Spectacular oscillations in the dark and photocurrent in thiol-capped CdS quantum dots embedded in PMMA matrix

Abstract

In this work, experimental results of low-frequency, time-dependent current oscillations (under both dark and photoexcitation) in the sample cells of thiol-capped CdS quantum dots (QDs) embedded in poly(methyl methacrylate) (PMMA) matrix have been presented. Dark and photocurrent oscillatory behaviours have been studied under diverse conditions, such as sample cell temperature, applied bias voltage across the electrodes and concentration of thiol-capped CdS QDs. Oscillation amplitude gradually increases with applied bias voltage at a particular temperature and also with increasing sample cell temperature. The period of oscillation (i.e., frequency) can be tuned over a wide range either by changing the sample temperature, applied bias voltage, or varying concentration of thiol-capped CdS QDs. Existing theories of current oscillations in semiconductors have failed to explain the observed low-frequency current oscillations in these semiconductors. Such current oscillations are possibly associated with the formation of some kind of charge density waves in the PMMA films embedded with thiol-capped CdS QDs.

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1. Introduction

Polymer nanocomposites exhibiting interesting optical, electrical, mechanical, or other properties have attracted great interest in recent years.^1–13 Previous studies indicate that the major obstacle³ in large-scale production and commercialization of nanocomposites is the serious lack of cost-effective methods for controlling dispersion of nanocomposites in polymeric hosts. The absence of the structure–property relationship is another obstacle³ to the extensive use of nanocomposites. In fact, to resolve the above issues, studies of various aspects of polymer nanocomposites are needed.

Although it is known that PMMA is an insulator, the metallic behaviour of PMMA nanocomposites with single-walled carbon nanotubes (SWCNTs) has been noted.¹³ Interactions of nanoparticles with polymers are mediated by ligands attached to the nanoparticle, and the ligands markedly influence particle behaviour and spatial distribution.³ Studies of the synthesis and photophysical properties of CdS nanoparticles attached/capped with thiols as ligands have attracted much attention in recent years.^14–16 Our earlier study¹⁰ showed bistable electrical conductivity, that is, conductance switching between two conducting states in the nanocomposites of PMMA with benzyl mercaptan (thiol)-capped CdS QDs.

Fluctuations in physical properties have been reported near the phase transition temperature in many cases.^17,18 On the basis of the switching¹⁰ in electrical conductivity in thiol (BM1)-capped CdS QDs embedded in PMMA thin films from one conduction state to another conduction state, at a certain threshold bias voltage (different for forward- and reverse-bias voltage sweeps in the current–voltage (I–V) characteristics, depending on sample cell temperature and other experimental conditions), it was thought worthwhile to check fluctuation occurrence in electrical conductivity in thiol-capped CdS QDs embedded in PMMA thin films under suitable experimental conditions. The electron correlation effects on the charge transport properties of a QD under the influence of an external potential have been studied in the recent past.¹⁹ Because of the small dimensions of the nanostructure device, a very small bias applied on the device can cause a very strong internal electric field. Thus, the carriers in the device in presence of a field can exist in a state far from the equilibrium state and cause current oscillations. A current oscillation is a very interesting phenomenon that was usually observed in weakly coupled, compound semiconductor super-lattices (SLs).²⁰ Current oscillations in ferrocene and its derivatives under vapor adsorption had been studied in our laboratory.²¹ Recently, Majumder et al.⁹ have reported current instabilities in poly(methyl methacrylate) (PMMA) films with dispersed silver nanoparticles. Carrier relaxation dynamics in semiconductor quantum dots is important both for understanding the fundamental physics of nanoscale materials as well as for their incorporation into optoelectronic devices.²² In the course of studies in the electrical conductivity of PMMA embedded with thiol capped-CdS QDs, we have observed a bias voltage-dependent self-sustained current oscillations or so-called current self-oscillations in thiol-capped CdS QDs embedded in PMMA matrix at room temperature and also at higher temperatures. Thiol-capped CdS in the PMMA matrix caused the observed current oscillations in the presence of suitable bias voltage. Interestingly, without any thiol-capped CdS doping in the PMMA films, no such oscillations were observed under identical conditions. The experimental results have also shown that regular oscillations were noted only at a certain temperature and with a particular bias voltage. At other temperatures and applied voltages, the regularity of oscillations was not maintained. We must mention that between the two conduction switching events at different voltages in forward and reverse bias voltage sweeps in BM1-capped CdS QDs embedded in PMMA thin films, the current–voltage characteristic curves exhibited a hysteresis loop hierarchy.¹⁰ The threshold voltage and hysteresis loop voltage were observed to decrease with increasing sample cell temperature in the range 303–333 K. Ultimately, the loop almost vanished around 333 K and at this temperature, conduction switching was not clear. In fact, the temperature is near the glass transition temperature²³ of PMMA (isotactic). Therefore, to perform the experiment in the same phase of the polymer, the sample temperature was kept lower than this glass transition temperature for studies of current oscillatory behaviour. Experimental results of current oscillation dependence on several parameters such as sample cell temperature, bias voltage, and concentration of thiol-capped CdS in the PMMA matrix are presented in this paper. The possible mechanism of such current oscillations has been discussed.

2. Experimental

Benzyl mercaptan (BM)-capped CdS QDs were synthesized¹⁶ by using the microwave irradiation method. For the preparation of BM-capped CdS nanocrystallites/QDs, 50 mM each of cadmium acetate and thiourea and 30 mM of benzyl mercaptan (thiol) were taken.¹⁶ The prepared BM-capped CdS QDs were designated as BM1 in our earlier paper.¹⁶ The sizes (diameter: 2R) of the CdS QDs capped with BM1 were estimated¹⁶ at 2.734 nm. A measured amount of synthesized BM1-capped CdS QDs was dispersed into PMMA (MW 120 000) matrix. The mixture was dissolved in chloroform (spectroscopic grade; SRL India) solvent (100 mg/3 mL) and sonicated for 0.5 h to obtain a homogeneous mixture of PMMA/BM1-capped CdS QDs. In the mixed solution, concentration of BM1-capped CdS QDs in PMMA varied from 1 to 15 wt%. The QDs were characterized through standard procedures, namely, using UV-vis spectroscopy, FESEM (model JSM-6700F, JEOL, Japan, operating voltage 5 kV), EDX analyses, SAED patterns, transmission electron microscopy (TEM) (model JEM-2010, JEOL, Japan, operating voltage 200 kV), HRTEM images, XRD, and so on. To make the thin films, freshly prepared solution was used. Using this solution several almost identical thin films were prepared via the spin-coating (2000 rpm) method using a programmable spin coater (SCU-2008C, Apex Instruments Co., Kolkata, India). Solutions were spun on glass/quartz substrates at a speed of 2000 rpm for 60 s. All of these films were made in ambient air with relative humidity of 65–70%. To allow the evaporation of solvent from the films, they were kept at room temperature for 24 h. To study the electrical characteristics, a sample cell with the film was placed in a specially designed brass conductivity chamber coated with Teflon.^9,24 Measurements were carried out at room temperature and also at higher temperatures. Silver paste was used for electrical contact. The area of each electrode was 2 × 9 mm², and the separation gap between two electrodes was 4 mm in a surface-type cell configuration.^9,24 For the current–voltage measurement, the sweeping direction of the applied bias was −ve → +ve → −ve. The voltage sweep was 500 mV for every set of readings after 2 s, resulting in a scan speed of 250 mV s⁻¹. Temperature of the sample film was controlled via a PID temperature controller (model 2404, Eurotherm, UK). The current versus time and current–voltage characteristics were recorded with a programmable electrometer (Model 6517A, Keithley Inst. Inc., USA). The photoexcitation was done by using a xenon lamp source (Newport, Optical Instruments, USA). A monochromator (H-20 UV, Ins. SA, Jobin Yvon division, France) was used for obtaining monochromatic radiation of required wavelengths (200–800 nm) for photoexcitation. Before measuring conductivity, heating and cooling treatments were also carried out. The rate of heating and cooling was 4 °C min⁻¹ and 2.5 °C min⁻¹, respectively. All the measurements were performed in a vacuum chamber.^9,24 Between two successive measurements, the sample was allowed to relax for at least 30 minutes. The instrument was interfaced with a computer.

3. Results and discussion

3.1. TEM and FESEM images of PMMA composites with BM1-capped CdS QDs

TEM image of the composites of PMMA with BM1-capped CdS QDs is shown in Fig. 1(a). Similar TEM images and corresponding histograms were presented in our earlier article.¹⁰ For ready reference the histogram is shown in Fig. 1(b). The TEM image in Fig. 1(a) shows that spherical and isolated particles were obtained. From the HRTEM analysis (Fig. 1(b)), the size distribution of the QDs was estimated at nearly 3.35 nm. Clear crystalline lattices of the QDs were observed (shown in Fig. 1(c)), the spacing between two crystal planes was estimated at nearly 2.3 Å. Fig. 2 shows the FESEM images of BM1-capped CdS QDs embedded in PMMA matrix, having diverse concentrations of BM1-capped CdS QDs. With increasing concentration of BM1-capped CdS QDs in the PMMA matrix, the size of the nanocrystallites/particles increased. At higher concentrations (15 wt%), aggregation of particles was noted (Fig. 2(e)).

Fig. 1 (a) TEM image of CdS quantum dots embedded in PMMA matrix. (b) Histogram for particle distribution. (c) HRTEM image of BM1-capped CdS QDs.

Fig. 2 FESEM images of BM1-capped CdS QDs embedded in PMMA matrix with different concentrations of BM1-capped CdS QDs: (a) 1, (b) 5, (c) 7, (d) 10, and (e) 15 wt%, respectively.

3.2. Dependence of current–voltage (I–V) characteristics on amounts of BM1-capped CdS QDs in nanocomposites with PMMA

The current versus voltage (I–V) characteristics in the voltage range −5 V to +5 V (at room temperature, 303 K) of the sample PMMA cell with different concentrations of embedded BM1-capped CdS QDs, is shown in Fig. 3. During the forward sweep direction (from negative to positive voltage), the current at a particular voltage was different (except for the two junction positions) from the reverse direction (from positive to negative voltage) as demonstrated in Fig. 3. The current in cases of different concentrations under study shows hysteresis behaviour (clockwise) with respect to the applied bias voltage. The hysteresis behaviour presented in Fig. 3 showed an interesting dependence on the concentration of BM1-capped CdS QDs in the PMMA film. A drastic change in current was noted for the 20 and 50 wt% concentrations (Fig. 3(b)). It is also noted that areas within the hysteresis loops (at room temperature) and separation gaps between the forward and reverse currents at V = 0 V increased almost linearly with increasing concentration of BM1-capped CdS QDs up to 15 wt%° as shown in the Fig. 3(a) inset. It should be mentioned here that the separation gap increased abruptly for 20 and 50 wt% concentration (not shown). In addition, the separation gaps between forward- and reverse-applied voltages corresponding to I = 0 were observed to vary with the concentration of BM1-capped CdS QDs in the PMMA matrix and the variation in separation gap with the concentration was observed to be almost systematic at lower concentrations. The estimated separation gap for 1, 5, 7, 10, and 15 wt% concentration of BM1-capped CdS QDs embedded in PMMA was found to be 0.652, 0.88, 1.34, 1.52, and 1.02 V, respectively. From the observed results, the role of the amount of BM1-capped CdS QDs in PMMA matrix for the occurrence of hysteresis behaviour is clearly established. The hysteresis in I–V characteristics may be caused by the mobile ions, rearrangement of space charge, interfacial polarization, and crystal orientations.²⁵ The cause of variation in the observed currents (forward and reverse sweeps) might be due to asymmetry in the density of charge carrier production during the two sweeps. The asymmetric characteristics in the production of charge carriers during the forward and reverse sweeps seem repeatable, as their effects were understood via the difference in current values during the two sweeps observed in each case. This observation reflects the intrinsic properties of the contained material.

Fig. 3 Current–voltage (I–V) characteristics of BM1-capped CdS QDs embedded in PMMA matrix (thin film) measured at room temperature (303 K) under dark condition with different amounts of BM1-capped CdS QDs: (a) (1) 0, (2) 1, (3) 5, (4) 7, (5) 10, and (6) 15 wt%, respectively. Arrows indicate direction of voltage sweeps. Inset represents plot of separation gap at V = 0 V bias as a function of BM1-capped CdS QDs content in PMMA thin film. (b) (7) 20 and (8) 50 wt%, respectively.

In the experiment²⁴ with thin films of pure (un-doped) PMMA, and using both electrodes of the same material (silver) in a surface-type cell configuration, hysteresis behaviour (clockwise) was noted. The hysteresis did not depend on the starting position of the applied bias for I–V characteristic measurement, which indicated that field-induced slow polarization in PMMA was not responsible for the observed hysteresis. Similarly, in the present case the observed hysteresis was also not related to field-induced slow polarization in the system. Again, as the electrodes were of the same material and the hysteresis loops were symmetric about current axis, the hysteresis behaviour was not related to the tunneling of charge carriers from the electrodes.

Temperature dependence of the steady-state dark current (I_d) in the sample cells of organic and organometallic materials can be expressed by the relation^10,24

I_d = I_o exp(−E_d/2kT) (1)

where I_o is the pre-exponential factor, E_d the activation energy for dark conduction, k the Boltzmann constant, and T the absolute temperature. The plots of log I_d versus 1/T were expected to be linear, following eqn (1). On the other hand, the electrical current I_d (dark condition) in polymers is known to be controlled by variable range hopping (VRH),^10,24 which can be expressed as follows:^10,24

I_d = I_o exp[−(T_o/T)^1/γ] (2)

where I_d is the value of the dark current at temperature T, I_o and T_o are constants, and γ depends on the hopping dimension; γ is 2, 3, and 4 for one-, two- and three-dimensional hopping, respectively. We discussed in a recent publication²⁴ the temperature dependence of electrical conductivity (proportional to the measured current) for PMMA thin films prepared from chloroform. It was noted that the variation of log I_d versus 1/T^1/4 for bias voltage lower than a threshold voltage (V_th) showed the best linear fitting (based on the values of linear plot regression coefficient), indicating the validity of VRH model. But the variation of log I_d versus 1/T for bias voltage higher than V_th showed the best linear fitting. In the case of BM1-capped CdS embedded in PMMA, the validity of eqn (1) or (2) was also checked as discussed in an earlier paper.¹⁰ It was noted that the variation of log I_d versus 1/T^1/4 for bias voltage lower than a threshold voltage (V_th) showed the best linear fitting and the variation of log I_d versus 1/T for bias voltage higher than V_th showed the best linear fitting. Thus the conduction mechanisms for the bias voltage lower and higher than V_th were observed to be significantly different. The value of V_th for BM1-capped CdS QDs studied¹⁰ under the dark condition and at room temperature was around 28 V. It should be mentioned here that the voltage range for observing hysteresis behaviour is lower than V_th, which suggests the validity of the VRH model in the current case.

Various physical and chemical characteristics²⁶ of polymers are determined by molecular movements, which depend on sample temperature and other experimental conditions. The ester side groups of PMMA are known²⁷ to have high electron density, and possibly different conduction paths are activated¹⁰ in the PMMA films through the ester side groups following different types of voltage/electric field-induced conformation changes during application of forward and reverse bias. It is known that PMMA can be an electron donor.²⁸ The value of electron affinity of isotactic PMMA has been reported²⁹ to be 0.521 eV and that of CdS has been reported³⁰ as 4.5 eV. From studies⁶ of electric field–induced absorption (E–A) and photoluminescence (E–PL) spectra of BM-capped CdS quantum dots embedded in a PMMA film, it was suggested that BM-capped CdS QDs have a significant charge-transfer character in the emitting state, and the dipole moment of the QDs may be aligned along the applied electric field. Also, field-assisted dissociation into hole and electron at the photoexcited state having a charge-separated character was indicated.⁶ Hence, based on our experimental results, we propose charge transfer from polymer (PMMA) to BM1-capped CdS QDs under a high electric field for the electronic transition. At lower values of voltage, the concentration of free charge carriers is low, so the device shows a low current. However, when the electrical field increases to a certain value, an electron on the highest-occupied molecular orbital (HOMO) of PMMA³¹ may gain enough energy to be transferred into the BM1-capped CdS QDs forming additional conduction paths. At present it is not possible to support the above statement with theoretical calculations, and in future it may be done by other theoretical groups.

3.3. Temperature-dependent current–time (I–t) characteristics of PMMA nanocomposites with BM1-capped CdS QDs

It has been stated³² that the hysteresis phenomenon may be related to oscillations in the system. The current versus time (I–t) characteristics of BM1-capped CdS QDs embedded in the PMMA matrix (thin films) under dark conditions at different sample cell temperatures, with a constant bias voltage of 27 V, are shown in Fig. 4(a). In this figure, the profiles of the dark current as a function of time at different temperatures in the range of 303–328 K reveal the strong temperature dependence of the oscillatory nature of current with the variation of frequency and amplitude. Fig. 4(b) shows the I–t curves for PMMA films, embedded with BM1-capped CdS QDs under photoexcitation having 555 nm wavelength, with other conditions remaining the same as the experiment shown in Fig. 4(a). Observed currents under photoexcitation are lower than those for dark conditions. Fig. 4(c) represents the magnified plot of curve (3) of Fig. 4(a). At room temperature (303 K), both in dark condition (Fig. 4(a)) and under photoexcitation (Fig. 4(b)), the oscillatory behaviour in current is not very clear. At 328 K, the oscillatory behaviour in current under photoexcitation (Fig. 4(b)) was not so regular compared to that of the oscillatory behaviour in current measured in dark condition (Fig. 4(a)). Plots of peak to peak height (peak height) and time interval between two consecutive current peaks (peak interval) as a function of temperature are shown in Fig. 5(a) and (b), for under the dark condition and photoexcitation, respectively. In general an increase in peak height and decrease in peak interval with temperature were noted for dark conditions as well as under photoexcitation.

Fig. 4 Plots of (a) dark and (b) photocurrent (bias 27 V) versus time profile for BM1-capped CdS QDs (amount: 1 wt%) embedded in PMMA matrix (thin films), measured at different temperatures (1) 303, (2) 308, (3) 313, (4) 318, (5) 323, and (6) 328 K, respectively. (c) Curve (3) in (a), magnified scale.

Fig. 5 (a) Plots of peak height and (b) time interval between two consecutive current peaks (peak interval) versus sample cell temperature for BM1-capped CdS QDs embedded in PMMA matrix (thin films) in the case of surface-type cell configuration (at 303 K), curves (1) under dark and (2) photoexcitation at 555 nm wavelength.

3.4. Bias voltage-dependent, current–time (I–t) characteristics of PMMA nanocomposites with BM1-capped CdS QDs

To study the effect of bias voltage on the above mentioned oscillatory behaviour of the current, the current values were recorded at a fixed temperature as a function of time at different magnitudes of bias voltage (in the range of ∼5 V to 50 V) under dark conditions (Fig. 6(a)) and also under photoexcitation with a 555 nm wavelength (Fig. 6(b)). In Fig. 6(a), curves (4) to (6) show that the current–time profiles (oscillatory behaviour) for the bias voltages of 27–50 V were significantly different from the current–time profiles corresponding to lower bias voltages (curves (2) and (3), Fig. 6(a)). This observation is possibly related to the occurrence of voltage-induced conductivity switching¹⁰ (under dark condition) around 28 V in PMMA films embedded with BM1-capped CdS QDs. In contrast, the oscillatory behaviour in current under photoexcitation (Fig. 6(b), curves (1)–(5)) is regular and also similar for bias voltage of 5–40 V and oscillatory behaviour in current for bias voltage of 50 V (Fig. 6(b), curve (6)) is different. Observed regular oscillatory behaviour in current up to higher bias voltage under photoexcitation is related to the fact that voltage-induced conductivity switching under photoexcitation occurs¹⁰ at higher bias voltage in PMMA films embedded with BM1-capped CdS QDs. Variation in current peak heights and time interval between the two consecutive current peaks (peak interval) with increasing bias for both under dark condition and photoexcitation are shown in Fig. 7(a) and (b), respectively. With increasing bias voltage, peak height was observed to increase for under dark condition as well as photoexcitation. But the peak interval was found to increase up to 40 V, and for higher voltage of 50 V a decrease in peak interval was noted for both dark condition and photoexcitation. According to previous publications,^21,33 when the frequency varies with field at all, it should increase, whereas we found that in general the frequency decreases with increasing field strength.

Fig. 6 Current versus time profile for BM1-capped CdS QDs (amount: 1 wt%) embedded in PMMA matrix (thin film) measured at 313 K under (a) dark and (b) photoexcitation at 555 nm wavelength for different bias voltages: (1) 5, (2) 10, (3) 20, (4) 27, (5) 40, and (6) 50 V, respectively.

Fig. 7 Plots of (a) peak height and (b) time interval between the two consecutive current peaks (peak interval) versus bias voltage for BM1-capped CdS QDs embedded in PMMA matrix (thin film) in case of surface-type cell configuration (at 313 K), curves (1) under dark and (2) photoexcitation at 555 nm wavelength.

3.5. Dependence of I–t characteristics on amounts of BM1-capped CdS QDs in nanocomposites with PMMA

The oscillatory behaviour of current in the BM1-capped CdS QDs embedded in PMMA thin film under dark and photoexcitation with a 555 nm wavelength is shown in Fig. 8(a) and (b), respectively, for different concentrations of BM1-capped CdS QDs. The changes in peak height and peak interval with varied concentrations of BM1-capped CdS QDs are clearly observed in Fig. 8. Note that the current self-oscillations occurred even at lower concentrations of BM1-capped CdS QDs in the PMMA matrix. However, at higher concentrations, the current–time profiles for these PMMA films embedded with BM1-capped CdS QDs exhibited discontinuities on sequential current oscillations and showed a series of saw tooth or plateau like behaviour. In our experimental results, the stability of current oscillations was quite high (e.g., wave-form oscillations), showing no observable change for more than 25 min. Fig. 9(a) and (b) show the changes in peak height and peak interval with various concentrations of BM1-capped CdS QDs. Both peak height and peak interval generally increase with increasing concentration of BM1-capped CdS QDs in PMMA films and ultimately trend towards saturation or attain saturation. Given the nature of systematic variation in current oscillations as a function of various parameters—such as sample temperature, bias voltage, concentration of BM1-capped CdS QDs, and so on—it is clear that such current oscillations must have a physical basis.

Fig. 8 Current versus time profile measured at a fixed bias voltage of 10 V for BM1-capped CdS QDs embedded in PMMA matrix (thin film) measured at 313 K, (a) under dark and (b) photoexcitation at 555 nm wavelength with different concentrations of BM1-capped CdS QDs: (1) 1, (2) 5, (3) 7, (4) 10, and (5) 15 wt%, respectively.

Fig. 9 (a) Plots of peak height and (b) average time interval between the two consecutive current peaks versus amounts of BM1-capped CdS QDs content in PMMA matrix (thin film) in case of surface-type cell configuration (1) under dark and (2) photoexcitation at 550 nm wavelength.

3.6. Discussion of relevant models for current oscillation in semiconductors

Oscillatory behaviour of current has been reported in many semiconductors.^33–38 In some reported experiments, in addition to electric field, external illumination and/or an external magnetic field was applied to the semiconductors to obtain the current oscillations. A large number of semiconductors have shown oscillatory behaviour in electric current as a result of an inherent specific negative resistance as observed in the case of GaAs.³⁴ In contrast, in the present experiment, oscillatory behaviour in current has been noted by application of electric field only and without external illumination or an external magnetic field, and the system has shown positive resistance. Oscillatory behaviour of current in the positive resistance region has been reported without application of external illumination and/or an external magnetic field in the case of cobalt- (or gold-)compensated n-type silicon³⁵ and iodine-doped polyethylene.³⁶ In these cases^35,36 propagation of low field domains and perturbation across the sample, respectively, have been proposed. A theory of domain formation and propagation based on the difference in capture times for electrons and holes by traps was proposed by Konstantinov and Perel'.³³ The latter predicts that if the frequency of oscillations varies with the electric field at all, it should increase along with increasing field. But the opposite result was found in the present experiment, which confirms that the theory is not valid in respective conditions. The frequency range of semiconductor instabilities generally extends from a few hertz to gigahertz.^35,36,38 In the present experiment the frequency of current oscillations was very low. The phenomenon of current oscillations with a low frequency as observed in the present case is relatively rare^21,37 and poorly understood. Kispeter et al.³⁷ reported low-frequency oscillations in photocurrent in polycrystalline selenium samples. In this material, the frequency of oscillations in photocurrent was observed to increase with increasing bias voltage. But in the present case, the frequency of current oscillations decreased with increasing bias voltages as mentioned previously. Thus, the modified barrier model in the case of selenium³⁷ is not applicable in the present study. It should be mentioned here that low-frequency current oscillations have been reported²¹ in sample cells of ferrocene and its derivatives with adsorbed vapours, and the frequency of current oscillations was found to decrease with increasing bias voltages. In these cases the current oscillations were thought to originate from the adsorption-induced phase transition in the system.

As mentioned earlier fluctuations in physical properties have been reported near the phase-transition temperature in many cases.^17,18,21 The switching in electrical conductivity in BM1-capped CdS QDs embedded in PMMA thin films has been reported,¹⁰ and transition from one conduction state to another was discussed. The observed current oscillation in BM1-capped CdS QDs embedded in PMMA films, under various experimental conditions as discussed in this article, is expected to be related to the conductivity switching reported in our earlier paper.¹⁰ Instability in current versus time profile was recently reported⁹ in silver nanoparticles dispersed in PMMA thin films. Such instability in current versus time profiles was explained in terms of charge density wave (CDW) formation in the nanocomposite films of dispersed silver nanoparticles in PMMA films. The versatile polymer PMMA has been observed to manifest charge density waves.³⁹ Current oscillation reported in this article is possibly related to formation of charge density waves in the PMMA films embedded with BM1-capped CdS QDs. Such nanocomposites are expected to be useful for various nanotechnology-based devices.

4. Conclusions

The observed periodic current oscillations in the current versus time characteristics of BM1-capped CdS QDs embedded PMMA thin films was related to the voltage-induced conductivity switching in this material. Systematic variations of the current peak height and time interval between the two consecutive current peaks (peak interval) with various parameters such as sample cell temperature, bias voltage and concentration of BM1-capped CdS QDs embedded in PMMA thin films indicated that the oscillations in current must have some physical basis. The period of oscillation (i.e., frequency) can be tuned over a wide range either by changing the sample temperature, applying bias voltage or varying concentration of BM1-capped CdS QDs. Current oscillations reported in this paper were possibly related to the formation of charge density waves in the PMMA films embedded with BM1-capped CdS QDs. The hysteresis phenomenon is related to current oscillations in the system. Such polymer nanocomposites are expected to be useful for various nanotechnology-based devices.

Acknowledgements

Bipul Biswas thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing financial assistance in the form of a NET CSIR fellowship (09/080(0628)/2008/EMR-I). For this work the TEM facility at the Nanoscience Project (DST) of IACS was used.

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Footnotes

† Current affiliation: Department of Physics, National Institute of Technology, Silchar, P.O.-REC, Silchar-788 010, Assam, India.

‡ Former senior professor. E-mail: E-mail: spbm07@gmail.com; ; spbm@iacs.res.in

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