Nanomechanics of Low-Dimensional Materials for Functional Applications

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Nanomechanics of low-dimensional materials for functional applications Cite this: Nanoscale Horiz., 2019, 4,781 Sufeng Fan, †a Xiaobin Feng, †a Ying Han, †a Zhengjie Fanab and Yang Lu *acd

When materials’ characteristic dimensions are reduced to the nanoscale regime, their mechanical properties will vary significantly to that of their bulk counterparts. Recently low-dimensional materials, including one-dimensional (1D) and two-dimensional (2D) nanomaterials, have attracted the widespread attention of academia and industry because of their unique (e.g., thermal, optical, electrical, catalytic) properties. These outstanding properties give them a wide variety of functional applications; however, reliable devices and practical applications call for high structural reliability and mechanical robustness of these nanoscale building blocks. Therefore, there is a need to investigate and characterize the nanomechanical properties and deformation mechanisms of low-dimensional materials but this remains highly challenging. In this Focus article, we summarize the recent progress made in the nanomechanical studies on some representative 1D/2D crystalline nanomaterials, with a special emphasis on experimental research. Furthermore, the unconventional mechanical properties, such as the significantly enhanced elasticity, of these low-dimensional crystals can lead to unprecedented physical and chemical property changes, which may fundamentally change the way such materials conduct electricity/heat, transmit/emit Received 24th February 2019, light, and their involvement in chemical reactions. Therefore, the nanomechanical approach can be also Accepted 11th April 2019 used to tailor the materials’ functional properties and performance, by so-called strain engineering, which DOI: 10.1039/c9nh00118b can open up new avenues to explore how devices can be designed and fabricated with even more dramatic changes in low-dimensional crystalline materials for information processing, communications, rsc.li/nanoscale-horizons biomedical, and energy applications. Published on 26 April 2019. Downloaded 9/27/2021 11:15:40 AM. Introduction nanomechanical and nanoelectromechanical systems (NEMS). However, compared with their fancy functional properties, their With the rapid progress toward miniaturization and scalable mechanical property, which is also an important factor in most integration in the microelectronics industry, the design and practical applications, has drawn relatively less attention in fabrication of small-scale materials with desired mechanical, nanomaterials research. Actually, many existing studies, including electrical, thermal, and optical properties are still great challenges. our recent works,2–4 have clearly shown that miniaturization can Meanwhile, in the past decades, with the rapid development of give entirely different mechanical properties to conventional nanoscience and nanotechnology, low-dimensional materials crystalline materials because of the reduced size and high including one-dimensional (1D) nanomaterials (nanotubes, nano- surface-to-volume, creating great impacts on the design, manu- wires, etc.) and two-dimensional (2D) materials (graphene, layered facturing, and service life of the relevant components, which are

MoS2, etc.), have become promising candidates for functional device vital in the practical application of low-dimensional materials- components.1 As shown in Fig. 1, these emerging nanomaterials based functional devices. The size-dependent mechanical and have become key building blocks for future nanodevices and fracture behavior of crystalline materials at the nanoscale have thus generated great interest in the solid mechanics community

a Department of Mechanical Engineering, City University of Hong Kong, Kowloon, because of their importance to the assembly, performance, and Hong Kong. E-mail: [email protected] reliability of functional nanodevices and NEMS. Apart from b State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong reliable functional device applications, recent studies suggest University, 710049, Xi’an, China that the nanomechanics study of low-dimensional crystalline c Department of Materials Science and Engineering, City University of Hong Kong, materials could also provide a venue to manipulate and precisely Kowloon, Hong Kong d CityU-Xidian Joint Laboratory of Micro/Nano-Manufacturing, Shenzhen 518057, tune the physical properties of the nanomaterials under applied China mechanical loading, which is related to the concept of ‘‘strain † These authors contributed equally to this work. engineering,’’ which is based on the fact that a material’s physical

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device applications are also discussed herein. More importantly, we also introduce some most recent development on the ‘‘elastic strain engineering’’ (ESE) of low-dimensional crystalline materials, which may bring enormous new opportunities in nanomechanics research.

One-dimensional metallic crystalline materials

Metallic nanopillars, nanowires (NWs) and nanorods (NRs), with diameters ranging from a few to hundreds of nanometers, have stimulated great interest recently as important building blocks for future micro/nanoelectronics and electromechanical devices in various industrial applications. Therefore, their mechanical performance plays a key role in their reliability and other functional applications. Among various research studies, the size effect of metallic nanocrystals has received most interest in the nanomechanics community during the past decades. In the mechanical behavior of metallic materials, Fig. 1 Classification and some examples of low-dimensional (1D/2D) the size effect includes both an internal size effect and external crystalline nanomaterials. size effect. Among these, the internal size effect is primarily related to boundaries and is depicted as the classic ‘‘Hall– Petch’’ relationship, with the principle being ‘‘smaller is stronger.’’ and chemical properties are functions of the lattice parameters of However, when grain sizes decrease into the nanocrystalline

the underlying crystal lattice or the elastic strain, ee,withrespect regime, their limited ductility/deformability becomes the Achilles’ to the stress-free reference state. Fundamentally, the electronic heel of these ultrastrong nanocrystals. At the nanoscale, the

structure (band gap) changes with ee.Therefore,manyphysicaland sample size or the external size has an even greater effect and chemical properties can depend on the mechanical strain: strongly influences the mechanical properties, and here, even thermal, magnetic, transport, and electro-optical characteristics, nanolattices have demonstrated superior compressive specific

and the catalytic activities, which vary sensitively with ee.Moreover, strength and recoverability that are quite different from their bulk unlike some other ways of changing a material’s properties, such counterparts.12,13 Crystalline metallic pillars with diameters span- as by chemical doping, which produce a permanent, static change, ning from tens of micrometers to hundreds of nanometers Published on 26 April 2019. Downloaded 9/27/2021 11:15:40 AM. mechanical straining allows the properties to be changed on the demonstrated strong size effects when subjected to uniaxial fly. So, the ‘‘strain engineering’’ of low-dimensional crystalline compression,14,15 and it was found that dislocations would emit materials has become a new trend in the design and fabrication from the free surfaces instead of the interior when the diameter of novel nanoelectronics and optoelectronic devices. decreases to hundreds of nanometers.16 Consequently, unconven- Past study of both the mechanical properties and deformation- tional deformation mechanisms, including dislocation starvation,17 induced physical property changes of low-dimensional crystalline surface dislocation self-multiplication18 and mechanical annealing,19 materials were mostly performed using theoretical and com- may occur. In this case, on the basis of the 1D metallic characteristic, putational methods because of the great challenges involved in some new applications arise from nano-fabrication; for instance, experiments. Over the past two decades, due to the rapid LIGA (a German acronym for lithographie, galvanoformung, development and advancement of nano-fabrication and char- abformung, which means lithography, electroplating, and molding) acterization techniques, such as focused ion beam (FIB), in situ nickel with predominant mechanical performance has been electron microscopy (SEM/TEM), atomic force microscopy, and used in microelectromechanical systems (MEMS) for elevated nanoindentation, researchers can now perform real experiments temperatures.20 Crystalline metallic NWs exhibit remarkably to observe and quantify the intriguing mechanical properties, enhanced strengths, even ultrahigh yield strength that reaches including the strength, elastic strain, and plasticity, of individual their ideal strength, which is attributed to their reduced surface nanostructures, by using methods such as nanoindentation,5 areas, dense nanotwins, uniform dislocation nucleation, and resonance,6 uniaxial loading,2,7 bending tests,4,8,9 and fatigue them being low or even defect free, etc.12,13,21 For characterizing tests.10,11 So this Focus article reviews the state-of-art progress such tiny specimens, advanced in situ nanomechanical measure- in the experimental nanomechanics of 1D metallic and covalent ment instruments have been developed inside SEM or TEM, such crystallinematerialsaswellas2Dcrystals,setagainsttheback- asshowninFig.2(a).22 Metallic crystalline NWs also show ground of their functional applications. Also, the challenges that dramatic elastic and plastic deformation behavior. By performing need to be solved in the near future, opportunities for nano- the in situ tensile test in high-resolution electron microscopy, an mechanics of low-dimensional materials, and their functional exceedingly large elastic strain (B7.2%) was achieved in copper

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Fig. 2 Characterization of ultrathin metallic nanowires and their device applications: (a) the in situ TEM mechanical testing of an ultrathin nanowire.22 (b) ‘‘Cold welding’’ of ultrathin gold nanowires2 and (c) their scalable assembly for flexible electronics. (d) ‘‘Rayleigh instability’’ of ultrathin gold nanowires upon moderate heating,34 and (e) mechanically assisted self-healing for nanodevice recovery.35

(Cu) NWs, approaching its theoretical elastic strain limit.23 and experimental observations have indicated that the deformation Similarly, a remarkable elongation (269%) of niobium (Nb) NWs mechanisms of ultrathin NWs can be radically diﬀerent to their was observed experimentally in tensile test.24 These excellent larger counterparts. For example, a transition from ductile to brittle properties make flexible electric devices and sensors possible. fractures was observed for ultrathin gold NWs,29 which would Published on 26 April 2019. Downloaded 9/27/2021 11:15:40 AM. For example, scalable electrodes based on highly conductive greatly influence the reliability of devices. Apart from the Ag-NW networks on plastic substrates demonstrated good trans- deformation mechanisms, some unique behaviors are also very parency and superior conductivity, comparable to the ITO and important for the usage of ultrathin NWs. The phenomenon the carbon nanotube (CNT) electrode;25 also, crack-induced Ag called ‘‘cold welding’’ is found in ultrathin gold NWs with NW networks was used to fabricate a highly stretchable and diameters ranging from 3 to 10 nm, as shown in Fig. 2(b), and transparent strain sensor, which was encapsulated by a stretch- this process does not need heating or high load, which gives it able and transparent polymer.26 Meanwhile, the Cu-NW-based potential applications in the future bottom-up assembly of transparent conductor endured an extremely large strain exceeding metallic 1D nanostructures and next-generation interconnects 250% without substantial cracks.27 In addition to the functional for extremely dense logic circuits. Moreover, the cold welding of applications, 1D metallic nanomaterials are also widely exploited ultrathin metallic nanowires also allows them to be used to in nanocomposites because of their high strength, and large elastic fabricate flexible transparent films, such as gold NW networks and plastic strain. For example, a NiTi–Nb nanocomposite for use in flexible electronics and biointegrated electronics, as presented exceptional mechanical properties that could over- showninFig.2(c).30–33 Another interesting phenomenon is called come the intrinsic trade-off relationships among the elastic the ‘‘Rayleigh instability,’’ which indicates that the morphologies of strain, Young’s modulus, and strength, which indicated it could ultrathin NWs can be dramatically changed when subjected to be regarded as a promising shape memory composite for used moderate heating, as shown in Fig. 2(d), which may impede their in aerospace and biomedical areas.28 applications as interconnects.34 However, upon random mechanical As the critical feature sizes of modern electronic devices perturbation of the NWs, the phenomenon of Rayleigh instability continue to shrink, ultrathin metallic nanowires, in which and other damages can be healed in situ (Fig. 2(e)). Therefore, as the diameters approach the sub-10 nm regime, have emerged as size of the critical components in integrated circuits approach a new kind of 1D metallic crystals. Due to their exceedingly sub-10 nm, even the inner structures and surface morphologies of small sizes, the reliability of ultrathin NWs in the functional ultrathin NWs are more prone to be damaged in practical services. devices has become a great issue, as both theoretical predictions However, with a mechanically assisted self-healing mechanism,

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people can directly repair defective ultrathin metallic NWs inside micro/nanosized covalent crystals can withstand extremely functional nanowire-based devices.35 large deformation without mechanical inelastic relaxation or Lastly, for many flexible electronics and mechatronics appli- failure compared with their conventional counterparts. This is cations that involve cyclic loading, the repetitive straining because of the nature of covalent bonding, and as fewer defects conditions during long-term service require a comprehensive exist inside the nanoscale sample so that the inelastic deformation understanding of their fatigue behavior. Therefore, recently can be suppressed greatly under confining loading conditions. Si researchers have started to pay attention to the fatigue testing NW is one of the most important 1D covalent crystalline materials of NWs. For example, a novel high-cycle bending and torsion because of its availability and attractive physical properties, which straining micromachine, based on a digital micromirror device are vital for applications in nanoelectronics and nanoelectro- (DMD), has been developed for the bending or torsional fatigue mechanicalsystemdevicesaswellasforotherfunctionalapplica- study of various 1D nanostructures.36 The millions of independent tions. Recent in situ TEM tensile research has shown that VLS-growth movable micromirrors on a single DMD chip make the plat- single crystalline Si NWs with diameters of B100 nm exhibit 410% form particularly suitable for high-throughput fatigue testing.37 elastic strain at room temperature, which is getting close to the For low-cycle fatigue study, our patented micro-mechanical theoretical elastic limit.3 Fig. 3a shows the basic configuration of the device (MMD) actuated by a quantitative nanoindenter within tensile testing, and the result shows that the Si NW can fully recover a high-resolution SEM could provide critical insights into the its original length after strain values were applied of up to 13% deformation and failure mechanisms of metallic nanowires (Fig. 3b), showing its value for potential applications such as under cyclic straining.11 epidermal electronics, flexible electronics, and recent bio-nano interfaces,38–41 which involve large deformation. Another important covalent crystalline material is diamond, the hardest natural One-dimensional covalent crystalline and extreme material with high carrier mobility, in where nega- materials tive or positive carriers of electric current can move freely through it and which has a multitude of applications in high-frequency Covalent crystals refer to a class of crystalline solids in which electronic devices and power electronics. However, the disadvan- the atoms are bonded by covalent bonds in a continuous network tages of its poor deformability and high brittleness seriously extending throughout the entire material. Some of them are well- affect its functional applications. Recently, research has shown known semiconductor materials, such as pure crystals of silicon that single crystalline diamond nanoneedles can undergo elastic (Si), germanium (Ge), while many others are in fact seen as more bending deformation approaching the theoretical elastic strain attractive in recent years as ‘‘wide-bandgap semiconductors’’, limit of diamond (9%).4 AsshowninFig.3(candd),bydeveloping such as cubic boron nitride (c-BN) and silicon carbide (SiC), as a unique nanoindenter-based push-to-bend testing method, we were well as diamond. Because of their strong and directional able to demonstrate that a diamond nanoneedle can fully recover covalent bonding nature, they are usually very hard and brittle after ultralarge bending deformation without fracture, suggest- at the macroscopic scale, but recent studies have suggested that ing entirely new applications for nanodiamonds. Published on 26 April 2019. Downloaded 9/27/2021 11:15:40 AM.

Fig. 3 Ultralarge elasticity in nanoscale covalent crystals: (a) the in situ SEM tensile testing of a single nanowire based on a push-to-pull micromechanical device (MMD).3 (b) Loading–unloading cyclic test of a VLS-grown silicon nanowire showing near ideal elastic limit. (c) Fully recoverable bending deformation of a diamond nanoneedle by a (d) nanoindenter-induced bending test.4

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Not limited to elemental covalent crystal 1D nanomaterials, have been done to investigate the elastic and failure properties compound covalent nanostructures have also been found to of ZnO NWs,48 while the Young’s modulus and the bending have a much larger deformation ability as compared with their strength of CuO and ZnO NWs were measured with an in situ bulk counterparts. For example, elastic strains of B2%42 and SEM technique.49 4.5%43 were respectively found in SiC NWs through in situ tensile tests, which are close to the theoretical value (5%) obtained by first principle calculations.43 The elastic behavior Two-dimensional crystalline materials of diﬀerent types of single crystal GaAs NWs, another important third-generation semiconductor material, has been investigated Since the discovery of graphene, a single layer of carbon atoms by in situ TEM compression tests,44 showing fracture strains as arranged in a hexagonal lattice,50 more and more other 2D high as 10%. Overall, it has been proved that at the nanoscale, layered materials have been discovered and fabricated in the covalent crystal nanostructures’ deformability is much better past decades,51 including black phosphorous or phosphorene, than their bulk counterparts, in particular their elasticity, due to hexagonal boron-nitride (h-BN), transition metal-dichalcogenides

the difficulty to enter into plastic deformation as in the cases of (TMDs), metal oxides, graphitic carbon nitride (g-C3N4), and metallic nanowires. Combined with their unique band struc- MXenes.52 2D metal nanomaterials have also received extensive tures, these nanomechanical behaviors provide the foundation research interest because of their anisotropic structure, and to experimentally achieve ‘‘strain engineering,’’ or ESE, which promising applications in catalysis, bioimaging, sensing, and demonstrates the potential to significantly accelerate the engineering solar cells.53 MXene is a new member in the class of 2D of exotic electronic properties in ordinary materials via large elastic/ materials, and are promising candidates in the fields of sensors, lattice strains: by applying just a bit of strain to a piece of covalent catalysts, and batteries due to their metallically conductive and crystalline material, researchers can deform the orderly arrangement hydrophilia natures.54 Because of the unusual and excellent of atoms in its structure enough to cause dramatic changes in its physical, chemical properties of 2D materials, such as high properties, such as the way it conducts electricity, transmits light, or electron mobility, high strength, high transmittance, and high conducts heat. In fact, the potential commercial value of strain thermal conductivity,55,56 they can be the ideal candidates for engineering has been already demonstrated by the billion-dollar the fabrication of next-generation thinner and faster electronic strain-Si semiconductor industry, inwhicheven1%straininsilicon and optoelectronic components, such as transistors, transparent processor chips can in some cases improve the speed of the device touch screens, light panels, and solar cells.57,58 Either being by 50%, by allowing electrons to move through the material structural components or functional devices, mechanical loading much faster. Some recent research studies have been done and and deformations, like bending, distortion, vibration, and thermal demonstrated the potential for ESE to dramatically improve the expansion, are often involved in their real device applications during device performance; for example, Cui et al. discovered that the service. Therefore, it is crucial to obtain a clear understanding of the carrier mobility change could result in a giant piezoresistance mechanical properties and deformation behavior of 2D crystalline effect in p-type Si nanowires during stretching.38 Another example materials for the reliability of different functional applications. Published on 26 April 2019. Downloaded 9/27/2021 11:15:40 AM. is that a recent calculation suggested that the bandgap structure of However, the current understanding of their mechanical property germanium (Ge) could be fundamentally changed by a large tensile and physic-chemical performance under loading is limited since strain, changing from an indirect bandgap to a direct bandgap.45 experimental studies on 2D materials still face challenges, i.e., Thus, understanding the nanomechanical properties of 1D material preparation and transfer, need for suitable clamping covalent crystalline solids is not only important for the reliability device, in situ characterization, etc., and this is also the reason and properties of different kinds of micro/nanomechanical for the fact that there are some computational research studies devices and MEMS, but also brings about new possibilities and published but few experiments. solutions for creating materials tuned precisely for electronic, Despite the experimental mechanical study of 2D materials optoelectronic, and photonic devices that could find uses for remaining extremely challenging, exciting progress has been communications, information processing, and energy applica- made in the past decades. Different methods to explore the tions based on ESE. These studies also provide theoretical and mechanical properties of these materials have been developed, experimental support, so that novel functional properties based such as atomic force microscopy (AFM)-based nanoindentation on exotic properties through extremely strained covalent crystal- tests, pressurized blister tests or bulge tests, and micromechanical line materials for highly tunable electronic and optoelectronic testing device-based tests. Lee et al. were the first to report research devices can be developed in the near future. 1D covalent crystals, into the measurement of the elastic properties of an intrinsic such as diamond and SiC, could even make a difference in the fracture strength of free-standing monolayer graphene with the emerging ‘‘quantum computing’’ field, because of the diamond AFM nanoindentation technique, as shown in Fig. 4a.59 According nitrogen-vacancy center and silicon-vacancy related defects in to their report, the Young’s modulus and intrinsic strength of the hexagonal polytypes as leading qubit candidates.46,47 graphene were 1 TPa and 130 GPa, respectively. With the same Similarly, ionic crystals, due to their bonding nature, share a method, the elastic stiffness, intrinsic strength, and mechanical lot of similar mechanical properties, such as high brittleness, behavior of polycrystalline graphene films grown by chemical but are less strong. Some studies have already been done on vapor deposition (CVD) were studied.60 The same experiments ionic nanomaterials, for example, tension and buckling tests by AFM nanoindentation were also conducted to explore the

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Fig. 4 Nanomechanics of 2D materials: (a) measuring the intrinsic mechanical properties of suspended graphene by AFM indentation.59 (b) Measuring the interlayer shear stress of bilayer graphene based on pressurized microscale bubble loading.63 (c) Tensile testing of graphene films on PDMS substrate; (d) MMD-based fracture 65 67 testing of graphene with pre-crack. (e) Our in situ TEM fracture testing of multilayer graphene films, and (f) tensile testing of a free-standing MoS2 nanosheet.

mechanical properties and strain engineering of other 2D theory (DFT) calculations in zigzag BN nanoribbons upon elastic 61 62 materials, such as MoS2 and h-BN, and the results showed strain, their bandgap could be significantly tuned, resulting from 70 the local max strain of both could be very high (0.25 for MoS2 a reduced orbital hybridization; Andres et al. recently controlled

and 0.22 for h-BN). Despite the success of the AFM-based the band structure of MoS2 through local-strain engineering for 69 nanoindentation test, pressurized blister tests or bulge tests tailoring the optoelectronic properties of MoS2; Levy et al. were also developed for testing such ultrathin film layers. The explored the strain-induced pseudo-magnetic field in strained Published on 26 April 2019. Downloaded 9/27/2021 11:15:40 AM. interlayer shear stress of bilayer graphene was first measured graphene nanobubbles.68 However, this emerging field needs to based on pressurized microscale bubble loading devices, as be further explored with joint eﬀorts from mechanical engineers, shown in Fig. 4b.63 The biaxial straining of a suspended materials scientists, and physicists. A recent example is how twisted 64 71 monolayer MoS2 was conducted by Lloyd et al. Some indirect bilayer graphene demonstrated a tunable bandgap structure and methods have also been developed to test the tensile property of superconductivity,71 suggesting that there is still big room to explore 2D crystalline materials, such as by depositing them on the unprecedent functional applications based on the nanomechanical surface of flexible PDMS thin film (Fig. 4c) and then testing manipulation of existing 2D materials. them by conducting conventional tensile tests. Recently, Zhang et al. used a microfabricated micromechanical device (MMD), as shown in Fig. 4d, to measure the fracture toughness of CVD- Conclusions synthesized graphene with a pre-crack65 in a general agreement with Griﬃth theory.66 A brittle fracture was observed after Low-dimensional crystalline nanomaterials, including 1D metallic applying a load with a pre-crack introduced by FIB, and the fracture nanomaterials, covalent nanocrystals, and 2D materials, are morphology was shown. By using a similar micromechanical device, promising functional materials and consequently have attracted

the tensile fracture behaviors of multi-layer graphene and MoS2 the attention of numerous researchers, and their studies are membranes were also studied by Li et al., with Fig. 4e and f showing raising new possibilities for the development of low-dimensional the morphologies of the samples upon fracture.67 materials-based functional devices, including flexible electronics/ Similar to covalent crystals, the elastic straining of 2D energy devices nanoelectronics, optoelectronics, and nanoelectro- crystals can also be a practical and facile strategy for adjusting mechanical system (NEMS) devices. Despite this, even though this the bandgap and for engineering the electronic properties. Thus paper could only review some of the recent experimental research the so-called strain engineering or ESE of 2D materials has studies concerning a few representative low-dimensional crystalline attracted more and more research eﬀorts in recent years.68,69 nanomaterials, we found that they can have a huge impact in For example, by a performing first-principle density functional associated functional applications, which will provide critical

Nanoscale Horizons Focus

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