Fund Project:Project supported by National Natural Science Foundation of China (Grant No. 51905089) and the Fundamental Research Funds for the Central Universities of Ministry of Education of China (Grant No. 2232020D-31)
Received Date:28 February 2021
Accepted Date:15 April 2021
Available Online:07 June 2021
Published Online:20 August 2021
Abstract:Hexagonal boron nitride (h-BN) has huge potential applications in micro-nano electromechanical system due to its good lubricity and insulation. In this paper, a microporous array is prepared on a SiO2/Si substrate by the substrate etching process, and then the h-BN is transferred to the microporous substrate to form a suspension structure. The effect of electric field on tribological properties of suspended h-BN is studied by atomic force microscopy. The results show that the friction of the suspended h-BN is smaller than the friction on the h-BN supported by the substrate, because the greater in-plane stretch weakens the puckering effect. The electric field increases the friction of the suspended h-BN, and the influence of positive bias is greater than that of negative bias. The application of the electric field increases the electrostatic force on the tip, thereby increasing the additional load and the interface barrier in the friction process. The electric field causes the stick-slip behavior to change from single-slip to multi-slip. Compared with the h-BN supported by the substrate, h-BN in the suspended state is strongly affected by the electric field. The reduction of the interface distance and the absence of the substrate oxide layer lead the electrostatic force to increase. This paper proposes a method to adjust h-BN’s friction by electric field, which provides theoretical guidance for studying the friction characteristics of two-dimensional materials. Keywords:friction force/ suspended hexagonal boron nitride/ atomic force microscope/ electrostatic force
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3.1.悬浮h-BN的摩擦特性
h-BN晶体经机械剥离后覆盖在微孔阵列的基底上, 在光学显微镜下确定悬浮状态的h-BN的位置. 图2(a)显示了样品的光学图, 红色方框处所对应的AFM形貌图如图2(b)所示, 可以看出左下角绿色虚线圈内的微孔上覆盖了h-BN. h-BN的形貌表征是在AFM的轻敲模式下进行的, 扫描范围为20 μm × 20 μm, 扫描频率为1 Hz. 从图2(b)的插图中可知红色虚横截线处的h-BN的高度为23 nm. 由于h-BN样品与基底贴合并非完全紧密, 实验测得的样品厚度可能会略大于理论值[14]. 图 2 样品的光学图和形貌图 (a) 微孔基底上h-BN的光学图; (b) 微孔基底上h-BN的AFM形貌图, 插图为h-BN的高度轮廓图 Figure2. Optical image and topography of the sample: (a) Optical image of h-BN on microporous substrate; (b) AFM topography of h-BN on microporous substrate, the illustration shows the height profile of h-BN.
其中, K代表库伦常数, Qt和Qs分别表示针尖上和Si基底上针尖垂直对应区域上产生的感应电荷, d表示针尖与Si基底之间的距离. 施加电场后, 硅针尖会在电场的作用下会产生感应电荷, 针尖与Si基底之间的静电相互作用增强, 使得针尖所受到的静电力变大. 静电力的增大会产生额外的载荷, 进而使得摩擦力增大. 尖端与样品表面的相互作用可以反映在原子尺度的黏滑运动中. 黏滑运动(stick-slip)是纳米摩擦的一个特性, 可以反映出摩擦过程中能量的变化过程. 针尖开始运动时需要克服原子间的势垒, 发生滑移, 从一个稳态跃迁到下一个局部势能最低点. 在测量摩擦的区域内选取了3 nm × 3 nm的区域进行黏滑运动实验, 施加固定载荷10 nN, 扫描频率1 Hz. 由图7(a)所示, 在不施加电场的条件下, 测量出10—11个峰, 这与h-BN的理论晶格常数0.25040 nm保持一致. 当施加+5 V偏压以后, 峰的数量减少到如图7(b)所示的6—7个. 电场的施加会减少单位长度内黏滑运动的周期, 并增大黏滑运动的幅值. 该现象表示在电场作用下h-BN表面的黏滑运动发生了从单步黏滑向多步黏滑的转变, 针尖与h-BN之间的相互作用增强[24]. 电场的施加增大了针尖原子与h-BN之间的势垒, 针尖在h-BN表面的摩擦会产生更大的能量耗散. 图 7 悬浮h-BN的黏滑运动在电场下的变化 (a) 无电场时悬浮h-BN的侧向力曲线; (b) +5 V偏压下悬浮h-BN的侧向力曲线 Figure7. Variation of stick-slip behavior of suspended h-BN under electric field: (a) Lateral force curves measured on suspended h-BN without bias; (b) lateral force curves measured on suspended h-BN under +5 V bias.
23.3.电场下悬浮与支撑状态的h-BN摩擦特性对比 -->
3.3.电场下悬浮与支撑状态的h-BN摩擦特性对比
本文进一步对比了有基底支撑与悬浮状态的h-BN在电场下的摩擦特性. 如图8(a)和8(b)所示, 在有基底支撑的h-BN上施加–4 V和+4 V的偏压, 发现电场所引起的摩擦力变化并不明显, 这是由于该h-BN样品具有较厚的厚度, 静电力的影响被大大削弱. 然而在悬浮的h-BN上, 摩擦力却仍然受电场的影响较大. 由图8(c)和8(d)所示, 在+4 V偏压下的摩擦力较无偏压下的摩擦力相比已经有了明显的增大, –4 V偏压下的摩擦力也略有增大. 图 8 电场下支撑与悬浮状态的h-BN的摩擦力对比 (a) 电场下有基底支撑的h-BN的摩擦力图; (b) 不同偏压下有基底支撑的h-BN的摩擦力柱状图; (c) 电场下悬浮h-BN的摩擦力图; (d) 不同偏压下悬浮h-BN的摩擦力柱状图 Figure8. Comparison of the friction on the supported and suspended h-BN under electric fields: (a) Friction on supported h-BN under biases; (b) histogram of the friction on supported h-BN under different biases; (c) friction on suspended h-BN under biases; (d) histogram of the friction on suspended h-BN under different biases.
图9(a)和9(b)分别为电场下有基底支撑的和悬浮状态的h-BN的示意图. 悬浮h-BN的下落高度缩短了针尖与Si基底之间的距离d, 针尖加载后又会使悬浮h-BN再产生一定的向下变形量, 如图9(b)中虚线所示. 图 9 电场下支撑与悬浮状态的h-BN示意图对比 (a) 电场下有基底支撑的h-BN的示意图; (b)电场下悬浮h-BN的示意图 Figure9. Comparison of schematic diagram of h-BN in supported and suspended state under electric field: (a) Schematic diagram of supported h-BN under electric field; (b) schematic diagram of suspended h-BN under electric field.