Supporting Information for. Designing High-Performance PbS and PbSe Nanocrystal. Electronic Devices through Stepwise, Post-Synthesis, Colloidal

Transcription

1 Supporting Information for Designing High-Performance PbS and PbSe Nanocrystal Electronic Devices through Stepwise, Post-Synthesis, Colloidal Atomic Layer Deposition Soong Ju Oh, 1 Nathaniel E. Berry, 1 Ji-Hyuk Choi 1,4, E. Ashley Gaulding, 1 Hangfei Lin, 1 Taejong Paik, 3 Benjamin T. Diroll, 3 Shin Muramoto, 5 Christopher B. Murray, 1,3 and Cherie R. Kagan 1,2,3,* 1 Department of Materials Science and Engineering, 2 Department of Electrical and Systems Engineering, 3 Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA 4 Complex Assemblies of Soft Matter, CNRS-Rhodia-UPenn UMI 3254, Bristol, PA USA 5 National Institute of Standards and Technology, Gaithersburg, MD USA *To whom correspondence should be addressed. kagan@seas.upenn.edu S1

2 Materials : Tri-n-octylphosphine (further referred to as TOP, 90%), oleic acid (OA, 90%), 1- octadecene (ODE, 90%), oleylamine (70%), lead oxide (PbO, %), selenium pellets (Se, %), diphenylphosphine (DPP, 98%), (3-mercaptopropyl)trimethoxysilane (MPTS, 95%), lead chloride (PbCl 2, 99%), anhydrous chloroform, anhydrous hexane, anhydrous iso-propanol, anhydrous methanol, anhydrous toluene, and anhydrous acetonitrile are purchased from Aldrich. Anhydrous acetone and bis(trimethylsilyl) sulfide (TMS, 95%) are purchased from Acros. Bis(trimethylsilyl) selenide ((TMS) 2 Se) is bought from Gelest. Anhyrdrous sodium selenide (Na 2 Se, 99.8%) and sodium sulfide (Na 2 S) are bought from Alfa Aesar. Anhydrous potassium hydrogen sulfide (KHS, 95%) is purchased from Strem chemical. Methods Synthesis of PbSe and PbS NCs Synthesis of 5.9 nm PbSe NCs is performed using a slight modification of a previously reported procedure. 1 NC synthesis is carried out under nitrogen atmosphere with a Schlenk line system using standard air-free procedures. A solution of 892 mg PbO, 3 ml of oleic acid, and 20 ml of ODE is heated to 120 C and degassed for 1.5 hours under vacuum. The temperature is then raised to 180 C, at which point 8 ml of the Se precursor (1M TOP:Se and 60 µl of DPP) is rapidly injected into the hot solution. After sec of reaction time, the heating source is removed and the solution is allowed to cool to room temperature. The NCs are purified within a nitrogen glove box by first adding 2 ml of hexane to the reaction flask, then precipitating the solution with ethanol/isopropanol and centrifuging at 8000 rpm for 3 min and then redispersing in 4 ml of hexane. The wash process is repeated three more times using ethanol, S2

3 acetone/isopropanol, and then isopronapol as the antisolvents. Finally, the NCs are dispersed in 10 ml of hexane and stored in the glove box. Before the NCs are deposited, the solution is dried under vacuum to remove the hexane, and the NCs are re-dispersed in octane. 6.0 nm PbS NCs are synthesized using a previously reported method mg of PbO and 15 ml of oleic acid are heated in a 50 ml flask to 120 C and degassed under vacuum on a Schlenk line for two hours. During this time, a 42 µl TMS/2 ml ODE solution is prepared in a nitrogen glove box. After degassing, the reaction solution is put under nitrogen and the temperature lowered to 110 C at which point 5 ml of the TMS/ODE sulfur precursor solution is rapidly injected. After a growth time of 30 seconds, the reaction solution is quenched in a water bath. The NCs are then washed by adding 2 ml of hexane to the solution, then precipitating with 30 ml of acetone and centrifuging at 8000 rpm for 3 min and decanting the supernatant. NCs are washed a second and third time by redispersing the pellet in 6 ml of hexane, precipitating with 8 ml of acetone, and centrifuging at 8000 rpm for 3 min. NCs are then redispersed in 10 ml of hexane and stored in a nitrogen glove box. Before the NCs are deposited, the solution is dried under vacuum to remove the hexane, and the NCs are re-dispersed in octane. Preparation of PbCl 2 solution mm PbCl 2 in oleylamine is heated to 150 C and degassed for 1 h under vacuum. The solution is cooled to 70 C and transferred to the glove box. While PbCl 2 solutions at concentrations higher than 100 mm are cloudy-white, solutions at mm are transparent and yellowish without any precipitates. Device fabrication S3

4 All fabrication steps are carried out in the nitrogen glove box after cleaning the substrate and treating their surfaces with a self assembled monolayer, unless specified. Devices for FET and C-V measurements are made on heavily n-doped silicon wafers with 250 nm of thermally grown SiO 2, which serve as the back gate and part of the gate dielectric stack, respectively. The substrate is washed with DI water, acetone and IPA and cleaned with UVozone for 20 min. A 20 nm layer of Al 2 O 3 is deposited using atomic layer deposition (ALD) on top of the SiO 2. The substrate is then immersed in MPTS in IPA for 12 h to assemble a MPTS organic monolayer. When the substrate is removed from the solution, it is rinsed with clean toluene, sonicated with ethanol for 5 min, transferred to the nitrogen glove box and heated to 90 C for ten minutes. A solution of 10 mg/ml PbSe NCs in octane is spincoated at rpm to deposit a uniform NC thin film solid. For chalcogenide treatment, the device is soaked in mm Na 2 Se, Na 2 S or KHS in methanol for 10s, and washed with methanol twice. For lead treatment, the device is soaked in the PbCl 2 in oleylamine solution for the desired time at temperatures between C, and washed with hexane three times and further washed by isopropanol. 20 nm Cr and 30 nm Au source and drain top contacts separated by μm are defined by thermal evaporation through a shadow mask. Channel length (L) and width (W) is at constant W/L of 15 for all devices. Finally, each device is mechanically isolated both from each other and from the edge of the substrate to minimize leakage currents. Devices for Hall measurements are made on quartz, instead of on silicon wafers. The same process of cleaning, MPTS treatment and NC thin film fabrication is conducted as described above for FETs. Four square 40 nm thick Au electrodes (1 mm 2 ) separated from each other by 5 mm in a square pattern are defined by thermal evaporation through a shadow mask. Each device S4

5 is isolated by carefully scraping away the film around the square defined by the electrodes with a toothpick. Measurement and Characterization FET characterization is conducted on a Model 4156C semiconductor parameter analyzer (Agilent) in combination with a Karl Suss PM5 probe station mounted in a nitrogen glove box. Capacitance-Voltage characterization is carried out on a Model 4192A LF impedance Analyzer (Agilent) in a nitrogen glove box. Source and drain electrodes are electrically shorted and connected to the low terminal and the gate is connected to the high terminal of the LCR meter. C-V curves are measured in the range of 100 Hz to 5 khz and representative data is obtain at 1 khz. Hall measurements are carried out using an MMR Technologies H-50 measurement system with a 0.5 T permanent magnet inside a nitrogen glove box. Each measurement is preceded by a four point probe resistivity measurement using the Van der Pauw geometry followed by field and current reversal Hall measurements. Small-angle x-ray scattering (SAXS) is performed in transmission under vacuum on a multi-angle x-ray diffractometer system with a Bruker FR591 rotating anode at 40 kv and 85 ma, Osmic confocal optics, Rigaku pinhole collimation, and Bruker HiStar Multiwire detector at a fixed distance of 54.0 cm. Wide-angle x-ray scattering (WAXS) is performed in reflection geometry using a Rigaku Smartlab diffractometer operating at 40 kv and 30 ma. Absorption and FTIR spectroscopy are performed in transmission mode and samples are prepared on quartz and double side polished silicon substrates, respectively. S5

6 X-ray photoelectron spectroscopy (XPS) data is acquired on a Kratos Axis Ultra delayline detector (DLD) instrument* (Kratos, Manchester, England) in the hybrid mode, using a monochromatic Al Kα 1,2 x-ray source (hv = ev). The axis of the analyzer lens is oriented at 0 from the surface normal to capture the emitted photoelectrons. Atomic compositions are obtained from a survey scan covering 0 ev to 1300 ev using a pass energy of 40 ev with an energy resolution of 0.5 ev. High-resolution spectra of Se 3d (44 ev to 68 ev), Pb 4f (127 ev to 151 ev), C 1s (278 ev to 298 ev), and O 1s regions (525 ev to 540 ev) are acquired using a pass energy of 40 ev with an energy resolution of 0.1 ev. At least 3 scans are performed on each sample. Energy scales are calibrated by normalizing the large CH x peak in the C 1s region to ev and a linear background is subtracted for all peak quantifications. The peak areas are normalized by the manufacturer supplied sensitivity factors. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) experiments are performed using an ION-TOF TOFSIMS IV (IONTOF GmbH, Münster, Germany) equipped with a 25 kev Bi + 3 primary ion source oriented at an angle of 45 from the surface normal. Mass spectra are obtained in both positive and negative polarities using a high-current bunched mode for high mass resolutions. Analysis is confined to an area within 200 µm 200 µm with a pixel density of 128 pixels 128 pixels. At least 3 measurements are taken per sample per polarity. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is conducted on a Spectro Genesis ICP-OES. *Certain commercial equipment, instruments, or materials are identified in this paper to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor S6

7 does it imply that the materials or equipment identified are necessarily the best available for the purpose. Discussion We study the effect of air exposure for both Se-rich and Pb-rich PbSe NC FETs [Supporting Information Figure S13]. After a few minutes of air exposure, Se-rich NC FETs show the same semi-metallic behavior, but with an increase in conductance. We attribute the conductance increase to the increased number of midgap and acceptor states due to oxidation. 3,4 After a few minutes of air exposure, Pb-rich NC FETs switch polarity from n-type to p-type consistent with oxygen doping. 3,4 S7

8 Figure S1. FT-IR spectra of (A) as-prepared (black), Na 2 Se treated (blue), and Na 2 Se followed by PbCl 2 treated (red) PbSe NC thin films and (B) as-prepared (black), Na 2 S treated (red), KHS treated (orange), Na 2 S followed by PbCl 2 (green), and KHS followed by PbCl 2 (blue) treated PbSe NC thin films. S8

9 Figure S2. (A) HRTEM image and inset, diffraction pattern of Na 2 S followed by PbCl 2 treated PbSe NC thin films. TEM images of (B) Na 2 Se followed by PbCl 2 treated PbSe NC thin film, (C) S9

10 Na 2 S followed by PbCl 2 treated PbSe NC thin film, (D) Na 2 S followed by PbCl 2 treated PbS NC thin film, and (E) Na 2 Se followed by PbCl 2 treated thicker, 2-3 layer PbSe NC thin film. (F) SEM image of a continuous 2-3 layer PbSe NC thin film treated by Na 2 Se followed by PbCl 2. Scale bars : (A-E) 10nm (F) 1um. S10

11 Figure S3. SIMS data for the relative amount of (A) chlorine and (B) sulfur in as-prepared (OA), Na 2 Se treated, Na 2 Se followed by PbCl 2 treated, Na 2 S treated, and Na 2 S followed by PbCl 2 treated PbSe NC thin films. S11

12 Figure S4. (A) Pb:Se ratio characterized by EDX as a function of PbCl 2 treatment time for Na 2 Se treated (black), Na 2 S treated (red), and KHS treated (blue) PbSe NC thin films. (B) Pb:Se ratio as a function of PbCl 2 treatment time for Na 2 Se treated PbSe NC thin films fit to with the characteristic time of hours. S12

13 Intensity [a.u.] [deg] Figure S5. Wide angle X-ray scattering for Na 2 S (red) and both PbCl 2 and Na 2 S (black) treated PbSe NC thin films. S13

14 Figure S6. X-Ray photoemission spectroscopy scans for (A) the Se 3d and (B) Pb 4f in asprepared PbSe NC thin films, and for the Pb 4f for (C) Na 2 Se treated, and (D) both Na 2 Se followed by PbCl 2 treated PbSe NCs solids. S14

15 Intensity [a.u.] Intensity [a.u.] (A) Se Se_Pb (B) S S_Pb [deg] [deg] Figure S7. SAXS data for (A) Na 2 Se treated (black) and Na 2 Se followed by PbCl 2 treated (red) PbSe NC thin films, and (B) Na 2 S treated (black), and Na 2 S followed by PbCl 2 treated (red) PbSe NC thin films. S15

16 Figure S8. Capacitance-Voltage measurements of (A) Na 2 Se treated PbSe NC thin films (black) and after 1h (blue), 2h (green), 6h (orange), and 12h (red) of PbCl 2 treatment and (B) Na 2 S treated PbSe NC thin films before (black) and after 1h (blue), 6h (green), and 12h (red) of PbCl 2 treatment. S16

17 Figure S9. Transfer curves of Na 2 S treated PbSe NC thin films before (black) and after PbCl 2 treatment for 2 h (red) and 12 h (blue). S17

18 Figure S10. Transfer curves of Na 2 Se treated PbSe NC FETs before (black) and after PbCl 2 treatment at 65 C for 1 min (blue), 10 min (green), and 30 min (red). S18

19 Figure S11. Transfer curves of (A) Na 2 Se followed by PbCl 2 treated PbSe NC FETs with the mobility of 4.5 cm 2 /Vs,(B) Na 2 S followed by PbCl 2 treated PbSe NC FETs with the mobility of 1.8 cm 2 /Vs. PbCl 2 treatment was carried out at 65 C for 12 h. S19

20 Figure S12. (A) Transfer curves for PbSe NC FETs before annealing (black), after annealing in oleylamine at 65 C for 1h (red), and after annealing under N 2 at 65 C (green). TBAC treated PbSe NC FETs (blue). (B) Transfer curves of Se-enriched, PbSe NC FETs treated with PbCl 2 at 0.01 mm (black), 0.5 mm (red), and 10 mm (blue). S20

21 Figure S13. Transfer curves of (A) a Na 2 Se treated PbSe NC FET before (black) and after (red) a few minutes of air exposure, and (B) a Na 2 Se followed by PbCl 2 treated PbSe NC FET before (black) and after (red) a few minutes of air exposure. S21

22 REFERENCES (1) Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L. Chem. Mater. 2004, 35, (2) Hines, M. a.; Scholes, G. D. Adv. Mater. 2003, 15, (3) Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J. ACS nano 2008, 2, (4) Leschkies, K. S.; Kang, M. S.; Aydil, E. S.; Norris, D. J. J. Phys. Chem. C 2010, 114, S22