Size-dependent visible absorption and fast photoluminescence decay dynamics from freestanding strained silicon nanocrystals

In this article, we report on the visible absorption, photoluminescence (PL), and fast PL decay dynamics from freestanding Si nanocrystals (NCs) that are anisotropically strained. Direct evidence of strain-induced dislocations is shown from high-resolution transmission electron microscopy images. Si NCs with sizes in the range of approximately 5-40 nm show size-dependent visible absorption in the range of 575-722 nm, while NCs of average size <10 nm exhibit strong PL emission at 580-585 nm. The PL decay shows an exponential decay in the nanosecond time scale. The Raman scattering studies show non-monotonic shift of the TO phonon modes as a function of size because of competing effect of strain and phonon confinement. Our studies rule out the influence of defects in the PL emission, and we propose that owing to the combined effect of strain and quantum confinement, the strained Si NCs exhibit direct band gap-like behavior

Second Harmonic Generation in ZnO Nanowires

Second harmonic generation (SHG) is one of the most researched nonlinear material properties and finds applications in many fields ranging from laser projection to cancer detection to future optical switches for molecular devices. Studying SHG in ZnO nanostructures started few years ago and there is a long way to go to compete with the existing nonlinear crystals. Information gathered over the past few years in research on SHG of ZnO nanowires (NWs) is summarized in this chapter. Recent advancement in the growth techniques for various types of ZnO NWs used for SHG studies is also discussed. We present an extensive analysis and discussion on some key parameters that directly modify the efficiency of SHG in ZnO NWs. The key parameters considered for discussion are aspect ratio of NWs, doping, and external strain. At the end, current standing on the reported values of nonlinear coefficients and future outlook are presented

Bioimpedimetric analysis in conjunction with growth dynamics to differentiate aggressiveness of cancer cells

Determination of cancer aggressiveness is mainly assessed in tissues by looking at the grade of cancer. There is a lack of specific method to determine aggressiveness of cancer cells in vitro. In our present work, we have proposed a bio-impedance based non-invasive method to differentiate aggressive property of two breast cancer cell lines. Real-time impedance analysis of MCF-7 (less aggressive) and MDA-MB-231 cells (more aggressive) demonstrated unique growth pattern. Detailed slope-analysis of impedance curves at different growth phases showed that MDA-MB-231 had higher proliferation rate and intrinsic resistance to cell death, when allowed to grow in nutrient and space limiting conditions. This intrinsic nature of death resistance of MDA-MB-231 was due to modulation and elongation of filopodia, which was also observed during scanning electron microscopy. Results were also similar when validated by cell cycle analysis. Additionally, wavelet based analysis was used to demonstrate that MCF-7 had lesser micromotion based cellular activity, when compared with MDA-MB-231. Combined together, we hypothesize that analysis of growth rate, death resistance and cellular energy, through bioimpedance based analysis can be used to determine and compare aggressiveness of multiple cancer cell lines. This further opens avenues for extrapolation of present work to human tumor tissue samples

Photoconductive Laser Spectroscopy as a Method to Enhance Defect Spectral Signatures in Amorphous Oxide Semiconductor Thin-film Transistors

Defects in semiconductor thin-films often leave optical spectral signatures that can be used for their identification. In this letter, we report on spectrally resolved photoconductivity measurements of amorphous oxide semiconductor thin-film transistors. In contrast to previously reported photoconductive spectroscopy measurements recorded using spectrally filtered broadband light sources, we used a wavelength tunable picosecond laser to illuminate the thin-film. We extracted the absorption coeficient as a function of wavelength from the photocurrent measurement and showed that it followed the typical characteristic behaviour previously reported for amorphous oxide semiconductor thin-films. However, in addition, we observed several sharp spectral peaks in the photoconductivity spectrum which can be associated with sub-bandgap defects. These enhanced peaks are not normally visible in previously reported photoconductivity spectra. Furthermore, we show that we can control the sensitivity of our measurement by changing the applied gate bias voltage when the thin-films were fabricated into transistors. The enhancement achieved by using the wavelength tunable laser makes this a particularly sensitive characterisation tool and can additionally be used to discriminate between defects which have been incorporated after device fabrication

Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

Abstract: We report on the appearance of a strong persistent photoconductivity (PPC) and conductor-like behaviour in zinc tin oxide (ZTO) thinfilm phototransistors. The active ZTO channel layer was prepared by remote plasma reactive sputtering and possesses an amorphous structure. Under sub-bandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~ 10−4 A (a photo-to-dark current ratio of ~ 107) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits strong PPC with long-lasting recovery time, which leads the appearance of the conductor-like behaviour in ZTO semiconductor. In the present case, the conductivity changes over six orders of magnitude, from ~ 10−7 to 0.92/Ω/cm. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep states and tail states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-bandgap tail states of the ZTO in the strong PPC, while deep states contribute to mild PPC

Photoconductive laser spectroscopy as a method to enhance defect spectral signatures in amorphous oxide semiconductor thin- film transistors

Defects in semiconductor thin-films often leave optical spectral signatures that can be used for their identification. In this letter, we report on spectrally resolved photoconductivity measurements of amorphous oxide semiconductor thin-film transistors. In contrast to previously reported photoconductive spectroscopy measurements recorded using spectrally filtered broadband light sources, we used a wavelength tunable picosecond laser to illuminate the thin-film. We extracted the absorption coefficient as a function of wavelength from the photocurrent measurement and showed that it followed the typical characteristic behaviour previously reported for amorphous oxide semiconductor thin-films. However, in addition, we observed several sharp spectral peaks in the photoconductivity spectrum which can be associated with sub-bandgap defects. These enhanced peaks are not normally visible in previously reported photoconductivity spectra. Furthermore, we show that we can control the sensitivity of our measurement by changing the applied gate bias voltage when the thin-films were fabricated into transistors. The enhancement achieved by using the wavelength tunable laser makes this a particularly sensitive characterisation tool and can additionally be used to discriminate between defects which have been incorporated after device fabrication

Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires

We report on the major improvement in UV photosensitivity and faster photoresponse from vertically aligned ZnO nanowires (NWs) by means of rapid thermal annealing (RTA). The ZnO NWs were grown by vapor-liquid-solid method and subsequently RTA treated at 700°C and 800°C for 120 s. The UV photosensitivity (photo-to-dark current ratio) is 4.5 × 103 for the as-grown NWs and after RTA treatment it is enhanced by a factor of five. The photocurrent (PC) spectra of the as-grown and RTA-treated NWs show a strong peak in the UV region and two other relatively weak peaks in the visible region. The photoresponse measurement shows a bi-exponential growth and bi-exponential decay of the PC from as-grown as well as RTA-treated ZnO NWs. The growth and decay time constants are reduced after the RTA treatment indicating a faster photoresponse. The dark current-voltage characteristics clearly show the presence of surface defects-related trap centers on the as-grown ZnO NWs and after RTA treatment it is significantly reduced. The RTA processing diminishes the surface defect-related trap centers and modifies the surface of the ZnO NWs, resulting in enhanced PC and faster photoresponse. These results demonstrated the effectiveness of RTA processing for achieving improved photosensitivity of ZnO NWs

Size-dependent visible absorption and fast photoluminescence decay dynamics from freestanding strained silicon nanocrystals

Abstract In this article, we report on the visible absorption, photoluminescence (PL), and fast PL decay dynamics from freestanding Si nanocrystals (NCs) that are anisotropically strained. Direct evidence of strain-induced dislocations is shown from high-resolution transmission electron microscopy images. Si NCs with sizes in the range of approximately 5-40 nm show size-dependent visible absorption in the range of 575-722 nm, while NCs of average size &lt;10 nm exhibit strong PL emission at 580-585 nm. The PL decay shows an exponential decay in the nanosecond time scale. The Raman scattering studies show non-monotonic shift of the TO phonon modes as a function of size because of competing effect of strain and phonon confinement. Our studies rule out the influence of defects in the PL emission, and we propose that owing to the combined effect of strain and quantum confinement, the strained Si NCs exhibit direct band gap-like behavior.</p