Modern electronics have been geared toward exploring novel electronic materials that can encompass a broad set of unusual functionalities absent in conventional platforms. In this regard, two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors are highly promising, owing to their large mechanical resilience coupled with superior transport properties and van der Waals (vdW) attraction-enabled relaxed assembly. Moreover, 2D TMD heterolayers based on chemically distinct constituent layers exhibit even more intriguing properties beyond their mono-component counterparts, which can materialize only when they are manufactured on a technologically practical wafer-scale. This mini-review provides a comprehensive overview of recent progress in exploring wafer-scale 2D TMD heterolayers of various kinds. It extensively surveys a variety of manufacturing strategies and discusses their scientific working principles, resulting 2D TMD heterolayers, their material properties, and device applications. Moreover, it offers extended discussion on remaining challenges and future outlooks toward further improving the material quality of 2D TMD heterolayers in both material and manufacturing aspects.

Atomically thin 2D materials exhibit strong intralayer covalent bonding and weak interlayer van der Waals interactions, offering unique high in-plane strength and out-of-plane flexibility. While atom-thick nature of 2D materials may cause uncontrolled intrinsic/extrinsic deformation in multiple length scales, it also provides new opportunities for exploring coupling between heterogeneous deformations and emerging functionalities in controllable and scalable ways for electronic, optical, and optoelectronic applications. In this review, we discuss (i) the mechanical characteristics of 2D materials, (ii) uncontrolled inherent deformation and extrinsic heterogeneity present in 2D materials, (iii) experimental strategies for controlled heterogeneous deformation of 2D materials, (iv) 3D structure-induced novel functionalities via crumple/wrinkle structure or kirigami structures, and (v) heterogeneous strain-induced emerging functionalities in exciton and phase engineering. Overall, heterogeneous deformation offers unique advantages for 2D materials research by enabling spatial tunability of 2D materials' interactions with photons, electrons, and molecules in a programmable and controlled manner.

Epitaxial SnSxSe2−x films with tunable band gap energies (1.0–2.2 eV) are of growing interest for photodetectors and 2D heterostructures for nanoscale electronics. In this study, powder vapor transport growth of SnSxSe2−x was investigated on c-plane sapphire and epitaxial graphene (EG)/6H–SiC substrates using tin, sulfur, and selenium powder sources in a heated tube furnace. The SnSxSe2−x composition was controlled by varying the sulfur and selenium source temperatures and the corresponding chalcogen vapor pressure ratio. Raman spectroscopy was used to determine the alloy composition of the films, and the optical properties were characterized using UV-Vis-NIR spectroscopy. SnSxSe2−x grown on sapphire consisted of vertically oriented platelets. By contrast, large-area, planar coalesced SnSxSe2−x films grew on EG with low surface roughness indicative of a van der Waals growth mode. High-resolution X-ray diffraction θ–2θ scans and pole figure analysis confirm that the SnSxSe2−x films are c-axis oriented with epitaxial relation being $\left[ {11\bar{2}0} \right]$ SnSxSe2−x‖$\left[ {10\bar{1}0} \right]$ 6H–SiC.

Ab initio calculations are performed to investigate the structural, vibrational, electronic, and piezoelectric properties of functionalized single layers of TaS2. We find that single-layer TaS2 is a suitable host material for functionalization via fluorination and hydrogenation. The one-side fluorinated (FTaS2) and hydrogenated (HTaS2) single layers display indirect gap semiconducting behavior in contrast to bare metallic TaS2. On the other hand, it is shown that as both surfaces of TaS2 are saturated anti-symmetrically, the formed Janus structure is a dynamically stable metallic single layer. In addition, it is revealed that out-of-plane piezoelectricity is created in all anti-symmetric structures. Furthermore, the Janus-type single-layer has the highest specific heat capacity to which longitudinal and transverse acoustical phonon modes have contribution at low temperatures. Our findings indicate that single-layer TaS2 is suitable for functionalization via H and F atoms that the formed, anti-symmetric structures display distinctive electronic, vibrational, and piezoelectric properties.

We show interlayer stacking shifts occur in transition metal dichalcogenides (TMD) bilayers due to the strain introduced during sample heating, and attributed to rippling of one layer relative to the other. The atomic structure of the interlayer stacking is studied using annular dark field scanning transmission electron microscopy with an in situ heating holder. Before heating, bilayers show uniform interlayer stacking of AA′ and AB. When heated, contrast change is seen and associated with interlayer stacking changes at the atomic scale due to ripples. When cooled down to room temperature, these contrast features disappear, confirming it is a reversible process that is not related to defects or vacancies. Because the bottom layer is attached to the in situ heating chip made from Si3N4 and the top layer is in contact with the underlying TMD layer with weak van der Waals interaction, the two layers experience different forces during thermal expansion.

We fabricated a van der Waals heterostructure of WS2–ReSe2 and studied its charge-transfer properties. Monolayers of WS2 and ReSe2 were obtained by mechanical exfoliation and chemical vapor deposition, respectively. The heterostructure sample was fabricated by transferring the WS2 monolayer on top of ReSe2 by a dry transfer process. Photoluminescence quenching was observed in the heterostructure, indicating efficient interlayer charge transfer. Transient absorption measurements show that holes can efficiently transfer from WS2 to ReSe2 on an ultrafast timescale. Meanwhile, electron transfer from ReSe2 to WS2 was also observed. The charge-transfer properties show that monolayers of ReSe2 and WS2 form a type-II band alignment, instead of type-I as predicted by theory. The type-II alignment is further confirmed by the observation of extended photocarrier lifetimes in the heterostructure. These results provide useful information for developing van der Waals heterostructure involving ReSe2 for novel electronic and optoelectronic applications and introduce ReSe2 to the family of two-dimensional materials to construct van der Waals heterostructures.

Metal–organic frameworks (MOFs) possess tuneable properties and a variety of important applications in the areas of catalysis, adsorption, gas storage, and separation, among others. Herein, recent computational studies by density functional theory (DFT) applied for simulations of MOF structure and complex architecture determination, prediction of properties, and computational characterization, including large-scale screening and geometrical properties of hypothetical MOFs, diffusion and adsorption processes in MOFs, are reviewed. DFT calculations have been applied in the MOF area to study chemical stability; mechanical, photophysical, optical, and magnetic properties; photoluminescence; porosity; and semiconductor or metallic character. The prediction of MOF analogs with open-metal sites, studies of chemical bonding and the prediction of energies by quantum mechanics allows reducing experimental efforts in the creation of MOF/polymer membranes, adsorbents for CO2 uptake, separation of C2H2/CH4, C2H2/CO2, and inert gases, radionuclides sequestration, and water adsorption, as well as other promising advances. For the MOF-derived carbons, a lack of profound DFT investigations is currently observed, being mainly restricted to the electrocatalysis area (nitrogen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction), resulting applications in batteries and other storage devices, CO2 sequestration, and absorbance of organic substances.

As electrode materials, metal-organic frameworks always have low electrical conductivity and poor structural stability, which limits its applications in electrochemical fields. Here, Ni-BPDC/GO composites are synthesized using graphene oxide (GO) as a substrate and 4,4′-biphenyldicarboxylic acid (BPDC) as an organic ligand via a hydrothermal approach. The growth mechanism of the Ni-BPDC and Ni-BPDC/GO composites is proposed. In the composites, highly dispersed Ni-BPDC macro-nanostrips are supported on the GO surface in parallel. The presence of GO does not affect the growth and crystalline structure of Ni-BPDC. Compared with the Ni-BPDC, Ni-BPDC/GO composites exhibit higher specific capacitance, rate capability, and operating current density through lowering intrinsic resistance, charge-transfer resistance, and ion diffusion impedance. Moreover, the assembled Ni-BPDC/GO-3//reduced graphene oxide (rGO) asymmetric supercapacitor has large specific capacitance, good cycling stability, and high energy density (16.5 W h/kg at 250 W/kg). Hence, Ni-BPDC/GO composites are a potential electrode material for supercapacitors.

Nanosized, well-dispersed titania particles were synthesized via a hydrothermal method using multiwalled carbon nanotubes (MWCNTs) as structural modifiers during the nucleation process to decrease aggregation. Synthesized TiO2/MWCNT composites containing different amounts of MWCNTs were characterized using N2 physisorption, XRD, spectroscopic techniques (Raman, UV-visible, and X-ray photoelectron), and electron microscopy to illuminate the morphology, crystal structure, and surface chemistry of the composites. Photocatalytic performance was evaluated by measuring the degradation of acetaldehyde in a batch reactor under UV illumination. Average rate constants decrease in the following order: TiO2/MWCNT-1% > TiO2 > TiO2/MWCNT-5%. Addition of MWCNTs beyond the optimum loading ratio of 1:100 (MWCNT:TiO2) diminishes the effectiveness of the photocatalyst and the synergistic effect between MWCNTs and TiO2. The primary mechanism for photocatalytic activity enhancement in TiO2/MWCNT-1% is thought to be due to increased porosity, hydroxyl enrichment on the surface, and high dispersion of TiO2 particles.

The nitrogen-decorated CeO2/reduced graphene oxide nanocomposite (CeO2/N-rGO) was one-step synthesized by a facile hydrothermal technique and applied as counter electrode materials for dye-sensitized solar cells (DSSCs). For comparison, CeO2/rGO and rGO were also synthesized by adjusting corresponding reactants. It was found that the as-synthesized CeO2/N-rGO shows better electrocatalytic activity for triiodide/iodide reduction than that of pure rGO and CeO2/rGO, and a synergistic effect of nitrogen and CeO2 on the rGO sheets was observed. The photoelectric conversion efficiency of DSSCs based on CeO2/N-rGO counter electrode was 3.20%, which is higher than that of CeO2/rGO (2.45%) and rGO counter electrode (1.37%). Furthermore, the synergistic effect of nitrogen and CeO2 on the rGO sheets was also discussed in detail with different CeO2 amount levels. It is believed that this one-step synthetic method is a potential way to synthesize low-cost and efficient rGO-based multiple composited counter electrode materials to replace more expensive Pt.

Developing easy and effective surface functionalization approaches has required to facilitate the processability of graphene while seeking novel application areas. Herein, an in situ single-step reductive covalent bromination of graphene has been reported for the first time. Highly brominated graphene flakes (>3% Br) were prepared by only subjecting the bromine-intercalated graphite flakes to a reduction reaction with reactive lithium naphthalide. The bromine-functionalized graphene was characterized by X-ray photoelectron spectroscopy and thermogravimetric analysis. Results revealed that Br2 molecules acted as both an intercalating agent for the graphite and a reactant for the surface functionalization of the graphene. After brominating, the remaining negative charges on the reduced graphene surface were further used for the dual surface functionalization of graphene with a long-chain alkyl group (~1% dodecyl group addition). The functionalized graphenes were also characterized by Fourier transform infrared and Raman spectroscopy.

Double-layer absorbers have recently been extensively studied because single-layer absorbers can hardly meet the requirements of advanced absorbing materials. However, determining how to couple the matching and absorption layers remains a challenge. In the present work, we applied the hydrothermal method to prepare an ultrasmall Fe3O4 nanoparticle and a hierarchical MXene/Fe3O4 composite and then studied the microwave attenuation capabilities of single- and double-layer absorbers containing these two materials with different thicknesses. Absorbers with well-coupled layers showed improved absorption performance on account of the excellent impedance matching behavior of the Fe3O4 layer and the high microwave attenuation capability of the MXene/Fe3O4 layer. When the thickness of the matching layer filled with Fe3O4 was 0.1 mm and that of the absorption layer filled with MXene/Fe3O4 was 1.9 mm, a maximum reflection loss of −48.7 dB was achieved at 9.9 GHz. More importantly, when the thicknesses of the matching and absorption layers were 0.9 and 1.1 mm, respectively, the effective bandwidth was nearly 3.9 GHz. The double-layer absorbers with enhanced absorption properties may be regarded as a new generation of materials for electromagnetic wave absorption.