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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Energy Level Evolution and Oxygen Exposure of Fullerene/Black Phosphorus Interface Can Wang, Dongmei Niu, Shitan Wang, Yuan Zhao, Wenjun Tan, Lin Li, Han Huang, Haipeng Xie, Yunlai Deng, and Yongli Gao J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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The Journal of Physical Chemistry Letters

Energy Level Evolution and Oxygen Exposure of Fullerene/Black Phosphorus Interface Can Wang,†,‡ Dongmei Niu,∗,† Shitan Wang,† Yuan Zhao,† Wenjun Tan,† Lin Li,†,¶ Han Huang,† Haipeng Xie,† Yunlai Deng,‡,§ and Yongli Gao∗,†,k †Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410012, China ‡Light Alloy Research Institute, Central South University, Changsha 410083, China ¶School of Electronics and Information Engineering, Central South University of Forestry and Technology, Changsha, Hunan 410004, China §School of Materials Science and Engineering, Central South University, Changsha 410083, China kDepartment of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States E-mail: [email protected]; [email protected]

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Abstract The hetero-epitaxial growth of fullerene (C60 ) on single crystal black phosphorus (BP) has been studied using low-energy electron diffraction, X-ray and ultraviolet photoelectron spectroscopy, and density functional theory simulation. The occupied orbital features from C60 observed in the photoelectron spectra for C60 /BP interface are slightly broadened at higher coverages of C60 , and exhibit no direct evidence of hybridization, demonstrating that the C60 /BP interaction is physisorption. Oxygen exposure of interface leads to obvious oxidation of BP in which C60 bridges the large electron transfer barrier from BP to oxygen, and plays an important role for the production of O− 2 and oxidation of BP. Our findings suggest that C60 does not form an ideal protection layer as the other n-type semiconductors. With the assistance of density functional theory calculations, the oxidized phosphorus at the interface prevents further charge transfer from BP to C60 .

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Black phosphorus (BP) has received broad interests for its promising mechanical, electric, optical, and thermoelectric properties. 1–4 Ultrathin two-dimensional (2D) BP, also known as phosphorene, exhibits high carrier mobility and nice on/off ratios for field effect transistors (FETs) with a finite direct band gap. 5,6 The band gap of BP displays a strong thicknessdependent that shifts from 0.35 eV to 1.73 eV by lowering the number of layers in BP. 7 Moreover, BP presents a prodigious in-plane anisotropy, 8–10 which has been applied in photodetectors and thermoelectric devices. 11–13 Additionally, based on the fundamental properties of BP, the suitability of BP in organic-inorganic heterjunction photovoltaics devices have been explored. 14–17 Surface functionalization and molecular doping have been employed to manipulate the electronic structures of layered 2D materials, including graphene 18,19 and MoS2 . 20,21 In a previous literature, Xiang and coworkers utilized metal oxides (Cs2 CO3 and MoO3 ) to dope BP-based FETs. 22 Besides, in order to control and protect BP, a variety of organic species can be introduced to BP surface. 23–27 For example, organic molecular interlayer (F4TCNQ) on BP can improve the performance of BP-based FETs 24 and self-assembled monolayers (SAMs) can enhance the stability of BP. 27 Carbon doped BP-based FETs can obtain a condiderable mobility (1995 cm2 /V·s) via stable phosphorus-carbon (P-C) bonds at the interface. 28 The stable P-C bonds play a critical role in the electrochemical performance of BP-graphite composite ion batteries. 29 Recently, Jie at al. have utilized a phosphorenegraphene sandwich structure as anode materials to achieved impressive capacity in sodiumion batteries. 30 In addition, hybrid heterojunctions using BP/PCBM (phenyl-C61-butyric acid methyl ester) solar cells have been shown to be promising for photovoltaic conversion. 17 BP-carbon composites have a great potential for high-energy storage devices. Fullerene (C60 ), a typical carbonic organic semiconductor, has been studied as versatile building blocks in the field of organic optoelectronic and spintronic devices in past years. 31,32 For example, moulding C60 molecules on MoS2 surface forms hybrid organic-inorganic heterojunctions that exhibits a remarkable high-performance in memory device. 33 Moreover, using C60 /graphene hetero-

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structures, the fabricated graphene-based transistors combining n-type C60 achieve an high on/off ratio (3×103 ). 34 Notably, Shi et al. have made donor-acceptor blends incorporated of BP and C60 , which were implanted into a poly(methyl-methacrylate) (PMMA) matrix constructing BP/C60 /PMMA film with good optical quality. 35 Despite all the promising applications, the knowledge about the electronic properties of the BP-carbon composites is still lacking. The organic/inorganic interface, and particularly its energy-level alignment (ELA), plays an important role as they critically determine the device functionality and efficiency. In this letter, we performed detailed investigations on the interfacial properties and electronic structure between C60 and BP. We used ultraviolet photoemission spectroscopy (UPS) to get the energy-level ELA at the C60 /BP interface and X-ray photoemission spectroscopy (XPS) to determine the interaction of C60 molecules on BP. Combining atomic force microscopy (AFM), we also characterized the growth mode of C60 on BP surface. In particular, we observed that oxygen exposure of interface can lead to obvious oxidation of BP in which C60 bridges the large electron transfer barrier from BP to oxygen. Our results demonstrated that the ELA at C60 /BP interface can be tuned with oxygen exposure. The photoelectron spectra measurements (XPS and UPS) were performed with a hemispherical analyzer (Phoibos 150, SPECS ) in an ultrahigh vacuum (UHV) system. A detailed description of the set-up can be found in our previous literatures. 36–38 A -5.0 V bias was applied to the sample in order to obtain the low energy secondary cutoff in UPS measurements. AFM images were captured ex situ with Agilent 5500 SPM. All spectra were recorded in normal emission with the sample at room temperature. BP (HQ Graphene) was cleaved in load-lock chamber (see Supporting Information Figure S1), the base pressure was better than 6 × 10−8 mbar. C60 (Sigma-Aldrich) molecules were deposited on the BP substrate in organic preparation chamber with a base pressure 4.0×10−8 mbar. The deposition rate was 0.1 nm/min. The deposition rate was monitored by a quartz crystal microbalance (QCM). Oxygen exposure experiments were performed in the

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preparation chamber by controlling the inject O2 gas of 99.9999% purity through a variable leak valve. One Langmuir (L) is equal to the 10−6 Torr sec. exposure. The calculations were performed by DFT as implemented in Vienna Ab initio Simulation Package (VASP) code, 39 with the projector-augmented wave (PAW) method. 40 Exchangecorrelation interactions were described by the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional. 41 The wave functions were expanded into plane waves with an energy cut-off of 500 eV. A Monkhorst-Pack 10×10×4 k-point grid was used to sample the first Brillouin zone (BZ) for the bulk BP. A 6×6×1 k-point sampling mesh was adopted to the supercell (3×3×1) of C60 /BP and O2 -treated C60 /BP systems. The convergence criteria for electron and ionic minimization are 10−5 eV and 0.05 eV/Å, respectively. In order to account for the van der Waals (vdW) interactions, we used the the optB86b vdW functional. 42 Prior to depositing C60 on BP, the surface of the vacuum-cleaved BP is characterized with XPS, UPS and LEED. As shown in Figure S2, Supporting Information, the P 2p peak can be deconvoluted into two components with Gaussian-Lorentzian function (2p3/2 and 2p1/2 at 129.94 and 130.79 eV, respectively), originated from the spin-orbital coupling, which is consistent with the previous literature. 43 There is no O 1s and C 1s components in XPS survey scanning spectrum (see Supporting Information Figure S3). The secondary electron cutoff (SECO) region and the valence band (VB) region measured with He I UPS are shown in Figure S2, Supporting Information. From the leading edge of the cutoff region, the WF of BP is calculated to be ΦBP = 4.0 eV. The onset of the valence band maximum (VBM) is located at 0.15 eV below the Fermi level, as shown in the right part of the VB region. Taking the 0.35 eV band gap of the bulk BP into consideration, the freshly cleaved BP is p-doping semiconductor, which is in consistent with the reports of other group. 7 Next, we prepared the organic-inorganic heterostructure by depositing different thickness C60 thin film on the clean BP. Figure 1 shows the evolution of the UPS spectra at the SECO region and VB region during the deposition. The inset of Figure 1a shows the clear LEED

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Figure 1: (a) The change of SECO region during the deposition of C60 on cleaved BP. The inset of (a): the LEED pattern at 36 eV electron energy. (b) The change of VB or HOMO region during the deposition of C60 on BP. Inset: Zoom-in VB region at 0.2 nm C60 . (c) The evolutions of WF, IP, and HOMO leading edge with different C60 film thickness.

pattern of the cleaved BP in UHV recorded with electron beam energy of 36 eV. Similar LEED patterns have been observed in previous report, 44 which is indicative of the high quality of the BP surface. With 0.2 nm C60 deposition C60 on BP, the WF increased from 4.0 eV (clean BP) to 4.15 eV, indicating an upward shift of vacuum level (VL) by 0.15 eV (see Figure 1a). The WF increased by 0.34 eV when the thickness is 0.4 nm. A total upward shift of the WF by 0.52 eV was observed with the deposition of 3.2 nm C60 thin film. In general, the substantial WF increase can be attributed to charge transfer (CT) from inorganic substrate (BP) to organic thin film (C60 ). This can be understood, as the WF of C60 (5.7 eV) is considerably higher than the ionization energy of the BP substrate (4.05 eV), and, therefore, occupied molecular states are initially located below the Fermi

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level. Consequently, electrons are transferred from BP to C60 for establishing new electronic equilibrium. In the VB region (Figure 1b), two new peaks at binding energies of 2.2 eV and 3.9 eV appear with 0.2 nm C60 , which are assigned to the electrons emission from the highest occupied molecular orbital (HOMO) and HOMO-1 of C60 . At 0.2 nm C60 coverage, the VB region is amplified to reveal the evolution of VBM in BP, as can be seen in the inset of Figure 1b. Clearly, the VBM peak feature of BP is visible at 0.4 eV below the Fermi level when 0.2 nm C60 absorbed on BP. However, the VBM of BP was very sensitive to the C60 molecules and upon further deposition of 0.4 nm C60 thin film it has become indiscernible. As shown in Figure 1b, the C60 HOMO edge is located at 1.7 eV below the Fermi level. With thickness increasing to 3.2 nm, the HOMO region of C60 shows no apparent shape and position change except the increasing intensity. This indicates the HOMO level was pinned at the C60 /BP interface. This pinning may be attribute to the CT from the BP substrate to the C60 molecule. The mechanisms of the HOMO pinning at C60 /BP interface is in analogy with the electron-injecting across organic polymer/metal interface, 45,46 due to the BP substrate WF is smaller than the negative polaron/bi-polaron formation energy in C60 thin film. Figure 1c summarize the shifts of WF, ionization potential (IP), and HOMO leading edge as a function of thin film thickness. Upon the C60 deposition, an upward IP shift (0.48 eV) can be observed. Further insight in the ELA at C60 /BP interface is obtained from XPS measurements. Figure 2 shows the XPS spectra of P 2p and C 1s core levels as the thickness of C60 increases. For a clear view and comparison, we normalized all the spectra. Upon deposition of increasing amounts of C60 , the P 2p core-level of BP shifts upward to smaller binding energy (Figure 2a) by a total of 0.16 eV. This rigid shift indicates a C60 -induced band bending in the BP surface. Besides, the intensity of the P 2p core level decrease with the increase of the thickness of C60 (see Supporting Information Figure S3). Shown in Figure 2b is the evolution of C 1s peak. The C 1s core-level shifts to lower binding energy by a total of 0.14 eV. At 3.2 nm

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Figure 2: (a) P 2p core level as a function of C60 thickness. Note that the peak position of P 2p moves toward the lower binding energy, evidencing an bend bending at BP surface occur. (b) C 1s core level as a function of C60 thickness deposited on BP. (c) The ELA at the C60 /BP interface. AFM images of (d) bare BP, (e) 0.2 nm C60 /BP, (f) 0.4 nm C60 /BP, (g) 3.2 nm C60 /BP and (h) 10 nm C60 /BP. The line profile in (d) indicates a atomic level smooth (1.1 nm, 2 layers) BP surface with cleaving. The line profiles in (e) and (f) indicate about 1.2 nm and 2.1 nm height 60 islands on BP, respectively. The line profile in (g) indicates about 3.3 nm height C60 islands with flower shape.

C60 thickness, the shape of the C 1s and P 2p peaks remains almost unchanged. This XPS investigation suggested that there is no chemical interaction between C60 and BP. Therefore, the interaction between C60 and BP is mainly dominated by vdW bonding. The large WF difference between C60 and BP results in CT at C60 /BP interface that leads

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to the accumulation of electrons in the C60 thin film. Naturally, an upward shift of the entire band level in BP occurs when approaching the interface. A similar behaviour was observed at PTCDA/BP interface. 44 Generally, two separate effects can be considered as causes for the shift of VL (∆Φ) on inorganic/organic interfaces: Band bending (∆ΦBB ) at the surface of the inorganic semiconductor and a interface dipole (∆ΦID ). For C60 on BP, ∆ΦBB and ∆ΦID can be calculated quantitatively by checking the shift of the UPS and XPS spectra. The band bending can be directly obtained from the shift of P 2p core level in Figure 2a (∆ΦBB = 0.16 eV). Thus, the remaining WF shift is the interface dipole (∆ΦID = 0.36 eV). Therefore, the interface dipole dominates the total WF change at C60 /BP interface, because of electron transfer to C60 acceptor. We deduced the ELA of C60 /BP interface in Figure 3c. The WF increases by 0.52 eV upon deposition of 3.2 nm C60 thin film on BP. A negative C− 60 thin film layer was produced, owing to the strong attraction in C60 layer. Initially, the HOMO onset of C60 was pinned to EF . With the deposition of C60 an interface dipole of 0.52 eV was observed, which suggests strong interface n-doping of C60 by transfer of electrons from BP side to the C60 . The morphological structures of the organic thin film have an important influence on its electronic structure and interfacial ELA. Figure 2d-h displays the AFM topography images for bare BP, 0.2 nm C60 /BP, 0.4 nm C60 /BP, 3.2 nm C60 /BP, and 10 nm C60 /BP samples, respectively. The line profile in Figure 2d indicates an atomic level smooth (1.1 nm, 2 layers) BP surface. There are some small white dots (3-4 nm) at the step edge which may be ascribed to oxidation phosphorus in BP defects. For ultrathin films on BP, C60 molecules firstly accumulated into islands. The height of isolated islands is about 1.2 nm for 0.2 nm C60 coverage and 2.1 nm for 0.4 nm C60 coverage, as displayed in Figure 2e-f. The image of the 3.2 nm thickness C60 /BP thin film is characterized by islands with irregular shape, size and fractures. The line profile in Figure 2g indicates that the height of the C60 islands is about 3 nm. With 10 nm of C60 coverage, the boundaries between the islands become more blurred, which is expected for general island growth (Figure 2h). Clearly, the more thicker

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C60 film completely cover the BP surface. (a) Cutoff region

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Figure 3: (a) SECO region and (b) HOMO region in the UPS spectra for C60 /BP thin film with oxygen exposure. Upon oxygen to expose, the SECO onset shifts to the higher binding energy, revealing an downward VL shift. (c) UPS spectra after subtraction of background photoelectrons with Gaussian convoluted HOMO bands. Variation in (d) FWHM of HOMO band and (e) IP, WF, and HOMO edge after oxygen exposure.

The environment gases may impose significant impact on interface electronic properties and thereby the performance of the organic devices. 47–50 As is known, BP is intrinsically vulnerable to the effects of ambient environment (including oxygen, water, and illumination). 51–53 Particularly, oxygen exposure can modulate the electron and hole mobility in thin BP-based FETs. 54 To explore the effect of the environment exposure on C60 /BP interface, we examined the XPS and UPS of the C60 /BP after controlled pure oxygen exposure and then the ambient environment exposure. Considering the light irradiation can induce oxidation of BP, 54,55 the oxygen exposure experiments were performed under dark environment. Firstly, we did the control experiment of exposing bare BP to oxygen. It should be noted that oxygen exposure did not induce any obvious change in the UPS spectra of bare BP. The SECO and VB region show no visible change upon oxygen exposure from 0 L to 1600 L (Figure S4, Supporting Information). Additionally, XPS measurements were carried on

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oxygen exposed bare BP to reveal the physisorption nature of oxygen on BP (Figure S5, Supporting Information). This is clearly stated that pure oxygen can hardly oxidize BP. Figure 3 presents the evolution of the UPS spectra of 3.2 nm C60 /BP interface at the increasing O2 exposure from 100 L to 2000 L. As can be seen, the O2 exposure modifies the position of the C60 /BP interface in cutoff region (Figure 3a) greatly. For 100 L O2 exposure, the WF decrease by 0.15 eV as evaluated from the cutoff edge position. Further exposure to 2000 L produces a total downward shift of the WF of 0.3 eV. On the contrary, the position of the HOMO peak (Figure 3b) does not shift apparently, but the shape become broaden with increasing oxygen exposure, as shown in Figure 3c. For visual clarity the background of the HOMO region has been substracted. The full width at half maximum (FWHM) values of the HOMO band as a function of the oxygen exposure are summarized in Figure 3d. The FWHM of the HOMO band increase from initial 0.47 eV to 0.64 eV under 2000 L exposure, which causes the leading edge of HOMO increased by 0.05eV. The IP, calculated by the difference of VL (equal to WF) and HOMO, is therefore changed upon the various O2 exposure treatments, as summarized in Figure 3e. For further insight of the electronic structure changes, XPS measurements were performed for chemical analysis at the C60 /BP interface. Figure 4a shows the evolution of the P 2p core level after 100 L - 2000 L O2 exposure. The oxidation of BP is confirmed by the appearance of the weak components at deeper binding energy of 134-135 eV, labelled as Oxides. The C 1s does not change much at the O2 exposure, as shown in Figure 4b. There is an obvious O 1s peak with two components at 532 eV after oxygen exposure, as shown in Figure 4c. As has been predicted by theoretical research and confirmed by experimental studies, pure BP does not react with oxygen. 54,56 We also check the XPS of bare BP after oxygen exposure and found neither phosphorus oxide nor oxygen peaks, as shown in Figure S5, Supporting Information. Thus, the appearance of O 1s in XPS spectra clearly indicates that C60 plays an important role for the absorption of oxygen and the oxidation of phosphorus. Fitting of the O 1s indicates that there are two components of the dangling (O-P=O) one in red at

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532.3 eV and bridging (P-O-P) one in blue at 533.7 eV, respectively. 57 The observed peak intensity ratio of two oxygen components is by in accordance with their relative population (2:3) in the P2 O5 molecules, as shown in the upper panel of Figure 4d. For visual clarity, we expand the energy region and enlarge the vertical coordinate to present the P 2p peak after O2 exposure for 2000 L in the bottom panel of Figure 4d. The weak doublet with a large FWHM at 135 eV can be attributed to phosphorus oxidation species. (a)

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Figure 4: XPS spectra of P 2p (a), C 1s (b), and O 1s (c) core levels of the 3.2 nm C60 /BP thin film as a function of the gradual oxygen exposure. (d) The O 1s (up) and P 2p (bottom) core levels of C60 /BP thin film after 2000 L oxygen exposure.

It is interesting that the C60 covered BP is more vulnerable to oxygen, contrary to other n-type organic molecular films such as PTCDA 44 and TCNQ 25 which have been proved able to protect BP from oxygen attack. To understand the role of C60 in the oxidization process, 12


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it is instructive to compare the electron affinity (EA) of the BP, C60 (atomic) and O2 , which are 4.0 eV, 2.68 eV and 0.48 eV, respectively. For pure BP exposed to O2 , it is unlikely that the electrons can transfer from the BP to O2 directly and induce chemical and/or physical interaction, which has been proved both theoretically and experimentally. 54,56 While the EA of C60 is much larger, it is energetically easier for the electrons to transfer from BP to C60 , as reflected in the aforementioned CT across the interface. The partially negative C− 60 may 48 react with the oxygen molecules through CT to produce transient O− because of the smaller 2

difference of their EA value. The C60 is more likely a ladder for the CT process from the 55 BP to O2 . As is known, O− The C60 -induced 2 plays a key role in the BP oxidation process. − oxidation mechanism of BP surface can be expressed as follows: BP + C60 → BP + h+ + C60 , − O2 + C60 → O2− + C60 , then, O2− + P + h+ → Px Oy . The O− 2 forms two P-O bonds at the

BP surface, resulting in a native surface oxide. To examine the effect of ambient environment we exposed the C60 /BP to air for 2 hours and examined the electronic structures with XPS. We found an obvious oxidation P 2p peak appears at 135 eV binding energy, about 5 eV deeper than the 130 eV of the neutral P 2p peak. To localize the distribution of the oxidized P atoms we performed angle-resolved XPS (AR-XPS). Shown in Figure 5a are P 2p spectra which are collected by changing the take-off angle (θ) values (0◦ , 20◦ , 40◦ , and 60◦ ). We can see that the relative XPS intensity of phosphours signal from the phosphorus oxide surfaces increases compared to that of the neutral P 2p ones as the θ increases, which indicates that the oxidized P atoms mainly locates near the surface. Assuming a uniform BP oxidization layer thickness, we can obtain the oxide thickness through d = (−λ/cosθ)ln[IBP /(IBP + Ioxide )] by fitting to the data in Figure 5b, where λ is an inelastic mean-free path (in this equation, assumed λ = 2 nm), IBP and Ioxide are the intensities of phosphorus and phosphorus oxide XPS signals, respectively. We calculated the values of the oxide phosphorus thickness is d = 0.66 nm, in agreement with previous literature. 58 For the ambient environment exposure on the C60 /BP interface, we observed more obvious

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(a)

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Figure 5: (a) Angle-resolved XPS measurements of phosphorus oxide collected over θ values from 0 to 60◦ . (b) Relation between 1/cosθ and ln[IBP /(IBP +Ioxide ) for a uniform phosphorus oxide on BP surface.

oxidation of BP than oxygen exposure (Figure 5a). On the one hand, C60 molecules can not be assemble into stable and compact thin film on BP surface, which can be see from our previous AFM measurements in Figure 2. On the other hand, the ambient air is mixed with oxygen and water, even with light illumination, which yields the rapid deterioration of the BP surface. 52 This result suggests that C60 does not form an ideal protection layer as an n-type organic semiconductor, in contrast to TCNQ and PTCDA thin films. 23,44 To help understand the oxidization process, we performed DFT calculations on the band structures of the isolated and O2 -treated C60 /BP interface. Because the HOMO of C60 is much lower than VBM of both monolayer and bulk BP, so we use monolayer BP-C60 system for the DFT calculation to qualitatively describe the CT process. The degree of electron transfer will increase with the increase of BP layers at organic/BP interfaces. 23,44 The relaxed structures of C60 /BP and O2 -treated C60 /BP, together with the relative total energy of the lowest-energy configuration are shown in Supporting Information Table S1. The isolated C60 /BP system is a direct-band-gap semiconductor, with its VBM and conduction band minimum (CBM) both located at the Γ point (Figure S6, Supporting Information). The CBM and VBM dispersions display high similarity to the initial BP (Figure S7, Supporting 14


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Information).

Figure 6: The side views of partial charge density for CBM (a) and VBM (b) of the isolated C60 /BP system. (c) Charge density difference for C60 molecule interacting with BP. The side views of partial charge density for CBM (d) and VBM (e) of the O2 -treated C60 /BP system. (f) Charge density difference for C60 molecule interacting with O2 -treated BP.

We computed the partial charge density distribution of the CBM and VBM for isolated and O2 -treated C60 /BP interface. For the isolated C60 /BP interface, the CBM is solely contributed by the C60 (Figure 6a) with the VBM mainly stems from BP (Figure 6b). In contrast, the VBM of O2 -treated C60 /BP system is solely provided by the C60 , while the CBM comes from BP, as shown in Figure 6d-e. Obviously, the oxidized phosphorus layer between C60 and BP interface prevents CT from BP to C60 . The electrons are localized at PO bonds, as can be seen in Figure 6e. Thus, the formation of oxidized phosphorus thin layer between BP and C60 can serve as a barrier layer to change the ELA at the C60 /BP interface. The native oxide layer of BP plays multiple critical roles in the surface functionalization process. For examine how oxygen exposure impact on the CT between C60 and BP interface, we compared the charge density difference between adsorbed C60 molecule on BP and O2 treated BP. The charge density difference is defined as: ∆ρ = ρC60/BP − ρC60 − ρBP , where ρC60/BP , ρC60 , and ρBP represent the charge densities of C60 -adsorbed systems, the isolated C60 molecule, and pristine BP (and with O2 -treated BP), respectively. The red region 15



denotes the charge accumulation and the blue region denotes the charge depletion. For C60 adsorbed BP system, there is a strong electron accumulation at the interface by subtracting the electron of the hybrid system from that of isolated C60 and BP (see Figure 6c). Our results clearly indicate the CT at the C60 /BP interface. In contrast, for O2 -treated C60 /BP system, as shown in Figure 6f, there is less electron accumulation at the interface, and electron accumulates mostly at the bottom of C60 molecule. Thus, the formed thin oxidized phosphorus at the interface prevents further CT from BP to C60 . In summary, the electronic structure at the organic-inorganic interface of C60 on BP was investigated with UPS, XPS and DFT calculations. The WF increase from 4.0 eV of pure BP to 4.52 eV for a 3.2 nm C60 thin film on BP, which is mainly attributed to CT from BP to C60 . The upward shift of the P 2p core level indicates that the band bending occurs at the interface. Oxygen exposure experiment of pristine BP does not lead to observable oxidation of BP, while oxygen exposure of C60 /BP leads to obvious oxidation of BP. The oxidized P 2p signal comes from the topmost layer with a thickness of 0.66 nm. C60 plays an important role for the production of O− 2 and hence the oxidation of BP. Our findings suggest that C60 is not the ideal protection layer as other n-type semiconductors. Based on our DFT results, the oxidized phosphorus thin film can prevent further CT from BP to C60 at the interface.

Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51173205 and 11334014), the Fundamental Research Funds for the Central Universities of Central South University (2016zzts049). Y.D. thanks the support from the Chinese National Science Foundation (Project No. 51375503). Y.G. thanks the support by the National Science Foundation (Grant Nos. DMR-1303742 and CBET-1437656).

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Supporting Information Available Single crystal BP cleavage preparation (Figure S1). XPS and UPS spectra for vacuumcleaved BP (Figure S2). The survey XPS spectra (Figure S3). UPS spectra evolution of pure BP with oxygen exposure for cutoff region and VB region (Figure S4). XPS spectra of pure BP as with (Figure S5). Optimization structural parameters on C60 /BP systems (Table S1). The band structure of the isolated C60 /BP and O2 -treated C60 /BP systems (Figure S6). The band structure and electronic wave function of BP (Figure S7).

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