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ConspectusLayered metal halide perovskites represent a natural quantum well system for charge carriers that provides rich physics, and the organic encapsulation of the inorganic metal halide layers not only increases their stability in devices but also provides an immense freedom to design their functionality. Intriguingly, these organic moieties strongly impact the optical, electrical, and mechanical properties, not only through their dielectric, elastic, and chemical properties but also because of induced mechanical distortions in the inorganic lattice. This tunability makes two-dimensional layered perovskites (2DLPs) highly attractive as light emitters. Common consensus is that exciton-phonon coupling plays an important role in radiative recombination. For bulk and some two-dimensional (2D) materials, the band edge emission broadening can be described by the classic models for polar inorganic semiconductors, while for the temperature dependence of the self-trapped exciton emission, an analysis developed for color centers has been successfully applied. For many 2DLPs these approaches do not work because of the complexity of their vibrational spectra. However, their emission is still strongly determined by phonons, and therefore, an adequate understanding of the electron-phonon coupling needs to be developed.With polarized and angle-resolved Raman spectroscopy studies on single 2DLP flakes based on different ammonium molecules as organic cations, in 2020 we revealed very rich phonon spectra in the low-frequency regime. Although the phonon bands at low frequency can generally be attributed to the vibrations of the inorganic lattice, we found very different responses by only changing the type of organic cations. In addition, the intensity of the different phonon modes depended strongly on the angle of the linearly polarized excitation beam with respect to the in-plane axes of the octahedron lattice. In 2022, we mapped this angular dependence of the phonon modes, which allowed identification of the directionality of the different lattice vibrations. By correlating the phonon spectra with the temperature-dependent emission for a set of 2DLPs that featured very different self-trapped exciton (STE) emission, we demonstrated that the exciton relaxation cannot be related to coupling with a single (longitudinal-optical) phonon band and that several phonon bands should be involved in the emission process. To gain insights into the exciton-phonon coupling effects on the band edge emission, we performed both angle-resolved polarized emission and Raman spectroscopy on single 2D lead iodide perovskite microcrystals. These experiments revealed the impact of the organic cations on the linear polarization of the emission and corroborated that multiple phonon bands should be involved in the radiative recombination process. Analysis of the temperature-dependent line width broadening of the band edge emission showed that for many systems, the behavior cannot be described by assuming the involvement of only one phonon mode in the electron-phonon coupling process. Our studies revealed a wealth of highly directional low-frequency phonons in 2DLPs from which several bands are involved in the emission process, which leads to diverse optical and vibrational properties depending on the type of organic cation in the material.

Raman spectroscopy is a powerful technique to probe structural and doping behaviors of two-dimensional (2D) materials. In MoS 2 , the always coexisting in-plane (E 2g 1 ) and out-of-plane (A 1g ) vibrational modes are used as reliable fingerprints to distinguish the number of layers, strains, and doping levels. In this work, however, we report an abnormal Raman behavior, i.e., the absence of the A 1g mode in cetyltrimethylammonium bromide (CTAB)-intercalated MoS 2 superlattice. This unusual behavior is quite different from the softening of the A 1g mode induced by surface engineering or electric-field gating. Interestingly, under a strong laser illumination, heating, or mechanical indentation, an A 1g peak gradually appears, accompanied by the migration of intercalated CTA + cations. The abnormal Raman behavior is mainly attributed to the constraint of the out-of-plane vibration due to intercalations and resulting severe electron doping. Our work renews the understanding of Raman spectra of 2D semiconducting materials and sheds light on developing next-generation devices with tunable structures.

The lead halide perovskite has become a promising candidate for the study of exciton polaritons due to their excellent optical properties. Here, both experimental and simulated results confirm the existence of two kinds of Fabry-Pérot microcavities in a single CsPbBr 3 microwire with an isosceles right triangle cross section, and we experimentally demonstrate that confined photons in a straight and a folded Fabry-Pérot microcavity are strongly coupled with excitons to form exciton polaritons. Furthermore, we reveal the polarization characteristic and double-cavity modulation of exciton polaritons emission by polarization-resolved fluorescence spectroscopy. Our results not only prove that the modulation of exciton polaritons emission can occur in this simple double-cavity system but also provide a possibility to develop related polariton devices.

The emergence of superconductivity in two-dimensional (2D) materials has attracted tremendous research efforts because the origins and mechanisms behind the unexpected and fascinating superconducting phenomena remain unclear. In particular, the superconductivity can survive in 2D systems even with weakened disorder and broken spatial inversion symmetry. Here, structural and superconducting transitions of 2D van der Waals (vdW) hydrogenated germanene (GeH) are observed under compression and decompression processes. GeH possesses a superconducting transition with a critical temperature ( T c ) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa, while superconductivity remains. An abnormal increase of T c up to 6.11 K was observed during the decompression process, while the GeH remained in the 2D amorphous phase. A combination study of in situ high-pressure synchrotron X-ray diffraction, in situ high-pressure Raman spectroscopy, transition electron microscopy, and density functional theory simulations suggests that the superconductivity in 2D vdW GeH is attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure even in the form of an amorphous phase. The unique pressure-induced phase transition of GeH from 2D crystalline to 2D amorphous metal hydride provides a promising platform to study the mechanisms of amorphous hydride superconductivity.

Chiral optical metamaterials have attracted a great deal of attention due to their intriguing properties with respect to fundamental research and practical applications. For metamaterials with achiral structures, the system composed of metamaterials and obliquely incident light has extrinsic chirality and can produce circular dichroism (CD) effect. However, there have been few studies on the azimuth-dependent CD spectra of achiral metamaterials that have greatly improved our understanding of optical phenomena caused by external chirality. In this work, we experimentally studied the azimuth-dependent CD that originated from the extrinsic chirality of the metamaterials in an asymmetric-U shape and a U-bar-shape gold unit structure, separately. We explain the origin of the CD in the coupling of the macro-electric dipole and magnetic dipole, and the simulation results are in good agreement with the experiment. Our results provide a possible way to build an on-chip azimuth sensor based on azimuth-dependent CD spectra of metamaterials.

Recently, the coupling between magnons (quanta of spin waves) and phonons (quanta of lattice vibrations) in two-dimensional (2D) antiferromagnet FePS 3 offers a myriad of applications ranging from spintronic devices to quantum information technologies. However, the reported magnon-phonon coupling in the FePS 3 flake using Raman measurements requires an ultrahigh magnetic field up to 30 T. Here, we investigate the magnon-phonon coupling in FePS 3 by near-resonant magneto-Raman spectroscopy under a relatively small magnetic field (| H 0 | ≤ 9 T). Under near-resonant excitation, we find more pronounced coupling effects that are absent in non-resonant excitation: three optical phonons sensitive to the applied magnetic field are resolved, two of which show a frequency anti-crossing coupling with magnon, while the other coupled phonon exhibits only a polarization-coupled character without frequency anti-crossing. Besides, our polarized Raman results also show the polarization transferring between coupled magnon-phonon modes. On the basis of a modified theoretical model, we can well explain the measured Raman spectra.

Quantum emitters are needed for a myriad of applications ranging from quantum sensing to quantum computing. Hexagonal boron nitride (hBN) quantum emitters are one of the most promising solid-state platforms to date due to their high brightness and stability and the possibility of a spin-photon interface. However, the understanding of the physical origins of the single-photon emitters (SPEs) is still limited. Here we report dense SPEs in hBN across the entire visible spectrum and present evidence that most of these SPEs can be well explained by donor-acceptor pairs (DAPs). On the basis of the DAP transition generation mechanism, we calculated their wavelength fingerprint, matching well with the experimentally observed photoluminescence spectrum. Our work serves as a step forward for the physical understanding of SPEs in hBN and their applications in quantum technologies.

Materials with a quasi-one-dimensional stripy magnetic order often exhibit low crystal and magnetic symmetries, thus allowing the presence of various energy coupling terms and giving rise to macroscopic interplay between spin, charge, and phonon. In this work, we performed optical, electrical and magnetic characterizations combined with first-principles calculations on a van der Waals antiferromagnetic insulator chromium oxychloride (CrOCl). We detected the subtle phase transition behaviors of exfoliated CrOCl under varying temperature and magnetic field and clarified its controversial spin structures. We found that the antiferromagnetism and its air stability persist down to few-layer samples, making it a promising candidate for future 2D spintronic devices. Additionally, we verified the magnetoelastic coupling effect in CrOCl, allowing for the potential manipulation of the magnetic states via electric field or strain. These virtues of CrOCl provide us with an ideal platform for fundamental research on spin-charge, spin-phonon coupling, and spin-interactions.

Among many phase-changing materials, graphite is probably the most studied and interesting: the rhombohedral (3R) and hexagonal (2H) phases exhibit dramatically different electronic properties. However, up to now the only way to promote 3R to 2H phase transition is through exposure to elevated temperatures (above 1000 °C); thus, it is not feasible for modern technology. In this work, we demonstrate that 3R to 2H phase transition can be promoted by changing the charged state of 3D graphite, which promotes the repulsion between the layers and significantly reduces the energy barrier between the 3R and 2H phases. In particular, we show that charge transfer from lithium nitride (α-Li 3 N) to graphite can lower the transition temperature down to 350 °C. The proposed interlayer slipping model potentially offers the control over topological states at the interfaces between different phases, making this system even more attractive for future electronic applications.

Interlayer coupling in two-dimensional (2D) layered materials plays an important role in controlling their properties. 2H- and 3R-MoS 2 with different stacking orders and the resulting interlayer coupling have been recently discovered to have different band structures and a contrast behavior in valley physics. However, the role of carrier doping in interlayer coupling in 2D materials remains elusive. Here, based on the electric double layer interface, we demonstrated the experimental observation of carrier doping-enhanced interlayer coupling in 3R-MoS 2 . A remarkable tuning of interlayer Raman modes can be observed by changing the stacking sequence and carrier doping near their monolayer limit. The modulated interlayer vibration modes originated from the interlayer coupling show a doping-induced blue shift and are supposed to be associated with the interlayer coupling enhancement, which is further verified using our first-principles calculations. Such an electrical control of interlayer coupling of layered materials in an electrical gating geometry provides a new degree of freedom to modify the physical properties in 2D materials.

A comprehensive understanding of the ultrafast electron dynamics in two-dimensional transition metal dichalcogenides (TMDs) is necessary for their applications in optoelectronic devices. In this work, we contribute a study of ultrafast electron cooling and decay dynamics in the supported and suspended monolayer WS 2 by time- and energy-resolved photoemission electron microscopy (PEEM). Electron cooling in the Q valley of the conduction band is clearly resolved in energy and time, on a time scale of 0.3 ps. Electron decay is mainly via a defect trapping process on a time scale of several picoseconds. We observed that the trap states can be produced and increased by laser illumination under an ultrahigh vacuum, and the higher local optical-field intensity led to the faster increase of trap states. The enhanced defect trapping could significantly modify the carrier dynamics and should be paid attention to in photoemission experiments for two-dimensional materials.

The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden-Popper phase of these materials allows for distinct directionality of the vibrational modes with respect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at ultralow frequencies. A wealth of Raman bands is distinguished in the range from 15 to 150 cm -1 (2-15 meV), whose features depend on the organic cation species, on temperature, and on the direction of the linear polarization of the incident light. By controlling the angle of the linear polarization of the excitation laser with respect to the in-plane axes of the octahedral layer, we gain detailed information on the symmetry of the vibrational modes. The choice of two different organic moieties, phenethylammonium (PEA) and butylammonium (BA), allows us to discern the influence of the linker molecules, evidencing strong anisotropy of the vibrations for the (PEA) 2 PbBr 4 samples. Temperature-dependent Raman measurements reveal that the broad phonon bands observed at room temperature consist of a series of sharp modes and that such mode splitting strongly differs for the different organic moieties and vibrational bands. Softer molecules such as BA result in lower vibrational frequencies and splitting into fewer modes, while more rigid molecules such as PEA lead to higher frequency oscillations and larger number of Raman peaks at low temperature. Interestingly, in distinct bands the number of peaks in the Raman bands is doubled for the rigid PEA compared to the soft BA linkers. Our work shows that the coupling to specific vibrational modes can be controlled by the incident light polarization and choice of the organic moiety, which could be exploited for tailoring exciton-phonon interaction, and for optical switching of the optoelectronic properties of such 2D layered materials.

The successful fabrication of a two-dimensional boron sheet, which features a triangular lattice with periodic hole arrays, has stimulated great interest in its specific structure as well as properties such as possible superconductivity. Here, we report a study on the vibrational spectra and electron-phonon coupling (EPC) in monolayer boron sheets by in situ Raman and tip-enhanced Raman spectroscopy (TERS) at low temperature and ultrahigh vacuum. The gap-mode TERS gives a 3 × 10 9 selective enhancement on vertical vibrational Raman modes. A spatial resolution of 1 nm is achieved in this system. Combined with first-principle calculations, the vibrational properties as well as EPC in borophene are determined. The results are helpful for further study on the mechanical, electronic, and possible superconducting properties of two-dimensional boron.

Phonon-assisted anti-Stokes photoluminescence (ASPL) up-conversion lies at the heart of optical refrigeration in solids. The thermal energy contained in the lattice vibrations is taken away by the emitted anti-Stokes photons' ASPL process, resulting in laser cooling of solids. To date, net laser cooling of solids is limited in rare-earth (RE)-doped crystals, glasses, and direct band gap semiconductors. Searching more solid materials with efficient phonon-assisted photoluminescence up-conversion is important to enrich optical refrigeration research. Here, we demonstrate the phonon-assisted PL up-conversion process from the silicon vacancy (SiV) center in diamond for the first time by studying ASPL spectra for the dependence of temperature, laser power, and excitation energy. Although net cooling has not been observed, our results show that net laser cooling might be eventually achieved in diamond by improving the external quantum efficiency to higher than 95%. Our work provides a promising route to investigate the laser cooling effect in diamond.

The material choice, layer thickness, and twist angle widely enrich the family of van der Waals heterostructures (vdWHs), providing multiple degrees of freedom to engineer their optical and electronic properties. The moiré patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena, such as Hofstadter butterfly spectrum and moiré excitons. Here, in the as-grown/transferred twisted bilayer MoS 2 (tBLMs), one of the simplest prototypes of vdWHs, we show that the periodic potentials of moiré patterns also modify the properties of phonons of its monolayer MoS 2 constituent to generate Raman modes related to moiré phonons. These Raman modes correspond to zone-center phonons in tBLMs, which are folded from the off-center phonons in monolayer MoS 2 . However, the folded phonons related to crystallographic superlattices are not observed in the Raman spectra. By varying the twist angle, the moiré phonons of tBLM can be exploited to map the phonon dispersions of the monolayer constituent. The lattice dynamics of the moiré phonons are modulated by the patterned interlayer coupling resulting from periodic potential of moiré patterns, as confirmed by density functional theory calculations. The Raman intensity related to moiré phonons in all tBLMs are strongly enhanced when the excitation energy approaches the C exciton energy. This study can be extended to various vdWHs to deeply understand their Raman spectra, moiré phonons, lattice dynamics, excitonic effects, and interlayer coupling.

The atomic structures of self-assembled silicon nanoribbons and magic clusters on Ag(110) substrate have been studied by high-resolution noncontact atomic force microscopy (nc-AFM) and tip-enhanced Raman spectroscopy (TERS). Pentagon-ring structures in Si nanoribbons and clusters have been directly visualized. Moreover, the vibrational fingerprints of individual Si nanoribbon and cluster retrieved by subnanometer resolution TERS confirm the pentagonal nature of both Si nanoribbons and clusters. This work demonstrates that Si pentagon can be an important element in building silicon nanostructures, which may find important applications for future nanoelectronic devices based on silicon.

Two-dimensional (2D) materials with high anisotropic properties, such as black phosphorus and ReS 2 , show amazing potential for applications in future nanoelectronic and optoelectronic devices. However, degradation of black phosphorus under ambient conditions and the expensiveness of Re block their application. In this study, another layered material, KP 15 , that has highly anisotropic properties was successfully prepared. The detailed crystal structure and electron-density distribution calculation reveal that KP 15 exhibits an anisotropic layered structure with two rows of P tubes connected by K atoms that are antiparallel in a single layer. Outstanding chemical stability, angular dependence of the Raman response, excitation, and exciton emission at room temperature have been found in exfoliated KP 15 nanoribbons. Importantly, the exciton emission at room temperature suggests the existence of a large exciton binding energy. Our results indicate that, because this layered material, KP 15 , has high anisotropic properties and ultrachemical stability and is derived from abundant raw materials, it has great potential for applications in optoelectronic devices.

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.

Interfacial coupling between neighboring layers of van der Waals heterostructures (vdWHs), formed by vertically stacking more than two types of two-dimensional materials (2DMs), greatly affects their physical properties and device performance. Although high-resolution cross-sectional scanning tunneling electron microscopy can directly image the atomically sharp interfaces in the vdWHs, the interfacial coupling and lattice dynamics of vdWHs formed by two different types of 2DMs, such as semimetal and semiconductor, are not clear so far. Here, we report the ultralow-frequency Raman spectroscopy investigation on interfacial couplings in the vdWHs formed by graphene and MoS 2 flakes. Because of the significant interfacial layer-breathing couplings between MoS 2 and graphene flakes, a series of layer-breathing modes with frequencies dependent on their layer numbers are observed in the vdWHs, which can be described by the linear chain model. It is found that the interfacial layer-breathing force constant between MoS 2 and graphene, α 0 ⊥ (I) = 60 × 10 18 N/m 3 , is comparable with the layer-breathing force constant of multilayer MoS 2 and graphene. The results suggest that the interfacial layer-breathing couplings in the vdWHs formed by MoS 2 and graphene flakes are not sensitive to their stacking order and twist angle between the two constituents. Our results demonstrate that the interfacial interlayer coupling in vdWHs formed by two-dimensional semimetals and semiconductors can lead to new lattice vibration modes, which not only can be used to measure the interfacial interactions in vdWHs but also is beneficial to fundamentally understand the properties of vdWHs for further engineering the vdWHs-based electronic and photonic devices.

Lattice vibrational modes in cadmium chalcogenide nanocrystals (NCs) have a strong impact on the carrier dynamics of excitons in such confined systems and on the optical properties of these nanomaterials. A prominent material for light emitting applications are CdSe/CdS core-shell dot-in-rods. Here we present a detailed investigation of the acoustic phonon modes in such dot-in-rods by nonresonant Raman spectroscopy with laser excitation energy lower than their bandgap. With high signal-to-noise ratio in the frequency range from 5-50 cm -1 , we reveal distinct Raman bands that can be related to confined extensional and radial-breathing modes (RBM). Comparison of the experimental results with finite elements simulation and analytical analysis gives detailed insight into the localized nature of the acoustic vibration modes and their resonant frequencies. In particular, the RBM of dot-in-rods cannot be understood by an oscillation of a CdSe sphere embedded in a CdS rod matrix. Instead, the dot-in-rod architecture leads to a reduction of the sound velocity in the core region of the rod, which results in a redshift of the rod RBM frequency and localization of the phonon induced strain in vicinity of the core where optical transitions occur. Such localized effects potentially can be exploited as a tool to tune exciton-phonon coupling in nanocrystal heterostructures.

Raman spectroscopy is the prime nondestructive characterization tool for graphene and related layered materials. The shear (C) and layer breathing modes (LBMs) are due to relative motions of the planes, either perpendicular or parallel to their normal. This allows one to directly probe the interlayer interactions in multilayer samples. Graphene and other two-dimensional (2d) crystals can be combined to form various hybrids and heterostructures, creating materials on demand with properties determined by the interlayer interaction. This is the case even for a single material, where multilayer stacks with different relative orientations have different optical and electronic properties. In twisted multilayer graphene there is a significant enhancement of the C modes due to resonance with new optically allowed electronic transitions, determined by the relative orientation of the layers. Here we show that this applies also to the LBMs, which can be now directly measured at room temperature. We find that twisting has a small effect on LBMs, quite different from the case of the C modes. This implies that the periodicity mismatch between two twisted layers mostly affects shear interactions. Our work shows that ultralow-frequency Raman spectroscopy is an ideal tool to uncover the interface coupling of 2d hybrids and heterostructures.

Molybdenum disulfide (MoS2) is back in the spotlight because of the indirect-to-direct bandgap tunability and valley related physics emerging in the monolayer regime. However, rigorous control of the monolayer thickness is still a huge challenge for commonly utilized physical exfoliation and chemical synthesis methods. Herein, we have successfully grown predominantly monolayer MoS2 on an inert and nearly lattice-matching mica substrate by using a low-pressure chemical vapor deposition method. The growth is proposed to be mediated by an epitaxial mechanism, and the epitaxial monolayer MoS2 is intrinsically strained on mica due to a small adlayer-substrate lattice mismatch (~2.7%). Photoluminescence (PL) measurements indicate strong single-exciton emission in as-grown MoS2 and room-temperature PL helicity (circular polarization ~0.35) on transferred samples, providing straightforward proof of the high quality of the prepared monolayer crystals. The homogeneously strained high-quality monolayer MoS2 prepared in this study could competitively be exploited for a variety of future applications.

Technological progress is determined, to a great extent, by developments in material science. Breakthroughs can happen when a new type of material or new combinations of known materials with different dimensionality and functionality are created. Multilayered structures, being planar or concentric, are now emerging as major players at the forefront of research. Raman spectroscopy is a well-established characterization technique for carbon nanomaterials and is being developed for layered materials. In this issue of ACS Nano, Hirschmann et al. investigate triple-wall carbon nanotubes via resonant Raman spectroscopy, showing how a wealth of information can be derived about these complex structures. The next challenge is to tackle hybrid heterostructures, consisting of different planar or concentric materials, arranged "on demand" to achieve targeted properties.

Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS(2)) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS(2) opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS(2) and WSe(2) that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS(2) and WSe(2), respectively. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by a chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe(2).

We use anhydrous ferric chloride (FeCl(3)) to intercalate graphite flakes consisting of 2-4 graphene layers and to dope graphene monolayers. The intercalant, staging, stability, and doping of the resulting intercalation compounds (ICs) are characterized by Raman scattering. The G peak of heavily doped monolayer graphene upshifts to ∼1627 cm(-1). The 2-4 layer ICs have similar upshifts, and a Lorentzian line shape for the 2D band, indicating that each layer behaves as a decoupled heavily doped monolayer. By performing Raman measurements at different excitation energies, we show that, for a given doping level, the 2D peak can be suppressed by Pauli blocking for laser energy below the doping level. Thus, multiwavelength Raman spectroscopy allows a direct measurement of the Fermi level, complementary to that derived by performing measurements at fixed excitation energy significantly higher than the doping level. This allows us to estimate a Fermi level shift of up to ∼0.9 eV. These ICs are thus ideal test-beds for the physical and chemical properties of heavily doped graphenes.