Strain tolerance of two-dimensional crystal growth on curved surfaces

Giant Second Harmonic Generation in Supertwisted WS2 Spirals Grown in Step-Edge Particle-Induced Non-Euclidean Surfaces

In moiré crystals resulting from the stacking of twisted two-dimensional (2D) layered materials, a subtle adjustment in the twist angle surprisingly gives rise to a wide range of correlated optical and electrical properties. Herein, we report the synthesis of supertwisted WS2 spirals and the observation of giant second harmonic generation (SHG) in these spirals. Supertwisted WS2 spirals featuring different twist angles are synthesized on a Euclidean or step-edge particle-induced non-Euclidean surface using carefully designed water-assisted chemical vapor deposition. We observed an oscillatory dependence of SHG intensity on layer number, attributed to atomically phase-matched nonlinear dipoles within layers of supertwisted spiral crystals where inversion symmetry is restored. Through an investigation into the twist angle evolution of SHG intensity, we discovered that the stacking model between layers plays a crucial role in determining the nonlinearity, and the SHG signals in supertwisted spirals exhibit enhancements by a factor of 2 to 136 when compared with the SHG of the single-layer structure. These findings provide helpful perspectives on the rational growth of 2D twisted structures and the implementation of twist angle adjustable endowing them great potential for exploring strong coupling correlation physics and applications in the field of twistronics.

Locally Strained 2D Materials: Preparation, Properties, and Applications

2D materials are promising for strain engineering due to their atomic thickness and exceptional mechanical properties. In particular, non-uniform and localized strain can be induced in 2D materials by generating out-of-plane deformations, resulting in novel phenomena and properties, as witnessed in recent years. Therefore, the locally strained 2D materials are of great value for both fundamental studies and practical applications. This review discusses techniques for introducing local strains to 2D materials, and their feasibility, advantages, and challenges. Then, the unique effects and properties that arise from local strain are explored. The representative applications based on locally strained 2D materials are illustrated, including memristor, single photon emitter, and photodetector. Finally, concluding remarks on the challenges and opportunities in the emerging field of locally strained 2D materials are provided.

Substrate Engineering for Chemical Vapor Deposition Growth of Large-Scale 2D Transition Metal Dichalcogenides

The large-scale production of 2D transition metal dichalcogenides (TMDs) is essential to realize their industrial applications. Chemical vapor deposition (CVD) has been considered as a promising method for the controlled growth of high-quality and large-scale 2D TMDs. During a CVD process, the substrate plays a crucial role in anchoring the source materials, promoting the nucleation and stimulating the epitaxial growth. It thus significantly affects the thickness, microstructure, and crystal quality of the products, which are particularly important for obtaining 2D TMDs with expected morphology and size. Here, an insightful review is provided by focusing on the recent development associated with the substrate engineering strategies for CVD preparation of large-scale 2D TMDs. First, the interaction between 2D TMDs and substrates, a key factor for the growth of high-quality materials, is systematically discussed by combining the latest theoretical calculations. Based on this, the effect of various substrate engineering approaches on the growth of large-area 2D TMDs is summarized in detail. Finally, the opportunities and challenges of substrate engineering for the future development of 2D TMDs are discussed. This review might provide deep insight into the controllable growth of high-quality 2D TMDs toward their industrial-scale practical applications.

Built-in tensile strain dependence on the lateral size of monolayer MoS2 synthesized by liquid precursor chemical vapor deposition

Strain engineering is an efficient tool to tune and tailor the electrical and optical properties of 2D materials. The built-in strain can be tuned during the synthesis process of a two-dimensional semiconductor, such as molybdenum disulfide, by employing different growth substrates with peculiar thermal properties. In this work, we demonstrate that the built-in strain of MoS2 monolayers, grown on a SiO2/Si substrate by liquid precursor chemical vapor deposition, is mainly dependent on the size of the monolayer. In fact, we identify a critical size equal to 20 μm, from which the built-in strain increases drastically. The built-in strain is the maximum for a 60 μm sized monolayer, leading to 1.2% tensile strain with a partial release of strain close to the monolayer triangular vertexes due to the formation of nanocracks. These findings also imply that the standard method for evaluation of the number of layers based on the Raman mode separation can become unreliable for highly strained monolayers with a lateral size above 20 μm.

Curvature geometry in 2D materials

The two-dimensional (2D) material family can be regarded as the extreme externalization form of the matter in the planar 2D space. These atomically thin materials have abundant curvature structures, which will significantly affect their atomic configurations and physicochemical properties. Curvature engineering offers a new tuning freedom beyond the thoroughly studied layer number, grain boundaries, stacking order, etc. The precise control of the curvature geometry in 2D materials can redefine this material family. Special attention will be given to this emerging field and highlight possible future directions. With the step-by-step achievement in understanding the curvature engineering effect in 2D materials and establishing reliable delicate curvature controlling strategies, a brand-new era of 2D materials research could be developed.

Single-Photon Emission from Two-Dimensional Materials, to a Brighter Future

Single photons, often called flying qubits, have enormous promise to realize scalable quantum technologies ranging from an unhackable communication network to quantum computers. However, finding an ideal single-photon emitter (SPE) is a great challenge. Recently, two-dimensional (2D) materials have shown great potential as hosts for SPEs that are bright and operate under ambient conditions. This Perspective enumerates the metrics required for an SPE source and highlights that 2D materials, because of reduced dimensionality, exhibit interesting physical effects and satisfy several metrics, making them excellent candidates to host SPEs. The performance of SPE candidates discovered in 2D materials, hexagonal boron nitride and transition metal dichalcogenides, will be assessed based on the metrics, and the remaining challenges will be highlighted. Lastly, strategies to mitigate such challenges by developing design rules to deterministically create SPE sources will be presented.

Breaking Rotational Symmetry in Supertwisted WS2 Spirals via Moiré Magnification of Intrinsic Heterostrain

Twisted stacking of van der Waals materials with moiré superlattices offers a new way to tailor their physical properties via engineering of the crystal symmetry. Unlike well-studied twisted bilayers, little is known about the overall symmetry and symmetry-driven physical properties of continuously supertwisted multilayer structures. Here, using polarization-resolved second harmonic generation (SHG) microscopy, we report threefold (C3) rotational symmetry breaking in supertwisted WS2 spirals grown on non-Euclidean surfaces, contrasting the intact symmetry of individual monolayers. This symmetry breaking is attributed to a geometrical magnifying effect in which small relative strain between adjacent twisted layers (heterostrain), verified by Raman spectroscopy and multiphysics simulations, generates significant distortion in the moiré pattern. Density-functional theory calculations can explain the C3 symmetry breaking and unusual SHG response by the interlayer wave function coupling. These findings thus pave the way for further developments in the so-called "3D twistronics".

Extreme Bendability of Atomically Thin MoS2 Grown by Chemical Vapor Deposition Assisted by Perylene-Based Promoter

Shaping two-dimensional (2D) materials in arbitrarily complex geometries is a key to designing their unique physical properties in a controlled fashion. This is an elegant solution, taking benefit from the extreme flexibility of the 2D layers but requiring the ability to force their spatial arrangement from flat to curved geometries in a delicate balance among free-energy contributions from strain, slip-and-shear mechanisms, and adhesion to the substrate. Here, we report on a chemical vapor deposition approach, which takes advantage of the surfactant effects of organic molecules, namely the tetrapotassium salt of perylene-3,4,9,10-tetracarboxylic acid (PTAS), to conformally grow atomically thin layers of molybdenum disulphide (MoS₂) on arbitrarily nanopatterned substrates. Using atomically resolved transmission electron microscope images and density functional theory calculations, we show that the most energetically favorable condition for the MoS₂ layers consists of its adaptation to the local curvature of the patterned substrate through a shear-and-slip mechanism rather than strain accumulation. This conclusion also reveals that the perylene-based molecules have a role in promoting the adhesion of the layers onto the substrate, no matter the local-scale geometry.

Radial bound states in the continuum for polarization-invariant nanophotonics

All-dielectric nanophotonics underpinned by the physics of bound states in the continuum (BICs) have demonstrated breakthrough applications in nanoscale light manipulation, frequency conversion and optical sensing. Leading BIC implementations range from isolated nanoantennas with localized electromagnetic fields to symmetry-protected metasurfaces with controllable resonance quality (Q) factors. However, they either require structured light illumination with complex beam-shaping optics or large, fabrication-intense arrays of polarization-sensitive unit cells, hindering tailored nanophotonic applications and on-chip integration. Here, we introduce radial quasi-bound states in the continuum (radial BICs) as a new class of radially distributed electromagnetic modes controlled by structural asymmetry in a ring of dielectric rod pair resonators. The radial BIC platform provides polarization-invariant and tunable high-Q resonances with strongly enhanced near fields in an ultracompact footprint as low as 2 µm². We demonstrate radial BIC realizations in the visible for sensitive biomolecular detection and enhanced second-harmonic generation from monolayers of transition metal dichalcogenides, opening new perspectives for compact, spectrally selective, and polarization-invariant metadevices for multi-functional light-matter coupling, multiplexed sensing, and high-density on-chip photonics.

In their work on radial BICs, the authors realize a nanophotonic platform with high resonance Q factors and drastically reduced spatial footprint ideally suited for enhanced on-chip biomolecular sensing and nonlinear light generation.

Designing 1D correlated-electron states by non-Euclidean topography of 2D monolayers

Two-dimensional (2D) bilayers, twisted to particular angles to display electronic flat bands, are being extensively explored for physics of strongly correlated 2D systems. However, the similar rich physics of one-dimensional (1D) strongly correlated systems remains elusive as it is largely inaccessible by twists. Here, a distinctive way to create 1D flat bands is proposed, by either stamping or growing a 2D monolayer on a non-Euclidean topography-patterned surface. Using boron nitride (hBN) as an example, our analysis employing elastic plate theory, density-functional and coarse-grained tight-binding method reveals that hBN's bi-periodic sinusoidal deformation creates pseudo- electric and magnetic fields with unexpected spatial dependence. A combination of these fields leads to anisotropic confinement and 1D flat bands. Moreover, changing the periodic undulations can tune the bandwidth, to drive the system to different strongly correlated regimes such as density waves, Luttinger liquid, and Mott insulator. The 1D nature of these states differs from those obtained in twisted materials and can be exploited to study the exciting physics of 1D quantum systems.

Electron Optics and Valley Hall Effect of Undulated Graphene

Electron optics is the systematic use of electromagnetic (EM) fields to control electron motions. In graphene, strain induces pseudo-electromagnetic fields to guide electron motion. Here we demonstrate the use of substrate topography to impart desirable strain on graphene to induce static pseudo-EM fields. We derive the quasi-classical equation of motion for Dirac Fermions in a pseudo-EM field in graphene and establish the correspondence between the quasi-classical and quantum mechanical snake states. Based on the trajectory analysis, we design sculpted substrates to realize various "optical devices" such as a converging lens or a collimator, and further propose a setup to achieve valley Hall effect solely through substrate patterning, without any external fields, to be used in valleytronics applications. Finally, we discuss how the predicted strain/pseudo-EM field patterns can be experimentally sustained by typical substrates and generalized to other 2D materials.

Ambient Pressure Chemical Vapor Deposition of Flat and Vertically Aligned MoS2 Nanosheets

Molybdenum disulfide (MoS₂) got tremendous attention due to its atomically thin body, rich physics, and high carrier mobility. The controlled synthesis of large area and high crystalline monolayer MoS₂ nanosheets on diverse substrates remains a challenge for potential practical applications. Synthesizing different structured MoS₂ nanosheets with horizontal and vertical orientations with respect to the substrate surface would bring a configurational versatility with benefit for numerous applications, including nanoelectronics, optoelectronics, and energy technologies. Among the proposed methods, ambient pressure chemical vapor deposition (AP-CVD) is a promising way for developing large-scale MoS₂ nanosheets because of its high flexibility and facile approach. Here, we show an effective way for synthesizing large-scale horizontally and vertically aligned MoS₂ on different substrates such as flat SiO₂/Si, pre-patterned SiO₂ and conductive substrates (TaN) benefit various direct TMDs production. In particular, we show precise control of CVD optimization for yielding high-quality MoS₂ layers by changing growth zone configuration and the process steps. We demonstrated that the influence of configuration variability by local changes of the S to MoO₃ precursor positions in the growth zones inside the CVD reactor is a key factor that results in differently oriented MoS₂ formation. Finally, we show the layer quality and physical properties of as-grown MoS₂ by means of different characterizations: Raman spectroscopy, scanning electron microscopy (SEM), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). These experimental findings provide a strong pathway for conformally recasting AP-CVD grown MoS₂ in many different configurations (i.e., substrate variability) or motifs (i.e., vertical or planar alignment) with potential for flexible electronics, optoelectronics, memories to energy storage devices.

Selective Antisite Defect Formation in WS2 Monolayers via Reactive Growth on Dilute W-Au Alloy Substrates

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form S_W and S2_W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS₂ monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS₂ . The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS₂ monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Strain Engineering of Low-Dimensional Materials for Emerging Quantum Phenomena and Functionalities

Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron-electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.

Theoretical Study of Chemical Vapor Deposition Synthesis of Graphene and Beyond: Challenges and Perspectives

Two-dimensional (2D) materials have attracted great attention in recent years because of their unique dimensionality and related properties. Chemical vapor deposition (CVD), a crucial technique for thin-film epitaxial growth, has become the most promising method of synthesizing 2D materials. Different from traditional thin-film growth, where strong chemical bonds are involved in both thin films and substrates, the interaction in 2D materials and substrates involves the van der Waals force and is highly anisotropic, and therefore, traditional thin-film growth theories cannot be applied to 2D material CVD synthesis. During the last 15 years, extensive theoretical studies were devoted to the CVD synthesis of 2D materials. This Perspective attempts to present a theoretical framework for 2D material CVD synthesis as well as the challenges and opportunities in exploring CVD mechanisms. We hope that this Perspective can provide an in-depth understanding of 2D material CVD synthesis and can further stimulate 2D material synthesis.

Defect-Enabled Phase Programming of Transition Metal Dichalcogenide Monolayers

The ability to tune the local electronic transport properties of group VI transition metal dichalcogenide (TMD) monolayers by strain-induced structural phase transformations ("phase programming") has stimulated much interest in the potential applications of such layers as ultrathin programmable and dynamically switchable nanoelectronics components. In this manuscript, we propose a new approach toward controlling TMD monolayer phases by employing macroscopic in-plane strains to amplify heterogeneous strains arising from tailored, spatially extended defects within the monolayer. The efficacy of our proposed approach is demonstrated via numerical simulations of emerging domains localized around arrays of holes, grain boundaries, and compositional heterointerfaces. Quantitative relations between the macroscopic strains required, spatial resolution of domain patterns, and defect configurations are developed. In particular, the introduction of arrays of holes is identified as the most feasible phase programming route.

Strategies, Status, and Challenges in Wafer Scale Single Crystalline Two-Dimensional Materials Synthesis

The successful exfoliation of graphene has given a tremendous boost to research on various two-dimensional (2D) materials in the last 15 years. Different from traditional thin films, a 2D material is composed of one to a few atomic layers. While atoms within a layer are chemically bonded, interactions between layers are generally weak van der Waals (vdW) interactions. Due to their particular dimensionality, 2D materials exhibit special electronic, magnetic, mechanical, and thermal properties, not found in their 3D counterparts, and therefore they have great potential in various applications, such as 2D materials-based devices. To fully realize their large-scale practical applications, especially in devices, wafer scale single crystalline (WSSC) 2D materials are indispensable. In this review, we present a detailed overview on strategies toward the synthesis of WSSC 2D materials while highlighting the recent progress on WSSC graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenide (TMDC) synthesis. The challenges that need to be addressed in future studies have also been described. In general, there have been two distinct routes to synthesize WSSC 2D materials: (i) allowing only one nucleus on a wafer scale substrate to be formed and developed into a large single crystal and (ii) seamlessly stitching a large number of unidirectionally aligned 2D islands on a wafer scale substrate, which is generally single crystalline. Currently, the synthesis of WSSC graphene has been realized by both routes, and WSSC hBN and MoS₂ have been synthesized by route (ii). On the other hand, the growth of other WSSC 2D materials and WSSC multilayer 2D materials still remains a big challenge. In the last section, we wrap up this review by summarizing the future challenges and opportunities in the synthesis of various WSSC 2D materials.

Tip-Induced Nano-Engineering of Strain, Bandgap, and Exciton Funneling in 2D Semiconductors

The tunability of the bandgap, absorption and emission energies, photoluminescence (PL) quantum yield, exciton transport, and energy transfer in transition metal dichalcogenide (TMD) monolayers provides a new class of functions for a wide range of ultrathin photonic devices. Recent strain-engineering approaches have enabled to tune some of these properties, yet dynamic control at the nanoscale with real-time and -space characterizations remains a challenge. Here, a dynamic nano-mechanical strain-engineering of naturally-formed wrinkles in a WSe₂ monolayer, with real-time investigation of nano-spectroscopic properties is demonstrated using hyperspectral adaptive tip-enhanced PL (a-TEPL) spectroscopy. First, nanoscale wrinkles are characterized through hyperspectral a-TEPL nano-imaging with <15 nm spatial resolution, which reveals the modified nano-excitonic properties by the induced tensile strain at the wrinkle apex, for example, an increase in the quantum yield due to the exciton funneling, decrease in PL energy up to ≈10 meV, and a symmetry change in the TEPL spectra caused by the reconfigured electronic bandstructure. Then the local strain is dynamically engineered by pressing and releasing the wrinkle apex through an atomic force tip control. This nano-mechanical strain-engineering allows to tune the exciton dynamics and emission properties at the nanoscale in a reversible fashion. In addition, a systematic switching and modulation platform of the wrinkle emission is demonstrated, which provides a new strategy for robust, tunable, and ultracompact nano-optical sources in atomically thin semiconductors.

Strain-Induced Growth of Twisted Bilayers during the Coalescence of Monolayer MoS2 Crystals

Tailoring the grain boundaries (GBs) and twist angles between two-dimensional (2D) crystals are two crucial synthetic challenges to deterministically enable envisioned applications such as moiré excitons, emerging magnetism, or single-photon emission. Here, we reveal how twisted 2D bilayers can be synthesized from the collision and coalescence of two growing monolayer MoS₂ crystals during chemical vapor deposition. The twisted bilayer (TB) moiré angles are found to preserve the misorientation angle (θ) of the colliding crystals. The shapes of the TB regions are rationalized by a kink propagation model that predicts the GB formed by the coalescing crystals. Optical spectroscopy measurements reveal a θ-dependent long-range strain in crystals with stitched grain boundaries and a sharp (θ > 20°) threshold for the appearance of TBs, which relieves this strain. Reactive molecular dynamics simulations explain this strain from the continued growth of the crystals during coalescence due to the insertion of atoms at unsaturated defects along the GB, a process that self-terminates when the defects become saturated. The simulations also reproduce atomic-resolution electron microscopy observations of faceting along the GB, which is shown to arise from the growth-induced long-range strain. These facets align with the axes of the colliding crystals to provide favorable nucleation sites for second-layer growth of a TB with twist angles that preserve the misorientation angle θ. This interplay between strain generation and aligned nucleation provides a synthetic pathway for the growth of TBs with deterministic angles.

Topology of transition metal dichalcogenides: the case of the core-shell architecture

Non-planar architectures of the traditionally flat 2D materials are emerging as an intriguing paradigm to realize nascent properties within the family of transition metal dichalcogenides (TMDs). These non-planar forms encompass a diversity of curvatures, morphologies, and overall 3D architectures that exhibit unusual characteristics across the hierarchy of length-scales. Topology offers an integrated and unified approach to describe, harness, and eventually tailor non-planar architectures through both local and higher order geometry. Topological design of layered materials intrinsically invokes elements highly relevant to property manipulation in TMDs, such as the origin of strain and its accommodation by defects and interfaces, which have broad implications for improved material design. In this review, we discuss the importance and impact of geometry on the structure and properties of TMDs. We present a generalized geometric framework to classify and relate the diversity of possible non-planar TMD forms. We then examine the nature of curvature in the emerging core-shell architecture, which has attracted high interest due to its versatility and design potential. We consider the local structure of curved TMDs, including defect formation, strain, and crystal growth dynamics, and factors affecting the morphology of core-shell structures, such as synthesis conditions and substrate morphology. We conclude by discussing unique aspects of TMD architectures that can be leveraged to engineer targeted, exotic properties and detail how advanced characterization tools enable detection of these features. Varying the topology of nanomaterials has long served as a potent methodology to engineer unusual and exotic properties, and the time is ripe to apply topological design principles to TMDs to drive future nanotechnology innovation.

Strain Effects of Vertical Separation and Horizontal Sliding in Commensurate Two-Dimensional Homojunctions

Strain, as an economic yet controllable approach for structural modulation, frequently plays a vital role in the preparation and performance optimization of two-dimensional nanomaterials (TNMs). Here, utilizing first-principles simulations, the analysis of energetics shows that the biaxial stretching and compressing could facilitate the vertical separation and horizontal sliding in graphene (Gr/Gr), hexagonal boron nitride (h-BN/h-BN), and molybdenum disulfide (MoS₂/MoS₂) bilayers. The quantification of electron redistribution between layers confirmed that the shifts of interlayer charge density (ρ_inter-) and its relative values (Δρ_inter-) are responsible for the vertical separation and horizontal sliding facilitated by biaxial strain. More effortless horizontal sliding was enabled by a smoother potential energy surface because a smaller Δρ_inter- can be acquired under compression, whereas more effortless vertical separation followed a more vulnerable surface energy because a lower ρ_inter- occurs under tensile strain. The vertical and horizontal division of strain effect provides a novel idea for further understanding its pivotal roles in strain engineering of commensurate-contact TNMs, such as mechanical exfoliation and solid lubrication.

Two-Dimensional Palladium Diselenide with Strong In-Plane Optical Anisotropy and High Mobility Grown by Chemical Vapor Deposition

Two-dimensional (2D) palladium diselenide (PdSe₂ ) has strong interlayer coupling and a puckered pentagonal structure, leading to remarkable layer-dependent electronic structures and highly anisotropic in-plane optical and electronic properties. However, the lack of high-quality, 2D PdSe₂ crystals grown by bottom-up approaches limits the study of their exotic properties and practical applications. In this work, chemical vapor deposition growth of highly crystalline few-layer (≥2 layers) PdSe₂ crystals on various substrates is reported. The high quality of the PdSe₂ crystals is confirmed by low-frequency Raman spectroscopy, scanning transmission electron microscopy, and electrical characterization. In addition, strong in-plane optical anisotropy is demonstrated via polarized Raman spectroscopy and second-harmonic generation maps of the PdSe₂ flakes. A theoretical model based on kinetic Wulff construction theory and density functional theory calculations is developed and described the observed evolution of "square-like" shaped PdSe₂ crystals into rhombus due to the higher nucleation barriers for stable attachment on the (1,1) and (1,-1) edges, which results in their slower growth rates. Few-layer PdSe₂ field-effect transistors reveal tunable ambipolar charge carrier conduction with an electron mobility up to ≈294 cm² V^-1 s^-1 , which is comparable to that of exfoliated PdSe₂ , indicating the promise of this anisotropic 2D material for electronics.