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A new 2H-2H'/1T co-phase in polycrystalline MoS2 and MoSe2 thin films Mehwish Naz, Toby Hallam, Nina C. Berner, Niall McEvoy, Riley Gatensby, John B. McManus, Zareen Akhter, and Georg S. Duesberg ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10972 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016
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ACS Applied Materials & Interfaces
A new 2H-2H'/1T co-phase in polycrystalline MoS2 and MoSe2 thin films
Mehwish Naz, a,b Toby Hallam, b,c Nina C. Berner, b Niall McEvoy, b,d Riley Gatensby, b,d John McManus, b,d , Zareen Akhter,a Georg S. Duesberg. b,d a
Department of Chemistry; Quaid-i-Azam University, 45320 Islamabad , Pakistan
b
Centre for Research on Adaptive Nanostructures and Nanodevices & Advanced Materials
BioEngineering Research Centre (AMBER),Trinity College Dublin, Ireland c
School of Physics, Trinity College Dublin, Ireland
d
School of Chemistry, Trinity College Dublin, Ireland
Abstract We report on 2H-2H'/1T phase conversion of MoS2 and MoSe2 polycrystalline films grown by thermally assisted conversion. The structural conversion of the transition metal dichalcogenides was successfully carried out by organolithium treatment on chip. As a result we obtained a new 2H-2H'/1T co-phase system of the TMDs thin films which was verified by Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The conversion was successfully carried to on selected areas yielding a lateral heterostructure between the pristine 2H phase and the 2H'/1T co-phase regions. Scanning Electron Microscopy and Atomic Force Microscopy revealed changes in the surface morphology and work function of the co-phase system in comparison to the pristine films, with a surprisingly sharp lateral interface region. Key Words: Heterostructure, transition metal dichalcogenides, molybdenum disulfide, 2D
materials, polytype, butyl-lithium
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1. Introduction Layered inorganic 2D transition metal dichalcogenides (TMDs) materials such as MoS2, WS2, MoSe2 and WSe2, have been recognized lately for their attractive chemical, optical and electronic properties. They have been suggested for numerous applications in electronics,
1,2
optoelectronics, 3,4 gas sensing 5,6 and energy storage and conversion. 7-8 TMDs can exist in several different structural phases due to different stacking patterns of the layers. The polymorphs 2H and 3R have a AbA BaB AbA BaB and AbA BcB CaC AbA, or the inverted AbA CaC BcB AbA stacking configuration respectively, while 1T has AbC AbC stacking order. Thus far, MoS2 is the most widely studied TMD material because of its relative stability.9,10 Its 2H and 3R phases are semiconducting in nature, while 1T-MoS2 is metallic. nbutyl lithium (n-BuLi) can be utilized as chemical intercalation and exfoliation to tune the electronic structure and induce the phase transition 2H to 1T.
11-13
Lithium intercalation into the
MoS2 structure leads to a phase transition from the trigonal symmetry (2H phase) into octahedral symmetry (1T phase).
11
This type of transformation can lead to interesting defect phenomena
between the layers due to the breaking of crystalline symmetry.14,15 Previously, n-BuLi has been used for phase conversion in powder, 16 CVD,
17
and mechanically exfoliated flakes.18 In this
work, we report on the on-chip reaction of large scale TAC derived polycrystalline TMD films with n-BuLi. This is the first time that a continuous multilayer TMD film has shown chemically induced phase conversion, which is an important step towards the understanding and applications of phase engineering in TMD materials. The conversion of 2H TMD films to the 1T phase is utilized for subsequent chemical functionalization, as it is more reactive. That opens various synthetic routes for chemical functionalization to tune material properties and surface chemistry of TMDs to broaden their applicability in areas such as energy storage, catalysis for hydrogen evolution, opto-electronics, photoluminescence and gate modulation in field-effect transistors.19-21 It is envisaged that TMD functionalization via 2H-1T conversion will add to the flexibility of other covalent routes such as the functionalization of 2H-MoS2 with Cu (11) acetate salt which we have reported upon recently.22 Already chemical phase conversion of TMDs has shown reduction in contact resistance in monolayer FET devices. 17
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ACS Applied Materials & Interfaces
Recently, the use of Chemical Vapor Deposition (CVD) has been explored for fabricating TMD multilayer/monolayer films.23–25 CVD is considered one of the most favorable techniques for the growth of highly crystalline MoS2 monolayers, however it remains a challenge to produce uniform closed films over large areas. An alternative synthesis route capable of this is Thermally Assisted Conversion (TAC) of thin metal films yielding homogenous but poly-crystalline TMD films.26 We have reported on the synthesis of the sulfides and selenides of Mo and W with TAC, but recently extended the method to less explored material such as PtSe2.27 This straight forward method has also the advantage that the thickness of the films can be easily controlled by the deposition of the metal layer, typically performed by sputtering or evaporation.27 Typically, 20 nm thick homogenous MoS2 and MoSe2 on Si/SiO2 substrates converted to heterostructure system of the 2H-2H'/1T phase. The coexistence of the two phases was confirmed by Raman spectroscopy, wide angle X-Ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). To our knowledge the co-phase system is reported for the first time, and will be useful for successive functionalisation as well as interfacing TMD films. In this regard the conversion was successfully carried out on selected areas of the chip yielding a lateral interface between the pristine 2H phase and the 2H'/1T regions. Surface morphology and electronic nature of both phases as well as the interface regions were investigated by Scanning Electron Microscopy (SEM) and Scanning Kelvin Probe Microscopy (SKPM).
2. Methods Metal films of Mo, (99.99% MaTecK) were sputtered onto substrates (300 nm SiO2 on Si Alfa Aesar) using a Gatan Precision Etching Coating System (PECS). The film deposition rate, and thickness, was monitored using a quartz crystal microbalance, maintaining a deposition rate of
