Reverse Switching Phenomena in Hybrid Organic–Inorganic Thin Film

Article pubs.acs.org/JPCC

Reverse Switching Phenomena in Hybrid Organic−Inorganic Thin Film Composite Material Kallol Mohanta, Jose Rivas, and Ranjith Krishna Pai* Nanostructured Hybrid Functional Materials (NHFM), International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga, 4715-330 Braga, Portugal S Supporting Information *

ABSTRACT: A systematic approach was followed to develop a new hybrid organic−inorganic composite material with intriguing electrical and fluorescence properties into one ultrathin film system. Providing facile and cost-effective synthesis, this method utilizes a double decomposition reaction to introduce electric and fluorescence as an intrinsic property into one ultrathin film system, through dihydrolipoic acidcoated core/shell CdSe/ZnS quantum dots. Scanning tunneling microscope was used to asses, at the microstructured level, electrical properties of the composite material. Thin film composite devices exhibit higher conductivity with the application of a lower electrical field and inversely show lower conductivity when applying higher electrical bias point. The prospect of this feature solely lies in band gap engineering inherent to the device structure and geometric properties. The merits of such a device, paired with the ease of chemical functionalization provided by the water-soluble quantum dots, make the obtained hybrid organic−inorganic thin film composite material a viable candidate to be used as sensors, optolectronic devices, as well as pathogenic detectors.

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INTRODUCTION Hybrid organic−inorganic thin films have attracted a great deal of interest among materials scientists due to their wide-range applications in optoelectronic devices and biological sensors.1−5Double decomposition reaction6 (DDR) has been demonstrated to be a promising method for combining different types of materials such as anionic and cationic bipolar species,7 inorganic nanoparticles,8,9 polyelectrolytes, proteins, and polymers10 into one ultrathin film system. In addition, the DDR method is applied in many applications due to its low cost and simplicity in organic−inorganic composite preparation and its ability in achieving good reproducibility for the composite.9,11 Hybrid organic−inorganic thin film composite materials are not simply physical mixtures. They can be broadly defined as molecular or nanocomposites with organic and inorganic components, intimately mixed where at least one of the components domains has a dimension ranging from a few angstroms to several nanometers.12 Consequently, not only are the properties of hybrid composite materials the sum of the individual contributions of both phases but also the role of their inner interfaces could be predominant.13 Spherical shell arrays consisting of organic−inorganic nanoparticles have shown sizedependent diffraction properties due to their periodic modulation of the dielectric function, which can inhibit the propagation of certain frequencies of light through specific crystal orientations.14,15 It may be noted that the spherical shell arrays with organic−inorganic nanoparticles exhibit quite different surface topographies, which may lead to considerable differences in optical and surface properties.16 To fabricate such © 2012 American Chemical Society

highly ordered organic−inorganic arrays with feature sizes down to the submicrometer length scale, composite thin film preparation involving vaterite as templates have been recognized to be a cost-efficient bottom-up strategy.10,17 Vaterite is the least thermodynamically stable form of calcium carbonate (CaCO3). It appears as 1−10 μm spherulitic crystals composed of nanoparticles 20−30 nm in size.18 Vaterite can be manufactured in the laboratory by mixing concentrated solutions of calcium and carbonate containing salts. Vaterite has attracted particular attention due to high specific area, low density, high solubility, and high dispersion compared with other forms of calcium carbonate. Although stable under dry conditions, vaterite transforms easily and irreversibly into a thermodynamically stable polymorph of CaCO3 (calcite), thus limiting its applications. Stability can be achieved by the incorporation of polyelectrolytes into the vaterite crystal lattice.8,10 There has been increased attention to the fabrication of patterned CaCO3 thin films due to the importance of patterning for both scientific and industrial applications. Recently, fabrication of spherical vaterite consisting of CdSe/ ZnS nanoparticles with interesting anomalous optical properties has been reported.8,10 However, the development of mild, facile, and low-cost solution approaches toward such microstructured composite films made of desirable materials with Received: October 2, 2012 Revised: December 5, 2012 Published: December 11, 2012 124

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before being focused onto two single-photon-counting avalanche photodiodes (MPD Picoquant). FLIM images were acquired with a piezo-scanner (Physics Instrumente) interfaced with a TCSPC PicoHarp 300 multichannel analyzer. FLIM images and pixel-by-pixel fluorescence decays were analyzed using the Symphotime software (Picoquant).

anomalous electrical and optical properties still remains a challenge. Herein, we report on the spontaneous formation of hybrid organic−inorganic composite based on vaterite and dihydrolipoic acid coated core/shell CdSe/ZnS semiconductor quantum dots (DHLA-Qdots) with amazing electrical and fluorescence properties, into one ultrathin film system. Various approaches have been proposed for the preparation of organic−inorganic thin film materials.16,19−21 Organic−inorganic22 composites that have controlled hierarchical structures, called biominerals, can be found in nature.23 The formation processes of biominerals have inspired many researchers to use mild conditions for the development of novel organic−inorganic composite materials.24−26 We are able to obtain ideas from these structures and their biomineralization processes. Biomimetic syntheses of crystalline CaCO3 particles and thin films via transient amorphous CaCO3 have been reported by many researchers.27−31In contrast, spherical vaterite shell arrays coated DHLA-Qdots in bulk, and thin film states with anomalous electrical and optical properties have not yet been reported.

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RESULTS AND DISCUSSION In the present study, we have successfully prepared organic− inorganic composites consisting of vaterite stabilized by poly(ethylene glycol) (Vaterite-PEG) coated with DHLAQdots. The bulk and thin film of the composites introduced by us demonstrate good electrical, thermal, and bright fluorescence properties. We also show the functionalization of the composites through the incorporation of DHLA-Qdots. Recently, we reported similar fluorescent vaterite microspheres that showed good dispersibility in buffers of various pH values, including physiological pH.10 In the present study, they were also dispersed when deposited from ethanol on freshly cleaved highly ordered pyrolytic graphite (HOPG). After drying, the resultant composites form thin film system (Figure 1 left and right).

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EXPERIMENTAL SECTION The composite in the bulk and thin film state was synthesized by mixing of two solutions. An aqueous solution containing DHLA-Qdots (1.33 × 10−9 mol/dm3) was poured into the precursor solution containing buffered calcium chloride (0.125 mol/dm3) and PEG (3 wt % polymer, which corresponds to about 6 to 7 vol %) at room temperature. The resulting calcium-PEG-DHLA-Qdots complexes were converted into white precipitate by adding sodium bicarbonate solution (0.125 mol/dm3). The mixed solutions were stirred for 30 min, turning the mixture almost instantaneously opaque. The relatively small size (∼2 μm) and homogeneous size distribution of the vaterite microspheres (see Figure S1 left in the Supporting Information) was achieved by controlling the intensity of agitation of the reaction mixture (∼1100 rpm). Afterward, the dispersions were aged for 24 h under stagnant conditions. The resulting microspheres were isolated by centrifugation and repeatedly washed with milli-Q water and were thereafter dispersed in ethanol for further characterization. This procedure helped remove unbound DHLA-Qdots from porous particles to a level such that the supernatant solution became colorless, thus indicating very little or no DHLA-Qdots leaching from isolated vaterites. All reactions were carried out under a nitrogen atmosphere. STM measurements were conducted in air under ambient condition by a Park XE100 attached with scanning tunnelling microscope head. The etched tungsten tip made from 0.25 mm thick wire was used as a probe. Steady-state fluorescence spectroscopy was performed using a Cary Varian fluorimeter. Fluorescence spectra were recorded at a right angle using 488nm excitation. Time-resolved fluorescence spectroscopy was performed at 488 nm laser excitation by using the frequencydoubled output of a pulse-picked (8 MHz) femtosecond Ti:sapphire laser (80 MHz repetition rate) and a FluoTime 200 lifetime spectrometer (Picoquant, 45 ps instrumental response time). Fluorescence decays were analyzed using the FluoFit Pro software from Picoquant. Confocal fluorescence lifetime imaging microscopy (FLIM) was performed with 488 nm pulsed light using a home-built scanning-stage inverted microscope equipped with a 1.2 NA 60 X water-immersion objective lens. FLIM (Chroma 545 DRLP) was performed

Figure 1. Left: Confocal fluorescence microscopy image (scale bar = 10 μm) of thin film composite material on a cover glass. Right: Scanning tunneling microscope topography image of the thin film on HOPG, tip bias −1 V, and tip current 1 nA (range of colors from dark to light, 0 to 2.85 μm).

The thickness of the film ca. 2 μm was measured by atomic force microscope (AFM) tip used to create a furrow in the film, whereby the thickness was determined by scanning the sample across the furrow with the AFM (data not shown). The scanning tunneling microscope (STM) topography image shows that the organic−inorganic thin film composite exhibits a rough surface with grain boundary on micrometer scales (Figure 1 right). The presence of nanograins 10−15 nm in size is observed by dark-field transmission electron microscopy (TEM; Figure 2 upper left). This indicates vaterite spherulitic crystals (confirmed using the information from the selected area diffraction pattern in the inset in Figure 2, upper left) composed of nanoparticles 10−15 nm in size. The TEM image of a crushed composite suggests loading of DHLA-Qdots at the external surface of the vaterites (see Figure S2 left and right in the Supporting Information). The presence of DHLA-Qdots was confirmed by confocal fluorescence microscopy (Figure 1 left) and by energy-dispersive spectroscopy (see Figure S8 in the Supporting Information). The XRD pattern of the sample shows the presence of vaterite (see Figure S3 in the Supporting Information). The FTIR spectra support the presence of PEG in the composite materials (see Figure S9 in the Supporting Information). On the basis of these results, we conclude that the composites form nanosegregated thin film structures consisting of vaterite and PEG with the incorporation of 125

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Figure 2. Upper left: Transmission electron micrograph of crushed composite material. The bright white spots are nanoparticles featuring size of 10−15 nm, whereas gray areas are due to amorphous polymer and powdered material. At the inset, the selected area diffraction spectrum shows well crystallinity of the material. Upper right: Current map of thin film scanned by STM. Blue hills are more conductive while red areas form the background. The image has been normalized with respect to background current (range of colors from red to blue, 0 to 25.49 nA). Bottom panel: Structural model for the nanosegregated thin film composite material consisting of vaterite, PEG, and DHLA-Qdots. PEG stabilizes Vaterite structure, and the CdSe/ZnS nanoparticles are capped with DHLA. The interaction between open ends of PEG and DHLA immobilizes nanoparticles on the surface of CaCO3 microspheres.

of the composite material. Scanning tunneling spectroscopy (STS) of the thin film constructed from composite material deposited on HOPG substrate was recorded. The current− voltage (I−V) measurements were performed as follows: For approaching, −1 V was applied to the sample, and the tip current was fixed at 1 nA. Constant current mode was employed for recording the topography and current images of the thin film. Identical approaching conditions were used to obtain data from different domains of the sample to ensure comparable acquiring points between the probe and the sample. For combination of probe and sample, approximately five I−V profiles were obtained from each point of at least eight different surface positions (see the inset of Figure 3; the “+” sign is solely a guide for the eye), which gives a total of not less than 40 I−V profiles from different surface points. Figure 2 (upper left) shows a TEM image of core−shell nanoparticles and current mapping (Figure 2 upper right) of a thin film of the microspheres. A thin film for current mapping is produced from 1 mg/mL dispersion of microspheres in ethanol and deposited on highly oriented pyrolytic graphite (HOPG). The current map has been acquired by STM, with set bias: −1 V; tip current: 1 nA condition. In the TEM image, the 10−15 nm bright white spots represent isolated nanoparticles distinguished well in a dark background. We exclude the large spots, which may arise from a lump of aggregation of nanoparticles. The background is not completely dark but instead slightly gray, which may arise from the powdered CaCO3 and

DHLA-Qdots (Figure 2 bottom panels A−C). The stability of vaterite in the composites is attributed to the interaction between vaterite and PEG. Vaterite-PEG-DHLA-Qdots thin film composite, hereafter called composite thin film material, emits around 525 nm, with a photoluminescence spectrum similar to that of free DHLA-Qdot525 nanocrystals in TRIS buffer (see Figure S4 bottom left in the Supporting Information). The composite has good thermal stability and bright fluorescence even after being refluxed in water for 1 day (see Figure S4 bottom right in the Supporting Information). Photoluminescence decays measured from composite thin film material on a cover glass are multiexponential (see Figure S4 upper left in the Supporting Information), with average lifetime similar to those of free DHLA-Qdots525 nanocrystals dispersed in TRIS buffer (4 ns; see Figure S1 right and Figure S4 upper right in the Supporting Information). The relatively high brightness of the composite thin film material, surpassing that of commercially available water-soluble Qdots emitting around 525 nm, makes composite thin film material a promising candidate for biological imaging. Moreover, the large loading surface of vaterites is occluded within composite, and the fact that DHLA-Qdots can be easily functionalized with biologically relevant ligands such as biotin or carbohydrates or with protein makes composite thin film materials promising for biosensing and optoelectronic devices application. STM was used to assess, at the microstructured level, electrical properties 126

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Table 1. Statistics of Data Shown in Figure S5 Upper Left in the Supporting Informationa

Figure 3. Average I−V characteristics of the thin film. Bias scanned in cycle from −3 to +3 V, then again to −3 V. Measurement time was 1 min. Arrows show the direction of current loop. Inset: Topography of the thin film scanned by STM. The I−V characteristics are recorded at points shown by “+” marks.

point

no. of measurements

highest I−to+/I+to− ratio

switching bias

A B C D E F G H

4 6 5 4 3 5 4 2

10.6 9.1 5.4 4.2 3.6 38.2 8.3 21.0

1.07 1.22 2.30 1.44 1.86 0.72 1.28 1.18

Points A and B are shown by “+” marks in the picture shown as inset of the Figure 3; these are the positions at which the I−V profiles have been acquired.

a

rate of increase is reduced with additional applied +ve bias. We find that the increment of current decreases significantly and finally becomes the same as the low conducting state with the application of higher +ve bias. The ratio between current at +ve bias direction to current of −ve bias direction indicates the difference between the two conductive states starts from +1.2 V, as the ratio deviates from its unity value after this bias. This is quite dissimilar from the behavior of a simple metallic or conductive surface. STS of naked HOPG is more conductive, identical in both bias direction and retraceable over many cycles. We also carried out STS on thin film of Qdots as a control experiment. The STS of nanoparticles is shown in the Supporting Information (Figure S6). This is an average characteristic, and it shows typical semiconducting behavior, but on closer observation, it may be seen that there is very thin hysteresis between currents in both directions. When there exists two (or more) electrical conductive states for the same bias, then this phenomenon is known as “electrical bi(multi)-stability” or as “switching effect”.32−37 For this type of occurrence, electrical, magnetic, or optical field, chemical energy transfer, and so on act as stimuli.38−45 There may be various kinds of switching processes. When the difference in two states continues for long after withdrawing the stimulation, then it is called “memory”,34 but if the difference fades away just after stimulation, then it is a threshold46 type of switching. Depending on the erasing process, the memory switching also can be subdivided in different classes.34,47 However, the reason behind the switching phenomenon may be different. Researchers say charge trapping within defect sites47,48 or formation of percolating paths,49 some say oxidation− reduction22 or distortion in molecular configuration,34 whereas many people argue that “switching” may be an effect of metallic filament formation49 due to high field. In particular, for bistability effects in nanoparticle thin films, people explain as charge storage,48 but in general the higher electrical conductive states are established by applying a greater electrical field, and these higher conductive states continue even after withdrawal of the field. In contrast, we can achieve higher conductive states with application of a lower electrical field. The device shows lower conductivity when applying higher electrical bias point, and for this reason we call the observation “reverse switching”. In our case also to explain this “reverse switching” we consider charging the nanoparticles within a high dielectric media such as CaCO3. We ignore the possibility of metallic filament formation because we are using STM tip to probe microspheres.

amorphous polymer as the TEM samples are collected by crushing the microspheres. The current image mapping (see Figure 2 upper right) of composite thin film shows that the current is reasonably constant over the film varying only within a range of 30 nA. The color scale in the picture shows the range of current, whereas the dark blue indicates higher current; the red regions are for low current. This current map was recorded simultaneously with the surface topography; when we compare the map with the topography then we see that the elevated regions have higher current than the lower areas. This is attributed to scanning the tip on an uneven surface because it comes closer to high areas than lower ones, and thus higher regions have more current, but from the topography as well as from confocal fluorescent microscope image (Figure 1) we see that the thin film is uniformly distributed over a considerably large area. Now from the discussion we consider that the microspheres are on the HOPG substrate sitting mere regularly. The resolution from both the topography and current mapping is well enough to differentiate single particles and thus to probe individual microspheres. We probed eight different points, as indicated in the picture (inset of Figure 3) by “+” signs on the thin film; these are essentially individual microspheres. The composite thin film was subjected to a voltage scan in a loop from −3 to +3 V performed at each point for several times; there is a significant hysteresis in the I−V characteristic depending on the direction of the voltage sweep (data shown in Figure 3). The arrows in Figure 3 show the direction of the current, and it does not follow the same path in both bias directions. A high current was established when sweeping the voltage from −3 to +3 V, and low current was recognized when sweeping back from +3 to −3 V. Several I−V characteristics were recorded at different points throughout the surface within the probing area to strengthen the statistical foundation of the observation discussed above (see Figure S5 in Supporting Information). The statistical analysis (Table 1) confirms that the two different conductive states were observed for the thin film of the composite material. Furthermore, it is seen that the two different conductive states are distinguishable only after +1.2 V (see Figure S5 upper right in Supporting Information). This means that the inflection point of +1.2 V defines the onset bias for the formation of the high conductive states of the composite thin film. Note that the current starts to increase at this point (+1.2 V) when sweeping the voltage from −ve to +ve bias; then, the 127

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To comprehend this reverse switching more directly we have carried out the pulse application experiment and varied the range of the scanning voltage measurement. The scheme of pulse application experiment has been depicted, as shown in the bottom of Figure S5 in the Supporting Information. Specifically, we measured current two times at +2 V, once before application of pulse and another after the pulse; the 3 V pulse had a width of 2 min. The current before the pulse was 125 nA, whereas current after the pulse was 87 nA. As expected, the current for the second measurement (recorded after applying the pulse) is much lower than the current before applying the pulse. This again proves that conductivity of the device is reduced after it subjected to a bias of +3 V. When we altered the limit of the scanning voltage, we obtained the results shown in the Figure 4.

Figure 5. Band diagram of composite particles. The particles are assembled on HOPG substrate and are being probed by tungsten (W) tip. Dotted area shows the part more effective when tip bias swept. A and B are the different positions of Fermi level of W when tip bias is around −3 and +2 V respectively.

band structure (marked by dotted line), which is alongside with tungsten (tip) electrode. From the diagram, we can see that energy well forms at the position of CdSe, which has a much lower band gap than ZnS. Again on the right of ZnS within our area of interest is the much higher band gap of CaCO3. So charge could be trapped at the sites of CdSe. Now what happens when we start scanning from −3 V? That is, the fermi level of tungsten has been elevated (as indicated by A). In this situation, the electrons with higher energy from tip can cross the barrier, and some of the electrons get trapped within the energy well and fill it up. When bias is scanned from −ve to +ve direction then for the electrons the barrier seems to get much higher as the energy of electrons becomes less. As an effect, current is becoming lower. This continues until the applied bias reaches the inflection point; at this point (indicated as B in picture), the attractive force toward the tip for electrons is so strong that the electrons inside the energy well of CdSe begin to jump over the barrier due to thin ZnS shell. As the bias is becoming more positive, a greater number of electrons come out of the well. That is the reason for the increase in current after +1.2 V, and thus the trapped charges are released from the energy well. Now returning back from +ve to −ve direction, the well is almost empty and there are no more electrons to come out, so the current becomes lower again. Now when we apply a pulse of 3 V, during the pulsewidth of 2 min duration, the electrons leave the energy well, making it unfilled. So the next measurement after applying the pulse gives lesser current than the measurement preceding it. The same thing happens when conduction takes place in films made of only Qdots. Sensitive observation to the STS of Qdotsonly film shows minute (very small) hysteresis at the positive bias direction is noticeable. This small hysteresis can be attributed to the formation of energy well due to the core−shell structure of the Qdot, but the broadening of hysteresis is very small because the well is more shallow than the compound structure of the microsphere. To prove our consideration about the current conduction through the coated microspheres, and thus for reverse switching mechanism, we have fitted the data with existing theoretical formulas. As a rough approximation, the Fowler− Nordheim function is often applied to describe the tunneling current from one material to another. For strict considerations, this equation should only be applicable for bulk conduction, but here, because our material shows rather bulk phenomena, we employ this function to evaluate the current from +ve to −ve bias direction; we believe this is a valid assumption because our

Figure 4. Range of applying bias been varied. The scanning limits are 1, 1.5, 2, 2.5, and 3 V (different shades of colors indicating varying scan limits). The graph shows that hysteresis is visible only for scanning range of 2 V or more.

Here we varied the limit from 1 to 3 V in a step of 0.5 V and have noticed that for a limiting voltage value of more than 1.5 V the divergence between the two conducting state is spotted. The two different conductive states can only be distinguished after this point of inflection. As we said previously the thickness of the film is around 2 μm, and we know from SEM and TEM that the size of our composite particle is also within that range. Hence it will not be a major error if we consider that the film is made up of single layer of composite particles. In that case, probing on any point of the films essentially means that the conduction is taking place through the thin film composite material (see Figure S7 in the Supporting Information). Now contemplating the band-diagram of the composite, we would have an idea and probable explanation of the results discussed above. Simply stated, the composite particle is a larger CaCO3 covered with small CdSe/ZnS core−shell nanoparticles. Figure 5 shows a schematic illustration of the band diagram of a composite particle. In general, when the coating layer has a smaller band gap, as in our case, then charge carriers pass through that layer, which contributes more in conduction, and the core material has lesser impact, but here the thickness of the coating layer is very thin compared with the size of CaCO3, and also the coating layer is made of discrete particles. So we consider that charge carriers conduct through layers of ZnS, CdSe, and then CaCO3 despite the core material CaCO3 having a wider band gap than the coating particles. The band diagram50−55 is symmetric between the two electrodes, tungsten and HOPG, because CdSe/ZnS coating layer covers CaCO3 isotopically. The tip bias was scanned with respect to substrate, so our interest lies within that half of the 128

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lower conductivity when applying higher electrical bias point. The prospect of this feature solely lies in band gap engineering, inherent to the device structure and geometric properties. The merits of such a device, paired with the ease of chemical functionalization provided by the water-soluble quantum dots, make the obtained hybrid organic−inorganic thin film composite material a viable candidate to be used as sensors, optolectronic devices, as well as pathogenic detectors.

idea that in this direction no trapped charge contributed to current. The Fowler−Nordheim equation56 is as follows: ⎡ 8π 2m*q ⎤ ⎥ J ∝ E2exp⎢ − Ø3/2 B 3hE ⎢⎣ ⎥⎦

where J is the current density, E is the electric field, øB is the barrier at emission end, and m*, q, and h are effective electron mass, electronic charge, and Plank constant, respectively. Here, because area of contact is not known, we plotted only (I/V2) versus V−1, which fits nicely in a straight line (Figure 6 right).

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ASSOCIATED CONTENT

S Supporting Information *

SEM image, FLIM image, TEM image, XRD data, fluorescence decay, photoluminescence spectra, I−V curves, plot of ratio of current, Schematic plotting of pulse experiment and device, EDS, and FTIR. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +351 253 140 112. Fax: +351 253 140 119. E-mail: [email protected]; [email protected].

Figure 6. Fitting to (left) −ve to +ve bias direction current with Poole−Frenkel equation and (right) +ve to −ve bias direction current with Fowler−Nordheim equation.

Notes

The authors declare no competing financial interest.

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Similarly we tried to fit the current from −ve to +ve bias direction with the Poole−Frenkel equation. The Poole− Frenkel equation describes current−voltage characteristics of trap charge-dominated phenomenon. The Poole−Frenkel equation56 is given as: ⎡ q ⎧ J ∝ E exp⎢ − ⎨φT − ⎢⎣ kT ⎩

qE π∈

ACKNOWLEDGMENTS Research carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract no. DE-AC02-98CH10886. We thank Dr. Thomas J. La Tempa for language polishing on manuscript.

⎫⎤ ⎬⎥ ⎭⎥⎦

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All of the parameters have usual meaning as before only φT, k, T, and ∈ are tunneling energy level, Boltzmann constant, absolute temperature, and permittivity of active material, respectively. Now according to Poole-Frenkel equation, ln(I/ V) versus V0.5 would be a straight line if the current is due to trap charge flowing. From Figure 6 left we can see that our experimental data matches nicely with the Poole−Frenkel equation. This is convincing due to the fact that when going from −ve to +ve bias, trapped charges, mostly electrons, contribute to current. In contrast, when coming back from +ve to −ve bias, they follow conventional tunnelling.

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CONCLUSIONS In summary, we have obtained hybrid organic−inorganic thin film composite material exhibiting good electrical and bright fluorescence properties. Stabilization was achieved by the use of PEG and DHLA-Qdots, which also introduce fluorescence as an intrinsic property to the thin film composite material. These materials comprise vaterite, PEG, and DHLA-Qdots that spontaneously form nanosegregated structures. Adding the large loading surfaces of vaterites, and the fact that DHLAQdots can be easily functionalized with biomacromolecules, thin film composite materials are a promising scaffold for biosensing applications. Hybrid organic−inorganic composite thin films display bistable conductive states, showing contrasting difference with usual observations. Thin film composite devices exhibit higher conductivity with the application of a lower electrical field and inversely show 129

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dx.doi.org/10.1021/jp309750p | J. Phys. Chem. C 2013, 117, 124−130