upscalable methods that can control pore size from sub-nm to >5 nm, a pore size range required for membranes with effective molecular separation. Here we report a dry, facile, and scalable process introducing atomic defects by design, followed by selective etching of graphene edge atoms able to controllably expand the nanopore dimensions from sub-nm to 5 nm. The attainable average pore sizes at 10¹⁵ m^-2 pore density promise applicability to various separation applications. We investigate the gas permeation and separation mechanisms, finding that these membranes display molecular sieving (H₂/CH₄ separation factor = 9.3; H₂ permeance = 3370 gas permeation units (GPU)) and reveal the presence of interweaved transport phenomena of pore chemistry, surface flow, and gas molecule momentum transfer. We observe the smooth transition from molecular sieving to effusion at unprecedented permeance (H₂/CH₄ separation factor = 3.7; H₂ permeance = 10⁷ GPU). Our scalable graphene membrane fabrication approach in combination with sub-5 nm pores opens a new route employing 2D membranes to study gas transport and effectively paving the way to industrial applications.

Preparation of Carbon Nanomembranes without Chemically Active Groups

The electron-irradiation-induced synthesis of carbon nanomembranes (CNMs) from aromatic thiol-based self-assembled monolayers (SAMs) on gold substrate is a well-established method to form molecular thin nanosheets. These molecular two-dimensional materials can be prepared with tunable properties; therefore, they find a variety of applications in nanotechnology ranging from ultrafiltration to nanobiosensors. However, no chemically inert CNM was fabricated up to now, as the reactive thiol group is present on the membrane surface even after transferring it to other substrates. Here, we study the electron irradiation of carboxylic acid-based SAMs on a silver substrate as an alternative route for CNM formation. Our analysis, based on a combination of X-ray photoelectron spectroscopy and scanning electron microscopy demonstrates that for this type of SAMs, purely carbonaceous CNMs with tunable porosity can be obtained.