Synthesis of nano-sized PbSe from octeno-1,2,3-selenadiazole
Introduction
Semiconductor nano-particles (quantum dots) have varied application in areas, such as luminescent devices, biological markers, lasers and catalysis [1] including light emitting diodes and in telecommunication as well as in nano-electronics, photonics and photovoltaics [2], [3], [4].
The electron, hole and exciton Bohr radii of PbSe are 18, 23 and 46 nm, respectively [5]. Because of the larger Bohr radii of PbSe than CdSe, it offers scope for strong quantum confinement. Materials with pronounced quantum confinement effect would open avenues for band gap engineering with respect to particle size, surface morphology and other experimental conditions, such as time and temperature [6]. In PbSe nano-particles, energy gap between HOMO and LUMO can be tuned due to weak electron–phonon coupling [7]. In fact, tunable electroluminescence in the range 1.3–1.5 μm has been reported from PbSe quantum dots, thus, making them suitable candidates for near infra-red (NIR) applications [8].
Despite great advantage associated with PbSe, the research in this area is not so much extensive in comparison to metal and other semiconductor nano-particles. In recent time, efforts have been mostly limited to colloidal synthesis of PbSe nano-crystals but the efforts to fabricate morphologically different PbSe, such as nano-rods, cubes, pyramids and or cones are rather limited. It has been opined by researchers that rod like semiconductors would be better suited fore electronics application [4]. Of late, several methods of preparation of PbSe nano-particles have appeared in literature including single source precursors derived from organometallic reagents [9], [10], [11], [12], [13]. Surface capping of semiconductor particles has been described by use of suitable stabilizers, such as carboxylic acids, amines or phosphines [6], [14], [15]. Recently, organo-lead compounds have been described for obtaining nano-sized PbSe but not without contamination of selenium particles in the end product [12]. For preparation of surface capped metal selenides semiconductor particles, one normally uses freshly prepared selenide (TOPSe) as a source of selenium which can be synthesized by the reaction of Se in tri-octylphosphine (TOP) at about 100 °C for a couple of hours [6]. We have earlier reported direct synthesis of PbSe pyramids from the neat reaction of selenium, lead acetate and TOP [16]. Though the preparation of bulk lead selenide is rather simple [17], isolation of nano-particles often involves multi-stages handling. It would therefore be advantageous to have a simple one-pot method for their synthesis.
Synthetic chemistry of 1,2,3-selenadiazoles and release of selenium from them upon thermolysis has been well studied [18]. We have earlier described that reactions of cycloalkeno-1,2,3-selenadiazole with metal salts can lead to fine powders of semiconductors [19], [20], [21]. As an on-going efforts to our present research, it is our objective to appropriately extend the use of octeno-1,2,3-selenadiazole to synthesize the nano-crystals of semiconductors. Thus, selandiazole pre-combined with TOP would should essentially act as TOPSe in the current preparation of PbSe nano-crystals. Therefore, by the present method, one can avoid complex synthesis of single source precursors of lead, toxic hydrogen selenide gas and preparation of time consuming TOPSe from elemental selenium.
Section snippets
Experimental
Cycloocteno-1,2,3-selenadiazole was prepared by a method described elsewhere [18], lead acetate (anhydrous, 99.9%) and selenium powder (99%) were purchased from Qualigen India and were used as received. TOP was obtained from Sigma–Aldrich.
In a typical synthesis, lead acetate (1.75 g), oleic acid (6 mL) and diphenyl ether (25 mL) were heated at 140 °C for about an hour and to it, was added so-called Se-precursor (prepared by complete dissolution of desired quantity of octeno-1,2,3-selenadiazole in 12
Results and discussion
Thermal decomposition of cycloocteno-1,2,3-selenadiazole leads to generation of either free selenium or 1,4-diselenin due to dimerization of intermediate radical that is formed due to loss of a nitrogen molecule [18]. Availability of free selenium due to thermolysis of selenadiazole has been exploited in the present work. Thus, in situ generated selenium from octeno-1,2,3-selenadiazole reacts with lead ions via formation of a well known phosphine selenide (TOPSe) precursor reaction of metal
Conclusion
In the present study, we have shown that organo-selenium compound, i.e. octeno-1,2,3-selenadiazole with TOP is a useful precursor for synthesis of semiconductor nano-crystals. It is shown that the higher temperature results in completion of reaction and did not show the presence of lead-precursor but it also increases particle size. The optical studies show that the materials have bands in the near infra-red region. The NIR spectrum of PbSe indicates that the product has wide distribution of
Acknowledgment
We thank Executive Director, C-MET, Pune for encouragement and permission. This work was supported by DST, Govt. of India.
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