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Chem.Soc.Rev.,2014-层状硫化物的物理和化学改性

Cite this:DOI:10.1039/c4cs00287c

Physical and chemical tuning of two-dimensional transition metal dichalcogenides

Haotian Wang,a Hongtao Yuan,abc Seung Sae Hong,a Yanbin Li b and Yi Cui*bc

The development of two-dimensional (2D)materials has been experiencing a renaissance since the adventure of https://www.wendangku.net/doc/0f5653729.html,yered transition metal dichalcogenides (TMDs)are now playing increasingly important roles in both fundamental studies and technological applications due to their wide range of material properties from semiconductors,metals to superconductors.However,a material with fixed properties may not exhibit

versatile applications.Due to the unique crystal structures,the physical and chemical properties of 2D TMDs can be e?ectively tuned through di?erent strategies such as reducing dimensions,intercalation,heterostructure,alloying,and gating.With the flexible tuning of properties 2D TMDs become attractive candidates for a variety of applications including electronics,optoelectronics,catalysis,and energy.

1.Introduction

Two-dimensional (2D)layered materials with important physical and chemical properties have been studied for decades.Since the recently successful preparation and characterization of graphene,1–32D materials have attracted a great deal of attention since they exhibit novel and intriguing properties with potential applications in field effect transistors,optoelectronic devices,

topological insulators,electrocatalysts,and so on.4–16In the rich family of 2D materials,layered transition metal dichalcogenides (TMDs)become the focus of fundamental research and techno-logical applications due to their unique crystal structures,a wide range of chemical compositions,and a variety of material properties.6,7,11,12,172D TMDs are usually denoted MX 2,where M represents a transition metal (such as Ti,V,Nb,Mo,Hf,Ta,W),and X represents the chalcogen (S,Se,and Te).Transition metals ranging from group 4to group 10have different numbers of d-electrons,which fill up the non-bonding d bands to different levels,resulting in varied electronic properties including insulating,semiconducting,metallic,and superconducting.11The wide range of electronic structures not only boosts the development of 2D TMD electronic and optoelectronic

devices,

a Department of Applied Physics,Stanford University,Stanford,CA 94305,USA b

Department of Materials Science and Engineering,Stanford University,Stanford,CA 94305,USA.E-mail:yicui@https://www.wendangku.net/doc/0f5653729.html, c

Stanford Institute for Materials and Energy Sciences,SLAC National Accelerator Laboratory,2575Sand Hill Road,Menlo Park,California 94025,

USA

Haotian Wang

Haotian Wang is a graduate student in the Department of Applied Physics at Stanford Uni-versity.He is currently a Stanford Interdisciplinary Graduate Fellow (SIGF).He obtained BS (2011)in Physics from the University of Science and Technology of China.He is now supervised by Prof.Yi Cui and focused on applications of low-dimensional materials in catalysis,energy storage,and

electronics/optoelectronics.

Hongtao Yuan

Hongtao Yuan obtained his PhD (2007)degree from Institute of Physics,Chinese Academic of Sciences.He was a postdoc researcher (2007–2009)at Tohoku University,and a research associ-ate (2010–2011)and an assistant professor (2011–2012)at Quan-tum Phase Electronics Center in the University of Tokyo.Since 2012he has been a physics scien-tist research associate at Geballe Laboratory for Advanced Materi-als in Stanford University and at

Stanford Institute for Materials and Energy Sciences in SLAC National Accelerator Laboratory,USA.

Received 28th August 2014DOI:10.1039/c4cs00287c

https://www.wendangku.net/doc/0f5653729.html,/csr

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but also facilitates the applications into catalysis due to the strong correlation between electronic and catalytic properties. In addition to chemical composition of2D TMDs,atomic arrangements also play important roles in determining material properties.A representative atomic structure of2D TMDs is shown in Fig.1(a).The metals and chalcogens are covalently bonded within the molecular layers,which stack together via the weak van der Waals(vdW)interactions along the z axis to form the bulk material.The strong intra-and weak inter-layer inter-actions induce the high anisotropy of2D TMDs,with properties such as facile single layer exfoliation for electronic devices.8 However,the explosive popularity of2D TMDs does not only rely on the intrinsic material properties themselves,but highly depends on the tunable electronic and catalytic properties.6,7,17 Due to the high anisotropy and unique crystal structure,the material properties of2D TMDs can be e?ectively tuned in a wide regime through di?erent methodologies including reducing dimensions,intercalation,heterostructure,alloying,gating,pressure, and lighting as illustrated in Fig.1.For example,the band structures are significantly changed as we thin down the2D layers to the single-layer limit.8,15–17Another example is that through the intercalation of guest ions,the carrier densities of2D TMDs can be tuned by multiple orders of magnitude.18Modern technologies and applications require a wide range of high-quality material properties,which can be hardly realized in a single material without any modifications.Therefore,2D TMDs provide a great platform of tuning material properties towards desired func-tions,further attracting a great deal of attention and opening up opportunities for a wide range of applications.

There are a number of important studies and summaries focused on the attractive properties of2D TMDs already,6,7,11,19–21 however,the topical review of2D TMDs’tunability has not been proposed yet.Understanding how the material properties can be tuned and how these tunable properties can be utilized becomes increasingly important.In this review,we focus on different physical and chemical strategies for tuning2D TMDs properties, such as band structures,carrier densities,catalytic activities,optical properties.In each strategy,how the electronic and catalytic properties of2D TMDs are modified is explained in detail,with representative applications included.

2.Dimension tuning

2.1Dimension tuning along the z direction

2D TMD semiconductors with tunable bandgaps through thinning down the bulk materials to few-or single-layer limits have recently become a powerful approach for electronic and optical applications.8,22–26As an example,the bulk MoS2crystal,an indirect bandgap semiconductor of1.29eV,consists of vdW bonded S–Mo–S layer units.Each of these stable units(referred to as a MoS2monolayer)consists of two hexagonal planes

of Yanbin Li

Yanbin Li is a PhD student in the

Department of Materials Science

and Engineering at Stanford

University supervised by Prof.Yi

Cui.He received his BS degree in

physics from University of Science

and Technology of China in2013.

His graduate research focuses on

the controlled synthesis of low-

dimensional

materials.

Yi Cui

Yi Cui is an Associate Professor in

the Department of Materials

Science and Engineering at

Stanford University.He obtained

BS(1998)in Chemistry from the

University of Science and

Technology of China,PhD(2002)

from Harvard University.He was

a Miller Postdoctoral Fellow at the

University of California,Berkeley

from2003–2005.He holds a joint

appointment in SLAC National

Accelerator Laboratory.His

current research is on two-

dimensional layered materials,energy storage,photovoltaic,

topological insulators,biology and environment.Yi Cui is an

Associate Editor of Nano Letters.He founded Amprius Inc.in

2008,a company to commercialize the high-energy battery

technology.

Seung Sae Hong

Seung Sae Hong is currently a

postdoctoral fellow in the group

of Prof.Harold Y.Hwang at

Stanford University.He received

his BS degree in physics from

Seoul National University and

completed his PhD degree in

applied physics from Stanford

University,supervised by Prof.Yi

Cui.His research interests

include physics and materials

science of low-dimensional

materials,with an emphasis on

chalcogenide and oxide2D

materials.

Review Article Chem Soc Rev P

S atoms and an intermediate hexagonal plane of Mo atoms coordinated through ionic-covalent interactions with the S atoms in a trigonal prismatic arrangement.Due to the high anisotropy,single-layer MoS 2can be easily achieved by mechanical cleavage,directly providing the possibility to tune the physical properties at the 2D monolayer limit.Chemical vapor deposition enables large-area,uniform,and high-quality single-layer 2D TMDs.6,11,27–33In addition,chemical and electrochemical exfoliation of mono-layered TMDs makes it possible to achieve large-scale single layer production.34,35As early as 2007,theorists have predicted the scaling properties in the band structure:36,37the MoS 2crystals exhibit a crossover from an indirect to a direct gap semiconductor from bulk to its monolayer limit.As an early experimental report on the thickness dependent physical properties of the MX 2system (Fig.2),38,39the evolution of the electronic structure and resulting optical properties of ultrathin MoS 2crystals was observed as a function of layer number from 1to 6layers by using spectroscopic techniques:optical absorption,photoluminescence (PL),and photoconductivity (with the thickness characterized by atomic-force microscopy).The combination of these spectroscopic methods allowed researchers to optically trace the evolution from the indirect to direct bandgaps of the material with the layer thickness decreasing to the monolayer.The crossover from an indirect gap material to a direct gap material naturally accounts for the great enhancement of the luminescence observed in monolayer MoS 2.The observed dependence of the bandgap on the layer number is also in qualitative agreement with band calculations.The controllability of the bandgap may also be used to optimize the material’s use as a photocatalyst and for photovoltaic applications.To experimentally understand the origin of these extraordinary PL properties,angle-resolved photoemission spectroscopy (ARPES)measurements are carried out to investigate the detailed para-meters of the band structure and also the evolution of the band dispersion with the layer thickness (Fig.2).40,41One example is ARPES measurement based on monolayer MoSe 2thin films with variable thickness,grown by molecular beam epitaxy.40The band structure measured experimentally indicates a stronger tendency of monolayer MoSe 2towards a direct bandgap.As an important feature of the monolayer MX 2,a significant spin-splitting of B 180meV at the valence band maximum of a monolayer MoSe 2film,related to the strong spin orbital inter-action,could be clearly observed experimentally for the first time.Other examples are observations on the evolution of the thickness-dependent electronic band structure of the MoS 2and WSe 2,based on the combination of the spatial resolution ARPES with the tape-cleaved or chemical-vapor-deposition-grown ultra-thin samples.41All these observations provide direct evidence for indirect-to-direct band gap transition in both MoS 2and WSe 2cases (the shifting of the valence band maximum from G to K )as thinning down the sample from the bulk to

monolayer.

Fig.1(a)Schematic of 2D TMDs.M represents a transition metal,and X represents a chalcogen.(b)Schematic of tuning 2D TMD properties by reducing dimension along the z direction and xy directions.(c)Schematic of tuning 2D TMD properties by guest ion intercalation.(d)Schematic of tuning 2D TMD properties by constructing heterostructures and alloying.(e)Schematic of tuning 2D TMD properties by gating.(f)Schematic of tuning 2D TMD properties by applying high pressure.(g)Schematic of tuning 2D TMD properties by illuminating circularly-polarized light.

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Such an unusual electronic structure evolution stems from the characters and spatial distribution of d-electron orbitals of MoS 2.Theoretical calculations show that electronic states of di?erent wave vectors have electron orbitals with di?erent spatial distributions.38Specifically,conduction band states at the K point are primarily composed of strongly localized d orbitals at Mo atom sites,and they have minimal interlayer coupling since Mo atoms are located in the middle of the S–Mo–S unit cell.The states near the G point and the point of indirect band-gap originate from a linear combination of d orbitals on Mo atoms and antibonding p z

orbitals on S atoms.The orbitals have strong interlayer coupling and their energies depend sensitively on layer thickness.Such an understanding on the direct bandgap feature makes the optical measurement of the band structure available and further paves a way for the optical realization of the circularly-polarized light pumped valley polarization around the band edge at K points.2.2

Dimension tuning along xy directions

Due to the highly anisotropic structures,2D TMD materials tend to grow fast within the layer to form di?erent morphologies

such

Fig.2(a)Calculated band structures of bulk MoS 2,quadrilayer MoS 2,bilayer MoS 2and monolayer MoS 2.The solid arrows indicate the lowest energy transitions.Bulk MoS 2is characterized by an indirect bandgap.The direct excitonic transitions occur at high energies at the K point.With reduced layer thickness,the indirect bandgap becomes larger,while the direct excitonic transition barely changes.For monolayer MoS 2,it becomes a direct bandgap semiconductor.This dramatic change in electronic structure in monolayer MoS 2can explain the observed jump in monolayer photoluminescence e?ciency.Reproduced with permission from ref.38.Copyright 2010,American Chemical Society.(b)PL spectra of mono-and bilayer MoS 2samples in the photon energy range from 1.3to 2.2eV.Inset:PL quantum yield of thin layers (1–6layers).Reproduced with permission from ref.39.Copyright 2010,American Physical Society.(c)Band evolution with increasing thickness of MoSe 2thin films.ARPES spectra and second-derivative spectra of monolayer,bilayer,trilayer and 8ML MoSe 2thin films along the G –K direction.White and green dotted lines indicate the energy positions of the apices of valence bands at the G and K points,respectively,with energy values written in the same colors.Reproduced with permission from ref.41.Copyright 2013,Nature Publishing Group.

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as nanoplates,nanoribbons,nanotubes,and inorganic fullerene-like nanoparticles.27,29–31,42–44Those structures extend the dimension along the xy directions of 2D TMD atomic structures shown in Fig.1(a),as an effective way to minimize the exposure of the edge sites.17However,the atomic sites on the edges of 2D TMD nanosheets have unsaturated coordination and dangling bonds,which offers interesting and important properties and applications.17,45–49By reducing the dimension along the in-plane direction,the edges of 2D TMD are largely exposed and the electronic and catalytic properties are effectively changed.Here,we choose MoS 2as a typical 2D TMD material to explain the extraordinary edge properties through the xy dimension tuning.The edge sites of MoS 2have been demonstrated to have metallic electronic states,which are absent from the basal plane.45More importantly,as illustrated in Fig.3(a)and (b),the edge sites of MoS 2are demonstrated to be active catalytic centers for hydrogen evolution reaction (HER),in sharp con-trast to the HER inert terrace sites.12,47,50Other types of 2D TMDs,such as MoSe 2,WS 2,and WSe 2,show similar edge site activities.17,51,52To make full use of the catalytic centers and enhance the HER activity,the dimension of the xy direction should be significantly shrunk to increase the ratio of edge sites to terrace sites.A successful example is the MoS 2and MoSe 2edge-terminated nanofilms shown in Fig.3(c).17,51The TEM images show the densely packed,stripe-like grains,indicating that the molecular layers are all vertically standing on the substrate.17,51The edge-terminated surface is considered to be thermodynamically unstable due to the large surface energy of the edges;however,through rapid sulfurization synthesis,this metastable morphology is likely to be obtained by kineti-cally overcoming the free energy barrier.17,51The layer vertically aligned structure with reduced xy dimensions fully exposes the active edge sites on the substrate,providing a large number of catalytic reaction sites and thus boosting the overall HER activity.

Another way to have more active edge sites exposed is to reduce the dimension of the bulk MoS 2material into ultra-small nanoparticles or nanowires.53,54The large surface curvature of the nanosized structures is likely to induce a high surface energy to force the edges to expose.55Dai and his coauthors developed a solvothermal synthesis of MoS 2ultra small nanoparticles on reduced graphene oxide (RGO)sheets as illustrated in Fig.3(d).53The particle size is only around 5nm,exposing a large amount of edge sites.Free MoS 2nanoparticles with particle size around 100nm were also prepared for HER activity comparison with the smaller particles on RGO.The polarization curves and Tafel plots in Fig.3(d)show that the HER performance of MoS 2nano-particles on RGO is significantly improved compared to the large sized particles.The excellent Tafel slope of 41mV per decade indicates a facile reaction pathway with small activation barriers.53The dimensions of MoS 2nanosheets along the xy direction can also be reduced by introducing defects into MoS 2surfaces.56The defect-rich structure creates additional active edge sites in the MoS 2nanosheets,significantly enhancing the HER performance with a low onset potential and a small Tafel slope.To further reduce the dimension of 2D MoS 2,zero-dimension molecular MoS 2

edge site mimic is successfully prepared for high HER activity,again demonstrating the importance of edge sites.57

3.Intercalation tuning

3.1Intercalation tuning of electronic,optical,and thermal properties

Intercalation into layered structures changes a number of physical properties of the host materials,which has received research attention for decades.18The earlier investigations have focused on the bulk form of layered structures,and those works which are revisited as the 2D counterparts are widely studied recently.In the cases of transition metal dichalcogenides,the most common intercalants are alkali metal (Li,Na,K,...)and 3d transition metal (V,Cr,Mn,Fe,...)atoms,of which charges (electrons)can be easily transferred to the chalcogenide layers.19The charge transfer induced by metal atom intercalation increases the Fermi energy and density of states at the Fermi level.18Such ‘‘electron doping’’results in a huge increase in carrier density –several orders of magnitude larger than the modulation by electrostatic gating and traditional impurity doping –which alters the electronic and optical properties of layered materials substantially.18,19The existence of metallic intercalants also implies that the weak interaction between layers gets stronger and the electronic structure of the whole material becomes more three-dimensional.

In addition,intercalated compounds in vdW gaps cause structural changes in the layered structure.Intercalation of organic molecules,such as n -octadecylamine and pyridine,is a fine example to show how the interlayer spacing between layers can be enlarged.58The increase of layer separation leads to (1)smaller bonding strength and sound velocity,and (2)the weakening of interaction among layers,which makes each layer as a nearly-isolated single layer.We introduce a few experimental examples of intercalation of layered chalcogenides,showing how structural and electronic changes make an impact on the fundamental nature of metal chalcogenides,as well as practical applications.Some of the examples,especially the optical prop-erties,are demonstrated in two-dimensional layers,suggesting an intriguing direction for current studies of 2D materials.

Although most of the intercalation studies listed in this part are from transition metal chalcogenides,a few interesting exam-ples can also be found in the studies of chalcogenides of non-transition metal.As their physical concepts and tuning strategy can be applied in the same way,we include those special cases as well,expecting that such conceptual approaches provide an exciting direction in transition metal chalcogenide studies.

Thermoelectricity.The thermoelectric e?ect is defined as a direct conversion between electrical energy (electric voltage)and thermal energy (temperature gradient),which is useful to extract electricity from heat,or vice versa .There have been a number of studies to make use of the thermoelectric e?ect to collect energy from waste heat,but the low e?ciency of thermo-electric materials becomes a bottleneck for broad applications.One way to improve the conversion e?ciency is to reduce

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thermal conductivity.Intercalation into the vdW gaps in layered structures can naturally introduce atoms/molecules to perturb phonon propagation,thereby reducing thermal con-ductivity.One of the earliest examples is demonstrated in non-transition metal chalcogenides,CsBi 4Te 6,showing how the intercalated atoms in the layered structure can help to improve the e?ciency.59Cs +ions in the vdW gaps localize vibrations,generating resonant scattering of phonons.A more recent example is from the transition metal’s case –titanium disulfide (TiS 2).A natural superlattice of (SnS)1.2(TiS 2)2is formed by intercalation of SnS layers into TiS 2layered structures.60The intercalated SnS layer in the vdW gap shown in Fig.4(a)weakens interlayer bonding,reducing the transverse sound velocity.In addition,SnS layers can function as a translational disorder in the crystal-line lattice,resulting in photon localization.Such thermal conductivity control by intercalation is not limited to only transition metal chalcogenides but also can be generalized to other layered materials,61yielding an intriguing strategy to design thermoelectric materials made of layered structures.Superconductivity.Introducing intercalants into layered structures often induces a new type of collective electronic phenomena which does not exist in the original host material.Titanium diselenide (TiS 2)exhibits charge density waves (CDWs)at low temperatures,but once Cu is intercalated (Cu x TiS 2),a new superconducting state emerges near x =0.04,and the CDW transition is continuously suppressed as illustrated in Fig.4(b).62Such normal-superconductor transition by intercalation has been generalized to many other types of layered materials,as shown by an emerging superconductivity via intercalation is Bi 2Se 3,a candidate material for topological insulators.It has

been

Fig.3(a)Schematic of H adsorption on the MoS 2Mo-edge sites (top)and theoretical simulation of the adsorption free energies on di?erent materials (bottom).Reproduced with permission from ref.12.Copyright 2005,American Chemical Society.(b)Experimental demonstration of the active edge site of MoS 2for HER.The exchange current density scales with the length of the edges of the MoS 2nanoplates.Reproduced with permission from ref.47.Copyright 2007,the American Association for the Advancement of Science.(c)Schematic and TEM images of edge-terminated MoS 2and MoSe 2nanofilms.Reproduced with permission from ref.17.Copyright 2013,American Chemical Society.(d)MoS 2nanoparticles on reduced graphene oxide with excellent HER performance.Reproduced with permission from ref.53.Copyright 2011,American Chemical Society.

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demonstrated that both bulk crystal synthesis 63and electro-chemical intercalation 64of Cu can induce superconductivity in Bi 2Se 3.In the case of layered materials normally superconducting,intercalation can be used to enhance the superconducting transi-tion temperature (T c ).A well-known example of an iron-based superconductor –iron selenide (FeSe)has a T c of 8K.However,intercalation of metal atoms (K)and lithium amide/ammonia molecules (Li x (NH 2)y (NH 3)1ày )increases T c dramatically,exceed-ing 30–40K.65,66The microscopic origin of superconductivity and intercalation is not fully understood yet.Structural changes by intercalants seem to be closely related to the superconductivity,such as tetrahedral shape deformation 65and increasing layer separation by the spacer layer.66Fundamental electronic para-meters,such as high carrier density and the change in dimen-sionality of the Fermi surface,will also be important factors to explain the competition among different ground states tuned by intercalation.62

Optical and plasmonic properties.The large amount of metal atoms intercalated is also expected to modify optical properties of the host layered structures.Such a change in optical properties will be more evident in 2D materials,of which optical transmission and interference between interfaces can be observed by experi-ments.One of the interesting examples is from the non-transition metal chalcogenide’s study:Cu-intercalated Bi 2Se 3nanoribbons.Bi 2Se 3nanoribbons are usually synthesized as n-doped semi-conductors,showing silvery white color.But large amounts of intercalated Cu atoms,up to 60atomic percent,turn the color of the nanoribbons to orange color,due to such high concentration of copper inside.67In addition,Cu intercalation into very thin 2D layers of Bi 2Se 3manifests an unexpected optical transition in visible light wavelengths.68Fig.4(c)contains optical transmission

images of ultrathin Bi 2Se 3nanoplates before (top)and after (bottom)Cu intercalations,showing a dramatic change in optical transmit-tance of 2D materials via intercalation.The increasing transmittance of 2D materials is attributed to the increase of the e?ective bandgap due to the large free electron density introduced by metal atom intercalation.The concept of optical property tuning via intercalation can be generalized to the organic molecules.For example,a plasmonic peak shift by molecular intercalation has been demonstrated.69Such intercalation tuning methods of optical and plasmonic properties are expected to be applied to many TMD structures in similar passion.3.2

Intercalation tuning of catalytic activity

In catalysis,scientists have revealed the strong correlation between the electronic structure and the catalytic activity through theoretical simulations as well as experimental demonstrations.12,45–49Proper electronic structures of the active atomic sites should be designed to create suitable chemical bonding with the reactants (not too weak and not too strong),which ensures both a good electron transfer between the catalysts and the reagents and a facile products-releasing process.12Therefore,the tunable electronic structure through electrochemical or chemical intercalation makes 2D TMD materials very attractive candidates for catalysis optimization.Electrochemical intercalation can effectively shift the chemical potentials of 2D TMD materials within a wide range,getting to the optimized position for efficient catalysis.Sometimes the intercalation process introduces a phase transi-tion of the host matrix,significantly changing the electronic structure to another form which may perform excellent catalytic activity.70–74In addition,the intercalated guest atoms and the matrix material may have charge transfers between each other

Fig.4(a)Top:high-resolution transmission electron microscope (HRTEM)image of TiS 2layers after SnS intercalation.Bottom:lattice thermal conductivity change by SnS intercalation.Reproduced with permission from ref.60.Copyright 2011,Springer.(b)Phase diagram of Cu intercalated TiSe 2,of which transition among metal,charge density wave (CDW),and superconductivity (SC)phases is presented.Reproduced with permission from ref.62.Copyright 2006,Nature Publishing Group.(c)Crystal structure and optical transmission images of a Bi 2Se 3nanoplate before (top)and after (bottom)Cu intercalation.The scale bars indicate 4microns.Reproduced with permission from ref.68.Copyright 2014,Nature Publishing Group.

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illustrated in Fig.1(c),which increases the carrier density and thus improves the conductivity of the catalyst.71A successful example is lithium electrochemical tuning of 2H MoS 2for enhanced HER activity developed in our group.71,75As shown in Fig.5(a),the as-synthesized edge-terminated MoS 2nanofilm and a piece of Li foil were made into a pouch battery cell to perform Li intercalation.71A galvanostatic discharge curve is shown in Fig.5(b),providing useful information regarding the MoS 2electronic structure change such as the lowered Mo oxidation state and the MoS 22H to 1T phase transition.The intercalated Li transfers excess charge carriers to MoS 2,reducing the oxidation state of Mo,increasing the electronic energy of the whole system,and thus inducing the structure phase transition for a more stable octahedral coordination.This electronic structure change helps to improve the HER catalytic activity signifi-cantly as shown in Fig.5(c).The chemical potential of Li intercalated MoS 2was continuously shifted from the open circuit voltage to 1.1V vs.Li +/Li and stopped at different intermediate voltages.Consequently,the HER performance is continuously improved,with the Tafel slopes improved from 123mV per decade all the way to 44mV per decade.It should be noted that the 1T phase MoS 2is still stable even though the Li inside reacts with air and water.71,75,76

Another way to tune the electronic structure of MoS 2and WS 2nanosheets for improved HER activity is chemical inter-calation.72,73,76,77Chhowalla and coworkers successfully pre-pared 1T phase WS 2nanosheets by a chemical intercalation and exfoliation process.73The TEM images of 2H and 1T WS 2single layers show the atomic structure change.2H WS 2shows the hexagonal lattice,which undergoes a phase transition to

form a distorted 1T structure with a 2a 0?a 0superlattice.The HER performance of 2H WS 2was significantly improved after the phase transition.The reason for this change was revealed by their theoretical simulation that showed the strains in the 1T nanosheets help to lower the reaction free energy.73Jin and his coworkers’study of 1T MoS 2nanosheets obtained similar results.72The n -butyl lithium was utilized as the intercalation solution.This solution helps to chemically intercalate a large amount of Li atoms into the MoS 2,shift the MoS 2to low chemical potential,exfoliate the nanosheets into single or few layers,and induce the 2H to 1T phase transition.72The perfor-mance of the metallic 1T MoS 2was tremendously improved from the semiconducting 2H phase,with a Tafel slope of 43mV per decade.Several follow-up studies employed both electro-chemical and chemical intercalation methods for further improved HER activities.75–77It should be mentioned that the electrochemical tuning method has also been demonstrated to be effective to enhance electrocatalysis of other forms of 2D materials such as lithium transition metal oxides.74Therefore,2D TMD materials with the intercalation tuning properties constitute a vast screening pool of efficient catalysts.

4.Heterostructure and alloying tuning

4.1

Heterostructure tuning

As the leading 2D material,graphene is reported to achieve high electronic mobility exceeding 100000cm 2V à1s à1

Fig.5(a)Schematic of the edge-terminated MoS 2lithiation process.(b)Galvanostatic discharge curve with schematics of charge transfer and phase transition.(c)Improved HER performance after the Li tuning process.(d)The corresponding Tafel slopes of MoS 2lithiated to di?erent voltages.Reproduced with permission from ref.71.Copyright 2013,National Academy of Sciences,USA.

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artificially stacking graphene on top of hBN to form a hetero-structure.78This work first attracted attention to tuning the properties of 2D materials via forming heterostructures from materials with di?erent bandgaps and work functions.6In addition,graphene/h-BN heterostructure also demonstrates more exotic physics,known as Hofstadter’s butterfly.Similar to graphite,TMDs MX 2crystallizes in a vertically stacked,weakly interacted layered structure.Hence,the individual monolayers can be mechanically exfoliated by the simple Scotch-tape technique,2which makes it easy to obtain building blocks for heterostructure assembly.Due to the sulfur/selenium terminated surface with no dangling bond,the heterostructures can have clean and atomically sharp interfaces as shown in Fig.6(b).79The tunability of properties via assembling high-quality heterostructures,together with the impressive characterization of TMDs mentioned in previous sections,make TMDs promising candidates for band engineering 80and functional heterostructure design.2,8,81–84

Vertical heterostructure.The first observation of such tunability is reported as field e?ect tunneling transistors 79,85with multilayer MoS 2or WS 2serving as a vertical transport barrier on top of graphene as shown in Fig.6(a).Both theoretical 86and experimental results 87show that partially ionized donors in MoS 2transfer charge to the graphene to shift the Fermi level of graphene and lower the height of the tunnel barrier.Hence,with MoS 2or WS 2serving as barrier materials,the ON/OFF ratio of the FETT will increase,since the changes in the Fermi level of the graphene can be equal to or larger than the barrier height.For example,due to the advantage of switching between tunneling and thermionic transport regimes,transistors exceeding a 106ON/OFF ratio are reported at room temperature in a graphene–WS 2transistor.79The gate tunability in vertical heterostructures between TMDs and graphene allows devices with different functions be constructed such as nonvolatile memory cells,88–90complementary inverters,91photoresponsive memory devices.92Further,based on the enhancement on photon absorption and electron–hole creation due to the Van Hove singu-larities in the electronic density of states of TMDs,93flexible photovoltaic devices were also demonstrated.93,94Besides,hetero-structures between TMDs and other 2D materials such as carbon nanotubes 95and amorphous silicon 96were also reported to tune the properties of TMDs.97

Another example of tuning band structure is the hetero-structure formed by two di?erent TMDs.98Researchers created an atomically sharp,type II heterostructure with atomically thin layers of WSe 2(p-type)and MoS 2(n-type).This type of heterostructure introduces highly asymmetric charge transfer rates for electrons and holes,which drive the spontaneous dissociation of a photogenerated exciton into free carriers.It is reported that photoluminescence decreased dramatically in the p–n junction area.To further tune the device properties via forming heterostructures,researchers sandwiched the p–n junction with two graphene electrodes and obtained a five times increase in the photoresponsivity than typical laterally-contacted devices.

Besides simply forming vertical heterostructures,in recent studies,changing the interlayer twist angle has been demon-strated to tune material properties.99They reported that the energy for the valence band edge state at the G point is sensitive to interlayer interactions,while the energies at K point are relatively insensitive.Besides,owing to repulsion between sulfur atoms,the interlayer distance varies with di?erent twists,providing a potential tool to tune the physical properties of 2D heterostructures,including indirect optical transition energy and second-harmonic generation e?ects.

In previous studies,the small size and low yield of manually stacking assemblies limited the industrial application of TMD heterostructure devices.A scalable approach to these hetero-structures was reported in a recent work.100By selenization of MoO 3via a chemical vapor deposition method,the authors demonstrated large-area monolayer MoSe 2/graphene heterostructure,which might open up the industrial applications of heterostructures in optoelectronics,electronics and photocatalytics.

Lateral heterostructures.Beyond these great e?orts made on vertical designed heterostructures,it is also possible to tune the material properties via forming p–n heterostructures in the lateral direction.The first trial of this design was demonstrated by forming lateral p–n homojunctions by using electrostatic doping.With this method three groups reported similar studies on the WSe 2based lateral p–n junction,showing impressive optoelectronic properties via this lateral tuning.101–103However lateral heterostructures can only be achieved through direct growth between two different materials.As that happened in the development of vertical heterostructures,this design was first presented by a ‘patterned regrowth’process between graphene and hBN to demonstrate atomically thin circuitry.

104

Fig.6Graphene–WS 2vertical heterostructure.(a)Optical microscopy image of the device (scale bar,10m m).(b)Cross-section high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM)image (scale bar,5nm).(c)Schematic of vertical architecture of the transistor.Reproduced with permission from ref.79.Copyright 2013,Nature Publishing Group.

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Recently,three groups reported successful synthesis of such lateral heterosturctures in TMDs via atomic vapour pressure variation led sequential growth,105in situ reactant modulation,106and growth rate difference selected sequential growth 107respectively.It is also shown that the precise reaction temperature can select the final structure between vertically stacked bilayers and in-plane lateral heterojunctions.107All three studies reported the observation of a seamless and atomically sharp interface as shown in Fig.7a.EDS and Raman elemental mapping shows the existence of lateral heteroepitaxy growth between TMDs with high crystalline quality.Besides,strong localized photoluminescence enhancement is observed around the interface as shown in Fig.7b and c.Based on the successful obtainment of such p–n heterostructures researchers demonstrated a CMOS inverter by integrating a p-type WSe 2and an n-type WS 2FET and reported a voltage gain as large as 24.As a new tuning method,more interesting properties are expected from TMD lateral heterostructures.4.2

Alloying tuning

Ternary two-dimensional dichalcogenide alloys exhibit compo-sitionally modulated electronic structure,and hence,control of alloy concentration within each individual layer of these com-pounds provides a powerful tool to e?ciently modify their physical and chemical properties,including the carrier e?ective

mass and bandgap.108–111As mentioned above,the electronic properties of TMDs are qualitatively determined by the localization behavior of the d-bands of the transition metal.Depending on the degree of localization,these materials can be insulators,semi-conductors,semimetals,or metals.The degree of d-state mixing depends on the nature of the transition metal and its chalcogen ligand environment,and is expected to be influenced by its substitutions in an alloy.108By using alloys of two of these MX 2materials (either metal elements or chalcogens)one could achieve an even greater flexibility and access an almost continuous range of properties.The ability to tune continuously the bandgap of this distinctive class of atomically thin materials through the growth of S/Se alloys opens up many new possibilities for basic studies and device concepts.

Theoretical band calculation can always give us the rational design on bandgap engineering.Fig.8(a)gives us the guidance,in which the di?erences in theoretical bandgaps are plotted versus the mismatch of the lattice constants before the alloying of two layered MX 2semiconductors.108The data points in the upper corner of Fig.8(a)correspond to the largest di?erence in bandgaps and the smallest lattice mismatch.Generally,when mixing the electron-deficient V (also for group 5metals,such as Nb,and Ta)with group 6semiconductors such as CrX 2,MoX 2,or WX 2,the decrease in the electron number leads to

metallic

Fig.7TMD lateral heterostructures.(a)Atomic-resolution Z-contrast STEM images of the in-plane interface between WS 2and MoS 2domains.The red dashed lines highlight the atomically sharp interface along the zigzag-edge direction.Scale bar,1nm.Reproduced with permission from ref.107.Copyright 2014,Nature Publishing Group.(b)2D Photoluminescence intensity map of lateral heterostructure.Intense emission is seen from the 1D interface.Scanning micro-photoluminescence was performed with 532nm laser excitation at room temperature.Scale bars,2m m.Reproduced with permission from ref.105.Copyright 2014,Nature Publishing Group.(c)Photoluminescence spectra taken at the points indicated by the corresponding coloured arrows in (b).Reproduced with permission from ref.105.Copyright 2014,Nature Publishing Group.

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behavior and the shift of the Fermi level into the valence band.Mo–W dichalcogenide alloys are located at the bottom corner of the plot,indicating good lattice matching and moderate band-gap variation with concentration.The bandgaps in the thermo-dynamic ground states are shown with larger symbols.The LDA gap varies from 1.87to 2.0eV in Mo 1àx W x S 2with concen-tration,close to the experimentally reported variation from 1.85to 1.99eV in this alloy.112,113

Unlike alloying by mixing metals,chalcogen alloys generally have a more limited range of possible variations in the bandgap than in the case of a mixing metal alloy.The synthesis of molybdenum disulfide,substitutionally doped with a broad range of selenium concentrations,results in optical bandgap modulations in atomic layers.Based on the CVD grown mono-layer films,the photoluminescence and Raman measurements show that the band structure of the alloy films can be tuned continuously with composition.Both the experimental and theoretical studies show that the material exhibits a direct gap for all alloy compositions.The room temperature optical bandgap changes smoothly between the limits of 1.87eV (for pure single-layer MoS 2)and 1.55eV (for pure single-layer MoSe 2).Similarly,based on a one-step direct synthesis of MoS 2(1àx )Se 2x atomic mono-and bilayers with tunable compositions through CVD controlled selenium doping,the band structure of MoS 2could be modified and the optical bandgap could be continuously tuned by over https://www.wendangku.net/doc/0f5653729.html,ing atomic resolution Z-contrast imaging,direct atomic identification of Se dopants within the MoS 2lattice with almost 100%detection e?ciency was demonstrated.114

In order to better understand and fine-tune the desired properties by alloying,quantifying and locating the alloy atoms within each layer is of great importance.The statistics of the homo-and hetero-atomic coordinates in single-layered Mo 1àx W x S 2was obtained from the atomically resolved scanning transmission electron microscope images.These images successfully quan-tify the degree of alloying for the transition metal elements (Mo or W).110,115The direct visualization of the atomic species Mo and W in Mo 1àx W x S 2compounds via chemical analysis using Z-contrast imaging with scanning transmission electron microscope (STEM)annular dark-field (ADF)clearly shows how the two elements can be mixed in a single layer.Also,it was used to count the neighboring atoms of two transition metal components that show either mutual attraction or repulsion of the constituent species.The random alloying of such a mixed dichalcogenide system throughout the chemical

compositions

Fig.8(a)Theoretical band gaps of binary alloys as a function of their lattice constants.Triangles designate oxides,squares –sulfides,circles –selenides,and diamonds –tellurides.Reproduced with permission from ref.108.Copyright 2014,Royal Society of Chemistry.(b)LDA band gaps in Mo 1àx W x S 2(downward triangles,red)and MoSe 2(1àx )S 2x (upward triangles,blue)alloys as a function of concentration x .The band gaps in the thermodynamic ground states are shown with larger symbols.Reproduced with permission from ref.108.Copyright 2014,Royal Society of Chemistry.(c)Normalized RT PL spectra of MoS 2(1àx )Se 2x films of different composition.Right panel:variation of the photon energy of the PL emission peak as a function of sample composition,as determined by XPS (the inset shows a representative XPS spectrum of the Se-3p and S-2p peaks).The black line indicates a linear variation between the values of the stoichiometric compounds as found in DFT calculations.Reproduced with permission from ref.109.Copyright 2013,John Wiley and Sons.(d)PL spectra for a MoS 0.42Se 1.58film at different temperatures between 277and 5K.All spectra are scaled to the same height.The bottom shows the exciton emission energy as a function of the temperature.Reproduced with permission from ref.109.Copyright 2013,John Wiley and Sons.(e)HAADF-STEM image in false color.Mo sites:yellow;S 2sites:light blue;SSe sites:yellow,and Se 2sites:red.The red square in (e)outlines the part of the image shown in (f).The color scale shown in (d)is HAADF-STEM intensity in arbitrary units,in which different intensity ranges are shown in different colors.Reproduced with permission from ref.114.Copyright 2014,John Wiley and Sons.

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provides a direct proof of the alloying degree on individual atomic basis.

5.Gating,pressure,and lighting tuning

5.1

Electric field gating e?ect

Electric-field control of charge carrier density has attracted much attention since it is remarkably simple for modulating physical properties of condensed matters and for exploring new functionalities with a transistor configuration.8,116,117Adding carriers to layered chalcogenides can produce remarkable collective electronic e?ects owing to their enriched electronic phases,such as charge density wave and superconductivity.19Besides the chemical doping/intercalation mentioned above,electric-field tuning of surface carrier density through carrier accumulation or depletion in TMD-based field-e?ect-transistors is free from additional structural disorder and operating in a controllable/reversible way,especially at their monolayer limit.Therefore,to control electronic phases of layered TMDs in electrical means is attracting growing interest not only for understanding the fundamental physics but also practical electronics and spintronics applications.118

The 2D nature of the layered TMDs and the resulting carrier quantum confinement 38decides that the layered TMDs can serve as the ideal charge transport channel for a high perfor-mance transistor.As an example,the field e?ect transistors based on a single layer MoS 2with oxide as a gate insulator exhibit a large current on/o?ratio exceeding 1?108at room temperature 8,119,120which serve as the platform towards the realization of electronics and low-standby-power integrated circuits based on two-dimensional materials.The presence of the quantum mechanical confinement and further modifica-tion of the interfacial band alignment with the external gate electric field 121provide us with the room-temperature field e?ect transistors with a very large on/o?ratio and thus allow the observation of a metal–insulator transition in monolayer MoS 2.116It should be addressed that the 2D nature of the TMDs results in charge transport that is highly sensitive to a number of factors such as substrates/dielectric,122–125cleanliness (sur-face adsorbates),126–132and contact metals,133–139providing the possibilities of the mobility enhancement/engineering with these factors.

To realize novel field-e?ect modulated electronic phenom-ena in solids,a broad range of attainable carrier density is always required.The power of an electric double layer transistor with ionic liquid gating to achieve high sheet carrier densities beyond the maximum attainable range of conventional oxide gate dielectrics has been recently exploited to modulate novel electronic properties and quantum phenomena on chalco-genides.140–142The access of field-effect induced emergent phenomena such as superconductivity,ferromagnetism and metal–insulator transitions with EDLTs provides a powerful way to study novel physical phenomena at such highly-charged interfaces.141–144The recent application of the ionic gating technique allows the achievement of the ultrathin carrier

density on MX 2surfaces and large regime tuning of the Fermi level in the band structure.This powerful technique further plays a vital role in the experimental realization of the electric field induced Zeeman polarization,23the ambipolar operated transistor,145–148and the gating induced superconductivity in MX 2systems (Fig.9a–c).149,150

As another advantage of the transistor configuration,the intrinsic inversion symmetry can be broken simply by applying a perpendicular electric field in bilayer or bulk two-dimensional electron systems where the crystal symmetry governs the nature of electronic Bloch states.145,151For example,in bilayer MoS 2transistors,the circularly polarized photoluminescence can be continuously tuned from à15to 15%as a function of gate voltage,whereas in the structurally non-centrosymmetric monolayer MoS 2case the photoluminescence polarization is gate independent.151The observations demonstrated the con-tinuous tuning of orbital magnetic moments between positive and negative values through symmetry control by an electric field.More interestingly,with the electric field induced inver-sion symmetry breaking,a spin-coupled valley photocurrent whose direction and magnitude depend on the degree of circular polarization of the incident radiation can be further greatly modulated with an external electric field in an electric-double-layer transistor based on WSe 2.152Such room tempera-ture generation and electric control of valley/spin photocurrent provide a new property of electrons in MX 2systems,thereby enabling new degrees of control for quantum-confined spin-tronics devices.152–1545.2

Pressure induced insulator–metal transition

Unlike mono-atomic multilayered graphene with sp 2hybridiza-tion,multilayered MoS 2coupled with its d-orbital electronic states and small vdW gap raises the prospects of strong S–S interlayer interactions under axial compression that might lead to an electronic phase transition (Fig.9d).155With high pres-sure experiments on exfoliated single crystalline MoS 2up to 35GPa,it was found that the pressure induced a lattice distortion involving anisotropic c /a axial compression begin-ning at B 10GPa in multilayered MoS 2.This compression leads to an intermediate state followed by a pressure-induced insu-lator–metal transition at B 19GPa.First-principle theoretical calculations attribute the origin of the metallic electronic states to S–S interactions as the vdW gap closes at high pressures.The pressure control provides a possibility for the development of nanoscale pressure switches,sensors and multi-physics devices with coupled electrical,vibrational,optical and structural prop-erties using multilayered MoS 2and semiconducting TMDs.1555.3

Valley polarization by circularly-polarized light

From the physics viewpoint,the recent emergence of transition metal dichalcogenides provides a new platform for exploring the internal quantum degrees of freedom of electrons including the electron spin,the layer pseudospin,and the valley pseudospin.156Owing to hexagonal in-plane lattice structure,where valleys of energy-momentum dispersion are generally expected at the corners of the hexagonal Brillouin zone (at the K and àK points),new

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methods for the quantum control of the spin and these pseudospins arise from the existence of Berry phase-related physical properties and strong spin–orbit coupling.157Following theoretical discoveries of the intrinsic physical properties asso-ciated with valley pseudospin,experimental progress has been made in the control and tuning of valley polarization and coher-ence that allows manipulation in ways similar to real spin.Valley polarization,as the selective population of one valley is designated,has been demonstrated in monolayer MoS 2by optical pumping with circularly polarized light (Fig.9e and f).23–25

Several groups have independently reported the selective photoexcitation of the degenerate valleys by circularly polarized optical pumping in MoS 2monolayers.The inversion symmetry present in bulk and in thin films with an even number of layers is explicitly broken in thin films with an odd number of layers,giving rise to a valley-contrasting optical selection rule,158,159where the inter-band transitions in the vicinity of the K (K )point couple exclusively to right (left)-handed circularly polarized light

s +(s à).23–25The direct-bandgap transition at the two degenerate valleys,together with this valley-contrasting selection rule,sug-gests that one can optically generate and detect valley polariza-tions in a MoS 2monolayer.

6.Conclusions and outlook

In conclusion,we present di?erent physical and chemical strategies,including reducing dimension along z and xy direc-tions,intercalation of guest ions,stacking heterostructures,alloying di?erent transition metals or chalcogens,electrical field e?ects,high pressure,and lighting,for the property tuning of 2D TMDs.The tuning methods and corresponding tunable properties are summarized in Table 1.The tunability of these layered materials leads to not only fundamental studies of material properties but also versatile applications in di?erent areas.However,there is still much room for the development

Fig.9(a)Schematic structure of a typical WSe 2EDLT.By applying a negative V G through a Pt gate electrode,anions in the ionic liquid are electrostatically driven to the WSe 2surface,forming a highly charged electric double layer (EDL)interface.Most of potential drop occurs at the EDL interface and almost no potential is distributed in the liquid.Reproduced with permission from ref.145.Copyright 2013,Nature Publishing Group.(b)Ambipolar operation in the transfer characteristics of WSe 2EDLTs.Reproduced with permission from ref.145.Copyright 2013,Nature Publishing Group.(c)Unified phase diagram of superconductivity of both electrostatically and chemically doped MoS 2as a function of doping concentration x (upper horizontal axis)and carrier density n 2D (bottom horizontal axis).The field-induced superconducting data were from four di?erent samples,each marked with a di?erently shaped filled symbol.Filled circles of the same color correspond to the superconducting states at a fixed V LG but di?erent V BG ’s.Open circles show T c of MoS 2chemically intercalated with di?erent alkali metal dopants.Solid bars denote the range of doping showing the same T c .The structure of all intercalated compounds is 2H-type within the indicated carrier density region.Reproduced with permission from ref.150.Copyright 2012,the American Association for the Advancement of Science.(d)Theoretical calculation of the pressure-dependent band gap of multilayered MoS 2.The bandgap–pressure dependence can be modeled as a quadratic function,E g =E go +aP +bP 2was used,where a =à70meV GPa à1and b =1.13meV GPa à2.Reproduced with permission from ref.155.Copyright 2014,Nature Publishing Group.(e)Schematic of the MoS 2monolayer and optical selection rules at the K (K 0)point.Left panel:schematic of the MoS 2monolayer structure with the spatial inversion symmetry breaking in monolayers.Right panel,Schematic of proposed valley-dependent selection rules at K and K 0points in crystal momentum space:left (right)-handed circularly polarized light s +(s à)only couples to the band-edge transition at K (K 0)points for the sake of angular moment conservation and time reversal symmetry.Reproduced with permission from ref.25.Copyright 2012,Nature Publishing Group.(f)Polarization-sensitive photoluminescence spectra from MoS 2monolayers.Up-left:representative optical image of the MoS 2monolayer,bilayer and thin film flakes.Down-left:characteristic Raman spectra from different MoS 2flakes (monolayer,bilayer and thin film).Right panel:polarization resolved luminescence spectra under circularly polarized excitation from a He–Ne laser at 1.96eV and 10K.Reproduced with permission from ref.25.Copyright 2012,Nature Publishing Group.

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tunable 2D TMDs.For example,the synthesis of large scale and high quality single layer 2D TMDs needs to be further boosted for high performance devices;band structure and Fermi level tuning may become powerful tools in photoelectrocatalysis;carrier density tuning has the potential for optimizing the superconducting temperature.A number of important and exciting studies and applications employing the tunable proper-ties of 2D TMDs will certainly come out in the near future.

Acknowledgements

We acknowledge support by the Department of Energy,O?ce of Basic Energy Sciences,Materials Sciences and Engineering Division,under Contract DE-AC02-76-SFO0515.

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Table 1

Summary of 2D TMD tuning strategies and tunable properties

Tuning strategies Tunable properties

z direction dimension Electronic band structure,optical property xy directions dimension Electronic band structure,catalytic activity Intercalation Thermal conductivity,super conductivity,

optical property,catalytic activity

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