Size quantization effect in water-dispersible LEEH capped ZnSe nanocrystals

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Abstract

Herein we report the synthesis of LEEH capped water-dispersible ZnSe (Zinc Selenide) nanocrystals using a low temp (80 °C) solution processable chemical route. The reaction parameters such as reaction time, temperature, concentrations of reactants and capping agent were tuned to achieve water dispersible ZnSe nanocrystals of average size, 2 nm with improved photophysical properties. The structure, size and optical properties of the nanocrystals are evaluated from XRD, HRTEM, optical absorption and photoluminescence studies. Both HRTEM and XRD analysis indicate polycrystalline cubic phase of ZnSe with size in the range of 2.0 nm. A strong excitonic absorption peak appearing at 385 nm in the absorption spectrum suggests the size quantization effect in ZnSe nanocrystals marked by a large blue shift from its bulk band gap. Photoluminescence of ZnSe nanocrystals at 300 K exhibit multiband spectral feature appearing with a low intensity peak at 414 nm leading to a large nonresonant stokes shift by 29 nm.

Introduction

Synthesis of nanocrystal semiconductors with controlled size, shape, chemical composition and choice of surface ligand have become challenging in nanoscience and nanotechnology due to their unique properties [1], [2], [3] arising due to size quantization effect(SQE) for size less than or comparable to Bohr exciton radius (a0). SQE associated with semiconductor nanocrystals results in discretization of electronic energy levels there by widen the band gap, increases the oscillator strength and reduces the radiative recombination life time of carriers from a few nano- to few picoseconds and paves the way for many applications in the areas of laser, sensor, light emitting diode (LED), photovoltaic and bio-medical applications [4], [5], [6]. The selection of appropriate semiconductor is not only an important criterion to achieve the required applications but also the right synthesis methodologies to attain required size, stability and electro-optical properties. Most studied nanocrystal semiconductors belong to the II-VI group of materials because of their relatively ease of sample synthesis procedures [7], [8], [9]. Although the II–VI group nanomaterials can be synthesized by different techniques [10], [11], [12], [13], [14], [15], [16] such as microemulsions, thermal decomposition, hydrothermal and solvothermal synthesis, microwave synthesis, phase-transfer method and many more but the synthesis of solvent processable, low temperature aqueous synthesis of nanomaterials is attractive because such aqueous based synthesis procedures are less toxic and can reach the targeted requirements [17] by easily controlling the synthesis parameters. ZnSe belongs to the II-VI group of semiconductors with bulk band gap as 2.68 eV [18] which has remarkable electro-optical applications [19]. Large surface to volume ratio in semiconductor nanocrystals give rise to a large number of surface trap sites generated by unsaturated dangling bonds which needs to be passivated to attain charge neutrality conditions so that recombination losses at the surface can be reduced. Surface passivation of semiconductor nanocrystals is generally achieved by capping either with hydrophobic or hydrophilic organic ligands depending upon specific applications. Passivation [12], [13] by hydrophobic organic surfactants such as trioctylphosphine(TOP), trioctylphosphine oxide(TOPO), and thioglycerol are often not compatible with biological applications because of toxicity problems. Zn-based chalcogenides (S, Se & Te) semiconductor nanocrystals synthesized [20], [21], [22], [23], [24] using TOP and TOPO often shows toxicity due to the presence of phosphene. On the other hand TOPO is less reactive to Zn-salts [25]. Semiconductor nanocrystals capped with hydrophilic [15] biomolecules such as starch, proteins, and nucleic acid enables selective interaction with target molecules or biochemical species which can lead to many biological applications. Thus, thiol containing aminoacid i.e 2-amino-3-mercaptopropionic acid (L-cystein ethyle ester hydrochloride (LEEH)) is expected to be a better capping agent for low temperature ZnSe synthesis because of its direct interaction of thiol group with Zn2+ ion. In addition, with LEEH, (i) the energy levels of thiol act as inhibitors for hole-trapping processes associated with the Nanomaterials [26] and (ii) sulphur containing amino acids have the electron mediating abilities leading to potential biological functions through a metal-sulphur linkage.

In this work, we report the synthesis of LEEH capped ZnSe nanocrystals using a bottom up aqueous chemical route at relatively low temperature, 80 °C. Structural characterization by glancing angle X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) show cubic phase polycrystalline ZnSe nanocrystals. The importance of this work is that relatively stable and very small ZnSe nanocrystals with size, 2.0 nm can be prepared by the present techniques which show optical activities in UV region and expected to have applications in biological systems.

Section snippets

Chemical

Zinc acetate (99.99%) & selenium powder (99.9%), and Sodium borohydrate (NaBH4) & LEEH (99%) were purchased from Aldrich and Fluka respectively were as such used for sample synthesis without further purification. Millipore water was used as the solvent for preparing the aqueous salt solutions.

Synthesis

As shown in Figure 1, the synthesis of capped ZnSe was carried out in a single step chemical reaction at 80 °C. 10 ml of aqueous stock solution of NaBH4 (1 M) was prepared to which Selenium powder (0.5

Results and discussion

A typical; X-ray diffraction pattern of ZnSe sample is shown in Fig. 2 which exhibits dominant (1 0 1) reflection at the scattering angle, 2θ= 31.91° along with three other low intense (0 0 2), (1 1 0) & (2 0 2) reflections corresponding to their respective scattering angles 27.63, 43.66 & 60.45° which suggests that the ZnSe nanocrystals exhibit polycrystalline cubic phase[27]. The structure and phase of the samples were confirmed after comparing the lattice spacing(d) values with those of the standard

Conclusion:

The synthesis of water dispersible LEEH capped ZnSe nanocrystals prepared at 80oC using a nontoxic chemical method is reported. The ZnSe nanocrystals showed sharp absorption marked by the excitonic peak at 385 nm. A blue shift of 0.52 eV compared to its bulk band gap has been observed for ZnSe nanocrystals as a result of size quantization effect. Structural and size of the ZnSe nanocrystals were obtained from XRD and HRTEM measurements exhibited polycrystalline cubic phase of the sample with

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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