values. [10] Therefore, many research groups have proposed emerging methods to improve contact properties, for example, inserting functional layers between 2D semiconductors and electrodes, [11,12] thermal annealing at contact regions, [13] phase engineering with high-energy beam irradiation, [14] and employing graphene electrodes [15,16] low-work function metals, [17,18] or a direct chemical vapor deposition (CVD)-growth method of 2D heterostructures in lateral. [19,20] However, thermal annealing and high-energy beam irradiation may damage 2D materials, and direct CVDgrowth method of lateral 2D heterostructures is difficult to define accurate junction areas between 2D materials. Therefore, more facile approach to improve the contact properties between metal electrodes and 2D semiconductors is desired for the typical 2D field-effect transistor structure. Among these methods, introducing a thin tunneling barrier between the MoS 2 channel and electrodes is an efficient approach to improve contact properties by reducing the activation energy. [12,21,22] For example, graphene or hexagonal boron nitride (h-BN) have been vertically employed between metal contacts and 2D semiconductors as a thin tunneling barrier for the lower contact resistance. [21,22] Moreover, the use of cobalt electrodes facilitated Ohmic contact even in the extremely low temperature regime. [12] However, relatively complex, high-cost, and low-yield transfer processes were required to make vertically stacked 2D heterostructures. [11,12] Additionally, it is difficult to introduce atomically thin tunneling barriers using conventional transfer methods to the exact contact location on flake-type MoS 2 with dimensions of sub-micrometer size. Here, we demonstrate a simple strategy for inserting a thin tunneling barrier by depositing thiol-molecules between MoS 2 semiconductors and conventional metal electrodes. Vaporized thiol-molecules are chemically adsorbed on MoS 2 with covalent bonding. The inserted thiol-molecules at the contact region create additional tunneling paths, resulting in a drastically reduced activation energy; therefore, the primary injection mechanism of the contact-engineered MoS 2 field-effect transistors (FETs) changes from thermionic emission to field emission, allowing better contact properties without a temperature dependency. In addition, by defining contact regions on MoS 2 using conventional lithography, where injection engineering is desirable, a selective introduction of thiol-molecules is feasible with ≈100% yield Although 2D molybdenum disulfide (MoS 2 ) has gained much attention due to its unique electrical and optical properties, the limited electrical contact to 2D semiconductors still impedes the realization of high-performance 2D MoS 2based devices. In this regard, many studies have been conducted to improve the carrier-injection properties by inserting functional paths, such as graphene or hexagonal boron nitride, between the electrodes and 2D semiconductors. The reported strategies, however, require relatively time...

As two-dimensional (2D) transition metal dichalcogenides electronic devices are scaled down to the sub-micrometer regime, the active layers of these materials are exposed to high lateral electric fields, resulting in electrical breakdown. In this regard, understanding the intrinsic nature in layer-stacked 2D semiconducting materials under high lateral electric fields is necessary for the reliable applications of their field-effect transistors. Here, we explore the electrical breakdown phenomena originating from avalanche multiplication in MoS field-effect transistors with different layer thicknesses and channel lengths. Modulating the band structure and bandgap energy in MoS allows the avalanche multiplication to be controlled by adjusting the number of stacking layers. This phenomenon could be observed in transition metal dichalcogenide semiconducting systems due to its quantum confinement effect on the band structure. The relationship between the critical electric field for avalanche breakdown and bandgap energy is well fitted to a power law curve in both monolayer and multilayer MoS.

Red, green, blue, and natural white upconversion (UC) luminescence colors are realized from the tetragonal-structured LiGdF4-based core/triple-shell (C/T-S) upconversion nanophosphors (UCNPs) and the C/T-S UCNP-incorporated polymer composites. The LiYF4:Yb cores are used as sensitized seeds for the formation of LiGdF4:Yb,Tm UC shell followed by the growth of LiGdF4:Tb,Eu color tuning shell. Finally, LiYF4 inert shell is grown on the core/shell/shell UCNPs, and LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs exhibit enhanced UC luminescence. The single tetragonal-phased C/T-S UCNPs exhibit blue, green, and red UC luminescence, which is attributed to the electronic transitions in Tm3+ via energy transfer UC process and Tb3+ and Eu3+ via energy migration UC process, respectively. The multicolor UC emissions, including natural white, medium aquamarine, purple, and thistle color, are created by fine-tuning of the ratio of Tb3+ and Eu3+ in the color tuning shell. The transparent polymer composites are prepared by incorporating the C/T-S UCNPs into polydimethylsiloxane, and the polymer composites also exhibit red, green, blue, and natural white light UC emissions, indicating that these multicolor tunable LiYF4:Yb/LiGdF4:Yb,Tm/LiGdF4:Tb,Eu/LiYF4 C/T-S UCNPs have potential to be applied to transparent volumetric displays.

DNA-functionalized gold nanoparticles (AuNPs) are one of the most commonly used reagents in nanobiotechnology. They are important not only for practical applications in analytical chemistry and drug delivery but also for fundamental understanding of nanoscience. For biological samples such as blood serum or for intracellular applications, the effects of crowded cellular proteins and nucleic acids need to be considered. The thermodynamic effect of crowding is to induce nanoparticle aggregation.But before such aggregation can take place, there might also be a depletion repulsive barrier.Polyethylene glycol (PEG) is one of the most frequently used polymers to mimic the crowded cellular environment. We show herein that while DNA-functionalized AuNPs are very stable in buffer (e.g. no PEG), and citrate-capped AuNPs are very stable in PEG, DNA-functionalized AuNPs are unstable in PEG and are easily aggregated. Although such aggregation in PEG is mediated by DNA, no sharp melting transition typical for DNA-linked AuNPs is observed. We attribute this broad melting to depletion force instead of DNA base pairing. The effects of PEG molecular weight, concentration and temperature have been studied in detail and we also find an interesting PEG phase separation and AuNP partition into the water-rich phase at high temperature.

The melting temperature of duplex DNA is much higher in 5 polyanions than that in non-ionic polymers with similar ionic strength, suggesting an additional electrostatic contribution on top of the excluded volume effect.Biological fluids and the cytoplasm contain concentrated biopolymers such as nucleic acids and proteins. They occupy 10 ~20-40% of a live cell's volume, creating a crowded environment because of their mutual impenetrable property. 1A thermodynamic consequence of macromolecular crowding is to favor reactions that produce reduced excluded volumes, such as DNA hybridization and protein oligomerization.