Sanjit Sarkar

One-step nano-engineering of dispersed Ag–ZnO nanoparticles' hybrid in reduced graphene oxide matrix and its superior photocatalytic property

Reduced graphene oxide (rGO) has been engineered with a hybrid of dispersed Ag and ZnO nanoparticles by a one-step hydrothermal technique using graphene oxide, AgNO3 and Zn(CH3COO)2 precursors without adding any external toxic reagent. Formation of the Ag–ZnO–rGO hybrid is confirmed from X-ray diffractometry, high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The hybrid shows an enhanced and faster ultraviolet photocatalytic property i.e. 97.2% degradation of methyl orange in 90 minutes as compared to the values of 56%, 84% and 60% by bare ZnO, Ag–ZnO and ZnO–rGO, respectively. The enhanced photocatalytic property is due to an efficient charge transfer process from ZnO to both Ag and rGO. This result would be beneficial for synthesizing efficient ZnO-based ternary photocatalysts with different combinations of metal and graphene, and understanding their photocatalytic performance in photoreaction processes.

Self Powered Highly Enhanced Dual Wavelength ZnO@CdS Core–Shell Nanorod Arrays Photodetector: An Intelligent Pair

On the face of the impending energy crisis, developing low-energy or even zero-energy photoelectronic devices is extremely important. A multispectral photosensitivity feature of a self-powered device provides an additional powerful tool. We have developed an unprecedented high performance dual wavelength self-powered ZnO@CdS/PEDOT:PSS core-shell nanorods array photodetector through a simple aqueous chemical method wherein a suitable band alignment between an intelligent material pair, i.e. ZnO and CdS, has been utilized. Besides a noteworthy advantage of the devices being that they show a very sharp and prominent dual wavelength photosensitivity, both the ultraviolet and visible light sensitivity (ratio of current under illumination (Iphoto)/current under dark (Idark)) of the device are two orders of higher magnitude than those of pristine ZnO, attaining values of 2.8 × 10(3) and 1.07 × 10(3), respectively. At the same time, temporal responses faster than 20 ms could be achieved with these solution-processed photodetectors. The present study provides a very important direction to engineer core-shell nanostructured devices for dual wavelength high photosensitivity.

Defect controlled ultra high ultraviolet photocurrent gain in Cu-doped ZnO nanorod arrays: De-trapping yield

Understanding the mechanism behind high photocurrent gain is very important to realize a highly functional material for photodetector devices. Herein, we report a very high ultraviolet photocurrent gain of 2.8 × 105 in hydrothermally grown Cu-doped ZnO nanorod arrays which is two orders of magnitude higher as compared to the undoped sample. Trapping of carriers under dark and de-trapping them under illumination by Cu-related defects is responsible for high gain. The trap state at ∼1.65 eV is attributed to the [{CuZn+(3d10)}− − Zni+(4s1)]0 type of defects. A model is shown to explain the dark and photocurrent states in the doped samples.

A Novel Spectroscopic Method for the Investigation of Structural Phase Transitions in Molecular Crystals

Abstract A simple experimental technique for the investigation of structural phase transitions in molecular crystals in the temperature range 77-300 K has been developed and applied to a wide variety of organic molecular crystals. The ‘Photokinematical Approach’, as the method has been termed, utilizes the responsiveness of photoluminescence of guest aromatic ketone molecules to the changes in crystalline structure of host molecular crystals as sensitive probe for the identification of structural phase transformations in host molecular crystals. In this method the prediction of the number of distinct polymorphs a given molecular crystal may exist, their corresponding optimum temperatures of existence and the transition temperatures is based on the observation of characteristic changes in the luminescence intensity of the guest ketone molecules that occur in response to host crystalline modifications. The reliability of the method has been established from the conformity and reproducibility of the results ...

On the origin of some anomalous luminescence behavior of isomeric cyanopyridines

The results of a comparative study of the absorption and emission properties and the triplet yields of the isomeric cyanopyridines (CNP) and benzonitrile under various environmental conditions are reported. All the three CNP isomers are found to be nonfluorescent but exhibit a fairly strong phosphorescence emission of π, π character in rigid polar media at 77 K. ΦP values of the CNP are found to be sensitively dependent on the rigidity (or viscosity) and crystalline structure of the solvent medium. Whereas the 3‐CNP molecule phosphoresces with moderately strong intensity in hydrocarbon solvents, the 2 and 4 isomers are completely nonluminescent in glassy or highly disordered crystalline phases of hydrocarbons, e.g., n‐alkanes, cyclohexane (CH), and methylcyclohexane (MCH). A dramatic reappearance of the phosphorescence emission from these two isomers (with anomalously large red‐shifted spectral positions) has, however, been observed in crystalline CH and MCH matrices. Unlike the 3 isomer, both the 2‐ and ...

A successive photocurrent transient study to probe the sub-band gap electron and hole traps in ZnO nanorods

Though persistent photoconductivity and spectral response characteristics corresponding to sub-band gap energies of ZnO as a result of different defects are certainly problematic, study on this is quite useful in the sense that it provides information about the defects levels. A developed understanding of the energetic distributions of sub-band gap electron and hole traps in ZnO nanorods is pre-requisite for device applications and needs to be acquired. Through a study on the simple successive photocurrent transients of as-grown and annealed aqueous chemically grown ZnO nanorods, we have revealed the sub-band gap electron and hole traps responsible for their visible photoresponse. The evolutions in the room temperature photoluminescence properties of ZnO nanorods with annealing temperature have also been investigated critically to correlate the corresponding photocurrent results. As-grown samples show fewer excess O related electron trap states with energy corresponds to (Ec − 1.77 eV). A greater amount of O vacancy related hole traps, as well as fewer O interstitial related electron traps with energy corresponds to (Ec − 2.34 eV) were detected in the as-grown as compared to the annealed samples. These findings also provide general guidelines for probing sub-band gap traps, and engineering the optoelectronic properties of similar oxide semiconducting nanoparticles.

Phosphorescence enhancement in diphenyl ether in rigid ethanol glass at 77 K

Abstract The phosphorescence yield of diphenyl ether increases markedly relative to phenol in ethanol glass at 77 K. Analysis of the results reveals that the phosphorescence enhancement is attributable to increased spin—orbit coupling arising from the non-planarity of the lone-pair orbital of the oxygen atom in diphenyl ether.

Excited state charge-transfer effect on the phosphorescence emissions of 4-aminopyridine at 77 K

Absorption and phosphorescence spectral characteristics of 4-aminopyridine (AMP) have been studied in polar and non-polar solvents at 77 K. Absorption data provide evidence that the excited singlet π, π * states of the molecule possess substantial charge-transfer (CT) character. The 4-AMP molecule is nonfluorescent in all the solvents used in the investigation but phosphoresces strongly in ethanol glass at 77 K. The phosphorescence emissions have a lifetime and negative polarization characteristic of π,π * origin. From the observations that in acidic ethanol glassy medium the phosphorescence quantum yield somewhat decreases and the lifetime moderately increases as compared to that in neutral ethanol glass but the negative polarization persists, it has been concluded that all the coupling schemes π 1 , π * (CT) SO ¯ π 3 , π * , π 1 , π * SO ¯ n 3 , π * vib ¯ π 3 , π * and π 1 , π * vib ¯ n 1 , π * SO ¯ π 3 , π * are responsible for the phosphorescence characteristics of 4-AMP molecule. The substantial reductions in the values of the phosphorescence quantum yield and lifetime of 4-AMP in non-polar rigid matrix at 77 K have been attributed to enhanced nonradiative process arising from increased vibronic interactions between the closely spaced 3 n,π * and 3 π,π * states of the molecule in such environment.

Defect mediated highly enhanced ultraviolet emission in P-doped ZnO nanorods

P-doped ZnO one-dimensional (1D) nanorods (NRs), grown by simple hydrothermal method, show highly enhanced ultraviolet photoluminescence (UV PL) property as compared to the undoped ZnO sample. A detailed in-depth understanding on the mechanism behind the enhancement has been conducted by performing experimental analyses on power dependence of and annealing effect on the emissions, low temperature PL (LTPL), and electrical I–V characteristics of various P-doped ZnO NR samples. The doped NR samples retain hexagonal wurtzite structure and similar morphology as that of the undoped one. The UV/vis PL intensity ratio becomes 9 for as-grown sample and 22 for annealed 2% P-doped sample, which is one order higher than that of the undoped ZnO sample. Correlation of the LTPL and electrical I–V characteristics reveals that the high enhancement in the UVPL is due to recombination via shallow acceptor complex defects of PZn–2VZn type.

Effect of ring aza substitution on the luminescence of aromatic ketones. Luminescence of isomeric acetylpyridines

The absorption and luminescence characteristics of 2-,3-and 4-acetyl-pyridines (ACP) and acetophenone have been examined in polar and nonpolar media at room temperature and low temperature (77K). All the ACP exhibit intense and structured phosphorescence emission ofnπ* character. Though the triplet yields of these ketones are very high, the phosphorescence quantum yield and lifetime of ACP decrease significantly in the order 4-ACP>3-ACP>2-ACP> acetophenone. The distinctive differences in the spectral properties and the luminescence characteristics of ACP from their phenylanalogue have been interpreted in terms of the perturbing influences of the inductive effect of the pyridinic nitrogen, the vibronic interaction between thenπ* andππ* states and the photochemical reaction occurring from the lowest triplet state.