Investigating Sequential Vapor Infiltration Synthesis on Block

Subscriber access provided by GAZI UNIV

Article

Investigating Sequential Vapor Infiltration Synthesis on Block Copolymer Templated Titania Nanoarrays Olga M. Ishchenko, Sivashankar Krishnamoorthy, Nathalie Valle, Jérôme Guillot, Philippe Turek, Ioana Fechete, and Damien Lenoble J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10415 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 9, 2016

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 free 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 accessible to all readers and 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.

The Journal of Physical Chemistry C 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 32

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

The Journal of Physical Chemistry

Investigating Sequential Vapor Infiltration Synthesis on Block Copolymer Templated Titania Nanoarrays Olga M. Ishchenko,a,bSivashankar Krishnamoorthy, a* Nathalie Valle,a Jérôme Guillot,a Philippe Turek,c,d Ioana Fechete,b Damien Lenoblea* a

Luxembourg Institute of Science and Technology (LIST), 41 rue du Brill, Belvaux,

Luxembourg b

Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé - ICPEES, UMR

7515 CNRS, Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France c

Institut de Chimie, UMR 7177 CNRS - Université de Strasbourg, 1 rue Blaise Pascal, 67008

Strasbourg cedex, France d

Fondation IcFRC International Center for Frontier Research in Chemistry, 8 allée Gaspard

Monge, F-67000 Strasbourg, France KEYWORDS Sequential vapor infiltration synthesis (SVIS, SIS), block copolymers, directed self-assembly (DSA), titania, nanostructures, XPS, SIMS

ACS Paragon Plus Environment

1


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 32

ABSTRACT

Sequential vapor infiltration synthesis (SVIS) within block copolymer templates has emerged as an attractive means for the controlled formation of metal oxide nanoarrays on arbitrary substrates. This approach takes advantage of molecular-level controls that are inherent to the production of the template and the exposure tools that are available for vapor phase growth of materials. To take adequate advantage of these controls and their dependencies on any environmental factors, it is essential to understand the mechanisms that govern nanostructure morphology at different stages of the growth process. To this end, our work correlates the evolution in internal structure with the chemical functionality of block copolymer templates in response to different conditions of exposure to volatile titania precursors. The evolution is followed by mapping structural and functional information at the lateral and vertical resolutions down to a few nanometers through a combination of electron microscopies (SEM, TEM - crosssections), photoelectron spectroscopy (XPS) and mass spectrometry (SIMS).

Introduction Over the past decade, numerous approaches have been developed to grow nanostructures of different materials, both in solution and surface-initiated, with or without templates, and with varying degrees of control over geometry. Exercising fine control over nanostructure growth is critical to defining the resultant material properties and maximizing their function within devices/interfaces. Nanostructures attached to a substrate, as required for device interfaces, can be attained with high control over both the feature and spatial resolutions using direct-write techniques such as electron or focused ion beam lithographies. These techniques, however, are limited by tradeoffs among resolution, cost and throughput. Other techniques, including


2

Page 3 of 32

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


nanoimprint lithography (NIL), nanostencils, and block copolymer (BCP) templates , are capable of the efficient and large-scale fabrication of nanostructures.1-7 Block copolymers use molecules as building blocks, which enables high feature densities (>109 cm-2) with spatial resolutions down to a few nanometers. BCP templates can be converted into nanoarrays of different materials by either lithography8-10 or selective chemistry of the polymer blocks.11-15 BCP templates have been demonstrated using solution-based micelles of amphiphilic copolymers that were deposited as thin films on a surface, instead of phase-separated thin film morphologies. Micelle-based templates constitute an attractive alternative to other lithographic approaches due to their ease of formation, orthogonal controls to vary their feature size and spacing, quick processing times, and compatibility with a range of substrates. BCP reverse micelles have been exploited to create a range of inorganic nanostructures, either in solution or directly on surfaces, for multiple applications.16-24 These approaches typically take advantage of reverse micelles by treating their functional cores as nanoscale reactors to initiate the chemical reactions of inorganic precursors. The incorporation of precursors has been demonstrated both by pre-exposure in solution phase prior to coating on surfaces, and by post-exposure of the micelle arrays on surfaces to solution or vapor phase precursors. For metal oxide nanoparticles, post-exposure to vapor phase precursors is most suitable. The handling of metal oxide precursors in the solution phase is less straight forward and prone to non-selective, secondary reactions. Furthermore, the post-exposure route offers an independent means to control feature and spatial resolutions of the resulting nanoarrays by adjusting the precursor concentration and the speed of micelle deposition, respectively.25-27 However, controlled vapor phase exposure carries challenges due to the highly reactive nature of the precursors, as well as difficulties in exercising fine control over their concentrations. The latter has been overcome by performing the processing within an ALD


3


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 4 of 32

reactor that offers excellent control of the key growth parameters, viz. temperature, pressure, chemical environment and dosing of the precursors.16,

17, 28-30

Recently, several reports have

shown the sequential infiltration (SI) of volatile organic precursors for controlled growth of nanostructure assemblies of different materials, including alumina, TiO2,9,

29, 37, 38

and W.

31

31-35

silica,

31, 36

ZnO,

17, 31

This approach has been found to work well with block copolymer

templates in general16, 29, 30 and, more specifically, for creating nanoparticle arrays using block copolymer reverse micelles. The recent investigation by Biswas et al.31 on the mechanism of the SVIS of alumina within Poly(methyl methacrylate) (PMMA) films demonstrated the precursor incorporation as mediated by formation of weak non-covalent interactions followed by covalent bond formation with the carbonyl and ester functionalities within the polymer. Reports on reverse micelles had showed the selective growth of metal oxide within the hydrophilic blocks of the copolymer, upon alternating exposure to precursor and water pulses within the ALD reactor, indicating the micelle cores act as a confined reaction environment.32 The diblock copolymer reverse micelles present a composite chemical structure encompassing functionality from both the blocks, and thus presenting opportunities for precursor incorporation and subsequent material growth via both, coordination bond formation and moisture induced decomposition. The lack of understanding in this respect opens a knowledge gap to be addressed to enable rational understanding of the material growth within BCP reverse micelles, or composite polymeric films in general. The understanding drawn is from mapping the influence of multiple contributing factors (e.g., precursor chemistries in relation to the chemistry of the blocks, relative humidity levels, precursor concentration, and ageing of samples) on the nanoparticle geometries would help ensure high integrity and reproducibility of the resulting nanostructured assemblies. Among the key challenges involved in these investigations is the ability to probe growth events at


4

Page 5 of 32

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


ultrahigh resolutions and to arrive at rational design rules based on robust growthparameter‹─›geometry correlations. To this end, our work investigates the mechanism underlying the vapor phase growth of well-organized titanium dioxide nanoparticle arrays that were created using a reverse micelle PS-b-P2VP template within sequential infiltration (SI). Titania nanoparticle assemblies are sought after to achieve higher efficiency in rationally designed lightbased electronic devices, e.g., photovoltaics, photocatalysts, electrochromics and gas sensors.27, 40-43

The successful performance enhancements in these devices require the constituent titania

arrays to be produced with high integrity. This sets low tolerance limits for the quality control specifications demanded from the approaches used. Meeting these expectations requires an indepth understanding of the approach and its co-dependencies on factors that affect the final quality of the arrays. We demonstrate titania nanoparticles grown via two different growth modes, which differed in their growth rate and their susceptibility to environmental humidity. Uncovering the growth mechanism involved significant challenges in characterizing and monitoring the different stages of growth at spatial resolutions down to a few nanometers. This was achieved by correlating the information obtained from X-ray photoelectron spectrometry (XPS) and secondary ion mass spectrometry (SIMS) with the geometries revealed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Experimental Section Materials Polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) with a molecular weight of 40500-b-41000 Da and a polydispersity index (PDI) of 1.10 was purchased from Polymer Source, Inc. (Canada) and used without further purification. m-Xylene (≥99%, anhydrous), which was used for


5


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 6 of 32

dissolving the BCP into micelle solutions, and the titanium precursor (TiCl4, ≥99.995%) for ALD experiments were purchased from Sigma Aldrich. Methods Sample preparation The BCP micellar solutions with an optimized concentration 0.3-1.5%wt. were obtained by dissolving the block copolymers in m-Xylene and vigorously stirring for 24 h.25 Silicon substrates were cleaned using a UV-Ozone cleaner (UVO-Cleaner® 42, Jelight Company, USA) for 30 minutes. The reversed micellar films were spin-coated with a rotation speed of 8000 rpm for 30 seconds. All experiments were carried out in a clean room class 100, with an ambient humidity of 45-50%. During the experiments, a hygrometer was used to monitor the humidity at the site of spin-coating. Exposures to the precursors were performed in an ALD reactor TFS-200 (Beneq, Finland), that was equipped within a load-lock system. The ALD reactor was conditioned by heating at 400°C for 3 hours to remove residual moisture within the chamber. The parameters of ALD deposition were chosen according to an initial study of TiO2 films deposited on planar substrates: 200 ms pulses for TiCl4 and H2O, followed by 30 s of purging with nitrogen between pulses of precursors at a reactor pressure of 1-2 mbar. The growth rate of titania on silicon, as determined by ellipsometry and SEM cross-sections, was found to be ~ 0.125 nm/cycle. TiCl4 exposures without additional water pulses were realized by sequential pulses of TiCl4 (200 ms) and the nitrogen purge (30 s). The exposure to the precursors vapors was performed below the glass transition temperature of the BCP (~ 97°C). Before exposure to the precursor, the samples were degassed by purging the ALD reactor with nitrogen for 30 min. The TiO2 nanoparticles arrays were subsequently obtained by oxygen plasma treatment (0.4 mbar, 200 W, 30 min) in order to strip the polymer template.


6

Page 7 of 32

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


Characterization The morphology of the samples was measured by scanning electron microscopy (SEM) (Helios 50 FIB-SEM from FEI Company, USA, operated at 2kV), transmission electron microscopy (TEM) (Jeol 2100F) and atomic force microscopy (AFM) (Innova, Brüker). The micelles profile was measured on the areas with clear micelle’s separation in order to the tipconvolution effects. The control of samples’ microstructure was performed by Transmission Electron Microscopy (TEM). The TEM samples were prepared by FIB-SEM following a standard procedure of lamella preparation using gold and platinum protective layers. TEM analysis was performed at 200 kV. The images were recorded in TEM, scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) and bright field (BF) modes. Chemical analyses of the micelle films, with and without exposure to the precursors, were investigated using X-ray photoelectron spectrometry (XPS) (Axis Ultra DLD, from Kratos Analytical Ltd) and secondary ion mass spectrometry (SIMS) depth profiling (CAMECA SCUltra). XPS was performed using an X-ray source (Al Kα monochromated, E=1486.6eV) at 150 W, a pass energy of 20 eV for narrow spectra and a step size of 0.1eV. The analyzed zone size was 300 µm × 700 µm. SIMS depth profiles were optimized for depth resolution using a Cs+ primary ion beam with an energy of 1 keV. The analyzed area was limited to a circle that was centered in the scanned area and was 33 µm in diameter, ultimately encompassing ~104 micelles. The different elements of interest were analyzed as MCsx+ clusters (M= C, Si, Ti, N, O and Cl; x=1 or 2) to circumvent matrix effects. Results and Discussion


7


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 8 of 32

Ordered arrays of block copolymer reversed micelles with a hexagonal organization were obtained by spin-coating without any further solvent annealing, as is illustrated in Scheme 1.

Scheme 1. Illustration of steps involved in the formation of titania nanoparticles guided by BCP templates. The average feature diameter of 42.6 ± 2.5 nm was determined by statistical analysis of the of micelle features as observed in SEM images. AFM revealed the height of the features to be 2530 nm, which indicated a roughly hemispherical shape (Figure S1, Supporting Information). These ordered films were subsequently used for further investigations of the BCP-templated growth of TiO2 nanoparticles. Two exposure modes were performed on the micellar films: mode I consisted of exposure to sequential pulses of TiCl4 and water, and in mode II, the samples were exposed exclusively to TiCl4. SEM imaging of the sample’s morphology after exposure in mode I showed a significant increase in micelle size as a function of precursor exposure (given by the number of SVI cycles). Swelling of the micelle was observed even after a relatively low number


8

Page 9 of 32

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


of SVI cycles (Figure 1A), as expected due to the formation and growth of titania within the micelle’s core. After a low number of TiCl4 and water SVI cycles (mode I) and stripping of the polymer template by O2 plasma, the samples exhibited an array of self-assembled nanoparticles. However, for samples that were exposed to a greater number of SVI cycles (more than 50 cycles), no change in the micelle’s size after O2 plasma treatment was observed. This finding could indicate that the polymer was covered by titanium dioxide and was therefore inaccessible to the oxygen plasma. Exposure of the micelle’s film to a low number of cycles in mode I forms confined the nanoparticles inside the micelles (in-growth). An increase of SVI cycles leads to an over-growth through the micelle’s corona and ultimately forms a TiO2 shell around the micelle via further classical ALD growth. The periodicity of micelles (center-to-center distance) limits the space for the shell’s formation. A similar growth regime after exposure to TiCl4 and H2O was investigated by Yin and co-workers.38 According to the authors, the PVP-micelle’s core, which contains pyridyl groups, was saturated by metal-organic complexes that formed after a few SVI cycles. Then, further deposition from TiCl4 and H2O continued to grow on top of the preformed and polymer-encapsulated nanoparticles, which led to the micelle’s size extension.38


9


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 10 of 32

Figure 1. Micellar film after SVI of TiCl4 + H2O (mode I-(A)) (a) and after O2 plasma treatment (b), and micellar film after SVI of TiCl4 (mode II (B)) (c) and after O2 plasma treatment (d). The samples fabricated by exposure to a different number of TiCl4 pulses in mode II showed that the micelle’s diameter did not change (according to SEM), even after exposure to a high number of SVI cycles. The O2 plasma treatment removed the BCP micelles on all samples, and the developed arrays showed significantly smaller nanoparticles than obtained with mode I; however, the nanoparticles did slowly scale with the number of SVI, eventually reaching saturation at a diameter less than the initial micelle (Figure 1B). This observation could indicate that the growth of TiO2 was limited by one or several parameters, such as the micelle’s core size, accessibility to pyridyl groups, and quantity of water trapped during formation of the micellar film. Comparisons of the micelle’s size after exposure to different modes and the nanostructures obtained after oxygen plasma treatment are illustrated in Figure 2 and Scheme 2.


10

Page 11 of 32

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. Micelles’ size modification after two exposure modes and nanoparticles (NPs) size after O2 plasma treatment.

Scheme 2. Different growth regimes of TiO2 nanoparticles: exposure to TiCl4 + H2O (mode I) and exposure to only TiCl4 (mode II). The outcome of growth mode I was in accordance with what has been reported in earlier publications.9,31 The growth rate of TiO2 (plot in Figure 2) was found to be higher at low number


11


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 12 of 32

of exposure cycles (

Investigating Sequential Vapor Infiltration Synthesis on Block

Recommend Documents