Scanning cathodoluminescence microscopy: applications in semiconductor and metallic nanostructures

Transcription

1 Opto-Eletroni Advnes Review 218, Vol. 1, No. 4 DOI: /oe Snning thodoluminesene mirosopy: pplitions in semiondutor nd metlli nnostrutures Zhixin Liu, Meiling Jing, Ynglin Hu, Feng Lin, Bo Shen, Xing Zhu nd Zheyu Fng* Cthodoluminesene (CL) s rditive light produed y n eletron em exiting luminesent mteril, hs een widely used in imging nd spetrosopi detetion of semiondutor, minerl nd iologil smples with n ultrhigh sptil resolution. Conventionl CL spetrosopy shows n exellent performne in hrteriztion of trditionl mteril luminesene, suh s sptil omposition vritions nd fluoresent displys. With the development of nnotehnology, dvnes of modern mirosopy enle CL tehnique to otin deep vlule insight of the testing smple, nd further extend its pplitions in the mteril siene, espeilly for opto-eletroni investigtions t nnosle. In this rtile, we review the study of CL mirosopy pplied in semiondutor nnostrutures for the dislotion, rrier diffusion, nd struture, doping level nd exiton reomintion. Then dvntges of CL in reveling nd mnipulting surfe plsmon resonnes of metlli nnontenns re disussed. Finlly, the hllenge of CL tehnology is summrized, nd potentil CL pplitions for the future opto-eletroni study re proposed. Keywords: thodoluminesene; mirosopy; semiondutor; metlli nnostrutures; surfe plsmons Liu Z X, Jing M L, Hu Y L, Lin F, Shen B et l. Snning thodoluminesene mirosopy: pplitions in semiondutor nd metlli nnostrutures. Opto-Eletroni Advnes 1, 187 (218). Introdution Cthodoluminesene (CL) s n optil nd eletromgneti phenomenon referring to the rdition in form of fluoresene 1, ws first disovered in the mid-nineteenth entury from thode eletron rys hitting glss sustrte. When inident eletrons intert with luminesent mtter, mteril properties mke differene in CL spetr. As the development of nnotehnology nd nnosiene, the demnd for high-resolution mirosopy nd hrteriztion tehniques hs tremendously inresed. In ddition to trditionl optil mesurements, the snning ner-field optil mirosopy (SNOM) presents higher sptil resolution 2, ut suffers from n unvoidle intertion etween the snning proe nd the mteril surfe. CL mirosopy, on the other side, providing lel-free optil imging tehnique with deep suwvelength resolution, hs hieved signifint interests for the minerlogy 3 5, semiondutor physis 6 8, nd mny other reserh fields like the trking of ellulr proesses in iology 9,1. CL ws lso found extensive pplitions s the emission soure in thode-ry-tue omputer monitors nd televisions for the industry. CL spetrosopy is usully operted in snning eletron mirosopy (SEM) 11, lthough it is lso possile to perform mesurements in trnsmission eletron mirosopy (TEM) nd snning trnsmission eletron mirosopy (STEM) 12. When the foused eletron em psses through the perture hole of the proli mirror nd hits on the smple surfe, CL generted from the luminesent mteril n e refleted y the mirror nd olleted y photomultiplier tue (PMT) or hrge-oupled devie (CCD) where CL imges n e otined. Another se is tht CL is diretly reeived y CCD insted of onverging through lens. The system n interpret the Shool of Physis, Stte Key L for Mesosopi Physis, Ademy for Advned Interdisiplinry Studies, Collortive Innovtion Center of Quntum Mtter, Peking University, Beijing 1871, Chin These uthors ontriuted eqully to this work. *Correspondene: Z Y Fng, E-mil: zhyfng@pku.edu.n Reeived 17 April 218; epted 24 June 218; epted rtile preview online 29 June

2 rdition ngle distriution of the smple ording to the CCD imge, sine the light signl reeived t different positions on the CCD reflets the light informtion rdited from the smple in different diretions. Therefore, ngle-resolved CL mesurement n e relized. In ddition, the system supports the use of n optil fier to derive the CL signl nd ouple it to n externl spetrometer (Fig. 1). The spetrometer is used to seprte CL emissions in different wvelength rnges nd enefit the spetrl nlysis. The minerl omposition 13, nd struture nd impurity defets of the mteril n e effetively inferred y the oserved CL intensity, wvelength rnge nd the degree of light polriztion. 3 kev e - SEM Proli mirror Smple Lens Flip mirror Fier Filter hnges from 5 to 19 kv in ZnO nnoullet 21. The ultrhigh sptil resolution pilities of CL mirosopy n e used to nlyze the distriution of surfe dislotions, the property of mteril nd strutures, nd the optil emission of quntum dots Reently, CL mirosopy hs een lso suessfully pplied for two-dimensionl (2D) mterils 24, where the investigtion of exiton oupling ws relized with higher optil resolution. On the other hnd, trnsition rdition hppens when n eletron psses through oundry etween two medi with different dieletri onstnts. It is reted y the time dependent vrition nd eventul ollpse of the dipole moment tht generted y the inident eletron nd its imge hrge in the dieletri. This effet ws predited nd oserved for metls. Surfe plsmons (SPs), s the olletive osilltion of surfe eletrons t the interfe etween dieletri nd metl, hve ttrted huge ttentions for pplitions like super-imging 28, nnofousing 29, wveguiding 3 33, nd light hrvesting 34. With the development of metlli nnostrutures sed plsmonis, CL hs een widely pplied for the SPs hrteriztion, suh s the light emission from the metlli grting struture nd nnoprtile 38 4 under the eletron exittion. The emitted fluoresene during the plsmon oupling n e pplied to the imging of the resonne mode 41,42 nd further relize the ngle-resolved spetrosopy 43,44. CL mirosopy s n effetive hrteriztion method, n reflet the distriution of rditive lol density of sttes (LDOS), whih diretly determine the light-mtter intertion. With properly designing metlli nnostrutures, it is possile to ontrol nd mnipulte the light emission t deep su-wvelength sle for future informtion nd quntum studies. In ddition, oth eletron energy loss spetrosopy (EELS) nd photoemission eletron mirosopy (PEEM) hve extrordinry performne on high-resolution hrteriztion nd provide distintive diretions for investigting plsmons. EELS is n nlytil method for otining the physil nd hemil informtion of surfe toms y the loss of energy due to inelsti sttering of eletron ems inident on the surfe of the smple. It n mesure drk plsmon modes nd hrterize dipoles nd high-order modes 45,46. The omintion of CL mirosopy nd EELS to mesure drk plsmon modes n ompenste for the inility of EELS to distinguish plsmon rdition modes from non-rditive modes. Bsed on the photoeletri effet, PEEM funtions on n eletron mirosope tht imges the photoeletron emission distriution of smple surfe, otins ner-field mpping nd dynmi properties of plsmoni nnostrutures 47,48. In this review, we fous on CL investigtions for semiondutors nd metlli nnostrutures, respetively. In ddition, CL mirosopy t low-temperture, nd with ngle-resolved, time-resolved tehniques re disussed for the modern multi-funtionl high-resolution hr- Spetrometer CCD Fig. 1 Shemti overview of CL imging spetrometer with spetrl nlysis funtion. Figure reprodued from ref. 11, CUP Pulishing. Cherenkov nd trnsition rditions re two of min diret emission proesses tht involved in CL, whih re oherent with the externl field of the inoming eletron, s ll of the eletromgneti fields re desried y the sme set of Mxwell s equtions. Inoherent emission is generlly ssoited with eletron-hole reomintion in semiondutors whih is stronger nd does not interfere with oherent rdition. Generlly, these two rdition mehnisms n e distinguished y their hrteristi ngle profiles, dipolr-like loes for trnsition rdition nd lmertin ngulr distriution for inoherent luminesene 7. Cherenkov rdition is minly oserved in semiondutors, when n eletron pssing through trnsprent smple with its speed fster thn the phse veloity of light in tht medium. Trnsition rdition ours during the reonstrution of the Coulom field, when the point hrge of uniform liner motion moves in non-uniform ondition suh s medium interfe. For semiondutors, CL mirosopy possesses the ility of deep penetrtion depth nd ultrhigh sptil resolution. For exmple, in one-dimensionl CdS nnostruture, the penetrtion depth of the injeted eletrons my reh hundreds of nnometers or even severl mirometers under n elerting voltge of 1 kv, nd nnosle resolution is hieved in CL mpping 2. The optil ehvior of different depths of mteril n e deteted t vrious elerting voltges of the eletron proe, for exmple, CL intensity inreses s the pplied voltge CCD 187-2

3 teriztion. In onlusion, hllenges nd perspetives for the future CL spetrosopy pplitions re presented CL mirosopy for semiondutors Chrteriztion of lolized luminesene properties in semiondutors is importnt for the investigtion of semiondutor physis nd the development of opto-eletroni devies. CL mirosopy with its unique fetures of lrge snning re nd short totl mesuring time, n e used to study ny point on smple with n eletron em. Besides, the high intensity of the inident eletron em redues the exposure time, mking CL mirosopy suitle to explore the internl struture nd interfe dynmis of the mtter. Luminesene from semiondutor is indued y the eletron-hole pir reomintion tht n e generted y light or the eletron em exittion. For eletron exittion, primry eletrons n exite plsmons, vlene eletrons nd inner shell eletrons y losing energy. Eh of these proesses ontriutes to the genertion of eletron-hole pirs. Plsmons n dey into exitons, exittion of vlene eletrons n produe eletron-hole pirs, ut the exittion of seondry eletrons is the min soure of eletron-hole pirs 49. With sde proess, the seondry eletrons whih hve kineti energies of 5 1 ev, n e exited nd diffused in spheril region with depth of few hundred nnometers for generl semiondutor under n eletron em of severl kev. The eletron-hole pir generted y seondry eletrons reomines nd further emits the luminesene, thus the sptil resolution of CL mirosopy is t the level of the diffusion depth of seondry eletrons. CL mirosopy hs een suessfully used to hrterize the morphology of semiondutors, nd other insightful mteril physis like the sptil distriution of threding dislotions 5,51, length of rrier diffusions 52,53, nd doping level of the eletri nd struture 54, whih gives full desription of the smple. Dislotions s the mirosopi defet of rystlline semiondutors, re the lol irregulrity within rystl struture. The presene of dislotions strongly influenes mny mteril properties. For exmple, the defet of GP surfe ws hrterized y the CL mirosopy in deep su-wvelength sptil resolution, where the CL imge shows lk dots t the lotion of dislotions 5. These lk dots were nlyzed s non-rditive reomintion enters for free rriers in the semiondutor, nd this non-rditive property ws used y the dislotion of the smple. Experimentl results further show tht different types of dislotions suh s srews, edges nd mixed dislotions n result CL spots with similr size nd intensity. From the CL mesurement, the dopnt nd the orienttion of the rystl, dislotions grown in the epitxil semiondutor re ll non-rditive reomintion enters. But in piezoeletri rystls suh s GN, uniform strin produes piezoeletri polriztion, piezoeletri fields n dissoite exitons nd seprte the eletron hole pirs 55. This suggests tht the rrier reomintion my e inherent in the dislotion, nd independent with the impurity nd other point defets in the mteril. The em urrent n ffet the degree of non-rditive reomintion tivities in dislotions, nd lrge em urrent orresponds to the tive non-rditive reomintion stte s well s the lrge drk ontrst round dislotions in CL imges 56 (Fig. 2). Figures 2d 2f shows signifint drop in luminous effiieny due to urrent inrese t lower mgnifition. This theory shows tht the derese in luminous effiieny of the white light emitting diodes (LED) n e effetively redued y signifintly reduing the dislotion density of the mteril. SEM PCL I=.11 na PCL I=3.73 na 5 μm 5 μm 5 μm SEM d PCL I=.11 na e PCL I=3.73 na f 1 μm 1 μm 1 μm Fig. 2 () SEM imge of the GN/InGN multiple quntum well surfe. (,) CL imge of the sme re of () t eletron em urrents of.11 na nd 3.73 na, respetively. (d f) The sme imges t lower mgnifition lerly demonstrte the deline in luminous effiieny. Figure reprodued from ref. 56, AIP Pulishing. The rrier diffusion length L d is nother importnt ftor for the semiondutor physis nd light-emitting devie pplition. CL mirosopy provides n effiient wy to determine the rrier nd free exiton diffusion length in semiondutors 57. The shemti of the mesurement is shown in Fig. 3, nd the eletron em exites exess rriers of the smple through metl msk whih is opque to the CL emission. Thus, the CL signl generted y the rrier reomintion n only e otined outside of the metl msk, nd the diffusion length / d of exited rriers n e lulted y ICL = I e x L, where I CL nd I represent the CL intensity mesured t 58 point x nd mesured t distne L d, respetively. x is defined s the distne etween the exittion point nd the edge of the metl msk. Figure 3 shows tht t low temperture, euse of the tunneling-ssisted trnsport effet in onjuntion with lolized nd-til sttes, the rrier diffusion length of the InG quntum wells strongly depends on the eletron-em urrent i. The diffusion length inreses with the enhnement of the exittion eletron energy, where the growth of the

4 Metl msk Eletron x I CL =I exp(-x/l d ) CL detetion Ld (μm) 1.5 Exittion Detetion (i) (ii) Cross-setion sheme SQW 1. T=5 K E=.973 ev 1 1 i (na) Fig. 3 () Shemti of CL method for the mesurement of L d. () L d s funtion of the eletron-em urrent i t 5 K. Inset: Shemti of the rrier trnsport in onjuntion with lolized sttes for low (i) nd high (ii) densities of lolized sttes. Figure reprodued from ref. 57, John Wiley & Son. exited stte density enlrges the tunneling proility. Thus, it is essentil to study the trnsmission mehnism of high lol density semiondutors y using CL. Energy nd trnsition is vitl physil proess of semiondutor luminesene. CL tehnology, s mens of deep suwvelength sptil resolution, n hrterize luminesent properties of nnosle semiondutor strutures nd nlyze the energy nd trnsition proess. For exmple, CL mirosopy n urtely distinguish luminesent properties t different positions of single AlGN nnoolumn ontining series of GN quntum disks t the top of the olumn 59. The SEM imges nd CL imges of the smple re shown in Fig. 4. CL imges ontin three emission sttes orresponding to three detetion energies in Fig. 4, solely. Luminesene spetr of the quntum disk region nd the AlGN region re lso plotted in Fig. 4, where the rodening of the CL1 pek with respet to the CL2 pek is minly due to the lterl strin distriution, hnge of the nd-filled, inhomogeneous eletri-field nd vritions mong the quntum disks. Therefore, using CL mirosopy to study energy nd trnsitions of nnosle semiondutor strutures ontining omplex omponents is of gret signifine. Doping level is onerned physil prmeter of the semiondutor, whih ffets its hrteristi prmeters suh s the foridden nd width, resistivity, rrier moility, nd unlned rrier lifetime. The CL tehnique provides n effetive mens for mesuring the lol rrier onentrtion t the nnosle resolution. For exmple, the doping level of silion-doped GAs semiondutor nnowires t nnometer sle hs een mesured y CL spetrosopy 54 (Fig. 5). Bsed on generlized Plnk's lw, Disks AlGN GN CL1 CL2 CL3 2 nm SE CL intensity (ounts/1) x6 CL3 CL1 CL Detetion energy (ev) Fig. 4 () Shemti, CL imges nd SEM imge of the single nnoolumn. () CL spetr of two single GN/AlGN nnoolumns. Figure reprodued from ref. 59, AIP Pulishing. E-em Luminesene d 1 NW CL intensity (. u.) Deep level fit 5 nm Photon energy (ev) Fig. 5 () Shemti of the eletron em intertion with nnowire nd luminesene emission. () SEM imge of single nnowire. () CL imge of single GAs nnowire mesured t 3 K. (d) CL spetrum from nnowire t room temperture nd the fit (red urve). Figure reprodued from ref. 54, ACS Pulitions

5 fitting model ws demonstrted to fit the CL spetrum of nnowires, in whih Fermi energy ould e distinguished. Experimentl results indite tht the rrier onentrtion lirted y Hll mesurement is onsistent with the one extrted from the CL spetrum (Fig. 5d). Furthermore, the influene of temperture nd externl field strength on the rrier onentrtion n lso e nlyzed ording to CL imges. This theory shows tht CL mirosopy provides n effetive nd high-resolution method for deteting the rrier onentrtion of semiondutor nnowires, nd n lso e pplied to other semiondutor nnostrutures with definite nd strutures. The exiton effet hs n essentil influene on the physil proesses nd optil properties in semiondutors The sorption nd reomintion of exitons diretly ffet the light sorption nd luminesene of semiondutors. In 2D semiondutor mterils 63 66, due to their low dimensionlity nd surfe defets, the sptil resolution must e redued to the nnometer sle in order to study exiton reomintion nd optil properties. However, the trditionl photoluminesene is restrited t su-nnosle due to the diffrtion limit of inident light. CL mirosopy is promising method for studying the exiton effet in 2D semiondutor mterils. For 2D mteril systems, the eletron em n only stimulte eletroni trnsitions in tens of nnometers rnge resulting tht olleted CL signls re wek. This prolem n e effetively solved, for exmple, y using n hbn/wse 2 /hbn vn der wls heterostruture 67 (Fig. 6). The luminesent WSe 2 lyer is sndwihed etween hbn lyers with higher energy ndgps. The diffusion length of eletrons nd holes in hbn lyer is in the mironmeter rnge nd the thikness of the heterojuntion lyer is tenths of mironmeters. The eletron n reh the WSe 2 lyer efore reomintion to hieve eletron injetion (Fig. 6). Optil mirosope imges nd monohromti CL imges of the heterostruture re shown in Figs. 6 nd 6d, from whih the CL signl enhnement n e lerly oserved. The sme phenomenon is lso oserved in MoS 2 nd WS 2 lyers. Enhned CL spetrosopy n e used to nlyze strin-indued exiton pek shift in 2D semiondutor mterils with this struture. This report pves the wy for studying exiton reomintion in the 2D mteril y CL mirosopy. Heterostrutures my hve potentil pplitions in high-energy prtile detetors, field emission disply tehnologies nd trnsmission eletron mirosope displys. CL mirosopy for metlli nnostrutures As the development of modern physis nd nnofrition tehniques, plsmonis hs mtured signifintly in the pst few dedes. The olletive exittion of free eletrons in metlli nnostrutures, known s SPs, hs een demonstrted to show fsinting optil properties like light loliztion, field enhnement nd the improvement of LDOS, whih gretly enefits the development of nnophotonis nd finds pplitions in mny res inluding io-sensing 68, light hrvesting nd optil ommunitions 69. As the optil properties of metlli nnostrutures ruilly depend on the exploittion of plsmoni resonnes, numerous light tehnologies nd devies require preise knowledge of eletromgneti mode distriutions, whih diretly determine the effiieny of light-mtter intertions. Reently, CL miros- h-bn Eletron em h-bn Eletron loud WSe 2 Hot e-h pirs reted y multiple sttering of free eletrons in the loud h-bn h-bn Luminesene Condution nd Energy Vlene nd WSe 2 Optil imge top hbn d Cthodoluminesene 8 WSe μm 5 μm Fig. 6 () Shemti of hbn/wse 2 /hbn vn der wls heterojuntion exited y eletron em. () Proess of the genertion, diffusion, nd reomintion of eletron hole (e-h) pirs in heterojuntion. () Optil mirosope imge of hbn/ WSe 2 /hbn heterostruture. (d) Monohromti CL imge of the heterostruture t WSe 2 emission energy (1.66 ev). Figure reprodued from ref. 67, ACS Pulitions

6 opy with nnosle resolution ws suessfully used in the investigtion of plsmoni modes in vrious nnostrutures suh s metlli nnontenns 7 nd photoni rystls 71. Under the eletron em stimultion, the CL signl n e generted from metlli nnostrutures with the exittion of plsmoni modes, whih is dominted y eletron-indued rdition emission (EIRE). In this se, the indued eletromgneti field is oherent with the externl evnesent field reted y moving eletrons. However, the rditive inoherent dey in metlli nnostrutures only hs minor ontriution to CL emissions, euse the oherent eletroni relxtion is severl orders of mgnitude fster. The snning eletron em funtions s liner urrent soure, enling flexile mode exittion nd ontrol, nd plsmoni mode distriutions n e simultneously proed with CL emissions. Metlli nnostrutures lso provide verstile pltform for the mnipultion of light-mtter intertions. By utilizing the CL mirosopy, the rditive LDOS distriution tht governs the rditive spontneous dey of quntum emitters, n e proed with deep-suwvelength resolution, filitting the investigtion of quntum optis. Moreover, vrious physil systems nd devies n e exploited nd mnipulted t deep-suwvelength sle y using the CL mirosopy, whih n e used to find unreveled physil phenomen suh s the hidden hirlity in hirl nnontenns. With the polriztion-resolved nd ngle-resolved CL mirosopy, the polriztion stte nd rdition informtion in momentum spe n e otined, promoting the design of nnophotoni light emission devies. By exploiting the time-resolved funtion, the ultr-fst dynmis of quntum emission n e further quired, whih enefits the knowledge of underlying quntum physil priniples. Resolving the stnding wve mode of single nnontenn is fundmentl to further plsmoni reserh. Conventionlly, optil exittion only wekly produes even-order modes under the off-norml exittion ngle s illustrted in sttering spetrum 42 (Fig. 7). By ontrst, in CL spetrosopy, there re oth odd nd even-order modes indited s l=3, nd l=4 (ottom spetr of Figs. 7 nd 7). Beuse eletrons possess higher energy thn light, higher-order modes re llowed to exist. As Figs. 7 nd 7 show, CL emission intensity is proportionl to the rditive LDOS. Furthermore, these two modes n e seprted y dding ndpss filter in the olletion optil pth, whih is esy to e hieved. The oupling struture of nnowires nd nnontenns (Fig. 7d) ws disussed s well. Compring two kinds of strutures, it is shown tht for the symmetri mode, the totl oupling effiieny is low nd only the hrteristis of its hyrid mode re preserved (Fig. 7e). It is possile to djust the oupling effiieny of the hyrid mode y djusting two rms of the nnowire, therey ffeting l=1 E-em position (nm) E-em position (nm) CL emission/sttering (.u.) l=2 l=3 l= Wvelength (μm) l=4 l=3 l=2 Integrted CL intensity (.u.) 65 nm 75 nm λ sp /2 l=4 l=3 d Wvelength (μm) Symmetri Asymmetri x E-em position (nm) e E-em position (nm) 85 nm 85 nm 1 nm 1 nm Fig. 7 () FDTD (red) simulted, drk-field (lk) sttering nd CL emission spetrum of Au ntenn indite the stnding wve eigenmodes. CL imges of nnontenn with ndpss filter t () 65 nm nd () 75 nm. Integrted CL intensity shows the stnding wve plsmon resonnes. (d) SEM imges of symmetri nd symmetri oupling struture of nnowire nd nnontenns. (e) CL imges of two oupling strutures with ndpss filter t 85 nm. Figure reprodued from ref. 42, ACS Pulitions

7 E-em CL intensity (.u.) LCP RCP CL hirlity (%) -2 Mx CL intensity Au heptmer Impinging positions 3 nm CL intensity (.u.) CL intensity (.u.) LCP RCP LCP RCP Wvelength (nm) CL hirlity (%) CL hirlity (%) Fig. 8 () Shemti of the eletron stimulted heptmer nnostruture. () Left: pnhromti CL imge of Au heptmer nnostrutures. Right: prt of SEM imge of Au heptmer. () CL spetr mesured from impinging positions s indited in (). Figure reprodued from ref. 72, ACS Pulitions. the property of the nnowire plsmon. The ner-field oupling revels the influene of plsmon hyridiztion on the eletron density of sttes of nnowire luminesene, the strong oupling etween two strutures here is due to the oinidene of the lol plsmoni energy of the nnowire nd the energy of the djent dipole of the nnontenn. The CL tehnique enles the hrteriztion of oupling properties t nnosle, onfirming the position ording to SEM while performing LDOS nlysis t ny position. The hirl light emission of nnontenns is ruil to nnophotonis 73. CL tehnology provides fvorle pltform for the tuning of hirl t su-nnosle. Figure 8 shows n exmple of Au heptmer to produe gint hirl CL response under eletron em exittion 72. The orresponding hirl CL emission n e otined y exiting different positions mrked s i iii (Fig. 8) of the heptmer. Right-hnded irulrly polrized (RCP) nd left-hnded irulrly polrized (LCP) CL ontriutions of the whole struture stimulted t different impinging positions hve een oserved. From mesured CL spetr, the intensity of LCP CL olleted under position i is lrger thn RCP CL. On the other side, opposite irulr polriztion emission sttes n e otined y stimulting positions ii nd iii (Fig. 8). Furthermore, during the movement of the eletron em from point ii to point i, the swith of hirlity ours within 1.86 nm. Aording to this phenomenon, the ternry nottion informtion oding ws demonstrted t su-nnosle. This report opens the gte to further expnsions of CL tehnology in quntum ommunition. Exept for resolving nd mnipultion, some phenomen hrdly visile under optil exittion, like hidden hirlity in hirl nnostrutures, were disovered Due to the interferene etween symmetri nd ntisymmetri modes of the struture, the generted hirlity of symmetril Al nnontenn stimulted y the norml inident light, is hidden in the ner field. Trditionl optil hrteriztion method is diffiult to hrterize the hirlity effet in the ner-field of the nnostruture. However, using irulrly polrized resolved CL mirosopy n revel the hidden hirlity of LCP-CL Eletron em Al strutures 1 LCP-CL 2 nm 8 45 Fig. 9 () Shemti of irulr polriztion resolved Al nnontenn with eletron em exittion. () LCP CL imge of single nnostruture with ndpss of 657 nm. () LCP CL imge for the heliity-dependent 2D disply of pitl letters CL. Figure reprodued from ref. 74, ACS Pulitions. 1 CL intensity (.u.)

8 the single symmetril Al nnontenn in the ner-field 74 (Fig. 9). The mesured LCP or RCP CL, generted y stimulting rm-ends (left-rm or right-rm) with eletron em, n reflet the hirl rditive LDOS distriution. Therefore, the hnge of eletron em exittion position llows tive swithing of CL heliity t su-nnosle. Figure 9 shows tht light nd shde of rm-ends orrespond to or 1 known s inry enoding. In ddition, these nnontenn units n e used to design heliity-dependent oding rrys to hieve 2D disply, s shown in Fig. 9. This demonstrtion is expeted to enourge CL mirosopy pplied in quntum informtion field. The optil nnontenn medites the optil oupling etween emitters nd the fr field for etter light emission nd reeption. Exploring the response of the nnontenn requires urte positioning of the su-wvelength sle trnsmitter with known orienttion. Sine the size of the nnontenn is muh smller thn the wvelength of light. The use of high-energy eletron em s the exittion soure to study the emission hrteristis of the Ygi-Ud ntenn omposed of Au nnoprtile rrys hs een proposed 75. It is inferred tht the emission diretion of nnontenns depends on the ner-field nd fr-field intertion of plsmoni resonne y olleting the ngulr resolution spetrum of nnontenns t different wvelengths. When the leftmost prtile is exited y the eletron em, most of the CL is olleted in loe pointing to the right of the ntenn (Fig. 1). If tking the lue line in Fig. 1 s ross-ut, the intensity nd rnge size of left nd right loe n e otined s shown in Fig. 1d. Furthermore, y moving the exittion point to detet the diretion of nnontenns t different wvelengths, the results show tht emission hrteristis vry with different wvelengths. The experimentl result indites tht Au nnoprtiles n e regrded s vertil dipoles perpendiulr to the sustrte when they re exited y eletron ems, whih well explins the rdition pttern used y the olletive interferene of light rdited y five oherently oupled dipole moments in nnontenns. In onlusion, s high-resolution tehnique, the ngle-resolved CL spetrosopy n e effetively used to hrterize the emission diretivity of simple nd even omposite strutures. The rditive lifetime of nnontenn is lso onerned for plsmoni reserh. The introdution of ultrfst pump detetion tehnology to the CL mirosopy tehnology enles the dynmi proess detetion of quntum phenomen suh s hrge trnsfer, quntum tunneling, simultneously on the nnometer sle nd the femtoseond time sle. The proli mirror is now used s ondenser to fous the femtoseond lser onto the smple nd pump the eletrons in the smple to the exited stte. At the sme time, using the femtoseond eletron em pulse generted y the sme femtoseond lser s proe to detet the smple, the relxtion of eletrons in the smple n e otined y ontrolling the time dely of the pump light nd the detetion eletron em. Time-resolved CL tehnology n reord spetrl hnges t different time, show the instntneous stte nd mke up for the defet of integrl spetrum. For exmple, time-resolved tehniques provide powerful mens of studying effets of plting Au or Al lyer on the surfe of GAs/AlAs/GAs nnowires 76. In this struture, high energy eletron em irrdition n generte exessive eletron-hole pirs in the semiondutor nnostruture, exitons derived from the eletron-hole pirs re oupled with SPs of the nnontenn. In this proess, time-resolved tehnology n diretly mesure the instntneous rrier lifetime whih hnges with temperture (Fig. 11). Time-resolved tehnology is more rel-time thn the physil informtion refleted y the ordinry spetrum, whih is n effetive wy to nlyze e - θ= θ= = d = 1 θ= = Fig. 1 () Shemti of the exittion geometry. () CL emission intensity s funtion of ngle. () 3D representtion of theoretil rdition pttern for this exittion position nd wvelength, together with projetion onto 2D grid. (d) Cross-ut through the ngulr dt showing CL intensity s funtion of θ (lue urve) together with theory (lk dshed urve). Figure reprodued from ref. 75, ACS Pulitions

9 8 7 6 GAs/AlAs/GAs Core-shell nnowire Bre Al oting Au oting GAs/AlAs/GAs Core-shell nnowire Bre Al oting Au oting τ (ns) 5 4 E =15 kev I =3 pa τr (ns) 1 E =15 kev I =3 pa Temperture (K) Temperture (K) Fig. 11 Mesured rrier dey times τ(t) in () nd rditive lifetime τ R (T) in () with time-resolved CL spetrosopy. Figure reprodued from ref. 76, ACS Pulitions. the instntneous light-emitting stte of CL. Conlusions nd outlook In this pper, we minly introdue dvntges nd pplitions of CL s hrteriztion tool in different mterils system nd physil perspetives. Bsed on the CL priniple, some vlule informtion in the smple n e otined, suh s energy sttes of eletrons nd holes, plsmon ehvior, rrier diffusion, exiton reomintion kinetis. In the erly development of semiondutors, CL tehnology n provide powerful support for the mesurement of struturl dislotions, rrier diffusion length, nd struture nd doping level in nnowires. With the flourishing development of miro-nno photonis nd plsmonis, CL mirosopy plys n essentil role in the study of metlli nnostrutures. It provides tehnil gurntee for the visuliztion nd the nlysis of plsmon resonne modes. In ddition to mesuring pek wvelength nd intensity-relted informtion out the luminesene of mteril, CL spetrosopy n lso e used to otin sptil distriution, polriztion, time-domin informtion nd ngulr distriution in omintion with time-resolved nd ngle-resolved tehniques. Despite tremendous progress of CL mirosopy, there re still mny unresolved prolems, suh s wek CL signls nd poor olletion effiieny. Aording to orreltive reports, utilizing dditionl fluoresene lyer nd system optimiztion n inrese the CL intensity nd improve the olletion effiieny. With the inresing integrtion of equipments nd smples, CL mirosopy hs ontinuously rodened its sope of pplitions, espeilly for semiondutors nd metlli nnostrutures. As n ultrhigh resolution imging nd nlysis tool, CL mirosopy provides diret guidelines for studying the physil mehnism of plsmonis in nnostrutures. This n eventully promote prtil pplitions in future optil iruits, quntum emitters nd informtion fields. Referenes 1. Coenen T, Hegel N M. Cthodoluminesene for the 21st entury: Lerning more from light. Appl Phys Rev 4, 3113 (217). 2. Pohl D W, Fisher U C, Dürig U T. Snning ner-field optil mirosopy (Snom). J Miros 152, (1988). 3. Shieer J, Krinsley D, Riiputi L. Digeneti origin of qurtz silt in mudstones nd implitions for sili yling. Nture 46, (2). 4. Prtesi G, Giudie A L, Vishnevsky S, Mnfredotti C, Ciprini C. Cthodoluminesene investigtions on the Popigi, Ries, nd Lppjrvi impt dimonds. Am Minerl 88, (23). 5. Pennyook S J. Investigting the optil properties of dislotions y snning trnsmission eletron mirosopy. Snning 3, (28). 6. Wtne K, Ngt T, Wkym Y, Sekiguhi T, Erdélyi R et l. Bnd-Gp Deformtion Potentil nd Elstiity Limit of Semiondutor Free-Stnding Nnorods Chrterized in Situ y Snning Eletron Mirosope-Cthodoluminesene Nnospetrosopy. ACS Nno 9, (215). 7. Brenny B J M, Coenen T, Polmn A. Quntifying oherent nd inoherent thodoluminesene in semiondutors nd metls. J Appl Phys 115, (214). 8. Storm K, Hlvrdsson F, Heurlin M, Lindgren D, Gustfsson A et l. Sptilly resolved Hll effet mesurement in single semiondutor nnowire. Nt Nnotehnol 7, (212). 9. Niiok H, Furukw T, Ihimiy M, Ashid M, Arki T et l. Multiolor Cthodoluminesene Mirosopy for Biologil Imging with Nnophosphors. Appl Phys Express 4, (211). 1. Brnett W A, Wise M L H, Jones E C. Cthodoluminesene of iologil moleules, mromoleules nd ells. J Miros 15, (1975). 11. Coenen T, Brenny B J M, Vesseur E J, Polmn A. Cthodoluminesene mirosopy: Optil imging nd spetrosopy with deep-suwvelength resolution. MRS Bull 4, (215). 12. Koik M, Zgonel L F. Cthodoluminesene in the snning trnsmission eletron mirosope. Ultrmirosopy 176, (217). 13. Gotze J. Potentil of thodoluminesene (CL) mirosopy nd spetrosopy for the nlysis of minerls nd mterils. Anl Bionl Chem 374, (22)

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