Thermal desorption of the structured water layer on graphene was observed in this study via electrical conductivity measurements. Specifically, a structured water layer was formed on the graphene surface via deionized water treatment, following which we examined the thermal desorption process of the layer using sheet resistance measurements. The water molecules acting as a p-type dopant were strongly adsorbed on graphene, forming a solid layer. Consequently, the layer was completely removed from the graphene surface at 300⁡°C. The thermal desorption spectrum of the structured water layer on graphene was quantitatively obtained by converting the measured sheet resistance to carrier density change

Kinoshita, Tomohiro

Minami, Tomoki

Nagase, Masao

Nakai, Hiroki

Ochi, Shuta

Ohno, Yasuhide

English

Tokushima University Institutional Repository

AIP Advances ARTICLE scitation.org/journal/advThermal desorption of structured water layeron epitaxial grapheneCite as: AIP Advances 11, 125012 (2021); doi: 10.1063/5.0075191Submitted: 19 October 2021 • Accepted: 22 November 2021 •Published Online: 9 December 2021Tomoki Minami,1,a) Shuta Ochi,1 Hiroki Nakai,1 Tomohiro Kinoshita,1 Yasuhide Ohno,2 and Masao Nagase2AFFILIATIONS1 Graduate School of Advanced Technology and Science, Tokushima University, 2-1 Minamijyousanjima,Tokushima 770-8506, Japan2 Institute of Post LED Photonics, Tokushima University, 2-1 Minamijyousanjima, Tokushima 770-8506, Japana)Author to whom correspondence should be addressed: t_minami@ee.tokushima-u.ac.jpABSTRACTThermal desorption of the structured water layer on graphene was observed in this study via electrical conductivity measurements. Specifi-cally, a structured water layer was formed on the graphene surface via deionized water treatment, following which we examined the thermaldesorption process of the layer using sheet resistance measurements. The water molecules acting as a p-type dopant were strongly adsorbedon graphene, forming a solid layer. Consequently, the layer was completely removed from the graphene surface at 300 ○C. The thermal des-orption spectrum of the structured water layer on graphene was quantitatively obtained by converting the measured sheet resistance to carrierdensity change.© 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0075191Graphene, a two-dimensional carbon nanomaterial, has excel-lent properties, such as high electrical mobility and a highsurface–volume ratio. Therefore, this material is expected to beapplied to various electronic devices.1–3 In particular, its conductiveand stable surface properties, owing to its two-dimensional nature,are expected to be exploited for applications in highly sensitive gassensors4–13 and biosensor devices.14–16 As gas molecules physicallyadsorb on the graphene surface and directly interact with theπ-electron system, the adsorbed molecules act as dopants,4–13 chang-ing the graphene carrier density. Numerous results on graphene gassensors have been reported for various gas species and a widerange of concentrations.4–13 Graphene gas sensors are promisingfor use in environmental pollutant sensing because of their highsensitivity. For example, nitrogen dioxide (NO2), whose environ-mental standard concentration is 40 ppb, can be easily detected.5–7In previous reports, the sensitivities of graphene sensors forvarious gases were seen to be inconsistent with each other becauseof their initial properties, such as doping concentration, defect, andsurface contamination. Furthermore, the graphene channel resis-tance or conductance change by adsorption or desorption of gasmolecules does not provide any information regarding gas species.A gas sensing method with selectivity and quantitative sensitivityfor adsorbed gas molecules is therefore required. To solve thisproblem, we propose thermal desorption spectroscopy (TDS) basedon the sheet resistance change of graphene by heating. The TDS ortemperature programmable desorption method is known as a con-ventional method for identifying the adsorbed gas molecules.17–20The conventional TDS method uses the mass number information ofdesorbed molecules measured using a quadrupole mass analyzer anda desorption rate temperature dependence fingerprint to identify themolecular species. In this study, we attempt to implement the con-ventional TDS concept to the TDS of graphene sheet resistance. Theresults must be compared with the conventional TDS to verify theconcept of the electrical TDS. As reported in several papers, almostall gas molecules are easily desorbed from graphene surfaces invacuum. For example, water molecules desorb from CVD grapheneat ∼130 K in vacuum.21–23 Moreover, previous studies reported thata structured water layer on epitaxial graphene formed by deionized(DI) water treatment was stable in vacuum at room temperature.The solid water layer that was observed by scanning probe micro-scopy acted as a strong p-type dopant for epitaxial graphene on theSiC substrate and was removed by N2 annealing at 300 ○C.4 Thus,measuring the resistance change by the thermal desorption of struc-tured water on epitaxial graphene could be expected. Furthermore,AIP Advances 11, 125012 (2021); doi: 10.1063/5.0075191 11, 125012-1© Author(s) 2021AIP Advances ARTICLE scitation.org/journal/advthe conventional TDS was also measurable. A clear graphene sur-face water desorption fingerprint was successfully observed in car-rier concentration change per rising temperature. The desorptionfingerprint was similar to be that of the conventional TDS.Here, epitaxial graphene was fabricated on a 4H-SiC(0001)semi-insulating substrate by thermal SiC desorption. The samplesize was 100 mm2, and the sample was diced using stealth dic-ing technology. The SiC substrate was annealed at 1575 ○C for5 min in Ar at 100 Torr for single-layer graphene delineation.24 Twographene sample types were prepared. One was a clean graphenesample (annealed) subjected to annealing in dry N2 at 300 ○C afterdeionized (DI) water cleaning. The surface adsorbent causing thedoping was completely removed by annealing up to 300 ○C (seeFig. S1 in the supplementary material). The other was with a struc-tured water layer (with water). The structured water layer on epi-taxial graphene was formed by immersion in DI water at 23 ○C for60 min. Previous studies have reported that the thickness of thestructured water layer is ∼0.5 nm, and it acts as a p-type dopantagainst graphene on SiC.4,25 The desorption process was evaluatedloading the sample in a small chamber with an N2 gas inlet, as shownin Fig. 1. The AlN ceramic heater was placed under the sample forheating to 400 ○C. Four electrodes were attached to each corner ofthe square sample to measure the sheet resistance using the van derPauw method,26 and the heater temperature was monitored using aK-type thermocouple. The graphene sheet resistance was measuredat a 1 s interval at a constant rising temperature rate of 0.25 ○C/s inambient N2.Figure 2 shows the temperature dependences of sheet resis-tance for two samples, annealed and with water. The sheet resistanceof the annealed sample increased monotonically with the risingtemperature. The initial sheet resistance at room temperature was598 Ω/sq. The average temperature coefficient of epitaxial graphenewas estimated to be 4.1 × 10−3 for a temperature range from 23 to300 ○C. The temperature dependence of sheet resistance should beattributed to the temperature dependence of mobility as predictedusing Matthiessen’s law. There are numerous reports on a strongmobility decrease owing to the temperature rising in the approxi-mate room temperature range.27–29 Conversely, the carrier densityof graphene on a semi-insulating substrate had a constant value upto and including 300 K.30 Furthermore, a mobility temperature fac-tor of β27–29 giving μ(T) = μ0(T0/T)β, where μ0 and μ are graphenemobilities at 23 and 300 ○C, respectively, can be estimated as 2.66,which is consistent with the previously reported values27–29 (seeTable S1 in the supplementary material). Therefore, the fundamentalcause of the sheet resistance change of the annealed sample was thetemperature dependence of mobility rather than that of the carrierdensity. The sheet resistance of graphene with the structured waterlayer (with water) was 1288 Ω/sq at 23 ○C. Because graphene on theSiC substrate is of intrinsically n-type doped by the charge transferbetween the SiC interface and graphene layers,31–33 the cause of theincreasing sheet resistance of DI water treated graphene is p-typedoping from the structured water layer on the graphene.4,25 Thedifference between the two curves shown in Fig. 2 originates fromthe doping amount due to the structured water layer. In particular,the graphene conductivity was used to derive the doping amount.The conductivity for both samples is expressed asσA(T) = enA(T)μA(T), (1)FIG. 1. Thermal desorption measurement system using epitaxial graphene on aSiC substrate. (a) Schematic and (b) photograph of the system. The 100 mm2graphene sample was set on an AlN heater with a thermocouple in the environ-mental control chamber. The sample was fixed using four electrodes to measurethe sheet resistance by the van der Pauw method.σW(T) = enW(T)μW(T), (2)where σA, nA, and μA are the conductivity, sheet carrier density, andmobility of the annealed sample, respectively. σW , nW , and μW arethe conductivity, sheet carrier density, and mobility of the samplewith the water layer, respectively. e is the elemental charge. If nA isconstant for temperature and the temperature dependency of mobil-ity for both samples, μA(T) and μW(T), is the same, the dopingamount originating from the structured water layer, ∆n(T), can beexpressed as follows:Δn(T) = nW(T) − nA = (σW(T)σA(T) − 1)nA. (3)AIP Advances 11, 125012 (2021); doi: 10.1063/5.0075191 11, 125012-2© Author(s) 2021AIP Advances ARTICLE scitation.org/journal/advFIG. 2. Temperature dependence of the sheet resistance of graphene on SiC. Thered line represents the graphene with a structured water layer (with water), whilethe blue line represents the annealed graphene (annealed).Figure 3 shows the temperature dependence of the doping amount,∆n(T). At room temperature, the structured water layer doping forepitaxial graphene was ∼6.6 × 1012 cm−2 and of p-type. The dopingamount was monotonically reduced by the rising temperature dueto water desorption. At 170 ○C, half of the structured water layerwas desorbed. When the sample temperature reached 300 ○C, thestructured water layer was nearly removed from the graphene sur-face. This result suggests that the structured water layer is stableFIG. 3. Temperature dependence of the doping amount, ∆n(T), originated fromthe structured water layer.at room temperature in dry nitrogen ambient compared with nor-mally adsorbed water molecules22 and is strongly adsorbed for thegraphene surface at over 100 ○C, which is the boiling temperature ofwater. By differentiation of the temperature dependence of the dop-ing amount in Fig. 3, the temperature dependence of the structuredwater layer desorption rate was estimated, as shown in Fig. 4(a). Thedesorption rate temperature dependence is maximized at 150 ○C.This figure shows a weak peak around 250 ○C. The result suggeststhat there are two desorption processes for strongly adsorbed wateron epitaxial graphene. The conventional method for the desorptionprocess measurement with material definition is TDS. Figure 4(b)FIG. 4. Thermal desorption spectra of the structured water layer obtained using(a) the graphene sheet resistance measured by the van der Pauw method and (b)conventional TDS of water molecules (m/z = 18) measured using a quadrupolemass spectrometer.AIP Advances 11, 125012 (2021); doi: 10.1063/5.0075191 11, 125012-3© Author(s) 2021AIP Advances ARTICLE scitation.org/journal/advshows the TDS result of water molecules (m/z = 18) from DI watertreated graphene on SiC. Two desorption peaks are clearly observedat ∼270 and 360 ○C. The main peak desorption rate is three timesthat of the sub-peak. These water desorption process fingerprints,shown in Fig. 4(b), coincide with the desorption rate estimated fromsheet resistance change shown in Fig. 4(a). The temperature valuecannot be identical for each system because the sample surface tem-perature is different from the stage thermocouple temperature. Gen-erally, the sample surface temperature value in vacuum should beremarkably lower than that of the stage because of insufficient ther-mal contact. Alternatively, the sample temperature in atmosphericpressure was expected to be approximately the same as the stagetemperature, as shown in Fig. S2. Nevertheless, the TDS of the struc-tured water layer on epitaxial graphene was successfully performedby electronically measuring the sheet resistance. This simple TDSmethod could be adaptable for environmental sensor devices. Pre-vious studies on graphene gas sensors focused on changes in elec-trical properties due to gas molecule adsorption to graphene. Thus,they did not focus on the selectivity of the adsorbed gas molecules.There are a few reports concerning sample heating, the purposeof which was simply the desorption of gas molecules. The sampletemperature is a very important factor for conventional gas sensorsbecause of selectivity and sensitivity. However, we can acquire lim-ited information regarding the amount of adsorbed water moleculesat a constant temperature rising rate in the previous discussions.If the desorption fingerprint of various gas species could be mea-sured, a graphene sensor system consisting of a few graphene chipsof different temperatures with high selectivity and sensitivity couldbe constructed.8 Further experiments for various gas molecules,such as nitrogen dioxide or sulfur dioxide, are important for estab-lishing the environmental monitoring graphene TDS gas sensorsystem.In conclusion, the sheet resistance change of epitaxial grapheneat a constant temperature rise rate provides information regardingthe desorption process of the structured water layer. The temper-ature dependence of the adsorbed water desorption rate estimatedfrom the carrier density of graphene coincided with the result mea-sured by conventional TDS. The structured water layer on epitaxialgraphene was nearly stable below 100 ○C. The maximum desorp-tion rate was observed at ∼150 ○C. The desorption fingerprint willplay a crucial role in identifying adsorbed molecules and realizinggraphene gas sensors in the future.See the supplementary material for additional informationregarding the change in sheet resistance due to heating.This work was supported by JSPS KAKENHI Grant Nos.JP19H02582 and JP21H01394.We would like to acknowledge Editage (www.editage.com) forEnglish language editing.AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts of interest directly relevant to thecontent of this article.Author ContributionsAll authors contributed equally to this work.DATA AVAILABILITYThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.REFERENCES1K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306(5696), 666–669(2004).2K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I.V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature 438(7065), 197–200(2005).3C. Lee, X. Wei, J. W. Kysar, and J. Hone, Science 321(5887), 385–388 (2008).4M. Kitaoka, T. Nagahama, K. Nakamura, T. Aritsuki, K. Takashima, Y. Ohno,and M. Nagase, Jpn. J. Appl. Phys., Part 1 56(8), 085102 (2017).5F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, andK. S. Novoselov, Nat. Mater. 6(9), 652–655 (2007).6S. Novikov, N. Lebedeva, and A. Satrapinski, J. Sens. 2015, 108581.7S. Novikov, N. Lebedeva, A. Satrapinski, J. Walden, V. Davydov, and A. Lebedev,Sens. Actuators, B 236, 1054 (2016).8C. Melios, V. Panchal, K. Edmonds, A. Larrrtsev, R. Yakimova, and O. Kazakova,ACS Sens. 3, 1666–1674 (2018).9C. W. Chen, S. C. Hung, M. D. Yang, C. W. Yeh, C. H. Wu, G. C. Chi, F. Ren,and S. J. Pearton, Appl. Phys. Lett. 99, 243502 (2011).10R. Jaaniso, T. Kahro, J. Kozlova, J. Aarik, L. Aarik, H. Alles, A. Floren, A. Gerst,A. Kasikov, A. Niilisk, and V. Sammelselg, Sens. Actuators, B 190, 1006–1013(2014).11T. Wang, D. Huang, Z. Yang, S. Xu, G. He, X. Li, N. Hu, G. Yin, D. He, and L.Zhang, Nano-Micro Lett. 8(2), 95–119 (2016).12H. J. Yoon, D. H. Jun, J. H. Yang, Z. Zhou, S. S. Yang, and M. M.-C. Cheng,Sens. Actuators, B 157(1), 310–313 (2011).13F. Ricciardella, S. Vollebregt, T. Polichetti, M. Miscuglio, B. Alfano, M. L.Miglietta, E. Massera, G. Di Francia, and P. M. Sarro, Nanoscale 9(18), 6085–6093(2017).14G. Seo, G. Lee, M. J. Kim, S.-H. Baek, M. Choi, K. B. Ku, C.-S. Lee, S. Jun, D.Park, H. G. Kim, S.-J. Kim, J.-O. Lee, B. T. Kim, E. C. Park, and S. I. Kim, ACSNano 14(4), 5135–5142 (2020).15Y. Ohno, K. Maehashi, Y. Yamashiro, and K. Matsumoto, Nano Lett. 9(9),3318–3322 (2009).16Y. Taniguchi, T. Miki, Y. Ohno, M. Nagase, Y. Arakawa, Y. Imada,K. Minagawa, and M. Yasuzawa, Jpn. J. Appl. Phys., Part 1 58, 055001(2019).17R. S. Smith and B. D. Kay, J. Phys. Chem. B 122(2), 587–594 (2018).18J. P. Meyburg, I. I. Nedrygailov, E. Hasselbrink, and D. Diesing, Rev. Sci.Instrum. 85(10), 104102 (2014).19S. Abe, H. Handa, R. Takahashi, K. Imaizumi, H. Fukidome, and M. Suemitsu,Jpn. J. Appl. Phys., Part 1 50(7R), 070102 (2011).20H. Atsumi, Y. Takemura, T. Konishi, T. Tanabe, and T. Shikama, J. Nucl. Mater.438, S963–S966 (2013).21U. Burghaus, Inorganics 4(2), 10 (2016).22U. Burghaus, J. Mater. Res. 36, 129 (2020).23A. Chakradhar and U. Burghaus, Chem. Commun. 50(57), 7698–7701(2014).24T. Aritsuki, T. Nakashima, K. Kobayashi, Y. Ohno, and M. Nagase, Jpn. J. Appl.Phys., Part 1 55, 06GF03 (2016).25J. Du, Y. Kimura, M. Tahara, K. Matsui, H. Teratani, Y. Ohno, and M. Nagase,Jpn. J. Appl. Phys., Part 1 58, SDDE01 (2019).26L. J. van der Pauw, Philips Res. Rep. 13, 174 (1958).AIP Advances 11, 125012 (2021); doi: 10.1063/5.0075191 11, 125012-4© Author(s) 2021AIP Advances ARTICLE scitation.org/journal/adv27S. Tanabe, Y. Sekine, H. Kageshima, M. Nagase, and H. Hibino, Phys. Rev. B 84,115458 (2011).28R.-J. Shiue, Y. Gao, C. Tan, C. Peng, J. Zheng, D. K. Efetov, Y. D. Kim, J. Hone,and D. Englund, Nat. Commun. 10(1), 109 (2019).29V. E. Dorgan, M.-H. Bae, and E. Pop, Appl. Phys. Lett. 97(8), 082112 (2010).30A. J. M. Giesbers, P. Procházka, and C. F. J. Flipse, Phys. Rev. B 87(19), 195405(2013).31F. Varchon, R. Feng, J. Hass, X. Li, B. N. Nguyen, C. Naud, P. Mallet, J.-Y.Veuillen, C. Berger, E. H. Conrad, and L. Magaud, Phys. Rev. Lett. 99(12), 126805(2007).32J. Hass, W. A. De Heer, and E. H. Conrad, J. Phys.: Condens. Matter 20, 323202(2008).33S. Kopylov, A. Tzalenchuk, S. Kubatkin, and V. I. Fal’ko, Appl. Phys. Lett.97(11), 112109 (2010).AIP Advances 11, 125012 (2021); doi: 10.1063/5.0075191 11, 125012-5© Author(s) 2021

Thermal desorption of structured water layer on epitaxial graphene

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Thermal desorption of structured water layer on epitaxial graphene

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