原子级制造的关键基础科学问题

人类的制造技术逐步向原子级推进,原子作为化学反应中的最小粒子,虽然它可以分成更小的原子核、电子等,但是从制造的角度看,原子级制造可以说是人类制造的最底层技术,也是继微纳制造之后新的制造范式,可将制造精度以及产品性能推向极致水平,代表着人类对物质世界认知和制造能力发展的新阶段,是引领未来产业变革发展的战略性技术,也是保障国家安全和推动国计民生重大装备跨代升级的重要前沿方向。本文基于第330期双清论坛总结了原子级制造的研究现状、发展趋势及机遇挑战,凝练出未来5~10年原子级制造的焦点问题和亟需解决的关键科学问题,探讨了相关领域的前沿发展方向和科学基金资助战略。

基于原子/分子团簇结构的材料与器件制造

原子/分子团簇是物质结构的一种新形态,具有独特的本征性质。从原子/分子团簇到器件的跨尺度制造,将为国防高端装备和新兴电子等产业发展带来深刻变革。团簇的多物质构效关系、宏量制造、团簇结构跨尺度构筑以及团簇器件的高性能制造等是原子/分子团簇器件制造的关键发展方向,主导着从原子到产品制造的发展历程。把握这些发展背后的重要机遇,将有助于占领原子级制造研究的制高点,引领原子级制造方法的变革。本文从团簇新材料的宏量制造、新型功能器件的原子/分子团簇构筑、团簇—器件的跨尺度制造工艺和装备等三个方面概括了原子/分子团簇与器件制造领域的主要研究进展,总结了原子/分子团簇与器件领域的关键科学问题及面临的挑战,并对其未来发展方向和发展战略给出了建议。

Comparative analysis of frictional behavior and mechanism of molybdenum ditelluride with different structures

Two-dimensional(2D)transition metal dichalcogenides(TMDCs)have layered structures with excellent tribological properties.Since the energy difference between hexagonal-molybdenum ditelluride(2H-MoTe_(2))and distorted octahedral-molybdenum ditelluride(1T′-MoTe_(2))is very small among the transition metal dichalcogenides(TMDCs),MoTe_(2) becomes one of the most promising candidates for phase engineering.In our experiment,we found that the friction force and friction coefficient(COF)of 2H-MoTe_(2) were an order of magnitude smaller than those of 1T′-MoTe_(2) by the atomic force microscope(AFM)experiments.The friction difference between 1T′-MoTe_(2) and 2H-MoTe_(2) was further verified in molecular dynamics(MD)simulations.The density functional theory(DFT)calculations suggest that the friction contrast is related to the difference in sliding energy barrier of the potential energy surface(PES)for a tip sliding across the surface.The PES obtained from the DFT calculation indicates that the maximum energy barrier and the minimum energy path(MEP)energy barrier of 2H-MoTe_(2) are both smaller than those of 1T′-MoTe_(2),which means that less energy needs to be dissipated during the sliding process.The difference in energy barrier of the PES could be ascribed to its larger interlayer spacing and weaker Mo–Te interatomic interactions within the layers of 2H-MoTe_(2) than those of 1T′-MoTe_(2).The obvious friction difference between 1T′-MoTe_(2) and 2H-MoTe_(2) not only provides a new non-destructive means to detect the phase transition by the AFM,but also provides a possibility to tune friction by controlling the phase transition,which has the potential to be applied in extreme environments such as space lubrication.

Active control of friction realized by vibrational excitation:Numerical simulation based on the Prandtl-Tomlinson model and molecular dynamics

Superlubricity and active friction control have been extensively researched in order to reduce the consumption of fossil energy,the failure of moving parts,and the waste of materials.The vibration-induced superlubricity(VIS)presents a promising solution for friction reduction since it does not require high-standard environment.However,the mechanism underlying the VIS remains unclear since the atomic-scale information in a buried interface is unavailable to experimental methods.In this paper,the mechanism of VIS was examined via numerical calculation based on the Prandtl-Tomlinson(PT)model and molecular dynamics(MD)simulations.The results revealed that the pushing effect of stick-slip is one of the direct sources of friction reduction ability under vibrational excitation,which was affected by the response amplitude,frequency,and the trace of the tip.Moreover,the proportion of this pushing effect could be modulated by changing the phase difference when applying coupled vibrational excitation in x-and z-axis.This results in a significant change in friction reduction ability with phase.By this way,active friction control from the stick-slip to superlubricity can be achieved conveniently.

Interlayer friction behavior of molybdenum ditelluride with different structures

The interlayer friction behavior of two-dimensional transition metal dichalcogenides(TMDCs)as crucial solid lubricants has attracted extensive attention in the field of tribology.In this study,the interlayer friction is measured by laterally pushing the MoTe_(2)powder on the MoTe_(2)substrate with the atomic force microscope(AFM)tip,and density functional theory(DFT)simulations are used to rationalize the experimental results.The experimental results indicate that the friction coefficient of the 1T'-MoTe_(2)/1T'-MoTe_(2)interface is 2.025×10^(−4),which is lower than that of the 2H-MoTe_(2)/2H-MoTe_(2)interface(3.086×10^(−4)),while the friction coefficient of the 1T'-MoTe_(2)/2H-MoTe_(2)interface is the lowest at 6.875×10^(−5).The lower interfacial friction of 1T'-MoTe_(2)/1T'-MoTe_(2)compared to 2H-MoTe_(2)/2H-MoTe_(2)interface can be explained by considering the relative magnitudes of the ideal average shear strengths and maximum shear strengths based on the interlayer potential energy.Additionally,the smallest interlayer friction observed at the 1T'-MoTe_(2)/2H-MoTe_(2)heterojunction is attributed to the weak interlayer electrostatic interaction and reduction in potential energy corrugation caused by the incommensurate contact.This work suggests that MoTe_(2)has comparable interlayer friction properties to MoS_(2)and is expected to reduce interlayer friction in the future by inducing the 2H-1T'phase transition.

Dynamic friction energy dissipation and enhanced contrast in high frequency bimodal atomic force microscopy

Dynamic friction occurs not only between two contact objects sliding against each other,but also between two relative sliding surfaces several nanometres apart.Many emerging micro-and nano-mechanical systems that promise new applications in sensors or information technology may suffer or benefit from noncontact friction.Herein we demonstrate the distance-dependent friction energy dissipation between the tip and the heterogeneous polymers by the bimodal atomic force microscopy(AFM)method driving the second order flexural and the first order torsional vibration simultaneously.The pull-in problem caused by the attractive force is avoided,and the friction dissipation can be imaged near the surface.The friction dissipation coefficient concept is proposed and three different contact states are determined from phase and energy dissipation curves.Image contrast is enhanced in the intermediate setpoint region.The work offers an effective method for directly detecting the friction dissipation and high resolution images,which overcomes the disadvantages of existing methods such as contact mode AFM or other contact friction and wear measuring instruments.