Supporting Information

Transcription

1 Supporting Information Colloidal Monolayer β-in 2 Se 3 Nanosheets with High Photoresponsivity Guilherme Almeida,&, Sedat Dogan, Giovanni Bertoni,, Cinzia Giannini, Roberto Gaspari,, Stefano Perissinotto, Roman Krahne, Sandeep Ghosh,* and Liberato Manna,* Department of Nanochemistry and Compunet, Istituto Italiano di Tecnologia (IIT), via Morego 30, I Genova, Italy Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, I Milan, Italy & Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso, 31, 16146, Genova, Italy IMEM-CNR, Parco Area delle Scienze 37/A, I Parma, Italy Istituto di Cristallografia, Consiglio Nazionale delle Ricerche, via Amendola 122/O, Bari, Italy S1

2 Table of Contents S1. Control syntheses using double injection procedure...3 S2. Syntheses using a single-injection procedure...5 S3. Additional x-ray diffraction simulations...6 S4. Surface composition, TGA and AFM topography...7 S5. Raman spectroscopy and thermal stability S6. Results of DFT Calculations S7. External quantum efficiency and detectivity of photodetectors S2

3 S1. Control syntheses using double injection procedure Figure S1. (a) TEM image of nanocrystals with an average composition of In 3 Se 2 assessed by HRTEM-EDS resulting from the control experiment where a DMF blank was employed as second injection. (b) HAADF-STEM image highlighting large selenium particles formed along with the In 3 Se 2 nanocrystals. (c) TEM image of nanosheets grown with cyanamide instead of dicyandiamide (in inset: size-distribution histogram). (d) XRD patterns for the nanocrystals formed upon injection of selenourea (denoted as t=0) and resulting from the control experiment, along with that obtained for the -In 2 Se 3 nanosheets for comparison. The extinction spectra of dispersions of nanocrystals and nanosheets in hexane are shown in (e) and (f) respectively. S3

4 Figure S2. TEM images of nanocrystals obtained by replacing InCl 3 with a) InBr 3 and b) InI 3 and maintaining all other reaction conditions identical to those described for the synthesis of 900 nm -In 2 Se 3 nanosheets. Figure S3. (a) TEM image (inset: size-distribution histogram), (b) large-area SAED pattern and (c) extinction spectrum of 300 nm nanosheets synthesized with 60 % volume fraction of oleylamine (all other reaction conditions were kept the same as those used to synthesize the 900 nm -In 2 Se 3 nanosheets). S4

5 S2. Syntheses using a single-injection procedure Experimental: InCl 3 (10mg, 45µmol), oleylamine (1.0 ml) and 1-octadecene (4.0 ml) were loaded into a 25 ml 3-neck round-bottomed flask equipped with a thermocouple and a magnetic stirrer and degassed at 100 C for 1 h. Thereafter, the temperature was raised to 215 C (or 200 C) under a dry nitrogen flow and a mixture of selenourea (12 mg, 97 µmol) and dicyandiamide (3.7 mg, 45 µmol) dissolved in DMF (200 µl) were injected. The reaction mixture was allowed to stir for additional 10 minutes and finally the heating mantle was removed to cool the reaction mixture. The work-up procedure was identical to the one reported in the main text for the 900 nm nanosheets. Figure S4. TEM images of indium selenide nanocrystals and nanosheets obtained with a singleinjection procedure at 215 C (a,b) and at 200 C (c). S5

6 S3. Additional x-ray diffraction simulations Experimental Sim. 1L Sim. 2L Sim. 3L * (, Cu K ) Figure S5. Experimental XRD pattern along with simulated patterns of -In 2 Se 3 crystals 1 to 3 layers thick (* the peak labeled with an asterisk (*) in the 3-layer pattern is assigned to the superposition of (101) and (111) reflections, not to the (100) peak). S6

7 S4. Surface composition, TGA and AFM topography The presence of ligands was confirmed by elemental and thermogravimetric (TGA) analyses. Electron energy loss spectroscopy (EELS) on as-synthesized single nanosheets confirmed the presence of carbon and nitrogen in considerable amounts (Figure S5). SEM-EDS analysis yielded similar results (12% In, 6% N, 57% C). Upon extensive washing with methanol, the amount of organic (ligands) content is decreased as can be inferred from the smaller weight loss observed in the TGA curve after washing (Figure S6) and from EDS analysis (17% In, 7% N, 47% C). Moreover, upon washing, the nanosheets cannot be dispersed anymore and were found to be much aggregated into stacks, confirming the stripping of ligands. Thus, it is expected that the close-range stacking gives signatures in the XRD pattern at positions close to the (005) and (006) reflections observed in bulk at 2 = and 18.54, respectively. Therefore, we assign the reflections observed in the experimental XRD pattern at 15.7 and 19.6 to the random closerange stacking of the nanosheets. Figure S6. EELS spectrum of -In 2 Se 3 nanosheets. S7

8 Weight (%) Washed Unwashed Temperature ( C) Figure S7. Thermogravimetric analysis of washed (as described for XRD) and unwashed (asdescribed in the synthetic procedure) In 2 Se 3 nanosheet powders. Figure S8. a) Typical AFM topography image of a single In 2 Se 3 nanosheet, and b) the heightprofile of the corresponding line-scan of the same nanosheet. S8

9 Figure S9. Thickness-distribution histograms as determined by AFM topographic measurement for (a) 900 nm nanosheets, and (b) 300 nm nanosheets. S9

10 S5. Raman spectroscopy and thermal stability A 2 1g a-se 60 s 50 s 40 s 30 s 20 s 10 s Raman shift (cm -1 ) Figure S10. Raman spectra of -In 2 Se 3 nanosheets after increasing exposure times ( = 514 nm, P = 0.4 mw). The peaks are attributed to the A 2 1g mode (205 cm -1 ) of -In 2 Se 3 and amorphous selenium (a-se, 250 cm -1 ) S10

11 Figure S11. TEM images of In 2 Se 3 nanosheets annealed under vacuum (P 10-3 bar) at 150 C, 200 C and 300 C. S11

12 S6. Results of DFT Calculations Figure S12. (a) Electronic band structure and (b) density of states (DOS) of the In 2 Se 3 β-phase monolayer. For all cases the conduction band was rigidly shifted up in energy by 0.7 ev with respect to the DFT result, to match the onset of the calculated absorption with the experimental value. S12

13 S7. External quantum efficiency and detectivity of photodetectors Figure S13. (a) Current-voltage curves recorded from a single monolayer β-in 2 Se 3 nanosheet contacted with Ti/Al electrodes in the dark and upon illumination with a 473 nm laser. (b) External quantum efficiencies (EQE) and (c) detectivities (D*) of several β-in 2 Se 3 monolayer devices at 5V bias. S13