Quantitative imaging of anion exchange kinetics inhalide perovskitesYe Zhanga,b,1, Dylan Lua,b,1, Mengyu Gaob,c, Minliang Laia, Jia Lina, Teng Leia, Zhenni Linb,c, Li Na Quana,b,and Peidong Yanga,b,c,d,2

aDepartment of Chemistry, University of California, Berkeley, CA 94720; bMaterials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA94720; cDepartment of Materials Science and Engineering, University of California, Berkeley, CA 94720; and dKavli Energy Nano Science Institute, Berkeley,CA 94720

Contributed by Peidong Yang, May 6, 2019 (sent for review February 27, 2019; reviewed by Maksym Kovalenko and Nathan R. Neale)

Ion exchange, as a postsynthetic transformation strategy, offersmore flexibilities in controlling material compositions and struc-tures beyond direct synthetic methodology. Observation of suchtransformation kinetics on the single-particle level with rich spatialand spectroscopic information has never been achieved. We reportthe quantitative imaging of anion exchange kinetics in individualsingle-crystalline halide perovskite nanoplates using confocalphotoluminescence microscopy. We have systematically observed asymmetrical anion exchange pathway on the nanoplates with depen-dence on reaction time and plate thickness, which is governed by thecrystal structure and the diffusion-limited transformation mechanism.Based on a reaction–diffusion model, the halide diffusion coefficientwas estimated to be on the order of 10−14cm2 · s−1. This diffusion-controlled mechanism leads to the formation of 2D perovskite hetero-structures with spatially resolved coherent interface through the pre-cisely controlled anion exchange reaction, offering a design protocolfor tailoring functionalities of semiconductors at the nano-/microscale.

halide perovskites | anion exchange and diffusion | reaction kinetics |quantitative imaging | 2D heterostructures

The potential of chemical transformations has been extensivelyexploited in modifying the structures, compositions, and mor-

phologies of materials to discover novel properties for varioususeful applications (1). Ion exchange is one such transformationthat provides an additional pathway to achieve controllability inmaterials engineering which is potentially limited by direct syn-thetic routes (2–4). Significant efforts on utilizing partial cationexchange on metal chalcogenides in colloidal nanoscience as wellas in solid-state chemistry have enabled the design and productionof a wide range of crystalline nanomaterials with various structuralcomplexities, such as segmented superlattice (5) or core–shellheterostructure (6) for efficient light-emitting or photovoltaic ap-plications. Recently, a rapid exchange behavior on anion sites ofhalide perovskites has been reported (7–11). These studies havedemonstrated the versatility of anion exchange in realizing thebandgap tunability and accessing otherwise metastable phases (12).Along with these phenomenological investigations on ion ex-

change for expanding the library of materials, mechanistic insightinto this transformation is also of great interest and fundamentalimportance. Work on studying the ion exchange kinetics has beenpredominantly carried out for cation exchange in metal chalco-genides nanocrystals (NCs) (4, 13, 14). By contrast, there have beenlimited efforts on studying the analogous anion exchange beforethe recent emergence of halide perovskites as promising opto-electronic materials (15–19). It is thus of our interest to understandthe anion exchange mechanism which would provide guidelines fortailoring the material functionalities. On the other hand, kineticalanalysis based on a spatially resolved transformation process insingle particle has never been achieved due to the limitations im-posed by imaging techniques and material dimensions. Conven-tional methods for probing cation exchange in those NCs typicallyrely on transmission electron microscopy (TEM) and TEM-basedenergy-dispersive X-ray spectroscopy (5, 20–22). These techniques,

although helpful for resolving the structural or compositionalchanges in single NCs of different exchange extents (23, 24), poseinconvenience due to the operation under vacuum. Moreover, theelectron beam would be destructive to exposed materials (25) andeven triggers additional changes to the as-formed NCs structures(26), which is especially true for the halide perovskites. In contrastto that, the emerging halide perovskites will enable fast and non-invasive probing of anion exchange mechanism by optical charac-terizations thanks to their high photoluminescence efficiency andcomposition-dependent bandgap tunability. Initial investigationshave developed a microkinetic description of nanoscale anion ex-change for halide perovskites NCs (27). However, the rapid reac-tion rate in NCs and limitations in temporal and spatial resolutionmake it elusive to directly observe the anion exchange kinetics onsingle-particle level for demonstrating the plausible mechanism.We observed anion exchange kinetics in cesium lead halide

perovskites (CsPbX3, X = Cl, Br, I) by spatially mapping theemission evolution of individual single-crystalline nanoplates,through confocal photoluminescence microscopy. Our improvedchemical vapor transport (CVT) growth method (28) provided dry,solid perovskite nanoplates with low defect densities and symmet-rical features for better resolving intrinsic material properties, andalso enabled the tunability in lateral size and thickness for systematicstudies. An experimental design of the vapor-phase anion exchangeprocedure additionally contributed to the rapid reaction quenching

Significance

Fundamental understanding of chemical transformation mecha-nism in solid-state semiconductors can offer guidelines for engi-neering thesematerials with various functionalities. Microscopicallyresolving such transformation process on the single-particle levelcan provide a direct picture of the dynamics, but has always beenchallenging due to the deficiencies of characterization techniquesand material dimensions. This work presents a unique design onrealizing the quantitative imaging of anion exchange reaction ki-netics in halide perovskites by taking advantage of their high andtunable photoluminescence and characteristic lattice dynamics,coupled with controllable chemical treatments and optical micro-scopic techniques. Our study on kinetically visualizing ion exchangebehavior in solid-state materials could also inspire mechanistic in-sights into other phenomena in a broad context of physical science.

Author contributions: Y.Z. and P.Y. designed research; Y.Z., D.L., M.G., M.L., J.L., T.L., Z.L.,and L.N.Q. performed research; Y.Z., D.L., M.G., and P.Y. analyzed data; and Y.Z. and P.Y.wrote the paper.

Reviewers: M.K., ETH Zurich; and N.R.N., National Renewable Energy Laboratory.

The authors declare no conflict of interest.

Published under the PNAS license.1Y.Z. and D.L. contributed equally to this work.2To whom correspondence may be addressed. Email: p_yang@berkeley.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1903448116/-/DCSupplemental.

Published online June 12, 2019.

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for immediate characterizations of their optical behaviors. More-over, the micrometer-scale lateral size of these nanoplates andcorresponding minute-long reaction time made the experimentalsystem compatible with both the spatial and temporal resolutionsneeded for microscopic studies.As the starting material for subsequent anion exchange stud-

ies, CsPbBr3 nanoplates were heteroepitaxially grown on micasubstrates by a one-step atmosphere–pressure CVT method(Methods). The optical bright-field image of the as-grown CsPbBr3nanoplates on mica substrates shows a well-defined square shapewith typical lateral width of 10–20 μm (SI Appendix, Fig. S1A).High-resolution TEM (HRTEM) image of a thin nanoplate (SIAppendix, Fig. S1B) and the corresponding fast Fourier trans-form (FFT) pattern (SI Appendix, Fig. S1C) indicate its single-crystalline structure which can be indexed to the crystal symme-try of the [001] zone plane of CsPbBr3 lattice. Further analysis ofthe FFT pattern implies the highly symmetrical nature of ournanoplates (see details in SI Appendix, Fig. S1C). Fig. 1A shows awide-field photoluminescence spectrum of CsPbBr3 nanoplate aswell as its optical image (Bottom Inset) under laser excitation. TheCsPbBr3 nanoplates are highly emissive with an emission peakcentering at around 526 nm, in agreement with the reportedbandgap (29). A corresponding scanning electron microscopy

(SEM) image (Fig. 1 A, Top Inset) of the nanoplate furtherconfirms its morphology.A facile approach was developed for controllable vapor-phase

anion exchange reaction on the as-grown CsPbBr3 nanoplates, asschematically illustrated in Fig. 1B (see details in Methods); theiodide vapor transformed CsPbBr3 to CsPb(Br1−xIx)3 and even-tually to CsPbI3 (12) due to the rapid anion exchange process(7–9, 11). The whole exchange reaction proceeds under a con-stant temperature quantified as around 170 °C for the nanoplatessample itself (see details in SI Appendix). Rapid quenching of thereaction enabled us to capture different anion exchange stages asexamined by photoluminescence measurement. As shown in SIAppendix, Fig. S2, the emission color and peak wavelength evolvewith anion exchange reaction time. And, the coexistence of greenand red emission colors and the emergence of two distinctivephotoluminescence peaks indicate the formation of a hetero-structure with Br- and I-rich regions on a single nanoplate, implyinga diffusion-limited pathway of the anion exchange process. Addi-tional characterizations including SEM and atomic force micros-copy (AFM) (SI Appendix, Fig. S3) demonstrate the well-preservedmorphology of the nanoplates after anion exchange reaction.To elucidate the anion diffusion pathway, time evolution studies

were performed by confocal microscopic photoluminescence im-aging. Confocal laser scanning microscopy in lambda (wavelength)

CsPbBr3plates

n-C4H9NH3I (s)

vapor

HotplateHotplate

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12

3

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10 min10 min

15 min15 min

20 min20 min

25 min25 min

A

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Fig. 1. Anion exchange process in single-crystalline halide perovskite nanoplates and its characterizations. (A) Photoluminescence (PL) spectrum and image(Bottom Inset), and SEM image (Top Inset) for one starting CsPbBr3 nanoplate. (Scale bars, 5 μm.) (B) Schematic of the experimental approach for performingvapor-phase anion exchange reaction on CsPbBr3 nanoplates (heating temperature of the hotplate was calibrated as constant within 175–180 °C). (C–F) Thedifferent anion exchange status of individual nanoplates (thickness: 352, 374, 352, and 360 nm) with reaction time of 10, 15, 20, and 25 min, respectively. (LeftColumn) Confocal PL mapping of the nanoplates exhibiting different anion distributions. (Scale bars, 5 μm.) (Center Column) Local emission spectra collected(collection spot <500 nm in diameter) for each nanoplate at the face site (position 1), the edge site (position 2), and the corner site (position 3). (Right Column)Profiles of the iodine ratio (converted from the emission wavelength) along the dashed lines in each of the nanoplates.

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scan mode allows the emission from individual nanoplates to bespatially resolved with each pixel (≤500 nm) of the scanned imagecorrelated to a specific emission wavelength (30). The anion ex-change at four different intermediate reaction stages was visual-ized through the evolution of emission color on individualnanoplates (Fig. 1 C–F, Left Column). The emission color evolu-tion indicates decrease in bandgap with increase in I/Br ratio.Based on this observation, we identified a step-by-step pathway ofthe anion exchange reaction, which initially starts from the pe-ripheral sites of the nanoplates before propagating symmetricallyinto the center, along with certain crystalline direction of CsPbX3structure (suggested by HRTEM and FFT pattern shown in SIAppendix, Fig. S1). To analyze the reaction pathway quantitatively,local emission spectra of three representative positions (face,edge, and corner) were acquired for the nanoplates at each stageto further demonstrate the diffusion pathway (Fig. 1 C–F, CenterColumn). Composition profiles along each horizontal dashed lineacross the nanoplates (Fig. 1 C–F, Right Column) were obtainedby converting the I ratio at each pixel from emission wavelengthfollowing the bowing relation with the bandgaps of CsPb(Br1−xIx)3(SI Appendix, Fig. S4). These line profiles prove the formation ofheterojunction on single nanoplates and further suggest a possiblediffusion-determining anion exchange rate.

These observations led to a systematic investigation for acomprehensive picture of the anion exchange kinetics. Based onour further-modified CVT method, we were able to obtainthickness-controllable perovskite nanoplates (SI Appendix, Fig.S5) to examine the effect of thickness on the anion exchangekinetics; we found anion exchange rates varied with platethickness. A matrix of confocal photoluminescence images inFig. 2 A–E reveals the time evolution of the anion exchangeprocess for five groups of nanoplates with different thicknessranges (20–40 nm; 50–80 nm; 100–130 nm; 220–260 nm; 350–380nm). Statistical experiments verified the comparable reactionrates for nanoplates in the same thickness range, thus individualnanoplates can be chosen as the representatives for our timeevolution studies. The qualitative observation clearly identifiedthat the anion exchange on nanoplates among different thicknessgroups follows the similar trend of a symmetrical propagationpathway but with different rates. To quantitatively study thethickness effect on anion exchange kinetics, the time evolution ofI ratio at three sites (face, edge, corner) of nanoplates wasplotted for each thickness group (Fig. 2F). We found that thereaction time for a complete anion exchange increases with platethickness; for example, a 20-nm-thick nanoplate requires only 10min to complete the anion exchange reaction whereas 25 min is

time

thic

knes

s

A F

B

C

D

E

Fig. 2. Illustration of the thickness- and time dependence of anion exchange. Confocal PL mapping on individual nanoplates of different thicknessesand reaction times. (A) Thickness range: 20–40 nm (38, 30, 27, 28, and 30 nm, respectively). Reaction times are 0, 3, 4, 7, and 10 min from left to right, re-spectively. (B) Thickness range: 50–80 nm (65, 73, 64, 55, and 50 nm, respectively). Reaction times are 0, 6, 8, 10, and 15 min from left to right, respectively.(C) Thickness range: 100–130 nm (110, 114, 116, 112, and 127 nm, respectively). Reaction times are 0, 10, 15, 18, and 25 min from left to right, respectively. (D)Thickness range: 220–260 nm (260, 240, 253, 220, and 236 nm, respectively). Reaction times are 0, 10, 15, 20, and 25 min from left to right, respectively. (E)Thickness range: 350–380 nm (362, 352, 374, 352, and 360 nm, respectively). Reaction times are 0, 10, 15, 20, and 25 min from left to right, respectively. (F)Plots of iodine ratio changing with reaction time for the three different sites (face, edge, and corner denoted in Fig. 1) of the nanoplates in identifiedthickness ranges from top to bottom, corresponding to those in images from A to E, respectively. (All scale bars, 5 μm.)

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needed for a nanoplate thicker than 120 nm. This feature can beattributed to the simple scaling of the size in a diffusion-controlled reaction scheme, providing further support for theproposed anion exchange mechanism.Motivated by these experimental observations, we performed

theoretical analysis and modeling to simulate the anion exchangeprocess. A detailed hypothesis was put forward that the anionexchange in halide perovskites contains two steps: (i) initialsurface reaction; and (ii) subsequent solid-state diffusion (Fig.3A). To evaluate the rates of the exchange reaction and aniondiffusion in perovskite structures, a model of one-dimensional(1D) diffusion coupled with surface reaction was constructed tomimic the vertical diffusion channel extracted from the centralcolumn of the nanoplates (Fig. 3B). Correspondingly, the ex-perimental information of local emission spectrum from thiscentral column can also be obtained through confocal laserprobe. This can be further converted to I ratio and taken as theaverage value over the entire vertical length, given that thevertical resolution of confocal microscope (∼500 nm) exceedsthe plate thickness (Fig. 3B). Analysis of the anion diffusionpathway in this channel is also based on the assumption that the Iratio change is only due to diffusion in the z direction from thetop surface in a short time period (not longer than 10 min in ourcase). We collected the experimental I ratio of the central sur-face spot for a series of nanoplates with different thickness andsame reaction time, as represented by some of those nanoplatesshown in SI Appendix, Fig. S6. Theoretically, a modified finite-difference method was utilized for simulation, combined with thefirst-order reaction mechanism and Fick’s second law in our 1Dmodel, thus giving the theoretical average I ratio with de-pendence on plate thickness (see details in SI Appendix). Fromthe simulation, a reaction constant k of ð1.5∼ 3.5Þ× 10−3s−1 anddiffusion coefficient D of ð1∼ 5Þ× 10−14cm2 · s−1 can be well fit-ted with the experiment data (Fig. 3C).To elucidate the mechanism of anion exchange reaction on

perovskite nanoplates, we further analyzed our simulation resultsalongside experimental observations. The halide diffusion coefficientD on the order of 10−14cm2 · s−1 estimated from simulation is

comparable with the values from previous reports and in agreementwith the halide vacancy-assisted diffusion mechanism (31, 32). Bycontrast, the surface reaction constant k is not dependent on spatialparameters, and its high value suggests the surface reaction can bevery rapid. We thus concluded the anion diffusion is the rate-determining step that gives rise to the gradual transformation toheterogeneous Br/I mixed perovskites and ultimately to CsPbI3,enabling the anion exchange propagation to be visualized with thetemporal resolution at a minute level. Furthermore, the diffusionlength L is approximately proportional to

ffiffiffiffiffi

Dtp

(t means time)matching the experimental results. For example, a reaction time of10 min yields an L value of around 40–50 nm in our vertical diffu-sion channel (z direction) (SI Appendix, Fig. S6). As extended to thein-plane (x, y) direction, such diffusion length (below or around 100nm) is much shorter than the large lateral size of the nanoplates(typically around 10 μm). Therefore, such inconsiderable diffusionlength scale of ions in perovskite nanoplates with high aspect ratio(i.e., width is much greater than height) contributes to the ob-servable anisotropic characteristic that lateral anion exchangepathway outcompetes the vertical way. This leads to the formationof 2D heterostructures with Br- and I-rich separated regions on asingle-crystalline nanoplate at intermediate anion exchange stages.As a proof-of-concept study for the as-formed perovskite

heterostructures, we further investigated the evolution of carrierbehavior and recombination pathway associated with the anionexchange process. Photoluminescence lifetime imaging micros-copy was used as another quantitative imaging technique to probethe carrier dynamics of the perovskite nanoplates throughout theanion exchange process. The photoluminescence lifetime map-pings for individual nanoplates at different anion exchange stageswere shown in Fig. 4 A–D. No significant changes of lifetime wereobserved during this transformation, demonstrating that the op-tical quality of perovskite is retained. Within each of individualnanoplates, the lifetime mappings show homogeneous distributionfor both of CsPbBr3 (Fig. 4A) and CsPbI3 (Fig. 4D), whereas theheterostructures (Fig. 4 B and C) exhibit strong contrast at dif-ferent sites. Quantitative fitting to the photoluminescence decaycurves with biexponential function at three denoted sites in Fig. 4B

n-C4H9NH3+

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solliddidididdddidididdddssoosssooo

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Fig. 3. Schematics and simulations of the anion exchange process. (A) Schematic of the anion exchange process which contains: (i) surface reaction for anionexchange between n-C4H9NH3I vapor and CsPbBr3 nanoplates; and (ii) solid-state diffusion of I− in CsPbBr3 nanoplates. (B) Schematics illustrating the re-alization of obtaining both the experimental (left side, from confocal laser probe) and theoretical (right side, from 1D diffusion model) value of I ratio at thecenter of nanoplates surface. (C) Simulation of the relation between I ratio (average) at the center of plates surface versus plates thickness. The dashed areaexhibits simulation result with the reaction constant k given as ð1.5∼ 3.5Þ×10−3s−1 and the diffusion coefficient D as ð1∼ 5Þ× 10−14cm2 · s−1, well fitted withthe experimental data points shown as the hollow dots.

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identifies the lifetime as 0.524 ns for face, 1.726 ns for edge, and10.11 ns for corner, respectively (Fig. 4E). The lifetime increasefrom central to peripheral region of nanoplates is associated withthe increasing I/Br ratio (Fig. 4F), which is consistent with ourprevious report on the photoluminescence lifetime in mixed Brand I perovskites (9). Such increases in lifetime could also be at-tributed to the carrier flow from the Br-rich to I-rich region drivenby the bandgap energy funneling effect (33, 34), due to the for-mation of coherent heterojunction with built-in halide gradientand bandgap decrease from CsPbBr3 to CsPbI3. The carrier dy-namics at such heterointerface resulting from the diffusion-limitedmechanism of anion exchange may imply areas of interest inphotophysics and inspire further investigations.In conclusion, we showed that the anion exchange kinetics in in-

dividual halide perovskite nanoplates was visually and quantitativelyresolved by microscopic photoluminescence imaging, revealing adiffusion-controlled mechanism that was also validated by theoreti-cal simulations. We further demonstrated that the 2D perovskiteheterostructures can be created through the precisely controlledanion exchange approach that we developed here, which would

generate fundamental research interest in photophysics, such as thephotoinduced carrier dynamics at the coherent heterointerface.Overall, the presented study not only provides a quantitativemechanistic insight into understanding ion exchange behaviorin solid-state semiconductors, but also stands out as a creative designby combining the strengths of materials properties and dimensionwith chemical experiment control and optical microscopic techniquesfor quantitatively imaging a chemical reaction and the process of 2Dheterostructure formation. Such concept and strategy can be furthergeneralized to the study of similar properties in other material sys-tems and various phenomena in the context of physical science.

MethodsAdditional details regarding the materials and methods may be found inSI Appendix.

Growth and Vapor-Phase Anion Exchange of Perovskites. The CsPbBr3 perov-skite nanoplates were grown by a CVT method (28, 35–37). The vapor-phaseanion exchange reaction of the as-grown CsPbBr3 nanoplates was carriedout in a capped 20-mL glass vial as shown in Fig. 1B. The iodine source of10 mg n-C4H9NH3I (n-Butylammonium iodide) powder was first placed inside

1 2

3

1 2

3

6 ns

1 ns

A B C D

E F

Fig. 4. Time-resolved PL imaging for different anion exchange stages. A–D (Top Row) PL lifetime imaging of nanoplates. A–D (Bottom Row) Correspondingconfocal PL mapping of nanoplates. The thickness values are 110, 374, 250, and 30 nm, respectively. (A) The starting CsPbBr3 nanoplate. (B) Heterostructureformed at early reaction stage. (C) Heterostructure formed at later reaction stage. (D) CsPbI3 nanoplate after conversion from CsPbBr3. (E) Time-resolved PLdecay curves in square dots and their fitting curves in solid lines for the nanoplate in B at the face, edge, and corner sites, respectively. (F) Steady-state PLspectra for the nanoplate in B at the face, edge, and corner sites, respectively. (All scale bars, 5 μm.)

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the vial and was evenly distributed at the bottom. The mica substrate (withCsPbBr3 nanoplates grown on the top surface) was then placed on top of theevenly distributed tiny powder layer at the bottom of the vial. The thicknessof mica substrate is between 50 and 75 μm. Then, the reaction vial wasplaced on a hotplate (with constant temperature calibrated as within 175–180 °C) for heating; the exact temperature of the sample on substrate hasbeen measured by an infrared pyrometer as maintaining 170 ± 2 °C duringthe experiment (see quantification details in SI Appendix). The anion ex-change reaction was allowed to proceed between the CsPbBr3 nanoplatesand continuously evaporated n-C4H9NH3I vapor for different periods of re-action time to study the time evolution of the reaction. The reaction waseffectively quenched by removing the vials from the hotplate and taking thesample out immediately. This entire procedure was conducted in an argon-filled glovebox.

Confocal Laser Scanning Photoluminescence Microscopy. Confocal laser scan-ning photoluminescence microscopy was performed using a Carl Zeiss 710LSM confocal microscope with a 50× 0.6 numerical aperture (N.A.) objectiveand analyzed by Zen software. A laser excitation at 405 nm was used withthe laser power and the electric gain optimized for the dynamic range of theemission intensity from different samples. All images were 512×512 pixelscollected at 10ms per line with a pinhole size of 1 Airy unit. Lambda scans wereperformed by collecting a series of photoluminescence images while scanningthe wavelength range within 470–710 nm with a 5-nm spectral window.

Photoluminescence Lifetime Imaging Microscopy. The carrier recombinationrates and lifetime distributions over the perovskite nanoplates at differentanion exchange stages were measured by a time-resolved microscopic

imaging system (Zeiss 510 NLO AxioVert 200M). A Ti:sapphire laser system(Spectra-PhysicsMai Tai) with a pulsewidth of less than 100 fs and a repetitionrate of 80 MHz was applied to excite the samples through a 50× objectivewith 0.6 N.A. at an excitation wavelength of 405 nm (second-harmonic laserwavelength). The illuminating power was controlled by the neutral densityfilter. The photoluminescence signal was collected by the same objective andfiltered by a long-pass filter (488 nm) and a bandpass filter (530 nm/30 nm)before entering a controllable pinhole with a diameter of 50 μm in front of aHamamatsu photomultiplier tube. The lifetime images were taken with anacquisition time of 60 s.

ACKNOWLEDGMENTS. We thank L. Dou, D. Zhang, and Q. Kong for fruitfuldiscussions on designing research; C. Jackson for discussion on manuscriptpreparation; C. Kley for discussion on AFM characterization; H. Aaron forhelp on lifetime imaging facilities. This work was supported by the USDepartment of Energy, Office of Science, Office of Basic Energy Sciences,Materials Sciences and Engineering Division, under Contract DE-AC02-05CH11231 within the Physical Chemistry of Inorganic Nanostructures Pro-gram (KC3103). Confocal laser scanning microscopic study was conducted atthe College of Natural Resources Biological Imaging Facility, supported in partby the National Institutes of Health S10 Program under Award 1S10RR026866-01. The content is solely the responsibility of the authors and does not neces-sarily represent the official views of the National Institute of Health. Photo-luminescence lifetime imaging experiments were conducted at the CancerResearch Laboratory Molecular Imaging Center, supported by NSF DBI-0116016. Y.Z., M.L., and T.L. acknowledge the fellowship from Suzhou Indus-trial Park. M.G. acknowledges the Ning Fellowship granted by University ofCalifornia, Berkeley.

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