Synthesis and Characterization of CdSe Quantum Dot via Pyrolysis of

Journal of Modern Education Review, ISSN 2155-7993, USA October 2011, Volume 1, No. 1, pp. 63-73  Academic Star Publishing Company, 2011 http://www.academicstar.us

Synthesis and Characterization of CdSe Quantum Dot via Pyrolysis of Organometalic Reagent Charles Ahamefula Ubani1, M. Y. Sulaiman2, Z. Ibarahim1, N. B. Ibrahim1, M. Y. Othman3  (1. Pusat Pengajian Fizik Gunaan, Fakulti Sains dan Teknologi Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia; 2. Solar Energy Research InstituteUniversiti Kebangsaan Malaysia, 43600 Bangi, Malaysia; 3. Institute of Islam Hadari, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia)

Abstract: We report a low cost and reproducible synthesis of cadmium selenide (CdSe) quantum dot (QD) via pyrolysis of organometallic reagent. The synthesis was performed in a fume-hood starting with the injection of one week old selenium precursor (organometallic source) into cadmium oxide (CdO) solution (pyrolyfic source) at 195oC to produce discrete homogeneous nucleation under slow growth. Differences in the injection time resulted to dissimilarity in the nanocrystal growth, shape and size. Oleic acid (OA) was used to protect the CdSe QD against oxidation and photon emission loss while organic ligand layer of trioctylphosphine (TOPO) was used to protect the QDs from agglomeration. The optical properties of the CdSe QD with size ranges between 2.09 and 4.90 nm were characterized using transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive x-ray spectroscopy (EDX) optical photoluminescence (PL) and absorption spectroscopy (UV-vis). The peak-to-peak values, irregularities in the surface texture and the root-mean-square values of the surface roughness of the CdSe QDs were determined. The CdSe QDs were found to be stable under ambient temperature for about three months. Key words: CdSe quantum dot; nanocrystals; photoluminescence; solar cell

1. Introduction Quantum dots (QDs) are electromagnetic radiation emitters with tunable bandgap. This novel semiconductors nanoparticle exhibit size-dependent optical properties and tunable bandgaps that can be engineered to perfectly match the solar spectrum than the operating capacities of traditional semiconductors (Alivisatos, 1996). Strong stimulus causes valence band electron to take residence in the conduction band resulting in the creation of a positively charged hole in the valence band (McClean and Thomas, 1993). The photon energy absorbed by QDs particles into the conduction band corresponds to the bandgap energy (Weinhardt, Gleim, Fuchs, and Heske et al., 2003). The percentage of absorbed photons that result to the emission of photon energy is referred to as quantum yield (QY) and can be controlled by nonradiative transition of electrons and holes between energy levels that Charles Ahamefula Ubani, Ph.D., Pusat Pengajian Fizik Gunaan, Fakulti Sains dan Teknologi, Universiti Kebangsaan Malaysia; research areas: quantum dot sensitized solar cell. E-mail: [email protected]. M. Y. Sulaiman, Professor, Solar Energy Research Institute, Universiti Kebangsaan Malaysia. Z. Ibarahim, Pusat Pengajian Fizik Gunaan, Fakulti Sains dan Teknologi Universiti Kebangsaan Malaysia. N. B. Ibrahim, Pusat Pengajian Fizik Gunaan, Fakulti Sains dan Teknologi Universiti Kebangsaan Malaysia. M. Y. Othman, Institute of Islam Hadari, Universiti Kebangsaan Malaysia. 63

Electronic copy available at: http://ssrn.com/abstract=1963222

Synthesis and Characterization of CdSe Quantum Dot via Pyrolysis of Organometalic Reagent

produce no electromagnetic radiation. These excellent properties of QDs have been used in light-emitting diodes (Coe, Woo, Bawendi and Bulovic, 2002), biological labels (Zhang, Wang, Xiong, Hu, Yang and Li, 2003; Sato, Tachibana, Hattori, Chiba and Kuwabata, 2008), optoelectronic devices (Duan, Huang, Cui and Lieber, 2003) and in solar cells (Huynh, Dittmer and Alivisatos, 2002). QDs respond differently to solar spectrum however; their peak emission frequencies solely depend on their respective particle sizes. Long-lived electron/hole pair generation allows the ejection of conduction band electrons to the electrode or the injection of electrons from the electrode into the valence band of the particle. Thus, a photo current can be detected which is much enhanced when electron donors or acceptors are present in solution. The specific detail of the QD electronic structure and the excitonic nature of the photoexcited states solely depend on the semiconductor properties and differ significantly across different materials as have been shown in PbSe and CdSe (Dimroth and Kurtz, 2007). The structure of the CdSe-TOPO QDs comprises the core which is the CdSe, the outside which is the vacuum barriers and the TOPO passivation layer located between them. Experimental results show that the decrease of passivation effect increases the effective barrier around the CdSe core causing remarkable blue shift (Mao and Wu, 2005). QDs are cost effectively produced in bulk via chemical manufacturing techniques leading to lower cost and efficient photovoltaic cells (Murray, Kagan and Bawendi, 2000). The photoelectric potential of QDs and their incorporation into photovoltaic cell fabrication has been intensively investigated (Lin, Liedl, and Sperling et al., 2007; Katz, Willner and Wang, 2004). Various approaches for CdSe QDs synthesis have also shown notable potential (Baughman, Zakhidov and Heer, 2007). Interest in the use of QDs is partly due to the progress in chemical synthesis and their respective surface modification properties (Clapp, Medintz, and Uyeda et al., 2005; Chan, Maxwell, Gao, Bailey, Han and Shuming, 2002). Another feature of QD is their electronic structure which facilitates the generation of electron–hole pairs during photoexcitation facilitates their use as electrodes (Bakkers, Roest, and Marsman et al., 2000; Sharma, Pillai and Kamat, 2003). CdSe QDs synthetic approach that relies on the injection of Se precursor into a mixture of solvent and ligands has been successful. This approach entails precise temperature control to separate the nucleation and growth of the QDs which solely depend on the temperature of the mixture, injection process utilized as well as the concentration gradient under control (Prezhdo and Rossky, 1998). Study revealed that room temperature injection of cadmium acetate and sodium seleno-sulphite (Na2SeSO3) in oleic acid showed controlled fluorescence of CdSe QDs (Liu, Howarth, and Greytak et al., 2008). Murcia et al. (2006) reported sono-chemical synthesis of CdSe quantum dots using TOPO, cadmium acetate and hexadecylamine. Mono-disperse zinc blende CdSe QD ranging from 2.5 to 4.3 nm have been synthesized in an open-air microfluidic reactor with PTFE (Polytetrafluoroethylene) capillaries as the reaction channels (Luan, Yang, Tu and Wang, 2007). Similarly, our synthesis reported here produced comparable grain size range (2.09 to 4.90 nm). This paper present the synthesis of CdSe QD via pyrolysis of organometallic reagents injected into a hot non-coordinating solvent and the characterization of the optical and microstructure properties. Octadecene (ODE) is used as the non-coordinating solvent owing to its excellent properties. ODE is stable in air and has a low melting point (below 20°C) and this makes it easier to be handled at room temperature. Besides, it has a high boiling point (about 360°C) coupled with its inert nature to the selenium precursor, less toxicity, low cost and good dissolving power which makes it an ideal solvent for the growth of high quality QDs (Yu and Peng, 2002). The synthetic approach to produce monodisperse CdSe QDs using octadecene as solvent is cheaper and environmentally friendlier (Wang, Jiang, and Chen et al., 2009). Oleic acid (OA) is used to protect the CdSe QDs against oxidation and emission loss instead of the costlier HPA (heteropolyacid) or TDPA (thiodipropionic acid) 64

Electronic copy available at: http://ssrn.com/abstract=1963222


(Peng and Peng, 2001) to dissolve CdO powder to form homogeneous cadmium oleate solution and also as capping ligand for the CdSe QDs. The CdSe QDs covered with organic ligand layer of TOPO so as to protect the samples from predominant defects associated with QDs, such as agglomeration which has been shown to affect the optoelectrical and microstructure properties of CdSe QDs (Himani Sharma, Shailesh N. Sharma, Gurmeet Singh and Shivaprasad, 2006).

2. Experiment 2.1 Chemicals Organometallic selenium (Se, SCR, 99.5+%, Sigma-Aldrich), with trioctylphosphine (TOPO, 90%, Sigma-Aldrich) and octadecene (ODE, Fisher, 90%) were used to prepare the precursor while the pyrolyfic solution which comprises metal base cadmium oxide (CdO, SCR, 99.9+%, Sigma-Aldrich), oleic acid (OA, SCR, 90%, Sigma-Aldrich) and octadecene. All the materials were not subjected to further purification or treatment and were stored at 23°C. 2.2 Methodology Two different steps used for the synthesis of the CdSe QD are firstly, the preparation of the Se precursor and secondly, the preparation of CdSe quantum dots. Different reaction times were chosen in other to obtain different sizes of the CdSe quantum dots. 2.2.1 Selenium solution 30mg of selenium powder (Se) and 5 ml octadecene were added to a 10 ml flat bottom flask over a stirrer hot plate in a fume hood. 0.4 ml trioctylphosphine was measured by syringe from its sure-seal bottle and were added to the flask. A magnetic stir bar was inserted and the mixture was stir-heated to completely dissolve the selenium after which it was cooled to room temperature. The solution was stored in a sealed container for one week to allow the precursor to form. 2.2.2 Synthesis of CdSe quantum dot Table 1 Transition in Physical Properties of CdSe QDs

Selenium solution

CdO

Properties

Before

After

Color

Black

Transparent on cooling

Appearance

Colloidal heterogeneous

Homogenous

Solution

Watery-heterogonous

Sticky transparent

Color

Dark brown

Orange at 195°C

Appearance

Viscous colloidal

Homogenous at 195°C

Solution

Heterogonous

Oily at 195°C

Color CdSe

Light red to dark brown

Texture

Oily Jelly-like

Solvent

Octadecene and toluene

13 mg of CdO was added to 25 ml round bottom flask clamped on a heating mantle. This operation was done in a fume hood to avoid inhalation hazard associated with cadmium compound. To the same flask, pipette was used to add 0.6 ml oleic acid and 10 ml octadecene after which a thermometer capable of measuring 195° was inserted into the mixture. The cadmium solution was heated until its temperature reaches 195°. At this temperature, a clean dry Pasteur pipette was used to quickly transfer 1 ml of the one week old room temperature selenium 65


solution into the 195° heated cadmium solution. The introduction of selenium precursor into the heated solution at 195°C evolved smoke which is believed to be as a result of temperature gradient and rapid formation of CdSe QDs. The samples were removed at 10 seconds intervals using a 9 inch glass pasteur pipette as the CdSe particles grow in size. The samples were synonymous with color transition from light red to dark brown coloration which was as a result of different reaction time and particles size growth. We noted the visible properties transition before and after the formation of the CdSe QDs which are as shown in Table 1.

3. Sample Characterization A one-cm path length quartz curette was used for the spectral study. Perkin Elmer Lambda-20 UV-vis spectrometer was used to carry out the optical measurement in the range of 200–800nm wavelength at room temperature. The absorption peaks were signatory to the absorption properties of the CdSe QDs. The PL spectra were recorded using Perkin Elmer Ls-55 Luminescence Spectrometer with zenon lamp over 350–700 nm range. For SEM, AFM, FESEM and EDX sample preparation, transparent glass measuring 25.2 x 22.2mm (1 “x3”) having thickness ranging between 1mm to 1.2mm was ultrasonically cleaned with distilled water for 10 minutes and the process was repeated using methanol. The glass was dried in nitrogen gas to keep moisture away from the glass. The sample was annealed at 300°C for 120 minutes so as to get rid of the solvent that used for the synthesis and impurities. Samples for the TEM analysis was prepared by dissolving the CdSe QDs with ODE in a ratio of 1:3. However, the presence of black dots in the TEM image could be attributed to the impurities which are not such as dust which were also revealed in the AFM microstructure image (Figure 3b). The atomic force microscopy (AFM) technique brings three-dimensional images including surface roughness, grain size, step height and pitch of CdSe QDs sample surface topography and enabled us to view the QDs distributions with the resolution similar to that obtained in the SEM. The AFM technique is carried out for CdSe nanoparticles samples at room temperature and atmospheric pressure. AFM analyzed surface parameters such as the peak-to-peak value, average surface roughness and surface root-mean-square values. The TEM and FESEM were used to evaluate the micro-structure of the CdSe QDs. The TEM analysis was carried out using CdSe QDs solution. For the FESEM and EDX analysis, a drop of CdSe QDs was dried using carbon-copper grid in ODE dispersed solution and was left to dry at room temperature.

4. Result and Discussion The UV-vis absorption and PL emission spectra of CdSe nanocrystals with diameter range of 2.09 to 4.90 nm are shown in Figure (1a), (1b) and (1c) respectively. The illustration in Figure (1a) shows that the absorption spectrum of CdSe QDs is a function of the growth time as the colour of the CdSe QDs changes from light red to dark-brown in 1 minute with respect to the discharge time. The fast decolouration in the CdSe QDs indicated rapid nuclei growth which was verified by the corresponding absorption peak shift. The small CdSe QDs with an absorption peak of 482 nm was formed in 10 seconds and grew bigger QDs with an absorption peak of 542nm in 1 minute. The numbering shown on the absorption peaks (Figure 1a) illustrated the transition in size range (1 represent smallest and 6 the largest) as the temperature of the CdSe QDs increases, the sizes of the QDs increases with respect to time while the absorption peaks shift to longer wavelength as a result of transition in the interband of the CdSe QDs (Kim, Fisher, Eisler, and Bawendi, 2003). It became obvious that the particle size of CdSe QDs can be tuned by varying the reaction time during synthesis. 66


The corresponding PL spectra of the CdSe nanocrystals in Figure (1b) show the extension time of reaction with emission wavelength shifts ranging from 442 nm to 533 nm. The intensity of the PL for the CdSe QDs increased with an increase in the growth time from 0 to 1 min. Initially, the PL peak was broad and asymmetric and became narrower and symmetric as the reaction time increases. Similar nanocrystal size distribution has been reported by Peng et al. (Peng, Wickham and Alivisatos, 1998) where 1.5 minute reaction time resulted in the increase of particle size owing to the depletion of the monomer concentration in the reaction solution.

Figure 1a UV–vis Spectral Peaks of the CdSe QDs

Figure 1b PL Spectra Emission of CdSe QDs

The peak difference between the absorbed and emitted photons as illustrated in Figure (1c) is thought to be caused by thermal losses. The PL properties of CdSe QDs are important because the tunability of the emission wavelength and nanocrystal size peaks could be used to determine their application because their optoelectronics properties are size-related. Spectral relationship of UV-vis and PL in (Figure 1c) shows that the PL spectra are typical of CdSe QDs consisting of two peaks: one with the position closer to the absorption peak called band-edge PL and the other is red-shifted peak referred to as trap-related PL. The peak difference between the absorbed and emitted photons is as illustrated in Figure (1c) and this is

67


thought to be caused by thermal losses. The PL properties of CdSe QDs reveal the ability of the QDs to emit absorbed photon energy and however; influence their applications since their optoelectronics properties varies with respect to particle sizes of the QD. Spectral relationship of UV-vis and PL shown in Figure 1c revealed that the PL spectra are typical of CdSe QDs consisting of two peaks: one with the position closer to the absorption peak called band-edge PL and the other is red-shifted peak referred to as trap-related PL.

Figure 1c

Figure 2

UV-vis and PL Spectra Relationship

SEM Microstructure Image of the CdSe QD

SEM diffraction pattern of the CdSe QDs shows regular arrangement of CdSe QDs images. Each bright spot correspond to the crystal plane and the symmetry of the dots reveals the crystal structure. Figure 3a and b shows AFM 3D 1micron surface orientation of the CdSe QDs while Figure 3c illustrated the surface morphology of CdSe QD. Figure 3a revealed the 3D surface colour image of the CdSe QD while Figure 3b shows the surface roughness profile of the CdSe QD sample. Figure 3c depicts the QD particle distribution network at the surface of the synthesized sample. Figure 3d is an illustration showing the surface roughness histogram of the synthesized CdSe QDs.

68


Figure 3a 3D surface Colour Image of the CdSe QD

Figure 3b 3D Surface Roughness Profile of the Synthesized CdSe QD

Figure 3c

QD Particle Distribution Network at the Surface of the Sample

Figure 3d Surface Roughness Histogram of the CdSe QDs 69


For ten point count height of AFM images with scan size of 609.089 nm containing 65536 CdSe nanoparticles have peak-to-peak value of 1218.65 nm, surface skewness of -0.0396428 and surface kurtosis of -1.23324 respectively. The irregularities in the surface texture associated with surface roughness have average roughness value of 265.749 nm and surface root-mean-square values of 303.945 nm.

Figure 4a

TEM Image of the CdSe QDs

Figure 4b FESEM Image of the CdSe QDs

Figure (4a) and (4b) show the TEM and FESEM images of the CdSe QDs. The information obtain from the TEM and FESEM are useful in determining the QD particle size and orientation. The TEM reveal that the QDs size distribution possesses almost spherical morphology. The line length between the QD particles size at the surface of the sample is as shown in Figure 5. Energy-dispersive X-ray spectroscopy (EDX) analysis (Figure 6a and b) revealed CdSe QDs interactions with electromagnetic radiation and matter as they response to charged particles. This analytical approach allows energy of the X-rays to be characterized through energy differences existing between the dissimilar shells and of the atomic structure of the element from which they were emitted allowing the compositional elements of the specimen to be measured by EDX. Its fundamental principle of characterization relies on the uniqueness of the atomic structure which allows X-rays of an element’s atomic 70


structure to be uniquely identified individually. Carbon tap used for the analysis were believed to be the dominant source of carbon (C) as illustrated in Figure 6(a) and (b). The presence of oxygen (O) could be due to exposure to the atmosphere during when the sample was characterized. Selenium (Se) and cadmium (Cd) was the major compound used during the synthesis leading to the formation of CdSe QD. Other unidentified peaks could be as a result of dust and oxygen interaction with the sample. Figure 4a (TEM) and 4b (FESEM) images showing CdSe QD lattice plane spacing and particles sizes ranging from 2.09 to 4.9 nm respectively. Owing to this effect, appropriate care should be taken during the preparation and storage of QDs to reduce contamination and the influence of oxygen which can result to the agglomeration of the particles. Since the CdSe is inorganic compound, prolonged contact with oxygen could result to agglomeration of the QDs. Precaution to this form of defect includes storing the freshly prepared sample in toluene, and the annealed sample in desiccators to prevent moisture interaction with the sample. Compositional description of the CdSe QDs with respect to concentration, intensity of the ejected electron, weight of the compositional elements and the atomic percentage of a point probe of the sample are as shown in table 2. The concentration ranges from carbon (189.73), oxygen (18.92), selenium (0.89) and cadmium (0.11). Their respective intensity to X-ray was 1.6459, 0.5415, 0.6721, and 0.7200 for carbon (C), oxygen (O), selenium (Se) and cadmium (Cd). Weight in percentage for the composed elements were 115.28 (C), 34.95 (O), 1.33 (Se) and 0.15 (Cd). The atomic percentage were 8134 (C), 18.51 (O), 0.14 (Se) and 0.01 (Cd) respectively. Cadmium was the least in weight and concentration besides; its optoelectric property is better than that of oxygen and selenium.

Figure 5

Line Length Distribution between Successive CdSe QDs

Figure 6a EDX Spectrum of the CdSe QD 71


Figure 6b Scale Showing the Composition of the CdSe QDs by Weight Table 2

Composition of a Point Probed CdSe QDs Sample

Element

Concentration

Intensity

Weight %

Atomic %

C

189.73

1.6459

115.28

81.34

O

18.92

0.5415

34.95

18.51

Se

0.89

0.6721

1.33

0.14

Cd

0.11

0.7200

0.15

0.01

Total

151.71

5. Conclusion We demonstrate cheap and efficient method for CdSe QDs synthesis via pyrolysis of organometallic reagents. Although cheap and convenient method was used, the synthesis produced high quality CdSe QDs. The synthesized CdSe nanocrystals showed narrow size distribution with particles sizes ranging from 2.09 nm to 4.9 nm. OA was used to protect the CdSe QDs from oxidation and emission loss. Their vulnerability to this effect degrades the photoluminescence quantum yield of nanocrystals semiconductors. The organic ligand layer TOPO was added to improve the optical properties of the CdSe QDs. Spectral relationship of UV-vis and PL peak shows that the PL spectra are typical of CdSe QDs consisting of band-edge photoluminescence and trap-related photoluminescence. The quantitatively analyzed CdSe QDs surface parameters for ten point count height of atomic force microscopy images with scan size of 609.1 nm containing 65536 CdSe nanoparticles have peak-to-peak value of 1218.7 nm, surface skewness of -0.04 and surface kurtosis of -1.23. The irregularities in the surface texture have an average surface roughness value of 265.7 nm and a root-mean-square value of 303.9 nm. The surface topography reveals the particle distributions with resolution similar to that obtained with scanning electron microscopy. SEM was used to evaluate the morphology of the CdSe QD sample. Transmission electron microscopy and FESEM images revealed size distribution pattern of the CdSe QDs, its spherical morphology and the lattice plane spacing. The electron dispersed X-ray shows the compositional scale of a point probe sample of the CdSe QD by concentration, intensity, weight percentage and the atomic percentage respectively.

Acknowledgement This work is supported by the Ministry of Science Innovation and Technology Malaysia (Sciencefund Grant N0: 06-01-02-SF0534). 72

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