Monolayer GaN functionalized with alkali metal and alkaline earth metal atoms: A first-principles study

Superlattices and Microstructures 130 (2019) 428–436

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Monolayer GaN functionalized with alkali metal and alkaline earth metal atoms: A first-principles study Keat Hoe Yeoh a, *, Tiem Leong Yoon b, **, Thong Leng Lim c, Rusi d, Duu Sheng Ong e a

Department of Electrical and Electronic Engineering, Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Kajang, Selangor, 43000, Malaysia School of Physics, Universiti Sains Malaysia, USM, Penang, 11800, Malaysia c Faculty of Engineering and Technology, Multimedia University, Jalan Ayer Keroh Lama, Melaka, 75450, Malaysia d Center for Foundation Studies, International University of Malaya-Wales, Kuala Lumpur, 50480, Malaysia e Faculty of Engineering, Multimedia University, Persiaran Multimedia, Cyberjaya, Selangor, 63100, Malaysia b

A R T I C L E I N F O

A B S T R A C T

Keywords: Alkali metal atom Alkaline earth metal atom Density functional theory Monolayer GaN Work function

Based on first-principles calculations, we have carried out a systematic study on the geometric, electronic and magnetic properties of free-standing monolayer GaN (ML GaN) functionalized with Lithium (Li), Sodium (Na), Beryllium (Be) or Calcium (Ca) atoms. We consider three different levels of concentrations i.e. θ ¼ 1=8,1=18 and 1=32. Within the tested θ, metallization of ML GaN only occurs with the adsorption of Li or Na atoms. The adsorption of Be or Ca atoms preserves the semiconducting characteristics of ML GaN. The ML GaN remains non-magnetic with the adsorption of Be or Ca atom. In contrast, the total magnetization of the Li-adsorbed ML GaN decreases as Li atoms concentration increases. For the case of Na adsorption, the ML GaN exhibits ferromagnetism only at θ ¼ 1=18. In addition, we found the work function of the functionalized ML GaN can be controlled by varying the concentrations of the adatoms. Our findings here suggest that by selective adsorption of Group I and Group II element, ML GaN is a promising material for the development of spintronic and field emission devices.

1. Introduction Over the years, gallium nitride (GaN) has received much attention owing to its excellent material properties such as large breakdown voltage, wide bandgap and high saturation drift velocity which enable this material to find its applications in high power transistor and blue light emitting devices [1,2]. In year 2014, the Nobel Prize of Physics was awarded to Shuji Nakamura, Hiroshi Amano and Isamu Akasaki for the invention of efficient blue light-emitting diodes (LED) which have enabled bright and energy-saving white light sources [3]. These blue light-emitting diodes were fabricated from high-quality GaN crystal. Their achievements have encouraged researchers to look into low dimensional GaN in the form of monolayer (ML) [4,5], nanowire [6–8] and quantum dots [9–11]. In recent years, GaN nanowire has been successfully synthesized by using various techniques such as Chemical Vapour Deposition (CVD), Metal-Organic Chemical Vapour Deposition (MOCVD), Molecular Beam Epitaxy (MBE) and Hydride Vapour Phase Epitaxy (HVPE) [12–15]. In fact, various GaN nanowire applications have been reported such as LED, photocatalyst, field effect

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.H. Yeoh), [email protected] (T.L. Yoon). https://doi.org/10.1016/j.spmi.2019.05.011 Received 3 December 2018; Received in revised form 23 April 2019; Accepted 8 May 2019 Available online 9 May 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.

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transistor and nanogenerators [16–18]. Similarly, GaN quantum dots and nanodiscs are widely researched and hold promising po­ tential in light emission and detection for wavelength range in the ultraviolet (UV) region [19,20]. However, research work on ML GaN is not widely reported. The ML hexagonal BN (h-BN), a 2D analogs of ML GaN, has a wide band-gap of 5.56 eV, calculated using Heyd-Scuseria-Ernzerhof (HSE) approach while the bandgaps obtained using GW approximation vary from 7.4 to 8.43 eV [21]. ML h-BN has high surface smoothness, promising potential applications in tunneling devices and insulating layer for graphene [22,23]. Large area ML h-BN has been grown using HVPE method [24]. This development has motivated researchers to explore the potential of ML GaN. Through first-principles calculations, pristine free-standing ML GaN was predicted to be a planar honeycomb structure with an indirect bandgap of 1.95 eV–2.27 eV and it has a very low thermal conductivity (14.93 WmK 1) [4,5,23–28]. Experimentally, Yeh et al. have used GaN 2D nanosheets as vertical nonpolar growth template for LED [29]. Hydrogenated ML GaN which takes the form of a buckled honeycomb structure has been synthesized using the migration-enhanced encapsulated growth (MEEG) technique [30]. Experimental and theoretical studies have shown that the physical and chemical properties of a 2D material can be controlled through atom adsorption or substitutional doping [31–37]. For example, fluorographene sheet has been experimentally demonstrated [36]. Additionally, graphene deposited with Li atom has been shown to exhibit superconductivity with Tc � 5.9 K [37]. Theoretically, Sun et al. have investigated the geometric structures, energetics, electronic and magnetic properties of a blue phosphorene substituted with B, C, N, O or F [38]. Through first-principles studies, Luo et al found that the adsorption of Fe, Co, or Au can induce magnetic moments on black phosphorene [39]. Using Density Functional Theory (DFT) calculations, Sun et al. predicted that the solar energy absorption of arsenene improves significantly with the adsorption of TCNQ molecule [40]. For ML GaN, Mu revealed that the magnetic and electronic properties of the ML GaN can be changed by O, F or N atom functionalization [28]. Recently, Tang et al. carried out a systematic investigation on the magnetism induced by non-metal atoms adsorbed on a ML GaN through first-principles study [41]. Zhao et al. showed that the magnetism of a ML GaN can be tuned by vacancies or Mg/Si doping [42]. Additionally, the effects of hydrogenation, vacancies defects, atom adsorption and substitutional doping on ML GaN has been reported by Kadioglu et al. [43]. Recently Cui et al. have carried out first-principles studies on the adsorption of alkali metals on a pristine ML GaN [44]. Modulation to the physical properties of ML GaN as reported in Ref. [44], e.g., the stability, work function, static dielectric constant and absorption spectrum is attributed to the adsorption of single alkali metal atom per unit supercell. The effects due to variation in the concentration of the alkali metal atoms were not investigated in Ref. [44]. It is well known that the concentration of atoms adsorbed on a 2D material has a substantial impact on its physical properties especially when the concentration is high due to the coupling effects between the adjacent atoms. In this work, in addition to studying the effects of alkali metal adsorption at different concentrations, we also extend

Fig. 1. (a) Top view of the optimized structure of a free-standing ML GaN. The black dotted box indicates the primitive unit cell with lattice constant, a. (b) Fig. 1(a) is tilted to give a clearer visualization on the possible adsorption sites. The yellow-color dots indicate the possible adsorption sites. The purple and bluish gray ball denotes Ga and N atom respectively. (c) The energy band diagram and (d) density of states (DOS) of a pristine ML GaN. 429

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our effort to investigate the physical properties of a ML GaN when it is adsorbed with alkaline earth metal atom. Interestingly, the adsorption of alkaline earth metal atoms on ML GaN displays electronic properties that are quite distinct from that of alkali metal atoms. From the density of states (DOS) calculations, the adsorption of alkali metal atoms changes the semiconducting characteristic of ML GaN into metallic. This is in contrast to the adsorption of alkaline earth metal atoms where the semiconducting characteristic of the ML GaN is preserved. 2. Methodology All DFT calculations were carried out using the Perdew Burke -Ernzerhof (PBE) form of the generalized gradient approximation (GGA) for the exchange correlation functional implemented in the Quantum ESPRESSO simulation package [45–47]. The ULTRA­ SOFT pseudopotential was used. The plane-waves kinetic energy and charge densities cutoff were set to 40 Ry and 320 Ry respectively. An 18 Å vacuum layer was inserted in the perpendicular direction to avoid interaction that could occur from the adjacent GaN layer along this direction. Dipole correction was taken into consideration in our calculations. Atomic position and cell parameters were optimized until all the forces on the atoms were less than 0.01 eV/Å. For all the calculations, the momentum space was sampled on a

12 � 12 � 1 Monkhorst-Pack grid. Geometric structures and spin charge density were plotted using XCRYSDEN [48]. Charge transfer, ρ, was calculated using Lo€wdin charge analysis. ρ > 0 denotes the transfer of electron from the atom to the ML GaN and vice versa if ρ < 0. The partial density of states (PDOS) was broadened by Gaussian smearing of 0.01 eV and all the projected states were magnified 8� for better viewing convenience. van der Waals interaction (DFT-D) was added to the calculations to accurately describe the dispersion [49]. 3. Results and discussions

For validation purposes, a free-standing pristine ML GaN is relaxed to a flat honeycomb structure as shown in Fig. 1(a). The calculated bandgap, the lattice constant and Ga–N bond length of the optimized ML GaN are 2.16 eV, 3.21 Å and 1.86 Å respectively. These values are in close agreement to those previously reported values [25–28]. In contrast to graphene, ML GaN has four possible adsorption sites as shown in Fig. 1(b), namely, on top of the Ga atom (Top-Ga), on top of the N atom (Top-N), on top of the Ga–N bond (Bridge) and above the center of the hexagonal ring (Hollow). In the present work, we use Li and Na as the representative atom for alkali metal atom while Be and Ca are considered for the case of alkaline earth metal atom. The Li, Na, Be or Ca atom is placed on one of these adsorption sites and variable-cell relaxation is carried out. After the structural relaxation, the binding energy of the atom on the ML GaN is calculated as: EB ¼ EGaNþatom

EGaN

(1)

Eatom

where EGaNþatom is the total energy of the ML GaN adsorbed with adatom, EGaN is the total energy of the pristine ML GaN and Eatom is the energy of the isolated adatom calculated using the same supercell parameters of the ML GaN. Spin polarization is taken into consideration when calculating the binding energy of the system. From equation (1), the ML GaN system is considered stable when the calculated EB yields a negative value. The most favorable adsorption site for each type of atom is the site with the largestjEB j. In the present work, we consider three different levels of adatom coverage (a.k.a. concentration) i.e. θ ¼ 1=8, 1=18 and 1=32, which correspond to the adsorption of a single adatom on a 2 � 2, 3 � 3 and 4 � 4 supercell respectively. As shown in Table 1, all binding energies yield a negative value which indicates that these systems are stable. For θ ¼ 1=32, the adsorption of Be raises the adjacent Ga atom. Other than this, the ML GaN preserves its honeycomb structure. Regardless of the coverage, all the Li, Na, Be and Ca atoms act as electron donor. This is reasonably expected because alkali metal and alkaline earth metal atoms are in the Group I and Group II of the Periodic Table. Elements in these groups are known to be electro­ positive. For Li, Na and Ca adsorption, EB becomes stronger as the concentrations of the adatom increases. However, for Be adsorption, Table 1 Electronics, magnetic and structural properties of ML GaN: EB , μ, ρ* and h denote the binding energy, total magnetization, charge transfer, average vertical distance of the atom from the GaN layer respectively. Δd is the localized tensile strain of the Ga–N atom near the adatom, calculated using equation (2). M and SC denotes metallic and semiconducting respectively. Coverage (θ)

Adatom

1/8

Li Na Be Ca Li Na Be Ca Li Na Be Ca

1/18

1/32

EB (eV)

μ(μB )

ρ*

h(Å)

Δd(%)

Characteristic

0.40 1.04 1.61 2.62 0.62 1.07 1.58 2.96 0.71 1.18 1.94 3.08

0.24 0 0 0 0.44 0.14 0 0 0.46 0.00 0.00 0.00

0.08 0.53 0.14 0.75 0.17 0.59 0.09 0.83 0.21 0.64 0.18 0.85

2.55 2.16 1.77 1.62 2.65 2.25 1.97 1.61 2.72 2.30 2.20 1.66

6.0 2.0 9.7 22.7 3.9 1.7 6.4 22.9 4.0 1.7 11.3 21.9

M M SC SC M M SC SC M M SC SC

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as θ is increased from 1=18 to 1=8 , it is expected that the EB will be weaker. Nevertheless, we observed a reverse trend where EB changes from 1.58 eV to 1.61 eV. A closer inspection reveals that the charge transfer ρ for Be/ML GaN system is much smaller for θ ¼ 1=18 (0.09e) compared to θ ¼ 1=8 (0.14e). A higher ρ further enhances the electrostatic attraction between the Be atom and the ML GaN which results in a stronger EB : Δd in Table 1 is the average tensile strain near the adatom, calculated as:

Fig. 2. DOS and PDOS of ML GaN adsorbed with (a) Li, (b) Na, (c) Be and (d) Ca with concentration of ¼ 1=8. The projected s-states and p-states for the adatoms are denoted by the red and blue line respectively. 431

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Fig. 3. DOS and PDOS of ML GaN adsorbed with (a) Li, (b) Na, (c) Be and (d) Ca with concentration of ¼ 1=18. The projected s-states and p-states for the adatoms are denoted by the red and blue line respectively.

432

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Fig. 4. DOS and PDOS of ML GaN adsorbed with (a) Li, (b) Na, (c) Be and (d) Ca with concentration of ¼ 1=32. The projected s-states and p-states for the adatoms are denoted by the red and blue line respectively.

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Δd ¼

d

d0 d0

(2)

� 100

where d is the average bond length between Ga and N atoms in the vicinity of the adatom while d0 is the Ga–N bond length of the pristine free-standing ML GaN. As shown in Table 1, the adsorption of the alkali metal or alkaline earth metal atoms stretches the neighboring Ga–N bond, resulting in localized tensile strain. The increase of the Ga–N bond length may weaken the π bonds of the ML GaN and promote interaction of Ga and N atoms with the adatom. This leads to the formation of new chemical bonds between the adatom and the Ga and/or N atom. In fact, similar effects have been observed in halogenated graphene [50]. When the localized tensile strain of the GaN is sufficiently high, we observed a marked increment in the binding energy and charge transfer. For example, for the case of θ ¼ 1=32 the Ca atom is bounded to the ML GaN with EB of 3.08 eV and charge transfer of 0.85e. This corresponds to the increment of the neighboring Ga–N bond length by 22%. N-Top is the preferred adsorption site for Li atom for all values of θ. The Ga-N-Ga and Li-Ga-N angle for Li adsorption is about 114� and 104� respectively. Comparing these angles to those of the ideal sp2 (120� ) and sp3 (109.5� ), one can observe that these angles are getting closer to the sp3 hybridization. The electronic band dispersions (see supplementary data), the density of states (DOS) and partial density of states of the pristine ML GaN and ML GaN adsorbed with different alkali atoms and alkaline earth metal atoms are shown in Figs. 2–4. For all the tested θ, the adsorption of Li and Na changed the semiconducting characteristics of ML GaN to metallic as evident by the presence of Fermi level in the DOS. Unlike Li- ML GaN and Na- ML GaN system, the semiconducting characteristics of Be- ML GaN and Ca- ML GaN system are retained with the presence of gap states. The work function of the ML GaN is calculated as the energy difference between the elec­ trostatic potential in the vacuum and the Fermi energy, given as: ϕ ¼ Evac

(3)

EF

where EF and Evac is the Fermi level and vacuum level respectively. Evac is obtained from the planar-averaged electrostatic potential energy in the vacuum direction. Using this approach, the work function of the pristine ML GaN is calculated to be 5.25 eV. We note that the work function of the pristine ML GaN predicted by Cui et al. [44]. is about 1 eV lower than ours. The calculation of work function is dependent on the simulation conditions such as dipole correction, the type of functional and van der Waals correction being used [51,52]. It is possible that the discrepancy in the work function could be due to the simulation conditions between our work and those by Cui et al. [44]. It is also worth mentioning that the work function of a pristine ML GaN has not been experimentally measured before. Nevertheless, we can analyze the work functions obtained in terms of the percentage change with respect to the pristine condition, which is defined as: Δϕ ¼

ϕ

ϕo � 100 ϕo

(4)

where ϕ is the work of the ML GaN adsorbed with adatom and ϕ0 is the work function of the pristine ML GaN. In Fig. 5, we plotted the Δϕ of the ML GaN adsorbed with various alkali metal or alkaline earth metals atoms at different values of θ. Compared to the pristine ML GaN, the work functions of these functionalized ML GaNs are lower. This is understandable because naturally alkali metal and alkaline earth metal atoms are known to have low work function. In general, the Be- ML GaN systems have the highest work function followed by Ca- ML GaN, Li- ML GaN and Na- ML GaN system. Depending on the type of adatom and θ, the work function of the ML GaN can be reduced by 11%–53% as compared to the pristine ML GaN. Apart from Be adsorption, the work function decreases gradually with increasing θ; albeit it is still lower than the work function of the pristine free-standing ML GaN. Particularly, for Be adsorption, we observe that the work function is reduced by 15.5% at θ ¼ 1=8 and 24.8% at θ ¼ 1=18 eV. Thereafter it starts to increase when the Be atom are further diluted to θ ¼ 1=32. Recall that similar trend has been observed for EB at different values of θ and the ρ is relatively lower at θ ¼ 1=18 compared to that at θ ¼ 1=8. Such a low ρ could result in a different adatom-induced surface charge distribution that may further reduce the work function instead of increasing it. Overall, these work functions increase with the binding energy and charge transfer. ML GaN in its pristine form is non-magnetic. When the ML GaN is adsorbed with Li atom, the system is turned into ferromagnetic. The total magnetization is increased from 0.24 μB to 0.46 μB as θ is reduced from 1=8 to 1=32: For all the tested θ, no magnetism is

Fig. 5. Percentage change of the functionalized ML GaN’s work function with respect to the pristine condition at different adatom coverages of θ ¼ 1=8, 1=18 and 1=32. The hollow square symbol denotes the work function of the pristine ML GaN. 434

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Fig. 6. Spin charge density distribution of a ML GaN adsorbed with Li at (a) ¼ 1=32 , (b) θ ¼ 1=18 , (d) θ ¼ 1=8 and (c) Be at θ ¼ 1=18. Red (blue) shaded region denotes net spin-up (spin-down) charge density. The iso-surface is set at 8 � 10 4 e/bohr3.

observed when ML GaN is adsorbed with either Be or Ca atom, while the ML GaN is weakly magnetized upon the adsorption of Na at θ ¼ 1=18. The spin charge density for the alkali or alkaline earth metal doped ML GaN is illustrated in Fig. 6. Clearly, the spin charge density distributions are localized near the atom. This suggests that the ferromagnetism observed in the ML GaN is attributed to the adsorption of Li or Na atom. 4. Conclusions In summary, we have carried out first-principles calculations to investigate the electronic, magnetic and structural properties of a pristine free-standing ML GaN with the adsorption of alkali metal (Li, Na) and alkaline earth metal atoms (Be and Ca). Depending on the type of the atom adsorption, the semiconducting ML GaN can either change into metallic characteristics or remain as a semi­ conductor. For adatom coverage of θ ¼ 1=8, 1=18 and 1=32, the ML GaN exhibits metallic characteristics with the adsorption of Li and Na. However, the Be or Ca doped ML GaN remains as semiconductor with the presence of gap states. For all cases, the adatom acts as a donor with electron transfer always in the direction from the adatom to the ML GaN. Analysis of the magnetic effect reveals that the LiML GaN system is magnetized with total magnetization increases with θ while Na doped ML GaN is weakly magnetized when θ ¼ 1=18. Other than these, the remaining systems are not magnetized. We also found that the work function of ML GaN depends on the type of adatom adsorbed on it and can be controlled by adjusting the adatom coverage. Overall, the alkali metal or alkaline earth metal atoms doped ML GaN systems have lower work function as compared to their pristine counterpart. Our findings here suggest that ML GaN can be used as an interlayer to alter the Schottky barrier height either by selective adsorption of alkali metal or alkaline earth metal atom. In addition, the work function also can be adjusted by controlling these adatom concentrations adsorbed on the surface of the ML GaN. Due to its high selectivity and sensitivity, ML GaN could be a promising material for chemical sensor and photocatalyst. Acknowledgment Prof. Mohd. Zubir Mat Jafri from USM School of Physics are gratefully acknowledged for providing us computing resources to carry out part of the calculations done in this paper. We acknowledge the financial support from MOHE under grant number FRGS/1/2017/ STG07/UTAR/02/2. YTL acknowledges the financial support of USM Bridging (2018). We also gratefully acknowledge the support of NVIDIA Corporation with the donation of the Quadro P6000 used for this research. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.05.011.

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