二维过渡金属硫族化合物的环境稳定性.pdf

第 43 卷第 4 期2023 年 8 月物理学进展PROGRESS IN PHYSICSVol.43 No.4Aug.2023Environmental Stability of 2D Transition Metal DichalcogenidesZHOU Zhen-jia1,XU Jie1,GAO Li-bo1,21.National Laboratory of Solid State Microstructures&Department of Physics,Nanjing University,Nanjing 210093,China2.Collaborative Innovation Center of Advanced Microstructures,Nanjing University,Nanjing 210093,ChinaAbstract:Two-dimensional(2D)transition metal dichalcogenides(TMDCs)with a uniqueunity of favorable electronic and mechanical properties have been developed for fundamen-tal studies and applications in electronics,spintronics,optoelectronics,energy harvesting andcatalysis.However,as they are unstable under harsh conditions,and prone to degradation inthe ambient environment,most TMDCs applications are limited.In this review,we analyzethe recent advances in the research of environmental stability in TMDCs,covering the latestgrowth methods,the fundamental mechanisms for stability and kinds of routes to protect 2DTMDCs materials from aging and deterioration.By analyzing key factors that affect TMDCsstability from the growth process,we present a short review of optimizing growth methods forimproving the stability of TMDCs.Finally,by providing insights into existing factors,thisreview is expected to guide the growth of stable TMDCs,which could lead to a new potentialapproach to growing advanced materials and designing more unexplored heterostructures.Key words:two-dimensional materials;transition metal dichalcogenides;stability;vacancyCLC number:O469Document code:ADOI:10.13725/ki.pip.2023.04.001CONTENTSI.Introduction97II.The Origin of Enviromental Instability of TMDCs98A.Preparation methods for TMDCs materials981.Atomic structure982.Optical properties1003.Electronic properties101B.Harsh environment for TMDC materials1021.Electron irradiation1022.Ion irradiation104C.Degradation mechanisms of TMDCs1041.Oxygen adsorption reaction1042.Hydrogen adsorption reaction104III.Strategies for Improving Stablity106A.Surface protection and substrate modification1061.h-BN encapsulation1062.Graphene substrate106B.Induced doping and chemical bonding1071.High-oxides doping1072.Li atom doping109Received date:2023-6-6 E-mail: E-mail:C.Modulated growth methods1091.Two-step vapor deposition1092.Growing on Au foil110IV.Conclusion and Outlook111References111I.INTRODUCTIONOn the evening of 29 December 1959,underthe title Theres plenty of room at the bottomRichard Feynman envisioned the bright future of nan-otechnology and inspired generations of researchersto explore the possibilities of manipulating and con-trolling on the nanoscale.Since the discovery ofgraphene1,two-dimensional(2D)materials analo-gous,including 2D transition metal dichalcogenides(TMDCs)2-6,hexagonal boron nitride7-8,blackphosphorus9,silicene10-11,germanene12-13andother 2D materials have been extensively researched,in which TMDCs have attracted increasing atten-Article ID:1000-0542(2023)04-0097-209798ZHOU Zhen-jia et al.:Environmental Stability of 2D Transition Metal Dichalcogenidestion.Compared with their bulk counterparts,2DTMDCs have abundant properties,such as semi-conducting14-17,superconductivity18-21,ferromag-netism22,ferroelectricity23,electrocatalysis24-25,etc.,indicating the promising application in exoticphysical phenomena in low dimensions,low-power,high-performance nanoelectronic devices26-30,32-33,37.Similar to graphene,TMDCs can be prepared byeither bottom-up routes or the top-down exfoliationof bulk layered materials34-40,extremely contribut-ing to the fundamental studies and the development ofdevice applications.Whatever the developed prepara-tion methods are,the atomically thin TMDCs mate-rials are susceptible to the ambient environment anddeteriorate after exposure for a short time,leadingto remarkable changes in their physical and chem-ical properties41-44.Environmental degradation ofTMDCs significantly impedes further research,suchas material characterization,performance testing anddevice fabrication,which also hinders their practi-cal application including high-performance electronics,optoelectronics and flexible devices.Consequently,inorder to extract the intrinsic properties and promotedevice performance,it is necessary to explore the fac-tors which influence the stabilities of 2D materials andthe way to enhance their stabilities.In this review,we focus on the environmental sta-bilities of TMDCs,as well as their performance inelectrical and optical properties.We start discussingthe factors that contribute to affecting the stabili-ties by introducing the various growth methods ofTMDCs,demonstrating the nucleation process of thesamples grown by each method and their correspond-ing atomic structure20,42,45-46.We notice that theproperties of TMDCs exhibit various degrees of degen-eration due to the reaction with oxygen species fromthe ambient environment34,41,45.In addition,we dis-cuss the storage conditions and the degeneration mech-anisms47-49.Later,we also suggest correspondingapproaches for improving the stability,such as sur-face protection47,substrate modification50,chem-ical bonding and the optimization of growth meth-ods20,51.Finally,we summarize the challenges andperspectives in this research field,expecting to exploremore approaches to enhance the stability of 2D mate-rials,as well as the reliability and lifetime of relateddevices.II.THE ORIGIN OF ENVIROMENTALINSTABILITY OF TMDCSA.Preparation methods for TMDCs materials1.Atomic structureSimilar to graphene,TMDCs monolayer can beobtained from high-quality natural or synthetic bulkcrystals via mechanical exfoliation3,44,52-54,as shownin Fig.1(a).Despite its simplicity,mechanical exfolia-tion is a popular method for rapid fabrication of func-tional devices.However,when exfoliated down to a fewlayers,some TMDCs suffer performance degradationonce exposed to ambient conditions.In order to protectthese highly sensitive 2D materials,a typical method isapplied by encapsulating with environmentally stablematerial41,55-56.For example,as shown in Fig.1(b),exfoliated NbSe2flakes are encapsulated by graphene tosuppress aging and degradation44.It is noted that thenumber of the structural defects in Fig.1(c)(markedin red circles)still exist in the few-layer flakes althoughthe inert atmosphere is employed during the wholemechanical|exfoliation process.Indeed,most studieson TMDCs layered material still focus on exfoliatedsamples.In contrast to the other available preparationmethods,mechanical exfoliation is deemed less effi-cient for functional device applications.Currently,anavailable preparation method for relatively large-scaleTMDCs layers with high crystal integrity is limited.One of the scalable methods for growing high-qualityTMDCs is molecular beam epitaxy(MBE),of whichthe operation principle is demonstrated in Fig.1(d).MBE involves the synthesis from molecular beams oftransition metal(M)and chalcogen(X)by thermalor electron beam evaporation and direct deposition ofthe synthetic material onto a growth substrate in anultrahigh vacuum(UHV)chamber.During the growthprocess,the thickness and substrate crystallinity canbe precisely monitored in situ by reflection high-energyZHOU Zhen-jia et al.:Environmental Stability of 2D Transition Metal Dichalcogenides99Fig.1.Various preparation methods for TMDCs.(a)Mechanical exfoliation.MX2film is obtained by mechanical peel-offwith scotch tape.Optical image and atomic structure of mechanically exfoliated NbSe2are shown in(b),(c)44.(d)Molecular beam epitaxy.High-purity elemental source(X and M)are co-evaporated in ultrahigh vacuum.(e)and(f)respectively show large-scale STM images and atomic structure of the monolayer NbSe2grown on bilayer graphene58.(g)One-step CVD(CVD).Triangular single-crystalline MX2is grown on the substrate after the direct chemical reactionbetween transition metal oxide(MO3)and chalcogen source.Optical image and atomic structure of CVD grown NbSe2are shown in(h)and(i)21.(j)Two-step vapor deposition(T-VD).The transition metal film is directly sulphurized orselenized by chalcogen source.Optical image and atomic structure are displayed in(k)and(l)20.electron diffraction(RHEED)and low-energy electrondiffraction(LEED).In addition,benefiting from theweak van der Waals interaction,epitaxial growth cannot only initialize on the substrate but also occur onthe other layered materials regardless of lattice match-ing condition57,e.g.the example of the growth ofNbSe2on graphene,as shown in Fig.1(e)58.Thepurity of MBE-grown material is relatively high,due tothe use of high-purity source materials(99.999 9%)andthe growth process under UHV(typically lower than106Pa).However,the UHV condition and expensiveapparatus cause MBE difficult to popularize.Addi-100ZHOU Zhen-jia et al.:Environmental Stability of 2D Transition Metal Dichalcogenidestionally,MBE-grown samples are also unstable in theair and easily oxidized once exposed to an atmosphericenvironment.Therefore,interconnection by vacuum orinert gas protection is highly desirable for MBE meth-ods.There are still other strategies to explore efficientroutes for the growth of high-quality TMDCs.One-step chemical vapor deposition(CVD)is an effectivemethod to grow large-scale TMDCs under atmosphericpressure35.A typical experimental scheme is shownin Fig.1(g).The synthesis process starts from transi-tion metal oxides and chalcogen vapors supplied with acarrier gas,and then the metal oxides are decomposedat high temperature and subsequently converted intothe resulting TMDCs domains or films59-60.Mean-while,the residual oxygen derived from oxide precur-sors still exists in the grown samples,contributing tothe inevitable existence of heterogeneous doping or oxy-gen bonds.Such a heterogeneous structure presentedin Fig.1(i)(marked in red circles)can also be con-sidered that residual O atoms replace the position ofchalcogen atoms in the grown TMDCs21,61-62.Inorder to prevent the residual O atoms derived fromoxide precursor,two-step vapor deposition(T-VD)pro-vides a fully oxygen-free growth condition to avoidthe formation of oxygen bonds during the growth pro-cess,which uses the pure metal film as a precursor(Fig.1(j)20.Recently,wafer-size growth of NbSe2film has been controllably realized,and the formed sam-ples show good superconductivity and stability aftera variety of harsh treatments.The atomic imagesin Fig.1(l)exhibit near-perfect crystalline structure,indicating that the absence of redundant O elementscontributes to improving the quality of samples.Fig.1 summarizes that different growth condi-tions and methods result in differences in atomic struc-ture in the case of 2D superconductor NbSe2.Onceoxygen exclusion occurs in the whole growth process,the atomic structure of the sample will exhibit a rela-tively low concentration of defects(Figs.1(f)and(l).It has been demonstrated that each method has itsadvantages and drawbacks,so the quality of the sam-ples will be highly dependent on specific growth meth-ods.Therefore,growing structurally stable TMDCswith fewer defects is a crucial step to exploring materialproperties and fabricating functional devices,in partic-ular for materials that are susceptible to the ambientenvironment.2.Optical propertiesAs discussed in the beginning,the quality of thesamples is highly dependent on their growth condi-tions.As-prepared samples are prone to be defec-tive once exposed to any oxidation environment dur-ing the growth,especially for the mechanically exfo-liated samples and CVD-grown samples.In this sec-tion,we mainly focus on the effects of different growthmethods on the stability of TMDCs samples.Opti-cal images and AFM images in Figs.2(a)-(c)showthe morphological changes of exfoliated HfS2thin filmafter exposure to air for a given period46.Duringthe oxidation process,the thickness is increased due tothe expansion of the interlayer distance by oxidation,which is simultaneously accelerated in the defect sitesof the flake63.The aged results show that HfOxisnearly formed due to the oxidation of exfoliated HfS2while exposed to air.Notably,such an oxidation pro-cess also occurs in grown samples41.Monolayer WS2single crystals grown by the CVD method are charac-terized in Figs.2(d)-(i)after aging time64.As grownand two-month-aged crystals present uniform opticalcontrast,while the eight-month-aged WS2exhibits vis-ible degradation.Similar to optical images,as-grownand two-month-aged crystals display uniform surfacemorphology in AFM characterization.Small parti-cles on the two-month-aged WS2in Fig.2(h)can bedemonstrated as the unintentional contaminant fromthe ambient environment during storage.However,inFig.2(i),the light-contrast grain is 3.5 nm higher thanmonolayer WS2,which is consistent with the opticalimages.In Figs.2(j)-(q),typical monolayer MoS2prepared via mechanical exfoliation and CVD growthmethod are adopted to demonstrate the relationshipbetween the stability of as-prepared samples and thegrowth process41.The results of the accelerated agingtest demonstrate that the single crystalline exfoliatedMoS2and CVD-grown MoS2exhibit different degreesof degradation under the same conditions in Figs.2(j)-(q).The reason is that the oxygen element is normallyZHOU Zhen-jia et al.:Environmental Stability of 2D Transition Metal Dichalcogenides101Fig.2.Optical characterization of TMDCs grown by various methods.(a-c)Optical and AFM images of mechanicallyexfoliated HfS2flake exposed in air46.(d-f)Optical images of(d)as grown,(e)two-month-aged,and(f)eight-month-agedmonolayer WS264.(g-i)AFM images of(g)as grown,(h)two-month aged,and(i)eight-month-aged monolayer WS264.(j-q)Exfoliated and CVD-grown MoS2during the accelerated aging evolution41.PL mapping of exfoliated MoS2flakebefore(k)and after(l)the accelerated aging evolution.(m)Typical PL spectra of the monolayer region before(black)and after(red)the accelerated aging treatment.PL mapping of CVD-grown monolayer MoS2before(o)and after(p)theaccelerated aging evolution.(q)Change in the PL spectra of the CVD-grown MoS2sheet during the accelerated aging test.introduced into CVD-grown MoS2via the form of tran-sition metal oxides(e.g.MoO3),oxide growth sub-strates or the oxidizing reaction atmosphere(e.g.H2O,O2)during the CVD process63,65-66.Strong photo-luminescence(PL)quenching occurs in a CVD-grownsample,while only a slight reduction is observed inthe monocrystalline exfoliated sample.This indicatesthat the presence of S vacancies in CVD-grown samplesplays a crucial role during the oxidation process.This isconsistent with the observation of massive degradationfrom WS2samples grown by CVD method41,64.Sincechalcogen vacancies are thermodynamically favorableto be replaced by substitutional oxygen atoms67-68,the transition metal atoms in TMDCs are more reac-tive under ambient environments69.It can be demon-strated that defects cause dangling bonds and producemore reactive sites on the surface.The absence ofdefects reduces chemical instability and protects atomsfrom reacting with environmental sp