Chandan Biswas

Layer-by-layer doping of few-layer graphene film.

We propose a new method of layer-by-layer (LbL) doping of thin graphene films. Large area monolayer graphene was synthesized on Cu foil by using the chemical vapor deposition method. Each layer was transferred on a polyethylene terephthalate substrate followed by a salt-solution casting, where the whole process was repeated several times to get LbL-doped thin layers. With this method, sheet resistance was significantly decreased up to approximately 80% with little sacrifice in transmittance. Unlike samples fabricated by topmost layer doping, our sample shows better environmental stability due to the presence of dominant neutral Au atoms on the surface which was confirmed by angle-resolved X-ray photoelectron spectroscopy. The sheet resistance of the LbL-doped four-layer graphene (11 x 11 cm(2)) was 54 Omega/sq at 85% transmittance, which meets the technical target for industrial applications.

Chemically Doped Random Network Carbon Nanotube p–n Junction Diode for Rectifier

Semiconductors with higher carrier mobility and carrier density are required to fabricate a p-n junction diode for high-speed device operation and high-frequency signal processing. Here, we use a chemically doped semiconducting single-walled carbon nanotube (SWCNT) random network for a field effect transistor (FET) and demonstrate a rectifier operated at a wide range of frequencies by fabricating a p-n junction diode. The p-n diode was fabricated by using a pristine p-type SWCNT-FET where half was covered by SiO(2) and the other half was chemically doped by using benzyl viologen molecules, which was converted into an n-type channel. The half-wave rectifier of the random network SWCNT p-n junction diode clearly highlights the device operation under high input signal frequencies up to 10 MHz with very low output distortion, which a commercial silicon p-n junction diode cannot access. These results indicate that the random network SWCNT p-n junction diodes can be used as building blocks of complex circuits in a range of applications in microelectronics, optoelectronics, sensors, and other systems.

Negative and Positive Persistent Photoconductance in Graphene

Persistent photoconductance, a prolonged light-induced conducting behavior that lasts several hundred seconds, has been observed in semiconductors. Here we report persistent negative photoconductance and consecutive prominent persistent positive photoconductance in graphene. Unusually large yields of negative PC (34%) and positive PC (1652%) and remarkably long negative transient response time (several hours) were observed. Such high yields were reduced in multilayer graphene and were quenched under vacuum conditions. Two-dimensional metallic graphene strongly interacts with environment and/or substrate, causing this phenomenon, which is markedly different from that in three-dimensional semiconductors and nanoparticles.

Atmospheric and Aqueous Deposition of Polycrystalline Metal Oxides Using Mist-CVD for Highly Efficient Inverted Polymer Solar Cells.

Large scale, cost-effective processing of metal oxide thin films is critical for the fabrication of many novel thin film electronics. To date, however, most of the reported solution-based techniques require either extended thermal anneals or additional synthetic steps. Here we report mist chemical vapor deposition as a solution-based, readily scalable, and open-air method to produce high-quality polycrystalline metal oxide thin films. Continuous, smooth, and conformal deposition of metal oxide thin films is achieved by tuning the solvent chemistry of Leidenfrost droplets to promote finer control over the surface-local dissociation process of the atomized zinc-bearing precursors. We demonstrate the deposited ZnO as highly efficient electron transport layers for inverted polymer solar cells to show the power of the approach. A highest efficiency of 8.7% is achieved with a fill factor of 73%, comparable to that of conventional so-gel ZnO, which serves as an indication of the efficient vertical transport and electron collection achievable using this material.

Determining the Fermi level by absorption quenching of monolayer graphene by charge transfer doping

While optical properties of graphene in the visible region are solely defined by the frequency-independent fine structure constant, an onset of absorption has been observed in the infrared region due to Pauli blocking of interband transitions. Here, we report a complete absorption quenching in the infrared region by coating graphene with bis(trifluoromethanesulfonyl)amine (TFSA), an optically transparent p-type chemical dopant. The Fermi level downshift due to TFSA doping results in enhanced transmission in the infrared region proportional to the doping concentration. An absorption quenching onset method, developed in our work, to extract the Fermi level shift in pristine and doped graphene agrees with values extracted from Raman G-band and 2D-band shifts, Hall measurements and the binding energy shift observed in X-ray photo-electron spectroscopy. Performing simple UV-visible transmittance spectroscopy to obtain the absorption quenching onset of graphene also allows detection of environmental and substrate effects via Fermi level shift. Our method opens up the practical implementation of this unique phenomenon of graphene in future optoelectronic devices.

Efficacious photocurrent generation and carrier transport by quantum dot decorated carbon nanotubes

CdSe/ZnS core/shell quantum dots have been decorated on thin multiwalled and singlewalled carbon nanotubes (CNTs) by chemical functionalization and substrate gate-bias control. CdSe quantum dots were negatively charged by adding mercaptoacetic acid (MAA). The silicon oxide substrate was decorated by octadecyltrichlorosilane (OTS) and converted to hydrophobic surface. The negatively charged CdSe/ZnS NCs were adsorbed on the SWCNT surface by applying the negative gate bias. The selective adsorption of CdSe/ZnS quantum dots on SWCNTs was confirmed by confocal laser scanning microscope. Quantum dots decorated carbon nanotubes have been used for effective photogenaration and carrier transport through the organic photovoltaic device which has fabricated using effective polymers. The results clearly indicate the efficient photocurrent generation and carrier transport which effectively increased the efficiency of the device for the next generation organic solar cell applications.

Layer-by-layer hybrid chemical doping for high transmittance uniformity in graphene-polymer flexible transparent conductive nanocomposite

A traditional transparent conducting film (TCF) such as indium tin oxide (ITO) exhibits poor mechanical flexibility and inconsistent transmittance throughout the UV-VIS-NIR spectrum. Recent TCFs like graphene films exhibit high sheet resistance (Rs) due to defect induced carrier scattering. Here we show a unique hybrid chemical doping method that results in high transmittance uniformity in a layered graphene-polymer nanocomposite with suppressed defect-induced carrier scattering. This layer-by-layer hybrid chemical doping results in low Rs (15 Ω/sq at >90% transmittance) and 3.6% transmittance uniformity (300–1000 nm) compared with graphene (17%), polymer (8%) and ITO (46%) films. The weak localization effect in our nanocomposite was reduced to 0.5%, compared with pristine (4.25%) and doped graphene films (1.2%). Furthermore, negligible Rs change (1.2 times compared to 12.6 × 103 times in ITO) and nearly unaltered transmittance spectra were observed up to 24 GPa of applied stress highlighting mechanical flexibility of the nanocomposite film.

Boosting photoresponse in silicon metal-semiconductor-metal photodetector using semiconducting quantum dots.

Silicon based metal-semiconductor-metal (MSM) photodetectors have faster photogeneration and carrier collection across the metal-semiconductor Schottky contacts, and CMOS integratibility compared to conventional p-n junction photodetectors. However, its operations are limited by low photogeneration, inefficient carrier-separation, and low mobility. Here, we show a simple and highly effective approach for boosting Si MSM photodetector efficiency by uniformly decorating semiconducting CdSe quantum dots on Si channel (Si-QD). Significantly higher photocurrent on/off ratio was achieved up to over 500 compared to conventional Si MSM photodetector (on/off ratio ~5) by increasing photogeneration and improving carrier separation. Furthermore, a substrate-biasing technique invoked wide range of tunable photocurrent on/off ratio in Si-QD photodetector (ranging from 2.7 to 562) by applying suitable combinations of source-drain and substrate biasing conditions. Strong photogeneration and carrier separation were achieved by employing Stark effect into the Si-QD hybrid system. These results highlight a promising method for enhancing Si MSM photodetector efficiency more than 100 times and simultaneously compatible with current silicon technologies.

Inverted polymer solar cells based on thin ZnO films grown by Mist chemical vapor deposition system

Extensive investigations have been conducted in order to synthesize high quality Zinc oxide (ZnO) thin films for numerous applications. These methods are either expensive to make or result polycrystalline thin films with low optoelectronic properties. Here we demonstrated a simple and inexpensive method to grow high quality ZnO thin films by a mist chemical vapor assisted depositing (Mist-CVD) system for inverted polymer solar cell (IPSC) application. The IPSC performance fabricated by Mist-CVD grown ZnO thin films were compared with two different Zn precursors (Zinc acetylacetonate hydrate and Zinc acetate dehydrate). Variations in IPSC performance on the growth temperature and growth time of the ZnO thin films were prominently demonstrated. The surface morphology of the ZnO films was investigated using scanning electron microscopy, atomic force microscopy and correlated with IPSC performance. The IPSC performance using two different precursors has been compared thoroughly. A 24% increase in solar cell efficiency (contributed from 21% increase in fill factor and 151% increase in shunt resistance) was achieved using Zinc acetate dehydrate compare to Zinc acetylacetonate hydrate precursor. The transmittance of ZnO thin films was evaluated by transmission spectroscopy. High performance IPSC can be fabricated using this simple and inexpensive method by synthesizing high quality thin ZnO films.

Highly concentrated diameter selective nanodispersion of single-walled carbon nanotubes in water

Nanodispersion of single-walled carbon nanotubes (SWCNTs) has been systematically investigated with the use of sodium dodecyl sulfate (SDS) and poly(vinylpyrrolidone) (PVP) surfactant in de-ionized water. A high concentration of nanodispersed SWCNTs up to 0.08 mg/mL was achieved with introduction of an additional dispersant of PVP by optimizing surfactant concentration, sonication time, and centrifugation speed, which was crucial to obtaining a high concentration of SWCNTs in the supernatant solution. We also demonstrate that diameters of the nanodispersed nanotubes can be sorted out by controlling the centrifugation speed and furthermore the saturated SWCNT concentration was nearly constant, independent of the initial concentration at high centrifugation speed. Two dispersion states were identified depending on the centrifugation speed: an intermediate dispersion of nanodispersion mixed with macrodispersion (I) and nanodispersion (II). This was verified by Raman spectroscopy, scanning probe microscopy, optical absorption spectroscopy, and photoluminescence measurements. The obtained SWCNT solution was stable up to about ten days. Some aggregated SWCNT solution after a long period of time was fully recovered to initial state of dispersion after re-sonication for a few minutes. Our systematic study on high concentration nanodispersion of SWCNTs with selective diameters provides an opportunity to extend the application areas of high quality SWCNTs in large quantity.