Nitrogen ion induced nitridation of Si(111) surface: Energy and fluence dependence

We present the surface modification of Si(111) into silicon nitride by exposure to energetic N₂⁺ ions. In-situ UHV experiments have been performed to optimize the energy and fluence of the N₂⁺ ions to form silicon nitride at room temperature (RT) and characterized in-situ by X-ray photoelectron spectroscopy. We have used N₂⁺ ion beams in the energy range of 0.2–5.0 keV of different fluence to induce surface reactions, which lead to the formation of Si_xN_y on the Si(111) surface. The XPS core level spectra of Si(2p) and N(1s) have been deconvoluted into different oxidation states to extract qualitative information, while survey scans have been used for quantifying of the silicon nitride formation, valence band spectra show that as the N₂⁺ ion fluence increases, there is an increase in the band gap. The secondary electron emission spectra region of photoemission is used to evaluate the change in the work function during the nitridation process. The results show that surface nitridation initially increases rapidly with ion fluence and then saturates.     

Transformation of c-oriented nanowall network to a flat morphology in GaN films on c-plane sapphire

The work significantly optimizes growth parameters for nanostructured and flat GaN film in the 480–830 °C temperature range. The growth of ordered, high quality GaN nanowall hexagonal honeycomb like network on c-plane sapphire under nitrogen rich (N/Ga ratio of 100) conditions at temperatures below 700 °C is demonstrated. The walls are c-oriented wurtzite structures 200 nm wide at base and taper to 10 nm at apex, manifesting electron confinement effects to tune optoelectronic properties. For substrate temperatures above 700 °C the nanowalls thicken to a flat morphology with a dislocation density of 10¹⁰/cm². The role of misfit dislocations in the GaN overlayer evolution is discussed in terms of growth kinetics being influenced by adatom diffusion, interactions and bonding at different temperatures. The GaN films are characterized by reflection high energy electron diffraction (RHEED), field emission scanning electron (FESEM), high resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL).     

Size-induced changes in optical and X-ray photoelectron spectra of GaN nanoparticles deposited at lower substrate temperature

This study reports the synthesis of GaN nanoparticles having hexagonal structure by a simple technique of activated reactive evaporation with substrates kept at comparatively lower temperatures than usually reported. By varying the substrate temperature from 30 degrees C to 350 degrees C, it is possible to vary nanoparticle sizes from 5-30 nm. X-ray diffraction and X-ray photoelectron spectroscopy analysis confirm the formation of GaN on quartz and silicon substrates at room temperature. The observed size dependent shift in energy position, large increase in full width at half maximum value of Ga 3d and N 1s X-ray photoelectron spectroscopy peaks and blue shift in the optical absorption edge are related to nanoparticle character.     

Studies of interaction of amines with TOPO/TOP capped CdSe quantum dots: Role of crystallite size and oxidation potential

This work reports the interaction of aliphatic (triethyl amine, butyl amine) and aromatic amines (PPD, aniline) with CdSe quantum dots of varied sizes. The emission properties and lifetime values of CdSe quantum dots were found to be dependent on the oxidation potential of amines and crystallite sizes. Smaller CdSe quantum dots (size 5 nm) ensure better surface coverage of amines and hence higher quenching efficiency of amines could be realized as compared to larger CdSe quantum dots (size 14 nm). Heterogeneous quenching of amines due to the presence of accessible and inaccessible set of CdSe fluorophores is indicated. PPD owing to its lowest oxidation potential (0.26 V) has been found to have higher quenching efficiency as compared to other amines TEA and aniline having oxidation potentials 0.66 and >1.0 V, respectively. Butyl amine on the other hand, plays a dual role: its post-addition acts as a quencher for smaller and enhances emission for larger CdSe quantum dots, respectively. The beneficial effect of butyl amine in enhancing emission intensity could be attributed to enhance capping effect and better passivation of surface-traps.     

Structural transition in nanostructured Eu2O3 under high pressures

We report here studies on the effect of high pressure on the structural properties of nano-sized Europium sesquioxide (Eu2O3) up to a pressure of about 16.4 GPa. At ambient conditions, the starting sample was found to be predominantly cubic type Eu2O3 or in Eu3+ state with a trace of Eu2+. The presence of Eu2+ state is assumed to be arising due to the non-stoichiometric Eu(1-x)O phase which is obtained from XPS studies by the deconvolution of the Eu 3d-core levels. The Raman studies at ambient show a strong peak at about 333 cm(-1), which is known to occur due to the Fg mode of cubic Eu2O3 and in a similar way, the XRD data shows major peaks corresponding to the cubic phase of Eu2O3. A Mao-Bell type diamond anvil cell (DAC) was used to generate high pressures for XRD and Raman spectroscopy studies. It was observed that the material undergoes a structural change from cubic to monoclinic structure with an on set transition pressure at around 2 GPa and completes at around 8 GPa. This has been inferred from the fact that above about 2.0 GPa pressure, Raman studies show the emergence of a new peak corresponding to the monoclinic phase which increases in intensity and shifts further with increase in pressure, while the XRD studies show that above about 2.0 GPa, the peaks corresponding to monoclinic phase emerge, which show a slight increase in preferred orientation as the pressure is increased. A detailed discussion has been provided to explain this fact.     

Geometric and electronic changes during interface alloy formation in Cu/Pd bimetal layers

This study involves monitoring the interface evolution with increasing annealing temperatures in a Cu-Pd bimetal layer structure. The changes due to interdiffusion and ensuing charge transfer are monitored by extensive X-ray photoelectron spectroscopy (XPS) and glancing angle X-ray diffraction (GAXRD) studies. The Pd and Cu core level fingerprinting and change in lattice parameter provide evidence for alloy formation. The changes in the valence band features indicate the formation of new states that are different from the density of states of individual metal surfaces. The study demonstrates the possibility of tuning interface properties by alloy formation that may have specific applications in catalysis, hydrogen sensing and storage.     

Stability and Hydrogenation of “Bare” Gadolinium Nanoparticles

Gadolinium nanoparticles, deposited via an inert gas evaporation method, show improved stability towards oxidation and it is therefore possible to carry out an ex‐situ investigation on “bare” Gd nanoparticles, i.e., in the absence of a protective Pd layer, for the first time. A size‐induced structural transformation from hexagonal close packing to the higher‐symmetry face‐centered cubic structure is observed. The important observation of hydrogen–Gd‐nanoparticle interaction at room temperature and atmospheric pressure, without a Pd catalytic layer, makes Gd nanoparticles a potential candidate for hydrogen‐sensing, switching, and storage applications.     

Some transport properties of palladium films

The effect of deposition rate and substrate temperature on the electrical resistivity (), temperature coefficient of resistance (TCR) and thermolelectric power (TEP) of palladium films, in the thickness range 2 to 25 nm, is found to be marked. Higher rates of deposition and substrate temperatures are found to result in larger grains and hence changes in transport properties. The Fuchs-Sondheimer theory is used to explain the size effect in palladium films while the Mayadas-Shatzkes and Meyer relations are employed to study these effects on, TCR and TEP.