Determination of Band Offsets in Heterostructured Colloidal Nanorods

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NANO LETTERS

Determination of Band Offsets in Heterostructured Colloidal Nanorods Using Scanning Tunneling Spectroscopy

2008 Vol. 8, No. 9 2954-2958

Dov Steiner,†,‡ Dirk Dorfs,‡,§ Uri Banin,‡,§ Fabio Della Sala,| Liberato Manna,| and Oded Millo*,†,‡ Racah Institute of Physics, the Center for Nanoscience and Nanotechnology, and Institute of Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel, and NNL-National Nanotechnology Laboratory of CNR-INFM, Via per Arnesano km 5, 73100 Lecce, Italy Received June 26, 2008; Revised Manuscript Received July 14, 2008

ABSTRACT The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine bandoffsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.

Colloidal core/shell semiconductor nanocrystals (NCs) have attracted considerable interest in the past few years due to their unique and intricate optical and electrical properties, which depend on the specific material, size and shape of each of their components.1-6 By tailoring the potential landscape through material choice, the wave functions of the electron and hole ground states can each either be confined to the core or extend out to the shell, affecting the degree of overlap and consequently the band gap photoluminescence (PL) energy, quantum yield, lifetime and stability. For example, core/shells with type I band-offsets exhibit bright and stable fluorescence that is widely applied in biological tagging7,8 and light emitting diodes,9,10 as both electron and hole can be confined to the core. Multiexciton properties, important in optical gain applications, are also controlled by the band offsets where type II alignment, leading to charge separation, may prove advantageous.4,5,11 It is therefore obvious that knowledge of band-offsets in colloidal nanostructures is necessary for predictive heterostructure design. In lack of * Corresponding author. E-mail: [email protected]. † Racah Institute of Physics, The Hebrew University of Jerusalem. ‡ The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem. § Institute of Chemistry, The Hebrew University of Jerusalem. | NNL-National Nanotechnology Laboratory of CNR-INFM. 10.1021/nl801848x CCC: $40.75 Published on Web 08/09/2008

 2008 American Chemical Society

direct experimental measurements, a common approach taken in this context so far has been to assume band-offset values of the bulk materials or of epitaxial superlattices. Obviously, in nanoscale colloidal heterostructures, the band offsets can differ considerably. While optical measurements provide energies of allowed transitions between the bands, scanning tunneling spectroscopy (STS) provides information on the conduction and valence states separately.12,13 This, along with the capability to map the local density of states (DOS) with nanometer resolution in composite NCs,14 is used here to extract level offsets between different components of colloidal heterostructures. Along with theoretical modeling, these data allow the determination of the band offsets. We applied this approach to type I CdSe/CdS and type II ZnSe/CdS quantum-dot/nanorod (QD/NR) core/shell NCs synthesized via a seeded growth method.15-18 These NCs exhibit intriguing properties, such as pronounced linearly polarized photoluminescence (PL) that can be tuned by external electric field.15,16,19-21 The experiments performed so far on the QD/NR core/shell NCs were based on optical methods, from which conclusions on the level structure, band offsets and charge separation were deduced. In particular, such optical data provided a possible signature for charge separation in CdSe/CdS QD/NR core/shell NCs, where the hole is located in the CdSe core (positioned close to one

Figure 1. Illustration of the (most common) previously predicted band structure and the electron and hole ground-state wave functions of type I CdSe/CdS (a) and type II ZnSe/CdS (b) QD/NR core/ shell heterostructures. TEM images of the CdSe/CdS (c) and ZnSe/ CdS (d) core/shell nanorods. Scale bars are 50 nm. PL (dashed blue curves) and absorption (black curves) spectra, measured at room temperature on ensembles of CdSe/CdS (e) and ZnSe/CdS (f) NCs dissolved in toluene. Insets show a magnification of the absorption in the band gap regions.

end of the NR), while the electron extends over the CdS shell19-21 (see illustration in Figure 1a). The separation between electrons and holes is expected to be even stronger in type II QD/NR core/shell structures, similar to the case of their spherically shaped counterparts, as was recently shown for CdS/ZnSe QDs via optical measurements.5 However, no direct measurement of the band offsets in such NCs was reported so far. In particular, the spatially resolved electronic level structure within a single QD/NR heterostructure was not yet probed using STS. Our STS data presented here portray spatial variations of the bands’ gaps and edges along single NCs, reflecting their local compositional structure and thus providing further insight into their electronic and optical properties, which could not be deduced from optical data alone. In particular, the electron groundstate of the CdSe/CdS NCs is shown to be localized in the core, in contrast to previous reports, and the type II gap of the ZnSe NC is directly measured. The band-offset of different II-VI semiconductor heterostructures were previously calculated using various theoretical methods and compared with optical data.1,22-26 While the reported values for the CdSe/CdS valence-band offset (∆v) are always positive, the CdSe/CdS conduction band-offset (∆c) vary considerably between different reports, from -0.25 eV25 (i.e., a type II band alignment) up to +0.27 eV23,24 (a type I band alignment, as shown in Figure 1a). In the latter case the smaller CdSe energy gap (Eg(CdSe) ) 1.84 eV at 4 K27) resides inside the larger CdS energy gap ( Eg(CdS) ) 2.58 eV at 4 K27), with ∆v larger than ∆c. Note, in particular, Nano Lett., Vol. 8, No. 9, 2008

that in previous reports on QD/NR core/shell heterostructures a relatively small ∆c has been estimated.17,19 Consequently, the ground-state wave function of the hole was predicted to be localized inside the CdSe core, whereas that of the (much lighter) electron is commonly assumed to extend along the entire rod. The ZnSe/CdS system exhibits, on the other hand, a well-established type II electronic structure with a “type II energy-gap” (between the CdS conduction-band edge and the ZnSe valance-band edge) predicted to be ∼1.8 eV,25,26 much smaller than the energy gaps of both CdS and ZnSe (Eg(ZnSe) ∼2.82 eV at 4.2 K27) (see Figure 1b). CdSe/CdS NCs consisting of a CdSe spherical core of ∼4 nm diameter embedded inside an elongated CdS shell, ∼6 nm in diameter and ∼40 nm long, were prepared as described in ref 16. The recently developed type II ZnSe/CdS QD/NR core/shell NCs (60 nm long, 6 nm wide and 4 nm core diameter) were synthesized following a similar approach, as reported in ref 18. Transmission electron microscopy (TEM) images of the CdSe/CdS and ZnSe/CdS core/shell NRs are shown in Figures 1c and 1d, respectively. It is important to note here that high resolution TEM data show that the QD cores are located close to one apex of the NR shell (usually at about 1/3 to 1/4 of its length),15,16,18 consistent with ref 19. In Figure 1e we present the absorption and PL spectra measured on a dilute ensemble of CdSe/CdS QDs/NRs dissolved in toluene. The significant rise in the absorption spectrum at about 2.6 eV is attributed to the band gap of the CdS shell. The absorption onset at 2.09 eV, is close to the PL peak (2.05 eV), with a very small Stokes shift,16 and is ascribed to the band gap of the system related with the presence of the CdSe core. Turning now to the optical spectra measured on the ZnSe/CdS NCs (see Figure 1f), the absorption peak at ∼2.6-2.7 eV is attributed to the band gap of the CdS shell, and the PL peak at ∼2.1 eV can be associated with the type II gap of the NC. From the optical absorption and PL spectra alone it is not possible, however, to directly determine the band-offsets or the electron localization. Theoretical calculations (see Supporting Information) show that the lowest excitation energy of CdSe/CdS QD/NR systems changes by only 50 meV when ∆c is varied from 0 to 0.3 eV, due to a balance between quantization energies and electron-hole Coulomb interaction. Such small shifts cannot be detected due to sizedistribution broadening. Intraband energies are also insensitive to band-offset.1 The most elaborate analysis of electron localization so far was provided by the elegant optical measurements under electric field where lifetime measurements were performed and compared with calculated e-h overlaps for different aspect ratios.19 However the exciton lifetime is not directly related to the band-offsets. In contrast, direct information on the band-offsets and degree of electron localization can be obtained from STS measurements, as shown in the following. For STM measurements, the NCs were deposited on a flat Au(111) film by letting a drop of toluene-NCs solution slowly dry on the substrate. The samples were mounted inside a homemade cryogenic STM that was cooled down 2955

Figure 2. (a) Three dI/dV vs V tunneling spectra, acquired at 4.2 K on a single CdSe/CdS NC at different locations (black curves). The corresponding theoretical curves are shown above the measured spectra (red curves). The dashed lines are guides to the eye, in order to highlight the energy-gap variations. The spectra are shifted vertically for clarity. (b) 3D topographic STM image of the measured rod. The locations where the tunneling spectra were measured are indicated by numbers. (c) Cross-sections of a current image taken at a voltage of 1.2 V (upper green curve) and of the calculated electron ground-state probability density (blue curve, arbitrary units), manifesting the localization of the electron ground state in the CdSe core.

to 4.2 K for data acquisition. Spatially resolved tunneling spectra (dI/dV vs V characteristics, proportional to the local DOS), were measured by first positioning the STM (Pt/Ir) tip above a single core/shell NC, thus forming a double barrier tunnel junction. Care was taken to retract the STM tip as much as possible from the NC in order to minimize the effect of voltage division between the junctions, which tends to broaden the measured energy-gap and level spacings.12,13 Tunneling spectra were then taken on different regions of the particle by moving the tip along the rod. Theoretical tunneling spectra at different positions along the rod (z-direction) were computed as [dI(eV;z)]/dV ) ∑i|Mi(z)|2 g(ηeV - εi + µf) where Mi(z) are the tunneling matrix elements related to the overlap between the tip-wave function and the electron (hole) wave functions with eigenvalues εi as obtained from effective-mass calculations (see Supporting Information), g is a Gaussian broadening function and µf is the Fermi level. In the calculations we fixed the voltage division factor12,13 (η ) 0.95) and modified ∆c to fit the experimental data. This procedure thereby provides the band-offsets in a self-consistent manner. Representative dI/dV spectra acquired at 4.2 K on a single CdSe/CdS core/shell rod are presented in Figure 2a. The spectra were measured at different locations along the rod, as marked in Figure 2b, and the corresponding theoretical curves are shown above the experimental spectra (red curves). Curve 1 was measured near one apex of the NR, and curve 2 was taken on the other side of the NR, but closer to the middle. Both curves exhibit a single-particle band gap (between the electron and hole ground states) of about 2.9 eV. The band gap measured along the CdS NR, far enough from the CdSe core, was nearly constant, and varied sporadically between 2.8 and 2.9 eV. These small nonsystematic variations can be accounted for by local changes in the voltage division between the junctions.12,13 This measured gap is in very good agreement with the computed one (2.85 eV), and it is larger than the bulk CdS gap due to quantization 2956

effects in the CdS rod. These values are also consistent with the measured optical gap of the CdS shell (Figure 1e), taking into account the red shift of the optical transition at room temperature compared with low temperature and the electron-hole Coulomb interaction. In addition, the tunneling gaps are enlarged due to the voltage division effect. Curve 3, however, acquired about 15 nm from the right edge of the NC, exhibits a significantly smaller band gap (∼2.3 eV), a value consistent with (slightly larger than, due to the aforementioned factors) the measured optical band gap of the system. At this position, the first tunneling peak on the negative bias side, corresponding to the valence band ground state, is red-shifted with respect to curves 1 and 2, consistent with the expected large ∆v between the CdSe core and CdS shell. It is thus reasonable to assume that spectrum 3 was measured above the CdSe core. This position is also consistent with the location of the core deduced previously from TEM data,16 and the STM topographic image indeed portrays there a slight broadening of the NR diameter, as frequently observed in the TEM images. The doublet appearing at the onset of tunneling at the positive bias side is assigned to the ground state of the electron within the CdSe core, which in a spherical particle corresponds to a doubly (spin) degenerate S state.6 Surprisingly, a red shift with respect to the CdS is seen also for the ground state of the electron. The reduction of the band gap in the CdSe core with respect to the CdS shell was predicted to take place asymmetrically, with a very small shift in the CB edge.22-24 This, in turn, was assumed to yield delocalization of the electron ground state all over the NR. Such a scenario was suggested in ref 19, based on optical measurements. In contrast, the above experimental STS results depict a different picture for our NCs. As can be seen in Figure 2a, both the electron ground state and the hole ground state are red-shifted when measured at the core position compared to the spectra taken on the “pure” CdS NR regions. The red shift of the Nano Lett., Vol. 8, No. 9, 2008

electron ground state is, however, somewhat smaller compared to that of the hole. This behavior could be revealed directly here due to the local nature of the STS technique and its ability to detect directly the single-particle level structure. The optical spectra, in contrast, portray the energy level separation and can reflect only indirectly the electron and hole spatial distribution along the NC. Moreover, the measurement, along with the theoretical model, allows the extraction of the band-offsets. The theoretical curves, which reproduce very well the measured band-gaps and the level offsets, have been obtained using ∆c ) 0.30 eV (∆v ) 0.44 eV). This conduction-band offset is large enough to allow localization of the electronic ground state in the CdSe seed. Further experimental evidence for the above electron localization scenario is provided by current imaging tunneling spectroscopy measurements, which yield information on the shape and extent of the electronic wave functions, as detailed in ref 6. This is shown in Figure 2c, where a cross-section of a current image acquired at a bias of 1.2 V is presented. This voltage corresponds to the energetic position of the conduction-band (electron) ground state, as depicted by the upper curve in Figure 2a. The peak in this current crosssection curve, located about 15 nm from one end of the NR, clearly manifests the localized electron ground-state wave function, consistent with (although obviously broader than) the calculated wave function shown also in the figure. We note here that such a pronounced peak did not appear in current images taken at bias voltages larger than 1.5 V, indicating that the higher conduction-band excited states extend over the whole particle. The current images measured at negative sample bias were too noisy to enable arriving at any conclusion regarding the valence-band wave functions. As manifested by Figure 2a, the electron and hole groundstate energies deduced from the simulated spectra are very close to the measured values. However, the higher energy peaks in the STS spectra do not conform to the calculated excited states. Computation of the STS theoretical spectra at higher energies is a challenging task (see Supporting Information), due to the intricate level structure, on one hand, and various factors complicating the experimental spectra, such as charging effects and intrinsic peak-broadening,28 on the other. It is important to note, however, that we focus here on the issue of measuring and obtaining the band-offsets between the two materials. This information is deduced from the lowest lying states and not affected by these issues. The findings concerning the conduction-band band-offsets and the resultant electron localization are in agreement with a previous report on CdSe/CdS core-shell quantum dot.23,24 However, as mentioned above, they differ from the picture given in ref 19 where a smaller conduction-band offset and electron delocalization were concluded from the sound analysis of elegant time-resolved photoluminescence data measured on the seeded rod structure. Nonetheless, lifetime measurements are also affected by additional factors such as the exciton binding energy, its fine-structure and coupling with acoustic phonons, unlike the direct STS approach used here. Nano Lett., Vol. 8, No. 9, 2008

Figure 3. Three experimental spectra (black lines), measured on the NC shown in the STM image presented in the inset. The positions where the spectra were acquired are marked near the rod. The corresponding calculated spectra are shown above the measured curves (red lines).

To demonstrate the generality of our approach, we next apply it to the type II ZnSe/CdS QD/NR core/shell system. Figure 3 shows that the band gap values observed along most of the NR were around 2.7-2.9 eV (e.g., spectra 1 and 2), corresponding to the CdS gap (enlarged with respect to the bulk value due to quantum confinement and voltage division effects), and are in good agreement with the calculated gaps (2.7 eV), and the optical absorption spectrum. However, smaller gaps, of around 2 eV, were measured in position 3, at a distance of about 1/4 rod length from the edge, where the rod is locally broadened. This is indeed the position where the ZnSe core (encapsulated by the CdS shell) is expected to be located.18 Interestingly, this value does not conform to the band gap of ZnSe, but rather corresponds to the type II band gap of this system as measured optically. Namely, in that region the electron first tunnels into the conduction-band edge of the CdS shell (first peak at positive bias of spectrum 3) and the hole first tunnels to the valenceband edge of the ZnSe core (first peak at negative bias). This scenario is supported by the calculated STS curve in this point which reproduce the experimental profile taking the band-offsets ∆c ) -1.02 eV and ∆v ) 0.78 eV. Notably, as above, the experimental STS data, along with an optimized calculation, allowed extraction of the band-offsets. The STS results provide direct information on the internal electronic structure of the complex QD/NR heterstructures studied, thus yielding further insight into this system beyond that obtained from previous optical measurements and allowing, along with theoretical modeling, the extraction of the band-offsets. The spatially resolved tunneling spectroscopy measurements of type I CdSe/CdS and type II ZnSe/ CdS quantum-dot/nanorod core/shell nanocrystals reveal the evolution of the energy-gap along these particles and directly portray the band-offset scheme in these heterostructured NCs. Specifically, the tunneling spectra indicate that both electron and hole ground states reside in the CdSe core, and they also reveal the type II band gap of the ZnSe/CdS nanocrystal. The extracted band-offset obtained from the direct measurements of level offsets provides useful data for engineering the potential landscape and hence the optical and electronic 2957

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NL801848X

Nano Lett., Vol. 8, No. 9, 2008