Mechanical self-assembly of deformed 2D materials for advanced functionalities

Introducing three-dimensionality to two-dimensional (2D) materials opened new possibilities and novel applications in material system design. Extraordinary intrinsic properties of 2D materials have attracted scientific research communities, and various 2D material – based structures and devices have been proposed and demonstrated. Given the fascinating electrical, optical, and thermal properties of 2D materials, constructing three-dimensional (3D) structures out of 2D materials would make 2D systems even more interesting by providing means of extrinsic modulation of material properties of 2D materials. 2D materials frequently undergo out-of-plane deformation because the atomic thinness of 2D materials leads to rippling, wrinkling, and folding at the surface during synthesis and transfer processes. Those out-of-plane deformations are neither organized nor controllable structures, and therefore, they have often been considered inevitable defects. However, recent studies have reported tunable electrical, chemical, optical, and mechanical properties when a controlled in-plane strain gradient is applied to 2D lattice structures. This implies that such out-of-plane deformations with structural control can be useful tools for material engineering. Therefore, we seek ways to manipulate the deformation of 2D materials to modulate material properties for advanced functionalities. One of our strategies to architecture 2D materials is inspired by fine wrinkle formation that occurs when there is a strain mismatch between a thin film and an underlying substrate. We use extreme-case strain mismatch, often up to 300 percent or more. In addition, we control the direction of prestrains to create uniaxially or biaxially crumpled 2D materials with a uniaxial prestrain or biaxial prestrains, respectively. The resulting structure is a mechanically self-assembled, buckle-delaminated structure with delocalized crumples and inhomogeneous strain in the crumpled 2D lattice. Furthermore, we create mixed-dimensional structures by combining our architecturing strategy with unconventional crack lithography. Through the crack lithography – inspired strategy, heterogeneous 2D-3D mixed-dimensional structures are formed with localized crumples. These strategies are universally applicable to 2D materials and 2D material – based hybrid structures, such as graphene, molybdenum disulfide (MoS2), and graphene – gold nanoparticles (Au NPs) hybrid structures. We further explore advanced functionalities of buckle-delaminated crumpled structures of 2D materials and 2D materials – based hybrid structures. The crumpled graphene – Au NPs hybrid structure demonstrates advanced functionality based on the topography of deformed 2D materials. In the hybrid structure, Au NPs are formed on a graphene surface, through thermal dewetting of gold thin film. The graphene is then deformed into a 3D crumpled structure. The crumpled structure effectively enhances localized electromagnetic fields between adjacent Au NPs by reducing the gap between Au NPs and helps utilize the 3D focal volume of the incident laser. Therefore, the crumpled hybrid structure – based surface-enhanced Raman spectroscopy (SERS) sensor exhibits an order of magnitude higher sensitivity, compared to a flat hybrid structure – based SERS sensor. Additionally, the crumpled hybrid structure – based SERS sensor can be easily applied on an arbitrary curvilinear surface demonstrating its potential for in situ SERS assays. We further applied the crumpled hybrid structure of graphene – Au NPs to create a photodetector with plasmonically enhanced photoresponsivity and high stretchability. Gold nanoparticles in the hybrid structure effectively enhance photoresponsivity, and further enhancement is achieved by material densification by crumpling the hybrid structure. As a result, we demonstrate 1200% enhanced photoresponsivity over a flat graphene device by combining the plasmonic effect and material densification. The crumpled structure also provides an exceptional 200% stretchability. The fabricated structure is mechanically robust, with no failure observed after 1000 cycles of stretching and releasing. The deformed 2D material system also provides a material platform for strain engineering of 2D materials. We created a 3D crumpled structure with a semiconducting 2D material, MoS2, to create inhomogeneous strain in the MoS2 lattice. This continuous strain change across the deformed structure induces a bandgap energy gradient, and therefore, provides efficient transport paths for photoexcited excitons in the deformed 2D lattice. We further demonstrated dynamic photoresponsivity modulation by structural modulation of a deformed 2D material, implying exciton drift modulation in a crumpled and flattened lattice structure. In conclusion, we have demonstrated effective strategies to create deformed 2D material systems with various levels of structural complexity, and related applications utilizing the unique topography and the strain gradient simultaneously created in the deformed lattice. We believe our approach to deforming 2D materials and achieving advanced functionalities contribute to the related research communities by demonstrating a way to extrinsically manipulate 2D materials' topography, and therefore modulate intrinsic properties.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste

Mechanically Self-Assembled, Three-Dimensional Graphene–Gold Hybrid Nanostructures for Advanced Nanoplasmonic Sensors

Hybrid structures of graphene and metal nanoparticles (NPs) have been actively investigated as higher quality surface enhanced Raman spectroscopy (SERS) substrates. Compared with SERS substrates, which only contain metal NPs, the additional graphene layer provides structural, chemical, and optical advantages. However, the two-dimensional (2D) nature of graphene limits the fabrication of the hybrid structure of graphene and NPs to 2D. Introducing three-dimensionality to the hybrid structure would allow higher detection sensitivity of target analytes by utilizing the three-dimensional (3D) focal volume. Here, we report a mechanical self-assembly strategy to enable a new class of 3D crumpled graphene–gold (Au) NPs hybrid nanoplasmonic structures for SERS applications. We achieve a 3D crumpled graphene–Au NPs hybrid structure by the delamination and buckling of graphene on a thermally activated, shrinking polymer substrate. We also show the precise control and optimization of the size and spacing of integrated Au NPs on crumpled graphene and demonstrate the optimized NPs’ size and spacing for higher SERS enhancement. The 3D crumpled graphene–Au NPs exhibits at least 1 order of magnitude higher SERS detection sensitivity than that of conventional, flat graphene–Au NPs. The hybrid structure is further adapted to arbitrary curvilinear structures for advanced, in situ, nonconventional, nanoplasmonic sensing applications. We believe that our approach shows a promising material platform for universally adaptable SERS substrate with high sensitivity

Large scale self-assembly of plasmonic nanoparticles on deformed graphene templates

Abstract Hierarchical heterostructures of two-dimensional (2D) nanomaterials are versatile platforms for nanoscale optoelectronics. Further coupling of these 2D materials with plasmonic nanostructures, especially in non-close-packed morphologies, imparts new metastructural properties such as increased photosensitivity as well as spectral selectivity and range. However, the integration of plasmonic nanoparticles with 2D materials has largely been limited to lithographic patterning and/or undefined deposition of metallic structures. Here we show that colloidally synthesized zero-dimensional (0D) gold nanoparticles of various sizes can be deterministically self-assembled in highly-ordered, anisotropic, non-close-packed, multi-scale morphologies with templates designed from instability-driven, deformed 2D nanomaterials. The anisotropic plasmonic coupling of the particle arrays exhibits emergent polarization-dependent absorbance in the visible to near-IR regions. Additionally, controllable metasurface arrays of nanoparticles by functionalization with varying polymer brushes modulate the plasmonic coupling between polarization dependent and independent assemblies. This self-assembly method shows potential for bottom-up nanomanufacturing of diverse optoelectronic components and can potentially be adapted to a wide array of nanoscale 0D, 1D, and 2D materials

Three-Dimensional Integration of Graphene via Swelling, Shrinking, and Adaptation

The transfer of graphene from its growth substrate to a target substrate has been widely investigated for its decisive role in subsequent device integration and performance. Thus far, various reported methods of graphene transfer have been mostly limited to planar or curvilinear surfaces due to the challenges associated with fractures from local stress during transfer onto three-dimensional (3D) microstructured surfaces. Here, we report a robust approach to integrate graphene onto 3D microstructured surfaces while maintaining the structural integrity of graphene, where the out-of-plane dimensions of the 3D features vary from 3.5 to 50 μm. We utilized three sequential steps: (1) substrate swelling, (2) shrinking, and (3) adaptation, in order to achieve damage-free, large area integration of graphene on 3D microstructures. Detailed scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and electrical resistance measurement studies show that the amount of substrate swelling as well as the flexural rigidities of the transfer film affect the integration yield and quality of the integrated graphene. We also demonstrate the versatility of our approach by extension to a variety of 3D microstructured geometries. Lastly, we show the integration of hybrid structures of graphene decorated with gold nanoparticles onto 3D microstructure substrates, demonstrating the compatibility of our integration method with other hybrid nanomaterials. We believe that the versatile, damage-free integration method based on swelling, shrinking, and adaptation will pave the way for 3D integration of two-dimensional (2D) materials and expand potential applications of graphene and 2D materials in the future

Synergistic effects of mixing and strain in high entropy spinel oxides for oxygen evolution reaction

Abstract Developing stable and efficient electrocatalysts is vital for boosting oxygen evolution reaction (OER) rates in sustainable hydrogen production. High-entropy oxides (HEOs) consist of five or more metal cations, providing opportunities to tune their catalytic properties toward high OER efficiency. This work combines theoretical and experimental studies to scrutinize the OER activity and stability for spinel-type HEOs. Density functional theory confirms that randomly mixed metal sites show thermodynamic stability, with intermediate adsorption energies displaying wider distributions due to mixing-induced equatorial strain in active metal-oxygen bonds. The rapid sol-flame method is employed to synthesize HEO, comprising five 3d-transition metal cations, which exhibits superior OER activity and durability under alkaline conditions, outperforming lower-entropy oxides, even with partial surface oxidations. The study highlights that the enhanced activity of HEO is primarily attributed to the mixing of multiple elements, leading to strain effects near the active site, as well as surface composition and coverage

High thermoelectric figure of merit of porous Si nanowires from 300 to 700 K.

Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4 nm. Concurrent measurements of thermal conductivity (κ), electrical conductivity (σ), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700 K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700 K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000 K