Batu Ghosh

Color tunable light-emitting diodes based on copper doped semiconducting nanocrystals

We have introduced copper-doped semiconducting nanocrystals in light-emitting diodes (LEDs). Characteristics of the devices show that electroluminescence (EL) emission in these LEDs is color tunable. In copper-doped ZnS nanocrystals in the core and Zn1−xCdxS host as a shell-layer, photoluminescence (PL) arises from a transition from conduction band-edge of the host to 3d-levels of copper-ions. The PL of the nanocrystals and hence the EL of LEDs based on such nanostructures become tunable by varying the Cd-content in Zn-Cd-S alloys, that is, Zn1−xCdxS with different values of x, which changes the conduction band-edge of the host.

Al-doped ZnO nanocrystals: Electronic states through scanning tunneling spectroscopy

We form a monolayer of undoped and Al-doped ZnOnanocrystals and measure tunneling current with a scanning tunneling microscope tip. From the density of states, we determine the location of conduction and valence band edges with respect to the Fermi energy. We show that with n-doping, Fermi energy of ZnOnanoparticles shifts toward the conduction band edge. The difference between the electronic states, that is, the bandgap of the nanocrystals does not change upon doping. This is in agreement with the optical absorption spectra of the nanomaterials. We also find that inhomogeneity of doping in nanoparticles is reflected in density of states. With an increase in doping concentration, the distribution of dopants among particles becomes broader. We characterize the monolayers also with Hgelectrodes to comment on electrical conductivity versus doping concentration behavior.

Transport gap of nanoparticle-passivated silicon substrates.

The transport gap of nanoparticle-passivated Si substrates is measured by scanning tunneling microscopy. Passivation is achieved using a monolayer of CdSe nanoparticles. It is shown that the transport gap and conduction-band edge of the system change upon passivation. The size of the nanoparticles that passivate the Si substrate is varied to study its effect on the transport gap of the system. Plots of the tunneling current versus voltage show that the transport gap of the system can be tuned by the binding of just a monolayer of suitable nanoparticles. From the normalized density of states, it is shown that the conduction-band edge of the system responds to the size of the nanoparticles. Here, a monolayer of the nanoparticles, which were capped with suitable functional groups, has been formed via electrostatic adsorption with the substrate.

Electrical and optical properties of silver core-lithium niobate shell nanostrucures within a glass medium

Silver nanoparticles of diameters in the range of 4.2–46nm have been used to grow nanoshells of lithium niobate with thicknesses in the range of 2.2–22nm in a silicate glass matrix. The lithium niobate shells form a percolative network and show electronic conduction. This has been explained as due to Mott’s variable range hopping conduction mechanism. The hopping is interparticle because the density of localized states is found to be in reasonable agreement with the density of nanoparticles with a core-shell structure. The ac conductivity is shown to be dependent on frequency with the exponent having a value around 0.5. This arises due to the charge motion’s being operative in a two-dimensional structure with one of these dimensions involved in such motion. Optical absorption of the core-shell structured nanoparticles exhibits two peaks—one around 350nm is ascribed to plasmon resonance of silver nanoparticles which are not covered by a niobate shell, and the other in the range of 550–650nm is caused by li...

ac conductivity analysis for a metal core-silver orthosilicate shell nanostructure

Nanocomposites containing silver particles of diameter of 20nm with silver orthosilicate crystals forming the shell with thickness around 21nm closely packed in a silicate glass were prepared. The ac conductivity of samples subjected to different heat treatments were measured over the frequency range of 100Hzto6MHz in the temperature range of 500–570K. The data were analyzed by the Macdonald model based on Kohlrausch-related frequency response formalism designated as CK0. The reference system was taken as the glass-crystal composite containing a lithium orthosilicate crystalline phase. The shape parameter βo for the reference system was found to be 0.33, whereas that for the nanocomposites was extracted to be 0.46. The former implied a one-dimensional lithium ion motion along the grain boundaries of the orthosilicate crystals, whereas the latter indicated that there was a one-dimensional silver ion motion in an effective two-dimensional structure in the shell surface because of a high stress condition alo...

Pulsed Electroluminescence from Quantum Dot Light-Emitting Diodes

We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell layer and characterize the devices under dc and ac voltages. By applying an ac voltage, we achieve pulsed electroluminescence (EL) from these quantum dot LEDs (QDLEDs). As frequency of the applied ac voltage increases, we observe a phase-lag between the pulsed EL and applied sinusoidal voltage. From frequency response of the pulsed emission, we determine −3 dB frequency of the ac-QDLEDs. The frequency response of EL also provides a route to study mechanism of the LEDs. The EL spectra under dc and ac voltages provide information on carrier transport and exciton generation in these devices. From the resemblance between EL and photoluminescence spectra, which appear due to transitions between d-states of Mn-ions in doped nanocrystals, we infer that excitons form directly in the quantum dots in these QDLEDs.

Core–shell nanotubes to enhance electrical bistability for 2-bit memory

We have grown core-shell nanotubes with CNTs in the core and CdS as the shell. The thickness of the shell has been varied by controlling the reaction parameters. In devices based on such core-shell nanotubes, the nanotubes act as carrier-transporting channels to augment charge confinement in the shell. Increasing the density of confined carriers results in enhanced electrical bistability and memory phenomena in CdS. With widely separated low- and high-conducting states, we were able to scale the high-state, so that different high-conducting states could be achieved for multi-level memory applications.