Surface Review and Letters

The application areas of two-dimensional (2D) materials, their heterostructures and composites are rapidly expanding — thanks to their extraordinary mechanical, optical, electronic and thermal properties. The family of 2D materials, which are finding a use in applications, is rapidly growing, and now includes graphene, graphene oxide, hexagonal boron nitride and transition-metal dichalcogenide, as well as the recently obtained chemical derivatives, physically modified hybrids and allotropes. New applications also trigger the development of new and more efficient methods for the synthesis of 2D materials and the formulation of composites. In this paper, we focus on the relation between the structure of the 2D materials and their heterostructures on the one hand and their properties on the other, summarize recent improvements in the synthesis approaches and discuss the perspectives in applications. We highlight existing applications in electronics, optoelectronics, electrocatalysis and energy storage and discuss new potential areas such as functional coatings and biomedical materials and devices.

During the last decades, the bottom–up strategy of on-surface molecular reactions has been extensively investigated in order to fulfill controllable fabrication of covalent interlinking nanostructures/nanomaterials at atomic scale. A variety of organic reactions have been introduced to substrates, such as Ullmann coupling, Glaser coupling, cyclodehydrogenation and so on. In this paper, these on-surface molecular reactions will be reviewed from three aspects: the precursor, surface and external stimuli. Finally, a summary of past achievements and an outlook of future scientific challenges will be discussed.

In this paper, we give an overview of different chemical modifications that can be done on the surface of two-dimensional (2D) layered materials. We place emphasis on the diversity of reactions that have been proposed and are now available to surface scientists working in 2D materials field. Using mainly, but not exclusively, MoS₂ as example, reactions involving covalent and non-covalent interactions are discussed.

Organic–2D material heterostructures have attracted intensive research interest due to their intriguing properties, with a wide range of potential applications in multifunctional flexible electronic and optoelectronic devices. Central to the realization of such devices is a fundamental understanding of the electronic structures at organic–2D material heterointerfaces. The energy level alignment (ELA) at the interface is of paramount importance because it determines the charge transfer barriers between the two materials in contact. In this paper, we discuss the physical mechanisms determining the ELAs, with special attention on interfacial charge transfer at the heterostructures. We review the current understanding of electronic properties at the heterointerfaces formed by the integration of organics with graphene and 2D transition metal dichalcogenides (TMDs), and conclude with a perspective on the future development of organic–2D material heterostructure.

Organic molecule–two-dimensional material (OM–2D material) heterostructures are relatively an emergent class of hybrid-based electronic devices and yet have gained enormous interests. These systems hold promise towards application in the realization of electronic, optoelectronic and energy conversion devices, due to their tunable thickness, rich raw materials, flexibility, and also their capability to merge the high light absorption efficiency of organic molecules and the high carrier mobility of 2D materials. Dielectric screening effects have a large influence on electronic structure and optical properties at the OM–2D material interface due to the low intrinsic dielectric polarizability of both organic molecules and 2D materials. Here, we will review current knowledge about the dielectric screening behaviors in 2D materials and the screening effect at the OM–2D material heterointerfaces, including the physical origin of the dielectric screening effect across such interfaces, how to investigate it, and the influence of the screening effect on OM–2D material interfaces. Engineering of the dielectric screening effects in OM–2D material systems is also discussed. We conclude the review by illustrating a brief introduction of other important factors, e.g. defects, charge transfer, and hybridization, that will contribute to the screening effect and these features should be considered for the future scientific and device development of OM–2D material heterostructures.

The discovery of two-dimensional (2D) materials with single or a few-atom-thick layers has presented unprecedented opportunities to study, control and harness the properties of van der Waals (vdW) materials at their 2D limit. It also offers a platform that allows the arbitrary creation of heterostructures in a materials-by-design approach with properties tailored by the constituting components. Combining organic molecules with 2D materials to form hybrid heterointerfaces has recently emerged as a facile and versatile approach to engineer the electronic, optical, magnetic and chemical properties of 2D materials for new or optimized device applications. Underpinning the development of organic–2D materials heterostructures is the ability to interrogate the interfacial electronic structures and properties at multiple dimensions. This review provides a timely update on the application of synchrotron-based soft X-ray spectroscopies (SR-SXS) in the characterization of organic–2D materials interfaces. By harnessing the unparalleled high energy resolution, tunable energy, high brilliance and tunable polarization inherent to synchrotron radiation, electronic structures, charge transfer dynamics, molecular orientations and spin configurations at the hybrid interfaces can be examined, which helps us to formularize a coherent understanding of the organic–2D materials interfaces that will guide the design of new hybrid vdW structures and devices. We also offer our perspective on the future application of SR-SXS in the exploration of organic–2D materials heterostructures.

Recent advances in bond-resolved scanning tunneling microscopy (BRSTM) have demonstrated the tremendous potential of this characterization technique to attain an ultra-high spatial resolution at the level of a single chemical bond. Due to such a unique ability to visualize chemical bonds, BRSTM has been recognized as a valuable characterization tool in the rapidly developing field of on-surface chemistry. In this paper, we discuss the recent experimental advances in BRSTM imaging techniques and their applications in the characterization of a wide scope of functional nanostructures, including individual molecules, elusive nanographenes fabricated by means of surface-assisted synthetic strategies and extended supramolecular self-assemblies.

Convergent beam electron diffraction (CBED) performed on two-dimensional (2D) materials recently emerged as a powerful tool to study structural and stacking defects, adsorbates, atomic 3D displacements in the layers, and the interlayer distances. The formation of the interference patterns in individual CBED spots of 2D crystals can be considered as a hologram, thus the CBED patterns can be directly reconstructed by conventional reconstruction methods adapted from holography. In this study, we review recent results applying CBED to 2D crystals and their heterostructures: holographic CBED on bilayers with the reconstruction of defects and the determination of interlayer distance, CBED on 2D crystal monolayers to reveal adsorbates, and CBED on multilayered van der Waals systems with moiré patterns for local structural determination.

Renewed interest has brought significant attention to tune coherently the electronic and optical properties of hybrid organic–inorganic perovskites (HOIPs) in recent years. Tailoring the intimate structure–property relationship is a primary target toward the advancement of light-harvesting technologies. These constructive progresses are expected to promote staggering endeavors within the solar cells community that needs to be revisited. Several considerations and strategies are introduced mainly to illustrate the importance of structural stability, interfacial alignment, and photo-generated carriers extraction across the perovskite heterostructures. Here, we review recent strides of such vast compelling diversity in order to shed some light on the interplay of the interfacial chemistry, photophysics, and light-emitting properties of HOIPs via molecular engineering or doping approach. In addition, we outline several fundamental knowledge processes across the role of charge transfer, charge carrier extraction, passivation agent, bandgap, and emission tunability at two-dimensional (2D) level of HOIPs/molecule heterointerfaces. An extensive range of the relevant work is illustrated to embrace new research directions for employing organic molecules as targeted active layer in perovskite-based devices. Ultimately, we address important insights related to the physical phenomena at the active molecules/perovskites interfaces that deserve careful considerations. This review specifically outlines a comprehensive overview of surface-based interactions that fundamentally challenges the delicate balance between organic materials and perovskites, which promotes bright future of desired practical applications.

Graphene oxide (GO) is an amphiphilic, water dispersible, chemical derivative of graphene. Widely used as a pathway to obtain graphene, it also has a number of interesting applications by itself due to its ability to form covalently and non-covalently bonded organic–inorganic hybrids and polymer composites. Thus, GO-based composites are used in numerous applications in membrane and coating technologies. It is important that due to the presence of functional acidic groups, GO possesses tunable physicochemical properties like a negatively charged polyelectrolyte and can be used as stimuli responsive membranes, membranes that can interact with environment and switch their properties on demand. Thus, ionic/molecular separation, water purification, selective sensing, and stimuli responsive properties have already been demonstrated in the laboratory. Good mechanical strength and conductivity (in its partially reduced form) make it attractive for the construction of the membranes for energy devices and sensors. However, concentration and distribution of the functional groups on GO molecules is difficult to control. It makes GO materials difficult to standardize, produce, and apply in industry. To this end, it is important to highlight recent achievement in the synthesis of GO as well as in design of GO-based energy devices, corrosion inhibiting coatings, and biomedical devices with improved working performances to evoke interest on mass production of GO with improved formulation.

Reduced graphene oxide (rGO) has attracted interest in its potential application in large area photodetectors owing to its ease of manufacture and wideband optical absorbance. Here, we report that thin rGO films produced via vacuum filtration of GO followed by reduction by immersion in L-ascorbic acid are capable of sensing light through a bolometric mechanism. The photoresponse of these rGO thin films can be further enhanced by dropcasting graphene quantum dots (GQDs) on the rGO surface. These GQDs were observed to increase the opacity of the rGO film and hence its absorptivity of light, thereby enabling a significant increase in the photoresponse of the device.