J. Phys. Chem. C 2010, 114, 1539–1546
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Binary Amine-Phosphine Passivation of Surface Traps on CdSe Nanocrystals Wonjung Kim, Sung Jun Lim, Sunghan Jung, and Seung Koo Shin* Bio-Nanotechnology Center, Department of Chemistry, Pohang UniVersity of Science and Technology, San31 Hyoja-dong Nam-gu, Pohang, Kyungbuk 790-784, Korea ReceiVed: NoVember 25, 2009
Surface traps, such as electron and hole traps, quench the photoluminescence (PL) of semiconductor nanocrystals. We find a binary ligand system that effectively passivates those surface traps on bare CdSe nanocrystals, thereby making the nanocrystals highly luminescent. Zinc-blende CdSe nanocrystals are prepared by colloidal synthesis, and their optical properties are monitored by varying the amounts of propylamine (PA) and tributylphosphine (TBP) in chloroform at room temperature. The starting CdSe nanocrystals that are mostly covered with fatty acid carboxylates show very low quantum efficiency with a multiexponential PL decay. Addition of excess PA induces blueshifts in both absorption and emission spectra with a slight increase in quantum efficiency and PL lifetime, whereas that of TBP reduces the PL intensity. Surprisingly, addition of both PA and TBP makes nanocrystals emit light with an ∼50% quantum efficiency and a nearly single-exponential PL decay, regardless of the sequence of ligand addition. We further characterize the chemical species dissolved into the solution after amine-phosphine passivation by using X-ray photoelectron spectroscopy (XPS) and electrospray ionization mass spectrometry (ESIMS). The XPS data indicate that fatty acid carboxylates are covered on the surface of as-prepared nanocrystals and both amine and phosphine remove carboxylates from the surface. The ESI mass spectra identify the chemical species dissolved into the solvent by ligand. Taken all together, our results suggest that primary amine and tertiary phosphine dissolve surface adatoms into the solution and cooperatively passivate surface dangling bonds on CdSe nanocrystals, thereby reproducibly yielding bright CdSe nanocrystals. Introduction Nanocrystals having a high surface-to-volume ratio are covered with many surface dangling bonds that can trap either electrons or holes of the exciton.1-5 On the surface of CdSe nanocrystals, unoccupied valence orbitals of cadmium and nonbonding valence electrons of selenium act as electron and hole traps, respectively.4 These surface traps lead to deep-trap emission while suppressing band-edge emission,6 and their heterogeneous distributions on the surface cause significant variations in optical properties of single nanocrystals.7 Thus, the photoluminescence (PL) quantum efficiency (QE) strongly depends on the extent of surface passivation removing these traps.1-5 Surface traps have typically been removed by growing the shell of larger band-gap materials, such as CdS,8-10 ZnSe,9-11 and ZnS,9,10,12-14 on the CdSe core. The epitaxial growth of these inorganic shells covers up both cadmium and selenium dangling bonds on the CdSe surface, thereby suppressing deep-trap emission while enhancing band-edge emission.8-14 However, the surface heterogeneity of the core is not washed out nor homogenized by successive overcoating of shell materials,7 which results in nonuniform shells with interfacial defects and/ or stacking faults.13 Consequently, optical properties of CdSebased core/shell nanocrystals vary from batch to batch, no matter how elaborately they are synthesized.7 One convenient way of preparing CdSe nanocrystals with reproducible optical properties is to use organic ligands to clean up and passivate the surface of the nanocrystals. Organic ligands can coordinate weakly bound adatoms on the surface to dissolve them into the solvent and coordinately passivate the surface * To whom correspondence should be addressed. E-mail: skshin@ postech.ac.kr.
dangling bonds. In this paper, we present a binary aminephosphine ligand system to clean up and passivate the surface of CdSe nanocrystals and describe its remarkable effects on the enhancement of the QE and the elongation of the PL lifetime. We have prepared zinc-blende (ZB) CdSe nanocrystals using a noncoordinating solvent system.10 The binary ligand system is composed of a primary amine and a tertiary phosphine. Primary amines have been used as an additive to focus the size in colloidal synthesis of CdSe nanocrystals.14-16 Tertiary phosphines have been used in preparation of chalcogen precursors by reacting with chalcogens.8,9,11-18 Both amine and phosphine can coordinate cadmium, but only phosphine can form phosphine chalcogenides with the Group VI elements by valenceshell expansion of phosphorus. We have obtained the optical spectra of CdSe nanocrystals and their PL decay profile by varying the amounts of surface-passivating ligands in chloroform at room temperature. The absorption spectra provide information about the degree of surface cleanup and/or size reduction, the emission spectra and QE yield information about the efficacy of surface passivation on the light-emitting bright state, while the PL decay provides information on the dynamics of exciton relaxation. We have also varied the Cd/Se ratio on the surface of the nanocrystals to differentiate the role of the two ligands on surface passivation. Further, we have examined the effects of amine-phosphine passivation on five different sized nanocrystals emitting at 520-600 nm. We have also compared the passivation efficacy of amine using primary, secondary, and tertiary amines. Lastly, we have characterized the chemical species dissolved into the solution by using X-ray photoelectron spectroscopy (XPS) and electrospray ionization mass spectrometry (ESIMS).
10.1021/jp911207v 2010 American Chemical Society Published on Web 01/05/2010
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J. Phys. Chem. C, Vol. 114, No. 3, 2010
Experimental Section Chemicals. Cadmium oxide (CdO, 99.99%) was purchased from Kojundo Chemical Laboratory (Saitama, Japan). Selenium powder (Se, 100 mesh, g99.99%), stearic acid (95%), oleic acid (technical grade, 90%), 1-octadecene (ODE, technical grade, 90%), tributylphosphine (TBP, 97%), trioctylphosphine (TOP, technical grade, 90%), tributylphosphine oxide (TBPO, 95%), trioctylphosphine oxide (TOPO, 99%), triphenylphosphine (TPP, 99%), propylamine (PA, 98%), tributylamine (TBA, g98.5%), dodecylamine (DDA, 98%), octylamine (OA, 99%), dioctylamine (DOA, 98%), trioctylamine (TOA, 98%), anhydrous hexane (95%), and anhydrous chloroform (>99%) were purchased from Sigma-Aldrich (St. Louis, MO). Methanol (HPLC grade) and acetone (HPLC grade) were obtained from J. T. Baker (Phillipsburg, NJ). Anhydrous chloroform was degassed by freeze-pump-thaw before use. All other chemicals were used without further purification. Synthesis of CdSe Nanocrystals. CdSe nanocrystals were synthesized by the method described earlier.10 In a typical synthesis, CdO (4 mmol) and stearic acid (8-8.2 mmol) were mixed in ODE (∼5 mL) and heated to 200 °C under Ar. After the solution became clear, the temperature was reduced to 150 °C and ODE was added to make the volume of 50 mL. After adding Se powder (2 mmol), we raised the temperature to 220-240 °C. CdSe nanocrystals began to form as Se powder dissolved at ∼210 °C. When the desired emission color was reached, the growth was quenched by lowering the temperature. Unconsumed precursors and byproduct were washed out with methanol after adding oleic acid (a few milliliters). CdSe nanocrystals were harvested by precipitation in acetone, and the resulting nanocrystals were kept in anhydrous hexane under Ar. The concentration of the nanocrystal stock solution was determined by measuring the absorbance at the band edge. Preparation of Cd-Rich CdSe Nanocrystals. CdO (0.5 mmol) and oleic acid (∼1.5 mmol) were mixed in ODE (∼6 mL) and heated to 200 °C under Ar. After the solution became clear and colorless, the temperature was reduced to below 50 °C. The nanocrystal stock solution containing purified CdSe nanocrystals (∼850 µmol) was added to the cadmium oleate solution, and hexane was pumped out. The temperature was raised back to ∼200 °C and held there for ∼14 h. Cd-rich CdSe nanocrystals were washed with methanol and harvested by precipitation in acetone. Ligand-Dependent Optical Properties of CdSe Nanocrystals. As-prepared CdSe nanocrystals were dissolved in anhydrous chloroform (3 mL) in a 4 mL quartz cell, and the cell was sealed with a rubber septum under Ar. The concentration of the nanocrystals was kept at the band-edge absorbance of ∼0.1, corresponding to 0.5-2 µM. A controlled amount (from 1 to 500-2000 equiv excess) of surface-passivating ligand, such as PA, TBA, DDA, OA, DOA, TOA, TBP, TOP, and TPP, was injected into the nanocrystal solution using a syringe. One equivalent refers to the calculated amount of ligand that can passivate either Cd or Se atoms on the surface of the nanocrystals. The solution was kept in the dark. The absorption and emission spectra as well as the PL decay were measured before ligand addition and ∼14 h thereafter. The second ligand was added ∼14 h after addition of the first ligand, and then the optical properties were measured 5-8 h thereafter. Stability of Binary Amine-Phosphine Passivation and Reproducibility of Optical Properties. Nanocrystals passivated by binary ligands were stored in the dark for 3-4 days either under Ar or under air, and the stability was examined. The reproducibility of optical properties was tested by repeating the
Kim et al. following “wash-dry-passivation” cycle. Nanocrystals passivated by PA-TBP or PA-TPP in chloroform were mixed with excess anhydrous methanol to wash out the ligands and precipitate the nanocrystals. The precipitates were dried under vacuum and dissolved in oxygen-free anhydrous chloroform. Excess PA and a controlled amount of TBP or TPP were then injected into the solution to passivate the nanocrystals again. Powder X-ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM). An XRD sample was prepared by drying a drop of a CdSe nanocrystal stock solution onto a 1 × 1 cm2 silicon wafer. XRD patterns were obtained with a Rigaku D/MAX-2500 diffractometer. A HRTEM sample was made by placing a drop of a diluted CdSe nanocrystal stock solution onto a 400-mesh copper grid with a lacey carbon supporting film (PELCO). Excess liquid was wicked away. HRTEM images were taken with a JEOL JEM2100F Cs-corrected TEM operating at 300 kV. Absorption, Emission, and PL Decay Measurements. The absorption spectra were obtained with an Agilent 8453 UV/vis spectrometer. The emission spectra and PL decay were measured using a home-built bifurcated optical setup.5 The excitation light source was the 407 nm picosecond laser (PicoQuant, LDH-PC-405) operating at 2.5 MHz with 35 pJ per pulse. The same lens was used to focus the excitation light onto the sample and to collect the emission light. The emission spectra were taken with a spectrograph (Chromex, 250-IS, 300 gr/mm) equipped with a liquid-nitrogen-cooled charge-coupled device (LN/CCD) (Princeton Instruments, 1024E), while the PL decay was monitored using a monochromator (Acton, SP-150, 1200 gr/ mm) equipped with a photomultiplier tube (PMT) (Becker & Hickl, PMC-100-1). Both the signal from a single-photon counting PMT and the trigger signal from a driver (PicoQuant, PDL-800-B) were sent to a reverse start-stop time-correlated single-photon counting module (Becker & Hickl, SPC-630). XPS and ESIMS Analyses. An aliquot (1 mL) of CdSe solution (the band-edge absorbance of 0.4) in anhydrous chloroform was mixed with excess DDA (30 µL) or excess TBP (30 µL) and kept in the dark for ∼4 h under Ar. We used DDA in place of PA because PA was too volatile to study with XPS under ultrahigh vacuum. Each sample containing ligand and the control sample containing no ligand were washed with anhydrous methanol (15 mL). Resulting nanocrystals were precipitated by centrifuge (11 000-13 000 rpm for 10-30 min). Supernatants were saved for XPS and ESIMS analyses, whereas precipitates were washed twice with anhydrous methanol (2-3 mL) and dissolved in anhydrous chloroform. An XPS sample was prepared by drying a drop of a sample solution on a 200 nm thick gold-coated silicon wafer. The XPS spectra were taken with an XPS instrument (Thermo Scientific, K-Alpha) using aluminum KR radiation before and after 3 keV argon ion sputtering for 10 s. An area of ∼0.4 mm in diameter was exposed to X-ray radiation under ∼5 × 10-9 Torr. Each scan was recorded over a 20-30 eV range centered on the peak of interest with the pass energy of 50 at 0.1 eV increments and 50 ms dwell time per step. The spectra were averaged over 10 scans and calibrated against either the Au 4f or C 1s peak. Supernatants from DDA or TBP passivation in methanol were sprayed as prepared for ESIMS. The ESI mass spectra were acquired using a quadrupole time-of-flight (Q-TOF) mass spectrometer (Applied Biosystems, QSTAR Pulsar-i) equipped with a turbo ESI source operating at the electrospray voltage of 4 kV with a 5 µL min-1 flow rate.
Passivation of Surface Traps on CdSe Nanocrystals
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Figure 1. XRD patterns of as-prepared (black) and Cd-rich CdSe nanocrystals (red) and a HRTEM image of CdSe nanocrystals. Lines represent the XRD patterns of bulk zinc-blende CdSe.
Results and Discussion Lattice Structure, Size, and Shape of CdSe Nanocrystals. We characterized as-prepared nanocrystals synthesized from the 2:1 Cd/Se mixture and Cd-rich nanocrystals obtained from annealing of as-prepared nanocrystals in 3:1 to 4:1 Cd/Se mixtures. Se-rich nanocrystals were not characterized because annealing of as-prepared nanocrystals in ODE containing excess Se powder resulted in nonfluorescent aggregates. The highly crystalline ZB lattice patterns are apparent in Figure 1 (left) for both as-prepared and Cd-rich CdSe nanocrystals. The HRTEM image presented in Figure 1 (right) displays two-dimensional hexagonal packing of spherical nanocrystals with a diameter of ∼3.5 nm, indicating a narrow size distribution. Notably, Cdrich nanocrystals exhibit an extra shoulder peak at 2θ ) ∼20°, presumably due to cadmium oleate assembled on the surface of the nanocrystals.19,20 Further washing of Cd-rich nanocrystals with propylamine significantly reduces the height of this shoulder peak (data not shown).10 Both as-prepared and Cdrich nanocrystals are considered to be covered with fatty acid carboxylates, as shown in our recent report on mass spectrometric imaging of nanocrystal surfaces.21 Binary Amine-Phosphine Passivation of CdSe Nanocrystals. We examined the effects of the ligand on the optical properties of both as-prepared and Cd-rich CdSe nanocrystals dissolved in oxygen-free anhydrous chloroform. First, we varied the amounts of PA and TBP, observed the absorption and emission spectra, and monitored the PL decay. Results are shown in Figure 2a,c for as-prepared CdSe nanocrystals and in Figure 2b,d for Cd-rich CdSe nanocrystals. Spectral properties, such as the band-edge absorption and emission maxima (λabs and λem), full width at half-maximum (fwhm), QE, and PL lifetime (τeff), are listed in Table 1. The fitting parameters for PL decay curves are given in Table S1 in the Supporting Information (SI). The absorption spectra of as-prepared nanocrystals show the characteristic features of ZB CdSe nanocrystals (Figure 2a, top),10 but the emission spectra are broad and asymmetric presumably due to surface heterogeneity (λabs ) 576 nm, λem ) 585 nm, fwhm ) 37 nm). Weak emission with ∼3% QE and multiexponential PL decay imply the quenching of the PL by surface traps. In the case of the Cd-rich sample, the absorption spectra display the same ZB features as those of the as-prepared sample (Figure 2b, top), but the emission spectra are narrow and symmetric (λabs ) 576 nm, λem ) 583 nm, fwhm ) 25 nm) and QE increases to 15% with an increase in PL lifetime (τeff) from 5.5 to 10.1 ns. This result indicates that annealing of nanocrystals in excess cadmium oleate reduces the surface heterogeneity. The amounts of PA and TBP were varied as follows: 1 equiv of PA, excess PA, 1 equiv of TBP, excess TBP, 1 equiv of
Figure 2. Absorption spectra of (a) as-prepared and (b) Cd-rich CdSe nanocrystals in chloroform. PL decay profile (top) and emission spectra (bottom) of (c) as-prepared and (d) Cd-rich CdSe nanocrystals in chloroform before and after propylamine (PA)-tributylphosphine (TBP) passivation. Vertical lines in the absorption spectra indicate the bandedge absorption maxima before (solid) and after (dashed) binary passivation. Arrows show the directions of spectral shift. One equivalent refers to the calculated amount of ligand (∼0.3 µmol) that can passivate either Cd or Se atoms on the surface of the nanocrystals with a diameter of ∼3.5 nm. Excess PA equals 500 equiv, and excess TBP equals ∼2000 equiv. One equivalent of TBP/excess PA denotes addition of 1 equiv of TBP, followed by excess PA. Data shown in red represent addition of ligands in a reverse order.
TBP plus excess PA, and excess TBP plus excess PA. For asprepared CdSe nanocrystals, addition of 1 equiv of PA induces almost no change in both the optical spectra and the PL lifetime (λabs ) 576 nm, λem ) 585 nm, fwhm ) 37 nm, QE ) 3%, τeff ) 5.7 ns; data not shown). Excess PA results in some changes, blue-shifting both the absorption and the emission spectra by 5-6 nm, narrowing the bandwidth to 26 nm, slightly increasing the QE to 9%, and extending the lifetime to 8.2 ns. In contrast, addition of 1 equiv of TBP leads to almost no change in the optical spectra, but it reduces the QE to
