Weak vdW Interactions Research Articles

(Invited) Deformable Electronics Based on Strain-Engineered Van Der Waals Materials

Materials exhibit drastically different physical and chemical properties as their dimensionality varies. Atomically-thin van der Waals (vdW) materials possess unique mechanical, electrical, optical, and thermal characteristics, originated from quantum confined low-dimensionality and a lack of dangling bond. In particular, strong in-plane covalent bonding and weak interlayer vdW interaction of two-dimensional (2D) vdW materials enable exceptional combination of mechanical properties, such as high Young’s modulus, high in-plane strength, and ultralow bending stiffness at a level of cell membrane. Moreover, mechanical strain can induce modulation of electronic and phononic band structures of 2D vdW materials more effectively due to mechanical resilience and high surface-to-volume ratio. For these reasons, deformation engineering has garnered substantial interest in mechanics, materials and electronics communities as a new tuning knob for emerging phenomena and functionalities of 2D vdW materials. In this talk, I will discuss deformation engineering of 2D vdW materials – (1) active and dynamic deformation strategies, including in-plane interfacial slippage and out-of-plane structural deformation, (2) deformation strain induced phenomena, and (3) emerging device concepts based on deformation engineering of 2D vdW materials.

Unveiling the electronic properties of BiP[formula omitted] triphosphide from bulk to graphene-based heterostructures by first-principles calculations

In our study, we investigated the structural and electronic properties of bismuth triphosphide (BiP3) in its bulk, few-layer, and monolayer forms. We found that BiP3 in bulk exhibits a metallic stable layered structure. The exfoliation energy of 1.07 J/m2 indicates ease exfoliation, comparable to other triphosphides. The band gap varies with thickness, transitioning from semiconductor–metal between four to five layers, influenced by interlayer coupling and quantum confinement. We also investigated the heterostructure created by depositing graphene (G) on few-layer BiP3. In monolayer (G/m-BiP3) and bilayer (G/2L-BiP3) forms, a metal–semiconductor junction is formed, characterized by weak vdW interactions at the interface and exhibiting p-type Schottky contacts. We observed that the Schottky Barrier Height (SBH) can be modulated by altering the interlayer distance between G and BiP3. This adjustment allows transitions between n-type and p-type Schottky contacts in G/m-BiP3 and the formation of an ohmic contact in G/2L-BiP3. Furthermore, applying an electric field affects the SBH, leading to similar transitions and the development of an ohmic contact. Additionally, our study shows that n-doping in graphene increases with the number of BiP3 layers and external electric field application. These properties position BiP3 few-layer as a promising material for nanoelectronic, optoelectronic, and graphene-based devices.

First-principles investigations of the controllable electronic properties and contact types of type II MoTe2/MoS2 van der Waals heterostructures.

Two-dimensional (2D) van der Waals (vdW) heterostructures are considered as promising candidates for realizing multifunctional applications, including photodetectors, field effect transistors and solar cells. In this work, we performed first-principles calculations to design a 2D vdW MoTe2/MoS2 heterostructure and investigate its electronic properties, contact types and the impact of an electric field and in-plane biaxial strain. We find that the MoTe2/MoS2 heterostructure is predicted to be structurally, thermally and mechanically stable. It is obvious that the weak vdW interactions are mainly dominated at the interface of the MoTe2/MoS2 heterostructure and thus it can be synthesized in recent experiments by the transfer method or chemical vapor deposition. The construction of the vdW MoTe2/MoS2 heterostructure forms a staggered type II band alignment, effectively separating the electrons and holes at the interface and thereby extending the carrier lifetime. Interestingly, the electronic properties and contact types of the type II vdW MoTe2/MoS2 heterostructure can be tailored under the application of external conditions, including an electric field and in-plane biaxial strain. The semiconductor-semimetal-metal transition and type II-type I conversion can be achieved in the vdW MoTe2/MoS2 heterostructure. Our findings underscore the potential of the vdW MoTe2/MoS2 heterostructure for the design and fabrication of multifunctional applications, including electronics and optoelectronics.

Theoretical prediction of electronic properties and contact barriers in a metal/semiconductor NbS2/Janus MoSSe van der Waals heterostructure.

The emergence of van der Waals (vdW) heterostructures, which consist of vertically stacked two-dimensional (2D) materials held together by weak vdW interactions, has introduced an innovative avenue for tailoring nanoelectronic devices. In this study, we have theoretically designed a metal/semiconductor heterostructure composed of NbS2 and Janus MoSSe, and conducted a thorough investigation of its electronic properties and the formation of contact barriers through first-principles calculations. The effects of stacking configurations and the influence of external electric fields in enhancing the tunability of the NbS2/Janus MoSSe heterostructure are also explored. Our findings demonstrate that the NbS2/MoSSe heterostructure is not only structurally and thermally stable but also exfoliable, making it a promising candidate for experimental realization. In its ground state, this heterostructure exhibits p-type Schottky contacts characterized by small Schottky barriers and low tunneling barrier resistance, showing its considerable potential for utilization in electronic devices. Additionally, our findings reveal that the electronic properties, contact barriers and contact types of the NbS2/MoSSe heterostructure can be tuned by applying electric fields. A negative electric field leads to a conversion from a p-type Schottky contact to an n-type Schottky contact, whereas a positive electric field gives rise to a transformation from a Schottky into an ohmic contact. These insights offer valuable theoretical guidance for the practical utilization of the NbS2/MoSSe heterostructure in the development of next-generation electronic and optoelectronic devices.

Computational investigations of the metal/semiconductor NbS2/boron phosphide van der Waals heterostructure: effects of an electric field.

In this work, we design computationally the metal-semiconductor NbS2/BP heterostructure and investigate its atomic structure, electronic properties and contact barrier using first-principles prediction. Our results show that the M-S NbS2/BP heterostructure is energetically stable and is characterized by weak vdW interactions. Interestingly, we find that the combination of the metallic NbS2 and semiconducting BP layers leads to the formation of a M-S contact. The M-S NbS2/BP heterostructure exhibits a p-type Schottky contact and a low tunneling-specific resistivity of 3.98 × 10-10 Ω cm2, indicating that the metallic NbS2 can be considered as an efficient 2D electrical contact to the semiconducting BP layer to design NbS2/BP heterostructure-based electronic devices with high charge injection efficiency. The contact barrier and contact type in the M-S NbS2/BP heterostructure can be adjusted by applying an external electric field. The conversion from p-type ShC to n-type ShC can be achieved by applying a negative electric field, while the transformation from ShC to OhC type can be achieved under the application of a positive electric field. The conversion between p-type and n-type ShC and ShC to OhC type in the NbS2/BP heterostructure demonstrates that it can be considered as a promising material for next-generation electronic devices.

Reduction of thermal conductivity in carbon nanotubes by fullerene encapsulation from machine-learning molecular dynamics simulations

The carbon nano-peapod is a representative structure with interlayer van der Waals (vdW) interactions, in which encapsulated fullerene molecules play a critical role in modulating the transport properties of the carbon nanotubes (CNTs). In particular, their influence on the thermal transport characteristics has been the focal point of considerable attention. In this study, we trained an accurate machine learning potential for fullerene-encapsulated CNTs based on the efficient NEP model to investigate their thermal properties. Using equilibrium molecular dynamics simulation along with the spectral decomposition method for thermal conductivity, we find that the thermal conductivity of fullerene-encapsulated CNTs is roughly 55% lower than that of empty CNTs, aligning with experimental observations for CNT bundles with fullerene encapsulation [Kodama et al., Nat. Mater. 16, 892 (2017)]. The research suggests that weak vdW interactions between both the fullerene and CNTs, as well as between fullerene molecules themselves, hinder phonon propagation. The encapsulated fullerene contributes to an increase in phonon scattering within the CNTs, ultimately leading to a reduction in thermal conductivity. We utilized machine learning potential to investigate the structure of fullerene-encapsulated CNTs and their heat transport property. This approach provides valuable insights for performance research of complex systems featuring interlayer vdW interactions.

Introduction of hydrogen bond recognition sites in ZIF‐71 for effective separation of bio‐diols in aqueous solutions

AbstractThe bio‐based diols 1,3‐propanediol (1,3‐PDO) and 2,3‐butanediol (2,3‐BDO) can form strong hydrogen bonds with water, which increases the separation difficulty of diols in aqueous solutions. Herein, a series of mixed‐linker ZIF‐71‐clnImx samples with hydrogen bond–van der Waals (vdW) dual recognition sites have been constructed using 4‐chloro‐5‐nitroimidazole (clnIm) linkers to substitute partially for the original 4,5‐dichloroimidazole (dclIm) linkers of the ZIF‐71 framework. The experimental and calculated results show that ZIF‐71‐clnImx with appropriate clnIm substitution proportion (x = 7.6 and 12.8 mol%) can strengthen the interaction between ZIF‐71‐clnImx and 2,3‐BDO and enhance the adsorption and mass transfer of 2,3‐BDO, thus achieving the efficient separation of 2,3‐BDO/1,3‐PDO. Molecular simulation results further verify that 2,3‐BDO can simultaneously form strong H bonds with the –NO2 of the clnIm linker and weak vdW interactions with the –Cl of the dclIm linker, which are the mixed recognition sites for 2,3‐BDO adsorption intensification in aqueous solution for ZIF‐71‐clnImx.

Bonding-Driven Crystallization of Dimensionally Resolved Nanostructures Based on Quasi-1D Van Der Waals Building Blocks

The discovery of complex phenomena and strongly correlated behavior in the solid state has incessantly relied upon the creation of stable low-dimensional lattices approaching the atomic limit. Whereas 2D van der Waals (vdW) materials with atomic scale thicknesses have gained widespread interest, very little has been known about the chemistry and physics of their more confined 1D counterparts in the solid state. In these length scales and dimensionalities, unique physical properties arising from finite size effects such as ballistic electrical transport, size-dependent optical resonance modes, long carrier lifetimes, and exotic higher-order topological states become realizable. To this end, I will present the recent efforts in my group towards elucidating the chemical bonding interactions which define the structure and nanoscale dimensionality of structures derived from lattices which are comprised of 1D chains with sub-nm diameters that are held together by weak vdW interactions. By gaining anisotropy-driven control over these interactions via bottom-up chemical vapor growth, we demonstrate the creation of dimensionally-resolved nanostructures with various thicknesses ranging from 1D nanowire bundles to quasi-2D nanoribbons and nanosheets, all based on a the same 1D vdW lattice motif. Extensive electron diffraction and atomic-resolution microscopy of these resulting nanostructures reveal the strong dependence of the size and dimensionality of the 1D and quasi-2D nanostructures on anisotropic inter-chain vdW bonding interactions, precursor ratios, and growth temperatures. Using these results, we propose a crystallization mechanism defined by the competition between covalent and vdW bonding interactions and epitaxy. As physical properties directly correlate to the structure, dimensionality, and size in the solid state, I will also talk about how the unique topographies of resulting freestanding nanostructures dictate their electronic properties. I will describe how the synthetic control over the structural attributes led to our discovery of emergent properties such as the elusive indirect-to-direct band gap transition and higher-order topological states in 1D vdW nanostructures. Through these efforts, we define for the first time the anisotropy-driven growth of these emergent 1D vdW-derived nanostructures and how nanoscale morphologies in these nanostructures dictate their electronic structures. Altogether, this precise control over crystalline and electronic structure poises these emergent low-dimensional solids as building blocks towards programmable electrocatalysts, densified electronic and quantum devices in sensing, photonic, energy storage and conversion, and spintronic technologies.

Ternary GePdS3: 1D van der Waals Nanowires for Integration of High-Performance Flexible Photodetectors

One-dimensional (1D) van der Waals (vdW) materials are anticipated to leverage for high-performance, giant polarized, and hybrid-dimension photodetection owing to their dangling-bond free surface, intrinsic crystal structure, and weak vdW interaction. However, only a few related explorations have been conducted, especially in the field of flexible and integrated applications. Here, high-quality 1D vdW GePdS3 nanowires were synthesized and proven to be an n-type semiconductor. The Raman vibration and band gap (1.37-1.68 eV, varying from bulk to single chain) of GePdS3 were systemically studied by experimental and theoretical methods. The photodetector based on a single GePdS3 nanowire possesses fast photoresponse at a broadband spectrum of 254-1550 nm. The highest responsivity and detectivity reach up to ∼219 A/W and ∼2.7 × 1010 Jones (under 254 nm light illumination), respectively. Furthermore, an image sensor with 6 × 6 pixels based on GePdS3 nanowires is integrated on a flexible polyethylene terephthalate (PET) substrate and exhibits sensitive and homogeneous detection at 808 nm light. These results indicate that the ternary noble metal chalcogenides show great potential in flexible and broadband optoelectronics applications.

Tunable Schottky barrier in a graphene/AlP van der Waals heterostructure

The controllable modulation of the electronic properties of two-dimensional van der Waals (vdW) heterostructures is crucial for their applications in the future nanoelectronic and optoelectronic devices. In this paper, the electronic properties of a graphene/AlP heterostructure are theoretically studied by first-principles calculation. The results show that due to the weak vdW interaction between graphene and the AlP monolayer, both the Dirac semi-metallic properties of graphene and the semiconductivity properties of monolayer AlP are well retained. The graphene/AlP heterostructure forms a 0.41 eV p-type Schottky contact, and the barrier height and contact type can be successively controlled by the applied external electric field and vertical stress. When the applied electric field exceeds −0.5 V Å−1, the heterostructure interface changes from a p-type Schottky contact to an n-type Schottky contact. When the applied electric field exceeds 0.4 V Å−1 or the interlayer spacing is less than 3.1 Å, the interface contact type changes to Ohmic contact. These results indicate that the graphene/AlP heterostucture behaves as tunable Schottky barrier for potential applications in nano-devices.

Free-standing [0 0 1]-oriented one-dimensional crystal-structured antimony selenide films for self-powered flexible near-infrared photodetectors

Flexible optoelectronic is one of the key components for acquiring biological signals in wearable devices and implantable systems. Recently, antimony selenide (Sb2Se3) has shown an exciting potential for flexible optoelectronic applications due to its unique one-dimensional (1D) ribbon-typed crystal structure, low-cost constituents, and superior optoelectronic properties. However, the growth of [001]-oriented Sb2Se3 film, which can efficiently enhance the carrier transport, is still challenging, particularly for its application in flexible electronics. Herein, we propose to grow the free-standing [001]-oriented Sb2Se3 film for self-powered flexible photodiode through selenizing the highly (0001)-textured Sb film on mica substrate. The theoretical calculation reveals that Sb(0001) plane provides a high adsorption energy for Se atoms, which promotes the [001]-oriented growth of Sb2Se3 film. The weak vdW interaction between Sb2Se3 film and mica substrate enables the film separation, and results in a free-standing Sb2Se3 film. Benefiting from the enhanced carrier transport in [001]-oriented Sb2Se3 film, the flexible photodiode exhibits a high responsivity up to 0.74 A W−1, a specific detectivity of 5.1 × 1013 Jones, a linear dynamic range of 102 dB, rise/fall time of 0.16/0.15 ms for near-infrared light detection without any external bias, and excellent folding endurance after 1000 bending cycles. It has been also successfully employed as a wearable photodetector to detect a person’s heart rate. This work demonstrates a strategy for the orientation control of low-dimensional crystal-structured materials and a great potential of free-standing films in flexible electronics applications.

Van der Waals integration of artificial heterostructures and high-order superlattices

<p indent="0mm">The recent blossom in 2D atomic crystals (2DACs) has ignited intense interest in a new type of bond-free van der Waals heterostructures (vdWHs), in which distinct material components are physically brought together within a vdW distance and held together by weak vdW interactions. Without direct chemical bonding between the constituent materials, the vdWHs negate the lattice matching requirements in typical epitaxially bonded heterostructures. Here we briefly summarize the key advances in the construction and fundamental investigation of versatile vdWHs from diverse 2DACs and beyond, and highlight a unique class of vdW superlattices (vdWSLs) consisting of alternating 2D atomic layers and/or self-assembled molecular layers, with tailored structural symmetry, electronic band modulation, interlayer coupling, and chirality. Lastly, we conclude with a brief outlook on the opportunities in exploring such artificial materials to unlock previously inaccessible physical limits and enable new device concepts beyond the reach of the existing materials.

In-situ scanning tunneling microscopy observation of thickness-dependent air-sensitive layered materials and heterodevices

Quasi-two-dimensional (Quasi-2D) van der Waals (vdW) materials can be mechanically or chemically exfoliated down to monolayer because of their strong intralayer bonding and the weak interlayer vdW interaction. Thanks to this unique property, one can often find exotic thickness-dependent electronic properties from these quasi-2D vdW materials, which can lead to bandgap opening, emerging superconductivity, or enhanced charge density waves with decreasing thickness. Surface-sensitive scanning tunneling microscopy (STM) can provide direct observation of structural and electronic characteristics of such layered materials with atomic precision in real space. However, it is very challenging to preserve the intrinsic surfaces of air-sensitive quasi-2D materials between preparation and measurement. In addition, vdW 2D crystals after exfoliation are extremely hard to explore with a typical STM setup due to their small size (≤ 10 μm). Here, we present a straightforward method compatible with any STM setup having optical access: (1) exfoliating and/or stacking layered materials in a glove box, (2) transferring them to an ultra-high vacuum STM chamber using a suitcase without exposure to air, and (3) navigating surface to locate exfoliated vdW 2D flakes with different thicknesses. We successfully demonstrated that the clean surfaces of the air-sensitive $$\hbox {Fe}_3\hbox {GeTe}_2$$ can be effectively protected from unwanted oxidation during transfer. Furthermore, our method provides a simple but useful way to access a specific tiny stack of layered materials without any ex-situ fabrication processes for STM navigation. Our experimental improvement will open up a new way to investigate air-sensitive layered vdW materials with various thicknesses via surface-sensitive techniques including STM.

Dihydrolevoglycosenone as a novel bio-based nanofluid for thermal energy storage: Physiochemical and quantum chemical insights

Dihydrolevoglycosenone, commercially known as Cyrene, is a biodegradable solvent with a multitude of potential applications such as chemical reactions and heat transfer media. The current work reports a nanofluid comprising cyrene as a potential bio-organic thermal base media dispersed with Multi-walled carbon nanotube (MWCNT) nanoparticles. Two volume fractions (0.0016 and 0.0032) of nanoparticles were added to enhance the heat transfer capacity of the resulting nanofluid. Thereafter, thermophysical properties were reported in the temperature range of 30–85 °C for both base fluid and nanofluid. The measured values were then compared with a commercial heat transfer fluid, Paramtherm GLT, within the temperature range of 30–85 °C. Further, the stability of the nanofluid was investigated by a combination of visual observation, microscopic analysis, and zeta potential measurements. Thereafter, forced convection experiments were performed in a circular tube section under laminar conditions to measure the local heat transfer coefficient alongside temperature profiles. Results showed that the nanofluid possessing 0.0032 volume fraction of MWCNT-Cyrene nanofluid at NRe = 1881 gave the highest heat transfer coefficient. Density Functional Theory was also used to investigate the microstructure formed by Cyrene on the surface of Single-walled carbon nanotube (SWCNT). The orbital energy and influence of interaction energy on the van der Waals (vdW) interactions of Cyrene and ethylene glycol with SWCNT were examined and correlated with the dispersive forces within the fluid. The orbital energy revealed the fact that the HOMO-LUMO energy gap of the Cyrene/SWCNT was reduced due to the approach of Cyrene towards SWCNT surface, making it a stable nanofluid system. Reduced density gradient (RDG) analysis further demonstrated that weak vdW interactions were the primary driving force between solvent-SWCNT systems, primarily through X…π interactions.

A general theory for the bending of multilayer van der Waals materials

Multilayer van der Waals (vdW) materials exhibit nontrivial out-of-plane responses and deformability due to their inherent atomic scale thickness coupled with weak interlayer vdW interactions, which call for a general mechanical framework to quantify such extraordinary characteristics given the structure and material properties. Here, we demonstrate that by considering the competing mechanisms between intralayer stretching, interlayer shearing, and monolayer bending, a general centerline-based theory for multilayered structures is developed to portray the bending landscape of multilayer vdW materials. Combined with large-scale molecular dynamics simulations of three-point bending, we find that when the slenderness ratio or the interlayer shear rigidity is smaller or the intralayer elasticity is higher, the multilayered structures exhibit pronounced nonplanar section deformation due to the interlayer shear capacity. To elucidate quantitative correlations between deformation distribution and structural dimensions and material properties, a critical criterion is proposed to depict the transformation from planar to nonplanar section deformation through a dimensionless characteristic parameter. The effective bending stiffness is then systematically explored in the space of characteristic geometries and material properties to reveal the underlying mechanism of the anomalous size effect. We identify that the effective bending stiffness follows a unified scaling law regarding the dimensionless characteristic parameter. More importantly, a series of deformation-mode phase diagrams are constructed using two universal characteristic lengths, intuitively illustrating the transition of dominated deformation modes and the synergism of geometries and inherent material properties on the bending responses. Validated by several typical vdW materials and novel semiconducting two-dimensional materials, the proposed diagrams not only deepen the understanding of bending in multilayer vdW materials but also provide guidelines for the design and optimization of vdW material-based devices.

Graphene/Cs2PbI2Cl2 van der Waals heterostructure with tunable Schottky barriers and contact types

Two-dimensional halide perovskite Cs2PbI2Cl2 with the Ruddlesden–Popper structure has attracted much interest in both experiment and theory, owing to its excellent structural stability and electronic and optical properties. Here, we design the graphene/Cs2PbI2Cl2 van der Waals (vdW) heterostructure (HS) and comprehensively investigate its structural, electronic, and contact properties by using first principle calculations. Four types of graphene/Cs2PbI2Cl2 HSs are considered, and the most stable one is identified. Because the composed system has weak vdW interaction, the intrinsic band structures of both graphene and Cs2PbI2Cl2 are well maintained. Meanwhile, the graphene opens a minute energy gap of about 68 meV, which may have resulted from a broken sublattice inversion symmetry and tiny structure distortion. Moreover, it is found that graphene/Cs2PbI2Cl2 forms a p-type Schottky contact. The HS undergoes a contact-type transition to p-type Ohmic contact and n-type Ohmic contact from the original p-type Schottky contact under positive and negative electric fields, respectively. When interlayer coupling strength increases or decreases, a contact-type transition to the p-type Ohmic contact from the original p-type Schottky contact occurs. These findings provide a meaningful guidance for tuning the electronic properties and constructing high-performance graphene/Cs2PbI2Cl2 HS-based Schottky devices.

α−As2Te3 as a platform for the exploration of the electronic band structure of single layer β−tellurene

Arsenic telluride, ${\mathrm{As}}_{2}{\mathrm{Te}}_{3}$, is a layered van der Waals (vdW) semiconducting material usually known for its thermoelectric properties. It is composed of layers stacked together via weak vdW interactions, which can consequently be exfoliated into thin two-dimensional layers. Here, we studied the electronic properties of the $\ensuremath{\alpha}$ phase of ${\mathrm{As}}_{2}{\mathrm{Te}}_{3}$ by using angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT). In addition to the spectroscopic signature of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Te}}_{3}$, we were able to isolate anisotropic 2D electronic states, decoupled from the $\ensuremath{\alpha}\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Te}}_{3}$ electronic structure, that we propose to ascribe to single layer (SL) $\ensuremath{\beta}\ensuremath{-}\mathrm{tellurene}$. Our findings are supported by theoretical investigations using DFT, which reproduce the main ARPES experimental features. Our work thereby proposes $\ensuremath{\alpha}\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Te}}_{3}$ (100) surface as an interesting platform for the experimental exploration of the electronic band structure of SL $\ensuremath{\beta}\ensuremath{-}\mathrm{tellurene}$, which has been difficult to experimentally access otherwise.

Bending deformation and self-restoration of submicron-sized graphite cantilevers

Van der Waals (vdW)-stacked materials exhibit unique mechanical properties owing to their strong in-plane atomic bonds and weak interlayer vdW interactions. In this study, we experimentally demonstrate that submicron-sized graphite cantilevers can accommodate large out-of-plane deformations without fracturing and undergo self-restoration upon unloading. Submicron-sized cantilever beam specimens were fabricated from highly oriented pyrolytic graphite using the focused ion beam method and subjected to in situ transmission electron microscopy-based bending tests. The cantilevers exhibited a nonlinear load–displacement relationship, and their deformation was almost completely restored upon unloading, even though a large hysteresis loop was observed during the loading–unloading process. Moreover, similar loading–unloading curves were obtained during the repeated loading of the same specimen, indicating that the mechanical properties were restored by unloading even in the case of large nonlinear deformations. For larger deformations, plastic deformation occurred, and the self-restoration property was lost. However, no clear fracture was observed in the cantilevers even when they were deformed to a deflection angle of 62°. Even in this case, the deformation could be restored by electron-beam irradiation. Based on the obtained results, we propose a mechanism to explain the nonlinear and fully reversible deformation behavior of the specimens.

Strain engineering on the electronic properties and interface contact of graphene/GeN3 van der Waals heterostructure

Graphene-based van der Waals (vdW) heterostructures have shown great potential in electronic and optoelectronic nanodevices. Herein, we investigate the electronic property and Schottky barrier of graphene/GeN3 vdW heterostructure by first-principles calculations. It is noted that the electronic natures of graphene and GeN3 monolayers are well preserved in the heterostructure lattice due to the weak vdW interaction. Interestingly, the p-type Schottky contact in graphene/GeN3 heterostructure with a barrier height of 0.21 eV can be effectively tuned by both vertical and horizontal strains. Herein, the carrier concentration in the graphene layer reaches ∼1013 cm−2 level by strain engineering. It is noteworthy that the designed optoelectronic field-effect transistor based on graphene/GeN3 heterostructure exhibits distinguished responsivity of 0.297 AW−1 and impressive external quantum efficiency of 54.5% under illumination based on further non-equilibrium Green's function simulations. Our findings are of utmost significance for the metal–semiconductor vdW contact and corresponding applications in high-performance electronic and optoelectronic devices.

DFT simulation of interfacial interaction of graphene/SiO2 composites

Thoroughly understanding the graphene/SiO2 interfacial properties is of great significance for improving the performance of graphene-based nanoelectronic devices. In this work, the first-principle density functional theory (DFT) method was applied to systematically investigate the interfacial interaction and electronic properties of graphene on the β-cristobalite SiO2 surface. It was first demonstrated that the Si-terminated β-cristobalite SiO2 substrate has a strong chemical covalent interaction with the pristine graphene, whereas the hydroxylated SiO2 surface just shows weak vdW interaction with graphene. The interlayer water molecules between graphene and SiO2 can reduce the adhesion energy for the graphene/SiO2 composites, in which the water layers provide the main contribution to the graphene adhesion with the water-coated SiO2. The effects of the SiO2 surface polarity and the interfacial water geometry on the electronic properties of graphene were investigated. The electron transfer between hydroxylated SiO2 and graphene is highly related to the surface silanol density. Meanwhile, the interlayer water molecules could induce a minor electron transfer from graphene to the surfaces, resulting in the hole-doping of graphene. Our simulation provides a new understanding of the interaction for graphene/SiO2 interfaces.