- Email: [email protected]
Tracing the initial state of surface oxidation in black phosphorus
Journal Pre-proofs Full Length Article Tracing the initial state of surface oxidation in black phosphorus Kyoung Hun Oh, Sung Won Jung, Keun Su Kim PII: DOI: Reference:
S0169-4332(19)33157-5 https://doi.org/10.1016/j.apsusc.2019.144341 APSUSC 144341
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
23 July 2019 3 October 2019 8 October 2019
Please cite this article as: K. Hun Oh, S. Won Jung, K. Su Kim, Tracing the initial state of surface oxidation in black phosphorus, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144341
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Tracing the initial state of surface oxidation in black phosphorus Kyoung Hun Oha,b, Sung Won Junga,c, Keun Su Kima,* a
Department of Physics, Yonsei University, Seoul 03722, Korea
b
c
4-2-2, Agency for Defense Development, Daejeon 34186, Korea
Diamond Light Source, Harwell Campus, Didcot, OX11 0DE, UK
Black phosphorus has emerged as a class of two-dimensional semiconductors, but its degradation caused by surface oxidation upon exposure to ambient conditions has been a serious issue. A key to understanding the mechanism of surface oxidation is the initial-state structure that has remained elusive. We study the initial state of surface oxidation in black phosphorus by lowtemperature core-level photoelectron spectroscopy with the in situ dosage of O2 in the ultrahighvacuum condition. Our high-resolution P 2p core-level spectra show two clearly distinct initialstate components of P atoms that have one and two neighboring O atoms, respectively. It is followed by the rapid growth of other higher binding-energy components originating from incomplete P2O5 bonded to black phosphorus with one or two less bonds to O atoms. The variation in the proportion of these components reveals the initial-state structure of dissociative adsorption and its evolution to the final form of phosphorus oxides. Keywords: Black phosphorus, surface oxidation, photoelectron spectroscopy
*E-mail: [email protected].
1
1. Introduction Black phosphorus (BP) has attracted renewed interest as a class of two-dimensional layered semiconductors [1]. It has been shown that atomically thin (or few-layer) BP exhibits promising device characteristics, such as high carrier mobility and on-off ratio [2–4], and highly anisotropic optical responses [5–7]. The band structure of BP is widely tunable by various parameters, such as thickness [8], strain [9], and electric field [10]. The last has been recently exploited to induce Dirac fermions protected by crystal symmetries [11,12]. BP is thus an interesting system not only for device applications, but also for the fundamental study of gap-closing transitions and topological phases. Even though BP is the most stable allotrope of phosphorus (Fig. 1(a)), its surface is unstable and subject to degradation under ambient conditions [13–20]. This becomes even more serious, when BP is thinned down to the few-layer regime [16]. Carrier mobility in transistors made of atomically thin BP degrades more rapidly, which is accompanied by the formation of bubble-like structures [17,19]. It was proposed that light-induced excitons generate superoxide anions that react spontaneously with BP to form phosphorus oxides (Fig. 1(b)) [14–16]. This process is known to be further expedited in the presence of water molecules that interact preferentially with defects and step edges [17–19]. The initial state of surface oxidation is key to understanding the microscopic mechanism so as to develop a way to protect BP against degradation. In the dissociative adsorption of O2 on the surface of BP, theoretical calculations have predicted two energetically stable adsorption structures as illustrated in Fig. 1(c) and (d). However, despite intensive studies on the degradation of BP, little is known experimentally about the initial-state (or low-density) structure
2
of dissociative adsorption and how it progressively evolves to the final state of phosphorus oxides. An experimental technique that can be employed to trace the structural evolution in the process of surface oxidation is core-level photoelectron spectroscopy. In recent experiments, it was commonly observed that the P 2p core-level spectra of degraded BP show broad peaks at the binding energy in the range of 132136 eV [21–25]. Based on theoretical models, the broad peaks have been attributed to the formation of P2O5 (and/or H3PO4 in the presence of H2O), which is the most stable molecular form of phosphorus oxides (Fig. 1(b)). However, the previous core-level spectroscopy studies were performed with BP exposed to O2 either at atmospheric pressure or at room temperature (RT), where precise control of the surface-oxidation process was rather difficult. We overcome this limitation by employing low-temperature (LT) core-level photoelectron spectroscopy with the in situ dosage of O2 in the ultrahigh-vacuum condition. At the O2 dosage less than 2400 L, which is small enough to see little oxidation at RT, we found a clear onset of surface oxidation at 15 K. This allows us to precisely trace the evolution of P 2p core-level spectra along the process of surface oxidation in BP. At the beginning, there are two clearly distinct components originating from P atoms bonded to one and two O atoms, respectively. It is followed by the rapid growth of other higher binding-energy components that originate from incomplete P2O5 chemically bonded to BP with one or two less bonds to O atoms. The variation in the proportion of these components with the dosage of O2 reveals the structural evolution from the initial state of dissociative adsorption to the final state of phosphorus oxides.
3
2. Experimental Methods X-ray photoelectron spectroscopy experiments were conducted at Beamline 7.0.2 of the Advanced Light Source (ALS) and at Beamline 6U of the Ultraviolet Synchrotron Orbital Radiation Facility (UVSOR). These experimental stations were equipped with hemispherical electron analyzers whose energy resolutions were better than 30 meV. We used monochromatic photons in the range of 160–200 eV at the normal emission, and the samples was kept at RT or cryogenically cooled down to 15 K by means of liquid helium. The single-crystal BP (HQ graphene) was in situ cleaved in the ultrahigh vacuum chamber with the base pressure better than 1.5 × 10–10 torr. The clean surface of BP was exposed to O2 with the purity of 99.999 % and without blocking the natural light coming through view ports in the vacuum chamber. The partial pressure of O2 was kept at 2 × 10–7 torr, and the time of exposure was adjusted to control the dosage in unit of Langmuir. The time of exposure was in the range of a few ten seconds, which is long enough to ignore the effect of time-dynamic processes at a given O2 density.
3. Results and discussion The black line in Fig. 2(a) shows P 2p core-level spectra taken from pristine BP. Consistent with the previous reports [21–27], we found a well-defined 2p doublet at the binding energy of 130.14 eV for 2p3/2 and 131.01 eV for 2p1/2. Their spectral width is as narrow as 0.2 eV, which confirms high energy resolution and sample quality. The same data taken after exposing the samples to O2 at the dosage of 2400 L (about 5 orders of magnitude smaller than exposing to the air for 1 second) at RT is shown by the overlaid red line in Fig. 2(a). We found little difference between the two data, indicating that the surface oxidation at RT is negligible at this dosage.
4
On the other hand, exactly the same measurements at LT shows a dramatic difference. Fig. 2(b) shows P 2p core-level spectra taken for BP exposed to the O2 dosage of 2400 L at 15 K. The binding energy of the main doublet is nearly unchanged, whereas their spectral width is significantly broadened about 3 times greater than that of pristine BP (Fig. 1(a)). More importantly, extra components indicated by arrows in Fig. 2(b) appear at higher binding energies, which is the clear signature of the formation of phosphorus oxides. This indicates that the adsorption of O2 on the surface of BP is more efficient at LT at a given dosage. Thus, our experiments at LT allow us to precisely control the process of surface oxidation and to trace the structural evolution. We employ a least-squares fitting procedure based on the Voigt function, the convolution of Lorentzian and Gaussian functions. The red line overlaid in Fig. 2(b) is a best fit composed of the main doublet denoted as P0 and four extra peaks denoted in the order of the binding-energy shifts as P1–P4. The binding energies of P1–P4 and their relative intensities are similar to the previously reported components of phosphorus oxides for BP exposed to ambient conditions [21,25] or to atmospheric pressure O2 at RT [22–24]. Unlike P1–P4 that are rather broad in energy and closely spaced to each other, the binding energy of P0 can be reliably determined from the raw data. We thus subtracted the P0 peak obtained by the curve fit to see more closely how P1–P4 (related to phosphorus oxides) change at the initial stage of surface oxidation. Fig. 3 shows a series of P 2p core-level spectra taken with increasing the O2 dosage up to 2000 L and plotted as a function of the binding-energy shift with respect to that of P0 2p3/2. In the O2 dosage of 200–400 L, there are two components that correspond to P1 and P2, respectively. As the peaks of P1 and P2 were clearly resolved, their binding-energy shifts could be reliably determined without involving curve fit analysis as EP1 = 1.41 eV and EP2 = 2.33 eV.
5
In the dissociative adsorption of O2, several stable structures of O2 have been suggested based on first-principles calculations: A possible adsorption structure is that the two O atoms are bonded to an outermost P atom in the zigzag chain of BP [20], as shown by blue balls in Fig. 1(c). This structure with two O atoms at the lone-pair site was termed the “LL-2O” model [18]. The binding-energy shift of P atoms that have one neighboring O atom was predicted to be 1.40 eV [22–24], which is in agreement with P1. Another, energetically more stable structure is shown in Fig. 1(d), where an O atom binds to an outermost P atom, and the other O atom is bridging in between two P atoms. This structure with an O atom at the interstitial site and an O atom at the lone-pair site as the “IL-2O” model [18]. The binding-energy shifts of P atoms with one and two neighboring O atoms, represented by blue and green balls in Fig. 1(d), were predicted to be 1.38–1.40 eV and 2.30–2.41 eV [21,22], which are in agreement with P1 and P2, respectively. In the O2 dosage greater than 400 L (Fig. 3), while P1 and P2 keep growing in intensity, we found at least two more components corresponding to P3 and P4, respectively. As the peaks of P3 and P4 were clearly resolved at 1600–2000 L, their binding-energy shifts could be determined reliably without involving any curve fitting analysis as EP3 = 3.0 eV and EP4 = 4.4 eV. In the earlier core-level spectroscopy studies on BP exposed to ambient conditions or O2 at atmospheric pressure at RT, a dominant component of phosphorus oxides was found at the similar bindingenergy shift to P4, and attributed to the formation of molecular P2O5 (Fig. 1(b)) [21,22] (and/or H3PO4 in the presence of H2O [22,28]). However, the binding-energy shifts for P atoms in the molecular form of P4O10 and H3PO4 were 5.56 eV and 5.87 eV, respectively [22], which are more than 1 eV greater than that of P4.
6
An important clue to understand the origin of P3 and P4 can be obtained from the time evolution of P 2p core-level spectra taken from BP exposed to ambient conditions for more than a day. In the first data taken right after exposing samples to the synchrotron radiation (as-exp. in Fig. 4), we found a previously unobserved peak denoted as P5 at the binding-energy shift greater than P4. The asymmetric shape of this spectrum is due to the mixture of P4 and P5. As the time of exposure to the synchrotron radiation goes by, we found that P5 rapidly transfers its weight to P4. Eventually, it becomes similar shape to that for the O2 dosage of over 2400 L (Fig. 2(b)). The disappearance of P5 in an hour indicates that the relevant structure is unstable in exposure to the synchrotron radiation. This may be the reason why P5 has never been observed in the previous core-level spectroscopy experiments [21–25]. The binding-energy shift of P5 was estimated to be 6.1 eV, which is closer to those predicted for P4O10 and H3PO4 [22]. The molecular form of P4O10 and H3PO4 are expected to be weakly adsorbed (or physisorbed) on the surface of BP, which naturally explains the metastability of P5 upon exposure to the synchrotron radiation. Then, the origin for P3 and P4 at the binding-energy shifts smaller than that of P5 and larger than those of P1 and P2 can be understood from P atoms that have three and four neighboring O atoms, respectively. Indeed, the binding-energy shifts for those represented by yellow and red balls in Fig. 1(e) and (f) were predicted to be 3.1 and 4.7–4.9 eV, respectively [22,24], which is in good agreement with our observations for P3 and P4. The structural models for P3 and P4 can be viewed as incomplete P2O5 that has one or two less bonds to O atoms and instead is chemically bonded to BP. The remaining bonds of P3 and P4 to BP accounts for their stability under exposure to the synchrotron radiation.
7
With P0–P5 assigned to their respective chemical species in Fig. 1, we now discuss the structural evolution in the process of surface oxidation in BP. A curve fit to the spectra consisting of multiple peaks as in Fig. 3 may overestimate or underestimate the area of each peak depending on their spectral widths. To avoid such ambiguity, we plot the peak height of P1–P4 from the raw data as a function of the O2 dosage in Fig. 5. In the dissociative adsorption of O2 on the surface of BP, the LL-2O model (Fig. 1(c)) is expected to produce two P1 atoms, but the IL-2O model (Fig. 1(d)) is expected to produce one P1 and one P2 atoms simultaneously. As indicated by the arrow in Fig. 5, our results at the O2 dosage less than 400 L show the simultaneous growth of P1 and P2, which confirms the IL-2O model as the initial-state structure. This can be further supported by recent molecular-dynamics simulations [16], where the O2 molecule adsorbed on BP is dissociated to form the IL-2O structure rather than that in the LL-2O structure. On the other hand, as the rate of increase in the number of P1 and P2 reduces from 800 L, that of P3 and P4 grows more rapidly. As the surface layer of BP is saturated by P1 and P2 atoms, excess O atoms start penetrating into the outer zigzag chain of BP, as shown in Fig. 1(e) and (f), leading to the formation of P3 and P4. This should be accompanied by the formation of extra P1 atoms (blue balls in Fig. 1(e) and (f)) that explains the higher intensity of P1 than P2 in the range of 800–1600 L. At the O2 dosage over 2000 L, the number of P4 increases with even a higher rate than that of P3. This suggests that the surface layer of BP is being saturated by incomplete P2O5, as the complete molecular form of P2O5 is unstable under exposure to the synchrotron radiation. We found little dependence of P1–P5 on the escape depth of photoelectrons (tuned by the photon energy and grazing angle), ensuring that the oxidation of BP starts from the surface and develops in a layer-by-layer manner. Therefore, the progressive structural evolution revealed
8
in this study is likely to be generic to BP regardless of thickness, although the superoxide anions were reported to be more easily formed at the monolayer limit [14,17].
4. Conclusions In summary, we study the process of surface oxidation in BP by using LT core-level photoelectron spectroscopy with the in situ dosage of O2 in the ultrahigh-vacuum condition. Our high-resolution core-level spectra reveal five components of phosphorus oxides, and their structural origins were identified based on binding-energy shifts and by comparison to those taken after exposure to ambient conditions. From these findings, we confirm the initial state of surface oxidation as the IL-2O model (Fig. 1(d)), and disentangle incomplete P2O5 from the complete form of phosphorus oxides.
Acknowledgements This work was supported from the National Research Foundation (NRF) of Korea (Grants No. 2017R1A2B3011368, No. 2017R1A5A1014862, and No. 2018K1A3A7A09027641), and the Future-leading Research Initiative of 2019-22-0079 of Yonsei University. The works at the ALS were supported by the U. S. Department of Energy, Office of Sciences under Contract No. DEAC02-05CH11231. We thank C. Jozwiak, A. Bostwick, E. Rotenberg for help in experiments at the ALS, and J. K. Kim, H. Yamane, and N. Kosugi for help in experiments at the UVSOR.
9
References [1] X. Ling, H. Wang, S. Huang, F. Xia, M. S. Dresselhaus, The Renaissance of Black Phosphorus. Proc. Natl. Acad. Sci. U.S.A. 112 (2015) 4523–4530. [2] L. Li, Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X. H. Chen, Y. Zhang, Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 9 (2014) 372–377. [3] H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tománek, P. D. Ye, Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 8 (2014) 4033–4041. [4] S. P. Koenig, R. A. Doganov, H. Schmidt, A. H. Castro Neto, B. Özyilmaz, Electric Field Effect in Ultrathin Black Phosphorus. Appl. Phys. Lett. 104 (2014) 103106. [5] F. Xia, H. Wang, Y. Jia, Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 5 (2014) 4458. [6] J. Qiao, X. Kong, Z.-X. Hu, F. Yang, W. Ji, High-Mobility Transport Anisotropy and Linear Dichroism in Few-Layer Black Phosphorus. Nat. Commun. 5 (2014) 4475. [7] X. Wang, A. M. Jones, K. L. Seyler, V. Tran, Y. Jia, H. Zhao, H. Wang, L. Yang, X. Xu, F. Xia, Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus. Nat. Nanotechnol. 10 (2015) 517–521. [8] L. Li, J. Kim, C. Jin, G. J. Ye, D. Y. Qiu, F. H. da Jornada, Z. Shi, L. Chen, Z. Zhang, F. Yang, K. Watanabe, T. Taniguchi, W. Ren, S. G. Louie, X. H. Chen, Y. Zhang, F. Wang, Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene. Nat. Nanotechnol. 12 (2017) 21–25.
10
[9] A. S. Rodin, A. Carvalho, A. H. Castro Neto, Strain-Induced Gap Modification in Black Phosphorus. Phys. Rev. Lett. 112 (2014) 176801. [10] Q. Liu, X. Zhang, L. B. Abdalla, A. Fazzio, A. Zunger, Switching a Normal Insulator into a Topological Insulator via Electric Field with Application to Phosphorene. Nano Lett. 15 (2015) 1222–1228. [11] J. Kim, S. S. Baik, S. H. Ryu, Y. Sohn, S. Park, B. -G. Park, J. Denlinger, Y. Yi, H. J. Choi, K. S. Kim, Observation of Tunable Band Gap and Anisotropic Dirac Semimetal State in Black Phosphorus. Science 349 (2015) 723–726. [12] J. Kim, S. S. Baik, S. W. Jung, Y. Sohn, S. H. Ryu, H. J. Choi, B. -J. Yang, K. S. Kim, Two-Dimensional Dirac Fermions Protected by Space-Time Inversion Symmetry in Black Phosphorus. Phys. Rev. Lett. 119 (2017) 226801. [13] J. D. Wood, S. A. Wells, D. Jariwala, K. -S. Chen, E. Cho, V. K. Sangwan, X. Liu, L. J. Lauhon, T. J. Marks, M. C. Hersam, Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation. Nano Lett. 14 (2014) 6964–6970. [14] A. Favron, E. Gaufrès, F. Fossard, A. -L. Phaneuf-L’Heureux, N. Y. -W. Tang, P. L. Lévesque, A. Loiseau, R. Leonelli, S. Francoeur, R. Martel, Photooxidation and Quantum Confinement Effects in Exfoliated Black Phosphorus. Nature Materials 14 (2015) 826–832. [15] A. Ziletti, A. Carvalho, D. K. Campbell, D. F. Coker, A. H. Castro Neto, Oxygen Defects in Phosphorene. Phys. Rev. Lett. 114 (2015) 046801. [16] G. Wang, W. J. Slough, R. Pandey, S. P. Karna, Degradation of Phosphorene in Air: Understanding at Atomic Level. 2D Mater. 3 (2016) 025011.
11
[17] Q. Zhou, Q. Chen, Y. Tong, J. Wang, Light-Induced Ambient Degradation of Few-Layer Black Phosphorus: Mechanism and Protection. Angew. Chem. Int. Ed. 55 (2016) 11437-11441. [18] Y. Huang, J. Qiao, K. He, S. Bliznakov, E. Sutter, X. Chen, D. Luo, F. Meng, D. Su, J. Decker, W. Ji, R. S. Ruoff, P. Sutter, Interaction of Black Phosphorus with Oxygen and Water. Chem. Mater. 28 (2016) 8330-8339. [19] C. Han, Z. Hu, A. Carvalho, N. Guo, J. Zhang, F. Hu, D. Xiang, J. Wu, B. Lei, L. Wang, C. Zhang, A. H. Castro Neto, W. Chen, Oxygen Induced Strong Mobility Modulation in Few-Layer Black Phosphorus. 2D Mater. 4 (2017) 021007. [20] J. Gómez-Pérez, B. Barna, I. Y. Tóth, Z. Kónya, Á. Kukovecz, Quantitative Tracking of the Oxidation of Black Phosphorus in the Few-Layer Regime. ACS Omega 3 (2018) 12482–12488. [21] M. T. Edmonds, A. Tadich, A. Carvalho, A. Ziletti, K. M. O’Donnell, S. P. Koenig, D. F. Coker, B. Özyilmaz, A. H. Castro Neto, M. S. Fuhrer, Creating a Stable Oxide at the Surface of Black Phosphorus. ACS Appl. Mater. Interfaces 7 (2015) 14557-14562. [22] T. Yang, B. Dong, J. Wang, Z. Zhang, J. Guan, K. Kuntz, S. C. Warren, D. Tománek, Interpreting Core-Level Spectra of Oxidizing Phosphorene: Theory and Experiment. Phys. Rev. B 92 (2015) 125412. [23] W. Luo, D. Y. Zemlyanov, C. A. Milligan, Y. Du, L. Yang, Y. Wu, P. D. Ye, Surface Chemistry of Black Phosphorus under a Controlled Oxidative Environment. Nanotechnology 27 (2016) 434002.
12
[24] K. L. Kuntz, R. A. Wells, J. Hu, T. Yang, B. Dong, H. Guo, A. H. Woomer, D. L. Druffel, A. Alabanza, D. Tománek, S. C. Warren, Control of Surface and Edge Oxidation on Phosphorene. ACS Appl. Mater. Interfaces 9 (2017) 9126-9135. [25] M. van Druenen, F. Davitt, T. Collins, C. Glynn, C. O’Dwyer, J. D. Holmes, G. Collins, Evaluating the Surface Chemistry of Black Phosphorus during Ambient Degradation. Langmuir 35 (2019) 2172-2178. [26] Y. Harada, K. Murano, I. Shirotani, T. Takahashi, Y. Maruyama, Electronic Structure of Black Phosphorus Studied by X-Ray Photoelectron Spectroscopy. Solid State Commun. 44 (1982) 877-879. [27] N. B. Goodman, L. Ley, D. W. Bullett, Valence-Band Structures of Phosphorus Allotropes. Phys. Rev. B 27 (1983) 7440-7450. [28] Y. Wang, B. Yang, B. Wan, X. Xi, Z. Zeng, E. Liu, G. Wu, Z. Liu, W. Wang, Degradation of Black Phosphorus: A Real-Time 31P NMR Study. 2D Mater. 3 (2016) 035025.
13
Figure Captions Fig. 1. Ball-and-stick model of (a) single-layer BP and (b) molecular P4O10. Those in (c)–(f) show the evolution of phosphorus oxides with the increasing number of O atoms bonded to BP. Black balls are P atoms, and white balls are O atoms. The blue, green, yellow, and red balls show P atoms that have one, two, three, and four neighboring O atoms, respectively. Fig. 2. (a) P 2p core-level spectroscopy data taken from pristine BP (black line) and after exposure to the O2 dosage of 2400 L at RT (red line). (b) P 2p core-level spectra of BP taken after exposure to the O2 dosage of 2400 L at 15 K (black dots). The arrows indicate components of phosphorus oxides. The overlaid red line is a best fit composed of five components (P0–P4) shown below. Fig. 3. P 2p core-level spectra taken from BP exposed to the O2 dosage indicated at the upper left. Data were recorded with the photon energy of 180 eV at the sample temperature held at 15 K. Each spectra is shown after subtracting the P0 doublet obtained by a best fit to the raw data. The horizontal axis is plotted as a function of binding-energy shifts from the peak position of P0 2p3/2. Fig. 4. P 2p core-level spectra taken from BP exposed to ambient conditions. Data were recorded with the photon energy of 180 eV at the sample temperature of 15 K. A series of data taken at the elapsed time from exposing samples to the synchrotron radiation (marked on the left) is plotted with the offset for clarity. Fig. 5. Peak height of P1–P4 taken from the data in Fig. 3 and plotted as a function of O2 dosage. The arrow indicates the simultaneous growth of P1 and P2 at the initial state of surface oxidation in BP.
14
Fig. 1
15
Fig. 2
16
Fig. 3
17
Fig. 4
18
Fig. 5
19
Highlights y
The initial state of surface oxidation was studied by low-temperature core-level spectros copy with the in situ deposition of oxygen molecules.
y
Our high-resolution spectra reveal the progressive structural evolution from the initial st ate to the final form of phosphorus oxides.
y
We could disentangle the chemical component of phosphorus oxides from incomplete P2 O5 with one or two less bonds to oxygen atoms.
20
S0169-4332(19)33157-5 https://doi.org/10.1016/j.apsusc.2019.144341 APSUSC 144341
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
23 July 2019 3 October 2019 8 October 2019
Please cite this article as: K. Hun Oh, S. Won Jung, K. Su Kim, Tracing the initial state of surface oxidation in black phosphorus, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144341
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Tracing the initial state of surface oxidation in black phosphorus Kyoung Hun Oha,b, Sung Won Junga,c, Keun Su Kima,* a
Department of Physics, Yonsei University, Seoul 03722, Korea
b
c
4-2-2, Agency for Defense Development, Daejeon 34186, Korea
Diamond Light Source, Harwell Campus, Didcot, OX11 0DE, UK
Black phosphorus has emerged as a class of two-dimensional semiconductors, but its degradation caused by surface oxidation upon exposure to ambient conditions has been a serious issue. A key to understanding the mechanism of surface oxidation is the initial-state structure that has remained elusive. We study the initial state of surface oxidation in black phosphorus by lowtemperature core-level photoelectron spectroscopy with the in situ dosage of O2 in the ultrahighvacuum condition. Our high-resolution P 2p core-level spectra show two clearly distinct initialstate components of P atoms that have one and two neighboring O atoms, respectively. It is followed by the rapid growth of other higher binding-energy components originating from incomplete P2O5 bonded to black phosphorus with one or two less bonds to O atoms. The variation in the proportion of these components reveals the initial-state structure of dissociative adsorption and its evolution to the final form of phosphorus oxides. Keywords: Black phosphorus, surface oxidation, photoelectron spectroscopy
*E-mail: [email protected].
1
1. Introduction Black phosphorus (BP) has attracted renewed interest as a class of two-dimensional layered semiconductors [1]. It has been shown that atomically thin (or few-layer) BP exhibits promising device characteristics, such as high carrier mobility and on-off ratio [2–4], and highly anisotropic optical responses [5–7]. The band structure of BP is widely tunable by various parameters, such as thickness [8], strain [9], and electric field [10]. The last has been recently exploited to induce Dirac fermions protected by crystal symmetries [11,12]. BP is thus an interesting system not only for device applications, but also for the fundamental study of gap-closing transitions and topological phases. Even though BP is the most stable allotrope of phosphorus (Fig. 1(a)), its surface is unstable and subject to degradation under ambient conditions [13–20]. This becomes even more serious, when BP is thinned down to the few-layer regime [16]. Carrier mobility in transistors made of atomically thin BP degrades more rapidly, which is accompanied by the formation of bubble-like structures [17,19]. It was proposed that light-induced excitons generate superoxide anions that react spontaneously with BP to form phosphorus oxides (Fig. 1(b)) [14–16]. This process is known to be further expedited in the presence of water molecules that interact preferentially with defects and step edges [17–19]. The initial state of surface oxidation is key to understanding the microscopic mechanism so as to develop a way to protect BP against degradation. In the dissociative adsorption of O2 on the surface of BP, theoretical calculations have predicted two energetically stable adsorption structures as illustrated in Fig. 1(c) and (d). However, despite intensive studies on the degradation of BP, little is known experimentally about the initial-state (or low-density) structure
2
of dissociative adsorption and how it progressively evolves to the final state of phosphorus oxides. An experimental technique that can be employed to trace the structural evolution in the process of surface oxidation is core-level photoelectron spectroscopy. In recent experiments, it was commonly observed that the P 2p core-level spectra of degraded BP show broad peaks at the binding energy in the range of 132136 eV [21–25]. Based on theoretical models, the broad peaks have been attributed to the formation of P2O5 (and/or H3PO4 in the presence of H2O), which is the most stable molecular form of phosphorus oxides (Fig. 1(b)). However, the previous core-level spectroscopy studies were performed with BP exposed to O2 either at atmospheric pressure or at room temperature (RT), where precise control of the surface-oxidation process was rather difficult. We overcome this limitation by employing low-temperature (LT) core-level photoelectron spectroscopy with the in situ dosage of O2 in the ultrahigh-vacuum condition. At the O2 dosage less than 2400 L, which is small enough to see little oxidation at RT, we found a clear onset of surface oxidation at 15 K. This allows us to precisely trace the evolution of P 2p core-level spectra along the process of surface oxidation in BP. At the beginning, there are two clearly distinct components originating from P atoms bonded to one and two O atoms, respectively. It is followed by the rapid growth of other higher binding-energy components that originate from incomplete P2O5 chemically bonded to BP with one or two less bonds to O atoms. The variation in the proportion of these components with the dosage of O2 reveals the structural evolution from the initial state of dissociative adsorption to the final state of phosphorus oxides.
3
2. Experimental Methods X-ray photoelectron spectroscopy experiments were conducted at Beamline 7.0.2 of the Advanced Light Source (ALS) and at Beamline 6U of the Ultraviolet Synchrotron Orbital Radiation Facility (UVSOR). These experimental stations were equipped with hemispherical electron analyzers whose energy resolutions were better than 30 meV. We used monochromatic photons in the range of 160–200 eV at the normal emission, and the samples was kept at RT or cryogenically cooled down to 15 K by means of liquid helium. The single-crystal BP (HQ graphene) was in situ cleaved in the ultrahigh vacuum chamber with the base pressure better than 1.5 × 10–10 torr. The clean surface of BP was exposed to O2 with the purity of 99.999 % and without blocking the natural light coming through view ports in the vacuum chamber. The partial pressure of O2 was kept at 2 × 10–7 torr, and the time of exposure was adjusted to control the dosage in unit of Langmuir. The time of exposure was in the range of a few ten seconds, which is long enough to ignore the effect of time-dynamic processes at a given O2 density.
3. Results and discussion The black line in Fig. 2(a) shows P 2p core-level spectra taken from pristine BP. Consistent with the previous reports [21–27], we found a well-defined 2p doublet at the binding energy of 130.14 eV for 2p3/2 and 131.01 eV for 2p1/2. Their spectral width is as narrow as 0.2 eV, which confirms high energy resolution and sample quality. The same data taken after exposing the samples to O2 at the dosage of 2400 L (about 5 orders of magnitude smaller than exposing to the air for 1 second) at RT is shown by the overlaid red line in Fig. 2(a). We found little difference between the two data, indicating that the surface oxidation at RT is negligible at this dosage.
4
On the other hand, exactly the same measurements at LT shows a dramatic difference. Fig. 2(b) shows P 2p core-level spectra taken for BP exposed to the O2 dosage of 2400 L at 15 K. The binding energy of the main doublet is nearly unchanged, whereas their spectral width is significantly broadened about 3 times greater than that of pristine BP (Fig. 1(a)). More importantly, extra components indicated by arrows in Fig. 2(b) appear at higher binding energies, which is the clear signature of the formation of phosphorus oxides. This indicates that the adsorption of O2 on the surface of BP is more efficient at LT at a given dosage. Thus, our experiments at LT allow us to precisely control the process of surface oxidation and to trace the structural evolution. We employ a least-squares fitting procedure based on the Voigt function, the convolution of Lorentzian and Gaussian functions. The red line overlaid in Fig. 2(b) is a best fit composed of the main doublet denoted as P0 and four extra peaks denoted in the order of the binding-energy shifts as P1–P4. The binding energies of P1–P4 and their relative intensities are similar to the previously reported components of phosphorus oxides for BP exposed to ambient conditions [21,25] or to atmospheric pressure O2 at RT [22–24]. Unlike P1–P4 that are rather broad in energy and closely spaced to each other, the binding energy of P0 can be reliably determined from the raw data. We thus subtracted the P0 peak obtained by the curve fit to see more closely how P1–P4 (related to phosphorus oxides) change at the initial stage of surface oxidation. Fig. 3 shows a series of P 2p core-level spectra taken with increasing the O2 dosage up to 2000 L and plotted as a function of the binding-energy shift with respect to that of P0 2p3/2. In the O2 dosage of 200–400 L, there are two components that correspond to P1 and P2, respectively. As the peaks of P1 and P2 were clearly resolved, their binding-energy shifts could be reliably determined without involving curve fit analysis as EP1 = 1.41 eV and EP2 = 2.33 eV.
5
In the dissociative adsorption of O2, several stable structures of O2 have been suggested based on first-principles calculations: A possible adsorption structure is that the two O atoms are bonded to an outermost P atom in the zigzag chain of BP [20], as shown by blue balls in Fig. 1(c). This structure with two O atoms at the lone-pair site was termed the “LL-2O” model [18]. The binding-energy shift of P atoms that have one neighboring O atom was predicted to be 1.40 eV [22–24], which is in agreement with P1. Another, energetically more stable structure is shown in Fig. 1(d), where an O atom binds to an outermost P atom, and the other O atom is bridging in between two P atoms. This structure with an O atom at the interstitial site and an O atom at the lone-pair site as the “IL-2O” model [18]. The binding-energy shifts of P atoms with one and two neighboring O atoms, represented by blue and green balls in Fig. 1(d), were predicted to be 1.38–1.40 eV and 2.30–2.41 eV [21,22], which are in agreement with P1 and P2, respectively. In the O2 dosage greater than 400 L (Fig. 3), while P1 and P2 keep growing in intensity, we found at least two more components corresponding to P3 and P4, respectively. As the peaks of P3 and P4 were clearly resolved at 1600–2000 L, their binding-energy shifts could be determined reliably without involving any curve fitting analysis as EP3 = 3.0 eV and EP4 = 4.4 eV. In the earlier core-level spectroscopy studies on BP exposed to ambient conditions or O2 at atmospheric pressure at RT, a dominant component of phosphorus oxides was found at the similar bindingenergy shift to P4, and attributed to the formation of molecular P2O5 (Fig. 1(b)) [21,22] (and/or H3PO4 in the presence of H2O [22,28]). However, the binding-energy shifts for P atoms in the molecular form of P4O10 and H3PO4 were 5.56 eV and 5.87 eV, respectively [22], which are more than 1 eV greater than that of P4.
6
An important clue to understand the origin of P3 and P4 can be obtained from the time evolution of P 2p core-level spectra taken from BP exposed to ambient conditions for more than a day. In the first data taken right after exposing samples to the synchrotron radiation (as-exp. in Fig. 4), we found a previously unobserved peak denoted as P5 at the binding-energy shift greater than P4. The asymmetric shape of this spectrum is due to the mixture of P4 and P5. As the time of exposure to the synchrotron radiation goes by, we found that P5 rapidly transfers its weight to P4. Eventually, it becomes similar shape to that for the O2 dosage of over 2400 L (Fig. 2(b)). The disappearance of P5 in an hour indicates that the relevant structure is unstable in exposure to the synchrotron radiation. This may be the reason why P5 has never been observed in the previous core-level spectroscopy experiments [21–25]. The binding-energy shift of P5 was estimated to be 6.1 eV, which is closer to those predicted for P4O10 and H3PO4 [22]. The molecular form of P4O10 and H3PO4 are expected to be weakly adsorbed (or physisorbed) on the surface of BP, which naturally explains the metastability of P5 upon exposure to the synchrotron radiation. Then, the origin for P3 and P4 at the binding-energy shifts smaller than that of P5 and larger than those of P1 and P2 can be understood from P atoms that have three and four neighboring O atoms, respectively. Indeed, the binding-energy shifts for those represented by yellow and red balls in Fig. 1(e) and (f) were predicted to be 3.1 and 4.7–4.9 eV, respectively [22,24], which is in good agreement with our observations for P3 and P4. The structural models for P3 and P4 can be viewed as incomplete P2O5 that has one or two less bonds to O atoms and instead is chemically bonded to BP. The remaining bonds of P3 and P4 to BP accounts for their stability under exposure to the synchrotron radiation.
7
With P0–P5 assigned to their respective chemical species in Fig. 1, we now discuss the structural evolution in the process of surface oxidation in BP. A curve fit to the spectra consisting of multiple peaks as in Fig. 3 may overestimate or underestimate the area of each peak depending on their spectral widths. To avoid such ambiguity, we plot the peak height of P1–P4 from the raw data as a function of the O2 dosage in Fig. 5. In the dissociative adsorption of O2 on the surface of BP, the LL-2O model (Fig. 1(c)) is expected to produce two P1 atoms, but the IL-2O model (Fig. 1(d)) is expected to produce one P1 and one P2 atoms simultaneously. As indicated by the arrow in Fig. 5, our results at the O2 dosage less than 400 L show the simultaneous growth of P1 and P2, which confirms the IL-2O model as the initial-state structure. This can be further supported by recent molecular-dynamics simulations [16], where the O2 molecule adsorbed on BP is dissociated to form the IL-2O structure rather than that in the LL-2O structure. On the other hand, as the rate of increase in the number of P1 and P2 reduces from 800 L, that of P3 and P4 grows more rapidly. As the surface layer of BP is saturated by P1 and P2 atoms, excess O atoms start penetrating into the outer zigzag chain of BP, as shown in Fig. 1(e) and (f), leading to the formation of P3 and P4. This should be accompanied by the formation of extra P1 atoms (blue balls in Fig. 1(e) and (f)) that explains the higher intensity of P1 than P2 in the range of 800–1600 L. At the O2 dosage over 2000 L, the number of P4 increases with even a higher rate than that of P3. This suggests that the surface layer of BP is being saturated by incomplete P2O5, as the complete molecular form of P2O5 is unstable under exposure to the synchrotron radiation. We found little dependence of P1–P5 on the escape depth of photoelectrons (tuned by the photon energy and grazing angle), ensuring that the oxidation of BP starts from the surface and develops in a layer-by-layer manner. Therefore, the progressive structural evolution revealed
8
in this study is likely to be generic to BP regardless of thickness, although the superoxide anions were reported to be more easily formed at the monolayer limit [14,17].
4. Conclusions In summary, we study the process of surface oxidation in BP by using LT core-level photoelectron spectroscopy with the in situ dosage of O2 in the ultrahigh-vacuum condition. Our high-resolution core-level spectra reveal five components of phosphorus oxides, and their structural origins were identified based on binding-energy shifts and by comparison to those taken after exposure to ambient conditions. From these findings, we confirm the initial state of surface oxidation as the IL-2O model (Fig. 1(d)), and disentangle incomplete P2O5 from the complete form of phosphorus oxides.
Acknowledgements This work was supported from the National Research Foundation (NRF) of Korea (Grants No. 2017R1A2B3011368, No. 2017R1A5A1014862, and No. 2018K1A3A7A09027641), and the Future-leading Research Initiative of 2019-22-0079 of Yonsei University. The works at the ALS were supported by the U. S. Department of Energy, Office of Sciences under Contract No. DEAC02-05CH11231. We thank C. Jozwiak, A. Bostwick, E. Rotenberg for help in experiments at the ALS, and J. K. Kim, H. Yamane, and N. Kosugi for help in experiments at the UVSOR.
9
References [1] X. Ling, H. Wang, S. Huang, F. Xia, M. S. Dresselhaus, The Renaissance of Black Phosphorus. Proc. Natl. Acad. Sci. U.S.A. 112 (2015) 4523–4530. [2] L. Li, Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X. H. Chen, Y. Zhang, Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 9 (2014) 372–377. [3] H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tománek, P. D. Ye, Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 8 (2014) 4033–4041. [4] S. P. Koenig, R. A. Doganov, H. Schmidt, A. H. Castro Neto, B. Özyilmaz, Electric Field Effect in Ultrathin Black Phosphorus. Appl. Phys. Lett. 104 (2014) 103106. [5] F. Xia, H. Wang, Y. Jia, Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 5 (2014) 4458. [6] J. Qiao, X. Kong, Z.-X. Hu, F. Yang, W. Ji, High-Mobility Transport Anisotropy and Linear Dichroism in Few-Layer Black Phosphorus. Nat. Commun. 5 (2014) 4475. [7] X. Wang, A. M. Jones, K. L. Seyler, V. Tran, Y. Jia, H. Zhao, H. Wang, L. Yang, X. Xu, F. Xia, Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus. Nat. Nanotechnol. 10 (2015) 517–521. [8] L. Li, J. Kim, C. Jin, G. J. Ye, D. Y. Qiu, F. H. da Jornada, Z. Shi, L. Chen, Z. Zhang, F. Yang, K. Watanabe, T. Taniguchi, W. Ren, S. G. Louie, X. H. Chen, Y. Zhang, F. Wang, Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene. Nat. Nanotechnol. 12 (2017) 21–25.
10
[9] A. S. Rodin, A. Carvalho, A. H. Castro Neto, Strain-Induced Gap Modification in Black Phosphorus. Phys. Rev. Lett. 112 (2014) 176801. [10] Q. Liu, X. Zhang, L. B. Abdalla, A. Fazzio, A. Zunger, Switching a Normal Insulator into a Topological Insulator via Electric Field with Application to Phosphorene. Nano Lett. 15 (2015) 1222–1228. [11] J. Kim, S. S. Baik, S. H. Ryu, Y. Sohn, S. Park, B. -G. Park, J. Denlinger, Y. Yi, H. J. Choi, K. S. Kim, Observation of Tunable Band Gap and Anisotropic Dirac Semimetal State in Black Phosphorus. Science 349 (2015) 723–726. [12] J. Kim, S. S. Baik, S. W. Jung, Y. Sohn, S. H. Ryu, H. J. Choi, B. -J. Yang, K. S. Kim, Two-Dimensional Dirac Fermions Protected by Space-Time Inversion Symmetry in Black Phosphorus. Phys. Rev. Lett. 119 (2017) 226801. [13] J. D. Wood, S. A. Wells, D. Jariwala, K. -S. Chen, E. Cho, V. K. Sangwan, X. Liu, L. J. Lauhon, T. J. Marks, M. C. Hersam, Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation. Nano Lett. 14 (2014) 6964–6970. [14] A. Favron, E. Gaufrès, F. Fossard, A. -L. Phaneuf-L’Heureux, N. Y. -W. Tang, P. L. Lévesque, A. Loiseau, R. Leonelli, S. Francoeur, R. Martel, Photooxidation and Quantum Confinement Effects in Exfoliated Black Phosphorus. Nature Materials 14 (2015) 826–832. [15] A. Ziletti, A. Carvalho, D. K. Campbell, D. F. Coker, A. H. Castro Neto, Oxygen Defects in Phosphorene. Phys. Rev. Lett. 114 (2015) 046801. [16] G. Wang, W. J. Slough, R. Pandey, S. P. Karna, Degradation of Phosphorene in Air: Understanding at Atomic Level. 2D Mater. 3 (2016) 025011.
11
[17] Q. Zhou, Q. Chen, Y. Tong, J. Wang, Light-Induced Ambient Degradation of Few-Layer Black Phosphorus: Mechanism and Protection. Angew. Chem. Int. Ed. 55 (2016) 11437-11441. [18] Y. Huang, J. Qiao, K. He, S. Bliznakov, E. Sutter, X. Chen, D. Luo, F. Meng, D. Su, J. Decker, W. Ji, R. S. Ruoff, P. Sutter, Interaction of Black Phosphorus with Oxygen and Water. Chem. Mater. 28 (2016) 8330-8339. [19] C. Han, Z. Hu, A. Carvalho, N. Guo, J. Zhang, F. Hu, D. Xiang, J. Wu, B. Lei, L. Wang, C. Zhang, A. H. Castro Neto, W. Chen, Oxygen Induced Strong Mobility Modulation in Few-Layer Black Phosphorus. 2D Mater. 4 (2017) 021007. [20] J. Gómez-Pérez, B. Barna, I. Y. Tóth, Z. Kónya, Á. Kukovecz, Quantitative Tracking of the Oxidation of Black Phosphorus in the Few-Layer Regime. ACS Omega 3 (2018) 12482–12488. [21] M. T. Edmonds, A. Tadich, A. Carvalho, A. Ziletti, K. M. O’Donnell, S. P. Koenig, D. F. Coker, B. Özyilmaz, A. H. Castro Neto, M. S. Fuhrer, Creating a Stable Oxide at the Surface of Black Phosphorus. ACS Appl. Mater. Interfaces 7 (2015) 14557-14562. [22] T. Yang, B. Dong, J. Wang, Z. Zhang, J. Guan, K. Kuntz, S. C. Warren, D. Tománek, Interpreting Core-Level Spectra of Oxidizing Phosphorene: Theory and Experiment. Phys. Rev. B 92 (2015) 125412. [23] W. Luo, D. Y. Zemlyanov, C. A. Milligan, Y. Du, L. Yang, Y. Wu, P. D. Ye, Surface Chemistry of Black Phosphorus under a Controlled Oxidative Environment. Nanotechnology 27 (2016) 434002.
12
[24] K. L. Kuntz, R. A. Wells, J. Hu, T. Yang, B. Dong, H. Guo, A. H. Woomer, D. L. Druffel, A. Alabanza, D. Tománek, S. C. Warren, Control of Surface and Edge Oxidation on Phosphorene. ACS Appl. Mater. Interfaces 9 (2017) 9126-9135. [25] M. van Druenen, F. Davitt, T. Collins, C. Glynn, C. O’Dwyer, J. D. Holmes, G. Collins, Evaluating the Surface Chemistry of Black Phosphorus during Ambient Degradation. Langmuir 35 (2019) 2172-2178. [26] Y. Harada, K. Murano, I. Shirotani, T. Takahashi, Y. Maruyama, Electronic Structure of Black Phosphorus Studied by X-Ray Photoelectron Spectroscopy. Solid State Commun. 44 (1982) 877-879. [27] N. B. Goodman, L. Ley, D. W. Bullett, Valence-Band Structures of Phosphorus Allotropes. Phys. Rev. B 27 (1983) 7440-7450. [28] Y. Wang, B. Yang, B. Wan, X. Xi, Z. Zeng, E. Liu, G. Wu, Z. Liu, W. Wang, Degradation of Black Phosphorus: A Real-Time 31P NMR Study. 2D Mater. 3 (2016) 035025.
13
Figure Captions Fig. 1. Ball-and-stick model of (a) single-layer BP and (b) molecular P4O10. Those in (c)–(f) show the evolution of phosphorus oxides with the increasing number of O atoms bonded to BP. Black balls are P atoms, and white balls are O atoms. The blue, green, yellow, and red balls show P atoms that have one, two, three, and four neighboring O atoms, respectively. Fig. 2. (a) P 2p core-level spectroscopy data taken from pristine BP (black line) and after exposure to the O2 dosage of 2400 L at RT (red line). (b) P 2p core-level spectra of BP taken after exposure to the O2 dosage of 2400 L at 15 K (black dots). The arrows indicate components of phosphorus oxides. The overlaid red line is a best fit composed of five components (P0–P4) shown below. Fig. 3. P 2p core-level spectra taken from BP exposed to the O2 dosage indicated at the upper left. Data were recorded with the photon energy of 180 eV at the sample temperature held at 15 K. Each spectra is shown after subtracting the P0 doublet obtained by a best fit to the raw data. The horizontal axis is plotted as a function of binding-energy shifts from the peak position of P0 2p3/2. Fig. 4. P 2p core-level spectra taken from BP exposed to ambient conditions. Data were recorded with the photon energy of 180 eV at the sample temperature of 15 K. A series of data taken at the elapsed time from exposing samples to the synchrotron radiation (marked on the left) is plotted with the offset for clarity. Fig. 5. Peak height of P1–P4 taken from the data in Fig. 3 and plotted as a function of O2 dosage. The arrow indicates the simultaneous growth of P1 and P2 at the initial state of surface oxidation in BP.
14
Fig. 1
15
Fig. 2
16
Fig. 3
17
Fig. 4
18
Fig. 5
19
Highlights y
The initial state of surface oxidation was studied by low-temperature core-level spectros copy with the in situ deposition of oxygen molecules.
y
Our high-resolution spectra reveal the progressive structural evolution from the initial st ate to the final form of phosphorus oxides.
y
We could disentangle the chemical component of phosphorus oxides from incomplete P2 O5 with one or two less bonds to oxygen atoms.
20