Dopant-Tunable Ultrathin Transparent Conductive Oxides for Efficient Energy Conversion Devices.

Ultrathin film-based transparent conductive oxides (TCOs) with a broad work function (WF) tunability are highly demanded for efficient energy conversion devices. However, reducing the film thickness below 50 nm is limited due to rapidly increasing resistance; furthermore, introducing dopants into TCOs such as indium tin oxide (ITO) to reduce the resistance decreases the transparency due to a trade-off between the two quantities. Herein, we demonstrate dopant-tunable ultrathin (≤ 50 nm) TCOs fabricated via electric field-driven metal implantation (m-TCOs; m = Ni, Ag, and Cu) without compromising their innate electrical and optical properties. The m-TCOs exhibit a broad WF variation (0.97 eV), high transmittance in the UV to visible range (89-93% at 365 nm), and low sheet resistance (30-60 Ω cm-2). Experimental and theoretical analyses show that interstitial metal atoms mainly affect the change in the WF without substantial losses in optical transparency. The m-ITOs are employed as anode or cathode electrodes for organic light-emitting diodes (LEDs), inorganic UV LEDs, and organic photovoltaics for their universal use, leading to outstanding performances, even without hole injection layer for OLED through the WF-tailored Ni-ITO. These results verify the proposed m-TCOs enable effective carrier transport and light extraction beyond the limits of traditional TCOs.

Graphene Nanoribbons with Atomically Sharp Edges Produced by AFM Induced Self-Folding.

The ability to create graphene nanoribbons with atomically sharp edges is important for various graphene applications because these edges significantly influence the overall electronic properties and support unique magnetic edge states. The discovery of graphene self-folding induced by traveling wave excitation through atomic force microscope scanning under a normal force of less than 15 nN is reported. Most remarkably, the crystallographic direction of self-folding may be either along a chosen direction defined by the scan line or along the zigzag or armchair direction in the presence of a pre-existing crack in the vicinity. The crystalline direction of the atomically sharp edge is confirmed via careful lateral force microscopy measurements. Multilayer nanoribbons with lateral dimensions of a few tens of nanometers are realized on the same graphene sheet with different folding types (e.g., z-type or double parallel). Molecular dynamics simulations reveal the folding dynamics and suggest a monotonic increase of the folded area with the applied normal force. This method may be extended to other 2D van der Waals materials and lead to nanostructures that exhibit novel edge properties without the chemical instability that typically hinders applications of etched or patterned graphene nanostructures.

Prediction of Green Phosphorus with Tunable Direct Band Gap and High Mobility.

Black phosphorus is an emerging material in nanoelectronics and nanophotonics due to its high carrier mobility and anisotropic in-plane properties. In addition, the polymorphism of phosphorus leads to numerous searches for new allotropes that are more attractive than black phosphorus in a variety of applications. On the basis of ab initio evolutionary crystal structure search computation, we report the prediction of a phosphorus allotrope called green phosphorus (λ-P), which exhibits direct band gaps ranging from 0.7 to 2.4 eV and strong anisotropy in optical and transport properties. Free-energy calculations show that a single-layer form, termed green phosphorene, is energetically more stable than blue phosphorene, and a phase transition from black to green phosphorene can occur at temperatures above 87 K. We suggest that green phosphorene can be synthesized on corrugated metal surfaces rather than clean surfaces due to its buckled structure, providing guidance to achieving epitaxial growth.

Dipole-allowed direct band gap silicon superlattices.

Silicon is the most popular material used in electronic devices. However, its poor optical properties owing to its indirect band gap nature limit its usage in optoelectronic devices. Here we present the discovery of super-stable pure-silicon superlattice structures that can serve as promising materials for solar cell applications and can lead to the realization of pure Si-based optoelectronic devices. The structures are almost identical to that of bulk Si except that defective layers are intercalated in the diamond lattice. The superlattices exhibit dipole-allowed direct band gaps as well as indirect band gaps, providing ideal conditions for the investigation of a direct-to-indirect band gap transition. The fact that almost all structural portions of the superlattices originate from bulk Si warrants their stability and good lattice matching with bulk Si. Through first-principles molecular dynamics simulations, we confirmed their thermal stability and propose a possible method to synthesize the defective layer through wafer bonding.

Bandgap Widening of Phase Quilted, 2D MoS2 by Oxidative Intercalation.

Controllable bandgap widening from 1.8 to 2.6 eV is reported from oxidized MoS2 sheets that are composed of quilted phases of various MoSxOy flakes. The exfoliated flakes have large size (≥100 µm × 100 µm) sheets with average thickness of 1.7 nm. Remarkably, fine reversible tuning of the bandgap is achieved by postprocessing sulfurization of the MoSxOy sheets.

The effects of electric field and gate bias pulse on the migration and stability of ionized oxygen vacancies in amorphous In-Ga-Zn-O thin film transistors.

Oxygen vacancies have been considered as the origin of threshold voltage instability under negative bias illumination stress in amorphous oxide thin film transistors. Here we report the results of first-principles molecular dynamics simulations for the drift motion of oxygen vacancies. We show that oxygen vacancies, which are initially ionized by trapping photoexcited hole carriers, can easily migrate under an external electric field. Thus, accumulated hole traps near the channel/dielectric interface cause negative shift of the threshold voltage, supporting the oxygen vacancy model. In addition, we find that ionized oxygen vacancies easily recover their neutral defect configurations by capturing electrons when the Fermi level increases. Our results are in good agreement with the experimental observation that applying a positive gate bias pulse of short duration eliminates hole traps and thus leads to the recovery of device stability from persistent photoconductivity.