Substrate-mediated growth of oriented, vertically aligned MoS2 nanosheets on vicinal and on-axis SiC substrates

The layer- and morphology-dependent properties of two-dimensional molybdenum disulfide (MoS2) have established its relevance across broad applications in electronics, optoelectronics, sensing, and catalysis. Understanding how to manipulate the material growth to achieve the desired properties is the key to tailoring the material towards a specific application. In this work, we investigate the growth of vertically standing MoS2 nanosheets by chemical vapor deposition on vicinal and on-axis 4H-SiC (0001) substrates. In both cases the MoS2 flakes exhibit three preferred orientations, aligning with the substrate directions due to strain minimization of a MoO2 intermediate phase. Whereas MoS2 grown on vicinal SiC substrates exhibits strict near-vertical alignment, scanning electron microscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements indicate a near-random vertical orientation when MoS2 is grown on on-axis SiC. Photoemission spectroscopy and NEXAFS measurements indicate the presence of defects and disordered edges which establish the suitability of the material for applications in sensing and catalysis

Van der Waals epitaxy of transition metal dichalcogenides on graphene

This research is a fundamental study of the direct epitaxial growth of MoS2 on diverse types of graphene substrates, including self-standing and graphene/SiC, using chemical vapour deposition (CVD). The size, quality, and growth rate of epitaxial graphene on SiC, subsequently used as a substrate for the formation of MoS2, is controlled with high precision using a novel face-to-face technique in ultra-high vacuum. The effect of growth parameters and substrate characteristics on the CVD growth of MoS2 are investigated to design an optimised viable approach for the growth of high-quality MoS2/Graphene heterostructures

Poly (vinylidene fluoride-co-hexafluoro propylene) / polyethylene oxide-based nanoparticles reinforced gel polymer electrolytes for dye-sensitized solar cell / Negar Zebardastan

Solar energy is the most abundant and clean source of energy on the earth. Recently scientists have been able to develop a technology to harvest solar energy and today we are able to convert the sunlight directly to the electricity. Dye-sensitized solar cells (DSSCs) are one of the promising solar harvesting technologies with numerous advantages over the other technologies such as silicon based solar cells. Usually high performance DSSCs are obtained using liquid electrolyte which face several drawbacks for long term usage, such as leakage, electrolyte evaporation and interface corrosion. Gel polymer electrolyte can be an alternative to overcome these issues but the ionic conductivity of this gel polymer electrolytes must be improved to achieve high energy conversion efficiency. In this work we studied three gel polymer electrolyte (GPE) systems and the performance of DSSCs using GPEs have been analyzed. These GPEs are formulated by blending Poly(vinylidene fluoride-co-hexafluoro propylene) copolymer (PVdF-HFP) and polyethylene oxide (PEO) polymers. First, incorporation of sodium iodide (NaI) salt in different concentrations in the GPE system is investigated and later the addition of fumed silica (SiO2) and zinc oxide (ZnO) nanofiller into the GPE system are studied. GPEs are examined using electrochemical impedance spectroscopy (EIS) to determine ionic conductivity values. The highest ionic conductivities of 6.38, 8.84 and 8.36 mS cm−1 are achieved after the incorporation of 100 wt.% of sodium iodide (NaI), 13 wt.% of fumed silica (SiO2) and 3 wt.% of ZnO in each system, respectively. Temperature-dependent ionic conductivity study confirms that GPE systems follow Arrhenius thermal activated model. GPEs are characterized for structural studies using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. DSSCs are fabricated using GPEs and need to be recorded under 1 Sun simulator which produced the significant highest energy conversion efficiency of 5.67, 9.44 and 9.08 % with incorporation of 100 wt.% of sodium iodide (NaI) with respect to the total weight of PEO:PVdF-HFP polymers, 13 wt.% of fumed silica (SiO2) and 3 wt.% of ZnO in each system, respectively

High quality epitaxial graphene on 4H-SiC by face-to-face growth in ultra-high vacuum

Epitaxial graphene on SiC is the most promising substrate for the next generation 2D electronics, due to the possibility to fabricate 2D heterostructures directly on it, opening the door to the use of all technological processes developed for silicon electronics. To obtain a suitable material for large scale applications, it is essential to achieve perfect control of size, quality, growth rate and thickness. Here we show that this control on epitaxial graphene can be achieved by exploiting the face-to-face annealing of SiC in ultra-high vacuum. With this method, Si atoms trapped in the narrow space between two SiC wafers at high temperatures contribute to the reduction of the Si sublimation rate, allowing to achieve smooth and virtually defect free single graphene layers. We analyse the products obtained on both on-axis and off-axis 4H-SiC substrates in a wide range of temperatures (1300 °C-1500 °C), determining the growth law with the help of x-ray photoelectron spectroscopy (XPS). Our epitaxial graphene on SiC has terrace widths up to 10 μm (on-axis) and 500 nm (off-axis) as demonstrated by atomic force microscopy and scanning tunnelling microscopy, while XPS and Raman spectroscopy confirm high purity and crystalline quality.</p

High quality epitaxial graphene on 4H-SiC by face-to-face growth in ultra-high vacuum

Epitaxial graphene on SiC is the most promising substrate for the next generation 2D electronics, due to the possibility to fabricate 2D heterostructures directly on it, opening the door to the use of all technological processes developed for silicon electronics. To obtain a suitable material for large scale applications, it is essential to achieve perfect control of size, quality, growth rate and thickness. Here we show that this control on epitaxial graphene can be achieved by exploiting the face-to-face annealing of SiC in ultra-high vacuum. With this method, Si atoms trapped in the narrow space between two SiC wafers at high temperatures contribute to the reduction of the Si sublimation rate, allowing to achieve smooth and virtually defect free single graphene layers. We analyse the products obtained on both on-axis and off-axis 4H-SiC substrates in a wide range of temperatures (1300 °C-1500 °C), determining the growth law with the help of x-ray photoelectron spectroscopy (XPS). Our epitaxial graphene on SiC has terrace widths up to 10 m (on-axis) and 500 nm (off-axis) as demonstrated by atomic force microscopy and scanning tunnelling microscopy, while XPS and Raman spectroscopy confirm high purity and crystalline quality

2D MoS2 Heterostructures on Epitaxial and Self-Standing Graphene for Energy Storage : From Growth Mechanism to Application

Layered molybdenum disulphide (MoS2) crystals in combination with graphene create the opportunity for the development of heterostructures with tailored surface and structural properties for energy storage applications. Herein, 2D heterostructures are developed by growing MoS2 on epitaxial and self-standing nanoporous graphene (NPG) using chemical vapor deposition (CVD). The effect of substrate as well as different CVD growth parameters such as temperature, amount of sulfur and MoO3 precursors, and argon flow on the growth of MoS2 is systematically investigated. Interestingly, various structures of MoS2 such as monolayer triangular islands, spirals, standing sheets, and irregular stacked multilayered MoS2 are successfully developed. The growth mechanism is proposed using different advanced characterization techniques. The formation of a continuous wetting layer with grain boundaries over the surface prior to formation of any other structures is detected. As a proof of principle, MoS2/NPG is employed for the first time as anode material in potassium ion battery. The electrode delivers a specific capacity of 389 mAh g−1 with over 98% stability after 200 cycles. The porous structures clearly facilitate the ion transport which is beneficial for the ion battery. These encouraging results open new opportunities to develop hierarchical heterostructures of 2D-materials for next-generation energy storage technologies.</p