Session Z72: Structural and Phononic Properties of 2D Materials

Measuring strain fields in moiré superlattices

Atomically thin or two-dimensional (2D) materials can be assembled into bespoke heterostructures to produce some extraordinary physical phenomena. One exciting and relatively recent example is the formation of moiré superlattices from azimuthally misoriented (twisted) layers. These moiré superlattices result in flat bands that lead to an array of correlated electronic phases. However, in these systems, complex strain relaxation can also strongly influence the fragile electronic states of the material. Precise characterization of these materials and their properties is therefore critical to the field's understanding of the behavior of these novel moiré materials (and 2D heterostructures in general).This talk will discuss how spontaneous mechanical deformations (atomic reconstruction) and resultant intralayer strain fields at twisted bilayer graphene and twisted transition metal dichalcogenides have been quantitatively imaged using Bragg interferometry, based on four-dimensional scanning transmission electron microscopy, and the impact of these mechanical deformations and strain fields to the electronic properties of these moiré superlattices.   

Simultaneous cryogenic nanomanipulation and measurement of van der Waals moires

Van der Waals materials can be combined into layered heterostructures exhibiting new properties which can be tuned by changing the relative alignment between individual layers. Furthermore, their atomically flat surfaces and weak out-of-plane bonding result in low interface friction, enabling heterostructures to be modified at room temperature by sliding layers relative to each other. So far such interlayer motion has not been reported at liquid helium temperatures, which are necessary to observe fragile quantum phenomena. Cryogenic interlayer sliding would enable dynamic studies by allowing simultaneous manipulation and measurement of heterostructures. It would also facilitate rapid, position-sensitive experiments, such as insitu twist angle studies. To this end, we have built a cryogenic nano-manipulation apparatus to slide our mechanically reconfigurable van der Waals heterostructures while at cryogenic temperatures. In this talk, I will discuss our progress towards developing this technique and utilizing it to achieve insitu van der Waals sliding studies.   

Characterizing Networks of Graphene Nanomechanical Resonators

Programmable nanomechanical networks have a wide range of potential applications, from phononic circuits to quantum simulation. To realize these networks in the lab, it is essential to characterize the network's resonator nodes and connectivity. While spectroscopy is commonly used to characterize small assemblies—like a pair of coupled resonators—it does not provide spatially resolved, node-specific information and readily fails to detect weak coupling. To overcome this challenge, we use scanning interference microscopy (SIM) to spatially image the amplitude and phase of hybridized vibrational modes in an array of suspended graphene nanomechanical resonators. We use these SIM measurements to algebraically characterize the network's complete mechanical parameters including node elasticity, mass, and damping in addition to nearest-neighbor coupling constants. In combination with existing methods to tune resonators, our diagnostic tool and resonator array form a viable programmable nanomechanical network platform and may be used to pattern acoustic waveguides and lattices or to realize dynamic phononic metamaterials.   

Mechanisms of sliding in bulk and bilayer transition-metal-doped MoS2

MoS₂ is a useful solid lubricant owing to the ease of shearing along the weakly-bonded basal planes, and dopants such as Ni improve its lubrication properties. The atomistic mechanisms of sliding have been studied in pristine MoS₂, but not in doped MoS₂. We use density functional theory to study the structure and energetics of Ni-doped MoS₂ during sliding, in both bulk and bilayer form. We consider the four reasonable dopant sites of our previous work (J. Phys. Chem. C 125, 13401-13412 (2021)): Mo/S substitutions or octahedral/tetrahedral intercalations. The shape of the sliding potential is similar to that of undoped MoS₂, but we find overall increases in the sliding barrier, particular in intercalated MoS₂ which has interlayer covalent bonds. The energetics in bulk can be well described by pairwise interactions between layers from our bilayer calculations. We study bond breaking, symmetry breaking, and structural changes during sliding, and compute forces as a function of out-of-plane load. The increased interlayer interactions in the presence of a dopant lead to shearing preferentially between undoped layers. Our results provide an atomistic view of how sliding occurs in a doped transition-metal dichalogenide.   

Controlling Electronic Phase of MoTe2 by Piezoelectric Strain Engineering

A new type of strain-based phase change transistor has been introduced in our previous work [1].  Transition metal dichalcogenide MoTe₂ can be reversibly switched between the 1T’ semimetallic phase and a semiconducting phase under application of gate controllable ferroelastic strain, achieving large non-volatile control of the channel conductivity. The phase change was achieved by strain engineering of MoTe₂ through the combination of a higher static strain set by a thin film stressor with a smaller electric-field controllable strain from the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (PMN-PT). Here, we show that the thin film stressor can also result in large strain gradients near the surface of the ferroelectric, causing a flexoelectric field comparable with the coercive field of PMN-PT. By controlling the film force of the thin film stressor, we can continuously tune the internal bias to control ferroelastic strain applied by the ferroelectric vs. applied electric field, thereby achieving control of ferroelastic non-volatility. Moreover, by combining thin film strain engineering and PMN-PT with different intrinsic internal biases, we show MoTe₂ phase change devices with the same engineered non-volatility.   

Panoply of doping-induced reconstructions and electronic phases in Ni-doped 1T-MoS2

Transition metal doping of monolayer MoS₂ can tune its structure and optoelectronic properties. We use density functional theory to investigate the effect of Ni-doping in metastable metallic 1T-MoS₂, considering adatom and substitutional sites, and find an array of distorted phases induced by Ni-doping [arXiv:2107.07541]. Doping is thermodynamically favored compared to the undistorted 1T. Depending on concentration and site, Ni-doping induces reconstructions to 2×2, √3×√3 (in two distinct phases), 3×3, and 4×4. The doped phases become semiconducting in most cases, and a few are also magnetic. These phases are metastable after removal of the dopant, offering a potential route to the experimental synthesis of pristine distorted phases.  These distorted pristine phases show spontaneous polarization, so they are potential ferroelectric materials. We find that the pristine phases have distinct semiconducting electronic structures, including several higher gaps in the conduction bands, which could be interesting for transparent conductors and intermediate band solar cells. Our calculations show that Ni-doping of 1T offers a systematic route to different distorted phases of 1T-MoS₂, both doped and pristine, with a variety of electronic properties.   

Towards Dynamic Trapping of Excitations in van der Waals Heterostructures

Surface acoustic waves (SAWs) have been previously used to probe the electronic properties of quantum systems. Recently, there has been significant interest in the application of SAWs to manipulate and control quantum excitations, particularly in two-dimensional materials and strongly correlated systems. Here, we demonstrate the use of SAWs to control and trap charge carriers within a van der Waals heterostructure device. Our findings show that SAW systems are an excellent platform for studying the dynamics of trapped excitations, and are well suited to use with two dimensional material systems.   

Suspended PVDF/Quasi-1-D TiS3 (001) Interfaces for Applications in Nanoelectromechanical Systems: Accelerometer Prototype

Quasi-1-D materials have been garnering increased attention due to a multitude of interesting properties ranging from chiral photocurrents, to charge density waves, and anisotropic band structures. These exciting properties make quasi-1-D materials potentially good candidates in microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS).  The current Si based MEMS/NEMS suffer from large device area size. One way to reduce the area is to use suspended 2-D or quasi-1-D materials with an attached proof mass which can also enhance the sensitivity and performance. In this presentation the characterization and fabrication of a prototype accelerometer based on a suspended quasi-1-D TiS₃ (001) nanowhisker coated with a PVDF ferroelectric micro bead are discussed. Single crystal TiS₃ (001) nanowhiskers are grown via direct transport reactions in an evacuated quartz ampule. The resulting nanowhiskers are coated in a PVDF solution by a dip-pull method which forms a PVDF micro bead which acts as the proof mass. Then the device is suspended from silver electrodes. Current versus voltage (I-V) measurements of devices under acceleration suggest that the ferroelectric PVDF micro bead enhances the electrical response compared to a proof mass made from conductive silver paste.   

Molecular arrangement of Cu-Pthalocyanine on a graphene/h-BN heterostructure

Metal phthalocyanine (M-Pc) molecules have a cross-like shape and host a metal atom at their center. These versatile molecules are compatible with almost all metal elements. When combined with graphene, interesting properties can emerge, adding functionalities derived from electronic correlations, spin-orbit coupling or magnetism from the central atom, as demonstrated recently for FePc [1]. Here, we present electronic transport measurements as well as transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterization of a copper phthalocyanine/graphene/hexagonal boron nitride (CuPc/graphene/h-BN) heterostructure. Preliminary electronic transport data indicate that the molecular arrangement of CuPc on graphene has a profound effect on the differential conductance characteristics of the h-BN/Gr/CuPc.

[1]Chin. Phys. Lett, 38, 087201 (2021)   

Silicene Straintronics

The tremendous success of graphene has laid the foundation for exploring various properties of graphene-like 2-D materials, commonly referred to as Xenes.  Silicene (single-layer silicon) is a front runner amongst them due to its compatibility with the current silicon fabrication technology. Recent works on silicene have unveiled its exceptional electronic and mechanical properties. The rapid miniaturization of silicon devices, along with the useful electro-mechanical properties of silicene, have paved the way for exploration of straintronics in silicene [1] in nano electro-mechanical systems (NEMS). In this work, we develop a model to investigate straintronics in nanoscale silicene using density functional theory and quantum transport theory.  We demonstrate that the directional piezoresistance of silicene is very small and is sinusoidally dependent on transport angle like graphene [2]. Based on the obtained results, we propose application of silicene as a strain sensor and as an interconnect in flexible electronics. The model developed herein can be used for similar applications in other Xenes.

References:            

[1] Zhao, H.  Strain and chirality effects on the mechanical and electronic properties of silicene and silicane under uniaxial tension. Physics Letters A, 376(46), 3546-3550, (2012).

[2] Sinha, A., Sharma, A., Priyadarshi, P., Tulapurkar, A., & Muralidharan, B., Graphene as a nanoelectromechanical reference piezoresistor. Physical Review Research, 2(4), 043041, (2020).

    

Relative Stability of Bernal and Rhombohedral Stackings in Trilayer Graphene under Distortions

Stackings in graphene have a pivotal role in properties to be discussed in the future, as seen in the recently found superconductivity of twisted bilayer graphene[1]. Beyond bilayer graphene, the stacking order of multilayer graphene can be rhombohedral, which shows flat bands near the Fermi level that are associated with interesting phenomena, such as tunable conducting surface states[2] expected to exhibit spontaneous quantum Hall effect[3], surface superconductivity[4], and even topological order[5]. However, the difficulty in exploring rhombohedral graphenes is that in experiments, the alternative, hexagonal stacking is the most commonly found geometry and has been considered the most stable configuration for many years. Here we reexamine this stability issue in line with current ongoing studies in various laboratories. We conducted a detailed investigation of the relative stability of trilayer graphene stackings and showed how delicate this subject is. These few-layer graphenes appear to have two basic stackings with similar energies. The rhombohedral and Bernal stackings are selected using not only compressions but anisotropic in-plane distortions. Furthermore, switching between stable stackings is more clearly induced by deformations such as shear and breaking of the symmetries between graphene sublattices, which can be accessed during selective synthesis approaches. We seek a guide on how to better control – by preserving and changing - the stackings in multilayer graphene samples [6].

[1] Y. Cao,  et al. Nature 2018, 556, 43.

[2] Y. Shi,  et al. Nature 2020, 584, 210–214.

[3] F. Zhang,  et al. Phys. Rev. Lett. 2011, 106, 156801.

[4] N. Kopnin,  et al. Phys. Rev. B 2013, 87, 140503.

[5] S. Slizovskiy, et al. Comm. Phys. 2019, 2, 1-10.

[6] F.R. Geisenhof, et al. ACS Appl. Nano. Mater. 2019, 2, 6067–6075.

[7] R. Guerrero-Avilés, M. Pelc, F. Geisenhof, T. Weitz, A. Ayuela. 2021. https://arxiv.org/abs/2110.06590   

Ultra high pressure sensing using graphene

Thin-film MEMS sensors are extremely useful for pressure sensing due to their high sensitivity, high-pressure range, and compact size. The sensitivity of MEMS pressure sensors strongly depends on the membrane thickness [1]. Atomically thin 2-D materials are the foremost contenders that are expected to replace conventional membranes in the near future. Graphene is one of the prime contender amongst them due to its high sensitivity, high elastic limit, high adhesivity, and impermeability to gases [1,2].

At present, the size of pressure sensors has approached the sub-micron scale, which motivates us, in this work, to explore the pressure-sensing characteristics of graphene in nano-electro-mechanical systems (NEMS). This work uses the thin-film approximation to calculate graphene's critical pressure on various substrates along with the membrane theory to calculate its deflection and pressure sensitivity.  This work shows that the graphene’s pressure sensitivity depends only on its radius, while the pressure range depends on its radius and adhesivity.  From the results, we infer that graphene can be used to design highly sensitive NEMS pressure sensors with an ultra-high-pressure range (~10⁹ Pa) in the nanoscale regime.

 

References: 

 

 1. Smith, A. D., et al., Nano letters 13.7 (2013): 3237-3242.

 

2. Koenig, S. P., et al., Nature Nanotechnology 6.9 (2011): 543-546.   

Study of homogeneous and heterogeneous junctions of 2D MoS2 on flexible substrates for improved functionalities in piezotronics

Strain gated electronics have gained attention in research for the development of interactive electronic systems. Combined electronic and sensor capabilities for interactive electronics can be achieved using semiconducting materials that are piezoelectric, also called piezotronic materials. Two-dimensional (2D) materials are apt for piezotronic devices because of their tunable electronic properties, and many of them are found to exhibit piezoelectricity.  The piezo-potentials generated from a single material have limited functionalities. The junctions of 2D materials can tap the be piezo-potentials effectively and can suffice the switching behavior. A homogenous junction can be formed between a monolayer and a multilayer of the same material, whereas the heterojunction with two different 2D materials. Here, we present the comparison of homogeneous and heterogeneous junctions of 2D MoS₂ for piezotronics in terms of process complexity and efficiency. Devices are fabricated on flexible substrates to study the strain-dependent electrical and optical characteristics. We have also proposed detailed methods to fabricate and test these devices on flexible substrates reliably. These devices have shown improved performance and can help realize efficient strain-gated electronics.