Enhanced Sensitivity of Epitaxial Graphene to NO2 by Water

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Enhanced Sensitivity of Epitaxial Graphene to NO2 by Water Co-adsorption Mohamed Ridane, Igor Iezhokin, Peter Offermans, and C F J Flipse J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03495 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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The Journal of Physical Chemistry

Enhanced Sensitivity of Epitaxial Graphene to NO2 by Water coAdsorption M. Ridene1, I. Iezhokin1, P. Offermans2 and C. F. J. Flipse*1 1

Molecular Materials and Nanosystems, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands 2

Holst Centre/IMEC-NL, 5605 KN Eindhoven, The Netherlands

corresponding author: [email protected]

ABSTRACT. Epitaxial graphene has shown a high sensitivity to NO2 down to ppb-level. However, the surrounding environment can strongly influence its sensing capability. Since water is naturally present in the atmosphere, understanding the effect of humidity on the gas sensitivity of epitaxial graphene is of paramount interest for creating novel gas sensors. Here, we show experimentally that epitaxial graphene exhibits an enhanced electrical response of a factor more than two for NO2 under humid condition compared to dry condition due to a strong enhancement of the hole doping of the graphene layer. Using electronic structure calculations, we explain the sensitivity enhancement by a water dipole screening of the graphene-NO2 dipole causing an increase of the charge transfer and consequently of the hole doping of the graphene layer. Keywords: sensing, epitaxial graphene, NO2, co-adsorption, electronic structure calculations.

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INTRODUCTION Graphene1 is known for its high sensitivity to gases by virtue of its two-dimensional nature, consisting mainly of a surface with a high mobility and a doping dependent resistance2,3. Therefore, it has successfully been demonstrated as a sensing material for common environmental pollutants, such as nitrogen dioxide (NO2). The ability to detect NO2 in sub-partsper-billion (ppb) levels remains challenging since it is highly dependent on the surrounding environment. However, it has been argued that graphene may even detect a single NO2 molecule with charge transfer as the main mechanism behind the sensing2-4, i.e., surface absorbed NO2 molecules act as electron acceptors and change the charge carrier concentration as well as the resistance of graphene2. Several studies have shown that graphene devices can be fabricated by different procedures, such as mechanical exfoliation5,6, chemical vapor deposition (CVD)7, or thermal decomposition of SiC substrates3,8. A major advantage of using the latter technique is the easy up-scaling possibility: since epitaxial graphene (EG) is grown on SiC, there is the potential of compatibility with current silicon-based device manufacturing technology.

Despite the extensive experimental and theoretical works on the molecular adsorption on graphene, there is a lack of information regarding the co-adsorption of molecules on graphene. In particular, understanding the impact of humidity and/or adsorbed water molecules (H2O) on the sensitivity of graphene to NO2 has received limited experimental and theoretical attention so far. Therefore, in this paper, we explore the effect of co-adsorption of H2O and NO2 on the electrical properties of epitaxial graphene by performing Hall effect measurements under precisely controlled conditions, using a Hall effect measurement setup equipped with a gas flow chamber. We then interpret the observed changes in the sheet resistance (RS) and majority carrier concentration measurements with Density Functional Theory (DFT) calculations. Firstly, we investigate the effect of humidity itself on the carrier concentration and the resistance of epitaxial graphene. Secondly, the response of epitaxial graphene to NO2 in both dry and humid conditions is studied, showing a significant enhancement of the graphene sensitivity to NO2. Complementary DFT calculations provide an explanation of the mechanism behind this enhancement by elucidating the effect of NO2/H2O co-adsorption on the electronic properties of


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graphene. This mechanism could be more generally applied to understand the effectiveness of molecular co-adsorption on graphene toward its sensitivity improvement.

Materials and Methods Device fabrication EG and QFMG samples are grown by thermal decomposition on the silicon (0001) side of a 4×4 mm2 insulating 6H-SiC wafer piece from II-VI Inc, similar as in Ref. (3). After sample growth, metal Au (50nm)/Cr(2nm) contacts were evaporated (Angstrom Engineering) for all samples through a shadow mask and the devices were wire-bonded to a chip carrier providing contact pads for Hall measurements. Gas sensing The resistance and major carrier concentration of the device were measured in the Van der Pauw configuration using a Hall effect measurement system equipped with a gas flow chamber (Ecopia HMS-5500). The gas flow chamber was supplied with a constant 3slm gas flow at atmospheric pressure and room temperature (25 °C). Nitrogen (99.9995%, 5.5 N purity) was used as a carrier gas and NO2 was supplied from a certified permeation tube (KIN-TEK, 581 ng/min at 45 °C) and diluted using a gas calibration system (CGM, MCZ). For the sensing experiments with humidity, the carrier gas was humidified by a controlled evaporation module (Bronkhorst) and added 1:1 with the diluted gas flow. The resistance and major carrier concentration of the device were measured in the Van der Pauw configuration using a Hall Effect measurement system (Ecopia HMS-5500) with 0.546 Tesla permanent magnet. A constant current of 0.1 mA was applied to the sensor and the relative resistance change and major carrier concentration were recorded during gas exposure (signal-to-noise ratio at base line is ∆R/R=10-4). Prior to the measurements, the sample was annealed in-situ at a temperature of 150oC for 30 minutes under 3 slm pure N2 flow to get the sample in its pristine condition. DFT calculations We conducted spin polarized calculations within the Local Spin Density Approximation (LSDA) taking into account electron correlations where an extra Hubbard-U term is introduced following the standard Duradev implementation for an on-site Coulomb interaction between localized orbitals, parameterized by Ueff = U – J, where U and J are the Coulomb and the exchange parameters as implemented in the SIESTA computational code9. A double-ζ basis set of 3 ACS Paragon Plus Environment


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localized atomic orbitals was used for the valence electrons. Sampling of the Brilouin zone takes place by a (5 x 5 x 3) shifted Monkhorst-Pack grid10, while a mesh cutoff energy of 200 Ry has been imposed for real-space integration. The structure has been relaxed until forces were less than 0.05 eV/Å.

RESULTS & DISCUSSION. In Fig. 1, the change in carrier concentration and the response of an epitaxial graphene sample when exposed for 30 minutes to 40% relative humidity (RH) is shown. The response is expressed in (%) and is defined as the relative change of the sample resistance (R-R0)/R0, where R0 is the resistance before NO2 exposure. The change in carrier concentration (∆n = |n – n0|) is defined as the difference between the measured and the initial (clean epitaxial graphene) concentration. Initially, the sample is electron-doped with an electron concentration n0 = 1013 cm-2, which is equivalent to the Dirac point situated about 0.5 eV below the Fermi level11. Upon humidity exposure, the carrier concentration decreases and ∆n increases as function of exposure time up to about 1011 cm-2 after 30 minutes of exposure. When water vapor is introduced, the resistance first decreases about 0.1% of its baseline value and then increases to 0.3% after 30 minutes of exposure. The decrease of the carrier concentration, accompanied by the increase of the resistance, indicates that the water vapor induces a hole doping of the EG layer. This is not surprising since it is known that water acts as an electron acceptor2,12,13. After switching off the water vapor source, the carrier concentration continues to decrease, while a jump in the resistance is observed that is followed by a gradual increase, indicating that the adsorbed water molecules strongly interact with the graphene device, as we will see below, this interaction becomes much more evident in the presence of co-adsorbed NO2. See also the supplementary information file.

Subsequently, the sample is annealed at 150 °C to recover to its pristine state before humidity exposure and NO2 is added to the flow. Fig. 1 (c) and (d) shows the change in carrier concentration and the resistance response to NO2 in dry and humid conditions, respectively. Firstly, in dry conditions (orange solid lines), the electron concentration decreases by almost 4x1012 cm-2 within 5 minutes. The relative change in resistance exhibits a strong initial increase up to 60% within 5 minutes. Within 30 minutes, the response approaches a steady-state with an 4 ACS Paragon Plus Environment

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electron concentration of almost 5 x1012 cm-2 (this is equivalent to a Fermi level shift of 260 meV toward Dirac point). After 30 minutes of exposure, gas desorption is apparent by a gradual recovery of the resistance and carrier concentration. However, the desorption rate is relatively low and the initial resistance value is not reached. These results are in agreement with previous experimental2,3 and theoretical reports14,4, and it highlights the fact that NO2 is an electron acceptor. The hole doping of epitaxial graphene induced by NO2 is almost two orders of magnitude higher than that induced by water only (blue solid lines) with a corresponding much higher change in resistance.

Secondly, under humid conditions (40% RH, green solid lines) the sensitivity of the sample is more enhanced.Within only 5 minutes, the electron concentration decreases by about 5x1012 cm-2 and the response exhibits an enhancement reaching almost 100% of the values within 30 minutes in dry conditions. The NO2 gas supply is turned off after 30 minutes; at this time, the electron concentration reduces by 7x1012 cm-2 (this corresponds to Fermi level shift by about 300 meV toward Dirac point) and the relative change of the resistance reaches 200%. These results show a large enhancement of the sensitivity of epitaxial graphene to NO2 in humid conditions by about 100 % compared to dry conditions. The desorption rate after stopping the gas supply is improved by 30% under humid conditions, but the initial state is not reached anymore. The co-adsorption of water and NO2 leads to two effects: firstly, it increases the sensitivity, and, secondly, it induces a faster NO2 desorption during recovery. Both effects may be explained by a shift of the Fermi level to the Dirac point, as we will show below. It should be noted that we exclude a chemical reaction between water and NO2 towards the formation of nitric acid (HNO3). Recently, it has been reported experimentally and by means of DFT calculations that HNO3 induces an electron doping of EG15 which we do not observe in our experimental results.

Epitaxial graphene on SiC is characterized by the presence of silicon dangling bonds (DBs) below the carbon buffer layer providing intrinsic n-type doping. It has been argued that the adsorption of water molecules not only influences the graphene layer but also may have an impact on impurities or defect states in the substrate12. To elucidate qualitatively the effect of water and NO2 molecules on epitaxial graphene as well as the large enhancement of the sensitivity in the presence of both water and NO2 molecules, we have performed Density 5 ACS Paragon Plus Environment


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Functional Theory (DFT) calculations. Modelling epitaxial graphene is quite challenging due to the large unit cell including the buffer layer. It has been shown that for a good understanding of the epitaxial graphene properties, the on-site Coulomb interaction in the silicon dangling bonds must be taken into account16. For this reason, the DFT calculations were performed using the LSDA+U method where an extra term Ueff is introduced, following the standard Duradev formulation as implemented in the SIESTA code17. The spin polarized band structure of epitaxial graphene is presented in Fig.2a. With Ueff = 3 eV, a good comparison to the experimental results of epitaxial graphene is obtained16. The intrinsic n-type doping of graphene is evident from the position of the Dirac point lying 0.5 eV below the Fermi energy and the Coulomb induced splitting of the Si DB gives rise to two localized states at ~ ± 0.2 eV16 .

Several theoretical studies of water adsorption on pure graphene (no substrate included), have found the most favorable configuration of water adsorption, i.e. the orientation of the dipole with respect to graphene plane, by comparing the adsorption energy for each configuration14,13. In our case, initially, the water molecule is placed with the oxygen atom pointing toward the graphene plane. After geometry relaxation, until the atomic forces reached less than 0.05 eV/Å, the dipole moment of the water molecule rotates over 180 degrees, indicating that the water dipole is strongly interacting with the EG layer (Figure S1 in supplementary information file). This results from the initial n-type doping of the graphene layer, caused by the presence of the Si DBs underneath. In this case, we expect a screening of the Si DBs by the water vapor and therefore a decrease of the Ueff value from 3 eV to 1 eV is applied, which results in a non-spin polarized Si DB state17. The band structure of epitaxial graphene with an adsorbed water molecule on top of its surface is shown in Fig. 2b using Ueff = 1 eV. No shift of the Fermi level with respect to the Dirac point is observed, indicating that the charge transfer is not large enough to dope the graphene layer. However, as claimed in ref (4), a charge transfer between water and graphene is still possible through orbital mixing. Furthermore, in our case, the relaxation of the system leads to water molecules with hydrogen atoms pointing toward the graphene surface. According to ref (4), this situation induces a small charge transfer to the water molecule which explains the small hole doping (1011 cm-2) of EG (Fig. 1(a)). Moreover, it has been shown that a water dipole layer modifies the work function of graphene18 and recently, Giusca & al. reported a modification of the work function of EG upon humidity exposure19. The rotation of the water dipole moment 6 ACS Paragon Plus Environment

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towards the graphene layer, as shown by our calculations, supports the formation of a dipole layer on top of EG and the modification of the work function.

In contrast to water, NO2 molecules are paramagnetic, which might lead to a larger doping effect of graphene due to the presence of molecular orbitals close to the Fermi level14,4. Figure 2c and d show the band structure calculations of a NO2 molecule adsorbed on epitaxial graphene without and with a water molecule using Ueff = 3 eV and 1 eV respectively. Besides the Si-DB splitted states at ~ ± 0.2 eV, molecular orbitals of NO2 appear in the band structure hybridized with the graphene bands. The highest occupied molecular orbital (HOMO) is lying at about -1 eV and the lowest unoccupied molecular orbital (LUMO) is very close to Fermi energy. As expected, the Fermi energy shifts towards the Dirac cone indicating that NO2 acts as an electron acceptor14,4. This shift is about 150 meV, which remains in agreement with the experimental results in Fig. 1 (c).

When the NO2 molecule is adsorbed on epitaxial graphene in the presence of a water molecule, the band structure exhibits more changes as shown in Fig. 2(d). The Fermi energy, in this case, is lying at the Dirac point indicating an overall shift of 500 meV from its initial position for epitaxial graphene. All these results indicate that the presence of a water molecule leads to a significant enhancement of the charge transfer from the graphene layer to the NO2 molecule and therefore to a significant hole doping of the graphene layer as experimentally obtained (Fig. 2(d)). Furthermore, the magnetic moment of the NO2 molecule decreased from 0.9 µB to 0.69 µB (deduced from Mulliken population analysis) giving a rough estimate of the charge transfer to the NO2 molecule of 0.31e and 0.1e with and without water, respectively. The projected density of states (PDOS) on the nitrogen and oxygen orbitals is depicted in Fig. 3. Without water (Fig. 3(a)), the HOMO of NO2 is lying at about -1 eV and the LUMO is very close to Fermi energy (0.1 eV above). When the water molecule is taken into account in the calculations (Fig. 3(b)), the LUMO-level of NO2 lies almost at the Fermi level and the HOMO is 0.7 eV below it. This indicates that there is a significant reduction of the HOMO-LUMO gap (EHL). Interestingly, after relaxing the geometry with NO2 and H2O co-adsorbed on EG, the H2O dipole rotates toward the NO2 molecule while, in the beginning, it was pointing toward the 7 ACS Paragon Plus Environment


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graphene layer. Consequently, the reduction of EHL is well explained by a screening effect of the water molecules adsorbed on EG20.

A qualitative model is illustrated in Fig. 4 in order to better explain the water induced enhancement of the charge transfer from EG to NO2. EG is initially electron doped and its Fermi level lies above the Dirac point. Firstly, when only NO2 is adsorbed on EG (Fig. 4(a)), the LUMO of NO2 (acceptor level) lies below the EG Fermi level causing a charge transfer. This is a p-type doping and induces a downward shift of the Fermi level by ∆EF1 which is in the order of 150 meV from the band structure calculations (Fig. 2). Secondly, when NO2 is co-adsorbed with water, the water dipole screens the dipole of the graphene-NO2 system, resulting from the charge transfer between the two. This screening causes a larger charge transfer. This mechanism is supported by the observation that the water molecule changes its orientation (fig S3) to cancel the graphene-NO2 dipole. The reduction of the HOMO-LUMO gap (EHL) of NO2 with the LUMO shifting downwards in energy, more below the EG Fermi level than in the previous case (Fig. 4(a)) is a consequence of the partial occupation of the NO2 orbital level (spin down) which leads to a decreased spin interaction. More charge transfer occurs from EG to NO2 and the Fermi level shifts downwards by ∆EF2 in the order of 500 meV, so ∆EF2 > ∆EF1. Generally, a shift of the Fermi level towards the Dirac point leads to a decrease in carrier density since it is also the charge neutrality point (CNP). Therefore the observed large enhanced EG resistance, when NO2 is adsorbed under humid conditions (Fig. 1(d)), can be explained by the fact the Fermi level is lying close to the Dirac point.

CONCLUSIONS. In conclusion, the sensitivity of epitaxial graphene on SiC(0001) for NO2 under dry and humid conditions is studied by means of electrical transport measurements and density functional theory. In dry conditions, NO2 is acting as an electron acceptor as revealed by the decrease of the electron concentration and the increase of the resistance. In humid conditions a large sensitivity enhancement of a factor more than two of epitaxial graphene on SiC for NO2 is obtained resulting in a response that is two times higher compared to dry conditionns. This is best explained by a water dipole screening of the graphene-NO2 dipole causing an increase of the charge transfer. 8 ACS Paragon Plus Environment

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Supporting information In the SI, the supercell and the relaxed atomic structures after adsorption of molecules are provided. The results of the same experiment on the quasi free standing graphene is also shown. This material is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgement This work has been supported by the Dutch Organization for Scientific Research (NWO) under Project No. 11447 and by the Foundation for Fundamental Research on Materials (FOM). Competing financial interests: The authors declare no competing financial interest.

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Figure Captions: Figure 1. Sensitivity of epitaxial graphene to water vapor and to NO2. (a) Change in carrier concentration and (b) the relative change in resistance of EG exposed to 40% RH. (c) Change in carrier concentration and (d) the relative change in resistance of EG exposed to 100 ppb NO2 in dry (orange dots) and humid conditions (green dots) and 40% RH (blue dots).

Figure 2. Evolution of the band structure of graphene. LSDA+U band structure of epitaxial graphene (a) bare with Ueff = 3 eV, (b) with one H2O molecule per one Si DB and Ueff = 1 eV, (c) with 1 NO2 molecule per one Si DB and Ueff = 3 eV and (d) with one H2O and one NO2 molecule per one Si DB and Ueff = 1 eV. Red (blue) lines correspond to majority (minority) spin bands.

Figure 3. Projected density of states (PDOS) on nitrogen and oxygen atoms. (a) for a single NO2 molecule adsorbed on EG. (b) for a NO2 molecule co-adsorbed with a H2O molecule.

Figure 4. Qualitative model describing the water induced enhancement of the charge transfer from EG to NO2. The Fermi level of EG lies above the Dirac point.(a) when only NO2 is adsorbed on EG, the LUMO of NO2 lies below the EG Fermi level causing a charge transfer. This induces a downward shift of the Fermi level by ∆EF1.(b) When NO2 is co-adsorbed with water, screening of this latter dipole induces a reduction of the HOMO-LUMO gap (EHL) of NO2. More charge transfer occurs from EG to NO2 and the Fermi level shifts downwards by ∆EF2 > ∆EF1.


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Figure 1. Sensitivity of epitaxial graphene to water vapor and to NO2. (a) Change in carrier concentration and (b) the relative change in resistance of EG exposed to 40% RH. (c) Change in carrier concentration and (d) the relative change in resistance of EG exposed to 100 ppb NO2 in dry (orange dots) and humid conditions (green dots) and 40% RH (blue dots).



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Figure 2: Evolution of the band structure of graphene. LSDA+U band structure of epitaxial graphene (a) bare with Ueff = 3 eV, (b) with one H2O molecule per one Si DB and Ueff = 1 eV, (c) with 1 NO2 molecule per one Si DB and Ueff = 3 eV and (d) with one H2O and one NO2 molecule per one Si DB and Ueff = 1 eV. Red (blue) lines correspond to majority (minority) spin bands.


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Figure 3: Projected density of states (PDOS) on nitrogen and oxygen atoms. (a) for a single NO2 molecule adsorbed on EG. (b) for a NO2 molecule co-adsorbed with a H2O molecule.



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Figure 4: Qualitative model describing the water induced enhancement of the charge transfer from EG to NO2. The Fermi level of EG lies above the Dirac point.(a) when only NO2 is adsorbed on EG, the LUMO of NO2 lies below the EG Fermi level causing a charge transfer. This induces a downward shift of the Fermi level by ∆EF1.(b) When NO2 is co-adsorbed with water, screening of this latter dipole induces a reduction of the HOMO-LUMO gap (EHL) of NO2. More charge transfer occurs from EG to NO2 and the Fermi level shifts downwards by ∆EF2 > ∆EF1.


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Enhanced Sensitivity of Epitaxial Graphene to NO2 by Water

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