CHENG Fangliang, YU Miao, JIA Linyuan, TIAN Qihang, ZHANG Jihong＊,Bokhyeon Kim, ZHAO Xiujian

(1. State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China; 2. Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Oryongdong, Buk-gu, Gwangju 500-712, Korea)

Abstract: Ultra-small PbSe quantum dots (QDs) were synthesized using conventional hot-injection method. A small amount of Sn was used as a nucleation promotion agent to control nucleation and growth during the QDs synthesis process. The average diameter of the QDs is about 1.6 nm, of which absorption peak centers at 550 nm and photoluminescence peak centers at 750 nm under 350 nm laser excitation with power as low as 500 µW. Transmission electron microscopy images confirm that the QDs size well matches with the calculated diameter from Brus equation. This match and electron energy loss spectroscopy analysis proves that Sn is not involved into the final structure of the ultra-small PbSe QDs. An ion-exchange process was proposed for the nucleation control and ultra-small QDs synthesis. The prepared ultra-small QDs could be a promising candidate for luminescence, solar cell devices, and others.

Key words: quantum dots; hot injection synthesis; photoluminescence; ion-exchange

1 Introduction

Lead chalcogenide (PbS, PbSe, and PbTe)semiconductor nanocrystals (quantum dots, QDs) have attracted considerable attentions for their potential applications in infrared photonic devices[1,2], biology[3],solar cells[4], and others[5], due to their unique optical properties originated from quantum confinement effect when the quantum dots sizes are smaller than the Bohr radii[6,7]. Among them, PbSe QDs possess narrow band gap (0.28 eV), large Bohr radius (46 nm), and strong quantum confinement effect, thus PbSe QDs can be a promising ingredient for above applications[8,9].Up to now, the synthesis methods, optical properties,and applications of PbSe QDs have been extensively investigated[10,11,12]. In general, the quantum dots can be synthesized by chemically hot injection methods and dispersed into organic solvent[13], or precipitated into glass matrix by thermal treatment-based method[14].The diameter of PbSe QDs was normally ranged from 3 to 10 nm, corresponding near infrared luminescence ranged from 1 to 2 µm[15,16]. It is also known that the photoluminescence efficiency of the QDs dramatically decreases with the QDs size further increases over 10 nm[17].

The relationship between band gap and QDs size can be described by Brus equation[18,19]. The smaller QDs size leads to the larger band gap, the shorter wavelength emission, and the higher quantum efficiency. Recent studies discovered that PbS[20]and PbSe QDs[21]could provide visible absorption and emission when their sizes decreased to smaller than 2 nm. In addition, small sized QDs could provide large open circuit voltages (Voc) and high photocurrent in QDs based solar cell, which resulted in improved conversion efficiency[21]. Therefore, finding practical preparation method and understanding optical properties of ultra-small PbSe QDs (＜ 2 nm) become very important and interesting issues.

The formation of QDs includes nucleation and nanocrystal growth processes[22]. The nanocrystal growth process can be controlled by synthesis temperature and duration, while the nucleation process is generally too fast to be controlled even though it’s necessary for ultra-small QDs formation[23]. In fact, fine control of nucleation process to form ultrasmall QDs is not easy work in conventional chemical synthesis process and heat-treatment method. Recently,several approaches have been investigated to solve this problem. Ultra-small PbS and PbSe QDs were synthesized by physical control including hot fluid quenching to interrupt nano-crystals growth[20,21]and low temperature synthesis[24]. It is thought that chemical control using a passivation agent during the nucleation process can be an alternative method to prepare ultra-small QDs. Recent research[25]proved that SnCl2addition could be effective method to promote nucleation, for small and highly photostable PbSe QDs.it provided the possibility of ultra-small QDs synthesis using chemical nucleation promoting.

In this paper, ultra-small PbSe QDs was synthesized using chemical nucleation control. Tin was used as a nucleation promotion agent. In results, the obtained ultra-small PbSe QDs diameters ranged from 1.2 to 2.0 nm and about 1.6 nm in average. Visible absorption around 550 nm and strong infrared emission band centered at 750 nm were observed. An ionexchange process was proposed for the nucleation and ultra-small QDs formation.

2 Experimental

2.1 QDs preparation

Ultra-small PbSe and regular-sized PbSe QDs were prepared by conventional hot-injection method using a homemade Shlenk line. Raw materials, lead acetate tri-hydrate (Pb(Ac)2·3H2O, 99.9%, high purity chemicals), diphynel ether (90%, Sigma Aldrich),oleic acid (OA, 90%, Sigma Aldrich), Tri-n-octyl phosphine (TOP, 99%, Sigma Aldrich), selenium powder (99.99%, high purity chemicals), Tin oleate(85%, Gelest), were used without further purification.2 mmol Pb(Ac)2·3H2O were weighed into a 3-neck round bottom flask, and dissolved into a mixture of 10 ml diphynel ether and 1.28 mL OA. As a nucleation promotion agent, extra 6mol% Sn-oleate (compare to Pb(Ac)2·3H2O), 78 µL) was added into the flask containing Pb(Ac)2·3H2O, diphenyl ether, and oleic acid. The solution was degassed at 80 ℃ for 1 hour to remove dissolved oxygen and moisture, then filled with high purity N2as protection atmosphere. Meanwhile,the TOP-Se as a Se precursor was prepared by mixing 0.02 mol Se powder and 20 mL TOP and stirring until the solution became clear, and Se powder was completely dissolved, in a N2atmosphere glove-box.The Pb precursor solution was heated to 120 ℃, then 5 mL Se precursor (TOP-Se) was quickly injected into the solution and maintained at the temperature for 2 minutes for the nucleation and growth of ultra-small PbSe QDs. Then, the solution was slowly cooled down to room temperature. The obtained QDs were purified at least 2 times using a high speed centrifuge after mixing with methanol and Tetrachloroethylene (TCE).Finally, the QDs were dispersed into 20 mL TCE. The obtained ultra-small QDs sample was designated as U-PbSe hereafter.As a reference, conventional PbSe QDs with regular size sample was prepared by the same procedure and parameters for U-PbSe QDs preparation and except Sn-Oleate addition above. The reference sample was named as C-PbSe QDs. In addition, one more sample was prepared with exact same process except the nucleation and growth duration extended to 5 minutes, to obtain larger sizes QDs, for clear elements distribution analysis using electron energy loss spectroscopy (EELS) mapping due to resolution limitation.

2.2 Characterization

The absorption spectra of colloidal U-PbSe and C-PbSe QDs in the range from 400 to 2 000 nm were recorded using a UV-Vis-Nir spectrophotometer (Jasco,V570). The photoluminescence spectra of the QDs were measured upon excitation using 350 nm UV laser diode with the output powder below 5 mW (for U-PbSe QDs) and 800 nm tunable Ti3+: sapphire laser with the output powder of 200 mW (for C-PbSe QDs).m focal length monochromator equipped with a photomultiplier tube detector (for U-PbSe QDs) or a liquid N2cooled InSb detector (for C-PbSe QDs), lock in amplifier, and mechanical chopper were used for the measurement. The instruments for absorption and PL measurement were calibrated in order to control the wavelength error within 5 nm. The transmission electron microscope (TEM) images were recorded to investigate the distribution of QDs sizes using a JEOL system (JEM-2200F). Carbon coated copper grid (400 mesh) was used for the TEM sample preparation. EELS mapping associated with the Pb (O2,3), Se (M4,5), and Sn (M4,5) shell electrons were measured using the same system, to analyze the distribution of each element in the specimens. A three-window technique[26]was used to visualize the distribution of Pb, Se, and Sn by investigating one post-edge image and two pre-edges images for background signal. Sizes of the windows were selected to avoid overlap of signals from different elements. All measurements were conducted at room temperature.

3 Results and discussion

The color of the Pb precursor solution for conventional PbSe QDs turned black immediately after Se-precursor (TOP-Se) injection. While the solution color with nucleation promotion agent (Sn-oleate) for U-PbSe QDs changed much slowly from light yellow,to yellow, brown, dark brown, then to black, after TOPSe was injected. It took longer than 3 minutes to finish the color changing process at the synthesis temperature(120 ℃). This slow color change indicated that this method could provide opportunity in controlling the speed of nanocrystal nucleation process to make ultrasmall PbSe QDs.

The measured absorption spectra for synthesized U-PbSe and C-PbSe are presented in Fig.1. The absorption peaks centers at 1 130 nm for C-PbSe QDs,whereas it is found that the absorption peaks center at much shorter wavelength of 550 nm for U-PbSe QDs.This visible absorption peak indicates that U-PbSe QDs have larger band gap than that of C-PbSe QDs.Inset compares the colors of the diluted PbSe QDs. It is observed that C- PbSe QDs is brown while U-PbSe QDs is dark red, which is consistent with the measured absorption spectra. From the measured absorption peaks, the average diameters of U-PbSe QDs and C-PbSe QDs were estimated using the simplified Brus equation[18,19,27]:

Fig.1 Absorption spectra of U-PbSe and its reference C-PbSe QDs. Inset shows the photo-images of the diluted colloidal QDs samples (left: U-PbSe QDs; right: C-PbSe QDs)

where,Eg(D),Eg(∞), andDare the band gap of QDs,the bulk PbSe bandgap (0.28 eV), and the diameter of QDs, respectively. The estimated diameters for U-PbSe and C-PbSe QDs are 1.6 and 3.7 nm, respectively.

The photoluminescence spectrum from U-PbSe QDs and C-PbSe QDs is shown in Fig.2(a). In the C-PbSe QDs, emission under 800 nm excitation is found at a conventional near infrared wavelength range centered at 1210 nm and the excitation laser power is 200 mW. In the case of U-PbSe QDs, on the other hand,emission band is found at shorter wavelength centered at 750 nm upon 350 nm laser excitation. The excitation power is as low as 500 µW.

Fig.2 Photoluminescence spectrum of U-PbSe and C-PbSe QDs.U-PbSe was excited by a 350 nm UV laser at 500 µW,while C-PbSe was excited by 800 nm NIR laser at 200 mW(a); Photoluminescence spectra of the U-PbSe QDs with the different excitation powers of the 350 nm laser in the range from 0.5 to 3 mW(b); the relationship between excitation power and luminescence intensities(c)

The emission intensity increases with excitation power increase, while the emission band positions do not change due to specific quantum dots size (Fig.2(b)).Moreover, the fitting line from the relationship between excitation power and emission intensity (Fig.2(c))shows the threshold excitation power for U-PbSe QDs is as low as 300 µW, which is obtained by prolong the fitting line, that indicates the lowest excitation power that ultra-small quantum dots may be excited. The low power emission property indicates that U-PbSe QDs have high luminescence efficiency and this result is consistent with that smaller QDs have strong quantum confinement effect[28]. It is worth to note that the Stokes shift for U-PbSe QDs is 200 nm and much larger than that of PbSe QDs(80 nm). It is well known that Stokes shift originates from the surface defect, dangling bond,and others in QDs, thus the large Stokes shift can be made in the U-PbSe QDs due to the large amount of un-passivated surface atoms resulted from the large surface area to volume ratio of extremely small QDs[29,30].

Fig.3 shows the TEM images and the particle size distribution diagrams of C-PbSe QDs and U-PbSe QDs, respectively. As shown in the TEM images,nanoparticles are well mono-dispersed in all samples.In the size distribution diagram of C-PbSe QDs, the particle size is in the range of 3.2 to 4.4 (±0.4) nm, and the average diameter is 3.8 ± 0.4 nm. As a comparison,the diameter calculated from the Brus equation is 3.7 nm. Whereas in the case of U-PbSe QDs, the particle size is distributed in the range, 1.2 to 2.0 (±0.2) nm,and the average diameter is 1.6 ± 0.2 nm, which is well match the calculated result (1.6 nm) from the Brus equation.

Fig.3 TEM images and particle size distribution diagrams of(a) U-PbSe and (b) C-PbSe QDs

In order to understand the origin of tin oleate addition on the formation of ultra-small nanoparticles,compositional investigation of PbSe QDs was carried out using EELS analysis. Fig.4 represents the EELS mapping images of U-PbSe synthesized with longer synthesis duration (5 min, with diameter about 5 nm)and C-PbSe QDs dispersed samples. It is noted that the blue stands for high concentration, while red indicates low, and the background of Sn distribution image is enhanced for clear Sn analysis. Only Pb and Se are found from C-PbSe QDs (Fig.4(a)). For U-PbSe QDs,Pb and Se are also concentrated at the QDs positon,while Sn is difficult to be found (Fig.4(b)). In addition,the consistent of calculated sizes using PbSe band gap and measured diameters also indicate that obtained ultra-small QDs is PbSe, no Sn elements contains in the QDs structure. All these evidences prove that only PbSe QDs form after hot-injection process. The addition of Tin-oleate only controlls the QDs nucleation and growth processes, finally determines the QDs sizes.

Fig.4 Electron energy loss spectra (EELS) mapping images of Pb(O2,3), Se (M4,5), and Sn (M4,5) shell electrons for C-PbSe QDs(a) and U-PbSe QDs synthesized with extended duration (5 min)(b) with their reference TEM images

According to the theories[6,18,27], the quantum confinement effect of QDs is strongly influenced by the sizes. In general, small QDs sizes generate stronger quantum confinement effect. The relationship between QDs diameters and the QDs bandgap is shown in Fig.5, the line is originated from equation (1), while the dots represent synthesized U-PbSe and PbSe QDs,red cubic for U-PbSe and black circle for C-PbSe.The whole region can be manually divided into high,mediate and low effect region. When the QDs diameter is smaller than 2 nm, the QDs bang gap is sharply dependent on QDs diameters. After that the diameters dependency on QDs bandgap decrease in mediate range. In fact, the QDs bandgap almost has no change after the QDs diameters are larger than 10 nm. The diameters of U-PbSe QDs is in the highly effect region.In additional, the study of PbSe QDs sizes dependency on quantum yield indicate that smaller QDs generate higher efficiency[28]. Therefore, the synthesized U-PbSe QDs could be excited by 350 nm laser with very low powder even below than 500 µW. The center of luminescence bands do not shift, only emission intensity increases, with excitation power increase(Fig.2 (b)), since the QDs size has no change.

Fig.5 The relationship between PbSe QDs diameters and band gap, from Brus equation. The cubic dot represented U-PbSe QDs, while circle dot for C-PbSe QDs synthesized at 120℃ for 2 minutes

Since the addition of Sn plays important role in the formation of U-PbSe QDs, and there is no Sn found in final U-PbSe product (Fig.4), the formation process of U-PbSe could be proposed. Zhanget al[25]studied the addition of Sn into raw material could promote the nucleation and obtain smaller and more stable PbSe QDs, using SnCl2as Tin precursor. In general, PbSe QDs formation process, the Se reacted with Pb immediately after the injection of TOP-Se,as Se precursor. Small number of PbSe nuclei (1-2 nm) formed quickly with small number, and grow at synthesis temperatures and durations, as shown in Fig.6(a). The nucleation process is a quick process and difficult to control. But ultra-small QDs still could be formed by external parameters change, such as synthesis temperature and duration[23]. Therefore, ultrasmall QDs were synthesized by quenching reaction fluid[20], and decrease reaction temperature[24]. When Snoleate was added into the raw materials, [SnSex] group was firstly formed due to the much larger reactivity of Sn in metal source fluid and Se in TOP-Se precursor than that of Pb in metal source and Se. In general,higher reactive energy precursor made it easier to form small QDs due to large number of nuclei[31]. Therefore,small, but large amount of SnSe nuclei formed, along with possible [SnSex] group. The further growth was interrupt because of its low concentration in solution.The fluid kept clear and light yellow for the wide bandgap of SnSe (0.9 eV indirect and 1.3 eV direct) and extremely small nuclei sizes. Because Pb concentration in solution was much higher than Sn, an ion exchange process could be proposed. Pb ions slowly substituted Sn ions in SnSe nuclei and [SnSex] groups, until Sn was completely returned to the solution, at 120 ℃.PbSe nuclei were formed, and continuously grow.The fluid color slowly turn red. Ultra-small PbSe QDs can be obtained by stop the reaction, cooling down the reaction to room temperature (Fig.6(b)). More experimental evidences and detailed process are being investigated.

Fig.6 The schematic of proposed (a) C-PbSe QDs and (b) U-PbSe QDs formation process

4 Conclusions

In this paper, ultra-small PbSe QDs as small as 1.6 nm were fabricated via conventional hot injection method, and Sn addition for nucleation control.Visible absorption peak centered at 550 nm, while luminescence band centered at 750 nm, while the absorption and luminescence of normal PbSe QDs synthesized without Sn addition centered at 1 130 nm and 1 210 nm respectively. The ultra-small QDs could be excited by UV laser with low power and luminescence bands center did not change with excitation power increase. Measured PbSe QDs from TEM images and diameters diagram well matched with calculated results from Brus equation. In addition,elements distribution mapping from EELS analysis confirmed the formation of PbSe QDs, without Sn in QDs. An ion exchange process between Sn and Pb was proposed for the nucleation process control and formation of ultra-small PbSe QDs. The synthesized ultra-small QDs can be promising material for light source and soar cell.