Interface and surface engineering of black phosphorus: a review for optoelectronic and photonic applications

Since being rediscovered as an emerging 2D material, black phosphorus (BP), with an extraordinary energy structure and unusually strong interlayer interactions, offers new opportunities for optoelectronics and photonics. However, due to the thin atomic body and the ease of degradation with water and oxides, BP is highly sensitive to the surrounding environment. Therefore, high-quality engineering of interfaces and surfaces plays an essential role in BP-based applications. In this review, begun with a review of properties of BP, different strategies of interface and surfaces engineering for high ON-OFF ratio, enhanced optical absorption, and fast optical response are reviewed and highlighted, and recent state-of-the-art advances on optoelectronic and photonic devices are demonstrated. Finally, the opportunities and challenges for future BP-related research are considered.

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For the fabrication of these devices, interface and surface engineering play a vital role [53][54][55].Due to its atomic thickness nature like other 2D materials, few-layer BP is essentially an 'interface type' material, in which both the properties of BP and the performance of BP-based devices are susceptible to the interface and surface [56,57].First, intrinsic BP is a p-type semiconductor with an unbalanced hole-dominated transport behavior [14,29].By interface engineering or surface functionalization, the dominant types and transport properties of BP can be tuned for the device requirements, hence realizing complementary device applications [19,58].In addition, when BP is contacted with different semiconductor, metal, or dielectric materials, different types of interfaces are formed, which affects abundant physical mechanisms for multifunctional devices, including band alignments, carrier transfer, contact barriers, dielectric screening, and so on [29,59,60].By engineering the interface between BP and other materials with strong light-matter interactions, different exotic effects can also be obtained, including anisotropic interlayer exciton, exciton-plasmon polaritons (EPP), plasmon-phonon polaritons, and so on, which can simulate the development of future photonics [61][62][63].Last but not least, BP degrades very fast in the ambient environment.Through interface engineering and surface passivation, BP can be protected or modified for practical applications in the atmosphere [64][65][66].
In this review, we begin with a comprehensive overview of the electronic and optical properties of BP, as well as its degradation mechanism.Then we discuss the different interfaces between BP and other materials and their engineering strategies.The surface modifications and functionalization of BP are also discussed in the section.Subsequently, the interface and surface engineering of BP for different types of applications are demonstrated.Finally, we provide the conclusions and highlight the challenges and opportunities of BPbased optoelectronics and photonics.

Band structure and electronic properties
BP has a layered structure composed of P atoms, in which each P atom is covalently bonded with each of its three adjacent phosphorus atoms [10,25,67].Among them, two P-P bonds form in zigzag (ZZ, perpendicular to the pucker) direction, and a bond is formed in armchair (AC, parallel to the pucker) direction, where P atoms are not arranged in the same plane and create the unique puckered structure (see figure 1(a)).Using the GW (G is the Green's function, W is the screened coulomb interaction) calculation, the bond length along the ZZ direction is ∼2.24Å, while the P-P bond length between the two P atoms in the upper and lower sublayer is ∼2.28 Å [25].Because of the reduced-symmetry orthorhombic lattice structure of BP, the lattice parameter along the x-and z-directions are different (∼4.58 Å and 3.32 Å, respectively).The unique in-plane anisotropy provides BP with anisotropic physical, electronic, and optical properties [15,19].Moreover, BP has a relatively large interlayer distance of ∼5.25 Å, which makes the feasibility of bulk BP exfoliating into few-layer BP.The distance between the top and bottom planes of BP is ∼2.1 Å.
The band structure of bulk BP has long been studied since its discovery in 1914 [69,70].According to theory calculations and experimental characterizations, BP has a direct bandgap of ∼0.3 eV at Z-point in the Brillouin zone (see  [16] by permission from Springer Nature Customer Service Centre GmbH.(d), (e) Band structures of monolayer (d) and bilayer BP (e), which are calculated by the tight-binding parametrization.Reprinted with permission from [68], copyright (2014) by the American Physical Society.figures 1(b) and (c)) [16].When decreasing the layer numbers to few-layers, the interlayer coupling is weaker, resulting in a larger bandgap, and both conduction band minimum (CBM) and valence band maximum (VBM) shift to Γ point in the Brillouin zone [11,16,68,71].In monolayer BP, the direct bandgap at Γ point is ∼1.6 eV, while in bilayer BP, both valence band (VB) and conduction band (CB) undergo band splitting due to the introduction of interlayer interaction and the companying wavefunction overlap.According to the tightbinding calculation, the bandgap reduces by over 0.5 eV from monolayer to bilayer (as shown in figures 1(d) and (e)).In fewlayer BP system, with stronger interlayer interaction and larger dispersion of VB and CB, the bandgap is smaller (∼0.53 eV in five-layer BP).Besides, BP represents anisotropic VB and CB dispersion [16].The energy band is very steep in the Γ-X direction (corresponding to the AC direction in real space), with the carrier effective masses of 0.15 m 0 (hole) and 0.17 m 0 (electron).While in the Γ-Y direction (the ZZ direction), the energy band is nearly flat, where the hole effective mass is heavily at 6.35 m 0 and the electron effective mass is 1.12 m 0 .The results indicate that the carrier transport along AC direction is much faster than along ZZ direction.
Due to the direct bandgap feature and steep energy band structure, monolayer BP has high carrier mobility over 1000 cm 2 V −1 S −1 , and the hole mobility is theoretically predicted to be more than 26 000 cm 2 V −1 S −1 .In particular, mechanically exploited BP sandwiched between two hBN flakes shows ultrahigh hole mobility of 45 000 cm 2 V −1 S −1 at cryogenic temperature [14].The high carrier mobility provides BP with fascinating phenomena such as quantum oscillations and quantum Hall effects, and field effect transistor (FET) applications with high performance.

Optical properties
2.2.1.Optical absorption.Due to the direct bandgap characteristics regardless of its thickness, BP has tunable optical properties enormously depending on the number of layers.In 2014, Low et al systematically calculated the optical properties of bulk BP by the Kubo formula, including optical conductivities and optical absorption with a thickness of more than 4 nm (8 L) [18].The absorption edge witnesses an increase from 0.3 to 0.6 eV with the thickness decrease, as shown in figure 2(a).When further decreasing the film thickness, the dielectric screening is significant and the excitonic effect cannot be excluded, the enhanced optical absorption appears near the absorption edge.Monolayer and few-layer BP as 2D semiconductors, show step-like absorption characteristics in the single electron interaction picture, and the quasi-particle bandgap E g would be changed due to the onset of band-to-band transitions [72,73].Photoluminescence (PL) spectroscopy has been widely applied to experimentally observing E g of 2D BP [14,22,45,74,75].However, due to the instability of BP and the high sensitivity of PL spectra to defects and doping, the value of E g reported in these papers are quite different.In 2017, Li et al probed the optical bandgap in 1-5 L BP by a simple absorption spectroscopy [20].The light absorption is insensitive to impurities and defects in 2D  cm −2 and momentum q paralleled with x (right) and y (left) directions, respectively.(i) Scaling of plasmon frequency with electron concentration of monolayer and 20 nm thick BP, where graphene is for comparison.(j) Distribution of anisotropic plasmons in k surface.(h) and (i) are reprinted with permission from [78], copyright (2014) by the American Physical Society.(j) is reprinted with permission from [68], copyright (2014) by the American Physical Society.crystals, so absorption spectroscopy is a reliable method for determining the optical bandgap of monolayer and few-layer BP.Combing with the rigorous ab initio GW Bethe-Salpeter equation (GW-BSE) calculations, the direct optical bandgap energies evolve from 1.73 eV to 0.75 eV in 1-5 L BP.A rich set of sub-band optical resonances are also revealed, which covering a broad spectral range.
Owing to the orthorhombic crystal structure with low symmetry, BP exhibits highly optical in-plane anisotropy [15,16].Qiao et al predicted the polarized optical absorption by calculating the dielectric function [16].The experimental geometry is in figure 1(a), in which the linearly polarized light near-normally illuminates into the sample, and both the sample and the light polarization can be rotated.Two absorption spectra are obtained under the light incidence in the z-direction along with x (armchair) and y (zigzag) directions, respectively, as shown in figures 2(b) and (c).The first absorption peak in figure 2(b) corresponds to the optical bandgap of BP and the absorption edge is at 1.55 eV in the monolayer, which remarkably decreases with thickness.By contrast, under y-polarized light incidence, the cut-off energy is found at 3.14 eV in one-layer BP and its position is insensitive to the thickness, remaining at around 2.76 eV in bulk BP.The dichroism phenomenon is explained by a spatially structural model of wavefunctions in BP.The dipole operator along the armchair direction connects the VB and CB states, which allows the direct inter-band transition, while in the zigzag direction, the direct bandgap process is forbidden and the energy transition from VB to CB states occurs at much higher photon energy [16].Lan et al and Ling et al studied the behavior of anisotropic electron-photon interactions in few-layer BP using angle-resolved light absorption spectroscopy.According to the previous reports, the similar experimental results are obtained by polarized absorption measurement at 1.96 eV.Both the thin (9 nm) and thick (225 nm) BP flakes are measured, in which the imaginary part of the refractive index (i.e. the absorption coefficient) along the armchair direction is significantly larger than that along the Zigzag direction.

Excitonic behavior.
Similar to other 2D semiconductors.Few-layer BP has strong many-electron effects of excited states due to the reduced dimensionality and compressed dielectric screening [21,76].Therefore, when discussing the band structure of few-layer BP, excitonic behavior as well as the related electron-hole and electron-electron interactions should be considered.In few-layer BP, due to the quantum confinement along the out-of-plane direction, the valence and CBs split into quantized sub-bands (figure 2(d)) [18,23,33,77].Optical transitions are allowed when subbands index has no difference, which leads to the exciton bound state.Figure 2(e) is the illustration of its optical absorption [22].In free carrier states, the optical absorption of BP is characteristic of step-like features with a quasi-particle bandgap E g .The transitions from the exciton ground state (1s) to the exciton excited states (2s) lead to new spectral features below E g .The energy binding energy represents the difference between the lowest energy absorption peak and E g .Tran et al obtained the layer-dependent energy structure of fewlayer BP by calculating a GW-BSE [21].Compared to standard Density Functional (DFT) calculations, the effects of electronelectron interactions are considered in the GW calculation and the underrated quasi-particle bandgap is corrected to 2 eV in Monolayer BP.Furthermore, the exciton binding energy is predicted to largely be at 800 meV, and the lowest-energy absorption peak is 1.2 eV.However, because of the difference between the few-layer BP's optical quality and the effect of substrates, the experimental value of exciton binding energy differs in different reports [20,22,24].For example, according to the first experimental report about monolayer BP's excitons, the exciton binding energy is largely at 900 meV [22], but in another experiment, the value is only 0.1 eV [20].In 2018, the layer-dependent exciton binding energies in BP from monolayer to over 10-layers were experimentally determined by IR extinction spectra [23].Here 4L BP is taken as an example.The BP sample is exfoliated on polydimethylsiloxane (PDMS) substrates and the exposure time in the ambient environment is typically <5 min, which promises the high optical quality of BP.A solid and sharp absorption peak (∼20 meV linewidth) is observed at room temperature for armchair light polarization, indicating the huge light-matter interactions at the E 11 resonance.While for zigzag light polarization, the resonance peak is featureless, originating from unique quasi-one-dimensional (1D) band dispersions of BP.By using a polarization-resolved infrared absorption spectra, the significant anisotropic optical conductivity is observed on a 6L BP sample, as shown in figure 2(f).In addition to the anisotropy, the layer-controlled E b is also observed in their experiments (figure 2(g)) [77].For freestanding monolayer BP, E b is at 762 meV, and the value would be reduced to 316 meV for monolayer BP on PDMS due to the influence of substrate screening [79].
2.2.3.Plasmonic behavior.Surface plasmons in 2D materials provide light confinement and electric-field localization at sub-wavelength scales and can be widely tunable by electrical, which have attracted enormous attention.Low et al theoretically reported collective plasmonic excitations in BP [78].Due to the thickness-dependent optical energy band, BP exhibits tunable optical conductivity with different thicknesses.Figure 2(h) shows the calculated plasmon dispersion in monolayer BP, where the electromagnetic response of BP includes two types of electron motions: the intraband motion (motions of electrons in the CB or holes in the valence band) and the interband transitions (electron transitions between the valence and the CB).As the calculated results in figure 2(i) show, the plasmon frequency (ω) in monolayer BP is determined by carrier density (n), where ω ∝ n 1/2 .While in multilayer BP, due to the interband coupling, the plasmon dispersion relation is modified to ω ∝ n β , where β < 1  2 instead [80].The strength of the interlayer screening is between that of graphene and the TMD semiconductors, such as MoS 2 .In addition, due to different effective mass along with armchair and Zigzag crystal direction, the plasmonic dispersion shows anisotropic characteristics.The plasmonic propagation along the armchair direction is Landau damped only, which shows long-lived propagation time (figure 2(j)).Corresponding to the reduced symmetry of the atomic arrangement, the hyperbolic plasmon polaritons are predicted to naturally exist in BP, which is the result of the coupling between the inter-band and intra-band optical conductivities [81].

Nonlinear optics.
Owing to the quantum confinement, 2D materials commonly have larger nonlinear optical coefficients than their bulk counterparts.Different kinds of the novel nonlinear optical phenomenon have been found in 2D materials, such as strong second-order nonlinear optical effects and second-harmonic generation in MoS 2 [82,83], WSe 2 [84,85], and hBN [86], third-order nonlinear optical effects and large Kerr nonlinearity in graphene [87,88], and so on.Due to the centrosymmetric crystalline structure in intrinsic BP, the second-order nonlinearity is prohibited, but the strong anisotropy and layer-dependently energy band structure allow BP with intriguing third-order nonlinear effects.For example, Third-harmonic generation (THG) has been achieved in multilayer BP samples, demonstrating the substantial third-order nonlinear (χ (3) ) process and the promising possibility in mid-infra (MIR) nonlinear optics [89].Excited by a 1557 nm pump laser, a significant THG signal is obtained in a mechanically exfoliated BP flake (as shown in the microscopy image of figure 3(a)) at the center wavelength of 519 nm.According to the THG mapping results in figure 3(b), the THG intensity is highly dependent on the BP's thickness and sensitive to the wrinkles and boundaries.When the thickness is around 15 nm, the THG reaches peak intensity.Two competing mechanisms that the depletion of the THG signal originating from the inherently strong absorption at visible wavelengths and the phase mismatch between the THG and fundamental signal, are responsible for the thickness dependence [34,90].
The real part of the complex χ (3) process is responsible for the intensity-dependent refractive index (Optical Kerr effect, n 2 (ω)), which has been studied in BP [91,92].As the calculation results in figure 3(c), the n 2 (ω) of multilayer BP along armchair direction varies from positive (corresponding to the self-focusing effect) to negative (the selfdefocusing of the optical beam) in infra-wavelengths [92].Furthermore, BP shows anisotropic n 2 (ω) dispersion along armchair and Zigzag directions in which the maximum positive values are about 5.6 × 10 −10 cm 2 W −1 (in armchair direction) and 6 × 10 −11 cm 2 W −1 (in armchair direction), respectively.The strong two-photon absorption from visible to infra wavelengths has also been predicted in BP in the paper.
The saturable absorption, originating from the imaginary part of the χ (3) process, has been systematically investigated in BP from ultraviolet to MIR wavelengths [91,[93][94][95][96].According to Wang et al BP dispersions show a large nonlinear absorptive coefficient (α NL ) and the value of imaginary part of the χ (3) process (Imχ (3) ), which is comparable to that of graphene, and much more significant than that of MoS 2 dispersions at 800, 1550, and 2000 nm (figures 3(d) and (e)) [93].In particular, the bandgap of 3-4 L BP is around 0.8 eV, corresponding to a wavelength of 1550 nm.As shown in figure 3(f), 3-4 L BP has ultrafast excited carrier dynamics, where the carrier-to-carrier scattering decay time is only ∼35 fs when the excited light is 1550 nm, which is even faster than that of graphene under the same experimental conditions, indicating the potential of BP as an excellent candidate for telecommunication photonics.

Degradation
Despite the most stable allotrope in the phosphorus family, few-layer BP still has poor stability in an ambient environment.According to some pioneering studies [97,98], there are five valence electrons in each P atom, but only three of them form covalent bonds with adjacent atoms.An unbonded lone pair is formed by the other two electrons, which are liable to be degraded in air. Figure 4(a) is the atomic force microscope (AFM) image of freshly exfoliated BP exposed in air, in which small bumps are seen.After a few days, droplet-like structures were observed on the surface.
To explaining the degradation mechanism, there are a large number of papers [67,[99][100][101][102][103].Favron et al firstly investigated the origin of degradation by in situ Raman and transmission electron spectroscopies [104], where the effect of visible light in the degradation process is emphasized.Huang et al discuss the results of a joint experimental and theoretical study to comprehensively address the interactions of BP with O 2 and H 2 O and their effects on the electronic properties and the wetting by aqueous solutions.BP flakes with exposure to air, water with dissolved oxygen, and deuterated water were monitored simultaneously.They found that oxygen plays a crucial role in the chemical modification of BP and in changing its electronic properties, whereas deoxygenated water in the absence of O 2 has a negligible effect.According to research works [67,[99][100][101][102][103], water, oxygen, and light are combinedly responsible for the chemical breakdown of BP, and even hBN encapsulation was unable to eliminate the degradation.
Wang et al studied the degradation-induced changes in the energy structures of few-layer BP [105].To characterize the impacts of air exposure on the electronic structure of FL-BP, we monitored the evolution of IR and PL spectra of few-layer BP samples in the air over time.All samples were deliberately exposed to air for a specific time and subsequently protected in a chamber by purging nitrogen gas for optical measurements.From the measured IR absorption and PL spectra, the degradation induces a blueshift of all-optical resonances, with shift rates sub-band and layer dependent.They simulate the blueshift of the optical transitions numerically by numerically solving the one-dimensional Schrodinger equation under the effective-mass approximation.The band structure of pristine FL-BP can be approximated as that of an infinite square quantum well.By the same token, the band structure will also be sensitive to the modification of the quantum well profile induced by the oxidation of the surface layer.

Interface engineering
For conventional bulk materials, the main approach of changing their physical properties is to modify their chemical bonds or crystalline structure, such as introducing dopant atoms.However, for layered materials like BP, owing to the naturally atomic body thickness, the changes of the material interfaces or interlayer stacking configurations can highly affect materials' properties without changing in-plane chemical bonds.Furthermore, unlike bulk materials with strong covalent or ionic bonds between every neighboring atom, BP has relatively weak interlayer interactions, in which various stacking configurations with different materials (including different semiconductors, metals, and dielectric materials) are allowed.In this section, the engineering strategies of three kinds of BPbased interfaces (BP-semiconductor, -metal, dielectric interfaces) are demonstrated to be not only efficient but convenient for modifying both electronic and optical properties of BP.

Homojunctions.
The design of 2D homojunction is a great strategy since the unique geometries and properties, such as homogeneous components, efficient charge transfer at the interface, and perfect lattice matching.Till now, various preparation approaches have been studied for constructing 2D homojunction, including vapor-phase deposition, chemical doping, electrostatic doping, laser irradiation, and so on [106].In 2015, Yuan et al constructed a BP-based vertical p-n junction structure (figure 5(a)) [30].By ionic gel gating, the surface of the BP flake is heavily n-doped, where electrons are strongly confined, while in the deeper bulk regime, the p-type behavior is preserved.The photogenerated carriers are separated into surface and bulk layers driven by the perpendicular electric field, which significantly enhances the value of photocurrent.The lateral BP p-n junction was also formed by chemical doping methods.Benzyl viologen (BV) as the surface electron dopant was partially doped on the top surface of a fewlayer BP [58].Because the reduction potential of BV is higher than the CBM of few-layer BP, the Fermi-level of BP occurs an upshift to the CB and the carrier type is converted from p-type to n-type.Al 2 O 3 with 10 nm thickness was deposited on the other half region of the BP sample as the hole dopant layer and the lateral p-n junction with ambient stability is formed (figure 5(b)).By calculating a modified Schottky diode equation, the homojunction based FET has a low diode ideality factor and small series resistance, showing the advantages compared to other chemically doped p-n structures.Moreover, photodetectors with high detectivity and photovoltaic devices with a power conversion efficiency of ∼0.75% were demonstrated based on the fabricated BP homojunction diode.
A new type of homo-structure is obtained based on twisted BP.According to the first-principles calculations, twisted bilayer BP surprisingly displays isotropic electronic dispersion [107].In addition, the interlayer couplings in the 90 • twisted structure are much weaker than in the individual BP with high anisotropy [107,108].Srivastava et al systematically studied the twisted trilayer BP (figure 5(c)).Through a standard PDMS stamping transfer technique, a multilayer BP (7-14 L) flake was stacked between two degenerate BP, where the crystal orientation of middle BP is twisted 90 • compared to the top and bottom BP flakes.The negative differential resistance and resonant tunneling effects are observed in the orthogonally stacked homo-structure.Furthermore, different from heterostructures, the absence of a physical barrier provides twisted BP with higher current densities and larger peak-tovalley current ratios [109].

Heterojunction engineering.
Van der Waals materials display a small lattice mismatch compared to their traditional semiconductor counterparts due to the absence of dangling bonds at the interface.Hence, BP can be staked with other 2D semiconductors with different energy structures flexibly, and different types of heterojunctions are formed at the interface [29,110,111].Such variety in BP-based heterojunctions opens up opportunities for multifunctional and scalable nextgeneration optoelectronic systems [110,112].
Experimental evidence of BP-based type-I heterostructure is shown in figure 5(a), where the few-layer BP flake (6 nm) was transferred onto a monolayer WSe 2 thin-film [113].Since the layer number of BP is over six layers, the direct bandgap is remarkably narrower than that of monolayer WSe 2 (∼2 eV bandgap).From the DFT simulation results, a straddlinggap (type-I) alignment is formed, where BP is regarded as a quantum well and collects the separated electron-hole pairs from the adjacent WSe 2 layer, while WSe 2 as an optical absorption layer which can enhance the PL of BP at MIR wavelengths.Under the laser excitation of 2.33 eV, the heterostructure sample has a PL peak at ∼3.18 µm, and the PL intensity is 1.65 times that observed in pure BP sample, indicating the efficient energy transfer from monolayer WSe 2 to BP.Furthermore, due to the straddling-gap alignment, the electrons and holes are confined in BP layer, and the PL peak of the heterostructure is at the same position compared to that of the pure BP.Based on the type-I heterostructure, an efficient energy transfer is realized, and the enhancement effect of PL emission is realized.
A typical demonstration of BP-based staggered-gap ((type-II) heterojunction is the BP-monolayer MoS 2 heterojunction [29].As shown in figure 4, an exfoliated multilayer BP is transferred onto the chemical vapor deposited MoS 2 film, and the overlapped junction region is formed.BP is an inherently p-doped narrow-bandgap semiconductor with high hole mobility, while MoS 2 is an n-type semiconductor with a large bandgap and high electron mobility.The p-n junction is formed in the interface of BP and MoS 2 layers, in which the energy band diagram is in figure 4(e).Under forward, zero, and negative drain bias, the energy barrier between BP and MoS 2 can be adjusted, and different transfer characteristics with high carrier mobility and a large on-off ratio are achieved for multifunctional applications.
The type-III heterojunction has also been realized by choosing BP and ReS 2 flakes, as shown in figure 4(c) [114].According to the estimating of Kelvin probe force microscopy measurements, the VBM of ReS 2 (∼4.68 eV) is higher than the CBM of BP, manifesting a broken-gap at the interface.The band alignment of the heterostructure at thermal equilibrium is obtained in figure 4(f).A built-in potential barrier (V bi ) is formed because the smaller work function of BP compared to that of ReS 2 and electrons transfer from the BP layer to the ReS 2 layer leads to an accumulation layer.Different from the traditional type-I and type-II p-n heterojunctions, no depletion region forms in either side of the type-III heterojunction.By calculations, the width of the hole accumulation layer in the BP region is 43 nm, and that of the electron accumulation layer in ReS 2 is 9 nm.Since the accumulation width in BP is larger than the thickness, BP is demonstrated to be fully accumulated with donors, with the V bi of 0.47 eV.Meanwhile, the ReS 2 layer is partly collected and the V bi is 0.17 eV.Different from the conventional type-I and II p-n heterojunctions, where the diffusion of carriers is temperature-dependent, forward current in the broken-gap band alignments is a temperatureindependent tunneling process, making it potentially exploited to create high-speed and integrated devices.

Excitons in heterojunctions.
Due to the quasi-one dimensionally electronic dispersion, few-layer BP has unique subband structures, providing exciton-dominated photoexcitation processes.An efficient approach to dissociate excitons into free charges is constructing heterojunctions with type-II band alignments [115].Figure 6(a) is the schematic diagram that excitons created in the BP layer are dissociated at the interface between BP and P3HT, which greatly enhances free charge carriers.Due to the in-built electric field in the type-II heterostructure, holes flow into P3HT and electrons can be transferred to BP.The photocurrent measurements are performed to indicate the effect of exciton dissociation further.Compared to pure BP, the photocurrent of the BP/P3HT hybrid structure shows a drastic enhancement more than 18.3 times (figure 6(c)).In addition, BP can promote the exciton dissociation of other direct-bandgap semiconductors, in which a significant PL quenching is observed in a BP-MoS 2 heterojunction system [61].The transfer of interlayer photocarriers between BP and MoS 2 has an ultrafast time of ∼5 ps, which is much shorter than the interband recombination processes of pure BP and MoS 2 [59].The construction of heterojunctions can promote exciton dissociation and boost the development of ultrafast optoelectronics.
Multiple exciton generation (MEG), where a high-energy photon creates one or more extra electron-hole pairs, is considered a feasible and promising way to overcome the Shockley-Queisser limit and evaluate the light conversion efficiency.According to Zhou et al multi-excitons achieved in BP-based MEG systems can be efficiently harvested by constructing heterostructures [116].They performed transient absorption (TA) measurements for a 4 L BP-1 L MoS 2 heterostructure sample.The MEG occurs within 300 fs when the excitation photon energy is more than two times the bandgap of 4 L BP.Based on theoretical calculations, the type II band alignment is formed, and the photoinduced electrons in BP flow into MoS 2 (figure 6(d)).According to the 2D plot of TA spectra in figure 6(e), the photoinduced bleaching of A and B excitons of monolayer MoS 2 is achieved while the excitation energy is lower than the bandgap of monolayer MoS 2 , indicating the carrier transfer from BP to MoS 2 .The electron density, as well as transfer process, is probed using photoinduced bleaching kinetics of MoS2.Interestingly, the transmission difference at the excitation energy of 2.25 eV is 1.32 times that of 1.36 eV under the same absorbed photon density, which is coincident with the contribution of MEG, confirming the multielectron transfer process (figure 6(f)).
Like bulk quantum well structures, interlayer (or indirect) excitons can also be hosted in 2D heterostructures [117].These excitons have a prolonged lifetime and are controllable by electric fields, so they are of particular interest.Till now, studies on interlayer excitons have mainly focused on TMD materials and gapped graphene [118][119][120].BP has a tunable and direct bandgap, which can be an ideal platform for interlayer excitons.However, there have been no experimental studies about BP-based heterostructures for interlayer excitons till now.Chen et al designed BP-GeS and BP-hBN heterojunction systems and predicted their exciton properties [121].Both of these systems support the existence of bright interlayer excitons.In addition, in the BP-GeS heterostructure, the exciton effective mass is changed due to the strong hybridization, and the exciton lifetime is prolonged at room temperature.While in the BP-hBN-BP heterostructure, the hBN decouples the BP layers and allows the appearance of low energy dark excitons with very small oscillator strengths.

BP-metal interface
3.2.1.Metal-semiconductor contact.The contact engineering between 2D semiconductors and metal electrodes also plays a vital role for optoelectronic devices.Traditionally, the metal is evaporated onto the contact area of the 2D functional layer, which is based on the non-van der Waals junction interface.The barrier height in metal-semiconductor contact is mainly decided by semiconductor electron affinity and metal work function.By aligning suitable metal and semiconductor layers and designing band edges, the barrier height and the contact resistance can be reduced, and the optoelectronic performance is effectively modified.Figure 7(a) is the schematic diagram of a BP-based FET device, where different metals were adopted as electrodes [122].The BP/metal interface shows the typical Schottky barrier behavior, but with different contact metals, the barrier height and contact resistance are significantly different.As shown in figure 7(b), the contact resistance of BP/ Pd interface (1.75 ± 0.06 Ω mm at −40 V gate voltage) is much smaller than that of BP/ Ni interface (3.15 ± 0.15 Ω mm at the same gate voltage).Metal Pd has a higher work function (∼5.4 eV), making it easier to inject holes from the metal into the VB of BP and reducing contact resistance [123,124].By gating modulating the two Schottky barriers, the device can be alternated from p-type to n-type characteristics.
Another strategy for constructing metal/semiconductor interface is using 2D semimetals as graphene and bismuth [125] as the buffer layer or electrodes.Unlike the traditional processes, the van der Waals heterogeneous contact technique avoids the damage of the evaporation process without chemical bonding and reduces the Schottky barrier width.Figure 4(c) is the BP-based LED device, where two graphite films sandwich the BP flake with 87 nm thickness [46].A high current up to 6.7 mA can be injected into the BP flake with ∼1200 µm 2 region, indicating the narrow barrier width (figure 7(d)).

Enhanced light-matter interactions.
Although fewlayer BP has extraordinary optical properties, an inherent drawback is their weak light absorption due to the thinness.Constructing hybrid nanostructures with metals is an effective approach for enhancing light-matter interactions.For example, the effects of gold-based nanostructures have been studied to boost light absorption and the IR light emission in BP [126].The schematic illustration is shown in figure 7(e), where the Al 2 O 3 /Au substrate acts as a spacer/mirror structure and BP is stacked on the metallic mirror.Then the periodically arranged array of T-shaped Au nano-antennas is placed on the top of BP.According to the simulation results, the thickness and length of each nano-antennas are 30 and 400 nm, respectively.The antenna gap is 40 nm, which is aligned along the armchair direction of BP.Due to the light confinement of the metallic substrate and the effective manipulation of the Plasmonic nano-antennas, the light absorption of BP achieved a huge enhancement at the localized surface plasmon resonances wavelength (∼3.08 µm, corresponding to the As the thickness of BP decreases to atomic layers, the excitons show great electromagnetic response and strong coupling capability, in which the energy exchange rate between BP and metals is faster than the respective dissipation rate.The separated exciton and plasmonic states would be coupled into the EPP state [127].According to Liu et al the EPP coupling and Rabi splitting at the interface between monolayer BP and Au is theoretically predicted [63].They constructed a Drude model of the dielectric function of 1 L BP, where the exciton emission is at 1.73 eV with the binding energy of ∼270 meV.By a transfer matrix of the optical characteristics of BP and Au, the EPP coupling occurs under the particular excitation angle and distance of a prime.The strong EPP coupling in the BP-Au heterostructure platform will have application potentials, including optical sensors, photodetectors and slow-light devices [128,129].

BP natural quantum wells and electro-optical effect.
Quantum well semiconductors, such as III-V quantum wells and Si/SiGe quantum wells, have been widely studied to have superior electro-optical properties [130][131][132].The quantum-confined Stark effect acts as the dominant electro-optical effect in semiconductor quantum wells, where the absorption band edge of III-V heterostructures experiences a red-shift under a transverse electric field.When few-layer BP is placed on the SiO 2 dielectric layer, it can be regarded as a natural quantum well structure, in which the quantum-confined Stark effect is also presented.According to Liu et al the giant Stark effect coefficient is achieved in electrostatically gated few-layer BP, where a semiconductor-to-metal transition is allowed under a low critical field (∼0.68 V nm −1 for ten-layer BP) [133].
In addition, other than traditional quantum wells, most of which cannot be operated beyond the telecommunication spectrum (0.8-1.7 µm), few-layer BP shows excellent electro-optical characteristics in the MIR frequencies [33].An intrinsic 5 nm BP film was exploited on the SiO 2 substrate.The upper air and bottom oxide interfaces make BP a quantum well structure.Applied an out-of-plane electric field, the evolution of the energy band diagram is shown in figure 8(a).The bandgap of intrinsic 5 nm BP is around 0.62 eV, which is determined by the energy levels of the first valence and conduction sub-bands, respectively.When a positive gate bias is applied, the field-induced quantum-confined Franz-Keldysh effect leads to the upshift of the Fermi level E f , where the band bending occurs in the BP quantum well and the electron and hole sub-bands become closer in energy [19].On the other hand, 2D electron gas becomes more degenerate at the BP-oxide interface by the induced electron carrier density N s , leading to the upshift of the Fermi level E f and the broadening of the bandgap, which originates from the Pauli-blocked Burstein-Moss shift (BMS), as shown in figure 8(b).The two mechanisms make optical bandgap of gate biased BP may undergo blue-shift, red-shift, or bidirectionalshift, which cannot be realized in graphene and traditional III-V semiconductors.Besides the optical bandgap as well as optical absorption changes, the optical anisotropy in few-layer BP is also tunable by an externally applied field, in which the highly anisotropic absorption is transformed to a nearly isotropic feature [134].Since the disallowance of inter-subband optical transitions along the Zigzag direction of BP, the influence of BMS shows a significant difference between in armchair and in Zigzag direction.The quantum-confined Stark effect is also anisotropic in BP.When sandwiched BP between two dielectric layers (such as SiO 2 and Al 2 O 3 ) and applied a symmetrybreaking electric field, both linear dichroism and birefringence are electrically controlled.

Dielectric screening and many-body effects.
It is known that the dielectric screening between 2D semiconductors and the surrounding environment plays an important role in the quasi-particle gaps and exciton-binding energies.According to previous reports, when other fused silica or oxides materials, including SiO 2 , quartz, Al 2 O 3 , and hBN are used as substrates or capping layers, the optical gap of monolayer BP is experimentally obtained ranging from 1.3 eV to 1.73 eV, where the bandgap shifts raise from the dielectric screeninginduced effects [60,135].
By calculating the GW-BSE equations, Qiu et al systemically studied the effects of the encapsulating environment in few-layer BP and their calculation matches well with the previous experimental results [60].At first, they modeled a nearly commensurate supercell that monolayer BP is sandwiched between (0001) Al 2 O 3 substrate and hBN capping layer (figure 8(c)).Different factors such as the relaxed distance between BP, substrate and the capping layer, lattice mismatch, and the anisotropic effective mass along different directions are considered in the calculation.The changes of the QP gaps, optical gaps, and exciton binding energies of 1-4 L BP under the effects of the substrate and encapsulation are depicted in figure 8(d).When monolayer BP is sandwiched by hBN and Al 2 O 3 , the QP gap reduces by 0.48 eV and the exciton binding energy decreases by as much as 70% (from 0.48 to 0.14 eV).BP is more sensitive to the surrounding environments than TMDs, owing to BP's small intrinsic dielectric constant (a static dielectric constant of six for BP and 13-16 for bulk MoS 2 ).Similarly, in 4 L BP, the renormalization of the QP gaps (by 0.09 eV) and exciton binding energies (from 0.07 to 0.01 eV) is achieved by the encapsulation.

Surface engineering
Surface engineering strategies, such as surface modification by introducing dopant atoms, absorbers and impurities, and plasma etching, have long been studied in conventional bulk materials and have now been widely developed in 2D materials.For few-layer BP, the two most inherent shortcomings are relatively low electron mobility and poor air stability.Both of these two shortcomings can be optimized through surface engineering, as discussed in this section.

Charge transfer doping
BP is inherently an ambipolar semiconductor with strong asymmetry between electron and hole transport, where both the mobility and concentration of electrons are orders of magnitude lower than that of holes, limiting the applications in optoelectronics [136,137].Since the transport behavior of BP is sensitive to surface impurities, absorbates, and adatoms, surface functionalization is a practical approach to tune the p-or n-doping level of BP.Unlike the conventional substitutional doping or plasma treatment, surface functionalization as a nondestructive doping scheme, does not introduce a significant number of defects or even destroy the crystal structure of the underlying semiconductors.In 2015, Du et al reported the surface functionalization of BP with two metal oxides, Cs 2 CO 3 and MoO 3 [136].As a substantial electron donor, Cs 2 CO 3 significantly increases the electron concentration in BP, thereby enhancing electron mobility.As shown in figure 4(a), with the increase of the n-type doping level, the transfer characteristics of BP have a transition from hole-dominated to electrondominated behavior.While in MoO 3 covered BP, owing to the extremely high work function of MoO 3 in a high vacuum, a giant p-doping effect is observed.The photoresponsivity of BP-based photodetectors was increased more than four folds for both MoO 3 and Cs 2 CO 3 doping cases.
Following the functionalization study, some dopants such as Cu, Al, K, BV, and self-assembled layers have been used to modulate the electronic and optical properties of BP [19,42,[138][139][140][141][142].For example, by contacting Cu adatoms, the controllably n-doped few-layer BP is achieved, lowering the threshold voltage for n-type conduction [138].Both Cu adatoms bond to the top surface and Cu atoms are intercalated into the few-layer crystal.According to the DFT calculation, a massive Fermi level shift is observed, where the donated electron originates from the Cu 4s shell.In the Cu as configuration, the bandgap is reduced to 0.09 eV due to an internal electric field, while for the intercalated Cu int configuration, besides the n-type doping the distance between the two BP layers increases, decoupling the system into two BP bilayers, leading to a rise in the bandgap from 0.18 to 0.32 eV.
Al atoms have been doped at the surface and the intercalated site of few-layer BP nanosheets, which induce n-type conductance [42].Due to the hybridization of the Al atom with the lone electron pair in phosphorus atoms, the binding energy is large, which leads to large outward and in-plane atomic displacements of phosphorus atoms around the Al dopant.The Fermi level in the Al-BP system shifts upwardly into the CB.
Organic absorption on van der Waals materials has long been studied and shows a pronounced doping effect [64][65][66][143][144][145].Dopants with a bipolar pole or low redox potential lead to the n-doping effect.For example, −NH 2 containing organics has long electron pairs to withdraw holes from the host material [143].
In 2016, Hersam et al reported a pioneering work on the covalent functionalization of BP via aryl diazonium chemistry, triggering the research hotspot of chemical functionalization of BP [64].By reacting few-layer BP with different aryl diazonium salts, namely 4-nitrobenzenediazonium (4-NBD) and 4-methoxybenzenediazonium tetrafluoroborate salts, BP was successfully passivated by covalently attaching the aryl groups through P-C bonds.As a result, phosphorus atoms are four-coordinate bonded, with one lone pair of the electrons forming P-C bonds; thus, its reactivity with oxygen is inhibited.
Molecule doping is a flexible and effective method for optimizing the electronic properties of 2D materials [58,145].Yu et al employ BV as the surface electron dopant on the top layer a (p-type) few-layer BP flake and achieve an ambient stable, in-plane P-N junction [58].N-and p-doping of BP was achieved with 3-amino-propyltriethoxysilane (APTES) and octadecyltrichlorosilane (OTS).The functional groups in APTES (NH 3 −) and OTS (CH 3 −) have negative and positive charges, respectively, which are able to adjust the carrier density in the BP channel.The photocurrent of the APTES-doped BP photodetector decreased from 77.9 to 65.9 nA µm −1 .Conversely, for OTS-doping, the recombination rate of photocarriers appears to be reduced in the BP channel.As a result, the photocurrent increased from 133 to 187 nA µm −1 after OTS doping.
Covalent functionalization of BP generally has a limited degree of functionalization, and in some cases, the grafted functional groups may even degrade the electronic properties of BP.There are several non-covalent functionalization approaches of its 2D surface that were pioneered in order to protect BP from degradation under ambient conditions.For example, Hirsch et al reported a noncovalent hybrid of BP with 7,7,8,8-tetracyano-pquinodimethane (TCNQ) and perylene bisimide via van der Waals interactions [66].The ambient stability of BP was enhanced significantly.Adsorption of stable molecules on the surface of BP by electrostatic interactions has been developed as an alternative strategy to protect BP.

Improvement of ambient stability
As discussed in section 2.3, the unbonded lone pairs of phosphorus atoms is easily reacted with oxygen, few-layer BP suffers from oxidization and degradation when exposed to air.For effectively improving the air stability of BP, the most two common approaches are physical coating and chemical functionalization.Plasma etching and thermal annealing have also proven to be effective for preparing stable BP.Table 1 summaries variety of passivation techniques of BP as well as characterization methods.
Physical coating or covering can serve as a physical barrier to encapsulate BP, which avoids the environmental reaction of BP's lone pair of electrons.In 2014, Wood et al deposited 10 nm thick Al 2 O 3 onto a 9 nm thick exfoliated BP flake.The inorganic coating method effectively suppresses the ambient degradation of BP and the as-prepared FET maintained high On/Off ratio and mobility over two weeks.Followed by the O 2 plasma etching + hBN covering + rapid thermal annealing 7 months STEM, PL, Raman [153] research, different capping methods have been studied, such as organic coating, Al 2 O 3 -organic encapsulation and 2D flakes.
In addition to the considerable protecting ability, the most merit of physical coating methods is that the intrinsic properties of BP can be highly preserved because any chemical process is not involved in the whole passivation processes.Chemical functionalization is another effective way to enhance the air stability of BP.A pioneer work is the noncovalent functionalization of BP's 2D surface with TCNQ.The stable hybrids are formed between BP and TCNQ molecules and electron transfer occurs from BP to the organic layer, which can highly stabilize the BP flake but also affects BP's mobility.The covalent functionalization of BP is developed as an alternative strategy to protect BP.For example, Hersam's group developed a wet chemical approach in which aryl diazonium 4-NBD is covalent modified with 10 nm thick BP.An invariant morphology is exhibited in BP over three weeks of ambient exposure and the electronic properties is controllable due to the covalent bonds between BP and the passivation component.However, the as-protected BP still has low functionalization degrees in this way, and a clear spectroscopic fingerprint is absent for characterizing the covalent functionalization degrees.In 2020, by introducing insert solvent and non-time-consuming statistical Raman Spectroscopy, Wild et al established the systematic quantification method to dramatically increase the functionalization degree of methylated BP.Both P-C and C-H covalently vibrational modes are exhibited from the spectroscopy analysis, indicating the construction of a phosphine like P 2 P-CH 3 -species.The functionalization degree is quantitated and highly at 4.7% for NaP 4 and 7% for KP 6 BPICs.
The combination of physical and chemical passivation can furtherly enhance BP's air stability.For example, Pei et al fabricated air-stable phosphorene flakes through oxygen plasma etching and the following Al 2 O 3 coating (figures 9(d)-(f)).Using the oxygen plasma etching method, P-O covalent bonds are formed on the BP surface and the P x O y capping layer is produced as a protective coating.Then a 5 nm thick Al 2 O 3 layer is deposited as an extra passivation layer.As shown in figure 9(f), the BP flake capped with P x O y and Al 2 O 3 protected layers maintains clean topography over two months under air exposure.Followed by this work, Li et al developed a systematic strategy by combing O 2 plasma etching, boron nitride passivation, and thermal annealing process.The rapid thermal annealing method can effectively reconstruct the surface defect and amorphous P x O y and maintain BP's intrinsic properties.According to their PL degradation results, the PL intensity of their BP sample remains at ∼80% of the original intensity after 209 days (seven months) under ambient environment, indicating that the combined passivation strategy is effective for air-stable and high-quality BP samples.

Applications
Nowadays, different kinds of high-performance BP-based devices have been developed, including photodetectors, LEDs, electro-optic modulators, all-optical modulators, and so on.Interface and surface engineering play an essential role for all of these devices.For example, the responsivity can be improved, and the working wavelength can be broadened by constructing P-N junctions in BP-based photodetectors.Through designing a BP-WSe 2 based straddling-gap alignment, carriers are efficiently confined in BP, and the PL intensity of BP-based LEDs achieves a significant enhancement.By surface coordination, BP with good air stability for air-stable optoelectronic and photonic devices.This section focuses on the interface and surface engineering strategies in these promising applications.

Photodetectors
Photodetectors are optoelectronic devices that can capture the light signal and convert it to an electric signal, which have been indispensable devices in modern photonics and optoelectronics.The moderate and direct bandgap of BP makes it suitable for photodetection applications, especially in nearinfra (NIR) and MIR wavelengths [26,32,39,[154][155][156]. BPbased photodetectors with different structures and working modes have been studied, showing promising device performance such as ultrahigh responsivity and fast response time [30,37,38,110,[157][158][159][160][161].Furthermore, the unique anisotropy makes BP sensitive to light polarization and the possibility for polarization-sensitive detection [30,155].Figure 10(a) is the optical image of a BP-based polarization-sensitive detector, where a ring-shaped gating electrode is used for excluding the influence from the geometric edge at the BP-metal interface [30].From the spatial photocurrent mapping image at 1550 nm (figure 10(b)), the device works at photoconductive mode and strong photocurrent for AC polarization is observed in BP between two electrodes.By contrast, the photocurrent for ZZ polarization from 400 nm to 1700 nm is at least an order of magnitude smaller than that for AC polarization, demonstrating the linear dichroism of the device.Shortly after, Bullock et al realized the linearly polarized detection of BP at MIR waveband by constructing a vertically stacked BP-MoS 2 heterostructure.The MoS 2 layer acts as an electron-selective contact which allows the flow of electrons while blocks the flow of holes from BP to the top electrode and BP has proved to be promising for polarized mid-IR detection [159].
Tunable BP-based MIR photodetectors have been realized by designing a dual-gate device configuration on the interface of BP [32].By supplying a high back-gate voltage and a considerate top-gate voltage, strong photoresponse at room temperature is observed with an extrinsic responsivity of 518, 30, and 2.2 mA W −1 at 3.4, 5, and 7.7 µm, respectively.The light absorption edge as well as photoresponse witnesses a significant redshift, originating from the shift of the charge-neutrality position of BP and the Franz-Keldysh effect.Yuan et al improved the dual-gate transistor structure above, where the top gate layer uses monolayer graphene to replace metals, and the thickness of the BP functional layer is increased to 13 nm (see figure 10(c)) [162].The transparency of graphene ensures more than 99% transmission of incident light at MIR wavelengths, while the thickness of 10-15 nm of BP is a compromising result considering the light absorption and the free-carrier screening effect.A matrix learning process from a temperature tunable blackbody source allows an ultracompactly optical spectrometer to be realized in the spectral range from 2 to 9 µm.[163], copyright (2017) American Chemical Society.(g), (h) Waveguide-integrated MIR LEDs based on the BP functional layer.Reprinted with permission from [46], copyright (2020) American Chemical Society.(g) SEM image of BP LED integrated on a waveguide.The stacked van der Waals layers including BP functional layer, two graphite electrodes, and an hBN encapsulation layer.(h) The spatially resolved electroluminescence mapping image of the LED, where the white dash lines draw two electrodes and the dark dash line draws the silicon waveguide.
(I ds ) mapping with top and bottom gate biases, in which four regions can be divided based on the polarities in the sourcedrain channel.By a learning process under 41 different displacements fields, spectral responsivity vectors are generated and a spectral responsivity matrix is constructed.As a demonstration, the single-detector-based spectrometer can successfully capture the optical absorption spectrum of carbon dioxide (CO 2 ) with moderate resolution (figure 10(e)).
Waveguide-based photodetectors have been very promising due to their compatibility with CMOS circuits and the ultrahigh integration [38,155,158,160,161].By integrating 2D materials with low-loss waveguides, the BP-dielectric interface is formed and the optical interaction length of 2D materials is extended, which can enhance the optical absorption of 2D materials, so waveguide-based 2D photodetectors commonly have high responsivity and detectivity.In 2015, Si-waveguide integrated BP photodetector is firstly realized with a high photoresponsivity of ∼135 mA W −1 , a low dark current of 220 nA, and a fast response with a cut-off frequency up to 2.8 GHz [38].Compared to the graphene photodetector with the same device structure, the dark current of BP photodetector is three orders of magnitude lower due to the open of the direct bandgap.Chen et al exploit the plasmonic structures into the waveguide integrated BP photodetector system [155].The Au plasmonic nanostructure is aligned between the BP functional layer and the silicon dielectric waveguide to form a BP-metal interface, which achieves the low-loss of light propagation, confines the optical field below the diffraction limit, and drastically enhances the optical field of the light-BP interaction.The responsivity is highly at 10 A W −1 while the 3 dB cut-off frequency is ∼150 MHz.According to Huang et al, the work wavelength of waveguide integrated BP photodetectors is extended to MIR region [29].The waveguide system contains an input grating and an output grating, then 200 nm planarized SiO 2 is used as the insulation and passivation layer.BP with 23 nm thickness is transferred onto the SiO 2 layer.The Al 2 O 3 layer with 20 nm thickness acts the dielectric for separating the channel and gate tuning.Due to the small bandgap of BP (∼0.3 eV), the cutoff wavelength of the device is 4.13 µm.The responsivity is 23 A W −1 and shows anisotropic characteristics.By adopting a silicon-oninsulator (SOI) ridge waveguide, Yin et al presents a hybrid silicon/BP waveguide photodetector and achieves the highspeed response with the 3 dB cut-off frequency of 1.33 GHz at 2 µm [161].The response speed is restricted by the RCconstant and can be further improved by reducing the BP capacitance and the BP-metal contact resistance.

Electro-optical modulators
Owing to the solid electro-optical response and relatively small bandgap, BP is an excellent midinfrared Electro-optic material for modulation applications [33,133].In 2016, Lin et al proposed a proof-of-concept of BP-assisted electroabsorption modulator, in which BP thin film is placed on the top of Silicon waveguide and encapsulated by a 7 nm Al 2 O 3 spacer, where the BP-dielectric interface is engineered and the electro-optical effects are studied [33].By choosing a suitable thickness of BP, the device can operate at 2.1-3.3 µm.According to the simulation results, the BP-assisted modulator has a large modulation level (0.05 dB µm −1 ), a low work voltage (62% reduction compared to graphene-based device), and a high ON-OFF ratio of extinction (exceeding the performance of SiGe modulators).Soon after, Peng et al experimentally demonstrated a BP-based MIR modulator by using the quantum confined Franz-Keldysh effect as the dominant mechanism [163].The exfoliated multilayer BP is transferred on the SiO 2 substrate.The thickness of SiO 2 is set to be 450 nm for enhanced mid-IR absorption in BP.A 10 nm Al 2 O 3 is deposited on BP as the encapsulated and protective layer.The optical absorption modulation is achieved up to 3% with the applied gate bias adjusting from 0 to 150 V (figure 10(f)).The performance of the device would be further improved if using high-k dielectric materials as the gate oxide.Besides, due to the anisotropic optical-electric effects, BP also has application potential to modulate light polarization and propagation direction [134].

Light emission diodes
Van der Waals based light-emission devices (LEDs) is attractive due to the high degree of confinement of 2D materials and the ease of integration with silicon chips.The heterojunction constructions of BP with other 2D semiconductors extend the operation wavelength of Van der Waals based LEDs to the MIR waveband [113,164].For example, Wang et al demonstrate a BP-MoS 2 based LED with the type-II heterojunction alignment, in which the polarized electroluminescence is emitted at 3.68 µm under room-temperature [164].When applied a negative drain-source bias, the hot electrons in MoS 2 inject into the BP layer, which enhances the luminescence efficiency of BP.An on-off ratio highly at 10 5 and A room temperature internal quantum efficiencies of ∼1% have been achieved in the BP-based MIR LED.The anisotropic characteristics in BP-based LEDs have been observed that the electroluminescence (EL) intensity along the armchair axis is over seven times higher than that along the Zigzag axis [113].
Waveguide-integrated LEDs based on BP have also been realized, which are compatible with the silicon photonic platform [46].The BP functional layer sandwiched by two graphite electrodes is transferred on a silicon waveguide fabricated on an SOI chip.By the encapsulation of hBN, an onchip LED is constructed, where the emission light from BP can be evanescently coupled and then propagated through the silicon waveguide.The schematic diagram is in figure 10(g).The electroluminescence power at 3.6 µm is highly at 1.8 µW with excellent stability over 100 h, and the coupling efficiency is 10%.After further optimization, the measured emission intensity from the grating coupler would be more than 40% with respect to that from the BP emission region (figure 10(h)).Recently, using a dual-gating configuration, few-layer BP sandwiched by bottom and top hBN flakes can generate bright and wide-tunable PL from 3.7 to 7.7 µm, further indicating the potential of BP for the realization of future widely-tunable and polarized MIR LEDs [47].

All-optical modulators
All-optical modulators have been extensively utilized in ultrahigh bit rate communication systems due to the all-optical signal process with an ultrafast response and broad bandwidth.Furthermore, the all-optical modulation can be realized in simple configurations such as optical fibers or silicon waveguides, which have a miniature size for compact and integrated operation [165].There are many different all-opticalmodulators till now, including saturable absorbers [166][167][168], polarization controllers [169,170], wavelength converters [171] and optical limiters [172].The operation mechanisms of most of these devices are based on the strong optical nonlinearities of 2D materials (especially the third-order susceptibility).For example, the saturable absorption is originated from the imaginary part of the complex χ (3) process, which is for passive mode-locking and Q-switching operations of lasers.In comparison, the real part of the complex χ (3) process is responsible for Kerr effects, which can be employed for wavelength converters such as four-wave mixing (FWM) devices.
BP-based saturable absorbers are the earliest and most widely investigated photonic devices due to the growing need for ultrafast lasers.Till now, both mode-locking and Qswitching operations in NIR and MIR wavelengths have been achieved by BP-based saturable absorbers [48,50,[173][174][175][176].To realize a practical saturable absorption device, the instability of BP in the ambient environment is a problem.To avoid the oxidization of BP, Mu et al introduced a semi-industry approach of electrospinning to prepare robust BP-polymer networked membranes (figure 11(a)), in which BP nanosheets are uniformly distributed in PVP matrix [48].The ultrafast and high-energy pulse generation is achieved in a 1550 nm fiber laser based on the optically transparent membrane with strong saturable absorption effects.Inkjet print is also a practical approach to achieve stable BP membranes [50,176].BP is dispersed in a binary solvent composed of isopropyl alcohol as a binder-free inkjet ink, allowing stable jetting with high printing consistency and spatial uniformity.As shown in figure 11(b), the as-printed BP membrane encapsulated by PET and perylene-C is integrated into the optical fiber system for mode-locking operations.The self-starting mode-locking with the pulse duration shortly at 102 fs is obtained in a dispersion-managed fiber laser cavity by the inkjet-printed BP, as shown in figure 11(c).When the laser continually operates over 30 days, the optical spectra still show no variations (figure 11(d)), indicating the long stability of inkjet-printed BP under ambient conditions.BP wavelength converters have also been realized based on the large optical Kerr nonlinearity and the derivative FWM effects [52,177].BP is deposited on the D-shaped fiber to form BP-dielectric interface and interacts with the propagating light in the fiber through the evanescent field [52].When two light beams with high power and different wavelength are incident into the nonlinear optical media BP, the FWM occurs and signals with new frequencies are converted.As shown in figure 11(e), the signal wavelength is fixed at 1552.6 nm, and the pump wavelength is tuned to 1559 nm, where a new signal at 1565.4 nm with two sub-bands is clearly observed.The modulation frequency is highly at 20 GHz, indicating that the ultrafast modulation is achieved by BP.When tuning the pump wavelength by 0.1 nm each time, the accordingly shift by 0.1 nm the generated signals is obtained in figure 11(f), which is the strong evidence of the FWM effects.However, due to the rapid degradation of BP under ambient conditions, the FWM-based wavelength converter lacks sufficient stability.Soon after, Zheng et al modified BP with metal-ion, in which the ambient stability is greatly enhanced [177].Both the optical Kerr switching with an extinction ratio highly at 26 dB and the FWM operations with a conversion efficiency of −59.15 dB are experimentally realized by the BP-coated microfiber.By surface modifications, BP with a moderate direct bandgap and strong optical nonlinearity offers new opportunities to develop all-optical switches.
All-optical plasmonic modulators are new kinds of alloptical modulators with ultrahigh light confinement under the diffraction limit and ultra-low power consumption.The interface polaritons have realized the femtosecond photo-switching in SiO 2 /BP/SiO 2 heterostructures [51].The experimental setup is in figure 11(g), in which a 1560 nm laser pumps the heterostructure with a pulse duration of ∼40 fs, and a MIR signal light is as the probe light.The photo-induced interband excitation is driven in BP but cannot be supported in SiO 2 due to its large-bandgap.The hybridization between the surface plasmon modes in BP and the surface phonon modes in SiO 2 is activated at around 38 THz by the interband excitation.The hybrid phonon-plasmon polariton mode is ultrafast switchable.As the recorded near-field snapshot images within 15 ps timescale in figure 11(h), the narrowband polaritons are launched with significant fringes.The response time of the amplitude saturation from zero to half its maximum is only ∼90 fs.The ultrafast switchable interface mode has a confined region and long propagation length, owing to the phonon-like nature within the Reststrahlen band [178].BP-based heterostructures as a robust technological platform are promising for polariton-based mid-infrared photonics.

Conclusion and perspectives
In conclusion, we highlight and review the recent progress of interface and surface engineering strategies based on BP towards high-performance optoelectronics and photonics.Outstanding issues, including the construction of homoand heterostructures with tailored band alignment, the reduction of Schottky barriers in metal-semiconductor contacts, and the achievement of giant quantum-confined Stark effects, have been made on different optoelectronic devices, such as photodetectors, spectrometers, electro-optical modulators, and LEDs.In addition, due to the solid dielectric screening, BP has strong exciton absorption with large binding energy.By introducing some strategies, including ultrafast exciton dissociation by constructing heterostructure, enhanced light-matter interactions by metal nanostructure, and surface modifications, BP is promising for photonic applications such as all-optical modulators and polariton devices.
Despite significant advances, BP-based applications are still in the early stages and have many challenges.Firstly, most current approaches, including surface passivation and encapsulation for achieving stable BP, are based on exfoliated BP samples, in which the flake size is minimal.Therefore, it is still very challenging to realize the high stability of large-scale BP and related devices in ambient conditions.In our opinion, the combination of chemical functional BP with a high functionalization degree and atomic layer deposition (ALD) physical passivation layers would be a promising approach for producing large-scale and large-area air-stabilized BP.Secondly, though different BP-based optoelectronic devices have been demonstrated with high performance, integrating LEDs, modulators, and photodetectors as a versatile system is still a problem.Finally, in spite of the fact that many-body interactions such as anisotropic interlayer exciton plasmon-phonon polaritons have been observed at the interface of BP, the novel polariton devices based on BP are still in concept, which needs to be developed in future BP-based photonics.Overall, as an ideal 2D platform with intriguing properties, BP is promising for future optoelectronics and photonics by a practical approach to interface and surface engineering.

Figure 1 .
Figure 1.Lattice and electronic band structures of BP.(a) Schematic illustration of the puckered lattice structure of BP.Reprinted from [20] by permission from Springer Nature Customer Service Centre GmbH.(b) Brillouin zone of the primitive cell of BP.(c) Electronic band structure for BP by the mBJ potential (dashed blue line) and the HSE06 functional (solid red line) calculations, respectively.The zoom-in plot in the right of the figure shows the bandgap of BP at z point.(b) and (c) are reprinted from [16] by permission from Springer Nature Customer Service Centre GmbH.(d), (e) Band structures of monolayer (d) and bilayer BP (e), which are calculated by the tight-binding parametrization.Reprinted with permission from [68], copyright (2014) by the American Physical Society.

Figure 2 .
Figure 2. Optical properties of BP.(a) The tunable optical energy gap of multilayer BP with different thicknesses.Reprinted with permission from [18], copyright (2014) by the American Physical Society.(b), (c) Optical absorption spectra for the incident light with the polarization direction along armchair (b) and zigzag (c) directions.The thicknesses of BP vary from one layer to five layers.Reprinted from [16] by permission from Springer Nature Customer Service Centre GmbH.(d)-(g) Exciton effects as well as exciton binding energies in BP.(d) Optical transitions between two quantized sub-bands of BP, based on the quasi-one-dimensional tight-binding model.(e) The model of optical absorption of few-layer BP when considering the exciton resonances, including exciton ground (1s), excited (2s) states, and continuum (step-like) states.Reprinted from [22] by permission from Springer Nature Customer Service Centre GmbH.(f) Real part of the optical conductivity of 6L BP on a quartz substrate with incident light polarizations from 15 • to 90 • .(g) Theoretical values of exciton binding energies of free-standing BP (black dots) and BP (red dots) on the PDMS substrate, respectively.(f) and (g) are reproduced from [77], CC BY 4.0.(h)-(j) Plasmons in BP.(h) Calculated energy loss dispersion for electron doping in BP of 1 × 10 13 cm −2 and momentum q paralleled with x (right) and y (left) directions, respectively.(i) Scaling of plasmon frequency with electron concentration of monolayer and 20 nm thick BP, where graphene is for comparison.(j) Distribution of anisotropic plasmons in k surface.(h) and (i) are reprinted with permission from [78], copyright (2014) by the American Physical Society.(j) is reprinted with permission from [68], copyright (2014) by the American Physical Society.

Figure 3 .
Figure 3. Nonlinear optics in BP. (a), (b) Third-harmonic generation (THG) in a multilayer BP flake.Reprinted with permission from [89], copyright (2017) American Chemical Society.(a) Optical image of the BP flake.(b) THG mapping image excited by a 1557 nm pump laser.(c) Nonlinear refractive index n 2 (ω) of BP along the armchair direction by the numerical simulation, demonstrating the Kerr nonlinearity.Reproduced from [92].© IOP Publishing Ltd.All rights reserved.(d)-(e) Nonlinear optical absorption of BP.Reprinted with permission from [93], copyright (2016) American Chemical Society.(d) Normalized differential absorptivity of BP, graphene and MoS 2 nanosheets under different incident light fluence.(e) The values of the imaginary part of the third-order nonlinear optical susceptibility Im χ (3) of BP and graphene measured at different light wavelengths.(f) Transient absorption curve of the BP dispersion at 1550 nm, where the inset illustrates the dynamic carrier process.Reproduced with permission from [48], John Wiley & Sons [© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].

Figure 4 .
Figure 4. Degradation of BP. (a), (b) AFM image of BP immediately exfoliated on the SiO 2 substrate (a) and placed under ambient conditions after a few days (b).Reprinted from [104] by permission from Springer Nature Customer Service Centre GmbH.(c) Schematic illustration of the light-induced ambient degradation mechanism of BP.Reproduced with permission from [102], John Wiley & Sons [© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].(d)-(f) Degradation-induced changes in optical absorption spectra as well as electronic band structures of BP. (d), (e) Blueshift of exciton resonances of 3 L (d) and 8 L (e) BP.(f) Changes of E 11 and E 22 peak energies as function with exposure time at the ambiance of 8 L BP. Reprinted with permission from [105], copyright (2019) by the American Physical Society.

Figure 5 .
Figure 5. (a)-(c) Schematic diagrams of Homojunctions based on BP.(a) A vertical BP p-n homo-structure based on a typical ionic gel gating configuration.Reprinted from [30] by permission from Springer Nature Customer Service Centre GmbH.(b) A lateral BP p-n structure by chemical doping methods, where Al 2 O 3 is the surface hole dopant and BV is the surface electron dopant.Reprinted from [58], copyright (2016), with permission from Elsevier.(c) A trilayer homo-structure based on three BP flakes with orthogonally crystal directions.Reprinted from [109] by permission from Springer Nature Customer Service Centre GmbH.(d)-(i) Schematic diagrams of three different BP-semiconductor heterojunctions and the corresponding energy structures.(d), (g) BP-WSe 2 Type-I heterojunction.Reproduced from [113].CC BY 4.0.(e), (h) BP-MoS 2 Type-II heterojunction.Reprinted from [29] by permission from Springer Nature Customer Service Centre GmbH.(f), (i) BP-ReS 2 Type-III heterojunction.Reprinted with permission from [114], copyright (2019) American Chemical Society.

Figure 6 .
Figure 6.Exciton generation, dissociation, and harvesting mechanisms in BP-semiconductor heterostructures.(a)-(c) Exciton dissociation at the interface of BP/P3HT and the enhancement of photocurrent generation.Reprinted with permission from [115], copyright (2019) American Chemical Society.(a) Schematic diagram of the transfer of electrons and holes at the BP/P3HT interface.(b) Migration of electrons and holes adjacent to the p-n heterojunction.(c) I-V characteristics of BP (left) and BP/P3HT heterostructure (right) under dark and 650 nm light illumination conditions reflect the enhancement of photocurrent in the heterostructure.(d)-(f) Multiple exciton generation and harvesting in BP-MoS 2 heterostructure.Reprinted with permission from [116], copyright (2020) American Chemical Society.(d) Theoretically calculated band alignment indicates the generation of multiple excitons from 4 L BP and the exciton dissociation by interfacial electron transfer to MoS 2 .(e) Transient optical absorption spectra of heterostructure under 1.36 eV excitation.(f) Transient absorption kinetics of MoS 2 A exciton within 1.5 ps time scale under 1.36 and 2.25 eV excitation energies, respectively.

Figure 7 .
Figure 7. Interface engineering between BP and metals.(a), (b) Ni/Au and Pd/Au contacts depositing on BP flakes.Reprinted with permission from [122], copyright (2014) American Chemical Society.(a) Schematic illustration.(b) Contact resistance characteristics with different back-gate voltage.(c), (d) Graphene as electrodes for BP functional layer.Reprinted with permission from [46], copyright (2020) American Chemical Society.(c) Schematic illustration of a BP flake sandwiched by two graphite films.(d) Typical I-V characteristics of the graphite-BP-graphite device.(e), (f) Metals on BP for light-matter interaction enhancement.Reprinted with permission from [126], copyright (2021) American Chemical Society.(e) T-shaped Au nano-antennas with polarization-tailoring alignments on BP.(f) Enhanced optical absorption of BP for x-and y-polarized light incidence.

Figure 8 .
Figure 8. Interface engineering between BP and dielectric materials.(a) and (b) BP between oxide and air layers as natural quantum wells for electro-optical effects [33, 133].(a) Schematic diagram of a BP flake on SiO 2 substrate as a gate-tunable device for electro-optical effects.Reprinted with permission from [133], copyright (2017) American Chemical Society.(b) Energy band structure illustrating the change of bandgap of BP natural quantum wells under intrinsic, QCFK, and BMS regimes, respectively.Reprinted with permission from [33], copyright (2016) American Chemical Society.(c) Schematic diagram of BP sandwiched by two hBN layers.Reprinted from [135] by permission from Springer Nature Customer Service Centre GmbH.(d) Exciton characteristics including quasi-particle gaps, optical gaps, and binding energy in 1-4 L BP with and without dielectric encapsulation.Reprinted with permission from [60], copyright (2017) American Chemical Society.

Figure 9 .
Figure 9. Surface engineering of BP. (a), (b) Al-doped BP as n-doped functional layers for transistor characterizations.Reproduced with permission from [42], John Wiley & Sons.[© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].(a) Schematic diagram that shows Al atoms as electron dopants for the BP host lattice.(b) Transfer characteristics of BP transistors, which indicate the transform from p-type to n-type of BP when doping Al atoms.The inset is the same plotted data but using a logarithmic scale.(c) Energy band alignment of BP and BV, which exhibits the n-type chemical doping effect of few-layer BP.Reprinted from [58], copyright (2016), with permission from Elsevier.(d)-(f) Oxidation and surface coating for ambiently stable BP.Reproduced from [146].CC BY 4.0.(d) O 2 plasma etching and Al 2 O 3 coating on a BP flake.(e) Optical images of fresh exfoliated (left) and O 2 plasma etched (middle) BP, as well as the PL mapping image.(f) Visual images of an O 2 plasma etched BP flake before and after three days and a PxOy + Al 2 O 3 coated BP flake before and after 30 days.
Figure 10(d)  is the source-drain current

Figure 10 .
Figure 10.Interface and surface engineering of BP for optoelectronic applications.(a), (b) BP vertical p-n junction-based photodetector with polarization sensitivity.Reprinted from [30] by permission from Springer Nature Customer Service Centre GmbH.(a) Optical microscope image of the device that a ring-shaped Ti/Au electrode is applied to avoid the extra polarization originating from the straight edge of the metal.(b) Photocurrent mapping images of the BP inside the ring electrode under a linear-polarized light with the polarization direction from 0 • to 90 • .(c)-(e) A concept of proof of a BP-based MIR spectrometer.Reprinted from [162] by permission from Springer Nature Customer Service Centre GmbH.(c) Schematic illustration of the hBN/BP/hBN heterostructure-based spectrometer.(d) An array of source-drain current with different values of the top-gate (Vtg) and back-gate (V bg ) voltages.(e) The optical absorption spectrum of CO 2 which is captured and reconstructed by the BP spectrometer.(f) Optical extinction spectra of a BP-based electro-optical modulator, indicating an up to 6% modulation depth with the gate bias from −150 to 150 V. Reprinted with permission from[163], copyright (2017) American Chemical Society.(g), (h) Waveguide-integrated MIR LEDs based on the BP functional layer.Reprinted with permission from[46], copyright (2020) American Chemical Society.(g) SEM image of BP LED integrated on a waveguide.The stacked van der Waals layers including BP functional layer, two graphite electrodes, and an hBN encapsulation layer.(h) The spatially resolved electroluminescence mapping image of the LED, where the white dash lines draw two electrodes and the dark dash line draws the silicon waveguide.

Figure 11 .
Figure 11.Interface and surface engineering of BP for photonic applications.(a) Electro-spun BP-PVP membrane for ultrafast pulse generations.Reproduced with permission from [48], John Wiley & Sons.[© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].(b)-(d) Ink-printed BP as saturable absorbers for mode-locked fiber lasers.Reproduced from [50].CC BY 4.0.(b) Schematic illustration that shows two fiber end-facets sandwich the ink-printed BP membrane for fiber integration.(c) Autocorrelation trace of the obtained mode-locking pulses.(d) Wavelength spectra of long-term stable operation across 30 d. (e), (f) Four-wave mixing devices based on BP-deposited nonlinear fibers for all-optical modulation.Reproduced from [52].CC BY 4.0.(e) Optical spectra of the generated signals by the four-wave mixing effects with the modulation frequency from 0.13 to 20 GHz.(f) Optical spectra of the signals evolving by tuning the channel distance from 0.1 to 1.3 nm.(g), (h) Ultrafast optical switch based on interface polaritons in the SiO 2 /BP/SiO 2 heterostructure.Reprinted from [51] by permission from Springer Nature Customer Service Centre GmbH.(g) Schematic diagram of the setup and the device where two SiO 2 layers sandwich BP.(h) Scattered near-field intensity images in which the excitation and decay process of a hybrid phonon-plasmon-polariton mode are observed within a ten ps time scale. 13

Table 1 .
Passivation techniques for improving air stability of BP.The monitored time only refers to the duration it is monitored using characterization techniques, which is not the maximum survival time of BP.