Fitted PL decay lifetime components.a
Semiconductor nanocrystals (NCs), also referred to as semiconductor quantum dots (QDs), which are small compared to the bulk exciton radius have unique properties associated with the spatial confinement of the electronic excitations. These semiconductor QDs have discrete electronic states, in contrast to the bulk band structure, with an effective band gap blue shifted from that of the bulk. Due to their unique properties, QDs have been of great interest for fundamental research and industrial development in recent years [1-2]. The size-dependent optical properties of QDs have been actively studied during the pass decade. The synthesis and subsequent functionality of QDs for a variety of applications include photostable luminescent biological labels [3-8], light harvesters in photovoltaic devices [9-15] and as the emissive material in light-emitting devices (LEDs) [16-23]. They are also characterized by large surface to volume ratios. Unlike quantum wells and wires, experimental studies suggest that the surfaces of QDs may play a crucial role in their electronic and optical properties. However, these semiconductor QDs with large surface to volume ratios are metastable species in comparison to the corresponding bulk crystal and must be kinetically stabilized. The most common method to maintain their stability is by chemically attaching a monolayer of organic molecules to the atoms on the surfaces of QDs. These organic molecules are often called surfactants, capping groups, or ligands. In addition to the protection function, this monolayer of ligands on the surfaces of QDs provides the necessary chemical accessibility for the QDs by varying the terminal groups of the ligands pointing to the outside environment. For example, the QDs covered with hydrophobic ligands cannot be used directly in applications that require aqueous solubility or an effective charge transport property.
For both the protection and solubility function, the photoluminescence quantum yield (PL-QY) is another crucial factor for semiconductor QDs. The QDs have a large fraction of surface atoms because of their small volume; therefore, several inhomogeneous defect points occur on the large surface area. The processes that determine the luminescence QYs in semiconductor QDs have been investigated for several years. In this topic, the surface chemistry plays a crucial role in the manipulation of semiconductor QDs because it determines the dispersion interactions of the QDs in the medium, and high quantum yields and long term photostability can be achieved only through an improved understanding of surface recombination processes. Therefore, proper passivation of the surfaces of QDs is necessary to achieve a high PL-QY.
QDs of II-VI semiconductors have been extensively studied during the past two decades [24-30]. To date, the light-emitting core part of most QDs is cadmium selenide (CdSe), which can be prepared under mild conditions using well-known precursors. However, CdSe QDs have a spectral limitation at emission wavelengths shorter than 490 nm. To achieve high quantum efficiency in the blue region, cadmium sulfide (CdS) is a suitable candidate that can be prepared under mild conditions. Because the bulk CdS has an energy band gap of approximately 2.5 eV, it is easier to enable CdS QDs to emit blue light than the same sized CdSe. In connection with the improvement of the emission efficiency of CdSe and CdS QDs, several studies focused on the capping ligands introduced to the surfaces of CdS and CdSe QDs to study the variation of PL-QY [31-35]. In general, the usual method for surface modification of CdS QDs is to cap the synthesized CdS QDs with thiolate ligands during the growth period . Uchihara et al. investigated the pH dependent photostability of thioglycerol-capped CdS (TG-CdS) and mercaptoacetate-capped CdS (MA-CdS) in colloidal solutions under stationary irradiation . Because of the various charge properties of TG and MA, the carboxyl group of MA is neutralized by the addition of proton in the lower pH region, and the photostability decreases by lowering the pH of the solutions for MA-CdS; however, the photostability of TG-CdS are slightly influenced by the pH of the solution. Thangadurai et al. used 1.4-dithiothreitol (DTT), 2-mercaptoethanol (ME), cysteine (Cys), methionine (Meth), and glutathione (GSH) as ligands to cap the surfaces of CdS QDs to study the photo-initiated surface degradation . It is noteworthy that the band edge emission of DDT capped CdS shifted to a higher energy, and this shift was in conformity with the lowest grain size. In addition, the intensity of the broadband related to the surface defect states of CdS QDs exhibited a reduced trend compared to the other samples. The surface coating with suitable thiol molecules can yield a lower grain size in the cubic phase and obtain excellent fluorescence properties with efficient quenching of the surface traps.
In contrast to the photochemical stability of thiol capped-CdS QDs, the photochemical instability of CdSe QDs capped with thiol molecules was reported by Peng et al. . Based on their research, they proposed that the photochemical instability of CdSe QDs capped with thiol molecules included three distinguishable processes, as follows: (1) the photocatalytic oxidation of the thiol molecules on the surfaces of CdSe QDs; (2) the photooxidation of CdSe QDs; and (3) the precipitation of CdSe QDs. Thiols are the most widely used ligands for stabilizing semiconductors [38-39]. However, the stability of the thiol-stabilized CdSe QDs is not satisfactory because of the photooxidation of the QDs-ligand complex using CdSe QDs as the photocatalysts. It is difficult to reproducibly apply chemical and biochemical procedures to these QDs because of their unstable nature. In addition to the use of thiol molecules to cap the surfaces of CdSe QDs, most researchers used various amines as ligands to modify the optical properties of CdSe QDs [40-44]. Talapin et al. synthesized CdSe QDs in a three-component hexadecylamine (HDA)-trioctylphosphine oxide (TOPO)-trioctylphosphine (TOP) mixture . The room temperature PL-QY of as-synthesized CdSe QDs without adding HDA was in the range of 10-25%. However, the PL-QY of CdSe QDs can be improved substantially by surface passivation with HDA molecules. This indicates that PL efficiency losses are caused by insufficient passivation of the surface traps. Murray et al. examined the effect of the surface modification of CdSe QDs on the optical properties . They interpreted the substantial increase in PL intensity following the addition of HDA molecules as the elimination of nonradiative decay pathways. Similar results were reported by Bullen and Mulvaney with primary, secondary, and tertiary amine .
Two types of alkylamine (
2. The surface modification of CdS and CdSe QDs via organic ligands
The capping of CdS and CdSe QDs consisting of semiconductor cores surrounded by organic ligands has attracted considerable interest for applications in materials science and nanotechnology [46-48]. Although these studies enabled syntheses of stable capping semiconductor QDs with various sizes, shapes, and compositions, few studies have been conducted on the surface structures and properties of capping ligands. Information regarding the nature and chemical properties of the binding between QDs and their ligands is limited. However, compared to atoms on the flat surface of bulk substrates, the binding abilities of atoms on curved surfaces may be affected by their diverse structural environments and size-dependent electron configuration. In this study, the synthesis of CdS and CdSe QDs was conducted in a noncoordinating solvent using 1-ODE. The CdO (0.16 mmol) was mixed with 0.7 mmol OA and 4.8 g of 1-ODE in a 25 mL three-neck flask. The mixture was heated to 300 oC under Ar flow for 30 min, and subsequently injected with Se stock solution (0.1 mmol of S or Se powder dissolved in 0.62 mmol of TBP and 1 g of ODE). The solution mixture was cooled, and the nanocrystals were allowed to grow at 260 oC to reach the desired size, as determined by UV-visible absorption. To monitor the growth of QDs, a small amount of the sample (approximately 0.2 mL) was obtained through a syringe and diluted to exhibit an optical density between 0.1 and 0.2 by the addition of anhydrous toluene. The resulting CdS and CdSe QDs were suspended in toluene, and the unreacted starting materials and side products were removed by extraction and precipitation procedures. Size sorting was not performed in any of the samples. An aliquot of CdS or CdSe QDs solution was diluted with toluene to yield an optical density of approximately 0.1 at a wavelength of 350 nm (the excitation wavelength of PL). A 3 mL portion of the nanocrystals solution was mixed with various capping molecules at a fixed concentration of 5 mM for surface modification. The solution mixture was stirred in the dark at room temperature for 1 h. The CdS and CdSe QDs were subsequently precipitated with methanol and re-dispersed in toluene for characterization of the change of PL quantum yield by UV/vis absorption and photoluminescence spectroscopy.
Figure 1 shows the transmission electron microscopic (TEM) images of the as-grown CdSe and CdS QDs synthesized by the non-coordinate method. It shows that the material has a uniform size distribution and regular shape with 5 nm and formed close-packed arrays.
The absorption and PL spectra of the CdSe QDs varying with growth time are shown in Figure 2. The luminescence spectra from these CdSe QDs are symmetric and narrow. However, the PL quantum yields decreased in conjunction with the growth time to 5%, as shown in Figure 2(c). The absorption and PL spectra of CdS QDs recorded for samples grown at various times are shown in Figure 3. The PL-QY increased rapidly in conjunction with the growth time initially and reached a steady value of approximately 60% after 200 s. The increasing PL-QY with crystal growth for CdS QDs exhibited contrasting behavior to several other QDs, such as CdSe, which exhibited a decreasing PL QY with crystal growth time . A characteristic peak for CdS QDs was not observed in the UV and PL spectra in the initial 60 s after S precursor injection to the Cd2+ solution. The UV absorption spectrum and the narrow emission peak with full-width at half maximum (approximately 21 nm) were observed after 60 s reaction. This behavior can be attributed to the slow growth of CdS QDs in the initial period, and their size was too small in this time domain for identification by the spectroscopy of UV-visible absorption and PL emission. Therefore, because of the slow growth process in the formation of CdS QDs, high quality QDs with large quantities of radiative surface-states with low nonradiative surface quenching defects can be obtained with the increase in PL-QY in conjunction with the growth time.
Based on these observations, the control of the surface, probably a reconstructed surface of QDs, may be crucial for controlling and improving their PL properties. To understand the changes in the optical properties of the CdSe and CdS QDs upon modification by the capping ligands,
Generally, it is believed that the capping ligands effectively passivate the surface states and suppress the non-radiative recombination at surface vacancies, leading to enhanced PL quantum yield. However, among the three capping agents in the case of CdSe QDs, the OA ligand exhibited contrasting behavior. In addition, the three ligands were used to quench the emission properties of CdS QDs. Generally, the relaxation process of QDs is radiative recombination. Otherwise, competing radiation-less relaxation processes are used, including carrier trapping at QD defects, charge transfer between QDs and ligand-based orbitals, and inter-QDs energy transfer. The observations in this study clearly indicate that the passivation effect of the capping ligands for CdSe and CdS QDs are more complex, indicating the requirement for a careful examination of the photo-induced charge transfer between CdSe QDs and the capping ligands.
To resolve the various behaviors of CdS QDs, time-resolved photoluminescence (TRPL) was used to probe the decay kinetics of the exciton emission of bare QDs and ligand capping QDs by
3. The study of time resolved photoluminescence technique on surface modification of CdS and CdSe QDs.
For TRPL analysis, this study used a system with a single picosecond diode laser driver with a 375cnm laser head (integrated collimator and TE cooler for temperature stabilization was integrated by Protrustech Co., Ltd). An Andor iDus CCD with 1024 × 128 pixels was used to obtain the PL signal and the Pico Quant PMT Detector head with 200820 nm and <250 ps IRF was integrated to obtain the TRPL signal. A theoretical model with multi-exponential analysis is often used to determine the lifetime of QDs by PL decay data. Typically, the feature of the decay of the PL intensity for QDs is a universal occurrence of a biexponential time distribution in the radiative lifetime, as shown in the following equation,
where S and L represent the shorter lifetime and longer lifetime, respectively, and Ai (i = S, L) is the amplitude of the components at t = 0. The shorter lifetime is on a time scale of several nanoseconds, and the longer lifetime is on a time scale of tens of nanoseconds. The shorter lifetime is generally attributed to the intrinsic recombination of initially populated internal core states; however, the possible origin of the longer lifetime remains relatively uncertain [51-53]. Recently, Xiao et al. reported that the longer lifetime component in the PL decay is caused by the radiative recombination of electrons and holes on the surface involving surface-localized states . For QDs with high PL-QY, the amplitude AL with longer lifetime dominants the total PL. In other words, electrons and holes have an increased probability to be presented on the surfaces of QDs with high QY to contribute to this surface-related emission with a longer lifetime.
The representative PL decay curves recorded for the bare CdSe and CdS QDs and their surfaces treated by
|Sample ID||YS (%)||S (ns)||YL (%)||L (ns)||<"/> (ns)||2||QY (%)|
As shown in Table 1, for bare CdSe and CdS QDs, S equals to 2.93 ns and 2.27 ns, and L equals to 16.99 ns and 22.60 ns. The value of S for the short lifetime constant is consistent with the theoretical value of approximately 3 ns, which was calculated by considering the screening of the radiating field inside the QD . Moreover, for the bare CdS QD samples with 60% QY, the amplitude of AL with a longer lifetime accounts for nearly 57% of the total PL. Similar results were observed in the capping ligands for modified CdSe QDs. This larger share of the longer lifetime component is a clear indication of major surface-related emission caused by the radiative recombination of charge carriers involving surface states. Although it is difficult to identify the real origin for such radiative recombination through surface state centers in this stage, it may result from the net residual charges on atoms of CdSe and CdS QDs caused by the bonding of Cd-Se and Cd-S and the nearest neighboring atoms in the crystal lattice.
4. The investigation of density function theory (DFT) on capping CdS and CdSe QDs.
Here, we proposed to use the density function theory (DFT) calculations result to provide atomistic information on the adsorption energy and charge transfer of small QDs capped by different types of ligands. Theoretical modeling could provide valuable insight in atomic scale. Unfortunately, efforts in this field have been rather limited due to the high computational cost and uncertainty related to the chemical composition and morphology of the nanocrystals. Early theoretical studies of QDs simulated the surface by assuming an infinite potential barrier around the QDs. . More sophisticated models of capping QDs have represented the QD core through the bulk atoms using semiempirical tight-binding  and pseudopotential  approaches, while the passivating molecules have been modeled through either single oxygen atoms or simplified model potentials . Any realistic model, however, has to explicitly describe bonding between the QD and the ligands, which is lacking from the approaches mentioned above. First-principle quantum-chemical methods, such as density functional theory (DFT), are able to provide this information with a reasonable level of accuracy. Unfortunately, DFT is numerically expensive. Therefore, most DFT calculations simulate the core atoms of QDs on the basis of bulk structures, while dangling bonds on the surface are artificially terminated wit covalently bonded hydrogen atoms [60-61]. Only a few reports have focused the specifics of the adsorption energies and charge transfer of small cluster of QD interacting with ligand molecule. [46, 62]
The experimentally studied QDs retained the original crystal structure in the core. Figure 7 shows the construction of CdSe from a zinc blended lattice with bulk Cd-Se bond lengths. The analogous construction of CdSe clusters from the bulk semiconductor has been used in theoretical studies. [46, 63-65]. In addition to the uncapped (“bare”) CdSe and CdS, we simulated clusters with ligands attached to the surface of the QDs. The selected capping groups, that is,
In the previous studies have shown computationally that the dominant binding interaction occur between N atoms of the ligand and Cd atoms on the QD surface. We start our modeling with constructing three systems, Cd4Se4, Cd4Se4HA, Cd4Se4BA, Cd4Se4OA, Cd3S5, Cd3S5HA, Cd3S5BA, Cd3S5OA, and CdSe(111) slab (see Figure 8 (b) and (c)). The ligands were attached to the most chemically active surface atoms (all 2-coordinated Cd atoms on the cluster surface) as described the previous studies [49, 66]. Recent experiments have shown that such small “magic”-size QDs with diameter than ~2nm demonstrate great stability, the controllable size and shape, and reproducible optical properties, including an efficient blue-light emission . The Cd33Se33 “magic” structure with diameter of 1.3 nm has been experimentally shown to be very stable , while it is the smallest cluster that supports a crystalline-like core-shell.
Two systems were applied in this work. To save computation resources, the Cd4M4 (M = Se and S) cluster is a reliable model for quantum-chemical studies of physical chemistry properties of CdSe and CdS QDs. The system requires reasonable computational efforts for atomistic modeling based on DFT, even when it is passivated with multiple ligands. A cluster model was used to simulate the interaction between the ligand and QD. This cluster was cut from the CdSe supercell optimized by DFT. Figure 9 shows the cluster model, in which the formula of the cluster is Cd4Se4 and the nitrogen atoms are bonded on the Cd atoms.
As the CdSe nanostructures were modified by the capping ligands, the capping effect of
Table 2 lists the
Puzder et al. obtained ca. 0.91 eV from DFT calculations for trimethylamine on a (CdSe)15 NC . Similar values (0.89-1.02 eV) have been reported in the DFT study of amines at a CdSe surface . Our results for organic amines are in good agreement with these values. It must be noted that the more negative the
The Bader charge analyses were carried out for CdSe,
The charges for Cd in CdSe–BA, CdSe–HA, and CdSe–OA exhibited a decrease from 11.404e for a bare CdSe, indicating a net electron transfer from Cd atom to Se for the three capping molecules. Cd has lower electronegativity (1.7) than that of nitrogen (3.0); therefore, greater electron donation occurs to Se from the N atom of the capping molecules than that from Cd. Although the amine molecules adsorb strongly with a facile electron donation from their “–NH2” functional group to Se of CdSe nanocrystals (with higher
|BA||N: 7.787||BA||N: 7.787|
|HA||N: 7.801||HA||N: 7.801|
|OA||O: 7.921||OA||O: 7.921|
|CdSe-BA||Se: 6.696||Se: 6.614-6.696 = -0.082||CdS-BA||S: 6.763||S: 6.723-6.763 = -0.040|
|Cd: 11.388||Cd: 11.404-11.388 = + 0.016||Cd: 11.194||Cd: 11.227-11.194 = + 0.033|
|N: 7.561||N: 7.787-7.561 = +0.226||N: 7.764||N: 7.787-7.764 = +0.023|
|CdSe-HA||Se: 6.700||Se: 6.614-6.700 = -0.086||CdS-HA||S: 6.775||Se: 6.723-6.775 = -0.052|
|Cd: 11.375||Cd: 11.404-11.375 = +0.029||Cd: 11.207||Cd: 11.227-11.207 = +0.020|
|N: 7.561||N: 7.801-7.561 = +0.24||N: 7.791||N: 7.801-7.791 = +0.010|
|CdSe-OA||Se: 6.637||Se:6.614- 6.637 = -0.023||CdS-OA||S: 6.727||S: 6.723- 6.727 = -0.004|
|Cd: 11.267||Cd: 11.404- 11.267 = +0.137||Cd: 11.184||Cd: 11.227- 11.184 = +0.043|
|O: 7.906||O: 7.921-7.906 = +0.015||O: 7.900||O: 7.921-7.900 = +0.021|
Surface modification of CdS nanostructures by the organics was treated in regard to the adsorption of
Bader charge analyses were performed for CdS,
The proposed mechanism can be extended to explain the behavior of the other amine,
Based on the TRPL analysis and DFT computation, this study demonstrated that the amine can be used as the capping ligands for CdSe QDs to enhance PL-QYs, whereas the amine used as the capping ligands for CdS QDs can be used to PL-QYs. The PL-QYs of CdSe and CdS QDs decreased considerably when using oleic acid as capping ligands. We propose that the interactions between capping ligands and CdSe or CdS QDs may be attributed to NH2 group for amine molecules and –COOH group for fatty acid molecules. An improved understanding of this interaction will facilitate the design of superior conjugates for CdSe and CdS QDs for various applications.Acknowledgement
We thank the NCHC HPC and NTUST for providing massive computing time. Financially support from the National Science Council under Contract No. NSC 99-2811-M-011-005 is gratefully acknowledged.