Size Control and Photophysical Properties of Quantum Dots Prepared

Oct 22, 2010 - The use of quantum dot (qdot) nanomaterials in aqueous media for biosensing, imaging, and energy conversion typically requires multiste...
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Size Control and Photophysical Properties of Quantum Dots Prepared via a Novel Tunable Hydrothermal Route Hyunjoo Han, Gianna Di Francesco, and Mathew M. Maye* Department of Chemistry, Syracuse UniVersity, Syracuse, New York 13244, United States ReceiVed: August 14, 2010; ReVised Manuscript ReceiVed: October 4, 2010

The use of quantum dot (qdot) nanomaterials in aqueous media for biosensing, imaging, and energy conversion typically requires multistep phase transfer routes based on tailoring surface chemistry. Such surface modification can lead to instability, and increased hydrodynamic diameters, which affect utility. Thus, the ability to synthesize qdots under aqueous conditions with improved photophysical properties that are comparable to the state of the art would be very beneficial. One limitation to this is the availability of high temperature aqueous protocols, which limits size control and crystalline annealing. Here, we show the ability to fabricate highly emissive CdSe, CdSe/CdS, and CdSe/CdS/ZnS qdots under fine-tuned hydrothermal conditions. The novelty of this approach is the use of a synthetic microwave reactor for dielectric heating that provides both kinetic control, and in situ monitoring of temperature and pressure. Results indicate the dramatic improvement for core and core-shell qdot luminescence at hydrothermal temperatures, as indicated by increased monodispersity, quantum yields, qdot brightness, and lifetimes. Introduction The synthetic methodology for colloidal nanoparticles, such as for semiconductive quantum dots (qdots), is a fascinating synergy of traditional wet-chemical approaches that employ organic and inorganic reactions, with that of solid-state processing; where key parameters include high temperature annealing, nucleation and growth, and epitaxial deposition.1-14 Much work has been accomplished since the seminal reports,1-6 and the knowledge base for qdot synthesis and the remarkable final photophysical properties has grown considerably.7 The fabricated qdots are crystalline, monodisperse, and highly hydrophobic due to dense shells of self-assembled monolayers.1-3 While such encapsulation is crucial for controlling nucleation and growth, and for preserving quantum confinement effects, the resulting qdots are notorious to functionalize for a number of studies, including those related to energy transfer,15 biological imaging,16,17 biodiagnostics,18 and biomimetic self-assembly.19-21 In many of these studies, the qdots must be processed under aqueous conditions, which requires multiple functionalization steps.16-21 This functionalization can lead to increases in hydrodynamic radii, as well as to instability when ligand exchange in not complete. Moreover, a loss of fluorescence quantum yields is often observed. Because of this, the synthesis of qdots under aqueous conditions,4,5,7-10,13,14 the original fabrication route,2,5,6 is still pursued in hopes of providing a material that is easily functionalized and fabricated. Recently, qdots synthesized under aqueous conditions have found utility in a number of studies, including: bioimaging,22 FRET energy transfer,23,24 photoelectrode assemblies25 and dye sensitized solar cells.26 In these studies, CdTe qdots and nanorods have proven especially popular,7,13,14,22 and utilize noninjection protocols where qdot size is tailored by precursor ratios or reaction time, and not by temperature control, due to low reflux temperatures.8-10,13,14 One emerging protocol for the aqueous synthesis of nanomaterials is the use of MW-irradiation (MWI) as a heating * To whom correspondence should be addressed. E-mail: mmmaye@ syr.edu.

source. Unlike traditional mantle or oil based heating which rely on conduction, convection, and radiation; MWI based heating affords direct energy transfer from MW electromagnetic radiation and the dipole moment of solvent, chemical, or material at high frequency.27-29 This dielectric heating acts simultaneously over the entire reaction volume, via absorption of energy (i.e., 10-1000 W) selectively to high dielectric materials, namely; dipole containing solvents or monomers. This allows for vastly decreased thermal gradients in a reaction, therefore providing a uniform thermal activation, which is ideal for nuclei formation and uniform growth for nanomaterials. The interaction between a material and electromagnetic radiation is best described in terms of dielectric constants.29 Briefly, energy transfer from the microwave electromagnetic radiation can be described as a dielectric loss e′′, which is dependent upon a materials dielectric constant e′ (e′ ) ereO, er ) dielectric constant, eO ) permittivity). A dissipation factor, γ )e′′/e′, where γ is the loss tangent, then broadly defines a materials dielectric heating.28 Such energy absorption is drastically enhanced due to the high frequency of the low-energy MWI. The promise for MWI use in organic chemistry was described in 1986 by Rousell30 and Giguere;31 and since there have been a number of classical thermodynamic and or kinetic descriptions of how low energy electromagnetic radiation can potentially effect reaction yields and trajectories. Because the energy transferred from from a 2.45 GHz alternating MWI source is only ∼0.3 cal/mol, increased number of monomer collisions and increased entropy are thought to account for increases in kinetics and yields. Kinetically, k ) A exp(-∆GAct/RT), MWI has also been hypothesized to increase reaction kinetics via: (i) increasing the probability of impacts (A), and (ii) decreasing activation energy (∆GAct) due to entropic effects of the induced high frequency rotation.18 Taken together these interactions lead to dramatically decreased reaction times, and increased yields. In addition, the opportunity does exist for MWI to induce a third, so-called nonequilibrium condition, such as local heating, and super heating.32-35

10.1021/jp107702b  2010 American Chemical Society Published on Web 10/22/2010

Quantum Dots Prepared via a Novel Route Such nonequilibrium conditions are particularly interesting from a colloidal synthesis point of view, since the local temperature of the growing nuclei might be at much higher temperatures than the surrounding medium, either solvent or ligand shell. Not only would this increase reaction kinetics, it may also increase nanoparticle temperature above that of its melting point, which is possible since a nanomaterials melting temperature is drastically decreased at nanometer grain sizes.36 This may therefore affect crystallinity, and perhaps lead to interesting phase behavior of the nanocrystal itself. The direct probing of nonequilibrium effects is challenging and under intense investigation. For example, Wada and co-workers recently developed an experimental setup to probe the localheating phenomena at metal surfaces under MWI irradiation using a novel single molecule Raman investigation, which was used to understand heterogeneous catalysis.33 The use of MWI based heating is now the preferred method in industry, especially in the preparation of many commercially available small molecules, oligonucleotides, peptides, or polymers, which take advantage of automated synthesis at high throughput levels,37 for example. While the use of MWI in synthetic chemistry is well established, its use in solid-state, and colloidal chemistry is less understood by comparison. However, recently the number examples using MWI has grown rapidly. El-Shall and co-workers has shown that by using short pulses of MWI, 1D semiconductive rods can be synthesized in organic solvents,38 as well as gold nanoparticles,39 rare earth oxide nanomaterials40 and noble metal catalysts for CO oxidation.40 Other noble metal catalysts have also been prepared via MWI, such as Pt and Pd, as well as PtRu-alloys on carbon black support.41,42 Researchers have also used MWI to fabricate phosphonate nanostructures,43 metal-organic frameworks,44 nanostructured Ferrites,45 nanosized Titania,46 and silica shell formations.8,47 MWI has also been exploited to functionalize the surface chemistry of nanomaterials, such as carbon nanotubes.48,49 In addition to oxide and metallic nanoparticles,50-52 qdots have also been a focus of MWI based synthesis methodology,8-14,38,53-57 especially that of aqueous based synthesis.55-57 One exciting opportunity that the MWI based heating affords is the potential for automation, high-throughput screening, and ease of scalability. This was shown recently by Strouse and co-workers, who used a highly automated MWI condition, where delicate control of MW power, temperature, and reaction time allowed for improved control over nucleation and growth of InP, CdTe, and CdSe qdots.11,12 Such control may allow researchers to better delineate the effects of MWI on colloidal synthesis. In addition to MWI based aqueous one-pot synthesis of nanomaterials, the use of hydrothermal conditions has also been explored, but examples are much more limited.10,58-60 For instance, the synthesis of CdSe,10 CdTe,59 CdTe/ZnS,60 as well as FeS261 has recently been shown under hydrothermal conditions. Hydrothermal processing is intriguing, as it provides the high temperatures typically required for crystalline annealing of nanomaterials, such as qdots. However, fine control of reaction kinetics or heating and cooling rates is challenged in hydrothermal routes, due in large part to experimental set-ups. Herein, we describe the synthesis of CdSe cores, as well as CdSe/CdS, CdSe/ZnS, and CdSe/CdS/ZnS core/shell qdots using hydrothermal conditions with well-defined temperatures between 120-210 °C. The novelty of this work lies in the ability to rapidly tune hydrothermal temperature (TH), kinetic ramping, and temperature quenching via the use of a synthetic microwave reactor (Discovery-S, CEM Inc.). Moreover, this scalable MWI-

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19271 based hydrothermal protocol produces high quality, and easily functionalized qdots in minutes. The MW reactor also serves to monitor growth conditions with in situ monitoring of reaction temperature and pressures, thus facilitating highly fine-tunable and reproducible results. Experimental Section Chemicals. Cadmium perchlorate hydrate (Cd(ClO4)2•XH2O, 99.999%), zinc perchlorate hexahydrate (Zn(ClO4)2•6H2O, >99%), sodium citrate tribasic dihydrate (Cit, >99%) was purchased from Sigma. N,N-dimethylselenourea (Me2NCSeNH2, 97%) was obtained from Acros organics and sodium hydroxide was from Fisher scientific. Thioacetamide (MeCSNH2, >99.0%) was obtained from Fluka. Ultrapure water (18.2 MΩ) was provided from a Sartorius Stedim Arium 61316 reverse osmosis unit combined with a Arium 611DI polishing unit. All chemicals were used as received. Synthesis. The synthesis of CdSe quantum dots (qdots) as well as CdSe/ZnS, CdSe/CdS, and CdSe/CdS/ZnS core/shell qdots was carried out in an aqueous system using well-defined hydrothermal temperatures (TH). Here, TH is achieved using a synthetic microwave reactor (Discovery-S, CEM Inc.) that facilitated rapid heating, stable set-points, and temperature quenching. CdSe Qdots. The precursor chemicals and initial synthesis ratios were inspired by Kotov and co-workers,8,9 and used with modification. In a typical synthesis, an aliquot (0.25-1.0 mL) of 40 mM of Cd(ClO4)2 was diluted in 1-1.8 mL of ultrapure water (18.2 MΩ). Next, an aliquot (100-300 µL) of 0.1 M Cit and 20 mM of Me2NCSeNH2 (0.5-1.0 mL) was added. Before dilution, the freshly prepared Cd and Se stock solutions were deaerated with N2. Finally, the pH was adjusted to ∼12 by addition of 1.0 M NaOH. The final solution was then sealed in 10 mL glass microwave reaction vials, hermetically sealed, and deaerated via N2 before MWI processing. In a typical experiment, the total heating time at the desired hydrothermal set point (120-180 °C) was 2 min. A number of synthetic parameters were varied to best optimize and tailor the nucleation and growth. A main parameter is the synthetic ratio r, r ) [Cd]/ [Se]. Here, we show the results of r ) 4 and 8. In general, we found that r ) 4 leads to greatest TH dependent size tunability, where r ) 8 results in qdots with higher QY. In initial experiments, synthesis was carried out in 3-10 mL scales. The synthesis was then extended to 25-30 mL scales without significant changes to the qdot characteristics, owing in large part to the direct dielectric heating the MWI provides. Moreover, changes to scale do not dramatically alter heating or cooling kinetics. Core/Shell Qdots. For the preparation of core/shell CdSe/ CdS, CdSe/ZnS, and CdSe/CdS/ZnS qdots, the as-synthesized Cit-capped CdSe qdots synthesized above were combined with MeCSNH2 as a sulfur source and Zn(ClO4)2 as a zinc source in quantities required to epitaxially grow a 2-4 monolayer ZnS shell. Briefly, to the 1.5 mL of CdSe qdot solution of known concentration, 50-150 µL of 20 mM MeCSNH2 was added, depending upon core concentration (calculated using the first absorption maxima),62 and desired shell thickness, and then purged with N2. Next, the sample was hydrothermally processed for 2 min at either 120 or 160 °C. In this case, the excess Cd2+ from core growth is also used for shell formation. For the growth of ZnS shells, 50-150 µL of 20 mM of Zn(ClO4)2 was added to the CdSe qdots, in which the excess Cd2+ was first removed by ion exchange filtration using 10 KDa molecular weight cutoff centrifugation filter and redispersed in the 6.0 mM sodium citrate

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solution with pH ) 10. Next, the sample was then processed for 2 min at hydrothermal temperatures of either 120 or 160 °C. The qdots were typically stored in the synthesis mother liquor, however purification could also be performed via overnight dialysis using a 500 Da membrane (Spectrum Laboratories Inc.). The qdot QY was found to increase dramatically at hydrothermal processing temperatures (T > 120 °C). However, we also observed the slow increase in QY over 10-50 days, due to an aging and self-annealing process. Such an annealing process was recently described,8,9 and likely involves the photoactivated annealing of qdot surface, resulting in less surface defect sites, as well as the potential growth of thin layers of higher bandgap CdO shells. Instrumentation. Synthetic MicrowaWe Reactor. A Discovery-S (CEM Inc.) synthetic microwave reactor was employed. The instrument is computer controlled, and operates at 0300W, from 30-300 °C, and from 0-200 PSI. Temperature is monitored in situ during synthesis via the use of an integrated IR-sensor, or via an immersed fiber optic temperature probe. The instrument is equipped with an active pressure monitoring system, which provides both pressure monitoring and added safety during synthesis. Pressure rated glass reaction vials with volumes of 10 or 35 mL were employed during synthesis. Active cooling was provided by the influx of the MW cavity with compressed N2, which rapidly cools the sample at a controlled rate. UV-Wisible Absorption (UV-Wis). The UV-vis measurements were collected on a Varian Cary100 Bio UV-vis spectrophotometer between 200-900 nm. The instrument is equipped with an 8-cell automated holder with high precision Peltier heating controller. Photoluminescence (PL). The PL emission and excitation measurements were collected on a Fluoromax-4 photon counting spectrofluorometer (Horiba Jobin Yvon). The instrument is equipped with a 150W xenon white light excitation source and computer controlled monochromator. The detector is a R928P high sensitivity photon counting detector that is calibrated to emission wavelength. All PL emission and excitation spectra were collected using both wavelength correction of source intensity and detector sensitivity. The excitation wavelength is 400 nm using 3 nm excitation and emission slits unless otherwise noted, and excitation spectra were collected at the qdot emission peak using 1 nm excitation and emission slits. The instrument is equipped with a computer-controlled temperature controller provided by a Thermo NESLAB temperature recirculator (Thermo Scientific). Transmission Electron Microscopy (TEM). TEM measurements were performed on either a FEI T12 Twin TEM operated at 120 kV with a LaB6 filament and Gatan Orius dual-scan CCD camera (Cornell Center for Materials Research), or a JEOL 2000EX instrument operated at 120 kV with a tungsten filament (SUNY-ESF, N.C. Brown Center for Ultrastructure Studies). Particle size was analyzed manually by modeling each qdot as a sphere, with statistical analsysis performed using ImageJ software on populations of at least 100 counts. Time Correlated Single Photon Counting (TCSPC). The TCSPC measurements were performed at Brookhaven National Laboratory (BNL) in the Center for Functional Nanomaterials (CFN) facility. Photoluminescence decays were measured by the time-correlated single photon counting (TCSPC) method by using 420 nm pulsed laser excitation. The setup is based on a frequency doubled diode-pumped Ti:sapphire laser system (Newport Spectra Physics, 8 MHz repetition rate, 60 fs pulse

Han et al. width) and a Fluotime 200 time-resolved fluorescence spectrometer (Picoquant GmbH). Fluorescence decays were collected at magic angle in the spectral range 520-600 nm, detected by a microchannel plate photomultiplier (Hamamatsu, 25 ps response) and registered by a TCSPC module (Picoharp 300, Picoquant GmbH). Decay histograms were collected with a time resolution of 4 ps per channel and analyzed by reiterative convolution of the instrumental response function (45 ps) with an exponential model (eq 1) function using the FluoFit software (Picoquant, GmbH). n

I(t) )

(

)

∫-∞t IRF(t′) ∑ Aiexp - t -τi t′ dt′ i)1

(1)

Fluorescence Correlation Spectroscopy (FCS). Fluorescence correlation spectroscopy (FCS) was performed at the CFN in BNL. FCS was performed with a homemade confocal fluorescence microscope based on an Olympus IX 81 microscope (1.2 NA 60× water immersion lens) by using the 457 nm laser light from an Ar-ion laser (Melles-Griot, 10 µW average power at the sample). Photoluminescence emitted by freely diffusing qdots was collected by the same lens, spectrally filtered from excitation by a dichroic mirror (DRLP455, Omega Filters) and a band bass (HQ605/40, Omega Filters) and imaged, via a 75 µm pinhole and a 50/50 beam splitter, onto two single photon counting avalanche photodiodes (MPD, Picoquant GmbH, Germany). FCS (intensity correlation) curves were acquired in cross-correlation mode using a real-time hardware correlator (time-correlated single photon counting analyzer, PicoHarp300, Picoquant GmbH, Germany). Autocorrelation data (AC(τ)) were recorded for 1 min and collected and processed using SymPhoTime software. FCS curves were then fit via a simple model accounting for 3D diffusion and blinking (eq 2):

AC(τ) ) N-1(

1 × (1 + τ/τDiff)(1 + τ/(r0 /ω0)2τDiff)1/2 [1 + (FT /(1 - FT)]exp(-τ/τT)

(2)

where N is the average number of molecules in the confoca volume, τDiff is the diffusion time, r0 and ω0 are the radial and axial dimensions of the excitation volume, and FT and τT are the fraction of molecules in the triplet state and the triplet lifetime, respectively. Diffusion coefficients and hydrodynamic radius (rH) were estimated by using the Stokes-Einstein equation. A structural parameter related to the probe volume was estimated based on FCS measurements of rhodamine 110 in water. Calculations The qdot concentration were calculated based on UV-vis optical absorption measurements of the qdot first band edge absorption (1s-1s) intensity using qdot size dependent optical extinction coefficients (εqdot). Qdot size was correlated to absorption wavelength using the Peng calibration method,62 which was then used to estimate εqdot. For instance, a CdSe qdot with band edge absorption of 555 nm corresponds to a core diameter ∼3.2 nm, which in turn determines the εqdot ) 1.9 × 105 cm-1 M-1. The final qdot concentration was then obtained using the Beers-lambert relationship Abs ) εbc; where ε is the estimated extinction coefficient (M-1cm-1), b is the path length, and c is concentration.

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Figure 1. A schematic illustration (a) of the synthesis strategy employed. Microwave irradition (MWI) based dielectric heating to hydrothermal temperatures allows for automated synthesis, high throughput, and in situ monitoring of reaction temperature (b), and pressure (c) during hydrothermal qdot synthesis.

The qdot photoluminescence quantum yields (QY) were calculated based on comparison to a reference dye using standard methods,64 via eq 3:

(

QYqdot(%) ) QYR

)( )(

2 AbsR PLqdot ηqdot Absqdot PLR ηR2

)

(3)

where QYR is the reference dye quantum yield (Rhodamine)31%, Rhodamine 6G ) 95%), AbsR and Absqdot are the optical absorption at specific excitation for the reference dye and qdot samples respectively. Here, careful attention was paid to prepare samples with optical absorption below 0.05-0.10 in order to limit inner filter effects.64 PLR and PLqdot correspond to the total area of the PL emission after wavelength dependent calibration of both the excitation source, and photoluminescence detector, as well as after PL spectra baseline correction. The emission is fit to a Gaussian profile. For samples exhibiting a trap-state emission (lower energy), only the band-edge emission PL area is used in QY calculations. The refractive index of the reference and qdot solvent, ηR and ηqdot, where also taken into account when required. Results and Discussion In this section we first describe the fine-tunable hydrothermal synthesis of CdSe qdots using microwave irradiation (MWI), and then reveal the corresponding temperature dependent photophysical properties. We then demonstrate the ease of growth of CdS or ZnS shells at the CdSe cores. The purpose of this study is to investigate the hypothesis that hydrothermal temperatures, if highly controlled, can improve the properties of nanomaterials synthesized under aqueous protocols, due to the potential for higher uniformity and improved crystallinity. As a proof-of-principle example, we chose CdSe qdots due to the wealth of structural information that can be obtained from their photophysical characteristics. Figure 1 shows an idealized illustration of the automated MWI based synthesis setup employed (a), as well as the in situ temperature (b) and pressure (c) profiles obtained during a typical synthesis of CdSe qdots at temperatures of 60-180 °C. In our system, MWI heating is computer controlled, with MW power being dynamically attenuated by temperature feedback measured via an integrated infrared detector or fiber optic probe, allowing for control of heating and cooling rates (Figure 1, Figure S1 of the Supporting Information). Since both heating and cooling rates are rapid, this protocol can produce materials

in a generally high-throughput manner, with the potential for automation. Figure 1b shows the rapid increase in hydrothermal temperature (TH) to a desired set-point (region-i), followed by a stable annealing temperature (region-ii), and the rapid temperature quenching (region-iii) due to absence of MWI coupled with high flow rate purging of the MW cavity by compressed N2. A similar profile is shown for the systems pressure characteristics (Figure 1c). Figure 2 shows representative UV-visible absorption (UVvis, a), photoluminescence emission (PL, b) and PL excitation spectroscopy (c) results for the CdSe qdots synthesized at 60 (i), 90 (ii), 120 (iii), 150 (iv), and 180 °C (v). In this work, cadmium perchlorate is first complexed with trisodium citrate (Cit) at pH ) 12 in deareated ultrapure water (18.2 MΩ) forming the Cit-Cd complex precursor. While seemingly benign, Cit possesses a rich set of complexation chemistry with transition metals, such as with titanium,65 and provides a stable Cd2+ precursor. Next, an aqueous aliquot of freshly prepared, deareated, selenourea is added at molar ratios (r ) [Cd]/[Se]) of 4 or 8, with a concentration of Cit of 1-4r. Prior to heating, the precursor solution was rapidly deaerated with N2 or Ar, and hermetically sealed in glass reaction vessels equipped with stirring capability. In this study, the annealing time was held constant for each synthesis at 5 min. The observed UV-vis and PL red-shift with increasing TH (i-V) in Figure 2 is consistent for an increase in qdot diameter (d) from ∼2.5 (i) to ∼4.0 nm (V).62,63 Here, nearly identical temperature ramping (1-2 °C/s) was utilized, providing comparable kinetic growth pathways between samples. As expected, CdSe prepared at low temperature possessed poor optical properties. At a modest TH ) 60 °C (i) a first band edge absorption of 526 nm and PL emission of 564 nm is observed. A second emission at lower energy (∼625 nm) was also observed, indicating either a polydisperse size disribution, a strong trapped-state emission due to poor crystallinity, or a combination thereof. A slight improvement was observed when TH is increased to 90 °C (ii), where a symmetric band edge emission emerges with a fwhm of ∼45 nm is observed. In contrast, CdSe prepared at elevated TH (>120 °C) show much improved emission properties. When prepared at TH ) 180 °C (v) the qdots reveal a symmetric band edge emission with decreased fwhm of 35-40 nm, and lack of trap emission, thus demonstrating improved monodispersity and crystallinity. The improved monodispersity is apparent in the observed PL excitation measurements (Figure 2c), which possess increasing numbers of distinguishable energy levels.

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Han et al.

Figure 2. Representative UV-vis absorption (a), PL emission (b) and PL excitation (c) spectra for CdSe qdots synthesized at 60 (i), 90 (ii), 120 (iii), 150 (iv), and 180 °C (v) with r ) [Cd]:[Se] ) 4, and 5 min reaction time (see Figure 1b). An excitation of 400 nm was used for PL emission; each spectrum is normalized and offset for comparison.

Figure 4. Representative TEM micrographs and statistical analysis for CdSe qdots synthesized at TH ) 120 °C (a,b) revealing average diameter of 3.4 ( 0.6 nm (n ) 192), and at TH ) 180 °C (c,d) revealing average diameter of 3.7 ( 0.4 nm (n ) 150).

Figure 3. Summary of TH dependence of band edge absorption (λAbs, a), PL emission (λPL, b), and QY (c) for CdSe qdots prepared at r ) 8 (i), and 4 (ii).

The qdot size range and its dependence on TH was further tailored by r. For instance, when the r was increased from 4 to 8, we observed a decrease in the qdot size at identical TH and reaction times (Figure S2 of the Supporting Information). Interestingly, we also observed a decrease in fwhm to ∼35 nm at TH> 120 °C. Figure 3a-b summarizes these results and reveals a near linear dependence of first band edge absorption (a) and Stokes shifted PL emission wavelength (b) with TH. A decrease in fwhm indicates further improvement in qdot quality, especially in terms of monodispersity and crystallinity. This was further substantiated via Transmission Electron Microscopy (TEM). Figure 4 shows representative TEM micrographs of CdSe prepared at 120 °C (a) and 180 °C (c) with r ) 8. Statistical analysis of the qdots (Figure 4b,c) reveals a modest size distribution of 3.4 ( 0.6 nm when prepared at 120 °C, whereas an increase in diameter and narrowing of distribution to 3.8 ( 0.4 nm is measured at 180 °C. These diameters are in close agreement with the corresponding optical properties, and also reveal the qdots to have highly truncated morphology.

To quantify the improved optical properties of the hydrothermally prepared qdots, the PL quantum yield (QY) was carefully calculated. Figure 3c shows the calculated QY for the CdSe prepared at both r of 8 (i) and 4 (ii). We observed a dramatic increase in QY for CdSe prepared at TH > 120 °C when synthesized at r ) 8, which typically have higher QY than the r ) 4 counterparts. For instance, qdots prepared at 60-100 °C conistently have QY of only ∼1% directly after synthesis, while samples prepared at TH g 120 °C showed consistent increases of ∼3.0, ∼6.6, and ∼9.9% when prepared at 120, 150, and 180 °C respectively at r ) 8. These QY values in particular are high for CdSe, especially for those synthesized via aqueous protocols. An improved QY is indicative of high core crystallinity, and a decrease in surface defects of a qdot. Such structural insights can be best investigated by lifetime measurements, as described next. To further probe the photophysical properties of the hydrothermally prepared qdots, we performed time correlated single photon counting (TCSPC) experiments to probe characteristic PL decay. The TCSPC delay signatures for qdots synthesized at r ) 8 and TH of: 60 (i), 90 (ii), 120 (iii), 150 (iv), and 180 °C (v) are shown in Figure 5a. From these PL decays, it is evident that CdSe prepared at high TH possess longer lifetimes (τ) with a more pronounced single exponential character. The PL decay was first modeled as a triexponential response (τ1 +

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Figure 5. (a) TCSPC measurements CdSe prepared at 60 (i), 90 (ii), 120 (iii), 150 (iv), and 180 °C (v) at r ) 8. An excitation of 420 nm was provided by a Ti:sapphire laser system with a 60 fs pulse, and a 45 ps instrument response function. (b) A corresponding plot of calculated τAve from TCSPC measurements indicating a systematic increase in τAve with increased hydrothermal temperature.

τ2 + τ3) and the results of the least-squares fitting is listed in Table 1. For example, the CdSe prepared at TH ) 90 °C (ii) possessed lifetimes with decays of τ1 ≈ 13, τ2 ≈ 2.1, and τ3 ≈ .08 ns, with intensity weighted percent contributions of 66%, 30%, and 4% respectively, indicating the strong influence of surface trapping by the increased ultrafast contributions (τ2,τ3). On the contrary, qdots prepared at TH g 120 (iii-v), revealed longer lifetimes with high τ1 contributions. For instance, τ1 of 17.4 (77%) and 19.4 ns (84%) were measured from qdots prepared at TH ) 150 and 180 °C respectively. To better compare results, an intensity weighted average PL lifetime (τAve) was calculated. As shown in Table 1, a τAve of; 8.6, 9.4, 12.8, 14.0, and 17.3 ns are calculated for CdSe prepared at 60, 90, 120, 150, and 180 °C, respectively. Figure 6b summarizes the results, and shows the significant increase in τAve with TH.

τAve )

1 Γ+

(4)

∑ kNR

The fluorescent τ of a molecule or material provides valuable insights into the electronic structure, and is inversely proportional to the sum of emission rate (Γ) and the sum of nonradiative decay rates (knr),64 as shown ideally in eq 4. If we make the assumption that Γ is unchanged in the qdots due to the identical composition, excitation, environment (shell and solvent), then the increase in τAVe for qdots synthesized at high TH is indicative of a decrease in knr. Here, knr is taken to be the sum of multiple decay channels. A number of factors are known to influence qdot knr,7,66,67 including composition, band structure (diameter, shell type), crystallinity, and surface trapping type.67

Figure 6. A representative set of FCS results for CdSe qdots fabricated at 150 °C (i) and a Rhodamine 110 (ii) standard normalized to qdot concentration (a). Fitting with a triplet diffusion model results in ∼17 qdots in the focal volume during measurement, with a diffusion constant ∼24.5 mm2/s. Compared with corresponding intensity time traces (b), average qdot brightness is estimated. A corresponding plot of the trend between qdot brightness and processing temperature (c).

Taken together, along with the increase in diameter, the increase in τAve and the corresponding single exponent contribution, is indicative of a well-defined band structure and decrease in crystalline or surface defects. This provides further evidence of the improvement of CdSe at high hydrothermal temperatures. In addition, the ultrafast component is likely also influenced by exciton-solvent interactions, given the weak encapsulating shells, and the polar media (water) employed.68 Qdot photophysical characteristics can also be probed at the single-qdot level, using fluorescence correlation spectroscopy (FCS). FCS probes diffusion constants () for the qdot emitters, as well as quantifying the number of emitters (N) in a known confocal volume.68 Combined with intensities obtained from time traces, one can obtain average single qdot brightness (counts/N). Qdot brightness, as well as PL intermittency (i.e., blinking) provides further insights into photophysical behavior.69 Figure 6 shows a representative FCS trace for a CdSe qdot prepared at TH ) 150 °C compared to a Rhodamine standard. When analyzed via eq 2, a of 24.5 µm2/s and N of ∼17 was calculated, corresponding to a brightness of 5.9 kHz (counts/ ms). Figure 6c reveals the brightness dependence on TH. Compared to qdots synthesized at TH < 100 °C with brightness between 1-2 kHz, those prepared at TH > 120 °C revealed a systematic increase in brightness to >5 kHz. In addition to brightness, FCS provides information pertaining to the qdot emitter hydrodynamic diameter (Dh) via the measurement of diffusion constants.70 Average Dh values obtained from FCS reveal Dh ≈ 6 nm, which are in good agreement with the qdot

TABLE 1: Analysis of QY, TCSPC, and FCS fitting for CdSe and CdSe/ZnS qdots

CdSea

CdSe/ZnS

TH

PL λmax

QYb

(°C)

(nm)

(%)

60 90 120 150 180 120/120e

529 545 560 569 583 557

0.14 0.50 2.94 6.58 9.81 38.00

TCSPCc

FCSd

τ1 /ns (%)

τ2 /ns (%)

τ3 /ns (%)

13.2 13.2 16.8 17.4 19.7 20.4

1.88 2.20 3.39 3.43 4.07 6.93

0.018 0.081 0.246 0.239 0.281

(69%) (66%) (77%) (77%) (84%) (93%)

(25%) (30%) (21%) (22%) (15%) (7%)

(6%) (4%) (2%) (1%) (1%)

τave (ns)

D (µm2/s)

PL (kHz)

8.65 9.40 12.8 14.0 17.3 19.5

24.3 29.2 23.9 24.5 28.8 35.4

1.21 1.89 2.12 5.9 4.86 8.13

a CdSe in these studies were prepared at r ) 8 exclusively. b QY calculated by comparison to dye standard using eq 3. c TCSPC fitting performed using multiexponential decay models of decay histograms with correction for the instrumental response function (∼45 ps) using eq 1. Individual lifetimes (τ1,τ2, τ3) are shown with intensity weighted percentages, which can be used to estimate average lifetime τave. d FCS correlation curves were fit using a standard triplet diffusion model using eq 2. e CdSe cores prepared at 120 °C, followed by ZnS shell growth at 120 °C.

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Figure 7. A representative set of the UV-vis (a) and PL emission (b) for CdSe-cores synthesized at 160 °C (i), and after CdS (ii), ZnS (iii), and CdS/ZnS (iv) shell growth at TH ) 120 °C. The TCSPC results (c) for the as-synthesized CdSe-cores before (i) and after ZnS shell growth (iii) with 420 nm excitation.

increase in QY is complemented by an increases in τAve from 12.8 nm to 19.5 ns (Figure 7c), and a FCS measured brightness increase from ∼2.1 to ∼8.3 kHz for CdSe/ZnS qdots (Table 1). Shell growth was confirmed by TEM. A set of representative TEM micrographs from CdSe/ZnS qdots are shown in Figure 8 after shell growth at TH ) 120 °C with CdSe cores synthesized at 120 (a) and 180 °C (c). Compared to the core (Figure 4), and core/shell diameters increased to 5.9 ( 1.0 nm (b), and 5.2 ( 0.5 (d) repectively. These results show that qdot monodispersity is largely retained even after a relatively thick ZnS shell is grown. The higher dispersity for the cores prepared at 120 °C may be due to the initially higher core polydispersity, slight variation in growth conditions and concentrations, or increased faceting, the latter of which is challenging to account for at the current resolution. Figure 8. Representative TEM micrographs and statistical analysis for CdSe/ZnS qdots with CdSe cores synthesized at TH ) 120 °C (a,b) revealing average diameter of 5.9 ( 1.0 nm (n ) 110), and at TH ) 180 °C revealing average diameter of 5.2 ( 0.5 nm (n ) 129) (c,d). ZnS shells prepared at TH ) 120 °C.

sizes and the thin Cit-encapsulating layer. This value is important, as a number of studies have shown the importance of small qdot Dh on applications in bioimaging and in vivo transport. It was recently shown that NIR emitting qdots with thin encapsulating shells of mercaptopropionic acid (MPA) can facilitate effective transport in mice models for tumor imaging.71 While the size tunability and photophysical properties of the hydrothermally prepared qdots are promising, further steps must be taken to improve QY to the 30-50% range to best find utility in a number of applications. This requires limiting exciton surface trapping sites and exciton-solvent interactions. This is best achieved by epitaxially encasing the CdSe core within a larger band gap semiconductor, such as CdS, ZnS, and combinations thereof.1-7 This step passivates the unsaturated surface dangling bonds, and sequesters excitons within the core.67 Our MWI-based hydrothermal method also facilitates this approach. Figure 7 shows the optical characteristics after growth of CdS (ii), ZnS (iii), and CdS/ZnS (iv) shells at CdSe cores (i). Briefly, CdSe cores were fabricated at TH > 120 °C at r ) 8, and then reprocessed in the presence of either sulfur precursors, or sulfur and zinc precursors at TH > 120 °C. Successful shell growth was confirmed by UV-vis (a) and PL emission (b), which showed characteristic red shifts, and notable increases in QY from ∼3% to 20-40%, depending upon conditions such as r, as well as sample annealing time. This

Conclusions These results show that qdot synthesis under aqueous processing is greatly facilitated by the presented MWI-based hydrothermal protocol. The increased throughput and processing temperature allows for size control, narrowing of size distributions, and improved quantum yields. The resulting core and core-shell qdots are bright and possess small hydrodynamic diameters. Work is still needed in order to optimize all conditions, and broaden the protocol for additional qdot classes, such as alloyed qdots, and Cd-free d-dots. Nevertheless, the size control, and high quantum yields show promise for the tailorable hydrothermal processing of qdots, the method of which may be adapted for other nanomaterials. An added novelty of these qdots is the accessible qdot interface, which facilitates ligand exchange and biofunctionalization, which may aid in biomimetic self-assembly and FRET based sensing and imaging, all part of our ongoing work. Acknowledgment. M.M.M. acknowledges Professor ChuanJian Zhong for training. We thank the Department of Chemistry, the Syracuse Biomaterials Institute, and Syracuse University for generous start-up support. This work is partially supported by a Department of Defense PECASE award, sponsored by AFOSR (FA9550-10-1-0033). G.D.F. acknowledges an NSF-REU fellowship at SU. TCSPC and FCS measurements were carried out with Mircea Cotlet as Users of the Center for Functional Nanomaterials, Brookhaven National Laboratory (US-DOE DEAC02-98CH10886). We acknowledge thoughtful TEM assistance and training from Yuanming Zhang at the Cornell Center for Materials Research (CCMR) and Robert Smith at SUNY-ESF.

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