Enhanced oscillator strengths and optical parameters of doped ZnS bulk and nanophosphors

We synthesize and investigate the oscillator-strength (OS), dipole-moment (DM), and integrated cross-section values (ICSVs) of singly (Mn) and doubly (Mn and Co or Ni) doped samples of ZnS bulk and nanophosphors. The oscillator-strength values (OSVs) are found to enhance by three orders of magnitude when Co or Ni dopant is incorporated in ZnS:Mn bulk phosphors, which strongly suggests that a quencher dopant triggered an energy-transfer process in the host ZnS material. Nevertheless, although these quencher dopants were previously considered to kill the luminescence from the host material, we used these dopants in ZnS:Mn to create an additional pathway for the relaxation of the carrier, and to initiate the energy-transfer mechanism. On the other hand, a four orders of magnitude enhancement in the OSVs was observed on incorporating the quencher dopants in ZnS:Mn nanophosphors, which is attributed to the fact that our nanosamples are related to a strong-confinement case while the quencher dopant played a significant role in the variation of OSVs. Moreover, the analysis of OSVs showed that excitonic/defect level emission from ZnS host is due to a weak electric dipole transition (WEDT), while a magnetic dipole transition (MDT) dominates in the case of ZnS:Mn nanophosphors. Based on the present investigations, we clearly obtained an origin of excitonic- and impurity-related emission from the doped ZnS bulk and nanophosphor samples, which were almost vague in the previous studies of other researchers.     

Solvent-assisted optical modulation of FRET-induced fluorescence for efficient conjugated polymer-based DNA detection

The solvent effects were studied in fluorescence resonance energy transfer (FRET) from a cationic polyfluorene copolymer (FHQ, FPQ) to a fluorescein (Fl)-labelled oligonucleotide (ssDNA-Fl). Upon addition of dimethyl sulfoxide (DMSO), the optical properties of polymers and the probe dye were substantially modified and the FRET-induced PL signal was enhanced 3.8-37 times, relative to that in phosphate buffer solution (PBS). The hydrophobic interaction between polymers and ssDNA-Fl is expected to decrease in the presence of DMSO, which induces the weaker polymer/ssDNA-Fl complexation with longer intermolecular donor-acceptor separation and perturbs the competition between the FRET and PL quenching processes such as photo-induced charge transfer. The gradual decrease in Fl PL quenching with increasing the DMSO content was investigated by measuring the Stern-Volmer quenching constants (3.3-4.2 × 10(6) M(-1) in PBS, 0.56-1.1 × 10(6) M(-1) in 80 vol% DMSO) and PL lifetime of the excited Fl* in polymer/ssDNA-Fl (600 ps in PBS and 2120 ps in 80 vol% DMSO for FHQ/ssDNA-Fl) in PBS/DMSO mixtures. The substantially reduced PL quenching would amplify the resulting FRET Fl signal. The signal amplification in real DNA detection was also demonstrated with fluorescein-labelled PNA (probe PNA) in the presence of a complementary target DNA and noncomplementary DNA in aqueous DMSO solutions. This approach suggests a simple way of modifying the fine-structure of polymer/ssDNA-Fl and improving the detection sensitivity in conjugated polymer-based FRET bioassays.     

An alternative method for measurement of charge carrier mobility in semiconductors using photocurrent transient response

We report here a simple alternative method for measuring charge carrier drift mobilities in semiconductor devices. A typical falling photocurrent transient formula for switch-off-state was adjusted to obtain simultaneously electron and hole mobilities. For both undoped ZnO film and InAlAs/InGaAs quantum well structure, electron mobilities extracted from our model were compared with those obtained from maximum-entropy mobility-spectrum analysis method (ME-MSA). Our results demonstrated that electron mobility obtained from our photocurrent response model could serve as substitutes for a representative mobility obtained from ME-MSA.