Hydrogen addition for centimeter-sized monolayer tungsten disulfide continuous films by ambient pressure chemical vapor deposition

Monolayer tungsten disulfide (WS 2 ) offers great prospects for use in optoelectronic devices due to its direct bandgap and high photoluminescence intensity. Here, we show how the controlled addition of hydrogen into the chemical vapor deposition growth of WS 2 can lead to the formation of centimeter scale continuous monolayer films at ambient pressure without the need for seed molecules, specially prepared substrates, or low pressure vacuum systems. Modifications of the reaction conditions, including growth time and hydrogen to argon ratio, allow for control over the domain size, film coverage, and film uniformity of the prepared WS 2 film. The combined control of hydrogen and a double-furnace system enables an increases in the growth rate of WS 2 , which results in fully merged films with cm 2 coverage and reduced multilayer content. Field effect transistors are fabricated to demonstrate that WS 2 has high quality for electronic applications. Our ambient pressure chemical vapor deposition reaction is simple and efficient, ideal for mass-production of large area monolayer WS 2

Chemically Tailored Growth of 2D Semiconductors via Hybrid Metal-Organic Chemical Vapor Deposition

Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) are an exciting platform for new excitonic physics and next-generation electronics, creating a strong demand to understand their growth, doping, and heterostructures. Despite significant progress in solid-source (SS-) and metal-organic chemical vapor deposition (MOCVD), further optimization is necessary to grow highly crystalline 2D TMDCs with controlled doping. Here, we report a hybrid MOCVD growth method that combines liquid-phase metal precursor deposition and vapor-phase organo-chalcogen delivery to leverage the advantages of both MOCVD and SS-CVD. Using our hybrid approach, we demonstrate WS$_2$ growth with tunable morphologies - from separated single-crystal domains to continuous monolayer films - on a variety of substrates, including sapphire, SiO$_2$, and Au. These WS$_2$ films exhibit narrow neutral exciton photoluminescence linewidths down to 33 meV and room-temperature mobility up to 34 - 36 cm$^2$V$^-$$^1$s$^-$$^1$). Through simple modifications to the liquid precursor composition, we demonstrate the growth of V-doped WS$_2$, MoxW$_1$$_-$$_x$S$_2$ alloys, and in-plane WS$_2$-MoS$_2$ heterostructures. This work presents an efficient approach for addressing a variety of TMDC synthesis needs on a laboratory scale

Plasma enhanced atomic layer etching of high-k layers on WS2

D. Marinov has received funding from the European Unions Horizon 2020 research and innovation program under Marie Sklodowska-Curie, Grant Agreement No. 752164. J.-F. de Marneffe received funding from the Graphene Flagship, Grant Agreement No. 952792. All authors acknowledge the support of imecs beyond CMOS program, imecs pilot line, and imecs materials and characterization (MCA) group. The authors thank Dr. Laura Nyns (imec) for the ALD of ZrO 2 and HfO 2 films and Dr. Benjamin Groven (imec) for the deposition of WS 2 films by PEALD

Influence of nanotube length and density on the plasmonic terahertz response of single-walled carbon nanotubes

We measure the conductivity spectra of thin films comprising bundled single-walled carbon nanotubes (CNTs) of different average lengths in the frequency range 0.3-1000 THz and temperature interval 10-530 K. The observed temperature-induced changes in the terahertz conductivity spectra are shown to depend strongly on the average CNT length, with a conductivity around 1 THz that increases/decreases as the temperature increases for short/long tubes. This behaviour originates from the temperature dependence of the electron scattering rate, which we obtain from Drude fits of the measured conductivity in the range 0.3-2 THz for 10 $\mu$m length CNTs. This increasing scattering rate with temperature results in a subsequent broadening of the observed THz conductivity peak at higher temperatures and a shift to lower frequencies for increasing CNT length. Finally, we show that the change in conductivity with temperature depends not only on tube length, but also varies with tube density. We record the effective conductivities of composite films comprising mixtures of WS$_2$ nanotubes and CNTs vs CNT density for frequencies in the range 0.3-1 THz, finding that the conductivity increases/decreases for low/high density films as the temperature increases. This effect arises due to the density dependence of the effective length of conducting pathways in the composite films, which again leads to a shift and temperature dependent broadening of the THz conductivity peak.Comment: Submitted to Journal of Physics D. Main manuscript: 9 pages, 8 figures. Supplementary material: 5 pages, 6 figure

Tribological properties of PVD coatings with lubricating films

Abstract. This work reports on the tribological performance of three different commercial hard PVD coatings (TiN, TiCN and nACo) with lubricating extra films of Al 2 O 3 , Ni-WS 2 and diamondlike carbon (DLC). WC-Co hardmetal has been used as substrate material. Wear tests, employing two counter bodies of Al 2 O 3 and hardmetal WC-Co, were performed for the PVD coatings with and without the extra films. The results showed that the presence of DLC extra film reduces the coefficient of friction of the PVD hard coatings TiN and nACo. Furthermore, the wear of TiN coatings was reduced in the presence of an extra Ni-WS 2 lubricant film

Lessons from Oxypnictide Thin Films

First experiments on the growth of oxypnictide F-doped LaFeAsO thin films indicated an incomplete normal-to-superconducting transition and offered a work programme challenging to overcome possible difficulties in their fabrication. In this regard the possibility of an all in-situ epitaxial growth appeared to be a matter of time and growth parameters. The following review clarifies that F-doped oxypnictide thin films are extremely difficult to grow by in-situ PLD due to the formation of very stable impurity phases such as oxyfluorides (LaOF) and oxides (La2O3) and the loss of stoichiometry possibly due to incongruent evaporation of the target or re-evaporation of volatile elements at the substrate surface. However, the review also demonstrates that the employed two-step fabrication process for oxypnictide thin films has been successfully applied in the preparation of clean polycrystalline as well as of epitaxial thin films. Fundamental investigations on the upper critical field, its temperature dependence and its anisotropy contributed to an understanding of multiband superconductivity in oxypnictides.Comment: accepted, pre-print versio

Pairing in cuprates from high energy electronic states

The in-plane optical conductivity of Bi2Sr2CaCu2O8+d thin films with small carrier density (underdoped) up to large carrier density (overdoped) is analyzed with unprecedented accuracy. Integrating the conductivity up to increasingly higher energies points to the energy scale involved when the superfluid condensate builds up. In the underdoped sample, states extending up to 2 eV contribute to the superfluid. This anomalously large energy scale may be assigned to a change of in-plane kinetic energy at the superconducting transition, and is compatible with an electronic pairing mechanism.Comment: 11 pages, 3 figure

Probing the sp^2 dependence of elastic moduli in ultrahard diamond films

The structural and elastic properties of diamond nanocomposites and ultrananocrystalline diamond films (UNCD) are investigated using both empirical potentials and tight binding schemes. We find that both materials are extremely hard, but their superb diamondlike properties are limited by their sp^2 component. In diamond composites, the sp^2 atoms are found in the matrix and far from the interface with the inclusion, and they are responsible for the softening of the material. In UNCD, the sp^2 atoms are located in the grain boundaries. They offer relaxation mechanisms which relieve the strain but, on the other hand, impose deformations that lead to softening. The higher the sp^2 component the less rigid these materials are.Comment: 10 pages, 3 figures. to appear in Diamond and Relarted Material

Up-dating the Cholodny method using PET films to sample microbial communities in soil

The aim of this work was to investigate the use of PET (polyethylene terephtalate) films as a modern development of Cholodny’s glass slides, to enable microscopy and molecular-based analysis of soil communities where spatial detail at the scale of microbial habitats is essential to understand microbial associations and interactions in this complex environment. Methods. Classical microbiological methods; attachment assay; surface tension measurements; molecular techniques: DNA extraction, PCR; confocal laser scanning microscopy (CLSM); micro- focus X-ray computed tomography (μCT). Results. We first show, using the model soil and rhizosphere bacteria Pseudomonas fluorescens SBW25 and P. putida KT2440, that bacteria are able to attach and detach from PET films, and that pre-conditioning with a filtered soil suspension improved the levels of attachment. Bacteria attached to the films were viable and could develop substantial biofilms. PET films buried in soil were rapidly colonised by microorganisms which could be investigated by CLSM and recovered onto agar plates. Secondly, we demonstrate that μCT can be used to non-destructively visualise soil aggregate contact points and pore spaces across the surface of PET films buried in soil. Conclusions. PET films are a successful development of Cholodny’s glass slides and can be used to sample soil communities in which bacterial adherence, growth, biofilm and community development can be investigated. The use of these films with μCT imaging in soil will enable a better understanding of soil micro-habitats and the spatially-explicit nature of microbial interactions in this complex environment