Jerzy Radecki
email: j.radecki@pan.olsztyn.pl

Remalakshmy Poduval

Katarzyna Kurzatkowska

Magdalena Stobiecka

F A Wim

Wim F A Dehaen

Hanna Jadwiga Dehaen

Radecka

K Kurzątkowska

H Radecka

Systematic study of interaction of the neutral form of anilines with undecylcalix[4]resorcinarene derivatives by means of potentiometry

Keywords: undecylcalix[4]resorcinarene, aniline derivatives, molecular recognition, potentiometry, neutral analytes undecylcalix[4]resorcinarene, aniline derivatives, molecular recognition, potentiometry, neutral analytes

teaching and research institutions in France or abroad, or from public or private research centers.

hydrogen bonding, hydrophobic forces, van der Waals forces, π-π interactions, and electrostatic effects between two or more molecules (1,2).

A receptor (host) can recognize a substrate (guest) on the base of the structural complementarities of the two molecular species. The design and characterization of synthetic host-guest pairs still is an open challenge in supramolecular chemistry.

Different techniques like NMR, mass spectroscopy, X-Ray crystallography, laser technologies, isothermal titration calorimetry, cyclovoltammetry and also computational methods have been employed to study supramolecular interactions (3)(4)(5)(6)(7). Potentiometry also is of importance in this regard (8). Potentiometric techniques help to provide an insight into the recognition at the liquid-liquid interface where the mechanism of generation of a potentiometric signal depends on the charge separation between the two phases which in turn is the result of a selective transfer of guest molecule from the aqueous to the organic phase.

Vital components of such type of sensors are poly(vinyl chloride) (PVC) membranes modified with specific ionophores, which are responsible for important from analytical point of view parameters such as: selectivity and sensitivity as well as the time of response and reproducibility. Different types of molecules such as calixarenes (9,10), macrocyclic polyamines (11)(12)(13), calixpyrroles (14), corroles (15) and calix [4]phyrins (16) were applied as a host in PVC supported liquid membranes. The chemical structure complementarities between host and specific guest are crucial in selective molecular recognition. The structure of host is usual read out by the guest.

Potentiometric sensors have some outstanding advantages including simple design and operation, wide linear dynamic range, relatively fast response and rational selectivity.

Because of this, their construction is particularly interesting from the perspective of the supramolecular chemists. The potentiometric sensors could be applied as useful tools for observation of molecular recognition process at the border of two phases.

In classical potentiometry, the analytes are ionic species. So far, there are only few examples of the potentiometric recognition of neutral molecules. In a pioneering paper by Umezawa and co-workers, the possibilities of potentiometric signal generation by polymeric membranes modified with quaternary ammonium salts (17) and lipophilic polyamines (18,19), after their stimulation with uncharged phenol derivatives, were described for the first time. This research dealt with permanently charged ligands (tetra-alkylammonium salts), or formally neutral amines which gain a plus charge after being in contact with the aqueous phase (e.g. macrocyclic polyamines). Next, new classes of ligands which are able to recognize undissociated nitrophenols and generate changes of membrane potential were described by our group. These new ligands belong to the family of calix [4]pyrroles, corroles, calix [4]phyrins and metaloporphyrins (20)(21)(22)(23)(24)(25)(26). The first three of them are tetrapyrrolic macrocyclic compounds and are members of a very wide family of porphyrin analogues. They are well known as sensitive and selective receptors for anions (27)(28)(29)(30)(31)(32).

In general, the host-guest recognition process in the case of these compounds could be realized mainly by the creation of hydrogen bonds between the ligand and target molecules, and also by electrostatic interactions, Lewis acid-base and π-π electron interactions (33)(34)(35)(36). undecylcalix [4]resorcinarene generate cationic potentiometric signals in the presence of unprotonated diaminobenzene isomers (37). The para-isomer with the highest basicity generates the highest potentiometric response. We confirmed by spectroscopic methods that the potentiometric signals were generated based on the formation of a supramolecular complex between the undecylcalix [4]resorcinarene host and the diaminobenzene isomer guest at the organic-aqueous interfaces (37).

In the same paper we have suggested a hypothetical mechanism of cationic potential signal generation as a result of intermolecular recognition process between the undecylcalix [4]resorcinarene in the polymeric membrane and diaminobenzene isomers in solution. The observed recognition process is selective and allows distinguishing between the particular isomers (37).

The main goal of our research was showing the generation of potentiometric signals by uncharged molecules and possibility of changing the direction of potentiometric signals by changing the nature of host and guest molecules.

The anionic potentiometric signals were observed when macrocycles containing nitrogen atoms were applied as the host molecules and phenol derivatives play role of the guests (17)(18)(19)(20)(21)(22)(23)(24)(25). When macrocycles containing phenolic groups were applied as the host molecules and aniline derivatives play role of the guests -the cationic potentiometric responses were observed (37).

Here, we summarize the results of the systematic potentiometric investigation of the intermolecular recognition processes at the water/polymer membrane border between some derivatives of undecylcalix [4]resorcinarenes and neutral (unprotonated) forms of aniline derivatives such as: diaminobenzene, chloroaniline, hydroxyaniline, methylaniline, methoxyaniline and nitroaniline. In particular, we focused on the effect of ligand and analyte structure on the generation of the membrane potential changes. The structures of the host molecules are illustrated in Figure 1.

Experimental

Reagents and materials

The receptor (host) molecules used for this study are tetra-undecyl-tetra-pnitrophenylazocalix [4]resorcinarene (Ligand 1), tetra-undecylcalix [4]resorcinarene (Ligand 2), tetra-undecyl-tetra-p-phenylazocalix [4]resorcinarene (Ligand 3), tetra-undecyl-tetrabromocalix [4]resorcinarene (Ligand 4), tetra-undecyl-tetra-hydroxycalix [4]resorcinarene (Ligand 5) (Figure 1). All ligands were synthesized by previously reported procedures (38)(39).

2-Nitrophenyl-octylether (NPOE) was purchased from Fluka-Aldrich (Poznań, Poland). High molecular-weight poly(vinyl chloride) (PVC; n av =1100) which was used as the polymer matrix was purchased from Wako (Japan). Tetrahydrofuran (THF) was obtained from POCh (Gliwice, Poland) and was distilled over solid sodium hydroxide prior to use. Aniline, isomers of anisidine, toluidine and chloroaniline, ortho-and meta-diaminobenzene, parahydroxyaniline and para-nitroaniline were purchased from Sigma-Aldrich (Poznań, Poland).

Para-aminoaniline and di-sodium hydrogen phosphate anhydrous were obtained from POCH (Gliwice, Poland). All solutions were prepared using deionised water with resistivity of 18.2 MΩcm just before the potentiometric measurements.

Electrode preparation and potential measurements

The PVC membranes were prepared with using the following components: 1% (wt.) of the host, 66% (wt.) of plasticizer, and 33% (wt.) of PVC. All components were dissolved in 2 ml freshly distilled THF and the homogenous mixture was poured into a glass ring of 30 mm diameter and left for 24 h to allow THF to evaporate. The membranes obtained were of 100 µm (ca.) thickness. Circles of 6 mm diameter were cut from the membrane and directly mounted on to a liquid membrane-type Philips ISE body (Glasbläserie Möller, Zürich, Switzerland). All potentiometric measurements were performed at room temperature (~20° C) on a multi-channel station, EMF-16 (Lawson Labs). A double junction Ag│AgCl electrode was used as the reference electrode. The electrode cell assembly is presented bellow:

Ag / AgCl | 3 M KCl | 1 M CH 3 COOLi || sample solution | membrane |0.1 M KCl | Ag / AgCl
The electrodes were conditioned in 1.0 x 10 -1 M phosphate buffer (pH 7.0) for 24 h.

The calibrating solutions of the guest molecules were prepared in 1.0 x 10 -1 M phosphate buffer, pH 7.0.

Results and discussion

Cationic potentiometric responses of PVC supported liquid membranes incorporating undecylcalix[4]resorcinare hosts in the presence of neutral forms of aniline derivatives

Our previous study on the of potentiometric responses of liquid membrane electrodes incorporating with undecylcalix [4]-resorcinarene in the presence of diaminobenzene isomers , where all isomers exist in the cationic form (37). Therefore, all potentiometric measurements presented here were performed at pH 7.0. Under such conditions all aniline derivatives investigated exist in water almost entirely as the neutral (unprotonated) compounds (Table 1). Figure 2 illustrates an example of potentiometric responses of PVC membranes containing Ligand 1 towards of diaminobenzene isomers. The results obtained for all aniline derivatives and all type of membranes studied are collected in Table 2. It has been proved that all types of membranes generate cationic responses towards uncharged aniline derivatives.

Comparison of the results for all analytes showed the general tendency: with the increase of the analyte basicity, the response increases. In most cases, the para isomer generates the highest response. For ortho and meta isomers it is difficult to estimate which one generates higher signal (Table 2).

Based on the results obtained we can show only general qualitative relationship between potentiometric signals and acidity of the guests. This is because of the others parameters such as lipophilicity, steric hindrance also have some influence of the phenomenon studied. Similar trends we have observed in the case of membranes modified with pyrrole containing macrocycles stimulated with isomers of dihydroxybenzene (21,23).

An explanation of the weak response of membranes after stimulation with ortho isomers is the possibility to form intramolecular hydrogen bonds. Because of that, the formation of intermolecular (host-guest) hydrogen bonds is relatively more difficult.

We have not observed the relationship between potentiometric responses and partition coefficients of the aniline guests. This is evident that phenomenon observed is govern by acidity / basicity of the host /guest. The hydrophobicity of the guests plays a secondary role.

The weakest response was observed for nitro-and chloro-derivatives of aniline. These guests are the strongest acids among the investigated compounds. In the case of membranes modified with pyrrole containing macrocycles stimulated with nitro phenol, the opposite phenomenon was observed. The strongest responses were observed in the case of guests with the strongest acidity (21).

The ligands under study have different structures of the upper rim. In Ligand 1 there are dihydroxybenzene substituents in which the OH groups are in position 1 and 3 in relation to each other. These distances make intramolecular hydrogen bonds very weak. Additionally Similarly as in Ligand 5 this system allows to create an intramolecular hydrogen bonds network. Additionally, the bromine atom is rather large atom and because of this hinders the approach of guest molecules towards the host.

A comparison of the values of potentiometric response for each ligand showed a general tendency, which is as follows:

Ligand 1 > Ligand 2 > Ligand 3, Ligand 4 > Ligand 5
This sequence indicates that an increase of the acidity of the phenol groups causes an increase of the potentiometric response value.

We observed the strongest response for the membrane modified with Ligand 1 containing in its structure dihydroxybenzene substituted with the electron-withdrawing azonitrobenzene.

The acidity of these OH groups is the highest.

Next in this sequence is Ligand 2 with unsubstituted resorcinol. In this case the acidity of the OH group is lower than in Ligand 1, but the accessibility of Ligand 2 for analytes is much easier. In consequence, the creation of a hydrogen bond (H…O…H…N) between analyte and ligand is relatively easy. The Ligand 3 contains in its structure a resorcinol substituted with azobenzene. This causes an increase of the phenolic acidity, but at the same time, the presence of a rather large substituent hinders the access of OH group for analyte molecules. The signal value generated by membrane modified with Ligand 3 is a result of these two opposite effects.

Ligand 4 containing the bromo-derivatives of resorcinol in their structure is next in the sequence of the response value. In this case, similarly as it was in the previous one, two factors compete: from one side the inductive and mezomeric effects, which causes an increase of the acidity of the phenol group and the large bromine atom, which is a significant hindrance for the approach of the analyte.

The lowest response we have obtained for the membrane modified with Ligand 5 which possesses pyrogallol (trihydroxybenzene) in its structure. In this ligand, the strong intramolecular hydrogen bonds are present. As the consequence, this lowers overall phenolic acidity. Because of this, the formation of a supramolecular complex with the analyte is relatively the most difficult among the ligands under study.

Analysis of the data obtained shows that there is no direct relation between the lipophilicity of the amine guests and the values of the signal generated by them.

However, the low lipophilicity of para-hydroxyaniline may be one of the reasons of its weak potentiometric response in spite of its high basicity (Table 2).

Checking of the possibility of transport of para-anisidine through the PVC membrane modified with tetra-undecyl-tetra-p-nitrophenylazocalix[4]resorcinarene

The possibility of transporting para-anisidine across the tetra-undecyl-tetra-pnitrophenylazocalix [4]resorcinarene (Ligand 1) membrane was checked by performing a long-term experiment in two solutions (1) 1.0 x 10 -1 M KCl and solution (2) 1.0 x 10 -4 M of para-anisidine in 1.0 x 10 -2 M phosphate buffer pH 7.0. The cells were separated by a piece of membrane containing Ligand 1. The transport of para-anisidine through the modified membrane was monitored by UV-Vis measurements. After 24h the KCl solution was free of para-anisidine. This means that para-anisidine is not able to penetrate through a PVC membrane modified with Ligand 1 during this time interval. Therefore, it might be concluded that the potentiometric signals observed in the present study were not generated as a result of diffusion of the analyte through the polymeric membranes, but there are a consequence of the supramolecular recognition phenomena occurring at the organic/aqueous interface.

The mechanism of the generation of the potentiometric signal of undecylcalix[4]resorcinare modified membranes after stimulation by unprotonated derivatives of anilines

Based on the obtained results and data from the literature we propose the following mechanism of cationic potentiometric signal generation by membranes modified with derivatives of undecylcalix [4]resorcinarene after stimulation with unprotonated aniline derivatives. In the first step, during conditioning, some of the phenolic groups of undecylcalix [4]resorcinarene derivatives, located at the surface of polymeric membranes, dissociate and the membranes gain a minus charge. Such type of dissociation of OH groups at the upper rim of undecylcalix [4]resorcinarene was already reported (40)(41)(42). In the next step, a network of hydrogen bonds between the aniline derivatives and the phenolate groups is created. Thus, a supramolecular complex of undecylcalix [4]resorcinarene and aniline derivatives is formed at the water/membrane interface. The amino groups are donors of hydrogen atoms in hydrogen bonds. Their polarity correlates with the acidity of phenol groups.

The formation of such a hydrogen bond network in one phase was already reported (43 -45).

The measurements were carried out at pH 7.0. Under this condition, all of the investigated anilines exist in solution mostly as unprotonated compounds (Table 1). The increase of the electron density at the amine nitrogen atom upon complex formation causes an increase of its basicity and this is the driving force for transferring the proton from the surface of the aqueous phase to the surface of the organic (membrane) phase. Proton transfer leads to an increase of the plus charge on the membrane surface, and we can observe the generation of a potentiometric cationic signal.

Generally, the proposed mechanism is based on three steps.

The first one concerns dissociation of some phenolic groups from the upper rim of the investigated resorcinarene ligands.

(I) HostOH membrane + H 2 O ↔ HostO -+ H 3 O +
The next step consists of the transfer of the analyte from the bulk solution to the interface and formation of a supramolecular complex between the ligand and the analyte through hydrogen bonds.

(II) HostO - interface + NH 2 -Ar ↔ HostO -----H----NH-AR interface A consequence of this is the increase of the basicity of the nitrogen atom of the supramolecular complex, and its protonation. undecylcalix [4]resorcinarene shows that the accessibility of phenols groups is a very important parameter for the intermolecular recognition process which is taking place between the investigated ligands and the analytes.

The weakest response we have observed in case of membranes modified with Ligand 5 which possesses in its structure trihydroxybenzene could be explain by the low acidity and the possibility to form the intramolecular hydrogen bonds. This is a relatively strong energetic barrier for the described phenomenon.

The second step of the proposed mechanism, which relies on proton transfer from the water to the organic phase, is supported by the fact that regardless of the ligand, the strongest signal was observed for the strongest bases between compounds under study. The results obtained

show that the lipophilicity of the analytes is not a crucial parameter.

Conclusions

The systematic work we have carried out (21)(22)(23)(24)(25) on the study of the potentiometric signal generated by uncharged molecules was inspired by the first pioneer papers by Umezawa (18)(19)(20) on the anionic potentiometric responses of membranes modified with macrocyclic polyamines or quaternary ammonium salts stimulated with undissociated dihydroxybenzenes.

It was proved that PVC supported membranes incorporating hosts with amino groups upon stimulation with phenolic guests generate anionic potentiometric signals. The base of this phenomenon is a proton transfer from supramolecular complexes located at the interface towards the aqueous phase.

In this paper we proved that membranes modified with hosts possessing phenolic groups generate cationic potentiometric responses upon stimulation by uncharged guests possessing NH 2 groups. In this case, the base of the potentiometric signal generation relies on the proton transfer from the aqueous phase on the supramolecular complexes located at the interface.

In both cases, the acidity and basicity of the ligands, as well as the analytes, are crucial parameters, which govern these phenomena. The shape of the cavity of the macrocyclic ligand is also significant. The lipophilicity of the analytes plays a secondary role.

Based on the Umezawa papers, as well as our already published work and the research

Introduction

  Molecular recognition refers to the specific noncovalent interactions such as:

  In our previous paper, concerning the generation of a potentiometric signals by neutral analytes, we have shown that PVC supported liquid membrane electrode incorporated with

  performed at pH 3.0 and pH 7.0. The potentiometric signals were observed only at pH 7.0, where all guests exist in uncharged form. The potentiometric signals were not observed at pH 3.0

Ligand 1

 1 is substituted in position 2 with azonitrobenzene. The presence of this substituent because of its inductive and mezomeric effect causes a substantial increase of the acidity of the phenol groups from the upper rim. Ligand 2 possesses in its structure dihydroxybenzene in positions 1 and 3. Next, Ligand 3 consists in its upper rim the dihydroxyazobenzene. The OH groups are in position 1 and 3. The azobenzene substituent is in the position 2. This substituent because of its inductive and mezomeric effect increasing the acidity of phenolic groups in relation to Ligand 2. Ligand 5 contains in its structure pyrogallol units (1, 2, 3trihydroxybenzene). The close vicinity of OH groups lead to the formation of intramolecular hydrogen bonds. Ligand 4 possesses in the upper rim 2-bromo-1,3-hydroxybenzene.

(

  III) HostO -----H----NH-AR interface + H 3 O + ↔ HostO -----H----NH-AR interface + H 2 O H +The confirmation of the first step is based on the results we have obtained with the membrane modified with Ligand 1 in which the acidity of phenolic groups is the highest. The results we

Figure 1 .Page 14 of 17 URLFigure 2 .

 1172 Figure 1.

Table 1 .

 1 Percentage of protonated and neutral species at pH 7.0.

	pH 7.0

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F o r P e e r R e v i e w O n l y

Table 2 .

 2 The potentiometric response of PVC liquid membranes incorporating calixarene host generated in the presence of aniline derivatives; ∆E [mV]. = E i -E 0 ; E 0 -potential recorded in buffer solution, E i -potential recorded in the presence of analyte.

	Guests	pK 1	Log P o/w	Ligand 1	Ligand 2	Ligand 3	Ligand 4	Ligand 5
	p-diaminobenzene	6.22	-0.26	148.9	138.4	88.4	86.9	41.5
	p-anisidine	5.36	1.15	118.6	68.9	53.9	59.4	37.4
	m-anisidine	4.20	1.32	92.6	62.2	55.1	20.1	28.4
	o-diaminobenzene	4.61	0.37	66.8	57.2	64.2	31.0	31.5
	m-diaminobenzene	5.01	0.03	36.2	42.1	35.8	13.8	7.6
	p-toluidine	5.08	1.43	92.3	36.2	44.1	27.0	30.2
	p-chloroaniline	3.98	1.81	31.5	24.2	22.8	-0.6	6.2
	o-anisidine	4.53	1.65	62.7	20.1	35.7	19.8	7.8
	p-hydroxyaniline	5.48	-0.24	27.6	15.5	25.2	-2.1	2.4
	aniline	4.87	1.24	57.7	12.8	34.5	10.7	13.1
	m-toluidine	4.71	1.59	84.2	9.7	32.9	14.9	22.0
	o-toluidine	4.45	1.61	75.8	9.4	20.8	10.7	7.7
	p-nitroaniline	1.02	1.19	4.7	3.0	-	-	2.4
	m-chloroaniline	3.52	1.88	19.8	0.7	2.0	-	9.2
	o-chloroaniline	2.66	2.02	8.6	-4.5	1.2	-	1.9
	∆ ∆ ∆ ∆E[mV]

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Acknowledgements

The work was supported by European Union Marie Curie Transfer of Knowledge Research Grant MTKD-CT-2006-042708, the Polish Ministry of Sciences and Higher Education no. 105/6.PR. UE/2007/7, and Institute of Animal Reproduction and Food Research of the Polish Academy of Science, Olsztyn, Poland. W.F.A.D. and W.D. thank the University of Leuven, the Ministerie voor Wetenschapsbeleid and the F.W.O. for continuing support.


W P Leroy

S Konstantinidis

S Mahieu

R Snyders

D Depla

Angular-Resolved Energy Flux Measurements of a DC-and HIPIMS-powered rotating cylindrical magnetron in reactive and non-reactive atmosphere

published or not. The documents may come

Angular-resolved energy flux measurements of a dc-and HIPIMS-powered rotating cylindrical magnetron in reactive and non-reactive atmosphere

Introduction

Nowadays, the use of functional coatings is widely spread and numerous applications and tools rely on the properties and stability of the functional coating. To apply these functional thin films, a wide variety of deposition techniques can be used. For many applications, physical vapor deposition (PVD) has prevailed as the method of choice. Moreover, magnetron sputtering can be considered as the most industrially favored PVD technique, due to its high-throughput production of high quality thin films.

In the 1980s, a rotating cylindrical magnetron was developed [START_REF] Mckelvey | Magnetron Cathode Sputtering Apparatus[END_REF]2] to improve target utilization. As magnetron sputtering has many industrially relevant advantages, the erosion profile due to the presence of a stationary racetrack often limits the target consumption and increases its cost of ownership [3]. A rotating cylindrical magnetron has a superior target utilization (up to 70% or more), as compared to planar magnetrons [2]. Furthermore, higher power levels (and hence higher deposition rates) are possible due to its better target cooling. Therefore, these magnetrons are now the workhorse of many industries that are doing large-area and in-line coatings. Products of these industries include flat panel displays, low-emissivity and solar control windows and thin film photovoltaics. Furthermore, rotating cylindrical magnetrons have proven their worth in research, as to exhibit certain fundamental effects in the magnetron sputtering process (e.g. the influence of redeposition on the behavior in reactive sputtering), which would not have been possible for a planar magnetron [4][5][6].

Recently, there has been a growing interest in the process of high power impulse magnetron sputtering (HIPIMS) [7][8][9][10], both among academics as industrials. HIPIMS discharges generate a dense plasma, allowing a high degree of ionization of the sputtered (film-forming) atoms. With this PVD technique, smooth coatings are being deposited with a high density, even on complex-shaped substrates [7]. However, some drawbacks to the technique include a lower deposition rate as compared to conventional DC sputtering, a complex and not well-understood behavior in reactive atmosphere, and technological complications for upscaling [10]. Still, HIPIMS can be regarded as a very interesting and promising technique for industrial applications.

The combination of these two industrially favored techniques, namely HIPIMS using a rotating cylindrical magnetron, seems like a winning combination for different industries. Recently, the first successful combination was reported [11]. Key to the use of a certain PVD technique in a process is its energy flux towards the substrate, as these substrates are often sensitive to high temperature (e.g. flexible solar panels). Furthermore, the energy flux towards the substrate plays a vital role in the growth behavior of the thin film [12,13]. For the latter, the energy per arriving adparticle is of high importance. Therefore, the arriving adparticles (or the deposition rate) will be compared for both DC and HIPIMS mode. Additionally, both operating modes will be tested in oxide regime ('full poisoning'), i.e. when the target surface is completely oxidized [14,[START_REF] Berg | Reactive Sputter Deposition[END_REF]. The aim of the present work is to give more detailed analysis of the behavior of a rotating cylindrical magnetron, when applying HIPIMS.

Experimental

All experiments were performed in a stainless-steel vacuum system of which a sketch is shown in figure 1. The chamber was evacuated using a turbomolecular pump to a base pressure of 10 -4 Pa (10 -6 mbar). Before the experiments, the pumping speed was set to 48 L/s using a valve. The working pressure of Ar for the experiments was kept constant at 0.7 Pa. The oxygen flow was regulated using a mass flow controller (Aera) and in oxide regime it was set at 4 sccm (standard cubic centimeter per minute) yielding a total pressure of ca. 0.8Pa. In the oxide regime, the target surface was fully poisoned, as was evidenced by the DC target voltage and the quartz microbalance measurements.

A next generation of the small-scaled, rotating cylindrical magnetron as originally described in previous work [START_REF] Depla | [END_REF] has been used. The most important change to the setup was the use of a different magnet system, which produces a higher maximal, radial magnetic field strength of 1000 G. The magnet configuration can be rotated to any fixed position within the target, which enables angularresolved measurements (see figure 1). The area of the racetrack was ca. 10 cm 2 . The Ti target is a tube with a length of 185 mm and an outer diameter of 48 mm. The target rotated at a speed of 20 rpm.

The HIPIMS power supply used in this work [17] allows the generation of pulses whose length was varied between 5µs, 15µs and 20µs with a repetition frequency of 5700Hz, 700Hz and 500Hz respectively. These are relatively short HIPIMS pulses compared to other groups [8,10,18]. For all HIPIMS experiments, the applied voltage was set around 600V, resulting in a peak current of about 75 -100A and obtaining a constant average power of 300W (± 5%). The current and voltage waveforms were recorded at the magnetron side. For the DC measurements, a ENI RPG 50 power supply was used to deliver constant current (ca. 0.9 A) to match the average power in the HIPIMS discharge. Hence, the peak power obtained with the HIPIMS power supply is typically two orders of magnitude larger than the one reached during the DC experiments. However, to have the best description of the processes, all measurements will be normalized to the average power. This will allow for a well-founded comparison.

A passive thermal probe measured the total energy flux towards the substrate [19,20]. Deposition rates were measured with a quartz crystal microbalance (Maxteck, Inc. Thickness monitor model TM-350) facing the target, at the same distance as the thermal probe (12cm). Both probes were grounded during the measurements.

Results and Discussion

HIPIMS pulse

In previous work [11], it was shown that a stable operation of a rotating cylindrical magnetron during HIPIMS sputtering can be obtained. Based on the I-V characteristics (see figure 2) from the HIPIMS discharges, this can be confirmed for both metallic as well as oxide mode.

When comparing the shape of the voltage-current pulses for the two sputter regimes (metallic and oxide), a difference in the current behavior can be distinguished. For the metallic regime, there is a linear increase of the current with time, as long as the voltage pulse is running. This is expected [21], and as shown by Lundin et al. [22], the discharge current would drop back to a lower plateau value (around the DC value), if the pulse duration would be extended (i.e. more than 100 µs).

When examining the current pulse in oxide regime, there is at first a slower, linear increase of the current with pulse duration. However, after a few microseconds the current starts to increase more rapidly and finally overtakes the one of the metallic regime. This different shape of the current curve, namely a sudden, sharp increase and "bent" form of the current pulse is typical for the oxide regime and was noticed throughout all oxide measurements.

When the target is operated in oxide regime, the surface completely consists of oxide material. This oxide layer is not removed within one HIPIMS pulse. As a rough estimate, it can be considered that during one pulse about 1.5 x 10 15 ions arrive at the target surface, if the discharge current pulse is considered to be supplied solely by singly charged ions. In this estimate, the contribution of negatively charged species (e.g. the secondary electrons, negative oxygen ions) to the current is neglected and hence the calculated amount of arriving ions is an upper limit. Furthermore, it is assumed that the target only gets sputtered by Ar ions. This is a good approximation, as the metal selfsputtering is negligible for short pulse lengths [21]. With the area of the racetrack region being 10 cm², the flux of ions per pulse towards the target surface is 1.5 x 10 14 ions/cm². Considering that the sputter yield of titanium oxide is 10 to 20 times lower than the one of metallic titanium, which is 0.67 [23] for these conditions, it is clear that no complete monolayer of the target surface (around 10 15 atoms/cm 2 , [START_REF] Ohring | Materials Science of Thin Films[END_REF]) gets sputtered off during one HIPIMS pulse. Therefore, the target surface will remain oxidized during the pulse and hence this cannot explain the sharp increase of the current. Some different mechanisms must kick in, which should originate from the plasma chemistry. The plasma has a different composition due to the added oxygen. These oxygen molecules are also ionized and produce some oxygen atoms by dissociation, which are in turn ionized. Time-resolved OESmeasurements support the notion of increased oxygen dissociation during the high-power pulse [START_REF] Konstantinidis | Reactive Sputter Deposition[END_REF]. This concentration of atomic oxygen increases with a certain delay from the start of the pulse, as compared to the argon concentration. Hence these oxygen-related processes start contributing to the current with a certain delay and then make the current increase more than linearly. The same behavior was observed and discussed by Benzeggouta et al. [START_REF] Benzeggouta | [END_REF] for the efficiency of pre-ionization in the case of HIPIMS-sputtering of a ruthenium target in an Ar/O 2 atmosphere.

Deposition Rates

In this section, the advantage of the rotating cylindrical magnetron is that the magnets can be turned within the cathode. This allows for a complete angular description of the plasma parameters. Hence, deposition rate measurements were carried out by systematically moving the cathode magnets by steps of 10°. Figure 3(a) shows that the deposition rate drops when the magnets (and hence the racetrack) are rotated away from the quartz balance. For DC, this shape can be perfectly simulated using the test-particle Monte Carlo code SIMTRA [27]. Furthermore, a more than 75% decrease can be observed for the deposition rate in HIPIMS mode (with a 20µs long pulse), compared to DC mode.

The ratio of the deposition rate as a function of angle is shown in figure 3(b), where the deposition rate for each sputter mode is normalized to the deposition rate of the DC mode. This allows for a good comparison of the angular distribution for each of the sputter modes.

Among the different HIPIMS modes, there is a decrease in deposition rate with the increase of the pulse length. This phenomenon has been discussed by e.g. Konstantinidis et al. [21], and is often attributed to the self-sputtering regime. With longer pulse durations, a higher density of ionized metal particles is obtained and self-sputtering becomes important to dominant in the process. However, other processes will contribute to this phenomenon: the depletion of the local gas density ("gas rarefaction") [28,29], a different emission profile, different particle transport throughout the gas phase [30,31]. The latter two processes can cause a different material distribution throughout the deposition chamber, hence yielding a locally changed deposition rate.

However, Figure 3(b) clearly demonstrates that the shape of the angular distribution is identical for all sputter modes. This inclines that the emission profile and the transport of the particles is the same for all sputter modes. Horwat and Anders noticed the same when making an angular resolved study around a planar magnetron with a Cu HIPIMS discharge [32]. This behavior would imply that the anomalous transport evidenced by Lundin et al. [30], which enhances the sideways ejection of metal ions in the case of a circular magnetron geometry wouldn't apply for a rectangular magnet configuration, as used in the present study. Or that sideway emission is in a different direction than measured here. One could also assume that another effect counterbalances the phenomenon discussed by Lundin et al. in such a way that the HIPIMS and DC profiles are identical for our working conditions.

Furthermore, the normalized ratios of the deposition rate are shown in the legend of figure 3(b). Compared to DC, the deposition rate drops significantly (to half) when using the HIPIMS 5µs mode. For longer pulse times, there seems to be a stabilization and the decrease in deposition rate is less pronounced, which is consistent with earlier published results [21]. For 100µs pulses, Lundin et al. [33] observed a decrease to 20% of the DC-deposition rate. This observation agrees with the trend noticed here.

When the cathode is sputtered in oxide regime, the deposition rate drops significantly for both modes (DC and HIPIMS). This phenomenon is already well-known [14,[START_REF] Berg | Reactive Sputter Deposition[END_REF], but the deposition rate for the HIPIMS oxide regime is again lower than for the DC oxide regime, as can be observed in figure 3(a). The deposition rate in DC-oxide regime drops to 6% of the normal DC deposition rate. For the HIPIMS-oxide regime this drop is as expected even bigger, namely to 3%. Comparing the oxide regimes to their respective metallic regimes, i.e. the HIPIMS 5µs oxide regime to the HIPIMS 5µs regime, the drop in deposition rate is equal (for HIPIMS: 0.03/0.47=0.06). This indicates that the change in sputter behavior (i.e. sputter yield and electron emission coefficient) is the same for both sputtering modes (DC and HIPIMS), when going from a metallic target to an oxidized target.

Energy Flux

The energy flux towards the substrate plays an important role in the deposition process. It can influence the growth behavior of the thin film, or damage temperature-sensitive substrates. HIPIMS has been proposed as an ideal deposition technique to avoid temperature damage to the substrate [18]. Therefore, the total energy flux for the rotating cylindrical magnetron was measured for the different deposition modes. The values for the arriving energy flux (E flux ) have been normalized to the average power (P av ) of the used deposition mode. Overall, the angular dependence is similar for all operating modes, with the highest arriving total energy flux for the DC-mode. All sputter modes exhibit a maximum energy flux around 40 degrees. The difference in total arriving energy flux is however small between the DCmode and the HIPIMS mode, especially for the short pulse durations (e.g. 5µs). The DC mode of operation could therefore be considered as the limit of the HIPIMS mode for which the pulse duration is infinitely short (0 µs) and the frequency infinitely large.

For the different HIPIMS modes, there is a small decrease in total energy flux with increasing pulse length. Lundin et al. [33] and West et al. [18] also reported on the arriving energy flux during HIPIMS. However, different HIPIMS parameters were used, which makes it hard to compare. To study the several measurements, the total energy flux of the HIPIMS mode is normalized to the one of the DC mode. These ratios can be seen in figure 4(b), as plotted as a function of their pulse length. Figure 4(b) shows a decrease in this ratio with increasing pulse lengths. Furthermore, the values reported in literature confirm our findings, if the observed trend is extrapolated to a 100µs pulse length (dashed line). Overall, the total energy flux is lower for the HIPIMS modes than for the DC mode. However, for short pulse lengths, this difference becomes quite small. This behavior could originate from the total amount of particles (Ar fast neutrals and ions, sputtered metal neutrals and ions, electrons, …) that reach the probe. Indeed, it has been shown that both the ionization and deposition rates depend on the pulse duration (at constant discharge voltage) [21]. The larger the pulse duration, the larger is the ionization rate but the lower is the deposition rate. Hence the energy must be calculated with respect to the number of incoming adparticles.

Energy flux per adparticle

Not only the total energy flux arriving at the substrate plays a vital role, but the energy per arriving adparticle plays a crucial role in the growth behavior of the thin film [12,13]. It can be either desirable (e.g. interface engineering [34]) or detrimental (e.g. high-energy negative oxygen ions [35]), depending on the material and the property wanted. Therefore, figure 5 shows the energy flux per arriving Ti adparticle for the different sputter modes, with figure 5(b) showing an enlarged view for the metallic regimes.

Considering the overall values for the energy flux per adparticle, the DC mode has the lowest values. This is followed by the HIPIMS mode with 5µs pulse length, which already exhibits 1.5 times more energy per adparticle than the DC mode. For the longer pulse lengths, even higher amounts are observed, raising to about 2.8 times and 3 times the energy of the DC mode, for the 15µs and 20µs pulse length respectively. The two oxide regimes clearly have the highest arriving energy flux per arriving adparticle. Compared to their metallic regime, the DC oxide regime exhibits a 12 times higher energy flux per arriving adparticle, while for the HIPIMS oxide regime this is almost 19 times higher. The dramatic increase of the overall energy per adparticle when working in the oxide regime could be attributed to the significant amount of negative ions such as O -reaching the probe surface. These ions have been detected in a DC magnetron discharges [35,36] and in HIPIMS plasmas [37]. Negative ions are sputtered from the oxidized target and accelerated by the cathode sheath. Hence they may reach the probe with very high energies of the order of 300eV during our DC magnetron sputtering experiments and 600eV during HIPIMS.

The angular shape of this energy flux per adparticle looks quite puzzling. The overall trend is a rise in the energy flux per adparticle with higher angles. However, a decrease occurs for all sputter modes, albeit at different angles. For the DC mode this decrease happens around 50 degrees, while the HIPIMS 20µs (and roughly speaking the 5µs and 15µs too) have their maximum at 60 degrees.

Mahieu et al. [12] measured and calculated the different contributions to the total energy flux per incoming adparticle for reactive DC magnetron sputtering of TiN. It is evidenced that the main contribution (depending on the working conditions used) originates from the ion flux, and to a lesser extent from the electrons and the sputtered particles. It is therefore obvious to observe that the HIPIMS discharge, characterized by an increased ionization rate, exhibits i) an increased energy flux per adparticles as compared to DC and ii) that during HIPIMS the energy flux increases when increasing the pulse duration. The ionization rate of the sputtered metal vapor was found to increase as the pulse duration was increased (at constant discharge voltage) [21].

As concluded from the first section, the angular emission profile of the sputtered particles is similar for DC and HIPIMS mode. Therefore the difference in the angular shape of the total energy per adparticle probably arises from a changed electron and (positive and negative) ion distribution. More research will be conducted to compare these distributions for DC and HIPIMS mode on a rotating cylindrical magnetron by e.g. Langmuir probe, optical absorption spectroscopy and mass spectrometry. There exists some data in literature which already indicate a difference between the two modes of operation. Lundin et al. [30] observed a radial acceleration of the ions over the target surface, which they attribute to an anomalous electron transport. However, the data reported by Lundin et al. were acquired for a circular target (and racetrack), not for a rectangular one. It is known from the literature that the rectangular target geometry exhibits some discrepancies regarding the motion of the electrons (e.g. the cross-corner effect). Furthermore, Bohlmark et al. [38] showed a two-stage deformation of the magnetic field during the HIPIMS pulse. This could an effect which causes a change in the transport of the charged particles, hence causing a change in the angular shape of the arriving energy flux per adparticle. These example highlight the need for more comparative experiments, in which the option for angular distributions using the rotating cylindrical magnetron will definitely prove its worth.

Conclusions

A rotating cylindrical magnetron is successfully operated in HIPIMS mode, both in metallic as in oxide regime. A difference in the shape of the current pulse is ascribed to a different plasma chemistry when sputtering in oxide regime, namely a delayed dissociation-ionization of the oxygen molecules. This gives rise to a faster-than-linear increase of the current.

A comparison is presented of the energy flux from a rotating cylindrical magnetron for sputtering in DC mode and in HIPIMS mode. This is done for both the metallic as the oxide regime in function of angle around the cylindrical target. The total energy flux is quantitatively compared by determining the adparticle flux (from the deposition rate) as a function of angle, for each mode.

The deposition rate of the HIPIMS mode is lower than for the DC mode, and it decreases with increasing pulse length. The decrease in deposition rate stabilizes for higher pulse lengths. Furthermore, the angular emission profile and the transport of the particles is the same for all sputter modes.

The angular dependence of the total energy flux is similar for all operating modes, with the highest arriving total energy flux for the DC-mode. The difference in total arriving energy flux is however small between the DC-mode and the HIPIMS mode, especially for the short pulse durations (e.g. 5µs). The decrease in total energy flux with increasing pulse length confirms previous work by other groups.

The energy which arrives per adparticle is the highest for both oxide regimes. Among the metallic regimes, the HIPIMS modes exhibit a much higher energy per adparticle, with the highest values for the longer pulse lengths. This effect would originate from the increased ionization rate of the sputtered material in HIPIMS as the pulse duration is increased. Furthermore, a difference in the angular shape of the energy per arriving adparticle is noticed between the DC and the HIPIMS modes. The DC mode has a maximum arriving energy per adparticle around 50 degrees, while this is at 60 degrees for the HIPIMS mode.

List of Figures

Figure 4 (

 4 Figure 4(a) shows the angular dependence of the total energy flux for the different operating modes.The values for the arriving energy flux (E flux ) have been normalized to the average power (P av ) of the used deposition mode. Overall, the angular dependence is similar for all operating modes, with the highest arriving total energy flux for the DC-mode. All sputter modes exhibit a maximum energy flux around 40 degrees. The difference in total arriving energy flux is however small between the DCmode and the HIPIMS mode, especially for the short pulse durations (e.g. 5µs). The DC mode of operation could therefore be considered as the limit of the HIPIMS mode for which the pulse duration is infinitely short (0 µs) and the frequency infinitely large.

Figure 1 (

 1 Figure1(color online): Sketch of the experimental setup. The magnet array (and hence the racetrack) can be positioned at any angle α to the used process monitor. The tubular Ti target rotates around the stationary magnet array with a cerain speed.

Figure 2 (

 2 Figure 2 (color online): Current and voltage waveforms during sputtering of a rotating cylindrical magnetron in HIPIMS mode (5µs pulse width), in metallic and oxide regime.

Figure 3 (

 3 Figure 3 (color online): (a) The deposition rate (R d ) as measured by a quartz microbalance, as a function of angle around the rotating cylindrical magnetron, for different sputter modes. (b) The deposition rates relative to the deposition rate of the DC sputter mode, as a function of angle around the rotating cylindrical magnetron.

Figure 4 (

 4 Figure 4 (color online): (a) Polar plot of the total arriving energy flux for the different sputter modes, as a function of angle of rotation around the rotating cylindrical magnetron, normalized to the average sputtering power. (b) Ratio of the total energy flux for the HIPIMS mode to the DC mode, plotted as a function of pulse length. A comparison to literature values (West et al.[18] and Lundin et al.[33]) is given. The dashed line is a guide to the eye.

Figure 5 (

 5 Figure 5 (color online): Energy flux per arriving adparticle for the different sputer modes. (b) is a magnification of the region boxed in (a).

Figure 1 (Figure 2 Figure 3 (Figure 4 (Figure 5 (

 12345 Figure 1 (WouterLeroy_Revision_Figure1.eps)

Acknowledgements

This work was partially funded by the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen), under SBO Project No. 60030. Furthermore, the WOG: "Structural and chemical characterization of materials at the micro-and nanoscale" is acknowledged for its support. S.M. is a Postdoctoral Fellow of the Research Foundation-Flanders (FWO) and S.K. is Postdoctoral Fellow of the National Funds for Scientific Research (FNRS). S.K. and R.S. thank the Interuniversity Attraction Poles Program of the Belgian Science Policy (Project "PSI: Fundamentals of Plasma Surface Interactions") for financial support.