In this study, we present an investigation on the growth of thin Mo2C crystals via chemical vapor deposition using CH4. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy(AFM), and Raman spectroscopy studies show that the morphology and the thickness of Mo2C crystals are strongly affected by the impurities in the system, the thickness of the copper substrate, and the graphene presence on Cu surface prior to Mo2C formation. Our studies show that during the CVD process, orthorhombic Mo2C crystals grow along the [100] direction on two different regions: directly on Cu surface or on graphene covered regions. Mo2C crystals that form on graphene are found to be thinner and less defective compared to the ones formed on the Cu surface. This is attributed to graphene acting as an additional diffusion barrier for Mo atoms diffusing through the copper. In addition to the graphene beneath the Mo2C crystal, Raman studies indicate that graphene may grow also on top of the Mo2C crystal, forming a graphene/Mo2C/graphene sandwich structure which may offer interesting properties for electronic applications.

We investigated the chemical vapor deposition synthesis of Mo 2 C/graphene heterostructures on a partially wetted liquid copper surface, studied the morphology of resulting phases using electron and optical microscopy, and determined the rate-limiting step for the growth of Mo 2 C on graphene. The morphology of the Mo 2 C crystals varied from the center to the edge of the copper substrate because of the change in the Mo diffusion pathways owing to the variation in the thickness of the Cu substrate. Thin, hexagonal-shaped crystals of Mo 2 C were found in the central region, where Cu is the thickest. In addition, the growth pressure substantially affects the nucleation and growth kinetics of both Mo 2 C and graphene.At high pressures (750 Torr), the graphene layer fully covered the Cu surface and Mo 2 C crystals formed with a regular shape, while at low pressures (5 Torr), the nucleation of both domains was suppressed, leading to the evolution of Mo 2 C crystals with irregular shapes. The activation energy for the growth of Mo 2 C on graphene was calculated to be 3.76 ± 0.3 eV, and the diffusion of Mo to the Cu surface through uncovered Cu or graphene vacancies/defects was determined to be the rate-limiting step.

Chemical vapor deposition is a promising technique to produce Mo2C crystals with large area, controlled thickness, and reduced defect density. Typically, liquid Cu is used as a catalyst substrate; however, its high melting temperature (1085 °C) prompted research groups to search for alternatives. In this study, we report the synthesis of large-area thin Mo2C crystals at lower temperatures using liquid In, which is also advantageous with respect to the transfer process due to its facile etching. SEM, EDS, Raman spectroscopy, XPS, and XRD studies show that hexagonal Mo2C crystals, which are orthorhombic, grow along the [100] direction together with an amorphous carbon thin film on In. The growth mechanism is examined and discussed in detail, and a model is proposed. AFM studies agree well with the proposed model, showing that the vertical thickness of the Mo2C crystals decreases inversely with the thickness of In for a given reaction time.

Observation of thermally activated delayed fluorescence (TADF) in conjugated systems redefined the molecular design approach to realize highly efficient organic light emitting diodes (OLEDs) in the early 2010s. Enabling effective reverse intersystem crossing (RISC) by minimizing the difference between singlet and triplet excited state energies (ΔE ST ) is proven to be a widely applicable and fruitful approach, which results in remarkable external quantum efficiencies (EQE). The efficacy of RISC in these systems is mainly dictated by the first-order mixing coefficient (λ), which is proportional to spin-orbit coupling (H SO ) and inversely proportional to ΔE ST . While minimizing ΔE ST has been the focus of the OLED community over the last decade, the effect of H SO in these systems is largely overlooked. Here, molecular systems with increased H SO are designed and synthesized by substituting selected heteroatoms of high-performance TADF materials with heavy-atom selenium. A new series of multicolor TADF materials with remarkable EQEs are achieved. One of these materials, SeDF-B, results in pure blue emission with EQEs approaching 20%. Additionally, flexible graphene-based electrodes are developed for OLEDs and revealed to have similar performance as standard indium tin oxide (ITO) in most cases. These devices are the first report of TADF based OLEDs that utilize graphene-based anodes.