Synthesis of cubic CdSe nanocrystals and their spectral properties

Abstract

The cadmium selenide nanocrystals are prepared by colloidal chemistry under mild conditions. X-ray diffraction and high-resolution transmission electron microscopy measurements indicate that as-prepared cadmium selenide nanocrystals are zinc blende cubic structure. We carry out an analysis of quantum size effect in the Raman spectra of cadmium selenide nanocrystals performed by utilizing the chemical bond theory of Raman peak shift developed recently. It is revealed that the shifts of Raman peaks in cadmium selenide nanocrystals result from the overlapping of the quantum effect shifts and surface effect shifts. The sizes of the as-prepared cadmium selenide nanocrystals obtained by employing the Raman peak shift theory are in good agreement with the nanocrystal sizes determined by high-resolution transmission electron microscopy.

Introduction

In the past decade, photoluminescent colloidal cadmium selenide (CdSe) nanocrystals have drawn much attention due to their promising applications in light-emitting diodes, living cells’ fluorescent labels, photovoltaic devices and so on. The optical properties of CdSe nanocrystals are different from bulk materials due to quantum confinement effect. ^{1 –5} When the nanocrystals are small compared to the exciton Bohr radius (5.4 nm) of CdSe nanocrystals, interesting size-dependent optical properties arise in CdSe nanocrystals. The marked experimental effect of the quantum confinement is the blue shift of the absorption edge. ² So far, many literatures are available on relatively large CdSe nanocrystals, while reports on smaller CdSe nanocrystals are relatively in deficiency. ^6,7 For typical crystalline CdSe, Raman spectral features of nanocrystals were considerably different from the corresponding pattern and features. ^8,9 Studies on the vibrational properties of nanocrystals are focused on an understanding of the fundamental physical properties of strongly confined phonons. ^{10 –13} The Raman spectra of bulk CdSe are dominated by the longest wavelength longitudinal optical (LO) phonons. In bulk CdSe, the LO phonon mode generally appears as a narrow feature at 212 cm⁻¹. ¹⁴ The fundamental LO phonon of CdSe nanocrystals shows a similar feature, shifted several wave numbers to lower frequency from the bulk value induced by size-dependent effect. ¹⁵ In this work, zinc blende CdSe nanocrystals with diameters down to the exciton Bohr radius were prepared, and Raman spectra of CdSe nanocrystals were obtained. A new theoretical method for the quantum confinement effects on the Raman spectra developed recently ¹³ was employed to determine Raman spectra of CdSe nanocrystals. It is found that the shift of Raman spectra in cubic CdSe nanocrystals could be result from two overlapping effects: the quantum effect shift and the surface effect shift.

Materials and methods

CdSe nanocrystals were obtained by heating the mixture of cadmium oxide and stearic acid. The details of the growth technique are given in the study by Zhu et al. ¹⁶ and Yu and Peng. ¹⁷ Chemicals were purchased from Beijing Chemical Reagent Company (Beijing, China). X-ray diffraction (XRD) patterns were recorded using powder XRD measurements (Dmax-2500/PC) using copper K _a radiation (k = 1.5406 Å). High-resolution transmission electron microscopy (HRTEM) of CdSe nanocrystallites was carried out using a JEOL 2010 high-resolution transmission electron microscope operated at 200 kV. Absorption spectra were recorded using a WFZ-26A UV–vis spectrophotometer. Our sample was characterized by adding DMSO until 3-mL aliquots, and DMSO was used as the reference.

A standard quartz cell (Fisher) with a 10-mm path length was used and rinsed with DMSO (dimethyl sulfoxide) before each run. The Raman spectra of the samples were measured using ‘inVia’ Renishaw’s micro-Raman spectrometer with the 514.5-nm lines of the Ar⁺ laser as the excitation source. The spectra were collected in the backscattering geometry using the 50× Leica objective (NA = 0.78) and normalized on a spectral sensitivity of the inVia Renishaw measured with a black-body radiation unit.

Results and discussion

Figure 1 shows selected HRTEM images of the CdSe nanocrystals. The HRTEM observation (JEOL-JEM 2010 operated at 200 kV) show the spacing between adjacent lattice planes is about 0.35 nm, corresponding to the distance between (111) planes of zinc blende CdSe nanocrystals. Good crystallinity can be indicated by the clear lattice plane observations on the HRTEM. Four as-prepared samples shown in Figure 1(a) to (d) are denoted by CdSe<1>, CdSe<2>, CdSe<3> and CdSe<4>, respectively. However, the dispersion for nanocrystals in a sample of CdSe<1> is not homogeneous. The average diameters of CdSe<1>, CdSe<2>, CdSe<3> and CdSe<4>, as measured by HRTEM, are 1.8, 3.9, 4.7 and 5.2 nm, respectively. The relative mean deviation size of nanocrystals is about 18%.

Figure 1.

HRTEM of the synthesized samples. (a) CdSe<1>, (b) CdSe<2>, (c) CdSe<3> and (d) CdSe<4>. HRTEM: high-resolution transmission electron microscopy. CdSe: cadmium selenide.

XRD patterns for four as-prepared samples are shown in Figure 2. The diffraction peaks corresponding to the (111), (220) and (311) lattice planes of the CdSe match well with those of the zinc blende CdSe peaks. These nanocrystals have a zinc blende structure but not a wurtzite structure. The peaks are broadened owing to the finite size of the nanocrystals.

Figure 2.

XRD pattern of the synthesized samples. (a) CdSe<1>, (b) CdSe<2>, (c) CdSe<3> and (d) CdSe<4>. XRD: X-ray diffraction; CdSe: cadmium selenide.

Figure 3 shows a series of room temperature absorption spectra for CdSe nanocrystallites ranging from 1.8 nm to 5.2 nm in diameter. CdSe absorptions are shifted dramatically from 716-nm bulk band gaps. ¹ The optical gaps of CdSe<1>, CdSe<2>, CdSe<3> and CdSe<4> nanocrystals are 2.59, 2.0, 1.93 and 1.89 eV, respectively, exceeding the value of the bulk CdSe band gap (E_{g Bulk} = 1.74 eV). All four samples clearly present the effect of quantum confinement, which can be quantitatively explained using a recent chemical bond model. ¹⁸ From Figure 4, it can be seen that the experimental results are in good agreement with calculated values. The effective mass models based on the exciton confinement are found to indeed overestimate the band gaps.

Figure 3.

UV−vis spectra of as-prepared CdSe nanocrystals. CdSe: cadmium selenide.

Figure 4.

Experimental and calculated energy-gap shifts of CdSe nanocrystals. CdSe: cadmium selenide.

Figure 5 shows Raman spectra of different sizes of CdSe nanocrystals in the range of 150–450 cm⁻¹. Raman spectra of these samples illustrate that as the grain size decreases, the Raman peaks shift to lower wave number. Figure 5(a) shows relatively weak Raman peaks near 202.4 cm⁻¹, indicating that the PL background is a very serious problem in the case of highly luminescent colloidal nanocrystals. The main peak at 202.4 cm⁻¹ can be reliably assigned to scattering by the LO phonons in the CdSe<1> sample. In Figure 5(b) to (d), the Raman spectra consist of a strong mode centred at 207.7–208.9 cm⁻¹, arising from the LO phonon with a weaker mode arising from the second-order LO phonon appearing at about 415 cm⁻¹.

Figure 5.

Raman spectra of the four samples. (a) CdSe<1>, (b) CdSe<2>, (c) CdSe<3> and (d) CdSe<4>. CdSe: cadmium selenide.

It is known that the bulk value of the CdSe LO phonon frequency is 212 cm⁻¹. Generally, the difference between the measured values and bulk may be attributed to a phonon confinement effect observed in a wide range of semiconductor nanocrystal. To explain these observations, Richter et al. proposed a phenomenological model using a relaxation of the wave-vector selection rule. ¹⁹ The modified model by Faraci et al. ²⁰ showed better agreement with the values in the smaller nanocrystals. More sophisticated techniques such as bond polarizability approximation were also used to investigate the Raman spectra nanocrystals. ²¹ Recently, we proposed a new method from chemical bond point of view. According to theory, ¹³ the shift of Raman spectra in nanocrystals can be result from the quantum effect and the surface effect. The size-dependent Raman shift of nanocrystal is expressed as

$\Delta \omega \left(D\right)=\omega \left(\infty \right)-\omega \left(D\right)=\left(\frac{m_e{e}^4}{2{ħ}^2}\right){\left(4N\right)}^{-\frac{1}{3}}\left(\frac{Q{m}_e}{M_p}\right)+F^{\text{surface}}\omega \left(\infty \right)\left[{\left(\frac{d^{\text{surface}}}{d^{\text{bulk}}}\right)}^2-1\right],\left(\text{eV}\right)$

1

where ω(∞) is the Raman peak position of bulk and ω(D) is the size-dependent Raman frequency. N is the number of atoms in nanocrystals, m_e is the electronic mass and M_p is the mass of proton. Q = F ^surface Q ^surface +(1 − F ^surface)Q ^bulk. F ^surface is the fraction of surface bonds composing the nanocrystal, Q ^surface and Q ^bulk are an effective number of electrons bounding in a bond for surface and bulk molecule, respectively. d ^surface is the bond length of surface atoms and d ^bulk is the bond length of bulk. Using equation (1), we calculate the Raman shifts of cubic CdSe nanocrystals, which are shown in Figure 6. For comparison, wurtzite CdSe nanocrystal is also listed in Figure 6. It can be seen from Figure 6 that our measured Raman peak positions for as-prepared CdSe<1>, CdSe<2>, CdSe<3> and CdSe<4> are in agreement with the calculation values for zinc blende CdSe nanocrystals.

Figure 6.

Raman peak position as a function of the nanocrystal diameter for zinc blende and wurtzite CdSe nanocrystals. CdSe: cadmium selenide.

The detailed parameters for the spherical CdSe nanocrystals are listed in Table 1. From Table 1, it can be seen that the atoms at the surface of the nanocrystals have a lower average coordination number N_c than that of the bulk. The shift of Raman spectra in ultrasmall CdSe nanocrystals can be separated into the quantum effect shift and surface effect shift. The calculated quantum effect shifts in the 1.8, 3.9, 4.7 and 5.2-nm CdSe nanocrystals are 7.74, 3.69, 3.09 and 2.79 cm⁻¹, respectively. The calculated surface effect shifts in the 1.8, 3.9, 4.7 and 5.2-nm CdSe nanocrystals are 2.62, 0.72, 0.55 and 0.47 cm⁻¹, respectively. The calculated proportion of surface effect shift in Raman shift, η, is ranged from 14% to 25% for various sizes of CdSe nanocrystals. As a result, the contribution of surface effect shift in the ultrasmall CdSe nanocrystals is so great that cannot be ignored. The calculated sizes for as-prepared samples are 1.9, 4.0, 4.0 and 5.2 nm using equation (1), which can be applied to determine the size of nanocrystals according to the study by Gao et al.¹³ It shows good agreement between the crystallite sizes evaluated from the Raman peak shift theory and those determined by HRTEM.

Table 1.

Theoretical parameters for the spherical nano CdSe.

D (nm)	N		nb	d (Å)	Nc	Δω (cm−1)	η (%)	ω(D)cal. (cm−1)	ω(D)expt. (cm−1)
1.8	123	Inside	118	2.62	4	7.74	25	201.6	202.4
		Surface	78	2.63	2.44	2.62
3.9	1130	Inside	1630	2.62	4	3.69	16	207.6	207.7
		Surface	402	2.63	2.58	0.72
4.7	1928	Inside	2932	2.62	4	3.09	15	208.3	207.7
		Surface	600	2.63	2.63	0.55
5.2	2616	Inside	4102	2.62	4	2.79	14	208.7	208.9
		Surface	726	2.63	2.61	0.47

CdSe: cadmium selenide; N: number of atoms; n_b : number of bonds; N_c : average coordination number; η: proportion of surface effect shift in Raman shift; ω(D)_cal.: calculated value of Raman peak positions; ω(D)_expt.: experimental value of Raman peak positions.

Conclusion

In summary, cubic CdSe nanocrystals are synthesized. The UV–vis absorption spectra of these CdSe nanocrystals indicate a shift to higher energy of the band gap, as the particle size is lowered. The effect of quantum confinement is quantitatively explained using a chemical bond model. The Raman spectra of the CdSe nanocrystals are obtained. For comparison, a Raman peak shift theory is employed to determine Raman shifts. The calculated quantum effect shifts in the 1.8, 3.9, 4.7 and 5.2-nm CdSe nanocrystals are 7.74, 3.69, 3.09 and 2.79 cm⁻¹, respectively. The calculated proportion of surface effect shift in Raman shift is ranged from 14% to 25% for various sizes of CdSe nanocrystals. The calculated sizes of nanocrystals by Raman shifts are in good agreement with observed values.