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Vacuum 86 (2012) 599e602

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Vacuum

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Temperature enhanced gas sensing properties of diamond films

Marina Davydova a,b,*, Martin Stuchlik c, Bohuslav Rezek a, Alexander Kromka a

a Institute of Physics, Academy of Science of the Czech Republic, Cukrovarnicka 10, 16253 Prague, Czech RepublicbDepartment of Physics, Faculty of Civil Engineering, CTU in Prague, Thakurova 7, 16629 Prague, Czech RepubliccDepartment of Chemistry, TU Liberec, Studentska 2, 46117 Liberec, Czech Republic

Keywords:Nanocrystalline diamondPhosgeneSurface conductivityGas sensorSEM

* Corresponding author. Institute of Physics, AcadRepublic, Cukrovarnicka 10, 16253 Prague, Czech Rep

E-mail address: davydova@fzu.cz (M. Davydova).

0042-207X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.vacuum.2011.07.025

a b s t r a c t

Nanocrystalline diamond (NCD) film was used as a functional part of gas sensor. The gas sensingproperties of H-terminated nanocrystalline diamond films were examined to oxidizing gases (i.e., COCl2and humid air). Pronounced increase in the surface conductivity (3 orders of magnitude) was found aftersensor exposure to phosgene gas and was explained by the surface transfer doping effect. We alsopresent a possible way how to achieve sensor selectivity, i.e. how to distinguish between phosgene andhumid air (the mostly present background gas in a common environment).

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Detection of gases and determination of their composition hasbeen constantly increasing in recent decades. Toxic and combustiblegas detection is a critical issue in diverse fields including vehicleemission control, household security, environmental monitoring,etc. Numerous studies focused on detecting NH3, CO2, NO, NO2, andO3 due to their toxicity [1e4]. Phosgene (COCl2) is another verycommon irritant and toxic gas. It is a highly reactive non-flammablecolorless gas at room temperature and ambient pressure and it hasa suffocating odor similar to moldy hay. It is a lung irritant and veryinsidious poison, which does not irritate immediately, even whenfatal concentrations are inhaled. The odormay be detected between1.6 and 6 mg/m3. Environmental phosgene levels arise from indus-trial emissions and thermal decomposition of some chlorinatedsolvents and chlorinated polymers (especially, polyvinyl chloride)[5,6]. Phosgene can be also a byproduct of burning process, inparticular when buildings are on fire.

The main requirements for phosgene sensor devices are: highsensitivity and selectivity, reproducibility, long term reliability, andportability. Numerous attempts have been made to use various gassensing materials. Although, they have satisfying sensitivity andreproducible results, they suffer from a lack of selectivity. Ourprevious study pointed out that the surface conductivity (SC) ofH-terminated nanocrystalline diamond (NCD) layers is modulatedby its exposure to different gases. Especially high response wasfound for phosgene [7].

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In the present study we show that the optimal working temp-erature of the NCD-based sensor for humid air and COCl2 isdifferent which allows for obtaining clear gas selectivity. To explainvariation of the surface conductivity after sensor exposure todifferent gas environments we propose a sensingmechanism basedon phosgene dissolution in the adsorbate layer and subsequentsurface transfer doping of H-terminated diamond.

2. Experimental

The gas sensor fabrication started with the deposition of metalinterdigitated electrodes (Au and Ti/Au) on Al2O3 substrate bya thermal evaporation method, and using a standard UV-lithography and lift-off technique. Each pair of interdigitated elec-trodes (IDEs) was separated by a 50 mm wide gap.

Next, the NCD films have been grown on metal electrodes. Atfirst, substrates were seeded by a dispersed detonation nano-crystalline diamond powder with an average grains in size of 5 nmusing an ultrasonic treatment procedure for 2 min. Then, the NCDfilm was grown by MW-plasma enhanced chemical vapor deposi-tion process (Aixtron P6). Deposition of NCD layer was realized atfollowing parameters: 1% methane concentration in hydrogen, gaspressure of 30 mbar, microwave power of 1000 W, and depositiontime of 5 h. The substrate temperature was as low as 400 �C. Dia-mond character of such films has been previously confirmed byRaman measurements [7]. After the CVD deposition, the methaneflow was switched off and the as-grown diamond was kept inhydrogen plasma for 10 min to induce p-type surface conductivity[8]. The surface morphology of the nanocrystalline diamond filmwas characterized by scanning electron microscopy.

The gas sensing properties (as specified by electrical conduc-tivity measurements) were measured at room temperature and

Fig. 1. (a) SEMmicrograph of surface morphology of nanocrystalline diamond layer on the substrate with electrodes. (b) High magnification SEM image of diamond grains on sensordevice.

M. Davydova et al. / Vacuum 86 (2012) 599e602600

140 �C, at voltage of 1 V, frequency of 3 kHz, and measurementperiod of 5 s (LCR meter HIOKI 3532-50). The following testinggases were used: i) humid air (RH¼ 22%), and ii) phosgene. The gassensing setup was described in our previous work [9].

3. Results

Fig. 1 shows SEM images of surface morphology of the Al2O3substrate with electrodes coated with NCD diamond film (Fig. 1a).The detailed SEM scan (Fig. 1b) shows 3D surface roughness andwell faceted grains in size up to 200 nm. Our previous study hasdemonstrated that the NCD growth is predominately localized onthe metal interdigitated electrodes, whereas Al2O3 substrateexhibited grains at lower size and density [7]. A similar result,where the diamond growth is enhanced on metal, was presentedby Daenen et al. [10].

Fig. 2 represents the response of H-terminated NCD surface tophosgene (5 and 20 ppm) and to humid air (RH ¼ 22%). Thesemeasurements were provided at relatively high temperature(140 �C) assuming that the higher temperature will lead to a fasterresponse and recovery behavior than at the room temperature [2].This testing temperature was not overcome to save the hydroge-nated diamond surface from degradation. As illustrated in Fig. 2, anexposure of the sensor element to 5 ppm of the phosgene gas leadsto increase in surface conductivity from 5.5 � 10�7 S up to2.2 � 10�6 S. A steep increase in the SC up to 6.5 � 10�5 S isobserved for higher phosgene content (20 ppm). After exposing thesensing element to humid air, the surface conductivity increasesnegligibly approximately to the value of 2.7 � 10�7 S.

Fig. 2. The time dependence of surface conductivity of H-terminated NCD layer onphosgene (5 and 20 ppm) and on humid air (22%). Conductivity measurements wereprovided at 140 �C.

Fig. 3 shows time and temperature dependencies of the surfaceconductivity of the H-terminated diamond sensor on the presenceof phosgene and humid air. Both curves show maximum conduc-tivity for a certain temperature. This temperature is 82 �C (T1) and91 �C (T2) for humid air and COCl2, respectively.

Moreover, for humid air it is found that the surface conductivityvalue at 140 �C is lower (4.5 � 10�7 S) than the value at roomtemperature (1.5�10�6 S). Themaximumvalue of SC is 2.4�10�6 Sat 82 �C. In the case of phosgene, a steeper increase of the surfaceconductivity is observed during the heating up period. Themaximum value of SC is 1 � 10�5 S which corresponds to thetemperature value of 91 �C (T2).

4. Discussion

The presence of gas at the solid state material surface caninitialize the transfer of electrons from (or into) the material bulk.In the case of the H-terminated diamond surface, the surfaceconductivity was reported to increase after exposure to oxidizinggases [11]. Similar results were observed also in our study wherethe surface conductivity rose up after exposure to humid air andphosgene at room temperature.

The gas detection mechanism can be explained by the surfacetransfer doping effect. Fig. 4a schematically shows the situation atthe diamond surface as described by Maier et al. [12]. First, a thinlayer of adsorbed water is formed at the H-terminated surface afterits exposure to ambient air. Assuming that this adsorbed water

Fig. 3. The temperature dependence of surface conductivity of H-terminated NCDlayer for phosgene (20 ppm) and humid air (22%).

Fig. 4. Schematic illustration of electron transfer from the diamond subsurface to the adsorbed liquid electrolyte layer in air (according to Maier et al. [12]), and in phosgeneenvironment (b).

M. Davydova et al. / Vacuum 86 (2012) 599e602 601

layer is initially clean and neutral, low and equal densities ofhydronium (H3Oþ) and hydroxide (OH�) ions are formed:

2H2O4H3Oþ þ OH� (1)

The surface transfer doping model is based on the electronstransfer from the diamond subsurface to the H3Oþ ions at thediamond top surface [13]. Thus, the reaming holes in the diamondsubsurface behave as mobile charge carriers which results in the p-type conductivity. Snidero et al. have experimentally shown thatexposing of diamond surface to water or water-like adsorbentsplays a vital role in the promotion of surface conductivity [14].Additional increase in the induced surface conductivity can occur ifacid-forming molecules become dissolved in the newly formedelectrolyte layer. For example, acidic electrolyte surface layers mayarise by dissolution of atmospheric CO2 in such adsorbed water [15]as follows:

CO2 þ 2H2O/H2CO3 þ H2O/H3Oþþ HCO�

3 (2)

The situation is very close to real conditions where the sensorelement has to be operational also in humid air.

Fig. 4b presents the sensing mechanism when the diamondsurface is exposed to phosgene. Phosgene is known as an oxidizinggas which is well dissoluble in water whereas carbon dioxide (CO2)and hydrogen chloride (HCl) are formed [16]:

COCl2 þ H2O/CO2 þ 2HCl (3)

Furthermore, CO2 and HCl molecules can be further dissociatedin the adsorbed water resulting in an increase in hydronium ions byfollowing reactions:

CO2 þ 2H2O/H2CO3 þ H2O/ H3OþþHCO�

3 (4)

HClþ 2H2O/H3Oþ þ Cl� (5)

As shown in Fig. 4b, due to double reaction of two acid-formingmolecules (e.g. CO2 and HCl) at the diamond surface, the density ofH3Oþ ions becomes higher than by only CO2 molecule, whichnaturally comes from the ambient air.

Consequently, adopting the above mentioned electron transfermechanism, the concentration of holes in diamond subsurfaceincreases and results in the observed higher surface conductivity. Itis noteworthy that the phosgene molecules contribute to thisincrease through two parallel pathways (i.e. dissociation of CO2 andHCl, both molecules coming from phosgene).

We have observed that for humid air the change of the surfaceconductivity (i.e. ΔSC) measured at high temperature (T ¼ 140 �C)

was negative (ΔSC < 0) with respect to the starting point (SC at20 �C). Conversely, exposing the diamond-based sensor to thephosgene gas gave a positive value (ΔSC > 0). Thus the curve ofsurface conductivity as the function of temperature is a valuablecharacteristic to distinguish the sensor selectivity. In our case, theselectivity is defined by the specific temperature (T1 or T2) at whichthe maximum surface conductivity is achieved. Further increasingof temperature above this temperature (i.e. 82 �C for humid air and91 �C for phosgene) resulted in decreasing of the surface conduc-tivity. Understanding the SC curve (i.e. slope and gradient) areadditional and valuable parameters to define the diamond-basedsensor selectivity to different gases. The presented results withappropriate model are under intensive study.

5. Conclusions

Nanocrystalline diamond-based gas sensor was fabricated ontoAu/Ti/Al2O3 substrate.We have shown that the surface conductivityof hydrogenated diamond exhibited sensitivity to the testing gases(COCl2 and humid air) and the gas sensing principle was attributedto the surface transfer doping mechanism which is commonlyaccepted for H-terminated diamond surface. Our proposed gassensing mechanism of phosgene included its dissolution into CO2and HCl by-products due to the presence of adsorbed water layer.Further dissociation of CO2 and HCl rose the concentration ofhydrionium ions, and thus resulting in a quite steep increase ofsurface conductivity. Moreover, the optimal working temperaturesat 82 �C and 91 �C were determined for humid air and phosgene,respectively. These differences were assigned as specific property ofeach individual gas which indicated the possible way to control theselectivity between the background gas (i.e. humid air) and thetesting gas (phosgene).

Acknowledgments

The research work at the Institute of Physics was supported bythe grants IAAX00100902, KAN400100701, KAN400480701, SGS10/125/OHK1/2T/11, by the projects No. LC510, by the Fellowship J. E.Purkyne and by the Grant Agency of ASCR.

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