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 cmÀ1. 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 LÀ1 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.