20244
J. Phys. Chem. B 2005, 109, 20244-20250
Electrostatic and Covalent Interactions in CdTe Nanocrystalline Assemblies Ruth Osovsky,*,† Alexey Shavel,‡ Nikolai Gaponik,‡ Lilac Amirav,† Alexander Eychmu1 ller,‡ Horst Weller,‡ and Efrat Lifshitz*,† Department of Chemistry and Solid State Institute and the Russell Berrie Nanotechnology Institute, Technion, Haifa 32000, Israel, and Institute of Physical Chemistry, UniVersity of Hamburg, 20146 Hamburg, Germany ReceiVed: May 22, 2005; In Final Form: July 28, 2005
This paper focuses on the interactions between cysteamine-stabilized CdTe nanocrystals [CdTe(CA) NCs] and thioglycolic-acid-stabilized CdTe nanocrystals [CdTe(TGA) NCs]. These interactions were examined by the absorption, continuous, and time-resolved photoluminescence (PL) spectra of the electrostatically mixed and the covalently linked NCs assemblies comprised of the oppositely surface charged CdTe(CA) and CdTe(TGA) NCs and by a comparison with those of the corresponding pristine NCs. The CdTe(CA)-CdTe(TGA) coupling is dictated by the surfactant spacer, ranging between 0.93 and 1.14 nm and by electrostatic and covalent interactions, enabling a Fo¨rster resonance energy transfer (FRET) process among the NCs. The results revealed an excellent spectral overlap between the emission of the CdTe(TGA) NCs and the absorption of the CdTe(CA) NCs as well as a PL spectral red shift on the formation of electrostatic and covalent interactions. Furthermore, the measurements showed a lifetime ranging between 1.2 and 3 ns for the electrostatically mixed and the covalently linked assemblies, shorter than those of the pristine CdTe(CA) NCs and CdTe(TGA) NCs, both of which measured as ∼5.5 ns. When CdTe(TGA) NCs performed as the most efficient donors, FRET rates of 1010-1011 s-1 were calculated for the electrostatically mixed NCs or covalently linked NCs.
I. Introduction Semiconductor nanocrystals (NCs) (colloidal quantum dots) are promising materials for light emitting diodes,1 low threshold lasers,2 photovoltaic cells,3 electronic circuitry,4 and biological markers.5 Individual NCs exhibit unique physical properties associated with size confinement and surface coating.6-13 The present synthetic procedures produce dispersed NCs of uniform shape and size (1 nm results in efficient Fo¨rster resonance energy transfer (FRET) from donor to acceptor NCs. This energy transfer occurs as a result of the electrostatic interaction of the emission dipole moment of an exciton generated in one NC with the absorption dipole moment of another NC. This process leads to the * To whom correspondence should be addressed. E-mail: rutho@ tx.technion.ac.il;
[email protected]. Tel: +972 4 8293987. Fax: +972 4 8235107. † Department of Chemistry and Solid State Institute and the Russell Berrie Nanotechnology Institute. ‡ University of Hamburg.
migration of excitons from the smaller NCs to the larger NCs in an assembly. The dipole-dipole interaction relies on the interNCs distance and arrangement; inter-NCs distance and arrangement are controlled by the surfactants’ molecular length and nature, producing either van der Waals attraction, electrostatic attraction, or covalent bonds among the NCs. Covalently coupled NCs form irreversible cross-linkages without long-range ordering,20 however, with stable bonding of NCs of identical or different sizes, either in solution or in a solid phase. Various examples of NCs assemblies exhibiting different inter-NCs interactions were investigated during the past couple of years. Recent studies of epitaxial quantum dots showed Coulombic blockade and electron pairing,21 interesting for quantum logic and computation.22 Bawendi et al.23 showed the formation of closely packed three-dimensional assemblies of CdSe NCs named “artificial solids”. This work revealed the existence of FRET among the NCs with an inter-NCs distance of 1-10 nm. These authors24 also demonstrated a photoconductivity property of those closely packed solids characterized by charge resonant tunneling between adjacent CdSe NCs. Vossmeyer et al., Do¨llefeld et al.,25 and Fenske et al.26 showed the formation of CdS clusters with covalent inter-NCs linking. Nabiev et al.27 reported an energy transfer process between oppositely charged CdSe/ZnS core-shell NCs, attracted by electrostatic forces in an aqueous solution. Woggon et al.18 examined the collective effects in closely packed, ultrasmall CdSe NCs, where the dense ensemble developed electron states extending into minibands, analogous to Anderson transitions in disordered solids. Rogach et al.28 reported an efficient FRET in bilayers of water-soluble CdTe NCs capped with thiol surfactants, forming a layer-bylayer assembly. Semiconductor NCs were also packed into ordered or disordered assemblies, using organic templates. Lahav et al.29
10.1021/jp0526795 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/05/2005
Interactions in CdTe Nanocrystalline Assemblies prepared assemblies of CdS or PbS NCs templated in an ordered phenylene-acetylene-dicarboxylates matrix, exhibiting an energy transfer via an exciton diffusion mechanism from the conjugated organic matrix into the NCs moieties.30 Alphandery et al.31 exhibited a FRET mechanism between CdTe NCs and Rhodamine B in mixed solid films. Anni et al.32 proposed a FRET process in an organic/NCs assembly from the blue emitting polymer into colloidal CdSe/ZnS NCs. Klimov et al.33 investigated the two-dimensional arrangement of CdSe NCs in Langmuir-Blodgett (LB) assemblies. This arrangement enabled an efficient FRET within a monolayer at a time scale of 50 ps to 10 ns, dominated by the interaction of a donor NC with acceptor NCs from the first three surrounding shells. However, assembling of bilayers with different NCs diameters showed an effective vertical energy transfer with a time constant of 120 ps. Semiconductor NCs were used recently34 in new microscopic techniques, combining near-field scanning optical microscopy (NSOM) and a FRET process. The method relies on attaching an acceptor or donor NC at the end of a NSOM fiber tip and scanning molecular dyes on a substrate. This method offers a dramatic reduction in the sample’s volume probed by a NSOM tip. While previous studies discussed various inter-NCs interactions, there is a lack of comparison between electrostatic coupling and covalent coupling, using the same constituents. Thus, this paper focuses on the interactions between thioamine (cysteamine [CA])-stabilized CdTe NCs, containing NH2 terminating groups, and thio-acid (thioglycolic acid [TGA])stabilized CdTe NCs, containing COOH terminating groups,35 both named hereon “pristine” NCs. The physical mixture of the thioamine and thio-acid-capped NCs, called a “mixed” assembly, is controlled by the electrostatic interactions among the constituents.36 NCs that attract each other via the formation of a peptide bond directly via the surfactants’ functional groups, without using an additional bridging molecule, are called “linked” assemblies. Through the use of absorption, continuous, and time-resolved photoluminescence (PL) spectroscopy, the inter-NC interactions were examined by investigating the changes in the optical properties of the assemblies with respect to those of the pristine NCs. It is anticipated that the knowledge gained in this study will be utilized in certain applications where assembly of NCs of different sizes or different semiconductors takes place and the electrostatic or covalent bonding is preferred over the dispersion method.37 Furthermore, covalent linking could induce chemically selective bonding and additional interNCs coupling. II. Experimentation II.1. Preparation of NCs. Thiol-capped CdTe NCs syntheses were carried out in aqueous media, following the procedure given in ref 35. Cd(ClO4)2‚6H2O was dissolved in water, and the thiol stabilizers were added while stirring. Then the pH was adjusted to the appropriate values (depending on the nature of the stabilizer) by the dropwise addition of NaOH solution. The solution was then placed in a three-necked flask fitted with a septum and valves and was deaerated by bubbling N2. H2Te gas (generated by the reaction of Al2Te3 lumps with a H2SO4 solution under N2 atmosphere) was passed through the solution while stirring together with a slow nitrogen flow. CdTe precursors were formed at this stage, accompanied by a change of the solution color to orange (using TGA) or dark red (using CA). The precursors were converted to CdTe NCs by refluxing the reaction mixture at 100 °C under open-air conditions with a condenser attached. The size of the CdTe NCs was controlled
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20245 by the duration of the reflux. A post-preparative size-selective precipitation procedure was successfully applied for the isolation of the TGA-capped CdTe NCs [CdTe(TGA)] from their crude solutions and for the reduction of the size distribution. The CAstabilized CdTe NCs [CdTe(CA)] were used directly from the crude solution without size-selective precipitation. Instead, the crude solution was dialyzed against water to remove excess thiols and by-products of the reaction. In addition, a calibration curve given by Peng et al. was used to compare the exciton energy in the measured absorption spectra vs size.41 The covalently linked CdTe NCs were generated by a direct reaction between the CdTe(TGA) and CdTe(CA) NCs surfactants’ functional groups, without using an additional bridging molecule, when the addition of carbodiimide molecules acts as a mediator only during the reaction. This standard peptide synthesis with water soluble carbodiimides offered the possibility of mild reaction conditions without changing the solvent used for the preparation of pristine NCs. The formation of the peptide bond has been proven by IR absorption spectroscopy, as was shown in Figure 2 of ref 36. The formation of an amide bond between the CA-stabilized NCs and TGA-stabilized NCs is seen by the occurrence of the band at 3250 cm-1 (ν NH (CONH)) in the spectra of the linked NCs. By the use of the same solution medium, the electrostatically mixed assembly was produced by mixing the CdTe(TGA) and CdTe(CA) NCs solutions without the addition of carbodiimide. In this case, the band corresponding to the formation of the peptide bonds was not observed in the IR spectra (Figure 2 in ref 36). In both the mixed and linked NCs assemblies, the molar ratio between the different pristine NCs was approximately 1:1. The inter-NCs distance was measured by small-angle X-ray scattering (SAXS) showing diffraction bands that are associated with the NCs center-to-center distances. For example, the centerto-center distance of 4.4 ( 0.2 nm was found between two identical CdTe(TGA) NCs, each with an average diameter of 3.4 ( 0.2 nm, as described in ref 38. This reveals a surfaceto-surface distance of 1.0 ( 0.2 nm between NCs of the same kind. It should be noted that the various samples (pristine, mixed, and linked NCs) were examined either in their powder form or when dispersed in a polymethyl-metacrylate (PMMA) polymer, a transparent and solid medium suitable for optical measurements at cryogenic temperatures. II.2. Instrumentation. The absorption spectra were recorded using a Shimadzu UV-vis spectrometer and a UV-vis-NIR spectrometer JASCO V-570. The PL spectra were recorded either at room temperature or at liquid helium temperature by immersing the samples either in a continuous-flow or in a capillary-type Janis cryogenic Dewar. The continuous-wave PL spectra were obtained by exciting the samples with a 2.7-3.7 eV Ar+ laser. The emitted light was selected by a holographic grating monochromator (Jobin Yvon Model THR300 or THR1000) and detected with either a Princeton Instrument intensified CCD or a Hamamatsu R666 photomultiplier tube. Time-resolved PL measurements were performed by exciting the samples with a 4 ns Nd:YAG pulsed laser and by detecting the delayed fluorescence with the intensified CCD. The roomtemperature PL quantum yield (QY) of CdTe NCs was estimated according to the procedure given in ref 39 by comparison with Rhodamine 6G in ethanol solution, which exhibits a PL QY of 95%. This procedure revealed that the PL QY of the pristine CdTe(TGA) NCs is 15%; that of the pristine CdTe(CA) NCs is 3%.
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Figure 1. (A) Normalized room-temperature absorbance (right curves) and PL spectra (left curves) of dispersed pristine CdTe(TGA) (upper spectra) and CdTe(CA) (bottom spectra) NCs in diluted solutions and (B) normalized room-temperature PL spectra of CdTe(TGA) (upper curves) and of CdTe(CA) (bottom curves) of dispersed NCs (solid lines), NCs powders (dashed lines), and a concentrated blend of NCs in a polymer medium (dotted lines).
TABLE 1: NCs Surface-to-Surface Distance, the Fo1 rster Critical Radius (R0), and the Rate of the FRET Process donor NC f acceptorNC
surface-to-surface distance (nm)
R0 (nm)
KD-A (109 s-1)
CdTe(TGA) f CdTe(TGA) CdTe(CA) f CdTe(CA)
1.12 1.14
9.7 7.5
26 4.9 KD-A (109 s-1)
surface-to-surface distance (nm) donor NC f acceptor NC
electrostatically mixed NCs
covalently linked NCs
R0 (nm)
electrostatically mixed NCs
covalently linked NCs
CdTe(TGA) f CdTe(CA) CdTe(CA) f CdTe(TGA)
1.13 1.13
0.93 0.93
10.9 5.5
73 1.2
240 4.0
III. Results and Discussion Figure 1A shows the room-temperature absorption and PL spectra of pristine CdTe(TGA) (with a diameter of 3.2 nm) and CdTe(CA) (with a diameter of 3.4 nm) NCs dispersed in diluted solutions. The PL spectra are dominated by an exciton emission, with full width at half maximum of about 170 meV, and a significant global Stokes shift (140-180 meV) with respect to the 1S exciton absorption. This Stokes shift produces a partial spectral overlap between the exciton absorption and emission bands of NCs of the same kind. However, Figure 1A exhibits a substantially larger spectral overlap between the emission band of CdTe(TGA) NCs and the 1S exciton absorption of CdTe(CA) NCs (solid lines in Figure 1A) and a small overlap between the 1S exciton absorption band of CdTe(TGA) and the emission of CdTe(CA) NCs (dashed lines in Figure 1A). Figure 1B represents the room-temperature PL spectra of concentrated polymer blends of CdTe(TGA) and CdTe(CA) NCs (dashed lines) and that of the corresponding NCs powders (dotted lines). These spectra exhibit an exciton emission, red shifted by 80 meV in CdTe(TGA) and 20 meV in CdTe(CA), with respect to the corresponding PL bands in diluted solutions (solid lines in Figure 1B). A FRET process from small CdTe NCs to larger ones could lead to an exciton red shift. The PL intensity of the “red” side of the spectrum becomes enhanced with respect to the “blue” side, as a result of the enhancement of the absorption cross section of the large NCs.33 The exciton emission energies of the powder and polymer blend of the CdTe(TGA) and CdTe(CA) pristine NCs are nearly identical, supporting the use of the polymer medium in the measurements presented below and subsequently avoiding the scattered light that often disturbs measurements of powder samples. The Fo¨rster radius R0 (also known as the critical radius) is an important parameter involved in FRET analysis, defined as the distance at which energy transfer is 50% efficient, and is directly related to the spectral overlap integral between the
acceptor absorption and donor emission spectra, shown in Figure 1A. The R0 expression is given in eq 140
R06 )
9000(ln 10)k2φD 5 4
128π n N
∫0∞ fD(λ)FA(λ)λ4 dλ
(1)
φD corresponds to the luminescence quantum yield of the donors (given in the Experimentation section as ∼15% for the CdTe(TGA) and ∼3% for the CdTe(CA)), n ) 2.87 is a volumeweighted average of the refractive index of the NCs and surrounding surfactants and polymer,23 fD(λ) is the normalized spectrum of the donor emission, and FA is the acceptor’s NCs absorption spectrum, expressed by its extinction coefficient and calculated according to the expression given by Peng et al.41 N is Avogadro’s number, and k is the orientation factor associated with the dipole-dipole interaction between donor and acceptor, given as k2 ) 2/3 for random orientations. Thus, the R0 values concerning the interactions between identically or differently capped NCs were evaluated and have been summarized in Table 1. These R0 values range between 5.5 and 10.9 nm, exceeding the surface-to-surface distance between NCs and predicting FRET processes between neighboring NCs with an increase in probability as R0 increases. Figure 2 shows the PL spectra of the pristine NCs assemblies and those of the mixed and linked NCs assemblies recorded at 4.2 K; all are embedded in polymer films. Figure 2 shows that the PL band of the mixed assembly is red shifted by 19 meV and that of the linked assembly is red shifted by 36 meV from the PL band of the pristine CdTe(CA) NCs assemblies. The absorption and emission spectral overlap (Figure 1A) and the R0 values suggest that the red shift is mainly controlled by the FRET process from the CdTe(TGA) NCs donors into the CdTe(CA) NCs acceptors. The PL band of the mixed assembly shows reduction of the intensity of the CdTe(TGA) and enhancement at the “red” side of the CdTe(CA) component. This behavior
Interactions in CdTe Nanocrystalline Assemblies
Figure 2. Normalized PL spectra of pristine CdTe(TGA) assembly (black), pristine CdTe(CA) assembly (blue), electrostatically mixed assembly (red), and covalently linked assembly (light blue) all embedded in a polymer film and recorded at 4.2 K.
may suggest a preferential energy transfer from the CdTe(TGA) into the largest CdTe(CA) NCs. The larger red shift observed in the linked assembly is associated with the stronger dipoledipole coupling due to a shrinking of the inter-NCs interaction upon the formation of the peptide bond as calculated in the following paragraph and due to the additional interaction via the chemical bonds. The distance between two adjacent NCs surfaces, discussed in the Experimentation section, is restricted by the surfactant spacers, as measured by SAXS. Considering a relatively condensed packing of the organic ligands at the NCs’ surfaces and a molecular length of 0.56 nm for TGA or 0.57 nm for CA, the closest surface-to-surface distance between identical or mixed NCs produces an inter-NCs distance of 1.12-1.14 nm, although a peptide bond between CdTe(TGA) and CdTe(CA) shrinks the indicated distance to 0.93 nm. The relevant inter-NCs distances are summarized in Table 1. Identical pristine NCs, with the same surfactant, may attract each other by hydrogen bonds, while NCs with different surfactants (e.g., amine and carboxyl functional groups) interact electrostatically.
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20247 In the case of a nearly complete surfactant cover at the NCs surfaces, the estimated surface-to-surface spacing is the lower limit for the inter-NCs distance. These distances will be the basis for dipole-dipole interactions. The luminescence decay curves of pristine CdTe(TGA) and CdTe(CA) NCs assemblies, embedded in a polymer, were measured at 4.2 K. Figure 3A displays representative decay curves, deconvoluted with respect to the laser and system response, of the CdTe(TGA) NCs monitoring various energies within the exciton band. The corresponding time-resolved PL spectra, recorded at various delay times after the laser pulse, are shown in Figure 3B. These time-resolved spectra exhibit a red shift of the band with increasing delay time (see the guiding line in figure). Hence, the time-resolved PL spectra and the decay curves suggest a faster decay process at the “blue” side of the band. This is related to an intrinstic size quantization process42,43 and due to a FRET from the small to large NCs in the assembly.44 We fail to fit the decay curves to simple exponential or biexponential functions. Instead, the decay curves were best fitted to a modified Kohlrausch-Williams-Watts (KWW) function, as given in eq 245,46
I(t) ) I0 exp(-t/τ0) + I1 exp(-t/τ1)β
(2)
This equation combines a single-exponential function with decay time τ0 and a stretched exponential component with decay time τ1. The parameter β (0 < β < 1) is inversely related to the distribution of decay times present in the stretched exponential term (as β becomes smaller, the distribution of decay times becomes broader). The best fit parameters concealed a dominant contribution of the stretch exponent component with the decay times of the CdTe(TGA) NCs, between 4 and 8 ns, and an a β value of ∼0.7, given in Table 2, and a smaller contribution of an additional single exponent with τ0 between 80 and 91 ns. The slow component may be associated with a minor contribution from trapped carriers recombination. Similar treatment was given to the PL decay curves of CdTe(CA) NCs either in the polymer medium or in the powder form (not shown), and the fitted parameters are summarized in Table 2. The PL decay curves and the corresponding time-resolved spectra of both the electrostatically mixed and covalently mixed assemblies were recorded at 4.2 K, and the decay curves were deconvoluted with respect to the system response. Representa-
Figure 3. (A) Grassy lines represent the PL decay curves of the pristine CdTe(TGA) NCs assembly, embedded in polymer and recorded at various energies within the exciton band (as marked by the color-coded arrows in the inset) and at 4.2 K. The smooth lines are associated with the simulated curves. (B) Time-resolved PL spectra of the pristine CdTe(TGA) assembly recorded at various delay times after a laser pulse as indicated in the figure (the solid line guides the eye to distinguish the red shift developed with time).
20248 J. Phys. Chem. B, Vol. 109, No. 43, 2005
Osovsky et al.
Figure 4. (A) Grassy lines represent the PL decay curves of electrostatically mixed NCs assembly, embedded in polymer and recorded at various energies within the exciton band (as marked by the color-coded arrows in the inset) and at 4.2 K, while the solid lines are the corresponding simulated curves. (B) Time-resolved PL spectra of the mixed NCs assembly recorded at various delay times after a laser pulse as indicated in the figure (the solid line guides the eye to distinguish the red shift developed with time).
TABLE 2: Best Fitted Parameters (used in eq 2) for the PL Decay Curves of the Pristine, Mixed, and Linked Assemblies sample CdTe(TGA)
CdTe(CA)
electrostatically linked
covalently linked
detection energy (eV)
I0
τ0 (ns)
I1
τ1 (ns)
β
2.2 2.18 2.16 2.14 2.12 2.105 2.087 2.07 2.053 2.036 2.10 2.05 2.04 2.02 2.07
0.09 0.10 0.14 0.14 0.17 0.17 0.16 0.17 0.21 0.29 0.25 0.28 0.33 0.38 0.12
82.5 72.9 64.9 79.4 91.4 51.6 51.6 57.0 48.1 52.3 27.9 23.9 25.2 25.6 24.9
1.18 1.13 1.05 1.00 1.02 1.25 1.13 1.15 1.17 1.06 2.01 1.55 1.43 1.31 1.47
4.71 5.05 6.56 7.99 8.49 4.15 4.55 5.08 5.18 5.19 1.33 1.88 2.49 3.03 1.235
0.65 0.63 0.71 0.76 0.75 0.79 0.71 0.74 0.80 0.65 0.59 0.68 0.77 0.99 0.38
tive best fit decay curves of the electrostatically mixed NCs are shown in Figure 4A, and the time-resolved spectra of the electrostatically mixed NCs are shown in Figure 4B. Table 2 represents the relevant best fit parameters when using eq 2. The dominating decay time of the mixed assemblies is between 1.3 and 3.0 ns, and that of the covalently linked NCs is ∼1.2 ns; both are shorter than the corresponding decay times (∼5.5 ns) of the pristine assemblies. Figure 5 represents a plot of the red shift (∆E ) Et)0 - Et)x) in the time-resolved PL spectra vs the time delay (t ) x) after the laser pulse for the pristine CdTe(TGA), the pristine CdTe(CA), the electrostatically mixed, and the covalently linked assemblies. These plots indicate a faster progression of the red shift with time for the mixed and linked assemblies when compared with that of the pristine samples. In Figure 6, the best fit curves of the deconvoluted PL decay processes of the electrostatically mixed assemblies and covalently linked assemblies, monitored at the PL apex, are compared with those of the pristine assemblies. Once again, a shorter decay time in the mixed and linked assemblies is indicated when compared with the corresponding pristine constituents. Assuming that the pristine, mixed, and linked NCs assemblies do not show long-range ordering, we compare the inter-NCs interactions with only the next neighbor. Thus, the rate of energy transfer for an isolated single donor-acceptor NCs pair (KD-A), separated by a distance r, can be expressed by the Fo¨rster
Figure 5. Energy red shift observed in the time-resolved spectra (∆E ) Et)0 - Et)x) vs the delay time (t ) x) after the laser pulse of the pristine, electrostatically mixed, and covalently linked assemblies (the continous lines are drawn to guide the eye).
formalism given in eq 340
KD-A )
()
1 R0 (τD) r
6
(3)
where τD is the lifetime of the excited donor NC, and r is the center-to-center distance between the donor and acceptor NCs. Considering the NCs’ radius, the surface-to-surface distances, the R0 values (see Table 1), and the decay times (see Table 2), the rates of the FRET processes between identical NCs and between mixed and linked NCs could be calculated, as listed in the right column of Table 1. The FRET rate among identical NCs of ∼109 s-1 cannot be ignored, and such an interaction could have enhanced the FRET rate of the mixed and linked assemblies. Therefore, instead of considering τD values of dispersed donors in solution, we substituted the fastest lifetime of an assembly (τ1, monitored at the “blue” side of the PL band). Then, the observed rates of the FRET processes from the CdTe(TGA) donor NCs into the CdTe(CA) acceptor were estimated around 1010-1011 s-1, 1 to 2 orders of magnitude faster than the other processes. A careful look at Table 1 reveals that the
Interactions in CdTe Nanocrystalline Assemblies
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20249 Acknowledgment. The authors express their deep gratitude to Prof. Shammai Speiser and to Dr. Aldona Sashchiuk for the stimulating discussion regarding the interpretation of the results and to Dr. Dimitri Talapin for carrying out the SAXS measurements. The authors express their appreciation to Mrs. Angelica Berrie, the Russell Berrie Foundation, and the Israeli Ministry of Industry, Trade, and Labor for the common efforts on behalf of and generous contributions to the establishment of the Russell Berrie Nanotechnology Institute at the Technion. The authors also express their appreciation to Gabriel and Matilda Barnett for the donation of the Semiconductor Nanocrystals Research Laboratory. This project was supported by the German-Israel Foundation (GIF) Project #156/03-12.6 and by the GermanIsrael Project Cooperation (DIP) Project #D 3.2. References and Notes
Figure 6. PL intensity decay curves of the pristine and the corresponding electrostatically mixed and covalently linked NCs assemblies, as indicated in the inset, each measured at the apex of the relevant PL bands.
rates deduced for the linked NCs are three times faster than those for the corresponding mixed NCs. A FRET produced between CdTe(CA) NCs donors and the CdTe(TGA) NCs acceptors exhibits the slowest energy transfer rate (∼109 s-1), although the covalently linked assembly is relatively faster than the corresponding electrostatically mixed alternative. It should be noted that the observed transfer rates of the mixed and linked assemblies exceed the values reported recently in the literature. Klimov et al.33 showed an energy transfer of about 2 × 1010 s-1 for the interactions among one, two, or three next neighbor CdSe/ZnS NCs shells. Rogach et al.28b reported a rate of 2 × 1010 s-1 for the FRET process between a monolayer of 4 nm CdTe(CA) NCs and unidirectionally aligned monolayer of 2.5 nm CdTe(TGA) NCs. It should be noted that the indicated coupling was between NCs with substantially different NCs sizes, reducing the spectral overlap, with CdTe(CA) acting as the donor. This is in contrast to our case when CdTe(TGA), having a better QY than CdTe(CA) NCs, acted as the most efficient donor (see Table 1) and the interacting NCs had a similar size (3.2 and 3.4 nm), and thus, a slightly faster FRET rate (7.3 × 1010 s-1) is expected. In addition, the analogous covalently linked 3.2 and 3.4 nm NCs showed even faster FRETs due to the shorter inter-NCs distance and presumably also due to the extra coupling via the chemical bonds, althought this argument should be the subject of further investigations in the future. In summary, we compared the inter-NCs interactions among pristine assemblies, electrostatically mixed assemblies, and covalently linked NCs assemblies of CdTe NCs stabilized with thioamine and thio acid (CA and TGA) surfactants. The spectral overlaps between the absorption and emission curves, the PL red shift, the PL decay curves, and the calculated FRET rates revealed that covalently linked NCs have the strongest interNCs interaction, while CdTe(TGA) NCs act as the most efficient donors. The knowledge gained in this study is essential for certain applications where assembling NCs of different sizes or different semiconductors takes place and where electrostatic or covalent bonding is preferred over the dispersion method.37 Furthermore, covalent linking induces chemically selective bonding and additional inter-NCs coupling.
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