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Transport Gap vis-a`-vis Electrical Bistability of Alloyed ZnxCd1-xS (x ) 0 to 1) Quantum Dots Batu Ghosh and Amlan J. Pal* Indian Association for the CultiVation of Science, Department of Solid State Physics, JadaVpur, Kolkata 700032, India ReceiVed: May 26, 2010; ReVised Manuscript ReceiVed: July 12, 2010
We study a correlation between electrical bistability and transport gap of II-VI semiconducting quantum dots. We first grow alloyed ZnxCd1-xS (x ) 0 to 1) quantum dots for different values of x, functionalize them with suitable (anionic) stabilizers, and form their monolayer on an electrode surface via electrostatic assembly. We characterize the monolayers of the quantum dots by scanning tunneling microscopy. Current-voltage characteristics of the monolayers evidence electrical bistability and memory phenomena that depend on composition or Zn-content of the quantum dots. The dependence is due to the fact that an addition of Zn in ZnxCd1-xS introduces trap-states, which assist the process of electrical bistability and play a major role in the conduction process of high-conducting states of quantum dots. Transport gap, which depends on the composition of the quantum dots, also responds to the electrical bistability; for all the quantum dots, the gap decreases during the transition from a low- to a high-conducting state. We here correlate the transport gap of ZnxCd1-xS (x ) 0 to 1) and its change upon conductance-switching with the electrical bistability of quantum dots. 1. Introduction One of the major challenges in today’s memory technology is to make denser, faster, and less energy consuming nonvolatile memory. Quantum dots, being nanoregime in size with unique optical and electronic properties,1 are considered as potential elements for data storage. Apart from metallic nanoparticles,2-6 oxide quantum dots7-9 and in particular the II-VI semiconducting nanomaterials10-13 are being considered in this direction. Electrical conductivity of the nanostructures is used to “read” the memory elements. To “write” a high bit or state, a voltage pulse is generally applied; to “erase” the high state to a low one, an opposite voltage is applied.3,10,12 In principle, reversible electrical bistability of quantum dots has been the key process for data-storage applications. In electrically bistable devices, at least two current values are observed at a given voltage; a voltage pulse in general changes the state or conductivity of the material. Electrical bistability in quantum dots has so far been explained in terms of electric-field-induced charge confinement in the nanoparticles or trap-state-assisted conduction.6-8,10,11,13 Apart from the advantage of providing high density in datastorage applications, the nanomaterials provide size-dependent opto-electronic properties. To tune electrical bistability and memory phenomena of semiconducting quantum dots, the diameter of the nanoparticles11 and nature of coordinating ligands7 have been some of the parameters in this direction. The parameters of bistability include On/Off ratio, which is defined as the ratio between current of high- and low-conducting states at a voltage and threshold voltage of switching (VTh). Apart from the diameter, composition of nanocrystals can also be considered as a parameter to control electrical bistability. Since charge confinement in the nanoparticles or trap-state-assisted conduction yields electrical bistability,6-8,10,11,13 alloyed quantum dots can be a unique material in this direction. We hence synthesized alloyed ZnxCd1-xS (x ) 0 to 1) nanoparticles,14,15 * To whom correspondence should be addressed. E-mail: sspajp@iacs. res.in. Phone: +91-33-24734971. Fax: +91-33-24732805.
characterized them, functionalized them to form a monolayer on a suitable electrode through layer-by-layer (LbL) electrostatic assembly,16-19 and studied electrical properties (and correspondingly electrical bistability) as a function of composition. The alloys in their bulk and thin-film forms have earlier exhibited nonvolatile memory applications.20,21 The studies of electrical properties, namely current-voltage (I-V) characteristics of a monolayer of nanoparticles with a scanning tunneling microscope (STM) tip, in general yield the transport gap of the material.22,23 We hence investigate if the process of electrical bistability or conductance-switching that induces charge confinement or trapping of carriers has an effect on the transport gap of quantum dots. 2. Experimental Section 2.1. Growth of Nanoparticles. We have grown ZnxCd1-xS (x ) 0 to 1) nanoparticles following the procedure reported by Zhong et al.14 In a typical synthesis for ZnxCd1-xS (x ) 0.36), CdO (6.4 mg 0.05 mM), ZnO (24.3 mg 0.2 mM), octadecene (5 mL), and oleic acid (0.5 mL) were mixed together and warmed to 90 °C under argon atmosphere for 30 min. The temperature of the solution was then increased to 310 °C at which CdO and ZnO dissolved completely to form a clear colorless solution. The temperature of the solution was then lowered slightly (300 °C); 1 mL of sulfur solution (0.2 mM) in octadecene was quickly injected into the hot solution. The photoluminescence (PL) spectrum of the solution was recorded at regular intervals to monitor the growth process. A narrow PL emission confirmed completion of the growth; the growth process was terminated by spraying alcohol in the wall of the reaction vessel. To vary the composition of the nanoparticles, different content of ZnO and sulfur were used. Synthesized nanoparticles were repeatedly washed in a separating funnel by adding methanol in hexane solution of the nanoparticles. They were finally precipitated by adding acetone followed by methanol.
10.1021/jp1048056 2010 American Chemical Society Published on Web 07/28/2010
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Figure 1. (a) TEM image of ZnxCd1-xS (x ) 0.36). The inset shows the HR-TEM image of the particles. (b) XRD patterns of ZnxCd1-xS with different Zn-content (as specified in the legend).
2.2. Ligand Exchange. Since we required forming a monolayer of the nanoparticles via electrostatic assembly, we carried out the ligand exchange reaction. We replaced the hydrophobic long-chain oleic acids by water-soluble short-chain mercaptopropaionic acid (MPA). To do so, we followed the procedure reported by Peng’s group.24 A 5 mg sample of MPA was added to 5 mL of methanol in a reaction vessel; the pH of the mixture was adjusted to >10 with tetramethylammonium hydroxide (TMAH). A 5 mg sample of ZnxCd1-xS nanocrystals was then added to the mixture, which was then stirred overnight at 60 °C under argon environment. The reaction was stopped by adding ethyl acetate and ether that precipitated the nanoparticles. The product was centrifuged out, dissolved in methanol, and finally reprecipitated with ethyl acetate. 2.3. Characterization of Nanoparticles. Electronic absorption and PL spectra of the nanoparticles in dispersed solution were recorded and compared with standard spectra to establish formation of the alloyed nanocrystals. The spectra were recorded with Shimadzu 2550 UV and Fluoromax 4P, respectively. Lattice parameters of the nanostructures were determined from powder XRD patterns (Rich-Seifert XRD 3000P). The nanoparticles were also characterized with a TEM transmission electron microscope (TEM) and high-resolution TEM (Jeol JSM 2010). 2.4. Monolayer Formation. To probe the nanoparticles with Hg-drop or STM, a monolayer of the nanoparticles was formed on doped Si 〈111〉 substrates. The Si wafers were n-type in nature due to arsenic dopants with a resistivity of 3-10 mΩ · cm. Formation of a monolayer of the nanoparticles was a part of the LbL electrostatic adsorption process. That is, the electrodes were suitably protonated to electrostatically adsorb anionic nanoparticles that were capped with MPA during the growth of the particles. The adsorption process of the nanoparticles was monitored by electronic absorption of one and multiple layers via the LbL adsorption process on quartz substrates. Deposition of LbL layers was done by sequential adsorption of a monolayer of the nanoparticles and a monolayer of a polycation, namely, poly(allylaminehydrochloride) (PAH), in cycles. In practice, quartz substrates were dipped in nanoparticle and PAH solutions in sequence with washing in deionzied water baths three times after electrostatic adsorption of each layer. The electronic absorption spectrum of the films was recorded after assembly of every bilayer. Adsorption of a monolayer of the nanoparticles on Si substrates was evidenced by STM topography.
2.5. I-V with Hg Drop. To record I-V characteristics of the quantum dots, the monolayer of the nanoparticles on Si was placed in a vacuum bell-jar with the monolayer facing downward. The bell-jar was equipped with a syringe containing mercury, which could be pushed through its metallic needle from outside so that the mercury blob could just touch the film. Here, the LbL deposition process was cycled only once to obtain a monolayer of quantum dots on doped Si substrates. Moreover, mechanism of LbL deposition ensured formation of a monolayer for electrical characterization. I-V characteristics were recorded by making contacts between Si and Hg (through the metallic needle of the syringe). Bias was applied to Hg in loops with a Yokogawa 7651 dc voltage source with a scan speed of 50 mV/ s; current was measured with a Keithley 486 Picoammeter. The instruments were controlled with a PC through GPIB. 2.6. I-V with STM Tip. I-V characteristics of the monolayers were also recorded with a STM. The measurements were carried out with a Nanosurf EasyScan2 in ambient condition. Here, while the doped Si was the base electrode, Pt/Ir was the top one. Apart from characterizing pristine films, I-V characteristics were recorded after applying a voltage pulse of suitable amplitudes and directions that induce a high-conducting state or reinstate a low one. STM topography of the monolayer was recorded to evidence nanoparticles on Si substrates. 3. Results and Discussion 3.1. Characterization of Nanoparticles. Transmission electron microscope (TEM) images of the nanoparticles were recorded. A typical case is shown in Figure 1a. The image shows that the particles were fairly monodispersed. High-resolution TEM (HR-TEM) images display the lattice fringes evidencing the crystalline nature of the alloys. The diameter of the nanoparticles ranged between 4.5 and 5.0 nm. X-ray diffraction (XRD) patterns of the nanoparticles of different compositions are shown in Figure 1b. The patterns returned lattice parameters that matched the reported results.14,25 With a variation of x in ZnxCd1-xS nanoparticles, the diameter ranged between 3.0 and 6.0 nm. Electronic absorption and photoluminescence (PL) spectra of all the nanoparticles of different Zn-content were recorded. Spectra of ZnxCd1-xS nanoparticles with different Zn mole fractions in the alloy (along with the two extremes, namely, CdS and ZnS) are shown in Figure 2. For each material except ZnS, the peak emission is located very close to the excitonic
Transport Gap and Electrical Bistability of Quantum Dots
Figure 2. Photoluminescence spectra of ZnxCd1-xS (x ) 0 to 1) in dispersed solution. Electronic absorption spectra of the corresponding particles in dispersed solution are shown in the inset.
Figure 3. Electronic absorption spectra of ZnxCd1-xS (x ) 0.36) LbL films deposited with PAH as the polycation. Spectra for 2 to 12 bilayers of ZnxCd1-xS with PAH are shown. The insets show (i) absorbance at 400 nm versus number of bilayers of the LbL films along with typical STM topographies of (ii) a monolayer of ZnxCd1-xS (x ) 0.36) nanoparticles on a doped Si substrate and (iii) the substrate itself. Total scan area of insets ii and iii is 40 nm × 40 and 80 nm × 80 nm, respectively.
absorption maximum. The PL is sharp with a full width at halfmaximum (fwhm) of 18-27 nm. This shows that the PL is dominated by band-edge emissions. The PL of ZnS is expectedly a broad one due to its high bandgap and corresponding trapstates below the band-edge. In ZnxCd1-xS (x ) 0.36), the PL at 540 nm can be identified to originate from the traps sites.15 3.2. Formation of a Monolayer. To study the electrical properties of the quantum dots required the formation of a monolayer of the nanoparticles on suitable electrode surfaces. We formed such a monolayer via the electrostatic adsorption process. To establish monolayer formation, we relied on electronic absorption spectra of multiple bilayers, since a monolayer would return a very small value of absorbance in the spectrum. That is, we deposited LbL films of the anionic nanoparticles with PAH as the polycation for a different number of bilayers and recorded electronic absorption spectra of the films. Such spectra for a particular composition, namely ZnxCd1-xS (x ) 0.36), are shown in Figure 3. A plot of absorbance at a particular wavelength versus number of bilayers is shown in the inset. The plot shows that the absorbance increases linearly with the number of bilayers. This shows that the nanoparticles become adsorbed uniformly during the LbL deposition process. The finite absorbance at one bilayer, as obtained from the extrapolated region toward the origin (inset of Figure 3), shows that nanoparticles were adsorbed even during dipping of the first layer or a monolayer.
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Figure 4. Current-voltage characteristics of a monolayer of ZnxCd1-xS (x ) 0 to 1) quantum dots with Hg as the top electrode. Bias was applied in loops as indicated by arrows. For each of the monolayers, I-V characteristics under three consecutive voltage loops are shown. The inset shows the On/Off ratio of the five cases obtained from the first loop of I-V characteristics.
Monolayer formation during adsorption of nanoparticles has also been supported by STM topography. We recorded the topography of a monolayer on Si (inset of Figure 3). The size of the nanoparticles matched reasonably well with the diameter obtained from HR-TEM images. Control topography without the monolayer, that is, a bare Si, has also been recorded and shown in another inset of the figure. A comparison of the topographies clearly shows that the nanoparticles were adsorbed on the substrate during the dipping process. The adsorption expectedly occurs due to electrostatic assembly process. The topography provides a decent view of the surface visualizing the nanoparticles on the substrate. It also offers the scale of compactness of the nanoparticles in the monolayer. 3.3. Electrical Bistability: Measurement with a Hg Drop. We have characterized the monolayers of different nanoparticles with mercury electrodes. The doped Si substrate acted as the bottom electrode. I-V characteristics of the monolayers under a voltage loop are shown in Figure 4. Characteristics of all the monolayers show electrical bistability. The magnitude of the current at a given voltage is higher during the sweep from a negative voltage than that from a positive voltage. The bistability is prominent in the negative voltage region. For each of the cases, characteristics under three voltage loops have been included in the figure to show the reproducibility of the bistability. The figure shows that parameters of electrical bistability depend on the composition. That is, the threshold voltage (VTh) of switching, the voltage at which current increases sharply to a higher value, depends on the composition. With the addition of Zn, the system requires higher (magnitude of) VTh to switch to a higher conducting state. The On/Off ratio in the Zn-rich system is, however, more. The On/Off ratio versus voltage plots of different monolayers are shown in the inset of Figure 4. The figure first of all shows the possibility of tuning of electrical bistability by the addition of Zn in the system. Since the addition of Zn in ZnxCd1-xS alloys creates surface and interior trap-states below the band-edge15 and we have observed a higher On/Off ratio in Zn-rich ZnxCd1-xS (x ) 0.36) quantum dot alloys, a correlation between them (trap-states and bistability) can be envisaged. That is, the high-conducting state may appear when the trap-states are filled. Under application of a suitable (positive) voltage, the traps may become empty reinstating the low-conducting off-state. Such a mechanism has earlier been proposed in CdSe/ZnS systems.13
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Figure 5. Read-only memory (ROM) applications of monolayers of ZnxCd1-xS (x ) 0 to 1) quantum dots with Hg as the top electrode. While -1.3 (30 s) and +1.3 V (30 s) voltage pulses were applied to switch the monolayers to a high and a low state, respectively, current under a probe voltage (-0.3 V) was measured as a function of time to “read” the states. Absolute values of current are plotted in the figure. Legends specify Zn-content in the nanostructures. Open and filled symbols represent low and high state, respectively.
Carrier mobility and the corresponding electrical conductivity of the quantum dots may play a major role in yielding an optimum On/Off ratio in the system with an intermediate Zncontent (inset of Figure 4). Addition of Zn in the system that increases bandgap is expected to decrease carrier mobility. This has been manifested as a decrease in both Off- and On-state currents. Addition of Zn also introduces trap-states that support an electrical switching to a high-conducting state. These two effects played a role in tandem to yield a high On/Off ratio at an intermediate Zn-content in the ZnxCd1-xS alloys. Apart from their reversibility, the two conducting states are stable over time. That is, once a high-conducting state is activated, the device retains the state. The state can be nondestructively probed by measuring current under a small voltage. In practice, we probed both high- and low-conducting states separately. To do so, we switched the devices to a highand a low-conducting state by applying suitable voltages. We probed the states as a function of time. Current under a probevoltage for the two states is plotted in Figure 5, which exemplifies read-only memory (ROM) applications. The figure shows results from devices based on nanoparticles of different Zn-contents. The two states were distinguishable over a considerable period of time. Devices based on all the nanoparticles exhibited ROM applications. The scale of On/Off ratio, obtained from I-V characteristics in loops, is reflected in measurements for ROM applications. There was no clear correlation between composition of the nanoparticles and retention time of the memory states within the time range of our experiment. We hence can conclude from the figure that though the parameters of electrical bistability depend on the Zn-content, all the alloyed nanoparticles are suitable for ROM applications. 3.4. Electrical Bistability: Measurement with a STM Tip. To reduce the area of I-V measurements in the monolayer, we have characterized the monolayers with a STM tip. STM measurements moreover gave us the possibility to characterize the monolayers with another electrode, namely Pt/Ir, which has a different workfunction than Hg. With the Pt/Ir, we have recorded I-V characteristics of the monolayers in their lowand high-conducting states. In practice, we first characterized a pristine monolayer between -2.0 and +2.0 V. The range of voltage-scan was kept small so as to ensure that the scanning does not itself induce a higher conducting state. To deliberately
Ghosh and Pal
Figure 6. Current-voltage characteristics of a monolayer of ZnxCd1-xS (x ) 0.36) quantum dots with Pt/Ir tip of STM as the top electrode. Measurements were carried out at three different points on the monolayer as represented by traces of different colors. At each position, bias was scanned on pristine films, after applying a voltage pulse of -6.0 V (10 ms) that induces a high-conducting state, and after applying a pulse of +6.0 V (10 ms) that reinstates the initial low-conducting state. For all the measurements, bias was applied from -2.0 to +2.0 V. Separate measurements by scanning bias from +2.0 to -2.0 V yielded the same results. The inset shows On/Off ratio versus voltage plots of different nanoparticles, namely ZnxCd1-xS (x ) 0 to 1) monolayers.
induce a high-state, a voltage pulse (-6.0 V, 10 ms) was applied. I-V characteristics of the high-state were then recorded in the same voltage range. Conductivity of the pristine film or a lowconducting state was then reinstated by applying a voltage pulse of opposite bias (+6.0 V, 10 ms). I-V characteristics of the low-conducting state were scanned in a similar fashion. Measurements were carried out many times in cycles and also at different points on the monolayers to test the reproducibility. Such a sequence of STM measurements was carried out for all the monolayers having different content of Zn in CdS nanoparticles. I-V characteristics of a typical case for ZnxCd1-xS (x ) 0.36) monolayer at three different points are shown in Figure 6. For each point on the monolayer, we have presented characteristics of (i) the pristine sample, (ii) the high-state, and (iii) the reinstated low-state. The results exhibit electrical bistability in the monolayer. That is, conductivity of the monolayer at a voltage increases due to application of a -6.0 V pulse. The bistability was reversible. Upon application of a +6.0 V pulse, conductivity of the monolayer decreases and the I-V curve matches that of the pristine film. Though all the ZnxCd1-xS (x ) 0 to 1) nanoparticles exhibited electrical bistability measured with STM tip, it depended on the Zn-content in the quantum dots. The dependence can be visualized best by plotting On/Off ratio versus voltage for all the compositions of the nanoparticles. The inset of Figure 6 shows such a plot. The ratio, which is as usual voltage dependent, responded to the Zn-content. At an intermediate Zncontent (x ) 0.36), the ratio has been higher than that of the other compositions. This reaffirms the role of trap-states in achieving a higher-conducting state in these electrically bistable quantum dots. The On/Off ratio rose to 250 for ZnxCd1-xS (x ) 0.36) at -1.5 V. 3.5. Transport Gap vis-a`-vis Electrical Bistability. I-V characteristics of a monolayer with the Pt/Ir tip of STM as the top electrode can provide important information about the transport gap of the quantum dots. To do so, we have replotted the I-V characteristics of pristine monolayers in a log-linear scale. The voltages at which current rises from the background value indicate the location of conduction and valence bandedges of the quantum dots on doped Si. In other words, at these
Transport Gap and Electrical Bistability of Quantum Dots
Figure 7. Current-voltage characteristics of monolayers of ZnxCd1-xS (x ) 0.36) quantum dots with Pt/Ir tip of STM as the top electrode, as presented in Figure 6, in a log-linear scale. Measurements carried out at three different points on the monolayer are represented by traces of different colors. The arrow shows the method to determine the transport gap of the nanoparticles in their high state. The inset of the figure shows transport gap of low- and high-conducting states of different ZnxCd1-xS nanoparticles with Zn-content varying from 0 to 1.
voltages, electron injection occurs from the tip to the conduction band of the quantum dot (negative voltage) and from the valence band to the STM tip (positive voltage). From the difference between the voltages, the transport gap of the quantum dots on doped Si can be obtained. In Figure 7 we show ln(I) versus V plots of a pristine monolayer at three different points on the film. We have also included characteristics of its highconducting state (after application of a -6.0 V pulse) at the three points. In both cases, there is little or no point-to-point variation in the transport gap. The transport gap of the highstate is clearly smaller than that of the pristine low-conducting nanoparticles. In other words, we can infer that electrical bistability in quantum dots is associated with a change or decrease in transport gap. The decrease in the transport gap upon switching to a high-state is associated with a decrease in its conduction band and an increase in the valence band energy. The filled-in trap-states may, however, act as effective conduction and valence bands in the system for carrier transport processes. The change in the transport gap is reversible in nature. That is, upon switching back to a low-conducting state (by applying a positive voltage pulse), the initial value of the gap is reinstated. Such characteristics recorded again at the three different points on the monolayer have been included in Figure 7. A change or decrease in the transport gap upon conductanceswitching is true for all the nanoparticles of different Zn-content. That is, we measured the transport gap of all five different nanoparticles, namely, ZnxCd1-xS (x ) 0 to 1) in their lowand high-conducting states. A plot of transport gap of ZnxCd1-xS quantum dots as a function of x is shown in the inset of Figure 7. The plot shows that the transport gap of pristine nanoparticles increases with Zn-content in the alloys. The results are in unison with the results obtained from the electronic absorption spectra (inset of Figure 2); it may be recalled that the optical gap of the alloys increased with an increase in Zn-content. The transport gap of the nanoparticles on doped Si responds to electrical bistability. For all the nanoparticles, the transport gap always decreases during conductance-switching from a low- to a highconducting state. To substantiate the role of trap-states (in ZnxCd1-xS alloys) in the observed electrical bistability, we have calculated the density of states (DOS) of all the nanoparticles in their lowand high-conducting states. The results from a monolayer of ZnxCd1-xS (x ) 0.36) are shown in Figure 8; as in the other
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Figure 8. Density of states (DOS) versus voltage plot of a monolayer of ZnxCd1-xS (x ) 0.36) quantum dots in their high and low states with Pt/Ir tip of STM as the top electrode. DOS has been calculated from the current-voltage characteristics, as presented in Figure 6. Measurements carried out at three different points on the monolayer are represented by traces of different colors.
plots, results from three different points on the monolayer are shown to evidence reproducibility. The plot shows that upon conductance-switching to a high-conducting state, DOS of the nanoparticles in the monolayer increases, particularly near the band-edges. Plots for other nanoparticles have yielded a similar increase in the DOS upon conductance-switching. An increase in DOS points toward the appearance of energy or trap levels within the bandgap. The results hence add support to the mechanism that the high-conducting state of electrically bistable quantum dots arises due to the trap-state-assisted conduction process. 4. Conclusions In conclusion, we have correlated electrical bistability and change in transport gap of ZnxCd1-xS (x ) 0 to 1) nanoparticles. We have observed that electrical bistability or conductanceswitching to a high-state is associated with a decrease in transport gap of the nanoparticles on doped Si substrates. Here, we grew the alloyed nanoparticles and functionalized them to form a monolayer through the electrostatic adsorption process. The monolayers of the quantum dots have been characterized by STM. Electrical bistability, which has been manifested in the I-V characteristics recorded with Hg drop or Pr/Ir tip of the STM, has depended on Zn-content in the nanoparticles. Since addition of Zn in CdS introduces trap-states, we could analyze conductance-switching in terms of the trap-state-assisted conduction process. DOS of high- and low-conducting states supported such a mechanism of electrical bistability. Acknowledgment. The authors acknowledge financial assistance from DST Nano Mission project SR/NM/NS-55/2009 and Ramanna Fellowship SR/S2/RFCMP-01/2009. References and Notes (1) Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887– 2894. (2) Paul, S.; Pearson, C.; Molloy, A.; Cousins, M. A.; Green, M.; Kolliopoulou, S.; Dimitrakis, P.; Normand, P.; Tsoukalas, D.; Petty, M. C. Nano Lett. 2003, 3, 533–536. (3) Tseng, R. J.; Huang, J. X.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077–1080. (4) Wang, H. P.; Pigeon, S.; Izquierdo, R.; Martel, R. Appl. Phys. Lett. 2006, 89, 183502. (5) Chandra, A.; Clemens, B. M. Appl. Phys. Lett. 2005, 87, 253113. (6) Lee, J. S.; Cho, J.; Lee, C.; Kim, I.; Park, J.; Kim, Y. M.; Shin, H.; Lee, J.; Caruso, F. Nat. Nanotechnol. 2007, 2, 790–795.
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