Electric-Field- and Hydrogen-Passivation-Induced Band Modulations

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Electric-Field- and Hydrogen-Passivation-Induced Band Modulations in Armchair ZnO Nanoribbons Liangzhi Kou,† Chun Li,‡ Zhuhua Zhang,† and Wanlin Guo*,† Institute of Nano Science, Nanjing UniVersity of Aeronautics and Astronautics, Nanjing 210016, China, and School of Mechanics, CiVil Engineering and Architecture, Northwestern Polytechnical UniVersity, Xi’an 710072, People’s Republic of China ReceiVed: October 7, 2009; ReVised Manuscript ReceiVed: NoVember 13, 2009

We report on the electric-field- and H chemical-absorption-induced band manipulations of armchair ZnO nanoribbons using first-principles calculations. It is shown that the band gap of a semiconducting armchair nanoribbon can be reduced monotonically with increasing transverse field strength, demonstrating a giant Stark effect. The critical field strength to completely close the band gap decreases with increasing ribbon width, while it is almost independent of the stacking thickness. On the other hand, the nanoribbon with the edges fully passivated shows an enhanced gap but a slightly weaker Stark effect. We also observe hydrogentermination-induced metallization of the ribbons when only the edge O atoms are passivated, which results from an n-type doping effect. These findings suggest potential ways of band engineering in armchair ZnO nanoribbons. Introduction Since the discovery of nanoribbons of semiconducting oxides in 2001,1 it has sparked an intense research effort toward the understanding of these novel materials with promising applications in nanoelectronic and spintronic devices.2,3 The ZnO nanoribbon (ZNNR) is found to be one of the most typical and successful examples of these oxide nanoribbons. Owing to the excellent optical, piezoelectric, and biocompatible properties inherited from its bulk material,4,5 ZNNRs have been successfully applied in field effect transistors,6 ultrasensitive nanosize gas sensors,7 nanoresonators,8 nanocantilevers,9 and so on. As functional building blocks in various nanoscale devices, especially in field effect transistors, a tunable band gap in nanostructures would be highly desirable because it would entail great flexibility in the design and optimization of nanodevices, in particular, if it could be tuned by applying a well-controlled external electric field. In the past decades, an applied external electric field has been extensively proved to efficiently modulate the electronic properties and even enable insulator-metal transitions in numerous low-dimensional nanostructures, such as boron nitride nanoribbons,10 carbon nanotubes, and boron nitride nanotubes,11 etc.12 However, the effects of electric fields on the electronic properties of semiconducting armchair ZnO nanoribbons (A-ZNNRs) remain to be investigated. On the other hand, chemical absorptions at the edges of nanostructures, especially hydrogen absorptions, have been found to be able to affect the electronic structures of nanoscale materials.13-15 In the fabrication process of ZNNRs, hydrogen is frequently present during the growth using different techniques.16 Even in highvacuum systems, H2O always exists as a residual gas, which may serve as a source of hydrogen. In addition, the response of ZNNRs to gas atmosphere, especially vapor, is also an important issue that needs to be addressed for applications in functional * To whom correspondence should be addressed. E-mail: wlguo@ nuaa.edu.cn. † Nanjing University of Aeronautics and Astronautics. ‡ Northwestern Polytechnical University.

devices. The knowledge obtained may be used not only to assess precisely the operating performance and life of ZNNR-based devices but also to derive more meaningful applications in gas sensing.7 Moreover, exposure to hydrogen can strongly affect the electronic properties of other ZnO nanostructures, as revealed in recent investigations.17-20 Recently, using ab initio calculations, Jia et al. demonstrated that the hydrogen adsorption on ZnO nanowires will convert a semiconducting nanowire21,22 into metal.23 In addition, some current calculations have shown that hydrogen absorption to the Zn-terminated edges will result in ferromagnetic half-metal zigzag ZnO nanoribbons (Z-ZNNRs),24 whereas passivating the edge with sulfur will significant affect the electronic and magnetic properties.25,26 Therefore, the interaction between hydrogen and A-ZNNRs is fundamentally interesting as well, and a deep understanding of the band modulation in the potential A-ZNNRs by hydrogen absorptions at the edges is thereby necessary for the development of nanoscale optical and electrical devices. In this paper, we present from density functional theory (DFT) calculations strong band modulations by external electric fields and atomic hydrogen chemical absorption in A-ZNNRs. It is shown that the band gap of an A-ZNNR can be decreased by a transversely applied electric field and eventually closed under field strengths beyond a critical value. This electric field effect is more remarkable in wider A-ZNNRs and is robust to the stacking of ribbon layers and full hydrogen termination. Most interesting, partially passivating all the edge O atoms of A-ZNNRs could induce metallization of the systems. Models and Technique Details The A-ZNNRs in our studies are originally constructed by cutting a monolayer along armchair lines, as shown in Figure 1. For the structure with a few number of ZnO layers, it has been recently demonstrated that each layer prefers a planar configuration in which both cations and anions are coplanar after relaxation.27 The multilayer ribbons are constructed by placing the planar layers on top of each other with AB stacking. Following conventional custom, A-ZNNRs are classified by the

10.1021/jp909584j  2010 American Chemical Society Published on Web 12/10/2009

Band Modulations in Armchair ZnO Nanoribbons

Figure 1. Schematic diagrams of (a) an A-ZNNR under an external electric field, (b) a monolayer A-ZNNR-2H, and (c) a monolayer A-ZNNR-HO. The suffixes 2H and HO stand for the hydrogen termination of all edge atoms and only edge O atoms, respectively. The monolayer ribbons are assumed to be infinitely long along the y direction. The region contained by the dashed rectangle is a unit cell calculated in this work.

number of dimer lines (n) across the ribbon width and denoted as An-ZNNRs (Figure 1a). Calculations are carried out with the linear combination of atomic orbital basis implemented in the SIESTA package,28 using the Perdew-Burke-Ernzerhof (PBE)29 of generalized gradient approximation (GGA) for the exchange-correlation energy and norm-conserving pseudopotentials for the corevalence interactions. The double-ζ polarized numerical atomicorbital basis sets for Zn, O, and H are used. Single k-point and 2 × 2 × 6 Monkhorst-Pack k-point grids are used in the structural optimizations and energy calculations, respectively. An energy cutoff of 500 Ry is sufficient to converge the grid integration of the charge density, and atomic positions are fully relaxed under applied electric fields using a conjugate gradient method so that the force on each atom is less than 0.02 eV/Å. Vacuum layers of at least 10 Å are chosen in width and thickness directions in order to guarantee negligible interactions between the neighboring ZNNRs. All these pseudopotentials and parameters adopted here have been successfully used in other ZnO nanostructures in our previous studies,30,31 and their validity has been proven. The external electric field is modeled by transversely adding a sawtooth-like potential to the nanoribbon.32 It should be noted that the most commonly used DFT functional always underestimates the band gap, but it is powerful to predict a correct trend toward the band gap change and properly unveil the physical mechanism.10-12 More precise prediction of the field-induced gap modulation needs GW or some hybrid DFT methods, which are expected in the future studies. Results and Discussion 1. Electric-Field-Induced Band Gap Modulations in Bare A-ZNNRs. All the bare A-ZNNRs involved in our calculations exhibit a semiconducting nature; the electronic structure always displays a direct band gap of about 2 eV, which is well consistent with the previous theoretical prediction.33 Similar to other electronic modulations, external electric fields are transversely applied to modulate band gaps of the nanoribbons, as shown in Figure 1a. Figure 2 shows the evolution of the band gaps for monolayer A-ZNNRs with different widths under external electric fields (solid lines). Here, only the modulations under electric fields with one specified direction are presented because no pronounced differences are found when the field direction is reversed, owing to the symmetrical structure in A-ZNNRs. Unlike the cases in single-walled carbon nanotubes,

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Figure 2. Band gap evolution of monolayer A-ZNNRs (solid lines) and A-ZNNRs-2H (dashed line) as a function of ribbon width under various external electric fields. In the plotting, 2H represents the fullpassivated ribbons, whereas Bare represents the ribbons without passivation.

Figure 3. Band gap evolution of bare A-ZNNRs as a function of ribbon thickness (number of layers) under applied fields. The corresponding case for the A-ZNNRs-2H with edges fully passivated is shown in the inset. Here, 2H and Bare have the same meanings as in the stipulations of Figure 2.

where the variations of band gaps are quit small below a threshold electric field,11 the semiconducting A-ZNNRs, on the other hand, demonstrate a remarkable reduction in band gap once an external electric field is applied. This difference results from reduced screening of the electric field in the A-ZNNRs. It is clearly shown that the band gap reduces nearly linearly with increasing electric field strength and eventually closes when the electric field strength reaches a critical value for each specified A-ZNNR. Meanwhile, it can be noted that the semiconductormetal transition occurs at a lower value of critical electric field with increasing width. For example, the A13-ZNNR experiences a semiconductor-metal transition at an external electric field of 0.3 V/Å compared with that of 0.45 V/Å for the A11-ZNNR. This width dependence is induced by the different field-induced electrostatic potential differences between two ribbon edges that are proportional to the width of the A-ZNNRs, as will be specially discussed below. When an A-ZNNR is stacked over another one, the bilayer A-ZNNR remains semiconducting, but with a slightly reduced band gap, which is due to the increased dispersion of the subbands induced by the interlayer interaction. Nevertheless, the variation curve of the band gap with electric field is almost parallel to that of the monolayer system with the same width, as shown in Figure 3. When the number of layers further increases to three or four, both the band gaps and their variations under applied electric fields are nearly similar to the bilayer

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Kou et al.

Figure 5. (a) Band structure (left) and density of state (right) of A11ZNNR-HO. The Fermi level is set to zero. (b) Charge distribution of the bands crossing the Fermi level is exhibited. Figure 4. Band structures (left) and the corresponding charge densities for CBM (right upper) and VBM (right lower) of A13-ZNNR at (a) 0 V/Å and (b) 0.3 V/Å. The isosurface is 3 × 10-4 e/Å3. The Fermi level is shifted to zero.

case. The thickness independence of gap modulations originates from the fact that the electrostatic potential difference between the edges is proportional to the ribbon width but rarely changes with the number of ribbon layers, which induces essentially the same charge redistribution. To further investigate the underlying physics, we study the charge redistributions of A-ZNNRs driven by external electric fields. Figure 4 exhibits the plots of charge densities for the valence band maximum (VBM) and conduction band minimum (CBM) states in the A13-ZNNR with and without an external electric field, respectively. For the A13-ZNNR under zero field, the charge density of the VBM is found to be an edge state with wave function localized at the edge O atoms, whereas the CBM is a different localized state with a wave function mainly concentrated at the Zn atoms of the ribbon center (top right panel of Figure 4a). When an external electric field is applied, the charges are redistributed due to the break of electrostatic potential symmetry, which is known as giant Stark effects.34 The wave functions of the VBM and CBM states now are localized at the opposite edges, where the external electrostatic potential felt by a charge is higher (VBM) or lower (CBM) (right panel of Figure 4b). This charge redistribution results in splitting of the sub-band levels and decreasing of the band gaps of A-ZNNRs (left panel of Figure 4b). It should be noted that the width range of the ribbons included in the present investigations is limited due to high computational costs, and the critical electric field required to close the band gap of an A-ZNNR in our studied range is relatively high and seems to be difficult to achieve in practical applications. Nevertheless, as discussed above, the critical field strength will decrease dramatically with further increasing ribbon width. In fact, the ribbon width obtained in experiments usually reaches up to tens or even hundreds of nanometers, which would make the band gap sensitively modulated and easy to be closed by an electric field available in practice. The sensitive field-induced band gap modulation features the A-ZNNR as a promising material for diverse applications in nanodevices. 2. Hydrogen-Passivation-Induced Band Modulations. Now we turn our attention to the effect of edge hydrogen termination on the electronic properties of the A-ZNNRs. Different from the 3-fold coordination in ZnO nanosheets, each Zn or O atom at the edges of bare A-ZNNRs is only 2-fold

coordinated. Upon hydrogen adsorption, the edge atoms become 3-fold coordinated with the same bond order as those in the ribbon interior. For simplicity, we choose two representative situations: (i) both edge Zn and O atoms are passivated and (ii) only the edge O atoms are passivated with H atoms (denoted as A-ZNNRs-2H and A-ZNNRs-HO), as shown in Figure 1b,c, respectively. To evaluate the relative stabilities of passivated ribbons, the binding energy Eb is computed as Eb ) ETotal Ebare - nHEH, where ETotal and Ebare are the calculated total energies of the nanoribbon with and without passivated edges, respectively, and nHEH is the sum of the free atom energy for H in a unit cell. As expected, the binding energy of the nanoribbons (both passivated cases) decreases with increasing widths and eventually approaches to a constant. For the two passivated ribbons with the same width, the A-ZNNR-HO is slightly more stable. The case of only edge Zn atoms being passivated is not considered because of the large positive binding energy, indicating an endothermic process. When all the edge atoms are passivated using atomic hydrogen, the system remains semiconducting in behavior, but with an enhanced band gap. Taking monolayer A11-ZNNR-2H as an example, the calculated band gap is 2.66 eV, slightly larger than 2.34 eV of the bare ribbon due to the full passivation of the dangling bonds of the A-ZNNR that removes the localized edge states around the Fermi level.23 This gap difference decreases with increasing width, as indicated in Figure 2. However, when only the edge O atoms are passivated, the A-ZNNR is strikingly metallized. As shown in the band structure of Figure 5a, a band crossing the Fermi level clearly indicates its metallicity, similar to the results found in other ZnO nanostructures.18-20 Because each H atom absorbed to O atoms will lose electrons, it acts as an n-doping source, which provides electrons to partially occupy the 2p state of edge O atoms and the 4s state of edge Zn atoms, inducing the metallic behavior. This assumption is supported by the total and partial density of states (DOS) of the valence electrons and its charge distributions. As displayed in the right plot of Figure 5a, the electronic states crossing the Fermi level mainly come from the 2p state of O atoms and the 4s state of Zn atoms, mostly localized at the O and Zn atoms of the edge/subedge (Figure 5b), while the contribution from the other valence states is much less significant (not shown here). In reality, the gained electrons of O atoms from the absorbed H atoms and that of the neighboring Zn atoms through a back-donation from O atoms eventually result in more delocalized and active electronic states of the edges and consequently lead to an enhanced electrical conductivity of

Band Modulations in Armchair ZnO Nanoribbons

Figure 6. Band structures (left) and the corresponding charge densities for CBM (right upper) and VBM (right lower) of A13-ZNNR-2H at (a) 0 V/Å and (b) 0.3 V/Å. The isosurface is 5 × 10-4 e/ Å3. The Fermi level is shifted to zero.

A-ZNNRs-HO.18 Owing to the metallicity, no significant variation in the electronic property is found when an electric field is applied. The present results provide a new opportunity to tailor the electronic band structures of A-ZNNRs by careful control of the hydrogen adsorption ratio on the nanoribbon edges. 3. Electric-Field-Induced Band Gap Modulations in A-ZNNRs-2H. Since we have studied the band gap modulations induced by external electric fields and H passivation, it should be interesting to investigate the combined effect of the two factors. Here, the band gap variation of monolayer A-ZNNRs2H under transversely applied external electric fields is presented, as shown in Figure 2 (the dashed line). It is shown that each A-ZNNR-2H has a slower decreasing rate of the band gap and requires a larger critical electric field to close its band gap compared with the bare A-ZNNR of the same width. A noticeable reason is that the H-passivated nanoribbons have higher band gaps than the bare ones. To further reveal the underlying physical mechanism, the charge redistributions of A-ZNNRs-2H induced by external electric fields are examined. Figure 6 shows the results of A13-ZNNR-2H as an example. Similar to the bare A13-ZNNR, the distributions of the CBM and VBM states in the A13-ZNNR-2H with and without applied electric fields are also exhibited, respectively. When no electric field is applied, the CBM exhibits a similar distribution as that in the bare A13-ZNNR where the charge densities are mainly concentrated at Zn atoms of the ribbon center (upper panel in Figure 6a). In contrast, the charge distribution of VBM is totally different from that of the corresponding bare system, which is almost uniformly distributed on O atoms throughout the ribbon except the edge O atoms due to removing of the edge states after passivation (lower panel in Figure 6a). Once an external electric field is applied, the charge distributions of the CBM and VBM are driven to localize at opposite edges owing to the different electrostatic potential, as analyzed above. However, the degree of the charge distribution seems to be less sensitive to the applied electric field for the passivated ribbon (Figure 6b). This is due to the reduced distance between the charge density centers of the VBM and CBM; as in this case, the unperturbed VBM state of A-ZNNRs-2H is concentrated on the ribbon center, thus reducing effective electrostatic potential

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1329 change induced by electric fields.35 For instance, under an electric field of 0.3 V/Å, both the energy upshift (downshift) and the degree of the charge redistribution to higher (lower) electrostatic potential edge of VBM (CBM) in the passivated ribbon are much less than the counterparts in the bare system with the same width (n ) 13), as evidenced by comparing Figures 4b and 6b. This gives a higher critical electric field for semiconductor-metal transition, rendering a weaker Stark effect in A-ZNNRs-2H. Nevertheless, the difference in decreasing rate of the gap modulation decreases with increasing ribbon width, as indicated in Figure 2. Therefore, for ribbons of experimental width, it would be the same for gap modulations in both bare and passivated ZNNRs, highlighting the robustness of the band engineering by electric fields. On the other hand, we also calculate the band gap variation of stacked multilayer A-ZNNRs-2H induced by applied electric fields, as shown in the inset of Figure 3. It is shown that the gap modulations are robust to the number of stacked layers, and this is quite similar to the case of the corresponding bare systems with the same size. The only different point is that the band gap value is enhanced due to the full edge passivation. To sum up, the thickness effect is insignificant for the present field-induced gap modulation on both cases with bare and passivated edges, at least in the investigated range of thickness and ribbon width. Conclusions In conclusion, our first-principles calculations show that the electronic properties of A-ZNNRs can be effectively modulated by external electric fields and H chemical absorptions. The band gaps exhibit a gradual reduction once an external electric field is applied due to the symmetrical structure, and the semiconductor-metal transition can be finally realized as a result of Stark effects. In addition, the critical electric field for this transition decreases with increasing width, whereas the thickness effect on the band gap modulation is negligible. On the other hand, the hydrogen passivation at the edges in A-ZNNRs can also efficiently modify the electronic properties. The nanoribbons with dangling bonds completely passivated by H atoms remain semiconducting, exhibiting a remarkably enhanced electronic gap, which is slightly more difficult to be modulated by applied fields. However, the A-ZNNRs with hydrogen only terminated on the 2-fold coordinated O atoms are metallic. The electronic states associated with the metallicity mainly originate from the 2p state of O atoms and the 4s state of Zn atoms at the nanoribbon edges. The ample methods for electronic property modulation methods provide some new ways of band engineering in A-ZNNRs and could be used in the future nanodevices based on the ZNNRs. Acknowledgment. This work is supported by the 973 Program (No. 2007CB936204), the National NSF (No. 10732040), the Jiangsu Province NSF (BK2008042), and the Ministry of Education (Nos. 705021 and IRT0534) of China. References and Notes (1) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (2) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P.; Hughes, W. L.; Yang, R. AdV. Funct. Mater. 2004, 14, 943. ¨ zgu¨r, U ¨ .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (3) O Dog˘an, S.; Avrutin, V.; Cho, S.-J.; Morkoc¸, H. J. Appl. Phys. 2005, 98, 041301. (4) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829. (5) Li, C.; Guo, W.; Kong, Y.; Gao, H. Appl. Phys. Lett. 2007, 90, 033108.

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