Hierarchically Structured Thermoelectric Materials ... - ACS Publications

Subscriber access provided by UNIV OF CAMBRIDGE

Article

Hierarchically Structured Thermoelectric Materials in Quaternary System Cu-Zn-Sn-S Featuring a Mosaic-type Nanostructure Chao Li, Yawei Shen, Rong Huang, Akihito Kumamoto, Shiyou Chen, Chenmin Dai, Masato Yoshiya, Susumu Fujii, kohei funai, Craig A.J. Fisher, Yifeng Wang, Ruijuan Qi, Chun-Gang Duan, Lin Pan, Junhao Chu, Tsukasa Hirayama, and Yuichi Ikuhara ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00278 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Hierarchically Structured Thermoelectric Materials in Quaternary System Cu-Zn-Sn-S Featuring a Mosaic-type Nanostructure ⊥

Chao Li,† Yawei Shen,‡ Rong Huang,*, †, §, ǁ Akihito Kumamoto, Shiyou Chen,† Chenmin Dai, † Masato Yoshiya,§, # Susumu Fujii, #

Kohei Funai, # Craig A. J.

Fisher, § Yifeng Wang,*, ‡ Ruijuan Qi,† Chun-Gang Duan,† Lin Pan,‡ Junhao Chu,† Tsukasa Hirayama§ and Yuichi Ikuhara§, ⊥ †

Key Laboratory of Polar Materials and Devices, Ministry of Education, East China

Normal University, Shanghai 200062, China. ‡

College of Materials Science and Engineering, Nanjing Tech University, Nanjing

210009, China. §

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587,

Japan ǁ

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan,

Shanxi 030006, China ⊥

#

Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan

Department of Adaptive Machine Systems, Osaka University, Osaka 565-0871, Japan

1

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

ABSTRACT Multinary chalcogenide semiconductors in the Cu-Zn-Sn-S system have numerous potential applications in the fields of energy production, photocatalysis and nonlinear optics, but characterisation and control of their microstructures remains a challenge because of the complexity resulting from the many mutually soluble metallic elements. Here, using state-of-the-art scanning transmission electron microscopy, energy dispersive spectroscopy, first-principles calculations and classical molecular dynamics simulations, we characterise the structures of promising thermoelectric materials Cu2(Zn,Sn)S3 at different length scales to gain a better understanding of how the various components influence the thermoelectric behaviour. We report the discovery of a mosaic-type domain nanostructure in the matrix grains comprising well-defined cation-disordered domains (the “tesserae”) coherently bonded to a surrounding network phase with semi-ordered cations. The network phase is found to have composition Cu4+xZnxSn2S7, a previously unknown phase in the Cu-Zn-Sn-S system, while the tesserae have compositions closer to that of the nominal composition.

This

nanostructure

represents

a

new

kind

of

phonon-glass

electron-crystal, the cation-disordered tesserae and the abrupt domain walls damping the thermal conductivity while the cation-(semi)ordered network phase supports a high electronic conductivity. Optimisation of the hierarchical architecture of these materials represents a new strategy for designing environmentally benign, low-cost thermoelectrics with high figures of merit.

2


Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Keywords: scanning transmission microscopy, thermoelectricity, mosaic-like nanodomain, Cu4ZnSn2S7, hierarchical features INTRODUCTION Multinary chalcogenide semiconductor systems1 such as I2-II-IV-VI4, I2-III-VI4, I-III-IV-VI4 and I-III-VI2, where I = Cu or Ag; II = Zn, Cd, Mn, Fe, Co or Ni; III = In, Ga or Sb; IV = Si, Ge, Sn or Ti; and VI = S, Se or Te, constitute a large family of materials and are of intense interest because of their many potential applications in thermoelectric conversion,2-5 solar-cell absorbers,6-7 photocatalysts for solar water splitting,8-9

nonlinear

optics,10

topological

insulators,11

magneto-optics

and

magneto-ferroics.12-13 The crystals of these materials are formed from cation-anion tetrahedral units in accordance with the valence-octet rule.

14

The distribution of

cations over interstices within the group VI anion sublattice mainly depends on a balance between the cation valences and radii, and different distributions can result in very different materials properties. Some of the major difficulties encountered when characterising these materials are (i) the presence of impurity phases; (ii) varying degrees of cation intersite exchange and ordering within and between grains; and (iii) non-uniform distributions and concentrations of other point defects. One of the chief reasons for these difficulties is that the constituent atoms have similar masses, ion radii and electronic configurations, so differences between crystal structures with different compositions and/or degrees of cation ordering are often beneath the limit of detection by experiment.15 Nevertheless, in order to optimise these materials for high-performance, energy-efficient applications, it is important to obtain an 3



Page 4 of 34

understanding of the relationship between their compositions, nanostructures, microstructures and materials properties.

The quaternary system Cu-Zn-Sn-S contains some of the most studied chalcogenides for use in solar cell thin films.16-18 For example, ternary compound Cu2SnS3 has a melting point of 1129 K19 and a direct band gap of 0.9~1.3 eV,20 existing as one of three possible polymorphs with monoclinic (Figure 1(a)), cubic (Figure 1(b)), or tetragonal symmetry depending on the arrangement of its corner-sharing MS4 (M = Cu or Sn) tetrahedra. The particular crystal structure adopted depends on the degree of Cu/Sn disorder in the zinc-blende parent structure.20 According to first-principles band-structure calculations, Cu2SnS3 should also be a good candidate for thermoelectric applications at intermediate temperatures.21-22 Indeed, recent measurements have confirmed that it can be used as a p-type thermoelectric material with a relatively high thermopower (up to 300 mV/K) and low thermal conductivity (below 1.0 W/m K) at temperatures above 700 K.23 A figure of merit (ZT) close to 0.6 at a relatively low temperature (773 K) has been achieved by replacing 10 mol% Sn with In, holding out the promise of even higher values once the structure and composition have been optimised. When alloyed with ZnS, the microstructures of these materials become considerably more complex. First-principles calculations have shown that the energy required for Zn and Cu cations to exchange positions in (001) layers of Cu2ZnSnS4 (CZTS) crystals is very small, so that such exchange should occur facilely under normal synthesis conditions24. This explains why multiple secondary phases, partially 4


Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ordered/disordered structures, and a high concentration of intrinsic defects (isolated or clustered) have been observed to coexist in CZTS crystals.16 The quaternary alloy also typically exhibits relatively low thermal conductivity, making it another attractive material for thermoelectric applications.25-30 In other thermoelectric materials, complex nano/microstructures, such as modulated structures and nanodots,31-32 interlaced domains,15 mosaic crystals,33 and multiscale hierarchical architectures,34 are known to be important for enhancing phonon scattering to achieve lower thermal conductivities and higher ZT. Recently we reported that Cu2ZnxSn1-xS3 (x = 0.00, 0.05, 0.10, 0.15, 0.20) polycrystalline ceramics exhibit good thermoelectric performance.35 The lattice thermal conductivity was found to decrease monotonically with increasing Zn content, making it an excellent system for investigating any correlations between microstructure and thermal conductivity. However, because the secondary phases share the same zinc-blende-type structure as the parent phase, it is very difficult to determine their precise structural parameters, compositions and relative abundance using X-ray diffraction (XRD), neutron diffraction and Raman scattering analysis alone. In this study, atomic resolution spherical aberration-corrected scanning transmission electron microscopy (STEM) and X-ray energy dispersive spectroscopy (EDS) mapping techniques have been used to characterise the micro- and nanostructures, including secondary phases, local cation ordering, and crystal domains, of polycrystalline Cu2(Zn,Sn)S3 ceramics. The matrix phase was found to have a 5



Page 6 of 34

distinctive mosaic-type nanostructure consisting of coherently bonded domains with fully disordered and semi-ordered cation sublattices. The semi-ordered domains correspond to a previously unknown phase in the Cu-Zn-Sn-S system. Lattice stabilities and thermal conductivities of these and other multinary sulfides were examined using a combination of first-principles calculations and classical perturbed molecular dynamics simulations to better understand the mechanisms responsible for the very low lattice thermal conductivity and good thermoelectric performance of these materials. Experimental Section Synthesis of the compounds and preparation ceramic of preparation. Ingots with nominal compositions Cu2ZnxSn1-xS3, with x = 0, 0.05, 0.10, 0.15 or 0.20, were synthesized by direct reaction of high purity powders in the appropriate molar ratio in a sealed silica tube at high temperature. Each ingot was ground into a powder and then consolidated into pellets by spark plasma sintering under 50 MPa at 773 K for 5 min. Relative densities of the sintered pellets were determined by the Archimedes method to be ~ 94% of the theoretical value in all cases. More details can be found in ref. 34.

STEM observation. Transmission electron microscopy specimens were prepared by a standard procedure which included mechanical grinding, polishing, precision dimpling, and ion milling. To minimise damage, samples were held at 100 K during milling. 6


Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electron diffraction analyses and HRTEM observations were carried out using a field emission microscope (JEM-2100F, JEOL, Co., Tokyo, Japan) operated at 200 kV. STEM observations were performed using an aberration-corrected scanning transmission electron microscope (JEM-ARM200CF, JEOL Co. Ltd) operated at 200 keV. A probe convergence angle of 25 mrad and a detection angle of 73-194 mrad were used.

STEM-EDS maps were acquired by scanning the beam over different regions, and using the NSS3 software developed by Thermo Fisher Scientific Inc. The STEM-EDS system was equipped with a silicon drift detector (SDD) and the solid angle for the whole collection system was about 1.7 sr. The probe size was 1.2 Å with a probe current of about 60 pA. Spectral peaks used for EDS mapping were Kα = 8.040 keV for Cu, Kα = 8.637 keV for Zn, Lα = 3.443 keV for Sn and Kα = 2.307 keV for S. The individual element maps were also summed to produce a combined map. In order to obtain sufficiently strong signals for individual spectra, the total acquisition time for each map was about 100 min with a dwell time of 20 ms. Measurements were repeated to confirm reproducibility and were found to be little affected by varying the experimental conditions reported here.

Conductive atomic force microscopy (C-AFM) measurements. C-AFM measurements were performed in air using an Asylum MFP-3D atomic force microscope. Current detection was achieved with an Asylum Research ORCA cantilever holder, which has a current resolution of 1 pA up to a maximum of 20 nA. 7



Page 8 of 34

Conducting diamond-coated silicon probes (Nanoworld CDT-NCHR) with a nominal 300 kHz resonant frequency and 40 Nm-1 nominal spring constant were used for all AFM images. A microscopy specimen containing the mosaic nanostructure was used for the C-AFM measurements so that the electrical conductivities of different domains could be distinguished.

First-principles calculations. Total energy calculations were performed within the framework of density-functional theory as implemented in the Vienna Ab Initio Simulation Package (VASP).36 The generalized gradient approximation to the exchange-correlation functional and projector augmented wave pseudopotentials were used with an energy cut-off of 500 eV for the planewave basis set and a 10 × 10 × 10 Monkhorst-Pack grid for k-point sampling. To correct for bandgap underestimation using the GGA functional, the HSE hybrid functional was also used to calculate band structures. Structural relaxations were performed until the Hellman-Feynman forces on all atoms were smaller than 1 meV/Å. Other settings were the same as reported elsewhere.20,24,37

Thermal conductivity calculations. A perturbed molecular dynamics method was used to calculate thermal conductivities of a number of different zinc-blende-related structures and compositions. The Stillinger-Weber force field38 for Si was used for all atomic interactions, with only the atomic masses changed according to the type of element.

8


Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A 38 × 38 × 38 Å3 supercell was used to model ordered and disordered binary (SiS, CaS, CuS, ZnS, MoS, SnS, LaS and PbS), ternary (Cu2SnS3), and quaternary (Cu4ZnSn2S7 and Cu2ZnSnS4) zinc-blende-type structures with the types and positions of the cations changed to match the target composition and distribution. To model the mosaic structure, a 190 × 190 × 38 Å3 zinc-blende supercell containing 68,600 atoms was used, with a diamond-shaped cation-disordered region embedded within a cation-ordered region. The volume ratio of the disordered region to the simulation box (supercell) was 0.405. For simplicity, a fully ordered cation sublattice was used to represent the network phase rather than a semi-ordered cation sublattice. As the atomic masses of Cu and Zn are close to each other, the thermal conductivities of the semi-ordered and fully ordered structures should be very similar, so that this approximation should not unduly affect interpretation of the results. Equilibration under constant pressure and temperature conditions (NPT ensemble) was performed for each model for 2 million time steps, long enough to determine the equilibrium lattice constants reliably, at 700 K with a time step of 1 fs. Next, thermal equilibration under constant volume conditions (NVT ensemble) with the equilibrium lattice constants obtained from the previous stage was performed for 500,000 steps using temperature scaling, followed by a further 1.5 million steps using a Nosé-Hoover thermostat to achieve full thermal equilibrium of all atoms in the supercell. The system was then subjected to a thermal perturbation that produced a small heat flow during a production run of 15 million time steps, ensuring that the system remained in a steady state. 9



Page 10 of 34

The thermal conductivity of each system was calculated from the heat flux using the last 10 million steps of the production run. In each case, more than three perturbations of different magnitudes were used to confirm that the system remained within the linear response regime and to improve the statistics. Thermoelectric properties measurements: The Seebeck coefficients and electrical conductivities were measured in the radial direction of a bar shaped specimen with dimensions of 10mm×3mm×3mm by a conventional steady state method and a four-probe method, respectively, in He atmosphere at 323 to 723 K with a commercial system (LSR-3, Linseis, Germany). Thermal diffusivity (D) was measured in the axial direction of a disk-shaped sample of Φ10 mm × 1 mm using a Netzsch laser flash diffusivity instrument (LFA457, Netzsch, Germany). Thermal conductivity (κ) was calculated by κ = DdCp, where Cp is the specific heat capacity measured by differential scanning calorimetry (DSC: 2910, TA instruments, America), and d is the mass density measured using the Archimedes method. RESULTS AND DISCUSSION Structural characterization and chemical analysis. The crystal structure was firstly analyzed by powder X-ray diffraction (XRD) and Raman spectrum shown in Figure S1. The results revealed an interesting structural evolution in Cu2ZnxSn1−xS3 with x from 0 to 0.20. For undoped Cu2SnS3 sample, XRD and Raman spectra show a monoclinic Cu2SnS3 .39 high resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) of the undoped (parent) phase Cu2SnS3 along the [100] zone axis also confirmed that it had a monoclinic crystal structure 10


Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(space group: C c, a=6.653 Å, b=11.537 Å, c= 6.665 Å and β=109.39º) shown in Figure. S2 (a). Figure 1(c) shows a typical HAADF-STEM image of Cu2SnS3, with ordered columns of Sn and Cu atoms. The sample was confirmed to be single phase.

In contrast to the parent phase, HRTEM and SAED along the [110] cubic zone axis (shown in Figure. S2(b)) showed that crystals in the 5 mol% Zn-doped sample had cubic symmetry (space group: F -43m, a=5.430 Å). This increase in symmetry is consistent with the Cu, Zn and Sn atoms being randomly distributed over cation sites of the zinc-blende structure. In the samples with higher Zn doping level, the XRD and Raman spectra shown in Figure S1 cannot match with the reported phases in the Cu-Zn-Sn-S quaternary system. In order to reveal the detailed phase information, advanced electron microscopy techniques with high spatial resolution are necessary.

Figure 1(d) shows a typical low-magnification bright field TEM image of a crystal grain in a 20 mol% Zn-doped sample; dark elongated nanoparticles (precipitates) are visible embedded in a grey matrix phase. EDS maps in Figure 1(e) show that the precipitates are rich in Cu but poor in Zn and Sn, while the S content is uniform throughout. The height of the Cu peak in the EDS spectrum of the precipitate (lower-right graph of Figure 1(e)) is nearly twice that of Cu in the matrix phase, leading us to conclude that the precipitates are platelets of composition Cu2S with almost no Zn or Sn. It is further confirmed as tetragonal Cu2S phase (Space Group: P 43 21 2, a = b = 3.9962 Å and c = 11.2870 Å) by HRTEM and corresponding fast Fourier transform pattern of the nanoprecipitate, as shown in Figure S4. 11



Page 12 of 34

To determine the structure of the matrix crystals, the uniform grey area within the red rectangle in Figure 1(d) was imaged at atomic resolution using HAADF-STEM (Figure 2(a)). This revealed a distinctive mosaic nanostructure with roughly 10-nm wide facetted domains (the “tesserae”) surrounded by a ~5-nm wide connective phase forming a continuous crystalline network. Remarkably, the crystal lattices of the “tessera” domains and network domain were fully coherent.

Because the contrast of the HAADF image is roughly proportional to the square of atomic number (Z), and thus very sensitive to heavy elements,40-41 the image contrast in Figure 2(a) is dominated by the cations, namely Cu, Zn and Sn. The centres of the tesserae can be seen to consist of square arrays of cation columns with uniform contrast, whose lattices produce the main diffraction spots in the SAED pattern (Figure 2(a) inset), corresponding to overall cubic symmetry. The SAED pattern also revealed many weak diffraction spots (marked with red arrows) distributed periodically between the main spots, indicating the presence of regions of lower symmetry. The bright atom columns, corresponding to Sn (Z = 50), distributed periodically among darker atom columns of Cu (Z = 29) and Zn (Z = 30) in the connective layers in Figure 2(a), reveal a partially ordered structure. Because cation ordering is known to produce a monoclinic distortion, the weak superlattice spots in the SAED pattern can be attributed to this network phase. However, the superlattice spots do not correspond to the monoclinic structure of ordered Cu2SnS3, suggesting that this phase is different to any of those reported to date in the quaternary Cu-Zn-Sn-S system. 12


Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To obtain chemical information about the different domains in the mosaic nanostructure, we used a recently developed atomic-resolution EDS mapping technique.42-43 Figure 2(b) shows a HAADF-STEM image and STEM-EDS maps from the regions enclosed by the yellow squares marked ‘1’ and ‘2’ in Figure 2(a). The bright columns in the HAADF image of region 1 indicate a high concentration of Sn. From the Zn map, it can be seen that Zn atoms are not distributed uniformly. Instead they preferentially occupy certain sites (connected by the white dashed lines in the figure) that are modulated by columns of Sn atoms. Concomitantly, in the Cu map, Sn-rich columns show almost no contrast, while Zn-rich columns are slightly darker than Zn-poor columns, and the S map shows uniform contrast. EDS mapping thus indicates that Sn atoms are ordered in a diamond arrangement (when viewed down [010]cubic) within the network phase, while Zn and Cu atoms are distributed over the remaining cation sites, but with a higher concentration of Zn in the central columns between Sn diamonds than Cu.

In contrast, atomic-resolution STEM-EDS mapping of region 2 shows that all three metal elements (Sn, Zn and Cu) are randomly distributed over the cubic cation sites (Figure 2(b)). EDS mapping thus revealed that cations in the facetted domains (tesserae) are fully disordered, while in the surrounding network phase they are semi-ordered, with ordering strongest for Sn and weakest for Cu. Quantitative EDS analysis also showed that the semi-ordered and disordered domains are similar in composition (Figure S3), Cu1.90Zn0.19Sn0.74S3 and Cu2.25Zn0.20Sn0.71S3, respectively, the main difference being that there is slightly more Cu in the latter than the former. The 13



Page 14 of 34

average composition is richer in Zn and Sn than the nominal sample composition, consistent with the formation of Cu2S precipitates. This suggests that Cu deficiency facilitates formation of the mosaic nanostructure.

Structural modelling. For quaternary Cu-Zn-Sn-S materials, the octet rule can be used to assess the energetic stability of any given structure.24, 37 This rule states that if the sum of the valence electrons of cations surrounding an anion divided by the anion’s coordination number plus the number of the anion’s valence electrons is equal to eight, then that structure will be in a low energy state because the anion has a closed shell. In the case of kesterite-structured Cu2ZnSnS4 (CZTS), with its network of MX4 tetrahedral units, this requires the sum of the valence electrons of the cations surrounding each anion to be equal to eight, since S has six valence electrons. Cu, Zn and Sn have one, two and four valence electrons, respectively, so when an S atom is bonded to two Cu, one Zn and one Sn atom, as is the case in CZTS, the octet rule is satisfied, consistent with the structure’s low lattice energy. However, in the case of Cu2SnS3 the rule is not satisfied: there are five possible configurations around an S atom, namely, 4Cu, 3Cu-Sn, 2Cu-2Sn, Cu-3Sn, and 4Sn, whose valence electron sums are 4, 7, 10, 13, and 16, respectively. In this situation, the structure with the smallest deviation from the octet rule is expected to be most stable. In addition, to preserve local charge neutrality, electron-rich clusters (e.g., 2Cu-2Sn) should be close to electron-deficient clusters (e.g., 3Cu-Sn). Indeed, pure Cu2SnS3 has a monoclinic structure whose unit cell is a supercell of the zinc-blende structure containing eight

14


Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Cu3Sn)-S and four (Cu2Sn2)-S tetrahedra; this is the network with the smallest deviation from the octet rule,20 and is consistent with our experimental results.

When small amounts of Sn in the 2Cu-2Sn clusters are replaced with Zn in Cu2SnS3, 2Cu-Zn-Sn clusters, which satisfy the octet rule, preferentially form as this lowers the total energy of the system. 2Cu-Zn-Sn clusters do not preferentially occupy any particular sites, disrupting the ordering of the 3Cu-Sn and 2Cu-2Sn clusters so that all cations become disordered, i.e., (Cu2ZnSn)-S, (Cu3Sn)-S and (Cu2Sn2)-S tetrahedra are distributed uniformly throughout the crystal. This is consistent with the overall cubic symmetry of the matrix crystals in Zn-doped Cu2SnS3 according to electron diffraction.

If sufficient Zn is added to give a composition close to Cu2ZnSnS4 (CZTS), (Cu2ZnSn)-S tetrahedra appear throughout the crystal structure because they are low in energy. Networks of these tetrahedra form the kesterite structure, illustrated in Figure 3(a), which is a (Cu-S)2(Zn-S)(Sn-S)(201) superstructure of the zinc-blende structure. To highlight the relationship between the two structures, the Sn atom columns in Figure 3(a) are connected by black lines, which form diamonds each enclosing two Cu columns and one Zn column. However, if half the Zn atoms are removed, an ordered structure, illustrated in Figure 3(b), can be derived from that of Cu2ZnSnS4 by removing half the (Zn-S)(201) layers, and shearing half the (Sn-S)(201) layers of the zinc-blende unit cell in the direction of the red arrows in Figure 3(a). This results in a (Cu-S)2(Zn-S)(Sn-S)(Cu-S)2(Sn-S)(201) superstructure, whose overall composition is 15



Page 16 of 34

Cu4ZnSn2S7 (Cu2SnS3:ZnS = 2:1). This structure has the minimum deviation from the octet rule and thus the lowest energy for this composition, with the Sn positions matching those observed in the network domains of the mosaic nanostructure from HAADF-STEM imaging (Figure 2(b), Region 1). The positions of (Zn-S) (201) layers are different in the derived and observed structures, however, as the EDS maps in Figure 2(b) show that all three [010] columns within an Sn diamond contain some Cu atoms, i.e., the Zn and Cu atoms are distributed over all three cation positions rather than being fully ordered, which is reasonable given the low formation energy of Cu/Zn exchange defects. Re-writing the composition determined by EDS analysis of the semi-ordered region in Figure 2(b) as Cu4.50Zn0.45Sn1.72S7 suggests that it may be a Cu-rich form of Cu4ZnSn2S7. If so, this new phase appears to be tolerant to relatively large deviations from the ideal Cu/Zn ratio of 4:1. Based on these results, the Cu2SnS3-ZnS pseudo-binary phase diagram in Figure 3(c) was constructed to show where Cu4ZnSn2S7 lies relative to other phases in the Cu-Zn-Sn-S system. According to the Raman spectra shown in Figure S1 (c), the two peaks located at 305 and 355 cm−1 may originate from the cubic Cu2SnS3 structure (fully disordered region) in the Cu2Zn0.2Sn0.8S3 sample. In addition, the peak at 339 cm-1 is related to the third A mode in CZTS,44 which is corresponding to the vibrations of the S anion sublattice and is influenced by the local S coordination. This characteristic peak indicates the formation of S-Cu2ZnSn ordering in the Cu2Zn0.2Sn0.8S3 sample, consistent with the proposed Cu4ZnSn2S7 structure.

16


Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


First-principles calculations were performed to determine the relative stabilities of disordered and ordered forms of this new Cu4ZnSn2S7 structure, whose lattice parameters are a=8.6013 Å, b=5.4400 Å, c=12.1641 Å, β= 81.86º, and space group P1. Structure models of twelve possible cation orderings within monoclinic unit cells and composition Cu4ZnSn2S7 were constructed as illustrated in Figure 4(a). Their total energies and bandgap widths are listed in Figure 4(b). Structures 1, 4, 8, and 9 (highlighted with dashed rectangles in the table) have the lowest energies out of the twelve, each with the same ordering of Sn atoms as the observed network structure. In these four structures, there are two symmetrically nonequivalent Cu-Zn configurations, illustrated in Figure 4(c). Configuration 1 is shared by structures 1 and 8, and configuration 2 is shared by structures 4 and 9. The energies of all four structures, however, are almost the same, indicating that the different Cu/Zn configurations have little effect on the total energy, consistent with Cu and Zn atoms being disordered over all three sites within the ordered Sn framework as deduced from atomic-resolution EDS mapping (Figure 2(b)).

Electronic properties. Band structure calculations revealed that the four lowest-energy structures all have relatively narrow bandgaps of about 0.84 eV. Figure 4(d) shows the band structure diagram of structure 1, with its high curvature conduction band at the Γ point, suggesting good semiconductivity. Taking an average of all twelve cation configurations to represent the disordered configuration indicates it will have a smaller band gap. However, because disordering increases the scattering of electrons and holes, carrier mobility in the disordered domains is likely lower than 17



Page 18 of 34

that in the semi-ordered domains. Measurement of the bandgap may itself serve as a “fingerprint” of the degree of cation ordering in the system. The larger bandgap of the semi-ordered structure may also make it attractive as a photovoltaic material.

Experimentally, Zn-doped Cu2SnS3 samples exhibit a good combination of high electronic conductivity and ultralow thermal conductivity, as listed in Table S2. The electrical conductivity of the samples increases with the Zn content. In addition to the increase of carriers contents due to the charge doping effect, our first-principles calculations also suggest that the semi-ordered Cu4ZnSn2S7 phase may contribute to the good electrical conductivity because of its high carrier mobility, similar to the experimentally measured properties of Cu5Sn2S7.45 To determine the electrical properties of the different phases in Zn-doped Cu2SnS3, conductive atomic force microscopy (C-AFM) was performed on a TEM specimen of the 20 mol% Zn-doped sample. Figure 5(a) shows a height map of the observed area, with the height gradually increasing from left to right. No domain structure is apparent. However, alternately bright and dark contrasts are visible in the corresponding current map in Figure 5(b), consistent with the distribution of semi-ordered and disordered domains observed by HAADF-STEM. The widths of the high current areas in Figure 5(c) are 8-10 nm, whereas the widths of the low current areas are only 2-4 nm. These sizes match closely the widths of the domains observed by HAADF-STEM, with the semi-disordered domains having higher conductivity than the fully disordered domains, as inferred from

the first-principles calculations. The narrowness of the 18


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electron-transport (boundary) domain may also induce quantum confinement and/or electron filtering effects that enhance the Seebeck coefficient.46 This mosaic nanostructure thus represents a new type of phonon-glass electron-crystal topology, with a high-mobility semiconducting phase interwoven with a phonon scattering, low thermal conductivity phase.

Thermal conductivity calculations. To investigate the impact of cation disorder and the mosaic nanostructure on thermal conduction at the submicron scale, theoretical calculations of thermal conductivity in a number of sulfides were carried out using perturbed molecular dynamics47-48 at 700 K. For simplicity, all structures were assumed to be tetragonal, with no monoclinic distortion, so that only differences in the atomic masses of the cations affected the lattice thermal conductivity. Thermal conductivities of eight binary sulfides, SiS, CaS, CuS, ZnS, MoS, SnS, LaS and PbS, were calculated using this method, with values decreasing from 42.5 to 25.7 W/mK with increasing cation mass (see Table S1). All these values are high, as expected, including that of the heaviest sulfide in the Cu-Zn-Sn-S system, SnS, at 32.5 W/mK. Lower values were obtained for ordered ternary and quaternary sulfides: 16.6, 17.7 and 21.9 W/mK for Cu2SnS3, Cu4ZnSn2S7 and Cu2ZnSnS4, respectively. The lower thermal conductivities of these compounds compared to the binary compounds is consistent with the well-known alloying effect whereby mixtures of atoms with different atomic masses, even when they are ordered, produce lower thermal conductivities than their end members. Cation disorder, however, results in a striking decrease in thermal conduction even in the absence of monoclinic distortion; in the case of disordered 19



Page 20 of 34

Cu4ZnSn2S7, the lattice thermal conductivity decreased by ca. 94% to 1.0 W/mK. This can be attributed to the strong dispersion of short-wavelength phonons when atoms of different mass are distributed over the cation sublattice, making heat transfer on the nanometre scale highly inefficient. All the thermal conductivities calculated for ternary and quaternary sulfides are compared with those of SnS and CuS in Table 1.

Table 1. Thermal conductivities of five multinary compounds and their ordered and disordered forms calculated using perturbed molecular dynamics.

[Cu,Zn]/([Cu,Zn]+[Sn]) Thermal Conductivit y [W/mK]

ordered

SnS

Cu2SnS3

Cu4ZnSn2S7

Cu2ZnSnS4

CuS

0.00

0.67

0.71

0.75

1.00

32.5±2.9

16.6±0.9

17.7±1.3

21.9±1.3

34.2±2.2

0.9±0.4

1.0±0.4

1.2±0.3

disordered

The thermal conductivity of the

68,000-atom

supercell containing a

diamond-shaped cation-disordered region embedded in a cation-ordered region was calculated to be 4.5 W/mK, as much as 59% less than the thermal conductivity expected by taking the volume average of the thermal conductivities of the ordered and disordered structures calculated separately, as shown in Table 2. This decrease in thermal conductivity originates from the abrupt interfaces between the two chemically similar

but

phononically

dissimilar

regions,

effectively

scattering

intermediate-wavelength phonons. The mosaic nanostructure thus plays an important role in suppressing thermal conductivity in these sulfides.

20


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Thermal conductivity of the mosaic structure compared with the arithmetic average by volume fraction of the separate phases calculated using perturbed molecular dynamics. Mosaic structure

Cation-Order ed Cu4ZnSn2S7

Cation-Disord ered Cu4ZnSn2S7

Average

0.60

0.40

1.00

17.7±1.3

1.0±0.4

11.0

Volume Fraction Thermal Conductivity [W/mK]

4.5 ±0.3

In the actual material, in addition to the intragrain mosaic nanostructure, Cu2S precipitates also serve to reduce the thermal conductivity by scattering long-wavelength phonons. Based on our microstructural and nanostructural analysis, the three main mechanisms responsible for the much lower thermal conductivity in Cu2(Zn,Sn)S3 samples compared to more conventional polycrystalline materials appear to be (i) disorder over the cation sublattice within tessera domains in the matrix grains; (ii) abrupt interfaces between the cation-(semi)ordered network domains and the cation-disordered tessera domains; and (iii) large lattice strain within matrix grains introduced by platelet-like Cu2S precipitates. This structural hierarchy and the different phonon scattering mechanisms are summarized schematically in Figure 6. The combination of mechanisms at different length scales results in a thermal conductivity as low as 0.4 W/mK at 723 K for the 20 mol% Zn-doped Cu2SnS3 sample in Table S2, which is comparable to those of other Cu-containing sulfides with complex microstructures and low thermal conductivities, 21



Page 22 of 34

such as Cu4Sn7S16,25 colusite Cu26V2Sn6S3227 and tetraedrite Cu12Sb4S13.28 With no toxic elements, Cu2Zn0.2Sn0.8S3 is already a competitive candidate material for low-cost, environmentally friendly thermoelectric applications, with further avenues for improvement available by optimising the volume fractions and compositions of the various hierarchical components.

CONCLUSIONS

Atomic-resolution aberration-corrected STEM of crystals in Cu2SnS3-ZnS alloys revealed a mosaic nanostructure comprising ~10 nm-wide facetted tessera domains coherently bonded to a surrounding connective (network) phase with branches ~5 nm thick. Cations in the tesserae of the mosaic structure were found to be fully disordered, while those in the network phase were semi-ordered. The distributions of Sn, Zn and Cu atoms in the connective phase were determined by atomic-resolution EDS mapping, and the structure and composition found to correspond to that of a previously unknown phase in the Cu-Zn-Sn-S system, Cu4+xZn1-xSn2S7. Interweaving of semi-ordered and fully disordered domains within the crystal grains, combined with other hierarchical features such as Cu2S precipitates and grain boundaries, explains the observed combination of good electronic conductivity and low thermal conductivity of Cu2(Zn,Sn)S3 ceramics. Our findings suggest new ways of designing nanostructures to optimise the physical properties of multinary chalcogenide semiconductors by tuning site occupancies of cations, size and widths of mosaic domains, and the amount of precipitates. The methods used here can be readily 22


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


extended to the study of other complex inorganic materials, and are expected to aid in the development of novel and improved functionalities through materials design at the atomic scale upwards.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.XXXXXX.

The structure characterization of Cu2ZnxSn1-xS3 ceramic samples with x = 0, 0.05, 0.10,

0.15 and 0.20. component analysis of the cation-(semi)ordered and cation-disordered domains by EDS. Thermal conductivities of binary phases. Electrical properties and thermal conductivity. (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2017YFA0303403), the National Key Project for Basic Research of China (Grants No. 2014CB921104), NSFC under Grants No. 51572085 and 61574059, Natural Science Foundation of Shanghai (16ZR1409500) and CC of ECNU. RH was 23



Page 24 of 34

supported partially by the Public Foundation of Chubu Science and Technology Center, Japan. REFERENCES (1) Coughlan, C.; Ibanez, M.; Dobrozhan, O.; Singh, A.; Cabot, A.; Ryan, K. M., Compound Copper Chalcogenide Nanocrystals. Chem. Rev. 2017, 117, 5865-6109. (2) Fan, F. J.; Wang, Y. X.; Liu, X. J.; Wu, L.; Yu, S. H., Large-scale colloidal synthesis of non-stoichiometric Cu2ZnSnSe4 nanocrystals for thermoelectric applications. Adv. Mater. 2012, 24, 6158-6163. (3) Fan, F.-J.; Wu, L.; Yu, S.-H., Energetic I-III-VI2and I2-II-IV-VI4nanocrystals: synthesis, photovoltaic and thermoelectric applications. Energy Environ. Sci. 2014, 7, 190-208. (4) Qiu, P.; Zhang, T.; Qiu, Y.; Shi, X.; Chen, L., Sulfide bornite thermoelectric material: a natural mineral with ultralow thermal conductivity. Energy Environ. Sci. 2014, 7, 4000-4006. (5) Heinrich, C. P.; Day, T. W.; Zeier, W. G.; Snyder, G. J.; Tremel, W., Effect of isovalent substitution on the thermoelectric properties of the Cu2ZnGeSe4-xSx series of solid solutions. J. Am. Chem. Soc. 2014, 136, 442-448. (6) Tanaka, K.; Oonuki, M.; Moritake, N.; Uchiki, H., Cu2ZnSnS4 thin film solar cells prepared by non-vacuum processing. Sol. Energy Mater. Sol. C. 2009, 93, 583-587. (7) Guo, Q.; Ford, G. M.; Yang, W. C.; Walker, B. C.; Stach, E. A.; Hillhouse, H. W.; Agrawal, R., Fabrication of 7.2% efficient CZTSSe solar cells using CZTS nanocrystals. J. Am. Chem. Soc. 2010, 132, 17384-17386. (8) Tsuji, I.; Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A., Novel Stannite-type Complex Sulfide Photocatalysts AI2-Zn-AIV-S4 (AI= Cu and Ag; AIV= Sn and Ge) for Hydrogen Evolution under Visible-Light Irradiation. Chem. Mater. 2010, 22, 1402-1409. (9) Regulacio, M. D.; Han, M. Y., Multinary I-III-VI2 and I2-II-IV-VI4 Semiconductor Nanostructures for Photocatalytic Applications. Acc. Chem. Res. 2016, 49, 511-519. (10) Samanta, L. K.; Bhar, G. C., Optical nonlinearity of some stannite and famatinite crystals. Phys. Status Solidi (a) 1977, 41, 331-337. (11) Chen, S.; Gong, X. G.; Duan, C.-G.; Zhu, Z.-Q.; Chu, J.-H.; Walsh, A.; Yao, Y.-G.; Ma, J.; Wei, S.-H., Band structure engineering of multinary chalcogenide topological insulators. Phys. Rev. B 2011, 83, 245202. (12) Fries, T.; Shapira, Y.; Palacio, F.; Morón, M. C.; McIntyre, G. J.; Kershaw, R.; Wold, A.; McNiff, E. J., Magnetic ordering of the antiferromagnet Cu2MnSnS4 from magnetization and neutron-scattering measurements. Phys. Rev. B 1997, 56, 5424-5431. (13) Nenert, G.; Palstra, T. T., Magnetoelectric and multiferroic properties of ternary copper chalcogenides Cu2M(II)M(IV)S4. J. Phys. Condens. Matter. 2009, 21, 176002. (14) Pamplin, B. R., Super-Cell Structure of Semiconductors. Nature 1960, 188, 136-137. 24


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Shen, X.; Hernandez-Pagan, E. A.; Zhou, W.; Puzyrev, Y. S.; Idrobo, J. C.; Macdonald, J. E.; Pennycook, S. J.; Pantelides, S. T., Interlaced crystals having a perfect Bravais lattice and complex chemical order revealed by real-space crystallography. Nat. Commun. 2014, 5, 5431. (16) Chen, S.; Walsh, A.; Gong, X. G.; Wei, S. H., Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers. Adv. Mater. 2013, 25, 1522-1539. (17) Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B., Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Adv. Energy Mater. 2014, 4, 1301465. (18) Kaur, K.; Kumar, N.; Kumar, M., Strategic review of interface carrier recombination in earth abundant Cu–Zn–Sn–S–Se solar cells: current challenges and future prospects. J. Mater. Chem. A 2017, 5, 3069-3090. (19) Fiechter, S.; Martinez, M.; Schmidt, G.; Henrion, W.; Tomm, Y., Phase relations and optical properties of semiconducting ternary sulfides in the system Cu-Sn-S. J. Phys. Chem. Solids 2003, 64, 1859-1862. (20) Zhai, Y.-T.; Chen, S.; Yang, J.-H.; Xiang, H.-J.; Gong, X.-G.; Walsh, A.; Kang, J.; Wei, S.-H., Structural diversity and electronic properties of Cu2SnX3(X=S, Se): A first-principles investigation. Phys. Rev. B 2011, 84, 075213. (21) Shi, X.; Xi, L.; Fan, J.; Zhang, W.; Chen, L., Cu−Se Bond Network and Thermoelectric Compounds with Complex Diamondlike Structure. Chem. Mater. 2010, 22, 6029-6031. (22) Xi, L.; Zhang, Y. B.; Shi, X. Y.; Yang, J.; Shi, X.; Chen, L. D.; Zhang, W.; Yang, J.; Singh, D. J., Chemical bonding, conductive network, and thermoelectric performance of the ternary semiconductors Cu2SnX3(X=Se, S) from first principles. Phys. Rev. B 2012, 86, 155201. (23) Tan, Q.; Sun, W.; Li, Z.; Li, J.-F., Enhanced thermoelectric properties of earth-abundant Cu2SnS3 via In doping effect. J. Alloys Compd. 2016, 672, 558-563. (24) Chen, S.; Gong, X. G.; Walsh, A.; Wei, S.-H., Crystal and electronic band structure of Cu2ZnSnX4 (X=S and Se) photovoltaic absorbers: First-principles insights. Appl. Phys. Lett. 2009, 94, 041903. (25) Bourgès, C.; Lemoine, P.; Lebedev, O. I.; Daou, R.; Hardy, V.; Malaman, B.; Guilmeau, E., Low thermal conductivity in ternary Cu4Sn7S16 compound. Acta Mater. 2015, 97, 180-190. (26) He, T.; Lin, N.; Du, Z.; Chao, Y.; Cui, J., The role of excess Sn in Cu4Sn7S16 for modification of the band structure and a reduction in lattice thermal conductivity. J. Mater. Chem. C 2017, 5, 4206-4213. (27) Kim, F. S.; Suekuni, K.; Nishiate, H.; Ohta, M.; Tanaka, H. I.; Takabatake, T., Tuning the charge carrier density in the thermoelectric colusite. J. Appl. Phys. 2016, 119, 175105. (28) Lai, W.; Wang, Y.; Morelli, D. T.; Lu, X., From Bonding Asymmetry to Anharmonic Rattling in Cu12Sb4S13Tetrahedrites: When Lone-Pair Electrons Are Not So Lonely. Adv. Funct. Mater. 2015, 25, 3648-3657. 25



Page 26 of 34

(29) Skoug, E. J.; Morelli, D. T., Role of lone-pair electrons in producing minimum thermal conductivity in nitrogen-group chalcogenide compounds. Phys. Rev. Lett. 2011, 107, 235901. (30) Chen, D.; Zhao, Y.; Chen, Y.; Wang, B.; Wang, Y.; Zhou, J.; Liang, Z., Hot-Injection Synthesis of Cu-Doped Cu2ZnSnSe4 Nanocrystals to Reach Thermoelectric zT of 0.70 at 450 degrees C. ACS Appl. Mater. Inter. 2015, 7, 24403-24408. (31) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G., Cubic AgPbmSbTe2+m: bulk thermoelectric materials with high figure of merit. Science 2004, 303, 818-821. (32) Androulakis, J.; Lin, C. H.; Kong, H. J.; Uher, C.; Wu, C. I.; Hogan, T.; Cook, B. A.; Caillat, T.; Paraskevopoulos, K. M.; Kanatzidis, M. G., Spinodal decomposition and nucleation and growth as a means to bulk nanostructured thermoelectrics: enhanced performance in Pb1-xSnxTe-PbS. J. Am. Chem. Soc. 2007, 129, 9780-9788. (33) He, Y.; Lu, P.; Shi, X.; Xu, F.; Zhang, T.; Snyder, G. J.; Uher, C.; Chen, L., Ultrahigh Thermoelectric Performance in Mosaic Crystals. Adv. Mater. 2015, 27, 3639-3644. (34) Biswas, K.; He, J. Q.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G., High-performance bulk thermoelectrics with all-scale hierarchical architectures. Nature 2012, 489, 414-418. (35) Shen, Y.; Li, C.; Huang, R.; Tian, R.; Ye, Y.; Pan, L.; Koumoto, K.; Zhang, R.; Wan, C.; Wang, Y., Eco-friendly p-type Cu2SnS3 thermoelectric material: crystal structure and transport properties. Sci. Rep. 2016, 6, 32501. (36) Kresse, G.; Joubert, D., From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758-1775. (37) Chen, S.; Gong, X. G.; Walsh, A.; Wei, S.-H., Defect physics of the kesterite thin-film solar cell absorber Cu2ZnSnS4. Appl. Phys. Lett. 2010, 96, 021902. (38) Stillinger, F. H.; Weber, T. A., Computer simulation of local order in condensed phases of silicon. Phys. Rev. B 1985, 31, 5262-5271. (39) Guan, H.; Shen, H.; Gao, C.; He, X., Structural and optical properties of Cu2SnS3 and Cu3SnS4 thin films by successive ionic layer adsorption and reaction. J. Mater. Sci.: Mater. Electron. 2012, 24, 1490-1494. (40) Pennycook, S. J.; Jesson, D. E., High-resolution Z-contrast imaging of crystals. Ultramicroscopy 1991, 37, 14-38. (41) Buban, J. P.; Matsunaga, K.; Chen, J.; Shibata, N.; Ching, W. Y.; Yamamoto, T.; Ikuhara, Y., Grain boundary strengthening in alumina by rare earth impurities. Science 2006, 311, 212-215. (42) Feng, B.; Yokoi, T.; Kumamoto, A.; Yoshiya, M.; Ikuhara, Y.; Shibata, N., Atomically ordered solute segregation behaviour in an oxide grain boundary. Nat. Commun. 2016, 7, 11079. (43) Lugg, N. R.; Kothleitner, G.; Shibata, N.; Ikuhara, Y., On the quantitativeness of EDS STEM. Ultramicroscopy 2015, 151, 150-159.

26


Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Dumcenco, D.; Huang, Y.-S., The vibrational properties study of kesterite Cu2ZnSnS4 single crystals by using polarization dependent Raman spectroscopy. Optical Materials 2013, 35, 419-425. (45) Fan, J.; Carrillo-Cabrera, W.; Antonyshyn, I.; Prots, Y.; Veremchuk, I.; Schnelle, W.; Drathen, C.; Chen, L.; Grin, Y., Crystal Structure and Physical Properties of Ternary Phases around the Composition Cu5Sn2Se7with Tetrahedral Coordination of Atoms. Chem. Mater. 2014, 26, 5244-5251. (46) Snyder, G. J.; Toberer, E. S., Complex thermoelectric materials. Nat. Mater. 2008, 7, 105-114. (47) Fujii, S.; Yoshiya, M., Manipulating Thermal Conductivity by Interfacial Modification of Misfit-Layered Cobaltites Ca3Co4O9. J. Electron. Mater. 2015, 45, 1217-1226. (48) Yoshiya, M.; Harada, A.; Takeuchi, M.; Matsunaga, K.; Matsubara, H., Perturbed Molecular Dynamics for Calculating Thermal Conductivity of Zirconia. Mol. Simul. 2004, 30, 953-961.

27



Page 28 of 34

Figure and captions

Figure 1. Cation-ordered and -disordered unit cells of Cu2SnS3 and typical microstructures in 20 mol% Zn-doped Cu2SnS3. (a) Crystal structure of monoclinic Cu2SnS3, with ordered Cu and Sn atoms. (b) Crystal structure of cubic Cu2SnS3, with randomly distributed Cu and Sn atoms. (c) HAADF-STEM image of monoclinic Cu2SnS3 taken along the [100] zone axis. The bright spots correspond to columns of Sn atoms.

(d) Low-magnification bright-field TEM image of the nominally

Cu2Zn0.2Sn0.8S3 sample showing platelet-like Cu2S precipitates in a matrix grain. The area enclosed by the red rectangle was subsequently imaged using HAADF-STEM. (e) EDS maps of a portion the grain in (d) showing that precipitates are Cu-rich. 28


Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Atomic-resolution HAADF-STEM images and EDS maps of the mosaic nanostructure. (a) HAADF-STEM image of the area enclosed by the red square in Figure 1(d) showing the mosaic nanostructure. The inset is the corresponding SAED pattern showing the main diffraction spots (connected by yellow dashed lines) from the zinc-blende lattice and superlattice reflections (indicated by red arrows). (b) Atomic resolution EDS maps of the semi-ordered and disordered regions enclosed by yellow squares marked ‘1’ and ‘2’, respectively, in (a). Sites at the centres of the diamonds formed by the bright (Sn) spots are preferentially occupied by Zn (highlighted by white dashed lines), while in the disordered phase all cations produce a uniform contrast.

29



Figure 3.

Page 30 of 34

Evolution of the ordered Cu4ZnSn2S7 structure and the Cu2SnS3-ZnS

phase diagram. (a) Kesterite-structured Cu2ZnSnS4 projected along the [010]zinc-blende direction showing the typical Sn diamond configuration with two Cu columns and one Zn column within each diamond, with the (Cu-S)2(Zn-S)(Sn-S)(201) superlattice indicated by solid black lines. Red arrows show the direction of shearing to form the semi-ordered structure in (b). (b) The semi-ordered structure of Cu4ZnSn2S7 projected along the [010]zinc-blende direction, with the (Cu-S)2(Zn-S)(Sn-S)(Cu-S)2(Sn-S)(201) superlattice indicated by solid black lines. (c) Cu2SnS3-ZnS pseudo-binary phase diagram including the new semi-ordered Cu4ZnSn2S7 phase and the fully disordered Cu2Zn0.05Sn0.95S3 cubic phase. 30


Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Structural models, corresponding total energies and band structure diagram of Cu4ZnSn2S7 from first-principles calculations. (a) Twelve possible ordered structures with composition Cu4ZnSn2S7 and monoclinic symmetry. All structures are drawn viewed down the [100] axis except for structure 7, which is down the [010] axis. (b) Total energies and bandgaps of the twelve structures in (a) from first-principles calculations. (c) Zn and Cu configurations at the centre of the Sn diamonds of the four lowest total energy structures; the small energy difference is consistent with a disordered arrangement of Cu and Zn at normal temperatures. (d) Calculated band structure diagram of structure 1.

31



Page 32 of 34

Figure 5. Conductivities of different domains in the mosaic nanostructure. (a) Topography of the mosaic structure; (b) Current map showing the change in conductivity across the area in (a); (c) Intensity profile along the red arrow in (b) showing fluctuations in the electrical conductivity.

32


Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Mechanisms responsible for ultralow thermal conductivity at different length scales. Phonon scattering greater than that in a defect-free single crystal is achieved by (i) disorder over the cation sublattice of tessera domains; (ii) abrupt interfaces between the cation-(semi)ordered network domains and the fully disordered tessera domains; (iii) large lattice strain introduced by platelet-like Cu2S precipitates within matrix grains; and (iv) lattice strain at grain boundaries.

33



Page 34 of 34

Table of Contents

34


Hierarchically Structured Thermoelectric Materials ... - ACS Publications

Recommend Documents