ARTICLE pubs.acs.org/JPCC
Surface Related Emission in CdS Quantum Dots. DFT Simulation Studies Hung-Lung Chou,† Chia-Hung Tseng,† K. Chandrasekara Pillai,† Bing-Joe Hwang,†,‡ and Liang-Yih Chen*,† †
Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Section 4, Keelung Road, Taipei, 106, Taiwan ‡ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
bS Supporting Information ABSTRACT: In general, organic capping molecules are applied to passivate the surface of semiconductor nanomaterials to modulate the optical properties of these nanostructures. In this work, two alkylamines (n-butylamine (n-BA) and n-hexylamine (n-HA)) and oleic acid (OA) were used to modify the surface of moderately high luminescent CdS quantum dots (QDs). From the photoluminescence (PL) spectra and the quantum yield (QY) analyses, we observed that the PL QY of the CdS QDs decreased after introduction of the alkylamine and oleic acid molecules. The PL decay kinetics for these CdS-capping molecule systems were followed by time-resolved photoluminescence (TRPL), and the spectra were analyzed in terms of a biexponential model identifying two lifetime values, shorter lifetime (τS) and longer lifetime (τL). Compared to bare CdS QDs, for the CdS QDs surface modified by alkylamine or fatty acid, both the shorter and the longer excited state lifetimes were decreased; the fractional contribution by the longer-lifetime component became reduced and the shorterlifetime component accounted for most of the total PL. Density function theory (DFT) simulation was employed using a Cd3S5 cluster to model the adsorption of organics to calculate the binding energy and the charge on Cd and S of CdS. By comparing the elemental charges of the bare CdS with those of the CdS modified by the organic molecules, it is suggested that n-BA, n-HA, and OA could decrease the surface related radiative charge-recombination process and the PL-QY of the CdS QDs.
’ INTRODUCTION Semiconductor quantum dots (QDs) show quantum size effects distinct from those of the corresponding bulk materials, and they are of great interest for both fundamental research and industrial development.17 The semiconductor nanomaterials are widely used in light-emitting diodes (LED),3,8 thin film transistors,9 solar cells,5,7 biological labeling,6,10,11 and so on. Among these materials, group IIVI semiconductor QDs, especially CdS and CdSe, have been extensively researched due to the ease with which their emission in the visible range can be simply tuned by changing their size and the advances in their preparation methods. Due to their superior optical properties including narrow emission, broad absorption, and high photostability, they have great potential in optoelectronic devices.12,13 Additionally, CdS QDs are excellent as candidates of blue photoluminescence (PL) emitters.14 Numerous methods have been developed for the preparation of CdS QDs,1518 and the relatively successful approaches, including the organometallic approach,19,20 and its alternatives,21,22 are exclusively performed in coordinating solvents. The tunable electronic band gaps of semiconductor QDs by controlling their size may provide an advantage in a variety of photoactive applications. However, QDs with significant surfaceto-volume ratio characteristics can be influenced by the surface conditions and environment. Recently, Jones and Scholes reported r 2011 American Chemical Society
the photophysics of QDs to demonstrate that the electronic structure of QDs comprises intrinsic exciton states, delocalized states, and trap states.23,24 One of the most important radiationless charge recombination is the quenching process of the exciton states at the trap states on the surface. To overcome this difficulty, proper electronic passivation of the QD surface with organic or inorganic capping agents serves to reduce the number of surface defects to achieve a high QY. In previous reports, organic-molecule-protected CdS QDs consisting of semiconductor cores surrounded by an organic monolayer have been employed for applications in materials science.2527 Uchihara et al. reported the pH dependence of photostability of aqueous colloidal solutions of CdS QDs protected by thioglycerol and mercaptoacetate under stationary irradiation.25 These results demonstrated that the negative charge of the capping agents acts on the hole-trapping process in primary photochemical events of the surface modified CdS particles and their photostability. Patra et al. studied the optical properties of CdS QDs capping by silica and thiosalicylic acid using steady-state and time-resolved photoluminescence (TRPL) spectroscopy and showed delayed radiative recombination of Received: May 18, 2011 Revised: August 3, 2011 Published: September 20, 2011 20856
dx.doi.org/10.1021/jp2046382 | J. Phys. Chem. C 2011, 115, 20856–20863
The Journal of Physical Chemistry C carriers due to electron or hole trapped on the capping molecules, resulting in strong quenching of PL efficiency.26 Thangadurai et al. studied the capping effect of various organic thiol molecules on CdS QDs, and the results showed that 1,4dithiothreitol is more efficient to influence the optical properties of CdS QDs by reducing the particle size and quenching the trap state fluorescence emission.27 Thiols have been used more frequently as efficient capping molecules for CdS QDs, but very few studies have been conducted on the other capping molecules, such as fatty acids and alkylamines.28 Indeed, alkylamines have been often used to passivate surface trapping states of CdSe QDs, and high PL QYs have been reported in their presence.2932 In selection of the capping molecules for the QDs, a general practice is to extend the knowledge from traditional solution-coordination chemistry and surface chemistry on bulk materials. It must, however, be noted that compared to atoms on the flat surface of bulk substrates, the binding abilities of the atoms on the curved surfaces may be affected by their diverse structural environments and sizedependent electron configuration. Despite the importance of surface exchange reactions with organic capping molecules for functionalization of semiconductor QDs, information on the influence of the organic ligands on the optical properties of CdS QDs is fragmented. The knowledge of binding details is crucial for understanding of size and shape control during QD growth. In some cases, this lack of understanding may even hinder the correct interpretation of experimental data. Computational approaches provide a useful tool for studying ligand adsorption.3335 Classical simulation studies of bare CdSe QDs have resolved the mechanism of surface relaxation35,36 and of the pressure induced phase transitions.37,38 Several methods have been reported for the relationship between QD and ligands. For example, Kilina et al. exchanged amine and phosphine oxide model ligands and fabricated the strong surfaceligand interactions leading to substantial charge redistribution and polarization effects on the surface via DFT.39 Ab initio calculations have the potential to deliver reliable results, but they are limited to very small systems and do not account, typically, for thermal fluctuations. Thus, binding energies for capping molecules have been computed for different surface sites of QDs by classical simulation33,34 and density functional theory (DFT).4042 Adsorption of n-butylamine (n-BA), n-hexylamine (n-HA), and oleic acid (OA) on bulk CdSe planes by DFT has also been investigated.32 In our continued interest in the rational design of organiccapped semiconductor QDs with predictable optical properties for optoelectronic applications, we recently prepared CdS QDs modified by n-BA, n-HA, and OA. For these systems, the adsorbed molecules were found to decrease the PL QYs of bare CdS QDs, instead of the expected increase. To unravel this complex behavior, TRPL studies were carried out on these systems, and the spectra were analyzed in terms of a biexponential model identifying two lifetime values, shorter lifetime (τS) and longer lifetime (τL). DFT simulation was employed, and the calculated binding energies and the changes in the residual charges on Cd and S of CdS due to organic adsorption were analyzed to understand the PL behavior of CdS QDs in the presence of the added organics. Extensive theoretical work has been performed by DFT on CdS and CdSe nanoparticles, including semiempirical tight-binding methods.4348 Ab initio calculations have been employed on only relatively small systems because the computational cost is much higher than that for semiempirical methods. Because of the computational efficiencies
ARTICLE
that are built into the VASP package, that program was able to perform calculations with clusters of CdS with ligands.
’ EXPERIMENTAL SECTION Chemicals. Cadmium oxide (CdO, Alfa Aesar, 99.98%), sulfur powder (S, Alfa Aesar, 99.999%), 1-octadecene (1-ODE, Acros, 90%), tri-n-butylphosphine (TBP, Strem, 99%), oleic acid (OA, Showa, ACS), n-butylamine (n-BA, TCI, ACS reagent), n-hexylamine (n-HA, TCI, ACS reagent), LD466 (Acros), toluene (Tedia), methanol (Tedia), and hexane (Tedia) were used without further purification. Synthesis of CdS Quantum Dots. The synthesis of CdS QDs was conducted in a noncoordinating solvent, and 1-ODE was used in this work. A typical procedure for the synthesis of CdS QDs is as follows. CdO (0.15 mmol) was mixed with 0.7 mmol of OA and 4.8 g of 1-ODE in a 25 mL three-neck flask. The mixture was heated to 300 C under Ar flow for 30 min, and a stock solution of S (0.1 mmol of S powder dissolved in 0.62 mmol of TBP and 1 g of ODE) was then injected. The solution mixture was cooled, and the QDs were allowed to grow at 260 C. Crystal growth was monitored by UVvisible absorption. At each stage of CdS QD growth, a small amount of the sample (∼0.2 mL) was taken via a syringe and diluted with sufficient amount of anhydrous toluene to show an optical density between 0.1 and 0.2. The resulting CdS QDs were purified by dissolving in toluene, and removing the unreacted starting materials and side products by extraction and precipitation procedures reported previously.49 No size sorting was performed in any of the samples reported here. Ligand Modification of the CdS QD Surface. An aliquot of CdS QDs solution was diluted with toluene to yield an optical density of approximately 0.1. Three milliliters of QDs solution was mixed with various capping molecules at a fixed concentration of 5 mM. The solution mixture was stirred in the dark at room temperature for 1 h. The QDs were precipitated with methanol, redispersed in toluene, and characterized by UV/visible absorption and photoluminescence spectroscopy for PL QY. Characterization. The room temperature UVvisible absorption spectroscopy measurements were carried out using Jasco V-560 on samples with low optical density in order to minimize reabsorption and avoid absorption saturation. PL spectra were acquired from a Hitachi HF-7100 upon excitation at 350 nm, using a 300 W Xe lamp as the excitation source and a double grating monochromator. PL QYs were obtained according to reported procedures, using LD466 dye as standard with QY of 80% in ethanol.50 The size, size distribution, structure, and orientation of the CdS QDs were analyzed by high-resolution transmission electron microscopy (HR-TEM) in a Philip TecnaiG2 operating at 200 kV. The TEM specimens were prepared by placing a drop of diluted QDs solution onto a carbon/pioloform film supported on a copper mesh grid and allowing it to dry under air at room temperature. XPS was performed using a VG ESCA Scientific Theta Probe with a monochromated Al X-ray source. Purified CdS QDs deposited on a silicon substrate were used for XPS measurements. The pressure in the analysis chamber was 109 Torr. All analyses were calibrated to Au 4f7/2 at a binding energy (BE) of 83.8 eV. The system of time-resolved photoluminescence (TRPL) with one lasers as picosecond diode laser driver with 375 nm laser head (with integrated collimator and TE cooler for temperature stabilization) was integrated by 20857
dx.doi.org/10.1021/jp2046382 |J. Phys. Chem. C 2011, 115, 20856–20863
The Journal of Physical Chemistry C
Figure 1. (a) TEM image of a sample of CdS QDs. (b) HRTEM image of a single CdS QDs. (c) The corresponding fast Fourier transfer (FFT) diffraction pattern.
Protrustech Co., Ltd. Andor iDus CCD with 1024 128 pixels was used to take the PL signal, and the Pico Quant PMT Detector head with 200820 nm,
