Accepted Article Title: Doping phosphorene by holes and electrons through molecular charge transfer Authors: Pratap Vishnoi, S. Rajesh, S. Manjunatha, Arkamita Bandyopadhyay, Manaswee Barua, Swapan K. Pati, and C.N.R. Rao This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: ChemPhysChem 10.1002/cphc.201700789 Link to VoR: http://dx.doi.org/10.1002/cphc.201700789
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COMMUNICATION Doping phosphorene by holes and electrons through molecular charge transfer Pratap Vishnoi, S. Rajesh, S. Manjunatha, Arkamita Bandyopadhyay, Manaswee Barua, Swapan K. Pati, and C. N. R. Rao* Abstract: An important aspect of phosphorene, the novel twodimensional semiconductor, is whether holes and electrons can both be doped in this material. Some reports found that only electrons can be preferentially doped into phosphorene. There are some theoretical calculations showing charge-transfer interaction with both tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE). We have carried out an investigation of chemical doping of phosphorene by a variety of electron donor and acceptor molecules, employing both experiment and theory, Raman scattering being a crucial aspect of the study. We find that both electron acceptors and donors interact with phosphorene by charge-transfer, with the acceptors having more marked effects. All the three Raman bands of phosphorene soften and exhibit band broadening on interaction with both donor and acceptor molecules. First-principles calculations establish the occurrence of charge-transfer between phosphorene with donors as well as acceptors. The absence of electron-hole asymmetry is noteworthy.
Phosphorene is a novel two-dimensional material that has come into the fore since the advent of graphene. Phosphorene, unlike graphene, has a direct band-gap.[1-3] A sizable band gap (~ 1.5 eV for monolayer phosphorene) and high charge-carrier mobility (~1,000 cm2 V−1 s−1) have attracted increasing attention in the recent past.[4] Phosphorene has a puckered two-dimensional structure wherein each phosphorus atom is covalently linked to three neighbouring atoms. One of the properties of phosphorene that has aroused much interest is the possibility of chargetransfer doping. Based on density functional theory (DFT) calculations, Zhang et al.[5] found that phosphorene transfers electrons to the electron with-drawing tetracyano-pquinodimethane (TCNQ), and that the electron-donating tetrathiafulvalene (TTF) creates only deep donor states due to weak charge-transfer. Interaction of a TCNQ derivative is reported to enhance the p-type conductance of phosphorene due to charge transfer.[6] Non-covalent functionalization of black phosphorous occurs with a TCNQ derivative but not with a perylene diimide.[7] DFT calculations of Yu et al.[8], predict that the n-type donor benzyl viologen injects electrons to phosphorene while TCNQ injects holes. Jing et al.[9] based on DFT calculations, report that TCNE withdraws 0.40 electrons
Dr. P. Vishnoi, S. Rajesh, S. Manjunatha, A. Bandyopadhyay, M. Barua, Prof. S. K. Pati and Prof. C. N. R. Rao New Chemistry Unit, Theoretical Sciences Unit International Centre for Materials Science and Sheikh Saqr Laboratory Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P. O., Bangalore-560064 (India) E-mail: [email protected] Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
from phosphorene while TTF donates 0.15 electrons. These results differ from the report of Chakraborty et al.[10] who examined Raman scattering of phosphorene in an electrochemically top-gated field effect transistor. These workers find that the Raman Ag phonons are more affected by electrons than holes. Furthermore, the Bg band was insensitive to electrochemical doping. In view of the somewhat uncertain situation with regard to the electron-withdrawing or donating property of phosphorene, we have examined its interaction with TCNE and TTF by Raman spectroscopy. It is to be noted that Raman scattering has been immensely successful in unraveling the nature of interaction of donor and acceptor molecules with graphene and carbon nanotubes.[11] Equally importantly, we have carried out a detailed spectroscopic investigation of the interaction of phosphorene with a few aromatic donor and acceptor molecules shown below. Furthermore, we have carried out first-principles calculations to throw light on the nature of interaction of phosphorene with the donor and acceptor molecules. The present study clearly establishes that both electrons and holes can be doped into phosphorene, with holedoping causing greater changes in the spectra of phosphorene. The nature of charge-transfer interaction between phosphorene and donor/acceptor molecules has been unraveled by experiments as well as calculations. N S
S
S
S
COOK
KOOC
COOK
KOOC
S
S
C N
N
N A1
D1
S
C
TCNE
KOOC
S
C
N
TTF
KOOC
N C
COOK COOK
D2
HOOC HN N
O
O
N
N
N NH COOH
O
O A2
Phosphorene, prepared by sonication of black phosphorus in N-methyl-2-pyrrolidone (NMP) or dimethylformamide (DMF) was characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and Raman spectroscopy (Scheme 1). The AFM images measured on Si/SiO2 substrate indicate that the exfoliated sample contains micron size sheets of ca. 0.85-2.70 nm thickness (Figure S1, Supporting Information). Interaction of phosphorene with TTF and TCNE which are classic donor and acceptor molecules respectively has been carefully examined by Raman spectroscopy. The spectra clearly show softening of the A , A and B Raman bands on interaction with both TTF and TCNE (Figure 1). It is
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Figure 1. A , B and A Raman bands of phosphorene on interaction with varying concentrations of (a) TTF (1; 0.0, 2; 0.4, 3; 0.8 and 4; 1.6 mM) and (b) TCNE (1; 0.0, 2; 0.4, 3; 0.8 and 4; 1.2 mM). Vertical dashed lines (red) indicate the peak position and numbers (blue) indicate the corresponding Raman shifts. Scheme 1. (a) Schematic presentation of exfoliation of black phosphorus (BP) to phosphorene sheets by using sonication, (b) ultra-high resolution-TEM micrograph of phosphorene sheet (inset: fast Fourier transform) and (c) Raman spectrum of phosphorene (inset: three optical phonon modes of phosphorene).
noteworthy that all the three Raman bands including B are affected (Figure 2). The magnitude of the shift, however, remains almost same. In addition to band softening, the Raman bands also show broadening on interaction with TTF as well as TCNE, just as in the case of graphene.[11,12] The broadening, however, is much more with TCNE than TTF. TCNE also shows greater softening of the B and A bands and marginal softening of the A band. Unlike with TCNE, softening of all the three bands is comparable in the case of TTF. These observations clearly show the absence of electron-hole asymmetry, although interaction with the acceptor TCNE has a greater effect on the Raman spectra. It should be noted that the changes in the Raman spectra caused by the interaction with donor and acceptor molecules arise mainly due to phononelectron coupling just as in the case of graphene.[12] Theoretical studies have shown that broadening arises from phononelectron coupling.[10] The N(1s) core level XPS spectrum of TCNE doped phosphorene is shown in Figure S5. Deconvolution of the spectrum shows three distinct species of nitrogen atoms at binding energies of 398.9, 400.2 and 401.7 eV. The peak at 400.2 eV corresponds to the neutral N atoms of pristine TCNE molecules. The low-binding-energy peak at 398.9 eV, due to negatively charged TCNE molecules, clearly indicates that the TCNE molecules acts as electron acceptor in this case. These binding energies are in good agreement with the reported peak positions for TCNE doped graphene.[13] The small signal observed at 401.7 eV is due to the oxidation of nitrogen atoms of TCNE. Our study of the interaction of phosphorene with aromatic donors and acceptors has yielded interesting results. In Figure 3, we show the effect of the donor D2 and the acceptor A2 on the Raman spectrum of phosphorene. These molecules also cause softening of the Raman bands along with the broadening of the bands (Figure 4). In fact the frequency shifts of all the three bands due to the donor D2 are comparable, and also small as compared to the shifts caused by the acceptor A2. However the effects of acceptor are significant causing much larger frequency shift as well as broadening.
Figure 2. Shifts of the phonon frequencies () and the full width at half maxima (FWHM) of the A , B and A bands of phosphorene as the function of concentration of (a-b) TCNE and (c-d) TTF. Increase in concentration of TCNE and TTF corresponds to increase in hole and electron doping respectively.
Figure 3. A , B and A Raman bands of phosphorene on interaction with varying concentrations of (a) electron donor D2 (1; 0.0, 2; 0.5, 3; 0.8 and 4; 1.0 mM) and (b) electron acceptor A2 (1; 0.0, 2; 0.5 and 3; 1.0 µM). Vertical dashed lines (red) indicate the peak position and numbers (blue) indicate the corresponding Raman shifts.
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COMMUNICATION We have investigated the UV-visible spectra of aromatic donors and acceptors in the presence of phosphorene. In Figure 5a, we show the absorption spectra of D2 as a function of concentration of phosphorene. Figure 5b shows the absorption spectra of A2 as a function of the concentration of phosphorene. The changes in the band intensities with increasing concentration of phosphorene confirm the interaction of the donor and acceptor molecules with phosphorene. Making use of absorption band intensities, we could calculate the association constants (Ka) of D1, D2, A1 and A2 with phosphorene. The association constants of donors D1 and D2 are 7.2 x 102 and 5.6 x 102 M-1 respectively while those of the acceptors A1 and A2 are 6.5 x 103 and 1.9 x 103 M-1 respectively. Clearly, acceptors interact much strongly than the donors, in accordance with the conclusions from Raman studies. Our first-principles calculations show that TCNE and TTF molecules lie flat on the phosphorene at distances of 3.32 and 3.15 Å respectively. Charge-transfer in the case of TCNE is 0.346e while in the case of TTF it is 0.187e. Thus, TCNE as an acceptor interacts more strongly through charge transfer. Interestingly, the distance of the central C=C bond in TCNE deceases from 1.43 Å to 1.40 Å because of its electron accepting character. The C=C bond of the donor TTF, on the other hand, increases from 1.35 Å to 1.37 Å. In the case of aromatic donors, the acceptor A2 shows large charge-transfer (0.17e) compared to the donor D2 (0.04e). The distance between A2 and phosphorene is 3.33 Å and that between D2 and phosphorene is 3.32 Å. Charge difference maps reveal the nature of charge transfer between phosphorene and the donors as well acceptor molecules. In Figure 6, we show the charge density difference maps depicting the interaction between phosphorene and (a) TCNE (b) TTF (c) A2 and (d) D2. Here the green lobes represent charge accumulation and the purple lobes depict charge depletion. In the density of states (DOS) (Figure 7), D2 shows a donor level just below the Fermi energy, while for A2, a molecular acceptor level arises. This confirms the occurrence of charge transfer interaction. Acceptors take electrons from the filled pz orbitals of phosphorus. These electrons are delocalized in nature, whereas donor molecules give electrons to the narrow
Figure 4. Shifts of the phonon frequencies () and FWHM of A , B and A Raman bands of phosphorene as the function of concentration of (a-b) A2 and (c-d) D2. Increase in concentrations of A2 and D2 corresponds to increase in hole and electron doping respectively. Similar variations were found with D1 and A1 as well.
Figure 5. (a) Absorption spectra of (a) D2 (30 µM) and (b) A2 (9.0 µM) with increasing concentration of phosphorene exfoliated in NMP.
Figure 6. Charge density difference maps of phosphorene with (a) TCNE, (b) TTF, (c) A2 and (d) D2.
Figure 7. Projected density of states (pDOS) plots for A2 and D2 on phosphorene.
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COMMUNICATION vacant 3d-bands of phosphorene. Here, the electrons are donated to the localized orbitals, and because of this, the donor molecule-phosphorene interaction becomes unfavourable compared to that of the acceptors. In conclusion, we have been able to establish that both holes and electrons can be doped into phosphorene by molecular charge-transfer. Although electron acceptor or hole doping causes a marginally greater effect on the spectral characteristics and magnitude of association, there is no gross electron-hole asymmetry as reported by some of the studies.
Experimental Section Materials. Black phosphorus crystals of 99.998% purity were purchased (Smart Elements, UK) and exfoliated by using a probe sonicator (Sonics Vibra cell™ VCX 750Ultrasonic processor). Raman spectroscopic studies were conducted by using a LabRAM HR high-resolution Raman spectrometer (Horiba-Jobin Yvon) using a Ar laser (λ 514.5 nm). Transmission Electron Microscope (TEM) images were collected with a FEI Titan3TM with an accelerating voltage of 80 kV. Atomic Force Microscope (AFM) images were collected with a Bruker Innova Microscope. X-ray photoelectron spectra (XPS) were collected with an Omicron Nanotechnology spectrometer by using Al-K (1486.6 eV) X-ray radiation. Absorption spectra were recorded with a Perkin–Elmer UV/VIS/NIR lambda-750 spectrometers. Preparation of phosphorene. Phosphorene was prepared by solvent exfoliation of black phosphorus (BP) in NMP/DMF under a nitrogen atmosphere. Anhydrous NMP/DMF was purged with UHP grade N2 gas for 2 h to remove dissolved oxygen. BP crystals were stored in an MBraun N2 glove box with O2 and H2O levels of < 1.0 ppm. The BP crystals were ground and transformed to thin flakes. A 50 mL centrifuge vessel, containing small magnetic stir bar, was charged with 20 mg of BP flakes. The vessel was tightly closed with the cap. Prior to closing, a tapered microtip (6 mm diameter and 142 mm length) of sonicator was inserted in the center of the cap through a suitable sized hole created in it. Interface between the cap and the tip was sealed with silicone grease. 20 mL of deoxygenated NMP/DMF was added to the vessel. To avoid exposure of BP to ambient, Teflon tapes were wrapped around the interfaces of vessel, cap and sonicator tip. The vessel was immersed in an ice water bath and connected to sonicator operating at 20% amplitude and 20 kHz, 4 sec on / 4 sec off pulses for 4 h. The BP-NMP/DMF mixture was magnetically stirred at 300 rpm. After sonication, the BP dispersion was centrifuged at 2000 rpm for 20 minutes. The supernatants were collected and further centrifuged at 5000 rpm for 1 h. The light yellow supernatants were stored under nitrogen environment in a Schlenk flask covered with aluminium foil. The as-prepared phosphorene sample was characterized by Raman spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Samples for Raman studies were prepared by sonication of phosphorene and donor/acceptor compounds. Typically, an aliquot amount of the solution of respective donor/acceptor compound was mixed with 50 µL of as-prepared phosphorene dispersion and bath sonicated for 2 h. After sonication, the mixture was drop casted on glass slide and dried at 150 ᴼC in an oven. The samples were vacuum dried in case of TCNE and TTF. Raman data for every sample were collected at ~ 20 mW.
Angstrom away from each other. The systems are optimized until the forces acting on per atom are less than 0.02 eV/Angstrom. We have considered the sampling of the Brillouin zone using a Monkhorst Pack grid of 5 X 5 X 1, and for the electronic property calculations, we have considered a 21 X 21 X 1 grid. For the Raman mode of molecules, we have used Gaussian 09 software package.[17]
Acknowledgements P.V. thanks the DST Nano Mission for a postdoctoral fellowship. Authors thank Prof. Subi J. George and Prof. T. Govindaraju for providing aromatic donor and acceptor compounds. Keywords: phosphorene . 2D materials . chemical doping . donor-acceptor interaction [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15] [16] [17]
H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tománek, P. D. Ye, ACS Nano, 2014, 8, 4033. F. Xia, H. Wang, Y. Jia, Nature Commun. 2014, 5, 4458. L. Liang, J. Wang, W. Lin, B. G. Sumpter, V. Meunier, M. Pan, Nano Lett., 2014, 14, 6400. Y. Cai, Q. Ke, G. Zhang, Y. P. Feng, V. B. Shenoy, Y.-W. Zhang, Adv. Funct. Mater. 2015, 25, 2230. R. Zhang, B. Li, J. Yang, J. Phys. Chem. C, 2015, 119, 2871. Y. He, F. Xia, Z. Shao, J. Zhao, J. Jie, J. Phys. Chem. Lett., 2015, 6, 4701. G. Abellán, V. Lloret, U. Mundloch, M. Marcia, C. Neiss, A. Görling, M. Varela, F. Hauke, A. Hirsch, Angew. Chem., Int. Ed., 2016, 55, 14557. G. Yu, Y.-W. Zhang, B. I. Yakobson, Nano Energy, 2016, 23, 34. J. Yu, T. Qing, H. Peng, Z. Zhen, S. Panwen, Nanotechnology, 2015, 26, 095201. C. Biswanath, G. Satyendra Nath, S. Anjali, K. Manabendra, K. Chandan, D. V. S. Muthu, D. Anindya, U. V. Waghmare, A. K. Sood, 2D Materials, 2016, 3, 015008. C. N. R. Rao, R. Voggu, Materials Today, 2010, 13, 34. D. J. Late, A. Ghosh, B. Chakraborty, A. K. Sood, U. V. Waghmare, C. N. R. Rao, J. Exp. Nanosci. 2011, 6, 641. D. Choudhury, B. Das, D. D. Sarma, C. N. R. Rao, Chem. Phys. Lett. 2010, 497, 66. J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758. P. E. Blöchl, Phys. Rev. B 1994, 50, 17953. K. E. Riley, M. Pitoňák, et al., Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford CT, 2010.
Computational details. Density Functional Theory based calculations were performed using Vienna Ab initio Simulation Package (VASP). To describe the exchange and correlation of electrons Perdew-BurkeErnzerhof (PBE) functional within the generalized gradient approximation (GGA) has been considered.[14] Projected augmented-wave (PAW) potential has been used in all the calculations.[15,16] A plane wave basis set with a sufficient energy cutoff of 500 eV has been used to represent valence electrons. To avoid the spurious interactions in the nonperiodic direction, we created a vacuum of 30 Angstrom along the nonperiodic direction and our supercell consideration keeps the molecules 10
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COMMUNICATION Interaction of phosphorene with electron acceptor tetracyanoethylene (TCNE) and donor tetrathiafulvalene (TTF) causes charge depletion and accumulation on the phosphorene sheet through charge-transfer (see figure). This leads to softening as well broadening of all the three Raman bands of phosphorene. The study has been extended to several aromatic donors and acceptors.
Pratap Vishnoi, S. Rajesh, S. Manjunatha, Arkamita Bandyopadhyay, Manaswee Barua, Swapan K. Pati, and C. N. R. Rao* Page No. – Page No. Doping phosphorene by holes and electrons through molecular charge transfer
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COMMUNICATION Doping phosphorene by holes and electrons through molecular charge transfer Pratap Vishnoi, S. Rajesh, S. Manjunatha, Arkamita Bandyopadhyay, Manaswee Barua, Swapan K. Pati, and C. N. R. Rao* Abstract: An important aspect of phosphorene, the novel twodimensional semiconductor, is whether holes and electrons can both be doped in this material. Some reports found that only electrons can be preferentially doped into phosphorene. There are some theoretical calculations showing charge-transfer interaction with both tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE). We have carried out an investigation of chemical doping of phosphorene by a variety of electron donor and acceptor molecules, employing both experiment and theory, Raman scattering being a crucial aspect of the study. We find that both electron acceptors and donors interact with phosphorene by charge-transfer, with the acceptors having more marked effects. All the three Raman bands of phosphorene soften and exhibit band broadening on interaction with both donor and acceptor molecules. First-principles calculations establish the occurrence of charge-transfer between phosphorene with donors as well as acceptors. The absence of electron-hole asymmetry is noteworthy.
Phosphorene is a novel two-dimensional material that has come into the fore since the advent of graphene. Phosphorene, unlike graphene, has a direct band-gap.[1-3] A sizable band gap (~ 1.5 eV for monolayer phosphorene) and high charge-carrier mobility (~1,000 cm2 V−1 s−1) have attracted increasing attention in the recent past.[4] Phosphorene has a puckered two-dimensional structure wherein each phosphorus atom is covalently linked to three neighbouring atoms. One of the properties of phosphorene that has aroused much interest is the possibility of chargetransfer doping. Based on density functional theory (DFT) calculations, Zhang et al.[5] found that phosphorene transfers electrons to the electron with-drawing tetracyano-pquinodimethane (TCNQ), and that the electron-donating tetrathiafulvalene (TTF) creates only deep donor states due to weak charge-transfer. Interaction of a TCNQ derivative is reported to enhance the p-type conductance of phosphorene due to charge transfer.[6] Non-covalent functionalization of black phosphorous occurs with a TCNQ derivative but not with a perylene diimide.[7] DFT calculations of Yu et al.[8], predict that the n-type donor benzyl viologen injects electrons to phosphorene while TCNQ injects holes. Jing et al.[9] based on DFT calculations, report that TCNE withdraws 0.40 electrons
Dr. P. Vishnoi, S. Rajesh, S. Manjunatha, A. Bandyopadhyay, M. Barua, Prof. S. K. Pati and Prof. C. N. R. Rao New Chemistry Unit, Theoretical Sciences Unit International Centre for Materials Science and Sheikh Saqr Laboratory Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P. O., Bangalore-560064 (India) E-mail: [email protected] Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
from phosphorene while TTF donates 0.15 electrons. These results differ from the report of Chakraborty et al.[10] who examined Raman scattering of phosphorene in an electrochemically top-gated field effect transistor. These workers find that the Raman Ag phonons are more affected by electrons than holes. Furthermore, the Bg band was insensitive to electrochemical doping. In view of the somewhat uncertain situation with regard to the electron-withdrawing or donating property of phosphorene, we have examined its interaction with TCNE and TTF by Raman spectroscopy. It is to be noted that Raman scattering has been immensely successful in unraveling the nature of interaction of donor and acceptor molecules with graphene and carbon nanotubes.[11] Equally importantly, we have carried out a detailed spectroscopic investigation of the interaction of phosphorene with a few aromatic donor and acceptor molecules shown below. Furthermore, we have carried out first-principles calculations to throw light on the nature of interaction of phosphorene with the donor and acceptor molecules. The present study clearly establishes that both electrons and holes can be doped into phosphorene, with holedoping causing greater changes in the spectra of phosphorene. The nature of charge-transfer interaction between phosphorene and donor/acceptor molecules has been unraveled by experiments as well as calculations. N S
S
S
S
COOK
KOOC
COOK
KOOC
S
S
C N
N
N A1
D1
S
C
TCNE
KOOC
S
C
N
TTF
KOOC
N C
COOK COOK
D2
HOOC HN N
O
O
N
N
N NH COOH
O
O A2
Phosphorene, prepared by sonication of black phosphorus in N-methyl-2-pyrrolidone (NMP) or dimethylformamide (DMF) was characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and Raman spectroscopy (Scheme 1). The AFM images measured on Si/SiO2 substrate indicate that the exfoliated sample contains micron size sheets of ca. 0.85-2.70 nm thickness (Figure S1, Supporting Information). Interaction of phosphorene with TTF and TCNE which are classic donor and acceptor molecules respectively has been carefully examined by Raman spectroscopy. The spectra clearly show softening of the A , A and B Raman bands on interaction with both TTF and TCNE (Figure 1). It is
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Figure 1. A , B and A Raman bands of phosphorene on interaction with varying concentrations of (a) TTF (1; 0.0, 2; 0.4, 3; 0.8 and 4; 1.6 mM) and (b) TCNE (1; 0.0, 2; 0.4, 3; 0.8 and 4; 1.2 mM). Vertical dashed lines (red) indicate the peak position and numbers (blue) indicate the corresponding Raman shifts. Scheme 1. (a) Schematic presentation of exfoliation of black phosphorus (BP) to phosphorene sheets by using sonication, (b) ultra-high resolution-TEM micrograph of phosphorene sheet (inset: fast Fourier transform) and (c) Raman spectrum of phosphorene (inset: three optical phonon modes of phosphorene).
noteworthy that all the three Raman bands including B are affected (Figure 2). The magnitude of the shift, however, remains almost same. In addition to band softening, the Raman bands also show broadening on interaction with TTF as well as TCNE, just as in the case of graphene.[11,12] The broadening, however, is much more with TCNE than TTF. TCNE also shows greater softening of the B and A bands and marginal softening of the A band. Unlike with TCNE, softening of all the three bands is comparable in the case of TTF. These observations clearly show the absence of electron-hole asymmetry, although interaction with the acceptor TCNE has a greater effect on the Raman spectra. It should be noted that the changes in the Raman spectra caused by the interaction with donor and acceptor molecules arise mainly due to phononelectron coupling just as in the case of graphene.[12] Theoretical studies have shown that broadening arises from phononelectron coupling.[10] The N(1s) core level XPS spectrum of TCNE doped phosphorene is shown in Figure S5. Deconvolution of the spectrum shows three distinct species of nitrogen atoms at binding energies of 398.9, 400.2 and 401.7 eV. The peak at 400.2 eV corresponds to the neutral N atoms of pristine TCNE molecules. The low-binding-energy peak at 398.9 eV, due to negatively charged TCNE molecules, clearly indicates that the TCNE molecules acts as electron acceptor in this case. These binding energies are in good agreement with the reported peak positions for TCNE doped graphene.[13] The small signal observed at 401.7 eV is due to the oxidation of nitrogen atoms of TCNE. Our study of the interaction of phosphorene with aromatic donors and acceptors has yielded interesting results. In Figure 3, we show the effect of the donor D2 and the acceptor A2 on the Raman spectrum of phosphorene. These molecules also cause softening of the Raman bands along with the broadening of the bands (Figure 4). In fact the frequency shifts of all the three bands due to the donor D2 are comparable, and also small as compared to the shifts caused by the acceptor A2. However the effects of acceptor are significant causing much larger frequency shift as well as broadening.
Figure 2. Shifts of the phonon frequencies () and the full width at half maxima (FWHM) of the A , B and A bands of phosphorene as the function of concentration of (a-b) TCNE and (c-d) TTF. Increase in concentration of TCNE and TTF corresponds to increase in hole and electron doping respectively.
Figure 3. A , B and A Raman bands of phosphorene on interaction with varying concentrations of (a) electron donor D2 (1; 0.0, 2; 0.5, 3; 0.8 and 4; 1.0 mM) and (b) electron acceptor A2 (1; 0.0, 2; 0.5 and 3; 1.0 µM). Vertical dashed lines (red) indicate the peak position and numbers (blue) indicate the corresponding Raman shifts.
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COMMUNICATION We have investigated the UV-visible spectra of aromatic donors and acceptors in the presence of phosphorene. In Figure 5a, we show the absorption spectra of D2 as a function of concentration of phosphorene. Figure 5b shows the absorption spectra of A2 as a function of the concentration of phosphorene. The changes in the band intensities with increasing concentration of phosphorene confirm the interaction of the donor and acceptor molecules with phosphorene. Making use of absorption band intensities, we could calculate the association constants (Ka) of D1, D2, A1 and A2 with phosphorene. The association constants of donors D1 and D2 are 7.2 x 102 and 5.6 x 102 M-1 respectively while those of the acceptors A1 and A2 are 6.5 x 103 and 1.9 x 103 M-1 respectively. Clearly, acceptors interact much strongly than the donors, in accordance with the conclusions from Raman studies. Our first-principles calculations show that TCNE and TTF molecules lie flat on the phosphorene at distances of 3.32 and 3.15 Å respectively. Charge-transfer in the case of TCNE is 0.346e while in the case of TTF it is 0.187e. Thus, TCNE as an acceptor interacts more strongly through charge transfer. Interestingly, the distance of the central C=C bond in TCNE deceases from 1.43 Å to 1.40 Å because of its electron accepting character. The C=C bond of the donor TTF, on the other hand, increases from 1.35 Å to 1.37 Å. In the case of aromatic donors, the acceptor A2 shows large charge-transfer (0.17e) compared to the donor D2 (0.04e). The distance between A2 and phosphorene is 3.33 Å and that between D2 and phosphorene is 3.32 Å. Charge difference maps reveal the nature of charge transfer between phosphorene and the donors as well acceptor molecules. In Figure 6, we show the charge density difference maps depicting the interaction between phosphorene and (a) TCNE (b) TTF (c) A2 and (d) D2. Here the green lobes represent charge accumulation and the purple lobes depict charge depletion. In the density of states (DOS) (Figure 7), D2 shows a donor level just below the Fermi energy, while for A2, a molecular acceptor level arises. This confirms the occurrence of charge transfer interaction. Acceptors take electrons from the filled pz orbitals of phosphorus. These electrons are delocalized in nature, whereas donor molecules give electrons to the narrow
Figure 4. Shifts of the phonon frequencies () and FWHM of A , B and A Raman bands of phosphorene as the function of concentration of (a-b) A2 and (c-d) D2. Increase in concentrations of A2 and D2 corresponds to increase in hole and electron doping respectively. Similar variations were found with D1 and A1 as well.
Figure 5. (a) Absorption spectra of (a) D2 (30 µM) and (b) A2 (9.0 µM) with increasing concentration of phosphorene exfoliated in NMP.
Figure 6. Charge density difference maps of phosphorene with (a) TCNE, (b) TTF, (c) A2 and (d) D2.
Figure 7. Projected density of states (pDOS) plots for A2 and D2 on phosphorene.
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10.1002/cphc.201700789
ChemPhysChem
COMMUNICATION vacant 3d-bands of phosphorene. Here, the electrons are donated to the localized orbitals, and because of this, the donor molecule-phosphorene interaction becomes unfavourable compared to that of the acceptors. In conclusion, we have been able to establish that both holes and electrons can be doped into phosphorene by molecular charge-transfer. Although electron acceptor or hole doping causes a marginally greater effect on the spectral characteristics and magnitude of association, there is no gross electron-hole asymmetry as reported by some of the studies.
Experimental Section Materials. Black phosphorus crystals of 99.998% purity were purchased (Smart Elements, UK) and exfoliated by using a probe sonicator (Sonics Vibra cell™ VCX 750Ultrasonic processor). Raman spectroscopic studies were conducted by using a LabRAM HR high-resolution Raman spectrometer (Horiba-Jobin Yvon) using a Ar laser (λ 514.5 nm). Transmission Electron Microscope (TEM) images were collected with a FEI Titan3TM with an accelerating voltage of 80 kV. Atomic Force Microscope (AFM) images were collected with a Bruker Innova Microscope. X-ray photoelectron spectra (XPS) were collected with an Omicron Nanotechnology spectrometer by using Al-K (1486.6 eV) X-ray radiation. Absorption spectra were recorded with a Perkin–Elmer UV/VIS/NIR lambda-750 spectrometers. Preparation of phosphorene. Phosphorene was prepared by solvent exfoliation of black phosphorus (BP) in NMP/DMF under a nitrogen atmosphere. Anhydrous NMP/DMF was purged with UHP grade N2 gas for 2 h to remove dissolved oxygen. BP crystals were stored in an MBraun N2 glove box with O2 and H2O levels of < 1.0 ppm. The BP crystals were ground and transformed to thin flakes. A 50 mL centrifuge vessel, containing small magnetic stir bar, was charged with 20 mg of BP flakes. The vessel was tightly closed with the cap. Prior to closing, a tapered microtip (6 mm diameter and 142 mm length) of sonicator was inserted in the center of the cap through a suitable sized hole created in it. Interface between the cap and the tip was sealed with silicone grease. 20 mL of deoxygenated NMP/DMF was added to the vessel. To avoid exposure of BP to ambient, Teflon tapes were wrapped around the interfaces of vessel, cap and sonicator tip. The vessel was immersed in an ice water bath and connected to sonicator operating at 20% amplitude and 20 kHz, 4 sec on / 4 sec off pulses for 4 h. The BP-NMP/DMF mixture was magnetically stirred at 300 rpm. After sonication, the BP dispersion was centrifuged at 2000 rpm for 20 minutes. The supernatants were collected and further centrifuged at 5000 rpm for 1 h. The light yellow supernatants were stored under nitrogen environment in a Schlenk flask covered with aluminium foil. The as-prepared phosphorene sample was characterized by Raman spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM). Samples for Raman studies were prepared by sonication of phosphorene and donor/acceptor compounds. Typically, an aliquot amount of the solution of respective donor/acceptor compound was mixed with 50 µL of as-prepared phosphorene dispersion and bath sonicated for 2 h. After sonication, the mixture was drop casted on glass slide and dried at 150 ᴼC in an oven. The samples were vacuum dried in case of TCNE and TTF. Raman data for every sample were collected at ~ 20 mW.
Angstrom away from each other. The systems are optimized until the forces acting on per atom are less than 0.02 eV/Angstrom. We have considered the sampling of the Brillouin zone using a Monkhorst Pack grid of 5 X 5 X 1, and for the electronic property calculations, we have considered a 21 X 21 X 1 grid. For the Raman mode of molecules, we have used Gaussian 09 software package.[17]
Acknowledgements P.V. thanks the DST Nano Mission for a postdoctoral fellowship. Authors thank Prof. Subi J. George and Prof. T. Govindaraju for providing aromatic donor and acceptor compounds. Keywords: phosphorene . 2D materials . chemical doping . donor-acceptor interaction [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15] [16] [17]
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Computational details. Density Functional Theory based calculations were performed using Vienna Ab initio Simulation Package (VASP). To describe the exchange and correlation of electrons Perdew-BurkeErnzerhof (PBE) functional within the generalized gradient approximation (GGA) has been considered.[14] Projected augmented-wave (PAW) potential has been used in all the calculations.[15,16] A plane wave basis set with a sufficient energy cutoff of 500 eV has been used to represent valence electrons. To avoid the spurious interactions in the nonperiodic direction, we created a vacuum of 30 Angstrom along the nonperiodic direction and our supercell consideration keeps the molecules 10
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COMMUNICATION Interaction of phosphorene with electron acceptor tetracyanoethylene (TCNE) and donor tetrathiafulvalene (TTF) causes charge depletion and accumulation on the phosphorene sheet through charge-transfer (see figure). This leads to softening as well broadening of all the three Raman bands of phosphorene. The study has been extended to several aromatic donors and acceptors.
Pratap Vishnoi, S. Rajesh, S. Manjunatha, Arkamita Bandyopadhyay, Manaswee Barua, Swapan K. Pati, and C. N. R. Rao* Page No. – Page No. Doping phosphorene by holes and electrons through molecular charge transfer
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