coatings Article
Structural and Optical Properties of Luminescent Copper(I) Chloride Thin Films Deposited by Sequentially Pulsed Chemical Vapour Deposition Richard Krumpolec 1 , Tomáš Homola 1 , David C. Cameron 1, * , Josef Humlíˇcek 2 , Ondˇrej Caha 2 , Karla Kuldová 3 , Raul Zazpe 4,5 , Jan Pˇrikryl 4 and Jan M. Macak 4,5 1
2 3 4
5
*
R & D Center for Low-Cost Plasma and Nanotechnology Surface Modifications (CEPLANT), Department of Physical Electronics, Faculty of Science, Masaryk University, Kotláˇrská 267/2, 611 37 Brno, Czech Republic;
[email protected] (R.K.);
[email protected] (T.H.) Department of Condensed Matter Physics, Masaryk University, Kotláˇrská 267/2, 611 37 Brno, Czech Republic;
[email protected] (J.H.);
[email protected] (O.C.) Institute of Physics of the Czech Academy of Sciences, v.v.i., Cukrovarnická 10, 162 00 Prague 6, Czech Republic;
[email protected] Centre of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, Nám. ˇ Legií 565, 530 02 Pardubice, Czech Republic;
[email protected] (R.Z.);
[email protected] (J.P.); Cs.
[email protected] (J.M.M.) Central European Institute of Technology, Brno University of Technology, Purkynova ˇ 123, 612 00 Brno, Czech Republic Correspondence:
[email protected]; Tel.: +420-549-49-1411
Received: 31 August 2018; Accepted: 15 October 2018; Published: 18 October 2018
Abstract: Sequentially pulsed chemical vapour deposition was used to successfully deposit thin nanocrystalline films of copper(I) chloride using an atomic layer deposition system in order to investigate their application to UV optoelectronics. The films were deposited at 125 ◦ C using [Bis(trimethylsilyl)acetylene](hexafluoroacetylacetonato)copper(I) as a Cu precursor and pyridine hydrochloride as a new Cl precursor. The films were analysed by XRD, X-ray photoelectron spectroscopy (XPS), SEM, photoluminescence, and spectroscopic reflectance. Capping layers of aluminium oxide were deposited in situ by ALD (atomic layer deposition) to avoid environmental degradation. The film adopted a polycrystalline zinc blende-structure. The main contaminants were found to be organic materials from the precursor. Photoluminescence showed the characteristic free and bound exciton emissions from CuCl and the characteristic exciton absorption peaks could also be detected by reflectance measurements. Keywords: vapour deposition; copper chloride; characterization; optical properties; XPS; crystal structure
1. Introduction Copper(I) chloride (CuCl) is a wide-bandgap, I–VII ionic semiconductor. γ-CuCl with zinc blende structure, which is stable below 407 ◦ C, has a direct bandgap with Eg = 3.395 eV at 4 K [1]. It has a high exciton binding energy of ~190 meV [2], which is higher than both GaN (25 meV) [3] or ZnO (60 meV) [4]. CuCl has been investigated for some time for its application to optoelectronic devices [5–8]. The high binding energy gives the possibility of stable, room temperature, UV emission which, together with high biexciton binding energies, enables optoelectronic effects such as bistability and four-wave mixing with the potential for new short wavelength devices [9,10]. In order to use CuCl in optoelectronic devices it has to be deposited in thin films or in arrays of nanoparticles. Thin films of CuCl have been deposited by thermal evaporation, molecular beam Coatings 2018, 8, 369; doi:10.3390/coatings8100369
www.mdpi.com/journal/coatings
Coatings 2018, 8, 369
2 of 16
epitaxy, and magnetron sputtering [11–13]. Arrays of nanoparticles of Cu halides have been produced in a matrix of glass, silicon, or organic compounds by gas or liquid phase methods, typically in a three dimensional form [6,14–17] but the size of the crystallites has been difficult to control. Atomic layer deposition (ALD) has recently been shown to be able to produce two-dimensional nanocrystalline arrays of CuCl on substrates and the size and distribution of the crystallites could be controlled by the parameters of the deposition process [18,19]. These films have shown the crystal structure of γ-CuCl and demonstrated the luminescent and optical characteristics typical of CuCl. One important feature of CuCl is that it is sensitive to moisture in its environment. Films of CuCl will hydrolyse to oxy- or hydroxy-halides after a short time and they need hermetic protection for long-term stability. Successful encapsulation by spun-on organic materials such as polysilsesquioxanes and cycloolefin copolymers has proved successful while plasma-enhanced chemical vapour deposition of SiO2 has not provided adequate encapsulation [20]. These spun-on techniques required short-term exposure of the CuCl to the atmosphere so there is still the possibility of some degradation before encapsulation. In addition, the relative thickness and probable lack of uniformity of spun-on films will be a drawback in device construction. ALD is a chemical vapour deposition technique characterised by its ability to controllably deposit ultrathin layers with extreme uniformity. In ALD, the substrate is exposed sequentially to pulses of reactant gases or vapours and each pulse forms an additional chemisorbed molecular layer on it. Between the reactant pulses, an inert gas is used as a purge gas for removing all the excess precursor molecules that have not chemisorbed or undergone exchange reactions with the surface groups, and removing the reaction byproducts [21]. A single sequence of precursor and purge pulses is known as an ALD cycle. Initially, the film formation may proceed by the nucleation and growth of individual nanocrystallites, whose size and density varies according to the number of ALD cycles used in the process [22]. The details of this nucleation process are dependent on the chemical interaction between the precursors and the initial surface chemical state of the substrate, which is affected by its pretreatment. This process of controlled nucleation provides a possible method of producing plasmonic structures in copper halides in a much more repeatable way. In the previous ALD work [18,19], the Cl precursor used, i.e., a solution of HCl in butanol, had limitations with respect to questionable stability of the vapour pressure and possible bi-reactions with butanol itself. In this publication, we report on the sequentially pulsed chemical vapour deposition of CuCl using an alternative Cl precursor which circumvents the problems of the one used in previous ALD of CuCl. We study the change in the distribution of nanocrystallites as deposition progresses. We characterise the structural, optoelectronic and chemical properties of the film and explore the use of ultrathin diffusion barrier layers deposited in situ to provide environmental protection. 2. Materials and Methods The films were deposited simultaneously on quartz glass, Si, and soda-lime glass substrates by thermal pulsed chemical vapour deposition using a TFS 200 ALD system (Beneq Oy, Espoo, Finland). The deposition temperature and chamber pressure were 125 ◦ C and 2 mbar, respectively, with 100, 200, 500, or 1000 deposition cycles. The precursor for copper was [Bis(trimethylsilyl)acetylene]-(hexafluoroacetylacetonato)copper(I) (Sigma-Aldrich, St. Louis, MO, USA), abbreviated to CuBTMSA for convenience heated to 80 ◦ C, and the precursor for chlorine was pyridine hydrochloride (Sigma-Aldrich, ≥98%) heated to 50 ◦ C. One CuCl deposition cycle was defined by the following sequence. Cu pulse (2 s) → N2 purge (4 s) → Cl pulse (3 s) → N2 purge (6 s). Nitrogen was also used as a carrier gas. Some samples were protected against oxidation and atmospheric moisture by an Al2 O3 capping layer. This capping layer was deposited in situ by ALD from trimethylaluminium (TMA, Strem Chemicals, Newburyport, MA, USA; electronic grade) and ozone (ozone generator, BMT Messtechnik, Stahnsdorf, Germany; 8 g/h output) at 150 ◦ C. One Al2 O3 ALD deposition cycle was defined by the following sequence. TMA pulse (0.1 s) → N2 purge (3s) → O3 pulse (0.3 s) → N2 purge (3 s). Ozone was used as the oxidiser here because of the fear that
Coatings 2018, 8, 369
3 of 16
using water might hydrolyse the CuCl. The capping layer consisted of 47 ALD cycles giving an Al2 O3 thickness of approximately 5 nm. The deposition chamber was connected to a nitrogen glove box with moisture and oxygen content values < 0.1 ppm. The samples were stored in the glove box and only removed for the time necessary for analyses except for those capped samples deliberately exposed to the atmosphere. ALD is characterised by growth saturation with increasing precursor dose and the presence of an ALD temperature window. In the process described here, the full range of deposition parameters has not yet been explored with these precursors to definitively prove that the CuCl deposition was purely an ALD process. Therefore, the process can only strictly be called sequentially pulsed chemical vapour deposition. We consider, however, that in comparing the growth of different samples it is in order to refer to the number of “ALD cycles” as a shorthand descriptor rather than saying “sequentially pulsed precursor-purge cycles”. The CuCl was deposited on various types of substrate: (i) silicon wafers (100) (P/Boron doped, thickness = 525 ± 25 µm, Ω = 10–30 Ω·cm); (ii) soda-lime glass; and (iii) quartz glass Herasil 102 (Heraeus, Hanau, Germany). Before the deposition the samples of size approximately 1 × 1 cm2 were cleaned in an ultrasonic bath: 5 min in acetone, 5 min in isopropanol, and dried with nitrogen. Hydrofluoric acid was used to etch native oxide from the Si wafer surfaces. Transparent samples (soda-lime glass and quartz glass) were coated with Kapton tape on the back side to allow deposition only on one side. The Kapton tape was removed prior to optical measurements. X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250Xi apparatus (ThermoFisher Scientific, Waltham, MA, USA) with monochromated Al Kα radiation (1486.6 eV) to determine the chemical composition of CuCl thin films. All samples were measured at two spots at a takeoff angle of 90◦ in 10−8 mbar vacuum at 20 ◦ C. An electron flood gun was used to compensate for charges on sample surfaces. The spectra were referenced to C–C at 284.8 eV. The spot size was 650 × 650 µm2 and the pass energy was 50 eV for the survey and 20 eV for the high resolution scan. The elemental composition was estimated from the survey spectra using Avantage ver. 5.938 software. The measurements on sample Q8 after atmospheric exposure were carried out using an Axis Supra instrument (Kratos Analytical, Manchester, UK) under slightly different conditions; spot size 700 × 300 µm2 and the pass energy was 120 eV for the survey and 20 eV for the high resolution scan. The analysis was done by CasaXPS software ver. 2.3.19PR1.0. The high-resolution spectra were fitted using XPSPEAK software (ver. 4.1) with Shirley and/or linear background. Grazing incidence X-ray diffractometry (GIXRD) was used to measure the crystalline properties of the CuCl. The measurements were taken with a SmartLab diffractometer (Rigaku, Tokyo, Japan) at grazing incidence angle α = 0.2◦ using a copper X-ray tube (1.542 Å). Normal-incidence reflectance was measured in the Vis–UV range using an Avantes 2048 pixel spectrometer (Avantes BV, Apeldoorn, The Netherlands; spectral range from 1.9 to 5.1 eV), equipped with a fibre reflectance probe [23]. The angles of incidence covered the range of 0.2◦ –1.7◦ and the nearly circular spot had a measured diameter of 0.8 mm for the 400 µm detection fibre. The small measurement spot allowed us to test the in-plane homogeneity of the samples. Bare substrates were used to collect reference signals. The photoluminescence (PL) data were obtained with using a LabRam HR Evolution (HORIBA Scientific, Kyoto, Japan) 0.8 m focal length single-stage spectrometer with CW HeCd laser excitation at 325 nm (3.8 eV). The mirror lens collected signals from approximately a 1 µm circular spot; the excitation power was kept low enough to prevent heating of the layers (~200 W cm−2 ). The PL spectra were recorded with a Peltier-cooled back-illuminated UV-sensitive CCD detector Synapse 2048 × 512. The instrument spectral range sensitivity has been corrected by the Intensity Correction System (HORIBA Scientific, Kyoto, Japan) delivered with the system. All reflectance and PL measurements were performed at room temperature (300 K).
Coatings 2018, 8, 369 Coatings 2018, 8, x FOR PEER REVIEW
4 of 16 4 of 16
3. Results and Discussion 3.1. Deposition Table Table 11 shows shows the the deposition deposition conditions conditions for for quartz quartz glass glass samples. samples. The growth habit of the films on all substrates showed individual nuclei growing in number and size as SEM images of of thethe uncapped films on glass for as the the number numberof ofcycles cyclesincreased. increased.Figure Figure1 1shows shows SEM images uncapped films on glass 100, 200,200, 500,500, andand 10001000 ALDALD cycles (abbreviated to c. to hereafter) deposited on soda-lime glass.glass. The films for 100, cycles (abbreviated c. hereafter) deposited on soda-lime The on Si substrates were similar. films on Si substrates were similar. Table 1. Table 1. Deposition Deposition parameters parameters for for films films on on quartz quartz substrates. substrates. Sample ALD Cycles Sample No. No. No.No. of of ALD Cycles 100 Q1 Q1 100 Q5 200 Q5 200 Q7 500 Q7 Q10 500 1000 Q10 Q2 1000 100 200 Q2 Q6 100 500 Q6 Q8 200 Q8 500
Capping Layer Capping Layer nono nono no no no yesno yesyes yesyes
yes
(a)(a) 100100 c.; c.; (b)(b) 200200 c.; (c) 500500 c.; and (d) Figure 1. SEM SEMimages imagesofofCuCl CuCldeposited depositedon onsoda-lime soda-limeglass: glass: c.; (c) c.; and 1000 c. Scale barsbars represent 1 μm. (d) 1000 c. Scale represent 1 µm.
As the number number of of cycles cyclesincreased, increased,the thediameter diameterofofthe thecrystallites crystallites also increased. There also increased. There waswas an an initial nucleation phase and then the surface was covered in an array of crystallites. The areal initial nucleation phase and then the surface was covered in an array of crystallites. The areal density density of the crystallites a decrease withnumber the number of cycles; some form of agglomeration of the crystallites showedshowed a decrease with the of cycles; some form of agglomeration or or ripening evident. Dimensional change of the crystallites place processes: ripening waswas evident. Dimensional change of the crystallites cancan taketake place by by twotwo processes: (i) (i) Coalescence, where there pooradhesion adhesionbetween betweenthe thenanocrystallites nanocrystallitesand andthe the substrate substrate and and they Coalescence, where there is is poor are mobile enough enough to move across it and to coalesce and (ii) Ostwald ripening, in which there is strong adhesion of the nanocrystallites to the surface and material transfer takes place by the diffusion of atoms across the surface. In this case, there is no significant coalescence of nanocrystallites therefore there is place. Initially, thethe crystallites hadhad somewhat “liquid-like” is an anOstwald Ostwaldripening ripeningprocess processtaking taking place. Initially, crystallites somewhat “liquidlike” shapes whereas the crystallites in the 1000 c. films had a more facetted appearance. This change in habit was accompanied by differences in the chemical composition of the films as discussed later.
Coatings 2018, 8, 369
5 of 16
shapes2018, whereas thePEER crystallites Coatings 2018, 8, x x FOR FOR PEER REVIEW Coatings 8, REVIEW
in the 1000 c. films had a more facetted appearance. This change of in 16 55 of 16 habit was accompanied by differences in the chemical composition of the films as discussed later. It is It is is possible possible that was this caused was caused caused bylonger the longer longer deposition time required required for the thicker thicker film giving giving possible that that this by the deposition time required for the thicker film giving more It this was by the deposition time for the film more time for the The the the increases time for reconfiguration of theof crystallites. The size theof in thein filmsfilms increases with more time for reconfiguration reconfiguration of the crystallites. crystallites. Theofsize size ofcrystallites the crystallites crystallites inCuCl the CuCl CuCl films increases with the number of cycles as shown in Figure 2a, while Figure 2b shows this behaviour in comparison the number of cycles as shown in Figure 2a, while Figure 2b shows this behaviour in comparison with with the number of cycles as shown in Figure 2a, while Figure 2b shows this behaviour in comparison with that reported reported previously different precursors [19]. that reported previously usingusing different precursors [19]. [19]. with that previously using different precursors Figure 22 demonstrates demonstrates that that the the rate rate of of increase of crystallite crystallite diameter is similar with both demonstrates Figure increase of diameter is similar with both the the current and the previous precursors, once the initial nucleation has taken place, and that the growth has taken place, and that thethe growth on current and and the the previous previousprecursors, precursors,once oncethe theinitial initialnucleation nucleation has taken place, and that growth on crystalline and noncrystalline substrates is similar. similar. This similarity similarity is not not unexpected unexpected since the crystalline andand noncrystalline substrates is similar. This similarity is not unexpected since thesince reactant on crystalline noncrystalline substrates is This is the reactant is HCl in here both itcases; cases; hereby it decomposition is derived derived by byofdecomposition decomposition of pyridine pyridine hydrochloride, is HCl in is both cases; is derived pyridine hydrochloride, previously the HCl reactant HCl in both here it is of hydrochloride, previously the HCl vapour came from an HCl/butanol solution. The nucleation behaviour is vapour came from anvapour HCl/butanol nucleationsolution. behaviour is somewhat with previously the HCl came solution. from an The HCl/butanol The nucleationdifferent: behaviour is somewhat different: with PyrHCl there is iswith slower nucleation. precursor However,the with the HCl/butanol HCl/butanol PyrHCl there is slowerwith nucleation. However, the HCl/butanol nucleation behaviour somewhat different: PyrHCl there slower nucleation. However, with the precursor the nucleation behaviour depended on the growth cycle parameters so further exploration depended on the growth cycle parameters so further exploration of the deposition parameters with precursor the nucleation behaviour depended on the growth cycle parameters so further exploration of the deposition deposition parameters with may show differences PyrHCl may also parameters show differences in nucleation behaviour. of the with PyrHCl PyrHCl may also also show differences in in nucleation nucleation behaviour. behaviour.
(a) (a)
(b) (b)
Figure 2. 2. (a) (a) Crystallite Crystallite diameter diameter vs. vs. number number of atomic atomic layer layer deposition deposition (ALD) (ALD) cycles cycles using using pyridine pyridine number of Figure hydrochloride (PyrHCl) precursor and (b) comparison with previous results using HCl/butanol hydrochloride (PyrHCl) precursor and (b) comparison with previous results using HCl/butanol as hydrochloride (PyrHCl) precursor comparison previous results using HCl/butanol as precursor (HCl). PyrHCl–Si, PyrHCl–glass, HCl (short purge), HCl (long In (b) error precursor (HCl). PyrHCl–Si, ++ PyrHCl–glass, PyrHCl–glass, HCl HCl (short (short purge), purge), HCl HCl (long (long purge). purge). In In (b) (b) error error bars have have been been omitted omitted for for clarity. clarity. bars have been omitted for clarity. bars
3.2. Crystal Crystal Structure Structure 3.2. 3.2. Crystal Structure Films for for XRD XRD analysis analysis were were deposited deposited on on quartz quartz glass glass both both without without aa capping capping layer layer and and with with aa Films Films for XRD analysis were deposited on quartz glass both without a capping layer and with a capping layer of ~5 nm of aluminium oxide deposited in situ in order to prevent the films undergoing capping layer of ~5 nm of aluminium oxide deposited in situ in order to prevent the films undergoing capping layer of ~5 nm of aluminium oxide deposited in situ in order to prevent the films undergoing hydrolysis in atmospheric moisture. The crystal crystal structure structure was was determined determined by by X-ray X-ray diffraction diffraction (XRD) (XRD) hydrolysis in in atmospheric atmospheric moisture. moisture. The The hydrolysis crystal structure was determined by X-ray diffraction (XRD) using GIXRD. GIXRD. The The results results are are shown shown in Figure Figure 3a,b. 3a,b. The The films films show show the zinc zinc blende crystal crystal structure structure using using GIXRD. The results are shown in in Figure 3a,b. The films show the the zinc blende blende crystal structure of γ-CuCl with no evidence of any other crystalline phases. The films with an Al O capping layer of γ-CuCl γ-CuCl with with no no evidence evidence of of any any other other crystalline crystalline phases. phases. The The films films with with an Al2222O O3333 capping capping layer layer of an Al ◦ ◦ also showed showed the the same additional peaks at ~36 and and 45 (marked (marked with also same structure structure with with small small additional broad broad also showed the same structure with small additional broad peaks peaks at at ~36° ~36° and 45° 45° (marked with with asterisks). These do not fit CuCl (PDF 9001506), CuO (PDF 1011148), Cu O (PDF 1010926), or other other 2 2 asterisks). These These do do not not fit fit CuCl CuCl222 (PDF (PDF 9001506), 9001506), CuO CuO (PDF (PDF 1011148), 1011148), Cu Cu22O O (PDF (PDF 1010926), 1010926), or or asterisks). other likely compounds. notnot clear what material they they represent. The crystallites showedshowed largely random likely compounds.ItIt Itis is clear what material represent. The crystallites largely likely compounds. is not clear what material they represent. The crystallites showed largely orientation although with some preferential (220) orientation for thinner films. For randomly oriented random orientation orientation although although with with some some preferential preferential (220) (220) orientation orientation for for thinner thinner films. films. For For randomly randomly random films the films intensity ratio I220 /I 0.60 and the ratio here ishere much higher.higher. However, it mustitbe borne 111 I≈ 220 111 oriented the intensity ratio 220 /I 111 ≈ 0.60 and the ratio is much However, must be oriented films the intensity ratio I220/I111 ≈ 0.60 and the ratio here is much higher. However, it must be in mind that because of the glancing angle X-ray incidence, any preferential orientation is not parallel borne in mind that because of the glancing angle X-ray incidence, any preferential orientation is not borne in mind that because of the glancing angle X-ray incidence, any preferential orientation is not to the substrate. From these results can conclude that the bulk of the film is γ-CuCl. parallel to the the substrate. substrate. From thesewe results we can can conclude conclude that γ-CuCl. parallel to From these results we that the the bulk bulk of of the the film film is is γ-CuCl.
Coatings 2018, 8, 369 Coatings 2018, 8, x FOR PEER REVIEW
(a)
6 of 16 6 of 16
(b)
Figure Figure 3. 3. XRD XRD spectra spectra of of (a) (a) uncapped uncapped and and (b) (b) capped capped CuCl CuCl films films on on quartz quartz substrates. substrates. Unidentified Unidentified peaks peaks marked marked with with asterisks. asterisks. Note: Note: The “missing” “missing” CuCl CuCl peaks peaks ((200), ((200), (222), (222), (400), (400), and and (420)) (420)) have have low intensities intensities compared compared to to the the (111) (111) peak peak for for randomly randomlyoriented orientedmaterial material(PDF (PDF1010991). 1010991).
3.3. Chemical Composition 3.3. Chemical Composition The chemical composition was investigated by XPS. The scans were taken before and after Ar The chemical composition was investigated by XPS. The scans were taken before and after Ar sputter-cleaning of the substrate to investigate the extent of surface contamination. Table 2 shows the sputter-cleaning of the substrate to investigate the extent of surface contamination. Table 2 shows the analysis of survey scans of the deposited material for the unsputtered samples while Table 3 shows analysis of survey scans of the deposited material for the unsputtered samples while Table 3 shows the relative concentrations of Cu and Cl after Ar sputter cleaning for 150 s. The XPS analysis was the relative concentrations of Cu and Cl after Ar sputter cleaning for 150 s. The XPS analysis was not not performed in situ so there will be significant amounts of adventitious carbon from environmental performed in situ so there will be significant amounts of adventitious carbon from environmental contamination between deposition and analysis for the unsputtered films. The sputtered samples contamination between deposition and analysis for the unsputtered films. The sputtered samples show significantly reduced C 1s signal. There is evidence, however, of the presence of carbon as a show significantly reduced C 1s signal. There is evidence, however, of the presence of carbon as a residue from the organic precursors (see below). There is also some fluorine content in all films which residue from the organic precursors (see below). There is also some fluorine content in all films which must come from the CuBTMSA precursor. There is significant uncertainty in the Cu/Cl ration due to must come from the CuBTMSA precursor. There is significant uncertainty in the Cu/Cl ration due to the dispersed crystallites which allow a large signal from the substrate material to be detected and the dispersed crystallites which allow a large signal from the substrate material to be detected and because the relative sensitivity factor for the XPS quantification for the Cl 2p peak is much lower than because the relative sensitivity factor for the XPS quantification for the Cl 2p peak is much lower than for for the Cu 2p peaks by a factor of ~11. Nevertheless, for the unsputtered samples the Cu/Cl ratio the Cu 2p peaks by a factor of ~11. Nevertheless, for the unsputtered samples the Cu/Cl ratio is lower is lower than the expected 1/1 ratio for CuCl; however, the sputter-cleaned samples show close to than the expected 1/1 ratio for CuCl; however, the sputter-cleaned samples show close to the the stoichiometric ratio. The low CuCl ratio for the unsputtered samples arises from the different stoichiometric ratio. The low CuCl ratio for the unsputtered samples arises from the different composition of the surface layer. composition of the surface layer. Table 2. Element concentration obtained from X-ray photoelectron spectroscopy (XPS) survey scan Table 2. Element concentration obtained from X-ray photoelectron spectroscopy (XPS) survey scan (not Ar-sputtered). (not Ar-sputtered). Concentration of Elements (at.%) Concentration of Elements (at.%) Cycles Cycles Cu 2p 2p2p AlAl 2p2p OO1s1s Cu 2p Cl 2p Cl 2p C 1sC 1s Si Si Q1Q1 100100 0.5 0.5 3.1 3.1 19.619.6 29.9 0.00.0 46.6 29.9 46.6 Q5Q5 200200 1.9 1.9 2.7 2.7 12.912.9 31.3 0.00.0 50.5 31.3 50.5 NoNo cap cap Q7 500500 1.4 1.4 2.7 2.7 15.715.7 31.1 0.00.0 48.6 Q7 31.1 48.6 Q10 1000 3.8 10.2 50.7 12.8 0.0 21.0 Q10 1000 3.8 10.2 50.7 12.8 0.0 21.0 Q2Q2 100100 0.3 0.3 0.8 0.8 22.122.1 8.48.4 21.9 46.3 21.9 46.3 Capped Q6 200 0.2 0.5 23.9 6.0 23.2 45.7 Q6 200 0.2 0.5 23.9 6.0 23.2 45.7 Capped Q8 500 0.2 0.7 25.9 5.5 24.0 43.5 Q8 500 0.2 0.7 25.9 5.5 24.0 43.5 Sample Sample
F 1s 1s F 0.4 0.7 0.7 0.5 0.5 0.6 0.6 0.3 0.3 0.4 0.4 0.3 0.3
Cu/Cl Cu/Cl 0.1 0.1 0.7 0.7 0.5 0.5 0.4 0.4 0.3 0.3 0.4 0.4 0.3 0.3
Coatings 2018, 8, 369
7 of 16
Table 3. Relative concentrations of Cu and Cl obtained from XPS (after Ar sputtering). Coatings 2018, 8, x FOR PEER REVIEW
7 of 16
Relative Concentration of Cu and Cl Table 3. Relative concentrations Sample Cycles of Cu and Cl obtained from XPS (after Ar sputtering). Cu 2p Cl 2p Cu/Cl Relative Concentration of Cu and Cl Sample Cycles Q7 500 55 45 1.2 Cu 2p Cl 2p Cu/Cl Q10 1000 49 51 1.0 Q7 500 5547 4553 1.2 Q8 (capped) 500 0.9 Q10 1000 49 51 1.0 Q8 (capped) 500 47 53 0.9
High-resolution scans of the Cl 2p peak from the uncapped samples with peak fittings are shown in Figure 4 for the unsputtered sputter-cleaned samples. Forsamples Q1, Q5, with and Q7 the Cl 2p are peak is well High-resolution scans ofand the Cl 2p peak from the uncapped peak fittings shown fitted by one4 2p consisting of sputter-cleaned 2p3/2 and 2p1/2samples. components separated in Figure fordoublet the unsputtered and For Q1, Q5, and by Q71.6 theeV. Cl Figure 2p peak4a is is representative ofone these. The binding energy are2p consistent with those givenbyfor chlorides well fitted by 2p doublet consisting ofvalues 2p3/2 and 1/2 components separated 1.6copper eV. Figure 4a is representative of these. binding between energy values with those given forthe copper although they cannot easily The distinguish Cu(I) are andconsistent Cu(II) chloride. In general, CuCl2 chlorides although they cannot easily distinguish Cu(I) and Cu(II) chloride. In general, the should appear at a higher binding energy [24–26]. between Taking into account the XRD results which show CuCl 2 should appear atthat a higher energy Taking account theClO XRD results which only CuCl we conclude peak binding at ~199.0 eV is [24–26]. CuCl. There isinto no significant formation: this x show only CuCl we conclude that peak at ~199.0 eV is CuCl. There is no significant ClO x formation: would give rise to 2p3/2 peaks at energies approximately 206–208 eV [27], and there is no evidence this would risespectra. to 2p3/2 peaks at energies 206–208 and there evidence of these in thegive Cl 2p The peak detailsapproximately are given in Table 4. eV For[27], sample Q10, is if no a single peak of these in the Cl 2p spectra. The peak details are given in Table 4. For sample Q10, if a single peak fitting is used, the fitting error is significantly higher. However, the measured Q10 signal can be well fitting is used, the fitting error is significantly higher. However, the measured Q10 signal can be well fitted with two 2p components separated by 0.6 eV (Figure 4b). This is consistent with the emergence fitted with two 2p components separated by 0.6 eV (Figure 4b). This is consistent with the emergence of two different bonding environments for Cl. The lower energy peak is characteristic of HCl·nH2 O of two different bonding environments for Cl. The lower energy peak is characteristic of HCl·nH2O (198.4 eV) [28] and is also consistent with CuCl bound to an organic ligand [29]. The peak area ratio (198.4 eV) [28] and is also consistent with CuCl bound to an organic ligand [29]. The peak area ratio between this component and the one at slightly lower energy is 1.4 indicating significant surface between this component and the one at slightly lower energy is 1.4 indicating significant surface contamination. Figure 4c shows the Cl 2p peak for Q7 after Ar sputter-cleaning. For the Ar-sputtered contamination. Figure 4c shows the Cl 2p peak for Q7 after Ar sputter-cleaning. For the Ar-sputtered Q10Q10 sample, again thethe best case, the theadditional additionalpeak peakappears appears higher sample, again bestfitfitisiswith withtwo twodoublets. doublets. In In this this case, atat higher energy, consistent with the existence of an organic chloride [28,30]. energy, consistent with the existence of an organic chloride [28,30].
(a)
(b)
(c)
(d)
Figure 4. XPS spectrafor foruncapped uncapped quartz quartz samples peaks: (a)(a) Unsputtered Q7 Q7 Figure 4. XPS ClCl 2p2pspectra samplesshowing showingfitted fitted peaks: Unsputtered 500 c.; (b) unsputtered Q10 1000 c.; (c) Ar-sputtered Q7 500 c.; and (d) Ar-sputtered Q10 1000 c. 500 c.; (b) unsputtered Q10 1000 c.; (c) Ar-sputtered Q7 500 c.; Ar-sputtered Q10 1000 c.
Coatings 2018, 8, 369
8 of 16
Table 4. Components of Cl 2p XPS peaks. Coatings 2018, 8, x FOR PEER REVIEW
Sample
Q7, unsputtered Q7, Ar-sputtered Sample Q10, unsputtered Q7, unsputtered Q10, Q7,Ar-sputtered Ar-sputtered Q8 (capped), Ar-sputtered Q10, unsputtered
Cycles
Peak
2p3/2 Binding Energy (eV)
Area P2/P1
500 TableP1 – 4. Components of199.0 Cl 2p XPS peaks 500 P1 199.6 – P1 198.9 Cycles Peak 2p3/2 Binding Energy (eV) Area P2/P1 1000 1.0 P2 198.3 500 P1 199.0 – P1 199.1 500 P1 199.6 – 1000 0.2 P2 200.5 P1 198.9 500 P1 199.3 – 1000 1.0 P2 198.3
P1
199.1
8 of 16
Peak FWHM (eV) 1.2 1.2 1.2 (eV) Peak FWHM 1.21.1 1.2 1.2 1.4 1.22.2 1.1
1.2
Q10, Figure 5 Ar-sputtered shows the high1000 resolution 2p3/2 and 2p1/2 raw data0.2scans scan of P2 200.5 1.4 Cu and the decomposition ofAr-sputtered the 2p3/2 peaks Q7 is– representative Q8 (capped), 500 intoP1individual components. 199.3 2.2 of the other samples with less than 1000 c. deposition. Table 5 shows the peak positions and full width half Figure 5 shows the high resolution 2p3/2 and 2p1/2 raw data scans scan of Cu and the decomposition maxima (FWHM). of the 2p3/2 peaks individual Q7 isCu representative of the other samples with less than but 2+ peaks solely It is difficult tointo separate the components. Cu0 , Cu+ , and by their binding energies 1000 c. deposition. Table 5 shows the peak positions and full width half maxima (FWHM). taking into account the XRD spectra,0 peak P1 is assigned to CuCl which has been previously seen at It is difficult to separate the Cu , Cu+, and Cu2+ peaks solely by their binding energies but taking into 932.4–932.6 eV [31,32]. Peak P2 (~934 eV) could be indicative of CuO, CuCl2 , or Cu with an organic account the XRD spectra, peak P1 is assigned to CuCl which has been previously seen at 932.4–932.6 eV 2+ species should be indicated by an or organo-chlorate ligand [33–36]. presence [31,32]. Peak P2 (~934 eV) could beHowever, indicative the of CuO, CuCl2,of or Cu Cu with an organic or organo-chlorate obvious shake-up satellite peak at ~940 eV [37] and there is no evidence in Q7 both with and 2+ ligand [33–36]. However, the presence of Cu species should be indicated of by this an obvious shake-up without sputtering (Figure 5a,b). Neither is there evidence for chlorate bonding in the Cl 2p spectra. satellite peak at ~940 eV [37] and there is no evidence of this in Q7 both with and without sputtering + Therefore, contains Cu evidence bonds and is ascribed toinCu(I) organic bonding. The amount (FigureQ7 5a,b). Neitheronly is there for P2 chlorate bonding the Cl 2p spectra. Therefore, Q7 + bonds and P2 is ascribed to Cu(I) organic bonding. The amount of the Cu organic only Cu of thecontains Cu organic contamination in both Q7 and Q10 is clearly less after Ar sputtering (Figure 5d,f). contamination in both Q7 and Q10 is clearly after satellite Ar sputtering 5d,f). Figure 5a,e, Q10 Figure 5a,e, Q10 (1000 c., unsputtered), showsless a large peak(Figure at the expected energy for Cu2+ 2+ (1000 unsputtered), large satellite peak at the may expected energy forfor Cusome at ~940 eV together at ~940 eVc.,together with shows a peaka at 936.7 eV, P3, which be evidence fluoride bound to with a peak at 936.7 eV, P3, which may be evidence for some fluoride bound to organic material [38]. organic material [38]. However, the Q10 Ar-sputtered sample, Figure 5f, shows the same three peaks However, the Q10 Ar-sputtered sample, Figure 5f, shows the same three peaks as for the Q10 as for the Q10 unsputtered sample with no evidence of a satellite peak. Therefore, it is not clear which unsputtered sample with no evidence of a satellite peak. Therefore, it is not clear which of the peaks of theispeaks is to related to2+the Cu2+ satellite peak.beIt that maythe be that the relatively lower intensity of Ar P3 after related the Cu satellite peak. It may relatively lower intensity of P3 after Ar sputtering gives rise to a much smaller satellite peak which is not visible in Figure 5f. sputtering gives rise to a much smaller satellite peak which is not visible in Figure 5f.
(a)
(b)
(c)
(d) Figure 5. Cont.
Coatings 2018, 8, 369
9 of 16
Coatings 2018, 8, x FOR PEER REVIEW
9 of 16
(e)
(f)
Figure 5. XPS Cupeaks, 2p peaks, uncapped quartz samples:(a) (a)Q7 Q7500 500c.c. and and Q10 1000 Figure 5. XPS Cu 2p uncapped quartz samples: 1000 c., c.,unsputtered; unsputtered;(b) (b) Q7 3/2 peak, Q7, unsputtered; (d) decomposed 500Q10 c. and Q10c., 1000 c., Ar-sputtered; (c) decomposed 500 c.Q7 and 1000 Ar-sputtered; (c) decomposed 2p2p 3/2 peak, Q7, unsputtered; (d) decomposed 3/2 peak, Q7, Ar-sputtered; (e) decomposed 2p3/2 peak, Q10, unsputtered; and (f) decomposed 2p3/2 2p3/2 2p peak, Q7, Ar-sputtered; (e) decomposed 2p3/2 peak, Q10, unsputtered; and (f) decomposed 2p3/2 peak, Ar-sputtered. peak, Q10,Q10, Ar-sputtered.
Table 5. 5. Peak parameters parameters for for Cu Cu 2p 2p XPS. XPS. Table Sample Sample
Cycles Peak PeakCu Cu2p 2p3/2 Cycles 3/2 P1 P1 Q7, unsputtered 500 Q7, unsputtered 500 P2 P2 Q7, Ar-sputtered 500 Q7, Ar-sputtered 500
P1 P1 P2 P2
Q10, unsputtered 10001000 Q10, unsputtered
P1 P1 P2 P2 P3 P3 SS
Q10, Ar-sputtered 10001000 Q10, Ar-sputtered
P1 P1 P2 P2 P3 P3
Q8, Ar-sputtered Q8, Ar-sputtered 500500
P1 P1 P2 P2
Binding Energy (eV) Binding Energy (eV) FWHM FWHMAssignment Assignment 932.6 1.3 CuCl 932.6 1.3 CuCl 933.7 2.1 2.1 Cu(I) org. 933.7 Cu(I) org. 932.8 1.3 CuCl 932.8 1.3 CuCl 933.7 2.1 2.1 Cu(I) org. 933.7 Cu(I) org. 932.6 1.3 CuCl 932.6 1.3 CuCl 934.2 2.3 2.3 Cu(I) org. 934.2 Cu(I) org. 936.7 3.0 3.0 organofluoride 936.7 organofluoride 940.3 940.3 3.5 3.5 Cu2+ Cu2+ 932.5 1.5 1.5 CuCl CuCl 932.5 933.8 2.0 2.0 Cu(I) org. 933.8 Cu(I) org. 936.7 organofluoride 936.7 3 3 organofluoride 933.2 933.2 934.7 934.7
1.8 1.8 2.1 2.1
CuCl CuCl Cu(I) org. Cu(I) org.
Observation of the O 1s spectra yields further information. The spectra for Q7 and Q10 before
Observation of the O 1s spectra yields further information. The spectra for Q7 and Q10 before and and for Q10 after Ar sputtering are shown in Figure 6. The samples are dominated by the peak at for Q10 after Ar sputtering are shown in Figure 6. The samples are dominated by the peak at 533.3 eV 533.3 eV due to SiO2 [39] from the quartz since the substrate coverage is not complete. Both due to SiO from theFigure quartz since substrate is notiscomplete. 2 [39] samples, unsputtered 6a,b, alsothe show a peak atcoverage ~534 eV which probably OBoth in anunsputtered organic samples, Figure 6a,b, also a peak at ~534 precursor eV whichmaterial is probably O film. in anIn organic ligand indicating ligand indicating someshow residual CuBTMSA in the addition, Figure 6b someshows residual CuBTMSA precursor the film. Figure shows a large a large additional peak atmaterial 535.8 eV,inwhich is at In anaddition, energy which has6bbeen ascribed toadditional an O peak atom at 535.8 eV, which at an energy which been ascribed an to O atom linked todecomposed a CF containing linked to a CFiscontaining ligand [30].has This again is likelytodue some partially CuBTMSA is representative of alldecomposed the sputtered samples and shows only the SiO2 6c is ligand [30]. Thisprecursor. again is Figure likely 6c due to some partially CuBTMSA precursor. Figure peak indicating oxygen bonded organic material only on2 the surface. representative of allthat the the sputtered samplestoand shows only isthe SiO peak indicating that the oxygen the material Cl 2p, Cu is 2p,only and on O 1sthe results are taken together with the XRD results, it is clear that bonded toWhen organic surface. the bulk of the films consists of γ-CuCl with a certain amount of organic contamination from the When the Cl 2p, Cu 2p, and O 1s results are taken together with the XRD results, it is clear that the precursor molecules. This is confirmed by the presence of significant carbon content even in the Arbulk of the films consists of γ-CuCl with a certain amount of organic contamination from the precursor sputtered samples. The unsputtered samples show, as is to be expected, greater organic molecules. This is confirmed by the presence of significant carbon even the Ar-sputtered contamination, including from F-containing organic fragments andcontent HCl from the in CuBTMSA and samples. The unsputtered samples show, as is to be expected, greater organic contamination, including PyrHCl precursors, respectively. The exact nature of the bonding is the subject of further from investigation. F-containing organic fragments and HCl from the CuBTMSA and PyrHCl precursors, respectively. The exact nature of the bonding is the subject of further investigation.
Coatings 2018, 8, 369 Coatings Coatings 2018, 2018, 8, 8, xx FOR FOR PEER PEER REVIEW REVIEW
(a)
10 of 16 10 10 of of 16 16
(b)
(c)
Figure 6. for uncapped quartz samples: (a) (b) Figure 6. 6. Decomposition Decomposition of OO1s 1s1speaks peaks for uncapped quartz samples: (a) Q7 Q7 500 c., unsputtered; unsputtered; (b) Figure Decompositionof ofO peaks for uncapped quartz samples: (a) 500 Q7 c., 500 c., unsputtered; Q10 1000 c., unsputtered; and (c) Q10 1000 c., Ar-sputtered. Q10 1000 c., unsputtered; and (c) (c) Q10 1000 c., c., Ar-sputtered. (b) Q10 1000 c., unsputtered; and Q10 1000 Ar-sputtered.
Thin Thin uncapped uncapped CuCl CuCl films films will will hydrolyse hydrolyse after after approximately approximately one one day. day. Figure Figure 77 and and Tables Tables 3–5 3–5 show show measurements measurementson onthe thesputter sputtercleaned cleanedsample sampleQ8 Q8(500 (500c., c.,Al Al222O O333 capped) capped) after after exposure exposure to to normal normal atmosphere and tabulated data areare very similar to those of the atmospherefor forapproximately approximately3 3weeks. weeks.The Thecurves curves and tabulated data very similar to those of uncapped samples with the same number of growth cycles kept in an inert atmosphere (Figures the uncapped samples with the same number of growth cycles kept in an inert atmosphere (Figures 4c 4c and and 5e). 5e). They evidence of of degradation, degradation, indicating indicating that that 55 nm nm of of oxide oxide capping capping layer layer provides provides They show show no no evidence an effective barrier to hydrolysis of the films. an effective barrier to hydrolysis of the films.
(a)
(b)
Figure peaksof ofQ8 Q8(500 (500c. capped,Ar-sputtered) Ar-sputtered)after after Figure 7. Decomposition of (a) Cl 2p and (b) Cu 2p 3/2 Figure7. 7.Decomposition Decompositionof of(a) (a)Cl Cl2p 2pand and(b) (b)Cu Cu2p 2p3/2 3/2 peaks peaks of Q8 (500 c.c.capped, capped, Ar-sputtered) after approximately approximately weeks in normal air. approximately333weeks weeksin innormal normalair. air.
3.4. Photoluminescence 3.4. Photoluminescence The photoluminescence signals obtained from sample Q10 (1000 c., uncapped) were easily The photoluminescence signals obtained from sample Q10 (1000 c., uncapped) were easily measurable at room temperature. Shown in Figure 8 is the experimental spectrum, decomposed measurable at room temperature. Shown in Figure 8 is the experimental spectrum, decomposed into into three Gaussian bands located on a linear background. The sum fits the measured data within three Gaussian bands located on a linear background. The sum fits the measured data within background noise. The band parameters are listed in Table 6. We use band area as the measure of background noise. The band parameters are listed in Table 6. We use band area as the measure of its its strength. The location of the PL maximum is very close to the position of the strongest PL2 band, strength. The location of the PL maximum is very close to the position of the strongest PL2 band, coinciding with the position of the absorption peak PL2 (3.248 eV) of Table 6 within experimental coinciding with the position of the absorption peak PL2 (3.248 eV) of Table 6 within experimental uncertainty. The weaker but discernible high-energy PL3 band at 3.308 eV lies 0.06 eV above PL2. uncertainty. The weaker but discernible high-energy PL3 band at 3.308 eV lies 0.06 eV above PL2. The lower energy band PL1 is situated at 3.225 eV, approximately 0.10 eV lower than PL2. These results The lower energy band PL1 is situated at 3.225 eV, approximately 0.10 eV lower than PL2. These are similar to those observed on CuCl films obtained by thermal evaporation [40] and magnetron results are similar to those observed on CuCl films obtained by thermal evaporation [40] and sputtering [41]. Those showed that at low temperature the emission peak could be resolved into magnetron sputtering [41]. Those showed that at low temperature the emission peak could be three components which broadened into one composite peak at room temperature. The main peak resolved into three components which broadened into one composite peak at room temperature. The was identified as the Z3 free exciton peak observed at energy 3.227 eV at 15 K. This peak shifted to main peak was identified as the Z33 free exciton peak observed at energy 3.227 eV at 15 K. This peak higher energy of 3.243 eV at room temperature, a shift of approximately 0.016 eV [42]. This peak is shifted to higher energy of 3.243 eV at room temperature, a shift of approximately 0.016 eV [42]. This peak is almost at the same energy as the main peak reported here (3.248 eV). The peak shown here
Coatings 2018, 8, 369
11 of 16
Coatings 2018, 8, x FOR PEER REVIEW Coatings 2018, 8, x FOR PEER REVIEW
11 of 16 11 of 16
almost thedecomposed same energyinto as the main peak reported here (3.248 eV). The peakresults shownand here can alsofor be can alsoatbe three components. By comparison with these allowing can also be decomposed into three components. By comparison with these results and allowing for decomposed into three components. By comparison with these results and allowing for the shift with the shift with temperature, the main peak indicating the most probable radiative channel is ascribed the shift temperature, mainpeak peak indicating theradiative most probable radiative channel temperature, the main peak the indicating the we most channel is be ascribed to the Z3ascribed exciton. to the Z3 with exciton. The lower energy seeprobable at 3.225 eV (385.4 nm) can ascribed to is the I1 peak to the Z 3 exciton. The lower energy peak we see at 3.225 eV (385.4 nm) can be ascribed to the I 1 peak The lower energy peak we see at 3.225 eV (385.4 nm) can be ascribed to the I peak due to an exciton due to an exciton bound to an impurity, probably a Cu vacancy [43]. The higher1 energy peak at 3.308 eV due to an exciton bound to an impurity, probably a Cu vacancy [43]. The higher energy peak at 3.308 eV9 bound to an impurity, probably a Cu vacancy [43]. The higher energy peak at 3.308 eV (375.8 nm) can (375.8 nm) can be ascribed to the Z1,2 exciton. PL mapping across the sample shown in Figure 2 step (375.8 nm)μm can be Zascribed to the Zintensity 1,2 exciton. PL the mapping across inµm Figure 9 be ascribed to2,the PL mapping across sample shown in sample Figure 9shown (32 2, × due 32 1,2 (32 × 32 step 4 exciton. μm) shows varying by a factor of the approximately to ,local 2 (32 × 32 μm , step 4 μm) shows varyinginby a factor approximately 2,ofdue to local 4 µm) shows intensity varying by aintensity factor approximately 2, dueof towidth local variations the effective variations of the effective film thickness. Noofchanges peak position, or shape were observed. variations of the effective film thickness. No changes in peak position, width or shape were observed. film thickness. No changes in peak position, width or shape were observed.
Figure 8. Photoluminescence emission intensity vs. photon energy showing resolved peaks for Q10 Figure 8. Photoluminescence Photoluminescence emission emission intensity vs. vs. photon photon energy energy showing showing resolved resolved peaks peaks for for Q10 Q10 Figure (1000 c. uncapped sample). The dashedintensity line is the fitted peak. (1000 c. uncapped uncapped sample). sample). The The dashed dashed line line is is the the fitted fitted peak. Table used in in modelling modelling the the PL PL spectrum spectrum of of Figure Figure 8. 8. Table 6. 6. Best-fit Best-fit parameters parameters of of the the Gaussian Gaussian bands bands used Table 6. Best-fit parameters of the Gaussian bands used in modelling the PL spectrum of Figure 8.
Band Band Band PL1 PL1PL1 PL2PL2 PL3 PL3PL2 PL3
(a) (a)
Strength(a.u.) (a.u.) Strength Strength (a.u.) 5.8 ± 0.3 5.8 5.8±±±0.3 0.3 13.1 0.3 13.1 ± 0.3 13.1 ± 0.3 0.7±±0.3 0.3 0.7 0.7 ± 0.3
Position (eV) Position (eV) Position (eV) 3.225 ± 0.010 3.225 ± 0.010 3.225 0.010 3.248±±± 0.010 0.010 3.248 3.248 ± 0.010 3.308±± 0.010 0.010 3.308 3.308 ± 0.010
Width (eV) Width (eV) Width (eV) 0.097 ± 0.010 0.097 ± 0.010 0.097 ±± 0.010 0.056 0.010 0.056 ± 0.010 0.056 ±±0.010 0.054 0.02 0.054 ± 0.02 0.054 ± 0.02
(b) (b)
Uncapped quartz quartz sample sample Q10, Q10, 1000 1000 c. (a) (a) White White light light photography photography and and (b) (b) integral integral PL Figure 9. Uncapped Figure 9. (arb. Uncapped quartz Q10, c. (a) White photography and (b) integral PL intensity units) over thesample same area of 1000 the substrate with light 4× ×44μm µm22resolution. resolution. intensity (arb. units) over the same area of the substrate with 4 × 4 μm2 resolution.
3.5. Reflectance Reflectance 3.5. 3.5. Reflectance The measured (Q10 1000 c.)c.) is shown in The measured reflectance reflectancespectrum spectrumofofthe thethickest thickestuncapped uncappedCuCl CuClfilm film (Q10 1000 is shown Figure 10a. It It shows a asignificant increase reflected intensity compared to bare substrate, The measured reflectance spectrum ofof the thickest uncapped CuCl film (Q10 1000 c.) substrate, is shown in Figure 10a. shows significant increase ofthe the reflected intensity compared to the the bare in Figure 10a. It shows a significant increase of the reflected intensity compared to the bare substrate, which is indicative of a larger optical density of the film. In addition, a fairly strong spectral structure which is indicative of a larger optical the film. In fairly strong spectralThe structure with maximum intensity at about 3.3density eV has of apparently ataddition, least twoanarrow components. latter with maximum intensity at about 3.3 eV has apparently at least two narrow components. The latter
Coatings 2018, 8, 369
12 of 16
which indicative of a larger optical density of the film. In addition, a fairly strong spectral structure Coatingsis2018, 8, x FOR PEER REVIEW 12 of 16 with maximum intensity at about 3.3 eV has apparently at least two narrow components. The latter are better seenseen in the differentiated spectra plotted in Figure 10b.10b. The The differentiation enhances the are better in twice the twice differentiated spectra plotted in Figure differentiation enhances sharper structures and suppresses the flatthe background. Based onBased the narrow in the derivative the sharper structures and suppresses flat background. on thestructures narrow structures in the spectra, we have fitted measured with areflectance model consisting a flat background derivative spectra, wethehave fitted reflectance the measured with aofmodel consisting dielectric of a flat function, withdielectric two superposed absorption bands. The model lineshape of Figure 10a,b background function,Gaussian with two superposed Gaussian absorption bands. The model represents theFigure best-fit10a,b results with the the (fitted) value of thewith filmthe thickness 29 nm, andfilm the background lineshape of represents best-fit results (fitted)ofvalue of the thickness of value of the of its dielectric of 3.2. Theofresulting parameters of the areresulting listed in 29 nm, andreal thepart background valuefunction of the real part its dielectric function of bands 3.2. The Table 7. Theof strength parameter proportional the areaparameter below theisabsorptive (imaginary) of parameters the bands are listedisin Table 7. Theto strength proportional to the areapart below the function, having theofunits of eV; however, wehaving are interested in of relative values and the dielectric absorptive (imaginary) part the dielectric function, the units eV; however, we use are arbitrary here. values and use arbitrary units here. interestedunits in relative
(a)
(b)
(c)
Figure Normal incidence incidence reflectance reflectancespectra spectraof ofthe theuncapped, uncapped,1000 1000c.c.sample; sample; the positions Figure 10. 10. (a) Normal the positions of of two prominent Gaussian bands are indicated by arrows, bare substrate (dashed line); (b) second two prominent Gaussian bands are indicated by arrows, bare substrate (dashed line); (b) second derivative; derivative; and and (c) (c) second second derivative derivative of of the the reflectance reflectance spectrum spectrum from from the the capped, capped, 500 500 c.c. sample. sample. Measured Measured data data (symbols), (symbols),model modellineshapes lineshapes(red (redsolid solidand anddashed dashedlines). lines). Table 7. Best-fit parameters of the Gaussian bands used in modelling the reflectance spectra of Figure 10. Table 7. Best-fit parameters of the Gaussian bands used in modelling the reflectance spectra of Figure 10. Band
Band R1 R1 R2 R2
Strength (a.u.)
Strength (a.u.) 0.03 ± 0.01 0.03 ± 0.01 0.23 ± 0.01 0.23 ± 0.01
Position (eV)
Position (eV) 3.25 ± 0.01 3.25 ± 0.01 3.33 ± 0.01 3.33 ± 0.01
Width (eV)
Width (eV) 0.06 ± 0.02 0.06 ± 0.02 0.14 ± 0.02 0.14 ± 0.02
The The two two Gaussian Gaussian bands bands are are 0.08 0.08 eV eV apart, apart, which which is is in in aa good good agreement agreement with with the the distance distance of of 0.06 0.06 eV eV seen seen in in the theabsorption absorptionspectra spectraof ofZZ33 (3.23 (3.23 eV) eV) and and ZZ1,2 1,2 (3.308 (3.308 eV) eV) excitons excitons at at in in the the PL PL results results and and those those seen seen at at 44 K K [44]. [44]. In In addition, addition, the the significantly significantly larger larger strength strength of of the the upper upper (3.30 (3.30 eV) eV) band band is is clearly clearly seen seen in in the the reflectance reflectance results, results, in in contrast contrast to to the the PL PL intensities. intensities. This This is is similar similar to to the the relative relative intensities intensities seen seen in in direct direct optical optical absorption absorption measurements measurements[18,45]. [18,45]. We Wehave havealso also observed observed aa weak weak and and broad luminescence band centred at about 2.7 eV, with the maximum signal below 4% of broad luminescence band centred at about 2.7 eV, with the maximum signal below 4% of the the main main PL PL band; band; its its high high energy energy tail tail forms forms the the smooth smooth background background seen seen in in Figure Figure 7. 7. This This may may be be due due to to radiative recombination of defect states in the CuCl crystallites, as well as other species present in the radiative recombination of defect states in the CuCl crystallites, as well as other species present in the deposited deposited layer. layer. A A film film of of the the sample sampleQ8 Q8(500 (500c.c. with with an an Al Al22O O33 capping capping layer) layer) displays displays aa reduced reduced intensity intensity excitonic structure in the differentiated reflectance, see Figure 10c. The results are compatible excitonic structure in the differentiated reflectance, see Figure 10c. The results are compatible with with reduced 1212 nm. InIn addition, thethe upper lineshape seems to be reducedeffective effectivefilm filmthickness thicknessofofapproximately approximately nm. addition, upper lineshape seems to narrower thanthan that of theofthicker samplesample Q10, probably due to the of the crystallite be narrower that the thicker Q10, probably duesmaller to thedispersion smaller dispersion of the sizes. It cansizes. be seen that measurements are a useful to identify presence of CuCl crystallite It can bereflectance seen that reflectance measurements areway a useful way tothe identify the presence in method would would be particularly usefuluseful wherewhere the film deposited on aon non-UV of thin CuClfilms. in thinThis films. This method be particularly the is film is deposited a nontransparent substrate. UV transparent substrate.
The reflectance of sample Q8 was remeasured after approximately three weeks in normal atmosphere. A comparison between the two measurements is shown in Figure 11. There is no significant change to the excitonic absorption confirming the XPS results that 5 nm of ALD Al2O3
Coatings 2018, 8, 369
13 of 16
The reflectance of sample Q8 was remeasured after approximately three weeks in normal atmosphere. comparison between the two measurements is shown in Figure 11. There 13 is of no16 Coatings 2018, 8, A x FOR PEER REVIEW significant change to the excitonic absorption confirming the XPS results that 5 nm of ALD Al2 O3 providesan aneffective effectiveprotection protectionlayer layerfor forthe theCuCl. CuCl.This Thisisisconsistent consistentwith withpublished publishedwork workshowing showing provides thatALD ALDAl Al2 O 2O3 constitutes constitutes a good moisture diffusion barrier when applied to other materials [46]. that a good moisture diffusion barrier when applied to other materials [46]. 3
Figure Figure11. 11.Comparison Comparisonofofreflectance reflectancecurves curves(2nd (2ndderivative) derivative)for foraluminium aluminiumoxide-capped oxide-cappedsample sampleQ8 Q8 before and after approximately 3-week exposure to ambient air. before and after approximately 3-week exposure to ambient air.
4. Conclusions 4. Conclusions It has been demonstrated that CuCl can be deposited by sequentially pulsed vapour deposition It has been demonstrated that CuCl can be deposited by sequentially pulsed vapour deposition process using the precursors [Bis(trimethylsilyl)acetylene](hexafluoroacetylacetonato)copper(I) and process using the precursors [Bis(trimethylsilyl)acetylene](hexafluoroacetylacetonato)copper(I) and pyridine hydrochloride and that an in situ ALD capping layer of aluminium oxide is an effective pyridine hydrochloride and that an in situ ALD capping layer of aluminium oxide is an effective barrier for preventing atmospheric degradation. The crystal structure has been shown to be the barrier for preventing atmospheric degradation. The crystal structure has been shown to be the zinc zinc blende-structure γ-CuCl by XRD. The crystallites become more facetted as the film thickness blende-structure γ-CuCl by XRD. The crystallites become more facetted as the film thickness increases. The bulk of films shows only some organic contamination from incomplete precursor increases. The bulk of films shows only some organic contamination from incomplete precursor reaction, however, there is heavier organic contamination on the surface, again from unreacted reaction, however, there is heavier organic contamination on the surface, again from unreacted precursor molecules. The characteristic photoluminescence behaviour of CuCl has been shown with precursor molecules. The characteristic photoluminescence behaviour of CuCl has been shown with emissions from the Z1,2 , Z3 and bound excitons. The chemical composition was investigated by XPS: emissions from the Z1,2, Z3 and bound excitons. The chemical composition was investigated by XPS: the 200 c. and 500 c. films show surface layers with predominantly +Cu+ bonding from CuCl with the 200 c. and 500 c. films show surface layers with predominantly Cu bonding from CuCl with some some organic contamination. The thicker films with 1000 deposition cycles have significant Cu2+ organic contamination. The thicker films with 1000 deposition cycles have significant Cu2+ content content from CuF2 or F containing organic fragments. Optical reflectance measurements have shown from CuF2 or F containing organic fragments. Optical reflectance measurements have shown that the that the characteristic exciton absorptions can be detected, at similar energies to optical absorption characteristic exciton absorptions can be detected, at similar energies to optical absorption features features measured by transmission. This enables exciton absorption measurements to be carried out measured by transmission. This enables exciton absorption measurements to be carried out on on nontransparent samples. nontransparent samples. The overall results show that this is a method for deposition of nanocrystalline arrays suitable The overall results show that this is a method for deposition of nanocrystalline arrays suitable for further investigation for the development of new optoelectronic structures and devices. Further for further investigation for the development of new optoelectronic structures and devices. Further studies are also required to clarify the difference in chemical composition between the surface layers studies are also required to clarify the difference in chemical composition between the surface layers and the bulk of the film. and the bulk of the film. Author Contributions: Conceptualization, D.C.C; Methodology, D.C.C.; Formal Analysis, T.H., R.K., and D.C.C.; Author Contributions: Methodology, D.C.C.; Formal Analysis, T.H., R.K., and Investigation, R.K., T.H., Conceptualization, O.C., K.K., R.Z., J.P.,D.C.C.; and J.M.M.; Resources, R.Z., J.P., and J.M.M.; Writing–Original Draft Preparation, T.H. and Writing–Review & Editing, R.Z., J.M.M., T.H.,J.M.M.; K.K., and O.C.; D.C.C.; Investigation, R.K.,J.H.; T.H., O.C., K.K., R.Z., J.P., andD.C.C., J.M.M.;J.H., Resources, R.Z., R.K., J.P., and Writing– Visualization, D.C.C. and J.H.; Supervision, D.C.C., J.H., and J.M.M.; Project Administration, D.C.C. and Original Draft Preparation, T.H. and J.H.; Writing–Review & Editing, D.C.C., J.H., R.Z., J.M.M., R.K., T.H.,R.K.; K.K., Funding Acquisition, D.C.C. and J.M.M. and O.C.; Visualization, D.C.C. and J.H.; Supervision, D.C.C., J.H., and J.M.M.; Project Administration, D.C.C. Funding: This work was financially supported and R.K.; Funding Acquisition, D.C.C. and J.M.M. by the Czech Science Foundation (No. 17-02328S), the Ministry of Youth, Education and Sports of the Czech Republic (Nos. LM2015082, LQ1601, LO1411 Funding: ThisCZ.02.1.01/0.0/0.0/16_013/0001829), work was financially supported by the Czech Science Foundation (No. 17-02328S), the (project Ministry (NPU I), and and the European Regional Development Fund CZ.1.05/2.1.00/03.0086). of Youth, Education and Sports of the Czech Republic (Nos. LM2015082, LQ1601, LO1411 (NPU I), and CZ.02.1.01/0.0/0.0/16_013/0001829), and the European Regional Development Fund (project CZ.1.05/2.1.00/03.0086).
Acknowledgement: The authors acknowledge the support of COST action MP1402 HERALD in the course of this work. Conflicts of Interest: The authors declare no conflicts of interest.
Coatings 2018, 8, 369
14 of 16
Acknowledgments: The authors acknowledge the support of COST action MP1402 HERALD in the course of this work. Conflicts of Interest: The authors declare no conflicts of interest.
References 1. 2. 3. 4. 5. 6.
7. 8.
9. 10. 11.
12. 13.
14. 15. 16.
17. 18. 19. 20.
Soga, M.; Imaizumi, R.; Kondo, Y.; Okabe, T. A method of growing CuCl single crystals with flux. J. Electrochem. Soc. 1967, 114, 388–390. [CrossRef] Nakayama, M.; Ichida, H.; Nishimura, H. Bound-biexciton photoluminescence in CuCl thin films grown by vacuum deposition. J. Phys. Cond. Matter 1999, 11, 7653–7662. [CrossRef] Monemar, B. Fundamental energy gap of GaN from photoluminescence excitation spectra. Phys. Rev. B 1974, 10, 676. [CrossRef] Ryu, Y.R.; Lee, T.S.; White, H.W. Properties of arsenic-doped-type ZnO grown by hybrid beam deposition. Appl. Phys. Lett. 2003, 83, 87–89. [CrossRef] Alam, M.M.; Lucas, F.O.; Danieluk, D.; Bradley, A.L.; Daniels, S.; McNally, P.J. Temperature dependent photoluminescence of nanocrystalline γ-CuCl hybrid films. Thin Solid Films 2014, 564, 104–109. [CrossRef] Alam, M.M.; Lucas, F.O.; Danieluk, D.; Bradley, A.L.; Rajani, K.V.; Daniels, S.; McNally, P.J. Hybrid organic–inorganic spin-on-glass CuCl films for optoelectronic applications. J. Phys. D Appl. Phys. 2009, 42, 225307. [CrossRef] Göbel, A.; Ruf, T.; Cardona, M.; Lin, C.T.; Wrzesinski, J.; Steube, M.; Reimann, K.; Merle, J.C.; Joucla, M. Effects of the isotopic composition on the fundamental gap of CuCl. Phys. Rev. B 1998, 57, 15183. [CrossRef] Mitra, A.; O’Reilly, L.; Lucas, O.F.; Natarajan, G.; Danieluk, D.; Bradley, A.L.; McNally, P.J.; Daniels, S.; Cameron, D.C.; Reader, A.; Martinez-Rosas, M. Optical properties of CuCl films on silicon substrates. Phys. Status Solidi B 2008, 245, 2808–2814. [CrossRef] Grun, J.B.; Hönerlage, B.; Levy, R. Dynamics of optical nonlinearities and bistability in 3-level systems (CuCl). Phys. Scr. 1986, T13, 184–188. [CrossRef] Gomes, M.; Kippelen, B.; Hönerlage, B. Time, intensity and energy dependence of four-wave mixing processes in CuCl. Phys. Stat. Solidi B 1990, 159, 101–106. [CrossRef] Natarajan, G.; Daniels, S.; Cameron, D.C.; O’Reilly, L.; Mitra, A.; McNally, P.J.; Lucas, O.; Rajendra Kumar, R.T.; Reid, I.; Bradley, A.L. Stoichiometry control of sputtered CuCl thin films: Influence on ultraviolet emission properties. J. Appl. Phys. 2006, 100, 033520. [CrossRef] Kawamori, A.; Edamatsu, K.; Itoh, T. Growth of CuCl nanostructures on CaF2 (111) substrates by MBE—Their morphology and optical spectra. J. Cryst. Growth 2002, 237–239, 1615–1619. [CrossRef] O’Reilly, L.; Lucas, O.F.; McNally, P.J.; Reader, A.; Natarajan, G.; Daniels, S.; Cameron, D.C.; Mitra, A.; Martinez-Rosas, M.; Bradley, A.L. Room-temperature ultraviolet luminescence from γ-CuCl grown on near lattice-matched silicon. J. Appl. Phys. 2005, 98, 113512. [CrossRef] Zhou, Y.Y.; Lu, M.K.; Zhou, G.J.; Wang, S.F. Preparation and photoluminescence of gamma-CuI nanoparticles. Mater. Lett. 2006, 60, 2184–2186. [CrossRef] Kurisu, H.; Nagoya, K.; Nakayama, N.; Yamamoto, S.; Matsura, M. Exciton and biexciton properties of CuCl microcrystals in an SiO2 matrix prepared by sputtering method. J. Lumin. 2000, 87–89, 390–392. [CrossRef] Fukumi, K.; Chayahara, A.; Kageyama, H.; Kadono, K.; Akai, T.; Mizoguchi, H.; Horino, Y.; Makihara, M.; Fujii, K.; Hayakawa, J. Formation process of CuCl nano-particles in silica glass by ion implantation. J. Non-Cryst. Sol. 1999, 259, 93–99. [CrossRef] Yingjie, Z.; Yitai, Q.; Youfou, C. γ-Radiation synthesis of nanocrystalline powders of copper (I) halides. Mater. Sci. Eng. B 1999, 57, 247–250. [CrossRef] Natarajan, G.; Maydannik, P.S.; Cameron, D.C.; Akopyan, I.; Novikov, B.V. Atomic layer deposition of CuCl nanoparticles. Appl. Phys. Lett. 2010, 97, 241905. [CrossRef] Maydannik, P.S.; Natarajan, G.; Cameron, D.C. Atomic layer deposition of nanocrystallite arrays of copper(I) chloride for optoelectronic structures. J. Mater. Sci.-Mater. Electron. 2017, 28, 11695–11701. [CrossRef] Lucas, O.; O’Reilly, L.; Natarajan, G.; McNally, P.J.; Daniels, S.; Taylor, D.M.; William, S.; Cameron, D.C.; Bradley, A.L.; Mitra, A. Encapsulation of the heteroepitaxial growth of wide bandgap-CuCl on silicon substrates. J. Cryst. Growth 2006, 287, 112–117. [CrossRef]
Coatings 2018, 8, 369
21. 22.
23. 24. 25. 26. 27. 28. 29.
30. 31. 32. 33.
34.
35. 36. 37.
38. 39. 40.
41.
42. 43.
15 of 16
George, S.M.; Ott, A.W.; Klaus, J.W. Surface chemistry for atomic layer growth. J. Phys. Chem. 1996, 100, 13121–13131. [CrossRef] ˇ Cameron, D.C.; Krumpolec, R.; Ivanova, T.V.; Homola, T.; Cernák, M. Nucleation and initial growth of atomic layer deposited titanium oxide determined by spectroscopic ellipsometry and the effect of pretreatment by surface barrier discharge. Appl. Surf. Sci. 2015, 345, 216–222. [CrossRef] Humlíˇcek, J.; Šik, J. Optical functions of silicon from reflectance and ellipsometry on silicon-on-insulator and homoepitaxial samples. J. Appl. Phys. 2015, 118, 195706. [CrossRef] Sesselmann, W.; Chuang, T.J. The interaction of chlorine with copper: I. Adsorption and surface reaction. Surf. Sci. 1993, 176, 32–66. [CrossRef] Vasquez, R.P. CuCl by XPS. Surf. Sci. Spect. 1993, 2, 138–143. [CrossRef] Vasquez, R.P. CuCl2 by XPS. Surf. Sci. Spect. 1993, 2, 160–164. [CrossRef] Martin-Vosshage, D.; Chowdari, B.V.R. XPS Studies on (PEO)n LiCIO4 and (PEO)n Cu(CIO4 )2 polymer electrolytes. J. Electrochem. Soc. 1995, 142, 1442–1446. [CrossRef] Wren, A.G.; Phillips, R.W.; Tolentino, C.U. Surface-reactions of chlorine molecules and atoms with water and sulfuric-acid at low-temperatures. J. Colloid Interface Sci. 1979, 70, 544–557. [CrossRef] Yamamoto, Y.; Konno, H. Ylide-metal complexes. X. An X-ray photoelectron spectroscopic study of triphenylmethylenephosphorane and gold- and copper-phosphorane complexes. Bull. Chem. Soc. Jpn. 1986, 59, 1327–1330. [CrossRef] Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; Wiley: New York, NY, USA, 1992. Klein, J.C.; Proctor, A.; Hercules, D.M.; Black, J.F. X-ray excited Auger intensity ratios for differentiating copper-compounds. Anal. Chem. 1983, 55, 2055–2059. [CrossRef] Wagner, C.D. Chemical-shifts of auger lines, and Auger parameter. Faraday Discuss. Chem. Soc. 1975, 60, 291–300. [CrossRef] Ressan, M.; Furlani, C.; Polzonetti, G. XPS of coordination-compounds—Additive ligand effect in some copper(I) and copper(II) chelates with 1,2 phosphino (or phosphine oxide)-sulfido ethane ligands. Polyhedron 1983, 2, 523–528. [CrossRef] Parmigiani, F.; Depero, L.E.; Minerva, T.; Torrance, J.B. The fine-structure of the Cu 2p3/2 X-ray photoelectron-spectra of copper-oxide based compounds. J. Electron. Spectrosc. Relat. Phenom. 1992, 58, 315–323. [CrossRef] Kishi, K.; Ikeda, S. X-ray photoelectron spectroscopic study of the reaction of evaporated metal films with chlorine gas. J. Phys. Chem. 1974, 78, 107–112. [CrossRef] Li, X.; Liu, Z.; Kim, J.; Lee, J-Y. Heteogeneous catalytic reaction of elemental mercury vapor over cupric chloride for mercury emissions control. Appl. Catal. B Environ. 2013, 132–133, 401–407. [CrossRef] Biesinger, M.C.; Lau, L.W.M.; Gerson, A.R.; Smart, R.S.C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 2010, 257, 887–898. [CrossRef] Van der Laan, G.; Westra, C.; Haas, C.; Sawatzky, G.A. Satellite structure in photoelectron and auger-spectra of copper dihalides. Phys. Rev. B 1981, 23, 4369. [CrossRef] Finster, J.; Klinkenberg, E.-D.; Heeg, J.; Braun, W. ESCA and SEXAFS investigations of insulating materials for ULSI microelectronics. Vacuum 1990, 41, 1586–1589. [CrossRef] O’Reilly, L.; Natarajan, G.; McNally, P.J.; Cameron, D.; Lucas, O.F.; Martinez-Rosas, M.; Bradley, L.; Reader, A.; Daniels, S. Growth and characterisation of wide-bandgap, I–VII optoelectronic materials on silicon. J. Mater. Sci. Mater. Electron. 2005, 16, 415–419. [CrossRef] Natarajan, G.; Mitra, A.; O’Reilly, L.; Daniels, S.; Cameron, D.C.; McNally, P.J.; Lucas, O.F.; Bradley, L. Optical investigations on sputtered CuCl thin films. In Progress in Semiconductor Materials V: Novel Materials and Electronic and Optoelectronic Applications, Proceedings of the MRS Symposium, Boston, MA, USA, 28 November–1 December 2005; Olafsen, L.J., Biefeld, R.M., Wanke, M.C., Saxler, A.M., Eds.; Cambridge University Press: New York, NY, USA, 2006; pp. 151–156. Natarajan, G.; Mitra, A.; Daniels, S.; Cameron, D.C.; McNally, P.J. Temperature dependent optical properties of UV emitting γ-CuCl thin films. Thin Solid Films 2008, 516, 1439–1442. [CrossRef] Certier, M.; Wecker, C.; Nikitine, S. Zeeman effect of free and bound excitons in CuCl. J. Phys. Chem. Sol. 1969, 30, 2135–2142. [CrossRef]
Coatings 2018, 8, 369
44. 45. 46.
16 of 16
Cardona, M. Optical properties of the silver and cuprous halides. Phys. Rev. 1963, 129, 69–78. [CrossRef] Akopyan, I.K.; Golubkov, V.V.; Dyatovla, O.A.; Novikov, B.V.; Tsagan-Mandzhiev, A.N. Structure of copper halide nanocrystals in photochromic glasses. Phys. Solid State 2008, 50, 1352–1356. [CrossRef] Carcia, P.F.; McLean, R.S.; Walls, D.J.; Reilly, M.H.; Wyre, J.P. Effect of early stage growth on moisture permeation of thin-film Al2 O3 grown by atomic layer deposition on polymers. J. Vac. Sci. Technol. A 2013, 31, 061507. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
