Article pubs.acs.org/JPCC
Ultra Narrow PbS Nanorod Field Emitter Umamahesh Thupakula, J. K. Bal, Anupam Debangshi, Ali Hossain Khan, Amit Dalui, and Somobrata Acharya* Centre for Advanced Materials (CAM), Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
ABSTRACT: Investigating the electronic properties of a single ultra narrow semiconductor nanorod remains a significant challenge in nanotechnology that may lead to the miniaturization of electronic devices. We explore the electronic properties of ultra narrow PbS nanorod using scanning tunneling microscopy and spectroscopy techniques. The tunneling spectroscopic properties of an isolated and self-assembled PbS nanorods exhibit nonresonant tunneling of electrons and showed large tunnel current after exceeding certain critical bias voltage. A crossover from the direct to the Fowler−Nordheim tunneling regime is observed after exceeding the critical bias voltage corresponding to the field emission properties at higher bias. Suppression in the field emission property is evidenced from the self-assembled PbS nanorods owing to the strong electronic coupling between the adjacent nanorods. Studying the electronic transport properties of a single nanorod with field emission properties provides an opportunity to recognize miniaturized device at nanoscale.
■
INTRODUCTION One dimensional (1D) nanomaterials are key components in nanotechnology for recognizing miniaturization of electronic devices down to a molecular scale.1−3 The two-dimensional (2D) confinement of the charge carriers for 1D nanomaterials substantially affect the electronic properties compared to the 0D dots or 2D sheets enabling them to illustrate novel device properties.4−8 A wide range of applications has already been realized from 1D nanomaterials in electronic, electro-optic, and photovoltaic applications.1,3,5,9 Thus, the study of the local electronic structure of a single semiconductor nanorod holds fundamental importance in understanding the operational conditions for miniaturized electronics. In this context, scanning tunneling microscopy and spectroscopy (STM and STS) offer unique advantages since it provides knowledge about the electronic properties of a material with nanometer spatial resolution. In particular, STM and STS on nanomaterials enable the spatial mapping of differentiated electronic structure which can be correlated with the local density of states (LDOS) providing direct access to the local electronic structure of the nanomaterials.10,11 The electronic properties of single semiconductor nanocrystals have been elucidated in double barrier tunnel junction (DBTJ) formalism, in which electrons or holes participate in the resonant tunneling through discretized conduction band (CB) or valence band (VB).10−12 The © 2012 American Chemical Society
resultant electronic structure deduced from the nanocrystals depends on the width of the barriers formed at two junctions, namely tip−nanocrystal and nanocrystal−substrate, and also on the relative alignment of valence and conduction levels with respect to the Fermi levels of electrodes.13 However, the relative voltage distribution across the tip−nanocrystal− substrate configuration determines types of electron transport mechanisms such as direct tunneling and Fowler−Nordheim (FN) tunneling, in addition to the resonant tunneling. In direct tunneling, the electrons tunnel through the rectangular barrier formed between the two metal electrodes contributing a low degree of tunneling current to flow through the band gap region of the semiconductor nanocrystal. However, in resonant tunneling, the electron transport phenomenon assumes most of the bias voltage distributes at tip−nanocrystal and nanocrystal− substrate junctions and the electrons tunnel resonantly through the discretized conduction and valence channels defining the peaks at both positive and negative bias respectively. In FNformulation, the finite bias voltage distributed across the nanocrystal bends the valence and conduction channels to create the trapezoidal barrier.14 When the bias voltage exceeds a critical value, the electrons tunnel from the emitting nanocrystal Received: July 28, 2012 Published: August 9, 2012 18564
dx.doi.org/10.1021/jp3074927 | J. Phys. Chem. C 2012, 116, 18564−18570
The Journal of Physical Chemistry C
Article
temperature was reduced to 70 °C and the rods were collected by washing two times with methanol (centrifuged at 3000 rpm for 3 min) and once with a mixture of dichloromethane and methanol (5: 40 by volume) to remove excess surfactants. TEM and HRTEM measurements were carried out using a JEOL JEM-2010 microscope with an accelerating voltage of 200 KV. For TEM imaging, the sample was prepared by drop casting the nanorods from chloroform suspension onto a lacey carbon-coated copper electron microscope grid. STM and STS measurements on PbS nanorods were performed using ultrahigh vacuum (∼ 10−11 mbar) with variable temperature STM instrument (VT-STM, Omicron Nanotechnology). Tunneling spectra (I−V characteristics) were acquired at room temperature by placing the STM tip above the nanorod while disabling the feedback loop. The sample was prepared by drop casting the PbS nanorods from a chloroform suspension on top of a highly oriented pyrolytic graphite (HOPG) substrate prior to the STM and STS measurements. The tunneling current (I) was then measured as a function of the tip−substrate potential difference, i.e., bias (V). Typically, a large number (∼25) of I−V curves were averaged to improve the signal-to-noise ratio for each data point.
through triangular barrier contributing large tunneling current. Hence, the field emission properties in FN tunneling regime at relatively higher bias voltages has been realized for selective nanocrystals showing field emission properties in its bulk form.14−17 Investigating such intrinsic electronic properties of single semiconductor nanocrystals with ultra narrow dimension utilizing STM and STS techniques is important to realize working mechanism of a miniaturized device. Additionally, most of the reports on field emission properties from nanomaterials have focused on the hexagonal phase compound semiconductors like ZnO or ZnS in vertical configuration.18−20 Relatively small progress has taken place on the field emission properties from cubic crystallographic phase compounds.21,22 Exploring new and suitable materials in this direction is always interesting since it delivers the versatility in choosing the materials which can encompass multifunctionality. Bulk lead sulfide (PbS) is a narrow band gap cubic phase semiconductor with a bulk band gap of 0.41 eV and with a large exciton Bohr radius (aB ≈ 18 nm), which make the material promising for photovoltaics, sensing, and detection applications.23 The low and similar effective mass for electrons (me = 0.08m0) and holes (mh = 0.08m0) in PbS offers strong confinement effects on charge carriers even for a moderate nanocrystal size in contrast to II−VI semiconductor compounds.24 Hence, the electronic properties of PbS nanocrystals can be precisely tuned over a broad range in an ease by tailoring the size or shape.25 In spite of such potential, the reports on the field emission from a single ultranarrow semiconductor leadchalcogenide nanorod is rare. The anisotropy, degree of ordering, crystalline quality, and small radius of curvature may play crucial role in the field emitting property of leadchalcogenide nanorods. Probing the electronic transport properties of such a unique material is of fundamental scientific importance in realizing a miniaturized device. We report here on the electronic properties of ultra narrow PbS nanorods using STM and STS techniques at ultrahigh vacuum conditions (∼10−11 mbar). Our synthesized PbS nanorods exhibit cubic rock salt structure and narrow size distribution as is evidenced from the transmission electron microscopy (TEM) and high resolution TEM (HRTEM) measurements. The ultra narrow PbS rods are highly single crystalline in nature with a diameter of 1.8 nm and 18−20 nm in length.26−28 The tunneling current (I) versus voltage (V) measurements on isolated PbS nanorod reveal a transition from the direct to the FN tunneling regime with an increasing bias voltage. At relatively low positive and negative bias voltages, direct tunneling with a low magnitude of current is observed followed by a large tunneling current owing to field emission from PbS nanorod at relatively higher bias voltages. When the I−V measurement is performed on a close packed arrayed assembly of PbS nanorods, a suppression of field emission properties has been observed pointing to different tunneling pathways of electrons induced by the electronic coupling between the adjacent nanorods.
■
RESULTS AND DISCUSSION The TEM images of the as-synthesized PbS nanorods show the monodispersed nature of the sample with average length ∼18− 20 nm of the nanorods (Figure 1a). The HRTEM image (Figure 1b) reveals an ∼1.8 nm width of the nanorod with the well-defined lattice spacings reflecting high degree of
Figure 1. (a) Low resolution dark field TEM image of PbS nanorods with dimensions of ∼18−20 nm in length and ∼1.8 nm in width. (b) HRTEM image of an isolated PbS nanorod showing well resolved lattice planes with atomic resolution. The lattice spacing’s of 0.29 ± 0.02 nm and 0.21 ± 0.02 nm are indexed to (200) and (220) atomic planes of cubic rock salt structure of PbS respectively. The growth direction is indicated by an arrow in the figure. (c) A threedimensional (3D) STM image showing PbS nanorod assembly on HOPG substrate. The image was obtained by using set-voltage of 0.4 V and set-current of 0.1 nA. (d) A 3D STM image of an isolated PbS nanorod showing the dimensions of ∼25 nm in length and ∼5 nm in diameter.
■
EXPERIMENTAL SECTION Ultra narrow PbS nanorods were synthesized using single step benchtop reaction conditions as reported by us in references (26−28). For the synthesis of ultra narrow PbS rods, lead hexadecylxanthate (0.065 g) was added in one shot to 1.6 mL of trioctylamine (Aldrich, 98%) at 65 °C with continuous stirring under N2. A grayish-milky color appeared after 5 min and then the temperature was increased to 80 °C. Annealing was carried out for 40 min at this temperature. Finally the 18565
dx.doi.org/10.1021/jp3074927 | J. Phys. Chem. C 2012, 116, 18564−18570
The Journal of Physical Chemistry C
Article
Figure 2. (a) Tunneling I−V curves of PbS nanorod measured at three different set-currents of 0.1 nA (black squares and line), 0.4 nA (red dots and line), and 0.8 nA (blue up-triangles and line) at a fixed set-voltage of 0.4 V. The I−V curves are collected by placing the STM tip on top of the nanorod while disabling the feedback loop. Typically ∼25 I−V curves are averaged to improve the signal-to-noise ratio. Inset shows the STM image of an isolated PbS nanorod with the reference position (green dot) of STS. (b) The dI/dV curves obtained from the numerical derivative of I−V curves. The apparent band gaps (zero conductance gaps) extracted from the onset positions at both positive and negative bias reveal 2.9, 2.6, and 2.0 eV for the three different set-currents of 0.1, 0.4, and 0.8 nA, respectively, at a set-voltage of 0.4 V.
any peak in the conductance spectra correlating the electronic or hole states. However, such electronic transitions from the valence to conduction levels were observed from optical spectroscopy earlier.10 Additionally, the optical spectra of PbS nanorods advised strong absorption and emission features, which are the characteristic of quantum confinement effect since the nanorod dimensions were well below aB.28 The apparent band gap calculated from the onset positions of tunneling current at both positive and negative bias reveal values of 2.9, 2.6, and 2.0 eV (Figure 2b) for the three different set-currents of 0.1, 0.4, and 0.8 nA respectively. These energy gaps are well below the actual band gap value of 3.41 eV calculated from the optical absorption spectrum.28 Additionally, the associated low tunneling current also suggests a direct tunneling mechanism through the sub gap region. The observed features of STS spectra suggest that the resonant tunneling through the LUMO and HOMO states may not be the tunneling mechanism, since the resonant tunneling corresponds to the peaks at both positive and negative bias originating from corresponding LUMO and HOMO states of the nanocrystal. However, the large tunnel current observed both at positive and negative bias points to the possibility of field emission of electrons from the PbS nanorod in FN tunneling regime. In order to ensure the FN tunneling mechanism, we have plotted the ln(I/V2) against 1/V (the FN-plot in Figure 3) at positive bias for three different set-currents (0.1, 0.4, and 0.8 nA). The FN curves reveal two different regimes (regime I and regime II depicted in Figure 3) with an inflection (Vtrans) separating the direct to FN tunneling regimes. Logarithmic behavior (regime I) in the FN-curves at low bias voltages indicate the direct tunneling of electrons from tip to substrate whereas the stiffer region at high bias voltages (regime II) associated with the FN tunneling from the PbS nanorod. The FN-plot for negative bias voltages (not shown here) also reveals similar behavior. The Vtrans positions (0.87, 0.7, and 0.63 V for 0.1, 0.4, and 0.8 nA, respectively) extracted from the inflection points from the FN curves suggest that the transition voltage decreases with the increase in the set-current for a particular set-voltage (0.4 V). The field emission onset positions reveal the low turn-on-field values which are indicative of superior field emitter properties. Designing such ultra narrow nanorods with a field emission property at low turn on voltages provides
crystallinity of the nanorods. The lattice planes correspond to the d-spacing’s of 0.29 ± 0.02 nm and 0.21 ± 0.02 nm consistent with the (200) and (220) planes of bulk rock salt structure of PbS. 3D STM topography images of the PbS nanorod assembly and an isolated PbS nanorod on the HOPG substrate is presented in Figure 1c,d, respectively. The STM imaging carried out in constant current mode by maintaining set-voltage at 0.4 V and set-current at 0.1 nA show several ordered PbS nanorods situating in side-by-side fashion on HOPG substrate (Figure 1c). Such small scale ordering of nanorods is also evidenced from the TEM observation (Figure 1a). The 3D STM image of an isolated PbS nanorod (Figure 1d) suggests a relatively large diameter ∼5 nm and length ∼25 nm in comparison to the size observed from HRTEM image (Figure 1b). A finite radius of the STM tip may contribute to the apparent enlargement of nanorod dimensions.29 Once a stable image was obtained, tunneling spectra were acquired by placing the tungsten (W) tip at the center of an isolated nanorod while disabling the feedback controls. The tunneling current (I) plotted as a function of bias voltage (V) collected for three different set-currents (0.1, 0.4, and 0.8 nA) by keeping the set-voltage (0.4 V) constant is shown in Figure 2a. All three I−V curves show symmetric nature in the positive and negative bias voltages and a sharp rise in the tunneling current beyond a certain bias voltages is observed. The degree of the tunneling current increases with the increase in the set-current at fixed set-voltage after the critical bias. Earlier reports on the tunneling spectroscopy performed on semiconductor nanocrystals using STM showed atomic like states with a step like behavior in the I−V curves at both positive and negative bias voltages representing the LUMO and HOMO states of the nanocrystals respectively.30,31 Generally, such I−V characteristics collected for single semiconductor nanocrystals have been elucidated in the DBTJ formalism with one barrier configured at the tip− nanocrystal junction whereas the other forming at the nanocrystal−substrate junction.10−12 In contrast our I−V measurements (Figure 2a) reveal continuous increase of tunnel current after exceeding the zero tunnel current regions at both positive and negative bias with a lack of step like behavior which could be correlated with the discretized VB and CB. Tunneling conductance (dI/dV), which represents the LDOS, was obtained by differentiating the experimental I−V curves (Figure 2b). The dI/dV spectra also show the absence of 18566
dx.doi.org/10.1021/jp3074927 | J. Phys. Chem. C 2012, 116, 18564−18570
The Journal of Physical Chemistry C
Article
PbS.34 The calculated shift in each CB and VB levels (bottom of the conduction band and top of the valence band) from the bulk band positions is 1.8 eV with respect to the vacuum level. Figure 4b shows the respective VB and CB positions of PbS nanorod accounting the shift due to quantum confinement effect and the relative Fermi level positions of substrate and tip with respect to the vacuum level. A comparison of calculated VB and CB positions of the PbS nanorod with respect to both EFHOPG and EFtip shows that the CB of PbS nanorod is situated near EFHOPG and EFtip. The position of the CB level of PbS nanorods with respect to EFHOPG and EFtip determines the height of the barrier (ϕ) in the single barrier approximation (Figure 4c). At zero bias, the electrons participate in the direct tunneling through the rectangular barrier formed between the substrate and tip, however, no net tunneling of electrons could occur owing to the near equal energies of EFHOPG and EFtip (Figure 4c). At low positive bias voltages ( Vtrans. At zero bias the electrons participate through the rectangular barrier (width = d1) with no net-tunneling (left panel). At low positive bias voltages (Vtrans) initiates the field emission of electrons through triangular region of the barrier (right panel, barrier width = d3). ϕ defines the height of the barrier of 1.55 eV (energy difference between the LUMO of PbS nanorod and Fermi level of HOPG/tip). Horizontal dashed arrow indicates the tunneling path for the electrons. The horizontal dashed line representing Vtrans, which separates the triangular region from trapezoidal barrier.
Figure 5. (a) Tunneling I−V curves of isolated PbS nanorods measured at three different set-currents of 0.1 nA (black squares and line), 0.8 nA (red dots and line), and 1.2 nA (blue up-triangles and line) at a fixed set-voltage of 1 V. (b) Corresponding FN curves plotted at positive bias show the clear transition from direct tunneling to FN tunneling. The black dotted line guides the transition voltages (Vtrans) between the direct and FN tunneling regimes for each set-current.
We have measured the I−V properties on ordered PbS nanorods assembled in side-by-side fashion. Figure 6a display the I−V characteristics collected for assembled PbS nanorods at two different set-currents of 0.1 and 0.4 nA for a fixed set-
voltage of 0.4 V. Interestingly, a considerable decrease in the tunneling current in comparison to the isolated nanorod for identical set-parameters is noted which is further reflected as reduced magnitude of logarithmic conductance in the FN18568
dx.doi.org/10.1021/jp3074927 | J. Phys. Chem. C 2012, 116, 18564−18570
The Journal of Physical Chemistry C
Article
Figure 6. (a) I−V curves measured on self-assembled PbS nanorods for two different set-currents 0.1 nA (black squares and line) and 0.4 nA (red dots and line) at a fixed set-voltage 0.4 V. Inset shows the STM image of self-assembled PbS nanorods. The green dot represents the reference position taken for STS measurements. (b) The corresponding FN curves plotted for two different set-currents of 0.1 nA (black squares and line) and 0.4 nA (red dots and line) at set-voltage 0.4 V. Inset, schematic showing the cross sectional view of possible tunneling processes through an array of nanorods where the field emitted electrons (ef−) are distributed (or leaked) through the adjacent nanorods suppressing the overall field emission.
Figure 7. (a) I−V curves measured on self-assembled PbS nanorods for two different set-currents 0.1 nA (black squares and line) and 0.1 nA (red dots and line) at a fixed set-voltage 1 V. (b) The corresponding FN curves show a clear transition in the electron conduction behavior similar to the FN plots measured at set-voltage of 0.4 V and also to FN plots of isolated PbS nanorod.
constant (∼1 V) with respect to change in the set parameters. These measurements reveal the need of a local electronic structure estimation on the relevant length scale to realize realistic device performance.
curves (Figure 6b). The low effective masses of electrons and holes in case of PbS defines the larger Bohr radii for both the charge carriers which may lead to spatial extension of wave functions beyond the nanorod boundary.12 Owing to the ultra narrow width, the electron and hole wave functions of neighboring PbS nanorods may couple significantly when the inter rod separation is small.35 During the tunneling measurements on the self-assembled closed packed ordered nanorods, the tunneling electrons could leak through the surrounding nanorods owing to the enhanced coupling between the adjacent PbS nanorods which further reduces the net tunneling between the tip and substrate (inset of Figure 6b). In fact such quantum mechanical coupling owing to the increased spatial extension of the wave functions of the carriers has been observed earlier for PbSe quantum dots.12 The electron leakage induced by strong electronic coupling further inhibits the strong FN tunneling behavior compared to an isolated nanorod. Figure 7a shows the I−V properties measured on selfassembly of PbS nanorods at set-currents of 0.1 and 1.2 nA at a fixed set-voltage of 1 V. The corresponding FN plots (Figure 7b) show a similar trend in the conductivity transition from direct to field emission tunneling of electrons as that measured at 0.4 V. From Figures 6 and 7, variation in the magnitude of tunneling current with respect to the set-parameters is less significant in case of self-assembled PbS nanorods. The Vtrans position which points out the transition of electron conduction mechanism from direct to FN tunneling also remains almost
■
CONCLUSION
In conclusion, we have reported on the tunneling spectroscopy performed on the ultra narrow PbS nanorods in isolated and self-assembled configurations. The I−V properties of isolated nanorod illustrated large tunnel current at higher bias confirming the nonresonant tunneling of the electrons through PbS nanorod. Measurements of I−V properties of PbS nanorod show a transition from direct tunneling regime at low bias to the FN-tunneling regime at the high bias voltages leading to the field emission phenomena. Transition voltage between these two tunneling regimes is found to vary with the set-currents. On the other hand, the I−V measured on self-assembled nanorods depicts a suppression of FN tunneling behavior due to the strong quantum coupling within the nearest neighbors. Such mutual interaction offers enormous versatility to control electronic properties over different scale lengths. The design of such ultra narrow PbS nanorods with intrinsic field emission properties might be an important step in realizing a miniaturized device. 18569
dx.doi.org/10.1021/jp3074927 | J. Phys. Chem. C 2012, 116, 18564−18570
The Journal of Physical Chemistry C
■
Article
(24) Dasgupta, N. P.; Lee, W.; Prinz, F. B. Chem. Mater. 2009, 21, 3973−3978. (25) Acharya, S.; Sarma, D. D.; Golan, Y.; Sengupta, S.; Ariga, K. J. Am. Chem. Soc. 2009, 131, 11282−11283. (26) Acharya, S.; Gautam, U. K.; Sasaki, T.; Bando, Y.; Golan, Y.; Ariga, K. J. Am. Chem. Soc. 2008, 130, 4594−4595. (27) Kundu, S.; Hill, J. P.; Richards, G. J.; Ariga, K.; Khan, A. H.; Thupakula, U.; Acharya, S. ACS Appl. Mater. Interfaces 2010, 2, 2759− 2766. (28) Khan, A. H.; Ji, Q.; Ariga, K.; Thupakula, U.; Acharya, S. J. Mater. Chem. 2011, 21, 5671−5676. (29) Hens, Z.; Vanmaekelbergh, D.; Stoffels, E. J. A. J.; Kempen, H. V. Phys. Rev. Lett. 2002, 88, 236803. (30) Banin, U.; Cao, Y. W.; Katz, D.; Millo, O. Nature 1999, 400, 542−544. (31) Millo, O.; Katz, D.; Cao, Y. W.; Banin, U. Phys. Rev. Lett. 2001, 86, 5751−5754. (32) Park, J.; Lee, K. H.; Galloway, J. F.; Searson, P. C. J. Phys. Chem. C 2008, 112, 17849−17854. (33) Thupakula, U.; Khan, A. H.; Bal, J. K.; Ariga, K.; Acharya, S. J. Nanosci. Nanotechnol. 2011, 11, 7709−7714. (34) Hyun, B. R.; Bartnik, A. C.; Lee, J. K.; Imoto, H.; Sun, L.; Choi, J. J.; Chujo, Y.; Hanrath, T.; Ober, C. K.; Wise, F. W. Nano Lett. 2010, 10, 318−323. (35) Ihly, R.; Tolentino, J.; Liu, Y.; Gibbs, M.; Law, M. ACS Nano 2011, 5, 8175−8186.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +91-9475085806. Fax: +91-33-2473-2805. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
REFERENCES
Financial support from DST, Government of India, Grant No. SR/S5/NM-47/2005 and Grant No. SR/NM/NS-49/2009 are gratefully acknowledged. We thank Dr. H. M. Jafri, Department of Engineering Sciences, Uppsala University for useful discussions.
(1) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66−69. (2) Bjork, M. T.; Ohlsson, B. J.; Thelander, C.; Persson, A. I.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2002, 81, 4458−4460. (3) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241−245. (4) Pradhan, N.; Acharya, S.; Ariga, K.; Karan, N. S.; Sarma, D. D.; Wada, Y.; Efrima, S.; Golan, Y. J. Am. Chem. Soc. 2010, 132, 1212− 1213. (5) Acharya, S.; Kundu, S.; Hill, J. P.; Richards, G. J.; Ariga, K. Adv. Mater. 2009, 21, 989−993. (6) Patla, I.; Acharya, S.; Zeiri, L.; Israelachvili, J.; Efrima, S.; Golan, Y. Nano Lett. 2007, 7, 1459−1462. (7) Acharya, S.; Patla, I.; Kost, J.; Efrima, S.; Golan, Y. J. Am. Chem. Soc. 2006, 128, 9294−9295. (8) Acharya, S.; Panda, A. B.; Efrima, S.; Golan, Y. Adv. Mater. 2007, 19, 1105−1108. (9) Tian, B.; Kempa, T. J.; Lieber, C. M. Chem. Soc. Rev. 2009, 38, 16−24. (10) Liljeroth, P.; Jdira, L.; Overgaag, K.; Grandidier, B.; Speller, S.; Vanmaekelbergh, D. Phys. Chem. Chem. Phys. 2006, 8, 3845−3850. (11) Liljeroth, P.; Emmichoven, P. A. Z. V.; Hickey, S. G.; Weller, H.; Grandidier, B.; Allan, G.; Vanmaekelbergh, D. Phys. Rev. Lett. 2005, 95, 086801. (12) Liljeroth, P.; Overgaag, K.; Urbieta, A.; Grandidier, B.; Hickey, S. G.; Vanmaekelbergh, D. Phys. Rev. Lett. 2006, 97, 096803. (13) Katz, D.; Millo, O.; Kan, S. H.; Banin, U. Appl. Phys. Lett. 2001, 79, 117−119. (14) Ikuno, T.; Okamoto, H.; Sugiyama, Y.; Nakano, H.; Yamada, F.; Kamiya, I. Appl. Phys. Lett. 2011, 99, 023107. (15) Zhai, T.; Fang, X.; Bando, Y.; Liao, Q.; Xu, X.; Zeng, H.; Ma, Y.; Yao, J.; Golberg, D. ACS Nano 2009, 3, 949−959. (16) Li, L.; Wu, P.; Fang, X.; Zhai, T.; Dai, L.; Liao, M.; Koide, Y.; Wang, H.; Bando, Y.; Golberg, D. Adv. Mater. 2010, 22, 3161−3165. (17) Liu, B.; Bando, Y.; Jiang, X.; Li, C.; Fang, X.; Zeng, H.; Terao, T.; Tang, C.; Mitome, M.; Golberg, D. Nanotechnology 2010, 21, 375601. (18) Maiti, U. N.; Maiti, S.; Goswami, S.; Sarkar, D.; Chattopadhyay, K. K. Cryst. Engg. Comm. 2011, 13, 1976−1983. (19) Fang, X.; Gautam, U. K.; Bando, Y.; Dierre, B.; Sekiguchi, T.; Golberg, D. J. Phys. Chem. C 2008, 112, 4735−4742. (20) Fang, X.; Bando, Y.; Ye, C.; Golberg, D. Chem. Commun. 2007, 3048−3050. (21) Fang, X.; Bando, Y.; Ye, C.; Shen, G.; Gautam, U. K.; Tang, C.; Golberg, D. Chem. Commun. 2007, 4093−4095. (22) Pan, Z. W.; Lai, H. L.; Au, F. C. K.; Duan, X.; Zhou, W.; Shi, W.; Wang, N.; Lee, C. S.; Wong, N. B.; Lee, S. T.; Xie, S. S. Adv. Mater. 2000, 12, 1186−1190. (23) Khan, A. H.; Maji, S.; Chakraborty, S.; Manik, N. B.; Acharya, S. RSC Adv. 2012, 2, 186−191. 18570
dx.doi.org/10.1021/jp3074927 | J. Phys. Chem. C 2012, 116, 18564−18570
