Pure and Oxidized Ag Substrate Influence on the Phase

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Pure and Oxidized Ag Substrate Influence on the Phase Transformation and Semiconducting Behaviors of Layered ZnO: A First Principles Study Changhong Wang, Wei-hua Wang, Feng Lu, Yahui Cheng, Liru Ren, Weichao Wang, and Hui Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512621r • Publication Date (Web): 26 Jan 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Pure

and

Oxidized

Ag

Substrate

Influence

on

the

Phase

Transformation and Semiconducting Behaviors of Layered ZnO: A First Principles Study Changhong Wang,† Wei-Hua Wang,† Feng Lu,† Yahui Cheng,† Liru Ren,‡ Weichao Wang,*,† Hui Liu*,†

†

Department of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin Film Device and

Technology, Nankai University, Tianjin, 300071, People’s Republic of China ‡

Department of Microelectronics, Nankai University, Tianjin, 300071, People’s Republic of China

* E-mail: [email protected]; Tel (fax): (86)-22-23509930 (W.W.). * E-mail: [email protected]; Tel (fax): (86)-22-23509930 (H.L.).

ACS Paragon Plus Environment

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ABSTRACT Graphitic-like ZnO layers had been experimentally synthesized on metal substrates over the past few years. Nevertheless, the impact of metal substrates on the structural and electric properties of ZnO is still unclear. Utilizing first-principle calculations with van der Waals correction, we found that the phase transformation from graphitic-like to wurtzite structure occurs when the thickness of freestanding ZnO exceeds seven layers. With the presence of pure Ag(111) substrate, the critical transformation thickness decreases to two layers due to the depolarization effect originating from the charge transfer from Ag substrate to ZnO. Band structure analysis displays the semiconducting behaviors for the freestanding graphitic-like ZnO layers. On the pure Ag substrate, monolayer and bilayer ZnO is n-doped by the substrate and a metallic character of ZnO is observed. Importantly, the semiconducting behavior of ZnO layers is maintained when ZnO contacting with oxidized Ag substrate because of less charge transfer between ZnO and Ag. And the metal/semiconductor contact results in a Schottky barrier of 0.8 eV. The simulation findings indicate that the few layered ZnO on oxidized Ag system possesses potential applications in the optoelectronic devices.

KEYWORDS Graphitic-like layered ZnO, phase transformation, ZnO interfacing with Ag substrate, charge transfer, first principles calculation


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1. INTRODUCTION In the post-silicon era, low-power devices are highly expected arising from the emerging of the recent two-dimensional (2D) materials. Graphene, a 2D honeycomb lattice structure, has been extensively studied due to its excellent electronic properties.1-4 Nevertheless, the absence of band gap sets limitations of graphene in the logic devices. Thus, it is important to search for other 2D non-zero band gap materials with graphitic-like structures, such as SiC5,6, BN7,8, transition metal dichalcogenides9,10, and transition metal oxides11,12. Among these numerous candidates for the novel 2D materials, graphitic-like ZnO has received attention over the past few years.11-18 As is known, bulk ZnO is naturally stable with a wurtzite structure. However, recent theoretical and experimental researches indicate that, due to the surface depolarization, the polar (0001)-oriented ZnO film with only a few atomic layers favors a stable graphitic-like structure.11-13,16-18 Whereas, for the phase transformation from 3D wurtzite to 2D structure, the critical thickness of the ZnO atomic layers is still under debating. Previous theoretical studies found that the planar freestanding ZnO is stable until eight atomic layers.11-13 Considering ZnO contacting with metal substrates, the critical thickness might varies. So far, there are rare theoretical insights into this critical thickness with the presence of metal substrates. Experimentally, the critical layers number is much smaller after contacting with the metal (Ag, Pd) substrates.16, 17 Tusche et al. experimentally confirmed the presence of two- and three-monolayer-thick graphitic-like ZnO on the Ag(111) substrate.16 Weirum et al. showed that, on Pd(111) surface, up to four graphitic-like ZnO layers are thermodynamically stable.17 They concluded that the stability of the planar phase of ZnO on metal substrate is mainly determined by layer confinement and/or depolarization effects rather than interactions between ZnO and metal substrates. The large discrepancy of ZnO critical layer thickness between theoretical and


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experimental results might arise from excluding the metal substrates in previous theoretical works.11-13 In this work, we perform DFT calculations including the long range dispersion between ZnO layers and introducing metal substrate to clarify debate of the critical thickness. Besides, for the device applications, electronic structures are crucial to determine whether the device successfully operates or not. Topsakal et al. observed a semiconducting character of freestanding graphitic-like monolayer (Eg = 1.68 eV) and bilayer (Eg = 1.44 eV) ZnO based on the first-principles calculations.14 When metal contacts with ZnO layers, charge transfer occurs because of the different electron negativity between ZnO and substrates. Consequently, ZnO would be n-type or p-type doped by the substrates. For an even stronger interaction at the interface, electronic structures of ZnO layers might be significantly modified, leading to the failure in the device operations. For instance, Deng et al. prepared graphitic-like monolayer ZnO on Au(111) substrate with a second ZnO layer partially coated on the first monolayer. Rather than semiconducting behaviors in free standing ZnO layers, the metallic performance was observed.18 To date, there is no further understanding why this metallic behaviors take place. It is thus of significance to theoretically explore the phase transformation (from 2D graphitic-like to wurtzite structure) and electronic structures of both freestanding ZnO and ZnO layers with substrates. 2. COMPUTATIONAL DETAILS First-principles calculations were performed within density-functional theory (DFT), as implemented in plane-wave basis code Vienna ab initio Simulation Package (VASP).19 Exchange-correlation potential was approximated by generalized gradient approximation (GGA). The GGA in the Perdew−Burke−Ernzerhof (PBE) forms and the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional were employed.20-22 A kinetic-energy cut off of 500 eV was used. The 15 Å vacuum region was used


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to minimize the spurious interactions between top and bottom images in the periodic slab model. The Brillouin zone was sampled by (11×11×1) k-points for ZnO freestanding layers and (8×8×1) k-points for ZnO layers on the Ag(111) substrate. All atomic positions and lattice constants were optimized using conjugate gradient method, where the total energy and atomic forces were minimized. The convergence criterion for energy was 10−4 eV between two electronic steps and the maximum Hellmann-Feynman force on each atom was less than 0.01 eV/Å during the ionic relaxation. Because the standard GGA-type density functionals are unable to describe the weak long distance interactions, an empirical dispersion-corrected density functional theory (DFT-D2) method proposed by Grimme was adopted.23 For electronic structure calculations, it is well known that DFT underestimates the band gap, thus, the hybrid functional (HSE) were used. The optimized lattice constant of bulk Ag is 4.15 Å, which agrees well with the 4.07 Å (exp. work24) and 4.17 Å (theoretical result25). ZnO layers on the pure Ag (ZnO/Ag(111)) and oxidized Ag (ZnO/O/Ag(111)) substrates were modeled by a periodic slab of ZnO(0001)-(

3

×

3

) and

Ag(111)-(2×2) . Ag slab with seven layers was adopted in this work and the two bottom layers were frozen during the structural optimizations. The monolayer and bilayer ZnO were expanded by ~3.2% and 1.7%, respectively, and rotated counterclockwise by 30° to match the Ag(111) surface (Figure 1). A vacuum spacing of 15 Å was used to eliminate the spurious interactions between top and bottom atoms in the periodic slab images. It is known that DFT could not locate the global energy minimum. To solve this problem, along c-direction, we changed the distance between the ZnO layer and the Ag(111) substrate until the lowest total energy was obtained. Subsequently, due to the identical symmetry along a and b directions, we only moved the ZnO layer along a-direction until the lowest energy was located. Subsequently, the full relaxation was carried out, starting from the above


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structure with the lowest energy along three vector directions (Figure 2).

Figure 1. (a) Ag monolayer for Ag bulk normal to (111) direction and (b) ZnO monolayer.


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Figure 2. Optimized geometry structures of (a) monolayer and (b) bilayer ZnO on top of pure Ag(111) surface; (c) total energy difference vs. distance between ZnO and Ag substrate; (d) total energy difference vs. movement of ZnO monolayer along primary vector a.

The (0001) layered ZnO slab model is constructed with a vacuum thickness of 15 Å. The average binding energy (per ZnO) between two monolayers can be expressed as: Eb =

nEmono − Etot N ( n − 1)

(1)

where Etot and Emono denote the total energy of n layered-ZnO and monolayer ZnO, respectively. n is


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the number of layers. The capital letter N is the number of primitive cells in each ZnO layer. The adhesion energy between ZnO layers and substrate was calculated by using the equation:

Ead =

Esub + EZnO − Etot N

(2)

where Esub, EZnO, and Etot are the total energy of substrate, the energy of freestanding ZnO layers, and the energy of ZnO/Ag(111) or ZnO/O/Ag(111) interface, respectively. N is the number of primitive cells of ZnO layers.

3. RESULTS AND DISCUSSION 3.1 Stability of ZnO layers with and without Ag substrates Previous theoretical work indicated that the freestanding planar ZnO was stable until a thickness more than eight atomic layers,11-13 while the experimental thickness was much smaller (~3 atomic layers) when including metal substrates.16-18 In this work, the initial layered ZnO structure is constructed by w-ZnO(0001) with the stacking ABAB... sequence. As expected, below a critical thickness, the initial wurtzite structure transforms into the graphitic-like structure after full relaxation. Within PBE calculations without the consideration of van der waals (vdW) correction, we found that the planar freestanding ZnO monolayers were energetically more favorable than bulk w-ZnO if atomic layers are less than 11 (Table 1), which are consistent with previous theoretical results.13 As is known, in the graphitic structure ZnO, the Zn and O atoms within the same atomic layer possesses a sp2-like hybridization. The strong covalent bonding causes the in-plane stability of 2D crystals while the weak vdW force ensures the stability between layers. In order to fully describe these two types of bonds, an advanced dispersion-corrected density functional theory (DFT-D2) approach, proposed by Grimme, was adopted.23 The long-range dispersion, vdW interaction is introduced by adding a semi-empirical dispersion potential to the conventional Kohn-Sham DFT energy:


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(3)

EDFT − D = EKS − DFT + Edisp and Edisp is described by: N −1

Edisp = − s6 ∑ i =1

C6ij ∑ 6 fdmp ( Rij ) j = i +1 Rij N

(4)

where N is the number of atoms and s6 is a global scaling factor. C6ij denotes the dispersion coefficient between any atom pair i and j, and Rij is the distance of the atom pair. fdmp(Rij) is a damping function, which avoids near-singularities within typical bonding distances.23

Table 1. Number of layers N, Lattice constant a, average interlayer spacing d, and binding energy Eb between two ZnO layers calculated based on PBE+vdW functionals. N

a (Å)

d (Å)

Eb (eV)

1

3.283

--

--

2

3.333

2.454

0.547

3

3.353

2.436

0.562

4

3.363

2.422

0.572

5

3.371

2.413

0.578

6

3.376

2.400

0.583

7

3.380

2.394

0.586

8

3.308

--

--

w-ZnO

3.265

2.636

--

With the DFT-D2 correction, for freestanding case, a critical thickness of seven atomic ZnO layers was obtained, comparing with ten layers without vdW correction. It indicates that vdW plays an important role to describe the interlayer bonding. In the following context, vdW corrections are adopted in layered ZnO with and without Ag substrate. In table 1, with vdW corrections, we find that the lattice constant a keeps increasing when the ZnO layer thickness increases from one atomic layer


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to the phase transition layers. When exceeding the critical thickness, the lattice constant begins reducing down to that of bulk w-ZnO. In the second column of table 1, the average distance between two adjacent layers slightly decreases as we increase the film thickness. For the binding energy in column III, as the film thickness increases, the binding energy tends to enhance as well and essentially lead to stronger interlayer interaction. In reality, the contact between the semiconductor and the substrate should be considered. Here, we explore the phase transformation of ZnO on the pure and oxidized Ag(111) substrates. Firstly, we focus on the ZnO/Ag system. After geometry optimization, the layered configuration of the monolayer and bilayer ZnO were maintained due to the weak interaction between ZnO layers and the Ag substrate. The average spacing between monolayer ZnO and Ag substrate was 2.66 Å. For bilayer ZnO on Ag surface, the distance between the bottom ZnO monolayer and the Ag substrate was 2.65 Å, while the average ZnO interlayer spacing was 2.35 Å (Table 2), which is smaller than that of the freestanding bilayer ZnO by 4.2% and is consisted with the experimental observation of 2.20 ± 0.1 Å.16 In term of the adhesion energy between ZnO layers and Ag substrate, it is 0.51 eV and 0.61 eV per ZnO unit cell for monolayer and bilayer ZnO on the pure Ag substrate, respectively. Thus, bilayer ZnO could be more favorable to stay on Ag substrate with regard to monolayer ZnO. Nevertheless, their adhesion energy difference per unit cell is 0.1 eV. Such a small energy difference might allow the coexistence of the monolayer and bilayer structure during the layered ZnO growth on Ag substrate. As one more layer is introduced on the bilayer ZnO, we found that the phase transformation from 2D graphitic-like to 3D wurtzite occurs, well being consistent with the experimental observations.16 The role of Ag substrate to impact the phase transition thickness could be explained as follows. Ultrathin w-ZnO films, with alternate positive and negative charge layers 10 ACS Paragon Plus Environment

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along (0001) direction, possess an intrinsic polar field, destabilizing the wurtzite structure. On the other hand, ZnO layers with few atomic layers thick prefer a graphitic-like structure. Comparing to the sp3-like hybridizing of w-ZnO, graphitic-like ZnO adopt sp2-like hybridizing, thus, the bond strength reduces between Zn and O atoms. Therefore, the stability of wurtzite and graphitic structures of ZnO depends on the competition between the polar field and the chemical binding.26 Kan et al. found that, by doping electrons on wutzite ZnO, the added carriers can screen the intrinsic polar field and thus depolarize the structure to make wurtzite structures more stable.26 In this work, electron transfer from Ag to ZnO is also observed (Table 2). Consequently, the phase transition from graphitic to wurtzite structure occurs when ZnO thickness exceeds two atomic layers on the pure Ag substrate.

Table 2. Number of layers N, lattice constant a, average interlayer spacing d between two ZnO layers, adhesion energy Ead between ZnO layers and Ag substrates of ZnO/Ag(111) and ZnO/O/Ag(111) systems, and electrons (etr, e per ZnO unit cell) transform from pure Ag substrate to ZnO, calculated based on PBE+vdW functionals.

ZnO/Ag(111)

ZnO/O/Ag(111)

N

a (Å)

d (Å)

Ead (eV)

etr

a (Å)

d (Å)

Ead (eV)

1

3.391

--

0.513

0.020

3.391

--

0.510

2

3.391

2.350

0.607

0.027

3.391

2.478

0.609

3

3.391

2.493

0.639

0.028

The influence of oxidized Ag substrate on the stability of ZnO layers was also explored. Figure 3 shows the possible oxidization sites, i.e. fcc, hcp, bridge and top sites, on the Ag(111) surface. Both fcc site and hcp site lie on the Ag atom triangular center of the top most layer. Their difference arises 11 ACS Paragon Plus Environment


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from the beneath Ag atom arrangements. Fcc site locates right on top of Ag atoms belong to the third Ag layer while hcp site on top of the second Ag layer. Bridge site and top site lie on top of the top-most Ag-Ag bridge and Ag atom, respectively. After full ionic relaxation, O atom on the bridge site and top site gradually shift to the fcc site. According to the adhesion energy calculations, we found that the fcc site was energetically more stable than hcp site (1.34 eV for fcc site and 1.21 eV for hcp site). On this specific oxidized surface, we found that the planar form of graphitic-like ZnO is destroyed (the roughness of ZnO is 0.25 Å). This deformation of ZnO originates from the formation of chemical bonds (bond length is 1.99 Å) between interfacial Zn and O atoms of the oxidized Ag substrate. The adhesion energy between monolayer ZnO and oxidized substrate is 0.51 eV per unit cell. If one more ZnO layer was added, the adhesion energy increased up to 0.61 eV per ZnO unit cell. Comparing with pure Ag substrate, the oxidized one showed almost no difference in terms of the adhesion energy. Similarly, on the oxidized surface, ZnO favors to stay bilayer rather than that of monolayer structure. Also, the adhesion energy difference between monolayer-ZnO/O/Ag and bilayer-ZnO/O/Ag is around 0.1 eV/unit cell. Likewise, on oxidized Ag substrate, the monolayer and bilayer ZnO are likely to coexist.

Figure 3. Oxidization sites on Ag(111) surface. Gray spheres are Ag atoms. 3.2 Electronic properties of freestanding ZnO layers 12 ACS Paragon Plus Environment

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To investigate the electronic properties of ZnO layers, we firstly calculated the band structure of bulk w-ZnO within vdW correction (Table 3). It is well known that DFT calculations usually

underestimate the band gap of semiconductors, the HSE hybrid functional was thus employed. The calculated lattice constants a and c were consistent with the experimental values (Table 3). Furthermore, Hartree-Fock mixing is tuned to achieve the reasonable band gap. With a value of 25% Hartree-Fock mixing parameter, the band gap is 2.45 eV. When the contribution increased up to 37.5%, the gap is 3.35 eV (exp. 3.44 eV27). Therefore, all following calculations were performed within HSE hybrid functional with vdW corrections and the Hartree-Fock mixing parameter is set to 37.5%.

Table 3. Lattice constant a, c and band gap Eg of bulk wurtzite ZnO calculated by different methods (with vdW correction) and experimental values27. a (Å)

c (Å)

Eg (eV)

PBE

3.265

5.272

0.762

HSE(AEXX=0.25)

3.265

5.272

2.451

HSE(AEXX=0.375)

3.265

5.272

3.354

Experiment

3.242

5.188

3.44

Figure 4 shows the band structures and partial density of states (PDOS) of monolayer and bilayer ZnO. The PDOS indicate that the highest valence band is mainly contributed by the O-2p orbitals with small Zn-3d, Zn-4s, and Zn-4p contributions. The lowest conduction band is mainly contributed by O-2p, Zn-4s, and Zn-4p orbitals. This indicate that the valence band maximum (VBM) is mainly constructed by p-d hybridization between O-2p and Zn-3d orbitals, and s-p hybridization between Zn-4s and O-2p orbitals. The conduction band minimum (CBM) is mainly contributed by s-p (Zn-4s 13 ACS Paragon Plus Environment


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and O-2p orbitals) and p-p (Zn-4p and O-2p orbitals) hybridizations. From the band structures, both VBM and CBM locate at the gamma point (G) in the first Brillouin zone (BZ), displaying a direct band gap character. Further calculations showed that all graphitic-like ZnO layers have the direct band gaps. The band gaps of layered ZnO decrease from 4.14 eV to 3.65 eV as the atomic layers increase from one to seven (Table 4). Table 4 also provides some previous calculated band gaps of layered-ZnO and bulk w-ZnO. Utilizing PW91 functionals, Kang et al. and Topsakal et al. presented a much smaller band gap of w-ZnO with GGA functionals (0.75 eV13,14) comparing with the experimental results (3.44 eV27). Topsakal et al. further performed frequency-dependent G0W0 and GW0 calculations, and the band gaps of w-ZnO turned to be 2.76 eV and 3.29 eV for G0W0 and GW0 calculations, respectively.14 The band gap of freestanding monolayer and bilayer ZnO were calculated to be 5.64 eV and 5.10 eV, respectively, within the GW0 calculations. The large band gap in regard to the bulk w-ZnO stems from the dimensional effect. Due to the large band gaps from 3.65 eV to 4.14 eV, ZnO few layers have the potential to be utilized in the ultraviolet optoelectronics.

Table 4. Band gaps of layered ZnO and w-ZnO. Number of layers

1

2

3

4

5

6

7

w-ZnO

Reference

HSE

4.143

3.930

3.810

3.744

3.684

3.667

3.650

3.354

GW0

5.64

5.10

--

--

--

--

--

3.29

13

G0W0

4.87

4.45

--

--

--

--

--

2.76

13

PW91

1.68

1.44

--

--

--

--

--

0.75

13

PW91

~1.68

~1.44

~1.32

~1.24

~1.19

--

--

0.755

14

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Figure 4. Band structures and partial density of states of freestanding (a) monolayer and (b) bilayer ZnO. Red dash line denotes the Fermi level.

3.3 Impact of pure and oxidized Ag substrates on the electronic properties of ZnO When metal comes to contact with semiconductors, the Schottky barrier will be formed. This Schottky barrier has rectifying characteristics, suitable for use as a diode.28 One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by ΦB . The value of

ΦB depends on the combined electronic structures of metal and semiconductor. In this present work, the barrier ΦB is defined as the energy difference between the Fermi level and the conduction band minimum. Figure 5(b) and (d) illustrate the band structures and projected density of states (PDOS) of monolayer and bilayer ZnO on the pure Ag(111) substrate, respectively. Figure 5(a) and (c) show the differential charge densities of monolayer and bilayer ZnO on the pure Ag substrate. When pure Ag interfaces with monolayer ZnO, owing to the charge transfer from Ag to ZnO, ZnO is n-doped and consequently the Fermi level moves up into the ZnO conduction band region. The band gap of ZnO layers has a reduction of ~1 eV. This large Eg reducing might be correlated two aspects. Firstly, it 15 ACS Paragon Plus Environment


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partially originates from the lattice constant enlargement of ZnO when including Ag substrate. Secondly, based on the tight binding approximation, the band width is proportional to Zt, where Z is the coordination number and t is hopping integral. When Ag substrate is included, the coordination number of the bottom O atoms increases and the O band width expand accordingly. As a result, combining with strain effect, the ZnO band gap reduces by ~1 eV with including Ag substrate. Overall speaking, the whole system shows a metallic character comparing with the semiconducting performance of the freestanding layered ZnO and bulk w-ZnO. For bilayer-ZnO/Ag system, almost identical electronic structures are observed. In both systems (Figure 5(b) and (d)), low density of gap states are found. Based on the local density states analysis, these gap states originate from the formation of weak bonding between O and Ag. Because of the existence of the gap states, it fails to tune the Schottky barrier between ZnO/Ag contacts. In fact, this type of electronic properties with gap states is not favorable for real device applications. Specifically, the barrier between a metal and semiconductor is predicted by the Schottky-Mott rule, which is proportional to the difference of the metal-vacuum work function and the semiconductor-vacuum electron affinity. In this work, however, ZnO/Ag interfaces do not follow this rule. Instead, the chemical interaction between ZnO and Ag creates electron states within its band gap. The nature of these metal-induced gap states and their occupation by electrons tends to pin the center of the band gap to the Fermi level. Thus the height of the Schottky barriers in ZnO/Ag contacts shows little dependence on the value of ZnO or Ag work functions, in contrast to the Schottky-Mott rule.


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Figure 5. Differential charge densities of (a) monolayer and (c) bilayer ZnO on pure Ag(111) substrate. Iso-surface value is 0.003 eÅ−3. Brown atoms are Ag, red atoms are O, and blue atoms are Zn. Band structures and density of states of (b) monolayer and (d) bilayer ZnO on pure Ag(111) substrate. The green lines show the total contribution of the ZnO/Ag system. Red points represent the contribution of ZnO layers. The point size denotes the contribution ratio. Blue dash lines denote the Fermi level. In order to suppress the gap states, Ag substrate is oxidized before interfacing with ZnO. As a result, O-Ag bonding between ZnO layer and Ag substrate do not exist. Instead, chemical bonding between interfacial Zn and O atoms of the oxidized Ag substrate is formed. This specific Zn atom 17 ACS Paragon Plus Environment


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forms four bonds with surrounding O atoms, just like that in the w-ZnO. Figure 6(a) and (c) show the differential charge densities distribution of the ZnO/O/Ag system. As expected, the charge transfer between Ag surface and adjacent ZnO layer is greatly decreased compared to the pure Ag substrate case. From the band structures in Figure 6(b) and (d), the semiconducting character of ZnO layers is maintained. The calculations show that the Schottky barrier is around 0.8 eV. In contrast to freestanding ZnO layers, the band gaps of ZnO in ZnO/O/Ag systems are also reduced. While the band gap narrowing mechanism here is different from that in the ZnO/Ag systems. Chemical bonding between interfacial O and Zn gives rise to the phase transformation of ZnO from graphitic-like to wurtzite-like structure. Combine with the lattice constant enlargement of ZnO when contacting with oxidized Ag substrate, the band gap of ZnO reduces by ~1 eV. Comparing with ZnO/Ag system, the ZnO/O/Ag is more suitable for device applications due to the absence of any gap states.


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Figure 6. Differential charge densities of (a) monolayer and (c) bilayer ZnO on oxidized Ag(111) substrate. Iso-surface value is 0.003 eÅ−3. Brown atoms are Ag, red atoms are O, and blue atoms are Zn. Band structures and density of states of (b) monolayer and (d) bilayer ZnO on top of oxidized Ag(111) surface. The green lines show the total contribution of the ZnO/O/Ag system. Red points represent the contribution of ZnO layers. The point size denotes the contribution ratio. Blue dash lines present the Fermi level.

4. CONCLUSION In summary, we have performed DFT calculations to study the structural and electronic properties of graphitic-like ZnO few layers with and without Ag(111) substrate. For freestanding ZnO layers, our 19 ACS Paragon Plus Environment


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results show that the graphitic-to-wurtzite phase transformation occurs with ZnO thickness exceeding seven layers. From one layer to seven layers, the direct band gaps vary from 4.14 eV to 3.65 eV. In the presence of pure Ag(111) substrate, the 2D graphitic-like structure maintains for monolayer and bilayer ZnO. While, when one more layer is introduced on the bilayer ZnO, phase transformation from 2D graphitic-like to 3D wurtzite occurs. This originates from the electron transfer from Ag to wurtzite ZnO, which depolarize the structure to make wurtzite structures more stable. Compared to monolayer ZnO, the bilayer shows more energetically favorable to grow on the pure Ag substrate. In terms of electronic structures, both monolayer and bilayer ZnO show an n-doped behavior owing to the charge transfer from Ag surface to ZnO layer. Simultaneously, the

low density of gap states appears because of the weak covalent bonding between Ag and O atoms. For the oxidized Ag(111) substrate, the planar shape of the ZnO monolayer and bilayer are deformed originating from the formation of chemical bonds between interfacial Zn and O atoms of the oxidized Ag substrate. Nevertheless, the semiconducting performance of ZnO is preserved well and the Schkotty barrier is 0.8 eV.

ACKNOWLEDGEMENTS This work is supported by National Natural Science Foundation of China (11304161, 11104148, and 51171082), Tianjin Natural Science Foundation (13JCYBJC41100, 14JCZDJC37700), the National Basic Research Program of China (973 Program with No. 2014CB931703), Specialized Research Fund for the Doctoral Program of Higher Education (20110031110034) and the Fundamental Research Funds for the Central Universities. We thank the technology support from the Texas Advanced

Computing

Center

(TACC)

at

the

University

of

Texas

at

Austin

(http://www.tacc.utexas.edu) for providing grid resources that have contributed to the research results 20 ACS Paragon Plus Environment

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reported within this paper.

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For Table of Contents Only

Differential charge densities and band structures of bilayer ZnO on pure (left side) and oxidized (right side) Ag(111) substrate.


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Pure and Oxidized Ag Substrate Influence on the Phase

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