Semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have been intensively investigated due to their well-known size dependent effects and solution processability, resulting in a big family of functional nanomaterials with tunable optical properties for many cutting-edge technologies, including display, lighting, photodetection, solar cells, etc1,2. In the field of display technology, CdSe and InP based nanocrystals have attracted significant attentions from scientific and industrial communities3–6. In particularly, they are considered as suitable printing inks for fabricating electroluminescence display panels7–8. Blue emitter with peaks of 455–475 nm is an urgent need to open up the industrialization of nanocrystals-based printing display technology9. ZnSe nanocrystals has been considered as potential blue emitters in view of its suitable band-gap (2.7 eV)10–13. Owing to the quantum confinement effect, the fabrication of large-sized ZnSe nanocrystals is of vital importance to tune their emission into pure blue region.
After more than 30 years’ efforts, a thorough investigation of nanocrystals synthesis resulted in the state-of-the-art CdSe nanocrystals as well as the corresponding nucleation and growth mechanisms14–21. Nevertheless, there are few reports on CdSe nanocrystals with average size over 10 nm22. Even applying a multiple precursor injection method, the average size of giant CdSe based core-shell nanocrystals was still limited to 20 nm23–25. With the increase of nanocrystal’s size, surface chemical potential decreases due to the reduction of surface to volume ratio19. Under high concentration or multiple injection conditions, monomers tend to grow epitaxially on crystal planes with higher chemical potential26,27, resulting in the formation of anisotropic nanocrystals such as nanorods28, nanosheets29, or branched-nanocrystals30. Similar anisotropic structures have been also observed in ZnSe-based nanocrystals31–33. Hence, it has been a great challenge to fabricate large-sized nanocrystals with isotropic shapes.
In this work, we investigated the influence of reactivity of Zn and Se precursors on the nucleation and growth of ZnSe nanocrystals via hot-injection method. Based on the experimental results and previous theoretical models, we developed a nuclei number-considered Lamer model to illustrate the correlation between the final size of nanocrystals and the nuclei number. A general strategy of reactivity-controlled epitaxial growth (RCEG) was proposed to synthesize large-sized ZnSe nanocrystals with average size over 35 nm. The resulted large-sized ZnSe nanocrystals show pure blue emission between 455–470 nm with narrow PL width (16–25 nm FWHM). After coating ZnS shell, we obtained nearly spherical ZnSe/ZnS core-shell nanocrystals with PLQY of 60%, which are promising candidates for display application. The RCEG strategy was successfully extended to synthesize large-sized CdSe and PbSe nanocrystals with average size over 70 nm. Except for the display application, the available large-sized nanocrystals combining the quantum confined effects and classical surface effects, will provide new opportunities for functionality-oriented studies such as photocatalysis, solar cells, and other optoelectronic devices.
Reactivity-control of ZnSe nanocrystals synthesis.
Considering the ZnSe nanocrystals synthesis, most of the previous studies adapted the reaction routes from CdSe nanocrystals, using zinc carboxylates (zinc oleate, zinc stearate) and selenium coordinates (Se-TOP, Se-ODE) as precursors9–13, 31–35. Because the high formation energy of ZnSe36, high reactivity precursors are much preferred for the synthesis of ZnSe nanocrystals. To our knowledges, there are few works considering how the reactivity of precursors affect the nucleation and growth of ZnSe nanocrystals. In general, the cationic precursors are metal-organic coordinated compounds. The reactivity is mainly determined by the bonds between cationic and coordinated functional group37. In this work, we design a simple route to regulate the reactivity of Zn precursors. A high reactive Zn precursor of [Zn(OAc)2]-OLA complex was prepared by dissolving Zn(OAc)2 into mixed solvents of oleylamine (OLA) and 1-octadecene (ODE), of which the reactivity can be modulated by adding oleic acid (OA) as co-ligands due to the strong binding between zinc and carboxyl (schematic diagram as shown in figure S1). The decomposition of OA-[Zn(OAc)2]-OLA complexes into ZnO nanoparticles can be applied to determine their reactivity by monitoring the UV-vis absorption spectra at different temperature intervals (detailed in figure S2). As shown in figure S6a, the decomposition temperature of OA-[Zn(OAc)2]-OLA complexes increased with the OA/OLA ratio increasing, implying the reactivity decreasing.
By regulating the reactivity of OA-[Zn(OAc)2]-OLA complex and Se-X coordinates (X = DPP, TOP, ODE), we then investigated the influence of precursors reactivity on the nucleation and growth of ZnSe nanocrystals. Nucleation temperature indicates the temperature at which the formation of ZnSe nanocrystals is initiated. The precursor reactivity can be characterized by comparing the nucleation temperature of the reaction between Zn and Se precursors. In our work, the nucleation temperature was recorded by monitoring the appearance of a distinct exciton absorption peak in the UV-vis absorption spectra (detailed in figure S3-S5). As shown in figure S6b, the nucleation temperature can be tuned over a wide range (140–280°C) by implementing different reactivity Zn and Se precursors. It is noticed that the nucleation temperature of the reaction between OA-[Zn(OAc)2]-OLA and Se-X coordinates increased with the concentration of OA increasing, confirming that the reactivity of OA-[Zn(OAc)2]-OLA reduce with the increase of OA concentration. Moreover, the nucleation temperature can also be varied using different Se-X precursors with a reactivity order of Se-DPP > Se-TOP > Se-ODE (see figure S6b).
We further studied the effects of the reactivity of precursors on the nucleation and growth of ZnSe nanocrystals via hot-injection approach. Figure 1a-1d show the evolution of absorption and PL spectra of ZnSe nanocrystals with reaction time under different Zn and Se precursors with various reactivity. The injection of Se-X precursor into Zn precursor solution at high temperature prompts the formation of ZnSe monomers as well as the subsequent formation of ZnSe nuclei. With the reaction prolonging, the ZnSe nuclei undergo a diffusion-controlled growth. When the ZnSe monomers are completely consumed, the growth of ZnSe nanocrystals stopped, which is evidenced by slight change of UV and PL spectra. As shown in Fig. 1e, the use of lower reactivity precursors can result in a narrower FWHM and shorter PL Peak wavelength. To track the size evolution with reaction time prolonging, an equation that correlating ZnSe spectra with particle size is feasible38. Here, we plotted the correlations between the average sizes of resulted ZnSe nanocrystals with their PL emission peaks (Fig. 1f, the fitting formula shown in the method section). For most of the synthesis, the resulted ZnSe nuclei underwent an exponential growth process at the beginning, and then approach to a critical diameter with the reaction prolonging. As summarized in Fig. 1g, the maximum size of resulted ZnSe nanocrystals was limited to ~ 5 nm with a corresponding PL peak at around 425 nm when using Se-DPP and OA-[Zn(OAc)2]-OLA (OA: OLA = 0.2) as precursors.
Insights into nucleation and growth of monodisperse nanocrystals.
To understand the key factors in determining the final size of ZnSe nanocrystals, we adapted the modified Lamer models that developed from classical CdSe nanocrystals synthesis39–42. As shown in Fig. 2a, a typical colloidal semiconductor nanocrystal synthesis includes three stages. In the first stage, the metal (A) and nonmetal (B) precursors react to form monomers ([AB]). In the second stage, the aggregation of monomers induces the formation of embryos, also termed as the burst nucleation stage43. The collision of embryos with surrounding monomers results in size increase. The embryos with size over the critical nucleation radius (r*) are stable in the reaction medium, denoted as nuclei. In the third stage, the nuclei are proceeded the subsequent diffusion-controlled growth into larger nanocrystals. These three stages correspond to the Ⅰ, Ⅱ, and Ⅲ in the Lamer model in Fig. 2b respectively. Assuming that when the monomer concentration is less than C*min, and the nucleation (stageⅡ) is effectively stopped. For monodisperse nanocrystals synthesis, if Ostwald ripening is not considered, the final size (Rf) of resulted nanocrystals is mainly determined by the ratio between total amount of monomers and the total number of nuclei (Nnuclei). And Rf can be calculate using Eq. 1.
where Mtotal is the total amount of monomers, φ is the proportion of atomic volume in unit cell.
Although, many previous works have derived the equation of nucleation rate (dN/dt) from chemical dynamics, it is still great challenge to calculate the Nnuclei due to the difficulty in determining the nucleation time17, 19–21, 44,45. In this work, we try to correlate the Nnuclei with r* from the viewpoint of thermodynamics. Based on the experimental results and previous theoretical models, we developed the nuclei number-considered Lamer model based on the Maxwell-Boltzmann distribution of embryos, which enable us to estimate the maximum final size of monodispersed nanocrystals.
From the viewpoint of thermodynamics, we here proposed that the size distribution of embryos (produced in the nucleation stage) follows a Maxwell-Boltzmann statistic as Eq. 246.
where ΔNr is the number of embryos of size r, Nm is the total number of monomers, A is the pre-exponential factor, vm is the molar volume of the crystal, γ is the specific surface energy, R is the gas constant, and T is the absolute temperature, ω is the supersaturation of reactional solution, k is the Boltzmann constant. Figure 2c shows the general profile of Maxwell-Boltzmann distribution of embryos sizes at a constant temperature at which the nucleation conducted. Hence, the Nnuclei can be estimated by the integral of the size distribution function (r* to ∞). Based on the above discussion, the value of r* is the key factor to determine the final size of nanocrystals using one-step injection method.
According to the classical Gibbs equation, the r* can be derived as Eq. 317, and according to the modified Gibbs-Thompson equation, the r* can also be described as Eq. 447, quantitatively, the r* found in Eq. 4 is 1/3 larger in size than the valve shown in Eq. 3. However, the Gibbs-based equations are not self-consistent, because the derived r* in the Gibbs methods neglect the size effect and solvent coordination effect of surface energy. In fact, the value of γ of nanoparticle is strongly correlated with its size, surface ligands and solvent medium48,49. Basically, ligands “push” on the surface of nucleus to stabilize it by moderating the surface energy48. The binding affinity between ligands and as-formed strongly affects the surface energy of nucleus. Whereas a more in-depth theoretical argument of r* is in development. Qualitatively, r* strongly relates to γ and the ω of reaction medium, which is proportional to γ and inversely proportional to ω if set reaction temperature as constant. Here, we proposed a modified formula to describe γ based on previous studies49,50, as shown in Eq. 5.
where γ(r) is the proportion of size dependent surface energy, it was described in different formula using different model51, NL is the number of ligands bonded to nanocrystalline surface atoms, is the surface energy correction factor, Ebond is the bond energy between ligand and nanocrystalline surface atom.
We further analyzed the experimental results of ZnSe nanocrystals synthesis. As shown in Fig. 1g, the final size of ZnSe nanocrystals decrease with the increasing of OA concentration. Based on the proposed model above, the strong binding affinity between OA and ZnSe nuclei induced the surface energy of ZnSe nuclei decreasing, resulting in the decrease of r*. As a sequence, the Nnuclei increased with the OA concentration increasing (as shown in figure S8 and Table S1). Therefore, according to Eq. 1, the increase of the total number of ZnSe nuclei accounts for the final size of ZnSe nanocrystals decreasing. As discussed above, except for γ, ω also determined the value of r*. We further investigated the influence of initial supersaturation (precursor concentration) on the as-formed total number of nuclei. As shown in figure S7, the final size of resulted ZnSe nanocrystals decreased with precursor concentration increasing. The increase of precursor concentration resulted in the increase of ω, thus induce the decrease of r*. As a sequence, the decrease of ZnSe nanocrystals’ size with precursors concentration increasing can be explained to the increase of Nnuclei. It is concluded that the nucleation process in ZnSe nanocrystals synthesis can be well described by the nuclei number-considered Lamer model based on the Maxwell-Boltzmann distribution of embryos.
In the stage Ⅲ, the diffusion-controlled growth is limited by the diffusion of monomers to the nuclei. For the growth of ZnSe nanocrystals, to address the existing challenges in the synthesis of large-sized ZnSe nanocrystals, we further analyzed the diffusion-controlled growth of ZnSe nanocrystals. Assuming nanocrystal grows layer-by-layer, all monomers in the diffusion sphere can diffuse to the surface of preformed ZnSe nanocrystal to satisfy the requirement of one layer’s growth, as shown in figure S16. Figure 2d illustrates the calculation result of evolution of the diffusion radius (RD) with R during one-step growth. When the diffusion spheres in the reaction system are tangent, the radius of diffusion sphere is defined as the critical diffusion radius. RD increases dramatically when it reaches the critical point, indicating the further growth of nanocrystal becomes extremely difficult. The size evolution of ZnSe nanocrystals with time prolonging can be predicted by considering the monomers concentration in solution (CL) over time (shown in SI, diffusion-controlled growth model section), which in line with our experimental results and the above discussions. Therefore, we draw a conclusion that the final size of ZnSe nanocrystals obtained from hot-injection synthesis has a critical value. The value of the final size can be varied by tunning the reactivity of Zn and Se precursors or their concentration. To our knowledge, there is no existed reaction system to obtain large-sized ZnSe nanocrystals with expected size (over 10 nm) for pure blue emitting applications (SI, Extended Data Table S2).
Reactivity-Controlled Epitaxial Growth (RCEG) of large-sized ZnSe nanocrystals.
Epitaxial growth is typical methodology to grow large-sized core shell nanocrystals, which is realized via continuous injection of precursors into small-sized seeds52–54. Nevertheless, secondary nucleation events are very easy to occur during the continuous injection of precursors, which leads to size-broadening. The epitaxial growth conditions need to be carefully chosen, especially the replenishment rate of the precursors55. In our work, by optimizing the reactivity of continuous injected precursors, we developed a versatile strategy of reactivity-controlled epitaxial growth for fabricating large-sized spherical ZnSe nanocrystals with pure-blue emission by sequential injection of high-reactivity and low-reactivity Zn (Se) precursors. The secondary nucleation during epitaxial growth was suppressed by adding low reactivity precursors. In a typical synthesis, high reactivity Zn and Se precursors are employed for fabricating ZnSe seeds, while low reactivity Zn and Se precursors are added for further epitaxial growth into large-sized ZnSe nanocrystals.
Figure 3a schematically shows the procedure of RCEG strategy. By injecting Se-DPP into the precursor solution of OA-Zn(OAc)2-OLA (OA : OLA = 0.2 : 1) at 280°C then followed the growth at 300°C, ZnSe seeds with small size of 4.0 nm can be fabricated. After that, a mixed precursor solution of Se-ODE and OA-Zn(OAc)2-OLA (OA : OLA = 1 : 1) was continuously added into the reaction medium to realize the epitaxial growth of ZnSe nanocrystals with diameter over 10 nm. Figure 3b-3c show the evolution of the UV-vis absorption and PL spectra with the RCEG synthesis prolonging, Excitingly, the PL peaks of these ZnSe nanocrystals shifted from 425 nm to 470 nm during the RCEG process, and a sample with FWHM of 25 nm at 460 nm was obtained. As shown in the TEM images (Fig. 3d-3f), the obtained large-sized ZnSe nanocrystals are monodispersed with narrow size distribution. The average sizes of these samples are 8.3 nm, 10.3 nm, 13.4 nm, 17.6 nm, 27.1 nm, and 35.2 nm, respectively. The XRD characterizations shown in figure S10 indicate that these samples are of zinc blende structure.
Fabricating of large-sized ZnSe/ZnS core-shell nanocrystals.
To enhance the PL efficiency and photostability of large-sized ZnSe nanocrystals, ZnS (bandgap, 3.7eV) was chosen as shell coating to passivate the surface defects of ZnSe nanocrystals. In our work, we used ZnSe nanocrystals with average diameter of ∼9 nm, PL peak of 455 nm, FWHM of 22 nm, and PLQY of 23% for shell coating. The ZnS shell coating was performed by successively injecting S-TOP (to form the first layer of ZnS shell, marked as ZnS1) and pre-prepared Zn-S precursors (to form the second layer of ZnS shell, marked as ZnS2). Figure 4a shows the evolution of the absorption and PL spectra after the ZnS shell growth on the ZnSe cores. The absorbance at 365 nm remained the same, and the absorbance at 300 nm increased after the epitaxial growth of ZnS on the surface of ZnSe cores, indicating that ZnS was successfully coated on ZnSe cores. Figure 4b shows the evolution of PLQY, PL peak, and FWHM during the ZnS shell growth. The PLQY reach to 60% as the shell thickness increased to ∼4 MLs and then started to decrease indicating the occurrence of interfacial strain-related defects12. Upon growth of the ZnS shell, three major diffraction peaks shifted to higher angles due to the smaller ZnS lattice constant compared with ZnSe (Fig. 4c). Figure 4d-4f provide the TEM, HRTEM and corresponding fast Fourier Transform (FFT) images of ZnSe, ZnSe/ZnS1 and ZnSe/ZnS2, respectively. Both of lattice structure and FFT correspond to the zinc-blende structure. Such large-sized ZnSe/ZnS core-shell nanocrystals show pure blue emission (∼455 nm) and narrow ensemble PL width (∼22 nm), which are ideal emitter for display applications.
RCEG of large-sized CdSe and PbSe nanocrystals.
To verify the generality of the RCEG strategy for large-sized nanocrystals synthesis, we further fabricated large-sized CdSe and PbSe nanocrystals. Firstly, monodispersed small-sized CdSe and PbSe seeds (< 5 nm, the TEM images were shown in figure S14a and figure S15a) were obtained through hot-injection method. Then, epitaxial growth of CdSe and PbSe seeds was achieved by continuous injection of moderate-reactivity cations and anions precursors. Note that during the nucleation and growth process, after employing our precise reactivity regulating strategy of precursors, secondary nucleation can be effectively suppressed. Figure 5 and figure S14-S15 show the TEM images of CdSe and PbSe nanocrystals that fabricated with different amount of precursors injection. The largest size of obtained CdSe and PbSe nanocrystals approach to 76.3 nm and 86.6 nm, respectively. Their size distribution is exceedingly narrow, about 10% in coefficient of variation (= standard deviation/mean size).