Substrate engineering for wafer-scale two-dimensional material growth: strategies, mechanisms, and perspectives

Enormous and Tunable Bulk Charge/Spin Photovoltaic Effect in Piezoelectric Binary Materials T-IV-VI and T-V-V

Understanding the nonlinear response of light and materials is crucial for fundamental physics and next-generation electronic devices. In this work, we have investigated the second-order nonlinear bulk photovoltaic (BPV) and bulk spin photovoltaic (BSPV) effects in the piezoelectric binary materials T-IV-VI and T-V-V (IV = Ge, Sn; VI = S, Se; and V = P, As, Sb, Bi). The independent nonzero conductivity tensors of charge current are derived for these binaries through the symmetry analysis, along with the mechanism for generating pure spin current. These binaries, with their unique folded structure, exhibit significant charge and spin currents under illumination. Furthermore, we find that strain engineering can effectively modulate charge/spin currents by influencing charge density distribution and built-in electric field due to the piezoelectric effect. Our research suggests that the piezoelectric binary materials possess enormous and tunable charge/spin currents, underscoring their potential for applications in nonlinear flexible optoelectronics and spintronics.

Large-Substrate-Terrace Confined Growth of Arrayed Ultrathin PtSe2 Ribbons on Step-Bunched Vicinal Au(001) Facets Toward Electrocatalytic Applications

Ultrathin PtSe2 ribbons can host spin-polarized edge states and distinct edge electrocatalytic activity, emerging as a promising candidate for versatile applications in various fields. However, the direct synthesis is still challenging and the growth mechanism is still unclear. Herein, the arrayed growth of ultrathin PtSe2 ribbons on bunched vicinal Au(001) facets, via a facile chemical vapor deposition (CVD) route is reported. The ultrathin PtSe2 flakes can transform from traditional irregular shapes to desired ribbon shapes by increasing the height of bunched and unidirectionally oriented Au steps (with step height hstep) is found. This crossover, occurring at hstep ≈ 3.0 nm, defines the tailored growth from step-flow to single-terrace-confined modes, as validated by density functional theory calculations of the different system energies. On the millimeter-scale single-crystal Au(001) films with aligned steps, the arrayed ultrathin PtSe2 ribbons with tunable width of ≈20-1000 nm, which are then served as prototype electrocatalysts for hydrogen evolution reaction (HER) is achieved. This work should represent a huge leap in the direct synthesis and the mechanism exploration of arrayed ultrathin transition-metal dichalcogenides (TMDCs) ribbons, which should stimulate further explorations of the edge-related physical properties and practical applications.

Electrochemical Generation of Te Vacancy Pairs in PtTe for Efficient Hydrogen Evolution

Two-dimensional (2D) van der Waals materials are increasingly seen as potential catalysts due to their unique structures and unmatched properties. However, achieving precise synthesis of these remarkable materials and regulating their atomic and electronic structures at the most fundamental level to enhance their catalytic performance remain a significant challenge. In this study, we synthesized single-crystal bulk PtTe crystals via chemical vapor transport and subsequently produced atomically thin, large PtTe nanosheets (NSs) through electrochemical cathode intercalation. These NSs are characterized by a significant presence of Te vacancy pairs, leading to undercoordinated Pt atoms on their basal planes. Experimental and theoretical studies together reveal that Te vacancy pairs effectively optimize and enhance the electronic properties (such as charge distribution, density of states near the Fermi level, and d-band center) of the resultant undercoordinated Pt atoms. This optimization results in a significantly higher percentage of dangling O-H water, a decreased energy barrier for water dissociation, and an increased binding affinity of these Pt atoms to active hydrogen intermediates. Consequently, PtTe NSs featuring exposed and undercoordinated Pt atoms demonstrate outstanding electrocatalytic activity in hydrogen evolution reactions, significantly surpassing the performance of standard commercial Pt/C catalysts.

Harnessing Smectic Ordering for Electric-Field-Driven Guided-Growth of Surface Topography in a Liquid Crystal Polymer

The guided-growth strategy has been widely explored and proved its efficacy in fabricating surface micro/nanostructures in a variety of systems. However, soft materials like polymers are much less investigated partly due to the lack of strong internal driving mechanisms. Herein, the possibility of utilizing liquid crystal (LC) ordering of smectic liquid crystal polymers (LCPs) to induce guided growth of surface topography during the formation of electrohydrodynamic (EHD) patterns is demonstrated. In a two-stage growth, regular stripes are first found to selectively emerge from the homogeneously aligned region of an initially flat LCP film, and then extend neatly along the normal direction of the boundary line between homogeneous and homeotropic alignments. The stripes can maintain their directions for quite a distance before deviating. Coupled with the advanced tools for controlling LC alignment, intricate surface topographies can be produced in LCP films starting from relatively simple designs. The regularity of grown pattern is determined by the LC ordering of the polymer material, and influenced by conditions of EHD growth. The proposed approach provides new opportunities to employ LCPs in optical and electrical applications.

Towards the selective growth of two-dimensional ordered CxNy compounds via epitaxial substrate mediation

Two-dimensional (2D) ordered carbon-nitrogen binary compounds (CxNy) show great potential in many fields owing to their diverse structures and outstanding properties. However, the scalable and selective synthesis of 2D CxNy compounds remain a challenge due to the variable C/N stoichiometry induced coexistence of graphitic, pyridinic, and pyrrolic N species and the competitive growth of graphene. Here, this work systematically explored the mechanism of selective growth of a series of 2D ordered CxNy compounds, namely, the g-C3N4, C2N, C3N, and C5N, on various epitaxial substrates via first-principles calculations. By establishing the thermodynamic phase diagram, it is revealed that the individualized surface interaction and symmetry match between 2D CxNy compounds and substrates together enable the selective epitaxial growth of single crystal 2D CxNy compounds within distinct chemical potential windows of feedstock. The kinetics behaviors of the diffusion and attachment of the decomposed feedstock C/N atoms to the growing CxNy clusters further confirmed the feasibility of the substrate mediated selective growth of 2D CxNy compounds. Moreover, the optimal experimental conditions, including the temperature and partial pressure of feedstock, are suggested for the selective growth of targeted 2D CxNy compound on individual epitaxial substrates by carefully considering the chemical potential of carbon/nitrogen as the functional of experimental parameters based on the standard thermochemical tables. This work provides an insightful understanding on the mechanism of selective epitaxial growth of 2D ordered CxNy compounds for guiding the future experimental design.

Interfacial Self-Assembly of Oriented Semiconductor Monolayer for Chemiresistive Sensing

Semiconductor nanofilm fabrication with advanced technology is of great importance for next-generation electronics/optoelectronics. Fabrication of high-quality and perfectly oriented semiconductor thin films and integration into high-performance electronic devices with low cost and high efficiency are huge challenges. Here we exquisitely utilized the Marangoni effect to perfectly guide tin disulfide (SnS2) nanocoins into an ordered assembly in milliseconds, resulting in an uniaxial-oriented monolayer semiconductor film. Further exploration revealed that the formed "crumple zone" at the interface caused by the Marangoni force endows the nanofilm with a rapid healable capability, which can be easily transferred to arbitrary substrates. As a proof of concept, the nanocoin-monolayer was transferred onto a micro-interdigitated electrode substrate to form a high-performance chemiresistive sensor that can effectively monitor the trace amounts of toxic gases. In addition, the assembled monolayer nanofilms can be conformally printed on freeform surfaces: both flat and nonflat substrates. This efficient and low-cost Marangoni force-assisted surface self-assembly (MFA-SSA) strategy is promising for advanced microelectronics and real industrial applications.

Wafer scale growth of single crystal two-dimensional van der Waals materials

Two-dimensional (2D) van der Waals (vdW) materials, including graphene, hexagonal boron nitride (hBN), and metal dichalcogenides (MCs), form the basis of modern electronics and optoelectronics due to their unique electronic structure, chemical activity, and mechanical strength. Despite many proof-of-concept demonstrations so far, to fully realize their large-scale practical applications, especially in devices, wafer-scale single crystal atomically thin highly uniform films are indispensable. In this minireview, we present an overview on the strategies and highlight recent significant advances toward the synthesis of wafer-scale single crystal graphene, hBN, and MC 2D thin films. Currently, there are five distinct routes to synthesize wafer-scale single crystal 2D vdW thin films: (i) nucleation-controlled growth by suppressing the nucleation density, (ii) unidirectional alignment of multiple epitaxial nuclei and their seamless coalescence, (iii) self-collimation of randomly oriented grains on a molten metal, (iv) surface diffusion and epitaxial self-planarization and (v) seed-mediated 2D vertical epitaxy. Finally, the challenges that need to be addressed in future studies have also been described.

The Roadmap of 2D Materials and Devices Toward Chips

Due to the constraints imposed by physical effects and performance degradation, silicon-based chip technology is facing certain limitations in sustaining the advancement of Moore's law. Two-dimensional (2D) materials have emerged as highly promising candidates for the post-Moore era, offering significant potential in domains such as integrated circuits and next-generation computing. Here, in this review, the progress of 2D semiconductors in process engineering and various electronic applications are summarized. A careful introduction of material synthesis, transistor engineering focused on device configuration, dielectric engineering, contact engineering, and material integration are given first. Then 2D transistors for certain electronic applications including digital and analog circuits, heterogeneous integration chips, and sensing circuits are discussed. Moreover, several promising applications (artificial intelligence chips and quantum chips) based on specific mechanism devices are introduced. Finally, the challenges for 2D materials encountered in achieving circuit-level or system-level applications are analyzed, and potential development pathways or roadmaps are further speculated and outlooked.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure-Property Interplay for Next-Generation Technologies

MXenes are a class of 2D materials that include layered transition metal carbides, nitrides, and carbonitrides. Since their inception in 2011, they have garnered significant attention due to their diverse compositions, unique structures, and extraordinary properties, such as high specific surface areas and excellent electrical conductivity. This versatility has opened up immense potential in various fields, catalyzing a surge in MXene research and leading to note worthy advancements. This review offers an in-depth overview of the evolution of MXenes over the past 5 years, with an emphasis on synthetic strategies, structure-property relationships, and technological prospects. A classification scheme for MXene structures based on entropy is presented and an updated summary of the elemental constituents of the MXene family is provided, as documented in recent literature. Delving into the microscopic structure and synthesis routes, the intricate structure-property relationships are explored at the nano/micro level that dictate the macroscopic applications of MXenes. Through an extensive review of the latest representative works, the utilization of MXenes in energy, environmental, electronic, and biomedical fields is showcased, offering a glimpse into the current technological bottlenecks, such asstability, scalability, and device integration. Moreover, potential pathways for advancing MXenes toward next-generation technologies are highlighted.

Growing sp2 materials on transition metals: calculated atomic adsorption energies of hydrogen, boron, carbon, nitrogen, and oxygen atoms, C2 and BN dimers, C6 and (BN)3 hexamers, graphene and h-BN with and without atomic vacancies

The growth of graphene and hexagonal boron nitride on hot transition metal surfaces involves the adsorption of precursor molecules, and their dissociation and assembly into two-dimensional honeycomb lattices. In a recent account it was found that h-BN may be distilled on a rhodium metal surface, which yields higher quality h-BN [Cun et al., ACS Nano, 2020, 15, 1351]. In this context, we calculated in a systematic approach the adsorption energies and sites of hydrogen, boron, carbon, nitrogen, and oxygen atoms and from the site dependence the activation energy for diffusion. Existing computed values of the solvation energy into the bulk were compared to the present ones with our calculation scheme and found to be in good agreement. For the distinction of different systems we introduce the concepts of epiphilicity and epiphobicity. The adsorption energies and stabilities of the C2 and BN dimers, the C6 and (BN)3 ring-hexamers and the graphene and h-BN monolayers allow the prediction of the performance of different substrates in chemical vapor deposition (CVD) processes for the growth of graphene and h-BN. Finally, vacancy creation energies were calculated as a criterion for the stability of graphene and h-BN on metallic substrates.

Study of the growth mechanism of a self-assembled and ordered multi-dimensional heterojunction at atomic resolution

Multi-dimensional heterojunction materials have attracted much attention due to their intriguing properties, such as high efficiency, wide band gap regulation, low dimensional limitation, versatility and scalability. To further improve the performance of materials, researchers have combined materials with various dimensions using a wide variety of techniques. However, research on growth mechanism of such composite materials is still lacking. In this paper, the growth mechanism of multi-dimensional heterojunction composite material is studied using quasi-two-dimensional (quasi-2D) antimonene and quasi-one-dimensional (quasi-1D) antimony sulfide as examples. These are synthesized by a simple thermal injection method. It is observed that the consequent nanorods are oriented along six-fold symmetric directions on the nanoplate, forming ordered quasi-1D/quasi-2D heterostructures. Comprehensive transmission electron microscopy (TEM) characterizations confirm the chemical information and reveal orientational relationship between Sb2S3 nanorods and the Sb nanoplate as substrate. Further density functional theory calculations indicate that interfacial binding energy is the primary deciding factor for the self-assembly of ordered structures. These details may fill the gaps in the research on multi-dimensional composite materials with ordered structures, and promote their future versatile applications.

Wafer-Scale Epitaxial Growth of Two-dimensional Organic Semiconductor Single Crystals toward High-Performance Transistors

The success of state-of-the-art electronics and optoelectronics relies heavily on the capability to fabricate semiconductor single-crystal wafers. However, the conventional epitaxial growth strategy for inorganic wafers is invalid for growing organic semiconductor single crystals due to the lack of lattice-matched epitaxial substrates and intricate nucleation behaviors, severely impeding the advancement of organic single-crystal electronics. Here, an anchored crystal-seed epitaxial growth method for wafer-scale growth of 2D organic semiconductor single crystals is developed for the first time. The crystal seed is firmly anchored on the viscous liquid surface, ensuring the steady epitaxial growth of organic single crystals from the crystal seed. The atomically flat liquid surface effectively eliminates the disturbance from substrate defects and greatly enhances the 2D growth of organic crystals. Using this approach, a wafer-scale few-layer bis(triethylsilythynyl)-anthradithphene (Dif-TES-ADT) single crystal is formed, yielding a breakthrough for organic field-effect transistors with a high reliable mobility up to 8.6 cm2 V-1 s-1 and an ultralow mobility variable coefficient of 8.9%. This work opens a new avenue to fabricate organic single-crystal wafers for high-performance organic electronics.

Controlled Synthesis and Accurate Doping of Wafer-Scale 2D Semiconducting Transition Metal Dichalcogenides

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.