朱 志剛

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application/pdf博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))518085 bytesothe

朱, 志剛

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Theoretical Study on Surface and ElectricalProperties of Solid-State Gas Sensor Materials著者 朱 志剛号 52学位授与番号 3962URL http://hdl.handle.net/10097/37678しゅしごう氏名朱志剛 授与学位博士(工学)` 学位授与年月日平成20年3月25β 学位授与の根拠法規学位規則第4条第1項 研究科,専攻の名称東北大学大学院工学研究科(博士課程)応用化学専攻 学位論文題目TheoreticalStudyonS秘rfaceandElectricalPropertiesof Soiid-StateGasSensorMaterials(固体ガスセンサー材料の表面および電気特性に関 する理論研究)指導教員東北大学教授宮本明論文審査委員主査東北大学教授宮本明東北大学教授浅井圭介東北大学教授滝澤博胤`論文内容要旨 The廿1es{sdevotedtothes舵Kiyof釦ndame葺talrela重ionshipofsur撚ce,elec頴ca1,aなdelec纐。獄圭cpro茎)erties ・fs・Hd-stategassens・rma纈alswi癒血e辻se簸singc鉦aごactedstまcby曲gva撤・usc・mputati・瞬。薮em圭s㎎methods. 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However, theoretical simula{ion on the Sn02 conductivity has not been reported yet, which might be hindered by the high computational effort of regular first-principle methods. A novel computational method has been developed and applied in chapter 3 to investigate electrical properties of Sn02(1 1 O) surface. The calculated -18 -5 -1 conductivities for stoichiometric surface and surface with oxygen vacancies are I .2x:10 and 2.3xlO S cm , respectively. The obtained conductivity of reduced surface is in the range of the available experimental surface conductrvrty of I o 7 to I 0~5 S cm~ , which suggested that surface oxygen vacancies have influence on the surface conductivity, The electrical conductivity of the reduced surface with dissociative water is calculated to be 2.2x I O~4 S cm~1 . This reproduced the trend of enhancement of conductivity by water adsorption on the Sn02 Surface. Despite the fact that gas-sensing properties of (*) H I ~ ~ (b) /H** Sn02 to CH4 can be improved by doping with Pt, there has been no investigation of the interaction of CH4 over the undoped and Pt doped Sn02 surface. The role of Pt playing on the activation of CH4 with Fig. I Final configurations of CH4 on the (a) lattice oxygen on the Pt doped Sn02(1 1 O) surface has undoped and (b) Pt doped Sn02(1 1 O) surface after been studied using DFT method in chapter 4. Fig. I first-principles moleeular dynamics simulation. shows the final configurations of CH4 on the undoped and Pt doped surface after first-principle molecular dynamics simulation at 300 K. It is observed that H1** become tightly bound to bridging oxygen (O~) while the CH3 fragment are lifted from the surface in the case of Pt doped surface. However. CH4 dissociation on the undoped Sn02(1 1 O) surface was not identified. The energy barrier results suggested that the activated barrier of hydrogen abstraction by Ob for CH4 on Pt doped suri~ce is around I . I eV Iower than that on the undoped Sn02(110) surface. On the basis of the ditference in electron density; it was found that upon CH4 adsorption a significant bond fonnation between Pt ions and CH4 takes place. It was concluded that CH cleavage on Pt doped surface is due to the injection of electrons into the antibonding acH orbital ofmethane from Pt. The behavior of Ru doped Sn02 as an effective gas sensor material has been extensively reported. However, the working principle of this type ofmaterials is not yet well understood. A theoretical study on the electronic and adsorptive properties of Sn02(1 10) surface by substituting a Ru atom on 5-fold Sn and 6-fold Sn sites has been reported in chapter 5. Partial density of states (PDOS) for Ru doped Sn02(1 lO) surface are depicted in Fig. 2. It is - 520 -~{}i ii 'i=~.; i~~ ,,*; }ii!:; }:;;{ }I *1}; i< found that Ru 4d orbitals are located at the (*) :: (b) : ~ .*,* ,' ,' edge of energy band gap regron of the ,-,* '* ," surfaces, which indicates that surface ~~ 'L e~ '" ~~~ : electronic properties are significantly changed g ' ~~* ~, ~: by Ru dopant. The difference in the electron ~ >, ~ -~... ~ ' ~ = "*' +p density for molecular oxygen adsorpted on I [ " - ~ ~*'" undoped and Ru doped stoichiometric ' ,' , Sn02(llO) surfaces are shown in Fig. 3. One ',' ** , *" *' " ' *, ~~.~y f~~*1 (*v) E~*~~ ~'*1 t.v) can note that electron density between oxygen Fig. 2 PDOS for (a) 5-fold Ru and 6-fold Ru doped Sn02 (1 1 O) surfaces. and both the 5-fold and 6-fold Ru sites are much larger than that of undoped surface, indicating that Ru on the Sn02 surface strongly influences the oxygen adsorption property Adsorption energies of oxygen molecule on the Ru6c/Sn02 and Ru5c/Sn02 surface are decreased to -0.26 eV and ~D.65 eV, respectively from 0.02 eV on the undoped surface. It is concluded that the modified electronic properties and oxygen adsorption properties of the surface through doping with Ru may both contribute to the improvement of sensing perfonnance for Sn02-02 02 o, (a) (b)  - o*2~   i. e.2p     !~; s !j;~ ~ ' f'~J~ I~: :! ~ ~ ~l  Sr~S p   ;. ~$       h    - R~'4s     I[ I Ij i~ F 11 ~t ~i ~j!:j ~! ~ ht Illl l II l,.l '.~ ~ Il I: ~ ~1'4 p - RL~~d (  - o･2s  e-~p   ~ i~'~     A~ ii; ~ ' ?~i ~, ~  ~r'~~p     li;' i l',   ? i*        .* l I~II l ' Ii i{{ ' - f2~,4S ~f 4~ - - P?U~d Fig. 3 The electron density differences for oxygen molecular adsorption on (a) undoped, (b) 6-fold Ru and (c) 5-fold Ru doped Sn02(1 10) surfaces. Ti deposited single wall carbon nanotube (SWNT) has been prepared using electron beam evaporation techniques, recently However, the application of this material as H2 gas sensor is still an open question. In chapter 6, by employing DFT combined with TBQC method, the interaction of H2 With Ti deposited SWNT has been investigated. The band gap, and conductivities calculated by TBQC method along with the band gap obtained by DFT method are given in Table I . The conductivity of pristine SWNT exhibits semiconductor property. A metallic conductivity is obtained after Ti deposition, and a decrease in conductivity of Ti-deposited carbon nanotube upon H2 adsorption is observed. The band gap simulation results agree well with those obtained by DFT method. In -521 -addition, the charge calculation Table I Band gaps and conductivities of Ti deposited SWNT and H2 adsorbed on Ti deposited SWNT calculated by TBQC as well as DFT suggested that H2 is negatrvely method. (Conductivities are calculated using TBQC method) charged and the Ti deposrted pristine SWNT Ti-SWNT H2 on Ti-SWNT SWNT Iose electron after H2 Band gap (eV) DFT 0.0 0.30 0.82 Band gap (eV) TBQC 0.0 0.21 0.89 The eharge adsorption. 7.2 Conductivity (S cm~1) 2.0>< I 02 1 . 8x I 0-5 redistributions between Ti and SWNT upon H2 adsorption is assumed to be due to the change in band gap values. It is concluded that Ti deposited carbon nanotube is a good candidate of novel sensor material to H2 molecule. To shed valuable insight into the reliability issues of metal-oxide semiconductor transistor gas sensor devices, In chapter 7, a novel method is addressed and employed to 1 o I o~o simulate dielectric breakdown of amorphous Si02 ~ _ 8 Io-20 ~~f (a-Si02) under high electric fields, as well as the role of >* 6 (1) ~ * . ^ Band gap l0-3* ~ ~ ~  hydrogen and oxygen vacancy in the same process. The ~ 4 ' conducttvrty ･,:~> . e~ ~~ 2 Io-50 ~: * o calculation model of a-Si02 used c~ntains 93 atoms. The ~ 1 o-'o o 1 4 o 2 3 5 calculated energy gap for a-Si02 at 300 K without electric Electric field ( x lolo wm) filed is 7.8 eV, which is in close agreement with the Fig. 4 The relationships of calculated band gap and conductivity with external electric field for experimental data. Fig. 4 shows the relationship of non-defective a-Si02-calculated band gap and conductivity with the electric field for non-defective a-Si02. It was found that the electrical conductivity increases with the increase of the electric field and a-Si02 exhibits conductor property under the electric field of 3 .9x 1010 V/m. This can be ascribed to the deerease of band gap with the inerease of the electric field. Under the experimentally observed breakdown fleld of I .Oxl09 V/m for a-Si02, it was found that aceumulation of three hydrogen atoms and three oxygen vacancies into the a-Si02 structure produced high conductivity values of I . I and 12 S cm~1 respectively. These results indicated that defects might have important relation to the triggering of dielectric breakdown. Summary and conclusion are given in chapter 8 . On the basis of above results, it is shown clearly how the important parameters such as, sensitivity, selectivity, and reliability of gas sensor materials are correlated to their surface, electronic and electrical properties. It is expected that the study of solid=sate gas sensor materials will be improved from the knowledge that is acquired with the computational chelnistry method as reported in this thesis. ~ 522 -i,~~* }I ~~ {{' ,,~~ ** ~~ ~ ~~ ~ ~~ ,{"' ~';+;* I *,~ **,~~' ,{'= ; {; li ~ {*; *,*=* {*i~; ii;; ilii {** ,{{* Ili;i:; ~ ,={ l  論文審査結果の要旨 固体ガスセンサーのセンサー機能特性とその表面および電気特性の関係を知ることは、高感度・高選 択性を有すガスセンサー開発において重要な課題である。本研究では様々な固体ガスセンサー材料の機 能において最も重要な表面特性に対するガス分子の吸着および表面修飾の影響を解明する目的で理論 的な解析を行っている。 本論文は、「TheoreticalStudyonSurfaceaadElectricalpropertiesofSolid-StateGasSensor Materials(固体ガスセンサー材料の表面および電気特性に関する理論研究)」と題し、以下の8章から 成り立つ。 第1章では、固体ガスセンサーの重要性について説明し、理論手法に基づく固体ガスセンサー材料に 関する既往の研究をまとめ、本論文の目的について述べている。 第2章では、本研究で用いた、密度汎関数理論(PFT)、Tight-Binding量子分子動力学法、古典分子動 力学法の理論的背景について述べている。さらに、Tig焼一BiRd鶏g量子分子動力学法に基づく電気伝導 度の定量予測手法について述べている。 第3章では、代表的固体ガスセンサー材料であるSnO2の機能を調べる目的で、SnO2(110)表面の電気 伝導特性を解析している。酸素欠陥を有する表面や水の吸着した表面に対する解析に基づき、それらの 電気伝導度の向上への影響を理論的に解明することに初めて成功している。 第4章では、DFT法を用いて、SRO2(110)表面におけるCH婆の活性化に対するPtドーピングの効果につ いて調べている。Ptドーピングのない表面において、CH4は表面と強く相互作用するものの解離はおき ないことを示している。一方、Ptがドーピングされた表面においては、PtとCH善の間に結合が生じ、Pt からCH4の反結合性軌道に電子が注入されることによってCH4の解離が促進されていることを理論的に 解明することに成功している。 第5章では、S簸ol(110)表面へのRuドーピングの効果について調べている。状態密度の解析からRu4d 軌道が表面準位に寄与していることを明らかにしており、Ruにより表面特性を大きく影響されるとの示 唆を得ている。また、酸素分子の吸着特性を調べ、Ruのない表面では見られなかったRuとの強い相互 作用を明らかにしており、ドーピングによる表面特性の改善効果を明らかにしている。 第6章ではTig批一Binding量子分子動力学法およびDFTを用いて、Ti修飾単層カーボンナノチューブ (swNT)のH2との相互作用を調べている。swNTへのTi修飾により電気伝導度が大きく向上すること、Ti 修飾SWNTへのH2吸着によって電気伝導度が低下することの理論的な予測に成功しており、Ti修飾SWNT のH2センサー材料としての適用可能性を理論的に示すことに成功している。 第7章では、分子動力学法とTight-Bi箆ding量子分子動力学法を常い、高電界下でのアモルファス Sio2(a-Sio2)の絶縁破壊に関する理論解析を行っています。絶縁体であったa-Sio2が電界の増加に伴い 導電性を示す傾向を予測することに成功するとともに、a-SiO2中の格子間Hや酸素欠損の存在により絶 縁破壊が誘発されることを理論的に予測することに成功している。 第8章は、本論文の総括である。 以上要するに、本研究は固体ガスセンサー分野における先駆的な理論解析を行っており、表面電子状 態および電気特性に関して多くの新規知見を得ることに成功している。 よって,本論文は博士(工学)の学位論文として合格と認める。 一523一

Theoretical Study on Surface and Electrical Properties of Solid-State Gas Sensor Materials

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固体ガスセンサー材料の表面および電気特性に関する理論研究

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