Communication Cite This: J. Am. Chem. Soc. 2019, 141, 10967−10971
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Solution-Processable Superatomic Thin-Films Jingjing Yang,*,† Boyuan Zhang,† Alexander D. Christodoulides,‡ Qizhi Xu,†,⊥ Amirali Zangiabadi,∥ Samuel R. Peurifoy,† Christine K. McGinn,§ Lingyun Dai,‡ Elena Meirzadeh,† Xavier Roy,† Michael L. Steigerwald,† Ioannis Kymissis,§ Jonathan A. Malen,‡ and Colin Nuckolls*,† †
Department of Chemistry, Columbia University, New York, New York 10027, United States Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ⊥ The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced Materials and Nanotechnology, Wuhan University of Science and Technology, Wuhan 430081, China ∥ Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States § Department of Electrical Engineering, Columbia University, New York, New York 10027, United States
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‡
S Supporting Information *
complementary clusters from solution is hindered, and the solution is easily spin-cast into amorphous and homogeneous thin-films containing the ionic pairs of clusters. We prepared a series of superatomic thin-films with varying compositions based on a metal chalcogenide cluster Co6Te8(PPr3)6 (PPr3, tri-n-propylphosphine)2b with propyl side-chains, and a fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester3 with butyric acid methyl ester side-chains (named [Co] and [PCBM] throughout the paper) (Figure 1a). The simple structural change of installing these side-chains allows for materials that are previously unattainable. We now achieve superatomic materials that are solution-processable and have the ability to adjust the ratios of the constituent clusters arbitrarily, neither of which is possible in crystals. These amorphous thin-films show very high electrical conductivities up to 300 S/m, ultralow thermal conductivities of 0.05 W m−1 K−1, and high optical transparency of up to 92% in the visible range (80% at 550 nm). Such high electrical conductivity is 2 orders of magnitude higher than those of crystalline superatomic solids or doped PCBMs,2a,b,4 and is several times higher than many conducting polymers5 including commercial-grade poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, 10−4−100 S/m). In addition, the thermal conductivities are 3−5 times lower than their crystalline counterparts (0.16 W m−1 K−1 of [Co 6 Te 8 (PEt 3 ) 6 ][C 6 0 ] 2 and 0.25 W m − 1 K − 1 of [Co6Se8(PEt3)6][C60]2),2c and are as lowest as the ever measured k associated with all dense solids (0.05 W m−1 K−1).6 Given the high electrical conductivity and the low thermal conductivity, we investigated their thermoelectric behavior and obtained an unoptimized ZT value of ∼0.02 at room temperature. This is higher than that of bulk silicon (0.01 at 300 K)7 and 1 order of magnitude higher than those of most types of organic thermoelectric materials.8 Such solution-processability and functionalities chart the path to the deployments of superatomic materials for transparent conductors, electronic and thermoelectric devices.9
ABSTRACT: Atomically precise nanoscale clusters could assemble into crystalline ionic crystals akin to the atomic ionic solids through the strong electrostatic interactions between the constituent clusters. Here we show that, unlike atomic ionic solids, the electrostatic interactions between nanoscale clusters could be frustrated by using large clusters with long and flexible side-chains so that the ionic cluster pairs do not crystallize. As such, we report ionic superatomic materials that can be easily solutionprocessed into completely amorphous and homogeneous thin-films. These new amorphous superatomic materials show tunable compositions and new properties that are not achievable in crystals, including very high electrical conductivities of up to 300 S per meter, ultra low thermal conductivities of 0.05 W per meter per degree kelvin, and high optical transparency of up to 92%. We also demonstrate thin-film thermoelectrics with unoptimized ZT values of 0.02 based on the superatomic thin-films. Such properties are competitive to state-of-the-art materials and make superatomic materials promising as a new class of electronic and thermoelectric materials for devices.
T
he use of nanoscale clusters as programmable superatomic building blocks allows for the assembly of functional materials with desirable properties.1 Like atoms, these nanoclusters could, through electron transfer between the constituent clusters, assemble into ionic crystals with collective electrical transport and magnetic properties,2 yet their powder formulations hindered their use as functional materials in devices. Formation of superatomic ionic crystals is dominated by the strong electrostatic interactions between the constituent clusters that result in the crystallization of ionic pairs. Here, we show that such interactions can be frustrated, leading to solution-processable superatomic thin-films that are completely amorphous. We use large clusters with flexible sidechains such that the charge could be dispersed over a large volume of space to weaken electrostatic interactions. As such, we demonstrate that the crystallization of two electron © 2019 American Chemical Society
Received: May 8, 2019 Published: July 1, 2019 10967
DOI: 10.1021/jacs.9b04705 J. Am. Chem. Soc. 2019, 141, 10967−10971
Communication
Journal of the American Chemical Society
Figure 1. (a) Atomic structures and sizes of [Co] and [PCBM] clusters. Co in neon blue, Te in green, P in yellow, C in white, and O in red, H atoms omitted. (b) Redox potentials of the isolated [Co]2b and [PCBM]10 clusters vs Fc/Fc+ (ferrocene/ferrocenium couple). Each redox couple is labeled above its corresponding potential level. (c) Schematic of a spin-coated superatomic thin-film with only one layer of superatoms shown.
Figure 2. Characterizations of superatomic solutions and the spin-coated superatomic thin-films. (a) Optical image of a superatomic solution [Co][PCBM]5 in a 4 mL vial. (b) Optical image of spin-coated superatomic film [Co][PCBM]5 on a silicon wafer with square Au microelectrode arrays. Inset is the original silicon wafer before spin-coating. (c) EDS mapping of Co and Te in [Co][PCBM]5 thin-film. (d) HRTEM image of thin-film of [Co][PCBM]5 with SAED patterns. (e) Electronic absorption spectrum of drop casted [Co][PCBM]5. (f) AFM image of [Co][PCBM]5.
S2 and S13). We then use atomic force microscopy (AFM) to probe the thickness and surface roughness of the films (Figure 2f, Figures S14−S19). All thin-films show similar thicknesses of ∼100 nm under the same spin-coat condition (Table S2). Assuming the height of a layer of [Co] and [PCBM] superatoms of 1.8 nm and van der Waals contacts of 0.3 nm, this thickness corresponds to 48 layers of superatoms. The root-mean-square (RMS) roughness of the films is on the order of 0.5 nm (Tables S2), which is even smaller than the size of a single superatom, indicating good smoothness and flatness of the surfaces. Such results demonstrate the outstanding film-forming capability of these materials. The outstanding film-forming capabilities come from the amorphous nature of the thin-films. In Figure 2d and Figures S20−S37, we show the high-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) patterns of each thin-film. All thin-films remain completely amorphous with no crystalline domains or
The two nanoscale clusters possess suitable redox potentials that allow one electron transfer from [Co] to [PCBM] (Figure 1b). We combine [Co] and [PCBM] clusters with different molar ratios (1:2, 1:3, 1:4, 1:5, 1:7, and 1:9) in chlorobenzene, and in each case we obtain a dark reddish brown solution (Figure 2a). These solutions remain stable with no precipitation even after 4 months. In contrast, combinations of clusters with shorter side-chains typically result in superatomic crystals within hours.2 The solutions containing mixtures of [Co] and [PCBM] can be easily spin-coated into thin-films on different substrates. In Figure 2b, we show the optical images of a [Co][PCBM]5 film on a silicon substrate; we can see a clear color change of the channel from blue to green after spin-coating, indicating the formation of a homogeneous thin-film. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) confirm the uniformity of the thin-films as well as the homogeneous distribution of [Co] clusters (Figure 2c, Figures 10968
DOI: 10.1021/jacs.9b04705 J. Am. Chem. Soc. 2019, 141, 10967−10971
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Journal of the American Chemical Society
Figure 3. (a) Optical image of superatomic films spin-coated on glass. From left to right are [Co][PCBM]x (x = 2, 3, 4, 5, 7, 9), respectively. (b) Transmittance spectra of superatomic thin-films on glass substrates. (c) σ of superatomic thin-films with uncertainties (also in Table S3). (d) Plot of the ln conductance (G) versus 1/T. The Arrhenius fits, shown as solid lines, are used to extract the activation energy Ea (with uncertainties in Figures S53−S55).
[PCBM] to [Co] ratios (Figure 3c). σ is observed to peak at a ratio of 5:1 with up to 301 S/m and to approach zero as the mixing ratios approach pure [PCBM] or [Co]. To illustrate the origin of differences in σ, we performed variable temperature electrical resistivity measurements (Figure 3d, Figures S53− S55). As shown in Figure 3d, [Co][PCBM]5 exhibits an Arrhenius-type activated electrical transport with an activation energy Ea of 87 meV. This activation energy is almost the same as that of [PCBM]-rich [Co][PCBM]9 (86 meV). However, the σ of [Co][PCBM]5 is 2.3 times higher than that of [Co][PCBM]9 (129 S/m). These results suggest that differences in conductivity are the result of a higher free electron density in [Co][PCBM]5 due to a higher ratio of electron donating [Co]. The change could also be due to the change of the electronic structures, for example, a wider conduction band in [Co][PCBM]5 due to the stronger interactions between clusters. Yet too much [Co] results in the increasing activation energy ([Co][PCBM]2 of 101 meV), leading to a decreased σ (19 S/m). An additional feature of these superatomic thin-films is their unique thermal transport properties. Using frequency domain thermoreflectance (FDTR), a noncontact optical pump−probe technique,2c,14 we measured room-temperature thermal conductivities (k) of the binary thin-films (Figure 4a, Table S6). Binary film k values are 0.05−0.07 W m−1 K−1 with no discernible trend as a function of composition. The [Co][PCBM]5 sample has both the highest k and σ, and the
large nanostructure agglomerates identifiable in the HRTEM images and no diffraction spots or sharp Debye-Scherre diffraction rings observable in SAED patterns. Such amorphous and homogeneous nature of the superatomic films clearly reveals that the electrostatic interactions between the ionic clusters are fully frustrated, which is in distinct contrast to the strong tendency to crystallize in atomic ionic solids or previous superatomic crystals.2,11 Electron absorption spectroscopy confirms the electron transfer from [Co] to [PCBM]. We observe an absorption band between 1020 and 1025 nm that is characteristic of the [PCBM] radical anion12 for all superatomic films (Figure 2e and Figures S38−S43). Neither [Co] nor [PCBM] have this feature on their own (Figures S44 and S45). This result is consistent with the redox potentials of the individual clusters, which corresponds to 1/x electron per [PCBM] in [Co][PCBM]x (x = 2, 3, 4, 5, 7, 9) (Figure 1b). All superatomic thin-films are highly transparent in the visible range (Figure 3a). Transmittance spectra in the wavelength range of 350−750 nm are shown in Figure 3b. Transmittances increase with wavelength, and up to 92% were observed. The transmittance of superatomic films at 550 nm is about 80%, which is comparable to that of the widely used transparent conductor material PEDOT:PSS (85%).13 We measured the electrical conductivities, σ, of superatomic thin-films using the two-probe electrical resistivity method (Figures S46−S51). The σ is between 19 and 301 S/m (Table S3), and possesses a volcano-type relationship with the 10969
DOI: 10.1021/jacs.9b04705 J. Am. Chem. Soc. 2019, 141, 10967−10971
Communication
Journal of the American Chemical Society
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Synthesis, optical images, SEM and EDS, AFM, TEM, UV−vis, electrical conductivity measurement, thermal conductivity measurement, thermoelectric measurement (PDF)
AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *
[email protected] ORCID
Jingjing Yang: 0000-0002-1192-7368 Samuel R. Peurifoy: 0000-0003-3875-3035 Xavier Roy: 0000-0002-8850-0725 Jonathan A. Malen: 0000-0003-4560-4476 Colin Nuckolls: 0000-0002-0384-5493 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.N. thanks Sheldon and Dorothea Buckler for their generous support. Support for this research was provided by the Center for Precision Assembly of Superstratic and Superatomic Solids, an NSF MRSEC (award number DMR-1420634), the Air Force Office of Scientific Research (award number FA9550-181-0020), and the Army Research Office (award number ARO#71641-MS). Shared Materials Characterization Laboratory at Columbia University, maintained using funding from Columbia University for which we are grateful. We thank Mr. Jose Antonio Bahamonde for assistance in thermoelectric measurement.
Figure 4. (a) Room-temperature k of superatomic thin-films with uncertainties (Table S6). Data for pure [Co] and [PCBM] are included for comparison. (b) Seebeck coefficients S [−129 (2) μV/K and −169 (4) μV K−1] and ZT values [0.0213 (0.0044) and 0.0203 (0.0066)] of [Co][PCBM]5 and [Co][PCBM]9 at room temperature.
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Weidemann Franz Law indicates that less than 3% of its k results from electrons. The high electrical conductivities and low thermal conductivities spurred us to investigate these superatomic thin-films as solution-processable thermoelectrics. We measured their thermoelectric susceptibility of [Co][PCBM]5 and [Co][PCBM]9 (Figure S58) and obtained Seebeck coefficients (S) of −129 and −169 μV K−1, respectively (Figure 4b, left). The negative Seebeck coefficients confirm that electrons are the major carriers in these thin-films. ZT values are calculated to be of ∼0.02 at room temperature (298 K) for both thin-film samples (Figure 4b, right). In summary, we demonstrate the chemistry of frustrating electrostatic forces that typically bond atoms, and clusters into crystalline solids in ionic superatomic material. We install flexible side-chains on nanoscale clusters to hinder the crystal formation. The result is that we can create superatomic materials that can be easily solution-processed into completely amorphous and homogeneous thin-films with tunable compositions and functions unattainable in crystals but highly desired for devices. In addition, such simple use of nanoclusters with appropriate redox potentials and long and flexible side-chains could be generally applied to create various solution-processable and multifunctional cluster solids.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b04705. 10970
DOI: 10.1021/jacs.9b04705 J. Am. Chem. Soc. 2019, 141, 10967−10971
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