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1 Materials. GdCl 3 xh 2 O, (99.99%), YCl 3 6H 2 O (99.99%), YbCl 3 6H 2 O (99.99%), ErCl 3 6H 2 O (99.9%), NaOH (98+%), NH 4 F (98+%), 1 octadecene (90%), and oleic acid (90%) were all purchased from Sigma Aldrich and used as starting materials without further purification. Synthesis of the nanorods. In a typical procedure to the synthesis of lanthanide doped NaYF 4 nanorods, a DI water solution (1.5 ml) of 0.3 g NaOH was mixed with 5 ml of ethanol and 5 ml of oleic acid under stirring. To the resulting mixture were selectively added 2 ml of RECl 3 (0.2 M, RE = Y, Yb, Er, Gd, Sm, Nd and La) and 1 ml of NH 4 F (2 M). Then the solution was transferred into a 20 ml of Teflon lined autoclave and heated at 200 o C for 2 h. The obtained nanorods were collected by centrifugation, washed with water and ethanol several times, and finally re dispersed in cyclohexane. Synthesis of the nanoparticles. In a typical experiment, 2 ml of RECl 3 (0.2 M, RE = Y, Yb, Er, Gd, and Tm) in methanol were added to a 50 ml flask containing 3 ml oleic acid and 7 ml 1 octadecene and the solution was heated to 160 o C for 30 min and then cooled down to room temperature. Thereafter, 5 ml methanol solution of NH 4 F (1.6 mmol) and NaOH (1 mmol) was added and the solution was stirred for 30 min. After methanol evaporated, the solution was heated to 300 o C under argon for 1.5 h and cooled down to room temperature. The resulting nanoparticles were precipitated by the addition of ethanol, collected by centrifugation, washed with methanol and ethanol several times, and finally re dispersed in cyclohexane. Characterization. XRD analysis was carried out on a Siemens D5005 X ray diffractometer with Cu Kα radiation (λ = Å). Low resolution TEM measurements were carried out on a JEOL 2010 transmission electron microscope operating at an acceleration voltage of 200 kv. High resolution TEM was performed on a FEI Tecnai G 2 F20 electron microscope operated at 200 kv. The luminescence spectra were obtained with a DM150i monochromator equipped with a R928 photon counting photomultiplier tube (PMT), in conjunction with a 980 nm diode laser. Photographs were taken with a Canon PowerShot G9 digital camera. 1

2 Figure S1. Iso surface of charge transfer in NaYF 4 and NaGdF 4 cystals. Positive and negative charges are drawn in blue and yellow, respectively. To further understand the effect of doping on electronic structures of the materials, we show the iso surface of charge difference between initial charge density (the combination of charges of isolated atoms) and converged charge density by DFT calculations for NaYF 4 and NaGdF 4 in both cubic and hexagonal phases. As expected, the charge transfer mainly occurs between Y 3+ /Gd 3+ and F atoms. In cubic phase, the doping of Gd 3+ decreases the charge transfer, resulting in the decrease of dipole polarizibility and formation energy. In hexagonal phase, the doping of Gd 3+ facilitates the charge transfer, and the calculation shows that the charge transfer to each F atom increases from 0.76 electrons for NaYF 4 to 0.85 electrons for NaGdY 4, resulting in an increase of polarizability and formation energy by Gd 3+ doping. 2

3 Figure S2. TEM characterization of the as synthesized NaYF 4 :Yb/Er/Gd (18/2/60 mol %) nanorods. (a) TEM image showing the formation of uniform nanorods. (b) HRTEM image taken in [210] incidence of a nanorod shown in (a) reveals a d spacing of 0.36 nm and c axis growth direction of the nanorod. (c) EDX pattern of a single nanorod reveals the appearance of doped elemental Yb, Er, and Gd. Note that the strong signals for Cu come from the copper TEM grid. 3

4 Figure S3. Cubic to hexagonal phase transformation of NaYF 4 :Yb/Er (18/2 mol %) nanocrystals by doping of various lanthanide ions. (a,b) XRD patterns and corresponding TEM images of the nanocrystals prepared after heating for 2 h at 200 o C in the presence of 30 mol % Sm 3+, Nd 3+, and La 3+. (c) XRD pattern of the nanocrystals prepared in the presence of 60 mol % La 3+. The bottom diffraction pattern is the literature reference for hexagonal LaF 3 crystals (Joint Committee on Powder Diffraction Standards file number ). Note that the nanocrystals prepared in the presence of 60 mol % La 3+ clearly show two distinct phases of cubic phase NaYF 4 and hexagonal phase LaF 3. The XRD peaks corresponding to hexagonal phase LaF 3 in (c) exhibit a spectral shift toward high diffraction angles relative to literature data, implying the reduction of the unit cell volume probably caused by the substitution of smaller Y 3+, Yb 3+, and Er 3+ ions into the LaF 3 crystal lattice. Scale bar in (b) is 200 nm. 4

5 Figure S4. Photoluminescence studies of NaYF 4 :Yb/Er (18/2 mol %) nanocrystals with varying dopant concentration of Gd 3+. The emssion spectra were normalized to Er 3+ emission at 540 nm. With increasing dopant concentration of Gd 3+, pure hexagonal NaYF 4 were formed, resulting in an increase in green to red emission ratio. 5

6 Figure S5. Gd 3+ induced size tuning of NaYF 4 :Yb/Er (18/2 mol %) nanoparticles obtained after heating for 1.5 h at 300 o C in 1 octadecene. (a d) TEM images of NaYF 4 :Yb/Er (18/2 mol %) nanoparticles doped with a Gd 3+ content of 0, 5, 15, and 30 mol%, respectively. The corresponding average diameter of the nanoparticles is 25, 20, 15, and 10 nm, respectively. Scale bar is 100 nm. 6

7 doi: /nature08777 Figure S6. Gd3+ induced nanocrystal growth in NaYF4:Yb/Er (18/2 mol %) upon heating for 1.5 h at varied temperatures in 1 octadecene. (a,b) XRD patterns and corresponding TEM images of the nanoparticles prepared in the absence of Gd3+. (c,d) XRD patterns and corresponding TEM images of the nanoparticles prepared in the presence of 30 mol % Gd3+. Note that in the absence of Gd3+ dopant ions heating at 230 oc yields mostly polydisperse NaF crystals rather than monodisperse NaYF4 nanoparticles. These direct data comparison at different temperatures further confirms that the doping of Gd3+ promotes cubic to hexagonal phase transformation in NaYF4 nanocrystals. Scale bar is 100 nm for TEM images. 8 7

8 Dynamic Beam Expander Galvanometer 980 nm Diode Laser F-theta Lens Z position X-Y Sample PC and Galvanometer Height adjustable Stage Figure S7. Schematic drawing of the experimental setup for computer controlled three dimensional NIR laser scanning. A 980 nm diode laser was directed into a fast scanning X Y galvanometer with an F theta lens (focal length of 198 mm) to focus the laser beam. A dynamic beam expander was attached to the galvanometer to control the Z position of the laser beam at a range of +/ 7 mm. The scanning of the laser beam was controlled through CyberLease scanning software from IDI Laser. 8

9 Figure S8. Photoluminescence studies of NaYF 4 nanocrystals embedded in PDMS composite materials. (a) Photograph showing physical dimension and transparency of a PDMS bar composed of 0.1 wt % of NaYF 4 :Yb/Er/Gd (18/2/5 mol %) nanoparticles. (b e) Luminescent photos of the PDMS bars comprising 0.1 wt % of NaYF 4 :Yb/Er/Gd (18/2/5 mol %), NaYF 4 :Yb/Tm/Er/Gd (20/0.2/0.1/5 mol %), NaYF 4 :Yb/Tm/Er/Gd (20/0.2/0.05/5 mol %), and NaYF 4 :Yb/Tm/Gd (20/0.2/5 mol %) nanoparticles, respectively. (f) Photograph showing physical dimension and transparency of a PDMS disk composed of 0.1 wt % of NaYF 4 :Yb/Er/Gd (18/2/5 mol %) nanoparticles. (g,h) Luminescent images generated in the PDMS disk via computer controlled NIR laser scanning. 9