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J. Phys. Chem. C 2007, 111, 2974-2979
CdSe:Te Nanocrystals: Band-Edge versus Te-Related Emission Thomas Franzl, Josef Mu1 ller, Thomas A. Klar, Andrey L. Rogach,* and Jochen Feldmann Photonics and Optoelectronics Group, Physics Department & Center for Nanoscience (CeNS), Ludwig-Maximilians-UniVersita¨t Mu¨nchen, 80799 Munich, Germany
Dmitri V. Talapin*,† and Horst Weller Institute of Physical Chemistry, UniVersity of Hamburg, Grindelallee 117, 20146 Hamburg, Germany ReceiVed: October 31, 2006; In Final Form: December 20, 2006
Strongly luminescent monodisperse CdSe nanocrystals in which a few Se atoms are substituted with Te atoms (CdSe:Te) provide a model system for studies of both band-edge and trap-related luminescence. Ensemble photoluminescence spectra of CdSe:Te nanocrystals are asymmetrically broadened and red-shifted in comparison to bare CdSe nanoparticles. Single particle luminescence measurements show that the bare CdSe and the CdSe:Te nanocrystals emit at distinctly different wavelengths and differ in line shape and line width. Individual CdSe:Te nanocrystals show two kinds of emission spectra, which have been ascribed by us to particles with one Te and with a few Te atoms per nanocrystal. Single particle measurements furthermore show that a single CdSe:Te nanocrystal can emit either from the band-edge states or from trap state(s) created by the Te atom(s), but not from both.
Introduction Colloidally synthesized semiconductor nanocrystals (NCs) are currently of particular interest for a variety of applications in optoelectronics1-3 and biolabeling,4,5 especially as emitters in the visible and NIR spectral regions because of their excellent luminescence efficiency, color purity, and stability. The size tunability of the optical and electronic properties of NCs originates from the quantum confinement effect.6 Apart from the size, the composition of the NCs influences their optical properties, as well. The composition can be controlled at the synthesis stage through alloying.7-9 For a variety of alloyed II-VI compounds, the size of the band gap varies approximately linearly between the bandgaps of the pure materials according to the composition.10 A third strategy to govern the optical properties of semiconductor NCs relies on impurity energy levels located inside the band gap of the host material, which, for instance, can be deliberately created by the substitution of host cations with metals like manganese, copper, or rare-earth elements.11-15 By the incorporation of impurity atoms into the lattice of the host semiconductor, the dominant recombination route can be transferred to the impurity related trap states,11 providing an alternative pathway to the band-edge emission, which involves the highest occupied and the lowest unoccupied quantum-confined orbitals. In contrast to the well-studied band-edge emission, only a few studies of trap-related luminescence in strongly quantumconfined semiconductors have been conducted.16,17 An interesting question arises whether a specific NC with trap states shows both band-edge and trap-related recombination routes simultaneously. In addition, studies of trap-related luminescence can provide a deeper insight into the carrier trapping processes that * Corresponding authors. E-mail:
[email protected] (A.L.R.);
[email protected] (D.V.T.). † Present address: The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA.
cause quenching of the band-edge luminescence in semiconductor NCs by trapping photoexcited carriers into midgap states associated with the surface dangling bonds.16,17 The above-mentioned issues are addressed in this paper by studying CdSe:Te NCs with a very low Te content. Despite numerous activities in the colloidal synthesis of II-VI NCs with variable cation stoichiometry7,8,11-15 and a large body of information on alloyed II-VI nanocrystallites with variable anion stoichiometry, which are widely used in commercial color cutoff glass filters and are fabricated by diffusion-controlled growth in supersaturated glass matrices,6 there is very little literature on the latter compounds grown as NCs in solution by methods of colloidal chemistry. Several groups introduced alloyed CdSexTe1-x NCs with a large Te content (x from 0.39 to 0.85) prepared by high-temperature colloidal synthesis.9,18 We report here on CdSe:Te NCs with an extremely low Te content (one to a few atoms per particle) synthesized via hightemperature pyrolysis of Cd, Se, and Te precursors in a threecomponent mixture of highly boiling coordinating solvents. The NCs contain trap levels of Te impurities, which are most probably located close to the valence band of CdSe formed by the occupied Se 4p atomic orbitals.16,19 Our single particle measurements are consistent with the presence of both bare CdSe NCs and CdSe:Te NCs in our samples, which emit at distinctly different wavelengths. Single CdSe:Te NCs show two kinds of emission spectra and have been ascribed by us to particles with one Te and with a few Te atoms per NC. Timeresolved luminescence measurements on ensembles of CdSe: Te NCs indicate extremely fast trapping of photogenerated charge carriers by Te sites, and slower PL decays for CdSe:Te NCs in comparison with bare CdSe NCs. Experimental Section Chemicals and Reactions. All reactions and manipulations were performed using standard air-free techniques. The chemi-
10.1021/jp067166s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/30/2007
CdSe:Te Nanocrystals cals used were of analytical grade or of the highest purity available. Tellurium (99.999%, ChemPur, powder), dimethylcadmium (EpiChem), selenium (99.999%, shots), diethylzinc, tri-n-octylphosphine oxide (TOPO, 90%), and bis-trimethylsilylsulfide (all from Aldrich) were used as received. Tri-noctylphosphine (TOP, Fluka) was additionally purified by distillation. Hexadecylamine (HDA, Merck) was purified and degassed in the reaction vessel by heating for several hours at 100 °C under vacuum. Bare CdSe and CdSe/ZnS core-shell NCs were synthesized according to the previously published procedure.20 CdSe:Te NCs were prepared in a similar fashion by introducing a small amount of tellurium precursor (molar ratio of Te to Se from 2% to 5%) in the form of TOPTe to the injection solution. The TOPTe solution was prepared by dissolving 0.65 g of tellurium in 10 mL of TOP by overnight heating at 150 °C. We denote the Te-substituted samples by providing in brackets ({}) the molar amount of Te used in the synthesis (like CdSe:Te{2%} or CdSe: Te{5%}) but stress that the actual amount of incorporated Te ions is much smaller, as it will be discussed below. In a typical synthesis of CdSe:Te NCs, 1 mmol of TOPSe and 1.35 mmol of dimethylcadmium were dissolved in 5 mL of TOP, Te precursor was added, and the mixture was rapidly injected into a vigorously stirred solution of 10 g of TOPO (55 mol %) and 5 g of HDA (45 mol %) heated to 300 °C. Further growth occurred at 250-310 °C depending on the desirable NC size. In a typical synthesis of CdSe/ZnS core-shell NCs with a CdSe:Te core, 2.5 mL of the crude solution of CdSe:Te NCs was mixed with toluene, and the NCs were precipitated with methanol. The isolated CdSe:Te NCs were mixed with 7 g of TOPO and 3.5 g of HDA and heated to 220 °C. The amount of ZnS stock solution (0.4 mmol of diethylzinc and 0.51 mmol of bis-trimethylsilylsulfide in 3 mL of TOP) necessary to obtain the desired shell thickness was calculated from the ratio between the core and shell volumes using bulk lattice parameters of CdSe and ZnS. This amount was added dropwise to a vigorously stirred solution of CdSe:Te NCs. In a control experiment, CdSe:Te NCs overgrown with CdSe shell were synthesized as follows. 25 mg of 3.1-nm CdSe:Te{5%} NCs was isolated from the crude solution, thoroughly washed with methanol, redispersed in 5 g of TOPO and 2.5 g of HDA, and heated to 240 °C. The growth of CdSe shells around Te-alloyed cores was achieved by dropwise addition of a CdSe stock solution (1 mmol of dimethylcadmium and 1.5 mmol of TOPSe in 4 mL of TOP). The growth of the CdSe shell was monitored by evolution of the absorption spectrum. When the NC size increased to 4.3 nm, the addition of CdSe precursors was stopped, and the reaction mixture was cooled to room temperature. Structural Characterization. Powder X-ray diffraction (XRD) spectra were taken on a Philips X’Pert diffractometer (Cu KR-radiation, variable entrance slit, Bragg-Brentano geometry, secondary monochromator). High-resolution transmission electron microscopy (HRTEM) and EDX spectroscopy were performed on a Phillips CM-300 microscope operating at 300 kV. The samples for TEM were prepared by dropping dilute solutions of NCs in toluene or hexane onto 400-mesh carboncoated copper grids and immediately evaporating the solvent. Optical Spectroscopy. UV-vis absorption, photoluminescence (PL), and photoluminescence excitation (PLE) spectra were measured on colloidal solutions of NCs with a Cary 50 spectrophotometer (Varian) and a Cary Eclipse (Varian) spectrofluorometer, respectively. Rhodamin 6G (laser grade, Lambda Physik) was used as a standard for determining the PL quantum
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Figure 1. TEM and HRTEM images of CdSe:Te{5%} NCs.
yields of the NC solutions. Both integrated and time-resolved PL measurements were performed on colloidal solutions with an optical density below 0.2 at the excitation wavelength of 400 nm. The frequency-doubled output (400 nm) of a Kerrlens mode-locked titanium-sapphire laser (120 fs, 76 MHz) was used as the optical excitation source in time-resolved PL measurements. To derive decays over hundreds of nanoseconds, a pulse picker (APE, Berlin) reduced the repetition rate of the TiSa laser down to 3.8 MHz. The emission was dispersed both temporally and spectrally by using a streak camera with a maximal temporal resolution of 2 ps (C 5680, Hamamatsu) in combination with a Cromex spectrometer. Appropriate spectral filters were used to block any scattered light from the excitation beam. Time correlated single photon counting was performed using a microchannel plate and a Becker & Hickl Spc300 module with a time resolution of 100 ps. All time-resolved PL measurements were performed at room temperature. PL measurements on single NCs were carried out on coreshell CdSe/ZnS and CdSe:Te/ZnS particles dispersed in a polystyrene matrix, with an appropriate dilution that provides an average distance between the particles of 5 µm. The sample was mounted in a cold finger helium cryostat at 10 K and excited using the 457.9-nm line of an argon ion laser at typical intensities of 50 W/cm2 in a dark field configuration. The PL light from single particles was split by a Wollaston prism into two perpendicularly polarized beams, collected with a longdistance microscope objective lens (NA: 0.55), spectrally resolved in a 0.3 m spectrometer, and detected in a charge coupled device with typical integration times of 30 s. Results and Discussion The growth dynamics of both CdSe and CdSe:Te NCs were monitored by measuring UV-vis and PL spectra on aliquots taken from the reaction mixtures at different time intervals (Figure S1 from the Supporting Information). A standard sizeselective precipitation procedure utilizing toluene-methanol “solvent-nonsolvent” mixtures applied to the crude solutions had only a moderate influence on the broadness of the absorption and PL spectra (Figure S2 from the Supporting Information) and indicated that this is an intrinsic property of the system and not a result of a broad size distribution. The narrow size distribution of the CdSe:Te NCs is directly confirmed by the TEM and HRTEM images (Figure 1). In contrast to the recently reported alloyed CdSe1-xTex NCs with x varying from 0.39 to 0.85,9 the powder X-ray diffraction patterns of our CdSe and CdSe:Te NCs (Figure 2) show
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Figure 2. Powder XRD spectra of 4.5-nm CdSe and CdSe:Te NCs. The molar percentage of Te precursor used in the synthesis is shown above each spectrum. The line patterns indicate the reflections of bulk wurtzite CdSe (bottom) and wurtzite CdTe (top) phases.
negligible distortion of the host lattice of CdSe by inclusion of the Te atoms. This demonstrates that the amount of Te incorporated into nanocrystal lattice is much lower than the amount of Te precursor added to the reaction mixture. This coincides well with the previously reported data from literature that the amount of impurity atoms that is incorporated into a host lattice is 1-2 orders of magnitude lower than the amount of the impurity atoms used in the synthesis.13,15,21 Se ions in CdSe NCs with wurtzite structure, like those under consideration in our work, have been shown to be particularly difficult to substitute.13,22 The difference in the amount of incorporated Te at the synthetic conditions of our work and those of ref 9 lies most probably in its different incorporation kinetics, determined by the difference in initial concentrations of Se and Te precursors. At the conditions of our synthesis with a large excess of Se precursors, pure CdSe NCs were pre-nucleated first, and the incorporation of Te into the CdSe lattice was hindered due to the large difference in Se and Te ionic radii. Additionally, the effects of NC “self-cleaning” by ejection of impurity atoms13 can further reduce concentration of Te atoms in the CdSe:Te NCs with initially low Te content. Therefore, we conclude that our CdSe:Te NCs have wurtzite structure, show the same lattice constant as pure CdSe NCs, and, consequently, have the same band gap as pure CdSe NCs, while the Te ions serve as impurities that provide trap states for charge carriers. Our attempt to estimate Te-content using EDX spectroscopy showed that actual content of Te in thoroughly washed CdSe:Te NCs is less than 1%. Detailed analysis of CdSe NCs doped with only several Te atoms is rather difficult and should be a subject of separate study. Figure 3 compares the ensemble absorption and PL spectra of CdSe (‚ ‚ ‚), CdSe:Te{2%} (- - -), and CdSe:Te{5%} NCs (-) of the same size. The absorption maximum becomes slightly broadened for the NCs with increasing Te content, while the positions of the absorption maxima corresponding to the first as well as the other electronic transitions remain the same for all three samples, indicating that the CdSe host is responsible for light absorption in the NCs. The PL spectra of the CdSe:Te NCs exhibit red shifts and an asymmetric broadening. Both effects become progressively pronounced when the amount of Te precursor used in the NC synthesis is increased. The longwavelength part of the emission spectrum of the NCs is commonly associated with the radiative recombination from trap states.16,17 In our samples, a pronounced long-wavelength tail in the emission spectra of CdSe:Te NCs (Figure 3b) can be assigned to trap states created through Te atoms, which are not present in the bare CdSe NCs.
Figure 3. (a) Absorption and (b) PL spectra of CdSe (‚ ‚ ‚), CdSe: Te{2%} (- - -), and CdSe:Te{5%} (-) NCs of 3.8 nm size.
Figure 4. (a) Spectral positions of the first electronic transitions in the PLE spectra of 3.8-nm CdSe NCs (f) and CdSe:Te{5%} NCs (9) taken at different detection wavelengths over the corresponding cw PL bands. An example of a cw PL spectrum and a PLE spectrum obtained at a detection wavelength of 625 nm, with a first electronic transition peak at 560 nm (indicated by arrows), is given in the inset. (b) cw PL spectra of CdSe (- - -) and CdSe:Te{5%} (-) NCs. The cw PL spectrum of CdSe:Te{5%} NCs is compared to the PL spectrum of the same NCs measured by a streak camera (gray line). The latter spectrum was obtained through integration over the first 10 ps after the excitation pulse.
A series of PLE spectra was measured at different emission wavelengths over the entire PL spectrum of both CdSe and CdSe:Te{5%} NCs of the same size (Figure 4a). For bare CdSe NCs, the electronic transitions in the PLE spectra gradually shift to the red with an increasing detection wavelength (f in Figure 4a). This is a signature that the band-edge emission originates from a narrow subset of NCs present in the residual size distribution in the sample.23 Similarly to the bare CdSe NCs,
CdSe:Te Nanocrystals
Figure 5. (a) Normalized PL decays of 3.8-nm CdSe, CdSe:Te{2%}, and CdSe:Te{5%} NCs integrated over the whole spectral range. (b) Normalized PL decays of CdSe NCs measured at the position of the PL maximum and of CdSe:Te{5%} NCs measured at the blue edge of the emission band @540 nm and at the long wavelength tail @650 nm.
the first electronic transition of the CdSe:Te{5%} NCs shifts to the red with an increasing detection wavelength up to a detection wavelength of 590 nm, which is the wavelength range of the PL maximum. However, the red shift of the peak wavelength of the PLE saturates for PL wavelengths longer than 590 nm, which correspond to the asymmetrically broadened red part of the PL spectrum (9 in Figure 4a). We explain the saturation in the PLE peak position for the long wavelength emission of the CdSe:Te NCs by an unaffected absorption of the CdSe core, whereas the red tail emission originates from the Te trap states, which are distributed among several energetic levels of an ensemble of CdSe:Te NCs. Figure 4b compares a cw PL spectrum of CdSe:Te{5%} NCs with a PL spectrum measured with a streak camera, which was integrated over the first 10 ps after excitation. There is almost no difference in the spectral shape. This indicates that the trapping of photogenerated charge carriers by Te impurities is a very fast process with a rate of more than (10 ps)-1. The time-resolved PL decays of CdSe, CdSe:Te{2%}, and CdSe:Te{5%} NCs of the same size are compared in Figure 5a. Wavelength binning is performed over all wavelengths, thus averaging over the ensemble of NCs. All PL decays are nonexponential and extend to tens of nanoseconds. A decrease of the decay rates for increasing Te content is clearly seen, which indicates that recombination of charge carriers trapped by Te impurities occurs on a slower time scale than for bare CdSe NCs. Figure 5b provides further evidence that this decrease in the PL decay rate is indeed related to the charge carriers trapped by the Te atoms. The spectra of CdSe:Te{5%} NCs shown in Figure 5b have been obtained by wavelength binning between 535 and 545 nm (dashed line), and between 645 and 655 nm (solid line). The decay at 538-542 nm corresponds to the emission of those NCs in the ensemble, which spectrally coincides with the emission of bare CdSe NCs. Indeed, this decay is very similar to the decay curve measured on the bare CdSe NC sample. The PL decay at 648-652 nm is clearly slower and is assigned to the emission through Te traps.
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2977 The room-temperature PL quantum yield of CdSe:Te NCs is comparable with that of the bare CdSe NCs being ∼5-8% at room temperature. A well-established method of increasing the PL quantum yield of colloidal NCs is to overgrow them with a shell of a wide-band gap material (typically ZnS or CdS). Coating of CdSe:Te NCs with ZnS shells leads to a pronounced increase of their emission that reaches a room-temperature quantum efficiency of 33% for a nominal shell thickness of ∼2 monolayers. The substitution of Se ions with Te implies incorporation of Te atoms into the CdSe lattice during the NC growth. Instead of being incorporated into the crystalline lattice, tellurium can adsorb onto the NC surface, a potential source of experimental artifacts.14 To rule out this possibility, we overcoated CdSe:Te NCs with a shell of pure CdSe in a control experiment (Figure S3 from the Supporting Information). Comparison of the optical properties of CdSe:Te{5%} and CdSe:Te{5%}/CdSe revealed no substantial difference between these samples, which allowed us to conclude that the Te atoms indeed are incorporated into the lattice of growing CdSe NCs, rather than are adsorbed on the NC surface. For the very small amount of Te atoms incorporated into the CdSe host lattice in our synthesis, we might expect that there would be statistically some pure CdSe NCs in the resulting ensemble, as well as NCs containing one or only a few Te atoms incorporated into the CdSe host lattice. Keeping this consideration in mind, we have applied single NC emission spectroscopy at a temperature of 4 K as a technique to eliminate the effects of the ensemble averaging and thermal broadening.24,25 NCs overcoated with ZnS shells have been used for these measurements to ensure a PL signal strong enough to detect single NCs. The on-off blinking criterion24 has been applied to prove that only single NCs were taken into account for the resulting statistics. The use of a Wollaston prism allowed us to observe two perpendicular polarizations simultaneously. The NCs were found to emit unpolarized light because the spectral shapes and positions of single particle emission spectra were almost independent of polarization. Figure 6 shows representative PL spectra of single NCs. Panels (a) and (b) show spectra of two single CdSe/ZnS NCs, with a typical form and line width that corresponds to the spectra previously reported in the literature very well.24,25 A slight broadening of the spectra is caused by spectral diffusion24 due to the long integration time. All other spectra in Figure 6 are from a sample of CdSe:Te{5%}/ZnS NCs. Approximately 40 single particle spectra have been measured and analyzed in each case. Single NC spectra obtained on a sample of CdSe:Te{5%}/ ZnS NCs can be ascribed to three different types according to their spectral position, line shape, and line width. About onehalf of all of the analyzed spectra are similar to those two presented in Figure 6c,d and show a narrow zero phonon line (10-12 meV full width at half-maximum, fwhm) in a spectral region of 2.15-2.25 eV. A phonon replica set off by ∼28 meV to the red coincides well with a corresponding bulk value of CdSe.26 These spectra clearly resemble the spectra of single bare CdSe/ZnS NCs (Figure 6a,b) and are ascribed by us to those particles in the ensemble that do not have any Te impurities. The remaining 50% of the single NC spectra can be, in turn, divided into two groups, which are presented by the typical spectra shown in Figure 6e,f and g,h, correspondingly. The spectra like those in Figure 6e,f show relatively narrow (1415 meV fwhm) zero-phonon lines, and well-resolved higher order phonon replicas. The spectral positions are, however, strongly red-shifted in comparison to the spectra of bare CdSe
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Figure 7. Model energy scheme of CdSe:Te NCs with trap state(s) within the band gap of CdSe determined by one (a) or several (b) Te atom(s). |a> is the ground state of the CdSe NCs; |b> represents an excited state of the CdSe NCs, which carries most of the oscillator strength in absorption; |c> represents a trap state created by a single Te atom in a CdSe lattice (panel a) or a distribution of trap states created by several Te atoms (panel b). kba is the radiative decay from |b> to |a>, that is, the band-edge emission as it takes place in bare CdSe NCs; kbc stays for extremely fast trapping of a photoexcited carrier (most probably a hole19); kca is the radiative decay through the trap state(s), as it takes place in CdSe:Te NCs. A nonradiative pathway knr is also indicated to account for all other processes leading to a luminescence quantum yield
