Entropic Ligands for Nanocrystals: From Unexpected Solution

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Entropic ligands for nanocrystals: from unexpected solution properties to outstanding processibility Yu Yang, Haiyan Qin, Maowei Jiang, Long Lin, Tao Fu, Xingliang Dai, Zhenxing Zhang, Yuan Niu, Hujia Cao, Yizheng Jin, Fei Zhao, and Xiaogang Peng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00730 • Publication Date (Web): 28 Feb 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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Entropic ligands for nanocrystals: from unexpected solution properties to outstanding processibility Yu Yang,1 Haiyan Qin,1 Maowei Jiang,1 Long Lin,1 Tao Fu,2 Xingliang Dai,3 Zhenxing Zhang,1 Yuan Niu,1 Hujia Cao,1 Yizheng Jin,1,3 Fei Zhao,2 Xiaogang Peng*,1 1

Center for Chemistry of Novel & High-Performance Materials, and Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China. 2

Najing Technology Corporation, 500 Qiuyi Road, Hangzhou 310052, China

3

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, P. R. China.

Abstract: Solution processiblity of nanocrystals coated with a stable monolayer of organic ligands (nanocrystal-ligands complexes) is the starting point for their applications, which is commonly measured by their solubility in media. A model described in the other report reveals that, instead of offering steric barrier between inorganic cores, it is the rotation/bending entropy of the C-C σ bonds within typical organic ligands that exponentially enhances solubility of the complexes in solution. Dramatic ligand chain-length effects on the solubility of CdSe-n-alkanoates complexes shall further reveal the power of the model. Subsequently, ―entropic ligands‖ are introduced to maximize the intra-molecular entropic effects, which increases solubility of various nanocrystals by 102-106. Entropic ligands can further offer means to greatly improve performance of nanocrystals-based electronic and optoelectronic devices. Keywords: nanocrystal-ligands complexes, entropic ligands, solubility, enthalpy change, entropy change, optoelectronic devices

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ACS Paragon Plus Environment

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Nanocrystal-ligands complexes are nanometer-sized fragments of bulk inorganic crystals stably coordinated with one monolayer of organic ligands,

1

which are commonly known as colloidal

nanocrystals. Their solution processibility and size-dependent properties are two foundations for promising applications, such as solution-processed high-performance optoelectronics, electronics,

4, 5

and bio-medical labeling.

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2, 3

printable

However, knowledge on their solution properties is scarce

and often controversial. 1, 7-9 While classic colloidal models suggest increased steric separation between nanocrystals by elongation of the n-alkyl chain of conventional ligands improves colloidal stability of the nanocrystals, 10 results here reveal that the trend could be on an opposite direction. This unexpected effect is attribute to crystalline chain-chain inter-digitation between adjacent particles in solid.

11, 12

Conversely, it is the C-C σ bond rotation/bending freedoms of the ligands released in dissolution—intra-molecular entropy—that promote colloidal stability of nanocrystals. Intra-molecular entropy

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and enthalpy for destructing crystalline packing

12

of n-alkanoate ligands are found to be

~100-1000 times greater than the main entropy and enthalpy terms in classic colloidal models.

10,14,15

These understandings invite us to design ―entropic ligands‖ to universally boost solubility of colloidal nanocrystals for ~102-106 yet to retain (or improve) their intrinsic functions. Results further imply that intra-molecular entropy, though being largely ignored, may play a decisive role in many organic, polymeric, and biological systems.

Monodisperse nanocrystals stably coordinated with a certain number of ligands—a class of special complexes—should possess defined solubility in a given solvent. With limited experimental data, traditional models treat the ligands as structureless and rigid barriers between inorganic cores, which 2


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suggest solubility increase by elongation of the hydrocarbon chains in organic ligands.

10, 12, 15-17

To

obtain a suited model system to quantitatively study their solution properties, monodisperse CdSe-alkanoates nanocrystal-ligands complexes (Figure S1) were synthesized

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and purified.

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Regardless of the number of carbon atoms (Cn) in n-alkanoates, surface ligands were confirmed bearing a fixed footprint per ligand (0.25 ±0.4 nm2) and bonded stably to the inorganic cores under experimental conditions.

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Solubility was measured by in situ laser scattering and/or determined with

saturated solution after either long-standing or centrifugation.

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The basic thermodynamic model was

developed in a separate report. 20

Figure 1a shows the saturated solutions of 2.9 nm CdSe nanocrystals coordinated with five types of n-alkanotes, namely myristate (C14), palmitate (C16), stearate (C18), eicosanoate (C20), and docosanoate (C22), in different solvents. The color of the nanocrystals in saturated solutions fades universally from left to right, and quantitative data in CCl4 are provided in Figure 1b. The trend in Figs. 1a&1b holds qualitatively for small CdSe nanocrystals (< ~3.0 nm) at all temperatures, which shows sharply decreased solubility of the nanocrystal-ligands complexes by elongation of the n-alkanoate ligands. Conversely, the trend is inverted for large CdSe nanocrystals (> ~6.5 nm) under elevated temperatures (Figure S2). For the medium core sizes, the solubility possesses a maximum (Figure S2). For inorganic cores greater than ~5 nm that are most relevant to applications, their complexes with typical n-alkanoate ligands are found to be nearly insoluble at room temperatures in typical organic solvents (see details below). These size- and ligand-dependent solubility trends share qualitatively among common organic solvents (see examples in Figure 1a). Because of its rigid and spherical shape as well 3


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as decent IR transparency (Figure S3), CCl4 is applied as the default solvent unless mentioned specifically.

Figure 1. (a) Digital photos of saturated solutions in four common solvents and (b) Solubility in CCl4 of CdSe (2.9 nm) nanocrystal-ligands complexes with different n-alkanoate ligands at 303 K. (c) Fitting of temperature-dependent solubility of 2.9 nm CdSe complexes with three types of n-alkanoates. (d, e, f) Size-dependent ΔmHNC and ΔmSNC for three types of the CdSe-alkanoates complexes. In this work, the error bar was within ± 5% for the solubility and within ±10% for ΔmHNC and ΔmSNC.

Bishop et al 15 have thoroughly considered all enthalpy and entropy terms specifically for nanoparticle systems using classic colloidal models. Numerous reports

10, 14, 15, 21, 22

have estimated the main

terms—enthalpy related to Van der Waals-London attraction and free-volume (steric repulsion) entropy—to be usually < RT (RT  2.5 kJ/mol at room temperature. R: gas constant. T: absolute

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temperature). Using similar methods, both terms for 3 nm CdSe-stearate complexes are estimated to be < 1 kJ/mol at room temperature (Figure S4).

Casado et al

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have quantitatively studied the ―crystal-to-rotator‖ solid-solid phase transition of lead

n-alkanoates, in which the C-C σ-bond internal-rotation/bending of n-alkane chains is activated but their translational freedoms are still locked by the layered solid structure. For ―crystal-to-rotator‖ transition of lead stearate, the entropy and enthalpy changes are respectively ~200 J/mol/K (equivalent to ~24 RT at 298 K) and ~80 kJ/mol. Furthermore, these large enthalpy and entropy changes both increase linearly with the n-alkyl chain length. Similarly, octadecane’s melting entropy and enthalpy are ~204 J/mol/K and ~62 kJ/mol, 23 respectively. These facts indicate that activation of conformational freedoms of long n-alkane chains would release huge intra-molecular entropy and the enthalpy cost for destructing the crystalline packing of the n-alkyl chains would also be dramatic.

The n-alkane chains in the solid of nanocrystal-ligands complexes are well known to pack in zig-zag crystalline form and inter-digitated between adjacent complexes.

11, 12

FTIR spectra (Figure S3) reveal

that, similar to melting at elevated temperatures, dissolution of the complexes at room temperatures would destruct such crystalline packing and activate conformational freedom of the ligand skeleton. These molecular features along with multiple ligands per nanocrystal suggest chain length-dependent and gigantic enthalpy and entropy changes during their dissolution, which should dominate both non-ideal dissolution entropy (ΔmSNC) and dissolution enthalpy (ΔmHNC) as discussed in the other report. 20

For dissolution, ideal mixing Gibbs free energy is quantitatively represented by solubility (molar 5


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fraction of the solute, χ). Accordingly, a quantitative and straightforward procedure

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results in

Equation (1).   e 

m

H NC / RT

e

m

S NC / R

(1)

Figure 1c shows that the temperature-dependent solubility of 2.9 nm CdSe-alkanoates complexes is well fitted by Equation (1), which is true for all cases if the solubility is not excessively low. 20 Fittings with differently sized cores and/or different n-alkanoates yield size-dependent and gigantic ΔmHNC and ΔmSNC for different ligands (see examples in Figs. 1d-f), which are 102-103 greater than those estimated using the classic models (Figure S4) but similar with the melting enthalpy and entropy of colloidal nanocrystals documented in literature. 12

The solid lines in Figs. 1d-f are fitting curves using Eqs. (2) and (3). The ΔmSNC should be proportional to the number of ligands per dot for a given ligand. Thus, it should increase linearly with the surface area per nanocrystal that is in turn proportional to the square of the nanocrystal core diameter (D), given the size-insensitive ligand footprint.

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For a given ligand, the ΔmHNC can be fitted with one

quadratic and one cubic terms.  m S NC  D 2

(1)

m H NC  D2  D3

(2)

The quadratic term in Equation (3) should be related to destructing the crystalline ligand-ligand inter-digitation between adjacent particles during dissolution. Similar to the entropic effects discussed 6


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in the above paragraph, energy cost on destructing the crystalline packing of the ligand chains should also be a quadratic function to the core diameter. The cubic term is tentatively ascribed to the unusually large (~100 Debye) and volume-dependent permanent dipole of CdSe nanocrystals. 24, 25 The core-core interaction is much larger than that predicted by the classic model though it is significantly smaller than the ligand-ligand interaction (quadratic term in Equation (3)) for small nanocrystals (< 10 nm).

Figure 2a plots three coefficients (α, β, and γ) in Equations (2) and (3) verses Cn in n-alkanoate ligands with fittings, whose intercepts suggest ~4-6 inactive -CH2- units per ligand during dissolution. This is consistent with the inter-particle distances in solid measured by NMR

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and X-ray scattering (Figure

S3). The power of super-linear function of γ is about -3 (Figure 2a, bottom plot), which is thought to be consistent with permanent dipole interactions in solid. 27

Equations (1)-(3) imply a universal solution to battle the processibility challenge of colloidal nanocrystals. If the ligands could maximize the intra-molecular entropy and minimize the enthalpy for destructing the crystalline chain-chain interactions in solid, they should substantially increase solubility of the resulting nanocrystal-ligands complexes. These ligands would be named as ―entropic ligands‖, which should interrupt crystalline ligand-ligand inter-digitation in solid but harvest conformational entropy in solution. Going through handbooks, one would identify that irregularly branched alkyl chains could substantially minimize the melting enthalpy in comparison to their n-alkyl isomers. For example, the melting enthalpy of hexane (13.08 kJ/mol) and 2,3-dimethylbutane (0.79 kJ/mol) 23—the same molecular formula and number of C-C σ bonds—are drastically different from each other. 7


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Several branched-alkanoic acids with sufficiently high boiling points are commercially available and explored as entropic ligands for synthesis of CdSe nanocrystals (Figure S5). Figure 2b demonstrates that all purified nanocrystal-ligands complexes with these entropic ligands are extremely soluble (~1000-10000 times of that with n-alkanoate ligands). Entropic ligands also greatly enhance

Figure 2. (a) Chain-length dependence of α, β, and γ (error bar, ± 10%) in Equations (2) and (3) for the CdSe nanocrystal-ligands complexes with fitting functions as solid lines (the functions being placed next to each line). (b) Solubility in logarithm scale of 3.8 nm CdSe nanocrystal-ligands complexes with five different alkonates as ligands at 303 K. (c) Solubility in logarithm scale of 4.7 nm CdSe nanocrystals coated with mixed ligands of myristate (C14) and docosanoate (C22) at 303 K. The x-axis is the molar ratio of docosanoate and total n-alkanoates. (d) TEM image of rod-like CdSe nanocrystals with 4-methyloctanoate ligands. Inset: digital photo of the saturated solution at 303 K and the molecular structure of the ligand. (e) The digital photos of 3.8 nm CdSe nanocrystal-ligands complexes with stearate (left column) and 2-hexyl-decanoate (right column) as ligands in four solvents at 303 K. All photos were taken after high-speed centrifugation to precipitate insoluble solid. 8


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high-temperature solubility during synthesis. Deposition on the flask wall and precipitation from the solution of large nanocrystals under reaction temperatures—safety and performance hazards—can be ready eliminated by replacing n-alkanoic acids with branched ones.

Mixing of different types of n-alkanoates ligands on a nanocrystal should also interrupt the crystalline packing of the hydrocarbon chains in solid and help release of the conformational freedom in solution. Results reveal that, while 4.7 nm CdSe-myristates complexes are practically insoluble at room temperature (< 0.001 mg/ml), by mixing myristate with 25-75% of docosanoate, solubility of the complexes increases by ~102-106 (Figure 2c).

Figure 2e shows that dramatic increase of solubility is generally reproducible in common organic solvents. Results in Figure 2d further indicate that 4-methyloctanoate could boost solubility of nearly monodisperse CdSe nanorods to ~250 mg/ml, which are known to be less soluble than the corresponding nanodots.

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By replacing stearate with iso-stearate, solubility of Fe3O4

nanocrystal-ligands complexes increases from 6 to 340 mg/ml at room temperature (Figure 3a).

2-ethyl-hexanethiol is selected as an example of entropic ligands for post-synthesis surface modification. Branched thiols have been applied to enhance stability of colloidal nanocrystals by assuming strong steric repulsion.

29, 30

Specifically, 2-ethyl-hexanethiol mixed with dodacanthiol

(~30%) was applied to stabilize CuInS2 nanocrystals.

30

Unfortunately, the authors did not report the

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degree of improvements and also not specify the effects of the mixing. Results in Figure 2e show that mixing of ligands could also contribute greatly to colloidal stability of the nanocrystals.

The original n-alkanoate ligands on CdSe nanocrystals are confirmed to be completely replaced by 2-ethyl-hexanethiolates.

20

The steric barrier of pure 2-ethyl-hexanethiol ligands (Table S1) is found to

be significantly smaller than that from n-alkanoates ligands. However, for 4.7 nm CdSe nanocrystals, 2-ethyl-hexanethiolates ligands increases their solubility by ~105 in common solvents in comparison to those with either stearate or octadacanthiolate ligands (Figures 3b & S6).

Figure 3b shows that solubility of Ag (11 nm) nanocrystals is substantially increased with 2-ethyl-hexanethiolate as the ligands, which simultaneously reduces the organic content in nanocrystal-ligands complexes (Table S1) beneficial for Ag conducting inks.

5

2-ethyl-hexanethiolate

also greatly boosts solubility of CdSe/CdS core/shell nanocrystals for preparation of printable luminescent inks (Figures 3c & S7) necessary for printable light-emitting-diodes. 3

Similar to the nanocrystal-ligands complexes, there are little solvent effects (Figure S8) for dissolution of n-alkanoic acid crystals with different chain length in small organic solvents. As long as the organic solvents can offer random and non-collective molecular interactions to compensate solute-solute interaction in their melted state, subtle differences between different solvents would be dominated by the gigantic intra-molecular entropy and crystalline-packing enthalpy of the long/multiple n-alkyl chains. Solubility of n-alkanoic acid crystals can be quantitatively accounted by Equation (1), which 10


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Figure 3. (a)Solubility of 12 nm Fe3O4 nanocrystal-ligands complexes at 295 K synthesized with stearic acid (n-C18 acid) and iso-stearic acid (i-C18 acid). (b) Solubility of 4.7 nm CdSe and 11 nm Ag nanocrystal-ligands complexes with n-octadecanethiolate (left) and 2-ethyl-hexanethiolate (right) as ligands in CCl4 at 303 K. (c) Digital photo of fluorescent inks of CdSe/CdS core/shell nanocrystals coated with 2-ethyl-hexanethiolate (≥100 mg/ml) at 295 K under UV radiation. (d) Current density verses applied potential for the thin films (~20 nm thickness) of 4.7 nm CdSe nanocrystals coated with 2-ethyl-hexanethiolate and n-octadecanethiolate. Inset: device structure. (e) External power efficiency (EPE) of the LEDs with the same CdSe/CdS core/shell nanocrystals and device structure but different ligands. (f) Electroluminescence spectrum at an applied voltage of 3V. Inset: photo of a working device with a pattern of “ΔS”. yields large dissolution enthalpy and entropy similar to the corresponding melting enthalpy and entropy in literature. 31 Importantly, their chain-length dependences of dissolution entropy and enthalpy (Figure S8) are similar to those of CdSe-n-alkanoates complexes in Figure 2a. These results not only strongly 11


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support the model but also imply that entropic ligands might be beyond colloidal nanocrystals. Metal alkanoates at present are the most common metal precursors for nanocrystal synthesis and the most troublesome impurities for removal.

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Figure S8 shows, in comparison to n-alkanoate ligands,

solubility of cadmium salts with 2-hexyldecanoate ligands (~1000 mg/ml) increases ~10000 times.

Entropic ligands can simultaneously improve chemical processibility in device fabrication and performance for printable electronics and optoelectronics. Figure 3d shows that, in comparison with octadecanethiolate ligands, 2-ethyl-hexanethiolate enables ~103 increase of conductance of an electron-only device of thin-film CdSe nanocrystals. Different from other strategies

4, 32

on improving

charge transport in nanocrystal solids, entropic ligands retain their outstanding optical and optoelectronic properties (Figures. 3e&3f). Figure 3e shows that 2-ethyl-hexanethiolate increases the external power efficiency of LEDs with CdSe/CdS core/shell nanocrystals as the emitters (Figure S9) by ~30%.

In conclusion, intra-molecular entropy is readily accessible in solution for molecules with long/multiple alkane chains. Entropic ligands not only universally address proccessibility challenges for colloidal nanocrystals but also retain (or improve) their performance in devices. Inter-molecular entropy is often ignored by scientists

33

and intra-molecular entropy is even less noticed. In reality, intra-molecular

entropy may be encountered commonly for organic, polymeric, and biological molecules, which can be dominating in solution but has been generally overlooked. For instance, it is the intra-molecular

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entropy that enables cellular phospholipids membrane to be flexible, responsible yet strong, given the melting enthalpy of phospholipid bilayers being ~2 times of that of NaCl crystals. 23,34

Methods CdSe nanocrystals coated with stearates with their size range between 2~7 nm were synthesized by injecting Se-octadecene suspension 18 or Se-octadecene solution 35 into a hot reaction mixture of ODE, stearic acid, and CdO. Nearly mono-disperse silver nanocrystals were synthesized using a published scheme 36 with some modifications. The phase-pure zinc blende structure CdSe–CdS core–shell quantum dots and colloidal ZnO nanocrystals were synthesized according to ref. 3 Preparation of nearly monodisperse iron oxide nanocrystals (~12 nm) was carried out with a scheme modified from the reference. 37 Thiolate-coated CdSe nanocrystals, silver nanocrystals and CdSe–CdS core–shell nanocrystals was prepared by ligand exchange. Device fabrication were prepared according to reference. 38 The fluorescent inks for ink-jet printing was a mixture of the colloidal nanocrystals dispersed in hexane and dodecane. Detailed synthesis, purification procedures and device fabrication procedures used in this work could be found in Supporting Information.

AUTHOR INFORMATION Corresponding Author: [email protected] Notes: The authors declare no competing financial interests. Supporting Information: Additional methods, table and figures. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grants 21233005, 91433204). References 1.

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Nano Letters

TOC figure

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Entropic Ligands for Nanocrystals: From Unexpected Solution

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