Transport and Charging in Single Semiconductor Nanocrystals

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NANO LETTERS

Transport and Charging in Single Semiconductor Nanocrystals Studied by Conductance Atomic Force Microscopy

2004 Vol. 4, No. 1 103-108

Eyal Nahum,†,§ Yuval Ebenstein,†,§ Assaf Aharoni,†,§ Taleb Mokari,†,§ Uri Banin,*,†,§ Nira Shimoni,§ and Oded Millo*,‡,§ Departments of Physical Chemistry and Physics, and the Center for Nanoscience and Nanotechnology, The Hebrew UniVersity, Jerusalem 91904, Israel Received October 23, 2003; Revised Manuscript Received November 13, 2003

ABSTRACT Electrical transport measurements through single InAs and CdSe semiconductor nanocrystals embedded in a thin polymer film were performed using conductance atomic force microscopy. The current and topography images showed excellent correlation, where current was detected only over the nanocrystals. A rapid current decay in consecutive scans was observed for positive sample bias, while remaining intact at negative bias. This current decay was accompanied by bias-dependent changes in the height of the nanocrystals. These phenomena, which were not observed for gold nanocrystals, are attributed to long-sustained charging of the nanocrystals.

Semiconductor nanocrystals (NCs) exhibit optical and electronic properties that can be tailored via control of particle size, shape, and composition.1-4 Understanding charge transport through semiconductor nanocrystals is of importance from both scientific and technological standpoints. The transport of charge through nanocrystals is sensitive to both the discrete electronic level structure that arises from quantum confinement, and to single electron charging phenomena.5 Understanding both aspects is required for improved implementation of nanocrystals in electrooptical devices such as light emitting diodes and solar cells.6,7 The interplay between charging and the level structure was clearly observed in tunneling spectra performed on single NCs in the double barrier tunnel junction (DBTJ) configuration.5,8 In these experiments, the charged state is maintained only while acquiring the tunneling spectra, i.e., while passing current through the NC. The number of excess electrons (or holes) is determined by various factors, in particular the applied voltage and the tunneling rates in the two junctions.9,10 However, the effect of a long-lived metastable NC charged state in these experiments was not yet considered. Such long-lived charged states were reported to form upon exposure of the NC to above band-gap light and may affect electrical transport through the NCs, as well as their optical properties.11,12 The dark-off state observed in single nanocrystal fluorescence studies is also commonly assigned to a * Corresponding authors. E-mail: [email protected]; milode@ vms.huji.ac.il. † Department of Physical Chemistry. ‡ Racah Institute of Physics. § The Center for Nanoscience and Nanotechnology. 10.1021/nl034928b CCC: $27.50 Published on Web 12/03/2003

© 2004 American Chemical Society

charged state of the nanocrystals.13,14 The relationship between nanocrystal luminescence and charging was directly seen in the on-off behavior of a nanocrystal solid-state device where while charging the nanocrystal emission was turned off.15 Studies of long-sustained charging of semiconductor NCs, were performed using electrostatic force microscopy (EFM), which enables the determination of the number of trapped electrons or holes on a single NC.11,12,16 The effect of this metastable charged state on the electrical transport through these particles, an issue of both fundamental and applicative importance, has not yet been investigated. This latter issue cannot be investigated efficiently using scanning tunneling microscopy (STM) since, as shown below, such a charged state may block the tunneling current, which is the input feedback signal in STM. In this work we employed conductance atomic force microscopy (C-AFM) to investigate conductance properties of single NCs embedded in poly(methyl methacrylate) (PMMA). In C-AFM, a conductive tip scans the surface in the contact mode (constant force) while applying a bias voltage between the tip and the sample, and the tunneling current is measured simultaneously with the surface topography. In contrast to STM, the topographic information in C-AFM is (nearly) decoupled from the local electrical conductance properties, thus allowing a reliable simultaneous measurement of both properties.17 Therefore, C-AFM is most suitable for samples with low conductance or composite materials containing insulating regions where an STM would lose its feedback signal. The C-AFM was previously applied to image current flow through carbon nanotubes,18 and most

Figure 1. Two sequential C-AFM measurements of InAs NCs (∼6.4 nm in diameter) embedded in PMMA. The sample bias was VB ) 4 V. Panels a and b are the topography and current images of the first scan, while panels d and e are of the second. Image c was obtained by subtracting b from a, showing good correlation between the current and topography images and that current was detected for most of the NCs (10 of 11). The NCs are clearly detected in the second topographic scan, image d, but have vanished in the second current image, e. Image sizes are 520 × 520 nm2, and the full-color scale for the topographic images is 7.5 nm and for the current images -10 pA to 10 pA. The schematic of the sample and experimental setup is shown in image f (tip size exaggerated).

recently to study carbon black/polymer composites.19 Here, by following the time evolution of the current and topographic images and their dependence on the polarity of the sample bias voltage, we found that semiconductor NCs (InAs and CdSe, dots and rods), form a metastable charged state during the conduction process, whereas Au NCs in PMMA do not exhibit such a behavior. A variety of semiconductor nanocrystals used in this experiment were prepared via pyrolisis of suitable precursors in organometallic solvents as detailed elsewhere for each system studied.20-24 These nanocrystals are overcoated by a layer of organic ligands. The samples for the AFM experiments were prepared by first dissolving the NCs in toluene solution containing PMMA at concentrations of ∼2-3 g/L. An 80 µL drop of the solution was spun onto highly oriented pyrolytic graphite (HOPG) for 30 s at 6000 to 7200 rpm, to achieve ∼5 nm thick PMMA films that contain electrically and spatially isolated NCs (see schematic in Figure 1f). The PMMA also effectively anchors the NCs, thus enabling reliable measurements of these particles in the contact AFM mode without dragging them.25 The thickness of the obtained 104

PMMA films was first characterized using elipsometry on Si/SiO2 substrates, and then by AFM where a scratch was intentionally induced on the film. The C-AFM measurements were performed at ambient conditions using a Digital Instrument Dimension 3100 scanning probe microscope with a Nanoscope IV controller. TiN-coated tips, having typically 35 nm radius and cantilever spring constant of 0.03 N/m, were used in all the measurements. The force load (determined from deflection-distance curves) was kept minimal, a few nN, to avoid damaging the soft PMMA substrate. Due to drift it was not possible to obtain reliable current-voltage curves on single nanocrystals, and this was partially compensated for by measuring scans at different bias voltages as detailed below. In Figure 1 we present typical C-AFM measurements on InAs dots,23,24 6.4 nm in diameter, at positive sample bias, VB ) 4 V. The topographic and current images of the first scan are shown in Figure 1a and 1b, respectively, whereas images 1d and 1e (topography and current) were obtained in a subsequent scan of the same area, starting immediately after the first one. The time interval between successive monitoring of each point is thus determined by the image acquisition time, ∼10 min. In the first scan, the current values detected on most (more than 90%) of the NCs (10 to 60 pA) were well above that typically measured on the PMMA (the noise level, ∼0.5 pA), reflecting the thinner barrier associated with tunneling through the NCs (see Figure 1f). The current level varied from one NC to another, probably due mostly to variations in the PMMA thickness under the NCs that affects the tunneling resistance through that junction. The excellent correlation between the current and topography images is clearly depicted in image 1c, obtained by subtracting (in arbitrary units) image 1b from 1a. We would like to note, however, that the fraction of the NCs that was detected in the current images was not always so high as in Figure 1b, and in some cases it was less than 40%, even at large bias voltages (up to 6-7 V where there is already measurable current flow through the PMMA layer itself - the exact bias value depends on the specific preparation conditions for the PMMA layer). Current suppression through a NC may be due to a thick underlying polymer layer or to NC charging, as will be discussed below. The topographic image obtained in the second scan, Figure 1d, looks similar to the first scan. The NCs are clearly observed without any apparent dragging effect. In contrast, there is a striking difference between the two corresponding current images, where the current vanished in the second scan for nearly all the NCs (Figure 1e). This rapid change in the conductance through the InAs NCs repeated itself many times (although not always as abruptly, sometimes taking two or three scans), at various locations on different samples, and for various positive bias values in the range VB ) 4 (the onset of detectable current) to VB ) 6 V (where detectable current flows also through the PMMA itself in this case). Moreover, the current images did not recover even after 30 min under zero bias. We verified that the current decay was not due to tip damage by scanning a “fresh” area and obtaining a current image similar to 1(b). Nano Lett., Vol. 4, No. 1, 2004

Figure 2. Three C-AFM measurements at VB ) -9.5 V performed sequentially on a sample consisting of InAs NCs (6.4 nm in diameter) in PMMA. Panels a and b are the corresponding topographic and current images of the first scan, showing good correlation. Panels c and d are current images acquired sequentially on the same area (up to drift induced shift). As can clearly be seen, the current images acquired at negative sample bias are relatively stable. The scan area is 350 × 350 nm2, the full color scale for the topographic image is 3.5 nm and for the current image -3 pA to 1 pA (the change in relative contrast, as compared to Figure 1, reflects the current sign reversal).

A different behavior was observed when initially scanning at negative sample bias. Here, the currents through the NCs did not decrease with repeated scans, resulting in relatively stable current (in addition to topographic) images. This can be seen in Figure 2, in which the three consecutive current images in frames b-d exhibit stable current values for several NCs. We note here that in the negative bias regime, the voltage needed for driving detectable current through the NCs was large (above 8 V) compared to that in positive bias (around 4 V). This may be due to nonsymmetric band offsets in the NCs and/or the PMMA with respect to the Fermi energy. To further examine the nature and generality of the above phenomena, we have investigated by a similar method other semiconductor NCs, namely CdSe dots20 and rods3,21,22 as well as metallic Au particles26,27 in PMMA. The results obtained on the CdSe NCs, 5.5 nm in diameter, were qualitatively similar to those observed for the InAs dots: the current flowing through the NCs decayed with time for positive VB (see Figure 3) but remained stable for negative bias, and the onset-bias of (detectable) current through the NCs was larger for negative bias. However, the decay of the current under positive bias was more gradual for the CdSe NCs. This was most notable in particular for CdSe rods as shown below (30 nm long and 6 nm in diameter). Additionally, the onset-bias values were smaller for both bias polarities, with CdSe NCs (about +2 V and -5 V). The smaller onset value (at positive bias) provided a large bias range that enabled us to observe current recovery for some of the dots upon further increase of the bias. This is seen in Nano Lett., Vol. 4, No. 1, 2004

Figure 3. Four sequential C-AFM measurements of CdSe dots (∼5.5 nm in diameter) embedded in PMMA. The topography images are shown at the left side and current images are at the right. The sample bias was VB ) 2.2 V in the first three scans (images a to f), showing current decay. The fourth scan (images g, h) was taken at a higher bias, VB ) 2.8 V, showing recovery of the current through one of the NCs. The images size are 235 × 235 nm2, color scale for the topographic images spans 3 nm, and for the current images (1.5 pA (images b, d, f) and (9 pA for image h. The 50 Hz line noise and its harmonics were filtered out from all the current images.

Figure 3, where the 4th scan (frames g and h) was measured at a higher bias level and the current through one of the NCs reappeared. In contrast to the semiconductor NCs, the tunneling currents measured on the gold nanoparticles (7.5 nm in diameter) were stable for both negative and positive bias polarities, showing no decay whatsoever. Moreover, most of the Au particles showed up in the current images, while for the semiconductor NCs, some of the nanocrystals did not exhibit current flow at all, as mentioned before. To quantify the current decay, we calculate the amount of charge flowing through a NC during a single sweep, by integrating the measured current over the NC area in the 105

Figure 4. The amount of charge flowing through a NC in a scan as a function of time under positive sample bias. The full circles and triangles represent data for two CdSe NCs, and full squares are for CdSe rods, all showing an exponential decay. No such decay was observed for gold NCs (empty symbols).

current image and taking into account the scan rate and pixel size. In Figure 4 we plot the amount of charge that flows through single NCs under positive bias as function of accumulative scan time over it, for several NCs, representing the different types studied in this work (the times are digitized to the middle of each scan). For the semiconductor NCs (filled symbols), the amount of charge decays exponentially with time. The decay rate is slower for CdSe rods (30 nm long and 6 nm in diameter, filled squares) as compared to CdSe dots. The gold NCs (empty symbols), on the other hand, do not exhibit any decay, and the charge flow fluctuates around the value observed in the first C-AFM scan. We attribute the current-decay phenomena described above to NC charging that may block further charge transport through the NC. The charging is specific to the semiconductor nanocrystals and is seen only for positive sample bias, while the metallic particles did not show this charging effect. Charging was also seen to be long-lived on the time scale of the AFM measurements (tens of minutes, limited by drift and sample degradation due to contact interaction with the tip), consistent with charging induced by illumination studied by EFM.11 This suggests that the trapped charge resides in a trap site possibly at the NC-PMMA interface. This assignment gains support from our observation that the current measured on CdSe NCs could be partially recovered after switch-off by increasing the (positive) bias (see Figure 3), allowing for additional charge flow either due to de-trapping or in a way analogous to a capacitor charging up. We note that a reminiscent current switching effect was recently observed in conductance-AFM measurements on carbon black/polymer composites, and was attributed to the electrothermal switching effect.19 However, in this case the switched-off (zero current) state was obtained for both bias polarities and current was recovered after less than a minute. Additionally, electrothermal switching should have been more pronounced for metallic particles where the current levels are higher. Further insight into the current-decay phenomena, as well as additional support to its assignment to a charging effect, 106

Figure 5. Histograms showing NC height differences between pairs of subsequent C-AFM measurements (a) under positive sample bias, between the scans shown in figures 1d (after charging) and 1a (before charging), depicting a height increase of 0.7 nm on average; (b) between a scan taken with VB ) -4 V; and scan 1d taken with VB ) 4 V, both for charged NCs, showing that most of the NCs were pushed back into the polymer nearly to the original position; (c) under negative sample bias, between the measurements corresponding to figures 2c and 2b, showing that for the uncharged NCs there is no change in height (on average). The lines represent best fits to normal distribution.

is gained by analyzing the NC height statistics extracted from the topographic images. This is presented in Figure 5, where the histogram in Figure 5a depicts the difference between NC heights measured at positive bias (VB ) 4 V) after and before current-decay (images 1d and 1a, respectively). It is evident that the measured NC heights increased by 0.7 nm on average (and, in fact, a height increase is observed for all NCs). This height distribution remained unaltered in following scans performed at zero bias. However, upon reversing the bias (VB ) -4 V), but still in the “current switched-off state”, the measured heights were very similar to those in the first scan. This is clearly shown in Figure 5b, presenting the difference in heights measured at VB ) -4 V and at VB ) 4 V (image 1d), where a relative downshift of Nano Lett., Vol. 4, No. 1, 2004

0.7 nm on average is observed. In the case where no current decay took place (Figure 2), there was also no apparent change in the NCs height. This is depicted in Figure 5c, where we plot the histogram of height variations between two consecutive negative-bias C-AFM scans, image 2a and (the corresponding topographic image of) image 2c. The Au nanoparticles, for which no current decay was observed, exhibited no change with time in their average height, for both positive and negative sample bias. Previous reports of effects of charging on apparent height of particles were limited, to the best of our knowledge, to particles imaged by tapping-mode AFM rather then contact mode. Such phenomenon was reported for silicon NCs with diameters of a few tens of nm and were attributed to additional electrostatic force affecting the tip-sample interaction and hence the measured particle heights.28 Tapping mode AFM inherently suffers from complications in interpreting the height data for particles, as was shown for nanocrystals in recent work.29 The height obtained from contact-mode AFM is more directly coupled to the actual topographic data. Indeed, we ascribe the height change that accompanies the charging to a real topographic effect. The results presented in Figure 5 are consistent with the picture that the semiconductor NCs become positively charged during the C-AFM scans at positive sample bias. The increase in the NC height when scanning with positive sample bias (Figure 5a) may be due to the electrostatic forces acting on the NC by the field that points from the substrate to the tip (in Figure 1f), thus pushing the NC out of the PMMA layer. This, in turn, may induce a plastic deformation in the PMMA, and consequently the NC did not return spontaneously to its original position, as manifested in following zero-bias scans. We note that this field-induced force (about 0.2 nN at VB ) 5V assuming charging by a single hole) is estimated to be an order of magnitude larger than those due to the image charges, yet smaller than the force applied by the cantilever. At negative bias, the electrostatic force pushes the NCs into the polymer, but due to the relatively large polymer (and possible ligand) stiffness, the NCs cannot be pushed significantly further into the PMMA, as compared to their original position. The tunneling current did not recover upon NC height recovery, indicating that the current switch-off is due to NC charging, and not only to the increase in the tunneling barrier when they are pushed out. Obviously, when the NCs are not charged (absence of current decay), the electrostatic forces are very weak, and consequently their height is not expected to change between scans (Figure 5c)]. Similarly, Au nanocrystals that do not charge, did not show height variations upon bias changes. Coupling of nanoparticles motion to charging was previously seen in conductance experiments on single C60 molecules.30 Interestingly, the increase in the NCs height upon scanning at positive sample bias (as compared to the initial scan performed at zero bias) was observed also for NCs that did not show any current signal in the first scan. One possible explanation for this behavior is that the currents passing through the NCs in the initial C-AFM scans were below our detection limit (0.5 pA), yet sufficient for NC charging as Nano Lett., Vol. 4, No. 1, 2004

Figure 6. Energy level scheme of the tip-NC-substrate system, portraying our model for charging at positive sample bias and absence of charging at negative bias. The empty circle signifies the hole trapping that leads to current blocking. See text.

even at this low current level about 104 electrons flow through the nanocrystal in each scan. Another possible scenario is that the NCs may have been already charged before the C-AFM measurements. Photoinduced charging of CdSe NCs under ambient light conditions (detected via electrical force microscopy) was recently reported.11,12 These latter measurements exhibited long-sustained NC charging by either positive charge (consistent with the vast majority of our data) or negative charge, depending on various sample parameters. Further support for the positive charging (i.e., that holes are trapped in long-lived states) is gained from considering the asymmetric current flow through the NC in our configuration. The case of the tip-QD-HOPG can be approximated as an asymmetric DBTJ where most of the bias drops on the HOPG-QD junction with the smaller capacitance (see scheme in Figure 6).5 Under negative sample bias, electrons flow through the empty conduction band states of the NC and stable current flow is seen. At positive sample bias, the current flow takes place through the valence band states of the NC, and therefore holes can be trapped in states that are within the NC band-gap. When holes occupy such traps the nanocrystal is positively charged and there is a change in the chemical potential that leads to blocking of further current flow. The nature of these hole traps is not clear. However, the common qualitative behavior for the different semiconductor nanocrystals studied here (InAs dots, CdSe dots, and CdSe rods), which are all capped by an organic layer, suggests that they are related with the interface of the nanocrystals, either at the organic overcoat layer, or at the interface with PMMA. The charging rate appeared also to depend on sample type, and the relatively slower rate of charging in CdSe rods may be related with the reduced capacitance in the larger rods. More subtle differences were observed between the InAs and CdSe dots and may be due to differences in surface passivation. Further investigation using other polymers and additional surface ligands may assist in better understanding of the trap sites. For Au nanoparticles, the density of states at the Fermi level of the electrodes is significantly higher compared with that present for the semiconductor NCs, and therefore current flow paths are readily available and no trapping is detected. In conclusion, conductance AFM study on single semiconductor NCs of various kinds embedded in PMMA showed asymmetric conductance properties, where the current was 107

stable at negative sample bias while a rapid irreversible current decay was observed for positive bias. The decay of the current at positive bias is associated with long-term charging of the nanocrystals where holes are believed to be trapped at the particle-polymer interface. The systematic increase in height of the NCs with respect to the PMMA layer upon scanning at positive bias, as well as the asymmetric current flow, are consistent with such charging scenario. Au nanocrystals prepared and measured via the same methods showed stable current flow without height variations, indicating that long-term charging did not take place. Although charge trapping in the semiconductor NCs is a rare event, its outcome on the conductance properties is very significant because of the long-lived nature of the charged state. The common behavior observed for various semiconductor nanocrystals, of different materials (InAs, CdSe) and different shapes (dots, rods), indicates that the trapping is associated with the interface that is similar for these different systems that are all capped by hydrophobic ligands. Expanding this study of conductance through nanocrystals in various polymer matrices such as semiconducting polymers will bear direct significance on the operation of polymer-nanocrystal electrooptical devices such as solar cells and LEDs that require charge transport. Acknowledgment. This work was supported in part by grants from the Israel-US Binational Foundation, the DeutcheIsrael Program (DIP), and the Israel Science Foundation. References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Norris, D. J.; Bawendi, M. G. Phys. ReV. B 1996, 53, 16338. (3) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (4) Kan, S. H.; Mokari, T.; Rothenberg, E.; Banin, U. Nature Mater. 2003, 2, 155. (5) Banin, U.; Millo, O. Annu. ReV. Phys. Chem. 2003, 54, 465. (6) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506.

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Nano Lett., Vol. 4, No. 1, 2004