Mohammad Kamal Hossain

Near-field Raman imaging and electromagnetic field confinement in the self-assembled monolayer array of gold nanoparticles.

Spatial distribution of surface enhanced Raman activity is visualized for two-dimensional (2D) nearly close-packed and well-ordered monolayer array of gold nanoparticles by using scanning near-field optical microscope. The 2D arrays exhibit highly nonuniform enhancement in Raman scattering, i.e., the regions along the edge of the 2D array are preferentially enhanced. We demonstrate that the spatial distribution of the localized electric field is also nonuniform and agrees well with that of the Raman enhancement.

A Study of Glutathione Molecules Adsorbed on Silver Surfaces under Different Chemical Environments by Surface-Enhanced Raman Scattering in Combination with the Heat-Induced Sensing Method

In this study, surface-enhanced Raman scattering (SERS) in combination with a heat-induced sensing technique has been applied for investigating molecular orientations of glutathione molecules adsorbed on silver colloidal nanoparticles under different chemical environments, which has enabled us to further study how glutathione molecules are adsorbed on the silver surfaces. Factors that may affect the configurations of glutathione molecules adsorbed on the silver nanocolloids have been investigated. By observing the relative enhancement factors of SERS bands due to individual functional groups contributed from different terminals, the affinity between the different functional groups of glutathione and the silver surfaces under different conditions has been sorted and the orientations of glutathione molecules adsorbed on the silver surfaces have been investigated.

Silver nanoparticles on Zinc Oxide thin film: An insight in fabrication and characterization

In this work, a simple two-steps process has been explained to fabricate silver (Ag) nanoparticles on Zinc Oxide (ZnO) thin film followed by their characterizations. The underneath layer ZnO thin film, as an example, was also investigated how the properties change during the course of nanoparticles fabrication. ZnO thin film was sputtered on standard glass substrate followed by further sputtering of an ultrathin Ag layer. Subsequently the specimen was treated at high temperature in inert environment. A periodic observation at specific temperature intervals confirmed the formation of Ag nanoparticles on ZnO thin film. Field-emission scanning electron microscopic (FESEM) observations revealed the size distribution of as-fabricated Ag nanoparticles in the range of 50-250 nm. Elemental analysis was also confirmed by SEM-aided energy dispersion spectroscopy. The underneath layer ZnO thin film was found to go through recrystallization, stress relaxation, and grain growth during the annealing process. Further treatment to ZnO only film showed a variation in surface topology with reference to those with Ag nanoparticles on ZnO. Such a system was also analysed with finite different time domain (FDTD) analysis. A typical model was considered and FDTD simulation was carried out to understand the trend of absorption depth profile within the absorbing layer involved in plasmonics solar cell.

Near-Field Scanning Optical Microscopy: Single Channel Imaging of Selected Gold Nanoparticles through Two Photon Induced Photoluminescence

Near-field scanning optical microscopy (NSOM) is known to be a technique of choice to investigate nanometric materials and their properties beyond far-field diffraction limit resulting high spatial, spectral and temporal resolution. Here in this report, a state of art facility, aperture-NSOM was used to probe single nanoparticle, dimer, trimer and small nanoaggregate of gold nanoparticles. Shear force topography and two photon induced photoluminescence (TPI-PL) images captured simultaneously by the system facilitated to clarify the correlation between the local geometry and the emitted photon of TPI-PL. Small gold aggregates including trimer showed strong optical confinement of TPI-PL with reference to that of single gold nanoparticles. It was also evident that the interparticle gap does have a great influence in localized electromagnetic (EM) field mediated optical confinement of TPI-PL. Such observations were also supported by finite different time domain (FDTD) analysis keeping the simulation parameter nearly identical to that of experiment. FDTD simulation demonstrated that incident excitation parallel to the interparticle axis induces strong near-field distribution at the interstitials whereas out of plane excitation modifies such confinement depending on the nanometric geometry of the nanoaggregates. Such an observation is indispensable to understand the localized EM field-mediated optical confinement in surface-enhanced spectroscopy.

Nanowires: a new pathway to nanotechnology-based applications

AbstractThe synthesis and the characterisation of silicon nanowires (SiNWs) have recently attracted great attention due to their potential applications in electronics and photonics. As yet, there are no practical uses of nanowires, except for research purposes, but certain properties and characteristics of nanowires look very promising for the future. Graphical abstractSemiconductor nanowires are attracting more and more interest for their applications in nanoscience and nanotechnology. The characteristic of the nanowires is their geometry with a diameter in the range of a few nanometers and a length far greater than their diameter. The structural defects often lead to mechanical defects. By reducing the number of defects per unit length, decreasing the lateral dimensions, crystalline nanowires are expected to be more resistant than the solid. Recently nanowires are attracting intense interest for solar energy conversion. In this review, we summarize the different methods of nanowires production and their applications. Special focus will be kept on silicon nanowires.

Fabrication and optical simulation of vertically aligned silicon nanowires

Silicon nanowires (Si-NWs) have been considered widely as a perfect light absorber with strong evidence of enhanced optical functionalities. Here we report finite-difference time-domain simulations for Si-NWs to elucidate the key factors that determine enhanced light absorption, energy flow behavior, electric field profile, and excitons generation rate distribution. To avoid further complexity, a single Si-NW of cylindrical shape was modeled on c-Si and optimized to elucidate the aforementioned characteristics. Light absorption and energy flow distribution confirmed that Si-NW facilitates to confine photon absorption of several orders of enhancement whereas the energy flow is also distributed along the wire itself. With reference to electric field and excitons generation distribution it was revealed that Si-NW possesses the sites of strongest field distributions compared to those of flat silicon wafer. To realize the potential of Si-NWs-based thin film solar cell, a simple process was adopted to acquire vertically aligned Si-NWs grown on c-Si wafer. Further topographic characterizations were conducted through scanning electron microscope and tunneling electron microscope-coupled energy-dispersive spectroscopy.

PLD Grown Polycrystalline Tungsten Disulphide (WS2) Films

Polycrystalline WS2 films were grown by pulsed laser deposition (PLD) system at relatively low temperature. The main objective of this study is to optimize the growth conditions for polycrystalline WS2 films at relatively low temperature to use them for photovoltaics (PVs). Different growth conditions and substrates are used and examined systematically. It is found out that films grown on strontium titanate SrTiO3 (STO) substrate have the best structural properties when compared to other substrates examined in this work. X-ray diffraction and optical characterizations of these films reveal crystallographic growth and very promising optical properties for PVs. Furthermore, it was observed that higher growth temperature (>300°C) has an unfavorable effect on the layers by creating some tungsten metallic droplets.

Silver Nanoparticles on Zinc Oxide: An Approach to Plasmonic PV solar cell

Efficient light management in solar cells can be achieved by incorporating plasmonic nanoscatterers that support surface plasmons: excitations of conduction electrons at the interface/surface. As known, light trapping increases the amount of light absorbed by bouncing the light within the cell, giving it a chance to be absorbed thereby increasing the absorption and scattering cross-section. The challenge is to fabricate these plasmonic nanoparticles in cost-effective method as well as without hampering optical, electrical and topographical properties of underneath layers. Here in this report a simple two step method was adopted to fabricate silver nanoparticles on zinc oxide followed by topographic and elemental analysis thereof. Numerical calculation was carried out to elucidate optical scattering of silver nanoparticles of various sizes as well as that of dimer. Near-electric field distribution of single silver nanoparticles and dimer along with the individual component of electric field was simulated by finite different time domain analysis. Using the benefit of increased scattering cross-section and ease of such nanoparticles fabrication, a cell configure is proposed herewith.

Metallic quantum dots as sensitizers for solar cells

The concept of using the metallic quantum dots (MQD) as sensitizer in sensitized solar cells is presented. This concept is only possible due to the nano-scale size effects like emerging energy gap, states redistribution, and phonon bottleneck. Also, the initial work to grow silver quantum dots on ZnO film by sputtering and then post annealing is presented and discussed. This growth procedure will be used later to realize the concept device.