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Small molecules make big differences: molecular doping effects on electronic and optical properties of phosphorene
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 095201 (http://iopscience.iop.org/0957-4484/26/9/095201) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 095201 (9pp)
doi:10.1088/0957-4484/26/9/095201
Small molecules make big differences: molecular doping effects on electronic and optical properties of phosphorene Yu Jing, Qing Tang, Peng He, Zhen Zhou and Panwen Shen Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Computational Centre for Molecular Science, Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, People’s Republic of China E-mail: [email protected] Received 15 December 2014 Accepted for publication 14 January 2015 Published 10 February 2015 Abstract
Systematical computations on the density functional theory were performed to investigate the adsorption of three typical organic molecules, tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF), on the surface of phosphorene monolayers and thicker layers. There exist considerable charge transfer and strong non-covalent interaction between these molecules and phosphorene. In particular, the band gap of phosphorene decreases dramatically due to the molecular modification and can be further tuned by applying an external electric field. Meanwhile, surface molecular modification has proven to be an effective way to enhance the light harvesting of phosphorene in different directions. Our results predict a flexible method toward modulating the electronic and optical properties of phosphorene and shed light on its experimental applications. S Online supplementary data available from stacks.iop.org/NANO/26/095201/mmedia Keywords: phosphorene, molecular doping, electronic, optical (Some figures may appear in colour only in the online journal) 1. Introduction
silicone [18–21], germanane [22, 23] and MXenes [24–26], have been obtained experimentally via chemical vapor deposition (CVD) or chemical exfoliation methods. These novel 2D structures present distinct properties from their bulk counterparts and have potential applications in electronics and energy storage devices [27–31]. Searching for novel 2D materials never stops. As one of the VA elements, phosphorous (P) is abundant in nature and plays an important role in the material world. Very recently, few-layered and even single-layered black phosphorous (BP) have been successfully prepared or exfoliated [32–39], which definitely enrich the family of 2D materials. Akin to graphene, single-layered BP is termed as ‘phosphorene’ due to its one-layer configuration, although one may argue that there are no double bonds in BP layers. Increasing studies have been accomplished to explore the intrinsic properties and potential applications of phosphorene. As revealed by
The advent of graphene promoted the explorations of twodimensional (2D) structures from many inorganic layered materials, including but not limited to h-BN, transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs) [1–10]. Those inorganic 2D structures possess a number of properties rivaling or surpassing those of graphene. For example, h-BN nanosheets exhibit higher thermal stability and chemical inertness than graphene [11], which would facilitate applications in some harsh environments. Many TMDs (such as MoS2 [12–15] and WS2 [16]) and TMOs (such as MoO3 [17]) have sizeable band gaps, rendering them more promising candidates for microelectronics than graphene, which is semi-metallic and lacks a band gap to turn off the electric current. Recently, some 2D structures, which were initially expected to exist only in the scope of theory, such as 0957-4484/15/095201+09$33.00
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computational and experimental investigations, phosphorene is semiconducting with a moderate direct band gap [40–42] and exhibits high carrier mobility (∼1000 cm2 V s−1) [12, 43], good mechanical flexibility [44, 45], high on/off ratios [13, 46], linear dichroism [21], strong excitonic effect [47, 48], spatially anisotropic electrical and thermal conductance [49], sensitive photo response and anisotropic Raman response [17–52]. These magnificent properties are dependent on its layer thickness [16, 53], stacking order [54, 55], applied strain force [56–61] and external electric field (E-field) [35], thus endowing phosphorene with great potential for electronics [31, 32, 62], energy storage [35, 63] and gas sensors [64]. Meanwhile, the specific properties of phosphorene will be altered after it is cut into nanoribbons [65–73] or curved into nanotubes [74] or after it forms heterojunctions with other 2D structures [75]. Despite the performed investigations, more accessible and facile approaches to tune or control the band structure of this new 2D material are still in demand, and more potential applications of phosphorene are highly expected. In recent years, computational and experimental studies have demonstrated that molecular doping is an effective and flexible method toward modulating the electronic properties of 2D materials, including graphene, BN monolayer/nanoribons, MoS2 monolayers and silicone [76–82]. For example, the band gap of a graphene bilayer can be opened to be 0.15 eV by doping with decamethylcobaltocene (DMC) [56], while epitaxial graphene can be tuned into a p-type semiconductor by modifying it with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) [57]. Similarly, by doping with tetracyanoquinodimethane (TCNQ) or tetrathiafulvalene (TTF), BN nanosheets and nanoribbons can be tuned into por n-type semiconductors [58]. In addition, a considerable charge transfer from electron-donating molecules (TTF or benzyl viologen (BV)) to 2D MoS2 was verified experimentally [60, 61]. On the basis of these investigations, molecular doping is highly attractive and is expected in tuning the electronic and optical properties of phospherene. In this work, we studied the interaction between phosphorene and three typical organic molecules, including TCNQ, tetracyanoethylene (TCNE) and TTF, on the basis of density functional theory (DFT) computations. Due to the charge transfer between phosphorene and electron-withdrawing or -donating molecules, the band gap of a modified phosphorene monolayer can be reduced significantly. The effect of the E-field was also studied for the pristine and modified phosphorene. These four systems respond differently to the external E-field, which can be comprehended by analyzing the corresponding electrostatic potential variations. In addition, we disclosed that all three molecules could enhance the optical properties of phosphorene for effective light harvesting.
computations [83]. The ion-electron interaction was described using the projector-augmented plane wave (PAW) approach [84, 85]. In order to achieve an accurate description of weak interactions, we employed the PBE+D2 (PBE is the abbreviation of the functional of Perdew, Burke and Ernzerhof, and D stands for dispersion) method with the Grimme vdW correction [86, 87] on the basis of the generalized gradient approximation (GGA) [88]. A 420 eV cutoff for the planewave basis set was adopted in all the computations. The geometry optimizations were performed using the conjugated gradient method, and the convergence threshold was set to be 10−4 eV in energy and 10−3 eV Å−1 in force. We set the x and y directions parallel and the z direction perpendicular to the basal plane of phosphorene and adopted a supercell length of 20 Å for the isolated and modified phosphorene monolayers and a length of 30 Å for those of the bilayers in the z direction. The Brillouin zone was represented by a Monkhorst– Pack special k-point mesh of 4 × 4 × 1 for geometry optimizations, while a larger grid (6 × 6 × 1) was used for band structure computations. The electric field was applied along the z direction of the isolated and modified phosphorene, in the range of −0.5 V Å−1 ∼ +0.5 V Å−1, and the same k-point mesh (6 × 6 × 1) was applied for the band structure computations. The frequency-dependent dielectric matrixes of the isolated and modified phospherene were computed using the PBE functional and were expanded over an 8 × 8 × 1 k-point mesh. The imaginary part was determined by a summation over empty states using the equation [89]:
ε2 (ω) =
4π 2 e2 1 lim 2 Σ 2wk δ ( εck − εvk − ω) Ω q → 0 q c, v, k × μ ck + eαq μ vk
μ ck + e βq μ vk *
where the indices c and v represent conduction and valence band states, respectively, and μck is the cell periodic part of the wavefunctions at the given k-point.
3. Results and discussion The atomic structure of the phosphorene monolayer is shown in figures 1(a) and (b) with both the top and side views. The phosphorene monolayer has a tetragonal lattice with the lattice parameters a = 3.294 Å and b = 4.571 Å, optimized at the GGA-PBE level. Each unit cell of the phosphorene monolayer consists of four P atoms, which are all bonded with three neighboring P atoms. Different from graphene, which has an exact planar structure, the phosphorene monolayer consists of two atomic planes. The length of the in-plane P-P bonds is 2.216 Å, while the length of those interlayer bridges is 2.253 Å. The rather complicated configuration of phosphorene results in low symmetry. The x direction, labeled in figure 1(a), can be termed as the zigzag direction, while the y direction can be regarded as the armchair direction. All of these results are in good accordance with previous reports [35, 52].
2. Computational details The plane-wave technique, implemented in the Vienna ab initio simulation package (VASP), was used for the DFT 2
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Bader charge-population analysis reveals that there is 0.40 |e| charge transfer from phosphorene to TCNE. For the modification of electron-donating TTF on the surface of phosphorene, the molecule tends to be stabilized along the y direction, with a slight incline. In this configuration, the two five-membered rings of TTF are stabilized on top of two P atoms of the upper P atomic plane. The adsorption energy of TTF on phosphorene is −0.82 eV, and the vertical distance is 3.39 Å. Similarly, the strong noncovalent interaction between TTF and phosphorene also contributes to a slight surface bend to phosphorene. As a typical electron donor, TTF donates about 0.15 |e| charge to phosphorene, implied by the Bader charge-population analysis. Therefore, all three small molecules have strong interaction and a considerable charge transfer with the phosphorene monolayer, which will make certain changes to the electronic and optical properties of phosphorene. To investigate the effect of molecular doping on the electronic properties of phosphorene, we computed the band structures of the isolated and modified phosphorene under the same computational condition. As shown in figure 3(a), the phosphorene monolayer is a direct-band-gap semiconductor, with its valence band maximum (VBM) and conduction band minimum (CBM) both located at the Γ point. The band gap of phosphorene is computed as 0.86 eV, which agrees well with the previous report [48]. However, since GGA-PBE usually underestimates the band gap of materials, we also adopted HSE06 to verify the band gap of isolated phosphorene. As revealed by HSE06, the band gap of isolated phosphorene is 1.47 eV (as shown in figure S2), in accordance with previous studies [21, 22]. Considering the large computation consumption for a 5 × 4 supercell, we computed and compared the band gaps of various phosphorene systems at the same PBE theoretical level. Although the band gaps are underestimated by PBE to some degree, the physics predicted here should be reliable. Due to the strong non-covalent interaction and considerable charge transfer between phosphorene and the molecules, the three modified phosphorene systems exhibit significantly decreased band gaps, as shown in figures 3(b)– (d). The new levels in the gap region of phosphorene, which are introduced by the adsorbed molecules, result in sharp reduced band gaps of 0.16 eV, 0.22 eV and 0.42 eV for TCNQ/phosphorene, TCNE/phosphorene and TTF/phosphorene, respectively. The renewed band structure of molecule-modified phosphorene can be regarded as the recombination of the levels of phosphorene and those of the molecules. Consequently, all three molecules contribute to a transformation from direct- to indirect-band-gap semiconductors of modified phosphorene. However, for TCNQ and TCNE modified phosphorene (figures 3(b) and (c)), the new shallow levels appear on top of the VBM after doping with electron-withdrawing molecules. In contrast, in TTF/ phosphorene (figure 3(d)), the new level appears in the middle of the band gap region as a deep gap level. Similar results have been found for TCNQ/phosphorene and TTF/phosphorene in a very recent work [90]. In particular, due to the relatively large charge transfer between TCNQ or TCNE and
Figure 1. Atomic structures of phosphorene (5 × 4 supercell) in the
top view (a) and side view along the y direction (b), TCNQ (c), TCNE (d) and TTF (e).
To explore the surface modification effects on phosphorene, three typical charge-transfer organic molecules, including two electron-withdrawing molecules (TCNQ and TCNE) and one electron-donating molecule (TTF), were selected in this study, as shown in figures 1(c)–(e). The phosphorene systems modified with these three molecules were abbreviated as TCNQ/phosphorene, TCNE/phosphorene and TTF/phosphorene. First, the available adsorption sites of these three organic molecules on the surface of the phosphorene monolayer were explored by comparing the adsorption energies. The adsorption energy per molecule is determined as: Ea = Ephosphorene/molecule − Ephosphorene − Emolecule, where Ephosphorene/molecule, Ephosphorene and Emolecule stand for the total energy of the molecule-modified phosphorene, the isolated phosphorene and the single molecule, respectively. According to this definition, a more negative Ea indicates a more favorable interaction. We considered three possible adsorption sites for each organic molecule on the surface of phosphorene in a 5 × 4 supercell, and the most favorable adsorption sites for TCNQ, TCNE and TTF are presented in figure 2 (see other considered configurations in the Supporting Information). First, electron-withdrawing TCNQ prefers adsorption on the surface of phosphorene, with its long axis along the x direction of phosphorene, with a vertical distance of ∼3.30 Å. The adsorption energy of TCNQ is −0.97 eV, indicating a rather strong non-covalent interaction between TCNQ and phosphorene. The strong interaction between TCNQ and phosphorene also causes a slight bending to the phosphorene monolayer (as shown in figure 2(b)). Since TCNQ is a strong electron acceptor, there is 0.405 |e| charge transfer from phosphorene to TCNQ, indicated by the Bader chargepopulation analysis. As another electron-withdrawing molecule, TCNE prefers to be stabilized diagonally, with its centered carboncarbon double bond adsorbed vertically to a P-P bond of phosphorene. The vertical distance between TCNE and phosphorene is ∼3.27 Å, and the adsorption energy is −0.74 eV, a little lower than that of TCNQ. Due to the strong interaction between TCNE and phosphorene, the surface bend also occurs to phosphorene. Akin to TCNQ/phosphorene, the 3
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Figure 2. Structural schematics of TCNQ/phosphorene (a), (b), TCNE/phosphorene (c), (d) and TTF/phosphorene (e), (f). The upper and
lower ones represent the top and side views, respectively.
Figure 3. Band structures of isolated phosphorene (a), TCNQ/phosphorene (b), TCNE/phosphorene (c) and TTF/phosphorene (d). The Fermi level in each system is labeled by the red line.
Figure 4. Partial charge density of the CBM and VBM for TCNQ/phosphorene (a), TCNE/phosphorene (b) and TTF/phosphorene (c).
phosphorene, the molecular levels of TCNQ and TCNE distort more severely than those of TTF. Similar results are found in other 2D structures modified with organic molecules such as BN and MoS2 sheets [58, 59]. To achieve deeper insight into the electronic properties of the molecule-modified phosphorene systems, we computed the partial charge density of CBM and VBM for different
modified phosphorene systems. For phosphorene modified with electron-withdrawing molecules, as shown in figures 4(a) and (b), the CBM is mainly contributed by the electron-withdrawing molecules, TCNQ and TCNE. Slight charge densities of CBM are also found on phosphorene for both TCNQ/phosphorene and TCNE/phosphorene. However, for TCNQ/phosphorene, the VBM is located on both TCNQ 4
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Figure 5. Structural schematics of PBL (a), TCNQ/PBL (b), TCNE/PBL (c) and TTF/PBL (d). The upper and lower ones represent the top and side views, respectively.
Figure 6. Band structures of PBL (a), TCNQ/PBL (b), TCNE/PBL (c) and TTF/PBL (d). The Fermi level in each system is labeled by a
red line.
energy of PBL is −0.30 eV/unit. As shown in figures 5(b)–(d), the adsorption energies of TCNQ, TCNE and TTF on the surface of PBL are 1.06 eV, 0.76 eV and 0.65 eV, respectively, indicating that these three molecules can be stabilized on the surface of PBL with strong non-covalent interactions as well. As shown in figures 6(a)–(d), the GGA-PBE computed band gaps of PBL, TCNQ/PBL, TCNE/PBL and TTF/PBL are 0.48 eV, 0.003 eV, 0.05 eV and 0.25 eV, respectively, demonstrating that molecular modification is also effective in reducing the band gap of thicker phosphorene layers. Applying an external E-field has proven an efficient method toward tuning the electronic properties of many 2D materials [9, 91, 92]. We also studied the responses of band gaps for the isolated and modified phosphorene under the Efield at different intensities. Two directions of the E-field (+z, −z) perpendicular to the plane of phosphorene were considered. Note that the +z direction is equivalent to the −z direction for isolated phosphorene but is not equal to those of molecule-modified phosphorene. The band gaps of different phosphorene systems are shown in figure 7 as a function of the E-field.
and phosphorene, whereas the VBM is only contributed by phosphorene for TCNE/phosphorene. The partial charge density analysis indicates a rather effective interaction between TCNQ and phosphorene compared to that of TCNE/ phosphorene. In contrast, the partial charge density of TTF/phosphorene (figure 4(c)) shows different distributions than those of TCNQ/phosphorene or TCNE/phosphorene. The CBM is contributed by both phosphorene and TTF, while the VBM is only contributed by TTF. The extensive CBM distributions on TTF and phosphorene indicate rather strong interactions between TTF and phosphorene. Therefore, the partial charge density analysis demonstrates the significant interactions between these molecules and phosphorene, which will highly influence the corresponding electronic and optical properties of phosphorene. In order to examine the molecular modification effects on thicker phosphorene layers, we also explored the electronic properties of the phosphorene bilayer (PBL), modified with TCNQ, TCNE or TTF. The PBL was considered in an AAstacking order, as in the bulk counterpart, shown in figure 5(a). GGA-PBE computations indicate that the binding 5
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field the electrostatic potential of TCNQ would be lowered, while that on phosphorene would be lifted. As a result, the levels of TCNQ move downward gradually with the increasing E-field. Note that the CBM of TCNQ/phosphorene is localized totally on TCNQ; thus, the band gap of TCNQ/ phosphorene would decrease with the increasing positive Efield. In contrast, with a negative E-field, the electrostatic potential of TCNQ is lifted, while that on phosphorene is lowered. As a result, the levels of TCNQ move upward with the increase of the E-field, which enlarges the band gap correspondingly. The above analysis for TCNQ/phosphorene also applies to TCNE/phosphorene, but the situation for TTF/phosphorene is different. With a positive E-field, the electrostatic potential of TTF would be lowered, while that on the phosphorene layer would be lifted. Therefore, the levels of TTF would move downward gradually with the increasing E-field. The VBM of TTF/phosphorene is localized on the TTF; thus, the band gap of TTF/phosphorene increases with the increasing positive E-field. With a negative E-field, the levels of TTF would move upward due to the lifted electrostatic potential, and the band gap would decrease correspondingly. Since the band gap of phosphorene can be effectively tuned by molecular doping, some effects are highly expected on the optical absorption of phosphorene. Finally, we studied the optical properties of different molecule-modified phosphorene systems in comparison with the isolated phosphorene. The imaginary parts of the dielectric functions (ε2) along three directions for isolated and molecular-modified phosphorene are shown in figure 8. ε2 is an effective parameter to measure the optical absorption of materials. Due to the anisotropy of phosphorene along the x, y and z directions, the computed ε2 in the three directions presents different characteristics. For the light polarized along the x direction (figure 8(a)), isolated phosphorene shows absorption starting from 1.0 eV, covering strong absorption of ultraviolet light and partial visible light. After molecular modifications, the absorption in the ultraviolet range is significantly heightened. In particular, TCNQ/phosphorene exhibits rather enhanced light absorptions in the infrared range. For the light polarized along the y direction (figure 8(b)), the isolated phosphorene exhibits a wide range of light absorption, covering ultraviolet, visible and infrared regions. The impressive absorption in the infrared range can be ascribed to the strong excitonic effects of phosphorene, which have been demonstrated in previous studies [27, 28]. Molecular doping also contributes to the absorption intensity enhancement of phosphorene along the y direction. For the light polarized along the z direction (figure 8(c)), the isolated phosphorene shows strong ultraviolet light absorption from the energy of 2.2 eV. After molecular doping, the absorption of ultraviolet light is also enhanced. Therefore, the isolated phosphorene exhibits good potential for light harvesting, and all three molecules turn out to be effective in further enhancing the optical absorption of phosphorene.
Figure 7. Field-dependent band gaps of phosphorene, TCNQ/
phosphorene, TCNE/phosphorene and TTF/phosphorene. The direction of the applied positive E-field is labeled inside by taking TCNQ/phosphorene as an example.
It is interesting to note that the electronic properties of isolated phosphorene are rather robust in response to the external E-field. As shown in figure 7, the band gap of phosphorene only decreases from 0.86 to 0.77 eV when the external E-field reaches 0.5 V Å−1. In contrast, the electronic properties of the modified phosphorene are rather sensitive to the external E-field, and the variations of band gaps are dependent on the type of adsorbed molecules. For phosphorene modified with electron-withdrawing molecules TCNQ/phosphorene and TCNE/phosphorene, the band gap decreases with the increasing external electric field. When the external electric field is +0.5 V Å−1, the band gap of TCNQ/phosphorene is only 0.04 eV, while that of TCNE/phosphorene is tuned to be 0.10 eV. When the external electric field reaches −0.5 V Å−1, the band gap of TCNQ/phosphorene and TCNE/phosphorene is 0.46 eV and 0.47 eV, respectively. Moreover, the band gap decrease of TCNQ/phosphorene shows a more sensitive response to the external E-field than that of TCNE/phosphorene. In the case of phosphorene modified with electrondonating TTF, the band gap responds in a concordant way to the variation of the external E-field, which is definitely contrary to those of TCNQ/phosphorene and TCNE/phosphorene. The band gap of TTF/phosphorene changes from 0.19 eV at −0.5 V Å−1 to 0.84 eV at +0.5 V−Å−1. Therefore, both the isolated phosphorene and the molecule-modified phosphorene show robust band gap responses to the external E-field. However, the isolated phosphorene, electron-withdrawing molecule (TCNQ/TCNE)-modified phosphorene and electron-donating molecule (TTF)-modified phosphorene respond differently to the variation of the external E-field. These differences can be expected since these three systems have different band structures. What is the underlying mechanism for E-field tunable electronic properties of molecule-modified phosphorene? Actually, it can be simply attributed to the well-known Stark effect. Explicitly, for TCNQ/phosphorene, under a positive E-
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Figure 8. Computed imaginary dielectric functions versus energy for isolated and molecule-modified phosphorene in the x (a), y (b) and z (c) directions.
4. Conclusion
References
Here, we systematically investigated the doping effects of three typical organic molecules, TCNQ, TCNE and TTF, on the electronic and optical properties of a phosphorene monolayer by dispersion-corrected DFT computations. All three molecules can be effectively adsorbed on the surface of the phosphorene monolayer via strong non-covalent interactions. The band gap of the phosphorene monolayer can be significantly reduced by modifying it with TCNQ, TCNE and TTF due to the strong interaction and significant charge transfer between phosphorene and these molecules. In particular, the band gap of the phosphorene bilayer can also be reduced by doping with TCNQ, TCNE or TTF, which indicates that molecular modification is also effective for tuning the electronic properties of thicker phosphorene layers. By applying an external E-field, the band gap of moleculemodified phosphorene can further increase or decrease, which endows wider molecule-modified phosphorene applications. Finally, the optical properties of molecule-modified phosphorene were compared with those of the isolated phosphorene. As a result, the isolated phosphorene exhibits wide light absorption in three light polarized directions, and molecular modifications can further improve the optical absorption of phosphorene significantly in each direction. Therefore, molecular doping proves facile and effective in modulating the electronic and optical properties of phosphorene. Our findings would facilitate phosphorene for wider applications.
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Acknowledgments This work was supported by the Tianjin Municipal Science and Technology Commission (12JCZDJC28100), NFFTBS (J1103306), NSFC (21273118 and 21421001) and the MOE Innovation Team (IRT13022) in China. The computations were performed on Magic Cube at Shanghai Supercomputer Center. 7
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[57] Fei R and Yang L 2014 Strain-engineering the anisotropic electrical conductance of few layer black phosphorus Nano Lett. 14 2884 [58] Fei R and Yang L 2014 Lattice vibrational modes and raman scattering spectra of strained phosphorene Appl. Phys. Lett. 105 083120 [59] Qin G, Yan Q B, Qin Z, Yue S Y, Cui H J, Zheng Q R and Su G 2014 Hinge-like structure induced unusual properties of black phosphorus and new strategies to improve thermoelectric performance Sci. Rep. 4 4946 [60] Peng X, Wei Q and Copple A 2014 Strain-engineered directindirect band gap transition and its mechanism in twodimensional phosphorene Phys. Rev. B 90 085402 [61] Lv H Y, Lu W J, Shao D F and Sun Y P 2014 Enhanced thermoelectric performance of phosphorene by straininduced band convergence Phys. Rev. B 90 085433 [62] Du Y, Liu H, Deng Y and Ye P D 2014 Device perspective for black phosphorus field-effect transistors: contact resistance, ambipolar behavior, and scaling ACS Nano. 8 10035 [63] Zhao S, Kang W and Xue J 2014 The potential application of phosphorene as an anode material in li-ion batteries J. Mater. Chem. A 2 19046 [64] Kou L, Frauenheim T and Chen C 2014 Phosphorene as a superior gas sensor: selective adsorption and distinct I−V response J. Phys. Chem. Lett. 5 2675 [65] Zhu Z, Li C, Yu W, Chang D, Sun Q and Jia Y 2014 Magnetism of zigzag edge phosphorene nanoribbons Appl. Phys. Lett. 105 113105 [66] Maity A, Singh A and Sen P 2014 Peierls transition and edge reconstruction in phosphorene nanoribbons (arXiv: 1404.2469) [67] Ezawa M 2014 Topological origin of quasi-flat edge band in phosphorene New J. Phys. 16 115004 [68] Peng X and Wei Q 2014 Chemical scissors cut phosphorene nanostructures Mater. Res. Express 1 045041 [69] Li W, Zhang G and Zhang Y W 2014 Electronic properties of edge-hydrogenated phosphorene nanoribbons: a firstprinciples study J. Phys. Chem. C 118 22368 [70] Tran V and Yang L 2014 Scaling laws for the band gap and optical response of phosphorene nanoribbons Phys. Rev. B 89 245407 [71] Ramasubramaniam A 2014 Ab initio studies of thermodynamic and electronic properties of phosphorene nanoribbons Phys. Rev. B 90 085424 [72] Han X, Stewart H M, Shevlin S A, Catlow C R A and Guo Z X 2014 Strain and orientation modulated bandgaps and effective masses of phosphorene nanoribbons Nano Lett. 14 4607 [73] Zhang J, Liu H J, Cheng L, Wei J, Liang J H, Fan D D, Shi J, Tang X F and Zhang Q J 2014 Phosphorene nanoribbon as a promising candidate for thermoelectric applications Sci. Rep. 4 6452 [74] Guo H, Lu N, Dai J, Wu X and Zeng X C 2014 Phosphorene nanoribbons, phosphorous nanotubes, and van der waals multilayers J. Phys. Chem. C 118 14051
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Small molecules make big differences: molecular doping effects on electronic and optical properties of phosphorene
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 095201 (http://iopscience.iop.org/0957-4484/26/9/095201) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 095201 (9pp)
doi:10.1088/0957-4484/26/9/095201
Small molecules make big differences: molecular doping effects on electronic and optical properties of phosphorene Yu Jing, Qing Tang, Peng He, Zhen Zhou and Panwen Shen Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Computational Centre for Molecular Science, Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, People’s Republic of China E-mail: [email protected] Received 15 December 2014 Accepted for publication 14 January 2015 Published 10 February 2015 Abstract
Systematical computations on the density functional theory were performed to investigate the adsorption of three typical organic molecules, tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF), on the surface of phosphorene monolayers and thicker layers. There exist considerable charge transfer and strong non-covalent interaction between these molecules and phosphorene. In particular, the band gap of phosphorene decreases dramatically due to the molecular modification and can be further tuned by applying an external electric field. Meanwhile, surface molecular modification has proven to be an effective way to enhance the light harvesting of phosphorene in different directions. Our results predict a flexible method toward modulating the electronic and optical properties of phosphorene and shed light on its experimental applications. S Online supplementary data available from stacks.iop.org/NANO/26/095201/mmedia Keywords: phosphorene, molecular doping, electronic, optical (Some figures may appear in colour only in the online journal) 1. Introduction
silicone [18–21], germanane [22, 23] and MXenes [24–26], have been obtained experimentally via chemical vapor deposition (CVD) or chemical exfoliation methods. These novel 2D structures present distinct properties from their bulk counterparts and have potential applications in electronics and energy storage devices [27–31]. Searching for novel 2D materials never stops. As one of the VA elements, phosphorous (P) is abundant in nature and plays an important role in the material world. Very recently, few-layered and even single-layered black phosphorous (BP) have been successfully prepared or exfoliated [32–39], which definitely enrich the family of 2D materials. Akin to graphene, single-layered BP is termed as ‘phosphorene’ due to its one-layer configuration, although one may argue that there are no double bonds in BP layers. Increasing studies have been accomplished to explore the intrinsic properties and potential applications of phosphorene. As revealed by
The advent of graphene promoted the explorations of twodimensional (2D) structures from many inorganic layered materials, including but not limited to h-BN, transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs) [1–10]. Those inorganic 2D structures possess a number of properties rivaling or surpassing those of graphene. For example, h-BN nanosheets exhibit higher thermal stability and chemical inertness than graphene [11], which would facilitate applications in some harsh environments. Many TMDs (such as MoS2 [12–15] and WS2 [16]) and TMOs (such as MoO3 [17]) have sizeable band gaps, rendering them more promising candidates for microelectronics than graphene, which is semi-metallic and lacks a band gap to turn off the electric current. Recently, some 2D structures, which were initially expected to exist only in the scope of theory, such as 0957-4484/15/095201+09$33.00
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Nanotechnology 26 (2015) 095201
computational and experimental investigations, phosphorene is semiconducting with a moderate direct band gap [40–42] and exhibits high carrier mobility (∼1000 cm2 V s−1) [12, 43], good mechanical flexibility [44, 45], high on/off ratios [13, 46], linear dichroism [21], strong excitonic effect [47, 48], spatially anisotropic electrical and thermal conductance [49], sensitive photo response and anisotropic Raman response [17–52]. These magnificent properties are dependent on its layer thickness [16, 53], stacking order [54, 55], applied strain force [56–61] and external electric field (E-field) [35], thus endowing phosphorene with great potential for electronics [31, 32, 62], energy storage [35, 63] and gas sensors [64]. Meanwhile, the specific properties of phosphorene will be altered after it is cut into nanoribbons [65–73] or curved into nanotubes [74] or after it forms heterojunctions with other 2D structures [75]. Despite the performed investigations, more accessible and facile approaches to tune or control the band structure of this new 2D material are still in demand, and more potential applications of phosphorene are highly expected. In recent years, computational and experimental studies have demonstrated that molecular doping is an effective and flexible method toward modulating the electronic properties of 2D materials, including graphene, BN monolayer/nanoribons, MoS2 monolayers and silicone [76–82]. For example, the band gap of a graphene bilayer can be opened to be 0.15 eV by doping with decamethylcobaltocene (DMC) [56], while epitaxial graphene can be tuned into a p-type semiconductor by modifying it with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) [57]. Similarly, by doping with tetracyanoquinodimethane (TCNQ) or tetrathiafulvalene (TTF), BN nanosheets and nanoribbons can be tuned into por n-type semiconductors [58]. In addition, a considerable charge transfer from electron-donating molecules (TTF or benzyl viologen (BV)) to 2D MoS2 was verified experimentally [60, 61]. On the basis of these investigations, molecular doping is highly attractive and is expected in tuning the electronic and optical properties of phospherene. In this work, we studied the interaction between phosphorene and three typical organic molecules, including TCNQ, tetracyanoethylene (TCNE) and TTF, on the basis of density functional theory (DFT) computations. Due to the charge transfer between phosphorene and electron-withdrawing or -donating molecules, the band gap of a modified phosphorene monolayer can be reduced significantly. The effect of the E-field was also studied for the pristine and modified phosphorene. These four systems respond differently to the external E-field, which can be comprehended by analyzing the corresponding electrostatic potential variations. In addition, we disclosed that all three molecules could enhance the optical properties of phosphorene for effective light harvesting.
computations [83]. The ion-electron interaction was described using the projector-augmented plane wave (PAW) approach [84, 85]. In order to achieve an accurate description of weak interactions, we employed the PBE+D2 (PBE is the abbreviation of the functional of Perdew, Burke and Ernzerhof, and D stands for dispersion) method with the Grimme vdW correction [86, 87] on the basis of the generalized gradient approximation (GGA) [88]. A 420 eV cutoff for the planewave basis set was adopted in all the computations. The geometry optimizations were performed using the conjugated gradient method, and the convergence threshold was set to be 10−4 eV in energy and 10−3 eV Å−1 in force. We set the x and y directions parallel and the z direction perpendicular to the basal plane of phosphorene and adopted a supercell length of 20 Å for the isolated and modified phosphorene monolayers and a length of 30 Å for those of the bilayers in the z direction. The Brillouin zone was represented by a Monkhorst– Pack special k-point mesh of 4 × 4 × 1 for geometry optimizations, while a larger grid (6 × 6 × 1) was used for band structure computations. The electric field was applied along the z direction of the isolated and modified phosphorene, in the range of −0.5 V Å−1 ∼ +0.5 V Å−1, and the same k-point mesh (6 × 6 × 1) was applied for the band structure computations. The frequency-dependent dielectric matrixes of the isolated and modified phospherene were computed using the PBE functional and were expanded over an 8 × 8 × 1 k-point mesh. The imaginary part was determined by a summation over empty states using the equation [89]:
ε2 (ω) =
4π 2 e2 1 lim 2 Σ 2wk δ ( εck − εvk − ω) Ω q → 0 q c, v, k × μ ck + eαq μ vk
μ ck + e βq μ vk *
where the indices c and v represent conduction and valence band states, respectively, and μck is the cell periodic part of the wavefunctions at the given k-point.
3. Results and discussion The atomic structure of the phosphorene monolayer is shown in figures 1(a) and (b) with both the top and side views. The phosphorene monolayer has a tetragonal lattice with the lattice parameters a = 3.294 Å and b = 4.571 Å, optimized at the GGA-PBE level. Each unit cell of the phosphorene monolayer consists of four P atoms, which are all bonded with three neighboring P atoms. Different from graphene, which has an exact planar structure, the phosphorene monolayer consists of two atomic planes. The length of the in-plane P-P bonds is 2.216 Å, while the length of those interlayer bridges is 2.253 Å. The rather complicated configuration of phosphorene results in low symmetry. The x direction, labeled in figure 1(a), can be termed as the zigzag direction, while the y direction can be regarded as the armchair direction. All of these results are in good accordance with previous reports [35, 52].
2. Computational details The plane-wave technique, implemented in the Vienna ab initio simulation package (VASP), was used for the DFT 2
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Bader charge-population analysis reveals that there is 0.40 |e| charge transfer from phosphorene to TCNE. For the modification of electron-donating TTF on the surface of phosphorene, the molecule tends to be stabilized along the y direction, with a slight incline. In this configuration, the two five-membered rings of TTF are stabilized on top of two P atoms of the upper P atomic plane. The adsorption energy of TTF on phosphorene is −0.82 eV, and the vertical distance is 3.39 Å. Similarly, the strong noncovalent interaction between TTF and phosphorene also contributes to a slight surface bend to phosphorene. As a typical electron donor, TTF donates about 0.15 |e| charge to phosphorene, implied by the Bader charge-population analysis. Therefore, all three small molecules have strong interaction and a considerable charge transfer with the phosphorene monolayer, which will make certain changes to the electronic and optical properties of phosphorene. To investigate the effect of molecular doping on the electronic properties of phosphorene, we computed the band structures of the isolated and modified phosphorene under the same computational condition. As shown in figure 3(a), the phosphorene monolayer is a direct-band-gap semiconductor, with its valence band maximum (VBM) and conduction band minimum (CBM) both located at the Γ point. The band gap of phosphorene is computed as 0.86 eV, which agrees well with the previous report [48]. However, since GGA-PBE usually underestimates the band gap of materials, we also adopted HSE06 to verify the band gap of isolated phosphorene. As revealed by HSE06, the band gap of isolated phosphorene is 1.47 eV (as shown in figure S2), in accordance with previous studies [21, 22]. Considering the large computation consumption for a 5 × 4 supercell, we computed and compared the band gaps of various phosphorene systems at the same PBE theoretical level. Although the band gaps are underestimated by PBE to some degree, the physics predicted here should be reliable. Due to the strong non-covalent interaction and considerable charge transfer between phosphorene and the molecules, the three modified phosphorene systems exhibit significantly decreased band gaps, as shown in figures 3(b)– (d). The new levels in the gap region of phosphorene, which are introduced by the adsorbed molecules, result in sharp reduced band gaps of 0.16 eV, 0.22 eV and 0.42 eV for TCNQ/phosphorene, TCNE/phosphorene and TTF/phosphorene, respectively. The renewed band structure of molecule-modified phosphorene can be regarded as the recombination of the levels of phosphorene and those of the molecules. Consequently, all three molecules contribute to a transformation from direct- to indirect-band-gap semiconductors of modified phosphorene. However, for TCNQ and TCNE modified phosphorene (figures 3(b) and (c)), the new shallow levels appear on top of the VBM after doping with electron-withdrawing molecules. In contrast, in TTF/ phosphorene (figure 3(d)), the new level appears in the middle of the band gap region as a deep gap level. Similar results have been found for TCNQ/phosphorene and TTF/phosphorene in a very recent work [90]. In particular, due to the relatively large charge transfer between TCNQ or TCNE and
Figure 1. Atomic structures of phosphorene (5 × 4 supercell) in the
top view (a) and side view along the y direction (b), TCNQ (c), TCNE (d) and TTF (e).
To explore the surface modification effects on phosphorene, three typical charge-transfer organic molecules, including two electron-withdrawing molecules (TCNQ and TCNE) and one electron-donating molecule (TTF), were selected in this study, as shown in figures 1(c)–(e). The phosphorene systems modified with these three molecules were abbreviated as TCNQ/phosphorene, TCNE/phosphorene and TTF/phosphorene. First, the available adsorption sites of these three organic molecules on the surface of the phosphorene monolayer were explored by comparing the adsorption energies. The adsorption energy per molecule is determined as: Ea = Ephosphorene/molecule − Ephosphorene − Emolecule, where Ephosphorene/molecule, Ephosphorene and Emolecule stand for the total energy of the molecule-modified phosphorene, the isolated phosphorene and the single molecule, respectively. According to this definition, a more negative Ea indicates a more favorable interaction. We considered three possible adsorption sites for each organic molecule on the surface of phosphorene in a 5 × 4 supercell, and the most favorable adsorption sites for TCNQ, TCNE and TTF are presented in figure 2 (see other considered configurations in the Supporting Information). First, electron-withdrawing TCNQ prefers adsorption on the surface of phosphorene, with its long axis along the x direction of phosphorene, with a vertical distance of ∼3.30 Å. The adsorption energy of TCNQ is −0.97 eV, indicating a rather strong non-covalent interaction between TCNQ and phosphorene. The strong interaction between TCNQ and phosphorene also causes a slight bending to the phosphorene monolayer (as shown in figure 2(b)). Since TCNQ is a strong electron acceptor, there is 0.405 |e| charge transfer from phosphorene to TCNQ, indicated by the Bader chargepopulation analysis. As another electron-withdrawing molecule, TCNE prefers to be stabilized diagonally, with its centered carboncarbon double bond adsorbed vertically to a P-P bond of phosphorene. The vertical distance between TCNE and phosphorene is ∼3.27 Å, and the adsorption energy is −0.74 eV, a little lower than that of TCNQ. Due to the strong interaction between TCNE and phosphorene, the surface bend also occurs to phosphorene. Akin to TCNQ/phosphorene, the 3
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Figure 2. Structural schematics of TCNQ/phosphorene (a), (b), TCNE/phosphorene (c), (d) and TTF/phosphorene (e), (f). The upper and
lower ones represent the top and side views, respectively.
Figure 3. Band structures of isolated phosphorene (a), TCNQ/phosphorene (b), TCNE/phosphorene (c) and TTF/phosphorene (d). The Fermi level in each system is labeled by the red line.
Figure 4. Partial charge density of the CBM and VBM for TCNQ/phosphorene (a), TCNE/phosphorene (b) and TTF/phosphorene (c).
phosphorene, the molecular levels of TCNQ and TCNE distort more severely than those of TTF. Similar results are found in other 2D structures modified with organic molecules such as BN and MoS2 sheets [58, 59]. To achieve deeper insight into the electronic properties of the molecule-modified phosphorene systems, we computed the partial charge density of CBM and VBM for different
modified phosphorene systems. For phosphorene modified with electron-withdrawing molecules, as shown in figures 4(a) and (b), the CBM is mainly contributed by the electron-withdrawing molecules, TCNQ and TCNE. Slight charge densities of CBM are also found on phosphorene for both TCNQ/phosphorene and TCNE/phosphorene. However, for TCNQ/phosphorene, the VBM is located on both TCNQ 4
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Figure 5. Structural schematics of PBL (a), TCNQ/PBL (b), TCNE/PBL (c) and TTF/PBL (d). The upper and lower ones represent the top and side views, respectively.
Figure 6. Band structures of PBL (a), TCNQ/PBL (b), TCNE/PBL (c) and TTF/PBL (d). The Fermi level in each system is labeled by a
red line.
energy of PBL is −0.30 eV/unit. As shown in figures 5(b)–(d), the adsorption energies of TCNQ, TCNE and TTF on the surface of PBL are 1.06 eV, 0.76 eV and 0.65 eV, respectively, indicating that these three molecules can be stabilized on the surface of PBL with strong non-covalent interactions as well. As shown in figures 6(a)–(d), the GGA-PBE computed band gaps of PBL, TCNQ/PBL, TCNE/PBL and TTF/PBL are 0.48 eV, 0.003 eV, 0.05 eV and 0.25 eV, respectively, demonstrating that molecular modification is also effective in reducing the band gap of thicker phosphorene layers. Applying an external E-field has proven an efficient method toward tuning the electronic properties of many 2D materials [9, 91, 92]. We also studied the responses of band gaps for the isolated and modified phosphorene under the Efield at different intensities. Two directions of the E-field (+z, −z) perpendicular to the plane of phosphorene were considered. Note that the +z direction is equivalent to the −z direction for isolated phosphorene but is not equal to those of molecule-modified phosphorene. The band gaps of different phosphorene systems are shown in figure 7 as a function of the E-field.
and phosphorene, whereas the VBM is only contributed by phosphorene for TCNE/phosphorene. The partial charge density analysis indicates a rather effective interaction between TCNQ and phosphorene compared to that of TCNE/ phosphorene. In contrast, the partial charge density of TTF/phosphorene (figure 4(c)) shows different distributions than those of TCNQ/phosphorene or TCNE/phosphorene. The CBM is contributed by both phosphorene and TTF, while the VBM is only contributed by TTF. The extensive CBM distributions on TTF and phosphorene indicate rather strong interactions between TTF and phosphorene. Therefore, the partial charge density analysis demonstrates the significant interactions between these molecules and phosphorene, which will highly influence the corresponding electronic and optical properties of phosphorene. In order to examine the molecular modification effects on thicker phosphorene layers, we also explored the electronic properties of the phosphorene bilayer (PBL), modified with TCNQ, TCNE or TTF. The PBL was considered in an AAstacking order, as in the bulk counterpart, shown in figure 5(a). GGA-PBE computations indicate that the binding 5
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field the electrostatic potential of TCNQ would be lowered, while that on phosphorene would be lifted. As a result, the levels of TCNQ move downward gradually with the increasing E-field. Note that the CBM of TCNQ/phosphorene is localized totally on TCNQ; thus, the band gap of TCNQ/ phosphorene would decrease with the increasing positive Efield. In contrast, with a negative E-field, the electrostatic potential of TCNQ is lifted, while that on phosphorene is lowered. As a result, the levels of TCNQ move upward with the increase of the E-field, which enlarges the band gap correspondingly. The above analysis for TCNQ/phosphorene also applies to TCNE/phosphorene, but the situation for TTF/phosphorene is different. With a positive E-field, the electrostatic potential of TTF would be lowered, while that on the phosphorene layer would be lifted. Therefore, the levels of TTF would move downward gradually with the increasing E-field. The VBM of TTF/phosphorene is localized on the TTF; thus, the band gap of TTF/phosphorene increases with the increasing positive E-field. With a negative E-field, the levels of TTF would move upward due to the lifted electrostatic potential, and the band gap would decrease correspondingly. Since the band gap of phosphorene can be effectively tuned by molecular doping, some effects are highly expected on the optical absorption of phosphorene. Finally, we studied the optical properties of different molecule-modified phosphorene systems in comparison with the isolated phosphorene. The imaginary parts of the dielectric functions (ε2) along three directions for isolated and molecular-modified phosphorene are shown in figure 8. ε2 is an effective parameter to measure the optical absorption of materials. Due to the anisotropy of phosphorene along the x, y and z directions, the computed ε2 in the three directions presents different characteristics. For the light polarized along the x direction (figure 8(a)), isolated phosphorene shows absorption starting from 1.0 eV, covering strong absorption of ultraviolet light and partial visible light. After molecular modifications, the absorption in the ultraviolet range is significantly heightened. In particular, TCNQ/phosphorene exhibits rather enhanced light absorptions in the infrared range. For the light polarized along the y direction (figure 8(b)), the isolated phosphorene exhibits a wide range of light absorption, covering ultraviolet, visible and infrared regions. The impressive absorption in the infrared range can be ascribed to the strong excitonic effects of phosphorene, which have been demonstrated in previous studies [27, 28]. Molecular doping also contributes to the absorption intensity enhancement of phosphorene along the y direction. For the light polarized along the z direction (figure 8(c)), the isolated phosphorene shows strong ultraviolet light absorption from the energy of 2.2 eV. After molecular doping, the absorption of ultraviolet light is also enhanced. Therefore, the isolated phosphorene exhibits good potential for light harvesting, and all three molecules turn out to be effective in further enhancing the optical absorption of phosphorene.
Figure 7. Field-dependent band gaps of phosphorene, TCNQ/
phosphorene, TCNE/phosphorene and TTF/phosphorene. The direction of the applied positive E-field is labeled inside by taking TCNQ/phosphorene as an example.
It is interesting to note that the electronic properties of isolated phosphorene are rather robust in response to the external E-field. As shown in figure 7, the band gap of phosphorene only decreases from 0.86 to 0.77 eV when the external E-field reaches 0.5 V Å−1. In contrast, the electronic properties of the modified phosphorene are rather sensitive to the external E-field, and the variations of band gaps are dependent on the type of adsorbed molecules. For phosphorene modified with electron-withdrawing molecules TCNQ/phosphorene and TCNE/phosphorene, the band gap decreases with the increasing external electric field. When the external electric field is +0.5 V Å−1, the band gap of TCNQ/phosphorene is only 0.04 eV, while that of TCNE/phosphorene is tuned to be 0.10 eV. When the external electric field reaches −0.5 V Å−1, the band gap of TCNQ/phosphorene and TCNE/phosphorene is 0.46 eV and 0.47 eV, respectively. Moreover, the band gap decrease of TCNQ/phosphorene shows a more sensitive response to the external E-field than that of TCNE/phosphorene. In the case of phosphorene modified with electrondonating TTF, the band gap responds in a concordant way to the variation of the external E-field, which is definitely contrary to those of TCNQ/phosphorene and TCNE/phosphorene. The band gap of TTF/phosphorene changes from 0.19 eV at −0.5 V Å−1 to 0.84 eV at +0.5 V−Å−1. Therefore, both the isolated phosphorene and the molecule-modified phosphorene show robust band gap responses to the external E-field. However, the isolated phosphorene, electron-withdrawing molecule (TCNQ/TCNE)-modified phosphorene and electron-donating molecule (TTF)-modified phosphorene respond differently to the variation of the external E-field. These differences can be expected since these three systems have different band structures. What is the underlying mechanism for E-field tunable electronic properties of molecule-modified phosphorene? Actually, it can be simply attributed to the well-known Stark effect. Explicitly, for TCNQ/phosphorene, under a positive E-
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Figure 8. Computed imaginary dielectric functions versus energy for isolated and molecule-modified phosphorene in the x (a), y (b) and z (c) directions.
4. Conclusion
References
Here, we systematically investigated the doping effects of three typical organic molecules, TCNQ, TCNE and TTF, on the electronic and optical properties of a phosphorene monolayer by dispersion-corrected DFT computations. All three molecules can be effectively adsorbed on the surface of the phosphorene monolayer via strong non-covalent interactions. The band gap of the phosphorene monolayer can be significantly reduced by modifying it with TCNQ, TCNE and TTF due to the strong interaction and significant charge transfer between phosphorene and these molecules. In particular, the band gap of the phosphorene bilayer can also be reduced by doping with TCNQ, TCNE or TTF, which indicates that molecular modification is also effective for tuning the electronic properties of thicker phosphorene layers. By applying an external E-field, the band gap of moleculemodified phosphorene can further increase or decrease, which endows wider molecule-modified phosphorene applications. Finally, the optical properties of molecule-modified phosphorene were compared with those of the isolated phosphorene. As a result, the isolated phosphorene exhibits wide light absorption in three light polarized directions, and molecular modifications can further improve the optical absorption of phosphorene significantly in each direction. Therefore, molecular doping proves facile and effective in modulating the electronic and optical properties of phosphorene. Our findings would facilitate phosphorene for wider applications.
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Acknowledgments This work was supported by the Tianjin Municipal Science and Technology Commission (12JCZDJC28100), NFFTBS (J1103306), NSFC (21273118 and 21421001) and the MOE Innovation Team (IRT13022) in China. The computations were performed on Magic Cube at Shanghai Supercomputer Center. 7
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