Molecular dynamics simulation of contact behaviors between multiwall carbon nanotube and metal surface
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摘要: 碳纳米管与金属表面间的界面接触行为对碳基电子器件的性能研究具有重要意义. 本文利用分子动力学模拟方法研究了端部开口和闭口的多壁碳纳米管与金属表面间的界面接触构型和粘着强度. 模拟结果表明, 多壁碳纳米管在金属表面上的位置和形状变化是因为减少的范德瓦耳斯能转变成碳纳米管的内能; 多壁碳纳米管与金属表面的结合能为负值, 表明两者存在粘着作用; 多壁碳纳米管水平地接触金属表面时的粘着强度受初始间距、接触长度和金属种类影响, 即范德瓦耳斯能作用的初始间距阈值约为1 nm且最终平衡状态时的两者间距约为0.3 nm; 随着两者接触长度增加, 粘着强度增大, 铂与碳纳米管的粘着强度比钨和铝的大; 端部开口和闭口的碳纳米管与金属表面垂直接触时的粘着强度都比水平接触时的小, 两端开口的碳纳米管在金属表面上发生径向压缩变形, 最终形成带状结构; 而两端封闭的碳纳米管在金属表面上发生轴向压缩变形; 在碳纳米管场效应晶体管中, 两端开口的多壁碳纳米管与单壁碳纳米管一样变形成带状结构, 并且各个管壁之间以及最外层管壁与金属电极之间的间距相当, 该原子尺度的间距(约0.34 nm)保证电子从金属隧穿到最外层管壁, 并在内层管壁之间径向迁移.Abstract: The interfacial contact configuration and contact intensity between carbon nanotube and metal surface play an important role in the electrical performance of carbon nanotube field effect transistors and nanoscale carbon nanotube robotic manipulation. In this paper, we investigate numerically the contact configuration and the contact intensity between multiwall carbon nanotube with open ends or capped ends and various metal surfaces in carbon nanotube field effect transistor assembly by the molecular dynamics simulation. The simulation results show that the change in the position and shape of multiwall carbon nanotube on the metal surface are mainly due to the decrease of van der Waals energy reduction: the decrement of van der Waals energy is converted into the internal energy and kinetic energy of carbon nanotubes. Moreover, the binding energy between multiwall carbon nanotube and metal surface is negative, which indicates that multiwall carbon nanotube adheres to the metal surface. In addition, the contact intensity of multiwall carbon nanotube in horizontally contacting metal surface is influenced by initial distance, contact length and metal materials. The final equilibrium distance is around ~0.3 nm when the initial distance is less than ~1 nm. And the contact intensity increases with the augment of contact length between carbon nanotube and metal. The contact intensity between platinum and carbon nanotube is larger than that between tungsten and aluminum, therefore, platinum-coated probe is generally utilized for picking carbon nanotube up. The contact intensity of the carbon nanotubes with the open ends and closed ends in the vertical contact with the metal surface are both lower than those in the horizontal contact. The interfacial contact configuration of carbon nanotube and metal materials mainly include the displacement and geometric deformation of carbon nanotube. The displacement and geometric deformation of multiwall carbon nanotube with open ends on the metal surface finally result in its radial nanoscale ribbon structure. But the closed-end three-wall carbon nanotube has the small axial geometric deformation through comparing the concentration profiles between the initial carbon nanotube and the collapsed carbon nanotube. In a carbon nanotube field effect transistor, the collapsed multiwall carbon nanotube forms the ribbon structure like a single wall carbon nanotube. And the distance between carbon nanotube walls and between the outermost carbon nanotube wall and the metal electrode are both about ~0.34 nm. The atomic scale spacing ensures that electrons tunnel from the metal to the outermost carbon nanotube wall and migrate radially between the inner carbon nanotube walls.
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Key words:
- carbon nanotube /
- van der Waals energy /
- molecular dynamic
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图 1 多壁碳管及其分子动力学模型 (a) 碳纳米管和AFM悬臂梁的SEM图像; (b) 原子模型侧视图; (c) 主视图; (d) 范德瓦耳斯能差值
Figure 1. Picked CNT and its molecular dynamic model; (a) SEM images of CNT and AFM cantilever; (b) side view; (c) front view; (d) vdW energy reduction.
图 2 碳纳米管与金界面的能量变化 (a)总能; (b)势能; (c)内能; (d)动能
Figure 2. Energy changing at the interface of CNT and gold durface: (a) Total energy; (b) potential energy; (c) internal energy; (d) kinetic energy.
图 3 碳纳米管与金属表面的接触模式 (a)水平接触; (b)垂直接触
Figure 3. Contact model between CNT and metal surface: (a) Horizontal contact; (b) vertical contact.
图 4 碳纳米管与金表面在不同初始间距下的界面接触构型及范德瓦耳斯能差 初始间距: (a) 1.1999 nm; (b) 0.9489 nm; (c) 0.1378 nm; 平衡间距: (d) 1.16 nm; (e) 0.3081 nm; (f) 0.2933 nm; 范德瓦耳斯能差: (g) 1.1999 nm; (b) 0.9489 nm; (c) 0.1378 nm
Figure 4. Contact configuration and Van der Waals energy of CNT with different initial gap: (a) Initial separate gap of 1.1999 nm; (b) 0.9489 nm; (c) 0.1378 nm; (d) final gap of 1.16 nm; (e) 0.3081 nm; (f) 0.2933 nm; Van der Waals energy at distance of (g) 1.1999 nm; (b) 0.9489 nm; (c) 0.1378 nm.
图 5 碳纳米管与不同金属间的界面接触构型和范德瓦耳斯能差值. 接触构型： (a) 铂; (c) 铝; (e)钨; 范德瓦耳斯能差值： (d) 铂; (e) 铝; (f) 钨
Figure 5. Contact behavior and VdW energy reduction of three-walled CNT on different metal: (a) Pt; (c) Al; (c) W; VdW energy reduction contacting with different metal: (d) Pt; (e) Al; (f) W.
图 6 碳纳米管与金在不同接触长度下的接触构型和范德瓦耳斯能差值. 接触构型： (a) 2.46 nm; (b) 3.689 nm; (c) 4.919 nm; 范德瓦耳斯能差值： (d) 2.46 nm; (e) 3.689 nm; (f) 4.919 nm
Figure 6. Contact behavior and VdW energy reduction with different contact length. Contact behavior: (a) 2.46 nm; (b) 3.689 nm; (c) 4.919 nm; and VdW energy reduction under contact length: (d) 2.46 nm; (e) 3.689 nm; (f) 4.919 nm.
图 7 碳纳米管与金表面垂直接触下的接触构型和范德瓦耳斯能差值 接触构型：(a) 两端开口; (b) 一端开口和闭口; (c) 两端闭口; 范德瓦耳斯能差值： (d) 两端开口; (e) 一端开口和闭口; (f) 两端闭口
Figure 7. Contact behavior in vertical contact and VdW energy reduction: (a) 2 open ends; (b) capped and open end; (c) 2 capped ends; VdW energy reduction with: (d) 2 open ends; (e) capped and open end; (f) 2 capped ends.
图 8 初始和变形的端部开口的三壁碳纳米管的浓度分布 (a) xz平面上; (b) x方向; (c) yz平面上; (d) y方向; (e) xz平面上; (f) z方向
Figure 8. Concentration profile of initial and collapsed capped-ends three-walled CNT: (a) xz plane; (b) x; (c) yz plane; (d) y; (e) xz plane; (f) z.
图 9 初始和变形的端部闭口的三壁碳纳米管的浓度分布 (a) xz平面上; (b) x方向; (c) yz平面上; (d) y方向; (e) xz平面上; (f) z方向
Figure 9. Concentration profile of initial and collapsed open-ends three-walled CNT: (a) xz plane; (b) x; (c) yz plane; (d) y; (e) xz plane; (f) z.
图 10 三壁闭口碳纳米管的场效应晶体管 (a)原子模型; (b)正视图; (c)侧视图; (d)俯视图
Figure 10. Three-walled capped-ends CNTFET: (a) Molecular dynamic modeling of CNTFET; (b) front view; (c) side view; (d) top view
图 11 三壁开口碳纳米管场效应晶体管 (a)原子模型; (b)正视图; (c)侧视图; (d)俯视图
Figure 11. Three-walled open-ends CNTFET: (a) Molecular dynamic modeling of CNTFET; (b) front view; (c) side view; (d) top view
图 12 单壁两端闭口碳纳米管场效应晶体管 (a)原子模型; (b)正视图; (c)侧视图; (d)俯视图
Figure 12. Single-walled open-ends CNTFET: (a) Molecular dynamic modeling of CNTFET; (b) front view; (c) side view; (d) top view.
图 13 两端开口或闭口碳纳米管与钯电极接触的构型和范德瓦耳斯能差值 接触构型：(a)单壁开口; (b)单壁闭口; (c)三壁开口; 范德瓦耳斯能差值： (d)单壁开口; (e)单壁闭口; (f)三壁开口
Figure 13. Interface configuration and Van der Waals energy of CNTs contacting with palladium: (a) Single-walled open-ends CNT; (b) single-walled closed-ends CNT; (c) three-walled open-ends CNT; Van der Waals energy of (d) single-walled open-ends CNT; (e) single-walled closed-ends CNT; (f) three-walled open-ends CNT.
表 1 多壁碳纳米管与金表面接触系统径向压缩变形前后的能量组成
Table 1. The energy components of multi-walled CNT and gold surface before and after collapse.
能量组成 初始状态 稳定状态 能量差值 总能/Mcal/mol 144.3 约143 –1.3 势能/Mcal/mol 约140 约138.7 –1.3 动能/Mcal/mol 4.5 ± 0.03 4.5 ± 0.03 — 内能/Mcal/mol 约139.3 约139.5 0.2 范德瓦耳斯能/Mcal/mol 约0.7 约–0.83 –1.53 
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表 2 多壁碳纳米管与金表面上径向压缩变形前后的初始间距及其对应能量
Table 2. VDW energy of multi-walled CNT and gold surface at different original distance.
能量对应间距 初始状态 最终状态 范德瓦
耳斯
能差能量/
(Mcal/mol)
(间距/ nm)约0.7(1.1999) 约0.7(1.16) 0 约0.7(0.9489) –0.8(0.3081) 1.5 –3.6(0.1378) –3.6(0.2933) 0
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表 3 多壁碳纳米管与金表面在不同金属种类、接触长度和接触模式下范德瓦耳斯能差
Table 3. VDW energy of MWCNT and gold surface with different metal, contact length and model.
能量/(Mcal/mol) 金属种类 水平接触长度/nm 垂直接触 铂 铝 钨 2.46 3.689 4.919 两端开口 开口封闭 两端封闭 范德瓦耳斯能差 2 1.15 1.05 1.2 1.8 2.4 0 0.5 0
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表 4 多壁碳纳米管最外层管壁与金表面的接触距离
Table 4. Contacting distance of outmost layer of MWCNT with gold surface.
碳纳米
管结构源漏初始间距/nm 源漏最终
间距/nm栅极初始
间距/nm栅极最终
间距/nm三壁两
端闭口0.6374 0.29 0.9233 0.36 三壁两
端开口0.6729 0.3204 0.932 0.3235 单壁两
端开口0.67 0.3 0.93 0.34
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