Henry Medina

Wafer Scale Phase-Engineered 1T-and 2H-MoSe2/Mo Core-Shell 3D-Hierarchical Nanostructures toward Efficient Electrocatalytic Hydrogen Evolution Reaction

The necessity for new sources for greener and cleaner energy production to replace the existing ones has been increasingly growing in recent years. Of those new sources, the hydrogen evolution reaction has a large potential. In this work, for the first time, MoSe2 /Mo core-shell 3D-hierarchical nanostructures are created, which are derived from the Mo 3D-hierarchical nanostructures through a low-temperature plasma-assisted selenization process with controlled shapes grown by a glancing angle deposition system.

Scalable graphite/copper bishell composite for high-performance interconnects.

We present the fabrication and characterizations of novel electrical interconnect test lines made of a Cu/graphite bishell composite with the graphite cap layer grown by electron cyclotron resonance chemical vapor deposition. Through this technique, conformal multilayer graphene can be formed on the predeposited Cu interconnects under CMOS-friendly conditions. The low-temperature (400 °C) deposition also renders the process unlimitedly scalable. The graphite layer can boost the current-carrying capacity of the composite structure to 10(8) A/cm(2), more than an order of magnitude higher than that of bare metal lines, and reduces resistivity of fine test lines by ∼10%. Raman measurements reveal that physical breakdown occurs at ∼680-720 °C. Modeling the current vs voltage curves up to breakdown shows that the maximum current density of the composites is limited by self-heating of the graphite, suggesting the strong roles of phonon scattering at high fields and highlighting the significance of a metal counterpart for enhanced thermal dissipation.

Ultrafast and low temperature synthesis of highly crystalline and patternable few-layers tungsten diselenide by laser irradiation assisted selenization process.

Recently, a few attempts to synthesize monolayers of transition metal dichalcogenides (TMDs) using the chemical vapor deposition (CVD) process had been demonstrated. However, the development of alternative processes to synthesize TMDs is an important step because of the time-consuming, required transfer and low thermal efficiency of the CVD process. Here, we demonstrate a method to achieve few-layers WSe2 on an insulator via laser irradiation assisted selenization (LIAS) process directly, for which the amorphous WO3 film undergoes a reduction process in the presence of selenium gaseous vapors to form WSe2, utilizing laser annealing as a heating source. Detailed growth parameters such as laser power and laser irradiation time were investigated. In addition, microstructures, optical and electrical properties were investigated. Furthermore, a patternable WSe2 concept was demonstrated by patterning the WO3 film followed by the laser irradiation. By combining the patternable process, the transfer-free WSe2 back gate field effect transistor (FET) devices are realized on 300 nm-thick SiO2/P(+)Si substrate with extracted field effect mobility of ∼0.2 cm(2) V(-1) s(-1). Similarly, the reduction process by the laser irradiation can be also applied for the synthesis of other TMDs such as MoSe2 from other metal oxides such as MO3 film, suggesting that the process can be further extended to other TMDs. The method ensures one-step process to fabricate patternable TMDs, highlighting the uniqueness of the laser irradiation for the synthesis of different TMDs.

Conformal graphene coating on high-aspect ratio Si nanorod arrays by a vapor assisted method for field emitter

Carbon materials like nanotubes and graphene have been previously used for field emission application due to their high emission currents and low turn-on voltages. However, in most cases, these devices show low reliability and poor endurance after a few hours of testing. The poor performance is usually attributed to lack of alignment, poor structure quality, and/or non-conformal coating. In this paper, a hybrid structure of graphene–silicon nanorod arrays (NRAs) was demonstrated by direct growth of self-crystallized graphene with Ni vapor-assisted growth via a conventional chemical vapor deposition (CVD) system. By carefully adjusting parameters and reducing the deposition rate, thicknesses of graphene layers can be systematically coated in a controllable manner, even on high aspect ratio surfaces such as aligned silicon NRAs. Detailed surface morphologies and microstructures of the graphene–NRAs core–shell hybrid structures were investigated. Findings in field emission measurements indicate that the graphene coating exhibits a remarkable enhancement by lowering the turn-on field, increasing the current density over 4 orders of magnitude, and greatly improving the endurance. The endurance test shows a stable current density of 1000 μA cm−2 after more than 15 hours of operation under a constant applied high bias stress.

Plasma-Assisted Synthesis of High-Mobility Atomically Layered Violet Phosphorus

Two-dimensional layered materials such as graphene, transition metal dichalcogenides, and black phosphorus have demonstrated outstanding properties due to electron confinement as the thickness is reduced to atomic scale. Among the phosphorus allotropes, black phosphorus, and violet phosphorus possess layer structure with the potential to be scaled down to atomically thin film. For the first time, the plasma-assisted synthesis of atomically layered violet phosphorus has been achieved. Material characterization supports the formation of violet phosphorus/InN over InP substrate where the layer structure of violet phosphorus is clearly observed. The identification of the crystal structure and lattice constant ratifies the formation of violet phosphorus indeed. The critical concept of this synthesis method is the selective reaction induced by different variations of Gibbs free energy (ΔG) of reactions. Besides, the Hall mobility of the violet phosphorus on the InP substrate greatly increases over the theoretical values of InP bulk material without much reduction in the carrier concentration, suggesting that the mobility enhancement results from the violet phosphorus layers. Furthermore, this study demonstrates a low-cost technique with high compatibility to synthesize the high-mobility atomically layered violet phosphorus and open the space for the study of the fundamental properties of this intriguing material as a new member of the fast growing family of 2D crystals.

Thermally Strained Band Gap Engineering of Transition Metal Dichalcogenide Bilayers with Enhanced Light-Matter Interaction toward Excellent Photodetectors

Integration of strain engineering of two-dimensional (2D) materials in order to enhance device performance is still a challenge. Here, we successfully demonstrated the thermally strained band gap engineering of transition-metal dichalcogenide bilayers by different thermal expansion coefficients between 2D materials and patterned sapphire structures, where MoS2 bilayers were chosen as the demonstrated materials. In particular, a blue shift in the band gap of the MoS2 bilayers can be tunable, displaying an extraordinary capability to drive electrons toward the electrode under the smaller driven bias, and the results were confirmed by simulation. A model to explain the thermal strain in the MoS2 bilayers during the synthesis was proposed, which enables us to precisely predict the band gap-shifted behaviors on patterned sapphire structures with different angles. Furthermore, photodetectors with enhancement of 286% and 897% based on the strained MoS2 on cone- and pyramid-patterned sapphire substrates were demonstrated, respectively.

Transfer-Free Growth of Atomically Thin Transition Metal Disulfides Using a Solution Precursor by a Laser Irradiation Process and Their Application in Low-Power Photodetectors.

Although chemical vapor deposition is the most common method to synthesize transition metal dichalcogenides (TMDs), several obstacles, such as the high annealing temperature restricting the substrates used in the process and the required transfer causing the formation of wrinkles and defects, must be resolved. Here, we present a novel method to grow patternable two-dimensional (2D) transition metal disulfides (MS2) directly underneath a protective coating layer by spin-coating a liquid chalcogen precursor onto the transition metal oxide layer, followed by a laser irradiation annealing process. Two metal sulfides, molybdenum disulfide (MoS2) and tungsten disulfide (WS2), are investigated in this work. Material characterization reveals the diffusion of sulfur into the oxide layer prior to the formation of the MS2. By controlling the sulfur diffusion, we are able to synthesize continuous MS2 layers beneath the top oxide layer, creating a protective coating layer for the newly formed TMD. Air-stable and low-power photosensing devices fabricated on the synthesized 2D WS2 without the need for a further transfer process demonstrate the potential applicability of TMDs generated via a laser irradiation process.

Tunable defect engineering in TiON thin films by multi-step sputtering processes: from a Schottky diode to resistive switching memory

The role of defect engineering is essential in resistive switching memory. In this study, multi-step sputtering processes to fabricate TiON for a resistive random access memory (ReRAM) device were demonstrated and detailed mechanisms were systematically investigated. The multi-step sputtered TiON film shows asymmetric defect distribution, exhibiting rectifying characteristics as a Schottky diode and resistive switching behavior as memory, depending on the applied bias. Rectifying properties, including a rectifying ratio of 102 at ±1.5 V, a forward current of ∼2 mA at 1.5 V, a turn-on voltage of 1.5 V and an ideality factor of 4.5, were measured. In addition, compared to a TiON ReRAM device prepared via a single-step sputtering process, TiON film with a gradient distribution of defects exhibits stable switching behavior with a better uniform SET voltage (VSET) and a coefficient of variation (σ/μ) which improves from 0.49 to 0.17. The conduction mechanisms of two kinds of device were investigated via a trap-controlled space charge limit conduction (SCLC) process. The mechanisms of how the distribution of asymmetric defects affects the resistive switching behavior were discussed in detail. The results disclose the possibility of the modulation of defect engineering toward Schottky diode applications, leading to the improvement of ReRAM performance for one-diode one-resistor (1D1R) applications in the future.

Graphene-coated copper nanowire networks as a highly stable transparent electrode in harsh environments toward efficient electrocatalytic hydrogen evolution reactions

Copper nanowire networks (NWs) coated with a graphene layer through a carbon-enclosed chemical vapor deposition technique at a low temperature of 400 °C with a low sheet resistance of 23.2 Ω sq−1 and a high transmittance of 83.4%, which is comparable to the typical values of tin-doped indium oxide (ITO), as a transparent conducting electrode were demonstrated. The graphene-coated copper NW networks retain a low sheet resistance of less than 25 Ω sq−1 even after annealing at a temperature of 240 °C in a pure oxygen environment for 1 h, while a sheet resistance less than 100 Ω sq−1 can still be maintained in natural sea water, and acidic and basic solutions. Their highly stable features in harsh environments make these graphene-coated copper nanowire networks suitable as a catalyst toward high-efficiency hydrogen evolution reactions (HERs) with a low overpotential of 252 mV at 10 mA Cm−2 and a low Tafel slope of 67 mV dec−1. The non-corrosive and anti-oxidant graphene-coated copper nanowire networks could be used as an alternative transparent conducting electrode in harsh environments, such as in tandem photocatalytic water splitting.

Modifying optical properties of GaN nanowires by Ga2O3 overgrowth

The authors report on the modification of optical properties of GaN nanowires by growing a thin Ga2O3 overlayer on GaN surface, forming a core/shell heterostructure. The GaN/Ga2O3 core/shell nanowires were formed first by the axial growth of GaN nanowires, followed by the radical growth of the Ga2O3 overlayer. The GaN core possesses single crystalline wurtzite structure, whereas the Ga2O3 shell layer is monoclinic polycrystalline. For the GaN/Ga2O3 core/shell nanowires, a pronounced blueshift of the Raman A1(LO) mode was found, indicating a compressive stress on the core wire due to the lattice mismatch with the outer shell. This picture is also supported by the photoluminescence spectrum, in which the peak shifts to higher energy after the overgrowth of Ga2O3 on GaN.