Interface Electronic Structure between Au and Black Phosphorus

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Interface Electronic Structure between Au and Black Phosphorus Baoxing Liu, Haipeng Xie, Dongmei Niu, Han Huang, Can Wang, Shitan Wang, Yuan Zhao, Yuquan Liu, and Yongli Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03146 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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The Journal of Physical Chemistry

Interface Electronic Structure between Au and Black Phosphorus Baoxing Liu1) Haipeng Xie1),* Dongmei Niu1) Han Huang1) Can Wang1) Shitan Wang1) Yuan Zhao1) Yuquan Liu1) Yongli Gao1),2),* 1) Hunan Key Laboratory for Super-microstructure and Ultrafast Process, College of Physics and Electronics, Central South University, Changsha, Hunan 410012, P. R. China 2) Department of Physics and Astronomy, University of Rochester, Rochester 14627, USA

Abstract The interface electronic structure between Au and black phosphorus have been investigated with ultraviolet and X-ray photoemission spectroscopy (UPS and XPS). We observed that Au clusters form at the initial Au deposition and an interface dipole is observed at the interface between Au and BP. The outermost BP lattice is destroyed and unbonded P appears, which is due to the formation of metallic Au by the deposition of more than 8 Å Au. The unbonded P is surface segregated at 8~15 Å and it is covered with further increasing of the Au thickness. These observations reveal the processes in Au/BP interface and provide possible directions to fabricate high performance Au/BP-based device.

* Correspondence: [email protected]; [email protected]

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1. Introduction Two-dimensional materials have attracted extensive research interest due to their remarkable physical, chemical, mechanical, and electrical properties. The first discovered 2D material graphene has exhibited extremely high carrier mobility and a wealth of fundamental physical properties.1~5 However, graphene has zero-band gap, impeded the development of its potential functionalities and applications.6 Black phosphorus (BP) has received great attention due to its excellent optical and electronic properties.7,8 Elemental phosphorus has three main allotropes, namely, white, red, and black phosphorus. The most thermodynamically stable at ambient temperature and pressure of them is BP. Bulk BP has a layered structure with eight atoms per unit cell and the atomic layers stack via weak van der Waals interactions like graphite. Compared with the zero gap of graphene, the significant difference is BP has intrinsic direct band gap which depends on its thickness and changes from 0.3 eV in the bulk to 2.0 eV of single layer BP (phosphorene).9~12 In addition, strain and electric fields can modified the band gap of phosphorene.13,14 BP possesses good electrical properties with electron and hole mobility of 220 and 350 cm2⋅V−1⋅s−1 at room temperature.15,16 In addition, BP possesses distinctive anisotropic physical properties, which stem from the non-equal bond angles and bond strengths in its crystal structure along its orthogonal in-plane directions.17~19 During the past four years, the physical and chemical properties of BP have been intensively studied, and various BP based devices have been demonstrated,20~33 such as field-effect transistor (FET),25~27 tunable infrared

photo-detectors,31,32

thin-film

solar

cells,34,35

optical

polarizers,36

nano-electromechanical oscillators,37 thermal management38 and gas sensors, 39 etc. Despite numerous achievements in the device application of BP, some challenges facing BP applications.40~42 Compare to graphene, the systematic studies of BP are still lacking. There is a grand task to take and a long way to go for the exploration of BP toward practical application. Therefore, understanding the exact role of each interface in the BP-based devices is a task of top priority. In general, Au has been widely used as electrode, the typical structure of Au/BP had been used to build an 2


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effective planar device with different function. Miao et al.26 reported black phosphorus field-effect transistors, using Au as source/drain contacts and with an ultrashort channel length, exhibits excellent properties. Li et al.27 have compared Auand Sc- contacted BP transistors, result display that Au is more stable and there is almost no impurity residue in Au/BP contacts. The hysteresis BP transistors with gold contacts is significantly smaller than scandium. Hu et al.43 have investigated Au adsorption on phosphorene using first principles density-functional theory and found that the spin polarized band structures appeared in Au doped phosphorene system. Despite the interesting observations, how the electronic structure at Au/BP interfaces is unclear, and it is necessary to investigate the electronic structure in order to understand and explore the device behavior. In this article, the electronic structure at the Au/BP interface has been studied by using ultraviolet UPS and X-ray photoemission spectroscopy (UPS and XPS). The results show that the Au clusters form at the Au/BP interface and BP lattice is destroyed by deposition of more than 8 Å Au. An interface dipole is observed at the Au/BP interface. These studies provide some insight into understanding the Au/BP-based device.

2. Experimental method Au films were deposited from a rod heated by electron beam bombardment in the MBE chamber whose based pressure was better than 1.5×10-10 mbar. A quartz crystal microbalance has been used to monitor the thickness of Au during deposition process. In the preparation chamber, a scotch tape-based mechanical exfoliation method was used to get a freshly cleaved bulk BP crystal at pressure of ~2×10-8 mbar. Then, it was transferred immediately to the analysis chamber and the quality was verified by XPS and low energy electron diffraction (LEED). The analysis chamber equipped with a Microwave UV Light Source (He I, hν = 21.22 eV), and a monochromatic Microfocus X-ray Source (Al Kα, hν = 1486.7 eV). The UV light spot is about 1 mm in diameter.44,45 A total energy resolution is about 70 meV for the present UPS as 3



determined from the Fermi edge of clean Au (111).46,47 For XPS measurement, the X-ray source was operated at 100 W with 40 eV pass energy. In our system, we use monochromatized Al Kα X-Rays for excitation and its best resolution is 0.44 eV FWHM (full width at half maximum). The measured FWHM is given by the convolution of that of the light source and electron energy analyzer. In order to obtain the optimized resolution and throughput, the value of the FWHM of the electron energy analyzer should be close to that of the Al Kα X-Ray source. We used 40 eV pass energy, and the FWHM of the electron energy analyzer is about 0.45 eV. More detailed descriptions can be found in Ref. [44-47]. All measurements were taken at room temperature unless otherwise specified.

3. Results and discussion Layered crystal structure of bulk black phosphorus is shown in Fig. 1(a), the layer-to-layer spacing of BP approximately 5 Å. The XPS full scan spectra displays only phosphorus, as shown in Fig. 1(b). The P 2p peak is located at ca. 130.32 eV and no peak or shoulder detected at higher binding energies, confirming that no BP oxidation occurred after cleaving in the preparation chamber. As shown in Fig. 1(c), a clear LEED pattern can be seen, indicating that the BP surface has been prepared successfully by mechanical exfoliation in ultrahigh vacuum.

Fig. 1. (a) Layered crystal structure of bulk BP. (b) XPS full spectra of black phosphorus. (c) LEED patterns of bulk black phosphorus.

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In Fig. 2, the evolution of the UPS spectra is presented as a function of the thickness of Au deposited on BP. In order to more intuitive and clear vision, all the spectra will be normalized to the same height. As shown in Fig. 2(a), the work function (WF) is obtained from the difference between the photon energy and the cut-off energy. Linear extrapolation method determines the cut-off energy. The WF of BP is 4.24 eV, which is agree with the previous reports.48,49 The WF at 1 Å Au thickness is 4.15 eV, indicates that there is an interface dipole of ca. 0.09 eV at the interface between Au and BP. After the deposition of 15 Å Au, the WF decreases at first, approaching to the minimum 3.77 eV and the total shift is 0.47 eV. With further increasing of the Au thickness, the WF increases slowly and saturates at 4.45 eV at 120 Å. The valence band maximum (VBM) of BP is 0.1 eV, as shown in Fig. 2(b). Shown in the inset of Fig. 2(b) is the enlarged view of the VB edge region near EF. At first, the BP VBM locates at 0.1 eV below the EF. It shifts toward the EF with the Au deposition of up to 15 Å. The metallic Fermi level cutoff starts to appear as Au thickness reaches 8 Å.

Fig. 2. Evolution of UPS spectra as a function of Au thickness deposited on BP. (a) Cut-off region. (b) VB edge region. Inset: the local enlarged view of the VB edge region near EF.

To find out the reasons that caused the change of WF and VB with increasing Au thickness, we investigated the chemical characteristics using XPS. The Au 4f 7/2 and P 2p core levels with the increasing of Au coverage are shown in Fig. 3. All the spectra 5



were normalized to the same height. As shown in Fig. 3(a), the Au 4f7/2 peak is located at 84.75 eV for 1 to 4 Å Au thickness. With the subsequent deposition, the Au 4f7/2 peak shifts toward lower BE and reaches the minimum BE of 84.26 eV at 90 Å. This behavior is similar to that observed for the growth of Au clusters on an inert substrate (as graphite). Berry et al.50,51 measured the evolution of the electronic state of size-selected clusters deposited onto amorphous carbon. The largest shift 0.7 eV was observed for Au clusters composed by five atoms only. Increasing the cluster dimension, the Au 4f peak moved toward lower binding energy values. The same behavior is observed in our case, suggesting the presence of gold clusters in Au/BP interface. The size of Au clusters increases with the Au film thickness increases. As shown in Fig. 3(b), the position of the P 2p peak of cleaved BP is located at 130.32 eV and stays basically unchanged with the increasing Au thickness until 4 Å. With the subsequent deposition, it shifts toward lower BE and saturates at 129.98 eV at 60 Å. This is consistent with the appearance of the Au Fermi edge at the same coverage in the UPS spectrum. As shown in Fig. 3(c), the relative shifts of Au 4f7/2 and P 2p core levels at the interface deduced from XPS. The total shift of Au 4f 7/2 core level is about 0.49 eV and that of P 2p is about 0.34 eV.

Fig. 3. Evolution of XPS spectra as a function of Au thickness deposited on BP. (a) Au 4f7/2, (b) P 6


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2p, (c) The relative shifts of the core levels at the Au/BP interface.

We have also considered that the charge transfer occurred between Au and BP. According to the P 2p core level shift to lower binding energy, it can be speculate that the charge transfer from the BP substrate to the Au clusters. Huang et al reported that the charge transfer from the reduced TiO2 substrate to the Au clusters due to the Au 4f shifts to higher binding energy when the Au thickness increases from 1.5 Å to 2 Å,52 which is not consistent with our result as shown in Fig. 3(a). Otherwise, Lu et al reported that the excited holes will move toward the gold surface and then annihilate with the electrons from gold.53 Thus, we can exclude the charge transfer between BP and Au. To further learn the process of deposition Au on BP, we have analyzed the P 2p spectra for different thickness. The P 2p core level is a doublet consisting of P 2p3/2 and P 2p1/2 components with an energy separation of 0.87 eV and a branching ratio of 2,54 as shown in Fig. 4(a). For convenience, only the P 2p3/2 peak is analyzed in detail. For the P 2p3/2 spectra, only one peak is observed at 130.27 eV before Au deposition, which can be attribute to lattice phosphorus components in BP.55 The position of the Au 4f 7/2 and P 2p peak stays unchanged with the increasing Au thickness until 4 Å. After the deposition of 8Å Au, the other peak of P 2p3/2 was observed at 129.88 eV, corresponding to the unbonded P atoms.56 First-principles calculations reveal that, in pristine BP, the energy of a phosphorus atom replaced by an Au atom is 1.647 eV, the energy of Au atoms adsorbed to the BP surface are -1.938 eV.57 The metal electrode material deposition on device usually lead to additional chemical disorders, ‘high-energy’ metal atom or cluster bombardment and strong local heating to the contact region, which damage the crystal lattice at or near the interface.58-60 The core level spectra suggest that in the initial stages of deposition, there is little Au interaction with the BP film, and the crystalline structure of BP stays intact. With increasing Au coverage, metallic Au layer forms and the energy released in the formation ignites an interface disruption between Au and BP. As a result, the outermost BP lattice is destroyed by deposition of more than 8 Å Au atomic and some 7



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unbonded P atoms appeared. The unbonded P component increases as the thickness increases from 8 to 15 Å. Combining with the above UPS data, it is logical to deduce that the change of WF is caused by the formation of unbonded P atoms. These observations reveal that interfacial disruption between the metal layer and BP film is a serious problem that may hurt the device performance.

Fig. 4. (a) P 2p spectra evolution of Au thickness deposited on BP. (b) The function relationship between the photoelectron intensity and the thickness of Au film. The dotted (dashed dotted) line marks the slopes for the P 2p (Au 4f7/2) before the Au deposition of 8 Å.

In order to further analyze the process of deposition Au on BP, we concentrate on the XPS intensity attenuation by the Au overlayer. The intensity attenuation of photoelectrons after passing through an Au overlayer (IAu)61 IAu= I60⋅[1-exp (-d/λAu)]

(1)

or that from the BP substrate (IBP) IBP= I0⋅exp (-d/λBP)

(2)

Where I60 is the photoelectron intensities from the 60 Å Au overlayer and I0 is the photoelectron intensities from the BP substrate without the Au overlayer, d is the Au overlayer thickness, and λAu (λBP) is photoexcited electrons mean free path in Au (P). The intensity is on account of the elements peak area which was obtained by fitting Gaussian-Lorentzian peaks, then normalization with corresponding relative atomic sensitivity factors. Based on Eq. (1) and (2), the attenuation of the XPS peaks 8


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of the Au overlayer or BP substrate can be used to figure out the growth mode of Au deposition onto BP substrate.62,63 Fig. 4(b) shows that the intensities of the photoelectrons ejected from the Au 4f7/2 decrease slowly when the Au thickness increase up to 8 Å, and attenuate sharply with further increasing the thickness of Au film, which can be attributed to the effect of Au clusters, in agreement with the above discussion. At lower Au coverages, i.e. Θ < 8 Å, the formation of Au clusters exposes a larger surface area of the underlying BP film than a uniformly distributed Au film, thus the photoelectron intensity decreased slowly at the beginning, the small slope of the Au 4f7/2 as shown in the dash dotted line in the figure can be used to confirm. By fitting the experimental data of Θ < 8 Å to obtain a linear slope of the curve. However, on the top of the BP substrate, the Au clusters percolate and a continuous metal Au surface coverage is formed after enough Au cover thickness, i.e. Θ > 8Å. In Supporting Information Fig. S1~ S3, the AFM image show that Au clusters form at the initial Au deposition, the size of Au clusters increases with the Au film thickness increases to 8 Å. After enough Au cover thickness, i.e. Θ = 15 Å, disruption becomes sever as Au clusters coalesce into a full layer. As previously mentioned, the P 2p spectra was fitted by with two peaks, correspond respectively with cleaved BP and unbonded P. For the photoelectrons ejected from the BP, the intensities decrease sharply when the thickness of Au increase up to 8 Å, and attenuate gradually when the thickness of Au increase from 8 to 15 Å, a relatively large slope (dash line) for the BP as shown in Fig. 4(b). The data lead to an electron escape depth of ~ 10 Å, not agrees with an expected value.64 It can be attributed to the outermost BP flake lattice is destroyed by the deposition of more than 8Å Au. At the initial deposition, i.e. Θ < 8 Å, as Au clusters, the contact to BP is minimal and little disruption takes place, the molecular structure of BP stays intact. After enough Au cover thickness, i.e. Θ > 8 Å, disruption becomes sever as Au clusters coalesce into a full layer and the metallic Au forms, then the outermost BP lattice is destroyed and turn to unbonded P. With the subsequent deposition, the BP signal is reduced because of the photoelectrons are suppressed by the Au coverage. As 9



shown in Fig. 4(b), the photoelectrons intensities of the unbound P increase from 8 to 15 Å, which can be attributed to some unbonded P atoms segregate at the surface. The surface segregated unbonded P will further reduce the WF and reach to the minimum after the deposition of 15 Å Au, as mentioned in discussing the UPS data. The photoelectrons intensities of unbonded P attenuate sharply and the WF increases again, indicating that the amount of unbonded P is the most at 8~15 Å. The interaction between Au and bulk BP has been investigated experimentally, which should contain the most of the interactions between Au and a van der Waals surface such as that of BP. Future investigations on the electronic structure of Au/few-layer BP interface may provide further details of contributions from sub-surface van der Waals layers, for example, more energy level shift because of the wider band gap.

4. Conclusions In conclusion, we have studied the electronic structure of the Au/BP interface using UPS and XPS. The results show that Au clusters form at the Au/BP interface during the initial Au deposition. The unbonded P appears when the outermost BP lattice is destroyed at the deposition of more than 8 Å Au. The unbonded P is surface segregated at 8~15 Å, which lead to the reduction of the work function. As Au thickness further increases, the amount of surface segregated P reduces due to the coverage of Au film, and the work function increases again. An interface dipole is observed at Au/BP interface. Those observations provide important new information to improve the performance of Au/BP-based devices.

Supporting information AFM images of 4 Å, 8 Å, and 15 Å Au deposited on BP.

Acknowledgments We thank the financial support by the National Natural Science Foundation of China 10


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(Grant Nos. 11334014 and 51173205). H.X. acknowledges the support by the Natural Science Foundation of Hunan Province (Grant No. 2018JJ3625). Y.G. acknowledges the support by the National Science Foundation (Grant Nos. DMR-1303742 and CBET-1437656).

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Interface Electronic Structure between Au and Black Phosphorus

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