PAPER www.rsc.org/crystengcomm | CrystEngComm Growth orientation and shape evolution of colloidal lead selenide nanocrystals with different shapes
Daoli Zhang,*ab Guangmei Zhai,a Jianbing Zhang,ab Lin Yuan,a Xiangshui Miao,ab Siyao Zhua and Ying Wanga
Received 4th January 2010, Accepted 16th April 2010 DOI: 10.1039/b927238k
Lead selenide (PbSe) nanocrystals with different shapes were synthesized via solution-processing by adjustment of the reaction conditions. The prepared PbSe nanocrystals were characterized by X-ray diffraction and transmission electron microscopy. X-Ray diffraction pattern shows that the synthesized PbSe nanocrystals have cubic rock salt structure. The initial injection of precursors into hot reaction solvent immediately results in the formation of truncated octahedron-shaped nuclei, which are terminated by {100} faces and {111} faces. Acetate in the reaction mixture plays an important role in the formation of star-shaped PbSe nanocrystals. In addition, the reaction solvent also influences the shape of PbSe nanocrystals. Spindle-shaped PbSe nanocrystals are formed by spontaneous alignment and fusion of small quasi-spherical PbSe nanoparticles, namely, oriented attachment. The occurrence of oriented attachment of PbSe nanocrystals is the result of competition between the steric hindrance force and the orientation force. If the orientation force along some axes can overcome the steric hindrance force, the small PbSe nanocrystals with certain size distribution can self-assemble and evolve to spindle-shaped PbSe nanocrystals, vice versa, the small PbSe nanocrystals will grow individually to bigger quasi-spherical PbSe nanocrystals.
Introduction changing the reaction conditions, including injection and growth temperature, growth time, precursors, reaction solvent It is well known that the properties of semiconductor nano- and capping agents. For example, Wehrenberg21 reported the crystals strongly depend on the size of structures, but new synthesis of quasi-spherical nanocrystals, cubic nanocrystals, 1–4 studies show that the shape is also as important as the size. octahedral nanocrystals and star-shaped nanocrystals. More- Therefore it is very important to synthesize semiconductor over, through oriented attachment of nanocrystal building nanocrystals with different shapes not only for fundamental blocks, they obtained straight nanowires, undulated nanowires, studies but also for various applications. Lead selenium materials zigzag nanowires, tapered nanowires and nanorings. But in their have attracted much attention because of their infrared optical report, the star-shaped PbSe nanocrystals were synthesized properties. PbSe material in normal form, with a narrow band using hexadecylamine as co-surfactant. Moreover, to the best of gap energy of 0.21–0.40 eV, is sensitive to infrared light with our knowledge, the spindle-shaped PbSe nanocrystals have not a wavelength of 3–5 mm and is extensively used in the field of been reported yet. infrared detection. PbSe nanoparticles, with a Bohr radius of In this work, we synthesized star-shaped and spindle-shaped about 46 nm, have extensive prospects as potential applications PbSe nanocrystals via solution-processing by carefully adjust- 5 6 7 such as near-infrared lasers, solar cells, and photodetectors, ment of reaction conditions. Their growth mechanisms as well as especially due to the carrier multiplication or multiexciton shape evolution are discussed preliminarily. generation in PbSe nanocrystals.8–11 It can be used to create a new kind of thin film field-effect transistor,12 to prepare mid-infrared emitting devices13 and to assemble new kinds of Experimental superlattices.14 2.1 Chemicals To date, PbSe nanocrystals with a variety of shapes have been synthesized by chemical vapor deposition method15–17 and All manipulations were carried out using standard Schlenk line solution-phase synthetic method.18,19 Since the synthesis of high- techniques under argon. Tri-n-octylphosphine (further referred quality colloidal PbSe nanocrystals was developed from lead to as TOP, technical grade, 90%), selenium power (99.95%), oleic oleate and trioctylphosphine selenide by Murray and co- acid (analytical grade), lead(II) acetate trihydrate (analytical workers in 2001,20 PbSe nanocrystals and nanowires with grade, 99.5%) and lead(II) oxide yellow (analytical grade, 99%) different sizes and morphologies have been prepared by were used as purchased without further purification. Anhydrous ethanol, n-hexane, and tetrachloroethylene were purchased from a variety of sources. aDepartment of Electronic Science and Technology, Huazhong University of Science and Technology, No. 1037 Luoyu Road, Hongshan District, Wuhan City, Hubei Province, 430074, P. R. China. E-mail: 2.2 Synthesis of star-shaped PbSe nanocrystals [email protected] bWuhan National Laboratory for Optoelectronics, 1037 Luoyu Road, The nanoparticles of PbSe were grown by conventional colloidal Hongshan District, Wuhan City, Hubei Province, 430074, P. R. China method (hereinafter referred to as scheme 1). To prepare 1.5 M
This journal is ª The Royal Society of Chemistry 2010 CrystEngComm, 2010, 12, 3243–3248 | 3243 stock solution of TOPSe, 0.47 g of selenium was dissolved in a drop of the colloid in tetrachloroethylene on a carbon-coated 4 mL of tri-n-octylphosphine (TOP) over 2 h at 50 C. For copper grid and allowing the solvent to evaporate at room a typical synthesis, 2 mmol of lead acetate trihydrate, 3 mL of temperature. A FEI Tecnai G 220 electron microscope operating oleic acid and 2 mL of TOP were dissolved in 25 mL of phenyl at 200 kV was used to acquire the TEM images. Fourier trans- ether in 100 mL three-necked flask equipped with a condenser. form infrared spectra were taken with a Bruker VERTEX 70 The reaction mixture was then heated to 150 C for 1.5 h under Fourier transform infrared spectrometer operating from 11 765– vigorous stirring and under a continuous flow of argon. During 3704 cm 1. Samples were prepared by placing a drop of a dilute this process, lead acetate trihydrate decomposed and lead oleate tetrachloroethylene dispersion of nanocrystals on the surface of formed. After 30 min, the stock was heated again until the a KBr plate. temperature was at the needed temperature of 165 C. Then 4 mL of 1.5 M TOPSe was loaded into a 5 mL syringe and rapidly Results and discussions injected into the reaction solution. Upon injection, the reaction solution turned from colorless to brown, and then to black in 5 s, 3.1 Formation of star-shaped PbSe nanocrystals. indicating that small PbSe cluster nucleated immediately and Upon injection, small PbSe clusters immediately nucleated and began to grow. This was also accompanied by a sudden decrease started to grow with stability provided by the capping ligands in temperature to 145 C. The reaction mixture was kept at (TOP and oleic acid). Star-shaped PbSe nanocrystals shown in 145 C for about 30–60 min, and then the reaction was stopped Fig. 1 are produced by injecting trioctylphosphine selenide into and the reactant was cooled to room temperature under a stream the mixture of lead oleate and phenyl ether preheated to 165 C. of argon. At various time intervals, 5 mL of aliquots of the The growth times of the nanocrystals were 3 min, 5 min, 15 min reaction mixture were taken from the reaction flask. The aliquots and 30 min respectively. Star-shaped nanocrystals with well should be immediately cooled to room temperature to quench dispersion are obtained. The main diameters of these nano- the reaction by mixing with 5 mL cold tetrachloroethylene. crystals are 27 nm, 29 nm, and 43 nm respectively with standard PbSe nanocrystals were precipitated from other components by deviations of 7.8%, 10.6% and 12.3% respectively. The star shape adding 50 mL of anhydrous alcohol to the crude solution and can be pictured as truncated octahedron where six symmetric separated by centrifugation, and then re-dispersed in proper (100) faces have grown into horns.22 The star shape is in fact a 2D solvent. Washed several times by alcohol, the pure outgrowth projection of a 3D hexapod real structure. The PbSe nanocrystal was stored in n-hexane. looks like a star through the zone axis of [111] and a cross through the zone axis of [100]. 2.3 Synthesis of spindle-shaped PbSe nanocrystals Lee and co-workers reported the architectural control of PbS nanocrystals which evolved from rod-based structures through For a typical synthesis (hereinafter referred to as scheme 2), star-shaped structures as a transient species to stable truncated 1.5 mmol of lead(II) oxide yellow was dissolved in a solution of octahedron and cubes.22 Low-temperature conditions ( 140 C) phenyl ether (2 mL), oleic acid (1.4 mL), and tri-n-octylphos- phine (TOP, 5 mL) (flask 1). This flask was heated under an inert atmosphere of argon for an hour at 85 C and then cooled to 45 C. 1.5 mL of 1 mol L 1 tri-n-octylphosphine selenium (TOPSe) was then added to flask 1. In another flask (flask 2), 10 mL of phenyl ether was heated to 180 C. The contents of flask 1 were then rapidly injected under vigorous stirring into flask 2. After injection, flask 2 cooled to about 135 C and the yellow solution rapidly turned black. Then the products were allowed to grow for 1–10 min at certain growth temperature. A portion of the hot reaction mixture (2 mL) was extracted from the flask after a certain time of growth and cooled to room temperature. After that the as-synthesized PbSe nanocrystals were precipitated out of the reaction mixture with ethanol and redispersed in hexane. After repeating this procedure twice, the PbSe nanocrystals were dissolved in tetrachloroethylene, forming stable colloidal solutions.
2.4 Sample characterization The as-synthesized nanocrystals were characterized by X-ray diffraction and transmission electron microscopy (TEM). Powder X-ray diffraction was performed at room temperature on an X’ Pert PRO X-ray diffractometer with Cu-Ka1 line (l ¼ Fig. 1 Well-defined star-shaped PbSe nanocrystals were prepared using 0.154 nm) in the 2q range from 20 to 80 . Samples were prepared scheme 1, the growth times of the nanocrystals were 3 min (a), 5 min (b), by evaporating several drops of the PbSe nanocrystals sample 15 min(c) and 30 min (d) respectively. Longer growth time results in onto a glass slide. TEM samples were prepared by depositing larger average nanocrystal size.
3244 | CrystEngComm, 2010, 12, 3243–3248 This journal is ª The Royal Society of Chemistry 2010 with a high flux of monomers resulted in kinetically controlled growth, and growth on the {100} faces was preferred, a variety of 1D rod-based PbS nanocrystals were obtained. When the temperature was increased to 180 C, the enhanced growth rate on the {100} faces finally resulted in star-shaped PbS nano- crystals. After further increasing the growth temperature ( 250 C), the growth process appeared to shift into the ther- modynamic regime, and the more thermodynamically stable cube shape with marginal truncation on its corners was favored. Cho and co-workers19 observed the formation of star-shaped PbSe nanocrystals, and they indicated that the star shape was typical for reactions employing primary amines, high concen- trations of the lead precursor, and phenyl ether serving as a reaction medium. The {100} faces consist of both Pb and Se atoms, while the {111} faces consist of Pb or Se atoms. As a result, because of the different electro-negativities between Pb and Se, {111} faces are polar and their arrangement will deter- mine the dipole moment in PbSe nanocrystals. Cho predicted a high fraction of PbSe nanocrystals with permanent dipole moments. The highest probability and the largest magnitude of the dipole moment are predicted along the <100> direction, and the chemical tend to deposit on {100} faces. In our experiment, in the absence of a primary amine, it clearly appears that the initial injection of TOPSe into hot lead oleate solution immedi- ately gives rise to the formation of truncated octahedron-shaped nuclei, which are terminated by two characteristic faces: {100} faces and {111} faces. Since the formation of the 2D nuclei on {100} faces has a low activation energy in our reaction condition, the growth rate along <100> direction is faster than <111> direction, which induces the shrinking of the six {100} faces into sharp corners and finally gives rise to star-shaped nanocrystals (Fig. 2). When lead oxide is used as lead source, PbSe nanocrystals are irregular multipods, not well-defined stars, as shown in Fig. 3. Fig. 3 TEM images of star-shaped PbSe nanocrystals with irregular This result suggests that the acetate in the reaction mixture, multipods, 15 min (a) and 30 min (b). which promotes the formation of star like geometries, plays an important role in controlling the growth mechanism and the resulting shape of nanocrystals.23 Houtepen indicated that the 3.2 Spindle-shaped PbSe nanocrystals from zero-dimensional acetate concentration had a decisive impact on the final size and nanoparticles. shape of nanocrystals when the growth temperature was 110– 230 C. In addition, Colvin synthesized spherical PbSe nano- The X-ray diffraction trace of the 10 min growth and 30 min crystals in a solvent of 1-octadecene utilizing PbO as a lead growth products of spindle-shaped PbSe nanocrystals is pre- source.24 However, in our experiment, irregular multipods are sented in Fig. 4, in which all of the detectable peaks are indexed synthesized in a solvent of phenyl ether involving the same lead to almost the same positions as those from a standard bulk of source PbO and the same reaction temperature. This suggests PbSe (JCPDS-ICDD card 06-0354). The XRD pattern indicates that the reaction solvent is also responsible for the shape of that the as-synthesized PbSe nanocrystals have cubic rock salt nanocrystals. lattice structure and there is no obvious impurity in sample. To reveal the growth process of spindle-shaped PbSe nanocrystals, we analyzed the products with different growth time by trans- mission electron microscopy. Fig. 5(a) and (b) demonstrate the images of 7 min growth and 10 min growth products, respec- tively. From Fig. 5(a) we can find that there are some chain-like aggregations composed of the PbSe nanocrystals and dispersed PbSe nanocrystals. As shown in Fig. 5(b), the products are the mixture of spindle-shaped nanocrystals and quasi-spherical nanocrystals. The average length of spindle-shaped nanocrystals is about 165 nm; the average size of the biggest diameter of Fig. 2 Scheme showing the formation of a star-shaped nanocrystal due spindle-shaped nanocrystals is about 10 nm. The average diam- to a faster growth on the {100} faces. eter of the quasi-spherical nanocrystals is also about 10 nm, with
This journal is ª The Royal Society of Chemistry 2010 CrystEngComm, 2010, 12, 3243–3248 | 3245 Fig. 4 X-Ray diffraction pattern of PbSe nanocrystals indexed to the bulkrock salt crystal structure. The growth times of the nanocrystals were 10 min (a) and 30 min (b), respectively (*: impurity). a standard deviation of about 11.1%. These observations suggest that spindle-shaped PbSe nanocrystals are probably formed by spontaneous alignment and fusion of small quasi-spherical PbSe nanocrystals, namely, oriented attachment. On the basis of ref. 19, the majority of PbSe nanocrystals Fig. 5 Well-defined spindle-shaped PbSe nanocrystals were prepared ( 89%) have non-zero dipole moments due to the different atomic using scheme 2. TEM images of (a) 7 min growth PbSe nanocrystals and distribution on the nanocrystals surface, the relative magnitude of (b) 10 min growth PbSe nanocrystals (the scale bars are 200 nm and 50 nm, respectively). dipole moment along different
3246 | CrystEngComm, 2010, 12, 3243–3248 This journal is ª The Royal Society of Chemistry 2010 Fig. 7 Near-infrared absorption spectrum of spindle-shaped PbSe Fig. 9 The room temperature photoluminescence (PL) spectra of spindle-shaped PbSe nanocrystals prepared by scheme 2. nanocrystals prepared by scheme 2. then evolve to spindle-shaped PbSe nanocrystals with minimal the fundamental band of PbSe nanocrystals and later should be surface roughness, vice versa, the small PbSe nanocrystals will the overtone band. grow individually to bigger quasi-spherical PbSe nanocrystals. Conclusions Star- and spindle-shaped PbSe nanocrystals were synthesized in 3.3 Optical properties of PbSe nanocrystals solution from lead oleate and TOPSe precursors by rigorous The near-infrared absorption spectrum and Fourier transform control of the reaction conditions. It is conformed that the infrared (FTIR) spectrum of typical spindle-shaped PbSe nano- acetate in the reaction mixture plays an important role in crystals prepared using scheme 2 are shown in Fig. 7 and 8. Four controlling the formation of star-shaped PbSe nanocrystals. In discrete absorption peaks appear at about 2300 nm, 2020 nm, addition, the reaction solvent is also responsible for the shape of 1830 nm, and 1675 nm, respectively. Compared with bulk PbSe nanocrystals. Due to the orientation force caused by non-zero (about 4430 nm), the first absorption peak (about 2300 nm) of polar moment, spindle-shaped PbSe nanocrystals synthesized by PbSe nanocrystals exhibit dramatic blue shift. The first absorp- oriented attachment of collections of nanoparticles that attached tion feature corresponds to an interband transition. However, and fused along identical crystal faces formed oriented chains. It the assignment of other features is still in debate, especially for is worth pointing out that at present the spindle-shaped PbSe the second optical transition.25 The appearance of four well- nanocrystals are synthesized with the presence of a volume of separated absorption peaks and blue shift of the first absorption quasi-spherical PbSe quantum dots. The study aimed to improve are the results of quantum confinement effects. The room the volume fraction of spindle-shaped PbSe nanocrystals is under temperature photoluminescence (PL) spectra of PbSe nano- way. crystals were prepared using scheme 2 is shown in Fig. 9. Fig. 9 Due to the effects of quantum confinement, the Fourier consists of two bands centered at 980 nm and 1590 nm. Former is transform infrared absorption spectrum of nanocrystals exhibits dramatic blue shift from the band gap of bulk PbSe. The special morphology and optical properties of PbSe nanocrystals with different shapes show the potential applications in the optoelectronic devices and biomedical fields.
Acknowledgements We thank the financial support of Natural Science Foundation of Hubei Province (no. 2007ABA275), the International Coopera- tion Fund of Wuhan City (no. 201070934333) and the Graduate Innovation Fund of HUST (no. HF-07-06-2010-185). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology for sample characterization.
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3248 | CrystEngComm, 2010, 12, 3243–3248 This journal is ª The Royal Society of Chemistry 2010