Spontaneous Folding of CdTe Nanosheets Induced by Ligand Exchange

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05324 • Publication Date (Web): 25 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Chemistry of Materials

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, Lomonosov Moscow State University, 119991, Moscow,

Russia ‡

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 *

E-mail: [email protected]

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 [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

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CdTe (001) basal planes as ultrathin CdS layer is formed. Possibility to precisely tune the nanostructure shape simply by ligand-induced strain can evolve in new synthetic strategies to control a spatial morphology of 2D materials.

Two-dimensional semiconductor nanomaterials have attracted a great attention because of unique electronic structure shown for graphene,1 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 riches 30% for bare CdSe nanoplatelets and further may be increased to unity7 by wide bandgap 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 phototransistors.15 To date synthetic approaches were reported to grow CdSe,16 CdTe,17 CdS,18 CdSxSe1-x,19 CdTexSe1-x20 nanoplatelets, and also CdSe21-24 and CdS25 nanoribbons and nanobelts with quasi-two-dimensional morphology. Colloidal growth allows to significantly increase 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 Brillouin zone27 and was not observed for other cadmium chalcogenide nanoparticles. It was found that ultrathin 5 ML CdSe and CdSxSe1-x 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 2


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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 to 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 both in a nonpolar and polar phase with hexadecanethiol (HDT) and thioglycolic acid (TGA), respectively. We provide a study of the morphology of CdTe folded nanostructures compliment with analysis of crystal structure, elemental composition, and ligand coverage. Analysis of optical properties shows the presence of pronounced exciton transitions which are specific for quasi-two-dimensional nanoparticles. We explain the nanosheet folding mechanism by the contribution from misfit strain appeared when thiol groups are attached to basal planes of NS, that was supported by analysis of scroll radii and energy shift of exciton bands.

MATERIALS AND METHODS. Synthesis Protocols. Chemicals. Cadmium acetate dihydrate (Cd(OOCCH3)2·2H2O, ≥98%), oleic acid 3



(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 ºС (5 ML CdTe), 200 ºС (6 ML CdTe) or 230 ºС (7 ML CdTe) under argon flow. After that, 100 µl of 1M solution of tellurium in trioctylphosphine was injected rapidly (5 ML CdTe) or slowly drop-wise added (6 ML and 7 ML CdTe) and the growth of the NSs was continued for 30 min. Then 1 ml of OA was injected and 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 2-3 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-synthetized nanosheet in hexane was diluted in 2 mL hexane and 200 µL hexadecanethiol was added under argon flow. The solution turned red or dark red as ligand exchange started. The mixture was stirred during 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 assynthetized nanosheets in hexane was diluted in 1 mL 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 4


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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 200kV. Scanning transmission electron microscopy (STEM) images in high-angle 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 Instrument). 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 drop-wise 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 on CuKα radiation on D/Max 2500V/PC Rigaku diffractometer. XRD patterns were recorded in the reflection mode in the 2Θ range 10º- 60º with a step of 0.1º. 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 Perkin-Elmer Frontier spectrometer at room temperature in 400-4000 cm-1

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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 on Carry50 (Varian) spectrophotometer in 200-800 nm wavelength range with scanning speed 60 nm/min. Spectra were taken from colloidal solutions of nanosheets in hexane or methanol diluted to an optical density

Spontaneous Folding of CdTe Nanosheets Induced by Ligand Exchange

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