利用原子力顯微術探討二硫化鉬表面的奈米摩擦伏特效應

Abstract

本研究利用原子力顯微鏡（AFM）探討二硫化鉬（MoS₂）與鉑（Pt）探針接觸下的摩擦伏特效應（Tribovoltaic Effect），並分析不同探針所施正向力、二硫化鉬層數、實驗時的溫濕度對局部摩擦伏特電流的影響。首先，我們透過赫茲接觸模型推估實驗中探針與二硫化鉬表面的接觸面積，以及因正向力造成的垂直方向的壓縮量與，作為後續摩擦伏特電流變異分析的物理基礎。隨正向力增加，我們觀察到摩擦伏特電流隨之上升。這主要歸因於正向力提升導致探針與樣品間的接觸面積擴大，增加了界面區域內電子–電洞對的激發機會。同時，壓縮作用促使二硫化鉬表面產生更高的機械變形與應力集中，進一步降低肖特基能障（Schottky Barrier, SSB），提高載子的激發與分離效率，從而使摩擦伏特電流隨正向力增加而增大。我們進一步的研究顯示，樣品不同區域在相同正向力下會呈現正電流、負電流或無明顯電流的行為，反映出二硫化鉬表面態密度、缺陷及局部PN型變異造成的電子性質的非均勻性。克氏表面電位顯微鏡(Kelvin probe force microscopy, KPFM) 的量測進一步驗證了樣品表面接觸電位（contact potential difference CPD）分佈的不均勻性。 此外，隨二硫化鉬層數增加，其能隙轉增加機械剛性上升對電流表現產生影響，溫度提升則提高費米能階與載子激發效率，溫度梯度形成的熱電勢，這些因子都會對摩擦電流產生影響。本研究系統性地揭示了多因素交互作用下摩擦伏特效應的行為特徵，並提供理解界面能量轉換與電荷分離機制的重要依據，對於未來摩擦能量收集器件的設計具有參考價值。This study employs atomic force microscopy (AFM) to investigate the tribovoltaic effect at the contact interface between molybdenum disulfide (MoS₂) and a platinum (Pt) probe, and analyzes how local tribovoltaic current is influenced by factors including applied normal force, MoS₂ layer thickness, and environmental temperature and humidity. First, the Hertz contact model is used to estimate the contact area and indentation depth between the AFM probe and the MoS₂ surface, establishing the physical basis for subsequent analysis of tribovoltaic current variations. As the normal force increases, the measured tribovoltaic current also increases. This enhancement is primarily attributed to the expansion of the contact area, which promotes more frequent generation of electron–hole pairs within the interfacial region. In addition, the increased compressive stress at the interface leads to greater mechanical deformation and stress concentration in the MoS₂, which further reduces the Schottky barrier height (SBH), thereby enhancing carrier excitation and separation efficiency. Further investigation reveals that under the same normal force, different regions of the MoS₂ sample exhibit positive current, negative current, or negligible current, reflecting local variations in surface states, defect distributions, and p–n-type characteristics. Measurements from Kelvin probe force microscopy (KPFM) confirm the non-uniform distribution of contact potential difference (CPD) across the MoS₂ surface. Moreover, an increase in the number of MoS₂ layers leads to a transition in the bandgap and a rise in mechanical stiffness, both of which influence current output. Elevated temperature raises the Fermi level and enhances carrier excitation efficiency, while the resulting vertical temperature gradient introduces additional thermoelectric contributions. This study systematically elucidates the tribovoltaic behavior under multiple interacting factors and provides key insights into interfacial energy conversion and carrier dynamics. These findings offer a valuable reference for the future design of triboelectric energy harvesting devices.