In print at Nature Chemistry (2016): http://dx.doi.org/10.1038/nchem.2505 AbstractFunctionalization of atomically thin nanomaterials enables the tailoring of their chemical, optical, and electronic properties. Exfoliated black phosphorus -a layered two-dimensional semiconductor exhibiting favorable charge carrier mobility, tunable bandgap, and highly anisotropic properties -is chemically reactive and degrades rapidly in ambient conditions. In contrast, here we show that covalent aryl diazonium functionalization suppresses the chemical degradation of exfoliated black phosphorus even following weeks of ambient exposure. This chemical modification scheme spontaneously forms phosphorus-carbon bonds, has a reaction rate sensitive to the aryl diazonium substituent, and alters the electronic properties of exfoliated black phosphorus, ultimately yielding a strong, tunable p-type doping that simultaneously improves field-effect transistor mobility and on/off current ratio. This chemical functionalization pathway controllably modifies the properties of exfoliated black phosphorus, thus improving its prospects for nanoelectronic applications.

We ascertain the anisotropic thermal conductivity of passivated black phosphorus (BP), a reactive 2D nanomaterial with strong in-plane anisotropy. We measure the room temperature thermal conductivity by time-domain thermoreflectance for three crystalline axes of exfoliated BP. The thermal conductivity along the zigzag direction (86 ± 8 W m −1 K −1 ) is ~2.5 times higher than that of the armchair direction (34 ± 4 W m −1 K −1 ). TOC Figure 2Black phosphorus (BP), a stable phosphorus allotrope at ambient temperature and pressure, [1] is a two-dimensional electronic material with desirable properties for transistor, [2, 3] thermoelectric, [4] and optical sensing [5] applications. Few-layer BP flakes can be exfoliated from bulk crystals due to weak interlayer bonding. [2, 3,6] In contrast to the planar character of graphite and transition metal dichalcogenides, BP has a puckered, honeycomb structure, leading to heightened chemical reactivity [7] and pronounced in-plane anisotropy.Experimental and theoretical examinations of the electrical, [3, 4,6] optical, [3,8] mechanical, [9] and thermal [4,10,11,12] properties reveal distinct anisotropy along BP's two high-symmetry, inplane directions. These symmetry axes are commonly referred to as the zigzag and armchair directions, with lattice constants of a = 3.314 Å and c = 4.376 Å, respectively. [1] Understanding an electronic material's thermal conductivity is critical for the thermal management of small-scale devices and for exploring potential thermoelectric applications.Despite extensive electrical characterization of exfoliated BP, experimental measurements of BP's thermal properties are few. [12,13] First-principles calculations of the anisotropic thermal conductivity of monolayer BP, that is, phosphorene, predict that the thermal conductivity along the zigzag direction is two-or three-fold higher [10,11] than along the armchair direction; for example, ref. 11 finds 110 and 36 W m −1 K −1 , respectively, in the two directions. Of this, the electronic contribution to the thermal conductivity is markedly small, less than 3 W m −1 K −1 , even at a high carrier concentration of ~10 12 cm −2 . [11] Experimentally, Slack found the thermal conductivity of bulk, polycrystalline BP to be 10 W m −1 K −1 at room temperature, [13] but no anisotropic effects were examined. Only mechanically exfoliated BP flakes, with defined symmetry axes, allow an assessment of anisotropic thermal properties in all three high symmetry directions of the crystal. A recent preprint [12] reports the in-plane, anisotropic thermal transport for exfoliated, few-layer BP using micro-Raman spectroscopy; still, the extracted values were much smaller than theoretically predicted for phosphorene. For thinner (<15 nm) BP samples, the measured BP thermal conductivity is modified by phonon scattering from oxidized regions, substrates, and surface imperfections. By contrast, thermal measurements on thicker (>100 nm) BP flakes, especially those protected against ambient oxidation, provide an intrinsic ...

Understanding and exploiting the remarkable optical and electronic properties of phosphorene require mass production methods that avoid chemical degradation. Although solution-based strategies have been developed for scalable exfoliation of black phosphorus, these techniques have thus far used anhydrous organic solvents in an effort to minimize exposure to known oxidants, but at the cost of limited exfoliation yield and flake size distribution. Here, we present an alternative phosphorene production method based on surfactant-assisted exfoliation and postprocessing of black phosphorus in deoxygenated water. From comprehensive microscopic and spectroscopic analysis, this approach is shown to yield phosphorene dispersions that are stable, highly concentrated, and comparable to micromechanically exfoliated phosphorene in structure and chemistry. Due to the high exfoliation efficiency of this process, the resulting phosphorene flakes are thinner than anhydrous organic solvent dispersions, thus allowing the observation of layer-dependent photoluminescence down to the monolayer limit. Furthermore, to demonstrate preservation of electronic properties following solution processing, the aqueous-exfoliated phosphorene flakes are used in field-effect transistors with high drive currents and current modulation ratios. Overall, this method enables the isolation and mass production of few-layer phosphorene, which will accelerate ongoing efforts to realize a diverse range of phosphorenebased applications.black phosphorus | deoxygenated water | liquid phase exfoliation | photoluminescence | field-effect transistor F ew-layer phosphorene (FL-P) isolated by micromechanical exfoliation has been widely studied both fundamentally and in applications such as high-performance electronic and optoelectronic devices (1-11). Although micromechanical exfoliation provides individual, high-quality FL-P flakes, this technique lacks scalability and is not amenable to large-area applications. Conventional approaches for mass production of 2D nanomaterials involve chemical vapor deposition (CVD) and liquid phase exfoliation (LPE). Whereas CVD growth of black phosphorus (BP) thin films is hindered by challenges with molecular precursors and extreme growth conditions (12), LPE of BP has been demonstrated and used for the large-scale deposition of thin films akin to the approaches for other 2D nanomaterials (13-17). Specifically, stable BP dispersions have been produced by LPE using high-boiling-point solvents including N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethyl sulfoxide, and N-cyclohexyl-2-pyrrolidone (18-21). With these anhydrous organic solvents, chemical degradation from ambient O 2 and water (11) are avoided, but the exfoliation yield and flake size distribution are suboptimal, especially compared with the results that have been achieved with other 2D nanomaterials using stabilizing surfactants in aqueous solutions (16). Furthermore, organic solvents have limited compatibility with methods such as ultracentrifugation for structu...

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) and black phosphorus (BP) have beneficial electronic, optical, and physical properties at the few-layer limit. As atomically thin materials, 2D TMDCs and BP are highly sensitive to their environment and chemical modification, resulting in a strong dependence of their properties on substrate effects, intrinsic defects, and extrinsic adsorbates. Furthermore, the integration of 2D semiconductors into electronic and optoelectronic devices introduces unique challenges at metal-semiconductor and dielectric-semiconductor interfaces. Here, we review emerging efforts to understand and exploit chemical effects to influence the properties of 2D TMDCs and BP. In some cases, surface chemistry leads to significant degradation, thus necessitating the development of robust passivation schemes. On the other hand, appropriately designed chemical modification can be used to beneficially tailor electronic properties, such as controlling doping levels and charge carrier concentrations. Overall, chemical methods allow substantial tunability of the properties of 2D TMDCs and BP, thereby enabling significant future opportunities to optimize performance for device applications.

Black phosphorus (BP) has recently emerged as a promising narrow band gap layered semiconductor with optoelectronic properties that bridge the gap between semimetallic graphene and wide band gap transition metal dichalcogenides such as MoS2. To date, BP field-effect transistors have utilized a lateral geometry with in-plane transport dominating device characteristics. In contrast, we present here a vertical field-effect transistor geometry based on a graphene/BP van der Waals heterostructure. The resulting device characteristics include high on-state current densities (>1600 A/cm(2)) and current on/off ratios exceeding 800 at low temperature. Two distinct charge transport mechanisms are identified, which are dominant for different regimes of temperature and gate voltage. In particular, the Schottky barrier between graphene and BP determines charge transport at high temperatures and positive gate voltages, whereas tunneling dominates at low temperatures and negative gate voltages. These results elucidate out-of-plane electronic transport in BP and thus have implications for the design and operation of BP-based van der Waals heterostructures.

Black phosphorus (BP) is a layered semiconductor that recently has been the subject of intense research due to its novel electrical and optical properties, which compare favorably to those of graphene and the transition metal dichalcogenides. In particular, BP has a direct bandgap that is thickness-dependent and highly anisotropic, making BP an interesting material for nanoscale optical and optoelectronic applications. Here, we present a study of the anisotropic third-harmonic generation (THG) in exfoliated BP using a fast scanning multiphoton characterization method. We find that the anisotropic THG arises directly from the crystal structure of BP. We calculate the effective third-order susceptibility of BP to be ∼1.64 × 10 m V. Further, we demonstrate that multiphoton microscopy can be used for rapid, large-area characterization indexing of the crystallographic orientations of many exfoliated BP flakes from one set of multiphoton images. This method is therefore beneficial for samples of areas ∼1 cm in future investigations of the properties and growth of BP.

which provides opportunities for fundamental studies and applications that are otherwise not possible with highersymmetry materials. [9] As the prototypical 2D nanomaterial with in-plane anisotropy, BP also serves as a model system that is informing investigations of other emerging anisotropic 2D nanomaterials including ReS 2 , [10,11] GeS, [12] and borophene. [13,14] However, a review of BP anisotropic properties with an emphasis on the experimental techniques capable of resolving such anisotropy has not yet been performed. Herein, starting from the highly anisotropic atomic structure of BP, we review methods for characterizing the anisotropic properties of exfoliated BP including optical, vibrational, electronic, thermal, and mechanical anisotropy. We also provide insight into potential applications that are derived from 2D in-plane anisotropy and suggest how the results for BP can be generalized to other anisotropic 2D materials. Structural AnisotropyBP is the most stable phosphorus allotrope, and exists as a layered van der Waals crystal with an orthorhombic structure at room temperature and pressure. As shown in Figure 1a, the two in-plane directions are defined as a and c, and the out-ofplane direction is defined as b. The unit cell dimensions of bulk BP are a = 3.314 Å, b = 10.478 Å, and c = 4.376 Å, with the interlayer distance being 5.239 Å. [5] Similar to graphene, each phosphorus atom is bound to three neighbors. However, unlike graphene, BP displays out-of-plane distortion resulting in a ridge structure along the zigzag direction (i.e., the a direction) and a puckered structure along the armchair direction (i.e., the c direction), and thus non-equal bond lengths (d 1 = 2.244 Å, d 2 = 2.224 Å) and bond angles (α = 96.34°, β = 102.09°). This structure leads to substantial in-plane anisotropy and resembles the hinge-like structure seen in artificially engineered auxetic materials. [15] As a result, monolayer BP exhibits a negative Poisson ratio. [16,17] The anisotropic in-plane structure of BP can be visualized via microscopy and diffraction methods. By cleaving a bulk crystal to expose the a-c plane, scanning tunneling micro scopy (STM) images reveal atomic chains of BP along the zigzag direction as demonstrated in the left portion of Figure 1b. [18][19][20] Exfoliated BP layers can also be visualized with high-resolution transmission electron microscopy (HRTEM) (right portion of Figure 1b). [21][22][23][24] The in-plane orientation of BP sheets is Two-dimensional (2D) black phosphorus (BP) has recently attracted significant attention due to its favorable semiconducting properties and promise for nanoscale devices. Additionally, BP possesses distinctive anisotropic physical properties, which stem from the non-equal bond angles and bond strengths in its crystal structure along its orthogonal in-plane directions. Here, the anisotropic properties of BP are summarized, along with the methods to characterize them and the opportunities they present for novel technological applications. This survey of the pro...

ReS represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single-crystalline ReS in a distorted 1T phase is determined at room temperature for the in-plane directions parallel and perpendicular to the Re-chains, and the through-plane direction using time-domain thermoreflectance. ReS is prepared in the form of flakes having thicknesses of 60-450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in-plane thermal conductivity is higher along the Re-chains, (70 ± 18) W m K , as compared to transverse to the chains, (50 ± 13) W m K . As expected from the weak interlayer bonding, the through-plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m K , resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in-plane directions. The thermal conductivity and interface thermal conductance of ReS are discussed relative to the other 2D materials.