Black phosphorus: narrow gap, wide applications

Black phosphorus: narrow gap, wide applications Andres Castellanos-Gomez*. Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-nanociencia), Campus de Cantoblanco, E-18049 Madrid, Spain. ([email protected]) The recent isolation of atomically thin black phosphorus by mechanical exfoliation of bulk layered crystals has triggered an unprecedented interest, even higher than that raised by the first works on graphene and other two-dimensional, in the nanoscience and nanotechnology community. In this Perspective we critically analyze the reasons behind the surge of experimental and theoretical works on this novel two-dimensional material. We believe that the fact that black phosphorus band gap value spans over a wide range of the electromagnetic spectrum that was not covered by any other two-dimensional material isolated to date (with remarkable industrial interest such as thermal imaging, thermoelectrics, fiber optics communication, photovoltaics, etc), its high carrier mobility, its ambipolar fieldeffect and its rather unusual in-plane anisotropy drew the attention of the scientific community towards this two-dimensional material. Here we also review the current advances, the future directions and the challenges in this young research field. 1 TEXT Followed by the isolation of graphene by mechanical exfoliation of graphite,1 many other twodimensional materials have been produced by exfoliation of bulk layered crystals whose layers are hold together by weak van der Waals forces.2 Up to 2013, more than 15 different two-dimensional materials were isolated by that simple yet effective fabrication method. But there was something odd however, all those two-dimensional materials (beyond graphene) were composed by two or more elements.2,3 This statement might seem not accurate as silicene, a 2D layer made of silicon atoms, was synthesized in 2012.4 Nonetheless silicene, unlike the other two-dimensional materials referred to above, cannot be isolated by exfoliation of bulk silicon but it is epitaxially grown onto specific substrates. Therefore, for almost 10 years people thought that graphene was a rara avis, the only elemental two-dimensional materials stable in free-standing form. In 2013, a talk in the American Physical Society March meeting entitled ‘Electronic Properties of Few-layer Black Phosphorus’ by Yuanbo Zhang and co-workers dethrone graphene as the only elemental two-dimensional material isolated by mechanical exfoliation.5 According to Zhang et al., atomically thin layers (and eventually one-atom thick layers) of phosphorus can be prepared by mechanical exfoliation of bulk black phosphorus (a layered allotrope of phosphorus synthesized by the first time in 1914 by Bridgman 6). That presentation unleashed a great deal of attention on atomically thin black phosphorus, unprecedented even in the rapidly moving community of twodimensional materials,7–12 but why? What makes black phosphorus more special than other recently isolated 2D materials? This Perspective analyses the facts that motivated the surge of interest on black phosphorus and critically discusses the current advances, future directions and challenges in this recent field. 2 Crystal structure Similarly to graphite, black phosphorus atoms are strongly bonded in-plane forming layers while the layers weakly interact through van der Waals forces. But unlike in graphite, the phosphorus atoms are not arranged in a completely planar lattice but they form a puckered honeycomb lattice (orthorhombic, with space group Cmca). Figure 1a shows a schematic diagram of the black phosphorus crystal structure. This atomic arrangement yields two inequivalent directions within the black phosphorus lattice: the zigzag (parallel to the atomic ridges) and the armchair (perpendicular to the ridges). This strong structural anisotropy is the stem of its particular in-plane anisotropic electrical and optical properties that will be discussed later on in this Perspective. Figure 1. (a) Schematic diagram of the crystal structure of black phosphorus. (b) Calculated band structure of single-, bi- and tri-layer black phosphorus. Data extracted from Ref. [13]. (c) Thickness dependence of the black phosphorus band gap, calculated with different approaches. Data extracted from References [11,13,14]. Band structure: ‘the right gap’ Apart from the crystal structural difference, black phosphorus also differs from graphene in the electronic band structure. While graphene is a zero-gap semiconductor (the conduction and valence 3 bands touch at the so-called Dirac or neutrality point), black phosphorus is a semiconductor with a sizeable gap.15–17 Figure 1b shows the calculated band structure for monolayer, bilayer and trilayer black phosphorus by ab initio calculations with the GW approximation.18 Independently of the number of layers the band gap remains direct at the Γ point of the Brillouin zone. This constitutes a big difference with respect to semiconducting transition metal dichalcogenides that present a gap at the K point and it is only direct for single-layers.19,20 Interestingly, the band gap value strongly depends on the number of layers. Figure 1c summarizes the thickness dependence of the black phosphorus band gap, calculated with different ab initio methods.11,14,21 Eventhough the magnitude of band gap depends on the approximation employed to calculate the band structure, in all the cases there is a marked thickness dependence: from a large gap (close to 2 eV) for singlelayer that monotonically decreases to a narrow band gap value (about 0.3 eV) for bulk black phosphorus. This thickness dependent band gap, due to quantum confinement of the charge carriers in the out-of-plane direction, is stronger that that observed in other 2D semiconductor materials 19,20 and it provides an exceptional degree of tunability: one can select the thickness of black phosphorus to have a material with a band gap value that is optimized for a certain application. This is a very appealing feature as the band gap value spans over a wide energy range that was not covered by any of the other 2D materials isolated to date (see Figure 2). In fact, while graphene can cover the range from 0 eV to 0.2 eV (by patterning graphene into nanoribbon shape or applying a perpendicular electric field to bilayer graphene in order to open a band gap),22–25 transition metal dichalcogenides present a band gap in the range of 1.0-2.0 eV (depending on the thickness, strain level and chemical composition).26,27 Thus black phosphorus bridges the gap between graphene (a zero or nearly-zero band gap semiconductor) and transition metal dichalcogenides (wide band gap semiconductors). But what is so special about that energy range? As it can be seen at the 4 bottom of Figure 2, a lot of applications require semiconductor materials with a band gap value in that range of energies. For example, although other two-dimensional semiconductors could be of potential use in photovoltaic energy harvesting and photocatalisys applications (which are optimized for semiconductors with 1.2 eV - 1.6 eV band gap),28 fiber optic telecommunications (that employs wavelength in the range of 1.2 µm -1.5 µm, corresponding to photon energies of 0.8 eV - 1 eV),29 thermal imaging (typically requires semiconductors with gaps spanning from 0.1 eV to 1.0 eV) and thermoelectric power generation (that uses materials with band gaps in the order of 0.2 eV to 0.3 eV) are out of reach of the two-dimensional semiconductors isolated to date. These applications are covered with conventional 3D semiconductors such as Pb- chalcogenides, In1xGaxAs, In1-xGaxSb and Si1-xGex. Therefore, the band gap values range spanned by black phosphorus nanolayers makes this 2D material especially suitable for thermal imaging, thermoelectric, telecom and photovoltaic applications and also makes it a prospective replacement of conventional narrow gap semiconductors in applications requiring thin, flexible and quasitransparent material where using those 3D materials might be challenging. 5 Figure 2. Comparison of the band gap values for different 2D semiconductor materials. The band gap values for conventional semiconductors have been also included for comparison. The horizontal bars spanning a range of band-gap values indicate that the band gap can be tuned over that range by changing the number of layers, straining or alloying. In conventional semiconductors, the bar indicates that the band gap can be continuously tuned by alloying the semiconductos (e.g. Si 1-xGex or In1-xGaxAs). The range of band gap values required for certain applications have been highlighted at the bottom part of the figure to illustrate the potential applications of the different semiconductors. Bridging the gap between graphene and transition metal dichalcogenides Very rapidly after the first isolating black phosphorus by mechanical exfoliation,7–12 all the expertise build up during 10 years in graphene research was exploited to fabricate and to characterize black phosphorus based nanodevices. Figure 3 shows several examples of black phosphorus nanodevices with different functionalities. The field-effect characteristics of a black 6 phosphorus transistor are shown in Figure 3a.7 These kind of devices show ambipolar field-effect, high hole mobility (in the order of 10 cm2/Vs - 1000 cm2/Vs) and current switching ratios of 10010000.7–12 The ambipolarity of black phosphorus field-effect transistors, rather uncommon in most of 2D semiconductors materials that typically show a unipolar n-type behavior,30 is an asset in the fabrication of more complex nanodevices such as PN junctions. Figure 3b shows an example where the ambipolar field-effect has been exploited to fabricate a PN junction with electrostatic split-gate geometry.31 The built-in electric field at the PN junction can be used to separate photogenerated electron-hole pairs giving rise to a short-circuit current, a feature of photovoltaic effect in solar cells. Alternatively to the split-gate architecture, another concept of black phosphorus based photovoltaic device relies on artificial vertical stacking of two nanosheets with n-type and p-type doping to form a vertical PN junction.32 Figure 3c shows a different example of nanodevice where the reduced dimensions and the low mass of black phosphorus nanosheets are exploited to fabricate nanoelectromechanical resonators with a natural resonance frequency in the MHz regime.33 Coupling black phosphorus with other electronic and optoelectronic devices can be used to add new functionalities and/or to improve the performance of the fabricated black phosphorus nanodevices. Figure 3d shows an example of ultrafast photodetector fabricated by coupling a black phosphorus transistors to a silicon waveguide optical resonator.34 To date, several photodetector devices based on black phosphorus have been demonstrated and characterized as this is a very active research area in the black phosphorus field.12,32,34–41 Figure 3e displays the electrical characteristics of an inverter amplifier device fabricated by coupling an n-type MoS2 transistor and a p-type black phosphorus transistor,9 demonstrating the potential of ultrathin semiconductor materials for logic circuit applications.9,42,43 7 Figure 3. Black phosphorus based nanodevices. (a) Ambipolar field-effect transistor, adapted from Ref. [7] with permission. (b) Electrostatically-gated PN junction displaying photovoltaic effect, adapted from Ref. [31] with permission. (c) Nanoelectromechanical resonator vibrating in the very-high frequency regime, adapted from Ref. [33] with permission. (d) High frequency photodetector coupled to a silicon waveguide, adapted from Ref. [34] with permission. (e) Inverter amplifier based on MoS2 and black phosphorus transistors, demonstrating the potential of black phosphorus for logic circuits, adapted from Ref. [9] with permission. Figure 4 compares the performances of black phosphorus nanodevices with those reported for other two-dimensional materials: graphene and transition metal dichalcogenides. Figure 4a displays the charge carrier mobility vs. the current on/off ratio measured for field-effect transistors made out of graphene,44–49 transition metal dichalcogenides 50–58 and black phosphorus.7,8,11,12,35,40,42,43,59–63 Interestingly, the field-effect performance of black phosphorus 8 FETs also bridges the gap between graphene (very high mobility and poor current on/off ratio) and transition metal dichalcogenides (low mobility and excellent on/off ratio). Note that the data displayed in Figure 4a is an updated version of Figure 2b from Ref. [64] where several recent relevant references have been added. A similar behaviour is also observed for the performance of photodetectors. Figure 4b summarizes the photoresponse and the response time measured for photodetector based on graphene,65–69 transition metal dichalcogenides 70–83 and black phosphorus.12,31,34,38,39 While graphene detectors typically display a fast but rather weak photoresponse, transition metal dichalcogenides can have photoresponse values as high as 103 A/W (104 times larger than that of pristine graphene based photodetectors) but they tend to suffer from very low response time, hampering their use in video imaging applications. Black phosphorus photodetectors, on the other hand, show a balance between speed and photoresponse. The data displayed in Figure 4b has been adapted from Figure 12a in Ref. [37], including some recent relevant references. Finally, Figure 4 also presents a comparison between two of the most important physical properties for thermoelectric power generation applications: the Seebeck coefficient and the electrical resistivity. An ideal thermoelectric material should present a high Seebeck coefficient and a low electrical resistivity. Note that the amount of reported data for thermoelectric devices is still very limited (and mostly limited to theoretical studies) in comparison with electronic and optoelectronic devices.84–90 According to Figure 4c, transition metal dichalcogenides can reach sky high values of the Seebeck coefficient but with high electrical resistivity and graphene, on the other hand, presents rather low Seebeck coefficient but with a small electrical resistivity.91–94 For black phosphorus, the only direct measurement of the thermoelectric power has been done for bulk samples finding a Seebeck coefficient value of +345 µV/K with an electrical resistance in between that of graphene and transition metal 9 dichalcogenides.89 Figure 4c also represents theoretical data calculated for black phosphorus nanolayers with different density of charge carriers,85,86,90 which shows that using electrostatic field-effect one could substantially reduce electrical resistance while keeping a high enough Seebeck coefficient which motivates the exploration of black phosphorus as a thermoelectric material. Figure 4. Comparison between the performances of 2D-based nanodevices. (a) Carrier mobility vs. current on/off ratio reported for field-effect transistors based on graphene, TMDCs and black phosphorus. (b) Summary of the responsivity and time response of photodetectors based on graphene, TMDCs and black phosphorus. The data displayed in (a) and (b) are updated versions of Figure 2b in Ref. [64] and Figure 12a in Ref. [37], including recent relevant references. (c) Seebeck coefficient vs. electrical responsivity of graphene, TMDCs and black phosphorus. A rather unusual in-plane anisotropy As discussed above, the crystal structure of black phosphorus (sketched in Figure 1a) is the stem of its in-plane anisotropic properties.7,9,10,14,21,35,39,87,88,95–97,97–117 Graphene, boron nitride or Moand W- based transition metal dichalcogenides, on the other hand, do not present noticeable inplane anisotropy. Figure 5 summarizes the in-plane angular distribution of electrical, optical and mechanical properties of black phosphorus. Field-effect devices with electrodes arranged at 10 different angles have been used to probe the anisotropy of the charge carrier mobility and the electrical conductance, finding a higher mobility and conductance along the armchair direction (up to 50% higher than along the zigzag direction at room temperature). 7,9,10,105 Figure 5a shows a polar plot with the angular dependence of the DC conductivity measured in a black phosphorus device with electrodes patterned with an angular spacing of 30º (see the inset). 10 The relative optical extinction, measured on the black phosphorus device in Figure 5a, is shown in Figure 5b for different polarization angles (relative to the horizontal axis) showing an even stronger anisotropy.10 This marked linear dichroism of black phosphorus, optical absorption that depends on the relative orientation between the materials lattice and an incident linearly polarized light, has also strong implications in its Raman spectra, photoresponse and photoluminescence emission yield.21,35,39,105,110,115 For example, Figure 5c shows a polar plot with the polarization angle of the photoluminescence yield of a black phosphorus flake excited with a laser linearly polarized along the zigzag direction, armchair direction and a direction at 45º between the zigzag and armchair directions. For all the different excitation laser polarization angles, the photoluminescence yield always shows a high degree of polarization along the armchair direction of the flake.110 Finally, Figure 5e shows the calculated angular dependence of the Young’s modulus of black phosphorus which is predicted to be up to 4 times higher along the zigzag direction (Ezz = 166 GPa) than along the armchair (Eac = 44 GPa),118 although no direct experimental evidence of this anisotropic mechanical properties has been reported yet. This characteristic in-plane anisotropy of black phosphorus can be exploited to study quasi-1D physics in a system with 2D geometry, which is easier to integrate with other devices and to interact with them because of their large surface area. Moreover, the in-plane anisotropy can also provide with novel functionality to devices based on black phosphorus (see the Outlook and Summary section). 11 Figure 5. In-plane anisotropic properties of black phosphorus. (a) Angular dependence of the DC electrical conductance of a black phosphorus transistor. (b) Angular dependence of the relative optical extinction measured on the same flake displayed in (a). Panels (a) and (b) have been adapted from Ref. [ 10] with permission. (c) Angular dependent photoluminescence yield of a black phosphorus flake with laser excitation (λ = 532 nm) polarized along the zigzag direction, armchair direction and a 45º direction. Adapted from Ref. [110] with permission. (d) Calculated angular dependence of the black phosphorus Young’s modulus. The calculation predicts that the zigzag direction is about 4 times stiffer than the armchair direction. Adapted from Ref. [117] with permission. Challenges: isolation and stability In the following, we will discuss the main two challenges that the black phosphorus research is facing so far: the fabrication of large area samples and the environmental instability of black phosphorus. Most of the experiments reported on black phosphorus in the literature relied on mechanically exfoliated samples. This fabrication method, although it provides high quality 12 material for research, it is not scalable in industrial applications. Recently demonstrated liquidphase exfoliation, on the other hand, offers a cheap and scalable alternative to isolate ultrathin black phosphorus layers but the quality of the produced material might not be high enough for certain electronic or optical applications.119–124 Very recently, another alternative method to fabricate black phosphorus thin films based on pulsed laser deposition, a technology that can be scaled up, has been presented.125 The fabricated layers, however, lack of crystallinity and their electrical properties are still way poorer that those of pristine black phosphorus material. Therefore in order to transfer the black phosphorus from research laboratories to the first real-life applications one should solve the challenge of developing a large area synthesis method to fabricate highly crystalline black phosphorus layers. For graphene, it took 5 years since the first mechanical exfoliation experiments till the development of the first chemical vapor deposition recipes to fabricate large area single-layer graphene.126,127 This time was significantly reduced to about 2 years for transition metal dichalcogenides,128,129 indicating that the scientific community intensified the effort to develop large-area fabrication techniques to grow high quality wafer-scale two-dimensional materials. Thus the fabrication of large area black phosphorus nanolayers is surely motivating the groups specialized on synthesis of nanomaterials nowadays as a reliable recipe to fabricate large area crystalline black phosphorus thin films will undoubtedly have a strong impact in the two-dimensional materials community. Another current challenge in the black phosphorus research community is the environmentalinduced degradation of the black phosphorus.8,11,130–133 Most of the two-dimensional materials studied to date (graphene, h-BN, MoS2, etc) are rather stable at atmospheric conditions which facilitated the fabrication of devices based on these materials. Back phosphorus, however, shows a relatively large reactivity. Exfoliated flakes of black phosphorus are highly hygroscopic and tend 13 to uptake moisture from air (see Figure 6a).130 The long term contact with the water condensed on the surface seems to degrade the black phosphorus, as it can be seen by measuring the electrical performance of transistors 130,131 or the sheet resistance (measured with an atomic force microscopy based microwave impedance microscope) 132 as a function of time (Figure 6b and 6c). In the work by Favron et al.133 a systematic study of the degradation mechanisms combining Raman and transmission electron spectroscopies allowed to determine that the most plausible mechanism behind the aging of black phosphorus in air is a photoassisted oxidation reaction with oxygen dissolved in the adsorbed on its surface. Nonetheless, the timescale at which noticeable degradation takes place is long enough to allow the fabrication of black phosphorus devices but they should be operated in vacuum conditions to ensure their reliability. A different approach relies on encapsulating the black phosphorus devices at an early stage during the fabrication.60,131,132 Different encapsulation options have been explored so far: polymer dielectrics,38,134 atomic layer deposited oxides 139 43,59,131,132 and van der Waals heterostructures with boron nitride flakes.60,63,135– Figure 6b and 6c show the time evolution of the electrical properties of black phosphorus flakes encapsulated with an aluminum oxide layer.131,132 While the pristine, unencapsulated, flakes degrade in a time scale of 1-2 days, the encapsulated flakes remains unaltered for several days. Figure 6d shows the fabrication steps necessary to encapsulate black phosphorus flakes between two dielectric flakes of hexagonal boron nitride and to make electrical contacts to the edges of black phosphorus.138 This encapsulation and contacting method has been recently developed to fabricate high electronic quality graphene and MoS2 samples, which showed record-high charge carrier mobilities.140,141 The black phosphorus devices fabricated by this method also present charge carrier mobilities higher than conventional black phosphorus field-effect transistors and their excellent electronic properties have been probed by studying quantum oscillations at low 14 temperatures.135–139 Therefore, despite of the youth of the black phosphorus research, the experimental efforts to overcome its environmental instability by encapsulation are at a very advanced stage. Figure 6. Environmental-induced degradation of black phosphorus and its encapsulation as a route to preserve its pristine properties. (a) Atomic force microscopy topography images acquired as a function of time. Due to the high hygroscopic character of black phosphorus it uptakes moisture from air. Adapted from Ref. [130] with permission. (b) Time evolution of the charge carrier mobility of black phosphorus field-effect transistors. Unencapsulated devices rapidly degrade upon air exposure while devices encapsulated with an aluminum oxide layer shows almost time independent performance for several days. Adapted from Ref. [131] with permission. (c) Microwave impedance microscopy map of the local sheet resistance of two black phosphorus flakes: one unencapsulated and one encapsulated with a thick film of aluminum oxide. While the encapsulated device remains unaltered for several days, the unencapsulated device resistance strongly changes with time, starting at the edges of the flake and growing towards the center. Adapted from Ref. [132] with permission. (d) Sandwiching of black phosphorus between h-BN fakes to fabricate high-quality encapsulated devices with edge electrical contacts. Adapted from Ref. [138] with permission. 15 Outlook and summary In the last part of this Perspective, we will briefly discuss the possible future avenues in the black phosphorus research. One can anticipate that among the most urgent topics one will be, as previously discussed in the paragraph devoted to the challenges, the development of a large-area fabrication technique to isolate high quality wafer scale thin films of black phosphorus as that result could have a big impact not only in the scientific community but also in the first applications based on black phosphorus. Because of its narrow band gap value, multilayered black phosphorus will also find a place in photodetection in the near- and mid-infrared part of the spectrum and in thermoelectric power generation applications. Although some black phosphorus photodetector devices have been already demonstrated and characterized, only simple architectures have been studied so far and there is plenty of room to optimize the performance of the devices and to explore unconventional device architectures. For instance, the combination of graphene with semiconducting quantum dots have demonstrated to improve the detectivity of graphene-based photodetectors by a factor of 109.142 Regarding the application of black phosphorus on thermoelectric applications, despite of the recent theoretical interest on the thermoelectric properties of black phosphorus 84–88,90 the amount of experimental works is still very scarce and it is basically a completely open field that will bloom in the near future because of the promising properties of this material for thermoelectrics. For instance the inherent in-plane anisotropic properties of black phosphorus can be exploited to increase the performance of thermoelectric power generation devices as the high electrical conductance and with high thermal conductance crystal directions are orthogonal.90 Therefore, in black phosphorus one can engineer a device with low electrical resistance that can keep a large thermal gradient along the device, which are the requirements for a highly efficient thermopower generator. 16 Another interesting aspect in black phosphorus research is the use of strain engineering, the modification of the electronic and optical properties of a material by subjecting it to a mechanical deformation. In fact, strain engineering in 2D materials,143 not only in black phosphorus, is recently gaining more and more attention as they can typically stand very large deformations before the rupture (around 10%) and thus one can widely tune the electrical/optical properties through strain engineering.144–149 Conventional 3D semiconductors, on the contrary, tend to break for moderate deformations (<1.5%). For black phosphorus, strain engineering might be even more interesting than for other 2D semiconductors because of its in-plane anisotropy.85,87,96,98,114,150 Recent theoretical works predicted that one can control the preferable direction for charge carriers by changing the level of strain on the black phosphorus lattice.98 Nonetheless, the amount of works on strain engineering in black phosphorus is still limited and mainly restricted to theoretical studies. In summary, this Perspective article has reviewed the recent advances in the research on black phosphorus. The aspects that motivated the large surge of experimental and theoretical works on this recently isolated material has been critically discussed finding several factors that justify the recent interest on this material: (1) its band gap spans over a range of the spectrum not covered by any other 2D material isolated to date, (2) it presents a good trade-off between charge carrier mobility and current on/off switching ratios which makes it a prospective material for digital and high frequency electronics, (3) it shows a rather exotic in-plane anisotropy that can be exploited to engineer new concepts of devices. Another remarkable characteristic of this material is that black phosphorus electronic, optoelectronic and thermoelectric nanodevices display performances that bridge those of graphene and transition metal dichalcogenides. This trade-off between graphene-like (rather metallic) and transition metal dichalcogenide-like (wide band gap 17 semiconductor) can be optimal (or even crucial) for many different applications such as thermal imaging, photodetection in the telecom range or thermopower generation from themal waste. The challenges and the open questions in this young research field have been also discussed in the last part of this Perspective. We consider that the two main challenges are (1) the fabrication of waferscale crystalline black phosphorus thin films and (2) overcoming the environmental induced degradation of black phosphorus by finding encapsulation methods that preserve its pristine properties at long-term. Among the possible future directions of the field, we highlighted here (1) the integration of black phosphorus in complex device architectures or combining it with other nanomaterials to enhance the performance, (2) the experimental exploration of black phosphorus as a thermoelectric material (especially exploiting its anisotropy to decouple the high electrical and thermal conductance directions) and (3) the use of strain engineering to control the electrical and optical properties of black phosphorus (and even to control the preferential in-plane conduction directions for charge carriers). 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