Boris Colin

Olivier Lavastre
email: olivier.lavastre@univ-rennes1.fr

Stéphane Fouquay

Guillaume Michaud

Frédéric Simon

Olivier Laferte

Jean-Michel Brusson

Development of new high-throughput screening method to compare and to detect efficient catalysts for adhesive materials

Keywords: Silyl modified polymers, Cross-linking, High-throughput screening, catalysts

INTRODUCTION

Silyl modified polymers (SMP) are key materials, applied in many applications such as binders for elastic adhesives [1,2], composites [3,4], electrolytes [5] or for coating [6,7] applications. These hybrid polymers are also becoming increasingly common in aeronautical and aerospacial applications [8]. They can be used in several other applications due to their adjustable properties [9] such as Young's modulus, tensile strength, thermal, gas barrier and photonic properties. Cross-linking of silyl modified polymers takes place with alkoxysilane groups attached to the ends of polymer chains by hydrolysis and polycondensation with moisture [10,11]. The hydrolysis and condensation reactions occur almost simultaneously and they are in competition with each other. Organotin compounds such as dibutyltin dilaurate (DBTDL) are widely used as catalysts to accelerate the cross-linking of silyl modified polymers with moisture [12]. However, in recent years, the high toxicity of organotin compounds has been proved and the environmental concerns should lead before 2020 to ban organotin catalysts. Thus, several studies were performed in basic or acidic conditions allowing cross-linking of silyl modified polymers without tin catalysts. Indeed, strong acids like CF 3 SO 3 H [13] or HCl [14] efficiently accelerate the process of hydrolysis. Under basic conditions to deprotonate water, hydroxyl OH -ions attack the most positively charged atom by nucleophilic reactions [15]. Thus super-bases, like DBU or TBD amines are very efficient to accelerate the reaction and to increase the density of cross-linking. Metal alkoxide catalysts were also reported, in order to generate different cross-linked matrix by comparison with acid or basic organic catalysts. In all of these studies, the discovery of efficient catalysts has always been obtained by an iterative approach. Each potential catalyst is tested one per one and sometimes at different time scale. This can be time-consuming and does not allow comparison of a large number of catalysts in the same time period, for instance for the same batch of starting material. This classical iterative method has already been described by Bostik [16] for different applications. On the other hand, the high-throughput Here, we report a semi-quantitative, fast and inexpensive high-throughput screening method based on the visual difference of polymer surfaces when undergoing cross-linking reactions. Indeed, several steps appear during cross-linking reaction (Fig. 1). Prepolymers were used here as starting materials. These stable materials are reactive intermediates which can be cross-linked later. At the beginning of the hydrolysis process, the prepolymer is viscous and when condensation occurs, the viscosity increases. Then, a three dimensional cross-linked structure is obtained. At the end of polycondensation, the surface is solid with a gum texture. According to this point of view, the efficiency of a catalyst could be determined just by touching the surface after a given time t. Indeed, a catalyst will be efficient when the surface is not tacky with a solid gum texture at time t. On the other hand, if the mixture is viscous at time t, the catalyst is inefficient or not sufficiently efficient for the target time t.

EXPERIMENTAL

Adhesive materials

Silyl modified polymers are silane based chemicals that contain inorganic and organic moieties which can form polymers exhibiting the beneficial characteristics of both often without many of their disadvantages. Two prepolymers were used for the study: SMP1 and SMP2 which were provided by BOSTIK. They are dimethoxymethylsilane terminated polyethers with a urethane moiety for SMP1 and an alkyl chain for SMP2 (Fig. 2).

SMP1

Chemical products

All chemicals were purchased from Sigma Aldrich and were used as received without further purification.

Acid catalysts: Trifluoromethanesulfonic acid (HOTf, >99%wt), Hydrochloric acid (HCl, 37%wt), Methanesulfonic acid (MSA, >99,5%wt), p-Toluenesulfonic acid monohydrate (PTSA, 98%wt), Dodecylbenzenesulfonic acid (DDBSA, 95%wt), Oxalic acid (98%wt), Maleic acid (>99%wt), Malonic acid (99%wt), Chloroacetic acid (99%wt), Glycolic acid (99%wt), Benzoic Acid (>99,5%wt), Acetic acid (>99,7%wt), Propionic acid (>99,5%wt), Lauric acid (>98%wt), Stearic acid (95%wt), Amine catalysts: ), Tributylamine (TBA, >98,5%wt), Triethylamine (TEA, ≥99%wt), Pyrazole (98%wt), 1-Methylimidazole (99%wt), Imidazole (>99%wt), Pyridine (>99%wt), 1,2,4-Triazole (98%wt) and Triethanolamine (>99%wt) and Metal-based catalysts: Dibutyltin dilaurate (DBTDL, 95%wt), Zirconium(IV) propoxide (Zr(IV) n OPr, 70%wt), Zirconium(IV) isopropoxide (Zr(IV) i OPr, 99,9%wt), Zirconium(IV) butoxide (Zr(IV) n OBu, 80%wt), Titanium(IV) propoxide (Ti(IV) n OPr, 98%wt), Titanium(IV) isopropoxide (Ti(IV) i OPr, 97%wt), Titanium(IV) butoxide (Ti(IV) n OBu, 97%wt), Zirconium(IV) acetylacetonate (Zr(IV)acac, 98%wt), Aluminum acetylacetonate (Al(III)acac, 99%wt), Iron(III) acetylacetonate (Fe(III)acac, 97%wt), Copper(II) acetylacetonate (Cu(II)acac, 97%wt), Calcium acetylacetonate (Ca(II)acac, 99,95%wt), Manganese(II) acetylacetonate (Mn(II)acac).

In order to simplify tables and figures, the different catalysts are not reported in full name. When available, only corresponding abbreviations are used.

Development of high-throughput experiment method with DBU

SMP1 (55g) was introduced into a large plastic vessel (185*95*15mm). As indicated in Fig. 3, a plate containing 96 holes was slowly disposed on the plastic tank to allow the viscous liquid to fill the 96 cavities by capillarity. By this way, 96 identical samples of prepolymer are generated without any transfer by multichannel pipette or individual weighing. Each reactor has an 8mm diameter and 8mm height. Various rates of DBU 0.05%wt (rows G and H), 0.1%wt (rows E and F), 0.3%wt (rows C and D) and 0.5%wt (rows A and B) (0.032mmol to 0.32mmol) were added at the surface of each well with a multichannel pipette. The reaction begins when the mixture is in contact with moisture to allow the development of cross-linking reactions.

General procedure for parallel screening of catalysts (HTS)

Typically, 1mL tubes (from VWR) are placed in a plate A, containing 96 holes. In a glove box, catalysts (2,25.10 -5 mol) and THF (50 L) were added in each tubes. After closing tubes, mixtures were stirred during 1h at room temperature. SMP1 (55g) was introduced in a large plastic vessel (185*95*15mm). Another plate B, containing 96 holes, was slowly disposed on the plastic tank to allow the viscous liquid to fill to 96 cavities by capillarity. By this way, 96 identical samples of prepolymer are generated without any transfer by multichannel pipette or individual weighting. Each reactor has an 8mm diameter and 8mm height. Catalytic systems were then transferred from the plate A into the 96 micro-reactors of plate B with a multichannel pipette to form a film at the surface of

General procedure for polymerization of SMP1 in bulk

Catalysts (4,5.10 -4 mol) and THF (100 L) were added in 1mL tubes (from VWR). Mixtures were stirred during 1h at room temperature. SMP1 (10g) was introduced into a plastic cup (diameter 50mm and height 30mm). Catalytic systems were introduced into the plastic cup and stirred with prepolymer for 1 minute. Surface texture was evaluated by touching the surface every 5 min over a period of one hour, followed by 1 hour for 8hrs and every 24h for 1 week. 2.6. General procedure for polymerization of SMP2 in bulk Catalysts (9,5.10 -5 mol) and THF (100 L) were added in 1mL tubes (from VWR). Mixtures were stirred over a period of 1h at room temperature. SMP2 prepolymer (10g) was introduced into a plastic cup (diameter 50mm and height 30mm). Catalytic systems were introduced into the plastic cup and stirred with prepolymer for 1 minute. Surface texture was evaluated by touching the surface every 5 min over a period of one hour, every 1 hour for 8hs and every 24h for 1 week.

RESULTS AND DISCUSSION

Development and validation of high-throughput screening method

The key point for any HTS approach is to be sure to compare all experiments, accurately performed in the same conditions: a) the amount of material must be identical in each micro-reactor, b) the ratio of catalyst/prepolymer should be the same, c) when different components are mixed together the homogeneity should be guaranteed, d) one should pay attention to avoid cross contamination between micro-reactors. We have developed a new experimental protocol specific for extremely viscous systems. The prepolymer was introduced into a large plastic vessel (185*95*15mm). A plate containing 96 holes was slowly disposed on the plastic tank to allow the viscous liquid to fill the 96 cavities by capillarity. By this way, 96 identical samples of prepolymer are generated without any transfer by multichannel pipette or individual weighing. Catalytic systems were then transferred from a 96 wells plate A to the 96 micro-reactors of plate B with a multichannel pipette. The small size of the individual reactors do not allow an efficient stirring, thus there is a strong risk of having an inhomogeneous reaction mixture. Therefore, we proposed to create a film of catalysts solution at the surface of each well, instead mixing of components. All experiments were able to allow the crosslinking reactions to occur at the surface in contact with moisture under the same conditions for all experiments. Each reactor has an 8mm diameter and 8mm height. Thus, the height prevents any risk of catalyst migration from the surface to the bottom of each well. The screening was performed by touching the surface of each well with a multichannel tip system to visualize the physical state (viscous or gum solid state) every 5 minutes during 1 hour at room temperature. For a given time and only if the catalyst is active, the effective curing of polymers will affect both the surface and the top part of the micro-reactor as the cure depth is related to the square root of time. Thus when the surface is viscous, this means that the catalyst is inefficient. But when a gum aspect occurs (indicating surface and deep curing) , this means that the catalyst is efficient. We have validated this high-throughput screening by the classification of catalysts in 3 categories (i.e.; fast, medium and slow activity). Three different rates of a well-known catalyst DBU were applied to simulate three different activity levels of catalysts (Fig. 3). 0,5% and 0,3%wt DBU = Fast (efficient between 0 and 15minutes) 0,1%wt DBU = Medium (efficient between 15 and 30 minutes) 0,05%wt DBU = Slow (inefficient after 1 hour) This method allows the fast detection of catalytic systems with different activities. Indeed, we can see the difference with different rates of DBU, 0.5%wt DBU is faster than 0.05%wt DBU which is slower. Moreover, the good reproducibility of the HTS method was validated because for each DBU rate, similar results were obtained. After this validation step, the HTS method was applied on several libraries to compare and to detect efficient catalysts on silyl modified polymers.

Screening of amines library

Amines and especially tertiary amines are well-known to catalyze polyurethane synthesis, such as DBU and TBD. These catalysts and other amines are also described by Wakabayashi et al. in their patent on alkoxysilane terminated polymers [23]. The activation mechanism is similar in these two cases. In the first case, Schwetlick et al. [24] showed that tertiary amine allows the activation of alcohol through a base catalysis mechanism to facilitate the addition of this alcohol onto an isocyanate. In the second case, Riegel et al. [25] suggested that an amine catalyst increases the dissociation of water to produce nucleophilic hydroxylanions which can attack alkoxysilane groups on a 3glycidoxylpropyl-trimethoxysilane (GPTS) molecule to accelerate hydrolysis. We can expect that the hydrolysis and condensation mechanisms to crosslink alkoxysilane terminated polymers are the same as base catalysis. To our knowledge, a comparison of the different catalytic systems under the same conditions has not been reported on alkoxysilane terminated polyethers. That is why the efficiency of several amine catalysts was determined by HTS method under the same conditions (temperature and humidity). DBTDL catalyst was used as a reference to compare its activity with amine catalysts (Fig. 4). proposed a mechanism which is based on hydrogen bonding between a guanidine like TBD and an alcohol to provide nucleophilic activation. Similarly, in addition to the basicity, the potential participation of the amidinium and guanidinium moiety in the catalytic activation of the alkoxysilane hydrolysis and condensation could be proposed (Fig. 5). A guanidine or amidine system such as TBD, DBU or DBN can activate a water molecule (1). The nucleophilic character of hydroxyl was enriched to favor the nucleophilic substitution with an alkoxysilane group to provide a hydroxyl alkoxysilane group (2) and the regeneration of catalyst (3). Then, the catalyst system can also increase the reactivity of the hydroxyl alkoxysilane by hydrogen bonding (4). The condensation between this system and another hydroxy or alkoxysilane (5) leads to the formation of polymer or a 3D network structure (6).

Screening with multichannel tips system by parallel approach

Screening of acids library

Acids are also able to catalyze the hydrolysis and condensation of terminal alkoxysilyl groups. Brinker et al. [32] showed that in acid catalysis (HCl) on tetraethylorthosilicate (TEOS), the hydrolysis is favored compared to condensation by H + electrophilic attack on alkoxide oxygen. In this case, the silicate gel is not highly crosslinked as compared to base catalysis. Pope et al. [33] reported the effect of several inorganic acids such as HF, HCl, HNO 3 , H 2 SO 4 on TEOS gel time, considering both hydrolysis and condensation steps. Organic acids catalysts are also very interesting with the opportunity to modulate the Bronsted or Lewis acid properties by playing with the acid functional part (carboxylic, sulfonic groups, ..) or with the other part of the molecule (aliphatic or aromatic acids, electroattractive or electrodonating substituents, ..). For instance, Ni et al. [34] showed that -toluene sulfonic acid (PTSA) can be used to catalyze the moisture curing of an organic/inorganic mixture composed of an alkoxysilane terminated isocyanurate, 1,6-hexamethylene diisocyanate (HDI) and TEOS. Thus, an HTS method could be certainly useful to investigate a large variety of organic acid catalysts and to compare them with DBTDL as reference (Fig. 6). A variety of organic acids were tested, including strong and weak acids, monocarboxylic acids with short and long aliphatic chains or aromatic groups, di-carboxylic acids with different methylene units as well as acids with hydroxyl or chlorine substituents. The different weak acids with relative pKa higher than 1, were medium or not efficient for the cross-linking of SMP1 by comparison with the tin reference catalyst tested under the same conditions. At room temperature and after 1 hour of reaction, all the systems involving weak acids stayed more or less viscous but did not generate the same solid gum aspect like the reference DBDTL. As expected, strong acids with low pKa [35] value such as HOTf (-12), HCl (-7), MSA (-2.6) and PTSA (-1.34) are very efficient crosslinking silyl modified polymers in less than 15 min. These results are coherent with the fact that generally speaking and according to our HTS results, acids with a pKa value equal or less than -1 are efficient systems. Interestingly, the HTS method led us to detect catalysts with intermediate activity able to generate the solid gum aspect but only after 15 to 30 min. For instance, the HTS method showed that a substitution of the methyl group of PTSA (pKa -1.34) [36] by a longer dodecyl group for the DDBSA (pKa -1)

[37] has a great impact on the activity. This effect could be tentatively attributed to higher hydrophobicity induced by the longer dodecyl group by comparison with a group. However, the HTS method also gave an unexpected result as benzoic acid was also detected as a catalyst with medium activity like DDBSA. Only the acidic property seems not enough to explain this result because benzoic acid has a high pKa value (4.2). In comparison with other carboxylic acids like oxalic or maleic acids, it seems that the electronic delocalization through the aromatic ring for the benzoate anionic is a key factor for the catalyst efficiency. Based on the generally accepted hypothesis, indicating that the cross-linking of alkoxysiloxane is favored when using acids with low pKa value, the benzoic acid would not be evaluated. This result indicated that the HTS approach could be really interesting to detect new acidic catalyst families.

Screening of M-(OR) and M(acac) libraries

In some cases, organic catalysts could present some side effects due to the residual catalyst, such as odor diffusion from the material, color modification versus time as well as migration of the catalyst from the 3D cross-linked polymer to another matrix in contact with the polymer. In addition, the organic catalysts could also affect the structure and the morphology of the final reticulated material. For instance, acid catalysts accelerate the hydrolysis step and generate more linear units than base Fig. 6: a) pKa values of acid compounds and b) their efficiency on SMP1 obtained by HTS method at room temperature catalysts. Basic catalysts generate more dense structures and sometimes colloidal containing structures. Thus, the use of metal-containing catalysts, more particularly metal alkoxides, is also of great interest to generate polymers. Blank et al. [38] reported several organometallic compounds such as Zr(IV)acac, Zr(IV)O n Bu and Ti(IV)O n Bu for the reaction between an isocyanate and a hydroxyl group. Wen et al. [39] showed that metallic alkoxides were used as precursors to form an organic/inorganic hybrid network material. In fact, the use of metal alkoxide compounds as efficient catalysts for silyl modified polymers has already been described [40]. Acetylacetonate has often been used as a chelating ligand to decrease hydrolysis and condensation reactions. Several catalysts, such as Al(III)acac, have also been reported to increase the hydrolysis on methyltrimethoxysilane and tetramethoxysilane [41], as well as Fe(III)acac on silyl terminated polymers [42]. However, these reports do not provide a clear comparison between each catalyst for the same substrate, thus it is practically impossible to extract relative reactivity data. In order to compare the efficiency of several metal-containing catalysts we have applied the HTS method and compared to DBTDL as reference under the same conditions (Fig. 7).

All metallic alkoxides tested here are very efficient on silyl terminated polyethers except for Zr(IV)O i Pr which is inefficient. It seems that the Zr(IV)O i Pr is so reactive with moisture that it is too quickly deactivated and has no time to react with Si-OMe bonds inducing an heterogeneous system. In the literature, Pandey et al. In conclusion, the development of a semi-quantitative high-throughput screening method allows the classification of different catalytic systems according to their efficiency. All catalysts were compared under rigorous identical conditions: geometry and size of micro-reactors, moisture exposure and time for parallel testing. Thus, only differences in catalytic properties will affect the surface or depth curing [45] of polymer. This method is applicable to different types of catalysts like organic bases or acids as Fig. 7 Efficiency of metallic compounds on SMP1 obtained by HTS method at room temperature *= heterogeneous mixture well as for metal-containing catalysts. This can be applied to detect new catalysts by testing 96 catalysts under the same conditions. It is also to spend time on more focused studies, but only on the best candidates for target applications.

Skinning time of some efficient catalysts in bulk

The HTS method allowed the ranking of different catalytic systems into 3 categories (fast, medium and slow catalysts) according to their relative activity versus the reference catalyst DBDTL. The second step is to measure their 3D solid cross-linking time values compared to DBTDL (Table 1) in order to conclude if DBTDL can be substituted. The hits detected by the HTS approach were also tested for another prepolymer SMP2 (Fig. 2). In fact, the SMP2 and SMP1 are methoxysilane terminated polymers. In the case of urethanedimethoxysilane (SMP1), the electron donor (nitrogen atom with free electron pair) is attached to a silicon atom via only one methylene group as spacer. In addition, the methyl group attached to the silicon atom can play an important role by inductive effect. Indeed, the electron density is shifted to the silicon atom and the Si-O(CH 3 ) bond is weakened. Thus for SMP1, the time of cross-linking reaction decreases due to the activation of alkoxide group. That is why the alkoxide groups split off more easily and hydrolysis is accelerated for SMP1. We envisioned that if the SMP2 is less reactive than SMP1, it is not necessary to screen all of the catalysts again for the SMP2, because an inefficient catalyst on SMP1 will be inefficient too on SMP2. Times to reach the gum aspect of different hits catalysts were determined on SMP2 (Table 2). HOTf, PTSA and DDBSA were no longer usable as they induced out of control reticulation. However, HCl and MSA are approximately 200 to 300 times more efficient than DBTDL for the cross-linking of SMP2 and benzoic acid which is slow on SMP1 is inefficient on SMP2. Benzoic acid and DBU did not allow the cross-linking, these results are not surprising as these two catalysts presented a medium activity for the SMP1. In the same way, the TBD, much active for SMP1 is under control for the SMP2 and is 16 times more efficient than DBTDL. Metallic alkoxides and metal acetylacetonate are also more efficient than the tin catalyst, excepted for Zr(IV) acac and Mn(II)acac which are inactive.

Catalysts

several metal acetylacetonate systems have shown catalytic activity. This HTS method will be used to detect new catalysts to substitute organotin catalysts to generate ecofriendly adhesive materials.

  powerful approach for the fast screening of a large diversity of compounds such as biomaterials [17], inorganic materials [18], polymers [19] or catalysts [20, 21, 22].

Fig. 1 :

 1 Fig. 1: General scheme of hydrolysis and condensation reactions and intermediate states: viscous or solid 3D cross-linked gum.

Fig. 2 :

 2 Fig. 2: Scheme of silyl terminated polyethers

Fig. 3 :

 3 Fig. 3: Scheme of the results obtained during the development of high-throughput screening method

Fig. 4 :

 4 Fig. 4: a) pKa values of amine compounds and b) their efficiency on SMP1 obtained by HTS method at room temperature

Fig. 5 :

 5 Fig. 5: Catalytic activation of hydrolysis (a) and condensation (b) step of SMP1 using guanidine as organocatalysts

[ 43 ]

 43 used a combination of Zr(IV)OiPr and dichloroacetic acid to decrease the reaction of zirconium tetraisopropoxide with moisture. Ti(IV)O i Pr is effective as a catalyst but this result could be explained by the lower reactivity of Ti(IV)OR than Zr(IV)OR reported by Wen et al.[44]. As expected, the results show that acetylacetonate systems with Zr, Ti, Al and Fe are very efficient. Copper and Calcium acetylacetonate systems are inefficient after 1 hour. Interestingly, a manganese system has medium efficiency. This result was very difficult to predict, thus showing the potential benefits of a fast parallel comparison method.

Table 1 .

 1 3D

			Using the same conditions, temperature,
			humidity and rate of catalytic system, the
	crosslinked gum solid state time of efficient catalysts in bulk by classical method at room temperature on SMP1 prepolymer	reactions were performed in bulk condition for 10 g of prepolymer SMP1 and a fast mixing of
			the mixture. Concerning the acids catalysts
		Gum solid state time (min)	HOTf, HCl, MSA, PTSA and DDBSA are much more active than DBDTL as they induced an
	DBTDL	150	instantaneous 3D cross-linking. However, these
	HOTf	instantaneous	systems are not very useful in bulk conditions or
	HCl	instantaneous	for adhesive applications as a setting time is necessary to generate a homogeneous mixture for
	MSA	instantaneous	bulk reactions and/or to apply the adhesive
	PTSA	instantaneous	materials onto a surface. If the final application
	DDBSA	instantaneous	does not need a very short setting time, the
	Benzoic acid	4320	benzoic acid could be applied according to a lower toxicity than the tin catalyst DBDTL. In
	DBU	10	our application, a setting time of few minutes is
	TBD	instantaneous	necessary to apply adhesive materials on a
	Zr(IV) n OPr Ti(IV) n OPr	30 30	surface. Benzoic acid could be interesting due to the high toxicity of DBTDL. For bases systems, if TBD generated an instantaneous cross-linking
	Zr(IV)acac	120	very difficult to control, the DBU catalyst
	Ti(IV)acac	60	presented very interesting properties with a
	Al(III)acac	105	shorter time for reticulation than DBDTL and a lower toxicity. Metal alkoxides also present high
	Fe(III)acac	75	potential as they are faster than the tin catalysts
	Mn(II)acac	390	with a lower toxicity. For longer setting time it
			could also be interesting to use the less reactive
			metal acac complexes as they are less moisture
			sensitive than metal alkoxides.

also for metal containing complexes. The method allows to compare and to extract different hits which were analysed in a second step to extract their relative solid state reaching time. DBU, metallic alkoxides and metal acetylacetonate systems could be used to substitute organotin catalysts on silylated activated prepolymers. For inactivated ones, only TBD, HCl, MSA, metal alkoxides and

Table 2 .

 2 Skinning time of efficient catalysts in bulk by classical method at room temperature on SMP2 Solid gum state reaching time was evaluated at room temperature every 5 min during one hour and every 1 hour until 8h. Days after and early morning, skinning time was evaluated every 24h until 1 week.

*:

ACKNOWLEDGEMENTS

The authors gratefully acknowledge CNRS, ANRT, BOSTIK and TOTAL for their financial support.

REFERENCES AND NOTES


J Khaymovich

H Pekola

C Courtois

Winkelmann

D M T Van Zanten

D M Basko

I M Khaymovich

J P Pekola

H Courtois

C B Winkelmann

Single Quantum Level Electron Turnstile

  L'archive ouverte pluridisciplinaire

The ability to control current flow down to the single electron level in nanodevices has triggered a vast activity on quantum metrological current sources in recent years [START_REF] Geerligs | [END_REF][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In a quantum current source, electrons are conveyed one by one across a mesoscopic conductor, which is achieved by Coulomb repulsion. Early device geometries relied on two or more Coulomb blockaded islands in series [3]. Among the most promising recent approaches are islands with tunable barriers in 2D electron gases [2,6,11,17] along with superconducting single electron transistors [7]. Beyond metrological applications, the development of on-demand single-electron sources opens broad perspectives in the field of quantum coherent electronics and electron optics [18][19][20][21][22].

The superconducting single-electron transistor (SINIS) turnstile [7,23] takes advantage of the sharply defined energy gap in the density of states in superconductors, as an energy filter. A normal metallic region (N) is weakly coupled to two superconducting leads (S) through tunnel barriers. N has to be sufficiently small to have a Coulomb charging energy U , which should be at least on the order of the superconducting gap in the leads, ∆. Nevertheless, N displays a dense set of states, appearing as continuous at accessible temperatures. A finite island temperature then allows for an entire energy window ∼ k B T of available states in N for tunneling, which leads to turnstile operation errors associated to double occupation or tunneling into the wrong lead [23].

In this Letter, we demonstrate the first realization of a source of quantized dc current based on a single quantum energy level. The physical operation principle is similar to the SINIS turnstile, with the important difference that electrons are here carried by a single energy level of a quantum dot (Q). After demonstrating the expected principal turnstile operation characteristics, we focus on novel electronic transport features of the SQS turnstile. In particular, we show that tunneling can be tuned to occur within a narrow energy window. We theoretically compare the dominant turnstile error processes in the SQS and SINIS devices, concluding that the former has a lower sensitivity to out-of-equilibrium quasiparticles.

The fabrication of the SQS junctions, described in [24], relies on the in situ creation of a nanometer-sized fracture in superconducting constrictions by electromigration [25], a proven technique for connecting single molecules [26] and nano-particles [27]. Randomly dispersed gold nanoparticles of about 5 nm diameter can occasionally bridge the nanometric fractures, providing thereby welldefined quantum dot junctions. By using superconducting aluminum electrodes, SQS junctions can be obtained [24,28]. Because higher-order processes are detrimental to turnstile operation accuracy, we restrict ourselves to rather weakly coupled devices.

The relevant device parameters of the quantum dot junction are its charging energy U , the quantum dot orbital level spacing δE, the tunnel couplings γ and the capacitances C to the three terminals source, drain and gate, which we denote by indices S, D and G, respectively. All these can be determined from transport data in static conditions, that is, measuring the current I as a function of the applied bias voltage V B and gate voltage V G . The I(V B , V G ) maps show typical Coulomb blockade behavior in which only a single or at most a few charge degeneracy points (Fig. 1a) are accessible in the available gate voltage range. We find charging energies U > 50 meV and orbital energy level spacings δE on the order of 1 meV or higher. Because δE k B T , the thermal population beyond the ground state is vanishingly small and electron transport occurs uniquely through a single orbital quantum level [29]. We focus on two devices with quite different tunnel couplings: S has rather symmetric tunnel couplings (γ S = 2.1 µeV, γ D = 1.4 µeV), while A is strongly asymmetric (γ S = 5.2 µeV, γ D = 0.4 µeV).

Here and further, we set h = 1. The determination of all dc transport characteristics of both devices was described in detail in [24].

Superconductivity in the leads provides a hard energy filter for tunneling. The absence of quasi-particle states at energies |E| < ∆ ≈ 260 µeV in the leads results in a suppression of conductance for |V B | < 2∆/e at any gate voltage, as is seen in Fig. 1a. For turnstile operation, a small constant bias 0 < |V B | < 2∆/e is applied and a periodic modulation signal with frequency f and variable amplitude is added to the static gate potential. The energy difference between the n + 1 and n electron occupation numbers in the quantum dot, (t), varies between ¯ ±A , where ¯ is controlled by the static voltages V G and V B . A single electron can tunnel into the quantum dot as soon as (t) reaches the occupied states of the contact with the higher chemical potential (Fig. 1b; right grey triangle in Fig. 1a). By raising (t) via the back gate to reach the empty states above the upper gap edge in the opposite lead (left grey triangle in Fig. 1a), the level is emptied to that lead. By driving (t) cyclically, one electron is conveyed per cycle from the higher chemical potential lead to the other, giving rise to a dc current I = ef .

The combination of both above tunneling processes, in and out of the quantum dot, corresponds to the desired operation mode of the turnstile and will be named forward tunneling in the remainder. As shown in Fig. 1, forward tunneling requires the amplitude A of the modulation of (t) to be A > ∆ -e|V B |/2. On the other hand, a too large modulation amplitude A > ∆+e|V B |/2 will eventually allow for tunneling into/from the opposite lead. Such backtunneling processes are detrimental to current quantization, and their signature will be discussed later on.

Throughout this work, a square wave signal, with a rise time τ ≈ 1.6 ns associated to the finite bandwidth of the generator, is used for modulating (t). The experimental dc current I(V B ) measured for (t) with an amplitude A around ¯ = ¯ 0 ≡ (µ S + µ D )/2 is shown in Fig. 2a. Here µ S,D are the leads' chemical potentials, with µ S -µ D = eV B . Above the threshold voltage for forward tunneling, V f w B = ±2(∆ -A )/e, a broad current plateau at I = ef develops. Turnstile operation is only effective for a restricted range of ¯ (Fig. 2b). The value of the turnstile current, determined at the inflection point, follows the predicted linear dependence on frequency (Fig. 2c), with a standard deviation of about 1 %, to which adds a systematic deficit of about 0.7 % at higher frequencies. The plateaus show a small residual slope at all frequencies. This feature has instrumental origin, which is discussed in the Supplemental Material file.

At charge degeneracy, the thresholds for the onset of both forward and backtunneling can be seen as the narrow blue stripes in Fig. 3a. Both thresholds cross at V B = 0 when A = ∆. Whereas the frequency-dependent transmission of the ac gate signal to the device is not pre- cisely known, this crossing is used to calibrate A . The bright color identifies regions of voltage-independent current, corresponding to I = 0 and I = ±ef , respectively. When ¯ is slightly detuned from ¯ 0 by the static gate potential, the onset of forward tunneling is linearly shifted towards larger A (Fig. 1b). Note that turnstile operation requires two successive tunneling events to occur. This is visible in Fig. 3b, where the current is shown as function of gate detuning and modulation amplitude. For larger amplitudes A , an increasing tolerance of the turnstile operation with respect to the proper tuning of ¯ -¯ 0 develops.

Having evidenced electron turnstile operation, let us now identify the hallmarks of transport through a single quantum energy level. In SINIS turnstiles, backtunneling can be occasioned by electrons from the high-energy tail of the thermal energy distribution in N. The backtunneling probability increases thus steadily and smoothly as A is cranked up [30]. Conversely, in a SQS turnstile backtunneling sets in abruptly, when the threshold A = ∆ + |V B |/2e is exceeded. This is seen in Fig. 4a, where at high enough modulation amplitudes, the current drops suddenly from ef . We numerically model the turnstile current dependence on A , both for the SINIS and the SQS turnstile, by solving the time-dependent rate equations using the measured output of the ac signal generator. In the SQS case, the instantaneous tunneling rates to each lead are found from the retarded Green's function's pole [24,31,32], that is, beyond Fermi's golden rule. This is particularly important near the singularities in the superconducting density of states (see Supplemental Information file). The calculation (continuous line in Fig. 4a) nicely captures the abrupt decrease of the current as soon as the backtunneling threshold is met. For comparison, in a SINIS device with parameters taken from the most precise devices presently studied [33,34], the onset of backtunneling is markedly smoother (dashed line).

This particularly sharp onset of backtunneling is all the more pronounced if the rise time τ of (t) is short, or more precisely, if the time available for forward tunneling only is brief. If is raised to the backtunneling threshold within τ γ -1 S,D , the probability of backtunneling may actually exceed that of forward tunneling. This means that a current inversion, of magnitude up to ef , might eventually be produced with proper parameter combinations. This could however not be observed in our experiment, because the square wave rise time is of the same order of magnitude as the inverse tunneling rate (τ ∼ γ -1 S,D ). To highlight the energy selectivity of the tunneling process, we calculate the energy resolved transferred charge dq/d over a half-period of an ac gate cycle, using the assumptions and parameters of the calculation in Fig. 4a. The results are shown for different values of A and for both forward and backward processes in Fig. 4c. While a certain fraction of forward tunneling occurs near the superconducting gap edge (where the lead's density of states is largest), good energy selectivity of the tunneling is achieved for sufficiently large values of A . For even larger A , backtunneling is possible, which we represent using negative values of dq/d . The accuracy of the energy selectivity is ultimately limited by the tunnel coupling, but in the present experiment it is dominated by deviations of the ac drive signal from a perfect square wave. The SQS device is expected to yield about the same energy resolution as the semiconducting ac single electron source [18], assuming identical gate drive and tunnel couplings [39]. For comparison, the energy distribution of levitons [21] is pinned to the Fermi level.

We now move to the discussion of possible error processes of the SQS turnstile. One obvious source of error is the missed tunneling event. As the tunneling rate is finite, tunneling may be missed during the corresponding half-period, leading to I < ef . For a single-level quantum dot, the Fermi golden rule tunneling rate for each lead (α = S, D) can be written as Γ α = (2) γ α n s ( (t)±eV b /2), where n s (E) is the normalized quasi-particle density of states in the superconducting leads. The factor of 2 takes into account the possibility of tunneling for two spin projections, and is present only for tunneling at one of the leads. For a symmetric square wave modulation of (t), the probability of missed tunneling at one of the leads can be roughly estimated as e -Γαt eff . Here, the effective time available for tunneling t eff ≈ 1/(2f ) -τ , takes into account the signal rise time. At frequencies around 200 MHz, this estimate gives a current deficit of 0.8 % for the device parameters of sample S, which agrees well with the experimental value of about 0.7 % (Fig. 2c inset). For device A, the missed tunneling rates at high frequencies are higher because of the tunneling bottleneck at its less transparent tunnel junction.

In turnstile operation with a normal metal island and at finite temperature, a fraction ∼ exp(-∆/k B T ) of electrons has sufficiently high energy for backtunneling. In aluminum-based SINIS turnstiles, the associated error is rapidly dominant above about 300 mK [34]. An expected hallmark of energy quantization in the turnstile operation should be a rather marked temperature insensitivity as long as δE k B T and Pauli blocking of states in the leads can be neglected. We have followed the turnstile operation of device A as a function of temperature up to 0.5 K and we indeed observe the turnstile plateau to subsist through the entire temperature range, with only a rather moderate increase in error rate. We quantify the error by the I = ef plateau slope dI/dV B . As seen in Fig. 4b, this slope shows only little dependence on temperature. For comparison, the calculation of the same for a SINIS turnstile, shows a rapid divergence above about 300 mK. While thermal errors are negligible only in the low mK range in most reported turnstiles, the SQS device can operate up to relatively high temperatures without suffering from thermal tunneling.

An important source of errors in superconducting turnstiles is related to the presence of non-equilibrium quasiparticles in the leads, with concentration x qp = n qp /(2ν∆). Here, n qp is the quasiparticle density in the lead and ν is the density of states (per spin projection) at the Fermi level in the normal state. Such quasiparticles can accumulate as a consequence of noise and, in particular, of the turnstile operation itself and are well known to be difficult to evacuate [33,36]. Using the diffusion model described in [33] we estimate n qp ∼ 10 µm -3 near the SQS junction, yielding x qp ≈ 2 × 10 -6 . In the SINIS turnstile, direct tunneling of such quasi-particles between one lead and the central island occurs with a rate ∼ x qp g ∆ leading to a frequency-independent leakage current. Here g is the dimensionless conductance of the tunnel junctions, in units of the conductance quantum. Crucially, this leakage, which is at present the main source of errors in SINIS turnstiles [33], is suppressed for the SQS device by the lack of states in the quantum dot at the quasiparticle energy [40]. For tunneling processes of higher order in γ S,D , the limitations to accuracy of the SINIS and the SQS devices are comparable. Details of the above derivations are given in the Supplemental Information file.

To conclude, a metallic quantum dot embedded between superconducting leads allows for turnstile operation in which the charges are conveyed by a single quantum level. As a consequence, tunneling occurs within a narrow energy window, determined by the level energy and broadening. Under realistic assumptions, the SQS turnstile can serve as a monochromatic on-demand single electron source. As a next step, one can explore the possibility of spin-polarized turnstile operation by Zeeman splitting in a moderate magnetic field [41]. This work was funded by the European Union Seventh Framework Programme INFERNOS (FP7/2007-2013) under grant agreement no. 308850 and partially by the Russian Foundation for Basic Research (IMK) and the Nanosciences Foundation (JPP and IMK). Samples were fabricated at the Nanofab facility at Institut Néel. We also thank F. Hekking, X. Waintal, and B. Sacépé for discussions and C. Hoarau, A. Nabet, F. Balestro, S. Samaddar, and A. de Cecco for help with the experiments. We are indebted to E. Bonet, C. Thirion, and W. Wernsdorfer for developing and sharing NanoQT.

FIG. 1 :

 1 FIG.1:(a) Experimental current map of a superconductorquantum dot hybrid device as function of gate and bias potential, in absence of periodic gate drive (device S). Coloured solid lines correspond to the four superconducting gap edges as illustrated in (b). The device is operated as a single level turnstile when its state is modulated periodically around its (n, n + 1) charge degeneracy point. The on-state currents are I+ = 290 pA (red) and I-= -250 pA (blue). (b) Energy diagram of the device with a small bias applied, illustrating electron tunneling events in and out the quantum dot. Grey areas indicate the amplitude range for solely forward tunneling, also seen in (a). Driving the turnstile with a square wave signal allows for tunneling to occur within a narrow energy window.

FIG. 2 :

 2 FIG. 2: (a) Current-bias traces measured near the charge degeneracy point. Characteristic plateaus appear with I = ef (indicated by dashed lines) when applying a small modulation signal (magenta: A ≈ 0.64∆, f = 190 MHz, blue: A ≈ 1.0∆, f = 60 MHz) to the gate. The black trace shows the current response with no ac gate drive. (b) Current-gate traces measured for A ≈ ∆ and the same frequencies as in (a), at VB = 3 2 ∆/e (magenta) and VB = ∆/e (blue). (c) Current at the inflection point of the plateaus shown in (a) as function of operation signal frequency. The insets highlight deviations of the normalized current I/ef from 1 in both the low and high frequency ranges (all data are from device S).

FIG. 3 :

 3 FIG. 3: (a) Colormap of ∂I/∂VB as a function of bias and gate modulation amplitude (f = 56 MHz, ¯ = ¯ 0). Narrow blue regions, corresponding to rapid increase in current, separate areas of voltage independent current (white), with values I = 0 and I = ±ef . (b) Colormap of turnstile current as a function of static gate offset from degeneracy point and gate modulation amplitude (f = 60 MHz, VB = 1.5∆/e). All data are from device A.

FIG. 4 :

 4 FIG. 4: (a) Turnstile current as a function of operation signal amplitude (device A, f = 56 MHz, ¯ = ¯ 0 and eVB = 0.7∆). The sharp decrease in current indicates the sudden onset of backtunneling. The continuous line is the numerical calculation for the SQS with all parameters determined by the device dc transport properties (see text). The dashed line is the analogous calculation for an SINIS device with normal state resistance RN = 300 kΩ, U = 3.0∆ and assuming quasiequilibrium of electrons in N by electron-phonon relaxation [35]. The arrows indicate the values of A used in (c). (b) Slope at inflection point of I(V b ) on the turnstile plateaus, averaged over A , as a function of temperature (device A). The dashed line is the calculation for the SINIS device, with parameters as in (a). (c) Calculation of the energy distribution of the delivered charge per cycle, for different gate drive amplitudes A , with parameters as in (a). The negative part of the panel displays the backtunneling contribution. The highest position of the quantum dot level, as determined by the gate modulation, is represented in the inset by the lines of corresponding colors.