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Exploring the high stability of NEA GaN nanowire photocathodes by activation methods: First principles
Applied Surface Science 508 (2020) 145250
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Exploring the high stability of NEA GaN nanowire photocathodes by activation methods: First principles
T
⁎
Lei Liu , Feifei Lu, Jian Tian Department of Optoelectronic Technology, School of Electronic and Optical, Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
A R T I C LE I N FO
A B S T R A C T
Keywords: GaN nanowire NEA surface 3Cs-3Li-NF3 activation Residual gas adsorption Photoemission properties First-principles
In order to investigate the effect of different activation modes on the stability of GaN nanowire photocathodes, we studied the system stability and electronic properties of nanowire surface activated by 3Cs, 3Cs-O, 3Cs-NF3 and 3Cs-3Li-NF3 methods after adsorption of residual gases (CH4, CO, CO2 and H2O molecules) by first-principles. The results show that the residual gas is more readily adsorbed on the activated surface than on the pure P-type surface. The 3Cs-3Li-NF3 activation mode can significantly reduce the work function and electron affinity of the GaN nanowire cathode. Even though residual gas can damage the electronic properties of the activated surface, it still has better photoemission characteristics than the pristine P-type surface. The introduction of Li atoms is more favorable for the higher stability of 3Cs-NF3 activated photocathode. This study is conducive to exploring ways to improve the lifetime of nanowire photocathodes from the activation modes.
1. Introduction Since GaN nanowires have quantum size effects and surface effects, they have been widely used in devices such as photodetection, photocatalysis, photovoltaics, lasers, and sensors, exhibiting better optoelectronic properties than devices based on GaN bulk materials [1–5]. At present, the research on vacuum type nanowire photocathode is also progressing steadily, Zou et al. have preliminarily prepared and successfully activated nanowire array photocathode [6–9]. Activation methods such as Cs/O activation can reduce the electron affinity of surface, and even achieve a negative electron affinity (NEA) surface, which greatly improves the quantum efficiency of the photocathode [10,11]. It is well known that the current activation methods for photocathodes mainly include the following: Cs activation [12], Cs-O coactivation [13,14], Cs-NF3 coactivation [15], Cs-Li-NF3 coactivation [16]. The commonly used NEA activation mode is Cs-O activation, while the Cs-NF3 activated photocathode has high stability, and the introduction of Li atoms further improves the lifetime of the photocathode [16–18]. NEA photocathode is also an important component of vacuum electron source, whose stability/lifetime and vacuum degree directly restrict the development of large-scale physical detection equipment such as linear accelerator, free electron laser and fourth-generation light source [19]. Therefore, it is important to study how to improve and maintain the performance of NEA GaN photocathodes. The residual
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gas in the vacuum chamber not only reduces the degree of vacuum, but different types of vacuum residual gas also have various effects on the stability of the cathode [20]. It has been investigated that the quantum efficiency of NEA photocathodes exposed to O2 and CO2 is continuously attenuated under ultra-high vacuum conditions [21]. More importantly, the residual gas exhibits lower damage to the Cs-NF3 co-activated NEA surface than to the Cs-O activated surface, so the Cs-NF3 activated NEA cathode has higher stability [22]. In order to study the stability of NEA GaN nanowire photocathodes activated by four activation methods, we calculated the adsorption energy, work function and dipole moment parameters of NEA GaN nanowires adsorbed by CH4, CO, CO2 and H2O based on the first principle. The results have important implications for the practical application of NEA GaN nanowire photocathodes in ultra-high vacuum environments. 2. Computational details All total energy and electronic structure calculations are based on the first principle software (CASTEP) of density functional theory (DFT) [23,24]. Generalized gradient approximation PBE schemes and pseudopotential methods are used to deal with exchange-related interactions [25]. The energy cutoff of the plane wave expansion is set to 400 eV. The integration of the Brillouin region is performed by applying a Gamma-centric Monkhorst-Pack scheme with 2 × 2 × 4 k-points.
Corresponding author. E-mail address: [email protected] (L. Liu).
https://doi.org/10.1016/j.apsusc.2020.145250 Received 8 October 2019; Received in revised form 5 December 2019; Accepted 1 January 2020 Available online 03 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
Applied Surface Science 508 (2020) 145250
L. Liu, et al.
20 Å vacuum layer is used to avoid interaction between adjacent nanowires. All models have been optimized until the convergence accuracy is below 2 × 10−6 eV/atom, the interatomic force is less than 0.01 eV/s, the stress is less than 0.1 GPa, and the maximum atomic displacement during the iteration is less than 0.0002 nm. Detailed modeling of NEA P-type GaN nanowires with doping elements being Mg atoms can be found elsewhere [26]. The initial crystal parameters of wurtzite GaN were set to a = b = 3.189 Å, c = 5.185 Å. According to the study of GaN nanowire diameters and crystal planes reported by Wang and Carter et al., it is found that [0 0 1] oriented nanowires with hexagonal cross sections are more advantageous than [1 0 0] and [1–10] oriented nanowires with triangle cross sections [27,28]. This is mainly because the considered [0 0 1] oriented nanowires have non-polar (1 0 0) side facets, appearing as hexagonal sections, and each of Ga and N surface atoms has one dangling bond. [1–10] oriented nanowires are surrounded by (1 1 2), (−1 −1 2) and (0 0 1) sides, while [1 1 0] oriented nanowires are surrounded by (1 −1 2), (−1 1 2) and (0 0 1) sides facets, appearing as triangle sections, and the surface Ga and N atoms have more than two dangling bonds. Therefore, the (1 0 0) surface of GaN nanowires with [0 0 1] orientation is selected as the research target [29]. The obtained GaN nanowire model has six (1 0 0) crystal planes and exhibits a hexagonal cross section. To simplify the computational complexity, only a specific (1 0 0) plane is selected to perform the adsorption process of the residual gas, and the dangling bonds on other surfaces are all passivated by hydrogen atoms to eliminate surface effects. Although the average relative stability of the nanowires increases as the diameter of the nanowires increases, due to the limitations of computing equipment, we set the diameter of the GaN nanowires to 9.5 Å. The change of electronic characteristics after adsorption can still predict experimental observations [30]. Top and side views of the NEA GaN nanowire activated by P-type, 3Cs, 3Cs-O, 3Cs-NF3, 3Cs-3Li-NF3 are shown in Fig. 1. The ratio and site selected of the 3Cs, 3Cs/O, 3Cs/NF3 and 3Cs/3Li/NF3 activation atoms is based on Refs. [7,26,31]. Residual gases (CH4, CO, CO2, H2O) that may be present in the ultra-high vacuum system are adsorbed to each activated (1 0 0) surface [32].
process on the surface of the GaN nanowires, and the obtained adsorption model is stable. The H2O molecule is the most easily adsorbed gas on both the P-type surface and the activated surface, while the adsorption of CH4 gas is more difficult. The order of difficulty in forming the four activation surfaces is: 3Cs > 3Cs-NF3 > 3Cs-3LiNF3 > 3Cs-O activation. In other words, the insertion of Li atoms into the surface of the nanowire can promote the formation of the 3Cs-NF3 activation mode. Compared to a pure P-type surface, the surface of the nanowire after activation is more easily adsorbed by residual gas, so the vacuum environment of the NEA cathode is more demanding. In particular, the adsorption energy of the surface activated by 3Cs-O is most increased after adsorption of residual gas, and its stability is most susceptible compared to all NEA surfaces. However, the introduction of Li atoms makes the 3Cs-NF3 activated NEA surface more stable, reducing the possibility of adsorption by residual gases. The stability of the photocathode is mainly determined by the surface electronic properties derived from the arithmetic between the system levels, such as bandgap (Eg), electron affinity (χ), work function (φ), and dipole moment (μ). There is a close relationship between the above parameters, mainly from the vacuum energy level (Evac), the conduction band minimum energy level (ECBM), the Fermi level (EF) and the valence band maximum energy level (EVBM). The band gap, electron affinity and work function of the nanowire can be obtained by χ= the following equations: Eg = ECBM − EVBM , Evacuum − ECBM , φ = Evacuum − EF , and the results are shown in Fig. 3(a), (b) and (c). The calculated bandgap of bulk wurtzite GaN is 1.43 eV, which is much smaller than the experimental value (3.4 eV). It is believed that the bandgap calculated by the DFT-GGA and PBE methods is always underestimated [34,35]. However, it does not affect the qualitative analysis of the electronic properties of the residual gas adsorption surface [36]. Undoubtedly, the adsorption of the residual gas can cause the band gap, the electron affinity and the work function of the activated surface to rise to some extent, which largely affects the photoelectric performance of the NEA photocathode. For NEA photocathodes, we want to reduce the electron affinity and work function as much as possible to extend their lifetime and improve their stability. The four activation methods all reduce the band gap of the P-type surface, which makes the photoelectrons on the valence band easier to transiti to the conduction band. The electron affinity and work function also show the same trend, which means that the possibility of photoelectrons escaping from the conduction band to the vacuum is increased, which is beneficial to the improvement of the cathode photoelectric performance. It can be seen from Fig. 3(b) that only 3Cs-O, 3Cs-NF3 and 3Cs-3Li-NF3 can activate the surface of the GaN nanowire from the positron affinity to NEA. However, relying on pure Cs activation can only reduce the electron affinity of the P-type surface, so the performance of a
3. Results and discussion The system stability of different residual gas-adsorbed GaN nanowire models is evaluated by calculating the average adsorption energy. The adsorption energy is defined according to the following formula: Eads = EGaN − res − EGaN − Eres [33]. Where EGaN and EGaN − res represent the total energy of the activated GaN nanowire before and after the residual gas adsorption, respectively. Eres is the total energy of the residual gas. As shown in Fig. 2, all the adsorption energies are negative, indicating that the residual gas is exothermic during the adsorption
Fig. 1. Top and side views of a NEA GaN nanowire activated by P-type, 3Cs, 3Cs-O, 3Cs-NF3, 3Cs-3Li-NF3. All of the models shown have undergone structural optimization. 2
Applied Surface Science 508 (2020) 145250
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Fig. 2. Adsorption energy after adsorption of residual gas on the surface of (a) P type, (b) 3C, (c) 3Cs-O, (d) 3Cs-NF3 and (e) 3Cs-3Li-NF3 activated nanowires. The illustrations are top views of the optimized surfaces of the adsorbed residual gases, respectively.
Fig. 3. (a) Band gap, (b) electron affinity, (c) work function, (d) dipole moment of P-type and four activated nanowire surfaces with the adsorption of residual gas. 3
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the activating substance and residual gases on each activated surface, as shown in Fig. 6. On a pure P-type surface, the residual gas has a small transfer charge obtained from the nanowire, so it can be explained that the dipole moment from the P-type nanowire to the residual gas is relatively short. After the 3Cs activation, when the nanowires adsorb residual gas, the charge of the residual gas obtained from the activated nanowires rises remarkably. It is even observed that the amount of transferred charge of residual gas adsorbed in the Cs-activated surface is the highest in all activation modes, especially CO2 gas. Moreover, from the structural optimization of the 3Cs-activated nanowire model adsorbed by the residual gas, it can be found that the distance between the residual gas and the active surface is too short, and the two factors together result in the extension of the dipole moment. The introduction of the auxiliary gas O2 and NF3 increases the average positive charge of Cs atom. Meanwhile, the charge of the active surface is not sharply transferred to residual gas like the 3Cs activation method. The insertion of the Li atom reduces the average charge amount of the Cs atom and increases the negative charge amount of the NF3 molecule. Surprisingly, the negative charge of the residual gas adsorbed on the 3Cs-3Li-NF3 activated surface is significantly reduced. In general, the charge transfer of the activated nanowire surface is more intense than the pure P-type surface.
Fig. 4. (a) Schematic diagram of the band structure of the GaN nanowire activated by the four activation methods, (b) a dipole moment caused by the residual gas on the activated GaN nanowire.
photocathode activated by a single Cs may be far less than the other three activation modes. The reduction of the work function of the GaN nanowire surface by different activation modes is shown in Fig. 4(a). According to the calculation results, the insertion of Li atoms makes the activation mode of 3Cs-3Li-NF3 have the best photoemission performance, especially the work function is drastically reduced. In addition, it can be clearly observed that the activation surface is not as stable as the P-type surface in any activation mode, because the increasement of the band gap, the electron affinity and the work function of the residual gas on the activation surface are much more intense than the P-type surface. However, it has to be mentioned that even so, the activated surface after adsorption of the residual gas still has better photoelectric emission properties than the pure surface. Among the residual gas, the stability of activated GaN nanowires adsorbed by CO2 molecules is most severely impaired. The residual gas itself carries a negative charge, a reverse dipole moment is generated between the activated GaN nanowires and residual gas, as shown in Fig. 4(b), so that electrons on the surface of the nanowire are transferred to the residual gas. It is not conducive to the escape of photoelectrons to vacuum. The formula for calculating the dipole moment is described as: μ = (1/12π )AΔφ / θ , where μ is the dipole moment, A is the area of the surface, Δφ is the change in the work function, and θ is the coverage of the gas molecules. As shown in Fig. 3(d), residual gas adsorption causes a dipole moment in the direction from the nanowire to the adsorbate. Compared with the P-type surface, the dipole moment formed between the 3Cs, 3Cs-O, 3CsNF3 mode activated surface and residual gas is significantly increased, but the dipole moment between the residual gas and 3Cs-3Li-NF3 activated surface is smaller than that of the P-type surface. We suspect that this may be due to the fact that the Li atom carrying more positive charges can neutralize the negative charge of the residual gas to some extent. In summary, the activation mode of 3Cs-3Li-NF3 can improve the photoelectric emission efficiency and exhibit high stability in terms of work function. Herein, we detail the band structure of various surfaces before and after CO2 adsorption, as shown in Fig. 5. Due to the new energy levels in the band structure of the nanowire surface after activation of 3Cs-NF3 and 3Cs-3Li-NF3, the band gap is further reduced, which promotes the transition of electrons between the valence band and the conduction band. Compared with the P-type surface, regardless of the activation mode, the degree of bending of the conduction band is intensified, and a band bending zone (BBR) is formed at the edge of the conduction band. However, when the residual gas is adsorbed on the activated surface, not only the rise of the conduction band and the vacuum level increases the work function and electron affinity, but also the degree of bending of BBR also decreases, and the combination of these two factors hinders the transmission of electrons between the valence band, the conduction band and the vacuum. To further understand the charge of residual gases on each activated surface, we additionally analyzed the Mulliken charge distribution of
4. Conclusion In summary, we used a first-principles calculation to systematically study the electronic properties of the surface of activated GaN nanowires adsorbed by residual gases (CH4, CO, CO2, H2O). Firstly, NEA GaN nanowires models obtained by four activation methods were constructed (3Cs, 3Cs-O, 3Cs-NF3 and 3Cs-3Li-NF3). The adsorption energy, electron affinity, work function, dipole moment, band structure and Mulliken charge distribution of various activated GaN nanowires after residual gas adsorption were calculated and analyzed. The results show that the adsorption process of all residual gases on the surface of GaN nanowires is exothermic, and the surface adsorption model is stable. Residual gases are more readily present on the activated surface than on P-type pure surfaces, especially H2O molecules. In addition, the band gap of 3Cs-NF3 mode activated nanowire is the smallest, but the band gap after residual gas adsorption has different degrees of increase. Residual gases also reduce the bending of the BBR. 3Cs-O and 3Cs-NF3 co-adsorption can produce NEA surfaces, and Li insertion can further reduce surface work function and electron affinity. Although the residual gas causes a significant increase in electron affinity and work function, it is still much lower than that of pure P-type surface. The charge transfer caused by the adsorption of residual gas produces a dipole moment directed from the surface of the nanowire to the residual gas, which is detrimental to the escape of photogenerated electrons into the vacuum. Compared to the other three activation modes, the dipole moment generated by the residual gas on the 3Cs-3Li-NF3 activation surface is much shorter. All calculations show that the 3Cs-3Li-NF3 activation mode can greatly improve the photoemission performance and stability of GaN nanowire cathodes, which is beneficial to the use of GaN nanowire cathodes in electron source devices. CRediT authorship contribution statement Lei Liu: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Feifei Lu: Software, Formal analysis, Investigation, Writing original draft. Jian Tian: Data curation, Validation, Visualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 4
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Fig. 5. The band structure of P-type and activated GaN nanowire with the adsorption of the CO2 molecule. The pink dotted line and the green solid line indicate the band structure of the GaN nanowires before and after the adsorption of CO2 molecules, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Mulliken charge distribution of residual gases and activating substances of P-type and four activated surfaces.
Acknowledgements
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This work is supported by Qing Lan Project of Jiangsu ProvinceChina (Grant No. 2017-AD41779), the Fundamental Research Funds for the Central Universities-China (Grant No. 30916011206) and the Six Talent Peaks Project in Jiangsu Province-China (Grant No. 2015-XCL008). Meishan Wang of Ludong University is greatly appreciated for the help of first principle calculations. References [1] J. Lahnemann, A. Ajay, M.I. Den Hertog, E. Monroy, Near-infrared intersubband photodetection in GaN/AlN nanowires, Nano Lett. 17 (11) (2017) 6954–6960. [2] H.S. Jung, Y.J. Hong, Y. Li, J. Cho, Y.J. Kim, G.C. Yi, Photocatalysis using GaN nanowires, ACS Nano 2 (4) (2008) 637–642. [3] Y. Dong, B. Tian, T.J. Kempa, C.M. Lieber, Coaxial group III-nitride nanowire photovoltaics, Nano Lett. 9 (5) (2009) 2183–2187. [4] C. Li, J.B. Wright, S. Liu, P. Lu, J.J. Figiel, B. Leung, W.W. Chow, I. Brener, D.D. Koleske, T.-S. Luk, D.F. Feezell, S.R.J. Brueck, G.T. Wang, Nonpolar InGaN/ GaN core-shell single nanowire lasers, Nano Lett. 17 (2) (2017) 1049–1055. [5] T. He, X. Zhang, X. Ding, C. Sun, Y. Zhao, Q. Yu, J. Ning, R. Wang, G. Yu, S. Lu, K. Zhang, X. Zhang, B. Zhang, Broadband ultraviolet photodetector based on vertical Ga2O3/GaN nanowire array with high responsivity, Adv. Opt. Mater. 7 (7)
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Exploring the high stability of NEA GaN nanowire photocathodes by activation methods: First principles
T
⁎
Lei Liu , Feifei Lu, Jian Tian Department of Optoelectronic Technology, School of Electronic and Optical, Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
A R T I C LE I N FO
A B S T R A C T
Keywords: GaN nanowire NEA surface 3Cs-3Li-NF3 activation Residual gas adsorption Photoemission properties First-principles
In order to investigate the effect of different activation modes on the stability of GaN nanowire photocathodes, we studied the system stability and electronic properties of nanowire surface activated by 3Cs, 3Cs-O, 3Cs-NF3 and 3Cs-3Li-NF3 methods after adsorption of residual gases (CH4, CO, CO2 and H2O molecules) by first-principles. The results show that the residual gas is more readily adsorbed on the activated surface than on the pure P-type surface. The 3Cs-3Li-NF3 activation mode can significantly reduce the work function and electron affinity of the GaN nanowire cathode. Even though residual gas can damage the electronic properties of the activated surface, it still has better photoemission characteristics than the pristine P-type surface. The introduction of Li atoms is more favorable for the higher stability of 3Cs-NF3 activated photocathode. This study is conducive to exploring ways to improve the lifetime of nanowire photocathodes from the activation modes.
1. Introduction Since GaN nanowires have quantum size effects and surface effects, they have been widely used in devices such as photodetection, photocatalysis, photovoltaics, lasers, and sensors, exhibiting better optoelectronic properties than devices based on GaN bulk materials [1–5]. At present, the research on vacuum type nanowire photocathode is also progressing steadily, Zou et al. have preliminarily prepared and successfully activated nanowire array photocathode [6–9]. Activation methods such as Cs/O activation can reduce the electron affinity of surface, and even achieve a negative electron affinity (NEA) surface, which greatly improves the quantum efficiency of the photocathode [10,11]. It is well known that the current activation methods for photocathodes mainly include the following: Cs activation [12], Cs-O coactivation [13,14], Cs-NF3 coactivation [15], Cs-Li-NF3 coactivation [16]. The commonly used NEA activation mode is Cs-O activation, while the Cs-NF3 activated photocathode has high stability, and the introduction of Li atoms further improves the lifetime of the photocathode [16–18]. NEA photocathode is also an important component of vacuum electron source, whose stability/lifetime and vacuum degree directly restrict the development of large-scale physical detection equipment such as linear accelerator, free electron laser and fourth-generation light source [19]. Therefore, it is important to study how to improve and maintain the performance of NEA GaN photocathodes. The residual
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gas in the vacuum chamber not only reduces the degree of vacuum, but different types of vacuum residual gas also have various effects on the stability of the cathode [20]. It has been investigated that the quantum efficiency of NEA photocathodes exposed to O2 and CO2 is continuously attenuated under ultra-high vacuum conditions [21]. More importantly, the residual gas exhibits lower damage to the Cs-NF3 co-activated NEA surface than to the Cs-O activated surface, so the Cs-NF3 activated NEA cathode has higher stability [22]. In order to study the stability of NEA GaN nanowire photocathodes activated by four activation methods, we calculated the adsorption energy, work function and dipole moment parameters of NEA GaN nanowires adsorbed by CH4, CO, CO2 and H2O based on the first principle. The results have important implications for the practical application of NEA GaN nanowire photocathodes in ultra-high vacuum environments. 2. Computational details All total energy and electronic structure calculations are based on the first principle software (CASTEP) of density functional theory (DFT) [23,24]. Generalized gradient approximation PBE schemes and pseudopotential methods are used to deal with exchange-related interactions [25]. The energy cutoff of the plane wave expansion is set to 400 eV. The integration of the Brillouin region is performed by applying a Gamma-centric Monkhorst-Pack scheme with 2 × 2 × 4 k-points.
Corresponding author. E-mail address: [email protected] (L. Liu).
https://doi.org/10.1016/j.apsusc.2020.145250 Received 8 October 2019; Received in revised form 5 December 2019; Accepted 1 January 2020 Available online 03 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
Applied Surface Science 508 (2020) 145250
L. Liu, et al.
20 Å vacuum layer is used to avoid interaction between adjacent nanowires. All models have been optimized until the convergence accuracy is below 2 × 10−6 eV/atom, the interatomic force is less than 0.01 eV/s, the stress is less than 0.1 GPa, and the maximum atomic displacement during the iteration is less than 0.0002 nm. Detailed modeling of NEA P-type GaN nanowires with doping elements being Mg atoms can be found elsewhere [26]. The initial crystal parameters of wurtzite GaN were set to a = b = 3.189 Å, c = 5.185 Å. According to the study of GaN nanowire diameters and crystal planes reported by Wang and Carter et al., it is found that [0 0 1] oriented nanowires with hexagonal cross sections are more advantageous than [1 0 0] and [1–10] oriented nanowires with triangle cross sections [27,28]. This is mainly because the considered [0 0 1] oriented nanowires have non-polar (1 0 0) side facets, appearing as hexagonal sections, and each of Ga and N surface atoms has one dangling bond. [1–10] oriented nanowires are surrounded by (1 1 2), (−1 −1 2) and (0 0 1) sides, while [1 1 0] oriented nanowires are surrounded by (1 −1 2), (−1 1 2) and (0 0 1) sides facets, appearing as triangle sections, and the surface Ga and N atoms have more than two dangling bonds. Therefore, the (1 0 0) surface of GaN nanowires with [0 0 1] orientation is selected as the research target [29]. The obtained GaN nanowire model has six (1 0 0) crystal planes and exhibits a hexagonal cross section. To simplify the computational complexity, only a specific (1 0 0) plane is selected to perform the adsorption process of the residual gas, and the dangling bonds on other surfaces are all passivated by hydrogen atoms to eliminate surface effects. Although the average relative stability of the nanowires increases as the diameter of the nanowires increases, due to the limitations of computing equipment, we set the diameter of the GaN nanowires to 9.5 Å. The change of electronic characteristics after adsorption can still predict experimental observations [30]. Top and side views of the NEA GaN nanowire activated by P-type, 3Cs, 3Cs-O, 3Cs-NF3, 3Cs-3Li-NF3 are shown in Fig. 1. The ratio and site selected of the 3Cs, 3Cs/O, 3Cs/NF3 and 3Cs/3Li/NF3 activation atoms is based on Refs. [7,26,31]. Residual gases (CH4, CO, CO2, H2O) that may be present in the ultra-high vacuum system are adsorbed to each activated (1 0 0) surface [32].
process on the surface of the GaN nanowires, and the obtained adsorption model is stable. The H2O molecule is the most easily adsorbed gas on both the P-type surface and the activated surface, while the adsorption of CH4 gas is more difficult. The order of difficulty in forming the four activation surfaces is: 3Cs > 3Cs-NF3 > 3Cs-3LiNF3 > 3Cs-O activation. In other words, the insertion of Li atoms into the surface of the nanowire can promote the formation of the 3Cs-NF3 activation mode. Compared to a pure P-type surface, the surface of the nanowire after activation is more easily adsorbed by residual gas, so the vacuum environment of the NEA cathode is more demanding. In particular, the adsorption energy of the surface activated by 3Cs-O is most increased after adsorption of residual gas, and its stability is most susceptible compared to all NEA surfaces. However, the introduction of Li atoms makes the 3Cs-NF3 activated NEA surface more stable, reducing the possibility of adsorption by residual gases. The stability of the photocathode is mainly determined by the surface electronic properties derived from the arithmetic between the system levels, such as bandgap (Eg), electron affinity (χ), work function (φ), and dipole moment (μ). There is a close relationship between the above parameters, mainly from the vacuum energy level (Evac), the conduction band minimum energy level (ECBM), the Fermi level (EF) and the valence band maximum energy level (EVBM). The band gap, electron affinity and work function of the nanowire can be obtained by χ= the following equations: Eg = ECBM − EVBM , Evacuum − ECBM , φ = Evacuum − EF , and the results are shown in Fig. 3(a), (b) and (c). The calculated bandgap of bulk wurtzite GaN is 1.43 eV, which is much smaller than the experimental value (3.4 eV). It is believed that the bandgap calculated by the DFT-GGA and PBE methods is always underestimated [34,35]. However, it does not affect the qualitative analysis of the electronic properties of the residual gas adsorption surface [36]. Undoubtedly, the adsorption of the residual gas can cause the band gap, the electron affinity and the work function of the activated surface to rise to some extent, which largely affects the photoelectric performance of the NEA photocathode. For NEA photocathodes, we want to reduce the electron affinity and work function as much as possible to extend their lifetime and improve their stability. The four activation methods all reduce the band gap of the P-type surface, which makes the photoelectrons on the valence band easier to transiti to the conduction band. The electron affinity and work function also show the same trend, which means that the possibility of photoelectrons escaping from the conduction band to the vacuum is increased, which is beneficial to the improvement of the cathode photoelectric performance. It can be seen from Fig. 3(b) that only 3Cs-O, 3Cs-NF3 and 3Cs-3Li-NF3 can activate the surface of the GaN nanowire from the positron affinity to NEA. However, relying on pure Cs activation can only reduce the electron affinity of the P-type surface, so the performance of a
3. Results and discussion The system stability of different residual gas-adsorbed GaN nanowire models is evaluated by calculating the average adsorption energy. The adsorption energy is defined according to the following formula: Eads = EGaN − res − EGaN − Eres [33]. Where EGaN and EGaN − res represent the total energy of the activated GaN nanowire before and after the residual gas adsorption, respectively. Eres is the total energy of the residual gas. As shown in Fig. 2, all the adsorption energies are negative, indicating that the residual gas is exothermic during the adsorption
Fig. 1. Top and side views of a NEA GaN nanowire activated by P-type, 3Cs, 3Cs-O, 3Cs-NF3, 3Cs-3Li-NF3. All of the models shown have undergone structural optimization. 2
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Fig. 2. Adsorption energy after adsorption of residual gas on the surface of (a) P type, (b) 3C, (c) 3Cs-O, (d) 3Cs-NF3 and (e) 3Cs-3Li-NF3 activated nanowires. The illustrations are top views of the optimized surfaces of the adsorbed residual gases, respectively.
Fig. 3. (a) Band gap, (b) electron affinity, (c) work function, (d) dipole moment of P-type and four activated nanowire surfaces with the adsorption of residual gas. 3
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the activating substance and residual gases on each activated surface, as shown in Fig. 6. On a pure P-type surface, the residual gas has a small transfer charge obtained from the nanowire, so it can be explained that the dipole moment from the P-type nanowire to the residual gas is relatively short. After the 3Cs activation, when the nanowires adsorb residual gas, the charge of the residual gas obtained from the activated nanowires rises remarkably. It is even observed that the amount of transferred charge of residual gas adsorbed in the Cs-activated surface is the highest in all activation modes, especially CO2 gas. Moreover, from the structural optimization of the 3Cs-activated nanowire model adsorbed by the residual gas, it can be found that the distance between the residual gas and the active surface is too short, and the two factors together result in the extension of the dipole moment. The introduction of the auxiliary gas O2 and NF3 increases the average positive charge of Cs atom. Meanwhile, the charge of the active surface is not sharply transferred to residual gas like the 3Cs activation method. The insertion of the Li atom reduces the average charge amount of the Cs atom and increases the negative charge amount of the NF3 molecule. Surprisingly, the negative charge of the residual gas adsorbed on the 3Cs-3Li-NF3 activated surface is significantly reduced. In general, the charge transfer of the activated nanowire surface is more intense than the pure P-type surface.
Fig. 4. (a) Schematic diagram of the band structure of the GaN nanowire activated by the four activation methods, (b) a dipole moment caused by the residual gas on the activated GaN nanowire.
photocathode activated by a single Cs may be far less than the other three activation modes. The reduction of the work function of the GaN nanowire surface by different activation modes is shown in Fig. 4(a). According to the calculation results, the insertion of Li atoms makes the activation mode of 3Cs-3Li-NF3 have the best photoemission performance, especially the work function is drastically reduced. In addition, it can be clearly observed that the activation surface is not as stable as the P-type surface in any activation mode, because the increasement of the band gap, the electron affinity and the work function of the residual gas on the activation surface are much more intense than the P-type surface. However, it has to be mentioned that even so, the activated surface after adsorption of the residual gas still has better photoelectric emission properties than the pure surface. Among the residual gas, the stability of activated GaN nanowires adsorbed by CO2 molecules is most severely impaired. The residual gas itself carries a negative charge, a reverse dipole moment is generated between the activated GaN nanowires and residual gas, as shown in Fig. 4(b), so that electrons on the surface of the nanowire are transferred to the residual gas. It is not conducive to the escape of photoelectrons to vacuum. The formula for calculating the dipole moment is described as: μ = (1/12π )AΔφ / θ , where μ is the dipole moment, A is the area of the surface, Δφ is the change in the work function, and θ is the coverage of the gas molecules. As shown in Fig. 3(d), residual gas adsorption causes a dipole moment in the direction from the nanowire to the adsorbate. Compared with the P-type surface, the dipole moment formed between the 3Cs, 3Cs-O, 3CsNF3 mode activated surface and residual gas is significantly increased, but the dipole moment between the residual gas and 3Cs-3Li-NF3 activated surface is smaller than that of the P-type surface. We suspect that this may be due to the fact that the Li atom carrying more positive charges can neutralize the negative charge of the residual gas to some extent. In summary, the activation mode of 3Cs-3Li-NF3 can improve the photoelectric emission efficiency and exhibit high stability in terms of work function. Herein, we detail the band structure of various surfaces before and after CO2 adsorption, as shown in Fig. 5. Due to the new energy levels in the band structure of the nanowire surface after activation of 3Cs-NF3 and 3Cs-3Li-NF3, the band gap is further reduced, which promotes the transition of electrons between the valence band and the conduction band. Compared with the P-type surface, regardless of the activation mode, the degree of bending of the conduction band is intensified, and a band bending zone (BBR) is formed at the edge of the conduction band. However, when the residual gas is adsorbed on the activated surface, not only the rise of the conduction band and the vacuum level increases the work function and electron affinity, but also the degree of bending of BBR also decreases, and the combination of these two factors hinders the transmission of electrons between the valence band, the conduction band and the vacuum. To further understand the charge of residual gases on each activated surface, we additionally analyzed the Mulliken charge distribution of
4. Conclusion In summary, we used a first-principles calculation to systematically study the electronic properties of the surface of activated GaN nanowires adsorbed by residual gases (CH4, CO, CO2, H2O). Firstly, NEA GaN nanowires models obtained by four activation methods were constructed (3Cs, 3Cs-O, 3Cs-NF3 and 3Cs-3Li-NF3). The adsorption energy, electron affinity, work function, dipole moment, band structure and Mulliken charge distribution of various activated GaN nanowires after residual gas adsorption were calculated and analyzed. The results show that the adsorption process of all residual gases on the surface of GaN nanowires is exothermic, and the surface adsorption model is stable. Residual gases are more readily present on the activated surface than on P-type pure surfaces, especially H2O molecules. In addition, the band gap of 3Cs-NF3 mode activated nanowire is the smallest, but the band gap after residual gas adsorption has different degrees of increase. Residual gases also reduce the bending of the BBR. 3Cs-O and 3Cs-NF3 co-adsorption can produce NEA surfaces, and Li insertion can further reduce surface work function and electron affinity. Although the residual gas causes a significant increase in electron affinity and work function, it is still much lower than that of pure P-type surface. The charge transfer caused by the adsorption of residual gas produces a dipole moment directed from the surface of the nanowire to the residual gas, which is detrimental to the escape of photogenerated electrons into the vacuum. Compared to the other three activation modes, the dipole moment generated by the residual gas on the 3Cs-3Li-NF3 activation surface is much shorter. All calculations show that the 3Cs-3Li-NF3 activation mode can greatly improve the photoemission performance and stability of GaN nanowire cathodes, which is beneficial to the use of GaN nanowire cathodes in electron source devices. CRediT authorship contribution statement Lei Liu: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Feifei Lu: Software, Formal analysis, Investigation, Writing original draft. Jian Tian: Data curation, Validation, Visualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 4
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Fig. 5. The band structure of P-type and activated GaN nanowire with the adsorption of the CO2 molecule. The pink dotted line and the green solid line indicate the band structure of the GaN nanowires before and after the adsorption of CO2 molecules, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Mulliken charge distribution of residual gases and activating substances of P-type and four activated surfaces.
Acknowledgements
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