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Large scale synthesis, doping of two-dimensional (2D) transition metal dichalcogenides and their heterostructures

Author
Zhang, Kehao
Published
[University Park, Pennsylvania] : Pennsylvania State University, 2019.
Physical Description
1 electronic document
Additional Creators
Robinson, J. A. (Josh A.)
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Graduate Program
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Open Access.
Summary
Two dimensional materials receive tremendous attention since the discovery of a single layer crystalline carbon film known as graphene, due to the great potential to be applied in the next generation electronic, photonic, optoelectronic and sensing devices. However, the current devices are mainly based on the state-of-the-art demonstration on exfoliated 2D materials (<10 m lateral size) because of the lack of large-scale synthesis. The first part of this dissertation addresses the large-scale synthesis of 2D transition metal dichalcogenides (TMDs). Specifically, multiple techniques including ion-exchange, powder vaporization (PV) and metal organic chemical vapor deposition (MOCVD) are developed. Ion-exchange is based on the physical vapor deposited (PVD) tungsten trioxide (WO3) thin films, followed by the selenization, which converts WO3 to WSe2. This process achieves uniform, few layer and polycrystalline WSe2. The co-evaporation of metal oxides and chalcogen powders is utilized in the PV technique that achieves single crystalline, monolayer MoS2 with >50 m lateral size on dielectric substrates such as sapphire and SiO2, with as-grown monolayer films exhibiting >30 cm2/V.s field effect mobility. Meanwhile, the epitaxy of monolayer MoS2 is achieved by choosing the lattice matched substrate such as sapphire and gallium nitride (GaN). Additionally, it grows monolayer MoS2 on various substrates including graphene, gallium nitride, mica and glass ect, enabling collaborative study focuses on various topics of 2D materials such as defect passivation, nanocrystal charge transfer ect. The inch-scale uniform, epitaxial growth of monolayer MoS2 is realized by alkali-free MOCVD, enabling the scalable fabrication of field effect transistors (FETs). A direct comparison between alkali-assisted growth and alkali-free growth reveals the heterogeneities in growth mechanism is induced by using alkali halides, along with degradation of the FETs performance in as-grown and transferred MoS2 films. One of the important aspects in semiconductors is doping. However, in the field of 2D semiconductors, especially for monolayer, the doping process and properties remain unclear. The second part of this dissertation describes substitution doping of MoS2 by PV and MOCVD. Three dopants: manganese (Mn), rhenium (Re) and niobium (Nb) are used. 2 atomic percent (at%) Mn substitutional doping in monolayer MoS2 lattice is exclusively realized on graphene due to the inert surface of graphene. Attempts are made to synthesize Mn into MoS2 on conventional 3D dielectric substrates to further probe the magnetic properties, but the Mn is widely spread on the substrates instead of being incorporated in the lattice. Re doping of MoS2 induces significant modulation in the electronic and photonic properties. 1 at% Re doped MoS2 shifts the Fermi level towards the conduction band minima by 0.5 eV and therefore exhibits nearly degenerately-doped electrical behavior. Interestingly, the Re doped MoS2 shows significantly reduced defect photoluminescence (PL) emission at low temperatures (<77K), which is likely attributed to the suppression of Mo-O defects and removal of sulfur vacancies related sub-gap states in the synthetic MoS2. Tunable Nb doping of one-layer to few-layer MoS2 is developed. The Fermi level of Nb MoS2 is tuned from 0.2 eV below the conduction band (n-type) to 0.2 eV below the valence band (degenerate p-type doping) via increasing Nb dopant concentration. The 5 at% Nb doped MoS2 enhances the chemical sensitivity to triethylamine (TEA) by >60 at 100ppm TEA concentration and improving the detection limit to <15 ppb. Benefitting from the layered structure and ultra-thin nature of TMDs, integrating TMDs with conventional 3D semiconductors holds unique promise for the novel design of high power, high frequency heterojunction bipolar transistors and quantum wells. The third part of this thesis demonstrates the epitaxial growth of MoS2 on n-type and p-type GaN by PV and MOCVD. It is elucidated that the 2D/3D heterostructure is electrically active in the vertical direction, but the transport characteristics are strongly dependent on the thickness of the TMDs. One-layer TMDs lead rectifying behavior due to the Schottky barrier, while few layer TMDs on GaN build p-n junctions. It is worth noting that the dopants of p-type GaN can be passivated during the H2 involved MOCVD growth. Additionally, the 2D layers can serve as a tunable substrate that enhances the biosensing properties. Stacking FeSx on graphene (n-type epitaxial graphene, p-type epitaxial graphene and CVD graphene enables ultra-high sensitivity to hydroperoxides, in which CVD graphene provides the highest sensitivity (<500pmol). Further electrochemical analysis suggests that CVD graphene stands out due to its slight p-doping as well as the high conductivity.
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Dissertation Note
Ph.D. Pennsylvania State University 2019.
Reproduction Note
Microfilm (positive). 1 reel ; 35 mm. (University Microfilms 28929423)
Technical Details
The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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