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 [36]. 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 [37]. 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 [33]. 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. [34]. 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 [35]. 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 [45]. 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 [41].

Two types of alkylamine (n-butylamine (n-BA) and n-hexylamine (n-HA)) and oleic acid (OA) were used to modify the surfaces of the CdS and CdSe nanocrystals. To understand the changes of optical properties of un-modified QDs and surface modified QDs, PL spectra and PL-QY were used to characterize the emission peak position and emission efficiency after surface modification of these ligands for CdSe and CdS QDs. The PL decay kinetics for these ligand capped-QDs systems were followed by time-resolved photoluminescence (TRPL), and the spectra were analyzed in regard to a biexponential model to identify two lifetime values, that is, shorter-lifetime (S) and longer-lifetime (L). The detailed mechanism was studied by density function theory (DFT) simulation to demonstrate the binding energy and charge analyses of CdS or CdSe QDs with n-BA, n-HA, and OA.

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.

Transmission electron microscopic images of samples of (a) CdSe QDs; (b) CdS QDs.

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 [49]. 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.

Temporal evolution of (a) UV-vis; (b) PL spectra and (c) PL quantum yield of a growth reaction of CdSe QDs.

Temporal evolution of (a) UV-visible spectra; (b) PL spectra and (c) PL quantum yield of a growth reaction of CdS QDs.

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, n-BA, n-HA,and OA were added to a solution of as-grown CdSe and CdS QDs solution, and the PL and the UV/vis spectra were recorded before and after the addition. The PL spectra and the quantum yields of the CdSe QDs modified by n-BA, n-HA, and OA are shown in Figure 4. As shown in Fig. 4, the positions of the luminescence emission peaks of the ligand-modified CdSe QDs shifted to lower wavelengths for the three capping molecules compared to the as-grown CdSe QDs. Moreover, Figure 4(b) shows that the PL quantum yield increased to 45% and 61% for the amines n-BA and n-HA capping CdSe QDs, respectively, whereas OA capping of CdSe QDs exhibited a decrease to 5%. For ligand capping of CdS QDs, the PL spectra and the QYs of the CdS QDs modified by n-BA, n-HA, and OA are shown in Figure 5. As shown in Figure 5(a), the PL intensity was decreased substantially by the additives in the entire wavelength region, whereas the PL spectral maximum occurred at the same wavelength for all systems. Figure 5(b) shows that the PL QY of CdS QDs decreased from 60% to 6% and 3% for n-BA and n-HA, respectively, whereas OA exhibited a decrease to 2%. Compared to the case of CdSe QDs, the three organic additives in this study exhibited contrasting behavior for the CdS QDs system.

a) Room temperature PL spectra of as-grown, n-BA, n-HA and OA modified CdSe QDs; (b) The corresponding PL quantum yield variety of CdSe QDs after capping ligand modification.

a) Room temperature PL spectra of as-grown, n-BA, n-HA and OA capping molecule modified CdS QDs; (b) PL quantum yield of CdS QDs versus 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 n-BA, n-HA, and OA molecules. Because of the high sensitivity of TRPL for ensemble and single particle PL analysis, it is often used to determine the transient population of one or more radiative excited states [50].

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