Polarization-Resolved Raman Study of Bulk-like and Davydov-Induced Vibrational Modes of Exfoliated Black Phosphorus

Fabrication of Ternary Titanium Dioxide/Polypyrrole/Phosphorene Nanocomposite for Supercapacitor Electrode Applications

In this paper, we report a titanium dioxide/polypyrrole/phosphorene (TiO2/PPy/phosphorene) nanocomposite as an active material for supercapacitor electrodes. Black phosphorus (BP) was fabricated by ball milling to induce a phase transition from red phosphorus, and urea-functionalized phosphorene (urea-FP) was obtained by urea-assisted ball milling of BP, followed by sonication. TiO2/PPy/phosphorene nanocomposites can be prepared via chemical oxidative polymerization, which has the advantage of mass production for a one-pot synthesis. The specific capacitance of the ternary nanocomposite was 502.6 F g-1, which was higher than those of the phosphorene/PPy (286.25 F g-1) and TiO2/PPy (150 F g-1) nanocomposites. The PPy fully wrapped around the urea-FP substrate provides an electron transport pathway, resulting in the enhanced electrical conductivity of phosphorene. Furthermore, the assistance of anatase TiO2 nanoparticles enhanced the structural stability and also improved the specific capacitance of the phosphorene. To the best of our knowledge, this is the first report on the potential of phosphorene hybridized with conducting polymers and metal oxides for practical supercapacitor applications.

Optical Effect Modulation in Polarized Raman Spectroscopy of Transparent Layered α-MoO3

Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO₃ as an excellent platform. It is found that there are several anomalous ARPRS responses in α-MoO₃ that cannot be reproduced by the real Raman tensor widely used in non-absorbing materials; however, they can be well explained by considering the birefringence-induced Raman selection rules. Moreover, the systematic thickness-dependent study indicates that birefringence modulates the ARPRS response to render an interference pattern; however, the amplitude of modulation is considerably lower than that by linear dichroism as occurred in black phosphorous. This weak modulation brings convenience to the crystal orientation determination of transparent ALMs. Combining the atomic vibrational pattern and bond polarizability model, the intrinsic ARPRS response of α-MoO₃ is analyzed, giving the physical origins of the Raman anisotropy. This study employs α-MoO₃ as an example, although it is generally applicable to all transparent birefringent ALMs.

Visible Out-of-plane Polarized Luminescence and Electronic Resonance in Black Phosphorus

Black phosphorus (BP) is unique among layered materials because of its homonuclear lattice and strong structural anisotropy. While recent investigations on few-layer BP have extensively explored the in-plane (a, c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE), and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. An unexpected b-polarized luminescence emission is detected in the visible, far above the fundamental gap. PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman with the enhancement of the A_g phonon modes scattering efficiency. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.

Strong In-Plane Anisotropic SiP2 as a IV-V 2D Semiconductor for Polarized Photodetection

In-plane anisotropic two-dimensional (2D) materials, emerging as an intriguing type of 2D family, provide an ideal platform for designing and fabrication of optoelectronic devices. Exploring air-stable and strong in-plane anisotropic 2D materials is still challenging and promising for polarized photodetection. Herein, SiP₂, a 2D IV-V semiconductor, is successfully prepared and introduced into an in-plane anisotropic 2D family. The basic characterizations combined with theoretical calculations reveal 2D SiP₂ to exhibit an intrinsically low-symmetry structure, the in-plane anisotropy of phonon vibrations, and an anisotropically dispersed band structure. Moreover, the photodetector based on 2D SiP₂ exhibits high performance with a high detectivity of 10¹² Jones, a large light on/off ratio of 10³, a low dark current of 10^-13 A, and a fast response speed of 3 ms. Furthermore, 2D SiP₂ demonstrates a high anisotropic photodetection with an anisotropic ratio up to 2. In addition, the polarization-sensitive photodetector presents a dichroic ratio of 1.6 due to the intrinsic linear dichroism. These good characteristics make 2D SiP₂ a promising candidate as an in-plane anisotropic semiconductor for high-sensitivity and polarized optoelectronic applications.

Polarized Raman spectroscopy in low-symmetry 2D materials: angle-resolved experiments and complex number tensor elements

In this perspective review, we discuss the power of polarized Raman spectroscopy to study optically anisotropic 2D materials, belonging to the orthorhombic, monoclinic and triclinic crystal families. We start by showing that the polarization dependence of the peak intensities is described by the Raman tensor that is unique for each phonon mode, and then we discuss how to determine the tensor elements from the angle-resolved polarized measurements by analyzing the intensities in both the parallel- and cross-polarized scattering configurations. We present specific examples of orthorhombic black phosphorus and monoclinic 1T'-MoTe₂, where the Raman tensors have null elements and their principal axes coincide with the crystallographic ones, followed by a discussion on the results for triclinic ReS₂ and ReSe₂, where the axes of the Raman tensor do not coincide with the crystallographic axes and all elements are non-zero. We show that the Raman tensor elements are, in general, given by complex numbers and that phase differences between tensor elements are needed to describe the experimental results. We discuss the dependence of the Raman tensors on the excitation laser energy and thickness of the sample within the framework of the quantum model for the Raman intensities. We show that the wavevector dependence of the electron-phonon interaction is essential for explaining the distinct Raman tensor for each phonon mode. Finally, we close with our concluding remarks and perspectives to be explored using angle-resolved polarized Raman spectroscopy in optically anisotropic 2D materials.

Unambiguous determination of crystal orientation in black phosphorus by angle-resolved polarized Raman spectroscopy

Angle-resolved polarized Raman spectroscopy (ARPRS) is widely used to determine the crystal orientations of anisotropic layered materials (ALMs), which is an essential step to study all of their anisotropic properties. However, the understanding of the ARPRS response of black phosphorous (BP) as a most widely studied ALM is still unsatisfactory. Here, we clarify two key controversies about the physical origin of the intricate ARPRS response and the determination of crystal orientations in BP. Through systematic ARPRS measurements, we show that the degree of anisotropy of the response evolves gradually and periodically with the BP thickness, eventually leading to the intricate response. Meanwhile, we find that using the Raman peak intensity ratio of the two A_g phonon modes, the crystal orientations of BP can be unambiguously distinguished via a concise inequality . Comprehensive analysis and first-principles calculations reveal that the external anisotropic interference effect and the intrinsic electron-phonon coupling are responsible for the observations.

Resonance-Enhanced Excitation of Interlayer Vibrations in Atomically Thin Black Phosphorus

The strength of interlayer coupling critically affects the physical properties of 2D materials such as black phosphorus (BP), where the electronic structure depends sensitively on layer thickness. Rigid-layer vibrations reflect directly the interlayer coupling strength in 2D van der Waals solids, but measurement of these characteristic frequencies is made difficult by sample instability and small Raman scattering cross sections in atomically thin elemental crystals. Here, we overcome these challenges in BP by performing resonance-enhanced low-frequency Raman scattering under an argon-protective environment. Interlayer breathing modes for atomically thin BP were previously unobservable under conventional (nonresonant) excitation but became strongly enhanced when the excitation energy matched the sub-band electronic transitions of few-layer BP, down to bilayer thicknesses. The measured out-of-plane interlayer force constant was found to be 10.1 × 10¹⁹ N/m³ in BP, which is comparable to graphene. Accurate characterization of the interlayer coupling strength lays the foundation for future exploration of BP twisted structures and heterostructures.

Anisotropic Generation and Detection of Coherent Ag Phonons in Black Phosphorus

Black phosphorus (BP) has attracted great attention due to its layer-tuned direct bandgap, in-plane anisotropic properties, and novel optoelectronic applications. In this work, the anisotropic characteristics of BP crystal in terms of the Raman tensor and birefringence are studied by investigating polarization dependence in both the generation and detection of A_g mode coherent phonons. While the generated coherent phonons exhibit the typical linear dichroism of BP crystal, the detection process is found here to be influenced by anisotropic multiple thin film interference, showing wavelength and sample thickness sensitive behaviors. We additionally find that the A_g¹ and A_g² optical phonons decay into lower frequency acoustic phonons through the temperature-dependent anharmonic process.

In-Plane Optical and Electrical Anisotropy of 2D Black Arsenic

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

Synthesis of highly crystalline black phosphorus thin films on GaN

Black phosphorus (BP) has recently garnered significant attention due to its specific physical properties. At present, high-quality few-layer and thin-film BP is obtained principally by mechanical exfoliation, restricting its device applications in the future. Here, a facile, direct synthesis of highly crystalline thin-film BP on GaN(001) substrates is achieved by conversion of red phosphorus to BP under atmospheric pressure. The synthesized ≈100-500 nm thick BP thin films with a length ranging from 4 to 15 μm can maintain long-term stability with no sign of oxidation after 5 months of exposure to ambient conditions, as indicated by energy dispersive spectroscopy (EDS). Cross-sectional spherical aberration correction transmission electron microscopy (STEM) analysis of the entire thin-film BP sample did not show any aggregation nucleation through the selected sample. The interface of the BP/GaN heterostructure is atomically sharp, which is very critical for high-performance device fabrication using a direct step in the future. And it is worth noting that there are fluctuations of a few atoms on the surface of GaN. Moreover, using first-principles approaches, here we establish a novel kinetic pathway for fabricating thin-film BP via epitaxial growth. The step of fluctuations with a few atoms on the GaN surface are first preferentially covered by P adatoms, then P adatoms cover the remaining part. Once formed, such a structure of thin-film BP is stable, as tested using EDS and STEM. Combining the results of the experiment and simulation, it can be revealed that the P adatom on undulatory GaN is sufficiently mobile and the undulating surface of GaN plays a major role in forming high-quality thin-films of BP. The preferentially covered nearby step growth mechanism discovered here may enable the mass production of high-quality thin-film BP, and could also be instrumental in achieving the epitaxial growth of thin-film BP on GaN and other 2D materials.

Polarized Raman spectroscopy for studying two-dimensional materials

Raman spectroscopy has been established as one of the core experimental tools to study two-dimensional materials (2DMs) including graphene, black phosphorus, transitional metal chalcogenides, and other layered materials. If the polarization of the incident photons and the scattered photons are carefully controlled, the selection rules for the Raman scattering from phonon modes allow accurate mode assignments, which is not always possible in Raman scattering measurements using unpolarized light. Furthermore, polarized Raman spectroscopy can be used to determine the crystallographic orientation of isotropic 2DMs with in-plane strain or anisotropic 2DMs. This review explains the basics of polarized Raman spectroscopy, especially in the context of 2DMs research, and survey some of the most important applications of polarized Raman spectroscopy in isotropic and anisotropic 2DMs studies.

Phonon Anharmonicity in Few-Layer Black Phosphorus

We report a temperature-dependent Raman spectroscopy study of few-layer black phosphorus (BP) with varied incident polarization and sample thickness. The Raman-active modes A_g¹, B_2g, and A_g² exhibit a frequency downshift, while their line width tends to increase with increasing temperature. To understand the details of these phenomena, we perform first-principles density functional theory calculations on freestanding monolayer BP. The effect of thermal expansion is included by constraining the temperature-dependent lattice constant. The study of the temperature-induced shift of the phonon frequencies is carried out using ab initio molecular dynamics simulations. The normal-mode frequencies are calculated by identifying the peak positions from the magnitude of the Fourier transform of the total velocity autocorrelation. Anharmonicity induces a frequency shift for each individual mode, and the three- and four-phonon process coefficients are extracted. These results are compared with those obtained from many-body perturbation theory, giving access to phonon lifetimes and lattice thermal conductivity. We establish that the frequency downshift is primarily due to phonon-phonon scattering while thermal expansion only contributes indirectly by renormalizing the phonon-phonon scattering. Overall, the theoretical results are in excellent agreement with experiment, thus showing that controlling phonon scattering in BP could result in better thermoelectric devices or transistors that dissipate heat more effectively when confined to the nanoscale.

Anisotropic Thermal Boundary Resistance across 2D Black Phosphorus: Experiment and Atomistic Modeling of Interfacial Energy Transport

Interfacial thermal boundary resistance (TBR) plays a critical role in near-junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal-orientation-dependent interfacial transport study. The measurements show that the metal-semiconductor TBR of the cross-plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction-oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature-dependent TBR using density functional theory (DFT)-derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR-structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces.

Strain-tunable van der Waals interactions in few-layer black phosphorus

Interlayer interactions in 2D materials, also known as van der Waals (vdWs) interactions, play a critical role in the physical properties of layered materials. It is fascinating to manipulate the vdWs interaction, and hence to "redefine" the material properties. Here, we demonstrate that in-plane biaxial strain can effectively tune the vdWs interaction of few-layer black phosphorus with thickness of 2-10 layers, using infrared spectroscopy. Surprisingly, our results reveal that in-plane tensile strain efficiently weakens the interlayer coupling, even though the sample shrinks in the vertical direction due to the Poisson effect, in sharp contrast to one's intuition. Moreover, density functional theory (DFT) calculations further confirm our observations and indicate a dominant role of the puckered lattice structure. Our study highlights the important role played by vdWs interactions in 2D materials during external physical perturbations.

The impact of hexagonal boron nitride encapsulation on the structural and vibrational properties of few layer black phosphorus

The encapsulation of two-dimensional layered materials such as black phosphorus is of paramount importance for their stability in air. However, the encapsulation poses several questions, namely, how it affects, via the weak van der Waals forces, the properties of the black phosphorus and whether these properties can be tuned on demand. Prompted by these questions, we have investigated the impact of hexagonal boron nitride encapsulation on the structural and vibrational properties of few layer black phosphorus, using a first-principles method in the framework of density functional theory. We demonstrate that the encapsulation with hexagonal boron nitride imposes biaxial strain on the black phosphorus material, flattening its puckered structure, by decreasing the thickness of the layers via the increase of the puckered angle and the intra-layer P-P bonds. This work exemplifies the evolution of structural parameters in layered materials after the encapsulation process. We find that after encapsulation, phosphorene (single layer black phosphorous) contracts by 1.1% in the armchair direction and stretches by 1.3% in the zigzag direction, whereas few layer black phosphorus mainly expands by up to 3% in the armchair direction. However, these relatively small strains induced by the hexagonal BN, lead to significant changes in the vibrational properties of black phosphorus, with the redshifts of up to 10 cm^-1 of the high frequency optical mode A _g ¹. In general, structural changes induced by the encapsulation process open the door to substrate controlled strain engineering in two-dimensional crystals.

Thermoelectric terahertz photodetectors based on selenium-doped black phosphorus flakes

Chemical doping of bulk black phosphorus is a well-recognized way to reduce surface oxidation and degradation. Here, we report on the fabrication of terahertz frequency detectors consisting of an antenna-coupled field-effect transistor (FET) with an active channel of Se-doped black phosphorus. Our devices show a maximum room-temperature hole mobility of 1780 cm2 V-1 s-1 in a SiO2-encapsulated FET. A room-temperature responsivity of 3 V W-1 was observed, with a noise-equivalent power of 7 nW Hz-1/2 at 3.4 THz, comparable with the state-of-the-art room-temperature photodetectors operating in the same frequency range. The inclusion of Se dopants in the growth process of black phosphorus crystals enables the optimization of the transport and optical performances of FETs in the far-infrared with a high potential for the development of BP-based electro-optical devices. We also demonstrate that the flake thickness can be tuned according to the target application. Specifically, thicker flakes (>80 nm) are suitable for applications in which high mobility and high speed are essential, thinner flakes (<10 nm) are more appropriate for applications requiring high on/off current ratios, while THz photodetection is optimal with flakes 30-40 nm thick, due to the larger carrier density tunability.

Two-dimensional black phosphorus: its fabrication, functionalization and applications

Phosphorus, one of the most abundant elements in the Earth (∼0.1%), has attracted much attention in the last five years since the rediscovery of two-dimensional (2D) black phosphorus (BP) in 2014. The successful scaling down of BP endows this 'old material' with new vitality, resulting from the intriguing semiconducting properties in the atomic scale limit, i.e. layer-dependent bandgap that covers from the visible light to mid-infrared light spectrum as well as hole-dominated ambipolar transport characteristics. Intensive research effort has been devoted to the fabrication, characterization, functionalization and application of BP and other phosphorus allotropes. In this review article, we summarize the fundamental properties and fabrication techniques of BP, with particular emphasis on the recent progress in molecular beam epitaxy growth of 2D phosphorus. Subsequently, we highlight recent progress in BP (opto)electronic device applications achieved via customized manipulation methods, such as interface, defect and bandgap engineering as well as forming Lego-like stacked heterostructures.

Anomalous Phonon Modes in Black Phosphorus Revealed by Resonant Raman Scattering

Black phosphorus (BP), a layered material with puckered crystalline structure in each layer, has drawn intense interest due to its unique optical and electronic properties. In particular, the intricate Raman scattering effect in BP is intriguing and provides a platform for researchers to probe the physical properties of BP in depth. Here we report the first observation of anomalous modes with the frequency in the range of 100-900 cm^-1 in BP due to the resonant Raman effect. The origin and assignment of the anomalous modes are discussed based on the excitation energy- and angle-dependent Raman measurements. Density functional theory (DFT) calculated electronic band structure is used to support our understanding. The newly observed phonon modes could serve as a unique probe for the fine electronic structures and the exciton-phonon couplings, which promote a better understanding of BP for potential nanoelectronic and nanophotonic applications in the future.

Second-Order Raman Scattering in Exfoliated Black Phosphorus

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp²-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D₁, D₁^', D₂ and D₂^', in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the A_g¹ and A_g² modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D' and D modes occur through intravalley scatterings with split contributions in the armchair and zigzag directions, respectively. The high sensitivity of these D modes to disorder helps explaining several discrepancies found in the literature.

Synthesis of Crystalline Black Phosphorus Thin Film on Sapphire

Black phosphorus (BP) has recently attracted significant attention due to its exceptional physical properties. Currently, high-quality few-layer and thin-film BP are produced primarily by mechanical exfoliation, limiting their potential in future applications. Here, the synthesis of highly crystalline thin-film BP on 5 mm sapphire substrates by conversion from red to black phosphorus at 700 °C and 1.5 GPa is demonstrated. The synthesized ≈50 nm thick BP thin films are polycrystalline with a crystal domain size ranging from 40 to 70 µm long, as indicated by Raman mapping and infrared extinction spectroscopy. At room temperature, field-effect mobility of the synthesized BP thin film is found to be around 160 cm² V^-1 s^-1 along armchair direction and reaches up to about 200 cm² V^-1 s^-1 at around 90 K. Moreover, red phosphorus (RP) covered by exfoliated hexagonal boron nitride (hBN) before conversion shows atomically sharp hBN/BP interface and perfectly layered BP after the conversion. This demonstration represents a critical step toward the future realization of large scale, high-quality BP devices and circuits.

Spotting the differences in two-dimensional materials – the Raman scattering perspective

This review discusses the Raman spectroscopic characterization of 2D materials with a focus on the “differences” from primitive 2D materials.

Low-Frequency Shear and Layer-Breathing Modes in Raman Scattering of Two-Dimensional Materials

Ever since the isolation of single-layer graphene in 2004, two-dimensional layered structures have been among the most extensively studied classes of materials. To date, the pool of two-dimensional materials (2DMs) continues to grow at an accelerated pace and already covers an extensive range of fascinating and technologically relevant properties. An array of experimental techniques have been developed and used to characterize and understand these properties. In particular, Raman spectroscopy has proven to be a key experimental technique, thanks to its capability to identify minute structural and electronic effects in nondestructive measurements. While high-frequency (HF) intralayer Raman modes have been extensively employed for 2DMs, recent experimental and theoretical progress has demonstrated that low-frequency (LF) interlayer Raman modes are more effective at determining layer numbers and stacking configurations and provide a unique opportunity to study interlayer coupling. These advantages are due to 2DMs' unique interlayer vibration patterns where each layer behaves as an almost rigidly moving object with restoring forces corresponding to weak interlayer interactions. Compared to HF Raman modes, the relatively small attention originally devoted to LF Raman modes is largely due to their weaker signal and their proximity to the strong Rayleigh line background, which previously made their detection challenging. Recent progress in Raman spectroscopy with technical and hardware upgrades now makes it possible to probe LF modes with a standard single-stage Raman system and has proven crucial to characterize and understand properties of 2DMs. Here, we present a comprehensive and forward-looking review on the current status of exploiting LF Raman modes of 2DMs from both experimental and theoretical perspectives, revealing the fundamental physics and technological significance of LF Raman modes in advancing the field of 2DMs. We review a broad array of materials, with varying thickness and stacking configurations, discuss the effect of in-plane anisotropy, and present a generalized linear chain model and interlayer bond polarizability model to rationalize the experimental findings. We also discuss the instrumental improvements of Raman spectroscopy to enhance and separate LF Raman signals from the Rayleigh line. Finally, we highlight the opportunities and challenges ahead in this fast-developing field.

Observation of A Raman mode splitting in few layer black phosphorus encapsulated with hexagonal boron nitride

We investigate the impact of encapsulation with hexagonal boron nitride (h-BN) on the Raman spectrum of few layer black phosphorus. The encapsulation results in a significant reduction of the line width of the Raman modes of black phosphorus, due to a reduced phonon scattering rate. We observe a so far elusive peak in the Raman spectra ∼4 cm^-1 above the A mode in trilayer and thicker flakes, which had not been observed experimentally. The newly observed mode originates from the strong black phosphorus inter-layer interaction, which induces a hardening of the surface atom vibration with respect to the corresponding modes of the inner layers. The observation of this mode suggests a significant impact of h-BN encapsulation on the properties of black phosphorus and can serve as an indicator of the quality of its surface.

Layered SnSe nano-plates with excellent in-plane anisotropic properties of Raman spectrum and photo-response

In-plane anisotropy in optical, electronic and thermal properties of two-dimensional (2D) materials has attracted significant interest because of the huge potential applications for developing novel devices. In this work, outstanding angle-dependent Raman property of layered SnSe nano-plates is obtained via polarized Raman system and it is confirmed that the Raman polarization directions of two A_g modes (130 cm^-1 and 150 cm^-1) are consistent with specific crystalline directions (zigzag direction or armchair direction) of SnSe flakes under parallel polarization configuration at home temperature and low temperature. Furthermore, the SnSe nano-plate devices show excellent angle-resolved photo-response at home temperature and low temperature (150 K) with a 90° cycle period and the polarized directions are also along zigzag direction and armchair direction, which is ascribed to the unique in-plane asymmetric crystal structure. These prominent in-plane anisotropic properties provide a precise and rapid method to determine the crystal orientation of SnSe nano-flakes and open up the new applications of 2D asymmetric structure materials.

Black Phosphorus: Optical Characterization, Properties and Applications

The rise of black phosphorus (BP) as a new family member of two-dimensional (2D) materials brings new concepts and applications to the field, because of the infrared band gap and anisotropic properties of such materials. Among many excellent properties of BP, the optical property attracts special attention in recent years. Optical methods have been widely and successfully used in characterizing BP, not only to obtain the structural information (such as thickness and crystalline orientation), but also to probe the fundamental properties of BP in terms of the behavior of electrons, phonons, excitons etc. In this Review, a comprehensive understanding about the optical characterization of BP such as Raman, absorption, and photoluminescence is presented. Also, the unique optical properties and applications explored in recent years are reviewed.

Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus

As an emerging single elemental layered material with a low symmetry in-plane crystal lattice, black phosphorus (BP) has attracted significant research interest owing to its unique electronic and optoelectronic properties, including its widely tunable bandgap, polarization-dependent photoresponse and highly anisotropic in-plane charge transport. Despite extensive study of the steady-state charge transport in BP, there has not been direct characterization and visualization of the hot carriers dynamics in BP immediately after photoexcitation, which is crucial to understanding the performance of BP-based optoelectronic devices. Here we use the newly developed scanning ultrafast electron microscopy (SUEM) to directly visualize the motion of photoexcited hot carriers on the surface of BP in both space and time. We observe highly anisotropic in-plane diffusion of hot holes with a 15 times higher diffusivity along the armchair (x-) direction than that along the zigzag (y-) direction. Our results provide direct evidence of anisotropic hot carrier transport in BP and demonstrate the capability of SUEM to resolve ultrafast hot carrier dynamics in layered two-dimensional materials.

Two-Dimensional SnS: A Phosphorene Analogue with Strong In-Plane Electronic Anisotropy

We study the anisotropic electronic properties of two-dimensional (2D) SnS, an analogue of phosphorene, grown by physical vapor transport. With transmission electron microscopy and polarized Raman spectroscopy, we identify the zigzag and armchair directions of the as-grown 2D crystals. The 2D SnS field-effect transistors with a cross-Hall-bar structure are fabricated. They show heavily hole-doped (∼10¹⁹ cm^-3) conductivity with strong in-plane anisotropy. At room temperature, the mobility along the zigzag direction exceeds 20 cm² V^-1 s^-1, which can be up to 1.7 times that in the armchair direction. This strong anisotropy is then explained by the effective mass ratio along the two directions and agrees well with previous theoretical predictions. Temperature-dependent carrier density determined the acceptor energy level to be ∼45 meV above the valence band maximum. This value matches a calculated defect level of 42 meV for Sn vacancies, indicating that Sn deficiency is the main cause of the p-type conductivity.

Revealing anisotropy and thickness dependence of Raman spectra for SnS flakes

The anisotropic Raman behavior of SnS flake is found to be strongly dependent on the thickness of flake.