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Charge Transfer at the PTCDA/Black Phosphorus Interface Can Wang, Dongmei Niu, Baoxing Liu, Shitan Wang, Xuhui Wei, Yuquan Liu, Haipeng Xie, and Yongli Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06678 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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The Journal of Physical Chemistry

Charge Transfer at the PTCDA/Black Phosphorus Interface Can Wang,†,‡ Dongmei Niu,∗,† Baoxing Liu,† Shitan Wang,† Xuhui Wei,† Yuquan Liu,† 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]; [email protected]

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Abstract The interfacial electronic structure at the organic-inorganic semiconductor interface plays an important role in determining the electrical and optical performance of organicbased devices. Here, we studied the molecular alignment and electronic structure of thermally deposited 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on cleaved black phosphorus using photoelectron spectroscopy. The work function of black phosphorus is substantially upped with an organic thin film, originating from the charge transfer from black phosphorus to PTCDA. According to our photoemission spectrum and theoretical simulation, we also define the interaction between PTCDA and black phosphorus is weak van de Waals physisorption, rather than bonding chemisorption. Furthermore, we show that PTCDA thin film can effectively isolate reactive oxygen species, thereby protecting BP surface oxidation and deterioration under ambient condition. Our results suggest the possibility of manipulating interfacial electronic structures of black phosphorus interface by noncovalent with organic semiconductor, in particular for applications in high-performance organic-inorganic hybrid photovoltaic.

Introduction Black phosphorus (BP), the most stable allotrope of phosphorus, has attracted more and more attention in recent years. 1–3 BP is a layered semiconductor material consisting of stacks held together by van der Waals (vdW) forces. In a single BP layer (phosphorene), each phosphorus atoms is covalently bonded with three adjacent phosphorus atoms to form a puckered honeycomb structure due to sp3 hybridization. 4 Fascinating physics can be found in this layered two-dimensional (2D) material, including ultrahigh carrier mobility, 5–7 quantum oscillations, 8,9 quantum Hall effect, 10 Dirac semimetal, 11 and anisotropic thermal/electrical conductances. 12–14 Especially, the strongly layer-dependent direct band gap of BP (from a value of 1.5-2.0 eV for phosphorene to 0.3 eV for the bulk phase 15 ) make it suitable for


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optical devices covering from visible to mid-infrared. 16,17 However, the degradation of BP in air moisture limited its device applications. 18 On the other hand, various capping layers have been implemented to protect the BP against moist air, such as hexgonal boron nitride (h-BN), 19,20 aluminum oxide (Al2 O3 ), 21,22 metal nanopaticles, 23 and partial oxidized Px Oy layer. 24 Organic molecule passivation technique is a common method to tailor the electrical properties of 2D materials through charge transfer. Covalent and noncovalent functionalization is an effective way to protect BP surface and improve the performance of BP-based devices. 2 Recently, Abellán et al. reported that the noncovalent functionalization of bulk BP with 7,7,8,8-tetracyano-p-quinodimerthane (TCNQ) and a perylene bisimide (PDI) can result pronounced charge transfer and vdW interactions. 25 It was shown that the performance of BP-based field effect transistor (FET) can be enhanced with the help of small organic (F4TCNQ) molecular interlayer. 26,27 More recently, organic functionalization of BP with layers of aryl diazonium has been shown to provide effective chemical passivation, although this is accompanied by p-doping of the channel material. 28 Artel et al. reported BP passivation with self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS), thus gaining important advantages in terms of stability, oxidation resistance, and elimination of electronic devices degradation. 29 In the field of theoretical calculation, a wide range of organic molecules, including TCNQ, F4TNCQ, tetracyanoethylene (TCNE), tetrathiafulvalene (TTF), and phenylenevinylene (PV), have been introduced onto the BP surface to modify the electrical, chemical and optical properties of BP. 30–33 Hybrid inorganic/organic semiconductor systems have attracted attention because of the high carrier mobility and high excitation densities of inorganic semiconductors and strong light-matter coupling featured by the organic counterparts. The aromatic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), a typical planar n-type organic semiconductor, has long been studied as model system for understanding organic molecular thin film growth on metal 34–37 or inorganic semiconductor 38–41 surfaces. Very recently, Zhao et al. found that PTCDA does not react with



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BP and can self-assemble into stable network on BP surface via vdW interaction by density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations. 42 Moreover, they conclude that the charge transfer between PTCDA and BP is very small. Their theoretical research is based on the single layer BP, while in the realistic studies, the few layers BP and even bulk BP based devices are usually the cases. As the electronic energy structure of few layers of BP is very different from the single layer BP and very close to the bulk ones, we would like to study the interface of PTCDA and bulk BP. On one hand, the experimental electronic properties of organic/BP interface remains unexplored, on the other hand, the interface of the n-type PTCDA and p-type BP provide prototype device for organic-inorganic PN junction and might be very helpful to understand related devices. In this paper, we combined angle-resolved photoemission spectroscopy (ARPES), lowenergy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and density functional theory (DFT) to study the electronic structure of as well as the morphology the PTCDA/BP interface. Using photoemission spectroscopy measurements and DFT calculations, the mechanism of work function change was identified as electron transfer. To gain qualitative understanding of the charge transfer at PTCDA/BP, we used an effective electrostatic model which included the contributions from band bending and interface dipole. We show that the energy-level alignment at an organic/BP semiconductor interface can be modified with a PTCDA interlayer.

Experimental section Sample preparations The PTCDA was purchased from Sigma-Aldrich and BP single crystal from HQ Graphene. The PTCDA thin films were evaporated in the organic evaporation chamber. The base pressure of the organic evaporation chamber was typically p ≤ 2.0 × 10−8 mbar. Evaporation rate of PTCDA was 1 Å/min. Film mass thickness-


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es were monitored via a quartz crystal microbalance. All depositions were performed with the substrate at room temperature. A commercial BP single crystal was fixed on substrate with silver paste. To prevent chemical degradation in ambient conditions, it was cleaved with a scotch tape in the loadlock with pressure about 10−9 mbar and transferred to the main chamber. The AFM images of the samples were recorded using an ex situ an Agilent 5500AFM/SPM system in the trapping mode.

Photoemission measurements The sample was transferred into an ultrahigh vacuum (UHV) system, which included standard tools for surface characterization, such as Ar+ ion sputter gun, LEED, UPS, and XPS. The UPS and XPS spectra were measured with a SPECS Phoibos 150 hemispherical energy analyzer, equipped with a Microwave UV Light Source (He Iα , hν = 21.2 eV ) and a monochromatic Microfocus X-ray Source (Al Kα , hν = 1486.7 eV) as previouly described. 43–45 The base pressure in main chamber was 4.0 × 10−10 mbar. UPS spectra were recorded with a bias of -5.0 V to enable the observation of the low energy secondary cutoff. The work function (φ) of the sample can be obtained through the following equation, φ = hν(21.21) − Eb , where Eb is determined by extending the spectra baseline and secondary electron onset. A total energy resolution is about 70 meV for the present UPS as determined from the Fermi edge of clean Au. For XPS measurements, all core-level spectra were obtained with a pass energy of 40 eV. All photoemission measurements were performed with the substrate at room temperature Theoretical simulations First-principles calculations were performed based on DFT 46 implemented in the VASP package. The generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) and projector augmented wave (PAW) potentials were used. 47,48 The Monkhorst-Pack scheme was used to sample the Brillouin zone. 49 A mesh 10×10×1 k-point sampling was used for bulk BP. A mesh 3×2×1 k-point sampling was used for PTCDA/BP systems. A plane wave cutoff 500 eV was used for all the calculations. A sizable vacuum layer of 20 Å isolates neighboring periodic images of PTCDA/BP slabs. All 5 ACS Paragon Plus Environment


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atoms were relaxed and the final force on each atom was less than 0.05 eV/Å for each ionic step and the convergence criterion for the self-consistent field energy was set to be 10−5 eV. We considered the effect of vdW interactions, as implemented in the optimized exchange vdW functional B86b of Becke functional. 50 The optB86b-vdW functional was applied, since vdW interactions play a significant role in noncovalent functionalization. The monolayer (bilayer) BP was built by a periodic slab geometry with a 4×4×1 supercell containing 64 (128) phosphorus atoms to ensure at least 10 Å vacuum separations in all directions for the PTCDA molecule adsorbed on BP surface.

Figure 1: (a) The puckered honeycomb lattice of monolayer BP; x and y denote the armchair and zigzag crystal orientations, respectively. (b) and (c) the LEED pattern of cleaved BP measured with 90 eV electron energy, the reciprocal lattice of BP is illustrated by the dashed rectangle. (d) Band structure of cleaved BP, obtained by ARPES measurements (color mapping) and from ab inito band calculations (blue circle line) along the kx direction (left) and along the ky direction (right).

Results and discussion The band dispersion of inorganic semiconductors plays an important role in understanding the electronic properties and transport characteristic. Therefore, at first, we perform ARPES 6 ACS Paragon Plus Environment

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and theoretical calculations to study the band dispersion of single crystal BP. The crystal surface was prepared in situ in a vacuum better than 10−9 mbar by cleaving with a scotch tape inside the load lock chamber. Immediately after the cleaving, the surface quality of BP is confirmed by XPS and clear pattern in LEED. In the survey XPS spectrum of cleaved BP, no O 1s and C 1s peaks are observed (see Figure S1, Supporting Information). Using the LEED, we determine the high-symmetry directions of the single-crystalline BP. We observe a single domain with well-defined spots and weak diffuse background from the LEED pattern at 90 eV, which is consistent with reported earlier. 51 In the upper plane of Figure 1(b-c), the long axis is along the ky direction (zigzag) and the short axis along the kx direction (armchair). The measured reciprocal unit cell vector ratio kx /ky =0.74 is in excellent agreement with our ab initio calculated crystal unit cell vector ratio a/b = 0.75. The bulk BP band structure is obtained in a straightforward way from the photoemission measurements by calculating kk , the electron momentum in the plane of the surface, with √ respect to the photoelectron kinetic energy (Ekin ): h ¯ kk = 2mEkin · sinθ, with Ekin = hν − Eb − Φ, where Eb is the binding energy of the electron relative to the Fermi energy and Φ the work function of the material, θ is the photoemission angle relative to the surface normal. We measured the band dispersion along the Γ-X and Γ-Y high-symmetry directions, as shown in Figure 1d, left and right. Obviously, there are three bands near the Fermi level. The surface band dispersions in ARPES along the armchair direction and zigzag direction are consistent with the DFT calculations. However, the measured inner band dispersions are not well accord with the calculated band structures. The inconformity may be partly attributed to the temperature of APRES which is at room temperature while the DFT calculations assume the zero absolute temperature. The band dispersion of valence band is nearly linear along the armchair direction and quadratic along the zigzag direction in Figure 1d. The anisotropic band dispersions have also been observed by ARPES in previous reports. 11 The corresponding effective mass of holes is highly anisotropic because it is proportional to the inverse of the curvature of the band dispersion. This anisotropic band



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dispersion is a direct experimental confirmation for the highly anisotropic effective mass of bulk BP predicted in theoretical band calculations. The effective mass of hole at the valence band maximum (VBM) can be obtained using:

1 m∗

=

1 ∂2E , ¯ ∂k2 h

where E and k correspond to

the binding energy and the reciprocal lattice vector along the high symmetric direction. The effective mass of holes located at VBM is 0.45(±0.15) m0 along the armchair direction, and 1.07(±0.05) m0 along the zigzag direction (m0 is the bare electron mass). These values are in satisfactory agreement with the previous reported experimental data. 52 Neglecting the quasiparticle scattering, the carrier mobility is inversely proportional to the effective mass. Therefore, the hole mobility along the armchair is higher than along the zigzag direction, which has been confirmed in BP-based FETs. 53

Figure 2: (a) P 2p XPS spectra of cleaved BP. (b) Evolution of the core-level spectra of P 2p with respect to the thickness of PTCDA on BP. (c) C 1s XPS spectra of a multilayer equivalent deposit of 3.2 nm PTCDA on BP. (d) Evolution of the core-level spectra of C 1s with respect to the thickness of PTCDA on BP. (e) O 1s XPS spectra of a multilayer equivalent deposit of 3.2 nm PTCDA on BP. (f) Evolution of the core-level spectra of O 1s with respect to the thickness of PTCDA on BP.

To examine the molecule-substrate interaction, XPS measurement is used for a chemical


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analysis of the BP surface after PTCDA deposition. The survey XPS spectrum of the different thickness PTCDA on cleaved BP can be found in Figure S2 of Supporting Information. It is found that the intensity of P 2p and 2s decrease monotonically with increasing PTCDA coverage from 0.2 nm to 3.2 nm. Inversely, the C 1s and O 1s core-level intensity are consistent with typical organic thin film growth with increasing signal intensity in the raw data (see Figure S2, Supporting Information). The LEED patterns of BP and PTCDA-covered BP are also shown in Figure S3 of Supporting Information. Figure 2a shows P 2p core-level photoemission spectra for the cleaved BP. The P 2p spectrum exhibits a single doublet (fitted with two Lorentzian-Gaussion peaks), 2p3/2 and 2p1/2 (resulting from the spin-orbit splitting of 0.8 eV), located at respectively 129.9 and 130.7 eV, characteristic of crystalline BP. 54 Owing to cleave in vacuum, no oxidized phosphorus peaks are observed in P 2p core-level spectrum. The ratio of the intensity of 2p3/2 (Ip3/2 ) to 2p1/2 (Ip1/2 ) is Ip3/2 /Ip1/2 =2.1(±0.2), very close to the theoretical value of 2.0. Figure 2b summarizes the binding energy shifts as a function of PTCDA thickness. It can be noted that the intensity ratio of Ip3/2 /Ip1/2 decrease to 1.9(±0.2) at 3.2 nm PTCDA deposition. The possible reason for the slightly deviation from the 2.0 might be attributed to the raw data noise casued by the low intensity. The P 2p emission from the PTCDA-adsorbed BP shows no extra features, indicating that the PTCDA molecule does not interact chemically with the cleaved BP substrate. From the Figure 2b, however, the P 2p3/2 peak gradually shifts to higher binding energy region, implying that a downward band bending of about 0.15 eV existed at the interface. The C 1s component spectrum of PTCDA at 3.2 nm is shown in Figure 2c. The splitting of the C 1s peak is due to the different chemical environment of carbon atoms in the conjugated π-system of the PTCDA. The major peak at a binding energy of 285.1 eV (labele 1) is attributed to the carbon atoms in the perylene core of the molecule. The carboxylic carbon atoms are represented by the smaller peak at 288.7 eV (labeled 2). The C 1s spectrum is consistent with reported experimental results and theoretical calculations. 40,41 The evolution of C 1s with different PTCDA coverage is shown in Figure 2d. The stable peak



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shape implies that no chemical environment of carbon atoms changes. It is worthwhile to note that there is a 0.4 eV shift of C 1s core-level in Figure 2d. Therefore, the gradually shift caused by PTCDA deposition implies that band bending or interface dipole exists at PTCDA/BP interface. The O 1s spectrum of a multilayer equivalent deposit of 3.2 nm PTCDA on BP is shown in Figure 2e. The O 1s spectrum is also dominated by two peaks (fitting by Lorentzian-Gaussion functions). The peak at lower binding energy (labeled 4) at 531.8 eV is attributed to O= of the anhydride, and the component (labeled 3) at a binding energy of 533.8 eV is caused by the bridging -O- from the anhydride group. The ratio between the 3rd peak and the 4th is about 0.98, which agree with the chemical composition of PTCDA. The O 1s component shifts to higher binding energy with increasing the PTCDA thickness, which is similar to the shift of C 1s spectra, shown in Figure 2f. Based on the XPS measurements, there is no evidence that any chemical reaction is taking place at the PTCDA/BP interface. We continue our discussions on the energy-level alignment and interaction mechanism of PTCDA deposited on the BP surface. Figure 3a shows the UPS spectra of the onset of the secondary photoelectrons, also known as the secondary electron cutoff (SECO). The UPS spectra have been standardly referred to the binding energy with respect to the Fermi level (EF ). Upon incremental deposition of PTCDA, the SECO shifts from 17.20 eV for cleaved BP to 16.55 eV for 3.2 nm PTCDA/BP. The cutoff region in Figure 3a shows an increase of the work function by ∆Φ = 0.65 eV in the 3.2 nm PTCDA treated BP surface, which also means an interface dipole or charge transfer exist.. Charge transfer across the interface is mainly driven by the fact that the electron affinity of the one material is higher than or similar to the ionization energy of the other one. For example, deposition of F4TCNQ, a strong electron acceptor molecule, on metals gives rise to a considerable work function increase, which originates from charge transfer from the metal to F4TCNQ. 55 Charge transfer also occurs at the organic/inorganic interface, including F4TCNQ/ZnO and F4TCNQ/GaN interfaces. 56,57 The electron affinity of PTCDA is 4.2 eV 58 while the ionization energy of BP is 4.0 eV. Therefore, the electron can transfer from BP to PTCDA causing the positive and


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Figure 3: UPS measurement of the (a) secondary electron cutoff region and (b) HOMO region of the PTCDA/BP interface as a function of PTCDA coverage. Binding energy relative to the Fermi level (EF =0 eV) is used in the figure. The UPS cutoff spectra of bulk BP gradually evolved towards the lower binding energy region with increasing PTCDA thickness, suggesting that the work function increase from 4.0 to 4.65 eV after the deposition of 3.2 nm PTCDA. All spectra are measured under the normal emission condition.

negative charges separated across the interface. The charge transfer can also be reflected from the swift decline of the VBM of BP upon the deposition of PTCDA film, as shown in Figure 3b (HOMO region). Upon 0.2 nm of PTCDA deposition, the VBM of BP almost disappear and the HOMO of PTCDA appear at 1.3 eV. However, the effect of BP electronic state at 1.0 eV still can be seen from UPS spectra for 0.2 nm PTCDA/BP. With the increasing the thickness of PTCDA, the VBM of BP feature completely disappear. The diminishing of the VBM of BP feature clearly suggests that electrons flow from the VBM of BP to PTCDA. The HOMO level of PTCDA is measured at 1.85 eV below the cleaved BP VBM. It is worth noted that C 1s core-level shifts (in XPS) and HOMO shifts (in UPS) occur in parallel. It indicates that the charge transfer at PTCDA/BP interfac is a main contributing factor to the band bending, leading



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to energy-level redistribution in the PTCDA thin film. Figure 4 summarizes the relevant energy shifts obtained from both UPS and XPS data. With increasing PTCDA coverage the BP features vanish while the distinct peaks of PTCDA arise. In order to show how fast the BP features disappear, we subtract the background of HOMO region in Figure 4a. Therefore, we can clearly see the evolution of BP features with increasing the thickness of PTCDA. The feature of BP is visible at 0.2 and 0.4 nm PTCDAadsorbed on BP. However, the feature of BP completely disappears at 0.8 nm PTCDA, is shown in Figure 4a. The development of the work function with different PTCDA film thickness, is presented in the up plane of Figure 4b. The shifts of C 1s, O 1s, and P 2p corelevels with increasing coverage are plotted in the bottom plane of Figure 4b. From Figure 4b we can determine the band bending across the PTCDA/BP interface. The evolution of band bending at the BP surface with increasing PTCDA coverage is evident from the slight shift to higher binding energy of the P 2p. The total shifts of the C 1s and O 1s amounts to 0.4 eV which means to the interface dipole or band bending occurring in the PTCDA thin film.

Figure 4: (a) The HOMO region of PTCDA/BP interface after substract the background signals. (b) Work function (top) and XPS core-level line shifts (bottom) as a function of the PTCDA layer thickness. (c) AFM image of cleaved BP substrate. The image size is 10×10 micrometer. (d) AFM picture of 3.2 nm PTCDA on BP surface. The image size is 0.5×0.5 micrometer.


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Figure 4(c-d) shows the AFM topography images for the cleaved BP surface and 3.2 nm PTCDA on BP. As shown in Figure 4c, the substrate BP surface is very smooth with a root mean square (RMS) roughness of 0.35 nm. After exposure in ambient air for 3 hours, many small bump-like structures with a height of 20 nm are observed on BP surface (Figure S4, Supporting Information), which agrees well with previous studies on the ambient oxidation of BP. 18 The BP substrate is completely covered by the 3.2nm PTCDA film, as is shown in Figure 4d. The RMS roughness of PTCDA-adsorbed on BP is about 3.4 nm, indicating a reasonably flat surface. The film now grows in regularly shaped domains with a size of about 10 nm. Upon 3 hours air exposure for PTCDA/BP, no bump-like structure is observed. To determine the chemical composition of the samples during the ambient treatment, we carried out XPS measurements for the cleaved BP and PTCDA/BP samples (Figure S4, Supporting Information). Upon air exposure, oxidation of BP is confirmed by the appearance of phosphorus oxide species at binding energy 134∼135 eV, labeled as Px Oy . However, no oxidized phosphorus peaks are observed for BP with 3.2 nm PTCDA protection layer after air exposure. This indicates that PTCDA thin film can protect BP surface against ambient oxidation. To further understand the interfacial interaction between PTCDA and BP, we performed first-principles calculations on the hybrid organic-inorganic PTCDA/BP system. To gain the best adsorption site of PTCDA molecule adsorbed on BP, we choose three structures of the PTCDA/BP hybrid system: PTCDA molecule along the zigzag direction, armchair direction, and diagonal direction. The geometry parameters of the three hybrid PTCAD/BP systems are shown in Figure S5, Supporting Information. Note that there is a critical distortion in the PTCDA/BP system along the zigzag direction. The diagonal PTCDA/BP has lower total energy than other directions from our optimized calculations. This implies the diagonal PTCDA/BP configuration is more stable than others configurations. Our results indicate that there is no chemical bond formed between PTCDA and BP, which is well consistent with our core-level photoemission measurements. The electronic band structures of the



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PTCDA/BP hybrid system along the diagonal direction are presented in Figure 5. As clearly seen from Figure 5a, the dispersion of the CBM and the VBM are very similar with the bulk BP. However, a flat-band emerges within the band gap, which is the localized electronic state and arises from the LUMO of PTCDA. Such flat-band can serve as local scattering centers, which reduce the mobility of charge carriers and enhance the recombination of photo excited electron-hole pairs.

Figure 5: (a) Band structure of PTCDA/BP composite, and the optimized structure of PTCAD molecule along the diagonal direction. The electronic wave function of three energy bands near the Fermi level at Γ (0, 0, 0). The electronic wave function of (b) VBM, (c) Flat-band, and (d) CBM from top view (up) and side view (down), respectively.

To gain more information of the interfacial electronic properties of PTCDA/BP system, the energy states at Γ point are explored by analyzing their charge distribution and wave functions. The electronic wave functions of the VBM, mid-gap state, and CBM are presented in Figure 5(b-c), respectively. Clearly, the VBM is dominated by pz orbitals which are antibonding along the x direction and non-bonding along the y direction. It is worthwhile mentioning that the charge distribution along the armchair direction completely disappears (Figure 5b and 5d). This indicates that charge at the surface of BP with PTCDA locates


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on the armchair direction, leading to electron or hole cannot transport along the zigzag direction. This decrease is mainly due to the hybridization between px orbitals in BP and σ orbitals in PTCDA. For the mid-gap state, the charge distribution is localized in PTCDA molecule which is contributed to LUMO of PTCDA (Figure 5c). Meanwhile, the CBM consists of pz orbitals which are non-bonding along the z direction and bonding along the y direction (Figure 5d). In order to estimate charge transfer between the PTCDA molecules and BP, we adopt Bader charge analysis method, using the Bader program of Henkelman’s group. 59,60 According to our Bader charge analysis, the charge transfer between PTCDA and monolayer BP is about 0.12 electron per molecule from BP to PTCDA. To examine the molecular modification effects on thicker BP layers, we also explored the electronic properties of the bilayer BP, modified with PTCDA. The charge distribution of the bilayer BP with PTCDA is similar to that of the monolayer (Figure S6, Supporting Information). However, between PTCDA and bilayer BP system, the charge transfer increase to 0.34 electron per molecule. Such electron transfer indicates that the PTCDA-adsorbed BP system is typical p-type doping manner, similar to TCNQ-adsorbed on BP. 25 Furthermore, with increasing the thickness of BP, the charge transfer may also significantly increase. However, based on our DFT simulations, we find the charge transfer between PTCDA and monolayer BP interface is minute, consistent to Zhao’s report. 42 Obviously, the DFT results of monolayer BP is in contradiction with our spectra results. It might origin from the significant difference of work function and band gap between the single layer BP and the bulk BP. From our UPS spectra, we can conclude that the work function of bulk BP is 4.0 eV. However, the work function of single layer BP is 5.2 eV. The work funtion as well as the VBM and CBM are dependent with the thickness of BP. 61 There is a significant shift of VBM and CBM upon thickness variation, which may be useful for tuning Schottky barrier to promote charge injection efficiency. For further understanding of the PTCDA/BP interface formation, the energy-level dia-



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gram of this hybrid organic/inorganic semiconductor structure is plotted in Figure 6. The position of the HOMO level of PTCDA is determined to be 1.9 eV below the Fermi level from the UPS spectra (Figure 3b). Meanwhile, from the SECO of the UPS spectra, the work function of PTCDA on BP is found to be 4.65 eV. The cleaved BP substrate showed the VBM of 0.05 eV and work function of 4.0 eV. Using a simple band picture where the LUMO level of PTCDA is determined by the optical gap (2.2 eV) to HOMO; 58 similarly, the CBM of BP is the sum of VBM and BP band gap (0.35 eV). 62 The configuration of PTCDA/BP energy-level alignment facilitate electron transfer from BP to PTCDA film because of the barrier between CBM of BP and LUMO of the PTCDA film. The higher work function of the hybrid organic-inorganic PTCDA/BP system compared with the cleaved BP substrate can result in the buildup of interfacial dipole barrier.

Figure 6: Schematics for the energy-level diagrams of PTCDA thin films on cleaved BP. (a) The left plane represents the DFT calculated band-level alignment of BP from one layer (1 L) to five layers (5 L). (b) The right plane represents the UPS measured band-level alignment between PTCDA and BP interface. ∆Φ = 0.65 eV, owing to charge transfer from the BP to PTCDA.

The calculated band alignment is also plotted in Figure 6a. Clearly, the calculated work function (5.2 eV) and band gap (1.5 eV) of BP are larger than the measured work function 16 ACS Paragon Plus Environment

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(4.0 eV) and band gap (0.3 eV). Furthermore, the CBM of few layers BP is away from the LUMO of PTCDA. For single layer (1 L) BP, there is a large electron injection barrier (> 0.8 eV) between CBM and LUMO which prevent excited electrons by some photo-excited process from PTCDA to BP. The VBM of bulk BP is close to the LUMO of PTCDA, which means that this heterojunction facilate the excited electrons transfer across the interface in some optoelectronic applications. From the energy-level alignment of PTCDA/BP interface, the electron acceptor has an electron affinity higher than the work function of the BP substrate. This remarkable change in the work function and band gap with the number of layers offers a practical route to tune the injection barrier height, which can lead to more efficient electron injection and electron transport across the n-type organic semiconductors. Actually, the surface potential, screening effects, Fermi level, and work function show strongly thickness-dependent in low-dimensional materials. In graphene, the surface potential decreases exponentially with the increasing the number of graphene layers. 63 For example, the surface potential of the MoS2 films can increases by 0.15 eV when the number of layers increased from 2 to 12 L. 64 However, the real contacts between low-dimensional BP and organic molecules have not been observed experimentally, which may mainly limit to the in situ growth and unstable of BP thin film. On an organic/inorganic semiconductor interface, two independent effects must be considered as causes for the change of work function (∆Φ): Band bending (∆ΦBB ) at the surface of the inorganic semiconductor and the formation of a more localized interface dipole (∆ΦID ). For the PTCAD/BP interface, these effects can be isolated quantitatively by close examination of the UPS spectrum. Electron transfer from the bulk BP to the PTCDA molecules generates both band bending within the semiconductor as well as an interface dipole with an effective length (def f ). 56,57 Then, the ∆Φ is given by

∆Φ = ∆ΦBB + ∆ΦID


(1)


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the band-bending contribution (∆ΦBB ) is determined by the Schottky-depletion-layer approximation, ∆ΦBB =

δq 2 2ε0 εND

(2)

and the interface dipole contribution (∆ΦID ) can be expressed

∆ΦID = e

δq def f ε0

(3)

where δq is the area charge density on the acceptor, ND is the donor concentration in the inorganic semiconductor, ε0 is the vacuum permittivity, ε is the dielectric constant of the inorganic semiconductor. To solve Eqs.(1)-(3), the appropriate parameters for our PTCDA/BP sample are: def f = 3.8 Å(estimated by our DFT calculation), ε = 14.5 (refer to Nagahama’s et al result 65 ), ε0 = 8.85 × 10−12 F/m. The solution of Eqs.(1)-(3) is summarized in Figure 7. The charge transfer giving rise to ∆Φ strongly changes with ND as show in Figure 7a, the BP doping level used in the experimental (1016 -1019 m−3 ). 66 With low ND , one obtains a thick depletion layer of the depletion layer and a large surface band bending (∆ΦBB ), leading to only minute electron transfer necessary. However, with high doping concentrations, ∆ΦBB is largely supressed and the total potential change ∆Φ is dominated by the interface dipole ∆ΦID . Thus, further increasing ND yields no change in the necessary δq, since ∆ΦBB is only determined by ND . In our experiments, the downward band bending ∆ΦBB in BP is 0.10∼0.15 eV by measuring the P 2p core-level shift in XPS spectrum. Except the band bending in PTCDA, the interface dipole ∆ΦID is 0.30∼0.35 eV. Therefore, the main contribution is interface dipole in organic semiconductor PTCDA in the total potential change ∆Φ.


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(a)

(b)

0.050

BP crystal

N

0.010

0.5

D 0=

010 1

9

0.005

0(c

m -3 )

ND 0

=01

0.001

5 ´ 10- 4

0 18 0(cm -3 )

ND 0=

010 17 0(cm -3 )

0.4

0.3

0.2

deff

0.1

ND0=010 16 0(cm -3)

1 ´ 10- 4

Electric0potential0(V)

-3 Charge0transfer0(C/m0)

0.6

PTCDA molecules

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1016

1018

1020

-3 Doping0concentration0N00(cm00) D

1022

ΔΦID

ΔΦBB

0.0

1014

- 3. ´ 10- 7

- 2.5 ´ 10- 7

- 2. ´ 10- 7

- 1.5 ´ 10- 7

- 1. ´ 10- 7

- 5. ´ 10- 8

0

Distance0from0BP0surface0(m)

Figure 7: (a) Calculated molecular acceptor-induced charge transfer δq to reach a given total ∆Φ versus donor concentration in the inorganic. Charge transfer to the acceptor depletes the BP crystal, which is modeled in Schottky-depletion-layer approximation. (b) Corresponding electrical potential curves for selected points of (a). The quadratic (linear) behavior of the potential inside (outside) the crystal are clearly visible.

Conclusions In summary, a clean surface is obtained through cleaving BP in vaccum, which exhibits a welldefined LEED pattern. We have demonstrated that the bulk BP has a feature surface state and anisotropic effective mass by performing ARPES measurements. The PTCDA molecule was then thermally deposited on the clean surface of BP and the evolution of the electronic structures with film thickness was investigated with UPS and XPS. A sharp decrease of the VBM peak of BP on the deposition of PTCDA demonstrates the facilitated electron transfer from BP valence band to PTCDA. A combination of photoelectron spectroscopy and an electrostatic model is used to explain the band alignment, band bending, and interface dipole of PTCDA/BP interface. The P 2p, C 1s and O 1s peak shift in the XPS and corresponding DFT calculations indicate that interactions of PTCDA molecules with the substrate BP are vdW interaction. Moreover, a stable and compact PTCDA thin film was determined with AFM at the surface of BP. Thus, the hybrid interface opens a new avenue to protection the surface of BP from degradation, which is crucial for novel optoelectronic devices. These results are likely general for n-type organic semiconductors adsorbed on BP and point the way towards a rational design for efficient level engineering at hybrid organic/BP interfaces. 19 ACS Paragon Plus Environment


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Acknowledgement We thank the financial support by the National Natural Science Foundation of China (Grant Nos. 51173205 and 11334014). Y.G. acknowledges the support by the National Science Foundation (Grant Nos. DMR-1303742 and CBET-1437656). C.Wang was supported by the Fundamental Research Funds for the Central Universities of Central South University (2016zzts049).

Supporting Information Available The Supporting Information is available free of charge. • The survey XPS spectrum of the cleaved BP and PTCDA/BP systems (Figure S1 and S2); The LEED patterns (Figure S3); AFM images and P 2p core-level XPS of bulk BP upon exposure air (Figure S4); Optimization structural informations about PTCDA/BP systems, as obtained by DFT calculations (Figure S5 and Table S1); The electronic wave function of PTCDA on bilayer BP (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org/.

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