In this work, a chemical vapor deposition (CVD) method was developed for the synthesis of transition metal dichalcogenide alloy monolayers, with a composition gradient in the radial direction. The composition gradient was achieved by controlling the substrate cooling rate during the CVD growth. The two types of alloys, namely, WS2(1−x)Se2x and MoS2(1−x)Se2x, were found to exhibit an opposite composition gradient. This is attributed to their different cohesive energies. A two-dimensional Ising model is used to explain the growth mechanism, where two ends of the composition were modeled as a magnetically ordered phase and a paramagnetic phase. The composition as a function of substrate temperature is then represented by the thermal magnetization curve.

We studied the optical properties of a composition graded WS2(1−x)Se2x alloy monolayer. A symmetric gradual composition gradient from a Se-rich center to the relatively less Se-rich edges of an equilateral triangle shaped flake is confirmed by Raman mapping. Photoluminescence (PL) mapping shows a large 100 meV variation in the exciton energy, resulting from the composition dependent bandgap variation and carrier localization. The alloying leads to symmetry breaking and large nonlinear optical susceptibility. Second harmonic generation (SHG) mapping was carried out to study the non-linear properties and additionally to determine the lattice strain of the alloy flake. In contrast to PL and Raman mappings, SHG intensity is found to be spatially uniform. However, polarization dependent SHG reveals a unidirectional strain parallel to the (zigzag) edge of the flake, in addition to the sixfold symmetry expected from the transition metal dichalcogenide (TMD) lattice. Our results suggest potential applications of composition graded TMD alloys as ultra-compact color-tunable light sources and miniaturized spectrometers.

Flexible electronics for harsh and hazardous environments could offer a broad range of technological applications from conformal structural health monitoring, hypersonics, to telecommunication systems. However, advanced materials with the capability of additive manufacturing and the tolerance to extreme operating conditions are imperative. Here, we report hightemperature radiofrequency electronics with thermal management by printing copper hybrid conductors onto flexible thin alumina ribbon ceramic and ceramic fiber/silica aerogel composite. Regulating thermal stability, tuning resonance frequency, and increasing current-carrying ability of printed electronics are synergistically achieved using a flexible thermal-insulation ceramic fiber/silica aerogel composite or thermally conductive alumina ribbon ceramic substrates and high-temperature copper−graphene conductors. The printed copper conductor coatings exhibit tunable antenna resonance and electromagnetic interference effectiveness of 70 dB at a thickness of 5 μm, opening a pathway toward flexible hybrid radiofrequency electronics with thermal management.

As one of the conductive ink materials with high electric conductivity, elemental copper (Cu) based nanocrystals promise for printable electronics. Here, single crystalline Cu nanoplates were synthesized using a facile hydrothermal method. Size engineering of Cu nanoplates can be rationalized by using the LaMer model and the versatile Cu conductive ink materials are suitable for different printing technologies. The printed Cu traces show high electric conductivity of 6 MS/m, exhibiting electro-magnetic interference shielding efficiency value of 75 dB at an average thicknesses of 11 μm. Together with flexible alumina ceramic aerogel substrates, it kept 87% conductivity at the environmental temperature of 400 ℃, demonstrating the potential of Cu conductive ink for high-temperature printable electronics applications.

We map the optical properties with Raman, PL and SHG methods of a single gradient 2D alloy flake with spatially varying composition of CVD grown WSe2-WS2 monolayers that can be used in novel opto-electronics devices.