F

MOa

b

c

a

AA

P76886

KNDSGS

1

ninhmtdano

Gdgfc

Rf

0d

Applied Surface Science 256 (2010) 5602–5605

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

abrication of diamond nanorods for gas sensing applications

arina Davydovaa,b,∗, Alexander Kromkaa, Bohuslav Rezeka,leg Babchenkoa, Martin Stuchlikc, Karel Hruskaa

Institute of Physics, Academy of Science of the Czech Republic, Cukrovarnicka 10, 16200 Prague, Czech RepublicDepartment of Physics, Faculty of Civil Engineering, CTU in Prague, Thakurova 7, 16629 Prague, Czech RepublicDepartment of Chemistry, TU Liberec, Studentska 2, 46117 Liberec, Czech Republic

r t i c l e i n f o

rticle history:vailable online 11 March 2010

ACS:3.63.Bd1.46.Km1.05.ug1.15.Gh

a b s t r a c t

Diamond nanorods were fabricated for a sensing device by utilizing reactive ion etching in CF4/O2 radiofrequency plasma. The length of the nanorods has been controlled by the ion etching time. The obtainedmorphologies were investigated by scanning electron microscopy. The gas sensing properties of the H-terminated diamond-based sensor structures are indicating that we have achieved high sensitivity todetect phosgene gas. Also, our sensor exhibited good selectivity between humid air and phosgene gasif the measurement is conducted at elevated temperatures, such as 140 ◦C. Furthermore, such sensorresponse rating could reach as high value as 4344 for the phosgene gas, which was evaluated for the

8.55.A

eywords:anocrystalline diamondiamond nanorodsurface conductivity

sample consisting of the longest nanorods (up to 200 nm).© 2010 Elsevier B.V. All rights reserved.

as sensorEM

. Introduction

Recently, one-dimensional nanostructures such as nanowires,anotubes, nanocones and nanorods, respectively, have becom-

ng widespread [1–4]. These structures are being used in a largeumber of electronic and gas sensing devices among others. Itas been found that the one-dimensional nanostructures, amongany possible geometrical configurations, exhibit distinctive fea-

ures that are not present in the bulk films, for instance [5]. The mainifference between the two sensing nanostructures, i.e. nanorodsnd thin films, is in the higher surface-to-volume ratio values inanorods as compared to thin films, which has significant impactn the number of possible reaction sites.

In comparison to common sensitive materials, such as SnO2,a2O3, TiO2, etc., diamond is very attractive material for this field

ue to its inherent physical and chemical properties [6–8]. Theeometrical diversity of 1D diamond structures has been alreadyabricated using reactive ion etching and by employing varioushemistries [9,10]. For example, Rakha et al. [11] demonstrated

∗ Corresponding author at: Institute of Physics, Academy of Science of the Czechepublic, Cukrovarnicka 10, 16200 Prague, Czech Republic. Tel.: +420 220 318 511;

ax: +420 233 343 184.E-mail address: davydova@fzu.cz (M. Davydova).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.03.034

fabrication of diamond nanorods (DNRs) by hydrogen plasma post-treatment of nanocrystalline diamond (NCD) films, where their “topsurface” diameter was approx. 5 nm. Nanorods of single crystallinediamond with a diameter of ∼200 nm have also been producedcombining a microwave plasma treatment with a reactive ion etch-ing method [4]. A uniform diamond nanocones array was formedwith a density of 2 × 108 cones/cm2 by using selective ion sputter-ing process [3].

We have pointed out in our previous publication that theincreased surface-to-volume ratio in the porous-like structure for-mations is playing a crucial role in enhancing the sensitivity of suchsystems [12]. In this work, we are presenting routes for fabricationof diamond nanorods using a dry plasma etching process. We areinvestigating the impacts of the etching time on the surface geom-etry of the formed diamond nanorods and on their gas sensingproperties. The gas sensing properties of the H-terminated dia-mond device are examined using various probe gases. The influenceof the probe gas concentration and the sensor operation tempera-ture is also investigated.

2. Experimental

The process of fabricating diamond nanorods was carried out inthe following steps: First, before NCD growth, the Ti/Au electrodes

rface S

(mUewptfiboh3tRptwtp[

qIsttegit

Fn

M. Davydova et al. / Applied Su

Ti/Au = 30/50 nm) were deposited on the Al2O3 substrates by ther-al evaporation method and these were patterned by a standardV-lithography and lift-off technique. Each pair of interdigitatedlectrodes is separated by a 200 �m wide gap. Then, the substratesere seeded by a dispersed detonation nanocrystalline diamondowder with an average grains size of 5 nm using an ultrasonicreatment procedure for 40 min. Nanocrystalline diamond (NCD)lms have been directly grown on Ti/Au interdigitated electrodesy microwave plasma enhanced CVD process in n ellipsoidal res-nator (Aixtron P6). The process parameters were the following:ydrogen gas flow 300 sccm, methane gas flow 3 sccm, gas pressure0 mbar, and microwave power 1000 W. The final film thickness ofhe diamond films was 400 nm. Previously we have shown that theaman spectrum of such NCD films consist of a dominant sharpeak, centered at the wavenumber of 1330 cm−1, which representshe optical phonon in diamond [12]. Then, the grown NCD filmsere coated with 2.5 nm of Ni metal layer using thermal evapora-

ion and treated in hydrogen plasma for 5 min. After a hydrogenlasma treatment, nano-sized particles of Ni layers were formed13].

Etching and realization of DNRs was provided by radio fre-uency (RF) plasma, in CF4/O2 gas mixture (Phantom LT Reactive

on Etch (RIE) System, Trion Technology). Our recent study hashown that the etching at low RF power (100 W) results in a forma-ion of DNRs with diameter ranging from 15 to 40 nm and height in

he range of 150–210 nm [13]. Samples presented in this study weretched in gas mixture of 2 sccm CF4 diluted in 50 sccm O2; the totalas pressure was 150 mTorr and RF power 100 W. The various etch-ng times were used (0, 1 and 5 min). After the etching procedure,he remaining Ni nanoparticles were carefully removed by wet

ig. 1. SEM images (left) and schematic illustration (right) of surface morphology of thanoparticles on the top diamond surface), (b) 1 min and (c) 5 min. The samples were me

cience 256 (2010) 5602–5605 5603

etching in a diluted nitric acid mixture (1:1 of HNO3 and H2O). Thewet etching time was 4 min at room temperature. Finally, all sam-ples (without and with DNRs) were exposed to hydrogen plasmafor 10 min in order to induce a p-type surface conductivity [14].The surface morphology of the dry-etched diamond nanostructurebefore and after removing of the Ni nanoparticles, respectively, wasinvestigated by scanning electron microscopy (SEM, Raith e LiNE).

The gas sensing properties (as specified by electrical conductiv-ity measurements) were conducted at two different temperatures.Firstly, at room temperature and secondly at the temperature of140 ◦C, both at voltage of 1 V, frequency of 3 kHz, and measurementperiod of 5 s (LCR meter HIOKI 3532-50). The following testing gaseswere used: (a) humid air (RH = 23%); (b) phosgene (COCl2). The gassensing setup has been described in Ref. [15].

3. Results and discussion

Fig. 1 shows the SEM images of surface morphologies of the sen-sor substrates (ceramic + electrodes) coated with NCD layer (Fig. 1a)and structured diamond films by reactive ion etching in CF4/O2plasma for different etching time (Fig. 1b and c). In Fig. 1a, SEMmicrograph of non-etched NCD film also shows Ni nanoparticleswhich were used as the testing material for the nanorods fabri-cation. The primary NCD film (i.e. non-etched) is continuous andreveals grains sizes of up to 200 nm. After etching for 1 min by

the RIE process, the flat NCD film changed to an array of nanorods(Fig. 1b). The diameter of nanorods varies in the range between20 and 30 nm and their length was up to 40 nm. When the etch-ing time was extended to 5 min, the length of nanorods increasesto 150–200 nm (Fig. 1c). It must be noted that this DNRs length

e used NCD films as the sensing part where the etching time was (a) 0 min (Niasured at angle of 45◦ towards the SEM detector.

5604 M. Davydova et al. / Applied Surface Science 256 (2010) 5602–5605

F nt cot

inits

tra

sc

S

wsa

gtgetTt

Ca

F

ig. 2. Time dependence of surface conductivity of H-terminated DNRs to differeemperature and at 140 ◦C, respectively.

s assumed from on our previous study [13]. The diameter of theanorods is approx. 30 nm, i.e. the same as that had been observed

n the previous case (Fig. 1b). The SEM images clearly show thathe diamond nanorods are aligned well vertically to the base of theensor substrate and their density is ∼4 × 1010 rods/cm2.

Fig. 2 illustrates the surface conductivity of hydrogenated DNRso phosgene in different concentrations, i.e. 5, 10 and 20 ppm,espectively. The samples were measured at room temperature andt 140 ◦C, respectively.

The sensor response is defined as the relative variation in theurface conductivity due to the presence of a specific gas and it isalculated as the following:

RNS =(

�g

�0− 1

)× 100% (1)

here �g represents the conductivity value in the presence of apecific gas and �0 represents the conductivity value measured indry air.

As it is illustrated in Fig. 2, exposure of the sensor to a phosgeneas leads to an increase in the surface conductivity as compared tohe initial surface conductivity value. The non-etched (0′) hydro-enated NCD surface has the highest conductivity (1 × 10−5 S) afterxposure to 20 ppm of COCl2 at room temperature. However, it hashe weakest sensor response value to the phosgene gas SRNS = 28%.

he SNRS value has been increased up to 150% at higher tempera-ure measurements.

Generally, the highest SNRS response is found for 20 ppm ofOCl2 for sample etched for 5 min. The SNRS values for this samplere 677% and 4344% for temperatures 25 and 140 ◦C, respectively.

ig. 3. Time dependence of H-terminated DNRs surface conductivity to 23% of relative hu

ncentration of phosgene gas. During measurements samples were kept at room

It is evident that the gas response value at ambient is relativelysmall as comparing to the gas response value measured at 140 ◦C(Fig. 2).

Fig. 3 shows the time dependence of surface conductivity of theH-terminated sensor devices to the humid air (RH = 23%) measuredat 25 and 140 ◦C, respectively. Once again, the non-etched sampleexhibits the highest surface conductivity. The room temperaturemeasurements are revealing increase in the surface conductivityapprox. by up to 8 × 10−6 S for the non-etched sample and by upto 9 × 10−7 S for the sample etched for 5 min, respectively. On theother hand, measurements conducted at 140 ◦C are resulting inlowering the surface conductivity values for all samples. It hasbeen observed that the surface conductivity of H-terminated DNRsin humid environment decreases as the measurement tempera-ture increases from 25 to 140 ◦C, respectively. It should be notedthat this is an opposite trend as compared to the measurementsobserved using the phosgene gas, for which the SRNS responsevalue is increases when the measurement is conducted at 140 ◦C.This indicates that the selectivity of the diamond-based sensorto phosgene is good versus the humid air if it is measured at140 ◦C.

The origin behind this behavior is quite complex and it is stillunder studies. It is known that for specific gas environments thetransfer of electrons can be changed with the type of the gas. For

example, if the H-terminated DNRs surface is exposed to “oxidiz-ing” gases, its surface conductivity will increase. In our case, suchan increase has been observed for COCl2 or humidity. The surfaceconductivity rose up on exposing the hydrogenated DNRs sensordevices to phosgene. Adopting the previously published model for

midity (RH) as a function of the temperature of the measurement (25 and 140 ◦C).

rface S

ofapcefita

mnodtaD

4

sddocbfstr

[[

[

M. Davydova et al. / Applied Su

xidizing gases [3], we suppose that the electrons are transferredrom the DNRs surface into the adsorbed “aqueous COCl2” interfacend thus, the surface conductivity increases. The chemical sensingroperties of the hydrogenated DNR array sensor are enhanced in-omparison with the as-formed diamond film. Nanorods exhibitxceedingly high surface-to-volume ratios and considerable sur-ace activity. As a result, better gas response and selectivity arendicated. Besides, we found that that increasing measurementemperature enhanced the sensor selectivity between humiditynd phosgene (Figs. 2 and 3).

Finally, it can be noted that the same electrical conductivityeasurements were provided for the hydrogenated DNRs with Ni

anoparticles before removing them from the structure. The resultsbtained were almost identical. The surface conductivity of suchevice is decreased by an order of magnitude from the non-etchedo the etched (5 min) sensor substrate. Based on these results were concluding that the gas sensing properties of the H-terminatedNRs are not influenced by the presence of Ni nanoparticles.

. Conclusions

Nanocrystalline diamond nanorods were formed on the sensorubstrate Au/Ti/Al2O3 covered NCD layer by using RIE process forifferent etching time. The morphology of the DNRs showed depen-ence on the etching time. We observed that due to the formationf column structure the increased surface-to-volume ratio plays arucial role in enhancing the sensitivity of the H-terminated DNRs

ased gas sensors. The SNRS value has been increased significantlyor the etching time of 5 min, which was assigned to the enlargedurface area. The sensor response, i.e. the SRNS value, increased upo 4344 in 20 ppm of phosgene for NCD layer etched 5 min. Theseesults indicate that the H-terminated DNRs device has prospects as

[

[[

cience 256 (2010) 5602–5605 5605

gas sensing element for industrial uses, especially in COCl2 detec-tion.

Acknowledgements

The research work at the Institute of Physics was supportedby the Institutional Research Plan No. AV0Z10200521, by theprojects Nos. IAAX00100902, KAN400100701, KAN400100652,KAN400480701, by the Fellowship J. E. Purkyne, and by the CzechMinistry of Education Youth and Sport projects LC-510 (CzechRepublic).

References

[1] R. Arenal, P. Bruno, D.J. Miller, M. Bleuel, J. Lal, D.M. Gruen, Phys. Rev. B 75(2007) 195431.

[2] T. Ueda, K. Katsuki, H.A. Narges, T. Ikegami, F. Mitsugi, Diamond Relat. Mater.13 (2008) 1586–1589.

[3] Q. Wang, S.L. Qu, S.Y. Fu, W.J. Liu, J.J. Li, C.Z. Gu, J. Appl. Phys. 102 (2007) 103714.[4] Y. Ando, Y. Nishibayashi, A. Sawabe, Diamond Relat. Mater. 13 (2004) 633–637.[5] G. Carter, V. Vishnyakov, Phys. Rev. B 54 (1996) 17647–17653.[6] P.J. Gielisse, V.I. Ivanov-Omskii, G. Popovici, M. Prelas, Diamond and diamond-

like film applications, Technomic Pub Co, 1998.[7] R.J. Hamers, J.E. Butler, T. Lasseter, B.M. Nichols, J.N. Russell Jr., K.Y. Tse, W. Yang,

Diamond Relat. Mater. 14 (2005) 661–668.[8] A. Helwig, G. Muller, J.A. Garrido, M. Eickhoff, Sens. Actuators B 133 (2008)

156–165.[9] P.W. Leech, G.K. Reeves, A.S. Holland, J. Mater. Sci. 36 (2001) 3453–3459.10] R. Otterbach, U. Hilleringmann, Diamond Relat. Mater. 11 (2002) 841–844.11] S.A. Rakha, Y. Guojun, Z. Xingtai, I. Ahmed, D. Zhu, J. Gong, J. Cryst. Growth 311

(2009) 3332–3336.12] A. Kromka, M. Davydova, B. Rezek, M. Vanecek, M. Stuchlik, P. Exnar, Diamond

Relat. Mater. 19 (2010) 196–200.13] O. Babchenko, A. Kromka, K. Hruska, M. Michalka, J. Potmesil, M. Vanecek, Cent.

Eur. J. Phys. 7 (2) (2009) 310–314.14] K. Hayashi, et al., J. Appl. Phys. 81 (1997) 744–753.15] M. Davydova, A. Kromka, P. Exnar, M. stuchlik, K. Hruska, M. Vanecek, M. Kalbac,

Phys. Status Solidi A 206 (2009) 2070–2073.