Anisotropic Two-dimensional Materials Research Articles

Enhanced Photodetection Performance of InBiSe3/ReS2 Polarization-Sensitive Heterostructure Photodetectors.

Individual anisotropic two-dimensional (2D) materials have been widely applied for developing polarization-sensitive photodetectors, but they often suffer from limitations in photoresponsivity, detection range, etc. To overcome these challenges, van der Waals (vdW) heterostructures created by stacking different 2D materials provide a promising solution to enhance the performance of the photoelectronic device. In this work, a novel polarization-sensitive photodetector is developed by leveraging a heterojunction formed by InBiSe3 and anisotropic ReS2 nanoflakes. The InBiSe3/ReS2 vdW heterostructure devices exhibit excellent photodetection performance with a high photoresponsivity (R)of 7.68 AW-1 and a specific detectivity (D*)up to 1.26×1011 Jones as well as an external quantum efficiency (EQE) of 1790% under 532 nm laser irradiation. Additionally, benefiting from the broadband light absorption of InBiSe3 crystals together with the pronounced anisotropic electronic and optical characteristics of ReS2 flakes, the devices demonstrate a broad spectral response range from 402 to 1006 nm with a distinct polarization sensitivity of 1.24. Moreover, the devices exhibit extraordinary optical communication and high contrast polarimetric imaging capacity. This work demonstrates the enhanced photodetection performance with the InBiSe3/ReS2 vdW heterostructures operating in a photoconductive mode and illustrates promising application of these heterostructures in integrated optoelectronic systems.

High performance self-driven broadband photodetector for polarized imaging based on novel ZrS3/ReSe2 van der Waals heterojunction

The distinctive characteristics of anisotropic two-dimensional (2D) materials, including in-plane anisotropy of optical absorption and carrier mobility, render them exceptionally suitable for application in the field of polarization detection and as a novel platform for the polarization imaging. Meanwhile, the consolidation of diverse functionalities within a single photodetector is highly anticipated to meet the demands of some special scenarios. Herein, a novel ZrS3/ReSe2 van der Waals (vdWs) heterostructure device was successfully constructed to realize polarization-sensitive, self-powered, and broadband photodetection and imaging. Owing to the built-in electric field of the type-II band alignment within the heterojunction, the device achieves a self-powered photoresponse ranging from 300 to 980 nm, an ultralow dark currentt ∼1 pA, and a commendable rise/decay time of 0.35/0.28 ms. Additionally, it has been demonstrated that the self-driven photodetector possesses a polarization-sensitivity with a notable anisotropic ratio about 2.02 (1.98) under 490 nm (980 nm) light illumination with zero bias, coupled with an excellent repeatability and stability. Furthermore, we also demonstrate the polarization imaging capabilities of the device in visible and near-infrared spectrum, realizing a contrast-enhanced degree of linear polarization imaging. This work paves a new platform to develop heterojunction photodetectors for high performance polarization-sensitive photodetection and next-generation polarized imaging.

Sub-picosecond, strain-tunable, polarization-selective optical switching via anisotropic exciton dynamics in quasi-1D ZrSe3

In cutting-edge optical technologies, polarization is a key for encoding and transmitting vast information, highlighting the importance of selectively switching and modulating polarized light. Recently, anisotropic two-dimensional materials have emerged for ultrafast switching of polarization-multiplexed optical signals, but face challenges with low polarization ratios and limited spectral ranges. Here, we apply strain to quasi-one-dimensional layered ZrSe3 to enhance polarization selectivity and tune operational energies in ultrafast all-optical switching. Initially, transient absorption on unstrained ZrSe3 reveals a sub-picosecond switching response in polarization along a specific crystal axis, attributed to shifting-recovery dynamics of an anisotropic exciton. However, its polarization selectivity is weakened by a slow non-excitonic response in the perpendicular polarization. To overcome this limitation, we apply strain to ZrSe3 by bending its flexible substrate. The compressive strain spectrally decouples the excitonic and non-excitonic components, doubling the polarization selectivity of the sub-picosecond switching and tripling it compared to that in the tensile-strained ZrSe3. It also effectively tunes the switching energy at a shift rate of ~93 meV %-1. This strain-tunable switching is repeatable, reversible, and robustly maintains the sub-picosecond operation. First-principles calculations reveal that the strain control is enabled by momentum- and band-dependent modulations of the electronic band structure, causing opposite shifts in the excitonic and non-excitonic transitions. Our findings offer a novel approach for high-performance, wavelength-tunable, polarization-selective ultrafast optical switching.

Anisotropic van der Waals Tellurene-Based Multifunctional, Polarization-Sensitive, In-Line Optical Device.

Polarization plays a paramount role in scaling the optical network capacity. Anisotropic two-dimensional (2D) materials offer opportunities to exploit optical polarization-sensitive responses in various photonic and optoelectronic applications. However, the exploration of optical anisotropy in fiber in-line devices, critical for ultrafast pulse generation and modulation, remains limited. In this study, we present a fiber-integrated device based on a single-crystalline tellurene nanosheet. Benefiting from the chiral-chain crystal lattice and distinct optical dichroism of tellurene, multifunctional optical devices possessing diverse excellent properties can be achieved. By inserting the in-line device into a 1.5 μm fiber laser cavity, we generated both linearly polarized and dual-wavelength mode-locking pulses with a degree of polarization of 98% and exceptional long-term stability. Through a twisted configuration of two tellurene nanosheets, we realized an all-optical switching operation with a fast response. The multifunctional device also serves as a broadband photodetector. Notably, bipolar polarization encoding communication at 1550 nm can be achieved without any external voltage. The device's multifunctionality and stability in ambient environments established a promising prototype for integrating polarization as an additional physical dimension in fiber optical networks, encompassing diverse applications in light generation, modulation, and detection.

Photodetectors Based on ZrS3/MoS2 Heterostructures.

High-performance photodetectors with the detection capability of linearly polarized light have broad applications in both military and civilian fields. Quasi-one-dimensional ZrS3 as an emerging anisotropic two-dimensional material has come under the spotlight owing to its intriguing properties. However, the performance of the ZrS3 photodetector is seriously restricted by its low responsivity. Herein, a novel high-performance photodetector based on the van der Waals ZrS3/MoS2 heterostructure is proposed. Attributed to the charge trapping-assisted photogating effect, interlayer carrier transitions, and fast spatial separation of the photogenerated electron-hole pairs, the device displays superior photoresponse characteristics ranging from the ultraviolet to the visible spectrum in terms of high responsivity up to 212 A/W, an extraordinary external quantum efficiency of 8.5 × 104%, and a prompt rise/decay time of 0.19/0.38 ms. In addition, owing to the profound birefringence and dichroism effects in ZrS3 together with strong light-matter interactions in the heterostructure, profound linear-polarization sensitivity is demonstrated with a dichroic ratio of about 2.8. Overall, this photodetector not only is integrated with the excellent properties of ZrS3 and monolayer MoS2 but also further enhances the advantages through interlayer couplings, which demonstrate the strong potential of the ZrS3-based devices for high-performance, ultrafast, and polarization-sensitive photodetection.

Non-thermal phonon dynamics and a quenched exciton condensate probed by surface-sensitive electron diffraction

Interactions among and between electrons and phonons steer the energy flow in photo-excited materials and govern the emergence of correlated phases. The strength of electron–phonon interactions, decay channels of strongly coupled modes and the evolution of three-dimensional order are revealed by electron or X-ray pulses tracking non-equilibrium structural dynamics. Despite such capabilities, the growing relevance of inherently anisotropic two-dimensional materials and functional heterostructures still calls for techniques with monolayer sensitivity and, specifically, access to out-of-plane phonon polarizations. Here, we resolve non-equilibrium phonon dynamics and quantify the excitonic contribution to the structural order parameter in 1T-TiSe2. To this end, we introduce ultrafast low-energy electron diffuse scattering and trace strongly momentum- and fluence-dependent phonon populations. Mediated by phonon–phonon scattering, a few-picosecond build-up near the zone boundary precedes a far slower generation of zone-centre acoustic modes. These weakly coupled phonons are shown to substantially delay overall equilibration in layered materials. Moreover, we record the surface structural response to a quench of the material’s widely investigated exciton condensate, identifying an approximate 30:70 ratio of excitonic versus Peierls contributions to the total lattice distortion in the charge density wave phase. The surface-sensitive approach complements the ultrafast structural toolbox and may further elucidate the impact of phonon scattering in numerous other phenomena within two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.

Anisotropic Fracture of Two-Dimensional Ta2NiSe5.

Anisotropic two-dimensional materials present a diverse range of physical characteristics, making them well-suited for applications in photonics and optoelectronics. While mechanical properties play a crucial role in determining the reliability and efficacy of 2D material-based devices, the fracture behavior of anisotropic 2D crystals remains relatively unexplored. Toward this end, we herein present the first measurement of the anisotropic fracture toughness of 2D Ta2NiSe5 by microelectromechanical system-based tensile tests. Our findings reveal a significant in-plane anisotropic ratio (∼3.0), accounting for crystal orientation-dependent crack paths. As the thickness increases, we observe an intriguing intraplanar-to-interplanar transition of fracture along the a-axis, manifesting as stepwise crack features attributed to interlayer slippage. In contrast, ruptures along the c-axis surprisingly exhibit persistent straightness and smoothness regardless of thickness, owing to the robust interlayer shear resistance. Our work affords a promising avenue for the construction of future electronics based on nanoribbons with atomically sharp edges.

Experimental Observation of Highly Anisotropic Elastic Properties of Two-Dimensional Black Arsenic.

Anisotropic two-dimensional layered materials with low-symmetry lattices have attracted increasing attention due to their unique orientation-dependent mechanical properties. Black arsenic (b-As), with the puckered structure, exhibits extreme in-plane anisotropy in optical, electrical, and thermal properties. However, experimental research on mechanical properties of b-As is very rare, although theoretical calculations predicted the exotic elastic properties of b-As, such as the anisotropic Young's modulus and negative Poisson's ratio. Herein, experimental observations on highly anisotropic elastic properties of b-As were demonstrated using our developed in situ tensile straining setup based on the effective microelectromechanical system. The cyclic and repeatable load-displacement curves proved that Young's modulus along the zigzag direction was ∼1.6 times greater than that along the armchair direction, while the anisotropic ratio of ultimate strain reached ∼2.5, attributed to the hinge structure in the armchair direction. This study could provide significant insights into the design of novel anisotropic materials and explore their potential applications in nanomechanics and nanodevices.

Anisotropic energy transfer near multi-layer black phosphorus

Energy transfer (ET) between quantum emitters is a key process for many scientific domains and technological applications, and can be influenced by strategic placement of appropriate materials in the vicinity. However, all explored conventional isotropic materials lacks directional control over this process. Here, we show that multilayered black phosphorus (bP), a novel anisotropic two-dimensional material, can simultaneously dramatically boost and directionally control ET rates in the near-field regime. We find that bP exhibits a critical thickness above which the ET rates increase by several orders of magnitude compared to vacuum. Moreover, we demonstrate that bP can manipulate the ET in specific in-plane directions due to its strong in-plane anisotropy. Our results build the framework and provide fundamental insights into the mechanisms of ET near anisotropic materials, and open up new possibilities for designing and optimizing ET-based devices, systems and applications.

Tunable Optical Activity in Twisted Anisotropic Two-Dimensional Materials.

Twisted van der Waals structures exhibit a variety of unusual electrical and optical phenomena and could provide a powerful means for designing nanodevices with tunable chiral properties. However, programming intrinsic chiral properties of the film on the atomic scale remains a great challenge due to the limitations of fabrication and measurement techniques. Here, we report a highly tunable large optical activity of twisted anisotropic two-dimensional (2D) materials, including black phosphorus (BP), ReS2, PdSe2, and α-MoO3, by varying the twist angle between the stacked layers. The chirality can be deliberately tailored through the engineering of the symmetry, band structure, and anisotropy of 2D materials, demonstrating the high tunability of the chirality. The results show the highest thickness-normalized ellipticity value (13.8 deg μm-1, twisted ReS2) and ellipticity value (1581 mdeg, twisted BP) among the systems based on 2D materials. It is also shown that the chiroptical response exists in an extremely large spectral range from the visible to the infrared. Furthermore, the twisted ReS2 enabled spin-selective control of the information transformation. These results show that highly controllable chirality in twisted 2D anisotropic materials has considerable potential in on-chip polarization optics, nano-optoelectronics, and biology.

Polarized vdW Schottky photodiode with high fill-factor and polarization ratio

Against the background of a variety of polarized optical applications, a great number of polarization-sensitive photodetectors based on anisotropic two-dimensional (2D) materials and their heterojunctions have been proposed. However, the low polarization ratio (PR), describing the response sensitivity on the polarization angle of the incident light, is still demanding to be solved. In this work, we demonstrate a van der Waals (vdW) Schottky photodiode comprising 2D semiconductor InSe and semimetal 1T′-MoTe2 for the linearly polarized photodetection. The device exhibits obvious photovoltaic characters with a maximum open circuit voltage (Voc) of 0.51 V and fill factor (FF) up to 0.78, which is the highest value in the vdW heterojunction-based photovoltaic device. Due to the anisotropic in-plane structure of 1T′-MoTe2, our photodiode also shows a strong polarization-sensitive photodetection with a PR value up to ∼22, enabling the practical applications in angle-dependent photodetector and image sensors. This work reports a vdW Schottky heterojunction for polarized photodiode with high FF and polarization ratio, toward the polarization-dependent photovoltaic applications.

Universal visualization of crystalline orientation for black phosphorus by angle-resolved polarized photoacoustic microscopy.

Anisotropic two-dimensional (2D) materials, such as black phosphorus (BP), normally possess unique directional in-plane electrical, optical, and thermal properties that are highly correlated with their crystalline orientations. Nondestructive visualization of their crystalline orientation is an indispensable premise for the 2D materials to harness their distinctive strengths in optoelectronic and thermoelectric applications. Here, by photoacoustically recording the anisotropic optical absorption variation under linearly polarized laser beams, an angle-resolved polarized photoacoustic microscopy (AnR-PPAM) is developed, capable of non-invasively determining and visualizing BP's crystalline orientation. We theoretically deduced the physical relationship between the crystalline orientation and polarized photoacoustic (PA) signals, and experimentally proved the ability of AnR-PPAM to universally visualize BP's crystalline orientation regardless of its thickness, substrate, and encapsulation layer. This method provides a new, to the best of our knowledge, strategy for crystalline orientation recognition of 2D materials with flexible measurement conditions, prefiguring important potential for the applications of anisotropic 2D materials.

Impedance modeling for excluding contact resistance from cross-plane electronic conductivity measurement of anisotropic two-dimensional Ti3C2Tx MXenes

We propose impedance spectroscopy modeling as a convenient tool for estimating the cross-plane conductivity of anisotropic two-dimensional materials by excluding contact resistance with the underlying metal electrodes. We showcase this idea on two types of titanium carbide Ti3C2Tx MXenes, wherein Tx represents surface functional termination. These MXenes are synthesized from the same parent MAX phase material (Ti3AlC2) by different synthesis protocols and are studied in their plain as-synthesized states as well as after short annealing. The cross-plane conductivity of MXenes estimated by our approach appears to be up to an order of magnitude higher than is estimated by a conventional one. We validate our impedance modeling approach by comparing the specific contact resistance results derived from it to a direct estimation by transmission line measurement. We also deduce which functional group has a dominant footprint on the conductivity in each studied MXene by performing numerical simulations on cross-plane conductivity as well as on electronic band structures. The novel equivalent circuit developed herein may be extended to other anisotropic thin films to aid their application in various electronic devices.

Low symmetric sub-wavelength array enhanced lensless polarization-sensitivity photodetector of germanium selenium

Polarization-sensitive photodetectors, with the ability of identifying the texture-, stress-, and roughness-induced light polarization state variation, displace unique advantages in the fields of national security, medical diagnosis, and aerospace. The utilization of in-plane anisotropic two-dimensional (2D) materials has led the polarization photodetector into a polarizer-free regime, and facilitated the miniaturization of optoelectronic device integration. However, the insufficient polarization ratio (usually less than 10) restricts the detection resolution of polarized signals. Here, we designed a sub-wavelength array (SWA) structure of 2D germanium selenium (GeSe) to further improve its anisotropic sensitivity, which boosts the polarized photocurrent ratio from 1.6 to 18. This enhancement comes from the combination of nano-scale arrays with atomic-scale lattice arrangement at the low-symmetric direction, while the polarization-sensitive photoresponse along the high-symmetric direction is strongly suppressed due to the SWA-caused depolarization effect. Our mechanism study revealed that the SWA can improve the asymmetry of charge distribution, attenuate the matrix element in zigzag direction, and the localized surface plasma, which elevates the photo absorption and photoelectric transition probability along the armchair direction, therefore accounts for the enhanced polarization sensitivity. In addition, the photodetector based on GeSe SWA exhibited a broad power range of 40 dB at a near-infrared wavelength of 808 nm and the ability of weak-light detection under 0.1 LUX of white light (two orders of magnitude smaller than pristine 2D GeSe). This work provides a feasible guideline to improve the polarization sensitivity of 2D materials, and will greatly benefit the development of polarized imaging sensors.

Strong interlayer coupling and unusual antisite defect-mediated p-type conductivity in GePx (x = 1, 2).

As an emerging candidate for anisotropic two-dimensional materials, the group IV-V family (e.g. GeP, GeP2) has appealing applications in photoelectronics. However, their intrinsic point defect properties, which largely determine the device performance and optimization, are still poorly explored. In our study, through density functional theory (DFT) calculations, antisite defects were affirmed to be dominant with the lowest formation energies in 2D GePx semiconductors because of the similar atomic size and electronegativity of elemental components, which is in contrast to previous calculations and experimental speculation. These antisite defects could introduce relatively shallow states within the bandgap in bulk cases. The transition energy levels and electronic structures of defects reveal that GeP and PGe antisites act as dominant acceptors and donors, respectively. Strong interlayer coupling between anions results in a significant upshift of the valence band maximum (VBM) and shallower acceptor behaviors of GePx. Together with the dominant GeP antisite defect, the large upshift of the VBM in GeP leads to a remarkable transition of conductivity from intrinsic in the monolayer to p-type in the bulk. Such a synergistic effect in GeP2 is rather weak due to the strong inherent intralayer coupling of anions. Our research provides deep insights into the strong anion coupling effects on the electronic structures and defect properties of GeP and GeP2, which sheds light on defect engineering and electronic applications of GePx based semiconductors.

Strong anisotropy of Sc2X2Se2 (X = Cl, Br, I) monolayers contributes to high thermoelectric performance.

As a novel type of anisotropic two-dimensional material, extensive attention has been paid to the thermoelectric (TE) properties of FeOCl-type monolayers, such as Al2X2Se2 (X = Cl, Br, I), Sc2I2S2, and Ir2Cl2O2. Recently, theoretical works based on first-principles calculations have been powerful driving forces in field of TE research. In this work, we perform an investigation into the TE properties of Sc2X2Se2 (X = Cl, Br, I) monolayers based on density functional theory (DFT). A study on the stability, including AIMD simulation and phonon calculation, shows the stable structure of Sc2Cl2Se2, Sc2Br2Se2, and Sc2I2Se2 monolayers. Additionally, the electronic and thermal transport properties of Sc2X2Se2 monolayers are anisotropic along the x and y directions. Moreover, the combination of excellent Seebeck coefficient and ultralow lattice thermal conductivity contributes to outstanding ZT values, and the ZT values follow the order: Sc2I2Se2 > Sc2Br2Se2 > Sc2Cl2Se2. At 300 K, we obtained maximum ZT of 0.34, 0.77, and 1.97 for Sc2Cl2Se2, Sc2Br2Se2, and Sc2I2Se2, respectively, by n-type doping in the x direction. These results demonstrate that monolayer Sc2X2Se2 (X = Cl, Br, I) materials are promising thermoelectric materials, Sc2I2Se2 has more desirable properties along the x direction, and n-type doping can significantly enhance the ZT values. Our work lays a foundation for exploring the TE transport properties of FeOCl-type monolayers.

Disorder-driven transition to tubular phase in anisotropic two-dimensional materials

We develop a theory of anomalous elasticity in disordered two-dimensional flexible materials with orthorhombic crystal symmetry. Similar to the clean case, we predict existence of infinitely many flat phases with anisotropic bending rigidity and Young's modulus showing power-law scaling with momentum controlled by a single universal exponent the very same as in the clean isotropic case. With increase of temperature or disorder these flat phases undergo crumpling transition. Remarkably, in contrast to the isotropic materials where crumpling occurs in all spatial directions simultaneously, the anisotropic materials crumple into tubular phase. In distinction to clean case in which crumpling transition happens at unphysically high temperatures, a disorder-induced tubular crumpled phase can exist even at room-temperature conditions. Our results are applied to anisotropic atomic single layers doped by adatoms or disordered by heavy ions bombarding.

Anomalous narrow-band optical anisotropy of MoO2 crystal in the visible regime

The emergence of anisotropic two-dimensional (2D) materials provides a platform for the cutting-edge nano- and optoelectronic devices. Exploring low-dimensional materials and revealing their anisotropic behavior are crucial for designing angle-dependent nanodevices. The metallicity of molybdenum dioxide (MoO2) crystal differentiates it from the most commonly studied semi-conductive anisotropic 2D materials. However, the studies on its optical anisotropy are still lacking. Here, two most commonly obtained shapes of rhombic and hexagonal MoO2 were synthesized by one step method of chemical vapor deposition. The rhombic and hexagonal MoO2 display a slight frequency shift of 1–5 cm−1 depending on the variation modes, but the Raman modes at 366 cm−1 remain unaltered. Using a combination of differential reflectance spectroscopy and reflectance difference spectroscopy, we revealed the unusual narrow-band optical anisotropies of rhombic and hexagonal MoO2 crystals in the visible wavelength region due to its unique metallic properties. Furthermore, it is found that the center wavelengths of the narrow-band optical anisotropy of the MoO2 crystal can be effectively adjusted by coherent optical interference. Our results present an interesting anisotropic metallic 2D candidate and an effective cavity-based approach to regulate the center wavelengths of as-obtained narrow-band optical anisotropy, which is highly beneficial for the wavelength-selected devices.

Polarization-sensitive infrared photodetectors based on van der Waals heterojunction with the unilateral depletion region

In recent years, polarization infrared detection technology has made great progress and has been successfully applied in various fields such as national defense, astronomy, and civil applications. However, so far, the integration of micro polarizers and infrared photodetectors is still a big challenge since metal microstructures are severely limited in their size. As anisotropic two-dimensional (2D) materials rapidly appear, this distinctive polarization characteristic regardless of the size provides new freedom to polarization infrared photodetectors. In this paper, a van der Walls heterojunction with the unilateral depletion region was constructed to obtain polarization-sensitive infrared photodetection, in which black phosphorus (BP) serves as a natural polarizer. BP/PtS2 infrared photodetectors possess a fast response speed (the rise time is 56 μs and the fall time is 64 μs). At the same time, these photovoltaic detectors achieve a high detectivity of 4.47 × 1010 cm Hz1/2 W−1 at 830 nm. Using the anisotropic structure of BP, the photovoltaic polarization detector was realized, and its polarization extinction ratio at 830 nm was 2.66 (3.18 @ 1310 nm and 2.63 @ 1550 nm). This work discovers the potential value of anisotropic 2D materials in infrared polarization-sensitive photodetection and provides a possibility for a new infrared polarization detection system.

Strong Anisotropic Optical Properties by Rolling up MoS2 Nanoflake.

Although anisotropic two-dimensional materials have attracted great scientific interest, the anisotropy of those materials is limited to particular crystallographic directions. Herein, with dimension confining, MoS2 nanoscrolls are successfully fabricated by a rolling-up process after dropping an ethanol-water solution on a chemical vapor deposition-grown MoS2 monolayer. The anisotropic vibrational and optical properties are systematically studied by angle-resolved polarized spectroscopy, including Raman, photoluminescence, and reflection measurements. Upon comparing the photoluminescence results between MoS2 nanoscrolls and nanosheets, an obvious PL quenching phenomenon is observed, indicating the efficient separation of photon-induced carriers. Moreover, the time-resolved PL test identifying the lifetime of the carriers is decreased to 303 ps in the nanoscrolls, indicating a higher carrier-transfer efficiency. In summary, our work demonstrates the strong anisotropic optical properties of MoS2 nanorolls, showing the nanoscrolls are a promising candidate for the fabrication of multifunctional devices.