High yield synthesis of matchstick-like PbS nanocrystals using mesoporous organosilica as template

Abstract

A simple hard template method has been developed to prepare uniform matchstick-like PbS nanocrystals. The approach combines functionalization of the mesoporous walls and channel surface with thioether groups, adsorption of Pb²⁺, and heating in an N₂ atmosphere at high temperature. The structure, morphology and composition of the nanocrystals have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The optical properties of the matchstick-like PbS nanocrystals have been systematically investigated by Raman spectroscopy, UV-visible absorption spectroscopy (UV-vis), and photoluminescence spectroscopy (PL). These results demonstrate that these matchstick-like PbS nanocrystals are single crystals and possess novel optical properties, suggesting that they may have many potential applications. A large blue shift is observed in the photoluminescence spectrum, and this clearly shows the quantum size effects of the matchstick-like PbS . Furthermore, a growth mechanism of the PbS heteronanostructure is proposed.

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Introduction

In recent years, interest in the development of the semiconductor nanostructured materials has grown rapidly owing to their unique physical and chemical properties and potential applications in nanoscale devices.¹ In principle, the properties of semiconductor nanocrystals depend not only on their chemical composition but also their synthetic route, particle size and specific nanostructure.² The exploration of various shape-controlled synthetic methods and studies on their unusual properties will inevitably drive the progress of nanoscience and nanotechnology. Therefore, many studies on shaped-controlled synthesis of semiconductor nanocrystals with different nanostructures have been reported.^3–5

Nanoscale lead chalcogenides have been an intensive research interest because of their wide variety of applications in sensors, lasers, solar cells, infrared detectors and thermoelectric cooling materials.⁶ As one of the important II–VI semiconductor compounds, PbS is an important π–π semiconductor material, with a narrow band gap energy (0.41 eV) and a large exciton Bohr radius (18 nm).⁷ Furthermore, quantum sized PbS also has an exceptional third-order nonlinear optical property, meaning that it can be useful for optical devices such as high-speed optical switches.⁸ Consequently, the synthesis of PbS nanocrystals with different morphologies is of great significance in the search for novel applications. Up to now, PbS nanocrystals with various nanostructures have been synthesized, for example, nanorods, nanobelts, dendritic nanostructures,⁹ nanocubes,¹⁰ star-shaped nanocrystals,¹¹ well-aligned nanoporous nanowire architectures,¹²etc. However, most of these fabrication techniques produced nanocrystals which were too large to exhibit quantum confinement effects, which limited their properties and applications. The “ship-in-bottle” synthesis is a promising route for the confined growth of nanorods, nanowires, metal complexes and nanocomposites in micro- and mesoporous materials.^13–15 Previous studies have confirmed that it is possible to prepare some novel nanostructures of semiconductor nanocrystals using this hard template method.

Heteronanostructures, which are nanosized single structures consisting of more than one component, can have multiple properties simultaneously.¹⁶ For example, highly ordered arrays of the (-ZnS-CdS-ZnS-)_n superlattice nanowires have been synthesized by a chemically programmed approach, and show an unusual ultranarrow laserlike PL band at 380 nm.^16g Since semiconductor nanostructures exhibit shape- and size-dependent properties, single-component semiconductor nanostructures with one or more domains of different shapes or sizes extended epitaxially can be categorized as another type of heteronanostructure.¹⁷ In this study, we report the synthesis of matchstick-like PbS nanocrystals with well defined crystal structures in large quantities by a hard template route. The hard template used here is one of periodic mesoporous organosilicas (PMOs) whose pore walls contain rich thioether groups, which exhibit high adsorption affinity for Pb²⁺ ions.¹⁸ The syntheses of functionalized mesoporous organosilica with thioether groups involve the co-condensation of tetraethoxysilane (TEOS) and bis[3-(triethoxysilyl)propyl]tetrasulfide ((C₂H₅O)₃Si(CH₂)₃S₄(CH₂)₃Si(OC₂H₅)₃, TESPTS) in the presence of block copolymer (P123) under acidic conditions. Matchstick-like PbS nanocrystals were obtained by heating Pb²⁺-adsorbed mesoporous organosilica in N₂ atmosphere at high temperature. Finally, a possible formation mechanism of the matchstick-like PbS nanostructure was also discussed.

Experimental

Periodic mesoporous organosilica synthesis

Mesoporous organosilica template was synthesized according to a published procedure using Pluronic P123 (EO₂₀PO₇₀EO₂₀, M_av = 5800, Aldrich), tetraethyl orthosilicate (TEOS, 98%, Aldrich) and bis[3-(triethoxysilyl)propyl]tetrasulfide ((C₂H₅O)₃Si(CH₂)₃S₄(CH₂)₃Si(OC₂H₅)₃, named TESPTS, Shanghai Chemical Reagents Co.) under acid conditions.¹⁹ In a typical synthesis, 4 g of P123 was dissolved into 30 mL of distilled water and 120 mL of 2 M HCl aqueous solution while stirring. Then a mixed solution composed of 7.76 g TEOS and 2.23 g TESPTS was slowly added into the above solution. After stirring at 35–40 °C for 24 h and ageing statically at 100 °C for 48 h, the solid powder was collected by washing and filtering. The P123 triblock copolymer was removed by reflux extraction with ethanol for 48 h. The final mesoporous organosilica materials (denoted TESPTS-PMO) were obtained by filtration and dried at 60 °C for 24 h.

Matchstick-like PbS nanocrystals synthesis

The synthesis procedure of matchstick-like PbS nanostructure is as follows: 1.0 g of mesoporous organosilica (TESPTS-PMO) was dried in a vacuum oven at 150 °C for 2 h, and then immersed in 200 mL of ethanol solution containing 7.59 g of Pb(COO)₂·3H₂O. After stirring at room temperature for 48 h, the mixture was filtered and washed with distilled water for several times to remove extra Pb²⁺ on the outside of the template. A solid gray product was obtained after washing with ethanol and dried. Subsequently, the as-prepared solid was heated at 300 °C for 2 h under the protection of a nitrogen gas current to form PbS nanocrystals in the mesoporous silica template (named as PbS-TESPTS-PMO). The obtained products were washed with 2 M NaOH aqueous solution several times to remove the silica template, the template-free products were filtered, washed with absolute ethanol, and dried at 60 °C for 12 h to produce black matchstick-like PbS nanocrystals for characterization.

Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku/Max-3A X-ray diffractometer using Cu-Kα radiation (λ = 1.542 Å). TEM, HRTEM images and SAED patterns were obtained on a JEOL-2010 microscope with an accelerating voltage of 200 kV. EDS is attached to the JEOL 2010. Sample grids were prepared by sonicating powdered samples in ethanol for 20 min and evaporating one drop of the suspension onto a carbon-coated, holey film supported on a copper grid for TEM measurements. The adsorption–desorption isotherms of nitrogen at the temperature of liquid nitrogen were measured using a Micromeritics ASAP2010 system. The samples were outgassed at 100 °C for more than 10 h before the measurements. The Brumauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas. The pore-size distributions were calculated using the Barrett–Joyner–Halenda (BJH) model from the adsorption branch isotherms. XPS measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh-vaccum (UHV) chambers. All binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. Raman spectra were recorded with an InVia microscopic confocal Raman spectrometer (Renishaw, England) using a 514.5 nm laser beam. UV-Vis absorption spectra were recorded on a Shimadzu spectrophotometer (Model 2501 PC). Photoluminescence (PL) measurements were carried out on a Perkin-Elmer LS-55 luminescence spectrometer using a pulsed Xe lamp.

Results and discussion

X-Ray diffraction (XRD) patterns of the mesoporous organosilica template (TESPTS-PMO), hybrid materials before calcination (Pb²⁺-TESPTS-PMO) and calcined samples (PbS-TESPTS-PMO) are displayed in Fig. 1. Fig. 1a shows that the mesoporous organosilica template TESPTS-PMO has a strong low-angle reflection peak which is characteristic of ordered mesoporous silica with uniform cylindrical channels.¹⁵ The d-spacing, calculated from the (100) peak, is 7.1 nm. The disappearance of the (110) and (200) diffraction peaks implies that the ordered mesoporous assembly was disturbed, which may be ascribed to the “soft” propanetetrasulfide groups bridging the mesoporous framework not being rigid enough to uphold the mesoporous structure.¹⁸Fig. 1b shows a low-angle XRD pattern of the hybrid materials (Pb²⁺-TESPTS-PMO) before calcination, which implies that filling electron rich materials within regularly arrayed pores provides strong XRD peaks. Fig. 1c shows low-angle XRD pattern of the calcined sample PbS-TESPTS-PMO. It can be seen that the low-angle reflection peaks almost disappeared after calcination, suggesting that the ordered mesostructure had almost completely collapsed. Additionally, the wide-angle XRD pattern of PbS-TESPTS-PMO indicates that the PbS nanocrystals formed in the template (Fig. 1d). What is more, an amorphous phase with a wide peak at about 27° is also observed, which comes from the diffraction of the amorphous silica walls. The other sharp peaks could be indexed as PbS crystals with lattice constants comparable to the values of JCPDS card (NO 5–592). It is the formation of PbS nanocrystals inside the channels that leads to the decrease in the intensity of the low-angle XRD peaks, since pore filling can reduce the scattering contrast between the pores and the walls of mesoporous materials.¹³Fig. 2 shows the nitrogen adsorption–desorption isotherms (Fig. 2A) and pore-size distribution plots (Fig. 2B) for the TESPTS-PMO template (a) and PbS-TESPTS-PMO (b). All of the isotherms are of type IV classification, which is typical of adsorption isotherms of mesoporous materials. A well-defined step occurs at approximately P/P₀ = 0.6–0.8 for TESPTS-PMO, which is associated with the filling of the mesopores due to capillary condensation, and exhibits a high surface area of 697.86 m² g⁻¹, and a narrow pore size distribution with a mean value of 6.6 nm. After formation of nanocrystals in the nanopores (PbS-TESPTS-PMO), the amount of adsorbed nitrogen decreases and the inflection point of the step shifts to a smaller P/P₀. Also, the surface area and pore size decrease to 231.69 m² g⁻¹ and 3.7 nm, respectively. The effects can be attributed to the introduction of PbS nanocrystals into the channels of mesopores. The reduced amount of nitrogen absorption is caused by a smaller specific surface, while the shift of the step to lower relative pressure is indicative of smaller pore sizes, also suggesting that the pores have been filled up by PbS nanocrystals.

Fig. 1 (a) Low-angle XRD pattern of the TESPTS-PMO template. (b) Low-angle XRD pattern of the Pb²⁺-TESPTS-PMO sample. (c) Low-angle XRD pattern of PbS-TESPTS-PMO sample. (d) Wide-angle XRD pattern of the PbS-TESPTS-PMO sample.

Fig. 2 Nitrogen adsorption–desorption isotherms (A) and pore-size distribution curves (B) for the TESPTS-PMO template (a), and PbS-TESPTS-PMO sample (b).

Transmission electron microscopy (TEM) was employed to achieve greater insight into the microstructure of the materials. Fig. 3 shows typical TEM images of the TESPTS-PMO template (Fig. 3a) and the PbS-TESPTS-PMO sample (Fig. 3b, 3c). The parallel channels of the mesoporous template, with a mesopore size of 6.5 nm, can be clearly seen in Fig. 3a. It can be seen from Fig. 3b and 3c that PbS nanocrystals appear as dark objects between the walls of the mesoporous template and the ordered structures become very poor, suggesting destruction of the mesostructure framework after calcination, which is consistent with the low-angle XRD results. The observed results further confirm our successful synthesis of PbS nanocrystals in the mesoporous template. Matchstick-like PbS nanorods with a diameter of 6–20 nm and a length of 30–50 nm can be obtained by this template route. Growth of PbS nanocrystals takes place not only in the channels (as clearly shown in Fig. 3c) but also in the walls of mesopores due to the distribution of thioether in the channel surface and walls. Fused growth of nanorods in the channels leads to the formation of matchstick-like PbS nanorods with a diameter larger than that of the mesopores, and collapse of the mesopores (Fig. 3b).

Fig. 3 (a) TEM image of TESPTS-PMO template recorded along the direction perpendicular to the pore axis; (b, c) TEM images of the obtained PbS-TESPTS-PMO composites.

To obtain pure PbS nanocrystals, the PbS-TESPTS-PMO composites were washed repeatedly with 2 M NaOH aqueous solution to completely remove the silica template. The phase purity of the final products was examined by XRD measurements. From Fig. 4 it can be seen that all the peaks of the XRD pattern could be readily indexed to a face-centered-cubic PbS structure with a lattice constants a = 5.933 Å, consistent with the standard value from the JCPDS card 5–592 (a = 5.936 Å). No obvious characteristic diffraction peaks from other impurities could be detected. The strong peaks indicate that PbS nanocrystals were highly crystalline. It is worth noting that the ratio between the intensity of the (200) and (220) diffraction peak is higher than the bulk PbS, implying that the obtained PbS nanorods grow along the [100] direction.

Fig. 4 XRD pattern of the as-synthesized matchstick-like PbS nanocrystals.

The morphology and structure of the template-free products were further examined by TEM and high-resolution TEM (HRTEM). Fig. 5a shows a typical TEM image of the as-prepared PbS nanocrystals. The products almost entirely consist of uniform matchstick-like nanocrystals with the diameters of 6–20 nm and lengths ranging from 30 to 50 nm. The diameter of most matchstick-like nanocrystals is larger than the pore size of the nanoscale pore channels (6.5 nm). This implies that fused growth occurred in the pore channels to form a larger nanorod, leading to the collapse of the mesopores, as already demonstrated in low-angle XRD patterns and BET measurements. The average diameter of nanorods is about 15 nm, which suggests that severe sintering had taken place during thermal decomposition. The electron diffraction (ED) pattern recorded from a large area of the nanocrystals indicates that the matchstick-like PbS has a face-centered cubic structure and high crystallinity (inset in Fig. 5a), in agreement with XRD analysis. The HRTEM image of a single matchstick-like PbS nanocrystal reveals clear lattice fringes for the stick and the head, and confirms that each PbS nanocrystal is a single crystal. The fringe spacing of the lattice is determined to be about 0.297 nm, consistent with the (200) d-spacing of PbS crystals, indicating that the crystal growth of the stick is preferential in the [100] direction. The fast Fourier transformation (FFT) patterns of the stick and the spherical tip (inset in Fig. 5b) further confirm the single-crystalline nature of PbS self-tipped nanocrystal. It has to be mentioned that a small amount of dumbbell-like PbS nanocrystals can also be observed in the final product, as indicated in the red arrows in Fig. 4a and HRTEM image (Fig. 5c). The match stick heads in Fig. 5b and 5c are brighter than the stick possibly due to that the crystallinity degree of the heads being lower than that of the stick. Energy-dispersive X-ray spectroscopy (EDS) analysis shows that these matchstick-like nanocrystals consist of stoichiometric PbS with a Pb/S ratio of ∼1.10 : 1.00 (Fig. 5d). No elemental silicon was detected, indicating the complete removal of silica.

Fig. 5 (a) Typical TEM image of as-synthesized matchstick-like PbS nanocrystals (inset: a magnified TEM image (down-right); SAED pattern taken from large area nanocrystals (upper-right)). (b) HRTEM image of a single matchstick-like PbS nanocrystal (inset: down-left FFT pattern corresponding to the stick and upper-right FFT pattern corresponding to the head. (c) TEM image of a single dumbbell PbS nanorod. (d) EDS spectrum of matchstick-like PbS nanostructure. The Cu and Cr peaks came from the TEM grid.

The as-synthesized matchstick-like PbS nanocrystals were further investigated by X-ray photoelectron spectroscopy (XPS) to evaluate their purity and composition. Fig. 6 shows the XPS spectrum of the as-synthesized matchstick-like PbS nanocrystals. The XPS spectrum indicates the presence of Pb and S as well as minor C and O. The peaks located at 18, 137, 142, 412, 435, and 644 eV are assigned to the binding energies of Pb5d_5/2, Pb4f_7/2, Pb4f_5/2, Pb4d_5/2, Pb4d_3/2, and Pb4p_3/2, respectively; and those at 160 and 224 eV are ascribed to the binding energies of S2p_3/2 and S2s_1/2. All of the observed binding energy values for Pb and S are consistent with the data reported in the literature.²⁰ 284 eV for C1s as well as 531 eV for O1s are also observed, which may come from adsorbed gaseous molecules due to the high surface-to-volume ratio of PbS nanocrystals.²¹ Moreover, no other impurities were found on the surfaces of PbS, suggesting that the as-synthesized PbS nanocrystals are relatively pure. After quantitative calculation from the peak areas of Pb4f and S2p, we obtain a molar ratio of the product of 1.12 : 1.00 for Pb : S, indicating that the surfaces of the samples are slightly rich in Pb. This favors the evidence that no silica template adsorbed on the surface of matchstick-like PbS nanocrystals, and the result is ultimately in good agreement with the given formula for the as-prepared product and also basically consistent with the EDS measurements.

Fig. 6 XPS spectrum of as-synthesized matchstick-like PbS nanocrystals.

It is well known that Raman spectroscopy is a fast and nondestructive tool for estimating the quality of crystalline materials. It can provide valuable structural information with regard to semiconductor nanostructures. The optical properties of nanocrystals are generally impacted by many factors, such as the size, shape, defects and size distribution of the crystals. So it is worth further measuring Raman spectra of the as-synthesized matchstick-like PbS nanocrystals to reveal any possible shape- (or defects)-dependent effects. Fig. 7 shows the Raman spectrum of the obtained matchstick-like PbS nanocrystals using the 514.5 nm excitation line at room temperature, which reveals several bands, at 134, 175.66, 270.89, 431.47, 603.78, and 964 cm⁻¹. Generally speaking, the observed Raman shifts usually correspond to the longitudinal optical (LO) modes in a crystalline semiconductor or insulator, whereas other modes such as the transverse optical (TO) and the surface phonon (SP) modes are mostly invisible due to symmetry restrictions and their low intensities.²² The strong band below 150 cm⁻¹ is tentatively attributed to a combination of longitudinal and transverse acoustic modes [LA(L) + TA(L)].²³ The weak band at ∼176 cm⁻¹ is attributed to the fundamental longitudinal optical phonon (LO) mode, and those at 432 cm⁻¹ and 604 cm⁻¹ are assigned to its first and second overtones (2LO and 3LO, respectively),²⁰ with a shift in the band maximum to low wavenumbers. Krauss et al. reported bands at ∼215 cm⁻¹ (LO), ∼415 cm⁻¹ (2LO) and 630 cm⁻¹ (3LO), recorded at 4.2 K with an excitation of 584 nm.²⁴ It was reported that the ∼204 cm⁻¹ (LO) band near the E₁ resonance was due to coupling of the E₁ gap through the so-called Fröhlich interaction, while a 454 cm⁻¹ (2LO) band was induced by the Fröhlich interaction recorded over the temperature range 80–373 K using 632.8 nm excitation.²³ The small broad band centred at 271 cm⁻¹ can also be observed, which is similar to the 270 cm⁻¹ band recorded at 4.2 K with excitation at 584 nm.²⁴ The band centred at 964 cm⁻¹ may be attributed to oxy-sulfates in the sample or laser-induced degradation. However, the XRD pattern of PbS only indicates a cubic PbS structure ,without the presence of PbSO₄, so the 960 cm⁻¹ band should be attributed to the laser-induced degradation previously reported by Chen et al.²⁵

Fig. 7 Raman spectrum of as-synthesized matchstick-like PbS sample recorded at room temperature with 514.5 nm excitation.

The UV–vis absorption spectrum of as-prepared PbS nanocrystals dispersed in ethanol solution is shown in Fig. 8a. From this spectrum it can be clearly seen that three well defined peaks appear at 284 nm, 453 nm and 627 nm, similar to that of PbS nanocubes.²⁶ There is a possibility that the weakest peak at 284 nm is caused by the crystal defects, the peak at 453 nm may be correspond to transition into high-energy band, and another peak at 627 nm is an excitonic transition.²⁷ The absorption onset of matchstick-like PbS nanocrystals is roughly estimated to be 1.53 eV. This result shows a remarkable blue shift from the direct band gap 0.41 eV of bulk PbS crystals, due to quantum-confinement effects, which may be attributed to the relatively small head and sharp edges of the stick of matchstick-like PbS nanocrystals.⁷

Fig. 8 (a) UV–vis absorption spectrum of sample dispersed in ethanol solution. (b) Room-temperature of photoluminescence spectrum (λ_ex = 325 nm) of as-synthesized matchstick-like PbS nanostructures.

A photoluminescence (PL) investigation of the as-prepared matchstick-like PbS nanocrystal was also carried out at room temperature with an excitation wavelength of 325 nm. As shown in Fig. 8b, the PL spectrum of the product shows a broad emission with a maximum at ∼430 nm that shifts toward short wavelength as compared with its bulk counterpart. This massive blue shift is observed in PL spectrum, clearly indicating the quantum size effect of the obtained PbS nanocrystals. Emission bands at 430 nm are usually related to the transition of electrons from the conduction band edge to holes, tapped at interstitial Pb²⁺ sites.²⁸ Different models have been applied to interpret the blue shift of the band edge as a function of the particle size, such as the hyperbolic band mode, the square-well potential, and the finite-depth square-well model, etc.^20,29,30 Another possibility is that it is a consequence of intermolecular exciton interactions.³¹ It is also possible that the newly observed peaks result from crystal defects, such as sulfur vacancies or lead interstitials in the obtained PbS nanocrystals. The origin of these surprising optical properties is still far from well-understood, and more detailed investigations are needed.

On the basis of the experimental results mentioned above, we are inclined to describe the formation process of the matchstick-like PbS nanocrystals as an adsorption–nucleation–directional growth route, as illustrated in Fig. 9. Firstly, Pb²⁺ ions in ethanol solution chemically or physically adsorbed on the surfaces of the mesoporous organosilica channels whose walls possess plenty of thioether groups which can interact with Pb²⁺ by coordination behavior to form Pb-TESPTS-PMO precursors.¹⁸ Secondly, during the calcination treatment, with increasing temperature and the prolonging of the reaction time, the Pb-TESPTS-PMO precursors decomposed gradually to release S species which could react with Pb²⁺, and PbS nucleated subsequently. During the nucleation, PbS nanocrystals formed due to the aggregation of the nuclei and grew along the (100) direction to form the stick of matchstick-like PbS nanocrystals in the channels of mesoporous materials, owing to the template-confinement effects. The head of matchstick-like PbS nanocrystals was formed at the entrance of mesopore channels, which was determined by synthetical conditions, such as N₂ flow rate, capillarity, partial blockage and end-capping influence. Finally, the matchstick-like PbS nanocrystals were obtained after the template was dissolved with NaOH solution. In the formation process of matchstick-like PbS nanocrystals, we presume that the usage of TESPTS as a sulfur source should play a critical role, because TESPTS molecules can provide a lot of thioether groups in the mesoporous framework of the template, which result in a strong adsorption capacity for Pb²⁺. Thus the nucleation process and crystal growth were facilitated. Moreover, our results show that the growth of matchstick-like PbS nanocrystals was influenced by experimental conditions such as calcination temperature, calcination time, nitrogen gas flow rate and lead source. The detailed studies on the formation of this amazing heteronanostructure are now being carried out.

Fig. 9 Schematic illustration of the formation process of the matchstick-like PbS nanocrystals.

Conclusion

In conclusion, we have developed a simple hard template route to prepare matchstick-like PbS nanocrystals. The present strategy utilizes one of the periodic mesoporous organosilica materials as a template whose walls and channel surface contain rich thioether functional groups, which exhibit a high adsorption affinity for lead ions and act as the S source of PbS. The obtained products show distinct optical behaviors in comparison with bulk PbS crystals. The Raman spectrum, UV-vis absorption spectrum and the fluorescence spectrum are quite interesting. A massive blue shift is observed in photoluminescence spectra, and this clearly shows the quantum size effects of the matchstick-like PbS nanocrystals. Because of their interesting and unique structures and properties, the matchstick-like PbS nanocrystals provide a new candidate for building blocks in constructing future nanoscale electronic and optoelectronic devices such as photovoltaics, solar absorbers, IR photodetectors, mode-locking in lasers, light-emitting diodes, high-speed optical switches, and so on. In addition, this simple hard template technique can be further extended to the shape-controlled synthesis of other semiconductors with unique nanostructures. The origin of the observed optical properties is still far from well-understood, and more detailed investigations are needed in future.

Acknowledgements

This work was supported by the National Basic Research Program of China (2010CB934700, 2011CB933700), the 100 Talents program of the Chinese Academy of Sciences, the National Natural Science Foundation of China (20971118), and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS.

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