CdTe nanocrystals were synthesized by the hot-injection method with a mixture of oleylamine and octadecene as a solvent. The influence of the composition of the solvent and of the injection solution on the shape of CdTe nanoparticles was investigated. Various shapes of CdTe nanocrystals, such as nanodots, nanorods, multipods, and nanowires, could be obtained by changing the reaction conditions. Tuning the reactivity of both the cadmium and the tellurium precursors at the same time was found to be the main reason for the shape control of CdTe nanocrystals in this reaction system. The reactivity of the Cd precursor was controlled by the composition of the solvent, while the activity of the Te precursor could be influenced by using trioctylphosphine and tributylphosphine in the injection solution.
CdTe is a direct semiconductor with a bandgap in the bulk of 1.475 eV and a Bohr exciton radius of 7.3 nm. CdTe nanocrystals attract interest due to their pronounced quantum size effect and optical activity both in the visible and near-infrared spectral regimes. This makes them interesting candidates for a variety of applications, for example, in solar energy conversion [
In the hot-injection method, monomer activity during the nucleation and growth processes plays a key role for shape control of nanocrystals. Several parameters influence the monomer activity, like precursor concentration, precursor activation, and ligands effect. Ligands as an important part in the synthesis of nanocrystals influence the monomer activity in several ways. Yu et al. [
All chemicals were used as received without further purification: cadmium oxide (CdO, 99%, Fluka), tri-n-octylphosphine (TOP, 90%, Aldrich), tri-n-butylphosphine (TBP, 90%, Aldrich), oleic acid (OA, 90%, Aldrich), 1-octadecene (ODE, Merck), tellurium powder (Te, purum p., Fluka), and oleylamine (OLAM, approximately C18-content 80–90%, Acros Organics).
The general procedure for preparing Te precursor solutions is as follows. Te was added to TOP, TBP, or the mixture of them in the glove box. The mixture was further heated to 200°C under nitrogen flow till the dissolution of Te. The Te injection solution was obtained by diluting this Te solution with OLAM or ODE (Table
The composition of the Te precursor solutions.
Injection solutions | Te (g) | TOP (g) | TBP (g) | Te solution (g) | OLAM mL | ODE mL |
---|---|---|---|---|---|---|
Te-TOP-OLAM | 0.5 | 4.5 | 0 | 0.0638 | 2 | 0 |
Te-TOP-ODE | 0.5 | 4.5 | 0 | 0.0638 | 0 | 2 |
Te-TBP-OLAM | 0.5 | 0 | 4.5 | 0.0638 | 2 | 0 |
Te-TBP-ODE | 0.5 | 0 | 4.5 | 0.0638 | 0 | 2 |
Te-TOP-TBP-OLAM | 0.7018 | 6.5428 | 2.1054 | 0.1190 | 2 | 0 |
Te-TOP-TBP-ODE | 0.7018 | 6.5428 | 2.1054 | 0.1190 | 0 | 2 |
A mixture of CdO (0.1 mmol), OA (0.16 mL), and OLAM (10 mL) was degassed at 100°C for 1 hour in a 50 mL three-necked flask connected to a condenser. The mixture was heated slowly under nitrogen until most of the CdO decomposed at ~290°C, then another 0.2 mL OA was added to form a clear and colourless solution. Te precursor solution (including 0.05 mmol tellurium) was injected quickly at 300°C. The temperature dropped to 260°C and maintained this value throughout the synthesis. All syntheses were stopped after 5 minutes by removing the heating mantle and by rapidly cooling the flask. Aliquots were taken out at different reaction times to monitor the reaction process by measuring the UV-Vis absorbance and TEM images.
All manipulations were performed using standard air-free techniques. A mixture of CdO (0.1 mmol), OA (0.16 mL), and OLAM + ODE (10 mL) was degassed at 110°C for 40 minutes in a 50 mL three-necked flask connected to a condenser. The mixture was heated slowly under nitrogen until most of the CdO decomposed at ~290°C, then another 0.2 mL OA was added to form a clear and colourless solution. A Te-TOP-TBP-ODE (0.124 mL, Cd/Te = 10 : 7) precursor solution was injected quickly at 300°C. The temperature dropped to 260°C and was kept at this value throughout the synthesis. Samples were taken out at different reaction times between 1 and 5 minutes.
UV/Vis spectra were recorded using a Carry 100 absorption spectrophotometer. The samples were dispersed in 1 cm path length quartz cells filled with hexane. The absorbance values were used to calculate the molar particle extinction coefficients, the NCs diameters, and their concentration.
The size and morphology of the nanocrystals were studied with a Zeiss EM 902 A transmission electron microscope with an accelerating voltage of 80 kV. TEM specimens were prepared by putting a 10
Powder X-ray diffraction (XRD) was measured with a PANalytical X’Pert PRO MPD diffractometer operating with Cu K
In this work, we investigated the influence of the reaction conditions on the shape of CdTe nanoparticles, which were synthesized by a hot-injection method in reaction of Cd-oleate with trialkylphoshine telluride in OLAM as solvent. OLAM is a common organic compound which can be surfactant and solvent at the same time. OLAM reacts with OA to form an amide in the presence of metal ions [
Various shapes of CdTe nanocrystals, such as nanodots, nanorods, multipods, and nanowires, can be synthesized in pure OLAM solution, only by changing the composition of the tellurium injection solution (Figure
TEM images of CdTe NCs synthesized from different tellurium precursor solutions. (a) Te-TOP-OLAM; (b) Te-TOP-TBP-OLAM; (c) Te-TBP-OLAM; (d) Te-TOP-ODE; (e) Te-TOP-TBP-ODE; (f) Te-TBP-ODE.
Comparison between the upper and the lower panel in Figure
The use of TOP and TBP as ligands in tellurium precursor solutions is a good way to control the activity of the Te monomers. TOP and TBP are similar organic compounds, with C8 chains in TOP and C4 chains in TBP. Ligands with shorter carbon chains have a higher diffusion coefficient, which results in a higher monomer activity, compared to compounds with longer carbon chains. Furthermore, TOP-Te and TBP-Te differ in the strength of their Te-P bonds; TOP-Te being more stable. Thus, TBP-Te is more reactive than TOP-Te. In spite of that, the results of reaction with TBP-Te and TOP-Te show some similarities; in OLAM both Te-precursors lead to the formation of nanorods, while the growth of quasispherical particles can be observed in ODE. However, a closer look at the development of the reaction shows that the underlying mechanisms are different. We compared the absorption onsets of aliquots taken during reactions with Te-TOP-ODE and Te-TBP-ODE (Figure
Absorption onsets of CdTe NCs synthesized with different tellurium precursor solutions: Te-TOP-ODE and Te-TBP-ODE.
In reactions with Te-TOP-TBP, two Te precursors with different reactivities are present in one reaction. The more reactive Te-TBP precursor should react first and be preferentially involved in the seed formation process. The amount of monomers used up during this growth stage will be smaller, than in the case of pure Te-TBP precursor, because of the lower concentration of Te-TBP. Thus, the chemical potential of the solution remains relatively high, and also enough monomers are left for further one-dimensional growth of the nanocrystals. In the reactions with a more reactive Cd precursor (in pure OLAM), this leads to the formation of nanowires, while a slightly lower Cd-precursor reactivity (with ODE in the injection solution) result, in the formation of tetrapods.
With TOP as ligand and OLAM as solvent in tellurium precursor solution, uniform CdTe nanorods were synthesized. Their growth was studied in more detail by TEM (Figure
TEM images and histogram of CdTe nanorods at different growth time. (a) 3 min; (b) 6 min; (c) 8 min.
In this section, we investigated the possibility to influence further the morphology of CdTe tetrapods. We used a Te-TOP-TBP-ODE injection solution, which turned out to be suitable to synthesize uniform tetrapods (see Figure
Figure
TEM images of CdTe NCs synthesized with different OLAM : OA ratio. (a) No OLAM; (b) OLAM : OA = 1; (c) OLAM : OA = 4; (d) OLAM : OA = 20.
Figure
XRD pattern of CdTe NCs synthesized from OLAM : OA = 4 (TEM image in Figure
In conclusion, we demonstrated that various shapes of CdTe nanocrystals could be synthesized by controlling the composition of the solvent and of the tellurium precursor solution. Tuning the reactivity of both the cadmium and the tellurium precursors is the main reason for the shape control of CdTe nanocrystals in such a simple system. The activity of the Cd precursor can be controlled by the concentration of OLAM in the solvent, while the use of TOP, TBP, or a mixture of both phosphines in the tellurium injection solution turned out to be a suitable way to control the reactivity of the Te monomers. Changing the reactivity of the Cd and Te precursors at the same time opens up the possibility to influence the seed formation and the growth process of the CdTe nanocrystals and, thus, to control their shape.
The authors would like to thank Erhard Rhiel and Heike Oetting (University of Oldenburg) for assistance in obtaining TEM images and Marta Kruszynska (University of Oldenburg) for taking the XRD data. They gratefully acknowledge funding of the EWE Research Group “Thin Film Photovoltaics” by the EWE AG, Oldenburg.