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Spin coating epitaxial films

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Science  12 Apr 2019:
Vol. 364, Issue 6436, pp. 166-169
DOI: 10.1126/science.aaw6184

Epitaxial films through spin coating

A simple way to coat a surface with a uniform film is by spin coating. The substrate is spun at high speed, and a droplet of solution containing the coating is added at the center, spreads out, and evaporates. This method is used to make polycrystalline inorganic coatings and amorphous films, such as polymers used in lithography. Kelso et al. performed spin coating with single-crystal substrates, carefully controlling the thickness of the spreading solution on the basis of its viscosity and the rotation rate. In this way, they achieved epitaxial growth—in which the crystallites are oriented by the substrate—for perovskites, zinc oxide, and sodium chloride.

Science, this issue p. 166

Abstract

Spin-coated films, such as photoresists for lithography or perovskite films for solar cells, are either amorphous or polycrystalline. We show that epitaxial films of inorganic materials such as cesium lead bromide (CsPbBr3), lead(II) iodide (PbI2), zinc oxide (ZnO), and sodium chloride (NaCl) can be deposited onto a variety of single-crystal and single-crystal–like substrates by simply spin coating either solutions of the material or precursors to the material. The out-of-plane and in-plane orientations of the spin-coated films are determined by the substrate. The thin stagnant layer of supersaturated solution produced during spin coating promotes heterogeneous nucleation of the material onto the single-crystal substrate over homogeneous nucleation in the bulk solution, and ordered anion adlayers may lower the activation energy for nucleation on the surface. The method can be used to produce functional materials such as inorganic semiconductors or to deposit water-soluble materials such as NaCl that can serve as growth templates.

Epitaxy is the growth of crystals whose orientation is determined by their crystalline substrate (1, 2). Epitaxy can produce thin films with atomic perfection that rivals that of single crystals as well as metastable phases (such as body-centered cubic Ni on Fe and face-centered cubic Fe on Cu), superlattices, quantum wells, and strained-layer architectures with tunable properties (2). Although epitaxial growth is usually constrained to ultrahigh vacuum or high temperatures by techniques such as molecular beam epitaxy, chemical vapor deposition, and liquid-phase epitaxy, it was first demonstrated in aqueous solution in 1836 when Frankenheim showed that sodium nitrate crystals could be grown epitaxially on a freshly cleaved calcite substrate (3). In 1950, Johnson also used aqueous solution to grow NaCl crystals epitaxially on single-crystal Ag (4). Other examples of solution-based deposition of epitaxial films are hydrothermal processing (5), chemical bath deposition (68), and electrodeposition (911).

Each of these solution methods has limitations. Hydrothermal processing requires high temperature and pressure, chemical bath deposition requires specific reactions to occur at the substrate surface, and electrodeposition requires conducting or semiconducting substrates. Here, we show that epitaxial films can be deposited by a simple, rapid, and inexpensive spin-coating method directly from solution precursors. We demonstrate the epitaxial growth of a diverse array of inorganic materials such as CsPbBr3, PbI2, NaCl, and ZnO onto a variety of single-crystal and single-crystal–like substrates such as Au, Ag, SrTiO3, and mica by spin coating with low-temperature in situ heating.

Spin coating is used commercially to deposit polymer films for lithography. It has also been used to deposit organic semiconductor films (12, 13) and has become the preferred deposition method for perovskite materials such as CH3NH3PbI3 and CsPbBr3 for use in solar cells, photodetectors, and light-emitting diodes (LEDs) (1419). For example, dense and uniform films of CH3NH3PbI3−xBrx have been spin coated by engineering the solvent mix (17), and CH3NH3PbI3−xClx films with millimeter-scale crystal grains have been spin coated by dispensing the precursor solution onto a heated substrate (16). However, the substrates in this earlier work were polycrystalline or amorphous, so the films were not epitaxial. Ji et al. have spin coated CH3NH3PbI3 onto single-crystal KCl and crystallized the as-deposited amorphous material into an epitaxial film with a final annealing step (19). There has also been previous work on the spin coating of amorphous sol-gel precursors for oxides onto single-crystal surfaces that were then converted into epitaxial films by a high-temperature burn off of organics (2022). In the present work, the epitaxial films were directly spin coated from solution precursors without the need for postannealing or a high-temperature conversion step.

We propose that the very thin supersaturated solution layer that is formed in spin coating serves to promote heterogeneous nucleation of the material onto the single-crystal substrate over homogeneous nucleation in the bulk solution that would occur during crystallization by conventional solvent evaporation methods. The activation energy for nucleation on the single-crystal surface may also be lowered by the production of ordered adlayers of anions on the surface. Anions specifically adsorb on single-crystal metal surfaces to form monolayer or submonolayer adlayers (23). For example, I forms a (√3 by √3)R30° adlayer on Au(111), and Cl forms a c(2 by 2) adlayer on Ag(100). These anion adlayers could mimic the anion-terminated surfaces of compounds to serve as seed crystals for the nucleation of the spin-coated material.

The general mechanism for epitaxial spin coating that we outline in Fig. 1 assumes that the formation of a hydrodynamic boundary layer and the subsequent evaporation of the solvent occur in steps, as opposed to more rigorous models in which the two processes occur simultaneously (24, 25). A drop of solution containing dissolved material or precursors is dispensed onto the rotating substrate (Fig. 1A). After spinning, the solution forms a stagnant hydrodynamic boundary layer (Fig. 1B) whose thickness, yh, is determined by the kinematic viscosity, v, of the solution and the rotation rate, ω (in terms of angular frequency of rotation, measured in s−1), of the substrate (26): yh=3.6(vω)1/2 (1)For example, a rotation rate of 3000 rpm and a kinematic viscosity of 0.01 cm2 s−1 will generate a boundary layer ~200 μm thick. An ordered adlayer that lowers the activation energy for nucleation also forms at this stage (Fig. 1C). Because of the in situ heating and the spinning of the substrate, the solvent evaporates and the solution reaches supersaturation (Fig. 1C). Once the supersaturation reaches the critical value for nucleation, the nuclei form on the substrate, and the concentration at the solid-liquid interface is reduced to the saturation concentration of the solution, csat’d. A concentration gradient is then established that serves as the driving force for the ion or molecule diffusion (Fig. 1D). The diffusion layer thickness, δ, is given by Eq. 2, where D is the diffusion coefficient of dissolved species (cm2 s−1) (26):δ=1.61D1/3ω1/2v1/6 (2)For a rotation rate of 3000 rpm and a diffusion coefficient of 1 × 10−6 cm2 s−1, the diffusion layer would be ~4 μm thick. An epitaxial film continues to grow until the solvent completely evaporates (Fig. 1E). A similar model is observed in chemical vapor deposition and liquid phase epitaxy, in which film growth occurs by mass transport across a diffusion layer (1).

Fig. 1 Epitaxial spin coating schematic.

(A) Each precursor solution is dispensed on a room temperature or preheated single-crystal or single-crystal–like substrate as it begins to spin. y, cylindrical axis; r, radius; ϕ, azimuthal angle. (B) When the solution reaches the spin speed of the sample, a hydrodynamic boundary layer forms with thickness yh. (C) An ordered anion adlayer forms at the substrate-solution interface, and the solution concentration reaches supersaturation owing to evaporation. (D) Nucleation occurs at the solution-substrate interface, and a concentration gradient and diffusion layer form with thickness δ. (E) The nuclei grow into a film, and the interface between solution and substrate continues to shift until solvent evaporation is complete.

The spin coating of epitaxial films requires either that the material to be spin coated is soluble or that a soluble precursor to the material is available that can be converted to the material after drying at slightly increased temperatures. For the first case, we have spin coated PbI2 from N,N-dimethylformamide (DMF) solution, CsPbBr3 from dimethyl sulfoxide (DMSO) solution, and NaCl from aqueous solution. For the second case, we have spin coated ZnO from an aqueous ammonia solution of Zn(II) (27). In the ZnO case, the NH3 evolved during the spin-coating process. The concentrations and conditions for spin coating are given in the supplementary materials. The substrates used for spin coating were either single crystals such as SrTiO3 and mica or proxies for single crystals that were thin epitaxial films of Au or Ag that were electrodeposited onto single-crystal Si wafers (11, 28, 29). PbI2, CsPbBr3, and ZnO are all functional materials that can serve as semiconductors in solar cells and LEDs (9, 15, 27, 30). Epitaxial NaCl can serve as a water-soluble template for large-scale epitaxial lift-off of flexible single-crystal–like materials for electronics, solar cells, and displays (31).

The in-plane order of the spin-coated materials can be seen in the scanning electron micrographs (SEMs) and optical micrographs in Fig. 2. CsPbBr3 deposited with a diamond-like morphology on single-crystal SrTiO3 (Fig. 2A). Although the CsPbBr3 typically deposited as islands, it may be possible to produce dense films by multiple spin-coating applications or by using additives such as HBr or solvent mixtures (17, 32). PbI2, NaCl, and ZnO were all deposited onto epitaxial films of either Au or Ag on single-crystal Si. PbI2 showed a mesh-like morphology with the expected threefold symmetry of the Au(111) surface (Fig. 2B). NaCl deposited as perfectly aligned cubes on the surface of Ag(100) (Fig. 2C). Optical micrographs of NaCl on Ag(111) and Ag(110) are shown in fig. S1. The plan view of ZnO on Au(111) was featureless (fig. S2), so the SEM in Fig. 2D is a cross-sectional view of the ~500-nm-thick ZnO film.

Fig. 2 Morphology of spin-coated materials.

(A and B) SEM micrograph of CsPbBr3 on SrTiO3(100) (A) and PbI2 on Au/Si(111) (B). (C) Optical micrograph of NaCl on Ag/Au/Si(100). (D) SEM cross section of ZnO on Au/Si(111).

The crystallographic orientation of the spin-coated materials relative to the single-crystal and single-crystal–like substrates was determined by x-ray diffraction. The single-crystal–like substrates were thin epitaxial layers of Au or Ag that were electrochemically deposited onto single-crystal Si to serve as large-area, inexpensive proxies for a single-crystal surface (29). The out-of-plane orientation of the deposits was determined by x-ray 2θ scans using CuKα1 radiation. Figure 3A shows the 2θ scan of CsPbBr3 on single-crystal SrTiO3. Only the {100} family of peaks was observed for the cubic material, consistent with a [100] preferred out-of-plane orientation. The 2θ scans of CsPbBr3 on Mica(001), Au(100)/Si(100), and Au(111)/Si(111) are shown in fig. S3. In all cases, the out-of-plane orientation of the CsPbBr3 was determined by the substrate. Figure 3B shows the 2θ scan of PbI2 on Au(111)/Si(111). Only the {001} family of peaks was observed for the material, consistent with a [001] orientation for the material. The triclinic PbI2 grew with the (001) basal plane parallel with the Au(111) surface. Similarly, Fig. 3C shows that cubic NaCl grew with a [100] orientation on Ag(100)/Au(100)/Si(100), and Fig. 3D shows that hexagonal ZnO grew with a [001] orientation on Au(111)/Si(111).

Fig. 3 Out-of-plane orientation by x-ray diffraction.

(A to D) 2θ scans of CsPbBr3 on single crystal SrTiO3(100) (A), PbI2 on Au/Si(111) (B), NaCl on Ag/Au/Si(100) (C), and ZnO on Au/Si(111) (D). c/s, counts per second.

The in-plane orientation of the deposits relative to the substrate was determined by x-ray pole figures. In these measurements, the 2θ value was fixed for either the film or the substrate, and the sample was rotated azimuthally from 0° to 360° at a series of tilt angles from 0° to 90°. Discrete spots in the pole figures instead of rings indicate that the material had both out-of-plane and in-plane order. Figure 4 shows pole figures for both films and substrates for CsPbBr3 on SrTiO3(100), PbI2 on Au(111)/Si(111), NaCl on Ag(111)/Au(111)/Si(111), and ZnO on Au(111)/Si(111). Figure 4A shows a (220) pole figure of CsPbBr3, and Fig. 4B shows a (110) pole figure of SrTiO3, both with four spots separated azimuthally by 90° at a tilt angle of 45°, corresponding to the fourfold symmetry of the (100) plane. The tilt angle of 45° corresponds to the angle between the (100) and (220) planes in a cubic system. The pole figure of the CsPbBr3 was rotated 45° in-plane in relation to SrTiO3. This rotation could be explained with the lower mismatch caused by the rotation, as shown in interface models in fig. S4. The epitaxial relation is given by CsPbBr3(100)[011] || SrTiO3(100)[001].

Fig. 4 In-plane orientation by x-ray pole figures.

(A and B) (220) pole figure of CsPbBr3 (A) and (110) pole figure of SrTiO3(100) (B) substrate show that CsPbBr3 is epitaxial with a 45° in-plane rotation from SrTiO3. (C and D) (102) pole figure of PbI2 (C) and (200) pole figure of Au(111) (D) film show that PbI2 is epitaxial with a 30° in-plane rotation from Au. (E and F) (422) pole of NaCl (E) and (422) pole of Si(100) (F) substrate show that NaCl is epitaxial with no in-plane rotation. (G and H) (101) pole of ZnO (G) and (200) pole of Au(111) (H) film show that ZnO is epitaxial with no in-plane rotation. The radial grid lines represent 30° increments of the tilt angle.

Figure 4C shows a (102) pole figure of PbI2 on the Au(111)/Si(111) substrate. The pole figure showed six spots separated azimuthally by 60° at a tilt angle of 42°, corresponding to two domains of threefold symmetry for trigonal PbI2. The presence of the second domain is caused by the second in-plane domain in the epitaxial Au(111) film on Si(111) substrate, as previously studied (29) and shown in the Au substrate (200) pole figure in Fig. 4D. The tilt angle of 42° corresponds to the angle between (001) and (102) planes in a trigonal system. The Au pole figure shows six spots (two domains with three spots) separated azimuthally by 60° at a tilt angle of 55°, corresponding to the threefold symmetry of the cubic (111) plane. There is also a minor set of six spots corresponding to the (103) pole of PbI2, which had a 2θ close enough to the (200) pole of Au to appear in the same projection. Comparing the Au and PbI2 pole figures showed that PbI2 was rotated 30° in-plane in relation to its Au substrate. The epitaxial relation is given by PbI2(001)[100]Au(111)[101¯]Si(111)[101¯].

Unlike CsPbBr3 and PbI2, there is no in-plane rotation between the spin-coated material and substrate for NaCl and ZnO. The (422) pole figure of NaCl is shown in Fig. 4E, and the (422) pole figure for the Si(100) substrate is shown in Fig. 4F. Both pole figures showed four spots separated azimuthally by 90° at a tilt angle of 35° and eight spots (four sets separated 53° azimuthally of two spots separated 37° azimuthally) at a tilt angle of 66°, corresponding to the fourfold symmetry of the (100) plane. All of the spots corresponded to the angle between the (100) and (422) planes in a cubic system. The epitaxial relation is given by NaCl(100)[001] || Ag(100)[001] || Au(100)[001] || Si(100)[001].

Similarly, the in-plane order of ZnO is indicated by the (101) pole figure in Fig. 4G and Au substrate pole figure in Fig. 4H. The ZnO pole figure showed six spots, separated azimuthally by 60° at a tilt angle of 62°, corresponding to the angle between (001) and (101) and the sixfold symmetry in the hexagonal system. The Au substrate pole figure again showed six spots separated azimuthally by 60° at a tilt angle of 55°, corresponding to two in-plane domains of the threefold symmetric cubic (111) plane. The epitaxial relation is given by ZnO(001)[110]Au(111)[101¯]Si(111)[101¯].

The spin coating of epitaxial films offers an inexpensive and readily accessible route to single-crystal–like materials that should exhibit superior electronic and optical properties owing to the absence of high-angle grain boundaries. A wide range of materials can be deposited onto a variety of wafer-sized substrates with unprecedented simplicity. The films were deposited from solutions of the material or from precursors of the material that readily converted to the final product with only volatile side products. The precursor route used for depositing ZnO from an ammine complex should be applicable to other metal oxides. Spin coating also offers two avenues to highly-ordered semiconductors for flexible electronics, displays, and solar cells. The materials can be spin coated onto flexible single-crystal–like metal foils (11, 33), or they can be deposited by more conventional vapor deposition techniques onto spin-coated water-soluble salts such as NaCl that serve as sacrificial templates for epitaxial lift-off of free-standing semiconductor foils (31).

Supplementary Materials

www.sciencemag.org/content/364/6436/166/suppl/DC1

Materials and Methods

Figs. S1 to S4

Table S1

References and Notes

Acknowledgments: Funding: The material is based on work supported by the U.S. Department of Energy, Office of Basic Sciences, Division of Materials Sciences and Engineering, under grant DE-FG02-08ER46518. Author contributions: M.V.K. and J.A.S. wrote the article with input, edits, and approval from all the authors. N.K.M. and J.Z.T. deposited and characterized PbI2 and perovskite films, Q.C. deposited and characterized NaCl films, and M.V.K. deposited and characterized ZnO films. The idea was conceived and the research was directed by J.A.S. Competing interests: The authors declare no competing interests. Data and materials availability: All data are presented in the main paper and supplementary materials.
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