Defective Graphene/Plasmonic Nanoparticle Hybrids for Surface-Enhanced Raman Scattering Sensors.

Two-dimensional-zero-dimensional plasmonic hybrids involving defective graphene and transition metals (DGR-TM) have drawn significant interest due to their near-field plasmonic effects in the wide range of the UV-vis-NIR spectrum. In the present work, we carried out extensive investigations on resonance Raman spectroscopy (RRS) and localized surface plasmon resonance (LSPR) from the various DGR-TM hybrids (Au, Ag, and Cu) using micro-Raman, spatial Raman mapping analysis, high-resolution transmission electron microscopy (HRTEM), and LSPR absorption measurements on defective CVD graphene layers. Further, electric field (E) mappings of samples were calculated using the finite domain time difference (FDTD) method to support the experimental findings. The spatial distribution of various in-plane and edge defects and defect-mediated interaction of plasmonic nanoparticles (NPs) with graphene were investigated on the basis of the RRS and LSPR and correlated with the quantitative analysis from HRTEM, excitation wavelength-dependent micro-Raman, and E-field enhancement features of defective graphene and defective graphene-Au hybrids before and after rapid thermal annealing (RTA). Excitation wavelength-dependent surface-enhanced Raman scattering (SERS) and LSPR-induced broadband absorption from DGR-Au plasmonic hybrids reveal the electron and phonon interaction on the graphene surface, which leads to the charge transfer from TM NPs to graphene. This is believed to be responsible for the reduction in the SERS signal, which was observed from the wavelength-dependent Raman spectroscopy/mappings. We implemented defective graphene and DGR-Au plasmonic hybrids as efficient SERS sensors to detect the Fluorescein and Rhodamine 6G molecules with a detection limit down to 10-9 M. Defective graphene and Au plasmonic hybrids showed an impressive Raman enhancement in the order of 108, which is significant for its practical application.

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

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.

Strain dependence of the nonlinear optical properties of strained Si nanoparticles.

We report on the lattice strain dependence of the nonlinear optical (NLO) parameters of strained Si nanoparticles (NPs), which are prepared in a controlled way by a mechanical ball milling process. X-ray diffraction analysis shows that the nature of strain is compressive and is primarily caused by milling-induced lattice dislocations, which is further supported by high-resolution transmission electron microscopy imaging. It is found that the nonlinear refractive index (n2) and nonlinear absorption coefficient (ß) are strongly influenced by the associated lattice strain present in Si NPs. With the increase of lattice strain, the ß gradually decreases while n2 increases slowly. The strain-dependent observed changes in the NLO parameters of Si NPs are found to be advantageous for application purpose, and it is explained on the basis of strain-induced modification in the electronic structure of the highest occupied molecular orbital and lowest unoccupied molecular orbital states of Si NPs. These results demonstrate the potential of strain-dependent enhancement of nonlinearities for silicon photonics applications.

Europium doping induced symmetry deviation and its impact on the second harmonic generation of doped ZnO nanowires.

In this work, we investigated the effects of europium doping on the second harmonic generation (SHG) of ZnO nanowires (NWs). A non-monotonic enhancement in the SHG is observed with the increase of the europium concentration. Maximum SHG is observed from the 1 at.% europium doped ZnO NWs with an enhancement factor of 4.5. To understand the underlying mechanism, the effective second order non-linear coefficient (deff) is calculated from the theoretical fitting with consideration of the absorption effect. Microstructural characterization reveals the structural deformation of the ZnO NWs caused by europium doping. We estimated the deviation in the crystal site symmetry around the Eu(3+) ions (defined as the asymmetric factor) from photoluminescence measurement and it is found to be strongly correlated with the calculated deff value. A strong linear dependence between the magnitudes of deff and the asymmetric factor suggests that deviation in the local site symmetry of the ZnO crystal by europium doping could be the most probable origin of the observed large second order non-linearity.

Graphene-assisted controlled growth of highly aligned ZnO nanorods and nanoribbons: growth mechanism and photoluminescence properties.

We demonstrate graphene-assisted controlled fabrication of various ZnO 1D nanostructures on the SiO2/graphene substrate at a low temperature (540 °C) and elucidate the growth mechanism. Monolayer and a few layer graphene prepared by chemical vapor deposition (CVD) and subsequently coated with a thin Au layer followed by rapid thermal annealing is shown to result in highly aligned wurtzite ZnO nanorods (NRs) with clear hexagonal facets. On the other hand, direct growth on CVD graphene without a Au catalyst layer resulted in a randomly oriented growth of dense ZnO nanoribbons (NRBs). The role of in-plane defects and preferential clustering of Au atoms on the defect sites of graphene on the growth of highly aligned ZnO NRs/nanowires (NWs) on graphene was established from micro-Raman and high-resolution transmission electron microscopy analyses. Further, we demonstrate strong UV and visible photoluminescence (PL) from the as-grown and post-growth annealed ZnO NRs, NWs, and NRBs, and the origin of the PL emission is correlated well with the X-ray photoelectron spectroscopy analysis. Our results hint toward an epitaxial growth of aligned ZnO NRs on graphene by a vapor-liquid-solid mechanism and establish the importance of defect engineering in graphene for controlled fabrication of graphene-semiconductor NW hybrids with improved optoelectronic functionalities.

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.

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.

Size dependent anisotropic strain and optical properties of strained Si nanocrystals.

We report on the growth of strained Si nanocrystals (NCs) of sizes in the range 5-43 nm and analyze the detailed nature of strain and its influence on the optical properties of the NCs as a function of size. Freestanding Si NPs were prepared in a controlled way using a contamination free mechanical ball milling for duration 2-40 hrs. Structural analysis based on X-ray diffraction (XRD) pattern and high resolution transmission electron microscopy (HRTEM) confirms the good crystalline nature of these Si NCs. A detailed analysis of XRD line profile reveals that nature of the strain is anisotropic and the screw type dislocations are the main contributors to the lattice strain. The dislocation density and corresponding strain changes non-monotonically, while the crystallite size changes monotonically with milling time. Direct evidence of dislocations is shown from HRTEM images. The UV-vis-NIR absorption spectra of the Si NCs show an enhanced absorption band in the visible region that shows a systematic blue shift with reduced NC sizes. Si NCs with size approximately 5-10 nm exhibits a distinct photoluminescence (PL) band in the visible region at 580-585 nm at room temperature, while higher size NCs does not exhibit any visible emission. PL excitation measurement shows a very small Stokes shift for the visible emission band indicating no involvement of defects/interface in the emission. We argue that the observed absorption and emission can be explained based on the enhanced confinement effect on the strained Si NCs due to the combined effect of strain and size quantization.