Effects of Post-Synthesis Processing on CdSe Nanocrystals and Their

Oct 21, 2014 - Department of Electrical and Systems Engineering, University of Pennsylvania, 200 South 33rd Street, Philadelphia, Pennsylvania. 19104 ...
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Effects of Post-Synthesis Processing on CdSe Nanocrystals and Their Solids: Correlation between Surface Chemistry and Optoelectronic Properties E. D. Goodwin,† Benjamin T. Diroll,† Soong Ju Oh,‡ Taejong Paik,§ Christopher B. Murray,†,‡ and Cherie R. Kagan*,†,‡,§ †

Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States § Department of Electrical and Systems Engineering, University of Pennsylvania, 200 South 33rd Street, Philadelphia, Pennsylvania 19104, United States ‡

S Supporting Information *

ABSTRACT: In this work, we report the effects on CdSe nanocrystal (NC) surface chemistry of acetone and methanol when used as the antisolvents for NC washing and as the solvents for ligand exchange of NC solids with ammonium thiocyanate (NH4SCN). We find that NCs washed with methanol have significantly fewer remaining organic ligands and lower photoluminescence quantum yield than those washed with acetone. When used as the ligand exchange solvent, methanol leaves more organic ligands and introduces fewer bound thiocyanates on the NC surface than when acetone is used. We demonstrate the effect of these different surface chemistries on NC solid optoelectronic properties through photoconductivity measurements, showing a greater photocurrent in NC solids with greater organic ligand coverage. We also show that NC washing with methanol or ligand exchange with NH4SCN in methanol removes a significant number of surface Cd atoms from the NCs, creating Cd vacancies that act as traps for recombination. Independent of the wash and exchange process, the NC surface may be repaired by introducing CdCl2 to the NC surface, enhancing the measured photocurrent.

1. INTRODUCTION

The electronic and optoelectronic properties of NC solids are highly sensitive to NC composition and surface chemistry, making it essential to have accurate knowledge and control of these NC characteristics for comprehensive device studies. It has long been known that exposing surfactant-capped NCs to solvents, such as is done during NC washing, can remove surfactant molecules and thereby reduce the photoluminescence (PL) quantum yield (QY), but it has been shown recently by Hassinen et al.33 that this effect on NC surface chemistry is enhanced for protic versus aprotic solvents. Similarly, Kirmani et al.34 and Ning et al.35 have shown that when used as a ligand exchange solvent, methanol can remove passivating atomic chlorine from the surface of PbS NCs. In addition, Anderson et al.36 have shown that carboxylate−metal complexes can leave the NC surface in the presence of Lewis bases, changing the NC composition. There is increased recognition that the solvents selected for NC washing and ligand exchange reactions impact the NC surface chemistry and therefore ultimately affect the electronic and optoelectronic

Colloidal semiconductor nanocrystals (NCs) are promising materials for electronic1 and optoelectronic2−4 devices, such as transistors,5−7 photovoltaics,8−10 photodetectors,11−13 and light-emitting diodes,14,15 because of their size, shape, and compositionally tunable optical and electronic properties16−19 and the simple, solution-based, low-cost processing techniques available for NC synthesis and device integration. Common wet-chemical methods for the synthesis of colloidal NCs use organic media and long-chain surfactants to control NC size, shape, monodispersity, and solubility in organic solvents. To isolate the NCs from the growth medium and remove excess surfactant, the NC synthesis products are washed using a polar organic antisolvent, often a short-chain alcohol. When these NCs are assembled into a thin-film solid for device integration, the long-insulating ligands must be removed or replaced by a ligand exchange reaction to reduce the spacing between neighboring NCs and thereby increase charge carrier transport. Recent efforts20 have explored a large range of ligand exchange agents, such as short-chain thiols,21,22 amines,23,24 and small organic acids25,26 with increased emphasis most recently on inorganic agents, for example, metal chalcogenides,6,7,27−29 halides,6,7,9,30 and thiocyanate (SCN).5,31,32 © 2014 American Chemical Society

Received: July 30, 2014 Revised: October 10, 2014 Published: October 21, 2014 27097

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with the same antisolvent are performed. These multiple washings in antisolvent are done carefully to ensure complete removal of unreacted Cd precursor. The NCs are then dispersed in hexanes. For brevity, in this text, CdSe NCs washed with acetone during purification will be referred to as AW (for acetone wash) and those washed with methanol will be referred to as MW (for methanol wash). 2.3. Solution-Phase Optical Measurements. UV−vis absorption spectra of dilute AW or MW CdSe NCs in hexane are taken in air on a Cary 5000 spectrophotometer at 2 nm spectral bandwidth. The same samples are then transferred to a FluoroLog-3 Horiba Jobin-Yvon spectrofluorometer with a Si PMT detector for PL and time-correlated single-photon counting (TCSPC) measurements. PL measurements are made with 450 nm excitation, 1 nm spectral slit widths, and 1 s integration time. TCSPC measurements are carried out by excitation with a 405 nm Picoquant pulsed diode laser and collection at the NC PL peak (651 nm) using 2 nm spectral slit widths and a Si PMT detector connected to a Horiba JobinYvon single photon counting controller (Fluorohub). To extract PL lifetimes, TCSPC data are fit by nonlinear leastsquares regression to the sum of two exponential decays. 2.4. NC Solid Assembly and Treatments. AW or MW CdSe NCs in octane are spincast in a nitrogen glovebox as a single layer at 500 rpm for 30 s, then 800 rpm for 30 s onto substrates that are treated with a self-assembled monolayer (SAM) of (3-mercaptopropyl)-trimethoxysilane (MPTS) according to modified literature procedures.38 Substrates are cleaned by ultrasonication in 5% detergent in DI water, DI water, 1 M hydrochloric acid, and ethanol before being dried in an oven at 100 °C for at least 60 min. Substrates are then immersed in a 5% MPTS in anhydrous toluene solution, sealed, and left for at least 16 h to allow for self-assembly. Coated substrates are removed from the MPTS solution and rinsed by ultrasonication three times each with toluene and ethanol. All substrate preparation is carried out in air. NC solid assembly and processing is performed in a nitrogen glovebox. The NH4SCN ligand exchange reaction is carried out by soaking the NC solids in the exchange solution for 2 min, followed by two 30 s soaks in pure solvent to rinse away excess NH4SCN. Ten mg/mL NH4SCN in acetone is used for acetone exchange of NC solids (AEX). 400 mg/mL NH4SCN in methanol is used for methanol exchange of NC solids (MEX). (See Supporting Information Figure S1 for a discussion of the NH4SCN exchange concentration dependence in acetone and methanol.) Note that NH4SCN concentrations in this work are mg solute/ mL solvent for experimental ease, with the volume of a solution prepared at the high solute concentration of 400 mg of NH4SCN and 1 mL of methanol rising to 1.3 mL upon mixing. Four wash/exchange solvent combinations are studied in this work: CdSe NCs washed with acetone and exchanged in acetone (AWAEX), NCs washed with acetone and exchanged in methanol (A W M EX ), NCs washed with methanol and exchanged in acetone (MWAEX), and NCs washed with methanol and exchanged in methanol (MWMEX). For CdCl2 treatment, NC solids are soaked for 10 min in a saturated solution of CdCl2 in acetone made by stirring excess CdCl2 in acetone for 10 min, followed by two 1 min soaks in pure acetone to remove excess CdCl2. The solubility of CdCl2 in acetone is found to be MWAEX because the AWAEX NC solid has more remaining organic ligand than the MWAEX NC solid and neither NC solid exhibits a large red shift and broadening of the first absorption maximum. The photocurrent is not increased by a greater amount of SCN on the NC surface which is consistent with SCN introducing midgap trap states. After CdCl2 treatment, the photocurrent increases greatly regardless of wash and exchange solvent combination because added Cd atoms repair the NC surface, filling vacancies and doping the NC solid. The photocurrent increase is also attributed to the passivation and doping introduced by Cl as well as a reduction of trap states as Cl removes/replaces SCN. The photocurrent enhancement, despite a blue shift and narrowing of all of the NC solids upon CdCl2 treatment, further supports that lifetime and not mobility governs optoelectronic properties in these materials.

introducing impurities that block Cd sites, akin to the methanol blocking of oleate addition,33 or from fewer available Cd sites suggested by the fusion of neighboring NCs (discussed later), as seen without annealing in solution-based treatment of lead chalcogenide NC solids.7,42 It has been suggested that Cdblocking impurities could be hydroxide groups from a condensation reaction between oleic acid and methanol.33 Such a reaction would occur to a much larger extent in the methanol wash step because there is an excess of oleic acid leftover from the NC synthesis, consistent with the severely reduced SCN content on the MWAEX and MWMEX NC solids. Treatment with CdCl2 reintroduces surface Cd atoms and at least in part replaces SCN by Cl, with a uniformly small decrease in bound organic ligands. Figure 5 schematically summarizes the evolution in NC surface chemistry for antisolvent wash, ligand exchange, and CdCl2 treatment, providing a picture of the NC surface chemistry probed in optoelectronic measurements. To summarize the NC solid optoelectronic properties, Figure 4d,e presents the wavelength and width, respectively, of the first UV−vis absorption maximum, and Figure 4f shows the photocurrent at 2.5 × 104 V/cm for each of the NC solids. An interesting correlation between the NC solid surface chemistry and optical properties is seen by comparing solids exposed to methanol. The MWAEX NC solid has similar Cd content and much fewer organic ligands compared with the AWMEX NC solid but also much less SCN, suggesting that surface Cd sites are blocked by impurities introduced in the methanol wash step. The surface Cd site availability in the MWAEX NC solid is not likely affected by NC fusing, as the more dramatic red shift and broadening consistent with fusing is not observed. Such a red shift and broadening is observed for NC solids exchanged in methanol, as seen in the AWMEX and MWMEX NC solids. The loss of available surface Cd sites from impurities during washing and from impurities and NC fusion during exchange results in the least SCN in the MWMEX NC solid. Photocurrent is maximized in a NC solid with the highest product of mobility and lifetime, which are known to be increased by electronic coupling and trap passivation. Before CdCl2 treatment, the greatest photocurrent is measured in the AWMEX NC solid, which exhibits a large red shift and broadening of the first absorption maximum, consistent with appreciable electronic coupling, and the most remaining organic ligand, consistent with increased trap passivation. The

4. CONCLUSIONS We demonstrate the important role of solvent selection used in NC wash and ligand exchange processes on the NC surface chemistry and therefore NC solid optoelectronic properties. Postsynthesis washing of CdSe NCs with methanol removes a significant amount of organic ligand and Cd atoms from the NC surface as compared with washing with acetone. Exchange of the NC solids with the compact NH4SCN ligand in methanol removes additional Cd atoms from the NC surface, not seen when acetone is selected as the ligand exchange solvent. Exchange with NH4SCN in methanol also results in increased electronic coupling through NC fusing, not usually seen in CdSe NC solids without thermal annealing. The loss of organic ligands and surface Cd atoms and the introduction of SCN reduces the carrier lifetime and limits photoconductivity in CdSe NC solids. Cd vacancies created during NC wash and exchange can be refilled by treatment in a Cd-salt, repairing the NC surface and greatly enhancing the charge carrier lifetime and photoconductivity. This is similar to the increased carrier lifetime and mobility seen upon metal halide treatment of NC solids constructed from materials such as PbS30 and PbSe,7 suggesting metal halide treatment may generally be used to repair the surface of NC materials after processing. These results emphasize the importance of careful solvent choice for 27103

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NC wash and ligand exchange and stress the thorough understanding of NC composition and surface chemistry when designing NC solids for electronic and optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

HRTEM images, EDX spectra, additional IR absorption spectra, current versus electric field data, time-resolved current data, and UV−vis diffuse reflectance spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSF CBET program under award no. CBET1236406 for primary support of this work for NC solid preparation and ligand exchange; FTIR, UV−vis, and cw and time-resolved PL spectroscopies; and photoconductivity measurements. We thank the U.S. Department of Energy Office of Basic Energy Sciences, Division of Materials Science and Engineering under award no. DE-SC0002158 for support of NC synthesis, TEM, and EDX spectroscopy.

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