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Tuning Electronic Structures of ZnO Nanowires by Surface Functionalization: A First-Principles Study Shu-Ping Huang, Hu Xu, I. Bello, and R. Q. Zhang* Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China ReceiVed: March 16, 2010; ReVised Manuscript ReceiVed: April 12, 2010
Using first-principles calculations, we systematically investigated electronic structures of ZnO nanowires of various sizes modified with different surface coverages of H, F, Cl, NH2, and NH3. We found that the 50% H(O), 50% F(Zn); 100% F; 50% H(O), 50% Cl(Zn); and 50% NH3(Zn) passivations are energetically very favorable compared with 100% H and 100% Cl passivations. The surface chemistry involved presents a strong effect on the band structure of ZnO nanowires as significant as that of quantum confinement. The 100% F passivation of the surfaces of ZnO nanowires leads to a decrease in the band gap, whereas the 100% H, 50% H(O), 50% F(Zn); 50% H(O), 50% Cl(Zn); 100% Cl; 50% H(O), 50% NH2(Zn); and 50% NH3(Zn) passivations increase the band gap when compared with the bare wires. Like the size effect, the surface passivation is an additional option to engineer electronic properties of ZnO nanowires. 1. Introduction All II-VI nanostructured semiconductors are wide-band-gap materials, offering potential applications in both nanoscale science and nanotechnology.1-9 In particular, ZnO-nanostructured materials are attractive and have been the materials of interest in many experimental investigations over recent years owing to their unique optical, electrical, and optoelectronic properties as well as to their potential technological applications. ZnO nanomaterials can be synthesized with diverse morphologies, such as nanotubes, nanorings, nanobelts, nanowires, and nanocages. However, ZnO nanowires (ZnONWs) are the dominant one-dimensional (1D) nanostructures. Using a bottomup approach, they can be assembled into larger functional blocks and nanoscale electronic,10 electro-optical devices,11 gas12 and biochemical sensors,13 field effect transistors,14 nanogenerators,15 and so forth. The properties of ZnONWs have been intensely studied both experimentally and theoretically.16-23 However, the successful integration of ZnONWs into devices depends on the degree of controlling their structures and physical properties. These properties can considerably vary with a change in diameter, morphology, doping, and surface functionalization. Evidently, synergetically studying ZnONWs’ properties and their structures as well as their treatment are important for further investigation and design of functional nanosystems and nanodevices. The surface is crucial for determining the physical properties of nanostructures because of the significant surface-to-volume ratio. The surface behavior greatly affects the nature of nanostructures. Guo et al. have shown that surface passivation doping can be an alternative approach to conventional volume doping for modulating the conductivity of silicon nanowires (SiNWs).24 This finding referring to SiNWs implies that surface passivation may also apply to modulating the electronic properties of a wide range of nanomaterials. Despite that ZnO nanomaterials have been of interest for construction of many gas- and biosensing devices because of ZnO’s biocompatibility, * To whom correspondence should be addressed. E-mail: aprqz@ cityu.edu.hk.
its wide-band-gap properties, high chemical stability, and low cost,25-27 few theoretical studies have been carried out on the adsorption of chemical species on both ZnO surfaces and ZnONWs. Therefore, the scope of this paper is systematically studying the structural stability and electronic properties of ZnONWs with different surface coverages of H, F, Cl, NH2, and NH3, respectively, resulting from simulated dissociative adsorption of H2, HF, F2, HCl, Cl2, and NH3 and the molecular adsorption of NH3. The considered surface coverages include 100% H, 50% H(O), 50% F(Zn); 100% F; 50% H(O), 50% Cl(Zn); 100% Cl; 50% H(O), 50% NH2(Zn); and 50% NH3(Zn) passivations. It is expected that such theoretical studies on the electronic properties of surface-decorated ZnONWs will lay the foundation for developing and fabricating new devices and will have serious implications for the realization of quantum computation and communication. 2. Computational Details Our calculations were performed using density functional theory (DFT) within the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP).28 We adopted the Perdew and Wang (PW91) form29 of the GGA with the projected augmented wave (PAW)30 potentials. We placed the ZnONW in a supercell and separated it from its image in the neighboring supercell by a vacuum region of 15 Å. We then performed a full relaxation of the ionic positions with no symmetry constraints on all ZnONW structures. Cutoff energies of 400 and 500 eV were used for the atomic relaxation and the energy and electronic property calculations, respectively. In addition, we adopted 1 × 1 × 4 and 1 × 1 × 8 k-point Monkhorst-Pack meshes for the geometric relaxations and electronic density calculation of the final configuration, respectively. All atoms were allowed to relax during geometry relaxation until Hellmann-Feynman forces smaller than 0.02 eV/Å were reached. We studied ZnONWs ranging from 0.6 to 1 nm in diameter and having different cross sections, as shown in Figure 1. We constructed the initial atomic structures of the ZnONWs along
10.1021/jp102388g 2010 American Chemical Society Published on Web 04/27/2010
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Figure 1. Side views (the upper panels) and top views (the botton panels) of relaxed ZnONWs with different sizes and cross-sectional patterns: (a) triangular, (b) quadrangular, and (c) hexangular. The red (gray) spheres represent O (Zn) atoms. Zn(s) and O(s) are the atoms in the outermost surface layer, and Zn(sub) and O(sub) are those in the second surface layer. Zn(c) and O(c) are the atoms in the core.
Figure 2. Top views of triangular ZnONWs with different passivations: (a) 100% H, (b) 50% H(O), 50% F(Zn), (c) 100% F, (d) 50% H(O), 50% Cl (Zn), (e) 50% H(O), 50% NH2(Zn), and (f) 50% NH3(Zn). The red, gray, azure, green, blue, and white spheres represent O, Zn, F, Cl, N, and H atoms, respectively.
the 〈0001〉 directions from the bulk wurtzite structure. The ZnONWs with 100% H; 50%H(O), 50%F(Zn); 100% F; 50% H(O), 50% Cl(Zn); 100% Cl; 50% H(O), 50% NH2(Zn); and 50% NH3(Zn) passivations were then built, as shown in Figure 2. The optimized structural parameters of the bulk ZnO were a ) 3.292 Å, c ) 5.292 Å, and u ) 0.380, which were in good agreement with the experimental values,31 indicating a high accuracy of the present computational method. In our calculations, the bulk ZnO showed a direct band gap of 0.76 eV, which was much smaller than the experimental value of 3.37 eV.32 The underestimation of the band gap might result from the selfinteraction problem,33 the discontinuity of the exchangecorrelation potential,34 and incorrect description of the asymptotic behavior by LDA/GGA exchange-correlation potentials,35 which often decay too fast. However, this underestimation will
not affect the trends of band gaps for ZnONWs with different sizes and coverages. 3. Results and Discussion Table 1 lists the structural parameters of relaxed ZnONWs with different surface passivations. Clearly, the surface relaxation plays an important role in the bare wires.28 The surface Zn(s) atoms relax inward, whereas the surface O(s) atoms relax outward. The surface Zn(s)-Zn(s) distances (2.80 Å) and the subsurface Zn(sub)-Zn(sub) distances (2.99 Å) for the bare triangular ZnONWs are significantly smaller than those in ZnO bulk (3.26 Å) but are close to the bond length of bulk hcp-Zn, which is 2.66 Å;36 also, all surface Zn-O bond lengths are smaller than those in the bulk ZnO.
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TABLE 1: Selected Bond Lengths of Relaxed Triangular ZnONWs with Various Surface Passivationsa triangularb
bare
100% H
Zn(sub)-Zn(sub) Zn(s)-Zn(s) (1) Zn(s)-Zn(s) (2) Zn(s)-O(s)|
2.99 2.80 3.35 1.90 1.88 1.92
3.24 3.82 3.17 2.07 2.03 2.14
1.99 1.97 2.04
2.03 2.03 1.98 1.55 0.99
Zn(s)-O(s)⊥ Zn(s)-O(sub) Zn(sub)-O(s) Zn(sub)-O(sub)| Zn-H O-H F-F/Cl-Cl O-F/Cl Zn-F/Cl/N H · · · F/Cl H· · ·N H· · ·O N-H
50% H 50% F
100% F
50% H 50% Cl
3.28 3.56 3.27 1.98 1.95 2.09 2.07 2.02 2.02 1.99
3.17 3.86 3.11 2.16 1.99 2.21 2.15 2.03 2.03 1.97
3.25 3.62 3.22 2.03 2.00 2.12 2.08 2.02 2.03 1.99
1.04
1.88
100% Cl 3.22 3.53 3.23 2.02 2.00 2.06 2.08 2.03 2.02 2.03
1.01 2.22 2.60 1.49 1.79
1.48 1.50
2.20
50% H 50% NH2
50% NH3
3.26 3.60 3.22 2.02
3.34 3.33 3.16 1.95
2.09 2.08 2.02 2.03 2.00
2.00 1.99 2.00 1.99 2.03
1.09 2.35 2.41 1.98 2.35, 2.58
1.97
2.12
2.07 2.10 1.54 1.02
1.66 1.02 1.08
a
All units are in angstroms. b Zn(s) and O(s) are the atoms in the outermost surface layer, and Zn(sub) and O(sub) are those in the second surface layer. Zn(s)-O(s)| denotes the length of the bond parallel to the 〈0001〉 directions, whereas Zn(s)-O(s)⊥ is the length of the bond perpendicular to the 〈0001〉 directions.
Additionally, the situation in the quadrangular and hexangular ZnONWs is similar to that in the triangular ZnONWs. This inward contraction of the surface atoms results in an increase in the lattice constant c of the bare wires. The optimized lattice constants c of various ZnONWs are given in Table 4. The optimized c for the triangular, quadrangular, and hexangular bare nanowires are 5.40, 5.41, and 5.42 Å, respectively, which are larger than the value (5.29 Å) in ZnO bulk. The Zn-O bond lengths and bond angles in the core of the nanowires are less affected and are close to bulk ZnO values, indicating that the effect of passivation is localized. When the surface is passivated, the Zn(s)-Zn(s) distances of ZnONWs are much larger than those in ZnO bulk, but the subsurface Zn(sub)-Zn(sub) distances are recovered and are close to those in the bulk. After passivation by 100% H; 50% H, 50% F; 50% H, 50% Cl; 100% Cl; 50% H, 50 %NH2; or 50% NH3, the lengths of surface Zn-O bonds along the axial direction of NWs (Zn(s)-O(s)|) are close to the value in ZnO bulk. When the surface is fully passivated by F, one of the Zn(s)-O(s)| bonds is stretched from 2.00 to 2.16 Å. Note that there are two nonequivalent surface Zn(s)-O(s)| bonds. It can be seen from Figure 2 that, in the case of full F saturation, the surface Zn(s) atoms relax outward, while the surface O(s) atoms relax inward, which differs from the case of bare wires. The distances of surface Zn-O bonds perpendicular to the axial direction of NWs (Zn(s)-O(s)⊥) become larger after the surface is passivated by all the chemical species we studied, except for 50% NH3passivated Zn, whereas the effect of F is strongest because of its high electron affinity. The Zn(s)-O(sub) and Zn(sub)-O(s) bond distances are close to those in bulk ZnO. Additionally, the bond lengths of O-H (around 1.0 Å) and Zn-H (1.55 Å) in full H passivation, N-H (1.02 Å) in 50% H, 50% NH2 passivation, and O-F (1.49 Å) in full F passivation are close to those corresponding to H2O (0.96 Å), gas-phase ZnH (1.59 Å), and OF2 (1.41 Å). The O-Cl bond length is 1.98 Å, much larger than that in HClO (1.69 Å) because of weaker interactions. Also, the lattice constant c value is smaller than the value in ZnO
bulk when the surface of ZnONWs are passivated by 100% H; 50% H, 50% F(Cl, NH2); or 50% NH3. The F or Cl full passivations make the c value 0.02-0.04 or 0.10-0.13 Å larger than it is in the bare wires, which is the result of the closest F or Cl atoms on the surfaces of the NWs tending to maintain a distance between each other because of the repulsion. In addition, intramolecular hydrogen bonds O-H · · · F/Cl are formed in the 50% H, 50% F/Cl/NH2 and 50% NH3 surfacepassivated ZnONWs. The average H · · · F, H · · · Cl, H · · · N, and H · · · O distances are 1.49, 1.54, 1.66, and 2.09 Å, respectively. These hydrogen bonds elongate the Zn-F, Zn-Cl, or O-H compared with those corresponding to ZnF2, ZnCl2, or H2O, as well as one-third of the N-H bonds in 50% NH3, compared with those in NH3. The adsorption energy per molecule for the passivated ZnONW is calculated using equation wire Ea ) [Etot - Ewire bare - nEmolecule]/n wire wire is the total energy of a passivated ZnONW, Ebare is where Etot the total energy of a bare NW, n is the number of the chemically or physically adsorbed gas molecules (H2, HF, F2, HCl, Cl2, or NH3), and Emolecule is the total energy of the involved gas molecules. Table 2 presents the adsorption energies for the ZnONWs adsorbed with different molecules. All the adsorption energies are negative, indicating that all adsorptions are exothermic and implying that 100% coverage on these ZnONWs should be possible to obtain. The absolute value of the adsorption energy for the triangular ZnONWs is larger than those for the quadrangular and hexangular ZnONWs because the triangular ZnONW is the most unfavorable of the bare wires. The absolute value of the adsorption energy of Cl2 is the smallest, which agrees with the observation that pure ZnO thick films are almost insensitive to chlorine gas at room temperature.26 Moreover, the adsorption energy of H2 is significantly smaller than the
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TABLE 2: Adsorption Energies (eV/molecule) for the ZnONWs Adsorbed with Various Molecules adsorption energy
100% H
50% H 50% F
100% F
50% H 50% Cl
100% Cl
50% H 50% NH2
50% NH3
triangular quadrangular hexangular
-0.52 -0.50 -0.47
-1.41 -1.39 -1.35
-1.66 -1.62 -1.59
-1.73 -1.71 -1.74
-0.41 -0.39 -0.35
-0.73 -0.71 -0.69
-0.89 -0.87 -0.84
TABLE 3: Atomic Charges from Bader Charge Analysis for Triangular ZnONWs with Different Passivations
Zn(s) Zn(sub) Zn(c) O(s) O(sub) O(c) H(Zn) H(O)
bare
100% H
1.13∼ 1.14 1.13∼ 1.16 1.13 -1.15∼ -1.13 -1.17∼ -1.16 -1.17
0.97∼ 0.98 1.23∼ 1.24 1.20 -1.64∼ -1.26 -1.23∼ -1.22 -1.20 -0.35 -0.32 0.63∼ 1.0
50% H 50% F 1.28∼ 1.30 1.17∼ 1.18 1.25 -1.45∼ -1.23 -1.16∼ -1.15 -1.26
100% F 1.28∼ 1.35 1.25∼ 1.26 1.18 -0.59∼ -0.52 -1.20 -1.15
0.65∼ 0.87
50% H 50% Cl
100% Cl
1.16∼ 1.26 1.18∼ 1.26 1.23 -1.43∼ -1.27 -1.25∼ -1.15 -1.22
1.21∼ 1.24 1.20∼ 1.21 1.25 -0.97∼ -0.92 -1.16∼ -1.15 -1.23
0.66∼ 0.81
H(N) F(Zn) F(O)
-0.72∼ -0.69
-0.62∼ -0.57 -0.19∼ -0.16
Cl(Zn) Cl(O) N
adsorption energy of HCl, HF, F2, or NH3, suggesting that ZnONWs with surfaces passivated by HCl, HF, F2, or NH3 are energetically favorable. The absolute value of the adsorption energy of the completely or partly dissociated adsorption of NH3 on ZnONW surfaces is also smaller than that of molecular NH3 adsorption. Therefore, NH3 molecules tend to adsorb on the surfaces of ZnONWs at low temperature mainly nondissociatively, This is in good agreement with previous results on ZnO (101j0) surfaces,37,38 but this differs from the half dissociation of water molecules on ZnONW surfaces27 and the dissociative adsorption of NH3 on GaNNW surfaces.39 We also calculated the charge transfers for the studied systems using the Bader analysis40 based on the calculated charge densities. Table 3 shows the results of the Bader charge analysis on the studied ZnONWs. In the bulk ZnO, the charges for Zn and O are 1.2345 and -1.2345, respectively. It can be seen that the change in the coordination of surface atoms for the bare ZnONWs weakly affects the charge distribution on both Zn and O. The hydrogen passivation, however, causes substantial changes in the charge on the surface Zn and O atoms. The H atoms on the O-H bonds lose an electron to develop a positive charge, while the H atoms on the Zn-H bonds gain an electron and carry a slightly negative charge. The H atoms on the O-H bonds and the H atoms on Zn-H bonds are able to form “dihydrogen bonds,” the distance of H · · · H being 2.02-2.12 Å, which is smaller than the 2.4 Å distance corresponding to twice the van der Waals radius of a hydrogen atom. Because the electronegativity of F (3.98) is larger than that of O (3.44), the F atoms on the Zn-F bonds render the charge of Zn larger, while the F on the O-F bonds render the
-0.57∼ -0.56
-0.35∼ -0.26 0.01∼ 0.04
50% H 50% NH2
50% NH3
1.16∼ 1.17 1.16
1.17∼ 1.20 1.17
1.16 -1.28∼ -1.24 -1.16
1.19 -1.25∼ -1.23 -1.19∼ -1.18 -1.19
-1.15
0.6∼ 0.64 0.32∼ 0.44
0.37∼ 0.58
-1.37∼ -1.27
-1.43∼ -1.28
absolute charge of O smaller; thus, the absolute charge of F bonded to Zn is larger than the F bonded to O. The electronegativity of Cl (3.16) is close to that of O, so the charge transfer between O and Cl is small. The Cl atoms bonded to O also weakly bond with the surface Zn, with a Zn-Cl distance of 2.58 Å. The Cl2 is not completely decomposed on the surface of ZnONWs, and the distance of Cl-Cl is 2.35 Å, which is consistent with the low adsorption energy. For the 50% H, 50% NH2 passivation, the charge of H(O) is larger than that of H(N) because O is more electronegative than N. The changes in the charges of Zn(s) and N for the 50% H, 50% NH2 and 50% NH3 passivations are insignificant. Thus, when the passivations change the surface structures, the electronic structures of the ZnONWs are subsequently altered. Now we turn to the electronic properties of the studied ZnONWs. Table 4 gives the energy gaps for ZnONWs of different sizes, cross sections, and passivations. All the studied ZnONWs are direct band-gap semiconductors. The surface chemical species obviously alter the band gaps. The calculated band gaps for the bare triangular, quadrangular, and hexangular ZnONWs are 1.67, 1.62, and 1.47 eV, respectively, which are much larger than the band-gap value of the bulk ZnO owing to the quantum confinement effects. The fully hydrogen-passivated triangular ZnONW presents the largest band gap (3.22 eV). The 50% H, 50% F; 50% H, 50% Cl; and 50% H, 50% NH2 passivations also increase the gap. Full fluorine passivation, on the other hand, leads to a gap narrower than that of the bare wire due to the strong electronegativity of fluorine. The gap changes for the 100% Cl- and 50% NH3-passivated ZnONWs are small as a result of the weaker interaction between Cl/NH3
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TABLE 4: Lattice Constants c (Å) and Band Gap (eV) of Various ZnONWs cross-sectional pattern triangular
quadrangular
hexangular
chemical species
c (Å)
band gap (eV)
bare 100% H 100% F 50% H, 50% 100% Cl 50% H, 50% 50% H, 50% 50% NH3 bare 100% H 100% F 50% H, 50% 50% H, 50% 100% Cl 50% H, 50% 50% NH3 bare 100% H 100% F 50% H, 50% 50% H, 50% 100% Cl 50% H, 50% 50% NH3
5.40 5.10 5.44 5.13 5.50 5.27 5.27 5.25 5.41 5.13 5.44 5.14 5.27 5.52 5.27 5.25 5.42 5.17 5.44 5.18 5.31 5.53 5.27 5.25
1.67 3.22 1.21 2.84 1.70 2.44 2.54 1.77 1.62 2.95 1.13 2.60 2.28 1.67 2.41 1.70 1.47 2.46 0.92 2.16 1.88 1.56 2.11 1.58
F Cl NH2
F Cl NH2
F Cl NH2
and ZnONWs. The ZnONWs with different cross sections and sizes exhibit similar trends. To obtain further insight into the effects of passivation on the band gaps of ZnONWs, we used the partial electronic densities of states (PDOS) to analyze the interactions between the terminating groups and the ZnONWs, as displayed in Figure 3. For the bare wire, the top of the valence bands is located mainly at the surfaces (O 2p states with a small mixing of Zn 3d states), whereas the bottom of the conduction bands spans the whole wire (mainly the Zn 4s states).41 This implies that the bare ZnONWs may be chemically reactive. As shown in Figure 3b, for the hydrogen-passivated ZnONWs, the energy levels at the valence band maximum mainly consist of O(sub) 2p, O(c) 2p, and H(Zn) 1s states, whereas those at the conduction band minimum are primarily assembled from the Zn 4s and O 2p states. The H saturation stabilizes the bonding molecular orbitals, pushes the antibonding molecular orbitals upward, removes the localized electronic states at the surfaces, and increases the gap;42 the ZnONWs thus become kinetically more stable. When the H(Zn) atoms are substituted by F atoms (i.e., 50% H, 50% F passivation), the top of the valence bands is derived mainly from the O(c) 2p, O(sub) 2p, and Zn(c) 3d states, with small contributions from the F 2p states; the bottom of the conduction bands, on the other hand, mainly results from the Zn(c) 4s and Zn(sub) 4s states, with a small mixing of F 2p states. The high electron affinity of the fluorine atoms lowers the energies of both the highest occupied valence band (HOVB) and the lowest unoccupied conduction band (LUCB), with the LUCB shifted by a greater extent. As shown in Figure 3c, for fully F-passivated ZnONWs, the top of the valence bands is derived mainly from the F(Zn) 2p and Zn 3d states. The contributions from the O atoms are negligible due to the outstanding electron-attractive force of fluorine, whereas the bottom of the conduction bands comes from the F(O) 2p, O(s) p, and Zn 4s states. The full F passivation further lowers the energies of the HOVB and LUCB, with the LUCB shifted by a greater extent, resulting in a smaller gap than that in the bare NWs.
Figure 3. PDOS of the triangular ZnONWs with different passivations: (a) bare, (b) 100% H, (c)100% F, (d) 100% Cl, (e) 50% H(O), 50% NH2(Zn), and (f) 50% NH3(Zn). The Fermi level is set to zero. O(s) and Zn(s) are the surface O and Zn atoms, O(sub) and Zn(sub) are the subsurface O and Zn atoms, H(Zn) and H(O) are the H atoms bonded to surface Zn or O atoms, H(N) is the H atom on NH2, and H(O · · · H) is the H atom on NH3, which formed a hydrogen bond with a surface O atom. F(O) and F(Zn) are the F atoms bonded to surface O or Zn atoms, and Cl(O) and Cl(Zn) are the Cl atoms bonded to surface O or Zn atoms.
When the H(Zn) atoms are replaced by Cl atoms (i.e., 50% H, 50% Cl passivation), the top of the valence bands is dominated by the Cl 2p states, whereas the bottom of the conduction bands derive mainly from the Zn 4s states. When all H atoms are replaced by Cl atoms, the valence band maximum states come mainly from the Cl 2p and O(s) 2p, whereas the conduction band minimum states are based on the Cl 2p and Zn(c) 4s states, as shown in Figure 3d. The energies of both the HOVB and the LUCB increase, with the HOVB shifted by a greater extent. When the H(Zn) atoms are substituted by -NH2 (i.e., 50% H, 50% NH2 passivation), the top of the valence bands is dominated by the N 2p, O 2p, and Zn 3d states, whereas the bottom of the conduction bands comes from the Zn 4s and H(O) 1s states. The N 2p states hybridize with the Zn 3d states, pushing the HOVB to higher energy levels and enforcing a reduction of the band gap. A similar force effect is exerted on the LUCB to push them to lower energy levels, but to a lesser extent.
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Finally, for the NH3-undissociated adsorption on the ZnONWs surfaces, the weights of the N 2p states near the Fermi level are negligible, thus causing little change in the band gap compared with that of the bare wires. 4. Summary In summary, we have studied the effects of various surface passivations on the geometric and electronic structures of ZnONWs by DFT calculations. For a passivation with a given chemical species, the atomic relaxations of the NWs exhibit similar bond length and bond angle changes across the range of diameters and shapes examined. All the passivations of H, F, Cl, and NH2 are energetically favorable and allow for engineering the electronic states of ZnONWs. The band gaps of ZnONWs with different types of surface passivation can differ by up to a few electronvolts, being comparable to those induced by quantum confinement, which is similar to SiNWs.43 Such variation is especially more pronounced for smaller ZnONWs. The magnitude of the band gap is determined by the interaction between the passivating group and the wire surface. In particular, the incompletely dissociative adsorption of Cl2 and undissociated adsorption of NH3 on ZnONW surfaces lead to smaller changes in energy band gaps. Acknowledgment. The work described in this paper was supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (CityU3/CRF/ 08 and CityU 110209). References and Notes (1) Garcia, M. A.; Merino, J. M.; Pinel, E. F.; Quesada, A.; de la Venta, J.; Gonza´lez, M. L. R.; Castro, G. R.; Crespo, P.; Llopis, J.; Gonza´lezCalbet, J. M.; et al. Nano Lett. 2007, 7, 1489–1494. (2) Mclaren, A.; Valdes-Soles, T.; Li, G.; Tsang, S. C. J. Am. Chem. Soc. 2009, 131, 12540–12541. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Hu, J. T.; Li, L. S.; Yang, W. D.; Manna, L.; Wang, L. W.; Alivisatos, A. P. Science 2001, 292, 2060. (5) Li, L. S.; Hu, J. T.; Yang, W. D.; Alivisatos, A. P. Nano Lett. 2001, 1, 349. (6) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (7) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (8) Weller, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1079. (9) Huang, S. P.; Cheng, W. D.; Wu, D. S.; Hu, J. M.; Shen, J.; Xie, Z.; Zhang, H.; Gong, Y. J. Appl. Phys. Lett. 2007, 90, 031904. (10) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (11) Kind, H.; Yan, H. Q.; Messer, B.; Law, M.; Yang, P. D. AdV. Mater. 2002, 14, 158.
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