Patterning and Substrate Adhesion Efficiencies of Solid Films

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J. Phys. Chem. C 2010, 114, 19782–19791

Patterning and Substrate Adhesion Efficiencies of Solid Films Photodeposited from the Liquid Phase Jean-Pierre Delville,*,† Emmanuel Hugonnot,† Christine Labruge`re,‡ Touria Cohen-Bouhacina,† and Marie-He´le`ne Delville*,§ Centre de Physique Molećulaire Optique et Hertzienne, UMR CNRS/UniVersite´ No. 5798, UniVersite´ Bordeaux I, 351 Cours de la Libe´ration, F-33405 Talence cedex, France, CeCAMA, 87 AVenue du Docteur A. Schweitzer, F-33608 Pessac cedex, France, and Institut de Chimie de la Matie`re Condenseé de Bordeaux, UPR CNRS No 9048, UniVersite´ Bordeaux I, 87 AVenue du Docteur A. Schweitzer, F-33608 Pessac cedex, France ReceiVed: September 28, 2010; ReVised Manuscript ReceiVed: October 7, 2010

We experimentally and theoretically investigated the patterning and adhesion, always assumed and almost never discussed, of coatings photochemically deposited on substrates from photoactive solutions of different compositions and pHs. Considering the well-known deposition of Cr(III) layers from potassium chromate solutions, we analyzed the morphology and properties of the deposit when induced by two interfering continuous Ar+ laser waves. The solubility, patterning, and adhesion are investigated in both organic (acetic acid) and inorganic (HCl) acidic solutions. The photodeposition process is also compared for several types of substrates usually found in the literature (glass, silanized glass, PMMA, silicon wafer, indium tin oxide (ITO), and stainless steel). We demonstrate the major role played by the interaction between the generated coating and the substrate and propose a strategy to find the best conditions for photochemical deposition from the liquid phase, an approach that is mandatory for any application requiring optical recording developments. Introduction Laser deposition is now commonly used as an alternative photolithographic method to generate coatings with controlled profiles.1,2 The resulting rapid prototyping is particularly efficient to create or repair microelectronic components,3,4 to build and integrate optoelectronic elements on chips,5 or to functionalize surfaces for catalysis and chemical sensoring at small scale.6 Among the reported methods, including thermal decomposition or photolysis1,7 under strong laser irradiation to generate vapors of elemental atoms or small molecules that further adsorb onto a substrate, photochemical deposition by excitation of photosensitive liquid solutions received increasing attention over the past years8 and is still in progress, for the three following reasons. First, photochemical deposition from liquid phases can be applied to a very broad range of materials because precursors are simply soluble in solutions. Because of liquid density, the photoabsorption cross sections of precursors then are much larger than in gas phase, and thus photodeposition often requires very moderate beam intensities. Finally, experiments are extremely simple to set up because photodeposition is performed in classical homemade tight cells. This flexibility9 offers the opportunity to deposit a large variety of materials: noble metals (such as Au,10,11 Ag,12,13 Pd,14 Pt,15,16 or Cu17), semiconductors (such as CdS or ZnS,18-20 CdSe or ZnSe,21,22 (Bi, Sb)2S3,23 SexTe1-x,24 InS,25 PbS,26 CuxS,27 CuInS2,28 Sd1-xZnxS,29 FeSxOy30), metal oxides and hydroxide (CrO2,31 Cr(OH)3,32 MnO2,33 SnO2,34 ZnO35), polymers,36 and bio-organic37 or molecular complexes.38,39 * Corresponding author. Phone: (33) 5 40 00 22 07. Fax: (33) 5 40 00 27 67. E-mail: [email protected] (J.-P.D.); [email protected] (M.-H.D.). † Centre de Physique Molećulaire Optique et Hertzienne, Universite´ Bordeaux I. ‡ CeCAMA. § Institut de Chimie de la Matie`re Condenseé de Bordeaux, Universite´ Bordeaux I.

Moreover, irradiating solutions with a well-defined light intensity distribution makes it possible to write in a single step a large variety of patterns in serial (dot40 and line41 assemblies, for example) and in parallel (such as holographic gratings42) onto both flat43 and curved44 solid substrates in contact with the photosensitive solution. Laser light thus behaves as a “smart” pencil, which tailors the material deposition by simply modifying the excitation wavelength, the intensity distribution, and its spatial extension.45 Finally, when driven by a one-photon chemical reaction, photodeposition is supported by systemindependent predictions, which make the description of deposit growth universal and allow for the quantitative sorting of photochemical reactions in terms of deposition efficiency.33 However, as illustrated by the above-mentioned examples, up to now most investigations have concerned the versatility of the technique in either photosensitive precursors or deposited materials. Photodeposition nonetheless requires fulfilling two additional crucial conditions: nucleation at the substrate and adhesion. Indeed, as far as substrate patterning is concerned, the nucleation of the deposit must obviously take place on or close to the surface of the chosen substrate. If the nucleation barrier on the substrate is of the order of or larger than that in the liquid, the photoprecipitates are also likely generated in the bulk and their diffusion and/or sedimentation eventually blur the desired surface patterning. Moreover, an efficient process also requires adhesion between dissimilar materials (the photodeposit and the substrate), which strongly depends on the chemical, physical, and morphological properties of the interface. Even if these two fundamental conditions are always implicitly assumed in photodeposition investigations, there is no obvious reason to accept the generality of such a statement, and attention must be paid to the crucial role played by the surface of the substrate on both nucleation conditions and bonding/adhesion of the deposit. Surprisingly, these aspects, even if essential in terms of applicability and reliability of the

10.1021/jp109283v  2010 American Chemical Society Published on Web 11/02/2010

Patterning and Substrate Adhesion Efficiencies of Solid Films technique, have scarcely been investigated in photochemical deposition from the liquid phase. In most publications, the substrates (glass, In2O3 and indium-tin oxide (ITO) coated glass, silicone, plastic sheets, elastomers) are indicated without further explanations. In a few of them, the relative adsorption on different substrates is mentioned.46 The pretreatment of the substrate to enhance heterogeneous nucleation was presented by one group in the case of Pd activation and Sn sensitization of glass plates, for instance.23,26 Finally, beyond feasibility and property analysis of the deposit nature, almost no work has been done on deposition improvements versus variation of experimental conditions (spreading enhancement by chelating agents,47 influence of ultrasonication48). The goal of the present investigation is to analyze in details all these effects to enlighten their respective importance in photopatterning. To illustrate the different behaviors taking place when varying the experimental conditions, we performed experiments on photodeposition of chromium hydroxide from aqueous solutions of potassium chromate. This choice was motivated by the five following reasons: (i) Historically, the photoreduction of Cr(VI) ions into Cr(III) ones has been widely used for hologram recording in dichromated gelatins;49,50 (ii) chromium hydroxide precipitates appear in numerous technological areas;51-53 (iii) chromium hydroxide is a gel, with a structure switching from polymeric to crystalline with increasing pH;54 (iv) chromium hydroxide is a semiconductor of band gap 2.40 ( 0.05 eV;55 and (v) Cr(III) behaves as Fe(III) in the environment, being readily precipitated or sorbed onto a variety of inorganic and organic substrates at near-neutral pH.,56,57 whereas the Cr(VI) derivatives interact with biomolecules and have been recognized as important factors in genotoxicity processes.58-60 Moreover, because metal oxide slides (such as SiO2, TiO2, ZnO, ITO...) are classically used as substrates, we chose the most common one, glass, to illustrate most of our purpose. Glass is considered as a dielectric substrate, and results obtained with other types of substrates, considered as semiconducting, are also presented. By varying the composition and the pH of the initial chromate solution,61 we compared the chromium hydroxide photodeposition yields, the corresponding substrate patterning, and the photodeposit adhesions, and discussed the correlation between these three behaviors. Experimental Section Sample Preparation. Standard chemicals were used as received without any further purification: potassium chromate (K2CrO4, Aldrich, 99% ACS reagent), ethanol, hydrochloric acid HCl 37%, acetic acid (CH3CO2H puriss. g 99%, Fluka), and ultrapure H2O (resistivity of 18.2 MΩ cm at 20 °C). The pH of the final solutions was adjusted using the respective acids at various concentrations to vary from 1.5 and 9.3. The experiments were all carried out with freshly prepared solutions. The presence of an organic compound ROH is necessary as an organic quencher providing electrons to the activated Cr(VI).62 Even if the organic acid plays this role in some solutions used, the alcohol enhances the photoredox process as will be illustrated later on. Solutions are enclosed in tight homemade cells composed of a glass slide and a coverslip separated by 30 µm-thickness mylar spacers. The coverslip is as well silanized to prevent any photochemical deposition on the entrance window of the cell. Photodeposition Setup. The experimental setup is presented in Figure 1. The photochemical excitation of solutions is driven by two interfering beams (from a linearly polarized continuous Ar+ laser, Innova 90-5 Coherent, working at the wavelength λ

J. Phys. Chem. C, Vol. 114, No. 46, 2010 19783

Figure 1. (a) Schematics of the experimental setup for laser-induced photochemical deposition. The size of the waist a0 of the two interfering pump beams at the entrance of the cell is set by the lens L1, L2, and L3. The lens L4 is used to image the diffraction pattern of the two pump beams associated with the produced photodeposit. (b) Side view of the imaging setup. (c) Cr(III) pattern photodeposited in solution (I) at pH ) 0.6, see Table 1, from interfering beams of waists a0 ) 156 µm, total beam power P ) 10 mW, time exposure texp ) 20 s, and fringe spacing Λ0 ) 5 µm.

TABLE 1: Mass Concentrations (in % wt) of the Different Solutions Used for Photodeposition Experiments solution solution solution solution solution

(I) (II) (III) (IV) (V)

chromate (% wt)

ethanol (% wt)

acid

9 9 2 9 9

8 2 8 8 0

HCl HCl HCl CH3CO2H CH3CO2H

) 514 nm), which are focused and intersect at the entrance of the cell containing the sample. The beam waist is a0 ) 156 µm, and the chosen fringe-spacing Λ0 of the interference pattern is Λ0 ) 5 µm (Figure 1a). This beam waist a0 is monitored by imaging the intermediate beam waist generated at the focus of the lens L1 with the couple (L2-L3), which behaves as a beam expander; this setup is necessary to combine both beam intersection and focusing at the same location. As illustrated by the side view of the setup (Figure 1b), the sample is illuminated with a white light source, and the observation of the deposit growth is performed in situ with a CCD video camera and an ocular microscope of focal length 30 mm; an example of modulated photodeposit induced by the interfering beams is presented in Figure 1c. The sample is placed between two beam splitters. The first one collinearly injects the laser beams and the white light illumination in the sample, while the second one rejects the laser beams to protect the recording CCD video camera against laser radiation. Finally, the lens L4 is used to image on a screen or a camera, the diffraction of the two pump beams by the photodeposit (Figure 1a,b). Observation of diffraction patterns is indeed a particularly efficient way to characterize the onset and the efficiency of surface patterning. Characterization Methods. X-ray photoelectron spectra of photodeposits were recorded with a VG 220i-XL ESCALAB spectrometer. Irradiation was performed with a nonmonochromatized Mg KR source (hν ) 1253.6 eV) operated at 300 W. Acquisition was done at 10 V voltage under a current of 20 mA. The analyzed area was about 150 µm in diameter. The prepared samples were degassed in the introduction chamber to 10-7 Torr before entering the main chamber, in which the

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Delville et al.

vacuum was kept at 5 × 10-9 Torr during XPS data acquisitions. All the binding energies were referenced to the Si2p peak of silica slide at 103.3 eV to correct for the charging effect during acquisition63 and confirmed by the right positioning of the impurity C1s contribution at 284.6 eV. The survey scans were performed under survey acquisition mode between 0 and 700 eV within 2 min. Precise multiplet fittings for deconvolution of the Cr2p XPS curves by the Gaussian-Lorentzian method were carried out when necessary. X-ray diffraction was performed with a Philips PW 1050 diffractometer, Cu anticathode under 40 kV and 40 mA, with a Bragg-Brentanon θ-2θ. The AFM experiments were performed in a glovebox (with typical O2 and H2O concentrations as low as a few ppm) with a Nanoscope IIIa from Digital Instruments (Santa Barbara, CA) operating in tapping mode at room temperature. In the present experiments, the main interest of the use of a glovebox was to keep experimental conditions stable over a week. A commercial silicon tip-cantilever was used, with a specified stiffness of about 40 N m-1, a measured resonance frequency of 290 kHz, and a quality factor of 440. Finally, adhesion of the photodeposit was investigated by the scotch tape test.64 Results and Discussion Photochemical Deposition of Cr(III). Based on the wellknown photoreduction of Cr(VI) ions into Cr(III) ions, used in dichromated gelatins for optical recording,50 the experiment relies on the photochemical deposition of chromium hydroxide layers. It is performed at room temperature in acidic aqueous solutions of K2CrO4 containing a small amount of alcohol as illustrated in Table 1. The photoreduction of Cr(VI) ions into Cr(III)49 starts with a light-induced excitation of Cr(VI) followed by a reduction to intermediate Cr(V)65 and further reaction of Cr(V) leading to Cr(III), here amorphous chromium hydroxide, as shown by eq 1:

Cr(VI) + hν f Cr(VI)*

(1a)

Cr(VI)* + RCH2OH f “Cr(V)” + RCH2O · + H+ (1b) “Cr(V)” f f Cr(III)

(1c)

RCH2O · f f RCHO

(1d)

The star indicates the photoexcited state, and hν is the energy of the absorbed photon. Cr(III) Photodeposit. X-ray photoelectron spectra of a photodeposited sample obtained from solution (IV) at pH ) 6.2 (for which adhesion on glass is obtained) are given in Figure 2 after etching with an Ar+ ion gun to clean the specimen surface; 100 s etching corresponds to a penetration depth of 7 nm. They show that the peaks due to both chromium and oxygen are much higher than the others peaks (silicon, sodium, calcium, and potassium), which likely come from the glass substrate contribution, because the size of the photodeposited coatings is of the order of magnitude of the XPS analyzed area. The contribution of the carbon peak was observed for all the samples; it is mainly due to the unavoidable contaminant impurities during the material processing and could not be completely

Figure 2. XPS spectrum of a Cr(III) coating issued from photoactivation of solution (IV) and etched with Ar+ ion bombardment. Inset: High resolution acquisition spectrum and curve fit of the Cr(III) coating in the Cr region (a) before and (b) after etching with Ar+ ion bombardment. Experimental conditions for deposition are: pH ) 6.2, P ) 35 mW, a0 ) 156 µm, and texp ) 30 min.

removed even after etching, suggesting that it is part of the Cr(III) layer composition through coordination of the acetate to Cr(III). A typical high resolution spectrum centered on the chromium (inset of Figure 2) confirms that Cr is in its trivalent oxidation state Cr(III) with a 2p3/2 level centered at 576.8 ( 0.1 eV and a 2p1/2 level centered at 585.9 ( 0.1 eV with respectively narrow full width at half-maximum (fwhm) values (2.9 and 3.2 eV)66,67 and in the same range as Cr2O3.68 We never detected any trace of Cr(VI) species (centered at 580.0 eV for K2CrO4,69 at 579.1 eV for CrO370 and 580-581 eV for Phillips catalysts CrOx/SiO271,72), showing that the photochemical reaction does not trap unreduced species during the formation of the layer. Special care in experimental conditions was taken to avoid the possibility of the reduction of trapped Cr(VI) by the X-ray beam.70,73,74 By analogy to deposition obtained in the case of the reduction of Cr(VI) to Cr(III) on metallic surfaces,75 we considered that the product that is formed is chromium trihydroxide Cr(OH)3. High resolution XPS spectra of coatings from solutions (I) and (IV) were compared to those of the starting material K2CrO4, Cr2O3, and Cr(CH3CO2)3. Results gathered in Table 2 confirm the oxidation state of the resulting coatings. Finally, for the crystallographic structure of the photodeposited coating, a spectrum at grazing incidence (θ ) 2.5°) performed on the same sample as that used for XPS characterizations only revealed a broad peak, indicating that the coatings have an amorphous structure as expected for pH e 10.77 Cr(VI) in Aqueous Solutions. The most common oxidation states in solution are Cr6+, Cr3+, and Cr2+.78 The specific chromium oxide species that can exist depend on the pH of the solution, the chromium oxide concentration, and the redox potential. Cr6+, for example, may be present in water as chromate (CrO42-), dichromate (Cr2O72-), hydrogen chromate (HCrO4-), dihydrogen chromate (H2CrO4), hydrogen dichromate (HCr2O72-), trichromate (Cr3O102-), and tetrachromate (Cr4O132-).79 The last three ions, detected only in pH < 0 solutions or at Cr(VI) concentrations higher than 1 M, will not be taken into consideration in this work. Above pH ) 8, only CrO42- is stable, and in the 2-6 pH region, the equilibrium shifts to dichromate according to: 2+ 2CrO24 + 2H f Cr2O7 + H2O

(2)

Patterning and Substrate Adhesion Efficiencies of Solid Films


TABLE 2: XPS Peak Energy Assignments of Starting Materials and Coatings Prepared from Solutions (I) and (IV) product

Cr2p3/2 (eV)

fwhm (eV)

Cr2p1/2 (eV)

fwhm (eV)

Cr2O3 Cr(acetate)3 coating from solution (I) coating from solution (IV)

580.03 578.30 576.05 576.76 576.62 576.40

1.44 2.56 2.57 3.6 3.12 2.90

589.31 587.60 585.72 586.20 586.32 585.90

1.73 3.02 3.00 3.53 3.50 3.30

K2CrO4

satellite peaka (eV)

fwhm (eV)

597.10

3.91

597.10 597.1

2.64 3.35

a Satellite peak due to outgoing photoelectron that can interact with valence electrons, which gives rise to additional energy loss processes that lower the measured kinetic energy of the electron and yield additional features in the XPS spectrum at a binding energy slightly greater than the core level peak.76

The Cr(VI) species distribution in pure aqueous solution as a function of the solution pH can therefore be described by the following reactions:52 + H2CrO4 h H + HCrO4 K1 ) 0.18 (pK1 ) 0.74)

(3) 2+ HCrOK2 ) 3.2 × 10-7 (pK2 ) 6.49) 4 h H + CrO4 (4) 22HCrO4 h Cr2O7 + H2O K3 ) 98

(5)

where pK1 and pK2 are the respective acid dissociation constants of the diacid H2CrO4. The equilibrium constants actually vary with the ionic strength of solutions,80 but the K values given above are reasonably accurate for the used chromate concentrations. Furthermore, HCrO4- is the only Cr(VI) form that can be light-activated in the blue-green wavelength range of a continuous Ar+ laser. Assuming x ) [HCrO4-], one gets: [H2CrO4] ) x[H+]/K1, [CrO42-] ) xK2/[H+], and [Cr2O72-] ) K3x2, x being determined by mass balance, from conservation of the total chromium concentration [Cr(VI)]:

2K3x2 + (1 + [H+]/K1 + K2 /[H+])x - [Cr(VI)] ) 0

(6) Considering [Cr(VI)] ) 0.51 M, corresponding to 9 wt % in K2CrO4 as in solutions (I), (II), (IV), and (V), the abundance of Cr(VI), illustrated in the inset of Figure 3, shows that (i) the neutral chromic acid, H2CrO4, rapidly decreases, (ii) negatively charged HCrO4- and Cr2O72- are the major species between pH ) 1 and 6.5, and (iii) CrO42- prevails for pH higher than 8. The same behavior is observed for [Cr(VI)] ) 0.1 M (solution (III)), except that the ratio [HCrO4-]/[Cr2O72-] slightly increases from 10/90 to 20/80. Photochemical Deposition of Cr(III). Experimental Determination of the Photodeposition Threshold. In acid solution, Cr3+ is always an octahedral hexaquo ion. It tends to hydrolyze with increasing pH, resulting in the formation of polynuclear complexes containing OH bridges.81 This is thought to occur by the loss of a proton from coordinated water, followed by coordination of the OH- to a second cation. The final product of this hydrolysis is hydrated Cr(III) oxide or chromium hydroxide (Cr(OH)3). The following modifications are thus observed with increasing pH: 2+ Cr(H2O)3+ f [Cr(H2O)4(OH)2]+ f 6 f [Cr(H2O)5OH] Cr(H2O)3(OH)3 (7)

Figure 3. Laser beam power required to reach the Cr(III) solubility versus pH values for the photoactive solutions presented in Table 1. Inset: Variation of the compositions of the different forms Cr(VI) species (H2CrO4, HCrO4-, CrO42-, and Cr2O72) in aqueous solution between pH ) 0 and 10 for a chromium concentration [Cr(VI)] ) 0.51 M (9 wt % in K2CrO4).

Besides the relatively fast hydrolysis reactions, olation and oxolation reactions resulting in the formation of oligomers, cleavage, and slow structural transformations of isopoly ions also take place in aqueous solution of Cr(III), but they are much slower.82 According to the reaction scheme given in eq 1a, the Cr(III) concentration in the solution increases with laser exposure. The nucleation of the Cr(III) precipitate takes place as soon as the concentration reaches the solubility product of Cr(OH)3 in solution.83 The corresponding precipitation onset under laser excitation at a given pH was measured using a dichotomy in beam power, and setting the maximum exposure time to 1 h. The accurate determination of the deposition onset was performed using the self-diffraction of the pump beams on the induced deposit as grating methods are known to be very efficient for detecting very weak variations of the refraction index. This procedure leads to an accurate determination of the variations of the beam power threshold for the Cr(OH)3 precipitation with respect to the pH for the different initial solutions. Figure 3 shows that the power threshold required to precipitate Cr(OH)3 qualitatively follows the variation of the HCrO4- concentration. Therefore, a significant increase in beam power is necessary in both acid and basic conditions to produce and precipitate Cr(OH)3 whatever the composition of the initial solution. The photochemical deposition also depends on the behavior of the produced Cr(III) because its solubility is high at both very acid and very basic pHs and is low in the intermediate range with a solubility of molality 10-6.84 M/kg.84,85 Consequently, the increase of power necessary for the photoprecipitation at high and low pHs depends on both the ratio of HCrO4in the Cr(VI) solution and the variation of solubility of the produced Cr(III) with the pH.

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The presence of an organic species is also essential,62 and the precipitation threshold is significantly lowered whatever the pH when ethanol is added to the aqueous solution (compare threshold powers between solutions (I) and (II) for HCl, and between solutions (IV) and (V) for acetic acid). It acts as a photoreducer (an electron reservoir) for the photoactivated chromated species86 and favors the precipitation of Cr(III) derivatives. In the case of organic acid solutions, the alcohol is no more crucial because an oxidizable species is already present in solution; it nonetheless enhances precipitation as illustrated by the comparison between solutions (IV) and (V) (Figure 3). Theoretical Description of the Photodeposition Threshold. Let us first briefly summarize the basic equations for the photoproduction of Cr(III) species.45 According to eq 1a, the production of Cr(III) results from a reaction-diffusion process involving a one-photon photochemical reaction. Within this framework, the reaction scheme is reduced to its two main steps:

{

kv

A + hν y\z A* [Cr(VI)excitation/relaxation]

) 0, is not affected by the fringe pattern. For a large beamwaist a0 and thin samples, as in experiments, the beam intensity has a cylindrical symmetry. Therefore, at a radial distance r from the propagation axis and z from the entrance of the cell containing the photosensitive mixture, its variation is given by the following expression:

[(

I(r, z) ) I0 exp -

kA*B

(8)

DC∇2[C] + kA*B[A]0[B]0

I(r, z) 1 )0 2IS 1 + I(r, z)/2IS

(9) where r is the radial distance from the beam propagation axis z. DC is the mass diffusion constant of C molecules. The first and the second terms of the left-hand side of eq 9, respectively, represent the diffusion and the production of species C. According to eq 9, the production in Cr(III) is an increasing function of the optical excitation. Photoprecipitation then occurs as soon as the Cr(III) solubility [C]S is reached. To describe this process, let us consider that the electromagnetic field is a classical continuous TEM00 Gaussian laser beam propagating vertically along the z-axis; note that we did not choose two interfering Gaussian beams45 because calculations are simpler and the photodeposition onset, obtained on beam axis at r ) z

( )

∞

I

∑ (-1)n 2I0S

DC∇2[C] + kA*B[A]0[B]0

where A, A*, B, and C are, respectively, Cr(VI), excited Cr(VI), alcohol, and Cr(III) species, while kv, kV, and kA*B are the reaction rates associated with the different processes. Assuming the kinetics of excitation/relaxation of Cr(VI) to be much faster than any mass diffusion involved in the dynamics of photoprecipitation, the concentration [A] of Cr(VI) and [A*] of Cr(VI)* are estimated at steady state and are simply related by [A*] ) (kv/kV)[A]. To calculate the ratio kv/kV, we suppose that A + hν hA* is mainly governed by a one-photon electronic transition, and we use the standard Einstein coefficients for the one-photon absorption, and the spontaneous and stimulated emissions.87 We find kv/kV ) (I/(2IS))/(1 + I/(2IS)) and dI/dz ) -εA([A] - [A*])I along the propagation axis z. I is the exciting beam intensity, and IS ) NAhν/(2εAτ*) is the saturation intensity related to the lifetime τ* of the excited state A*, to the molar extinction coefficient εA and to the Avogadro number NA. As the irradiated area in experiments is always small as compared to the sample extension, we assume that [A] ≈ [A]0 and [B] ≈ [B]0 (i.e., the bulk concentration of A and B). Using these hypotheses, the stationary concentration [C] of the produced Cr(III) is described by the following field-modified reaction-diffusion equation:45

(10)

where I0 ) P/(πa02) is the intensity on beam axis at the entrance of the photosensitive solution, P is the incident power and σ ) εA[A]0 is the optical absorption of the mixture. Note that eq 9 was previously solved in the linear excitation regime I , 2IS.45 However, as beam power thresholds are of the order of a few tens of milliwatts in both very acid and basic solutions (see Figure 3), a linear approximation becomes insufficient, and the source term has to be taken into account in full. In these conditions (i.e. for I < 2IS), eq 9 becomes:

kV

A* + B y\z C [Cr(III)production/decomposition]

)]

r2 + σz a20

n)0 2

[

exp -(n + 1)

]

n+1

×

r exp[-(n + 1)σz] ) 0 (11) a20

Rescaling the variables as R ) (n + 1)1/2(r)/(a0), Z ) (n + 1) (z)/(a0), and W(n) ) (n + 1)1/2σa0, and setting [C] ) ∑∞n ) 0[C](n) and K(n) ) (kA*B[A]0[B]0)/(DC)(a02)/(n + 1)(-1)n((I0)/ (2IS))n+1, we get: 1/2

(

∇R2 +

)

∂2 [C](n) + K(n) exp[-R2] exp[-W(n)Z] ) 0 ∂Z2 (12)

According to Hugonnot et al.,45 the solution at R ) Z ) 0 is:

[C](n)(R ) 0, Z ) 0, t ) ∞) )

K(n) 2

-λ2/4

∫0∞ λ e+ W(n) dλ (13)

If σa0 > 2 as in experiments (σa0 ) 2.028 with σ ) 1.3 × 10-4 m-1 and a0 ) 156 µm), we finally obtain:

√πK(n) K(n) ∞ -λ2/4 e dλ ) ) (n) 0 2W 2W(n) I0 √π kA*B[A]0[B]0 a0 (-1)n 3/2 2 DC σ (n + 1) 2IS

[C](n)(0, 0, ∞) ≈

∫

( ) ]

[

n+1

(14)

The precipitation threshold, given by [C](0,0,∞) g [C]S, is thus found by solving the following set of equations:

[C](0, 0, ∞) ≡ [Cr(III)] ) ∞

∑

n)0

[

√π kA*B[A]0[B]0 a0 × 2 DC σ

( ) ]

(-1)n P (n + 1)3/2 PS

n+1

g [C]S

(15a)


[A]0 ≡ [HCrO4-] ) 1 4K3

{(

1+

K2 [H+] + + K1 [H ]

(

)

2

1+


+ 8K3[Cr(VI)]0 K2 [H+] + + K1 [H ]

[B]0 ≡ [R-OH]

)}

(15b) (15c)

I/2IS has been rewritten as P/PS, where PS ) 2πa02IS is the saturation power associated with IS. Data Fit and Discussion. To fit the measurements presented in Figure 3, we used the following data set. The beam waist was experimentally chosen to be a0 ) 156 µm. Initial concentrations of Cr(VI) were [Cr(VI)] ≈ 0.51 M (9 wt %) or [Cr(VI)] ≈ 0.10 M (2 wt %). Concentrations of ethanol were [C2H5OH] ≈ 1.91 M (8 wt %) or [C2H5OH] ≈ 0.48 M (2 wt %). The estimation of the mass diffusion constant of Cr(III) molecules DC ≈ 0.5 × 10-10 m2/s and the measurement of the optical absorption of the Cr(VI) solution at the used wavelength σ ) 1.3 × 104 m-1 were given previously.88 The pH dependence of the Cr(III) solubility in water is still controversial89 because it seems to depend on the precursors used. Measurements,84 nevertheless, show a decrease up to pH ) 6 toward a plateau value at [C]S ≈ 10-6.84 M over the pH range 6-10, followed by another increase; because data on different Cr(III) precursors or on the influence of alcohol on the solubility are missing, we will consider [C]S ≈ 10-6.84 M as a first approximation over the full range of investigated pH. Finally, according to Mytych et al.,62 kA*B ≈ 5 × 10-6 s-1 M-1 L per M L-1 of ethanol at pH ) 4. Thus, the fitting parameters are PS ) 2πa2IS, and K1 and K2 are the equilibrium constants, which are expected to vary with the addition of alcohol; we also assumed that the value K3 ) 98 in aqueous solution at ionic strength 1 M in the pH range 1-10 is preserved in our high ionic strength alcoholic solutions. A fit of the whole set of experiments performed in hydrochloric acid (solutions (I)-(III)) gives pK1 ) 2.9, pK2 ) 6.9, and PS ) 250 mW. The fit of the data set performed in acetic acid (solutions (IV) and (V)) gives pK1 ) pK2 ) 7.0, and PS ) 250 mW for an average 0.5 wt % organic fraction associated with acetic acid in solution (IV) and 8.5 wt % in solution (V) (Figure 3). A clear shift of the pK’s as compared to values obtained in pure aqueous solution is observed, likely due to the presence of alcohol; these pK values are limited to 7. Moreover, Ps is significantly higher than any measured power threshold. This is totally consistent with the present model and previous photodeposit growth characterizations33,45 performed at I/2IS , 1. Consequently, experimental data can reasonably be theoretically well-predicted with a simple model coupling a one-photon photochemical reaction and chemical equilibria, even if uncertainties on several data remain. This demonstrates that a chemical investigation of photochemical deposition is necessary as soon as a quantitative description is required for dedicated applications. Such an approach is performed in the following sections. Adhesion and Patterning of the Induced Deposit. Up to now, the investigation was mainly focused on the mechanism of photodeposition and very little on the substrate on which this photodeposition was taking place. However, as stressed in the Introduction, the quality of the photodeposited layer is strongly dependent on the environmental conditions and on the type and composition of the supports (SiO2, Al2O3, semiconductors, metals, and polymers). Therefore, we undertook a system-

Figure 4. Optical microscopy images showing the influence of the pH and the nature of the solutions on glass patterning. (a) Solution (I), pH ) 0.6, P ) 35 mW, texp ) 150 s; (b) solution (I), pH ) 6.8, P ) 3.5 mW, texp ) 300 s; (c) solution (IV), pH ) 6.2, P ) 30 mW, texp ) 300 s; (d) solution (IV), pH) 7.2, P ) 30 mW, texp ) 200 s.

atic study of the photodeposition of Cr(III) layers on various substrates in terms of patterning (to investigate the relation between the optical pattern and the resulting deposit profile) and adhesion (scotch tape test). Deposition on Glass Slides. The first photolayer study was performed on cleaned microscope glass slides (silica). The surface patterning induced by the two interfering beams and the adhesion of the coating were studied with respect to the pH for the different solutions (Table 1). Figure 4 shows typical examples of the resulting photodeposits obtained from solutions (I) and (IV) at different pH values. As illustrated, the aspect of the deposit is strongly dependent on (i) the pH of the solution, (ii) the chemical nature of the acid, and (iii) the presence of an electron source whether it is the acid itself (acetic acid) or an alcohol. Relief patterns are systematically obtained at low pH (where hydrolysis prevails over condensation reactions), while the influence of light distribution is lost at pH > 6.8 (where condensation is faster). Moreover, the influence of the pH is so dramatic in the case of solution (I) that the laser power had to be reduced by a factor of 10 between Figure 4a and b to prevent the sample destruction. These observations made in the 1.5-9.3 pH range led to the curves presented in Figure 5. Besides the patterning aspects, we also point out the adhesion ones. As opposed to its patterning, the photodeposit adhesion is strongly dependent on both the pH and the nature of the acid used to prepare the solutions. With hydrochloric acid (solutions (I) and (II)), the photodeposit does not adhere to the surface at either low or high pH’s; in the zone of intermediate pH values, the layer when still on the slide can be removed by simple washings with distilled water. For acetic acid solutions (IV) and (V), the scotch tape test had to be used with a test-resistant zone between pH 5 and 7.8. This difference of behavior accounts for the interactions of the coating with the surface of the substrate. Indeed, because the photoreaction takes place in water, the surface of the silica slide exhibits a hydroxyl population of surface silanols. These vicinal, geminal, and isolated surface silanols are subjected to the following pH-dependent equilibria:90,91 + Si-OH+ 2 a Si-OH + Hs

(16)

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Delville et al.

Si-OH a Si-OH- + Hs+

(17)

Hs+ a H+

(18)

where Hs+ and H+ are, respectively, the surface and solution protons. The positioning of the surface isoelectric point (IEP), which represents the pH of surface net zero charge, with respect to the pH of the Cr(VI) solution and Cr(III) species is crucial. The lower is the IEP, the more the equilibria 16-18 are driven to the right and those of reactions 4 and 7 are driven toward the formation of dichromate and a Cr(H2O)63+ complex, respectively. Adhesion on Glass Slides. In the case of silica, the IEP of the order of 2.3 in water can be shifted upward by nearly 2 pH units with the introduction of alcohols;92 that of Cr(III) species is between 6.49 and 8.4 depending on authors.91,93 The silica at the interface is of the form SiOH2+ and thus positively charged below pH 2, just like the Cr(III) species whose rate of hydrolysis, condensation (olation and oxolation), depends on its photochemically generated local concentration. Furthermore, the laser wave heats the solution and raises the temperature by a few tens of degrees,45 which enhances the aging of the gel and generates polynuclear species such as Cr2(OH)24+, Cr3(OH)43+, and Cr4(OH)66+, some of them already known individually.94 These positively charged polynuclear species suffer from electrostatic repulsions with the positive surface of silica, thus preventing adhesion of the coating. Between pH 2 and 8, the photoactive HCrO4- species is at its highest concentration generating a high concentration of Cr(III) species, which itself leads to insoluble neutral form Cr(OH)3(H2O)3. Further condensation then contributes to the nucleation of hydrated hydroxide gels with a weak interaction with the surface (low stability toward the scotch tape test). The interaction of the CrOH bonds of these gels with the O- and OH groups of the silica surface was shown to take place through an inner-sphere monodentate Cr(III) surface complexation in the early stage95 even if it could not be observed in HCl solutions due to Si-O-Cr bonds instability with respect to this acid.96 In acetic acid (absence of chloride anion), the Si-O-Cr chemical bond seems more stable, and the layer exhibits a much better adhesion.97 This was already observed by Matijevic,98 who

Figure 5. Influence of the pH on the adhesion of the photodeposit and on its patterning for (a) hydrochloric acid solution (I); (b) solution (II); (c) acetic acid solution (IV) with ethanol; and (d) acetic acid solution (V) without ethanol.

Figure 6. Optical images of dried photodeposited coating of modulation Λ0 ) 5 µm. (a) Solution (I) at very low pH, pH ) 0.8, P ) 35 mW, texp ) 150 s. Inset: Magnification of the shrinking. (b) Solution (I) at pH ) 1.2, P ) 15 mW, texp ) 150 s. Inset: Large field of view showing breaking of the coating during cell opening as well as its poor adhesion. (c) Solution (V) at pH ) 5.8, P ) 35 mW, texp ) 300 s. The deposit cracks but adheres to the substrate. (d) Atomic force microscopy mapping of an adhesive photodeposit produced from solution (IV) at pH ) 6.2, P ) 30 mW, texp ) 300 s.

found that Cr(III) hydroxide, with an IEP of 8, can strongly adhere to silica surface over the range 4-11 in pH, even if at some point the two systems silica-surface/Cr(III)-nanoaggregates both exhibit negative charges. As an overall result, the limit for the adhesion of the coating is pH 7.8-8 depending on the presence or not of the alcohol scavenger (Figure 5c,d). Again, the influence of the alcohol is outlined by the ratio of power threshold required to precipitate the Cr(III) between solutions (IV) and (V). However, the nature of the organic acid scavenger is also rationalized because such a photochemical reaction does not take place in pure HCl solutions. Finally, at much higher pH, typically pH > 8, the chromium hydroxide gel gives the soluble [Cr(OH)4(H2O)2]- species.94 To check the adhesion of the photodeposited layer, the cells were carefully opened, and the Cr(III) coatings were observed after contact to air (Figure 6). The drying is dramatic when HCl is used (Figure 6a and b), whereas it much less affects or even does not affect at all the deposit in CH3CO2H, depending on the pH (Figure 6c and d). The formation of a gel with thicker regions in bright fringes of the interference pattern is clearly exemplified as well as a strong size shrinking at low pH (Figure 6a). This shrinkage illustrating the highly hydrated nature of the deposited gel is highly pH dependent as shown by the difference between Figure 6a and b for a variation of only 0.4 pH unit. Figure 6c illustrates the formation of the Si-O-Cr chemical bonds leading to a first layer of monodentate Cr(III) complexation in acetic acid. In this case, the drying process induces cracks in the deposit due to a weak shrinkage, but adhesion is preserved. Finally, at pH g 6, the deposit does not shrink, and its adhesion is strong enough to allow a characterization by AFM (Figure 6d), which shows a 3D image of the modulated layer. This corresponds to the pH region for which photochemical deposition can be confidently used to realize surface relief coatings or any other type of stiff pattern. Patterning on Glass Slides. After adhesion, the dependence of the morphology and the structuring of the deposit on the pH, that is, wavy pattern versus rough coating, is the second key point. The film aspect is modulated by both the optical intensity distribution and the Cr(III) concentration, which itself



Figure 7. Photodeposition of Cr(III) layers on (a) silanized glass, P ) 17.5 mW, texp ) 150 s, solution (IV), pH ) 6.4; and (b) non silanized glass, P ) 35 mW, texp ) 150 s, solution (IV), pH ) 6.4. Insets are magnifications of the patterns.

depends on the pH. In the 0.8-6 pH range, a thin monolayer of Cr(III) hydroxide forms on the whole enlightened silica surface and serves as a basis for the laser-induced nucleation and growth of the subsequent patterning (Figure 6a-c). Next, nucleation mainly occurs and progresses in the laser fringes (Figure 4a and c) outward from the surface because polymerization cannot progress across the silica surface due to the differences in the interatomic distances of silica and chromium hydroxide. Epitaxial growth is inhibited; photogeneration and adsorption seem to govern the overall process. For pH values >7.0, the Cr(III) concentration is at its highest level, and the deposits lose the memory of the optical pattern as illustrated in Figure 4b and d by the formation of a rough coating (with surface colloid particles clustering). At intermediate pH values, a progression from isolated site binding at low coverage to surface Cr(III) nucleation is observed, and the structure of the coating shifts progressively from spatially modulated to colloidal clustering (Figure 4c). The mechanism of this transition from modulated to rough coating is not well understood because there is also a large crossover regime at intermediate pH where the modulated gel and colloid forms of Cr(III) coexist, as illustrated in Figure 4c. Influence of the Substrate. To evidence the influence of the nature of the substrate surface, we first performed a silanization of the glass slide surface with a hydrophobic organic silane ((CH3)3SiCl) to produce a hydrophobic non wetting surface. The resulting photodeposit on such a substrate from acetic acid solutions, in which adhesion of the photodeposit to glass substrates has shown to be effective, is shown in Figure 7 and compared to that obtained on nonmodified glass. The comparison of the two sets of pictures highlights the strong influence of the substrate surface modification. Both the

Figure 8. (a) Pattern formed from solution (IV) at pH ) 6.4, P ) 35 mW, texp ) 120 s on a p-type silicon wafer. Inset: Magnification of the patterning. (b) AFM image and topography (inset) of the produced pattern at P ) 7 mW, texp ) 20 s.

modulation and the contrast of the photodeposited pattern are deeply altered by silanization due to the hydrophobic character of the modified surface and the absence of grafting functional groups. The adhesion of the deposit also disappears with silanization. The same type of experiments was carried out on p-type silicon wafers (Figure 8a) to avoid dark field deposition. The patterned coatings exhibit a very high contrast and a behavior analogous to those observed on glass in HCl and CH3CO2H solutions with, respectively, no and strong adhesion after washings. Such a result can be explained considering the native pure thin layer of silicon dioxide at the surface of the wafers. The quality of the patterning and its granular structure determined by AFM imaging are illustrated in Figure 8b; the optically forced coating modulation is retrieved experimentally. Other attempts were performed on substrates as different as PMMA, stainless steel, and semi conductive surfaces. In the first case, coatings behaved exactly as when formed on silanized surfaces, showing a very weak interaction between organic functions and inorganic Cr(III) precipitates. On stainless steel surface (source of electron), the Cr(III) precipitation obviously occurred without illumination; however, the reaction rate was greatly increased with a laser beam. The possibility of generating well-structured patterns at the surface of conductive materials is thus almost impossible due to the abundance of electrons at the surface. We solved this problem by performing photodeposition on a 300 nm thick transparent conductive ITO layer

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Figure 9. Pattern formed from solution (I) at pH ) 0.6, P ) 35 mW, texp ) 10 s on an ITO substrate. Inset: Magnification of the patterning.

deposited on a glass slide. The layer is composed of transparent indium oxide (In2O3) doped by 1% SnO2 (sheet resistance is 10 Ω/0). No deposition spontaneously occurred, and illumination was required. The resulting pattern is shown in Figure 9 for solution (I) at pH ) 0.6. The contrast of the induced photodeposit is excellent. It also demonstrates that photodeposition can be implemented on conductive substrates as far as the surface electron density is not too high.

Delville et al. adhesion to the substrate. Adhesion also depends indeed on the pH in the sense that the right affinity between surface and deposit in a liquid phase needs to be found. The answers to these questions constituted the corpus of the present work. We analyzed the adhesion and morphology of chromium hydroxide layers photodeposited on various substrates versus the composition and the pH of the initial photoactive solutions. For increasing pH, the deposit morphology switches from a light-modulated gel film to a compact colloid aggregate, with a crossover regime. Nice adhesion to the substrate only occurs at intermediate pH and in organic acids. In conclusion, deposition from the liquid phase has many advantages, as claimed in the Introduction. For instance, this is obviously the easiest method to create patterns of molecular compounds. However, as opposed to chemical or physical vapor deposition, where the atmosphere is simply made of the material to be deposited, the liquid phase cannot be considered as inert because reagents and photoproduct must satisfy different chemical equilibria to exist in the right forms, to be soluble and interact efficiently with the substrate surface. Strictly speaking, the liquid phase plays a major role, and that is what we demonstrated with the photodeposition of Cr(III). Finally, our investigation shows that maps such as those presented in Figure 5 constitute efficient tools to optimize surface patterning driven by photochemical reactions in liquids as soon as highquality deposits are required.

Conclusion We have experimentally explored the adhesion and morphology of coatings driven by laser-induced photochemical deposition in solution. Using the historical example of Cr(III) deposition from chromates, our goal was to understand the different conditions required to control the patterning of the photodeposited layer and its adhesion on different substrates. Indeed, generally speaking, most of the results on photodeposition presented in the literature paid much more attention to the novelty of the nature of the deposited layer than to the conditions required for its deposition and always implicitly assumed the patterning of the induced deposit. However, when we performed experiments on deposit growth,33,45 it appeared that none of these properties were automatically satisfied. Patterning, investigated here using interfering beams, and adhesion were shown to be strongly dependent on the pH of the solutions, on the type of acid, and on the nature of the substrate. Surprisingly, no explanation had been given yet, while reliable applications were shown to require both control over the optically written structure and its adhesion. Three key points were therefore important to be addressed. The first one concerned the influence of the properties of the initial solution on the quantity of photoexcitable precursor, and the opportunity to prevent any deposition even at significant initial concentrations. We demonstrated that when a chemical equilibrium between different species evolves with the pH, the concentration of photoexcitable species can vary by orders of magnitude despite a large amount of precursors. The pattern of the induced coatings can be different from that of the light field distribution. The second key point to be addressed was then the possibility to find the right experimental conditions so as to print a targeted pattern. By a simple variation of the pH of the solution, it appeared that the morphology of the induced pattern could change dramatically even if the photoproduction of Cr(III) was still the same. Eventually, even if we are able to find the right conditions to get adequate concentrations of the photoexcitable compound and to control substrate patterning, we still need to take care of deposit

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