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Effect of He ion irradiation on microstructure and electrical properties of graphene

Zhang Na Liu Bo Lin Li-Wei

Effect of He ion irradiation on microstructure and electrical properties of graphene

Zhang Na, Liu Bo, Lin Li-Wei
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  • Graphene is a planar two-dimensional material composed of sp2-bonded carbon atoms with extraordinary electrical, optical and mechanical properties, and considered as one of the revolutionary electronic component materials in the future. Some studies have shown that the inert gas ion irradiation as a defect introducing technique can change the structure and properties of graphene without introducing additional effects. In this paper, the 5.4 keV He ion irradiation at the dose ranging from 0.7 × 1013 cm–2 to 2.5 × 1013 cm–2 has a strong effect on graphene deposited by CVD technology. The X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman) and semi-conductor parameter analysis instrument are used to study the changes in the microstructure and electrical properties of graphene before and after irradiation. Detailed analysis shows that the defect density increases gradually with the irradiation dose increasing. Raman spectrum shows that when the irradiation dose increases to 1.6 × 1013 cm–2, the value of ID/IG begins to decrease, and XPS shows that the irradiation changes the structure of C chemical bond in graphene which causes the bonding state of C—C sp2 to be destroyed and partly converted into the C—C sp3 bonding state. Therefore, the structure of graphene begins to transform from nano-crystalline structure into sp3 amorphous structure. Simultaneously, increasing defects causes the graphene conductivity to continuously decrease, and also gives rise to the electrical transition from defect scattering mechanism based on Boltzmann transport to the hopping transport. The positive voltage direction offset of Vdirac increases nearly in direct proportion, which is due to the enhancement of graphene’s p-type doping effect caused by defects and adsorbed impurities. This work conduces to the understanding the mechanism of He ion interaction with graphene, and also provides an effective way of controlling the electronic properties.
      Corresponding author: Liu Bo, liubo2009@scu.edu.cn
    [1]

    Karimi H, Yusof R, Rahmani R, Ahmadi M T 2013 J. Nanomater. 2013 789454

    [2]

    Geim A K, Novoselov K S 2007 Nat. Mater. 6 183

    [3]

    张辉, 蔡晓明, 郝振亮, 阮子林, 卢建臣, 蔡金明 2017 物理学报 66 218103

    Zhang H, Cai X M, Hao Z L, Ruan Z L, Lu J C, Cai J M 2017 Acta Phys. Sin. 66 218103

    [4]

    Li M, Qu G F, Wang Y Z, Zhu Z S, Shi M G, Zhou M L, Liu D, Xu Z X, Song M J, Zhang J, Bai F, Liao X D, Han J F 2019 Chin. Phys. B 28 093401

    [5]

    Zeng J, Liu J, Yao H J, Zhai P F, Zhang S X, Guo H, Hu P P, Duan J L, Mo D, Hou M D, Sun Y M 2016 Carbon 100 16

    [6]

    Kumar S, Kumar A, Tripathi A, Tyagi C, AvasthiCitation D K 2018 J. Appl. Phys. 123 161533

    [7]

    Hang S J, Moktadir Z, Mizuta H 2014 Carbon 72 233

    [8]

    Tapasztó L, Dobrik G, Nemes-Incze P, Vertesy G, Lambin Ph, Biró L P 2008 Phys. Rev. B 78 233407

    [9]

    Lucchese M M, Stavale F, Ferreira E M, Vilani C, Moutinho M, Capaz R B, Achete C, Jorio A 2010 Carbon 48 1592

    [10]

    Al-Harthi S H, Kara’a A, Hysen T, Elzain M, Al-Hinai A T, Myint M T Z 2012 Appl. Phys. Lett. 101 213107

    [11]

    Chen J H, Cullen W G, Jang C, Fuhrer M S, Williams E D 2009 Phys. Rev. Lett. 102 236805

    [12]

    Amor S B, Baud G, Jacquet M, Nansé G, Fioux P, Nardin M 2000 Appl. Surf. Sci. 153 172

    [13]

    王淑芬 2018 博士学位论文 (合肥: 中国科学技术大学)

    Wang S F 2018 Ph. D. Dissertation (Hefei: University of Science and Technology of China) (in Chinese)

    [14]

    曾健 2014 博士学位论文 (兰州: 兰州大学)

    Zeng J 2014 Ph. D. Dissertation (Lanzhou: Lanzhou University) (in Chinese)

    [15]

    Ferrari A C, Robertson J 2000 Phys. Rev. B 61 14095

    [16]

    吴娟霞, 徐华, 张锦 2014 化学学报 72 301

    Wu J X, Xiu H, Zhang J 2014 Acta Chim. Sin. 72 301

    [17]

    Kim J H, Hwang J H, Suh J, Tongay S, Kwon S, Hwang C C, Wu J Q, Park J Y 2013 Appl. Phys. Lett. 103 171604

    [18]

    Wang H, Wu Y, Cong C, Shang J, Yu T 2010 ACS Nano 4 7221

    [19]

    Pimenta M A, Dresselhaus G, Dresselhaus M S, Cancado L G, Jorio A, Saito R 2007 Phys. Chem. Chem. Phys. 9 1276

    [20]

    Ferrari A C 2007 Solid State Commun. 143 47

    [21]

    白冰 2016 博士学位论文 (镇江: 江苏大学)

    Bai B 2016 Ph. D. Dissertation (Zhenjiang: Jiangsu University) (in Chinese)

    [22]

    宋航, 刘杰, 陈超, 巴龙 2019 物理学报 68 097301

    Song H, Liu J, Chen C, Ba L 2019 Acta Phys. Sin. 68 097301

    [23]

    Guermoune A, Chari T, Popescu F, Sabri S S, Guillemette J, Skulason H S, Szkopek T, Siaj M 2011 Carbon 49 4204

    [24]

    Yuan H Y, Chang S, Bargatin I 2015 Nano Lett. 15 6475

    [25]

    Stauber T, Peres N M R, Guinea F 2007 Phys. Rev. B 76 205423

    [26]

    Chen C F, Park C H, Boudouris B W, Horng J, Geng B, Girit C, Zettl A, Crommie M F, Segalman R A, Louie S G, Wang F 2011 Nature 471 617

    [27]

    Wang Q, Liu S, Ren N F 2014 Appl. Phys. Lett. 105 133506

    [28]

    Zhou Y B, Liao Z M, Wang Y F, Duesberg G S, Xu J, Fu Q, Wu X S, Yu D P 2010 J. Chem. Phys. 133 234703

    [29]

    Cancado L G, Jorio A, Ferreira E H M, Stavale F, Achete C A, Capaz R B, Moutinho M V O, Lombardo A, Kulmala T S, Ferrari A C 2011 Nano Lett. 11 3190

    [30]

    Zhou Y B, Han B H, Liao Z M, Wu H C, Yu D P 2011 Appl. Phys. Lett. 98 222502

  • 图 1  石墨烯样品XPS C1 s峰谱图 (a)未辐照; (b) 0.7 × 1013 He+/cm2; (c) 1.6 × 1013 He+/cm2; (d) 2.5 × 1013 He+/cm2

    Figure 1.  XPS C1 s peak spectra of graphene samples: (a) Unirradiated; (b) 0.7 × 1013 He+/cm2; (c) 1.6 × 1013 He+/cm2; (d) 2.5 × 1013 He+/cm2.

    图 2  未辐照石墨烯Raman光谱图

    Figure 2.  The Raman spectra of unirradiated graphene.

    图 3  He+辐照前后石墨烯Raman光谱图

    Figure 3.  The Raman spectra of graphene before and after He+ irradiation.

    图 4  Raman峰强ID/IG, I2D/IG比值与辐照剂量的关系

    Figure 4.  The relationship between Raman peak strength ID/IG, I2D/IG ratio and irradiation dose.

    图 5  不同辐照剂量下电导率随栅极电压变化曲线

    Figure 5.  Electrical conductivity versus gate voltage at different irradiation doses.

    图 6  狄拉克电压偏移量与辐照剂量的关系

    Figure 6.  The relation between Dirac voltage variation and irradiation dose.

    表 1  辐照前后石墨烯样品C1 s峰面积比

    Table 1.  C1 s peak area ratio of graphene samples before and after irradiation.

    辐照剂量C-C sp2C—C sp3C—O—HC—O—CO—C=O
    未辐照0.520.190.100.090.10
    0.7 × 1013 He+/cm20.500.300.130.050.02
    1.6 × 1013 He+/cm20.370.320.150.100.06
    2.5 × 1013 He+/cm20.280.350.200.120.05
    DownLoad: CSV
  • [1]

    Karimi H, Yusof R, Rahmani R, Ahmadi M T 2013 J. Nanomater. 2013 789454

    [2]

    Geim A K, Novoselov K S 2007 Nat. Mater. 6 183

    [3]

    张辉, 蔡晓明, 郝振亮, 阮子林, 卢建臣, 蔡金明 2017 物理学报 66 218103

    Zhang H, Cai X M, Hao Z L, Ruan Z L, Lu J C, Cai J M 2017 Acta Phys. Sin. 66 218103

    [4]

    Li M, Qu G F, Wang Y Z, Zhu Z S, Shi M G, Zhou M L, Liu D, Xu Z X, Song M J, Zhang J, Bai F, Liao X D, Han J F 2019 Chin. Phys. B 28 093401

    [5]

    Zeng J, Liu J, Yao H J, Zhai P F, Zhang S X, Guo H, Hu P P, Duan J L, Mo D, Hou M D, Sun Y M 2016 Carbon 100 16

    [6]

    Kumar S, Kumar A, Tripathi A, Tyagi C, AvasthiCitation D K 2018 J. Appl. Phys. 123 161533

    [7]

    Hang S J, Moktadir Z, Mizuta H 2014 Carbon 72 233

    [8]

    Tapasztó L, Dobrik G, Nemes-Incze P, Vertesy G, Lambin Ph, Biró L P 2008 Phys. Rev. B 78 233407

    [9]

    Lucchese M M, Stavale F, Ferreira E M, Vilani C, Moutinho M, Capaz R B, Achete C, Jorio A 2010 Carbon 48 1592

    [10]

    Al-Harthi S H, Kara’a A, Hysen T, Elzain M, Al-Hinai A T, Myint M T Z 2012 Appl. Phys. Lett. 101 213107

    [11]

    Chen J H, Cullen W G, Jang C, Fuhrer M S, Williams E D 2009 Phys. Rev. Lett. 102 236805

    [12]

    Amor S B, Baud G, Jacquet M, Nansé G, Fioux P, Nardin M 2000 Appl. Surf. Sci. 153 172

    [13]

    王淑芬 2018 博士学位论文 (合肥: 中国科学技术大学)

    Wang S F 2018 Ph. D. Dissertation (Hefei: University of Science and Technology of China) (in Chinese)

    [14]

    曾健 2014 博士学位论文 (兰州: 兰州大学)

    Zeng J 2014 Ph. D. Dissertation (Lanzhou: Lanzhou University) (in Chinese)

    [15]

    Ferrari A C, Robertson J 2000 Phys. Rev. B 61 14095

    [16]

    吴娟霞, 徐华, 张锦 2014 化学学报 72 301

    Wu J X, Xiu H, Zhang J 2014 Acta Chim. Sin. 72 301

    [17]

    Kim J H, Hwang J H, Suh J, Tongay S, Kwon S, Hwang C C, Wu J Q, Park J Y 2013 Appl. Phys. Lett. 103 171604

    [18]

    Wang H, Wu Y, Cong C, Shang J, Yu T 2010 ACS Nano 4 7221

    [19]

    Pimenta M A, Dresselhaus G, Dresselhaus M S, Cancado L G, Jorio A, Saito R 2007 Phys. Chem. Chem. Phys. 9 1276

    [20]

    Ferrari A C 2007 Solid State Commun. 143 47

    [21]

    白冰 2016 博士学位论文 (镇江: 江苏大学)

    Bai B 2016 Ph. D. Dissertation (Zhenjiang: Jiangsu University) (in Chinese)

    [22]

    宋航, 刘杰, 陈超, 巴龙 2019 物理学报 68 097301

    Song H, Liu J, Chen C, Ba L 2019 Acta Phys. Sin. 68 097301

    [23]

    Guermoune A, Chari T, Popescu F, Sabri S S, Guillemette J, Skulason H S, Szkopek T, Siaj M 2011 Carbon 49 4204

    [24]

    Yuan H Y, Chang S, Bargatin I 2015 Nano Lett. 15 6475

    [25]

    Stauber T, Peres N M R, Guinea F 2007 Phys. Rev. B 76 205423

    [26]

    Chen C F, Park C H, Boudouris B W, Horng J, Geng B, Girit C, Zettl A, Crommie M F, Segalman R A, Louie S G, Wang F 2011 Nature 471 617

    [27]

    Wang Q, Liu S, Ren N F 2014 Appl. Phys. Lett. 105 133506

    [28]

    Zhou Y B, Liao Z M, Wang Y F, Duesberg G S, Xu J, Fu Q, Wu X S, Yu D P 2010 J. Chem. Phys. 133 234703

    [29]

    Cancado L G, Jorio A, Ferreira E H M, Stavale F, Achete C A, Capaz R B, Moutinho M V O, Lombardo A, Kulmala T S, Ferrari A C 2011 Nano Lett. 11 3190

    [30]

    Zhou Y B, Han B H, Liao Z M, Wu H C, Yu D P 2011 Appl. Phys. Lett. 98 222502

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  • Received Date:  06 September 2019
  • Accepted Date:  17 October 2019
  • Available Online:  13 December 2019
  • Published Online:  01 January 2020

Effect of He ion irradiation on microstructure and electrical properties of graphene

    Corresponding author: Liu Bo, liubo2009@scu.edu.cn
  • Key Laboratory of Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China

Abstract: Graphene is a planar two-dimensional material composed of sp2-bonded carbon atoms with extraordinary electrical, optical and mechanical properties, and considered as one of the revolutionary electronic component materials in the future. Some studies have shown that the inert gas ion irradiation as a defect introducing technique can change the structure and properties of graphene without introducing additional effects. In this paper, the 5.4 keV He ion irradiation at the dose ranging from 0.7 × 1013 cm–2 to 2.5 × 1013 cm–2 has a strong effect on graphene deposited by CVD technology. The X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman) and semi-conductor parameter analysis instrument are used to study the changes in the microstructure and electrical properties of graphene before and after irradiation. Detailed analysis shows that the defect density increases gradually with the irradiation dose increasing. Raman spectrum shows that when the irradiation dose increases to 1.6 × 1013 cm–2, the value of ID/IG begins to decrease, and XPS shows that the irradiation changes the structure of C chemical bond in graphene which causes the bonding state of C—C sp2 to be destroyed and partly converted into the C—C sp3 bonding state. Therefore, the structure of graphene begins to transform from nano-crystalline structure into sp3 amorphous structure. Simultaneously, increasing defects causes the graphene conductivity to continuously decrease, and also gives rise to the electrical transition from defect scattering mechanism based on Boltzmann transport to the hopping transport. The positive voltage direction offset of Vdirac increases nearly in direct proportion, which is due to the enhancement of graphene’s p-type doping effect caused by defects and adsorbed impurities. This work conduces to the understanding the mechanism of He ion interaction with graphene, and also provides an effective way of controlling the electronic properties.

    • 石墨烯是一种由碳原子以sp2杂化方式构成的平面二维材料, 拥有优异的电学、光学、力学性能, 被认为是未来革命性的电子元件材料之一[1-3]. 但本征石墨烯为零带隙材料, 用其直接制成的场效应晶体管不具备开关特性, 限制了其在纳米电子器件中的广泛应用.

      迄今为止, 大量研究表明通过离子束辐照石墨烯引入缺陷是一种打开石墨烯带隙并调控其电学性能的有效方法[4-6], 其中低能惰性气体离子辐照不仅能避免在石墨烯中引入其他杂质, 且具有低成本和大面积可控操作等特点, 因此采用惰性气体离子辐照实施对石墨烯结构和电学性能调控的研究引起了广泛关注[7,8], 如Lucchese等[9]观察到随Ar+辐照剂量增加, 石墨烯微观结构由纳米晶向无定形结构转变; Al-Harthi等[10]发现Ar+辐照下石墨烯部分碳原子杂化类型由sp2向sp3转变, 进而影响层间电子耦合作用致使电子能带变宽; Chen等[11]通过Ne+辐照石墨烯发现其电导率随辐照剂量增大而减小, 在高剂量辐照时石墨烯的最小电导率低于无谷间散射时电导率的理论值, 且缺陷散射引起载流子迁移率的降低量是同剂量密度下电荷杂质散射的4倍以上. 然而, 有关He+辐照后石墨烯微观结构转变及其电导率、电子输运机制、狄拉克电压等电学性能的系统性研究相对较少.

      本文采用化学气相沉积法制备单层石墨烯, 结合XPS, Raman对辐照前后石墨烯的结构表征, 系统地分析和讨论了不同剂量He+辐照对单层石墨烯微观结构变化与电学性能的影响规律及相关机理.

    2.   实 验
    • 石墨烯样品尺寸为5 mm × 5 mm, 制备过程具体如下: 采用化学气相沉积法在铜衬底上生长制得石墨烯, 表面涂覆一层转移介质聚甲基丙烯酸甲酯 (PMMA)并用腐蚀液腐蚀铜衬底, 去离子水清洗后转移至表面SiO2厚度约300 nm的Si片上, 使用丙酮清洗表面PMMA得到最终所需的SiO2衬底单层石墨烯样品. 利用掩膜工艺和电子束蒸镀技术在石墨烯表面沉积60 nm厚的金膜电极作为源、漏极, 获得Si/SiO2/GR/Au测试结构. 辐照实验He+能量为5.4 keV, 离子束流强度为20 µA, 辐照靶室真空度为1 × 10–4 Pa, 辐照剂量分别为0.7 × 1013, 1.6 × 1013和2.5 × 1013 He+/cm2.

      利用XPS, Raman对石墨烯的结构进行表征. XPS表征采用X射线光源为Al Kα (1486.6 eV), 光斑直径400 μm. Raman表征选用Witec alpha 300共聚焦拉曼光谱仪, 采用100 × 物镜进行分析, 激发光波长为532 nm(对应的能量为2.33 eV), 光斑直径600 nm. 电学测量采用Keithley 2400测量背栅工艺下不同剂量辐照后石墨烯器件的电导率, 源漏极电压为0.1 V. 所有样品在测量前均用丙酮清洗并干燥.

    3.   结果与讨论
    • 图1所示为辐照前后石墨烯的C1s峰XPS谱图的Gaussian-Lorentzian拟合结果, 表1列出辐照前后各拟合峰的面积占比. 如图1(a)所示, 未辐照石墨烯样品C1s峰拟合为C—C键的sp2杂化(284.5 eV); C—C键的sp3杂化(285.1 eV); C—O—H键(285.6 eV); C—O—C键(286.7 eV); O—C=O键(289.1 eV)[12,13]. 结合图1表1定量分析结果可知, 各个化学键的峰位随辐照剂量增加变动不大, 但其相对含量发生明显改变. 当辐照剂量由0增至2.5 × 1013 He+/cm2, C—C sp2峰面积比从0.52减小至0.28, C—C sp3峰面积比从0.19增大至0.35, C—O—H峰面积比从0.1增大至0.2. 辐照导致晶格缺陷增多, 从而加剧杂化相的产生, sp2键部分转化成sp3[14], C—C sp3峰面积比增大; 并且, 辐照造成石墨烯表面C悬挂键出现并与环境中的O2, H2O相互作用, 致使C—O—H峰面积比增大.

      Figure 1.  XPS C1 s peak spectra of graphene samples: (a) Unirradiated; (b) 0.7 × 1013 He+/cm2; (c) 1.6 × 1013 He+/cm2; (d) 2.5 × 1013 He+/cm2.

      辐照剂量C-C sp2C—C sp3C—O—HC—O—CO—C=O
      未辐照0.520.190.100.090.10
      0.7 × 1013 He+/cm20.500.300.130.050.02
      1.6 × 1013 He+/cm20.370.320.150.100.06
      2.5 × 1013 He+/cm20.280.350.200.120.05

      Table 1.  C1 s peak area ratio of graphene samples before and after irradiation.

      图2所示为未辐照石墨烯的Raman光谱, 从图可知在1583和2671 cm–1处分别有明显的G峰和2D峰, G峰通常产生于sp2碳原子的面内振动, 2D峰是与与碳原子的层间堆垛方式有关的双声子共振拉曼峰[15]. 图2插图为2D峰洛伦兹拟合图, 从图可知2D峰拟合为单洛伦兹峰, 且2D峰和G峰的峰强比值I2D/IG大于1.5, 说明该样品是单层石墨烯[16]. D峰峰强与无序程度有关, 常用于表征石墨烯的缺陷密度[17], 谱图中微弱的D峰表明该石墨烯样品质量很高.

      Figure 2.  The Raman spectra of unirradiated graphene.

      图3所示为He+辐照前后石墨烯的Raman光谱图. 由图可见, 辐照剂量为0.7 × 1012 He+/cm2时, 在1340和1620 cm–1[18]处分别出现明显的D, D’峰, 表明辐照在石墨烯体内已引入缺陷; 当辐照剂量增至1.6 × 1013 He+/cm2, D峰强度强烈增加, 表明石墨烯的缺陷密度显著增加; 当辐照剂量进一步增加到2.5 × 1013 He+/cm2, D峰、G峰、2D峰峰强均减弱.

      Figure 3.  The Raman spectra of graphene before and after He+ irradiation.

      D峰与G峰强度之比ID/IG是用来表征石墨烯缺陷密度的重要参数, ID/IG, I2D/IG与辐照剂量的关系如图4所示. 随着辐照剂量增大, ID/IG比值由0增至1.35再减至0.42, I2D/IG比值由2.71减至1.31. 当辐照剂量由0增至1.6 × 1013 He+/cm2, 石墨烯结构无序程度增加, 呈现纳米晶结构[15], 此时石墨烯的缺陷密度$ n_{\rm d} $可表示为[19]

      Figure 4.  The relationship between Raman peak strength ID/IG, I2D/IG ratio and irradiation dose.

      其中λL为激光波长; 石墨烯经过He+辐照后, 由于缺陷密度增大ID/IG逐渐增大至1.35. 当辐照剂量继续增大至2.5 × 1013 He+/cm2, 由XPS和Raman分析结果可知辐照后石墨烯C—C sp3键含量不断增多但ID/IG开始减小且拉曼特征峰强减弱, 因此石墨烯由纳米晶结构向sp3无定形碳结构转变[20], 此时ndID/IG不再符合(1)式的关系. 其比值减小的原因可从Raman特征峰的来源及变化分析, D峰与2D峰是二阶双共振拉曼散射峰, 与碳环中sp2原子呼吸振动模式相关[21], 辐照剂量由1.6 × 1013 He+/cm2增至2.5 × 1013 He+/cm2时, 离子辐照打断C—C键使大量碳环断裂, 即无序程度的增加抑制了二阶拉曼散射过程, 造成D峰、2D峰的强度降低; 而G峰源于一阶E2 g声子的平面振动, 与sp2杂化对的伸缩振动模式相关, 辐照后伸缩振动减弱较慢, 即G峰强度降低较慢. 因此, ID/IGI2D/IG均减小.

      图5图6分别给出石墨烯器件辐照前后电导率随栅极电压(Vg)的变化曲线和Vdirac偏移量随辐照剂量的变化率. 如图6所示, Vdirac向正电压方向的偏移量由未辐照样品的6.7 V近线性增加至2.5 × 1013 He+/cm2剂量辐照时的10.3 V. 未辐照石墨烯样品出现Vdirac正向偏移的主因是制备过程中其表面吸附了空气中的O2和H2O等杂质, 电子输运时极易被吸附的杂质捕获致使石墨烯的空穴浓度变大[22], 产生P型掺杂效应. 随着辐照剂量增大, 不断增多的缺陷通过捕获电子引起费米能级的逐渐下降[23], 导致辐照缺陷引起的P型掺杂效应逐步加强, 这与Raman光谱中观察到2D峰峰位发生蓝移的现象相对应[17, 24], 故Vdirac向正电压方向的偏移量继续增大; 同时, 结合XPS中C—O—H峰面积变化结果可知, 离子辐照打断部分C—C键使C悬键的数量增多以致石墨烯吸附杂质作用增强, 这说明辐照后吸附杂质对石墨烯的P型掺杂效应增强仍有少量贡献.

      Figure 5.  Electrical conductivity versus gate voltage at different irradiation doses.

      Figure 6.  The relation between Dirac voltage variation and irradiation dose.

      辐照后石墨烯微观结构的转变对其电子输运机制将产生重要的影响. 辐照剂量低于1.6 × 1013 He+/cm2时石墨烯为纳米晶结构, 此时载流子的主导散射机制是基于经典玻尔兹曼扩散输运理论的带电杂质散射或缺陷散射[25]. Chen等[26]发现Raman G峰峰位偏移量与费米能级偏移量呈线性关系, 由吸附杂质引起的陷阱电荷密度变化${n_{\rm{c}}}$表示为:

      式中$\Delta {\varOmega _{\rm{G}}}$为G峰峰位偏移量, 本实验测得辐照剂量由0至1.6 × 1013 He+/cm2期间G峰峰位蓝移为4 cm–1; ${E_{\rm{F}}}$为费米能级偏移量, 由(3)式得出费米能级偏移量为0.095 eV; h为普朗克常数; VF为费米速度. 最终估算出杂质吸附引起的陷阱电荷密度${n_{\rm c}}$为1.68 × 1010 cm–2, 而总的陷阱电荷密度${n_{\rm{t}}}$可由狄拉克电压偏移量计算[27]:

      式中${C_{{\rm{ox}}}}$为单位面积SiO2的电容; $\Delta {V_{{\rm{dirac}}}}$为狄拉克电压偏移量, 此辐照剂量区间的$\Delta {V_{{\rm{dirac}}}}$为2.21 V; q为单位电荷带电量; 对应总的陷阱电荷密度${n_{\rm{t}}}$为1.59 × 1011 cm–2. 由于${n_{\rm{c}}}$值仅占${n_{\rm{t}}}$值的10%, 表明辐照引起的缺陷散射是此阶段载流子输运主要的散射机制.

      根据Zhou等[28]的研究, 1.5 × 1013 cm–2剂量的Ga+辐照使单层石墨烯形成无定形碳结构且电导率降低了近一个数量级, 此时载流子输运受变程跃迁机制控制; 而在本工作中, 离子辐照剂量由1.6 × 1013 He+/cm2增至2.5 × 1013 He+/cm2时, 石墨烯纳米晶结构同样向无定形碳结构转变, 同时石墨烯的电导率随辐照剂量增大而逐渐减小, 2.5 × 1013 He+/cm2剂量辐照后最小电导率减小至0.5 e2/h, 仅为未辐照石墨烯的最小电导率的10%, 由此推断该剂量阶段辐照引起石墨烯结构的强烈无序化使电子的主要输运机制转变为相邻缺陷态间的跃迁输运. 另一方面, 此阶段石墨烯晶粒尺寸${L_{\rm{a}}}$表示为[29]

      其中$C'\left( \lambda \right)$是与激光波长相关的常数, 该剂量区间ID/IG随辐照剂量增大而减小, 即晶粒尺寸不断减小, 晶粒间距不断增大使原本连续的石墨烯导电沟道引入能量势垒[30], 电子在输运过程中必须克服该势垒, 则电子的主要输运方式需通过相邻缺陷间的跃迁传输完成. 因此, 随着辐照剂量增大, 石墨烯发生从纳米晶结构向无定形碳结构转变的同时, 其电子输运机制也从以缺陷散射为主的玻尔兹曼扩散输运转变为跃迁输运.

    4.   结 论
    • 本文采用化学气相沉积法制备单层石墨烯, 研究了不同剂量He+辐照对其微观结构、电导率、电子输运机制及狄拉克电压的影响. 得出如下结论:

      1)低于1.6 × 1013 He+/cm2剂量时, ID/IG随辐照剂量增大而增大, 石墨烯呈现纳米晶结构, 电子输运机制为缺陷散射为主的玻尔兹曼扩散输运; 辐照剂量由1.6 × 1013 He+/cm2增至2.5 × 1013 He+/cm2时, ID/IG减小, 拉曼特征峰峰强开始减弱, 且部分C—C sp2键转化成C—C sp3键, 石墨烯由纳米晶结构向sp3无定形碳结构转变的同时电导率持续降低, 电子主导输运机制转变为跃迁输运;

      2)随着辐照剂量增大(0—2.5 × 1013 He+/cm2), 石墨烯的缺陷密度增大, C-O-H键含量相对增加, 辐照缺陷和吸附杂质使石墨烯P型掺杂效应增强, Vdirac向正电压方向的偏移量由6.7 V增至10.3 V.

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