Covalent Functionalization of Black Phosphorus from First-Principles

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Covalent Functionalization of Black Phosphorus from First Principles Qiang Li, QiongHua Zhou, Xianghong Niu, Yinghe Zhao, Qian Chen, and Jinlan Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02192 • Publication Date (Web): 29 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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The Journal of Physical Chemistry Letters

Covalent Functionalization of Black Phosphorus from First Principles Qiang Li, Qionghua Zhou, Xianghong Niu, Yinghe Zhao, Qian Chen, and Jinlan Wang* Department of Physics, Southeast University, Nanjing 211189, China AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ABSTRACT

The chemical functionalization is proven to be an effective and controllable

approach to modify the properties of black phosphorus (BP), and improves the air-stability of BP and its nanoelectronic applications [Nat. Chem., 2016, 8, 597]. However, covalent functionalization of BP and related properties are poorly understood. Here we present a theoretical investigation on the electronic structure and transport property of chemically modified BP. Our calculations reveal that the molecule modification generates a rather flat energy band within the bandgap, which leads to a reduced hole mobility of BP. Alternatively, we propose to use polymers bonded to BP surface, aiming at a balance between functionality and carrier mobility. The polymer-BP composites preserve both electron and hole mobility of pristine BP; Meanwhile, the stability of polymer-BP composites in ambient condition is enhanced as well.

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TOC GRAPHICS

Like graphite, black phosphorus (BP) atoms strongly bonded in-plane to form layers, and the layers weakly interact through van der Waals forces.1 It possesses high carrier mobility of ∼1000 cm2 V-1 s -1 at room temperature.2-4 Unlike graphene, BP layer is a semiconductor with a sizable band gap (∼0.3 - 2.0 eV) and on-off current ratios exceeding 105, and it also exhibits anisotropic optical properties.3, 5-7 Consequently, BP shows promising electronic, spintronic, and optoelectronic properties that could complement or exceed graphene. Nevertheless, the main obstacle for wide applications of BP is the rapid degradation in ambient conditions, due to the lone pair of phosphorous atoms on the surface, making them very reactive to air.8 Studies show that the degradation process occurs firstly to the oxidation of phosphorous atoms involving reactive dangling oxygen atoms, and dangling oxygen atoms increase the hydrophilicity of phosphorene step further.9,10 Encapsulation methods that preserve pristine properties of BP at long-term at ambient conditions have been carried out, Al2O311,12 and h-BN13,14 in most cases. Such encapsulations, however, also trap defects and impurities, which have negative effects on carrier mobility;15 besides, oxygen and water may enter through the interfaces causing eventual breakdown.16


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Chemical modification schemes, determined to be efficient in manipulating the chemical and electronic properties of nanomaterials, are barely reported regarding to BP.17 Very recently, Ryder et al. showed that covalent functionalization limits the chemical degradation of exfoliated BP up to three weeks, with an enhanced semiconductor performance.18 However, the covalent functionalization of BP and related properties are poorly understood so far. We initiate the electronic properties of covalent functionalization of BP in this study from first principles. The carrier mobility is much of concern, since extrinsic adsorbates commonly adversely affect the mobility of semiconductors.19 To continue, we propose surface functionalization of BP via polymers, based on the grounds that a) polymers are relatively cheap and easy to process; b) abundant polymers lead polymer nanocomposites to desirable properties, such as thermal stability, dispersibility, optical properties etc.;20-24 and c) ideally, polymer functionalization would provide the desired chemical activity without adversely affecting electrical transport property.25 Poly(phenylenevinylene) (PPV) is chosen as an example, since its derivatives are one of the most studied classes of conjugated polymers, served as a benchmark system for electroluminescent and photovoltaic material design.26,27 In the end, the stability of the designed systems is assessed. Covalent modification of BP. We first examine a single molecule bonded to the BP surface with a 4x4 supercell, three different kinds of molecules are chosen, metoxybenzene (MB), nitrobenzene (NB) and phenylenevinylene (PV) which the former two were employed experimentally.18 Given that the three molecules show similar changes in both geometry and electronic structure, we focus on the PV molecule in the following (the other two can be seen in Figure S1). The in-plane BP structure is anisotropic, the alkenyl group of PV stretches either along armchair or zigzag direction (Figure 1a). The energy difference between the two


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orientations is trivial, within 0.15 eV at PBE level. The optimized structures show that a newcoming bond from PV molecule breaks the P-P bond between the surface phosphorus and the one in the second layer, which is further pushed away since the lone pair of surface P atom redirects to the downside. The attraction of PV lifts the surface phosphorus atom 0.2 Å and the sublayer P is deviated by 0.3 Å, the broken P-P distance is 2.84 Å. Such chemical modification on the BP surface has influence not only on the surface atoms, but also or even more profound on the sublayer positions. The movements indicate that high level of functionalization results in collapse of BP, as observed in experiment.18 Binding energy, defined as EBE = EPV-BP - EPV - EBP, is calculated to be -1.74 eV. The relevant electronic properties include band structure, and decomposed density of states (DDOS) near the Fermi level are shown in Figure 1b. The highest occupied molecular orbital (HOMO) originates from P(3pz) orbitals that associated with lone pairs of electrons, also with very less contribution from C atom which connected to the BP. The lowest unoccupied molecular orbital (LUMO), in another case, is contributed by hybridized orbitals. Notably, a singly occupied molecular orbital (SOMO) lies in between the band gap, and the density is mainly localized on the sublayer phosphorus atom, which has been pushed away (P-pushed), thus a magnetic center is formed. Adopting a second PV molecule, the two magnetic centers prefer to be coupled anti-ferromagnetically, and the energy difference is 14 meV at PBE level between high and low spin states, or broken symmetry state for the later. The spin density is plotted in Figure S3.


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Figure 1. a) Optimized structures of covalent bonding of PV molecule to BP along armchair (top) and zigzag (down) directions, the broken P-P bond is denoted by a dash line. b) Band structure (spin up) of PV molecule along armchair direction at PBE+D2 level, together with decomposed density of states of three energy bands near the Fermi level at Γ (0, 0, 0) point. c) Band structure of H-saturated PV-BP, optimized structure and the decomposed density of state of VBM are shown inside.

In pristine BP, each phosphorus atom connects to three neighboring phosphorus atoms through sp3 hybridized orbitals, making the phosphorus atoms be arranged in a puckered honeycomb structure. Each phosphorus atom also has a lone pair, which can be easily oxidized in the air and forms P=O double bonds. Previous studies suggest that molecules with dangling bonds can directly bond to BP.18, 28 Our results indicate that unlike stable P=O double bond, the formed single P-C bond replaces one of the P-P bond, leaving a dangling bond at P atom on the sublayer, which can be unstable and reactive to the environment. From another perspective, the localized electronic state (Figure 1b), that inside the band gap of BP due to the dangling bond of the sublayer phosphorus atom, is similar to crystal defect states. Such localized states, especially in the mid-gap, can serve as local scattering centers which reduce the mobility of charge carriers and enhance the nonradiative recombination of photoexcited electron−hole pairs.29 Our previous studies, combined with experiments and density functional theory, showed that the charge transport of few-layer MoS2 in low-carrier-density regime can be explained by hopping through defect-induced localized states, which results in a reduced carrier mobility.30,31 Under these


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circumstances, the unsaturated P atoms in the sublayer can easily react with cations or anions in the solutions, H+ for example. The light hydrogen atom migrates into the valley of BP layers, thus stabilizes the unsaturated P atom in the sublayer. The binding energy, defined as EBE = EHPV-BP

- EPV-BP – EH, is determined to be -0.41 eV, where EH is taken from half of H2. Figure 1c

clearly shows that the mid-gap state disappears (shift down to the deep valence bands) upon hydrogenation, and the VBM now becomes mainly P(3pz) character (Figure S1). However, since the molecules are quite localized, even with very less contribution from C atom (Figure 1c), the modification effectively flattens the landscape of top valence band near Γ point. Based on the non-curving valence band, the effective hole mass increases to 9.48 m0 for molecule modified BP, much larger than the value of 6.23 m0 for pristine BP, which is expected to result in a reduced carrier mobility of BP.32,33 Polymerization and functionalization. One possible solution to remove the localization of the molecules is chaining the individuals, i.e. polymerization. We expect the incorporation polymers with BP can bring desirable properties and retain the high carrier mobility of BP. In fact, polymers have been incorporated with graphene and other 2D materials in previous studies, through both covalent and noncovalent functionalization.34-36 For example, a double layer capping of Al2O3 and hydrophobic fluoropolymer affords BP devices and transistors with long air-stability;12 poly(amidoamine) modified graphene oxide has superior adsorption ability towards heavy metal ions;37 tetrathiafulvalene (TTF)-based polymers coordinated with MoS2 affords significantly enhanced solution stabilization of the nanosheets;23 Nylon-6 polymer covalently linked to MoS2 shows enhancement in thermal and mechanical properties;38 incorporation of BP as electron transfer layer in the organic photovoltaics leads to pronounced enhancement of the device performance.39 Non-covalent modification, at one point, is difficult to


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control and quantify due to the inherent instability of the resulting super molecular systems. Covalent molecular binding, at the other point, can lead to significant defect generation which adversely affects electrical transport in the material. In this respect, chemical methods should be controllable towards a balance between functionality and carrier mobility. Herein, we study electrical transport properties of PPV polymers-BP nano-composites. We note that the zigzag direction of BP is perfect matching the PPV polymer periodicity, while the armchair direction shows less compatibility. Certainly, the positions and directions of the polymers on BP can be random and arbitrary. To simplify our calculations, we only consider PPV along the zigzag direction in the following (Figure 2a). Actually, phosphorene’s peculiar structure of parallel zigzag rows leads to very anisotropic electron and hole masses, optical absorption, and mobility.9 The deformation potential theory has been extensively applied to study carrier mobility in 2D materials, including graphene, MoS2 and BP,4, 40-44 and are exploited to study the mobility of functionalized BP here. Three most relevant factors determining the carrier mobility are listed in Table 1, namely the effective mass, the deformation potential E1 and the elastic modulus C2D in the propagation direction of the longitudinal acoustic wave. Figure 2 represents the band structures of PPV-BP composites with monolayer, two and three layers of BP (denoted as PPV-1 ML BP, PPV-2 ML BP and PPV-3 ML BP), together with partial density of states (PDOS). Obviously, the top of VBM for PPV-1 ML BP is mainly distributed on the PPV, while the second VBM delocalizes on the BP layer. Therefore, the latter is chosen for further carrier mobility calculations, for better comparison with pristine BP. With two and three layers of BP, the VBM and conduction band minimum (CBM) of PPV-2 ML BP and PPV-3 ML BP are dominantly contributed from BP layers, as shown in Figure 2b-d.


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The hole mobility of monolayer BP along zigzag direction is an exceptional case according to the PBE and HSE calculations.4, 44 It has an extraordinary large hole effective mass, since the VB appears to be nearly flat close to the Γ point. Moreover, it possesses a small deformation potential, 0.15 eV at HSE level4 and 0.42 from PBE, which leads to an attractive hole mobility, up to ten times of few layers of BP. The electron mobility of BP is found to be much smaller than the hole mobility, suffering from the large deformation potentials of electrons. For thicker BP, the carrier mobility sustains increasing trend, since the elastic modulus keeps rising. We consider PPV polymer decorated monolayer BP first. As clearly seen from Table 1, the PPV covered BP shows a reduced hole mobility and an increased electron mobility comparing with the isolate one. In detail, the hole effective mass drops sharply to 1.23, while the deformation potential goes from 0.42 up to 1.38 eV and the elastic modulus increases from 102 to 173 J m-2, which induce a smaller hole mobility of 3.58 × 103 cm2 V-1 s-1. On the other hand, a reduced deformation potential leads to an increased electron mobility, from 160 to 500 cm2 V-1 s-1. Comparing 2ML BP with PPV-2ML BP, a reduced hole effective mass and an increased elastic modulus bring about a higher hole mobility; meanwhile, the combined action of a decreased electron deformation potential and an increased elastic modulus results in a higher electron mobility. For PPV-3ML BP composites, the effective mass and deformation potential are consistent with those of 3ML BP, as denoted in Table 1. However, the elastic modulus of PPV3ML BP corresponds to 4ML BP. Therefore, the carrier mobility of PPV-nML BP is higher than that of simplex nML BP. The hole effective mass of PPV-BP along armchair direction (G-X) is presented in Figure S7. Clearly, it is quite smaller than the mass of the free electron (me), which means that the PPV-BP composites have considerably high carrier mobility.4,44 Furthermore, the hole effective masses from PPV-BP composites and pristine BP along the armchair direction are


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in the same order of magnitude, which are over an order of magnitude smaller than that along the zigzag direction, showing highly anisotropic. Considering the uncertainty of effective mass approximation and deformation theory4, we conclude that PPV covered BP preserves both hole and electron mobilities of pristine BP in principle. Table 1. Predicted carrier mobility along zigzag (G-Y) direction at PBE+vdW level.a) m*/m0 E1 (eV) C2D (J m-2) µ (103 cm2 V-1 s-1) hole electron hole electron hole electron ML BP 6.23 1.23 0.42 5.22 102.4 5.22 0.16 2ML BP 2.08 1.31 1.57 5.75 199.1 1.78 0.26 3ML BP 1.41 1.29 1.99 5.95 296.7 3.46 0.43 4ML BP 1.18 1.31 2.29 6.06 394.1 5.48 0.65 PPV-1L BP 1.23 1.05 1.38 3.13 173.3 3.58 0.50 PPV-2L BP 1.39 1.30 2.13 4.17 271.7 1.84 0.38 PPV-3L BP 1.03 1.28 1.83 4.65 374.9 5.38 0.46 NO2PPV-1L BP 1.21 0.93 1.45 3.08 186.1 3.49 0.45 a) * m , E1 and C2D denotes for effective mass, deformation potential and 2D elastic modulus, respectively. The mobilities µ are calculated with the temperature of 300K.

Figure 2. a) Structure of PPV-BP composites along zigzag direction from armchair view (top) and zigzag view (down); b-d) Band structure of PPV-BP composites with monolayer, two and three layers of BP (denoted as PPV-1 ML BP, PPV-2 ML BP and PPV-3 ML BP) based on PBE+vdW calculations, together with partial density of states (PDOS) of BP (red line) and PPV (blue line).

Passivation effect. Above we showed that PPV-BP composites maintain the electronic properties of BP well. Next, we examine the stability of the composites in ambient conditions, which remains a big challenge for wide applications of BP. Indeed, covalent modifications occupy surface phosphorous atoms, thereby avoid the contact between the air mixture and BP


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surface. Nevertheless, the polymers (or molecules) hardly reach to a 100% seamless coverage. Therefore, there are naked phosphorous atoms which can still be reactive to air. Recent experimental and theoretical studies have shown that light, oxygen and water are three indispensable parameters for BP degradation, 9,10, 45-46 and this process involves generation of superoxide under light, dissociation of the superoxide and eventual breakdown under the action of water.47 Among them, light illumination is the primary step for the degradation of BP. In monolayer or few layers (less than 5) of BP, the photo-generated electrons can easily transfer from the conduction band of phosphorene to the O2 on the surface and form superoxide anion O2, since the redox potential of O2/O2- lies in between the bandgap as shown in Figure 3a. Nevertheless, the NB modification successfully shifts the CBM of BP below the redox potential of O2/O2- (Figure 3a) and thereby it is hard to produce O2- and improves the ambient stability of BP. This well explains why the NB modified BP could be persisted for as long as 25 days.18 Regarding PPV functionalization, it is found that the CBM and VBM of BP are shifted up by ~0.45 eV, and the band gap stays ~1.5 eV. Nevertheless, the CBM is still above the redox potential of O2/O2-, suggesting that the generation of superoxide anion remains possible. In another word, the PPV covered BP is not stable enough at the ambient conditions. A major merit of organic modification is the variability of the ligands, and the variation, in turn, can efficiently manipulate the electronic and optical properties of materials. Taking advantages of diversity of PPV family, we model several derivatives of PPV-BP composites with four electron withdrawing groups, OCH3-, F-, CN- and NO2-, towards the controllable energy levels of the system.


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As shown in Figure 3a, the OCH3-, F-, CN- and NO2- decorated PPV shifts down the single particle energy levels with respect to the PPV-BP composite, at different levels. The energy level shift is highly relied on the decoration groups, basically following the order of the electrophilic ability, OCH3-

Covalent Functionalization of Black Phosphorus from First-Principles

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