Effect of Molecular Passivation on the Doping of InAs Nanowires

Aug 2, 2012 - College of Optical and Electronic Technology, China Jiliang University, 310018 ... ABSTRACT: The molecular passivation effect on the dop...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Effect of Molecular Passivation on the Doping of InAs Nanowires Haibo Shu,*,†,‡ Dan Cao,§ Pei Liang,† Shangzhong Jin,† Xiaoshuang Chen,*,‡ and Wei Lu‡ †

College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, 200083 Shanghai, China § College of Science, China Jiliang University, 310018 Hangzhou ‡

ABSTRACT: The molecular passivation effect on the doping of InAs nanowires is explored by the first-principles calculation within the density-functional theory. We demonstrate the passivation of two different molecules that is implemented by the adsorption of NH3 and NO2 on the sidewall of InAs nanowire, respectively, but the two molecular passivations indicate different roles in the doping of InAs nanowires. The NH3 adsorption is a physisorption process which makes InAs nanowire with surface dangling bonds (SDBs) stay at n-type, while the adsorption of NO2 molecule is chemisorption that can reactivate the Zn dopant in InAs nanowires and yields a p-type doping characteristic. The difference of molecular passivation effect is attributed to the charge-compensation ability of passivating molecules to the SDBs of nanowires. This mechanism can be applied to explain the experimental observations on the doping of InAs nanowires through the surface passivation.

1. INTRODUCTION III−V semiconductor nanowires (SNWs) have received increasing interest in recent years due to their superiority as the building blocks in the nanoscale electronic and optoelectronic devices.1−6 Among these semiconductor nanowires, InAs nanowires are particularly intriguing since bulk InAs exhibits many outstanding physical properties, such as small electron effective mass, high electron mobility (more than 6000 cm2/(V s)), and high-saturation drift velocity.7 The applications of InAs nanowires in electronic and optoelectric devices require high carrier concentrations so as to allow for sufficient conductivity and/or the displacement of Fermi level. Doping is a fundamental way to provide free carries. However, the doping efficiency of InAs nanowires is far less than that expected, in particular for the p-type doping. Therefore, the deep understanding of the doping limitations of InAs nanowires and the exploration of new schemes to improve the doping efficiency are highly desired. The doping in various SNWs has been demonstrated in two ways, one is in situ doping by incorporating the foreign atoms into the host lattice during synthesis, and another is ex-situ doping by adding the doping agents in a later processing. Insitu doping has been widely used in InAs nanowires, but it is still a challenge for obtaining high hole carriers. The low doping efficiency is mainly ascribed to the following two factors: (i) quantum size ef fect, inducing the increase of ionization energy of dopants with decreasing nanowire size due to the dielectric confinement,8,9 and (ii) Fermi-level pinning in conduction band,10,11 resulting in a surface accumulation layer of electrons. The latter, therefore, is the main factor for limiting the p-type doping of InAs nanowires. To overcome the p-type doping difficulty and improve the doping efficiency of InAs nanowires, extensive efforts have been © 2012 American Chemical Society

made. Owing to the large surface-volume-ratio of SNWs, the doping is thus easily implemented on nanowire surface by the adsorption and/or incorporation of doping agents and molecules. The doping method has been successfully demonstrated in diamond, Si and Ge nanowires by a so-called electron-transfer mechanism (or named surface-transfer doping).12−17 For instance, Guo et al.14 have first demonstrated the surface passivation doping in Si nanowires, and they have found that the surface transfer doping can provide a considerable concentration of majority carriers in Si nanowires with appropriate passivants. Yuan et al.16 reported tunable and reversible transition of conduction in intrinsic Si nanowires via changing surface conditions. Similar experiments on the doping of InAs nanowires implemented by the surface passivations have also been reported in recent years.18−21 For example, Ford et al.18 have demonstrated that the p-type InAs nanowires can be realized by gas-phase surface diffusion of Zn in a postgrowth process. Hang et al.19,20 have reported that surface passivation of Cd-doped InAs nanowires with the organic ligands induces the transition from n-type conductivity to the p-type one. Li et al.21 have proposed a remote p-type doping concept in InAs/ InP core−shell nanowires. In their experiment, the Zn substituting impurity atoms introduced into the InP shell drive the hole carriers confined in InAs core nanowires. All these experimental studies suggest that the doping carried out on the surface of InAs nanowires is a feasible way to obtain the p-type conductivity. On the theoretical side, most of the studies have focused on the electronic properties of pure InAs nanowires;22−25 there are few studies on the doping of InAs Received: May 5, 2012 Revised: July 18, 2012 Published: August 2, 2012 17928

dx.doi.org/10.1021/jp304350f | J. Phys. Chem. C 2012, 116, 17928−17933

The Journal of Physical Chemistry C

Article

nanowires. Recently, dos Santos et al.26 have reported that the Cd-doped or Zn-doped InAs nanowires with a good surface passivation can behave as an acceptor character. Very recently, our first-principles calculations27 have shown that surface dangling bonds could be a limiting factor for the p-type doping of InAs nanowires. It is noteworthy that, although it has been proved that the passivation of molecules or the organic ligands has a positive effect for improving the doping efficiency and obtaining p-type conductivity in InAs nanowires, its microscopic mechanism is far from clear. For example, what is the relationship between the passivating molecules and the doping effect of InAs nanowires? What is the mechanism of molecular adsorption inducing the transition of conductivity from n-type to p-type in InAs nanowires? Insight into these questions is essential for realizing reliable and well-controlled p-type doping of InAs nanowires. Here we report a systematic study of the molecular passivation effect on the doping of InAs nanowires and the reactivation mechanism of Zn dopant in InAs nanowires. Our results first demonstrate that the surface states of the unpassivated InAs nanowires are an important factor for limiting the p-type doping. Utilizing the adsorption of NH3 and NO2 molecules on InAs nanowires induces two different features of band structures. The NH3 adsorption on the nanowire surface is a physisorption process and leads to an ntype doping, and the adsorption of NO2 can induce the reactivation of Zn dopant in InAs nanowires by a chargecompensation mechanism. The present results can be used to explain the experimental observations18−21 in which the surface passivation increases the conductivity of p-type-doped InAs nanowires.

Table 1. Diameter (in nm) and Atomic Numbers of InAs Nanowires Considered in the Present Studya nanowires

diameter (nm)

N(InAs)

N(H-InAs)

1 2 3 4

0.93 1.32 1.85 2.35

38 74 122 182

68 116 176 248

a

N(InAs) and N(H-InAs) represent the atomic numbers of the bare and pseudo-H passivating nanowires, respectively.

× 2 supercell of the surface unit cell in the slab geometry, sampling the Brillouin zone with a grid of 4 × 4 of k-points with the Monkhorst−Pack scheme. For the geometry optimization, all of the atoms in the supercell except for the bottommost InAs layer and the pseudo-H atoms have been relaxed until the remaining forces acting on the atoms are less than 10−3 eV/Å.

3. RESULTS AND DISCUSSION We start our discussion with the electronic properties of bare InAs nanowires (bare-NWs) and pseudo-H passivating ones (H-NWs). It is useful to provide a reference for the subsequent calculations on the molecule-passivation doping of InAs nanowires. Figure 1a shows the optimized atomic structures and band structures of 1.85-nm bare-NWs. Owing to the dangling bonds enriched in surface atoms, the band structure of the bare InAs nanowire exhibits a typical n-type character. Similar results have also been observed in other sized nanowires. It can be found that there are several bands around the Fermi level (see Figure 1a). The charge-density isosurface indicates that the surface electronic states mainly localize in In atoms of the nanowire surface (see Figure 1c). For the InAs nanowires without the treatment of passivation in the experiment, there should be a great deal of surface states that contribute to the Femi-level pinning in the conduction band. When a p-type dopant, such as Zn, is introduced into the InAs nanowires, the surface states of nanowires can trap the holes from Zn dopant and limit the p-type doping. To inhibit the effect of surface dangling bonds (SDBs), the bare-NWs need to be passivated. Figure 1b indicates the optimized atomic structure and the band structure of 1.85-nm H-NW. The passivating InAs nanowire exhibits an intrinsic semiconductor feature that originates from the suppression of surface states by H atoms. The band gaps of H-NWs indicate a strong dependence on nanowire size. We have extracted the band gap variation ΔEg, which has defined the band gap difference between InAs nanowires and bulk as a function of nanowire diameter shown in Figure 1d. It is found that the larger the nanowire size, the smaller the value of ΔEg. To give a quantitative relationship, these data points are fitted by the following expression,

2. MODELS AND COMPUTATIONAL METHODS All calculations in the present study are performed within density-functional theory (DFT) as implemented by the Vienna ab initio simulation package (VASP).28,29 The exchangecorrelation energy is treated in the generalized-gradient approximation (GGA) using the PW91 functional.30 The energy cutoff for the plane-wave expansion is set to 360 eV, using the projector augmented wave (PAW) potentials31 to describe the electron−ion interaction. The Monkhorst−Pack kpoint mesh of 1 × 1 × 6 is found to provide sufficient accuracy in the Brillouin-zone integration. For the geometry optimization, all atoms have been relaxed until the force acting on each atom is less than 0.01 eV/Å. The nanowires are created on the basis of geometryoptimization zinc-blende InAs bulk, and they are oriented along the [111] direction with diameters ranging from 0.93 to 2.32 nm. All nanowires present six {110} planes, forming hexagonal cross sections. In the present study, both bare and pseudo-H32 passivating InAs nanowires have been considered. The structural information of nanowires including their size and atomic number has been listed in Table 1. The distance between two neighboring nanowires is set to larger than 10 Å for eliminating the interactions between adjacent nanowires. To study the effect of NH3 and NO2 molecules on the doping of InAs nanowires, these molecules are put on the surface of dangling bonds by removing hydrogen atoms from the surface of the nanowires. To explore the effect of NH3 and NO2 molecules on the doping of InAs nanowires, we have also considered the adsorption of the molecules on InAs(110) thin slab. We use a 2

ΔEg (d) = A /d n

(1)

where d is the diameter of nanowires, and A is a scale factor which corresponds to the variation of band gap for a 1-nm nanowire relative to the bulk value. The fitted parameters A and n are 1.84 and 0.87, respectively. The result is in good agreement with the previous theoretical studies,23,33 and the fitted curve shown in Figure 1d also matches the experimental data34 very well. Ignoring the surface states (see Figure 1a), the band gap of bare-NWs only depends on the electron-levels position of bulk atoms in these nanowires. As shown in Figure 1d, the ΔEg values of bare-NWs are larger than that of H-NWs 17929

dx.doi.org/10.1021/jp304350f | J. Phys. Chem. C 2012, 116, 17928−17933

The Journal of Physical Chemistry C

Article

Figure 1. Optimized atomic structures (top view) and band structures of (a) bare and (b) H-passivating InAs nanowires. The large, medium, and small balls represent In, As, and H atoms, respectively. (c) The charge density isosurface of surface states around Fermi level in the bare InAs nanowire. (d) The band gap vibration ΔEg of bare (bare-NWs) and H-passivating (H-NWs) InAs nanowires as a function of nanowire diameters (in nm); the experimental data is from ref 34, Wang et al.

Figure 2. Band structures of (a) NH3 and (b) NO2 adsorption on the In-SDB of InAs nanowires, respectively. The horizontal dashed lines denote the position of the Fermi level. (c) The charge-density isosurface of the donor state (shown in panel a) for NH3 adsorption on InAs nanowires. (d) The schematic process that the adsorption of NO2 induces in the intrinsic character of InAs nanowire.

implemented by the exposure of the sample to NH3 (or NO2) molecules. To evaluate their effect on the doping of InAs nanowires, we first investigate the intrinsic doping tendency of NH3 and NO2 molecular adsorption on the nanowire surface. The adsorption site of molecules is considered to be on In surface dangling bonds (In-SDB). Both adsorption processes are energetically favorable, and the adsorption of NH3 and NO2 molecules on the nanowire surface induces energy reduction of 0.37 and 1.94 eV relative to the H-passivating nanowire, respectively. We have also studied the adsorption of NH3 and NO2 molecules on As-SDB of nanowires, but both adsorption processes are energetically unfavorable. Moreover, the As atoms of the nanowire surface can form strong bonds with H atoms in a hydrogen atomosphere.27 Therefore, we only consider the molecular adsorption on In-SDB of the nanowire surface in the following discussions.

in the same size, correspondingly. On the basis of eq 1, the fitted parameters A and n in bare-NWs are 1.77 and 0.42, respectively. We find that these parameters are correspondingly smaller than that of H-NWs. The results suggest that the band gaps of nanowires depend on not only the quantum confined effect but also the surface effect.24,35 Although hydrogen is a good passivating material, the H2 molecule has a relatively high dissociated energy barrier (4.52 eV) for the exposure of InAs nanowires in H2 atmosphere. Moreover, our previous study27 has shown that H atoms bonded with In atoms of the nanowire surface are not very stable, and the dangling bonds of In atoms could be a defect that limits the p-type doping of InAs nanowires. In contrast, NH3 and NO2 molecules are proved to be effective passivating materials in Si and Ge nanowires.36 The adsorption of the two molecules on the nanowire surface does not involve the dissociation of molecules, and the passivation is thus easily 17930

dx.doi.org/10.1021/jp304350f | J. Phys. Chem. C 2012, 116, 17928−17933

The Journal of Physical Chemistry C

Article

conduction bands, respectively. Such an adsorption effect of NO2 provides a premise to reactivate the p-type dopant of InAs nanowires. To study the passivation effect of NH3 and NO2 molecules on the p-type doping of InAs nanowires, a substitutional Zn dopant is introduced into the nanowires. The band structures of NH3 and NO2 adsorption on Zn-doped InAs nanowires are shown in a and b of Figure 3, respectively. It is found that the NH3 adsorption on the SDB-Zn complex does not lead to a ptype behavior of the nanowire and thus shows an intrinsic semiconductor characteristic (see Figure 3a). The result arises because the hole produced by the substitutional Zn dopant is recombined by the unpaired electron of In-SDB (see Figure 3c). Therefore, the NH3 adsorption cannot reactivate the Zn impurity atom of InAs nanowires. On the other hand, the NO2 adsorption on SDB-Zn complex induces the InAs nanowire to exhibit a typical p-type character, as shown in Figure 3b. The origin of the p-type InAs nanowire can be understood by the electron-transfer mechanism.12−17 Owing to the unpaired electron of In-SDB compensated by NO2, the Zn dopant is thus reactivated. The charge-density isosurface indicates that the acceptor states are mainly localized at the Zn dopant and NO2 molecule. The result suggests that the NO2 molecule does not supply itself a hole but can induce the reactivation of the Zn dopant. We also have put the substituting Zn atom on several other different sites of the nanowire, but the calculations indicate that the position of the Zn dopant on a nanowire does not change the role of NH3 and NO2 molecules. It needs to be mentioned that the size of nanowires in the present calculations is smaller than 2.5 nm, but the sizes of most InAs nanowires in the experiments are about 20−40 nm. However, the calculation of nanowires with the larger sizes is obviously beyond the scope of the present work. To prove the fundamental effect of surface passivation of NH3 and NO2 molecules on the doping of InAs nanowires, we have studied the electronic properties of NH3 and NO2 adsorption on InAs(110) thin slab, respectively. The two-dimension InAs(110) thin slab can be analogized to the larger-size InAs nanowires due to the considerable quantum confinement effect. The band structures of NH3 adsorption on InAs(110) and NO2 adsorption on Zn-doped InAs(110) surface have been shown in

Panels a and b of Figure 2 respectively show the band structures of NH3 and NO2 adsorption on In-SDB of 1.85-nm InAs nanowires. The adsorption of NH3 leads to an n-type doping character, as shown in Figure 2a. Interestingly, the charge-density distribution shows that the donor-type electronic states are not from the NH3 molecule, and they are mainly localized around the In-SDB (see Figure 2c). The further Bader charges analysis shows that the charge transfer from In-SDB to the NH3 molecule is only 0.05 e, as listed in Table 2. The calculated In−N bond length in the nanowire is Table 2. Charge Transfer ΔQIn (e) from In-SDB to the Passivating Moleculesa ΔQIn (e) NW-NH3 NW-NO2 slab-NH3 slab-NO2

0.05 0.18 0.04 0.15

a ΔQIn is calculated by comparing the Bader charges of the In atom with those of SDB on the InAs nanowire or InAs(110) slab before and after the passivation of molecules. NW-NH3 and NW-NO2 represent the InAs nanowire passivated by NH3 and NO2 molecules, respectively. Slab-NH3 and Slab-NO2 represent the InAs(110) slab with the passivation of NH3 and NO2 molecules, respectively.

2.41 Å larger than that of NO2 adsorbed on InAs nanowires (2.28 Å). Hence, the adsorption of an NH3 molecule is a physisorption process that does not change the n-type character of InAs nanowires with SDB. The role of NH3 in InAs nanowires is different from that in Si nanowires where the adsorption of NH3 is a chemisorption process.36 In contrast, the adsorption of NO2 yields an intrinsic semiconductor characteristic (see Figure 2b). It originates from the adsorption of NO2 as a chemisorption process which induces the charge transfers of 0.18 e from the In-SDB to the NO2 molecule from the Bader charges analysis, as listed in Table 2. The result suggests that the passivating NO2 molecule can accept the unpaired electron of In-SDB to reach a charge balance (see Figure 2d), resulting in the bonding and antibonding states of the SDB-NO2 complex being pushed into the valence and

Figure 3. Band structures of (a) NH3 and (b) NO2 adsorption on In-SDB of Zn-doped InAs nanowires, respectively. The horizontal dashed lines denote the position of the Fermi level. (c) The schematic process where the adsorption of NH3 induces an intrinsic semiconductor character of the Zn-doped InAs nanowire. (d) The charge-density isosurface of the acceptor state (shown in panel b) for the adsorption of NO2 on Zn-doped InAs nanowires. 17931

dx.doi.org/10.1021/jp304350f | J. Phys. Chem. C 2012, 116, 17928−17933

The Journal of Physical Chemistry C

Article

Figure 4. Band structures of (a) NH3 adsorption on In-SDB of InAs(110) surface and (b) NO2 adsorption on In-SDB of Zn-doped InAs(110) surface, respectively. The horizontal dashed lines denote the position of the Fermi level. The charge-density isosurface of the donor state (shown in panel a) for (c) the NH3 adsorption on In-SDB of the InAs(110) surface and (d) that of the acceptor state (shown in panel b) for the NO2 adsorption on In-SDB of the Zn-doped InAs(110) surface, respectively.

of NO2 molecules in reactivating the Cd dopant of InAs nanowires.

Figure 4a and b, respectively. We found that the role of NH3 and NO2 on InAs(110) is similar to that on InAs nanowires. In Figure 4a, InAs(110) with the passivating NH3 exhibits a donor-type doping tendency, and the donor state is mainly localized at the In-SDB on InAs(110) surface (see Figure 4c) due to the physisorption of NH3. The Bader charges analysis also indicated that there is no obvious charge transfer (0.04 e) from In-SDB to NH3, as listed in Table 2. As shown in Figure 4b, the passivation of NO2 on Zn-doped InAs(110) surface yields an acceptor-type band structure, and the acceptor state is mainly localized around the Zn dopant and NO2 molecule (see Figure 4d). The result originates from the chemisorption of NO2 in which the passivating NO2 molecule can accept the unpaired electrons of In-SDB (0.15 e listed in Table 2) and induces the reactivation of Zn impurity. Hence, the moleculepassivation effect on the doping of InAs nanowires does not depend on the nanowire size. On the basis of the above results, the surface adsorption of NH3 and NO2 molecules present different role in the doping of InAs nanowires: the NH3-adsorption itself does not change the doping tendency of InAs nanowires, but it protects the n-type character of InAs nanowires with SDBs against the influences of the extrinsic molecules and ligands. In contrast, the NO2 adsorption can reactivate the p-type Zn dopant in InAs nanowires by means of a charge-compensation mechanism. Therefore, the molecule-passivation effect on the doping of InAs nanowires actually depends on the charge compensation between the adsorbed molecules and the surface defects (e.g., In-SDBs). In a physisorption process, there is almost no charge transfer between the adsorbed molecules and SDBs, and thus InAs nanowires stay at n-type. This may be why the observed InAs nanowires in most experiments exhibit n-type character in spite of the nanowires under a gas atmosphere.37−39 For a chemisorption, the adsorbed molecules can accept the unpaired electron of SDBs and results in the transition of electronic structure of InAs nanwires from n-type to intrinsic type. The mechanism can be applied to explain the experimental observations by Hang et al.19,20 in which the surface passivation of organic ligands induces the conductivity transition of Cddoped InAs nanowires from n-type to p-type. In their experiment, the passivating organic ligands play a similar role

4. CONCLUSIONS In summary, we have performed the first-principles calculations to investigate the passivation effect of NH3 and NO2 molecules on the doping of InAs nanowires. The adsorptions of NH3 and NO2 on the sidewall of nanowires present different roles in the doping of InAs nanowires: the NH3 adsorption is a physisorption process that results in InAs nanowires with SDBs remaining as n-type, while the adsorption of NO2 molecule is a chemisorption process that leads to the reactivation of Zn dopant in InAs nanowires, yielding a ptype doping characteristic. The differences in the molecular passivation effect on the doping of InAs nanowires originate from the different charge-compensation ability of the adsorbed molecules to the SDBs of nanowires. This mechanism can be applied to explain and guide the doping of InAs nanowires through molecular passivations in experiment.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.S.); [email protected] (X.C.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported in part by Zhejiang Provincial National Natural Science Foundation of China (Y1110777), the National Natural Science Foundation of China (61006051, 61006090, 10990104, and 60976092), and Shanghai Science and T echnology Foundation (09DJ1400203 and 10JC1416100). Computational resources from the Shanghai Supercomputer Center are alsoacknowledged.

■

REFERENCES

(1) Hayden, O.; Agarwal, R.; Lieber, C. M. Nat. Mater. 2006, 5, 352. (2) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885. (3) Agarwal, R.; Lieber, C. M. Appl. Phys. A 2006, 85, 209. 17932

dx.doi.org/10.1021/jp304350f | J. Phys. Chem. C 2012, 116, 17928−17933

The Journal of Physical Chemistry C

Article

(4) Lind, E.; Persson, A. I.; Samuelson, L.; Wernersson, L. E. Nano Lett. 2006, 6, 1842. (5) Dayeh, S. A.; Aplin, D. P. R.; Zhou, X. T.; Yu, P. K. L.; Yu, E. T.; Wang, D. L. Small 2007, 3, 326. (6) Heremans, J. Spinger Handb. Nanotechnol. 2007, 345, 345. (7) Tomioka, K.; Motohisa, J.; Hara, S.; Fukui, T. Nano Lett. 2008, 8, 3475. (8) Scheffler, M.; Nadj-Perge, S.; Kouwenhoven, L. P.; Borgström, M. T.; Bakkers, E. P. A. M. J. Appl. Phys. 2009, 106, 124303. (9) Leu, P. W.; Shan, B.; Cho, K. Phys. Rev. B 2006, 73, 195320. (10) Noguchi, M.; Hirakawa, K.; Ikoma, T. Phys. Rev. Lett. 1991, 66, 2243. (11) Khanal, D. R.; Yim, J. W. L.; Walukiewicz, W.; Wu, J. Nano Lett. 2007, 7, 1186. (12) Ristein, J. Science 2006, 313, 1057. (13) Strobel, P.; Riedel, M.; Ristein, J.; Ley, L. Nature 2004, 430, 439. (14) Guo, C. S.; Luo, L. B.; Yuan, G. D.; Yang, X. B.; Zhang, R. Q.; Zhang, W. J.; Lee, S. T. Angew. Chem., Int. Ed. 2009, 48, 9896. (15) Collins, G.; Holmes, J. D. J. Mater. Chem. 2011, 21, 11052. (16) Yuan, G. D.; Zhou, Y. B.; Guo, C. S.; Zhang, W. J.; Tang, Y. B.; Li, Y. Q.; Chen, Z. H.; He, Z. B.; Zhang, X. J.; Wang, P. F.; et al. ACS Nano 2010, 4, 3045. (17) Yang, X. B.; Guo, C. S.; Zhang, R. Q. Appl. Phys. Lett. 2009, 95, 193105. (18) Ford, A. C.; Chuang, S.; Ho, J. C.; Chueh, Y. L.; Fan, Z. Y.; Javey, A. Nano Lett. 2010, 10, 509. (19) Hang, Q. L.; Wang, F. D.; Carpenter, P. D.; Zemlyanov, D.; Zakharov, D.; Stach, E. A.; Buhro, W. E.; Janes, D. B. Nano Lett. 2008, 8, 49. (20) Hang, Q. L.; Wang, F. D.; Buhro, W. E.; Janes, D. B. Appl. Phys. Lett. 2007, 90, 062108. (21) Li, H.-Y.; Wunnicke, O.; Borgström, M. T.; Immink, W. G. G.; van Weert, M. H. M.; Verheijen, M. A.; Bakkers, E. P. A. M. Nano Lett. 2007, 7, 1144. (22) dos Santos, C. L.; Piquini, P.; Lima, E. N.; Schmidt, T. M. Appl. Phys. Lett. 2010, 96, 043111. (23) dos Santos, C. L.; Piquini, P. Phys. Rev. B 2010, 81, 075408. (24) Shu, H. B.; Chen, X. S.; Zhao, H. X.; Zhou, X. H.; Lu, W. J. Phys. Chem. C 2010, 114, 17514. (25) Shu, H. B.; Liang, P.; Wang, L.; Chen, X. S.; Lu, W. J. Appl. Phys. 2011, 110, 103713. (26) dos Santos, C. L.; Schmidt, T. M.; Piquini, P. Nanotechnology 2011, 22, 265203. (27) Shu, H. B.; Chen, X. S.; Ding, Z. L.; Dong, R. B.; Lu, W. J. Phys. Chem. C 2011, 115, 14449. (28) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953. (29) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. (30) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. Rev. Mod. Phys. 1992, 64, 1045. (31) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (32) Shiraishi, K. J. Phys. Soc. Jpn. 1990, 59, 3455. (33) Niquet, Y. M.; Lherbier, A.; Quang, N. H.; Fernandez-Serra, M. V.; Blase, X.; Delerue, C. Phys. Rev. B 2006, 73, 165319. (34) Wang, F. D.; Yu, H.; Jeong, S.; Pietryga, J. M.; Hollingsworth, J. A.; Gibbons, P. C.; Buhro, W. E. ACS Nano 2008, 2, 1903. (35) Pan, H.; Feng, Y. P. ACS Nano 2008, 2, 2410. (36) Miranda-Durán, A; Cartoixà, X; Irisson, M. C.; Rurali, R. Nano Lett. 2010, 10, 3590. (37) Pfund, A.; Shorubalko, I.; Leturcq, R.; Ensslin, K. Appl. Phys. Lett. 2006, 89, 252106. (38) Jespersen, T. S.; Agesen, M.; Sørensen, C.; Lindelof, P. E.; Nygrdård, J. Phys. Rev. B 2006, 74, 233304. (39) Thelander, C.; Dick, K. A.; Borgström, M. T.; Fröberg, L. E.; Caroff, P.; Nilsson, H. A.; Samuelson, L. Nanotechnology 2010, 21, 205703.

17933

dx.doi.org/10.1021/jp304350f | J. Phys. Chem. C 2012, 116, 17928−17933