Hemen Kalita

Work function modulation and thermal stability of reduced graphene oxide gate electrodes in MOS devices.

Work function (WF) tuning of the contact electrodes is a key requirement in several device technologies, including organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and complementary metal oxide semiconductor (CMOS) transistors. Here, we demonstrate that the WF of the gate electrode in an MOS structure can be modulated from 4.35 eV (n-type metal) to 5.28 eV (p-type metal) by sandwiching different thicknesses of reduced graphene oxide (rGO) layers between top contact metals and gate dielectric SiO2. The WF of the gate electrode shows strong dependence on the rGO thickness and is seen to be nearly independent of the contact metals used. The observed WF modulation is attributed to the different amounts of oxygen concentrations in different thicknesses of rGO layers. Importantly, this oxygen concentration can also be varied by the reduction extent of the graphene oxide as experimentally demonstrated. The results are verified by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses. The obtained WF values are thermally stable up to 800 °C. At further high temperatures, diffusion of metal through the rGO sheets is the main cause for WF instability, as confirmed by cross-sectional high-resolution transmission electron microscopy analysis. These findings are not limited to MOS devices, and the WF modulation technique has the potential for applications in other technologies such as OLEDs and OPVs involving graphene as conducting electrodes.

Hysteresis and charge trapping in graphene quantum dots

We report current hysteresis in response to applied voltage in graphene quantum dots of average diameter 4.5 ± 0.55 nm, synthesized electrochemically using multiwalled carbon nanotubes. In response to step voltages, transient current decay, characteristic of deep and shallow level charge traps with time constants 186 ms and 6 s, is observed. Discharging current transients indicate charge storage of the order of 100 μC. Trap states are believed to arise due to the fast physisorption of external adsorbates, which are found to have a significant effect on charge transport and changes the resistance of the prepared device by an order of 3.

Work function tuning and improved gate dielectric reliability with multilayer graphene as a gate electrode for metal oxide semiconductor field effect device applications

Graphene with varying number of layers is explored as metal gate electrode in metal oxide semiconductor structure by inserting it between the dielectric (SiO2) and contact metal (TiN) and results are compared with TiN gate electrode. We demonstrate an effective work function tuning of gate electrode upto 0.5 eV by varying the number of graphene layers. Inclusion of even 1-3 layers of graphene results in significantly improved dielectric reliability as measured by breakdown characteristics, charge to breakdown, and interface state density. These improvements are attributed to the impermeability of graphene for TiN and hence reduced metallic contamination in the dielectric.

Photophysical and photoconductivity properties of thiol-functionalized graphene–CdSe QD composites

Graphene–semiconductor QD hybrid nanostructure materials have recently emerged as a new class of functional materials because of their potential applications in solar energy conversion, optoelectronic devices, sensing etc. Here, oleic acid-capped CdSe QDs are attached to –PhSH functionalized graphene by ligand exchange via bonding with the –SH group. The shifting of the G-band and D-band due to structural changes for the attachment of QD with graphene has been evaluated by using Raman spectroscopy. Steady state photoluminescence (PL) and time resolved fluorescence measurements have been employed to understand the electronic interactions between graphene and CdSe QDs. A time resolved fluorescence spectroscopic study has been used to understand the fluorescence dynamics of the photoexcitated CdSe QDs in the presence of graphene. It is evident that the electron transfer occurs from photoexcited QDs to graphene and the electron transfer rate is found to be 12.8 × 108 s−1 for 3.8 nm CdSe QDs. Photoconductivity properties of the graphene–QD device under illumination have been examined and it is to be noted that 2–3 fold increase in the photocurrent is found in this composite device in presence of 1.5 AM solar simulated light. The enhancement of the photocurrent in this hybrid device is found to be suitable for potential applications in optoelectronic and solar cell systems.

Multilayer Graphene as Charge Storage Layer in Floating Gate Flash Memory

Charge storage capability of multilayer graphene (MLG) in floating gate flash memory structure is demonstrated. MLG sheets are considered for this purpose because of the higher work function and higher density of states compared to single layer graphene (SLG) and lower conductivity along c-axis. A memory window of 6.8V for 1 second programming is obtained at ±18V program/erase voltage. Number of electrons stored in MLG sheets after 18V programming voltage is calculated as 9.1 × 1012 cm-2 which is higher than the density of states in SLG, suggesting the suitability of MLG for multi level data storage flash memory devices.

Reduced Multilayer Graphene Oxide Floating Gate Flash Memory With Large Memory Window and Robust Retention Characteristics

Reduced multilayer graphene (rMLG) is investigated as a charge storage layer (CSL) in conventional floating gate (FG) flash memory structure. A large memory window of 9.4 V at ±20-V program/erase and robust 10-years data retention at 150°C is demonstrated. Significant over-erase observed in these memory devices signifies hole storage in the rMLG sheets. Fast programming and clear saturation of the program transients observed with the rMLG CSL memory devices suggest reduced ballistic transport in the plane perpendicular to the graphene. Activation energies for programmed and erased state retention losses are calculated as 1.05 and 1.11 eV, respectively. These observations establish the potential of rMLG sheets as a replacement of conventionally used polycrystalline silicon FG.

Efficient synthesis of rice based graphene quantum dots and their fluorescent properties

We present a facile green approach to synthesize monodisperse graphene quantum dots (GQDs) of sizes 2–6.5 nm using rice grains as a carbon source. As the size of the GQDs increases from 2–6.5 nm, a red shift (blue to cyan) in the photoluminescence emission spectra is observed due to quantum confinement effect. The colloidal solution of as synthesized GQDs is highly luminescent under 336 nm illumination. The quantum yield (QY) of the as-prepared GQDs in water is size dependent and increases from 16 to 24% with the decrease in size from 6.5 to 2 nm. The potential of these GQDs as biomarkers for cell imaging is explored further. The cytoxicity study with different concentrations of the GQDs confirms the excellent biocompatibility of the GQDs.

Field effect transport properties of chemically treated graphene quantum dots

We report the field effect properties of lithographically fabricated FET with as-prepared graphene quantum dots (GQDs) and hydrazine treated GQDs as channel material. GQDs of 4.5 ± 0.55 nm average diameter are synthesised via an electrochemical approach using multiwalled carbon nanotubes (MWCNTs) as precursor. After treatment in hydrazine vapour for 24 h, the field effect measurements yield hole mobility of 0.01 cm 2 V –1 s –1 and I on / I off ratio of about 45. Hydrazine treated channel shows a significant decrease in resistance in comparison to the channel with as-prepared GQDs and is p-type under ambient conditions.

Field effect transport properties of electrochemically prepared graphene quantum dots

Herein we report the field effect properties of lithographically fabricated FET with graphene quantum dots (GQDs) as channel. GQDs are synthesized via an electrochemical avenue using multiwall carbon nanotubes. As-prepared dots of 4.5±0.55 nm average diameter are found to be p-type in nature under ambient conditions. Field effect measurements yield hole mobility of 0.01 cm² V^-1s^-1 and I_on/I_off ratio of 45. After annealing of devices in Argon atmosphere at 300°C for 20 min, the channel is found to show ambipolar transport with significant increase in resistance.

The exclusive response of LSPR in uncapped gold nanoparticles towards silver ions and gold chloride ions

We report a significant modulation of the plasmon peak of a bare gold nanoparticle system after treatment with Ag+ or [AuCl4]− ions. The shift is highly selective to the presence of Ag+/[AuCl4]− metal ions and is not observed when other ions (Cu2+, Cd2+, Ce2+, Hg2+, Na+ and K+) are tested. The value of the red shift depends on the concentration of ions. As compared to Ag+ ions, the shift is larger in the presence of [AuCl4]− ions. We attribute the red-shift in plasmon to the formation of atomic clusters of Ag or Au and a change in relaxation times of hot electrons at the gold nanoparticle surface after treatment with Ag+ or [AuCl4]− ions. The red-shift is also sensitive to capping on gold nanoparticles, and silver ions do not shift the LSPR when gold nanoparticles are capped with citrate. The study of the effect of ions on the plasmon of AuNPs is significant from the viewpoint of designing highly selective AuNP based gold and silver ion sensors. The role of charge transfer and changes in electron dynamics, which lead to shifting of the localized surface plasmon resonance in AuNPs, has been exemplified as well. The versatility of the sensing approach has been tested in both solution-based and substrate-based configurations.