Interface Energy-Level Alignment between Black Phosphorus and F

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Interface Energy-Level Alignment between Black Phosphorus and F16CuPc Molecular Films Can Wang, Dongmei Niu, Yuan Zhao, Shitan Wang, Chuan Qian, Han Huang, Haipeng Xie, and Yongli Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01541 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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The Journal of Physical Chemistry

Interface Energy-Level Alignment between Black Phosphorus and F16CuPc Molecular Films Can Wang,†,‡ Dongmei Niu,∗,† Yuan Zhao,† Shitan Wang,† Chuan Qian,† Han Huang,† Haipeng Xie,† and Yongli Gao∗,†,¶ †Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410012, People’s Republic of China ‡Light Alloy Research Institute, Central South University, Changsha 410083, People’s Republic of China ¶Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA E-mail: [email protected]; [email protected]

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Abstract The organic/inorganic semiconductor interfaces play a key role in determining the functionality of charge injection devices. The energy-level alignment at interfaces between two in-plane organic materials (F16 CuPc and CuPc) and single-crystal black phosphorus was systematically studied by in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). In this work, we reveal that the deposition of F16 CuPc onto black phosphorus surface leads to a considerable charge transfer across interface, resulting in the pronounced increment of work function. As for the molecule-substrate interaction, both F16 CuPc and CuPc interact with black phosphorus via van de Waals physisorption, as confirmed by the UPS and XPS measurements. Our results demonstrates the versatile use of organic molecules to tailor the energy levels of black phosphorus-based hybrid structures, which can be potentially used to optimize its function in future devices.

Introduction Black phosphorus (BP) has gained a lot of attentions due to its unique electronic properties and potential applications in nanodevices. 1–4 BP is a layered semiconductor possessing direct band gap covering from 0.3 eV to 2 eV depending on the number of layers. 5 Particularly, the band gap in BP can be modulated via strain or electric field. 6–9 Unlike graphene or other 2D materials, BP presents strong in-plane anisotropic behaviour in the electric and thermal conductances. 10 The BP-based field effect transistors (FETs) is found to have an on/off ratio of 104 and a high mobility up to 6×103 cm2 V−1 s−1 . 11 From the perspective of spintronics, long spin relaxation lifetimes (4 ns) and spin relaxation lengths (6 µm) were observed in BP-based spin valve. 12 With the help of giant Stark effect, the band structure of BP can be engineered via potassium doping and the resulting anisotropic Dirac cone with semimetal feature. 13,14 Surface functionalization of BP-based FETs via potassium doping can also improve the electron transport, leading to the remarkble enhancement of electron 2


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mobility. 15 Until now, some previous reports were dedicated to improve the electron mobility by employing metal elements, including Cu adatoms diffusion, 16 Al adatoms doping, 17,18 and Te adatoms 19 on BP-based FETs devices. However, metal adatoms have a large atomic size that are deemed challenging to diffuse into the BP lattice. Additionally, the thermal evaporation metal atoms with high temperature may potentially result in crystal defects, even BP surface damage by our previous study. 20 Non-metallic materials doping, like Cs2 CO3 and MoO3 , palys a practical way to modify the ambipolar characteristics in BP-based FETs. 21 Treating flakes of BP with a benzenediazonium derivative passivates can protects BP and enhance the electronic properties in BP-based device. 22 The bulk noncovalent organic functionalization with perplene dimide provided an effect method to improve the resistance of the BP flakes against oxygen degradation. 23 On the other hand, the covalent chemical functionalization also offered an important route for the improvement of BP chemical stability. 24 Recently, He et al. reported that dioctybenzothienobenzothiophene (C8-BTBT) thin films can improve the performance and air stability of BP-based FETs by using van der Waals epitaxy approach. 25 Particularly, the intercalation of BP with organic molecules can produce monolayer BP molecular superlattices with excellent mobility and superior stability. 26 Previously, we have studied the degradation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) covered BP to the air exposure and found that the PTCDA can protect the BP from oxidation. 27 In contrast, C60 thin film can not protect the BP surface from degradation in the oxygen/air exposure because of its appropriate electron affinity. 28 However, C60 molecules as a sacrificial shield can protect BP nanosheets from oxidation via edge-selective bonding. 29 Therefore, understanding the electronic properties and energy-level alignment at organic/BP interface is of crucial importance since it determines the BP-based devices performance. In this paper, we reported on a first characterization of novel organic-inorganic heterojunctions built from BP and the typical n-type organic molecule of F16 CuPc where F16 CuPc acted as rather strong electron acceptors. We compared the interfacial electronic proper-

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ties at F16 CuPc/BP and CuPc/BP interfaces by in situ photoelectron spectroscopy (PES). Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) reveal that the deposition of F16 CuPc onto vacuum-cleaved BP surface leads to strong charge transfer between F16 CuPc layer and the BP substrate, resulting in a pronounced increment of work function (WF). In addition, the morphology of different thickness of organic molecules on BP was studied by atomic force microscopy (AFM). Finally, we discussed the energy-level alignments at the F16 CuPc/BP and CuPc/BP interfaces and their possible application in the BP related electronic devices.

Experimental section Experimental set-up The PES measurements were taken using a electron analyzer (Phoibos 150, SPECS) with a UV source (He Iα , hν = 21.2 eV) for UPS and a monochromatic X-ray source (Al Kα , hν = 1486.7 eV) for XPS in our main ultrahigh vacuum (UHV) chamber (base pressure, 4.0×10−10 mbar). 30–32 A -5.0 V bias was applied to obtain the low energy secondary cutoff (SECO) in UPS measurement. The WF (Φ) were obtained from the SECO measurements of UPS spectra with bias. The total energy resolution was determined to be 70 meV (UPS) and 0.35 eV (XPS), respectively. AFM images (ex situ) were taken on an Agilent 5500AFM/SPM system with tapping mode. 33

Sample preparation The single crystal of BP (HQ Graphene) was pasted onto a substrate. In order to obtain the fresh and clean surface required for PES studied in BP, in situ exfoliation in UHV conditions were performed. Low-energy electron diffraction (LEED) and XPS were taken to confirm the surface cleanliness after cleaving. The F16 CuPc and CuPc thin films were thermally evaporated on clean BP surface in the organic evaporation chamber (base pressure, 4.0 × 10−8 mbar). The organic materials were purchased from Sigma-Aldrich. The deposition rate during evaporation is about 0.1 nm/min. Each evaporation process was controlled with a calibrated quartz crystal microbalance (QCM) close to the substrate. The 4


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substrate was kept at room temperature for the whole depositions and PES measurements.

Figure 1: (a) Molecular structures of CuPc, F16 CuPc. Crystal structure of bulk BP, which is composed of puckered honeycomb layers. (b) Optical image of the cleaved BP at air condition. Scale bar, 2 µm. (c) AFM image (10 µm × 10 µm) of the cleaved BP at the same condition. (d) UPS photoelectron spectra of the cleaved BP measured at normal emission. Insert: The LEED pattern for bare BP with 90 eV energy.

Results and discussion The quality of the BP surface is comfirmed by AFM, optical measurements, XPS, UPS, and LEED before depositing organic molecules on BP. The bulk BP crystal structure with puckered honeycomb layers is shown in Figure 1a. The optical and AFM images of freshly exfoliated BP were taken in ambient conditions within 30 min. The optical image (Figure 1b) indicates the surface of the cleaved BP is highly clean without obvious oxidation area. Correspondingly, the AFM image (Figure 1c), exhibits highly flatness with a root-meansquare (Rms) of 0.05 nm and no apparent droplets and bumps. The height of the line profile in the inset of Figure 1c is measured to be 0.55 nm, close to the thickness of BP monolayer. Figure 1d shows the SECO and the valence band (VB) region of BP taken with UPS. From the cutoff region, we can obtain the WF of BP is 4.2 eV. In the VB region spectrum, the 5



valence band maximum (VBM) can be identified (0.15 eV). Thus, the bulk BP is p-type semiconductor if one takes the 0.35 eV band gap into consideration. 5 Note that the UHVcleaved BP exhibts apparent VB features compared to ambient-cleaved BP. 34 The LEED pattern for bare BP (UHV-cleaved) with 90 eV energy is shown in the inset of Figure 1d. The LEED spots form a clear and sharp pattern as also seen in previous literature, 27,28 indicating the highly quality of the BP surface. The ionization energy (IE) is determined by combining VBM and the WF. The electron affinity (EA) is the energy difference between the conduction band minius (CBM) and VL. Thus, we can obtain IE=4.35 eV and EA=4.0 eV in BP, respectively, in agreement with previously published data. 35 The XPS measurements were also carried out at survey region and P 2p core-level region on the cleaved BP. Benefiting from vacuum-cleaved, no C 1s and O 1s components can be seen in the survey XPS spectrum (see Figure S1, Supporting Information). The deconvoluted P 2p peak show two components at around 129.8 eV (P 2p3/2 ) and 130.7 eV (P 2p1/2 ), 36 see Figure S2 in Supporting Information. These results demonstrate the successful cleavage of the BP and the high quality of the sample surface. Firstly, we give a thorough investigation of the electronic properties at F16 CuPc/BP interface. Figure 2 show the UPS spectra of the F16 CuPc/BP system as a function of the incremental deposition of the F16 CuPc thickness (Θ) in the SECO and HOMO region. The cutoff gradually shifts toward the lower binding energy with the increasing of F16 CuPc coverage, as seen in Figure 2a. On the initial stage of deposition (Θ=0.2 nm), the cutoff shifts by 0.5 eV, yielding Φ= 4.7 eV. Upon depositing 5.2 nm F16 CuPc on BP results in an increase of WF to Φ= 5.2 eV. The WF seems to be saturated after 5.2 nm F16 CuPc deposition on BP. Note that the sudden shift of the cutoff upon initial F16 CuPc deposition (Θ=0.2 nm) suggests the formation of an interface dipole due to charge transfer at F16 CuPc/BP interface. F16 CuPc as rather strong electron acceptors, can be used to improve charge transfer at interfaces, since the fluorination enhances the EA substantially. 37 The EA of F16 CuPc can be calculated as 5.0 eV at Θ=5.2 nm with a band gap of 1.5 eV. 38 Therefore, electrons in BP

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Figure 2: (a) SECO and (b) HOMO region for BP with different thickness F16 CuPc. The inset of (b) is the HOMO edge and peak shifts with thickness of F16 CuPc thin film. (c) Background subtraction of HOMO region with Gaussian convoluted HOMO peaks at F16 CuPc/BP interface. The value of FWHM of HOMO with different F16 CuPc coverages are listed.

(IE=4.35 eV) can easily transfer into F16 CuPc thin film. The remarkable charge transfer also be found between others n-type organic semiconductor and BP interface, like PTCDA/BP, 27 C60 /BP, 28 and TCNQ/BP interface. 23,39 In Figure 2b, the HOMO peaks (HOMO and HOMO-1) of F16 CuPc also gradually shifts toward the lower binding energy, leading to an increase in upward band bending at F16 CuPc/BP interface. In the HOMO region of the F16 CuPc/BP system, one can see a weak HOMO peak located at 1.6 eV below Fermi level at Θ=0.2 nm. 40 The spectral features of F16 CuPc molecules become visible around 1.7 eV below Fermi level at Θ=0.4 nm. The inset of Figure 2b displays the HOMO edge and peak shifts with thickness of F16 CuPc thin film. It can be clearly seen that HOMO edge and peak move toward the Fermi level by 0.35 and 0.63 eV with Θ=5.2 nm, respectively. Notably, the trend of shift in HOMO peak and edge displays slightly differs at the lower F16 CuPc coverages (Θ ≤ 0.8 nm). This means that the full width at half maximum (FWH7



M) of the HOMO band changes with molecular coverages. Figure 2c shows the background subtract UPS spectra in HOMO region with Gaussian convoluted HOMO peaks. After background subtracting, one can clearly see the evolution of HOMO component as a function of F16 CuPc layer thickness. The VB features in BP still can be seen at Θ=0.2 nm. Interestingly, the FWHM of the HOMO for low coverage is much larger than for higher coverage. The value of FWHM of HOMO is about 1.44 eV at Θ=0.4 nm. However, the FWHM decreases into 0.69 eV at Θ=5.2 nm. It could indicate that the F16 CuPc layer is reorganizing or selforganizing at the initial stage of layer formation. The change of the width of HOMO can be related to the molecular ordering of thin film. 41 On the initial growth stages, F16 CuPc preferred to assemble into well-ordered thin layers on SiO2 due to the the relatively moleculesubstrate interfacial interactions. In contrast, the nucleation and the growth of island taken palce within the intermolecular interaction after the formation of interfacial layer. 42 For example, the FWHM of the HOMO for ultrathin CuPc films on Au(100) or Si(111) is quiet different with bulk CuPc, due to the interplay interaction between molecule-molecule and molecule-substrate. 43,44 On the other hand, the admixture between VB features in BP and HOMO feature in F16 CuPc may attribute to the broadening of FWHM at ultrathin film. In order to further analysis the molecule-substrate interactions between F16 CuPc and BP, the P 2p and C 1s XPS measurements for F16 CuPc with different thickness on BP were performed. In Figure 3, the P 2p and C 1s XPS spectra are presented as a function of different thickness of F16 CuPc. It is noteworthy that the P 2p shifts toward lower binding energy as F16 CuPc thickness increases, see Figure 3a. The shift of P 2p amounts to 0.25 eV at Θ=5.2 nm. This behaviour indicates that the electronic property of BP is obviously modified with F16 CuPc. The deconvolution of C 1s at Θ=5.2 nm F16 CuPc on BP with four peaks can be seen in Figure 4a. The XPS measurement at Θ=5.2 nm F16 CuPc on BP shows four main peaks in the C 1s spectrum at 285.22 (C-C), 286.43 (C-N), 287.28 (C-F), and 289.12 (shake-up) eV. 45 Note that the shifts of C-C bonds and C-F bonds of C 1s are slightly different in Figure 3b. The shift of C-C bonds is 0.1 eV at Θ=0.4 nm. The shift of C-F

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bonds is 0.12 eV at Θ=0.4 nm. This can be understood by the formation of interface dipole. The main contribution of interface dipole at F16 CuPc/BP interface is exposed C-F polar bond in molecular film. 46 Further deposition of F16 CuPc leads to an intensity increase in the higher binding energy component in the C 1s spectrum. The main C 1s peak (C-F) shifts to lower binding energy by a total of 0.48 eV at Θ=5.2 nm (Figure 3b). The total shifts of the Cu 2p3/2 , N 1s, and F 1s core-levels are 0.23, 0.24, and 0.28 eV, respectively (Figure S3, Supporting Information). Accordingly, the Cu 2p3/2 , N 1s, and F 1s core-levels all shift to lower binding energy with the formation of an upward band bending at F16 CuPc/BP interface. The fact that HOMO shifts (in Figure 2b) and core-level shifts (in Figure 3) occurred in parallel indicated that photo-voltage charging effects did not contribute to the energy-level shifts at the F16 CuPc/BP interface. 47 (a)

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Figure 3: (a) P 2p XPS spectra of BP with different thickness F16 CuPc coverage. (b) C 1s XPS spectra of F16 CuPc with different thickness molecular films on BP. (c) The development of sample WF and P 2p core-level versus F16 CuPc thickness.

The developments of the WF and P 2p peak with different F16 CuPc film thickness are depicted in Figure 3c. Notably, the WF increased from 4.2 eV (pristine BP) to 5.2 eV (nominally 5.2 nm F16 CuPc). Shifts of this kind and magnitude can be attributed to charge transfer formation between BP and F16 CuPc. Charge transfer is greatly enhanced at the F16 CuPc/BP interface, because the EA of F16 CuPc is 5.0 eV greater than the IE of BP 9



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5.2 nm F16CuPc/BP 290

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Figure 4: (a) The deconvolution of C 1s XPS core-level spectrum at 5.2 nm F16 CuPc/BP interface. The dash lines labelled the peak positions of C-C, C-N, and C-F core-levels in F16 CuPc film. (b) The deconvolution of C 1s XPS core-level spectrum at 4.8 nm CuPc/BP interface. The dash lines labelled the peak positions of C-C and C-N core-levels in CuPc film.

of 4.25 eV. Thus, the electron can easily transfer from BP to F16 CuPc and leading to the upward band bending in the BP. Excepted the band bending, the main contribution for the increasing WF is the interface dipole (0.75 eV) between BP and F16 CuPc. It originates from the charge transfer from BP and the resulting electron accumulation at the side of organic thin film, which forms the space charging region between BP and F16 CuPc. We can confirm this assumption by depositing other electron acceptor material with high WF and EA (MoO3 ) on BP. The shift of P 2p core-level indicates a F16 CuPc-induced band bending in BP, consistent with the deposition of MoO3 as shown in Supporting Information Figure S4. Upon deposition of 0.4 nm MoO3 , the WF largely increase from 4.2 eV (cleaved BP) to 5.54 eV, or a upward shift of VL by 1.34 eV, owing to the substantial electron transfer from BP to MoO3 (or hole transfer from MoO3 to BP). 21 This produced charge transfer was further confirmed by the shift of P 2p core-level (0.42 eV) with 0.4 nm MoO3 , see Figure S4 10


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in Supporting Information. Actually, the formation of organic-inorganic p-n heterojunctions at F16 CuPc/BP or MoO3 /BP can lead to new electron equilibrium by pronounced charge transfer and band bending. Considering two independent contributions in the total change of WF (∆Φ) at an organic/inorganic semiconductor interface: (i) band bending (∆ΦBB ), (ii) interface dipole (∆ΦID ). 47,48 At the F16 CuPc/BP interface, the band bending can be measured from P 2p core-level shift in Figure 3c (∆ΦBB = 0.25 eV). Consequently, the interface dipole at F16 CuPc can be obtained (∆ΦID = 0.75 eV). The formation of large interfacial dipole can be mainly attribute to the charge transfer from BP to F16 CuPc. Except that charge transfer, other factor can play role in determining the interface dipole. The exposed C-F polar bond in F16 CuPc may also have an apparent effect on the interface dipole, due to the exposed polarized intramolecular bonds on thin film surfaces. 37,49

Figure 5: (a) SECO and (b) HOMO spectra with different thickness CuPc on BP. The inset of (b) is the HOMO edge and peak shift with thickness of CuPc thin film. (c) Background subtraction of HOMO region with Gaussian convoluted HOMO peaks at CuPc/BP interface. The value of FWHM of HOMO with different F16 CuPc coverages are listed.

To compare with F16 CuPc/BP interface, we also study the electronic structure at CuPc/BP interface by depositing CuPc molecules on vacuum-cleaved BP. Figure 5 displays the He I 11



UPS spectra of the CuPc/BP system with the different deposition amount of CuPc (Ω) in the SECO region and HOMO region. In contrast to F16 CuPc/BP interface, the SECO shifts toward higher binding energy by about 0.36 eV, as can be seen in Figure 5a. It indicates that the WF decreases by 0.4 eV at Ω=4.8 nm, resulting in a final WF of 3.8 eV. In Figure 5b, the HOMO band onset of CuPc is located at about 0.65 eV below the Fermi level at Ω=0.2 nm. It should be noticed that the VBM of BP dose not completely disappear upon initial deposition of 0.2 nm CuPc. At Ω=0.2 nm, the features of single-crystal BP (VBM) and CuPc (HOMO, HOMO-1) all can be seen in the HOMO region. However, at Ω=0.4 nm, the VBM features of single-crystal BP can not be seen in the HOMO region. The evolutions of HOMO edge and HOMO peak with CuPc molecular film thickness are shown in inset of Figure 5b. With the increasing of CuPc deposition, the HOMO moves forward to the higher binding energy side until it saturates after 4.8 nm of CuPc deposition. Figure 5c shows the background subtract UPS spectra in HOMO region for CuPc on BP. Unlike the case of F16 CuPc/BP interface, we observe no significant change in the FWHM of the HOMO at different CuPc coverages on BP. At Ω=0.4 nm, the FWHM of the CuPc HOMO is about 0.65 eV. At Ω=4.8 nm, the FWHM of the CuPc HOMO is about 0.44 eV. This trend may suggest that the physical properties at low coverage are similar to the bulk materials. It is worth noting that WF shifts and HOMO edge shifts occur in parallel. This indicates that the IE of CuPc/BP interface remains unchanged with a value 4.45 eV. Figure 6 displays the evolution of XPS core-level with different CuPc coverages on BP. Figure 6a shows the P 2p for BP as a function of CuPc molecular thickness. The P 2p peak shows no apparent shifts, contrary to the upward bending of F16 CuPc case. The evolution of the C 1s XPS spectra as a function of CuPc molecular thickness is dispalyed in Figure 6b. The decomposition of C 1s at final CuPc coverage (Ω=4.8 nm) is presented in Figure 4b. The C 1s spectrum exihibts of three peaks with binding energies 284.43 (C-C), 285.62 (C-N), and 287.60 (shake-up) eV. 50 The intensity of C 1s, Cu 2p3/2 , and N 1s increases with the increasing the CuPc coverage (Figure S5, Supporting Information). The evolution

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Figure 6: (a) P 2p XPS spectra of BP with different thickness CuPc coverage. (b) C 1s XPS spectra of CuPc with different thickness molecular films on BP. (c) The development of sample WF and P 2p core-level with CuPc thickness.

of Cu 2p3/2 and N 1s spectra in CuPc with thickness are shown in Figure S6, Supporting Information. Upon CuPc deposition, C 1s, Cu 2p3/2 , and N 1s shifts toward slightly higher binding energies. In contrast to the case of F16 CuPc/BP interface, the C 1s spectra in CuPc shifts toward higher binding energy, as shown in Figure 6b. No extra feature occurs in the C 1s peak of CuPc/BP and F16 CuPc/BP, indicating that both CuPc and F16 CuPc molecules does not interact chemically with single-crystal BP surface. Thus, CuPc and F16 CuPc molecules interact with BP via van der Waals interaction. Figure 6c shows the shifts of WF and P 2p spectra versus CuPc thickness. The band bending can be measured from the shift of P 2p (∆ΦBB =0.09 eV) at CuPc/BP interface. Unlike F16 CuPc/BP interface, the band bending for CuPc on single-crystal BP surface is very weak. The main reason is that no charge transfer between CuPc and BP interface. Thus, the remaining WF shift comes from the formation of an interface dipole (∆ΦBB =0.27 eV). Clearly, the main contribution to the change of WF is the formation of the interface dipole at CuPc/BP interface. Molecular orientation and molecular packing may induce the formation of the interface dipole, as described in the previous literatures. 51 The film morphology of F16 CuPc and CuPc molecular films on BP are presented in 13



Figure 7: AFM images of (a) 1.6 nm and (b) 5.2 nm thick F16 CuPc films on cleaved BP. AFM images of (c) 1.6 nm and (d) 4.8 nm thick CuPc films on cleaved BP. Size: 10 µm × 10 µm. The line profile in panel (b) and (d) indicates about 2.5 and 5.0 nm height organic thin films on BP.

Figure 7. Figure 7a shows an AFM image of 1.6 nm F16 CuPc film on BP, which showing that F16 CuPc forms very smooth, amorphous films on BP surface with a Rms roughness of 0.85 nm. With further deposition of F16 CuPc to 5.2 nm (Figure 7b), the surface morphology is characterized by needle-like feature randomly oriented in the film plane without islands formed. Such film morphology is similar to the needle-like nanostructures of F16 CoPc on SiO2 . 52 The Rms roughness is 1.25 nm for 5.2 nm F16 CuPc/BP sample. These two AFM images could indicate a Frank-van der Merve growth mode for F16 CuPc/BP. In the initial stages of deposition, the molecule-substrate interactions outweigh the molecule-molecule interactions. Then, molecules adsorb preferentially to BP surface resulting in fully covered layers. The evolution of morphology may lead to change the FWHM of the HOMO in UPS spectra, as described previously in Figure 2c. As shown in Figure 7c, the 1.6 nm CuPc/BP is smooth with a Rms roughness of 1.12 nm. We can observe the formation of the 2D islands in the more coverage CuPc molecular films growth. This the island growth 14


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behavior is very similar to CuPc on bare SiO2 . 53 Upon depositing 4.8 nm CuPc (Figure 7d), the molecules nucleate into islands with a Rms roughness of 2.64 nm. The line profile shows that the height of the CuPc islands is 5.0 nm in Figure 7d. This layer then island growth is Stranski-Krastanov mode. Since the interactions of molecules with the substrate the molecules completely covers the BP surface, but in a thicker regime clusters or islands are formed. This clearly demonstrates the anisotropy of lattice structure of BP may induce the formation of islands for CuPc and F16 CuPc thin films. Additionally, the LEED patterns for different thickness CuPc and F16 CuPc on BP also indicate no perfect thin film or domain formed on BP surface (Figure S7, Supporting Information). (a)

VL

4.2 eV

(b) Δ = +1.0 eV

VL

4.2 eV

5.21 eV

Δ = -0.36 eV

3.86 eV

LUMO

eCBM

CBM

LUMO

0.35 eV

1.5 eV

VBM

1.2 eV

EF

VBM

0.59 eV

HOMO

HOMO

BP

1.6 eV

0.35 eV

F16CuPc thickness

BP

CuPc thickness

Figure 8: Schematic illustration for the energy-level diagrams of (a) F16 CuPc/BP interface and (b) CuPc/BP interface. The electronic process of charge transfer between F16 CuPc and BP is indicated in (a). The main difference between F16 CuPc/BP interface and CuPc/BP interface is the formation and size of an band bending and interface dipole.

Finally, the energy-level alignments at F16 CuPc/BP and CuPc/BP interfaces are depicted in Figure 8. The lowest unoccupied molecualr orbital (LUMO) energy position is determined by assuming a band gap of 1.5 eV for F16 CuPc and 1.6 eV for CuPc from inverse 15



photoemission spectroscopy (IPES), respectively. 38,54 The formation of band bending and interface dipole in the interfical regions can be clearly seen in the energy-level alignment diagram for F16 CuPc/BP interface, as shown in Figure 8a. The F16 CuPc/BP interface shows a large WF change of 1.01 eV as indicated by VL offsets. Especially, the electron injection barrier (0.1 eV) is small enough and lowered by band bending at the F16 CuPc/BP interface . Consequently, the electrons in the CBM of BP will diffuse into the LUMO of F16 CuPc. Actually, a typical organic-inorganic p-n heterojunction can be built up by combining F16 CuPc molecular film and BP. At the same time, the energy levels of F16 CuPc bend down towards the interface facilitates electron transfer from BP to F16 CuPc thin layer. As in the case of CuPc/BP interface (Figure 8b), the cutoff shifts downward by 0.36 eV upon deposition of 4.8 nm CuPc, indicating the formation of an interface dipole. Considering the electron or hole barrier at CuPc/BP interface, charge transfer at this interface will become difficult. Thus, we can tune the WF by inserting F16 CuPc molecular layer between BP and metal oxides in BP-based devices.

Conclusion In summary, we have detailed studied the electronic properties of the organic-inorganic heterostructure formed by F16 CuPc and CuPc on vacuum-cleaved single-crystal BP by using UPS and XPS. The evolution of the electronic states at F16 CuPc/BP and CuPc/BP interfaces with different thickness of thin film was thoroughly investigated. We directly observed that the pronounced charge transfer at F16 CuPc/BP interface in UPS spectra, and resulted in remarkable WF increase. Furthermore, F16 CuPc adsorbed on BP can modify the energylevel alignment as ideal an electron acceptor. On the other hand, we also found that the heterostructure of CuPc/BP was mainly characterized by interface dipole, due to the evolution of thin film morphology confirmed by AFM. Especially, the formation of band bending at F16 CuPc/BP can facilitate the efficiency of electron injection from BP to organic molec-

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ular film. Our results demonstrated that the energy-level alignment at organic/BP interface can be tailored and potentially used to manipulate carrier transport in organic/BP hybrid devices.

Acknowledgement We thank the financial support by the National Natural Science Foundation of China (Grant Nos. 51173205 and 11334014) and National Science Foundation (Grant Nos. DMR-1303742 and CBET-1437656).

Supporting Information Available The survey XPS spectra of F16 CuPc on BP (Figure S1). The deconvoluted P 2p XPS corelevel for BP (Figure S2). N 1s, Cu 2p3/2 , and F 1s XPS spectra of F16 CuPc/BP (Figure S3). P 2p XPS spectrum and UPS spectra evolution of BP with MoO3 (Figure S4). The survey XPS spectra of CuPc on BP (Figure S5). Cu 2p3/2 , N 1s spectra at CuPc/BP interface (Figure S6). LEED patterns of F16 CuPc and CuPc on BP (Figure S7).

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TOC Graphic

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Interface Energy-Level Alignment between Black Phosphorus and F

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