Fei
Wang
a,
Minyi
Zhang
b,
Wei
Chen
a,
Shaghraf
Javaid
a,
Heng
Yang
c,
Sheng
Wang
c,
Xuyong
Yang
c,
Lai-Chang
Zhang
d,
Mark A.
Buntine
a,
Chunsen
Li
*b and
Guohua
Jia
*a
aCurtin Institute of Functional Molecules and Interfaces, School of Molecular and Life Science, Curtin University, Bentley, WA 6102, Australia. E-mail: guohua.jia@curtin.edu.au
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. E-mail: chunsen.li@fjirsm.ac.cn
cKey Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai 200072, P. R. China
dSchool of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia
First published on 27th May 2020
Atomically thin colloidal quasi-two-dimensional (2D) semiconductor nanoplatelets (NPLs) have attracted tremendous attention due to their excellent properties and stimulating applications. Although some advances have been achieved in Cd- and Pb-based semiconductor NPLs, research into heavy-metal-free NPLs has been reported less due to the difficulties in the synthesis and the knowledge gap in the understanding of the growth mechanism. Herein wurtzite ZnTe NPLs with an atomic thickness of about 1.5 nm have been successfully synthesized by using Superhydride (LiEt3BH) reduced tributylphosphine–Te (TBP-Te) as the tellurium precursor. Mechanistic studies, both experimentally and theoretically, elucidate the transformation from metastable ZnTe MSC-323 magic-size nanoclusters (MSCs) to metastable ZnTe MSC-398, which then forms wurtzite ZnTe NPLs via an oriented attachment mechanism along the [100] and [002] directions of the wurtzite structure. This work not only provides insightful views into the growth mechanism of 2D NPLs but also opens an avenue for their applications in optoelectronics.
Among ZnX (X = S, Se, and Te) semiconductors, ZnTe with a direct bandgap of 2.26 eV in bulk and excellent physical properties, such as ultrafast charge separation and transfer dynamics, has important applications such as in photodetectors,21 solar cells,22 and terahertz imaging.23 Although bulk ZnTe materials are stable under ambient conditions, their counterparts such as ZnTe nanocrystals are extremely vulnerable to air and moisture and tend to undergo decomposition as they have a large number of surface atoms. This makes the study on ZnTe nanocrystals challenging because of the instability of ZnTe nanocrystals and the rigid oxygen- and moisture-free conditions required in post-synthesis handling. Furthermore, the chemistry of tellurium precursors is not well-developed and therefore substantial effort in this field is highly demanded.
Zhang et al. developed a synthetic approach for producing wurtzite ZnTe nanorods with a controllable aspect ratio by tuning the reaction temperature and time.24 In addition, they reported the self-assembly of ZnTe MSCs that evolved into one-dimensional (1D) wurtzite ZnTe ultrathin nanowires via alignment and fusion along the [002] crystallographic direction at a prolonged reaction time,25 which is different from the growth behaviour of CdSe MSCs as a nucleant for the growth of wurtzite CdSe NPLs.1,13,14 Hyeon et al. suggested that the growth of wurtzite CdSe NPLs preferred a mild reaction temperature because it can differentiate the subtle surface energy difference between (100) and (110) facets of the wurtzite structure, and also stabilize the MSCs and the soft colloidal templates.26 However, because of the metallic characteristics of both zinc and tellurium, a reaction for the synthesis of ZnTe nanocrystals conducted at a mild temperature may not be able to overcome the reaction activation energy barrier; therefore, tellurium precursors with appropriate reactivity are required. This suggests that care should be taken to keep the balance between the reactivity of tellurium precursors and the reaction temperature in the preparation of ZnTe nanocrystals with one-dimensional or two-dimensional shapes. Tellurium precursors such as bis(tert-butyldimethylsilyl) telluride,1 trioctylphosphine telluride (TOP-Te),27 polytellurides reduced from TOP-Te by Superhydride,24 tributylphosphine telluride (TBP-Te)28 and tris(dimethylamino)phosphine telluride29 were used to synthesize CdTe or ZnTe nanocrystals. For the synthesis conducted at a mild reaction temperature, high-quality ZnTe or CdTe nanocrystals can only be obtained by using tris(dimethylamino)phosphine telluride or polytellurides reduced from TOP-Te.
Herein we report the synthesis and growth mechanism of free-standing layered wurtzite ZnTe NPLs with a thickness of ∼1.5 nm by using Superhydride (LiEt3BH) reduced tributylphosphine-Te (TBP-Te) as the tellurium precursor. From both experimental and theoretical perspectives, we reveal that the growth mechanism of wurtzite ZnTe NPLs involves a stepwise transition from metastable self-assembled ZnTe MSC-323 to ZnTe MSC-398 and then oriented attachment of ZnTe MSC-398 along [100] and [002] directions to produce NPLs.
Fig. 1 presents the absorption spectra of the aliquots taken at various reaction stages and clearly shows the sharp absorption peaks of ZnTe MSCs and NPLs. After hot-injection of the tellurium precursor at 60 °C, the temperature of the reaction mixture was increased to 120 °C in 6 minutes. The mild hot-injection temperature is necessary to stabilize the MSCs because we found that ZnTe MSCs were not produced if the injection temperature was higher than 60 °C. Some reactions may have already taken place at 60 °C,30,31 producing precursor compounds and monomers that are indispensable for the formation of MSC-323 at 120 °C.30,31
Fig. 1 UV-Vis absorption spectra (red curve) of ZnTe MSC-323, ZnTe MSC-398 (green curve) and ZnTe NPLs (blue curve). |
The first aliquot was taken from the turbid white solution when the reaction proceeded for 2 hours at 120 °C. The corresponding absorption spectrum (red curve in Fig. 1) presents a sharp peak at 323 nm along with a shoulder at 297 nm, which matches well with the characteristic absorption peak locations of ZnTe MSC-323.25 Within 5 minutes, the reaction temperature further increased to 200 °C and the reaction mixture turned turbid and light-yellow gradually. As the reaction proceeded for 2 minutes at 200 °C, the second aliquot was taken and its absorption spectrum (green curve in Fig. 1) exhibits two peaks at 362 and 398 nm, matching the unique absorption peak locations of ZnTe MSC-398.25 As the reaction proceeded for 30 minutes at 200 °C, ZnTe NPLs were obtained and the exciton absorption peaks of ZnTe NPLs (blue curve in Fig. 1) appear at 360 and 396 nm. In addition, the first exciton absorption peak of ZnTe NPLs is much sharper than that of ZnTe MSC-398. Here, the “double absorption peaks” feature is usually observed for both zinc blende and wurtzite II–VI semiconductor nanoplatelets due to the electron-light hole (e-lh) and electron-heavy hole (e-hh) transitions.1,6,9
To analyse the transition from metastable ZnTe MSCs to ZnTe NPLs in detail, more aliquots were taken to investigate this process. The temporal evolution of the UV-Vis absorption spectra of the aliquots is compared in Fig. 2 as the reaction temperature increases from 120 °C to 200 °C. As the temperature increases to 120 °C, the absorption spectrum of the aliquot shows a broad peak at 325 nm (curve 1, Fig. 2). As the reaction evolves from 10 to 120 min at this temperature, this peak becomes sharper (curve 1–4, Fig. 2). The absorption spectrum (curve 4, Fig. 2) of ZnTe MSC-323 displays two distinctive exciton peaks at 302 nm and 323 nm. As the reaction temperature increases from 120 °C to 200 °C, the absorption peaks of ZnTe MSC-323 fade along with the appearance of the absorption peaks of ZnTe MSC-398 (curve 5 and 6, Fig. 2). The coincidence of the characteristic absorption features of both ZnTe MSC-323 and MSC-398 confirms the stepwise transition from metastable ZnTe MSC-323 to ZnTe MSC-398, which is similar to that observed in CdSe MSCs reported elsewhere.32 Interestingly, the exciton absorption peak of ZnTe NPLs (curve 8, Fig. 2) shows a slight blue shift of 2 nm with respect to that of ZnTe MSC-398 (curve 7, Fig. 2). This may indicate that ZnTe NPLs form an atomically flat basal plane; therefore the excitons are confined only in the thickness direction of ZnTe NPLs. Both ZnTe MSCs and NPLs do not have detectable PL, which results from surface traps and the dynamic nature of alkylamine ligands.33
The TEM image of ZnTe MSC-323 (Fig. 3a) obtained at 120 °C for 120 minutes shows that the MSCs self-assembled in coin-like plates with a lateral size of ∼20 nm. Interestingly, these self-assembled MSCs (Fig. S5†) can stack with each other due to weak interactions between the bilayer mesophase template.1 As the reaction temperature increased from 120 °C to 200 °C and proceeded for 2 minutes at this temperature, the first exciton absorption peak has a profound red-shift from 323 nm to 398 nm (red and green curve in Fig. 1), which corresponds to the transition from metastable ZnTe MSC-323 to ZnTe MSC-398. This suggests that the increase of the reaction temperature broke the structural stability of metastable self-assembled ZnTe MSC-323 and resulted in the formation of ZnTe MSC-398 from the transition of self-assembled ZnTe MSC-323. Our observation showed that the transition from MSC-323 to MSC-398 manifesting a significant red shift of the absorption onset could be induced by a kinetics driven reconstruction of self-assembled MSC-323.
Fig. 3b presents the TEM image of self-assembled ZnTe MSC-398 with an irregular plate-like shape. In addition, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Fig. S6†) reveals that this self-assembled ZnTe MSC-398 was composed of individual small particles, being similar to those reported by Zhang et al.25 After the reaction proceeded for 30 minutes at 200 °C, well-defined rectangle-shaped ZnTe NPLs (Fig. 3c) with lateral dimensions of ∼20 nm × ∼60 nm were formed. Fig. 3d presents a schematic illustration to summarise the synthesis of ZnTe NPLs converted from the stepwise transition of metastable self-assembled ZnTe MSC-323 and MSC-398.
Fig. 4a shows the TEM image of single-layered ZnTe NPLs. ZnTe NPLs with a two dimensional rectangular shape are observed alongside with black dots. These black dots are Te metal dots due to the oxidation of ZnTe NPLs. The high-resolution TEM (HRTEM) image (Fig. 4b), the Fast Fourier Transform (FFT) of the HRTEM image (Fig. 4c) and selected-area electron diffraction (SAED) pattern (Fig. 4d) of ZnTe NPLs reveal that ZnTe NPLs are single-crystalline with a hexagonal wurtzite structure. Lattice spacing values extracted from the FFT of the HRTEM image are 0.355 nm and 0.374 nm for (002) and (100) crystal facets, respectively, confirming that the long lateral direction of ZnTe NPLs corresponds to the c-axis of the wurtzite structure. To further analyze the crystal structure of the as-prepared ZnTe NPLs, the SAED pattern of ZnTe NPLs was resolved by using PASAD tools (Fig. S5†). The corresponding Miller indices were annotated to the diffraction peaks (Fig. S6†) according to the resolved lattice spacing values as listed in Table S1,† which confirms the as-prepared ZnTe NPLs with a wurtzite structure. The fitted lattice spacing values of ZnTe NPLs were a little bit smaller than the standard values for wurtzite bulk CdTe (JCPDS No. 19-1482), which results from the lattice contraction phenomenon that was also observed in CdSe nanosheets1 and CdTe quantum belts.14Fig. 4e shows that each ZnTe NPL stands on its edge. The thickness of a single ZnTe NPL is estimated to be ∼1.5 nm, which is close to the ∼1.4 nm thickness of ZnSe and CdSe nanosheets.1,34Fig. 4f–i present the HAADF-STEM image and the corresponding STEM-EDX elemental maps of ZnTe NPLs. The element maps show that the obtained NPLs contain both Zn and Te elements, with both elements being evenly distributed throughout the ZnTe NPLs. The elemental maps shown in Fig. 4i also confirm the formation of the Te metal dots, which is attributed to the oxidation of ZnTe NPLs during the preparation or purification or characterization of the TEM samples.
To elucidate the growth mechanism of wurtzite ZnTe NPLs, density functional theory (DFT) calculations were performed (see the ESI for more details†). We studied the surface energies of (110), (100) and (002)/(00) facets, which are relevant to the growth of wurtzite ZnTe NPLs. The (110), (100) and (002)/(00) terminated facets of wurtzite ZnTe NPLs are shown in Fig. S7.† The surface energy calculations show that the polar (002)/(00) facets have significantly higher surface energy than any of the non-polar (110) and (100) facets over almost the entire thermodynamically allowed range (Fig. 5), being similar to other systems with a hexagonal wurtzite crystal structure.35 In this sense, fast growth or oriented-attachment of ZnTe nanocrystals or MSCs along polar [002]/[00] directions is thermodynamically favoured, dominating the growth of wurtzite ZnTe NPLs. Since the surface energies of (110) and (100) facets are almost equal, the size difference between (110) and (100) facets of the as-synthesized wurtzite ZnTe NPLs cannot be explained.
Fig. 5 Surface energy as a function of the chemical potential ΔμZn of the wurtzite ZnTe slab surface. |
To further investigate the growth behaviour along [110] and [100] directions, we chose a wurtzite (ZnTe)34 MSC as a nucleate because the magic number 34 has been experimentally determined for other II–IV MSCs.14,36 Three isomers of (ZnTe)34 MSCs with a stacking morphology are shown in Fig. 6. Structure I in Fig. 6a presents a bilayer wurtzite stack which contains ten hexagonal rings and one complementary atom connecting two hexagonal edges on each layer with calculated energies of −69611.6675 a.u.; structure II in Fig. 6b also corresponds to the bilayer wurtzite stack with ten hexagons on each layer and a complementary atom connecting two hexagonal edges on each layer with calculated energies of −69611.6302 a.u.; structure III in Fig. 6c corresponds to a four-layer wurtzite stack with four hexagons on each layer and a complementary atom connecting two hexagonal edges on each layer with calculated energies of −69611.4918 au. Among these three isomers of (ZnTe)34, structure I is the most stable structure. Furthermore, we have used the time-dependent DFT method to calculate the UV-Vis absorption spectra of these (ZnTe)34 isomers (Fig. 6). Among all the simulated structures, the calculated adsorption spectrum of structure I has two strongest peaks located at 367 nm and 397 nm, respectively, agreeing well with the two distinct absorption peaks of as-prepared ZnTe MSC-398 located at 362 nm and 398 nm. However, the absorption peak locations of the calculated adsorption spectrum of structure II and III do not match with the experimental results. Therefore, structure I was chosen as the structural model of (ZnTe)34 MSCs.
Fig. 6 The simulated cluster structure I (a), II (b) and III (c) and the simulated UV-Vis absorption spectrum of (ZnTe)34 MSCs. Zn and Te atoms are labeled in blue and yellow, respectively. |
Then we stacked two structural models of (ZnTe)34 MSCs along [110] and [100] directions to simulate the growth of wurtzite ZnTe NPLs and calculated the formation energy of the resulting (ZnTe)68 MSCs (Fig. 7a and b). The binding energy was calculated as Ebinding = E68 − 2E34, where E68 and E34 are the total binding energy of (ZnTe)68 nanoclusters and the energy of (ZnTe)34 nanoclusters, respectively. The binding energy of [110] stacking and [100] stacking is −0.12 eV and −1.18 eV, respectively. For [100] stacking, the rhombic ring of one (ZnTe)34 nanocluster is opened and bonded to another (ZnTe)34 cluster, which leads to two new hexagonal rings and enhances the stability of (ZnTe)68 clusters. The calculation results show that [100] stacking is a thermodynamically spontaneous process and has more energy preferable than that of [110] stacking. Thus, we can speculate that (ZnTe)68 nanoclusters will continue to attach orientally along the [100] direction, which leads to the formation of a zigzag lateral structure (Fig. 7c) with a larger size along the [100] direction. In addition, the thickness of the zigzag lateral structure is determined to be ∼1.4 nm from the eight layers of (ZnTe)68 clusters, which is close to the experimental estimation of 1.5 nm. In summary, experimental results and DFT calculations reveal oriented attachment growth of ZnTe MSC-398 along [100] and [002] directions to form wurtzite ZnTe NPLs.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0na00409j |
This journal is © The Royal Society of Chemistry 2020 |