Supplementary Materials to Addressable Photo-Charging of Single Quantum Dots Assisted with Atomic Force Microscopy Probe

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1 Supplementary Materials to Addressable Photo-Charging of Single Quantum Dots Assisted with Atomic Force Microscopy Probe M. Dokukin 1, R. Olac-Vaw 2, N. Guz 1, V. Mitin 2, and I. Sokolov 1,* 1 Dept. of Physics, Clarkson University, Potsdam, NY Dept. of Electrical Engineering, University of Buffalo, The State University of New York, Buffalo, NY * Contact information: 8 Clarkson ave. Department of Physics, Clarkson University, Potsdam, NY 13699, USA; Phone: ; isokolov@clarkson.edu Materials and Methods Sample preparation. To prepare the samples, highly ordered pyrolytic graphite films (graphene) with lateral size of 5-7 mm were attached to AFM metal sample holders with double sticky tape. The film was glued to the AFM sample holder with silver glue to provide electrical connection to the holder. CdSe QDs with the diameter of 4.79 nm, capped with ~ 1-3nm thick tri-noctylphosphine oxide (TOPO) were purchased commercially (Evident Technologies, Inc.). As purchased, QDs were diluted to the concentration of 0.041nM in toluene (class 1B by Fisher Scientific, Inc.). A droplet of this solution was put onto the prepared graphene film. After several minutes toluene was removed with a filter paper, and the rest was air dried. 1

2 Instrumentation. A green laser with wavelength of 532 nm and intensity of 0.12 W/cm 2 was used for photo excitation. A Nanoscope IIIa Multimode AFM (Veeco/Digitals Instruments, Inc) was used for both charge assistant photo-induction and the charge measurements. Chromium coated ATM probes (MSTP tips by Veeco, Inc., resonance frequency is ~60KHz) were used in the AFM measurements. An Environizer (model 90200) air purifier with negative ion source by Kaz, Inc (Southborongh, MA 01772, USA) was used to treat the surface of the charged QDs. AFM mode of operation. Visualization is done by collecting of height images with oscillating AFM probe (so-called tapping or AC mode). Electrical charges on QDs were detected using a Lift mode with AC/tapping scanning using multiple lift heights (from 3 to 10 nm) 1. After scanning of a line in tapping mode, the probe lifts up at a preset distance, and the scanning repeats while keeping constant the lift distance and oscillation phrase, and measuring the shift f of resonance frequency f of the oscillating probe (so-called the Lift mode). The shift of the cantilever resonance frequency f is proportional to the gradient of electrostatic force acting between the conducting AFM probe and a localized electrical charge. Such shift occurs due to induction of the mirror charge in the conducting volume of the AFM probe. Topographic images were acquired in the tapping mode at a scanning frequency of 2 Hz. The AFM probe was biased with -0.15V to eliminate the contact potential difference (capacitive force) between probe and graphene film ( td ~0.15V is the work function difference between the chromium tip and graphite surface). The size of the scanned area was between 1x1 to 5x5 um 2. Possible contribution of mirror images When we calculating the amount of photo charge induced in each quantum dot, in principle, we had to take into account the mirror charges induced in the substrate. These charges can also contribute 2

3 to the force experienced by the AFM probe. It should also be noted that the mirror charge in the substrate, which was not taken into account in eq.(1), brings a negligible contribution. Even if we ignore the screening of the mirror charge by TOPO coating 2 and a relatively high dielectric constant of QD, which is ~ 9 3, its contribution does not exceed q f a(3 z a dz) f, (1) k ( z a dz) eff 0 (d is the QD diameter, eff is an effective dielectric constant of the probe-mirror charge gap), which is ~ 5% of the frequency shift given by eq.(1). As a result, we will use just eq.(1) is a good approximation for evaluation of the charge. The induced charges are positive Eq.(1) does not predict the sign of the charge. Despite it is expected to be positive 4, we confirm it independently because the nature of our photo ionization is a bit different (assisted now with an AFM probe). To do that, we used an air flow of negative ions created by an air purifier with negative ion source. If the charges on QDs were negative, the effect of negative ions would be negligible, and vice versa. Supplementary Fig.1a shows an area with charged QDs. The charges were clearly seen during the scanning from the top to the bottom. When the probe was in the middle of the image, the flow of negative ions was introduced (shown with an arrow). One can see instant disappearance of the charges. This indicates that we are dealing with positive charges on QDs. The charges are robust in ambient conditions An intriguing behavior was observed within several seconds after switching off the ion source, Supplementary Fig.1b. As scanning goes from the top to the bottom, one can clearly see the 3

4 reoccurrence of the charges. The whole scan took about 90 seconds. One can see that the recovery of the charges happened within approximately 1 minute. This behavior can presumably be explained by the diffusion of the negative ions on the TOPO layer to the conductive graphene substrate, where the ions are neutralized/reduced. The same area was measured 10 hours later. The presence of the majority of charges was confirmed. Typically charges disappeared after 24 hours. The same mechanism of diffusion can explain this longevity of the charges in ambient conditions, where a number of negative ions are typically present. Supplementary Figure 1. Images of the resonance frequency shift/charge as a function of time and action of a flow of negative ions. The slow scan direction is shown by the arrows. (a) The sample was subjected to a negative ion beam shot at the middle of the scan. (b) The charges are recovering after turning off the ion beam. (c) The frequency shift after 10h of storage in ambient conditions. 4

5 Comparison with the previously AFM measured charges on quantum dots It is also worth of comparing our observation of exactly +1e charge on QDs with the AFM measurements of photo-induced charges on CdSe QDs reported in 5. A non-uniform distribution of charge on each QD was reported there with a significant fraction of the QD possessing a positive charge, with the average value of +0.1e (later it was corrected to +0.5e 4 ). Moreover, the observed positive charge demonstrated changes while being imaged. Here we observed a single electron charge which was quite stable during the measurements. In particular, this robustness allowed us to measure the dependence on the probe-qd distance. The difference in our results comes presumably from the used AFM methods. To measure the charges, the AFM method utilized in 5 relied on oscillating voltage of ~ 6V amplitude applied to the AFM probe. Comparing to our estimation of the height of the effective potential barrier (~3.7 ev), one can conclude that ±6V amplitude could provide sufficient energy for electrons to tunnel back to the charged QDs or even recharging it. In this work, we utilize another AFM method, in which there is essentially no voltage applied to the AFM probe. Supplementary references 1 I. Sokolov, D. S. Smith, G. S. Henderson et al., "Cell surface electrochemical heterogeneity of the Fe(III)-reducing bacteria Shewanella putrefaciens," Environmental Science & Technology 35 (2), (2001). 2 J. Jiang, T. D. Krauss, and L. E. Brus, "Electrostatic force microscopy characterization of trioctylphosphine oxide self-assembled monolayers on graphite," Journal of Physical Chemistry B 104 (50), (2000). 5

6 3 L. W. Wang and A. Zunger, "Pseudopotential calculations of nanoscale CdSe quantum dots," Physical Review B 53 (15), (1996). 4 T. D. Krauss and L. E. Brus, "Charge, polarizability, and photoionization of single semiconductor nanocrystals (vol 83, pg 4840, 1999)," Physical Review Letters 84 (7), (2000). 5 T. D. Krauss and L. E. Brus, "Charge, polarizability, and photoionization of single semiconductor nanocrystals," Physical Review Letters 83 (23), (1999). 6