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Electronic and structural properties of black phosphorene doped with Si, B and N
Physics Letters A 383 (2019) 125945
Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Electronic and structural properties of black phosphorene doped with Si, B and N Luiza Goulart ∗ , Liana da S. Fernandes, Cláudia Lange dos Santos, Jussane Rossato Área de Ciências Tecnológicas, Universidade Franciscana, Santa Maria, Brazil
a r t i c l e
i n f o
Article history: Received 22 July 2019 Received in revised form 27 August 2019 Accepted 3 September 2019 Available online 6 September 2019 Communicated by R. Wu Keywords: Functionalization Phosphorene Semiconductor
a b s t r a c t The electronic and structural properties of substitutional and doped phosphorene with B, N and Si were studied using first principles calculations based on density functional theory. Moreover, electronic and structural properties of functionalized phosphorene slowly increasing the concentration of doping was investigated. Phosphorene strongly binds with doped functionalization; B doped phosphorene is the most stable configuration studied. Si doped phosphorene maintains the semiconductor characteristic. B and N doped phosphorene present n-type and p-type semiconductors, respectively. Doped phosphorene with odd number of Si is a semiconductor material, doped phosphorene with an odd number of B has ntype semiconductor characteristic, and doped phosphorene with odd number of N atoms has a p-type semiconductor behaviour. Doped phosphorene with even number of Si has a metallic characteristic, while B and N doped phosphorene with even number present a semiconductor behaviour. This work reveals that phosphorene electronic properties could be changed by introducing the dopants on the system, and the properties are affected by the increasing number of dopants on phosphorene sheet. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of nanostructures based on carbon atoms, such as fullerene, carbon nanotube, and graphene [1–3], exploration of other elements that could lead into new nanostructures has yielded many nanostructures such as boron nitride, silicene, zironium diboride, black phosphorene and others [4–9]. Among these, black phosphorene, a 2D material with a single layer phosphorus sheet exfoliated from bulk black phosphorus has recently attracted scientific attention due to its properties [10–12]. Black phosphorene is a puckered layer of phosphorus atoms, in which each atom bonds to other 3 in an orthorhombic structure [4, 11]. Its main electronic characteristic is the semiconductor character, with a gap around 1.0 eV [13,14] and its high carrier mobility comparable to graphene (1000 cm2 V−1 s−1 at room temperature) [15,16], due to these properties, it has been extensively applied on the electronic field [17–20]. However, to modify and improve its properties, 2D materials often require chemical functionalization [21–25]. For instance, B and N doped graphene exhibit both p-type and n-type semiconductor characteristics, respectively [26–28]. Moreover, the band structures of phosphorene can be modified by doped effects [29–31].
*
Corresponding author. E-mail address: [email protected] (L. Goulart).
https://doi.org/10.1016/j.physleta.2019.125945 0375-9601/© 2019 Elsevier B.V. All rights reserved.
Doped phosphorene can exhibit both n-type and p-type semiconductor characteristics [30], while doped phosphorus nanostructures such as Si doped blue phosphorene changes the electronic character of this semiconductor nanostructure to a metallic character [29]. Zigzag blue phosphorene nanoribbons passivated with −H and −OH present a wide-gap semiconductor property, while with −O, it presents a metallic characteristic; also, when passivated in both the edges with various functional groups, it presents some important functional characteristics [30]. Decorated phosphorene has better adsorption capability than other 2D materials such as BN, SiC, MoS2 and graphene, surface adsorption functionalized phosphorene improves the system potential applications in nanoelectrics and spintronics [31]. Substitutionally doped phosphorene affects the electronic structures of phosphorene, enhancing chemical activity for capture gas molecules, suggesting the system potential application on molecular adsorption and desorption processes, such as catalysis, gas sensing and storage [32]. While the mentioned studies investigate the decorated doped phosphorene, or the substitutionally doped phosphorene, systematic studies that compared the two types of functionalization are rare. Moreover, despite the significant effect of doping on phosphorene electronic properties, no study has been performed to examine the effect of doping phosphorene with several dopants atoms so far. Thus, the aim of this study is to analyze the electronic and structural properties of black phosphorene decorated and substitu-
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Fig. 1. Electronic band structure are shown in the bottom panel and the optimized geometries are shown in the top panel for pristine phosphorene (a), Si doped phosphorene (b), B doped phosphorene (c), and N doped phosphorene (d). Fermi energy level is represented in red. (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.)
tionally with Si, B and N atoms. Moreover, it aims to investigate the electronic and structural properties of black phosphorene slowly increasing the concentration of doping until the amount of five adsorbed atoms. The chosen atoms have been extensively used in theoretical studies of carbon materials [33–39]. The results indicate that the doping with Si decreases the gap of phosphorene when compared to the pristine system, while the doping with B and N presents characteristics of an n-type and p-type semiconductor, respectively. Also, doping of the black phosphorene with an odd number of these atoms exhibits different behaviour than those with an even number. The results presented here may be useful for the study of phosphorene application in nano-electrics, catalysis, gas sensing and storage.
Table 1 The adsorption energies (Ead ) for the substitutional and decorated sites with the different dopants, Si, B and N. Dopant
Site
Ead (eV)
Silicon
T B H
−0.32 −2.06 −2.46
Boron
T B H
−2.21 −2.51 −3.62
Nitrogen
T B H
−0.48 −2.62 −2.36
2. Computational methods 3. Results and discussion The density functional theory (DFT) [40] calculations were performed with SIESTA [41] using Double-Zeta base with spin polarization and generalized gradient approximation (GGA) for the exchange and correlation term [42]. We used periodic boundary conditions fs a 9.95 Å × 13.56 Å × 30.00 Å phosphorene supercell (36 P atoms). The k-point was set to (10 × 8 × 1) for the Brillouin zone representation and a kinetic energy cut off of 300 eV was used. The radius cut off was defined with an energy shift of 0.01 eV and all the structures are relaxed until the forces on each atom were less than 0.05 eV Å−1 . The adsorption energy of doped black phosphorene was calculated as
Ead = E(DopedPhosphorene) − E(Phosphorene) − nE(Atom) /n
(1)
where E(DopedPhosphorene) is the total energy of the relaxed phosphorene doped with Si, B or N, E(Phosphorene) , E(Atom) are the total energies of the relaxed pristine phosphorene and the Si, B or N atoms, respectively, and n is the number of dopants atoms on the phosphorene sheet.
Substitutional and decorated doped phosphorene were studied. To find the most favourable configuration, the dopants under investigation were initially placed at three different positions on phosphorene sheet: the top site (T) is when a P atom of the top layer is replaced by a dopant, the hollow site (H) at the middle of buckled hexagon, and the bridge site (B) at the midpoint of P−P bond [24,25]. The adsorption energies found are presented in Table 1. All the atomic species bind to the phosphorene with adsorption energies that range from −0.32 eV to −3.62 eV. Phosphorene strongly binds with doped functionalization. The most favourable adsorption is with Boron by the H site. Electronic and structural properties are presented in the following sections. 3.1. Si doped phosphorene Table 1 shows that the most favourable configuration of Si doped phosphorene (BP_Si) is H site with Ead of −2.46 eV. This result agrees with the results reported by Ding and Wang [31] who
L. Goulart et al. / Physics Letters A 383 (2019) 125945
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Fig. 2. Non-optimized geometries are shown in the top panel (a)-(e), and optimized geometries are shown in the bottom of the panel (f)-(j) for Si doped phosphorene with increased concentration of Si atoms.
Fig. 3. Electronic band structure and adsorption energy of pristine black phosphorene (a), BP_Si (b), BP_2Si (c), BP_3Si (d), BP_4Si (e), and BP_5Si (f).
theoretically described that the most stable site for doping phosphorene with Si is the H site, with a similar adsorption energy of −2.52 eV. Fig. 1 (b) shows the geometry of Si doped phosphorene, and the band structure for this configuration. The distance between the Si atom and the nearest P atom is 2.32 Å. Fig. 2 (a) and (b) show the electronic band structure of pristine phosphorene and Si doped phosphorene, respectively. Through a comparison of the electronic band structures, the band gap of Si doped phosphorene is 0.23 eV, while the band gap of pristine phosphorene is 0.80 eV. This indicates that the black phosphorene remains as a semiconductor after doping with Si. It can be explained due to Si and P atom similar electronegativity, around 2. Considering the obtained results after the doping of the phosphorene sheet with a Si atom, it was investigated the properties of phosphorene sheet doped with an increased number of Si atoms (BP_nSi), where n ranges from one to five. Fig. 2 (a)-(e) shows in the top panel the initial geometries chosen by symmetry based on the most favourable site for one atom (H site). The optimized geometries of these configurations are shown in the bottom of Fig. 2 (f)-(j). Doped phosphorene with two or more Si atoms causes dis-
tortions in P–P bonds near the dopants, resulting in the rupture of these bonds, as shown in Fig. 2. The addition of more Si atoms promoted different distortion patterns in the phosphorene structure, as can be seen in Fig. 2 (g)-(j). A similar result was also reported by Ying, who showed that the increase of Si atom density in the medium caused structural changes in the 2D material tested [43]. Fig. 3 shows electronic band structures, and adsorption energies per atom. Doped phosphorene with an even number of Si atoms causes a metallic behaviour, as shown in Fig. 3 (c) and (e). On the other hand, doped phosphorene with an odd number of Si atoms causes a semiconductor behaviour with indirect gap of 0.23 eV for BP_Si, 0.68 eV for BP_3Si, and 0.48 eV for BP_5Si, as observed in the electronic band structure shown in Fig. 3 (b), (d) and (f), respectively. Also, it is clear from Fig. 3 (b), (d) and (f) that the position of the valence band maximum (VBM) is altered. For the doping with one Si atom, the VBM is localized near the X-point while that for the doping with three and five Si atoms, the VBM position shifts to Spoint. On the other hand, the conduction band minimum (CBM) is always in G-point.
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Fig. 4. Non-optimized geometries are shown in the top panel (a)-(e), and optimized geometries are shown in the bottom of the panel (f)-(j) for B doped phosphorene with increased concentration of B atoms.
Fig. 5. Electronic band structure and adsorption energy of: pristine phosphorene (a), BP_B (b), BP_2B (c), BP_3B (d), BP_4B (e), and BP_5B (f).
3.2. B doped phosphorene The most favourable configuration of B doped phosphorene (BP_B) is H site with Ead of −3.62 eV, as presented in Table 1. Similar to the previous case where the most favourable site for Si doped phosphorene is H, this result is in agreement with the results reported by Ding and Wang [31]. The geometry of B doped phosphorene, and the band structure for this configuration are shown in Fig. 1 (c). The shortest B–P distance after optimization is 1.9 Å, which is the typical value of B–P bond distance, due to the electron deficiency of B atom, it bonds with three P atoms forming a trigonal planar structure. From Fig. 1 (a) and (c), it is possible to observe that black phosphorene changes its character after doping from semiconductor with 0.80 eV gap to n-type semiconductor since Fermi level moves the conduction band region upwards. This behaviour is due to the fact that B atom has one half filled orbital and two p empty orbitals, which causes a strong attraction with P atom.
Considering the obtained results after the doping of the black phosphorene sheet with a B atom, it was investigated the properties of the phosphorene doped an increased number of B atoms (BP_nB), ranging from one to five. Similar to the previous case (Sidoped), the positions were chosen by symmetry based on the most favourable site (H site). Fig. 4 (a)-(e) shows in the top the nonoptimized geometry of black phosphorene doped with a different number of B atoms. The optimized geometries of these configurations are shown in the bottom of Fig. 4 (f)-(j). Doped phosphorene with two or more B atoms causes distortions on P–P bonds near the dopants, rupturing these bonds, as shown in Fig. 4. The B atom will make three bonds, but because it has an empty orbital, it will tend to approach the largest number of P atoms to stabilize. Some P–P bonds are broken so that B atoms are inserted into the structure, making it stable. Fig. 5 presents the electronic band structure and adsorption energies per atom of those configurations. Doped phosphorene with even number of B atoms causes a semiconductor behaviour with an indirect gap of 0.41 eV for BP_2B and 0.65 eV for BP_4B, as shown in Fig. 5. In this case, on the
L. Goulart et al. / Physics Letters A 383 (2019) 125945
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Fig. 6. Non-optimized geometries are shown in the top panel (a)-(e), and optimized geometries are shown in the bottom of the panel (f)-(j) for N doped phosphorene with the increase of N atoms concentration.
Fig. 7. Electronic structure of bands and adsorption energy of: pristine phosphorene (a), BP_N (b), BP_2N (c), BP_3N (d), BP_4N (e) e BP_5N (f).
contrary to the previous case (Si doped phosphorene), no significant changes are seen on the VBM position which keeps near the X-point. However, the CBM changes from X-point to Y-point when the number of B atoms varies from two to four atoms, as shown in Fig. 5 (c) and (e), respectively. On the other hand, doped phosphorene with odd number of B atoms causes a n-type semiconductor behaviour, as observed in the electronic structure of BP_B, BP_3B and BP_5B, shown in Fig. 5 (b), (d) and (f), respectively. 3.3. N doped phosphorene The most favourable configuration of N doped phosphorene (BP_N) is B site with Ead of −2.62 eV, as presented in Table 1, similar to the results reported by Ding and Wang [24]. The optimized geometry and the electronic band structure for this configuration are shown in Fig. 1 (d). The smallest N–P distance after optimization is 1.65 Å, approximately the typical value of the N–P bond (1.62 Å), the N adsorption ruptures the below P–P bond and forms a P–N–P configuration. Through a comparison of the electronic band structures, it can be
seen that N doped phosphorene is a p-type semiconductor since Fermi level moves the valence band region downwards (Fig. 1 (d)), while the band gap of pristine phosphorene is 0.80 eV (Fig. 1 (a)). N atom is the third element and the most electronegative from the periodic table, which causes it to be more electronegative than P atom. Thus, N and P atoms will share electrons. This result agrees with the literature, which indicates that N-doped phosphorene presents a p-type semiconductor characteristic [31]. Considering the result of N doped black phosphorene, it was investigated the properties of black phosphorene doped N increasing the number of this atom on the phosphorene sheet (BP_nN), where n ranges from one to five. Fig. 6 (a)-(e) shows in the top of the panel the non-relaxed configurations, and the optimized geometries are shown in the bottom of the panel of Fig. 6 (f)-(j). The positions N atoms were chosen by symmetry, based on the most favourable site for one N atom (B site). Electronic band structures and adsorption energies per atom are shown in Fig. 7. Fig. 6 (f)-(j) shows that doped phosphorene with two or more N atoms causes distortion of P–P bonds near doping, resulting in the rupture of these bonds. It is observed that doped phospho-
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rene with three or more than three atoms, as shown in Fig. 6 (h)-(j), presents the N atom binding differently to the P atom in the black phosphorene. Bond length differences may be attributed to the multiplicity of N–P bonds. The shortest N–P bond length is 1.65 Å, when compared to another N atom that distances from P with a single bond of 1.77 Å length, making it stable. Doped black phosphorene with an even number of N atoms causes a semiconductor behaviour with an indirect gap of 0.75 eV for the BP_2N and gap of 0.82 eV the BP_4N, as shown in Fig. 6 (c) and (e), respectively. However, black phosphorene doped with an odd number of N atoms causes a p-type semiconductor behaviour, as observed in the electronic structure of BP_N, BP_3N and BP_5N, as shown in Fig. 6 (b), (d) and (f), respectively. 4. Conclusions In summary, using first-principles DFT calculations the electronic and structural properties of substitutional and decorated Si-, B- and N- doped black phosphorene were investigated. The energy adsorption indicates that B doped phosphorene is the most favourable dopant. Those dopants affect the electronic structures of the host phosphorene as demonstrated by the electronic band structure. Si doped phosphorene decreases gap when compared to the pure one, while the B and N modify phosphorene characteristic of semiconductor with gap to n-type and p-type semiconductors, respectively. Increasing the concentration of doping until the amount of five adsorbed atoms, it was observed a symmetry with an odd number of dopants, and an even number of dopants. Doped phosphorene with an odd number of Si causes a semiconductor behaviour, and with an even number causes a metallic behaviour. Doped phosphorene with an odd number of B presents a n-type semiconductor characteristic, while the doped phosphorene with even number presents gap on the electronic band structure. Doped phosphorene with an odd number of N atoms causes a p-type semiconductor behaviour, and with an even number causes the characteristic of semiconductor with gap. This work demonstrates that the increase the number of dopants on phosphorene sheet can change its electronic properties. Acknowledgements The authors acknowledge the financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 and the Centro Nacional de Processamento de Alto Desempenho (CENAPAD) from São Paulo for the computational space. References [1] S. Iijima, Nature 354 (6348) (1991) 56. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (5696) (2004) 666. [3] H.W. Kroto, Nature 329 (6139) (1987) 529. [4] H. Liu, A.T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tománek, P.D. Ye, ACS Nano 8 (4) (2014) 4033.
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Contents lists available at ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Electronic and structural properties of black phosphorene doped with Si, B and N Luiza Goulart ∗ , Liana da S. Fernandes, Cláudia Lange dos Santos, Jussane Rossato Área de Ciências Tecnológicas, Universidade Franciscana, Santa Maria, Brazil
a r t i c l e
i n f o
Article history: Received 22 July 2019 Received in revised form 27 August 2019 Accepted 3 September 2019 Available online 6 September 2019 Communicated by R. Wu Keywords: Functionalization Phosphorene Semiconductor
a b s t r a c t The electronic and structural properties of substitutional and doped phosphorene with B, N and Si were studied using first principles calculations based on density functional theory. Moreover, electronic and structural properties of functionalized phosphorene slowly increasing the concentration of doping was investigated. Phosphorene strongly binds with doped functionalization; B doped phosphorene is the most stable configuration studied. Si doped phosphorene maintains the semiconductor characteristic. B and N doped phosphorene present n-type and p-type semiconductors, respectively. Doped phosphorene with odd number of Si is a semiconductor material, doped phosphorene with an odd number of B has ntype semiconductor characteristic, and doped phosphorene with odd number of N atoms has a p-type semiconductor behaviour. Doped phosphorene with even number of Si has a metallic characteristic, while B and N doped phosphorene with even number present a semiconductor behaviour. This work reveals that phosphorene electronic properties could be changed by introducing the dopants on the system, and the properties are affected by the increasing number of dopants on phosphorene sheet. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of nanostructures based on carbon atoms, such as fullerene, carbon nanotube, and graphene [1–3], exploration of other elements that could lead into new nanostructures has yielded many nanostructures such as boron nitride, silicene, zironium diboride, black phosphorene and others [4–9]. Among these, black phosphorene, a 2D material with a single layer phosphorus sheet exfoliated from bulk black phosphorus has recently attracted scientific attention due to its properties [10–12]. Black phosphorene is a puckered layer of phosphorus atoms, in which each atom bonds to other 3 in an orthorhombic structure [4, 11]. Its main electronic characteristic is the semiconductor character, with a gap around 1.0 eV [13,14] and its high carrier mobility comparable to graphene (1000 cm2 V−1 s−1 at room temperature) [15,16], due to these properties, it has been extensively applied on the electronic field [17–20]. However, to modify and improve its properties, 2D materials often require chemical functionalization [21–25]. For instance, B and N doped graphene exhibit both p-type and n-type semiconductor characteristics, respectively [26–28]. Moreover, the band structures of phosphorene can be modified by doped effects [29–31].
*
Corresponding author. E-mail address: [email protected] (L. Goulart).
https://doi.org/10.1016/j.physleta.2019.125945 0375-9601/© 2019 Elsevier B.V. All rights reserved.
Doped phosphorene can exhibit both n-type and p-type semiconductor characteristics [30], while doped phosphorus nanostructures such as Si doped blue phosphorene changes the electronic character of this semiconductor nanostructure to a metallic character [29]. Zigzag blue phosphorene nanoribbons passivated with −H and −OH present a wide-gap semiconductor property, while with −O, it presents a metallic characteristic; also, when passivated in both the edges with various functional groups, it presents some important functional characteristics [30]. Decorated phosphorene has better adsorption capability than other 2D materials such as BN, SiC, MoS2 and graphene, surface adsorption functionalized phosphorene improves the system potential applications in nanoelectrics and spintronics [31]. Substitutionally doped phosphorene affects the electronic structures of phosphorene, enhancing chemical activity for capture gas molecules, suggesting the system potential application on molecular adsorption and desorption processes, such as catalysis, gas sensing and storage [32]. While the mentioned studies investigate the decorated doped phosphorene, or the substitutionally doped phosphorene, systematic studies that compared the two types of functionalization are rare. Moreover, despite the significant effect of doping on phosphorene electronic properties, no study has been performed to examine the effect of doping phosphorene with several dopants atoms so far. Thus, the aim of this study is to analyze the electronic and structural properties of black phosphorene decorated and substitu-
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L. Goulart et al. / Physics Letters A 383 (2019) 125945
Fig. 1. Electronic band structure are shown in the bottom panel and the optimized geometries are shown in the top panel for pristine phosphorene (a), Si doped phosphorene (b), B doped phosphorene (c), and N doped phosphorene (d). Fermi energy level is represented in red. (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.)
tionally with Si, B and N atoms. Moreover, it aims to investigate the electronic and structural properties of black phosphorene slowly increasing the concentration of doping until the amount of five adsorbed atoms. The chosen atoms have been extensively used in theoretical studies of carbon materials [33–39]. The results indicate that the doping with Si decreases the gap of phosphorene when compared to the pristine system, while the doping with B and N presents characteristics of an n-type and p-type semiconductor, respectively. Also, doping of the black phosphorene with an odd number of these atoms exhibits different behaviour than those with an even number. The results presented here may be useful for the study of phosphorene application in nano-electrics, catalysis, gas sensing and storage.
Table 1 The adsorption energies (Ead ) for the substitutional and decorated sites with the different dopants, Si, B and N. Dopant
Site
Ead (eV)
Silicon
T B H
−0.32 −2.06 −2.46
Boron
T B H
−2.21 −2.51 −3.62
Nitrogen
T B H
−0.48 −2.62 −2.36
2. Computational methods 3. Results and discussion The density functional theory (DFT) [40] calculations were performed with SIESTA [41] using Double-Zeta base with spin polarization and generalized gradient approximation (GGA) for the exchange and correlation term [42]. We used periodic boundary conditions fs a 9.95 Å × 13.56 Å × 30.00 Å phosphorene supercell (36 P atoms). The k-point was set to (10 × 8 × 1) for the Brillouin zone representation and a kinetic energy cut off of 300 eV was used. The radius cut off was defined with an energy shift of 0.01 eV and all the structures are relaxed until the forces on each atom were less than 0.05 eV Å−1 . The adsorption energy of doped black phosphorene was calculated as
Ead = E(DopedPhosphorene) − E(Phosphorene) − nE(Atom) /n
(1)
where E(DopedPhosphorene) is the total energy of the relaxed phosphorene doped with Si, B or N, E(Phosphorene) , E(Atom) are the total energies of the relaxed pristine phosphorene and the Si, B or N atoms, respectively, and n is the number of dopants atoms on the phosphorene sheet.
Substitutional and decorated doped phosphorene were studied. To find the most favourable configuration, the dopants under investigation were initially placed at three different positions on phosphorene sheet: the top site (T) is when a P atom of the top layer is replaced by a dopant, the hollow site (H) at the middle of buckled hexagon, and the bridge site (B) at the midpoint of P−P bond [24,25]. The adsorption energies found are presented in Table 1. All the atomic species bind to the phosphorene with adsorption energies that range from −0.32 eV to −3.62 eV. Phosphorene strongly binds with doped functionalization. The most favourable adsorption is with Boron by the H site. Electronic and structural properties are presented in the following sections. 3.1. Si doped phosphorene Table 1 shows that the most favourable configuration of Si doped phosphorene (BP_Si) is H site with Ead of −2.46 eV. This result agrees with the results reported by Ding and Wang [31] who
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Fig. 2. Non-optimized geometries are shown in the top panel (a)-(e), and optimized geometries are shown in the bottom of the panel (f)-(j) for Si doped phosphorene with increased concentration of Si atoms.
Fig. 3. Electronic band structure and adsorption energy of pristine black phosphorene (a), BP_Si (b), BP_2Si (c), BP_3Si (d), BP_4Si (e), and BP_5Si (f).
theoretically described that the most stable site for doping phosphorene with Si is the H site, with a similar adsorption energy of −2.52 eV. Fig. 1 (b) shows the geometry of Si doped phosphorene, and the band structure for this configuration. The distance between the Si atom and the nearest P atom is 2.32 Å. Fig. 2 (a) and (b) show the electronic band structure of pristine phosphorene and Si doped phosphorene, respectively. Through a comparison of the electronic band structures, the band gap of Si doped phosphorene is 0.23 eV, while the band gap of pristine phosphorene is 0.80 eV. This indicates that the black phosphorene remains as a semiconductor after doping with Si. It can be explained due to Si and P atom similar electronegativity, around 2. Considering the obtained results after the doping of the phosphorene sheet with a Si atom, it was investigated the properties of phosphorene sheet doped with an increased number of Si atoms (BP_nSi), where n ranges from one to five. Fig. 2 (a)-(e) shows in the top panel the initial geometries chosen by symmetry based on the most favourable site for one atom (H site). The optimized geometries of these configurations are shown in the bottom of Fig. 2 (f)-(j). Doped phosphorene with two or more Si atoms causes dis-
tortions in P–P bonds near the dopants, resulting in the rupture of these bonds, as shown in Fig. 2. The addition of more Si atoms promoted different distortion patterns in the phosphorene structure, as can be seen in Fig. 2 (g)-(j). A similar result was also reported by Ying, who showed that the increase of Si atom density in the medium caused structural changes in the 2D material tested [43]. Fig. 3 shows electronic band structures, and adsorption energies per atom. Doped phosphorene with an even number of Si atoms causes a metallic behaviour, as shown in Fig. 3 (c) and (e). On the other hand, doped phosphorene with an odd number of Si atoms causes a semiconductor behaviour with indirect gap of 0.23 eV for BP_Si, 0.68 eV for BP_3Si, and 0.48 eV for BP_5Si, as observed in the electronic band structure shown in Fig. 3 (b), (d) and (f), respectively. Also, it is clear from Fig. 3 (b), (d) and (f) that the position of the valence band maximum (VBM) is altered. For the doping with one Si atom, the VBM is localized near the X-point while that for the doping with three and five Si atoms, the VBM position shifts to Spoint. On the other hand, the conduction band minimum (CBM) is always in G-point.
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Fig. 4. Non-optimized geometries are shown in the top panel (a)-(e), and optimized geometries are shown in the bottom of the panel (f)-(j) for B doped phosphorene with increased concentration of B atoms.
Fig. 5. Electronic band structure and adsorption energy of: pristine phosphorene (a), BP_B (b), BP_2B (c), BP_3B (d), BP_4B (e), and BP_5B (f).
3.2. B doped phosphorene The most favourable configuration of B doped phosphorene (BP_B) is H site with Ead of −3.62 eV, as presented in Table 1. Similar to the previous case where the most favourable site for Si doped phosphorene is H, this result is in agreement with the results reported by Ding and Wang [31]. The geometry of B doped phosphorene, and the band structure for this configuration are shown in Fig. 1 (c). The shortest B–P distance after optimization is 1.9 Å, which is the typical value of B–P bond distance, due to the electron deficiency of B atom, it bonds with three P atoms forming a trigonal planar structure. From Fig. 1 (a) and (c), it is possible to observe that black phosphorene changes its character after doping from semiconductor with 0.80 eV gap to n-type semiconductor since Fermi level moves the conduction band region upwards. This behaviour is due to the fact that B atom has one half filled orbital and two p empty orbitals, which causes a strong attraction with P atom.
Considering the obtained results after the doping of the black phosphorene sheet with a B atom, it was investigated the properties of the phosphorene doped an increased number of B atoms (BP_nB), ranging from one to five. Similar to the previous case (Sidoped), the positions were chosen by symmetry based on the most favourable site (H site). Fig. 4 (a)-(e) shows in the top the nonoptimized geometry of black phosphorene doped with a different number of B atoms. The optimized geometries of these configurations are shown in the bottom of Fig. 4 (f)-(j). Doped phosphorene with two or more B atoms causes distortions on P–P bonds near the dopants, rupturing these bonds, as shown in Fig. 4. The B atom will make three bonds, but because it has an empty orbital, it will tend to approach the largest number of P atoms to stabilize. Some P–P bonds are broken so that B atoms are inserted into the structure, making it stable. Fig. 5 presents the electronic band structure and adsorption energies per atom of those configurations. Doped phosphorene with even number of B atoms causes a semiconductor behaviour with an indirect gap of 0.41 eV for BP_2B and 0.65 eV for BP_4B, as shown in Fig. 5. In this case, on the
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Fig. 6. Non-optimized geometries are shown in the top panel (a)-(e), and optimized geometries are shown in the bottom of the panel (f)-(j) for N doped phosphorene with the increase of N atoms concentration.
Fig. 7. Electronic structure of bands and adsorption energy of: pristine phosphorene (a), BP_N (b), BP_2N (c), BP_3N (d), BP_4N (e) e BP_5N (f).
contrary to the previous case (Si doped phosphorene), no significant changes are seen on the VBM position which keeps near the X-point. However, the CBM changes from X-point to Y-point when the number of B atoms varies from two to four atoms, as shown in Fig. 5 (c) and (e), respectively. On the other hand, doped phosphorene with odd number of B atoms causes a n-type semiconductor behaviour, as observed in the electronic structure of BP_B, BP_3B and BP_5B, shown in Fig. 5 (b), (d) and (f), respectively. 3.3. N doped phosphorene The most favourable configuration of N doped phosphorene (BP_N) is B site with Ead of −2.62 eV, as presented in Table 1, similar to the results reported by Ding and Wang [24]. The optimized geometry and the electronic band structure for this configuration are shown in Fig. 1 (d). The smallest N–P distance after optimization is 1.65 Å, approximately the typical value of the N–P bond (1.62 Å), the N adsorption ruptures the below P–P bond and forms a P–N–P configuration. Through a comparison of the electronic band structures, it can be
seen that N doped phosphorene is a p-type semiconductor since Fermi level moves the valence band region downwards (Fig. 1 (d)), while the band gap of pristine phosphorene is 0.80 eV (Fig. 1 (a)). N atom is the third element and the most electronegative from the periodic table, which causes it to be more electronegative than P atom. Thus, N and P atoms will share electrons. This result agrees with the literature, which indicates that N-doped phosphorene presents a p-type semiconductor characteristic [31]. Considering the result of N doped black phosphorene, it was investigated the properties of black phosphorene doped N increasing the number of this atom on the phosphorene sheet (BP_nN), where n ranges from one to five. Fig. 6 (a)-(e) shows in the top of the panel the non-relaxed configurations, and the optimized geometries are shown in the bottom of the panel of Fig. 6 (f)-(j). The positions N atoms were chosen by symmetry, based on the most favourable site for one N atom (B site). Electronic band structures and adsorption energies per atom are shown in Fig. 7. Fig. 6 (f)-(j) shows that doped phosphorene with two or more N atoms causes distortion of P–P bonds near doping, resulting in the rupture of these bonds. It is observed that doped phospho-
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rene with three or more than three atoms, as shown in Fig. 6 (h)-(j), presents the N atom binding differently to the P atom in the black phosphorene. Bond length differences may be attributed to the multiplicity of N–P bonds. The shortest N–P bond length is 1.65 Å, when compared to another N atom that distances from P with a single bond of 1.77 Å length, making it stable. Doped black phosphorene with an even number of N atoms causes a semiconductor behaviour with an indirect gap of 0.75 eV for the BP_2N and gap of 0.82 eV the BP_4N, as shown in Fig. 6 (c) and (e), respectively. However, black phosphorene doped with an odd number of N atoms causes a p-type semiconductor behaviour, as observed in the electronic structure of BP_N, BP_3N and BP_5N, as shown in Fig. 6 (b), (d) and (f), respectively. 4. Conclusions In summary, using first-principles DFT calculations the electronic and structural properties of substitutional and decorated Si-, B- and N- doped black phosphorene were investigated. The energy adsorption indicates that B doped phosphorene is the most favourable dopant. Those dopants affect the electronic structures of the host phosphorene as demonstrated by the electronic band structure. Si doped phosphorene decreases gap when compared to the pure one, while the B and N modify phosphorene characteristic of semiconductor with gap to n-type and p-type semiconductors, respectively. Increasing the concentration of doping until the amount of five adsorbed atoms, it was observed a symmetry with an odd number of dopants, and an even number of dopants. Doped phosphorene with an odd number of Si causes a semiconductor behaviour, and with an even number causes a metallic behaviour. Doped phosphorene with an odd number of B presents a n-type semiconductor characteristic, while the doped phosphorene with even number presents gap on the electronic band structure. Doped phosphorene with an odd number of N atoms causes a p-type semiconductor behaviour, and with an even number causes the characteristic of semiconductor with gap. This work demonstrates that the increase the number of dopants on phosphorene sheet can change its electronic properties. Acknowledgements The authors acknowledge the financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 and the Centro Nacional de Processamento de Alto Desempenho (CENAPAD) from São Paulo for the computational space. References [1] S. Iijima, Nature 354 (6348) (1991) 56. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (5696) (2004) 666. [3] H.W. Kroto, Nature 329 (6139) (1987) 529. [4] H. Liu, A.T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tománek, P.D. Ye, ACS Nano 8 (4) (2014) 4033.
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