| In recent years,Microelectromechanical Systems(MEMS)have experienced rapid development and widespread application.MEMS integrate electrical and mechanical components at the micro and nanoscale,which introduces many new challenges due to the miniaturization of MEMS structures.Adhesion is a significant factor contributing to the failure of MEMS during manufacturing and usage.Silicon is the primary material used in MEMS devices,and graphene is extensively employed as a surface coating in Microelectromechanical Systems.It is common to use silicon as the supporting substrate for graphene.Therefore,a comprehensive study of the adhesion mechanisms at the silicon-silicon and silicon-graphene interfaces,along with the artificial control of adhesion based on adhesion principles,is crucial for advancing the development of MEMS and commercial applications of graphene.This paper investigates the time-dependent adhesion of silicon-silicon and silicon-graphene interfaces with varying hydrophilicity using an atomic force microscope under different relative humidity conditions.The findings are summarized as follows:(1)Adhesion between hydrophobic silicon micro cantilevers and partially hydrophobic silicon samples was measured using an atomic force microscope(AFM)at relative humidity ranging from 10% to 90%.The time dependence of adhesion forces was investigated.The experimental results indicated that the contact time dependence of adhesion at the hydrophobic silicon-partially hydrophobic silicon interface is influenced by relative humidity and contact history.Adhesion increased logarithmically with increasing dwell time from 10%to 80% relative humidity(with saturation occurring between 80 and 200 seconds),while at～90% relative humidity,adhesion became independent of dwell time.However,after exposure to 90% relative humidity,adhesion at this humidity level exhibited instability,and the behavior of adhesion transitioned from being independent to logarithmically increasing or showing large values at long dwell time.This instability was manifested as changes in the amplitude of adhesion with repeated contacts.The changes in adhesion behavior were attributed to the condensation reaction of orthosilicic acid leading to the formation of sols and gels,resulting in increased surface liquid film thickness and higher viscosity.Following exposure to high humidity,the behavior of adhesion with dwell time at 60% to 80% relative humidity showed either an "increase-decrease" or "no change-increase-no change" pattern.Additionally,exposure also led to a decrease in adhesion saturation time at relative humidity ranging from 30% to 10%.However,this exposure effect could be eliminated through repeated contacts at low relative humidity.Due to the exposure effect,the behavior of adhesion in terms of contact time dependence and amplitude variation exhibited differences when increasing and decreasing relative humidity.Therefore,the history of interface contact has a significant influence.(2)Adhesion on hydrophilic silica surfaces was measured using AFM with hydrophobic and hydrophilic unpatterned silicon micro cantilevers at different relative humidity levels.The aim was to investigate the influence of surface hydrophilicity on the time dependence of adhesion.Relative humidity was first increased in a step wise manner from 10%(with 10%increments)up to 90%,and then decreased back to 10%.The results revealed that surface hydrophilicity has a significant impact on the contact time dependence of adhesion at the silicon-silicon interface,and the contact history at high humidity levels may alter the time dependence behavior.From 10% to 80% relative humidity,adhesion at the hydrophobic silicon-hydrophilic silicon interface exhibited a logarithmic increase with increasing contact time(indicating contact time dependence).The magnitude of adhesion variation with repeated contacts was relatively small,demonstrating a stable behavior.In other words,the average adhesion forces obtained under increasing and decreasing dwell times overlap well.However,at 90% relative humidity,adhesion at the hydrophobic silicon-hydrophilic silicon interface may or may not exhibit contact time dependence,and the adhesion becomes unstable(varies with repeat contacts),occasionally showing a jump in adhesion force.Additionally,after exposure to 90% relative humidity,the contact time dependence of adhesion at high relative humidity(80% to 60%)becomes less evident,and the magnitude also becomes unstable.However,when the relative humidity is below 60%,adhesion regains contact time dependence and exhibits better stability.In contrast,adhesion at the hydrophilic silicon-hydrophilic silicon interface showed no contact time dependence at low humidity(10% to 40%),remaining unchanged or slightly increasing with increasing contact time.However,this adhesion exhibited contact time dependence at moderate to high humidity(50%to 90%).After exposure to high humidity,adhesion showed contact time dependence across the entire humidity range(90% to 10%).Furthermore,when relative humidity remained at10%,the adhesion behavior returned to a state without contact time dependence.(3)Adhesion on the monolayer graphene attached to silicon dioxide was measured using AFM with unpatterned silicon micro cantilevers under different relative humidity.The aim was to investigate the contact time dependence of adhesion and explore the influence of surface hydrophilicity on this dependence.The contact interfaces included hydrophilic silicon-hydrophobic graphene,hydrophobic silicon-hydrophilic graphene,and hydrophobic silicon-hydrophobic graphene.The results revealed that the contact time dependence of adhesion at the silicon-graphene interface varied under different relative humidity conditions,depending on the surface hydrophilicity.Specifically,(1)At relative humidity levels ranging from 10% to 90%,the adhesion between hydrophilic silicon and hydrophobic graphene showed no time dependence.The adhesion either remained constant,slightly increased(considered to be no time dependence due to the small magnitude of the increase),or exhibited an uncertain trend of both increasing and decreasing(varied with repeated contacts).It is worth noting that the adhesion displayed extremely unstable behavior at high humidity levels(70% to 90%),where the average adhesion obtained from increasing and decreasing contact times exhibited significant differences.After exposure to 90% relative humidity,the magnitude of the adhesion became substantial(up to 1 μN),whereas prior to that,the adhesion remained relatively small(30 to200 n N).(2)Generally,the magnitude of adhesion between hydrophobic silicon and hydrophilic graphene consistently decreased with repeated contacts: the maximum adhesion in the initial contacts exceeded 1 μN,while the minimum values in later contacts was less than 5 n N.In the first half of the contacts,as the relative humidity increased step wise from 10% to 90%,the adhesion generally exhibited no time dependence(except at 60% relative humidity).Furthermore,in the initial stages at lower humidity levels(10% to 40%),the adhesion displayed highly unstable behavior,and even sudden jumps occurred.However,in the second half of the contacts when decreasing the relative humidity(80% to 30%),the adhesion showed time dependence.Yet,at low humidity(20% to 10%),the adhesion once again became independent of contact time.(3)Similar to the adhesion between hydrophilic silicon and hydrophobic graphene,the magnitude of adhesion between hydrophobic silicon and hydrophobic graphene also reached its maximum value(up to 1.4 μN)after experiencing contact at 90% relative humidity.Before experiencing contact at 90% relative humidity,the adhesion at this interface showed no time dependence and remained stable at low humidity,but exhibited instability at moderate to high humidity levels.After experiencing contact at 90% relative humidity,the contact time dependence of the adhesion forces is intermittent,and this dependence is more commonly observed at medium to low humidity. |
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