Nano-particulate CuI film formed on porous copper substrate by iodination

Article abstract of DOI:10.1016/j.cplett.2004.02.058

A nano-particulate thin CuI film is fabricated by iodination of a porous copper substrate that was prepared by using an alumina mask and characteristic properties of these films are studied.

Full text of DOI:10.1016/j.cplett.2004.02.058

Chemical Physics Letters 387 (2004) 400–404
www.elsevier.com/locate/cplett
Nano-particulate CuI film formed on porous
copper substrate by iodination
a
b
a
a,
b
Yang Yang , Xuefei Li , Bin Zhao , Huilan Chen *, Ximao Bao
a
Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing University,
Hankou Road 22, Nanjing, Jiangsu 210093, PR China
Department of Physics, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, Jiangsu 210093, PR China
b
Received 13 June 2003; in final form 19 February 2004
Published online: 10 March 2004
Abstract
A nano-particulate thin CuI film is fabricated by iodination of a porous copper substrate that was prepared by using an alumina
mask and characteristic properties of these films are studied.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
iodination of a porous copper substrate, which was
prepared by using a PAA mask. To the best of our
knowledge, there are few reports relating to the prep-
aration of semiconductor thin films by this method
until now. The formation process, morphology, and
corresponding properties of the prepared CuI films are
investigated.
Nano-particulate semiconductor thin films which
present restricted size and regular space distribution have
been extensively studied [1,2]. Copper iodide (CuI) is one
of a few kinds of p-type high band gap materials, which
has been identified in the fabrication of conductive
transparent thin films. In Particular, nano-particulate
CuI thin films have potentiality for application in dif-
ferent kinds of electronic devices. Various techniques
have been used to prepare such a structure, including
radio frequency/direct current (rf–dc) coupled magne-
tron sputtering [3], pulse laser deposition (PLD) [4], hy-
brid electrochemical/chemical synthesis [5], and others
[6–8]. It is well known that semiconductor thin films can
be prepared using porous anodic alumina (PAA) as a
template. As a typical self-assembly film, PAA holds
nearly parallel nanoporous structure [9–13], thus func-
tional materials are usually deposited into its pores to
form nanostructured composite films [14–16]. Addi-
tionally, however, PAA film could also be utilized as a
mask for the underlying substrate to prepare certain
nanostructures. In this Letter, we describe a novel and
facile method to fabricate a nano-particulate CuI film by
2. Experimental
2.1. Synthesis of nano-particulate CuI thin film
A copper plate was mechanically polished and ul-
trasonically cleaned with a detergent. After removing
the grease and the native oxidation layer, highly pure
aluminum film (99.99%) with a thickness of about
400 nm was deposited on the pretreated copper plate
using the electron beam evaporation technique. The
vacuum chamber was maintained under a pressure of
2.5 ꢀ 10ꢁ6 Pa. The accelerating voltage of the electron
beam is kept constant near 10 kV and the electron gun
current of 0.5 A. The surface of Al film deposited on Cu
substrate does not exhibit any cracks or pinholes, and
was directly used for anodization.
The anodization process was conducted at a constant
cell potential in a 15 wt% sulfuric acid electrolyte at
room temperature (25 °C). The prepared Al/Cu plate
* Corresponding author. Fax: +86-2583317761.
E-mail address: hlchen@nju.edu.cn (H. Chen).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.02.058

Y. Yang et al. / Chemical Physics Letters 387 (2004) 400–404
401
was used as the working electrode in the electrochemical
3. Results and discussion
cell. Only the Al surface of Al/Cu plate contacted the
electrolyte in order to avoid anodizing the Al-absent
copper face in the acid solution. The distance between
the Pt counter electrode and the Al/Cu working elec-
trode was kept at 2 cm. The voltage was set at a constant
DC 5 V, allowing for copper’s good conductivity. Syn-
chronous current-time (I–t) curve during the anodizing
process was recorded to characterize the formation of
both PAA mask and porous Cu surface. The as-treated
Cu plate with porous surface was washed and dried for
the following iodination reaction [17].
After washed with 2-propanol, the Cu plate sample
was kept suspended in a chamber, where it reacted
with the iodine vapor mixed with 2-propanol gas car-
ried over by N2 steam (50 ml minꢁ1) for 5 min. A CuI
thin film was formed and its thickness is about 400 nm
determined by a chemical method [17]. The synthetic
procedure of the nano-particulate CuI thin film grown
on the surface of Cu plate is shown in Scheme 1. For
comparison, copper plate not treated by PAA mask
was also anodized in acid solution and then used to
prepare CuI film by the same method mentioned
above.
Fig. 1 presents typical anodization I–t curves of alu-
minum layer deposited on substrates copper (a) and
silicon (b), respectively. The shapes for both of the
curves in A–C range are very similar to that of bulk Al
plate [18], because at this stage the electrolyte only
contacts the Al film and the substrate only works as an
anode. With the exhaustion of Al film (around point C),
the alumina barrier layer will become thinner. When the
electrolyte passes through the porous layer of PAA film
and gradually extends to the aluminum/substrates in-
terface, different processes will take place, which de-
pends on the various types and/or properties of the
substrates. In case of chemically stable Au [19] or in-
dium tin oxide (ITO) [20], the substrates will just act as
the conductive media layer for the growth of PAA film.
If the substrate is metal Ta [21] or semiconductor Si
[22,23], the PAA film produced in the former stage will
function as a mask for the local anodization of the un-
derlying materials. As a result, pillared Ta2O5 [21], po-
rous silicon nanostructures [22], and/or nanoscaled
island-shaped SiO2 films [23] have been acquired by this
way. For our Al/Cu system, since the Cu substrate
contacts the electrolyte after point C, the current grad-
ually increases, as shown in curve a. At the same time,
the copper crystals at the bottom of PAA pores are lo-
cally oxidized. The formed copper ions depart from the
Cu substrate and enter the solution, leaving many fal-
lings on the Cu surface. With these fallings extending,
the PAA mask will peel off and the current intensity will
further increase. At this stage the anodization process is
ended (around point D). Therefore, we can detect and
control the growing process of porous Cu substrates by
using the I–t curve. For the Al/Si system (curve b), sil-
icon crystals at the bottom of PAA pores are locally
oxidized into SiO2, thus the current for curve b is evi-
dently reduced after point C.
2.2. Instruments and measurements
The microstructures of the prepared specimens were
observed by a scanning electron microscope (Hitachi
X650), and an atomic force microscope (Nanoscope
IIIa), respectively. Elemental analysis was made by X-
ray energy dispersive spectroscopy (EDAX PV9100)
equipped on SEM. The phase identification was char-
acterized by X-ray diffraction patterns (SHIMADZU
XD-3A diffractometer) with a Cu Ka radiation source
(35 kV) and X-ray photoelectron spectra (ESCALab
MK2 photoelectron spectrometer) using Al Ka as the
exciting source. Emission studies of the films were car-
ried out by a photoluminescence measurement (Flu-
oroMax-2 spectrometer) with a Xe lamp as the
excitation light source at room temperature. The exci-
tation wavelength of the samples was 340 nm.
Fig. 2a shows the SEM image of the Cu plate surface
anodized without PAA mask. It is observed that the
copper plate forms a continuous mountain-like surface.
CuI film was then produced on this copper surface by
Scheme 1. Fabrication process of the nano-particulate CuI thin film grown on the surface of Cu plate.

402
Y. Yang et al. / Chemical Physics Letters 387 (2004) 400–404
face. As observed from SEM image of Fig. 2b, average
D
size of CuI particles in the prepared film is about
300–400 nm and the particle shape is very irregular.
For the Cu film treated with PPA mask and its cor-
responding CuI film prepared by subsequent iodination
reaction, no Cu or CuI grains were observed even at the
maximum enlargement power of SEM. Thus, AFM
measurements were employed to detect their surface
morphology. Fig. 2c exhibits a typical three-dimensional
AFM picture of the Cu surface treated with PAA mask,
from which a homogeneous and uniform structure ap-
pears on Cu substrate. Each isolated Cu grain has an
average diameter of about ꢂ40 nm. The corresponding
X-ray energy dispersive spectroscopy (XEDS) analyses
prove that this surface is composed only of the element
Cu, not the residues of the peeled PAA film. It is no-
ticeable that some fallings are clearly seen among the Cu
grains. It might be supposed that each PAA pore orig-
inally occupied the position of each falling. Therefore,
the PAA mask ought to be responsible for the formation
of the porous Cu substrate. From Fig. 2d, it can be seen
that the CuI film prepared through the mask route ex-
hibits distinct packed nano-particulate structure with an
average diameter of about ꢂ40 nm.
A
C
a
B
B
A
b
C
D
0
20
40
60
80
100
Time/s
Fig. 1. Current density vs time (I–t) curve during anodization. (a) Al/
Cu system, (b) Al/Si system.
the interfacial iodination reaction: Cu + I2 ! 2CuI. The
existence of 2-proganol vapors in mixed I2 gas can lower
the vapor pressure of the iodine, which may assist the
formation of a uniform CuI layer [17]. After reaction, a
shallow-white thin layer could be found on the Cu sur-
The phase of the CuI film prepared through the PAA
mask route was checked by X-ray diffraction (XRD)
Fig. 2. SEM images of Cu plate anodized in acid solution without PAA mask (a) and CuI grains grown on the untreated Cu substrate surface (b).
Typical 3D AFM images of Cu plate treated by PAA mask (c) and CuI grains grown on the treated Cu substrate surface (d).

Y. Yang et al. / Chemical Physics Letters 387 (2004) 400–404
403
pattern. As shown in Fig. 3a, three characteristic peaks
at 42.9, 49.9 and 73.8 ° are attributed to Cu (1 1 1),
(2 0 0), and (2 2 0) reflections, respectively. Except for the
peaks originating from Cu substrate, a new and well-
resolved pattern appears due to the formation of a
c-phase of CuI crystals. The peak at 2h ¼ 25:2° is as-
signed as the (1 1 1) reflection line. Meanwhile, no peaks
from other species were observed. The inset of Fig. 3a
shows CuI (1 1 1) reflection of the samples fabricated
without (curve 1) and with (curve 2) the PAA mask.
Curve 2 affords an obvious broad reflection for the
(1 1 1) plane of CuI crystals, which indicates the for-
mation of ultra fine CuI grains through the mask route.
The average particle size is estimated to be 36 nm by the
Scherrer equation, conforming to the results of AFM
observations.
XPS spectra were also used to determine the chemical
composition of the prepared CuI film. The C1s peak
located at 284.6 eV was taken as the internal standard
and different peak positions were normalized with re-
spect to it. All the observed peaks could be assignable to
Cu, I, C, or O and no traces of other ions are revealed
(this figure is not shown in the text). The presence of C is
attributed to gaseous molecules adsorbed on the film
surface. Fig. 3b exhibits the higher-resolution spectra of
the Cu and I regions of the CuI film prepared with the
PAA mask (curve 2). The corresponding spectra for the
CuI film prepared without the PAA mask is also shown
as a reference (curve 1). From a quantitative standpoint
for both of the samples, the observed Cu 2p1=2 photo-
emission peak is at around 954.0 eV and the I 3d5=2
photoemission peak is at 619.3 eV. All of those observed
peaks match those of the standard and are in good
agreement with the values of 952.9 and 620.0 eV re-
ported for CuI film prepared by other methods [3]. In
addition, obvious shake-up peaks appear in curve 1 at
the position with the energy 8–10 eV higher than that of
Cu 2p3=2 or Cu 2p3=2, but no such a phenomenon ap-
pears in curve 2, which demonstrates that an amount of
Cu2þ (i.e., CuO) might exist in the CuI film prepared
without the PAA mask [24]. The absence of Cu2þ in the
sample with the PAA mask may be attributed to the
porous structure of Cu substrate, which could control
crystal growth of CuI, thus making the iodination re-
action moderate and protecting Cu near the surface
from oxidizing under the fierce reaction conditions.
However, Cu0 from the substrate and Cuþ from CuI
cannot be distinguished by the XPS data because their
photoemission lines are very close. Thus, we cannot
obtain the actual ratio of Cu/I for the prepared film by
comparing their corresponding peak areas.
Photoluminescence spectrum of the sample prepared
with the PAA mask was shown as curve a in Fig. 4. For
comparison, the PL spectrum of CuI film converted
from the Cu substrate without the PAA mask was also
determined (curve b). Both of the samples present a
characteristic emission peak at the position of 411 nm
(3.0 eV), which is located at lower energy position
compared to a previously reported bandgap of CuI
(about 3.1 eV) [5]. This observed emission peak might be
due to the surface trapping sites (ꢂ0.2 eV higher the
valence band edge of CuI) created by adsorbed iodine
because samples of CuI always contain a stoichiometric
excess of iodine [25]. At the same time, it can be found
that the emission intensity from the CuI film grown on
the Cu surface treated by PAA is higher than the one
from reference. Though the amount of excess iodine in
CuI films might influence the PL intensity of CuI [25],
formation of ultra fine CuI grains may be another im-
portant reason. It can be suggested that the porous Cu
substrate should hold larger surface areas compared to
the untreated Cu surface. So, the fabricated CuI film will
Fig. 3. (a) X-ray diffraction pattern of CuI film produced on the surface of Cu plate treated with the PAA mask. Inset is CuI (1 1 1) reflection of CuI
film prepared without (1) and with (2) the PAA mask. (b) XPS close-up survey spectrum of Cu 2p and I 3d cores of CuI film prepared without (1) and
with (2) the PAA mask.

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Y. Yang et al. / Chemical Physics Letters 387 (2004) 400–404
20071017) and the Specialized Research Fund for the
Doctoral Program of Higher Education (No.
200028401).
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