Biddut K. Sarker

Position dependent photodetector from large area reduced graphene oxide thin films

We fabricated large area infrared photodetector devices from thin film of chemically reduced graphene oxide (RGO) sheets and studied their photoresponse as a function of laser position. We found that the photocurrent either increases, decreases, or remain almost zero depending upon the position of the laser spot with respect to the electrodes. The position sensitive photoresponse is explained by Schottky barrier modulation at the RGO film-electrode interface. The time response of the photocurrent is dramatically slower than single sheet of graphene possibly due to disorder from the chemical synthesis and interconnecting sheets.

Semiconducting enriched carbon nanotube aligned arrays of tunable density and their electrical transport properties.

We demonstrate assembly of solution-processed semiconducting enriched (99%) single-walled carbon nanotubes (s-SWNTs) in an array with varying linear density via ac dielectrophoresis (DEP) and investigate detailed electronic transport properties of the fabricated devices. We show that (i) the quality of the alignment varies with frequency of the applied voltage and that (ii) by varying the frequency and concentration of the solution, we can control the linear density of the s-SWNTs in the array from 1/μm to 25/μm. The DEP assembled s-SWNT devices provide the opportunity to investigate the transport property of the arrays in the direct transport regime. Room temperature electron transport measurements of the fabricated devices show that with increasing nanotube density the device mobility increases while the current on-off ratio decreases dramatically. For the dense array, the device current density was 16 μA/μm, on-conductance was 390 μS, and sheet resistance was 30 kΩ/◻. These values are the best reported so far for any semiconducting nanotube array.

Thermionic Emission and Tunneling at Carbon Nanotube–Organic Semiconductor Interface

We study the charge carrier injection mechanism across the carbon nanotube (CNT)-organic semiconductor interface using a densely aligned carbon nanotube array as electrode and pentacene as organic semiconductor. The current density-voltage (J-V) characteristics measured at different temperatures show a transition from a thermal emission mechanism at high temperature (above 200 K) to a tunneling mechanism at low temperature (below 200 K). A barrier height of ∼0.16 eV is calculated from the thermal emission regime, which is much lower compared to the metal/pentacene devices. At low temperatures, the J-V curves exhibit a direct tunneling mechanism at low bias, corresponding to a trapezoidal barrier, while at high bias the mechanism is well described by Fowler-Nordheim tunneling, which corresponds to a triangular barrier. A transition from direct tunneling to Fowler-Nordheim tunneling further signifies a small injection barrier at the CNT/pentacene interface. Our results presented here are the first direct experimental evidence of low charge carrier injection barrier between CNT electrodes and an organic semiconductor and are a significant step forward in realizing the overall goal of using CNT electrodes in organic electronics.

Fabrication of organic field effect transistor by directly grown poly(3 hexylthiophene) crystalline nanowires on carbon nanotube aligned array electrode.

We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.13 cm(2)/(V s) and 3.1 × 10(5), respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect, which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline nanowire structure.

Diffusion mediated photoconduction in multiwalled carbon nanotube films

We present a near infrared photoresponse study of large area multiwalled carbon nanotube (MWNT) films with different electrode separations. We show that the photocurrent strongly depends on the position of the laser spot with maximum response occurring at the metal-film interface. The time constant of dynamic photoresponse is slow and increases with increasing electrode separations. The photoconduction mechanism can be explained by the Schottky barrier modulation at the metal-nanotube film interface and charge carrier diffusion through percolating MWNT networks.

High-performance short channel organic transistors using densely aligned carbon nanotube array electrodes

We report high-performance short channel pentacene field effect transistor (FET) using carbon nanotube aligned array electrodes. The devices show field effect mobility of up to 0.65 cm2/Vs and current on-off ratio of up to 1.7 × 106, which is the best for sub-micron pentacene FETs. The calculated cutoff frequency (fc) of the devices is up to 211 MHz which is among the best reported fc for organic transistors. The high performance of our short channel FET is attributed to improved charge injections from the aligned array carbon nanotube electrodes into the pentacene.

The effect of carbon nanotube/organic semiconductor interfacial area on the performance of organic transistors

We show that the performance of pentacene transistors can be significantly improved by maximizing the interfacial area at single walled carbon nanotube (SWCNT)/pentacene. The interfacial areas are varied by anchoring short SWCNTs of different densities (0–30/μm) to the Pd electrodes. The average mobility is increased three, six, and nine times for low, medium, and high SWCNT densities, respectively, compared to the devices with zero SWCNT. The current on-off ratio and on-current are increased up to 40 times and 20 times with increasing the SWCNT density. We explain the improved device performance using reduced barrier height of SWCNT/pentacene interface.

Position sensitivity of graphene field effect transistors to X-rays

Device architectures that incorporate graphene to realize detection of electromagnetic radiation typically utilize the direct absorbance of radiation by graphene. This limits their effective area to the size of the graphene and their applicability to lower-energy, less penetrating forms of radiation. In contrast, graphene-based transistor architectures that utilize the field effect as the detection mechanism can be sensitive to interactions of radiation not only with graphene but also with the surrounding substrate. Here, we report the study of the position sensitivity and response of a graphene-based field effect transistor (GFET) to penetrating, well-collimated radiation (micro-beam X-rays), producing ionization in the substrate primarily away from graphene. It is found that responsivity and response speed are strongly dependent on the X-ray beam distance from graphene and the gate voltage applied to the GFET. To develop an understanding of the spatially dependent response, a model is developed that inc...

Position-dependent and millimetre-range photodetection in phototransistors with micrometre-scale graphene on SiC

The extraordinary optical and electronic properties of graphene make it a promising component of high-performance photodetectors. However, in typical graphene-based photodetectors demonstrated to date, the photoresponse only comes from specific locations near graphene over an area much smaller than the device size. For many optoelectronic device applications, it is desirable to obtain the photoresponse and positional sensitivity over a much larger area. Here, we report the spatial dependence of the photoresponse in backgated graphene field-effect transistors (GFET) on silicon carbide (SiC) substrates by scanning a focused laser beam across the GFET. The GFET shows a nonlocal photoresponse even when the SiC substrate is illuminated at distances greater than 500 µm from the graphene. The photoresponsivity and photocurrent can be varied by more than one order of magnitude depending on the illumination position. Our observations are explained with a numerical model based on charge transport of photoexcited carriers in the substrate.

Substrate-Induced Photofield Effect in Graphene Phototransistors

A single atomic layer of graphene, integrated onto an undoped bulk substrate in a back-gated transistor configuration, demonstrates surprising strong photoconduction, and yet, the physical origin of the photoresponse is not fully understood. Here, we use a detailed computational model to demonstrate that the photoconductivity arises from the electrostatic doping of graphene, induced by the surface accumulation of photogenerated carriers at the graphene/substrate interface. The accumulated charge density depends strongly on the rate of charge transfer between the substrate and the graphene; the suppression of the transfer rate below that of carriers thermal velocity is an essential prerequisite for a substantial photoinduced doping in the graphene channel under this mechanism. The contact-to-graphene coupling (defined by the ratio of graphene-metal contact capacitance to graphenes quantum capacitance) determines the magnitude of photoinduced doping in graphene at the source/drain contacts. High-performance graphene phototransistors would, therefore, require careful engineering of the graphene-substrate interface and optimization of graphene-metal contacts.