Nanoparticle route for the synthesis of a stable and stoichiometric Cu2C2 phase - A semiconductor material

Article abstract of DOI:10.1063/1.1533852

The application of nanoparticle route for the synthesis of a stable and stoichiometric Cu₂C₂ phase, using activated reactive evaporation technique, was presented. Optical absorption coefficient of the copper carbide nanoparticle films was evaluated in the wavelength region 0.35-1.5 μm from the reflectance, transmittance, and thickness data. The observed stability and preferential growth of Cu₂C₂ was linked to the nanoparticle nature. Nanoparticle films exhibited good electrical conductivity, n-type semiconducting nature, high optical absorption coeffecient, and a size-dependent absorption edge.

Full text of DOI:10.1063/1.1533852

“Nanoparticle route” for the synthesis of a stable and stoichiometric Cu 2 C 2 phase—a
semiconductor material
B. Balamurugan, B. R. Mehta, and S. M. Shivaprasad
Citation: Applied Physics Letters 82, 115 (2003); doi: 10.1063/1.1533852
View online: http://dx.doi.org/10.1063/1.1533852
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APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 1
6 JANUARY 2003
‘‘Nanoparticle route’’ for the synthesis of a stable and stoichiometric Cu2C2
phase—a semiconductor material
B. Balamurugan and B. R. Mehtaa)
Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi,
New Delhi-110 016, India
S. M. Shivaprasad
Surface Physics Group, National Physical Laboratory, New Delhi-110 012, India
͑Received 12 June 2002; accepted 6 November 2002͒
A stable and stoichiometric Cu2C2 phase in nanoparticle form has been synthesized using activated
reactive evaporation technique. X-ray diffraction, transmission electron microscopy, and x-ray
photoelectron spectroscopy studies reveal the formation of a stoichiometric Cu2C2 nanophase
having a tetragonal structure. Cu2C2 samples have a high absorption coefficient with a
size-dependent optical absorption edge and n-type semiconducting nature. Due to its structural
stability, chemical compatibility with other low-cost semiconductor materials, and suitable electrical
and optical properties, the Cu2C2 phase has the potential of emerging as a semiconductor
material. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1533852͔
In recent years, many metastable and nonequilibrium
phases have been successfully synthesized using the ‘‘nano-
particle route.’’ The ability to stabilize nonthermodynamical
phases by synthesizing them in nanoparticle form has been
explained due to the influence of stresses, modifications due
to surface structure, large concentration of defects, alter-
ations in Gibbs free energy, and changes in bonding configu-
ration at small sizes.1–7 A number of nanophase materials
also exhibit better postdeposition stability in comparison
with the bulk and polycrystalline materials.8 In addition, the
quantum confinement and enhanced surface effects at nan-
odimensions make the nanoparticle route an excellent meth-
odology to tune optical and electronic properties.9,10 Optical
absorption edge, absorption coefficient, conductivity type,
conductivity, mobility, and carrier concentrations can be tai-
lored by varying nanoparticle size.
Various copper compounds such as copper oxides, ni-
trides, and sulphides have been studied extensively due to
their excellent optical and electronic properties.11–14 In com-
parison, there are only a few reports on copper carbides.
Copper carbide is an unusual material having two known
phases: dicopper acetylide (Cu2C2) and dicopper diacetylide
(Cu2C4).15 The electronic and Fourier transform infrared
spectroscopic studies on Cu2C2 have shown its polymeric
and semiconducting nature.16 Cu2C2 powder in the form of
cuprous acetylide polynuclear complexes has been prepared
using organometallic synthesis.17 Cu2C2 layers have been de-
posited by electrochemical deposition onto copper
electrodes.18 CunC2m (nϭ1–15 and mϭ1,2) clusters have
also been formed by gas phase aggregation.19 It has been
observed in previous studies that the freshly prepared Cu2C2
in bulk and polycrystalline forms is shock sensitive and con-
verts to Cu2C4 with a bright explosion.15 In comparison with
Cu2C4 , Cu2C2 seems to be an attractive material for opto-
electronic devices as the preliminary studies predict Cu2C2
to have good photoluminescence with a band gap of 1.5 eV
and high optical absorbing nature.16,17 Despite its useful
properties, Cu2C2 phase has not attracted much attention due
to its metastable nature and poor postdeposition stability.
In the present study, the nanoparticle route has been ap-
plied to synthesize a stable and stoichiometric Cu2C2 phase
in nanoparticle film form using an activated reactive evapo-
ration ͑ARE͒ technique.20 Prior to deposition, the chamber
having a base pressure of 5ϫ10Ϫ6 Torr was filled with a
mixture of methane and argon (CH4ϩAr) gases up to 1 Torr
followed by evacuation to 5ϫ10Ϫ6 Torr. This process was
repeated several times to eliminate the oxygen residual spe-
cies in the chamber for minimizing the possibility of copper
oxide formation. The evaporation of high-purity copper
through CH4ϩAr plasma leads to copper carbide growth.
High flow rate ͑FR͒ of CH4 and Ar, high gas pressure (P),
and low substrate temperature (Ts) resulted in the formation
of copper carbide nanoparticles on glass and indium doped
tin oxide ͑ITO͒ substrates. Copper carbide nanoparticle film
sample CC1, prepared at Pϭ5ϫ10Ϫ3 Torr, FRϭ20 sccm,
and Tsϭ30 °C, and sample CC2, prepared at Pϭ5
ϫ10Ϫ3 Torr, FRϭ20 sccm, and Tsϭ200 °C have been in-
vestigated. A glancing angle x-ray diffractometer ͑XRD͒
͑Geigerflex-D/max-RB-RU200, Rigaku͒, transmission elec-
tron microscope ͓͑TEM͒, JEOL TEM 200 CX͔, x-ray photo-
electron spectrometer ͑XPS͒ ͑Perkin Elmer-1257 using a Mg
K␣ radiation of Eϭ1253.6 eV and a hemispherical section
analyzer with 25 meV resolution͒, and optical spectropho-
tometer ͑Hitachi-330 UV–VIS–NIR͒ have been used to
characterize the copper carbide nanophase. A Keithley 224
programmable current source and Keithley 2182 nanovolt-
meter have been employed for the electrical measurements
using van der Pauw’s configuration. Schottky junctions were
formed by vacuum evaporation of silver ͑Ag͒ onto the
Cu2C2 nanoparticle films deposited on ITO substrates.
The x-ray diffractogram of the nanoparticle film sample
prepared at 30 °C ͑sample CC1͒ is shown in Fig. 1. The
crystal structure of the copper carbide lattice has been deter-
a͒
Author to whom correspondence should be addressed; electronic mail:
brmehta@physics.iitd.ernet.in
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116
Appl. Phys. Lett., Vol. 82, No. 1, 6 January 2003
Balamurugan, Mehta, and Shivaprasad
FIG. 1. X-ray diffractogram of copper carbide nanoparticle film sample
CC1. TEM micrograph ͑inset͒ of sample CC1 clearly shows the nanoparticle
nature.
FIG. 3. Optical absorption coefficient ͑␣͒ as a function of wavelength ͑␭͒ in
copper carbide nanoparticle films ͑a͒ sample CC1, thickness 150 nm, and ͑b͒
sample CC2, thickness 100 nm.
mined from the observed ‘‘d’’ values by using an analytical
method.21 This analysis shows that the copper carbide nano-
particles have tetragonal crystal structure with lattice con-
stants: aϭ4.94 Å and cϭ5.23 Å. The observed d and the
corresponding calculated (hkl) values are 2.93 Å, ͑111͒;
2.47 Å, ͑200͒; 2.09 Å, ͑112͒; 1.81 Å, ͑202͒; 1.55 Å, ͑310͒;
1.50Å, ͑311͒; 1.45 Å, ͑222͒; and 1.27 Å, ͑400͒. The broad
peak at about 2␪ϭ25° corresponds to the glass substrate.
Some low-intensity peaks could not be identified. The values
of the particle size estimated using Scherer’s equation for
samples CC1 and CC2 are 5.5 and 8.5 nm, respectively. The
TEM micrograph ͑inset of Fig. 1͒ shows that most of the
nanoparticles are in the size range 5–8 nm in sample CC1.
The XPS spectrum of the as-deposited samples has a
small oxygen peak due to adsorbed oxygen, which com-
pletely disappears on a short Ar ion sputter cleaning. The C
1s and Cu 2p3/2 core level XPS spectra of sample CC1 after
5 min sputtering ͑Fig. 2͒ seems to be comprised of two
peaks, indicating two different copper carbide ͑probably,
to the atoms having different electron negativity values in
comparison to identical coordination of carbon atoms in
Cu2C2 (CuuCwCuCu). Due to this, Cu2C4 is more ionic
compared to Cu2C2 . Thus, the higher-energy C 1s and Cu
2p3/2 XPS peaks ͑A2 and B2͒ are assigned to the Cu2C4
phase and lower-energy C 1s and Cu 2p3/2 peaks ͑peaks A1
and B1͒ are assigned to the Cu2C2 phase. The stoichiometric
ratio x/y in CuxCy (x/yϭ1.01, 0.51͒ ͑calculated using the
area of A1, B1; A2, B2 peaks and sensitivity factors for Cu
and C͒ are quite close to the stoichiometric values of 1.00
and 0.50 corresponding to Cu2C2 and Cu2C4 phases, respec-
tively. This, along with the absence of a Cu 2p3/2 peak cor-
responding to elemental copper ͑at 932.2 eV͒ in the XPS
spectra, confirms that Cu is present only in the form of cop-
per carbide. The nanoparticle sample CC1 has a predominant
Cu2C2 phase ͑88.8 at. %͒. Thus, the XPS data together with
the XRD results confirm the growth of copper carbide nano-
particles with a stable Cu2C2 phase. Cu2C2 nanoparticles
prepared using the ARE technique in the present study have
been observed to be quite stable in normal ambient condi-
tions and do not show any shock sensitive nature. Freshly
prepared Cu2C2 in bulk powder form is reported to be un-
stable. A solid-state reaction known as the Glaser oxidative
coupling reaction converts bulk Cu2C2 into Cu2C4 phase.15
The stability of the Cu2C2 observed in the present study is a
consequence of the nanoparticle nature. As already men-
tioned, the influence of nanoparticle size on the growth and
stability of various crystallographic phases has been studied
in various nanoparticle systems such as Cu2O, CeO2 ,
PbTiO3 , PbZrO3 , Al2O3 , Fe2O3 , BaTiO3 , and ZrO2 .4 The
additional surface energy term due to the large nanoparticle
surface area modifies the Gibbs energy, resulting in the sta-
bility of nonthermodynamical phases.22 The structural de-
fects, lattice distortions, and changes in the bond character at
small size have also been observed to enhance the nanophase
stability.4–7
Cu2C2 and Cu2C4) phases. Both peaks, C 1s and Cu 2p3/2
,
have been deconvoluted into two peaks at 285.3 eV ͑peak
A1͒, 288.2 eV ͑peak A2͒ and 933.1 eV ͑peak B1͒, 935.7 eV
͑peak B2͒, respectively. The absence of the satellite peaks in
Cu 2p3/2 spectra ͑which is a strong signature of the CuO
phase͒ and the complete disappearance of the adsorbed oxy-
gen peak upon sputter cleaning rules out the presence of any
oxide phase ͑CuO or Cu2O). The difference in the binding
energy of the C 1s and Cu 2p3/2 peaks ͑position of A1, A2
and B1, B2 peaks͒ from the value of elemental carbon ͑284.6
eV͒ and copper ͑932.2 eV͒, respectively, confirms the
copper–carbon bond formation. In Cu2C4 (CuuCwCuC
wCuCu), carbon atoms across the triple bond are attached
The optical absorption coefficient ͑␣͒ of the copper car-
bide nanoparticle films ͑Fig. 3͒ has been evaluated in the
wavelength region 0.35–1.5 ␮m from the reflectance, trans-
mittance, and thickness data.23 A high value of
␣
(ϳ105 cmϪ1) in these samples reveals the strong absorbing
nature of the copper carbide. As shown in Fig. 3, strong
excitonic peaks are observed at 2.0 eV ͑sample CC1͒ and
FIG. 2. Core level C 1s and Cu 2p3/2 XPS spectra of copper carbide nano-
particle film sample CC1 after 5 min sputtering. Note that the C 1s peak is
magnified (ϫ23) compared to the Cu 2p3/2 peak.
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1.80 eV ͑sample CC2͒. The delocalization of the band struc-
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Appl. Phys. Lett., Vol. 82, No. 1, 6 January 2003
Balamurugan, Mehta, and Shivaprasad
117
pound semiconductors ͑CdS, ZnS, ZnSe, CdO, ZnO, In2O3 ,
and SnO2) has been the most nagging problem affecting the
stability and efficiency of these devices.11 Use of Cu2C2 in
these devices will significantly reduce this problem.
In summary, this is a report on the application of the
nanoparticle route for the synthesis of a stable and stoichio-
metric Cu2C2 phase using the ARE technique. The observed
stability and preferential growth of Cu2C2 is linked to the
nanoparticle nature. It has been shown that Cu2C2 has a te-
tragonal unit cell with lattice parameters aϭ4.94 Å and c
ϭ5.23 Å. Cu2C2 nanoparticle films exhibit good electrical
conductivity, n-type semiconducting nature, high optical ab-
sorption coeffecient, and a size-dependent absorption edge.
These properties can be fine tuned by controlling the nano-
particle size and postdeposition treatments. The synthesis of
a stable nanophase of Cu2C2 , shown in the present study,
will result in Cu2C2 emerging as a semiconductor material
for various optical and electronic devices.
FIG. 4. Current–voltage characteristic of Cu2C2 –metal ͑Ag and Al͒ junc-
tions. Inset shows the device configuration.
The authors express gratitude to Professor K. L. Chopra,
Emeritus Professor, Indian Institute of Technology Delhi, In-
dia, for useful discussions and encouragement. The financial
support provided by the Ministry of Non-conventional En-
ergy Sources ͑MNES͒, Government of India, is gratefully
acknowledged.
ture at nanodimensions results in the predominant excitonic
peaks in the optical absorption spectra. The appearance of
excitonic features in the absorption spectra of nanoparticles
and nanocrystalline films is a signature of the discreteness in
the intraband structure due to quantum confinement, and the
energy position of the first excitonic peak is normally taken
as the absorption edge.24,25 The band gap of the bulk copper
carbide has been reported to be 1.5 eV. The observed blue-
shift in the optical absorption edge in the copper carbide
samples is attributed to the quantum size effect at nanodi-
mensions. The increase in the optical absorption edge from
1.80 to 2.0 eV with decreasing substrate temperature is thus
due to the decrease in crystallite size. Hall measurements
were carried out on these samples using van der Pauw’s con-
figuration. These measurements showed n-type electrical
conduction in copper carbide. Electrical conductivity, carrier
concentration, and mobility for sample CC1 are 1.3
ϫ102 ⍀Ϫ1 cmϪ1, 2.4ϫ1020 cmϪ3, and 3.4 cm2/V s, respec-
tively. The I–V characteristics of the ITO–Cu2C2 –metal
͑Ag and Al͒ structures are shown in Fig. 4. The I–V curve of
Cu2C2 –Ag shows a good rectifying behavior. The forward
bias characteristic is observed on applying negative bias to
the Cu2C2 layer with respect to Ag contact, and this also
confirms its n-type semiconducting nature. The linear and
symmetric I–V characteristic in the Cu2C2 junction with Al
metal indicates that the Cu2C2 –ITO ͑and, also, Cu2C2 –Al)
contact is Ohmic and the rectifying I–V curve in the case of
ITO–Cu2C2 –Ag is due to the Schottky junction formed at
the Cu2C2 –Ag contact. The observation of a relatively larger
current observed in these devices is due to the nanoparticle
nature of the Cu2C2 films. A large surface-to-volume ratio at
nanodimensions results in a large interfacial area at the junc-
tion interface, and this has been observed to result in a higher
forward current density in the CdS and PbS nanoparticle film
based Schottky and heterojunction devices.26,27
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