Available online at www.sciencedirect.com
Diamond & Related Materials 17 (2008) 1194 – 1198
www.elsevier.com/locate/diamond
Characterization of UV irradiated nanocrystalline diamond
G. Speranza a , S. Torrengo b , L. Minati a , M. Filippi a , M. Castellino b , Cl. Manfredotti b ,
Ch. Manfredotti b , M. Dipalo c , A. Pasquarelli c , E. Kohn c , Hayssam El-Hajj c , E. Vittone b,⁎
b
a
Physics and Chemistry of Surfaces and Interfaces, Fondazione “Bruno Kessler”, Trento (I), Italy
Department of Experimental Physics and “Nanostructured Interfaces and Surfaces” NIS Centre of Excellence,
University of Torino, via P.Giuria 1, 10125 Torino (I), Italy
c
Department of Electron Devices & Circuits, Ulm University (D), Germany
Available online 11 February 2008
Abstract
The exposure of H-terminated nanocrystalline diamond (NCD) to ultraviolet (UV) light in air and at room temperature modifies the features of
the diamond surface, in terms of wettability, electrical conductivity and chemical reactivity. This allows the development of a soft, dry and non
invasive method to tailor the surface properties for the development of new chem- or bio-sensors.
In this work, we report about the analysis of hydrogen terminated nanocrystalline diamond films grown by hot-filament technique and of their
surface modification following UV light exposure. This UV treatment induces an hydrophobic to hydrophilic transformation and an outstanding
increase of the electrical resistivity (N 105). X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy provide insight into the role of
chemisorbed oxygen on the modification of the valence band and on the transformation of the electron affinity (from negative to positive) at the
diamond surface.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline diamond; Surface properties; Electron affinity; Hydrogen termination; Oxygen termination
1. Introduction
Nanocrystalline diamond, because of its nanostructured
surface, mechanical, optical, electrical/electrochemical properties, as well as for its biocompatibility combined with the
possibility of controlled surface bio-modification, is considered
a suitable platform for biointerfaces and biosensors [1]. Two
surface terminations are usually considered: H-terminated
diamond surfaces are biocompatible, transparent, electrically
conductive and biomolecules can be attached covalently at the
surface through photochemical processes [2]; oxygen termination induces an optically transparent and hydrophilic surface
which promotes cell adhesion and growth [1].
Among the various methods to tailor the electrical properties
of the diamond surface, the exposure to ozone has recently
attracted the interest of various researchers for fundamental
studies [3], for the fabrication of highly sensitive chem-FET [4],
or for cell adhesion and biocompatibility studies [1]. Such
⁎ Corresponding author.
E-mail address: vittone@to.infn.it (E. Vittone).
0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.diamond.2008.01.098
treatment can be produced by the exposure of diamond to
ultraviolet (UV) radiation, even if controversial data are
reported on the synergic combination of UV and ozone on the
surface treatment [5].
In this paper we report about the effects of UV irradiation in
air on the surface electronic characteristics of NCD. The
chemisorbed species, in particular the role of oxygen, as well as
the modification of the valence band, were monitored by XPS
and UPS in order to evaluate the photochemical effects of the
UV treatment. To assess the role of the surface photochemical
modification on electrical and hydrophobic properties of the
surface, XPS/UPS analyses have been correlated to conductivity and contact angle measurements.
2. Experimental
2.1. Diamond samples
NCD samples preparation consisted of 3 steps. Firstly a 4”
silicon wafer (100) was nucleated in Hot Filament CVD by
mean of BEN (Bias Enhanced Nucleation) [6]. Subsequently a
G. Speranza et al. / Diamond & Related Materials 17 (2008) 1194–1198
1195
Table 1
Process parameters for bias enhanced nucleation (BEN), diamond growth and
hydrogen termination
Pressure (Pa)
Gas flow (sccm)
Methane (%)
Microwave Power (W)
HF
Power (W)
Filament temperature (K)
Substrate temperature (K)
Process time
BEN
Growth
H-termination
2000
400
0.5
2000
400
0.3
4000
200
1
700
4800
2290
1000
2h
4800
2290
970
30 h
920
5 min
NCD film was grown onto the nucleated silicon wafer by means
of Hot Filament CVD, the growth parameters being described in
Table 1. In the end nanocrystalline diamond samples (1 × 1 cm2
surface area) were cut from the overgrown silicon wafer.
The average thickness was 5.3 μm with a dispersion of
0.4 μm as evaluated by optical reflection interferometry.
The morphology of the sample surfaces was determined
using a PSIA XE-100 Atomic Force Microscope (Fig. 1a). The
root mean square roughness of the samples surfaces, evaluated
by analyzing several 5 × 5 μm2 AFM maps, was (56 ± 3) nm; the
maximum grain size was in the sub-micrometer range as
evaluated from SEM (Fig. 1b).
Fig. 2. Red and UV Raman spectra of a NCD sample. The peak at 520 cm− 1,
relevant to the Si substrate, is visible only with red laser excitation because of the
low absorption coefficient.
Fig. 2 shows Raman spectra measured using a 244 nm (UV)
and a 632 nm (red) laser excitation. Red laser Raman indicates a
very poor transpolyacetylene contamination in the film (no
peaks at 1150 and 1480 cm− 1 [7]), stating the high quality of the
NCD diamond samples as is demonstrated also by the first order
diamond Raman line which is neither hidden by the strong
luminescence nor by the sp2 broad band around 1520 cm− 1. The
UV Raman spectra show a very pronounced diamond peak at
1333 cm− 1 (FWHM ≈ 5 cm− 1) and a broad G-band around
1569 cm− 1. Due to the much higher absorption coefficient of
244 nm light [8], UV Raman spectroscopy is much more surface
sensitive than the previous one, even if a comparison of the two
spectra has to take into account the different relative Raman
cross-section for the non-diamond carbon band [9].
2.2. Post-growth processes
After the growth process, all the diamond samples were
cleaned in a 1:2 H2O2:H2SO4 solution (piranha) and hence were
hydrogen terminated in a microwave assisted hydrogen plasma
with the parameters listed in Table 1. After the H plasma
treatment, the sample was allowed to cool down to room
temperature under H2 flux for about 90 min.
Three hydrogen plasma diamond samples (H-NCD) were
finally exposed in air at room temperature to UV irradiation
(UV-NCD). A Xe arc lamp (400 W) was used to irradiate for 3 h
one sample for XPS/UPS measurements; the other two samples,
used for electrical/wettability measurements, were exposed at a
distance of 3 cm to UV light produced by a deuterium lamp
(30 W). In both cases, ozone was clearly detected in the
environment and approximately the same exposure (i.e. the time
integral of irradiance) was adopted for all the samples.
2.3. Surface characterization
Fig. 1. (a) AFM map and (b) SEM image of a NCD surface.
XPS was carried out using a Scienta ESCA 200 instrument
equipped with a monochromated Al Kα source and a 200 mm
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G. Speranza et al. / Diamond & Related Materials 17 (2008) 1194–1198
Fig. 5. He I UPS spectra of H-terminated and UV-treated diamond.
Fig. 3. XPS wide scan spectra of H-terminated and UV-treated diamond. The
binding energy (BE) scale was calibrated with respect to the Ag Fermi edge. The
inset shows detailed scans around the C1 s peak.
hemispherical analyzer, with a pass energy of 150 eV (energy
resolution 0.3 eV). UPS spectra were recorded in a Physical
Electronics (PHI 590/550) spectrometer equipped with a helium
discharge lamp (HeI, hν = 21.2 eV and HeII, hν = 40.8 eV) and
with a double-pass cilyndrical analyzer (energy resolution
~ 0.5 eV).
Before XPS or UPS characterisation of the H-NCD, the
sample was annealed in UHV for 30 min at 400 °C in order to
remove main surface contaminants. No thermal treatments were
adopted after UV treatment.
2.4. Electrical and contact angle measurements
Electrical measurements were carried out after several days of
air exposure from the H-plasma process, through a collinear fourpoint probe head (radii = 0.04 mm, relative distance = 0.635 mm)
with tungsten carbide tips was used [10].
Fig. 4. XPS valence band spectra of H-terminated and UV-treated diamond. The
continuous line is reported only for highlight the presence of the O2 s spectral
feature.
Measurements of wettability of the diamond surface were
performed using the sessile drop method on a DATAPHYSICS
OCA 20 contact angle meter. A 2 µl drop of ultra pure water and
CH2I2 was placed onto the prepared samples surface, taking
three different measurements for each sample.
3. Results
Fig. 3 shows XPS survey spectra of H-NCD and UV-NCD
surfaces. Both spectra present a dominant C1 s peak located at
285 eV and no detectable surface contamination, except for
oxygen. The increase of the O/C atomic ratio (from 1.7% to 6%,
for H-terminated and UV-treated samples, respectively) as well
as the widening in the high energy side of the C1 s peak relevant
to the UV-NCD surface, which is attributable to ether (C–O–C),
carbonyl (C = O) or carboxyl (O = C–OH) surface contamination, prove that the UV-treatment induces the oxidation of the
Fig. 6. He II UPS spectra of H-terminated and UV-treated diamond.
G. Speranza et al. / Diamond & Related Materials 17 (2008) 1194–1198
H-NCD surface, as reported by several authors on homoepitaxial or polycrystalline diamonds [3,4].
This is further verified by contact angle (C.A.) measurements
which show a remarkable increase of the polar component of the
surface energy (σ) from H-NCD (H2O C.A. = 85°, CH2I2 C.A. =
27°, Overall surface energy σ = 45.5 mJ/m2, polar contribution
σp = 1.5 mJ/m2) to UV-NCD (H2O C.A. = 58°, CH2I2 C.A. = 29°,
σ = 50.9 mJ/m2, σp = 15 mJ/m2).
The XPS valence bands for the two surface terminations are
shown in Fig. 4. Both spectra present a sharp peak (peak II) at
about 13.7 eV which has been considered the fingerprint of the
diamond phase, whereas the structures at about 6 (III) and 18 eV
(I) have dominant p or s fractional character, respectively [11].
The UV-NCD spectrum shows also a shoulder at 26 eV to be
ascribed to O 2 s, whereas O 2p orbitals play a role in the shape
definition of the component III. However, it is worth noticing
that the valence band spectra are very similar to those relevant to
natural or homoepitaxial samples and do not show any
noticeable amorphous carbon component, whereas such contribution is evident in UV Raman spectra. This has to be
ascribed to the different surface sensivities of the two probes
(the extinction length of 244 nm photons in diamond is about
50 nm 0, whereas photoelectrons of 1200 eV kinetic energy
leads to a sampling depth of ~ 5 nm). Differently from Raman,
which probes also the diamond bulk, XPS does not reveal any
graphitic contaminant since H plasma selectively removes sp2
hybridized carbon from the surface.
UPS spectra are shown in Figs. 5 and 6. With respect to the
XPS valence band, the HeII spectrum of the H-terminated
sample shows a well resolved peak at about 8 eV (due to the
higher cross section of the C 2p state) and the diamond sp3 peak
which occurs at a slightly higher binding energy (14.2 eV).
The HeII spectrum of the UV-treated surface shows clear
differences to be ascribed to C–O bonds. In particular the O 2p
components widens the peak at 8 eV and the barely perceptible
shoulder occurring at about 2 eV, attributable to H-terminated
surfaces[12], disappears.
The high binding energy peak (NEA peak [13]) is more
intense and is located at higher binding energies in the HeI
spectrum of H-NCD with respect to UV-NCD surfaces. These
facts are compatible with the existence of a negative electron
affinity (NEA) of the H-terminated surfaces which transforms
to a positive electron affinity (PEA) when the surface is UVtreated.
In fact, the energy difference between the vacuum level (E0)
and the Fermi level (EF) (which is considered the origin of the
energy scale in any spectra) can be evaluated from the HeI
spectra (Fig. 5) by a linear extrapolation of the NEA peak to
zero (Ecut-off), whereas the valence band maximum (EV) can be
calculated from the HeII spectra through the linear extrapolation
of the onset of emission (Fig. 6).
In the H-terminated diamond surface the energy cut-off occurs
at 17.3 eV from EF (i.e. E0 −EF = hν(HeI) −Ecut-off = 3.9 eV) and
EF −EV = 1.3 eV; consequently, the electron affinity is χ =E0 −EF +
(EF −EV) −EG = −0.3 eV.
For the UV-treated Ecut-off = 16.7 eV, i.e. E0 − EF = 4.5 eV and
EF − EV = 2.6 eV; consequently, χ = +1.6 eV. The different χ value
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of the hydrogenated/oxidized diamond leads to a different
efficiency in the electron emission form the surfaces as shown
by the high BE peak in the HeI spectra in agreement with [14,15].
The electrical measurements carried out on diamond samples
kept in air after H plasma treatment yield a sheet resistivity (ρS) of
the order of 1 MΩ/□. Assuming a hole mobility of 60 cm2·V− 1·s− 1
0, the sheet hole concentration is about 1011 cm− 2. After UV
treatment, a stable and highly resistive surface (ρS N 200 GΩ/□)
was observed.
Such measurements are compatible with the electrochemical
transfer doping model [16] which assume the negative electron
affinity of the H terminated surface as an essential requirement
for the surface conductivity induced by adsorbed molecules
from an atmospheric water. However, from our data, we cannot
rule out the hypothesis that the UV treatment can generate an
high surface density of hole traps, induced by the oxygen
termination.
4. Conclusions
High quality NCD samples have been investigated prior and
after UV exposure in air and at room temperatures. The UVtreatment induces an oxidation of the surface originally Hterminated. This is verified macroscopically by the change of
the surface wettability and on the increase of the O content as
evaluated by XPS. UPS spectra show a change of the vacuum
level position which moves from − 0.3 eV (in H-NCD) to 1.6 eV
(in UV-NCD) with respect to the conduction band minimum.
The transformation from NEA to PEA of the diamond surface
can be considered one of the fundamental reasons of the
outstanding increase of the surface electrical resistivity for
samples subjected to intense UV irradiation.
Acknowledgements
This work was made possible by support from the Regione
Piemonte (Progetto D14 “Biosensori e interazione neuronisuperfici nanostrutturate”). Dr. Paola Rivolo is kindly acknowledged for her help in contact angle measurements as well as the
CSMFO group of Trento which has provided the UV sources to
perform the diamond oxidation.
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