Molecular Control of Structural Dynamics and Conductance Switching

Article pubs.acs.org/JPCC

Molecular Control of Structural Dynamics and Conductance Switching in Bismuth Nanoparticles Debora Marchak,†,§ Denis Glozman,†,§ Yuri Vinshtein,† Sigal Jarby,‡ Yossi Lereah,‡ Ori Cheshnovsky,*,† and Yoram Selzer*,† †

School of Chemistry, the Beverly and Raymond Sackler Faculty of Exact Sciences, and ‡Wolfson Applied Materials Center, Tel Aviv University, Tel Aviv 69978, Israel S Supporting Information *

ABSTRACT: Bismuth nanoparticles, protected by two types of capping ligands, 1-dodecanethiol and ethylene diamine tetra-acetate, were probed by TEM and STM at 80 and 300 K. Both types of nanoparticles show temperature-dependent structural fluctuations leading to pronounced changes in their anisotropic conductance properties. We show that the different capping ligands dramatically alter the structural dynamics in these particles. This finding suggests that molecular control of structural and consequently electronic switching in anisotropic nanosystems is feasible.

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coherent transport properties9 and interesting lattice modifications10 and dynamics.11 Recently5 we reported on large anisotropic conductance in decanethiol-protected Bi NPs using combined TEM and STM measurements (Figure 1). Anisotropy becomes apparent in these measurements when conductance is coherent; that is, tunneling is faster than any interaction/relaxation time in the NPs that can take place as a result of transient charging. As a consequence there is no dephasing of the tunneling charge, and the anisotropy of the effective mass tensor of Bi imparts directionality to the tunneling probability. We have shown that at low temperatures (80 K) the apparent band gap, Eg, in an individual Bi NP varies with the probing tip position above it. In addition, with increasing temperature the NP, having enough energy to overcome the energetic barriers, starts fluctuating between different stable configurations. As a result Eg is fluctuating as well, with a variation that can be as high as 1 eV at elevated temperatures (Figure 2). In this work, we show that it is possible to modify the switching behavior between configurational states by using different capping agents. Specifically we compare 1-decanethiol (Thiol) and ethylenediamine-tetraacetate (EDTA) and account for the role of these surrounding ligands on the mechanism of stress relaxation and dynamic structural behavior in Bi NPs. As is shown below, our analysis and conclusions are mainly based

INTRODUCTION Semiconductor nanoparticles (NPs) are the focus of research due to their potential applicability in many technologies.1 A major advantage of NPs is the ability to fine-tune their properties by varying certain parameters in their synthesis protocols. One of these parameters is the capping agent, the surrounding ligand that covers the surface of the NPs, commonly a molecular layer. The capping agent influences crucial properties of the NPs such as their size, surface passivation, and shape. The effect on shape was elegantly demonstrated by using capping agents preferring certain planes over others in certain NPs, leading to symmetry-breaking geometries.2 Capping ligands can also tune the position of electronic energy levels in NPs, giving way to level adjustment for better device functionality.3 It was recently shown that by replacing organic with atomic ligands it is possible to significantly change surface passivation to enhance electronic transport and to raise the efficiency of quantum-dot photovoltaic devices.4 Following our previous studies,5 we wish to show that in the case of NPs that are characterized by large thermally activated structural fluctuations the capping ligands can be used to alter this dynamic behavior and, as a consequence, affect the related changes in the electronic structure of the NPs. We demonstrate this approach using Bismuth (Bi) NPs. Bi has a Fermi wavelength of ∼25 nm, an extremely anisotropic electron mass tensor,6 low melting point, and a Peierls distorted structure.7 As a result, it is expected to exhibit semimetal-to-semiconductor transition and quantum confinement effects on the nanoscale8 as well as enhanced anisotropic © XXXX American Chemical Society

Special Issue: Ron Naaman Festschrift Received: December 17, 2012 Revised: May 5, 2013

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Figure 1. STM anisotropic conductance measurements of single BiNPs. (a) Schematics of a BiNP in a double barrier tunneling junction. The NP is anchored to the Au substrate by thiol groups protruding out of a mixed monolayer of 1:9 nonanedithiol:decanethiol. (b) Energy band-gap value, Eg, is extracted from each I−V curve as the difference between the positive and negative bias voltages at which onset of current is observed. (c) Direct measurement of the anisotropic conductance properties of a typical BiNP at 80 K. The two I−V curves were taken at two different parking positions of the tip above the same NP, showing striking difference in the apparent band gap.

Figure 2. Changes in the apparent band gap, Eg, of a Bi NP with time as a function of temperature. After selecting an individual NP using set points of 1.5 V and 50 pA and after parking the tip above the NP and turning the feedback off, 500−1000 sequential I−V measurements were made, followed by plotting the apparent Eg value from each trace as a function of measurement number (time). The feedback was reset after each trace.

Figure 3. TEM images of a Bi-Thiol NP (a−c) and a Bi-EDTA NP (d−f) of comparable size (7 nm average diameter). The snapshots were extracted from movies taken at room temperature, where both kinds of NPs exhibit dynamic structural behavior. During the recordings a Bi-EDTA NP can be seen switching between configurations and orientations faster than the temporal resolution of the camera (25 Hz) and always showing a well-defined faceted shape. In contrast, for a Bi-Thiol NP, the shape is less defined and the transition between orientations happens gradually.

on the dynamic behavior of the Eg values of the NPs, as extracted from I−V curves measured in the double-barrier

configuration shown in Figure 1a. Thus, the analysis reveals differences in the dynamic behavior of the bulk of the NPs that B

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mechanical stress once again to initiate instability in search for another pseudostable configuration. During the transition, the whole lattice rearranges to anneal this mechanical stress and reorganizes as the next configuration. To study the dynamic-related changes in the electronic properties of the BiNPs as a function of capping ligand, we probed single BiNPs by STM in a double-barrier tunnel junction configuration14 at 80 and 300 K. Sample topography was scanned at 1.5 V bias voltage and 50 pA current set point. Room-temperature STS measurements of the conductance of the NPs are perturbed by their frequent structural instability of the NPs, making it impossible to determine their band gap (even during a single I−V sweep, which lasts 63 ms). This is demonstrated in the plot resulting from monitoring the current as a function of time at a certain constant voltage (I−t trace). For this kind of measurement the STM tip was parked above the center of a selected BiNP using the same scanning parameters (bias and current set point of 1.5 V and 50 pA, respectively). The z distance was fixed, and current measurements were taken over a period of several seconds. The plot in Figure 4 shows a typical time-trace of the tunneling current for a Bi-Thiol NP at room temperature and

depends on the type of capping agent. The effect of these ligands is not via induced changes in the electronic structure of the NPs because the HOMO−LUMO gap of Thiol and EDTA is larger than the band gap of the NPs. This conclusion is corroborated by the fact that the Eg values of the NPs, capped by both types of molecules, are essentially identical (see below). It is also imperative at this point to rule out the possibility that the different ligands change the apparent Eg values by a Coulomb blockade, that is, by affecting the charging energy of the NPs. If the latter parameter of a NP with radius R is estimated to be that of a sphere (e2/4πε0R), then the charging energy of a 5 nm NP is 0.57 eV. The presence of the nearby electrodes increases the capacitance, leading to an even smaller charging energy. This can be estimated by using the capacitance 4πε0/(1/R1 −1/R2) of two metallic shells whose radii are R1 and R2. The inner shell is the NP and the outer shell is defined by the tip and surface. When R1 = 2.5 nm and the second shell is a ligand-length longer, that is, R2 = 3.5 nm, the charging energy is calculated to be 0.16 eV. A shorter ligand will make the charging energy even smaller. Thus, the contribution of the charging energy to the apparent Eg values (see below) is negligible, and structural changes in the capping agents are not affecting Eg via changes in the charging energy. We therefore argue below that the different ligands affect the surfaces of the NPs and their dynamic faceting, which in turn affects their bulk electronic properties.

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EXPERIMENTAL SECTION Bi NPs were prepared by reduction of the appropriate Bi complex solution with sodium borohydride using either Thiol or EDTA as capping agents. (See the Supporting Information.) The resulting NPs are stable colloid solutions with particles that are 3−8 nm in size, which appear to be stable for months.

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RESULTS AND DISCUSSION Transmission electron microscopy (TEM) analysis reveals the important structural difference between the two kinds of NPs. (See Figure 3.) The upper row shows three images (a−c) taken from a movie (25 frames/sec; see the Supporting Information) recording structural fluctuations of a Bi-Thiol NP at 300 K. This NP shows a nearly round-shaped NP with structural changes taking place with almost no effect on the overall shape. In contrast, the lower row of images (d−f) is taken from a similar movie recording a Bi-EDTA NP, which arranges only in certain polygonal structures and switches abruptly between well-defined faceted configurations. At room temperature, both types of NPs exhibit a structural dynamic behavior consisting of stochastic alternating periods between an all-solid nature and a quasi-molten state.12 The latter state refers to a fluid-like state of a part or of a whole NP, which facilitates its fluctuations between similar energy configurations without actually melting (melting as a firstorder phase transition). The onset of fluctuations is temperature-dependent. These fluctuations relate to a configurational space where each NP conformation represents a local minimum in the free-energy landscape, dominated mainly by the surfacefree energy, sort of a Wulff construction structure.13 The driving force of the transition between configurations is the relaxation of external mechanical stress, most likely induced by surface atomic migration. Even when a pseudostable configuration is achieved, thermally activated surface atomic diffusion occurs, which in turn changes the balance in external

Figure 4. Room-temperature I−t trace showing an event of quasimelting, taken at constant voltage (1.5 V) while the feedback is turned off and the set-point current prior to the measurement set to 0.05 nA. During the first 0.39 s of this measurement the NP is undergoing minor structural changes, which are reflected in its orientation relative to the STM tip and the tip−NP distance. This results in current fluctuations above and below the set-point current. The second part of the plot shows abrupt increases in current that also return abruptly to the set-point current. We attribute these to the BiNPs being in a quasimolten state, resembling a nanodrop, and having an increased density if electronic states. The lower row shows side-view TEM BiNP images of the same NP taken at different times, showing two different solid configurations (a and b) and a melting event (c).

1.5 V bias. The first part of the plot, up to ∼0.4 s, presents almost step-like fluctuations about the set point value of 0.05 nA, with upper, lower, and baseline states. We attribute this behavior to the fast structural dynamics of the NP between different configurations. Under these conditions the changes in current result from changes in orientation and enhanced surface atomic diffusion; both of which bring atoms closer to or away from the tip relative to the distance at the beginning of the measurement. (Throughout one measurement this distance is C

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Figure 5. Energy band-gap measurements at 80 K as a function of time for a BiNP covered by Thiol molecules (inset illustration). On the left, the measurement series shows the evolution of the band gap in time. The main feature is the gradual variation in band gap between similar values. The plots on the right show two dI/dV curves extracted from the A and B regions in the series. The band gap in B is slightly larger than that in A.

locked.) At ∼0.4 s on the plot in Figure 4, a sudden pronounced increase in the current can be observed. These abrupt variations in current happen randomly at room temperature. One possibility is that these fluctuations originate from structural changes that lead to major changes in Eg.11 We note however, as demonstrated in the I−V curves in Figure 1 taken at 80 K, that under a potential of 1.5 V, that is, when the applied potential is above the band gap of the NPs, changes in Eg even as large as ∼1 eV affect the current only marginally compared to the large fluctuations in Figure 4. We therefore assign this abrupt increases in current to quasi-melting periods (as observed in TEM) during which the NP can be considered as a “nanodrop”. (See Figure 4 TEM image c.) It is known that Bi at the liquid phase has a higher (electronic) density of states by a factor of at least 2.5 relative to the solid phase,15 as liquid Bi is actually more dense than the solid phase. (Bi expands upon solidification by 3.32%.) Upon solidification the current goes back to the small fluctuations around the set point value and structurally to one of the stable configurations. Similar behavior is observed for EDTA-capped Bi NPs at room temperature. To discern between capping-agent-related effects, similar measurements were performed at lower temperatures. In these experiments, the STM tip was parked above the center of a selected BiNP using the same scanning parameters (1.5 V, 50 pA), the z distance was fixed, the feedback turned off, and sequential I−V curves were probed over 63 ms periods. 1000 I−V curves were taken for each BiNP while resetting the feedback between each I−V curve, as described above. The value of Eg, calculated as the difference between the positive and negative bias voltages at which onset of current is observed (see Figure 1), was extracted from each curve and plotted as a function of time. Figure 5 shows a typical Eg versus time experiment performed at 80 K with a Bi-Thiol NP. A small and slow change between two Eg values is observed. Given that the NPtip current is reset between measurements, the change in Eg arises purely from a change in the band gap. The right panel of Figure 5 presents two dI/dV plots, both belonging to the series presented on the left, where the small change in Eg is discernible. When similar measurements are performed with Bi-EDTA NPs, a different activity is observed arising from prompt changes in crystallographic orientation, leading to abrupt fluctuations in Eg value (Figure 6). The I−V plots (marked as A−C) show three pairs of subsequent I−V traces belonging

Figure 6. Band-gap evolution in time of a BiNP covered with EDTA molecules (upper illustration) at 80 K. The plots on the right column are single consecutive measurements from the series shown to the left. The I−V marked as B demonstrates the ability of the STM to explore the conductance anisotropy and dynamics ongoing in this complex system. The first sweep (black) has a smaller band-gap value belonging to the orientation existing before its change, while the second (red) sweep already exhibits an Eg value closer to the larger gap characteristic of the next orientation. The HR-TEM image on the left shows a larger EDTA NP where atomic diffusion is clearly appreciated at the corner line between facets. We attribute these sudden changes in Eg to swift orientational/configurational changes in Bi-EDTA NPs, which happen due to the restriction on the overall shape of the NP to well-defined faceted structures rendering a higher number of corners and aristae.

to the respectively marked regions on the Eg versus time plot on the left. Note that insert B presents an abrupt band-gap transition from ∼1 to ∼0.6 eV within ∼60 ms. The distinct difference between the Thiol-capped and EDTA-capped Bi NPs, as shown in Figures 5 and 6, can be D

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Figure 7. Eg evolution in time for a Bi-EDTA NP at 80 K. The series plot (upper left) shows the NP is fluctuating back and forth mainly between two defined configurations, which result in two recurring band-gap values (histogram lower left). The dI/dV curves on the right were extracted from the measurement series at the marked Eg regions; A and C have a larger band gap while B and D have a smaller band gap.

molecules hinder the noncorrelated atomic surface movement in comparison with thiol molecules, a fact that is also reflected in the overall shape of the particles. For Bi-EDTA NPs, the most stable configurations exist only in certain faceted structures due to favorite binding of the EDTA molecules on certain surface planes. The EDTA-capped NPs are characterized by well-defined corners and aristae (interfacets border lines). These structural characteristics result in faster diffusion of noncomplexed Bi atoms at these spots. (See HR-TEM images in Figure 6.) In this case, dislocation movement caused by the external stress produces a plastic deformation that, when relaxed, leaves the NP with a different orientation or structure configuration.17 Thus orientational/configurational changes are taking place on shorter time scales relative to thiol passivated NPs. Furthermore, when NPs are analyzed under STM, if the facets of a certain NP are small in area it resembles a sphere and the identity of each facet is experimentally blurred by vicinal facets. The tip/NP/substrate tunneling events cross a potential that is a time average of all proximate facets facing the tip. This is the case for Bi-Thiol NPs, where only substantial changes in orientation relative to the tip result in significant change in Eg. This effect adds to the dominant plane-by-plane gliding pathway for structural dynamics in Bi-Thiol NPs, resulting in the gradual change in Eg that is observed at lower temperatures. In contrast, for Bi-EDTA, where facets extend for larger areas, tunneling measurements probe more defined and specific facets. Figure 7 shows another 1000 I−V measurement series performed on a Bi-EDTA NP at 80 K, where the NP is fluctuating abruptly between two specific configurations, as shown in the band-gap histogram. We emphasize again that all of these phenomena are a consequence of the capping ligand.

rationalized by considering the surface chemistry in each type of NP, which is also related to the structural properties observed separately under TEM. In Bi-Thiol NPs, each capping molecule is linked strongly16 to the Bi surface at only one active site through the sulfur atom, forming a bismuth−sulfide bond. The dangling bonds terminating the lattice are passivated by bonding to a sulfur atom, one molecule per atom. When surface atomic diffusion takes place each Bi atom is quite free to move around the surface and diffuse with a certain diffusion coefficient, carrying with its movement the molecule attached to it which interacts with other thiol molecules through van der Waals forces. While Bi−Bi bonds are probably being broken and created continuously in this process, the molecules remain on the surface. (See the TEM image in the Supporting Information, where the capping molecules passivate the nanocrystal surface and prevent such a small particle from coalescing with the bigger Bismuth crystal on which it is adsorbed.) Analysis of TEM images suggests that the mechanism for lattice rearrangements in Bi-Thiol NPs is through a plane-by-plane gliding mechanism (see the movies in the Supporting Information), which allows relaxation of stress by annealing a dislocation that crosses the NP.11 Such a process is expected to yield a gradual orientational change in the NP. When the NPs are passivated with EDTA, each carboxylate group is linked to one Bi atom. Therefore, according to structural modeling done by us and depending on the geometry of the EDTA molecule and energetic considerations for its bending, one EDTA molecule can bind simultaneously to between two and four bismuth atoms at the surface in a sort of bismuth-oxide bond. Because of steric hindrance, not all surface atoms are attached to EDTA molecules. This certainly influences the way that surface diffusion occurs. EDTA E

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(12) Ajayan, P. M.; Marks, L. D. Quasimelting and Phases of Small Particles. Phys. Rev. Lett. 1988, 60, 585−587. (13) Ringe, E.; Van Duyne, R. P.; Marks, L. D. Wulff Construction for Alloy Nanoparticles. Nano Lett. 2011, 11, 3399−3403. (14) Banin, U.; Millo, O. Tunneling and Optical Spectroscopy of Semiconductor Nanocrystals. Annu. Rev. Phys. Chem. 2003, 54, 465− 492. (15) Kakizaki, A. A UPS Study of Liquid and Solid Bismuth using Synchrotron Radiation. J. Phys. F: Met. Phys. 1988, 18, 2617. (16) Adamovski, M.; Zaja, A.; Gründler, P.; Flechsig, G. U. SelfAssembled Monolayers on Bismuth Electrodes. Electrochem. Commun. 2006, 8, 932−936. (17) Based on TEM analysis by Sigal Jarby and Yossi Lereah (a private communication).

CONCLUSIONS We have shown the influence of surface chemistry on the structural and electronic properties of Bi NPs, which can be tuned by the choice of capping ligands. While the effects of the capping agent on the dynamics and surface morphology of Bi NPs are very pronounced due to the highly anisotropic nature of Bi NPs, the apparent behavior can be general and is probably taking place in other NPs as well. Work is currently in progress to explore the possibility of inducing electronically measurable anisotropic behavior by molecular capping in other types of semiconductor NPs at elevated temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

Methods of NPs preparation, description of the STM measurements, and TEM video movies of structural fluctuations in both types of NPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (O.C.), [email protected] (Y.S.). Author Contributions §

D.M. and D.Z. contributed equally to this study.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS O.C. acknowledges the support of this research by the Israel Science Foundation (grant no. 984/08) REFERENCES

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