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Indefinite and bidirectional near-infrared nanocrystal photoswitching


Materials whose luminescence can be switched by optical stimulation drive technologies ranging from superresolution imaging1,2,3,4, nanophotonics5, and optical data storage6,7, to targeted pharmacology, optogenetics, and chemical reactivity8. These photoswitchable probes, including organic fluorophores and proteins, can be prone to photodegradation and often operate in the ultraviolet or visible spectral regions. Colloidal inorganic nanoparticles6,9 can offer improved stability, but the ability to switch emission bidirectionally, particularly with near-infrared (NIR) light, has not, to our knowledge, been reported in such systems. Here, we present two-way, NIR photoswitching of avalanching nanoparticles (ANPs), showing full optical control of upconverted emission using phototriggers in the NIR-I and NIR-II spectral regions useful for subsurface imaging. Employing single-step photodarkening10,11,12,13 and photobrightening12,14,15,16, we demonstrate indefinite photoswitching of individual nanoparticles (more than 1,000 cycles over 7 h) in ambient or aqueous conditions without measurable photodegradation. Critical steps of the photoswitching mechanism are elucidated by modelling and by measuring the photon avalanche properties of single ANPs in both bright and dark states. Unlimited, reversible photoswitching of ANPs enables indefinitely rewritable two-dimensional and three-dimensional multilevel optical patterning of ANPs, as well as optical nanoscopy with sub-Å localization superresolution that allows us to distinguish individual ANPs within tightly packed clusters.

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Fig. 1: Photoswitchable PA nanoparticles.
Fig. 2: Consistent photoswitching of single ANPs.
Fig. 3: ANP photobrightening and photodarkening.
Fig. 4: 2D and 3D microscale optical write, erase and rewrite of stable NIR-photoswitchable ANPs.
Fig. 5: Indefinite NIR photon avalanching localization microscopy of ANPs.

Data availability

All data generated or analysed during this study, which support the plots within this paper and other findings of this study, are included in this published article and its Supplementary InformationSource data are provided with this paper.

Code availability

The code for modelling the PA behaviour using the differential rate equations described in the Supplementary Information is freely available at


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We thank L. J. Kaufman at Columbia University for her technical support and S. Kim at KRICT for performing the SEM imaging. P.J.S., Y.D.S., S.H.N. and C.L. acknowledge support from the Global Research Laboratory (GRL) Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (no. 2016911815) and KRICT (KK2361-10). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy (DOE) under contract no. DE-AC02-05CH11231. C.L., P.J.S., B.E.C. and E.M.C. thank the Defense Advanced Research Project Agency (DARPA) Enhanced Night Vision in Eyeglass Form (ENVision) program (no. HR00112220006) for supporting the infrared spectroscopic investigations. Infrared emitter lifetime measurements were supported by the National Science Foundation under grant no. DMR-2019444. E.Z.X. acknowledges support from the NSF Graduate Research Fellowship Program. K.W.C.K. acknowledges support from the DOE NNSA Laboratory Residency Graduate Fellowship program (no. DE-NA0003960). P.J.S. also acknowledges support from Programmable Quantum Materials, an Energy Frontier Research Center funded by the US DOE, Office of Science, Basic Energy Sciences (BES), under award DE-SC0019443. Work at UNIST (Y.D.S.) was supported by IBS-R019-D1 and 2022 UNIST Research Fund (1.220108.01). We acknowledge seed funding support from Columbia University’s Research Initiatives in Science & Engineering competition, started in 2004 to trigger high-risk, high-reward and innovative collaborations in the basic sciences, engineering and medicine: This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US Department of Energy under contract DE-AC02-05CH11231. We thank Gatan Inc. as well as P. Denes, A. Minor, J. Ciston, C. Ophus, J. Joseph and I. Johnson who contributed to the development of the 4D Camera. N.F.M. acknowledges support from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 893439, the Fulbright Scholarship Program, the Zuckerman-CHE STEM Leadership Program and the ISEF Foundation. B.U. acknowledges support by the National Science Foundation under grant no. CHE-2203510. S.D.P. and T.L. acknowledge funding by the German Research Foundation (DFG) through the Collaborative Research Center 1032, Project no. 201269156, A8.

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Authors and Affiliations



P.J.S., C.L., E.M.C., B.E.C. and Y.D.S. conceived of the study. Experimental measurements and associated analyses were conducted by C.L., E.Z.X., K.W.C.K., B.U., M.E.Z., Y.K., N.F.M., C.C.S.P., H.S.P., J.K., S.H.N., S.D.P., T.L., J.S.O., P.E. and E.M.C. Advanced nanoparticle synthesis and characterization was performed by Y.L., A.T., C.C.S.P., H.S.P., B.E.C. and E.M.C. Theoretical modelling and simulations of PA photophysics were carried out by C.L. and E.M.C. All authors contributed to the preparation of the manuscript.

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Correspondence to Yung Doug Suh, Bruce E. Cohen, Emory M. Chan or P. James Schuck.

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Extended data figures and tables

Extended Data Fig. 1 Photodarkening and photoblinking in single ANPs.

(a) Atomic force microscopy (AFM) and (b) confocal scanning images of a single and a cluster of 4 ANPs (NaYF4: 8% Tm3+@NaY0.8Gd0.2F4, 10 nm core/4 nm shell). Scale bars are 250 nm. Magnified AFM images of the ANPs are shown in the top left (single) and bottom right (4 singles) panels in a. Colour bar in b: normalized luminescence intensity. Luminescence and excitation intensity Iex time-traces of the single (c) and four-ANP cluster (d) in a under 1,064 nm excitation at increasing intensities. Ith, ANP avalanching threshold intensity. e, Time trace showing blinking luminescence from a single 8% Tm3+ 17/6 nm core/shell nanocrystal at Iex = 164 kW cm−2.

Extended Data Fig. 2 Photodarkening in ANP ensembles.

a, Time dependence of 800 nm emission intensity at various 1064-nm excitation intensities, for 4% Tm3+ 14.3/3.7 nm core/shell Tm3+ nanoparticle ensemble films. UCNPs with Tm3+ doping ≤4% do not show avalanching behaviour20, and photodarkening here. b, Time dependence of emission intensity at various 1,064-nm excitation intensity for 8% 10.2/4.0 nm core/shell Tm3+ ANP ensemble films.

Extended Data Fig. 3 Determination of photodarkening intensity IPD as a function of ANP composition.

ag, Each plot shows the ratio between 800 nm emission intensities (Iemi) at t = 0 and after 120 s of continuous 1,064 nm exposure versus excitation intensity. These data allow us to define a photodarkening threshold intensity (IPD) as the 1,064 nm pump intensity where emission at t = 120 s decreases below the initial value. Error bars are standard deviations of four data points measured at the same spot within 0.4 s. h, Photodarkening threshold versus pre-darkened photon avalanche threshold for various ANP compositions. Symbol definitions are the same as in ag. Error bars are standard deviations derived from the curve fitting of the power-dependent photodarkening analysis shown in ag.

Extended Data Fig. 4 PA threshold shift along with decrease of the 3F4 lifetime.

a, Measurement (light and dark green circles) and differential rate equation model fitting (green dashed lines) of 800-nm emission intensity versus 1,064-nm excitation intensity of a single 17.3/5.6 nm 8% Tm3+ (17 nm core with 5.6 nm shells) ANP before and after photodarkening. Error bars are standard deviations derived from four separate measurements on the same single ANP. b, Time-resolved photoluminescence of IR emission from the Tm3+ 3F43H6 transition in an ensemble film of 8% Tm3+ 17.3/5.6 nm core/shell ANPs under 1064 nm excitation before (top) and after (bottom) photodarkening. The black dashed lines are fits of exponential functions to the data. Emission wavelengths >1,750 nm were selected with a long-pass filter and collected using a superconducting nanowire single-photon detector (SNSPD; Single Quantum EOS 6). A single exponential decay is observed in the non-photodarkened ANPs (top panel). The lifetime curve from the photodarkened region (bottom panel) includes contributions from non-photodarkened ANPs within the film, resulting in a biexponential decay. The biexponential decay analysis suggests approximately two thirds of the ANPs are photodarkened in the measureed region, and the photodarkend ANPs have a 3F4 lifetime that is 5.2 times shorter, consistent with the rate equation analysis and fits in Fig. 3a.

Extended Data Fig. 5 Photobrightening recovery percentage of various ANP ensemble film samples.

a, Photobrightening recovery of darkened ANP films as a function of irradiation wavelength. The Tm3+ contents for green, red, blue, and purple markers are 8%, 20%, 30%, and 100%. Excitation (1,064 nm) and photobrightening intensities are 54 kW cm−2 and 277.87 kW cm−2, respectively, for green triangles (10.2/4 nm), red triangles (10.4/2.7), blue triangles (13.0/2.1 nm), and purple triangles (15.8/4.2 nm). Excitation (1,064 nm) and photobrightening intensities are 33 kW cm−2 and 167 kW cm−2, respectively, for green circles (17.3/5.6 nm), green squares (26.6/4.0 nm), and red circles (17.5/2.7 nm). Exposure times for photodarkening and photobrightening are 1.2 and 1.3 s, respectively. Error bars are standard deviations of data points measured at the four different spots in the same ensemble sample. The photobrightening laser power at the sample is set to 1 mW. Photobrightening recoveries as a function of ANP core size at 420 nm (b), Tm3+ concentration at 710 nm (c), shell thickness at 420 nm (d), and core size at 710 nm (e).

Extended Data Fig. 6 ANP photodarkening with 450 nm excitation.

a, Potential mechanistic pathways for photodarkening in ANPs under 450 nm excitation. b, Photobrightening recovery of darkened 20% Tm3+ 10.4/2.7 nm core diameter/shell thickness ANP films as a function of irradiation wavelength from 440 nm to 480 nm. The photobrightening laser power at the sample is set to 1 mW for all wavelengths. c, Emission spectra from an ensemble film of 20% Tm3+ 10.4/2.7 nm core diameter/shell thickness ANPs before photodarkening (top), during 450 nm exposure (middle), and after photodarkening with 450 nm exposure (bottom). 1,064 nm illumination is used for pumping the luminescence in the left and right panels. The 1,064 nm intensity is 75 kW cm−2, and the 450 nm excitation intensity is 222 kW cm−2. The emission intensities in b are normalized to the maximum intensity in the left panel. d, UV emission spectra of 20% Tm3+ 10.4/2.7 nm core diameter/shell thickness ANPs under blue excitation with a wavelength range from 440 nm to 480 nm. The excitation powers are 1 mW. The emission intensities in c are normalized to the maximum intensity in the panel obtained under 450 nm excitation. The 345 nm and 365 nm emission peaks are attributed to the excited-state transitions from the 1I6 state to the 3F4 state and the 1D2 state to the 3H6 state, respectively.

Extended Data Fig. 7 Photodarkening and photobrightening mechanisms and fast photobrightening with sub-20-ms exposure.

a, Ultraviolet emission spectra of 8% Tm3+ 17.3/5.6 nm ANP ensembles at 1,064-nm excitation intensities above photodarkening threshold intensity (IPD). Spectra are normalized to the 365 nm peak. The 345 nm and 365 nm emission peaks are attributed to the excited-state transitions from 1I6 to 3F4 and 1D2 to 3H6, respectively. b, Potential mechanistic pathways for photodarkening and photobrightening in ANPs via charge transfer from the host material, resulting in Tm2+ as an intermediate. CB: conduction band, VB: valence band, W2: 3F4 relaxation rate. Hole traps in addition to electron traps can be produced through the charge transfer process, which can both potentially quench the 3F4 excited state in Tm ions. c, Recovery was accomplished with external heating only (5 min equilibration at each temperature), rather than by photobrightening. Before and after heating, 20% Tm3+ 10.4/2.7 nm films were photodarkened and probed with 1064 nm intensities of 320 kW cm-2 and 64 kW cm-2, respectively. Percentage recovery is defined as the ratio of the recovered luminescence intensity after temperature increase to the reduced luminescence intensity after initial photodarkening. Error bars are standard deviations of data points measured at the four different spots in the same film. d, Photodarkened regions of a film of 20% Tm3+ 10.4/2.7 nm core/shell ANPs were exposed to a 700 nm photobrightening beam for short exposure times, and the luminescence recovery was measured as a function of exposure time. The 1064 nm pumping and photodarkening intensities are 51 kW cm−2 and 252 kW cm−2, respectively. 700 nm photobrightening excitation intensity is 240 kW cm−2. The photodarkening exposure time is 188 ms. Error bars are standard deviations of data points measured at the four different spots in the same ensemble sample.

Extended Data Fig. 8 Indefinite photoswitching of a single aqueous ANP.

a, Time-resolved luminescence and excitation intensities for a single 8% Tm3+ 17.3/5.6 nm aqueous ANP48 in water . A 1,064 nm pump intensity of 16.7 kW cm−2 is continuously applied, which excites detectable emission in the on state but not in the off state. Irradiation conditions for photodarkening (turning off) are 75.5 kW cm−2 at 1,064 nm and 5 s; and for recovery (turning on) are 164.0 kW cm−2 at 700 nm for 10 s. b, A probability histogram of the average emission intensity while turning on for 1,520 irradiation cycles. c, Trace of emission from the single aqueous ANP for the first 60 and last 35 irradiation cycles.

Extended Data Fig. 9 INPALM of ANPs.

Confocal scanning image (a) and INPALM image (b) (number of localizations, N = 43) of a single and a dimer of 8% Tm3+ 26.6/3.0 nm ANPs. The diameter of the localization in b represents the localization accuracy of the 2D Gaussian point spread function (PSF) fit per frame. Scale bars are 125 nm.

Extended Data Fig. 10 INPALM of ANPs with drift correction and intensity filtering.

a, Wide-field image of ANPs of 8% Tm3+ 26.6/3.0 nm ANPs. Scale bar is 2 µm. b, Frame-by-frame localizations of the ANPs marked with circles in a before drift correction. Only particles in the red circle (identical to the ANPs in Extended Data Fig. 9) are exposed to periodic 1,064 nm and 532 nm illumination for photoswitching. The other ANPs in the circles are in the on state while photoswitching the ANPs in the red circle and used for drift correction. Magnified frame-by-frame localizations of the ANPs marked with circles in a before (c) and after drift correction (d). The diameter of the localization in c represent the 10 times of the standard deviation of the 2D Gaussian point spread function (PSF), and that in d represent the standard deviation of the 2D Gaussian PSF. e. SEM images of ANPs in a region marked with dashed lines in a. Scale bar is 500 nm. f, Frame-by-frame localizations of the ANPs marked with a red circle in a before (left) and after (right) intensity filtering.

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Lee, C., Xu, E.Z., Kwock, K.W.C. et al. Indefinite and bidirectional near-infrared nanocrystal photoswitching. Nature 618, 951–958 (2023).

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