Film transfer enabled by nanosheet seed layers on arbitrary sacrificial substrates

An approach for film transfer is demonstrated that makes use of seed layers of nanosheets on arbitrary sacrificial substrates. Epitaxial SrTiO3, SrRuO3, and BiFeO3 films were grown on Ca2 Nb 3O10 nanosheet seed layers on phlogopite mica substrates. Cleavage of the mica substrates enabled film transfer to flexible polyethylene terephthalate substrates. Electron backscatter diffraction, X-ray diffraction, and atomic force microscopy confirmed that crystal orientation and film morphology remained intact during transfer. The generic nature of this approach is illustrated by growing films on zinc oxide substrates with a nanosheet seed layer. Film transfer to a flexible substrate was accomplished via acid etching

An approach for film transfer is demonstrated that makes use of seed layers of nanosheets on arbitrary sacrificial substrates.Epitaxial SrTiO 3 , SrRuO 3 , and BiFeO 3 films were grown on Ca 2 Nb 3 O 10 nanosheet seed layers on phlogopite mica substrates.Cleavage of the mica substrates enabled film transfer to flexible polyethylene terephthalate substrates.Electron backscatter diffraction, X-ray diffraction, and atomic force microscopy confirmed that crystal orientation and film morphology remained intact during transfer.The generic nature of this approach is illustrated by growing films on zinc oxide substrates with a nanosheet seed layer.Film transfer to a flexible substrate was accomplished via acid etching.C 2015 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4921070]A major challenge in the development of flexible electronics is to combine thin film growth that requires temperatures of several hundreds of degrees, with flexible substrate materials that have an organic nature and therefore limited thermal stability.Perovskite films possess useful electronic properties such as ferroelectricity and ferromagnetism.A common deposition technique for these materials is pulsed laser deposition (PLD), and temperatures as low as 350 • C are reported in very few cases, 1 but oriented growth of perovskite films generally requires temperatures of a few hundred degrees higher.Such a high deposition temperature makes it impossible to deposit the films directly onto arbitrary flexible substrates.An approach to overcome this problem is to grow the films on a substrate with high thermal stability and transfer them afterwards to the flexible material.Several available methods for the transfer of perovskite films are the laser lift-off process, 2,3 ion cutting, 4 the use of water-soluble substrates, 5,6 and the epifree process. 7,8n this letter, an approach for film transfer is demonstrated that makes use of seed layers of nanosheets on arbitrary sacrificial substrates.Nanosheets are essentially two-dimensional single crystals; they have a constant thickness of a few nanometers at most and their lateral size is mostly in the micrometer range, 9 while their oriented crystal structure enables them to direct epitaxial growth of thin films.At present, usually thick single crystalline substrates are used as templates to direct the growth of oriented films.Drawbacks of such substrates are not only their relatively high prices and size limitations but also the fact that they can only induce epitaxial growth for films with matching lattice parameters.1][12][13] Nanosheets covering a wide range of lattice parameters are known and can, in principle, be derived from any layered crystal.When using a seed layer of nanosheets, the substrate underneath does not need to have lattice parameters that match those of the film.Thus, the choice of substrate becomes a tool to engineer the subsequent process.Film transfer is possible by choosing a sacrificial substrate material that can be removed easily.
Films of epitaxial SrTiO 3 , SrRuO 3 , and BiFeO 3 were grown on a seed layer of Ca 2 Nb 3 O 10 nanosheets on phlogopite mica (001) substrates.Mica crystals have a layered structure with a Author to whom correspondence should be addressed.Electronic mail: j.e.tenelshof@utwente.nl.interlayer cations that enable cleavage between the layers. 14Little force is required for cleavage, and when films are grown on top of the mica substrates, the films can be peeled off easily.
To make devices that need only limited flexibility, mica substrates could even serve as flexible substrates by themselves.Ca 2 Nb 3 O 10 nanosheets were prepared and deposited as a seed layer by Langmuir-Blodgett (LB) deposition 15 following the procedure described elsewhere. 13,16In short, the layered parent compound HCa 2 Nb 3 O 10 was dispersed in demi-water and tetra-n-butyl ammonium hydroxide was added as an exfoliation agent to separate the crystals into discrete nanosheets.Monolayers of these nanosheets were deposited onto freshly cleaved phlogopite mica substrates by LB deposition.After deposition of the first monolayer, tetra-n-butyl ammonium residues on the substrates were decomposed by heating to 600 • C for 30 min in a microwave oven equipped with silicon carbide elements to convert the microwaves into heat.A second monolayer of nanosheets was deposited to improve the overall coverage.The substrates were annealed inside a PLD chamber at 700 • C in vacuum for 3 h prior to film deposition.This led to small irregularities in the nanosheets as observed with atomic force microscopy (AFM) but did not significantly destruct their crystal lattice since their film orienting function was maintained.A stack of SrTiO 3 , SrRuO 3 , and BiFeO 3 was grown by PLD at the same temperature with a fluency of 2.0-2.4J/cm −2 and spot size of 2.5 mm 2 in a pure oxygen atmosphere.on the wax.The bottom side of the film was then placed on a PET substrate of larger size (Figure 2(c)), the wax was molten by heating to 150 • C, and the silicon support was removed by carefully sliding it away over the PET substrate (Figure 2(d)).The PET substrate was cooled to room temperature and residual wax was removed by rinsing with acetone, followed by rinsing with ethanol and drying with nitrogen gas.Pictures of a grown and transferred film are shown as example in Figure S1 in the supplementary material. 17  The ease with which mica crystals could be cleaved makes it an interesting material for sacrificial substrates, but in principle such cleavage can occur between any two crystal layers in the substrate.Any stack of residual layers that remains attached to the film may continue to be a "weak spot" that can cause the transferred film to detach again from its new support.This possibility should be taken into account when optimizing the process for functional devices.Alternative sacrificial substrate materials are, for example, materials that can be removed by chemical etching.This option was shown successful by using zinc oxide instead of mica; films grown on zinc oxide substrates were transferred to PET substrates by mild etching of zinc oxide with 0.5 vol.% aqueous hydrochloric acid.Experimental details and results of this transfer procedure are shown in the supplementary material. 17When using a procedure that includes etching, the film material must be compatible with the etchant.As for the concept of using seed layers of nanosheets for oriented film growth, the bottleneck is in the degree of perfection of both the seed layer and subsequent film growth.Depositing nanosheets with perfect coverage and preferably with a full control over their in-plane orientation is currently the main challenge in the field.But improved control over the solid state synthesis of parent compounds and the process of exfoliation is also desirable.However, the conceptual value of nanosheets is clear and they exist in a wide variety of compositions and crystal structures, enabling epitaxial growth for many different film materials.As demonstrated in this letter, subsequent film transfer to arbitrary substrates can be achieved by choosing suitable sacrificial substrates.
Thanks to Mark Smithers of the MESA+ Nanolab for his technical assistance with EBSD analyses.This work was financially supported by the Chemical Sciences division of the Netherlands Organization for Scientific Research (NWO-CW) in the framework of the TOP program.

Film
transfer enabled by nanosheet seed layers on arbitrary sacrificial substrates A. P. Dral, M. Nijland, G. Koster, and J. E. ten Elshof a MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands (Received 13 February 2015; accepted 28 April 2015; published online 11 May 2015)

1 © 2 Dral
Author(s) 2015 This article is copyrighted as indicated in the article.Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissionsDownloaded to IP: 130.89.45.179On: Wed, 12 Aug 2015 11:43:11 056102-et al. APL Mater.3, 056102 (2015) FIG. 1. HR-SEM image (HE-SE2 detector) (a) and EBSD inverse pole figure maps with the out-of-plane (b) and in-plane (c) crystal orientations of the surface of stacked SrTiO 3 , SrRuO 3 , and BiFeO 3 grown on a mica substrate with a seed layer of Ca 2 Nb 3 O 10 nanosheets.Red striped lines indicate the areas where individual nanosheets are located.The legend indicates the crystal orientations corresponding to each color.

FIG. 2 .
FIG. 2. Schematic representation of the experimental method to transfer films from mica substrates to plastic substrates by mechanical cleavage.(a) The top side of the film was placed on a droplet of soft wax on a rigid support (e.g., silicon) at 120 • C and the wax was hardened by cooling to room temperature; (b) the mica substrate was peeled off, leaving the film behind on the wax; (c) the bottom side of the film was placed on a plastic substrate (e.g., PET); (d) the wax was molten by heating to 150 • C and the support was carefully pushed off the plastic substrate.

Figure 3 (
FIG. 3. AFM image (a) and XRD θ-2θ spectrum (b) of stacked SrTiO 3 , SrRuO 3 , and BiFeO 3 after transfer to a flexible PET substrate.The film was originally grown on a mica (001) substrate with a seed layer of Ca 2 Nb 3 O 10 nanosheets.The weak BiFeO 3 (200) peak at 2θ 45.3 • cannot be distinguished due to overlap with the mica substrate peak.

056102- 4 Dral
FIG. 4. EBSD inverse pole figure maps with the out-of-plane (a) and in-plane (b) crystal orientations of the surface of stacked SrTiO 3 , SrRuO 3 , and BiFeO 3 after transfer to a flexible PET substrate.The film was originally grown on a mica substrate with a seed layer of Ca 2 Nb 3 O 10 nanosheets.The legend indicates the crystal orientations corresponding to each color.