Isabelle Favier

Daniel Pla

Montserrat Gómez
email: gomez@chimie.ups-tlse.fr

Metal-based nanoparticles dispersed in glycerol: an efficient approach for catalysis

Keywords:

The present mini-review describes the use of glycerol as solvent for the synthesis of metal and metal oxide nanoparticles exhibiting catalytic properties. This contribution specially underlines the ability of glycerol to both disperse and immobilize metal-based nanoparticles, increasing their lifetime. In other words, glycerol can act as a liquid support for nanocatalysts.

Introduction

Glycerol, mainly generated as concomitant product in the production of biodiesel, shows attractive physico-chemical properties to be considered as an eco-friendly solvent for synthesis and catalysis purposes: a large temperature range in liquid state (from 17.8 to 290 °C), a negligible vapor pressure (0.003 mmHg at 50 °C), a high ability to solubilize both organic and inorganic polar compounds, and also its immiscibility with low polar solvents (selected reviews: [1][2][3]). In addition, its high polarity, long relaxation time, high loss tangent value and high acoustic impedance make it appropriate to be used under microwave and ultrasound irradiation conditions, bypassing its main disadvantage for chemical transformations, i.e. its high viscosity (1.4 Pa.s) [4,5].

Focusing on metal-based nanoparticles, glycerol, analogously to other polyols (e.g. ethylene glycol or polyethylene glycol), has been applied in polyol synthetic methodologies, where the alcohol acts as both solvent and reducing agent of metal precursors (for selected contributions, see [6][7][8]), with the consequent formation of glycerol oxidation products. With the aim of circumventing this problem, we have developed the synthesis of metal nanoparticles in glycerol applying a low pressure of dihydrogen (< 3 bar) [9], which avoids glycerol hydrogenolysis and glycerol oxidation side reactions [10]. It is important to underline that the supramolecular structure of glycerol [11], prompted by its three different types of hydrogen bonds, favors both dispersion and trapping of nanoparticles, interesting features for catalysis.

A R T I C L E I N F O A B S T R A C T

In this mini-review, we highlight the contributions concerning the synthesis and catalytic applications of metal-based nanoparticles in glycerol. Currently, the published works mainly involve transition metals from groups 10 and 11. Thus, we have organized this contribution in three main parts: i) Metal-based nanoparticles from group 10; ii) Metal-based nanoparticles from group 11; and iii) Miscellaneous.

Herein, we draw special attention to the reported works on nanocatalysis involving glycerol as a solvent from 2014 to January 2017, representing an update to previous literature reviews [1][2][3].

Metal-based nanoparticles from group 10

Nickel

Novel methodologies based on variations of the polyol method have been recently reported for the synthesis of metal nanoparticles (MNPs) providing new bottom-up strategies to gain control on the size and shape of the kinetically stabilized metal phases at the nanometric scale.

In particular, zero-valent NiNPs of 100-1000 nm size could be prepared from nickel nitrate using supercritical water (SCW) as a reaction medium and glycerol as an environmentally friendly reducing agent (in comparison to formic acid or hydrazine, commonly used), at 400 °C and 300 bar by a hydrothermal method [12]. In this work, the glycerol/metal precursor ratio was a crucial parameter for the full reduction of metal salts into Ni(0) and Cu(0) species. However, for low electronegative metals (such as Co, Fe and Mn), glycerol reduced the salts into low-valent metal oxides, acting as an anti-oxidant reagent.

The catalytic activity is highly dependent on the size of the metal nanoparticles. Wang and Liu recently reported the preparation of nanocatalysts based on silica supported Ni nanoparticles where the mean particle size could be efficiently tuned by using different alkanol solvents through a wet impregnation method. In particular, the use of glycerol as delivery conveyer and removable carbon template effectively yielded smaller size and uniform dispersions of metal particles of 6-8 nm [13]. This Ni/SiO 2 catalyst prepared by glycerol impregnation was proven to display higher activity and selectivity towards partial oxidation of CH 4 , giving conversions up to 94%, and CO selectivity of 88% due to the higher metal dispersion and more unsaturated surface atoms.

Despite the enhanced reactivity patterns of kinetically stabilized Ni nanoparticles and bimetallic systems in glycerol, the right choice of nanocatalyst in combination with the fine tuning of the reaction conditions is of paramount importance to preclude undesired sidereactions of glycerol itself as a substrate. Otherwise, glycerol dehydration and formation of hydroxyacetone [14], and hydrodeoxygenation [14,15] have been described. Furthermore, the use of glycerol in alkaline media has been reported towards membrane-less direct liquid fuel cell platforms of high performance energy generation systems for transportation and stationary storage [16,17].

Palladium

Recent advances on microwave-assisted synthesis of monometallic palladium nanoparticles have been reported in continuous-flow by employing the polyol methodology in glycerol as both reducing agent and solvent, using poly(N-vinyl-2-pyrrolidone) as additive, under microwave irradiation [18]. Furthermore, an analogous continuous synthesis of bimetallic Pt shell Pd core nanoparticles was carried out, where glycerol acts as a reducing agent [19]. Under microwave conditions, glycerol efficiently converts MW energy into thermal activation, also enhanced by its long relaxation time which is related to the supramolecular arrangement by intermolecular hydrogen bonds [20]. The core-shell structure and the nanoparticle size (6.5 ± 0.6 nm) corresponds to one atom layer thickness of Pt (0.25 nm)

encapsulating the Pd core metal as elucidated by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS elemental mapping.

Scheme 1. Synthesis of palladium nanoparticles in glycerol using phosphine and cinchona based ligands as stabilizers.

With the aim of using glycerol merely as solvent, we developed a methodology based on a bottom-up approach for the synthesis of metal and metal oxide nanoparticles. Actually, metal precursors (salts or well-defined organometallic complexes) were decomposed under dihydrogen atmosphere in the presence of stabilizers (for selected contributions, see [21][22][23]), using glycerol as solvent (Scheme 1). In the absence of any stabilizer other than glycerol, only agglomerates were obtained. The optimization of different parameters such as metal concentration, stabilizer nature, metal/stabilizer ratio, hydrogen pressure and temperature, led to the formation of stable colloidal solutions, which can be stored for several months without loss of catalytic activity. Phosphine and cinchona based ligands gave colloidal solutions constituted by small PdNPs showing a relative narrow size distribution. The nature of the stabilizer induces different surface reactivity, in agreement with the different coordination mode at the metallic surface (Scheme 1).

Following this strategy, Pd nanoparticles were prepared and fully characterized by conventional techniques, both in glycerol solution and in solid state. In particular, the negligible vapor pressure of glycerol enabled TEM and XPS analyses from the colloidal solutions, allowing a direct analysis of the metal-based nanoparticles without isolation at the solid state (Figure 1).

PdNPs stabilized by glycerol-soluble phosphines such as tris(3-sulfophenyl)phosphine trisodium salt (TPPTS) [9,[START_REF] Chahdoura | Metal nano-catalysts in glycerol and applications in organic synthesis[END_REF]25] and Nsubstituted 1,3,5-triaza-7-phosphaadamantane ionic ligands (PTA-R + Br -) [26], were efficiently applied in different Pd-catalyzed transformations: C-C and C-heteroatom bond formation, hydrogenation and carbonylation, also including multi-step processes for the synthesis of heterocycles [25]. In general, PdNPs stabilized by TPPTS exhibited higher activity than those coming from PTA derivatives. For this reason, here we have chosen to underline the main catalytic achievements of the PdNPs/TPPTS system in glycerol.

This colloidal solution was active for different C-C cross-couplings (Suzuki-Miyaura, Heck-Mizoroki and Sonogashira), Michael conjugate additions using amines, phosphines and thiols as reagents, and hydrogenations of nitro derivatives and a variety of C=C bonds [9]. This versatility led us to apply this system in Pd-catalyzed multi-step processes (e.g. C-C coupling followed by cyclisation;

carbonylative couplings followed by cyclization; sequential processes adding a last hydrogenation step) with the aim of synthesizing heterocycles by one-pot/one-catalyst approach (Figure 2). This methodology permitted to obtain the desired products without isolating intermediates. In addition, the catalytic phase was efficiently recycled thanks to the ability of glycerol to immobilize metallic species, giving, in consequence, metal-free isolated organic products. With the purpose of using stabilizers coming from biomass, we prepared PdNPs in glycerol stabilized by cinchona alkaloids [27].

These ligands, previously used in the literature for the synthesis of supported Pd and Pt nanoparticles [28][29][30][31], coordinate to the metal surface through the aromatic quinoline fragment by -interaction [32][33][34]. In contrast to phosphines, these ligands disfavor the formation of defined molecular complexes and consequently, privilege the stabilization of metal nanoparticles. Actually, this coordination fact is reflected by their reactivity behavior. While PdNPs stabilized by TPPTS are active for both C-C cross-couplings and hydrogenations, PdNPs containing cinchona ligands (cinchonidine, cinchonine, quinine, quinidine) are efficient catalysts for hydrogenations and hydrodehalogenations under low dihydrogen pressure (Scheme 2a), processes working under surface regime conditions. PdNPs/quinidine catalytic phase was efficiently applied to the synthesis of amines and N-substituted anilines by a one-pot tandem strategy starting from aldehydes and amines or nitrobenzene under H 2 atmosphere, taking advantage of the fact that this catalyst is inactive for the reduction of aldehydes (Scheme 2b).

Scheme 2. PdNPs stabilized by quinidine applied in hydrodehalogenations (a) and in the synthesis of amines (b) [27].

Furthermore, the oxidation of glycerol for its potential applications as a portable fuel source and H 2 production have been extensively studied using monometallic palladium nanoparticles [35], and bimetallic combinations thereof (Pd/TiO 2 [36], Pd/ZnO [37],

Pd/Ti nanotubes [38], Pd-Ce supported on a metal organic framework Fe-MIL-101-NH 2 [39], Pd-Au/C [40,41], Pd shell -Au core /C [42],

Pd shell -Au core /TiO 2 [43], PdCu alloy /N-doped multi-walled carbon nanotubes [44]). Besides, the upgrading of glycerol towards the synthesis of value-added molecules is a research field of raising interest. In particular, recent progress on palladium supported in metal organic frameworks (MOFs) and zeolites shows great potential [45]. These topics fall out of the aims of our present review.

Platinum

The polyol method for the synthesis of Pt nanoparticles has been upgraded thanks to the efficiency of microwave irradiation in batch [46] and continuous-flow processes [18]. Thus, 2-8 nm spherical-shaped platinum nanoparticles featuring (111) facets were prepared using a Pt(II) precursor salt, glycerol (both as solvent and reducing agent), and polyvinylpyrrolidone (PVP, as stabilizer) under microwave irradiation for 3-5 min at 300 °C [46]. Furthermore, an analogous microwave-assisted methodology for the continuous syntheses of Pd/Pt core-shell nanoparticles through core particle formation coupled with galvanic metal displacement has also been described [19].

The kinetically stabilized colloids prepared by these methodologies showed enhanced electrocatalytic performance towards alkanol transformations [46]. In particular, solvent-controlled platinum nanocrystal growth has rendered up to four tailor-made nano-crystalline structures depending on the growth rates along〈100〉to〈111〉axis, thanks to the inherent reduction rates of the diverse reaction solutions tested (Fig. 3) [47]. In addition, the branched Pt/C featuring the high-index facet in the edge and stability of facet (111) of the branched nanocrystal exhibits the best electrocatalytic performance due to the highest electrochemically active surface area, enhanced activity of methanol oxidation reaction, together with improved robustness [47].

Metal-based nanoparticles from group 11

Glycerol was used as solvent and reducing agent towards the synthesis of CuNPs by solvothermal reduction method. The reaction was carried out at 150 °C for 40 min starting from Cu(NO 3 ) 2 in the presence of Na 2 CO 3 and polyethylene glycol surfactants (PEG200, 6000, 20000) or polysorbate 80 (PS80). Depending on the surfactant nature, the size of the NPs increased from 38 to 50 nm. The authors propose initial formation of Cu 2 O and then the reduction to Cu(0) (confirmed by XRD analyses) that could be evidenced by a color change from blue-green to ocher yellow, and then red-violet. The largest particles were obtained with PEG20000, and the smallest with PS80 [48].

To the best of our knowledge, only few works (out of our contributions) concerning the synthesis of group 11 metal nanoparticles, have been described using neat glycerol as solvent [49,50]. Ag, Au and Cu nanoparticles were synthesized by laser irradiation, finding that the best way to control the size of MNPs (10-30 nm) was under continuous laser irradiation [49]. An and Zhang prepared hybrid nanoparticles consisting in AgX/Ag nanostructures, i.e. cube-tetrapod-like AgCl/Ag nanoparticles and AgBr/Ag nanoplates. Authors proved that the main role of glycerol in these syntheses was to control the shape of the nano-objects, thanks to its high viscosity at low temperature, controlling the diffusion of ions and consequently diminishing both nucleation and growth rates of AgX [50]. The strong adsorption in the UV-visible spectral region, due to the surface plasmon resonance of the AgX/Ag materials, prompted the authors to apply them as photocatalysts in the reduction of CO 2 to methanol.

The formation of CuNPs was monitored by UV-Vis spectrometry, XANES (X-ray Absorption Near Edge Structure) and TEM in glycerol. Copper-based NPs were prepared from CuCl 2 •2H 2 O dissolved in glycerol at room temperature using hydrazine as reducing agent [51]. XANES analyses indicated that these materials tend to form a mixture of Cu(0) and CuO NPs at a longer reaction times.

Glycerol has been also used as additive for chemical reduction of silver and gold salts in aqueous medium. In particular, glycerol favored the formation of silver nanoplates, inducing a high stability of the as-prepared nano-objects against ripening and oxidation processes, probably due to the reductive nature of the polyol [52]. However, glycerol appears less efficient for the synthesis of gold nanoparticles [53].

Ag, Au and Au@Ag core shell NPs were prepared by dissolving AgNO 3 or HAuCl 4 salt in aqueous solution with (0-40%vol) glycerol with triethylamine as reducing agent in the presence of PVP or SDS (sodium dodecyl sulfate) as stabilizing agents at room temperature. For the bimetallic system, AuNPs were firstly prepared and after AgNO 3 was added with triethylamine and glycerol [54].

The resulting bimetallic NPs showed better antibacterial activity against E. coli than AgNPs.

Ag-based nanofluids were prepared in glycerol in order to study their thermal conductivity and viscosity. Glycerol was chosen for its high heat transfer capacity. The nanofluids were constituted by mesoporous silica or hemiaminal grafted mesoporous silica decorated by AgNPs dispersed in glycerol obtained after ultrasonication. The presence of AgNPs diminishes the viscosity of the mesoporous SiO 2 suspensions due to the diminution of the hydrogen bonds between the silica and glycerol, also increasing the thermal conductivity of the material [55].

In relation to copper, we were interested in the synthesis of Cu 2 O nanoparticles by the methodology employed for the synthesis of PdNPs (Scheme 1), starting from Cu(OAc) 2 in the presence of polyvinylpyrrolidone as stabilizer [56]. The as-prepared material was fully characterized both in glycerol solution and solid state, confirming the cubic structure of Cu 2 O (Figure 4).

Preformed Cu 2 ONPs were active in C-heteroatom bond formation, azide-alkyne cycloaddition and one-pot multi-step processes. This catalytic system allowed the synthesis of secondary amines and thioethers by reaction of iodo-aryl derivatives with primary amines or thiols. Furthermore, the synthesis of primary amines could be achieved directly from aqueous ammonia and haloarene substrates, leading to the corresponding anilines, which were unreactive towards further arylation under these conditions. Products were extracted from the glycerol phase by a biphasic methodology using dichloromethane as extraction solvent, remaining the copper species immobilized in glycerol and in consequence, copper-free organic compounds were directly isolated from the catalytic solution. In contrast to the Cu(I) systems reported in the literature, no co-solvent was necessary to carry out this type of couplings [57,58]. Since the independent pioneering works from Sharpless [59] and Meldal [60], Cu(I)-based catalytic systems have been largely applied in Cu-catalyzed azide-alkyne cycloadditions (CuAAC) to afford 1,4-triazoles (selected reviews: [61][62][63][64][65][66]). Cu 2 ONPs in glycerol were efficient systems (isolated yields > 90%) for the reaction of benzylazide with different terminal alkynes including alkyl derivatives (Scheme 3a). The glycerol catalytic phase was recycled up to ten times without loss of activity. This robust catalytic system also permits the preparation of unsymmetrically substituted bis(triazoles) by a three-step process, from 1,7-octadiyne and sodium azide followed by sequential addition of benzyl bromide and 4-(methylthio)benzyl bromide.

Scheme 3. Cu 2 ONPs applied in azide-alkyne cycloaddition (a) and azide-alkyne cycloaddition/C-N coupling tandem processes (b) [56].

We were also interested in using Cu 2 ONPs as the only catalyst for tandem processes involving C-N bond formation and AAC for the synthesis of polyfunctional compounds, taking into account that the first process is faster than the triazole formation (monitored by GC). Actually, N-substituted anilines containing a triazole moiety were isolated in high yields (Scheme 3b).

With the aim of making easier and pursuing a "click" methodology [67], we studied the CuAAC reaction using CuI in glycerol as catalytic precursor and in the absence of any Lewis base, for the benchmark reaction between phenylacetylene and benzyl azide [68].

Unfortunately, the reaction did not work using the highly pure "home-made" benzyl azide (prepared following the reported protocol [69]). However, when the commercial one was employed the reaction worked at room temperature [70]. After a deep analysis of chemicals and conditions, we concluded that the impurities present on the commercially available BnN 3 were the responsible of the observed activity. GC-MS and NMR analyses proved that they consisted in amines bearing long alkyl chains. The addition of this kind of ligands to the reaction mixture, such as oleylamine, undecylamine, octylamine, dioctylamine or trioctylamine, led to the corresponding 1,4-triazole in nearly quantitative yields under smooth conditions. This behavior was also observed for other alkynes and azides, obtaining from moderate to high yields (Scheme 4).

Scheme 4. CuI/oleylamine catalytic system in glycerol for the synthesis of 1,4-triazoles [68].

The reason why the CuI/amine catalytic system is active lies in the in situ formation of Cu(I)-based nanocatalysts, as observed by (HR)TEM analyses (Figure 5), due to the stabilizing effect caused by the long chain amines [71]. The presence of short chain amines such as diisopropylethylamine or ethylenediamine, gave inactive CuI agglomerates. Reprinted with permission from reference [73]; license number 4040100302835. Copyright 2017, Royal Society of Chemistry.

A glassy carbon electrode can then be modified with this novel material providing an excellent binder-free non-enzymatic amperometric chemical sensor for the detection of p-nitrophenol pollutant with high sensitivity, selectivity and stability [73] (Figure 7). displayed significantly reduced viscosity and provided convenient particles dispersion and alignment with optimal thermal conductivity properties, since high thermal conductivity enhancement with minimal viscosity increase is the primary goal of heat transfer nanofluids [74].

Besides, the preparation of ZrO 2 nanomaterials by a sol-gel process with carbon as a phase transformation promoter has been discussed. Cubic and tetragonal ZrO 2 were synthesized from ZrOCl 2 •8H 2 O as zirconium source by a sol-gel method using ethanol and glycerol and characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) [75].

Thus far, C. Daiguebonne, O. Guillou et al. have demonstrated that micrometric beads of lanthanide-based coordination polymers can be divided into nanometric particles via solvation of the lanthanide ions by glycerol. This process brings forth reliable means of synthesizing kinetically stabilized colloidal solutions of nanoparticles with both tunable luminescence properties and size. This approach brings forth new ways for the easy solubilization of lanthanide-based coordination polymers and enables the exploitation of the intrinsic luminescence properties of such compounds immobilized in liquid phase [76] (Figure 8). Thorium oxide nanoparticles were synthesized by hydrothermal method. It was found that glycerol acts as a size-controlling agent.

The synthesis consists in the reaction between Th(NO 3 ) 4 •5H 2 O, urea and glycerol. In the absence of glycerol, only heterogeneous mixtures were obtained. The addition of glycerol serves to fine-tune the size of the homogeneous precursor due to the coordination of glycerol to Th(IV) species in solution. When a moderate volume of glycerol is added, the number of nuclei decreases, and the growth step becomes slower. Pure ThO 2 NPs were obtained after calcination at 800 °C for 5 h under air [77].

Outlook and Perspectives

The novel methodologies described in the recent literature reports open up new opportunities towards the preparation of nanoparticles with optimal surface area and high selectivity to exploit the full potential of these novel materials as nanocatalysts in liquid phase. The enhanced immobilization properties of glycerol concerning metal species enables the implementation of flow chemistry technologies, as well as adsorption of glycerol catalytic phases on nano-structured materials including polymeric catalytic membranes. Furthermore, solvent engineering through chemical modifications may preclude the intrinsic reactivity of hydroxyl groups of glycerol, allowing the preparation of upgraded next generation solvents with optimal physical properties towards sustainable applications in catalysis. These tunable nanostructured materials shall provide excellent means towards both fundamental reactivity understanding of nano-systems, and synthetic applications under green chemistry premises. The virtually limitless possibilities associated to these new generation materials confer a huge potential towards innovative industrial methodologies given their recyclability and catalyst immobilization capabilities.

Figure 1 .

 1 Figure 1. TEM images in glycerol corresponding to PdNPs stabilized by TPPTS (a), PTA-Bn + Br -(b), cinchonidine (c) and quinidine (d). See Scheme 1 for ligand structures [9, 26, 27] .

Figure 2 .

 2 Figure 2. Heterocycles synthesized by Pd-catalyzed multi-step processes in glycerol (using preformed PdNPs stabilized by TPPTS), following a one-pot/one-catalyst methodology (top). Recycling of the catalytic phase illustrated by the phthalimide synthesis (bottom) [25].

Figure 3 .

 3 Figure 3. Relationship between the reduction rate and the growth rate along 〈100〉 to 〈111〉 for Pt nanocrystals. Reprinted with permission from reference [47]; license number 4038741241859. Copyright 2017, Royal Society of Chemistry.

Figure 4 .

 4 Figure 4. TEM (Ni grid) micrograph of Cu 2 ONPs in glycerol (a); HR-TEM micrograph of one isolated particle dispersed in glycerol (b); electronic diffraction spots observed by Fast Fourier Transform of one single particle from image b (c); powder X-ray diffraction analysis of Cu 2 ONPs at solid state (d); filtered image showing the zone axis [001] (e), with the corresponding cartoon of the cubic structure of bulk Cu 2 O (f). Reprinted with permission from reference [56]; license number 4040820273142. Copyright 2017, John Wiley and Sons.

Figure 5 .

 5 Figure 5. TEM images for CuI-based systems containing oleylamine (a), dioctylamine (b), ethylenediamine (c) and diisopropylamine (d) in glycerol. Reprinted with permission from reference [68]; license number 4040821133598. Copyright 2017, John Wiley and Sons.

Figure 6 .

 6 Figure 6. Schematic representation for stepwise formation of flower-shaped zinc glycerolate and corresponding ZnO microstructure.

Figure 7 .

 7 Figure 7. Cyclic voltammogram traces of bare glassy carbon electrode (GCE) and modified GCEs (a) in the absence and (b) in the presence of 0.5 mM of p-nitrophenol in phosphate buffered saline (pH 7) at a scan rate of 50 mV s-1. (c) Probable electrochemical redox reactions involved for p-nitrophenol sensing. Reprinted with permission from reference [73]; license number 4040100302835. Copyright 2017, Royal Society of Chemistry.

Figure 8 .

 8 Figure 8. Lanthanide-based nanometric coordination polymers. Reprinted with permission from reference [76]; license number 4040121144382. Copyright 2017, John Wiley and Sons.

Acknowledgments

The Centre National de la Recherche Scientifique (CNRS), and the Université de Toulouse 3 -Paul Sabatier are gratefully acknowledged for their financial support.


J P Landesman

N Isik Goktas

R R Lapierre

S Ghanad-Tavakoli

E Pargon

C Petit-Etienne

C Levallois

J Jiménez

S Dadgostar

Effect of the Plasma Etching on InAsP/InP Quantum Well Structures Measured Through Low Temperature Micro-photoluminescence and Cathodoluminescence

Photoluminescence and cathodoluminescence spectral imaging were performed across rectangular stripes etched in samples with InAs x P 1-x quantum wells of constant thickness and variable composition grown on InP. In particular, the effects of different etching chemistries (CH 4 /H 2 /Ar and Cl 2 /CH 4 /Ar) were investigated. The results discussed deal with modifications of the luminescence line shapes (which differ with etching process) and with the intensity variation of the emissions associated with the quantum wells across the stripes. The possible origins of these effects are investigated in terms of carrier recombination on the vertical sidewalls of the stripes and lateral diffusion of species from the plasma during etching. Cathodoluminescence measurements on samples under DC-bias also show the quantum confined Stark effect which is correlated to the material modifications induced by the etching.

Introduction

Many fabrication processes for photonic devices in III-V semiconductors involve plasma etching, also designated as reactive ion etching (RIE), for the definition of features with anisotropic profile. For example, waveguides in InP-based materials with rectangular profiles are made using this kind of tool (1). However, plasma etching is also a possible source of defects in the material during the process. This issue has already been investigated by a number of groups. The induced disorder can be chemical, e.g. nonstoichiometry (2), crystallographic, e.g. creation of an amorphous layer or collision cascades (2,3), and/or creating defects that affect the electronic and optical properties (4 -6). Due to the preferential direction of ions impinging the material, horizontal etched surfaces and vertical sidewalls show different behaviors. Interaction mechanisms with the impinging ions and chemical radicals have been proposed to explain the disorder. Ion channeling and diffusion (5) in particular were suggested to explain disorder occurring at depths of hundreds of nanometers below the etched surface.

The details of the disordering effects still need to be understood, especially as new etching processes are being developed with enhanced performance (etch rate, selectivity, morphology, etc.).

We propose a study on the etching of InP using different etching processes. Our goal is to retrieve information about the effects on the optical properties of the etched features / devices through the use of luminescence-based techniques (photoluminescence -PL -and cathodoluminescence -CL). In particular, we exploit the potential for spatial resolution of these techniques (7). We use etched ridges with rectangular profiles and various widths. A specific objective is to discriminate between vertical effects (damage propagating perpendicular below the surface) and lateral affects (propagating parallel to the surface within the etched devices). In this aim, we have prepared samples with a series of quantum wells (QWs) at different depths below the surface. Each QW having its own identified optical signature allows in principle an analysis of the depth dependence of the disordering as was already shown (6). The spatial resolution of micro-PL (µPL) and CL yields information on the lateral effects. In addition to the changes in optical properties identified from modifications of the PL and CL spectra, we have also investigated the effects of an electrical DC bias and show that the quantum confined Stark effect (QCSE) can play a significant role in understanding the PL and CL spectra, and their changes upon etching (8).

Samples and Experiments

Sample Growth

QW samples were grown by gas source molecular beam epitaxy on InP (100) n-type substrates, using solid sources (In) and PH 3 / AsH 3 gases. First, an InP buffer is grown (thickness: 300 to 400 nm). This buffer is followed by InAs x P 1-x QWs with InP barriers. The QW thickness is fixed for a given sample (typically 7 to 8 nm), while the As/P composition is changed from one QW to the following. The first QW on top of the buffer has the highest As content (typically in the order of x  0.5), while the QW closest to the surface has the lowest As content (x  0.35). In this growth sequence, QWs are separated by InP barriers of 100 nm and the structure is finally covered by a 300 nm layer of InP. Figure 1 shows a schematic of the QW samples (with 8 QWs; samples with 9 QWs were also used). Decreasing the As content in the QWs from substrate / buffer to the surface increases the energy band-gap, ensuring that the PL or CL signal from deeper QWs will not be absorbed in the QWs above (see following sub-section). Some samples were grown without any intentional doping, whereas other samples were grown with Be doping added to the InP buffer and barrier layers (p-type doping level  10 18 cm -3 ).

Etching of Rectangular Ridges

Plasma enhanced chemical vapor deposition of silicon nitride SiN x was used as the etching mask. This layer was grown at 250 °C with a thickness of 500 nm. Stripes of various widths (1 to 50 µm) were defined by optical lithography in this mask. After mask opening, the etching of the different samples was performed by plasma etching. Two different processes were used: Process #1: capacitively coupled RIE reactor with CH 4 /H 2 /Ar gas mixture. Process #2: inductively coupled RIE reactor with Cl 2 /CH 4 /Ar gas mixture.

The etch depth was 2 µm to ensure etching of all QWs. Details for the Etching Process #1: After SiN x mask opening and resist stripping, the InP etching was done with 6.4 sccm CH 4 / 50 sccm H 2 / 8.4 sccm Ar (total pressure: 30 mTorr), and a power of 40 W. No external sample heating was used.

After etching the InP, the polymer formed during the etching was first etched in the plasma using 50 sccm O 2 (total pressure: 6 mTorr) with a power of 50 W, and the SiN x mask was subsequently stripped in HF.

Details for the Etching Process #2: After SiN x mask opening and resist stripping, the InP etching was done using 20 sccm Cl 2 / 100 sccm Ar / 32 sccm CH 4 (total pressure: 20 mTorr), a power of 800 W at the plasma source and 200 W for sample bias, and a sample temperature of 200 °C.

After etching the InP, the SiN x mask and polymer passivation were first etched in HF and then stripped in the plasma using 90 sccm O 2 (total pressure: 10 mTorr), and a power of 800 W.

µPL Setup

The µPL experiments were performed at 10 K using a specially designed optical cryostat (9). The chosen excitation wavelength was 1064 nm to avoid the generation of electron-hole pairs in the InP substrate, buffer and barrier layers. The excitation power density was approximately 5.107 W/cm 2 . The PL spectrum obtained under these conditions for an as-grown un-doped sample with 8 QWs, prior to etching, is shown in figure 2.

CL Setup

The CL spectral images were recorded in a Carl Zeiss (LEO 1530) field emission scanning electron microscope, equipped with a Gatan mono-CL2 system with a liquid nitrogen -cooled InGaAs array detector. These measurements were done on cooled samples (80 K). The acceleration voltage for the electron gun was set to 20 kV. The penetration depth for this voltage is in the order of 1 µm (10). A typical CL spectrum obtained for an as-grown, un-doped sample can be seen in figure 3.

Experimental Results

As-grown Samples

We first comment on the spectra measured for the as-grown samples, before any etching process, such as shown in figs. 4.4 meV for the sharpest transition). These spectra display for each line a single, symmetric transition, indicating that a single hole level is apparently confined in the QWs due to compressive stress. A very good fit to the experimental PL spectra is obtained using Gaussian profiles for each line, with adjustable position, amplitude and width, as illustrated in figure 4. This broadening effect is most probably due to a transfer of the carriers in the InP barriers due to Be doping into the InAs x P 1-x QWs. Because of this transfer, a range of energy levels in the valence band becomes populated by holes. Under photo-pumping, the electrons created can recombine with holes in this entire range, thus broadening the PL signal (11).

CL spectra at 80 K yield similar trends as the PL spectra for the reference un-etched samples (8). We now examine the spectral images produced by scanning the laser beam across etched stripes at 10 K in the µPL set-up. Raw data obtained for stripes (width: W = 50 µm) etched using the different processes are displayed in figure 6.

The spectra in figure 6 were recorded on stripes etched from un-doped samples. The corresponding reference spectrum is shown in figure 2. Each spectrum was fitted with the model curve described above. Comparing with the reference spectrum, the following statements can be done:

-the CH 4 /H 2 /Ar process increases the spectral line width (average line width: 7 nm) -the Cl 2 /CH 4 /Ar process slightly decreases the spectral line width (sharpest lines: 3.8 nm)

Fitting with the model curve also allows plotting the intensity variation for each line versus position of the laser beam across the stripes. Examples of intensity variations are shown in figure 7 for two different stripes (widths 30 and 50 µm respectively) etched with the CH 4 /H 2 /Ar process. The Cl 2 /CH 4 /Ar process gives similar trends for the intensity distribution across the stripes. The main information derived from the intensity variation curves is the gradual decrease on both edges, extending over more than 10 µm on each side. A plateau seems to be reached for the case of the 50 µm stripe, but not for the 30 µm stripe.

CL Spectroscopy on DC-Biased Samples

We focus here on the spectroscopic measurements performed in the CL set-up applying a DC bias on the surface of the sample, whereas the backside is grounded. Such measurements were performed only on the un-etched samples for the moment. Figure 8 illustrates the results for both an un-doped and a doped sample comprising 9 QWs. The surface voltage which could be applied varied from one sample to the other, depending on the magnitude of the generated current.

For both negative and positive applied voltage, the emission lines in the un-doped sample have a small red-shift. This red shift is due to the QCSE (12).

For the doped sample, no change is observed for voltages in the interval [-4 V, +3 V] but as the voltage reaches -5 V suddenly a strong emission dominates the spectrum. This emission is associated with QW 9, the closest QW to the InP buffer, i.e. in the zone where the p-n junction was created in the p-doped samples. With a negative bias on the surface of this sample a strong reverse situation is created, probably leading to a depletion of the minority carriers generated in the QWs closer to the surface. In order to better visualize the QCSE induced shifts of the QW lines for the un-doped sample, figures 9 and 10 give a 3-dimensional representation of the CL spectra under DC bias.

Figure 9. CL spectra measured at 80 K on the un-doped sample as a function of surface DC bias (-0.9 to 1.5 V applied on the surface in steps of 0.1 V).

Wavelength(µ m) O V + 0.4 V -0.4 V + 0.8 V -0.8 V + 1 V -1 V 1.15 1.25 1.35 Normalized CL intensity (a.u.) Wavelength(µm) O V + 1 V + 3 V + 2 V -1 V -2 V -3 V -4 V -5 V (a) (b)
Figure 10. Color map of the CL spectra shown in fig. 8.

Discussion

We first discuss the changes observed on the PL spectra upon etching with the different processes for the un-doped samples. Analyzing the results for the doped samples, however, was more difficult because of the line broadening.

The spectral positions for the different QWs in etched stripes are shown in figure 11 (same sample as figure 7). There is a significant trend towards increasing red shift for QWs located deeper in the sample (with an exception for QW5). On the other hand, the spectral positions are remarkably constant across the stripes, contrary to line intensities (figure 7).

The line intensities show strong variations across the etched stripes. We tried fitting using a function with the form:

A (1 -Bexp - x L -Bexp x-W L ) [1]
where A represents the PL signal which should be measured in an area not affected by the presence of the stripe edges, x the position along the stripe, W the stripe width, L the minority carrier diffusion length, and B a term that depends on the surface recombination velocity on the etched vertical walls (13,14). More precisely, B has the form B = S S+1 where S = v.τ L . srv is the surface recombination velocity on the vertical etched walls,  the radiative lifetime, and S is called the reduced surface recombination velocity. The resulting fits are shown on figure 12 for a 30 µm stripe etched with the CH 4 /H 2 /Ar process (same sample as figure 7). Figure 12. Curve fitting and experimental QW line intensities across a 30 µm stripe with etch depth 2 µm after the CH 4 /H 2 /Ar process. The symbols have the same meaning as in figure 7.

The parameters resulting from this fitting procedure are given in table II below. The best fit curves agree reasonably well with the experimental data. Assuming the same diffusion length for all QWs, we get L = 15 ± 3 µm. This value indicates that the QWs have a very high quality and have not been degraded by the etching process. The reduced surface recombination velocity appears rather constant for all QWs (2.8 ± 0.2). Determination of the true surface recombination velocity requires measurement of the radiative life time.

We applied the same fitting procedure for the 50 µm stripe of figure 7-a. Only the left half of the stripe was used for the fitting procedure as the right half shows some irregular behavior. The curve fitting is shown in figure 13, and the corresponding fitting parameters in table III. The diffusion length for this stripe is about half the diffusion length found for the 30 µm stripe, and the reduced surface recombination velocity is 30 % higher than found for the 30 µm stripe. We do not really have an explanation for this, as both stripes were etched during the same process. However, the results show the interest of the measurement methodology based on the µPL set-up at low temperature for the evaluation of the impact of the plasma etching process on the material's parameters. The bias-dependent CL spectra have shown that the QCSE can significantly affect the spectral positions and even line shapes in these samples. The next step will be to simulate this effect and evaluate if the penetration of some species from the etching plasma, for example ions formed either in the plasma phase or within the material as the stable form of these species, might affect the PL and CL spectra. This might, in particular, explain the changes in PL line shapes observed for the 2 different etching processes.

Conclusion

The µPL and CL spectroscopic measurements performed on InAsP/InP QW samples before and after plasma etching processes prove useful to evaluate the impact of the etching on some of the important parameters. These parameters are, in particular, the surface recombination velocity and the charge carrier diffusion length. Evaluating these parameters is important when fabricating such devices as photonic waveguides.

Figure 1 .

 1 Figure 1. Epitaxial sample structure (sample grown with 8 QWs).

  InP (d = 300 nm) InP (d = 100 nm) InP (d = 100 nm) InP (d = 100 nm) InP (d = 100 nm) InP (d = 100 nm) InP (d = 100 nm) InP (d = 100 nm) Surface InP buffer + substrate QW1 QW2 QW8

Figure 2 .

 2 Figure 2. Low temperature (10 K) PL spectrum of an un-doped sample after growth (8 QWs). The lines are labeled with the QW number in the structure.

Figure 3 .

 3 Figure 3. CL spectrum (80 K) of an un-doped sample after growth (9 QWs).

  2 and 3. The signature of each QW can clearly be identified. The peaks are sharp (spectral width on the PL spectra at 10 K: 4.5 nm / 1300 1350 1400 1450 CL intensity (a.u.)Wavelength (µm)

Figure 4 .

 4 Figure 4. Low temperature (10 K) PL spectrum. indicates measured data and red lines indicate the model fit.

Figure 5 .

 5 Figure 5. Low temperature (10 K) PL spectrum of (a) un-doped sample; (b) doped sample. Both samples have the same epitaxial structure with 9 QWs, except for the Be-doping in the InP barriers in the case of the doped sample.

Figure 6 .Figure 7 .

 67 Figure 6. Series of µPL spectra measured at low temperature (10 K) across 50 µm ridges. (a) etching with CH 4 /H 2 /Ar, and (b) etching with Cl 2 /CH 4 /Ar. Each color corresponds to a different position across the ridge as indicated by the legend in the center. The center of the ridge corresponds to the 25 µm position.

Figure 8 .

 8 Figure 8. CL spectra recorded at 80 K on samples DC-biased on the surface (9 QWs). (a) un-doped sample, and (b) doped sample. The spectra for different biases were vertically offset for clarity.

Figure 11 .

 11 Figure 11. Spectral positions for individual PL lines across stripes with etch depth 2 µm after the CH 4 /H 2 /Ar process. The straight lines represent the corresponding positions on the reference, un-etched, sample. (a) Stripe width 50 µm, and (b) stripe width 30 µm.

Figure 13 .

 13 Figure 13. Curve fitting and experimental QW line intensities across a 50 µm stripe (left edge) with etch depth 2 µm after the CH 4 /H 2 /Ar process. The symbols have the same meaning as in figure 7.

TABLE I .

 I Spectral shifts measured for the etched stripes (CH 4 /H 2 /Ar etching process).

	QW #	Stripe width 50 µm	Stripe width 30 µm
	1	0.8 ± 0.1 nm	0.8 ± 0.1 nm
	2	0.9 ± 0.1 nm	0.8 ± 0.1 nm
	3	1.3 ± 0.1 nm	1.2 ± 0.1 nm
	4	2.2 ± 0.1 nm	2.2 ± 0.1 nm
	5	1.7 ± 0.1 nm	1.6 ± 0.1 nm
	6	2.3 ± 0.1 nm	2.2 ± 0.1 nm
	7	2.6 ± 0.1 nm	2.5 ± 0.1 nm
	8	3.6 ± 0.1 nm	3.3 ± 0.1 nm

TABLE II .

 II Fitting parameters for PL line intensities measured on the 30 µm stripe (CH 4 /H 2 /Ar etching process).

	QW #	A	B	S (reduced surface	L (µm)
				recombination velocity)
	1	13.3	0.72	2.57	22
	2	8.4	0.72	2.57	10
	3	10.7	0.73	2.7	11.8
	4	18.5	0.73	2.7	19
	5	24	0.76	3.17	15.8
	6	22.5	0.74	2.85	16.7
	7	18.9	0.74	2.85	14.3
	8	20.5	0.75	3.0	14.1

TABLE III .

 III Fitting parameters for PL line intensities measured on the 50 µm stripe (CH 4 /H 2 /Ar etching process).

	QW #	A	B	S (reduced surface	L (µm)
				recombination velocity)
	1	6.9	0.78	3.55	7.7
	2	10.6	0.78	3.55	8.3
	3	11.6	0.78	3.55	7.8
	4	11.6	0.78	3.55	7.9
	5	19.0	0.8	4	8
	6	16.5	0.78	3.55	7.6
	7	16.3	0.78	3.55	7.5
	8	18.5	0.79	3.76	7.7

Acknowledgments

The authors would like to thank the Center for Emerging Device Technologies (CEDT) at McMaster University where epitaxial growth was performed. The authors also thank RENATECH+ (the French national network of facilities for micro-nanotechnology) with NanoRennes and the LTM etching platform in Grenoble.