Two-dimensional transition metal dichalcogenide(TMD) nanosheets
Manish Chhowalla,*a Zhongfan Liu*b and Hua Zhang*c
This special issue is about two-dimensionaltransition metal dichalcogenides (2D TMDs),
a family of materials consisting of over 40compounds with the generalized formula ofMX2, where M is a transition metal typicallyfrom groups 4–7, and X is a chalcogen suchas S, Se or Te. Bulk TMDs have been widelystudied over several decades because it ispossible to formulate compounds with dis-parate electronic structures. In the bulkform, MX2 compounds are layered materials(or van der Waals solids) in which there isstrong intralayer bonding and weakinterlayer bonding. Each individual layerof the TMDs consists of three atomiclayers in which the transition metal is
sandwiched by two chalcogens. Further-more, the chalcogen atoms are saturatedand therefore are not highly reactive.These features allow for the attainmentof individual layers of the TMDs by severalexfoliation or vapor deposition methods.The isolation of monolayers of TMDs leadsto the dramatic changes in their proper-ties, primarily due to the confinement ofcharge carriers in two dimensions (x- andy-directions) due to the absence of inter-actions in the z-direction. Thus, single-layered nanosheets are two-dimensionalmaterials that possess dramatically different
a Materials Science and Engineering, Rutgers
University, 607 Taylor Road, Piscataway, NJ 08854,
USA. E-mail: [email protected] Center for Nanochemistry (CNC), Beijing National
Laboratory for Molecular Sciences, College of
Chemistry and Molecular Engineering, Academy for
Advanced Interdisciplinary Studies, Peking
University, Beijing 100871, People’s Republic of
China. E-mail: [email protected] School of Materials Science and Engineering,
Manish Chhowalla is a Professorand Associate Chair of the MaterialsScience and Engineering Departmentat Rutgers University. He is also theDirector of Nanotechnology for CleanEnergy NSF IGERT Program and theDonald H. Jacobs Chair in AppliedPhysics (2009–2011). From June2009–July 2010, he was a Professorin the Department of Materials atImperial College London. BeforeRutgers, he was a Royal Academy ofEngineering Postdoctoral ResearchFellow at the University of Cambridge
after completing his PhD in Electrical Engineering there. Prior to his PhD,he worked for Multi-Arc Inc. (now Ion Bond) where he developed one of thefirst applications of ‘‘amorphous diamond’’ thin films. His technologicalinterests are in the synthesis and characterization of novel low dimensionalmaterials and their incorporation into devices for electrical, optical,thermal and mechanical applications.
Zhongfan Liu
Zhongfan Liu received his PhDfrom the University of Tokyo in1990 and did his postdoc work atInstitute for Molecular Science(IMS) in Japan from 1991 to1993. He became an AssociateProfessor and then Full Professorat the Department of Chemistry ofPeking University in 1993, andChangjiang Chair Professor in1999. He was elected as themember of Chinese Academy ofSciences (CAS) in 2011 and theoutstanding scientist of Ten-
thousand Talent Program in 2013. His research interest focuses onlow-dimensional carbon materials and novel 2D atomic crystalstargeting electronic and energy conversion devices together with theexploration of fundamental phenomena in nanoscale systems.
fundamental properties compared to theirbulk counterparts, making them potentiallyinteresting for wide-ranging applicationssuch as catalysis, electronics and photonics.This has generated a tremendous amount ofinterest in these promising materials.
Several key challenges must beaddressed if 2D TMDs are to realize theirfull scientific and technological potential.Fundamental experimental and theoreticalstudies elucidating their electronic struc-ture and vibrational properties cancontinuously provide new insight intothe novel properties of 2D TMDs. Synth-esis of high-quality and uniform TMDsover large areas continues to be an activeresearch area. Fabrication methods tai-lored to different applications of TMDsare being studied. For example, chemicalvapor deposition is being investigated formaking large, highly uniform and elec-tronic grade TMDs for fundamental con-densed matter studies and electronics,while chemical exfoliation techniquesare developed for large-scale productionof nanosheets for catalysis and electro-chemical storage applications. Efforts totune the properties of TMD nanosheetsby phase engineering, integration withother advanced materials to form novelcomposites, and functionalization duringtheir preparation or post-synthesis are alsoongoing. Finally, there is a substantialamount of effort being devoted to realizenovel proof-of-concept devices for energystorage, electronics, photonics, catalysisand biomedical applications. The knowl-edge developed from the flurry of activityin TMDs and graphene has also inspired
research into other atomically thin materi-als such as phosphorene, which holdspromise for electronics and photonics.The following reviews are published in thisthemed issue.
Chaoliang Tan and Hua Zhang (DOI:10.1039/C4CS00182F) report on recentadvancements in large-scale basedapproaches to the synthesis of 2D TMDnanosheets and their integration withother advanced materials to form compo-sites with functional properties. Theydescribe 2D TMD-based composites withnoble metals, metal chalcogenides, metaloxides, metal organic frameworks, organicmolecules, and polymers to realize func-tional materials for batteries, electro- andphoto-catalysis, electronics, sensors, andbiomedicine.
Yi Xie and coworkers (DOI: 10.1039/C4CS00236A) review the role of the atomicand electronic structures of 2D materialsfor catalysis. They review works describingthe characterization of defects and disorderin 2D materials using advanced analyticaltechniques. In addition, they also providean overview of the literature related tovarious catalytic processes, including CO2
reduction using 2D materials. Finally, theypresent two major challenges for catalysiswith 2D materials.
Yi Cui and coworkers (DOI: 10.1039/C4CS00287C) report on the versatility of2D TMDs by describing physical andchemical tuning of their properties. Speci-fically, they review methods such as inter-calation, alloying, confinement throughdecreasing dimensions, high pressure,integration with other materials to build
heterostructures, and gating in order totailor the properties of 2D materials in acontrolled manner. Moreover, they go onto detail how these properties can beexploited in a variety of applications.
Damien Voiry, Aditya Mohite and ManishChhowalla (DOI: 10.1039/C5CS00151J) reviewthe different crystallographic phases that canoccur in 2D TMDs. They report on how thesephases are stabilized and how they can becontrollably induced to realize new function-alities. They describe how phase engineeringin 2D TMDs provides another parameter tocontrol their properties.
Hua Zhang and coworkers (DOI: 10.1039/C4CS00399C) highlight the technol-ogically relevant field of resistive mem-ory devices based on 2D materials. Inparticular, they review recent works ongraphene oxide/reduced graphene oxideand MoS2 based resistive memoriesand their unique features that allow therealization of novel flexible and trans-parent resistive memory devices. More-over, they also provide an overview onhow facile integration of active 2D layerscan lead to the enhancement of deviceperformance.
Lianzhou Wang and coworkers (DOI:10.1039/C4CS00300D) highlight recentadvances in the implementation of 2Dmaterials in biomedical applications. Theyhighlight key properties of 2D materialsthat allow them to be utilized for biomedi-cal applications as therapeutic, diagnostic,or theranostic agents in oncology. Theyalso highlight key remaining challengesthat must be overcome if 2D materialsare to be implemented in biomedicalapplications.
Zhongfan Liu and coworkers (DOI: 10.1039/C4CS00258J) review recent progresson chemical vapor deposition of 2D TMDs.In particular, they highlight the depositionof 2D TMDs and their alloys using methodssuch as sulphurisation/decomposition ofpre-deposited metal-based precursors,or the one-step reaction and deposition ofgaseous metal and chalcogen feedstocks.They also elucidate how growth can varybetween the commonly used SiO2 sub-strates and single crystals such as sapphireor strontium titanate. They additionallysummarize the affect of growth parameterssuch as temperature, gas flow rate andsubstrate to source distance.
Hua Zhang
Hua Zhang obtained his BS and MS degrees at NanjingUniversity in 1992 and 1995, respectively, and completed hisPhD with Prof. Zhongfan Liu at Peking University in 1998. Asa Postdoctoral Fellow, he joined Prof. Frans C. De Schryver’sgroup at Katholieke Universiteit Leuven (Belgium) in1999, and then moved to Prof. Chad A. Mirkin’s group atNorthwestern University in 2001. After he worked atNanoInk Inc. (USA) and Institute of Bioengineeringand Nanotechnology (Singapore), he joined NanyangTechnological University in July 2006. His current researchinterests focus on the synthesis of two-dimensionalnanomaterials and carbon materials (graphene and carbonnanotubes), and their applications in nano- and bio-sensors,clean energy, water remediation, etc.
Lain-Jong Li and coworkers (DOI: 10.1039/C4CS00256C) report on the latestdevelopments in the synthesis of 2DTMDs using chemical vapor deposition.They highlight key progress that hasbeen made thus far on alloying, substitu-tional doping, and integration of differentTMDs. They also address remaining chal-lenges that must be overcome in realizinghigh-quality and large-area 2D TMDs.
Hualing Zeng and Xiaodong Cui (DOI:10.1039/C4CS00265B) provide a detailedtutorial review of the rapid progress beingmade in the fundamental understandingof exploiting inversion symmetry breakingin monolayer TMDs for accessing valleydegrees of freedom. They highlight recentprogress in using optical techniques forprobing valley states, excitonic effects, andinterplay between spin states and valleys.
Ping-Han Tan and co-workers (DOI: 10.1039/C4CS00282B) describe how Ramanspectroscopy can be used to characterizethe vibrational properties of a varietyof TMDs. They report on how Ramanspectroscopy provides fundamentalinsights into material properties such
as interlayer coupling and spin–orbitsplitting as a function of number of layers,affect of substrate, and external perturba-tions. Their work provides a foundationfor understanding the basic propertiesof 2D materials using a readily accessibletechnique.
Wang Yao and coworkers (DOI: 10.1039/C4CS00301B) review theoreticalwork on 2D TMDs. They review the richcondensed matter effects that arise fromthe electronic structure of 2D materials.In particular, they highlight the origin ofthe direct band gap, substantial spinorbit coupling, and valley degrees of free-dom. They explain the different multi-scale models presented in the literatureand theoretically explain experimental andcalculation results.
Agnieszcka Kuc and Thomas Heine(DOI: 10.1039/C4CS00276H) report on the-oretical work related to the elucidation ofelectronic properties. They highlight howthe large spin–orbit coupling and giantspin–orbit splitting in odd numbers oflayers make them well suited for spin-based devices. They describe how the
materials behave in the presence of elec-tric and magnetic fields. In particular, theydescribe that application of high magneticfields can lead to Hall effects and evenquantum spin Hall effects in some 2DTMD materials. They also describe valley-based effects using polarized light.
Peide D. Ye and coworkers (DOI: 10.1039/C4CS00257A) highlight recent andrapid developments on a related non-TMD 2D material called phosphorene orblack phosphorus. They trace the historyof the research on this material back to100 years ago and provide insight into itsunique properties. They describe how itcan be exfoliated into monolayers and itsnovel electronic and optical properties.
This themed issue attempts to con-cisely provide an overview of the rapidlydeveloping field of 2D TMD materials.We believe that the compilation of thereviews in a single issue will benefitexperts and non-experts in the field.Finally, we would like to thank theauthors for their contributions to thisthemed issue and the RSC editorial staffmembers for their strong support.
