Shell Nanorod

J. Phys. Chem. C 2009, 113, 19161–19171

19161

Static and Dynamic Emission Quenching in Core/Shell Nanorod Quantum Dots with Hole Acceptors Zhong-Jie Jiang, Valerie Leppert, and David F. Kelley* UniVersity of California, Merced, 5200 North Lake Road, Merced, California 95343 ReceiVed: August 10, 2009; ReVised Manuscript ReceiVed: September 14, 2009

CdSe/CdS and CdSe/ZnS core/shell nanorods have been synthesized with various shell thicknesses and characterized by TEM. The CdSe cores have an aspect ratio of 5.0 ( 0.7. The TEM results show that there are different axial and radial shell growth rates. Shell deposition maintains the rod morphology, with a final aspect ratio of 4.1. The presence of the CdS or ZnS shell greatly affects the particle nonradiative relaxation dynamics and hence the luminescence quantum yields. Both types of shells passivate the surface, reducing the extent of both static and dynamic quenching. Two types of hole acceptors, phenothiazine and hexadecane thiol, have been adsorbed onto these core/shell nanorods, and the effects on the luminescence dynamics are studied using static and time-resolved spectroscopy. ZnS- and CdS-coated particles with adsorbed hole acceptors show differing extents of static and dynamic quenching. These differences may be understood in terms of the different shell morphologies, valence band offsets, and the differences in the phenothiazine and thiol energetics. The extents of static and dynamic quenching are also studied as a function of the phenothiazine or thiol concentration. In both the ZnS- and CdS-coated particles, these concentration dependencies indicate that static quenching is associated with strongly binding sites, presumably where the quencher is bound at a defect in the shell. Dynamic quenching is associated with more weakly binding sites on the surface of the particle. Introduction Cadmium selenide quantum dots (QDs) have electronic properties that are strongly shape dependent. Unlike spherical particles, lowest excited state of CdSe nanorods is dipole allowed from the ground state,1-4 which is desirable for many different types of applications. However, exploiting the photophysics of these particles often requires that the excited state be long lived, i.e., that it not be subject to rapid nonradiative quenching. The absence of nonradiative quenching pathways results in long-lived excitons and a high luminescence quantum yield (QY). In contrast, a low luminescence QY often indicates the presence of rapid nonradiative electron-hole recombination pathways, which are detrimental to such applications as solar energy conversion, fluorescent markers, etc. The luminescence QY depends on both the radiative and nonradiative rates. Specifically, QY is given by

Φ)

krad krad + knonrad

(1)

where krad and knonrad are the radiative and nonradiative exciton decay rates, respectively. The radiative rate is determined by the intrinsic properties of the particles and their environment. Thus, the QY is controlled by rates of nonradiative electron-hole recombination, which typically takes place at electron or hole traps. These traps are crystal defects which often occur at the particle surface. Synthesis of semiconductor QDs with high luminescence QYs requires passivation of the “dangling” bonds that result in surface trap states. These traps can be partially passivated by the adsorption of organic ligands on the particle surface. Traps are usually more effectively passivated by the * Corresponding author. E-mail: [email protected].

presence of an epitaxial coating of a larger bandgap semiconductor, that is, by forming a core/shell particle. If the shell valence band is at lower energy and the conduction band is at higher energy than the core (a “type I” core/shell particle), the electron and hole are confined to the inside of the QD. In this case, the presence of surface defects has a greatly diminished effect on the electron-hole recombination dynamics. Such core/ shell particles have luminescence QYs that are quite high, often greater than 50%. Core/shell semiconductor nanorods are of particular interest for several reasons. As mentioned above, CdSe nanorods have a somewhat different electronic structure, compared to spherical QDs. This results from the combination of quantum confinement and crystal field effects. CdSe has a wurtzite crystal structure, and as such, the lowest excited state is split by the crystal field. In spherical particles, luminescence from the lowest energy exciton is forbidden, and this state is referred to as the “dark exciton”.5 In nanorods, the long axis of the nanorod is along the crystallographic c-axis, and there is less quantum confinement along this coordinate, altering the ordering of the exciton states.1 The result is that transitions between the ground state and the lowest energy exciton state are allowed and polarized along the long axis of the nanorod.2-4 The luminescence polarization can be exploited by alignment of the particles, which can be accomplished by several different means.6,7 The combination of aligned particles and this electronic structure allows for the production of highly polarized luminescence from particles that are almost isotropic absorbers at shorter wavelengths. CdSe nanorods have typically been grown by the reaction of dimethyl cadmium with trioctyl phosphine selenium in coordinating solvents,8-10 similar to the original syntheses of spherical QDs. More recently, “greener” syntheses starting with CdO have been developed and have largely replaced the use of dimethyl cadmium.11,12 The basic idea of these syntheses is to control the cadmium and selenium precursor reactivities as well

10.1021/jp907728h CCC: $40.75  2009 American Chemical Society Published on Web 10/09/2009

19162

J. Phys. Chem. C, Vol. 113, No. 44, 2009

as selectively inhibit radial as opposed to axial particle growth. This is accomplished by starting out with cadmium phosphonate as the cadmium precursor and/or adding alkyl phosphonic acids to the reaction mixture. Luminescence QYs of CdSe NRs are typically low, particularly with particles produced from CdObased syntheses. Luminescence QYs may then be improved by surface passivation, accomplished by overcoating the nanorods with a larger bandgap semiconductor.13 Even so, luminescence QYs for CdSe NRs have remained relatively low, significantly lower than in spherical QDs. This is probably due to the difference in surface chemistry needed to obtain the rod morphology and the fact that nanorods have a larger surface to volume ratio than comparably sized spheres. The observed QYs are morphology dependent, and can be significantly improved by growing a nanorod around a spherical particle, the “dot-ina-rod” morphology.14-16 Adsorption of electron or hole acceptors at the particle surface can dramatically alter the excited state dynamics. An additional nonradiative pathway is associated with electron or hole transfer to an adsorbed acceptor, followed by nonradiative recombination with the other carrier. Charge transfer between the excited state quantum dots and adsorbed electron or hole acceptors has been extensively studied. The quenching dynamics are relatively easy to study because the concentration of recombination sites can be varied while holding everything else constant. This is done by simply varying the concentration of electron or hole acceptors adsorbed on the surface. These nanoparticle/adsorbate systems show diminished luminescence QYs, resulting from a combination of “static” and “dynamic” quenching.17-20 These terms are used in a variety of contexts, and must be defined carefully. In this paper, static and dynamic quenching are defined solely in terms of the observed luminescence kinetics and quantum yields. Dynamic quenching occurs when the luminescence lifetime is reduced, resulting in a corresponding reduction of the observed luminescence QY. This may be quantitatively understood in terms of radiative and nonradiative rate constants. An increase in the nonradiative decay rate increases the observed decay rate (krad + knonrad), with a corresponding decrease in the luminescence quantum yield, Φ, given by eq 1. Static quenching occurs when there is a reduction of the luminescence QY, with no change in the observed luminescence decay kinetics. In this case, the particles that undergo quenching have such short lifetimes that they are gone on a time scale much faster than that of the timeresolved luminescence experiment, and are therefore not observed at all. The particles that undergo static quenching are “dark”. The extent of static quenching is therefore simply related to the fraction of the particles having either a deep carrier trap or an adsorbed quencher that very rapidly quenches the excited state. Thus, when only static quenching is observed, some fraction of the particles undergo quenching on a very fast time scale, but essentially none do so on the radiative (several nanoseconds to hundreds of nanoseconds) time scale. This phenomenon has also been observed in single particle studies, which have confirmed that some fraction of the particles in an ensemble have luminescence QYs near unity, while the remainder are dark.21 It has also been suggested that this is related to fluorescence intermittency, i.e., blinking.22 Whether quenching is static or dynamic depends on the time scale of the experiment. Transient absorption studies indicate that, in CdSe nanorods and CdSe/ZnS core/shell nanorods, trapping at surface states takes place on the 200-500 fs time scale.23 Time-resolved luminescence experiments are commonly performed using time-correlated single-photon counting, with

Jiang et al. a time resolution on the order of 30 ps. Clearly, in some cases, higher time resolution would result in static quenching being reclassified as dynamic quenching. However, the 30 ps time scale is much faster than that of luminescence. The distinction is therefore a very useful one and is commonly referred to throughout the literature. Static and dynamic quenching as so defined are not mutually exclusive, and in some cases, both are observed. In such cases, the observed luminescence lifetime is shortened but not enough to explain the observed reduction in the luminescence QY.24 These cases are characterized by both very fast and relatively slow quenching dynamics, often with an absence of intermediate decay components. Phenothiazine (PTZ) is a commonly used and well characterized electron donor, i.e., a hole acceptor.25,26 Luminescence quenching by hole transfer to adsorbed phenothiazine has been shown to occur in spherical CdSe and CdSe/ZnS core/shell nanoparticles.27,28 In CdSe particles,27 transient absorption spectroscopy was used to establish that the quenching mechanism is hole transfer to the PTZ. The transient absorption kinetics indicate that quenching occurs on a range of picosecond to nanosecond time scales. The hole transfer times were analyzed in terms of a model in which several PTZs can adsorb on each particle, and the net hole transfer rate is the sum of the rates to each of the individual PTZs. Luminescence quantum yield measurements were not made, and these results were analyzed in terms of dynamic quenching. In contrast, the quenching of the core/shell particles28 has been studied using a combination of static and time-resolved luminescence spectroscopy, and the results analyzed in terms of a Perrin model.29 The static and time-resolved results indicate that all of the quenching is static and there is no dynamic quenching. Adsorbed alkyl thiols can also quench the excited states of CdSe QDs by hole transfer.30-33 This does not occur with CdTe QDs, which brackets the energetics of the thiol with respect to the CdSe valence band.31 The above examples indicate that the quenching dynamics are strongly dependent on the composition of the particles, the particle surfaces, and the energetics involved in charge transfer. In this paper, we investigate static and dynamic quenching in core/shell nanorods and quenching by charge transfer to adsorbed hole acceptors, phenothiazine and hexadecane thiol. This is examined in well-characterized core/shell nanorods with variable thickness shells. Specifically, we use time-resolved and static luminescence spectroscopy to examine the quenching dynamics of CdSe/CdS and CdSe/ZnS nanorods. We vary the thickness of the shells with both types of hole acceptors and the quencher concentration in order to elucidate the mechanisms of the different types of quenching dynamics. In all cases, the same type of CdSe core particles are used and the particle morphologies are characterized by transmission electron microscopy (TEM) imaging and optical spectroscopy as shell growth occurs. This combination of variables allows us to elucidate the structural and electronic factors controlling the dynamics. The results are interpreted in the framework of the known electronic properties of the different semiconductors and the known properties of the semiconductor interfaces. Experimental Section Chemicals. Cadmium oxide (CdO, 99.5%), trioctylphosphine oxide (TOPO, 90%), trioctylphosphine (TOP, 97%), tributylphosphine (TBP, 97%), octadecene (ODE, 90%), methanol (MeOH, 98%), toluene (99%), and octadecylamine (ODA, 90%) were obtained from Aldrich. Sulfur (99.5%), zinc oxide (ZnO, 99.9%), selenium (99%), and chloroform (CHCl3, 99.8%) were obtained from Alfa Aesar. Octadecylphosphonic acid (ODPA,

Core/Shell Nanorod Quantum Dots with Hole Acceptors 99%) was obtained from PCI synthesis. ODPA and ODA were recrystallized from toluene before use. TOP, TBP, and ODE were purified by vacuum distillation. In some cases, TOPO was purified by repeated recrystallization from acetonitrile. Methanol, toluene, and chloroform were purified by distillation from appropriate drying agents. All other chemicals were used as received. Instrumentation. Static fluorescence spectra were obtained using a Jobin-Yvon Fluorolog-3 spectrometer using the Horiba J-Y software. The instrument consists of a xenon lamp/double monochromator excitation source and a CCD detector. Quantum yields are determined by comparison of the nanoparticle spectra with the spectrum of rhodamine B in methanol, with the appropriate spectral calibration factors. This comparison involves collection of the luminescence in a face-on geometry. The absorbances of the nanorod and rhodamine samples are small (typically about 0.1) and very close to the same at the excitation wavelength. The quantum yields are then determined by taking the ratio of areas under the luminescence spectra. These spectra are corrected for instrument response: monochromator throughput and detector efficiency. The nanorod and rhodamine spectra are at close to the same wavelengths, so the relative correction factors are close to unity. Time-resolved luminescence measurements were obtained by time-correlated single-photon counting (TCSPC), using a Hamamatsu red-enhanced 6 µm microchannel plate PMT and a Becker-Hickel SP-630 board. The light source was a cavitydumped Ti:sapphire laser (Coherent Mira) operating at 410 nm with a 1 MHz repetition rate. In all cases, the fluorescence was focused through a 0.25 m monochromator with a 150 groove/ mm grating and onto the microchannel plate PMT. The monochromator has a polarization scrambler in front of the entrance slit, which almost completely eliminates the polarization dependence of the monochromator throughput. TEM images were obtained on a JEOL JEM-2010 transmission electron microscope equipped with a LaB6 filament. This instrument is capable of 2.4 Å point-to-point image resolution and 1.4 Å lattice fringe resolution. Preparation of CdSe Core Nanorods. The synthesis of CdSe nanorods (cores) was done using a method described in the literature.12 This synthesis requires preparation of cadmium and selenium precursors, followed by nanorod synthesis. These nanorods are then used as the cores in the preparation of the core/shell particles. (a) Cadmium Precursor. CdO (0.2054 g, 1.6 mmol), ODPA (3.2 mmol), and technical (90%) grade TOPO (2.93 g) were loaded into a reaction flask (50 mL) and then heated to 300-320 °C under nitrogen flow until all the CdO reacted. This optically clear solution was heated at 320 °C and kept at this temperature for 10 min, and then, it was cooled to room temperature under N2 flow. After aging for at least 24 h, it was used directly without further purification. Alternatively, some cadmium precursors were prepared using purified TOPO, with an added 20 µL of 2 M H3PO4. This is the equivalent amount of acid as is found in the technical (90%) grade TOPO, and yields essentially identical particles.34 (b) Selenium Precursor. In a nitrogen glovebox, 0.063 g of Se-TBP (0.253 g with 25% Se by mass, 0.8 mmol of Se) was mixed with 1.447 g of TOP and 0.3 g of toluene to obtain the injection solution. (c) Cadmium Selenide CdSe Nanorods. The cadmium precursor was heated to 320 °C under N2 flow. The selenium precursor loaded was in a 10 mL syringe and rapidly injected into the reaction flask. In all syntheses, the crystals were allowed to grow

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19163 for 8 min at 300 °C, after which the heating mantle was removed. When the reaction vessel was cooled down to 100 °C, 5 mL of toluene was added to prevent solidification. The obtained nanorod solution was kept in a nitrogen glovebox at ambient temperature. Preparation of Core/Shell Particles. (a) Preparation of Shell Precursors. A 0.04 M cadmium shell precursor was prepared by heating the mixture of 102.9 mg of CdO, 14.16 g of ODE, and 1.8 g of oleic acid at 250 °C to get a clear solution. A 0.04 M zinc shell precursor was prepared by heating the mixture of 65.0 mg of ZnO, 14.16 g of ODE, and 1.8 g of oleic acid at 250 °C to get a clear solution. A 0.04 M sulfur precursor was prepared by dissolving 25.7 mg of elemental sulfur into 15.78 g of ODE at 200 °C to get a clear solution. All of the shell precursors were stored in the glovebox at ambient temperature prior to use. (b) Preparation of Core/Shell Particles. For the preparation of CdSe/ZnS core/shell particles, the 1 mL of CdSe nanorod solution obtained above was dissolved into 3 g of ODE, which was then washed by extraction with chloroform/methanol (volume 1:1) three times to remove the excess cadmium precursor. All operations were done in a nitrogen atmosphere. The washed particles were dissolved in a mixture of 3.0 g of ODE and 3.0 g of ODA. The mixture was then loaded into a reaction vessel and heated to 100 °C under vacuum to remove any residual chloroform or methanol. The reaction vessel was then flooded with N2. A 1.50 mL portion of zinc shell precursor was added dropwise, followed by 0.75 mL of sulfur shell precursor in 5 min. The presence of these precursors prevents particle etching upon heating. The reaction solution was heated to 240 °C. After 30 min, 0.75 mL of the zinc shell precursor was added dropwise followed by 0.75 mL of sulfur shell precursor, also added dropwise. Each precursor addition took about 5 min. The subsequent shell growth was done by the addition of the same amounts of zinc and sulfur shell precursors alternately. Using the known size of the CdSe cores (see below) and the bulk ZnS lattice constants, the first addition corresponds to a shell of about 0.74 of a monolayer. Aliquots were taken for measurements after each set of zinc and sulfur injections. Before spectroscopic analysis, all of the samples were precipitated by the addition of methanol and subsequently washed by several cycles of suspension in toluene, followed by precipitation by methanol and centrifugation. The CdSe/CdS core/shell particles were prepared in an exactly analogous way, except that zinc was replaced by cadmium. (c) Sample Preparation. Prior to sample preparation, the nanorods were precipitated by addition of methanol followed by centrifugation. The precipitated particles were redissolved in toluene or the toluene/quencher solution at low concentration, typically about 2 µM. Results and Discussion 1. Particle Characterization. The nanorods used in these studies have been characterized by TEM imaging. The initial core particles as well as core/shell particles having several different shell thicknesses have been imaged. In each case, the dimensions of about 20-30 particles were measured. The dimensions (in nm) of the cores are 3.55 ( 0.36 (diameter) and 17.7 ( 1.8 (length). The uncertainties indicate the standard deviations of the distributions. The same core particles were used to obtain all of the core/shell particles used for TEM imaging. Shell growth increases both the nanorod diameter and length, maintaining the rod morphology with an approximately constant aspect ratio. Figure 1 shows a typical TEM image, in this case, CdSe cores with three injections of ZnS as the shell.

19164

J. Phys. Chem. C, Vol. 113, No. 44, 2009

Figure 1. TEM image of CdSe/ZnS core/shell nanorods having three injections of the ZnS shell precursors. Also shown is a nanorod with higher resolution showing (0 0 2) lattice fringes.

Figure 2. Experimentally determined (from TEM images) and calculated particle diameter and length increases (compared to the original core particles) for CdSe/ZnS nanorods, upon each injection of shell precursors. The experimental diameter and length results are indicated with large square and round symbols, respectively. The calculated values are indicated with smaller symbols, connected by lines. This calculation assumes all of the precursors are incorporated into the shell.

These particles have dimensions (in nm) of 4.3 ( 0.5 and 20.7 ( 1.8. The increase in particle size with each injection can be calculated from the known initial particle sizes and the amounts of shell precursors injected. Calculated sizes can be compared with those obtained from TEM images. Figure 2 shows the measured increases in diameter and length for CdSe/ZnS nanorods as a function of the number of injections of ZnS shell precursors. Both dimensions are seen to increase as the shell is deposited. The growth of the radius (rn) and the length (ln) as a function of the number of injections (n) can be easily calculated in terms of the amounts of shell precursors injected, the concentration of particles, and the relative radial and axial growth rates. Specifically, for each injection,

rn+1 ) rn + (0.74)(0.33 nm)krA0 /An and ln+1 ) ln + (0.74)(0.66 nm)klA0 /An (2) where An is the surface area of the particle following n injections of the shell precursors and kr and kl are the relative (unitless) radial and axial growth rates, respectively. The relative growth rates are subject to the mass conservation constraint kr ) 1 + (1 - kl)rn/ln. The ratio of kl/kr is the ratio of axial to radial deposition rates, and the case where kr ) kl ) 1 corresponds to uniform shell growth. The images show that shell growth

Jiang et al. maintains the rod morphology but causes a modest decrease of the aspect ratio, ln/rn. The initial particle dimensions set r0 ) 1.775 nm and l0 ) 17.7 nm, resulting in A0 ) 2πr02 + 2πr0l0 ) 217 nm2. The constant (0.74) is set by the initial particle area and the amount of zinc and sulfur precursors in each injection: the amounts correspond to 0.74 of a monolayer on the first injection. In this model, the thickness of a ZnS monolayer is taken to be 0.33 nm. The only adjustable parameter in this model is kl/kr, the ratio of the radial and axial deposition rates. Particle diameters and lengths calculated using eq 2 with kl/kr ) 2.8 are also shown in Figure 2. Quantitative agreement with the particle dimensions obtained from the TEM images is obtained. We emphasize that this calculation has only a single adjustable parameter, and it determines only the relative magnitudes of the radial and axial size increases. The total volume of the growing particles is accurately predicted by their initial size and the amounts of shell precursors, independent of any adjustable parameter. This agreement demonstrates that these nanorods grow in an understandable and completely predictable way. It is of interest to note that the ratio kl/kr is not unity; axial growth occurs along the unique axis of the wurtzite crystal and is 2.8 times as rapid as radial growth. This is less than the relative axial versus radial growth rates involved in core synthesis, and causes the aspect ratio to decrease from an initial value of 5.0 to a value of 4.1 after 10 injections (the largest size imaged). The higher growth rate on this crystal face is due to preferential binding of the ligands in solution (ODPA, ODA, and oleic acid), and hence reaction inhibition on the different crystal faces. Preferential growth on different crystal faces has been observed previously, and is the basis for growth of nanorods in general and of the dot-in-a-rod type particles.14-16 Analogous results are obtained for the CdSe/CdS core/shell nanorods. TEM images show that, although growth of a CdS shell maintains the rod morphology and approximate aspect ratio of the nanorods, the CdS shells grow more slowly than do the ZnS shells. The growth of CdS shells can be accurately modeled by eq 2 only if the amount of the precursors added in each injection (0.74 of a monolayer) is assumed to be 35% of this value (0.26 of a monolayer). Calculated particle growth curves are shown in Figure 3. With this assumption, Figure 3 shows that the CdS shells also grow with an axial to radial relative growth rate of 2.8. The thinner shells in the CdS case indicate loss of some of the cadmium and/or sulfur precursors. Cadmium precursors are much more reactive than the analogous zinc precursors,35 and the cadmium precursors are almost surely lost to homogeneous nucleation of CdS particles that occurs on each injection. We conclude that, in addition to (diffusion limited) shell growth, many of the nascent CdS monomers result in nucleation of CdS particles. These small particles are removed upon sample washing and play no role in the optical and quenching studies reported here. 2. Dynamics in the Absence of Hole Acceptors. Uncoated particles are only weakly luminescent, and have a QY of about 0.88%. The luminescence decay kinetics are strongly nonexponential: the decay components are 65% 95 ps, 23% 1.15 ns, and 12% 11.5 ns, giving an average decay time of 1.7 ns.36 A typical luminescence decay curve for bare CdSe nanorods is shown in Figure 4. An analysis of the luminescence decay kinetics and overall QY indicates that some fraction of the particles are “dark” and are not observed in the time-resolved luminescence experiment. The remaining “bright” particles are those that are observed in the time-resolved luminescence experiment. The QY of this subset of the particles is given by the ratio of the observed average decay time to the radiative

Core/Shell Nanorod Quantum Dots with Hole Acceptors

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19165

Figure 3. Experimentally determined (from TEM images) and calculated particle diameter and length increases (compared to the original core particles) for CdSe/CdS nanorods, upon each injection of shell precursors. The experimental diameter and length results are indicated with large square and round symbols, respectively. The calculated values are indicated with smaller symbols, connected by lines. This calculation assumes 35% of the CdS of the precursors are incorporated into the shell.

Figure 4. Normalized luminescence decay of bare CdSe nanorods. Also shown is a calculated curve corresponding to 95 ps (65%), 1.15 ns (23%), and 11.5 ns (12%) components.

lifetime. A reasonably accurate determination of the radiative lifetime is crucial to this analysis, because this quantity enters directly into the QY calculations for this subset of particles. The longest decay component seen in any of the particles is about 11.5 ns. Varying amplitudes of this component are seen in all of the different core/shell samples. Longer components are not observed, and we infer that this is close to the radiative lifetime. To corroborate this inference, this value of the radiative lifetime may be compared to that calculated from the absorption and luminescence spectra. Specifically, the radiative lifetime, τrad, may be calculated from the absorption spectrum (absolute, not just relative frequency-dependent extinction coefficients), the luminescence spectrum, and the degeneracies of the emitting states, as given in eq 3.37

1/τrad )

8π0.2303n2 gl -3 -1 〈νf 〉 Navc2 gu

dν ∫ ε(ν) ν

(3)

where the integration variable, ν, is the optical frequency, ε(ν) is the frequency-dependent extinction coefficient in L mol-1 cm-1, νf is the frequency of the luminescence, gl and gu are the degeneracies of the lower (ground) and upper states, respectively, n is the refractive index of the surrounding medium, and c is the speed of light. Each of the terms in eq 3 has to be evaluated, and is considered below. Most critical (and most difficult to measure accurately) is the extinction coefficient. We have measured the absorption spectrum for a precisely measured mass of washed, TOPO-coated CdSe core nanorods in a known volume of toluene. Knowing the particle sizes (from the TEM images), it is possible to determine particle concentrations, and hence the extinction coefficients. The extinction coefficient at the first absorption maximum is calculated to be 3.83 × 105 L mol-1 cm-1. It would also be of interest to compare this value with literature values. No direct comparison is possible, because the extinction coefficients for CdSe nanorods having different sizes and aspect ratios do not appear to have been reported. However, extinction coefficients for different sizes of CdSe spheres have been reported,38 and a very approximate comparison can be made. Taking the oscillator strengths (and hence the integrated extinction coefficients) for the nanorods to be the same as those for spheres having the same bandgap should not be too bad an approximation for the fairly low aspect ratio nanorods used here. The absorption bandwidth for the spheres is about 25 nm (fwhm), which is only slightly less than the spectral widths seen for these nanorods, 30 nm (fwhm). Thus, the maximum extinction coefficients for the nanorods may be taken to be a factor of 25-30 less than that of the comparable spheres. Therefore, based on the measurements on CdSe spheres, a maximum extinction coefficient of 4.5 × 105 L mol-1 cm-1 for the core nanorods is calculated, which is close to our measured value. While certainly not quantitative, this comparison establishes that our measured nanorod value is a reasonable one. Using our measured value for the maximum extinction coefficient and taking the width of the absorption transition to be the same as that of the luminescence spectrum, the integral in eq 3 can be evaluated. The measured luminescence spectrum also permits evaluation of the 〈νf-3〉-1 term in eq 3. The luminescence spectrum is fairly narrow (30 nm), and 〈νf-3〉-1 is essentially evaluated from the frequency of the luminescence maximum, νf max3. The ground state is singly degenerate, and gu is determined from the known electronic structure of CdSe nanorods. Excitation close to the band edge of a CdSe nanorod corresponds to exciting an electron from the selenium 4p orbitals (either px,y or pz) to the cadmium 5s orbitals.1 There are four thermally accessible electronic states for nanorods with this size and aspect ratio: the 0b, 0d, and 1( states. The 0d state is dark, the 1( state is x,y-polarized and weak, and the 0b state is dipole allowed from the ground state, and has most of the oscillator strength.3,4,39 The result is that CdSe nanorods give rise to luminescence that is strongly polarized along the long axis of the nanorod.2 These states are close in energy, and gu may be taken to be 4. The refractive index of the toluene/nanorod solution is taken to be 1.5. Using the above values, evaluation of eq 3 gives a radiative lifetime of 9.5 ns, which is in good agreement with the measured value of 11.5 ns. Using 11.5 ns for the radiative lifetime, the QY of the particles that are detected in the time-resolved luminescence experiment is simply the ratio of the observed (radiative + nonradiative) and radiative lifetimes; see eq 1. For these particles, this is calculated to be 15% ()1.7 ns/11.5 ns). This is much greater than the observed (static) QY of 0.88%. Thus, from the average lifetime and the observed QY, we infer that only about 6%

19166

J. Phys. Chem. C, Vol. 113, No. 44, 2009

Jiang et al.

TABLE 1: Spectroscopic Results for Core/Shell Particlesa CdSe/CdS

CdSe/ZnS

INJ:

1

3

15

1

3

15

f τ, ns φ

0.20 5.0 0.085

0.39 6.7 0.23

0.45 8.7 0.34

0.15 1.7 0.023

0.37 3.4 0.11

0.51 4.7 0.21

a Results are given for the fraction of particles that are observed (f), the average lifetime (τ), and the overall luminescence QY (φ).

()0.0088/0.15) of the particles are observed, while the remaining 94% of the particles are not observed at all; they are the “dark” particles, and their luminescence is lost to “static quenching”. Bare nanorods have a high density of surface defects, some of which result in sufficiently deep traps that the electron or hole trapping causes very rapid quenching. These are the dark particles. The observed particles undergo radiative and nonradiative decay on varying time scales, ranging from almost instrument limited (under 100 ps) to the radiative lifetime. The kinetic decays indicate that about 12% of the observed particles decay with the radiative lifetime (11.5 ns). Since only about 6% of the particles are observed, this corresponds to only about 0.72% ()0.12 × 0.06) of the entire ensemble of particles. Since their decay is entirely radiative, the quantum yield for this small fraction of the ensemble is close to 100%. The fraction of the luminescence resulting from the jth decay component is given by Ajτj/ΣAjτj, where Aj and τj are the amplitudes and decay times of the jth component. In the case of the 12% of the decay which occurs at the radiative rate, we note that this small fraction of the particles (0.72% of the entire ensemble) contributes about 82% of the luminescence and therefore dominates the luminescence spectrum. The other 18% of the observed luminescence comes from the particles having shorter decay times. Coating the CdSe cores with CdS or ZnS greatly increases the luminescence QY. This occurs because of an increase in the fraction of particles that are observed and an increase in their average lifetime. That is, coating the CdSe cores with either CdS or ZnS decreases the extents of both static and dynamic quenching. The extent of surface passivation is reflected in the overall observed QY, the average observed luminescence lifetimes, and the fraction of the particles that are observed in a TCSPC experiment (the fraction of particles that are “bright”). These spectroscopic and dynamic results for CdSe/CdS and CdSe/ZnS core/shell nanorods coated with 1, 3, and 15 precursor injections (INJ) are given in Table 1. Comparison of the less (1 and 3 INJ) and more (15 INJ) coated particles reveals that in all cases the addition of more shell layers further passivates surface traps, increasing both the fraction of bright particles and their average lifetime. The values of each of these parameters depends on the nature of the shell semiconductor in a way that can be understood in terms of the extents of lattice mismatch (and hence shell morphology) and the relative band energies. Each injection results in about 0.76 and 0.26 of a shell monolayer for ZnS and CdS shells, respectively, (see Figures 2 and 3 and recall that almost twothirds of the CdS precursors are lost to homogeneous nucleation). However, the fraction of particles that are observed (i.e., the fraction of particles that do not undergo static quenching) at 1 injection coverage is slightly greater for CdS than ZnS. This may be understood in terms of different shell morphologies resulting from different lattice mismatches. CdS has a smaller lattice mismatch with CdSe than does ZnS, about 4 and 11%, respectively.40 As a result, much of the CdS can deposit epitaxially, whereas ZnS tends to deposit as islands on CdSe.15,41

It is suggested that small lattice mismatches result in StranskiKrastanov growth, whereas larger mismatches result in Volmer-Weber island growth.42 When the 3 INJ shells have been deposited, CdS and ZnS shells are on average about 0.78 and 2.3 monolayers thick. These shells result in comparable values of f, that is, comparable static quenching. We infer that, despite the larger amount of ZnS deposited, more of the surface remains uncoated with ZnS, compared to the CdS case. The ZnS shell is more irregular than the CdS shell, and this results in shorter exciton lifetimes and a lower luminescence quantum yield. Table 1 shows that static quenching persists even when the equivalent of many layers of shell semiconductor have been deposited. Only about one-half of the particles are observed for the thickest CdS or ZnS shells. The other half is quenched by defects, presumably at the core/shell interface or the shell surface. The differences in average decay times may also be understood in terms of the same considerations as discussed above. Because of the larger lattice mismatch, the ZnS shell is more irregular and has more grain boundaries than does CdS. These grain boundaries have defects that can act as deep traps and cause quenching. Defects that are adjacent to the CdSe interface would be expected to interact strongly with the electron and hole, and result in very rapid (static) quenching. However, grain boundary defects are expected to be distributed throughout the ZnS shell and therefore have a distribution of distances from the core/shell interface. This results in a distribution of quenching rates and a reduced average excited state lifetime. We suggest that a defect in the ZnS shell may cause either static or dynamic quenching, depending on its location in the shell. Specifically, the quenching rates are expected to vary with how much it interacts with the electrons and holes that are nominally confined to the CdSe core. The presence of an increased number of defects results in faster decay times for the ZnS-coated particles for all shell thicknesses. It is of interest to compare these results with those of Mokari and Banin.13 As in the results presented here, ZnS shells were grown on CdSe nanorods of comparable dimensions. Mokari and Banin find that the luminescence QY improves until the equivalent of about two monolayers have been deposited. Further shell deposition causes the QY to decrease. Similar results are observed with spherical particles.28,43 This is not seen in the data presented here; continued shell deposition causes the QY to level off at a limiting value (about 21% for ZnS and 34% for CdS). We note that, in the present case, the shells were deposited at relatively high temperature (240 °C) and in the presence of a small amount of the cadmium precursor. The presence of the cadmium precursor is to prevent particle etching upon heating but, along with diffusion at the higher temperature, may result in a Cd-Zn graded interface. Consistent with this suggestion, we find the CdSe/CdS/ZnS nanorods give considerably higher luminescence QYs (≈50%) than either CdSe/CdS or CdSe/ZnS core/shell particles. Similar results have been observed in spherical CdSe/ZnSe core/shell44 and Cd,Zn-Se,S graded particles.35 The graded, rather than sharp, interface can relieve some of the lattice strain in the more thickly coated particles, avoiding the defects responsible for reducing the luminescence QY. 3. Hole Transfer Quenching by Phenothiazine and Hexadecane Thiol: Shell Thickness. Phenothiazine (PTZ) and hexadecane thiol (HDT) readily adsorb onto II-IV nanoparticles and quench the excited states by hole transfer, followed by nonradiative electron-hole recombination.27,28,30-33 The extents of static and dynamic quenching depend on the composition

Core/Shell Nanorod Quantum Dots with Hole Acceptors

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19167

TABLE 2: Static and Dynamic Quenching Fractions by Phenothiazine on Different Types of Particlesa

exp(-βx), where x is the shell thickness and effective hole transfer β values can be obtained. Values of β ) 3.0 nm-1 and β ) 0.41 nm-1 are obtained for PTZ and HDT, respectively. This is a dramatic difference, that can be understood in terms of the hole transfer energetics. The energetics of the barrier and final states affect the extents to which the initial and final states overlap. We note that factors other than electronic coupling (densities of states and vibrational factors) are expected to be essentially independent of shell thickness. The energetics may be summarized as follows. The CdSe and CdS valence bands are at about 6.0 and 6.6 eV, respectively, compared to the vacuum level, or 1.5 and 2.1 V, respectively, more positive than NHE.45 The phenothiazine 0/+1 reduction potential is at +0.85 V versus NHE.46-48 Thus, in the case of hole transfer to PTZ, the hole transfer driving force is about 0.65 eV and there is a 0.60 eV barrier caused by the presence of the shell. Hole transfer to alkyl thiols is less energetically favorable, which is indicated by the absence of hole transfer in the CdTe/thiol system. The valence band of CdTe is at 5.5 eV below vacuum, or 1.0 V versus NHE, which is insufficiently positive to facilitate hole transfer to alkyl thiols.31 This brackets the reduction potential of HDT in the range 1.0-1.5 V versus NHE. Thus, in the case of HDT, hole transfer has a 0.0-0.5 eV driving force. Other estimates put this driving force at 0.34 V.27 We conclude that the energies of the final states (the oxidized PTZ or thiol acceptor) are about 1.5 and 0.84 eV (0.5-1.0 eV) below the CdS shell valence band for PTZ and HDT, respectively. The larger energy difference between the final state and the shell valence band will result in less penetration of the hole wave function into the shell in the case of PTZ. The hole transfer rate depends on the overlap of the initial and final state wave functions, and this overlap drops off less quickly if the final state wave function can penetrate into the barrier. We suggest that greater charge localization is the primary reason that the hole transfer rate to PTZ shows a stronger shell thickness dependence than does HDT. It is tempting to try to apply the same analysis to the particles with ZnS shells, and to compare the CdS and ZnS results. These comparisons are complicated by the extents to which PTZ and HDT bind to CdS and CdSe versus ZnS surfaces, which is considered next. 3. Hole Transfer Quenching: Concentration Dependence. The concentration dependence of the quenching ratios in Tables 2 and 3 gives insight into the nature of the quencher binding that results in static versus dynamic quenching. Different types of binding sites (resulting in static and dynamic quenching) are expected to have different binding strengths and therefore have different concentration-dependent binding probabilities. The different morphologies of the ZnS and CdS shells are also expected to result in different densities of each type of surface binding site. We first consider the CdS-coated particles. Figure 5 shows the total, static, and dynamic quenching factors for 3 INJ CdS-coated particles as a function of the PTZ concentration. Note that I0/I is the inverse of the total quenching quantities in Tables 2 and 3. Analogous quantities for the static and dynamic quenching are also shown in Figure 5. In the particles used to obtain these results, the presence of PTZ results in a slight reduction in the extent of static quenching. (This is different than in the nominally identical particles used to obtain the shell thickness results. As noted above, there are these sorts of variations from one batch of particles to another.) The decrease in static quenching is due to a larger effect from dark particle passivation than from bright particle quenching. The extent to

phenothiazine

CdS

ZnS

no. of injections:

2

5

9

14

2

5

9

14

total quenching dynamic quenching static quenching rate constant, ns-1

0.21 0.25 0.84 0.69

0.43 0.48 0.89 0.18

0.40 0.49 0.82 0.13

0.46 0.53 0.86 0.12

0.23 0.30 0.77 0.44

0.23 0.24 0.96 0.48

0.44 0.44 1.0 0.18

0.70 0.66 1.06 0.071

a The total quenching is the product of the static and dynamic quenching. The hole transfer rate constant is obtained from comparison of the decay curves with and without phenothiazine.

and thickness of the nanorod shell and on the quencher concentration. The dependence on shell thickness at relatively high quencher concentration (0.5 mM) is summarized in Tables 2 and 3. The static and dynamic quenching fractions are the ratio of luminescence intensities due to the respective quenching processes. In the case of dynamic quenching, this is the ratio of average lifetimes. In the case of static quenching, it is the ratio of the fractions of particles that are observed. As such, the total quenching fraction is the product of the static and dynamic quenching fractions. It is important to note that the static and dynamic quenching fractions are obtained by comparing the same particles without adsorbed quenchers. Changes in the quenching therefore indicate only the changes that occur in the particles that luminesce in the absence of the quencher and the small number of dark particles that, upon quencher absorption, are passivated to become bright. As a result, these experiments look primarily at a subset of the particles, those for which the surface is sufficiently passivated that they are not statically quenched. Table 1 shows that this is on the order of 35-40% of the particles. There are several important results that come from the comparisons in Tables 2 and 3. First, the static quenching ratios are close to unity in almost all cases. The extent of static quenching depends on two factors, which work in opposite directions: fast hole transfer and surface defect passivation. The presence of fast hole transfer increases the extent of static quenching. However, when HDT or PTZ bind to defects on dark particles, the defects may be passivated, turning the dark particle into a bright one, and thereby decreasing the extent of static quenching. The extent of each process depends on the specifics of the surface and shows only a slight tendency to approach unity with the thickest ZnS shells. We have made several nominally identical samples and found that the extent of static quenching is somewhat variable; the static quenching ratio can be above or below unity. Second, the extent of dynamic quenching consistently decreases with increased shell thickness. Dynamic quenching fractions are not the best measure of dynamic quenching because they depend on the ratio of the hole transfer rate and the total decay rate in the absence of a quencher. A better measure of dynamic quenching is the hole transfer rate constants, and these consistently decrease with increasing shell thickness. It is of particular interest to compare the rate constants for PTZ versus HDT on different thickness CdS shells. PTZ rate constants show a strong dependence on the shell thickness (a factor of almost 6 between the 2 and 14 injection shells), whereas HDT shows a much weaker dependence (a factor of 1.7 between the 2 and 14 injection shells). We emphasize that these results were obtained with aliquots of the same core/shell particles; it is a valid comparison, and very similar results are seen on other sets of particles. The hole transfer rates can be taken to have an exponential dependence on shell thickness; the rate is proportional to

19168

J. Phys. Chem. C, Vol. 113, No. 44, 2009

Jiang et al.

TABLE 3: Static and Dynamic Quenching Fractions by Hexadecane Thiol on Different Types of Particlesa hexadecane thiol

CdS

ZnS

no. of injections:

2

5

9

14

2

5

9

14

total quenching dynamics quenching static quenching rate constant, ns-1

0.27 0.37 0.73 0.39

0.15 0.32 0.47 0.36

0.21 0.31 0.68 0.27

0.15 0.34 0.44 0.23

0.089 0.22 0.40 0.67

0.26 0.25 1.04 0.30

0.64 0.66 0.97 0.076

0.94 0.93 1.01 0.0096

a The total quenching is the product of the static and dynamic quenching. The hole transfer rate constant is obtained from comparison of the decay curves with and without hexadecane thiol.

Figure 5. Phenothiazine concentration dependence of the static, dynamic, and total quenching factors for 3 INJ CdS-coated particles. The static (triangles), dynamic (squares), and total (circles) quenching ratios are shown.

which this decrease in static quenching occurs depends on the PTZ concentration, and is essentially saturated at a concentration of 0.1 mM. The extent of dynamic quenching has a very different concentration dependence. It also increases rapidly below 0.1 mM, and continues to increase between 0.5 and 5 mM. These observations suggest that CdS-coated particles have at least two different types of PTZ absorption sites. A few dark particles have defect sites that bind PTZ very tightly, passivating these sites. These are presumably at voids in the CdS shell and perhaps at CdS grain boundaries. There are also binding sites on bright particles that can adsorb PTZ, resulting in dynamic quenching. Some of these sites adsorb PTZ very strongly, and saturate at low PTZ concentrations. These sites give rise to the rapid increase in dynamic quenching observed between 0.0 and 0.5 mM. Other sites adsorb PTZ more weakly, giving rise to the continued increase in dynamic quenching observed between 0.5 and 5.0 mM. At three CdS injections, the CdS shell still has voids, and these binding sites are presumably at these voids and on the CdS surface. The concentration dependence can also be considered in terms of a simple Stern-Volmer analysis. This analysis assumes an equilibrium between particles without (P) and with (PQ) an adsorbed PTZ (quencher, Q). Specifically,

P + Q a PQ

with

KSV )

[PQ] [P][Q]

and

[PQ] + [P] ) [P]0

(4)

In this model, particles without an absorbed quencher luminesce with a quantum yield of Φ0 and those with an adsorbed quencher are dark. Under the conditions used in these experiments, the particle concentration is low and adsorption onto the particles does not significantly affect the quencher concentration, [P], [PQ] , [Q]. This leads to the simple result I0/I ) 1 + KSV[Q].

Figure 6. Phenothiazine concentration dependence of the static, dynamic, and total quenching factors for 3 INJ ZnS-coated particles. The static (triangles), dynamic (squares), and total (circles) quenching ratios are shown. Panels a and b display the results from two different sets of experiments that examine different concentration ranges.

Thus, a plot of I0/I versus the PTZ concentration yields a straight line with the slope equal to the equilibrium constant for the adsorption of the PTZ onto the particle surface. Figure 5 shows that this is obviously not the case; neither the static nor the dynamic quenching follows this behavior, consistent with the above conclusion that there are different types of binding sites which result in different quenching behavior. Similar results are seen for PTZ quenching of the CdSe/ ZnS particles, Figure 6. The extent of static quenching increases rapidly with PTZ concentration, and then saturates at a concentration of about 0.1-0.2 mM. As in the case of the CdSe/CdS particles, the interpretation of this behavior is straightforward. Only a fraction of the particles have sites on which a PTZ can bind and cause static quenching, and these sites are saturated at a PTZ concentration of 0.1-0.2 mM. At saturation, I0/I has a value of about 1.4, indicating that about 30% of the bright particles have a defect site that can adsorb PTZ and undergo static quenching. This simple analysis cannot be applied to dynamic quenching. Figure 6

Core/Shell Nanorod Quantum Dots with Hole Acceptors

Figure 7. Hexadecane thiol concentration dependence of the static, dynamic, and total quenching factors for 3 INJ CdS- (open symbols) and 3 INJ ZnS (solid symbols)-coated particles. The static (triangles), dynamic (squares), and total (circles) quenching ratios are shown.

shows that the extent of dynamic quenching increases very rapidly at concentrations below 0.1 mM, more slowly between 0.1 and 0.5 mM, and very gradually at higher concentrations. This concentration dependence indicates that there are at least two types of adsorption sites that can result in dynamic quenching. We suggest that the static quenching sites are associated with surface irregularities (voids and perhaps other discontinuities in the ZnS shell), onto which strong binding can occur. Similar types of sites (which are also saturated at 0.1-0.2 mM) can also result in dynamic quenching. In addition, there are less strongly binding PTZ adsorption sites associated with dynamic quenching that are likely on the ZnS surface. The concentration-dependent quenching behavior of HDT on 3 INJ CdS- and ZnS-coated nanorods is shown in Figure 7. The two types of core/shell particles show very similar HDT quenching behaviors. In both cases, the plot of I0/I versus concentration is not linear, indicating the presence of different types of binding sites. The comparison of ZnS-shell and CdS-shell concentration dependencies for both HDT and PTZ gives insight into the chemical nature of the different types of binding sites on CdSe/ZnS particles. The important point is that PTZ shows similar concentration dependencies on the CdSe/ZnS and CdSe/CdS nanorods, indicating comparable binding constants onto the two different types of particles. The same can be said of HDT. This is a remarkable result, because alkyl thiols are known to bind much more strongly to CdS than ZnS. The Stern-Volmer constants obtained for alkyl thiols are about 4 × 104 M-1 on CdS and 3 × 102 to 103 M-1 on ZnS, about 2 orders of magnitude different.30,33 The difference in binding constants makes sense in terms of Lewis acid-base considerations. The sulfur in an alkane thiol is a very “soft” Lewis base, and cadmium in CdSe or CdS is a soft Lewis acid. The soft-soft acid-base interaction results in strong bonding. The zinc in ZnS is a harder Lewis acid and is expected to result in weaker bonds with HDT. The similarity of the HDT quenching curves on CdSe/CdS and CdSe/ZnS particles can be understood only if the HDT is binding to the CdSe exposed in the voids of the ZnS shell. The data in Figures 5-7 put limits on the numerical values of these binding constant magnitudes, which are consistent with this conclusion. The low-concentration slope of the HDT quenching curve corresponds to a binding

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19169 constant of at least 3 × 103 M-1. This is considerably greater than that for alkyl thiols on ZnS (about 300-1000 M-1),33 indicating that, at low concentrations, quenchers cannot be binding to the ZnS covered part of the particle surface. We suspect that, as the exposed CdSe parts are saturated, an increasing amount of binding occurs to the ZnS surface. These considerations explain part of the very strong thickness dependence of the quenching rate constant observed for HDT on CdSe/ZnS; see Table 3. On the basis of these considerations, HDT is expected to preferentially bind to CdSe, compared to ZnS. At low ZnS injection numbers, much of the CdSe surface is still exposed and binds with HDT. At higher injection numbers, the CdSe core is effectively covered with ZnS and there is little HDT binding to this surface. The net effect is that, with further ZnS injections, the extent of surface ZnS coverage increases and the effective binding constant decreases, and this is in part responsible for the observed shell-thickness-dependent rate constant. Comparable binding would be expected for CdS and CdSe, because in both cases binding is to the same Lewis acid, the cadmium atoms. Such effects would therefore not be observed in CdSe/ CdS particles, and the PTZ versus HDT comparison on CdSe/ CdS particles described above is valid. We have studied the concentration dependencies of both PTZ and HDT in several batches of CdSe/CdS and CdSe/ ZnS core/shell nanorods. The results presented in Figures 5-7 are very typical, and the above qualitative concentration dependencies are observed in every batch of particles with both PTZ and HDT. The conclusion, that static and dynamic quenching are associated with adsorption on strong and weak binding sites, respectively, is not specific to a particular batch of particles. Rather, it seems to be a persistent observation. Binding of thiol quenchers to different types of sites has also been observed in spherical CdSe/ZnS particles.33 In these studies, several different types of thiols were adsorbed onto CdSe/ZnS particles having four monolayer thick shells. The concentration-dependent total quenching was accurately fit to a model in which there are two different types of binding sites, having different bonding constants. The behavior obtained at the highest quencher concentrations has also been observed and analyzed in spherical TOPO-capped CdSe particles with PTZ as the quencher.27 That analysis assigned the PTZ concentration dependence to multiple PTZs adsorbing onto the particles, providing parallel quenching pathways. This is consistent with our assignment that much of the dynamic quenching is due to PTZs adsorbed on the particle surface, rather than in specific surface defect sites. Conclusions Several conclusions can be drawn from the results presented here. (1) CdS or ZnS shells can be deposited on CdSe nanorods in such a way that the shell semiconductor grows primarily axially, maintaining the rod morphology. Under the conditions used here, quantitative deposition is obtained for the ZnS but not the CdS shell. (2) The presence of the CdS or ZnS shell greatly affects the particle nonradiative relaxation dynamics and hence the luminescence quantum yields. The presence of the semiconductor shell passivates the surface, reducing the extent of both static and dynamic quenching. Under the deposition conditions used here (240 °C), there is sufficient lattice strain relaxation that high luminescence QYs can be obtained with relatively thick shells.

19170

J. Phys. Chem. C, Vol. 113, No. 44, 2009

(3) Hole acceptors such as phenothiazine and hexadecane thiol readily adsorb onto these core/shell nanorods. Both phenothiazine and hexadecane thiol can quench the core excited state by hole transfer through sufficiently thin shells. Both static and dynamic quenching are observed. The shell thickness dependence observed with both types of shells may be understood in terms of the phenothiazine and hexadecane thiol redox potentials and the relative energetics of the different shell materials. (4) The extents of static and dynamic quenching depend on the quencher concentration. In both the ZnS- and CdScoated particles, these concentration dependencies indicate that static and dynamic quenching are associated with strongly and weakly binding sites, respectively. Acknowledgment. This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-04ER15502. The authors wish to acknowledge the use of the instrumentation and Mike Dunlap’s assistance at the Imaging and Microscopy Facility at UC Merced. References and Notes (1) Hu, J.; Wang, L.-w.; Li, L.-s.; Yang, e.; Alivisatos, A. P. Semiempirical Pseudopotential Calculation of Electronic States of CdSe Quantum Rods. J. Phys. Chem. B 2002, 106, 2447. (2) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.-W.; Alivisatos, A. P. Linearly polarized emission from colloidal semiconductor quantum rods. Science 2001, 292, 2060. (3) Shabaev, A.; Efros, A. L. 1D Exciton Spectroscopy of Semiconductor Nanorods. Nano Lett. 2004, 4, 1821. (4) Zhao, Q.; Graf, P. A.; Jones, W. B.; Franceschetti, A.; Li, J.; Wang, L.-W.; Kim, K. Shape Dependence of Band-Edge Exciton Fine Structure in CdSe Nanocrystals. Nano Lett. 2007, 7, 3274. (5) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states. Phys. ReV. B 1996, 54, 4843. (6) Artemyev, M.; Moller, B.; Woggon, U. Unidirectional Alignment of CdSe Nanorods. Nano Lett. 2003, 3, 509. (7) Williams, Y.; Chan, K.; Park, J. H.; Khoo, I. C.; Lewis, B.; Mallouk, T. E. Electro-optical and nonlinear optical properties of semiconductor nanorod doped liquid crystals. Proc. SPIE 2005, 5936, 593613. (8) Peng, X.; Manna, L.; Yang, W.; Juanita, W.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Shape control of CdSe nanocrystals. Nature 2000, 404, 59. (9) Peng, A.; Peng, X. Mechanisms of the Shape Evolution of CdSe Nanocrystals. J. Am. Chem. Soc. 2001, 123, 1389. (10) Manna, L.; Scher, E. C.; Alivisatos, A. P. Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, and Tetrapod-Shaped CdSe Nanocrystals. J. Am. Chem. Soc. 2000, 122, 12700. (11) Peng, Z. A.; Peng, X. Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth. J. Am. Chem. Soc. 2002, 124, 3343. (12) Wang, W.; Banerjee, S.; Jia, S.; Steigerwald, M. L.; Herman, I. P. Ligand Control of Growth, Morphology, and Capping Structure of Colloidal CdSe Nanorods. Chem. Mater. 2007, 19, 2573. (13) Mokari, T.; Banin, U. Synthesis and Properties of CdSe/ZnS Core/ Shell Nanorods. Chem. Mater. 2003, 15, 3955. (14) Carbone, L.; Nobile, C.; De Giorgi, M.; Sala, F. D.; Morello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.; Nadasan, M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingolani, R.; Krahne, R.; Manna, L. Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/ CdS Nanorods Prepared by a Seeded Growth Approach. Nano Lett. 2007, 7, 2942. (15) Deka, S.; Quarta, A.; Lupo, M. G.; Falqui, A.; Boninelli, S.; Giannini, C.; Morello, G.; Giorgi, M. D.; Lanzani, G.; Spinella, C.; Cingolani, R.; Pellegrino, T.; Manna, L. CdSe/CdS/ZnS Double Shell Nanorods with High Photoluminescence Efficiency and Their Exploitation As Biolabeling Probes. J. Am. Chem. Soc. 2009, 131, 2948. (16) Talapin, D. V.; Koeppe, R.; Gtzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Highly Emissive Colloidal CdSe/CdS Heterostructures of Mixed Dimensionality. Nano Lett. 2003, 3, 1677.

Jiang et al. (17) Zhang, Y.; Jing, P.; Zeng, Q.; Sun, Y.; Su, H.; Wang, A.; Kong, X.; Zhao, J.; Zhang, H. Photoluminescence Quenching of CdSe Core/Shell Quantum Dots by Hole Transporting Materials. J. Phys. Chem. C 2009, 113, 1886. (18) Landes, C.; Burda, C.; Braun, M.; El-Sayed, M. A. Photoluminescence of CdSe Nanoparticles in the Presence of a Hole Acceptor: n-Butylamine. J. Phys. Chem. B 2001, 105, 2981. (19) Landes, C. F.; Braun, M.; El-Sayed, M. A. On the Nanoparticle to Molecular Size Transition: Fluorescence Quenching Studies. J. Phys. Chem. B 2001, 105, 10554. (20) Selmarten, D.; Jones, M.; Rumbles, G.; Yu, P.; Nedeljkovic, J.; Shaheen, S. Quenching of Semiconductor Quantum Dot Photoluminescence by a π-Conjugated Polymer. J. Phys. Chem. B 2005, 109, 15927. (21) Ebenstein, Y.; Mokari, T.; Banin, U. Fluorescence quantum yield of CdSe/ZnS nanocrystals investigated by correlated atomic-force and singleparticle fluorescence microscopy. Appl. Phys. Lett. 2002, 80, 4033. (22) Issac, A.; Jin, S.; Lian, T. Intermittent Electron Transfer Activity From Single CdSe/ZnS Quantum Dots. J. Am. Chem. Soc. 2008, 130, 11280. (23) Cretı´, A.; Anni, M.; Rossi, M. Z.; Lanzani, G.; Leo, G.; Sala, F. D.; Manna, L.; Lomascolo, M. Ultrafast carrier dynamics in core and core/ shell CdSe quantum rods: Role of the surface and interface defects. Phys. ReV. B 2005, 72, 125346. (24) Sharma, S. N.; Pillai, Z. S.; Kamat, P. V. Photoinduced Charge Transfer between CdSe Quantum Dots and p-Phenylenediamine. J. Phys. Chem. B 2003, 107, 10088. (25) Danielson, E.; Elliott, C. M.; Merkert, J. W.; Meyer, T. J. Photochemically induced charge separation at the molecular level. A chromophore-quencher complex containing both an electron donor and an acceptor. J. Am. Chem. Soc. 1987, 109, 2519. (26) Tierney, M. T.; Grinstaff, M. W. Synthesis and Characterization of Fluorenone-, Anthraquinone-, and Phenothiazine-Labeled Oligodeoxynucleotides: 5‘-Probes for DNA Redox Chemistry. J. Org. Chem. 2000, 65, 5355. (27) Huang, J.; Huang, Z.; Jin, S.; Lian, T. Exciton Dissociation in CdSe Quantum Dots by Hole Transfer to Phenothiazine. J. Phys. Chem. C 2008, 112, 19734. (28) Vinayakan, R.; Shanmugapriya, T.; Nair, P. V.; Ramamurthy, P.; Thomas, K. G. An Approach for Optimizing the Shell Thickness of CoreShell Quantum Dots Using Photoinduced Charge Transfer. J. Phys. Chem. C 2007, 111, 10146. (29) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1978. (30) Bullen, C.; Mulvaney, P. The Effects of Chemisorption on the Luminescence of CdSe Quantum Dots. Langmuir 2006, 22, 3007. (31) Wuister, S. F.; Doneg, C. d. M.; Meijerink, A. Influence of Thiol Capping on the Exciton Luminescence and Decay Kinetics of CdTe and CdSe Quantum Dots. J. Phys. Chem. B 2004, 108, 17393. (32) Munro, A. M.; Plante, I. J.; Ng, M. S.; Ginger, D. S. Quantitative Study of the Effects of Surface Ligand Concentration on CdSe Nanocrystal Photoluminescence. J. Phys. Chem. C 2007, 111, 6220–6227. (33) Breus, V. V.; Heyes, C. D.; Nienhaus, G. U. Quenching of CdSeZnS Core-Shell Quantum Dot Luminescence by Water-Soluble Thiolated Ligands. J. Phys. Chem. C 2007, 111, 18589. (34) Jiang, Z.-J.; Kelley, D. F. To be published. (35) Bae, W. K.; Char, K.; Hur, H.; Lee, S. Single-Step Synthesis of Quantum Dots with Chemical Composition Gradients. Chem. Mater. 2008, 20, 531. (36) This is the particle-averaged lifetime. 〈τ〉 ) ∑Anτn/∑An. Often average lifetimes are expressed as the photon-averaged lifetimes. 〈τ〉 ) ∑Anτn2/∑Anτn. We will use the former throughout this paper. (37) Strickler, S. J.; Berg, R. A. Relationship between Absorption Intensity and Fluorescence Lifetime of Molecules. J. Chem. Phys. 1962, 37, 814. (38) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854. (39) Thomas, N. L.; Herz, E.; Schop¨s, O.; Woggon, U.; Artemyev, M. V. Exciton Fine Structure in Single CdSe Nanorods. Phys. ReV. Lett. 2005, 94, 016803. (40) Hotje, U.; Rose, C.; Binnewies, M. Lattice constants and molar volume in the system ZnS, ZnSe, CdS, CdSe. Solid State Sci. 2003, 5, 1259. (41) Xie, R.; Kolb, U.; Li, J.; Basche, T.; Mews, A. Synthesis and Characterization of Highly Luminescent CdSe-Core CdS/Zn0.5Cd0.5S/ZnS Multishell Nanocrystals. J. Am. Chem. Soc. 2005, 127, 7480. (42) Goebl, J. A.; Black, R. W.; Puthussery, J.; Giblin, J.; Kosel, T. H.; Kuno, M. Solution-Based II-VI Core/Shell Nanowire Heterostructures. J. Am. Chem. Soc. 2008, 130, 14822. (43) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. (CdSe)ZnS CoreShell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites. J. Phys. Chem. B 1997, 101, 9463.

Core/Shell Nanorod Quantum Dots with Hole Acceptors (44) Lee, H.; Holloway, P. H.; Yanga, H.; Hardison, L.; Kleiman, V. D. Synthesis and characterization of colloidal ternary ZnCdSe semiconductor nanorods. J. Chem. Phys. 2006, 125, 164711. (45) Embden, J.; Jasieniak, J.; Go´mez, D. E.; Mulvaney, P.; Giersig, M. Review of the Synthetic Chemistry Involved in the Production of Core/ Shell Semiconductor Nanocrystals. Aust. J. Chem. 2007, 60, 457. (46) Mecklenburg, S. L.; Peek, B. M.; Schoonover, J. R.; McCafferty, D. G.; Wall, C. G.; Erickson, B. W.; Meyer, T. J. Photoinduced electron transfer in amino acid assemblies. J. Am. Chem. Soc. 1993, 115, 5479.

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19171 (47) Larson, S. L.; Elliott, C. M.; Kelley, D. F. Electron Transfer in Phenothiazine/Ru(bpy)32+ Donor-Chromophore Complexes. Inorg. Chem. 1996, 35, 2070. (48) Thomas, K. G.; Biju, V.; Kamat, P. V.; George, M. V.; Guldi, D. M. Dynamics of Photoinduced Electron-Transfer Processes in Fullerene-Based Dyads: Effects of Varying the Donor Strength. ChemPhysChem 2003, 4, 1299.

JP907728H