preciselymonitoringand tailoringthe surface · 2019-07-04 · reviews 48 vol.25,no.4,2017...

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Reviews 48 Vol.25 No.4 2017 SCIENCEFOUNDATION INCHINA Precisel monitorin and tailorin the surface structures of nanomaterials in situ electron microsco py XU Tao 탬쳎WANG Wen ÍõÎÄ YIN KuiBo Òü¿ü²¨ & SUN LiTao ËïÁ¢ÌÎ SEUFEINano PicoCenter Collaborative InnovationCenter orMicro Nano Fabrication Device andstem Ke Lab MEMSMinistr Education Southeast Universit Nan in 210096 China Center orAdvanced Materials and Manu acture Joint ResearchInstitute Southeast Universit and MonashUniversit Suzhou 215123 China Center orAdvancedCarbon Materials Southeast Universit andJian nanGra hene Research Institute Chan zhou 213100 China ReceivedAugust 18 2017 acceptedOctober 20 2017 Correspondingauthor Email slt seu.edu.cn Abstract Thetransmission electron microscope TEM is now one ofthe mostpowerfultoolsfor structural characterizationdueto itshighspatial resolution especially withtheadventof thespherical aberration corrector.The interactionbetweenelectrons andspecimens allows tailoringsurfacestructures byelectron irradiation.Furthermore along with the development of special holders and specimen preparation techniques the structural evolution of surfaces interfaces canbe dynamicallymonitoredunder external fields or reaction environments at the nanoscale which promotes not only the establishment of the relationshipbetween atomic structures and novel properties but also potential applications in nanodevices. In this report we reviewsome of our recent results obtainedbyTEM including dynamical observation of surface mediated novel phenomena controllable fabrication of nanostructures and some potential applications innanodevices. Keywords Surface structures Dynamic characterization Transmission electronmicroscopy doi 10.16262 j.cnki.1005 0841.2017.04.001 Introduction Nanomaterials have attracted great deal of attentiondue to their uniquepropertieswhicharedifferent from theirbulkcounterparts.Therearetwo majorfactorsleadingtothesespecialpropertiesatthe nanoscale including large surface to volume ratioandquantumeffects associatedwithsmall dimensions. For instance semiconductornanoparticlesshow strongquantum confinement whentheparticlesizeis smaller than excitonBohr radius and themetal nanoparticles showsize dependentplasmonabsorption bandwhen their conduction electrons are confined todimensions smaller than the electron mean freepath 1-2 .Alongwith the reductionof dimensions the proportionof surface atoms increases.For 100nm particle onlyabout1%ofatomsoccupythesurfacepositions andthepercentageriseto 12% and 60%for 10nm and 2nm particle respectively.Furthermore surface atoms are preferably reconstructed for adapting their geometrical and electronic structure to the environment.Thus the surface phasebeginstodominate materialpropertiesrangingfrom electronicandstructuralaspects whenthe

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Page 1: Preciselymonitoringand tailoringthe surface · 2019-07-04 · Reviews 48 Vol.25,No.4,2017 SCIENCE FOUNDATION IN CHINA Preciselymonitoringand tailoringthe surface structures

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Preciselymonitoringand tailoringthe surfacestructures of nanomaterials byin-situ

electron microscopyXU Tao(徐涛)1,2,WANG Wen(王文)1,

YIN KuiBo(尹奎波)1,2 &SUN LiTao(孙立涛)1,2,3*1 SEU-FEI Nano-Pico Center,Collaborative Innovation Center for Micro/Nano Fabrication,Device and System,Key Lab of MEMS of Ministry of Education,Southeast University,

Nanjing 210096,China;2 Center for Advanced Materials and Manufacture,Joint Research Institute of Southeast

University and Monash University,Suzhou 215123,China;3 Center for Advanced Carbon Materials,Southeast University and Jiangnan Graphene

Research Institute,Changzhou 213100,ChinaReceived August 18,2017;accepted October 20,2017

*Corresponding author(Email:slt@seu.edu.cn)

Abstract The transmission electron microscope (TEM)is now one of the most powerful tools forstructural characterization due to its high spatial resolution,especially with the advent of the sphericalaberration corrector.The interaction between electrons and specimens allows tailoring surface structuresby electron irradiation.Furthermore,along with the development of special holders and specimenpreparation techniques,the structural evolution of surfaces/interfaces can be dynamically monitored underexternal fields or reaction environments at the nanoscale,which promotes not only the establishment of therelationship between atomic structures and novel properties,but also potential applications in nanodevices.In this report,we review some of our recent results obtained by TEM,including dynamical observation ofsurface-mediated novel phenomena,controllable fabrication of nanostructures,and some potentialapplications in nanodevices.

Keywords Surface structures;Dynamic characterization;Transmission electron microscopydoi:10.16262/j.cnki.1005-0841.2017.04.001

1 Introduction

Nanomaterials have attracted a great deal of attention due to their unique properties which are differentfrom their bulk counterparts.There are two major factors leading to these special properties at thenanoscale,including large surface-to-volume ratio and quantum effects associated with small dimensions.For instance,semiconductor nanoparticles show strong quantum confinement when the particle size issmaller than exciton Bohr radius,and the metal nanoparticles show a size-dependent plasmon absorptionband when their conduction electrons are confined to dimensions smaller than the electron mean free path[1-2].Along with the reduction of dimensions,the proportion of surface atoms increases.For a 100nmparticle,only about 1% of atoms occupy the surface positions,and the percentage rise to~12% and~60%for a 10nm and a 2nm particle respectively.Furthermore,surface atoms are preferablyreconstructed for adapting their geometrical and electronic structure to the environment.Thus,the surfacephase begins to dominate material properties ranging from electronic and structural aspects when the

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characteristic dimension is reduced to sub-10nm.Currently,the understanding of surface-dominated behaviors of sub-10nm crystals relies heavily on

computer simulation.Although the simulation results offer insight into the dynamic behaviors,it is stilldebatable whether or not these simulation results can be directly extrapolated to real conditions.Thereliability and trust of computer simulations depends on the validity of the simulation model,and dynamicsimulation iteratively performs numerical solution of differential equations or differential-algebraicequations.But sometimes,the parameters in these equations are not determined for sub-10nm objects,which make the simulated results significantly different even if those parameters change a little.Unfortunately,there are few experimental observations and measurements to evaluate the reliability ofsimulated results.Therefore,dynamic characterization methods need to be developed to experimentallyexplore the relationship between surface phase and their unique properties at sub-10nm scale.With significant technological advances in high spatial and temporal resolutions imaging,the

transmission electron microscopy(TEM)has become an indispensable tool for structural characterizationof nanomaterial with atomic resolution[3,4].Furthermore,the development of special sample holders(Fig.1),which can apply external field to individual nano-object or can mimic reactive gas/liquidenvironment,provides great opportunities to reveal intrinsic evolution mechanisms under external stimuli[5-10].In this principle,several surface-dominated phenomena have been discovered,such as discreteplastic deformation[11],cold welding [12],and ligand mobility-controlled facet development duringcolloidal growth[13].

Figure 1 In situ TEM primary based on specimen holder[3].Special specimen holders are designed to apply external fields(thermal field,light,voltage bias,mechanical stress,etc.)to the specimen area or to mimic reactive environment(liquidand gas).

Besides,electrons inside the TEM can not only be used for atomic resolution imaging but also interactdirectly with specimens,resulting in structure and property changes[14].A large number of radiationstudies have been carried out inside the TEM,which are conducive to understanding radiation-inducedphenomena,such as radiation-induced high pressure and phase transition[15-17].In addition to detailedunderstanding of radiation-induced structure evolution,top-down fabrication of low-dimensional structuresbased on electron irradiation has become feasible.Especially,electron beams inside the modern TEM canbe focused onto spots less than 1,which allows manipulation at atomic scale[18,19].By this method,both quasi-one-dimensional(1D)[20-24]and quasi-2Dcrystals[25-27]have been fabricated with highprecision.In this review,we focus on our recent in-situ TEM researches,concerning precisely monitoring and

tailoring the structures of sub-10nm nanocrystals and 2Dlayered materials.Subsequently,we expand ourfocus to some possible applications in nanodevice.

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2 Precise monitoring of surface-dominated phenomena

Both 2Dlayered materials and sub-10nm crystals are adopted to investigate surface effects,and theessential difference is that the surface of 2D materials without unsaturated bonds is inactive,whereassurface atoms of sub-10nm crystals are more likely to diffuse due to unsaturated bonds and lower barrier.As atomic sheets,2Dmaterials are accessible to be imaged directly in the advent of spherical aberrationcorrected TEM.The reconstruction of the lattice occurs in the 2Dplane,which is in response to stimulifrom the surrounding environment or electron irradiation,can be seen without projection artifacts,so thatthe detailed picture of atomic arrangement becomes visible[4].Besides,individual atoms adsorbed on 2Dlayers can be distinguished[28],making 2Dmaterials ideal platforms for exploring dynamical behaviors ofadsorbates[29-31].2.1 Evolution of S vacancies in MoS2Monolayer MoS2is an emerging 2Dmaterial wherein Mo atoms are sandwiched between S layers.Each

Mo atom is connected to six S atoms,which imparts various defect structures,such as single S vacancy,double S vacancy,Mo antisite,and so on[32,33].These dot defects can be clearly distinguished bycomparing intensity profile of experimental images and simulated images[34,35].Fig.2(a)presentsexperimental images and corresponding profiles along the lattice with(red symbol)and without(blacksymbol)single S vacancy,which is in consistent with simulated images(red line).

Figure 2 Evolution of S vacancies in monolayer of MoS2 [34].(a)Identification of single S vacancy(denoted by arrows)by comparing the intensity profile of experimental images(red and black symbol present lattice with and without S vacancyrespectively)and simulated vacancy images(red line).(b)Migration and agglomeration of S vacancies,forming extendedline defect(highlighted by dotted line).

It is noted that new S vacancies are continuously generated at the rate of 5×1010 cm-2s-1 during imagingat a beam energy of 80keV and beam current density of 3A/cm2[34].Previous calculations based ondensity-functional theory(DFT)predicted that the threshold energy for sputtering S atom from the bottomlayer is approximately 6.9eV.The corresponding incident electron energy is 90keV,as calculated throughthe relativistic binary collision formula by assuming a static MoS2lattice;while the value reduces to below80keV if lattice vibrations are accounted for[36].The threshold energy for the top S atom in MoS2isconsiderably higher even if the corresponding bottom S atom has been ejected,which results in theprominent formation of single S vacancies in comparison with double S vacancy[36].With the increase in vacancy concentration,migration and agglomeration are expected to form extended

line(Fig.2(b)).DFT calculations predicted that the migration barrier for single S vacancy is only 2.3eVand can further lower down to 0.8eV if additional S atom adjacent to the diffusion path is removed[37].

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Both of these two values are much less than the maximum energy transferred from 80keV electrons,whichmeans that the diffusion is significantly frequent.As the local vacancy concentration increases,thediffusion barrier would drop considerably,which accelerates the formation of extended line defects.Linedefects have several possible configurations,and the most frequently observed type is “staggered”configuration in which two lines of S vacancies are at the neighboring sites and opposite sides[37].Afterprolonged exposure to the electron beam,the lines become wider,and then transfer to holes and Moclusters[38],forming subnanometer quasi-1Dnanowires eventually[39,40],and more details will begiven in section 3.3.2.2 Growth of CuO monolayers on graphene substrateAmong all kinds of 2Dcrystals,monolayer oxide sheets are an especially interesting class due to many

coupled degrees of freedom(charge,spin,lattice,etc.).Copper oxide,as the only one in which the metalatom has a full d-shell,has received a great deal of attention due to its potential application in hightemperature superconductivity and supported catalyst[41,42].However,the separation of an individualcopper oxide layer remains a challenge due to strong interlayer bonding.It is noted that electron irradiation can cause Fe cluster to move into graphene nanopore and then

rearrange neatly into monolayer Fe sheet[26].Analogously,the 60keV electron beam is expected to drivecopper oxide cluster to transform into monolayer sheet with a square Cu sub-lattice[43].Typical scanningtransmission electron microscopy (STEM)annular dark field (ADF)images are shown in Fig.3tomanifest the formation process of a monolayer copper oxide on graphene substrate.Cu sub-lattice presentsa constant of 2.69±0.05,and chemical analysis via electron energy-loss spectroscopy(EELS)shows thepresence of oxygen and copper without other light elements,such as B,N,and F.DFT calculationsshowed that the projected Cu-Cu distance in relaxed configuration of CuO is 2.67,which is in consistentwith experimental data.Furthermore,simulations also confirmed that CuO is energetically preferredcompared with Cu2O by 0.26eV per Cu atom.As a result,the copper oxide patched on the graphenesubstrate is CuO.

Figure 3 Formation process of a monolayer CuO on graphene substrate[43].(a)-(f)Time series of STEM-ADF images,showing the migration of CuO and the formation of ordered monolayer structure on graphene surface.Scale bars are1nm.

In addition to the formation of monolayer CuO nanosheet on a graphene substrate,suspended copperoxide monolayer can form in graphene pores.The suspended monolayer crystal has a square Cu sub-latticesimilar to those observed for supported CuO nanosheets,with a nearest Cu-Cu distance of~2.65.DFTcalculations predict that the free-standing CuO monolayer is a semiconductor with a bandgap of~3eV.

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2.3 Liquid-like pseudoelasticity of sub-10nm silver particlesNanoelectronic devices require their components to stay unchanged over a long period of time once a

certain shape is formed.However,the shape stability has been questioned especially when thecharacteristic dimension is scaled to the sub-10nm regime.At this scale,more than 10%of atoms occupythe surface position where diffusion barrier is much lower than the inner position.It remains to beexamined whether or not the surface atoms would affect the shape stability.Sub-10nm silver crystal is a good study object not only because Ag has been used industrially in

electrical contacts and conductors due to excellent electrical conductivity and thermal conductivity,but alsobecause surface diffusion barrier of Ag is only 0.6eV,which is lower than most of metals[44].However,sub-10nm Ag crystals are synthesized by colloid method,and the as-prepared crystals are always coveredwith surfactant ligands.The ligands restrain the diffusion of surface atoms,thereby screening intrinsicsurface-dominated phenomena.A new method to prepare sub-10nm crystals with naked surface has beendeveloped by using TEM,then the as-prepared crystals are directly tested inside the TEM [45].

Figure 4 Reversible pseudoelastic deformation of sub-10nm Ag particles [45].(a)Dynamic shape evolution of Agnanoparticles during compression.(b)Shape evolution of Ag nanoparticles during stretching.HRTEM images indicate that theparticle is crystalline and the crystal orientation is maintained during deformation.Scale bars are 5nm.

Fig.4shows a typical cycle of compression and stretching of sub-10nm Ag particle.Once thecompressive stress is loaded,the Ag particle is deformed and eventually transferred to a flat pancake-likeshape(Fig.4(a)).When the stress is unloaded,the shape of the Ag nanoparticle changes rapidly from anelongated spire to a stable geometry similar to initial configuration(Fig.4(b)).Such a reversible changein shape is repeatable except for atoms loss caused by substrate scratches and electron irradiation.

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Furthermore,during the process,the outer morphology of the Ag nanoparticle changes similarly to aliquid drop,while the deforming Ag particle remains single crystalline inside.These deformations arepseudoelastic rather than plastic because the particle can be restored to a single rest shape at zero stress.The rest shape is the balance between Ag surface energy and Ag-substrate interfacial energy.Generally,the imposed stress is relieved by the displacive dislocation slip at room temperature.But the

observation indicates that the stress is relieved by atom plating/ablation from the Ag surface to Ag-substrate interfaces,as in diffusive Coble creep.The essential difference is that creep strain is expected tobe plastic,whereas the strain observed is fully recoverable as the applied load returns to zero.Therefore,the deformation behaviors are‘Coble pseudoelasticity’driven by surface diffusion,which is also differentfrom conventional pseudoelasticity driven by martensitic transformations.Capillary energy plays a critical role in the memory and recall of the single rest shape in Coble

pseudoelasticity.As shown in Fig.5(a),the base part of the Ag tip remains unchanged and no dislocationactivity is observed,while the upper part of the Ag tip undergoes a continuous shape change without anyexternal stress and reveals a significant capillary action at room temperature.Molecular dynamicssimulations,which are consistent with experimental observations,indicate that:the shape evolution isdominated by single adatom movements rather than chain or island processes;the shape change isaccomplished by atom ablation of the surface layers with greater curvature,while the atoms below surfacialbilayer always remain highly crystalline;the shape change is rate-controlled by atomic processes on non-(111)facets of the nanoparticle(Fig.5(b)).

Figure 5 Surface-energy-driven shape change[45].(a)Experimental shape evolution of Ag tip after fracture.Scale barsare 5nm.(b)Molecular dynamics simulations showing that shape evolution of Ag tip is consistent with the experiments in(a).

Besides,electron beam activation may also accelerate surface diffusion.MD and accelerated MDsimulations indicate that Coble pseudoelasticity should occur for sub-10nm Ag nanoparticles at roomtemperature even in the absence of an electron beam.The shape relaxation timescale is expected to be<100sfor sub-10nm Ag tips based on power laws[46],which indicates that surface diffusion indeedseverely threatens the shape stability of sub-10nm scale interconnects at room temperature.

3 Precisely tailoring the surface structures

As mentioned above,imaging electrons interact with the specimen atoms when they pass through thespecimen,resulting in structural evolution.The effects induced by electron irradiation consist of atomicdisplacement,surface sputtering,carbon deposition,electrostatic charging and so on.Both displacementand sputtering occur by elastic collisions of energetic electrons with atom nucleus in the specimen,whileother effects occur by inelastic collisions with atomic electrons surrounding nucleus.These two processesoccur simultaneously,and they both have the capability of tailoring surface structures.Elastic collisions

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can displace specimen atoms to the interstitial position if the energy transferred from the incident electronsto the nucleus exceeds the threshold energy Td.Surface atoms do not have to be squeezed into aninterstitial site,and they are free to leave the specimen and enter the vacuum of the microscope,leading tomass loss.Hence,energy required for displacement of surface atoms is much lower than that of bulkatoms,and almost all low-Z and medium-Z atoms can be knocked away from the surface lattice by 300keVelectrons[47].Inelastic collision can cause electronic excitations,which makes local atomic bonds instableand even broken.Meanwhile,hydrocarbon molecules adsorbed on the specimen surface can be dissociatedby incident electrons and the nonvolatile components would be deposited on the substrates,leading to massgain.Both mass loss and mass gain are necessary to obtain the designed morphology.Besides,the surfacestructures can be modified under other external stimuli,but in this section,we focus on electronirradiation-induced structure tailoring.3.1 In-situ nanofabrication of carbon nanostructuresFocused electron beam has been widely used to tailor carbon nanotubes[48,49],and has presented the

capability to modulate graphene sheets[50].For example,focused electron beam can be used tosculptsub-5nm nanopores in graphene,which attracts global attention due to its potential application in DNAsequencing and single-molecule analysis[51].During the sculpting process,both surface sputtering and carbon deposition perform.The sputtering is

the most substantial radiation effect leading to carbon atoms loss,while deposition results in carbon gain.Previous literatures indicated that the sputtering rate pscould be calculated as ps=σJ[47,52],whereσissputtering cross section and Jis the beam current density.According to a simple elastic collision model,σis related to the electron beam energy,the angle of scattering and the atomic number[47,52].Hence,thesputtering rate is proportional to beam current density for a particular specimen under electron beamirradiation with a specific energy.Electron beam-induced carbon deposition is a complex process that involves specimen-hydrocarbons

interaction,electron-specimen interaction and electron-hydrocarbon interaction,which can be welldescribed by the model given by W.F.van Dorp and C.W.Hagen[53].In the case of 300keV focusedelectron beam with diameter of several nanometers,deposition rate can be considered as a constant relatedto surface concentration of hydrocarbon if the current density exceeds 10A/cm2.Without doubt there is a critical current density,the sputtering and the deposition respectively dominate

at different intervals separated by the critical value.The deposition is much more obvious than thesputtering when beam current density is less than the critical value;otherwise,sputtering is significant.Experimental results indicate that the critical value for ultrasonically exfoliated graphene is 300A/cm2

[54].Therefore,the focused electron beam with current density larger than 300A/cm2 is suitable forsculpting graphene nanopores.However,the as-sculpted nanopores cannot always meet size requirementfor single molecule detection because real-time imaging cannot be synchronously performed with sculpting.Considering that thermal treatment can promote migration and reconstruction of defects,thermal heating

is expected to modulate the size of as-sculpted nanopores and promote crystallization[55].As shown inFig.6(a),some as-sculpted nanopores shrink after thermal heating while the others expand.Tens ofnanopores are investigated and the results are shown in Fig.6(b).More than 85% of nanopores withinitial diameter larger than membrane thickness expand after thermal heating,whereas almost allnanopores with initial diameter smaller than membrane thickness shrink.Such phenomena can be explainedby thermal-induced migration of uncombined carbon atoms,which energetically prefer to form a stablestructure with lower free surface energy F.The nanopore with diameter dembedded in the few-layer graphene with thickness t can be simplified as

cylinder,and the change in free surface energy can be calculated asΔF=γΔA=2πγΔd(t-d),whereγissurface tension of the mobile carbon atoms.This is reasonable when uncombined carbon atoms occupy theregion around nanopores and the migration of uncombined carbon atoms is isotropic.The nanopores alwaysdeform to reduce Funder thermal treatment,that meansΔF<0.If t<d,Δdshould be positive,thatmeans nanopore should expand;while if t>d,nanopore should shrink,which is in consistent with

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Figure 6 Size-dependent evolution of graphene nanopores under thermal excitation[55].(a)TEM images showing that thethermal excitation can shrink or expand the as-sculpted nanopores.(b)Statistical result indicating that nanopores with initialdiameter smaller than membrane thickness tend to shrink,otherwise expand.

experimental results.Furthermore,control experiments also indicate that hydrocarbons adsorbed on thespecimen surface play a significant role in pore shrinkage.In addition to nanopores,carbon tubular structures can also be fabricated by electron beam-induced

carbon deposition on metal nanowire/nanotube template and subsequent removal of template materials bythermal heating or voltage bias(Fig.7(a)).The extraction of the core template by thermal heating orvoltage bias can also improve the crystalline quality of as-formed tubular structures due to annealing effect(Fig.7(b)).Significantly,these partial graphitized tubes present excellent conductivity and mechanicalproperties[56].Furthermore,diverse tubular structures,such as co-axial hollow (Fig.7(c))and Y-shape connected tubes (Fig.7(d)),can be formed by controlling template shape.Meanwhile,thedeposited carbon shell on metal nanowires or nanocrystals can govern the morphological transitions underthermal heating[57,58].For instance,uncovered gold nanorods can transform their shape into sphereswell below the bulk melting temperature through surface diffusion,but carbon shell can prevent thistransition.Besides,electron beam induced deposition of amorphous carbon can also work as excellentadhesive for joining and shaping features to designed morphology[59].

Figure 7 Investment casting of carbon tubular structures[56].(a)Carbon tubular structures formed by electron beaminduced carbon deposition on Te nanowires/nanotubes and subsequent removal of the template materials by thermal heatingor voltage bias.(b)TEM image indicating that the tubular structure is partial graphitized after extraction of the template.(c)-(d)Co-axial hollow and Y-shape connected tubular structures.

3.2 Nanotubes derived from bilayer BNHexagonal boron nitride(h-BN)has a layered structure similar to graphite,where the layers are held

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together by weak van der Waals forces.Each basal sheet is an atomic-scale honeycomb lattice made of Band N atoms with strong covalent bonds.Previous experiments indicated that B atoms were ejected moreoften than N atoms in perfect BN monolayer because of smaller threshold beam energy for surfacedisplacement(about 74keV for B and 84keV for N).So a large number of B vacancies are expected to begenerated under 80keV electron beam irradiation[60,61].Then the unsaturated N atoms around Bvacancies or at edges can be displaced because their threshold energy reduces dramatically to below 80keV.Therefore,higher dose irradiation would cause the formation of triangle-shaped holes in BN monolayer,whose internal edges always show N-terminated zigzag configuration.On this basis,B and N atoms can beidentified by assigning the vacancies as missing B atoms and allocating N-terminated edges to large triangledefects.Then the two stable stacking orders(AB stacking and AA′stacking)of bilayer BN [62]can bedistinguished by the relative orientation of the triangle-shaped defects in different basal layer because defectstructures in each layer are the same as in monolayer.When AA′stacked bilayer sheets are exposed to electron bombardment,interlayer B-N bonds form as

long as triangular edges in different basal sheet overlap,resulting in closed edges along zigzag directions(Fig.8(a)).When all dangling bonds are saturated with interlayer bonds,a hexagonal hole with closedzigzag edges is formed.On this basis,tubular structures form between adjacent holes.Furthermore,thehexagonal holes grow via formation and motion of kinks under ongoing electron irradiation(Fig.8(b)).Normally,kinks nucleate at the intersecting point between zigzag edges where non-hexagonal rings withhigher sputtering rate are required to accommodate curvature change[63].Analogously,atoms at thekinks are more likely to be ejected due to topological defects with higher sputtering rate,resulting in kinkmotion along zigzag edges until they vanish at the other intersecting point(Fig.8(b)).Hexagonal holesare enlarged as atomic rows are ejected.Accordingly,the tubular structures between adjacent holes arethinned simultaneously by removing atomic rows one by one.The projected diameter decreases in steps of0.14-0.15nm until 0.45nm corresponded to a(3,3)tube(Fig.8(c)),which is the smallest freestandingsingle-walled BN tube observed experimentally so far[64].

Figure 8 Creating single-walled BN nanotube from bilayer h-BN [64].(a)Formation of hexagonal holes in h-BN bilayerunder electron irradiation.The closed configuration nucleates at the crossing edge and grows along zigzag edges.(b)Hexagonal hole evolution via nucleation,propagation,and disappearance of kinks at closed zigzag edge.SK and DKdenote mono-and bi-atomic kinks,respectively.(c)Tube diameter evolution under electron irradiation.The projecteddiameter decreases in steps of 0.14-0.15nm until 0.45nm.The scale bars in the insets are 1nm.

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3.3 Nanoribbons derived from monolayer MoS2As mentioned in section 2.1,bottom S atoms are more likely to be knocked away from the lattice

position in monolayer of MoS2under 80keV electron beam,forming single S vacancies.After prolongedexposure to the electron beam,S vacancies migrate and then agglomerate into line configurations withlower energy.These line defects only appear and elongate along the directions of[100],[010],and[1-10]in MoS2sheet.Inevitably,nanopores are formed when these line defects are propagated to a point[38].Without doubt,nanopores can also be fabricated rapidly by focused electron beam irradiation,asdiscussed in section 3.1.The first-principles calculations predict that the energy for removing S atoms from zigzag or armchair

edge is much lower than the one for ejecting S atoms from perfect sheet due to dangling bonds[39].Meanwhile,the energy transferred from 80keV electrons to Mo atoms is still less than the one required toknock off Mo atoms from the edge.However,Mo atoms at edge can diffuse and reconstruct to low systemenergy.Therefore,nanopores will grow up quickly via knocking off S atoms under continuous electronbeam irradiation.Along with the growth of nanopores,the regions between adjacent pores are narrowed into nanoribbons

correspondingly(Fig.9(a)).The thinnest stable ribbon is crystal with lattice constants distinctlydifferent from MoS2and with width of about 0.35nm.Combining computer calculation with comparingintensity profile of experimental images and simulated images based on atomic model predicted by computercalculation,the as-formed quasi-1Dstructure is assigned as Mo5S4crystal with a band gap of 0.77eV(Fig.9(b)).Transition state simulation from MoS2to Mo5S4presents that the energy barrier of thisphase transition is only 0.05eV per atom (Fig.9(c)),indicating that Mo5S4nanoribbon can be formedspontaneously under electron irradiation.

Figure 9 Top-down fabrication of subnanometer nanowires derived from MoS2 monolayer[39].(a)Time series of theformation and growth for a suspended subnanometer wire under 80keV electron irradiation.(b)HRTEM image andsimulated image(labeled by the dotted rectangle)of Mo5S4and corresponding atomic model.(c)Transition state simulationfor forming Mo5S4nanowire from a S-impoverished armchair ribbon.

It is noted that Lin et al.also obtain similar subnanometer quasi-1Dstructures assigned as metallic MoScrystal using focused electron beam of a scanning TEM [40].No matter which atomic structure the final

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stable configuration is,the electron beam provides precise and flexible control for top-down fabrication.

4 Surface-based applications

The ultimate goal of surface structural characterization and modification is the efficient use of materials.Generally,there are two main strategies to investigate the application of nanomaterials.One is to explorecompletely new applications based on the unique properties of nanomaterials,the other is to replace thecomponent materials in existing devices to promote their performance.Both of them are based on theprofound understanding of the relationship between surface structure and their novel properties,and can beset up inside the TEM based on material’s novel properties,special TEM specimen holder and specimenpreparation techniques.One excellent example is the use of carbon cages to study the mechanical behaviors of the encapsulated

materials or the phase transition inside the cages at the atomic resolution.As mentioned in section 3.1,theenergetic electron with energy larger than 100keV can knock off C atoms from the lattice of carbonnanotubes or carbon onions inside a TEM,generating point defects;meanwhile thermal treatment canpromote the diffusion and reconstruction of defects.Under the effects of electron irradiation and thermalannealing,continuous loss of C atoms and surface reconstruction will cause the shrinkage of graphite shells(Fig.10).The self-contraction of outer shells causes an enormous inward pressure up to tens of GPa inthe interior,particularly when the cages are filled with C or other materials[15-17].On this basis,slowplastic deformation of nanometer-sized Au and Pt crystals is observed,which is considered to be governedby the activity of short-lived dislocations in a wide temperature range[16].

Figure 10 Electron beam irradiation-induced evolution of the encapsulated Fe3C nanocrystal inside the carbon nanotube[15].(a)- (d)Time series of electron irradiation-induced collapse of carbon tube and the deformation of Fe3C crystal.(e)Schematic diagram of the tube’s evolution and encapsulated crystal’s deformation.

Graphene,as a 2Dallotrope of carbon,also has attracted considerable attention due to its atomicallythin,chemical inert,structurally stable,electrically and thermally conductive.As a near-invisible supportfilm for TEM imaging,graphene has enabled the unsurpassed imaging of organic molecules and theinterfaces between hard material core and its surrounding coatings,such as ligands,proteins and so on[65,66];as a transparent and flexible conductor,graphene holds great promise for various devices,suchas memoristors and actuators[67,68];graphene-based composites can be used in energy storage andconversion,catalysis [69-72].Besides,graphene-based macrostructure,which is assembled byhydrothermal reduction of graphene oxide(GO),can be used as a sorbent material for environmental

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detection and protection.The pH plays a critical role during the agglomeration of graphene sheets,and hasa further impact on the performance of graphene macrostructure[73].When the pH is adjusted to around10by the addition of ammonia,GO sheets come together simultaneously as a whole.As most of thecarboxylic acid groups attached on GO sheets are removed,the reduced GO sheets agglomerate together ina compact manner driven by Van der Waals forces,leading to the enhancement of strength up to severaltimes higher than conventional graphite products[73].When the pH is not in above range,the GO sheetsagglomerate partially during the reaction,forming aporous self-assembled reduced-GO hydrogel.Thenfreeze drying or critical point drying can evaporate water molecular inside the hydrogel and maintain porousstructures,forming macroscale graphene sponges with high surface area.Spongy graphene with highlyhydrophobic and oleophilic surfaces are feasible candidates for oil and toxic organic adsorbers(Fig.11),and have shown great improvement in adsorption efficiency up to hundreds of their weights[74-78].Furthermore,the graphene sponges can be heated to remove almost all(>99%)adsorbates and thenreused without affecting their adsorption performance (>10cycles)[74],which suggests spongygraphene a widespread potential for applications in environmental protection.Though great progress hasbeen made in design and synthesis of adsorption materials,the mechanism of oil adsorption is not clear.Without doubt,the interaction between adsorption material and oil molecular at the solid-liquid interfacehas a significant influence on the adsorption performance.Graphene liquid cell holds great promise forinvestigating solid-liquid-vapor interface at the nanoscale inside the TEM [79].

Figure 11 Adsorption performance and structural characterization of spongy graphene macrostructures[74].(a-e)Adsorptionof dodecane in graphene sponge at intervals of 20s.(f)Efficiency of oil adsorption.(g)Contact angle of spongy graphenesurface and water/dodecane.(h-j)SEM and TEM images of graphene skeleton.

In addition,catalysis performance of sub-10nm nanoparticles can also be investigated at the nanoscale byTEM especially when gas and light are successfully introduced into the specimen area[80].Charge anddischarge process for layered-material-cathode can be dynamically observed when nanobatteries are set upwith the help of voltage biasing specimen holder[81,82].All these studies give new insights into workingmechanism and promote the applications of nanomaterials with high surface area in these fields.

5 Conclusions and outlook

The TEM has become one of the most powerful tools for structure characterization with atomicresolution.The atomic structure of nanocrystals has been studied in detail,and some defect configurationsare determined as well.The development of special holders and specimen preparation techniques makes itpossible to dynamically monitor the evolution of specimen under external fields or reaction environments,which promotes establishment of the relationship between atomic structures and novel properties.Combining this method with computer simulations,some novel phenomena have been observed dynamicallyand surface-dominated mechanism have been proposed,such as surface-diffusion-dominated Coblepseudoelasticity.Besides,TEM allows not only structural characterization but also structural modification

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by electron irradiation.Quasi-1Dstructures,such as Mo5S4nanowires,single-walled BN nanotubes,havebeen controllably fabricated from corresponding 2Dsheets.In addition,prototype devices can be set upbased on material’s novel properties,special TEM holders and specimen preparation techniques,whichmay promote the application of nanomaterials with high surface area in environmental protection,energystorage and conversion.Although a lot of experiments have been carried out inside the TEM to determine surface structures,to

monitor nanocrystal’s response to external stimuli,and to fabricate nanostructures from top to down,many important issues in surface-mediated phenomena and potential applications are still open.Thereshould be plenty of new chemistry for nanomaterials in reactive environments when many chemicalprocesses occur on the surface.Structural and chemical evolution would be crucial for understandingmechanisms such as catalytic mechanisms and designing new and efficient materials for a wide range ofapplications.Therefore,it is highly desirable to further develop additional techniques with the ability toprovide information with accurate chemical composition and high temporal resolution under reactionconditions.

Acknowledgements

This work was supported by the National Natural Science Fund for Distinguished Young Scholar(GrantNo.11525415),and the National Natural Science Foundation of China (Grant Nos.51420105003,61274114,11327901,61601116 &11674052).

References

[1]Alivisatos A P.Semiconductor Clusters,Nanocrystals,and Quantum Dots.Science,1996,271(5251):933-937.

[2]Link S,El-Sayed M A.Optical properties and ultrafast dynamics of metallic nanocrystals.Annu.Rev.Phys.Chem.,2003,54(1):331

-366.

[3]Shen Y,Sun L.Setting up a nanolab inside a transmission electron microscope for two-dimensional materials research.J.Mater.Res.,

2015,30(21):3153-3176.

[4]Sun L,Banhart F,Warner J.Two-dimensional materials under electron irradiation.MRS Bull.,2015,40(01):29-37.

[5]Xu T,Sun L.Dynamic In-Situ Experimentation on Nanomaterials at the Atomic Scale.Small,2015,11(27):3247-3262.

[6]Xu T,Sun L.Investigation on material behavior in liquid by in situ TEM.Superlattice Microst.,2016,99:24-34.

[7]Sun J,Xu F,Sun L-T.In situ investigation of the mechanical properties of nanomaterials by transmission electron microscopy.Acta Mech.

Sinica,2012,28(6):1513-1527.

[8]He X,Xu T,Xu X,et al.In Situ Atom Scale Visualization of Domain Wall Dynamics in VO2Insulator-Metal Phase Transition.Sci.

Rep.,2014,4:6544.

[9]Mei S,He L,Wu X,et al.Dynamic investigation of interface atom migration during heterostructure nanojoining.Nanoscale,2014,6(1):

405-411.

[10]Wan N,Sun L,Ding S,et al.Synthesis of graphene– CNT hybrids via joule heating:Structural characterization and electrical

transport.Carbon,2013,53(0):260-268.

[11]Zheng H,Cao A,Weinberger C R,et al.Discrete plasticity in sub-10-nm-sized gold crystals.Nat.Commun.,2010,1:144.

[12]Lu Y,Huang J Y,Wang C,et al.Cold welding of ultrathin gold nanowires.Nat.Nanotechnol.,2010,5(3):218-224.

[13]Liao H-G,Zherebetskyy D,Xin H,et al.Facet development during platinum nanocube growth.Science,2014,345(6199):916-919.

[14]Neng W,Jun X,Tao X,et al.Surface energy guided sub-10nm hierarchy structures fabrication by direct e-beam etching.RSC Adv.,

2013,3(39):17860-17865.

[15]Sun L,Banhart F,Krasheninnikov A,et al.Carbon nanotubes as high-pressure cylinders and nanoextruders.Science,2006,312

(5777):1199-1202.

[16]Sun L,Krasheninnikov A V,Ahlgren T,et al.Plastic deformation of single nanometer-sized crystals.Phys.Rev.Lett.,2008,101

(15):156101.

Page 14: Preciselymonitoringand tailoringthe surface · 2019-07-04 · Reviews 48 Vol.25,No.4,2017 SCIENCE FOUNDATION IN CHINA Preciselymonitoringand tailoringthe surface structures

Reviews

SCIENCE FOUNDATION IN CHINA  Vol.25,No.4,2017  61   

[17]Sun L,Rodriguez-Manzo J,Banhart F.Elastic deformation of nanometer-sized metal crystals in graphitic shells.Appl.Phys.Lett.,

2006,89(26):263104.

[18]Rodriguez-Manzo J A,Banhart F.Creation of individual vacancies in carbon nanotubes by using an electron beam of 1diameter.Nano

Lett.,2009,9(6):2285-2289.

[19]Rodríguez-Manzo J A,Cretu O,Banhart F.Trapping of metal atoms in vacancies of carbon nanotubes and graphene.ACS Nano,2010,

4(6):3422-3428.

[20]Rodrigues V,Fuhrer T,Ugarte D.Signature of Atomic Structure in the Quantum Conductance of Gold Nanowires.Phys.Rev.Lett.,

2000,85(19):4124-4127.

[21]Rodrigues V,Ugarte D.Real-time imaging of atomistic process in one-atom-thick metal junctions.Phys.Rev.B,2001,63(7):073405.

[22]Cretu O,Botello-Mendez A R,Janowska I,et al.Electrical Transport Measured in Atomic Carbon Chains.Nano Lett.,2013,13(8):

3487-3493.

[23]Cretu O,Komsa H-P,Lehtinen O,et al.Experimental Observation of Boron Nitride Chains.ACS Nano,2014,8(12):11950-11957.

[24]Jin C,Lan H,Peng L,et al.Deriving Carbon Atomic Chains from Graphene.Phys.Rev.Lett.,2009,102(20):205501.

[25]Ta H Q,Bachmatiuk A,Warner J H,et al.Electron-Driven Metal Oxide Effusion and Graphene Gasification at Room Temperature.ACS

Nano,2016,10(6):6323-6330.

[26]Zhao J,Deng Q,Bachmatiuk A,et al.Free-Standing Single-Atom-Thick Iron Membranes Suspended in Graphene Pores.Science,2014,

343(6176):1228-1232.

[27]Romdhane F B,Cretu O,Debbichi L,et al.Quasi‐2DCu2SCrystals on Graphene:In‐situ Growth and ab initio Calculations.Small,

2015,11(11):1253-1257.

[28]Meyer J C,Girit C O,Crommie M F,et al.Imaging and dynamics of light atoms and molecules on graphene.Nature,2008,454(7202):

319-322.

[29]Chuvilin A,Kaiser U,Bichoutskaia E,et al.Direct transformation of graphene to fullerene.Nat.Chem.,2010,2(6):450-453.

[30]Zhou W,Yin K,Wang C,et al.The observation of square ice in graphene questioned.Nature,2015,528(7583):E1-E2.

[31]Cretu O,Krasheninnikov A V,Rodríguez-Manzo J A,et al.Migration and Localization of Metal Atoms on Strained Graphene.Phys.

Rev.Lett.,2010,105(19):196102.

[32]Hong J,Hu Z,Probert M,et al.Exploring atomic defects in molybdenum disulphide monolayers.Nat.Commun.,2015,6:6293.

[33]Zhou W,Zou X,Najmaei S,et al.Intrinsic Structural Defects in Monolayer Molybdenum Disulfide.Nano Lett.,2013,13(6):2615-

2622.

[34]Qiu H,Xu T,Wang Z,et al.Hopping transport through defect-induced localized states in molybdenum disulphide.Nat.Commun.,

2013,4.

[35]Yu Z,Pan Y,Shen Y,et al.Towards intrinsic charge transport in monolayer molybdenum disulfide by defect and interface engineering.

Nat.Commun.,2014,5:5290.

[36]Komsa H-P,Kotakoski J,Kurasch S,et al.Two-Dimensional Transition Metal Dichalcogenides under Electron Irradiation:Defect

Production and Doping.Phys.Rev.Lett.,2012,109(3):035503.

[37]Komsa H-P,Kurasch S,Lehtinen O,et al.From point to extended defects in two-dimensional MoS2:Evolution of atomic structure under

electron irradiation.Phys.Rev.B,2013,88(3):035301.

[38]Ryu G H,Lee J,Kim N Y,et al.Line-defect mediated formation of hole and Mo clusters in monolayer molybdenum disulfide.2D

Mater.,2016,3(1):014002.

[39]Liu X,Xu T,Wu X,et al.Top–down fabrication of sub-nanometre semiconducting nanoribbons derived from molybdenum disulfide

sheets.Nat.Commun.,2013,4:1776.

[40]Lin J,Cretu O,Zhou W,et al.Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide

monolayers.Nat.Nanotechnol.,2014,9(6):436-442.

[41]Reitz J B,Solomon E I.Propylene Oxidation on Copper Oxide Surfaces:Electronic and Geometric Contributions to Reactivity and

Selectivity.Journal of the American Chemical Society,1998,120(44):11467-11478.

[42]Kastner M A,Birgeneau R J,Shirane G,et al.Magnetic,transport,and optical properties of monolayer copper oxides.Reviews of

Modern Physics,1998,70(3):897-928.

Page 15: Preciselymonitoringand tailoringthe surface · 2019-07-04 · Reviews 48 Vol.25,No.4,2017 SCIENCE FOUNDATION IN CHINA Preciselymonitoringand tailoringthe surface structures

Reviews

62    Vol.25,No.4,2017  SCIENCE FOUNDATION IN CHINA

[43]Yin K,Zhang Y-Y,Zhou Y,et al.Unsupported single-atom-thick copper oxide monolayers.2DMater.,2017,4(1):011001.

[44]Ayrault G,Ehrlich G.Surface self‐diffusion on an fcc crystal:An atomic view.J.Chem.Phys.,1974,60(1):281-294.

[45]Sun J,He L,Lo Y-C,et al.Liquid-like pseudoelasticity of sub-10-nm crystalline silver particles.Nat.Mater.,2014,13(11):1007-

1012.

[46]Combe N,Jensen P,Pimpinelli A.Changing shapes in the nanoworld.Phys.Rev.Lett.,2000,85(1):110.

[47]Egerton R F,Li P,Malac M.Radiation damage in the TEM and SEM.Micron,2004,35(6):399-409.

[48]Li J,Banhart F.The Engineering of Hot Carbon Nanotubes with a Focused Electron Beam.Nano Lett.,2004,4(6):1143-1146.

[49]Banhart F,Li J X,Terrones M.Cutting single-walled carbon nanotubes with an electron beam:Evidence for atom migration inside

nanotubes.Small,2005,1(10):953-956.

[50]Fischbein M D,Drndic M.Electron beam nanosculpting of suspended graphene sheets.Appl.Phys.Lett.,2008,93(11):113107.

[51]Garaj S,Hubbard W,Reina A,et al.Graphene as a subnanometre trans-electrode membrane.Nature,2010,467(7312):190-193.

[52]Banhart F.Irradiation effects in carbon nanostructures.Rep.Prog.Phys.,1999,62(8):1181-1221.

[53]van Dorp W F,Hagen C W.A critical literature review of focused electron beam induced deposition.J.Appl.Phys.,2008,104

(8):081301.

[54]Xu T,Xie X,Yin K,et al.Controllable Atomic-Scale Sculpting and Deposition of Carbon Nanostructures on Graphene.Small,2014,10

(9):1724-1728.

[55]Xu T,Yin K,Xie X,et al.Size-Dependent Evolution of Graphene Nanopores Under Thermal Excitation.Small,2012,8(22):3422-

3426.

[56]He L,Xu T,Sun J,et al.Investment casting of carbon tubular structures.Carbon,2012,50(8):2845-2852.

[57]Lian R,Yu H,He L,et al.Sublimation of Ag nanocrystals and their wetting behaviors with graphene and carbon nanotubes.Carbon,

2016,101:368-376.

[58]Khalavka Y,Ohm C,Sun L,et al.Enhanced thermal stability of gold and silver nanorods by thin surface layers.J.Phys.Chem.C,

2007,111(35):12886-12889.

[59]Wang M S,Wang J Y,Chen Q,et al.Fabrication and Electrical and Mechanical Properties of Carbon Nanotube Interconnections.Adv.

Funct.Mater.,2005,15(11):1825-1831.

[60]Jin C,Lin F,Suenaga K,et al.Fabrication of a Freestanding Boron Nitride Single Layer and Its Defect Assignments.Phys.Rev.Lett.,

2009,102(19):195505.

[61]Meyer J C,Chuvilin A,Algara-Siller G,et al.Selective Sputtering and Atomic Resolution Imaging of Atomically Thin Boron Nitride

Membranes.Nano Lett.,2009,9(7):2683-2689.

[62]Constantinescu G,Kuc A,Heine T.Stacking in bulk and bilayer hexagonal boron nitride.Phys.Rev.Lett.,2013,111(3):036104.

[63]Liu Y,Zou X,Yakobson B I.Dislocations and grain boundaries in two-dimensional boron nitride.ACS Nano,2012,6(8):7053-7058.

[64]Xu T,Zhou Y,Tan X,et al.Creating the Smallest BN Nanotube from Bilayer h-BN.Adv.Funct.Mater.,2017,27(19):1603897.

[65]Lee Z,Jeon K-J,Dato A,et al.Direct Imaging of Soft-Hard Interfaces Enabled by Graphene.Nano Lett.,2009,9(9):3365-3369.

[66]McBride J R,Lupini A R,Schreuder M A,et al.Few-Layer Graphene as a Support Film for Transmission Electron Microscopy Imaging

of Nanoparticles.ACS Appl.Mater.Inter.,2009,1(12):2886-2892.

[67]He C,Li J,Wu X,et al.Tunable Electroluminescence in Planar Graphene/SiO2Memristors.Advanced Materials,2013,25(39):5593

-5598.

[68]Bi H,Yin K,Xie X,et al.Microscopic bimetallic actuator based on a bilayer of graphene and graphene oxide.Nanoscale,2013,5(19):

9123-9128.

[69]Deng D,Chen X,Yu L,et al.A single iron site confined in a graphene matrix for the catalytic oxidation of benzene at room temperature.

Sci.Adv.,2015,1(11):e1500462.

[70]Zhu C,Min H,Xu F,et al.Ultrafast electrochemical preparation of graphene/CoS nanosheet counter electrodes for efficient dye-

sensitized solar cells.RSC Advances,2015,5(104):85822-85830.

[71]Li S,Min H,Xu F,et al.All electrochemical fabrication of MoS2/graphene counter electrodes for efficient dye-sensitized solar cells.RSC

Advances,2016,6(41):34546-34552.

[72]Liu Y,Shen Y,Sun L,et al.Elemental superdoping of graphene and carbon nanotubes.Nature Communications,2016,7:10921.

Page 16: Preciselymonitoringand tailoringthe surface · 2019-07-04 · Reviews 48 Vol.25,No.4,2017 SCIENCE FOUNDATION IN CHINA Preciselymonitoringand tailoringthe surface structures

Reviews

SCIENCE FOUNDATION IN CHINA  Vol.25,No.4,2017  63   

[73]Bi H,Yin K,Xie X,et al.Low Temperature Casting of Graphene with High Compressive Strength.Adv.Mater.,2012,24(37):5124

-5129.

[74]Bi H,Xie X,Yin K,et al.Spongy Graphene as a Highly Efficient and Recyclable Sorbent for Oils and Organic Solvents.Adv.Funct.

Mater.,2012,22(21):4421-4425.

[75]Bi H,Xie X,Yin K,et al.Highly enhanced performance of spongy graphene as an oil sorbent.J.Mater.Chem.A,2014,2(6):1652

-1656.

[76]Wan S,Bi H,Sun L.Graphene and carbon-based nanomaterials as highly efficient adsorbents for oils and organic solvents.Nanotechnol.

Rev.,2016,5(1):3.

[77]Wan S,Bi H,Xie X,et al.A facile strategy for rapid preparation of graphene spongy balls.Sci.Rep.,2016,6:32746.

[78]Xie X,Zhou Y,Bi H,et al.Large-range Control of the Microstructures and Properties of Three-dimensional Porous Graphene.Sci.

Rep.,2013,3:2117.

[79]Shin D,Park J B,Kim Y-J,et al.Growth dynamics and gas transport mechanism of nanobubbles in graphene liquid cells.Nat.

Commun.,2015,6:6068.

[80]Zhang L,Miller B K,Crozier P A.Atomic level in situ observation of surface amorphization in anatase nanocrystals during light

irradiation in water vapor.Nano Lett.,2013,13(2):679-684.

[81]Xia W,Zhang Q,Xu F,et al.Visualizing the Electrochemical Lithiation/Delithiation Behaviors of Black Phosphorus by in Situ

Transmission Electron Microscopy.The Journal of Physical Chemistry C,2016,120(11):5861-5868.

[82]Xu F,Ge B,Chen J,et al.Scalable shear-exfoliation of high-quality phosphorene nanoflakes with reliable electrochemical cycleability in

nano batteries.2DMaterials,2016,3(2):025005.

Litao Sun

Dr.Litao Sun (孙 立 涛)is Changjiang Distinguished Professor and

serves as the head of School of Electronic Science and Engineering,

Southeast University(SEU),the deputy director of Key Lab of MEMS

of Ministry of Education,and the director of SEU-FEI Nano-Pico

Center.He received his PhD from the Shanghai Institute of Applied

Physics,Chinese Academy of Sciences in 2005.He worked as a research

fellow at the University of Mainz,Germany from 2005to 2008,and a

visiting professor at the University of Strasbourg,France from 2009to

2010.Since 2008,he joined SEU and honored as a Distinguished

Professor.Currently,his research interests focus on:(1)Dynamic in-situ experimentation in the

electron microscope;(2)novel properties from sub-10nm materials;(3)applications of graphene-

related nanomaterials in environment,renewable energy and nanoelectromechanical systems.He is the

author and co-author of around 150papers in international journals including 2in Science,12in Nature

and Natureseries journals,etc.He holds around 70patents and has given more than 60invited talks at

international conferences.He is the founding chairman of IEEE Nanotechnology Council Nanjing

Chapter,the Review Panel member of Graphene Flagship,European Union and Member of European

Science Foundation College of Expert Reviewers.He has obtained the National Science Fund for

Distinguished Young Scholars of China,New Century Excellent Talents in University,Young Leading

Talent in Science and Technology Innovation,etc.