Negative differential resistance and photovoltaic phenomena observed in nanostructured organic heterojunction.

Single layer organic photovoltaic devices based on copper phthalocyanine (CuPc) sandwiched between indium tin oxide (ITO) coated glass substrate and aluminum (Al) electrode have been fabricated. The interface dipoles formed at the metal/organic interface play a key role in determining the barrier for charge (electron or hole) injection between the metal electrode and significantly affects the efficiency of organic based electronic and optoelectronic devices. The origin of interface dipoles formed at the metal/organic (Al/CuPc) interface is assumed to be the charge transfer process between the organics (CuPc) and the metal (Al) electrode. Such a device shows forward rectifying property under dark condition when ITO kept at positive bias and Al kept at negative bias. Negative differential resistance (NDR) effect has been observed at high reverse bias voltage. Under white light excitation the device shows reverse rectifying property with a high open circuit voltage. It appears that the interface dipoles formed at the Al/CuPc junction act as exciton dissociation centers. NDR effect can be explained due to charge accumulation layer at the AI/CuPc interface and hence interface dipoles and/or band bending in CuPc. Asymmetry in dark current-voltage (I-V) characteristics has been explained due to strengthening or weakening the dipole field on the application of reverse bias or forward bias at the junction interface, respectively. Various photovoltaic parameters have been calculated from the I-V characteristics of the devices under illumination through ITO electrode.

Observation of electrical switching, reverse rectification and hysteresis in nanostructured organic-organic heterojunction.

Nanostructured organic-organic (O-O) heterojunction was fabricated by using the thin films of a hole transporting material, copper phthalocyanine (CuPc) and an electron transporting material, copper hexadecafluoro-phthalocyanine (F16CuPc). The nanostructured thin films were characterized by optical absorption spectra, FESEM, AFM, X-ray diffraction, etc. Grain size of CuPc and F16CuPc on the substrate surface was different. XRD analysis shows that the crystallinity of the double layer films/heterojunction decreases as compared to the single layer film. The heterojunction sandwich structure ITO/F16CuPc/CuPc/Al, in the present study has shown a good diode like current-voltage (I-V) characteristics with reverse rectifying characteristics. In addition, electrical switching and hysteresis phenomena have also been observed in both sides of the voltage polarities. Interestingly, the single layer sandwich structure of the type ITO/CuPc/Al and ITO/F16CuPc/Al did not show any noticeable electrical switching and hysteresis in I-V characteristics as compared to double layer heterostructure. The reverse rectification has been explained on the basis of band bending due to the accumulation of charge carriers near the junction and the electrical switching has been explained considering the charge carriers trapping and detrapping at the O-O interface.

Controlled surface trap state photoluminescence from CdS QDs impregnated in poly(methyl methacrylate).

A method of microwave (MW) assisted synthesis was employed to prepare cadmium sulfide (CdS) quantum dots (QDs) in dimethylformamide in the presence of poly(methyl methacrylate) (PMMA). The MW irradiation was carried out for a fixed time of 20-30 s and the size of QDs varied from 2.9-5.5 nm. Before each irradiation the solution was cooled down to ambient temperature and the irradiation process was repeated six times. An increase in the intensity and red shift of the characteristic UV-vis absorption peak originating from CdS QDs were observed with repeated MW irradiation, suggesting that the amount of generated CdS QDs increased within the PMMA network and aggregated with repeated MW irradiation. MW irradiation could influence selectively the nucleation and growing rates of PMMA-CdS QDs systems. The broadness and large Stokes shift of the emission from Cd(2+)-rich PMMA-CdS QDs was due to the surface trap state photoluminescence. The recombination of shallow trapped electrons and shallow trapped holes has been considered as the primary source of the surface trap state photoluminescence in Cd(2+)-rich PMMA-CdS QDs. The photoluminescence lifetime was observed to be decreased sharply when the amount of QDs was less, showing the emission decay was dependent on the surface property of PMMA-CdS QDs. The origin of the longer lifetime was due to the involvement of surface trap states and dependent on the amount of CdS QDs present within PMMA and its environment. The effect of the concentration of Cd(2+), S(2-) and PMMA on the generation of CdS QDs within PMMA and the effect of repeated MW irradiation on the optical properties was studied and the results are discussed in this article.

Spectroscopic Studies on the Effect of Some Ferrocene Derivatives in the Formation of Silver Nanoparticles.

Silver nanoparticles were prepared by microwave assisted method using silver nitrate as precursor in the presence of some ferrocene derivatives. The formation of the silver nanoparticles was monitored using UV-Vis spectroscopy. The UV-Vis spectroscopy revealed the formation of silver nanoparticles by exhibiting typical surface plasmon absorption band. The position of plasmon band (406-429 nm) was observed to depend on the nature of a particular ferrocene derivative used. TEM images indicated that the nanoparticles were spherical in shape and well-dispersed. Quantum dots (3.2 nm) were prepared by using ferrocenecarboxylic acid. The surface plasmon absorption band has shown red shift with increasing concentration of ferrocene derivative. For different duration of microwave heating time, intensity of absorption spectra in general was found to increase except in presence of ferrocene carbaldehyde where it decreased. Time-dependent spectra have indicated almost stable position of the surface plasmon band with increasing time of observation confirming that the as prepared silver nanoparticles did not aggregate with lapse of time.

Fluctuations in absorbance observed for photoexcited ferrocene-doped poly(methyl methacrylate) thin films containing chloroform molecules as impurities.

The time-dependent changes in absorbance in the case of ferrocene-doped poly(methyl methacrylate) thin films containing chloroform molecules have been investigated after photoexcitation in nitrogen atmosphere. Photoexcitations have been made by the monochromatic light (using Xe-source and a monochromator) in the UV range. An increase in absorbance associated with spectacular fluctuations in its value, has been noticed. The nature of the variation in the fluctuations has been studied as a function of time (duration) of photoexcitation, photoexcitation wavelength and concentration of ferrocene in the film. The observed results have been discussed considering the role of secondary reaction after photoexcitation as well as the diffusion of small chemical species in the polymer matrix.

Photooxidation of some metallocenes in a polymer matrix.

The changes in the electronic absorption spectra (UV-Vis) of some metallocene-doped poly(methyl methacrylate) (PMMA) thin films containing chloroform molecules as impurities were studied after photoexcitation in the nitrogen atmosphere. Photoexcitations were made by monochromatic radiation (using a Xe-lamp source and a monochromator) at an interval of few nanometers in the spectral range 210-750 nm. The changes in spectra were studied as a function of photoexcitation time (duration), amount of metallocene in the film and the amount of chloroform molecules present in the film. Occurrence of photoinduced charge-transfer between some metallocenes and chloroform molecules confined in the PMMA thin films was observed, which indicated photooxidation of the metallocenes in the polymer matrix. Photoresponse in the case of ferrocene derivatives was observed to decrease with the increase in the value of para- Hammett constant for the substituent attached to the ferrocene unit and also with increasing half-wave potential for the ferrocene derivatives. Photoeffects on the metallocenes having different central metal atom were studied and it was noticed that the photoeffects on the metallocenes with "18 valence electrons", as in ferrocene and ruthenocene, favored the occurrence of photoinduced charge-transfer between the metallocene and chloroform molecules present in a PMMA film. The photooxidation of a metallocene in a PMMA thin film resulted in an enhanced photoconductivity of the polymeric film.

Spectroscopic studies on the photoeffects on some metallocenes in the presence of chloroform molecules confined in poly(methyl methacrylate) thin films.

The changes in the electronic absorption spectra (UV-Vis) (after photoexcitation) of poly(methyl methacrylate) (PMMA) thin films doped with a metallocene and containing chloroform molecules as impurities, have been studied as a function of photoexcitation wavelength (210-750 nm), photoexcitation time (duration), amount of metallocene in the film and amount of chloroform molecules present in the film. Photoeffects on the metallocenes have been observed, which depend significantly on the nature of the central metal atom of the metallocene and the results have been discussed on the basis of the electronic configuration of the metallocenes, which is under study.

Synthesis of thiol capped CdS nanocrystallites using microwave irradiation and studies on their steady state and time resolved photoluminescence.

Synthesis of cadmium sulfide (CdS) nanocrystallites has been performed through the microwave (MW) assisted reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF) in the presence of two capping agents, 1-butanethiol and 2-mercaptoethanol. Attempts were made to control the size and size distribution of the thiol capped CdS nanocrystallites by controlling the number of MW irradiations/exposures for a fixed time (duration). The prepared nanocrystallites have been characterized by UV-vis spectroscopy, FTIR, XRD, FESEM and TEM. The peak position of the absorption band of the 1-butanethiol caped CdS nanocrystals in DMF solution shifted towards longer wavelength with the increasing number of MW exposures indicating the growth of particle size. In contrast, the peak position of absorption band for the 2-mercaptoethanol capped CdS nanocrystals remained nearly at the same wavelength and only the intensity of the absorption band increased with the increasing number of MW exposures. The observed steady state photoluminescence (visible range) of the 1-butanethiol capped CdS nanocrystals in DMF solution shifted towards higher wavelength, showing a decrease in intensity, with the increase in the number of MW exposures. Whereas in the case of 2-mercaptoethanol capped CdS nanocrystals in DMF solution, the photoluminescence peak remained nearly at the same position showing a decrease in intensity with increase in the number of MW exposures. The interesting results on the size-dependent steady state and time resolved photoluminescence (PL) of the CdS nanocrystallites are discussed in the present article. Possible application of such studies in the area of biotechnology has been mentioned.

Time-dependent spectral changes of some ferrocene derivatives in halocarbon solvents after removal of photoexcitation.

The time-dependent changes in the electronic absorption spectra (UV-vis) of some ferrocene derivatives in the halocarbon solvents chloroform and carbontetrachloride have been investigated after removal of the photoexcitation by monochromatic radiation (UV) in nitrogen atmosphere. The spectral responses with time after removal of photoexcitations are entirely different for the two solvents. Except for a little exception, it has been observed that in general, in the case of the ferrocene derivatives under study the absorbance of new bands appeared due to photoexcitation increases in chloroform and decreases in carbontetrachloride with time after removal of photoexcitation. Our previous study has indicated the photoinduced charge-transfer between a particular ferrocene derivative and halocarbon solvent. The results of this present study indicate that some sort of secondary thermal reaction continued even after the removal of photoexcitation and the secondary thermal reaction depends significantly on the ferrocene derivative and also the nature of the halocarbon solvent used.

Power spectral density analysis and photoconducting behavior in copper(II) phthalocyanine nanostructured thin films.

Surface morphology of copper(II) phthalocyanine (CuPc) nanostructured thin films deposited on polished silicon surface was characterized by using atomic force microscopy (AFM). Characteristic topographic parameters like fractals contribute substantially to the thin film morphology, which directly or indirectly influences the physical and optical properties. Fractal geometry and scaling concepts can concisely as well as more effectively describe the complex rough surface morphology. The power values of power spectral density (PSD) for the AFM digital data were determined by the fast Fourier transform (FFT) algorithms instead of the root-mean-square (rms) and peak-to-valley value. The PSD provides a more reliable description to the topography than the rms roughness and imparts some useful information of the surface, including fractal contributions. Fractal analyses, including area-perimeter and PSD methods, have been used to evaluate surface morphology of a vacuum deposited CuPc thin film surface formed under various film deposition temperatures. Film deposition temperature has affected the microstructural and related morphological evolutions very differently. The PSD plots are successfully approximated by the k-correlation model. The film growth is interpreted by the bulk and surface diffusion using parameter C of the k-correlation model. The dark and photoconductivity of CuPc nanostructures in Si/CuPc/Ag configuration at different temperatures have been studied and their changes with roughness and temperature have been discussed. A significantly higher value of the rectification ratio (RR) and photoswitching behavior in such configuration have been noticed.

Nanostructured organic-inorganic photodiodes with high rectification ratio.

High quality organic-inorganic heterojunction photodiodes based on nanostructured copper (II) phthalocyanine (CuPc) and intrinsic zinc oxide (i-ZnO) have been fabricated. The i-ZnO thin films/layers were grown by RF magnetron sputtering on clean indium tin oxide (ITO) coated glass substrates. These films have been characterized by optical absorption and field emission scanning electron microscopy (FESEM). CuPc thin films deposited at room temperature on i-ZnO have exhibited a change in their surface morphology with the post-deposition annealing temperature under normal atmosphere. The electrical dark conductivity and the photoconductivity of ITO/i-ZnO/CuPc/Au sandwich structures have been measured under various photoexcitation intensities using a xenon light source. The devices have shown excellent reproducibility of their electrical characteristics and high rectification ratios. The highest rectification ratio is nearly 831 calculated above the threshold voltage at room temperature for the sample annealed at 250 °C (i.e. Pc 250). The effects of the annealing temperature of CuPc on the surface morphology, rectification ratio, and optical properties have been discussed.

A two-dimensional coordination compound as a zinc ion selective luminescent probe for biological applications.

A 2D coordination compound {[Cu2(HL)(N3)]ClO4}infinity (1; H3L = 2,6-bis(hydroxyethyliminoethyl)-4-methyl phenol) was synthesized and characterized by single-crystal X-ray diffraction to be a polymer in the crystalline state. Each [Cu2(HL)(N3)]+ species is connected to its adjacent unit by a bridging alkoxide oxygen atom of the ligand to form a helical propagation along the crystallographic a axis. The adjacent helical frameworks are connected by a ligand alcoholic oxygen atom along the crystallographic b axis to produce pleated 2D sheets. In solution, 1 dissociates into [Cu2(HL)2(H3L)]2H2O (2); the monomer displays high selectivity for Zn2+ and can be used in HEPES buffer (pH 7.4) as a zinc ion selective luminescent probe for biological application. The system shows a nearly 19-fold Zn2+-selective chelation-enhanced fluorescence response in the working buffer. Application of 2 to cultured living cells (B16F10 mouse melanoma and A375 human melanoma) and rat hippocampal slices was also studied by fluorescence microscopy.