Polyacrylonitrile as an Efficient Transfer Medium for Wafer-Scale Transfer of Graphene.

The disparity between growth substrates and application-specific substrates can be mediated by reliable graphene transfer, the lack of which currently strongly hinders the graphene applications. Conventionally, the removal of soft polymers, that support the graphene during the transfer, would contaminate graphene surface, produce cracks, and leave unprotected graphene surface sensitive to airborne contaminations. In this work, it is found that polyacrylonitrile (PAN) can function as polymer medium for transferring wafer-size graphene, and encapsulating layer to deliver high-performance graphene devices. Therefore, PAN, that is compatible with device fabrication, does not need to be removed for subsequent applications. The crack-free transfer of 4 in. graphene onto SiO2/Si wafers, and the wafer-scale fabrication of graphene-based field-effect transistor arrays with no observed clear doping, uniformly high carrier mobility (≈11 000 cm2 V-1 s-1), and long-term stability at room temperature, are achieved. This work presents new concept for designing the transfer process of 2D materials, in which multifunctional polymer can be retained, and offers a reliable method for fabricating wafer-scale devices of 2D materials with outstanding performance.

Recent trends in the transfer of graphene films.

Recent years have witnessed advances in chemical vapor deposition growth of graphene films on metal foils with fine scalability and thickness controllability. However, challenges for obtaining wrinkle-free, defect-free and large-area uniformity remain to be tackled. In addition, the real commercial applications of graphene films still require industrially compatible transfer techniques with reliable performance of transferred graphene, excellent production capacity, and suitable cost. Transferred graphene films, particularly with a large area, still suffer from the presence of transfer-related cracks, wrinkles and contaminants, which would strongly deteriorate the quality and uniformity of transferred graphene films. Potential applications of graphene films include moisture barrier films, transparent conductive films, electromagnetic shielding films, and optical communications; such applications call different requirements for the performance of transferred graphene, which, in turn, determine the suitable transfer techniques. Besides the reliable transfer process, automatic machines should be well developed for the future batch transfer of graphene films, ensuring the repeatability and scalability. This mini-review provides a summary of recent advances in the transfer of graphene films and offers a perspective for future directions of transfer techniques that are compatible for industrial batch transfer.

Tunable Adhesion for All-Dry Transfer of 2D Materials Enabled by the Freezing of Transfer Medium.

The real applications of chemical vapor deposition (CVD)-grown graphene films require the reliable techniques for transferring graphene from growth substrates onto application-specific substrates. The transfer approaches that avoid the use of organic solvents, etchants, and strong bases are compatible with industrial batch processing, in which graphene transfer should be conducted by dry exfoliation and lamination. However, all-dry transfer of graphene remains unachievable owing to the difficulty in precisely controlling interfacial adhesion to enable the crack- and contamination-free transfer. Herein, through controllable crosslinking of transfer medium polymer, the adhesion is successfully tuned between the polymer and graphene for all-dry transfer of graphene wafers. Stronger adhesion enables crack-free peeling of the graphene from growth substrates, while reduced adhesion facilitates the exfoliation of polymer from graphene surface leaving an ultraclean surface. This work provides an industrially compatible approach for transferring 2D materials, key for their future applications, and offers a route for tuning the interfacial adhesion that would allow for the transfer-enabled fabrication of van der Waals heterostructures.

Rapid and Scalable Transfer of Large-Area Graphene Wafers.

Recently, scalable production of large-area graphene films on metal foils with promising qualities is successfully achieved by eliminating grain boundaries, wrinkles, and adlayers. The transfer of graphene from growth metal substrates onto functional substrates remains one inescapable obstacle on the road to the real commercial applications of chemical vaport deposition (CVD) graphene films. Current transfer methods still require time-consuming chemical reactions, which hinders its mass production, and produces cracks and contamination that strongly impede performance reproducibility. Therefore, graphene transfer techniques with fine intactness and cleanness of transferred graphene, and improved production efficiency would be ideal for the mass production of graphene films on destination substrates. Herein, through the engineering of interfacial forces enabled by sophisticated design of transfer medium, the crack-free and clean transfer of 4-inch-sized graphene wafers onto silicon wafers within only 15 min is realized. The reported transfer method is an important leap over the long-lasting obstacle of the batch-scale graphene transfer without degrading the quality of graphene, bringing the graphene products close to the real applications.

Volatile organic compound removal by post plasma-catalysis over porous TiO2 with enriched oxygen vacancies in a dielectric barrier discharge reactor.

Non-thermal plasma (NTP) degradation of volatile organic compounds (VOCs) into CO2 and H2O is a promising strategy for addressing ever-growing environment pollution. However, its practical implementation is hindered by low conversion efficiency and emissions of noxious by-products. Herein, an advanced low-oxygen-pressure calcination process is developed to fine-tune the oxygen vacancy concentration of MOF-derived TiO2 nanocrystals. Vo-poor and Vo-rich TiO2 catalysts were placed in the back of an NTP reactor to convert harmful ozone molecules into ROS that decompose VOCs via heterogeneous catalytic ozonation processes. The results indicate that Vo-TiO2-5/NTP with the highest Vo concentration exhibited superior catalytic activity in the degradation of toluene compared to NTP-only and TiO2/NTP, achieving a maximum 96% elimination efficiency and 76% COx selectivity at an SIE of 540 J L-1. Mechanistic analysis reveals that the 1O2, ˙O2- and ˙OH species derived from the activation of O3 molecules on Vo sites contribute to the decomposition of toluene over the Vo-rich TiO2 surface. With the aid of advanced characterization and density functional theory calculations, the roles of oxygen vacancies in manipulating the synergistic capability of post-NTP systems were explored, and were attributed to increased O3 adsorption ability and enhanced charge transfer dynamics. This work presents novel insights into the design of high-efficiency NTP catalysts structured with active Vo sites.

Phase Engineering of Epitaxial Stanene on a Surface Alloy.

Stanene is a notable two-dimensional topological insulator with a large spin-orbit-coupling-induced band gap. However, the formation of surface alloy intermediates during the epitaxial growth on noble metal substrates prevents the as-grown stanene from preserving its intrinsic electronic states. Here, we show that an intentionally prepared 3×3Au2Sn(111) alloy surface is a suitable inert substrate for growing stanene without the further formation of a complicated surface alloy by scanning tunneling microscopy. The Sn tetramer and clover-shaped Sn pentamer are intermediates for the black-phosphorene-like Sn film at a substrate temperature of <420 K, which transforms to a blue-phosphorene-like stanene with a lattice constant of 0.50 nm above 500 K. First-principles calculations reveal that the epitaxial Sn layer exhibits a lattice registry growth mode and holds a direct energy gap of ∼0.4 eV. Furthermore, interfacial charge-transfer-induced significant Rashba splitting in its electronic structure gives it great potential in spintronic applications.

Catalyst-Free Growth of Two-Dimensional BCxN Materials on Dielectrics by Temperature-Dependent Plasma-Enhanced Chemical Vapor Deposition.

Traditional methods to prepare two-dimensional (2D) B-C-N ternary materials (BCxN), such as chemical vapor deposition (CVD), require sophisticated experimental conditions such as high temperature, delicate control of precursors, and postgrowth transfer from catalytic substrates, and the products are generally thick or bulky films without the atomically mixed phase of B-C-N, hampering practical applications of these materials. Here, for the first time, we develop a temperature-dependent plasma-enhanced chemical vapor deposition (PECVD) method to grow 2D BCxN materials directly on noncatalytic dielectrics at low temperature with high controllability. The C, N, and B compositions can be tuned by simply changing the growth temperature. Thus, the properties of the as-made materials including band gap and conductivity are modulated, which is hardly achieved by other methods. A 2D hybridized BC2N film with a mixed BC2N phase is produced, for the first time, with a band gap of about 2.3 eV. The growth temperature is 580-620 °C, much lower than that of traditional catalytic CVD for growing BCxN. The product has a p-type conducting property and can be directly applied in field-effect transistors and sensors without postgrowth transfer, showing great promise for this method in future applications.

One-pot synthesis of highly sintering- and coking-resistant Ni nanoparticles encapsulated in dendritic mesoporous SiO2 for methane dry reforming.

We present a one-pot method for the encapsulation of small Ni particles into a dendritic SiO2 skeleton. Such confined Ni particles are robust against metal particle sintering and preferentially grow amorphous carbon instead of graphitized carbon, which maintain high activity and stability towards methane dry reforming at 700 °C for 200 h.