From Chemical Solutions to Inorganic Nanostructured Materials: A

Subscriber access provided by University of Illinois at Urbana-Champaign Library

Review

From chemical solutions to inorganic nanostructured materials: a journey into evaporation-driven processes. Marco Faustini, Cédric Boissière, Lionel Nicole, and David Grosso Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm402132y • Publication Date (Web): 05 Sep 2013 Downloaded from http://pubs.acs.org on September 24, 2013

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.

Chemistry of Materials 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 45

Chemistry of 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

From chemical solutions to inorganic nanostructured materials: a journey into evaporation-driven processes. M. Faustini,1 C. Boissière,1 L. Nicole,1 D. Grosso1* 1

Laboratoire Chimie de la Matière Condensée de Paris (LCMCP), UMR-7574

UPMC-CNRS, Collège de France, 11, place Marcelin Berthelot, 75231 Paris Cedex 05, France. Abstract We present the conventional and emerging methods of preparation of nanostructured materials from chemical solution evaporation where precursors, reactants, and potential structuring agent are homogeneously dispersed or dissolved in a volatile solvent. The liquid is shaped as dictated by the selected process and undergoes evaporation that concentrates the non-volatile species triggering various chemical phenomena such as reaction, self-assembly, phase separation, nucleation growth, aggregation, etc. While the composition of the final material depends on the initial solution stoichiometry, the morphology is governed by the process and the internal structure is adjusted by the kinetic and thermodynamic aspects of the coupled process. Because many processes involving the later approach exist, we decided to describe some of the more important in terms of applicability and some of the more emerging in terms of added value. More precisely, thin films, 2D nano-in-micro patterns, hierarchical structures and powders presented here were prepared by combining the solution evaporation with dip-coating, micro and nano fabrication, infiltration-replication (especially of butterfly wings), and finally spray-drying respectively. Efficiency, throughput, compatibility and limitation of each process and

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 45

methods are discussed together with related priority developments to make in the future. KEY WORDS : Evaporation, process, aerosol, butterfly, liquid deposition, lithography Introduction Due to the fast evolution of nanotechnologies and the growing number of applications, material chemists have the challenge to synthesize and integrate smaller and smaller pieces of matter into more and more complex architectures and at the lowest possible cost. The challenge becomes even greater when all types of chemistry (organic, polymer, metals, ceramics, biomaterials) are involved synergistically.1,

2

When the size of the building units and/or the periodical

arrangement of matter shrinks towards the namometer scale, the control of all the dimensions, structures and textures of the whole material becomes a real challenge that is likely to be overcome by a precise mastering of processes coupled with a fine tuning of the involved chemical aspects.3, 4 In this respect and beyond, appropriated processes are usually selected to prepare a specific type of material and one may take as much as possible advantage of the maximal range of processing conditions to adjust the final characteristics of the targeted materials, whatever the chemical complexity of the system. While in some cases, some of these structural characteristics can only be obtained through a precise tuning of the chemical conditions (precursors, media, catalyst and inhibitor agents, molecular and supramolecular templates, etc.), one must not forget that a fair part of the control during the synthesis relies on the process, which is moreover seen as the first lever of control in industry.


2

Page 3 of 45


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

From R&D to production, many processing methods exist or are invented, that will further undergo adaption and upgrading. Each of these requires different state of precursors from molecular to pure solid monoliths. Here, we will concentrate on describing recent emerging processes, for which the initial precursor state is exclusively under the form of a chemical solution, and allowing not only the relative control of the material macroscopic characteristics, but simultaneously their nano structure. 5 In these cases the link between the two worlds of chemistry and process is assumed by the physical-chemistry of the system and more precisely by the initial solution rheology and the evaporation of the solvent at interfaces,

6

which governs

the first step of the transformation of solutes, and/or dispersed nano moieties, into a solid (nanostructured) material. This overall concept is shown in Figure 1, which is illustrated with the example of dip-coating processing of films. In this article, the generation of thin nanostructured coatings by Chemical Solution Deposition (CSD), which nowadays concerns a wider and wider range of applications, will first be treated though mainly dip-coating but also by mentioning other emerging methods. We will then report on the few recent examples of nano-micro patterns materials made when bottom-up (self-assembly) and top-down (dry/wet etching, Imprinting, Lithography) approaches are combined. “Bulky” materials with periodical and hierarchical structures at various length scales will be described from infiltrationreplication method. Because nanocasting methods have already been fairly reviewed, 7-9

we selected here to focalise on the complex and rich examples involving the

butterfly wing replication. Finally, powders will be treated through aerosol drying generation, a method that is becoming one of the more promising processes to continuously produce micro and sub-micro nanostructure spheres (powders). In most of these examples, the resulting


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 45

materials are intermediates and they will very often have to undergo subsequent treatments, aiming at modifying the structural and chemical characteristics to obtain the desired final material. Despite their great importance these post steps will not be discussed in the present article.

Fig. 1: Scheme describing the global concept of preparing nano structured materials from the controlled evaporation of an initial chemical solution and through specific processes that permits the selection of the macroscopic structural characteristics (illustration shows dip-coating followed by top-down nano patterning). The main advantage of such solvent-evaporation governed processes lies in the fast evaporation of the volatile species (mainly solvent) that forces all non-volatile moieties to cohabit within a more and more confined volume imposed by the process (sphere, fibres, pores, etc.). As a result, whatever the mutual chemical affinities of these moieties, they will have no choice but to self-assemble or simply organise together within the given space and with the confinement interface (air or hard template) at some point along the drying step. 10 The processing conditions here will also play a crucial role in the thermodynamics and the kinetics that govern the overall nano structuration of the system. 11 Indeed, the confinement and the presence


4

Page 5 of 45


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 interfaces with the atmosphere will affect the self-assembly together with the temperature and the rate at which the initial volume is concentrated, which are more or less controlled by the process and the dimension of the said volume. 4 Most of the time, the fast drying induces quenching of the system into a metastable state, before reaching the thermodynamic one. As a result, whatever non-volatile species are in the initial solution, they will be recovered into the final material under a degree of dispersion that depends on the involved chemical affinities and on the relative kinetics of the involved physical-chemical phenomena. These approaches are thus ideal to prepare materials requiring the intimate mixing of non-soluble compounds. Due to the fact that the evaporation is taking place at the atmosphere interface, creating gradients of concentrations from this interface towards the heart of the volume, a fine-tuning of the evaporation conditions must be applied to obtain materials with gradients of structural characteristics combined with anisotropy.

I. Liquid deposition. Thin films are extremely important systems that are used in many domains of application since they bring many different types of additional functions at the surface of any kind of material. The addition of coatings onto a surface is achieved either by a dry process (Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), Plasma Enhanced CVD (PECVD), etc.) or by a wet process (spray coating, brush casting, spin coating, dip coating, roll and roll-to-roll coating, etc.) also referred as Chemical Solution Deposition (CSD). The wet (CSD) methods are well adapted for the preparation of multiphased materials since any non-volatile compound (i.e. metal oxide precursors, organic functions, monomers, polymers or


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 45

various kinds of nanoparticles) that is dispersed or dissolved into the solution will be homogeneously distributed or organised into the final films, and with a fairly good control of the thickness depending on the process. They are also highly appropriate for the preparation of materials with controlled nanostructure and porosity since structuring agents can easily be added in the initial solution, self-assembled during evaporation, and potentially eliminated from the final material.

12-14.

Amongst the

various liquid deposition methods, dip-coating has always been the most versatile one because it offers the best control on the processing conditions (withdrawal speed combined with a controlled atmosphere temperature and composition) that govern the final thickness and internal structure. Even if it has been commonly used in R&D and fairly used in industries, exploitation of its full potentialities has only recently been envisaged. The most recent progresses made in the control of film characteristics (thickness, composition, structure and porosity) were thus made through a fine-tuning of the dip-coating atmosphere. 4 It has also been demonstrated that the withdrawal speed range can be extended towards ultra-low speeds, down to 0.001 mm.s-1 for instance, where the film construction is not anymore associated to the Landau-Levich, 15 regime but is governed by capillarity and evaporation-inducedconvection at the meniscus.16 While fast speeds are appropriate to produce optical quality films in the suitable range of thickness usually found between 50 and 300 nm, low speeds are more prone to produce ultra thin or ultra thick layers from ultra diluted solutions such as metallic colloidal solutions, and from poorly volatile solutions such as aqueous ones. 17 Figure 2 shows the typical log-log evolution of the thickness versus the withdrawal speed with the related fitted equation. Both main regime of deposition are clearly visible at the extremities, while the minimal thickness is obtained for the intermediate regime. Mechanisms and influence of main


6

Page 7 of 45


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

chemical and processing critical parameters have been described in detail somewhere else.

16

Briefly, disregarding all evaporation dependent quantitative parameters and

examining only the slopes, the viscous drag regime, characterized by the LandauLevich model, was verified for speeds above 1mm.s-1. Similarly, the capillary regime, where the deposition is governed by the combination of convective capillarity and evaporation effects, is found at speed below 0.1mm.s-1. The intermediate regime is thus located between 0.1 and 1mm.s-1, where the minimal thickness is obtained. For all speeds, the thickness is perfectly described by the sum of the contribution of each regime, which led to the semi experimental model (Equation Fig. 2).

16

It shows that one does not have to integrate time-dependant

phenomena such as viscosity variation, evaporation cooling, and thermal Marangoni flow, to predict the thickness. Ultra low to ultra thick films can be prepared using the same solution with a good control on the thickness.

E h0 = k i ( + Du 2 / 3 ) ! Lu

Thickness (nm)

Hybrid SiO2/Me mesoporous film. Experimental points

!

Capillarity regime Draining regime Capillarity + draining contributions

Withdrawal speed (mm.s-1)

Fig. 2 : Typical thickness vs dip–coating speed curve of a sol-gel film (mesoporous hybrid silica film). The equation links the final thickness to the solution chemical and processing conditions and is composed of 3 main terms: the solution composition constant ki, the rate of evaporation divided by the speed time, the dimension L of the


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 45

substrate (E.(L.u)-1) that describes the capillary regime of deposition, and the solution physical chemical characteristic time the speed (D.u2/3) that describes the viscous drag regime of deposition. Because the capillary regime is governed by the evaporation rate E, the speed of deposition can be considerably increased using warm air. It can also be used for preparing optical quality films from aqueous solutions, since water surface tension decreases with increasing temperature. A perfect control of processing parameters is easier to achieve in the dip-coating technique, where a highly accurate motion system combined to a dipping-drying chamber within which the atmosphere composition and temperature can be accurately adjusted, are only required. The present part refers to progresses made in dip coating where these external parameters have been controlled. While dip-coating can be used to deposit biomolecules, polymers or metallic nano particles,

18-21

it is also widely applied to process metallic oxide

coatings from sol-gel solutions. 22

The viscous drag regime of dip-coating (Landau-Levich) is extremely well suited to prepare films of optical quality with an excellent reproducibility and control on the thickness over very large surfaces. It has been utilized for decades to prepare more or less dense oxide coatings from colloidal solutions. When the method of producing porous thin films with controlled porosity using a sol-gel system combined with EISA (Evaporation Induced Self-Assembly)

23

emerged 15 years ago, one rapidly

realised that the evaporation rate of each volatile component of the solution (mainly H2O, EtOH, HCl) was governing the cooperative self-assembly of the mesophase template. A systematic in situ GI-SAXS study was realized for SiO2 and TiO2 mesostructured films.

4,11

It revealed that the humidity was a critical parameter to


8

Page 9 of 45


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

control in the chamber and that a single system can lead to radically different nanostructures by simply modulating the water vapour pressure. More recently, it was demonstrated that an optimal combination of humidity and temperature during drying is highly beneficial especially for transition metal oxide mesoporous thin films.

24

Such an evaporation control was then successfully used to combine phase

separation of polymers and salts with typical micellar templating.

25

This approach

has recently been used to go beyond by preparing nanostructured epitaxial alphaquartz films on silicon substrates.26 Some of the SEM images in Figure 3 show typical examples of nanostructured films prepared in the draining regime and in controlled environment. One of the most important advantages of this regime is the very accurate control on the film thickness, or more precisely on the quantity of material that is deposited onto the surface. This is critical in many cases. For instance, after a film has been prepared on a substrate, it may often undergo one, or several, additional deposition(s) aiming at impregnating the underlying porosity or at covering the first coating with a layer of any other material to form a stack. While the first one requires the homogeneous penetration of the full quantity of the deposited species within the porosity, the second one must absolutely prevent the impregnation to end up with the material laying on top of the first coating. Here again, dip coating has proven to be an adequate process as testified by the successful stacking of several pairs of mesoporous TiO2/organically-modified-SiO2 layers for photonic selective sensors

27

or anti-reflective, self-cleaning, anti-fogging and water repellent bilayer systems.

28

Impregnation has been reported for molecular functions,

29

metals

30-32

, salts and

ceramics33, 34 and polymers 35. Impregnation and replication of porous templates will be treated in more detail in the third part of the manuscript.


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 45

The capillary-convective regime. Contrary to the Landau-Levich regime, one can greatly increase the thickness in the capillarity regime by reducing the withdrawal speed. Indeed, one is only limited by the intrinsic lateral tensile constraints building into the film with increasing thickness, which eventually provokes cracks and/or delamination. This regime is thus well appropriate for preparing coatings up to the maximal thickness for mechanical stability in only one step. However, some drawbacks have to be underlined here. First due to the very low speed, it is only appropriate for small surfaces such as for electrodes or sensors. Second, due to the stick-and-slip phenomena,36 one often ends up with between 5 to 10% thickness nonhomogeneity, which may not be a problem for the latter applications, but is not suitable for interferometric systems. Beside these precisions, the regime is well suited for deposition of solutions that are difficult to deposit in the conventional draining regime. Indeed, depositing aqueous solutions in the Landau-Levich regime is difficult due to the high surface tension of water. On the other hand, it can easily be achieved in the capillary regime. For instance, crystalline mesoporous TiO2 coatings have been prepared from simple TiCl4, H2O and PEO-PPO-PEO block copolymer template solutions, with thickness above 500nm in one step37 never achieved from alcoholic solutions. This approach has also been successfully applied to prepare the first MOF films from colloidal solutions38 of MIL-10139 or ZIF-8.40 To finish, solutions with extremely low concentrations, such as dispersions of metallic, latex or Q-dot nanoparticles, which may not be stable at high concentrations, could be deposited using the convective effect. A similar convective deposition regime would be found using the controlled motion of a linear meniscus at the surface of a horizontal substrate18, 41, 42.


10

Page 11 of 45

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


Fig. 3 : SEM images of typical nanostructured films prepared by dip-coating. (a) anatase TiO2 ultra thin nano pattern prepared in the intermediate regime, (b) anatase TiO2 grid-like structure prepare in the draining regime, (c) anatase TiO2 films with closed pores prepared in the draining regime, (d) ultra thick and ultra porous anatase TiO2 foam like film prepared in the capillary regime, (e) PVCmesoporous silica-TiO2 nanoparticle stack prepared in the draining regime,

43

(f)

heptamer of silica-gold nanoshells nanoparticles assembled onto a prepatterned substrate,41 and (g) a silica pillared array-roof film prepared by nanocasting a porous polystyrene layer from infiltration with a sol-gel solution.44 (Scale bar = 50nm).


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 45

The intermediate regime. According to the relation in Figure 2, it is clear that the minimal thickness is obtained in the intermediate zone, when both regimes of deposition perfectly counterbalance each other. At this critical speed, reducing the thickness further down to a few nanometers necessitate the dilution of the initial solution. These extreme conditions have been recently exploited to produce nanostructured Inorganic Nano Patterns45 that are ultrathin (4-15nm thick) metal oxides (TiO2, Al2O3, ZrO2) layers bearing ordered nanoperforations (10-80nm in diameters) through which the surface of the substrate (e.g. Si, glass, FTO, ITO, Au, or any other porous or dense layer) is accessible (see Figure 3(a)). Because of their thinness, they can be seen as ordered heterogeneous nanostructured surfaces. They have been utilized as substrates for selective local growth of nanoobjects such as i) FePt

nanoparticles

using

electrodeposition,46

ii)

Prussian

Blue

Analogue

nanoparticles using layer by layer construction,47 iii) CoPt nanostructured multilayer using sputtering,48-50 or iv) Ge nanoparticles using solid dewetting51 for data storage systems. They have also been used as surfaces with controlled wetting properties,52, 53

as component for antireflective self-cleaning layers,

28

or to prepared ultra nano-

electrode arrays.54, 55 The fact that the conditions, where the minimum thickness can be obtained spreads over a wide range of withdrawal rates, can be exploited to assure a good reproducibility. Indeed, here one has only to adjust, if possible, the dilution of the initial solution to control the thickness. Alternative spray technologies. It is common knowledge that spraying is the most employed technique in the coating and painting industries since it is a fast and low cost process that is well-adapted to all substrate shapes. However, it does not meet the high degree of control yet associated with dip or spin coatings in terms of optical thickness homogeneity and structuration at the nanoscale. On the other hand, electro-


12

Page 13 of 45

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


spray technologies can be useful to prepare thick and/or very rough films for super hydrophobic silica based coatings56 or multimodal TiO2 electrodes for dye sensitive solar cells,57, 58 for instance. Concerning “drawing” on a surface, ink-jet and aerosol spray

59

are emerging technologies that have been successfully used to prepare sol-

gel based micro lenses and lines respectively. Even if they offer a relatively good control on the nanostructuration, one still has to account for the “coffee ring effect” creating thickness non-homogeneity at the material boundaries.60 Efforts are currently dedicated to the development of spray methods that would overcome these limitations and allow a sufficient control of droplet impaction, spreading and coalescence to make homogeneous in thickness optical thin films.

II. Nano-in-micro patterns using top-down techniques. In the previous sections we described some of the main “bottom-up” approaches to periodically assemble the matter at the sub 100 nm scales into mesostructured coatings and materials from inorganic solutions. On the other hand, “top-down” processes such as lithographic and micro/nanofabrication techniques allow fabrication of regular patterns from the micrometer down to nanometer size.61 Since the 70s, the development of these processes was crucial for the fabrication of most of the devices in the fields of microelectronics, fluidics, photonics, etc. The coupling between bottom-up and top-down processing is highly promising since complex hierarchical materials with additional functionalities can be potentially integrated into real devices.

62

The mastering of the micro and nanofabrication processes

permits theoretically the integration of almost all kind of materials, yet by applying numerous fabrication steps such as pattering, multiple co-deposition, etching, and


13


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 14 of 45

lift-off. Nowadays the real challenge is how to easily combine those strategies, adapting well-established lithographic techniques to “un-conventional” solution based materials (and vice-versa) in order to reduce the number of fabrication steps. In the last decade, several examples were reported on the utilization of micro and nanofabrication processing to pattern nanostructured inorganic materials.

62

Depending on the processing strategy, for simplicity, we can divide the lithographic techniques in three main categories: (i) processes involving exposure though a mask, (ii) molding/imprinting lithography and (iii) mask-less direct writing techniques.

Photolithography is probably the most common technique to pattern materials at the micrometric scale by UV exposing a sensitive resist through a mask (Figure 4(a)). The process was also adapted to fabricate mesoporous silica film micropatterns; 63, 64 in particular, the presence of a photoacid generator in the uncondensed film permitted the localized acid-catalyzed siloxane condensation and thus the selective etching of unexposed regions. 64 The same approach could be extended to other more energetic photonic radiations, such as in the case of deep UV

65, 66

and X-ray

lithography. 67 In general, the exposure of the hybrid organic/inorganic uncondensed layers to high energetic photons induces the decomposition of the organic porogen and the fast condensation of the silica inorganic network without the need of photoactive species. Recently Deep X-Ray lithography (DXRL) has also proved to be an effective and versatile tool for the patterning of other non-silica based films, achieved by both exposure through a mask or by direct writing from an X-ray beam. 68, 69,70

For instance Figure 4(b) shows an example of how nano-in-micro patterning

of ultrathin titania films could be obtained by applying DXRL to block copolymer templated Inorganic Nano Patterns (INP).

71

In addition to the simple patterning, the


14

Page 15 of 45

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


technique was also used to locally induce nucleation and growth of metallic particles, 72,73

formation of microdomains with tunable wettability

8,74

and conductivity71 and

for the fabrication of lab-on-a-chip devices. 44, 72 Shaping of the matter at the micro/nanometric scale can also be achieved by all the so-called “soft-lithography” approaches that consist in molding the material through an elastomeric stamp.75 The techniques usually involve utilization of a stamp (mold) in polydimethylsiloxane (PDMS) fabricated from a prepatterned substrate (master). The advantage of these approaches is that, from a single master, usually prepared through low throughput techniques such as Electron Beam lithography, many PDMS molds can be replicated and applied in a very easy and inexpensive way. In this category we can include Micromolding in capillaries (MIMIC) that consists in using the mold as a sort of microfluic circuit where a low viscosity precursor solution is convoyed into the channels by capillary forces.76 After gelation and evaporation of the volatile species the final material is selectively deposited into the microcavities making a faithful replica of the pattern. In this way, several hierarchically structured systems have been fabricated. For instance molding in linear channels was used to fabricate 1–3 µm width mesoporous silica waveguide arrays doped with fluorescent dye.77 A similar strategy was recently used to create hybrid 2D hexagonal mesoporous silica microtrenches and induce alignment of micelles along the channel direction.78 However the MIMIC process is quite slow and requires interconnected patterns that do not allow fabrication of isolated discrete features (dots arrays for instance). These issues are overcome by Soft Nano Imprinting lithography (s-NIL) that schematically illustrated in Figure 4(c) and consists in embossing the intermediate non-condensed layer with a patterned PDMS mold.

79-82

The

cooperative effect of the capillarity filling in the nanocavities and the applied


15


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 16 of 45

pressure allows the replica of the nanoreliefs on the desired coating. In addition flexible molds also permit homogeneous imprinting on slightly rough or curve substrates. The residual layer that often remains on the sample can be eventually removed by controlled dry etching. An example of s-NIL combined with selfassembled systems is shown in Figure 4(d): nanopatterned porous titania layers were obtained by direct embossing and calcination of hydrid films composed by Ti-based oligomers and block copolymer micelles that acted as porogen agent. The viscosity and condensation degree of the intermediate layers are the main critical parameters that have to be carefully adjusted for embossing. In fact, in the case of denser or more condensed layers, flexible PDMS molds cannot be used since a too high pressure provokes deformation of the PDMS reliefs. For this reason, Thermal Nano Imprinting lithography (t-NIL) with hard stamps (Si, SiO2, etc) at higher temperature and pressures was mostly used for embossing hybrid and sol-gel based inorganic layers. In particular the technique was used to process a wide range of metal-oxides 83

and porous materials.

84

Among the rich literature on silica based imprinted

materials, a remarkable example of direct embossing of hybrid sol-gel films was recently reported for the easy fabrication of all-silica micro and nanofluidic devices for single molecule studies.85 Despite the fact that t-NIL is quite mature and has been generalized to several systems, some issues remain critical such as the high cost of the apparatus and of the rigid masters, the damaging of the master after repeated imprinting cycles, the sticking and the inhomogeneous contacts with the substrate. For these reasons the further development of inexpensive s-NIL strategies applied to inorganic nanostructured systems remains crucial. The alternative of using a mask or a mold to pattern transfer into inorganic nanostructured materials is the direct writing of the nano or micromotifs. One widely


16

Page 17 of 45

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


exploited process is Electron Beam Lithography (EBL), which is based on the selective irradiation of sensitive films with an electron beam that is displaced in a controlled manner over the surface. By this technique, micropatterned mesoporous silica and titania films were obtained by either filling a prepatterned EBL resist film with the precursors solution 86 or direct exposure of the un-condensed layer to a low energy electron beam.

87

Another way to direct write features of nanostructured

materials is based on the localized deposition of the precursors solution droplets through a nozzle/probe displaced on the surface for instance in the case of Micro-Pen or Ink-Jet printing already mentioned in part I. Fan et al59 first demonstrated the possibility of applying those techniques to silica / surfactants based systems that allowed rapid fabrication of mesoporous micropatterns through localized deposition and EISA. Another appealing aspect that Ink-Jet printing also allows is tailoring the chemical composition of each motif. For instance, in addition to the precursor solution for mesoporous silica, each microdroplet can contain different functional molecules allowing the design of miniaturized sensors based on multi-array of functional mesoporous dots.88,

89

Another work demonstrated the fabrication of

mesoporous multicomponent metal oxides patterns with tunable composition used as high-throughput generation of catalyst libraries.

90

Following a similar strategy,

further miniaturization can be achieved and ultra-small nanopatterns can be directly written by Dip-Pen Nanolithography.91 The technique consists in dipping an AFM scanning probe into the precursor solution and in displacing the nanodroplet over the substrate.92 After evaporation and calcination, 100-200 nm large nanopatterned mesoporous films have been fabricated on silicon and glass.


17


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 18 of 45

Fig. 4: Schematic illustration of (a) UV or X-ray Lithography, (c) soft Nano Imprinting Lithography and (e) Reactive Ion Etching applied to inorganic nanostructured coatings. SEM images of (b) nano-in-micro titania patterned surface by DXRL,71 (d) nano-imprinted mesoporous titania by s-NIL93 and (f) silicon nanowells from RIE though a self-assembled mask. 94

In addition to the lithographic patterning techniques described above, other processes generally utilized in nanofabrication can be combined with of bottom-up derived materials in order to fabricate more complex and functional systems. This is the case of Reactive Ion Etching (RIE), the most common tool to perpendicularly etch materials and fabricate high aspect ratio nanostructures. It consists of exposing the system to an inductive plasma in the presence of reactive gases, typically fluorinebased, to induce the anisotropic etching, for instance of the silicon substrate. In this case, the inorganic nanostructured films prepared by bottom-up approach can act as 2D masks to be submitted to RIE process for the pattern transfer on the silicon substrate as schematically illustrated in Figure 4(e). In such a configuration the inorganic self-assembled films will play the role of hard masks allowing fabrication of high aspect ratio nanostructures on large surfaces overcoming the resolution limits


18

Page 19 of 45

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 most of the top-down lithographic techniques. One critical aspect in the fabrication of deep nanostructures concerns the composition of the nanomasks that needs be adjusted in order to maximize the difference in etching rate between the mask and the substrate. For instance, hard nanomasks consisting of iron oxide dot arrays were obtained by selective post-filling of self assembled block-copolymer layers and permitted the fabrication of 2D arrays of high aspect ratio silicon nanopillars.

95

A novel kind of Reactive Nano Masks (RNM), consisting of CaTiO3

INPs was fabricated by following the protocol described in part I. and by adding calcium chlorides as an inorganic precursor.

94

In this case, the role of calcium is to

increase the selectivity of the mask by reacting with the fluorinated species during etching to form a resistant CaF2 passivation layer. The method allowed faithful transfer pattern and fabrication of high aspect ratio silicon 50 nm diameter nanowells (as shown in Figure 4 (f)). Following the same approach 20 nm diameter pillars having aspect ratio equal to 8 were prepared by adjusting the composition and the morphology of the initial RNM.

In the last decade the combination between top-down and bottom-up approaches has represented a growing domain in nanotechnology for the fabrication of hierarchically structured inorganic films. The alliance between chemists, materials scientists and physicists has permitted the demonstration of patterning of self-assembled inorganic materials from the micrometric down to sub-100 nanometric resolution by exploiting several lithographic techniques. Those demonstrations represent a solid ground for further developments and for facing future challenges. From the technological point of view, the fabrication of such hierarchical materials on large surface following industrially compatible processes is crucial for going into real applications. In the


19


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 20 of 45

same way, many efforts need to be dedicated for the integration of different components (in terms of morphology, size and chemical composition) into single multi-functional devices. In addition the smart utilization of regular lithographic patterns to induce directed self-assembly (DSA) of mesophases and providing longrange ordered nanostructures must be fully explored also for inorganic based materials. At last, the fabrication of 3D complex patterns by upgrading the lithographic and etching techniques would significantly contribute to the development of complex fluidic and optical devices (including photonic crystals and metamaterials). Some techniques such as Two-Photons Polymerization lithography 96 are good candidates since they allow fabrication of complex 3D micro and nanostructures but generally limited on small surfaces. A long way needs to be crossed for reaching what Nature has made.

III. Replication of porous template via infiltration: example of the butterfly wing. As it was underlined in the previous parts, the current development of nanotechnologies implies the elaboration of complex, multi-functional, multi-scale and hierarchically organized architectures. Basically, two main routes are commonly employed by materials scientists to achieve such architectures: top-down and bottom-up strategies and their combination. 35, 62, 97-102 However, up to now, the most efficient way leading to multi-functional and hierarchical multi-scale systems (from nano-, micro- to macro-sizes) has been successfully experienced by Nature over billion years of evolution. 103-106


20

Page 21 of 45

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


Despite the astonishing diversity of smart and sophisticated hierarchical structures provided by Nature that are usually highlighted, one of the most important feature of natural architectures lies in the acceptable level of disorder at each scale.

107, 108

In

other words, natural architectures can exhibit optimized multi-functionalities despite the presence of imperfections.

109

It means that Nature has to optimize, with the

minimum of materials and energy expenditure, a set of various vital properties without optimizing a single function: a compromise between cost and efficiency. It explains that natural structures are often far from ideal, displaying a structural disorder, which finally (and surprisingly) strengthens all the properties at the macroscopic scale. This "robustness" of effects could be easily illustrated by natural photonic structures of butterfly wings.

108-110

Besides intra and inter-specific

communications (reproduction, camouflage, limited predation ...), the structure of wings contribute to the regulation of body temperature

111

while providing

superhydrophobic and self-cleaning properties. 112-115

These lessons from Nature, coupled to the lack of cost-effective engineering techniques able to copy natural architectures with their topological disorder, could explain the growing importance of bioreplication, biomineralization, biotemplating domains in nanotechnologies. Bioreplication consists of the direct replication of a natural structure and could be achieved via a biomineralization or a biotemplating approach. Generally, biomineralization gives rise to a positive or direct (true) replica while biotemplating induces a negative or inverse one. Various chemical and physical routes have been developed for replicating butterfly wings such as atomic layer deposition (ALD),

116, 117

chemical vapor deposition,


118

physical vapor

21


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

deposition (PVD)

119, 120

Page 22 of 45

and the present chemical solution deposition which is the

most widely used approach.

Various chemical and physical routes have been developed for replicating butterfly wings, however as a complete description of the topic is out of scope of this article, the readers are referred to more specialized reviews on bioreplication. 98, 103, 121-124

The following part will be dedicated to bioreplication of butterfly wings from inorganic chemical solutions. Several strategies have been employed to replicate such natural structures: (i) imprinting lithography, (ii) sol infiltration, (iii) layer-bylayer surface sol-gel deposition and (iv) single layer adsorption. The two first ones involve capillarity as driving force while the last two are based on an adsorption step followed by nucleation/growth or hydrolysis/condensation mechanisms. As natural architectures are uncommon substrates presenting several particular features, a brief description of butterfly wings is needed. With regard to the multi-scale structure, a five-level observation method has been adopted to describe butterfly wings (figure 5). 109 The first level is the macroscopic one and concerns the whole wing (cm). The second level deals with the scales covering wings (roughly hundred of µm). The third level is about striae or ridges on scales (roughly a few µm). The fourth level is in the range of 50-100 nm and concerns the structure of striae and inter-striae spacing called “Christmas tree”. The last level is the molecular one and is about the chitineous matrix; this matrix is composed of chitins molecules, which are organized into microfibrils coated with a proteinic matrix.


22

Page 23 of 45

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


Fig. 5: The different scales used to observe butterfly wings and their units of measure. 109

Imprinting lithography. This approach consists in the coating of the biotemplate with "liquid" materials to form a negative replica after a curing step. Once the removal of the biotemplate achieved, the negative mold could be used to pattern another material. This approach has been used to replicate the upper surface of butterfly wings with polydimethylsiloxane (PDMS) species.

115, 125

Although this approach

was efficient for replicating simple structures, 115 more complex striae fine structures such as the "Christmas tree" structures (figure 6) were not perfectly reproduced increasing thus the dimension of the scale features.

125

This observation was

explained by the low infiltration of the PDMS solution inside the nanometric treeshape structure. Finally, these PDMS stamps were successfully used for imprinting a liquid methyltriethoxysilane-based sol-gel film, which displayed a superhydrophobic behavior after curing.

115

An alternative approach based on soft-NIL process was

developed by Silver et al. 126 and Li et al.127 Briefly, replicas were obtained by a twostep process. The inorganic solution to be infiltrated was first coated onto quartz substrate and next, a section of a butterfly wing was placed between the coated substrate and a non-coated one and pressed together.

126

After a drying step, the

coated quartz substrate was heated at high temperature (700°C) in order to remove


23


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 24 of 45

the biotemplate. Here again, the striae fine structure of wings were not perfectly reproduced due to the poor wetting ability of the butterfly scale towards water-rich precursor solution.

126

Interestingly, depending on the experimental conditions, the

authors obtained either a negative or a thick positive replica, starting with the same initial solution. 126

Fig. 6: (left) General drawing of the cross section of a scale. (right) Examples of quasi-1D, 2D and 3D photonic structures of butterfly scales.

Imprint lithography should be the most suitable technique for the replication of biomaterials at the macroscopic scale since the whole fragile structure of wing is supported by a substrate. However, fine structures of the scales have been hardly accessible up to now and this technique is limited to the replication of simple structures (upper surface of biotemplates - figure 6).

Sol infiltration. Successfully developed for the replication of beetle scales,

128, 129

a

biological 3D photonic crystal (PhC), this approach has been recently employed for the replication of butterfly wings exhibiting similar 3D PhC structures (figure 2).


130

24

Page 25 of 45

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


In these studies, the pretreated biotemplate was clamped between two substrates followed by injection of sol-gel precursors solutions (∼ 1 µL) at the edge of the assembly. Depending on the biotemplate, i.e. butterfly wings or beetle scales, several pretreatment have been performed. For butterfly wings which present highly porous open-structures, a simple ethanol cleaning was done to eliminate loosely bound proteins and lipids on the surface.

130

In the case of beetle scales, the photonic

structure is embedded in an impermeable biopolymeric shell and thus is not directly accessible. 128, 129, 131

131

It means that the scales should be first cut before the infiltration step

and eventually treated in acidic medium for removing the thin waxy layer

coating the biopolymer structure.

131

The infilling step of biotemplates is driven by

capillary forces, which permit a high infiltration degree. By adjusting the concentration of non-volatile species in the solution and/or the processing conditions, solid or hollow negative replica were also obtained. 131 This strategy implies working with highly volatile solvents (such ethanol) presenting a low surface tension and thus a high wetting ability (we have to keep in mind that these natural structures present intrinsically a superhydrophobic behavior with a water contact angle around 160°). 115

Moreover, it has been observed that the "sandwich" capillary method limited

drastically the excess of inorganic materials around biotemplates. More surprisingly, in the case of butterfly wings, this method favors a selected infilling since only the interior of the scale (3D PhC) was filled by the sol and not the upper structure (ridges and cross-ribs).

130

This last observation underlines the main advantage of the sol

infilling strategy: this approach is self-limited since once voids are filled by the solgel material no more capillary diffusion can occur. Although sol-gel solutions are well-suited for the sol infiltration strategy, two main classical problems should be overcome: (i) the shrinkage of replicas due to high temperature treatments required


25


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 26 of 45

for biotemplate removal and eventually inorganic crystallization and (ii) the fragmentation of the macroscopic samples which limits the size of the final materials. To limit the natural shrinkage of inorganic replicas, Galusha et al.

128, 129, 131

developed an acid-etching technique at moderate temperature (130°C) for removing chitineous matrix. This approach results in a lattice contraction less than 5 % instead of 30 % observed with high temperature treatments (450°C). To circumvent the high fragmentation of the sample at the macroscopic scale, a SBA-based solution (a solution containing surfactant leading to mesostructured materials) was successfully tested. Besides an improvement of structural integrity, the use of surfactant increases the wetting properties of the solution and leads to mesostructured replicas with worm-like structure, providing a new scale of organization compared to the natural biotemplate.

130

This first negative SiO2-based replica, thermally and mechanically

more stable than the original biotemplate, was also used as sacrificial template for an anatase TiO2 replica following the same infilling method. 129

Layer-by-layer surface sol-gel (LbL SSG) deposition. This method consists in the deposition of a continuous thin oxide layer, which increases in thickness upon cycling. 132-134 Briefly, the biotemplate clipped on a glass/Si wafer substrate is ageing (10 minutes) in a dilute anhydrous alcoholic solution of metallic alkoxides precursors. The butterfly wings were then rinsed with anhydrous alcohol for removing the poorly bonded precursors and were immersed in water or hydroalcoholic solution for a short time (1-2 minutes) to promote the hydrolysis and condensation of the sol-gel precursors. The cycle is ended by a further rinsing with anhydrous alcohol and a drying step at moderate temperature. This method, which is automated (controlled by computer) implies usually between 20 and 50 cycles to


26

Page 27 of 45

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


achieve mechanically stable positive replicas (around 60 nm thick). This procedure should be achieved thanks to the presence of active side-groups at the surface of chitin (alcohol, amide, amine in case of previous deacetylation reactions), which favor the adsorption of the precursors via hydrogen bonding, ionic interactions or complexation reactions. 134, 135 This deposition technique followed by post-treatments (heating at high temperatures processing

132, 133

134

and eventually a microwave hydrothermal

) led to anatase, rutile TiO2, tetragonal BaTiO3 and Eu-doped

BaTiO3.

Single layer adsorption. This last strategy has been exclusively and widely developed by the Di Zhang's group since 2006. 123 This approach could be viewed as a kind of biomineralization process (biomorphic mineralization) since it favors the penetration of inorganic precursors inside the smallest gaps of the biotemplate structure and not only at the surface of butterfly wings. It explains thus why a pretreatment step is necessary before the adsorption step. 103, 136

1. Pre-treatment step. It is usually performed for demineralizing, deproteinizing the biotemplate, activating side-groups at the chitin surface (and thus adsorption mechanism) and removing pigments. A typical procedure consists in immersing, at moderate temperature, the wing several hours in a solution of HCl followed by ageing in a NaOH solution. Alternatively, a solution of EDTA/DMF favoring biomineralization process 137 or an acid acetic/ethanol mixture activating chitin sidegroups, were also used. 135


27


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 28 of 45

Fig.7. Left: photographs of samples at each step in the synthesis process. (a) Dissected fore wing of Ideopsis similis; (b) soaked wing templates; (c) the assynthesized white replica. Right: XRD patterns of the corresponding samples on the left column. 138

2. Adsorption step. After a washing-drying step, the wing slices are immersed for a long period (in general 12-24 hours at low temperature) in the precursor solution (anhydrous alcohol or hydro-alcoholic) and then washed-dried again. A sonochemical processing has been also investigated during the immersing step resulting in a shortening of the immersion time (3 hours) by using a high-intensity ultrasonic probe.

139

By locally increasing the temperature, ultrasound increases the

rate of hydrolysis and condensation reactions of inorganic precursors.


28

Page 29 of 45

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


3. Post-treatment step. The biotemplate is usually removed by a thermal treatment at high temperature. In order to retain the whole structure of the wing, the heating rate is quite slow (1°C/min) and the coated biotemplate is clamped between two glass substrates for keeping the soaked wing as flat as possible. The thermal treatment results in the crystallization of the inorganic network of bioreplica with a size reduction about 50 % due to shrinkage at high temperature (usually above 500°C). Usually performed in air, the heat treatment could be done in reducing conditions H2/Ar atmosphere

135

or carbon thermal reduction under vacuum

magnetophotonic replica (Fe3O4)

135

140

) leading to

and nanocomposite replica (Fe nanoparticles

embedded in a glassy carbon matrix).

135

Interestingly this nanocomposite replica

was used as master for PDMS-based imprinting lithography. Alternatively, in the case of metallic replicas, a long immersion (72 h) at room temperature of the metalcoated scales was performed in highly concentrated phosphoric acid (85 % wt). This alternative treatment was preferred because of the high surface energies of metallic nanoparticles which favor the formation of coarse precipitates at high temperature reducing thus the quality of metallic replicas. 141

This procedure led to full positive bioreplica from the macroscopic to the nanometric scale. Although the replica is rather brittle, it can still be handled with tweezers (see Figure 7). The thickness of the inorganic coating (below 100 nm) and thus the quality of replica could be adjusted by modifying the concentration in inorganic precursors and the immersing time in the precursor solution.

142

For example, it has been

observed that high concentration of inorganic precursors and long immersion time led to the collapse of the fine structure. 143 Another interesting feature of this "single layer adsorption" strategy lies in the synthesis of pure metallic replicas (Au, Ag, Co,


29


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 30 of 45

Cu, Ni, Pd and Pt) via inorganic chemical solution and thermal treatment at low temperature (

From Chemical Solutions to Inorganic Nanostructured Materials: A

Recommend Documents