Copper(II) Schiff base complexes and their mixed thin layers with ZnO nanoparticles

Article abstract of DOI:10.1007/s12039-016-1116-y

Cu(II) complexes with Schiff bases derived from ethylenediamine (en) and 2-pyridinecarboxaldehyde (pyca), 2,5-dimethoxybenzaldehyde (dmbaH) or 4-imidazolecarboxaldehyde (4Him) were obtained and studied by elemental analysis, UV-VIS and IR spectra. Zinc oxide was synthesized using a simple homogeneous precipitation method with zinc acetate as a starting material. Thin layers of the studied Cu(II) complexes were deposited on Si(111) or ZnO/Si(111) substrates by a spin coating method and characterized with a scanning electron microscopy (SEM/EDS), atomic force microscopy (AFM) and fluorescence spectroscopy. For Cu(II) layers the most intensive fluorescence bands due to intra-ligand transitions were observed between 462 and 503 nm. The fluorescence intensity of thin layers was corelated to the rotation speed. In the case of the [Cu(II)(en(4Him)₂)Cl₂](2a)/ZnO/Si and [Cu(en(dmbaH)₂)Cl₂](3a)/ZnO/Si layers the quenching of the emission band from ZnO at 440 nm (λ_ex = 330 nm) associated with various intrinsic or extrinsic lattice defects was noted. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1116-y

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1116-y
Copper(II) Schiff base complexes and their mixed thin layers with ZnO
nanoparticles
MAGDALENA BARWIOLEK∗, ROBERT SZCZE˛SNY and EDWARD SZŁYK
Faculty of Chemistry, Nicolaus Copernicus University in Torun, 87-100 Torun, Poland
e-mail: mbarwiolek@umk.pl
MS received 3 March 2016; revised 4 May 2016; accepted 24 May 2016
Abstract. Cu(II) complexes with Schiff bases derived from ethylenediamine (en) and 2-pyridinecarboxaldehyde
(pyca), 2,5-dimethoxybenzaldehyde (dmbaH) or 4-imidazolecarboxaldehyde (4Him) were obtained and stud-
ied by elemental analysis, UV-VIS and IR spectra. Zinc oxide was synthesized using a simple homogeneous
precipitation method with zinc acetate as a starting material. Thin layers of the studied Cu(II) complexes
were deposited on Si(111) or ZnO/Si(111) substrates by a spin coating method and characterized with a scan-
ning electron microscopy (SEM/EDS), atomic force microscopy (AFM) and fluorescence spectroscopy. For
Cu(II) layers the most intensive fluorescence bands due to intra-ligand transitions were observed between 462
and 503 nm. The fluorescence intensity of thin layers was corelated to the rotation speed. In the case of the
[Cu(II)(en(4Him)2)Cl2](2a)/ZnO/Si and [Cu(en(dmbaH)2)Cl2](3a)/ZnO/Si layers the quenching of the emis-
sion band from ZnO at 440 nm (λex = 330 nm) associated with various intrinsic or extrinsic lattice defects
was noted.
Keywords. Thin layer; ZnO nanoparticles; copper complexes; AFM; SEM; fluorescence.
1. Introduction
thin films of metal complexes, such as Zn(II), Pt(II),
Cu(II) or Ag(I) with Schiff bases (e.g., N,Nꢁ-bis(salicyli
dene)-1,2-ethylenediamine, bis(2-(2-hydroxyphenyl)
benzothiazole) were synthetised and their lumines-
cence properties have been employed in organic
optoelectronics.12–15
Study of copper(II) complexes of Schiff bases and
nitrogen donor ligands has become a focal point of
interest for researchers in recent times, not only for their
varied structural features, but also for their potential
application in various fields.1–5 These compounds are
also known for their biological role and fluorescence
properties. For instance, the copper(II) complexes with
Schiff bases derived from trans ( )cyclohexanediamine
and 2-pyridinecarboxaldehyde exhibited luminescence
properties in the solution and in the solid state.6 The
Cu(II) complexes [Cu(II)(L)](CF3SO3)2, where L is a
ligand obtained from cyclohexanediamine or ethylene-
diamine and aldehydes e.g., 2-quinolinecarboxaldehyde,
6-methyl-2-pyridinecarboxaldehyde or 2-thiazolecar-
boxaldehyde can be used as molecular memories for the
wet chemical computers.7 Furthermore, Schiff base
derivatives incorporating a fluorescent moiety are a
useful tool for optical sensing of metal ions. Thin orga-
nic and organometallic films have attracted research
interest due to their technologically important optical
and electronic properties.8 These materials exhibit lumi-
nescence; they are used as conductors, semi-conductors
and organic light emitting diodes (OLED).9–11 Many
The thin organic-inorganic layers of different metal
compounds were obtained by many different meth-
ods e.g., laser ablation, cathodoluminescence or spin
coating.16–19 However, there are only a few reports
on copper(II) complexes in materials obtained by spin
coating method. For example, [Cu3(opba)(pmdta)2]
(NO3)2 (pmdta = 1,1,4,7,7-pentamethyl-diethylenetria-
mine, opba = orthophenylenebis(oxamato)) complex
was used to get thin layer of Cu(II) complex on
SiO2/Si.10 This technique was also used for depo-
sition of [(nBu3P)3Cu-O2CCH2CO2-Cu(PnBu3)3] and
[(nBu3P)3Cu-O2C(CH2)2CO2-Cu(PnBu3)3] on SiO2/
TiN/Cu substrate. The layers were then heated up to
450◦C, and the result of this process was the con-
version of complexes to CuO by decomposition.20
In this case, SEM results showed incomplete homo-
geneous layers and defects in the surface structure.
This was explained by the different film thicknesses
obtained during the spin coating process, which upon
heating produced defects due to unequal evaporation
leading to partially cracked copper oxide films. These
∗For correspondence

Magdalena Barwiolek et al.
results showed the spin coating as a promising method by AFM and SEM microscopy, and the fluorescence
for deposition of thin and homogeneous layers of properties of the layers were studied. The mechanical
transition metal complexes. Previously, copper(II) and properties of some layers induced by microindentation
nickel(II) complexes with optically active Schiff bases were investigated as well.
derived from (1R, 2R)(-)cyclohexanediamine or 2-(2-
aminoethyl)pyridine were deposited by us on silicon or
2. Experimental
glass using the spin coating method.21–23 The obtained
thin materials exhibited fluorescence, which depends on
the spin coating parameters and the molecular structure
2.1 Materials and methods
of the complexes.
The ethylenediamine (en) (99%), 2-pyridinecarbo-
xaldehyde (99%) (pyca), 2,5-dimethoxybenzaldehyde
(dmbaH) (99%), 4-imidazolecarboxaldehyde (4Him)
(98%) and ethylene glycol (EG) (99%) were purchased
from Aldrich. CuCl2·2H2O, Cu(OOCCH3)2·H2O and
Zn(OOCCH3)2·2H2O were purchased from POCh
Gliwice. All reagents were used without further
purification.
It is also known, that the ZnO layers can be obtained
by sputtering processes, pulsed laser deposition tech-
nique or by chemical methods such as, microemul-
sion, hydrothermal, sol-gel and precipitation followed
by the deposition of nanoparticle suspensions using
wet coating methods.24–28 Those materials were also
used in optical technology. Moreover, the deposited
ZnO nanopowders29,30 and ZnO nanoparticles doped by
different metal ions such as, manganese31 or copper
exhibited photoluminescence bands, and the emission
intensity increased with increase in doping up to a
certain percentage of Cu2+ (5%).32,33
The advantages of using wet coating method are
two-fold: First, the layers can be formed by subse-
quent or simultaneous deposition of different materials.
Second, use of spin coating offers the opportunity for
facile fabrication of a uniformly dispersed nanopar-
ticles or/and solutions onto the substrates even for
large areas, and well-defined thickness of films in mild
conditions. Therefore, this type of deposition method
allows to fabricate a wide variety of composite mate-
rials. The facts mentioned above: the fluorescence
properties of the Schiff bases and their complexes,
the possibility of using the spin coating technique
for deposition of the copper(II) complexes and ZnO
nanoparticles on different substrates and the likely flu-
orescence behavior of the obtained layers, prompted us
to synthesize a series of new copper(II) complexes with
Schiff bases derived from ethylenediamine and several
aldehydes. These complexes were characterized spec-
troscopically and their fluorescence properties were
also studied.
Infrared spectra were recorded on a Perkin Elmer
SPECTRUM 2000 spectrophotometer. UV-Vis absorp-
tion spectra were recorded on a Milton Roy Spectronic
1201 spectrophotometer in CH3CN (1 × 10−5 M) solu-
tion. IR spectra were performed on a Spectrum 2000
Perkin/Elmer FT IR using KBr discs in the range 400–
4000 cm−1 and polyethylene IR sample cards in the
1
range 400-20 cm−1. 13C CPMAS NMR and H, 13C,
15N NMR spectra were collected with Bruker AMX -
300 spectrometer (300 MHz) in CDCl3 against TMS.
The fluorescence spectra were recorded on a spectroflu-
orometer F-7000 HITACHI in the range 900–200 nm
(5 × 10−4 mol dm−3 MeCN solution or silicon slides)
Elemental analyses were carried out using a Elemen-
tar Gmbh Vario Macro CHN analyzer. The films were
deposited on Si(111) wafers (10\10 mm) ∼500 um
thick using spin coating technique. Precursors were
dissolved in tetrahydrofuran and deposited on Si(111)
(900–2000 rpm/30 s–600 s; drying 5000 rpm/200 s, 1 ×
10−3 M in THF) using a Laurell 650SZ spin coater.
Zinc oxide layers were obtained from ethanol suspen-
sion (1.2 mg/cm3) by the same method. A suspension
of ZnO was spun onto the substrate at 1000–5000 rpm
for 20–360 s and each plate was subsequently dried at a
rotation speed of 5000 rpm for 20 s.
In this article, we would like to report how a sim-
The morphology and composition of the obtained
ple and efficient method as spin coating can be utilized materials were analyzed with a scanning electron
to produce composite films using both nanoparticle dis- microscopy (SEM) LEO Electron Microscopy Ltd,
persions and solutions of complexes. Moreover, the aim England, model 1430 VP equipped with detectors of
of the study was to investigate the microstructural and secondary electrons (SE) and energy dispersive X-ray
spectroscopic properties of obtained layers. Therefore, spectrometer (EDS) Quantax with detector XFlash
the new copper(II) complexes were used as precursors 4010 (Bruker AXS microanalysis GmbH). The atomic
of thin layers in the spin coating technique. In addi- force microscopy AFM studies in the tapping mode
tion, the zinc oxide and the Cu(II) complexes layers were performed on Veeco (Digital Instrument) micro-
doped with ZnO nanopowder were obtained. The mor- scope Type: MultiMode NanoScope IIIa. Powder X-ray
phology and homogeneity of the layers were analyzed diffraction (PXD) analysis was performed using a

Copper(II) complexes with ZnO nanoparticles thin layers
Philips (Almelo, The Netherlands) XPERT θ–2θ 2.3 Synthesis of the Cu(II) complexes
diffractometer with CuKα radiation. The indentation
properties of the layers were made on NHT2 microin- 2.3a [Cu(en(pyca)2)Cl2] (1a): Ethanolic solution of
dentation tester (TTX-NHT) from Anton Paar TriTec CuCl2 x 2H2O (0.0287 g, 0.16 mmol in 40 mL ethanol)
SA with a Berkovich indenter using loads 20 and was added to en(pyca)2 (1) (0.0396 g (0.16 mmol) in
100 mN and holding time of 5 s. The system hard- EtOH. The reaction mixture was stirred at 25◦C for
ness and elastic modulus were determined from the 4 h, then, the solvent was removed under reduced
indentation curves by the standard Oliver and Pharr pressure and a dark green solid was isolated.
method.34
[Cu(en(pyca)2)Cl2] (1a) (Yield 72.8%) Analysis: Cal-
culated for C14H14N4Cl2Cu: Cu 17.04, C 45.11, H
3.78, N 15.03. Found: Cu 16.61, C 45.48, H 4.11, N
15.34 [%]. IR (KBr, cm−1): 3386 m ν(N-H), 3068 m
ν(C=CH), 2922m ν(C-H), 1604vs ν(C=N), 1569m
ν(C-N), 1447s ν(C=C), 303s ν(Cu-Cl), 422s ν(Cu-
N).37 UV-visible in MeCN [λmax/nm (ε/[dm3· mol−1·
cm−1])] 255(8461), 320(2307).
2.2 Synthesis
2.2a Synthesis of en(pyca)2 (1): Ethylenediamine
(en) (60 μL, 1 mmol) in EtOH was added to pyridine-
2-carboxaldehyde (pyca) (190 μL, 2 mmol). The mix-
ture was stirred under reflux for 2 h. The orange
precipitate formed was recrystallized from ethanol.
In the same way [Cu(en(pyca)2)(OOCCH3)2] (1b),
[Cu(en(4Him)2)Cl2] (2a) and [Cu(en(dmbaH)2)Cl2]
(3a) were synthesized (figure 1).
1
(82.7% yield). H NMR [ppm]: δ 8.62 (d, 2H, arom.),
8.42 (s, 2H, CH=N), 7.98 (d, 2H, arom.), 7.73 (2H,
arom.), 7.30 (ddd, 2H, arom.), 4.07 (s, 4H, -CH2-).
13C NMR [ppm]: δ 163.4 C2, 154.4 C3, 149.4 C7,
136.5 C5, 124.7 C6, 121.3 C4, 61.3 C1(CH2). 15N
NMR [ppm]: -42.5 N(1), -67.3 CH=N. IR (KBr,
cm−1): 3010-3070s ν(C-HAr), 2922, 2853s ν(CH),
1645m ν(C=N), 1584s ν(C-N), 1434m ν(C=C).6,35–40
(vs, very strong; s, strong; m, medium; w, weak, vw
ver weak) (figure 1).
2.3b [Cu(en(pyca)2)(OOCCH3)2] (1b): (79.2% yield,
0.03194 g 0.16 mmol Cu(OOCCH3)2 x H2O, 0.0396 g
0.16 mmol en(pyca)2). Analysis: Calculated for
C18H20N4O4Cu: Cu 15.13, C 51.48, H 4.80, N 13.34;
Found: Cu 15.19, C 51.68, H 4.29, N 13.04 [%]. IR
(KBr, cm−1): 3422m ν(OH), 3074w ν(C=CH), 1598vs
ν(C=N), 1547vw ν(C-N), 1444m ν(C=C), 1710s
UV-visible in MeCN [λmax/nm(ε/[dm3·mol−1· cm−1])]
−
−
νasCOO , 1476m νsCOO , 1292m ν(C-O), 92w4 δ(O-H),
550m ν(Cu-O), 454m ν(Cu-N).6,7,36,37,44 UV-visible in
MeCN [λmax/nm (ε/[dm3·mol−1·cm−1])] 210(37795),
235(37520), 267(38120), 319(25326).
2.2b Synthesis of en(4Him)2 (2): (92.1% yield, 252(6299), 326(1574).
(60 μL, 1 mmol en, 0.1922 g, 2 mmol 4Him). 13C CP
MAS NMR [ppm]: δ 156.1 C2, 139.4 C5, 128.6 C3,
123.6 C4, 62.2 C1(CH2). IR (KBr, cm−1): 3436w
ν(N-H), 2913–3032w ν(-CH), 1648vs ν(C=N), 1512s
ν(C-N), 1450vs, 1472m ν(C=C).6,35–37,41 UV-visible in
MeCN [λmax/nm (ε/[dm3· mol−1· cm−1])] 273(31520),
318(16437) (figure 1).
2.3c [Cu(en(4Him)2)Cl2] (2a): (84.50% yield,
CuCl2 x 2H2O 0.0287 g, 0.16 mmol, 0.03459 g,
0.16 mmol en(4Him)2). Analysis: Calculated for
C10H12N6Cl2Cu: Cu 18.12, C 34.24, H 3.44, N 23.96;
Found: Cu 18.13, C 34.48, H 3.12, N 23.71 [%]. IR
(KBr, cm−1): 3422m ν(N-H), 2931w, 2853m ν(CH),
1635vs ν(C=N), 1508w ν(C-N), 1438s ν(C=C),
313m ν(Cu-Cl), 458 w ν(Cu-N).42,45 UV-visible in
MeCN [λmax/nm (ε/[dm3· mol−1· cm−1])] 264(9122),
322(1403).
2.2c Synthesis of en(dmbaH)2 (3): yield (92.2%,
1
60 μL, 1 mmol en, 0.3323 g, 2 mmol 4Him. H NMR
[ppm]: δ 8.71 (s, 2H, CH=N), 7.51 (d, 2H), 6.95
(dd, 2H), 6.85 (d, 2H), 3.99 (s, 4H, -CH2-), 3.82
(s, 6H) 3.79 (s, 6H) 13C NMR [ppm]: δ 158.6 C2,
153.8 C5, 153.4 (C8), 125.3 C3, 118.6 C6, 112.7 2.3d [Cu(en(dmbaH)2)Cl2] (3a): (76.4% yield,
C7, 110.6 C4, 61.9 C1(CH2), 56.3 C10(OCH3), 55.8 CuCl2 x 2H2O 0.0287 g, 0.16 mmol, 0.05702 g,
C9(OCH3). 15N NMR [ppm]: -69.6 -CH=N. IR (KBr, 0.16 mmol en(dmbaH)2). Analysis: Calculated for
cm−1): 3309 vs ν(N-H), 3009–3064m ν(C-HAr), 2906, C20H24N2O4Cl2Cu: Cu 12.94, C 48.93, H 3.92, N 5.70;
2831s ν(CH), 1637vs ν(C=N), 1582s ν(C-N), 1427s, Found: Cu 13.36, C 48.64, H 4.02, N 5.39 [%]. IR (KBr,
1496s ν(C=C), 1303s ν(Ph-O).36,37,42,43 UV-visible in cm−1): 3307vs ν(N-H), 3009-3064vs ν(C-HAr), 2953,
MeCN [λmax/nm (ε/[dm3·mol−1·cm−1])] 222(49732), 2967vs ν(CH), 1654vs ν(C=N), 1587vs ν(C-N), 1453s,
251(29946), 332(15508) (figure 1).
1496s ν(C=C), 477w ν(Cu-N), 373s ν(Cu-Cl).6,34

Magdalena Barwiolek et al.
1
1
1
1
2
2
2
2
N2
N2
N2
N2
N
N
1
1
3
3
3
3
4
4
4
7
7
4
5
N1
N1
NH
HN
5
5
6
5
6
(2)
(1)
1
1
2
2
N
N
4
4
H3CO
10
OCH3
9
OCH3
10
H3CO
9
5
7
6
(3)
N
N
2
2
N2
N2
Cu
N
N
Cu
N
N
OAc
OAc
Cl
Cl
(1b)
(1a)
N
N
N2
N2
Cu
Cu
Cl
Cl
H3CO
OCH3
N1
N1
HN
NH
H3CO
OCH3
Cl
Cl
(2a)
(3a)
Figure 1. Structures of en(pyca)2 (1), en(4Him)2 (2) and en(dmbaH)2 (3) ligands
and copper(II) complexes: [Cu(en(pyca)2)Cl2] (1a), [Cu(en(pyca)2)(OOCCH3)2] (1b),
[Cu(en(4Him)2)Cl2] (2a) and [Cu(en(dmbaH)2)Cl2] (3a).
UV-visible in MeCN [λmax/nm (ε/[dm3·mol−1·cm−1])]
shows the different experimental condition applied for
ZnO preparation.
211(45098), 251(18627), 350(3921).
2.4 Synthesis of ZnO nanoparticles
Table 1. Experimental conditions for ZnO synthesis.
In a typical synthesis procedure, Zn(OOCCH3)2·2H2O
(0.649 g, 3 mmol) and urea (0.995 g, 9 mmol) were
added to water/EG solution (1:8 v/v, 75 mL) and
kept under stirring solutions at 25◦C until a homoge-
neous solution was obtained. The resultant solution was
heated to 90◦C. The conditions were kept for 1.5 h and
then the mixture was cooled to room temperature. The
resulting white precipitate was washed and centrifuged
at 9000 rpm with water, acetone and ethanol. Table 1
Sample
R
T (◦C)
Time (h)
A1
A2
A3
A4
A5
A6
6:1
3:1
3:1
3:1
1:1
4:1
90
70
70
90
90
90
1.5
1.5
3
1.5
1.5
1.5
R = urea/Zn 2+ molar ratio.

Copper(II) complexes with ZnO nanoparticles thin layers
3. Results and Discussion
the most characteristic band derived from π → π*
transition in the azomethine group appeared at 319 nm
for (1), 318 nm for (2) and 332 nm for (3). The UV-Vis
spectra of the complexes showed absorption between
320–350 nm, which can be attributed to a LMCT transi-
tions. Additionally, in the spectra of the complexes, the
band from intraligand π → π* transition in imidazole
and phenolate shifted and appeared between 210 nm
and 264 nm.
3.1 Infrared spectroscopy
The band from stretching vibrations of the -C=N-
group, characteristic of the Schiff base, appeared in
the region between 1637 and 1648 cm−1. The band
of ν(C=N) vibrations are shifted by 13–41 cm−1 in
the spectra of copper(II) complexes towards lower
frequencies, as a result of Cu(II) coordination with
the ligand via the azomethine group.6,45 In the IR
spectrum of [Cu(en(pyca)2)(OOCCH3)2] (1b) bands at
1475 and 1711 cm−1 due to symmetrical and asym-
metrical stretching vibrations of acetate ion were
observed.44–48 In (1b) the monodentate coordination
of CH3COO− anion was noted. The similar type of
coordination was observed for the copper(II) complex
with en(pyca) and CF3SO3 ions7 or Cu(II) and trans
( )chxn(pyca).6 Furthermore, in the spectra of (1a),
(2a) and (3a), the band between 303 and 373 cm−1
are present due to the stretching vibration of the Cu-
Cl in these complexes.48 Additionally, in the IR spec-
trum of [Cu(en(pyca)2)(OOCCH3)2] (1b), the band
from Cu-O vibration at 550 cm−1 was observed. More-
over, because of the coordination via nitrogen, bands
from Cu-N vibrations appeared in the 422–477 cm−1
regions in the spectra of all complexes.6,48 Similar way
of coordination was already observed by us in the
copper(II) complexes with Schiff bases derived from
2-(2-pyridyl)ethylamine.23
3.3 ZnO nanoparticles characterization
The results (figure 2) demonstrated the presence of
particles with different particle sizes and shapes,
depending on the precipitation conditions. The most
homogeneous material with the smallest particle size
(sample A4) was stick-like in shape with widths of
40 nm and lengths of 100 nm (figure 2a). The other
crystallites reach the size of few hundred nanometers
(figure 2a, c). The XRD patterns of powders show
a full set of diffraction peaks of hexagonal wurtzite
structure (JCPDS 36-1451; figure 3a). These data were
also confirmed by IR spectra of the prepared samples.
A strong band at 422 cm−1 with a visible shoulder at
554 cm−1 corresponding with Zn-O vibrational mode
was observed (figure 3b).50 Moreover, infrared spec-
troscopy confirms the presence of functional groups
at the surface of ZnO particles. The broad absorp-
tion peaks at 3384 cm−1 and 2957/2925/2853 cm−1 are
attributed to stretching vibrations of -OH or -NH groups
and the CH stretching, respectively.51 The bands at 1585
and 885 cm−1 are ascribed to in and out of plane of O-
H and C=O stretching vibration.52,53 The absorbance
peaks at 1420, 1064 and 1037 cm−1 in ZnO can be
ascribed to C-O vibration and C-H bending modes. The
small bands at 2371 and 2341 cm−1 may arise from the
absorption of carbon dioxide.
The shift of the bands from stretching vibrations of
the azomethine group in all the compounds and the
presence of specific bands of the Cu-Cl or COOa−sym
/
sym
vibrations in the spectra of particular complexes indi-
cated the correctness of the proposed coordination
sphere.
3.2 Electronic absorption spectra
3.4 Thin films of Cu complexes
All ligands exhibit intense absorption bands between
222 and 273 nm assigned to intraligand π → π* transi- The SEM images of the Cu(II) complex layers showed
tion in imidazole or phenolate moieties.38,49 Moreover, a uniform distribution of the compounds on the silicon
Figure 2. SEM micrographs of ZnO powders: a) sample A1, b) sample A4, c) sample A5.

Magdalena Barwiolek et al.
Figure 3. (a) XRD pattern of sample A4 and (b) IR spectra of sample A6.
surface. The layers were amorphous, but small crystal- when the speed was set at 900 rpm (39.3 nm) for (1a)/Si
lites appeared in some places due to incomplete solidi- or 1100 rpm (12 nm) for (1b)/Si. Increasing the spin
fication of the complex solution during the spin coating speed to 2000 rpm resulted in a lowering of the surface
process, as previously reported.20,54
roughness, giving a relatively smooth material (Rq =
In the case of Cu(en(pyca)2)Cl2] (1a)/Si and 14.7 nm for (1a) and 7.26 nm for (1b)). Reducing the
[Cu(en(dmbaH)2)Cl2] (3a)/Si materials, the whole sil- rotation speed gave a layer of (1b)/Si with some struc-
icon surface was covered regularly with copper com- tures sticking out of the surface. Additionally, the latter
plexes (900, 1100 and 2000 rpm/min (1a)/Si and 1000, materials were irregularly covered by the compound.
1100 and 2000 rpm/min (3a)/Si). The EDS results con-
Similar to the (1b)/Si and (1a)/Si materials, the
firmed the presence of copper (0.45% - 900 rpm, 0.71% lowest roughness of [Cu(en(4Him)2)Cl2] (2a)/Si was
- 1000 rpm and 0.47% - 1100 rpm) in all (1a)/Si lay- observed at the spin speed of 2000 rpm (Rq = 25.8 nm).
ers. Whereas, the [Cu(en(4Him)2)Cl2] (2a)/Si material The complex was evenly spread on the Si(111) sub-
exhibited the highest copper content (3.11%) when spin strate. While, in contrast to (1b)/Si and (1a)/Si, in
rate 2000 rpm was set.
(2a)/Si lowering the spin speed did not cause impor-
The SEM/EDS analysis of [Cu(en(pyca)2)(OOCCH3)2] tant changes in the surface roughness. In the case
(1b)/Si(111) showed the evenly distributed complex on of [Cu(en(dmbaH)2)Cl2] (3a)/Si, the AFM analysis
the silicon surface. The copper content was the biggest showed that complex (3a) is evenly arranged on Si sub-
(1.44%) when spin rate was set at 1100 rpm. Addi- strate (figure 4). The compound formed islands of dif-
tionally, some crystallites were observed. The layers of ferent heights, resulting in a flat surface (Rq = 19.5 nm,
(1b)/Si obtained at the other spin speeds (900, 1000 1000 rpm). However, empty spaces in the surface were
and 2000 rpm) exhibited lowest copper content (about observed, which indicated layer discontinuity. Other
0.5%). Moreover, in these cases, the discontinuity of the spin speeds led to materials of the lowest roughness and
materials (1000 and 2000 rpm) was noted.
uniform layers.
The above results suggest that the quality of the layer
(uniformity, roughness) can be optimized by the varia-
tion of spin speed. In conclusion, layers obtained with
For the [Cu(en(pyca)2)(OOCCH3)2] (1b)/Si and [Cu
(en(pyca)2)Cl2] (1a)/Si layers, the surface roughness
(Rq - the root mean square roughness) was the highest
Figure 4. AFM images of [Cu(en(dmbaH)2)Cl2] (3a)/Si (1000 rpm, time 30 s).

Copper(II) complexes with ZnO nanoparticles thin layers
Figure 5. The ZnO layers: (a) optical microscopy and (b: cross-section) SEM images of
sample A4 (3000 rpm, 3 min); (c) SEM image of sample A6 (5000 rpm, 3 min).
spin coating method are, in general, thin and homo- with different tetraazaporphyrine derivatives on glass
geneous with complexes uniformly spread over the or quartz substrates had also shown porous structures
substrate surface.
with disordered array of island aggregation and thick-
The best parameters for the spin coating process ness between 40 and 250 nm.56–58 Those films, because
which provide smooth, thin and homogeneous cop- of their porous nature, showed gas sensing behavior.
per(II) materials with complexes uniformly spread over Summarizing, the multistage coating process allows
the substrate surface were: spin speed of 900, 1100 or obtaining thin, uniform copper complex-zinc oxide
2000 rpm for 30 s depending on the precursor used. The mixed layers.
layers reveal that the total thickness of film does not
exceed 100 nm.
3.6 Fluorescence properties of layers
The fluorescence properties of the prepared layers were
studied. [Cu(en(pyca)2)Cl2] (1a)/Si, [Cu(en(pyca)2)
3.5 [Cu(en(4Him)2)Cl2] (2a)/ZnO/Si
and [Cu(en(dmbaH)2)Cl2] (3a)/ZnO/Si thin films
(OOCCH3)2] (1b)/Si and [Cu(en(dmbaH)2)Cl2] (3a)/Si
The zinc oxide layers were examined by optical materials exhibited the highest fluorescence intensity
microscopy (OM), SEM and AFM. In most cases, we between 472 and 503 nm from intraligand π → π*
observed uniform coated surfaces (figure 5a,c). It was transitions (λex = 382 nm). The highest-intensity of
noticed that the inorganic material was deposited both emission band was noted for layers with relatively
as separate grains and agglomerates composed of sev- low roughness (1100 rpm for (1a)/Si and 900 rpm
eral nanoparticles (figure 5b). The AFM analysis of for (1b)/Si). Similar behavior was registered by us
sample A4 layers reveals that the total thickness of film for layers of copper(II) complexes with Schiff bases
does not exceed 500 nm.
derived from 2-(2-aminoethyl)pyridine.23 Materials
In the case of the mixed layers, the copper complex formed with [Cu(en(4Him)2)Cl2] (2a)/Si were excited
and ZnO form aggregates and grains which pointed to at 365 nm. In all cases, the emission from intraligand
porous nature of the layer (figure 6a,b). The total thick- π → π* transitions at 462 nm was observed. A sub-
ness of the layer does not exceed 700 nm. The mapping stantially higher intensity of the fluorescence band for
analysis indicated uniformly deposited copper com- (2a) was registered for layer obtained at 1000 rpm. The
plex and ZnO on the silicon surface (figure 6c). Sim- above results indicate that the copper layers exhibit
ilar situation for layers of complexes was observed.55 emission and that uniformly distributed film on sub-
Spin coated thin films of Cu, Co, Ni or Pb complexes strate increases the emission band intensity.
Figure 6. The SEM/EDS analysis of [Cu(II)((en)(4Him)2)Cl2] (2a)/ZnO/Si: SEM (a, b;
cross-section); EDS mapping (c) (Cu - green and Zn - red areas).

Magdalena Barwiolek et al.
The fluorescence properties of mixed copper com- fluorescence, a band coming from Rayleigh scatter-
plex/ZnO/Si films were studied as well. It is known ing at 659 nm and 649 nm for [Cu(II)((en)(4Him)2)Cl2]
that ZnO nanoparticles and the Schiff bases exhibit (2a)/ZnO/Si and [Cu(en(dmbaH)2)Cl2] (3a)/ZnO/Si,
photoluminescence (PL)32,59 and have varied opti- respectively, were registered.66 The quenching of fluo-
cal applications.33 In the PL spectrum of ZnO, typi- rescence by Cu(II) ion can be explained by process such
cally there are emission bands in the UV and visible as magnetic perturbation or redox activity.67–69
regions.60 The PL spectrum of ZnO nanorods excited
at 320 nm showed two UV emission bands at 356 nm
and 382 nm. The emission peak at 356 nm may be
3.7 The indentation of the layers
attributed to the band gap luminescence as it is blue Preliminary indentations were performed on uncoated
shifted compared to the optical absorption. The near- silicon substrates (Si(100)), Cu(complex)/Si and
band edge (NBE) emission peak at 382 nm is assigned Cu(complex)/ZnO (A4)/Si wafers (figure 8). The
to the recombination of free excitons.61 In addition, a calculated indentation hardness (HIT) and the inden-
broad shoulder in the range 400–425 nm and very weak tation modulus (EIT) for silicon ranged from 10.754
defect-related blue emissions at 445, 453 and 470 nm to 11.561 GPa and from 172.20 to 191.4 GPa, respec-
also appeared. Blue emission bands in ZnO nanostruc- tively, depending on the maximal load and loading
tures were also reported62–65 which are associated with rate values. The obtained values correspond well with
various intrinsic or extrinsic lattice defects. In order comparable literature data.70,71 In the case of fabricated
to check the influence of the copper complexes on the layers, the elastic modulus was, in general, similar to
PL properties of the ZnO layer, the emission spec- silicon substrate while hardness values were apparently
tra of ZnO/Si, [Cu(en(4Him)2)Cl2] (2a)/ZnO/Si, [Cu lower than that for Si(111). Moreover, we observed
(en(dmbaH)2)Cl2] (3a)/ZnO/Si, [Cu(en(dmbaH)2)Cl2] a significant decrease of both mechanical parameters
(3a)/Si, and [Cu(en(4Him)2)Cl2] (2a)/Si were recorded. for samples deposited at lower rotation speed coexist-
The intensity of the fluorescence band was the low- ing with longer deposition times (samples I and IV).
est for the copper(II) complex/Si layers. On the other The application of such coating parameters resulted in
hand, the highest intensity of the emission band was thicker layers, which obviously had serve impact on the
exhibited by the ZnO/Si film. For the ZnO nanopow- mechanical properties of our composites. Additionally,
der layers mixed with copper(II) complexes [Cu(II)(en the characteristic for monocrystalline silicon the pop-
(4Him)2)Cl2] (2a)/ZnO/Si and [Cu(en(dmbaH)2)Cl2] out phenomenon, which appeared during unloading for
(3a)/ZnO/Si, quenching of the PL band of ZnO most measurements, was observed.72 It may be related
at 440 nm was observed (λex = 330 nm, figure 7). to a substantial effect of the substrate on the overall
Additionally, because of the low intensity of the hardness measure. In general, the hardness response
Figure 7. The photoluminescence spectra of (a) ZnO/Si
(2000 rpm, 3 min), (b) [Cu(en(4Him)2)Cl2] (2a)/ZnO/Si
(1500 rpm, 1 min/3000 rpm, 3 min), (c) [Cu(en(dmbaH)2)
Cl2] (3a)/ZnO/Si (2000 rpm, 10 min/3000 rpm, 3 min),
(d) [Cu(en(dmbaH)2)Cl2] (3a)/Si (4000 rpm, 10 min),
(e) [Cu(en(4Him)2)Cl2] (2a)/Si (1500 rpm, 5 min).
Figure 8. The hardness (HIT) and elastic modulus (EIT)
values of fabricated layers registered under the following
conditions: loading rate 200 mN/min, max. load 100 mN
(dotted bars) and loading rate 40 mN/min, max. load 20 mN
(solid bars).

Copper(II) complexes with ZnO nanoparticles thin layers
of “soft film on hard substrate type” results from Acknowledgement
microstructures of layer substrate and physical phe-
Authors would like to thank the National Science Cen-
tre (NCN), Poland for financial support (grant no.
2013/09/B/ST5/03509).
nomena, such as the indentation size effect or cracking
around the indent.73,74 The pileup around the indenter
can also occur because of relative softness of deposited
layer in comparison to the harder silicon wafer. Due to
the complexity of the studied materials, it is difficult at
this point to clearly interpret the obtained results.
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