Graphene is a monolayer of carbon atoms packed into a two-dimensional (2D) honeycomb crystal structure, which is a special material with many excellent properties. In the present study, we will discuss the possibility that graphene can be used as a substrate for enhancing Raman signals of adsorbed molecules. Here, phthalocyanine (Pc), rhodamine 6G (R6G), protoporphyin IX (PPP), and crystal violet (CV), which are popular molecules widely used as a Raman probe, are deposited equally on graphene and a SiO(2)/Si substrate using vacuum evaporation or solution soaking. By comparing the Raman signals of molecules on monolayer graphene and on a SiO(2)/Si substrate, we observed that the intensities of the Raman signals on monolayer graphene are much stronger than on a SiO(2)/Si substrate, indicating a clear Raman enhancement effect on the surface of monolayer graphene. For solution soaking, the Raman signals of the molecules are visible even though the concentration is low to 10(-8) mol/L or less. What's more interesting, the enhanced efficiencies are quite different on monolayer, few-layer, multilayer graphene, graphite, and highly ordered pyrolytic graphite (HOPG). The Raman signals of molecules on multilayer graphene are even weaker than on a SiO(2)/Si substrate, and the signals are even invisible on graphite and HOPG. Taking the Raman signals on the SiO(2)/Si substrate as a reference, Raman enhancement factors on the surface of monolayer graphene can be obtained using Raman intensity ratios. The Raman enhancement factors are quite different for different peaks, changing from 2 to 17. Furthermore, we found that the Raman enhancement factors can be distinguished through three classes that correspond to the symmetry of vibrations of the molecule. We attribute this enhancement to the charge transfer between graphene and the molecules, which result in a chemical enhancement. This is a new phenomenon for graphene that will expand the application of graphene to microanalysis and is good for studying the basic properties of both graphene and SERS.

One hundred years after its first successful synthesis in the bulk form in 1914, black phosphorus (black P) was recently rediscovered from the perspective of a 2D layered material, attracting tremendous interest from condensed matter physicists, chemists, semiconductor device engineers, and material scientists. Similar to graphite and transition metal dichalcogenides (TMDs), black P has a layered structure but with a unique puckered single-layer geometry. Because the direct electronic band gap of thin film black P can be varied from 0.3 eV to around 2 eV, depending on its film thickness, and because of its high carrier mobility and anisotropic in-plane properties, black P is promising for novel applications in nanoelectronics and nanophotonics different from graphene and TMDs. Black P as a nanomaterial has already attracted much attention from researchers within the past year. Here, we offer our opinions on this emerging material with the goal of motivating and inspiring fellow researchers in the 2D materials community and the broad readership of PNAS to discuss and contribute to this exciting new field. We also give our perspectives on future 2D and thin film black P research directions, aiming to assist researchers coming from a variety of disciplines who are desirous of working in this exciting research field.At the beginning of 2014, a few research teams including the ones led by the authors reintroduced black phosphorus (black P) from the perspective of a layered thin film material (1-6), in which previously unidentified properties and applications have arisen. Since then, black P, the most stable allotrope of the phosphorus element, is emerging as a promising semiconductor with a moderate band gap for nanoelectronics and nanophotonics applications (7,8). Its single-and fewatomic layer forms can be isolated by techniques such as micromechanical exfoliation, giving rise to a type of 2D material with many unique properties not found in other members of the 2D materials family. Here, we present our perspectives on this latest addition to the 2D materials family, which can bridge the energy gap between that of graphene and transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS 2 ), molybdenum diselenide (MoSe 2 ), tungsten disulfide (WS 2 ), and tungsten diselenide (WSe 2 ). In addition, we also offer our viewpoint on using the in-plane anisotropy of black P to develop electronic, photonic, and thermoelectric devices.Black P is a single-elemental layered crystalline material consisting of only phosphorus atoms (9). Unlike in group IV elemental layered materials, such as graphene, silicene, or germanene, each phosphorus atom has five outer shell electrons. Black P has three crystalline structures (10): orthorhombic, simple cubic, and rhombohedral. Semiconducting puckered orthorhombic black P is of interest here and it belongs to the D 18 2h point group ( Fig. 1 A and B), which has reduced symmetry compared with its group IV counterparts (such as graphene) having the D 4 6h point group symmetr...

Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics.

The thinnest semiconductor, molybdenum disulfide (MoS2) monolayer, exhibits promising prospects in the applications of optoelectronics and valleytronics. A uniform and highly crystalline MoS2 monolayer in a large area is highly desirable for both fundamental studies and substantial applications. Here, utilizing various aromatic molecules as seeding promoters, a large-area, highly crystalline, and uniform MoS2 monolayer was achieved with chemical vapor deposition (CVD) at a relatively low growth temperature (650 °C). The dependence of the growth results on the seed concentration and on the use of different seeding promoters is further investigated. It is also found that an optimized concentration of seed molecules is helpful for the nucleation of the MoS2. The newly identified seed molecules can be easily deposited on various substrates and allows the direct growth of monolayer MoS2 on Au, hexagonal boron nitride (h-BN), and graphene to achieve various hybrid structures.

Recently, monolayers of layered transition metal dichalcogenides (LTMD), such as MX2 (M = Mo, W and X = S, Se), have been reported to exhibit significant spin-valley coupling and optoelectronic performances because of the unique structural symmetry and band structures. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics, and optoelectronics. However, most studies to date are hindered by great challenges on the synthesis and transfer of high-quality LTMD monolayers. Hence, a feasible synthetic process to overcome the challenges is essential. Here, we demonstrate the growth of high-quality MS2 (M = Mo, W) monolayers using ambient-pressure chemical vapor deposition (APCVD) with the seeding of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS). The growth of a MS2 monolayer is achieved on various surfaces with a significant flexibility to surface corrugation. Electronic transport and optical performances of the as-grown MS2 monolayers are comparable to those of exfoliated MS2 monolayers. We also demonstrate a robust technique in transferring the MS2 monolayer samples to diverse surfaces, which may stimulate the progress on the class of materials and open a new route toward the synthesis of various novel hybrid structures with LTMD monolayer and functional materials.

Realizing Raman enhancement on a flat surface has become increasingly attractive after the discovery of graphene-enhanced Raman scattering (GERS). Two-dimensional (2D) layered materials, exhibiting a flat surface without dangling bonds, were thought to be strong candidates for both fundamental studies of this Raman enhancement effect and its extension to meet practical applications requirements. Here, we study the Raman enhancement effect on graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2), by using the copper phthalocyanine (CuPc) molecule as a probe. This molecule can sit on these layered materials in a face-on configuration. However, it is found that the Raman enhancement effect, which is observable on graphene, hBN, and MoS2, has different enhancement factors for the different vibrational modes of CuPc, depending strongly on the surfaces. Higher-frequency phonon modes of CuPc (such as those at 1342, 1452, 1531 cm(-1)) are enhanced more strongly on graphene than that on h-BN, while the lower frequency phonon modes of CuPc (such as those at 682, 749, 1142, 1185 cm(-1)) are enhanced more strongly on h-BN than that on graphene. MoS2 demonstrated the weakest Raman enhancement effect as a substrate among these three 2D materials. These differences are attributed to the different enhancement mechanisms related to the different electronic properties and chemical bonds exhibited by the three substrates: (1) graphene is zero-gap semiconductor and has a nonpolar C-C bond, which induces charge transfer (2) h-BN is insulating and has a strong B-N bond, while (3) MoS2 is semiconducting with the sulfur atoms on the surface and has a polar covalent bond (Mo-S) with the polarity in the vertical direction to the surface. Therefore, the different Raman enhancement mechanisms differ for each material: (1) charge transfer may occur for graphene; (2) strong dipole-dipole coupling may occur for h-BN, and (3) both charge transfer and dipole-dipole coupling may occur, although weaker in magnitude, for MoS2. Consequently, this work studied the origin of the Raman enhancement (specifically, chemical enhancement) and identifies h-BN and MoS2 as two different types of 2D materials with potential for use as Raman enhancement substrates.

Orthorhombic black phosphorus (BP) and other layered materials, such as gallium telluride (GaTe) and tin selenide (SnSe), stand out among two-dimensional (2D) materials owing to their anisotropic in-plane structure. This anisotropy adds a new dimension to the properties of 2D materials and stimulates the development of angle-resolved photonics and electronics. However, understanding the effect of anisotropy has remained unsatisfactory to date, as shown by a number of inconsistencies in the recent literature. We use angle-resolved absorption and Raman spectroscopies to investigate the role of anisotropy on the electron-photon and electron-phonon interactions in BP. We highlight, both experimentally and theoretically, a nontrivial dependence between anisotropy and flake thickness and photon and phonon energies. We show that once understood, the anisotropic optical absorption appears to be a reliable and simple way to identify the crystalline orientation of BP, which cannot be determined from Raman spectroscopy without the explicit consideration of excitation wavelength and flake thickness, as commonly used previously.

Surface enhanced Raman spectroscopy (SERS) is an attractive analytical technique, which enables single-molecule sensitive detection and provides its special chemical fingerprints. During the past decades, researchers have made great efforts towards an ideal SERS substrate, mainly including pioneering works on the preparation of uniform metal nanostructure arrays by various nanoassembly and nanotailoring methods, which give better uniformity and reproducibility. Recently, nanoparticles coated with an inert shell were used to make the enhanced Raman signals cleaner. By depositing SERS-active metal nanoislands on an atomically flat graphene layer, here we designed a new kind of SERS substrate referred to as a graphene-mediated SERS (G-SERS) substrate. In the graphene/ metal combined structure, the electromagnetic "hot" spots (which is the origin of a huge SERS enhancement) created by the gapped metal nanoislands through the localized surface plasmon resonance effect are supposed to pass through the monolayer graphene, resulting in an atomically flat hot surface for Raman enhancement. Signals from a G-SERS substrate were also demonstrated to have interesting advantages over normal SERS, in terms of cleaner vibrational information free from various metal-molecule interactions and being more stable against photo-induced damage, but with a comparable enhancement factor. Furthermore, we demonstrate the use of a freestanding, transparent and flexible "G-SERS tape" (consisting of a polymer-layer-supported monolayer graphene with sandwiched metal nanoislands) to enable direct, real time and reliable detection of trace amounts of analytes in various systems, which imparts high efficiency and universality of analyses with G-SERS substrates.atomically smooth substrate | metal-molecule isolation | signal fluctuation | mediator | application F or almost all sorts of analytical methods, the dream to improve their sensitivity as well as their reproducibility and to optimize the analytical process (e.g., to simplify the sample preparation/ measurement procedures for quick analysis and to enable in situ and real time monitoring) is a long-term pursuit. Spectroscopic approaches based on fluorescence, infrared absorption and Raman scattering have been developed with rising importance for various sensing and imaging applications. Among them, Raman scattering provides more structural information (characteristic vibrational information of each chemical bond) over fluorescence, and higher spatial resolution (shorter excitation wavelength) over infrared absorption (1). However, as Raman scattering is an inelastic scattering process with a very low crosssection, it is not very sensitive and thus has limited analysis efficiency and applicability.Because of this, surface enhanced Raman spectroscopy (SERS) has been developed since the 1970s (2-4) to enable ultrasensitive characterization down to the single-molecule level (5, 6), comparable to single-molecule fluorescence spectroscopy (7). In a SERS experiment, a rough metal surface or colloid...