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Letter pubs.acs.org/NanoLett Band Mapping Across a pn-Junction in a Nanorod by Scanning Tunneling Microscopy Abhijit Bera, Sukumar Dey, and Amlan J. Pal* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information * ABSTRACT: We map band-edges across a pn-junction that was formed in a nanorod. We form a single junction between p-type Cu2S and n-type CdS through a controlled cationic exchange process of CdS nanorods. We characterize nanorods of the individual materials and the single junction in a nanorod with an ultrahigh vacuum scanning tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determine the conduction and valence band-edges at different points across the junction and the individual nanorods. We could map the band-diagram of nanorod-junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in the nanoscale. KEYWORDS: pn-junction in a nanorod, scanning tunneling spectroscopy, mapping of band-edges across a pn-junction in a nanorod, density of states B the surface, the ligands would perturb the interface between the two materials or the junction itself. We hence have targeted to form a pn-junction in a single nanorod instead. To do so, we have formed a junction between CdS and Cu2S in the nanorod through a cationic exchange process that acted as n- and p-type materials, respectively. We mapped the bands of the junction in a nanorod with an aim to visualize the electric field across the depletion region of the diode formed in a single nanorod. Characterization of Nanorods and Junctions: Optical Absorption, Transmission Electron Microscopy (TEM), High-Resolution TEM (HR-TEM) Images, and STM Topography. We have recorded optical absorption spectra of CdS and Cu2S nanorods and also of the CdS|Cu2S junctions (Figure 1). In characterizing nanorod-junctions, we chose two reaction times for the cationic exchange process that in effect controlled the length of Cu2S in the CdS|Cu2S junction. While the spectrum of CdS shows absorption in the short wavelength region, the one of Cu2S extends also to the near-IR region. The absorbance in the 475−650 nm region appeared solely due to Cu2S; the long wavelength region of the spectrum arises due to the localized surface plasmon resonance occurring out of copper deficiencies.27 Both the spectra matched the reported results of nanorods (and of other lower dimensional structures) of the respective materials.27−29 The spectra of the CdS|Cu2S junctions were mostly the sum of individual ones with the absorbance in the 475−650 nm regions increasing with the and-edges of lower dimensional structures have been one of the major parameters of importance in designing an electronic device. Accordingly, shrinking the length of active materials along one to three directions leading to quantum confinement effect,1−3 doping,4 and alloying5 of the semiconductors, and so forth have been considered as routes to alter the band-edges in one way or the other. Junctions between two semiconductors in the form of type-I,6,7 type-II or staggered,8,9 and type-III or type-II broken gap10 band-offset are inherently interesting systems so far as the energy-levels at the interfaces are concerned. Such band-offsets are formed in core−shell nanoparticles11−14 and nanorods/nanowires15 and in conventional pn-junctions in a nanorod.16,17 The offsets at the interface of a heterostructure, along with the location of bandedges, are hence important in engineering of nanostructures. In locating the band-edges of lower-dimensional systems, scanning tunneling spectroscopy (STS) employing a scanning tunneling microscope (STM) has so far been the most powerful technique.4,14,18−22 The ability to map band-edges of nanostructures has made the technique unique in many respects. The nanostructures in this direction ranged from simple quantum dots4,19 to complex core−shells14,18,21 and hybrid core−shell systems in which organic molecules formed the shell layer on inorganic nanocrystals.20,22 In research driven by applications, cross-sectional STM23−25 has been used to look at the interface between the materials of bulk-heterojunction solar cells.24 In this work, we aimed to map the bands in a pn-junction nanostructure. In a junction between two separately formed nanostructures, because they are grown through colloidal synthesis routes17,26 involving organic ligands as stabilizers at © 2014 American Chemical Society Received: January 8, 2014 Revised: February 15, 2014 Published: March 3, 2014 2000 dx.doi.org/10.1021/nl500081m | Nano Lett. 2014, 14, 2000−2005 Nano Letters Letter Figure 1. Optical absorption spectra of CdS and Cu2S nanorods and of CdS|Cu2S junctions. In the nanorod-junctions, CdS|Cu2S (1) and CdS|Cu2S (2) represent junctions formed with a reaction time of 7 and 10 min, respectively, for the cationic exchange process that in effect controlled the length of Cu2S section in the nanorod-junctions. Figure 3. Scanning tunneling microscopy (STM) topography of (a) CdS and (b) Cu2S nanorods and (c) Cu2S|CdS and (d) Cu2S|CdS| Cu2S nanorod junctions. Set-points for the approach of the tip were 0.4 nA at 2.0 V. These set-points were used during STS measurements. appeared brighter than the CdS presumably due to a higher conductivity of Cu2S that would have led to an increased tip-tonanorod distance as compared to CdS. From the difference in brightness and also wideness, we could identify the materials in a nanorod for further characterization under STM. The diameter of CdS and Cu2S nanorods or their sections in a junction were both around 5 nm. With the exciton Bohr radius in CdS and Cu2S being 3.0 nm and in the range of 3−5 nm, respectively, both the materials in nanorods that we characterized were close to their bulk form.30,31 In the CdS and Cu2S nanorods, while probing different points across the nanorods with a STM tip, our aim was to locate the conduction and valence band-edges at different points on the nanorods. To do so, we recorded tunneling current versus voltage (I−V) characteristics with the STM tip as the electrode for tunneling of charge carriers. Here, the tunneling current was recorded after positioning the STM tip above the nanostructure and disabling the scanning and feedback controls. Because the system resembled a doublebarrier tunnel junction (DBTJ) involving the tip−nanorod and nanorod−substrate tunnel barriers,32 we carried out the measurements at different tip−nanorod distances by using different set-points for the approach of the tip. We then calculated the density of states (DOS) so that the band-edges could be located. It may be stated here that because bias was applied with respect to the tip, the peaks at the positive voltage region, at which electrons can be injected from the tip to the nanorod, denote the location of conduction bands. Similarly, increase in the length of Cu2S section in CdS|Cu2S nanorod junctions. High-resolution TEM (HR-TEM) images of CdS and Cu2S nanorods separately, as presented in Figure 2a,b, respectively, show that the length of the nanorods was about 70 nm in both the cases. In the nanorod junctions, the dark-field scanning transmission electron microscopy (STEM) image, as shown in Figure 2c, confirms the presence of two phases in a nanorod. In some nanorods, Cu2S appeared to have formed from both ends of CdS. The images favorably implied that the Cu2S section of the nanorod-junction was bright and wider. The HR-TEM image further shows that though the nanorods were largely single crystalline in nature, two clear crystalline planes are visible in a single nanorod evidencing formation of a junction in a nanorod. An interlayer spacing of 0.338 and 0.340 nm at the two parts of a nanorod matched well with the ⟨002⟩ plane of CdS nanocrystals (hexagonal phase) and the ⟨002⟩ plane of Cu2S nanocrystals (chalcocite-type hexagonal phase), respectively. The coparison has been carried out with JCPDS files 840208 and 80-0006, respectively). STM Topography. We have recorded STM topography of CdS and Cu2S nanorods and also of the CdS|Cu2S junctions (Figure 3). Some of the junctions included Cu2S|CdS|Cu2S nanorods where Cu2S formed from both ends of CdS. While the topographies of CdS and Cu2S were uniform, the ones for the junctions exhibited a difference in topographies in the CdS and Cu2S sections of nanorod junctions. In imaging the junctions in a constant-current mode, the Cu2S section(s) Figure 2. (a) A TEM image of CdS nanorods, (b) that of Cu2S nanorods, (c) a dark-field TEM image of Cu2S|CdS and Cu2S|CdS|Cu2S junctions, and (d) HR-TEM image of a Cu2S|CdS junction showing interlayer spacing of different sections. 2001 dx.doi.org/10.1021/nl500081m | Nano Lett. 2014, 14, 2000−2005 Nano Letters Letter the peaks at negative voltages denote the valence band at which electrons can be withdrawn from the nanorods. The energies shown in the DOS spectra are with respect to the Fermi energy of the material. Tunneling Current and Density of States (DOS). The I−V characteristics at five different points on CdS and Cu2S nanorods are shown in Figures 4 and 5, respectively. STM Figure 5. (a) STM topography of a Cu2S nanorod showing the spots at which (b) tunneling current versus voltage characteristics were recorded. (c) DOS spectra of the I−V characteristics at the spots on the nanorod. The broken lines indicate the location of conduction and valence band-edges. tip was very close to the nanorods, the other tunnel-barrier of the DBTJ started to influence the results. With the positive and negative voltages at which peaks appeared in DOS spectra denoting the location of conduction and valence bands, respectively, we find that the conduction band in CdS is located closer to the Fermi energy that is fixed at 0 V. Similarly, for the Cu2S case the valence band is closer to its Fermi energy. The DOS spectra are hence in agreement with the n- and p-type nature of CdS and Cu2S nanorods, respectively. The position of the band-edges and also the intensity of DOS spectrum at different points on a nanorod did not vary when we scanned across a nanorod. The bandgap of CdS and Cu2S turned out to be 2.8 and 1.3 eV, respectively, that match reasonably well with the literature. The actual bandgap would be a little lower if the voltage drop at the nanorod−substrate junction of DBTJ was considered. From the DOS spectra of CdS and Cu2S nanorods, we find that the bandgap of Cu2S was clearly lower than that of CdS. The materials moreover would form a type-II band-alignment if a junction is formed between them. CdS|Cu2S Single Junction in a Nanorod. We then proceeded to characterize CdS|Cu2S nanorod junctions. To identify a nanorod with a single junction instead of Cu2S|CdS| Cu2S junctions, we relied on STM topographies, a typical of which is shown in Figure 6. We recorded I−V characteristics on such a single junction nanorod at many different points. We Figure 4. (a) STM topography of a CdS nanorod showing the spots at which (b) tunneling current versus voltage characteristics were recorded. (c) DOS spectra of the I−V characteristics at the spots on the nanorod. The broken lines indicate the location of conduction and valence band-edges. topography of the two nanorods with spots at which I−V characteristics were recorded is shown in the respective figures. The asymmetry in the I−V characteristics have arisen due to the difference in the work-function of the two electrodes. For CdS nanorods, the characteristics yielded a higher current in the positive voltage region at which electrons are injected to the conduction band of the n-type material, as compared to the current in the negative voltage section. Similarly, for the p-type Cu2S, the current remained higher at the negative voltage region due to facile hole injection to (electron extraction from) the valence band. The I−V characteristics expectedly did not vary from a point to another, because a single material was being characterized across the nanorod. The DOS spectra of the nanorods evidenced the location of conduction and valence band-edges in both the cases. The position of the band-edges did not depend on the set-points used for the approach of the tip (Figures S1a,b in the Supporting Information). When the 2002 dx.doi.org/10.1021/nl500081m | Nano Lett. 2014, 14, 2000−2005 Nano Letters Letter Figure 7. Mapping of a CdS|Cu2S nanorod junction for conduction and valence band-edges along the length of the nanorod. The figure in addition shows the STM topography of the nanorod junction and the spots at which tunneling current was measured. Conduction and valence band-edges (CB and VB, respectively) along with the Fermi energy (EF) across the junction and the depletion region of the pnjunction are also shown in the figure. nanoscale. The results enabled us to visualize the bandpositions along the interface between the CdS and Cu2S sections in a junction. The figure shows that in a 70-odd nanometer nanorod junction, the depletion region extends to around 42 nm. Schematic Band-Diagram of a pn-Junction in a Nanorod. Finally, we present schematic band-diagrams of Cu2S and CdS before and after formation of a pn-junction (Figure 8). While the “before-contact” diagram was based on Figure 6. (a) STM topography of a CdS|Cu2S nanorod junction showing the spots at which (b) tunneling current versus voltage characteristics were recorded. (c) DOS spectra of the I−V characteristics at the spots on the nanorod. The broken lines indicate the location of conduction and valence band-edges. focused at the interface between CdS and Cu2S sections of the nanorod by characterizing points at shorter interval of distance. Some such I−V characteristics are shown in Figure 6. The corresponding DOS are also shown in the figure. We looked at the characteristics and the DOS spectra for conduction and valence band-edges. We noted that while the ends of the nanorods resembled the individual materials, the interface brings out interesting features. In Figure 7, we have plotted the band-edges across the whole nanorod junction. The points at which measurements were carried out have also been marked in the STM topography of the CdS|Cu2S nanorod junction. On each point on the nanorod, we have recorded at least 50 I−V characteristics in determining the conduction and valence bandedges from the DOS spectra. The 50 measurements at each point have been summed up as error bars in energy values in Figure 7. The figure in essence is a mapping of the conduction and valence band-edges across a single pn-junction in a nanorod. The band-edges at the terminals resembled that of the individual nanorods, that is, CdS and Cu2S, as shown in Figures 4 and 5, respectively. The band-edges at the interface bring out the band-bending along with the depletion region of a pnjunction diode that formed in a single nanorod having a length of about 70 nm. In other words, we could map the conduction and valence bands along a single pn-junction formed in a Figure 8. Schematic band-diagrams of Cu2S and CdS nanorods before and after formation of a pn-junction. DOS of individual nanorods (Figures 5 and 6), the banddiagram of “after-contact” pn-junction was plotted from the band-mapping along the nanorod-junction. A type-II bandalignment of the materials along with the p- and the n-nature of Cu2S and CdS nanorods, respectively, were instrumental in forming a pn-junction in a nanorod. The position of band-edges in individual nanorods falls in the range of reported results for lower-dimensional structures of the respective materials. The band-edges at the extreme ends of the junction also matched that of individual components. The energy levels at the interface of the pn-junction matched well with a conventional diode junctions having a band-bending of around 0.3−0.4 eV. The results hence show the elegance of STM in determining 2003 dx.doi.org/10.1021/nl500081m | Nano Lett. 2014, 14, 2000−2005 Nano Letters Letter 1 mL) was added dropwise to the flask through a syringe, which contained 1.13 mg of CdS nanorods in toluene under a vigorous stirring condition at room temperature. The process of exchange of cadmium by copper ions from both ends of the CdS nanorods leading to the formation of Cu2S|CdS|Cu2S nanorods is manifested by an immediate change in color of the reaction bath from yellow to golden brown. The conversion process could further be slowed down for partial exchange of cadmium from one of the ends of CdS nanorods by a 5-fold dilution of the copper salt. In such a case, CdS|Cu2S junctions formed in the nanorods. The nanorods were washed with an addition of methanol followed by centrifugation and removal of the supernatant. To form Cu2S nanorod, we added a required amount of [Cu(CH3CN)4]PF6 so that all the cadmium atoms were replaced by copper. In practice, a methanol solution of the copper salt (4.8 mg in 0.7 mL) was added dropwise to a dispersed solution of CdS nanorods in toluene (0.113 mg in 2.0 mL). Characterization of CdS and Cu2S Nanorods and CdS| Cu2S Junctions. Optical absorption spectroscopy and TEM and HR-TEM images of CdS and Cu2S nanorods and CdS|Cu2S junctions in nanorods were recorded. Bright-field TEM images were obtained using a JEOL transmission electron microscope that was operated at 200 kV. For TEM measurements, a drop of nanorod solution was placed onto a carbon-coated copper grid in an ambient condition. Statistics for the length and diameter of the initial CdS nanorods and fully converted Cu2S nanorods were gathered from bright-field TEM images using Gatan software. At least 20 measurements were made for each sample. To characterize individual nanorods and CdS|Cu2S junctions through an ultrahigh vacuum STM (PAN style UHV-STM of M/s RHK Technology), ultradilute solution of the nanorods was drop-casted on highly ordered pyrolytic graphite (HOPG). While the pressure of the microscope was 1.2 × 10−10 Torr, the temperature of the substrate and the Pt/Ir tip both was 77 K. During approach of the tip, a current of 0.4 nA was targeted to achieve at 2.0 V. For smoothing of STM topographies, WSxM software was used. As per usual procedure,31 we used a low tipapproaching current to ensure that the bias was applied mainly on the tip-nanorod junction. and more importantly mapping the band-edges across a pnjunction in a single nanorod. In conclusion, we have grown a CdS|Cu2S junction in a nanorod through a cationic exchange process. Formation of such a junction in a single nanorod has been substantiated by TEM, HR-TEM, dark-field TEM images, and STM topographies. With UHV-STM, we have mapped the nanorodjunction and also individual nanorods across their length by recording tunneling current at different points on the nanorods at 77 K. From the DOS spectra, we could locate the conduction and valence band-edges across the Cu2S and the CdS nanorods and also along the pn-junction. The band-edges at the terminals of a pn-junction resembled that of CdS and of Cu2S nanorods, respectively. The DOS spectra across the pn-junction yielded a band-bending along with the depletion region in the junction. The results showed a unique method to map the conduction and valence band-edges along a single pn-junction nanorod based on CdS and Cu2S. Experimental Section. Growth of CdS Nanorods. CdS nanorods were grown following a colloidal synthetic method using a standard Schlenk line technique26 in an inert condition. In brief, CdO and measured amounts of ODPA (octadecylphosphonic acid), TDPA (tetradecylphosphonic acid), and TOPO (trioctylphosphine oxide) were added in a 25 mL threeneck flask. The flask was purged with nitrogen and heated to 120 °C that melted the components to remove water from the materials. The mixed content was heated to 320 °C for 20−30 min to enable the complexation of cadmium ions. The process of heating to 320 °C was repeated once more to complete the complexation process. At this stage, 2 g of TOP (trioctylphosphine) was injected into the flask that acted as the stabilizer. After stirring the liquid at 320 °C for 5 min, TOPS (trioctylphosphine sulfide), as a source of sulfur, was injected to the flask. Reaction was allowed to continue for 60 min to form CdS nanorods. Reaction was stopped by reducing the temperature of the flask down to room temperature. Washing of the nanorods was carried out by adding three solvents in sequence, namely toluene, toluene/octylamine (1:1 v/v), and toluene/acetone (1:1 v/v), followed by centrifugation after each step. Washing in toluene/acetone more importantly allowed separation of bipods, tripods, and tetrapods from the nanorods. Here, after centrifugation, the branched structures remained in the supernatant. Formation of a CdS|Cu2S Junction in Nanorods through a Cationic Exchange Process. We formed a junction between CdS and Cu2S in the nanorod following a reported route.26 Here copper-ions replaced the cadmium ions from one or both ends of the nanorods. To do so, tetrakis(acetonitrile)copper(I) hexafluorophosphate, [Cu(CH3CN)4]PF6 acted as the source of copper. Such a salt was used in the cationic exchange process due to a slow reactivity of copper in the salt so that the exchange of cadmium by copper ions remained an extremely slow process. Moreover, the replacement of cadmium occurred only at the end(s) of the nanorod since the surfactants are known to be weekly bound to the edges of a nanorod.33 In practice, while CdS nanorods were dispersed in toluene, [Cu(CH3CN)4]PF6 was dissolved in methanol. Concentration of copper ions was chosen to match the molar concentration of cadmium ions to be replaced in the nanorods. The concentration of cadmium ions in CdS nanorods was determined from inductively coupled plasma atomic emission spectroscopy (ICP-AES) Because toluene and methanol are miscible, a methanol solution of [Cu(CH3CN)4]PF6 (4.8 mg in ■ ASSOCIATED CONTENT S Supporting Information * DOS spectra calculated from I−V characteristics of CdS nanorod and Cu2S nanorod recorded at different set-points with a set-bias of 2.0 V. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +91-33-24734971. Fax: +9133-24732805. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors acknowledge financial support through Nano Mission projects. A.B. and S.D. acknowledge CSIR Fellowship Nos. 09/080(0779)/2011-EMR-I (Roll No. 510847) and 9/ 080(0647)/2009-EMR-I (Roll No. 507031), respectively. 2004 dx.doi.org/10.1021/nl500081m | Nano Lett. 2014, 14, 2000−2005 Nano Letters ■ Letter (23) Chien, T. Y.; Kourkoutis, L. F.; Chakhalian, J.; Gray, B.; Kareev, M.; Guisinger, N. P.; Muller, D. A.; Freeland, J. W. Visualizing shortrange charge transfer at the interfaces between ferromagnetic and superconducting oxides. Nat. Commun. 2013, 4, 2336. (24) Shih, M. C.; Huang, B. C.; Lin, C. C.; Li, S. S.; Chen, H. A.; Chiu, Y. P.; Chen, C. W. Atomic-scale interfacial band mapping across vertically phased-separated polymer/fullerene hybrid solar cells. Nano Lett. 2013, 13, 2387−2392. (25) Grandidier, B.; Niquet, Y. M.; Legrand, B.; Nys, J. P.; Priester, C.; Stievenard, D.; Gerard, J. M.; Thierry-Mieg, V. Imaging the wavefunction amplitudes in cleaved semiconductor quantum boxes. Phys. Rev. Lett. 2000, 85, 1068−1071. (26) Sadtler, B.; Demchenko, D. O.; Zheng, H.; Hughes, S. M.; Merkle, M. G.; Dahmen, U.; Wang, L. W.; Alivisatos, A. P. Selective facet reactivity during cation exchange in cadmium sulfide nanorods. J. Am. Chem. Soc. 2009, 131, 5285−5293. (27) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized surface plasmon resonances arising from free carriers in doped quantum dots. Nat. Mater. 2011, 10, 361−366. (28) Saunders, A. E.; Ghezelbash, A.; Sood, P.; Korgel, B. A. Synthesis of high aspect ratio quantum-size CdS nanorods and their surface-dependent photoluminescence. Langmuir 2008, 24, 9043− 9049. (29) Macdonald, J. E.; Bar Sadan, M.; Houben, L.; Popov, I.; Banin, U. Hybrid nanoscale inorganic cages. Nat. Mater. 2010, 9, 810−815. (30) Einevoll, G. T. Confinement of excitons in quantum dots. Phys. Rev. B 1992, 45, 3410−3417. (31) Bekenstein, Y.; Vinokurov, K.; Banin, U.; Millo, O. Electronic properties of hybrid Cu2S/Ru semiconductor/metallic-cage nanoparticles. Nanotechnology 2012, 23, 505710. (32) Banin, U.; Millo, O. Tunneling and optical spectroscopy of semiconductor nanocrystals. Annu. Rev. Phys. Chem. 2003, 54, 465− 492. (33) Park, J.; Koo, B.; Yoon, K. Y.; Hwang, Y.; Kang, M.; Park, J. G.; Hyeon, T. Generalized synthesis of metal phosphide nanorods via thermal decomposition of continuously delivered metal-phosphine complexes using a syringe pump. J. Am. Chem. Soc. 2005, 127, 8433− 8440. REFERENCES (1) Yoffe, A. D. Low-dimensional systems: Quantum-size effects and electronic-properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems. Adv. Phys. 1993, 42, 173−266. (2) Banin, U.; Cerullo, G.; Guzelian, A. A.; Alivisatos, A. P.; Shank, C. V. Quantum confinement and ultrafast dephasing dynamics in InP nanocrystals. Phys. Rev. B 1997, 55, 7059−7067. (3) Law, M.; Goldberger, J.; Yang, P. D. Semiconductor nanowires and nanotubes. Annu. Rev. Mater. Res. 2004, 34, 83−122. (4) Mocatta, D.; Cohen, G.; Schattner, J.; Millo, O.; Rabani, E.; Banin, U. Heavily doped semiconductor nanocrystal quantum dots. Science 2011, 332, 77−81. (5) Regulacio, M. D.; Han, M. Y. Composition-tunable alloyed semiconductor nanocrystals. Acc. Chem. Res. 2010, 43, 621−630. (6) Wei, S. H.; Zunger, A. Band offsets at the CdS/CuInSe2 heterojunction. Appl. Phys. Lett. 1993, 63, 2549−2551. (7) Tak, Y.; Kim, H.; Lee, D.; Yong, K. Type-II CdS nanoparticleZnO nanowire heterostructure arrays fabricated by a solution process: enhanced photocatalytic activity. Chem. Commun. 2008, 4585−4587. (8) Borys, N. J.; Walter, M. J.; Huang, J.; Talapin, D. V.; Lupton, J. M. The role of particle morphology in interfacial energy transfer in CdSe/CdS heterostructure nanocrystals. Science 2010, 330, 1371− 1374. (9) Teranishi, T.; Sakamoto, M. Charge separation in type-II semiconductor heterodimers. J. Phys. Chem. Lett. 2013, 4, 2867−2873. (10) Baranov, A. N.; Bertru, N.; Cuminal, Y.; Boissier, G.; Alibert, C.; Joullie, A. Observation of room-temperature laser emission from type III InAs/GaSb multiple quantum well structures. Appl. Phys. Lett. 1997, 71, 735−737. (11) Kim, S.; Fisher, B.; Eisler, H. Y.; Bawendi, M. G. Novel type-II quantum dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures. Abstr. Pap. Am. Chem. Soc. 2002, 224, U443−U443. (12) Balet, L. P.; Ivanov, S. A.; Piryatinski, A.; Achermann, M.; Klimov, V. I. Inverted core/shell nanocrystals continuously tunable between type-I and type-II localization regimes. Nano Lett. 2004, 4, 1485−1488. (13) Zhu, H. M.; Song, N. H.; Lian, T. Q. Wave function engineering for ultrafast charge separation and slow charge recombination in type II core/shell quantum dots. J. Am. Chem. Soc. 2011, 133, 8762−8771. (14) Dey, S.; Pal, A. J. Tuning of band-edges in type-I core-shell nanocrystals through band-offset engineering: Selective quantum confinement effect. RSC Adv. 2013, 3, 13225−13231. (15) Dorfs, D.; Salant, A.; Popov, I.; Banin, U. ZnSe quantum dots within CdS nanorods: A seeded-growth type-II system. Small 2008, 4, 1319−1323. (16) Pradhan, B.; Batabyal, S. K.; Pal, A. J. Rectifying junction in a single ZnO vertical nanowire. Appl. Phys. Lett. 2006, 89, 233109. (17) Rivest, J. B.; Swisher, S. L.; Fong, L. K.; Zheng, H. M.; Alivisatos, A. P. Assembled monolayer nanorod heterojunctions. ACS Nano 2011, 5, 3811−3816. (18) Millo, O.; Katz, D.; Cao, Y. W.; Banin, U. Imaging and spectroscopy of artificial-atom states in core/shell nanocrystal quantum dots. Phys. Rev. Lett. 2001, 86, 5751−5754. (19) Katz, D.; Wizansky, T.; Millo, O.; Rothenberg, E.; Mokari, T.; Banin, U. Size-dependent tunneling and optical spectroscopy of CdSe quantum rods. Phys. Rev. Lett. 2002, 89, 086801. (20) Wang, L.; Chen, Q.; Pan, G. B.; Wan, L. J.; Zhang, S. M.; Zhan, X. W.; Northrop, B. H.; Stang, P. J. Nanopatterning of donor/acceptor hybrid supramolecular architectures on highly oriented pyrolytic graphite: A scanning tunneling microscopy study. J. Am. Chem. Soc. 2008, 130, 13433−13441. (21) Steiner, D.; Dorfs, D.; Banin, U.; Della Sala, F.; Manna, L.; Millo, O. Determination of band offsets in heterostructured colloidal nanorods using scanning tunneling spectroscopy. Nano Lett. 2008, 8, 2954−2958. (22) Das, B. C.; Pal, A. J. To change transport gap of semiconducting nanoparticles without disturbing the optical one: Core-shell approach. Appl. Phys. Lett. 2009, 94, 082106. 2005 dx.doi.org/10.1021/nl500081m | Nano Lett. 2014, 14, 2000−2005