Article Cite This: Chem. Mater. 2018, 30, 1710−1717
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Spontaneous Folding of CdTe Nanosheets Induced by Ligand Exchange Roman B. Vasiliev,*,†,‡ Elizabeth P. Lazareva,† Daria A. Karlova,‡ Alexey V. Garshev,†,‡ Yuanzhao Yao,§ Takashi Kuroda,§ Alexander M. Gaskov,‡ and Kazuaki Sakoda§ †
Department of Materials Science and ‡Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia Photonic Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
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S Supporting Information *
ABSTRACT: Two-dimensional (2D) semiconductors exhibit unique electronic and optical properties arising from the atomic-scale thickness and two-dimensional electronic structure. However, it is usually limited by an intrinsically flat morphology of 2D materials. Here, we report an effect of spontaneous folding of quasi-2D CdTe nanosheets stimulated by ligand exchange. We show that initially flat CdTe nanosheets with 100−200 nm lateral size and 5−6 ML thickness are uniformly rolled up when oleic acid is replaced by thiol-containing ligands. Detailed study shows nanosheet folding along the [110] direction forming multiwall scroll-like structures with the diameter being dependent on sheet thickness. A pronounced red shift of the exciton transitions of CdTe nanosheets is found due to thickness increase and strain appearance under thiol attachment. The folding mechanism is likely related to misfit strain at CdTe (001) basal planes as ultrathin CdS layer is formed. Possibility to precisely tune the nanostructure shape simply by ligand-induced strain can evolve into new synthetic strategies to control a spatial morphology of 2D materials.
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nanoplatelets form scroll-like nanostructures when lateral size increases up to 100 nm and higher.19,26,28 Single or multiwall tube- and scroll-like structures are well-known for carbon-based materials or layered transition metal dichalcogenides grown by gas-phase methods.29−31 Rolled-up nanostructures of strained SiGe or InGaAs nanomembranes were demonstrated and found some interesting applications.32−34 However, tube or scroll formation is significantly less known for nanosheets synthesized by colloidal methods. In the case of cadmium chalcogenide quasi-two-dimensional nanoparticles only ultrathin CdSe and CdSxSe1−x nanoplates folded, while thicker 6 and 7 ML CdSe nanoplatelets were found to be flat because of lower elasticity.26 In the case of CdTe nanoplatelets, or nanosheets (NS), all reported populations of sheets are flat even for large lateral sizes.17,35 We have addressed this work with an analysis of ligand exchange on CdTe nanosheets. Recently ligand exchange was shown to promote reversible shift of excitonic bands for CdSe quantum belts36,37 and nanoplatelets38 accompanied by change of lattice parameters. In our work, we show an effect of spontaneous folding of CdTe nanosheets promoted by ligand exchange with thiols. Ligand exchange was performed in a nonpolar and polar phase with hexadecanethiol (HDT) and thioglycolic acid (TGA), respectively. We provide a study of the
wo-dimensional semiconductor nanomaterials have attracted great attention because of the unique electronic structure shown for graphene,1 for layered transition metal dichalcogenides,2 and later for colloidal quasi-two-dimensional nanoparticles.3 Cadmium chalcogenide quasi-two-dimensional nanoribbons,4 nanobelts,5 and nanoplatelets6 also known as colloidal quantum wells demonstrate the narrowest 5−8 nm wide exciton bands among other colloidal nanoparticles due to strictly uniform thickness. Narrow exciton transitions arising from fundamental band gap are easily tunable with varying nanoplatelet thickness.3,6 Quantum yield reaches 30% for bare CdSe nanoplatelets and further may be increased to unity7 by wide band gap semiconductor shell covering. Giant oscillator strength and short radiative lifetime make these nanoplatelets prominent for practical application, such as light-emitting diodes,8−10 lasers,11−13 photodetectors,14 and field effect transistors.15 To date, synthetic approaches were reported to grow CdSe,16 CdTe,17 CdS,18 CdSxSe1−x,19 CdTexSe1−x 20 nanoplatelets, and also CdSe 21−24 and CdS 25 nanoribbons and nanobelts with quasi-two-dimensional morphology. Colloidal growth allows for significant increase of lateral sizes of nanoplatelets up to hundreds of nanometers, transforming the platelets to sheets.26 Such laterally extended nanostructures exhibit pronounced two-dimensional character of electronic structure, which resulted in a set of high-energy exciton transitions at the boundary of the Brillouin zone,27 and were not observed for other cadmium chalcogenide nanoparticles. It was found that ultrathin 5 ML CdSe and CdS x Se 1−x © 2018 American Chemical Society
Received: December 22, 2017 Revised: February 25, 2018 Published: February 25, 2018 1710
DOI: 10.1021/acs.chemmater.7b05324 Chem. Mater. 2018, 30, 1710−1717
Article
Chemistry of Materials
Figure 1. Low-resolution TEM overview images of as-synthesized flat 6 ML CdTe NSs (a) and HDT covered scroll 5 ML (b, c) and 6 ML (d, e) CdTe NSs. Single nanoscroll top view is shown in (f). Insets to (c) and (e) show ED patterns. 7 days at room temperature for a ligand exchange. The CdTe NSs covered by HDT were precipitated by addition of an equal volume of acetone, separated by centrifugation, and dissolved in 2 mL of hexane. Ligand exchange with the thioglycolic acid was applied to synthesize CdTe NSs covered by TGA using phase-transfer method similar to ref 40. Briefly, 500 μL of the as-synthesized nanosheets in hexane was diluted in 1 mL of hexane and added to 100 μL/1 mL TGA/NMF mixture under argon flow. The NMF layer turned red or dark red as ligand exchange started, and TGA-covered CdTe NSs transferred to polar NMF phase. After 1 h of exchange a hexane layer was discarded and equal volume of toluene was added to precipitate the NSs. CdTe NSs covered by TGA were separated by centrifugation and dissolved in 2 mL of pure NMF. The solvent was replaced by methanol in the case of optical measurements or manipulation with solvent evaporation. Samples of CdTe NSs covered by HDT or TGA were stored under argon. Characterization Techniques. TEM Analysis. Transmission electron microscopy (TEM) images of nanosheets were recorded on Carl Zeiss LIBRA 200 microscope with accelerating voltage 200 kV. Scanning transmission electron microscopy (STEM) images in highangle annular dark field (HAADF) mode were acquired on the HAADF detector (Fischione). Energy-dispersive X-ray spectroscopy (EDX) signal was recorded on the silicon drift X-MAX 80 T detector (Oxford Instruments). STEM-EDX maps were generated from the intensity of the Cd (Lα1, Lβ1), S (Kα1, Kβ1), and Te (Lα1, Lβ1, Lβ2) lines. Diluted nanosheet solution in hexane was dropwise deposited onto the carbon holey coated 200 mesh copper grids with following solvent evaporation at room temperature. X-ray Powder Diffraction Analysis. X-ray diffraction (XRD) analysis was performed with Cu Kα radiation on a D/Max 2500V/ PC Rigaku diffractometer. XRD patterns were recorded in the reflection mode in the 2Θ range 10−60° with a step of 0.1°. Samples for analysis were prepared by depositing concentrated solutions of NSs on a Si substrate with [100] orientation resulting in the formation of nanosheet films after evaporation of the solvent at room temperature. FTIR Measurements. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a PerkinElmer Frontier
morphology of CdTe folded nanostructures complemented with analysis of crystal structure, elemental composition, and ligand coverage. Analysis of optical properties shows the presence of pronounced exciton transitions that are specific for quasi-two-dimensional nanoparticles. We explain the nanosheet folding mechanism by the contribution from misfit strain appearing when thiol groups are attached to basal planes of NS, which was supported by analysis of scroll radii and energy shift of exciton bands.
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MATERIALS AND METHODS
Synthesis Protocols. Chemicals. Cadmium acetate dihydrate (Cd(OOCCH3)2·2H2O, ≥98%), oleic acid (OA, 90%), tellurium powder (Te, 99.99%), trioctylphosphine (TOP, 90%), 1-octadecene (ODE, 90%), N-methylformamide (NMF, 90%), 1-hexadecanethiol (HDT, 95%), thioglycolic acid (TGA, 98%), and solvents were purchased from Sigma-Aldrich. CdTe Nanosheet Growth. CdTe nanosheets were synthesized by colloidal synthesis following procedures modified from ref 17. Briefly, a mixture containing 0.13 g of cadmuim acetate dihydrate, 0.08 mL of oleic acid, and 10 mL of octadecene was heated to 150 °C (5 ML CdTe), 200 °C (6 ML CdTe), or 230 °C (7 ML CdTe) under argon flow. After that, 100 μL of 1 M solution of tellurium in trioctylphosphine was injected rapidly (5 ML CdTe) or slowly dropwise added (6 and 7 ML CdTe) and the growth of the NSs was continued for 30 min. Then 1 mL of OA was injected and the flask was cooled down to room temperature. The NSs were precipitated by addition of an equal volume of acetone, separated by centrifugation, and dissolved in 2 mL of hexane. After two to three cycles of repeated precipitation and redispersion, the solutions of the CdTe NSs in hexane with minimum impurity content were obtained. Ligand Exchange. Ligand exchange protocol with hexadecanethiol ligand was adapted from ref 39. Briefly, 500 μL of the as-synthesized nanosheet in hexane was diluted in 2 mL of hexane and 200 μL of hexadecanethiol was added under argon flow. The solution turned red or dark red as ligand exchange started. The mixture was stirred during 1711
DOI: 10.1021/acs.chemmater.7b05324 Chem. Mater. 2018, 30, 1710−1717
Article
Chemistry of Materials spectrometer at room temperature in the 400−4000 cm−1 wavenumber range. Samples for analysis were prepared by mixing a drop of NS solution with KBr powder followed by pressing into pellets after solvent evaporation. Optical Measurements. Absorption spectroscopy at room temperature was performed with a Cary 50 (Varian) spectrophotometer in the 200−800 nm wavelength range with scanning speed of 60 nm/ min. Spectra were taken from colloidal solutions of nanosheets in hexane or methanol diluted to an optical density of
