Organic Hybrid (CdSe)n·Monoamine Nanobelts

Aug 23, 2018 - Synopsis. In this study, a series of (CdSe)n·monoamine (n = 1, 2, and 4, monoamine = butylamine, hexylamine, and octylamine) layered ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Layered Inorganic/Organic Hybrid (CdSe)n·Monoamine Nanobelts: Controllable Solvothermal Synthesis, Multiple Stage Amine DeIntercalation Transformation, and Two-Dimensional Exciton Quantum Confinement Effect Jing Li,† Xiufang Hao,† Jinhui Wang,† Xiaoyan Cui,† Xinxin Li,‡ Shuo Wei,*,† and Jun Lu§ †

College of Chemistry, and ‡Analytical Center, Beijing Normal University, Beijing, 100875, P. R. China State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, P. R. China

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/29/18. For personal use only.

§

S Supporting Information *

ABSTRACT: CdSe nanocrystals, including quantum dots and quantum rods, are a research hotspot in nanomaterials. Recently, it has been found that two-dimensional CdSe semiconductor nanobelts show a unique quantum confinement effect owing to their homogeneous nanoscale thickness. In this study, a series of (CdSe)n·monoamine (n = 1, 2, and 4, monoamine = butylamine, hexylamine, and octylamine) layered inorganic/organic hybrid nanobelts was synthesized by the solvothermal method. These hybrids have a lamellar structure with a few inorganic [CdSe] layers and monoamine layers alternatively stacked along the c axis of their crystallites, which exhibit the typical preferred orientation for layered crystallites and isomorphous [CdSe] layers according to their X-ray diffraction patterns. The [CdSe] layer is a contracted (110) superlattice cell of wurtzite (WZ)-type CdSe with unit cell parameter relations of a ≈ √3aWZ and b ≈ cWZ, and a lamellar nanobelt morphology with the [010] growth direction. These ordered hybrids exhibit prominent split exciton absorption peaks and sharp band edge luminescence owing to the homogeneous thickness of the [CdSe] layers within the hybrids, all of which are comparable with reported WZ-type CdSe colloidal nanosheets and exhibit a prominent two-dimensional exciton quantum confinement effect. The solvothermal reaction was systematically investigated. The multiple stage amine de-intercalation topotactic process was revealed for the WZ-type CdSe nanobelts with the (CdSe)n·monoamine (n = 1, 2, and 4) hybrids as the intermediate phases. The solvothermal synthesis method (70−150 °C) is facile and milder than the traditional hot injection method (>200 °C). Therefore, these hybrids can be considered to be intermediate phases or precursors of WZ-type CdSe nanosheets with a topotactic intralayer [CdSe] structure and also a type of multiple quantum well with extremely thin [CdSe] layers orderly stacked among the monoamine insulating blocking layers. co-workers9−13 developed the solution hot injection method to synthesize four- to seven-layer zinc blende (ZB)-type CdSe nanoplates. Hyeon and co-workers8 used the soft template method14−16 to synthesize free-standing single-layer wurtzite (WZ)-type CdSe nanosheets by controlling the interaction between the organic layers in the 2D templates of CdCl2 alkylamine complexes.17 Burho and co-workers18 used the same method to obtain WZ-type CdSe nanosheets with four to seven monolayers (1.4−2.6 nm). However, there are only a few reports of the 2D growth mechanism. Burho and coworkers19 presented a nucleation-based kinetic argument for the exceptional intra-nanocrystal thickness uniformity observed in pseudo-2D nanocrystals. They suggested that the discrete thicknesses of a few to several monolayers reflect the kinetic or thermodynamic predisposition for crystallographi-

1. INTRODUCTION Free-standing two-dimensional (2D) nanomaterials have attracted tremendous attention from many researchers in various fields. Not only can free-standing 2D nanocrystals with atomic thickness retain the inherent properties of their corresponding bulk materials, but they can also have novel optical, electronic, mechanical, and biocompatible properties.1 Furthermore, they play a bridging role between novel microscopic electronic structures and narrow, transparent, flexible, and small macroscopic electronic devices.2,3 Recently, 2D nanomaterials with atomic thickness have attracted interest, and many 2D nanomaterials of metals,4 metal oxides,5 rareearth oxides,6 and semiconductors7 have been synthesized. Compared with materials with layered crystal structures, such as graphite, synthesis of free-standing 2D nanocrystals of nonlayered materials, such as CdSe, is extremely challenging because selective exclusive growth along one specific facet among others with similar energies is required.8 Dubertret and © XXXX American Chemical Society

Received: May 24, 2018

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DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. PXRD patterns of (A) (CdSe)·(monoamine), (B) (CdSe)2·(monoamine), and (C) (CdSe)4·(monoamine). The red, green, and blue curves are for monoamine = ba, ha, and oa, respectively. The intensity in the high-angle region is magnified for clear presentation, and the three black solid lines with violet indices on top of each panel represent the diffraction positions of WZ-type CdSe (PDF no. 08-0459). (D) Unit cell parameters and unit area plotted against n for (CdSe)n·(monoamine) (n = 1, 2, and 4) and WZ-type CdSe. The dotted lines indicate the c and √3a parameters for WZ-type CdSe.

cally flat top and bottom surfaces, upon which steps, kinks, or other structural defects are minimized. They also found that many magic-size nanoclusters, such as (CdSe)13(RNH2)1320 and (CdSe)34(oleylamine)34,21,22 can be isolated as the intermediates of the flat CdSe nanosheets. Efros and coworkers13 systematically investigated the 2D electronic structures of CdE (E = S, Se, and Te) nanoplates and confirmed the electronic structure of the 2D quantum wells, which was the fastest colloidal fluorescent emitters with the giant oscillator strength transition. Buhro and co-workers23 reported that CdSe quantum ultrathin belts have high photoluminescence efficiency (30%). Artemyev and co-workers24 investigated the exaction transition energy shift of CdSe nanoplates induced by the organic ligand shell. Li and co-workers25−27 successfully synthesized a unique class of layered inorganic/organic hybrid compounds of the II−VI-based hybrid semiconductor family by the solvothermal method, namely, (MQ)2L, where M = Cd and Zn, Q = S and Se, and L = a primary alkylamine. All of the hybrids are composed of double-layered (MQ)2 slabs and organic monoamine spacers. In this study, we systematically investigated the solvothermal reaction involving monoamine, cadmium hydroxide, and selenium. We obtained the wellcrystalline layered inorganic/organic hybrids (CdSe)n·monoamine (n = 1, 2, and 4, monoamine = butylamine (ba), hexylamine (ha), and octylamine (oa)). The multiple stage amine de-intercalation process between these hybrids was identified and investigated. More importantly, compared with previously reported WZ-type/ZB-type CdSe colloidal nanosheets, the inorganic [CdSe] layers exhibit a prominent 2D exciton confinement effect and sharp band edge luminescence

owing to the homogeneous thickness of the [CdSe] layers within the hybrids. (CdSe)n·monoamine (n = 1, 2, and 4) can be regarded as the first three members of inorganic WZ-type CdSe nanoplates with the smallest thicknesses. Therefore, these hybrids can be considered to be intermediates of WZtype CdSe nanosheets with a multiple stage amine deintercalation topotactic process, as well as multiple quantum wells with extremely thin [CdSe] layers (0.27−1.07 nm).

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Cadmium hydroxide (Cd(OH)2), ba, ha, oa, oleylamine, trioctylphosphine, and elemental selenium powder (>99%) were purchased from Aladdin Chemical. Co., Ltd.,Shanghai, China. Analytically pure ethanol and hexane were purchased from Beijing Chemical Co., Ltd., Beijing, China. All of these reagents were used as received without further purification. Deionized water was used throughout the experiments. 2.2. Solvothermal Synthesis of (CdSe)n·Monoamine (n = 1, 2, and 4) Hybrids. The (CdSe)n·monoamine (n = 1, 2, and 4, monoamine = ba, ha, and oa) hybrids were synthesized by the solvothermal method with Cd(OH)2 and Se as the reactants in the presence of the respective alkylamine.26 In a typical experiment for (CdSe)2·oa, 0.1171 g (0.8 mmol) of Cd(OH)2 and 0.0789 g (1 mmol) of Se were mixed in 20 mL of octylamine, and the mixture was then sealed in a 25 mL Teflon-lined autoclave and heated at 100 °C for 4 days. After natural cooling to room temperature, the yellow product was centrifuged and washed with 90% ethanol and hexane, respectively, and vacuum-dried at 60 °C to give a pale orange wafer. The synthesis procedures for (CdSe)2·ba and (CdSe)2·ha were the same as that for (CdSe)2·oa, except the reaction period was 3 days. (CdSe)·monoamine and (CdSe)4·monoamine were analogously synthesized at 70 and 120 °C for various days, respectively. All of the elemental analysis results are listed in Table S1 in the Supporting Information. B

DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 2.3. Characterization. The ultraviolet−visible (UV−vis) absorption spectra were recorded with a Shimadzu UV-2450 spectrophotometer in the range 300−700 nm with a slit width of 1.0 nm. The powder X-ray diffraction (PXRD) patterns of all of the compounds were recorded with a PANalytical X’Pert Pro multipurpose diffractometer under the following conditions: 40 kV, 40 mA, and Cu Kα radiation (λ = 1.541844 Å) with step scanning (0.0330°/2θ per step) in the range 2−70° using a count time of 59.6900 s per step. The morphologies and microstructures of the compounds were investigated by scanning electron microscopy (SEM, Hitachi S-4800, Japan) with 20 kV accelerating voltage and high-resolution transmission electron microscopy (HRTEM, JEM 2100 JEOL, Japan) with 200 kV accelerating voltage. The steady-state fluorescence excitation and emission spectra were recorded with a fluorescence spectrophotometer (Edinburgh FS5, UK). Both the excitation and emission slits were set to 3.0 nm with excitation at 328, 377, or 467 nm. Thermogravimetric analysis (TGA) of the hybrids was performed with a computer-controlled TGA/differential scanning calorimetry analyzer (Mettler Toledo Instruments). Approximately 6−7 mg of the sample was loaded into a 70 μL aluminum pan for each run, which consisted of a 5 °C/min ramp from 30 to 600 °C and back to 30 °C in a N2 atmosphere. The Raman spectra of the samples were obtained with a Raman spectrometer (Renishaw inVia) by excitation with a 750 nm laser. 2.4. Rietveld Refinement of (CdSe)2·ba. The (CdSe)2·ba sample used for Rietveld structural refinement was checked by elemental analysis to confirm its purity. Its PXRD pattern was obtained with a PANalytical multipurpose diffractometer under the following conditions: 40 kV, 40 mA, and Cu Kα radiation (λ = 1.541844 Å) with step scanning (0.0170°/2θ per step) in the range 2.008−59.995° using a count time of 149.86 s per step. The PXRD data were used as the input data for Rietveld refinement by GSAS and EXPGUI software with the isomorphous (ZnSe)2·ba hybrid used as the initial structural model.27

compounds with extremely strong (002) diffraction peaks at 2θ < 5°. For (CdSe)2·ba, the d value of (002) diffraction is 20.83 Å, which is consistent with a previously reported value.25 The d values for (CdSe)2·ha and (CdSe)2·oa are 25.32 and 29.06 Å, respectively, which are larger than that of ba, indicating thicker monoamine layers. The intralayer unit cell parameters of the (CdSe)·ba hybrid (6.95 and 6.70 Å) are less than those of the (CdSe)·(en)0.5 single crystal (7.08 and 6.79 Å),25 which can be attributed to the nanometer size of the former with a contracted intralayer lattice (Figure 1D). Figure 2A showed the crystalline structure of (CdSe)·ba, which has an inorganic/ organic layered structure, the [CdSe] layer is capped by Cd− NH2C4H9 coordination bonding, and butylamine molecules aggregate to form hydrophobic bilayers with interdigital alignment. However, there are slight differences between (CdSe)·ba and (CdSe)2·ba; that is, the former has only one coordinate form ([CdSe3N] tetrahedra), and the latter has two ([CdSe3N] and [CdSe4] tetrahedra), as shown in Figure 2B. The inorganic [CdSe] layers have a structure similar to the (110) plane of WZ-type CdSe, which can be considered to be a √3aWZ × cWZ supercell,28 and the (200), (020), and (210) diffraction peaks correspond to the (100), (002), and (101) peaks of WZ-type CdSe (black lines on the top of Figure 1A− C). Although the (002) diffraction positions in the XRD patterns are different, the diffraction peaks for relatively high angles (2θ > 20°) are at similar positions with a small shift depending on the monoamine, indicating analogous intralayer structures. This indicates that all the of the hybrids of ha and oa are isomers of (CdSe)2·ba. The XRD patterns of the (CdSe)4·(monoamine) (monoamine = ba, ha, and oa) hybrids are shown in Figure 1C. They have similar diffraction patterns with extremely strong (002) diffraction peaks at low 2θ angles (d spacings of ba = 28.88 Å, ha = 33.20 Å, and oa = 37.7 Å) and similar peaks for relatively high angles (2θ > 20°). Therefore, it can be concluded that the (CdSe)4·(monoamine) hybrids have a lamellar structure similar to (CdSe)2·(monoamine), but the [CdSe] layers are thicker. According to the crystalline structures of (CdSe)2·ba and (CdSe)(en)0.5 and the lattice parameter of the stacking direction (c/2), it can be estimated that the thickness increases by 3.97 Å when n is doubled and by 4.49 Å when the monoamine is lengthened by one ethylene group (the all trans length is 2.12 Å). This 3.97 Å increase can be considered to be the monolayer thickness of the structure composed of [CdSe4] tetrahedra with the (110) superlattice of WZ-type CdSe. Therefore, the three hybrids can be classified as monolayer, bilayer, and tetralayer [CdSe] hybrids for n = 1, 2, and 4, respectively. According to the reported value of 2.735 Å for the [CdSe] monolayer,27 the thicknesses of the [CdSe] layers are estimated to be 6.72 and 10.69 Å for the bilayer and tetralayer hybrids, respectively. The 4.49 Å increase in the c direction for the hybrids with identical n indicates that all of these hybrids are isomorphous with the intercalated monoamine molecules adopting an all-trans configuration to orderly align between the two adjacent [CdSe] layers with a bilayer structure, like the lipid bilayers of biological membranes (Figure 2). The d spacings of the (200), (020), and (210) facets in the XRD patterns of the hybrids are less than those of the (100), (002), and (101) facets of WZ-type CdSe (3.72, 3.51, and 3.29 Å, respectively, Figure 1D), which indicates that the (110) superlattice [CdSe] layer in the hybrid is contracted compared with that in bulk WZ-type CdSe. (CdSe)·(monoamine) with all [CdSe3N] coordination tetrahedra contracts by at most

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. 3.1.1. Layered Structure Characterization. The pure solvothermal products were analyzed by elemental analysis and TGA to confirm their chemical formulas (Tables S1 and S2 and Figure S1 in the Supporting Information), and structurally characterized by PXRD. The typical PXRD patterns of the (CdSe)n·(monoamine) (n = 1, 2, and 4, monoamine = ba, ha, and oa) hybrids are shown in Figure 1A−C. The major diffraction peaks can be indexed according to the reported isomorphous compound (CdSe)(en)0.5 (en = ethylenediamine) with orthorhombic space group Pbca.27 The three main diffraction peaks (200), (020), and (002) were determined for all of the hybrids, and the unit cell parameters are listed in Table 1. The layered structure of the hybrids was confirmed by Rietveld refinement analysis of (CdSe)2·ba with (ZnSe)2·ba as the initial structure model (Table S3 and Figure S2 in the Supporting Information). All of the PXRD patterns in Figure 1A−C show the typical preferred orientation diffraction of layered Table 1. Unit Cell Parameters (Å) of (CdSe)n·(Monoamine) from the PXRD Dataa monoamine n

ba

ha

oa

1 2 4 WZ

6.98/6.68/16.86a 7.19/6.89/20.83 7.30/6.95/28.88

6.94/6.70/21.26 7.18/6.88/25.32 7.30/6.95/33.20 √3a = 7.446, c = 7.010

6.92/6.71/25.93 7.19/6.90/29.78 7.30/6.95/37.70

a

Unit cell parameters presented as a/b/0.5c. C

DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Crystal structures of (A) (CdSe)·ba and (B) (CdSe)2·ba. The side view from the b axis and the top view from the c axis are shown in the left and right panels, respectively. Cadmium, selenium, carbon, and nitrogen atoms are shown in blue, green, brown, and light silver, respectively. The [CdSe4] or [CdSe3N] coordination tetrahedra are highlighted to show the inorganic CdSe monolayers.25

Figure 3. SEM images of (A) (CdSe)2·oa and (B) (CdSe)4·ba.

6.66%/4.42% (a and b versus √3aWZ and cWZ), (CdSe)2· (monoamine) contracts by 3.44%/1.71%, and (CdSe)4· (monoamine) contracts by 1.96%/0.86%, which indicates gradual lattice expansion of the [CdSe] layers with stacking and the existence and propagation for the [CdSe 4 ] coordination tetrahedra in the latter two hybrids. The lattice contraction is consistent with the reported results for 1.4 nm-

thick CdSe nanosheets.8,29 This lattice contraction is in contrast to ZB-type CdSe nanoplates with almost identical unit parameters to bulk CdSe, and anisotropic lattice distortion is not detected in the XRD patterns of the hybrids.24,29 3.1.2. Morphology and Nanobelt Orientation. An SEM image of the (CdSe)2·oa nanohybrid is shown in Figure 3A. It shows lamellar structures composed of nanobelts with lengths D

DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. HRTEM characterization of the hybrids and a WZ-type CdSe single nanobelt. (A) HRTEM image of the (CdSe)4·ha hybrid. (B, C) Lattice fringe images of (CdSe)4·oa and WZ-type CdSe nanobelts, respectively.

layers alternatively packing along this direction (...Se2−-Cd2+Se2−-Cd2+-Se2−...). The lateral directions of these nanobelts are [100] for the former and [11̅00] for the latter, and the crystallographic axis orientations of these two phases are consistent with the superlattice relationship between the [CdSe] layers in the hybrid and WZ-type CdSe, that is, [100]hybrid//[11̅00]WZ and [010]hybrid//[0001]WZ. It is understandable that the [020]/[0002] directions are the extended directions for these nanobelts because the (020)hybrid/ (0002)WZ crystalline facets are electrostatic polar, which is not favorable for adsorption of nonpolar solvent molecules like monoamine. Therefore, growth of the (020)hybrid/(0002)WZ polar facets is faster than growth of the (200)hybrid/(11̅00)WZ nonpolar facets, due to the blocking effect of the nonpolar amine for the latter. Moreover, from crystallographic orientation analysis, it is speculated that the WZ-type CdSe nanobelts obtained by solvothermal synthesis are produced from (CdSe)n·(monoamine) by a multiple stage amine deintercalation process with the intact network of [CdSe] layers collapsing to form WZ-type CdSe nanobelts, which is a topotactic process. 3.1.3. Lattice Vibration. Raman spectroscopy is one of the most important techniques for characterizing the phonon spectra of nanoscale and bulk semiconducting materials.31

of 400−500 nm and widths of approximately 50 nm, which are the same dimensions as the nanobelts of (CdSe)2·ba and (CdSe)2·ha. As shown in Figure 3B, well-crystalline (CdSe)4· ba is composed of thin nanobelts with coarse surfaces, which is consistent with the XRD data, confirming the lamellar morphology. All of the hybrids exhibit a nanobelt morphology. (CdSe)4· ha is a typical example with nanobelt dimensions of 10−20 nm × 100−500 nm (Figure 4A), and the morphologies of (CdSe)· ha and (CdSe)2·ha are similar (Figure S3 in the Supporting Information). The lattice fringe images of (CdSe)4·oa and WZtype CdSe nanobelts obtained by 150 °C solvothermal synthesis are shown in Figure 4, panels B and C, respectively. The (CdSe)4·oa and WZ-type CdSe show (020) and (0002) lattice fringes vertical to the respective extended direction of the nanobelts. The fast Fourier transform patterns show an orthorhombic reciprocal lattice with a [001] zone axis, only the top view direction for (CdSe)4·oa, and the [1100] zone axis for WZ-type CdSe. There is also weak (010) diffraction, which indicates that a commensurable structure exists within the hybrid. The extended directions for the two nanobelts are [010] for the (CdSe)4·oa hybrid and [0001] for WZ-type CdSe, which is the electrostatic polar packing direction; that is, the Se2− anions and Cd2+ cations form the polar single-ionic E

DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Figure 5 shows the Raman spectra of (CdSe)n·oa (n = 1, 2, 4) in the range 100−500 cm−1 at room temperature. According to

previous studies,32,33 the prominent Raman peaks of bulk CdSe are the longitudinal optical (LO) mode at 210 cm−1 and its first overtone (2LO) at 418 cm−1. However, when the [CdSe] layers are connected with monoamines, the Raman spectra are considerably different. The Raman spectrum of (CdSe)4·oa and (CdSe)·oa shows the phonon frequencies of the LO mode at 207/205 cm−1, but the LO mode is not present in the (CdSe)2·oa spectrum, which may be because of its low intensity. The blueshift of the Raman scattering peaks for (CdSe)4·oa may be because of lattice contraction.32,34 For all the three hybrids, the enhanced modes at 148/150 cm−1, 184/ 186/187 cm−1, 233 cm−1, and 255 cm−1 have been identified as the transverse optical (TO) mode, surface optical (SO) phonons, the LO and longitudinal acoustic (LA) modes, and the out-of-phase Cd−Se−Cd modes, respectively.34−38 In addition, a SO phonon with a frequency between those of the TO and LO modes has been reported for CdSe nanospheres39

Figure 5. Raman spectra of (CdSe)n·oa (n = 1, 2, 4).

Figure 6. UV−vis absorption spectra and photoluminescence spectra of (A, B) (CdSe)·(monoamine), (C, D) (CdSe)2·(monoamine), and (E, F) (CdSe)4·(monoamine) in n-hexane dispersions. The black, red, and green curves are for ba, ha, and oa, respectively. The insets in (B), (D), and (F) show the overlap of the exciton absorption peaks (red) and emission/excitation (black/blue) peaks. The inset of (E) shows a plot of the band gap values for n = 1, 2, 4, and infinity (WZ-type CdSe). F

DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and CdSe quantum dots in a glass matrix.30 The existence of the SO mode is caused by the chemical connection between a dielectric material and nanocrystallites,37 proving that capping of the [CdSe] layers occurs during synthesis. 3.2. Optical Properties and Exciton Confinement Effect. The optical properties of the (CdSe)n·(monoamine) (monoamine = ba, ha, and oa) hybrids dispersed in n-hexane (Figure 6) were characterized by determining their absorption and emission spectra. All of the hybrids show split extinction peaks at 338 and 353 nm (Figure 6A) and a sharp emission peak (full-width at half-maximum (fwhm) = 58 meV) at 364 nm (Figure 6B). The extinction and emission spectra of the (CdSe)2·oa dispersion in n-hexane are shown in Figure 6C,D. The absorption peaks in Figure 6A,C have also been reported by Li and co-workers.25,26 (CdSe)2·oa exhibits three wellresolved exciton absorption peaks at 453, 426, and 378 nm (2.74, 2.91, and 3.28 eV). These are comparable with the reported values for 1.8/1.4 nm-thick CdSe nanosheets,8,18,21 and they can be attributed to the 2D quantum confinement effect exerted by the extremely thin [CdSe] bilayers (0.672 nm) in the hybrids. These can also be regarded as 2D exciton transitions for successive extremely thin quantum wells.18 According to Buhro and co-workers,23 the three features in the spectra of (CdSe)2·oa can be assigned to exciton transitions from the nEX = 1 exciton levels associated with the 1B−1e, 1A− 1e, and 2B−2e quantum-well transitions, where A is a light hole, B is a heavy hole, and e is an electron,23,40 as shown in the inset of Figure 6D. The emission spectrum of the (CdSe)2·oa in nhexane dispersion shows a sharp narrow emission peak at 454 nm (fwhm = 60 meV), which overlaps with the 1B−1e exciton optical absorption peak. Its excitation spectrum indicates that the higher energy exciton state (1A−1e, and 2B−2e) can relax into the 1B−1e exciton state to result in the PL. We attempted to simulate the exciton energy of (CdSe)2·oa with the effective mass approximation for CdSe quantum wells with 0.672 nm thickness,23 but we could not obtain satisfactory results, probably because of the improper effective masses of the electrons and holes in these unique hybrid quantum wells. Moreover, the emission intensity was measured under the same conditions, so they are comparable and the intensity enhancement with the amine spacing layer increasing between the adjacent [CdSe] bilayer. That is, the intensity for (CdSe)2· oa (interlayer spacing 23.06 Å) is about 3.6 times that for (CdSe)2·ba (interlayer spacing 14.11 Å), which is also the case for the monolayer and tetralayer hybrids. In Figure 6E, (CdSe)4·(monoamine) exhibits three wellresolved exciton absorption peaks at 568, 546, and 513 nm. These are comparable with the reported values for 2.6/2.2 nm free CdSe nanosheets,18 and they can be attributed to the 2D quantum confinement effect exerted by the extremely thin [CdSe] tetralayers (1.069 nm) in the hybrids. Furthermore, there are weak unknown features at 365 and 391 nm. The emission spectra (Figure 6F) are more complex and show exciton emission peaks at 573, 549, and 519 nm without a significant Stokes shift, which is also the case for ZB-type CdSe nanoplates.13 The 519/513 nm and 573/568 nm PL/ absorption peaks were of the 1B−1e, and 1A−1e exciton state due to the spin−orbital splitting effect for CdSe, according to the case in (CdSe)2·oa and its exciton spectrum (Figure 6F inset). The exact origin of the 549/546 nm PL/absorption peak was not very clearly. However, this possibility cannot be ruled out that there are some intermediate hybrids with the different n (maybe n = 3) or other crystalline structures for the

[CdSe] layers (maybe ZB-type), which shows this PL/ absorption peak. Therefore, it is demanded to investigate this complicated exciton absorption and PL spectra for n = 4 hybrid to reveal its origin. Considering all of the exciton emission peaks to be band edge transitions, the band gaps of the hybrids were estimated. The band gap is plotted against the layer number (n) in the inset of Figure 6E, which shows that there is a prominent quantum size effect for the band gap. 3.3. Solvothermal Synthesis and Stage Amine DeIntercalation Transformation. 3.3.1. Temperature and Reaction Period. Like hydrothermal synthesis, the solvothermal reaction is performed with a sealed and heated organic solvent system. All of the reactions for the (CdSe)n· (monoamine) hybrids were performed under relatively mild solvothermal conditions (70−120 °C) using primary monoamines (ba, ha, and oa) as the solvents and reactive reagents, which is favorable for hybrid crystallization during an extended reaction time. These reaction temperatures are much lower than that used in the hot injection method for free CdSe nanoplates.26 Three hybrids and one inorganic WZ-type CdSe product were obtained at different temperatures (Table 2). Table 2. Solvothermal Reaction Conditions and Products for the Cd(OH)2/Se/Monoamine Systems compounds (CdSe)·ba (CdSe)·ha (CdSe)·oa (CdSe)2·ba (CdSe)2·ha (CdSe)2·oa (CdSe)4·ba (CdSe)4·hal (CdSe)4·oa WZ-type CdSe NPs

temp (°C) 70 100

120

150

times (days) 9 9 10 3 3 4 9 11 16 1

CdSe:L 1.0

2.0

4.0 >15

By optimizing the reaction temperature and time, a singlephase product can be isolated. Relatively high solvothermal temperature (>140 °C) favors formation of inorganic WZ-type CdSe (Figure S4 in the Supporting Information), whereas an intermediate temperature (70−120 °C) gives the (CdSe)n· (monoamine) (n = 1, 2, and 4) nanohybrids as the major phases, and pure hybrids can even be obtained by controlling the reaction temperature and time. For example, wellcrystalline (CdSe)·oa nanobelts were obtained by solvothermal synthesis at 70 °C for 10 days, and (CdSe)2·oa and (CdSe)4·oa nanobelts were obtained at 100 °C for 4 days and 120 °C for 16 days, respectively. The ba and ha hybrids can be obtained under conditions similar to those for the oa hybrids, but the reaction times are different. Elemental analysis (Table S1 in the Supporting Information) indicates that all of the hybrids have the stoichiometric ratio, which confirms the chemical formulas (CdSe)n·(monoamine) (n = 1, 2, and 4) and also indicates that the as-prepared WZ-type CdSe nanobelts have some amine adsorbed on their surfaces. 3.3.2. Primary Monoamine Solvent. It is well-known that primary monoamines are appropriate solvents for synthesizing chalcogenide nanocrystals owing to their remarkable coordination capability for metal ions and dissolution capability for elemental chalcogens. Well-crystalline (CdSe)n·(monoamine) hybrids of primary monoamines, such as ba, ha, and oa, can be G

DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry obtained under solvothermal conditions. The monoamine molecules align within the hybrids to form an interdigital lamellar structure, like a cell membrane. These layered inorganic/organic hybrids can be considered to be extremely thin multiple quantum wells (0.27−1.07 nm) with the monoamine bilayers acting as energy barrier layers for the valence electrons of the [CdSe] layers. Furthermore, the isolation effect between adjacent [CdSe] layers can be easily adjusted by using monoamines with different lengths, which can be used to optimize the optoelectric properties. However, we failed to obtain crystalline (CdSe)n·(monoamine) hybrids with long primary monoamines, such as oleylamine. In addition, poorly crystalline hybrids were obtained for mixed monoamines, such as octylamine/oleylamine, which can be attributed to the difficulty of orderly alignment of the octadecyl groups between the [CdSe] layers. Moreover, stronger exciton luminescence was obtained for (CdSe)2·oa and (CdSe)4·oa compared with the ba and ha hybrids, which can be attributed to the good isolation effect of the oa layers inducing a more remarkable 2D exciton quantum confinement effect. 3.3.3. Multiple Stage Amine Deintercalation of the (CdSe)n·(Monoamine) Hybrids. For the solvothermal system with Cd(OH)2/Se/monoamine, we found that transformation between the three hybrids occurs. At 70 °C, the major products are the (CdSe)·(monoamine) hybrids, but they convert to (CdSe)2·(monoamine) at 80 °C and then to (CdSe)4·(monoamine) at 120 °C for an extended reaction time, and the WZ-type CdSe nanobelts are the major products for temperatures above 140 °C. The (CdSe)2·ha hybrid was chosen to investigate the transformation between (CdSe)2·ha and (CdSe)4·ha. In Figure 7A−B, the black and blue curves are the (CdSe)2·ha and (CdSe)4·ha hybrids, respectively, and the other curves are mixed products obtained at different reaction times. The small-angle XRD patterns of these compounds (Figure 7A) show that the d spacing of the (CdSe)2·ha hybrid is 25.32 Å. The Bragg peak shifts to lower angle as the reaction time increases from 12 h to 11 days, and the d spacing eventually changes to 33.29 Å for (CdSe)4·ha. The UV spectra can also be used to track the transformation process (Figure 7B). The characteristic split exciton extinction peaks of the (CdSe)2·ha hybrid (426 and 452 nm) slowly decrease, and those of (CdSe)4·ha simultaneously appear for an extended reaction time. The results for the (CdSe)2·ba and (CdSe)2·oa hybrids (Figure 7C−F) are similar to those for (CdSe)2·ha by adjusting the temperature and reaction time. This amine deintercalation transformation process also occurs between the (CdSe)·ba and (CdSe)2·ba hybrids at 80 °C (Figure 7G,H) and between the (CdSe)4·ba and WZ-type CdSe nanoparticles at 150 °C (Figure 8) with different solvothermal reaction times. Therefore, it can be concluded that formation of WZtype CdSe nanobelts, which can be regarded as (CdSe)n· (monoamine) with n → ∞, is a multiple stage amine deintercalation process with the hybrids as the intermediate phases (Scheme 1). During the amine de-intercalation process with concomitant collapse of the [CdSe] layers, the (110) superlattice [CdSe] layer remains intact, and it is present in the final WZ-type CdSe nanobelts; that is, the amine deintercalation process is topotactic for [CdSe] layer stacking. The hybrids with various [CdSe] layers can be considered to be produced by the competition between Cd−N(amine) and Cd−Se bond formation in the solvothermal system. High temperature favors the latter, which results in amine deintercalation with Cd2+ ions and formation of more Cd−Se

Figure 7. Small-angle XRD patterns and UV−vis absorption spectra of the (CdSe)2·ha hybrid with different solvothermal reaction times at 100 °C (A−B), of the (CdSe)2·ba and (CdSe)2·oa hybrids with different solvothermal reaction times at 120 °C (C−D, E−F), of the (CdSe)·ba hybrid with different solvothermal reaction times at 80 °C (G−H).

Figure 8. XRD patterns (A) and UV−vis absorption spectra (B) of the (CdSe)4·ba hybrid with different solvothermal reaction times at 150 °C. The intensity in the high-angle region in the XRD patterns is magnified for clarity.

bonds. It can be concluded that the different n hybrids are only stable at specific temperatures for specific reaction times. That is, a higher reaction temperature results in more layers or formation of a larger n hybrid as the thermodynamically phase. For a short reaction time, (CdSe)·monoamine and (CdSe)2· H

DOI: 10.1021/acs.inorgchem.8b01425 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Multiple Stage Amine De-Intercalation Topotactic Process for WZ-type CdSe NPs in Solvothermal Synthesis

monoamine coexist for the n = 2 hybrid, and (CdSe)2· monoamine and (CdSe)4·monoamine coexist for the n = 4 hybrid, which suggests that the hybrids are formed by the amine de-intercalation mechanism.

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4. CONCLUSION A series of layered inorganic/organic hybrid nanobelts [(CdSe)n·(monoamine), monoamine = ba, ha, and oa, n = 1, 2, and 4) with different thickness [CdSe] slabs has been controllably synthesized by the solvothermal method. With increasing temperature (70−120 °C), the [CdSe] layers change from monolayers (0.27 nm) to bilayers (0.67 nm) to tetralayers (1.07 nm), and WZ-type CdSe nanobelts are the final stable phase at 150 °C. These well-crystalline hybrids are isomorphous to (CdSe)(en)0.5, and all of their PXRD patterns can be indexed to the same orthorhombic Pbca space group. The [CdSe] layers can be regarded as the compressed √3aWZ × cWZ supercell of WZ-type CdSe with the [110] preferred orientation, layer stacking, and the [CdSe] supercell lattice dilated in plane toward the WZ-type CdSe terminal compounds. Lattice orientation analysis indicates that the hybrids and final WZ-type CdSe nanobelts have topotactic lattice stacking with the same nanobelt morphologies. The formation process can be considered to be initial formation of (CdSe)·(monoamine) and multiple stage amine de-intercalation with increasing reaction temperature, and the (CdSe)2· (monoamine) and (CdSe)4·(monoamine) hybrids are the intermediate phases. The monoamine molecules act as linkers by Cd−N coordination bonding and isolators for the [CdSe] slabs, which results in a prominent 2D exciton confinement effect in their extinction and emission spectra, comparable with the reported free-standing CdSe nanosheet counterparts. These nanohybrids are the first three members of 2D (110) CdSe quantum wells with atomic thickness. It can be concluded that the quantum confinement effect not only exists in the free-standing intact inorganic nanocrystals, but also in ordinary crystallites with dimensions of the functional units at the nanoscale. Finally, these hybrids realize ordered assembly of a-few-atoms-thick CdSe nanosheets by crystallization, which is a new route for assembly of nanocrystals and encourages investigation of other II−VI group amine hybrids.

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hybrids, XRD pattern and UV−vis extinction spectrum of WZ-type CdSe NPs from solvothermal synthesis at 150 °C and 1 day in butylamine (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuo Wei: 0000-0002-9860-3920 Jun Lu: 0000-0002-9347-7793 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Hui Ma at the analytical and test center of Beijing Normal University. REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01425. Elemental analysis of (CdSe)n·monoamine, thermostable properties, Rietveld refinement of the crystalline structure of (CdSe)2·ba, SEM of the (CdSe)·ha, TEM and HRTEM images of the (CdSe)·ha and (CdSe)2·ha I

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