Semiconductors are the basis of many vital technologies such as electronics, computing, communications, optoelectronics, and sensing. Modern semiconductor technology can trace its origins to the invention of the point contact transistor in 1947. This demonstration paved the way for the development of discrete and integrated semiconductor devices and circuits that has helped to build a modern society where semiconductors are ubiquitous components of everyday life. A key property that determines the semiconductor electrical and optical properties is the bandgap. Beyond graphene, recently discovered two-dimensional (2D) materials possess semiconducting bandgaps ranging from the terahertz and mid-infrared in bilayer graphene and black phosphorus, visible in transition metal dichalcogenides, to the ultraviolet in hexagonal boron nitride. In particular, these 2D materials were demonstrated to exhibit highly tunable bandgaps, achieved via the control of layers number, heterostructuring, strain engineering, chemical doping, alloying, intercalation, substrate engineering, as well as an external electric field. We provide a review of the basic physical principles of these various techniques on the engineering of quasi-particle and optical bandgaps, their bandgap tunability, potentials and limitations in practical realization in future 2D device technologies.

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Bandgap engineering of two-dimensional semiconductor materials

Phase has emerged as an important structural parameter — in addition to composition, morphology, architecture, facet, size and dimensionality — that determines the properties and functionalities of nanomaterials. In particular, unconventional phases in nanomaterials that are unattainable in the bulk state can potentially endow nanomaterials with intriguing properties and innovative applications. Great progress has been made in the phase engineering of nanomaterials (PEN), including synthesis of nanomaterials with unconventional phases and phase transformation of nanomaterials. This Review provides an overview on the recent progress in PEN. We discuss various strategies used to synthesize nanomaterials with unconventional phases and induce phase transformation of nanomaterials, by taking noble metals and layered transition metal dichalcogenides as typical examples. Moreover, we also highlight recent advances in the preparation of amorphous nanomaterials, amorphous–crystalline and crystal phase-based hetero-nanostructures. We also provide personal perspectives on challenges and opportunities in this emerging field, including exploration of phase-dependent properties and applications, rational design of phase-based heterostructures and extension of the concept of phase engineering to a wider range of materials. Properties of nanomaterials respond to changes in the material’s phase, as well as changes in size, composition and morphology. This Review discusses the most recent developments in phase engineering of nanomaterials to afford conventional and unconventional crystal phases, amorphous phases and amorphous–crystalline heterophases.

https://dr.ntu.edu.sg/bitstream/10356/144140/2/Nature%20Review%20Chemisty%20Phase%20engineerin.pdf

Phase engineering of nanomaterials

Two-dimensional (2D) layered metal chalcogenides (MXs) have significant potential for use in flexible transistors, optoelectronics, sensing and memory devices beyond the state-of-the-art technology. To pursue ultimate performance, precisely controlled doping engineering of 2D MXs is desired for tailoring their physical and chemical properties in functional devices. In this review, we highlight the recent progress in the doping engineering of 2D MXs, covering that enabled by substitution, exterior charge transfer, intercalation and the electrostatic doping mechanism. A variety of novel doping engineering examples leading to Janus structures, defect curing effects, zero-valent intercalation and deliberately devised floating gate modulation will be discussed together with their intriguing application prospects. The choice of doping strategies and sources for functionalizing MXs will be provided to facilitate ongoing research in this field toward multifunctional applications.

Doping engineering and functionalization of two-dimensional metal chalcogenides

The past decade has witnessed tremendous research efforts devoted to two-dimensional (2D) materials and great progress made in both their fundamental studies and technique development. 2D light-emitting materials such as transition-metal dichalcogenides (TMDs) and phosphorene are receiving particular attention as a result of their intriguing electronic, optical and optoelectronic properties. The ability to tune their layer numbers, engineer their dielectric environment, form alloys, create van der Waals heterostructures, etc. provides further means to modulate their properties which have led to a number of interesting luminescence features and potential applications in lighting, imaging and sensing. As the map of 2D materials has expanded over the past few years, 2D organic and organic-inorganic hybrid materials, such as 2D polymers, metal-organic frameworks (MOFs), and organic-inorganic hybrid perovskites, have also emerged with merits of low cost, chemical versatility and solution processibility. Importantly, the wide tunability of their functional properties is enabled by the rational design and controlled modification of their compositions and structures at the molecular level. Herein, we present a critical review of 2D light-emitting materials by categorizing them into three main groups, namely 2D inorganic light-emitting materials, 2D organic light-emitting materials, and 2D organic-inorganic hybrid light-emitting materials. Within each group, we will describe their preparation methods and discuss their structural and chemical properties. Significant emphasis will be placed on strategies that can tune their luminescence properties, and their potential applications in various fields. Finally, we will summarize by stating the current challenges and future opportunities in the development of 2D light-emitting materials.

Two-dimensional light-emitting materials: preparation, properties and applications

Two-dimensional (2D) materials1–5 offer a unique platform from which to explore the physics of topology and many-body phenomena. New properties can be generated by filling the van der Waals gap of 2D materials with intercalants6,7; however, post-growth intercalation has usually been limited to alkali metals8–10. Here we show that the self-intercalation of native atoms11,12 into bilayer transition metal dichalcogenides during growth generates a class of ultrathin, covalently bonded materials, which we name ic-2D. The stoichiometry of these materials is defined by periodic occupancy patterns of the octahedral vacancy sites in the van der Waals gap, and their properties can be tuned by varying the coverage and the spatial arrangement of the filled sites7,13. By performing growth under high metal chemical potential14,15 we can access a range of tantalum-intercalated TaS(Se)y, including 25% Ta-intercalated Ta9S16, 33.3% Ta-intercalated Ta7S12, 50% Ta-intercalated Ta10S16, 66.7% Ta-intercalated Ta8Se12 (which forms a Kagome lattice) and 100% Ta-intercalated Ta9Se12. Ferromagnetic order was detected in some of these intercalated phases. We also demonstrate that self-intercalated V11S16, In11Se16 and FexTey can be grown under metal-rich conditions. Our work establishes self-intercalation as an approach through which to grow a new class of 2D materials with stoichiometry- or composition-dependent properties. The intercalation of native atoms into bilayer transition metal dichalcogenides during growth generates ultrathin, covalently bonded materials into which ferromagnetic ordering can be introduced.

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Engineering covalently bonded 2D layered materials by self-intercalation.

Doped semiconductors are the most important building elements for modern electronic devices
 1
 . In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface2,3. Besides, the emergence of two-dimensional (2D) materials enables the realization of atomically thin integrated circuits4–9. However, the 2D nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development
 10
 . Here, we demonstrate a solvent-based intercalation method to achieve p-type, n-type and degenerately doped semiconductors in the same parent material at the atomically thin limit. In contrast to naturally grown n-type S-vacancy SnS2, Cu intercalated bilayer SnS2 obtained by this technique displays a hole field-effect mobility of ~40 cm2 V−1 s−1, and the obtained Co-SnS2 exhibits a metal-like behaviour with sheet resistance comparable to that of few-layer graphene
 5
 . Combining this intercalation technique with lithography, an atomically seamless p–n–metal junction could be further realized with precise size and spatial control, which makes in-plane heterostructures practically applicable for integrated devices and other 2D materials. Therefore, the presented intercalation method can open a new avenue connecting the previously disparate worlds of integrated circuits and atomically thin materials. Intercalation of copper and cobalt atoms into n-type SnS2 enables seamless integration of metal, and n- and p-type semiconductors in one parent 2D material.

Guodong Li

Papers

Spatially controlled doping of two-dimensional SnS 2 through intercalation for electronics